The role of secretory and endocytic pathways in the maintenance of cell polarity

Viral and cellular determinants of polarized trafficking of viral envelope proteins from insect-specific and insect-vectored viruses in insect midgut and salivary gland cells

Systemic viral infection of insects typically begins with the primary infection of midgut epithelial cells (enterocytes) and subsequent transit of the progeny virus in an apical-to-basal orientation into the hemocoel. For insect-vectored viruses, an oppositely oriented process (basal-to-apical transit) occurs upon secondary infection of salivary glands and is necessary for virus transmission to non-insect hosts. To examine this inversely oriented virus transit in these polarized tissues, we assessed the intracellular trafficking of two model viral envelope proteins (baculovirus GP64 and vesicular stomatitis virus G) in the midgut and salivary gland cells of the model insect, Drosophila melanogaster. Using fly lines that inducibly express either GP64 or VSV G, we found that each protein, expressed alone, was trafficked basally in midgut enterocytes. In salivary gland cells, VSV G was trafficked apically in most but not all cells, whereas GP64 was consistently trafficked basally. We demonstrated that a YxxØ motif present in both proteins was critical for basal trafficking in midgut enterocytes but dispensable for trafficking in salivary gland cells. Using RNAi, we found that clathrin adaptor protein complexes AP-1 and AP-3, as well as seven Rab GTPases, were involved in polarized VSV G trafficking in midgut enterocytes. Our results indicate that these viral envelope proteins encode the requisite information and require no other viral factors for appropriately polarized trafficking. In addition, they exploit tissue-specific differences in protein trafficking pathways to facilitate virus egress in the appropriate orientation for establishing systemic infections and vectoring infection to other hosts.

IMPORTANCE

Viruses that use insects as hosts must navigate specific routes through different insect tissues to complete their life cycles. The routes may differ substantially depending on the life cycle of the virus. Both insect pathogenic viruses and insect-vectored viruses must navigate through the polarized cells of the midgut epithelium to establish a systemic infection. In addition, insect-vectored viruses must also navigate through the polarized salivary gland epithelium for transmission. Thus, insect-vectored viruses appear to traffic in opposite directions in these two tissues. In this study, we asked whether two viral envelope proteins (VSV G and baculovirus GP64) alone encode the signals necessary for the polarized trafficking associated with their respective life cycles. Using Drosophila as a model to examine tissue-specific polarized trafficking of these viral envelope proteins, we identified one of the virus-encoded signals and several host proteins associated with regulating the polarized trafficking in the midgut epithelium.

RNF26 binds perinuclear vimentin filaments to integrate ER and endolysosomal responses to proteotoxic stress

Proteotoxic stress causes profound endoplasmic reticulum (ER) membrane remodeling into a perinuclear quality control compartment (ERQC) for the degradation of misfolded proteins. Subsequent return to homeostasis involves clearance of the ERQC by endolysosomes. However, the factors that control perinuclear ER integrity and dynamics remain unclear. Here, we identify vimentin intermediate filaments as perinuclear anchors for the ER and endolysosomes. We show that perinuclear vimentin filaments engage the ER-embedded RING finger protein 26 (RNF26) at the C-terminus of its RING domain. This restricts RNF26 to perinuclear ER subdomains and enables the corresponding spatial retention of endolysosomes through RNF26-mediated membrane contact sites (MCS). We find that both RNF26 and vimentin are required for the perinuclear coalescence of the ERQC and its juxtaposition with proteolytic compartments, which facilitates efficient recovery from ER stress via the Sec62-mediated ER-phagy pathway. Collectively, our findings reveal a scaffolding mechanism that underpins the spatiotemporal integration of organelles during cellular proteostasis.

Immunoprecipitation and Western Blot Analysis of AP-1 Clathrin-Coated Vesicles

The function and integrity of epithelial cells depends on the polarized localization of transmembrane proteins at either apical or basolateral plasma membrane domains. To facilitate sorting to the basolateral domain, columnar epithelial cells express the tissue-specific AP-1B complex in addition to the ubiquitously expressed AP-1A. Both AP-1A and AP-1B are heterotetrameric clathrin adaptor protein complexes that are closely related. Here we describe a biochemical method to separate AP-1B from AP-1A clathrin-coated vesicles by immunoprecipitation from clathrin-coated vesicle pellets that were obtained by ultracentrifugation and analyzed by SDS-PAGE and western blot using fluorescently labeled secondary antibodies.

Rho and Rab Family Small GTPases in the Regulation of Membrane Polarity in Epithelial Cells

Membrane polarity, defined as the asymmetric distribution of lipids and proteins in the plasma membrane, is a critical prerequisite for the development of multicellular tissues, such as epithelia and endothelia. Membrane polarity is regulated by polarized trafficking of membrane components to specific membrane domains and requires the presence of intramembrane diffusion barriers that prevent the intermixing of asymmetrically distributed membrane components. This intramembrane diffusion barrier is localized at the tight junctions (TJs) in these cells. Both the formation of cell-cell junctions and the polarized traffic of membrane proteins and lipids are regulated by Rho and Rab family small GTPases. In this review article, we will summarize the recent developments in the regulation of apico-basal membrane polarity by polarized membrane traffic and the formation of the intramembrane diffusion barrier in epithelial cells with a particular focus on the role of Rho and Rab family small GTPases.

Endocytosis at the Crossroad of Polarity and Signaling Regulation: Learning from Drosophila melanogaster and Beyond

Cellular trafficking through the endosomal–lysosomal system is essential for the transport of cargo proteins, receptors and lipids from the plasma membrane inside the cells and across membranous organelles. By acting as sorting stations, vesicle compartments direct the fate of their content for degradation, recycling to the membrane or transport to the trans-Golgi network. To effectively communicate with their neighbors, cells need to regulate their compartmentation and guide their signaling machineries to cortical membranes underlying these contact sites. Endosomal trafficking is indispensable for the polarized distribution of fate determinants, adaptors and junctional proteins. Conversely, endocytic machineries cooperate with polarity and scaffolding components to internalize receptors and target them to discrete membrane domains. Depending on the cell and tissue context, receptor endocytosis can terminate signaling responses but can also activate them within endosomes that act as signaling platforms. Therefore, cell homeostasis and responses to environmental cues rely on the dynamic cooperation of endosomal–lysosomal machineries with polarity and signaling cues. This review aims to address advances and emerging concepts on the cooperative regulation of endocytosis, polarity and signaling, primarily in Drosophila melanogaster and discuss some of the open questions across the different cell and tissue types that have not yet been fully explored.

Distinct mechanisms orchestrate the contra-polarity of IRK and KOIN, two LRR-receptor-kinases controlling root cell division

In plants, cell polarity plays key roles in coordinating developmental processes. Despite the characterization of several polarly localized plasma membrane proteins, the mechanisms connecting protein dynamics with cellular functions often remain unclear. Here, we introduce a polarized receptor, KOIN, that restricts cell divisions in the Arabidopsis root meristem. In the endodermis, KOIN polarity is opposite to IRK, a receptor that represses endodermal cell divisions. Their contra-polar localization facilitates dissection of polarity mechanisms and the links between polarity and function. We find that IRK and KOIN are recognized, sorted, and secreted through distinct pathways. IRK extracellular domains determine its polarity and partially rescue the mutant phenotype, whereas KOIN’s extracellular domains are insufficient for polar sorting and function. Endodermal expression of an IRK/KOIN chimera generates non-cell-autonomous misregulation of root cell divisions that impacts patterning. Altogether, we reveal two contrasting mechanisms determining these receptors’ polarity and link their polarity to cell divisions in root tissue patterning.

Protein polarization coordinates many plant developmental processes. Here the authors show that IRK and KOIN, two LRR-receptor-kinases polarized to opposite sides of cells in the root meristem, rely on distinct mechanisms to achieve polarity.

Novel function for AP-1B during cell migration

The epithelial cell-specific clathrin adaptor protein (AP)-1B has a well-established role in polarized sorting of cargos to the basolateral membrane. Here we show that β1 integrin was dependent on AP-1B and its coadaptor, autosomal recessive hypercholesterolemia protein (ARH), for sorting to the basolateral membrane. We further demonstrate an unprecedented role for AP-1B at the basal plasma membrane during collective cell migration of epithelial sheets. During wound healing, expression of AP-1B (and ARH in AP–1B-positive cells) slowed epithelial-cell migration. We show that AP-1B colocalized with β1 integrin in focal adhesions during cell migration using confocal microscopy and total internal reflection fluorescence microscopy on fixed specimens. Further, AP-1B labeling in cell protrusions was distinct from labeling for the endocytic adaptor complex AP-2. Using stochastic optical reconstruction microscopy we identified numerous AP–1B-coated structures at or close to the basal plasma membrane in cell protrusions. In addition, immunoelectron microscopy showed AP-1B in coated pits and vesicles at the plasma membrane during cell migration. Lastly, quantitative real-time reverse transcription PCR analysis of human epithelial-derived cell lines revealed a loss of AP-1B expression in highly migratory metastatic cancer cells suggesting that AP-1B’s novel role at the basal plasma membrane during cell migration might be an anticancer mechanism.

Fam20C regulates protein secretion by Cab45 phosphorylation

The TGN is a key compartment for the sorting and secretion of newly synthesized proteins. At the TGN, soluble proteins are sorted based on the instructions carried in their oligosaccharide backbones or by a Ca2+-mediated process that involves the cargo-sorting protein Cab45. Here, we show that Cab45 is phosphorylated by the Golgi-specific protein kinase Fam20C. Mimicking of phosphorylation translocates Cab45 into TGN-derived vesicles, which goes along with an increased export of LyzC, a Cab45 client. Our findings demonstrate that Fam20C plays a key role in the export of Cab45 clients by fine-tuning Cab45 oligomerization and thus impacts Cab45 retention in the TGN.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Exploring new routes for secretory protein export from the trans-Golgi network

Sorting of soluble proteins for transport to intracellular compartments and for secretion from cells is essential for cell and tissue homeostasis. The trans-Golgi network (TGN) is a major sorting station that sorts secretory proteins into specific carriers to transport them to their final destinations. The sorting of lysosomal hydrolases at the TGN by the mannose 6-phosphate receptor is well understood. The recent discovery of a Ca²⁺-based sorting of secretory cargo at the TGN is beginning to uncover the mechanism by which cells sort secretory cargoes from Golgi residents and cargoes destined to the other cellular compartments. This Ca²⁺-based sorting involves the cytoplasmic actin cytoskeleton, which through membrane anchored Ca²⁺ ATPase SPCA1 and the luminal Ca²⁺ binding protein Cab45 sorts of a subset of secretory proteins at the TGN. We present this discovery and highlight important challenges that remain unaddressed in the overall pathway of cargo sorting at the TGN.

Activity of the SPCA1 Calcium Pump Couples Sphingomyelin Synthesis to Sorting of Secretory Proteins in the Trans-Golgi Network

How the principal functions of the Golgi apparatus-protein processing, lipid synthesis, and sorting of macromolecules-are integrated to constitute cargo-specific trafficking pathways originating from the trans-Golgi network (TGN) is unknown. Here, we show that the activity of the Golgi localized SPCA1 calcium pump couples sorting and export of secreted proteins to synthesis of new lipid in the TGN membrane. A secreted Ca2+-binding protein, Cab45, constitutes the core component of a Ca2+-dependent, oligomerization-driven sorting mechanism whereby secreted proteins bound to Cab45 are packaged into a TGN-derived vesicular carrier whose membrane is enriched in sphingomyelin, a lipid implicated in TGN-to-cell surface transport. SPCA1 activity is controlled by the sphingomyelin content of the TGN membrane, such that local sphingomyelin synthesis promotes Ca2+ flux into the lumen of the TGN, which drives secretory protein sorting and export, thereby establishing a protein- and lipid-specific secretion pathway.

Membrane Transport across Polarized Epithelia

Polarized epithelial cells line diverse surfaces throughout the body forming selective barriers between the external environment and the internal milieu. To cross these epithelial barriers, large solutes and other cargoes must undergo transcytosis, an endocytic pathway unique to polarized cell types, and significant for the development of cell polarity, uptake of viral and bacterial pathogens, transepithelial signaling, and immunoglobulin transport. Here, we review recent advances in our knowledge of the transcytotic pathway for proteins and lipids. We also discuss briefly the promise of harnessing the molecules that undergo transcytosis as vehicles for clinical applications in drug delivery.

The Role of the Cytoskeleton and Myosin-Vc in the Targeting of KCa3.1 to the Basolateral Membrane of Polarized Epithelial Cells

Understanding the targeting of KCa3.1 to the basolateral membrane (BLM) of polarized epithelial cells is still emerging. Here, we examined the role of the cytoskeleton (microtubules and microfilaments) and Myosin-Vc (Myo-Vc) in the targeting of KCa3.1 in Fischer rat thyroid epithelial cells. We used a pharmacological approach with immunoblot (for the BLM expression of KCa3.1), Ussing chamber (functional BLM expression of KCa3.1) and siRNA experiments. The actin cytoskeleton inhibitors cytochalasin D (10 μM, 5 h) and latrunculin A (10 μM, 5 h) reduced the targeting of KCa3.1 to the BLM by 88 ± 4 and 70 ± 5%, respectively. Colchicine (10 μM, 5 h) a microtubule inhibitor reduced targeting of KCa3.1 to the BLM by 63 ± 7% and decreased 1-EBIO-stimulated KCa3.1 K⁺ current by 46 ± 18%, compared with control cells. ML9 (10 μM, 5 h), an inhibitor of myosin light chain kinase, decreased targeting of the channel by 83 ± 2% and reduced K⁺ current by 54 ± 8% compared to control cells. Inhibiting Myo-V with 2,3-butanedione monoxime (10 mM, 5 h) reduced targeting of the channel to the BLM by 58 ± 5% and decreased the stimulated current of KCa3.1 by 48 ± 12% compared with control cells. Finally, using siRNA for Myo-Vc, we demonstrated that knockdown of Myo-Vc reduced the BLM expression of KCa3.1 by 44 ± 7% and KCa3.1 K⁺ current by 1.04 ± 0.14 μA compared with control cells. These data suggest that the microtubule and microfilament cytoskeleton and Myo-Vc are critical for the targeting of KCa3.1.

Rab GTPase signaling in neurite outgrowth and axon specification

Neurons are highly polarized cells that contain specialized subcellular domains involved in information transmission in the nervous system. Specifically, the somatodendritic compartment receives neuronal inputs while the axons convey information through the synapse. The establishment of asymmetric domains requires a specific delivery of components, including organelles, proteins, and membrane. The Rab family of small GTPases plays an essential role in membrane trafficking. Signaling cascades triggered by extrinsic and intrinsic factors tightly regulate Rab functions in cells, with Rab protein activation depending on GDP/GTP binding to establish a binary mode of action. This review summarizes the contributions of several Rab family members involved in trans-Golgi, early/late endosomes, and recycling endosomes during neurite development and axonal outgrowth. The regulation of some Rabs by guanine exchanging factors and GTPase activating proteins will also be addressed. Finally, discussion will be provided on how specific effector-mediated Rab activation modifies several molecules essential to neuronal differentiation. © 2016 Wiley Periodicals, Inc.

N-linked glycans do not affect plasma membrane localization of multidrug resistance protein 4 (MRP4) but selectively alter its prostaglandin E2 transport activity

Multidrug resistance protein 4 (MRP4) is a member of subfamily C of the ATP-binding cassette superfamily of membrane transport proteins. MRP4 mediates the ATP-dependent efflux of many endogenous and exogenous solutes across the plasma membrane, and in polarized cells, it localizes to the apical or basolateral plasma membrane depending on the tissue type. MRP4 is a 170 kDa glycoprotein and here we show that MRP4 is simultaneously N-glycosylated at Asn746 and Asn754. Furthermore, confocal immunofluorescence studies showed that N-glycans do not affect MRP4's apical membrane localization in polarized LLC-PK1 cells or basolateral membrane localization in polarized MDCKI cells. However, vesicular transport assays showed that N-glycans differentially affect MRP4's ability to transport prostaglandin E2, but not estradiol glucuronide. Together these data indicate that N-glycosylation at Asn746 and Asn754 is not essential for plasma membrane localization of MRP4 but cause substrate-selective effects on its transport activity.

Role of the epithelial cell-specific clathrin adaptor complex AP-1B in cell polarity

Epithelial cells are important for organ development and function. To this end, they polarize their plasma membrane into biochemically and physically distinct membrane domains. The apical membrane faces the luminal site of an organ and the basolateral domain is in contact with the basement membrane and neighboring cells. To establish and maintain this polarity it is important that newly synthesized and endocytic cargos are correctly sorted according to their final destinations at either membrane. Sorting takes place at one of 2 major sorting stations in the cells, the trans-Golgi network (TGN) and recycling endosomes (REs). Polarized sorting may involve epithelial cell-specific sorting adaptors like the AP-1B clathrin adaptor complex. AP-1B facilitates basolateral sorting from REs. This review will discuss various aspects of basolateral sorting in epithelial cells with a special emphasis on AP-1B.

Analyzing the role of AP-1B in polarized sorting from recycling endosomes in epithelial cells

Epithelial cells polarize their plasma membrane into apical and basolateral domains where the apical membrane faces the luminal side of an organ and the basolateral membrane is in contact with neighboring cells and the basement membrane. To maintain this polarity, newly synthesized and internalized cargos must be sorted to their correct target domain. Over the last ten years, recycling endosomes have emerged as an important sorting station at which proteins destined for the apical membrane are segregated from those destined for the basolateral membrane. Essential for basolateral sorting from recycling endosomes is the tissue-specific adaptor complex AP-1B. This chapter describes experimental protocols to analyze the AP-1B function in epithelial cells including the analysis of protein sorting in LLC-PK1 cells lines, immunoprecipitation of cargo proteins after chemical crosslinking to AP-1B, and radioactive pulse-chase experiments in MDCK cells depleted of the AP-1B subunit μ1B.

Intraperitoneal Immunization with Cry1Ac Protoxin from Bacillus thuringiensis Provokes Upregulation of Fc-Gamma-II/and Fc-Gamma-III Receptors Associated with IgG in the Intestinal Epithelium of Mice

In humans, intestinal epithelial FcRn is expressed throughout life and mediates the bidirectional transport of IgG, but in mice, it is markedly expressed in neonatal intestine. In adults, its expression is only faintly upregulated after intestinal IgG induction such as that elicited by i.p. immunization with Cry1Ac protoxin (pCry1Ac) Bacillus thuringiensis. This led us to suggest that additional Fcγ receptors (Fcγ-R) may be participating in epithelial IgG uptake. So, first we determined whether CD16/32 [an epitope shared by Fcγ-RII (CD32) and Fcγ-RIII (CD16)] was expressed in the intestinal epithelia of mice. Using confocal microscopy and flow cytometry, we detected co-localization of IgG and CD16/32 in epithelial cells, whose frequency was increased by immunization with pCry1Ac. Western blot and cross-immunoprecipitation results with anti-CD16/32 and IgG antibodies in epithelial cell extracts suggested that epithelial cells bear both Fcγ-RII and Fcγ-RIII and contained IgG associated with Fcγ-RII/RIII. Using anti-CD32 and anti-CD16 antibodies, we confirmed by Western blot, confocal microscopy and flow cytometry that both Fcγ-RII and Fcγ-RIII were expressed and suggested that upregulation occurred upon immunization in intestinal epithelia. Finally, we examined the in vitro effect of anti-CD16/32, anti-CD16 and anti-CD32 antibodies on IgG uptake and transport by intestinal epithelial cells and found that it was partially reduced. Although further studies are still required, our results suggest that Fcγ-RII and Fcγ-RIII might participate in the uptake and/or transport of IgG through the intestinal epithelia of adult mice.

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.

Lgl regulates Notch signaling via endocytosis, independently of the apical aPKC-Par6-Baz polarity complex

BACKGROUND

The Drosophila melanogaster junctional neoplastic tumor suppressor, Lethal-2-giant larvae (Lgl), is a regulator of apicobasal cell polarity and tissue growth. We have previously shown in the developing Drosophila eye epithelium that, without affecting cell polarity, depletion of Lgl results in ectopic cell proliferation and blockage of developmental cell death due to deregulation of the Hippo signaling pathway.

RESULTS

Here, we show that Notch signaling is increased in lgl-depleted eye tissue, independently of Lgl's function in apicobasal cell polarity. The upregulation of Notch signaling is ligand dependent and correlates with accumulation of cleaved Notch. Concomitant with higher cleaved Notch levels in lgl- tissue, early endosomes (Avalanche [Avl+]), recycling endosomes (Rab11+), early multivesicular bodies (Hrs+), and acidified vesicles, but not late endosomal markers (Car+ and Rab7+), accumulate. Colocalization studies revealed that Lgl associates with early to late endosomes and lysosomes. Upregulation of Notch signaling in lgl- tissue requires dynamin- and Rab5-mediated endocytosis and vesicle acidification but is independent of Hrs/Stam or Rab11 activity. Furthermore, Lgl regulates Notch signaling independently of the aPKC-Par6-Baz apical polarity complex.

CONCLUSIONS

Altogether, our data show that Lgl regulates endocytosis to restrict vesicle acidification and prevent ectopic ligand-dependent Notch signaling. This Lgl function is independent of the aPKC-Par6-Baz polarity complex and uncovers a novel attenuation mechanism of ligand-activated Notch signaling during Drosophila eye development.

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.

A new role for Rab GTPases during early mitotic stages

A recent study revealed new roles for the Rab11 GTPase during mitosis. Rab11 is involved in recycling endosome localization to mitotic spindle poles via dynein-mediated transport. This process is in contrast to Golgi membranes, which disperse in mitosis and do not appear to directly contribute to mitotic functions. Rab11-depletion prevents recycling endosome organization at spindle poles, delays mitotic progression, and disrupts spindle pole protein recruitment, astral microtubule organization, and mitotic spindle orientation. However, Rab11 is not the only endocytic and/or trafficking protein that regulates mitotic progression. Clathrin and two small GTPases (Rab6A', Rab5) play key roles in spindle organization and function. In this commentary, we discuss the roles of all these canonical endocytic and membrane trafficking proteins during mitosis and speculate on possible cross-communication between them and their molecular pathways that ensure faithful progression through mitosis.

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.

Rab-mediated trafficking role in neurite formation

Neuronal cells are characterized by the presence of two confined domains, which are different in their cellular properties, biochemical functions and molecular identity. The generation of asymmetric domains in neurons should logically require specialized membrane trafficking to both promote neurite outgrowth and differential distribution of components. Members of the Rab family of small GTPases are key regulators of membrane trafficking involved in transport, tethering and docking of vesicles through their effectors. RabGTPases activity is coupled to the activity of guanine nucleotide exchange factors or GEFs, and GTPase-activating proteins known as GAPs. Since the overall spatiotemporal distribution of GEFs, GAPs and Rabs governs trafficking through the secretory and endocytic pathways, affecting exocytosis, endocytosis and endosome recycling, it is likely that RabGTPases could have a major role in neurite outgrowth, elongation and polarization. In this review we summarize the evidence linking the functions of several RabGTPases to axonal and dendritic development in primary neurons, as well as neurite formation in neuronal cell lines. We focused on the role of RabGTPases from the trans-Golgi network, early/late and recycling endosomes, as well as the function of some Rab effectors in neuritogenesis. Finally, we also discuss the participation of the ADP-ribosylation factor 6, a member of the ArfGTPase family, in neurite formation since it seems to have an important cross-talk with RabGTPases.

Endosomes derived from clathrin-independent endocytosis serve as precursors for endothelial lumen formation

Clathrin-independent endocytosis (CIE) is a form of bulk plasma membrane (PM) endocytosis that allows cells to sample and evaluate PM composition. Once in endosomes, the internalized proteins and lipids can be recycled back to the PM or delivered to lysosomes for degradation. Endosomes arising from CIE contain lipid and signaling molecules suggesting that they might be involved in important biological processes. During vasculogenesis, new blood vessels are formed from precursor cells in a process involving internalization and accumulation of endocytic vesicles. Here, we found that CIE has a role in endothelial lumen formation. Specifically, we found that human vascular endothelial cells (HUVECs) utilize CIE for internalization of distinct cargo molecules and that in three-dimensional cultures CIE membranes are delivered to the newly formed lumen.

Retromer maintains basolateral distribution of the type II TGF-β receptor via the recycling endosome

After basolateral (BL) cell surface delivery, retromer promotes type II TGF-β receptor exit and recycling to the BL plasma membrane. In the absence of retromer, however, type II receptors aberrantly sort and are mislocalized such that both BL and apical expression is observed independent of the Rab11-positive apical recycling endosome.

Transforming growth factor β (TGF-β) is critical for the development and maintenance of epithelial structures. Because receptor localization and trafficking affect the cellular and organismal response to TGF-β, the present study was designed to address how such homeostatic control is regulated. To that end, we identify a new role for the mammalian retromer complex in maintaining basolateral plasma membrane expression of the type II TGF-β receptor (TβRII). Retromer and TβRII associate in the presence or absence of TGF-β ligand. After retromer knockdown, although TβRII internalization and trafficking to a Rab5-positive compartment occur as in wild-type cells, receptor recycling is inhibited. This results in TβRII mislocalization from the basolateral to both the basolateral and apical plasma membranes independent of Golgi transit and the Rab11-positive apical recycling endosome. The data support a model in which, after initial basolateral TβRII delivery, steady-state polarized TβRII expression is maintained by retromer/TβRII binding and delivery to the common recycling endosome.