Trimeric G proteins control regulated exocytosis in bovine chromaffin cells: sequential involvement of Go associated with secretory granules and Gi3 bound to the plasma membrane

Microtubules and Gαo-signaling modulate the preferential secretion of young insulin secretory granules in islet β cells via independent pathways

For sustainable function, each pancreatic islet β cell maintains thousands of insulin secretory granules (SGs) at all times. Glucose stimulation induces the secretion of a small portion of these SGs and simultaneously boosts SG biosynthesis to sustain this stock. The failure of these processes, often induced by sustained high-insulin output, results in type 2 diabetes. Intriguingly, young insulin SGs are more likely secreted during glucose-stimulated insulin secretion (GSIS) for unknown reasons, while older SGs tend to lose releasability and be degraded. Here, we examine the roles of microtubule (MT) and Gαo-signaling in regulating the preferential secretion of young versus old SGs. We show that both MT-destabilization and Gαo inactivation results in more SGs localization near plasma membrane (PM) despite higher levels of GSIS and reduced SG biosynthesis. Intriguingly, MT-destabilization or Gαo-inactivation results in higher secretion probabilities of older SGs, while combining both having additive effects on boosting GSIS. Lastly, Gαo inactivation does not detectably destabilize the β-cell MT network. These findings suggest that Gαo and MT can modulate the preferential release of younger insulin SGs via largely parallel pathways.

Bovine Chromaffin Cells: Culture and Fluorescence Assay for Secretion

Over the last four decades, chromaffin cells originating from the adrenal medulla have been probably one of the most popular cell models to study neurosecretion at the molecular level. Accordingly, numerous seminal discoveries in the field, including the characterization of role of the cytoskeleton, fusogenic lipids, and soluble N-ethylmaleimide-sensitivefactor attachment protein receptor (SNARE) proteins, have been made using this model. In this chapter, we describe a standard method currently used to isolate and culture bovine chromaffin cells, and we illustrate a catecholamine secretion assay based on the successive transformation of adrenaline into adrenochrome and adrenolutine for fluorescence measurements. We also provide some guidelines for efficient cell recovery and for the use of this assay in the laboratory.

ENA/VASP proteins regulate exocytosis by mediating myosin VI-dependent recruitment of secretory granules to the cortical actin network

In neurosecretory cells, myosin VI associated with secretory granules (SGs) mediates their activity-dependent recruitment to the cortical actin network and is necessary to sustain exocytosis. The mechanism by which myosin VI interacts with SGs is unknown. Using a myosin VI pull-down assay and mass spectrometry we identified Mena, a member of the ENA/VASP family, as a myosin VI binding partner in PC12 cells, and confirmed that Mena colocalized with myosin VI on SGs. Using a knock-sideways approach to inactivate the ENA/VASP family members by mitochondrial relocation, we revealed a concomitant redistribution of myosin VI. This was ensued by a reduction in the association of myosin VI with SGs, a decreased SG mobility and density in proximity to the plasma membrane as well as decreased evoked exocytosis. These data demonstrate that ENA/VASP proteins regulate SG exocytosis through modulating the activity of myosin VI.

Dietary arachidonic acid as a risk factor for age-associated neurodegenerative diseases: Potential mechanisms

Alzheimer's disease and associated diseases constitute a major public health concern worldwide. Nutrition-based, preventive strategies could possibly be effective in delaying the occurrence of these diseases and lower their prevalence. Arachidonic acid is the second major polyunsaturated fatty acid (PUFA) and several studies support its involvement in Alzheimer's disease. The objective of this review is to examine how dietary arachidonic acid contributes to Alzheimer's disease mechanisms and therefore to its prevention. First, we explore the sources of neuronal arachidonic acid that could potentially originate from either the conversion of linoleic acid, or from dietary sources and transfer across the blood-brain-barrier. In a second part, a brief overview of the role of the two main agents of Alzheimer's disease, tau protein and Aβ peptide is given, followed by the examination of the relationship between arachidonic acid and the disease. Third, the putative mechanisms by which arachidonic acid could influence Alzheimer's disease occurrence and evolution are presented. The conclusion is devoted to what remains to be determined before integrating arachidonic acid in the design of preventive strategies against Alzheimer's disease and other neurodegenerative diseases.

Oligophrenin-1 Connects Exocytotic Fusion to Compensatory Endocytosis in Neuroendocrine Cells

Oligophrenin-1 (OPHN1) is a protein with multiple domains including a Rho family GTPase-activating (Rho-GAP) domain, and a Bin-Amphiphysin-Rvs (BAR) domain. Involved in X-linked intellectual disability, OPHN1 has been reported to control several synaptic functions, including synaptic plasticity, synaptic vesicle trafficking, and endocytosis. In neuroendocrine cells, hormones and neuropeptides stored in large dense core vesicles (secretory granules) are released through calcium-regulated exocytosis, a process that is tightly coupled to compensatory endocytosis, allowing secretory granule recycling. We show here that OPHN1 is expressed and mainly localized at the plasma membrane and in the cytosol in chromaffin cells from adrenal medulla. Using carbon fiber amperometry, we found that exocytosis is impaired at the late stage of membrane fusion in Ophn1 knock-out mice and OPHN1-silenced bovine chromaffin cells. Experiments performed with ectopically expressed OPHN1 mutants indicate that OPHN1 requires its Rho-GAP domain to control fusion pore dynamics. On the other hand, compensatory endocytosis assessed by measuring dopamine-β-hydroxylase (secretory granule membrane) internalization is severely inhibited in Ophn1 knock-out chromaffin cells. This inhibitory effect is mimicked by the expression of a truncated OPHN1 mutant lacking the BAR domain, demonstrating that the BAR domain implicates OPHN1 in granule membrane recapture after exocytosis. These findings reveal for the first time that OPHN1 is a bifunctional protein that is able, through distinct mechanisms, to regulate and most likely link exocytosis to compensatory endocytosis in chromaffin cells.

Myosin VI small insert isoform maintains exocytosis by tethering secretory granules to the cortical actin

Before undergoing neuroexocytosis, secretory granules (SGs) are mobilized and tethered to the cortical actin network by an unknown mechanism. Using an SG pull-down assay and mass spectrometry, we found that myosin VI was recruited to SGs in a Ca(2+)-dependent manner. Interfering with myosin VI function in PC12 cells reduced the density of SGs near the plasma membrane without affecting their biogenesis. Myosin VI knockdown selectively impaired a late phase of exocytosis, consistent with a replenishment defect. This exocytic defect was selectively rescued by expression of the myosin VI small insert (SI) isoform, which efficiently tethered SGs to the cortical actin network. These myosin VI SI-specific effects were prevented by deletion of a c-Src kinase phosphorylation DYD motif, identified in silico. Myosin VI SI thus recruits SGs to the cortical actin network, potentially via c-Src phosphorylation, thereby maintaining an active pool of SGs near the plasma membrane.

New insights into the role of the cortical cytoskeleton in exocytosis from neuroendocrine cells

The cortical cytoskeleton is a dense network of filamentous actin (F-actin) that participates in the events associated with secretion from neuroendocrine cells. This filamentous web traps secretory vesicles, acting as a retention system that blocks the access of vesicles to secretory sites during the resting state, and it mediates their active directional transport during stimulation. The changes in the cortical cytoskeleton that drive this functional transformation have been well documented, particularly in cultured chromaffin cells. At the biochemical level, alterations in F-actin are governed by the activity of molecular motors like myosins II and V and by other calcium-dependent proteins that influence the polymerization and cross-linking of F-actin structures. In addition to modulating vesicle transport, the F-actin cortical network and its associated motor proteins also influence the late phases of the secretory process, including membrane fusion and the release of active substances through the exocytotic fusion pore. Here, we discuss the potential interactions between the F-actin cortical web and proteins such as SNAREs during secretion. We also discuss the role of the cytoskeleton in organizing the molecular elements required to sustain regulated exocytosis, forming a molecular structure that foments the efficient release of neurotransmitters and hormones.

Melittin promotes exocytosis in neuroendocrine cells through the activation of phospholipase A₂

Regulated exocytosis requires the formation of trans-SNARE complexes that assemble at the interface between vesicles and the plasma membrane. Recent evidence has also highlighted the importance of lipid dynamic in this process. For instance, small cone-shaped lipids generating membrane curvature of the plasma membrane are synthesized at the exocytotic sites. Among those lipids, phosphatidic acid (PA) synthesized through the activity of phospholipase D (PLD) has been recently shown to be necessary to hormonal release in various cell types as well as in neurotransmitter release. In this paper we examined the possible role of arachidonic acid (AA), a fatty acid that is generated by the activity of phospholipase A₂ (PLA₂). Melittin a well-known activator of PLA₂ was found to concomitantly promote catecholamine and chromogranin A (CGA) release in a calcium-dependent manner and also to increase AA synthesis in chromaffin cells. The effects of melittin on exocytosis and AA synthesis did not involve heterotrimeric G protein activation, but were suppressed by PLA₂ inhibitors. Accordingly addition of exogenous PLA₂ stimulated AA synthesis and catecholamine release in permeabilized chromaffin cells, whereas provision of exogenous AA directly increased exocytosis. These results suggest that AA produced by PLA₂ activation during exocytosis may play an important regulatory role in hormonal and neurotransmitter release. The possibility that CGA-derived peptides released during exocytosis mimic the activity of melittin is discussed.

Calnuc plays a role in dynamic distribution of Galphai but not Gbeta subunits and modulates ACTH secretion in AtT-20 neuroendocrine secretory cells

In AtT-20 cells ACTH secretion is regulated by both Ca²⁺ and G proteins. We previously demonstrated that calnuc, an EF-hand Ca²⁺ binding protein which regulates Alzheimer's β-amyloid precursor protein (APP) biogenesis, binds both Ca²⁺ as well as Gα subunits. Here we investigate calnuc's role in G protein-mediated regulation of ACTH secretion in AtT-20 neuroendocrine secretory cells stably overexpressing calnuc-GFP. Similar to endogenous calnuc, calnuc-GFP is mainly found in the Golgi, on the plasma membrane (PM), and associated with regulated secretion granules (RSG). By deconvolution immunofluorescence, calnuc-GFP partially colocalizes with Gαi1/2 and Gαi3 at the PM and on RSG. Cytosolic calnuc(ΔSS)-CFP with the signal sequence deleted also partially colocalizes with RSG and partially cosediments with Gαi1/2 in fractions enriched in RSG. Overexpression of calnuc-GFP specifically increases the distribution of Gαi1/2 on the PM whereas the distribution of Gβ subunits and synaptobrevin 2 (Vamp 2) is unchanged. Overexpression of calnuc-GFP or cytosolic calnuc(ΔSS)-CFP enhances ACTH secretion two-fold triggered by mastoparan or GTPγS but does not significantly affect glycosaminoglycan (GAG) chain secretion along the constitutive pathway or basal secretion of ACTH. Calnuc's facilitating effects on ACTH secretion are decreased after introducing anti-Gαi1/2, Gαi3, Gβ or calnuc IgG into permeabilized cells but not when Gα12 or preimmune IgG is introduced. The results suggest that calnuc binds to Gα subunits on the Golgi and on RSG and that overexpression of calnuc causes redistribution of Gαi subunits to the PM and RSG, indicating that calnuc plays a role in dynamic distribution of only Gα but not Gβ subunits. Thus calnuc may connect G protein signaling and calcium signaling during regulated secretion.

Molecular mechanisms of go signaling

Go is the most abundant G protein in the central nervous system, where it comprises about 1% of membrane protein in mammalian brains. It functions to couple cell surface receptors to intercellular effectors, which is a critical process for cells to receive, interpret and respond to extracellular signals. Go protein belongs to the pertussis toxin-sensitive Gi/Go subfamily of G proteins. A number of G-protein-coupled receptors transmit stimuli to intercellular effectors through Go. Go regulates several cellular effectors, including ion channels, enzymes, and even small GTPases to modulate cellular function. This review summarizes some of the advances in Go research and proposes areas to be further addressed in exploring the functional role of Go.

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.

Ca2+-regulated pool of phosphatidylinositol-3-phosphate produced by phosphatidylinositol 3-kinase C2alpha on neurosecretory vesicles

Phosphatidylinositol-3-phosphate [PtdIns(3)P] is a key player in early endosomal trafficking and is mainly produced by class III phosphatidylinositol 3-kinase (PI3K). In neurosecretory cells, class II PI3K-C2alpha and its lipid product PtdIns(3)P have recently been shown to play a critical role during neuroexocytosis, suggesting that two distinct pools of PtdIns(3)P might coexist in these cells. However, the precise characterization of this additional pool of PtdIns(3)P remains to be established. Using a selective PtdIns(3)P probe, we have identified a novel PtdIns(3)P-positive pool localized on secretory vesicles, sensitive to PI3K-C2alpha knockdown and relatively resistant to wortmannin treatment. In neurosecretory cells, stimulation of exocytosis promoted a transient albeit large increase in PtdIns(3)P production localized on secretory vesicles sensitive to PI3K-C2alpha knockdown and expression of PI3K-C2alpha catalytically inactive mutant. Using purified chromaffin granules, we found that PtdIns(3)P production is controlled by Ca(2+). We confirmed that PtdIns(3)P production from recombinantly expressed PI3K-C2alpha is indeed regulated by Ca(2+). We provide evidence that a dynamic pool of PtdIns(3)P synthesized by PI3K-C2alpha occurs on secretory vesicles in neurosecretory cells, demonstrating that the activity of a member of the PI3K family is regulated by Ca(2+) in vitro and in living neurosecretory cells.

Receptor-mediated regulation of tomosyn-syntaxin 1A interactions in bovine adrenal chromaffin cells

Tomosyn, a soluble R-SNARE protein identified as a binding partner of the Q-SNARE syntaxin 1A, is thought to be critical in setting the level of fusion-competent SNARE complexes for neurosecretion. To date, there has been no direct evaluation of the dynamics in which tomosyn transits through tomosyn-SNARE complexes or of the extent to which tomosyn-SNARE complexes are regulated by secretory demand. Here, we employed biochemical and optical approaches to characterize the dynamic properties of tomosyn-syntaxin 1A complexes in live adrenal chromaffin cells. We demonstrate that secretagogue stimulation results in the rapid translocation of tomosyn from the cytosol to plasma membrane regions and that this translocation is associated with an increase in the tomosyn-syntaxin 1A interaction, including increased cycling of tomosyn into tomosyn-SNARE complexes. The secretagogue-induced interaction was strongly reduced by pharmacological inhibition of the Rho-associated coiled-coil forming kinase, a result consistent with findings demonstrating secretagogue-induced activation of RhoA. Stimulation of chromaffin cells with lysophosphatidic acid, a nonsecretory stimulus that strongly activates RhoA, resulted in effects on tomosyn similar to that of application of the secretagogue. In PC-12 cells overexpressing tomosyn, secretagogue stimulation in the presence of lysophosphatidic acid resulted in reduced evoked secretory responses, an effect that was eliminated upon inhibition of Rho-associated coiled-coil forming kinase. Moreover, this effect required an intact interaction between tomosyn and syntaxin 1A. Thus, modulation of the tomosyn-syntaxin 1A interaction in response to secretagogue activation is an important mechanism allowing for dynamic regulation of the secretory response.

The CSPα/G protein complex in PC12 cells

Cysteine string proteinalpha (CSPalpha) is a regulated vesicle protein and molecular chaperone that has been found to be critical for continuous synaptic transmission and is implicated in the defense against neurodegeneration. Previous work has revealed links between CSPalpha and heterotrimeric GTP binding protein (G protein) signal transduction pathways. We have shown that CSPalpha is a guanine nucleotide exchange factor (GEF) for Galphas. In vitro Hsc70 (70 kDa heat shock cognate protein) and SGT (small glutamine-rich tetratricopeptide repeat domain protein) switch CSPalpha from an inactive GEF to an active GEF. Here we have examined the cellular distribution of the CSPalpha system in the PC12 neuroendocrine cell line. CSPalpha, an established secretory vesicle protein, was found to concentrate in the processes of NGF-differentiated PC12 cells as expected. Gbeta subunits co-localized and Galphas subunits partially co-localized with CSPalpha. However, under the conditions examined, the GEF activity of CSPalpha is expected to be inactive, in that Hsc70 was not found in PC12 processes. These results indicate that CSPalpha activity is subject to regulation by factors that alter Hsc70 distribution and translocation within the cell.

Exocytosis in neuroendocrine cells: new tasks for actin

Most secretory cells undergoing calcium-regulated exocytosis in response to cell surface receptor stimulation display a dense subplasmalemmal actin network, which is remodeled during the exocytotic process. This review summarizes new insights into the role of the cortical actin cytoskeleton in exocytosis. Many earlier findings support the actin-physical-barrier model whereby transient depolymerization of cortical actin filaments permits vesicles to gain access to their appropriate docking and fusion sites at the plasma membrane. On the other hand, data from our laboratory and others now indicate that actin polymerization also plays a positive role in the exocytotic process. Here, we discuss the potential functions attributed to the actin cytoskeleton at each major step of the exocytotic process, including recruitment, docking and fusion of secretory granules with the plasma membrane. Moreover, we present actin-binding proteins, which are likely to link actin organization to calcium signals along the exocytotic pathway. The results cited in this review are derived primarily from investigations of the adrenal medullary chromaffin cell, a cell model that is since many years a source of information concerning the molecular machinery underlying exocytosis.

Intracellular translocation of the decapeptide carboxyl terminal of Gi3 alpha induces the dual phosphorylation of p42/p44 MAP kinases

The carboxyl terminal of heterotrimeric G protein alpha subunits binds both G protein-coupled receptors and mastoparan (MP), a tetradecapeptide allostere. Moreover, peptides corresponding to the carboxyl domains of G(i)3alpha and G(t) display intrinsic biological activities in cell-free systems. Thus, the purpose of this study was to develop a cell penetrant delivery system to further investigate the biological properties of a peptide mimetic of the G(i)3alpha carboxyl terminal (G(i)3alpha(346-355); H-KNNLKECGLY-NH2). Kinetic studies, using a CFDA-conjugated analogue of G(i)3alpha(346-355), confirmed the rapid and efficient intracellular translocation of TP10-G(i)3alpha(346-355) (t(0.5) = 3 min). Translocated G(i)3alpha(346-355), but not other bioactive cargoes derived from PKC and the CB1 cannabinoid receptor, promoted the dual phosphorylation of p42/p44 MAPK without adverse changes in cellular viability. The relative specificity of this novel biological activity was further confirmed by the observation that translocated G(i)3alpha(346-355) did not influence the exocytosis of beta-hexoseaminidase from RBL-2H3, a secretory event stimulated by other cell penetrant peptide cargoes and MP. We conclude that TP10-G(i)3alpha(346-355) is a valuable, non-toxic research tool with which to study and modulate signal transduction pathways mediated by heterotrimeric G proteins and MAPK.

Characterization of the G alpha(s) regulator cysteine string protein

Cysteine string protein (CSP) is an abundant regulated secretory vesicle protein that is composed of a string of cysteine residues, a linker domain, and an N-terminal J domain characteristic of the DnaJ/Hsp40 co-chaperone family. We have shown previously that CSP associates with heterotrimeric GTP-binding proteins (G proteins) and promotes G protein inhibition of N-type Ca2+ channels. To elucidate the mechanisms by which CSP modulates G protein signaling, we examined the effects of CSP(1-198) (full-length), CSP(1-112), and CSP(1-82) on the kinetics of guanine nucleotide exchange and GTP hydrolysis. In this report, we demonstrate that CSP selectively interacts with G alpha(s) and increases steady-state GTP hydrolysis. CSP(1-198) modulation of G alpha(s) was dependent on Hsc70 (70-kDa heat shock cognate protein) and SGT (small glutamine-rich tetratricopeptide repeat domain protein), whereas modulation by CSP(1-112) was Hsc70-SGT-independent. CSP(1-112) preferentially associated with the inactive GDP-bound conformation of G alpha(s). Consistent with the stimulation of GTP hydrolysis, CSP(1-112) increased guanine nucleotide exchange of G alpha(s). The interaction of native G alpha(s) and CSP was confirmed by coimmunoprecipitation and showed that G alpha(s) associates with CSP. Furthermore, transient expression of CSP in HEK cells increased cellular cAMP levels in the presence of the beta2 adrenergic agonist isoproterenol. Together, these results demonstrate that CSP modulates G protein function by preferentially targeting the inactive GDP-bound form of G alpha(s) and promoting GDP/GTP exchange. Our results show that the guanine nucleotide exchange activity of full-length CSP is, in turn, regulated by Hsc70-SGT.

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.

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.

Regulated secretion in chromaffin cells: an essential role for ARF6-regulated phospholipase D in the late stages of exocytosis

ARFs constitute a family of structurally related proteins that forms a subset of the ras GTPases. In chromaffin cells, secretagogue-evoked stimulation triggers the rapid translocation of ARF6 from secretory granules to the plasma membrane and the concomitant activation of PLD in the plasma membrane. Both PLD activation and catecholamine secretion are strongly inhibited by a synthetic peptide corresponding to the N-terminal domain of ARF6. ARNO, a potential guanine nucleotide exchange factor for ARF6, is expressed and localized in the plasma membrane of chromaffin cells. Using permeabilized cells, we found that the introduction of anti-ARNO antibodies into the cytosol inhibits both PLD activation and catecholamine secretion. Chromaffin cells express PLD1 at the plasma membrane. We found that microinjection of the catalytically inactive PLD1(K898R) dramatically reduces catecholamine secretion monitored by amperometry, most likely by interfering with a late postdocking step of calcium-regulated exocytosis. We propose that ARNO-ARF6 participate in the exocytotic reaction by controlling the plasma membrane-bound PLD1. By generating fusogenic lipids at the exocytotic sites, PLD1 may represent an essential component of the fusion machinery in neuroendocrine cells.

A direct inhibitory action of prostaglandins upon ACTH secretion at the late stages of the secretory pathway of AtT-20 cells

1. The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used as a model system for the study of the effects of prostaglandins upon the late stages of the adrenocorticotrophin (ACTH) secretory pathway. 2. Calcium (1 nM - 100 microM), guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) (1 - 100 microM) and mastoparan (1 and 10 microM) all stimulated ACTH secretion from permeabilized AtT-20 cells in a concentration-dependent manner. GTP-gamma-S and mastoparan stimulated ACTH secretion from permeabilized cells in the absence of calcium. Co-incubation with prostaglandins E(1) and E(2) (PGE(1), PGE(2)) (10 microM) but not prostaglandin F(2 alpha) (PGF(2 alpha)) (10 microM) significantly inhibited calcium-, GTP-gamma-S and mastoparan-evoked secretion by 30 - 50%. 3. The effects of PGE(1) and PGE(2) upon GTP-gamma-S (100 microM)-, calcium (10 microM)- and mastoparan (10 microM)-evoked secretion were concentration-dependent. PGE(1) significantly inhibited GTP-gamma-S- and calcium-evoked secretion at concentrations of PGE(1) above 1 microM but mastoparan-evoked secretion only at the highest concentration of PGE(1) investigated (10 microM). PGE(2) was much more potent than PGE(1) and significantly inhibited GTP-gamma-S- and calcium-evoked secretion at 10 nM and above and mastoparan-evoked secretion above 1 microM. 4. The inhibitory effects of PGE(1) and PGE(2) upon calcium-, GTP-gamma-S- and mastoparan-stimulated ACTH secretion from permeabilized cells were pertussis toxin (PTX) sensitive. 5. In intact cells PGE(1), PGE(2) and PGF(2 alpha) (1 nM - 10 microM) acting singly had little or no effect upon ACTH secretion. However, only PGE(2) (1 nM - 10 microM) significantly inhibited corticotrophin-releasing factor-41 (CRF-41) (100 nM)-evoked secretion in a concentration dependent manner. 6. The present study finds that prostaglandins of the E series exert an inhibitory action, via a pertussis toxin-sensitive GTP-binding (G)-protein, in the late stages of the ACTH secretory pathway distal to the G-exocytosis (Ge)/calcium point of control.

Effects of phospholipase A(2) activating peptides upon GTP-binding protein-evoked adrenocorticotrophin secretion

A GTP-binding protein (G-protein), termed G-exocytosis (Ge), mediates the effects of calcium ions in the late stages of the adrenocorticotrophin (ACTH) secretory pathway. An activator of Ge, mastoparan, also stimulates phospholipase A(2) and so a comparison of other phospholipase A(2)-activating peptides, melittin and phospholipase A(2)-activating peptide was made with mastoparan to assess whether phospholipase A(2)activation was an important component of Ge-evoked secretion. All three peptides stimulated ACTH secretion in the effective absence of calcium ions from permeabilised cells, actions potentiated by a phospholipase A(2)inhibitor. Ca(2+)-evoked secretion from permeabilised cells was similarly potentiated by a phospholipase A(2) inhibitor. Furthermore, arachidonic acid inhibited Ca(2+)- and Ge-evoked ACTH secretion, an action blocked by the cyclo-oxygenase inhibitor ibuprofen. This study suggests that the products of phospholipase A(2)-generated arachidonic metabolism may exert an inhibitory action on the late post-Ca(2+) stages of the ACTH secretory pathway and that prostaglandins may be the active agents in this capacity.

Identification of lysosomal and Golgi localization signals in GAP and ARF domains of ARF domain protein 1

ADP ribosylation factors (ARFs) are approximately 20-kDa guanine nucleotide-binding proteins that activate cholera toxin and phospholipase D and are critical components of vesicular trafficking pathways. ARF domain protein 1 (ARD1), a member of the ARF superfamily, contains a 46-kDa amino-terminal extension, which acts as a GTPase-activating protein (GAP) with activity towards its ARF domain. When overexpressed, ARD1 was associated with lysosomes and the Golgi apparatus. In agreement with this finding, lysosomal and Golgi membranes isolated from human liver by immunoaffinity contained native ARD1. ARD1, expressed as a green fluorescent fusion protein, was initially associated with the Golgi network and subsequently appeared on lysosomes, suggesting that ARD1 might undergo vectorial transport between the two organelles. Here we show by microscopic colocalization that GAP and ARF domains determine lysosomal and Golgi localization, respectively, consistent with the presence of more than one signal motif. Using truncated ARD1 molecules, expressed as green fluorescent fusion proteins, it was found that the signal for lysosomal localization was present in residues 301 to 402 of the GAP domain. Site-specific mutagenesis demonstrated that the sequence (369)KXXXQ(373) in the GAP domain was responsible for lysosomal localization. Association of ARD1 with the Golgi apparatus required tyrosine-based motifs. A green fluorescent fusion protein containing the QKQQQQF motif was partially associated with lysosomes, suggesting that this motif contains the information sufficient for lysosomal targeting. These results suggest that ARD1 is a multidomain protein with ARF and GAP regions, which contain Golgi and lysosomal localization signals, respectively, that could function in vesicular trafficking.

Presence of dynamin--syntaxin complexes associated with secretory granules in adrenal chromaffin cells

Dynamin proteins have been implicated in many aspects of endocytosis, including clathrin-mediated endocytosis, internalization of caveolae, synaptic vesicle recycling, and, more recently, vesicular trafficking to and from the Golgi complex. To provide further insight into the function(s) of dynamin in neuroendocrine cells, we have examined its intracellular distribution in cultured chromaffin cells by subcellular fractionation, immunoreplica analysis, and confocal immunofluorescence. We found that dynamin, presumably the dynamin-2 isoform, is associated specifically with the membrane of purified secretory chromaffin granules. Oligomerization state analysis by sucrose density velocity gradients indicated that the granule-associated dynamin is in a monomeric form. Immunoprecipitation experiments coupled to double-labeling immunofluorescence cytochemistry revealed that the granular dynamin is associated with a syntaxin component that is not involved in the granule-bound SNARE complex. The possibility that dynamin participates in the coupling of the exocytotic and endocytotic reaction through the building of a granular membrane subset of proteins is discussed.

Regulation of exocytosis in chromaffin cells by phosducin-like protein, a protein interacting with G protein betagamma subunits

Phosducin and related proteins have been identified as ubiquitous regulators of signalling mediated by betagamma subunits of trimeric G proteins. To explore a role for phosducin in regulated exocytosis, we have examined the distribution and putative function of phosducin-like protein (PhLP) in adrenal medullary chromaffin cells. The full-length cDNA encoding the short splice variant of PhLP (PhLPs) was cloned from cultured chromaffin cells. Native PhLPs was found associated with plasma membranes and detected in the subplasmalemmal area of resting chromaffin cells by confocal immunofluorescence analysis. Stimulation with secretagogues triggered a massive redistribution of PhLPs into the cytoplasm. When microinjected into individual chromaffin cells, recombinant PhLPs inhibited catecholamine secretion evoked by a depolarizing concentration of K+ without affecting calcium mobilization. Thus, PhLPs may participate directly in the regulation of calcium-evoked exocytosis.

Identification of a plasma membrane-associated guanine nucleotide exchange factor for ARF6 in chromaffin cells. Possible role in the regulated exocytotic pathway

ADP-ribosylation factors (ARFs) constitute a family of structurally related proteins that forms a subset of the Ras superfamily of regulatory GTP-binding proteins. Like other GTPases, activation of ARFs is facilitated by specific guanine nucleotide exchange factors (GEFs). In chromaffin cells, ARF6 is associated with the membrane of secretory granules. Stimulation of intact cells or direct elevation of cytosolic calcium in permeabilized cells triggers the rapid translocation of ARF6 to the plasma membrane and the concomitant activation of phospholipase D (PLD) in the plasma membrane. Both calcium-evoked PLD activation and catecholamine secretion in permeabilized cells are strongly inhibited by a synthetic peptide corresponding to the N-terminal domain of ARF6, suggesting that the ARF6-dependent PLD activation near the exocytotic sites represents a key event in the exocytotic reaction in chromaffin cells. In the present study, we demonstrate the occurrence of a brefeldin A-insensitive ARF6-GEF activity in the plasma membrane and in the cytosol of chromaffin cells. Furthermore, reverse transcriptase-polymerase chain reaction and immunoreplica analysis indicate that ARNO, a member of the brefeldin A-insensitive ARF-GEF family, is expressed and predominantly localized in the cytosol and in the plasma membrane of chromaffin cells. Using permeabilized chromaffin cells, we found that the introduction of anti-ARNO antibodies into the cytosol inhibits, in a dose-dependent manner, both PLD activation and catecholamine secretion in calcium-stimulated cells. Furthermore, co-expression in PC12 cells of a catalytically inactive ARNO mutant with human growth hormone as a marker of secretory granules in transfected cells resulted in a 50% inhibition of growth hormone secretion evoked by depolarization with high K(+). The possibility that the plasma membrane-associated ARNO participates in the exocytotic pathway by activating ARF6 and downstream PLD is discussed.

GTP-binding proteins and regulated exocytosis

Regulated exocytosis, which occurs in response to stimuli, is a two-step process involving the docking of secretory granules (SGs) at specific sites on the plasma membrane (PM), with subsequent fusion and release of granule contents. This process plays a crucial role in a number of tissues, including exocrine glands, chromaffin cells, platelets, and mast cells. Over the years, our understanding of the proteins involved in vesicular trafficking has increased dramatically. Evidence from genetic, biochemical, immunological, and functional assays supports a role for ras-like monomeric GTP-binding proteins (smgs) as well as heterotrimeric GTP-binding protein (G-protein) subunits in various steps of the vesicular trafficking pathway, including the transport of secretory vesicles to the PM. Data suggest that the function of GTP-binding proteins is likely related to their localization to specific cellular compartments. The presence of both G-proteins and smgs on secretory vesicles/granules implicates a role for these proteins in the final stages of exocytosis. Molecular mechanisms of exocytosis have been postulated, with the identification of a number of proteins that modify, regulate, and interact with GTP-binding proteins, and with the advent of approaches that assess the functional importance of GTP-binding proteins in downstream, exocytotic events. Further, insight into vesicle targeting and fusion has come from the characterization of a SNAP receptor (SNARE) complex composed of vesicle, PM, and soluble membrane trafficking components, and identification of a functional linkage between GTP-binding and SNARES.

The heterotrimeric Gi(3) protein acts in slow but not in fast exocytosis of rat melanotrophs

Besides having a role in signal transduction some trimeric G-proteins may be involved in a late stage of exocytosis. Using immunocytochemistry and confocal microscopy we found that Gi(3)-protein resides mainly in the plasma membrane, whereas Gi(1/2-)protein is preferentially associated with secretory granules. To study the function of trimeric Gi(3)- and Gi(1/2)-proteins, secretory responses in single rat melanotrophs were monitored by patch-clamp membrane capacitance measurements. We report here that mastoparan, an activator of trimeric G-proteins, enhances calcium-induced secretory activity in rat melanotrophs. The introduction of synthetic peptides corresponding to the C-terminal domain of the (α)-subunit of Gi(3)- and Gi(1/2)-proteins indicated that Gi(3)peptide specifically blocked the mastoparan-stimulated secretory activity, which indicates an involvement of a trimeric Gi(3)-protein in mastoparan-stimulated secretory activity. Flash photolysis of caged Ca(2+)-elicited biphasic capacitance increases consisting of a fast and a slower component. Injection of anti-Gi(3) antibodies selectively inhibited the slow but not the fast component of secretory activity in rat melanotrophs. We propose that the plasma membrane-bound Gi(3)-protein may be involved in regulated secretion by specifically controlling the slower kinetic component of exocytosis.

cAMP regulates Ca2+-dependent exocytosis of lysosomes and lysosome-mediated cell invasion by trypanosomes

Ca2+-regulated exocytosis, previously believed to be restricted to specialized cells, was recently recognized as a ubiquitous process. In mammalian fibroblasts and epithelial cells, exocytic vesicles mobilized by Ca2+ were identified as lysosomes. Here we show that elevation in intracellular cAMP potentiates Ca2+-dependent exocytosis of lysosomes in normal rat kidney fibroblasts. The process can be modulated by the heterotrimeric G proteins Gs and Gi, consistent with activation or inhibition of adenylyl cyclase. Normal rat kidney cell stimulation with isoproterenol, a beta-adrenergic agonist that activates adenylyl cyclase, enhances Ca2+-dependent lysosome exocytosis and cell invasion by Trypanosoma cruzi, a process that involves parasite-induced [Ca2+]i transients and fusion of host cell lysosomes with the plasma membrane. Similarly to what is observed for T. cruzi invasion, the actin cytoskeleton acts as a barrier for Ca2+-induced lysosomal exocytosis. In addition, infective stages of T. cruzi trigger elevation in host cell cAMP levels, whereas no effect is observed with noninfective forms of the parasite. These findings demonstrate that cAMP regulates lysosomal exocytosis triggered by Ca2+ and a parasite/host cell interaction known to involve Ca2+-dependent lysosomal fusion.

Mechanism of peptide-induced mast cell degranulation. Translocation and patch-clamp studies

Substance P and other polycationic peptides are thought to stimulate mast cell degranulation via direct activation of G proteins. We investigated the ability of extracellularly applied substance P to translocate into mast cells and the ability of intracellularly applied substance P to stimulate degranulation. In addition, we studied by reverse transcription--PCR whether substance P-specific receptors are present in the mast cell membrane. To study translocation, a biologically active and enzymatically stable fluorescent analogue of substance P was synthesized. A rapid, substance P receptor- and energy-independent uptake of this peptide into pertussis toxin-treated and -untreated mast cells was demonstrated using confocal laser scanning microscopy. The peptide was shown to localize preferentially on or inside the mast cell granules using electron microscopic autoradiography with 125I-labeled all-D substance P and 3H-labeled substance P. Cell membrane capacitance measurements using the patch-clamp technique demonstrated that intracellularly applied substance P induced calcium transients and activated mast cell exocytosis with a time delay that depended on peptide concentration (delay of 100-500 s at concentrations of substance P from 50 to 5 microM). Degranulation in response to intracellularly applied substance P was inhibited by GDPbetaS and pertussis toxin, suggesting that substance P acts via G protein activation. These results support the recently proposed model of a receptor-independent mechanism of peptide-induced mast cell degranulation, which assumes a direct interaction of peptides with G protein alpha subunits subsequent to their translocation across the plasma membrane.

Localization of ADP-ribosylation factor domain protein 1 (ARD1) in lysosomes and Golgi apparatus

ADP-ribosylation factor domain protein 1 (ARD1) is a member of the ADP ribosylation factor (ARF) family of guanine nucleotide-binding proteins that differs from other ARFs by the presence of a 46-kDa amino-terminal extension which acts as a GTPase-activating protein (GAP) for its ARF domain. Similar to ARF GAPs, the GAP domain of ARD1 contains a zinc finger motif and arginine residues that are critical for activity. It differs from other ARF GAPs in its covalent association with the GTP-binding domain and its specificity for the ARF domain of ARD1. ARFs are presumed to play a key role in the formation of intracellular transport vesicles and in their movement from one compartment to another. We report here that ARD1 overexpressed in cells, as a fusion or nonfusion protein, is localized in vesicular structures that are concentrated mainly in the perinuclear region, but are found also throughout the cytosol. Microscopic colocalization and subcellular fractionation studies showed that ARD1 was associated with the Golgi complex and lysosomal structures. ARD1 expressed as a green fluorescent fusion protein was initially associated with the Golgi network and subsequently localized to lysosomes. Lysosomal and Golgi membranes isolated from human liver by immunoaffinity contained native ARD1. Localization to these organelles, therefore, did not appear to be a result of overexpression. These observations suggest that the ARF-related protein ARD1 may play a role in the formation or function of lysosomes and in protein trafficking between Golgi and lysosomes.

Identification of a potential effector pathway for the trimeric Go protein associated with secretory granules. Go stimulates a granule-bound phosphatidylinositol 4-kinase by activating RhoA in chromaffin cells

Besides having a role in signal transduction, heterotrimeric G proteins may be involved in membrane trafficking events. In chromaffin cells, Go is associated with secretory organelles, and its activation inhibits the ATP-dependent priming of exocytosis. By using permeabilized cells, we previously described that the control exerted by the granule-bound Go on exocytosis may be related to effects on the cortical actin network through a sequence possibly involving Rho. To provide further insight into the function of Rho in exocytosis, we focus here on its intracellular localization in chromaffin cells. By immunoreplica analysis, immunoprecipitation, and confocal immunofluorescence, we found that RhoA is specifically associated with the membrane of secretory chromaffin granules. Parallel subcellular fractionation experiments revealed the occurrence of a mastoparan-stimulated phosphatidylinositol 4-kinase activity in purified chromaffin granule membranes. This stimulatory effect of mastoparan was mimicked by GAP-43, an activator of the granule-associated Go, and specifically inhibited by antibodies against Galphao. In addition, Clostridium botulinum C3 exoenzyme completely blocked the activation of phosphatidylinositol 4-kinase by mastoparan. We propose that the control exerted by Go on peripheral actin and exocytosis is related to the activation of a downstream RhoA-dependent phosphatidylinositol 4-kinase associated with the membrane of secretory granules.

The two phases of regulated exocytosis in permeabilized pancreatic acini are modulated differently by heterotrimeric G-proteins

In this study we examined the influence on AlF4- and GTP gamma S on amylase secretion from alpha toxin permeabilized pancreatic acini. AlF4- only activates heterotrimeric G-proteins, whereas GTP gamma S activates both small ras-like GTP-binding proteins and heterotrimeric G-proteins (Kahn, R. A., J. Biol. Chem., 266, 15595-15597, 1991). GTP gamma S, but not AlF4-, significantly stimulated Ca2(+)-independent amylase secretion, suggesting that a small GTP-binding protein controls regulated exocytosis distal to the site of action of Ca2+. In contrast, both AlF4- and GTP gamma S modulated Ca(2+)-dependent amylase secretion. AlF4- and GTP gamma S stimulated the initial rapid, ATP-independent, phase of Ca(2+)-dependent secretion but inhibited the second slower sustained, ATP-dependent, phase of release. There were significant differences in the GTP gamma S requirements for the stimulation and inhibition of Ca(2+)-dependent amylase secretion, consistent with GTP gamma S activating separate heterotrimeric G-proteins to modulate each phase of the Ca(2+)-dependent secretory response. Our studies also indicated that neither G-protein is a member of the Gi/o class of heterotrimeric G-proteins.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Molecular characterization of the GTPase-activating domain of ADP-ribosylation factor domain protein 1 (ARD1)

ADP-ribosylation factors (ARFs) are approximately 20-kDa guanine nucleotide-binding proteins recognized as critical components in intracellular vesicular transport and phospholipase D activation. Both guanine nucleotide-exchange proteins and GTPase-activating proteins (GAPs) for ARFs have been cloned recently. A zinc finger motif near the amino terminus of the ARF1 GAP was required for stimulation of GTP hydrolysis. ARD1 is an ARF family member that differs from other ARFs by the presence of a 46-kDa amino-terminal extension. We had reported that the ARF domain of ARD1 binds specifically GDP and GTP and that the amino-terminal extension acts as a GAP for the ARF domain of ARD1 but not for ARF proteins. The GAP domain of ARD1, synthesized in Escherichia coli, stimulated hydrolysis of GTP bound to the ARF domain of ARD1. Using ARD1 truncations, it appears that amino acids 101-190 are critical for GAP activity, whereas residues 190-333 are involved in physical interaction between the two domains of ARD1 and are required for GTP hydrolysis. The GAP function of the amino-terminal extension of ARD1 required two arginines, an intact zinc finger motif, and a group of residues which resembles a sequence present in Rho/Rac GAPs. Interaction between the two domains of ARD1 required two negatively charged residues (Asp427 and Glu428) located in the effector region of the ARF domain and two basic amino acids (Arg249 and Lys250) found in the amino-terminal extension. The GAP domain of ARD1 thus is similar to ARF GAPs but differs from other GAPs in its covalent association with the GTP-binding domain.

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

The ADP-ribosylation factor (ARF) GTP-binding proteins have been implicated in a wide range of vesicle transport and fusion steps along the secretory pathway. In chromaffin cells, ARF6 is specifically associated with the membrane of secretory chromaffin granules. Since ARF6 is an established regulator of phospholipase D (PLD), we have examined the intracellular distribution of ARF6 and PLD activity in resting and stimulated chromaffin cells. We found that stimulation of intact chromaffin cells or direct elevation of cytosolic calcium in permeabilized cells triggered the rapid translocation of ARF6 from secretory granules to the plasma membrane and the concomitant activation of PLD in the plasma membrane. To probe the existence of an ARF6-dependent PLD in chromaffin cells, we measured the PLD activity in purified plasma membranes. PLD could be activated by a nonhydrolyzable analogue of GTP and by recombinant myristoylated ARF6 and inhibited by specific anti-ARF6 antibodies. Furthermore, a synthetic myristoylated peptide corresponding to the N-terminal domain of ARF6 inhibited both PLD activity and catecholamine secretion in calcium-stimulated chromaffin cells. The possibility that ARF6 participates in the exocytotic reaction by controlling a plasma membrane-bound PLD and thereby generating fusogenic lipids at the exocytotic sites is discussed.

The heterotrimeric GTP-binding protein, GS, modulates the Cl- conductance of rat parotid acinar secretory granules

Gsalpha has been reported to be present in rat parotid acinar secretory granule membrane (SGM) fractions. In the present study, we evaluated epitope orientation of Gsalpha on the secretory granule (SG) and the ability of Gs to modulate the Cl- conductance of isolated granules by measuring granule lysis. Gsalpha was found to be associated with the cytoplasmic face of the SGM. Aluminum fluroide (AlF4-, 20 microM Al3+ and 10 mM F-) significantly increased granule lysis and this effect was blocked by GDPbetaS. Cholera toxin (5 microg/ml) mimicked the effects of AlF4- on granule lysis, whereas pertussis toxin (0.5 microg/ml) was without effect. GTPgammaS, however, reduced granule lysis in a concentration-dependent manner. The orientation of Gsalpha on the SGM as well as the effects of AlF4- and cholera toxin on granule lysis lends support for a role of Gs in the exocytotic process.

Trimeric G proteins control exocytosis in chromaffin cells. Go regulates the peripheral actin network and catecholamine secretion by a mechanism involving the small GTP-binding protein Rho

Besides having a role in signal transduction, heterotrimeric G proteins may be involved in membrane trafficking events. In chromaffin cells, Go is associated with secretory organelles and its activation by mastoparan inhibits the ATP-dependent priming of exocytosis. The effectors by which Go controls exocytosis are currently unknown. The subplasmalemmal actin network is one candidate, since it modulates secretion by controlling the movement of secretory granules to the plasma membrane. In streptolysin-O-permeabilized chromaffin cells, activation of exocytosis produces disassembly of cortical actin filaments. Mastoparan blocks the calcium-evoked disruption of cortical actin, and this effect is specifically inhibited by antibodies against Galphao and by a synthetic peptide corresponding to the COOH-terminal domain of Galphao. Disruption of actin filaments with cytochalasin E and Clostridium perfringens iota toxin partially reverses the mastoparan-induced inhibition of secretion. Furthermore, the effects of mastoparan on cortical actin and exocytosis are greatly reduced in cells treated with Clostridium botulinum C3 exoenzyme, which specifically inactivates the small G protein Rho. We propose that the control exerted by the granule-associated Go on exocytosis may be related to effects on the cortical actin network through a sequence of events which eventually involves the participation of Rho.

Immunocytochemical localization of synaptic proteins at vesicular organelles in PC12 cells

The distribution of the three synaptic vesicle proteins SV2, synaptophysin and synaptotagmin, and of SNAP-25, a component of the docking and fusion complex, was investigated in PC12 cells by immunocytochemistry. Colloidal gold particle-bound secondary antibodies and a preembedding protocol were applied. Granules were labeled for SV2 and synaptotagmin but not for synaptophysin. Electron-lucent vesicles were labeled most intensively for synaptophysin but also for SV2 and to a lesser extent for synaptotagmin. The t-SNARE SNAP-25 was found at the plasma membrane but also at the surface of granules. Labeling of Golgi vesicles was observed for all antigens investigated. Also components of the endosomal pathway such as multivesicular bodies and multilamellar bodies were occasionally marked. The results suggest that the three membrane-integral synaptic vesicle proteins can have a differential distribution between electron-lucent vesicles (of which PC12 cells may possess more than one type) and granules. The membrane compartment of granules appears not to be an immediate precursor of that of electron-lucent vesicles.

Regulated exocytosis in chromaffin cells. A potential role for a secretory granule-associated ARF6 protein

The ADP-ribosylation factor (ARF) GTP-binding proteins are believed to function as regulators of vesicular budding and fusion along the secretory pathway. To investigate the role of ARF in regulated exocytosis, we have examined its intracellular distribution in cultured chromaffin cells by subcellular fractionation and immunoreplica analysis. We found that ARF6 is specifically associated with the membrane of purified secretory chromaffin granules. Chemical cross-linking and immunoprecipitation experiments suggested that ARF6 may be part of a complex with betagamma subunits of trimeric G proteins. Stimulation of intact chromaffin cells or direct elevation of cytosolic calcium in permeabilized cells triggered the rapid dissociation of ARF6 from secretory granules. This effect could be inhibited by AlF4- which selectively activates trimeric G proteins. Furthermore, a synthetic myristoylated peptide corresponding to the N-terminal domain of ARF6 strongly inhibited calcium-evoked secretion in streptolysin-O-permeabilized chromaffin cells. The possibility that ARF6 plays a role in the effector pathway by which trimeric G proteins control exocytosis in chromaffin cells is discussed.