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Abstract
Catecholamines and peptides secreted from dense-core vesicles (DCVs) of adrenal chromaffin cells regulate a wide variety of physiological processes. For instance, the release of noradrenaline and adrenaline plays a key role in regulating heart rate and blood pressure. Thus understanding the mechanisms of secretory processes of DCVs is crucial for understanding the basis of diseases such as hypertension. DCVs undergo several stages of secretory processing before they are exocytosed. These include docking, priming and triggering of membrane fusion/exocytosis. Molecular studies of DCV exocytosis have identified many proteins critically involved in DCV secretion. These proteins include SNARE proteins, Munc18-1, phosphatidylinositol transfer protein, type I phosphatidylinositol-4-phosphate-5-kinases, NSF, Munc13, CAPS1, synaptotagmins, RalA/RalB GTPases and exocyst proteins. In this article, I will discuss the functions of these proteins within the context of the stages (i.e. docking, priming and triggering of membrane fusion/exocytosis) in DCV secretion.
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Affiliation(s)
- S Sugita
- Division of Fundamental Neurobiology, Toronto Western Research Institute, University Health Network and Department of Physiology, University of Toronto, Toronto, ON, Canada.
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Jeans AF, Oliver PL, Johnson R, Capogna M, Vikman J, Molnár Z, Babbs A, Partridge CJ, Salehi A, Bengtsson M, Eliasson L, Rorsman P, Davies KE. A dominant mutation in Snap25 causes impaired vesicle trafficking, sensorimotor gating, and ataxia in the blind-drunk mouse. Proc Natl Acad Sci U S A 2007; 104:2431-6. [PMID: 17283335 PMCID: PMC1793901 DOI: 10.1073/pnas.0610222104] [Citation(s) in RCA: 102] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
The neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex is essential for synaptic vesicle exocytosis, but its study has been limited by the neonatal lethality of murine SNARE knockouts. Here, we describe a viable mouse line carrying a mutation in the b-isoform of neuronal SNARE synaptosomal-associated protein of 25 kDa (SNAP-25). The causative I67T missense mutation results in increased binding affinities within the SNARE complex, impaired exocytotic vesicle recycling and granule exocytosis in pancreatic beta-cells, and a reduction in the amplitude of evoked cortical excitatory postsynaptic potentials. The mice also display ataxia and impaired sensorimotor gating, a phenotype which has been associated with psychiatric disorders in humans. These studies therefore provide insights into the role of the SNARE complex in both diabetes and psychiatric disease.
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Affiliation(s)
| | | | | | - Marco Capogna
- Medical Research Council Anatomical Neuropharmacological Unit, University of Oxford, Mansfield Road, Oxford, OX1 3TH, United Kingdom
| | - Jenny Vikman
- Department of Clinical Sciences in Malmö, Clinical Research Centre, Lund University, SE-205 02 Malmö, Sweden; and
| | - Zoltán Molnár
- Department of Physiology, Anatomy, and Genetics, University of Oxford, South Parks Road, Oxford, OX1 3QX, United Kingdom
| | - Arran Babbs
- *Medical Research Council Functional Genetics Unit
| | - Christopher J. Partridge
- Oxford Centre for Diabetes, Endocrinology, and Metabolism, University of Oxford, Churchill Hospital, Oxford OX3 7LJ, United Kingdom
| | - Albert Salehi
- Department of Clinical Sciences in Malmö, Clinical Research Centre, Lund University, SE-205 02 Malmö, Sweden; and
| | - Martin Bengtsson
- Department of Clinical Sciences in Malmö, Clinical Research Centre, Lund University, SE-205 02 Malmö, Sweden; and
| | - Lena Eliasson
- Department of Clinical Sciences in Malmö, Clinical Research Centre, Lund University, SE-205 02 Malmö, Sweden; and
| | - Patrik Rorsman
- Oxford Centre for Diabetes, Endocrinology, and Metabolism, University of Oxford, Churchill Hospital, Oxford OX3 7LJ, United Kingdom
| | - Kay E. Davies
- *Medical Research Council Functional Genetics Unit
- To whom correspondence should be addressed. E-mail:
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4
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Li G, Han L, Chou TC, Fujita Y, Arunachalam L, Xu A, Wong A, Chiew SK, Wan Q, Wang L, Sugita S. RalA and RalB function as the critical GTP sensors for GTP-dependent exocytosis. J Neurosci 2007; 27:190-202. [PMID: 17202486 PMCID: PMC6672288 DOI: 10.1523/jneurosci.2537-06.2007] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Although it has been established that the activation of GTPases by non-hydrolyzable GTP stimulates neurotransmitter release from many different secretory cell types, the underlying mechanisms remain unclear. In the present study we aimed to elucidate the functional role(s) for endogenous Ras-like protein A (RalA) and RalB GTPases in GTP-dependent exocytosis. For this purpose stable neuroendocrine pheochromocytoma 12 (PC12) cell lines were generated in which the expressions of both RalA and RalB were strongly downregulated. In these double knock-down cells GTP-dependent exocytosis was reduced severely and was restored after the expression of RalA or RalB was reintroduced by transfection. In contrast, Ca2+-dependent exocytosis and the docking of dense core vesicles analyzed by electron microscopy remained unchanged in the double knock-down cells. Furthermore, the transfected RalA and RalB appeared to be localized primarily on the dense core vesicles in undifferentiated and nerve growth factor-differentiated PC12 cells. Our results indicate that endogenous RalA and RalB function specifically as GTP sensors for the GTP-dependent exocytosis of dense core vesicles, but they are not required for the general secretory pathways, including tethering of vesicles to the plasma membrane and Ca2+-dependent exocytosis.
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Affiliation(s)
- Gang Li
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
- Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8
| | - Liping Han
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
- Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8
| | - Ting-Chieh Chou
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
- Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8
| | - Yoshihito Fujita
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
| | - Lakshmanan Arunachalam
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
- Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8
| | - Ainan Xu
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
| | - Aaron Wong
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
| | - Soon-Kwang Chiew
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
| | - Qi Wan
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
- Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8
| | - Li Wang
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
| | - Shuzo Sugita
- Division of Cellular and Molecular Biology, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8, and
- Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8
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5
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Abonyo BO, Gou D, Wang P, Narasaraju T, Wang Z, Liu L. Syntaxin 2 and SNAP-23 are required for regulated surfactant secretion. Biochemistry 2004; 43:3499-506. [PMID: 15035620 DOI: 10.1021/bi036338y] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The secretion of lung surfactant in alveolar type II cells is a complex process involving the fusion of lamellar bodies with the plasma membrane. This process is somewhat different from the exocytosis of hormones and neurotransmitters. For example, it is a relatively slower process, and lamellar bodies are very large vesicles with a diameter of approximately 1 microm. SNARE proteins are the conserved molecular machinery of exocytosis in the majority of secretory cells. However, their involvement in surfactant secretion has not been reported. Here, we showed that syntaxin 2 and SNAP-23 are expressed in alveolar type II cells. Both proteins are associated with the plasma membrane, and to some degree with lamellar bodies. An antisense oligonucleotide complementary to syntaxin 2 decreased its mRNA and protein levels. The same oligonucleotide also inhibited surfactant secretion, independent of secretagogues. A peptide derived from the N-terminus of syntaxin 2 or the C-terminus of SNAP-23 significantly inhibited Ca(2+)- and GTPgammaS-stimulated surfactant secretion from permeabilized type II cells in a dose-dependent manner. Furthermore, introduction of anti-syntaxin 2 or anti-SNAP-23 antibodies into permeabilized type II cells also inhibited surfactant release. Our results suggest that syntaxin 2 and SNAP-23 are required for regulated surfactant secretion.
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Affiliation(s)
- Barack O Abonyo
- Department of Physiological Sciences, Oklahoma State University, Stillwater, Oklahoma 74078, USA
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6
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Szule JA, Coorssen JR. Revisiting the role of SNAREs in exocytosis and membrane fusion. BIOCHIMICA ET BIOPHYSICA ACTA 2003; 1641:121-35. [PMID: 12914953 DOI: 10.1016/s0167-4889(03)00095-8] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
For over a decade SNARE hypotheses have been proposed to explain the mechanism of membrane fusion, yet the field still lacks sufficient evidence to conclusively identify the minimal components of native fusion. Consequently, debate concerning the postulated role(s) of SNAREs in membrane fusion continues. The focus of this review is to revisit original literature with a current perspective. Our analysis begins with the earliest studies of clostridial toxins, leading to various cellular and molecular approaches that have been used to test for the roles of SNAREs in exocytosis. We place much emphasis on distinguishing between specific effects on membrane fusion and effects on other critical steps in exocytosis. Although many systems can be used to study exocytosis, few permit selective access to specific steps in the pathway, such as membrane fusion. Thus, while SNARE proteins are essential to the physiology of exocytosis, assay limitations often prevent definitive conclusions concerning the molecular mechanism of membrane fusion. In all, the SNAREs are more likely to function upstream as modulators or priming factors of fusion.
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Affiliation(s)
- Joseph A Szule
- Cellular and Molecular Neurobiology Research Group, Department of Physiology and Biophysics, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada T2N 4N1.
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7
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Coorssen JR, Blank PS, Albertorio F, Bezrukov L, Kolosova I, Chen X, Backlund PS, Zimmerberg J. Regulated secretion: SNARE density, vesicle fusion and calcium dependence. J Cell Sci 2003; 116:2087-97. [PMID: 12692190 DOI: 10.1242/jcs.00374] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
SNAREs such as VAMP, SNAP-25 and syntaxin are essential for intracellular trafficking, but what are their exact molecular roles and how are their interactions with other proteins manifest? Capitalizing on the differential sensitivity of SNAREs to exogenous proteases, we quantified the selective removal of identified SNAREs from native secretory vesicles without loss of fusion competence. Using previously established fusion assays and a high sensitivity immunoblotting protocol, we analyzed the relationship between these SNARE proteins and Ca2+-triggered membrane fusion. Neither the extent of fusion nor the number of intermembrane fusion complexes per vesicle were correlated with the measured density of identified egg cortical vesicle (CV) SNAREs. Without syntaxin, CVs remained fusion competent. Surprisingly, for one (but not another) protease the Ca2+ dependence of fusion was correlated with CV SNARE density, suggesting a native protein complex that associates with SNAREs, the architecture of which ensures high Ca2+ sensitivity. As SNAREs may function during CV docking in vivo, and as further proteolysis after SNARE removal eventually ablates fusion, we hypothesize that the triggered steps of regulated fusion (Ca2+ sensitivity and the catalysis and execution of fusion) require additional proteins that function downstream of SNAREs.
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Affiliation(s)
- Jens R Coorssen
- Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892-1855, USA
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8
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Abstract
Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.
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Affiliation(s)
- Robert D Burgoyne
- The Physiological Laboratory, University of Liverpool, United Kingdom.
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9
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Bloch-Shilderman E, Abu-Raya S, Trembovler V, Boschwitz H, Gruzman A, Linial M, Lazarovici P. Pardaxin stimulation of phospholipases A2 and their involvement in exocytosis in PC-12 cells. J Pharmacol Exp Ther 2002; 301:953-62. [PMID: 12023524 DOI: 10.1124/jpet.301.3.953] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Pardaxin (PX) is a voltage-dependent ionophore that stimulates catecholamine exocytosis from PC-12 pheochromocytoma cells both in the presence and absence of extracellular calcium. Using a battery of phospholipase A(2) inhibitors we show that PX stimulation of phospholipase A(2) (PLA(2)) enzymes is coupled with induction of exocytosis. We investigated the relationship between PX-induced PLA(2) activity and neurotransmitter release by measuring the levels of arachidonic acid (AA), prostaglandin E(2) (PGE(2)), and dopamine release. In the presence of extracellular calcium, the cytosolic PLA(2) inhibitor arachidonyl trifluoromethyl ketone (AACOCF(3)) inhibited by 100, 70, and 73%, respectively, the release of AA, PGE(2), and dopamine induced by PX. The mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor 2'-amino-3'-methoxyflavone (PD98059) reduced by 100 and 82%, respectively, the release of AA and PGE(2) induced by PX. In the absence of extracellular calcium, the calcium-independent PLA(2) (iPLA(2)) inhibitors methyl arachidonyl fluorophosphonate, AACOCF(3), and bromoenol lactone (BEL) inhibited by 80 to 90% PX stimulation of AA release, by 65 to 85% PX stimulation of PGE(2) release, and by 80 to 90% PX-induced dopamine release. Using vesicle fusion-based enzyme-linked immunosorbent assay we found similar levels of inhibition of PX-induced exocytosis by these inhibitors. Also, PX induced the formation of soluble N-ethylmaleimide-sensitive factor attachment protein receptor complexes, an effect that was augmented by N-methylmaleimide. This complex formation was completely inhibited by BEL. Botulinum toxins type C1 and F significantly inhibited the release of AA, PGE(2), and dopamine induced by PX. Our data suggest that PX stimulates exocytosis by activating cystolic PLA(2) and iPLA(2), leading to the generation of AA and eicosanoids, which, in turn, stimulate vesicle competence for fusion and neurotransmitter release.
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Affiliation(s)
- Eugenia Bloch-Shilderman
- Department of Pharmacology and Experimental Therapeutics, School of Pharmacy, Faculty of Medicine, The Hebrew University of Jerusalem, Jerusalem 91120, Israel
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10
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Graham ME, Washbourne P, Wilson MC, Burgoyne RD. SNAP-25 with mutations in the zero layer supports normal membrane fusion kinetics. J Cell Sci 2001; 114:4397-405. [PMID: 11792805 DOI: 10.1242/jcs.114.24.4397] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Considerable data support the idea that intracellular membrane fusion involves a conserved machinery containing the SNARE proteins. SNAREs assembled in vitro form a stable 4-helix bundle and it has been suggested that formation of this complex provides the driving force for bilayer fusion. We have tested this possibility in assays of exocytosis in cells expressing a botulinum neurotoxin E (BoNT/E)-resistant mutant of SNAP-25 in which additional disruptive mutations have been introduced. Single or double mutations of glutamine to glutamate or to arginine in the central zero layer residues of SNAP-25 did not impair the extent, time course or Ca2+-dependency of exocytosis in PC12 cells. Using adrenal chromaffin cells, we found that exocytosis could be reconstituted in cells transfected to express BoNT/E. A double Q→E mutation did not prevent reconstitution and the kinetics of single granule release events were indistinguishable from control cells. This shows a high level of tolerance of changes in the zero layer indicating that the conservation of these residues is not due to an essential requirement in vesicle docking or fusion and suggests that formation of a fully stable SNARE complex may not be required to drive membrane fusion.
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Affiliation(s)
- M E Graham
- The Physiological Laboratory, University of Liverpool, Crown Street, Liverpool, L69 3BX, UK
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11
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Wagner ML, Tamm LK. Reconstituted syntaxin1a/SNAP25 interacts with negatively charged lipids as measured by lateral diffusion in planar supported bilayers. Biophys J 2001; 81:266-75. [PMID: 11423412 PMCID: PMC1301509 DOI: 10.1016/s0006-3495(01)75697-4] [Citation(s) in RCA: 130] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022] Open
Abstract
According to the soluble N-ethylmaleimide-sensitive factor (NSF)-attachment protein (SNAP) receptor hypothesis (SNARE hypothesis), interactions between target SNAREs and vesicle SNAREs (t- and v-SNAREs) are required for membrane fusion in intracellular vesicle transport and exocytosis. The precise role of the SNAREs in tethering, docking, and fusion is still disputed. Biophysical measurements of SNARE interactions in planar supported membranes could potentially resolve some of the key questions regarding the mechanism of SNARE-mediated membrane fusion. As a first step toward this goal, recombinant syntaxin1A/SNAP25 (t-SNARE) was reconstituted into polymer-supported planar lipid bilayers. Reconstituted t-SNAREs in supported bilayers bound soluble green fluorescent protein/vesicle-associated membrane protein (v-SNARE), and the SNARE complexes could be specifically dissociated by NSF/alpha-SNAP in the presence of ATP. The physiological activities of SNARE complex formation were thus well reproduced in this reconstituted planar model membrane system. A large fraction (~75%) of the reconstituted t-SNARE was laterally mobile with a lateral diffusion coefficient of 7.5 x 10(-9) cm(2)/s in a phosphatidylcholine lipid background. Negatively charged lipids reduced the mobile fraction of the t-SNARE and the lipids themselves. Phosphatidylinositol-4,5-bisphosphate was more effective than phosphatidylserine in reducing the lateral mobility of the complexes. A model of how acidic lipid-SNARE interactions might alter lipid fluidity is discussed.
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Affiliation(s)
- M L Wagner
- Department of Molecular Physiology and Biological Physics and Center for Structural Biology, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908, USA
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12
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Wendler F, Page L, Urbé S, Tooze SA. Homotypic fusion of immature secretory granules during maturation requires syntaxin 6. Mol Biol Cell 2001; 12:1699-709. [PMID: 11408578 PMCID: PMC37334 DOI: 10.1091/mbc.12.6.1699] [Citation(s) in RCA: 113] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Homotypic fusion of immature secretory granules (ISGs) gives rise to mature secretory granules (MSGs), the storage compartment in endocrine and neuroendocrine cells for hormones and neuropeptides. With the use of a cell-free fusion assay, we investigated which soluble N-ethylmaleimide-sensitive fusion protein attachment receptor (SNARE) molecules are involved in the homotypic fusion of ISGs. Interestingly, the SNARE molecules mediating the exocytosis of MSGs in neuroendocrine cells, syntaxin 1, SNAP-25, and VAMP2, were not involved in homotypic ISG fusion. Instead, we have identified syntaxin 6 as a component of the core machinery responsible for homotypic ISG fusion. Subcellular fractionation studies and indirect immunofluorescence microscopy show that syntaxin 6 is sorted away during the maturation of ISGs to MSGs. Although, syntaxin 6 on ISG membranes is associated with SNAP-25 and SNAP-29/GS32, we could not find evidence that these target (t)-SNARE molecules are involved in homotypic ISG fusion. Nor could we find any involvement for the vesicle (v)-SNARE VAMP4, which is known to be associated with syntaxin 6. Importantly, we have shown that homotypic fusion requires the function of syntaxin 6 on both donor as well as acceptor membranes, which suggests that t-t-SNARE interactions, either direct or indirect, may be required during fusion of ISG membranes.
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Affiliation(s)
- F Wendler
- Secretory Pathway Laboratory, Imperial Cancer Research Fund, London WC2A 3PX, UK
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13
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Weber T, Parlati F, McNew JA, Johnston RJ, Westermann B, Söllner TH, Rothman JE. SNAREpins are functionally resistant to disruption by NSF and alphaSNAP. J Cell Biol 2000; 149:1063-72. [PMID: 10831610 PMCID: PMC2174819 DOI: 10.1083/jcb.149.5.1063] [Citation(s) in RCA: 97] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
SNARE (SNAP [soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein] receptor) proteins are required for many fusion processes, and recent studies of isolated SNARE proteins reveal that they are inherently capable of fusing lipid bilayers. Cis-SNARE complexes (formed when vesicle SNAREs [v-SNAREs] and target membrane SNAREs [t-SNAREs] combine in the same membrane) are disrupted by the action of the abundant cytoplasmic ATPase NSF, which is necessary to maintain a supply of uncombined v- and t-SNAREs for fusion in cells. Fusion is mediated by these same SNARE proteins, forming trans-SNARE complexes between membranes. This raises an important question: why doesn't NSF disrupt these SNARE complexes as well, preventing fusion from occurring at all? Here, we report several lines of evidence that demonstrate that SNAREpins (trans-SNARE complexes) are in fact functionally resistant to NSF, and they become so at the moment they form and commit to fusion. This elegant design allows fusion to proceed locally in the face of an overall environment that massively favors SNARE disruption.
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Affiliation(s)
- Thomas Weber
- Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021
| | - Francesco Parlati
- Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021
| | - James A. McNew
- Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021
| | - Robert J. Johnston
- Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021
| | - Benedikt Westermann
- Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021
| | - Thomas H. Söllner
- Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021
| | - James E. Rothman
- Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021
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14
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Krantz DE, Waites C, Oorschot V, Liu Y, Wilson RI, Tan PK, Klumperman J, Edwards RH. A phosphorylation site regulates sorting of the vesicular acetylcholine transporter to dense core vesicles. J Cell Biol 2000; 149:379-96. [PMID: 10769030 PMCID: PMC2175167 DOI: 10.1083/jcb.149.2.379] [Citation(s) in RCA: 84] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Vesicular transport proteins package classical neurotransmitters for regulated exocytotic release, and localize to at least two distinct types of secretory vesicles. In PC12 cells, the vesicular acetylcholine transporter (VAChT) localizes preferentially to synaptic-like microvesicles (SLMVs), whereas the closely related vesicular monoamine transporters (VMATs) localize preferentially to large dense core vesicles (LDCVs). VAChT and the VMATs contain COOH-terminal, cytoplasmic dileucine motifs required for internalization from the plasma membrane. We now show that VAChT undergoes regulated phosphorylation by protein kinase C on a serine (Ser-480) five residues upstream of the dileucine motif. Replacement of Ser-480 by glutamate, to mimic the phosphorylation event, increases the localization of VAChT to LDCVs. Conversely, the VMATs contain two glutamates upstream of their dileucine-like motif, and replacement of these residues by alanine conversely reduces sorting to LDCVs. The results provide some of the first information about sequences involved in sorting to LDCVs. Since the location of the transporters determines which vesicles store classical neurotransmitters, a change in VAChT trafficking due to phosphorylation may also influence the mode of transmitter release.
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Affiliation(s)
- David E. Krantz
- Department of Neurology, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
- Department of Psychiatry, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
| | - Clarissa Waites
- Graduate Programs in Neuroscience, Cell Biology, and Biomedical Sciences, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
| | - Viola Oorschot
- Department of Cell Biology, University Medical Center and Institute of Biomembranes, Utrecht University, 3584 CX Utrecht, The Netherlands
| | - Yongjian Liu
- Department of Neurology, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
| | - Rachel I. Wilson
- Department of Physiology, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
| | - Philip K. Tan
- Department of Neurology, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
| | - Judith Klumperman
- Department of Cell Biology, University Medical Center and Institute of Biomembranes, Utrecht University, 3584 CX Utrecht, The Netherlands
| | - Robert H. Edwards
- Graduate Programs in Neuroscience, Cell Biology, and Biomedical Sciences, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
- Department of Neurology, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
- Department of Physiology, University of California at San Francisco School of Medicine, San Francisco, California 94143-0435
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15
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Abstract
Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.
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Affiliation(s)
- G Schiavo
- Imperial Cancer Research Fund, London, United Kingdom
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16
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Avery J, Jahn R, Edwardson JM. Reconstitution of regulated exocytosis in cell-free systems: a critical appraisal. Annu Rev Physiol 1999; 61:777-807. [PMID: 10099710 DOI: 10.1146/annurev.physiol.61.1.777] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Regulated exocytosis involves the tightly controlled fusion of a transport vesicle with the plasma membrane. It includes processes as diverse as the release of neurotransmitters from presynaptic nerve endings and the sperm-triggered deposition of a barrier preventing polyspermy in oocytes. Cell-free model systems have been developed for studying the biochemical events underlying exocytosis. They range from semi-intact permeabilized cells to the reconstitution of membrane fusion from isolated secretory vesicles and their target plasma membranes. Interest in such cell-free systems has recently been reinvigorated by new evidence suggesting that membrane fusion is mediated by a basic mechanism common to all intracellular fusion events. In this chapter, we review some of the literature in the light of these new developments and attempt to provide a critical discussion of the strengths and limitations of the various cell-free systems.
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Affiliation(s)
- J Avery
- Department of Neurobiology, Max-Planck-Institute for Biophysical Chemistry, Göttingen, Germany.
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17
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Banaschewski C, Höhne-Zell B, Ovtscharoff W, Gratzl M. Characterization of vesicular membrane-bound alpha-SNAP and NSF in adrenal chromaffin cells. Biochemistry 1998; 37:16719-27. [PMID: 9843441 DOI: 10.1021/bi981339p] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
alpha-SNAP and NSF are thought to act as soluble factors, which transiently bind to a complex formed between syntaxin and SNAP-25 located at the plasma membrane and synaptobrevin at the secretory vesicle membrane, at the moment of exocytosis. Here we present data which permit the novel conclusion that alpha-SNAP and NSF are not soluble in adrenal chromaffin cells but are rather membrane-bound in particular to undocked chromaffin vesicles. Evidence for this new paradigm is derived from several experimental approaches. First, alpha-SNAP and NSF were found predominantly at cellular membranes and not in the cytosol of cracked chromaffin cells. Second, alpha-SNAP and NSF were not released from membranes by Mg2+ATP, which causes priming of vesicles. Third, immune electron microscopy and immunoblotting of chromaffin vesicles purified by immunoisolation or density gradient centrifugation revealed the presence of alpha-SNAP and NSF together with typical vesicular proteins such as synaptobrevin and synaptotagmin. In the sucrose gradient 30% alpha-SNAP and 27% NSF were recovered with chromaffin vesicles. Bound alpha-SNAP was quantified (14 molecules/vesicle), and binding was characterized with recombinant his6-tagged alpha-SNAP. Overlay blots revealed that alpha-SNAP is bound to vesicular SNAP-25 and endogenous NSF. Our data show that mature chromaffin vesicles already contain specifically bound alpha-SNAP and NSF before docking at the plasmalemma.
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Affiliation(s)
- C Banaschewski
- Anatomisches Institut der Technischen Universität München, Germany
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18
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Abstract
The reconstitution of a membrane fusion event in a cell-free system makes possible a biochemical investigation of the molecular mechanisms underlying it. We have developed an in vitro assay for the fusion of pancreatic zymogen granules with the plasma membrane. The lipid-soluble fluorescent probe octadecylrhodamine is loaded into the granule membrane, and the granules are then incubated with unlabeled plasma membranes. Membrane fusion results in a dilution of the probe, which is detected through the dequenching of its fluorescence. The properties of the in vitro fusion event are impressively similar to those of exocytosis from permeabilized pancreatic acini, indicating that dequenching is detecting a physiologically relevant process. In particular, exocytotic membrane fusion both in vitro and in permeabilized acini is stimulated by Ca2+ with an EC50 of 1 microM, and enhanced by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) with an EC50 of 10-20 microM. Another parallel between the two systems is the incomplete inhibition of fusion/exocytosis by tetanus toxin, despite complete cleavage of synaptobrevin 2 on the zymogen granule membrane. Recently, the in vitro assay for membrane fusion has been used to indicate a role in the control of exocytosis for syncollin, a granule membrane protein that binds to syntaxin in a Ca2+-sensitive manner. The assay should continue to provide information about this exocytotic membrane fusion event in the future.
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Affiliation(s)
- J M Edwardson
- Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge, CB2 1QJ, United Kingdom
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Quintanar JL, Salinas E, Reig JA. Immunohistochemical demonstration of syntaxin and SNAP-25 in chromaffin cells of the frog adrenal gland. Gen Comp Endocrinol 1998; 111:119-22. [PMID: 9679084 DOI: 10.1006/gcen.1998.7110] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The release of catecholamines from chromaffin cells involves specific proteins such as synaptobrevin present in the secretory vesicles as well as syntaxin and synaptosomal-associated protein of 25 kDa (SNAP-25), both present in the plasma membrane. We have found syntaxin and SNAP-25 in chromaffin cells of the frog adrenal gland by immunohistochemistry. This result suggests that the secretion of catecholamines from chromaffin cells involves these proteins in the frog.
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Affiliation(s)
- J L Quintanar
- Centro de Ciencias Básicas, Universidad Autónoma de Aguascalientes, Aguascalientes, México
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20
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Abstract
Protein secretion from rat parotid acinar cells occurs in both the absence and presence of secretory agonists. Release takes place by four pathways that are distinguished by combined examination of their timing following biosynthetic labeling, their relative composition of salivary proteins, and their sensitivity to secretagogue stimulation. Following pulse-labeling with a radioactive amino acid, two unstimulated export pathways are detected--a constitutive-like pathway that is coupled to maturation of secretory granules and the later unstimulated exocytosis of secretory granules. In both cases, protein release is insensitive to secretory antagonists. Two regulated secretory pathways are also detected. The major regulated pathway comprises stimulated exocytosis of secretory granules and requires application of beta-adrenergic agonists (> or = 1 microM). A newly discovered minor regulated pathway resembles the constitutive-like pathway in secretory composition but requires low-dose stimulation by either beta-adrenergic or cholinergic agonists. The latter pathway may provide a significant component of basal secretion by the parotid gland during periods between meals.
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Affiliation(s)
- J D Castle
- Department of Cell Biology, University of Virginia Health Sciences Center, Charlottesville 22908, USA.
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21
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Aunis D. Exocytosis in chromaffin cells of the adrenal medulla. INTERNATIONAL REVIEW OF CYTOLOGY 1998; 181:213-320. [PMID: 9522458 DOI: 10.1016/s0074-7696(08)60419-2] [Citation(s) in RCA: 70] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
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.
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Affiliation(s)
- D Aunis
- Biologie de la Communication Cellulaire, Unité INSERM U-338, Strasbourg, France
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Weber T, Zemelman BV, McNew JA, Westermann B, Gmachl M, Parlati F, Söllner TH, Rothman JE. SNAREpins: minimal machinery for membrane fusion. Cell 1998; 92:759-72. [PMID: 9529252 DOI: 10.1016/s0092-8674(00)81404-x] [Citation(s) in RCA: 1901] [Impact Index Per Article: 73.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Recombinant v- and t-SNARE proteins reconstituted into separate lipid bilayer vesicles assemble into SNAREpins-SNARE complexes linking two membranes. This leads to spontaneous fusion of the docked membranes at physiological temperature. Docked unfused intermediates can accumulate at lower temperatures and can fuse when brought to physiological temperature. A supply of unassembled v- and t-SNAREs is needed for these intermediates to form, but not for the fusion that follows. These data imply that SNAREpins are the minimal machinery for cellular membrane fusion.
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Affiliation(s)
- T Weber
- Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA
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