Doc2 enhances Ca2+-dependent exocytosis from PC12 cells

Phosphorylation of Doc2 by EphB2 modulates Munc13-mediated SNARE complex assembly and neurotransmitter release

At the synapse, presynaptic neurotransmitter release is tightly controlled by release machinery, involving the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and Munc13. The Ca2+ sensor Doc2 cooperates with Munc13 to regulate neurotransmitter release, but the underlying mechanisms remain unclear. In our study, we have characterized the binding mode between Doc2 and Munc13 and found that Doc2 originally occludes Munc13 to inhibit SNARE complex assembly. Moreover, our investigation unveiled that EphB2, a presynaptic adhesion molecule (SAM) with inherent tyrosine kinase functionality, exhibits the capacity to phosphorylate Doc2. This phosphorylation attenuates Doc2 block on Munc13 to promote SNARE complex assembly, which functionally induces spontaneous release and synaptic augmentation. Consistently, application of a Doc2 peptide that interrupts Doc2-Munc13 interplay impairs excitatory synaptic transmission and leads to dysfunction in spatial learning and memory. These data provide evidence that SAMs modulate neurotransmitter release by controlling SNARE complex assembly.

16p11.2 CNV gene Doc2α functions in neurodevelopment and social behaviors through interaction with Secretagogin

Copy-number variations (CNVs) of the human 16p11.2 genetic locus are associated with neurodevelopmental disorders, including autism spectrum disorders (ASDs) and schizophrenia. However, it remains largely unclear how this locus is involved in the disease pathogenesis. Doc2α is localized within this locus. Here, using in vivo and ex vivo electrophysiological and morphological approaches, we show that Doc2α-deficient mice have neuronal morphological abnormalities and defects in neural activity. Moreover, the Doc2α-deficient mice exhibit social and repetitive behavioral deficits. Furthermore, we demonstrate that Doc2α functions in behavioral and neural phenotypes through interaction with Secretagogin (SCGN). Finally, we demonstrate that SCGN functions in social/repetitive behaviors, glutamate release, and neuronal morphology of the mice through its Doc2α-interacting activity. Therefore, Doc2α likely contributes to neurodevelopmental disorders through its interaction with SCGN.

Cytoskeletal Transport, Reorganization, and Fusion Regulation in Mast Cell-Stimulus Secretion Coupling

Mast cells are well known for their role in allergies and many chronic inflammatory diseases. They release upon stimulation, e.g., via the IgE receptor, numerous bioactive compounds from cytoplasmic secretory granules. The regulation of granule secretion and its interaction with the cytoskeleton and transport mechanisms has only recently begun to be understood. These studies have provided new insight into the interaction between the secretory machinery and cytoskeletal elements in the regulation of the degranulation process. They suggest a tight coupling of these two systems, implying a series of specific signaling effectors and adaptor molecules. Here we review recent knowledge describing the signaling events regulating cytoskeletal reorganization and secretory granule transport machinery in conjunction with the membrane fusion machinery that occur during mast cell degranulation. The new insight into MC biology offers novel strategies to treat human allergic and inflammatory diseases targeting the late steps that affect harmful release from granular stores leaving regulatory cytokine secretion intact.

Role of Skeletal Muscle in Insulin Resistance and Glucose Uptake

The skeletal muscle is the largest organ in the body, by mass. It is also the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation. Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in exercise and metabolic disease. In this article, we give an overview of the importance of skeletal muscle in metabolism, describing its role in glucose uptake and the diseases that are associated with skeletal muscle metabolic dysregulation. We focus on the role of skeletal muscle in peripheral insulin resistance and the potential for skeletal muscle-targeted therapeutics to combat insulin resistance and diabetes, as well as other metabolic diseases like aging and obesity. In particular, we outline the possibilities and pitfalls of the quest for exercise mimetics, which are intended to target the molecular mechanisms underlying the beneficial effects of exercise on metabolic disease. We also provide a description of the molecular mechanisms that regulate skeletal muscle glucose uptake, including a focus on the SNARE proteins, which are essential regulators of glucose transport into the skeletal muscle. © 2020 American Physiological Society. Compr Physiol 10:785-809, 2020.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Calcium sensing in exocytosis

Neurotransmitters, neuropeptides and hormones are released through regulated exocytosis of synaptic vesicles and large dense core vesicles. This complex and highly regulated process is orchestrated by SNAREs and their associated proteins. The triggering signal for regulated exocytosis is usually an increase in intracellular calcium levels. Besides the triggering role, calcium signaling modulates the precise amount and kinetics of vesicle release. Thus, it is a central question to understand the molecular machineries responsible for calcium sensing in exocytosis. Here we provide an overview of our current understanding of calcium sensing in neurotransmitter release and hormone secretion.

Doc2 is a Ca2+ sensor required for asynchronous neurotransmitter release

Synaptic transmission involves a fast synchronous phase and a slower asynchronous phase of neurotransmitter release that are regulated by distinct Ca(2+) sensors. Though the Ca(2+) sensor for rapid exocytosis, synaptotagmin I, has been studied in depth, the sensor for asynchronous release remains unknown. In a screen for neuronal Ca(2+) sensors that respond to changes in [Ca(2+)] with markedly slower kinetics than synaptotagmin I, we observed that Doc2--another Ca(2+), SNARE, and lipid-binding protein--operates on timescales consistent with asynchronous release. Moreover, up- and downregulation of Doc2 expression levels in hippocampal neurons increased or decreased, respectively, the slow phase of synaptic transmission. Synchronous release, when triggered by single action potentials, was unaffected by manipulation of Doc2 but was enhanced during repetitive stimulation in Doc2 knockdown neurons, potentially due to greater vesicle availability. In summary, we propose that Doc2 is a Ca(2+) sensor that is kinetically tuned to regulate asynchronous neurotransmitter release.

Reconstituted synaptotagmin I mediates vesicle docking, priming, and fusion

In vitro reconstitution fusion assays incorporating full-length membrane-anchored synaptotagmin I clarify its role in several steps in the secretory pathway.

The synaptic vesicle protein synaptotagmin I (syt) promotes exocytosis via its ability to penetrate membranes in response to binding Ca²⁺ and through direct interactions with SNARE proteins. However, studies using full-length (FL) membrane-embedded syt in reconstituted fusion assays have yielded conflicting results, including a lack of effect, or even inhibition of fusion, by Ca²⁺. In this paper, we show that reconstituted FL syt promoted rapid docking of vesicles (<1 min) followed by a priming step (3–9 min) that was required for subsequent Ca²⁺-triggered fusion between v- and t-SNARE liposomes. Moreover, fusion occurred only when phosphatidylinositol 4,5-bisphosphate was included in the target membrane. This system also recapitulates some of the effects of syt mutations that alter synaptic transmission in neurons. Finally, we demonstrate that the cytoplasmic domain of syt exhibited mixed agonist/antagonist activity during regulated membrane fusion in vitro and in cells. Together, these findings reveal further convergence of reconstituted and cell-based systems.

Doc2 supports spontaneous synaptic transmission by a Ca(2+)-independent mechanism

Two families of Ca(2+)-binding proteins have been proposed as Ca(2+) sensors for spontaneous release: synaptotagmins and Doc2s, with the intriguing possibility that Doc2s may represent high-affinity Ca(2+) sensors that are activated by deletion of synaptotagmins, thereby accounting for the increased spontaneous release in synaptotagmin-deficient synapses. Here, we use an shRNA-dependent quadruple knockdown of all four Ca(2+)-binding proteins of the Doc2 family to confirm that Doc2-deficient synapses exhibit a marked decrease in the frequency of spontaneous release events. Knockdown of Doc2s in synaptotagmin-1-deficient synapses, however, failed to reduce either the increased spontaneous release or the decreased evoked release of these synapses, suggesting that Doc2s do not constitute Ca(2+) sensors for asynchronous release. Moreover, rescue experiments revealed that the decrease in spontaneous release induced by the Doc2 knockdown in wild-type synapses is fully reversed by mutant Doc2B lacking Ca(2+)-binding sites. Thus, our data suggest that Doc2s are modulators of spontaneous synaptic transmission that act by a Ca(2+)-independent mechanism.

Role of the polybasic sequence in the Doc2alpha C2B domain in dense-core vesicle exocytosis in PC12 cells

The double C2 (Doc2) family is characterized by an N-terminal Munc13-1-interacting domain and C-terminal tandem C2 domains, and it comprises three isoforms, Doc2alpha, Doc2beta, and Doc2gamma, in humans and mice. Doc2alpha, the best-characterized, brain-specific isoform, exhibits Ca(2+)-dependent phospholipid-binding activity through its C2A domain, and the Ca(2+)-binding activity is thought to be important for the regulation of Ca(2+)-dependent exocytosis. In contrast to the C2A domain, however, nothing is known about the physiological functions of the C2B domain in regulated exocytosis. In this study, we demonstrated by a mutation analysis that the polybasic sequence in the C2B domain of Doc2alpha (306 KKSKHKTCVKKK 317) is required for binding of syntaxin-1a/synaptosome-associated protein of 25 kDa (SNAP-25) heterodimer. We also investigated the effect of Lys-to-Gln (named KQ) mutations in the polybasic sequence of the C2B domain on vesicle dynamics by total internal reflection fluorescence microscopy in PC12 cells. A Doc2alpha(KQ) mutant, which lacks binding activity toward syntaxin-1a/SNAP-25 heterodimer, significantly decreased the number of plasma membrane-docked vesicles before stimulation and strongly inhibited high-KCl-induced exocytosis from the plasma membrane-docked vesicles. These results indicate that the polybasic sequence in the C2B domain functions as a binding site for syntaxin-1a/SNAP-25 heterodimer and controls the number of 'readily releasable' vesicles in neuroendocrine cells.

Role of C2 domain proteins during synaptic vesicle exocytosis

Neurotransmitter release is mediated by the fusion of synaptic vesicles with the presynaptic plasma membrane. Fusion is triggered by a rise in the intracellular calcium concentration and is dependent on the neuronal SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) complex. A plethora of molecules such as members of the MUNC13, MUNC18, complexin and synaptotagmin families act along with the SNARE complex to enable calcium-regulated synaptic vesicle exocytosis. The synaptotagmins are localized to synaptic vesicles by an N-terminal transmembrane domain and contain two cytoplasmic C2 domains. Members of the synaptotagmin family are thought to translate the rise in intracellular calcium concentration into synaptic vesicle fusion. The C2 domains of synaptotagmin-1 bind membranes in a calcium-dependent manner and in response induce a high degree of membrane curvature, which is required for its ability to trigger membrane fusion in vitro and in vivo. Furthermore, members of the soluble DOC2 (double-C2 domain) protein family have similar properties. Taken together, these results suggest that C2 domain proteins such as the synaptotagmins and DOC2s promote membrane fusion by the induction of membrane curvature in the vicinity of the SNARE complex. Given the widespread expression of C2 domain proteins in secretory cells, it is proposed that promotion of SNARE-dependent membrane fusion by the induction of membrane curvature is a widespread phenomenon.

DOC2B, C2 domains, and calcium: A tale of intricate interactions

Ca(+2)-dependent exocytosis involves vesicle docking, priming, fusion, and recycling. This process is performed and regulated by a vast number of synaptic proteins and depends on proper protein-protein and protein-lipid interactions. Double C2 domain (DOC2) is a protein family of three isoforms found while screening DNA libraries with a C2 probe. DOC2 has three domains: the Munc13-interacting domain and tandem C2s (designated C2A and C2B) connected by a short polar linker. The C2 domain binds phospholipids in a Ca(2+)-dependent manner. This review focuses on the ubiquitously expressed isoform DOC2B. Sequence alignment of the tandem C2 protein family in mouse revealed high homology (81%) between rabphilin-3A and DOC2B proteins. We created a structural model of DOC2B's C2A based on the crystal structure of rabphilin-3A with and without calcium and found that the calcium-binding loops of DOC2B move upon calcium binding, enabling efficient plasma membrane penetration of its C2A. Here, we discuss the potential relation between the DOC2B bioinformatical model and its function and suggest a possible working model for its interaction with other proteins of the exocytotic machinery, including Munc13, Munc18, and syntaxin.

DOC2B acts as a calcium switch and enhances vesicle fusion

Calcium-dependent exocytosis is regulated by a vast number of proteins. DOC2B is a synaptic protein that translocates to the plasma membrane (PM) after small elevations in intracellular calcium concentration. The aim of this study was to investigate the role of DOC2B in calcium-triggered exocytosis. Using biochemical and biophysical measurements, we demonstrate that the C2A domain of DOC2B interacts directly with the PM in a calcium-dependent manner. Using a combination of electrophysiological, morphological, and total internal reflection fluorescent measurements, we found that DOC2B acts as a priming factor and increases the number of fusion-competent vesicles. Comparing secretion during repeated stimulation between wild-type DOC2B and a mutated DOC2B that is constantly at the PM showed that DOC2B enhances catecholamine secretion also during repeated stimulation and that DOC2B has to translocate to the PM to exert its facilitating effect, suggesting that its activity is dependent on calcium. The hypothesis that DOC2B exerts its effect at the PM was supported by the finding that DOC2B affects the fusion kinetics of single vesicles and interacts with the PM SNAREs (soluble NSF attachment receptors). We conclude that DOC2B is a calcium-dependent priming factor and its activity at the PM enables efficient expansion of the fusion pore, leading to increased catecholamine release.

Lysosomal exocytosis is impaired in mucolipidosis type IV

Mucolipidosis type IV (MLIV) is an autosomal recessive disease characterized by severe neurological impairment, ophthalmologic defects, and gastric dysfunction. MLIV cells have a deficiency in the late endosomal/lysosomal (LEL) pathway that results in the buildup of lysosomal inclusions. Using a Xenopus oocyte expression system, we previously showed that mucolipin-1 (MLN1), the protein encoded by the MCOLN1 gene is a Ca2+ -permeable non-selective cation channel that is transiently modulated by elevations in intracellular Ca2+. We further showed that MLN1 is translocated to the plasma membrane during lysosomal exocytosis. In this study we show that lysosomal exocytosis is impaired in fibroblasts from MLIV patients, indicating that MLN1 plays an active role in this process. Further, we show that transfection with wild type MLN1 cDNA rescues exocytosis, suggesting the possibility of treatments based on the restoration of this crucial cellular function.

DOC2A and DOC2B are sensors for neuronal activity with unique calcium-dependent and kinetic properties

Elevation of the intracellular calcium concentration ([Ca2+]i) to levels below 1 microm alters synaptic transmission and induces short-term plasticity. To identify calcium sensors involved in this signalling, we investigated soluble C2 domain-containing proteins and found that both DOC2A and DOC2B are modulated by submicromolar calcium levels. Fluorescent-tagged DOC2A and DOC2B translocated to plasma membranes after [Ca2+]i elevation. DOC2B translocation preceded DOC2A translocation in cells co-expressing both isoforms. Half-maximal translocation occurred at 450 and 175 nm[Ca2+]i for DOC2A and DOC2B, respectively. This large difference in calcium sensitivity was accompanied by a modest kinetic difference (halftimes, respectively, 2.6 and 2.0 s). The calcium sensitivity of DOC2 isoforms can be explained by predicted topologies of their C2A domains. Consistently, neutralization of aspartates D218 and D220 in DOC2B changed its calcium affinity. In neurones, both DOC2 isoforms were reversibly recruited to the plasma membrane during trains of action potentials. Consistent with its higher calcium sensitivity, DOC2B translocated at lower depolarization frequencies. Styryl dye uptake experiments in hippocampal neurones suggest that the overexpression of mutated DOC2B alters the synaptic activity. We conclude that both DOC2A and DOC2B are regulated by neuronal activity, and hypothesize that their calcium-dependent translocation may regulate synaptic activity.

Calcium concentration threshold and translocation kinetics of EGFP-DOC2B expressed in cultured Aplysia neurons

The double C2 domain protein family (DOC2) is characterized by two calcium-binding domains (C2). Upon binding to calcium, the affinity of the protein to phospholipids is significantly increased, leading to translocation of the protein from the cytosol to the plasma membrane. These properties, and the binding domain of DOC2B to Munc13, suggested that DOC2B could play a role in augmentation and potentiation of synaptic release. Nevertheless, the level of the free intracellular calcium concentration ([Ca(2+)](i)) which triggers its translocation under in vivo conditions, is not known. Using cultured Aplysia neurons that express rat EGFP-DOC2B, we found that the [Ca(2+)](i) increment necessary to induce EGFP-DOC2B translocation is approximately 200 nM in the bulk of the cytoplasm. The rate of EGFP-DOC2B recruitment to the plasma membrane is slower than the [Ca(2+)](i) elevation rate, while the detachment of EGFP-DOC2B from it is faster than the calcium removal. The extent of EGFP-DOC2B translocation to the plasma membrane reflects local submembrane [Ca(2+)](i). Our observations are consistent with the view that DOC2B can participate in the regulation of neurotransmitter release. It should be noted that EGFP-DOC2B could be used as a tool to map sub-membrane calcium dynamics under physiological conditions.

Calcium-dependent regulation of exocytosis

A rapid increase in intracellular calcium directly triggers regulated exocytosis. In addition, changes in intracellular calcium concentration can adjust the extent of exocytosis (quantal content) or the magnitude of individual release events (quantal size) in both the short- and long-term. It is generally agreed that calcium achieves this regulation via an interaction with a number of different molecular targets located at or near to the site of membrane fusion. We review here the synaptic proteins with defined calcium-binding domains and protein kinases activated by calcium, summarize what is known about their function in membrane fusion and the experimental evidence in support of their involvement in synaptic plasticity.

Munc18-1 regulates early and late stages of exocytosis via syntaxin-independent protein interactions

Sec1/Munc18 (SM) proteins are involved in various intracellular membrane trafficking steps. Many SM proteins bind to appropriate syntaxin homologues involved in these steps, suggesting that SM proteins function as syntaxin chaperones. Organisms with mutations in SM genes, however, exhibit defects in either early (docking) or late (fusion) stages of exocytosis, implying that SM proteins may have multiple functions. To gain insight into the role of SM proteins, we introduced mutations modeled on those identified in Caenorhabditis elegans, Drosophila melanogaster, and Saccharomyces cerevisiae into mammalian Munc18-1. As expected, several mutants exhibited reduced binding to syntaxin1A. However, three mutants displayed wild-type syntaxin binding affinities, indicating syntaxin-independent defects. Expression of these mutants in chromaffin cells either increased the rate and extent of exocytosis or altered the kinetics of individual release events. This latter effect was associated with a reduced Mint binding affinity in one mutant, implying a potential mechanism for the observed alteration in release kinetics. Furthermore, this phenotype persisted when the mutation was combined with a second mutation that greatly reduced syntaxin binding affinity. These results clarify the data on the function of SM proteins in mutant organisms and indicate that Munc18-1 controls multiple stages of exocytosis via both syntaxin-dependent and -independent protein interactions.

Ca(2+)-induced recruitment of the secretory vesicle protein DOC2B to the target membrane

Ca(2+)-dependent fusion of transport vesicles at their target can be enhanced by intracellular Ca2+ and diacylglycerol. Diacylglycerol induces translocation of the vesicle priming factor Munc13 and association of the secretory vesicle protein DOC2B to the membrane. Here we demonstrate that a rise in intracellular Ca2+ is sufficient for a Munc13-independent recruitment of DOC2B to the target membrane. This novel mechanism occurred readily in the absence of Munc13 and was not influenced by DOC2B mutations that abolish Munc13 binding. Purified DOC2B (expressed as a bacterial fusion protein) bound phospholipids in a Ca(2+)-dependent way, suggesting that the translocation is the result of a C2 domain activation mechanism. Ca(2+)-induced translocation was also observed in cultured neurons expressing DOC2B-enhanced green fluorescent protein. In this case, however, various degrees of membrane association occurred under resting conditions, suggesting that physiological Ca2+ concentrations modulate DOC2B localization. Depolarization of the neurons induced a complete translocation of DOC2B-enhanced green fluorescent protein to the target membrane within 5 s. We hypothesize that this novel Ca(2+)-induced activity of DOC2B functions synergistically with diacylglycerol-induced Munc13 binding to enhance exocytosis during episodes of high secretory activity.

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.

Taxilin; a novel syntaxin-binding protein that is involved in Ca2+-dependent exocytosis in neuroendocrine cells

BACKGROUND

The syntaxin family is a central coordinator and participates in multiple protein-protein interactions in the soluble N-ethyl maleimide-sensitive factor attachment protein receptor machinery, which is involved in intracellular vesicle traffic. However, the molecular mechanism by which the syntaxin family regulates intracellular vesicle transport is not well known.

RESULTS

We have identified and purified a novel binding partner of syntaxin-3 from rat lung, and isolated and sequenced the cDNA of its human homologue from a human brain cDNA library. The cDNA had an open reading frame encoding a protein of 546 amino acids with a calculated Mr of 61,890. We tentatively referred to this protein as taxilin. A structural analysis of taxilin revealed the existence of an extraordinarily long coiled-coil domain in its C-terminal half. Syntaxin-1a and -4, as well as syntaxin-3 interacted with taxilin, but syntaxin-7 or -8 did not. Northern blot analysis showed that taxilin was ubiquitously expressed. Over-expression of full-length taxilin inhibited Ca2+-dependent exocytosis in PC12 cells.

CONCLUSIONS

These results indicate that taxilin is a novel binding partner of several syntaxin family members and suggest that taxilin is involved in Ca2+-dependent exocytosis in neuroendocrine cells.

Synaptotagmin IX regulates Ca2+-dependent secretion in PC12 cells

Synaptotagmin (Syt) I-deficient phaeochromocytoma (PC12) cell lines show normal Ca(2+)-dependent norepinephrine (NE) release (Shoji-Kasai, Y., Yoshida, A., Sato, K., Hoshino, T., Ogura, A., Kondo, S., Fujimoto, Y., Kuwahara, R., Kato, R., and Takahashi, M. (1992) Science 256, 1821-1823). To identify an alternative Ca(2+) sensor, we searched for other Syt isoforms in Syt I-deficient PC12 cells and identified Syt IX, an isoform closely related to Syt I, as an abundantly expressed dense-core vesicle protein. Here we show that Syt IX is required for the Ca(2+)-dependent release of NE from PC12 cells. Antibodies directed against the C2A domain of either Syt IX or Syt I inhibited Ca(2+)-dependent NE release in permeable PC12 cells indicating that both Syt proteins function in dense-core vesicle exocytosis. Our results support the idea that Syt family proteins that co-reside on secretory vesicles may function cooperatively and redundantly as potential Ca(2+) sensors for exocytosis.

The C2A domain of double C2 protein gamma contains a functional nuclear localization signal

The C2 domain was originally defined as a homologous domain to the C2 regulatory region of Ca2+ -dependent protein kinase C and has been identified in more than 50 different signaling molecules. The original C2 domain of protein kinase Calpha functions as a Ca2+ binding module, and the Ca2+ binding to the C2 domain allows translocation of proteins to phospholipid membranes. By contrast, however, some C2 domains do not exhibit Ca2+ binding activity because of amino acid substitutions at Ca2+ -binding sites, and their physiological meanings remain largely unknown. In this study, we discovered an unexpected function of the Ca2+ -independent C2A domain of double C2 protein gamma (Doc2gamma) in nuclear localization. Deletion and mutation analyses revealed that the putative Ca2+ binding loop 3 of Doc2gamma contains six Arg residues ((177)RLRRRRR(183)) and that this basic cluster is both necessary and sufficient for nuclear localization of Doc2gamma. Because of the presence of the basic cluster, the C2A domain of Doc2gamma did not show Ca2+ -dependent phospholipid binding activity. Our findings indicate that by changing the nature of the putative Ca2+ binding loops the C2 domain has more diversified function in cellular signaling than a simple Ca2+ binding motif.

Physiological control of Xunc18 expression in neuroendocrine melanotrope cells of Xenopus laevis

In mammals, the brain-specific protein munc18-1 regulates synaptic vesicle exocytosis at the synaptic junction, in a step before vesicle fusion. We hypothesize that the rate of biosynthesis of munc18-1 messenger RNA (mRNA) and the amount of munc18-1 present in neurons and neuroendocrine cells are related to the physiologically controlled state of activity. To test this hypothesis, the homolog of munc18-1 in the clawed toad Xenopus laevis, xunc18, was studied in the brain and in the neuroendocrine melanotrope cells in the intermediate lobe of the pituitary gland, at both the mRNA and the protein level. In toads adapted to a black background, the melanotropes release the peptide alpha-melanophore-stimulating hormone (alpha-MSH), which induces darkening of the skin, whereas in animals adapted to a white background the cells hardly release but store alpha-MSH, making the animal's skin look pale. The intermediate pituitary lobe of black-adapted animals revealed a strong hybridization reaction with the xunc18 mRNA probe, whereas a much weaker hybridization was observed in the intermediate lobe of white-adapted animals (optical density black: 3.4 +/- 0.2 vs. white: 0.8 +/- 0.1; P < 0.02). Immunocytochemically, Xenopus munc18-like protein has been detected throughout the brain, in identified neuronal perikarya as well as in axon tracts. Western blot analysis and immunocytochemistry further demonstrated the presence of xunc18 in the neural, intermediate and distal lobe of the pituitary gland. Xunc18 protein was furthermore determined in immunoblots of homogenates of melanotropes dissociated from the pituitary gland. In melanotropes of toads adapted to a black background, the integrated optical density of the xunc18 immunosignal was 2.7 +/- 0.5 times higher than in cells of white-adapted toads (P < 0.0001). It is concluded that, in Xenopus melanotrope cells, the amounts of both xunc18 mRNA and xunc18 protein are up-regulated in conjunction with the induction of exocytosis of alpha-MSH as a result of a physiological stimulation (environmental light condition). We propose that xunc18 is involved in physiologically controlled exocytotic secretion of neuroendocrine messengers.

Nadrin, a novel neuron-specific GTPase-activating protein involved in regulated exocytosis

It has been proposed that the cortical actin filament networks act as a cortical barrier that must be reorganized to enable docking and fusion of the synaptic vesicles with the plasma membranes. We identified a novel neuron-associated developmentally regulated protein, designated as Nadrin. Expression of Nadrin is restricted to neurons and correlates well with the differentiation of neurons. Nadrin has a unique structure; it contains a GTPase-activating protein (GAP) domain for Rho family GTPases, a potential coiled-coil domain, and a succession of 29 glutamines. In vitro the GAP domain activates RhoA, Rac1, and Cdc42 GTPases. Expression of Nadrin in NIH3T3 cells markedly reduced the number of the actin stress fibers and the formation of the ruffled membranes, suggesting that Nadrin regulates actin filament reorganization. In PC12 cells, Nadrin colocalized with synaptotagmin in the neurite termini and also with cortical actin filaments in the subplasmalemmal regions. Expression of Nadrin or its mutant composed of the coiled-coil and GAP domain enhanced Ca(2+)-dependent exocytosis of PC12 cells, but a mutant lacking the GAP domain inhibited exocytosis. These results suggest that Nadrin plays a role in regulating Ca(2+)-dependent exocytosis, most likely by catalyzing GTPase activity of Rho family proteins and by inducing the reorganization of the cortical actin filaments.

Is double C2 protein (DOC2) expressed in bovine adrenal medulla? A commercial anti-DOC2 monoclonal antibody recognizes a major bovine mitochondrial antigen

We have examined the expression in bovine adrenal medulla of double C2 protein (DOC2), a vesicular protein which associates with intracellular phospholipid and Ca(2+) and is implicated in the modulation of regulated exocytosis. Extensive reverse transcription-PCR, Northern blot analyses and in vitro translation reactions have been combined with immunological studies to provide data to suggest that neither DOC2alpha nor DOC2beta is expressed at detectable levels in bovine adrenal chromaffin cells, and that a widely used monoclonal antibody directed against DOC2 also recognizes mitochondrial complex III core protein 2.

Doc2gamma, a third isoform of double C2 protein, lacking calcium-dependent phospholipid binding activity

The Doc2 (double C2) family consists of two isoforms (Doc2alpha and Doc2beta) characterized by an N-terminal Munc13-1 interacting domain (Mid) and two C2 domains that interact with Ca(2+) and phospholipid at the C-terminus. This Ca(2+)-binding property is thought to be important to the regulation of neurotransmitter release. In this paper, we report a third isoform of mouse Doc2, named Doc2gamma. Doc2gamma also contains a putative Mid domain and two C2 domains, and it is 45.6 and 43.2% identical to mouse Doc2alpha and Doc2beta, respectively, at the amino acid level. In contrast to the other Doc2 isoforms, the C2 domains of Doc2gamma impair Ca(2+)-dependent phospholipid binding activity. The highest expression of Doc2gamma mRNA was found in the heart, but occurs ubiquitously, the same as Doc2beta. These findings indicate that Doc2gamma may also function as an effector for Munc13-1 and that it may be involved in the regulation of vesicular trafficking.

Munc13-1 acts as a priming factor for large dense-core vesicles in bovine chromaffin cells

In chromaffin cells the number of large dense-core vesicles (LDCVs) which can be released by brief, intense stimuli represents only a small fraction of the 'morphologically docked' vesicles at the plasma membrane. Recently, it was shown that Munc13-1 is essential for a post-docking step of synaptic vesicle fusion. To investigate the role of Munc13-1 in LDCV exocytosis, we overexpressed Munc13-1 in chromaffin cells and stimulated secretion by flash photolysis of caged calcium. Both components of the exocytotic burst, which represent the fusion of release-competent vesicles, were increased by a factor of three. The sustained component, which represents vesicle maturation and subsequent fusion, was increased by the same factor. The response to a second flash, however, was greatly reduced, indicating a depletion of release-competent vesicles. Since there was no apparent change in the number of docked vesicles, we conclude that Munc13-1 acts as a priming factor by accelerating the rate constant of vesicle transfer from a pool of docked, but unprimed vesicles to a pool of release-competent, primed vesicles.

Hrs interacts with SNAP-25 and regulates Ca(2+)-dependent exocytosis

Synaptosome-associated protein of 25 kDa (SNAP-25) is a neuronal membrane protein essential for synaptic vesicle exocytosis. To investigate the mechanisms by which SNAP-25 mediates neurosecretion, we performed a search for proteins that interact with SNAP-25 using a yeast two-hybrid screen. Here, we report the isolation and characterization of a SNAP-25-interacting protein that is the rat homologue of mouse hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs). Hrs specifically interacts with SNAP-25, but not SNAP-23/syndet. The association of Hrs and SNAP-25 is mediated via coiled-coil interactions. Using an Hrs-specific antibody, we have shown that Hrs is highly enriched in brain, where it codistributes with SNAP-25 in most brain regions. Subcellular fractionation studies demonstrate that in brain, Hrs exists in both cytosolic and membrane-associated pools. Studies using indirect immunofluorescence and confocal microscopy reveal that, in addition to early endosomes, Hrs is also localized to large dense-core secretory granules and synaptic-like microvesicles in nerve growth factor-differentiated PC12 cells. Moreover, overexpression of Hrs in PC12 cells inhibits Ca(2+)-dependent exocytosis. These results suggest that Hrs is involved in regulation of neurosecretion through interaction with SNAP-25.

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

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

Double C2 protein. A review

Concerted effort has led to the identification of dozens of synaptic proteins and has thereby opened the door for the characterisation of the molecular mechanisms underlying regulated exocytosis. Calcium is known to play a number of roles in regulated exocytosis, acting as the trigger for fast synaptic transmission and also acting at some of the steps preceding vesicle fusion. Investigators have therefore focussed considerable attention on possible calcium sensors. What many of the candidate proteins have in common is a C2 domain, one of the four conserved domains originally described in protein kinase C. Such domains have been shown to bind calcium and phospholipid in a large number of intracellular proteins. Synaptotagmin, a C2-domain protein, is a very strong candidate for the protein involved in triggering fast calcium-dependent vesicle fusion. Recent attention has also concerned the other calcium sensors, which may play roles in the 'priming' or transport of vesicles. This review concerns one of these tentative calcium-binding proteins, double C2 or DOC2. DOC2 was originally isolated from nervous tissue but subsequently has been found to be more widely expressed. DOC2 is a vesicular protein that may be involved in the early stages of preparing vesicles for exocytosis.

SNIP, a novel SNAP-25-interacting protein implicated in regulated exocytosis

Synaptosome-associated protein of 25 kDa (SNAP-25) is a presynaptic membrane protein that has been clearly implicated in membrane fusion in both developing and mature neurons, although its mechanisms of action are unclear. We have now identified a novel SNAP-25-interacting protein named SNIP. SNIP is a hydrophilic, 145-kDa protein that comprises two predicted coiled-coil domains, two highly charged regions, and two proline-rich domains with multiple PPXY and PXXP motifs. SNIP is selectively expressed in brain where it co-distributes with SNAP-25 in most brain regions. Biochemical studies have revealed that SNIP is tightly associated with the brain cytoskeleton. Subcellular fractionation and immunofluorescence localization studies have demonstrated that SNIP co-localizes with SNAP-25 as well as the cortical actin cytoskeleton, suggesting that SNIP serves as a linker protein connecting SNAP-25 to the submembranous cytoskeleton. By using deletion analysis, we have mapped the binding domains of SNIP and SNAP-25, and we have demonstrated that the SNIP-SNAP-25 association is mediated via coiled-coil interactions. Moreover, we have shown that overexpression of SNIP or its SNAP-25-interacting domain inhibits Ca(2+)-dependent exocytosis from PC12 cells. These results indicate that SNIP is involved in regulation of neurosecretion, perhaps via its interaction with SNAP-25 and the cytoskeleton.

Doc2alpha is an activity-dependent modulator of excitatory synaptic transmission

Doc2alpha is a synaptic vesicle-associated Ca2 + -binding protein. To study the role of Doc2alpha in synaptic transmission and modulation, we generated homozygous null Doc2alpha mutant mice. In the CA1 region of hippocampal slices in the mutant mice, excitatory synaptic responses evoked with prolonged 5 Hz stimulation showed a significantly larger frequency facilitation followed by a steeper depression than those in wild-type mice, whereas there was no difference in synaptic transmission at lower frequencies or in paired-pulse facilitation. These results suggest that Doc2alpha regulates synaptic transmission when high Ca2 + concentrations in the presynaptic terminal are sustained. Furthermore, the mutant mice showed impairment in long-term potentiation and passive avoidance task. Thus, Doc2alpha may regulate transmitter release during repetitive synaptic activation, thereby contributing to memory formation.

Transfection analysis of functional roles of complexin I and II in the exocytosis of two different types of secretory vesicles

Classical neurotransmitters such as gamma-aminobutyric acid and glutamate are released from synaptic nerve terminals by exocytosis of synaptic vesicles. PC12 cells also have SSVs capable of storing acetylcholine (ACh). A novel method to examine the effect of transient transfection of any gene of interest on the exocytosis of SSVs was developed. The transfection of choline acetyltransferase (ChAT) into PC12 cells which have lost ACh synthesizing activity resulted in the accumulation of a substantial amount of ACh. Synthesized ACh was released in Ca(2+)-dependent manner. Release was thought to occur by an exocytosis of SSVs because: (1) release was abolished by treating the cells with vesamicol, a specific inhibitor of the vesicular ACh transporter (VAChT) localizing specifically in SSVs; and (2) the release was further increased by cotransfecting rat VAChT with the ChAT. By means of this method, we showed that overexpression of complexin I or II with ChAT markedly suppressed high-K(+)-dependent ACh release of SSVs.

Synaptogyrins regulate Ca2+-dependent exocytosis in PC12 cells

Synaptogyrins constitute a family of synaptic vesicle proteins of unknown function. With the full-length structure of a new brain synaptogyrin isoform, we now show that the synaptogyrin family in vertebrates includes two neuronal and one ubiquitous isoform. All of these synaptogyrins are composed of a short conserved N-terminal cytoplasmic sequence, four homologous transmembrane regions, and a variable cytoplasmic C-terminal tail that is tyrosine-phosphorylated. The localization, abundance, and conservation of synaptogyrins suggest a function in exocytosis. To test this, we employed a secretion assay in PC12 cells expressing transfected human growth hormone (hGH) as a reporter protein. When Ca2+-dependent hGH secretion from PC12 cells was triggered by high K+ or alpha-latrotoxin, co-transfection of all synaptogyrins with hGH inhibited hGH exocytosis as strongly as co-transfection of tetanus toxin light chain. Synaptophysin I, which is distantly related to synaptogyrins, was also inhibitory but less active. Inhibition was independent of the amount of hGH expressed but correlated with the amount of synaptogyrin transfected. Inhibition of exocytosis was not observed with several other synaptic proteins, suggesting specificity. Analysis of the regions of synaptogyrin required for inhibition revealed that the conserved N-terminal domain of synaptogyrin is essential for inhibition, whereas the long C-terminal cytoplasmic tail is largely dispensable. Our results suggest that synaptogyrins are conserved components of the exocytotic apparatus, which function as regulators of Ca2+-dependent exocytosis.

Ca2(+)-dependent interaction of N-copine, a member of the two C2 domain protein family, with OS-9, the product of a gene frequently amplified in osteosarcoma

N-copine is a novel two C2 domain protein that shows Ca2(+)-dependent phospholipid binding and membrane association. By using yeast two-hybrid assays, we identified OS-9 as a protein capable of interacting with N-copine. We further revealed that the second C2 domain of N-copine bound with the carboxy-terminal region of OS-9. Their interaction in vivo was also confirmed by co-immunoprecipitation from 293E cells co-expressing transfected N-copine and OS-9. In vitro binding assays showed that this interaction was Ca2(+)-dependent. By Northern blot analysis, N-copine and OS-9 were co-expressed in the same regions of human brain. These results reveal that OS-9 is a potential target of N-copine.

Localization and subcellular distribution of N-copine in mouse brain

N-Copine is a novel protein with two C2 domains. Its expression is brain specific and up-regulated by neuronal activity such as kainate stimulation and tetanus stimulation evoking hippocampal CA1 long-term potentiation. We examined the localization and subcellular distribution of N-copine in mouse brain. In situ hybridization analysis showed that N-copine mRNA was expressed exclusively in neurons of the hippocampus and in the main and accessory olfactory bulb, where various forms of synaptic plasticity and memory formation are known to occur. In immunohistochemical analyses, N-copine was detected mainly in the cell bodies and dendrites in the neurons, whereas presynaptic proteins such as synaptotagmin I and rab3A were detected in the regions where axons pass through. In fractionation experiments of brain homogenate, N-copine was associated with the membrane fraction in the presence of Ca2+ but not in its absence. As a GST-fusion protein with the second C2 domain of N-copine showed Ca2+-dependent binding to phosphatidylserine, this domain was considered to be responsible for the Ca2+-dependent association of N-copine with the membrane. Thus, N-copine may have a role as a Ca2+ sensor in postsynaptic events, in contrast to the known roles of "double C2 domain-containing proteins," including synaptotagmin I, in presynaptic events.

Localization of the Rab3 small G protein regulators in nerve terminals and their involvement in Ca2+-dependent exocytosis

The Rab3 small G protein subfamily (Rab3) consists of four members, Rab3A, -B, -C, and -D. We have recently isolated and characterized the Rab3 regulators, GDP/GTP exchange protein (GEP) and GTPase activating protein (GAP), both of which are specific for the Rab3 subfamily. Rab3 GEP stimulates the conversion of the GDP-bound inactive form to the GTP-bound active form, whereas Rab3 GAP stimulates the reverse reaction. Of the four members of the Rab3 subfamily, evidence is accumulating that Rab3A is involved in Ca2+-dependent exocytosis, particularly in neurotransmitter release. We first analyzed the subcellular localization of Rab3 GEP and GAP in rat brain. Subcellular fractionation analysis showed that both Rab3 GEP and GAP were enriched in the synaptic soluble fraction. Immunocytochemical analysis in primary cultured rat hippocampal neurons showed that both Rab3 GEP and GAP were concentrated at the presynaptic nerve terminals. We then examined whether Rab3 GEP and GAP were involved in Ca2+-dependent exocytosis by use of human growth hormone (GH) co-expression assay system of cultured PC12 cells. Overexpression of the deletion mutant of Rab3 GEP possessing the catalytic activity reduced the high K+-induced GH release without affecting the basal GH release, whereas that of the deletion mutant lacking the catalytic activity showed no effect on the high K+-induced GH release. In contrast, overexpression of Rab3 GAP or its deletion mutant possessing the catalytic activity did not affect the high K+-induced GH release or the basal GH release. These results indicate that Rab3 GEP and GAP are colocalized with Rab3A at the synaptic release sites and suggest that they regulate the activity of Rab3A and are involved in Ca2+-dependent exocytosis.

Interaction of Doc2 with tctex-1, a light chain of cytoplasmic dynein. Implication in dynein-dependent vesicle transport

Doc2 has one Munc13-interacting domain at the N-terminal region and two C2-like domains interacting with Ca2+ and phospholipid at the C-terminal region. Doc2 consists of two isoforms, Doc2alpha and -beta. Doc2alpha is specifically expressed in neuronal cells and implicated in Ca2+-dependent neurotransmitter release, whereas Doc2beta is ubiquitously expressed and its function is unknown. We show here that both Doc2alpha and -beta interact with rat tctex-1, a light chain of cytoplasmic dynein, in both cell-free and intact cell systems. Overexpression of the N-terminal fragment of Doc2 containing the tctex-1-interacting domain induces changes in the intracellular localization of cation-independent mannose 6-phosphate receptor and its ligand, cathepsin D, which are transported from trans-Golgi network to late endosomes. Overexpression of the C-terminal fragment containing two C2-like domains shows the similar effect, but to a lesser extent, whereas overexpression of full-length Doc2 or the C-terminal fragment of rabphilin3 containing two C2-like domains does not show this effect. Because dynein is a minus-end-directed microtubule-based motor protein, these results suggest that Doc2, especially Doc2beta, plays a role in dynein-dependent intracellular vesicle transport.

Calcium sensors in regulated exocytosis

Neurotransmitter release, hormone secretion and a variety of other secretory process are tightly regulated with exocytotic fusion of secretory vesicles being triggered by a rise in cytosolic Ca2+ concentration. A series of proteins that act as part of a conserved core machinery for vesicle docking and fusion throughout the cell have been identified. In regulated exocytosis this core machinery must be controlled by Ca(2+)-sensor proteins that allow rapid activation of the fusion process following elevation of cytosolic Ca2+ concentration. The properties of such Ca2+ sensors are known from physiological studies but their molecular identity remains to be unequivocally established. The multiple Ca(2+)-dependent steps in the exocytotic pathway suggest the likely involvement of several Ca(2+)-binding proteins with distinct properties. Functional evidence for the role of various Ca(2+)-binding proteins and their possible sites of action is accumulating but a definitive identification of the major Ca(2+)-sensor in the final step of Ca(2+)-triggered membrane fusion in different cell types awaits further analysis.

Molecular cloning and characterization of the noncatalytic subunit of the Rab3 subfamily-specific GTPase-activating protein

We recently purified and characterized from rat brain a GTPase-activating protein (GAP) specific for the Rab3 small G protein subfamily implicated in Ca2+-dependent exocytosis. Rab3 GAP showed two bands with Mr of about 130,000 (p130) and 150,000 (p150) on SDS-polyacrylamide gel electrophoresis. p130, but not p150, showed the catalytic activity. Because p150 was likely the subunit of Rab3 GAP, here we cloned the cDNA of p150, determined its primary structure, and characterized it. The tissue and subcellular distribution patterns of p150 and p130 were similar, and both the proteins were enriched in the synaptic soluble fraction. p150 was co-immunoprecipitated with p130 from this fraction. Recombinant p150 formed a heterodimer with recombinant p130 as estimated by sucrose density gradient ultracentrifugation. Recombinant p150 neither showed the Rab3A GAP activity nor affected the activity of recombinant p130. When p150 and p130 were co-expressed in the cells, the subcellular localization of each protein did not change. These results indicate that p150 is the noncatalytic subunit of Rab3 GAP.

Role of the Doc2 alpha-Munc13-1 interaction in the neurotransmitter release process

Doc2alpha and Munc13-1 proteins are highly concentrated on synaptic vesicles and the presynaptic plasma membrane, respectively, and have been implicated in Ca2+-dependent neurotransmitter release. Doc2alpha interacts with Munc13-1 through the N-terminal region of Doc2alpha (the Mid domain; amino acid residues 13-37). Here we examine whether the interaction between Doc2alpha and Munc13-1 is required for Ca2+-dependent neurotransmitter release from intact neuron. A synthetic Mid peptide (the Mid peptide), but not a control mutated Mid peptide or a scrambled Mid peptide, inhibited the interaction between Doc2alpha and Munc13-1 in vitro. Introduction of the Mid peptide into presynaptic neurons of cholinergic synapses, formed between rat superior cervical ganglion neurons, reversibly inhibited synaptic transmission evoked by action potentials. In contrast, the control peptides did not inhibit synaptic transmission. This inhibitory effect depended on the presynaptic activity and was affected by extracellular Ca2+ concentrations. The onset of the Mid peptide effect was shortened when the neuron was stimulated at a higher frequency, and the inhibition was more potent at 1 mM Ca2+ than at 5.1 mM Ca2+. These results suggest that the Doc2alpha-Munc13-1 interaction plays a role in a step before the final fusion step of synaptic vesicles with the presynaptic plasma membrane in the evoked neurotransmitter release process.

Neuronal Ca2+ sensor 1, the mammalian homologue of frequenin, is expressed in chromaffin and PC12 cells and regulates neurosecretion from dense-core granules

Neuronal Ca2+ sensor 1 (NCS-1) is the mammalian homologue of the Ca2+-binding protein frequenin previously implicated in regulation of neurotransmission in Drosophila (Pongs, O., Lindemeier, J., Zhu, X. R., Theil, T., Endelkamp, D., Krah-Jentgens, I., Lambrecht, H.-G., Koch, K. W., Schwemer, J., Rivosecchi, R., Mallart, A., Galceran, J. , Canal, I., Barbas, J. A., and Ferrus, A. (1993) Neuron 11, 15-28). NCS-1 has been considered to be expressed only in neurons, but we show that NCS-1 expression can be detected in bovine adrenal chromaffin and PC12 cells, two widely studied model neuroendocrine cells. NCS-1 was present in both cytosolic and membrane fractions including purified chromaffin granules, and in immunofluorescence, its distribution overlapped with peripheral punctate staining seen with the synaptic-like microvesicle marker synaptophysin in PC12 cells. The possible functional role of NCS-1 in exocytosis of dense-core granules was tested using transient transfection in PC12 cells and assay of co-transfected growth hormone (GH) release. Overexpression of NCS-1 increased evoked GH release in intact cells in response to ATP. No effect of overexpression was seen on GH release because of Ca2+ in permeabilized cells suggesting that NCS-1 may have a regulatory but not direct role in neurosecretion.

Interactions between presynaptic calcium channels and proteins implicated in synaptic vesicle trafficking and exocytosis

Monoclonal antibodies were generated by immunizing mice with chick brain synaptic membranes and screening for immunoprecipitation of solubilized omega conotoxin GVIA receptors (N-type calcium channels). Antibodies against two synaptic proteins (p35--syntaxin 1 and p58--synaptotagmin) were produced and used to purify and characterize a ternary complex containing N-type channels associated with these two proteins. These results provided the first evidence for a specific interaction between presynaptic calcium channels and SNARE proteins involved in synaptic vesicle docking and calcium-dependent exocytosis. Immunoprecipitation experiments supported the conclusion that syntaxin 1/SNAP-25/VAMP/synaptotagmin I or II complexes associate with N-type, P/Q-type, but not L-type calcium channels from rat brain nerve terminals. Immunofluorescent confocal microscopy at the frog neuromuscular junction was consistent with the co-localization of syntaxin 1, SNAP-25, and calcium channels, all of which are predominantly expressed at active zones of the presynaptic plasma membrane facing post-synaptic folds rich in acetylcholine receptors. The interaction of proteins implicated in calcium-dependent exocytosis with presynaptic calcium channels may locate the sensor(s) that trigger vesicle fusion within a microdomain of calcium entry.

A novel brain-specific isoform of beta spectrin: isolation and its interaction with Munc13

Munc13 is a component of the neurotransmitter release machinery which is specifically expressed in brain. Munc13 interacts with Doc2 and syntaxin which are also implicated in the neurotransmitter release process. Here we isolated another Munc13-interacting molecule from a rat brain cDNA library by use of the yeast two-hybrid system, identified it to be a novel type of beta spectrin, and named it beta SpIII sigma 1. beta SpIII sigma 1 was specifically expressed in brain, where it was enriched in the synaptic vesicle and plasma membrane fractions. Because spectrin has been shown to interact with the actin cytoskeleton which is involved in the exocytotic process, the present results suggest that the Munc13-beta SpIII sigma 1 interactions play a role in neurotransmitter release.