Toxins::SNARE (protein)

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Toxins Many neurotoxins directly affect SNARE complexes. Such toxins as the botulinum and tetanus toxins work by targeting the SNARE components. These toxins prevent proper vesicle recycling and result in poor muscle control, spasms, paralysis, and even death.

Botulinum neurotoxin

Botulinum Toxin (BoNT) is one of the most potent toxins to have ever been discovered.<ref name="Cytotoxicity">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> It is a proteolytic enzyme that cleaves SNARE proteins in neurons. Its protein structure is composed of two peptide subunits, a heavy chain (100kDas) and a light chain (50kDas), which are held together by a disulfide bond. The action of BoNT follows a 4-step mechanism including binding to the neuronal membrane, endocytosis, membrane translocation, and proteolysis of SNARE proteins.<ref name="Mechanism">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

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In its mechanism of action, the heavy chain of BoNT is first used to find its neuronal targets and bind to the gangliosides and membrane proteins of presynaptic neurons. Next, the toxin is then endocytosed into the cell membrane. The heavy chain undergoes a conformational change important for translocating the light chain into the cytosol of the neuron. Finally, after the light chain of BoNT is brought into the cytosol of the targeted neuron, it is released from the heavy chain so that it can reach its active cleavage sites on the SNARE proteins.<ref name="Mechanism" /> The light chain is released from the heavy chain by the reduction of the disulfide bond holding the two together. The reduction of this disulfide bond is mediated by the NADPH-thioredoxin reductase-thioredoxin system.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The light chain of BoNT acts as a metalloprotease on SNARE proteins that is dependent on Zn(II) ions,<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> cleaving them and eliminating their function in exocytosis.

There are 8 known isotypes of BoNT, BoNT/A - BoNT/H, each with different specific cleavage sites on SNARE proteins. SNAP25, a member of the SNARE protein family located in the membrane of cells, is cleaved by BoNT isotypes A, C, and E. The cleavage of SNAP-25 by these isotypes of BoNT greatly inhibits their function in forming the SNARE complex for fusion of vesicles to the synaptic membrane. BoNT/C also targets Syntaxin-1, another SNARE protein located in the synaptic membrane. It degenerates these Syntaxin proteins with a similar outcome as with SNAP-25. A third SNARE protein, Synaptobrevin (VAMP), is located on cell vesicles. VAMP2 is targeted and cleaved by BoNT isotypes B, D, and F in synaptic neurons.<ref name="Cytotoxicity" /> The targets of these various isotypes of BoNT as well as Tetanus Neurotoxin (TeNT) are shown in the figure to the right.

In each of these cases, Botulinum Neurotoxin causes functional damage to SNARE proteins, which has significant physiological and medical implications. By damaging SNARE proteins, the toxin prevents synaptic vesicles from fusing to the synaptic membrane and releasing their neurotransmitters into the synaptic cleft. With the inhibition of neurotransmitter release into the synaptic cleft, action potentials cannot be propagated to stimulate muscle cells. This result in paralysis of those infected and in serious cases, it can cause death. Although the effects of Botulinum Neurotoxin can be fatal, it has also been used as a therapeutic agent in medical and cosmetic treatments.<ref name="pmid9664753">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref name="pmid8556256">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Tetanus neurotoxin

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The breakdown of responsibilities and mechanisms of the heavy (HC) and light chain (LC) of tetanus neurotoxin: The HC assists in binding of TeNT to both the ganglioside receptor and the final receptor. Once TeNT is in the vesicle in the inhibitory interneuron space the HC assists in translocation of the LC into the cytoplasm. Then the LC, characterized by zinc endopeptidase activity, inhibits neurotransmission by cleavage of synaptobrevin 1.

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 endopeptidase or more specifically matrix metalloproteinase (MMP) activity through which cleaveage of synaptobrevin or VAMP is carried out.<ref name="VAMP cleavage">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

For the light chain of TeNT to be activated one atom of zinc must be bound to every molecule of toxin.<ref name="Zinc Activity">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> When zinc is bound reduction of the disulfide bond will be carried out primarily via the NADPH-thioredoxin reductase-thioredoxin redox system.<ref name=Thioredoxin>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> Then the light chain is free to cleave the Gln76-Phe77 bond of synaptobrevin.<ref name="VAMP cleavage" /> 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.<ref name="NSF target">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> This cleavage of synaptobrevin is the final target of TeNT and even in low doses the neurotoxin will inhibit neurotransmitter exocytosis.


SNARE (protein) sections
Intro   Types    Structure    Membrane fusion    Components    Mechanism of membrane fusion   Regulatory Effects on Exocytosis   Toxins    Role in neurotransmitter release    Role in autophagy   References  External links  

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