Mechanisms of synaptic vesicle exocytosis

Exocytotic release of alanine from cultured cerebellar neurons

Many neurons release a variety of amino acids in response to depolarizing stimuli. Although some of these amino acids, namely, glutamate, aspartate, and gamma-aminobutyric acid (GABA), have been qualified as neurotransmitters, functional roles of the other amino acids including alanine remain obscure. We investigated the mechanism and the origin of alanine release from cultured rat cerebellar cells. High-K(+)-induced depolarization produced a considerable amount (139+/-8 pmol/2 min/dish) of alanine release, comparable to that of glutamate (103+/-7 pmol/2 min/dish). Other depolarizing agents including veratridine or 4-aminopyridine also induced alanine release, suggesting that the major source is excitable neurons, rather than non-excitable glial cells. Depolarization-evoked alanine release was suppressed in the absence of extracellular Ca(2+), and was almost abolished by treating the cells with botulinum type B neurotoxin (BoNT/B), indicating that alanine is released by Ca(2+)-dependent exocytosis of vesicle-associated membrane protein-2 (VAMP-2)-containing vesicles. The properties of alanine release were different from those of glutamate and GABA in several aspects: (a) Depolarization-dependent alanine release appeared as early as 7 days in vitro, much earlier than that of GABA. (b) Fifty microM kainate, which causes selective cell death of GABAergic neurons in the culture, only partially reduced alanine release, whereas it had no effect on glutamate release. (c) Alanine release was not affected by phorbol ester, which enhanced glutamate and GABA release in a kinase-dependent manner. We therefore conclude that alanine release occurs via exocytosis of a pool of synaptic vesicles distinct from those containing glutamate or GABA.

Molecular analysis of SNAP-25 function in exocytosis

It is generally accepted that the SNARE proteins form the core of the machinery for intracellular membrane fusion and that formation of a SNARE complex is crucially important. Our aim is to dissect the molecular roles of the SNARE proteins and their regulators in physiological membrane fusion during exocytosis. We have developed approaches that allow us to manipulate protein expression in model secretory cells, PC12 and adrenal chromaffin cells, and to combine this with assay of exocytosis at high-time resolution using carbon-fiber amperometry. This technique allows us to assess the extent of exocytosis and to follow the kinetics of single secretory granule release events with millisecond time resolution. We established that manipulation of proteins involved in the exocytotic machinery can lead to detectable and interpretable changes in exocytosis kinetics that have revealed novel roles in late stages of exocytosis. Using this approach we have begun to analyze the function of SNAP-25B using a mutant resistant to the Clostridial neurotoxin BoNT/E. This SNAP-25 mutant can reconstitute exocytosis in BoNT/E-treated cells. With this construct it is possible to analyze the consequences of any introduced mutation in the absence of functional endogenous protein. We review here its use in the analysis of palmitoylated cysteines of SNAP-25 and the conserved residues of the 0 layer of the SNARE complex. The data suggest an important role of the cysteines, but not the 0 layer glutamines, in triggered exocytosis.

Neurosecretion competence. A comprehensive gene expression program identified in PC12 cells

The phenotype of neurosecretory cells is characterized by clear vesicles and dense granules, both discharged by regulated exocytosis. However, these organelles are lacking completely in a few neurosecretion-incompetent clones of the pheochromocytoma PC12 line, in which other specific features are maintained (incompetent clones). In view of the heterogeneity of PC12 cells, a differential characterization of the incompetent phenotype based on the comparison of a single incompetent and a single wild-type clone would have been inconclusive. Therefore, we have compared two pairs of PC12 clones, studying in parallel the transcript levels of 4,200 genes and 19,000 express sequence tags (ESTs) by high density oligonucleotide arrays. After accurate data processing for quality control and filtration, a total of 755 transcripts, corresponding to 448 genes and 307 ESTs, was found consistently changed, with 46% up-regulated and 54% down-regulated in incompetent versus wild-type clones. Many but not all neurosecretion genes were profoundly down-regulated in incompetent cells. Expression of endocytosis genes was normal, whereas that of many nuclear and transcription factors, including some previously shown to play key roles in neurogenesis, was profoundly changed. Additional differences appeared in genes involved in signaling and metabolism. Taken together these results demonstrate for the first time that expression of neurosecretory vesicles and granules is part of a complex gene expression program that includes many other features that so far have not been recognized.

Selective effects of calcium chelators on anterograde and retrograde protein transport in the cell

Calcium plays a regulatory role in several aspects of protein trafficking in the cell. Both vesicle fusion and vesicle formation can be inhibited by the addition of calcium chelators. Because the effects of calcium chelators have been studied predominantly in cell-free systems, it is not clear exactly which transport steps in the secretory pathway are sensitive to calcium levels. In this regard, we have studied the effects of calcium chelators on both anterograde and retrograde protein transport in whole cells. Using both cytochemical and biochemical analyses, we find that the anterograde-directed exit of vesicular stomatitis virus G protein and the retrograde-directed exit of Shiga toxin from the Golgi apparatus are both inhibited by calcium chelation. The exit of vesicular stomatitis virus G from a pre-Golgi compartment and the exit of Shiga toxin from an endosomal compartment are sensitive to the membrane-permeant calcium chelator 1,2-bis(2-amino phenoxy)ethane-N,N,N',N'-tetraacetic acid-tetrakis (acetoxymethyl ester) (BAPTA-AM). By contrast, endoplasmic reticulum exit and endocytic internalization from the plasma membrane are not affected by BAPTA. Together, our data show that some, but not all, trafficking steps in the cell may be regulated by calcium. These studies provide a framework for a more detailed analysis of the role of calcium as a regulatory agent during protein transport.

Staring, a novel E3 ubiquitin-protein ligase that targets syntaxin 1 for degradation

Syntaxin 1 is an essential component of the neurotransmitter release machinery, and regulation of syntaxin 1 expression levels is thought to contribute to the mechanism underlying learning and memory. However, the molecular events that control the degradation of syntaxin 1 remain undefined. Here we report the identification and characterization of a novel RING finger protein, Staring, that interacts with syntaxin 1. Staring is expressed throughout the brain, where it exists in both cytosolic and membrane-associated pools. Staring binds and recruits the brain-enriched E2 ubiquitin-conjugating enzyme UbcH8 to syntaxin 1 and facilitates the ubiquitination and proteasome-dependent degradation of syntaxin 1. These findings suggest that Staring is a novel E3 ubiquitin-protein ligase that targets syntaxin 1 for degradation by the ubiquitin-proteasome pathway.

Regulation of membrane fusion by the membrane-proximal coil of the t-SNARE during zippering of SNAREpins

We utilize structurally targeted peptides to identify a "tC fusion switch" inherent to the coil domains of the neuronal t-SNARE that pairs with the cognate v-SNARE. The tC fusion switch is located in the membrane-proximal portion of the t-SNARE and controls the rate at which the helical bundle that forms the SNAREpin can zip up to drive bilayer fusion. When the fusion switch is "off" (the intrinsic state of the t-SNARE), zippering of the helices from their membrane-distal ends is impeded and fusion is slow. When the tC fusion switch is "on," fusion is much faster. The tC fusion switch can be thrown by a peptide that corresponds to the membrane-proximal half of the cognate v-SNARE, and binds reversibly to the cognate region of the t-SNARE. This structures the coil in the membrane-proximal domain of the t-SNARE and accelerates fusion, implying that the intrinsically unstable coil in that region is a natural impediment to the completion of zippering, and thus, fusion. Proteins that stabilize or destabilize one or the other state of the tC fusion switch would exert fine temporal control over the rate of fusion after SNAREs have already partly zippered up.

A DrosophilaSNAP-25Null Mutant Reveals Context-Dependent Redundancy WithSNAP-24in Neurotransmission

The synaptic protein SNAP-25 is an important component of the neurotransmitter release machinery, although its precise function is still unknown. Genetic analysis of other synaptic proteins has yielded valuable information on their role in synaptic transmission. In this study, we performed a mutagenesis screen to identify new SNAP-25 alleles that fail to complement our previously isolated recessive temperature-sensitive allele of SNAP-25, SNAP-25ts. In a screen of 100,000 flies, 26 F1 progeny failed to complement SNAP-25ts and 21 of these were found to be null alleles of SNAP-25. These null alleles die at the pharate adult stage and electroretinogram recordings of these animals reveal that synaptic transmission is blocked. At the third instar larval stage, SNAP-25 nulls exhibit nearly normal neurotransmitter release at the neuromuscular junction. This is surprising since SNAP-25ts larvae exhibit a much stronger synaptic phenotype. Our evidence indicates that a related protein, SNAP-24, can substitute for SNAP-25 at the larval stage in SNAP-25 nulls. However, if a wild-type or mutant form of SNAP-25 is present, then SNAP-24 does not appear to take part in neurotransmitter release at the larval NMJ. These results suggest that the apparent redundancy between SNAP-25 and SNAP-24 is due to inappropriate genetic substitution.

Acrylamide neuropathy. II. Spatiotemporal characteristics of nerve cell damage in brainstem and spinal cord

Previous studies of acrylamide (ACR) neuropathy in rat PNS [Toxicol. Appl. Pharmacol. 151 (1998) 211] and cerebellum [Neurotoxicology, 2002a] have suggested that axon degeneration was not a primary effect and was, therefore, of unclear neurotoxicological significance. To continue morphological examination of ACR neurotoxicity in CNS, a cupric silver stain method was used to define spatiotemporal characteristics of nerve cell body, dendrite, axon and terminal degeneration in brainstem and spinal cord. Rats were exposed to ACR at a dose-rate of either 50 mg/kg per day (i.p.) or 21 mg/kg per day (p.o.), and at selected times brains and spinal cord were removed and processed for silver staining. Results show that intoxication at the higher ACR dose-rate produced a nearly pure terminalopathy in brainstem and spinal cord regions, ie. widespread nerve terminal degeneration and swelling were present in the absence of significant argyrophilic changes in neuronal cell bodies, dendrites or axons. Exposure to the lower ACR dose-rate caused initial nerve terminal argyrophilia in selected brainstem and spinal cord regions. As intoxication continued, axon degeneration developed in white matter of these CNS areas. At both dose-rates, argyrophilic changes in brainstem nerve terminals developed prior to the onset of significant gait abnormalities. In contrast, during exposure to the lower ACR dose-rate the appearance of axon degeneration in either brainstem or spinal cord was relatively delayed with respect to changes in gait. Thus, regardless of dose-rate, ACR intoxication produced early, progressive nerve terminal degeneration. Axon degeneration occurred primarily during exposure to the lower ACR dose-rate and developed after the appearance of terminal degeneration and neurotoxicity. Spatiotemporal analysis suggested that degeneration began at the nerve terminal and then moved as a function of time in a somal direction along the corresponding axon. These data suggest that nerve terminals are a primary site of ACR action and that expression of axonopathy is restricted to subchronic dosing-rates.

Vesicle-associated membrane protein-2/synaptobrevin binding to synaptotagmin I promotes O-glycosylation of synaptotagmin I

Synaptotagmin I (Syt I), an evolutionarily conserved integral membrane protein of synaptic vesicles, is now known to regulate Ca2+-dependent neurotransmitter release. Syt I protein should undergo several post-translational modifications before maturation and subsequent functioning on synaptic vesicles (e.g. N-glycosylation and fatty acylation in vertebrate Syt I), because the apparent molecular weight of Syt I on synaptic vesicles (mature form, 65,000) was much higher than the calculated molecular weight (47,400) predicted from the cDNA sequences both in vertebrates and invertebrates. Common post-translational modification(s) of Syt I conserved across phylogeny, however, have never been elucidated. In the present study, I discovered that dithreonine residues (Thr-15 and Thr-16) at the intravesicular domain of mouse Syt I are post-translationally modified by a complex form of O-linked sugar (i.e. the addition of sialic acids) in PC12 cells and that the O-glycosylation of Syt I in COS-7 cells depends on the coexpression of vesicle-associated membrane protein-2 (VAMP-2)/synaptobrevin. I also showed that a transmembrane domain of Syt I directly interacts with isolated VAMP-2, but not VAMP-2, in the heterotrimeric SNARE (SNAP receptor) complex (vesicle SNARE, VAMP-2, and two target SNAREs, syntaxin IA and SNAP-25). Since di-Thr or di-Ser residues are often found at the intravesicular domain of invertebrate Syt I, and VAMP-dependent O-glycosylation was also observed in squid Syt expressed in COS-7 cells, I propose that VAMP-dependent O-glycosylation of Syt I is a common modification during evolution and may have important role(s) in synaptic vesicle trafficking.

[Botulism: the agent, mode of action of the botulinum neurotoxins, forms of acquisition, treatment and prevention]

The botulinum neurotoxins are produced by anaerobic, spore-forming bacteria belonging to the Clostridium genus. They are synthesised as a single chain protein (150 kDa), which is not or weakly active. The active form results from a proteolysis cleaving the precursor in a light chain (about 50 kDa) and a heavy chain (about 100 kDa), which are linked by a disulfide bridge. The heavy chain is involved in the recognition of a specific neuronal surface receptor and mediates the internalization of the light chain into the cytosol. The light chain is responsible for the intracellular activity. It catalyses the proteolysis of SNARE proteins, which are involved in the exocytosis of synaptic vesicles containing acetylcholine. Hence, the release of acetylcholine at the neuromuscular junction is blocked, leading to a flaccid paralysis. Human botulism, usually type A, B or E, is associated with intoxination, ingestion of preformed toxin in food, with digestive toxi-infection, mainly in newborns (infant botulism), or with wound contamination (wound botulism). The treatment of botulism is usually symptomatic. The specific treatment is based on the serotherapy or on the use of purified specific antibodies. The vaccination against botulism is efficient. However, since the botulinum neurotoxins are widely used for the treatment of numerous dystonias, a generalised vaccination is not conceivable.

Calmodulin and lipid binding to synaptobrevin regulates calcium-dependent exocytosis

Neurotransmitter release involves the assembly of a heterotrimeric SNARE complex composed of the vesicle protein synaptobrevin (VAMP 2) and two plasma membrane partners, syntaxin 1 and SNAP-25. Calcium influx is thought to control this process via Ca(2+)-binding proteins that associate with components of the SNARE complex. Ca(2+)/calmodulin or phospholipids bind in a mutually exclusive fashion to a C-terminal domain of VAMP (VAMP(77-90)), and residues involved were identified by plasmon resonance spectroscopy. Microinjection of wild-type VAMP(77-90), but not mutant peptides, inhibited catecholamine release from chromaffin cells monitored by carbon fibre amperometry. Pre-incubation of PC12 pheochromocytoma cells with the irreversible calmodulin antagonist ophiobolin A inhibited Ca(2+)-dependent human growth hormone release in a permeabilized cell assay. Treatment of permeabilized cells with tetanus toxin light chain (TeNT) also suppressed secretion. In the presence of TeNT, exocytosis was restored by transfection of TeNT-resistant (Q(76)V, F(77)W) VAMP, but additional targeted mutations in VAMP(77-90) abolished its ability to rescue release. The calmodulin- and phospholipid-binding domain of VAMP 2 is thus required for Ca(2+)-dependent exocytosis, possibly to regulate SNARE complex assembly.

Non-polarized distribution of synaptotagmin IV in neurons: evidence that synaptotagmin IV is not a synaptic vesicle protein

Synaptotagmin IV (Syt IV) expression is regulated by neuronal development and by depolarization in the brain and in neuronal cell cultures. In cultures, immunocytochemical analysis has shown that Syt IV is localized at the Golgi and at the tips of growing neurites, but little was known about associations between Syt IV and vesicles or organelles [J. Neurochem. 74 (2000) 518]. In this study we performed an electron microscopic (EM) analysis of developing mouse neocortex to determine the exact localization of Syt IV in native mouse tissues. In neurons of layer II/III, Syt IV was found to be localized in the dendrites and axons, and at the Golgi in the cell body. Some Syt IV signals were clearly associated with vesicles and/or organelles, but EM and cell fractionation studies showed no Syt IV signals at synaptic vesicles. Detection of fluorescence protein-tagged Syt IV (Syt IV-EGFP) in hippocampal neurons also showed the presence of Syt IV-EGFP vesicles or organelles in the axons and dendrites. These results suggest that Syt IV regulates non-polarized membrane trafficking in neurons, which may be involved in synaptic plasticity or neuronal development.

Quantitative femto- to attomole immunodetection of regulated secretory vesicle proteins critical to exocytosis

Although immunoblotting (Western blotting) is widely used for the detection of specific proteins, it is often thought to be an inadequate technique for accurate and precise measurements of protein concentration. However, an accurate and precise technique is essential for quantitative testing of hypotheses, and thus for the analysis and understanding of proposed molecular mechanisms. The analysis of Ca(2+)-triggered exocytosis, the ubiquitous eukaryotic process by which vesicles fuse to the plasma membrane and release their contents, requires such an unambiguous identification and a quantitative assessment of the membrane surface density of specific molecules. Newly refined immunoblotting and analysis approaches permit a quantitative analysis of the SNARE protein complement (VAMP, SNAP-25, and syntaxin) of functional secretory vesicles. The method illustrates the feasibility of the routine quantification of femtomole to attomole amounts of known proteins by immunoblotting. The results indicate that sea urchin egg secretory vesicles and synaptic vesicles have markedly similar SNARE densities.

Differential expression of neuronal calcium sensor-1 in the developing chick retina

Neuronal calcium sensor-1 (NCS-1) is a Ca(2+) binding protein that has been implicated in the regulation of neurotransmission and synaptogenesis. In this study we investigated the developmental expression and localization of NCS-1 in the chick retina. Single- and double-labeling experiments with three-dimensional reconstruction as well as ultrastructural data of the distribution of NCS-1 suggest that this protein is also involved in axonal process outgrowth. We found an early expression of NCS-1 in ganglion cells and their axons, in amacrine, and in horizontal cells, whereas photoreceptors were immunonegative at embryonic stages. In the early posthatching days we found strong immunostaining for NCS-1 in horizontal cells and their processes in the outer plexiform layer. In contrast, synaptic vesicle protein 2 (SV2) was prominent only in photoreceptor synaptic terminals. Ultrastructural analysis confirmed that NCS-1 was localized postsynaptically in horizontal cell processes, whereas presynaptic terminals were immunonegative. However, at late posthatching days we observed that photoreceptor ribbon synapses (from rods and/or cones) also expressed NCS-1. Thus the results support the notion that NCS-1 is involved in neuronal process outgrowth and is localized in pre- and postsynaptic compartments including mature photoreceptor synapses.

The C(2)B Ca(2+)-binding motif of synaptotagmin is required for synaptic transmission in vivo

Synaptotagmin is a synaptic vesicle protein that is postulated to be the Ca(2+) sensor for fast, evoked neurotransmitter release. Deleting the gene for synaptotagmin (syt(null)) strongly suppresses synaptic transmission in every species examined, showing that synaptotagmin is central in the synaptic vesicle cycle. The cytoplasmic region of synaptotagmin contains two C(2) domains, C(2)A and C(2)B. Five, highly conserved, acidic residues in both the C(2)A and C(2)B domains of synaptotagmin coordinate the binding of Ca(2+) ions, and biochemical studies have characterized several in vitro Ca(2+)-dependent interactions between synaptotagmin and other nerve terminal molecules. But there has been no direct evidence that any of the Ca(2+)-binding sites within synaptotagmin are required in vivo. Here we show that mutating two of the Ca(2+)-binding aspartate residues in the C(2)B domain (D(416,418)N in Drosophila) decreased evoked transmitter release by >95%, and decreased the apparent Ca(2+) affinity of evoked transmitter release. These studies show that the Ca(2+)-binding motif of the C(2)B domain of synaptotagmin is essential for synaptic transmission.

How Tlg2p/syntaxin 16 'snares' Vps45

Soluble N-ethylmaleimide sensitive factor-attachment protein receptors (SNAREs) and Sec1p/Munc18-homologs (SM proteins) play key roles in intracellular membrane fusion. The SNAREs form tight four-helix bundles (core complexes) that bring the membranes together, but it is unclear how this activity is coupled to SM protein function. Studies of the yeast trans-Golgi network (TGN)/endosomal SNARE complex, which includes the syntaxin-like SNARE Tlg2p, have suggested that its assembly requires activation by binding of the SM protein Vps45p to the cytoplasmic region of Tlg2p folded into a closed conformation. Nuclear magnetic resonance and biochemical experiments now show that Tlg2p and Pep12p, a late- endosomal syntaxin that interacts functionally but not directly with Vps45p, have a domain structure characteristic of syntaxins but do not adopt a closed conformation. Tlg2p binds tightly to Vps45p via a short N-terminal peptide motif that is absent in Pep12p. The Tlg2p/Vps45p binding mode is shared by the mammalian syntaxin 16, confirming that it is a Tlg2p homolog, and resembles the mode of interaction between the SM protein Sly1p and the syntaxins Ufe1p and Sed5p. Thus, this mechanism represents the most widespread mode of coupling between syntaxins and SM proteins.

The four-helix bundle of the neuronal target membrane SNARE complex is neither disordered in the middle nor uncoiled at the C-terminal region

Assembly of the SNARE complex is an essential step for membrane fusion and neurotransmitter release in neurons. The plasma membrane SNAREs syntaxin 1A and SNAP-25 (t-SNAREs) and the delivery-vesicle SNARE VAMP2 (or v-SNARE) contain the "SNARE regions" that essentially mediate SNARE pairing. Using site-directed spin labeling and EPR distance measurement we show that two identical copies of the SNARE region from syntaxin 1A intertwine as a coiled coil near the "ionic layer" region. The structure of the t-SNARE complex appears to be virtually identical to that of the ternary SNARE complex, except that VAMP2 is substituted to the second copy of syntaxin 1A. Furthermore, it appears that the coiled coil structure is maintained up to residue 259 of syntaxin 1A, identical to that of the ternary complex. These results are somewhat contradictory to the previous reports, suggesting that the t-SNARE complex has the disordered midsection (Xiao, W. Z., Poirier, M. A., Bennett, M. K., and Shin, Y. K. (2001) Nat. Struc. Biol. 8, 308-311) and the uncoiled C-terminal region (Margittai, M., Fasshauer, D., Pabst, S., Jahn, R., and Langen, R. (2001) J. Biol. Chem. 276, 13169-13177). The newly refined structure of the t-SNARE complex provides a basis for the better understanding of the SNARE assembly process. It also provides possible structural-functional clues to the membrane fusion in the v-SNARE deleted fusion models.

Toluene-induced, Ca(2+)-dependent vesicular catecholamine release in rat PC12 cells

Acute effects of toluene on vesicular catecholamine release from intact PC12 phaeochromocytoma cells have been investigated using carbon fiber microelectrode amperometry. The frequency of vesicles released is low under basal conditions and is enhanced by depolarization. Toluene causes an increase in basal release frequency. The threshold effect is obtained at 30 microM toluene and the maximum enhancement of basal release is seven-fold at 1 mM toluene. Toluene-induced exocytosis depends on the influx of Ca(2+) through voltage-activated Ca(2+) channels, which are blocked by Cd(2+). Toluene neither affects depolarization-evoked exocytosis, nor the characteristics of release events. It is concluded that toluene-induced vesicular catecholamine release is due to an increase in intracellular Ca(2+) concentration, whereas basic processes underlying exocytosis do not appear to be affected by toluene at concentrations up to 300 microM.

Direct, Ca2+-dependent interaction between tubulin and synaptotagmin I: a possible mechanism for attaching synaptic vesicles to microtubules

The synaptic vesicle protein synaptotagmin I probably plays important roles in the synaptic vesicle cycle. However, the mechanisms of its action remain unclear. In this study, we have searched for cytoplasmic proteins that interact with synaptotagmin I. We found that the cytoskeletal protein tubulin directly and stoichiometrically bound to recombinant synaptotagmin I. The binding depended on mm Ca(2+), and 1 mol of tubulin dimer bound 2 mol of synaptotagmin I with half-maximal binding at 6.6 microm tubulin. The Ca(2+) dependence mainly resulted from Ca(2+) binding to the Ca(2+) ligands of synaptotagmin I. The C-terminal region of beta-tubulin and both C2 domains of synaptotagmin I were involved in the binding. The YVK motif in the C2 domains of synaptotagmin I was essential for tubulin binding. Tubulin and synaptotagmin I were co-precipitated from the synaptosome extract with monoclonal antibodies to tubulin and SNAP-25 (synaptosome-associated protein of 25 kDa), indicating the presence of tubulin/synaptotagmin I complex and tubulin binding to synaptotagmin I in SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complexes. Synaptotagmin I promoted tubulin polymerization and bundled microtubules in the presence of Ca(2+). These results suggest that direct interaction between synaptotagmin I and tubulin provides a mechanism for attaching synaptic vesicles to microtubules in high Ca(2+) concentrations.

Mechanisms through which ammonia regulates cortical collecting duct net proton secretion

Ammonia stimulates cortical collecting duct (CCD) net bicarbonate reabsorption by activating an apical H(+)-K(+)-ATPase through mechanisms that are independent of ammonia's known effects on intracellular pH and active sodium transport. The present studies examined whether this stimulation occurs through soluble N-ethylmaleimide-sensitive fusion attachment receptor (SNARE) protein-mediated vesicle fusion. Rabbit CCD segments were studied using in vitro microperfusion, and transepithelial bicarbonate transport was measured using microcalorimetry. Ammonia's stimulation of bicarbonate reabsorption was blocked by either chelating intracellular calcium with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester or by inhibiting microtubule polymerization with colchicine compared with parallel studies performed in the absence of these inhibitors. An inactive structural analog of colchicine, lumicolchicine, did not alter ammonia's stimulation of bicarbonate reabsorption. Tetanus toxin, a zinc endopeptidase specific for vesicle-associated SNARE (v-SNARE) proteins, prevented ammonia from stimulating net bicarbonate reabsorption. Consistent with the functional evidence for v-SNARE involvement, antibodies directed against a conserved region of isoforms 1-3 of the tetanus toxin-sensitive, vesicle-associated membrane protein (VAMP) members of v-SNARE proteins labeled the apical and subapical region of collecting duct intercalated cells. Similarly, antibodies to NSF protein, a protein involved in activation of SNARE proteins for subsequent vesicle fusion, localized to the apical and subapical region of collecting duct intercalated cells. These results indicate that ammonia stimulates CCD bicarbonate reabsorption through an intracellular calcium-dependent, microtubule-dependent, and v-SNARE-dependent mechanism that appears to involve insertion of cytoplasmic vesicles into the apical plasma membrane of CCD intercalated cells.

Rab27b association with melanosomes: dominant negative mutants disrupt melanosomal movement

The movement of melanosomes from post-Golgi compartments to the periphery of melanocytes is known to be regulated by factors including myosin Va and at least one Rab protein, Rab27a. Mutations in the genes encoding either protein in the mouse result in a hypopigmented phenotype mimicking the human disease Griscelli syndrome. Rab27b and Rab27a share 72% identity and they belong to the same melanocyte/platelet subfamily of Rab proteins. Rab27a orchestrates the transport of melanosomes by recruitment of the actin motor, myosin Va, onto melanosomes. By contrast, the function of Rab27b has remained elusive. In this study, we found that Rab27b mRNA is present in melanocytes and demonstrated the intrinsic GTPase activity of Rab27b protein. We explored the function of Rab27b by overexpression of two dominant negative mutants as well as the wild-type Rab27b in melan-a melanocytes. Green-fluorescent-protein-tagged Rab27b colocalizes with the melanosome marker tyrosinase-related protein 1 and with myosin Va at the cell periphery, whereas Rab27b mutants do not decorate melanosomes, and melanosomes in these mutant transfected cells redistribute from cell periphery to the perinuclear region. Furthermore, transient overexpression of the dominant negative forms of Rab27b caused diminution in both numbers and length of dendrites of melan-a cells. Our results suggest that Rab27b may regulate the outward movement of melanosomes and the formation or maintenance of dendritic extensions in melanocytes.

Ocsyn, a novel syntaxin-interacting protein enriched in the subapical region of inner hair cells

Sensory (hair) cells of the inner ear contain two specialized areas of membrane delivery. The first, located at the cell base, is the afferent synapse where rapid delivery of synaptic vesicles is required to convey information about auditory signals with exceedingly high temporal precision. The second area is at the apex. To accommodate the continuous movement of stereocilia and facilitate their repair, recycling of membrane components is required. Intense vesicular traffic is restricted to a narrow band of cytoplasm around the cuticular plate, which anchors stereocilia. Our previous analyses showed that SNARE proteins (syntaxin 1A/SNAP25/VAMP1) are concentrated at both poles of hair cells, consistent with their involvement in membrane delivery at both locations. To investigate further the molecules involved in membrane delivery at these two sites, we constructed a two-hybrid library of the organ of Corti and probed it with syntaxin 1A. Here we report the cloning of a novel syntaxin-binding protein that is concentrated in a previously uncharacterized organelle at the apex of inner hair cells.

Cloning and characterization of ZAP36, an annexin-like, zymogen granule membrane associated protein, in exocrine pancreas

ZAP36, a zymogen granule membrane associated protein with 36 kDa, was cloned from both canine and rat pancreas. ZAP36 is found to be a novel member of annexin IV, and showed an apical localization in exocrine pancreas and an ubiquitous expression in epithelial tissues. ZAP36 may be involved in exocytosis in apical regions of polarized cells.

Hyposmotic shock stimulates insulin secretion by two distinct mechanisms. Studies with the betaHC9 cell

Exposure of betaHC9 cells to a Krebs-Ringer bicarbonate-HEPES buffer (KRBH) made hypotonic by a reduction of 25 mM NaCl resulted in a prompt stimulation of insulin release. The stimulation was transient, and release rates returned to basal levels after 10 min. The response resembles that of the first phase of glucose-stimulated insulin release. The response did not occur if the reduction in NaCl was compensated for by the addition of an equivalent osmolar amount of sorbitol, so the stimulation of release was due to the osmolarity change and not the reduction in NaCl. The hyposmotic shock released insulin in KRBH with or without Ca(2+). The L-type Ca(2+) channel blocker nitrendipine inhibited the response in normal KRBH but had no effect in KRBH without Ca(2+) despite the latter response being larger than in the presence of extracellular Ca(2+). Similar data were obtained with calciseptine, which also blocks L-type channels. The T-type Ca(2+) channel blocker flunarizine was without effect, as was the chloride channel blocker DIDS. In parallel studies, the readily releasable pool of insulin-containing granules was monitored. Immunoprecipitation of the target-SNARE protein syntaxin and co-immunoprecipitation of the vesicle-SNARE VAMP-2 was used as an indicator of the readily releasable granule pool. After hypotonic shock in the presence of extracellular Ca(2+), the amount of VAMP-2 coimmunoprecipitated by antibodies against syntaxin was much reduced compared with controls. Therefore, under these conditions, hypotonic shock stimulates exocytosis of the readily releasable pool of insulin-containing granules. No such reduction was seen in the absence of extracellular Ca(2+). In conclusion, after reexamination of the effect of hyposmotic shock on insulin secretion in the presence and absence of Ca(2+) (with EGTA in the medium), it is clear that two different mechanisms are operative under these conditions. Moreover, these two mechanisms may be associated with the release of two distinct pools of insulin-containing granules.

Nerve terminals as the primary site of acrylamide action: a hypothesis

Acrylamide (ACR) is considered to be prototypical among chemicals that cause a central-peripheral distal axonopathy. Multifocal neurofilamentous swellings and eventual degeneration of distal axon regions in the CNS and PNS have been traditionally considered the hallmark morphological features of this axonopathy. However, ACR has also been shown to produce early nerve terminal degeneration of somatosensory, somatomotor and autonomic nerve fibers under a variety of dosing conditions. Recent research from our laboratory has demonstrated that terminal degeneration precedes axonopathy during low-dose subchronic induction of neurotoxicity and occurs in the absence of axonopathy during higher-dose subacute intoxication. This relationship suggests that nerve terminal degeneration, and not axonopathy, is the primary or most important pathophysiologic lesion produced by ACR. In this hypothesis paper, we review evidence suggesting that nerve terminal degeneration is the hallmark lesion of ACR neurotoxicity, and we propose that this effect is mediated by the direct actions of ACR at nerve terminal sites. ACR is an electrophile and, therefore, sulfhydryl groups on presynaptic proteins represent rational molecular targets. Several presynaptic thiol-containing proteins (e.g. SNAP-25, NSF) are critically involved in formation of SNARE (soluble N-ethylmaleimide (NEM)-sensitive fusion protein receptor) complexes that mediate membrane fusion processes such as exocytosis and turnover of plasmalemmal proteins and other constituents. We hypothesize that ACR adduction of SNARE proteins disrupts assembly of fusion core complexes and thereby interferes with neurotransmission and presynaptic membrane turnover. General retardation of membrane turnover and accumulation of unincorporated materials could result in nerve terminal swelling and degeneration. A similar mechanism involving the long-term consequences of defective SNARE-based turnover of Na+/K(+)-ATPase and other axolemmal constituents might explain subchronic induction of axon degeneration. The ACR literature occupies a prominent position in neurotoxicology and has significantly influenced development of mechanistic hypotheses and classification schemes for neurotoxicants. Our proposal suggests a reevaluation of current classification schemes and mechanistic hypotheses that regard ACR axonopathy as a primary lesion.

RIM binding proteins (RBPs) couple Rab3-interacting molecules (RIMs) to voltage-gated Ca(2+) channels

Ca(2+) influx through voltage-gated channels initiates the exocytotic fusion of synaptic vesicles to the plasma membrane. Here we show that RIM binding proteins (RBPs), which associate with Ca(2+) channels in hair cells, photoreceptors, and neurons, interact with alpha(1D) (L type) and alpha(1B) (N type) Ca(2+) channel subunits. RBPs contain three Src homology 3 domains that bind to proline-rich motifs in alpha(1) subunits and Rab3-interacting molecules (RIMs). Overexpression in PC12 cells of fusion proteins that suppress the interactions of RBPs with RIMs and alpha(1) augments the exocytosis triggered by depolarization. RBPs may regulate the strength of synaptic transmission by creating a functional link between the synaptic-vesicle tethering apparatus, which includes RIMs and Rab3, and the fusion machinery, which includes Ca(2+) channels and the SNARE complex.

Exocytosis, endocytosis, and development

During development, metazoans are faced with the daunting task of generating many different cell types in a temporally and spatially precise manner. This orderly process of cell generation relies on creating localized signals that activate or inhibit specific cellular pathways. Recent work has shown that some of these localized signals require the targeted secretion of proteins, or their uptake by endocytosis. The importance of these protein trafficking pathways in localized signal generation is further substantiated by endo- and exocytosis mutants which can phenocopy many developmental mutants. Genetic and molecular techniques that increase our ability to inhibit exocytosis and endocytosis in a temporal and cell-type specific manner are likely to further elucidate the complexities of development.

Protein kinase C-mediated translocation of secretory vesicles to plasma membrane and enhancement of neurotransmitter release from PC12 cells

In order to elucidate the molecular mechanism of phorbol ester-induced potentiation of neurotransmitter release, changes in the subcellular distribution of secretory vesicles were studied in PC12 cells. Dopamine (DA) and acetylcholine containing vesicles were selectively labelled by expressing green fluorescent protein-conjugated vesicular monoamine transporter and vesicular acetylcholine transporter, respectively. In the resting state, these vesicles were distributed throughout the cytoplasm. Phorbol-12-myristate-13-acetate (PMA), but not the inactive analogue 4 alpha-PMA, induced a redistribution of both types of secretory vesicles near the plasma membrane, and this change was abolished by a protein kinase C (PKC) inhibitor, bisindolylmaleimide I (BIS). PMA also induced a marked enhancement of depolarization-induced DA release and phosphorylation of SNAP-25 at Ser187. BIS completely inhibited PMA-induced SNAP-25 phosphorylation but suppressed PMA-induced enhancement of DA release only partially. These results suggest that PMA enhances neurotransmitter release from PC12 cells by both PKC-dependent and PKC-independent mechanisms, and PKC enhances neurotransmitter release by recruiting secretory vesicles to the plasma membrane.

In vivo membrane trafficking role for an insect N-ethylmaleimide-sensitive factor which is developmentally regulated in endocrine cells

The hexameric ATPase, N-ethylmaleimide-sensitive factor (NSF) is implicated in the release of neurotransmitters and in mediating fusion between intracellular membranes. Due to the conservation of proteins in constitutive and regulated membrane fusion reactions, NSF and its downstream targets have been predicted also to participate in fusion reactions underlying endocrine function, but there is little experimental evidence to support such a role for NSF in insect neuroendocrine secretion. Here we have characterized the NSF orthologue (MsNSF) from the endocrine model for development Manduca sexta. MsNSF is developmentally regulated in endocrine organs of the protocerebral complex. Enrichment of MsNSF in corpora cardiaca (CC) and not in corpora allata (CA) indicates that it might play a preferential role in releasing hormones produced in CC. Endocrine/paracrine cells of the enteric system in M. sexta exhibit selective MsNSF enrichment. Together the data point to a more selective participation of MsNSF in development of M. sexta by its involvement in a subset of factors, whereas other as-yet-unidentified homolog(s) might regulate secretion from CA and a large set of endocrine/paracrine cells. We further characterized the in vivo role of MsNSF by heterologous expression. In contrast to vertebrate NSF, MsNSF is functional in yeast membrane fusion in vivo. MsNSF rectifies defects in SEC18 (yeast NSF homologue) at nearly all discernible steps where Sec18p has been implicated in the biosynthetic route. This underscores the utility of our approach to delineate functional roles for proteins from systems that are not currently amenable to in vitroreconstitution.

NSF ATPase and alpha-/beta-SNAPs disassemble the AMPA receptor-PICK1 complex

AMPA receptor (AMPAR) trafficking is crucial for synaptic plasticity that may be important for learning and memory. NSF and PICK1 bind the AMPAR GluR2 subunit and are involved in trafficking of AMPARs. Here, we show that GluR2, PICK1, NSF, and alpha-/beta-SNAPs form a complex in the presence of ATPgammaS. Similar to SNARE complex disassembly, NSF ATPase activity disrupts PICK1-GluR2 interactions in this complex. Alpha- and beta-SNAP have differential effects on this reaction. SNAP overexpression in hippocampal neurons leads to corresponding changes in AMPAR trafficking by acting on GluR2-PICK1 complexes. This demonstrates that the previously reported synaptic stabilization of AMPARs by NSF involves disruption of GluR2-PICK1 interactions. Furthermore, we are reporting a non-SNARE substrate for NSF disassembly activity.

Distribution of soluble N-ethylmaleimide fusion protein attachment proteins (SNAPs) in the rat nervous system

Soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (SNAP) plays an essential role in vesicular transport and the release of neurotransmitters and hormones through associations with NSF and SNAP receptors (SNAREs). Three isoforms (alpha, beta and gamma) of SNAP are expressed in mammals. We have generated isoform-specific antibodies and studied the expression and distribution of these SNAP isoforms in the rat nervous system. Each antibody specifically recognized alpha-, beta- or gamma-SNAP in an isoform-specific manner in immunoblots of brain homogenate. Alpha- and gamma-SNAP were ubiquitously expressed in various tissues, whereas beta-SNAP was expressed only in brain. After subcellular fractionation of brain homogenates, all three isoforms were recovered in both soluble and particulate fractions. Immunohistochemistry revealed that alpha- and beta-SNAP were generally differentially distributed both in synaptic and non-synaptic regions, including brain white matter. The presynaptic location of both alpha- and beta-SNAP was confirmed by immunoelectron microscopy. At the neuromuscular junction, immunoreactive alpha-SNAP was identified in synaptic vesicles, while in the cerebellum, beta-SNAP was present in the presynaptic membranes of basket neuron and mossy fiber terminals. From these results we suggest that both alpha- and beta-SNAP may play an important role in neurotransmitter release as well as in constitutive vesicular transport.

Rapid and selective binding to the synaptic SNARE complex suggests a modulatory role of complexins in neuroexocytosis

The Ca(2+)-triggered release of neurotransmitters is mediated by fusion of synaptic vesicles with the plasma membrane. The molecular machinery that translates the Ca(2+) signal into exocytosis is only beginning to emerge. The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins syntaxin, SNAP-25, and synaptobrevin are central components of the fusion apparatus. Assembly of a membrane-bridging ternary SNARE complex is thought to initiate membrane merger, but the roles of other factors are less understood. Complexins are two highly conserved proteins that modulate the Ca(2+) responsiveness of neurotransmitter release. In vitro, they bind in a 1:1 stoichiometry to the assembled synaptic SNARE complex, making complexins attractive candidates for controlling the exocytotic fusion apparatus. We have now performed a detailed structural, kinetic, and thermodynamic analysis of complexin binding to the SNARE complex. We found that no major conformational changes occur upon binding and that the complexin helix is aligned antiparallel to the four-helix bundle of the SNARE complex. Complexins bound rapidly (approximately 5 x 10(7) m(-1) s(-1)) and with high affinity (approximately 10 nm), making it one of the fastest protein-protein interactions characterized so far in membrane trafficking. Interestingly, neither affinity nor binding kinetics was substantially altered by Ca(2+) ions. No interaction of complexins was detectable either with individual SNARE proteins or with the binary syntaxin x SNAP-25 complex. Furthermore, complexin did not promote the formation of SNARE complex oligomers. Together, our data suggest that complexins modulate neuroexocytosis after assembly of membrane-bridging SNARE complexes.

Down-regulated expression of exocytotic proteins in pancreatic islets of diabetic GK rats

Exocytosis is regulated by exocytotic proteins, which are present in insulin-secreting beta-cells and play regulatory roles in insulin secretion. Non-insulin dependent diabetes mellitus (type 2 diabetes) is a disease characterized by impaired insulin secretion and insulin resistance. Exocytotic protein immunoreactivities were studied in pancreatic islets of type 2 diabetic Goto-Kakizaki (GK) rats using immunofluorescence histochemistry. The immunoreactivities for vesicle-associated membrane protein-2 (VAMP-2), synaptotagmin III, cysteine string protein (CSP), mammalian homologue of the unc-18 gene (Munc-18), alpha-soluble N-ethylmaleimide-sensitive attachment protein (alpha-SNAP), N-ethylmaleimide-sensitive factor (NSF) and synaptosomal-associated protein of 25 kDa (SNAP-25) exhibited weaker immunofluorescence intensity in islets of GK rats as compared to control Wistar rats. Insulin immunoreactivity was also decreased in GK rat beta-cells, whereas no detectable alterations in the expression of actin immunoreactivity could be detected. The data suggest that reduced expression of exocytotic proteins and decreased insulin content may contribute to the diabetic syndrome in the GK rat.

Toward understanding vesicle traffic and the guard cell model

Vesicular trafficking and membrane fusion are integral to cell growth and development with SNARE proteins, RabGTPases and their associates implicated in membrane fusion and secretion throughout the plant endomembrane system. Although the overall pattern of function is similar to that of animals and yeast, many aspects of endomembrane organization and vesicle trafficking appear unique to plants, for example, the control of cell and vacuolar expansion, asymmetric growth and cell division. However, the dominant membrane trafficking pathways have yet to be defined. Comparative genomics provide important information about vesicle trafficking elements but assigning biological roles based on sequence similarities is extremely difficult. Cellular and genetic approaches are reviewed here that have allowed visualization of vesicle trafficking in plants, including capacitance and dye methods, imaging and marker techniques, protein interactions and reverse genetics. Stomatal guard cells are discussed as cell models for identifying vesicle trafficking pathways and evidence points to a role for vesicle trafficking in stomatal function. For plants generally, kinetic analyses and biochemical studies suggest that several different pools of vesicles, and possibly different mechanisms for delivery, are available for vesicle traffic between endomembrane compartments and the plasma membrane. Characterizing these pathways, their functions and controls provides a major challenge for the future.

Arresting developments in heptahelical receptor signaling and regulation

It is well established that the function of most heptahelical receptors (seven-transmembrane-span receptors; 7TMRs) is tightly regulated by the desensitizing actions of arrestins. Desensitization is the waning of 7TMR-mediated signals after prolonged exposure to agonist and occurs when arrestins bind to agonist-occupied and phosphorylated receptors, uncoupling the receptors from G proteins and preventing further signaling. Recently, there has been a marked shift in the focus of research into arrestin function because it has become clear that they not only prevent signaling from 7TMRs but also initiate and direct new signals from the very 7TMRs that they desensitize.

Targeted expression of tetanus neurotoxin interferes with behavioral responses to sensory input in Drosophila

Targeted inactivation of neurons by expression of toxic gene products is a useful tool to assign behavioral functions to specific neurons or brain structures. Of a variety of toxic gene products tested, tetanus neurotoxin light chain (TNT) has the least severe side effects and can completely block chemical synapses. By using the GAL4 system to drive TNT expression in a subset of chemo- and mechanosensory neurons, we detected walking and flight defects consistent with blocking of relevant sensory information. We also found, for the first time, an olfactory behavioral phenotype associated with blocking of a specific subset of antennal chemoreceptors. Similar behavioral experiments with GAL4 lines expressing in different subsets of antennal chemoreceptors should contribute to an understanding of olfactory coding in Drosophila. To increase the utility of the GAL4 system for such purposes, we have designed an inducible system that allows us to circumvent lethality caused by TNT expression during early development.

Vesicular restriction of synaptobrevin suggests a role for calcium in membrane fusion

Release of neurotransmitter occurs when synaptic vesicles fuse with the plasma membrane. This neuronal exocytosis is triggered by calcium and requires three SNARE (soluble-N-ethylmaleimide-sensitive factor attachment protein receptors) proteins: synaptobrevin (also known as VAMP) on the synaptic vesicle, and syntaxin and SNAP-25 on the plasma membrane. Neuronal SNARE proteins form a parallel four-helix bundle that is thought to drive the fusion of opposing membranes. As formation of this SNARE complex in solution does not require calcium, it is not clear what function calcium has in triggering SNARE-mediated membrane fusion. We now demonstrate that whereas syntaxin and SNAP-25 in target membranes are freely available for SNARE complex formation, availability of synaptobrevin on synaptic vesicles is very limited. Calcium at micromolar concentrations triggers SNARE complex formation and fusion between synaptic vesicles and reconstituted target membranes. Although calcium does promote interaction of SNARE proteins between opposing membranes, it does not act by releasing synaptobrevin from synaptic vesicle restriction. Rather, our data suggest a mechanism in which calcium-triggered membrane apposition enables syntaxin and SNAP-25 to engage synaptobrevin, leading to membrane fusion.

The SNARE protein SNAP-25 is linked to fast calcium triggering of exocytosis

Synchronous neurotransmission depends on the tight coupling between Ca(2+) influx and fusion of neurotransmitter-filled vesicles with the plasma membrane. The vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein synaptobrevin 2 and the plasma membrane SNAREs syntaxin 1 and synaptosomal protein of 25 kDa (SNAP-25) are essential for calcium-triggered exocytosis. However, the link between calcium triggering and SNARE function remains elusive. Here we describe mutations in two sites on the surface of the SNARE complex formed by acidic and hydrophilic residues of SNAP-25 and synaptobrevin 2, which were found to coordinate divalent cations in the neuronal SNARE complex crystal structure. By reducing the net charge of the site in SNAP-25 we identify a mutation that interferes with calcium triggering of exocytosis when overexpressed in chromaffin cells. Exocytosis was elicited by photorelease of calcium from a calcium cage and evaluated by using patch-clamp capacitance measurements at millisecond time resolution. We present a method for monitoring the dependence of exocytotic rate upon calcium concentration at the release site and demonstrate that the mutation decreased the steepness of this relationship, indicating that the number of sequential calcium-binding steps preceding exocytosis is reduced by one. We conclude that the SNARE complex is linked directly to calcium triggering of exocytosis, most likely in a complex with auxiliary proteins.

Secretion and assembly of zona pellucida glycoproteins by growing mouse oocytes microinjected with epitope-tagged cDNAs for mZP2 and mZP3

The zona pellucida (ZP) is a highly organized extracellular coat that surrounds all mammalian eggs. The mouse egg ZP is composed of three glycoproteins, called mZP1-3, that are synthesized, secreted, and assembled into a ZP exclusively by growing oocytes. Here, we microinjected epitope-tagged (Myc and Flag) cDNAs for mZP2 and mZP3 into the germinal vesicle (nucleus) of growing oocytes isolated from juvenile mice. Specific antibodies and laser scanning confocal microscopy were used to follow nascent, recombinant ZP glycoproteins in both permeabilized and nonpermeabilized oocytes. When such cDNAs were injected, epitope-tagged mZP2 (Myc-mZP2) and mZP3 (Flag-mZP3) were synthesized, packaged into large intracellular vesicles, and secreted by the vast majority of oocytes. Secreted glycoproteins were incorporated into only the innermost layer of the thickening ZP, and the amount of nascent glycoprotein in this region increased with increasing time of oocyte culture. Consistent with prior observations, the putative transmembrane domain at the C terminus of mZP2 and mZP3 was missing from nascent glycoprotein incorporated into the ZP. When the consensus furin cleavage site near the C terminus of mZP3 was mutated, such that it should not be cleaved by furin, secretion and assembly of mZP3 was reduced. On the other hand, mZP3 incorporated into the ZP lacked the transmembrane domain downstream of the mutated furin cleavage site, suggesting that some other protease(s) excised the domain. These results strongly suggest that nascent mZP2 and mZP3 are incorporated into only the innermost layer of the ZP and that excision of the C-terminal region of the glycoproteins is required for assembly into the oocyte ZP.

Identification of syntaxin-1A sites of phosphorylation by casein kinase I and casein kinase II

Casein kinases I (CKI) are serine/threonine protein kinases widely expressed in a range of eukaryotes including yeast, mammals and plants. They have been shown to play a role in diverse physiological events including membrane trafficking. CKI alpha is associated with synaptic vesicles and phosphorylates some synaptic vesicle associated proteins including SV2. In this report, we show that syntaxin-1A is phosphorylated in vitro by CKI on Thr21. Casein kinase II (CKII) has been shown previously to phosphorylate syntaxin-1A in vitro and we have identified Ser14 as the CKII phosphorylation site, which is known to be phosphorylated in vivo. As syntaxin-1A plays a key role in the regulation of neurotransmitter release by forming part of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, we propose that CKI may play a role in synaptic vesicle exocytosis.

Three-dimensional structure of the complexin/SNARE complex

During neurotransmitter release, the neuronal SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 form a four-helix bundle, the SNARE complex, that pulls the synaptic vesicle and plasma membranes together possibly causing membrane fusion. Complexin binds tightly to the SNARE complex and is essential for efficient Ca(2+)-evoked neurotransmitter release. A combined X-ray and TROSY-based NMR study now reveals the atomic structure of the complexin/SNARE complex. Complexin binds in an antiparallel alpha-helical conformation to the groove between the synaptobrevin and syntaxin helices. This interaction stabilizes the interface between these two helices, which bears the repulsive forces between the apposed membranes. These results suggest that complexin stabilizes the fully assembled SNARE complex as a key step that enables the exquisitely high speed of Ca(2+)-evoked neurotransmitter release.

Vesicle targeting in hair cells

The mammalian hair cell has two distinct plasma membrane domains separated by tight junctions, the apical domain which contains the stereocilia and the basolateral domain which contains the presynaptic region. Little is known concerning the mechanisms that regulate vesicle trafficking to these two domains. Using SNAP 25 and syntaxin as baits, we carried out a yeast two-hybrid screen of the organ of Corti. We identified a novel syntaxin interacting protein, ocsyn, that is enriched in inner hair cells and concentrated at the apical pole. Our results are consistent with ocsyn playing a role in vesicle trafficking to the apical membrane of the hair cell.

Phosphorylation of cysteine string protein by protein kinase A. Implications for the modulation of exocytosis

Cyclic AMP-dependent protein kinase (PKA) enhances regulated exocytosis in neurons and most other secretory cells. To explore the molecular basis of this effect, known exocytotic proteins were screened for PKA substrates. Both cysteine string protein (CSP) and soluble NSF attachment protein-alpha (alpha-SNAP) were phosphorylated by PKA in vitro, but immunoprecipitation of cellular alpha-SNAP failed to detect (32)P incorporation. In contrast, endogenous CSP was phosphorylated in synaptosomes, PC12 cells, and chromaffin cells. In-gel kinase assays confirmed PKA to be a cellular CSP kinase, with phosphorylation occurring on Ser(10). PKA phosphorylation of CSP reduced its binding to syntaxin by 10-fold but had little effect on its interaction with HSC70 or G-protein subunits. Furthermore, an in vivo role for Ser(10) phosphorylation at a late stage of exocytosis is suggested by analysis of chromaffin cells transfected with wild type or non-phosphorylatable mutant CSP. We propose that PKA phosphorylation of CSP could modulate the exocytotic machinery, by selectively altering its availability for protein-protein interactions.

The ionic layer is required for efficient dissociation of the SNARE complex by alpha-SNAP and NSF

The four-helical bundle soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein receptor (SNARE) complex that mediates intracellular membrane fusion events contains a highly conserved ionic layer at the center of an otherwise hydrophobic core. This layer has an undetermined function; it consists of glutamine (Q) residues in syntaxin and the two synaptosomal-associated protein of 25 kDa (SNAP-25) family helices, and an arginine (R) in vesicle-associated membrane protein (a 3Q:1R ratio). Here, we show that the ionic-layer glutamine of syntaxin is required for efficient alpha-SNAP and NSF-mediated dissociation of the complex. When this residue is mutated, the SNARE complex still binds to alpha-SNAP and NSF and is released through ATP hydrolysis by NSF, but the complex no longer dissociates into SNARE monomers. Thus, one function of the ionic layer--in particular, the glutamine of syntaxin--is to couple ATP hydrolysis by NSF to the dissociation of the fusion complex. We propose that alpha-SNAP and NSF drive conformational changes at the ionic layer through specific interactions with the syntaxin glutamine, resulting in the dissociation of the SNARE complex.

Two distinct effects on neurotransmission in a temperature-sensitive SNAP-25 mutant

Vesicle fusion in eukaryotic cells is mediated by SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors). In neurons, the t-SNARE SNAP-25 is essential for synaptic vesicle fusion but its exact role in this process is unknown. We have isolated a SNAP-25 temperature-sensitive paralytic mutant in Drosophila, SNAP-25(ts). The mutation causes a Gly50 to Glu change in SNAP-25's first amphipathic helix. A similar mutation in the yeast homologue SEC9 also results in temperature sensitivity, implying a conserved role for this domain in secretion. In vitro-generated 70 kDa SNARE complexes containing SNAP-25(ts) are thermally stable but the mutant SNARE multimers (of approximately 120 kDa) rapidly dissociate at 37 degrees C. The SNAP-25(ts) mutant has two effects on neurotransmitter release depending upon temperature. At 22 degrees C, evoked release of neurotransmitter in SNAP-25(ts) larvae is greatly increased, and at 37 degrees C, the release of neurotransmitter is reduced as compared with controls. Our data suggest that at 22 degrees C the mutation causes the SNARE complex to be more fusion competent but, at 37 degrees C the same mutation leads to SNARE multimer instability and fusion incompetence.