Rab35 regulates Arf6 activity through centaurin-β2 (ACAP2) during neurite outgrowth

SMAP2 regulates retrograde transport from recycling endosomes to the Golgi

Retrograde transport is where proteins and lipids are transported back from the plasma membrane (PM) and endosomes to the Golgi, and crucial for a diverse range of cellular functions. Recycling endosomes (REs) serve as a sorting station for the retrograde transport and we recently identified evection-2, an RE protein with a pleckstrin homology (PH) domain, as an essential factor of this pathway. How evection-2 regulates retrograde transport from REs to the Golgi is not well understood. Here, we report that evection-2 binds to SMAP2, an Arf GTPase-activating protein. Endogenous SMAP2 localized mostly in REs and to a lesser extent, the trans-Golgi network (TGN). SMAP2 binds evection-2, and the RE localization of SMAP2 was abolished in cells depleted of evection-2. Knockdown of SMAP2, like that of evection-2, impaired the retrograde transport of cholera toxin B subunit (CTxB) from REs. These findings suggest that evection-2 recruits SMAP2 to REs, thereby regulating the retrograde transport of CTxB from REs to the Golgi.

Small GTPases as regulators of cell division

The superfamily of small GTPases serves as a signal transducer to regulate a diverse array of cellular functions. The members of this superfamily are structurally and functionally classified into at least 5 groups (Ras, Rho/Rac, Rab, Arf, and Ran) and they are involved in the control of cell proliferation and differentiation, regulation of the actin cytoskeleton, membrane trafficking, and nuclear transport. It is widely reported that members of the Rab family participate in the control of intracellular membrane trafficking through the interaction with specific effector molecules. However, many Rabs and other small GTPases have also been shown to function in cell division. In this review, we discuss current knowledge about Rab proteins regulating different stages of the cell cycle, such as the congregation and segregation of chromosomes (during metaphase) and the final stage of cell division known as cytokinesis, in which a cell is cleaved originating 2 daughter cells.

Arf6, Rab11 and transferrin receptor define distinct populations of recycling endosomes

Recycling endosomes are key platforms for endocytic recycling that return internalized molecules back to the plasma membrane. To determine how recycling endosomes perform their functions, searching for proteins and lipids that specifically localized at recycling endosomes has often been performed by colocalization analyses between candidate molecules and conventional recycling endosome markers. However, it remains unclear whether all the conventional markers have identical localizations. Here we report finding that three well-known recycling endosome markers, i.e., Arf6, Rab11 and transferrin receptor (TfR), have different intracellular localizations in PC12 cells. The results of immunofluorescence analyses showed that the signals of endogenous Arf6, Rab11 and TfR in nerve growth factor-stimulated PC12 cells generally differed, although there was some overlapping. Our findings provide new information about recycling endosome markers, and they highlight the heterogeneity of recycling endosomes.

Rabex-5 protein regulates dendritic localization of small GTPase Rab17 and neurite morphogenesis in hippocampal neurons

Small GTPase Rab17 has recently been shown to regulate dendritic morphogenesis of mouse hippocampal neurons; however, the exact molecular mechanism of Rab17-mediated dendritogenesis remained to be determined, because no guanine nucleotide exchange factor (GEF) for Rab17 had been identified. In this study we screened for the Rab17-GEF by performing yeast two-hybrid assays with a GDP-locked Rab17 mutant as bait and found that Rabex-5 and ALS2, both of which were originally described as Rab5-GEFs, interact with Rab17. We also found that expression of Rabex-5, but not of ALS2, promotes translocation of Rab17 from the cell body to the dendrites of developing mouse hippocampal neurons. The shRNA-mediated knockdown of Rabex-5 or its known downstream target Rab5 in hippocampal neurons inhibited morphogenesis of both axons and dendrites, whereas knockdown of Rab17 affected dendrite morphogenesis alone. Based on these findings, we propose that Rabex-5 regulates neurite morphogenesis of hippocampal neurons by activating at least two downstream targets, Rab5, which is localized in both axons and dendrites, and Rab17, which is localized in dendrites alone.

Interactions between Rab and Arf GTPases regulate endosomal phosphatidylinositol-4,5-bisphosphate during endocytic recycling

After endocytosis, a selective endocytic recycling process returns many endocytosed molecules back to the plasma membrane. The RAB-10/Rab10 GTPase is known to be a key recycling regulator for specific cargo molecules. New evidence, focused on C. elegans RAB-10 in polarized epithelia, points to a key role of RAB-10 in the regulation of endosomal phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) levels. In turn, PI(4,5)P2 levels strongly influence the recruitment of many peripheral membrane proteins, including those important for vesicle budding through their membrane bending activities. Part of the effect of RAB-10 on endosomal PI(4,5)P2 is through its newly identified effector CNT-1, a predicted GTPase activating protein (GAP) of the small GTPase ARF-6/Arf6. In mammals PI(4,5)P2 generating enzymes are known Arf6 effectors. In C. elegans we found that RAB-10, CNT-1 and ARF-6 are present on the same endosomes, that RAB-10 recruits CNT-1 to endosomes, and that loss of CNT-1 or RAB-10 leads to overaccumulation of endosomal PI(4,5)P2, presumably via hyperactivation of endosomal ARF-6. In turn this leads to over-recruitment of PI(4,5)P2-dependent membrane-bending proteins RME-1/Ehd and SDPN-1/Syndapin/PACSIN. Conversely, in arf-6 mutants, endosomal PI(4,5)P2 levels were reduced and endosomal recruitment of RME-1 and SDPN-1 failed. This work makes an unexpected link between distinct classes of small GTPases that control endocytic recycling, and provides insight into how this interaction affects endosome function at the level of lipid phosphorylation.

In vivo quantitative proteomics of somatosensory cortical synapses shows which protein levels are modulated by sensory deprivation

Postnatal bilateral whisker trimming was used as a model system to test how synaptic proteomes are altered in barrel cortex by sensory deprivation during synaptogenesis. Using quantitative mass spectrometry, we quantified more than 7,000 synaptic proteins and identified 89 significantly reduced and 161 significantly elevated proteins in sensory-deprived synapses, 22 of which were validated by immunoblotting. More than 95% of quantified proteins, including abundant synaptic proteins such as PSD-95 and gephyrin, exhibited no significant difference under high- and low-activity rearing conditions, suggesting no tissue-wide changes in excitatory or inhibitory synaptic density. In contrast, several proteins that promote mature spine morphology and synaptic strength, such as excitatory glutamate receptors and known accessory factors, were reduced significantly in deprived synapses. Immunohistochemistry revealed that the reduction in SynGAP1, a postsynaptic scaffolding protein, was restricted largely to layer I of barrel cortex in sensory-deprived rats. In addition, protein-degradation machinery such as proteasome subunits, E2 ligases, and E3 ligases, accumulated significantly in deprived synapses, suggesting targeted synaptic protein degradation under sensory deprivation. Importantly, this screen identified synaptic proteins whose levels were affected by sensory deprivation but whose synaptic roles have not yet been characterized in mammalian neurons. These data demonstrate the feasibility of defining synaptic proteomes under different sensory rearing conditions and could be applied to elucidate further molecular mechanisms of sensory development.

Rab35 establishes the EHD1-association site by coordinating two distinct effectors during neurite outgrowth

Endocytic recycling is a process in which molecules have been internalized are recycled back to the plasma membrane, and although it is crucial for regulating various cellular events, the molecular nexus underlying this process remains poorly understood. Here we report a well-orchestrated molecular link between two gatekeepers for endocytic recycling, the molecular switch Rab35 and the molecular scissors EHD1, that is mediated by two distinct Rab35 effectors during neurite outgrowth of PC12 cells. Rab35 forms a tripartite complex with MICAL-L1 and centaurin-β2/ACAP2 and recruits them to perinuclear Arf6-positive endosomes in response to nerve growth factor stimulation. MICAL-L1 and centaurin-β2 then cooperatively recruit EHD1 to the same compartment by functioning as a scaffold for EHD1 and as an inactivator of Arf6, respectively. We propose that Rab35 regulates the formation of an EHD1-association site on Arf6-positive endosomes by integrating the functions of two distinct Rab35 effectors for successful neurite outgrowth.

Interplay between Rab35 and Arf6 controls cargo recycling to coordinate cell adhesion and migration

Cells inversely adjust the plasma membrane levels of integrins and cadherins during cell migration and cell-cell adhesion but the regulatory mechanisms that coordinate these trafficking events remain unknown. Here, we demonstrate that the small GTPase Rab35 maintains cadherins at the cell surface to promote cell-cell adhesion. Simultaneously, Rab35 supresses the activity of the GTPase Arf6 to downregulate an Arf6-dependent recycling pathway for β1-integrin and EGF receptors, resulting in inhibition of cell migration and attenuation of signaling downstream of these receptors. Importantly, the phenotypes of decreased cell adhesion and increased cell migration observed following Rab35 knock down are consistent with the epithelial-mesenchymal transition, a feature of invasive cancer cells, and we show that Rab35 expression is suppressed in a subset of cancers characterized by Arf6 hyperactivity. Our data thus identify a key molecular mechanism that efficiently coordinates the inverse intracellular sorting and cell surface levels of cadherin and integrin receptors for cell migration and differentiation.

All members of the EPI64 subfamily of TBC/RabGAPs also have GAP activities towards Ras

The importance of interconnective signalling networks between distinct GTPases and their regulators is being recognized. EPI64C/TBC1D10C/carabin, a haematopoietically enriched GTPase-activating protein (GAP) for Rab35, has been shown to exhibit RasGAP activity. Owing to the diverged Rab specificities among the EPI64 members (EPI64A-C) and the relatively weak sequence conservation between EPI64A/B and EPI64C in their catalytic TBC domains, it is difficult to predict whether EPI64A and B will also have RasGAP activities. Therefore, in this study, we examined the RasGAP activities of all three EPI64 subfamily members. We found that EPI64A-C exhibited in vivo GAP activities towards Ras using three independent methods, spectrofluorometry with Förster resonance energy transfer (FRET) sensors, the Bos' pull-down assay and time-lapse FRET imaging. EPI64A and B were predominantly localized at the periphery of COS-7 cells. In COS-7 cells, confocal FRET imaging showed that H-Ras activity was higher at the Golgi than at the plasma membrane. Thus, we propose that EPI64A and B, which are ubiquitously expressed members of the EPI64 subfamily, inactivate Ras and certain Rabs at the periphery of cells.

Rab35 regulates cadherin-mediated adherens junction formation and myoblast fusion

The small GTPase Rab35 is identified as a regulator of cadherin trafficking and stabilization at cell–cell contacts. Rab35 function is required for PIP5KIγ accumulation at cell contacts and PI(4,5)P2 production, and hence cadherin stabilization. Rab35 regulates myoblast fusion, a cellular process under the control of cadherin-dependent signaling.

Cadherins are homophilic cell–cell adhesion molecules implicated in many fundamental processes, such as morphogenesis, cell growth, and differentiation. They accumulate at cell–cell contact sites and assemble into large macromolecular complexes named adherens junctions (AJs). Cadherin targeting and function are regulated by various cellular processes, many players of which remain to be uncovered. Here we identify the small GTPase Rab35 as a new regulator of cadherin trafficking and stabilization at cell–cell contacts in C2C12 myoblasts and HeLa cells. We find that Rab35 accumulates at cell–cell contacts in a cadherin-dependent manner. Knockdown of Rab35 or expression of a dominant-negative form of Rab35 impaired N- and M-cadherin recruitment to cell–cell contacts, their stabilization at the plasma membrane, and association with p120 catenin and led to their accumulation in transferrin-, clathrin-, and AP-2–positive intracellular vesicles. We also find that Rab35 function is required for PIP5KIγ accumulation at cell–cell contacts and phosphatidyl inositol 4,5-bisphosphate production, which is involved in cadherin stabilization at contact sites. Finally, we show that Rab35 regulates myoblast fusion, a major cellular process under the control of cadherin-dependent signaling. Taken together, these results reveal that Rab35 regulates cadherin-dependent AJ formation and myoblast fusion.

Trafficking cascades mediated by Rab35 and its membrane hub effector, MICAL-L1

Various receptors navigate through the endocytic recycling compartment (ERC) on route to the plasma membrane. They are transported through recycling endosomes that emanate from the ERC that display distinct tubular morphology. A key question in the field is how the trafficking via these endosomes is regulated and how regulatory proteins such as Rab35, Rab8, Arf6 and EHD1 control this trafficking. Recent studies point to the protein MICAL-L1 as a major scaffold for these regulators. MICAL-L1 not only localizes to these tubular recycling endosomes and regulates trafficking, but it also controls the localization of EHD1 and Rab8 to these structures. It also connects its associated membranes to the motor proteins dynein and kinesin through its binding partner, CRMP2. Our recent study promotes MICAL-L1 as a Rab35 effector, where Rab35, both directly and indirectly through Arf6, controls the localization of MICAL-L1 and Rab8 to tubular membranes. We find that MICAL-L1 is a multi-tasking scaffold connecting various proteins to recycling endosomes for efficient trafficking.

RAB-10-GTPase-mediated regulation of endosomal phosphatidylinositol-4,5-bisphosphate

Caenorhabditis elegans RAB-10 and mammalian Rab10 are key regulators of endocytic recycling, especially in the basolateral recycling pathways of polarized epithelial cells. To understand better how RAB-10 contributes to recycling endosome function, we sought to identify RAB-10 effectors. One RAB-10-binding partner that we identified, CNT-1, is the only C. elegans homolog of the mammalian Arf6 GTPase-activating proteins ACAP1 and ACAP2. Arf6 is known to regulate endosome-to-plasma membrane transport, in part through activation of type I phophatidylinositol-4-phosphate 5 kinase. Here we show that CNT-1 binds to RAB-10 through its C-terminal ankyrin repeats and colocalizes with RAB-10 and ARF-6 on recycling endosomes in vivo. Furthermore, we find that RAB-10 is required for the recruitment of CNT-1 to endosomal membranes in the intestinal epithelium. Consistent with negative regulation of ARF-6 by RAB-10 and CNT-1, we found overaccumulation of endosomal phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] in cnt-1 and rab-10 mutants and reduced endosomal PI(4,5)P2 levels in arf-6 mutants. These mutants produced similar effects on endosomal recruitment of the PI(4,5)P2-dependent membrane-bending proteins RME-1/Ehd and SDPN-1/Syndapin/Pacsin and resulted in endosomal trapping of specific recycling cargo. Our studies identify a RAB-10-to-ARF-6 regulatory loop required to regulate endosomal PI(4,5)P2, a key phosphoinositide in membrane traffic.

Dictyostelium ACAP-A is an ArfGAP involved in cytokinesis, cell migration and actin cytoskeleton dynamics

ACAPs and ASAPs are Arf-GTPase-activating proteins with BAR, PH, GAP and ankyrin repeat domains and are known to regulate vesicular traffic and actin cytoskeleton dynamics in mammalian cells. The amoeba Dictyostelium has only two proteins with this domain organization instead of six in human, enabling a more precise functional analysis. Genetic invalidation of acapA, resulted in multinucleated cells with cytokinesis defects. Mutant acapA− cells were hardly motile and their multicellular development was significantly delayed. In addition, formation of filopodial protrusions was deficient in these cells. Conversely, re-expression of ACAP-A-GFP resulted in numerous and long filopodia-like protrusions. Mutagenesis studies showed that ACAP-A actin remodeling function was dependent on its ability to activate its substrate, the small GTPase ArfA. Likewise, the expression of a constitutively active ArfA•GTP mutant in wild-type cells led to a significant reduction of filopodia length. Together our data support a role for ACAP-A in the control of the actin cytoskeleton organization and dynamics through an ArfA-dependent mechanism.

Intrinsic mechanisms regulating axon regeneration: an integrin perspective

Adult central nervous system (CNS) axons fail to regenerate after injury because of inhibitory factors in the surrounding environment and a low intrinsic regenerative capacity. Axons in the adult peripheral nervous system have a higher regenerative capacity, due in part to the presence of certain integrins-receptors for the extracellular matrix. Integrins are critical for axon growth during the development of the nervous system but are absent from some adult CNS axons. Here, we discuss the intrinsic mechanisms that regulate axon regeneration and examine the role of integrins. As correct localization is paramount to integrin function, we further discuss the mechanisms that regulate integrin traffic toward the axonal growth cone.