Transmission at the squid giant synapse was blocked by tetanus toxin by affecting synaptobrevin, a vesicle-bound protein

Single synapse glutamate imaging reveals multiple levels of release mode regulation in mammalian synapses

Mammalian central synapses exhibit vast heterogeneity in signaling strength. To understand the extent of this diversity, how it is achieved, and its functional implications, characterization of a large number of individual synapses is required. Using glutamate imaging, we characterized the evoked release probability and spontaneous release frequency of over 24,000 individual synapses. We found striking variability and no correlation between action potential-evoked and spontaneous synaptic release strength, suggesting distinct regulatory mechanisms. Subpixel localization of individual evoked and spontaneous release events reveals tight spatial regulation of evoked release and enhanced spontaneous release outside of evoked release region. Using on-stage post hoc immune-labeling of vesicle-associated proteins, Ca²⁺-sensing proteins, and soluble presynaptic proteins we were able to show that distinct molecular ensembles are associated with evoked and spontaneous modes of synaptic release.

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

Synaptic activity regulates the abundance and binding of complexin

Nervous system function relies on precise chemical communication between neurons at specialized junctions known as synapses. Complexin (CPX) is one of a small number of cytoplasmic proteins that are indispensable in controlling neurotransmitter release through SNARE and synaptic vesicle interactions. However, the mechanisms that recruit and stabilize CPX are poorly understood. The mobility of CPX tagged with photoactivatable green fluorescent protein (pGFP) was quantified in vivo using Caenorhabditis elegans. Although pGFP escaped the synapse within seconds, CPX-pGFP displayed both fast and slow decay components, requiring minutes for complete exchange of the synaptic pool. The longer synaptic residence time of CPX arose from both synaptic vesicle and SNARE interactions, and surprisingly, CPX mobility depended on synaptic activity. Moreover, mouse CPX-GFP reversibly dispersed out of hippocampal presynaptic terminals during stimulation, and blockade of vesicle fusion prevented CPX dispersion. Hence, synaptic CPX can rapidly redistribute and this exchange is influenced by neuronal activity, potentially contributing to use-dependent plasticity.

Inhibition of exocytosis or endocytosis blocks activity-dependent redistribution of synapsin

The synaptic vesicle cycle encompasses the pre-synaptic events that drive neurotransmission. Influx of calcium leads to the fusion of synaptic vesicles with the plasma membrane and the release of neurotransmitter, closely followed by endocytosis. Vacated release sites are repopulated with vesicles which are then primed for release. When activity is intense, reserve vesicles may be mobilized to counteract an eventual decline in transmission. Recently, interplay between endocytosis and repopulation of the readily releasable pool of vesicles has been identified. In this study, we show that exo-endocytosis is necessary to enable detachment of synapsin from reserve pool vesicles during synaptic activity. We report that blockage of exocytosis in cultured mouse hippocampal neurons, either by tetanus toxin or by the deletion of munc13, inhibits the activity-dependent redistribution of synapsin from the pre-synaptic terminal into the axon. Likewise, perturbation of endocytosis with dynasore or by a dynamin dominant-negative mutant fully prevents synapsin redistribution. Such inhibition of synapsin redistribution occurred despite the efficient phosphorylation of synapsin at its protein kinase A/CaMKI site, indicating that disengagement of synapsin from the vesicles requires exocytosis and endocytosis in addition to phosphorylation. Our results therefore reveal hitherto unidentified feedback within the synaptic vesicle cycle involving the synapsin-managed reserve pool.

Synaptic structural modification following changes in activity induced by tetanus neurotoxin in cat abducens neurons

A low or a high dose of tetanus neurotoxin (TeNT) injected in the lateral rectus muscle of the cat causes respectively, functional block of inhibitory synapses only or of both inhibitory and excitatory synapses simultaneously in abducens neurons (González-Forero et al. [2003] J. Neurophysiol. 89:1878-1890). As a consequence, neuronal firing activity increases (at low dose) or decreases (at high dose). We investigated possible structural modifications of inhibitory synapses in response to these activity alterations induced by TeNT. We used immunofluorescence against postsynaptic (gephyrin) and presynaptic (vesicular gamma-aminobutyric acid [GABA] transporter [VGAT]) markers of inhibitory synapses in combination with cell type markers for abducens motoneurons (calcitonin gene-related peptide or choline acetyltransferase) or internuclear neurons (calretinin). Seven days after high-dose treatment, the number of gephyrin-immunoreactive (IR) clusters per 100 microm of membrane perimeter was reduced on the soma of abducens motoneurons by 55.3% and by 60.1% on internuclear neurons. Proximal dendritic gephyrin-IR clusters were also significantly altered but to a lesser degree. Partial synaptic re-establishment was observed 15 days post injection, and complete recovery occurred after 42 days. Coverage by VGAT-IR terminals was reduced in parallel. In contrast, a low dose of TeNT caused no structural alterations. With electron microscopy we estimated that overall synaptic coverage was reduced by 40% in both types of neurons after a high dose of TeNT. However, F-type terminals with postsynaptic gephyrin were preferentially lost. Thus, the ratio between F and S terminals diminished from 1.28 to 0.39 on motoneurons and from 1.26 to 0.47 on internuclear neurons. These results suggest that the maintenance of proximal inhibitory synaptic organization on central neurons is best related to neuronal activity and not to the level of inhibitory synaptic function, which was equally blocked by the high or low dose of TeNT.

Calcium-dependent acetylcholine release from Xenopus oocytes: simultaneous ionic currents and acetylcholine release recordings

The fusion of synaptic vesicles with presynaptic membranes is controlled by a complex network of protein-protein and protein-lipid interactions. SNAP-25, syntaxin and synaptobrevin (SNARE complex) are thought to participate in the formation of the core of the membrane fusion machine but the molecular basis of SNARE interactions is not completely understood. Thus, it would be interesting to design experiments to test those relationships in a new model. Xenopus laevis oocytes are valuable tools for studying the molecular structure and function of ionic channels and neurotransmitter receptors. Here we show that SNARE proteins are present in native Xenopus oocytes and that those oocytes injected with acetylcholine and presynaptic plasma membranes extracted from the electric organ of Torpedo marmorata assume some of the functions of a cholinergic nerve terminal. Neurotransmitter release and macroscopic currents were recorded and analysed simultaneously in a single oocyte electrically depolarized: acetylcholine release was detected using a chemiluminiscent method and calcium entry was measured by exploiting the endogenous Ca2+-activated chloride current of the oocyte with a two-electrode voltage-clamp system. Neurotransmitter release was calcium- and voltage-dependent and partially reduced in the presence of several calcium channel blockers. Clostridial neurotoxins, both holotoxin and injected light-chain forms, also inhibited acetylcholine release. We also studied the role of the SNARE complex in synaptic transmission and membrane currents by using monoclonal antibodies against SNAP-25, syntaxin or VAMP/synaptobrevin. The use of antibodies against VAMP/synaptobrevin, SNAP-25 and syntaxin inhibited acetylcholine release, as did clostridial toxins. However, macroscopic currents were only modified either by syntaxin antibody or by Botulinium-C1 neurotoxin. This model constitutes a new approach for understanding the vesicle exocytosis processes.

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.

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.

Calcium can disrupt the SNARE protein complex on sea urchin egg secretory vesicles without irreversibly blocking fusion

The homotypic fusion of sea urchin egg cortical vesicles (CV) is a system in which to correlate the biochemistry and physiology of membrane fusion. Homologues of vesicle-associated membrane protein (VAMP), syntaxin, and SNAP-25 were identified in CV membranes. A VAMP and syntaxin immunoreactive band at a higher apparent molecular mass (approximately 70 kDa) was detected; extraction and analysis confirmed that the band contained VAMP, SNAP-25, and syntaxin. This complex was also identified by immunoprecipitation and by sucrose gradient analysis. VAMP in the complex was insensitive to proteolysis by tetanus toxin. All criteria identify the SNARE complex as that described in other secretory systems. Complexes exist pre-formed on individual CV membranes and form between contacting CV. Most notably, CV SNARE complexes are disrupted in response to [Ca2+]free that trigger maximal fusion. N-Ethylmaleimide, which blocks fusion at or before the Ca2+-triggering step, blocks complex disruption by Ca2+. However, disruption is not blocked by lysophosphatidylcholine, which transiently arrests a late stage of fusion. Since removal of lysophosphatidylcholine from Ca2+-treated CV is known to allow fusion, complex disruption occurs independently from the membrane fusion step. As Ca2+ disrupts rather than stabilizes the complex, the presumably coiled-coil SNARE interactions are not needed at the time of fusion. These findings rule out models of fusion in which SNARE complex formation goes to completion ("zippers-up") after Ca2+ binding removes a "fusion-clamp."

Different VAMP/synaptobrevin complexes for spontaneous and evoked transmitter release at the crayfish neuromuscular junction

Different VAMP/synaptobrevin complexes for spontaneous and evoked transmitter release at the crayfish neuromuscular junction. J. Neurophysiol. 80: 3233-3246, 1998. Although vesicle-associated membrane protein (VAMP/synaptobrevin) is essential for evoked neurotransmitter release, its role in spontaneous transmitter release remains uncertain. For instance, many studies show that tetanus toxin (TeNT), which cleaves VAMP, blocks evoked transmitter release but leaves some spontaneous transmitter release. We used recombinant tetanus and botulinum neurotoxin catalytic light chains (TeNT-LC, BoNT/B-LC, and BoNT/D-LC) to examine the role of VAMP in spontaneous transmitter release at neuromuscular junctions (nmj) of crayfish. Injection of TeNT-LC into presynaptic axons removed most of the VAMP immunoreactivity and blocked evoked transmitter release without affecting nerve action potentials or Ca2+ influx. The frequency of spontaneous transmitter release was little affected by the TeNT-LC when the evoked transmitter release had been blocked by >95%. The spontaneous transmitter release left after TeNT-LC treatment was insensitive to increases in intracellular Ca2+. BoNT/B-LC, which cleaves VAMP at the same site as TeNT-LC but uses a different binding site, also blocked evoked release but had minimal effect on spontaneous release. However, BoNT/D-LC, which cleaves VAMP at a different site from the other two toxins but binds to the same position on VAMP as TeNT, blocked both evoked and spontaneous transmitter release at similar rates. The data indicate that different VAMP complexes are employed for evoked and spontaneous transmitter release; the VAMP used in spontaneous release is not readily cleaved by TeNT or BoNT/B. Because the exocytosis that occurs after the action of TeNT cannot be increased by increased intracellular Ca2+, the final steps in neurotransmitter release are Ca2+ independent.

Block of transmitter release by botulinum C1 action on syntaxin at the squid giant synapse

Electrophysiological, morphological, and biochemical approaches were combined to study the effect of the presynaptic injection of the light chain of botulinum toxin C1 into the squid giant synapse. Presynaptic injection was accompanied by synaptic block that occurred progressively as the toxin filled the presynaptic terminal. Neither the presynaptic action potential nor the Ca2+ currents in the presynaptic terminal were affected by the toxin. Biochemical analysis of syntaxin moiety in squid indicates that the light chain of botulinum toxin C1 lyses syntaxin in vitro, suggesting that this was the mechanism responsible for synaptic block. Ultrastructure of the injected synapses demonstrates an enormous increase in the number of presynaptic vesicles, suggesting that the release rather than the docking of vesicles is affected by biochemical lysing of the syntaxin molecule.

Inhibition of transmitter release correlates with the proteolytic activity of tetanus toxin and botulinus toxin A in individual cultured synapses of Hirudo medicinalis

We have studied the effects of tetanus toxin and botulinus toxin A on neurotransmitter release in the Retzius-->P-cell synapse of the leech and exploited the unique properties of this system, which allow for combined physiological and biochemical analyses in single-cell pairs. The sequences of Hirudo medicinalis synaptobrevin and synaptosomal-associated protein of 25 kDa (SNAP-25), deduced by cDNA cloning, are 61 and 55% identical, respectively, to their corresponding mammalian homologs. Whereas Hirudo synaptobrevin is proteolyzed by tetanus toxin, its SNAP-25 isoform is resistant to botulinus toxin A cleavage because of amino acid substitutions within and around the putative cleavage site. In close correlation, microinjection of tetanus toxin into the presynaptic neuron produced a block of transmitter release, whereas botulinus toxin A had no effect on synaptic transmission. Subsequent immunoblotting of single-cell pairs demonstrated directly that the tetanus toxin-mediated block of exocytosis is accompanied by cleavage of synaptobrevin in the injected neuron, resulting in the generation of a detectable C-terminal cleavage product. Immunoblotting also confirmed the resistance of SNAP-25 to botulinus toxin A cleavage in vivo. Using recombinant proteins, we show that the N-terminal fragment of synaptobrevin released by tetanus toxin, but not its C-terminal membrane-anchored cleavage product, participates with syntaxin and SNAP-25 in synaptic SNAP receptor (SNARE) ternary complex formation in Hirudo. Our data demonstrate a direct correlation between the inhibition of transmitter release and the ability of the neurotoxin to proteolyze its target protein and support the view that SNARE ternary complex formation is an important step leading to synaptic vesicle exocytosis.

Inhibition of transmitter release correlates with the proteolytic activity of tetanus toxin and botulinus toxin A in individual cultured synapses of Hirudo medicinalis

We have studied the effects of tetanus toxin and botulinus toxin A on neurotransmitter release in the Retzius-->P-cell synapse of the leech and exploited the unique properties of this system, which allow for combined physiological and biochemical analyses in single-cell pairs. The sequences of Hirudo medicinalis synaptobrevin and synaptosomal-associated protein of 25 kDa (SNAP-25), deduced by cDNA cloning, are 61 and 55% identical, respectively, to their corresponding mammalian homologs. Whereas Hirudo synaptobrevin is proteolyzed by tetanus toxin, its SNAP-25 isoform is resistant to botulinus toxin A cleavage because of amino acid substitutions within and around the putative cleavage site. In close correlation, microinjection of tetanus toxin into the presynaptic neuron produced a block of transmitter release, whereas botulinus toxin A had no effect on synaptic transmission. Subsequent immunoblotting of single-cell pairs demonstrated directly that the tetanus toxin-mediated block of exocytosis is accompanied by cleavage of synaptobrevin in the injected neuron, resulting in the generation of a detectable C-terminal cleavage product. Immunoblotting also confirmed the resistance of SNAP-25 to botulinus toxin A cleavage in vivo. Using recombinant proteins, we show that the N-terminal fragment of synaptobrevin released by tetanus toxin, but not its C-terminal membrane-anchored cleavage product, participates with syntaxin and SNAP-25 in synaptic SNAP receptor (SNARE) ternary complex formation in Hirudo. Our data demonstrate a direct correlation between the inhibition of transmitter release and the ability of the neurotoxin to proteolyze its target protein and support the view that SNARE ternary complex formation is an important step leading to synaptic vesicle exocytosis.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Tetanus toxin inhibits spontaneous quantal release and cleaves VAMP/synaptobrevin

Tetanus toxin decreased the frequency of spontaneous events at the electric organ of Torpedo marmorata. This reduction was up to 70% in poisoned electric organ. According to distribution analysis of miniature end plate currents, only a subpopulation of events which have small amplitudes were recorded after poisoning. Furthermore, isolated cholinergic nerve terminals showed a decrease in VAMP/synaptobrevin when poisoned with tetanus toxin under similar conditions. The relationship between the two effects of the toxin, i.e. inhibition of vesicle exocytosis and peptidase activity on synaptobrevin, is discussed.

A syntaxin-related protein controls acetylcholine release by different mechanisms in Aplysia

Polyclonal antibodies raised against rat syntaxin-1B and an affinity-purified fraction have been used to study the functional role of this protein in transmitter release from Aplysia neurons. In a ganglionic protein extract, this fraction recognized a 37,000 molecular weight protein which therefore might be the Aplysia homologue of rat brain syntaxin-1B. Immunoglobulins were injected in the presynaptic cell of an identified cholinergic synapse of the buccal ganglion of Aplysia californica. This treatment decreased the postsynaptic response due to a reduction of the number of quanta released in relation to a decline of presynaptic Ca2+ current. When antibodies were applied extracellularly, transmitter release also decreased. In contrast to intracellular injection, this reduction was not accompanied by a decrease of the Ca2+ current but by an increase of presynaptic outward current. When injected in the presynaptic neuron, syntaxin complementary RNA also depressed Ca2+ current and transmission. This work provides evidence that Aplysia neurons express a syntaxin-like protein which is involved in the control of the presynaptic Ca2+ influx triggering acetylcholine release from terminals. This protein appears to have an extracellular segment which might interact with outward current.

Inhibition of neurotransmitter release by synthetic proline-rich peptides shows that the N-terminal domain of vesicle-associated membrane protein/synaptobrevin is critical for neuro-exocytosis

Tetanus toxin and clostridial neurotoxins type B, D, F, and G inhibit intracellular Ca(2+)-dependent neurotransmitter release via the specific proteolytic cleavage of vesicle-associated membrane protein (VAMP)/synaptobrevin, a highly conserved 19-kDa integral protein of the small synaptic vesicle membrane. This results in the release of the larger part of the cytosolic domain of this synaptic protein into the cytoplasm. Microinjection of synthetic peptides corresponding to this fragment into identified presynaptic neurons of Aplysia californica led to a potent, long lasting, and dose-dependent inhibition (approximately 50% at 10 MicroM) of acetylcholine release, probably by hindering endogenous VAMP/synaptobrevin from interacting with synaptic proteins involved in exocytosis. Structure activity studies showed that this effect is confined to the N-terminal domain of VAMP/synaptobrevin isoform II and is related to the presence of a proline-rich motif (PGGPXGX3PP or PAAPXGX3PP). At higher concentrations, the inhibitory effect was lower and only transient, suggesting that the N-terminal proline-rich domain of VAMP/synaptobrevin plays opposing roles in neurotransmitter release very likely by interacting with different synaptic proteins. This probably occurs by disruption of the recently reported in vitro VAMP-synaptophysin interaction that involves the N-terminal domain of VAMP II and was proposed to hinder synatophysin-related formation of a fusion pore. The observed recovery of neurotransmitter release following injection of high concentration of N-terminal fragments of VAMP II brings a strong in vivo support to this hypothesis. The minimum active peptide GPGGPQGGMQPPREQS could be used for rationally designing potent synthetic blockers of neurotransmission.

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.