Posts Tagged ‘tight junctions’

Lesson 8 Cell Signaling and Motility: Lesson and Supplemental Information on Cell Junctions and ECM: #TUBiol3373

Curator: Stephen J. Williams, Ph.D.

Please click on the following link for the PowerPoint Presentation for Lecture 8 on Cell Junctions and the  Extracellular Matrix: (this is same lesson from 2018 so don’t worry that file says 2018)

cell signaling 8 lesson 2018


Some other reading on this lesson on this Open Access Journal Include:

On Cell Junctions:

Translational Research on the Mechanism of Water and Electrolyte Movements into the Cell     

(pay particular attention to article by Fischbarg on importance of tight junctions for proper water and electrolyte movement)

The Role of Tight Junction Proteins in Water and Electrolyte Transport

(pay attention to article of role of tight junction in kidney in the Loop of Henle and the collecting tubule)

EpCAM [7.4]

(a tight junction protein)

Signaling and Signaling Pathways

(for this lesson pay attention to the part that shows how Receptor Tyrosine Kinase activation (RTK) can lead to signaling to an integrin and also how the thrombin receptor leads to cellular signals both to GPCR (G-protein coupled receptors like the thrombin receptor, the ADP receptor; but also the signaling cascades that lead to integrin activation of integrins leading to adhesion to insoluble fibrin mesh of the newly formed clot and subsequent adhesion of platelets, forming the platelet plug during thrombosis.)

On the Extracellular Matrix

Three-Dimensional Fibroblast Matrix Improves Left Ventricular Function Post MI

Arteriogenesis and Cardiac Repair: Two Biomaterials – Injectable Thymosin beta4 and Myocardial Matrix Hydrogel



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Growth Factors, Suppressors and Receptors in Tumorigenesis

Writer and Curator: Larry H Bernstein, MD, FCAP

7.1 Growth Factors, Suppressors and Receptors in Tumorigenesis

7.1.1 Friend or Foe: Endoplasmic reticulum protein 29 (ERp29) in epithelial cancer

7.1.2 Putting together structures of epidermal growth factor receptors

7.1.3 Complex Relationship between Ligand Binding and Dimerization in the Epidermal Growth Factor Receptor

7.1.4 IGFBP-2.PTEN- A critical interaction for tumors and for general physiology

7.1.5 Emerging-roles-for-the-Ph-sensing-G-protein-coupled-receptor

7.1.6 Protein amino-terminal modifications and proteomic approaches for N-terminal profiling

7.1.7 Protein homeostasis networks in physiology and disease

7.1.8 Proteome sequencing goes deep

7.1.1 Friend or Foe: Endoplasmic reticulum protein 29 (ERp29) in epithelial cancer

Chen S1Zhang D2
FEBS Open Bio. 2015 Jan 30; 5:91-8

The endoplasmic reticulum (ER) protein 29 (ERp29) is a molecular chaperone that plays a critical role in protein secretion from the ER in eukaryotic cells. Recent studies have also shown that ERp29 plays a role in cancer. It has been demonstrated that ERp29 is inversely associated with primary tumor development and functions as a tumor suppressor by inducing cell growth arrest in breast cancer. However, ERp29 has also been reported to promote epithelial cell morphogenesis, cell survival against genotoxic stress and distant metastasis. In this review, we summarize the current understanding on the biological and pathological functions of ERp29 in cancer and discuss the pivotal aspects of ERp29 as “friend or foe” in epithelial cancer.

The endoplasmic reticulum (ER) is found in all eukaryotic cells and is complex membrane system constituting of an extensively interlinked network of membranous tubules, sacs and cisternae. It is the main subcellular organelle that transports different molecules to their subcellular destinations or to the cell surface [10,85].

The ER contains a number of molecular chaperones involved in protein synthesis and maturation. Of the ER chaperones, protein disulfide isomerase (PDI)-like proteins are characterized by the presence of a thioredoxin domain and function as oxido-reductases, isomerases and chaperones [33]. ERp29 lacks the active-site double-cysteine (CxxC) motif and does not belong to the redox-active PDIs [5,47]. ERp29 is recognized as a characterized resident of the cellular ER, and it is expressed ubiquitously and abundantly in mammalian tissues [50]. Protein structural analysis showed that ERp29 consists of N-terminal and C-terminal domains [5]: N-terminal domain involves dimerization whereas the C-terminal domain is essential for substrate binding and secretion [78]. The biological function of ERp29 in protein secretion has been well established in cells [8,63,67].

ERp9 is proposed to be involved in the unfolded protein response (UPR) as a factor facilitating transport of synthesized secretory proteins from the ER to Golgi [83]. The expression of ERp29 was demonstrated to be increased in cells exposed to radiation [108], sperm cells undergoing maturation [42,107], and in certain cell types both under the pharmacologically induced UPR and under the physiological conditions (e.g., lactation, differentiation of thyroid cells) [66,82]. Under ER stress, ERp29 translocates the precursor protein p90ATF6 from the ER to Golgi where it is cleaved to be a mature and active form p50ATF by protease (S1P and S2P) [48]. In most cases, ERp29 interacts with BiP/GRP78 to exert its function under ER stress [65].

ERp29 is considered to be a key player in both viral unfolding and secretion [63,67,77,78] Recent studies have also demonstrated that ERp29 is involved in intercellular communication by stabilizing the monomeric gap junction protein connexin43 [27] and trafficking of cystic fibrosis transmembrane conductance regulator to the plasma membrane in cystic fibrosis and non-cystic fibrosis epithelial cells [90]. It was recently reported that ERp29 directs epithelial Na(+) channel (ENaC) toward the Golgi, where it undergoes cleavage during its biogenesis and trafficking to the apical membrane [40]. ERp29 expression protects axotomized neurons from apoptosis and promotes neuronal regeneration [111]. These studies indicate a broad biological function of ERp29 in cells.

Recent studies demonstrated a tumor suppressive function of ERp29 in cancer. It was found that ERp29 expression inhibited tumor formation in mice [4,87] and the level of ERp29 in primary tumors is inversely associated with tumor development in breast, lung and gallbladder cancer [4,29].

However, its expression is also responsible for cancer cell survival against genotoxic stress induced by doxorubicin and radiation [34,76,109]. The most recent studies demonstrate other important roles of ERp29 in cancer cells such as the induction of mesenchymal–epithelial transition (MET) and epithelial morphogenesis [3,4]. MET is considered as an important process of transdifferentiation and restoration of epithelial phenotype during distant metastasis [23,52]. These findings implicate ERp29 in promoting the survival of cancer cells and also metastasis. Hence, the current review focuses on the novel functions of ERp29 and discusses its pathological importance as a “friend or foe” in epithelial cancer.

ERp29 regulates mesenchymal–epithelial transition

Epithelial–mesenchymal transition (EMT) and MET

The EMT is an essential process during embryogenesis [6] and tumor development [43,96]. The pathological conditions such as inflammation, organ fibrosis and cancer progression facilitate EMT [16]. The epithelial cells after undergoing EMT show typical features characterized as: (1) loss of adherens junctions (AJs) and tight junctions (TJs) and apical–basal polarity; (2) cytoskeletal reorganization and distribution; and (3) gain of aggressive phenotype of migration and invasion [98]. Therefore, EMT has been considered to be an important process in cancer progression and its pathological activation during tumor development induces primary tumor cells to metastasize [95]. However, recent studies showed that the EMT status was not unanimously correlated with poorer survival in cancer patients examined [92].

In addition to EMT in epithelial cells, mesenchymal-like cells have capability to regain a fully differentiated epithelial phenotype via the MET [6,35]. The key feature of MET is defined as a process of transdifferentiation of mesenchymal-like cells to polarized epithelial-like cells [23,52] and mediates the establishment of distant metastatic tumors at secondary sites [22]. Recent studies demonstrated that distant metastases in breast cancer expressed an equal or stronger E-cadherin signal than the respective primary tumors and the re-expression of E-cadherin was independent of the E-cadherin status of the primary tumors [58]. Similarly, it was found that E-cadherin is re-expressed in bone metastasis or distant metastatic tumors arising from E-cadherin-negative poorly differentiated primary breast carcinoma [81], or from E-cadherin-low primary tumors [25]. In prostate and bladder cancer cells, the nonmetastatic mesenchymal-like cells were interacted with metastatic epithelial-like cells to accelerate their metastatic colonization [20]. It is, therefore, suggested that the EMT/MET work co-operatively in driving metastasis.

Molecular regulation of EMT/MET

E-cadherin is considered to be a key molecule that provides the physical structure for both cell–cell attachment and recruitment of signaling complexes [75]. Loss of E-cadherin is a hallmark of EMT [53]. Therefore, characterizing transcriptional regulators of E-cadherin expression during EMT/MET has provided important insights into the molecular mechanisms underlying the loss of cell–cell adhesion and the acquisition of migratory properties during carcinoma progression [73].

Several known signaling pathways, such as those involving transforming growth factor-β (TGF-β), Notch, fibroblast growth factor and Wnt signaling pathways, have been shown to trigger epithelial dedifferentiation and EMT [28,97,110]. These signals repress transcription of epithelial genes, such as those encoding E-cadherin and cytokeratins, or activate transcription programs that facilitate fibroblast-like motility and invasion [73,97].

The involvement of microRNAs (miRNAs) in controlling EMT has been emphasized [11,12,18]. MiRNAs are small non-coding RNAs (∼23 nt) that silence gene expression by pairing to the 3′UTR of target mRNAs to cause their posttranscriptional repression [7]. MiRNAs can be characterized as “mesenchymal miRNA” and “epithelial miRNA” [68]. The “mesenchymal miRNA” plays an oncogenic role by promoting EMT in cancer cells. For instance, the well-known miR-21, miR-103/107 are EMT inducer by repressing Dicer and PTEN [44].

The miR-200 family has been shown to be major “epithelial miRNA” that regulate MET through silencing the EMT-transcriptional inducers ZEB1 and ZEB2 [13,17]. MiRNAs from this family are considered to be predisposing factors for cancer cell metastasis. For instance, the elevated levels of the epithelial miR-200 family in primary breast tumors associate with poorer outcomes and metastasis [57]. These findings support a potential role of “epithelial miRNAs” in MET to promote metastatic colonization [15].

ERp29 promotes MET in breast cancer

The role of ERp29 in regulating MET has been established in basal-like MDA-MB-231 breast cancer cells. It is known that myosin light chain (MLC) phosphorylation initiates to myosin-driven contraction, leading to reorganization of the actin cytoskeleton and formation of stress fibers [55,56]. ERp29 expression in this type of cells markedly reduced the level of phosphorylated MLC [3]. These results indicate that ERp29 regulates cortical actin formation through a mechanism involved in MLC phosphorylation (Fig. 1). In addition to the phenotypic change, ERp29 expression leads to: expression and membranous localization of epithelial cell marker E-cadherin; expression of epithelial differentiation marker cytokeratin 19; and loss of the mesenchymal cell marker vimentin and fibronectin [3] (Fig. 1). In contrast, knockdown of ERp29 in epithelial MCF-7 cells promotes acquisition of EMT traits including fibroblast-like phenotype, enhanced cell spreading, decreased expression of E-cadherin and increased expression of vimentin [3,4]. These findings further substantiate a role of ERp29 in modulating MET in breast cancer cells.

Fig. 1  ERp29 triggers mesenchymal–epithelial transition. Exogenous expression of ERp29 in mesenchymal MDA-MB-231 breast cancer cells inhibits stress fiber formation by suppressing MLC phosphorylation. In addition, the overexpressed ERp29 decreases the

ERp29 targets E-cadherin transcription repressors

The transcription repressors such as Snai1, Slug, ZEB1/2 and Twist have been considered to be the main regulators for E-cadherin expression [19,26,32]. Mechanistic studies revealed that ERp29 expression significantly down-regulated transcription of these repressors, leading to their reduced nuclear expression in MDA-MB-231 cells [3,4] (Fig. 2). Consistent with this, the extracellular signal-regulated kinase (ERK) pathway which is an important up-stream regulator of Slug and Ets1 was highly inhibited [4]. Apparently, ERp29 up-regulates the expressions of E-cadherin transcription repressors through repressing ERK pathway. Interestingly, ERp29 over-expression in basal-like BT549 cells resulted in incomplete MET and did not significantly affect the mRNA or protein expression of Snai1, ZEB2 and Twist, but increased the protein expression of Slug [3]. The differential regulation of these transcriptional repressors of E-cadherin by ERp29 in these two cell-types may occur in a cell-context-dependent manner.

Fig. 2  ERp29 decreases the expression of EMT inducers to promote MET. Exogenous expression of ERp29 in mesenchymal MDA-MB-231 breast cancer cells suppresses transcription and protein expression of E-cadherin transcription repressors (e.g., ZEB2, SNAI1 and Twist), ..

ERp29 antagonizes Wnt/ β-catenin signaling

Wnt proteins are a family of highly conserved secreted cysteine-rich glycoproteins. The Wnt pathway is activated via a binding of a family member to a frizzled receptor (Fzd) and the LDL-Receptor-related protein co-receptor (LRP5/6). There are three different cascades that are activated by Wnt proteins: namely canonical/β-catenin-dependent pathway and two non-canonical/β-catenin-independent pathways that include Wnt/Ca2+ and planar cell polarity [84]. Of note, the Wnt/β-catenin pathway has been extensively studied, due to its important role in cancer initiation and progression [79]. The presence of Wnt promotes formation of a Wnt–Fzd–LRP complex, recruitment of the cytoplasmic protein Disheveled (Dvl) to Fzd and the LRP phosphorylation-dependent recruitment of Axin to the membrane, thereby leading to release of β-catenin from membrane and accumulation in cytoplasm and nuclei. Nuclear β-catenin replaces TLE/Groucho co-repressors and recruits co-activators to activate expression of Wnt target genes. The most important genes regulated are those related to proliferation, such as Cyclin D1 and c-Myc [46,94], which are over-expressed in most β-catenin-dependent tumors. When β-catenin is absent in nucleus, the transcription factors T-cell factor/lymphoid enhancer factors (TCF/LEF) recruits co-repressors of the TLE/Groucho family and function as transcriptional repressors.

β-catenin is highly expressed in the nucleus of mesenchymal MDA-MB-231 cells. ERp29 over-expression in this type of cells led to translocation of nuclear β-catenin to membrane where it forms complex with E-cadherin [3] (Fig. 3). This causes a disruption of β-catenin/TCF/LEF complex and abolishes its transcription activity. Indeed, ERp29 significantly decreased the expression of cyclin D1/D2 [36], one of the downstream targets of activated Wnt/β-catenin signaling [94], indicating an inhibitory effect of ERp29 on this pathway. Meanwhile, expression of ERp29 in this cell type increased the nuclear expression of TCF3, a transcription factor regulating cancer cell differentiation while inhibiting self-renewal of cancer stem cells [102,106]. Hence, ERp29 may play dual functions in mesenchymal MDA-MB-231 breast cancer cells by: (1) suppressing activated Wnt/β-catenin signaling via β-catenin translocation; and (2) promoting cell differentiation via activating TCF3 (Fig. 3). Because β-catenin serves as a signaling hub for the Wnt pathway, it is particularly important to focus on β-catenin as the target of choice in Wnt-driven cancers. Though the mechanism by which ERp29 expression promotes the disassociation of β-catenin/TCF/LEF complex in MDA-MB-231 cells remains elusive, activating ERp29 expression may exert an inhibitory effect on the poorly differentiated, Wnt-driven tumors.

Fig. 3  ERp29 over-expression “turns-off” activated Wnt/β-catenin signaling. In mesenchymal MDA-MB-231 cells, high expression of nuclear β-catenin activates its downstream signaling involved in cell cycles and cancer stem cell

ERp29 regulates epithelial cell integrity

Cell adherens and tight junctions

Adherens junctions (AJs) and tight junctions (TJs) are composed of transmembrane proteins that adhere to similar proteins in the adjacent cell [69]. The transmembrane region of the TJs is composed mainly of claudins, tetraspan proteins with two extracellular loops [1]. AJs are mediated by Ca2+-dependent homophilic interactions of cadherins [71] which interact with cytoplasmic catenins that link the cadherin/catenin complex to the actin cytoskeleton [74].

The cytoplasmic domain of claudins in TJs interacts with occludin and several zona occludens proteins (ZO1-3) to form the plaque that associates with the cytoskeleton [99]. The AJs form and maintain intercellular adhesion, whereas the TJs serve as a diffusion barrier for solutes and define the boundary between apical and basolateral membrane domains [21]. The AJs and TJs are required for integrity of the epithelial phenotype, as well as for epithelial cells to function as a tissue [75].

The TJs are closely linked to the proper polarization of cells for the establishment of epithelial architecture[86]. During cancer development, epithelial cells lose the capability to form TJs and correct apico–basal polarity [59]. This subsequently causes the loss of contact inhibition of cell growth [91]. In addition, reduction of ZO-1 and occludin were found to be correlated with poorly defined differentiation, higher metastatic frequency and lower survival rates [49,64]. Hence, TJs proteins have a tumor suppressive function in cancer formation and progression.

Apical–basal cell polarity

The apical–basal polarity of epithelial cells in an epithelium is characterized by the presence of two specialized plasma membrane domains: namely, the apical surface and basolateral surface [30]. In general, the epithelial cell polarity is determined by three core complexes. These protein complexes include: (1) the partitioning-defective (PAR) complex; (2) the Crumbs (CRB) complex; and (3) the Scribble complex[2,30,45,51]. PAR complex is composed of two scaffold proteins (PAR6 and PAR3) and an atypical protein kinase C (aPKC) and is localized to the apical junction domain for the assembly of TJs [31,39]. The Crumbs complex is formed by the transmembrane protein Crumbs and the cytoplasmic scaffolding proteins such as the homologue of Drosophila Stardust (Pals1) and Pals-associated tight junction protein (Patj) and localizes to the apical [38]. The Scribble complex is comprised of three proteins, Scribble, Disc large (Dlg) and Lethal giant larvae (Lgl) and is localized in the basolateral domain of epithelial cells [100].

Fig. 4  ERp29 regulates epithelial cell morphogenesis. Over-expression of ERp29 in breast cancer cells induces the transition from a mesenchymal-like to epithelial-like phenotype and the restoration of tight junctions and cell polarity. Up-regulation and membrane

The current data from breast cancer cells supports the idea that ERp29 can function as a tumor suppressive protein, in terms of suppression of cell growth and primary tumor formation and inhibition of signaling pathways that facilitate EMT. Nevertheless, the significant role of ERp29 in cell survival against drugs, induction of cell differentiation and potential promotion of MET-related metastasis may lead us to re-assess its function in cancer progression, particularly in distant metastasis. Hence, it is important to explore in detail the ERp29’s role in cancer as a “friend or foe” and to elucidate its clinical significance in breast cancer and other epithelial cancers. Targeting ERp29 and/or its downstream molecules might be an alternative molecular therapeutic approach for chemo/radio-resistant metastatic cancer treatment

7.1.2 Putting together structures of epidermal growth factor receptors

Bessman NJ1Freed DM2Lemmon MA3
Curr Opin Struct Biol. 2014 Dec; 29:95-101


  • Several studies suggest flexible linkage between extracellular and intracellular regions.
  • Others imply more rigid connections, required for allosteric regulation of dimers.
  • Interactions with membrane lipids play important roles in EGFR regulation.
  • Cellular studies suggest half-of-the-sites negative cooperativity for human EGFR.

Numerous crystal structures have been reported for the isolated extracellular region and tyrosine kinase domain of the epidermal growth factor receptor (EGFR) and its relatives, in different states of activation and bound to a variety of inhibitors used in cancer therapy. The next challenge is to put these structures together accurately in functional models of the intact receptor in its membrane environment. The intact EGFR has been studied using electron microscopy, chemical biology methods, biochemically, and computationally. The distinct approaches yield different impressions about the structural modes of communication between extracellular and intracellular regions. They highlight possible differences between ligands, and also underline the need to understand how the receptor interacts with the membrane itself.

7.1.3 Complex Relationship between Ligand Binding and Dimerization in the Epidermal Growth Factor Receptor

Bessman NJ1Bagchi A2Ferguson KM2Lemmon MA3.
Cell Rep. 2014 Nov 20; 9(4):1306-17.


  • Preformed extracellular dimers of human EGFR are structurally heterogeneous
  • EGFR dimerization does not stabilize ligand binding
  • Extracellular mutations found in glioblastoma do not stabilize EGFR dimerization
  • Glioblastoma mutations in EGFR increase ligand-binding affinity


The epidermal growth factor receptor (EGFR) plays pivotal roles in development and is mutated or overexpressed in several cancers. Despite recent advances, the complex allosteric regulation of EGFR remains incompletely understood. Through efforts to understand why the negative cooperativity observed for intact EGFR is lost in studies of its isolated extracellular region (ECR), we uncovered unexpected relationships between ligand binding and receptor dimerization. The two processes appear to compete. Surprisingly, dimerization does not enhance ligand binding (although ligand binding promotes dimerization). We further show that simply forcing EGFR ECRs into preformed dimers without ligand yields ill-defined, heterogeneous structures. Finally, we demonstrate that extracellular EGFR-activating mutations in glioblastoma enhance ligand-binding affinity without directly promoting EGFR dimerization, suggesting that these oncogenic mutations alter the allosteric linkage between dimerization and ligand binding. Our findings have important implications for understanding how EGFR and its relatives are activated by specific ligands and pathological mutations.

X-ray crystal structures from 2002 and 2003 (Burgess et al., 2003) yielded the scheme for ligand-induced epidermal growth factor receptor (EGFR) dimerization shown in Figure 1. Binding of a single ligand to domains I and III within the same extracellular region (ECR) stabilizes an “extended” conformation and exposes a dimerization interface in domain II, promoting self-association with a KD in the micromolar range (Burgess et al., 2003, Dawson et al., 2005, Dawson et al., 2007). Although this model satisfyingly explains ligand-induced EGFR dimerization, it fails to capture the complex ligand-binding characteristics seen for cell-surface EGFR, with concave-up Scatchard plots indicating either negative cooperativity (De Meyts, 2008, Macdonald and Pike, 2008) or distinct affinity classes of EGF-binding site with high-affinity sites responsible for EGFR signaling (Defize et al., 1989). This cooperativity or heterogeneity is lost when the ECR from EGFR is studied in isolation, as also described for the insulin receptor (De Meyts, 2008).



Figure 1

Structural View of Ligand-Induced Dimerization of the hEGFR ECR

(A) Surface representation of tethered, unliganded, sEGFR from Protein Data Bank entry 1NQL (Ferguson et al., 2003). Ligand-binding domains I and III are green and cysteine-rich domains II and IV are cyan. The intramolecular domain II/IV tether is circled in red.

(B) Hypothetical model for an extended EGF-bound sEGFR monomer based on SAXS studies of an EGF-bound dimerization-defective sEGFR variant (Dawson et al., 2007) from PDB entry 3NJP (Lu et al., 2012). EGF is blue, and the red boundary represents the primary dimerization interface.

(C) 2:2 (EGF/sEGFR) dimer, from PDB entry 3NJP (Lu et al., 2012), colored as in (B). Dimerization arm contacts are circled in red.

Here, we describe studies of an artificially dimerized ECR from hEGFR that yield useful insight into the heterogeneous nature of preformed ECR dimers and into the origins of negative cooperativity. Our data also argue that extracellular structures induced by ligand binding are not “optimized” for dimerization and conversely that dimerization does not optimize the ligand-binding sites. We also analyzed the effects of oncogenic mutations found in glioblastoma patients (Lee et al., 2006), revealing that they affect allosteric linkage between ligand binding and dimerization rather than simply promoting EGFR dimerization. These studies have important implications for understanding extracellular activating mutations found in EGFR/ErbB family receptors in glioblastoma and other cancers and also for understanding specificity of ligand-induced ErbB receptor heterodimerization

Predimerizing the EGFR ECR Has Modest Effects on EGF Binding

To access preformed dimers of the hEGFR ECR (sEGFR) experimentally, we C-terminally fused (to residue 621 of the mature protein) either a dimerizing Fc domain (creating sEGFR-Fc) or the dimeric leucine zipper from S. cerevisiae GCN4 (creating sEGFR-Zip). Size exclusion chromatography (SEC) and/or sedimentation equilibrium analytical ultracentrifugation (AUC) confirmed that the resulting purified sEGFR fusion proteins are dimeric (Figure S1). To measure KD values for ligand binding to sEGFR-Fc and sEGFR-Zip, we labeled EGF with Alexa-488 and monitored binding in fluorescence anisotropy (FA) assays. As shown in Figure 2A, EGF binds approximately 10-fold more tightly to the dimeric sEGFR-Fc or sEGFR-Zip proteins than to monomeric sEGFR (Table 1). The curves obtained for EGF binding to sEGFR-Fc and sEGFR-Zip showed no signs of negative cooperativity, with sEGFR-Zip actually requiring a Hill coefficient (nH) greater than 1 for a good fit (nH = 1 for both sEGFRWT and sEGFR-Fc). Thus, our initial studies argued that simply dimerizing human sEGFR fails to restore the negatively cooperative ligand binding seen for the intact receptor in cells.

One surprise from these data was that forced sEGFR dimerization has only a modest (≤10-fold) effect on EGF-binding affinity. Under the conditions of the FA experiments, isolated sEGFR (without zipper or Fc fusion) remains monomeric; the FA assay contains just 60 nM EGF, so the maximum concentration of EGF-bound sEGFR is also limited to 60 nM, which is over 20-fold lower than the KD for dimerization of the EGF/sEGFR complex (Dawson et al., 2005, Lemmon et al., 1997). This ≤10-fold difference in affinity for dimeric and monomeric sEGFR seems small in light of the strict dependence of sEGFR dimerization on ligand binding (Dawson et al., 2005,Lax et al., 1991, Lemmon et al., 1997). Unliganded sEGFR does not dimerize detectably even at millimolar concentrations, whereas liganded sEGFR dimerizes with KD ∼1 μM, suggesting that ligand enhances dimerization by at least 104– to 106-fold. Straightforward linkage of dimerization and binding equilibria should stabilize EGF binding to dimeric sEGFR similarly (by 5.5–8.0 kcal/mol). The modest difference in EGF-binding affinity for dimeric and monomeric sEGFR is also significantly smaller than the 40- to 100-fold difference typically reported between high-affinity and low-affinity EGF binding on the cell surface when data are fit to two affinity classes of binding site (Burgess et al., 2003, Magun et al., 1980).

Mutations that Prevent sEGFR Dimerization Do Not Significantly Reduce Ligand-Binding Affinity

The fact that predimerizing sEGFR only modestly increased ligand-binding affinity led us to question the extent to which domain II-mediated sEGFR dimerization is linked to ligand binding. It is typically assumed that the domain II conformation stabilized upon forming the sEGFR dimer in Figure 1C optimizes the domain I and III positions for EGF binding. To test this hypothesis, we introduced a well-characterized pair of domain II mutations into sEGFRs that block dimerization: one at the tip of the dimerization arm (Y251A) and one at its “docking site” on the adjacent molecule in a dimer (R285S). The resulting (Y251A/R285S) mutation abolishes sEGFR dimerization and EGFR signaling (Dawson et al., 2005, Ogiso et al., 2002). Importantly, we chose isothermal titration calorimetry (ITC) for these studies, where all interacting components are free in solution. Previous surface plasmon resonance (SPR) studies have indicated that dimerization-defective sEGFR variants bind immobilized EGF with reduced affinity (Dawson et al., 2005), and we were concerned that this reflects avidity artifacts, where dimeric sEGFR binds more avidly than monomeric sEGFR to sensor chip-immobilized EGF.

Surprisingly, our ITC studies showed that the Y251A/R285S mutation has no significant effect on ligand-binding affinity for sEGFR in solution (Table 1). These experiments employed sEGFR (with no Fc fusion) at 10 μM—ten times higher than KD for dimerization of ligand-saturated WT sEGFR (sEGFRWT) (KD ∼1 μM). Dimerization of sEGFRWT should therefore be complete under these conditions, whereas the Y251A/R285S-mutated variant (sEGFRY251A/R285S) does not dimerize at all (Dawson et al., 2005). The KD value for EGF binding to dimeric sEGFRWT was essentially the same (within 2-fold) as that for sEGFRY251A/R285S (Figures 2B and 2C; Table 1), arguing that the favorable Gibbs free energy (ΔG) of liganded sEGFR dimerization (−5.5 to −8 kcal/mol) does not contribute significantly (<0.4 kcal/mol) to enhanced ligand binding. …

Thermodynamics of EGF Binding to sEGFR-Fc

If there is no discernible positive linkage between sEGFR dimerization and EGF binding, why do sEGFR-Fc and sEGFR-Zip bind EGF ∼10-fold more strongly than wild-type sEGFR? To investigate this, we used ITC to compare EGF binding to sEGFR-Fc and sEGFR-Zip (Figures 3A and 3B ) with binding to isolated (nonfusion) sEGFRWT. As shown in Table 1, the positive (unfavorable) ΔH for EGF binding is further elevated in predimerized sEGFR compared with sEGFRWT, suggesting that enforced dimerization may actually impair ligand/receptor interactions such as hydrogen bonds and salt bridges. The increased ΔH is more than compensated for, however, by a favorable increase in TΔS. This favorable entropic effect may reflect an “ordering” imposed on unliganded sEGFR when it is predimerized, such that it exhibits fewer degrees of freedom compared with monomeric sEGFR. In particular, since EGF binding does induce sEGFR dimerization, it is clear that predimerization will reduce the entropic cost of bringing two sEGFR molecules into a dimer upon ligand binding, possibly underlying this effect.

Possible Heterogeneity of Binding Sites in sEGFR-Fc

Close inspection of EGF/sEGFR-Fc titrations such as that in Figure 3A suggested some heterogeneity of sites, as evidenced by the slope in the early part of the experiment. To investigate this possibility further, we repeated titrations over a range of temperatures. We reasoned that if there are two different types of EGF-binding sites in an sEGFR-Fc dimer, they might have different values for heat capacity change (ΔCp), with differences that might become more evident at higher (or lower) temperatures. Indeed, ΔCp values correlate with the nonpolar surface area buried upon binding (Livingstone et al., 1991), and we know that this differs for the two Spitz-binding sites in the asymmetric Drosophila EGFR dimer (Alvarado et al., 2010). As shown in Figure 3C, the heterogeneity was indeed clearer at higher temperatures for sEGFR-Fc—especially at 25°C and 30°C—suggesting the possible presence of distinct classes of binding sites in the sEGFR-Fc dimer. We were not able to fit the two KD values (or ΔH values) uniquely with any precision because the experiment has insufficient information for unique fitting to a model with four variables. Whereas binding to sEGFRWT could be fit confidently with a single-site binding model throughout the temperature range, enforced sEGFR dimerization (by Fc fusion) creates apparent heterogeneity in binding sites, which may reflect negative cooperativity of the sort seen with dEGFR. …

Ligand Binding Is Required for Well-Defined Dimerization of the EGFR ECR

To investigate the structural nature of the preformed sEGFR-Fc dimer, we used negative stain electron microscopy (EM). We hypothesized that enforced dimerization might cause the unliganded ECR to form the same type of loose domain II-mediated dimer seen in crystals of unliganded Drosophila sEGFR (Alvarado et al., 2009). When bound to ligand (Figure 4A), the Fc-fused ECR clearly formed the characteristic heart-shape dimer seen by crystallography and EM (Lu et al., 2010, Mi et al., 2011). Figure 4B presents a structural model of an Fc-fused liganded sEGFR dimer, and Figure 4C shows a calculated 12 Å resolution projection of this model. The class averages for sEGFR-Fc plus EGF (Figure 4A) closely resemble this model, yielding clear densities for all four receptor domains, arranged as expected for the EGF-induced domain II-mediated back-to-back extracellular dimer shown in Figure 1 (Garrett et al., 2002, Lu et al., 2010). In a subset of classes, the Fc domain also appeared well resolved, indicating that these particular arrangements of the Fc domain relative to the ECR represent highly populated states, with the Fc domains occupying similar positions to those of the kinase domain in detergent-solubilized intact receptors (Mi et al., 2011). …

Our results and those of Lu et al. (2012)) argue that preformed extracellular dimers of hEGFR do not contain a well-defined domain II-mediated interface. Rather, the ECRs in these dimers likely sample a broad range of positions (and possibly conformations). This conclusion argues against recent suggestions that stable unliganded extracellular dimers “disfavor activation in preformed dimers by assuming conformations inconsistent with” productive dimerization of the rest of the receptor (Arkhipov et al., 2013). The ligand-free inactive dimeric ECR species modeled by Arkhipov et al. (2013) in their computational studies of the intact receptor do not appear to be stable. The isolated ECR from EGFR has a very low propensity for self-association without ligand, with KD in the millimolar range (or higher). Moreover, sEGFR does not form a defined structure even when forced to dimerize by Fc fusion. It is therefore difficult to envision how it might assume any particular autoinhibitory dimeric conformation in preformed dimers. …

Extracellular Oncogenic Mutations Observed in Glioblastoma May Alter Linkage between Ligand Binding and sEGFR Dimerization

Missense mutations in the hEGFR ECR were discovered in several human glioblastoma multiforme samples or cell lines and occur in 10%–15% of glioblastoma cases (Brennan et al., 2013, Lee et al., 2006). Several elevate basal receptor phosphorylation and cause EGFR to transform NIH 3T3 cells in the absence of EGF (Lee et al., 2006). Thus, these are constitutively activating oncogenic mutations, although the mutated receptors can be activated further by ligand (Lee et al., 2006, Vivanco et al., 2012). Two of the most commonly mutated sites in glioblastoma, R84 and A265 (R108 and A289 in pro-EGFR), are in domains I and II of the ECR, respectively, and contribute directly in inactive sEGFR to intramolecular interactions between these domains that are thought to be autoinhibitory (Figure 5). Domains I and II become separated from one another in this region upon ligand binding to EGFR (Alvarado et al., 2009), as illustrated in the lower part of Figure 5. Interestingly, analogous mutations in the EGFR relative ErbB3 were also found in colon and gastric cancers (Jaiswal et al., 2013).

We hypothesized that domain I/II interface mutations might activate EGFR by disrupting autoinhibitory interactions between these two domains, possibly promoting a domain II conformation that drives dimerization even in the absence of ligand. In contrast, however, sedimentation equilibrium AUC showed that sEGFR variants harboring R84K, A265D, or A265V mutations all remained completely monomeric in the absence of ligand (Figure 6A) at a concentration of 10 μM, which is similar to that experienced at the cell surface (Lemmon et al., 1997). As with WT sEGFR, however, addition of ligand promoted dimerization of each mutated sEGFR variant, with KD values that were indistinguishable from those of WT. Thus, extracellular EGFR mutations seen in glioblastoma do not simply promote ligand-independent ECR dimerization, consistent with our finding that even dimerized sEGFR-Fc requires ligand binding in order to form the characteristic heart-shaped dimer. …

We suggest that domain I is normally restrained by domain I/II interactions so that its orientation with respect to the ligand is compromised. When the domain I/II interface is weakened with mutations, this effect is mitigated. If this results simply in increased ligand-binding affinity of the monomeric receptor, the biological consequence might be to sensitize cells to lower concentrations of EGF or TGF-α (or other agonists). However, cellular studies of EGFR with glioblastoma-derived mutations (Lee et al., 2006, Vivanco et al., 2012) clearly show ligand-independent activation, arguing that this is not the key mechanism. The domain I/II interface mutations may also reduce restraints on domain II so as to permit dimerization of a small proportion of intact receptor, driven by the documented interactions that promote self-association of the transmembrane, juxtamembrane, and intracellular regions of EGFR (Endres et al., 2013, Lemmon et al., 2014, Red Brewer et al., 2009).

Setting out to test the hypothesis that simply dimerizing the EGFR ECR is sufficient to recover the negative cooperativity lost when it is removed from the intact receptor, we were led to revisit several central assumptions about this receptor. Our findings suggest three main conclusions. First, we find that enforcing dimerization of the hEGFR ECR does not drive formation of a well-defined domain II-mediated dimer that resembles ligand-bound ECRs or the unliganded ECR from Drosophila EGFR. Our EM and SAXS data show that ligand binding is necessary for formation of well-defined heart-shaped domain II-mediated dimers. This result argues that the unliganded extracellular dimers modeled by Arkhipov et al. (2013)) are not stable and that it is improbable that stable conformations of preformed extracellular dimers disfavor receptor activation by assuming conformations that counter activating dimerization of the rest of the receptor. Recent work from the Springer laboratory employing kinase inhibitors to drive dimerization of hEGFR (Lu et al., 2012) also showed that EGF binding is required to form heart-shaped ECR dimers. These findings leave open the question of the nature of the ECR in preformed EGFR dimers but certainly argue that it is unlikely to resemble the crystallographic dimer seen for unligandedDrosophila EGFR (Alvarado et al., 2009) or that suggested by computational studies (Arkhipov et al., 2013).

This result argues that ligand binding is required to permit dimerization but that domain II-mediated dimerization may compromise, rather than enhance, ligand binding. Assuming flexibility in domain II, we suggest that this domain serves to link dimerization and ligand binding allosterically. Optimal ligand binding may stabilize one conformation of domain II in the scheme shown in Figure 1 that is then distorted upon dimerization of the ECR, in turn reducing the strength of interactions with the ligand. Such a mechanism would give the appearance of a lack of positive linkage between ligand binding and ECR dimerization, and a good test of this model would be to determine the high-resolution structure of a liganded sEGFR monomer (which we expect to differ from a half dimer). This model also suggests a mechanism for selective heterodimerization over homodimerization of certain ErbB receptors. If a ligand-bound EGFR monomer has a domain II conformation that heterodimerizes with ErbB2 in preference to forming EGFR homodimers, this could explain several important observations. It could explain reports that ErbB2 is a preferred heterodimerization partner of EGFR (Graus-Porta et al., 1997) and might also explain why EGF binds more tightly to EGFR in cells where it can form heterodimers with ErbB2 than in cells lacking ErbB2, where only EGFR homodimers can form (Li et al., 2012).

7.1.4 IGFBP-2.PTEN- A critical interaction for tumors and for general physiology

Li ZengClaire M. PerksJeff M.P. Holly
Growth Hormone & IGF Research online 7 February 2015


  • IGFBP-2 is the second most abundant of the IGFBPs in the circulation.
  • IGFBP2 levels are increased in a variety of tumors and associated with progression and poor prognosis.
  • PTEN is a phosphatase that returns the PI3K/AKT/mTOR pathway to its inactivated state.
  • PTEN is the second most commonly mutated gene in a variety of common cancers.
  • Recent evidence indicates that IGFBP-2 regulates PTEN in a variety of normal and malignant cell types.
  • This review summarizes the evidence that these extracellular and intracellular modulators of the IGF-system are linked.


IGFBP-2 is an important modulator of IGF availability and activity. It is the second most abundant of the IGFBPs in the circulation and its levels are increased in a variety of tumors and associated with progression and poor prognosis. PTEN is a phosphatase that returns the PI3K/AKT/mTOR pathway to its inactivated state and is therefore a critical modulator of one of the main intracellular signaling pathways activated by the IGFs. Recent evidence has indicated that IGFBP-2 regulates PTEN in a variety of normal and malignant cell types. This review summarizes the recent evidence that these extracellular and intracellular modulators are linked to provide a synchronous system for cell regulation with coordinated control of both the ‘accelerator’ and the ‘brake’.



7.1.5 Emerging-roles-for-the-Ph-sensing-G-protein-coupled-receptor

Sanderlin EJ, Justus CR, Krewson EA, Yang LV
CHC March 2015 Volume 2015:7 Pages 99—109

Protons (hydrogen ions) are the simplest form of ions universally produced by cellular metabolism including aerobic respiration and glycolysis. Export of protons out of cells by a number of acid transporters is essential to maintain a stable intracellular pH that is critical for normal cell function. Acid products in the tissue interstitium are removed by blood perfusion and excreted from the body through the respiratory and renal systems. However, the pH homeostasis in tissues is frequently disrupted in many pathophysiologic conditions such as in ischemic tissues and tumors where protons are overproduced and blood perfusion is compromised. Consequently, accumulation of protons causes acidosis in the affected tissue. Although acidosis has profound effects on cell function and disease progression, little is known about the molecular mechanisms by which cells sense and respond to acidotic stress. Recently a family of pH-sensing G protein-coupled receptors (GPCRs), including GPR4, GPR65 (TDAG8), and GPR68 (OGR1), has been identified and characterized. These GPCRs can be activated by extracellular acidic pH through the protonation of histidine residues of the receptors. Upon activation by acidosis the pH-sensing GPCRs can transduce several downstream G protein pathways such as the Gs, Gq/11, and G12/13 pathways to regulate cell behavior. Studies have revealed the biological roles of the pH-sensing GPCRs in the immune, cardiovascular, respiratory, renal, skeletal, endocrine, and nervous systems, as well as the involvement of these receptors in a variety of pathological conditions such as cancer, inflammation, pain, and cardiovascular disease. As GPCRs are important drug targets, small molecule modulators of the pH-sensing GPCRs are being developed and evaluated for potential therapeutic applications in disease treatment.

Cellular metabolism produces acid as a byproduct. Metabolism of each glucose molecule by glycolysis generates two pyruvate molecules. Under anaerobic conditions the metabolism of pyruvate results in the production of the glycolytic end product lactic acid, which has a pKa of 3.9. Lactic acid is deprotonated at the carboxyl group and results in one lactate ion and one proton at the physiological pH. Under aerobic conditions pyruvate is converted into acetyl-CoA and CO2 in the mitochondria. CO2in water forms a chemical equilibrium of carbonic acid and bicarbonate, an important physiological pH buffering system. The body must maintain suitable pH for proper physiological functions. Some regulatory mechanisms to control systemic pH are respiration, renal excretion, bone buffering, and metabolism.14 The respiratory system can buffer the blood by excreting carbonic acid as CO2 while the kidney responds to decreased circulatory pH by excreting protons and electrolytes to stabilize the physiological pH. Bone buffering helps maintain systemic pH by Ca2+ reabsorption and mineral dissolution. Collectively, it is clear that several biological systems require tight regulation to maintain pH for normal physiological functions. Cells utilize vast varieties of acid-base transporters for proper pH homeostasis within each biological context.58 Some such transporters are H+-ATPase, Na+/H+exchanger, Na+-dependent HCO3/C1 exchanger, Na+-independent anion exchanger, and monocarboxylate transporters. Cells can also maintain short-term pH homeostasis of the intracellular pH by rapid H+ consuming mechanisms. Some such mechanisms utilize metabolic conversions that move acids from the cytosol into organelles. Despite these cellular mechanisms that tightly maintain proper pH homeostasis, there are many diseases whereby pH homeostasis is disrupted. These pathological conditions are characterized by either local or systemic acidosis. Systemic acidosis can occur from respiratory, renal, and metabolic diseases and septic shock.14,9 Additionally, local acidosis is characterized in ischemic tissues, tumors, and chronically inflamed conditions such as in asthma and arthritis caused by deregulated metabolism and hypoxia.1015

Acidosis is a stress for the cell. The ability of the cell to sense and modulate activity for adaptation to the stressful environment is critical. There are several mechanisms whereby cells sense acidosis and modulate cellular functions to facilitate adaptation. Cells can detect extracellular pH changes by acid sensing ion channels (ASICs) and transient receptor potential (TRP) channels.16 Apart from ASIC and TRP channels, extracellular acidic pH was shown to stimulate inositol polyphosphate formation and calcium efflux.17,18 This suggested the presence of an unknown cell surface receptor that may be activated by a certain functional group, namely the imidazole of a histidine residue. The identity of the acid-activated receptor was later unmasked by Ludwig et al as a family of proton-sensing G protein-coupled receptors (GPCRs). This group identified human ovarian cancer GPCR 1 (OGR1) which upon activation will produce inositol phosphate and calcium efflux through the Gq pathway.19 These pH-sensing GPCR family members, including GPR4, GPR65 (TDAG8), and GPR68 (OGR1), will be discussed in this review (Figure 1). The proton-sensing GPCRs sense extracellular pH by protonation of several histidine residues on their extracellular domain. The activation of these proton-sensing GPCRs facilitates the downstream signaling through the Gq/11, Gs, and G12/13 pathways. Their expression varies in different cell types and play critical roles in sensing extracellular acidity and modulating cellular functions in several biological systems.

Figure 1 Biological roles and G protein coupling of the pH-sensing GPCRs

Biological roles and G protein coupling of the pH-sensing GPCRs

Biological roles and G protein coupling of the pH-sensing GPCRs

Cells encounter acidotic stress in many pathophysiologic conditions such as inflammation, cancer, and ischemia. Intricate molecular mechanisms, including a large array of acid/base transporters and acid sensors, have evolved for cells to sense and respond to acidotic stress. Emerging evidence has demonstrated that a family of the pH-sensing GPCRs can be activated by extracellular acidotic stress and regulate the function of multiple physiological systems (Table 1). The pH-sensing GPCRs also play important roles in various pathological disorders. Agonists, antagonists and other modulators of the pH-sensing GPCRs are being actively developed and evaluated as potential novel treatment for acidosis-related diseases.

Table 1 The main biological functions of the pH-sensing GPCRs
Table1 The main biological functions of the pH-sensing GPCRs

Table1 The main biological functions of the pH-sensing GPCRs

7.1.6 Protein amino-terminal modifications and proteomic approaches for N-terminal profiling

Lai ZW1Petrera A2Schilling O3.
Curr Opin Chem Biol. 2015 Feb; 24:71-9


  • N-terminal acetylation, pyroglutamate formation, N-degrons and proteolysis are reviewed.
  • N-terminomics provide comprehensive profiling of modification at protein N-termini in a proteome-wide manner.
  • We outline a number of established methodologies for the enrichment of protein N-termini through positive and negative selection strategies.
  • Peptidomics-based approach is beneficial for the study of post-translational processing of protein N-termini.

Amino-/N-terminal processing is a crucial post-translational modification affecting almost all proteins. In addition to altering the chemical properties of the N-terminus, these modifications affect protein activation, conversion, and degradation, which subsequently lead to diversified biological functions. The study of N-terminal modifications is of increasing interest; especially since modifications such as proteolytic truncation or pyroglutamate formation have been linked to disease processes. During the past decade, mass spectrometry has played an important role in facilitating the investigation of N-terminal modifications. Continuous progress is being made in the development and application of robust methods for the dedicated analysis of native and modified protein N-termini in a proteome-wide manner. Here we highlight recent progress in our understanding of protein N-terminal biology as well as outlining present enrichment strategies for mass spectrometry-based studies of protein N-termini

7.1.7 Protein homeostasis networks in physiology and disease

Claudio Hetz1,2,3,* and Laurie H. Glimcher3,4,*
Curr Opin Cell Biol. 2011 Apr; 23(2): 123–125.

Although most text books of biochemistry describe the process of protein folding to a three dimensional native state as an intrinsic property of the primary sequence, it is becoming increasingly clear that this process can go wrong in an almost infinite number of ways. In fact, many different diseases are caused by the misfolding and aggregation of certain proteins without genetic mutations in the primary sequence. An integrative view of the mechanisms that maintain protein folding homeostasis is emerging, which could be thought as a balanced and dynamic network of interconnected processes tightly regulated by a series of quality control mechanisms. This protein homeostasis network involves families of folding catalysts, co-factors under specific environmental and metabolic conditions. Maintaining protein homeostasis is particularly challenging in specialized secretory cells where the high demand for protein synthesis generates a constant source of stress that could lead to proteotoxicity.

Protein folding is assisted and monitored by diverse interconnected processes that follow a sequential pattern over time. The calnexin/calreticulin cycle ensures the proper folding of glycosylated proteins through the secretory pathway, which establishes the final pattern of disulfide bond formation through interactions with the disulfide isomerase ERp57. Coupled to this cycle is the ER-associated degradation (ERAD) pathway, which translocates terminally misfolded proteins to the cytosol for degradation by proteasomes. In addition, macroautophagy is becoming a relevant mechanism for the clearance of damaged proteins and abnormal protein aggregates through lysosomal hydrolysis, a process also referred to as ERAD-II. The folding status at the ER is constantly monitored by the Unfolded Protein Response (UPR), a specialized signaling pathway initiated by the activation of three types of stress sensors. The process underlying the surveillance of protein folding stress by the UPR is not fully understood, but it may require coupling to key folding mediators such as BiP or the direct recognition of the misfolded peptides by stress sensors. The UPR regulates genes and processs related to almost every folding step in the secretory pathway to reduce the load of misfolded proteins, including protein translation into the ER, translocation, folding, quality control, ERAD, the redox status, and many other related functions. Protein folding stress is observed in many disease conditions such as cancer, diabetes, and neurodegeneration. For example, abnormal protein aggregation and the accumulation of protein inclusions is associated with Parkinson’s and Alzheimer’s Disease, and amyotrophic lateral sclerosis. In those diseases and many others, neuronal dysfunction and disease progression correlates with the presence of a strong ER stress response; however, the direct in vivo role of the UPR in the disease process has been experimentally defined in only a few cases. Therapeutic strategies are currently being developed to increase protein folding and clearance of misfolded proteins, with the goal of alleviating ER stress.

In this issue of Current Opinion in Cell Biology we present a series of focused reviews from recognized experts in the field, that provide an overview of mechanisms underlying protein folding and quality control, and how balance of protein homeostasis is maintained in physiology and deregulated in diseases. Daniela Roth and William Balch integrate the concept of protein homeostasis networks into an interesting model termed FoldFx, showing how the interconnection between different pathways in the context of the cellular proteome determines the energetic barrier required to generate a functional folded peptide. The authors have previously proposed the term Proteostasis to refer to the set of interacting activities that maintain the health of the proteome and the organism (protein homeostasis). The ER is a central subcellular compartment for protein synthesis and quality control in the secretory pathway. Yukio Kimata and Kenji Kohno give an overview of the signaling pathways that control adaptation to ER stress and maintenance of protein folding homeostasis. The authors summarize the models proposed so far for the activation of UPR stress sensors, and discuss how this directly or indirectly relates to the accumulation of unfolded proteins in the ER lumen. Chronic or irreversible ER stress triggers cell death by apoptosis. Gordon Shore, Feroz Papa, and Scott Oakes summarize the complex signaling pathways initiating apoptosis by ER stress, where cross talk between the ER and the mitochondria play a central role. The authors focus on addressing the role of the BCL-2 protein family on the activation of intrinsic mitochondrial apoptosis pathways, highlighting different cytosolic and transcriptional events that determine the transition between adaptive responses to apoptosis programmed by the UPR to eliminate irreversibly injured cells.

Although diverse families of chaperones, foldases and co-factors are expressed at the ER, only a few protein folding networks have been well defined. However, molecular explanations for specific substrate recognition and quality control mechanisms are poorly defined. Here we present a series of reviews covering different aspects of protein maturation. Amy Lee summarizes what is known about the biology of the key ER folding chaperone BiP/Grp78, and its emerging role in diverse pathological conditions including cancer. In two reviews, David B. Williams and Linda M. Hendershot describe the best characterized mechanism of protein quality control at the ER, the calnexin cycle. In addition, they give an overview of the function of a family of ER foldases, the protein disulfide isomerases (PDIs), in folding, quality control and degradation of abnormally folded proteins. PDIs are also becoming key factors in establishing the redox tone of the ER. Riccardo Bernasconi and Maurizio Molinari overview the ERAD process and how this pathway affects the efficiency of the protein folding process at the ER and its relation to pathological conditions.

Lysosomal-mediated degradation is becoming a fundamental process for the control of the haft-life of proteins and the degradation of misfolded, aggregate prone proteins. Ana Maria Cuervo reviews the relevance of Chaperone-mediated autophagy in the selective degradation of soluble cytosolic proteins in lysosomes, and also points out a key role for Chaperone-mediated autophagy in the cellular defense against proteotoxicity. David Rubinsztein and Guido Kroemer present two reviews highlighting the emerging relevance of macroautophagy in maintaining the homeostasis of the nervous system. They also discuss the actual impact of macroautophagy in the clearance of protein aggregates related to neurodegenerative diseases, including Parkinson’s disease, amyotrophic lateral sclerosis, Huntington’s disease among others. In addition, recent evidence suggesting an actual impairment of macroautophagy as a causative factor in aging-related disorders is also discussed.

Strategies to increase the efficiency of quality control mechanisms, to reduce protein aggregation and to enhance folding are suggested to be beneficial in the setting of diseases associated with the disruption of protein homeostasis.  Jeffery Kelly reviews recent chemical and biological therapeutic strategies to restore protein homeostasis, which could be achieved by enhancing the biological capacity of the proteostasis network or through small molecule to stabilize misfolding-prone proteins. In summary, this volume of Current Opinion in Cell Biology compiles the most recent advances in understanding the impact of protein folding stress in physiology and disease, and integrates a variety of complex mechanisms that evolved to maintain protein homeostasis in a dynamic way in the context of a changing environment. The biomedical applications of developing strategies to cope with protein folding stress have profound implications for the treatment of the most prevalent diseases in the human population.

7.1.8 Proteome sequencing goes deep

Richards AL1Merrill AE2Coon JJ3.
Curr Opin Chem Biol. 2015 Feb; 24:11-7


  • Recent MS advances have transformed the depth of coverage of the human proteome.
  • Expression of half the estimated human protein coding genes can be verified by MS.
  • MS sample preparation, instrumentation, and data analysis techniques are highlighted.

Advances in mass spectrometry (MS) have transformed the scope and impact of protein characterization efforts. Identifying hundreds of proteins from rather simple biological matrices, such as yeast, was a daunting task just a few decades ago. Now, expression of more than half of the estimated ∼20 000 human protein coding genes can be confirmed in record time and from minute sample quantities. Access to proteomic information at such unprecedented depths has been fueled by strides in every stage of the shotgun proteomics workflow — from sample processing to data analysis — and promises to revolutionize our understanding of the causes and consequences of proteome variation.

  1. Advances in proteomic sample preparation
  2. Advances in peptide separation and MS instrumentation
  3. Advances in computational proteomics
  4. Conclusions and outlook

Mg²+ is critical for maintaining the positional integrity of closely clustered phosphate groups. These clusters appear in numerous and distinct parts of the cell nucleus and cytoplasm. The Mg²+ ion maintains the integrity of nucleic acids, ribosomes and proteins. In addition, this ion acts as an oligo-element with role in energy catalysis. [6] Biological cell membranes and cell walls exhibit poly-anionic charges on the surface. This finding has important implications for the transport of ions, particularly because different membranes preferentially bind different ions. Both Mg²+ and Ca²+ regularly stabilize membranes by cross-linking the carboxylated and phosphorylated head groups of lipids.

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The Role of Tight Junction Proteins in Water and Electrolyte Transport


Reviewer and Curator: Larry H. Bernstein, MD, FCAP 


This article is Part II of a series that explores the physiology, genomics, and the proteomics of water and electrolytes in human and mammalian function in health and disease.  In this portion of curation, we examine the role of special proteins at the tight junctions of cells, including the claudins.  Consistent with the exploration of cation homeostasis, the last featured article is one of the altered handling of calcium (Ca2+) in CHF, and the closely regulated calcium efflux by the sodium-calcium exchanger (NCX).

The Role of Aquaporin and Tight Junction Proteins in the Regulation of Water Movement in Larval Zebrafish (Danio rerio).

Kwong RW, Kumai Y, Perry SF.
Department of Biology, University of Ottawa, Ottawa, Ontario, Canada.
PLoS One. 2013 Aug 14;8(8):e70764.   eCollection 2013.

Teleost fish living in freshwater are challenged by passive water influx; however the molecular mechanisms regulating water influx in fish are not well understood. The potential involvement of aquaporins (AQP) and epithelial tight junction proteins in the regulation of transcellular and paracellular water movement was investigated in larval zebrafish (Danio rerio).

We observed that the half-time for saturation of water influx (K u) was 4.3±0.9 min, and reached equilibrium at approximately 30 min. These findings suggest a high turnover rate of water between the fish and the environment. Water influx was reduced by the putative AQP inhibitor phloretin (100 or 500 μM). Immunohistochemistry and confocal microscopy revealed that AQP1a1 protein was expressed in cells on the yolk sac epithelium. A substantial number of these AQP1a1-positive cells were identified as ionocytes, either H(+)-ATPase-rich cells or Na(+)/K(+)-ATPase-rich cells. AQP1a1 appeared to be expressed predominantly on the basolateral membranes of ionocytes, suggesting its potential involvement in regulating ionocyte volume and/or water flux into the circulation.

Additionally, translational gene knockdown of AQP1a1 protein reduced water influx by approximately 30%, further indicating a role for AQP1a1 in facilitating transcellular water uptake. On the other hand, incubation with the Ca(2+)-chelator EDTA or knockdown of the epithelial tight junction protein claudin-b significantly increased water influx. These findings indicate that the epithelial tight junctions normally act to restrict paracellular water influx. Together, the results of the present study provide direct in vivo evidence that water movement can occur through transcellular routes (via AQP); the paracellular routes may become significant when the paracellular permeability is increased.

PMID:  23967101  PMCID: PMC3743848

The tight junction protein claudin-b regulates epithelial permeability and sodium handling in larval zebrafish, Danio rerio.

Kwong RW, Perry SF.
Department of Biology, University of Ottawa, Ottawa, Ontario, Canada.
Am J Physiol Regul Integr Comp Physiol. Apr 1, 2013; 304(7):R504-13.  Epub 2013 Jan 30.

The functional role of the tight junction protein claudin-b in larval zebrafish (Danio rerio) was investigated. We showed that claudin-b protein is expressed at epithelial cell-cell contacts on the skin. Translational gene knockdown of claudin-b protein expression caused developmental defects, including edema in the pericardial cavity and yolk sac.

Claudin-b morphants exhibited an increase in epithelial permeability to the paracellular marker polyethylene glycol (PEG-4000) and fluorescein isothiocyanate-dextran (FD-4). Accumulation of FD-4 was confined mainly to the yolk sac and pericardial cavity in the claudin-b morphants, suggesting these regions became particularly leaky in the absence of claudin-b expression.

Additionally, Na(+) efflux was substantially increased in the claudin-b morphants, which contributed to a significant reduction in whole-body Na(+) levels. These results indicate that claudin-b normally acts as a paracellular barrier to Na(+). Nevertheless, the elevated loss of Na(+) in the morphants was compensated by an increase in Na(+) uptake.

Notably, we observed that the increased Na(+) uptake in the morphants was attenuated in the presence of the selective Na(+)/Cl(-)-cotransporter (NCC) inhibitor metolazone, or during exposure to Cl(-)-free water. These results suggested that the increased Na(+) uptake in the morphants was, at least in part, mediated by NCC. Furthermore, treatment with an H(+)-ATPase inhibitor bafilomycin A1 was found to reduce Na(+) uptake in the morphants, suggesting that H(+)-ATPase activity was essential to provide a driving force for Na(+) uptake. Overall, the results suggest that claudin-b plays an important role in regulating epithelial permeability and Na(+) handling in zebrafish.
PMID: 23364531

Evidence for a role of tight junctions in regulating sodium permeability in zebrafish (Danio rerio) acclimated to ion-poor water.

Kwong RW, Kumai Y, Perry SF.
Department of Biology, University of Ottawa, Ottawa, ON, Canada.
J Comp Physiol B. Feb 2013 ;183(2):203-13.  Epub 2012 Jul 29.

Freshwater teleosts are challenged by diffusive ion loss across permeable epithelia including gills and skin. Although the mechanisms regulating ion loss are poorly understood, a significant component is thought to involve paracellular efflux through pathways formed via tight junction proteins. The mammalian orthologue (claudin-4) of zebrafish (Danio rerio) tight junction protein, claudin-b, has been proposed to form a cation-selective barrier regulating the paracellular loss of Na(+).

The present study investigated the cellular localization and regulation of claudin-b, as well as its potential contribution to Na(+) homeostasis in adult zebrafish acclimated to ion-poor water. Using a green fluorescent protein-expressing line of transgenic zebrafish, we found that claudin-b was expressed along the lamellar epithelium as well as on the filament in the inter-lamellar regions. Co-localization of claudin-b and Na(+)/K(+)-ATPase was observed, suggesting its interaction with mitochondrion-rich cells. Claudin-b also appeared to be associated with other cell types, including the pavement cells. In the kidney, claudin-b was expressed predominantly in the collecting tubules. In addition,

exposure to ion-poor water caused a significant increase in claudin-b abundance as well as a decrease in Na(+) efflux, suggesting a possible role for claudin-b in regulating paracellular Na(+) loss. Interestingly, the whole-body uptake of a paracellular permeability marker, polyethylene glycol-400, increased significantly after prolonged exposure to ion-poor water, indicating that an increase in epithelial permeability is not necessarily coupled with an increase in passive Na(+) loss. Overall, our study suggests that in ion-poor conditions, claudin-b may contribute to a selective reduction in passive Na(+) loss in zebrafish.
PMID: 22843140

Claudin-16 and claudin-19 function in the thick ascending limb.

Hou J, Goodenough DA.
Washington University School of Medicine, Div Renal Diseases, St Louis, Missouri
Curr Opin Nephrol Hypertens. Sep 2010; 19(5):483-8.

The thick ascending limb (TAL) of the loop of Henle is responsible for reabsorbing 25–40% of filtered Na+, 50–60% of filtered Mg2+ and 30–35% of filtered Ca2+. The dissociation of salt and water reabsorption in the TAL serves both to dilute the urine and to establish the corticomedullary osmolality gradient. Active transcellular salt reabsorption results in a lumen-positive transepithelial voltage that drives passive paracellular reabsorption of divalent cations. Claudins are the key components of the paracellular channel. The paracellular channels in the tight junction have properties of ion selectivity, pH dependence and anomalous mole fraction effects, similar to conventional transmembrane channels. Genetic mutations in claudin-16 and claudin-19 cause an inherited human renal disorder, familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC).

In the TAL of Henle’s loop, the epithelial cells form a water-impermeable barrier, actively transport Na+ and Cl− via the transcellular route, and provide a paracellular pathway for the selective absorption of cations. Na+ K+ and Cl− enter the cell through the Na-K-2Cl cotransporter (NKCC2) in the luminal membrane. Na+ exits the cell through the Na+/K+-ATPase, in exchange for K+ entry. K+ is secreted into the lumen through the renal outer medullary potassium channel. Cl− leaves the cell through the basolateral Cl− channel, made up of two subunits, ClCKb and barttin. The polarized distribution of luminal K+ versus basolateral Cl− conductance generates a spontaneous voltage source (Vsp) of +7−8mV , depending on active transcellular NaCl reabsorption. With continuous NaCl reabsorption along the axis of the TAL segment, the luminal fluid is diluted to 30–60mmol/l  and a large NaCl transepithelial chemical gradient develops at the end of the TAL. Because the paracellular permeability of the TAL is cation-selective (with a PNa/PCl value between 2 and 4), the diffusion voltage (Vdi) is superimposed onto the active transport voltage (Vsp) and becomes the major source of the transepithelial voltage (Vte), which now increases up to +30mV.

Early in-vivo micropuncture studies have shown that approximately 50–60% of the filtered Mg2+ is reabsorbed in the TAL. The flux–voltage relationship indicates that Mg2+ is passively reabsorbed from the lumen to the peritubular space through the paracellular pathway in this segment, driven by a lumen positive Vte.  Vte is made of the sum of Vsp and Vdi. There are two prerequisites required for the paracellular Mg2+ reabsorption in the TAL: the lumen-positive Vte as the driving force and the paracellular permeability for the divalent cation Mg2+.

Claudin-16 and claudin-19 underlie familial hypercalciuric hypomagnesemia with nephrocalcinosis

Claudin-16 and claudin-19 play a major role in the regulation of magnesium reabsorption in the thick ascending limb (TAL). This review describes recent findings of the physiological function of claudin-16 and claudin-19 underlying normal transport function for magnesium reabsorption in the TAL. Mutations in the genes encoding the tight junction proteins claudin-16 and claudin-19 cause the inherited human renal disorder familial hypomagnesemia with hypercalciuria and nephrocalcinosis. FHHNC, OMIM #248250, is a rare autosomal recessive tubular disorder. As a consequence of excessive renal Mg2+ and Ca2+ wasting, patients develop the characteristic triad of hypomagnesemia, hypercalciuria and nephrocalcinosis. Recurrent urinary tract infections and polyuria/polydipsia are frequent initial symptoms. Other clinical symptoms include nephrolithiasis, abdominal pain, convulsions, muscular tetany, and failure to thrive. Additional laboratory findings include elevated serum parathyroid hormone levels before the onset of chronic renal failure, incomplete distal tubular acidosis, hypocitraturia, and hyperuricemia. In contrast to hypomagnesemia and secondary hypocalcemia (HSH, OMIM #602014), FHHNC is generally complicated by end-stage renal failure in early childhood or adolescence.

Simon et al. used the positional cloning strategy and identified claudin-16 (formerly known as paracellin-1), which is mutated in patients with FHHNC. Most mutations reported to date in claudin-16 are missense mutations clustering in the first extracellular loop composing the putative ion selectivity filter. Konrad et al. have found mutations in another tight junction gene encoding claudin-19 from new cohorts of FHHNC patients (OMIM #248190). The renal tubular phenotypes are indistinguishable between patients with mutations in claudin-16 and those with mutations in claudin-19. Although claudin-16 and claudin-19 underlie FHHNC and paracellular Mg2+ reabsorption in the TAL, the transient receptor potential channel melastatin 6 (TRPM6) regulates the apical entry of Mg2+ into the distal convoluted tubule epithelia. Mutations in TRPM6 cause the HSH syndrome.

These above data suggested the hypothesis that claudin-16 and/or claudin-19 forms a selective paracellular Mg2+/Ca2+ channel, which was tested in a number of in-vitro studies. Ikari et al. transfected low-resistance Madin-Darby canine kidney (MDCK) cells with claudin-16 and reported that the Ca2+ flux in these cells was increased in the apical to basolateral direction, whereas the Ca2+ flux in the opposite direction remained unchanged. The Mg2+ flux was without any noticeable change. Kausalya et al.  transfected the high-resistance MDCK-C7 cell line and found that claudin-16 only moderately increased Mg2+ permeability without any directional preference. The effects of claudin-16 on Mg2+/Ca2+ permeation appeared so small (<50%) that the Mg2+/Ca2+ channel theory incompletely explains the dramatic effect of mutations in claudin-16 on Mg2+ and Ca2+ homoeostasis in FHHNC patients. However, , Hou et al.  transfected the anion-selective LLC-PK1 cell line with claudin-16 and found a large increase in Na+ permeability (PNa) accompanied by a moderately enhanced Mg2+ permeability (PMg). The permeability of claudin-16 to other alkali metal cations was found to be: K+ > Rb+ > Na+.  Yu et al. emphasized that these residue replacements can influence protein structures that may have impacts on ion permeability independent of amino acid charge.

The cation selectivity of the tight junction is vital for generating the lumen positive transepithelial potential in the TAL, which drives paracellular absorption of magnesium. Claudin-16 and claudin-19 require each other for assembly into tight junctions in the TAL. Heteromeric claudin-16 and claudin-19 interaction forms a cation selective tight junction paracellular channel. Loss of either claudin-16 or claudin-19 in the mouse kidney abolishes the cation selectivity for the TAL paracellular pathway, leading to excessive renal wasting of magnesium.

Claudins interact with each other both intracellularly and intercellularly: they copolymerize linearly within the plasma membrane of the cell, together with the integral protein occludin, to form the classical intramembrane fibrils or strands visible in freeze-fracture replicas. These intramembrane interactions (side-to-side) can involve one claudin protein (homomeric or homopolymeric) or different claudins (heteromeric or heteropolymeric). In the formation of the intercellular junction, claudins may interact head-to-head with claudins in an adjacent cell, generating both homotypic and heterotypic claudin–claudin interactions. Using the split-ubiquitin yeast 2-hybrid assay, Hou et al. found strong claudin-16 and claudin-19 heteromeric interaction. The point mutations in claudin-16 (L145P, L151F, G191R, A209T, and F232C) or claudin-19 (L90P and G123R) that are known to cause human FHHNC disrupted the claudin-16 and claudin-19 heteromeric interaction. In mammalian cells such as the human embryonic kidney 293 cells, claudin-16 can be coimmunoprecipitated with claudin-19. Freeze-fracture replicas revealed the assembly of tight junction strands in L cells coexpressing claudin-16 and claudin-19, supporting their heteromeric interaction.

Coexpression of claudin-16 and claudin-19 in LLC-PK1 cells resulted in a dramatic upregulation of PNa and down-regulation of PCl, generating a highly cation-selective paracellular pathway. Certain FHHNC mutations in claudin-16 (L145P, L151F, G191R, A209T, and F232C) or claudin-19 (L90P and G123R) that disrupted their heteromeric interaction abolished this physiological change. As claudin-16 colocalizes with claudin-19 in the TAL epithelia of the kidney, claudin-16 and claudin-19 association through heteromeric interactions confers cation selectivity to the tight junction in the TAL. Human FHHNC mutations in claudin-16 or claudin-19 that abolish the cation selectivity diminish the lumen-positive Vdi as the driving force for Mg2+ and Ca2+ reabsorption, readily explaining the devastating phenotypes in FHHNC patients.  Hou et al. generated claudin-16 deficient mouse models using lentiviral transgenesis of siRNA to knock down claudin-16 expression by more than 99% in mouse kidneys. Claudin-16 knockdown mice show significantly reduced plasma Mg2+ levels and excessive urinary excretions (approximately four-fold) of Mg2+ and Ca2+. Calcium deposits are observed in the basement membranes of the medullary tubules and the interstitium in the kidney of claudin-16 knockdown mice. These phenotypes of claudin-16 knockdown mice recapitulate the symptoms in human FHHNC patients.

The paracellular reabsorption of Mg2+ and Ca2+ is driven by a lumen-positive Vte made up of two components: Vsp and Vdi. When isolated TAL segments were perfused ex vivo with symmetrical NaCl solutions, there was no difference in Vsp between claudin-16 knockdown and wild-type mice, indicating Vsp was normal in claudin-16 knockdown. Blocking the NKCC2 channel with furosemide (thus dissipating VSP), the cation selectivity (PNa/PCl) was significantly reduced from3.1 ± 0.3 in wild type to 1.5 ± 0.1 in claudin-16 knockdown, resulting in the loss of Vdi. When perfused with a NaCl gradient of 145mmol/l (bath) versus 30mmol/l (lumen), the resulting Vdi was +18mV in wild type, but only +6.6mV in claudin-16 knockdown. Thus, the reduction in Vdi accounted for a substantive loss of the driving force for Mg2+ and Ca2+ reabsorption.

Renal handling of Na+ in claudin-16 knockdown mice is more complex. In the early TAL segment, the transcellular and paracellular pathways form a current loop in which the currents traversing the two pathways are of equal size but opposite direction. Net luminal K+ secretion and basolateral Cl− absorption polarize the TAL epithelium and generate Vsp. As the paracellular pathway is cation selective (PNa/PCl=2–4 , the majority of the current driven by Vsp through the paracellular pathway is carried by Na+ moving from the lumen to the interstitium. Hebert et al. estimated that, for each Na+ absorbed through the trans-cellular pathway, one Na+ is absorbed through the paracellular pathway. With the loss of claudin-16 and the concomitant loss of paracellular cation selectivity, Na+ absorption through the paracellular pathway is reduced.  In the late TAL segment, dilution of NaCl in the luminal space creates an increasing chemical transepithelial gradient; back diffusion of Na+ through the cation-selective tight junction generates a lumen-positive Vdi across the epithelium. The paracellular absorption of Na+ will be diminished when Vdi equals Vsp, and reversed when Vdi exceeds Vsp. As an equilibrium potential, Vdi blocks further Na+ backleak into the lumen. Without claudin-16, Vdi will be markedly reduced well below normal, providing a driving force for substantial Na+ secretion. Indeed, claudin-16 knockdown mice had increased fractional excretion of Na+ (FENa) and developed hypotension and secondary hyperaldosteronism. The observed Na+ and volume loss are consistent with human FHHNC phenotypes. For example, polyuria and polydipsia are the most frequently reported symptoms from FHHNC patients.

Epithelial paracellular channels are increasingly understood to be formed from claudin oligomeric complexes. In the mouse TAL, claudin-16 and claudin-19 cooperate to form cation-selective paracellular channels required for normal levels of magnesium reabsorption. Different subsets of the claudin family of tight junction proteins are found distributed throughout the nephron, and understanding their roles in paracellular ion transport will be fundamental to understanding renal ion homeostasis.

Keywords: claudin; hypomagnesemia; thick ascending limb; tight junction; transepithelial voltage.     PMID: 20616717  PMCID: PMC3378375

Function and regulation of claudins in the thick ascending limb of Henle.

Günzel D, Yu AS.
Depart Clin Physiol, Charité, Campus Benjamin Franklin, Berlin, Germany.
Pflugers Arch. May 2009; 458(1):77-88.  Epub 2008 Sep 16.

The thick ascending limb (TAL) of Henle mediates transcellular reabsorption of NaCl while generating a lumen-positive voltage that drives passive paracellular reabsorption of divalent cations. Disturbance of paracellular reabsorption leads to Ca(2+) and Mg(2+) wasting in patients with the rare inherited disorder of familial hypercalciuric hypomagnesemia with nephrocalcinosis (FHHNC). Recent work has shown that the claudin family of tight junction proteins form paracellular pores and determine the ion selectivity of paracellular permeability. Importantly, FHHNC has been found to be caused by mutations in two of these genes, claudin-16 and claudin-19, and mice with knockdown of claudin-16 reproduce many of the features of FHHNC. Here, we review the physiology of TAL ion transport, present the current view of the role and mechanism of claudins in determining paracellular permeability, and discuss the possible pathogenic mechanisms responsible for FHHNC.

Tight junctions form the paracellular barrier in epithelia. Claudins are ~22 kDa proteins that were first identified by Mikio Furuse in the laboratory of the late Shoichiro Tsukita as proteins that copurified in a tight junction fraction from the chicken liver [23]. The observation that they were transmembrane proteins with 4 predicted membrane domains and 2 extracellular domains raised early on the possibility that they could play a key role in intercellular adhesion and formation of the paracellular barrier. In 1999, Richard Lifton’s group identified mutations in a novel gene, which they called paracellin, as the cause of familial hypercalciuric hypomagnesemia, an inherited disorder believed to be due to failure of paracellular reabsorption of divalent cations in the thick ascending limb of the renal tubule. Paracellin turned out to be a claudin family member (claudin-16). This suggested that claudins in general might be directly involved in regulating paracellular transport in all epithelia. This is now supported by numerous studies demonstrating that overexpressing or ablating expression of various claudin isoforms in cultured cell lines or in mice affects both the degree of paracellular permeability and its selectivity (vide infra). Furthermore, in mammals alone there are ~24 claudin genes and each exhibits a distinct tissue-specific, pattern of expression. Thus, the specific claudin isoform(s) expressed in each tissue might explain its paracellular permeability properties.

Each nephron segment expresses a unique set of multiple claudin isoforms, and each isoform is expressed in multiple segments, thus making a complicated picture which even varies between different species. The role of combinations of claudins in determining paracellular permeability properties has hardly been studied yet. In mouse, rabbit and cattle, the thick ascending limb of Henle’s loop is thought to express claudins 3, 10, 11, 16  in adulthood, and, at least in mouse, additionally claudin-6 during development. In addition, claudin-4 has been found in cattle and claudin-8 in rabbit. To date, the distribution of Claudin-19 has been investigated in mouse, rat, and man where its presence in the TAL was demonstrated.

The thick ascending limb (TAL) of Henle’s loop, working as “diluting segment” of the nephron, is characterized by two major properties: high transepithelial, resorptive transport of electrolytes and low permeability to water. Major players to achieve electrolyte transport are the apical Na+-K+-2Cl−symporter (NKCC2), the apical K+ channel ROMK, the basolateral Cl− channel (CLC-Kb) together with its subunit barttin and the basolateral Na+/K+-ATPase . The combined actions of these transport systems have been extensively reviewed and are therefore only briefly summarized here. Na+ and Cl− are resorbed by entering the cells apically through NKCC2 and leaving the cells basolaterally through the Na+/K+-ATPase and CLC-Kb, respectively. In contrast, K+ is either recycled across the apical membrane as it is entering through NKCC2 and leaving through ROMK, or even secreted, as it is also entering the cells basolaterally through the Na+/K+-ATPase. Taking these ion movements together, there is a net movement of positive charge from the basolateral to the apical side of the epithelium, giving rise to a lumen positive voltage (3 – 9 mV [11]; about 5 – 7 mV [30,31]; 7 – 8 mV [57]). Over the length of the TAL, luminal NaCl concentration decreases gradually to concentrations of 30 – 60 mM at the transition to the distal tubule, depending on the flow rate within the tubule (low flow rates resulting in low concentrations).

To keep up such a high gradient, the TAL epithelium has to be tight to water and various studies summarized by Burg and Good report water permeability values from 28 µm/s down to values indistinguishable from zero. Tight junctions of the TAL are, however, highly permeable to cations with PNa being about 2 – 2.7 fold, 2.5 fold or even up to 6 fold that of PCl. Amongst the monovalent cations, a permeability sequence of PK > PNa > PRb = PLi > PCs > Porganic cation was observed which is similar to Eisenman sequence VIII or IX, indicating a strong interaction between the permeating ion and the paracellular pore that enables at least partial removal of the hydration shell (see below). As reviewed by Burg and Good, the transepithelial sodium and chloride permeabilities, estimated from radioisotope fluxes, are high (in the range of 10 – 63·10−6 cm/s) and the transepithelial electrical resistance is correspondingly low (21 – 25 W cm2 ; 30 – 40 W cm2 ; 11 – 34 W cm2 . Blocking active transport by the application of furosemide or ouabain increases transepithelial resistance only slightly, indicating that the low values are primarily due to a very high paracellular permeability. Due to these properties of TAL epithelial cells, Na+ ions leak back into the lumen of the tubule, creating a diffusion (dilution) potential that adds another 10 – 15 mV to the lumen positive potential, so that considerable potential differences (25 mV ; 30 mV; cTAL 23 mV, mTAL 17 mV ) may be reached at very slow flow rates.

Considerable proportions of the initially filtrated Mg2+ (50 – 60%; 50 – 70%; 65 – 75%) and Ca2+ (20%; 30 – 35% are resorbed in the TAL. The transepithelial potential is considered to provide the driving force for the predominantly paracellular resorption of Mg2+ and Ca2+ as in many studies, transport of both divalent ions in the TAL has been found to be strictly voltage dependent (resorbtive at lumen positive potentials, zero at 0 mV and secretory at lumen negative potentials) and permeability considerable (PCa 7.7·10−6 cm/s, i.e. approximately 25% of PNa). There is however, some conflicting evidence, e.g. by Suki et al. and Friedman. Both studies used cTAL (cortical TAL) and found that decreasing the transepithelial potential by applying furosemide did either not alter the unidirectional lumen to bath Ca2+ flux (rabbit) or left a substantial net Ca2+ resorption (mouse). Similarly, Rocha et al. found that bath application of ouabain almost abolished the transepithelial potential, but hardly affected net Ca2+ resorption and conclude that (a) all segments of Henle’s loop are relatively impermeable to calcium and (b) net calcium resorption occurs in the thick ascending limb which cannot be explained by passive mechanisms.  Mandon et al. conclude that both Mg2+ and Ca2+ are transported in the cTAL but not in the mTAL (medullary TAL) of rat and mouse, although transepithelial potential differences were similar in both segments, and even if the transepithelial potential was experimentally elevated to values above 20 mV. Wittner et al. even found evidence that in mouse mTAL the passive permeability to divalent cations is very low and that Ca2+ and Mg2+ can be secreted into the luminal fluid under conditions which elicit large lumen-positive transepithelial potential differences. They conclude that this Ca2+ and Mg2+ transport is most probably of cellular origin. In contrast, in rabbit, both ions are transported along the whole length of the TAL.

Both, Mg2+ and Ca2+ resorption are modulated through the action of the basolateral Ca (and Mg) sensing receptor (CaSR) which is found along the entire nephron but especially in the loop of Henle, distal convoluted tubule (DCT) and the inner medullary collecting duct. Different modes of action on Ca2+ and Mg2+ homeostasis exist, such as an indirect action through the modulation of PTH secretion or direct effects on the cells expressing CaSR. In the TAL the latter model is based on the assumptions depicted above, i.e. that Mg2+ and Ca2+ are resorbed paracellularly, driven by the lumen positive potential, so that a reduction in NaCl resorption causes a reduction in driving force for Mg2+ and Ca2+ resorption. As reviewed by Hebert and Ward, CaSR is activated through an increase in basolateral Ca2+ and/or Mg2+ concentration which triggers an increase in the intracellular Ca2+ concentration. This reduces the activity of the adenylate cyclase which, in turn, inhibits transcellular transport of Na+ and Cl−. In addition, the increase in intracellular Ca2+ activates phospholipase A2 (PLA2) and thus increases the intracellular concentration of arachidonic acid and its derivative, 20-HETE. 20-HETE inhibits NKCC2, ROMK and the Na+/K+-ATPase and by this Mg2+ and Ca2+ resorption. In keeping with this hypothesis, mutations in CaSR affect Ca/Mg resorption. Inactivating mutations cause hypercalcemia, hypocalciuria, hypomagnesiuria and, in some patients hypermagnesemia. Conversely, activating mutations (gain of function mutations) lead to hypocalcemia, hypercalciuria, hypermagnesiuria and in up to 50% of the patients mild hypomagnesemia.

Bartter syndrome type I (mutations in NKCC2), and type II (mutations in ROMK) lead to hypercalciuria and thus cause nephrocalcinosis, but no hypomagnesemia is observed. Reports on hypermagnesiuria are conflicting: while Kleta and Bockenhauer link it to nephrocalcinosis seen in these patients, Rodriguez-Soriano states that patients with neonatal Bartter syndrome (i.e. type I or II) show a lack of hypermagnesiuria that may be explained by compensation in the DCT. Hypomagnesemia is occationally present in Bartter type III (CLC-Kb). However, here it is believed to be mainly due to effects on DCT, where CLC-Kb shows highest expression. Patients with Bartter syndrome IV (CLC-Kbsubunit barttin) may or may not present nephrocalcinosis, while Mg2+ homeostasis appears undisturbed. Interestingly, the largest effects on Mg2+-homeostasis are observed in Gitelman syndrome, a defect in the Na+/Cl− symport (NCC) predominantly found in the DCT, where Mg2+ is transported along the transcellular route. Affected patients suffer from hypomagnesemia, hypermagnesuria and hypocalciuria. The effect on Mg2+ in Gitelman syndrome is still poorly understood and possibly due to a concomitant down-regulation of TRPM6, the apical Mg2+ uptake channel in DCT.

More than 30 different claudin-16 mutations have now been reported in families with FHHNC. Because of the large number of unique mutations, it has not been possible to identify any clear qualitative correlation between the phenotype and individual mutations, although certain mutations are associated with greater severity of disease. In 2006, a second locus was identified, CLDN19, which encodes claudin-19. In the initial report, the phenotype appeared similar to that due to claudin-16 mutations, with the exception that there was a high prevalence of ocular abnormalities, including macular colobomata, nystagmus and myopia. Claudin-19 is normally expressed at high levels in the retina, but why it causes these ocular disorders is unknown.

In vitro studies of claudin function comprise inducible or non-inducible transfection of various cells lines with cDNA for claudins that are not endogenously expressed by the cell line used. Alternatively, cells can be transfected with siRNA directed against an endogenous claudin. In both cases, cells are then grown to confluence on permeable filter supports that allow measurement of transepithelial permeabilities. Before the results of permeability studies can be interpreted, however, several parameters have to be controlled.

First, special care has to be taken to make sure that the exogenous claudin is correctly inserted into the tight junction. This can be achieved e.g. by confocal laser scanning microscopy, colocalizing the claudin of interest  with a tight junction marker protein such as occludin.

Second, it has to be ensured that endogenous claudin expression remains unaffected, as permeability changes always result from the combined effects of alterations in endogenous and exogenous claudins.

Third, it has to be kept in mind that, typically, epithelia express several different claudins that act together to produce tissue specific permeability properties. Thus, ideally, a cell line should be chosen that provides a claudin background resembling that usually experienced by the claudin investigated. The latter two points may be the reason for contradicting results obtained in permeability studies expressing a specific claudin in different cell lines.

Studies of paracellular permeabilities can be divided into two groups, those employing electrophysiological measurements (e.g. determination of diffusion potentials), and those measuring ion or solute flux, using either radioactive isotopes or various analytical methods to determine the amount transported.

Although transepithelial conductances depend on paracellular permeabilities of the predominant ions in the bath solution, conductance changes alone cannot be used to predict ion permeabilities.

Firstly, conductances always depend on both ion and counter-ion, not on one ion species alone.

Secondly, transepithelial conductances are the sum of the conductances of the transcellular and paracellular pathways.

Thus, they only reflect paracellular permeability, if paracellular conductance dominates transepithelial conductance and if transcellular conductance remains constant throughout the experiment. This, however, is often not the case, as concentration changes of the ions investigated may affect transcellular conductance, e.g. by activating ion transporters or by inhibiting ion channels. Thirdly, the specific conductance of each solution employed may differ and has therefore to be assessed and taken into account. Thus, comparison of results from diffusion potential measurements or flux studies and conductance measurements may even yield contradicting results. For the same reasons, other methods based on pure conductance/resistance measurements, including the more sophisticated conductance scanning method or one-path impedance spectroscopy are not ion specific and do not allow the measurement of paracellular permeabilities to single ions.

In contrast to electrophysiological measurements, flux measurements are not limited to ions but can also be extended to uncharged molecules.  Flux measurements per se do not distinguish between transcellular or paracellular transport. Therefore, to estimate paracellular permeabilities, inhibition or at least estimation of the transcellular flux is necessary. Assuming that transcellular flux for energetic reasons is not easily reversible while paracellular flux is passive and thus generally assumed to be symmetric, the transcellular proportion is often estimated by calculating the difference between apical to basolateral and basolateral to apical fluxes. All flux measurements are very sensitive to the development of diffusion zones (“unstirred layers”) near the cells. These layers are depleted/enriched in the compound transported and thus alter the driving forces acting on these compounds, if bath solutions are not continually circulated. If ionic fluxes are investigated, transepithelial potentials may develop that diminish or completely inhibit the flux investigated.

All the techniques described above have been employed to investigate the function of claudin-16 and -19, especially with respect to their ability to increase paracellular permeability to divalent cations. The hypothesis, that claudin-16 (then called paracellin-1) may be a paracellular Mg2+ and Ca2+ pore was originally expressed by Simon et al. It was based on the findings that mutations in claudin-16 were the cause of the severe disturbance in Mg2+ and Ca2+ homeostasis in FHHNC patients together with the observations that claudin-16 is a tight junction protein located in the TAL, i.e. the nephron segment responsible for bulk Mg2+ resoption along the paracellular pathway. When, recently, it was found that claudin-19 mutations were underlying hitherto unexplained cases of FHHNC and that claudin-19 co-localized with claudin-16, the hypothesis was extended to claudin-19.

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PMID: 18795318  PMCID:  PMC2666100

Deletion of claudin-10 (Cldn10) in the thick ascending limb impairs paracellular sodium permeability and leads to hypermagnesemia and nephrocalcinosis.

Breiderhoff T, Himmerkus N, Stuiver M, Mutig K, Will C, Meij IC et al.
Max Delbrück Center for Molec Med, Berlin, Germany.

Erratum in Proc Natl Acad Sci. 2012 Sep 11;109(37):15072.
Proc Natl Acad Sci. Aug 28, 2012; 109(35):14241-6. Epub 2012 Aug 13.

In the kidney, tight junction proteins contribute to segment specific selectivity and permeability of paracellular ion transport. In the thick ascending limb (TAL) of Henle’s loop, chloride is reabsorbed transcellularly, whereas sodium reabsorption takes transcellular and paracellular routes. TAL salt transport maintains the concentrating ability of the kidney and generates a transepithelial voltage that drives the reabsorption of calcium and magnesium. Thus, functionality of TAL ion transport depends strongly on the properties of the paracellular pathway. To elucidate the role of the tight junction protein claudin-10 in TAL function, we generated mice with a deletion of Cldn10 in this segment. We show that claudin-10 determines paracellular sodium permeability, and that its loss leads to hypermagnesemia and nephrocalcinosis. In isolated perfused TAL tubules of claudin-10-deficient mice, paracellular permeability of sodium is decreased, and the relative permeability of calcium and magnesium is increased. Moreover, furosemide-inhibitable transepithelial voltage is increased, leading to a shift from paracellular sodium transport to paracellular hyperabsorption of calcium and magnesium. These data identify claudin-10 as a key factor in control of cation selectivity and transport in the TAL, and deficiency in this pathway as a cause of nephrocalcinosis.

Whereas regulation of transporters and channels involved in trans-cellular ion transport has been characterized in much detail, the functional and molecular determinants of paracellular ion trans­port in the kidney remain incompletely understood. In the thick ascending limb (TAL) of Henle’s loop, both trans-cellular and paracellular ion transport pathways contribute to reabsorption of Na+, Cl, Mg2+, and Ca2+. Na+ and Clare reabsorbed mostly transcellularly by the concerted action of chan­nels and transporters. Mutations in five of the genes involved lead to Bartter syndrome, a disorder characterized by salt wasting and polyuria. Whereas Clis transported exclusively transcellularly, 50% of the Na+ load, as well as Ca2+ and Mg2+, are reabsorbed via paracellular pathways. In the TAL, this paracellular route is highly cation-selective. The paracellular passage is largely controlled by the tight junction (TJ), a supramolecular structure of membrane-spanning proteins, their intracellular adapters, and scaffolding proteins. Claudins, a family comprising 27 members, are the main components of the TJ defining the permeability properties. They interact via their extracellular loops with corre­sponding claudins of the neighboring cell to allow or restrict pas­sage of specific solutes (5, 6). In the kidney, their expression pattern is closely related to the corresponding segment-specific solute reabsorption profile. Several claudins are expressed in the TAL, including claudin-16, -19, -10, -3, and -18 The importance of claudin-16 and -19 in this tissue is documented by mutations in CLDN16 and CLDN19, which cause familial hypomagnesemia, hypercalciuria, and nephrocalcinosis, an autosomal recessive dis­order that leads to end-stage renal disease. The relevance of CLDN16 for paracellular reabsorption of Mg2+ and Ca2+ was confirmed in mouse models with targeted gene disruption. In addition, claudin-14, expressed in the TAL of mice on a high-calcium diet, was identified as negative regulator of claudin-16 function (15), and sequence variants in CLDN14 have been asso­ciated with human kidney stone disease. The functional significance of claudin-10, which is also ex­pressed in the TAL, remains unclear. This TJ protein is expressed in two isoforms, claudin-10a and claudin-10b, which differ in their first extracellular loop. In cultured epithelial cells, heter-ologous expression of claudin-10a increases paracellular anion transport, whereas claudin-10b expression increases paracellular cation transport. Both isoforms are expressed differentially along the nephron, with claudin-10a found predominantly in cortical segments, whereas claudin-10b is enriched in the medullary region.  In the present study we generated a mouse model with a TAL-specific Cldn10 gene defect to query the role of this protein in renal paracellular in transport in vivo. We found that claudin-10 is crucial to paracellular Na+ handling in the TAL, and that its absence leads to a shift from paracellular sodium transport to paracellular hyperreabsorption of Ca2+ and Mg2+.

Analysis of claudin-10 expression in the kidney. (A) Western blot analysis of kidney membrane extracts from control (ctr) and cKO mice. A dramatic reduction in claudin-10 protein can be seen in kidneys of cKO mice. Levels of the TJ marker occludin are unchanged. (B) Gene expression analysis of Cldn10 variants on cDNA from isolated segments of the nephron. (C) Immunohistological detection of claudin-10 and markers for PCT (NHE3) and TAL (NKCC2) on sections from control mice (ctr) and cKO mice demonstrates no difference in the signal for claudin-10 in the PCT between WT and cKO. Claudin-10 is expressed in TAL tubules positive for NKCC2. No specific clau-din-10 staining is evident in the TAL of cKO mice. Claudin-10 is detected in TJs positive for ZO-1. This signal is absent in cKO mice, whereas ZO-1 staining is unchanged. (Scale bar: 25 μm.)

In control animals, claudin-10 is located mainly in the TAL, as documented by coimmunostaining with the Na+K+2Clcotrans-porter (NKCC2). In this segment, a large portion of the claudin-10 immunofluorescence signal is located outside of the TJ; however, claudin-10 is present in the TJ, as demonstrated by colocalization with the TJ protein ZO-1. PCTs positive for the sodium-proton exchanger NHE-3 showed a considerably weaker signal restricted to the TJ area. Claudin-10 immunoreactivity was virtually absent in NKCC2-positive tubules of cKO mice, in line with the activity of Cre recombinase in this cell type. The immu-noreactivity of claudin-10 in PCTs of cKOs remained unchanged, however. ZO-1 staining in TAL sections of cKOs was unchanged compared with controls, indicating no unspecific effect on TJ structures. The TJ localization of claudin-16 and claudin-19 in medullary rays was similar in cKOs and controls.   To investigate the phenotypic consequences of renal claudin-10 deficiency, we per­formed a histological examination of the kidneys of 10-wk-old cKO mice and their respective controls. Kidneys from cKO mice contained extensive medullary calcium deposits, as revealed by von Kossa and alizarin red S staining. The deposits were found along the outer stripe of the outer medulla. The detection of extensive calcification suggests alterations in renal ion homeostasis in mice deficient for claudin-10.  Serum Na+ and Cllevels and their renal FE excretion rates were not different be­tween genotypes. In addition, serum creatinine and glomerular filtration rate were not altered compared with controls. Taken together, these findings indicate that calcium deposition does not nonpecifically affect overall glomerular or tubular function.

Fig 4. Gene expression analysis of renal claudins (A) and representative renal ion transporters and channels (B) by real-time PCR. Cldn10 deficiency results in differential gene expression of several genes. Values from cKO animals are shown relative to control mice (mean ± SEM). Wnk1, Wnk1-KS, Kcnj1, and Trpm6, n = 5/4; all other genes, n = 10/10. *P < 0.05; **P < 0.01; ***P < 0.001.    The thiazide-sensitive NaCl cotransporter NCC (Slc12a3), the protein involved in NaCl absorption in the DCT, and the respective inhibitory, kidney-specific kinase-defective KS-WNK1 were expressed at lower levels in the cKO mice. Taken together, these data suggest specific compensatory alterations in components of both paracellular and transcellular renal ion transport mechanisms in mice deficient in claudin-10 in the TAL.

Urinalysis demonstrated that the inhibition of TAL tubular transport by furosemide resulted in a completely differ­ent pattern of tubular Ca2+ and Mg2+ handling that identifies the TAL as the major nephron segment affected by claudin-10 deficiency.  Interestingly, the different effects on plasma Mg2+ and Ca2+ levels reflect the different major reabsorption sites of these ions. Some 60% of the filtered Mg2+ is reabsorbed in the TAL, compared with only 20% of the filtered Ca2+ load (20). Ca2+ hyperreabsorption in TAL seems to be balanced by reduced (proximal and) distal tubular Ca2+ transport. The hyperreabsorption of divalent cations in mice deficient in claudin-10 is in opposition to the loss of divalent cations seen in mouse models of claudin-16 deficiency and in human patients with mutation in CLDN16 or CLDN19. This finding indicates that claudins in the TAL have functions that differentially affect paracellular cation transport in this segment. Mice deficient for claudin-10b in the TAL exhibit decreased permeability for Na+ and increased permeability for Ca2+ and Mg2+, whereas in mice with claudin-16 or claudin-19 deficiency, decreased sodium per­meability in the TAL is paralleled by decreased reabsorption of Ca2+ and Mg2+.

1. Greger R (1981) Cation selectivity of the isolated perfused cortical thick ascending limb of Henle’s loop of rabbit kidney. Pflugers Arch 390:30–37.
2. Furuse M (2010) Molecular basis of the core structure of tight junctions. Cold Spring Harb Perspect Biol 2:a002907.
3. Konrad M, et al. (2006) Mutations in the tight-junction gene claudin 19 (CLDN19) are associated with renal magnesium wasting, renal failure, and severe ocular in­volvement. Am J Hum Genet 79:949–957.
4.  Simon DB, et al. (1999) Paracellin-1, a renal tight junction protein required for par-acellular Mg2+ resorption. Science 285:103–106.
5. Hou J, et al. (2007) Transgenic RNAi depletion of claudin-16 and the renal handling of magnesium. J Biol Chem 282:17114–17122.
6. Himmerkus N, et al. (2008) Salt and acid-base metabolism in claudin-16 knockdown mice: Impact for the pathophysiology of FHHNC patients. Am J Physiol Renal Physiol 295:F1641–F1647.

 PMID: 22891322  PMCID: PMC3435183

Paracellin-1 is critical for magnesium and calcium reabsorption in the human thick ascending limb of Henle.

Blanchard A, Jeunemaitre X, Coudol P, Dechaux M, Froissart M, et al.
Université Pierre et Marie Curie, INSERM and Laboratoire de Génétique Moléculaire, Hôpital Universitaire Européen Georges Pompidou, Paris, France.
Kidney Int. 2001 Jun; 59(6):2206-15.

A new protein, named paracellin 1 (PCLN-1), expressed in human thick ascending limb (TAL) tight junctions, possibly plays a critical role in the control of magnesium and calcium reabsorption, since mutations of PCLN-1 are present in the hypomagnesemia hypercalciuria syndrome (HHS).
No functional experiments have demonstrated that TAL magnesium and calcium reabsorption were actually impaired in patients with HHS.
Genetic studies were performed in the kindred of two unrelated patients with HHS.

We found two yet undescribed mutations of PCLN-1 (Gly 162 Val, Ala 139 Val). In patients with HHS, renal magnesium and calcium reabsorptions were impaired as expected; NaCl renal conservation during NaCl deprivation and NaCl tubular reabsorption in diluting segment were intact. Furosemide infusion in CS markedly increased NaCl, Mg, and Ca urinary excretion rates. In HHS patients, furosemide similarly increased NaCl excretion, but failed to increase Mg and Ca excretion. Acute MgCl(2) infusion in CS and ERH patient provoked a dramatic increase in urinary calcium excretion without change in NaCl excretion. When combined with MgCl(2) infusion, furosemide infusion remained able to induce normal natriuretic response, but was unable to increase urinary magnesium and calcium excretion further. In HHS patients, calciuric response to MgCl(2) infusion was blunted.

In patients with HHS, levels of circulating renin and aldosterone were normal, suggesting normal blood and extracellular volume. In addition, HHS patient 2 was normally able to lower her sodium excretion below 10 mmol/day during sodium deprivation, and in HHS pa­tient 1, sodium reabsorption in the diluting segment was normal as assessed by hypotonic saline infusion.  After oral NH4Cl load: Minimal urinary pH was 5.8 (normal value <5.4), and maximal net acid excretion reached only 24 pmol/min (normal value >80). Both subjects had hypocitraturia. The latter data suggested in the two probands distal defect of urinary acidification, probably related to nephrocalcinosis.

Because the filtered load of calcium but not the filtered load of magnesium remains unchanged during acute magnesium infusion in humans, the increase in calcium excretion is a better index of the inhibitory effect of peritubular magnesium on renal tubular divalent cation transport.  Urinary sodium excretion remained almost constant in both subjects during MgCl2 infusion (data not shown). Accordingly, the FECa/FENa ratio, which should remain constant if sodium reabsorption was primarily affected, increased in the CS and EHR patient.  Before the furosemide infusion, serum ultrafilterable (UF) Ca concentrations were similar in patients with HHS and the controls. However, Ca excretion markedly differed and was approximately five times higher in HHS patients than in controls.

In the two patients with homozygous mutations in the PCLN-1 gene, an impairment in renal tubular magne­sium and calcium reabsorption with normal NaCl recla­recla­mation was demonstrated. Accordingly, comparative studies performed under baseline condition in one pa­tient with ERH and in HHS patients demonstrated that the magnesium and calcium excretion in HHS patients were inappropriately high when compared with serum magnesium and calcium concentrations. However, renal NaCl reabsorption in HHS patients was intact. There was no clinical evidence of extracellular fluid volume  contraction. Furthermore, basal circulating renin and aldosterone concentrations were normal and adapted to the normal Na intake. Finally, abnormal NaCl reclama­tion in the diluting segment of the nephron was excluded in one patient, while the other was able to adapt normally to a sodium deprived diet.

This study is the first to our knowledge to demonstrate that homozygous mutations of PCLN-1 result in a selective defect in paracellular Mg and Ca reabsorption in the TAL, with intact NaCl reabsorption ability at this site. In addition, the study supports a selective physiological effect of basolateral Mg(2+) and Ca(2+) concentration on TAL divalent cation paracellular permeability, that is, PCLN-1 activity.   PMID: 11380823

Development of a Novel Sodium-Hydrogen Exchanger Inhibitor for Heart Failure

Elizabeth Juneman*, Reza Arsanjani, Hoang M Thai, Jordan Lancaster, Jeffrey B Madwed, Steven Goldman
Citation: Elizabeth Juneman, et al. (2013) Development of a Novel Sodium-Hydrogen Exchanger Inhibitor for Heart Failure. J Cardio Vasc Med 1: 1-6

This study was designed to determine the potential therapeutic effects of a new sodium-hydrogen exchanger (NHE-1) inhibitor in the rat coronary artery ligation model of chronic heart failure. After the induction of acute myocardial infarction, rats were entered randomly dose dranging from 0.3 mg/kg, 1.0 mg/kg, and 3.0 mg/kg. Solid state micrometer hemodynamics, echocardiographic, and pressure-volume relationships were measured after 6 weeks of treatment. Treatment with this NHE- 1 inhibitor at 3 mg/kg increased (P< 0.05) ejection fraction from 23±3% (N=6) to 33±2% (N=13) while the 1 mg/kg dose decreased (P< 0.05) the infarct size in CHF rats from 21.7±1.4% (N=7) to 15.9±0.7% (N=3) and prevented (P< 0.05) dilatation of the left ventricle in CHF rats in diastole (1.0±0.1 cm, N=6) to 0.9±0.1 cm, N=10) and in systole (0.9±0.1 cm, N=6) to 0.8±0.1, N=10). These study results suggest that this new NHE-1 inhibitor may be potentially useful in treating CHF with an improvement in maladaptive left ventricule remodeling. Because the mechanism of action of this agent is entirely different than the currently applied approach in treating CHF that focuses on aggressive neurohormonal blockade and because this agent does not adversely affect important hemodynamic variables, further investigations with this agent may be warranted.

Keywords: Congestive heart failure; Sodium/hydrogen exchange; Cardiovascular disease; Cardiovascular drugs; CHF: Chronic Heart Failure; NHE-1: Sodium-Hydrogen Exchanger; NCX: Sodium-Calcium Exchanger; Ca2+: Calcium; Na+: Sodium; Na+-K+ATPase: Sodium-Potassium ATPase; NKCC: Sodium-Potassium-Chloride co-transporter; MI: Myocardial Infarction; BI: Boehringer Ingelheim; LV: left Ventricle; EF: Ejection Fraction; LVD: left ventricular dysfunction; PV: Pressure-Volume; SE: Standard Error; ARB: Angiotensin Receptor Blocker; ACE: Angiotensin Converting Enzyme

Without reviewing the pathophysiology of CHF here, altered calcium (Ca2+) handling is a hallmark of CHF. Intracellular Ca2+ concentration is closely regulated by sodium-calcium exchanger (NCX) and Ca2+ efflux is dependent on the intracellular sodium (Na+) concentration and trans-sarcolemmal Na gradient. Multiple channels including sodium-potassium ATPase (Na+-K+ ATPase), sodium-hydrogen exporter (NHE), sodium-bicarbonate co-transporter, sodium-potassium-chloride co-transporter (NKCC), and sodium-magnesium exchanger are responsible for regulation of intracellular sodium in cardiac myocytes. The intracellular concentration of Na+ is significantly increased in heart failure, primarily due to influx of Na+. The NHE plays an integral part in rise of intracellular Na+ concentration and development of hypertrophy in heart failure. Because of its multifaceted role in myocardial function, there has been interest in examining the effects of NHE-1 inhibitors in heart failure.

In this study we report the physiologic responses of a new NHE-1 inhibitor, in a rodent model of heart failure. Previous evaluation of the pharmacokinetic properties of this agent in rat and dog revealed low clearance and robust oral bioavailability, suggesting a potential for once daily oral administration. This new compound was found to be potentially effective in preventing ischemic injury in isolated cells systems and in ischemic injury in isolated cells systems and in a Langendorff isolated heart preparation. Based on these encouraging a pharmacokinetic data, and the established preclinical roof of principle, the next step in new drug development was to test this inhibitor in an appropriate disease-relevant animal model. For this, we chose the rat coronary ligation model of CHF, which is the established model of chronic ischemic heart failure and well performed in our laboratory. The model with permanent occlusion of the left coronary artery is important because this model a similar to the clinical syndrome of CHF. This rat coronary artery model of CHF is the same model used in the classic study defining the beneficial use of angiotensin converting enzyme inhibition with captopril in the treatment of CHF. Thus results in this model have the potential to be predictive of the clinical response seen in patients.


In vivohemodynamic effect of NHE1: As noted previously by our laboratory, rats with severe CHF compared to Sham had changes (P< 0.05) in right ventricular weight, mean arterial pressure, tau, the time constant of LV relaxation, LV systolic pressure, LV end-diastolic pressure, +LVdP/dt, -LVdP/dt, dead volume and peak developed pressure. In this study, treatment resulted in no changes in body weight, chamber weight or hemodynamics. Because we stopped the lowest dose (0.3 mg/kg) there are only hemodynamic data with this dose in rats with CHF.

Echocardiographic changes in LV function and Dimensions with NHE1: Rats with CHF have decreases in EF accompanied by increases in LV systolic and diastolic dimensions. There was no change in anterior wallsystolic displacement. These data are consistent with other reports in this model showing that at 6 weeks after left coronary artery ligation, rats with large MIs have dilated left ventricles with LV remodeling and poor LV function (14,15). Treatment with the highest dose of 3 mg/kg increased (P< 0.05) ejection fraction from 23±3% (N=6) to 33±2% (N=13). Treatment with 1 mg/kg prevented maladaptive LV remodeling, it prevented (P< 0.05) dilatation of the LV in CHF rats in diastole (1.0±0.1 cm, N=6) to 0.9±0.1 cm, N=10) and in systole (0.9±0.1 cm, N=6) to 0.8±0.1, N=10) with no change anterior wall thickening.

Pressure-Volume relationships:  Although there are no significant changes in the PV relationships for either the Sham or CHF rats, there is a trend for the PV loop in CHF to be shifted toward the pressure axis with treatment. These data are consistent with the trend toward decreases in LV dimensions seen with treatment in CHF rats.


This study can be viewed as a corollary of a pilot Phase II clinical trial to look for a signal of a beneficial physiologic effect of this new NHE-1 inhibitor in CHF. In terms of drug development, this is an appropriate approach, i.e., take an agent with a therapeutic focus, with an acceptable toxicology profile, alter its pharmacokinetics to improve its oral delivery and bioavailability and then study the drug in an appropriate animal model. The administration of this agent to rats with CHF after left coronary artery ligation resulted in a therapeutic benefit with an increase in EF and a decrease in infarct size in rats with the largest infarcts. There is a suggestion of the prevention of LV remodeling with decreases in LV end-diastolic and end-systolic dimensions accompanied by a similar trend in the PV loop with a shift toward the pressure axis. There were no changes in hemodynamics.

Importantly, the decrease in infarct size with no changes in hemodynamicswould positively affect LV remodeling by minimizing LV dilatation without changes in LV afterload. From a therapeutic perspective, an agent like this may be advantageous in the treatment of heart failure after MI. The lack of hemodynamic changes is not a clinical problem because we already have agents that decrease afterload and lower LV end-diastolic pressure such as angiotensin converting enzyme (ACE) inhibitors and angiotens in receptor blockers (ARBs). In treating CHF, we also have diuretics to control blood volume, which in turn reduces LV end-diastolic pressure. The other potential advantage of an NHE-1 inhibitor is that as opposed to our current use of aggressive neurohormonal blockade, this represents a different approach to treating CHF. This is attractive because we essentially have exhausted or maximized our effects of neurohumoral blockade and with no real new treatments for CHF introduced in the last 10-15 years, need to look for other approaches to treat CHF.

Drug development is obviously a complicated and expensive undertaking. In exploring this agent, we would proposea stepwise approach. In this case with an agent whose analogs have been studied extensively, our thought would be to perform a larger dose ranging study in CHF rats to define dose response curves for systolic function as well as obtain more information on pharmacokinetics, as well as diastolic function and structural changes.

An attractive aspect of this work is that we are examining an agent with a different mechanisms of action that current treatments for heart failure. The stimulus to study the agent in an animal model of heart failure was based on multifaceted roles of sodium-hydrogen exchangers on myocardial function. Nine isoforms of NHE have currently been identified, with NHE- 1 being the predominant isoform in the plasma membrane of the myocardium [3,24]. Because NHE is activated by intracellular acidosis, angiotensin II, and catecholamines, its activity is expectedly increased in heart failure. Inhibition of NHE-1 has previously been associated with decreased fibrosis, apoptosis, preserved contractility, and attenuation of hypertrophy and development of heart failure.

1. Baartscheer A, van Borren MMGJ (2008) Sodium Ion Transporters as New Therapeutic Targets in Heart Failure. Cardiovasc Hematol Agents Med Chem 6: 229-236.
2. Murphy E, Eisner DA (2009) Regulation of Intracellular and Mitochondrial Sodium in Health and Disease. Circ Res 104: 292-303
3. Despa S, Islam MA, Weber CR, Pogwizd SM, Bers DM (2002) Intracellular Na(+) Concentration is Elevated in Heart Failure but Na/K Pump Function is Unchanged. Circulation 105: 2543-2548.
4. Baartscheer A, Schumacher CA, van Borren MMGJ, Belterman CNW, Coronel R, et al. (2003) Increased Na+/H+-Exchange Activity is the Cause of Increased [Na+]i and Underlies Disturbed Calcium Handling in the Rabbit Pressure and Volume Overload Heart Failure Model. Cardiovasc Res 57: 1015-1024.
5.  Pieske B, Houser SR (2003) [Na+]i Handling in the Failing Human Heart. Cardiovasc Res 57: 874-886.
6.  Engelhardt S, Hein L, Keller U, Klämbt K, Lohse MJ (2002) Inhibition of Na(+)-H(+) Exchange Prevents Hypertrophy, Fibrosis, and Heart Failure in Beta(1)- Adrenergic Receptor Transgenic Mice. Circ Res 90: 814-819.
7.  Chen L, Chen CX, Gan XT, Beier N, Scholz W, et al. (2004) Inhibition and Reversal of Myocardial Infarction-Induced Hypertrophy and Heart Failure by NHE-1 Inhibition. Am J Physiol Heart Circ Physiol 286: 381-387.
8.  Marano G, Vergari A, Catalano L, Gaudi S, Palazzesi S, et al. (2004) Na+/ H+ Exchange Inhibition Attenuates Left Ventricular Remodeling and Preserves Systolic Function in Pressure-Overloaded Hearts. Br J Pharmacol 141: 526-532.
9. Goldman S, Raya TE (1995) Rat Infarct Model of Myocardial Infarction and Heart Failure. J Card Fail 1: 169-177.
10. Gaballa MA, Goldman S (2002) Ventricular Remodeling in Heart Failure. J Card Fail. 8: 476-485.
11. Pfeffer MA, Pfeffer JM, Steinberg C, Finn P (1985) Survival After Experimental Myocardial Infarction: Beneficial Effects of Long-Term Therapy with Captopril. Circulation 72: 406-412.
12. Raya TE, Gay RG, Aguirre M, Goldman S (1989) Importance of Venodilatation in Prevention of Left Ventricular Dilatation after Chronic Large Myocardial Infarction in Rats: A Comparison of Captopril and Hydralazine. Circ Res 64: 330-337.

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pone.0070764.g006  Morpholino knockdown of aquaporin-1a1 reduces water influx.       NIHMS262281.html

nihms81087f1  Localization of claudin proteins in mammalian kidney.      F1.medium  intracellular Mg2+ in normal and Mg2+ depleted immortalized mouse distal convoluted tubule (MDCT) cells

F2.small  membrane voltage influences Mg2+ uptake in MDCT cells    pnas.1203834109fig04  Gene expression analysis of renal claudins

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Translational Research on the Mechanism of Water and Electrolyte Movements into the Cell

Reviewer and Curator: Larry H. Bernstein, MD, FACP


This article is the first in a three part curation covering work that has great importance to our understanding of hydration and possibly the effects of dehydration in cell physiology, and studied effects on renal function and brain, with possible implications for heart failure, myocardial contraction, heart rate, and arrhythmiagenesis.  The discovery of aquaporins and the elucidation of potassium channels and selective ion conduction was jointly awarded the Nobel Prize in Chemistry in 2003 to Peter Agre, at the Johns Hopkins School of Medicine, Baltimore, and Roderick Mac Kinnon, at the Howard Hughes Medical Institute, Laboratory of Molecular Neurobiology and Biophysics, Rockefeller University, New York, NY.  The transport of water, it was assumed, is associated with the movements of Na(+), K(+), Ca(2+), Mg(2+).  The calmodulin kinase, rhyanodine, and calcium sparks in the Ca(2+) release from sarcolemma is covered elsewhere in cardiac contraction, skeletal muscle, smooth muscle, and neural stimulation of muscle and adrenergic release.  The sodium/potassium exchange is depicted in diagrams, but not discussed.  In traditional chemistry we would think in terms of a cationic and anionic balance that has to be maintained in charge equivalents on both sides of a membrane.  However, the intricacies of membrane structure as well as active transporters has been delineated and has been a transformative factor in our understanding of organ function in health and disease.

Aquaporin Water Channels

AQUAPORIN WATER CHANNELS: Nobel Lecture, Dec 8, 2003, by Peter Agre. Fig1 Membrane orientation of AQP1

We have studied the aquaporin water channels for several years, and we now understand that they explain how water crosses biological membranes. Our bodies are 70% water, and all other vertebrates, invertebrates, microbes, and plants are also primarily water. The organization of water within biological compartments is fundamental to life, and the aquaporins serve as the plumbing systems for cells. Aquaporins explain how our
brains secrete and absorb spinal fluid, how we can generate aqueous humor within our eyes, how we can secrete tears, saliva, sweat, and bile, and how our kidneys can concentrate urine so effectively. These proteins are fundamental to mammalian physiology, but they are also very important in the lives of microorganisms and plants.
It was correctly proposed  in the 1920’s that water could move through the cell membrane simply by diffusing through the lipid bilayer. The current view is that the lipid bilayer has a finite permeability for water, but, in addition, a set of proteins exists that we now refer to as “aquaporins.” Their existence was suggested by a group of pioneers in the water transport field who preceded us by decades – people including Arthur K. Solomon in Boston, Alan Finkelstein in New York, Robert Macey in Berkeley, Gheorghe Benga in Romania, Guillermo Whittembury in Venezuela, Mario Parisi in Argentina – who by biophysical methods predicted that water channels must exist in certain cell types with high water permeability such as renal tubules, salivary glands, and red cells (reviewed by Finkelstein, 1987).
The difference between diffusional and channel-mediated water perme-ability is fairly distinct. Diffusion is a low capacity, bidirectional movement of water that occurs in all cell membranes, whereas the membranes of a subset of cells with aquaporin proteins have very high capacity for permeation by water.
This permeability is selective, since water (H O) crosses through the membranes with almost no resistance, while acid, the hydronium ion (H O ) does not permeate the proteins. This distinction is essential to life. The movement of water is directed by osmotic gradients, so aquaporins are not pumps or exchangers. They form a simple pore that allows water to rapidly pass through membranes by osmosis. There are also other differences between diffusion and channel-mediated water transport. No inhibitors are known for simple diffusion. In contrast, mercurials were discovered by Robert Macey to inhibit water transport in red cells but water permeability was restored by treatment with reducing agents (Macey and Farmer, 1970). These observations predicted that water channels must be proteins with sulfhydryls and characteristically low Arrhenius activation energy.
A number of investigators using ver y logical approaches attempted to identify the water channel molecule; identification proved a very difficult prolem. Isotopic mercurials labeled several membrane proteins – the anion exchanger (band 3). Solomon and a group of several proteins (band 4.5) by Benga. None of the proteins were isolated, reconstituted, and shown  to transport watter (reviewed by Agre et al., 1993a).


The field was essentially stuck, but following the well known scientific approach known as “sheer blind luck,” we stumbled upon the protein. Looking through our notebooks for the earliest studies that showed there was such a protein water channel. We were at that time attempting to raise antibodies in rabbits to the denatured partially purified Rh polypeptide.  The rabbits vigorously produced antibodies, but we failed to recognize initially that our antibody did not react with the core Rh polypeptide that migrated at 32 kDa, seen clearly by silver staining of sodium dodecyl sulfate polyacryamide electrophoresis gels (SDS-PAGE). Instead, our antibodies reacted only with a 28 kDa polypeptide. The 28 kDa was an unrelated protein.  Silver staining of SDS-PAGE migration of the isolated protein revealed a discrete band of 28 kDa in detergent insoluble extracts (it failed to stain with the conventional protein stains such as Coomassie blue). The protein was then purified in large amounts from human red cell membranes (Denker et al., 1988; Smith and Agre, 1991).  The 28 kDa protein was strikingly abundant. With approximately  copies per red cell, it was one of the major proteins in the membrane. The protein had features suggesting that it was a tetrameric membrane-spanning protein – suggesting that it was a channel, but a channel for what? The purified protein also provided us the N-terminal amino acid sequence that we used for cDNA cloning. Using our antibody, we looked at several other tissues and found the protein is also strikingly abundant in human kidney. We observed staining over the apical and basolateral membranes of proximal renal tubules and the descending thin limb of the loops of Henle, but we were still frustrated by our failure to recognize what the protein’s function might be.  My clinical mentor, John C. Parker (1935–1993) at the University of North Carolina at Chapel Hill, was the first to suggest to me that red cells and renal tubules were exceedingly permeable to water. He recommended that we consider a role in membrane water transport. While John did not live to see our later studies, he did live to see our initial discovery and we celebrated together.
Postdoctoral fellow Gregor y Preston cloned the cDNA from an erythroid brary (Preston and Agre, 1991). The coding region corresponded to a 269 amino acid polypeptide, predicted by hydropathy analysis to have six bilayer-spanning domains. Interestingly, the amino terminal half (repeat-1) and the carboxy terminal half of the molecule (repeat-2) were genetically related – about 20% identical. Two loops B and E were more highly related to each other, and each contained the signature motif – asparagine, proline, alanine (NPA) [Fig. 1]. Examining the genetics database, we recognized several sequence-related DNAs from diverse sources: lens of cow eyes, brains of fruit flies, bacteria, and plants. Nevertheless, none was functionally defined.
Figure 1. Membrane orientation of AQP1 predicted from primary amino acid sequence. Two tandem repeats each have three bilayer-spanning domains; the repeats are oriented 180˚ with respect to each other. The loops B and E each contain the conser ved motif, Asn- Pro-Ala (NPA)
These clues heightened our suspicion that the 28 kDa protein was a transporter, so we tested for possible water transport function with our colleague Bill Guggino at Johns Hopkins. We used oocytes the frog Xenopus laevis, a useful model, since frog oocytes have very low water permeability. Control oocytes were injected with water alone; oocytes were injected with 2 ng of cRNA encoding our protein. After days of protein synthesis, the oocytes appeared essentially identical. Then we stressed the oocytes by transferring them to distilled water, and an amazing difference was immediately apparent. Having exceedingly low water perme-
ability, the control oocytes failed to swell. In contrast, the test oocytes were highly permeable to water and exploded like popcorn [Fig. 2] (Preston et al., 1992).  The protein was christened “aquaporin” and is now officially designated “AQP1,” the first functionally defined water channel protein (Agre et al., 1993b).
Figure 2. Functional expression of AQP1 water channels in Xenopus laevis oocytes. Control oocyte (left) was injected with water; AQP1 oocyte (right) was injected with cRNA. The oocytes were transferred to hypotonic buffer. After 30 seconds (top) the AQP1 oocyte has begun to swell; after 3 minutes (bottom), the AQP1 oocyte has exploded. Modified and reprinted from Science with permission (Preston et al., 1992).
We  confirmed the function of this protein by studying the purified AQP1 reconstituted into synthetic lipid vesicles of ~0.1 micron diameter prepared by our colleague Suresh Ambudkar at Johns Hopkins (Zeidel et al., 1992). These simple membrane vesicles were examined by freeze fracture electron microscopy by our colleague Arvid Maunsbach, from the University of Aarhus. When lipid was reconstituted without protein, the membrane surfaces were smooth; however, membranes reconstituted with AQP1 contained many intramembraneous particles 0.01 micron diameter (Zeidel et al., 1994). We tested the membranes for water permeability in collaboration with Mark Zeidel at Har vard Medical School. Using stopped flow transfer to hypertonic buffer, the simple liposomes shrank, reaching equilibrium in about one half
second; this is believed to represent the baseline water permeability. When membranes reconstituted with AQP1 were examined, the shrinking occurred much more rapidly, reaching equilibrium in about 20 milliseconds. The channel-mediated flow of water was confirmed, since it was inhibited with mercurials. We calculated the Arrhenius activation energy (<5 kcal/mol), and we determined the unit permeability to be ~3×10 water molecules per subunit per second. Importantly, we attempted to measure proton permeation of AQP1, but despite massive water permeability, acid permeation was not detected. These studies verified that we had, in fact, isolated the long-sought water channel protein.


Subsequent efforts were devoted to identifying the mercurial inhibitory site predicted by the studies of Macey. Mercurials react with free sulfhydryls in the amino acid cysteine. Four cysteines are found in the AQP1 polypeptide, but only the residue in loop E (Cys-189 proximal to the second NPA motif) is inhibited by mercurials. We altered the AQP1 sequence by site-directed mutagenesis and expressed the recombinants in oocytes for water permeability studies. Mutation of this residue to serine (Cys-189-Ser) resulted in full water permeability without mercurial inhibition. When we then replaced the alanine in the corresponding position of loop B with a cysteine (Ala-73-Cys), the protein exhibited mercurial sensitive water permeability (Preston et al., 1993). Substitutions elsewhere in the AQP1 failed to produce this behavior. This suggested to us that loops B and E in opposite parts of the molecule must somehow form the aqueous pore. The model that we concocted turned out to be schematically correct and was termed “the hourglass.” The ancient timepiece allows sand to run from upper chamber to lower chamber; if inverted, the sand will flow in the opposite direction. Six bilayer spanning domains were predicted to surround a central domain containing loop B, dipping into the membrane from the cytoplasmic surface, and loop E, dipping into the membrane from the extracellular surface [Fig. 3 left and right].
Figure 3. Hourglass model for membrane topology of AQP1 subunit.
Left panel – Schematic folding of loops B and E overlap within the lipid bilayer to form a single aqueous pathway.
Right panel – Ribbon model of three dimensional structure of AQP1 subunit confirms hourglass with single aqueous pathway. Modified and reprinted with permission from Jour-
nal of Biological Chemistr y (Jung et al., 1994b) and Journal of Clinical Investigation (Kozono et al., 2002).
The overlap of loops B and E was predicted to form a single aqueous pore through the center of the molecule with the NPA motifs juxtaposed and mercurial inhibitory site alongside (Jung et al., 1994b). The AQP1 protein tetrameric with a central pore in each subunit. Thus, AQP1 is structurally like ion channel proteins where four subunits surround a single central as discussed by Rod MacKinnon in his lecture.
We  then sought to establish the high resolution structure of AQP1 in collaboration with Andreas Engel and his group at the Biozentrum in Basel. We were later joined with Yoshinori Fujiyoshi and his group at Kyoto University. Human red cell AQP1 protein was purified by Barb Smith in our lab; Andreas’s student Tom Walz reconstituted it into synthetic membranes at very high protein concentrations. Under these conditions, the AQP1 protein forms remarkably symmetrical arrays referred to as membrane crystals. By measuring the water permeability, we confirmed that the function was 100% retained, giving us confidence that the structure we deduced would be the biologically relevant structure (Walz et al., 1994).
Figure 4. Functional representation for selective water flow through AQP1 subunit and residues involved in human disease.
Left panel – Schematic of sagittal cross-section of AQP1 reveals bulk water in extracellu- lar and intracellular vestibules of hourglass. These are separated by a 20Å span where water passes in single file with transient interactions with pore-lining residues that prevent hy- drogen bonding between water molecules (bold colors). Two structures are believed to pre- vent permeation by protons (H O ): electrostatic repulsion is created by a fixed positive
charge from pore-lining arginine (R195) at a 2.8Å narrowing in the channel; water dipole reorientation occurs from simultaneous hydrogen bonding of water molecule with side chains of two asparagines residues in NPA motifs (N192 and N76). Two partial positive charges at the center of the channel result from orientation of two non-membrane span- ning alpha helices distal to the NPA motifs


While we were pursuing studies of AQP1, several other research groups from around the world became interested in what is now known to be a large family of related proteins. The combined efforts of these labs have led to the molecular identification of 12 mammalian aquaporin homologs, and several hundred related proteins have been recognized in other vertebrates as well as invertebrates, plants, and unicellular micro-organisms. The mammalian homologs may be loosely clustered into two subsets [Fig. 5]. The first is referred to as “classical aquaporins”, since they were initially considered to be exclusive water pores. The second is referred to as “aquaglyceroporins”, since they are permeated by water plus glycerol. Interestingly, E. coli has one member of
each – AqpZ (Calamita et al., 1995), and GlpF, isolated by other investigators much earlier. Together, the mammalian aquaporins and aquaglyceroporins are now known to contribute to multiple physiological processes that occur during our daily lives.
Figure 5. Human aquaporin gene family contains two subsets. Homologs freely permeated by water (classical aquaporins, blue) or water and glycerol (aquaglyceroporins, yellow) are represented. E. coli has one aquaporin (AqpZ) and one aquaglyceroporin (GlpF). Reprinted with permission from Journal of Physiology (Agre et al., 2002)
 The remainder of the Nobel Lecture (2003) can be found at the Nobel Prize site.  This portion is sufficient to cover the genesis and advancement of the water transport discovery.

Urinary Excretion of Aquaporin-2 Water Channel Differentiates Psychogenic Polydipsia from Central Diabetes Insipidus

T Saito, San-e Ishikawa, T Ito, H Oda, F Ando, … and T Saito Division of Endocrinology and Metabolism (Ta.S., S.I., F.A., Mi.H., S.N., To.S.), Department of Medicine, Jichi Medical School, Tochigi 329-0498; and Departments of Medicine and Psychiatry (T.I., H.O., Ma.H.), Tokyo Metropolitan Matsuzawa Hospital, Tokyo, Jp 
correspondence to: San-e Ishikawa, M.D., Division of Endocrinology and Metabolism, Department of Medicine, Jichi Medical School, Tochigi 329-0498, Japan. E-mail:
The present study was undertaken to determine whether urinary excretion of aquaporin-2 (AQP-2) water channel under ad libitum water intake is of value to differentiate polyuria caused by psychogenic polydipsia from central diabetes insipidus. A 30-min urine collection was made at 0900 h in 3 groups of: 11 patients with central diabetes insipidus (22–68 yr old), 10 patients with psychogenic polydipsia (28–60 yr old), and 15 normal subjects (21–38 yr old). In the patients with central diabetes insipidus, the plasma arginine vasopressin level was low despite hyperosmolality, resulting in hypotonic urine. Urinary excretion of AQP-2 was 37 ± 15 fmol/mg creatinine, a value one-fifth less than that in the normal subjects. In the patients with psychogenic polydipsia, plasma arginine vasopressin and urinary osmolality were as low as those in the patients with central diabetes insipidus. However, urinary excretion of AQP-2 of 187 ± 45 fmol/mg creatinine was not decreased, and its excretion was equal to that in the normal subjects. These results indicate that urinary excretion of AQP-2, under ad libitum water drinking, participates in the differentiation of psychogenic polydipsia from central diabetes insipidus. 
PSYCHOGENIC polydipsia causes a marked polyuria with hypotonic urine (1, 2). Arginine vasopressin (AVP) secretion is suppressed by hypoosmolality caused by excess intake of water. Suppression of AVP release obliges us to differentiate psychogenic polydipsia from central diabetes insipidus. Osmotic stimulation tests have been carried out to determine the reserve function of the posterior pituitary gland. Plasma AVP levels increase in response to an increase in plasma osmolality (Posm) in patients with psychogenic polydipsia but not in those with central diabetes insipidus.
In response to AVP, concentrated urine is produced by water reabsorption across the renal collecting duct (3, 4). Aquaporin-2 (AQP-2) is an AVP-regulated water channel of the collecting duct; it is translocated from the cytoplasmic vesicles to the apical plasma membranes by shuttle trafficking when the cells are stimulated by AVP (5, 6, 7), and it is again redistributed into the cytoplasmic vesicles after removal of AVP stimulation (8). Also, AQP-2 is, in part, excreted into the urine (9, 10). We demonstrated that urinary excretion of AQP-2 is of great value in diagnosing central diabetes insipidus in the hypertonic saline infusion test and impaired water excretion in the acute oral water load test (11, 12).   The present study was undertaken to determine whether urinary excretion of AQP-2, under ad libitum water intake, is a useful tool for diagnosing psychogenic polydipsia.

Subjects and study design

Three groups of subjects were examined in the present study.
[1]  11 patients who had been diagnosed as having idiopathic central diabetes insipidus. They had taken 1-deamino-8-D-AVP (DDAVP) intranasally, twice a day, and discontinued the DDAVP therapy 24 h before the study.
[2] 10 patients were diagnosed as having psychogenic polydipsia. They had been treated for psychiatric disorders, including schizophrenia, atypical psychiatric disorder, and chronic alcoholism.
[3] 15 normal volunteers, with ages ranging from 21–38 yr. (the age range of [1] and [2] reached 60)
All the subjects drank water ad libitum, and 30-min urine collection was made and blood drawn at 0900 h. Urine samples were subjected to measurements of urinary osmolality (Uosm) and urinary excretion of creatinine and AQP-2. Blood samples were used to measure Posm and plasma AVP levels. Uosm and Posm were measured by freezing-point depression (Model 3W2, Advanced Instruments, Needham Height, MA). Urinary creatinine was measured with an automatic clinical analyzer (Model 736, Hitachi Co., Tokyo, Jp). Plasma AVP levels were determined by RIA using AVP RIA kits (Mitsubishi Chemistry, Tokyo, Jp) (13). Urinary excretion of AQP-2 was measured as described below.

RIA of AQP-2

The RIA of urinary AQP-2 was performed by the method described in our previous reports (11, 12). Urinary AQP-2-like immunoreactivity was measured by a specific RIA that used the polyclonal antibody against a synthetic portion (Tyr0-AQP-2[ V257-A271]) of the C-terminal of human AQP-2 raised in rabbits. A synthetic peptide [Tyr0-AQP-2 (V257-A271)] was radioiodinated with iodine-125 (New England Nuclear, Boston, MA) by the chloramine-T method.  All samples were analyzed in duplicate. The intra- and interassay coefficients of variation were less than 10%. The minimal detectable quantity of AQP-2 was 0.86 pmol/tube, and an amount equivalent to 6.9 pmol/tube caused 50% inhibition of binding of the radiolabeled ligand.


In the patients with central diabetes insipidus, the plasma AVP level was low despite hyperosmolality of 297.8 ± 3.4 mosmol/kg H2O, resulting in hypotonic urine (Fig. 1⇓). Urinary excretion of AQP-2 was one-fifth less in the patients with central diabetes insipidus than in the normal subjects. AQP-2 is the AVP-dependent water channel of collecting duct cells and is recycling between the cytoplasmic vesicles and the apical plasma membranes in the cells (5, 6, 7, 8). AQP-2 is partly excreted into the urine, which is approximately 3% of AQP-2 in the collecting duct cells (14). In normal subjects, urinary excretion of AQP-2 is changeable in a wide range in physiological conditions (11). Because urinary excretion of AQP-2 has a positive correlation with plasma AVP levels in normal subjects (11), the reduced urinary excretion of AQP-2 was in concert with the impaired secretion of AVP in central diabetes insipidus.
Figure 1.
Posm, plasma AVP (Pavp), Uosm, and urinary excretion of AQP-2 (UAQP-2), under ad libitum water drinking, in 15 normal subjects (NL, •), 11 patients with central diabetes insipidus (CDI, ○) and 10 patients with psychogenic polydipsia (PP, □). *, P < 0.01; **, P < 0.05 vs. the normal subjects. Value are means ± sem.
In the patients with psychogenic polydipsia, Uosm was as low as that in the patients with central diabetes insipidus (Fig. 1⇑). The plasma AVP level was low because of the reduced Posm, which was derived from an exaggerated intake of water. Urinary excretion of AQP-2, however, was not decreased; and rather, its excretion kept the normal range. The relationship between plasma AVP levels and urinary excretion of AQP-2 is shown in Fig. 2⇓. The urinary excretion of AQP-2 in the patients with psychogenic polydipsia was dissociated from the positive correlation between plasma AVP and urinary excretion of AQP-2 in the normal subjects.
Figure 2.
Relationship between plasma AVP levels and UAQP-2. •, Normal subjects (n = 15); ○, patients with central diabetes insipidus (n = 11); □, patients with psychogenic polydipsia (n = 10). Values are means ± sem.


The present study demonstrated the clinical tool, of urinary excretion of AQP-2, in differentiating psychogenic polydipsia from central diabetes insipidus. What is involved in the marked difference in urinary excretion of AQP-2 in these two disorders? There is a possibility that, as patients with psychogenic polydipsia reduce water intake during sleep, antidiuresis may occur periodically at night and the production of AQP-2 be somewhat restored. Because approximately 3% of AQP-2 in collecting duct cells is excreted into the urine, urinary excretion of AQP-2 may keep relatively high, despite hypotonic urine. The difference may come from the periodicity of water intake in a day, in the patients with psychogenic polydipsia. As a whole, these changes may disrupt the positive relationship between urinary excretion of AQP-2 and plasma AVP levels. At the present time, however, other factors involved in urinary excretion of AQP-2 remain undetermined.
In conclusion, urinary excretion of AQP-2, under ad libitum water drinking, participates in the differentiation of polyuria caused by psychogenic polydipsia from central diabetes insipidus.


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Comparison of cardiovascular aquaporin-1 changes during water restriction between 25- and 50-day-old rats.

Netti VA, Vatrella MC, Chamorro MF, Rosón MI, Zotta E, Fellet AL, Balaszczuk AM.
Cátedra de Fisiología, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, IQUIMEFA, CONICET, Junín 956, C1113AAD, Buenos Aires, Argentina,
Eur J Nutr. Apr 27, 2013
Aquaporin-1 (AQP1) is the predominant water channel in the heart, linked to cardiovascular homeostasis. Our aim was to study cardiovascular AQP1 distribution and protein levels during osmotic stress and subsequent hydration during postnatal growth.
Rats aged 25 and 50 days were divided in: 3d-WR: water restriction 3 days; 3d-WAL: water ad libitum 3 days; 6d-WR+ORS: water restriction 3 days + oral rehydration solution (ORS) 3 days; and 6d-WAL: water ad libitum 6 days. AQP1 was evaluated by immunohistochemistry and western blot in left ventricle, right atrium and thoracic aorta.
Water restriction induced a hypohydration state in both age groups (40 and 25 % loss of body weight in 25- and 50-day-old rats, respectively), reversible with ORS therapy. Cardiac AQP1 was localized in the endocardium and endothelium in both age groups, being evident in cardiomyocytes membrane only in 50-day-old 3d-WR group, which presented increased protein levels of AQP1; no changes were observed in the ventricle of pups. In vascular tissue, AQP1 was present in the smooth muscle of pups; in the oldest group, it was found in the endothelium, increasing after rehydration in smooth muscle. No differences were observed between control groups 3d-WAL and 6d-WAL of both ages.
Our findings suggest that cardiovascular AQP1 can be differentially regulated in response to hydration status in vivo, being this response dependent on postnatal growth. The lack of adaptive mechanisms of mature animals in young pups may indicate an important role of this water channel in maintaining fluid balance during hypovolemic state.

 Clinical application of aquaporin research: aquaporin-1 in the peritoneal membrane

Nishino T, Devuyst O.
Division of Renal Care Unit, Second Department of Internal Medicine, Nagasaki University School of Medicine, Nagasaki, Jp
Peritoneal dialysis (PD) is an established mode of renal replacement therapy based on the exchange of fluid and solutes between blood and a dialysate that has been instilled in the peritoneal cavity. The dialysis process involves osmosis, as well as diffusive and convective transports through the highly vascularized peritoneal membrane. The membrane contains ultrasmall pores responsible for the selective transport of water across the capillary endothelium. The distribution of the water channel aquaporin-1 (AQP1), as well as its molecular structure ensuring an exquisite selectivity for water, fit with the characteristics of the ultrasmall pore. Peritoneal transport studies using AQP1 knockout mice demonstrated that the osmotic water flux across the peritoneal membrane is mediated by AQP1. This water transport accounts for 50% of the ultrafiltration during PD. Treatment with high-dose corticosteroids upregulates the expression of AQP1 in peritoneal capillaries, resulting in increased water transport and ultrafiltration in rats. These data illustrate the potential of the peritoneal membrane as an experimental model in the investigation of the role of AQP1 in the endothelium. They emphasize the critical role of AQP1 during PD and suggest that manipulating AQP1 expression could be clinically useful in PD patients.

Corticosteroids induce expression of aquaporin-1 and increase transcellular water transport in rat peritoneum

Stoenoiu MS, Ni J, Verkaeren C, Debaix H, Jonas JC, Lameire N, Verbavatz JM, Devuyst O.
Division of Nephrology and ENDO Unit, Université Catholique de Louvain Medical School, Brussels, Belgium
J Am Soc Nephrol. Mar 2003; 14(3):555-565.
The water channel aquaporin-1 (AQP1) is the molecular counterpart of the ultrasmall pore responsible for transcellular water permeability during peritoneal dialysis (PD). This water permeability accounts for up to 50% of ultrafiltration (UF) during a hypertonic dwell, and its loss can be a major clinical problem for PD patients. By analogy with the lung, the hypothesis was tested that corticosteroids may increase AQP1 expression in the peritoneal membrane (PM) and improve water permeability and UF in rats. First, the expression and distribution of the glucocorticoid receptor (GR) in the PM and capillary endothelium was documented. Time-course and dose-response analyses showed that a daily IM injection of dexamethasone (1 or 4 mg/kg) for 5 d induced an approximately twofold increase in the expression of AQP1 at the mRNA and protein levels. The GR antagonist RU-486 completely inhibited the dexamethasone effect. The functional counterpart of the increased AQP1 expression was a significant increase in sodium sieving and net UF across the PM, contrasting with a lack of effect on the osmotic gradient and permeability for small solutes. The latter observation reflected the lack of effect of corticosteroids on nitric oxide synthase (NOS) activity and endothelial NOS isoform expression in the PM. In conclusion, corticosteroids induce AQP1 expression in the capillary endothelium of the PM, which is reflected by increased transcellular water permeability and UF. These data emphasize the critical role of AQP1 during PD and suggest that pharmacologic regulation of AQP1 may provide a target for manipulating water permeability across the PM.

Aquaporins: relevance to cerebrospinal fluid physiology and therapeutic potential in hydrocephalus

Owler BK, Pitham T, Wang D.
Kids Neurosurgical Research Unit, Children’s Hospital at Westmead, Westmead NSW 2145, Australia.
Cerebrospinal Fluid Res.  Sep 22, 2010; 7:15.
The discovery of a family of membrane water channel proteins called aquaporins, and the finding that aquaporin 1 was located in the choroid plexus, has prompted interest in the role of aquaporins in cerebrospinal fluid (CSF) production and consequently hydrocephalus. While the role of aquaporin 1 in choroidal CSF production has been demonstrated, the relevance of aquaporin 1 to the pathophysiology of hydrocephalus remains debated. This has been further hampered by the lack of a non-toxic specific pharmacological blocking agent for aquaporin 1. In recent times aquaporin 4, the most abundant aquaporin within the brain itself, which has also been shown to have a role in brain water physiology and relevance to brain oedema in trauma and tumours, has become an alternative focus of attention for hydrocephalus research. This review summarises current knowledge and concepts in relation to aquaporins, specifically aquaporin 1 and 4, and hydrocephalus. It also examines the relevance of aquaporins as potential therapeutic targets in hydrocephalus and other CSF circulation disorders.
PMID: 20860832  PMCID:  PMC2949735

Pathophysiology of the aquaporin water channels

King LS, Agre P.
Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Annu Rev Physiol. 1996; 58:619-48.
Discovery of aquaporin water channel proteins has provided insight into the molecular mechanism of membrane water permeability. The distribution of known mammalian aquaporins predicts roles in physiology and disease.
Aquaporin-1 mediates proximal tubule fluid reabsorption, secretion of aqueous humor and cerebrospinal fluid, and lung water homeostasis.
Aquaporin-2 mediates vasopressin-dependent renal collecting duct water permeability; mutations or downregulation can cause nephrogenic diabetes insipidus.
Aquaporin-3 in the basolateral membrane of the collecting duct provides an exit pathway for reabsorbed water.
Aquaporin-4 is abundant in brain and probably participates in reabsorption of cerebrospinal fluid, osmoregulation, and regulation of brain edema.
Aquaporin-5 mediates fluid secretion in salivary and lacrimal glands and is abundant in alveolar epithelium of the lung.
Specific regulation of membrane water permeability will likely prove important to understanding edema formation and fluid balance in both normal physiology and disease.

Discovery of aquaporins: a breakthrough in research on renal water transport

van Lieburg AF, Knoers NV, Deen PM.
Department of Pediatrics, University of Nijmegen, The Netherlands.
Pediatr Nephrol. Apr 1995; 9(2):228-34.
Several membranes of the kidney are highly water permeable, thereby enabling this organ to retain large quantities of water. Recently, the molecular identification of water channels responsible for this high water permeability has finally been accomplished. At present, four distinct renal water channels have been identified, all members of the family of major intrinsic proteins.
Aquaporin 1 (AQP1), aquaporin 2 (AQP2) and the mercury-insensitive water channel (MIWC) are water-selective channel proteins, whereas the fourth,
Aquaporin 3 (AQP3), permits transport of urea and glycerol as well. Furthermore, a putative renal water channel (WCH3) has been found.
AQP1 is expressed in apical and basolateral membranes of proximal tubules and descending limbs of Henle,
AQP2 predominantly in apical membranes of principal and inner medullary collecting duct cells and
AQP3 in basolateral membranes of kidney collecting duct cells.
MIWC is expressed in the inner medulla of the kidney and has been suggested to be localised in the vasa recta.
The human genes encoding AQP1 and AQP2 have been cloned, permitting deduction of their amino acid sequence, prediction of their two-dimensional structure by hydropathy analysis, speculations on their way of functioning and DNA analysis in patients with diseases possibly caused by mutant aquaporins. Mutations in the AQP1 gene were recently detected in clinically normal individuals, a finding which contradicts the presumed vital importance of this protein. Mutations in the AQP2 gene were shown to cause autosomal recessive nephrogenic diabetes insipidus. The renal unresponsiveness to arginine vasopressin, which characterises this disease, is in accordance with the assumption that AQP2 is the effector protein of the renal vasopressin pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Selectivity of the renal collecting duct water channel aquaporin-3

Echevarría M, Windhager EE, Frindt G.
Depart Physiol Biophys, Cornell University Medical College, New York, NY
J Biol Chem. Oct 11, 1996; 271(41):25079-82.
Aquaporin-3 (AQP3) is a water channel found in the basolateral cell membrane of principal cells of the renal collecting tubule as well as in other epithelia. To examine the selectivity of AQP3, the permeability to water (Pf), urea (Pur), and glycerol (Pgly) of Xenopus oocytes injected with cRNA encoding AQP3 was measured. Oocytes injected with cRNA encoding either human or rat aquaporin-1 (AQP1) were used as controls. Although both aquaporins permit water flow across the cell membrane, only AQP3 was permeable to glycerol and urea (Pgly > Pur). The uptake of glycerol into oocytes expressing AQP3 was linear up to 165 mM. For AQP3 the Arrhenius energy of activation for Pf was 3 kcal/mol, whereas for Pgly and Pur it was >12 kcal/mol. The sulfhydryl reagent p-chloromercuriphenylsulfonate (1 mM) abolished Pf of AQP3, whereas it did not affect Pgly. In addition, phloretin (0.1 mM) inhibited Pf of AQP3 by 35%, whereas it did not alter Pgly or Pur. We conclude that water does not share the same pathway with glycerol or urea in AQP3 and that this aquaporin, therefore, forms a water-selective channel.

The aquaporin family of water channels in kidney

Agre P, Nielsen S.
Depart of Med, Johns Hopkins University School of Medicine, Baltimore, MD
Nephrologie. 1996;17(7):409-15.
The longstanding puzzle of membrane water-permeability was advanced by discovery of a new class of proteins known as the “aquaporins” (AQPs). First identified in red blood cells, AQP1 was shown to function as a water channel when expressed in Xenopus oocytes or when pure AQP1 protein was reconstituted into synthetic membranes. Analysis of the primary sequence revealed that the two halves of the AQP1 polypeptide are tandem repeats; site directed mutagenesis studies indicate that the repeats may fold into an obversely symmetric structure which resembles an hourglass. Electron crystallography elucidated the tetrameric organization of AQP1, and functional studies suggest that each tetramer contains multiple functionally independent aqueous pores.
AQP1 is abundant in the apical and basolateral membranes of renal proximal tubules and descending thin limbs, and is also present in multiple extra renal tissues.
AQP2 is expressed only in the principal cells of renal collecting duct where it is the predominant vasopressin (ADH, antidiuretic hormone) regulated water channel. AQP2 is localized in the apical membrane and in intracellular vesicles which are targeted to the apical plasma membranes when stimulated by ADH. Humans with mutations in genes encoding AQP1 and AQP2 exhibit contrasting clinical phenotypes.
AQP3 resides in the basolateral membranes of renal collecting duct principal cells providing an exit pathway for water;
AQP4 is abundant in brain where it may function as the hypothalamic osmoreceptor responsible for secretion of ADH. Continued analysis of the aquaporins is providing detailed molecular insight into the fundamental physiological problems of water balance and disorders of water balance.

Aquaporins in the kidney: from molecules to medicine

Nielsen S, Frøkiaer J, Marples D, Kwon TH, Agre P, Knepper MA.
The Water and Salt Res Center, Anatomy and Exper Clin Res Institutes, University of Aarhus, Aarhus, Denmark.
Physiol Rev. Jan 2002; 82(1):205-44.

The molecular identity of membrane water channels long-standing biophysical question of how water crosses long remained elusive until the pioneering discovery of biological membranes specifically, and provided insight, at the molecular level, of AQP1 by Agre and colleagues around 1989 –1991,  The discovery of aquaporin-1 (AQP1) answered the long-standing biophysical question of how water specifically crosses biological membranes. In the kidney, at least seven aquaporins are expressed at distinct sites. AQP1 is extremely abundant in the proximal tubule and descending thin limb and is essential for urinary concentration. AQP2 is exclusively expressed in the principal cells of the connecting tubule and collecting duct and is the predominant vasopressin-regulated water channel. AQP3 and AQP4 are both present in the basolateral plasma membrane of collecting duct principal cells and represent exit pathways for water reabsorbed apically via AQP2. Studies in patients and transgenic mice have demonstrated that both AQP2 and AQP3 are essential for urinary concentration.

Since the discovery of aquaporins, major efforts have been aimed at elucidating their structural organization. Hydropathy analysis of the deduced amino acid sequence of AQP1 led to the prediction that the protein resides primarily within the lipid bilayer (191), consistent with the initial studies of AQP1 in red cell membranes (46). AQP1 contains an internal repeat with the NH – and the first provided a molecular answer to the long-standing COOH-terminal halves being sequence related and each
containing the signature motif Asn-Pro-Ala (NPA) (181,252). This is consistent with earlier observations on the homologous major intrinsic protein from lens, (MIP, nowreferred to as AQP0). When evaluated by hydropathy analysis, six bilayer-spanning domains are apparent (Fig.1); however, the apparent interhelical loops B and E also exhibit significant hydrophobicity. Critical to the topology is the location of loop C which connects the two halves of the molecule. Preston et al. (194) demonstrated that loop C resides at the extracellular surface of the oocytes, confirming the obverse sym-metry of the NH – and COOH-terminal halves of the mol-lar surface of the oocytes, confirming the obverse symmetry of the NH – and COOH-terminal halves of the mol-ecule.The structural organization of other aquaporins such as bacterial aquaporin-Z and plant aquaporins have also been deduced. How can water channels avoid passage of protons (H O )? As predicted, loops B and E are associated by Van der Waals interactions between the two NPA motifs. Free hydrogen bonding occurs in the column of water within the pore, except at the very center where a single water molecule transiently reorients to bond with the two asparagines residues of the NPA motif. This results in minimum resistance to the flow of water, thus permitting kidneys to perform their important physiological roles of reabsorbing water while excreting acid.

FIG. 1. A: schematic representation of the structural organization of aquaporin-1 (AQP1) monomers in the membrane (top and bottom). Aquaporins have six membrane-spanning regions, both intracellular NH and COOH termini, and internal tandem repeats that, presumably, are due to an ancient gene duplication (top). The topology is consistent with an obverse symmetry for the two similar NH – and COOH- 2 terminal halves (bottom). The tandem repeat structure with two asparagine-proline-alanine (NPA) sequences has been proposed to form tight turn structures that interact in the membrane to form the pathway for translocation of water across the plasma membrane. Of the five loops in AQP1, the B and E loops dip into the lipid bilayer, and it has been proposed that they form “hemichannels” that connect between the leaflets to form a single aqueous pathway within a symmetric structure that resembles an “hourglass.” B: AQP1 is a multisubunit oligomer that is organized as a tetrameric assembly of four identical polypeptide subunits with a large glycan attached to only one.

Discovery and Biophysical Characterization of the First Molecular Water Channel AQP1 Expression of AQP1 in X. laevis oocytes by Preston et al. (192) demonstrated that AQP1-expressing oocytes exhibited remarkably high osmotic water permeability (P
cm/s), causing the cells to swell rapidly and explode in hypotonic buffer. The osmotically induced swelling of oocytes expressing AQP1 occurs with a low activation energy and is reversibly inhibited by HgCl or other mercurials. Only inward water flow (swelling) was examined, but it was predicted that the direction of water flow through AQP1 is determined by the orientation of the osmotic gradient. Consistent with this, it was later demonstrated that AQP1-expressing oocytes swell in hyposmolar buffers but shrink in hyperosmolar buffers (160).  Swelling of oocytes expressing AQP1 occurs with a low activation energy and is reversibly inhibited by HgCl or other mercurials. Only inward water flow (swelling) was examined, but it was predicted that the direction of water flow through AQP1 is determined by the orientation of the osmotic gradient. Consistent with this, it was later demonstrated that AQP1-expressing oocytes swell in hyposmolar buffers but shrink in hyperosmolar buffers (160).

Over the past 4 years a series of studies have explored the issues of selectivity and polytransport function of aquaporins. This has led to a division of aquaporins (4) into a group that transports water relatively selectively (the “orthodox” set or “aquaporins”) and a group of water channels that also conduct glycerol and other small solutes in addition to water (the “cocktail” set or aquaglyceroporins). This appears to represent an ancient phylogenetic divergence between glycerol transporters and pure water channels (185). Recently, it has become clear that transport properties are even more diverse, since AQP6 has been demonstrated to conduct anions as well (263), and it has also been demonstrated that aquaporins can be regulated by gating, as discussed below.

The signal transduction pathways have been de­scribed thoroughly in previous reviews. cAMP levels in collecting duct principal cells are in­creased by binding of vasopressin to V2 receptors. The synthesis of cAMP by adenylate cyclase is stim­ulated by a V2 receptor-coupled heterotrimeric GTP-bind-ing protein, Gs. Gs interconverts between an inactive responses to vasopressin. In this study it was demon­strated that changes in AQP2 labeling density of the apical plasma membrane correlated closely with the water per­meability in the same tubules, while there were reciprocal changes in the intracellular labeling for AQP2. In vivo studies using normal rats or vasopressin-deficient Brattleboro rats also showed a marked increase in apical plasma membrane labeling of AQP2 in response to vasopressin or dDAVP treatment.  The acute treatment of rats with vasopressin V2-receptor antagonist or acute water loading (to reduce endogenous vasopressin levels, both re­ducing vasopressin action, resulted in a prominent inter­nalization of AQP2 from the apical plasma membrane to small intracellular vesicles further underscoring the role of AQP2 trafficking in the regulation of collecting duct water permeability.

PGE2 inhibits vasopressin-induced water permeabil­ity by reducing cAMP levels. In preliminary studies, Zelenina et al. investigated the effect of PGE2 on PKA phosphorylation of AQP2 in kidney papilla, and the results suggest that the action of prostaglandins is associated with retrieval of AQP2 from the plasma membrane, but that this appears to be independent of AQP2 phosphorylation by PKA.  Phosphorylation of AQP2 by other kinases, e.g., pro­tein kinase C or casein kinase II, may potentially partici­pate in regulation of AQP2 trafficking (Fig. 9C). Phosphorylation of other cytoplasmic or vesicular regulatory proteins may also be involved. These issues remain to be investigated directly.

Since the fundamentals of the shuttle hypothesis have been confirmed, interest has turned to the cellular mechanisms mediating the vasopressin-induced transfer of AQP2 to the apical plasma membrane. The shuttle hypothesis has a number of features whose molecular basis remains poorly understood. First, AQP2 is delivered in a relatively rapid and coordinated fashion, and vesicles move from a distribution throughout the cell to the apical region of the cell in response to vasopressin stimulation. Furthermore, AQP2 is delivered specifically to the apical plasma membrane. Finally, AQP2-bearing vesicles fuse with the apical plasma membrane in response to vasopressin, but not to a significant degree in the absence of stimulation (e.g., in vasopressin-deficient Brattleboro rats where < 5% of total AQP2 is present in the apical plasma membrane. Thus there must be some kind of a “clamp” preventing fusion in the unstimulated state and/or a “trigger” when activation occurs.

The coordinated delivery of AQP2-bearing vesicles to the apical part of the cell appears to depend on the translocation of the vesicles along the cytoskeletal ele­ments. In particular, the microtubular network has been implicated in this process, since chemical disruption of microtubules inhibits the increase in permeability both in the toad bladder and in the mammalian collecting duct. Because microtubule-disruptive agents inhibit the development of the hydrosmotic response to vaso-pressin, but have no effect on the maintenance of an established response, and because they have been re­ported to slow the development of the response without affecting the final permeability in toad bladders , it has been deduced that microtubules appear to be involved in the coordinated delivery of water channels, without being involved in the actual insertion process.

In addition to increasing cAMP levels in collecting duct principal cells, vasopressin acting through the V2 receptor has also been demonstrated to transiently in­crease intracellular Ca2+. The increase occurs in the absence of activation of the phosphoinositide signaling pathway and has recently been dem­onstrated to be due to activation of ryanodine-sensitive calcium release channels in the collecting duct cells. Buffering intracellular calcium with BAPTA or inhibition of calmodulin completely blocked the water permeability response to vasopressin in isolated perfused inner med­ullary collecting ducts, suggesting a critical role for cal­cium at some step in the process of AQP2 vesicle traffick­ing.

In addition to the acute regulation of collecting duct water permeability brought about by the trafficking of AQP2 described above, it is now clear that there are longer term adaptational changes that modulate this acute response. These occur during prolonged changes in body hydration status and form an appropriate physiolog­ical response to such challenges. However, similar long ­term changes also appear to be important in a wide variety of pathological conditions,  and an understanding of the mechanisms involved in these adaptational responses may provide the basis both for a better understanding of, and for potential therapeutic ap­proaches to, pathological disorders of water balance.  Microtubules are polar structures, arising from microtubule organizing centers (MTOCs), at which their minus ends are anchored, and with the plus ends growing away “into” the cell. In fibroblastic cells, there is a single MTOC in the perinuclear region, and the plus ends project to the periphery of the cell. However, there is increasing evidence that in polarized epithelia microtubules arise from multiple MTOCs in the apical region, with their plus ends projecting down toward the basolateral membrane. If this is the case in collecting duct cells, and there is some evidence that it is , then a minus end-directed motor protein such as dynein would be expected to be involved in the movement of vesicles toward the apical plasma membrane.  Recently, it has been shown that dynein is present in the kidney of several mammalian spe­cies and that both dynein and dynactin, a protein complex believed to mediate the interaction of dynein with vesicles, associate with AQP2-bearing vesicles. It seems likely that dynein may drive the microtubule-dependent delivery of AQP2-bearing vesicles toward the apical plasma mem­brane.

The apical part of the collecting duct principal cells contains a prominent terminal web made up of actin filaments. These also appear to be involved in the hydrosmotic response, since disruption of microfilaments with cytochalasins inhibits the response in the toad bladder. Cytochalasins can also inhibit an estab­lished response, and even the offset of the response. From this it has been concluded that microfilaments are probably involved in the final movement of vesicles through the terminal web, their fusion with the plasma membrane, and the subsequent endocytic retrieval of the water channels. Interestingly, vasopressin itself causes actin depolymerization, suggesting that reor­ganization of the terminal web is an important part of the cellular response to vasopressin, a conclusion reached on morphological grounds by DiBona.

The problem of delivering vesicles to a particular domain and allowing them to fuse when, and only when, a signal arrives is conceptually very similar to the situa­tion in the neuronal synapse. It therefore seemed possible that a molecular apparatus similar to the SNAP/SNARE system described there might be present in the collecting duct principal cells.  There are specific proteins on the vesicles (vSNAREs) and the target plasma membrane (tSNAREs) that interact with components of a fusion complex to induce fusion of the vesicles only with the required target membrane. The process is thought to be regulated by other protein com­ponents that sense the signal for fusion (i.e., increased calcium in the synapse). Several groups have now shown that vSNAREs such as VAMP-2 are present in the collect­ing duct principal cells and colocalize with AQP2 in the same vesicles .

A putative tSNARE, SNAP23, has been found in collecting duct principal cells both in the apical plasma membrane and in AQP2-bearing vesicles. Some soluble components of the fusion complex, including NEM-sensitive factor (NSF) and a-soluble NSF-associated protein (SNAP), have also been identified in these cells. Thus it seems likely that the exocytic insertion of AQP2 is indeed controlled by a set of proteins similar to those involved in synaptic transmission, al­though considerable work remains to be done in isolating and characterizing the components, their regulation, and prime physiological function.

 Body water balance is tightly regulated by vasopressin, and multiple studies now have underscored the essential roles of AQP2 in this.
Vasopressin regulates acutely the water permeability of the kidney collecting duct by trafficking of AQP2 from intracellular vesicles to the apical plasma membrane.
The long-term adaptational changes in body water balance are controlled in part by regulated changes in AQP2 and AQP3 expression levels. Lack of functional AQP2 is seen in primary forms of diabetes insipidus, and reduced expression and targeting are seen in several diseases associated with urinary concentrating defects such as acquired nephrogenic diabetes insipidus, postobstructive polyuria, as well as acute and chronic renal failure. In contrast, in conditions with water retention such as severe congestive heart failure, pregnancy, and syndrome of inappropriate antidiuretic hormone secretion, both AQP2 expression levels and apical plasma membrane targetting are increased, suggesting a role for AQP2 in the development of water retention. Continued analysis of the aquaporins is providing detailed molecular insight into the fundamental physiology and pathophysiology of water balance and water balance disorders.
Three additional aquaporins are present in the kidney. AQP6 is present in intracellular vesicles in collecting duct intercalated cells, and AQP8 is present intracellularly at low abundance in proximal tubules and collecting duct principal cells, but the physiological function of these two channels remains undefined. AQP7 is abundant in the brush border of proximal tubule cells and is likely to be involved in proximal tubule water reabsorption.

Fluid transport across leaky epithelia: central role of the tight junction and supporting role of aquaporins.

Fischbarg J.
Institute of Cardiology Research , A. C. Taquini, University of Buenos Aires and National Council for Scientific and Technical Investigations, Buenos Aires, Argentina.
Physiol Rev. Oct 2010; 90(4):1271-90.
The mechanism of epithelial fluid transport remains unsolved, which is partly due to inherent experimental difficulties. However, a preparation with which our laboratory works, the corneal endothelium, is a simple leaky secretory epithelium in which we have made some experimental and theoretical headway. As we have reported, transendothelial fluid movements can be generated by electrical currents as long as there is tight junction integrity. The direction of the fluid movement can be reversed by current reversal or by changing junctional electrical charges by polylysine. Residual endothelial fluid transport persists even when no anions (hence no salt) are being transported by the tissue and is only eliminated when all local recirculating electrical currents are.   The notion that transepithelial movement of water depends on the movement of electrolytes arises from a finding by Peter Curran and Arthur K. Solomon that transintestinal water flow (“solvent” flow) depended on the transport of NaCl (“solute” flux) by that layer. That gave birth to the question of how the flow of solute (or “salt”) is linked to the movement of solvent (or “fluid”), or in the short jargon of the field, how solute-solvent cou­pling arises. 
To be noted, gradientless flow is different from transepithelial osmosis a` la Dutrochet. In this last one, in the presence of an osmotic gradient across an epithelial layer, water obligingly traverses the layer. This is well exempli­fied by the kidney collecting duct, a tight epithelium for which we accept nowadays that the water goes across both cell plasma membranes in series, traversing their aquaporins.  There is also the special case of the anuran skin epithelia, whose intercellular junctions are tight, and which water also appears to traverse through cell membrane aquaporins. As a rule, epithelia specialized to transport fluid do so in the absence of any external osmotic gradient across their layers; that is, fluid is transported between compartments of similar osmolarity.  That gave birth to the question of how the flow of solute (or “salt”) is linked to the movement of solvent (or “fluid”), or in the short jargon of the field, how solute-solvent cou­pling arises.
The progression of the ideas on fluid transport is linked to those in a parallel field, that of water channels.  After early advances in their characterization and isolation, they were molecularly identified by Peter Agre and co-workers in the early 1990s, who termed them aquaporins (AQPs). It was subsequently de­termined that AQPs were present in many fluid transport­ing epithelia  and were also present in water-perme­able kidney segments while absent in relatively water-impermeable ones . By then, the measurements of osmotic permeabilities of epithelial cell membranes had been refined using video microscopy techniques. The lab­oratories of Kenneth Spring (working on gallbladders)  and of the Welling brothers (working on kidney proximal tubule) found rather high osmotic perme­ability (or “filtration” permeability, Pf) values (Persson and Spring: 550 and 1,200 pm/s for the apical and baso-lateral membranes, respectively; Welling: -300 pm/s). Both laboratories suggested that, given such high Pf values, a few milliosmoles of osmotic pressure difference across the cell boundaries would suffice to drive the transported fluids through the cells.

There had been all along experimental evidence for the diverging view that fluid transport across leaky epi­thelia took place via paracellular, transjunctional water flow. That contrary evidence came from the laboratories of Adrian Hill using gallbladder, John Pappenheimer and his fellow James Madara using intestine, and Guillermo Whittembury and Gerhard Malnic using kidney proximal tubule. The contrary view of paracellular flow had remained a minority opinion. Still and all, these “rebels” stood their ground, led by an utterly unconvinced Adrian Hill. Con­sidering the divergent views, Kenneth Spring and col­leagues decided to take the bull by the horns and use confocal microscopy to look for evidence for or against transjunctional water flow in epithelia.
Paracellular, transjunctional fluid flow in an absorbing epithelium would lead to significant dilution of a paracellular fluorescent marker trapped in the inter­cellular spaces, which in turn would be detectable by the optical sectioning methods they mastered; all very ele­gant, for sure.

And so we come to the paper Spring and colleagues published in May of 1998  reporting that they had found no transjunctional water flow in cultured Madin-Darby canine kidney (MDCK) cell layers. Understandably, their statement had a very large impact. And yet, only some months afterwards, this notion had to be revised as it became clear that the preparation they had chosen presumably transported little if any water. By Spring’s own admission in October of the same 1998, “ . . . the fluid transport rate of MDCK cells is only about 1% of that of the renal proximal tubule… ”  To spell out the obvious, little or no fluid transport means no transjunctional (or trans-cellular) water flow either, so in perspective, the findings of Spring and colleagues (“absence of junctional flow”) bring no surprise and have no bearing on the issue of the route of fluid flow in general.

After the demise of the 1998 paper above, doubts about local osmosis continued to be fueled. Adrian Hill had been joined in his criticism of it by Thomas Zeuthen and Ernest Wright. In particular, Zeuthen and co-workers had developed an alternative model for transcellular water transfer based on molecular cotransport through transporters. Predictably, Hill’s views were newly sought out. In a thorough review written with his wife and colleague Bruria Shachar-Hill, they restated the evidence from theirs and collaborating laboratories for junctional flow for Necturus and rabbit gallbladder, Necturus intestine, Rhodnius Malpighian tubule, and rat and rabbit salivary gland. In addition, they gave a convincing account of the evidence consistent with junctional water flow for renal proximal tubule, exocrine gland (salivary, lacrimal), and small intestine. Here we will simply call attention to those arguments and will concentrate on other arguments plus additional evidence of our own.

By the end of the 1990s, Alan Verkman’s laboratory had been investigating the physiological effects of knock­ing out AQPs in mice.  The dele­tion of AQPs resulted in drastic decreases of cell mem­brane osmotic permeability, but only in rather mild decreases in rates of fluid transport, and this last to boot only in tissues that transported fluid at high rates. Verkman and colleagues generally discuss those results in a guarded manner, underlining the role of aquaporins as routes for cell water permeability without making pro­nouncements on the mechanism of transtissue fluid trans­port. Yet, paraphrasing the comments by Hill and col­leagues in another cogent review, the effects seen in the AQP knockouts are sometimes difficult to explain, and not commensurate with the deletion of what would be hypothetically a major route for transcellular transtissue water transfer.

Perhaps the existence and the location of electrogenic transporters and channels are telling us something very fundamental about the function of these layers. There does not seem to be an explanation of why epithelia in general, and specifically leaky epithelia, would have evolved to have an electrical potential difference across the layer. In principle, salts could simply be transported neutrally. In a similar vein, apical Na channels that allow Na to leak back into the cell would not make sense if the task of an epithelial cell would be to transport salt from the serosal (basal) to the luminal (apical) side. However, both of these apparent incongruencies suddenly make sense if the raison d’être of these epithelia is to perform tasks such as electro-osmosis. The electrical potential might not be an evolutionary leftover but a central fea­ture. The Na channel would not be apical by accident but to help build up the local current meant for electro-osmosis. As mentioned above, aside from the corneal endothelium , there is evidence for electro-osmosis in small intestine, kidney proximal tubule, and frog skin glands. Hence, it would be desirable if the presence of electro-osmosis would be explored in other fluid-transporting epithelia.

Electro-osmotic coupling would result in somewhat (perhaps 30%) hypotonic emerging fluid. This entails that the fluid left behind at the intercellular spaces might be correspondingly hypertonic. Such osmolarity difference in turn might be sensed by the cell and trigger mechanisms that would affect sites for regulation at basolateral and apical sites for HCO3  and Na transports, and perhaps also at the junction so as to modify the characteristics of the coupling. It is conceivable that such regulation might take place with some degree of period­icity. There may be a role for AQP1 in this regulation, which would explain the mild effects seen on fluid trans­port in this and other preparations in experiments done with AQP1 null cells. This would explain what has been noted by Verkman and colleagues, namely, that effects of AQP deletion are more pronounced in epithelia that gen­erate higher rates of fluid transport. Thus AQP deletion reduced near-isosmolar fluid transport in kidney proximal tubule and salivary gland, where fluid transport is rapid, but not in lung, lacrimal gland, sweat gland, or corneal endothelium where fluid trans­port is relatively slow.

Aquaporin (AQP) 1 is the only AQP present in these cells, and its deletion in AQP1 null mice significantly affects cell osmotic permeability (by > 40%) but fluid transport much less ( > 20%), which militates against the presence of sizable water movements across the cell. In contrast, AQP1 null mice cells have reduced regulatory volume decrease (only 60% of control), which suggests a possible involvement of AQP1 in either the function or the expression of volume-sensitive membrane channels/transporters. A mathematical model of corneal endothelium we have developed correctly predicts experimental results only when paracellular electro-osmosis is assumed rather than transcellular local osmosis. Our evidence therefore suggests that the fluid is transported across this layer via the paracellular route by a mechanism that we attribute to electro-osmotic coupling at the junctions. From our findings we have developed a novel paradigm for this preparation that includes

1) paracellular fluid flow;
2) a crucial role for the junctions;
3) hypotonicity of the primary secretion; and
4) an AQP role in regulation rather than as a significant water pathway.
These elements are remarkably similar to those proposed by the laboratory of Adrian Hill for fluid transport across other leaky epithelia.

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 1743-8454-7-15-1  Distribution in brain of aquaporin-1 (AQP1, blue) and AQP4 (orange), schematically illustrated on a sagittal section of a human brain
centralpore-small  Tetrameric Pore                     AQP-highlight
Created with The GIMP                           Gating of aquaporins
AQP-thumbnail  Gas Molecules Commute into Cell      aqpz-glpf  water channels
GlpF-ABF  Molecular Obstacle Course              nihms365271f1   Roles of water-selective aquaporins (AQPs, shown in purple).
building_a_model-02-full     nihms365271f2  Roles of water-glycerol-transporting aquaporins (aquaglyceroporins).

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