Botulinum Toxin channels

Bacterial botulinum toxin blocks presynaptic acetylcholine release 725 Snake, scorpion, spider, fish and snail peptide venoms act on multiple molecular targets at the neuromuscular junction 727 Electrolyte imbalances alter the voltage sensitivity of muscle ion channels 728... 

Schematic illustration of a generalized cholinergic junction (not to scale). Choline is transported into the presynaptic nerve terminal by a sodium-dependent choline transporter (CHT). This transporter can be inhibited by hemicholinium drugs. In the cytoplasm, acetylcholine is synthesized from choline and acetyl -A (AcCoA) by the enzyme choline acetyltransferase (ChAT). Acetylcholine is then transported into the storage vesicle by a second carrier, the vesicle-associated transporter (VAT), which can be inhibited by vesamicol. Peptides (P), adenosine triphosphate (ATP), and proteoglycan are also stored in the vesicle. Release of transmitter occurs when voltage-sensitive calcium channels in the terminal membrane are opened, allowing an influx of calcium. The resulting increase in intracellular calcium causes fusion of vesicles with the surface membrane and exocytotic expulsion of acetylcholine and cotransmitters into the junctional cleft (see text). This step can he blocked by botulinum toxin. Acetylcholine s action is terminated by metabolism by the enzyme acetylcholinesterase. Receptors on the presynaptic nerve ending modulate transmitter release. SNAPs, synaptosome-associated proteins VAMPs, vesicle-associated membrane proteins.

Splawski I, Yoo DS, Stotz SC, Cherry A, Clapham DE, Keating MT (2006) CACNA1H mutations in autism spectrum disorders. J Biol Chem 281 22085-91 Stanley EF (2003) Syntaxin I modulation of presynaptic calcium channel inactivation revealed by botulinum toxin Cl. Eur J Neurosci 17 1303-5... 

Sellin LC, Kauffman JA, Dasgupta BR (1983) Comparison of the effects of botulinum neurotoxin types A and E at the rat neuromuscular junction. Med Biol 61 120-5 Sheridan RE (1998) Gating and permeability of ion channels produced by botulinum toxin types A and E in PC12 cell membranes. Toxicon 36 703-17 Shone CC, Hambleton P, Melling J (1987) A 50-kDa fragment from the NH2-terminus of the heavy subunit of Clostridium botulinum type A neurotoxin forms channels in lipid vesicles. Eur J Biochem 167 175-80... 

Release of acetylcholine When an action potential propagated by the action of voltage-sensitive sodium channels arrives at a nerve ending, voltage-sensitive calcium channels in the presynaptic membrane open, causing an increase in the concentration of intracellular calcium. Elevated calcium levels promote the fusion of synaptic vesicles with the cell membrane and release of acetylcholine into the synapse. This release is blocked by botulinum toxin. By contrast, black widow spider venom causes all of the cellular acetylcholine stored in synaptic vesicles to spill into the synaptic gap. 

Treatment of tardive dyskinesia is often unsatisfactory, especially in severe cases. A large number of treatments have been proposed (SEDA-20,40), including antiparkinsonian drugs, benzodiazepines, baclofen, hormones, calcium channel blockers, valproate, propranolol, opiates, cyproheptadine, tryptophan, lithium, manganese, niacin, botulinum toxin, ECT, dietary control, and biofeedback training. In an open study, 20 patients (mean age 65 years) with severe unresponsive tardive dyskinesia (mean duration 44 months, mean exposure 52 months) were treated with tetrabenazine (mean dose 58 mg/day) (310). The mean score on the AIMS motor subset, determined from videotapes, improved by 54%. Sedation was the only subjective complaint. 

Toxins in general are potent poisons. Nevertheless, the selectivity of action of some of these toxins means they have been harnessed in medical therapeutics (and even more widely in experimental pharmacology and physiology). Toxins that have been, or still are. us in medicine include atropine, botulinum toxin, cardiac glycosides, coichidne, eserine, hyoscine, picrotoxin, morphine, ouabain, strychnine, veratridine, vinca alkaloids and many more. All these work by an action at a defined molecular site, whether ion channel, neurotransmitter receptor, enzyme, pump or intracellular organelle. Those toxins that work at nonneuronal, or not specifically at neuronal sites (e.g. cholera toxin, pertussis toxin, cardiac glycosides, phospholipases) are discussed under TOXINS. 

Presynaptic membrane depolarization opens voltage-dependent Ca2+ channels, and the influx of this ion causes fusion of the synaptic vesicle membranes with the presynaptic membrane, leading to exocytosis of ACh- Botulinum toxin ( in Figure II-2-1) interacts with synaptobrevin and other proteins to prevent ACh release. 

Fig. 9.5.6 Direct detection and amplification of the protein toxin Clostridium botulinum toxin A (toxoid). The upper line is an average of three channels with ant -botulinum antibody on the surface.

Those conditions that are predicted to up-regulate IGF-II gene expression in mature muscle are known to induce sprouting and synapse formation. By locally infusing tetrodotoxin in a cuff around nerves, Taxt (1983) was able to block Na+-channel-dependent action potentials and found that motor nerve terminals sprouted and formed polyneuronal synapses. Likewise, the sprouts induced by botulinum toxin form polyneuronal synapses (Duchen and Strich, 1968 Holland and Brown,... 

Santafe MM, Urbano FJ, Lanuza MA, Uchitel OD. Multiple types of calcium channels mediate transmitter release during functional recovery of botulinum toxin type A-poisoned mouse motor nerve terminals. Neuroscience 2000 95(1) 227-34. 

Blocker, D., Bachmeyer, C., Benz, R., Aktories, K. and Barth, H., Channel formation by the binding component of Clostridium botulinum C2 toxin glutamate 307 of C2II affects channel properties in vitro and pH-dependent C2I translocation in vivo, Biochem., 42, 5368-5377, 2003. 

Reuner KH, Schlegel K, Just I, et al. (1991) Autoregulatory control of actin synthesis in cultured rat hepatocytes. In FEBS Letters. 286 100-4 Schmid A, Benz R, Just I, et al. (1994) Interaction of Olostridium botulinum 02 toxin with lipid bilayer membranes. Formation of cation-selective channels and inhibition of channel function by chloroquine. In J Biol Chem. 269 16706-11 Simpson LL (1982) A comparison of the pharmacological properties of Clostridium botulinum type 01 and 02 toxins. In J Pharmacol Exp Then 223 695-701 Simpson LL (1989a) Botulinum Neurotoxin andTetanus Toxin, pp 1 -422, San Diego Academic Press... 

When an action potential approaches the axon terminal, voltage-gated Ca2+ channels (N-type) open and Ca2+ enters the presynapse. Ca2+ ions bind to proteins that connect the synaptic vesicle with the plasma membrane (acronym SNAP), inducing membrane fusion and consequently exocytosis of the neurotransmitter into the synaptic cleft. Botulinum b toxin contains a specific protease which interferes with synaptobrevin (a vesisle protein which interacts with the SNAP proteins) so that vesicles cannot fuse any longer. The inhibition of acetylcholine release can thus cause paralysis and death. 

Hollow structures can also be prepared by the self-assembly of stave or rod-like subunits into barrel or bundle-shaped frameworks. This is one of the most common strategies in nature for channel formation, where the rod-like molecules of the barrel-stave type are /S-sheets or a-helices of amphipathic character. The central cavity has hydrophilic properties, while the lipophilic area is oriented outward, in contact with the membrane of the cell. A natural example of this type of protein is a-hemolysin, a bacterium toxin formed by seven identical subunits that self-assemble when in contact with the cell membrane. This assembly gives rise to a mushroom-shaped structure, whose trunk is formed by a -barrel that is inserted into the cell membrane. The resulting channel has a diameter of 13 A at its narrowest point and can transport ions and other molecules. Other natural examples based on this model —but using an a-helix instead of f-sheets—include cholera toxin, potassium channels, porins, aquaporins, and the most powerful toxin known to date, botulinum neurotoxin (BoNT, known as Botox), broadly studied by Mental s group. ... 

Hoch, D. H., Romero-Mira, M., Ehlrich, B. E., Finkelstein, A., DasGupta, B. R. and Simpson, L. L., 1985, Channels formed by botulinum, tetanus and diphtheria toxins in planer lipid bilayers relevance to translocation of proteins across membranes. Proc. Natl. Acad. Sci. USA 82 1692-1696. 

Singh, B. R., Ledoux, D. N. and Fu, F.-N., 1994, An analysis of the protein structure of botulinum and tetanus neurotoxins to understand molecular basis of membrane channel formation. In Advances in Venom and Toxin Research (Tan, N. H., Oo, S. L., Thambyrajah, V. and Azila, N.. eds.), Malaysian Society on Toxinology, Kuala Lumpur, pp. 103-108. 

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