A regulatory role for ARF6 in receptor-mediated endocytosis

ARF6 Is an Actionable Node that Orchestrates Oncogenic GNAQ Signaling in Uveal Melanoma

Activating mutations in Gαq proteins, which form the α subunit of certain heterotrimeric G proteins, drive uveal melanoma oncogenesis by triggering multiple downstream signaling pathways, including PLC/PKC, Rho/Rac, and YAP. Here we show that the small GTPase ARF6 acts as a proximal node of oncogenic Gαq signaling to induce all of these downstream pathways as well as β-catenin signaling. ARF6 activates these diverse pathways through a common mechanism: the trafficking of GNAQ and β-catenin from the plasma membrane to cytoplasmic vesicles and the nucleus, respectively. Blocking ARF6 with a small-molecule inhibitor reduces uveal melanoma cell proliferation and tumorigenesis in a mouse model, confirming the functional relevance of this pathway and suggesting a therapeutic strategy for Gα-mediated diseases.

Molecular cloning and characterization of an ADP-ribosylation factor 6 gene (ptARF6) from Pisolithus tinctorius

ADP-ribosylation factor 6 (ARF6) is an evolutionarily conserved molecule that has an essential function in intracellular trafficking and organelle structure. To better understand its role during presymbiosis between plant roots and compatible filamentous fungi, the full-length cDNA sequence of ARF6 from Pisolithus tinctorius was cloned and a variety of bioinformatics analyses performed. The full-length sequence was 849 bp long and contained a 549 bp open reading frame encoding a protein of 182 amino acids. A phylogenetic analysis showed that ptARF6 was the ortholog of the ADP ribosylation factor 6/GTPase SAR1 gene from the white-rot basidiomycete Trametes versicolor. A domain architecture analysis of the ARF6 protein revealed a repeat region, which is a common feature of ARF6 in other species. Recombinant ARF6 protein was expressed with an N-terminal 6×His tag and purified using Ni(2+)-NTA affinity chromatography. The molecular mass of the recombinant protein was estimated by SDS-PAGE to be 25 kDa. The recombinant ARF6 protein bound strongly to 18:1 and 18:2 phosphatidic acids. Thus, ARF6 may participate in the signaling pathways involved in membrane phospholipid composition. The intracellular distribution of ptADP6 in HEK239T cells also indicates that ptADP6 may function not only in plasma membrane events but also in endosomal membranes events. Real-time quantitative PCR revealed that the differential expression of ptARF6 was associated with the presymbiotic stage. ptARF6 may be induced by presymbiosis during the regulation of mycorrhizal formation.

Which Way In? The RalF Arf-GEF Orchestrates Rickettsia Host Cell Invasion

Bacterial Sec7-domain-containing proteins (RalF) are known only from species of Legionella and Rickettsia, which have facultative and obligate intracellular lifestyles, respectively. L. pneumophila RalF, a type IV secretion system (T4SS) effector, is a guanine nucleotide exchange factor (GEF) of ADP-ribosylation factors (Arfs), activating and recruiting host Arf1 to the Legionella-containing vacuole. In contrast, previous in vitro studies showed R. prowazekii (Typhus Group) RalF is a functional Arf-GEF that localizes to the host plasma membrane and interacts with the actin cytoskeleton via a unique C-terminal domain. As RalF is differentially encoded across Rickettsia species (e.g., pseudogenized in all Spotted Fever Group species), it may function in lineage-specific biology and pathogenicity. Herein, we demonstrate RalF of R. typhi (Typhus Group) interacts with the Rickettsia T4SS coupling protein (RvhD4) via its proximal C-terminal sequence. RalF is expressed early during infection, with its inactivation via antibody blocking significantly reducing R. typhi host cell invasion. For R. typhi and R. felis (Transitional Group), RalF ectopic expression revealed subcellular localization with the host plasma membrane and actin cytoskeleton. Remarkably, R. bellii (Ancestral Group) RalF showed perinuclear localization reminiscent of ectopically expressed Legionella RalF, for which it shares several structural features. For R. typhi, RalF co-localization with Arf6 and PI(4,5)P₂ at entry foci on the host plasma membrane was determined to be critical for invasion. Thus, we propose recruitment of PI(4,5)P₂ at entry foci, mediated by RalF activation of Arf6, initiates actin remodeling and ultimately facilitates bacterial invasion. Collectively, our characterization of RalF as an invasin suggests that, despite carrying a similar Arf-GEF unknown from other bacteria, different intracellular lifestyles across Rickettsia and Legionella species have driven divergent roles for RalF during infection. Furthermore, our identification of lineage-specific Arf-GEF utilization across some rickettsial species illustrates different pathogenicity factors that define diverse agents of rickettsial diseases.

Phylogenomics analysis indicates divergent mechanisms for host cell invasion across diverse species of obligate intracellular Rickettsia. For instance, only some Rickettsia species carry RalF, the rare bacterial Arf-GEF effector utilized by Legionella pneumophila to facilitate fusion of ER-derived membranes with its host-derived vacuole. For R. prowazekii (Typhus Group, TG), prior in vitro studies suggested the Arf-GEF activity of RalF, which is absent from Spotted Fever Group species, might be spatially regulated at the host plasma membrane. Herein, we demonstrate RalF of R. typhi (TG) and R. felis (Transitional Group) localizes to the host plasma membrane, yet R. bellii (Ancestral Group) RalF shows perinuclear localization reminiscent of RalF-mediated recruitment of Arf1 by L. pneumophila to its vacuole. For R. typhi, RalF expression occurs early during infection, with RalF inactivation significantly reducing host cell invasion. Furthermore, RalF co-localization with Arf6 and the phosphoinositide PI(4,5)P₂ at the host plasma membrane was determined to be critical for R. typhi invasion. Thus, our work illustrates that different intracellular lifestyles across species of Rickettsia and Legionella have driven divergent roles for RalF during host cell infection. Collectively, we identify lineage-specific Arf-GEF utilization across diverse rickettsial species, previously unappreciated mechanisms for host cell invasion and infection.

Activation-Inactivation Cycling of Rab35 and ARF6 Is Required for Phagocytosis of Zymosan in RAW264 Macrophages

Phagocytosis of zymosan by phagocytes is a widely used model of microbial recognition by the innate immune system. Live-cell imaging showed that fluorescent protein-fused Rab35 accumulated in the membranes of phagocytic cups and then dissociated from the membranes of newly formed phagosomes. By our novel pull-down assay for Rab35 activity, we found that Rab35 is deactivated immediately after zymosan internalization into the cells. Phagosome formation was inhibited in cells expressing the GDP- or GTP-locked Rab35 mutant. Moreover, the simultaneous expression of ACAP2-a Rab35 effector protein-with GTP-locked Rab35 or the expression of plasma membrane-targeted ACAP2 showed a marked inhibitory effect on phagocytosis through ARF6 inactivation by the GAP activity of ACAP2. ARF6, a substrate for ACAP2, was also localized on the phagocytic cups and dissociated from the membranes of internalized phagosomes. In support of the microscopic observations, ARF6-GTP pull-down experiments showed that ARF6 is transiently activated during phagosome formation. Furthermore, the expression of GDP- or GTP-locked ARF6 mutants also suppresses the uptake of zymosan. These data suggest that the activation-inactivation cycles of Rab35 and ARF6 are required for the uptake of zymosan and that ACAP2 is an important component that links Rab35/ARF6 signaling during phagocytosis of zymosan.

Myristoylome profiling reveals a concerted mechanism of ARF GTPase deacylation by the bacterial protease IpaJ

N-myristoylation is an essential fatty acid modification that governs the localization and activity of cell signaling enzymes, architectural proteins, and immune regulatory factors. Despite its importance in health and disease, there are currently no methods for reversing protein myristoylation in vivo. Recently, the Shigella flexneri protease IpaJ was found to cleave myristoylated glycine of eukaryotic proteins, yet the discriminatory mechanisms of substrate selection required for targeted demyristoylation have not yet been evaluated. Here, we performed global myristoylome profiling of cells treated with IpaJ under distinct physiological conditions. The protease is highly promiscuous among diverse N-myristoylated proteins in vitro but is remarkably specific to Golgi-associated ARF/ARL family GTPases during Shigella infection. Reconstitution studies revealed a mechanistic framework for substrate discrimination based on IpaJ's function as a GTPase "effector" of bacterial origin. We now propose a concerted model for IpaJ function that highlights its potential for programmable demyristoylation in vivo.

Shigella flexneri regulation of ARF6 activation during bacterial entry via an IpgD-mediated positive feedback loop

Entry into cells is critical for virulence of the human bacterial pathogens Shigella spp. Shigella spp. induce membrane ruffle formation and macropinocytic uptake, but the events instigating this process are incompletely understood. The host small GTPase ADP-ribosylation factor 6 (ARF6) functions in membrane trafficking at the plasma membrane and activates membrane ruffle formation. We demonstrate that ARF6 is required for efficient Shigella flexneri entry, is activated by S. flexneri dependent on the phosphatase activity of the type III secreted effector IpgD, and depends on cytohesin guanine nucleotide exchange factors (GEFs) for recruitment to entry sites. The cytohesin GEF ARF nucleotide binding site opener (ARNO) is recruited to these sites, also dependent on IpgD phosphatase activity. ARNO recruitment is independent of ARF6, indicating that, in addition to the described recruitment of ARNO by ARF6, ARNO is recruited upstream of ARF6. Our data provide evidence that ARF6, IpgD, phosphoinositide species, and ARNO constitute a previously undescribed positive feedback loop that amplifies ARF6 activation at bacterial entry sites, thereby promoting efficient S. flexneri uptake.

IMPORTANCE

Shigella spp. cause diarrhea and dysentery by infection of epithelial cells in the human colon. Critical to disease is the ability of Shigella to enter into cells, yet the mechanisms involved in entry are incompletely understood. We demonstrate that the small GTPase ADP-ribosylation factor 6 (ARF6) is required for efficient cellular entry of Shigella flexneri and that activation of ARF6 depends on the phosphatase activity of the Shigella protein IpgD, which is introduced into cells via the bacterial type III secretion system. We further show that IpgD phosphatase activity is required for recruitment of the ARF6 guanine nucleotide exchange factor (GEF) ARF nucleotide binding site opener (ARNO) to bacterial entry sites and that ARNO lies upstream of ARF6 activation. These relationships define a positive feedback loop that contributes to activation of ARF6 at S. flexneri entry sites and leads to local amplification of signals that promote bacterial entry.

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.

Endocytosis as a mode to regulate functional expression of two-pore domain potassium (K₂p) channels

Two-pore domain potassium (K_2P) channels are implicated in an array of physiological and pathophysiological roles. As a result of their biophysical properties, these channels produce a background leak K⁺ current which has a direct effect on cellular membrane potential and activity. The regulation of potassium leak from cells through K_2P channels is of critical importance to cell function, development and survival. Controlling the cell surface expression of these channels is one mode to regulate their function and is achieved through a balance between regulated channel delivery to and retrieval from the cell surface. Here, we explore the modes of retrieval of K_2P channels from the plasma membrane and observe that K_2P channels are endocytosed in both a clathrin-mediated and clathrin-independent manner. K_2P channels use a variety of pathways and show altered internalisation and sorting in response to external cues. These pathways working in concert, equip the cell with a range of approaches to maintain steady state levels of channels and to respond rapidly should changes in channel density be required.

Clathrin-independent pathways do not contribute significantly to endocytic flux

Several different endocytic pathways have been proposed to function in mammalian cells. Clathrin-coated pits are well defined, but the identity, mechanism and function of alternative pathways have been controversial. Here we apply universal chemical labelling of plasma membrane proteins to define all primary endocytic vesicles, and labelling of specific proteins with a reducible SNAP-tag substrate. These approaches provide high temporal resolution and stringent discrimination between surface-connected and intracellular membranes. We find that at least 95% of the earliest detectable endocytic vesicles arise from clathrin-coated pits. GPI-anchored proteins, candidate cargoes for alternate pathways, are also found to enter the cell predominantly via coated pits. Experiments employing a mutated clathrin adaptor reveal distinct mechanisms for sorting into coated pits, and thereby explain differential effects on the uptake of transferrin and GPI-anchored proteins. These data call for a revision of models for the activity and diversity of endocytic pathways in mammalian cells.

DOI:http://dx.doi.org/10.7554/eLife.03970.001

Cells are enclosed by a ‘plasma membrane’ that allows nutrients and certain small molecules to move in and out of cells. Larger molecules—such as proteins—are carried into cells through a process known as endocytosis, where part of the plasma membrane engulfs the molecule and transports it through the cell inside a bubble-like compartment called a vesicle.

There may be several different ways by which endocytosis can occur. The most common method involves a protein known as clathrin, which coats part of the plasma membrane on the side facing the inside of the cell. This causes the membrane to deform into a pit. The pit grows around, and eventually completely surrounds, the molecule to be transported, at which point the clathrin-coated membrane pinches off from the rest of the plasma membrane to form a vesicle.

Other forms of endocytosis do not need clathrin to form vesicles, and so are collectively known as clathrin-independent endocytosis. However, the details of how these other types of endocytosis work and how important they are for moving molecules into the cell remain unclear. This is partly because it is difficult to identify particular types of endocytosis. Previous attempts to do this have involved trying to identify molecules that are specifically and solely associated with that type of endocytosis, and using these to track the vesicle. However, few—if any—such molecules are known for clathrin-independent methods of endocytosis. Another approach is to inhibit the formation of clathrin-coated pits and study those molecules that are still taken into cells. The problem here is that incomplete inhibition can make interpreting the results difficult. Furthermore, complete inhibition of an important process like clathrin-dependent endocytosis is likely to have severe effects on many other aspects of cell function.

Bitsikas et al. have developed a new method that allows a vesicle to be identified—regardless of how it forms—in cells that have not been treated with inhibitors. This method involves labelling proteins in the plasma membrane with a chemical that allows them to be traced, and so shows when they are included in vesicle membranes. Importantly, this new method can provide very accurate information as to whether or not proteins have been included in vesicles, and this may provide advantages over previous approaches.

Bitsikas et al. selected a group of proteins that are thought to only enter cells in a clathrin-independent manner, but unexpectedly found that these proteins predominantly enter cells through clathrin-coated vesicles. Further analysis revealed that approximately 95% of all molecules that enter cells by endocytosis are taken up via clathrin-coated endocytosis. Therefore, clathrin-independent endocytosis does not make a significant contribution to the transport of large molecules into cells.

These results are at odds with current thinking in the field. Future work could reveal whether the techniques applied by Bitsikas et al. detect more active clathrin-independent endocytosis in special situations, for example during cell migration, or in specific cell types.

DOI:http://dx.doi.org/10.7554/eLife.03970.002

Class I ADP-ribosylation factors are involved in enterovirus 71 replication

Enterovirus 71 is one of the major causative agents of hand, foot, and mouth disease in infants and children. Replication of enterovirus 71 depends on host cellular factors. The viral replication complex is formed in novel, cytoplasmic, vesicular compartments. It has not been elucidated which cellular pathways are hijacked by the virus to create these vesicles. Here, we investigated whether proteins associated with the cellular secretory pathway were involved in enterovirus 71 replication. We used a loss-of-function assay, based on small interfering RNA. We showed that enterovirus 71 RNA replication was dependent on the activity of Class I ADP-ribosylation factors. Simultaneous depletion of ADP-ribosylation factors 1 and 3, but not three others, inhibited viral replication in cells. We also demonstrated with various techniques that the brefeldin-A-sensitive guanidine nucleotide exchange factor, GBF1, was critically important for enterovirus 71 replication. Our results suggested that enterovirus 71 replication depended on GBF1-mediated activation of Class I ADP-ribosylation factors. These results revealed a connection between enterovirus 71 replication and the cellular secretory pathway; this pathway may represent a novel target for antiviral therapies.

EHD3-dependent endosome pathway regulates cardiac membrane excitability and physiology

RATIONALE

Cardiac function is dependent on the coordinate activities of membrane ion channels, transporters, pumps, and hormone receptors to tune the membrane electrochemical gradient dynamically in response to acute and chronic stress. Although our knowledge of membrane proteins has rapidly advanced during the past decade, our understanding of the subcellular pathways governing the trafficking and localization of integral membrane proteins is limited and essentially unstudied in vivo. In the heart, to our knowledge, there are no in vivo mechanistic studies that directly link endosome-based machinery with cardiac physiology.

OBJECTIVE

To define the in vivo roles of endosome-based cellular machinery for cardiac membrane protein trafficking, myocyte excitability, and cardiac physiology.

METHODS AND RESULTS

We identify the endosome-based Eps15 homology domain 3 (EHD3) pathway as essential for cardiac physiology. EHD3-deficient hearts display structural and functional defects including bradycardia and rate variability, conduction block, and blunted response to adrenergic stimulation. Mechanistically, EHD3 is critical for membrane protein trafficking, because EHD3-deficient myocytes display reduced expression/localization of Na/Ca exchanger and L-type Ca channel type 1.2 with a parallel reduction in Na/Ca exchanger-mediated membrane current and Cav1.2-mediated membrane current. Functionally, EHD3-deficient myocytes show increased sarcoplasmic reticulum [Ca], increased spark frequency, and reduced expression/localization of ankyrin-B, a binding partner for EHD3 and Na/Ca exchanger. Finally, we show that in vivo EHD3-deficient defects are attributable to cardiac-specific roles of EHD3 because mice with cardiac-selective EHD3 deficiency demonstrate both structural and electric phenotypes.

CONCLUSIONS

These data provide new insight into the critical role of endosome-based pathways in membrane protein targeting and cardiac physiology. EHD3 is a critical component of protein trafficking in heart and is essential for the proper membrane targeting of select cellular proteins that maintain excitability.

Systematic analysis of endocytosis by cellular perturbations

Endocytosis is an essential process of eukaryotic cells that facilitates numerous cellular and organismal functions. The formation of vesicles from the plasma membrane serves the internalization of ligands and receptors and leads to their degradation or recycling. A number of distinct mechanisms have been described over the years, several of which are only partially characterized in terms of mechanism and function. These are often referred to as novel endocytic pathways. The pathways differ in their mode of uptake and in their intracellular destination. Here, an overview of the set of cellular proteins that facilitate the different pathways is provided. Further, the approaches to distinguish between the pathways by different modes of perturbation are critically discussed, emphasizing the use of genetic tools such as dominant negative mutant proteins.

The small GTPase Arf6 is essential for the Tram/Trif pathway in TLR4 signaling

Recognition of lipopolysaccharides (LPS) by Toll-like receptor 4 (TLR4) at the plasma membrane triggers NF-κB activation through recruitment of the adaptor proteins Mal and MyD88. Endocytosis of the activated TLR4 allows recruitment of the adaptors Tram and Trif, leading to activation of the transcription factor IRF3 and interferon production. The small GTPase ADP-ribosylation factor 6 (Arf6) was shown to regulate the plasma membrane association of Mal. Here we demonstrate that inhibition of Arf6 also markedly reduced LPS-induced cytokine production in Mal(-/-) mouse macrophages. In this article, we focus on a novel role for Arf6 in the MyD88-independent TLR4 pathway. MyD88-independent IRF3 activation and IRF3-dependent gene transcription were strictly dependent on Arf6. Arf6 was involved in transport of Tram to the endocytic recycling compartment and internalization of LPS, possibly explaining its requirement for LPS-induced IRF3 activation. Together, these results show a critical role for Arf6 in regulating Tram/Trif-dependent TLR4 signaling.

A CREB3-ARF4 signalling pathway mediates the response to Golgi stress and susceptibility to pathogens

Treatment of cells with brefeldin A (BFA) blocks secretory vesicle transport and causes a collapse of the Golgi apparatus. To gain more insight into the cellular mechanisms mediating BFA toxicity, we conducted a genome-wide haploid genetic screen that led to the identification of the small G protein ADP-ribosylation factor 4 (ARF4). ARF4 depletion preserves viability, Golgi integrity and cargo trafficking in the presence of BFA, and these effects depend on the guanine nucleotide exchange factor GBF1 and other ARF isoforms including ARF1 and ARF5. ARF4 knockdown cells show increased resistance to several human pathogens including Chlamydia trachomatis and Shigella flexneri. Furthermore, ARF4 expression is induced when cells are exposed to several Golgi-disturbing agents and requires the CREB3 (also known as Luman or LZIP) transcription factor, whose downregulation mimics ARF4 loss. Thus, we have uncovered a CREB3-ARF4 signalling cascade that may be part of a Golgi stress response set in motion by stimuli compromising Golgi capacity.

Activation machinery of the small GTPase Arf6

The small GTPase ADP-ribosylation factor 6 (Arf6) plays pivotal roles in a wide variety of cellular events, including exocytosis, endocytosis, actin cytoskeleton reorganization and phosphoinositide metabolism, in various types of cells. To control such a wide variety of actions of Arf6, activation of Arf6 could be precisely controlled by its activators, guanine nucleotide exchange factors (GEFs), in spatial and temporal manners. In this manuscript, we summarize and discuss the characteristics of previously identified GEFs specific to Arf6 and activation machineries of Arf6.

Phosphorylation of the Bin, Amphiphysin, and RSV161/167 (BAR) domain of ACAP4 regulates membrane tubulation

ArfGAP With Coiled-Coil, Ankyrin Repeat And PH Domains 4 (ACAP4) is an ADP-ribosylation factor 6 (ARF6) GTPase-activating protein essential for EGF-elicited cell migration. However, how ACAP4 regulates membrane dynamics and curvature in response to EGF stimulation is unknown. Here, we show that phosphorylation of the N-terminal region of ACAP4, named the Bin, Amphiphysin, and RSV161/167 (BAR) domain, at Tyr34 is necessary for EGF-elicited membrane remodeling. Domain structure analysis demonstrates that the BAR domain regulates membrane curvature. EGF stimulation of cells causes phosphorylation of ACAP4 at Tyr34, which subsequently promotes ACAP4 homodimer curvature. The phospho-mimicking mutant of ACAP4 demonstrates lipid-binding activity and tubulation in vitro, and ARF6 enrichment at the membrane is associated with ruffles of EGF-stimulated cells. Expression of the phospho-mimicking ACAP4 mutant promotes ARF6-dependent cell migration. Thus, the results present a previously undefined mechanism by which EGF-elicited phosphorylation of the BAR domain controls ACAP4 molecular plasticity and plasma membrane dynamics during cell migration.

Endocytosis of gene delivery vectors: from clathrin-dependent to lipid raft-mediated endocytosis

The ideal nonviral vector delivers its nucleic acid cargo to a specific intracellular target. Vectors enter cells mainly through endocytosis and are distributed to various intracellular organelles. Recent advances in microscopy, lipidomics, and proteomics confirm that the cell membrane is composed of clusters of lipids, organized in the form of lipid raft domains, together with non-raft domains that comprise a generally disordered lipid milieu. The binding of a nonviral vector to either region can determine the pathway for its endocytic uptake and subsequent intracellular itinerary. Given this model of the cell membrane structure, endocytic pathways should be reclassified in relation to lipid rafts. In this review, we attempt to assess the currently recognized endocytic pathways in mammalian cells. The endocytic pathways are classified in relation to the membrane regions that make up the primary endocytic vesicles. This review covers the well-recognized clathrin-mediated endocytosis (CME), phagocytosis, and macropinocytosis in addition to the less addressed pathways that take place in lipid rafts. These include caveolae-mediated, flotillin-dependent, GTPase regulator associated with focal adhesion kinase-1 (GRAF1)-dependent, adenosine diphosphate-ribosylation factor 6 (Arf6)-dependent, and RhoA-dependent endocytic pathways. We summarize the regulators associated with each uptake pathway and methods for interfering with these regulators are discussed. The fate of endocytic vesicles resulting from each endocytic uptake pathway is highlighted.

Non-neuronal functions of the m2 muscarinic acetylcholine receptor

Acetylcholine is an important neurotransmitter whose effects are mediated by two classes of receptors. The nicotinic acetylcholine receptors are ion channels, whereas the muscarinic receptors belong to the large family of G protein coupled seven transmembrane helix receptors. Beyond its function in neuronal systems, it has become evident that acetylcholine also plays an important role in non-neuronal cells such as epithelial and immune cells. Furthermore, many cell types in the periphery are capable of synthesizing acetylcholine and express at least some of the receptors. In this review, we summarize the non-neuronal functions of the muscarinic acetylcholine receptors, especially those of the M₂ muscarinic receptor in epithelial cells. We will review the mechanisms of signaling by the M₂ receptor but also the cellular trafficking and ARF6 mediated endocytosis of this receptor, which play an important role in the regulation of signaling events. In addition, we provide an overview of the M₂ receptor in human pathological conditions such as autoimmune diseases and cancer.

ARF6 regulates neuron differentiation through glucosylceramide synthase

The small GTPase ADP ribosylation factor 6 (ARF6) mediates endocytosis and has in addition been shown to regulate neuron differentiation. Here we investigated whether ARF6 promotes differentiation of Neuro-2a neuronal cells by modifying the cellular lipid composition. We showed that knockdown of ARF6 by siRNA in Neuro-2a cells increased neuronal outgrowth as expected. ARF6 knockdown also resulted in increased glucosylceramide levels and decreased sphingomyelin levels, but did not affect the levels of ceramide or phospholipids. We speculated that the ARF6 knockdown-induced increase in glucosylceramide was caused by an effect on glucosylceramide synthase and, in agreement, showed that ARF6 knockdown increased the mRNA levels and activity of glucosylceramide synthase. Finally, we showed that incubation of Neuro-2a cells with the glucosylceramide synthase inhibitor D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) normalized the increased neuronal outgrowth induced by ARF6 knockdown. Our results thus show that ARF6 regulates neuronal differentiation through an effect on glucosylceramide synthase and glucosylceramide levels.

Syndecan-4 phosphorylation is a control point for integrin recycling

Precise spatiotemporal coordination of integrin adhesion complex dynamics is essential for efficient cell migration. For cells adherent to fibronectin, differential engagement of α₅β₁ and α_Vβ₃ integrins is used to elicit changes in adhesion complex stability, mechanosensation, matrix assembly, and migration, but the mechanisms responsible for receptor regulation have remained largely obscure. We identify phosphorylation of the membrane-intercalated proteoglycan syndecan-4 as an essential switch controlling integrin recycling. Src phosphorylates syndecan-4 and, by driving syntenin binding, leads to suppression of Arf6 activity and recycling of α_Vβ₃ to the plasma membrane at the expense of α₅β₁. The resultant elevation in α_Vβ₃ engagement promotes stabilization of focal adhesions. Conversely, abrogation of syndecan-4 phosphorylation drives surface expression of α₅β₁, destabilizes adhesion complexes, and disrupts cell migration. These data identify the dynamic spatiotemporal regulation of Src-mediated syndecan-4 phosphorylation as an essential switch controlling integrin trafficking and adhesion dynamics to promote efficient cell migration.

Interleukin receptor activates a MYD88-ARNO-ARF6 cascade to disrupt vascular stability

The innate immune response is essential for combating infectious disease. Macrophages and other cells respond to infection by releasing cytokines such as interleukin-1β (IL-1β), which in turn activate a well-described myeloid differentiation factor 88 (MYD88) -mediated, nuclear factor-κB (NF-κB) -dependent transcriptional pathway that results in inflammatory cell activation and recruitment^1–4. Endothelial cells, which usually serve as a barrier to the movement of inflammatory cells out of the blood and into tissue, are also critical mediators of the inflammatory response^5,6. Paradoxically, the same cytokines vital to a successful immune defense also have disruptive effects on endothelial cell-cell interactions and can trigger degradation of barrier function and dissociation of tissue architecture^7–9. The mechanism of this barrier dissolution and its relationship to the canonical NF-κB pathway remains ill defined. Here we show that the direct, immediate, and disruptive effects of IL-1β on endothelial stability are NF-κB independent and are instead the result of signaling via the small GTPase, ADP-ribosylation factor 6 (ARF6), and its activator, ARF nucleotide binding site opener (ARNO). Moreover, we show that ARNO binds directly to the adaptor protein MYD88, and thus propose MYD88-ARNO-ARF6 as a proximal IL-1β signaling pathway distinct from that mediated by NF-κB (Supplementary Fig. 1). Finally, we show that SecinH3, an inhibitor of ARF guanine nucleotide-exchange factors (GEFs) such as ARNO, enhances vascular stability and significantly improves outcomes in animal models of inflammatory arthritis and acute inflammation.

Arf6 promotes cell proliferation via the PLD-mTORC1 and p38MAPK pathways

The small G-protein ADP-ribosylation factor 6 (Arf6) belongs to the Ras GTPases superfamily and is mostly known for its actin remodeling functions and involvement in the processes of plasma membrane reorganization and vesicular transport. The majority of data indicates that Arf6 contributes to cancer progression through activation of cell motility and invasion. Alternatively, we found that the expression of a wild-type or a constitutively active Arf6 does not influence tumor cell motility and invasion but instead significantly stimulates cell proliferation and activates phospholipase D (PLD). Conversely the expression of a mutant Arf6 (Arf6N48I), that is, unable to interact with PLD has no effect on proliferation but promotes motility, invasion, and matrix degradation by uPA extracellular proteinase. Studying the mechanisms of Arf6-dependent stimulation of cell proliferation, we found some signaling pathways contributing to Arf6 promitogenic activity. Namely, we showed that Arf6 in a PLD-mTORC1-dependent manner activates S6K1 kinase, a well-known regulator of mitogen-stimulated translation initiation. Furthermore, we demonstrated an Arf6-dependent phosphorylation of mTORC1 downstream targets, 4E-BP1 and ribosomal S6 protein, confirming an existence of Arf6-PLD-mTORC1-S6K1/4E-BP1 signaling pathway and also demonstrated its impact on proliferation stimulation. Next, we found that Arf6 activation potentiates Erk1/2 and p38MAP kinases phosphorylation. Surprisingly, p38 opposite to Erk1/2 significantly contributes to Arf6-dependent proliferation increase promoting S6 ribosomal protein phosphorylation at Ser235/236 residues. Therefore, we demonstrated Arf6 proliferation stimulating activity and revealed PLD-mTORC1 and p38MAP kinase as Arf6 partners mediating promitogenic activity. These results highlight a new aspect of Arf6 functioning in cancer cell biology.

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.

Comparison of ultracentrifugation, density gradient separation, and immunoaffinity capture methods for isolating human colon cancer cell line LIM1863-derived exosomes

Exosomes are 40-100nm extracellular vesicles that are released from a multitude of cell types, and perform diverse cellular functions including intercellular communication, antigen presentation, and transfer of oncogenic proteins as well as mRNA and miRNA. Exosomes have been purified from biological fluids and in vitro cell cultures using a variety of strategies and techniques. However, all preparations invariably contain varying proportions of other membranous vesicles that co-purify with exosomes such as shed microvesicles and apoptotic blebs. Using the colorectal cancer cell line LIM1863 as a cell model, in this study we performed a comprehensive evaluation of current methods used for exosome isolation including ultracentrifugation (UC-Exos), OptiPrep™ density-based separation (DG-Exos), and immunoaffinity capture using anti-EpCAM coated magnetic beads (IAC-Exos). Notably, all isolations contained 40-100nm vesicles, and were positive for exosome markers (Alix, TSG101, HSP70) based on electron microscopy and Western blotting. We employed a proteomic approach to profile the protein composition of exosomes, and label-free spectral counting to evaluate the effectiveness of each method. Based on the number of MS/MS spectra identified for exosome markers and proteins associated with their biogenesis, trafficking, and release, we found IAC-Exos to be the most effective method to isolate exosomes. For example, Alix, TSG101, CD9 and CD81 were significantly higher (at least 2-fold) in IAC-Exos, compared to UG-Exos and DG-Exos. Application of immunoaffinity capture has enabled the identification of proteins including the ESCRT-III component VPS32C/CHMP4C, and the SNARE synaptobrevin 2 (VAMP2) in exosomes for the first time. Additionally, several cancer-related proteins were identified in IAC-Exos including various ephrins (EFNB1, EFNB2) and Eph receptors (EPHA2-8, EPHB1-4), and components involved in Wnt (CTNNB1, TNIK) and Ras (CRK, GRB2) signalling.

ADP ribosylation factor 1 activity is required to recruit AP-1 to the major histocompatibility complex class I (MHC-I) cytoplasmic tail and disrupt MHC-I trafficking in HIV-1-infected primary T cells

HIV-1-infected cells are partially resistant to anti-HIV cytotoxic T lymphocytes (CTLs) due to the effects of the HIV Nef protein on antigen presentation by major histocompatibility complex class I (MHC-I), and evidence has been accumulating that this function of Nef is important in vivo. HIV Nef disrupts the normal expression of MHC-I by stabilizing a protein-protein interaction between the clathrin adaptor protein AP-1 and the MHC-I cytoplasmic tail. There is also evidence that Nef activates a phosphatidylinositol 3 kinase (PI3K)-dependent GTPase, ADP ribosylation factor 6 (ARF-6), to stimulate MHC-I internalization. However, the relative importance of these two pathways is unclear. Here we report that a GTPase required for AP-1 activity (ARF-1) was needed for Nef to disrupt MHC-I surface levels, whereas no significant requirement for ARF-6 was observed in Nef-expressing T cell lines and in HIV-infected primary T cells. An ARF-1 inhibitor blocked the ability of Nef to recruit AP-1 to the MHC-I cytoplasmic tail, and a dominant active ARF-1 mutant stabilized the Nef-MHC-I-AP-1 complex. These data support a model in which Nef and ARF-1 stabilize an interaction between MHC-I and AP-1 to disrupt the presentation of HIV-1 epitopes to CTLs.

Several ADP-ribosylation factor (Arf) isoforms support COPI vesicle formation

Newly synthesized proteins and lipids are transported in vesicular carriers along the secretory pathway. Arfs (ADP-ribosylation factors), a family of highly conserved GTPases within the Ras superfamily, control recruitment of molecular coats to membranes, the initial step of coated vesicle biogenesis. Arf1 and coatomer constitute the minimal cytosolic machinery leading to COPI vesicle formation from Golgi membranes. Although some functional redundancies have been suggested, other Arf isoforms have been poorly analyzed in this context. In this study, we found that Arf1, Arf4, and Arf5, but not Arf3 and Arf6, associate with COPI vesicles generated in vitro from Golgi membranes and purified cytosol. Using recombinant myristoylated proteins, we show that Arf1, Arf4, and Arf5 each support COPI vesicle formation individually. Unexpectedly, we found that Arf3 could also mediate vesicle biogenesis. However, Arf3 was excluded from the vesicle fraction in the presence of the other isoforms, highlighting a functional competition between the different Arf members.

ADP-ribosylation factor 6 mediates E-cadherin recovery by chemical chaperones

E-cadherin plays a powerful tumor suppressor role. Germline E-cadherin mutations justify 30% of Hereditary Diffuse Gastric Cancer (HDGC) and missense mutations are found in 30% of these families. We found possible to restore in vitro mutant E-cadherin associated to HDGC syndrome by using Chemical Chaperones (CCs). Herein, our aim was to disclose the molecular mechanisms underlying the CCs effects in E-cadherin regulation. Using cells stably expressing WT E-cadherin or two HDGC-associated missense mutations, we show that upon DMSO treatment, not only mutant E-cadherin is restored and stabilized at the plasma membrane (PM), but also Arf6 and PIPKIγ expressions are altered. We show that modulation of Arf6 expression partially mimics the effect of CCs, suggesting that the cellular effects observed upon CCs treatment are mediated by Arf6. Further, we show that E-cadherin expression recovery is specifically linked to Arf6 due to its role on endocytosis and recycling pathways. Finally, we demonstrated that, as DMSO, several others CCs are able to modulate the trafficking machinery through an Arf6 dependent mechanism. Interestingly, the more effective compounds in E-cadherin recovery to PM are those that simultaneously inhibit Arf6 and stimulate PIPKIγ expression and binding to E-cadherin. Here, we present the first evidence of a direct influence of CCs in cellular trafficking machinery and we show that this effect is of crucial importance in the context of juxtamembrane E-cadherin missense mutations associated to HDGC. We propose that this influence should be taken into account when exploring the therapeutic potential of this type of chemicals in genetic diseases associated to protein-misfolding.

Viral infection: Moving through complex and dynamic cell-membrane structures

Viruses have developed different survival strategies in host cells by crossing cell-membrane compartments, during different steps of their viral life cycle. In fact, the non-regenerative viral membrane of enveloped viruses needs to encounter the dynamic cell-host membrane, during early steps of the infection process, in which both membranes fuse, either at cell-surface or in an endocytic compartment, to promote viral entry and infection. Once inside the cell, many viruses accomplish their replication process through exploiting or modulating membrane traffic, and generating specialized compartments to assure viral replication, viral budding and spreading, which also serve to evade the immune responses against the pathogen. In this review, we have attempted to present some data that highlight the importance of membrane dynamics during viral entry and replicative processes, in order to understand how viruses use and move through different complex and dynamic cell-membrane structures and how they use them to persist.

Structural remodeling, trafficking and functions of glycosylphosphatidylinositol-anchored proteins

Glycosylphosphatidylinositol (GPI) is a glycolipid that is covalently attached to proteins as a post-translational modification. Such modification leads to the anchoring of the protein to the outer leaflet of the plasma membrane. Proteins that are decorated with GPIs have unique properties in terms of their physical nature. In particular, these proteins tend to accumulate in lipid rafts, which are critical for the functions and trafficking of GPI-anchored proteins (GPI-APs). Recent studies mainly using mutant cells revealed that various structural remodeling reactions occur to GPIs present in GPI-APs as they are transported from the endoplasmic reticulum to the cell surface. This review examines the recent progress describing the mechanisms of structural remodeling of mammalian GPI-anchors, such as inositol deacylation, glycan remodeling and fatty acid remodeling, with particular focus on their trafficking and functions, as well as the pathogenesis involving GPI-APs and their deficiency.

Internalization of exogenous ADP-ribosylation factor 6 (Arf6) proteins into cells

Endogenous Arf6 is a myristoylated protein mainly involved in endosomal membrane traffic and structural organization at the plasma membrane. It has been shown that Arf6 mediates cancer cell invasion and shedding of plasma membrane microvesicles derived from tumor cells. In this article, we determined that Arf6 proteins both in the GDP and GTPγS bound forms can enter cells when simply added in the cell culture medium without requiring the myristoyl group. The GTPγS bound can enter cells at a faster rate than the GDP-bound Arf6. Despite the role of the endogenous Arf6 in endocytosis and membrane trafficking, the internalization of exogenous Arf6 may involve non-endocytic processes. As protein therapeutics is becoming important in medicine, we examined the effect of the uptake of Arf6 proteins on cellular functions and determined that exogenous Arf6 inhibits proliferation, invasion, and migration of cells. Future studies of the internalization of Arf6 mutants will reveal key residues that play a role in the internalization of Arf6 and its interaction and possible structural conformations bound to the plasma membrane.

Decoupling of activation and effector binding underlies ARF6 priming of fast endocytic recycling

The small GTP-binding protein ADP-ribosylation factor 6 (ARF6) controls the endocytic recycling pathway of several plasma membrane receptors. We analyzed the localization and GDP/GTP cycle of GFP-tagged ARF6 by total internal reflection fluorescent microscopy. We found that ARF6-GFP associates with clathrin-coated pits (CCPs) at the plasma membrane in a GTP-dependent manner in a mechanism requiring the adaptor protein complex AP-2. In CCP, GTP-ARF6 mediates the recruitment of the ARF-binding domain of downstream effectors including JNK-interacting proteins 3 and 4 (JIP3 and JIP4) after the burst recruitment of the clathrin uncoating component auxilin. ARF6 does not contribute to receptor-mediated clathrin-dependent endocytosis. In contrast, we found that interaction of ARF6 and JIPs on endocytic vesicles is required for trafficking of the transferrin receptor in the fast, microtubule-dependent endocytic recycling pathway. Our findings unravel a novel mechanism of separation of ARF6 activation and effector function, ensuring that fast recycling may be determined at the level of receptor incorporation into CCPs.

Arf6-dependent intracellular trafficking of Pasteurella multocida toxin and pH-dependent translocation from late endosomes

The potent mitogenic toxin from Pasteurella multocida (PMT) is the major virulence factor associated with a number of epizootic and zoonotic diseases caused by infection with this respiratory pathogen. PMT is a glutamine-specific protein deamidase that acts on its intracellular G-protein targets to increase intracellular calcium, cytoskeletal, and mitogenic signaling. PMT enters cells through receptor-mediated endocytosis and then translocates into the cytosol through a pH-dependent process that is inhibited by NH₄Cl or bafilomycin A1. However, the detailed mechanisms that govern cellular entry, trafficking, and translocation of PMT remain unclear. Co-localization studies described herein revealed that while PMT shares an initial entry pathway with transferrin (Tfn) and cholera toxin (CT), the trafficking pathways of Tfn, CT, and PMT subsequently diverge, as Tfn is trafficked to recycling endosomes, CT is trafficked retrograde to the ER, and PMT is trafficked to late endosomes. Our studies implicate the small regulatory GTPase Arf6 in the endocytic trafficking of PMT. Translocation of PMT from the endocytic vesicle occurs through a pH-dependent process that is also dependent on both microtubule and actin dynamics, as evidenced by inhibition of PMT activity in our SRE-based reporter assay, with nocodazole and cytochalasin D, respectively, suggesting that membrane translocation and cytotoxicity of PMT is dependent on its transfer to late endosomal compartments. In contrast, disruption of Golgi-ER trafficking with brefeldin A increased PMT activity, suggesting that inhibiting PMT trafficking to non-productive compartments that do not lead to translocation, while promoting formation of an acidic tubulovesicle system more conducive to translocation, enhances PMT translocation and activity.

HIV-1 requires Arf6-mediated membrane dynamics to efficiently enter and infect T lymphocytes

As the initial barrier to viral entry, the plasma membrane along with the membrane trafficking machinery and cytoskeleton are of fundamental importance in the viral cycle. However, little is known about the contribution of plasma membrane dynamics during early human immunodeficiency virus type 1 (HIV-1) infection. Considering that ADP ribosylation factor 6 (Arf6) regulates cellular invasion via several microorganisms by coordinating membrane trafficking, our aim was to study the function of Arf6-mediated membrane dynamics on HIV-1 entry and infection of T lymphocytes. We observed that an alteration of the Arf6-guanosine 5'-diphosphate/guanosine 5'-triphosphate (GTP/GDP) cycle, by GDP-bound or GTP-bound inactive mutants or by specific Arf6 silencing, inhibited HIV-1 envelope-induced membrane fusion, entry, and infection of T lymphocytes and permissive cells, regardless of viral tropism. Furthermore, cell-to-cell HIV-1 transmission of primary human CD4(+) T lymphocytes was inhibited by Arf6 knockdown. Total internal reflection fluorescence microscopy showed that Arf6 mutants provoked the accumulation of phosphatidylinositol-(4,5)-biphosphate-associated structures on the plasma membrane of permissive cells, without affecting CD4-viral attachment but impeding CD4-dependent HIV-1 entry. Arf6 silencing or its mutants did not affect fusion, entry, and infection of vesicular stomatitis virus G-pseudotyped viruses or ligand-induced CXCR4 or CCR5 endocytosis, both clathrin-dependent processes. Therefore we propose that efficient early HIV-1 infection of CD4(+) T lymphocytes requires Arf6-coordinated plasma membrane dynamics that promote viral fusion and entry.

SynArfGEF is a guanine nucleotide exchange factor for Arf6 and localizes preferentially at post-synaptic specializations of inhibitory synapses

SynArfGEF, also known as BRAG3 or IQSEC3, is a member of the brefeldin A-resistant Arf-GEF/IQSEC family and was originally identified by screening for mRNA species associated with the post-synaptic density fraction. In this study, we demonstrate that synArfGEF activates Arf6, using Arf pull down and transferrin incorporation assays. Immunohistochemical analysis reveals that synArfGEF is present in somata and dendrites as puncta in close association with inhibitory synapses, whereas immunoelectron microscopic analysis reveals that synArfGEF localizes preferentially at post-synaptic specializations of symmetric synapses. Using yeast two-hybrid and pull down assays, we show that synArfGEF is able to bind utrophin/dystrophin and S-SCAM/MAGI-2 scaffolding proteins that localize at inhibitory synapses. Double immunostaining reveals that synArfGEF co-localizes with dystrophin and S-SCAM in cultured hippocampal neurons and cerebellar cortex, respectively. Both β-dystroglycan and S-SCAM were immunoprecipitated from brain lysates using anti-synArfGEF IgG. Taken together, these findings suggest that synArfGEF functions as a novel regulator of Arf6 at inhibitory synapses and associates with the dystrophin-associated glycoprotein complex and S-SCAM.

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.

The human papillomavirus type 16 E5 oncoprotein inhibits epidermal growth factor trafficking independently of endosome acidification

The human papillomavirus type 16 E5 oncoprotein (16E5) enhances acute, ligand-dependent activation of the epidermal growth factor receptor (EGFR) and concomitantly alkalinizes endosomes, presumably by binding to the 16-kDa "c" subunit of the V-ATPase proton pump (16K) and inhibiting V-ATPase function. However, the relationship between 16K binding, endosome alkalinization, and altered EGFR signaling remains unclear. Using an antibody that we generated against 16K, we found that 16E5 associated with only a small fraction of endogenous 16K in keratinocytes, suggesting that it was unlikely that E5 could significantly affect V-ATPase function by direct inhibition. Nevertheless, E5 inhibited the acidification of endosomes, as determined by a new assay using a biologically active, pH-sensitive fluorescent EGF conjugate. Since we also found that 16E5 did not alter cell surface EGF binding, the number of EGFRs on the cell surface, or the endocytosis of prebound EGF, we postulated that it might be blocking the fusion of early endosomes with acidified vesicles. Our studies with pH-sensitive and -insensitive fluorescent EGF conjugates and fluorescent dextran confirmed that E5 prevented endosome maturation (acidification and enlargement) by inhibiting endosome fusion. The E5-dependent defect in vesicle fusion was not due to detectable disruption of actin, tubulin, vimentin, or cytokeratin filaments, suggesting that membrane fusion was being directly affected rather than vesicle transport. Perhaps most importantly, while bafilomycin A(1) (like E5) binds to 16K and inhibits endosome acidification, it did not mimic the ability of E5 to inhibit endosome enlargement or the trafficking of EGF. Thus, 16E5 alters EGF endocytic trafficking via a pH-independent inhibition of vesicle fusion.

Vascular endothelial growth factor receptor-2 activates ADP-ribosylation factor 1 to promote endothelial nitric-oxide synthase activation and nitric oxide release from endothelial cells

Vascular endothelial growth factor (VEGF) induces angiogenesis and regulates endothelial function via production and release of nitric oxide (NO), an important signaling molecule. The molecular basis leading to NO production involves phosphatidylinositiol-3 kinase (PI3K), Akt, and endothelial nitric-oxide synthase (eNOS) activation. In this study, we have examined whether small GTP-binding proteins of the ADP-ribosylation factor (ARF) family act as molecular switches to regulate signaling cascades activated by VEGF in endothelial cells. Our results show that this growth factor can promote the rapid and transient activation of ARF1. In endothelial cells, this GTPase is present on dynamic plasma membrane ruffles. Inhibition of ARF1 expression, using RNA interference, markedly impaired VEGF-dependent eNOS phosphorylation and NO production by preventing the activation of the PI3K/Akt signaling axis. Furthermore, our data indicate that phosphorylation of Tyr(801), on VEGF receptor 2, is essential for activating Src- and ARF1-dependent signaling events leading to NO release from endothelial cells. Lastly, this mediator is known to regulate a broad variety of endothelial cell functions. Depletion of ARF1 markedly inhibits VEGF-dependent increase of vascular permeability as well as capillary tubule formation, a process important for angiogenesis. Taken together, our data indicate that ARF1 is a novel modulator of VEGF-stimulated NO release and signaling in endothelial cells.

Arf3 is activated uniquely at the trans-Golgi network by brefeldin A-inhibited guanine nucleotide exchange factors

Arf3 associates with the TGN in a manner that is both temperature-sensitive and uniquely dependent on BIGs. TGN localization and release at 20°C are readily separated and depend on pairs of residues absolutely conserved and unique to Arf3 present at opposite ends of the protein. These results suggest that Arf3 plays a unique function at the TGN.

It is widely assumed that class I and II Arfs function interchangeably throughout the Golgi complex. However, we report here that in vivo, Arf3 displays several unexpected properties. Unlike other Golgi-localized Arfs, Arf3 associates selectively with membranes of the trans-Golgi network (TGN) in a manner that is both temperature-sensitive and uniquely dependent on guanine nucleotide exchange factors of the BIGs family. For example, BIGs knockdown redistributed Arf3 but not Arf1 from Golgi membranes. Furthermore, shifting temperature to 20°C, a temperature known to block cargo in the TGN, selectively redistributed Arf3 from Golgi membranes. Arf3 redistribution occurred slowly, suggesting it resulted from a change in membrane composition. Arf3 knockdown and overexpression experiments suggest that redistribution is not responsible for the 20°C block. To investigate in more detail the mechanism for Arf3 recruitment and temperature-dependent release, we characterized several mutant forms of Arf3. This analysis demonstrated that those properties are readily separated and depend on pairs of residues present at opposite ends of the protein. Furthermore, phylogenetic analysis established that all four critical residues were absolutely conserved and unique to Arf3. These results suggest that Arf3 plays a unique function at the TGN that likely involves recruitment by a specific receptor.

Importance of extracellular loop one of the neuropeptide S receptor for biogenesis and function

Neuropeptide S (NPS) is the endogenous ligand of a formerly orphan G protein-coupled receptor (GPCR). The NPS receptor (NPSR) belongs to the subfamily of peptide GPCRs and is widely expressed in the brain. NPS promotes arousal and induces anxiolytic-like effects after central administration in rodents. Previously, we have reported that the N107I polymorphism in the human NPS receptor results in a gain-of-function characterized by an increase in agonist potency without changing agonist binding affinity. We have extended our findings by investigating pharmacological and biochemical consequences of mutations in the vicinity of position 107. Alanine substitutions were made for D105 and N101, and stable clones were analyzed for agonist-induced changes of intracellular Ca(2+). Receptor protein expression was monitored by Western blot and flow cytometry. The mutation D105A produced receptors that have a approximately 200-fold higher EC(50) despite elevated total receptor protein and surface expression compared to cell lines expressing the parental receptor NPSR-N107. The mutation N101A resulted in slightly reduced agonist potency without affecting the ability of the protein to form functional receptors. Stable NPSR-A101 clones show little expression of the fully glycosylated form. However, NPSR-A101 receptors are expressed on the cell surface and are functional, suggesting that full glycosylation is not required for receptor function. Our studies suggest that N-linked glycosylation is not important for receptor biogenesis or function, and that residue D105 might be critical for receptor binding.

Small G proteins in peroxisome biogenesis: the potential involvement of ADP-ribosylation factor 6

BACKGROUND

Peroxisomes execute diverse and vital functions in virtually every eukaryote. New peroxisomes form by budding from pre-existing organelles or de novo by vesiculation of the ER. It has been suggested that ADP-ribosylation factors and COPI coatomer complexes are involved in these processes.

RESULTS

Here we show that all viable Saccharomyces cerevisiae strains deficient in one of the small GTPases which have an important role in the regulation of vesicular transport contain functional peroxisomes, and that the number of these organelles in oleate-grown cells is significantly upregulated in the arf1 and arf3 null strains compared to the wild-type strain. In addition, we provide evidence that a portion of endogenous Arf6, the mammalian orthologue of yeast Arf3, is associated with the cytoplasmic face of rat liver peroxisomes. Despite this, ablation of Arf6 did neither influence the regulation of peroxisome abundance nor affect the localization of peroxisomal proteins in cultured fetal hepatocytes. However, co-overexpression of wild-type, GTP hydrolysis-defective or (dominant-negative) GTP binding-defective forms of Arf1 and Arf6 caused mislocalization of newly-synthesized peroxisomal proteins and resulted in an alteration of peroxisome morphology.

CONCLUSION

These observations suggest that Arf6 is a key player in mammalian peroxisome biogenesis. In addition, they also lend strong support to and extend the concept that specific Arf isoform pairs may act in tandem to regulate exclusive trafficking pathways.

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.

Molecular mechanisms of clathrin-independent endocytosis

There is good evidence that, in addition to the canonical clathrin-associated endocytic machinery, mammalian cells possess multiple sets of proteins that are capable of mediating the formation of endocytic vesicles. The identity, mechanistic properties and function of these clathrin-independent endocytic pathways are currently under investigation. This Commentary briefly recounts how the field of clathrin-independent endocytosis has developed to date. It then highlights recent progress in identifying key proteins that might define alternative types of endocytosis. These proteins include CtBP (also known as BARS), flotillins (also known as reggies) and GRAF1. We argue that a combination of information about pathway-specific proteins and the ultrastructure of endocytic invaginations provides a means of beginning to classify endocytic pathways.