Genetic analysis of sorting nexins 1 and 2 reveals a redundant and essential function in mice

Out of the ESCPE room: Emerging roles of endosomal SNX-BARs in receptor transport and host-pathogen interaction

Several functions of the human cell, such as sensing nutrients, cell movement and interaction with the surrounding environment, depend on a myriad of transmembrane proteins and their associated proteins and lipids (collectively termed "cargoes"). To successfully perform their tasks, cargo must be sorted and delivered to the right place, at the right time, and in the right amount. To achieve this, eukaryotic cells have evolved a highly organized sorting platform, the endosomal network. Here, a variety of specialized multiprotein complexes sort cargo into itineraries leading to either their degradation or their recycling to various organelles for further rounds of reuse. A key sorting complex is the Endosomal SNX-BAR Sorting Complex for Promoting Exit (ESCPE-1) that promotes the recycling of an array of cargos to the plasma membrane and/or the trans-Golgi network. ESCPE-1 recognizes a hydrophobic-based sorting motif in numerous cargoes and orchestrates their packaging into tubular carriers that pinch off from the endosome and travel to the target organelle. A wide range of pathogens mimic this sorting motif to hijack ESCPE-1 transport to promote their invasion and survival within infected cells. In other instances, ESCPE-1 exerts restrictive functions against pathogens by limiting their replication and infection. In this review, we discuss ESCPE-1 assembly and functions, with a particular focus on recent advances in the understanding of its role in membrane trafficking, cellular homeostasis and host-pathogen interaction.

Aberrant lipid accumulation in the mouse visceral yolk sac resulting from maternal diabetes and obesity

Maternal diabetes and obesity in pregnancy are well-known risk factors for structural birth defects, including neural tube defects and congenital heart defects. Progeny from affected pregnancies are also predisposed to developing cardiometabolic disease in later life. Based upon in vitro embryo cultures of rat embryos, it was postulated that nutrient uptake by the yolk sac is deficient in diabetic pregnancies. In contrast, using two independent mouse models of maternal diabetes, and a high-fat diet-feeding model of maternal obesity, we observed excessive lipid accumulation at 8.5 days in the yolk sac. The numbers as well as sizes of intracellular lipid droplets were increased in yolk sacs of embryos from diabetic and obese pregnancies. Maternal metabolic disease did not affect expression of lipid transporter proteins, including ApoA1, ApoB and SR-B1, consistent with our earlier report that expression of glucose and fatty acid transporter genes was also unchanged in diabetic pregnancy-derived yolk sacs. Colocalization of lipid droplets with lysosomes was significantly reduced in the yolk sacs from diabetic and obese pregnancies compared to yolk sacs from normal pregnancies. We therefore conclude that processing of lipids is defective in pregnancies affected by maternal metabolic disease, which may lead to reduced availability of lipids to the developing embryo. The possible implications of insufficient supply of lipids -and potentially of other nutrients-to the embryos experiencing adverse pregnancy conditions are discussed.

Sorting nexins: role in the regulation of blood pressure

Sorting nexins (SNXs) are a family of proteins that regulate cellular cargo sorting and trafficking, maintain intracellular protein homeostasis, and participate in intracellular signaling. SNXs are also important in the regulation of blood pressure via several mechanisms. Aberrant expression and dysfunction of SNXs participate in the dysregulation of blood pressure. Genetic studies show a correlation between SNX gene variants and the response to antihypertensive drugs. In this review, we summarize the progress in SNX-mediated regulation of blood pressure, discuss the potential role of SNXs in the pathophysiology and treatment of hypertension, and propose novel strategies for the medical therapy of hypertension.

Sorting Out Sorting Nexins Functions in the Nervous System in Health and Disease

Endocytosis is a fundamental process that controls protein/lipid composition of the plasma membrane, thereby shaping cellular metabolism, sensing, adhesion, signaling, and nutrient uptake. Endocytosis is essential for the cell to adapt to its surrounding environment, and a tight regulation of the endocytic mechanisms is required to maintain cell function and survival. This is particularly significant in the central nervous system (CNS), where composition of neuronal cell surface is crucial for synaptic functioning. In fact, distinct pathologies of the CNS are tightly linked to abnormal endolysosomal function, and several genome wide association analysis (GWAS) and biochemical studies have identified intracellular trafficking regulators as genetic risk factors for such pathologies. The sorting nexins (SNXs) are a family of proteins involved in protein trafficking regulation and signaling. SNXs dysregulation occurs in patients with Alzheimer’s disease (AD), Down’s syndrome (DS), schizophrenia, ataxia and epilepsy, among others, establishing clear roles for this protein family in pathology. Interestingly, restoration of SNXs levels has been shown to trigger synaptic plasticity recovery in a DS mouse model. This review encompasses an historical and evolutionary overview of SNXs protein family, focusing on its organization, phyla conservation, and evolution throughout the development of the nervous system during speciation. We will also survey SNXs molecular interactions and highlight how defects on SNXs underlie distinct pathologies of the CNS. Ultimately, we discuss possible strategies of intervention, surveying how our knowledge about the fundamental processes regulated by SNXs can be applied to the identification of novel therapeutic avenues for SNXs-related disorders.

SNX27-retromer assembly recycles MT1-MMP to invadopodia and promotes breast cancer metastasis

Recycling of MT-MMPs to actin-rich membrane-protrusive structures promotes breast cancer invasion. This study shows that SNX27–retromer, an endosomal sorting and recycling machinery, interacts with MT1-MMP and regulates its transport to the cell surface, thus promoting matrix invasive activity of the breast cancer cells.

A variety of metastatic cancer cells use actin-rich membrane protrusions, known as invadopodia, for efficient ECM degradation, which involves trafficking of proteases from intracellular compartments to these structures. Here, we demonstrate that in the metastatic breast cancer cell line MDA-MB-231, retromer regulates the matrix invasion activity by recycling matrix metalloprotease, MT1-MMP. We further found that MT2-MMP, another abundantly expressed metalloprotease, is also invadopodia associated. MT1- and MT2-MMP showed a high degree of colocalization but were located on the distinct endosomal domains. Retromer and its associated sorting nexin, SNX27, phenocopied each other in matrix degradation via selectively recycling MT1-MMP but not MT2-MMP. ITC-based studies revealed that both SNX27 and retromer could directly interact with MT1-MMP. Analysis from a publicly available database showed SNX27 to be overexpressed or frequently altered in the patients having invasive breast cancer. In xenograft-based studies, SNX27-depleted cell lines showed prolonged survival of SCID mice, suggesting a possible implication for overexpression of the sorting nexin in tumor samples.

Sorting nexin 1 loss results in increased oxidative stress and hypertension

Acute renal depletion of sorting nexin 1 (SNX1) in mice results in blunted natriuretic response and hypertension due to impaired dopamine D₅ receptor (D₅ R) activity. We elucidated the molecular mechanisms for these phenotypes in Snx1^-/- mice. These mice had increased renal expressions of angiotensin II type 1 receptor (AT₁ R), NADPH oxidase (NOX) subunits, D₅ R, and NaCl cotransporter. Basal reactive oxygen species (ROS), NOX activity, and blood pressure (BP) were also higher in Snx1^-/- mice, which were normalized by apocynin, a drug that prevents NOX assembly. Renal proximal tubule (RPT) cells from hypertensive (HT) Euro-American males had deficient SNX1 activity, impaired D₅ R endocytosis, and increased ROS compared with cells from normotensive (NT) Euro-American males. siRNA-mediated depletion of SNX1 in RPT cells from NT subjects led to a blunting of D₅ R agonist-induced increase in cAMP production and decrease in Na⁺ transport, effects that were normalized by over-expression of SNX1. Among HT African-Americans, three of the 12 single nucleotide polymorphisms interrogated for the SNX1 gene were associated with a decrease in systolic BP in response to hydrochlorothiazide (HCTZ). The results illustrate a new paradigm for the development of hypertension and imply that the trafficking protein SNX1 may be a crucial determinant for hypertension and response to antihypertensive therapy.

Reverse-genetics studies of lncRNAs-what we have learnt and paths forward

Long non-coding RNAs (lncRNAs) represent a major fraction of the transcriptome in multicellular organisms. Although a handful of well-studied lncRNAs are broadly recognized as biologically meaningful, the fraction of such transcripts out of the entire collection of lncRNAs remains a subject of vigorous debate. Here we review the evidence for and against biological functionalities of lncRNAs and attempt to arrive at potential modes of lncRNA functionality that would reconcile the contradictory conclusions. Finally, we discuss different strategies of phenotypic analyses that could be used to investigate such modes of lncRNA functionality.

Loss of Cubilin, the intrinsic factor-vitamin B12 receptor, impairs visceral endoderm endocytosis and endodermal patterning in the mouse

The visceral endoderm is a polarized epithelial monolayer necessary for early embryonic development in rodents. A key feature of this epithelium is an active endocytosis and degradation of maternal nutrients, in addition to being the source of various signaling molecules or inhibitors required for the differentiation and patterning of adjacent embryonic tissues. Endocytosis across the visceral endoderm epithelium involves specific cell surface receptors and an extensive sub-membrane vesicular system with numerous apical vacuoles/lysosomes. We previously reported that Cubilin, the endocytic receptor for intrinsic factor-vitamin B12, albumin and apolipoproteinA-I/HDL allows maternal nutrient uptake by the visceral endoderm. In the present study, we show that the germline ablation of Cubilin impairs endodermal and mesodermal patterning, and results in developmental arrest at gastrulation. Notably, visceral endoderm dispersal is impeded in Cubilin null embryos. We further confirm the essential role of Cubilin in nutrient internalization by the early visceral endoderm and highlight its involvement in the formation of apical vacuoles. Our results reveal essential roles for Cubilin in early embryonic development, and suggest that in addition to its nutritive function, Cubilin sustains signaling pathways involved in embryonic differentiation and patterning.

Arl8b is required for lysosomal degradation of maternal proteins in the visceral yolk sac endoderm of mouse embryos

The small GTPase Arl8b localizes primarily to lysosomes and is involved in lysosomal motility and fusion. Here, we show that Arl8b is required for lysosomal degradation of maternal proteins in the visceral yolk sac endoderm (VYSE), an apical cell layer of the visceral yolk sac, of mouse embryos. The VYSE actively takes up maternal materials from uterine fluid and degrades them in lysosomes to provide breakdown products to the embryo as energy sources. Arl8b gene-trap mice (Arl8b−/−) displayed decreased early embryo body size. The Arl8b−/− VYSE exhibited defective endocytic trafficking to the lysosome and accumulation of maternal proteins such as albumin and immunoglobulin G in late endocytic organelles. Furthermore, Transthyretin-Cre;Arl8bflox/flox mice in which Arl8b was ablated specifically in the VYSE also showed decreased embryo body size, defects in trafficking to the lysosome, and reduction of the free amino acid level in the embryos. Taken together, these results suggest that Arl8b mediates lysosomal degradation of maternal proteins in the VYSE, thereby contributing to mouse embryonic development.

Retrograde trafficking from the endosome to the trans-Golgi network mediated by the retromer is required for fungal development and pathogenicity in Fusarium graminearum

In eukaryotes, the retromer is an endosome-localized complex involved in protein retrograde transport. However, the role of such intracellular trafficking events in pathogenic fungal development and pathogenicity remains unclear. The role of the retromer complex in Fusarium graminearum was investigated using cell biological and genetic methods. We observed the retromer core component FgVps35 (Vacuolar Protein Sorting 35) in the cytoplasm as fast-moving puncta. FgVps35-GFP co-localized with both early and late endosomes, and associated with the trans-Golgi network (TGN), suggesting that FgVps35 functions at the donor endosome membrane to mediate TGN trafficking. Disruption of microtubules with nocodazole significantly restricted the transportation of FgVps35-GFP and resulted in severe germination and growth defects. Mutation of FgVPS35 not only mimicked growth defects induced by pharmacological treatment, but also affected conidiation, ascospore formation and pathogenicity. Using yeast two-hybrid assays, we determined the interactions among FgVps35, FgVps26, FgVps29, FgVps17 and FgVps5 which are analogous to the yeast retromer complex components. Deletion of any one of these genes resulted in similar phenotypic defects to those of the ΔFgvps35 mutant and disrupted the stability of the complex. Overall, our results provide the first clear evidence of linkage between the retrograde transport mediated by the retromer complex and virulence in F. graminearum.

Membrane dynamics in mammalian embryogenesis: Implication in signal regulation

Eukaryotes have evolved an array of membrane compartments constituting secretory and endocytic pathways that allow the flow of materials. Both pathways perform important regulatory roles. The secretory pathway is essential for the production of extracellular, secreted signal molecules, but its function is not restricted to a mere route connecting intra- and extracellular compartments. Post-translational modifications also play an integral function in the secretory pathway and are implicated in developmental regulation. The endocytic pathway serves as a platform for relaying signals from the extracellular stimuli to intracellular mediators, and then ultimately inducing signal termination. Here, we discuss recent studies showing that dysfunction in membrane dynamics causes patterning defects in embryogenesis and tissue morphogenesis in mammals.

Retromer: Structure, function, and roles in mammalian disease

Retrograde transport from the endosome to the Golgi is mediated by a 5 protein complex known as the retromer. These five proteins (Vps5, Vps17, Vps26, Vps29, and Vps35 in yeast and SNX1/2, SNX5/6, Vps26, Vps29, and Vps35 in mammalian cells) act as a coat for vesicles budding off of the endosome, as well as perform cargo sorting at the endosome. The retromer is well conserved between yeast and mammalian systems, though variations exist within the mammalian retromer. Functionally, the retromer has been linked to prominent neurodegenerative diseases such as Alzheimer's and Parkinson's in human models as well as diabetes mellitus. However, the retromer also plays a role in the virulence of several microbial pathogens. Despite the current understanding of the retromer complex, there are still many questions to be answered in regards to its overall role in cell homeostasis.

PIP kinases define PI4,5P₂signaling specificity by association with effectors

Phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) is an essential lipid messenger with roles in all eukaryotes and most aspects of human physiology. By controlling the targeting and activity of its effectors, PI4,5P₂modulates processes, such as cell migration, vesicular trafficking, cellular morphogenesis, signaling and gene expression. In cells, PI4,5P₂has a much higher concentration than other phosphoinositide species and its total content is largely unchanged in response to extracellular stimuli. The discovery of a vast array of PI4,5P₂ binding proteins is consistent with data showing that the majority of cellular PI4,5P₂is sequestered. This supports a mechanism where PI4,5P₂functions as a localized and highly specific messenger. Further support of this mechanism comes from the de novo synthesis of PI4,5P₂which is often linked with PIP kinase interaction with PI4,5P₂effectors and is a mechanism to define specificity of PI4,5P₂signaling. The association of PI4,5P₂-generating enzymes with PI4,5P₂effectors regulate effector function both temporally and spatially in cells. In this review, the PI4,5P₂effectors whose functions are tightly regulated by associations with PI4,5P₂-generating enzymes will be discussed. This article is part of a Special Issue entitled Phosphoinositides.

Loss of PiT-1 results in abnormal endocytosis in the yolk sac visceral endoderm

PiT-1 protein is a transmembrane sodium-dependent phosphate (Pi) transporter. PiT-1 knock out (KO) embryos die from largely unknown causes by embryonic day (E) 12.5. We tested the hypothesis that PiT-1 is required for endocytosis in the embryonic yolk sac (YS) visceral endoderm (VE). Here we present data supporting that PiT-1 KO results in a YS remodeling defect and decreased endocytosis in the YS VE. The remodeling defect is not due to an upstream cardiomyocyte requirement for PiT-1, as SM22αCre-specific KO of PiT-1 in the developing heart and the YS mesodermal layer (ME) does not recapitulate the PiT-1 global KO phenotype. Furthermore, we find that high levels of PiT-1 protein localize to the YS VE apical membrane. Together these data support that PiT-1 is likely required in YS VE. During normal development maternal immunoglobulin (IgG) is endocytosed into YS VE and accumulates in the apical side of the VE in a specialized lysosome termed the apical vacuole (AV). We have identified a reduction in PiT-1 KO VE cell height and a striking loss of IgG accumulation in the PiT-1 KO VE. The endocytosis genes Tfeb, Lamtor2 and Snx2 are increased at the RNA level. Lysotracker Red staining reveals a loss of distinct AVs, and yolk sacs incubated ex vivo with phRODO Green Dextran for Endocytosis demonstrate a functional loss of endocytosis. As yolk sac endocytosis is controlled in part by microautophagy, but expression of LC3 had not been examined, we investigated LC3 expression during yolk sac development and found stage-specific LC3 RNA expression that is predominantly from the YS VE layer at E9.5. Normalized LC3-II protein levels are decreased in the PiT-1 KO YS, supporting a requirement for PiT-1 in autophagy in the YS. Therefore, we propose the novel idea that PiT-1 is central to the regulation of endocytosis and autophagy in the YS VE.

Positive and negative regulation of developmental signaling by the endocytic pathway

Multicellular organisms acquire complex architecture through highly regulated developmental processes in which cells are programmed to respond to a specific set of extracellular signals produced by themselves and others. Modulation of sensitivity or duration of response is controlled by a variety of intracellular mechanisms. The endoocytic pathway performs essential regulatory roles both for the activation as well as the inactivation of signal transduction. Early stage of endocytic pathway is required for the recruitment of cytosolic mediators for signal amplification of signaling, whereas signal termination by late endosomes/lysosomes is important for spatiotemporal regulation. Herein, we summarize recent studies showing that dysfunction in endocytic pathways causes patterning defects in early embryogenesis in mammals.

Sorting nexin 1 loss results in D5 dopamine receptor dysfunction in human renal proximal tubule cells and hypertension in mice

The peripheral dopaminergic system plays a crucial role in blood pressure regulation through its actions on renal hemodynamics and epithelial ion transport. The dopamine D5 receptor (D(5)R) interacts with sorting nexin 1 (SNX1), a protein involved in receptor retrieval from the trans-Golgi network. In this report, we elucidated the spatial, temporal, and functional significance of this interaction in human renal proximal tubule cells and HEK293 cells stably expressing human D(5)R and in mice. Silencing of SNX1 expression via RNAi resulted in the failure of D(5)R to internalize and bind GTP, blunting of the agonist-induced increase in cAMP production and decrease in sodium transport, and up-regulation of angiotensin II receptor expression, of which expression was previously shown to be negatively regulated by D(5)R. Moreover, siRNA-mediated depletion of renal SNX1 in C57BL/6J and BALB/cJ mice resulted in increased blood pressure and blunted natriuretic response to agonist in salt-loaded BALB/cJ mice. These data demonstrate a crucial role for SNX1 in D(5)R trafficking and that SNX1 depletion results in D(5)R dysfunction and thus may represent a novel mechanism for the pathogenesis of essential hypertension.

The retromer complex - endosomal protein recycling and beyond

The retromer complex is a vital element of the endosomal protein sorting machinery that is conserved across all eukaryotes. Retromer is most closely associated with the endosome-to-Golgi retrieval pathway and is necessary to maintain an active pool of hydrolase receptors in the trans-Golgi network. Recent progress in studies of retromer have identified new retromer-interacting proteins, including the WASH complex and cargo such as the Wntless/MIG-14 protein, which now extends the role of retromer beyond the endosome-to-Golgi pathway and has revealed that retromer is required for aspects of endosome-to-plasma membrane sorting and regulation of signalling events. The interactions between the retromer complex and other macromolecular protein complexes now show how endosomal protein sorting is coordinated with actin assembly and movement along microtubules, and place retromer squarely at the centre of a complex set of protein machinery that governs endosomal protein sorting. Dysregulation of retromer-mediated endosomal protein sorting leads to various pathologies, including neurodegenerative diseases such as Alzheimer disease and spastic paraplegia and the mechanisms underlying these pathologies are starting to be understood. In this Commentary, I will highlight recent advances in the understanding of retromer-mediated endosomal protein sorting and discuss how retromer contributes to a diverse set of physiological processes.

Molecular identification of t(w5): Vps52 promotes pluripotential cell differentiation through cell-cell interactions

After implantation, pluripotent epiblasts are converted to embryonic ectoderm through cell-cell interactions that significantly change the transcriptional and epigenetic networks. An entrée to understanding this vital developmental transition is the t(w5) mutation of the mouse t complex. This mutation produces highly specific defects in the embryonic ectoderm before gastrulation, leading to death of the embryonic ectoderm. Using a positional cloning approach, we have now identified the mutated gene, completing a decades-long search. The gene, vacuolar protein sorting 52 (Vps52), is a mouse homolog of yeast VPS52 that is involved in the retrograde trafficking of endosomes. Our data suggest that Vps52 acts in extraembryonic tissues to support the growth and differentiation of embryonic ectoderm via cell-cell interactions. It is also required in the formation of embryonic structures at a later stage of development, revealing hitherto unknown functions of Vps52 in the development of a multicellular organism.

Aberrant placenta gene expression pattern in bovine pregnancies established after transfer of cloned or in vitro produced embryos

In the present study, we used the global transcriptome profile approach to identify dysregulated genes, molecular pathways, and molecular functional alterations in bovine placentas derived from somatic cell nuclear transfer (SCNT) and in vitro embryo production (IVP) pregnancies compared with their artificial insemination (AI) counterparts at day 50 of gestation. For this, day 7 blastocysts derived from AI, IVP, or SCNT were transferred to oestrus-synchronized cows. The pregnant animals were slaughtered at day 50 of gestation, and the placentas were then recovered and used for transcriptome analysis using Affymetrix GeneChip bovine genome array. Results showed the SCNT placenta to be different from its AI counterpart in the expression of 1,196 transcripts. These genes were found to be associated with alterations in key biological processes and molecular pathways in SCNT placenta, and the dysregulation of 9% (n = 110) of these genes was due to transcriptional reprogramming error. IVP placenta also displayed alterations in the expression of 72 genes, of which 58 were common to SCNT placenta. Gene enrichment analysis revealed that the expression of genes involved in organ development, blood vessel development, extracellular matrix organization, and the immune system was affected in both SCNT and IVP placentas. However, 96% of the affected genes in SCNT were not significantly altered in IVP groups. Thus, the higher transcriptome dysregulation in SCNT placenta followed by IVP would reflect the degree of placental abnormality in SCNT and IVP pregnancies at day 50 of the gestation, which may have a profound effect on subsequent fetal development and health of the offspring.

SNX5 is essential for efficient macropinocytosis and antigen processing in primary macrophages

Macropinocytosis mediates the bulk endocytosis of solute molecules, nutrients and antigens. As this endocytic pathway is considered important in functions associated with immune responses, the molecular mechanisms regulating this pathway in immune cells is of particular significance. However, the regulators of macropinocytosis in primary cells remain poorly defined. Members of the sorting nexin (SNX) family have been implicated in macropinosome biogenesis in cultured cells and here we have analyzed the role of two SNX family members, SNX1 and its binding partner SNX5, in macropinocytosis of mouse primary macrophages. We show that endogenous SNX1 and SNX5 are localised to newly-formed macropinosomes in primary mouse macrophages and, moreover, demonstrate that SNX5 plays an essential role in macropinosome biogenesis. Depletion of SNX5 in bone marrow-derived macrophages dramatically decreased both the number and size of macropinosomes. Depletion of SNX5 also resulted in dramatic reduction in uptake and processing of soluble ovalbumin in macrophages, indicating that the majority of antigen uptake and delivery to late endosomes is via macropinocytosis. By contrast, the absence of SNX1 had no effect on endogenous SNX5 localisation and macropinosome biogenesis using macrophages from SNX1 knockout mice. Therefore, SNX5 can function independently of SNX1 and is a modulator of macropinocytosis that influences the uptake and processing of soluble antigen in primary mouse macrophages.

WNT secretion and signalling in human disease

Wnt signalling, a key pathway involved in various aspects of embryonic development, also underlies many human diseases, in particular, cancer. Research focused on signal transduction within signal-receiving cells led to the discovery of many Wnt pathway components, but study of the secretion of Wnt ligands themselves was neglected until recently. Attention was drawn to this highly regulated process by the association of aberrant Wnt levels with an increasing number of diseases. Studying the biogenesis and processing of active Wnt ligands will open new avenues for generating therapeutics to specifically target aberrant Wnt signalling. Here we review the proteins required for Wnt secretion and signalling at the plasma membrane, ending with a discussion on potential therapeutic approaches to treat Wnt-induced diseases.

An antiapoptotic role of sorting nexin 7 is required for liver development in zebrafish

Sorting nexin (SNX) family proteins are best characterized for their abilities to regulate protein trafficking during processes such as endocytosis of membrane receptors, endosomal sorting, and protein degradation, but their in vivo functions remain largely unknown. We started to investigate the biological functions of SNXs using the zebrafish model. In this study, we demonstrated that SNX7 was essential for embryonic liver development. Hepatoblasts were specified normally, and the proliferation of these cells was not affected when SNX7 was knocked down by gene-specific morpholinos; however, they underwent massive apoptosis during the early budding stage. SNX7 mainly regulated the survival of cells in the embryonic liver and did not affect the viability of cells in other endoderm-derived organs. We further demonstrated that down-regulation of SNX7 by short interfering RNAs induced apoptosis in cell culture. At the molecular level, the cellular FLICE-like inhibitory protein (c-FLIP)/caspase 8 pathway was activated when SNX7 was down-regulated. Furthermore, overexpression of c-FLIP(S) was able to rescue the SNX7 knockdown-induced liver defect.

CONCLUSION

SNX7 is a liver-enriched antiapoptotic protein that is indispensable for the survival of hepatoblasts during zebrafish early embryogenesis.

Retromer binds the FANSHY sorting motif in SorLA to regulate amyloid precursor protein sorting and processing

sorLA is a sorting receptor for amyloid precursor protein (APP) genetically linked to Alzheimer's disease (AD). Retromer, an adaptor complex in the endosome-to-Golgi retrieval pathway, has been implicated in APP transport because retromer deficiency leads to aberrant APP sorting and processing and levels of retromer proteins are altered in AD. Here we report that sorLA and retromer functionally interact in neurons to control trafficking and amyloidogenic processing of APP. We have identified a sequence (FANSHY) in the cytoplasmic domain of sorLA that is recognized by the VPS26 subunit of the retromer complex. Accordingly, we characterized the interaction between the retromer complex and sorLA and determined the role of retromer on sorLA-dependent sorting and processing of APP. Mutations in the VPS26 binding site resulted in receptor redistribution to the endosomal network, similar to the situation seen in cells with VPS26 knockdown. The sorLA mutant retained APP-binding activity but, as opposed to the wild-type receptor, misdirected APP into a distinct non-Golgi compartment, resulting in increased amyloid processing. In conclusion, our data provide a molecular link between reduced retromer expression and increased amyloidogenesis as seen in patients with sporadic AD.

Insights into the PX (phox-homology) domain and SNX (sorting nexin) protein families: structures, functions and roles in disease

The mammalian genome encodes 49 proteins that possess a PX (phox-homology) domain, responsible for membrane attachment to organelles of the secretory and endocytic system via binding of phosphoinositide lipids. The PX domain proteins, most of which are classified as SNXs (sorting nexins), constitute an extremely diverse family of molecules that play varied roles in membrane trafficking, cell signalling, membrane remodelling and organelle motility. In the present review, we present an overview of the family, incorporating recent functional and structural insights, and propose an updated classification of the proteins into distinct subfamilies on the basis of these insights. Almost all PX domain proteins bind PtdIns3P and are recruited to early endosomal membranes. Although other specificities and localizations have been reported for a select few family members, the molecular basis for binding to other lipids is still not clear. The PX domain is also emerging as an important protein–protein interaction domain, binding endocytic and exocytic machinery, transmembrane proteins and many other molecules. A comprehensive survey of the molecular interactions governed by PX proteins highlights the functional diversity of the family as trafficking cargo adaptors and membrane-associated scaffolds regulating cell signalling. Finally, we examine the mounting evidence linking PX proteins to different disorders, in particular focusing on their emerging importance in both pathogen invasion and amyloid production in Alzheimer's disease.

Evolutionary reconstruction of the retromer complex and its function in Trypanosoma brucei

Intracellular trafficking and protein sorting are mediated by various protein complexes, with the retromer complex being primarily involved in retrograde traffic from the endosome or lysosome to the Golgi complex. Here, comparative genomics, cell biology and phylogenetics were used to probe the early evolution of retromer and its function. Retromer subunits Vps26, Vps29 and Vps35 are near universal, and, by inference, the complex was an ancient feature of eukaryotic cells. Surprisingly, we found DSCR3, a Vps26 paralogue in humans associated with Down's syndrome, in at least four eukaryotic supergroups, implying a more ancient origin than previously suspected. By contrast, retromer cargo proteins showed considerable interlineage variability, with lineage-specific and broadly conserved examples found. Vps10 trafficking probably represents an ancestral role for the complex. Vps5, the BAR-domain-containing membrane-deformation subunit, was found in diverse eukaryotes, including in the divergent eukaryote Trypanosoma brucei, where it is the first example of a BAR-domain protein. To determine functional conservation, an initial characterisation of retromer was performed in T. brucei; the endosomal localisation and its role in endosomal targeting are conserved. Therefore retromer is identified as a further feature of the sophisticated intracellular trafficking machinery of the last eukaryotic common ancestor, with BAR domains representing a possible third independent mechanism of membrane-deformation arising in early eukaryotes.

A SNX10/V-ATPase pathway regulates ciliogenesis in vitro and in vivo

Sorting nexins (SNXs) are phosphoinositide-binding proteins implicated in the sorting of various membrane proteins in vitro, but the in vivo functions of them remain largely unknown. We reported previously that SNX10 is a unique member of the SNX family genes in that it has vacuolation activity in cells. We investigate the biological function of SNX10 by loss-of-function assay in this study and demonstrate that SNX10 is required for the formation of primary cilia in cultured cells. In zebrafish, SNX10 is involved in ciliogenesis in the Kupffer's vesicle and essential for left-right patterning of visceral organs. Mechanistically, SNX10 interacts with V-ATPase complex and targets it to the centrosome where ciliogenesis is initiated. Like SNX10, V-ATPase regulates ciliogenesis in vitro and in vivo and does so synergistically with SNX10. We further discover that SNX10 and V-ATPase regulate the ciliary trafficking of Rab8a, which is a critical regulator of ciliary membrane extension. These results identify an SNX10/V-ATPase-regulated vesicular trafficking pathway that is crucial for ciliogenesis, and reveal that SNX10/V-ATPase, through the regulation of cilia formation in various organs, play an essential role during early embryonic development.

Derlin-dependent retrograde transport from endosomes to the Golgi apparatus

Cells have to maintain stable plasma membrane protein and lipid compositions under normal conditions and to remodel their plasma membranes in response to stimuli. This maintenance and remodeling require that integral membrane proteins at the plasma membrane that become misfolded, because of the relatively harsher extracellular milieu or carbohydrate and amino acid sequence changes, are degraded. We had previously shown that Derlin proteins, required for quality control mechanisms in the endoplasmic reticulum, also localize to endosomes and function in the degradation of misfolded integral membrane proteins at the plasma membrane. In this study, we show that Derlin proteins physically associate with sorting nexins that function in retrograde membrane transport from endosomes to the Golgi apparatus. Using genetic studies in Caenorhabditis elegans and ricin pulse-chase analyses in murine RAW264.7 macrophages, we show that the Derlin-sorting nexin interaction is physiologically relevant. Our studies suggest that at least some integral membrane proteins that are misfolded at the plasma membrane are retrogradely transported to the Golgi apparatus and ultimately to the endoplasmic reticulum for degradation via resident quality control mechanisms.

Quantitative analysis of retromer complex-related genes during embryo development in the mouse

The retromer complex is a heteropentameric protein unit associated with retrograde transport of cargo proteins from endosomes to the trans-Golgi network. Functional silencing study of the Vps26a gene indicated the important role of the retromer complex during early developmental stages in the mouse. However, individual expression patterns and quantitative analysis of individual members of the retromer complex during the early developmental stages has not been investigated. In this study, we conducted quantitative expression analysis of six retromer complex genes (Vps26a, Vps26b, Vps29, Vps35, Snx1, and Snx2) and one related receptor gene (Ci-mpr) during the eleven embryonic stages with normal MEF (mouse embryonic fibroblast) and Vps26a(-/-) MEF cells. Remarkably, except for Vps26a (maternal expression pattern), all tested genes showed maternal-zygotic expression patterns. And five genes (Vps26b, Vps29, Vps35, Snx2, and Ci-mpr) showed a pattern of decreased expression in Vps26a(-/-) MEF cells by comparative analysis between normal MEF and Vps26a(-/-) MEF cells. However, the Snx1 gene showed a pattern of increased expression in Vps26a(-/-) MEF cells. From our results, we could assume that retromer complex-related genes have important roles during oocyte development. However, in the preimplantation stage, they did not have significant roles.

A novel, retromer-independent role for sorting nexins 1 and 2 in RhoG-dependent membrane remodeling

The sorting nexins SNX1 and SNX2 are members of the retromer complex involved in protein sorting within the endocytic pathway. While retromer-dependent functions of SNX1 and SNX2 have been well documented, potential retromer-independent roles remain unclear. Here, we show that SNX1 and SNX2 interact with the Rac1 and RhoG guanine nucleotide exchange factor Kalirin-7. Simultaneous overexpression of SNX1 or SNX2 and Kalirin-7 in epithelial cells causes partial redistribution of both SNX isoforms to the plasma membrane, and results in RhoG-dependent lamellipodia formation that requires functional Phox homology (PX) and Bin/Amphiphysin/Rvs (BAR) domains of SNX, but is Rac1- and retromer-independent. Conversely, depletion of endogenous SNX1 or SNX2 inhibits Kalirin-7-mediated lamellipodia formation. Finally, we demonstrate that SNX1 and SNX2 interact directly with inactive RhoG, suggesting a novel role for these SNX proteins in recruiting an inactive Rho GTPase to its exchange factor.

Deficiency of sorting nexin 27 (SNX27) leads to growth retardation and elevated levels of N-methyl-D-aspartate receptor 2C (NR2C)

Phox (PX) domain-containing sorting nexins (SNXs) are emerging as important regulators of endocytic trafficking. Sorting nexin 27 (SNX27) is unique, as it contains a PDZ (Psd-95/Dlg/ZO1) domain. We show here that SNX27 is primarily targeted to the early endosome by interaction of its PX domain with PtdIns(3)P. Although targeted ablation of the SNX27 gene in mice did not significantly affect growth and survival during embryonic development, SNX27 plays an essential role in postnatal growth and survival. N-Methyl-d-aspartate (NMDA) receptor 2C (NR2C) was identified as a novel SNX27-interacting protein, and this interaction is mediated by the PDZ domain of SNX27 and the C-terminal PDZ-binding motif of NR2C. Increased NR2C expression levels, together with impaired NR2C endocytosis in SNX27(-/-) neurons, indicate that SNX27 may function to regulate endocytosis and/or endosomal sorting of NR2C. This is consistent with a role of SNX27 as a general regulator for sorting of membrane proteins containing a PDZ-binding motif, and its absence may alter the trafficking of these proteins, leading to growth and survival defects.

Analyses of sorting nexins reveal distinct retromer-subcomplex functions in development and protein sorting in Arabidopsis thaliana

Sorting nexins (SNXs) are conserved eukaryotic proteins that associate with three types of vacuolar protein sorting (VPS) proteins to form the retromer complex. How SNXs act in this complex and whether they might work independently of the retromer remains elusive. Here, we show by genetic and cell imaging approaches that the Arabidopsis thaliana SNX1 protein recruits SNX2 at the endosomal membrane, a process required for SNX1-SNX2 dimer activity. We report that, in contrast with the mammalian retromer, SNXs are dispensable for membrane binding and function of the retromer complex. We also show that VPS retromer components can work with or independently of SNXs in the trafficking of seed storage proteins, which reveals distinct functions for subcomplexes of the plant retromer. Finally, we provide compelling evidence that the combined loss of function of SNXs and VPS29 leads to embryo or seedling lethality, underlining the essential role of these proteins in development.

Identification of different itineraries and retromer components for endosome-to-Golgi transport of TGN38 and Shiga toxin

The retrograde transport pathways from early/recycling endosomes are critical for recycling a range of endogenous cargo, as well as internalisation of bacterial and plant toxins. We have previously shown that the retrograde transport of the two model cargos, TGN38 and Shiga toxin, differs in the requirement for TGN golgins; transport of TGN38 requires the TGN golgin GCC88 whereas that of Shiga toxin requires GCC185. Here we have further defined the retrograde transport requirements of these two cargos. Tracking the transport of these cargos demonstrated that the bulk of Shiga toxin is transported from early endosomes to recycling endosomes en route to the TGN whereas the bulk of TGN38 is transported from early endosomes to the TGN with only low levels detected in recycling endosomes. In cells depleted of the TGN t-SNARE syntaxin 16, TGN38 accumulated predominantly in early endosomes whereas Shiga toxin accumulated in Rab11-positive recycling endosomes, suggesting distinct routes for each cargo. Retrograde transport of Shiga toxin and TGN38 requires retromer, however, whereas sorting nexin 1 (SNX1) is specifically required for transport of Shiga toxin, sorting nexin 2 (SNX2) is required for the transport of TGN38. Overall, our data have identified different itineraries for the retrograde transport of Shiga toxin and TGN38 and distinct retromer components that regulate the transport of these cargos.

Screening of chondrogenic factors with a real-time fluorescence-monitoring cell line ATDC5-C2ER: identification of sorting nexin 19 as a novel factor

OBJECTIVE

To establish a cell culture system for noninvasive and real-time monitoring of chondrogenic differentiation in order to screen for chondrogenic factors.

METHODS

The optimum reporter construct transfected into chondrogenic ATDC5 cells was selected by a luciferase reporter assay and fluorescence analysis during cultures with insulin. The established cell line was validated according to its fluorescence following stimulation with SOX proteins, bone morphogenetic protein 2 (BMP-2), or transforming growth factor beta (TGFbeta) and was compared with the level of messenger RNA for COL2A1 as well as with the degree of Alcian blue staining. Screening of chondrogenic factors was performed by expression cloning using a retroviral expression library prepared from human tracheal cartilage. The expression pattern of the identified molecule was examined by in situ hybridization and immunohistochemistry. Functional analysis was performed by transfection of the identified gene, the small interfering RNA, and the mutated gene.

RESULTS

We established an ATDC5 cell line with 4 repeats of a highly conserved enhancer ligated to a COL2A1 basal promoter and the DsRed2 reporter (ATDC5-C2ER). Fluorescence was induced under the stimulations with SOX proteins, BMP-2, or TGFbeta, showing good correspondence to the chondrogenic markers. Screening using the ATDC5-C2ER system identified several chondrogenic factors, including sorting nexin 19 (SNX19). SNX19 was expressed in the limb cartilage of mouse embryos and in the degraded cartilage of adult mouse knee joints during osteoarthritis progression. The gain-of-function and loss-of-function analyses revealed a potent chondrogenic activity of SNX19.

CONCLUSION

We established the ATDC5-C2ER system for efficient monitoring of chondrogenic differentiation by fluorescence analysis, and we identified a novel chondrogenic factor (SNX19) using this system. This system will be useful for elucidating the molecular network of chondrogenic differentiation.

SNX-BAR proteins in phosphoinositide-mediated, tubular-based endosomal sorting

The endocytic network is morphologically characterized by a wide variety of membrane bound compartments that are able to undergo dynamic re-modeling through tubular and vesicular structures. The precise molecular mechanisms governing such re-modeling, and the events that co-ordinated this with the major role of endosomes, cargo sorting, remain unclear. That said, recent work on a protein family of sorting nexins (SNX) - especially a subfamily of SNX that contain a BAR domain (SNX-BARs) - has begun to shed some much needed light on these issues and in particular the process of tubular-based endosomal sorting. SNX-BARs are evolutionary conserved in endosomal protein complexes such as retromer, where they co-ordinate membrane deformation with cargo selection. Furthermore a central theme emerges of SNX-BARs linking the forming membrane carrier to cytoskeletal elements for transport through motor proteins such as dynein. By studying these SNX-BARs, we are gaining an increasingly detailed appreciation of the mechanistic basis of endosomal sorting and how this highly dynamic process functions in health and disease.

Analysis of articulation between clathrin and retromer in retrograde sorting on early endosomes

Clathrin and retromer have key functions for retrograde trafficking between early endosomes and the trans-Golgi network (TGN). Previous studies on Shiga toxin suggested that these two coat complexes operate in a sequential manner. Here, we show that the curvature recognition subunit component sorting nexin 1 (SNX1) of retromer interacts with receptor-mediated endocytosis-8 (RME-8) protein, and that RME-8 and SNX1 colocalize on early endosomes together with a model cargo of the retrograde route, the receptor-binding B-subunit of Shiga toxin (STxB). RME-8 has previously been found to bind to the clathrin uncoating adenosine triphosphatase (ATPase) Hsc70, and we now report that depletion of RME-8 or Hsc70 affects retrograde trafficking at the early endosomes-TGN interface of STxB and the cation-independent mannose 6-phosphate receptor, an endogenous retrograde cargo protein. We also provide evidence that retromer interacts with the clathrin-binding protein hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) not only via SNX1, as previously published (Chin Raynor MC, Wei X, Chen HQ, Li L. Hrs interacts with sorting nexin 1 and regulates degradation of epidermal growth factor receptor. J Biol Chem 2001;276:7069-7078), but also via the core complex component Vps35. Hrs codistributes at the ultrastructural level with STxB on early endosomes, and interfering with Hrs function using antibodies or mild overexpression inhibits retrograde transport. Our combined data suggest a model according to which the functions in retrograde sorting on early endosomes of SNX1/retromer and clathrin are articulated by RME-8, and possibly also by Hrs.

SLIC-1/sorting nexin 20: a novel sorting nexin that directs subcellular distribution of PSGL-1

P-Selectin glycoprotein ligand-1 (PSGL-1) is a mucin-like glycoprotein expressed on the surface of leukocytes that serves as the major ligand for the selectin family of adhesion molecules and functions in leukocyte tethering and rolling on activated endothelium and platelets. Previous studies have implicated the highly conserved cytoplasmic domain of PSGL-1 in regulating outside-in signaling of integrin activation. However, molecules that physically and functionally interact with this domain are not completely defined. Using a yeast two-hybrid screen with the cytoplasmic domain of PSGL-1 as bait, a novel protein designated selectin ligand interactor cytoplasmic-1 (SLIC-1) was isolated. Computer-based homology search revealed that SLIC-1 was the human orthologue for the previously identified mouse sorting nexin 20. Direct interaction between SLIC-1 and PSGL-1 was specific as indicated by co-immunoprecipitation and motif mapping. Colocalization experiments demonstrated that SLIC-1 contains a Phox homology domain that binds phosphoinositides and targets the PSGL-1/SLIC-1 complex to endosomes. Deficiency in the murine homologue of SLIC-1 did not modulate PSGL-1-dependent signaling nor alter neutrophil adhesion through PSGL-1. We conclude that SLIC-1 serves as a sorting molecule that cycles PSGL-1 into endosomes with no impact on leukocyte recruitment.

Retromer: multipurpose sorting and specialization in polarized transport

Retromer is an evolutionary conserved protein complex required for endosome-to-Golgi retrieval of lysosomal hydrolases' receptors. A dimer of two sorting nexins-typically, SNX1 and/or SNX2-deforms the membrane and thus cooperates with retromer to ensure cargo sorting. Research in various model organisms indicates that retromer participates in sorting of additional molecules whose proper transport has important repercussions in development and disease. The role of retromer as well as SNXs in endosomal protein (re)cycling and protein targeting to specialized plasma membrane domains in polarized cells adds further complexity and has implications in growth control, the establishment of developmental patterns, cell adhesion, and migration. This chapter will discuss the functions of retromer described in various model systems and will focus on relevant aspects in polarized transport.

The chromatin-remodeling enzyme BRG1 plays an essential role in primitive erythropoiesis and vascular development

ATP-dependent chromatin-remodeling complexes contribute to the proper temporal and spatial patterns of gene expression in mammalian embryos and therefore play important roles in a number of developmental processes. SWI/SNF-like chromatin-remodeling complexes use one of two different ATPases as their catalytic subunit: brahma (BRM, also known as SMARCA2) and brahma-related gene 1 (BRG1, also known as SMARCA4). We have conditionally deleted a floxed Brg1 allele with a Tie2-Cre transgene, which is expressed in developing hematopoietic and endothelial cells. Brg1(fl/fl):Tie2-Cre(+) embryos die at midgestation from anemia, as mutant primitive erythrocytes fail to transcribe embryonic alpha- and beta-globins, and subsequently undergo apoptosis. Additionally, vascular remodeling of the extraembryonic yolk sac is abnormal in Brg1(fl/fl):Tie2-Cre(+) embryos. Importantly, Brm deficiency does not exacerbate the erythropoietic or vascular abnormalities found in Brg1(fl/fl):Tie2-Cre(+) embryos, implying that Brg1-containing SWI/SNF-like complexes, rather than Brm-containing complexes, play a crucial role in primitive erythropoiesis and in early vascular development.

The retromer component sorting nexin-1 is required for efficient retrograde transport of Shiga toxin from early endosome to the trans Golgi network

The mammalian retromer complex is a multi-protein complex that regulates retrograde transport of the cation-independent mannose 6-phosphate receptor (CI-MPR) from early endosomes to the trans Golgi network (TGN). It consists of two subcomplexes: a membrane-bound coat comprising sorting nexin-1 (SNX1) and possibly sorting nexin-2 (SNX2), and a cargo-selective subcomplex, composed of VPS26, VPS29 and VPS35. In addition to the retromer, a variety of other protein complexes has been suggested to regulate endosome-to-TGN transport of not only the CI-MPR but a wide range of other cargo proteins. Here, we have examined the role of SNX1 and SNX2 in endosomal sorting of Shiga and cholera toxins, two toxins that undergo endosome-to-TGN transport en route to their cellular targets located within the cytosol. By using small interfering RNA (siRNA)-mediated silencing combined with single-cell fluorescent-toxin-uptake assays and well-established biochemical assays to analyze toxin delivery to the TGN, we have established that suppression of SNX1 leads to a significant reduction in the efficiency of endosome-to-TGN transport of the Shiga toxin B-subunit. Furthermore, we show that for the B subunit of cholera toxin, retrograde endosome-to-TGN transport is less reliant upon SNX1. Overall, our data establish a role for SNX1 in the endosome-to-TGN transport of Shiga toxin and are indicative for a fundamental difference between endosomal sorting of Shiga and cholera toxins into endosome-to-TGN retrograde transport pathways.

Cellular prion protein interaction with vitronectin supports axonal growth and is compensated by integrins

The physiological functions of the cellular prion protein, PrP(C), as a cell surface pleiotropic receptor are under debate. We report that PrP(C) interacts with vitronectin but not with fibronectin or collagen. The binding sites mediating this PrP(C)-vitronectin interaction were mapped to residues 105-119 of PrP(C) and the residues 307-320 of vitronectin. The two proteins were co-localized in embryonic dorsal root ganglia from wild-type mice. Vitronectin addition to cultured dorsal root ganglia induced axonal growth, which could be mimicked by vitronectin peptide 307-320 and abrogated by anti-PrP(C) antibodies. Full-length vitronectin, but not the vitronectin peptide 307-320, induced axonal growth of dorsal root neurons from two strains of PrP(C)-null mice. Functional assays demonstrated that relative to wild-type cells, PrP(C)-null dorsal root neurons were more responsive to the Arg-Gly-Asp peptide (an integrin-binding site), and exhibited greater alphavbeta3 activity. Our findings indicate that PrP(C) plays an important role in axonal growth, and this function may be rescued in PrP(C)-knockout animals by integrin compensatory mechanisms.

SNX1 and SNX2 mediate retrograde transport of Shiga toxin

The bacterial toxin Shiga toxin (Stx) is transported retrogradely from early endosomes to the Golgi apparatus on its way to the endoplasmic reticulum (ER) and the cytosol. In this study we explored the functions of the two phosphoinositide binding proteins Sorting nexin 1 (SNX1) and Sorting nexin 2 (SNX2) in endosomal sorting of the toxin. When Vero cells were depleted of either SNX1 or SNX2 by small interfering RNA (siRNA), Stx transport to the trans-Golgi network (TGN) was impaired by > or = 40%, whereas combined depletion of SNX1 and SNX2 gave a total inhibition of approximately 80%. Inhibition of PI(3)P formation by wortmannin resulted in a similar reduction. Thus, although being partly redundant, both SNX1 and SNX2 are required for efficient Stx trafficking to the Golgi apparatus.

A loss-of-function screen reveals SNX5 and SNX6 as potential components of the mammalian retromer

The mammalian retromer is a multimeric protein complex involved in mediating endosome-to-trans-Golgi-network retrograde transport of the cation-independent mannose-6-phosphate receptor. The retromer is composed of two subcomplexes, one containing SNX1 and forming a membrane-bound coat, the other comprising VPS26, VPS29 and VPS35 and being cargo-selective. In yeast, an additional sorting nexin--Vps17p--is a component of the membrane bound coat. It remains unclear whether the mammalian retromer requires a functional equivalent of Vps17p. Here, we have used an RNAi loss-of-function screen to examine whether any of the other 30 mammalian sorting nexins are required for retromer-mediated endosome-to-trans-Golgi-network retrieval of the cation-independent mannose-6-phosphate receptor. Using this screen, we identified two proteins, SNX5 and SNX6, that, when suppressed, induced a phenotype similar to that observed upon suppression of known retromer components. Whereas SNX5 and SNX6 colocalised with SNX1 on early endosomes, in immunoprecipitation experiments only SNX6 appeared to exist in a complex with SNX1. Interestingly, suppression of SNX5 and/or SNX6 resulted in a significant loss of SNX1, an effect that seemed to result from post-translational regulation of the SNX1 level. Such data suggest that SNX1 and SNX6 exist in a stable, endosomally associated complex that is required for retromer-mediated retrieval of the cation-independent mannose-6-phosphate receptor. SNX5 and SNX6 may therefore constitute functional equivalents of Vps17p in mammals.

Interchangeable but essential functions of SNX1 and SNX2 in the association of retromer with endosomes and the trafficking of mannose 6-phosphate receptors

The retromer is a cytosolic/peripheral membrane protein complex that mediates the retrieval of the cation-independent mannose 6-phosphate receptor from endosomes to the trans-Golgi network (TGN) in mammalian cells. Previous studies showed that the mammalian retromer comprises three proteins, named Vps26, Vps29, and Vps35, plus the sorting nexin, SNX1. There is conflicting evidence, however, as to whether a homologous sorting nexin, SNX2, is truly a component of the retromer. In addition, the nature of the subunit interactions and assembly of the mammalian retromer complex are poorly understood. We have addressed these issues by performing biochemical and functional analyses of endogenous retromers in the human cell line HeLa. We found that the mammalian retromer complex consists of two autonomously assembling subcomplexes, namely, a Vps26-Vps29-Vps35 obligate heterotrimer and a SNX1/2 alternative heterodimer or homodimer. The association of Vps26-Vps29-Vps35 with endosomes requires the presence of either SNX1 or SNX2, whereas SNX1/2 can be recruited to endosomes independently of Vps26-Vps29-Vps35. We also found that the presence of either SNX1 or SNX2 is essential for the retrieval of the cation-independent mannose 6-phosphate receptor to the TGN. These observations indicate that the mammalian retromer complex assembles by sequential association of SNX1/2 and Vps26-Vps29-Vps35 subcomplexes on endosomal membranes and that SNX1 and SNX2 play interchangeable but essential roles in retromer structure and function.