ADP-ribosylation factor 6 and endocytosis at the apical surface of Madin-Darby canine kidney cells

Roles for ELMOD2 and Rootletin in ciliogenesis

ELMOD2 is a GTPase-activating protein with uniquely broad specificity for ARF family GTPases. We previously showed that it acts with ARL2 in mitochondrial fusion and microtubule stability and with ARF6 during cytokinesis. Mouse embryonic fibroblasts deleted for ELMOD2 also displayed changes in cilia-related processes including increased ciliation, multiciliation, ciliary morphology, ciliary signaling, centrin accumulation inside cilia, and loss of rootlets at centrosomes with loss of centrosome cohesion. Increasing ARL2 activity or overexpressing Rootletin reversed these defects, revealing close functional links between the three proteins. This was further supported by the findings that deletion of Rootletin yielded similar phenotypes, which were rescued upon increasing ARL2 activity but not ELMOD2 overexpression. Thus, we propose that ARL2, ELMOD2, and Rootletin all act in a common pathway that suppresses spurious ciliation and maintains centrosome cohesion. Screening a number of markers of steps in the ciliation pathway supports a model in which ELMOD2, Rootletin, and ARL2 act downstream of TTBK2 and upstream of CP110 to prevent spurious release of CP110 and to regulate ciliary vesicle docking. These data thus provide evidence supporting roles for ELMOD2, Rootletin, and ARL2 in the regulation of ciliary licensing.

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.

Live Imaging of HIV-1 Transfer across T Cell Virological Synapse to Epithelial Cells that Promotes Stromal Macrophage Infection

During sexual intercourse, HIV-1 crosses epithelial barriers composing the genital mucosa, a poorly understood feature that requires an HIV-1-infected cell vectoring efficient mucosal HIV-1 entry. Therefore, urethral mucosa comprising a polarized epithelium and a stroma composed of fibroblasts and macrophages were reconstructed in vitro. Using this system, we demonstrate by live imaging that efficient HIV-1 transmission to stromal macrophages depends on cell-mediated transfer of the virus through virological synapses formed between HIV-1-infected CD4⁺ T cells and the epithelial cell mucosal surface. We visualized HIV-1 translocation through mucosal epithelial cells via transcytosis in regions where virological synapses occurred. In turn, interleukin-13 is secreted and HIV-1 targets macrophages, which develop a latent state of infection reversed by lipopolysaccharide (LPS) activation. The live observation of virological synapse formation reported herein is key in the design of vaccines and antiretroviral therapies aimed at blocking HIV-1 access to cellular reservoirs in genital mucosa.

The small GTPase ARF-1.2 is a regulator of unicellular tube formation in Caenorhabditis elegans

The membrane trafficking events that regulate unicellular tube formation and maintenance are not well understood. Here, using an RNAi screen, we identified the small GTPase ARF1 homolog ARF-1.2 as a regulator of excretory tube formation in Caenorhabditis elegans. RNAi-mediated knockdown and knockout of the arf-1.2 gene resulted in the formation of large intracellular vacuoles at the growth sites (varicosities) of the excretory canals. arf-1.2 mutant animals were sensitive to hyperosmotic conditions. arf-1.2 RNAi affected the localization of the anion transporter SULP-8, which is expressed in the basal plasma membrane of the excretory canals, but did not affect the expression of SULP-4, which is expressed in the apical membrane. The phenotype of arf-1.2 mutants was suppressed by mutation of the small Rho GTPase CDC-42, a regulator of apical/basal traffic balance. These results suggest that ARF-1.2 plays an essential role in basal membrane traffic to regulate the formation of the unicellular excretory tube.

Insane in the apical membrane: Trafficking events mediating apicobasal epithelial polarity during tube morphogenesis

The creation of cellular tubes is one of the most vital developmental processes, resulting in the formation of most organ types. Cells have co-opted a number of different mechanisms for tube morphogenesis that vary among tissues and organisms; however, generation and maintenance of cell polarity is fundamental for successful lumenogenesis. Polarized membrane transport has emerged as a key driver not only for establishing individual epithelial cell polarity, but also for coordination of epithelial polarization during apical lumen formation and tissue morphogenesis. In recent years, much work has been dedicated to identifying membrane trafficking regulators required for lumenogenesis. In this review we will summarize the findings from the past couple of decades in defining the molecular machinery governing lumenogenesis both in 3D tissue culture models and during organ development in vivo.

ELMOD1 Stimulates ARF6-GTP Hydrolysis to Stabilize Apical Structures in Developing Vestibular Hair Cells

Sensory hair cells require control of physical properties of their apical plasma membranes for normal development and function. Members of the ADP-ribosylation factor (ARF) small GTPase family regulate membrane trafficking and cytoskeletal assembly in many cells. We identified ELMO domain-containing protein 1 (ELMOD1), a guanine nucleoside triphosphatase activating protein (GAP) for ARF6, as the most highly enriched ARF regulator in hair cells. To characterize ELMOD1 control of trafficking, we analyzed mice of both sexes from a strain lacking functional ELMOD1 [roundabout (rda)]. In rda/rda mice, cuticular plates of utricle hair cells initially formed normally, then degenerated after postnatal day 5; large numbers of vesicles invaded the compromised cuticular plate. Hair bundles initially developed normally, but the cell's apical membrane lifted away from the cuticular plate, and stereocilia elongated and fused. Membrane trafficking in type I hair cells, measured by FM1-43 dye labeling, was altered in rda/rda mice. Consistent with the proposed GAP role for ELMOD1, the ARF6 GTP/GDP ratio was significantly elevated in rda/rda utricles compared with controls, and the level of ARF6-GTP was correlated with the severity of the rda/rda phenotype. These results suggest that conversion of ARF6 to its GDP-bound form is necessary for final stabilization of the hair bundle.SIGNIFICANCE STATEMENT Assembly of the mechanically sensitive hair bundle of sensory hair cells requires growth and reorganization of apical actin and membrane structures. Hair bundles and apical membranes in mice with mutations in the Elmod1 gene degenerate after formation, suggesting that the ELMOD1 protein stabilizes these structures. We show that ELMOD1 is a GTPase-activating protein in hair cells for the small GTP-binding protein ARF6, known to participate in actin assembly and membrane trafficking. We propose that conversion of ARF6 into the GDP-bound form in the apical domain of hair cells is essential for stabilizing apical actin structures like the hair bundle and ensuring that the apical membrane forms appropriately around the stereocilia.

Cellular and subcellular localization of ADP-ribosylation factor 6 in mouse peripheral tissues

ADP-ribosylation factor 6 (Arf6) is a small GTPase that regulates endosomal trafficking and actin cytoskeleton remodeling. In the present study, we comprehensively examined the cellular and subcellular localization of Arf6 in adult mouse peripheral tissues by immunofluorescence and immunoelectron microscopy using the heat-induced antigen retrieval method with Tris-EDTA buffer (pH 9.0). Marked immunolabeling of Arf6 was observed particularly in epithelial cells of several tissues including the esophagus, stomach, small and large intestines, trachea, kidney, epididymis, oviduct, and uterus. In most epithelial cells of simple or pseudostratified epithelia, Arf6 exhibited predominant localization to the basolateral membrane and a subpopulation of endosomes. At an electron microscopic level, Arf6 was localized along the basolateral membrane, with dense accumulation at interdigitating processes and infoldings. Arf6 was present in a ring-like appearance at intercellular bridges in spermatogonia and spermatocytes in the testis and at the Flemming body of cytokinetic somatic cells in the ovarian follicle, thymus, and spleen. The present study provides anatomical clues to help understand the physiological roles of Arf6 at the whole animal level.

EFA6 regulates lumen formation through alpha-actinin 1

A key step of epithelial morphogenesis is the creation of the lumen. Luminogenesis by hollowing proceeds through the fusion of apical vesicles at cell-cell contact. The small nascent lumens grow through extension, coalescence and enlargement coordinated with cell division to give rise to a single central lumen. Here, using MDCK cells grown in 3D-culture, we show that EFA6A participates in luminogenesis. EFA6A recruits α-actinin 1 (ACTN1) through direct binding. In polarized cells, ACTN1 was found to be enriched at the tight junction where it acts as a primary effector of EFA6A for normal luminogenesis. Both proteins are essential for the lumen extension and enlargement, where they mediate their effect by regulating the cortical acto-myosin contractility. Finally, ACTN1 was also found to act as an effector for the isoform EFA6B in the human mammary tumoral MCF7 cell line. EFA6B restored the glandular morphology of this tumoral cell line in an ACTN1-dependent manner. Thus, we identified new regulators of cyst luminogenesis essential for the proper maturation of a newly-formed lumen into a single central lumen.

Rab14 specifies the apical membrane through Arf6-mediated regulation of lipid domains and Cdc42

The generation of cell polarity is essential for the development of multi-cellular organisms as well as for the function of epithelial organs in the mature animal. Small GTPases regulate the establishment and maintenance of polarity through effects on cytoskeleton, membrane trafficking, and signaling. Using short-term 3-dimensional culture of MDCK cells, we find that the small GTPase Rab14 is required for apical membrane specification. Rab14 knockdown results in disruption of polarized lipid domains and failure of the Par/aPKC/Cdc42 polarity complex to localize to the apical membrane. These effects are mediated through tight control of lipid localization, as overexpression of the phosphatidylinositol 4-phosphate 5-kinase α [PtdIns(4)P5K] activator Arf6 or PtdIns(4)P5K alone, or treatment with the phosphatidylinositol 3-kinase (PtdInsI3K) inhibitor wortmannin, rescued the multiple-apical domain phenotype observed after Rab14 knockdown. Rab14 also co-immunoprecipitates and colocalizes with the small GTPase Cdc42, and Rab14 knockdown results in increased Cdc42 activity. Furthermore, Rab14 regulates trafficking of vesicles to the apical domain, mitotic spindle orientation, and midbody position, consistent with Rab14’s reported localization to the midbody as well as its effects upon Cdc42. These results position Rab14 at the top of a molecular cascade that regulates the establishment of cell polarity.

Endocytosis and Signal Transduction: Basic Science Update

Endocytosis can be separated into the categories of phagocytosis and pinocytosis. Phagocytosis can be distinguished from pinocytosis primarily by the size of particle ingested and by its dependence on actin polymerization as a key step in particle ingestion. Several specific forms of pinocytosis have been identified that can be distinguished based on their dependence on clathrin or caveolin. Both clathrin and caveolin-dependent pinocytosis appear to require the participation of dynamin to internalize the plasma membrane. Other, less well-characterized forms of pinocytosis have also been described. Although endocytosis has long been known to affect receptor density, recent studies have demonstrated that endocytosis through clathrin- and caveolin-dependent processes plays a key role in receptor-mediated signal transduction. In some cases, blockade of these processes attenuates, or even prevents, signal transduction from taking place. This information, coupled with a better understanding of endocytosis mechanisms, will help advance the field of cell biology as well as present new targets for drug development and disease treatment.

PERP, a host tetraspanning membrane protein, is required for Salmonella-induced inflammation

Salmonella enterica  Typhimurium induces intestinal inflammation through the activity of type III secreted effector (T3SE) proteins. Our prior results indicate that the secretion of the T3SE SipA and the ability of SipA to induce epithelial cell responses that lead to induction of polymorphonuclear transepithelial migration are not coupled to its direct delivery into epithelial cells from Salmonella. We therefore tested the hypothesis that SipA interacts with a membrane protein located at the apical surface of intestinal epithelial cells. Employing a split ubiquitin yeast‐two‐hybrid screen, we identified the tetraspanning membrane protein, p53 effector related to PMP‐22 (PERP), as a SipA binding partner. SipA and PERP appear to have intersecting activities as we found PERP to be involved in proinflammatory pathways shown to be regulated by SipA. In sum, our studies reveal a critical role for PERP in the pathogenesis of S. Typhimurium, and for the first time demonstrate that SipA, a T3SE protein, can engage a host protein at the epithelial surface.

Arf6 exchange factor EFA6 and endophilin directly interact at the plasma membrane to control clathrin-mediated endocytosis

Members of the Arf family of small G proteins are involved in membrane traffic and organelle structure. They control the recruitment of coat proteins, and modulate the structure of actin filaments and the lipid composition of membranes. The ADP-ribosylation factor 6 (Arf6) isoform and the exchange factor for Arf6 (EFA6) are known to regulate the endocytic pathway of many different receptors. To determine the molecular mechanism of the EFA6/Arf6 function in vesicular transport, we searched for new EFA6 partners. In a two-hybrid screening using the catalytic Sec7 domain as a bait, we identified endophilin as a new partner of EFA6. Endophilin contains a Bin/Amphiphysin/Rvs (BAR) domain responsible for membrane bending, and an SH3 domain responsible for the recruitment of dynamin and synaptojanin, two proteins involved, respectively, in the fission and uncoating of clathrin-coated vesicles. By using purified proteins, we confirmed the direct interaction, and identified the N-BAR domain as the binding motif to EFA6A. We showed that endophilin stimulates the catalytic activity of EFA6A on Arf6. In addition, we observed that the Sec7 domain competes with flat but not with highly curved lipid membranes to bind the N-BAR. In cells, expression of EFA6A recruits endophilin to EFA6A-positive plasma membrane ruffles, whereas expression of endophilin rescues the EFA6A-mediated inhibition of transferrin internalization. Overall, our results support a model whereby EFA6 recruits endophilin on flat areas of the plasma membrane to control Arf6 activation and clathrin-mediated endocytosis.

Polar delivery in plants; commonalities and differences to animal epithelial cells

Although plant and animal cells use a similar core mechanism to deliver proteins to the plasma membrane, their different lifestyle, body organization and specific cell structures resulted in the acquisition of regulatory mechanisms that vary in the two kingdoms. In particular, cell polarity regulators do not seem to be conserved, because genes encoding key components are absent in plant genomes. In plants, the broad knowledge on polarity derives from the study of auxin transporters, the PIN-FORMED proteins, in the model plant Arabidopsis thaliana. In animals, much information is provided from the study of polarity in epithelial cells that exhibit basolateral and luminal apical polarities, separated by tight junctions. In this review, we summarize the similarities and differences of the polarization mechanisms between plants and animals and survey the main genetic approaches that have been used to characterize new genes involved in polarity establishment in plants, including the frequently used forward and reverse genetics screens as well as a novel chemical genetics approach that is expected to overcome the limitation of classical genetics methods.

Arf6 regulation of Gyrating-clathrin

'Gyrating-' or 'G'-clathrin are coated endocytic structures located near peripheral sorting endosomes (SEs), which exhibit highly dynamic but localized movements when visualized by live-cell microscopy. They have been implicated in recycling of transferrin from the sorting endosome directly to the cell surface, but there is no information about their formation or regulation. We show here that G-clathrin comprise a minority of clathrin-coated structures in the cell periphery and are brefeldin A (BFA)-resistant. Arf6-GTP substantially increases G-clathrin levels, probably by lengthening coated bud lifetimes as suggested by photobleaching and photoactivation results, and an Arf6(Q67L)-GTP mutant bearing an internal GFP tag can be directly visualized in G-clathrin structures in live cells. Upon siRNA-mediated depletion of Arf6 or expression of Arf6(T27N), G-clathrin levels rise and are primarily Arf1-dependent, yet still BFA-resistant. However, BFA-sensitive increased G-clathrin levels are observed upon acute incubation with cytohesin inhibitor SecinH3, indicating a shift in GEF usage. Depletion of both Arf6 and Arf1 abolishes G-clathrin, and results in partial inhibition of fast transferrin recycling consistent with the latter's participation in this pathway. Collectively, these results demonstrate that the dynamics of G-clathrin primarily requires completion of the Arf6 guanine nucleotide cycle, but can be regulated by multiple Arf and GEF proteins, reflecting both overlapping mechanisms operative in their regulation and the complexity of processes involved in endosomal sorting.

Toxicity mechanisms of amphotericin B and its neutralization by conjugation with arabinogalactan

Amphotericin B (AMB) is an effective antifungal agent. However, its therapeutic use is hampered by its toxicity, mainly due to channel formation across kidney cell membranes and the disruption of postendocytic trafficking. We previously described a safe injectable AMB-arabinogalactan (AG) conjugate with neutralized toxicity. Here we studied the mechanism of the toxicity of free AMB and its neutralization by conjugation with AG. AMB treatment of a kidney cell line modulated the trafficking of three receptors (C-X-C chemokine receptor type 4 [CXCR4], M1 receptor, and human transferrin receptor [hTfnR]) due to an increase in endosomal pH. Similar data were also obtained in yeast but with an increase in vacuolar pH and the perturbation of Hxt2-green fluorescent protein (GFP) trafficking. The conjugation of AMB with AG neutralized all elements of the toxic activity of AMB in mammalian but not in fungal cells. Based on these results, we provide an explanation of how the conjugation of AMB with AG neutralizes its toxicity in mammalian cells and add to the knowledge of the mechanism of action of free AMB in both fungal and mammalian cells.

Arf6 negatively controls the rapid recycling of the β2 adrenergic receptor

β2-adrenergic receptor (β2AR), a member of the GPCR (G-protein coupled receptor) family, is internalized in a ligand- and β-arrestin-dependent manner into early endosomes, and subsequently recycled back to the plasma membrane. Here, we report that β-arrestin promotes the activation of the small G protein Arf6, which regulates the recycling and degradation of β2AR. We demonstrate in vitro that the C-terminal region of β-arrestin1 interacts directly and simultaneously with Arf6GDP and its specific exchange factor EFA6, to promote Arf6 activation. Similarly, the ligand-mediated activation of β2AR leads to the formation of Arf6GTP in vivo in a β-arrestin-dependent manner. Expression of either EFA6 or an activated Arf6 mutant caused accumulation of β2AR in the degradation pathway. This phenotype could be rescued by the expression of an activated mutant of Rab4, suggesting that Arf6 acts upstream of Rab4. We propose a model in which Arf6 plays an essential role in β2AR desensitization. The ligand-mediated stimulation of β2AR relocates β-arrestin to the plasma membrane, and triggers the activation of Arf6 by EFA6. The activation of Arf6 leads to accumulation of β2AR in the degradation pathway, and negatively controls Rab4-dependent fast recycling to prevent the re-sensitization of β2AR.

Arf6 regulates AP-1B-dependent sorting in polarized epithelial cells

The epithelial cell-specific clathrin adaptor complex AP-1B facilitates the sorting of various transmembrane proteins from recycling endosomes (REs) to the basolateral plasma membrane. Despite AP-1B's clear importance in polarized epithelial cells, we still do not fully understand how AP-1B orchestrates basolateral targeting. Here we identify the ADP-ribosylation factor 6 (Arf6) as an important regulator of AP-1B. We show that activated Arf6 pulled down AP-1B in vitro. Furthermore, interfering with Arf6 function through overexpression of dominant-active Arf6Q67L or dominant-negative Arf6D125N, as well as depletion of Arf6 with short hairpin RNA (shRNA), led to apical missorting of AP-1B-dependent cargos. In agreement with these data, we found that Arf6 colocalized with AP-1B and transferrin receptor (TfnR) in REs. In addition, we observed specific recruitment of AP-1B into Arf6-induced membrane ruffles in nonpolarized cells. We conclude that activated Arf6 directs membrane recruitment of AP-1B, thus regulating AP-1B's functions in polarized epithelial cells.

Actin dynamics counteract membrane tension during clathrin-mediated endocytosis

Clathrin-mediated endocytosis is independent of actin dynamics in many circumstances but requires actin polymerization in others. We show that membrane tension determines the actin dependence of clathrin-coat assembly. As found previously, clathrin assembly supports formation of mature coated pits in the absence of actin polymerization on both dorsal and ventral surfaces of non-polarized mammalian cells, and also on basolateral surfaces of polarized cells. Actin engagement is necessary, however, to complete membrane deformation into a coated pit on apical surfaces of polarized cells and, more generally, on the surface of any cell in which the plasma membrane is under tension from osmotic swelling or mechanical stretching. We use these observations to alter actin dependence experimentally and show that resistance of the membrane to propagation of the clathrin lattice determines the distinction between 'actin dependent and 'actin independent'. We also find that light-chain-bound Hip1R mediates actin engagement. These data thus provide a unifying explanation for the role of actin dynamics in coated-pit budding.

Fission yeast ucp3 gene encodes a putative Arf6 GTPase-activating protein

In fission yeast Schizosaccharomyces pombe, the directions of cell growth change from a monopolar manner to a bipolar manner, which is known as 'New End Take Off' (NETO). We previously found that Arf6, a member (class III) of the ADP-ribosylation factor GTPase (Arf) family, is necessary for NETO in fission yeast. Here we report the characterization of a S. pombe gene, ucp3, encoding a putative Arf GTPase-activating protein (GAP) for Arf6. The Ucp3 contains Arf GAP domain, and has a high similarity to Gts1, which was identified as a GAP for Arf3 (class III Arf) in Saccharomyces cerevisiae. Overexpression of ucp3 inhibited growth from new end possibly by disturbing the GDP/GTP-cycling of Arf6. Gene disruption of ucp3 revealed that Ucp3 is essential for cell viability. Ucp3 uniformly localizes to the cell periphery. And its localization is not dependent on microtubules, actin cytoskeletons, Arf6 and Syt22 (guanine nucleotide exchange factor for Arf6). We hypothesize that Ucp3 functions as a GAP for Arf6. Moreover, Ucp3 might have another function important for cell viability.

From cradle to twilight: the carboxyl terminus directs the fate of the A(2A)-adenosine receptor

The extended carboxyl terminus of the A(2A)-adenosine receptor is known to engage several proteins other than those canonically involved in signalling by GPCRs (i.e., G proteins, G protein-coupled receptor kinases/GRKs, arrestins). The list includes the deubiquinating enzyme USP4, α-actinin, the guanine nucleotide exchange factor for ARF6 ARNO, translin-X-associated protein, calmodulin, the neuronal calcium binding protein NECAB2 and the synapse associated protein SAP102. However, if the fate of the A(2A)-receptor is taken into account - from its birthplace in the endoplasmic reticulum to its presumed site of disposal in the lysosome, it is evident that many more proteins must interact with the A(2A)-adenosine receptor. There are several arguments that support the conjecture that these interactions will preferentially occur with the carboxyl terminus of the A(2A)-adeonsine receptor: (i) the extended carboxyl terminus (of 122 residues=) offers the required space to accommodate companions; (ii) analogies can be drawn with other receptors, which engage several of these binding partners with their C-termini. This approach allows for defining the nature of the unknown territory. As an example, we posit a chaperone/coat protein complex-II (COPII) exchange model that must occur on the carboxyl terminus of the receptor. This model accounts for the observation that a minimum size of the C-terminus is required for correct folding of the receptor. It also precludes premature recruitment of the COPII-coat to a partially folded receptor.

alpha-Synuclein abnormalities in mouse models of peroxisome biogenesis disorders

alpha-Synuclein (alphaS) is a presynaptic protein implicated in Parkinson's disease (PD). Growing evidence implicates mitochondrial dysfunction, oxidative stress, and alphaS-lipid interactions in the gradual accumulation of alphaS in pathogenic forms and its deposition in Lewy bodies, the pathological hallmark of PD and related synucleinopathies. The peroxisomal biogenesis disorders (PBD), with Zellweger syndrome serving as the prototype of this group, are characterized by malformed and functionally impaired peroxisomes. Here we utilized the PBD mouse models Pex2-/-, Pex5-/-, and Pex13-/- to study the potential effects of peroxisomal dysfunction on alphaS-related pathogenesis. We found increased alphaS oligomerization and phosphorylation and its increased deposition in cytoplasmic inclusions in these PBD mouse models. Furthermore, we show that alphaS abnormalities correlate with the altered lipid metabolism and, specifically, with accumulation of long chain, n-6 polyunsaturated fatty acids that occurs in the PBD models.

Bardet-Biedl syndrome-associated small GTPase ARL6 (BBS3) functions at or near the ciliary gate and modulates Wnt signaling

The expansive family of metazoan ADP-ribosylation factor and ADP-ribosylation factor-like small GTPases is known to play essential roles in modulating membrane trafficking and cytoskeletal functions. Here, we present the crystal structure of ARL6, mutations in which cause Bardet-Biedl syndrome (BBS3), and reveal its unique ring-like localization at the distal end of basal bodies, in proximity to the so-called ciliary gate where vesicles carrying ciliary cargo fuse with the membrane. Overproduction of GDP- or GTP-locked variants of ARL6/BBS3 in vivo influences primary cilium length and abundance. ARL6/BBS3 also modulates Wnt signaling, a signal transduction pathway whose association with cilia in vertebrates is just emerging. Importantly, this signaling function is lost in ARL6 variants containing BBS-associated point mutations. By determining the structure of GTP-bound ARL6/BBS3, coupled with functional assays, we provide a mechanistic explanation for such pathogenic alterations, namely altered nucleotide binding. Our findings therefore establish a previously unknown role for ARL6/BBS3 in mammalian ciliary (dis)assembly and Wnt signaling and provide the first structural information for a BBS protein.

Adenosine diphosphate-ribosylation factor 6 is required for epidermal growth factor-induced glioblastoma cell proliferation

BACKGROUND

: Epidermal growth factor (EGF) signaling plays a pivotal role in gliomagenesis. The authors previously demonstrated that adenosine diphospate-ribosylation factor 6 (ARF6), a member of the Ras-related small guanosine-5'-triphospate-binding protein family, is required for EFA6A-induced glioma cell migration and invasion. However, the role of ARF6 in EGF signaling is unknown.

METHODS

: The authors analyzed messenger RNA (mRNA) levels of ARF6 and EGF receptor (EGFR) in 16 high-grade glioma samples and in 6 low-grade glioma samples by reverse transcriptase-polymerase chain reaction analysis. To determine whether EGF induces ARF6 expression in human glioblastoma U87 cells through transcriptional regulation and EGFR activation, the levels of ARF6 were assayed in EGF-treated U87 cells that were preincubated with a transcriptional inhibitor (actinomycin D) and an EGFR tyrosine kinase inhibitor (PD153035), respectively. The downstream signaling of EGFR-mediated ARF6 up-regulation also was investigated using specific inhibitors of mitogen-activated protein kinase (MEK), phosphatidylinositol 3' kinase (PI3K), and Janus kinase 2. The involvement of SP1 in the downstream signaling was studied by using an SP1 inhibitor (mithramycin A). Small-interfering RNAs (siRNAs) targeting ARF6 were used to investigate the effects of ARF6 on EGF-mediated glioma cell proliferation.

RESULTS

: The results demonstrated that ARF6 and EGFR mRNA levels were elevated in glioma tissues. Furthermore, EGF stimulated ARF6 expression in U87 cells in a dose-dependent and time-dependant manner. This stimulation was caused by increased transcription of ARF6 and by activation of the MEK/extracellular signal-regulated kinase 1 and 2 (ERK1/2) and PI3K signaling pathways. It is noteworthy that SP1 was essential for EGF-induced ARF6 up-regulation. Finally, EGF-induced glioblastoma cell proliferation depended on ARF6, because the suppression of ARF6 by siRNA or by a dominant-negative mutant significantly inhibited EGF-induced cell proliferation.

CONCLUSIONS

: The results of the current study suggested that EGF-induced ARF6 expression plays a significant role in glioma cell proliferation. Cancer 2009. (c) 2009 American Cancer Society.

Potassium channel silencing by constitutive endocytosis and intracellular sequestration

Tandem of P domains in a weak inwardly rectifying K(+) channel 1 (TWIK1) is a K(+) channel that produces unusually low levels of current. Replacement of lysine 274 by a glutamic acid (K274E) is associated with stronger currents. This mutation would prevent conjugation of a small ubiquitin modifier peptide to Lys-274, a mechanism proposed to be responsible for channel silencing. However, we found no biochemical evidence of TWIK1 sumoylation, and we showed that the conservative change K274R did not increase current, suggesting that K274E modifies TWIK1 gating through a charge effect. Now we rule out an eventual effect of K274E on TWIK1 trafficking, and we provide convincing evidence that TWIK1 silencing results from its rapid retrieval from the cell surface. TWIK1 is internalized via a dynamin-dependent mechanism and addressed to the recycling endosomal compartment. Mutation of a diisoleucine repeat located in its cytoplasmic C terminus (I293A,I294A) stabilizes TWIK1 at the plasma membrane, resulting in robust currents. The effects of I293A,I294A on channel trafficking and of K274E on channel activity are cumulative, promoting even more currents. Activation of serotoninergic receptor 5-HT(1)R or adrenoreceptor alpha2A-AR stimulates TWIK1 but has no effect on TWIK1I293A,I294A, suggesting that G(i) protein activation is a physiological signal for increasing the number of active channels at the plasma membrane.

Fission yeast syt22 protein, a putative Arf guanine nucleotide exchange factor, is necessary for new end take off

In fission yeast Schizosaccharomyces pombe, the directions of cell growth change from monopolar to bipolar in character, which is known as 'new end take off ' (NETO). We previously found that arf6p, a member (class III) of the ADP-ribosylation factor (Arf) family, is necessary for NETO in fission yeast. Here we report the characterization of an S. pombe gene, syt22(+), encoding a putative Arf guanine nucleotide exchange factor (GEF). The syt22 protein contains a Sec7 domain and a PH domain conserved in the mammalian EFA6 GEF family, and has high similarity to Yel1p, which was identified as a GEF for Arf3p (class III Arf) in Saccharomyces cerevisiae. syt22Delta cells, like arf6Delta cells, completely failed to undergo NETO. Syt22p uniformly localizes to the cell periphery. Its localization is not dependent on microtubules, actin cytoskeletons or arf6p. We hypothesize that syt22p functions as a GEF for arf6p.

Dual acylation is required for trafficking of growth-associated protein-43 (GAP-43) to endosomal recycling compartment via an Arf6-associated endocytic vesicular pathway

GAP-43 (growth-associated protein-43) is a dually palmitoylated protein, at cysteine residues at positions 3 and 4, that mostly localizes in plasma membrane both in neural and non-neural cells. In the present study, we have examined membrane association, subcellular distribution and intracellular trafficking of GAP-43 in CHO (Chinese hamster ovary)-K1 cells. Using biochemical assays and confocal and video microscopy in living cells we demonstrated that GAP-43, at steady state, localizes at the recycling endosome in addition to the cytoplasmic leaflet of the plasma membrane and TGN (trans-Golgi network). Pharmacological inhibition of newly synthesized GAP-43 acylation or double mutation of Cys3 and Cys4 of GAP-43 completely disrupts TGN, plasma membrane and recycling endosome association. A combination of selective photobleaching techniques and time-lapse fluorescence microscopy reveals a dynamic association of GAP-43 with recycling endosomes in equilibrium with the plasma membrane pool. Newly synthesized GAP-43 is found mainly associated with the TGN, but not with the pericentriolar recycling endosome, and traffics to the plasma membrane by a brefeldin A-insensitive pathway. Impairment of plasma membrane fusion and internalization by treatment with tannic acid does affect the trafficking of GAP-43 from plasma membrane to recycling endosomes which reveals a vesicle-mediated retrograde trafficking of GAP-43. Here, we also show that internalization of GAP-43 is regulated by Arf (ADP-ribosylation factor) 6. Taken together, these results demonstrate that dual acylation is required for sorting of peripheral membrane-associated GAP-43 to recycling endosome via an Arf6-associated endocytic vesicular pathway.

Dynamin participates in the maintenance of anterior polarity in the Caenorhabditis elegans embryo

Cell polarity is crucial for the generation of cell diversity. Recent evidence suggests that the actin cytoskeleton plays a key role in establishment of embryonic polarity, yet the mechanisms that maintain polarity cues in particular membrane domains during development remain unclear. Dynamin, a large GTPase, functions in both endocytosis and actin dynamics. Here, the Caenorhabditis elegans dynamin ortholog, DYN-1, maintains anterior polarity cues. DYN-1-GFP foci are enriched in the anterior cortex in a manner dependent on the anterior polarity proteins, PAR-6 and PKC-3. Membrane internalization and actin comet formation are enriched in the anterior, and are dependent on DYN-1. PAR-6-labeled puncta are also internalized from cortical accumulations of DYN-1-GFP. Our results demonstrate a mechanism for the spatial and temporal regulation of endocytosis in the anterior of the embryo, contributing to the precise localization and maintenance of polarity factors within a dynamic plasma membrane.

ADP-ribosylation factors regulate the development of CT signaling in immature human enterocytes

Diarrheal disease is a major cause of morbidity and mortality in infants and children worldwide. Evidence suggests that the interaction of immature human enterocytes with bacteria and their enterotoxins may account for the increased susceptibility of neonates to diarrheal diseases. However, the precise mechanisms that contribute to the excessive response to cholera toxin by the immature gut are largely unknown. Our aim was to characterize the cellular/molecular changes in Gs(alpha) during gut development. In this study, a colonic human epithelial cell line (T84) was used as representative of a mature enterocyte and a human fetal primary small intestinal cell line (H4) as representative of an immature enterocyte. Using our cell culture model of human intestinal development, we provide consistent evidence that cholera toxin (CT)-mediated Gs(alpha) activation in fetal enterocytes differs from that of mature enterocytes, and the difference may be related to ADP-ribosylation factor (ARF) interaction with the CT-signaling process. Here we demonstrated that ARF1 may play a critical role in clathrin-mediated CT trafficking through the endoplasmic reticulum and Golgi and that ARF6 may facilitate clathrin-mediated CT endocytosis that leads to enhanced Gs(alpha) activation by CT. Collectively, these findings support our hypothesis that there is a developmentally regulated intestinal cellular response to bacterial exotoxins involving complex cellular events that accounts for the increased incidence and severity of toxogenic diarrhea during infancy.

The adaptor complex AP-2 regulates post-endocytic trafficking through the non-clathrin Arf6-dependent endocytic pathway

The ADP-ribosylation factor 6 (Arf6) GTPase functions as a key regulator of endocytic trafficking, participating in clathrin-independent endocytosis in most cell types. Unexpectedly, we found that siRNA-mediated depletion of clathrin or of adaptor protein 2 (AP-2)-complex subunits alters trafficking of Arf6 pathway cargo proteins, such as major histocompatibility complex class I (MHCI) and beta1 integrin. Internalization of these cargoes from the plasma membrane was not affected in cells depleted of clathrin, but was modestly delayed in cells lacking AP-2. Furthermore, depletion of clathrin or AP-2 altered the intracellular distribution of MHCI and beta1 integrin, inducing clustering in a perinuclear region. Despite this altered localization in both depleted populations, enhanced lysosomal targeting of MHCI was observed uniquely in cells that lack AP-2. Total levels of MHCI were modestly but consistently reduced in AP-2-depleted cells, and restored by the lysosomal inhibitor bafilomycin A. Furthermore, the half-life of surface-derived MHCI was reduced in AP-2-depleted cells. Consistent with enhanced degradative sorting, colocalization of Arf6 cargo with the late endosome and lysosome markers CD63 and Lamp1 was increased in cells depleted of AP-2 but not clathrin. These studies indicate a role for AP-2 in maintaining normal post-endocytic trafficking through the Arf6-regulated, non-clathrin pathway, and reveal pervasive effects of clathrin and AP-2 depletion on the endosomal and lysosomal system.

Afi1p functions as an Arf3p polarization-specific docking factor for development of polarity

ADP-ribosylation factors (Arfs) are highly conserved small GTPases and are critical components of vesicle trafficking. Yeast Arf3p, despite its similarity to mammalian Arf6, is not required for endocytosis but is involved in polarity development. In this study, we identified an Arf3p interacting protein 1 (Afi1p), which, through its N-terminal conserved region, specifically interacts with GTP-bound Arf3p. Afi1p is distributed asymmetrically at the plasma membrane and is required for polarized distribution of Arf3p but not of an Arf3p guanine nucleotide-exchange factor, Yel1p. However, Afi1p is not required for targeting of Arf3p or Yel1p to the plasma membrane. Like arf3 mutant yeast, afi1 mutant yeast exhibited an abnormal budding pattern and partially delayed actin patch polarization. An Afi1p, (38)KLGP4A-Afi1p, mutated at the Arf3p-binding region, loses its ability to interact with Arf3p and maintain the polarized distribution of Arf3p. Although (38)KLGP4A-Afi1p still possessed a proper polarized distribution, it lost its ability to rescue actin patch polarization in afi1 mutant cells. Our findings demonstrate that Afi1p functions as an Arf3p polarization-specific adapter and participates in development of polarity.

Structural basis and mechanism of autoregulation in 3-phosphoinositide-dependent Grp1 family Arf GTPase exchange factors

Arf GTPases regulate membrane trafficking and actin dynamics. Grp1, ARNO, and Cytohesin-1 comprise a family of phosphoinositide-dependent Arf GTPase exchange factors with a Sec7-pleckstrin homology (PH) domain tandem. Here, we report that the exchange activity of the Sec7 domain is potently autoinhibited by conserved elements proximal to the PH domain. The crystal structure of the Grp1 Sec7-PH tandem reveals a pseudosubstrate mechanism of autoinhibition in which the linker region between domains and a C-terminal amphipathic helix physically block the docking sites for the switch regions of Arf GTPases. Mutations within either element result in partial or complete activation. Critical determinants of autoinhibition also contribute to insulin-stimulated plasma membrane recruitment. Autoinhibition can be largely reversed by binding of active Arf6 to Grp1 and by phosphorylation of tandem PKC sites in Cytohesin-1. These observations suggest that Grp1 family GEFs are autoregulated by mechanisms that depend on plasma membrane recruitment for activation.

Arf GAPs as regulators of the actin cytoskeleton

The Arf (ADP-ribosylation factor) GAPs (GTPase-activating proteins) are a family of proteins with a common catalytic domain that induces hydrolysis of GTP bound to Arf GTP-binding proteins. At least three groups of multidomain Arf GAPs affect the actin cytoskeleton and cellular activities, such as migration and movement, that depend on the cytoskeleton. One role of the Arf GAPs is to regulate membrane remodelling that accompanies actin polymerization. Regulation of membrane remodelling is mediated in part by the regulation of Arf proteins. However, Arf GAPs also regulate actin independently of effects on membranes or Arf. These functions include acting as upstream regulators of Rho family proteins and providing a scaffold for Rho effectors and exchange factors. With multiple functional elements, the Arf GAPs could integrate signals and biochemical activities that result in co-ordinated changes in actin and membranes necessary for a wide range of cellular functions.

Raft-dependent Endocytosis of Autocrine Motility Factor Is Phosphatidylinositol 3-Kinase-dependent in Breast Carcinoma Cells

Autocrine motility factor (AMF) is internalized via a receptor-mediated, dynamin-dependent, cholesterol-sensitive raft pathway to the smooth endoplasmic reticulum that is negatively regulated by caveolin-1. Expression of AMF and its receptor (AMFR) is associated with tumor progression and malignancy; however, the extent to which the raft-dependent uptake of AMF is tumor cell-specific has yet to be addressed. By Western blot and cell surface fluorescence-activated cell sorter (FACS) analysis, AMFR expression is increased in tumorigenic MCF7 and metastatic MDA-231 and MDA-435 breast cancer cell lines relative to dysplastic MCF10A mammary epithelial cells. AMF uptake, determined by FACS measurement of protease-insensitive internalized fluorescein-conjugated AMF, was increased in MCF7 and MDA-435 cells relative to MCF-10A and caveolin-1-expressing MDA-231 cells. Uptake of fluorescein-conjugated AMF was dynamin-dependent, methyl-beta-cyclodextrin- and genistein-sensitive, reduced upon overexpression of caveolin-1 in MDA-435 cells, and increased upon short hairpin RNA reduction of caveolin-1 in MDA-231 cells. Tissue microarray analysis of invasive primary human breast carcinomas showed that AMFR expression had no impact on survival but did correlate significantly with expression of phospho-Akt. Phospho-Akt expression was increased in AMF-internalizing MCF7 and MDA-435 breast carcinoma cells. AMF uptake in these cells was reduced by phosphatidylinositol 3-kinase inhibition but not by regulators of macropinocytosis such as amiloride, phorbol ester, or actin cytoskeleton disruption by cytochalasin D. The raft-dependent endocytosis of AMF therefore follows a distinct phosphatidylinositol 3-kinase-dependent pathway that is up-regulated in more aggressive tumor cells.

A Role for ARF6 and ARNO in the regulation of endosomal dynamics in neurons

During development, neuronal processes extend, branch and navigate to ultimately synapse with target tissue. We have shown a regulatory role for ARNO and ARF6 in dendritic branching and axonal elongation and branching during neuritogenesis, particularly with respect to cytoskeletal dynamics. Here, we have examined the role of ARF6 and the ARF GEF ARNO in endosomal dynamics during neurite elongation in hippocampal neurons. Axonal and dendritic endosomes were labeled by expression of the endosomal marker, endotubin. Expression of endotubin-green fluorescent protein resulted in targeting to tubular-vesicular structures throughout the somatodendritic and axonal domains. These endosomal structures did not colocalize with conventional early or late endosomal markers or with the synaptic vesicle marker, SV2. However, they did label with internalized lectin, indicating that they are endosomal structures. Expression of catalytically inactive ARNO (ARNO-E156K) or inactive ARF6 (ARF6-T27N) caused a redistribution of endotubin to the cell surface of the axons and dendrites. In contrast, expression of these constructs had no effect upon the distribution of SV2-positive structures. Furthermore, expression of inactive ARF1 (ARF1-T31N) did not change endotubin distribution. These results suggest that endotubin labels a distinct endosomal structure in neurons and that ARNO and ARF6 mediate neurite extension through the regulation of this compartment.

Beta-tubulin cofactor D and ARL2 take part in apical junctional complex disassembly and abrogate epithelial structure

In epithelial cells, the apical junctional complex (AJC), composed of tight junctions (TJs) and adherens junctions (AJs), maintains cell-surface polarity by forming a fence that prevents lateral movement and diffusion of proteins and lipids between the apical and basolateral PM and holds the epithelial monolayer intact through cell-cell contacts. Disassembly of this complex is a prime event in development and cell transformation. Maintenance of the AJC has been shown to involve mainly the actin cytoskeleton. Recent findings also point to the involvement of the microtubule (MT) system. Here we show the first evidence that in polarized epithelial MDCK cells, ARF-like protein 2 (ARL2) and beta-tubulin cofactor D, known to be involved in MT dynamics, have a role in disassembly of the AJC followed by cell dissociation from the epithelial monolayer, which is not dependent on MT depolymerization. In addition, we show that beta-tubulin cofactor D is partially localized to the lateral PM through its 15 C-terminal amino acids and intact MTs. ARL2 inhibited beta-tubulin cofactor D-dependent cell dissociation from the monolayer and AJC disassembly. To our knowledge, this is the first evidence that beta-tubulin cofactor D plays a role in cells independent of its presumed role in folding tubulin heterodimers. We conclude that ARL2 and beta-tubulin cofactor D participate in AJC disassembly and epithelial depolarization.

Does Sumoylation Control K2P1/TWIK1 Background K+ Channels?

A novel model for the regulation of cell excitability has recently been proposed. It originates from the observation that the background K(+) channel K2P1 (TWIK1) may be silenced by sumoylation in Xenopus oocytes and that inactivation of the putative sumoylation site (mutation K274E) gives rise to robust current expression in transfected COS-7 cells. Here, we show that only the mutation K274E, and not K274R, is associated with an increase of K2P1 current density, suggesting a charge effect of K274E. Furthermore, we failed to observe any band shift by western blot analysis that would confirm an eventual sumoylation of K2P1 in COS-7 cells and oocytes.

Identification of the suppressive factors for human immunodeficiency virus type-1 replication using the siRNA mini-library directed against host cellular genes

We performed the screening to find the novel host factors affecting human immunodeficiency virus type-1 (HIV-1) replication using the siRNA mini-library consisted with 257 siRNAs directed against cellular genes. J111 cells, a human acute monocytic leukemia cell line, were transfected with individual siRNA, followed by either infected or transfected with the HIV-1 molecular clone with luciferase reporter gene in 96-well plate format. The results showed that six siRNAs significantly enhanced the HIV-1 replication in J111 cells, indicating that the target cellular genes of those siRNAs may negatively regulate HIV-1 replication in normal cell culture condition. We also discuss the possible mechanisms by which those cellular proteins regulate viral replication.

Caveolin 2 regulates endocytosis and trafficking of the M1 muscarinic receptor in MDCK epithelial cells

Clathrin and caveolins are known for their involvement in the internalization of numerous receptors. Here we show that in polarized epithelial Madin-Darby canine kidney cells, both the clathrin machinery and caveolins are involved in the endocytosis and delivery to the plasma membrane (PM) of the M1 muscarinic acetylcholine receptor (mAChR). We initially localized this receptor to the lateral membrane, where it accumulates proximal to the tight junctions. From there it is internalized through the clathrin-mediated pathway. In addition, the receptor may associate on the PM with caveolin (cav) 2 or in intracellular compartments with either cav 2, or monomeric or oligomeric cav 1. Association of the PM M1 mAChR with cav 2 inhibits receptor endocytosis through the clathrin-mediated pathway or retains the receptor in an intracellular compartment. This intracellular association attenuates receptor trafficking. Expression of cav 1 with cav 2 rescues the latter's inhibitory effect. The caveolins stimulate M1 mAChR oligomerization thus maintaining a constant amount of monomeric receptor. These results provide evidence that caveolins play a role in the attenuation of the M1 muscarinic receptor's intracellular trafficking to and from the PM.

EPI64 regulates microvillar subdomains and structure

EPI64 is a TBC domain–containing protein that binds the PDZ domains of EBP50, which binds ezrin, a major actin-binding protein of microvilli. High-resolution light microscopy revealed that ezrin and EBP50 localize exclusively to the membrane-surrounded region of microvilli, whereas EPI64 localizes to variable regions in the structures. Overexpressing EPI64 results in its and EBP50's relocalization to the base of microvilli, including to the actin rootlet devoid of ezrin or plasma membrane. Uncoupling EPI64's binding to EBP50, expression of any construct mislocalizing its TBC domain, or knock down of EBP50 results in loss of microvilli. The TBC domain of EPI64 binds directly to Arf6-GTP. Overexpressing the TBC domain increases Arf6-GTP levels, and expressing dominant-active Arf6 results in microvillar loss. These data reveal that microvilli have distinct cytoskeletal subdomains and that EPI64 regulates microvillar structure.

Targeting of nanoparticles to the clathrin-mediated endocytic pathway

Nanoparticles (NPs) are considered attractive carriers for gene therapy and drug delivery owing to their minor toxic effect and their ability to associate and internalize into mammalian cells. In this study, we compared the endocytosis into HeLa cells of NPs exposing either a negative or positive charge on their surface. The exposed charge significantly affected their ability to internalize as well as the cellular endocytosis mechanism utilized. Negatively charged NPs show an inferior rate of endocytosis and do not utilize the clathrin-mediated endocytosis pathway. On the other hand, positively charged NPs internalize rapidly via the clathrin-mediated pathway. When this pathway is blocked, NPs activate a compensatory endocytosis pathway that results in even higher accumulation of NPs. Overall, the addition of a positive charge to NPs may improve their potential as nanoparticulate carriers for drug delivery.

EFA6 regulates endosomal trafficking and affects early endosomes in polarized MDCK cells

The small-GTPase family of ADP ribosylation factors (ARFs) recruit coat proteins to promote vesicle budding. ARFs are activated by an association with sec7-containing exchange factors which load them with GTP. In epithelial cells, the small GTPase ARF6 operates within the endocytic system and has been shown to associate with ARNO to promote apical endocytosis and early to late endosomal trafficking. EFA6 has been shown to stimulate tight-junction formation and maintenance. Here, we show that in polarized epithelial MDCK cells, EFA6 is localized to early endosomes, causes their dramatic enlargement, and promotes basolateral targeting of IgA, which is normally targeted to the apical PM. These results suggest that the physiological function of ARF6 within the endocytic system is regulated by the exchange factor it associates with.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

GEP100/BRAG2: activator of ADP-ribosylation factor 6 for regulation of cell adhesion and actin cytoskeleton via E-cadherin and alpha-catenin

GEP(100) (p100) was identified as an ADP-ribosylation factor (ARF) guanine nucleotide-exchange protein (GEP) that partially colocalized with ARF6 in the cell periphery. p100 preferentially accelerated guanosine 5[gamma-thio]triphosphate (GTPgammaS) binding by ARF6, which participates in protein trafficking near the plasma membrane, including receptor recycling, cell adhesion, and cell migration. Here we report that yeast two-hybrid screening of a human fetal brain cDNA library using p100 as bait revealed specific interaction with alpha-catenin, which is known as a regulator of adherens junctions and actin cytoskeleton remodeling. Interaction of p100 with alpha-catenin was confirmed by coimmunoprecipitation of the endogenous proteins from human HepG2 or CaSki cells, although colocalization was difficult to demonstrate microscopically. alpha-Catenin enhanced GTPgammaS binding by ARF6 in vitro in the presence of p100. Depletion of p100 by small interfering RNA (siRNA) treatment in HepG2 cells resulted in E-cadherin content 3-fold that in control cells and blocked hepatocyte growth factor-induced redistribution of E-cadherin, consistent with a known role of ARF6 in this process. F-actin was markedly decreased in normal rat kidney (NRK) cells overexpressing wild-type p100, but not its GEP-inactive mutants, also consistent with the conclusion that p100 has an important role in the activation of ARF6 for its functions in both E-cadherin recycling and actin remodeling.

ARF proteins: roles in membrane traffic and beyond

The ADP-ribosylation factor (ARF) small GTPases regulate vesicular traffic and organelle structure by recruiting coat proteins, regulating phospholipid metabolism and modulating the structure of actin at membrane surfaces. Recent advances in our understanding of the signalling pathways that are regulated by ARF1 and ARF6, two of the best characterized ARF proteins, provide a molecular context for ARF protein function in fundamental biological processes, such as secretion, endocytosis, phagocytosis, cytokinesis, cell adhesion and tumour-cell invasion.

ADP-ribosylation factor 6 regulation of phosphatidylinositol-4,5-bisphosphate synthesis, endocytosis, and exocytosis

Unlike other members of the ADP-ribosylation factor (ARF) family, Arf6 is localized to the plasma membrane and endosomes, and regulates membrane traffic from and into the plasma membrane. Arf6 regulates a clathrin-independent endocytic membrane recycling pathway in nonpolarized cells and clathrin-dependent endocytosis in polarized cells. It also regulates recycling endosome traffic back to the plasma membrane as well as dense-core vesicle exocytosis in neuroendocrine cells. A key effector for Arf6 is phosphatidylinositol 4-monophosphate 5-kinase, which catalyzes plasma membrane synthesis of phosphatidylinositol-4,5-bisphosphate (PIP2), a common required cofactor for several endocytic and exocytic membrane trafficking pathways. Long-term expression of a constitutively active Arf6 mutant in cells can lead to the depletion of PIP2 from the plasma membrane, its accumulation in intracellular vacuoles, and the inhibition of PIP2-dependent membrane trafficking at the plasma membrane.

Role of the Arf6 GDP/GTP cycle and Arf6 GTPase-activating proteins in actin remodeling and intracellular transport

We have analyzed both biochemically and functionally a series of Arf6 mutants, providing new insights into the molecular mode of action of the small G protein Arf6. First, by comparing a fast-cycling mutant (Arf6(T157N)) and a GTPase-deficient mutant (Arf6(Q67L)), we established the necessity for completion of the Arf6 GDP/GTP cycle for recycling of major histocompatibility complex molecules to the plasma membrane. Second, we found that aluminum fluoride (AlF), known for inducing membrane protrusion in cells expressing exogenous wild-type Arf6, stabilized a functional wild-type Arf6.AlF(x) . GTPase-activating protein (GAP) complex in vitro and in vivo. We also found that the tandem mutation Q37E/S38I prevented the binding of two Arf GAPs, but not the effector ARHGAP10, and blocked the formation of membrane protrusion and actin reorganization. Together, our results with AlF(x) and Arf6(Q37E/S38I) demonstrate the critical role of the Arf6 GAPs as effectors for Arf6-regulated actin cytoskeleton remodeling. Finally, competition experiments conducted in vivo suggest the existence of a membrane receptor for GDP-bound Arf6.