Adrenal chromaffin cells contain functionally different SNAP-25 monomers and SNAP-25/syntaxin heterodimers

v-SNARE function in chromaffin cells

Vesicle fusion is elementary for intracellular trafficking and release of signal molecules, thus providing the basis for diverse forms of intercellular communication like hormonal regulation or synaptic transmission. A detailed characterization of the mechanisms underlying exocytosis is key to understand how the nervous system integrates information and generates appropriate responses to stimuli. The machinery for vesicular release employs common molecular players in different model systems including neuronal and neuroendocrine cells, in particular members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) protein family, Sec1/Munc18-like proteins, and other accessory factors. To achieve temporal precision and speed, excitable cells utilize specialized regulatory proteins like synaptotagmin and complexin, whose interplay putatively synchronizes vesicle fusion and enhances stimulus-secretion coupling. In this review, we aim to highlight recent progress and emerging views on the molecular mechanisms, by which constitutively forming SNAREpins are organized in functional, tightly regulated units for synchronized release. Specifically, we will focus on the role of vesicle associated membrane proteins, also referred to as vesicular SNAREs, in fusion and rapid cargo discharge. We will further discuss the functions of SNARE regulators during exocytosis and focus on chromaffin cell as a model system of choice that allows for detailed structure-function analyses and direct measurements of vesicle fusion under precise control of intracellular [Ca]i.

SNAP-25 in hippocampal CA3 region is required for long-term memory formation

SNAP-25 is a synaptosomal protein of 25 kDa, a key component of synaptic vesicle-docking/fusion machinery, and plays a critical role in exocytosis and neurotransmitter release. We previously reported that SNAP-25 in the hippocampal CA1 region is involved in consolidation of contextual fear memory and water-maze spatial memory (Hou et al. European J Neuroscience, 20: 1593-1603, 2004). SNAP-25 is expressed not only in the CA1 region, but also in the CA3 region, and the SNAP-25 mRNA level in the CA3 region is higher than in the CA1 region. Here, we provide evidence that SNAP-25 in the CA3 region is also involved in learning/memory. Intra-CA3 infusion of SNAP-25 antisense oligonucleotide impaired both long-term contextual fear memory and water-maze spatial memory, with short-term memory intact. Furthermore, the SNAP-25 antisense oligonucleotide suppressed the long-term potentiation (LTP) of field excitatory post-synaptic potential (fEPSP) in the mossy-fiber pathway (DG-CA3 pathway), with no effect on paired-pulse facilitation of the fEPSP. These results are consistent with the notion that SNAP-25 in the hippocampal CA3 region is required for long-term memory formation.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Insights into GABA receptor signalling in TM3 Leydig cells

Gamma-aminobutyric acid (GABA) is an emerging signalling molecule in endocrine organs, since it is produced by endocrine cells and acts via GABA(A) receptors in a paracrine/autocrine fashion. Testicular Leydig cells are producers and targets for GABA. These cells express GABA(A) receptor subunits and in the murine Leydig cell line TM3 pharmacological activation leads to increased proliferation. The signalling pathway of GABA in these cells is not known in this study. We therefore attempted to elucidate details of GABA(A) signalling in TM3 and adult mouse Leydig cells using several experimental approaches. TM3 cells not only express GABA(A )receptor subunits, but also bind the GABA agonist [(3)H]muscimol with a binding affinity in the range reported for other endocrine cells (K(d) = 2.740 +/- 0.721 nM). However, they exhibit a low B(max) value of 28.08 fmol/mg protein. Typical GABA(A) receptor-associated events, including Cl(-) currents, changes in resting membrane potential, intracellular Ca(2+) or cAMP, were not measurable with the methods employed in TM3 cells, or, as studied in part, in primary mouse Leydig cells. GABA or GABA(A) agonist isoguvacine treatment resulted in increased or decreased levels of several mRNAs, including transcription factors (c-fos, hsf-1, egr-1) and cell cycle-associated genes (Cdk2, cyclin D1). In an attempt to verify the cDNA array results and because egr-1 was recently implied in Leydig cell development, we further studied this factor. RT-PCR and Western blotting confirmed a time-dependent regulation of egr-1 in TM3. In the postnatal testis egr-1 was seen in cytoplasmic and nuclear locations of developing Leydig cells, which bear GABA(A) receptors and correspond well to TM3 cells. Thus, GABA acts via an atypical novel signalling pathway in TM3 cells. Further details of this pathway remain to be elucidated.

Stimulation of TM3 Leydig cell proliferation via GABA(A) receptors: a new role for testicular GABA

The neurotransmitter gamma-aminobutyric acid (GABA) and subtypes of GABA receptors were recently identified in adult testes. Since adult Leydig cells possess both the GABA biosynthetic enzyme glutamate decarboxylase (GAD), as well as GABA(A) and GABA(B) receptors, it is possible that GABA may act as auto-/paracrine molecule to regulate Leydig cell function. The present study was aimed to examine effects of GABA, which may include trophic action. This assumption is based on reports pinpointing GABA as regulator of proliferation and differentiation of developing neurons via GABA(A) receptors. Assuming such a role for the developing testis, we studied whether GABA synthesis and GABA receptors are already present in the postnatal testis, where fetal Leydig cells and, to a much greater extend, cells of the adult Leydig cell lineage proliferate. Immunohistochemistry, RT-PCR, Western blotting and a radioactive enzymatic GAD assay evidenced that fetal Leydig cells of five-six days old rats possess active GAD protein, and that both fetal Leydig cells and cells of the adult Leydig cell lineage possess GABA(A) receptor subunits. TM3 cells, a proliferating mouse Leydig cell line, which we showed to possess GABA(A) receptor subunits by RT-PCR, served to study effects of GABA on proliferation. Using a colorimetric proliferation assay and Western Blotting for proliferating cell nuclear antigen (PCNA) we demonstrated that GABA or the GABA(A) agonist isoguvacine significantly increased TM3 cell number and PCNA content in TM3 cells. These effects were blocked by the GABA(A) antagonist bicuculline, implying a role for GABA(A) receptors. In conclusion, GABA increases proliferation of TM3 Leydig cells via GABA(A) receptor activation and proliferating Leydig cells in the postnatal rodent testis bear a GABAergic system. Thus testicular GABA may play an as yet unrecognized role in the development of Leydig cells during the differentiation of the testicular interstitial compartment.

Evidence for structural and functional diversity among SDS-resistant SNARE complexes in neuroendocrine cells

The core complex, formed by the SNARE proteins synaptobrevin 2, syntaxin 1 and SNAP-25, is an important component of the synaptic fusion machinery and shows remarkable in vitro stability, as exemplified by its SDS-resistance. In western blots, antibodies against one of these SNARE proteins reveal the existence of not only an SDS-resistant ternary complex but also as many as five bands between 60 and >200 kDa. Structural conformation as well as possible functions of these various complexes remained elusive. In western blots of protein extracts from PC12 cell membranes, an antibody against SNAP-25 detected two heat-sensitive SDS-resistant bands with apparent molecular weights of 100 and 230 kDa. A syntaxin antibody recognized only the 230 kDa band and required heat-treatment of the blotting membrane to detect the 100 kDa band. Various antibodies against synaptobrevin failed to detect SNARE complexes in conventional western blots and detected either the 100 kDa band or the 230 kDa band on heat-treated blotting membranes. When PC12 cells were exposed to various extracellular K(+)-concentrations (to evoke depolarization-induced Ca(2+) influx) or permeabilized in the presence of basal or elevated free Ca(2+), levels of these SNARE complexes were altered differentially: moderate Ca(2+) rises (</=1 microM) caused an increase, whereas Ca(2+) elevations of more than 1 microM led to a decrease in the 230 kDa band. Under both conditions the 100 kDa band was either increased or remained unchanged. Our data show that various SDS-resistant complexes occur in living cells and indicate that they represent SNARE complexes with different structures and diverging functions. The distinct behavior of these complexes under release-promoting conditions indicates that these SNARE structures have different roles in exocytosis.

Differential distribution and regulation of expression of synaptosomal-associated protein of 25 kDa isoforms in the Xenopus pituitary gland and brain

Synaptosomal-associated protein of 25 kDa (SNAP-25) regulates various membrane fusion processes including exocytosis by endocrine and neural cells. To increase our understanding of the occurrence and regulation of SNAP-25 isoforms, we identified and characterized SNAP-25a and SNAP-25b mRNAs in the pituitary gland and brain of the amphibian Xenopus laevis. The proteins are strongly conserved and are resistant to botulinum neurotoxin A but not to botulinum neurotoxin E, as shown by Western blotting. The spatial distribution of the two SNAP-25 isoforms was assessed with in situ hybridization. Both SNAP-25a mRNA and SNAP-25b mRNA reside in cells in the pituitary distal lobe and, particularly, in the endocrine melanotrope cells in the pituitary intermediate lobe. The melanotrope cells are involved in the background adaptation process of the skin by releasing alpha-melanophore-stimulating hormone. Quantitation of the respective in situ hybridization signals in the Xenopus pars intermedia indicated a differential response, SNAP-25b mRNA being more highly expressed in black-adapted animals than SNAP-25a mRNA, and more than in white-adapted toads. This differential upregulation was also studied by real-time reverse transcriptase polymerase chain reaction, showing that in the intermediate pituitary lobe, both isoforms are physiologically controlled by the background light intensity stimulus, but with different intensities; in black-adapted animals SNAP-25b mRNA is upregulated by 3.33 times compared with white-adapted animals, but SNAP-25a only by 1.96 times. As to neural tissue, in situ hybridization showed that both isoforms coexist throughout the brain, sometimes with similar strengths, but in various areas either SNAP-25a mRNA or SNAP-25b mRNA expression is prevalent. It is speculated that each of the SNAP-25 isoforms in the Xenopus pituitary and brain has a distinct function in cellular fusion processes including secretion, and that their occurrence and regulation depend on the type of secreted neurotransmitter/hormone and/or the activity state of the cell.

Secretory granule exocytosis

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.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

Homotypic fusion of immature secretory granules during maturation requires syntaxin 6

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.

Involvement of SNAP-23 and syntaxin 6 in human neutrophil exocytosis

To understand the molecular basis of exocytosis in human neutrophils, the role of syntaxin 6 and SNAP-23 in neutrophil degranulation was examined. Human syntaxin 6 was cloned and identified as a 255-amino acid protein with a carboxy-terminal transmembrane region and two coiled-coil domains. Syntaxin 6 was localized mainly in the plasma membrane of human resting neutrophils, whereas SNAP-23 was located primarily in the mobilizable tertiary and specific granules. SNAP-23 was translocated to the cell surface, colocalizing with syntaxin 6, on neutrophil activation. In vitro binding studies established that SNAP-23 binds to syntaxin 6. Coimmunoprecipitation assays indicated that SNAP-23 interacts with syntaxin 6 in vivo, and this interaction was dramatically increased on neutrophil activation. Antibodies against SNAP-23 inhibited Ca++ and GTP-γ-S–induced exocytosis of CD67-enriched specific granules, but they hardly affected exocytosis of the CD63-enriched azurophilic granules, when introduced into electropermeabilized neutrophils. Anti–syntaxin 6 antibodies prevented exocytosis of both CD67- and CD63-enriched granules in electropermeabilized neutrophils. These data show that syntaxin 6 and SNAP-23 are involved in human neutrophil exocytosis, demonstrating that vesicle SNAP receptor-target SNAP receptor (v-SNARE– t-SNARE) interactions modulate neutrophil secretion. Syntaxin 6 acts as a target for secretion of specific and azurophilic granules, whereas SNAP-23 mediates specific granule secretion.

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.

Involvement of SNAP-23 and syntaxin 6 in human neutrophil exocytosis

To understand the molecular basis of exocytosis in human neutrophils, the role of syntaxin 6 and SNAP-23 in neutrophil degranulation was examined. Human syntaxin 6 was cloned and identified as a 255-amino acid protein with a carboxy-terminal transmembrane region and two coiled-coil domains. Syntaxin 6 was localized mainly in the plasma membrane of human resting neutrophils, whereas SNAP-23 was located primarily in the mobilizable tertiary and specific granules. SNAP-23 was translocated to the cell surface, colocalizing with syntaxin 6, on neutrophil activation. In vitro binding studies established that SNAP-23 binds to syntaxin 6. Coimmunoprecipitation assays indicated that SNAP-23 interacts with syntaxin 6 in vivo, and this interaction was dramatically increased on neutrophil activation. Antibodies against SNAP-23 inhibited Ca++ and GTP-γ-S–induced exocytosis of CD67-enriched specific granules, but they hardly affected exocytosis of the CD63-enriched azurophilic granules, when introduced into electropermeabilized neutrophils. Anti–syntaxin 6 antibodies prevented exocytosis of both CD67- and CD63-enriched granules in electropermeabilized neutrophils. These data show that syntaxin 6 and SNAP-23 are involved in human neutrophil exocytosis, demonstrating that vesicle SNAP receptor-target SNAP receptor (v-SNARE– t-SNARE) interactions modulate neutrophil secretion. Syntaxin 6 acts as a target for secretion of specific and azurophilic granules, whereas SNAP-23 mediates specific granule secretion.

Taste cells with synapses in rat circumvallate papillae display SNAP-25-like immunoreactivity

SNAP-25 is a 25 kDa protein believed to be involved in the processes of membrane fusion and exocytosis associated with neurotransmitter release. In the present study we present evidence that SNAP-25-like immunoreactivity can be used as a marker for taste cells with synapses in rat circumvallate papillae. SNAP-25 immunoreactivity is present in most intragemmal nerve processes and a small subset of taste cells. Intense immunoreactivity is associated with the nerve plexus located below the base of the taste bud. Of a total of 87 taste cells with synapses onto nerve processes, 80 of the presynaptic taste cells had SNAP-25 immunoreactivity. The association of SNAP-25 immunoreactivity with taste cells possessing synapses suggests that these cells may be gustatory receptor cells. Because this SNAP-25 antibody can label taste cells with synapses, it may also serve as a useful tool for future studies correlating structure with function in the taste bud.

Synaptosome-associated protein of 25 kilodaltons in oocytes and steroid-producing cells of rat and human ovary: molecular analysis and regulation by gonadotropins

The synaptosome-associated protein of 25 kDa (SNAP-25) is crucially involved in exocytosis in neurons. The aim of this study was to investigate whether it is present in the ovary. We found SNAP-25 to be expressed in nonneuronal cells of the rat and human ovary, namely in all oocytes and in steroidogenic cells, including granulosa cells (GC) of large antral follicles and luteal cells. Both isoforms, SNAP-25a and b, were found in the ovary. Oocytes obtained by laser capture microdissection were shown to express SNAP-25b, whereas SNAP-25a was found in rat GC and human luteinized GC. Immunohistochemical observations of strong SNAP-25 staining in GC of large growing antral follicles compared with absent or weak staining in small follicles suggested a role in folliculogenesis. To study a presumed regulation of SNAP-25, we used a rat GC line (GFSHR-17), which expresses FSH receptors, and luteinizing human GC, which express LH receptors. FSH elevated SNAP-25 mRNA and protein levels about fivefold within 24 h in GFSHR-17 cells. The cAMP analogue dibutyryl-cAMP (db-cAMP) mimicked this action of FSH. The effects of both db-cAMP and FSH were inhibited by the protein kinase A (PKA) inhibitor H89. In contrast, SNAP-25 protein and mRNA-levels were not altered by LH/hCG in luteinized human GC. Our results for the first time identify SNAP-25b in oocytes and SNAP-25a in steroidogenic cells of the mammalian ovary. SNAP-25a and b may be involved in different exocytotic processes in these cell types.

Human testicular mast cells contain tryptase: increased mast cell number and altered distribution in the testes of infertile men

OBJECTIVE

To determine whether human testicular mast cells contain the potent fibroblast growth factor tryptase and to examine changes in mast cell morphology and intratesticular distribution in testes with normal spermatogenesis versus abnormal spermatogenesis.

DESIGN

Retrospective evaluation of testicular biopsies with the use of immunohistochemistry, morphometry, and electron microscopy.

SETTING

University research and clinical institutes.

PATIENT(S)

Infertile men (total of 24) with severe hypospermatogenesis, germ cell arrest syndrome, or Sertoli cell only syndrome, and men without pathologies.

INTERVENTION(S)

Diagnostic testicular biopsy.

MAIN OUTCOME MEASURE(S)

Location, number, and distribution of testicular mast cells.

RESULT(S)

All groups showed tryptase-positive mast cells. In specimens with normal spermatogenesis, mast cells were round and located mainly in the interstitial spaces close to Leydig cells. In germ cell arrest syndrome, a 2-fold increase was evident, and in Sertoli cell only syndrome, a >3-fold increase of tryptase-immunoreactive mast cells became evident. Moreover, there was a statistically significant shift of the cells from the interstitium to the tubular walls in Sertoli cell only syndrome and germ cell arrest syndrome. Mast cells in specimens of Sertoli cell only syndrome and germ cell arrest syndrome were heterogeneous, with rounded or elongated shapes and signs of degranulation. The thickness of the tubular walls was doubled in specimens of germ cell arrest syndrome and Sertoli cell only syndrome in comparison with normal specimens, and this increase was positively correlated with the number of mast cells in these patients.

CONCLUSION(S)

Our results suggest that mast cell products, including the potent fibroblast growth factor tryptase, are involved in the thickening of the tubular wall and other changes in infertile testes.

Localization and physiological regulation of the exocytosis protein SNAP-25 in the brain and pituitary gland of Xenopus laevis

In mammals, the synaptosomal-associated protein of 25 kDa, SNAP-25, is generally thought to play a role in synaptic exocytosis of neuronal messengers. Using a polyclonal antiserum against rat SNAP-25, we have shown the presence of a SNAP-25-like protein in the brain of the South-African clawed toad Xenopus laevis by Western blotting and immunocytochemistry. Xenopus SNAP-25 is ubiquitously present throughout the brain, where its distribution in various identified neuronal perikarya and axon tracts is described. Western blot analysis and immunocytochemistry also demonstrated the presence of SNAP-25 in the neural, intermediate and distal lobes of the pituitary gland. Intensity line plots of confocal laser scanning microscope images of isolated melanotropes indicated that SNAP-25 is produced and processed in the rough endoplasmatic reticulum and Golgi apparatus, and is associated with the plasma membrane. Immunoelectron microscopy substantiated the idea that SNAP-25 is present in the plasma membrane but also showed a close association of SNAP-25 with the bounding membrane of peptide-containing secretory granules in both the neurohemal axon terminals in the neural lobe and the endocrine melanotropes in the intermediate lobe. Quantitative Western blotting revealed that adapting Xenopus to a dark background has a clear stimulatory effect on the expression of SNAP-25 in the neural lobe and in the melanotrope cells. This background light intensity-dependent stimulation of SNAP-25 expression was confirmed by the demonstration of increased immunofluorescence recorded by confocal laser scanning microscopy of individual melanotropes of black background-adapted toads. On the basis of this study on Xenopus laevis, we conclude that SNAP-25 (i) plays a substantial role in the secretion of a wide variety of neuronal messengers; (ii) functions in the central nervous system but also in neurohormonal and endocrine systems; (iii) acts at the plasma membrane but possibly also at the membrane of synaptic vesicles and peptide-containing secretory granules; (iv) acts not only locally (as in synapses), but at various sites of the plasma membrane (as in the endocrine melanotrope cell); and (v) can be upregulated in its expression by physiological stimuli that increase the extent of the molecular machinery involved in exocytosis.

Identification of an ovarian voltage-activated Na+-channel type: hints to involvement in luteolysis

An endocrine type of voltage-activated sodium channel (eNaCh) was identified in the human ovary and human luteinized granulosa cells (GC). Whole-cell patch-clamp studies showed that the eNaCh in GC is functional and tetrodotoxin (TTX) sensitive. The luteotrophic hormone human CG (hCG) was found to decrease the peak amplitude of the sodium current within seconds. Treatment with hCG for 24-48 h suppressed not only eNaCh mRNA levels, but also mean Na+ peak currents and resting membrane potentials. An unexpected role for eNaChs in regulating cell morphology and function was indicated after pharmacological modulation of presumed eNaCh steady-state activity in GC cultures for 24-48 h using TTX (NaCh blocker) and veratridine (NaCh activator). TTX preserved a highly differentiated cellular phenotype. Veratridine not only increased the number of secondary lysosomes but also led to a significantly reduced progesterone production. Importantly, endocrine cells of the nonhuman primate corpus luteum (CL), which represent in vivo counterparts of luteinized GC, also contain eNaCh mRNA. Although the mechanism of channel activity under physiological conditions is not clear, it may include persistent Na+ currents. As observed in GC in culture, abundant secondary lysosomes were particularly evident in the regressing CL, suggesting a functional link between eNaCh activity and this form of cellular regression in vivo. Our results identify eNaCh in ovarian endocrine cells and demonstrate that their expression is under the inhibitory control of hCG. Activation of eNaChs in luteal cells, due to loss of gonadotropin support, may initiate a cascade of events leading to decreased CL function, a process that involves lysosomal activation and autophagy. These results imply that ovarian eNaChs are involved in the physiological demise of the temporary endocrine organ CL in the primate ovary during the menstrual cycle. Because commonly used drugs, including phenytoin, target NaChs, these results may be of clinical relevance.

How botulinum and tetanus neurotoxins block neurotransmitter release

Botulinum neurotoxins (BoNT, serotypes A-G) and tetanus neurotoxin (TeNT) are bacterial proteins that comprise a light chain (M(r) approximately 50) disulfide linked to a heavy chain (M(r) approximately 100). By inhibiting neurotransmitter release at distinct synapses, these toxins cause two severe neuroparalytic diseases, tetanus and botulism. The cellular and molecular modes of action of these toxins have almost been deciphered. After binding to specific membrane acceptors, BoNTs and TeNT are internalized via endocytosis into nerve terminals. Subsequently, their light chain (a zinc-dependent endopeptidase) is translocated into the cytosolic compartment where it cleaves one of three essential proteins involved in the exocytotic machinery: vesicle associated membrane protein (also termed synaptobrevin), syntaxin, and synaptosomal associated protein of 25 kDa. The aim of this review is to explain how the proteolytic attack at specific sites of the targets for BoNTs and TeNT induces perturbations of the fusogenic SNARE complex dynamics and how these alterations can account for the inhibition of spontaneous and evoked quantal neurotransmitter release by the neurotoxins.

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.

Neurotoxins affecting neuroexocytosis

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.

Comparison of cysteine string protein (Csp) and mutant alpha-SNAP overexpression reveals a role for csp in late steps of membrane fusion in dense-core granule exocytosis in adrenal chromaffin cells

Assembly of the SNARE complex and its disassembly caused by the action of soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) and NSF is crucial for the maintenance of vesicular traffic, including fusion of regulated exocytotic vesicles. Various other proteins may also have important roles in the processes leading to membrane fusion via interaction with the SNARE proteins, including the secretory vesicle cysteine string protein (Csp). Here we have examined the effect of overexpression of a dominant negative alpha-SNAP mutant or Csp on exocytosis of dense-core granules in single chromaffin cells monitored using amperometry to detect released catecholamine. Exocytosis of trans-Golgi network (TGN)-derived dense-core granules was substantially inhibited by expression of alpha-SNAP(L294A). The amplitude and characteristics of the individual release events were unaffected by expression of alpha-SNAP(L294A), consistent with an essential role for alpha-SNAP in early steps of priming but not in the fusion process. In contrast, Csp overexpression, which also inhibited the extent of exocytosis, also modified the kinetics of the individual release events seen as an increase in the rise time and a broadening of the residual amperometric spikes in Csp-transfected cells. These results suggest that unlike alpha-SNAP, Csp plays a key role in the protein interactions close to the fusion process or fusion pore opening during Ca(2+)-regulated exocytosis.

The mechanism of histamine secretion from gastric enterochromaffin-like cells

Enterochromaffin-like (ECL) cells play a pivotal role in the peripheral regulation of gastric acid secretion as they respond to the functionally important gastrointestinal hormones gastrin and somatostatin and neural mediators such as pituitary adenylate cyclase-activating peptide and galanin. Gastrin is the key stimulus of histamine release from ECL cells in vivo and in vitro. Voltage-gated K(+) and Ca(2+) channels have been detected on isolated ECL cells. Exocytosis of histamine following gastrin stimulation and Ca(2+) entry across the plasma membrane is catalyzed by synaptobrevin and synaptosomal-associated protein of 25 kDa, both characterized as a soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein. Histamine release occurs from different cellular pools: preexisting vacuolar histamine immediately released by Ca(2+) entry or newly synthesized histamine following induction of histidine decarboxylase (HDC) by gastrin stimulation. Histamine is synthesized by cytoplasmic HDC and accumulated in secretory vesicles by proton-histamine countertransport via the vesicular monoamine transporter subtype 2 (VMAT-2). The promoter region of HDC contains Ca(2+)-, cAMP-, and protein kinase C-responsive elements. The gene promoter for VMAT-2, however, lacks TATA boxes but contains regulatory elements for the hormones glucagon and somatostatin. Histamine secretion from ECL cells is thereby under a complex regulation of hormonal signals and can be targeted at several steps during the process of exocytosis.

Retention of cleaved synaptosome-associated protein of 25 kDa (SNAP-25) in neuromuscular junctions: a new hypothesis to explain persistence of botulinum A poisoning

Botulinum neurotoxins can block neurotransmitter release for several months. The molecular mechanism of these toxins' action is known, but the persistence of neuromuscular paralysis that they cause is unexplained. At frog neuromuscular junctions, application of botulinum toxin type A caused paralysis and reduced the C-terminus immunoreactivity of SNAP-25, but not that of the remaining N-terminus fragment. Botulinum toxin type C caused paralysis and reduced syntaxin immunoreactivity without affecting that of SNAP-25. Co-application of botulinum A and C reduced syntaxin immunoreactivity, and that of both C and N termini of SNAP-25. Application of hydroxylamine to de-palmitoylate SNAP-25 resulted in a slight reduction of the immunoreactivity of SNAP-25 N terminus, while it had no effect on immunoreactivity of botulinum A cleaved SNAP-25. In contrast, application of hydroxylamine to nerve terminals where syntaxin had been cleaved by botulinum C caused a considerable reduction in SNAP-25 N-terminus immunoreactivity. Hence the retention of immunoreactive SNAP-25 at the neuromuscular junction depends on its interactions with syntaxin and plasma membrane. Persistence of cleaved SNAP-25 in nerve terminals may prevent insertion of new SNAP-25 molecules, thereby contributing to the longevity of botulinum A effects.Key words: SNAP receptor, neurotoxin, dystonia, botulism, torticollis.

Early requirement for alpha-SNAP and NSF in the secretory cascade in chromaffin cells

NSF and alpha-SNAP have been shown to be required for SNARE complex disassembly and exocytosis. However, the exact requirement for NSF and alpha-SNAP in vesicular traffic through the secretory pathway remains controversial. We performed a study on the kinetics of exocytosis from bovine chromaffin cells using high time resolution capacitance measurement and electrochemical amperometry, combined with flash photolysis of caged Ca2+ as a fast stimulus. alpha-SNAP, a C-terminal mutant of alpha-SNAP, and NEM were assayed for their effects on secretion kinetics. Two kinetically distinct components of catecholamine release can be observed upon fast step-like elevation of [Ca2+]i. One is the exocytotic burst, thought to represent the readily releasable pool of vesicles. Following the exocytotic burst, secretion proceeds slowly at maintained high [Ca2+]i, which may represent vesicle maturation/recruitment, i.e. some priming steps after docking. alpha-SNAP increased the amplitude of both the exocytotic burst and the slow component but did not change their kinetics, which we examined with millisecond time resolution. In addition, NEM only partially inhibited the slow component without altering the exocytotic burst, fusion kinetics and the rate of endocytosis. These results suggest a role for alpha-SNAP/NSF in priming granules for release at an early step, but not modifying the fusion of readily releasable granules.

Organization of the secretory machinery in the rodent brain: distribution of the t-SNAREs, SNAP-25 and SNAP-23

Vesicular transport events appear to be facilitated by the VAMP/synaptobrevin family of membrane proteins in the vesicle (v-SNAREs) and a heterodimeric complex of syntaxin and SNAP-23/25 family members in the target membrane (t-SNAREs). In this manuscript we examine the tissue distribution and composition of the heterodimeric t-SNARE complexes in adult rodent brain. Analysis of protein extracts from brain regions shows that SNAP-25, syntaxin 1, and 4 are broadly distributed, while SNAP-23, syntaxin 3, and 7 show distinct patterns of expression. Further immunohistochemistry and fractionation studies show that while SNAP-25 is enriched in axons and nerve terminals, SNAP-23 is concentrated in cell bodies. Both SNAP-23 and SNAP-25 associate with the plasma membrane and can be metabolically labeled with [(3)H] palmitate in AtT-20 cells. Anti-SNAP-25 antibodies co-immunoprecipitate t-SNARE heterodimers from brain extracts that predominantly contain syntaxin 1 and 2. Contrary to results from in vitro binding assays, SNAP-23 was found predominantly associated with syntaxin 3. These observations suggest that t-SNARE, heterodimer composition is governed more by SNARE expression and localization than by simple protein-protein affinity.

SNAP-25 requirement for dendritic growth of hippocampal neurons

Structure and dimension of the dendritic arbor are important determinants of information processing by the nerve cell, but mechanisms and molecules involved in dendritic growth are essentially unknown. We investigated early mechanisms of dendritic growth using mouse fetal hippocampal neurons in primary culture, which form processes during the first week in vitro. We detected a key component of regulated exocytosis, SNAP-25 (synaptosomal associated protein of 25 kDa), in axons and axonal terminals as well as in dendrites identified by the occurrence of the dendritic markers transferrin receptor and MAP2. Selective inactivation of SNAP-25 by botulinum neurotoxin A (BoNTA) resulted in inhibition of axonal growth and of vesicle recycling in axonal terminals. In addition, dendritic growth of hippocampal pyramidal and granule neurons was significantly inhibited by BoNTA. In contrast, cleavage of synaptobrevin by tetanus toxin had an effect on neither axonal nor dendritic growth. Our observations indicate that SNAP-25, but not synaptobrevin, is involved in constitutive axonal growth and dendrite formation by hippocampal neurons.

Functional and molecular characterization of a muscarinic receptor type and evidence for expression of choline-acetyltransferase and vesicular acetylcholine transporter in human granulosa-luteal cells

Previously, we provided evidence for the presence of a class of muscarinic receptors on human luteinized granulosa cells (human GC) that is linked to transient increases in intracellular free calcium levels, but not to steroid production. The precise nature of the receptor is not known, and neither its function nor the source of its natural ligand acetylcholine (ACh) is clear. To address these issues we used RT-PCR approaches and isolated complementary DNAs corresponding to the M1 receptor subtype from reverse transcribed human GC messenger ribonucleic acids. M1 receptors were further shown by immunocytochemistry, using a M1 receptor antiserum. Single cell calcium measurements showed that the M1 receptor was functionally active and linked to acute increases in intracellular free calcium, as the M1 receptor specific antagonist pirenzepine blocked the Ca2+-mobilizing effect of oxotremorine M (a muscarinic agonist). An unexpected consequence of M1 receptor activation was evidenced by the ability of muscarinic agonists to stimulate the proliferation of human GC within 24 h. In vivo, ACh, the natural ligand of these receptors is thought to be contained in cholinergic nerve fibers innervating the ovary. Surprisingly, the prerequisite for the synthesis of ACh, the enzyme choline-acetyltransferase (ChAT), is also expressed by human GC, as shown by Western blotting and immunocytochemistry. In addition, these cells express another marker for ACh synthesis, namely the gene for the vesicular acetylcholine transporter, as evidenced by RT-PCR cloning, Western blotting, and immunocytochemistry. In conclusion, our data identify the M1 receptor in human GC and point to a novel, trophic role of the neurotransmitter ACh. Furthermore, the presence of the prerequisites of ACh synthesis in human GC indicate that an autocrine/paracrine regulatory loop also exists in the in vivo counterparts of these cells in the ovary, i.e. in the cells of the preovulatory follicle and/or of the young corpus luteum.

Subcellular localization of chromogranins, calcium channels, amine carriers, and proteins of the exocytotic machinery in bovine splenic nerve

Subcellular fractionation of bovine splenic nerves, which consist mainly of sympathetic nerve fibers, has been useful for characterizing cellular organelles en route to the terminal. In the present study we have characterized the subcellular distribution of both secretory and membrane proteins. A newly discovered chromogranin-like protein, NESP55, was found in large dense-core vesicles. The endogenous processing of NESP55 was comparable to that of chromogranins but more limited than that of secretogranin II and chromogranin B. For membrane proteins three major types of distribution were found. The amine carrier VMAT2 was confined to large dense-core vesicles. VAMP or synaptobrevin was present both in large dense-core vesicles and in lighter vesicles, whereas SNAP-25, syntaxin, and two types (N and L) of Ca2+ channels were found in a special population of lighter vesicles but were not present in large dense-core vesicles or at the most in very low concentrations. The plasma membrane norepinephrine transporter was apparently present in a separate type of vesicle, but this requires further study. These results further characterize vesicles en route to the terminal and establish for the first time that peptides involved in exocytosis (syntaxin, SNAP-25, and N- and L-type Ca2+ channels) are apparently transported to the terminal in a special type of vesicle. The exclusive presence of the amine carrier in large dense-core vesicles indicates that the formation of small dense-core vesicles in the terminals requires a reuse of membrane components of large dense-core vesicles.

Differential expression of SNAP-25 isoforms and SNAP-23 in the adrenal gland

In the rat adrenal gland, we previously observed that SNAP-25 is not restricted to the plasmalemma in noradrenergic cells as it is in adrenergic cells, and hypothesized that SNAP-25 isoform expression is different in the two phenotypes. Expression of SNAP-25 isoforms and SNAP-23 was examined by immunoblotting, immunofluorescence, and RT-PCR. Amplifications of SNAP-25 mRNAs were combined with Southern hybridization, restriction enzyme analysis, and sequencing of cloned PCR products to compare SNAP-25 isoform expression in rat and bovine adrenal glands. SNAP-25 and SNAP-23 mRNA and protein are expressed in the glands; SNAP-23 is enriched in the adrenal cortex, whereas SNAP-25 is restricted to the adrenal medulla. Furthermore, high levels of SNAP-25 and low levels of SNAP-23 are observed in the PC12 cells, whereas both SNAP-25 and SNAP-23 are expressed in adrenal medullary cultures. In all extracts, the SNAP-23 mRNA corresponded to SNAP-23a. SNAP-25a is the major form expressed in rat adrenal glands (75%), as it is in PC12 cells (80%), but both SNAP-25a and SNAP-25b (40% vs. 60%) are expressed in bovine adrenal medulla in situ and in culture. In addition, an enriched population of adrenergic cells (93%) expressed a higher level of SNAP-25b (70%), suggesting that this isoform may not be restricted to fast neurotransmission.

Characterization of vesicular membrane-bound alpha-SNAP and NSF in adrenal chromaffin cells

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.

SNAP-23 in rat kidney: colocalization with aquaporin-2 in collecting duct vesicles

Vesicle targeting proteins ("SNAREs") have been proposed to direct vasopressin-induced trafficking of aquaporin-2 water channels in kidney collecting ducts. A newly identified SNARE protein, SNAP-23, is proposed to mediate vesicle targeting to the plasma membrane in diverse tissues. The current studies were done to determine whether SNAP-23 is expressed in collecting ducts with an intracellular distribution compatible with a role in aquaporin-2 trafficking. RT-PCR demonstrated SNAP-23 mRNA in microdissected collecting ducts and other tubular segments including the proximal tubule and thick ascending limb. Immunoblotting using a polyclonal antibody raised against a COOH-terminal peptide revealed a solitary band at an apparent molecular mass of 30 kDa in renal medullary membrane fractions and inner medullary collecting duct suspensions. Differential centrifugation revealed that SNAP-23 is present in membrane fractions including the low-density fraction enriched in intracellular vesicles. Immunocytochemistry revealed SNAP-23 labeling at both the apex and the cytoplasm of collecting duct principal cells. Immunoblotting of intracellular vesicles immunoisolated using an aquaporin-2 antibody revealed the presence of both SNAP-23 and synaptobrevin-2 (VAMP-2) in aquaporin-2-bearing vesicles. We conclude that SNAP-23 is strongly expressed in collecting duct principal cells, consistent with a role in vasopressin-regulated trafficking of aquaporin-2. However, localization of SNAP-23 in both intracytoplasmic vesicles and plasma membranes suggests a function different from that originally proposed for SNAP-25 in synaptic vesicle targeting.

Differential extraction of proteins from paraformaldehyde-fixed cells: lessons from synaptophysin and other membrane proteins

While investigating the localization of synaptophysin in PC12 cells using immunofluorescence microscopy, we noticed a striking difference in its apparent subcellular distribution depending on whether digitonin or Triton X-100 was used as permeabilization agent of paraformaldehyde (PFA)-fixed cells. We found that this difference was due to epitope inaccessibility in the digitonin-treated cells combined with an almost quantitative extraction of the antigen on Triton X-100 permeabilization. Both phenomena were differential with respect to the various synaptophysin-containing compartments. The extraction of antigen from PFA-fixed cells was also seen with other membrane proteins but not with cytosolic proteins and proteins in the lumen of the secretory pathway. Significantly, some of the membrane proteins were extracted from the PFA-fixed cells in higher-molecular-weight forms which we believe represent their in vivo oligomeric states. The implications of our observations are discussed with respect to the method of immunofluorescence microscopy and also to the possible use of paraformaldehyde as an in vivo crosslinker for the study of membrane protein quaternary structure.

Relocation of the t-SNARE SNAP-23 from lamellipodia-like cell surface projections regulates compound exocytosis in mast cells

For regulated secretion, mast cells and several other cell types utilize compound exocytosis, a combination of granule-plasma membrane and granule-granule fusions. The molecular machinery that controls this massive export process has not been identified. We report that SNAP-23, a t-SNARE related to SNAP-25, relocates in response to stimulation from plasma membrane lamellipodia-like projections to granule membranes in permeabilized mast cells. While relocation is a prerequisite for secretion, it can occur without membrane fusion and will expedite a subsequent secretory response. After relocation, SNAP-23 is required for exocytosis, implying a crucial role in promoting membrane fusion. Thus, relocation of this SNARE regulates compound exocytosis and links granule-plasma membrane and granule-granule fusions.

Presynaptic protein interactions in vivo: evidence from botulinum A, C, D and E action at frog neuromuscular junction

The present study examines the paralytic action of botulinum neurotoxins at their natural target, the neuromuscular junction. We asked whether syntaxin, synaptosome-associated protein of 25 kDa (SNAP-25) and vesicle-associated membrane protein (VAMP/synaptobrevin), the proteins proteolysed by botulinum, are susceptible to cleavage in frog nerve terminals, and whether they form complexes in vivo. In control terminals, the three SNAREs were distributed in broad bands at 1 micrometer intervals, at sites consistent with presynaptic Ca2+ channels. Within 3 h, botulinum A, C, D and E (BoNT/A/C/D/E) blocked nerve-evoked muscle contractions but their effects on substrate immunoreactivity varied. The effect of BoNT/A on either C-terminus or N-terminus immunoreactivity of SNAP-25 was undetectable after 3-h incubation, although C-terminus immunoreactivity was reduced after 24 h; N-terminus immunoreactivity was not affected even after 36 h. BoNT/E reduced C-terminus immunoreactivity of SNAP-25 1.5 h after toxin application when transmitter release was blocked, but required 24 h to reduce N-terminus immunoreactivity. BoNT/C reduced syntaxin immunoreactivity after 24-h incubation but did not affect SNAP-25. BoNT/D reduced VAMP immunoreactivity at 3 h while it increased SNAP-25 C-terminal staining fourfold. BoNT/A and BoNT/C applied together for 24 h reduced syntaxin immunoreactivity and that of both C- and N-terminus of SNAP-25, indicating that retention of SNAP-25 N-terminus after cleavage by BoNT/A depended on intact syntaxin. Therefore, we infer that SNAP-25 interacts with VAMP and with syntaxin in vivo. Neurotoxin action abolished only 40-60% of SNAP-25, VAMP or syntaxin immunoreactivity suggesting that distinct pools of these proteins, not immediately involved in triggered exocytosis, are resistant to proteolysis.

Immunohistochemical demonstration of syntaxin and SNAP-25 in chromaffin cells of the frog adrenal gland

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.

Multiple kinetic components of exocytosis distinguished by neurotoxin sensitivity

The secretion of synaptic and other vesicles is a complex process involving multiple steps. Many molecular components of the secretory apparatus have been identified, but how they relate to the different stages of vesicle release is not clear. We examined this issue in adrenal chromaffin cells, where capacitance measurements and amperometry allow us to measure vesicle fusion and hormone release simultaneously. Using flash photolysis of caged intracellular calcium to induce exocytosis, we observed three distinct kinetic components to vesicle fusion, of which only two are related to catecholamine release. Intracellular dialysis with botulinum neurotoxin E, D or C1 or tetanus-toxin light chains abolishes the catecholamine-related components, but leaves the third component untouched. Botulinum neurotoxin A, which removes nine amino acids from the carboxy(C)-terminal end of SNAP-25, does not eliminate catecholamine release completely, but slows down both catecholamine-related components. Thus we assign a dual role to SNAP-25 and suggest that its nine C-terminal amino acids are directly involved in coupling the calcium sensor to the final step in exocytosis.

Formation and turnover of NSF- and SNAP-containing "fusion" complexes occur on undocked, clathrin-coated vesicle-derived membranes

Specificity of vesicular transport is determined by pair-wise interaction between receptors (SNAP receptors or SNAREs) associated with a transport vesicle and its target membrane. Two additional factors, N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment protein (SNAP) are ubiquitous components of vesicular transport pathways. However, the precise role they play is not known. On the basis that NSF and SNAP can be recruited to preformed SNARE complexes, it has been proposed that NSF- and SNAP-containing complexes are formed after SNARE-dependent docking of transport vesicles. This would enable ATPase-dependent complex disassembly to be coupled directly to membrane fusion. Alternatively, binding and release of NSF/SNAP may occur before vesicle docking, and perhaps be involved in the activation of SNAREs. To gain more information about the point at which so-called 20S complexes form during the transport vesicle cycle, we have examined NSF/SNAP/SNARE complex turnover on clathrin-coated vesicle-derived membranes in situ. This has been achieved under conditions in which the extent of membrane docking can be precisely monitored. We demonstrate by UV-dependent cross-linking experiments, coupled to laser light-scattering analysis of membranes, that complexes containing NSF, SNAP, and SNAREs will form and dissociate on the surface of undocked transport vesicles.

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.

Analysis of regulated exocytosis in adrenal chromaffin cells: insights into NSF/SNAP/SNARE function

Many of the proteins that function in regulated exocytosis have now been identified. Several proteins form part of a conserved core machinery that acts in many intracellular vesicular fusion steps and their essential roles confirmed by molecular genetic analysis. In addition, studies with adrenal chromaffin and PC12 cells have demonstrated the function of various proteins in regulated exocytosis and have permitted dissection of the stages of exocytosis in which they act. N-Ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) are key proteins in exocytosis. Examination of their function has indicated that they have a predocking role most likely as molecular chaperones to prepare the docking/fusion machinery. The exact site and time of action in exocytosis of many of the other identified proteins are unknown. A major emphasis for the future will be analysis of the molecular physiology of regulated exocytosis to permit the assignment of functions to identified proteins in particular stages of the regulated exocytotic pathway.

Targeting of SNAP-23 and SNAP-25 in polarized epithelial cells

SNAP-23 is the ubiquitously expressed homologue of the neuronal SNAP-25, which functions in synaptic vesicle fusion. We have investigated the subcellular localization of SNAP-23 in polarized epithelial cells. In hepatocyte-derived HepG2 cells and in Madin-Darby canine kidney (MDCK) cells, the majority of SNAP-23 was present at both the basolateral and apical plasma membrane domains with little intracellular localization. This suggests that SNAP-23 does not function in intracellular fusion events but rather as a general plasma membrane t-SNARE. Canine SNAP-23 is efficiently cleaved by the botulinum neurotoxin E, suggesting that it is the toxin-sensitive factor previously found to be involved in plasma membrane fusion in MDCK cells. The localization of SNAP-25 in transfected MDCK cells was studied for comparison and was found to be identical to SNAP-23 with the exception that SNAP-25 was transported to the primary cilia protruding from the apical plasma membrane, which suggests that subtle differences in the targeting signals of both proteins exist. In contrast to its behavior in neurons, the distribution of SNAP-25 in MDCK cells remained unaltered by treatment with dibutyryl cAMP or forskolin, which, however, caused an increased growth of the primary cilia. Finally, we found that SNAP-23/25 and syntaxin 1A, when co-expressed in MDCK cells, do not stably interact with each other but are independently targeted to the plasma membrane and lysosomes, respectively.

Functional importance of synaptobrevin and SNAP-25 during exocytosis of histamine by rat gastric enterochromaffin-like cells

Gastric enterochromaffin-like (ECL) cells release histamine upon stimulation with gastrin in a calcium-dependent manner. The intracellular mechanisms and proteins mediating exocytosis of histamine-containing vesicles in ECL cells have not been determined yet. We used immunocytochemistry to show the localization of SNAP-25 (synaptosome-associated protein of 25 kDa) and synaptobrevin VAMP (vesicle-associated membrane protein) in ECL cells of the rat gastric mucosa and in isolated, highly enriched ECL cells, which were identified with an antibody directed against the marker enzyme histidine decarboxylase. Immunoblots of isolated ECL cells demonstrated the presence of SNAP-25, synaptobrevin, synaptophysin, synaptotagmin, and syntaxin. Histamine release from isolated ECL cells permeabilized with 8 microM digitonin (2 min) was stimulated approximately 2.5-fold upon exposure to calcium (30 microM; 10-min incubation). Preincubation with 1 microM tetanus toxin light chain for 15 min attenuated calcium-induced histamine release by 40-50% and almost completely cleaved synaptobrevin. Botulinum neurotoxin A (100 nM) totally blocked calcium-induced histamine release and cleaved SNAP-25. We conclude that synaptobrevin, synaptophysin, synaptotagmin, SNAP-25, and syntaxin are present in gastric ECL cells. Inhibition of histamine secretion by clostridial neurotoxins associated with the cleavage of synaptobrevin and SNAP-25 implicates the functional importance of these proteins in the docking and fusion of histamine vesicles.

NSF and SNAP are present on adrenal chromaffin granules

N-ethylmaleimide sensitive fusion protein (NSF) and soluble NSF attachment proteins (SNAPs) are involved in many vesicular transport steps. It has been proposed that SNAPs and NSF associate with their membrane receptors only when vesicles dock on the target membrane. Analysis of NSF and alpha-SNAP distribution in fractionation of organelles from adrenal medulla indicated that a substantial amount of both proteins distributed with chromaffin granules. Further fractionation of intact granules and lysed granule membranes showed exact overlap of NSF and alpha-SNAP distribution with chromaffin granules. These results suggest that NSF and alpha-SNAP are associated with chromaffin granules and support the idea that they function prior to docking of the granules on the plasma membrane.

Assembly and disassembly of a ternary complex of synaptobrevin, syntaxin, and SNAP-25 in the membrane of synaptic vesicles

The synaptic membrane proteins synaptobrevin, syntaxin, and SNAP-25 form a ternary complex that can be disassembled by the ATPase N-ethylmaleimide-sensitive factor (NSF) in the presence of soluble cofactors (SNAP proteins). These steps are thought to represent molecular events involved in docking and subsequent exocytosis of synaptic vesicles. Using two independent and complementary approaches, we now report that such ternary complexes form in the membrane of highly purified and monodisperse synaptic vesicles in the absence of the plasma membrane. Furthermore, the complexes are reversibly dissociated by NSF and SNAP proteins. Thus, ternary complexes can be assembled and disassembled while all three proteins are anchored as neighbors in the same membrane, suggesting that NSF is involved in priming synaptic vesicles for exocytosis.

Neurotransmitter release - four years of SNARE complexes

Exocytosis in neurons requires proteins known as SNAREs, membrane proteins that have now been implicated in many intracellular fusion events. SNAREs assemble into stable ternary complexes that are dissociated by the ATPase NSF (N-ethylmaleimide-sensitive factor), working together with SNAPs (soluble NSF attachment proteins). Recent results have shed new light on the mechanisms underlying SNARE (SNAP receptor) complex assembly and disassembly, and suggest changes in models that relate these reactions to vesicle docking and fusion.