Posts Tagged ‘Molecular neuroscience’

Retromer in neurological disorders

Larry H. Bernstein, MD, FCAP, Curator




Retromer in Alzheimer disease, Parkinson disease and other neurological disorders.

Scott A. Small and Gregory A. Petsko
Nature Reviews Neuroscience 16; 126–132 (2015)


As discussed in the forum (see video here), there are many cellular pathways which are believed to be perturbed in Alzheimer’s Disease. Recent work has suggested that deficits in retromer complex function may underlie impairment of endosomal trafficking in neurons and may contribute to AD pathogenesis. This recent review illustrates the function of the retromer complex and discusses how its dysfunction may contribute to neurodegeneration.

By Tim Spencer on 24 Nov, 2015


Retromer is a protein assembly that has a central role in endosomal trafficking, and retromer dysfunction has been linked to a growing number of neurological disorders. First linked to Alzheimer disease, retromer dysfunction causes a range of pathophysiological consequences that have been shown to contribute to the core pathological features of the disease. Genetic studies have established that retromer dysfunction is also pathogenically linked to Parkinson disease, although the biological mechanisms that mediate this link are only now being elucidated. Most recently, studies have shown that retromer is a tractable target in drug discovery for these and other disorders of the nervous system.


Yeast has proved to be an informative model organism in cell biology and has provided early insight into much of the molecular machinery that mediates the intracellular transport of proteins1,2. Indeed, the term ‘retromer’ was first introduced in a yeast study in 1998 (Ref. 3). In this study, retromer was referred to as a complex of proteins that was dedicated to transporting cargo in a retrograde direction, from the yeast endosome back to the Golgi.

By 2004, a handful of studies had identified the molecular4 and the functional5, 6 homologies of the mammalian retromer, and in 2005 retromer was linked to its first human disorder, Alzheimer disease (AD)7. At the time, the available evidence suggested that the mammalian retromer might match the simplicity of its yeast homologue. Since then, a dramatic and exponential rise in research focusing on retromer has led to more than 300 publications. These studies have revealed the complexity of the mammalian retromer and its functional diversity in endosomal transport, and have implicated retromer in a growing number of neurological disorders.

New evidence indicates that retromer is a ‘master conductor’ of endosomal sorting and trafficking8. Synaptic function heavily depends on endosomal trafficking, as it contributes to the presynaptic release of neurotransmitters and regulates receptor density in the postsynaptic membrane, a process that is crucial for neuronal plasticity9. Therefore, it is not surprising that a growing number of studies are showing that retromer has an important role in synaptic biology10, 11, 12, 13. These observations may account for why the nervous system seems particularly sensitive to genetic and other defects in retromer. In this Progress article, we briefly review the molecular organization and the functional role of retromer, before discussing studies that have linked retromer dysfunction to several neurological diseases — notably, AD and Parkinson disease (PD).


The endosome is considered a hub for intracellular transport. From the endosome, transmembrane proteins can be actively sorted and trafficked to various intracellular sites via distinct transport routes (Fig. 1a). Studies have shown that the mammalian retromer mediates two of the three transport routes out of endosomes. First, retromer is involved in the retrieval of cargos from endosomes and in their delivery, in a retrograde direction, to the trans-Golgi network (TGN)5,6. Retrograde transport has many cellular functions but, as we describe, it is particularly important for the normal delivery of hydrolases and proteases to the endosomal–lysosomal system. The second transport route in which retromer functions is the recycling of cargos from endosomes back to the cell surface14, 15 (Fig. 1a). It is this transport route that is particularly important for neurons, as it mediates the normal delivery of glutamate and other receptors to the plasma membrane during synaptic remodelling and plasticity10, 11, 12, 13.

Figure 1: Retromer’s endosomal transport function and molecular organization.
Retromer's endosomal transport function and molecular organization.

a | Retromer mediates two transport routes out of endosomes via tubules that extend out of endosomal membranes. The first is the retrograde pathway in which cargo is retrieved from the endosome and trafficked to the trans-Golgi network (TGN). The second is the recycling pathway in which cargo is trafficked back from the endosome to the cell surface. The degradation pathway, which is not mediated by retromer, involves the trafficking of cargo from endosomes to lysosomes for degradation. b | The retromer assembly of proteins can be organized into distinct functional modules, all of which work together as part of retromer’s transport role. The ‘cargo-recognition core’ is the central module of the retromer assembly and comprises a trimer of proteins, in which vacuolar protein sorting-associated protein 26 (VPS26) and VPS29 bind VPS35. The ‘tubulation’ module includes protein complexes that bind the cargo-recognition core and aid in the formation and stabilization of tubules that extend out of endosomes, directing the transport of cargos towards their final destinations. The ‘membrane-recruiting’ proteins recruit the cargo-recognition core to the endosomal membrane. The WAS protein family homologue (WASH) complex of proteins also binds the cargo-recognition core and is involved in endosomal ‘actin remodelling’ to form actin patches, which are important for directing cargos towards retromer’s transport pathways. Retromer cargos includes a range of receptors — which bind the cargo-recognition core — and their ligands. PtdIns3P, phosphatidylinositol-3-phosphate.

As well as extending the endosomal transport routes, recent studies have considerably expanded the number of molecular constituents and what is known about the functional organization of the mammalian retromer. Following this expansion in knowledge of the molecular diversity and organizational complexity, retromer might be best described as a multimodular protein assembly. The protein or group of proteins that make up each module can vary, but each module is defined by its distinct function, and the modules work in unison in support of retromer’s transport role.

Two modules are considered central to the retromer assembly. First and foremost is a trimeric complex that functions as a ‘cargo-recognition core’, which selects and binds to the transmembrane proteins that need to be transported and that reside in endosomal membranes5, 6. This trimeric core comprises vacuolar protein sorting-associated protein 26 (VPS26), VPS29 and VPS35; VPS35 functions as the core’s backbone to which the other two proteins bind16. VPS26 is the only member of the core that has been found to have two paralogues, VPS26a and VPS26b17,18, and studies suggest that VPS26b might be differentially expressed in the brain19, 20. Some studies suggest that VPS26a and VPS26b are functionally redundant21, whereas others suggest that they might form distinct cargo-recognition cores20, 22.

The second central module of the retromer assembly is the ‘tubulation’ module, which is made up of proteins that work together in the formation and the stabilization of tubules that extend out of endosomes and that direct the transport of cargo towards its final destination (Fig. 1b). The proteins in this module, which directly binds the cargo-recognition core, are members of the subgroup of the sorting nexin (SNX) family that are characterized by the inclusion of a carboxy-terminal BIN–amphiphysin–RVS (BAR) domain23. These members include SNX1, SNX2, SNX5 and SNX6 (Refs 24,25). As part of the tubulation module, these SNX-BAR proteins exist in different dimeric combinations, but typically SNX1 interacts with SNX5 or SNX6, and SNX2 interacts with SNX5 or SNX6 (Refs 26,27). The EPS15-homology domain 1 (EHD1) protein can be included in this module, as it is involved in stabilizing the tubules formed by the SNX-BAR proteins28.

A third module of the retromer assembly functions to recruit the cargo-recognition core to endosomal membranes and to stabilize the core once it is there (Fig. 1b). Proteins that are part of this ‘membrane-recruiting’ module include SNX3 (Ref. 29), the RAS-related protein RAB7A30, 31,32 and TBC1 domain family member 5 (TBC1D5), which is a member of the TRE2–BUB2–CDC16 (TBC) family of RAB GTPase-activating proteins (GAPs)28. In addition, the lipid phosphatidylinositol-3-phosphate (PtdIns3P), which is found on endosomal membranes, contributes to recruiting most of the retromer-related SNXs through their phox homology domains33. Interestingly, another SNX with a phox homology domain, SNX27, was recently linked to retromer and its function15, 34. SNX27 functions as an adaptor for binding to PDZ ligand-containing cargos that are destined for transport to the cell surface via the recycling pathway. Thus, according to the functional organization of the retromer assembly, SNX27 belongs to the module that engages in cargo recognition and selection.

Recent studies have identified a fourth module of the retromer assembly. The five proteins in this module — WAS protein family homologue 1 (WASH1), FAM21, strumpellin, coiled-coil domain-containing protein 53 (CCDC53) and KIAA1033 (also known as WASH complex subunit 7) — form the WASH complex and function as an ‘actin-remodelling’ module28, 35, 36 (Fig. 1b). Specifically, the WASH complex functions in the rapid polymerization of actin to create patches of actin filaments on endosomal membranes. The complex is recruited to endosomal membranes by binding VPS35 (Ref. 28), and together they divert cargo towards retromer transport pathways and away from the degradation pathway.

The cargos that are transported by retromer include the receptors that directly bind the cargo-recognition core and the ligands of these receptors that are co-transported with the receptors. The receptors that are transported by retromer that have so far been identified to be the most relevant to neurological diseases are the family of VPS10 domain-containing receptors (including sortilin-related receptor 1 (SORL1; also known as SORLA), sortilin, and SORCS1, SORCS2 and SORCS3)7; the cation-independent mannose-6-phosphate receptor (CIM6PR)6, 5; glutamate receptors10; and phagocytic receptors that mediate the clearing function of microglia37. The most disease-relevant ligand to be identified that is trafficked as retromer cargo is the β-amyloid precursor protein (APP)7, 38, 39, 40, 41, which binds SORL1 and perhaps other VPS10 domain-containing receptors42 at the endosomal membrane.

Retromer dysfunction

Guided by retromer’s established function, and on the basis of empirical evidence, there are three well-defined pathophysiological consequences of retromer dysfunction that have proven to be relevant to AD and nervous system disorders. First, retromer dysfunction can cause cargos that typically transit rapidly through the endosome to reside in the endosome for longer than normal durations, such that they can be pathogenically processed into neurotoxic fragments (for example, APP, when stalled in the endosome, is more likely to be processed into amyloid-β, which is implicated in AD43 (Fig. 2a)). Second, by reducing endosomal outflow via impairment of the recycling pathway, retromer dysfunction can lead to a reduction in the number of cell surface receptors that are important for brain health (for example, microglia phagocytic receptors37 (Fig. 2b)).

Figure 2: The pathophysiology of retromer dysfunction.
The pathophysiology of retromer dysfunction.

Retromer dysfunction has three established pathophysiological consequences. In the examples shown, the left graphic represents a cell with normal retromer function and the right graphic represents a cell with a deficit in retromer function. a | Retromer dysfunction causes increased levels of cargo to reside in endosomes. For example, in primary neurons, retromer transports the β-amyloid precursor protein (APP) out of endosomes. Accordingly, retromer dysfunction increases APP levels in endosomes, leading to accelerated APP processing, resulting in an accumulation of neurotoxic fragments of APP (namely, β-carboxy-terminal fragment (βCTF) and amyloid-β) that are pathogenic in Alzheimer disease. b | Retromer dysfunction causes decreased cargo levels at the cell surface. For example, in microglia, retromer mediates the transport of phagocytic receptors to the cell surface and retromer dysfunction results in a decrease in the delivery of these receptors. Studies suggest that this cellular phenotype might have a pathogenic role in Alzheimer disease. c | Retromer dysfunction causes decreased delivery of proteases to the endosome. Retromer is required for the normal retrograde transport of the cation-independent mannose-6-phosphate receptor (CIM6PR) from the endosome back to the trans-Golgi network (TGN). It is in the TGN that this receptor binds cathepsin D and other proteases, and transports them to the endosome, to support the normal function of the endosomal–lysosomal system. By impairing the retrograde transport of the receptor, retromer dysfunction ultimately leads to reduced delivery of cathepsin D to this system. Cathepsin D deficiency has been shown to disrupt the endosomal–lysosomal system and to trigger tau pathology either within endosomes or secondarily in the cytosol.

The third consequence (Fig. 2c) is a result of the established role that retromer has in the retrograde transport of receptors, such as CIM6PR5, 6 or sortilin44, after these receptors transport proteases from the TGN to the endosome. Once at the endosome, the proteases disengage from the receptors, are released into endosomes and migrate to lysosomes. These proteases function in the endosomal–lysosomal system to degrade proteins, protein oligomers and aggregates45. Retromer functions to transfer the ‘naked’ receptor from the endosome back to the TGN via the retrograde pathway5, 6, allowing the receptors to continue in additional rounds of protease delivery. Accordingly, by reducing the normal retrograde transport of these receptors, retromer dysfunction has been shown to reduce the proper delivery of proteases to the endosomal–lysosomal system5,6, which, as discussed below, is a pathophysiological state linked to several brain disorders.

Although requiring further validation, recent studies suggest that retromer dysfunction might be involved in two other mechanisms that have a role in neurological disease. One study suggested that retromer might be involved in trafficking the transmembrane protein autophagy-related protein 9A (ATG9A) to recycling endosomes, from where it can then be trafficked to autophagosome precursors — a trafficking step that is crucial in the formation and the function of autophagosomes46. Autophagy is an important mechanism by which neurons clear neurotoxic aggregates that accumulate in numerous neurodegenerative diseases47. A second study has suggested that retromer dysfunction might enhance the seeding and the cell-to-cell spread of intracellular neurotoxic aggregates48, which have emerged as novel pathophysiological mechanisms that are relevant to AD49, PD50 and other neurodegenerative diseases.

Alzheimer disease

Retromer was first implicated in AD in a molecular profiling study that relied on functional imaging observations in patients and animal models to guide its molecular analysis7. Collectively, neuroimaging studies confirmed that the entorhinal cortex is the region of the hippocampal circuit that is affected first in AD, even in preclinical stages, and suggested that this effect was independent of ageing (as reviewed in Ref. 51). At the same time, neuroimaging studies identified a neighbouring hippocampal region, the dentate gyrus, that is relatively unaffected in AD52. Guided by this information, a study was carried out in which the two regions of the brain were harvested post mortem from patients with AD and from healthy individuals, intentionally covering a broad range of ages. A statistical analysis was applied to the determined molecular profiles of the regions that was designed to address the following question: among the thousands of profiled molecules, which are the ones that are differentially affected in the entorhinal cortex versus the dentate gyrus, in patients versus controls, but that are not affected by age? The final results led to the determination that the brains of patients with AD are deficient in two core retromer proteins — VPS26 and VPS35 (Ref. 7).

Little was known about the receptors of the neuronal retromer, so to understand how retromer deficiency might be mechanistically linked to AD, an analysis was carried out on the molecular data set that looked for transmembrane molecules for which expression levels correlated with VPS35 expression. The top ‘hit’ was the transcript encoding the transmembrane protein SORL1 (Ref. 43). As SORL1 belongs to the family of VPS10-containing receptors and as VPS10 is the main retromer receptor in yeast3, it was postulated that SORL1 and the family of other VPS10-containing proteins (sortillin, SORCS1, SORCS2 and SORCS3) might function as retromer receptors in neurons7. In addition, SORL1 had recently been reported to bind APP53, so if SORL1 was assumed to be a receptor that is trafficked by retromer, then APP might be the cargo that is co-trafficked by retromer. This led to a model in which retromer traffics APP out of endosomes7, which are the organelles in which APP is most likely to be cleaved by βAPP-cleaving enzyme 1 (BACE1; also known as β-secretase 1)43; this is the initial enzymatic step in the pathogenic processing of APP.

Subsequent studies were required to further establish the pathogenic link between retromer and AD, and to test the proposed model. The pathogenic link was further supported by human genetic studies. First, a genetic study investigating the association between AD, the genes encoding the components of the retromer cargo-recognition core and the family of VPS10-containing receptors found that variants of SORL1 increase the risk of developing AD38. This finding was confirmed by numerous studies, including a recent large-scale AD genome-wide association study54. Other genetic studies identified AD-associated variants in genes encoding proteins that are linked to nearly all modules of the retromer assembly55, including genes encoding proteins of the retromer tubulation module (SNX1), genes encoding proteins of the retromer membrane-recruiting module (SNX3 and RAB7A) and genes encoding proteins of the retromer actin-remodelling module (KIAA1033). In addition, nearly all of the genes encoding the family of VPS10-containing retromer receptors have been found to have variants that associate with AD56. Finally, a study found that brain regions that are differentially affected in AD are deficient in PtdIns3P, which is the phospholipid required for recruiting many sorting nexins to endosomal membranes57. Thus, together with the observation that the brains of patients with AD are deficient in VPS26a and VPS35 (Refs 7,37), all modules in the retromer assembly are implicated in AD.

Studies in mice39, 58, 59, flies39 and cells in culture34, 40, 41, 60, 61 have investigated how retromer dysfunction leads to the pathogenic processing of APP. Although rare discrepancies have been observed among these studies62, when viewed in total, the most consistent findings are that retromer dysfunction causes increased pathogenic processing of APP by increasing the time that APP resides in endosomes. Moreover, these studies have confirmed that SORL1 and other VPS10-containing proteins function as APP receptors that mediate APP trafficking out of endosomes.

Retromer has unexpectedly been linked to microglial abnormalities37 — another core feature of AD — which, on the basis of recent genetic findings, seem to have an upstream role in disease pathogenesis54, 63. A recent study found that microglia harvested from the brains of individuals with AD are deficient in VPS35 and provided evidence suggesting that retromer’s recycling pathway regulates the normal delivery of various phagocytic receptors to the cell surface of microglia37, including the phagocytic receptor triggering receptor expressed on myeloid cells 2 (TREM2) (Fig. 2b). Mutations in TREM2 have been linked to AD63, and a recent study indicates that these mutations cause a reduction in its cell surface delivery and accelerate TREM2 degradation, which suggests that the mutations are linked to a recycling defect64. While they are located at the microglial cell surface, these phagocytic receptors function in the clearance of extracellular proteins and other molecules from the extracellular space65. Taken together, these recent studies suggest that defects in the retromer’s recycling pathway can, at least in part, account for the microglial defects observed in the disease.

The microtubule-associated protein tau is the key element of neurofibrillary tangles, which are the other hallmark histological features of AD. Although a firm link between retromer dysfunction and tau toxicity remains to be established, recent insight into tau biology suggests several plausible mechanisms that are worth considering. Tau is a cytosolic protein, but nonetheless, through mechanisms that are still undetermined, it is released into the extracellular space from where it gains access to neuronal endosomes via endocytosis66, 67. In fact, recent studies suggest that the pathogenic processing of tau is triggered after it is endocytosed into neurons and while it resides in endosomes67. Of note, it still remains unknown which specific tau processing step — its phosphorylation, cleavage or aggregation — is an obligate step towards tau-related neurotoxicity. Accordingly, if defects in microglia or in other phagocytic cells reduce their capacity to clear extracellular tau, this would accelerate tau endocytosis in neurons and its pathogenic processing.

A second possibility comes from the established role retromer has in the proper delivery of cathepsin D and other proteases to the endosomal–lysosomal system via CIM6PR or sortilin (Fig. 2c). Studies in sheep, mice and flies68 have shown that cathepsin D deficiency can enhance tau toxicity and that this is mediated by a defective endosomal–lysosomal system68. Whether this mechanism leads to abnormal processing of tau within endosomes or in the cytosol via caspase activation68 remains unclear. As discussed above, retromer dysfunction will lead to a decrease in the normal delivery of cathepsin D to the endosome and will result in endosomal–lysosomal system defects. Retromer dysfunction can therefore be considered as a functional phenocopy of cathepsin D deficiency, which suggests a plausible link between retromer dysfunction and tau toxicity. Nevertheless, although these recent insights establish plausibility and support further investigation into the link between retromer and tau toxicity, whether this link exists and how it may be mediated remain open and outstanding questions.

Parkinson disease

The pathogenic link between retromer and PD is singular and straightforward: exome sequencing has identified autosomal-dominant mutations in VPS35 that cause late-onset PD69, 70, one of a handful of genetic causes of late-onset disease. However, the precise mechanism by which these mutations cause the disease is less clear.

Among a group of recent studies, all46, 48, 71, 72, 73, 74, 75, 76 but one77 strongly suggest that these mutations cause a loss of retromer function. At the molecular level, the mutations do not seem to disrupt mutant VPS35 from interacting normally with VPS26 and VPS29, and from forming the cargo-recognition core. Rather, two studies suggest that the mutations have a restricted effect on the retromer assembly but reduce the ability of VPS35 to associate with the WASH complex46, 75. Studies disagree about the pathophysiological consequences of the mutations. Four studies suggest that the mutations affect the normal retrograde transport of CIM6PR71, 73, 75, 76 from the endosome back to the TGN (Fig. 2c). In this scenario, the normal delivery of cathepsin D to the endosomal–lysosomal system should be reduced and this has been empirically shown73. Cathepsin D has been shown to be the dominant endosomal–lysosomal protease for the normal processing of α-synuclein76, and mutations could therefore lead to abnormal α-synuclein processing and to the formation of α-synuclein aggregates, which are thought to have a key pathogenic role in PD.

A separate study suggested that the mutation might cause a mistrafficking of ATG9, and thereby, as discussed above, reduce the formation and the function of autophagosomes46. Autophagosomes have also been implicated as an intracellular site in which α-synuclein aggregates are cleared. Thus, although future studies are needed to resolve these discrepant findings (which may in fact not be mutually exclusive), these studies are generally in agreement that retromer defects will probably increase the neurotoxic levels of α-synuclein aggregates48.

Several studies in flies71, 74 and in rat neuronal cultures71 provide strong evidence that increasing retromer function by overexpressing VPS35 rescues the neurotoxic effects of the most common PD-causing mutations in leucine-rich repeat kinase 2 (LRRK2). Moreover, a separate study has shown that increasing retromer levels rescues the neurotoxic effect of α-synuclein aggregates in a mouse model48. These findings have immediate therapeutic implications for drugs that increase VPS35 and retromer function, as discussed in the next section, but they also offer mechanistic insight. LRRK2 mutations were found to phenocopy the transport defects caused either by theVPS35 mutations or by knocking down VPS35 (Ref. 71). Together, this and other studies78suggest that LRRK2 might have a role in retromer-dependent transport, but future studies are required to clarify this role.

Other neurological disorders

Besides AD and PD, in which a convergence of findings has established a strong pathogenic link, retromer is being implicated in an increasing number of other neurological disorders. Below, we briefly review three disorders for which the evidence of the involvement of retromer in their pathophysiology is currently the most compelling.

The first of these disorders is Down syndrome (DS), which is caused by an additional copy of chromosome 21. Given the hundreds of genes that are duplicated in DS, it has been difficult to identify which ones drive the intellectual impairments that characterize this condition. A recent elegant study provides strong evidence that a deficiency in the retromer cargo-selection protein SNX27 might be a primary driver for some of these impairments79. This study found that the brains of individuals with DS were deficient in SNX27 and that this deficiency may be caused by an extra copy of a microRNA (miRNA) encoded by human chromosome 21 (the miRNA is produced at elevated levels and thereby decreases SNX27 expression). Consistent with the known role of SNX27 in retromer function, decreased expression of this protein in mice disrupted glutamate receptor recycling in the hippocampus and led to dendritic dysfunction. Importantly, overexpression of SNX27 rescued cognitive and other defects in animal models79, which not only strengthens the causal link between retromer dysfunction and cognitive impairment in DS but also has important therapeutic implications.

Hereditary spastic paraplegia (HSP) is another disorder linked to retromer. HSP is caused by genetic mutations that affect upper motor neurons and is characterized by progressive lower limb spasticity and weakness. Although there are numerous mutations that cause HSP, most are unified by their effects on intracellular transport80. One HSP-associated gene in particular encodes strumpellin81, which is a member of the WASH complex.

The third disorder linked to retromer is neuronal ceroid lipofuscinosis (NCL). NCL is a young-onset neurodegenerative disorder that is part of a larger family of lysosomal storage diseases and is caused by mutations in one of ten identified genes — nine neuronal ceroid lipofuscinosis (CLN) genes and the gene encoding cathepsin D82. Besides cathepsin D, for which the link to retromer has been discussed above, CLN3 seems to function in the normal trafficking of CIM6PR83. However, the most direct link to retromer has been recently described for CLN5, which seems to function, at least in part, as a retromer membrane-recruiting protein84.

Retromer as a therapeutic target

As suggested by the first study implicating retromer in AD7, and in several subsequent studies71,85, increasing the levels of retromer’s cargo-recognition core enhances retromer’s transport function. Motivated by this observation and after a decade-long search86, we identified a novel class of ‘retromer pharmacological chaperones’ that can bind and stabilize retromer’s cargo-recognition core and increase retromer levels in neurons61.

Validating the motivating hypothesis, the chaperones were found to enhance retromer function, as shown by the increased transport of APP out of endosomes and a reduction in the accumulation of APP-derived neurotoxic fragments61. Although there are numerous other pharmacological approaches for enhancing retromer function, this success provides the proof-of-principle that retromer is a tractable therapeutic target.

As retromer functions in all cells, a general concern is whether enhancing its function will have toxic adverse effects. However, studies have found that in stark contrast to even mild retromer deficiencies, increasing retromer levels has no obvious negative consequences in yeast, neuronal cultures, flies or mice40, 48, 61, 71. This might make sense because unlike drugs that, for example, function as inhibitors, simply increasing the normal flow of transport through the endosome might not be cytotoxic.

If retromer drugs are safe and can effectively enhance retromer function in the nervous system — which are still outstanding issues — there are two general indications for considering their clinical application. One rests on the idea that these agents will only be efficacious in patients who have predetermined evidence of retromer dysfunction. The most immediate example is that of individuals with PD that is caused by LRRK2 mutations. As discussed above, several ‘preclinical’ studies in flies and neuronal cultures have already established that increasing retromer levels71, 74can reverse the neurotoxic effects of such mutations and, thus, if this approach is proven to be safe, LRRK2-linked PD might be an appropriate indication for clinical trials.

Alternatively, the pathophysiology of a disease might be such that retromer-enhancing drugs would be efficacious regardless of whether there is documented evidence of retromer dysfunction. AD illustrates this point. As reviewed above, current evidence suggests that retromer-enhancing drugs will, at the very least, decrease pathogenic processing of APP in neurons and enhance microglial function, even if there are no pre-existing defects in retromer.

More generally, histological studies comparing the entorhinal cortex of patients with sporadic AD to age-matched controls have documented that enlarged endosomes are a defining cellular abnormality in AD87, 88. Importantly, enlarged endosomes are uniformly observed in a broad range of patients with sporadic AD, which suggests that enlarged endosomes reflect an intracellular site at which molecular aetiologies converge87. In addition, because they are observed in early stages of the disease in regions of the brain without evidence of amyloid pathology87, enlarged endosomes are thought to be an upstream event. Mechanistically, the most likely cause of enlarged endosomes is either too much cargo flowing into endosomes — as occurs, for example, with apolipoprotein E4 (APOE4), which has been shown to accelerate endocytosis89, 90 — or too little cargo flowing out, as observed in retromer dysfunction40, 61 and related transport defects57. By any mechanism, retromer-enhancing drugs might correct this unifying cellular defect and might be expected to be beneficial regardless of the specific aetiology.


The fact that retromer defects, including those derived from bona fide genetic mutations, seem to differentially target the nervous system suggests that the nervous system is differentially dependent on retromer for its normal function. We think that this reflects the unique cellular properties of neurons and how synaptic biology heavily depends on endosomal transport and trafficking. Although plausible, future studies are required to confirm and to test the details of this hypothesis.

However, currently, it is the clinical rather than the basic neuroscience of retromer that is much better understood, with the established pathophysiological consequences of retromer dysfunction providing a mechanistic link to the disorders in which retromer has been implicated. Nevertheless, many questions remain. The two most interesting questions, which are in fact inversions of each other, relate to regional vulnerability in the nervous system. First, why does retromer dysfunction target specific neuronal populations? Second, how can retromer dysfunction cause diseases that target different regions of the nervous system? Recent evidence hints at answers to both questions, which must somehow be rooted in the functional and molecular diversity of retromer.

The type and the extent of retromer defects linked to different disorders might provide pathophysiological clues as well as reasons for differential vulnerability. As discussed, in AD there seem to be across-the-board defects in retromer, such that each module of the retromer assembly as well as multiple retromer cargos have been pathogenically implicated. By contrast, the profile of retromer defects in PD seems to be more circumscribed, involving selective disruption of the interaction between VPS35 and the WASH complex. These insights might agree with histological87, 88 and large-scale genetic studies54 that suggest that endosomal dysfunction is a unifying focal point in the cellular pathogenesis of AD. In contrast, genetics and other studies91suggest that the cellular pathobiology of PD is more distributed, implicating the endosome but other organelles as well, in particular the mitochondria.

Interestingly, studies suggest that the entorhinal cortex — a region that is differentially vulnerable to AD — has unique dendritic structure and function92, which are highly dependent on endosomal transport. We speculate that it is the unique synaptic biology of the entorhinal cortex that can account for why it might be particularly sensitive to defects in endosomal transport in general and retromer dysfunction in particular, and for why this region is the early site of disease. Future studies are required to investigate this hypothesis, as well as to understand why the substantia nigra or other regions that are differentially vulnerable to PD would be particularly sensitive to the more circumscribed defect in retromer.

Perhaps the most important observation for clinical neuroscience is the now well-established fact that increasing levels of retromer proteins enhances retromer function and has already proved capable of reversing defects associated with AD, PD and DS in either cell culture or in animal models. The relationships between protein levels and function are not always simple, but emerging pharmaceutical technologies that selectively and safely increase protein levels are now a tractable goal in drug discovery93. With the evidence mounting that retromer has a pathogenic role in two of the most common neurodegenerative diseases, we think that targeting retromer to increase its functional activity is an important goal that has strong therapeutic promise.


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Layers of Human Brain

Larry H. Bernstein, MD, FCAP, Curator



Human Brain Peeled Back to Its Transcriptional Core

The Allen Human Brain Atlas, a data set derived from analyses of tissue samples such as the one shown here, was used in an investigation of differential transcription across 132 structures in six individual brains. The investigation revealed that a set of just 32 gene-expression signatures defines, in large part, a common network architecture that is conserved across the human population. [Allen Institute for Brain Science]


The human brain has so many organizational layers that you might wonder whether there is, deep down, a core that we all share, however diverse our brains are in other respects. It turns out that there is a core, report scientists at the Allen Institute for Brain Science. This core, the scientists say, is transcriptional and surprisingly compact—just 32 gene-expression signatures.

The Allen Institute scientists decided that the highly stereotyped character of the human brain implied that a conserved molecular program was responsible for the brain’s development, cellular structure, and function. “So much research focuses on the variations between individuals, but we turned that question on its head to ask, what makes us similar?” explained Ed Lein, Ph.D., investigator at the Allen Institute for Brain Science. “What is the conserved element among all of us that must give rise to our unique cognitive abilities and human traits?”

Using a microarray profiling dataset from the Allen Human Brain Atlas, Dr. Lein and colleagues found that many genes showed highly consistent patterns of transcriptional regulation across brain regions as quantified using a metric called differential stability (DS). DS is the tendency for a gene to exhibit reproducible differential expression relationships across brain structures.

This approach allowed the investigators to identify molecular patterns that dominate gene expression in the human brain and appear to be common to all individuals. The investigators detailed their work November 16 in the journal Nature Neuroscience, in an article entitled, “Canonical genetic signatures of the adult human brain.”

“[We assessed] reproducibility of gene expression patterning across 132 structures in six individual brains, revealing mesoscale genetic organization,” wrote the authors. “The genes with the highest differential stability are highly biologically relevant, with enrichment for brain-related annotations, disease associations, drug targets and literature citations.”

“[These genes appear to] represent a functionally critical set whose transcriptional regulation is tightly controlled,” the authors continued. “Taking this concept of conserved patterning from genes to gene networks, we demonstrate the existence of a relatively small (32) set of consensus coexpression gene networks that explain most (90.1%, ρ > 0.4) transcriptional variation across adult brain regions.”

In other words, most of the patterns of gene usage across all 20,000 genes could be characterized by just 32 expression patterns. While many of these patterns were similar in human and mouse, the dominant genetic model organism for biomedical research, many genes showed different patterns in human. Surprisingly, genes associated with neurons were most conserved across species, while those for the supporting glial cells showed larger differences.

The most highly stable genes—the genes that were most consistent across all brains—include those that are associated with diseases and disorders like autism and Alzheimer’s and include many existing drug targets. These patterns provide insights into what makes the human brain distinct and raise new opportunities to target therapeutics for treating disease.

Finally, the investigators noted that highly consistent transcriptional architecture in neocortex is correlated with resting state functional connectivity. According to the investigators, this suggests a link between conserved gene expression and functionally relevant circuitry.

“The human brain is phenomenally complex,” said Christof Koch, Ph.D., president and CSO at the Allen Institute for Brain Science. “There could easily have been thousands of patterns, or none at all. This gives us an exciting way to look further at the functional activity that underlies the uniquely human brain.”


Canonical genetic signatures of the adult human brain

Michael HawrylyczJeremy A MillerVilas MenonDavid FengTim DolbeareAngela L Guillozet-BongaartsAnil G Jegga, et al.

Nature Neuroscience (2015)

The structure and function of the human brain are highly stereotyped, implying a conserved molecular program responsible for its development, cellular structure and function. We applied a correlation-based metric called differential stability to assess reproducibility of gene expression patterning across 132 structures in six individual brains, revealing mesoscale genetic organization. The genes with the highest differential stability are highly biologically relevant, with enrichment for brain-related annotations, disease associations, drug targets and literature citations. Using genes with high differential stability, we identified 32 anatomically diverse and reproducible gene expression signatures, which represent distinct cell types, intracellular components and/or associations with neurodevelopmental and neurodegenerative disorders. Genes in neuron-associated compared to non-neuronal networks showed higher preservation between human and mouse; however, many diversely patterned genes displayed marked shifts in regulation between species. Finally, highly consistent transcriptional architecture in neocortex is correlated with resting state functional connectivity, suggesting a link between conserved gene expression and functionally relevant circuitry.


Genetic variability in the regulation of gene expression in ten regions of the human brain

Adaikalavan RamasamyDaniah TrabzuniSebastian GuelfiVibin VargheseColin SmithRobert WalkerTisham DeUK Brain Expression ConsortiumNorth American Brain Expression Consortium,  et al.

Nature Neuroscience  2014;  17; 1418–1428

Germ-line genetic control of gene expression occurs via expression quantitative trait loci (eQTLs). We present a large, exon-specific eQTL data set covering ten human brain regions. We found thatcis-eQTL signals (within 1 Mb of their target gene) were numerous, and many acted heterogeneously among regions and exons. Co-regulation analysis of shared eQTL signals produced well-defined modules of region-specific co-regulated genes, in contrast to standard coexpression analysis of the same samples. We report cis-eQTL signals for 23.1% of catalogued genome-wide association study hits for adult-onset neurological disorders. The data set is publicly available via public data repositories and via Our study increases our understanding of the regulation of gene expression in the human brain and will be of value to others pursuing functional follow-up of disease-associated variants.

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