Tetanus and botulism neurotoxins: a new group of zinc proteases

Functional characterisation of tetanus and botulinum neurotoxins binding domains

Tetanus and botulinum neurotoxins constitute a family of bacterial protein toxins responsible for two deadly syndromes in humans (tetanus and botulism, respectively). They bind with high affinity to neurons wherein they cause a complete inhibition of evoked neurotransmitter release. Here we report on the cloning, expression and use of the recombinant fragments of the heavy chains of tetanus neurotoxin and botulinum neurotoxin serotypes A, B and E as tools to study the neurospecific binding of the holotoxins. We found that the recombinant 50 kDa carboxy-terminal domains of tetanus and botulinum neurotoxins alone are responsible for the specific binding and internalisation into spinal cord cells in culture. Moreover, we provide evidence that the recombinant fragments block the internalization of the parental holotoxins in a dose-dependent manner, as determined by following the neurotoxin-dependent cleavage of their targets VAMP/synaptobrevin and SNAP-25. In addition, the recombinant binding fragments cause a significant delay in the paralysis induced by the corresponding holotoxin on the mouse phrenic nerve-hemidiaphragm preparation. Taken together, these results show that the carboxy-terminal domain of tetanus and botulinum neurotoxins is necessary and sufficient for the binding and internalisation of these proteins in neurons and open the possibility to use them as tools for the functional characterisation of the intracellular transport of clostridial neurotoxins.

Internalization and proteolytic action of botulinum toxins in CNS neurons and astrocytes

Tetanus and botulinum toxins bind and are internalized at the neuromuscular junction. Botulinum neurotoxins (BoNTs) enter the cytosol at the motor nerve terminal; tetanus neurotoxin (TeNT) proceeds retroaxonally inside the motor axon to reach the spinal cord inhibitory interneurons. Although the major target of BoNTs is the peripheral cholinergic terminals, CNS neurons are susceptible to intoxication as well. We investigated the route of entry and the proteolytic activity of BoNT/B and BoNT/F in cultured hippocampal neurons and astrocytes. We show that, differently from TeNT, which enters hippocampal neurons via the process of synaptic vesicle (SV) recycling, BoNTs are internalized and cleave the substrate synaptobrevin/VAMP2 via a process independent of synaptic activity. Labeling of living neurons with Texas Red-conjugated BoNTs and fluoresceinated dextran revealed that these toxins enter hippocampal neurons via endocytic processes not mediated by SV recycling. Botulinum toxins also exploit endocytosis to enter cultured astrocytes, where they partially cleave cellubrevin, a ubiquitous synaptobrevin/VAMP isoform. These results indicate that, in spite of their closely related protein structure, TeNT and BoNTs use different routes to penetrate hippocampal neurons. These findings bear important implications for the identification of the protein receptors of clostridial toxins.

In vitro evaluation methods for Clostridium botulinum type C and D vaccines

Reagents were prepared for use in ELISAs to determine the concentration of the antigenic components of Clostridium botulinum type C and D. The results obtained were compared with the L+dose assay and a good correlation was found between the two assays for measurement of the C and D neurotoxin concentration. These ELISAs were also used to determine the concentration of the neurotoxins in toxoid form. The relationship between the C neurotoxin dose, in toxoid form, and the immune response in guinea pigs could be deduced from the data obtained. The relationship for the D neurotoxin was not that clear, as the same concentration of the antigen resulted in variable potency values. However, these ELISAs can be used to formulate the concentration of the C and D components in the final bivalent vaccine. Replacement of the preliminary potency assay on the monovalent components after production with the in vitro assays will shorten the total production time of the vaccine by about 60 days. The economical and ethical implications are the reduction in the use of animals to evaluate the vaccine.

Tetanus and botulinum neurotoxins: mechanism of action and therapeutic uses

The clostridial neurotoxins responsible for tetanus and botulism are proteins consisting of three domains endowed with different functions: neurospecific binding, membrane translocation and proteolysis for specific components of the neuroexocytosis apparatus. Tetanus neurotoxin (TeNT) binds to the presynaptic membrane of the neuromuscular junction, is internalized and transported retroaxonally to the spinal cord. The spastic paralysis induced by the toxin is due to the blockade of neurotransmitter release from spinal inhibitory interneurons. In contrast, the seven serotypes of botulinum neurotoxins (BoNTs) act at the periphery by inducing a flaccid paralysis due to the inhibition of acetylcholine release at the neuromuscular junction. TeNT and BoNT serotypes B, D, F and G cleave specifically at single but different peptide bonds, of the vesicle associated membrane protein (VAMP) synaptobrevin, a membrane protein of small synaptic vesicles (SSVs). BoNT types A, C and E cleave SNAP-25 at different sites located within the carboxyl-terminus, while BoNT type C additionally cleaves syntaxin. The remarkable specificity of BoNTs is exploited in the treatment of human diseases characterized by a hyperfunction of cholinergic terminals.

Tetanus toxin impairs accessory and secretory functions in interferon-gamma-treated murine macrophages

Tetanus neurotoxin (TT), a product of microbial origin, acts as a zinc endopeptidase on vesicle-associated membrane proteins (VAMP). We have demonstrated that TT displays inhibitory effects on secretory and accessory functions in the murine macrophage (Mphi) cell line GG2EE. Nitric oxide (NO) secretion was decreased when interferon (IFN)-gamma-pretreated GG2EE Mphis were coincubated with a fungal costimulus (SMP200) and TT. When heat-inactivated TT was used this effect was not evident. The TT-mediated phenomenon was dose-dependent and specific since, under the same experimental conditions, it did not affect interleukin-6 or tumor necrosis factor-alpha secretion. Furthermore, IFN-gamma-induced major histocompatibility complex class II molecule expression and GG2EE accessory function, assessed by SMP200-stimulated lymphoproliferation, were also inhibited by TT. Such inhibition was incomplete, in line with our previous results showing that TT partially cleaves VAMP proteins in murine Mφ.

Inhibition by tetanus toxin of sodium-dependent, high-affinity [3H]5-hydroxytryptamine uptake in rat synaptosomes

Tetanus toxin (TeTx) is a powerful clostridial neurotoxin that inhibits Ca2+-dependent neurotransmitter secretion as do the botulinum neurotoxins (BoNTs). We found that TeTx (but not BoNT/A) produced a specific time- and dose-dependent inhibition of Na+-dependent [3H]5-hydroxytryptamine (serotonin, 5-HT) uptake in rat CNS synaptosomes. This effect was found in all CNS tryptaminergic areas, being maximal in the hippocampus and occipital cortex. TeTx produced the maximum reduction in [3H]5-HT uptake after 30 min of preincubation, being significant also at lower doses (10(-12) M) or shorter incubation times (10 min). Serotonin transport inhibitors such as fenfluramine (IC50, 11.0 +/- 0.9 microM), paroxetine (IC50, 33.5 +/- 0.1 microM), and imipramine (IC50, 89.9 +/- 5.7 microM) were 3 or 4 orders of magnitude less potent than TeTx (IC50, 8.7 +/- 1.0 nM). Of the two fragments of TeTx, (the C-terminal portion of the neurotoxin heavy chain, which is responsible for the binding to the nerve tissue) was consistently more effective than the L-H(N) fragment (the light neurotoxin chain disulfide linked to the N-terminal portion of the heavy chain, which is responsible for the toxic metalloprotease action) as inhibitor of [3H]5-HT uptake in synaptosomal preparations (56 +/- 5% and 95 +/- 3% with respect to control, respectively). Antagonism of the toxin-induced [3H]5-HT uptake blockade could not be reversed by zinc chelators but did have the ability to antagonize the TeTx inhibition of basal and K+-evoked [3H]5-HT release in rat synaptosomes. The reduction in serotonin accumulation induced by TeTx could be responsible for some tetanic symptoms that have been related to the serotonergic system.

In vitro translation of type A Clostridium botulinum neurotoxin heavy chain and analysis of its binding to rat synaptosomes

Botulinum neurotoxins (BoNTs) are highly potent toxins that inhibit neurotransmitter release from peripheral cholinergic synapses. BoNTs consist of a toxifying light chain (LC; 50 kDa) and a binding/translocating heavy chain (HC; 100 kDa) linked through a disulfide bond. A DNA fragment encoding type A Clostridium botulinum heavy chain (BoNT/A HC) was amplified by polymerase chain reaction and cloned into an E. coli PET-15b vector. In vitro translated [35S]BoNT/A HC was identified by anti-BoNT/A polyclonal antibodies, and was used to investigate the binding of the toxin to rat synaptosomes. The binding of [35S]BoNT/A HC to synaptosomes was abolished by 500-fold excess of cold BoNT/A, and by incubation with trypsin. Treatment of BoNT/A HC with anti-BoNT/A or G(T1b) blocked its binding to synaptosomes. The radioactive BoNT/A HC recognized three proteins corresponding to a molecular mass of 150 (P150), 120 (P120), and 75 (P75) kDa in rat and bovine synaptosomal preparations. These results represent the first successful expression of functional full-length BoNT heavy chain.

Streptococcal DNase B is immunologically identical to superantigen SpeF but involves separate domains

The previous suggestion that streptococcal superantigen SpeF might be identical to DNase B was confirmed in this study. Polyclonal SpeF-specific antisera were able to inhibit depolymerization of methyl-green DNA by DNase B. However, T-cell mitogenicity and nuclease activity appear to involve separate immune epitopes on SpeF, since sera with the capacity to neutralize the mitogenic activity of SpeF did not always inhibit the DNase activity.

Use of fluorescent probes to follow membrane traffic in nerve terminals

Optical tracers in conjunction with fluorescence microscopy have become widely used to follow the movement of synaptic vesicles in nerve terminals. The present review discusses the use of these optical methods to understand the regulation of exocytosis and endocytosis of synaptic vesicles. The maintenance of neurotransmission depends on the constant recycling of synaptic vesicles and important insights have been gained by visualization of vesicles with the vital dye FM1-43. A number of questions related to the control of recycling of synaptic vesicles by prolonged stimulation and the role of calcium to control membrane internalization are now being addressed. It is expected that optical monitoring of presynaptic activity coupled to appropriate genetic models will contribute to the understanding of membrane traffic in synaptic terminals.

The active site structure of tetanus neurotoxin resolved by multiple scattering analysis in X-Ray absorption spectroscopy

A detailed study of the x-ray absorption spectrum of tetanus neurotoxin in the K-edge EXAFS region of the zinc absorber is presented that allows the complete identification of the amino acid residues coordinated to the zinc active site. A very satisfactory interpretation of the experimental data can be given if multiple scattering contributions are included in the analysis. Comparing the absorption spectrum of tetanus neurotoxin to that of two other structurally similar zinc-endopeptidases, thermolysin and astacin, in which the zinc coordination mode is known from crystallographic data, we conclude that in tetanus neurotoxin, besides a water molecule, zinc is coordinated to two histidines and a tyrosine.

Neurotransmitter release from semi-intact synaptosomes

We have developed a secretion assay composed of semi-intact synaptosomes from which transmitter release is optimally evoked by micromolar Ca2+ in the presence of cytosol. Transmitter release from this preparation reconstitutes known characteristics of regulated exocytosis and is accompanied by a marked decrease in synaptic vesicles. The assay is useful in characterizing the components known to be involved in transmitter release, and should also facilitate the identification of additional factors that are important for this process.

Clostridial Neurotoxins in Synaptic Research

Research on clostridial neurotoxins provides a recent illustration of how medical research can spark fundamental breakthroughs in basic science. Clostridia are anaerobic bacilli that live in dust, soil, vegetation, and the gastrointestinal tract of animals and humans. Most are benign, but the pathogenic species produce potent neural toxins. In addition to their involvement in disease and germ warfare, interest in these toxins has increased within the basic research community with the appreciation that these agents can be used as precise experimental tools for dissecting the molecular basis of synaptic transmission, which is without question the most essential process in the nervous system.

Differential regulation of exocytosis by calcium and CAPS in semi-intact synaptosomes

Using a novel approach to measure exocytosis in vitro from semi-intact synaptosomes, we establish that the Ca2+-dependent release of glutamate requires cytosolic factors for mobilization from the reserve pool. The cytosolic activity for glutamate release was not satisfied by CAPS, a soluble component required for norepinephrine (NE) release. Moreover, the CAPS-independent glutamate release from synaptic vesicles (SVs) was 200-fold less sensitive to Ca2+ than that required for dense core vesicles (DCVs). The differential regulation of exocytosis by CAPS, Ca2+, and potential novel cytosolic factor(s) suggests that the docking and fusion machinery controlling DCVs has diverged from that regulating glutamate-containing SVs.

Efficacy of a novel metalloprotease inhibitor on botulinum neurotoxin B activity

The novel inhibitor 7-N-phenylcarbamoylamino-4-chloro-3-propyloxyisocoumarin (ICD 1578) was tested for its ability to antagonize the zinc metalloprotease activity of botulinum toxin B (BoNT/B). The efficacy of this compound was tested in a cell-free system using a 50-mer synaptobrevin peptide as substrate. The peptide, designated as [Pya88] S 39-88, had a fluorescent amino acid analog, L-pyrenylalanine (Pya), substituted for the normal Phe88 of synaptobrevin-2. Cleavage by BoNT light chain yielded fragments of 38 and 11 amino acids, respectively. The smaller fragment, containing the Pya fluorophore, was readily separated and quantified by fluorescence spectroscopy at 377 nm. In the presence of 7-200 microM ICD 1578, cleavage of [Pya88] S 39-88 was progressively reduced (IC50 = 27.6 microM), and 100 microM ICD 1578 produced >95% inhibition. For comparison, captopril, a well-known zinc metalloprotease inhibitor, generated less than 10% inhibition at a concentration of 5 mM. ICD 1578 is the most potent antagonist of BoNT/B light chain thus far described.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Expression of SNARE proteins in enteroendocrine cell lines and functional role of tetanus toxin-sensitive proteins in cholecystokinin release

In neurons, synaptic vesicle exocytosis involves the formation of a core complex particle including syntaxin-1, synaptosomal-associated protein of 25 kDa (SNAP-25) and vesicle-associated membrane protein (VAMP)-2/synaptobrevin. The expression of these proteins was investigated in a panel of cell lines, including lines of endocrine and intestinal origin, by Western blotting and/or immunocytochemistry. The three core complex proteins were detected in the enteroendocrine, cholecystokinin (CCK)-secreting, cell lines STC-1 and GLUTag, and in the endocrine non-intestinal cell lines CA-77 and HIT-T15. In contrast, SNAP-25 and syntaxin-1 were undetected in the intestinal non-endocrine cell lines IEC-6, HT-29 and Caco-2, whereas a slight expression of VAMP-2 was documented in IEC-6 and HT-29 cells. Co-immunoprecipitation experiments indicated that syntaxin-1, SNAP-25 and VAMP-2 were present in a complex similar to that identified in brain. In the STC-1 cell line, treatment of streptolysin-O-permeabilized cells with tetanus toxin (Tetx) selectively cleaved VAMP-2 and VAMP-3/cellubrevin, and simultaneously abolished Ca2+-induced CCK secretion (IC50 approximately 12 nM). These results show that endocrine cell lines of intestinal origin express syntaxin-1, SNAP-25 and VAMP-2, and suggest a key role for a Tetx-sensitive protein (for example VAMP-2 and/or VAMP-3) in the CCK secretion by STC-1 cells.

Recombinant SNAP-25 is an effective substrate for Clostridium botulinum type A toxin endopeptidase activity in vitro

Bacterial neurotoxins are now being used routinely for the treatment of neuromuscular conditions. Alternative assays to replace or to complement in vivo bioassay methods for assessment of the safety and potency of these botulinum neurotoxin-based therapeutic products are urgently needed. Advances made in understanding the mode of action of clostridial neurotoxins have provided the basis for the development of alternative mechanism-based assay methods. Thus, the identification of SNAP-25 (synaptosomal-associated protein of molecular mass 25 kDa) as the intracellular protein target which is selectively cleaved during poisoning by botulinum neurotoxin type A (BoNT/A) has enabled the development of a functional in vitro assay for this toxin. Using recombinant DNA methods, a segment of SNAP-25 (aa residues 134-206) spanning the toxin cleavage site was prepared as a fusion protein to the maltose-binding protein in Escherichia coli. The fusion protein was purified by affinity chromatography and the fragment isolated after cleavage with Factor Xa. Targeted antibodies specific for the N and C termini of SNAP-25, as well as the toxin cleavage site, were prepared and used in an immunoassay to demonstrate BoNT/A endopeptidase activity towards recombinant SNAP-25 substrates. The reaction required low concentrations of reducing agents which were inhibitory at higher concentrations as were metal chelators and some inhibitors of metallopeptidases. The endopeptidase assay has proved to be more sensitive than the mouse bioassay for detection of toxin in therapeutic preparations. A good correlation with results obtained in the in vivo bioassay (r = 0.95, n = 23) was demonstrated. The endopeptidase assay described here may provide a suitable replacement assay for the estimation of the potency of type A toxin in therapeutic preparations.

Pharmacologic characterization of botulinum toxin for basic science and medicine

The use of Botulinum neurotoxin (BoNT) is increasing in both clinical and basic science. Clinically, intramuscular injection of nanogram quantities of BoNT is fast becoming the treatment of choice for a spectrum of disorders including movement disorders such as torticollis, blepharospasm, Meige Disease, and hemifacial spasm (Borodic et al., 1991, 1994a; Jankovic and Brin, 1991; Clarke, 1992). Neuroscientists are using BoNTs as tools to develop a better understanding of the mechanisms underlying the neurotransmitter release process. Consequently, our ability to accurately and reliably quantify the biologic activity of botulinum toxin has become more important than ever. The accurate measurement of the pharmacologic activity of BoNTs has become somewhat problematic with the most significant problems occurring with the clinical use of the toxins. The biologic activity of BoNTs has been measured using a variety of techniques including assessment of whole animal responses to in vitro effects on neurotransmitter release. The purpose of this review is to examine the approaches employed to characterize, quantify and investigate the actions of the BoNTs and to provide a guide to aid investigators in determining which of these methods is most appropriate for their particular application or use.

Structural features of aminoquinolines necessary for antagonist activity against botulinum neurotoxin

Certain aminoquinoline antimalarial compounds, such as chloroquine, antagonize the paralytic actions of botulinum neurotoxins (BoNT). These studies have been extended to determine the critical structural groups necessary for synthetic aminoquinolines to have antagonist activity against BoNT. Isolated mouse hemidiaphragms were maintained at 36 degrees C and indirectly stimulated; the resulting isometric twitch tensions were recorded as a measure of synaptic function. The muscles were exposed to the test compounds before being treated with a challenge concentration of BoNT (typically 0.2 nM of serotype A). The time to onset of 50% muscle paralysis due to BoNT was used to assess quantitatively the efficacy of the test compounds, which were then ranked on the basis of the concentrations necessary to delay paralysis by a specified time increment. Of the compounds tested, those having a 7-chloro-4-aminoquinoline configuration, similar to chloroquine (or the structurally similar 6-chloro-9-amino acridine group in quinacrine), were most effective. Truncation of the alkyl-amino-alkyl group from chloroquine and conversion of the 4-amino nitrogen to a primary amine did not significantly alter its effectiveness as a BoNT antagonist. However, the 6-chloro- or 8-chloro- isomers of chloroquine were essentially ineffective. These results suggest that aminoquinolines antagonize the paralytic actions of BoNT through interaction with a selective, stereospecific site that is not well correlated with antimalarial activity.

Construction, expression and characterization of chimaeric toxins containing the ribonucleolytic toxin restrictocin: intracellular mechanism of action

Restrictocin is a ribonucleolytic toxin produced by the fungus Aspergillus restrictus. Two chimaeric toxins containing restrictocin directed at the human transferrin receptor have been constructed. Anti-TFR(scFv)-restrictocin is encoded by a gene produced by fusing the DNA encoding a single-chain antigen-combining region (scFv) of a monoclonal antibody, directed at the human transferrin receptor, at the 5' end of that encoding restrictocin. The other chimaeric toxin, restrictocin-anti-TFR(scFv), is encoded by a gene fusion containing the DNA encoding the single-chain antigen-combining region of antibody to human transferrin receptor at the 3' end of the DNA encoding restrictocin. These gene fusions were expressed in Escherichia coli, and fusion proteins purified from the inclusion bodies by simple chromatography techniques to near-homogeneity. The two chimaeric toxins were found to be equally active in inhibiting protein synthesis in a cell-free in vitro translation assay system. The chimaeric toxins were selectively toxic to the target cells in culture with potent cytotoxic activities. However, restrictocin-anti-TFR(scFv) was more active than anti-TFR(scFv)-restrictocin on all cell lines studied. By using protease and metabolic inhibitors, it can be shown that, to manifest their cytotoxic activity, the restrictocin-containing chimaeric toxins need to be proteolytically processed intracellularly and the free toxin or a fragment thereof thus generated is translocated to the target via a route involving the Golgi apparatus.

Localization of the regions on the C-terminal domain of the heavy chain of botulinum A recognized by T lymphocytes and by antibodies after immunization of mice with pentavalent toxoid

We have mapped the regions recognized by T and/or B cells (Abs) on the C-terminal domain (Hc) of the heavy chain of botulinum neurotoxin serotype A (BoNT/A) after immunization of two inbred mouse strains with pentavalent toxoid (BoNTs A, B, C, D and E). Using a set of synthetic overlapping peptides, encompassing the entire Hc domain (residues 855-1296), we demonstrated that T cells of Balb/c (H-2d) mice, primed with one injection of toxoid, recognized two major regions within residues 897-915 and 939-957. After multiple inoculations with toxoid, T cells of Balb/c expanded their recognition ability and responded very well to challenge with peptide 1261-1279 and moderately to stimulation with peptide 1149-1167. Unlike Balb/c T cells, those of toxoid-primed SJL (H-2s) mice exhibited a more complex profile and responded to challenge with a large number of overlapping peptides. After one toxoid injection, however, three peptides, 897-915, 939-957/953-971 overlap and 1051-1069, were the most potent T cells stimulators. After three toxoid injections, peptides 897-915 and 1051-1069 remained immunodominant while the third region was shifted upstream to 925-943/939-957 overlap. The immunodominant epitope within peptide 897-915 was recognized exclusively by T cells, since no Abs were detected against this region. The Ab binding profiles of the two mouse strains were quite similar, showing only small quantitative differences. Both, Balb/c and SJL anti-toxoid Abs displayed strong binding mainly to peptide 1177-1195, followed by peptides 869-887/883-901 overlap and 1275-1296. In addition, a significant amount of Balb/c anti-toxoid Abs was bound to peptide 1135-1153. Unlike Balb/c Abs, that interacted weakly with peptides 995-1013 and 1051-1069, the anti-toxoid Abs of SJL mice exhibited strong binding toward both peptides. The results showed that, in a given strain, the regions recognized by anti-toxoid Abs and T cells may coincide or may be uniquely B or T cell determinants.

The variability in the clinical effect induced by botulinum toxin type A: the role of muscle activity in humans

When botulinum toxin (BT) is administered for the first time at fixed doses, variable clinical responses can be observed in patients with the same form of dystonic disorder. Many factors may contribute to this phenomenon, including the variability rate of absorption of the drug. Animal experimental models (rat diaphragm preparation) have demonstrated an increased absorption of BT in the terminal nerve endings of the muscle under repetitive electrical stimulation, suggesting that "muscle activity" also may play an important role. The aim of our study was to evaluate in humans the role of the muscle activity on the variability of the effect induced by BT type A. Eleven patients with blepharospasm and idiopathic facial hemispasm were studied by using neurophysiologic techniques. In nine patients, both extensor digitorum brevis (EDB) muscles were injected with low (3 IU), fixed doses of type A BT. For the first 24 h after administration of the drug, periodic electrical stimulation of only one EDB was used. The subsequent percentage changes in compound muscle action potential (CMAP) amplitude was calculated at different intervals over a 30-day period. The percentage in the CMAP for the stimulated EDB was compared with that of the contralateral nonstimulated side. We found that the effect of the induced neuromuscular blockade was significantly greater for the stimulated side. In the other two patients, we injected BT in one EDB and the same volume of normal saline solution in the contralateral muscle to assess the stability of the CMAP in untreated muscle over time. We observed that the CMAP was unchanged in the untreated EDB; therefore we concluded that muscle activity plays an important role in the variability of clinical response often seen.

Bacterial protein toxins and cell vesicle trafficking

A group of bacterial protein toxins interfere with vesicular trafficking inside cells. Clostridial neurotoxins affect mainly the highly regulated fusion of neurotransmitter- and hormone-containing vesicles with the plasma membrane. They cleave the three SNARE proteins: VAMP, SNAP-25 and syntaxin, and this selective proteolysis results in a blockade of exocytosis. The Helicobacter pylori cytotoxin is implicated in the pathogenesis of gastroduodenal ulcers. It causes a progressive and extensive vacuolation of cells followed by necrosis, after a cytotoxin-induced alteration of membrane trafficking by late endosomes. Vacuoles originate from this compartment in a rab7-dependent process and swell because they are acidic and accumulate membrane-permeant amines.

Botulinum neurotoxins: mechanism of action and therapeutic applications

Recent studies have led to the discovery of the molecular lesions in motor neurons caused by botulinum neurotoxins. These neurotoxins are metalloproteinases that enter the cytosol and very specifically cleave protein components of the neuroexocytosis apparatus. Consequently, acetylcholine cannot be released and the muscle is paralysed. For this reason, botulinum neurotoxins are increasingly being used to treat a variety of conditions where a functional paralysis of neuromuscular junctions is useful as therapy.

SNAP-25 is required for a late postdocking step in Ca2+-dependent exocytosis

The Ca2+-activated fusion of large dense core vesicles (LDCVs) with the plasma membrane is reconstituted in mechanically permeabilized PC12 cells by provision of millimolar MgATP and cytosolic proteins. Ca2+-activated LDCV exocytosis was inhibited completely by the type E but not the type A botulinum neurotoxin (BoNT) even though both BoNTs were equally effective in proteolytically cleaving the synaptosome-associated protein of 25 kDa (SNAP-25). The greater inhibition of exocytosis by BoNT E correlated with a greater destabilization of detergent-extracted complexes consisting of SNAP-25, synaptobrevin, and syntaxin. LDCVs in permeable PC12 cells can be poised at a late postdocking, prefusion state by MgATP-dependent priming processes catalyzed by N-ethylmaleimide sensitive factor and priming in exocytosis proteins. BoNT E completely blocked Ca2+-activated LDCV exocytosis in ATP-primed cells, whereas BoNT A was only slightly inhibitory, implying that the C-terminal region of SNAP-25 (Ile181-Gln197) between the cleavage sites for BoNT E and BoNT A is essential for late postdocking steps. A required role for SNAP-25 at this stage was also indicated by inhibition of Ca2+-activated LDCV fusion in ATP-primed cells by a C-terminal peptide antibody. We conclude that plasma membrane SNAP-25, particularly residues 181-197, is required for Ca2+-regulated membrane fusion at a step beyond LDCV docking and ATP utilization.

Nitric oxide modulates synaptic vesicle docking fusion reactions

Nitric oxide (NO) stimulates calcium-independent neurotransmitter release from synaptosomes. NO-stimulated release was found to be inhibited by Botulinum neurotoxins that inactivate the core complex of synaptic proteins involved in the docking and fusion of synaptic vesicles. In experiments using recombinant proteins, NO donors increased formation of the VAMP/SNAP-25/syntaxin 1a core complex and inhibited the binding of n-sec1 to syntaxin 1a. The combined effects of these activities is predicted to promote vesicle docking/fusion. The sulfhydryl reagent NEM inhibited the binding of n-sec1 to syntaxin 1a, while beta-ME could reverse the NO-enhanced association of VAMP/SNAP-25/syntaxin 1a. These data suggest that post-translational modification of sulfhydryl groups by a nitrogen monoxide (likely to be NO+) alters the synaptic protein interactions that regulate neurotransmitter release and synaptic plasticity.

X-ray absorption spectroscopy study of zinc coordination in tetanus neurotoxin, astacin, alkaline protease and thermolysin

Tetanus and botulinum neurotoxins constitute a new group of Zn-endopeptidases which has been recently actively investigated with the purpose of correlating their biochemical properties to their neurobiocytosis inhibitory capacity. Crystallographic data show that Zn-endopeptidases are characterized by an active site with a Zn atom coordinated to two histidines and glutamate-bound water molecule. The two histidines and glutamate resides belong to the HEXXH motif which is characteristic of most Zn-endopeptidases. A forth metal ligand is a glutamate in thermolysin-like proteinases, but it is an histidine in the astacin family of proteinases and in alkaline protease. Astacin and alkaline protease possess a tyrosine as fifth Zn ligand, whose position in the case of alkaline protease could not be determined by X-ray crystallography. Not much is known about the atom arrangement around the active site in tetanus neurotoxin. In this work X-ray absorption spectroscopy has been used to obtain information on the Zn coordination mode in tetanus neurotoxin. The near-edge and extended fine-structure absorption spectra of this toxin are compared with those of astacin, alkaline protease and thermolysin. The present data and sequence information suggest a new pattern of Zn coordination in tetanus neurotoxin with one water molecule and three aromatic residues as metal ligands. These residues are the two histidines of the characteristic motif and a tyrosine which is tentatively identified with Tyr242, on the basis of sequence comparison and mutagenesis experiments. The mean distances of the Zn from the nearest coordinated atoms is reported. Our results indicate that alkaline protease, like astacin, also possesses a tyrosine as a fifth ligand.

VAMP/synaptobrevin isoforms 1 and 2 are widely and differentially expressed in nonneuronal tissues

VAMP/synaptobrevin is part of the synaptic vesicle docking and fusion complex and plays a central role in neuroexocytosis. Two VAMP (vesicle-associated membrane protein) isoforms are expressed in the nervous system and are differently distributed among the specialized parts of the tissue. Here, VAMP-1 and -2 are shown to be present in all rat tissues tested, including kidney, adrenal gland, liver, pancreas, thyroid, heart, and smooth muscle. The two isoforms are differentially expressed in various tissues and their level may depend on differentiation. VAMP-1 is restricted to exocrine pancreas and to kidney tubular cells, whereas VAMP-2 is the predominant isoform present in Langerhans islets and in glomerular cells. Both isoforms show a patchy vesicular intracellular distribution in confocal microscopy. The present results provide evidence for the importance of neuronal VAMP proteins in the physiology of all cells.

Review article: the use of botulinum toxin in the alimentary tract

New and future indications for the treatment of disorders of the alimentary tract using local injections of botulinum toxin are reviewed. Clinical experience shows that overactive smooth muscle sphincters may be weakened to treat disorders such as achalasia or chronic anal fissure. By contrast, injections placed into the sphincter of Oddi have proven less effective for postcholecystectomy pain syndrome. Experimental evidence suggests that food intake may be reduced by weakening the distal stomach with botulinum toxin. This approach may possibly lead to the treatment of obesity. There are some new possible indications for the use of botulinum toxin on the alimentary tract, and infantile hypertrophic pyloric stenosis seems to be the most promising new development.

Botulinum versus tetanus neurotoxins: why is botulinum neurotoxin but not tetanus neurotoxin a food poison?

Botulinum and tetanus neurotoxins, produced by Clostridium botulinum and Clostridium tetani, respectively, are the most poisonous poisons known to mankind. Although botulinum and tetanus neurotoxins share several characteristics, such as similar mol. wts, similar macrostructure, virtually identical mode of action, and a strong amino acid sequence homology, the two neurotoxins differ in one very significant way; only botulinum neurotoxin is a food poison. Factors responsible for the food poisoning potential of botulinum neurotoxins seem to be a group of complexing proteins that are also produced by C. botulinum, and are known to associate with the neurotoxin. Translation products of nucleotide sequences upstream to the neurotoxin genes of serotypes A, B, C, D, E and F botulinum neurotoxin reveal the location of genes for one of the complexing proteins that could be transcribed as polycistronic mRNA to include neurotoxin sequences. No such protein seems to be present in C. tetani, suggesting that the lack of complexing proteins might be responsible for tetanus not being a food poison.

Structure and function of tetanus and botulinum neurotoxins

Tetanus and botulinum neurotoxins are produced by Clostridia and cause the neuroparalytic syndromes of tetanus and botulism. Tetanus neurotoxin acts mainly at the CNS synapse, while the seven botulinum neurotoxins act peripherally. Clostridial neurotoxins share a similar mechanism of cell intoxication: they block the release of neurotransmitters. They are composed of two disulfide-linked polypeptide chains. The larger subunit is responsible for neurospecific binding and cell penetration. Reduction releases the smaller chain in the neuronal cytosol, where it displays its zinc-endopeptidase activity specific for protein components of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxins B, D, F and G recognize specifically VAMP/ synaptobrevin. This integral protein of the synaptic vesicle membrane is cleaved at single peptide bonds, which differ for each neurotoxin. Botulinum A, and E neurotoxins recognize and cleave specifically SNAP-25, a protein of the presynaptic membrane, at two different sites within the carboxyl-terminus. Botulinum neurotoxin type C cleaves syntaxin, another protein of the nerve plasmalemma. These results indicate that VAMP, SNAP-25 and syntaxin play a central role in neuroexocytosis. These three proteins are conserved from yeast to humans and are essential in a variety of docking and fusion events in every cell. Tetanus and botulinum neurotoxins form a new group of zinc-endopeptidases with characteristic sequence, mode of zinc coordination, mechanism of activation and target recognition. They will be of great value in the unravelling of the mechanisms of exocytosis and endocytosis, as they are in the clinical treatment of dystonias.

Crystal structure of the superantigen enterotoxin C2 from Staphylococcus aureus reveals a zinc-binding site

BACKGROUND

Staphylococcus aureus enterotoxin C2 (SEC2) belongs to a family of proteins, termed 'superantigens', that form complexes with class II MHC molecules enabling them to activate a substantial number of T cells. Although superantigens seem to act by a common mechanism, they vary in many of their specific interactions and biological properties. Comparison of the structure of SEC2 with those of two other superantigens--staphylococcal enterotoxin B (SEB) and toxic shock syndrome toxin-1 (TSST-1)--may provide insight into their mode of action.

RESULTS

The crystal structure of SEC2 has been determined at 2.0 A resolution. The overall topology of the molecule resembles that of SEB and TSST-1, and the regions corresponding to the MHC class II and T-cell receptor binding sites on SEB are quite similar in SEC2. A unique feature of SEC2 is the presence of a zinc ion located in a solvent-exposed region at the interface between the two domains of the molecule. The zinc ion is coordinated to Asp83, His118, His122 and Asp9* (from the neighbouring molecule in the crystal lattice). Atomic absorption spectrometry demonstrates that zinc is also bound to SEC2 in solution.

CONCLUSIONS

SEC2 appears to be capable of binding to MHC class II molecules in much the same manner as SEB. However, structure-function studies have suggested an alternative binding mode that involves a different site on the toxin. The zinc ion of SEC2 lies within this region and thus may be important for complex formation, for example by acting as a bridge between the two molecules. Other possible roles for the metal cation, including a catalytic one, are also considered.

Botulinum neurotoxin type C cleaves a single Lys-Ala bond within the carboxyl-terminal region of syntaxins

Botulinum neurotoxin serotype C (BoNT/C) is a 150-kDa protein produced by Clostridium botulinum, which causes animal botulism. In contrast to the other botulinum neurotoxins that contain one atom of zinc, highly purified preparations of BoNT/C bind two atoms of zinc per toxin molecule. BoNT/C is a zinc-endopeptidase that cleaves syntaxin 1A at the Lys253-Ala254 and syntaxin 1B at the Lys252-Ala253 peptide bonds, only when they are inserted into a lipid bilayer. The other Lys-Ala bond present within the carboxyl-terminal region is not hydrolyzed. Syntaxin isoforms 2 and 3 are also cleaved by BoNT/C, while syntaxin 4 is resistant. These data suggest that BoNT/C recognizes a specific spatial organization of syntaxin, adopted upon membrane insertion, which brings a selected Lys-Ala peptide bond of its carboxyl-terminal region to the active site of this novel metalloproteinase.

Tetanus toxin-induced seizures in infant rats and their effects on hippocampal excitability in adulthood

A new experimental model of developmental epilepsy is reported. Behavioral and EEG features of seizures produced by unilateral intrahippocampal injection of tetanus toxin in postnatal day 9-11 rats, are described. Within 24-72 h of tetanus toxin injection, rat pups developed frequent and often prolonged seizures which included combinations of repetitive wet dog shakes, and wild running-jumping seizures. Intrahippocampal and cortical surface EEG recordings showed that coincident with these behaviors, electrographic seizures occurred not only in the injected hippocampus, but also in the contralateral hippocampus and bilaterally in the neocortex. Analysis of the interictal EEG revealed multiple independent spike foci. One week following tetanus toxin injection, the number of seizures markedly decreased; however, interictal spiking persisted. After injection rats were allowed to mature some were observed to have unprovoked behavioral seizures and/or epileptiform EEG activity. Mature animals were also studied using in vitro slice techniques. Recordings from hippocampal slices demonstrated spontaneous epileptiform burst discharges in the majority of rats which had tetanus toxin induced seizures as infants. These events occurred in area CA3 and consisted of interictal spikes and intracellularly recorded paroxysmal depolarization shifts (PDSs). On rarer occasions, electrographic seizures were recorded. The use of the tetanus toxin model in developing rats may facilitate a better understanding of the unique features of epileptogenesis in the developing brain and the consequences early-life seizures have on brain maturation and the genesis of epileptic conditions in later life.

Structural studies on the zinc-endopeptidase light chain of tetanus neurotoxin

Tetanus neurotoxin (TeNT) blocks neuroexocytosis via a zinc-endopeptidase activity highly specific for vescicle-associated membrane protein(VAMP)/synaptobrevin. TeNT is the prototype of clostridial neurotoxins, a new family of metalloproteinases. They consist of three domains and the proteolytic activity is displayed by the 50-kDa light chain (L chain). The L chain was isolated here in the native state from bacterial filtrates of Clostridium tetani and its structure was studied via circular dichroism (CD) and fluorescence spectroscopy. The secondary structure content (27% alpha-helix and 43% beta-sheet), estimated by far-ultraviolet CD measurements, was in reasonable agreement with that obtained by standard predictive methods (25% alpha-helix and 49% beta-sheet). Moreover, the hypothetical zinc-binding motif, encompassing residues His-Glu-Leu-Ile-His, was correctly predicted to be in alpha-helical conformation, as also expected on the basis of the geometrical requirements for a correct coordination of the zinc ion. Both near-ultraviolet CD and fluorescence data strongly suggest that the single Trp43 residue is buried and constrained in a hydrophobic environment, likely distant from the zinc ion located in the active-site cleft. The contribution of the bound zinc ion to the overall conformation of TeNT L chain was investigated by different and complementary techniques, including spectroscopic (far- and near-ultraviolet CD, fluorescence, second derivative absorption spectroscopy) as well as proteolytic probes. The results indicate that the zinc ion plays little, if any, role in determining the structural properties of the L chain molecule. Similarly, the metal-free apo-enzyme and the holo-protein share common stability features evaluated in respect to different physico-chemical parameters (pH, temperature and urea concentration). These results parallel those obtained on thermolysin, a zinc-dependent neutral endoprotease from Bacillus thermoproteolyticus, where both conformational and stability properties are unchanged upon zinc removal.

SNAP-25 is expressed in islets of Langerhans and is involved in insulin release

SNAP-25 is known as a neuron specific molecule involved in the fusion of small synaptic vesicles with the presynaptic plasma membrane. By immunolocalization and Western blot analysis, it is now shown that SNAP-25 is also expressed in pancreatic endocrine cells. Botulinum neurotoxins (BoNT) A and E were used to study the role of SNAP-25 in insulin secretion. These neurotoxins inhibit transmitter release by cleaving SNAP-25 in neurons. Cells from a pancreatic B cell line (HIT) and primary rat islet cells were permeabilized with streptolysin-O to allow toxin entry. SNAP-25 was cleaved by BoNT/A and BoNT/E, resulting in a molecular mass shift of approximately 1 and 3 kD, respectively. Cleavage was accompanied by an inhibition of Ca(++)-stimulated insulin release in both cell types. In HIT cells, a concentration of 30-40 nM BoNT/E gave maximal inhibition of stimulated insulin secretion of approximately 60%, coinciding with essentially complete cleavage of SNAP-25. Half maximal effects in terms of cleavage and inhibition of insulin release were obtained at a concentration of 5-10 nM. The A type toxin showed maximal and half-maximal effects at concentrations of 4 and 2 nM, respectively. In conclusion, the results suggest a role for SNAP-25 in fusion of dense core secretory granules with the plasma membrane in an endocrine cell type- the pancreatic B cell.

The biochemistry of neurotransmitter secretion

The progress that has resulted from the convergence of biochemistry with yeast genetics has accelerated the pace at which the molecular events of membrane transport are being elucidated. Future research will focus not only on testing the proposed sequence of protein-protein interactions but also on identifying how calcium regulation is imposed on this system. As our understanding of the basic mechanisms of neurosecretion increases, attention will undoubtedly shift to how the molecules of release are modified to produce changes in synaptic efficacy.

Vesicle-associated membrane protein-2 (synaptobrevin-2) forms a complex with synaptophysin

Vesicle-associated membrane protein (VAMP) (or synaptobrevin), a type II membrane protein of small synaptic vesicles, is essential for neuroexocytosis because its proteolysis by tetanus and botulinum neurotoxins types B, D, F and G blocks neurotransmitter release. The addition of cross-linking reagents to isolated small synaptic vesicles induces the formation of 30 and 50 kDa complexes containing the isoform 2 of VAMP (VAMP-2). Whereas the 30 kDa band is a VAMP-2 homodimer, the 50 kDa species results from the cross-linking of VAMP-2 with synaptophysin. This heterodimer also forms in detergent-solubilized vesicles and involves the N-terminal part of VAMP-2. The implications of the existence of a synaptophysin-VAMP-2 complex in the processes of vesicle docking and fusion with the presynaptic membrane are discussed.

The metallo-proteinase activity of tetanus and botulism neurotoxins

Tetanus and botulinum neurotoxins are produced by several Clostridia and cause the paralytic syndromes of tetanus and botulism by blocking neurotransmitter release at central and peripheral synapses, respectively. They consist of two disulfide-linked polypeptides: H (100 kDa) is responsible for neurospecific binding and cell penetration of L (50 kDa), a zinc-endopeptidase specific for three protein subunits of the neuroexocytosis apparatus. Tetanus neurotoxin and botulinum neurotoxin serotypes B, D, F and G cleave at single sites, which differ for each neurotoxin, VAMP/synaptobrevin, a membrane protein of the synaptic vesicles. Botulinum A and E neurotoxins cleave SNAP-25, a protein of the presynaptic membrane, at two different carboxyl-terminal peptide bonds. Serotype C cleaves specifically syntaxin, another protein of the nerve plasmalemma. The target specificity of these metallo-proteinases relies on a double recognition of their substrates based on interactions with the cleavage site and with a non-contiguous segment that contains a structural motif common to VAMP, SNAP-25 and syntaxin.

The molecular machinery for fast and slow neurosecretion

Recent studies indicate that the molecular machinery for synaptic vesicle docking and fusion consists of a triad of botulinum/tetanus neurotoxin substrates (synaptobrevin, syntaxin, SNAP-25) that are homologues of proteins required for constitutive secretion. Proposed low-affinity Ca2+ sensors that regulate exocytosis remain to be identified, although recent studies on synaptotagmin suggest that it, along with other proteins, could play this role. Regulated peptide secretion from dense-core granules has been found to utilize a similar machinery for docking/fusion, and recent studies indicate that this pathway involves a pre-docking step that is regulated by a higher affinity Ca2+ sensor.