User:Kajax3/sandbox

This content will go in the SNARE (protein) page, under the Toxins heading I will add a Tetanus Neurotoxin (TeNT) subheading.

Tetanus Neurotoxin (TeNT)


Tetanus toxin, or TeNT, is composed of a heavy chain (100KDa) and a light chain (50kDa) connected by a disulfide bond. The heavy chain is responsible for neurospecific binding of TeNT to the nerve terminal membrane, endocytosis of the toxin, and translocation of the light chain into the cytosol. The light chain has zinc-dependent endopepdtidase or more specifically matrix metalloproteinase (MMP) activity through which cleaveage of synaptobrevin or VAMP is carried out.

For the light chain of TeNT to be activated one atom of zinc must be bound to every molecule of toxin. When zinc is bound reduction of the disulfide bond will be carried out primarily via the NADPH-thioredoxin reductase-thioredoxin redox system. Then the light chain is 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. This cleavage of synaptobrevin is the final target of TeNT and even in low doses the neurotoxin will inhibit neurotransmitter exocytosis.

Mechanism
Tetanus toxin causes violent spastic paralysis by blocking the release of γ-aminobutyric acid (GABA). GABA is a neurotransmitter that inhibits motor neurons.

The action of the A-chain stops the affected neurons from releasing the inhibitory neurotransmitters GABA and glycine, but also excitatory transmitters, by degrading the protein synaptobrevin 2. The consequence of this is dangerous overactivity in the muscles from the smallest stimulus—the failure of inhibition of motor reflexes by sensory stimulation. This causes generalized contractions of the agonist and antagonist musculature, termed a tetanic spasm.

Mechanism
The mechanism of TeNT action can be broken down and discussed in 6 different steps.


 * Transport


 * 1) Specific binding in the periphery neurons
 * 2) Retrograde axonal transport to the central nervous system (CNS) inhibitory interneurons
 * 3) Transcytosis from the axon into the inhibitory interneurons
 * Action


 * 1) Temperature and pH mediated translocation of the light chain into the cytosol
 * 2) Reduction of the disulphide bond between the light and heavy chain
 * 3) Cleavage of synaptobrevin

The first three steps outline the travel of tetanus 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 botulinum neurotoxin. It also binds 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 the disulfide bond is reduced, mainly by the NADPH-thioredoxin reductase-thioredoxin redox system and the light chain is 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 cleavage of synaptobrevin is the final target of TeNT and even in low doses the neurotoxin will inhibit neurotransmitter exocytosis in the inhibitory interneurons. The blockage of these neurotransmitters is what causes the physiological effects that accompany TeNT, specifically the blockage of the neurotransmitters GABA and glycine.

Tetanus toxin causes violent spastic paralysis by blocking the release of γ-aminobutyric acid (GABA). GABA is a neurotransmitter that inhibits motor neurons.

The action of the A-chain stops the affected neurons from releasing the inhibitory neurotransmitters GABA and glycine, but also excitatory transmitters, by degrading the protein synaptobrevin 2. The consequence of this is dangerous overactivity in the muscles from the smallest stimulus—the failure of inhibition of motor reflexes by sensory stimulation. This causes generalized contractions of the agonist and antagonist musculature, termed a tetanic spasm.

Tetanus Neurotoxin (TeNT)


Tetanus toxin or TeNT, is composed of a heavy chain (100KDa) and a light chain (50kDa) connected by a disulfide bond. The heavy chain is responsible for neurospecific binding of TeNT to the nerve terminal membrane, endocytosis of the toxin, and translocation of the light chain into the cytosol. The light chain has zinc-dependent endopepdtidase or more specifically matrix metalloproteinase (MMP) activity through which cleaveage of synaptobrevin or VAMP is carried out. TeNT causes tetanus through cleavage of synaptobrevin 2 and not synaptobrevin 1, similarly to botulinum neurotoxin B.

Transportation of TeNT into the nerve is mediated by the heavy chain which reversibly binds to the G1b series of polysialogangliosides on neuron membranes. TeNT then moves with the ganglioside through the lipid membrane to bind to a more specific protein receptor. The receptor with TeNT attached is then endocytosed and goes through retrograde axonal transport through the motor neurons to reach its final target, the inhibitory interneurons of the spinal cord. Translocation of the light chain into the cytosol follows and its subsequent synaptobrevin cleavage.

For the light chain of TeNT to be activated, one atom of zinc must be bound to every molecule of toxin. While the presence of zinc has almost no influence over light chain conformation or stability it is necessary for protease action and is thought to be catalytic. The disulfide bond between the light and heavy chains must also be intact after translocation into the interneuron. The difference between the acidic pH of the vesicle and the neutral pH of the cytosol allows the light chain to refold after translocation, and then it is ready for separation from the heavy chain. When these requirements are met the disulfide bond is reduced, mainly by the NADPH-thioredoxin reductase-thioredoxin redox system. Then the light chain is 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. This cleavage of synaptobrevin is the final target of TeNT, and even in low doses the neurotoxin will inhibit neurotransmitter exocytosis.