Targeting Kidney Glomerulus
Larry H. Bernstein, MD, FCAP, Curator
LPBI
Targeting the podocyte to treat glomerular kidney disease
Lal MA, Young KW, and Andag Uwe 1 CVMD iMED, AstraZeneca, Molndal, Sweden 2 Evotec AG, Germany
Drug Discovery Today http://www.e-ditionsbyfry.com/olive/ODE/DDT/Default.aspx?href=DDT/2015/10/01
The number of randomized clinical trials addressing renal disease continues to be outpaced by most other specialties and the failure of a number of recent kidney clinical trials suggests a need for new thinking and new strategies to address CKD [5,58]. Delineation of the crucial molecular species and signaling pathways underlying podocyte function will undoubtedly lead to the identification of cell-specific targets that can be exploited for treating various glomerulopathies and CKD.
The majority of chronic kidney disease (CKD) cases have their origin in the glomerulus, the microvascular unit of the nephron that serves as a filter tasked with forming primary urine. This selective filtration process is determined to a large extent by the functional capacity of the podocyte, a highly differentiated cell type that enwraps the outer aspect of the glomerular capillary wall. In this short review, we describe the biology of the podocyte, its central role in the etiology of various glomerulopathies and highlight current and future opportunities to exploit the unique properties of this cell type for developing kidney-specific therapeutics.
The kidneys The kidneys are a remarkable set of paired organs that receive › 20% of cardiac output and that filter some 180 l blood daily, which is subsequently modified to give a final urine volume of approximately 1.5 l. This phenomenal physiological capacity is carried out by the concerted actions of the glomerular filter and renal tubular systems that together represent the single functional unit of the kidney known as the nephron. In performing this outstanding feat, the kidneys participate in establishing wholebody ion homeostasis, acid-base balance and blood pressure regulation. It should therefore come as no surprise that loss of kidney function and a compromised ability to carry out such fundamental processes is associated with considerable risks to overall health [1].
Chronic kidney disease Chronic kidney disease (CKD) is a progressive malady defined by reduced glomerular filtration rate, increased urinary albumin excretion or both, and is a major global public health concern with an extremely high unmet medical need. CKD is estimated to occur in 8–16% of the worldwide population and results in a substantially reduced life expectancy [1]. Diabetes itself is recognized as the primary cause of kidney failure in almost half of all new CKD cases and, given the current global diabetes pandemic, the prevalence of renal complications will continue to grow.
There is a strong relationship between CKD and cardiovascular disease which itself is twice as common in CKD patients as compared with the general population [2]. Accordingly, the current clinical paradigm for treating CKD patients is primarily focused on reducing cardiovascular risk and is, not surprisingly, inadequate to prevent patients from continuing their inexorable loss of renal function because such strategies do not directly address the principal mechanisms accounting for renal disease itself. Despite overall advances in renal replacement therapy and dialysis for CKD patients ultimately succumbing to end-stage renal failure, these last resort options are insufficient in terms of limited organ availability and high mortality and/or cost. The potential identification and implementation of novel, kidney-specific treatment strategies therefore represents a significant untapped opportunity to improve the prognosis of these patients. The future development of such kidney-centric therapeutics will require the identification of suitable renal targets and can only be achieved through a comprehensive understanding of disease pathophysiology, the underlying molecular mechanisms of disease initiation and progression, and the implementation of translatable models and technologies [3].
The glomerular filtration barrier The vast majority of all cases of CKD have their origin in the glomerulus [4]. Clinically, although kidney disease progression shows the strongest correlation with the degree of fibrosis in the tubules and interstitium, it is lesions within the highly specialized microvascular unit that initiate disease. The primary function of the glomerulus is to filter blood selectively across the capillary wall and to elaborate an ultrafiltrate that is secondarily modified by the renal tubule system. The filtration capacity of the glomerulus is defined by the functional properties of the glomerular filtration barrier (GFB), a trilaminar molecular sieve comprising endothelial cells and visceral podocytes that medially elaborate the constituents of the glomerular basement membrane (GBM) that lies between them (Fig. 1). Although each individual component of the GFB is necessary for its concerted function, the podocyte is largely recognized as the final determinant of size-selective filtration and the ultimate barrier to albumin [4]. This is of particular clinical relevance because even small increases in albuminuria confer clinical risk and, at increasing levels of proteinuria, life expectancy is substantially reduced [5].
FIGURE 1 Ultrastructural images of the glomerular capillary and filtration barrier. (a,b) Scanning electron micrographs of the exterior surface of the glomerular capillary depicting the intricate pattern in which podocyte foot processes interdigitate, surround and enwrap the outer capillary surface. (c) Transmission electron micrograph of the glomerular filtration barrier in cross section. The * indicates the slit diaphragm. Size marker 1 mm (a) and 200 nm (b,c). Abbreviations: FP, foot process; GBM, glomerular basement membrane; EC, endothelial cell; RBC, red blood cell; CL, capillary lumen; US, urinary space.
http://www.e-ditionsbyfry.com/olive/ODE/DDT/server/GetContent.asp
The podocyte The podocyte is a terminally differentiated, highly specialized cell type with a remarkable morphology exquisitely designed to match its function. The podocyte establishes the integrity and function of the GFB and slit diaphragm, maintains capillary structure and resists intraglomerular blood pressure, contributes to the formation and modulation of the glomerular basement membrane and also determines endothelial cell homeostasis [6]. Ultrastructurally, the podocyte is like no other cell. From its voluminous cell body exposed in the urinary space extend primary and secondary processes that arborize into slender foot processes that interdigitate with those of neighboring podocytes to enwrap the outer surface of the glomerular capillary firmly (Fig. 1). One of the most unique features of the podocytes and their foot processes is the slit diaphragm, a modified cell–cell adherens junction that spans the length of their interaction. The slit diaphragm, visible upon electron microscopy as a bridge connecting juxtaposed foot processes, comprises a host of unique structural proteins, the founding member of which is nephrin [4]. It is the initial identification of nephrin and genetic mutations therein as the cause of the massive proteinuria that characterizes patients with congenital nephrotic syndrome of the Finnish type (CNSF) that has placed the podocyte at the center of current activities aimed at understanding the molecular and cellular determinants of proteinuria. Since the discovery of nephrin, the identification of a host of additional human genetic mutations in various proteins implicit in podocyte function has highlighted the central involvement of this cell type in the etiology of various inherited diseases of the glomerulus [7] (Table 1). Collectively, glomerular diseases can be classified as a related spectrum of podocytopathies where abnormalities in podocyte biology (i.e. dysfunction, injury and loss) are shared among them. They encompass purely genetic forms of podocyte disease [such as CNSF, Alport syndrome and some forms of focal and segmental glomerular sclerosis (FSGS) and membranous nephropathy] and also include nonhereditary glomerular diseases that occur secondary to conditions such as hypertension and diabetic nephropathy [8]. Taken together, glomerular diseases account for the vast majority of all end-stage kidney disease cases with diabetic nephropathy representing the single largest cause.
http://www.e-ditionsbyfry.com/olive/ODE/DDT/server/GetContent.asp
One of the earliest cellular lesions observed in the various podocytopathies that lead to glomerulosclerosis is a loss of podocytes or, more specifically, a reduction in the number of podocytes per glomerulus. Because podocytes are terminally differentiated and have a limited capacity for repair or regeneration, glomerular function is particularly sensitive to situations in which there is a mismatch between podocyte number and the glomerular filtration surface area. Experimental animal models show that a loss of <20% of podocytes causes transient proteinuria and that further podocyte loss results in progressive proteinuria, glomerulosclerosis and eventually loss of renal function [9]. In a similar manner, if glomerular volume exceeds the threshold for podocytes to maintain a functional GFB, glomerular disease will ensue. Data obtained from humans with diabetic nephropathy corroborate these preclinical observations [10]. Based on this understanding of disease etiology, therapeutic strategies aimed specifically at improving the health and function of the podocyte represent a promising avenue to address CKD [11].
Current therapeutic options with unpredicted effects on podocyte biology
The current state of the art for the treatment of CKD is focused on optimizing renal and cardiovascular risk factors and includes controlling blood pressure, albuminuria, blood glucose and blood lipids [5]. These current treatment options are based largely on repurposing of existing therapeutics and are not kidney-targetbased nor designed to address the underlying mechanisms of CKD progression. Nevertheless, there are now considerable amounts of data supporting the intriguing possibility that the beneficial effects on kidney function of a number of currently available drugs (including glucocorticoids, calcineurin inhibitors, rituximab, inhibitors of the renin–angiotensin–aldosterone system, among others) could be at least partially accounted for by their direct effects on the podocyte. Table 2 lists additional pathways and targets central to podocyte function for which therapeutic interventions have been described using preclinical models. Such examples will not be further described here so we invite the reader to consult some recent reviews on the subject [11–13].
http://www.e-ditionsbyfry.com/olive/ODE/DDT/server/GetContent.asp
The most intriguing and contentious clinical evidence available thus far supporting the possibility to improve renal function and prevent CKD progression by specifically targeting the podocyte comes from research examining the role of B7-1 (CD80) in this cell type [14–16]. Targeting of the podocyte at this molecular moiety with abatacept, an antibody-based inhibitor of B7-1, induced remission in five patients with FSGS [16]. Mechanistically, B7-1 overexpression in injured podocytes appears to interfere with b1-integrin-mediated cellular attachment to the glomerular basement membrane and abatacept exerts its protective effects by facilitating the stabilization of b1-integrin activation and podocyte adhesion. Although these results might be compelling, they are not without significant debate because the detection of B7-1 expression in podocytes as well as the efficacy of B7-1 blockers in treating FSGS patients are both highly controversial [17]. Future studies with sufficient sample size will hopefully clarify the situation.
Focusing on the podocyte as a specific therapeutic target Based on the above genetic evidence and efficacy of the described pharmacological approaches to targeting the podocyte clinically, combined with the plethora of preclinical animal studies conclusively demonstrating that podocyte dysfunction per se results in foot process effacement, albuminuria and glomerular disease, there is tantalizing hope of a future where the pursuit of podocentric therapeutic strategies will be realized. Although the ability to alter the podocyte genetically in mice has led to an explosion in the number of gene products identified as requisite for podocyte biology and potentially amenable to pharmacological manipulation, the ubiquitous nature of many of these targets and their pathways in other cell types makes them difficult to exploit. However, by identifying the molecular components that uniquely define and contribute to podocyte signaling and function, it should be possible to identify and develop cell-specific targeting opportunities.
One of the best-studied aspects of podocyte biology concerns that of the mechanisms and factors regulating its cytoskeleton. Indeed, podocyte effacement and loss of the slit diaphragm as seen in glomerular diseases are phenomena intricately tied to disrupted actin cytoskeletal homeostasis in this cell type [12,18–20]. Not surprisingly, podocyte-specific manipulation of the mouse genome clearly indicates that the RhoGTPase family of actin master regulators is intricately involved in determining podocyte function [21,22]. The ultimate question that remains to be addressed, however, is how to exploit such a ubiquitous cell biological process driven by canonical signaling pathways therapeutically. Perhaps, the best opportunity for such a scenario could come from further studies aimed at defining the unique, principal molecular components that impact podocyte actin cytoskeletal architecture. A number of actin cytoskeleton effectors with podocyte-enriched expression have been identified and they could provide novel entry points to facilitate the fine-tuning and manipulation of the actin cytoskeleton specifically in this cell type to promote the reestablishment of normal foot process architecture in disease [23–25].
Defining the molecular footprint of the podocyte The ability to isolate pure populations of glomeruli from the renal parenchyma and further selectively separate the podocyte from its neighboring endothelial and mesangial cells by lineage-tracking and fluorescently activated cell sorting combined with various omics technologies has allowed researchers to catalogue the podocyte transcriptome and proteome [26–28]. Further definition of the miRNA transcriptional profile, alternative gene splicing events, phosphoproteome and epigenome represent additional levels of molecular complexity that further enhance our understanding of the underpinnings of podocyte biology [26,29–31]. A computational, machine-learning-based approach has also been developed that is able to infer podocyte-specific expression from whole-tissue samples through iterative statistical analysis of shared gene expression patterns and will be particularly useful to elucidate the roles of individual cell-specific transcripts in human glomerular disease [32]. The daunting task remains to unravel the cellular role of these individual proteins in podocyte health and disease and to consider whether they represent viable therapeutic targets. Significant inroads have been made over the past two decades and it is not unreasonable to envision a not too distant future when the involvement of all individual genes in podocyte biology is achieved. Acquired glomerular diseases, such as diabetic nephropathy, are not due to genetic defects but rather can be seen as a consequence of crucial signaling pathways of the podocyte that have gone awry. Determining which of the proteins expressed in podocytes and their pathways that are most amenable to therapeutic intervention will require considerable efforts from the research community.
Use of podocytes in phenotypic screens The ability to propagate conditionally immortalized podocytes in culture combined with rapid advances in molecular biology has greatly complemented our knowledge of the molecular machinery that defines this cell type [33,34]. Although there are important limitations to consider when utilizing such podocyte cell lines, podocyte cell culture has been an instrumental tool for simulating various aspects of glomerular disease biology. From the drug discovery perspective, there remain significant opportunities to take advantage of this cellular system for target evaluation and discovery [13,35].
Target-agnostic high-content or phenotypic screening approaches are a recent addition to the drug discovery process [36]. Here, hit compounds are identified as having a positive phenotypic readout in a cell-based assay. Key to this process is the disease relevance of the phenotype being investigated and ultimately the translatability of the assay to human disease states. In this regard, we have used human conditionally immortalized podocytes [34] treated with a combination of palmate and high glucose to mimic conditions observed in diabetic nephropathy. The podocyte response to this ‘disease-relevant’ stimulus can be measured using high-content imaging technology, such as the OperaTM confocal imaging platform, and manifests as changes in the actin cytoskeleton and overall cell morphology, followed by onset of cell apoptosis (Fig. 2).
http://www.e-ditionsbyfry.com/olive/ODE/DDT/server/GetContent.asp
FIGURE 2 Podocyte high-content screening. Conditionally immortalized human podocytes were screened using a combination of palmitate and high glucose to mimic conditions observed in diabetic nephropathy. Small-molecule inhibitors of phenotypic changes were identified using the OperaTM high-content imaging platform. The primary screen was run using activation of caspase 3/7 as a marker of podocyte apoptosis. Hit compounds were also tested in an orthogonal assay which used Alexa-labeled phalloidin to detect changes in the actin cytoskeleton.
These events provide suitably robust outputs for compound screening. Evotec, in combination with AstraZeneca, has screened >120 000 compounds using protection from palmitate and highglucose-induced podocyte apoptosis as a primary readout. Protection against effects on the actin cytoskeleton was used as a secondary orthogonal assay. To this end, we have identified a number of targets, some previously recognized as important regulators of podocyte biology [e.g. mammalian target of rapamycin (mTOR), glucocorticoid receptor, all-trans retinoic acid, among others]. Interestingly , a number of compounds that had the capability to attenuate palmitate and high-glucose-driven podocyte apoptosis in the screen were identified as glycogen synthase kinase (GSK)3b inhibitors. 1-Azakenpaullone (1-AZK), a selective GSK3b inhibitor identified by the podocyte screening approach, was further analyzed for its potency to protect ex vivo isolated porcine glomeruli against palmitate and high-glucose injury. 1-AZK significantly increased cell viability of glomeruli exposed to palmitate and high glucose, highlighting again the protective effect of GSK3b inhibitors in this injury setting (Fig. 3). Recently, GSK3b inhibitors have been demonstrated to exert protective effects on injured podocytes in culture as well as in mouse models of kidney disease including diabetic nephropathy [37,38]. These findings suggest that pharmacologically targeting GSK3b could represent a therapeutic strategy to protect podocytes against injury.
http://www.e-ditionsbyfry.com/olive/ODE/DDT/server/GetContent.asp
FIGURE 3 Glycogen synthase kinase (GSK)3b inhibition protects against podocyte injury. Representative images of the degree of podocyte apoptosis measured as caspase 3/7 activation (green) under control (a) and 200 mM palmitate with 25 mM glucose (b) conditions. Podocyte nuclei are counterstained red. (c) 1-Azakenpaullone (1-AZK) dose-dependently reduces caspase 3/7 activation in human podocytes challenged with 200 mM palmitate and 25 mM glucose for 48 h. Podocyte apoptosis was quantified and normalized to confluence level by measuring caspase 3/7 activity and confluence using IncuCyte live cell imaging (n = 4). (d) Overall cell viability, quantified by AlamarBlueWassay (n = 3), of glomeruli freshly isolated from Gottingen minipig via graded sieving technique and cultured in 11 mM glucose ( › ) or 400 mM palmitate with 25 mM glucose (+) with indicated 1-AZK concentrations for three days.
The second, often more complex phase, of phenotypic screening is the need to identify the target or targets of hits found in the screening phase. Here, the process involves expansion of the chemical matter around the initial hit chemotype to understand the basic structure-related properties for the molecule. Next, chemical proteomics is employed to identify cellular binding partners for the hits and ultimately these targets need to be confirmed using the screening assay set up [39,40]. Together, Evotec and AstraZeneca have successfully progressed podocyte hit compounds to target identification and are currently developing compounds around these novel targets.
Tools to evaluate podocyte function Although the ability to culture podocytes has tremendously enhanced our understanding of the function of individual proteins in this cell type at a biochemical level, such methods and the conclusions drawn from their results need to be integrated within the complex framework of podocyte function in vivo as a requisite component of the GFB. This is important to consider because the podocyte in vitro is an imperfect representation of its in vivo counterpart at a molecular and functional level [25–27,41]. The isolated podocyte certainly mimics certain facets of podocyte biology observed in vivo, but there is a need for additional unbiased and integrated strategies that can facilitate our understanding of the mechanistic determinants of podocyte function in its native microenvironment. A number of novel and exciting strategies have been developed to fill the void between cell culture and whole-animal, mammalian studies of podocyte biology.
The concept of the podocyte as a dynamic, motile cell type in vitro has been around for some time, but the corollary of this phenomenon in vivo and its putative physiological significance in human health and disease is a subject of active debate. Through the use of transgenic animals (i.e. mice and zebrafish) expressing fluorescently labeled podocytes and advanced microscopic imaging techniques it is possible to study and interrogate the occurrence of podocyte motility in vivo and its relevance to glomerular disease [42,43]. The conclusions drawn from such studies are not in complete harmony and continue to fuel the debate over podocyte plasticity and the cellular nature of their origin. Regardless, such strategies are valuable tools that will be important to evaluating the efficacy of novel therapeutic approaches designed to exploit podocyte dynamics and the potential to control podocyte regeneration from progenitor cell niches.
Another novel and very interesting imaging-based approach for the study of glomerular function is a recently published technique based on repetitive noninvasive in vivo imaging of GFB integrity in isolated glomeruli transplanted into the anterior chamber of the mouse eye [44]. Podocytes from such glomeruli maintained their differentiation and displayed interdigitating podocyte foot processes for up to six months after transplantation and are amenable to repeated imaging. Transplanting healthy or diseased human glomeruli into mouse eyes might provide a means to image functioning human glomeruli and acutely examine the effect of therapeutics on glomerular function.
The fruit fly has also recently seen its debut as a model organism to study podocyte biology [45]. Despite gross anatomical differences in the excretory system of humans and fruit flies, the Drosophila pericardial nephrocyte bears a remarkable evolutionary conservation of molecular components and functional properties with the mammalian podocyte. To exploit this observation, and to harness the power of Drosophila genetics, a reporter system of nephrocyte filtration function has been combined with a large RNA interference genetic screen that enables the scanning of the entire genome for genes specifically required for pericardial nephrocyte function [46]. An initial genetic screen of about 1000 genes resulted in the identification of about 7% that were essential for nephrocyte function, notable among them the mammalian homologs of nephrin and podocin [46]. In the near future, the complete repertoire of genes required for nephrocyte-specific function will be defined and this will be an invaluable source of unbiased information ready to be translated to higher animal species.
The zebrafish, as mentioned above, has taken its place as an exceptional model for studying podocyte biology and glomerular function. The ability to knockdown individual genes and subsequently to follow functional parameters of filtration capacity has been an important tool in the armamentarium of the podocyte biologist to accelerate the study of podocyte gene function [47]. Reverse genetic screens can be readily used to investigate the potential role of novel podocyte proteins. Additionally, novel transgenic zebrafish models that incorporate inducible podocyte injury with a fluorescent tracer for proteinuria make it possible to use this functional model for the screening of therapeutic candidates that could improve overall podocyte health and GFB integrity [48].
Another strategy not yet fully realized for the kidney, but demonstrated in principal for other organ systems including that of the lung and liver, is the microfluidic organ-on-a-chip which aims to facilitate the in vitro organomimetic modeling of complex human physiology [49,50]. Similar to recapitulating the alveolarcapillary interface of the lung in a mechanical environment that mirrors physiological function, the GFB of the kidney would appear to be a particularly well-suited functional unit for such a strategy. One can envisage the culturing of human podocytes and glomerular endothelial cells on either side of a structural support in a self-contained unit where separated perfusion chambers bath cells in appropriate media and biomechanical properties such as hydrostatic pressure, fluid flow and shear stress can be modulated to simulate in vivo properties. Such a biomimetic device could be what is required to promote the differentiation of cultured podocytes to develop foot processes and slit diaphragms as is functionally required for GFB function. Interestingly, it has also been demonstrated through the cellular repopulation of decellularized kidneys that the native kidney extracellular matrix, including that of the GBM, provides cellular cues that might promote the potential differentiation of podocytes in the glomerulus [51]. The future development of a human GFB biomimetic represents a significant hurdle but holds great promise as a means to study podocyte function and to evaluate drug pharmacology and toxicity.
Podocyte delivery and targeting Briefly, cell-specific targeting of therapeutic agents to podocytes of the glomerulus might potentially be an attractive method to increase their efficacy and/or minimize their side-effects. Size and charge characteristics could be exploited to facilitate the capability of drug–carrier conjugates to reach and interact specifically with the correct target cells. Within the kidney, the podocyte is an attractive target cell because of its unique properties. Large molecules that accumulate in the GBM might not need to be filtered to reach the podocyte because of the robust endocytic capacity of this cell type [52]. This raises the possibility of using tissue-specific homing strategies to deliver large proteins, antibodies and siRNAs into cells in a biologically active form [53,54]. Ultrasound-microbubble-mediated delivery of gene therapies to the kidney is an interesting approach that has been used to achieve drug targeting to the kidney [55]. Efficacy of ultrasound-microbubble-mediated gene delivery has been demonstrated in rat models of renal fibrosis but upregulation of the transgene of interest was noted in all kidney tissues [56]. The ability to combine such ultrasound microbubble technology with cell-specific targeting should be possible for the podocyte as has been demonstrated for achieving endothelium-specific transgene expression of a shRNA [57].
Concluding remarks The number of randomized clinical trials addressing renal disease continues to be outpaced by most other specialties and the failure of a number of recent kidney clinical trials suggests a need for new thinking and new strategies to address CKD [5,58]. Delineation of the crucial molecular species and signaling pathways underlying podocyte function will undoubtedly lead to the identification of cell-specific targets that can be exploited for treating various glomerulopathies and CKD. Realizing this goal will require a holistic approach where the short-comings of any single strategy in isolation will be overcome only by integrating their individual strengths. With this in mind, the future for developing podocyte specific therapeutics looks bright.
1 Eckardt, K.U. et al. (2013) Evolving importance of kidney disease: from subspecialty to global health burden. Lancet 382, 158–169
2 de Zeeuw, D. et al. (2004) Proteinuria, a target for renoprotection in patients with type 2 diabetic nephropathy: lessons from RENAAL. Kidney Int. 65, 2309–2320
3 Gashaw, I. et al. (2011) What makes a good drug target? Drug Discov. Today 16, 1037–1043
4 Tryggvason, K. et al. (2006) Hereditary proteinuria syndromes and mechanisms of proteinuria. N. Engl. J. Med. 354, 1387–1401
5 Lambers Heerspink, H.J. and de Zeeuw, D. (2013) Novel drugs and intervention strategies for the treatment of chronic kidney disease. Br. J. Clin. Pharmacol. 76, 536–550
6 Pavenstadt, H. et al. (2003) Cell biology of the glomerular podocyte. Physiol. Rev. 83, 253–307
7 Fogo, A.B. (2014) Causes and pathogenesis of focal segmental glomerulosclerosis. Nat. Rev. Nephrol. 11, 76–87
8 Wiggins, R.C. (2007) The spectrum of podocytopathies: a unifying view of glomerular diseases. Kidney Int. 71, 1205–1214
9 Wharram, B.L. et al. (2005) Podocyte depletion causes glomerulosclerosis: diphtheria toxin-induced podocyte depletion in rats expressing human diphtheria toxin receptor transgene. J. Am. Soc. Nephrol. 16, 2941–2952
10 Pagtalunan, M.E. et al. (1997) Podocyte loss and progressive glomerular injury in type II diabetes. J. Clin. Invest. 99, 342–348
11 Mathieson, P.W. (2012) The podocyte as a target for therapies—new and old. Nat. Rev. Nephrol. 8, 52–56
12 Brinkkoetter, P.T. et al. (2013) The role of the podocyte in albumin filtration. Nat. Rev. Nephrol. 9, 328–336
13 Reiser, J. et al. (2010) Toward the development of podocyte-specific drugs. Kidney Int. 77, 662–668
14 Fiorina, P. et al. (2014) Role of podocyte B7-1 in diabetic nephropathy. J. Am. Soc. Nephrol. 25, 1415–1429
15 Reiser, J. et al. (2004) Induction of B7-1 in podocytes is associated with nephrotic syndrome. J. Clin. Invest. 113, 1390–1397
16 Yu, C.C. et al. (2013) Abatacept in B7-1-positive proteinuric kidney disease. N. Engl. J. Med. 369, 2416–2423
17 Benigni, A. et al. (2014) Abatacept in B7-1-positive proteinuric kidney disease. N. Engl. J. Med. 370, 1261–1263
18 Kriz, W. (2002) Podocyte is the major culprit accounting for the progression of chronic renal disease. Microsc. Res. Tech. 57, 189–195
19 Kriz, W. et al. (2013) The podocyte’s response to stress: the enigma of foot process effacement. Am. J. Physiol. Renal. Physiol. 304, F333–F347
20 Welsh, G.I. and Saleem, M.A. (2012) The podocyte cytoskeleton—key to a functioning glomerulus in health and disease. Nat. Rev. Nephrol. 8, 14–21
21 Lal, M.A. and Tryggvason, K. (2012) Knocking out podocyte rho GTPases: and the winner is. J. Am. Soc. Nephrol. 23, 1128–1129
22 Mouawad, F. et al. (2013) Role of Rho-GTPases and their regulatory proteins in glomerular podocyte function. Can. J. Physiol. Pharmacol. 91, 773–782
23 Akilesh, S. et al. (2011) Arhgap24 inactivates Rac1 in mouse podocytes, and a mutant form is associated with familial focal segmental glomerulosclerosis. J. Clin. Invest. 121, 4127–4137
24 Asanuma, K. et al. (2006) Synaptopodin orchestrates actin organization and cell motility via regulation of RhoA signalling. Nat. Cell Biol. 8, 485–491
25 Lal, M.A. et al. (2015) Rhophilin-1 is a key regulator of the podocyte cytoskeleton and is essential for glomerular filtration. J. Am. Soc. Nephrol. 26, 647–662
… more
Leave a Reply