A single mutation in the recombinant light chain of tetanus toxin abolishes its proteolytic activity and removes the toxicity seen after reconstitution with native heavy chain

Tetanus insensitive VAMP2 differentially restores synaptic and dense core vesicle fusion in tetanus neurotoxin treated neurons

The SNARE proteins involved in the secretion of neuromodulators from dense core vesicles (DCVs) in mammalian neurons are still poorly characterized. Here we use tetanus neurotoxin (TeNT) light chain, which cleaves VAMP1, 2 and 3, to study DCV fusion in hippocampal neurons and compare the effects on DCV fusion to those on synaptic vesicle (SV) fusion. Both DCV and SV fusion were abolished upon TeNT expression. Expression of tetanus insensitive (TI)-VAMP2 restored SV fusion in the presence of TeNT, but not DCV fusion. Expression of TI-VAMP1 or TI-VAMP3 also failed to restore DCV fusion. Co-transport assays revealed that both TI-VAMP1 and TI-VAMP2 are targeted to DCVs and travel together with DCVs in neurons. Furthermore, expression of the TeNT-cleaved VAMP2 fragment or a protease defective TeNT in wild type neurons did not affect DCV fusion and therefore cannot explain the lack of rescue of DCV fusion by TI-VAMP2. Finally, to test if two different VAMPs might both be required in the DCV secretory pathway, Vamp1 null mutants were tested. However, VAMP1 deficiency did not reduce DCV fusion. In conclusion, TeNT treatment combined with TI-VAMP2 expression differentially affects the two main regulated secretory pathways: while SV fusion is normal, DCV fusion is absent.

SNARE zippering and synaptic strength

Synapses vary widely in the probability of neurotransmitter release. We tested the hypothesis that the zippered state of the trans-SNARE (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) complex determines initial release probability. We tested this hypothesis at phasic and tonic synapses which differ by 100-1000-fold in neurotransmitter release probability. We injected, presynaptically, three Clostridial neurotoxins which bind and cleave at different sites on VAMP to determine whether these sites were occluded by the zippering of the SNARE complex or open to proteolytic attack. Under low stimulation conditions, the catalytic light-chain fragment of botulinum B (BoNT/B-LC) inhibited evoked release at both phasic and tonic synapses and cleaved VAMP; however, neither BoNT/D-LC nor tetanus neurotoxin (TeNT-LC) were effective in these conditions. The susceptibility of VAMP to only BoNT/B-LC indicated that SNARE complexes at both phasic and tonic synapses were partially zippered only at the N-terminal end to approximately the zero-layer with the C-terminal end exposed under resting state. Therefore, the existence of the same partially zippered state of the trans-SNARE complex at both phasic and tonic synapses indicates that release probability is not determined solely by the zippered state of the trans-SNARE complex at least to the zero-layer.

Improved lentiviral transduction of ALS motoneurons in vivo via dual targeting

Treatment of amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease, is hampered by its complex etiology and lack of efficient means for targeted transfer of therapeutics into motoneurons. The objective of this research was engineering of a versatile motoneuron targeting adapter--a full-length atoxic tetanus toxin fused to core-streptavidin (CS-TeTIM)--for retro-axonal transduction of viral vectors; validation of the targeting efficiency of CS-TeTIM in vivo, by expression of green fluorescence protein (GFP) reporter in motoneurons of presymptomatic and symptomatic ALS-like SOD1(G93A) mice, and comparison with age-matched controls; and appraisal of lentiviral transduction with CS-TeTIM relative to (1) a HC binding fragment of tetanus toxin CS-TeTx(HC), (2) rabies glycoprotein (RG), and (3) a CS-TeTIM-RG dual targeting approach. CS-TeTIM and CS-TeTx(HC) were engineered using recombinant technology and site-directed mutagenesis. Biotinylated vectors, pseudotyped with vesicular stomatitis virus glycoprotein (VSV-G) or RG, were linked to these adaptors and injected intraperitoneally (ip) into presymptomatic (12 weeks old), symptomatic SOD1(G93A) (22 weeks old) or wild type control mice, followed by monitoring of GFP expression in the spinal cord and supraspinal motor structures with quantitative PCR and immuno-histochemistry. Transcripts were detected in the spinal cord and supraspinal motor structures of all mice 2 weeks after receiving a single ip injection, although in symptomatic SOD1(G93A) animals reporter RNA levels were lower compared to presymptomatic and wild-type controls irrespective of the targeting approach. GFP transduction with CS-TeTIM proved more efficient than CS-TeTx(HC) across all groups while CS-TeTIM-RG dual-targeted vectors yielded the highest transcript numbers. Importantly, in both wild-type and presymptomatic SOD1(G93A) mice strong colabeling of choline-acetyltransferase (ChAT) and GFP was visualized in neurons of the brain stem and spinal cord. CS-TeTIM, a versatile adaptor protein for targeted lentiviral transduction of motoneurons, has been engineered and its competence assessed relative to CS-TeTx(HC) and RG. Evidence has been provided that highlights the potential usefulness of this novel recombinant tool for basic research with implications for improved transfer of therapeutic candidates into motoneurons for the amelioration of ALS and related diseases.

Tat-tetanus toxin fragment C: a novel protein delivery vector and its use with photochemical internalization

Protein delivery vectors can be grouped into two classes, those with specific membrane receptors undergoing conventional endocytosis and cell penetrating peptides (CPP) that have the capacity to cross cell or endosomal membranes. For both forms of vectors, translocation across a membrane is usually an inefficient process. In the current study, a novel vector combining the widely used CPP, Tat and the non-toxic neuronal binding domain of tetanus toxin (fragment C or TTC) was assessed for its capacity to deliver GFP as a test cargo protein to human neural progenitor cells (NPCs). These two functional membrane interacting domains dramatically enhanced internalization of the conjugated cargo protein. Tat-TTC-GFP was found to be bound or internalized at least 83-fold more than Tat-GFP and 33-fold more than TTC-GFP in NPCs by direct fluorimetry, and showed enhanced internalization by quantitative microscopy of 18 - and 14-fold, respectively. This preferential internalization was observed to be specific to neuronal cell types. Photochemical internalization (PCI) was utilized to facilitate escape of the endosome-sequestered proteins. The combined use of the Tat-TTC delivery vector with PCI led to both enhancement of neural cell type specific delivery to an endosomal target, followed by the option of efficient release to the cytosol.

Controlling synaptotagmin activity by electrostatic screening

Exocytosis of neurosecretory vesicles is mediated bythe SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins syntaxin-1, synaptobrevin, and SNAP-25, with synaptotagmin functioning as the major Ca²⁺-sensor for triggering membrane fusion. Here we show that bovine chromaffin granules readily fuse with large unilamellar liposomes in a SNARE-dependent manner. Fusion is enhanced by Ca²⁺ but only if the target liposomes contain PI(4,5)P₂ and if polyphosphate anions such as nucleotides or pyrophosphate are present. Ca²⁺-dependent enhancement is mediated by endogenous synaptotagmin-1. Polyphosphates operate by an electrostatic mechanism that reverses an inactivating cis-association of synaptotagmin-1 with its own membrane whereas trans-binding is not affected. Hence, balancing trans- and cis-membrane interactions of synaptotagmin may be a crucial element in the pathway of Ca²⁺-dependent exocytosis.

Molecular structures and functional relationships in clostridial neurotoxins

The seven serotypes of Clostridium botulinum neurotoxins (A-G) are the deadliest poison known to humans. They share significant sequence homology and hence possess similar structure-function relationships. Botulinum neurotoxins (BoNT) act via a four-step mechanism, viz., binding and internalization to neuronal cells, translocation of the catalytic domain into the cytosol and finally cleavage of one of the three soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) causing blockage of neurotransmitter release leading to flaccid paralysis. Crystal structures of three holotoxins, BoNT/A, B and E, are available to date. Although the individual domains are remarkably similar, their domain organization is different. These structures have helped in correlating the structural and functional domains. This has led to the determination of structures of individual domains and combinations of them. Crystal structures of catalytic domains of all serotypes and several binding domains are now available. The catalytic domains are zinc endopeptidases and share significant sequence and structural homology. The active site architecture and the catalytic mechanism are similar although the binding mode of individual substrates may be different, dictating substrate specificity and peptide cleavage selectivity. Crystal structures of catalytic domains with substrate peptides provide clues to specificity and selectivity unique to BoNTs. Crystal structures of the receptor domain in complex with ganglioside or the protein receptor have provided information about the binding of botulinum neurotoxin to the neuronal cell. An overview of the structure-function relationship correlating the 3D structures with biochemical and biophysical data and how they can be used for structure-based drug discovery is presented here.

The zinc-dependent protease activity of the botulinum neurotoxins

The botulinum neurotoxins (BoNT, serotypes A-G) are some of the most toxic proteins known and are the causative agents of botulism. Following exposure, the neurotoxin binds and enters peripheral cholinergic nerve endings and specifically and selectively cleaves one or more SNARE proteins to produce flaccid paralysis. This review centers on the kinetics of the Zn-dependent proteolytic activities of these neurotoxins, and briefly describes inhibitors, activators and factors underlying persistence of toxin action. Some of the structural, enzymatic and inhibitor data that are discussed here are available at the botulinum neurotoxin resource, BotDB (http://botdb.abcc.ncifcrf.gov).

Bacterial toxins and the nervous system: neurotoxins and multipotential toxins interacting with neuronal cells

Toxins are potent molecules used by various bacteria to interact with a host organism. Some of them specifically act on neuronal cells (clostridial neurotoxins) leading to characteristics neurological affections. But many other toxins are multifunctional and recognize a wider range of cell types including neuronal cells. Various enterotoxins interact with the enteric nervous system, for example by stimulating afferent neurons or inducing neurotransmitter release from enterochromaffin cells which result either in vomiting, in amplification of the diarrhea, or in intestinal inflammation process. Other toxins can pass the blood brain barrier and directly act on specific neurons.

Transglutaminase participates in the blockade of neurotransmitter release by tetanus toxin: evidence for a novel biological function

Inhibition of neuroexocytosis by tetanus neurotoxin (TeNT) involves VAMP-2/synaptobrevin-2 cleavage. However, deletion of the TeNT activity does not completely abolish its inhibitory action. TeNT is a potent activator of the cross-linking enzyme transglutaminase 2 (TGase 2) in vitro. The role of the latter mechanism in TeNT poisoning was investigated in isolated nerve terminals and intact neurons. TeNT-induced inhibition of glutamate release from rat cortical synaptosomes was associated with a simultaneous activation of neuronal transglutaminase (TGase) activity. The TeNT-induced blockade of neuroexocytosis was strongly attenuated by pretreatment of either live Aplysia neurons or isolated nerve terminals with specific TGase inhibitors or neutralizing antibodies. The same treatments completely abolished the residual blockade of neuroexocytosis of a non-proteolytic mutant of TeNT light chain. Electrophysiological studies indicated that TGase activation occurs at an early step of TeNT poisoning and contributes to the inhibition of transmitter release. Bioinformatics and biochemical analyses identified synapsin I and SNAP-25 as potential presynaptic TGase substrates in isolated nerve terminals, which are potentially involved in the inhibitory action of TeNT. The results suggest that neuronal TGase activity plays an important role in the regulation of neuroexocytosis and is one of the intracellular targets of TeNT in neurons.

The C-terminus of botulinum neurotoxin type A light chain contributes to solubility, catalysis, and stability

Botulinum neurotoxin type A (BoNT/A) is the etiological agent responsible for botulism, a disease characterized by peripheral neuromuscular blockade. BoNT/A is produced by Clostridium botulinum as a single chain protein that is activated by proteolytic cleavage to form a 50 kDa light chain (LC, 448 amino acids) and a disulfide bond-linked 100 kDa heavy chain (HC, 847 amino acids). Whilst HC comprises the receptor binding and translocation domains, LC is a Zn2+-endopeptidase that cleaves at a single glutaminyl-arginine bond corresponding to residues 197 and 198 at the C-terminus of SNAP25. Cleavage of SNAP25 uncouples the neural exocytosis docking/fusion machinery. LC/A (LC 1-448) and several C-terminal deletion proteins of LC/A were engineered and expressed as His-tagged fusion proteins in Escherichia coli. LC 1-448 was purified, but precipitated upon storage. Approximately 40% of LC 1-448 was a covalent dimer due to the formation of inter-chain disulfide bond formation at Cys430. Conversion of Cys430 to Ser abolished dimer formation of LC 1-448, but did not improve solubility. Three C-terminal deletion peptides were engineered; LC 1-425 and LC 1-418 were expressed and could be purified as soluble and stable proteins, whilst LC 1-398 was soluble, but not stable to storage. Kinetic studies showed that LC 1-448 and LC 1-425 efficiently cleaved GST-SNAP25 and the fluorescent substrate SNAPtide, while LC 1-418 catalyzed the cleavage of both the SNAP25 and the fluorescent substrate SNAPtide with a similar Km, but at a 10-fold slower kcat. Thus, regions within the C-terminus of LC/A contribute to solubility, stability, and catalysis.

Structural analysis of the catalytic domain of tetanus neurotoxin

Clostridium neurotoxins, comprising the tetanus neurotoxin and the seven antigenically distinct botulinum neurotoxins (BoNT/A-G), are among the known most potent bacterial protein toxins to humans. Although they have similar function, sequences and three-dimensional structures, the substrate specificity and the selectivity of peptide bond cleavage are different and unique. Tetanus and botulinum type B neurotoxins enzymatically cleave the same substrate, vesicle-associated membrane protein, at the same peptide bond though the optimum length of substrate peptide required for cleavage by them is different. Here, we present the first experimentally determined three-dimensional structure of the catalytic domain of tetanus neurotoxin and analyze its active site. The structure provides insight into the active site of tetanus toxin's proteolytic activity and the importance of the nucleophilic water and the role of the zinc ion. The probable reason for different modes of binding of vesicle-associated membrane protein to botulinum neurotoxin type B and the tetanus toxin is discussed. The structure provides a basis for designing a novel recombinant vaccine or structure-based drugs for tetanus.

Therapy and prophylaxis of inhaled biological toxins

This review highlights the current lack of therapeutic and prophylactic treatments for use against inhaled biological toxins, especially those considered as potential biological warfare (BW) or terrorist threats. Although vaccine development remains a priority, the use of rapidly deployable adjunctive therapeutic or prophylactic drugs could be life-saving in severe cases of intoxication or where vaccination has not been possible or immunity not established. The current lack of such drugs is due to many factors. Thus, methods involving molecular modelling are limited by the extent to which the cellular receptor sites and mode of action and structure of a toxin need to be known. There is also our general lack of knowledge of what effect individual toxins will have when inhaled into the lungs - whether and to what extent the action will be cell specific and cytotoxic or rather an acute inflammatory response requiring the use of immunomodulators. Possible sources of specific high-affinity toxin antagonists being investigated include monoclonal antibodies, selected oligonucleotides (aptamers) and derivatized dendritic polymers (dendrimers). The initial selection of suitable agents of these kinds can be made using cytotoxicity assays involving cultured normal human lung cells and a range of suitable indicators. The possibility that a mixture of selected antibody, aptamer or dendrimer-based materials for one or more toxins could be delivered simultaneously as injections or as inhaled aerosol sprays should be investigated.

Bacterial spores as vaccine vehicles

For the first time, bacterial spores have been evaluated as vaccine vehicles. Bacillus subtilis spores displaying the tetanus toxin fragment C (TTFC) antigen were used for oral and intranasal immunization and were shown to generate mucosal and systemic responses in a murine model. TTFC-specific immunoglobulin G titers in serum (determined by enzyme-linked immunosorbent assay) reached significant levels 33 days after oral dosing, while responses against the spore coat proteins were relatively low. Tetanus antitoxin levels were sufficient to protect against an otherwise lethal challenge of tetanus toxin (20 50% lethal doses). The robustness and long-term storage properties of bacterial spores, coupled with simplified genetic manipulation and cost-effective manufacturing, make them particularly attractive vehicles for oral and intranasal vaccination.

Targeted expression of tetanus toxin: a new tool to study the neurobiology of behavior

Over the past few decades, the explosion of molecular genetic knowledge, particularly in the fruit fly Drosophila melanogaster, has led to the identification of a large number of genes, which, when mutated, directly or indirectly affect fly behavior. Beyond the genetic and molecular characterization of genes and their associated molecular pathways, recent advances in molecular genetics also have allowed the development of new tools dedicated more directly to the dissection of the neural bases for various behaviors. In particular, the conjunction of the development of two techniques--the enhancer-trap detection system and the targeted gene expression system, based on the yeast GAL4 transcription factor--has led to the development of the binary enhancer-trap P[GAL4] expression system, which allows the selective activation of any cloned gene in a wide variety of tissue- and cell-specific patterns. Thus, this development, in addition to allowing the anatomical characterization of neuronal circuitry, also allows, via the expression of tetanus toxin light chain (known to specifically block synaptic transmission), an investigation of the role of specific neurons in certain behaviors. Using this system of "toxigenetics," several forms of behavior--from those mediated by sensory systems, such as olfaction, mechanoreception, and vision, to those mediated by higher brain function, such as learning, memory and locomotion--have been studied. These studies aim to map neuronal circuitry underlying specific behaviors and thereby unravel relevant neurophysiological mechanisms. The advantage of this approach is that it is noninvasive and permits the investigation of behavior in the free moving animal. We review a number of behavioral studies that have successfully employed this toxigenetic approach, and we hope to persuade the reader that transgenic tetanus toxin light chain is a useful and appropriate tool for the armory of neuroethologists.

Site-directed mutagenesis identifies active-site residues of the light chain of botulinum neurotoxin type A

Botulinum neurotoxins (BoNTs) are metalloproteases which block neuroexocytosis via specific cleavage and inactivation of SNARE proteins. Such proteolysis accounts for the extreme toxicity of these neurotoxins and of their prolonged effect. The recently determined structures of BoNT/A and/B allows one to design active-site mutants to probe the role of specific residues in the proteolysis of SNARE proteins. Here we present the results of mutations of the second glutamyl residue involved in zinc coordination and of a tyrosine and a phenylalanine residues that occupy critical positions within the active site of BoNT/A. The spectroscopic properties of the purified mutants are closely similar to those of the wild-type molecule indicating the acquisition of a correct tertiary structure. Mutation of the Glu-262* nearly abolishes SNAP-25 hydrolysis as expected for a residue involved in zinc coordination. The Phe-266 and Tyr-366 mutants have reduced proteolytic activity indicating a direct participation in the proteolytic reaction, and their possible role in catalysis is discussed.

Botulinum neurotoxin types B and E: purification, limited proteolysis by endoproteinase Glu-C and pepsin, and comparison of their identified cleaved sites relative to the three-dimensional structure of type A neurotoxin

Botulinum neurotoxin (NT) serotypes B and E are approximately 150 kDa proteins. Isolated from the liquid culture of Clostridium botulinum the NT type E is a single chain protein while the NT type B, from the proteolytic strain of the bacteria, is a mixture of dichain (nicked within a disulfide loop located about one-third the way from the N-terminus to the C-terminus) protein and its precursor single-chain protein. Endoproteinase Glu-C (EC 3.4.21.19) and pepsin (EC 3.4.23.1) were used for controlled digestion of NT types B and E; the amino acid residues flanking many of the cleavable peptide bonds were identified and the corresponding proteolytic fragments partially characterized. Chemical identification of 82 and 108 residues of types B and E NT, respectively, revealed that the residue 738 and 1098 in type E NT, identified as Leu and Asn, respectively, differ from those deduced from nucleotide sequences. Several fragments overlapped spanning various segments of the NT's functional domains; they appear to have potential for structure-function studies of the NT. The cleavage sites were compared with the previously determined proteolyzed sites on NT types A and E. The cleavage sites of the NT types A, B and E, all exposed on the protein surface, were scrutinized in the context of the three-dimensional structure of crystallized NT type A [Lacy, D.B., Stevens, R.C., 1999. J. Mol. Biol. 291, 1091-1104]. Detailed procedures for isolation of pure NT types B and E in large quantities (average yield 92 and 62 mg, respectively) suitable for crystallization are reported.

Recombinant forms of tetanus toxin engineered for examining and exploiting neuronal trafficking pathways

Tetanus toxin is a fascinating, multifunctional protein that binds to peripheral neurons, undergoes retrograde transport and trans-synaptic transfer to central inhibitory neurons where it blocks transmitter release, thereby, causing spastic paralysis. As a pre-requisite for exploiting its unique trafficking properties, a novel recombinant single chain was expressed at a high level in Escherichia coli as a soluble, easily purifiable protein. It could be activated with enterokinase to produce a dichain that matched native toxin in terms of proteolytic and neuroinhibitory activities, as well as induction of spastic paralysis in mice. Importantly, nicking was not essential for protease activity. Substitution of Glu(234) by Ala created a protease-deficient atoxic form, which blocked the neuroparalytic action of tetanus toxin in vitro, with equal potency to its heavy chain; but, the mutant proved >30-fold more potent in preventing tetanus in mice. This observation unveils differences between the intoxication processes resulting from retrograde transport of toxin in vivo and its local uptake into peripheral or central nerves in vitro, dispelling a popularly held belief that the heavy chain is the sole determinant for efficient trafficking. Thus, this innocuous mutant may be a useful vehicle, superior to the heavy chain, for drug delivery to central neurons.

Active-site mutagenesis of tetanus neurotoxin implicates TYR-375 and GLU-271 in metalloproteolytic activity

Tetanus neurotoxin (TeNT) blocks neurotransmitter release by cleaving VAMP/synaptobrevin, a membrane associated protein involved in synaptic vesicle fusion. Such activity is exerted by the N-terminal 50kDa domain of TeNT which is a zinc-dependent endopeptidase (TeNT-L-chain). Based on the three-dimensional structure of botulinum neurotoxin serotype A (BoNT/A) and serotype B (BoNT/B), two proteins closely related to TeNT, and on X-ray scattering studies of TeNT, we have designed mutations at two active site residues to probe their involvement in activity. The active site of metalloproteases is composed of a primary sphere of residues co-ordinating the zinc atom, and a secondary sphere of residues that determines proteolytic specificity and activity. Glu-261 and Glu-267 directly co-ordinates the zinc atom in BoNT/A and BoNT/B respectively and the corresponding residue of TeNT was replaced by Asp or by the non conservative residue Ala. Tyr-365 is 4.3A away from zinc in BoNT/A, and the corresponding residue of TeNT was replaced by Phe or by Ala. The purified mutants had CD, fluorescence and UV spectra closely similar to those of the wild-type molecule. The proteolytic activity of TeNT-Asp-271 (E271D) is similar to that of the native molecule, whereas that of TeNT-Phe-375 (Y375F) is lower than the control. Interestingly, the two Ala mutants are completely devoid of enzymatic activity. These results demonstrate that both Glu-271 and Tyr-375 are essential for the proteolytic activity of TeNT.

Light chain of botulinum A neurotoxin expressed as an inclusion body from a synthetic gene is catalytically and functionally active

Botulinum neurotoxins, the most potent of all toxins, induce lethal neuromuscular paralysis by inhibiting exocytosis at the neuromuscular junction. The light chains (LC) of these dichain neurotoxins are a new class of zinc-endopeptidases that specifically cleave the synaptosomal proteins, SNAP-25, VAMP, or syntaxin at discrete sites. To facilitate the structural and functional characterization of these unique endopeptidases, we constructed a synthetic gene for the LC of the botulinum neurotoxin serotype A (BoNT/A), overexpressed it in Escherichia coli, and purified the gene product from inclusion bodies. Our procedure can provide 1.1 g of the LC from 1 L of culture. The LC product was stable in solution at 4 degrees C for at least 6 months. This rBoNT/A LC was proteolytically active, specifically cleaving the Glu-Arg bond in a 17-residue synthetic peptide of SNAP-25, the reported cleavage site of BoNT/A. Its calculated catalytic efficiency kcat/Km was higher than that reported for the native BoNT/A dichain. Treating the rBoNT/A LC with mercuric compounds completely abolished its activity, most probably by modifying the cysteine-164 residue located in the vicinity of the active site. About 70% activity of the LC was restored by adding Zn2+ to a Zn2+-free, apo-LC preparation. The LC was nontoxic to mice and failed to elicit neutralizing epitope(s) when the animals were vaccinated with this protein. In addition, injecting rBoNT/A LC into sea urchin eggs inhibited exocytosis-dependent plasma membrane resealing. For the first time, results of our study make available a large amount of the biologically active toxin fragment in a soluble and stable form.

Protective effect of supplemental superoxide dismutase on survival of neuronal cells during starvation. Requirement for cytosolic distribution

There is evidence that raising cellular levels of Cu2+/Zn2+ superoxide dismutase (SOD1) can protect neurons from oxidative injury. We compared a novel method of elevating neuronal SOD activity using a recombinant hybrid protein composed of the atoxic neuronal binding domain of tetanus toxin (C fragment or TTC) and human SOD1 (hSOD1) with increasing cellular SOD levels through overexpression. Fetal murine cortical neurons or N18-RE-105 cells were incubated with the TTC-hSOD1 hybrid protein and compared to cells constitutively expressing hSOD1 for level of SOD activity, cellular localization of hSOD1, and capacity to survive glucose and pyruvate starvation. Cells incubated with TTC-hSOD1 showed a threefold increase in cellular SOD activity over control cells. This level of increase was comparable to fetal cortical neurons from transgenic mice constitutively expressing hSOD1 and transfected N18-RE-105 cells expressing a green fluorescent protein-hSOD1 fusion protein (GFP-hSOD1). Human SOD1 was distributed diffusely throughout the cytoplasm of the transgenic murine neurons and transfected N18-RE-105 cells. In contrast, cells incubated with TTC-hSOD1 showed hSOD1 localized to the cell surface and intra-cytoplasmic vesicles. The cells expressing hSOD1 showed enhanced survival in glucose- and pyruvate-free medium. Neither cortical neurons nor N18-RE-105 cells incubated in TTC-hSOD1 showed increased survival during starvation. Access to the site where toxic superoxides are generated or their targets may be necessary for the protective function of SOD1.

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.

Neurotoxins affecting neuroexocytosis

Nerve terminals are specific sites of action of a very large number of toxins produced by many different organisms. The mechanism of action of three groups of presynaptic neurotoxins that interfere directly with the process of neurotransmitter release is reviewed, whereas presynaptic neurotoxins acting on ion channels are not dealt with here. These neurotoxins can be grouped in three large families: 1) the clostridial neurotoxins that act inside nerves and block neurotransmitter release via their metalloproteolytic activity directed specifically on SNARE proteins; 2) the snake presynaptic neurotoxins with phospholipase A(2) activity, whose site of action is still undefined and which induce the release of acethylcholine followed by impairment of synaptic functions; and 3) the excitatory latrotoxin-like neurotoxins that induce a massive release of neurotransmitter at peripheral and central synapses. Their modes of binding, sites of action, and biochemical activities are discussed in relation to the symptoms of the diseases they cause. The use of these toxins in cell biology and neuroscience is considered as well as the therapeutic utilization of the botulinum neurotoxins in human diseases characterized by hyperfunction of cholinergic terminals.

High-level expression, purification, and characterization of recombinant type A botulinum neurotoxin light chain

Botulinum neurotoxin light chain (BoNT LC, 50 kDa) is responsible for the zinc endopeptidase activity specific for proteins of neuroexocytosis apparatus. We describe the expression of recombinant type A BoNT LC in Escherichia coli as well as the purification and characterization of the recombinant protein. A high level of expression of BoNT/A LC was obtained by an extended postinduction time of 15 h at 30 degrees C. Recombinant BoNT/A LC was isolated from an Ni(2+) column. Due to its high pI ( approximately 8.7), purification was achieved by a single step of passing the protein through anion-exchange chromatography at pH 8.0 without the need of elution. The purified recombinant BoNT/A LC retained proteolytic activity and had a secondary structure similar to that of native LC determined by CD measurement.

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.

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

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

Anthrax lethal factor cleaves the N-terminus of MAPKKs and induces tyrosine/threonine phosphorylation of MAPKs in cultured macrophages

Lethal factor (LF) is the major virulence factor produced by Bacillus anthracis. LF is sufficient to cause death in laboratory animals and cytolysis of peritoneal macrophages and macrophage cell lines. LF contains the characteristic zinc binding motif of metalloproteases and indirect evidence suggest that this hydrolytic activity is essential for its cytotoxicity. To identify the substrate(s) of LF, we have used the yeast two-hybrid system, employing a LF inactive mutant as bait. This approach has led to the identification of the MAP kinase kinases (MAPKKs) Mek1 and Mek2 as proteins capable of specific interaction with LF. LF cleaves Mek1 and Mek2 within their N-terminus in vitro and in vivo, hydrolyzing a Pro8-Ile9 and a Pro10-Arg11 peptide bond in Mek1 and Mek2 respectively. The removal of the amino terminus of MAPKKs eliminates the "docking site" for the MAPKs ERK1 and ERK2, which become phosphorylated in cultured macrophages following toxin challenge. The possible implications of these findings for the cytolysis of macrophage cells induced by LF are discussed. These results open the way to the design and screening of specific inhibitors of LF.

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.

Metal substitution of tetanus neurotoxin

Tetanus neurotoxin was depleted of its catalytic Zn2+ ion, and the apotoxin was reconstituted with different transition metal ions. The Mn2+- and Co2+-tetanus neurotoxins are highly active in the proteolysis of vesicle-associated membrane protein/synaptobrevin, the natural substrate of this toxin, whereas Cu2+ and Fe2+ minimally supported proteolytic activity. The visible absorbance spectrum of Co2+-tetanus neurotoxin shows a maximum at 538 nm with a molar absorption coefficient of 82 M(-1) x cm(-1). These results indicate that the Zn2+ environment at the active site of tetanus neurotoxin is different from those of known Zn2+-endopeptidases and provide a structural basis for the definition of tetanus neurotoxin, and the related clostridial neurotoxins, as an independent family of metalloproteases.

Cooperative exosite-dependent cleavage of synaptobrevin by tetanus toxin light chain

The light chain (L chain) of tetanus neurotoxin (TeNT) has been shown to have been endowed with zinc endopeptidase activity, selectively directed toward the Gln76-Phe77 bond of synaptobrevin, a vesicle-associated membrane protein (VAMP) critically involved in neuroexocytosis. In previous reports, truncations at the NH2 and COOH terminus of synaptobrevin have shown that the sequence 39-88 of synaptobrevin is the minimum substrate of TeNT, suggesting either the requirement of a well defined three-dimensional structure of synaptobrevin or a role in the mechanism of substrate hydrolysis for residues distal from the cleavage site. In this study, the addition of NH2- and COOH-terminal peptides of synaptobrevin, S 27-55 (S1) and S 82-93 (S2), to the synaptobrevin fragment S 56-81 allowed the cleavage of this latter peptide by TeNT to occur. This appears to result from an activation process mediated by the simultaneous binding of S1 and S2 with complementary sites present on TeNT as shown by surface plasmon resonance experiments and the determination of kinetic constants. All these results favor an exosite-controlled hydrolysis of synaptobrevin by TeNT, probably involving a conformational change of the toxin. This could account for the high degree of substrate specificity of TeNT and, probably, botulinum neurotoxins.

Immune response in mice following immunization with DNA encoding fragment C of tetanus toxin

Tetanus toxin is a potent neurotoxin synthesized by Clostridium tetani. Immunization with fragment C protein, the nontoxic C-terminal domain of tetanus toxin, will protect mice against lethal challenge with tetanus toxin. A synthetic gene encoding fragment C (tetC) had previously been shown to express high levels of fragment C in Saccharomyces cerevisiae. A plasmid, pcDNA3/tetC, which encodes the synthetic tetC gene expressed under the control of the human cytomegalovirus major intermediate-early promoter/enhancer region, was constructed. Expression of fragment C was observed in eukaryotic cells growing in vitro following transfection with pcDNA3/tetC. The immune response induced by intramuscular immunization with pure pcDNA3/tetC DNA was evaluated in a murine model. Anti-fragment C serum immunoglobulin and proliferative responses in splenocytes were observed in BALB/c mice following two immunizations with pcDNA3/tetC. The major immunoglobulin G subclass that recognized fragment C was immunoglobulin G2a, and the stimulated splenocytes secreted high levels of gamma interferon. Immunity to tetanus is dependent on the presence of neutralizing serum antibodies against tetanus toxin. Sufficient anti-fragment C serum immunoglobulins were induced by DNA-mediated immunization to protect mice against lethal challenge with tetanus toxin.

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.

Tetanus toxin inhibits neuroexocytosis even when its Zn(2+)-dependent protease activity is removed

Tetanus toxin (TeTX) is a dichain protein that blocks neuroexocytosis, an action attributed previously to Zn(2+)-dependent proteolysis of synaptobrevin (Sbr) by its light chain (LC). Herein, its cleavage of Sbr in rat cerebrocortical synaptosomes was shown to be minimized by captopril, an inhibitor of certain metalloendoproteases, whereas this agent only marginally antagonized the inhibition of noradrenaline release, implicating a second action of the toxin. This hypothesis was proven by preparing three mutants (H233A, E234A, H237A) of the LC lacking the ability to cleave Sbr and reconstituting them with native heavy chain. The resultant dichains were found to block synaptosomal transmitter release, albeit with lower potency than that made from wild type LC; as expected, captopril attenuated only the inhibition caused by the protease-active wild type toxin. Moreover, these protease-inactive toxins or their LCs blocked evoked quantal release of transmitter when micro-injected inside Aplysia neurons. TeTX was known to stimulate in vitro a Ca(2+)-dependent transglutaminase (TGase) (Facchiano, F., and Luini, A. (1992) J. Biol. Chem. 267, 13267-13271), an affect found here to be reduced by an inhibitor of this enzyme, monodansylcadaverine. Accordingly, treatment of synaptosomes with the latter antagonized the inhibition of noradrenaline release by TeTX while not affecting Sbr cleavage. This drug also attenuated the inhibitory action of all the mutants. Hence, it is concluded that TeTX inhibits neurotransmitter release by proteolysis of Sbr and a protease-independent activation of a neuronal TGase.

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.

Characterization of recombinant tetanus toxin derivatives suitable for vaccine development

Recombinant derivatives of tetanus toxin (TeTx) were isolated and used to immunize mice. Recombinant TeTx light chain, a derivative of fragment C that had lost the ability to bind neurons, and a recombinant TeTx holotoxoid that could protect mice against TeTx challenge were identified.

Targeted expression of tetanus toxin light chain in Drosophila specifically eliminates synaptic transmission and causes behavioral defects

Tetanus toxin cleaves the synaptic vesicle protein synaptobrevin, and the ensuing loss of neurotransmitter exocytosis has implicated synaptobrevin in this process. To further the study of synaptic function in a genetically tractable organism and to generate a tool to disable neuronal communication for behavioural studies, we have expressed a gene encoding tetanus toxin light chain in Drosophila. Toxin expression in embryonic neurons removes detectable synaptobrevin and eliminates evoked, but not spontaneous, synaptic vesicle release. No other developmental or morphological defects are detected. Correspondingly, only synaptobrevin (n-syb), but not the ubiquitously expressed syb protein, is cleaved by tetanus toxin in vitro. Targeted expression of toxin can produce specific behavioral defects; in one case, the olfactory escape response is reduced.