Inhibition of neurotransmitter release by clostridial neurotoxins correlates with specific proteolysis of synaptosomal proteins

Rat brain synaptosomes were used to study the effect of several clostridial neurotoxins on the neurotransmitter release. In this system the blockade of transmitter release correlated with the proteolytic activity of the toxins. Blockade of glutamate release was linked to selective proteolysis of one of the following synaptic proteins: synaptobrevin (BoNT/D, BoNT/F); SNAP-25 (BoNT/A, BoNT/E), or HPC-1/syntaxin (BoNT/C1). All the toxins used had an inhibitory effect on synaptosomes with the exception of BoNT/F. BoNT/F cleaved synaptobrevin in permeabilized synaptosomes but failed to produce the same effect on intact synaptosomes.

Botulinum neurotoxin C1 blocks neurotransmitter release by means of cleaving HPC-1/syntaxin

The anaerobic bacterium Clostridium botulinum produces several related neurotoxins that block exocytosis of synaptic vesicles in nerve terminals and that are responsible for the clinical manifestations of botulism. Recently, it was reported that botulinum neurotoxin type B as well as tetanus toxin act as zinc-dependent proteases that specifically cleave synaptobrevin, a membrane protein of synaptic vesicles (Link et al., Biochem. Biophys. Res. Commun., 189, 1017-1023; Schiavo et al., Nature, 359, 832-835). Here we report that inhibition of neurotransmitter release by botulinum neurotoxin type C1 was associated with the proteolysis of HPC-1 (= syntaxin), a membrane protein present in axonal and synaptic membranes. Breakdown of HPC-1/syntaxin was selective since no other protein degradation was detectable. In vitro studies showed that the breakdown was due to a direct interaction between HPC-1/syntaxin and the toxin light chain which acts as a metallo-endoprotease. Toxin-induced cleavage resulted in the generation of a soluble fragment of HPC-1/syntaxin that is 2-4 kDa smaller than the native protein. When HPC-1/syntaxin was translated in vitro, cleavage occurred only when translation was performed in the presence of microsomes, although a full-length product was obtained in the absence of membranes. However, susceptibility to toxin cleavage was restored when the product of membrane-free translation was subsequently incorporated into artificial proteoliposomes. In addition, a translated form of HPC-1/syntaxin, which lacked the putative transmembrane domain at the C-terminus, was soluble and resistant to toxin action. We conclude that HPC-1/syntaxin is involved in exocytotic membrane fusion.(ABSTRACT TRUNCATED AT 250 WORDS)

An anion binding site that regulates the glutamate transporter of synaptic vesicles

Glutamate, the major excitatory neurotransmitter of the mammalian central nervous system, is stored in synaptic vesicles and released by exocytosis upon depolarization of the presynaptic nerve terminal. Synaptic vesicles possess an active glutamate-specific transporter that is driven by an electrochemical proton gradient across the vesicle membrane and requires chloride for maximal activity. In this study, we have characterized the role of chloride in vesicular glutamate transport using 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), a potent inhibitor of anion translocators. DIDS inhibited glutamate uptake with an IC50 of 0.7 microM or less. In contrast, all energy gradient parameters (membrane potential, pH gradient, and ATPase activity) required at least 5-fold higher concentration of DIDS for inhibition. Furthermore, high concentrations of chloride but not of glutamate or other anions prevented DIDS inhibition of glutamate uptake. In contrast to uptake, glutamate efflux from glutamate-loaded vesicles was independent of chloride over a wide concentration range. However, efflux was still susceptible to DIDS inhibition. DIDS inhibition was prevented by excess chloride. We conclude that the vesicular glutamate transporter possesses a DIDS-sensitive chloride binding site on the cytoplasmic side, distinct from the substrate binding site, which regulates transport activity.

Relative properties and localizations of synaptic vesicle protein isoforms: the case of the synaptophysins

Synaptophysins are abundant synaptic vesicle proteins present in two forms: synaptophysin, also referred to as synaptophysin I (abbreviated syp I), and synaptoporin, also referred to as synaptophysin II (abbreviated syp II). In the present study, the properties and localizations of syp I and syp II were investigated to shed light on their relative functions. Our results reveal that syp II, similar to syp I, is an abundant, N-glycosylated membrane protein that is part of a heteromultimeric complex in synaptic vesicle membranes. Cross-linking studies indicate that syp II is linked to a low-molecular-weight protein in this complex as has been observed before for syp I. Furthermore, after transfection into CHO cells, syp II, similar to syp I, is targeted to the receptor-mediated endocytosis pathway. Immunocytochemistry of rat brain sections reveals that syp II expression is highly heterogeneous, with high concentrations of syp II only in selected neuronal populations, whereas syp I is more homogeneously expressed in most nerve terminals. In general, nerve terminals expressing syp II also express syp I. In addition to high levels of syp II observed in selected neurons, a rostrocaudal gradient of syp II expression was observed in the cerebellar cortex. Immunoelectron microscopy confirmed that syp II is localized to synaptic vesicles. Immunoprecipitations of synaptic vesicles from rat brain with antibodies to syp I demonstrated that syp II is colocalized with syp I on the same vesicles. However, after detergent solubilization, no coimmunoprecipitations of the two proteins were observed, suggesting that they are not complexed with each other although they are on the same vesicles. Together our results demonstrate that syp I and syp II have similar properties and are present on the same synaptic vesicles but do not coassemble. The presence of the two proteins in the same nerve terminal suggests that they have similar but nonidentical functions and that the relative abundance of the two proteins may contribute to the functional heterogeneity of nerve terminals.

Expression of endogenous NMDAR1 transcripts without receptor protein suggests post-transcriptional control in PC12 cells

Expression of RNA for the NMDAR1 subunit of the N-methyl-D-aspartate receptor was detected by Northern hybridization in both nerve growth factor-differentiated and undifferentiated rat pheochromocytoma (PC12) cells. The NMDA receptor type 1 (NMDAR1) message in PC12 cells was similar in size to that expressed in hippocampal neurons. PC12 cell cDNAs that were amplified by polymerase chain reaction with primers flanking the coding region of NMDAR1 corresponded to the NMDAR1 splice variant NMDA receptor type 1 isoform C (NMDAR1C). Using calcium imaging or patch-clamp recording, no functional NMDA-gated ion channels were found in PC12 cells. A monoclonal antibody against NMDAR1 was developed in order to investigate whether or not NMDAR1 protein was present in PC12 cells. Only trace amounts of NMDAR1 protein were found in native PC12 cells. However, expression of NMDAR1 protein was detected in PC12 cells that were transfected with an expression vector containing an NMDAR1C clone under control of a cytomegalovirus promoter. These findings suggest that the expression of NMDAR1 protein in PC12 cells may be controlled by post-transcriptional mechanisms. The PC12 cell line may serve as a model system for the study of the transcriptional, post-transcriptional, and translational regulation of NMDAR1. Furthermore, the presence of NMDAR1 RNA in a particular cell type may not necessarily indicate expression of NMDAR1 protein.

Botulinum neurotoxin A selectively cleaves the synaptic protein SNAP-25

Neurotransmitter release is potently blocked by a group of structurally related toxin proteins produced by Clostridium botulinum. Botulinum neurotoxin type B (BoNT/B) and tetanus toxin (TeTx) are zinc-dependent proteases that specifically cleave synaptobrevin (VAMP), a membrane protein of synaptic vesicles. Here we report that inhibition of transmitter release from synaptosomes caused by botulinum neurotoxin A (BoNT/A) is associated with the selective proteolysis of the synaptic protein SNAP-25. Furthermore, isolated or recombinant L chain of BoNT/A cleaves SNAP-25 in vitro. Cleavage occurred near the carboxyterminus and was sensitive to divalent cation chelators. In addition, a glutamate residue in the BoNT/A L chain, presumably required to stabilize a water molecule in the zinc-containing catalytic centre, was required for proteolytic activity. These findings demonstrate that BoNT/A acts as a zinc-dependent protease that selectively cleaves SNAP-25. Thus, a second component of the putative fusion complex mediating synaptic vesicle exocytosis is targeted by a clostridial neurotoxin.

Cleavage of cellubrevin by tetanus toxin does not affect fusion of early endosomes

Tetanus toxin is a potent inhibitor of neurotransmitter release, which acts as an intracellular metalloendoprotease that selectively cleaves synaptobrevin, a major membrane protein of synaptic vesicles. Recently, synaptobrevin has been found to form an ATP-dependent complex with N-ethylmaleimide-sensitive fusion protein (NSF) and soluble NSF attachment protein, which are known to function in endosome fusion. Furthermore, a highly homologous isoform of synaptobrevin, named cellubrevin, was identified that is expressed in virtually all tissues in the endocytic pathway and is cleaved by tetanus toxin light chain in vitro, suggesting that cellubrevin may have a general function in intracellular fusion events. In the present study, we have analyzed whether cleavage of cellubrevin by tetanus toxin influences the ATP-dependent, N-ethylmaleimide-sensitive fusion of early endosomes in vitro. Our results show that endosome fusion is not affected by tetanus toxin although cellubrevin is almost completely proteolyzed, suggesting that the function of NSF in endosome fusion does not involve cellubrevin.

Synaptic vesicle proteins and early endosomes in cultured hippocampal neurons: differential effects of Brefeldin A in axon and dendrites

The pathways of synaptic vesicle (SV) biogenesis and recycling are still poorly understood. We have studied the effects of Brefeldin A (BFA) on the distribution of several SV membrane proteins (synaptophysin, synaptotagmin, synaptobrevin, p29, SV2 and rab3A) and on endosomal markers to investigate the relationship between SVs and the membranes with which they interact in cultured hippocampal neurons developing in isolation. In these neurons, SV proteins are detected as punctate immunoreactivity that is concentrated in axons but is also present in perikarya and dendrites. In the same neurons, the transferrin receptor, a well established marker of early endosomes, is selectively concentrated in perikarya and dendrites. In the perikaryal-dendritic region, BFA induced a dramatic tubulation of transferrin receptors as well as a cotubulation of the bulk of synaptophysin. Synaptotagmin, synaptobrevin, p29 and SV2 immunoreactivities retained a primarily punctate distribution. No tubulation of rab3A was observed. In axons, BFA did not produce any obvious alteration of the distribution of SV proteins, nor of peroxidase- or Lucifer yellow-labeled early endosomes. The selective effect of BFA on dendritic membranes suggests the existence of functional differences between the endocytic systems in dendrites and axons. Cotubulation of transferrin receptors and synaptophysin in the perikaryal-dendritic region is consistent with a functional interconnection between the traffic of SV proteins and early endosomes. The heterogeneous effects of BFA on SV proteins in this cell region indicates that SV proteins are differentially sorted upon exit from the TGN and are coassembled into SVs at the cell periphery.

Synaptotagmin: a membrane constituent of neuropeptide-containing large dense-core vesicles

Synaptotagmin is known to be a major membrane protein of synaptic vesicles (SVs) in neurons. We have now used an immunoisolation procedure to demonstrate that synaptotagmin is also present in the membranes of peptide containing large dense-core vesicles (LDCVs) of rat hypothalamus and bovine posterior pituitary. Synaptotagmin bead-immunoisolated organelles from these tissues primarily consisted of SVs but contained occasionally larger structures reminiscent of LDCVs that were absent from vesicle populations immunoisolated with a synaptophysin antibody. Furthermore, the vesicles immunoisolated with synaptotagmin beads contained significant amounts of neuropeptide Y (NPY). In contrast, vesicles immunoisolated with synaptophysin beads did not contain detectable levels of NPY. Sucrose density gradient fractionation of postnuclear supernatants obtained from the bovine posterior pituitary resulted in a bimodal distribution of synaptotagmin, corresponding to the positions of both SVs and neurosecretory granules. A similar distribution was found for cytochrome b561 and the 116 kDa subunit of the vacuolar proton pump. In contrast, the SV proteins synaptophysin, SV2, and p29 were restricted to the SV-containing fractions. Immunoisolation of small and large vesicles from the sucrose gradient confirmed the differential distribution of synaptotagmin and synaptophysin in the two types of secretory vesicles in nerve endings of the posterior pituitary. We conclude that synaptotagmin is a constituent of both SVs and peptide-containing secretory vesicles in the nervous system. Since both types of organelles undergo Ca(2+)-dependent exocytosis, these findings support a general role of synaptotagmin as an exocytotic Ca2+ receptor.

Cellubrevin is a ubiquitous tetanus-toxin substrate homologous to a putative synaptic vesicle fusion protein

Tetanus toxin inhibits neurotransmitter release by selectively blocking fusion of synaptic vesicles. Recently tetanus toxin was shown to proteolytically degrade synaptobrevin II (also named VAMP-2), a synaptic vesicle-specific protein, in vitro and in nerve terminals. As targets of tetanus toxin, synaptobrevins probably function in the exocytotic fusion of synaptic vesicles. Here we describe a new synaptobrevin homologue, cellubrevin, that is present in all cells and tissues tested and demonstrate that it is a membrane trafficking protein of a constitutively recycling pathway. Like synaptobrevin II, cellubrevin is proteolysed by tetanus toxin light chain in vitro and after transfection. Our results suggest that constitutive and regulated vesicular pathways use homologous proteins for membrane trafficking, probably for membrane fusion at the plasma membrane, indicating a greater mechanistic and evolutionary similarity between these pathways than previously thought.

Structure of the murine rab3A gene: correlation of genomic organization with antibody epitopes

Rab3A is a neuronal low-molecular-mass GTP-binding protein that is modified post-translationally by two geranylgeranyl groups and specifically targeted to synaptic vesicles. We have now cloned and characterized the murine gene coding for rab3A. With a size of less than 8 kb including the promoter, the rab3A gene is relatively small. It contains five exons, the first of which is non-coding. The organization of the rab3A coding sequence into exons in the gene is different from that of ras proteins, the only other low-molecular-mass GTP-binding proteins with currently characterized gene structures. Nevertheless, the intron placement in the primary structure of rab3A may be indicative of a domain division of the protein, since each coding exon contains one of the four major conserved rab protein sequence motifs. The epitopes of monoclonal and polyclonal antibodies to rab3A were mapped with the hypothesis that antibody epitopes might represent distinct exposed protein domains and correlate with exon structures. Two monoclonal antibodies, named 42.1 and 42.2, were found to recognize epitopes with a different degree of conservation between different rab3 isoforms. These epitopes were mapped to relatively short amino acid sequences corresponding to exons 4 and 5 respectively, whereas a polyclonal antibody recognized a complex epitope that required the presence of intact rab3A. Comparison of the sequence of rab3A with that of ras, whose crystal structure has been determined, revealed that the epitopes for the monoclonal antibodies correspond to regions in ras that are highly exposed. Taken together, these results suggest that exons 4 and 5 at least represent distinct exposed protein domains that also form major natural epitopes in rab3A.

A gamma-aminobutyric acid transporter driven by a proton pump is present in synaptic-like microvesicles of pancreatic beta cells

A variety of peptide-secreting endocrine cells contain a population of recycling microvesicles that share several major membrane polypeptides with neuronal synaptic vesicles (SVs). The function of these synaptic-like microvesicles (SLMVs) remains to be elucidated. It was previously suggested that SLMVs of pancreatic beta cells may store and secrete gamma-aminobutyric acid (GABA). GABA, the major nonpeptide inhibitory neurotransmitter of the central nervous system, is stored in and secreted from SVs. GABA uptake into SVs is mediated by a transporter that is driven by a vacuolar proton ATPase. GABA is also present at high concentration in the endocrine pancreas where it is selectively localized in insulin-secreting beta cells, the core cells of pancreatic islets. GABA is not present in peripheral islet cells (mantle cells), represented primarily by glucagon-secreting alpha cells. In this study, an immunoisolation procedure was used to purify SLMVs from cell lines derived from mouse beta cells and alpha cells. SLMVs obtained from the beta-cell line, but not those obtained from the alpha-cell line, displayed a GABA-transport activity dependent upon a proton electrochemical gradient generated by a vacuolar proton ATPase. These data support the hypotheses that (i) SLMVs have a secretory function similar to that of SVs and (ii) beta-cell SLMVs are involved in the secretion of GABA, which in turn may have a paracrine function on mantle cells of the islet.

Synaptic vesicle proteins in exocytosis: what do we know?

Synaptic release of neurotransmitters is a fast process that is mediated by Ca(2+)-dependent exocytosis of synaptic vesicles. Several abundant membrane proteins of synaptic vesicles have been characterized at the molecular level but their function in synaptic vesicle traffic is poorly understood. Recent evidence suggests that some of these proteins are involved in exocytotic membrane fusion.

Phosphorylation of synaptotagmin I by casein kinase II

Synaptotagmin I is an abundant synaptic vesicle protein that binds Ca2+ in a phospholipid-dependent manner and is thought to function in synaptic vesicle exocytosis. We have now studied the phosphorylation of synaptotagmin I. Synaptotagmin I is one of the major substrates in brain for casein kinase II, which phosphorylates synaptotagmin at a single threonine. The phosphorylation site was mapped using recombinant proteins to threonine 128 of synaptotagmin I, which is located in the sequence between the transmembrane region and the C2 domain repeats of synaptotagmin I. The phosphorylation site of synaptotagmin I is also present in synaptotagmin II and is evolutionarily conserved between different species. Preceding the phosphorylation site, synaptotagmins I and II contain a lysine-rich sequence. Casein kinase II phosphorylation of many substrates is strongly stimulated by the addition of polylysine, but phosphorylation of synaptotagmin I by casein kinase II is not. In recombinant proteins, removal of the lysine-rich sequence of synaptotagmin I makes its phosphorylation dependent on exogenous polylysine, suggesting that the lysine-rich sequence in synaptotagmin serves as an endogenous polylysine stimulation signal for casein kinase II. Our data demonstrate that synaptotagmin I is an efficient substrate for casein kinase II at a conserved site with a possible modulatory role in nerve terminal function.

Interaction of synaptotagmin with the cytoplasmic domains of neurexins

Synaptotagmin, a major intrinsic membrane protein of synaptic vesicles that binds Ca2+, was purified from bovine brain and immobilized onto Sepharose 4B. Affinity chromatography of brain membrane proteins on immobilized synaptotagmin revealed binding of alpha- and beta-neurexins to synaptotagmin in a Ca(2+)-independent manner. Using a series of recombinant proteins in which glutathione S-transferase was fused to the cytoplasmic domains of three different neurexins or of control proteins, it was found that synaptotagmin specifically interacts with the cytoplasmic domains of neurexins but not of control proteins. This interaction is dependent on a highly conserved, 40 amino acid sequence that makes up most of the cytoplasmic tails of the neurexins. Our data suggest a direct interaction between the cytoplasmic domains of a plasma membrane protein (the neurexins) and a protein specific for a subcellular organelle (synaptotagmin). Such an interaction could have an important role in the docking and targeting of synaptic vesicles in the nerve terminal.

Tetanus toxin action: inhibition of neurotransmitter release linked to synaptobrevin proteolysis

Tetanus toxin is a potent neurotoxin that inhibits the release of neurotransmitters from presynaptic nerve endings. The mature toxin is composed of a heavy and a light chain that are linked via a disulfide bridge. After entry of tetanus toxin into the cytoplasm, the released light chain causes block of neurotransmitter release. Recent evidence suggests that the L-chain may act as a metalloendoprotease. Here we demonstrate that blockade of neurotransmission by tetanus toxin in isolated nerve terminals is associated with a selective proteolysis of synaptobrevin, an integral membrane protein of synaptic vesicles. No other proteins appear to be affected by tetanus toxin. In addition, recombinant light chain selectively cleaves synaptobrevin when incubated with purified synaptic vesicles. Our data suggest that cleavage of synaptobrevin is the molecular mechanism of tetanus toxin action.