Impairment of syntaxin by botulinum neurotoxin C1 or antibodies inhibits acetylcholine release but not Ca2+ channel activity

Neurotransmitter Release Site Replenishment and Presynaptic Plasticity

An action potential (AP) triggers neurotransmitter release from synaptic vesicles (SVs) docking to a specialized release site of presynaptic plasma membrane, the active zone (AZ). The AP simultaneously controls the release site replenishment with SV for sustainable synaptic transmission in response to incoming neuronal signals. Although many studies have suggested that the replenishment time is relatively slow, recent studies exploring high speed resolution have revealed SV dynamics with milliseconds timescale after an AP. Accurate regulation is conferred by proteins sensing Ca²⁺ entering through voltage-gated Ca²⁺ channels opened by an AP. This review summarizes how millisecond Ca²⁺ dynamics activate multiple protein cascades for control of the release site replenishment with release-ready SVs that underlie presynaptic short-term plasticity.

Presynaptic Calcium Channels

Presynaptic Ca²⁺ entry occurs through voltage-gated Ca²⁺ (Ca_V) channels which are activated by membrane depolarization. Depolarization accompanies neuronal firing and elevation of Ca²⁺ triggers neurotransmitter release from synaptic vesicles. For synchronization of efficient neurotransmitter release, synaptic vesicles are targeted by presynaptic Ca²⁺ channels forming a large signaling complex in the active zone. The presynaptic Ca_V2 channel gene family (comprising Ca_V2.1, Ca_V2.2, and Ca_V2.3 isoforms) encode the pore-forming α1 subunit. The cytoplasmic regions are responsible for channel modulation by interacting with regulatory proteins. This article overviews modulation of the activity of Ca_V2.1 and Ca_V2.2 channels in the control of synaptic strength and presynaptic plasticity.

Millisecond Ca2+ dynamics activate multiple protein cascades for synaptic vesicle control

For reliable transmission at chemical synapses, neurotransmitters must be released dynamically in response to neuronal activity in the form of action potentials. Stable synaptic transmission is dependent on the efficacy of transmitter release and the rate of resupplying synaptic vesicles to their release sites. Accurate regulation is conferred by proteins sensing Ca2+ entering through voltage-gated Ca2+ channels opened by an action potential. Presynaptic Ca2+ concentration changes are dynamic functions in space and time, with wide fluctuations associated with different rates of neuronal activity. Thus, regulation of transmitter release includes reactions involving multiple Ca2+-dependent proteins, each operating over a specific time window. Classically, studies of presynaptic proteins function favored large invertebrate presynaptic terminals. I have established a useful mammalian synapse model based on sympathetic neurons in culture. This review summarizes the use of this model synapse to study the roles of presynaptic proteins in neuronal activity for the control of transmitter release efficacy and synaptic vesicle recycling.

Fast, Ca2+-dependent exocytosis at nerve terminals: shortcomings of SNARE-based models

Investigations over the last two decades have made major inroads in clarifying the cellular and molecular events that underlie the fast, synchronous release of neurotransmitter at nerve endings. Thus, appreciable progress has been made in establishing the structural features and biophysical properties of the calcium (Ca2+) channels that mediate the entry into nerve endings of the Ca2+ ions that trigger neurotransmitter release. It is now clear that presynaptic Ca2+ channels are regulated at many levels and the interplay of these regulatory mechanisms is just beginning to be understood. At the same time, many lines of research have converged on the conclusion that members of the synaptotagmin family serve as the primary Ca2+ sensors for the action potential-dependent release of neurotransmitter. This identification of synaptotagmins as the proteins which bind Ca2+ and initiate the exocytotic fusion of synaptic vesicles with the plasma membrane has spurred widespread efforts to reveal molecular details of synaptotagmin's action. Currently, most models propose that synaptotagmin interfaces directly or indirectly with SNARE (soluble, N-ethylmaleimide sensitive factor attachment receptors) proteins to trigger membrane fusion. However, in spite of intensive efforts, the field has not achieved consensus on the mechanism by which synaptotagmins act. Concurrently, the precise sequence of steps underlying SNARE-dependent membrane fusion remains controversial. This review considers the pros and cons of the different models of SNARE-mediated membrane fusion and concludes by discussing a novel proposal in which synaptotagmins might directly elicit membrane fusion without the intervention of SNARE proteins in this final fusion step.

Kinetic organization of Ca2+ signals that regulate synaptic release efficacy in sympathetic neurons

Calcium regulation of neurotransmitter release is essential for maintenance of synaptic transmission. However, the temporal and spatial organization of Ca(2+) dynamics that regulate synaptic vesicle (SV) release efficacy in sympathetic neurons is poorly understood. Here, we investigate the N-type Ca(2+) channel-mediated kinetic structure of Ca(2+) regulation of cholinergic transmission of sympathetic neurons. We measured the effect of Ca(2+) chelation with fast 1,2-bis(2-aminophenoxy) ethane-tetraacetic acid (BAPTA) and slow ethyleneglycol-tetraacetic acid (EGTA) buffers on exocytosis, synaptic depression, and recovery of the readily releasable vesicle pool (RRP), after both single action potential (AP) and repetitive APs. Surprisingly, postsynaptic potentials peaking at ~12 milliseconds after the AP was inhibited by both rapid and slow Ca(2+) buffers suggests that, in addition to the well known fast Ca(2+) signals at the active zone (AZ), slow Ca(2+) signals at the peak of Ca(2+) entry also contribute to paired-pulse or repetitive AP responses. Following a single AP, discrete Ca(2+) transient increase regulated synaptic depression in rapid (<30-millisecond) and slow (<120-millisecond) phases. In contrast, following prolonged AP trains, synaptic depression was reduced by a slow Ca(2+) signal regulation lasting >200 milliseconds. Finally, after an AP burst, recovery of the RRP was mediated by an AP-dependent rapid Ca(2+) signal, and the expansion of releasable SV number by an AP firing activity-dependent slow Ca(2+) signal. These data indicate that local Ca(2+) signals operating near Ca(2+) sources in the AZ are organized into discrete fast and slow temporal phases that remodel exocytosis and short-term plasticity to ensure long-term stability in acetylcholine release efficacy.

The synaptic vesicle glycoprotein 2A ligand levetiracetam inhibits presynaptic Ca2+ channels through an intracellular pathway

Levetiracetam (LEV) is a prominent antiepileptic drug that binds to neuronal synaptic vesicle glycoprotein 2A protein and has reported effects on ion channels, but with a poorly defined mechanism of action. We investigated inhibition of voltage-dependent Ca(2+) (Ca(V)) channels as a potential mechanism through which LEV exerts effects on neuronal activity. We used electrophysiological methods to investigate the effects of LEV on cholinergic synaptic transmission and Ca(V) channel activity in superior cervical ganglion neurons (SCGNs). In parallel, we investigated the effects of the inactive LEV R-enantiomer, (R)-α-ethyl-2-oxo-1-pyrrolidine acetamide (UCB L060). LEV but not UCB L060 (each at 100 μM) inhibited synaptic transmission between SCGNs in long-term culture in a time-dependent manner, significantly reducing excitatory postsynaptic potentials after a ≥30-min application. In isolated SCGNs, LEV pretreatment (≥1 h) but not short-term application (5 min) significantly inhibited whole-cell Ba(2+) current (I(Ba)) amplitude. In current-clamp recordings, LEV reduced the amplitude of the afterhyperpolarizing potential in a Ca(2+)-dependent manner but also increased the action potential latency in a Ca(2+)-independent manner, which suggests additional mechanisms associated with reduced excitability. Intracellular LEV application (4-5 min) caused rapid inhibition of I(Ba) amplitude, to an extent comparable to that seen with extracellular LEV pretreatment (≥1 h). Neither pretreatment nor intracellular application of UCB L060 produced any inhibitory effects on I(Ba) amplitude. These results identify a stereospecific intracellular pathway through which LEV inhibits presynaptic Ca(V) channels; resultant reductions in neuronal excitability are proposed to contribute to the anticonvulsant effects of LEV.

Syntaxin 3B is essential for the exocytosis of synaptic vesicles in ribbon synapses of the retina

Ribbon synapses of the vertebrate retina are specialized synapses that release neurotransmitter by synaptic vesicle exocytosis in a manner that is proportional to the level of depolarization of the cell. This release property is different from conventional neurons, in which the release of neurotransmitter occurs as a short-lived burst triggered by an action potential. Synaptic vesicle exocytosis is a calcium regulated process that is dependent on a set of interacting synaptic proteins that form the so-called SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) complex. Syntaxin 3B has been identified as a specialized SNARE molecule in ribbon synapses of the rodent retina. However, the best physiologically-characterized neuron that forms ribbon-style synapses is the rod-dominant or Mb1 bipolar cell of the goldfish retina. We report here the molecular characterization of syntaxin 3B from the goldfish retina. Using a combination of reverse transcription (RT) polymerase chain reaction (PCR) and immunostaining with a specific antibody, we show that syntaxin 3B is highly enriched in the plasma membrane of bipolar cell synaptic terminals of the goldfish retina. Using membrane capacitance measurements we demonstrate that a peptide derived from goldfish syntaxin 3B inhibits synaptic vesicle exocytosis. These experiments demonstrate that syntaxin 3B is an important factor for synaptic vesicle exocytosis in ribbon synapses of the vertebrate retina.

Regulation of presynaptic Ca(V)2.1 channels by Ca2+ sensor proteins mediates short-term synaptic plasticity

Short-term synaptic plasticity shapes the postsynaptic response to bursts of impulses and is crucial for encoding information in neurons, but the molecular mechanisms are unknown. Here we show that activity-dependent modulation of presynaptic Ca(V)2.1 channels mediated by neuronal Ca(2+) sensor proteins (CaS) induces synaptic plasticity in cultured superior cervical ganglion (SCG) neurons. A mutation of the IQ-like motif in the C terminus that blocks Ca(2+)/CaS-dependent facilitation of the P/Q-type Ca(2+) current markedly reduces facilitation of synaptic transmission. Deletion of the nearby calmodulin-binding domain, which inhibits CaS-dependent inactivation, substantially reduces depression of synaptic transmission. These results demonstrate that residual Ca(2+) in presynaptic terminals can act through CaS-dependent regulation of Ca(V)2.1 channels to induce short-term synaptic facilitation and rapid synaptic depression. Activity-dependent regulation of presynaptic Ca(V)2.1 channels by CaS proteins may therefore be a primary determinant of short-term synaptic plasticity and information-processing in the nervous system.

A cholinergic model synapse to elucidate protein function at presynatic terminals

A large number of proteins have been identified at nerve terminals and a cascade of protein-protein interactions has been suggested to be involved in cycling of synaptic vesicle states. To explore protein function in presynaptic terminals, only a few unique synapses such as the squid giant synapse, the calyx of Held synapse and the hippocampal neuron autapse have been used. The squid giant synapse and the calyx of Held are useful to introduce reagents into their large presynaptic terminals and the hippocampal neuron autapse is a good system to modify a protein level by exogenous DNA or RNA. The cholinergic synapse formed between superior cervical ganglion (SCG) neurons in long-term culture is a useful model for a fast synapse. The axon of the large cell body contacts with soma of neighboring neurons. The architecture of synaptic connections makes it possible to introduce reagents into the presynaptic terminals by diffusion from a cell body within a short time. Introduction of exogenous cDNA or siRNA performed by microinjection into a SCG neuron allows us to modulate the level of the protein of interest or to express mutant proteins in the neuron. Here, we describe use of the model SCG neuronal synapse to elucidate function of presynaptic proteins in mediating synaptic transmission.

Syntaxin 1A occludes GABA(B) receptor-induced inhibition of exocytosis downstream of Ca(2+) entry in mouse hippocampal neurons

The mechanisms underlying gamma-amino butyric acid (GABA(B)) receptor-mediated inhibition of exocytosis have been characterized in a variety of synapses. Using patch-clamp recording methods, we attempted to clarify the intracellular mechanisms underlying presynaptic inhibition in autaptic synapses of isolated mouse hippocampal neurons. Baclofen, a selective GABA(B) receptor agonist, decreased the frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) without changing their amplitude in Ca(2+)-free extracellular solution, suggesting that baclofen inhibits exocytosis downstream of Ca(2+) entry. Syntaxin 1A is known to modulate exocytosis and suppress neuronal sprouting. Antisense oligonucleotide-mediated knockdown of syntaxin 1A increased the frequency of mEPSCs under Ca(2+)-free condition. Estimation of the number of functional release sites by staining with FM1-43 indicated that the increased frequency of mEPSCs was induced by facilitation of exocytosis at each site, rather than by an increased number of release sites due to neuronal sprouting. Baclofen reduced mEPSC frequency in syntaxin 1A-knockdown neurons to the same level as that in nonsense oligonucleotide transfected neurons under Ca(2+)-free condition. These results suggest that the GABA(B) receptor- and syntaxin 1A-induced inhibitions of exocytosis occlude one another and that the GABA(B) receptor shares a common intracellular pathway with syntaxin 1A in inhibiting transmitter release downstream of Ca(2+) entry.

SAD: a presynaptic kinase associated with synaptic vesicles and the active zone cytomatrix that regulates neurotransmitter release

A serine/threonine kinase SAD-1 in C. elegans regulates synapse development. We report here the isolation and characterization of mammalian orthologs of SAD-1, named SAD-A and SAD-B, which are specifically expressed in the brain. SAD-B is associated with synaptic vesicles and, like the active zone proteins CAST and Bassoon, is tightly associated with the presynaptic cytomatrix in nerve terminals. A short conserved region (SCR) in the COOH-terminus is required for the synaptic localization of SAD-B. Overexpression of SAD-B in cultured rat hippocampal neurons significantly increases the frequency of miniature excitatory postsynaptic current but not its amplitude. Introduction of SCR into presynaptic superior cervical ganglion neurons in culture significantly inhibits evoked synaptic transmission. Moreover, SCR decreases the size of the readily releasable pool measured by applying hypertonic sucrose. Furthermore, SAD-B phosphorylates the active zone protein RIM1 but not Munc13-1. These results suggest that mammalian SAD kinase presynaptically regulates neurotransmitter release.

Immunopharmacology--antibodies for specific modulation of proteins involved in neuronal function

The application of antibodies to living neurones has the potential to modulate function of specific proteins by virtue of their high specificity. This specificity has proven effective in determining the involvement of many proteins in neuronal function where specific agonists and antagonists do not exist, e.g. ion channel subunits. We discuss studies where antibodies modulate functions of voltage gated sodium, voltage gated potassium, voltage gated calcium hyperpolarisation activated cyclic nucleotide (HCN gated) and transient receptor potential (TRP) channels. Ligand gated channels studied in this way include nicotinic acetylcholine receptors, purinoceptors and GABA receptors. Antibodies have also helped reveal the involvement of different intracellular proteins in neuronal functions including G-proteins as well as other proteins involved in trafficking, phosphoinositide signalling and neurotransmitter release. Some suggestions for control experiments are made to help validate the method. We conclude that antibodies can be extremely valuable in determining the functions of specific proteins in living neurones in neuroscience research.

SV2A and SV2C contain a unique synaptotagmin-binding site

SV2 (Synaptic Vesicle Protein 2) is expressed in neurons and endocrine cells where it is required for normal calcium-evoked neurosecretion. In mammals, there are three SV2 genes, denoted SV2A, B and C. SV2A interacts with synaptotagmin, the prime candidate for the calcium sensor in exocytosis. Here, we report that all isoforms of native SV2 bind synaptotagmin and that binding is inhibited by calcium, indicating that all isoforms contain a common calcium-inhibited synaptotagmin-binding site. The isolated amino termini of SV2A and SV2C supported an additional interaction with synaptotagmin, and binding at this site was stimulated by calcium. The amino-terminal binding site was mapped to the first 57 amino acids of SV2A, and removal of this domain decreased calcium-mediated inhibition of binding to synaptotagmin, suggesting that it modulates calcium's effect on the SV2-synaptotagmin interaction. Introduction of the amino terminus of SV2A or SV2C into cultured superior cervical ganglion neurons inhibited neurotransmission, whereas the amino terminus of SV2B did not. These observations implicate the SV2-synaptotagmin interaction in regulated exocytosis and suggest that SV2A and SV2C, via their additional synaptotagmin binding site, function differently than SV2B.

G protein {beta}{gamma} subunits mediate presynaptic inhibition of transmitter release from rat superior cervical ganglion neurones in culture

The activation of presynaptic G protein-coupled receptors (GPCRs) is widely reported to inhibit transmitter release; however, the lack of accessibility of many presynaptic terminals has limited direct analysis of signalling mediators. We studied GPCR-mediated inhibition of fast cholinergic transmission between superior cervical ganglion neurones (SCGNs) in culture. The adrenoceptor agonist noradrenaline (NA) caused a dose-related reduction in evoked excitatory postsynaptic potentials (EPSPs). NA-induced EPSP decrease was accompanied by effects on the presynaptic action potential (AP), reducing AP duration and amplitude of the after-hyperpolarization (AHP), without affecting the pre- and postsynaptic membrane potential. All effects of NA were blocked by yohimbine and synaptic transmission was reduced by clonidine, consistent with an action at presynaptic alpha2-adrenoceptors. NA-induced inhibition of transmission was sensitive to pre-incubation of SCGNs with pertussis toxin (PTX), implicating the involvement of Galpha(i/o)betagamma subunits. Expression of Galpha transducin, an agent which sequesters G protein betagamma (Gbetagamma) subunits, in the presynaptic neurone caused a time-dependent attenuation of NA-induced inhibition. Injection of purified Gbetagamma subunits into the presynaptic neurone inhibited transmission, and also reduced the AHP amplitude. Furthermore, NA-induced inhibition was occluded by pre-injection of Gbetagamma subunits. The Ca(2+) channel blocker Cd(2+) mimicked NA effects on transmitter release. Cd(2+), NA and Gbetagamma subunits also inhibited somatic Ca(2+) current. In contrast to effects on AP-evoked transmitter release, NA had no clear action on AP-independent EPSPs induced by hypertonic solutions. These results demonstrate that Gbetagamma subunits functionally mediate inhibition of transmitter release by alpha2-adrenoceptors and represent important regulators of synaptic transmission at mammalian presynaptic terminals.

Requirement for the synaptic protein interaction site for reconstitution of synaptic transmission by P/Q-type calcium channels

Ca(v)2.1 channels, which conduct P/Q-type Ca(2+) currents, were expressed in superior cervical ganglion neurons in cell culture, and neurotransmission initiated by these exogenously expressed Ca(2+) channels was measured. Deletions in the synaptic protein interaction (synprint) site in the intracellular loop between domains II and III of Ca(v)2.1 channels reduced their effectiveness in synaptic transmission. Surprisingly, this effect was correlated with loss of presynaptic localization of the exogenously expressed channels. Ca(v)1.2 channels, which conduct L-type Ca(2+) currents, are ineffective in supporting synaptic transmission, but substitution of the synprint site from Ca(v)2.1 channels in Ca(v)1.2 was sufficient to establish synaptic transmission initiated by L-type Ca(2+) currents through the exogenous Ca(v)1.2 channels. Substitution of the synprint site from Ca(v)2.2 channels, which conduct N-type Ca(2+) currents, was even more effective than Ca(v)2.1. Our results show that localization and function of exogenous Ca(2+) channels in nerve terminals of superior cervical ganglion neurons require a functional synprint site and suggest that binding of soluble NSF attachment protein receptor (SNARE) proteins to the synprint site is a necessary permissive event for nerve terminal localization of presynaptic Ca(2+) channels.

Subtype-selective reconstitution of synaptic transmission in sympathetic ganglion neurons by expression of exogenous calcium channels

Fast cholinergic neurotransmission between superior cervical ganglion neurons (SCGNs) in cell culture is initiated by N-type Ca(2+) currents through Ca(v)2.2 channels. To test the ability of different Ca(2+)-channel subtypes to initiate synaptic transmission in these cells, SCGNs were injected with cDNAs encoding Ca(v)1.2 channels, which conduct L-type currents, Ca(v)2.1 channels, which conduct P/Q-type Ca(2+) currents, and Ca(v)2.3 channels, which conduct R-type Ca(2+) currents. Exogenously expressed Ca(v)2.1 channels were localized in nerve terminals, as assessed by immunocytochemistry with subtype-specific antibodies, and these channels effectively initiated synaptic transmission. Injection with cDNA encoding Ca(v)2.3 channels yielded a lower level of presynaptic labeling and synaptic transmission, whereas injection with cDNA encoding Ca(v)1.2 channels resulted in no presynaptic labeling and no synaptic transmission. Our results show that exogenously expressed Ca(2+) channels can mediate synaptic transmission in SCGNs and that the specificity of reconstitution of neurotransmission (Ca(v)2.1 > Ca(v)2.3 >> Ca(v)1.2) follows the same order as in neurons in vivo. The specificity of reconstitution of neurotransmission parallels the specificity of trafficking of these Ca(v) channels to nerve terminals.

SNAP-29: a general SNARE protein that inhibits SNARE disassembly and is implicated in synaptic transmission

Using the yeast two-hybrid system with syntaxin-1A as bait, we isolated soluble NSF attachment protein (SNAP)-29 from a human brain cDNA library. Synaptosomal fractionation and immunocytochemical staining of hippocampal neurons in culture showed that SNAP-29 is present at synapses and is predominantly associated with synaptic vesicles. The interaction of SNAP-29 with syntaxin-1 was further confirmed with immunoprecipitation analysis. Binding competition studies with SNAP-29 demonstrated that it could compete with alpha-SNAP for binding to synaptic SNAP receptors (SNAREs) and consequently inhibit disassembly of the SNARE complex. Introduction of SNAP-29 into presynaptic superior cervical ganglion neurons in culture significantly inhibited synaptic transmission in an activity-dependent manner. Although SNAP-29 has been suggested to be a general SNARE component in membrane trafficking, our findings suggest that it may function as a regulator of SNARE complex disassembly and modulate the process of postfusion recycling of the SNARE components.

SNARE proteins contribute to calcium cooperativity of synaptic transmission

A hallmark of calcium-triggered synaptic transmission is the cooperative relationship between calcium and the amount of transmitter released. This relationship is thought to be important for improving the efficiency of synaptic vesicle exocytosis. Although it is generally held that cooperativity arises from the interaction of multiple calcium ions with a single calcium-sensing molecule, the precise molecular basis of this phenomenon is not known. The SNARE proteins are known to be critical for synaptic vesicle exocytosis. We therefore tested for a contribution of SNARE proteins to cooperativity by genetically reducing the levels of syntaxin IA and neuronal-synaptobrevin in Drosophila. Surprisingly, we found that reducing these SNARE proteins also reduced Ca(2+) cooperativity. Thus, SNARE proteins are important for determining the cooperative relationship between calcium and synaptic transmission.

Differential contribution of syntaxin 1 and SNAP-25 to secretion in noradrenergic and adrenergic chromaffin cells

We used botulinum neurotoxins (BoNT) to examine whether differences in the secretory activity of noradrenergic and adrenergic chromaffin cells are related to differences in the exocytotic machinery of these two types of bovine adrenal medulla cells. Cleavage of syntaxin and SNAP-25 by BoNT/C1 decreased in a dose-dependent way the release of both noradrenaline and adrenaline, but noradrenaline release was more sensitive to BoNT/C1. Cleavage of SNAP-25 by BoNT/A also had a larger inhibitory effect on noradrenaline release than on adrenaline release. Neither BoNT/C1 nor BoNT/A affected the intracellular Ca2+ responses induced by K+-depolarisation, and the extent of the inhibition of K+-evoked catecholamine release by selective blockers of voltage-gated Ca2+ channels was not affected by BoNT/C1. Therefore, our data do not support the hypothesis of a regulatory effect of syntaxin or SNAP-25 on the activity of Ca2+ channels. The lower sensitivity of adrenaline release to BoNT was not due to a reduced ability of the toxins to enter or to cleave their protein targets in adrenergic cells, since immunoblot analysis showed the cleavage of a larger fraction of syntaxin 1A in adrenergic cells, as compared to the cleavage in noradrenergic cells. The immunoblot analysis also showed larger amounts of syntaxin 1A in noradrenergic chromaffin cells than in adrenergic cells. Thus, in spite of a greater cleavage of syntaxin 1A in adrenergic cells by BoNT/C1, adrenaline release was less sensitive to BoNT/C1, suggesting that the release process in noradrenergic cells might be more dependent on syntaxin 1A and SNAP-25, as compared to adrenergic cells.

Syntaxin modulation of slow inactivation of N-type calcium channels

Syntaxin, a membrane protein vital in triggering vesicle fusion, interacts with voltage-gated N- and P/Q-type Ca(2+) channels. This biochemical association is proposed to colocalize Ca(2+) channels and presynaptic release sites, thus supporting rapid and efficient initiation of neurotransmitter release. The syntaxin channel interaction may also support a novel signaling function, to modulate Ca(2+) channels according to the state of the associated release machinery (Bezprozvanny et al., 1995; Wiser et al., 1996; see also Mastrogiacomo et al., 1994). Here we report that syntaxin 1A (syn1A) coexpressed with N-type channels in Xenopus oocytes greatly promoted slow inactivation gating, but had little or no effect on the onset of and recovery from fast inactivation. Accordingly, the effectiveness of syntaxin depended strongly on voltage protocol. Slow inactivation was found for N-type channels even in the absence of syntaxin and could be distinguished from fast inactivation on the basis of its slow kinetics, distinct voltage dependence (voltage-independent at potentials higher than the level of half-inactivation), and temperature independence (Q(10), approximately 0.8). Trains of action potential-like stimuli were more effective than steady depolarizations in stabilizing the slowly inactivated condition. Agents that stimulate protein kinase C decreased the inhibitory effect of syntaxin on N-type channels. Application of BoNtC1 to cleave syntaxin sharply attenuated the modulatory effects on Ca(2+) channel gating, consistent with structural analysis of syntaxin modulation, supporting use of this toxin to test for the impact of syntaxin on Ca(2+) influx in nerve terminals.

Syntaxin modulation of calcium channels in cortical synaptosomes as revealed by botulinum toxin C1

When the presynaptic membrane protein syntaxin is coexpressed in Xenopus oocytes with N- or P/Q-type Ca(2+) channels, it promotes their inactivation (Bezprozvanny et al., 1995; Wiser et al., 1996, 1999; Degtiar et al., 2000) (I. B. Bezprozvanny, P. Zhong, R. H. Scheller, and R. W. Tsien, unpublished observations). These findings led to the hypothesis that syntaxin influences Ca(2+) channel function in presynaptic endings, in a reversal of the conventional flow of information from Ca(2+) channels to the release machinery. We examined this effect in isolated mammalian nerve terminals (synaptosomes). Botulinum neurotoxin type C1 (BoNtC1), which cleaves syntaxin, was applied to rat neocortical synaptosomes at concentrations that completely blocked neurotransmitter release. This treatment altered the pattern of Ca(2+) entry monitored with fura-2. Whereas the initial Ca(2+) rise induced by depolarization with K(+)-rich solution was unchanged, late Ca(2+) entry was strongly augmented by syntaxin cleavage. Similar results were obtained when Ca(2+) influx arose from repetitive firing induced by the K(+)-channel blocker 4-aminopyridine. Cleavage of vesicle-associated membrane protein with BoNtD or SNAP-25 with BoNtE failed to produce a significant change in Ca(2+) entry. The BoNtC1-induced alteration in Ca(2+) signaling was specific to voltage-gated Ca(2+) channels, not Ca(2+) extrusion or buffering, and it involved N-, P/Q- and R-type channels, the high voltage-activated channels most intimately associated with presynaptic release machinery. The modulatory effect of syntaxin was not immediately manifest when synaptosomes had been K(+)-predepolarized in the absence of external Ca(2+), but developed with a delay after admission of Ca(2+), suggesting that vesicular turnover may be necessary to make syntaxin available for its stabilizing effect on Ca(2+) channel inactivation.

How botulinum and tetanus neurotoxins block neurotransmitter release

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

Syntaphilin: a syntaxin-1 clamp that controls SNARE assembly

Syntaxin-1 is a key component of the synaptic vesicle docking/fusion machinery that forms the SNARE complex with VAMP/synaptobrevin and SNAP-25. Identifying proteins that modulate SNARE complex formation is critical for understanding the molecular mechanisms underlying neurotransmitter release and its modulation. We have cloned and characterized a protein called syntaphilin that is selectively expressed in brain. Syntaphilin competes with SNAP-25 for binding to syntaxin-1 and inhibits SNARE complex formation by absorbing free syntaxin-1. Transient overexpression of syntaphilin in cultured hippocampal neurons significantly reduces neurotransmitter release. Furthermore, introduction of syntaphilin into presynaptic superior cervical ganglion neurons in culture inhibits synaptic transmission. These findings suggest that syntaphilin may function as a molecular clamp that controls free syntaxin-1 availability for the assembly of the SNARE complex, and thereby regulates synaptic vesicle exocytosis.

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

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

The concept of a calcium sensor in transmitter release

The discovery was made in the 1940s that calcium is required for transmitter release at synapses, raising the question of the identity of the sensor molecule upon which this calcium acts. Subsequently it was shown in the 1960s that this calcium acts on the inside of the nerve terminal. The channels which mediate the influx of calcium ions into the nerve terminal were identified in the 1970s. This essay is concerned with tracing the development of the concept of a calcium sensor in nerve terminals and of recent work that identifies the sensor molecule as synaptotagmin.

Antibodies against cysteine string proteins inhibit evoked neurotransmitter release at Xenopus neuromuscular junctions

Cysteine string proteins (CSPs) are evolutionarily conserved proteins that are associated with synaptic vesicles and other regulated secretory organelles. To investigate the role of CSPs in vertebrate neuromuscular transmission, we introduced anti-CSP antibodies into the cell bodies of Xenopus spinal motor neurons that form synapses with embryonic muscle cells in culture. These antibodies produced a rapid (within 3-6 min), and in most cases complete, inhibition of stimulus-dependent neurotransmitter secretion. However, spontaneous neurotransmitter release was stable (both in frequency and amplitude) throughout the period of antibody exposure. Several control experiments validated the specificity of the anti-CSP antibody effects. First, the anti-CSP antibody actions were not mimicked either by antibodies against another synaptic vesicle protein SV2, or by nonspecific immunoglobins. Second, heat treatment of the anti-CSP antibodies eliminated their effect on evoked secretion. Third, immunoblot experiments showed that the anti-CSP and anti-SV2 antibodies were highly selective for their respective antigens in these Xenopus cultures. We conclude from these results that CSPs are vital constituents of the pathway for regulated neurotransmitter release in vertebrates. Moreover, the selective inhibition of evoked, but not spontaneous transmitter release by anti-CSP antibodies indicates that there is a fundamental difference in the machinery that mediates these secretory processes.

Interactions of presynaptic Ca2+ channels and snare proteins in neurotransmitter release

N- and P/Q-type Ca2+ channels are localized in high density in presynaptic nerve terminals and are crucial elements in neuronal excitation-secretion coupling. In addition to mediating Ca2+ entry to initiate transmitter release, they are thought to interact directly with proteins of the synaptic vesicle docking/fusion machinery. These Ca2+ channels can be purified from brain as a complex with SNARE proteins, which are involved in exocytosis. In addition, N-type and P/Q-type Ca2+ channels are colocalized with syntaxin in high-density clusters in nerve terminals. The synaptic protein interaction (synprint) sites in the intracellular loop II-III (LII-III) of both alpha 1B and alpha 1A subunits of N-type and P/Q-type Ca2+ channels bind to syntaxin, SNAP-25, and synaptotagmin. Ca2+ has a biphasic effect on the interactions of N-type Ca2+ channels with SNARE complexes, stimulating optimal binding in the range of 10-30 microM. PKC or CaM KII phosphorylation of the N-type synprint peptide inhibits interactions with SNARE complexes containing syntaxin and SNAP-25. Introduction of the synprint peptides into presynaptic superior cervical ganglion neurons reversibly inhibits EPSPs from synchronous transmitter release by 42%. At physiological Ca2+ concentrations, synprint peptides significantly reduce transmitter release in injected frog neuromuscular junctions in cell culture, consistent with detachment of 70% of the docked vesicles from Ca2+ channels as analyzed by a theoretical model. Together, these studies suggest that presynaptic Ca2+ channels not only provide the Ca2+ signal required by the exocytotic machinery, but also contain structural elements that are integral to vesicle docking, priming, and fusion processes.

Calcium transients and neurotransmitter release at an identified synapse

It is widely accepted that the modulation of the presynaptic Ca2+ influx is one of the main mechanisms by which neurotransmitter release can be controlled. The well-identified cholinergic synapse in the buccal ganglion of Aplysia has been used to study the modulations that affect presynaptic Ca2+ transients and to relate this to quantal evoked neurotransmitter release. Three types of Ca2+ channel (L, N and P) are present in the presynaptic neurone at this synapse. Influxes of Ca2+ through N- and P-type channels trigger the release of ACh with only N-type Ca2+ channels being regulated by presynaptic neuromodulator receptors. In addition, presynaptic Ca2+ stores, via complex mechanisms of Ca2+ uptake and Ca2+ release, control the Ca2+ concentration that triggers this evoked ACh release.

Snapin: a SNARE-associated protein implicated in synaptic transmission

Synaptic vesicle docking and fusion are mediated by the assembly of a stable SNARE core complex of proteins, which include the synaptic vesicle membrane protein VAMP/synaptobrevin and the plasmalemmal proteins syntaxin and SNAP-25. We have now identified another SNAP-25-binding protein, called Snapin. Snapin was enriched in neurons and exclusively located on synaptic vesicle membranes. It associated with the SNARE complex through direct interaction with SNAP-25. Binding of recombinant Snapin-CT to SNAP-25 blocked the association of the SNARE complex with synaptotagmin. Introduction of Snapin-CT and peptides containing the SNAP-25 binding sequence into presynaptic superior cervical ganglion neurons in culture reversibly inhibited synaptic transmission. These results suggest that Snapin is an important component of the neurotransmitter release process through its modulation of the sequential interactions between the SNAREs and synaptotagmin.

Structure and function of neuronal Ca2+ channels and their role in neurotransmitter release

Electrophysiological studies of neurons reveal different Ca2+ currents designated L-, N-, P-, Q-, R-, and T-type. High-voltage-activated neuronal Ca2+ channels are complexes of a pore-forming alpha 1 subunit of about 190-250 kDa, a transmembrane, disulfide-linked complex of alpha 2 and delta subunits, and an intracellular beta subunit, similar to the alpha 1, alpha 2 delta, and beta subunits previously described for skeletal muscle Ca2+ channels. The primary structures of these subunits have all been determined by homology cDNA cloning using the corresponding subunits of skeletal muscle Ca2+ channels as probes. In most neurons, L-type channels contain alpha 1C or alpha 1D subunits, N-type contain alpha 1B subunits, P- and Q-types contain alternatively spliced forms of alpha 1A subunits, R-type contain alpha 1E subunits, and T-type contain alpha 1G or alpha 1H subunits. Association with different beta subunits also influences Ca2+ channel gating substantially, yielding a remarkable diversity of functionally distinct molecular species of Ca2+ channels in neurons.

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.

Presynaptic injection of syntaxin-specific antibodies blocks transmission in the squid giant synapse

A polyclonal antibody, raised against the squid (Loligo pealei) syntaxin I, inhibited Ca2+-dependent interaction of syntaxin with synaptotagmin C2A domain in vitro. Presynaptic injection of the anti-Loligo syntaxin IgG into the squid giant synapse blocked synaptic transmission without affecting the presynaptic action potential or the voltage-gated calcium current responsible for transmitter release. Repetitive presynaptic stimulation produced a gradual decrease in the amplitude of the postsynaptic potential as the synaptic block progressed, indicating that the antibody interferes with vesicular fusion. Confocal microscopy of the fluorescein-labelled anti-Loligo syntaxin IgG showed binding at the synaptic active zone, while ultrastructurally, an increase in synaptic vesicular numbers in synapses blocked when this antibody was observed. These results implicate syntaxin in the vesicular fusion step of transmitter release in concert with synaptotagmin.

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

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

Presynaptic facilitation of synaptic transmission in the hippocampus

Biochemical and genetic characterization of proteins in presynaptic axon terminals have led to models of the biochemical pathways underlying synaptic vesicle docking, activation, and fusion. Several studies have attempted recently to assign a precise physiological role to these proteins. This review deals with some of these studies, concentrating on those performed with hippocampal synapses. It is shown that changes in the state of these presynaptic proteins, together with modifications in Ca2+ dynamics in axon terminals, functionally determine the level of basal synaptic transmission, and underlie pharmacologically induced and activity-dependent facilitation of transmitter release in the central nervous system.

Endogenous syntaxins 2, 3 and 4 exhibit distinct but overlapping patterns of expression at the hepatocyte plasma membrane

To investigate the mechanisms regulating polarized vesicle delivery to the cell surface in hepatocytes, we have characterized the endogenous plasma membrane (PM)-associated syntaxins. These integral membrane proteins are components of the membrane docking/fusion apparatus and are thought to function as vesicle receptors at the PM. In hepatocytes, the PM is divided into two domains, the apical and basolateral. If syntaxins are mediating the specific recognition of vesicles delivered to either membrane surface, the simple prediction is that each domain expresses one syntaxin isoform. However, we report that rat hepatocytes express three endogenous PM-associated syntaxin isoforms, syntaxins 2, 3 and 4. By biochemical subfractionation, we determined that the syntaxins exhibit distinct, but overlapping patterns of expression among the PM domains. Syntaxin 4 is primarily expressed at the basolateral surface while syntaxins 2 and 3 are enriched at the apical PM. The immunolocalization of syntaxins 2 and 4 in rat hepatocytes and PM sheets revealed similarly complex patterns of PM expression with enhanced apical staining for both. A significant proportion of syntaxin 3 (25%) was detected in subcellular fractions containing transport vesicles. We have used quantitative immunoblotting to determine that the syntaxins are relatively abundant PM molecules (11-260 nM) in rat liver, spleen and kidney. Also, we determined that the syntaxin binding protein, Munc-18, is present at concentrations from 1.5-20 nM in the same tissues. Although this fundamental quantitative and morphological information is lacking in other systems, it is critical not only for defining syntaxin function, but also for predicting the specific mechanisms that regulate vesicle targeting in hepatocytes and other tissues.

Disruption of syntaxin-mediated protein interactions blocks neurotransmitter secretion

The membrane protein syntaxin participates in several protein-protein interactions that have been implicated in neurotransmitter release. To probe the physiological importance of these interactions, we microinjected into the squid giant presynaptic terminal botulinum toxin C1, which cleaves syntaxin, and the H3 domain of syntaxin, which mediates binding to other proteins. Both reagents inhibited synaptic transmission yet did not affect the number or distribution of synaptic vesicles at the presynaptic active zone. Recombinant H3 domain inhibited the interactions between syntaxin and SNAP-25 that underlie the formation of stable SNARE complexes in vitro. These data support the notion that syntaxin-mediated SNARE complexes are necessary for docked synaptic vesicles to fuse.

Ca2+ or Sr2+ partially rescues synaptic transmission in hippocampal cultures treated with botulinum toxin A and C, but not tetanus toxin

Botulinum (BoNT/A-G) and tetanus toxins (TeNT) are zinc endopeptidases that cleave proteins associated with presynaptic terminals (SNAP-25, syntaxin, or VAMP/synaptobrevin) and block neurotransmitter release. Treatment of hippocampal slice cultures with BoNT/A, BoNT/C, BoNT/E, or TeNT prevented the occurrence of spontaneous or miniature EPSCs (sEPSCs or mEPSCs) as well as the [Ca2+]o-independent increase in their frequency induced by phorbol ester, 0.5 nM alpha-latrotoxin, or sucrose. [Ca2+]o-independent and -dependent release thus requires that the target proteins of clostridial neurotoxins be uncleaved. In contrast, significant increases in mEPSC frequency were produced in BoNT-treated, but not TeNT-treated, cultures by application of the Ca2+ ionophore ionomycin in the presence of 10 mM [Ca2+]o. The frequency of sEPSCs was increased in BoNT-treated, but not TeNT-treated, cultures by increasing [Ca2+]o from 2.8 to 5-10 mM or by applying 5 mM Sr2+. Large Ca2+ and Sr2+ influxes thus can rescue release after BoNT treatment, albeit less than in control cultures. The nature of the toxin-induced modification of Ca2+-dependent release was assessed by recordings from monosynaptically coupled CA3 cell pairs. The paired-pulse ratio of unitary EPSCs evoked by two presynaptic action potentials in close succession was 0.5 in control cultures, but it was 1.4 and 1.2 in BoNT/A- or BoNT/C-treated cultures when recorded in 10 mM [Ca2+]o. Log-log plots of unitary EPSC amplitude versus [Ca2+]o were shifted toward higher [Ca2+]o in BoNT/A- or BoNT/C-treated cultures, but their slope was unchanged and the maximal EPSC amplitudes were reduced. We conclude that BoNTs reduce the Ca2+ sensitivity of the exocytotic machinery and the number of quanta released.

Receptors controlling transmitter release from sympathetic neurons in vitro

Primary cultures of postganglionic sympathetic neurons were established more than 30 years ago. More recently, these cultures have been used to characterize various neurotransmitter receptors that govern sympathetic transmitter release. These receptors may be categorized into at least three groups: (1) receptors which evoke transmitter release: (2) receptors which facilitate; (3) receptors which inhibit, depolarization-evoked release. Group (1) comprises nicotinic and muscarinic acetylcholine receptors, P2X purinoceptors and pyrimidinoceptors. Group (2) currently harbours beta-adrenoceptors, P2 purinoceptors, receptors for PACAP and VIP, as well as prostanoid EP1 receptors. In group (3), muscarinic cholinoceptors, alpha 2- and beta-adrenoceptors, P2 purinoceptors, and receptors for the neuropeptides NPY, somatostatin (SRIF1) and LHRH, as well as opioid (delta and kappa) receptors can be found. Receptors which regulate transmitter release from neurons in cell culture may be located either at the somatodendritic region or at the sites of exocytosis, i.e. the presynaptic specializations of axons. Most of the receptors that evoke release are located at the soma. There ionotropic receptors cause depolarizations to generate action potentials which then trigger Ca(2+)-dependent exocytosis at axon terminals. The signalling mechanisms of metabotropic receptors which evoke release still remain to be identified. Receptors which facilitate depolarization-evoked release appear to be located preferentially at presynaptic sites and presumably act via an increase in cyclic AMP. Receptors which inhibit stimulation evoked release are also presynaptic origin and most commonly rely on a G protein-mediated blockade of voltage-gated Ca2+ channels. Results obtained with primary cell cultures of postganglionic sympathetic neurons have now supplemented previous data about neurotransmitter receptors involved in the regulation of ganglionic as well as sympatho-effector transmission. In the future, this technique may prove useful to identify yet unrecognized receptors which control the output of the sympathetic nervous system and to elucidate underlying signalling mechanisms.

Cleavage of syntaxin prevents G-protein regulation of presynaptic calcium channels

Neurotransmitter release into the synapse is stimulated by calcium influx through ion channels that are closely associated with the transmitter release sites. This link may involve the membrane protein syntaxin, which is known to be associated with the release sites and to bind to the calcium channels. There is evidence that presynaptic calcium channels are downregulated by second messenger pathways involving G proteins. Here we use the patch-clamp technique to test whether calcium current is regulated by G proteins in a vertebrate presynaptic nerve terminal, and whether this regulation is affected by the linkage to syntaxin. The calcium current in the nerve terminal showed typical G-protein-mediated changes in amplitude and activation kinetics which were reversed by a preceding depolarization. These effects of the G protein were virtually eliminated if syntaxin was first cleaved with botulinum toxin C1. Our findings indicate that this sensitivity of the current to modulation by G proteins requires the association of the presynaptic calcium channel with elements of the transmitter release site, which may ensure that channels tethered at release sites are preferentially regulated by the G-protein second messenger pathway.

Enhancement of neurite-sprouting by suppression of HPC-1/syntaxin 1A activity in cultured vertebrate nerve cells

HPC-1/syntaxin 1A is a C-terminal anchored neuronal membrane protein, of which all of the N-terminal regions are located on the intracellular side, and it interacts with presynaptic membrane proteins, synaptic vesicle proteins and soluble N-ethylmaleimide-sensitive fusion protein attachment proteins (SNAPs). HPC-1/syntaxin 1A has been proposed to act as a target SNAP receptor (t-SNARE) in the neuron and contributes to the vesicle docking/fusion process during the fast exocytosis at the presynaptic active zone. However, studies using an electron-microscope revealed that HPC-1/syntaxin 1A distributed not only at the presynaptic region but throughout the whole axonal membrane, and the functions of this axonal HPC-1/syntaxin 1A remain completely unknown. To investigate its physiological role, we attempted to inhibit the function of HPC-1/syntaxin 1A in cultured neural cells by following two methods. First, de novo synthesis of HPC-1/syntaxin 1A was inhibited by an application of antisense oligonucleotide in cultured adult rat dorsal root ganglion (DRG) neurons. Second, antibody against HPC-1/syntaxin 1A was applied intra-axonally in the cultured chick retinal ganglion neuron. Both treatments, which were expected to downregulate the function of HPC-1/syntaxin 1A, consistently elicited an enhancement of the axonal sprouting. These results suggest that the axonal HPC-1/syntaxin 1A would physiologically suppress the excess axon-collateral sprouting. Downregulation of HPC-1/syntaxin 1A expression may underlie the control of collateral sprouting and synapse formation during development and memory processes.

Inhibition of neurotransmission by peptides containing the synaptic protein interaction site of N-type Ca2+ channels

N-type Ca2+ channels bind directly to the synaptic core complex of VAMP/synaptobrevin, syntaxin, and SNAP-25. Peptides containing the synaptic protein interaction ("synprint") site caused dissociation of N-type Ca2+ channels from the synaptic core complex. Introduction of synprint peptides into presynaptic superior cervical ganglion neurons reversibly inhibited synaptic transmission. Fast EPSPs due to synchronous transmitter release were inhibited, while late EPSPs arising from asynchronous release following a train of action potentials were increased and paired-pulse facilitation was increased. The corresponding peptides from L-type Ca2+ channels had no effect, and the N-type peptides had no effect on Ca2+ currents through N-type Ca2+ channels. These results are consistent with the hypothesis that binding of the synaptic core complex to presynaptic N-type Ca2+ channels is required for Ca2+ influx to elicit rapid, synchronous neurotransmitter release.

Growth cone collapse and inhibition of neurite growth by Botulinum neurotoxin C1: a t-SNARE is involved in axonal growth

The growth cone is responsible for axonal growth, where membrane expansion is most likely to occur. Several recent reports have suggested that presynaptic proteins are involved in this process; however, the molecular mechanism details are unclear. We suggest that by cleaving a presynaptic protein syntaxin, which is essential in targeting synaptic vesicles as a target SNAP receptor (t-SNARE), neurotoxin C1 of Clostridium botulinum causes growth cone collapse and inhibits axonal growth. Video-enhanced microscopic studies showed (a) that neurotoxin C1 selectively blocked the activity of the central domain (the vesicle-rich region) at the initial stage, but not the lamellipodia in the growth cone; and (b) that large vacuole formation occurred probably through the fusion of smaller vesicles from the central domain to the most distal segments of the neurite. The total surface area of the accumulated vacuoles could explain the membrane expansion of normal neurite growth. The gradual disappearance of the surface labeling by FITC-WGA on the normal growth cone, suggesting membrane addition, was inhibited by neurotoxin C1. The experiments using the peptides derived from syntaxin, essential for interaction with VAMP or alpha-SNAP, supported the results using neurotoxin C1. Our results demonstrate that syntaxin is involved in axonal growth and indicate that syntaxin may participate directly in the membrane expansion that occurs in the central domain of the growth cone, probably through association with VAMP and SNAPs, in a SNARE-like way.

Role of myosin in neurotransmitter release: functional studies at synapses formed in culture

To determine the functional role of presynaptic proteins in the neurotransmitter release, I have employed cholinergic synapses formed between superior cervical ganglion neurons in culture. These synapses expressed proteins characteristic of mature synapses: immunofluorescence staining showed the presence of synaptophysin, synaptotagmin, VAMP/synaptobrevin-2, syntaxin and neurexin. The function of these proteins seems to be similar to that of mature synapses because botulinum neurotoxins A, E and C1 inhibited neurotransmitter release evoked by presynaptic action potentials. With this preparation, I have obtained evidence supporting roles for myosin II and myosin light chain kinase in neurotransmitter secretion. Acetylcholine release was inhibited by introduction of antibody against myosin II or inhibitors of myosin light chain kinase. This evidence suggests a model in which myosin light chain kinase phosphorylates myosin, and the resultant change in actin-myosin interactions is involved in some steps of transmitter release.