Regulated exocytosis in chromaffin cells. Translocation of ARF6 stimulates a plasma membrane-associated phospholipase D

Phospholipase D1 produces phosphatidic acid at sites of secretory vesicle docking and fusion

Phospholipase D1 (PLD1) activity is essential for the stimulated exocytosis of secretory vesicles where it acts as a lipid-modifying enzyme to produces phosphatidic acid (PA). PLD1 localizes to the plasma membrane and secretory vesicles, and PLD1 inhibition or knockdowns reduce the rate of fusion. However, temporal data resolving when and where PLD1 and PA are required during exocytosis is lacking. In this work, PLD1 and production of PA are measured during the trafficking, docking, and fusion of secretory vesicles in PC12 cells. Using fluorescently tagged PLD1 and a PA-binding protein, cells were imaged using TIRF microscopy to monitor the presence of PLD1 and the formation of PA throughout the stages of exocytosis. Single docking and fusion events were imaged to measure the recruitment of PLD1 and the formation of PA. PLD1 is present on mobile, docking, and fusing vesicles and also colocalizes with Syx1a clusters. Treatment of cells with PLD inhibitors significantly reduces fusion, but not PLD1 localization to secretory vesicles. Inhibitors also alter the formation of PA; when PLD1 is active, PA slowly accumulates on docked vesicles. During fusion, PA is reduced in cells treated with PLD1 inhibitors, indicating that PLD1 produces PA during exocytosis.

V-ATPase modulates exocytosis in neuroendocrine cells through the activation of the ARNO-Arf6-PLD pathway and the synthesis of phosphatidic acid

Although there is mounting evidence indicating that lipids serve crucial functions in cells and are implicated in a growing number of human diseases, their precise roles remain largely unknown. This is particularly true in the case of neurosecretion, where fusion with the plasma membrane of specific membrane organelles is essential. Yet, little attention has been given to the role of lipids. Recent groundbreaking research has emphasized the critical role of lipid localization at exocytotic sites and validated the essentiality of fusogenic lipids, such as phospholipase D (PLD)-generated phosphatidic acid (PA), during membrane fusion. Nevertheless, the regulatory mechanisms synchronizing the synthesis of these key lipids and neurosecretion remain poorly understood. The vacuolar ATPase (V-ATPase) has been involved both in vesicle neurotransmitter loading and in vesicle fusion. Thus, it represents an ideal candidate to regulate the fusogenic status of secretory vesicles according to their replenishment state. Indeed, the cytosolic V1 and vesicular membrane-associated V0 subdomains of V-ATPase were shown to dissociate during the stimulation of neurosecretory cells. This allows the subunits of the vesicular V0 to interact with different proteins of the secretory machinery. Here, we show that V0a1 interacts with the Arf nucleotide-binding site opener (ARNO) and promotes the activation of the Arf6 GTPase during the exocytosis in neuroendocrine cells. When the interaction between V0a1 and ARNO was disrupted, it resulted in the inhibition of PLD activation, synthesis of phosphatidic acid during exocytosis, and changes in the timing of fusion events. These findings indicate that the separation of V1 from V0 could function as a signal to initiate the ARNO-Arf6-PLD1 pathway and facilitate the production of phosphatidic acid, which is essential for effective exocytosis in neuroendocrine cells.

A two-subpopulation model that reflects heterogeneity of large dense core vesicles in exocytosis

Exocytosis of large dense core vesicles is responsible for hormone secretion in neuroendocrine cells. The population of primed vesicles ready to release upon cell excitation demonstrates large heterogeneity. However, there are currently no models that clearly reflect such heterogeneity. Here, we develop a novel model based on single vesicle release events from amperometry recordings of PC12 cells using carbon fiber microelectrode. In this model, releasable vesicles can be grouped into two subpopulations, namely, SP1 and SP2. SP1 vesicles replenish quickly, with kinetics of ~0.0368 s^-1, but likely undergo slow fusion pore expansion (amperometric signals rise at ~2.5 pA/ms), while SP2 vesicles demonstrate slow replenishment (kinetics of ~0.0048 s^-1) but prefer fast dilation of fusion pore, with an amperometric signal rising rate of ~9.1 pA/ms. Phorbol ester enlarges the size of SP2 partially via activation of protein kinase C and conveys SP1 vesicles into SP2. Inhibition of Rho GTPase-dependent actin rearrangement almost completely depletes SP2. We also propose that the phorbol ester-sensitive vesicle subpopulation (SP2) is analogous to the subset of superprimed synaptic vesicles in neurons. This model provides a meticulous description of the architecture of the readily releasable vesicle pool and elucidates the heterogeneity of the vesicle priming mechanism.

Phospholipase D1-generated phosphatidic acid modulates secretory granule trafficking from biogenesis to compensatory endocytosis in neuroendocrine cells

Calcium-regulated exocytosis is a multi-step process that allows specialized secretory cells to release informative molecules such as neurotransmitters, neuropeptides, and hormones for intercellular communication. The biogenesis of secretory vesicles from the Golgi cisternae is followed by their transport towards the cell periphery and their docking and fusion to the exocytic sites of the plasma membrane allowing release of vesicular content. Subsequent compensatory endocytosis of the protein and lipidic constituents of the vesicles maintains cell homeostasis. Despite the fact that lipids represent the majority of membrane constituents, little is known about their contribution to these processes. Using a combination of electrochemical measurement of single chromaffin cell catecholamine secretion and electron microscopy of roof-top membrane sheets associated with genetic, silencing and pharmacological approaches, we recently reported that diverse phosphatidic acid (PA) species regulates catecholamine release efficiency by controlling granule docking and fusion kinetics. The enzyme phospholipase D1 (PLD1), producing PA from phosphatidylcholine, seems to be the major responsible of these effects in this model. Here, we extended this work using spinning disk confocal microscopy showing that inhibition of PLD activity also reduced the velocity of granules undergoing a directed motion. Furthermore, a dopamine β-hydroxylase (DβH) internalization assay revealed that PA produced by PLD is required for an optimal recovery of vesicular membrane content by compensatory endocytosis. Thus, among numerous roles that have been attributed to PA our work gives core to the key regulatory role in secretion that has been proposed in different cell models. Few leads to explain these multiple functions of PA along the secretory pathway are discussed.

Phosphatidic acid: Mono- and poly-unsaturated forms regulate distinct stages of neuroendocrine exocytosis

Lipids have emerged as important actors in an ever-growing number of key functions in cell biology over the last few years. Among them, glycerophospholipids are major constituents of cellular membranes. Because of their amphiphilic nature, phospholipids form lipid bilayers that are particularly useful to isolate cellular content from the extracellular medium, but also to define intracellular compartments. Interestingly, phospholipids come in different flavors based on their fatty acyl chain composition. Indeed, lipidomic analyses have revealed the presence in cellular membranes of up to 50 different species of an individual class of phospholipid, opening the possibility of multiple functions for a single class of phospholipid. In this review we will focus on phosphatidic acid (PA), the simplest phospholipid, that plays both structural and signaling functions. Among the numerous roles that have been attributed to PA, a key regulatory role in secretion has been proposed in different cell models. We review here the evidences that support the idea that mono- and poly-unsaturated PA control distinct steps in hormone secretion from neuroendocrine cells.

Role of Phospholipase D-Derived Phosphatidic Acid in Regulated Exocytosis and Neurological Disease

Lipids play a vital role in numerous cellular functions starting from a structural role as major constituents of membranes to acting as signaling intracellular or extracellular entities. Accordingly, it has been known for decades that lipids, especially those coming from diet, are important to maintain normal physiological functions and good health. On the other side, the exact molecular nature of these beneficial or deleterious lipids, as well as their precise mode of action, is only starting to be unraveled. This recent improvement in our knowledge is largely resulting from novel pharmacological, molecular, cellular, and genetic tools to study lipids in vitro and in vivo. Among these important lipids, phosphatidic acid plays a unique and central role in a great variety of cellular functions. This review will focus on the proposed functions of phosphatidic acid generated by phospholipase D in the last steps of regulated exocytosis with a specific emphasis on hormonal and neurotransmitter release and its potential impact on different neurological diseases.

The Small GTPase Arf6: An Overview of Its Mechanisms of Action and of Its Role in Host⁻Pathogen Interactions and Innate Immunity

The small GTase Arf6 has several important functions in intracellular vesicular trafficking and regulates the recycling of different types of cargo internalized via clathrin-dependent or -independent endocytosis. It activates the lipid modifying enzymes PIP 5-kinase and phospholipase D, promotes actin polymerization, and affects several functionally distinct processes in the cell. Arf6 is used for the phagocytosis of pathogens and can be directly or indirectly targeted by various pathogens to block phagocytosis or induce the uptake of intracellular pathogens. Arf6 is also used in the signaling of Toll-like receptors and in the activation of NADPH oxidases. In this review, we first give an overview of the different roles and mechanisms of action of Arf6 and then focus on its role in innate immunity and host–pathogen interactions.

Phosphatidic acid induces conformational changes in Sec18 protomers that prevent SNARE priming

Eukaryotic cell homeostasis requires transfer of cellular components among organelles and relies on membrane fusion catalyzed by SNARE proteins. Inactive SNARE bundles are reactivated by hexameric N-ethylmaleimide-sensitive factor, vesicle-fusing ATPase (Sec18/NSF)-driven disassembly that enables a new round of membrane fusion. We previously found that phosphatidic acid (PA) binds Sec18 and thereby sequesters it from SNAREs and that PA dephosphorylation dissociates Sec18 from the membrane, allowing it to engage SNARE complexes. We now report that PA also induces conformational changes in Sec18 protomers and that hexameric Sec18 cannot bind PA membranes. Molecular dynamics (MD) analyses revealed that the D1 and D2 domains of Sec18 contain PA-binding sites and that the residues needed for PA binding are masked in hexameric Sec18. Importantly, these simulations also disclosed that a major conformational change occurs in the linker region between the D1 and D2 domains, which is distinct from the conformational changes that occur in hexameric Sec18 during SNARE priming. Together, these findings indicate that PA regulates Sec18 function by altering its architecture and stabilizing membrane-bound Sec18 protomers.

Phospholipase D and phosphatidic acid in the biogenesis and cargo loading of extracellular vesicles

Extracellular vesicles released by viable cells (exosomes and microvesicles) have emerged as important organelles supporting cell-cell communication. Because of their potential therapeutic significance, important efforts are being made toward characterizing the contents of these vesicles and the mechanisms that govern their biogenesis. It has been recently demonstrated that the lipid modifying enzyme, phospholipase D (PLD)2, is involved in exosome production and acts downstream of the small GTPase, ARF6. This review aims to recapitulate our current knowledge of the role of PLD2 and its product, phosphatidic acid, in the biogenesis of exosomes and to propose hypotheses for further investigation of a possible central role of these molecules in the biology of these organelles.

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.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Elucidation of the CHO Super-Ome (CHO-SO) by Proteoinformatics

Chinese hamster ovary (CHO) cells are the preferred host cell line for manufacturing a variety of complex biotherapeutic drugs including monoclonal antibodies. We performed a proteomics and bioinformatics analysis on the spent medium from adherent CHO cells. Supernatant from CHO-K1 culture was collected and subjected to in-solution digestion followed by LC/LC-MS/MS analysis, which allowed the identification of 3281 different host cell proteins (HCPs). To functionally categorize them, we applied multiple bioinformatics tools to the proteins identified in our study including SignalP, TargetP, SecretomeP, TMHMM, WoLF PSORT, and Phobius. This analysis provided information on the presence of signal peptides, transmembrane domains, and cellular localization and showed that both secreted and intracellular proteins were constituents of the supernatant. Identified proteins were shown to be localized to the secretory pathway including ones playing roles in cell growth, proliferation, and folding as well as those involved in protein degradation and removal. After combining proteins predicted to be secreted or having a signal peptide, we identified 1015 proteins, which we termed as CHO supernatant-ome (CHO-SO), or superome. As a part of this effort, we created a publically accessible web-based tool called GO-CHO to functionally categorize proteins found in CHO-SO and identify enriched molecular functions, biological processes, and cellular components. We also used a tool to evaluate the immunogenicity potential of high-abundance HCPs. Among enriched functions were catalytic activity and structural constituents of the cytoskeleton. Various transport related biological processes, such as vesicle mediated transport, were found to be highly enriched. Extracellular space and vesicular exosome associated proteins were found to be the most enriched cellular components. The superome also contained proteins secreted from both classical and nonclassical secretory pathways. The work and database described in our study will enable the CHO community to rapidly identify high-abundance HCPs in their cultures and therefore help assess process and purification methods used in the production of biologic drugs.

GGA3 mediates TrkA endocytic recycling to promote sustained Akt phosphorylation and cell survival

GGA3 binds directly to the TrkA internal DXXLL motif and mediates TrkA endocytic recycling. This effect is dependent on the activation of Arf6. GGA3 is a key player in a novel DXXLL-mediated recycling machinery for TrkA, where it prolongs the activation of Akt signaling and survival responses.

Although TrkA postendocytic sorting significantly influences neuronal cell survival and differentiation, the molecular mechanism underlying TrkA receptor sorting in the recycling or degradation pathways remains poorly understood. Here we demonstrate that Golgi-localized, γ adaptin-ear–containing ADP ribosylation factor-binding protein 3 (GGA3) interacts directly with the TrkA cytoplasmic tail through an internal DXXLL motif and mediates the functional recycling of TrkA to the plasma membrane. We find that GGA3 depletion by siRNA delays TrkA recycling, accelerates TrkA degradation, attenuates sustained NGF-induced Akt activation, and reduces cell survival. We also show that GGA3’s effect on TrkA recycling is dependent on the activation of Arf6. This work identifies GGA3 as a key player in a novel DXXLL-mediated endosomal sorting machinery that targets TrkA to the plasma membrane, where it prolongs the activation of Akt signaling and survival responses.

ADP ribosylation factor 6 (ARF6) promotes acrosomal exocytosis by modulating lipid turnover and Rab3A activation

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. ARF6 (ADP-ribosylation factor 6) is a small GTPase implicated in exocytosis, but its downstream effectors remain elusive in this process. We combined biochemical, functional, and microscopy-based methods to show that ARF6 is present in human sperm, localizes to the acrosomal region, and is required for calcium and diacylglycerol-induced exocytosis. Results from pulldown assays show that ARF6 exchanges GDP for GTP in sperm challenged with different exocytic stimuli. Myristoylated and guanosine 5'-3-O-(thio)triphosphate (GTPγS)-loaded ARF6 (active form) added to permeabilized sperm induces acrosome exocytosis even in the absence of extracellular calcium. We explore the ARF6 signaling cascade that promotes secretion. We demonstrate that ARF6 stimulates a sperm phospholipase D activity to produce phosphatidic acid and boosts the synthesis of phosphatidylinositol 4,5-bisphosphate. We present direct evidence showing that active ARF6 increases phospholipase C activity, causing phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol 1,4,5-trisphosphate-dependent intra-acrosomal calcium release. We show that active ARF6 increases the exchange of GDP for GTP on Rab3A, a prerequisite for secretion. We propose that exocytic stimuli activate ARF6, which is required for acrosomal calcium efflux and the assembly of the membrane fusion machinery. This report highlights the physiological importance of ARF6 as a key factor for human sperm exocytosis and fertilization.

Macrophages phagocytose nonopsonized silica particles using a unique microtubule-dependent pathway

Cells can take up particles by both opsonized and nonopsonized pathways. Silica and latex, but not zymosan, can be taken up by the nonopsonized pathway. Uptake of silica, but not latex, is toxic to macrophages. Nonopsonized phagocytosis is characterized and found to have key differences from the complement- and antibody-opsonized pathways.

Silica inhalation leads to the development of the chronic lung disease silicosis. Macrophages are killed by uptake of nonopsonized silica particles, and this is believed to play a critical role in the etiology of silicosis. However, the mechanism of nonopsonized-particle uptake is not well understood. We compared the molecular events associated with nonopsonized- and opsonized-particle phagocytosis. Both Rac and RhoA GTPases are activated upon nonopsonized-particle exposure, whereas opsonized particles activate either Rac or RhoA. All types of particles quickly generate a PI(3,4,5)P₃ and F-actin response at the particle attachment site. After formation of a phagosome, the events related to endolysosome-to-phagosome fusion do not significantly differ between the pathways. Inhibitors of tyrosine kinases, actin polymerization, and the phosphatidylinositol cascade prevent opsonized- and nonopsonized-particle uptake similarly. Inhibition of silica particle uptake prevents silica-induced cell death. Microtubule depolymerization abolished uptake of complement-opsonized and nonopsonized particles but not Ab-opsonized particles. Of interest, regrowth of microtubules allowed uptake of new nonopsonized particles but not ones bound to cells in the absence of microtubules. Although complement-mediated uptake requires macrophages to be PMA-primed, untreated cells phagocytose nonopsonized silica and latex. Thus it appears that nonopsonized-particle uptake is accomplished by a pathway with unique characteristics.

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.

The function of α-synuclein

Human genetics has indicated a causal role for the protein α-synuclein in the pathogenesis of familial Parkinson's disease (PD), and the aggregation of synuclein in essentially all patients with PD suggests a central role for this protein in the sporadic disorder. Indeed, the accumulation of misfolded α-synuclein now defines multiple forms of neural degeneration. Like many of the proteins that accumulate in other neurodegenerative disorders, however, the normal function of synuclein remains poorly understood. In this article, we review the role of synuclein at the nerve terminal and in membrane remodeling. We also consider the prion-like propagation of misfolded synuclein as a mechanism for the spread of degeneration through the neuraxis.

Endosomal trafficking of the receptor tyrosine kinase MuSK proceeds via clathrin-dependent pathways, Arf6 and actin

Muscle-specific kinase (MuSK), a receptor tyrosine kinase, is the key player during the formation of the neuromuscular junction. Signal transduction events downstream of MuSK activation induce both pre-and postsynaptic differentiation, which, most prominently, includes the clustering of acetylcholine receptors at synaptic sites. More recently, regulated MuSK endocytosis and degradation have been implicated as crucial events for MuSK signalling activity, implicating a cross-talk between signalling and endocytosis. In the present study, we use a live imaging approach to study MuSK endocytosis. We find that MuSK is internalized via a clathrin-, dynamin-dependent pathway. MuSK is transported to Rab7-positive endosomes for degradation and recycled via Rab4-and Rab11-positive vesicles. MuSK activation by Dok7 mildly affects the localization of MuSK on the cell surface but has no effect on the rate of MuSK internalization. Interestingly, MuSK colocalizes with actin and Arf6 at the cell surface and during endosomal trafficking. Disruption of the actin cytoskeleton or the proper function of Arf6 concentrates MuSK in cell protrusions. Moreover, inhibition of Arf6 or cytoskeletal rearrangements impairs acetylcholine receptor clustering and phosphorylation. These results suggest that MuSK uses both classical and nonclassical endosomal pathways that involve a variety of different components of the endosomal machinery.

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.

Phagocytic NADPH oxidase links ARNO-Arf6 signaling pathway in glucose-stimulated insulin secretion from the pancreatic β-cell

BACKGROUND

Recent findings from our laboratory have demonstrated that glucose-stimulated insulin secretion (GSIS) involves interplay between a variety of small G proteins belonging to the Rho (e.g., Cdc42 and Rac1) and ADP-ribosylation factor (e.g., Arf6) subfamilies. Using immunological, pharmacological and molecular biological approaches, we have also identified guanine nucleotide exchange factors (GEFs) for Rac1 (e.g., Tiam1) and Arf6 (e.g., ARNO) in clonal INS-1 832/13 cells, normal rat islets and human islets. As a logical extension to these studies, we investigated, herein, potential downstream signaling steps involved in Arf6/ARNO-mediated GSIS.

METHODS

Using a selective pharmacological inhibitor of ARNO/Arf6 signaling axis (e.g., secinH3) we assessed regulatory roles for Arf6/ARNO in promoting phospholipase D (PLD), phagocytic NADPH oxidase (Nox2), reactive oxygen species (ROS), extracellular-regulated kinases (ERK 1/2) and cofilin (actin-severing protein] signaling steps in clonal INS-1 832/13 cells.

RESULTS

Our data suggested a marked inhibition by secinH3 of glucose-induced PLD activation, ERK1/2 phosphorylation and dephosphorylation of cofilin, suggesting that Arf6/ ARNO signaling mediates PLD, ERK1/2 and cofilin activation in beta-cells. In addition, secinH3 blocked glucose-induced Nox2 activation and associated ROS generation, thus placing Nox downstream to Arf6/ARNO signaling step. Lastly, we also demonstrate a significantly higher cofilin phosphorylation (inactive) in islets derived from type 2 diabetic human donors as well as the Zucker Diabetic Fatty (ZDF) rat, a model for type 2 diabetes.

CONCLUSION

Together, our current findings identify signaling steps downstream to ARNO/Arf6 axis leading to insulin secretion.

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.

Genetically encoded probes for phosphatidic acid

In addition to forming bilayers to separate cellular compartments, lipids participate in vesicular trafficking and signal transduction. Among others, phosphatidic acid (PA) is emerging as an important signaling molecule. The spatiotemporal distribution of cellular PA appears to be tightly regulated by localized synthesis and a rapid metabolism. Although PA has been long proposed as a pleiotropic bioactive lipid, when and where PA is produced in the living cells have only recently been explored using biosensors that specifically bind to PA. The probes that we have generated are composed of the PA-binding domains of either Spo20p or Raf1 directly fused to GFP. In this chapter, we will describe the expression and purification of GST-fusion proteins of these probes, and the use of phospholipid strips to validate the specificity of their interaction with PA. We will then illustrate the use of GFP-tagged probes to visualize the synthesis of PA in the neurosecretory PC12 cells and RAW 267.4 macrophages. Interestingly, the two probes show a differential distribution in these cell types, indicating that they may have different affinities for PA or recognize different pools of PA. In conclusion, the development of a broader choice of probes may be required to adequately follow the complex dynamics of PA in different cell types, in order to determine the cellular distribution of PA and its role in various cellular processes.

Role of phospholipase D1 in glucose-induced insulin secretion in pancreatic Beta cells

As glucose is known to induce insulin secretion in pancreatic Beta cells, this study investigated the role of a phospholipase D (PLD)-related signaling pathway in insulin secretion caused by high glucose in the pancreatic Beta-cell line MIN6N8. It was found that the PLD activity and PLD1 expression were both increased by high glucose (33.3 mM) treatment. The dominant negative PLD1 inhibited glucose-induced Beta2 expression, and glucose-induced insulin secretion was blocked by treatment with 1-butanol or PLD1-siRNA. These results suggest that high glucose increased insulin secretion through a PLD1-related pathway. High glucose induced the binding of Arf6 to PLD1. Pretreatment with brefeldin A (BFA), an Arf inhibitor, decreased the PLD activity as well as the insulin secretion. Furthermore, BFA blocked the glucose-induced mTOR and p70S6K activation, while mTOR inhibition with rapamycin attenuated the glucose induced Beta2 expression and insulin secretion. Thus, when taken together, PLD1 would appear to be an important regulator of glucose-induced insulin secretion through an Arf6/PLD1/mTOR/p70S6K/ Beta2 pathway in MIN6N8 cells.

Small G proteins in islet beta-cell function

Glucose-stimulated insulin secretion from the islet beta-cell involves a sequence of metabolic events and an interplay between a wide range of signaling pathways leading to the generation of second messengers (e.g., cyclic nucleotides, adenine and guanine nucleotides, soluble lipid messengers) and mobilization of calcium ions. Consequent to the generation of necessary signals, the insulin-laden secretory granules are transported from distal sites to the plasma membrane for fusion and release of their cargo into the circulation. The secretory granule transport underlies precise changes in cytoskeletal architecture involving a well-coordinated cross-talk between various signaling proteins, including small molecular mass GTP-binding proteins (G proteins) and their respective effector proteins. The purpose of this article is to provide an overview of current understanding of the identity of small G proteins (e.g., Cdc42, Rac1, and ARF-6) and their corresponding regulatory factors (e.g., GDP/GTP-exchange factors, GDP-dissociation inhibitors) in the pancreatic beta-cell. Plausible mechanisms underlying regulation of these signaling proteins by insulin secretagogues are also discussed. In addition to their positive modulatory roles, certain small G proteins also contribute to the metabolic dysfunction and demise of the islet beta-cell seen in in vitro and in vivo models of impaired insulin secretion and diabetes. Emerging evidence also suggests significant insulin secretory abnormalities in small G protein knockout animals, further emphasizing vital roles for these proteins in normal health and function of the islet beta-cell. Potential significance of these experimental observations from multiple laboratories and possible avenues for future research in this area of islet research are highlighted.

Synthesis of fusogenic lipids through activation of phospholipase D1 by GTPases and the kinase RSK2 is required for calcium-regulated exocytosis in neuroendocrine cells

Exocytosis of hormones occurs through the fusion of large dense-core secretory vesicles with the plasma membrane. This highly regulated process involves key proteins such as SNAREs (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors) and also specific lipids at the site of membrane fusion. Among the different lipids required for exocytosis, our recent observations have highlighted the crucial role of PA (phosphatidic acid) in the late stages of membrane fusion in various exocytotic events. An RNAi (RNA interference) strategy coupled with the detection of PA in living cells has pointed to plasma membrane-associated PLD1 (phospholipase D1) as the main producer of PA in response to secretagogue stimulation. We have identified several GTPases which regulate the activation level of PLD1 in neuroendocrine cells. Finally, RSK2 (ribosomal S6 kinase 2) appears to phosphorylate and regulate the activity of PLD1 in a calcium-dependent manner. Altogether our results have unravelled a complex set of regulatory pathways controlling the synthesis of fusogenic lipids at the secretory granule fusion site by PLD1.

IQ-ArfGEF/BRAG1 is associated with synaptic ribbons in the mouse retina

IQ-ArfGEF/BRAG1 is a guanine nucleotide exchange factor for ADP ribosylation factors (Arfs), which are implicated in membrane trafficking and actin cytoskeleton dynamics. In this study, we examined the immunohistochemical localization of IQ-ArfGEF/BRAG1 in the adult mouse retina using light and electron microscopy. IQ-ArfGEF/BRAG1 was distributed in a punctate manner and colocalized well with RIBEYE in both the outer and inner plexiform layers. Immunoelectron microscopic analysis showed that IQ-ArfGEF/BRAG1 was localized at the synaptic ribbons of photoreceptors. When heterologously expressed in HeLa cells, IQ-ArfGEF/BRAG1 was recruited to RIBEYE-containing clusters and formed an immunoprecipitable complex with RIBEYE. Furthermore, immunoprecipitation analysis showed that anti-IQ-ArfGEF/BRAG1 antibody efficiently pulled down RIBEYE from retinal lysates. These findings indicate that IQ-ArfGEF/BRAG1 is a novel component of retinal synaptic ribbons and forms a protein complex with RIBEYE.

EFA6A encodes two isoforms with distinct biological activities in neuronal cells

The processes of neurite extension and remodeling require a close coordination between the cytoskeleton and the cell membranes. The small GTPase ARF6 (ADP-ribosylation factor 6) has a central role in regulating membrane traffic and actin dynamics, and its activity has been demonstrated to be involved in neurite elaboration. EFA6A has been shown to act as a guanine nucleotide exchange factor (GEF) for ARF6. Here, we report that two distinct isoforms of the EFA6A gene are expressed in murine neural tissue: a long isoform of 1025 amino acids (EFA6A), and a short isoform of 393 amino acids (EFA6As). EFA6A encompasses proline-rich regions, a Sec7 domain (mediating GEF activity on ARF6), a PH domain, and a C-terminal region with coiled-coil motifs. EFA6As lacks the Sec7 domain, and it comprises the PH domain and the C-terminal region. The transcript encoding EFA6As is the result of alternative promoter usage. EFA6A and EFA6As have distinct biological activities: upon overexpression in HeLa cells, EFA6A induces membrane ruffles, whereas EFA6As gives rise to cell elongation; in primary cortical neurons EFA6A promotes neurite extension, whereas EFA6As induces dendrite branching. Our findings suggest that EFA6A could participate in neuronal morphogenesis through the regulated expression of two functionally distinct isoforms.

ARF6 regulates the synthesis of fusogenic lipids for calcium-regulated exocytosis in neuroendocrine cells

An important role for specific lipids in membrane fusion has recently emerged, but regulation of their biosynthesis remains poorly understood. Among fusogenic lipids, phosphatidic acid and phosphoinositol 4,5-bisphosphate (PIP(2)) have been proposed to act at various steps of neurotransmitter and hormone exocytosis. Using real time FRET (fluorescence resonance energy transfer) measurements, we show here that the GTPase ARF6, potentially involved in the synthesis of these lipids, is activated at the exocytotic sites in PC12 cells stimulated for secretion. Depletion of endogenous ARF6 by siRNA dramatically inhibited secretagogue-evoked exocytosis. ARF6-siRNA greatly reduced secretagogue-evoked phospholipase D (PLD) activation and phosphatidic acid formation at the plasma membrane and moderately reduced constitutive levels of PIP(2) present at the plasma membrane in resting cells. Expression of an ARF6 insensitive to short interference RNA (siRNA) fully rescued secretion in ARF6-depleted cells. However, a mutated ARF6 protein specifically impaired in its ability to stimulate PLD had no effect. Finally, we show that the ARF6-siRNA-mediated inhibition of exocytosis could be rescued by an exogenous addition of lysophosphatidylcholine, a lipid that favors negative curvature on the inner leaflet of the plasma membrane. Altogether these data indicate that ARF6 is a critical upstream signaling element in the activation of PLD necessary to produce the fusogenic lipids required for exocytosis.

Predominant localization of EFA6A, a guanine nucleotide exchange factor for ARF6, at the perisynaptic photoreceptor processes

EFA6A is a guanine nucleotide exchange factor that is highly expressed in the nervous system with the ability to activate ADP ribosylation factor 6 (ARF6). In this study, we demonstrated the immunohistochemical localization of EFA6A in the adult mouse retina. Strong immunoreactivity for EFA6A was detected predominantly in the outer plexiform layer (OPL), where EFA6A was partially overlapped with dystrophin and synaptophysin. Immunoelectron microscopic analysis revealed that EFA6A was localized predominantly at the perisynaptic processes of photoreceptor terminals without association with synaptic ribbons. These findings suggest that EFA6A-ARF6 pathway may play a specific role at a subcompartment of perisynaptic photoreceptor processes.

The Coffin-Lowry syndrome-associated protein RSK2 is implicated in calcium-regulated exocytosis through the regulation of PLD1

Exocytosis of neurotransmitters and hormones occurs through the fusion of secretory vesicles with the plasma membrane. This highly regulated process involves key proteins, such as SNAREs, and specific lipids at the site of membrane fusion. Phospholipase D (PLD) has recently emerged as a promoter of membrane fusion in various exocytotic events potentially by providing fusogenic cone-shaped phosphatidic acid. We show here that PLD1 is regulated by ribosomal S6 kinase 2 (RSK2)-dependent phosphorylation. RSK2 is activated by a high K(+)-induced rise in cytosolic calcium. Expression of inactive RSK2 mutants or selective knockdown of endogenous RSK2 dramatically affects the different kinetic components of the exocytotic response in chromaffin cells. RSK2 physically interacts with and stimulates PLD activity through the phosphorylation of Thr-147 in the PLD1 amino-terminal phox homology domain. Expression of PLD1 phosphomimetic mutants fully restores secretion in cells depleted of RSK2, suggesting that RSK2 is a critical upstream signaling element in the activation of PLD1 to produce the lipids required for exocytosis. We propose that PLD-related defects in neuronal and endocrine activities could contribute to the effect observed after the loss-of-function mutations in Rsk2 that lead to Coffin-Lowry syndrome, an X-linked form of growth and mental retardation.

Podosomes and Invadopodia: Related structures with Common Protein Components that May Promote Breast Cancer Cellular Invasion

A rate-limiting step in breast cancer progression is acquisition of the invasive phenotype, which can precede metastasis. Expression of cell-surface proteases at the leading edge of a migrating cell provides cells with a mechanism to cross tissue barriers. A newly appreciated mechanism that may be relevant for breast cancer cell invasion is the formation of invadopodia, well-defined structures that project from the ventral membrane and promote degradation of the extracellular matrix, allowing the cell to cross a tissue barrier. Recently, there has been some controversy and discussion as to whether invadopodia, which are associated with carcinoma cells, are related to a similar structure called podosomes, which are associated with normal cells. Invadopodia and podosomes share many common characteristics, including a similar size, shape, subcellular localization and an ability to promote invasion. These two structures also share many common protein components, which we outline herein. It has been speculated that podosomes may be precursors to invadopodia and by extension both structures may be relevant to cancer cell invasion. Here, we compare and contrast the protein components of invadopodia and podosomes and discuss a potential role for these proteins and the evidence that supports a role for invadopodia and podosomes in breast cancer invasion.

Primary platelet signaling cascades and integrin-mediated signaling control ADP-ribosylation factor (Arf) 6-GTP levels during platelet activation and aggregation

Previous studies showed that ADP-ribosylation factor 6 (Arf6) is important for platelet function; however, little is known about which signaling events regulate this small GTP-binding protein. Arf6-GTP was monitored in platelets stimulated with a number of agonists (TRAP, thrombin, convulxin, collagen, PMA, thapsigargin, or A23187) and all led to a time-dependent decrease in Arf6-GTP. ADP and U46619 were without effect. Using inhibitors, it was shown that the decrease of Arf6-GTP is a direct consequence of known signaling cascades. Upon stimulation via PAR receptors, Arf6-GTP loss could be blocked by treatment with U-73122, BAPTA/AM, Ro-31-8220, or Gö6976, indicating requirements for phospholipase C, calcium, and protein kinase C (PKC) alpha/beta, respectively. The Arf6-GTP decrease in convulxin-stimulated platelets showed similar requirements and was also sensitive to piceatannol, wortmannin, and LY294002, indicating additional requirements for Syk and phosphatidylinositol 3-kinase. The convulxin-induced decrease was sensitive to both PKCalpha/beta and delta inhibitors. Outside-in signaling, potentially via integrin engagement, caused a second wave of signaling that affected Arf6. Inclusion of RGDS peptides or EGTA, during activation, led to a biphasic response; Arf6-GTP levels partially recovered upon continued incubation. A similar response was seen in beta3 integrin-null platelets. These data show that Arf6-GTP decreases in response to known signaling pathways associated with PAR and GPVI. They further reveal a second, aggregation-dependent, process that dampens Arf6-GTP recovery. This study demonstrates that the nucleotide state of Arf6 in platelets is regulated during the initial phases of activation and during the later stages of aggregation.

Nonredundant function of secretory carrier membrane protein isoforms in dense core vesicle exocytosis

Five secretory carrier membrane proteins (SCAMP-1, -2, -3, -4, and -5) have been characterized in mammalian cells. Previously, SCAMP-1 and -2 have been implicated to function in exocytosis. RNA inhibitor-mediated deficiency of one or both of these SCAMPs interferes with dense core vesicle (DCV) exocytosis in neuroendocrine PC12 cells as detected by amperometry. Knockdowns of these SCAMPs each decreased the number and frequency of depolarization-induced exocytotic events. SCAMP-2 but not SCAMP-1 depletion also delayed the onset of exocytosis. Both knockdowns, however, altered fusion pore dynamics, increasing rapid pore closure and decreasing pore dilation. In contrast, knockdowns of SCAMP-3 and -5 only interfered with the frequency of fusion pore opening and did not affect the dynamics of newly opened pores. None of the knockdowns noticeably affected upstream events, including the distribution of DCVs near the plasma membrane and calcium signaling kinetics, although norepinephrine uptake/storage was moderately decreased by deficiency of SCAMP-1 and -5. Thus, SCAMP-1 and -2 are most closely linked to the final events of exocytosis. Other SCAMPs collaborate in regulating fusion sites, but the roles of individual isoforms appear at least partially distinct.

Spermatocyte cytokinesis requires rapid membrane addition mediated by ARF6 on central spindle recycling endosomes

The dramatic cell shape changes during cytokinesis require the interplay between microtubules and the actomyosin contractile ring, and addition of membrane to the plasma membrane. Numerous membrane-trafficking components localize to the central spindle during cytokinesis, but it is still unclear how this machinery is targeted there and how membrane trafficking is coordinated with cleavage furrow ingression. Here we use an arf6 null mutant to show that the endosomal GTPase ARF6 is required for cytokinesis in Drosophila spermatocytes. ARF6 is enriched on recycling endosomes at the central spindle, but it is required neither for central spindle nor actomyosin contractile ring assembly, nor for targeting of recycling endosomes to the central spindle. However, in arf6 mutants the cleavage furrow regresses because of a failure in rapid membrane addition to the plasma membrane. We propose that ARF6 promotes rapid recycling of endosomal membrane stores during cytokinesis, which is critical for rapid cleavage furrow ingression.

Acidic extracellular pH increases calcium influx-triggered phospholipase D activity along with acidic sphingomyelinase activation to induce matrix metalloproteinase-9 expression in mouse metastatic melanoma

Acidic extracellular pH is a common feature of tumor tissues. We have reported that culturing cells at acidic pH (5.4-6.5) induced matrix metalloproteinase-9 expression through phospholipase D, extracellular signal regulated kinase 1/2 and p38 mitogen-activated protein kinases and nuclear factor-kappaB. Here, we show that acidic extracellular pH signaling involves both pathways of phospholipase D triggered by Ca2+ influx and acidic sphingomyelinase in mouse B16 melanoma cells. We found that BAPTA-AM [1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl) ester], a chelator of intracellular free calcium, and the voltage dependent Ca2+ channel blockers, mibefradil (for T-type) and nimodipine (for L-type), dose-dependently inhibited acidic extracellular pH-induced matrix metalloproteinase-9 expression. Intracellular free calcium concentration ([Ca2+]i) was transiently elevated by acidic extracellular pH, and this [Ca2+]i elevation was repressed by EGTA and the voltage dependent Ca2+ channel blockers but not by phospholipase C inhibitor, suggesting that acidic extracellular pH increased [Ca2+]i through voltage dependent Ca2+ channel. In contrast, SR33557, an L-type voltage dependent Ca2+ channel blocker and acidic sphingomyelinase inhibitor, attenuated matrix metalloproteinase-9 induction but did not affect calcium influx. We found that acidic sphingomyelinase activity was induced by acidic extracellular pH and that the specific acidic sphingomyelinase inhibitors (perhexiline and desipramine) and siRNA targeting aSMase/smpd1 could inhibit acidic extracellular pH-induced matrix metalloproteinase-9 expression. BAPTA-AM reduced acidic extracellular pH-induced phospholipase D but not acidic sphingomyelinase acitivity. The acidic sphingomyelinase inhibitors did not affect the phosphorylation of extracellular signal regulated kinase 1/2 and p38, but they suppressed nuclear factor-kappaB activity. These data suggest that the calcium influx-triggered phospholipase D and acidic sphingomyelinase pathways of acidic extracellular pH induced matrix metalloproteinase-9 expression, at least in part, through nuclear factor-kappaB activation.

Phospholipase D1 Production of Phosphatidic Acid at the Plasma Membrane Promotes Exocytosis of Large Dense-core Granules at a Late Stage

Substantial efforts have recently been made to demonstrate the importance of lipids and lipid-modifying enzymes in various membrane trafficking processes, including calcium-regulated exocytosis of hormones and neurotransmitters. Among bioactive lipids, phosphatidic acid (PA) is an attractive candidate to promote membrane fusion through its ability to change membrane topology. To date, however, the biosynthetic pathway, the dynamic location, and actual function of PA in secretory cells remain unknown. Using a short interference RNA strategy on chromaffin and PC12 cells, we demonstrate here that phospholipase D1 is activated in secretagogue-stimulated cells and that it produces PA at the plasma membrane at the secretory granule docking sites. We show that phospholipase D1 activation and PA production represent key events in the exocytotic progression. Membrane capacitance measurements indicate that reduction of endogenous PA impairs the formation of fusion-competent granules. Finally, we show that the PLD1 short interference RNA-mediated inhibition of exocytosis can be rescued by exogenous provision of a lipid that favors the transition of opposed bi-layer membranes to hemifused membranes having the outer leaflets fused. Our findings demonstrate that PA synthesis is required during exocytosis to facilitate a late event in the granule fusion pathway. We propose that the underlying mechanism is related to the ability of PA to alter membrane curvature and promote hemi-fusion.

Somatodendritic localization of EFA6A, a guanine nucleotide exchange factor for ADP-ribosylation factor 6, and its possible interaction with α-actinin in dendritic spines

EFA6A is a member of the guanine nucleotide exchange factors that can specifically activate ADP ribosylation factor 6 (ARF6). In this study, we identified alpha-actinin-1 as a possible interacting protein with EFA6A by the yeast two-hybrid screening with its C-terminal region as bait. The central region of alpha-actinin-1 containing a part of spectrin repeat 1 and spectrin repeats 2-3 is responsible for this interaction. In the hippocampal formation, EFA6A immunoreactivity occurred at a high level as numerous fine puncta in the strata oriens, radiatum, lacunosum-moleculare of the hippocampal CA1-3 subfields and the dentate molecular layer, whereas the immunoreactivity was faint in the neuronal cell layers and the stratum lucidum, the mossy fiber-recipient layer of the CA3 subfield. Double-immunofluorescent analyses revealed a partial overlapping of EFA6A and alpha-actinin at the dendritic spines of in vivo and cultured hippocampal neurons. Our present findings suggest that EFA6A may form a protein complex with alpha-actinin and activate ARF6 in close proximity of the actin cytoskeleton and membrane proteins in the dendritic spines.

Phosphoinositide regulation of neuroexocytosis: adding to the complexity

Exocytosis of neurotransmitter containing vesicles supports neuronal communication. The importance of molecular interactions involving specific lipids has become progressively more evident and the lipid composition of both the synaptic vesicle and the pre-synaptic plasma membrane at the active zone has significant functional consequences for neurotransmitter release. Several classes of lipids have been implicated in exocytosis including polyunsaturated fatty acids and phosphoinositides. This minireview will focus on recent developments regarding the role of phosphoinositides in neurosecretion.

Regulation of neuroendocrine exocytosis by the ARF6 GTPase-activating protein GIT1

Neuroendocrine cells release hormones and neuropeptides by exocytosis, a highly regulated process in which secretory granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Using chromaffin and PC12 cells, we have recently described that the granule-associated GTPase ARF6 plays a crucial role in exocytosis by activating phospholipase D1 at the plasma membrane and, presumably, promoting the fusion reaction between the two membrane bilayers. ARF6 is activated by the nucleotide exchange factor ARNO following docking of granules to the plasma membrane. We show here that GIT1, a GTPase-activating protein stimulating GTP hydrolysis on ARF6, is the second molecular partner that turns over the GDP/GTP cycle of ARF6 during cell stimulation. Western blot and immunofluorescence experiments indicated that GIT1 is cytosolic in resting cells but is recruited to the plasma membrane in stimulated cells, where it co-localizes with ARF6 at the granule docking sites. Over-expression of wild-type GIT1 inhibits growth hormone secretion from PC12 cells; this inhibitory effect was not observed in cells expressing a GIT1 mutant impaired in its ARF-GTPase-activating protein (GAP) activity or in cells expressing other ARF6-GAPs. Conversely reduction of GIT1 by RNA interference increased the exocytotic activity. Using a real time assay for individual chromaffin cells, we found that microinjection of GIT1 strongly reduced the number of exocytotic events. These results provide the first evidence that GIT1 plays a function in calcium-regulated exocytosis in neuroendocrine cells. We propose that GIT1 represents part of the pathway that inactivates ARF6-dependent reactions and thereby negatively regulates and/or terminates exocytotic release.

Arf6 plays an early role in platelet activation by collagen and convulxin

Small GTPases play critical roles in hemostasis, though the roster of such molecules in platelets is not complete. In this study, we report the presence of Ras-related GTPases of the ADP-ribosylation factor (Arf) family. Platelets contain Arf1 or 3 and Arf6, with the latter being predominantly membrane associated. Using effector domain pull-down assays, we show, counter to other GTPases, that Arf6-GTP is present in resting platelets and decreases rapidly upon activation with collagen or convulxin. This decrease does not completely rely on secondary agonists (ADP and thromboxane A2) or require integrin signaling. The decrease in free Arf6-GTP temporally precedes activation of Rho family GTPases (RhoA, Cdc42, and Rac1). Using a membrane-permeant, myristoylated peptide, which mimics the N-terminus of Arf6, we show that the Arf6-GTP decrease is essential for collagen- and convulxin-induced aggregation, platelet adherence, and spreading on collagen-coated glass. Treatment with this peptide also affects the activation of Rho family GTPases, but has little effect on RalA and Rap1 or on agonist-induced calcium mobilization. These data show that Arf6 is a key element in activation through GPVI, and is required for activation of the Rho family GTPases and the subsequent cytoskeletal rearrangements needed for full platelet function.

An effector domain mutant of Arf6 implicates phospholipase D in endosomal membrane recycling

In this study, we investigated the role of phospholipase D (PLD) in mediating Arf6 function in cells. Expression of Arf6 mutants that are defective in activating PLD, Arf6N48R and Arf6N48I, inhibited membrane recycling to the plasma membrane (PM), resulting in an accumulation of tubular endosomal membranes. Additionally, unlike wild-type Arf6, neither Arf6 mutant could generate protrusions or recruit the Arf6 GTPase activating protein (GAP) ACAP1 onto the endosome in the presence of aluminum fluoride. Remarkably, all of these phenotypes, including accumulated tubular endosomes, blocked recycling, and failure to make protrusions and recruit ACAP effectively, could be recreated in either untransfected cells or cells expressing wild-type Arf6 by treatment with 1-butanol to inhibit the formation of phosphatidic acid (PA), the product of PLD. Moreover, most of the defects present in cells expressing Arf6N48R or N48I could be reversed by treatment with agents expected to elevate PA levels in cells. Together, these observations provide compelling evidence that Arf6 stimulation of PLD is required for endosomal membrane recycling and GAP recruitment.