Retromer regulates apical-basal polarity through recycling Crumbs

CRB1 is required for recycling by RAB11A+ vesicles in human retinal organoids

CRB1 gene mutations can cause early- or late-onset retinitis pigmentosa, Leber congenital amaurosis, or maculopathy. Recapitulating human CRB1 phenotypes in animal models has proven challenging, necessitating the development of alternatives. We generated human induced pluripotent stem cell (iPSC)-derived retinal organoids of patients with retinitis pigmentosa caused by biallelic CRB1 mutations and evaluated them against autologous gene-corrected hiPSCs and hiPSCs from healthy individuals. Patient organoids show decreased levels of CRB1 and NOTCH1 expression at the retinal outer limiting membrane. Proximity ligation assays show that human CRB1 and NOTCH1 can interact via their extracellular domains. CRB1 patient organoids feature increased levels of WDFY1+ vesicles, fewer RAB11A+ recycling endosomes, decreased VPS35 retromer complex components, and more degradative endolysosomal compartments relative to isogenic control organoids. Taken together, our data demonstrate that patient-derived retinal organoids enable modeling of retinal degeneration and highlight the importance of CRB1 in early endosome maturation receptor recycling in the retina.

Crumbs complex-directed apical membrane dynamics in epithelial cell ingression

Epithelial cells often leave their tissue context and ingress to form new cell types or acquire migratory ability to move to distant sites during development and tumor progression. Cells lose their apical membrane and epithelial adherens junctions during ingression. However, how factors that organize apical-basal polarity contribute to ingression is unknown. Here, we show that the dynamic regulation of the apical Crumbs polarity complex is crucial for normal neural stem cell ingression. Crumbs endocytosis and recycling allow ingression to occur in a normal timeframe. During early ingression, Crumbs and its complex partner the RhoGEF Cysts support myosin and apical constriction to ensure robust ingression dynamics. During late ingression, the E3-ubiquitin ligase Neuralized facilitates the disassembly of the Crumbs complex and the rapid endocytic removal of the apical cell domain. Our findings reveal a mechanism integrating cell fate, apical polarity, endocytosis, vesicle trafficking, and actomyosin contractility to promote cell ingression, a fundamental morphogenetic process observed in animal development and cancer.

Phospholipase D and retromer promote recycling of TRPL ion channel via the endoplasmic reticulum

Plasma membrane protein trafficking is of fundamental importance for cell function and cell integrity of neurons and includes regulated protein recycling. In this work, we report a novel role of the endoplasmic reticulum (ER) for protein recycling as discovered in trafficking studies of the ion channel TRPL in photoreceptor cells of Drosophila. TRPL is located within the rhabdomeric membrane from where it is endocytosed upon light stimulation and stored in the cell body. Conventional immunohistochemistry as well as stimulated emission depletion super-resolution microscopy revealed TRPL storage at the ER after illumination, suggesting an unusual recycling route of TRPL. Our results also imply that both phospholipase D (PLD) and retromer complex are required for correct recycling of TRPL to the rhabdomeric membrane. Loss of PLD activity in PLD^3.1 mutants results in enhanced degradation of TRPL. In the retromer mutant vps35^MH20 , TRPL is trapped in a Rab5-positive compartment. Evidenced by epistatic analysis in the double mutant PLD^3.1 vps35^MH20 , PLD activity precedes retromer function. We propose a model in which PLD and retromer function play key roles in the transport of TRPL to an ER enriched compartment.

Dynamic Polarization of Rab11a Modulates Crb2a Localization and Impacts Signaling to Regulate Retinal Neurogenesis

Interkinetic nuclear migration (IKNM) is the process in which pseudostratified epithelial nuclei oscillate from the apical to basal surface and in phase with the mitotic cycle. In the zebrafish retina, neuroepithelial retinal progenitor cells (RPCs) increase Notch activity with apical movement of the nuclei, and the depth of nuclear migration correlates with the probability that the next cell division will be neurogenic. This study focuses on the mechanisms underlying the relationships between IKNM, cell signaling, and neurogenesis. In particular, we have explored the role IKNM has on endosome biology within RPCs. Through genetic manipulation and live imaging in zebrafish, we find that early (Rab5-positive) and recycling (Rab11a-positive) endosomes polarize in a dynamic fashion within RPCs and with reference to nuclear position. Functional analyses suggest that dynamic polarization of recycling endosomes and their activity within the neuroepithelia modulates the subcellular localization of Crb2a, consequently affecting multiple signaling pathways that impact neurogenesis including Notch, Hippo, and Wnt activities. As nuclear migration is heterogenous and asynchronous among RPCs, Rab11a-affected signaling within the neuroepithelia is modulated in a differential manner, providing mechanistic insight to the correlation of IKNM and selection of RPCs to undergo neurogenesis.

Explosion in the complexity of membrane protein recycling

For decades, recycling of membrane proteins has been represented in figures by arrows between the "endosome" and the plasma membrane, but recently there has been an explosion in the understanding of the mechanisms and protein complexes required to facilitate protein recycling. Here, some key discoveries will be introduced, including assigning function to a number of recently recognized protein complexes and linking their function to protein recycling. Furthermore, the importance of lipid interactions and links to diseases and epithelial polarity will be summarized.

Cytocortex-dependent dynamics of Drosophila Crumbs controls junctional stability and tension during germ band retraction

During morphogenesis, epithelia undergo dynamic rearrangements, which requires continuous remodelling of junctions and cell shape, but at the same time mechanisms preserving cell polarity and tissue integrity. Apico-basal polarity is key for the localisation of the machinery that enables cell shape changes. The evolutionarily conserved Drosophila Crumbs protein is critical for maintaining apico-basal polarity and epithelial integrity. How Crumbs is maintained in a dynamically developing embryo remains largely unknown. Here, we applied quantitative fluorescence techniques to show that, during germ band retraction, Crumbs dynamics correlates with the morphogenetic activity of the epithelium. Genetic and pharmacological perturbations revealed that the mobile pool of Crumbs is fine-tuned by the actomyosin cortex in a stage-dependent manner. Stabilisation of Crumbs at the plasma membrane depends on a proper link to the actomyosin cortex via an intact FERM-domain-binding site in its intracellular domain, loss of which leads to increased junctional tension and higher DE-cadherin (also known as Shotgun) turnover, resulting in impaired junctional rearrangements. These data define Crumbs as a mediator between polarity and junctional regulation to orchestrate epithelial remodelling in response to changes in actomyosin activity.This article has an associated First Person interview with the first author of the paper.

Retromer Controls Planar Polarity Protein Levels and Asymmetric Localization at Intercellular Junctions

The coordinated polarization of cells in the plane of a tissue, termed planar polarity, is a characteristic feature of epithelial tissues [1]. In the fly wing, trichome positioning is dependent on the core planar polarity proteins adopting asymmetric subcellular localizations at apical junctions, where they form intercellular complexes that link neighboring cells [1-3]. Specifically, the seven-pass transmembrane protein Frizzled and the cytoplasmic proteins Dishevelled and Diego localize to distal cell ends, the four-pass transmembrane protein Strabismus and the cytoplasmic protein Prickle localize proximally, and the seven-pass transmembrane spanning atypical cadherin Flamingo localizes both proximally and distally. To establish asymmetry, these core proteins are sorted from an initially uniform distribution; however, the mechanisms underlying this polarized trafficking remain poorly understood. Here, we describe the identification of retromer, a master controller of endosomal recycling [4-6], as a key component regulating core planar polarity protein localization in Drosophila. Through generation of mutants, we verify that loss of the retromer-associated Snx27 cargo adaptor, but notably not components of the Wash complex, reduces junctional levels of the core proteins Flamingo and Strabismus in the developing wing. We establish that Snx27 directly associates with Flamingo via its C-terminal PDZ binding motif, and we show that Snx27 is essential for normal Flamingo trafficking. We conclude that Wash-independent retromer function and the Snx27 cargo adaptor are important components in the endosomal recycling of Flamingo and Strabismus back to the plasma membrane and thus contribute to the establishment and maintenance of planar polarization.

The apical protein Apnoia interacts with Crumbs to regulate tracheal growth and inflation

Most organs of multicellular organisms are built from epithelial tubes. To exert their functions, tubes rely on apico-basal polarity, on junctions, which form a barrier to separate the inside from the outside, and on a proper lumen, required for gas or liquid transport. Here we identify apnoia (apn), a novel Drosophila gene required for tracheal tube elongation and lumen stability at larval stages. Larvae lacking Apn show abnormal tracheal inflation and twisted airway tubes, but no obvious defects in early steps of tracheal maturation. apn encodes a transmembrane protein, primarily expressed in the tracheae, which exerts its function by controlling the localization of Crumbs (Crb), an evolutionarily conserved apical determinant. Apn physically interacts with Crb to control its localization and maintenance at the apical membrane of developing airways. In apn mutant tracheal cells, Crb fails to localize apically and is trapped in retromer-positive vesicles. Consistent with the role of Crb in apical membrane growth, RNAi-mediated knockdown of Crb results in decreased apical surface growth of tracheal cells and impaired axial elongation of the dorsal trunk. We conclude that Apn is a novel regulator of tracheal tube expansion in larval tracheae, the function of which is mediated by Crb.

Expression and localization of the polarity protein CRB2 in adult mouse brain: a comparison with the CRB1rd8 mutant mouse model

Acquisition of cell polarization is essential for the performance of crucial functions, like a successful secretion and appropriate cell signaling in many tissues, and it depends on the correct functioning of polarity proteins, including the Crumbs complex. The CRB proteins, CRB1, CRB2 and CRB3, identified in mammals, are expressed in epithelial-derived tissues like brain, kidney and retina. CRB2 has a ubiquitous expression and has been detected in embryonic brain tissue; but currently there is no data regarding its localization in the adult brain. In our study, we characterized the presence of CRB2 in adult mice brain, where it is particularly enriched in cortex, hippocampus, hypothalamus and cerebellum. Double immunofluorescence analysis confirmed that CRB2 is a neuron-specific protein, present in both soma and projections where colocalizes with certain populations of exocytic and endocytic vesicles and with other members of the Crumbs complex. Finally, in the cortex of CRB1^rd8 mutant mice that contain a mutation in the Crb1 gene generating a truncated CRB1 protein, there is an abnormal increase in the expression levels of the CRB2 protein which suggests a possible compensatory mechanism for the malfunction of CRB1 in this mutant background.

VPS35 depletion does not impair presynaptic structure and function

The endosomal system is proposed as a mediator of synaptic vesicle recycling, but the molecular recycling mechanism remains largely unknown. Retromer is a key protein complex which mediates endosomal recycling in eukaryotic cells, including neurons. Retromer is important for brain function and mutations in retromer genes are linked to neurodegenerative diseases. In this study, we aimed to determine the role of retromer in presynaptic structure and function. We assessed the role of retromer by knocking down VPS35, the core subunit of retromer, in primary hippocampal mouse neurons. VPS35 depletion led to retromer dysfunction, measured as a decrease in GluA1 at the plasma membrane, and bypassed morphological defects previously described in chronic retromer depletion models. We found that retromer is localized at the mammalian presynaptic terminal. However, VPS35 depletion did not alter the presynaptic ultrastructure, synaptic vesicle release or retrieval. Hence, we conclude that retromer is present in the presynaptic terminal but it is not essential for the synaptic vesicle cycle. Nonetheless, the presynaptic localization of VPS35 suggests that retromer-dependent endosome sorting could take place for other presynaptic cargo.

Rab6-dependent retrograde traffic of LAT controls immune synapse formation and T cell activation

The adapter molecule linker for activation of T cells (LAT) orchestrates the formation of signalosomes upon T cell receptor (TCR) stimulation. LAT is present in different intracellular pools and is dynamically recruited to the immune synapse upon stimulation. However, the intracellular traffic of LAT and its function in T lymphocyte activation are ill defined. We show herein that LAT, once internalized, transits through the Golgi-trans-Golgi network (TGN), where it is repolarized to the immune synapse. This retrograde transport of LAT depends on the small GTPase Rab6 and the target soluble N-ethylmaleimide-sensitive factor attachment protein receptor (t-SNARE) Syntaxin-16, two regulators of the endosome-to-Golgi/TGN retrograde transport. We also show in vitro in Syntaxin-16- or Rab6-silenced human cells and in vivo in CD4⁺ T lymphocytes of the Rab6 knockout mouse that this retrograde traffic controls TCR stimulation. These results establish that the retrograde traffic of LAT from the plasma membrane to the Golgi-TGN controls the polarized delivery of LAT at the immune synapse and T lymphocyte activation.

A dual role of the extracellular domain of Drosophila Crumbs for morphogenesis of the embryonic neuroectoderm

Epithelia are highly polarised tissues and several highly conserved polarity protein complexes serve to establish and maintain polarity. The transmembrane protein Crumbs (Crb), the central component of the Crb protein complex, is required, among others, for the maintenance of polarity in most epithelia in the Drosophila embryo. However, different epithelia exhibit different phenotypic severity upon loss of crb. Using a transgenomic approach allowed us to more accurately define the role of crb in different epithelia. In particular, we provide evidence that the loss of epithelial tissue integrity in the ventral epidermis of crb mutant embryos is due to impaired actomyosin activity and an excess number of neuroblasts. We demonstrate that the intracellular domain of Crb could only partially rescue this phenotype, while it is able to completely restore tissue integrity in other epithelia. Based on these results we suggest a dual role of the extracellular domain of Crb in the ventral neuroectoderm. First, it is required for apical enrichment of the Crb protein, which in turn regulates actomyosin activity and thereby ensures tissue integrity; and second, the extracellular domain of Crb stabilises the Notch receptor and thereby ensures proper Notch signalling and specification of the correct number of neuroblasts.

Summary: Using a transgenomic approach we determine specific roles of the intra- and extracellular domain of the Crumbs protein for the maintenance of apico-basal epithelial polarity and epithelial morphogenesis in Drosophila embryos.

EGFR controls Drosophila tracheal tube elongation by intracellular trafficking regulation

Development is governed by a few conserved signalling pathways. Amongst them, the EGFR pathway is used reiteratively for organ and tissue formation, and when dysregulated can lead to cancer and metastasis. Given its relevance, identifying its downstream molecular machinery and understanding how it instructs cellular changes is crucial. Here we approach this issue in the respiratory system of Drosophila. We identify a new role for EGFR restricting the elongation of the tracheal Dorsal Trunk. We find that EGFR regulates the apical determinant Crb and the extracellular matrix regulator Serp, two factors previously known to control tube length. EGFR regulates the organisation of endosomes in which Crb and Serp proteins are loaded. Our results are consistent with a role of EGFR in regulating Retromer/WASH recycling routes. Furthermore, we provide new insights into Crb trafficking and recycling during organ formation. Our work connects cell signalling, trafficking mechanisms and morphogenesis and suggests that the regulation of cargo trafficking can be a general outcome of EGFR activation.

The control of organ size and shape is a critical aspect of morphogenesis, as miss-regulation can lead to pathologies and malformations. The tracheal system of Drosophila is a good model to investigate this issue as tube size is strictly regulated. In addition, tracheal system development represents also an excellent system to study the molecular mechanisms employed by signalling pathways to instruct cells to form tubular structures. Here we describe that EGFR, which triggers one of the principal conserved pathways acting reiteratively during development and homeostasis, is required to restrict tube elongation. We find that EGFR regulates the accumulation and subcellular localisation of Crumbs and Serpentine, two factors previously known to regulate tube length. We show that Crumbs and Serpentine are loaded in common endosomes, which require EGFR for proper organisation, ensuring delivery of both cargoes to their final destination. We also report that during tracheal development the apical determinant Crumbs undergoes a complex pattern of recycling, which involves internalisation and different sorting pathways. Our analysis identifies EGFR as a hub to coordinate both cell intrinsic properties, namely Crumbs-dependant apical membrane growth, and extrinsic mechanisms, Serpentine-mediated extracellular matrix modifications, which regulate tube elongation. We suggest that the regulation of the endocytic traffic of specific cargoes could be one of the molecular mechanisms downstream of the EGFR, and therefore could regulate different morphogenetic and pathological EGFR-mediated events.

CRB3 regulates contact inhibition by activating the Hippo pathway in mammary epithelial cells

The loss of contact inhibition is a hallmark of cancer cells. The Hippo pathway has recently been shown to be an important regulator of contact inhibition, and the cell apical polarity determinant protein CRB3 has been suggested to be involved in Hippo signalling. However, whether CRB3 regulates contact inhibition in mammary cells remains unclear, and the underlying mechanisms have not been elucidated. As shown in the present study, CRB3 decreases cell proliferation, promotes apoptosis, and enhances the formation of tight and adherens junctions. Furthermore, we report for the first time that CRB3 acts as an upstream regulator of the Hippo pathway to regulate contact inhibition by recruiting other Hippo molecules, such as Kibra and/or FRMD6, in mammary epithelial cells. In addition, CRB3 inhibits tumour growth in vivo. Collectively, the present study increases our understanding of the Hippo pathway and provides an important theoretical basis for exploring new avenues for breast cancer treatment.

Nuf and Rip11 requirement for polarity determinant recycling during Drosophila development

A tight relationship between apico-basal polarity and trafficking is essential for epithelial physiology and tissue homeostasis. Recent studies have described how some Rab GTPases, key components of the intracellular traffic machinery, contribute to the establishment of cell polarity in vertebrates. We have demonstrated a novel connection between cell polarity and trafficking: in Drosophila epithelia, the apical determinant aPKC is recycled via Rab11-Nuf-recycling endosomes to maintain cell polarity. Furthermore, the phosphorylation of Nuf by aPKC allows aPKC to control the sub-cellular localization of Nuf and its own membrane accumulation. Here we review these data and show the different contribution of the 2 Drosophila Rab11 adaptor proteins, Nuf and Rip11, to the maintenance of Drosophila embryonic ectoderm polarity.

Complex Polarity: Building Multicellular Tissues Through Apical Membrane Traffic

The formation of distinct subdomains of the cell surface is crucial for multicellular organism development. The most striking example of this is apical-basal polarization. What is much less appreciated is that underpinning an asymmetric cell surface is an equally dramatic intracellular endosome rearrangement. Here, we review the interplay between classical cell polarity proteins and membrane trafficking pathways, and discuss how this marriage gives rise to cell polarization. We focus on those mechanisms that regulate apical polarization, as this is providing a number of insights into how membrane traffic and polarity are regulated at the tissue level.

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.

Retromer-Mediated Protein Sorting and Vesicular Trafficking

Retromer is an evolutionarily conserved multimeric protein complex that mediates intracellular transport of various vesicular cargoes and functions in a wide variety of cellular processes including polarized trafficking, developmental signaling and lysosome biogenesis. Through its interaction with the Rab GTPases and their effectors, membrane lipids, molecular motors, the endocytic machinery and actin nucleation promoting factors, retromer regulates sorting and trafficking of transmembrane proteins from endosomes to the trans-Golgi network (TGN) and the plasma membrane. In this review, I highlight recent progress in the understanding of retromer-mediated protein sorting and vesicle trafficking and discuss how retromer contributes to a diverse set of developmental, physiological and pathological processes.

Retromer in Polarized Protein Transport

Retromer is an evolutionary conserved protein complex required for endosome-to-Golgi retrieval of receptors for lysosomal hydrolases. It is constituted by a heterotrimer encoded by the vacuolar protein sorting (VPS) gene products Vps26, Vps35, and Vps29, which selects cargo, and a dimer of phosphoinositide-binding sorting nexins, which deforms the membrane. Recent progress in the mechanism of retromer assembly and functioning has strengthened the link between sorting at the endosome and cytoskeleton dynamics. Retromer is implicated in endosomal sorting of many cargos and plays an essential role in plant and animal development. Although it is best known for endosome sorting to the trans-Golgi network, it also intervenes in recycling to the plasma membrane. In polarized cells, such as epithelial cells and neurons, retromer may also be utilized for transcytosis and long-range transport. Considerable evidence implicates retromer in establishment and maintenance of cell polarity. That includes sorting of the apical polarity module Crumbs; regulation of retromer function by the basolateral polarity module Scribble; and retromer-dependent recycling of various cargoes to a certain surface domain, thus controlling polarized location and cell homeostasis. Importantly, altered retromer function has been linked to neurodegeneration, such as in Alzheimer's or Parkinson's disease. This review will underline how alterations in retromer localization and function may affect polarized protein transport and polarity establishment, thereby causing developmental defects and disease.

AP-2-complex-mediated endocytosis of Drosophila Crumbs regulates polarity by antagonizing Stardust

Maintenance of epithelial polarity depends on the correct localization and levels of polarity determinants. The evolutionarily conserved transmembrane protein Crumbs is crucial for the size and identity of the apical membrane, yet little is known about the molecular mechanisms controlling the amount of Crumbs at the surface. Here, we show that Crumbs levels on the apical membrane depend on a well-balanced state of endocytosis and stabilization. The adaptor protein 2 (AP-2) complex binds to a motif in the cytoplasmic tail of Crumbs that overlaps with the binding site of Stardust, a protein known to stabilize Crumbs on the surface. Preventing endocytosis by mutation of AP-2 causes expansion of the Crumbs-positive plasma membrane domain and polarity defects, which can be partially rescued by removing one copy of crumbs. Strikingly, knocking down both AP-2 and Stardust leads to the retention of Crumbs on the membrane. This study provides evidence for a molecular mechanism, based on stabilization and endocytosis, to adjust surface levels of Crumbs, which are essential for maintaining epithelial polarity.

The Mammalian Blood-Testis Barrier: Its Biology and Regulation

Spermatogenesis is the cellular process by which spermatogonia develop into mature spermatids within seminiferous tubules, the functional unit of the mammalian testis, under the structural and nutritional support of Sertoli cells and the precise regulation of endocrine factors. As germ cells develop, they traverse the seminiferous epithelium, a process that involves restructuring of Sertoli-germ cell junctions, as well as Sertoli-Sertoli cell junctions at the blood-testis barrier. The blood-testis barrier, one of the tightest tissue barriers in the mammalian body, divides the seminiferous epithelium into 2 compartments, basal and adluminal. The blood-testis barrier is different from most other tissue barriers in that it is not only comprised of tight junctions. Instead, tight junctions coexist and cofunction with ectoplasmic specializations, desmosomes, and gap junctions to create a unique microenvironment for the completion of meiosis and the subsequent development of spermatids into spermatozoa via spermiogenesis. Studies from the past decade or so have identified the key structural, scaffolding, and signaling proteins of the blood-testis barrier. More recent studies have defined the regulatory mechanisms that underlie blood-testis barrier function. We review here the biology and regulation of the mammalian blood-testis barrier and highlight research areas that should be expanded in future studies.

Retromer and sorting nexins in endosomal sorting

The evolutionarily conserved endosomal retromer complex rescues transmembrane proteins from the lysosomal degradative pathway and facilitates their recycling to other cellular compartments. Retromer functions in conjunction with numerous associated proteins, including select members of the sorting nexin (SNX) family. In the present article, we review the molecular architecture and cellular roles of retromer and its various functional partners. The endosomal network is a crucial hub in the trafficking of proteins through the cellular endomembrane system. Transmembrane proteins, here termed cargos, enter endosomes by endocytosis from the plasma membrane or by trafficking from the trans-Golgi network (TGN). Endosomal cargo proteins face one of the two fates: retention in the endosome, leading ultimately to lysosomal degradation or export from the endosome for reuse ('recycling'). The balance of protein degradation and recycling is crucial to cellular homoeostasis; inappropriate sorting of proteins to either fate leads to cellular dysfunction. Retromer is an endosome-membrane-associated protein complex central to the recycling of many cargo proteins from endosomes, both to the TGN and the plasma membrane (and other specialized compartments, e.g. lysosome-related organelles). Retromer function is reliant on a number of proteins from the SNX family. In the present article, we discuss this inter-relationship and how defects in retromer function are increasingly being linked with human disease.

Progenitor Epithelium: Sorting Out Pancreatic Lineages

Insulin-producing β cells within the vertebrate fetal pancreas acquire their fate in a step-wise manner. Whereas the intrinsic factors dictating the transcriptional or epigenetic status of pancreatic lineages have been intensely examined, less is known about cell-cell interactions that might constitute a niche for the developing β cell lineage. It is becoming increasingly clear that understanding and recapitulating these steps may instruct in vitro differentiation of embryonic stem cells and/or therapeutic regeneration. Indeed, directed differentiation techniques have improved since transitioning from 2D to 3D cultures, suggesting that the 3D microenvironment in which β cells are born is critical. However, to date, it remains unknown whether the changing architecture of the pancreatic epithelium impacts the fate of cells therein. An emerging challenge in the field is to elucidate how progenitors are allocated during key events, such as the stratification and subsequent resolution of the pre-pancreatic epithelium, as well as the formation of lumens and branches. Here, we assess the progenitor epithelium and examine how it might influence the emergence of pancreatic multipotent progenitors (MPCs), which give rise to β cells and other pancreatic lineages.

The retromer complex in development and disease

The retromer complex is a multimeric protein complex involved in recycling proteins from endosomes to the trans-Golgi network or plasma membrane. It thus regulates the abundance and subcellular distribution of its cargo within cells. Studies using model organisms show that the retromer complex is involved in specific developmental processes. Moreover, a number of recent studies implicate aberrant retromer function in photoreceptor degeneration, Alzheimer's disease and Parkinson's disease. Here, and in the accompanying poster, we provide an overview of the molecular and cellular mechanisms of retromer-mediated protein trafficking, highlighting key examples of retromer function in vivo.

Formation of Tubulovesicular Carriers from Endosomes and Their Fusion to the trans-Golgi Network

Endosomes undergo extensive spatiotemporal rearrangements as proteins and lipids flux through them in a series of fusion and fission events. These controlled changes enable the concentration of cargo for eventual degradation while ensuring the proper recycling of other components. A growing body of studies has now defined multiple recycling pathways from endosomes to the trans-Golgi network (TGN) which differ in their molecular machineries. The recycling process requires specific sets of lipids, coats, adaptors, and accessory proteins that coordinate cargo selection with membrane deformation and its association with the cytoskeleton. Specific tethering factors and SNARE (SNAP (Soluble NSF Attachment Protein) Receptor) complexes are then required for the docking and fusion with the acceptor membrane. Herein, we summarize some of the current knowledge of the machineries that govern the retrograde transport from endosomes to the TGN.

The transmembrane protein Crumbs displays complex dynamics during follicular morphogenesis and is regulated competitively by Moesin and aPKC

The transmembrane protein Crumbs (Crb) functions in apical polarity and epithelial integrity. To better understand its role in epithelial morphogenesis, we examined Crb localization and dynamics in the late follicular epithelium of Drosophila. Crb was unexpectedly dynamic during middle-to-late stages of egg chamber development, being lost from the marginal zone (MZ) in stage 9 before abruptly returning at the end of stage 10b, then undergoing a pulse of endocytosis in stage 12. The reappearance of MZ Crb is necessary to maintain an intact adherens junction and MZ. Although Crb has been proposed to interact through its juxtamembrane domain with Moesin (Moe), a FERM domain protein that regulates the cortical actin cytoskeleton, the functional significance of this interaction is poorly understood. We found that whereas the Crb juxtamembrane domain was not required for adherens junction integrity, it was necessary for MZ localization of Moe, aPKC and F-actin. Furthermore, Moe and aPKC functioned antagonistically, suggesting that Moe limits Crb levels by reducing its interactions with the apical Par network. Additionally, Moe mutant cells lost Crb from the apical membrane and accumulated excess Crb at the MZ, suggesting that Moe regulates Crb distribution at the membrane. Together, these studies reveal reciprocal interactions between Crb, Moe and aPKC during cellular morphogenesis.

A Conserved Di-Basic Motif of Drosophila Crumbs Contributes to Efficient ER Export

The Drosophila type I transmembrane protein Crumbs is an apical determinant required for the maintenance of apico‐basal epithelial cell polarity. The level of Crumbs at the plasma membrane is crucial, but how it is regulated is poorly understood. In a genetic screen for regulators of Crumbs protein trafficking we identified Sar1, the core component of the coat protein complex II transport vesicles. sar1 mutant embryos show a reduced plasma membrane localization of Crumbs, a defect similar to that observed in haunted and ghost mutant embryos, which lack Sec23 and Sec24CD, respectively. By pulse‐chase assays in Drosophila Schneider cells and analysis of protein transport kinetics based on Endoglycosidase H resistance we identified an RNKR motif in Crumbs, which contributes to efficient ER export. The motif identified fits the highly conserved di‐basic RxKR motif and mediates interaction with Sar1. The RNKR motif is also required for plasma membrane delivery of transgene‐encoded Crumbs in epithelial cells of Drosophila embryos. Our data are the first to show that a di‐basic motif acts as a signal for ER exit of a type I plasma membrane protein in a metazoan organism.

A TOCA/CDC-42/PAR/WAVE functional module required for retrograde endocytic recycling

Endosome-to-Golgi transport is required for the function of many key membrane proteins and lipids, including signaling receptors, small-molecule transporters, and adhesion proteins. The retromer complex is well-known for its role in cargo sorting and vesicle budding from early endosomes, in most cases leading to cargo fusion with the trans-Golgi network (TGN). Transport from recycling endosomes to the TGN has also been reported, but much less is understood about the molecules that mediate this transport step. Here we provide evidence that the F-BAR domain proteins TOCA-1 and TOCA-2 (Transducer of Cdc42 dependent actin assembly), the small GTPase CDC-42 (Cell division control protein 42), associated polarity proteins PAR-6 (Partitioning defective 6) and PKC-3/atypical protein kinase C, and the WAVE actin nucleation complex mediate the transport of MIG-14/Wls and TGN-38/TGN38 cargo proteins from the recycling endosome to the TGN in Caenorhabditis elegans. Our results indicate that CDC-42, the TOCA proteins, and the WAVE component WVE-1 are enriched on RME-1-positive recycling endosomes in the intestine, unlike retromer components that act on early endosomes. Furthermore, we find that retrograde cargo TGN-38 is trapped in early endosomes after depletion of SNX-3 (a retromer component) but is mainly trapped in recycling endosomes after depletion of CDC-42, indicating that the CDC-42-associated complex functions after retromer in a distinct organelle. Thus, we identify a group of interacting proteins that mediate retrograde recycling, and link these proteins to a poorly understood trafficking step, recycling endosome-to-Golgi transport. We also provide evidence for the physiological importance of this pathway in WNT signaling.

Epithelial cell polarity determinant CRB3 in cancer development

Cell polarity, which is defined as asymmetry in cell shape, organelle distribution and cell function, is essential in numerous biological processes, including cell growth, cell migration and invasion, molecular transport, and cell fate. Epithelial cell polarity is mainly regulated by three conserved polarity protein complexes, the Crumbs (CRB) complex, partitioning defective (PAR) complex and Scribble (SCRIB) complex. Research evidence has indicated that dysregulation of cell polarity proteins may play an important role in cancer development. Crumbs homolog 3 (CRB3), a member of the CRB complex, may act as a cancer suppressor in mouse kidney epithelium and mouse mammary epithelium. In this review, we focus on the current data available on the roles of CRB3 in cancer development.

Retromer regulates HIV-1 envelope glycoprotein trafficking and incorporation into virions

The envelope glycoprotein (Env) of the Human Immunodeficiency Virus Type-1 (HIV-1) is a critical determinant of viral infectivity, tropism and is the main target for humoral immunity; however, little is known about the cellular machinery that directs Env trafficking and its incorporation into nascent virions. Here we identify the mammalian retromer complex as a novel and important cellular factor regulating Env trafficking. Retromer mediates endosomal sorting and is most closely associated with endosome-to-Golgi transport. Consistent with this function, inactivating retromer using RNAi targeting the cargo selective trimer complex inhibited retrograde trafficking of endocytosed Env to the Golgi. Notably, in HIV-1 infected cells, inactivating retromer modulated plasma membrane expression of Env, along with Env incorporation into virions and particle infectivity. Mutagenesis studies coupled with coimmunoprecipitations revealed that retromer-mediated trafficking requires the Env cytoplasmic tail that we show binds directly to retromer components Vps35 and Vps26. Taken together these results provide novel insight into regulation of HIV-1 Env trafficking and infectious HIV-1 morphogenesis and show for the first time a role for retromer in the late-steps of viral replication and assembly of a virus.

Virus assembly necessitates the hijacking of the host cell machinery in order for new infectious viral particles to be constructed and disseminate. The envelope glycoprotein (Env) of HIV is a critical determinant of viral infectivity and is also a major target for antiviral immune responses. The long cytoplasmic tail of HIV Env plays an essential role in the assembly of infectious virions and limiting exposure of Env to the immune system, but the cellular machinery that transports HIV Env in virus-infected cells remain poorly understood. Here we have identified the mammalian retromer complex involved in endosomal sorting as a novel cellular factor regulating Env trafficking in virus-infected cells. We show that inactivating retromer alters Env localization, cell surface expression and incorporation into virions and that retromer binds directly to the Env cytoplasmic tail to perform these functions. This study defines an important pathway of Env transport and describes for the first time a role for this highly conserved cellular complex in assembly of a virus.

The Scribble module regulates retromer-dependent endocytic trafficking during epithelial polarization

Scribble (Scrib) module proteins are major regulators of cell polarity, but how they influence membrane traffic is not known. Endocytosis is also a key regulator of polarity through roles that remain unclear. Here we link Scrib to a specific arm of the endocytic trafficking system. Drosophila mutants that block AP-2-dependent endocytosis share many phenotypes with Scrib module mutants, but Scrib module mutants show intact internalization and endolysosomal transport. However, defective traffic of retromer pathway cargo is seen, and retromer components show strong genetic interactions with the Scrib module. The Scrib module is required for proper retromer localization to endosomes and promotes appropriate cargo sorting into the retromer pathway via both aPKC-dependent and -independent mechanisms. We propose that the Scrib module regulates epithelial polarity by influencing endocytic itineraries of Crumbs and other retromer-dependent cargo.

Subcellular localisation of retromer in post-endocytic pathways of polarised Madin-Darby canine kidney cells

BACKGROUND INFORMATION

Retromer is required for endosome-to-Golgi retrieval of the cation-independent mannose 6-phosphate receptor (CI-MPR), allowing delivery of hydrolases into lysosomes. It is constituted by a conserved heterotrimer formed by vacuolar protein sorting (Vps) gene products Vps26, Vps35 and Vps29, which is in charge of cargo selection, and a dimer of phosphoinositide-binding sorting nexins (SNXs), which has a structural role. Retromer has been implicated in sorting of additional cargo. Thus, retromer also promotes polymeric immunoglobulin A (pIgA) transcytosis by the pIgA receptor (pIgR) in polarised cells, and considerable evidence implicates retromer in controlling epithelial cell polarity. However, the precise localisation of retromer along the endocytic pathway of polarised cells has not been studied in detail.

RESULTS

Our biochemical analysis using rat liver endosome fractions suggests a distinct distribution pattern. Although subunits of the cargo-selective complex were enriched in early endosomes (EEs), levels of SNX2 were greater in sorting endosomes. We then immunolocalised the retromer subunits in polarised Madin-Darby canine kidney (MDCK) cells by confocal microscopy. An estimated 25% of total Vps26 and SNX2 localised to EEs, with negligible portions in recycling endosomes as well as in late endosomes and lysosomes. Although Vps26 was in structures of more heterogeneous size and shape than SNX2, these markedly overlapped. In consequence, the two retromer subcomplexes mostly colocalised. When we analysed retromer overlap with its cargo, we found that structures retromer and pIgA(+) are independent of those structures retromer and CI-MPR(+) . Remarkably, retromer localised preferentially at the transcytotic pathway. Pharmacological inhibition of phosphoinositide 3-kinase affected the co-distribution of retromer with pIgA and the CI-MPR, delaying pIgA progress to the apical surface.

CONCLUSIONS

In polarised MDCK cells, we found retromer associated with certain specialised EE-derived pathways. Our data imply that retromer is largely engaged in pIgA transcytosis in pIgR-expressing MDCK cells, as opposed to endosome-to-Golgi retrieval.

Balance between apical membrane growth and luminal matrix resistance determines epithelial tubule shape

The morphological stability of biological tubes is crucial for the efficient circulation of fluids and gases. Failure of this stability causes irregularly shaped tubes found in multiple pathological conditions. Here, we report that Drosophila mutants of the ESCRT III component Shrub/Vps32 exhibit a strikingly elongated sinusoidal tube phenotype. This is caused by excessive apical membrane synthesis accompanied by the ectopic accumulation and overactivation of Crumbs in swollen endosomes. Furthermore, we demonstrate that the apical extracellular matrix (aECM) of the tracheal tube is a viscoelastic material coupled with the apical membrane. We present a simple mechanical model in which aECM elasticity, apical membrane growth, and their interaction are three vital parameters determining the stability of biological tubes. Our findings demonstrate a mechanical role for the extracellular matrix and suggest that the interaction of the apical membrane and an elastic aECM determines the final morphology of biological tubes independent of cell shape.

C. elegans as a model for membrane traffic

The counterbalancing action of the endocytosis and secretory pathways maintains a dynamic equilibrium that regulates the composition of the plasma membrane, allowing it to maintain homeostasis and to change rapidly in response to alterations in the extracellular environment and/or intracellular metabolism. These pathways are intimately integrated with intercellular signaling systems and play critical roles in all cells. Studies in Caenorhabditis elegans have revealed diverse roles of membrane trafficking in physiology and development and have also provided molecular insight into the fundamental mechanisms that direct cargo sorting, vesicle budding, and membrane fisson and fusion. In this review, we summarize progress in understanding membrane trafficking mechanisms derived from work in C. elegans, focusing mainly on work done in non-neuronal cell-types, especially the germline, early embryo, coelomocytes, and intestine.

From endosomes to the trans-Golgi network

The retrograde trafficking from endosomes to the trans-Golgi network (TGN) is one of the major endocytic pathways to divert proteins and lipids away from lysosomal degradation. Retrograde transported cargos enter the TGN via two itineraries from either the early endosome/recycling endosome or the late endosome and involve various machinery components such as retromer, sorting nexins, clathrin, small GTPases, tethering factors and SNAREs. Recently, the pathway has been recognized for its role in signal transduction, physiology and pathogenesis of human diseases.

Apical localisation of crumbs in the boundary cells of the Drosophila hindgut is independent of its canonical interaction partner stardust

The transmembrane protein Crumbs/Crb is a key regulator of apico-basal epithelial cell polarity, both in Drosophila and in vertebrates. In most cases studied so far, the apical localisation of Drosophila Crumbs depends on the interaction of its C-terminal amino acids with the scaffolding protein Stardust. Consequently, embryos lacking either Crumbs or Stardust develop a very similar phenotype, characterised by the loss of epithelial tissue integrity and cell polarity in many epithelia. An exception is the hindgut, which is not affected by the loss of either gene. The hindgut is a single layered epithelial tube composed of two cell populations, the boundary cells and the principal cells. Here we show that Crumbs localisation in the principal cells depends on Stardust, similarly to other embryonic epithelia. In contrast, localisation of Crumbs in the boundary cells does not require Stardust and is independent of its PDZ domain- and FERM-domain binding motifs. In line with this, the considerable upregulation of Crumbs in boundary cells is not followed by a corresponding upregulation of its canonical binding partners. Our data are the first to suggest a mechanism controlling apical Crumbs localisation, which is independent of its conserved FERM- and PDZ-domain binding motifs.

The retromer complex is required for rhodopsin recycling and its loss leads to photoreceptor degeneration

Rhodopsin mistrafficking can cause photoreceptor (PR) degeneration. Upon light exposure, activated rhodopsin 1 (Rh1) in Drosophila PRs is internalized via endocytosis and degraded in lysosomes. Whether internalized Rh1 can be recycled is unknown. Here, we show that the retromer complex is expressed in PRs where it is required for recycling endocytosed Rh1 upon light stimulation. In the absence of subunits of the retromer, Rh1 is processed in the endolysosomal pathway, leading to a dramatic increase in late endosomes, lysosomes, and light-dependent PR degeneration. Reducing Rh1 endocytosis or Rh1 levels in retromer mutants alleviates PR degeneration. In addition, increasing retromer abundance suppresses degenerative phenotypes of mutations that affect the endolysosomal system. Finally, expressing human Vps26 suppresses PR degeneration in Vps26 mutant PRs. We propose that the retromer plays a conserved role in recycling rhodopsins to maintain PR function and integrity.

Retromer promotes immune quiescence by suppressing Spätzle-Toll pathway in Drosophila

The Toll and Toll-like receptor signaling pathways are evolutionarily conserved pathways that regulate innate immunity in insects and mammals. While efforts have been made to clarify the signal transduction events that occur during infection, much less is known about the components that maintain immune quiescence. Here we show that retromer, an intracellular protein complex known for regulating vesicle trafficking, functions in modulating the Toll pathway in Drosophila melanogaster. In mutant animals lacking retromer function, the Toll pathway but not JAK-STAT or IMD pathway is activated, triggering both cellular and humoral responses. Genetic epistasis and clonal analysis suggest that retromer regulates a component that acts upstream of Toll. Our data further show that in the mutant the Toll ligand Spätzle has a processing pattern similar to that of after infection. Together, the results suggest a novel function of retromer in regulating Toll pathway and innate immunity at a step that modulates ligand processing or activity.

Retromer: a master conductor of endosome sorting

The endosomal network comprises an interconnected network of membranous compartments whose primary function is to receive, dissociate, and sort cargo that originates from the plasma membrane and the biosynthetic pathway. A major challenge in cell biology is to achieve a thorough molecular description of how this network operates, and in so doing, how defects contribute to the etiology and pathology of human disease. We discuss the increasing body of evidence that implicates an ancient evolutionary conserved complex, termed "retromer," as a master conductor in the complex orchestration of multiple cargo-sorting events within the endosomal network.

Complexities of Crumbs function and regulation in tissue morphogenesis

Establishing and maintaining epithelial polarity is crucial during development and for adult tissue homeostasis. A complex network of evolutionarily conserved proteins regulates this compartmentalization. One such protein is Crumbs, a type I transmembrane protein initially shown to be an important apical determinant in Drosophila. We discuss recent studies that have advanced our understanding of the function and regulation of Crumbs. New findings obtained in flies and fish, reporting homotypic interactions of the extracellular domain and retromer-mediated recycling, shed light on the regulation of Crumbs levels and activity. These results - obtained in different organisms, tissues and developmental stages - point to more complex functions and regulation than previously assumed.

Rab9 and retromer regulate retrograde trafficking of luminal protein required for epithelial tube length control

Apical extracellular matrix filling the lumen controls the morphology and geometry of epithelial tubes during development, yet the regulation of luminal protein composition and its role in tube morphogenesis are not well understood. Here we show that an endosomal-retrieval machinery consisting of Rab9, retromer and actin nucleator WASH (Wiskott–Aldrich Syndrome Protein and SCAR Homolog) regulates selective recycling of the luminal protein Serpentine in the Drosophila trachea. Secreted Serpentine is endocytosed and sorted into the late endosome. Vps35, WASH and actin filaments differentially localize at the Rab9-enriched subdomains of the endosomal membrane, where Serpentine containing vesicles bud off. In Rab9, Vps35 and WASH mutants, Serpentine was secreted normally into the tracheal lumen, but the luminal quantities were depleted at later stages, resulting in excessively elongated tubes. In contrast, secretion of many luminal proteins was unaffected, suggesting that retrograde trafficking of a specific class of luminal proteins is a pivotal rate-limiting mechanism for continuous tube length regulation.

The development of biological tubes is regulated by mutual interactions between cells and luminal extracellular matrix. Dong et al. show that retrograde recycling of luminal chitin deacetylase regulates Drosophila tracheal tubule geometry by restricting length independently of diameter.

Endosomal recruitment of the WASH complex: active sequences and mutations impairing interaction with the retromer

BACKGROUND INFORMATION

The Wiskott-Aldrich syndrome protein and scar homolog (WASH) complex is the major Arp2/3 activator at the surface of endosomes. The branched actin network, that the WASH complex induces, contributes to cargo sorting and scission of transport intermediates destined for most endosomal routes. A major challenge is to understand how the WASH molecular machine is recruited to the surface of endosomes. The retromer endosomal machinery has been proposed by us and others to play a role in this process.

RESULTS

In this work, we used an unbiased approach to identify the endosomal receptor of the WASH complex. We have delineated a short fragment of the FAM21 subunit that is able to displace the endogenous WASH complex from endosomes. Using a proteomic approach, we have identified the retromer cargo selective complex (CSC) as a partner of the active FAM21 sequence displacing the endogenous WASH complex. A point mutation in FAM21 that abolishes CSC interaction also impairs WASH complex displacement activity. The CSC is composed of three subunits, VPS35, VPS29 and VPS26. FAM21 directly binds the VPS35 subunit of the retromer CSC. Additionally, we show that a point mutant of VPS35 that blocks binding to VPS29 also prevents association with FAM21 and the WASH complex revealing a novel role for the VPS35-VPS29 interaction in regulating retromer association with the WASH complex.

CONCLUSIONS

This novel approach of endogenous WASH displacement confirms previous suggestions that the retromer is the receptor of the WASH complex at the surface of endosomes and identify key residues that mediate this interaction. The interaction between these two endosomal machineries, the WASH complex and the retromer, is likely to play a critical role in forming platforms at the surface of endosomes for efficient sorting of cargoes.

Cell polarity: models and mechanisms from yeast, worms and flies

Determinants of cell polarity orient the behaviour of many cell types during development. Pioneering genetic screens in yeast, worms and flies have identified key polarity determinants that are evolutionarily conserved across the animal kingdom. Recent work in these three model organisms has combined computer modelling with experimental analysis to reveal the molecular mechanisms that drive the polarisation of determinants. Two key principles have emerged: the first is the requirement for a positive-feedback loop to drive self-recruitment of determinants to the plasma membrane; the second is the requirement for mutual antagonism between determinants that localise to opposite ends of the cell.

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.

Tissue-specific function of Patj in regulating the Crumbs complex and epithelial polarity

Patj is described as a core component of the Crumbs complex. Along with the other components, Crumbs and Stardust, Patj has been proposed as essential for epithelial polarity. However, no proper in vivo genetic analysis of Patj function has been performed in any organism. We have generated the first null mutants for Drosophila Patj. These mutants are lethal. However, Patj is not required in all epithelia where the Crumbs complex is essential. Patj is dispensable for ectoderm polarity and embryonic development, whereas more severe defects are observed in the adult follicular epithelium, including mislocalisation of the Crumbs complex from the apical domain, as well as morphogenetic defects. These defects are similar to those observed with crumbs and stardust mutants, although weaker and less frequent. Also, gain-of-function of Crumbs and Patj mutation genetically suppress each other in follicular cells. We also show that the first PDZ domain of Patj associated with the Stardust-binding domain are sufficient to fully rescue both Drosophila viability and Crumbs localisation. We propose that the only crucial function of Patj hinges on the ability of its first two domains to positively regulate the Crumbs complex, defining a new developmental level of regulation of its dynamics.