A presynaptic role for the ADP ribosylation factor (ARF)-specific GDP/GTP exchange factor msec7-1

Small GTPase control of pituitary hormone secretion: Evidence from studies in the goldfish (Carassius auratus) neuroendocrine model

The secretion of vertebrate pituitary hormones is regulated by multiple hypothalamic factors, which, while generally activating unique receptor systems, ultimately propagate signals through interacting intracellular regulatory elements to modulate hormone exocytosis. One important family of intracellular regulators is the monomeric small GTPases, a subset of which (Arf1/6, Rac, RhoA, and Ras) is highly conserved across vertebrates and regulates secretory vesicle exocytosis in many cell types. In this study, we investigated the roles of these small GTPases in basal and agonist-dependent hormone release from dispersed goldfish (Carassius auratus) pituitary cells in perifusion experiments. Inhibition of these small GTPases elevated basal LH and GH secretion, except for Ras inhibition which only increased basal LH release. However, variable responses were observed with regard to LH and GH responses to the two goldfish native gonadotropin-releasing hormones (GnRH2 and GnRH3). GnRH-dependent LH release, but not GH secretion, was mediated by Arf1/6 GTPases. In contrast, inhibition of Rac and RhoA GTPases selectively enhanced GnRH3- and GnRH2-dependent GH release, respectively, while Ras inhibition only enhanced GnRH3-evoked LH secretion. Together, our results reveal novel divergent cell-type- and ligand-specific roles for small GTPases in the control of goldfish pituitary hormone exocytosis in unstimulated and GnRH-evoked release.

Physiological and Pathological Roles of the Cytohesin Family in Neurons

The cytohesin proteins, consisting of four closely related members (cytohesins-1, -2, -3, and -4), are a subfamily of the Sec7 domain-containing guanine nucleotide exchange factors for ADP ribosylation factors (Arfs), which are critical regulators of membrane trafficking and actin cytoskeleton remodeling. Recent advances in molecular biological techniques and the development of a specific pharmacological inhibitor for cytohesins, SecinH3, have revealed the functional involvement of the cytohesin-Arf pathway in diverse neuronal functions from the formation of axons and dendrites, axonal pathfinding, and synaptic vesicle recycling, to pathophysiological processes including chronic pain and neurotoxicity induced by proteins related to neurodegenerative disorders, such as amyotrophic lateral sclerosis and Alzheimer's disease. Here, we review the physiological and pathological roles of the cytohesin-Arf pathway in neurons and discuss the future directions of this research field.

The Recycling Endosome in Nerve Cell Development: One Rab to Rule Them All?

Endocytic recycling is an intracellular process that returns internalized molecules back to the plasma membrane and plays crucial roles not only in the reuse of receptor molecules but also in the remodeling of the different components of this membrane. This process is required for a diversity of cellular events, including neuronal morphology acquisition and functional regulation, among others. The recycling endosome (RE) is a key vesicular component involved in endocytic recycling. Recycling back to the cell surface may occur with the participation of several different Rab proteins, which are master regulators of membrane/protein trafficking in nerve cells. The RE consists of a network of interconnected and functionally distinct tubular subdomains that originate from sorting endosomes and transport their cargoes along microtubule tracks, by fast or slow recycling pathways. Different populations of REs, particularly those formed by Rab11, Rab35, and Arf6, are associated with a myriad of signaling proteins. In this review, we discuss the cumulative evidence suggesting the existence of heterogeneous domains of REs, controlling different aspects of neurogenesis, with a particular focus on the commonalities and singularities of these REs and their contribution to nerve development and differentiation in several animal models.

The small GTPase ARF6 regulates GABAergic synapse development

ADP ribosylation factors (ARFs) are a family of small GTPases composed of six members (ARF1-6) that control various cellular functions, including membrane trafficking and actin cytoskeletal rearrangement, in eukaryotic cells. Among them, ARF1 and ARF6 are the most studied in neurons, particularly at glutamatergic synapses, but their roles at GABAergic synapses have not been investigated. Here, we show that a subset of ARF6 protein is localized at GABAergic synapses in cultured hippocampal neurons. In addition, we found that knockdown (KD) of ARF6, but not ARF1, triggered a reduction in the number of GABAergic synaptic puncta in mature cultured neurons in an ARF activity-dependent manner. ARF6 KD also reduced GABAergic synaptic density in the mouse hippocampal dentate gyrus (DG) region. Furthermore, ARF6 KD in the DG increased seizure susceptibility in an induced epilepsy model. Viewed together, our results suggest that modulating ARF6 and its regulators could be a therapeutic strategy against brain pathologies involving hippocampal network dysfunction, such as epilepsy.

Pallidin is a novel interacting protein for cytohesin-2 and regulates the early endosomal pathway and dendritic formation in neurons

Cytohesin-2 is a member of the guanine nucleotide exchange factors for ADP ribosylation factor 1 (Arf1) and Arf6, which are small GTPases that regulate membrane traffic and actin dynamics. In this study, we first demonstrated that cytohesin-2 localized to the plasma membrane and vesicles in various subcellular compartment in hippocampal neurons by immunoelectron microscopy. Next, to understand the molecular network of cytohesin-2 in neurons, we conducted yeast two-hybrid screening of brain cDNA libraries using cytohesin-2 as bait and isolated pallidin, a component of the biogenesis of lysosome-related organelles complex 1 (BLOC-1) involved in endosomal trafficking. Pallidin interacted specifically with cytohesin-2 among cytohesin family members. Glutathione S-transferase pull-down and immunoprecipitation assays further confirmed the formation of a protein complex between cytohesin-2 and pallidin. Immunofluorescence demonstrated that cytohesin-2 and pallidin partially colocalized in various subsets of endosomes immunopositive for EEA1, syntaxin 12, and LAMP2 in hippocampal neurons. Knockdown of pallidin or cytohesin-2 reduced cytoplasmic EEA1-positive early endosomes. Furthermore, knockdown of pallidin increased the total dendritic length of cultured hippocampal neurons, which was rescued by co-expression of wild-type pallidin but not a mutant lacking the ability to interact with cytohesin-2. In contrast, knockdown of cytohesin-2 had the opposite effect on total dendritic length. The present results suggested that the interaction between pallidin and cytohesin-2 may participate in various neuronal functions such as endosomal trafficking and dendritic formation in hippocampal neurons. Cover Image for this issue: doi: 10.1111/jnc.14197.

Molecular Machines Determining the Fate of Endocytosed Synaptic Vesicles in Nerve Terminals

The cycle of a synaptic vesicle (SV) within the nerve terminal is a step-by-step journey with the final goal of ensuring the proper synaptic strength under changing environmental conditions. The SV cycle is a precisely regulated membrane traffic event in cells and, because of this, a plethora of membrane-bound and cytosolic proteins are devoted to assist SVs in each step of the journey. The cycling fate of endocytosed SVs determines both the availability for subsequent rounds of release and the lifetime of SVs in the terminal and is therefore crucial for synaptic function and plasticity. Molecular players that determine the destiny of SVs in nerve terminals after a round of exo-endocytosis are largely unknown. Here we review the functional role in SV fate of phosphorylation/dephosphorylation of SV proteins and of small GTPases acting on membrane trafficking at the synapse, as they are emerging as key molecules in determining the recycling route of SVs within the nerve terminal. In particular, we focus on: (i) the cyclin-dependent kinase-5 (cdk5) and calcineurin (CN) control of the recycling pool of SVs; (ii) the role of small GTPases of the Rab and ADP-ribosylation factor (Arf) families in defining the route followed by SV in their nerve terminal cycle. These regulatory proteins together with their synaptic regulators and effectors, are molecular nanomachines mediating homeostatic responses in synaptic plasticity and potential targets of drugs modulating the efficiency of synaptic transmission.

FRMD4A-cytohesin signaling modulates the cellular release of tau

One of the defining pathological features of Alzheimer's disease is the intraneuronal accumulation of tau (also known as MAPT) protein. Tau is also secreted from neurons in response to various stimuli and accumulates in the cerebrospinal fluid of Alzheimer's disease patients. Tau pathology might spread from cell to cell through a mechanism involving secretion and uptake. Here, we developed an assay to follow cellular release and uptake of tau dimers. Individual silencing of ten common late-onset Alzheimer's disease risk genes in HEK293T cells expressing the tau reporters suggested that FRMD4A is functionally linked to tau secretion. FRMD4A depletion by using RNA interference (RNAi) reduced and overexpression increased tau secretion. The activity of cytohesins, interactors of FRMD4A and guanine-nucleotide-exchange factors of Arf6, was necessary for FRMD4A-induced tau secretion. Increased Arf6 and cell polarity signaling through Par6 and atypical protein kinase Cζ (aPKCζ) stimulated tau secretion. In mature cortical neurons, FRMD4A RNAi or inhibition of cytohesins strongly upregulated secretion of endogenous tau. These results suggest that FRMD4A, a genetic risk factor for late-onset Alzheimer's disease, regulates tau secretion by activating cytohesin-Arf6 signaling. We conclude that genetic risk factors of Alzheimer's disease might modulate disease progression by altering tau secretion.

Arf6 regulates the cycling and the readily releasable pool of synaptic vesicles at hippocampal synapse

Recycling of synaptic vesicles (SVs) is a fundamental step in the process of neurotransmission. Endocytosed SV can travel directly into the recycling pool or recycle through endosomes but little is known about the molecular actors regulating the switch between these SV recycling routes. ADP ribosylation factor 6 (Arf6) is a small GTPase known to participate in constitutive trafficking between plasma membrane and early endosomes. Here, we have morphologically and functionally investigated Arf6-silenced hippocampal synapses and found an activity dependent accumulation of synaptic endosome-like organelles and increased release-competent docked SVs. These features were phenocopied by pharmacological blockage of Arf6 activation. The data reveal an unexpected role for this small GTPase in reducing the size of the readily releasable pool of SVs and in channeling retrieved SVs toward direct recycling rather than endosomal sorting. We propose that Arf6 acts at the presynapse to define the fate of an endocytosed SV.

DOI:http://dx.doi.org/10.7554/eLife.10116.001

Communication between neurons takes place at cell-to-cell contacts called synapses. Each synapse is formed between one neuron that sends the message, and another neuron that receives it. The neuron before the synapse – called the presynaptic neuron – contains packets called synaptic vesicles, which are full of chemical messengers ready to be released upon activity.

Accurate communication between neurons relies on the exact composition, and organized trafficking, of the synaptic vesicles when the neuron is active. Synapses also contain bigger structures, called endosomal structures, which may represent an intermediate station in which synaptic vesicle composition is controlled. However, the trafficking of synaptic vesicles through the endosomal structures is poorly understood.

Now, Tagliatti, Fadda et al. have revealed that a protein called Arf6 plays an important role in presynaptic neurons. The experiments involved rat neurons grown in the laboratory, and showed that Arf6 controls both the number of synaptic vesicles ready to be released and the trafficking of synaptic vesicles via endosomal structures in active neurons. The next step following on from these findings is to understand how Arf6 exerts its effects and how this protein is regulated in the presynaptic neuron.

DOI:http://dx.doi.org/10.7554/eLife.10116.002

Downregulation of Centaurin gamma1A increases synaptic transmission at Drosophila larval neuromuscular junctions

Adequate regulation of synaptic transmission is critical for appropriate neural circuit functioning. Although a number of molecules involved in synaptic neurotransmission have been identified, the molecular mechanisms regulating neurotransmission are not fully understood. Here, we focused on Centaurin gamma1A (CenG1A) and examined its role in synaptic transmission regulation using Drosophila larval neuromuscular junctions. CenG1A is a member of the Centaurin family, which contains Pleckstrin homology, ADP ribosylation factor GTPase-activating protein, and ankyrin repeat domains. Due to the existence of these functional domains, CenG1A is proposed to be involved in the process of synaptic release; however, no evidence for this has been found to date. In this study, we investigated the potential role for CenG1A in the process of synaptic release by performing intracellular recordings in larval muscle cells. We found that neurotransmitter release from presynaptic cells was enhanced in cenG1A mutants. This effect was also observed in larvae with reduced CenG1A function in either presynaptic or postsynaptic cells. In addition, we revealed that suppressing CenG1A function in postsynaptic muscle cells led to an increase in the probability of neurotransmitter release, whereas its suppression in presynaptic neurons led to an increase in neurotransmitter release probability and an increase in the number of synaptic vesicles. These results suggested that CenG1A functions at both presynaptic and postsynaptic sites as a negative regulator of neurotransmitter release. Our study provided evidence for a key role of CenG1A in proper synaptic transmission at neuromuscular junctions.

ARF6 directs axon transport and traffic of integrins and regulates axon growth in adult DRG neurons

Integrins are involved in axon growth and regeneration. Manipulation of integrins is a route to promoting axon regeneration and understanding regeneration failure in the CNS. Expression of α9 integrin promotes axon regeneration, so we have investigated α9β1 trafficking and transport in axons and at the growth cone. We have previously found that α9 and β1 integrins traffic via Rab11-positive recycling endosomes in peripheral axons and growth cones. However, transport via Rab11 is slow, while rapid transport occurs in vesicles lacking Rab11. We have further studied α9 and β1 integrin transport and traffic in adult rat dorsal root ganglion axons and PC12 cells. Integrins are in ARF6 vesicles during rapid axonal transport and during trafficking in the growth cone. We report that rapid axonal transport of these integrins and their trafficking at the cell surface is regulated by ARF6. ARF6 inactivation by expression of ACAP1 leads to increased recycling of β1 integrins to the neuronal surface and to increased anterograde axonal transport. ARF6 activation by expression of the neuronal guanine nucleotide exchange factors, ARNO or EFA6, increases retrograde integrin transport in axons and increases integrin internalization. ARF6 inactivation increases integrin-mediated outgrowth, while activation decreases it. The coordinated changes in integrin transport and recycling resulting from ARF6 activation or inactivation are the probable mechanism behind this regulation of axon growth. Our data suggest a novel mechanism of integrin traffic and transport in peripheral axons, regulated by the activation state of ARF6, and suggest that ARF6 might be targeted to enhance integrin-dependent axon regeneration after injury.

In vitro and in vivo analysis of neurotrophin-3 activation of Arf6 and Rac-1

Arf GTP-binding proteins and Rho-family GTPases play key roles in regulating membrane remodeling and cytoskeletal reorganization involved in cell movement. Several studies have implicated neurotrophins and their receptors as upstream activators of these small GTP-binding proteins, however, the mechanisms and the cell type specificity of this neurotrophin activity are still under investigation. Here we describe the rationale and protocols used for the dissection of an NT3 activated pathway that leads to the specific activation of Arf6 and Rac1.

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

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

ARF6 in the nervous system

Actin cytoskeleton dynamics and membrane trafficking are tightly connected and are among the most important driving forces of neuronal development, basic synaptic transmission events, and synaptic plasticity. One group of proteins involved in coordination of these two processes is the family of ADP ribosylation factors (ARFs) regulating actin dynamics, lipid modification and membrane trafficking. ARF6 is the only member of the ARF family that can simultaneously regulate actin cytoskeleton changes and membrane exchange between plasma membrane and endocytic compartments. The presence of ARF6 and its guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) in the brain, as well as its capability to regulate several aspects of neuronal development and synaptic plasticity, has been recently demonstrated. The main purpose of this review is to present the current knowledge about how ARF6 can influence morphological processes crucial for proper formation of the neuronal circuits in the brain, including dendrite and axon differentiation, development of dendritic arbor complexity and dendritic spine formation. Potential effects of ARF6 on synaptic events resulting from its ability to control exo- and endocytosis will be also discussed.

BRAG1, a Sec7 domain-containing protein, is a component of the postsynaptic density of excitatory synapses

The postsynaptic density (PSD) at excitatory synapses is a dynamic complex of glutamatergic receptors and associated proteins that governs synaptic structure and coordinates signal transduction. In this study, we report that BRAG1, a putative guanine nucleotide exchange factor for the Arf family of GTP-binding proteins, is a major component of the PSD. BRAG1 was identified in a 190 kDa band in the PSD fraction with the use of mass spectrometry coupled to searching of a protein sequence database. BRAG1 expression is abundant in the adult rat forebrain, and it is strongly enriched in the PSD fraction compared to forebrain homogenate and synaptosomes. Immunocytochemical localization of BRAG1 in dissociated hippocampal neurons shows that it forms discrete clusters that colocalize with the postsynaptic marker PSD-95 at sites along dendrites. BRAG1 contains a Sec7 domain, a domain that catalyzes exchange of GDP for GTP on the Arf family of small GTP-binding proteins. In their GTP-bound active state, Arfs regulate trafficking of vesicles and cytoskeletal structure. We demonstrate that the Sec7 domain of BRAG1 promotes binding of GTP to Arf in vitro. These data suggest that BRAG1 may modulate the functions of Arfs at synaptic sites.

ARF proteins: roles in membrane traffic and beyond

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

ARF6 and EFA6A regulate the development and maintenance of dendritic spines

The cellular and molecular mechanisms underlying the development and maintenance of dendritic spines are not fully understood. ADP-ribosylation factor 6 (ARF6) is a small GTPase known to regulate actin remodeling and membrane traffic. Here, we report involvement of ARF6 and exchange factor for ARF6 (EFA6A) in the regulation of spine development and maintenance. An active form of ARF6 promotes the formation of dendritic spines at the expense of filopodia. EFA6A promotes spine formation in an ARF6 activation-dependent manner. Knockdown of ARF6 and EFA6A by small interfering RNA decreases spine formation. Live imaging indicates that ARF6 knockdown decreases the conversion of filopodia to spines and the stability of early spines. The spine-promoting effect of ARF6 is partially blocked by Rac1. ARF6 and EFA6A protect mature spines from inactivity-induced destabilization. These results suggest that ARF6 and EFA6A may regulate the conversion of filopodia to spines and the stability of both early and mature spines.

Functional assay of effectors of ADP ribosylation factor 6 during clathrin/AP-2 coat recruitment to membranes

Arf proteins play pivotal roles in membrane traffic, cell signaling, and actin cytoskeletal rearrangements. We describe here methods to functionally analyze interacting partner proteins of recombinantly produced N-myristoylated Arf6. Combined evidence from affinity purification and chemical crosslinking experiments, in vitro recruitment assays, and the analysis of lipid kinase activities indicates that Arf6-GTP facilitates clathrin/AP-2 recruitment to synaptic membranes by direct binding and activation of the brain-specific phosphatidylinositol 4-phosphate 5-kinase type Igamma (PIPK Igamma). These methods shall help to mechanistically dissect the role of Arf6 in regulating exo-endocytic vesicle cycling at synapses and in related membrane trafficking events.

Phosphoinositide regulation of clathrin-mediated endocytosis

Endocytosis of transmembrane receptors largely occurs via clathrin-coated vesicles that bud from the plasma membrane and deliver their cargo to the endosomal system for recycling or degradation. PIs (phosphoinositides) control the timing and localization of endocytic membrane trafficking by recruiting adaptors and other components of the transport machinery, thereby being part of a coincidence detection system in adaptor-mediated vesicle transport. Activation of organelle- and substrate-specific PI kinases by small GTPases such as Arf (ADP-ribosylation factor) and other factors may result in local changes of PI content, thereby regulating activity-dependent endocytic events including the recycling of synaptic vesicle membranes at nerve terminals. One such example is the PtdIns(4)P 5-kinase-mediated formation of PI(4,5)P2 [PtdIns(4,5)P2], which is required for the exo- and endo-cytic cycling of presynaptic vesicles and secretory granules. Over the last few years, protein X-ray crystallography in combination with biochemical and cell biological assays has been used to investigate the structure and function of many PI-binding proteins, including protein components of the endocytic machinery. These studies have provided molecular insights into the mechanisms by which PI(4,5)P2 recruits and activates adaptor proteins and their binding partners. In this mini-review, I will discuss the pathways of PI(4,5)P2 formation and its interactions with endocytic trafficking adaptors.

SNAP25, but not syntaxin 1A, recycles via an ARF6-regulated pathway in neuroendocrine cells

Soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins mediate cellular membrane fusion events and provide a level of specificity to donor-acceptor membrane interactions. However, the trafficking pathways by which individual SNARE proteins are targeted to specific membrane compartments are not well understood. In neuroendocrine cells, synaptosome-associated protein of 25 kDa (SNAP25) is localized to the plasma membrane where it functions in regulated secretory vesicle exocytosis, but it is also found on intracellular membranes. We identified a dynamic recycling pathway for SNAP25 in PC12 cells through which plasma membrane SNAP25 recycles in approximately 3 h. Approximately 20% of the SNAP25 resides in a perinuclear recycling endosome-trans-Golgi network (TGN) compartment from which it recycles back to the plasma membrane. SNAP25 internalization occurs by constitutive, dynamin-independent endocytosis that is distinct from the dynamin-dependent endocytosis that retrieves secretory vesicle constituents after exocytosis. Endocytosis of SNAP25 is regulated by ADP-ribosylation factor (ARF)6 (through phosphatidylinositol bisphosphate synthesis) and is dependent upon F-actin. SNAP25 endosomes, which exclude the plasma membrane SNARE syntaxin 1A, merge with those derived from clathrin-dependent endocytosis containing endosomal syntaxin 13. Our results characterize a robust ARF6-dependent internalization mechanism that maintains an intracellular pool of SNAP25, which is compatible with possible intracellular roles for SNAP25 in neuroendocrine cells.

Identification of the minimal protein domain required for priming activity of Munc13-1

Most nerve cells communicate with each other through synaptic transmission at chemical synapses. The regulated exocytosis of neurotransmitters, hormones, and peptides occurs at specialized membrane areas through Ca2+-triggered fusion of secretory vesicles with the plasma membrane . Prior to fusion, vesicles are docked at the plasma membrane and must then be rendered fusion-competent through a process called priming. The molecular mechanism underlying this priming process is most likely the formation of the SNARE complex consisting of Syntaxin 1, SNAP-25, and Synaptobrevin 2. Members of the Munc13 protein family consisting of Munc13-1, -2, -3, and -4 were found to be absolutely required for this priming process . In the present study, we identified the minimal Munc13-1 domain that is responsible for its priming activity. Using Munc13-1 deletion constructs in an electrophysiological gain-of-function assay of chromaffin-granule secretion, we show that priming activity is mediated by the C-terminal residues 1100-1735 of Munc13-1, which contains both Munc13-homology domains and the C-terminal C2 domain. Priming by Munc13-1 appears to require its interaction with Syntaxin 1 because point mutants that do not bind Syntaxin 1 do not prime chromaffin granules.

Rab11-FIP3 and FIP4 interact with Arf6 and the exocyst to control membrane traffic in cytokinesis

The dual Rab11/Arf binding proteins, family of Rab11-interacting proteins FIP3 and FIP4 function in the delivery of recycling endosomes to the cleavage furrow and are, together with Rab11, essential for completion of abscission, the terminal step of cytokinesis. Here, we report that both FIP3 and FIP4 bind Arf6 in a nucleotide-dependent manner but exhibit differential affinities for Rab11 and Arf6. Both FIP3 and FIP4 can form ternary complexes with Rab11 and Arf6. Arf6 is localised to the furrow and midbody and we show that Arf6-GTP functions to localise FIP3 and FIP4 to midbodies during cytokinesis. Exo70p, a component of the Exocyst complex, also localises to the furrow of dividing cells and interacts with Arf6. We show that depletion of Exo70p leads to cytokinesis failure and an impairment of FIP3 and Rab11 localisation to the furrow and midbody. Moreover, Exo70p co-immunoprecipitates FIP3 and FIP4. Hence, we propose that FIP3 and FIP4 serve to couple Rab11-positive vesicle traffic from recycling endosomes to the cleavage furrow/midbody where they are tethered prior to fusion events via interactions with Arf6 and the Exocyst.

Multiple motifs regulate the trafficking of GABA(B) receptors at distinct checkpoints within the secretory pathway

gamma-Aminobutyric acid type B receptors (GABA(B)) are G-protein-coupled receptors that mediate GABAergic inhibition in the brain. Their functional expression is dependent upon the formation of heterodimers between GABA(B)R1 and GABA(B)R2 subunits, a process that occurs within the endoplasmic reticulum (ER). However, the mechanisms that regulate receptor surface expression remain largely unknown. Here, we demonstrate that access to the cell surface for GABA(B)R1 is sequentially controlled by an RSR(R) motif and a LL motif within its cytoplasmic domain. In addition, we reveal that msec7-1, a guanine-nucleotide-exchange factor (GEF) for the ADP-ribosylation factor (ARF) family of GTPases, critical regulators of vesicular membrane trafficking, interacts with GABA(B)R1 via the LL motif in this subunit. Finally, we establish that msec7-1 modulates the cell surface expression of GABA(B) receptors, a process that is dependent upon the integrity of the LL motif in GABA(B)R1. Together, our results demonstrate that the cell surface expression of the GABA(B)R1 subunit is regulated by multiple motifs, which act at distinct checkpoints in the secretory pathway, and also suggest a novel role for msec7-1 in regulating the membrane trafficking of GABA(B)R1 subunits.

Coupling actin and membrane dynamics during calcium-regulated exocytosis: a role for Rho and ARF GTPases

Release of neurotransmitters and hormones occurs by calcium-regulated exocytosis, a process that shares many similarities in neurons and neuroendocrine cells. Exocytosis is confined to specific regions in the plasma membrane, where actin remodelling, lipid modifications and protein-protein interactions take place to mediate vesicle/granule docking, priming and fusion. The spatial and temporal coordination of the various players to form a "fast and furious" machinery for secretion remain poorly understood. ARF and Rho GTPases play a central role in coupling actin dynamics to membrane trafficking events in eukaryotic cells. Here, we review the role of Rho and ARF GTPases in supplying actin and lipid structures required for synaptic vesicle and secretory granule exocytosis. Their possible functional interplay may provide the molecular cues for efficient and localized exocytotic fusion.

Reinvestigation of the Role of Snapin in Neurotransmitter Release

Snapin, a 15-kDa protein, has been identified recently as a binding partner of SNAP-25. Moreover, snapin is regulated by phosphorylation and enhances synaptotagmin binding to SNAREs. Furthermore, snapin and C-terminal snapin fragments have been effective in changing the release properties of neurons and chromaffin cells. Here we have reinvestigated the role of snapin using both biochemical and electrophysiological approaches. Snapin is ubiquitously expressed at low levels with no detectable enrichment in the brain or in synaptic vesicles. Using non-equilibrium and equilibrium assays including pulldown experiments, co-immunoprecipitations, and CD and fluorescence anisotropy spectroscopy, we were unable to detect any specific interaction between snapin and SNAP-25. Similarly, overexpression of a C-terminal snapin fragment in hippocampal neurons failed to influence any of the analyzed parameters of neurotransmitter release. Initial biochemical characterization of recombinant snapin revealed that the protein is a stable dimer with a predominantly alpha-helical secondary structure. We conclude that the postulated role of snapin as a SNARE regulator in neurotransmitter release needs reconsideration, leaving the true function of this evolutionarily conserved protein to be discovered.

Regulation of exocytosis in adrenal chromaffin cells: focus on ARF and Rho GTPases

Neurons and neuroendocrine cells release transmitters and hormones by exocytosis, a highly regulated process in which secretory vesicles or granules fuse with the plasma membrane to release their contents in response to a calcium trigger. Several stages have been recognized in exocytosis. After recruitment and docking at the plasma membrane, vesicles/granules enter a priming step, which is then followed by the fusion process. Cortical actin remodelling accompanies the exocytotic reaction, but the links between actin dynamics and trafficking events remain poorly understood. Here, we review the action of Rho and ADP-ribosylation factor (ARF) GTPases within the exocytotic pathway in adrenal chromaffin cells. Rho proteins are well known for their pivotal role in regulating the actin cytoskeleton. ARFs were originally identified as regulators of vesicle transport within cells. The possible interplay between these two families of GTPases and their downstream effectors provides novel insights into the mechanisms that govern exocytosis.

Synapsin I-associated phosphatidylinositol 3-kinase mediates synaptic vesicle delivery to the readily releasable pool

Maintaining synaptic transmission requires replenishment of docked synaptic vesicles within the readily releasable pool (RRP) from synaptic vesicle clusters in the synapsin-bound reserve pool. We show that synapsin forms a complex with phosphatidylinositol 3-kinase (PI 3-kinase) in intact nerve terminals and that synapsin-associated kinase activity increases on depolarization. Disruption of either PI 3-kinase activity or its interaction with synapsin inhibited replenishment of the RRP, but did not affect exocytosis from the RRP. Thus we conclude that a synapsin-associated PI 3-kinase activity plays a role in synaptic vesicle delivery to the RRP. This also suggests that PI 3-kinase contributes to the maintenance of synaptic transmission during periods of high activity, indicating a possible role in synaptic plasticity.

ARF6 stimulates clathrin/AP-2 recruitment to synaptic membranes by activating phosphatidylinositol phosphate kinase type Igamma

Clathrin-mediated endocytosis of synaptic vesicle membranes involves the recruitment of clathrin and AP-2 adaptor complexes to the presynaptic plasma membrane. Phosphoinositides have been implicated in nucleating coat assembly by directly binding to several endocytotic proteins including AP-2 and AP180. Here, we show that the stimulatory effect of ATP and GTPgammaS on clathrin coat recruitment is mediated at least in part by increased levels of PIP2. We also provide evidence for a role of ADP-ribosylation factor 6 (ARF6) via direct stimulation of a synaptically enriched phosphatidylinositol 4-phosphate 5-kinase type Igamma (PIPKIgamma), in this effect. These data suggest a model according to which activation of PIPKIgamma by ARF6-GTP facilitates clathrin-coated pit assembly at the synapse.

Regulation of neuritogenesis and synaptic transmission by msec7-1, a guanine nucleotide exchange factor, in cultured Aplysia neurons

msec7-1, a mammalian homologue of yeast sec7p, is known as a GDP/GTP exchange factor (GEF) for the ADP ribosylation factor (ARF) family of small GTPases. Here, we report that msec7-1 overexpression in cultured Aplysia neurons leads to an extensive neuritogenesis in a GEF activity-dependent manner through the modulation of the actin cytoskeleton. Similarly, the overexpression of ARNO, another mammalin GEF, produces extensive neuritogenesis in Aplysia neurons. In addition, msec7-1 overexpression increases the number of varicosities with an altered size and shape in a GEF activity-dependent manner. The overexpression of msec7-1 in pre-synaptic sensory neurons co-cultured with post-synaptic target motor neurons leads to an increase in the amplitude of the excitatory post-synaptic potential through its GEF activity. Our results demonstrate that msec7-1 regulates neuritogenesis and synaptic transmission.

The GIT family of proteins forms multimers and associates with the presynaptic cytomatrix protein Piccolo

The cytoskeletal matrix assembled at active zones (CAZ) is implicated in defining neurotransmitter release sites. However, little is known about the molecular mechanisms by which the CAZ is organized. Here we report a novel interaction between Piccolo, a core component of the CAZ, and GIT proteins, multidomain signaling integrators with GTPase-activating protein activity for ADP-ribosylation factor small GTPases. A small region (approximately 150 amino acid residues) in Piccolo, which is not conserved in the closely related CAZ protein Bassoon, mediates a direct interaction with the Spa2 homology domain (SHD) domain of GIT1. Piccolo and GIT1 colocalize at synaptic sites in cultured neurons. In brain, Piccolo forms a complex with GIT1 and various GIT-associated proteins, including betaPIX, focal adhesion kinase, liprin-alpha, and paxillin. Point mutations in the SHD of GIT1 differentially interfere with the association of GIT1 with Piccolo, betaPIX, and focal adhesion kinase, suggesting that these proteins bind to the SHD by different mechanisms. Intriguingly, GIT proteins form homo- and heteromultimers through their C-terminal G-protein-coupled receptor kinase-binding domain in a tail-to-tail fashion. This multimerization enables GIT1 to simultaneously interact with multiple SHD-binding proteins including Piccolo and betaPIX. These results suggest that, through their multimerization and interaction with Piccolo, the GIT family proteins are involved in the organization of the CAZ.

Localization of mRNAs for subfamily of guanine nucleotide-exchange proteins (GEP) for ARFs (ADP-ribosylation factors) in the brain of developing and mature rats under normal and postaxotomy conditions

ADP-ribosylation factors (ARFs) play important roles in vesicular trafficking and cytoskeletal regulation and its activation depends on guanine nucleotide-exchange proteins (GEPs). By way of in situ hybridization histochemistry, the localization of mRNAs for subfamily members of low-molecular-weight ARF-GEPs in the rat brain was studied at embryonic and postnatal stages. In the embryonic brain, the gene expression for msec7-1 was distinct in the ventricular zone while that for msec7-1, -3 and EFA6 in the mantle zone. In early postnatal brain, the expression for msec7-1, -2, -3 and EFA6 was seen widely in various loci of the gray matter with different intensity, and the expression of msec7-1 and -2 mRNAs was evident in the cerebellar external granule cell layer. In the adult brain, the gene expression for the four ARF-GEPs decreased more or less in most gray matter and the distinct expression was maintained mainly in the hippocampal and dentate neuronal layers and cerebellar cortex. The expression of EFA6 mRNA was also evident in the molecular layer of the hippocampus and dentate gyrus. No obvious gene expression for cytohesin-4 and ARF-GEP100 was detected in the brain at any stages of development. The present findings suggest that ARF-GEPs are differentially involved in some processes essential to neuronal differentiation and maturation in association with ARFs.

A role for phospholipase D1 in neurotransmitter release

Phosphatidic acid produced by phospholipase D (PLD) as a result of signaling activity is thought to play a role in membrane vesicle trafficking, either as an intracellular messenger or as a cone-shaped lipid that promotes membrane fusion. We recently described that, in neuroendocrine cells, plasma membrane-associated PLD1 operates at a stage of Ca(2+)-dependent exocytosis subsequent to cytoskeletal-mediated recruitment of secretory granules to exocytotic sites. We show here that PLD1 also plays a crucial role in neurotransmitter release. Using purified rat brain synaptosomes subjected to hypotonic lysis and centrifugation, we found that PLD1 is associated with the particulate fraction containing the plasma membrane. Immunostaining of rat cerebellar granule cells confirmed localization of PLD1 at the neuronal plasma membrane in zones specialized for neurotransmitter release (axonal neurites, varicosities, and growth cone-like structures). To determine the potential involvement of PLD1 in neurotransmitter release, we microinjected catalytically inactive PLD1(K898R) into Aplysia neurons and analyzed its effects on evoked acetylcholine (ACh) release. PLD1(K898R) produced a fast and potent dose-dependent inhibition of ACh release. By analyzing paired-pulse facilitation and postsynaptic responses evoked by high-frequency stimulations, we found that the exocytotic inhibition caused by PLD1(K898R) was not the result of an alteration in stimulus-secretion coupling or in vesicular trafficking. Analysis of the fluctuations in amplitude of the postsynaptic responses revealed that the PLD1(K898R) blocked ACh release by reducing the number of active presynaptic-releasing sites. Our results provide evidence that PLD1 plays a major role in neurotransmission, most likely by controlling the fusogenic status of presynaptic release sites.

Signaling complexes of the FERM domain-containing protein GRSP1 bound to ARF exchange factor GRP1

GRP1 is a member of a family of proteins that contain a coiled-coil region, a Sec7 homology domain with guanosine nucleotide exchange activity for the ARF GTP-binding proteins, and a pleckstrin homology domain at the C terminus. The pleckstrin homology domain of GRP1 binds phosphatidylinositol (3,4,5) trisphosphate and mediates the translocation of GRP1 to the plasma membrane upon agonist stimulation of PI 3-kinase activity. Using a (32)P-labeled GRP1 probe to screen a mouse brain cDNA expression library, we isolated a cDNA clone encoding a GRP1-binding partner (GRSP1) that exists as two different splice variants in brain and lung. The GRSP1 protein contains a FERM protein interaction domain as well as two coiled coil domains and may therefore function as a scaffolding protein. Mapping experiments revealed that the interaction of GRP1 and GRSP1 occurs through the coiled coil domains in the two proteins. Immunodepletion experiments indicate that virtually all of the endogenous GRSP1 protein exists as a complex with GRP1 in lung. When co-expressed in Chinese hamster ovary cells expressing the human insulin receptor, both proteins display a diffuse, cytoplasmic localization. Acute translocation and co-localization of GRSP1 and GRP1 to ruffles in the plasma membrane was evident after insulin stimulation. These results identify GRSP1 as a novel member of GRP1 signaling complexes that are acutely recruited to plasma membrane ruffles in response to insulin receptor signaling.

Expression of ARF6 mutants in neuroendocrine cells suggests a role for ARF6 in synaptic vesicle biogenesis

ARF6 regulates membrane trafficking between the plasma membrane and endosomes. We investigated the role of ARF6 in synaptic vesicle biogenesis as this process occurs both at the plasma membrane and at endosomes. We used a synaptic vesicle marker protein, p-selectin-horseradish peroxidase (HRP), to follow the effects of ARF6 expression on synaptic vesicle biogenesis in PC12 neuroendocrine cells. Expression of a constitutively active ARF6 mutant increased, while expression of a nucleotide-free ARF6 mutant decreased, p-selectin-HRP levels in the synaptic vesicle peak. These results provide the first direct evidence for a role for ARF6 in synaptic vesicle biogenesis.

The ADP ribosylation factor-nucleotide exchange factors Gea1p and Gea2p have overlapping, but not redundant functions in retrograde transport from the Golgi to the endoplasmic reticulum

The activation of the small ras-like GTPase Arf1p requires the action of guanine nucleotide exchange factors. Four Arf1p guanine nucleotide exchange factors have been identified in yeast: Sec7p, Syt1p, Gea1p, and its homologue Gea2p. We identified GEA2 as a multicopy suppressor of a sec21-3 temperature-sensitive mutant. SEC21 encodes the gamma-subunit of coatomer, a heptameric protein complex that together with Arf1p forms the COPI coat. GEA1 and GEA2 have at least partially overlapping functions, because deletion of either gene results in no obvious phenotype, whereas the double null mutant is inviable. Conditional mutants defective in both GEA1 and GEA2 accumulate endoplasmic reticulum and Golgi membranes under restrictive conditions. The two genes do not serve completely overlapping functions because a Deltagea1 Deltaarf1 mutant is not more sickly than a Deltaarf1 strain, whereas Deltagea2 Deltaarf1 is inviable. Biochemical experiments revealed similar distributions and activities for the two proteins. Gea1p and Gea2p exist both in membrane-bound and in soluble forms. The membrane-bound forms, at least one of which, Gea2p, can be visualized on Golgi structures, are both required for vesicle budding and protein transport from the Golgi to the endoplasmic reticulum. In contrast, Sec7p, which is required for protein transport within the Golgi, is not required for retrograde protein trafficking.

Phosphoinositides in membrane traffic at the synapse

Inositol phospholipids represent a minor fraction of membrane phospholipids; yet they play important regulatory functions in signaling pathways and membrane traffic. The phosphorylated inositol ring can act either as a precursor for soluble intracellular messengers or as a binding site for cytosolic or membrane proteins. Hence, phosphorylation-dephosphorylation of phosphoinositides represents a mechanism for regulation of recruitment to the membrane of coat proteins, cytoskeletal scaffolds or signaling complexes and for the regulation of membrane proteins. Recent work suggests that phosphoinositide metabolism has an important role in membrane traffic at the synapse. PtdIns(4,5)P(2) generation is implicated in the secretion of at least a subset of neurotransmitters. Furthermore, PtdIns(4,5)P(2) plays a role in the nucleation of clathrin coats and of an actin-based cytoskeletal scaffold at endocytic zones of synapses, and PtdIns(4,5)P(2) dephosphorylation accompanies the release of newly formed vesicles from these interactions. Thus, the reversible phosphorylation of inositol phospholipids may be one of the mechanisms governing the timing and vectorial progression of synaptic vesicle membranes during their exocytic-endocytic cycle.

Cytohesins and centaurins: mediators of PI 3-kinase-regulated Arf signaling

Receptor-activated phosphoinositide (PI) 3-kinases produce PtdIns(3, 4,5)P(3) and its metabolite PtdIns(3,4)P(2) that function as second messengers in membrane recruitment and activation of target proteins. The cytohesin and centaurin protein families are potential targets for PtdIns(3,4,5)P(3) that also regulate and interact with Arf GTPases. Consequently, these families are poised to transduce PI 3-kinase activation into coordinated control of Arf-dependent pathways. Proposed downstream events in PI 3-kinase-regulated Arf cascades include modulation of vesicular trafficking and the actin cytoskeleton.

The role of GTP-binding protein activity in fast central synaptic transmission

Guanosine 5'-triphosphate (GTP)-binding proteins (G proteins) are involved in exocytosis, endocytosis, and recycling of vesicles in yeast and mammalian secretory cells. However, little is known about their contribution to fast synaptic transmission. We loaded guanine nucleotide analogs directly into a giant nerve terminal in rat brainstem slices. Inhibition of G-protein activity had no effect on basal synaptic transmission, but augmented synaptic depression and significantly slowed recovery from depression. A nonhydrolyzable GTP analog blocked recovery of transmission from activity-dependent depression. Neither effect was accompanied by a change in presynaptic calcium currents. Thus, G proteins contribute to fast synaptic transmission by refilling synaptic vesicles depleted after massive exocytosis.

Specific functional interaction of human cytohesin-1 and ADP-ribosylation factor domain protein (ARD1)

Activation of ADP-ribosylation factors (ARFs) is mediated by guanine nucleotide-exchange proteins, which accelerate conversion of inactive ARF-GDP to active ARF-GTP. ARF domain protein (ARD1), a 64-kDa GTPase with a C-terminal ADP-ribosylation factor domain, is localized to lysosomes and the Golgi apparatus. When ARD1 was used as bait to screen a human liver cDNA library using the yeast two-hybrid system, a cDNA for cytohesin-1, a approximately 50-kDa protein with ARF guanine nucleotide-exchange protein activity, was isolated. In this system, ARD1-GDP interacted well with cytohesin-1 but very poorly with cytohesin-2. In agreement, cytohesin-1, but not cytohesin-2, markedly accelerated [(35)S]guanosine 5'-3-O-(thio)triphosphate binding to ARD1. The effector region of the ARF domain of ARD1 appeared to be critical for the specific interaction with cytohesin-1. Replacement of single amino acids in the Sec7 domains of cytohesin-1 and -2 showed that residue 30 is critical for specificity. In transfected COS-7 cells, overexpressed ARD1 and cytohesin-1 were partially colocalized, as determined by confocal fluorescence microscopy. It was concluded that cytohesin-1 is likely to be involved in ARD1 activation, consistent with a role for ARD1 in the regulation of vesicular trafficking.

N-terminal targeting of guanine nucleotide exchange factors (GEF) for ADP ribosylation factors (ARF) to the Golgi

B2-1 (cytohesin-1) is a member of a group of proteins (including ARNO and ARNO3) that are all of similar size and domain composition. The three proteins contain an N-terminal coiled-coil domain, followed by a Sec7 and a pleckstrin homology (PH) domain. While it is well established that the Sec7 domain functions as a guanine nucleotide exchange factor (GEF) for ADP-ribosylation factors (ARFs) and the PH domain anchors the proteins to membrane phosphoinositols, the function of the N-terminal domain is unknown. Here we show that the N terminus of B2-1 (residues 1–54) is necessary and sufficient to target the protein to the Golgi. The Sec7+PH domains of B2-1 (residues 55–398) are not sufficient for Golgi localization. Further deletion analysis and point mutagenesis indicate that the coiled-coil domain within the N terminus is responsible for Golgi targeting. Furthermore, ARNO and ARNO3 N termini also have the same capability of targeting to the Golgi. We conclude that the N-terminal, (α)-helical, coiled-coil domain is used to target this family of proteins to the Golgi complex.

Phosphoinositides determine specificity of the guanine-nucleotide exchange activity of cytohesin-1 for ADP-ribosylation factors derived from a mammalian expression system

ADP-ribosylation factors (ARFs) are small Ras-like GTPases which play important roles in intracellular vesicle transport and in the remodeling of the actin cytoskeleton. Guanine nucleotide exchange factors (GEFs) for ARFs have recently been identified. One of them, cytohesin-1, a 47-kDa cytoplasmic protein acts as an inside-out signaling molecule and regulates binding of the beta2 integrin leukocyte function antigen 1 (LFA-1) to its ligand intercellular adhesion molecule 1 (ICAM-1). In this study, we address the regulation of the GEF activity of cytohesin-1 by phosphoinositides, using mammalian expression of functional ARF-Ig chimeras. The fusion proteins, which can be quantitatively immunoprecipitated on protein A-Sepharose, target to the expected intracellular compartments, and they are readily induced to bind GTP in vitro. We show that both ARF1-Ig and ARF6-Ig chimeras are activated in vitro by cytohesin-1. However, GEF activity towards ARF6 is strongly suppressed by phosphatidylinositol-(3,4,5)-trisphosphate (PtdInsP3). In contrast, cytohesin-1-dependent GTP binding of ARF1 is significantly enhanced by PtdInsP3. We conclude that the membrane phospholipid PtdInsP3 determines the specificity of the GEF activity of cytohesin-1.

Regulation of transmitter release by Unc-13 and its homologues

Neurotransmitters are released by Ca(2+)-triggered exocytotic fusion of synaptic vesicles. Before fusion, vesicles dock at a specialised presynaptic plasma membrane region, the active zone, where they are primed to a fusion competent state. The nature of this priming reaction has long been enigmatic. Recent evidence demonstrates that priming is an essential and rate-limiting step in secretion from neurons and neuroendocrine cells. Members of the Unc-13 protein family, which are highly conserved during evolution and act as novel targets of the diacylglycerol second-messenger pathway, have been identified to play an essential role in this process.

A role for phospholipase D in GLUT4 glucose transporter translocation

Based on recent studies showing that phospholipase D (PLD)1 is associated with intracellular membranes and promotes membrane budding from the trans-Golgi, we tested its possible role in the membrane trafficking of GLUT4 glucose transporters. Using immunofluorescence confocal microscopy, expressed Myc epitope-tagged PLD1 was found to associate with intracellular vesicular structures by a mechanism that requires its N-terminal pleckstrin homology domain. Partial co-localization with expressed GLUT4 fused to green fluorescent protein in both 3T3-L1 adipocytes and Chinese hamster ovary cells was evident. Furthermore, microinjection of purified PLD into cultured adipocytes markedly potentiated the effect of a submaximal concentration of insulin to stimulate GLUT4 translocation to cell surface membranes. Insulin stimulated PLD activity in cells expressing high levels of insulin receptors but no such insulin effect was detected in 3T3-L1 adipocytes. Taken together, these results are consistent with the hypothesis that PLD1 associated with GLUT4-containing membranes acts in a constitutive manner to promote the mechanism of GLUT4 translocation by insulin.

Signalling via ADP-ribosylation factor 6 lies downstream of phosphatidylinositide 3-kinase

ADP-ribosylation factor (ARF) 6 regulates plasma membrane trafficking and cortical actin formation by cycling between inactive GDP and active GTP-bound conformations. Here we show that agonist stimulation of phosphatidylinositide 3-kinase (PI 3-kinase) activates a pathway that leads to ARF6 activation. We also describe experiments that propose a central role in this pathway for the PI 3-kinase-dependent plasma membrane recruitment of the cytohesin-1 family of PtdIns(3,4,5)P(3)-binding ARF-exchange factors.

Turning on ARF: the Sec7 family of guanine-nucleotide-exchange factors

ARF proteins are important regulators of membrane dynamics and protein transport within the eukaryotic cell. The Sec7 domain is approximately 200 amino acids in size and stimulates guanine-nucleotide exchange on members of the ARF class of small GTPases. The members of one subclass of Sec7-domain proteins are direct targets of the secretion-inhibiting drug brefeldin A, which blocks the exchange reaction by trapping a reaction intermediate in an inactive, abortive complex. A separate subclass of Sec7-domain proteins is involved in signal transduction and possess a domain that mediates membrane binding in response to extracellular signals.

ADP-ribosylation factor 6 as a target of guanine nucleotide exchange factor GRP1

The GRP1 protein contains a Sec7 homology domain that catalyzes guanine nucleotide exchange on ADP-ribosylation factors (ARF) 1 and 5 as well as a pleckstrin homology domain that binds phosphatidylinositol(3,4,5)P(3), an intermediate in cell signaling by insulin and other extracellular stimuli (Klarlund, J. K., Guilherme, A., Holik, J. J., Virbasius, J. V., Chawla, A., and Czech, M. P. (1997) Science 275, 1927-1930). Here we show that both endogenous GRP1 and ARF6 rapidly co-localize in plasma membrane ruffles in Chinese hamster ovary (CHO-T) cells expressing human insulin receptors and COS-1 cells in response to insulin and epidermal growth factor, respectively. The pleckstrin homology domain of GRP1 appears to be sufficient for regulated membrane localization. Using a novel method to estimate GTP loading of expressed HA epitope-tagged ARF proteins in intact cells, levels of biologically active, GTP-bound ARF6 as well as GTP-bound ARF1 were elevated when these ARF proteins were co-expressed with GRP1 or the related protein cytohesin-1. GTP loading of ARF6 in both control cells and in response to GRP1 or cytohesin-1 was insensitive to brefeldin A, consistent with previous data on endogenous ARF6 exchange activity. The ability of GRP1 to catalyze GTP/GDP exchange on ARF6 was confirmed using recombinant proteins in a cell-free system. Taken together, these results suggest that phosphatidylinositol(3,4,5)P(3) may be generated in cell membrane ruffles where receptor tyrosine kinases are concentrated in response to growth factors, causing recruitment of endogenous GRP1. Further, co-localization of GRP1 with ARF6, combined with its demonstrated ability to activate ARF6, suggests a physiological role for GRP1 in regulating ARF6 functions.

An efficient method for infection of adrenal chromaffin cells using the Semliki Forest virus gene expression system

We have expanded the use of the Semliki Forest virus (SFV) by infecting chromaffin cells with synaptic proteins at high efficiency. Using the SFV gene expression system, up to 40% of cultured bovine chromaffin cells express the protein of interest within 12-48 h after infection. In order to learn about the basic physiological properties of infected cells, we performed membrane capacitance measurements using the whole-cell patch-clamp technique and monitored catecholamine release with amperometry. We found that chromaffin cells infected with green fluorescent protein (GFP) were comparable to control cells in intracellular calcium concentrations ([Ca2+]i), leak currents and cell sizes. In response to depolarization, calcium currents were elicited and the cells secreted catecholamine. Comparison of the calcium current amplitude and the size of the readily releasable pool of vesicles revealed a small decrease in these parameters compared to control cells. The refilling kinetics after pool depletion, however, were not altered. Overexpressed munc13-1 translocates to the plasma membrane in response to phorbol esters, an effect that is also observed in fibroblasts transfected with conventional methods. Thus, the use of the SFV gene expression system to infect chromaffin cells represents a major improvement in infection efficiency compared to other methods. It opens up new opportunities to introduce synaptic proteins into chromaffin cells and study their role in secretion.

Direct interaction between the ARF-specific guanine nucleotide exchange factor msec7-1 and presynaptic Munc13-1

Msec7-1, a mammalian homologue of yeast sec7p, is a specific GDP/GTP exchange factor for small G-proteins of the ARF family. Overexpression of msec7-1 in Xenopus neuromuscular junctions leads to an increase in synaptic transmitter release that is most likely caused by an increase in the pool of readily releasable vesicles. However, the molecular mechanisms by which msec7-1 is targeted to presynaptic compartments and enhances neurotransmitter release are not known. In the present study, we demonstrate that msec7-1 interacts directly with Munc13-1, a phorbol ester-dependent enhancer of neurotransmitter release that is specifically localized to presynaptic transmitter release zones. Given that Munc13-1 and msec7-1 participate in very similar presynaptic processes and because Munc13-1 is specifically targeted to presynaptic active zones, we suggest that the msec7-1/Munc13-1 interaction serves to colocalize the two proteins at the active zone, a subcellular compartment with extremely high membrane turnover.