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Tetanus toxin ( TeNT) is an extremely potent neurotoxin produced by the vegetative cell of Clostridium tetani in anaerobic conditions, causing tetanus. It has no known function for clostridia in the soil environment where they are normally encountered. It is also called spasmogenic toxin, tentoxilysin, tetanospasmin, or tetanus neurotoxin. The LD50 of this toxin has been measured to be approximately 2.5–3 ng/kg, making it second only to the related botulinum toxin (LD50 2 ng/kg) as the deadliest toxin in the world. However, these tests are conducted solely on mice, which may react to the toxin differently from humans and other animals.
C. tetani also produces the exotoxin tetanolysin, a hemolysin, that causes destruction of tissues.
The TetX gene encoding this protein is located on the PE88 plasmid.
Several structures of the binding domain and the peptidase domain have been solved by X-ray crystallography and deposited in the PDB.
The first three steps outline the travel of tetanus toxin from the peripheral nervous system to where it is taken up to the CNS and has its final effect. The last three steps document the changes necessary for the final mechanism of the neurotoxin.
Transport to the CNS inhibitory interneurons begins with the B-chain mediating the neurospecific binding of TeNT to the nerve terminal membrane. It binds to GT1b polysialogangliosides, similarly to the C. botulinum neurotoxin. It also binds to another poorly characterized GPI-anchored protein receptor more specific to TeNT. Both the ganglioside and the GPI-anchored protein are located in lipid microdomains and both are requisite for specific TeNT binding. Once it is bound, the neurotoxin is then endocytosed into the nerve and begins to travel through the axon to the spinal neurons. The next step, transcytosis from the axon into the CNS inhibitory interneuron, is one of the least understood parts of TeNT action. At least two pathways are involved, one that relies on the recycling of synaptic vesicle 2 (SV2) system and one that does not.
Once the vesicle is in the inhibitory interneuron, its translocation is mediated by pH and temperature, specifically a low or acidic pH in the vesicle and standard physiological temperatures. Once the toxin has been translocated into the cytosol, chemical reduction of the disulfide bond to separate thiols occurs, mainly by the enzyme NADPH-thioredoxin reductase-thioredoxin. The light chain is then free to cleave the Gln76-Phe77 bond of synaptobrevin. Cleavage of synaptobrevin affects the stability of the SNARE core by restricting it from entering the low-energy conformation, which is the target for NSF binding. Synaptobrevin is an integral V-SNARE necessary for vesicle fusion to membranes. The final target of TeNT is the cleavage of synaptobrevin and, even in low doses, has the effect of interfering with exocytosis of from inhibitory . The blockage of the neurotransmitters γ-aminobutyric acid (GABA) and glycine is the direct cause of the physiological effects that TeNT induces. GABA inhibits motor neurons, so by blocking GABA, tetanus toxin causes violent spastic paralysis. The action of the A-chain also stops the affected neurons from releasing excitatory transmitters, by degrading the protein synaptobrevin 2. The combined consequence is dangerous overactivity in the from the smallest sensory stimuli, as the damping of is inhibited, leading to generalized contractions of the agonist and antagonist musculature, termed a "tetanic spasm".
Tetanic spasms can occur in a distinctive form called opisthotonos and be sufficiently severe to fracture long bones. The shorter nerves are the first to be inhibited, which leads to the characteristic early symptoms in the face and jaw, risus sardonicus and Tetanus.
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