Retromer and the cation-independent mannose 6-phosphate receptor-Time for a trial separation?

Cell-specific secretory granule sorting mechanisms: the role of MAGEL2 and retromer in hypothalamic regulated secretion

Intracellular protein trafficking and sorting are extremely arduous in endocrine and neuroendocrine cells, which synthesize and secrete on-demand substantial quantities of proteins. To ensure that neuroendocrine secretion operates correctly, each step in the secretion pathways is tightly regulated and coordinated both spatially and temporally. At the trans-Golgi network (TGN), intrinsic structural features of proteins and several sorting mechanisms and distinct signals direct newly synthesized proteins into proper membrane vesicles that enter either constitutive or regulated secretion pathways. Furthermore, this anterograde transport is counterbalanced by retrograde transport, which not only maintains membrane homeostasis but also recycles various proteins that function in the sorting of secretory cargo, formation of transport intermediates, or retrieval of resident proteins of secretory organelles. The retromer complex recycles proteins from the endocytic pathway back to the plasma membrane or TGN and was recently identified as a critical player in regulated secretion in the hypothalamus. Furthermore, melanoma antigen protein L2 (MAGEL2) was discovered to act as a tissue-specific regulator of the retromer-dependent endosomal protein recycling pathway and, by doing so, ensures proper secretory granule formation and maturation. MAGEL2 is a mammalian-specific and maternally imprinted gene implicated in Prader-Willi and Schaaf-Yang neurodevelopmental syndromes. In this review, we will briefly discuss the current understanding of the regulated secretion pathway, encompassing anterograde and retrograde traffic. Although our understanding of the retrograde trafficking and sorting in regulated secretion is not yet complete, we will review recent insights into the molecular role of MAGEL2 in hypothalamic neuroendocrine secretion and how its dysregulation contributes to the symptoms of Prader-Willi and Schaaf-Yang patients. Given that the activation of many secreted proteins occurs after they enter secretory granules, modulation of the sorting efficiency in a tissue-specific manner may represent an evolutionary adaptation to environmental cues.

Regulation of Endosomal Trafficking by Rab7 and Its Effectors in Neurons: Clues from Charcot-Marie-Tooth 2B Disease

Intracellular endosomal trafficking controls the balance between protein degradation and synthesis, i.e., proteostasis, but also many of the cellular signaling pathways that emanate from activated growth factor receptors after endocytosis. Endosomal trafficking, sorting, and motility are coordinated by the activity of small GTPases, including Rab proteins, whose function as molecular switches direct activity at endosomal membranes through effector proteins. Rab7 is particularly important in the coordination of the degradative functions of the pathway. Rab7 effectors control endosomal maturation and the properties of late endosomal and lysosomal compartments, such as coordination of recycling, motility, and fusion with downstream compartments. The spatiotemporal regulation of endosomal receptor trafficking is particularly challenging in neurons because of their enormous size, their distinct intracellular domains with unique requirements (dendrites vs. axons), and their long lifespans as postmitotic, differentiated cells. In Charcot-Marie-Tooth 2B disease (CMT2B), familial missense mutations in Rab7 cause alterations in GTPase cycling and trafficking, leading to an ulcero-mutilating peripheral neuropathy. The prevailing hypothesis to account for CMT2B pathologies is that CMT2B-associated Rab7 alleles alter endocytic trafficking of the neurotrophin NGF and its receptor TrkA and, thereby, disrupt normal trophic signaling in the peripheral nervous system, but other Rab7-dependent pathways are also impacted. Here, using TrkA as a prototypical endocytic cargo, we review physiologic Rab7 effector interactions and control in neurons. Since neurons are among the largest cells in the body, we place particular emphasis on the temporal and spatial regulation of endosomal sorting and trafficking in neuronal processes. We further discuss the current findings in CMT2B mutant Rab7 models, the impact of mutations on effector interactions or balance, and how this dysregulation may confer disease.

Receptor Recycling by Retromer

The highly conserved retromer complex controls the fate of hundreds of receptors that pass through the endolysosomal system and is a central regulatory node for diverse metabolic programs. More than 20 years ago, retromer was discovered as an essential regulator of endosome-to-Golgi transport in yeast; since then, significant progress has been made to characterize how metazoan retromer components assemble to enable its engagement with endosomal membranes, where it sorts cargo receptors from endosomes to the trans-Golgi network or plasma membrane through recognition of sorting motifs in their cytoplasmic tails. In this review, we examine retromer regulation by exploring its assembled structure with an emphasis on how a range of adaptor proteins shape the process of receptor trafficking. Specifically, we focus on how retromer is recruited to endosomes, selects cargoes, and generates tubulovesicular carriers that deliver cargoes to target membranes. We also examine how cells adapt to distinct metabolic states by coordinating retromer expression and function. We contrast similarities and differences between retromer and its related complexes: retriever and commander/CCC, as well as their interplay in receptor trafficking. We elucidate how loss of retromer regulation is central to the pathology of various neurogenerative and metabolic diseases, as well as microbial infections, and highlight both opportunities and cautions for therapeutics that target retromer. Finally, with a focus on understanding the mechanisms that govern retromer regulation, we outline new directions for the field moving forward.

Retrograde transport of CDMPR depends on several machineries as analyzed by sulfatable nanobodies

Nanobody toolkit enables the quantitative analysis of endosome-to-TGN transport of the mannose-6-phosphate receptor in cells depleted of retrograde transport machineries

Retrograde protein transport from the cell surface and endosomes to the TGN is essential for membrane homeostasis in general and for the recycling of mannose-6-phosphate receptors (MPRs) for sorting of lysosomal hydrolases in particular. We used a nanobody-based sulfation tool to more directly determine transport kinetics from the plasma membrane to the TGN for the cation-dependent MPR (CDMPR) with and without rapid or gradual inactivation of candidate machinery proteins. Although knockdown of retromer (Vps26), epsinR, or Rab9a reduced CDMPR arrival to the TGN, no effect was observed upon silencing of TIP47. Strikingly, when retrograde transport was analyzed by rapamycin-induced rapid depletion (knocksideways) or long-term depletion by knockdown of the clathrin adaptor AP-1 or of the GGA machinery, distinct phenotypes in sulfation kinetics were observed, suggesting a potential role of GGA adaptors in retrograde and anterograde transport. Our study illustrates the usefulness of derivatized, sulfation-competent nanobodies, reveals novel insights into CDMPR trafficking biology, and further outlines that the selection of machinery inactivation is critical for phenotype analysis.

Endosomal traffic and glutamate synapse activity are increased in VPS35 D620N mutant knock-in mouse neurons, and resistant to LRRK2 kinase inhibition

Vacuolar protein sorting 35 (VPS35) regulates neurotransmitter receptor recycling from endosomes. A missense mutation (D620N) in VPS35 leads to autosomal-dominant, late-onset Parkinson's disease. Here, we study the basic neurobiology of VPS35 and Parkinson's disease mutation effects in the D620N knock-in mouse and the effect of leucine-rich repeat kinase 2 (LRRK2) inhibition on synaptic phenotypes. The study was conducted using a VPS35 D620N knock-in mouse that expresses VPS35 at endogenous levels. Protein levels, phosphorylation states, and binding ratios in brain lysates from knock-in mice and wild-type littermates were assayed by co-immunoprecipitation and western blot. Dendritic protein co-localization, AMPA receptor surface expression, synapse density, and glutamatergic synapse activity in primary cortical cultures from knock-in and wild-type littermates were assayed using immunocytochemistry and whole-cell patch clamp electrophysiology. In brain tissue, we confirm VPS35 forms complexes with LRRK2 and AMPA-type glutamate receptor GluA1 subunits, in addition to NMDA-type glutamate receptor GluN1 subunits and D2-type dopamine receptors. Receptor and LRRK2 binding was unaltered in D620N knock-in mice, but we confirm the mutation results in reduced binding of VPS35 with WASH complex member FAM21, and increases phosphorylation of the LRRK2 kinase substrate Rab10, which is reversed by LRRK2 kinase inhibition in vivo. In cultured cortical neurons from knock-in mice, pRab10 is also increased, and reversed by LRRK2 inhibition. The mutation also results in increased endosomal recycling protein cluster density (VPS35-FAM21 co-clusters and Rab11 clusters), glutamate transmission, and GluA1 surface expression. LRRK2 kinase inhibition, which reversed Rab10 hyper-phosphorylation, did not rescue elevated glutamate release or surface GluA1 expression in knock-in neurons, but did alter AMPAR traffic in wild-type cells. The results improve our understanding of the cell biology of VPS35, and the consequences of the D620N mutation in developing neuronal networks. Together the data support a chronic synaptopathy model for latent neurodegeneration, providing phenotypes and candidate pathophysiological stresses that may drive eventual transition to late-stage parkinsonism in VPS35 PD. The study demonstrates the VPS35 mutation has effects that are independent of ongoing LRRK2 kinase activity, and that LRRK2 kinase inhibition alters basal physiology of glutamate synapses in vitro.

Retromer dependent changes in cellular homeostasis and Parkinson's disease

To date, mechanistic treatments targeting the initial cause of Parkinson's disease (PD) are limited due to the underlying biological cause(s) been unclear. Endosomes and their associated cellular homeostasis processes have emerged to have a significant role in the pathophysiology associated with PD. Several variants within retromer complex have been identified and characterised within familial PD patients. The retromer complex represents a key sorting platform within the endosomal system that regulates cargo sorting that maintains cellular homeostasis. In this review, we summarise the current understandings of how PD-associated retromer variants disrupt cellular trafficking and how the retromer complex can interact with other PD-associated genes to contribute to the disease progression.

Connecting the dots: combined control of endocytic recycling and degradation

Endocytosis is an essential process where proteins and lipids are internalised from the plasma membrane in membrane-bound carriers, such as clathrin-coated vesicles. Once internalised into the cell these vesicles fuse with the endocytic network where their contents are sorted towards degradation in the lysosome or recycling to their origin. Initially, it was thought that cargo recycling is a passive process, but in recent years the identification and characterisation of specialised recycling complexes has established a hitherto unthought-of level of complexity that actively opposes degradation. This review will summarise recent developments regarding the composition and regulation of the recycling machineries and their relationship with the degradative pathways of the endosome.

Targeting Endosomal Recycling Pathways by Bacterial and Viral Pathogens

Endosomes are essential cellular stations where endocytic and secretory trafficking routes converge. Proteins transiting at endosomes can be degraded via lysosome, or recycled to the plasma membrane, trans-Golgi network (TGN), or other cellular destinations. Pathways regulating endosomal recycling are tightly regulated in order to preserve organelle identity, to maintain lipid homeostasis, and to support other essential cellular functions. Recent studies have revealed that both pathogenic bacteria and viruses subvert host endosomal recycling pathways for their survival and replication. Several host factors that are frequently targeted by pathogens are being identified, including retromer, TBC1D5, SNX-BARs, and the WASH complex. In this review, we will focus on the recent advances in understanding how intracellular bacteria, human papillomavirus (HPV), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) hijack host endosomal recycling pathways. This exciting work not only reveals distinct mechanisms employed by pathogens to manipulate host signaling pathways, but also deepens our understanding of the molecular intricacies regulating endosomal receptor trafficking.

SNX-PXA-RGS-PXC Subfamily of SNXs in the Regulation of Receptor-Mediated Signaling and Membrane Trafficking

The SNX-PXA-RGS-PXC subfamily of sorting nexins (SNXs) belongs to the superfamily of SNX proteins. SNXs are characterized by the presence of a common phox-homology (PX) domain, along with other functional domains that play versatile roles in cellular signaling and membrane trafficking. In addition to the PX domain, the SNX-PXA-RGS-PXC subfamily, except for SNX19, contains a unique RGS (regulators of G protein signaling) domain that serves as GTPase activating proteins (GAPs), which accelerates GTP hydrolysis on the G protein α subunit, resulting in termination of G protein-coupled receptor (GPCR) signaling. Moreover, the PX domain selectively interacts with phosphatidylinositol-3-phosphate and other phosphoinositides found in endosomal membranes, while also associating with various intracellular proteins. Although SNX19 lacks an RGS domain, all members of the SNX-PXA-RGS-PXC subfamily serve as dual regulators of receptor cargo signaling and endosomal trafficking. This review discusses the known and proposed functions of the SNX-PXA-RGS-PXC subfamily and how it participates in receptor signaling (both GPCR and non-GPCR) and endosomal-based membrane trafficking. Furthermore, we discuss the difference of this subfamily of SNXs from other subfamilies, such as SNX-BAR nexins (Bin-Amphiphysin-Rvs) that are associated with retromer or other retrieval complexes for the regulation of receptor signaling and membrane trafficking. Emerging evidence has shown that the dysregulation and malfunction of this subfamily of sorting nexins lead to various pathophysiological processes and disorders, including hypertension.

Insulin-Like Growth Factor 2 As a Possible Neuroprotective Agent and Memory Enhancer-Its Comparative Expression, Processing and Signaling in Mammalian CNS

A number of studies performed on rodents suggest that insulin-like growth factor 2 (IGF-2) or its analogs may possibly be used for treating some conditions like Alzheimer’s disease, Huntington’s disease, autistic spectrum disorders or aging-related cognitive impairment. Still, for translational research a comparative knowledge about the function of IGF-2 and related molecules in model organisms (rats and mice) and humans is necessary. There is a number of important differences in IGF-2 signaling between species. In the present review we emphasize species-specific patterns of IGF-2 expression in rodents, humans and some other mammals, using, among other sources, publicly available transcriptomic data. We provide a detailed description of Igf2 mRNA expression regulation and pre-pro-IGF-2 protein processing in different species. We also summarize the function of IGF-binding proteins. We describe three different receptors able to bind IGF-2 and discuss the role of IGF-2 signaling in learning and memory, as well as in neuroprotection. We hope that comprehensive understanding of similarities and differences in IGF-2 signaling between model organisms and humans will be useful for development of more effective medicines targeting IGF-2 receptors.

The Retromer Complex: From Genesis to Revelations

The retromer complex has a well-established role in endosomal protein sorting, being necessary for maintaining the dynamic localisation of hundreds of membrane proteins that traverse the endocytic system. Retromer function and dysfunction is linked with neurodegenerative diseases, including Alzheimer's and Parkinson's disease, and many pathogens, both viral and bacterial, exploit or interfere in retromer function for their own ends. In this review, the history of retromer is distilled into a concentrated form that spans the identification of retromer to recent discoveries that have shed new light on how retromer functions in endosomal protein sorting and why retromer is increasingly being viewed as a potential therapeutic target in neurodegenerative disease.

Endosomal dysfunction in iPSC-derived neural cells from Parkinson's disease patients with VPS35 D620N

Mutations in the Vacuolar protein sorting 35 (VPS35) gene have been linked to familial Parkinson’s disease (PD), PARK17. VPS35 is a key component of the retromer complex, which plays a central role in endosomal trafficking. However, whether and how VPS35 deficiency or mutation contributes to PD pathogenesis remain unclear. Here, we analyzed human induced pluripotent stem cell (iPSC)-derived neurons from PD patients with the VPS35 D620N mutation and addressed relevant disease mechanisms. In the disease group, dopaminergic (DA) neurons underwent extensive apoptotic cell death. The movement of Rab5a- or Rab7a-positive endosomes was slower, and the endosome fission and fusion frequencies were lower in the PD group than in the healthy control group. Interestingly, vesicles positive for cation-independent mannose 6-phosphate receptor transported by retromers were abnormally localized in glial cells derived from patient iPSCs. Furthermore, we found α-synuclein accumulation in TH positive DA neurons. Our results demonstrate the induction of cell death, endosomal dysfunction and α -synuclein accumulation in neural cells of the PD group. PARK17 patient-derived iPSCs provide an excellent experimental tool for understanding the pathophysiology underlying PD.

Retromer regulates the lysosomal clearance of MAPT/tau

The macroautophagy/autophagy-lysosome axis enables the clearance and degradation of cytoplasmic components including protein aggregates, damaged organelles and invading pathogens. Protein aggregation and lysosomal system dysfunction in the brain are common features of several late-onset neurological disorders including Alzheimer disease. Spatial overlap between depletion of the endosomal-sorting complex retromer and MAPT/tau aggregation in the brain have been previously reported. However, whether retromer dysfunction plays a direct role in mediating MAPT aggregation remains unclear. Here, we demonstrate that the autophagy-lysosome axis is the primary mode for the clearance of aggregated species of MAPT using both chemical and genetic approaches in cell models of amyloid MAPT aggregation. We show that depletion of the central retromer component VPS35 causes a block in the resolution of autophagy. We establish that this defect underlies marked accumulation of cytoplasmic MAPT aggregates upon VPS35 depletion, and that VPS35 overexpression has the opposite effect. This work illustrates how retromer complex integrity regulates the autophagy-lysosome axis to suppress MAPT aggregation and spread.

Acute inactivation of retromer and ESCPE-1 leads to time-resolved defects in endosomal cargo sorting

Human retromer, a heterotrimer of VPS26 (VPS26A or VPS26B), VPS35 and VPS29, orchestrates the endosomal retrieval of internalised cargo and promotes their cell surface recycling, a prototypical cargo being the glucose transporter GLUT1 (also known as SLC2A1). The role of retromer in the retrograde sorting of the cation-independent mannose 6-phosphate receptor (CI-MPR, also known as IGF2R) from endosomes back to the trans-Golgi network remains controversial. Here, by applying knocksideways technology, we develop a method for acute retromer inactivation. While retromer knocksideways in HeLa and H4 human neuroglioma cells resulted in time-resolved defects in cell surface sorting of GLUT1, we failed to observe a quantifiable defect in CI-MPR sorting. In contrast, knocksideways of the ESCPE-1 complex – a key regulator of retrograde CI-MPR sorting – revealed time-resolved defects in CI-MPR sorting. Together, these data are consistent with a comparatively limited role for retromer in ESCPE-1-mediated CI-MPR retrograde sorting, and establish a methodology for acute retromer and ESCPE-1 inactivation that will aid the time-resolved dissection of their functional roles in endosomal cargo sorting.

Summary: Retromer, a master controller of endosomal cargo sorting, is deregulated in neurodegenerative disease. Here, we develop and apply a retromer knocksideways methodology to quantify endosomal cargo sorting upon acute perturbation.

An update on cellular and molecular determinants of Parkinson's disease with emphasis on the role of the retromer complex

Parkinson's disease (PD) is a highly prevalent neurodegenerative condition. The disease involves the progressive degeneration of dopaminergic neurons located in the substantia nigra pars compacta. Among late-onset, familial forms of Parkinson are cases with mutations in the PARK17 locus encoding the vacuolar protein sorting 35 (Vps35), a subunit of the retromer complex. The retromer complex is composed of a heterotrimeric protein core (Vps26-Vps35-Vps29). The best-known role of retromer is the retrieval of cargoes from endosomes to the Golgi complex or the plasma membrane. However, recent literature indicates that retromer performs roles associated with lysosomal and mitochondrial functions and degradative pathways such as autophagy. A common point mutation affecting the retromer subunit Vps35 is D620N, which has been linked to the alterations in the aforementioned cellular processes as well as with neurodegeneration. Here, we review the main aspects of the malfunction of the retromer complex and its implications for PD pathology. Besides, we highlight several controversies still awaiting clarification.

Involvement of CASP9 (caspase 9) in IGF2R/CI-MPR endosomal transport

Recently, we reported that increased expression of CASP9 pro-domain, at the endosomal membrane in response to HSP90 inhibition, mediates a cell-protective effect that does not involve CASP9 apoptotic activity. We report here that a non-apoptotic activity of endosomal membrane CASP9 facilitates the retrograde transport of IGF2R/CI-MPR from the endosomes to the trans-Golgi network, indicating the involvement of CASP9 in endosomal sorting and lysosomal biogenesis. CASP9-deficient cells demonstrate the missorting of CTSD (cathepsin D) and other acid hydrolases, accumulation of late endosomes, and reduced degradation of bafilomycin A₁-sensitive proteins. In the absence of CASP9, IGF2R undergoes significant degradation, and its rescue is achieved by the re-expression of a non-catalytic CASP9 mutant. This endosomal activity of CASP9 is potentially mediated by herein newly identified interactions of CASP9 with the components of the endosomal membrane transport complexes. These endosomal complexes include the retromer VPS35 and the SNX dimers, SNX1-SNX5 and SNX2-SNX6, which are involved in the IGF2R retrieval mechanism. Additionally, CASP9 interacts with HGS/HRS/ESCRT-0 and the CLTC (clathrin heavy chain) that participate in the initiation of the endosomal ESCRT degradation pathway. We propose that endosomal CASP9 inhibits the endosomal membrane degradative subdomain(s) from initiating the ESCRT-mediated degradation of IGF2R, allowing its retrieval to transport-designated endosomal membrane subdomain(s). These findings are the first to identify a cell survival, non-apoptotic function for CASP9 at the endosomal membrane, a site distinctly removed from the cytoplasmic apoptosome. Via its non-apoptotic endosomal function, CASP9 impacts the retrograde transport of IGF2R and, consequently, lysosomal biogenesis.Abbreviations: ACTB: actin beta; ATG7: autophagy related 7; BafA1: bafilomycin A₁; CASP: caspase; CLTC/CHC: clathrin, heavy chain; CTSD: cathepsin D; ESCRT: endosomal sorting complexes required for transport; HEXB: hexosaminidase subunit beta; HGS/HRS/ESCRT-0: hepatocyte growth factor-regulated tyrosine kinase substrate; IGF2R/CI-MPR: insulin like growth factor 2 receptor; ILV: intraluminal vesicles; KD: knockdown; KO: knockout; M6PR/CD-MPR: mannose-6-phosphate receptor, cation dependent; MEF: murine embryonic fibroblasts; MWU: Mann-Whitney U test; PepA: pepstatin A; RAB7A: RAB7, member RAS oncogene family; SNX-BAR: sorting nexin dimers with a Bin/Amphiphysin/Rvs (BAR) domain each; TGN: trans-Golgi network; TUBB: tubulin beta; VPS26: VPS26 retromer complex component; VPS29: VPS29 retromer complex component; VPS35: VPS35 retromer complex component.

MTV proteins unveil ER- and microtubule-associated compartments in the plant vacuolar trafficking pathway

The factors and mechanisms involved in vacuolar transport in plants, and in particular those directing vesicles to their target endomembrane compartment, remain largely unknown. To identify components of the vacuolar trafficking machinery, we searched for Arabidopsis modified transport to the vacuole (mtv) mutants that abnormally secrete the synthetic vacuolar cargo VAC2. We report here on the identification of 17 mtv mutations, corresponding to mutant alleles of MTV2/VSR4, MTV3/PTEN2A MTV7/EREL1, MTV8/ARFC1, MTV9/PUF2, MTV10/VPS3, MTV11/VPS15, MTV12/GRV2, MTV14/GFS10, MTV15/BET11, MTV16/VPS51, MTV17/VPS54, and MTV18/VSR1 Eight of the MTV proteins localize at the interface between the trans-Golgi network (TGN) and the multivesicular bodies (MVBs), supporting that the trafficking step between these compartments is essential for segregating vacuolar proteins from those destined for secretion. Importantly, the GARP tethering complex subunits MTV16/VPS51 and MTV17/VPS54 were found at endoplasmic reticulum (ER)- and microtubule-associated compartments (EMACs). Moreover, MTV16/VPS51 interacts with the motor domain of kinesins, suggesting that, in addition to tethering vesicles, the GARP complex may regulate the motors that transport them. Our findings unveil a previously uncharacterized compartment of the plant vacuolar trafficking pathway and support a role for microtubules and kinesins in GARP-dependent transport of soluble vacuolar cargo in plants.

Endosome-to-TGN Trafficking: Organelle-Vesicle and Organelle-Organelle Interactions

Retrograde transport from endosomes to the trans-Golgi network (TGN) diverts proteins and lipids away from lysosomal degradation. It is essential for maintaining cellular homeostasis and signaling. In recent years, significant advancements have been made in understanding this classical pathway, revealing new insights into multiple steps of vesicular trafficking as well as critical roles of ER-endosome contacts for endosomal trafficking. In this review, we summarize up-to-date knowledge about this trafficking pathway, in particular, mechanisms of cargo recognition at endosomes and vesicle tethering at the TGN, and contributions of ER-endosome contacts.

Molecular identification of a BAR domain-containing coat complex for endosomal recycling of transmembrane proteins

Protein trafficking requires coat complexes that couple recognition of sorting motifs in transmembrane cargos with biogenesis of transport carriers. The mechanisms of cargo transport through the endosomal network are poorly understood. Here, we identify a sorting motif for endosomal recycling of cargos including the cation-independent mannose-6-phosphate receptor and semaphorin 4C by the membrane tubulating BAR domain-containing sorting nexins SNX5 and SNX6. Crystal structures establish that this motif folds into a β-hairpin that binds a site in the SNX5/SNX6 phox homology domains. Over sixty cargos share this motif and require SNX5/SNX6 for their recycling. These include cargos involved in neuronal migration and a Drosophila snx6 mutant displays defects in axonal guidance. These studies identify a sorting motif and provide molecular insight into an evolutionary conserved coat complex, the ‘Endosomal SNX-BAR sorting complex for promoting exit 1’ (ESCPE-1), which couples sorting motif recognition to BAR domain-mediated biogenesis of cargo-enriched tubulo-vesicular transport carriers.

Retromer has a selective function in cargo sorting via endosome transport carriers

The molecular actions of retromer in the endolysosomal system remain unclear and controversial. Cui et al. demonstrate the essential role of retromer in the selective incorporation of cargo into a specific type of endosome transport carrier and the maintenance of lysosomal function.

Retromer is a peripheral membrane protein complex that coordinates multiple vesicular trafficking events within the endolysosomal system. Here, we demonstrate that retromer is required for the maintenance of normal lysosomal morphology and function. The knockout of retromer subunit Vps35 causes an ultrastructural alteration in lysosomal structure and aberrant lysosome function, leading to impaired autophagy. At the whole-cell level, knockout of retromer Vps35 subunit reduces lysosomal proteolytic capacity as a consequence of the improper processing of lysosomal hydrolases, which is dependent on the trafficking of the cation-independent mannose 6-phosphate receptor (CI-M6PR). Incorporation of CI-M6PR into endosome transport carriers via a retromer-dependent process is restricted to those tethered by GCC88 but not golgin-97 or golgin-245. Finally, we show that this retromer-dependent retrograde cargo trafficking pathway requires SNX3, but not other retromer-associated cargo binding proteins, such as SNX27 or SNX-BAR proteins. Therefore, retromer does contribute to the retrograde trafficking of CI-M6PR required for maturation of lysosomal hydrolases and lysosomal function.

The phosphorylation of sorting nexin 5 at serine 226 regulates retrograde transport and macropinocytosis

Sorting nexin 5 (SNX5), a member of sorting nexin family, plays an important role in membrane trafficking, including the retrograde trafficking of the cation independent mannose 6-phosphate receptor (CI-M6PR) and macropinocytosis. Using ESI-LCMS/MS analysis, we confirmed that SNX5 serine 226 is phosphorylated. Since SNX5 forms heterodimers with SNX1 or SNX2, we examined the effect of phosphorylation at S226 on the heterodimer formations. Wild-type and mutants of SNX5, in which S226 was mutated to a glutamic acid or an alanine, were expressed in 8505C cells. In pull-down assays using SNX5 as bait, only the S226E mutant failed to precipitate both SNX1 and SNX2. Confocal microscopy data indicated that the wild type and S226A mutant were colocalized with SNX1 and SNX2 in endosomes, but the S226E was not. SNX5 and SNX6 support each other's functions and are involved with CI-M6PR retrograde trafficking. In SNX5 and SNX6 double knockdown cells, CI-M6PR was dispersed and colocalized with the endosomal marker EEA1. In a rescue experiment using SNX5 mutants, the S226A rescued CI-M6PR localization, similar to control cells, but S226E did not. Furthermore, the decrease in the uptake of dextran by macropinocytosis in SNX5 knockdown cells was recovered by the expression of rescue-wild type or S226A mutant, but not by the rescue-S226E mutant. These observations indicate that SNX5 constitutive phosphorylation that mimics the mutant S226E decreases the active SNX5 in these cells. The phosphorylation of SNX5 regulates the dimerization with SNX1 or SNX2, and this suggests that it controls membrane trafficking and protein sorting.

Retromer in Synaptic Function and Pathology

The retromer complex mediates export of select transmembrane proteins from endosomes to the trans-Golgi network (TGN) or to the plasma membrane. Dysfunction of retromer has been linked with slowly progressing neurodegenerative disorders, including Alzheimer’s and Parkinson’s disease (AD and PD). As these disorders affect synapses it is of key importance to clarify the function of retromer-dependent protein trafficking pathways in pre- and postsynaptic compartments. Here we discuss recent insights into the roles of retromer in the trafficking of synaptic vesicle proteins, neurotransmitter receptors and other synaptic proteins. We also consider evidence that implies synapses as sites of early pathology in neurodegenerative disorders, pointing to a possible role of synaptic retromer dysfunction in the initiation of disease.

Emerging Role of Retromer in Modulating Pathogen Growth

Intracellular pathogens have developed elegant mechanisms to modulate host endosomal trafficking. The highly conserved retromer pathway has emerged as an important target of viruses and intravacuolar bacteria. Some pathogens require retromer function to survive. For others, retromer activity restricts intracellular growth; these pathogens must disrupt retromer function to survive. In this review, we discuss recent paradigm changes to the current model for retromer assembly and cargo selection. We highlight how the study of pathogen effectors has contributed to these fundamental insights, with a special focus on the biology and structure of two recently described bacterial effectors, Chlamydia trachomatis IncE and Legionella pneumophila RidL. These two pathogens employ distinct strategies to target retromer components and overcome restriction of intracellular growth imposed by retromer.

Retromer and the cation-independent mannose 6-phosphate receptor-Time for a trial separation?

The retromer cargo-selective complex (CSC) comprising Vps35, Vps29 and Vps26 mediates the endosome-to-Golgi retrieval of the cation-independent mannose 6-phosphate receptor (CIMPR). Or does it? Recently published data have questioned the validity of this long-established theory. Here, the evidence for and against a role for the retromer CSC in CIMPR endosome-to-Golgi retrieval is examined in the light of the new data that the SNX-BAR dimer is actually responsible for CIMPR retrieval.