Recombinant and truncated tetanus neurotoxin light chain: cloning, expression, purification, and proteolytic activity

Synthesis of BLF1-containing trimethyl chitosan nanoparticles and evaluation of its immunogenicity and protection in syrian mice by oral and subcutaneous injections

The bacterium Burkholderia pseudomallei is the cause of melioidosis infectious disease. In this bacterium, the BLF1 protein wide inhibits the synthesis of proteins in human cells. This disease is reported to cause a death rate of 40% in some parts of the world. Currently, no effective vaccine is available against this bacterial infection. In this study, therefore, a Nano vaccine was synthesized based on the trimethyl chitosan (TMC) polymer containing the BLF1 recombinant protein, and its immunogenicity and protection in Syrian mice were evaluated by oral and subcutaneous injections. The BLF1 recombinant protein expression was induced in Escherichia coli Bl21 (DE3) and purified by the affinity chromatography technique. Recombinant protein-containing nanoparticles (NPs) were then synthesized by the ionotropic gelation method. After oral and subcutaneous injections, antibody titration was assessed by the indirect ELISA assay. Finally, murine groups were challenged using the BLF1 toxin. The results indicated that the immune system showed more antibody titration in subcutaneous injection than in the oral form. However, the results were reversed in the challenge results, and the survival rate was more significant in the oral injection.

The blockade of the neurotransmitter release apparatus by botulinum neurotoxins

The high toxicity of the seven serotypes of botulinum neurotoxins (BoNT/A to G), together with their specificity and reversibility, includes them in the list A of potential bioterrorism weapons and, at the same time, among the therapeutics of choice for a variety of human syndromes. They invade nerve terminals and cleave specifically the three proteins which form the heterotrimeric SNAP REceptors (SNARE) complex that mediates neurotransmitter release. The BoNT-induced cleavage of the SNARE proteins explains by itself the paralysing activity of the BoNTs because the truncated proteins cannot form the SNARE complex. However, in the case of BoNT/A, the most widely used toxin in therapy, additional factors come into play as it only removes a few residues from the synaptosomal associate protein of 25 kDa C-terminus and this results in a long duration of action. To explain these facts and other experimental data, we present here a model for the assembly of the neuroexocytosis apparatus in which Synaptotagmin and Complexin first assist the zippering of the SNARE complex, and then stabilize and clamp an octameric radial assembly of the SNARE complexes.

Evidence for a radial SNARE super-complex mediating neurotransmitter release at the Drosophila neuromuscular junction

The SNARE proteins VAMP/synaptobrevin, SNAP-25 and syntaxin are core components of the apparatus that mediates neurotransmitter release. They form a heterotrimeric complex, and an undetermined number of SNARE complexes assemble to form a super-complex. Here, we present a radial model of this nanomachine. Experiments performed with botulinum neurotoxins led to the identification of one arginine residue in SNAP-25 and one aspartate residue in syntaxin (R206 and D253 in Drosophila melanogaster). These residues are highly conserved and predicted to play a major role in the protein-protein interactions between SNARE complexes by forming an ionic couple. Accordingly, we generated transgenic Drosophila lines expressing SNAREs mutated in these residues and performed an electrophysiological analysis of their neuromuscular junctions. Our results indicate that SNAP-25-R206 and syntaxin-D253 play a major role in neuroexocytosis and support a radial assembly of several SNARE complexes interacting via the ionic couple formed by these two residues.

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.

Adenoviral clostridial light chain gene-based synaptic inhibition through neuronal synaptobrevin elimination

Clostridial neurotoxins have assumed increasing importance in clinical application. The toxin's light chain component (LC) inhibits synaptic transmission by digesting vesicle-docking proteins without directly altering neuronal health. To study the properties of LC gene expression in the nervous system, an adenoviral vector containing the LC of tetanus toxin (AdLC) was constructed. LC expressed in differentiated neuronal PC12 cells was shown to induce time- and concentration-dependent digestion of mouse brain synaptobrevin in vitro as compared to control transgene products. LC gene expression in the rat lumbar spinal cord disrupted hindlimb sensorimotor function in comparison to control vectors as measured by the Basso-Beattie-Bresnahan (BBB) scale (P<0.001) and rotarod assay (P<0.003). Evoked electromyography (EMG) showed increased stimulus threshold and decreased response current amplitude in LC gene-transferred rats. At the peak of functional impairment, neither neuronal TUNEL staining nor reduced motor neuron density could be detected. Spontaneous functional recovery was observed to parallel the cessation of LC gene expression. These results suggest that light chain gene delivery within the nervous system may provide a nondestructive means for focused neural inhibition to treat a variety of disorders related to excessive synaptic activity, and prove useful for the study of neural circuitry.

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.

Clostridium botulinum and its neurotoxins: a metabolic and cellular perspective

Clostridium botulinum comprises a diverse assemblage of clostridia that have the common property of producing a distinctive protein neurotoxin (BoNT) of similar pharmacological activity and extraordinary potency. BoNTs are produced in culture as molecular complexes consisting of BoNT, hemagglutinin (HA) and associated subcomponent proteins, nontoxic nonhemagglutinin (NTNH), and RNA. The genes encoding the protein components reside as a cluster on the chromosome, on bacteriophages, or on plasmids depending on the C. botulinum serotype. A gene BotR coding for a regulatory protein has been detected in toxin gene clusters from certain strains, as well as ORFs coding for uncharacterized components. The gene encoding TeNT is located on a large plasmid, and expression of the structural gene is controlled by the regulatory gene, TetR, located immediately upstream of the TeNT structural gene. TeNT is not known to be assembled into a protein/nucleic acid complex in culture. Cellular synthesis of BoNT and TeNT have been demonstrated to be positively regulated by the homologous proteins, BotR/A and TetR. Evidence suggests that negative regulatory factors and general control cascades such as those involved in nitrogen regulation and carbon catabolite repression also regulate synthesis of BoNTs. Neurotoxigenic clostridia have attracted considerable attention from scientists and clinicians during the past decade, and many excellent reviews are available on various aspects of these organisms and their neurotoxins. However, certain areas have not been well-studied, including metabolic regulation of toxin formation and genetic tools to study neurotoxigenic clostridia. These topics are the focus of this review.

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.

Tetanus and botulinum neurotoxins: turning bad guys into good by research

The neuroparalytic syndromes of tetanus and botulism are caused by neurotoxins produced by bacteria of the genus Clostridium. They are 150 kDa proteins consisting of three-domains, endowed with different functions: neurospecific binding, membrane translocation and specific proteolysis of three key components of the neuroexocytosis apparatus. After binding to the presynaptic membrane of motoneurons, tetanus neurotoxin (TeNT) is internalized and transported retroaxonally to the spinal cord, where it blocks neurotransmitter release from spinal inhibitory interneurons. In contrast, the seven botulinum neurotoxins (BoNT) act at the periphery and inhibit acetylcholine release from peripheral cholinergic nerve terminals. TeNT and BoNT-B, -D, -F and -G cleave specifically at single but different peptide bonds, VAMP/synaptobrevin, a membrane protein of small synaptic vesicles. BoNT types -A, -C and -E cleave SNAP-25 at different sites within the COOH-terminus, whereas BoNT-C also cleaves syntaxin. BoNTs are increasingly used in medicine for the treatment of human diseases characterized by hyperfunction of cholinergic terminals.