Introduction

Botulism is a dangerous and sometimes fatal condition contracted from eating the contents of poorly home-canned foods or commercial improperly canned foods. It has been documented since the end if the 18th century, then termed "sausage poisoning" because afflicted individuals had dined on contaminated sausage. Botulism is caused by a toxin produced from the bacteria Clostridium botulinum. Infant botulism, discovered only in 1976 by S.S. Arnon, and suspected to be the cause of some cases of Sudden Infant Death Syndrome (SIDS), is caused by the ingestion of C. botulinum spores into the florally-immature intestinal tracts of infants. In these conditions the spores germinate and produce the toxin. One source of these spores is honey: a study by California researchers found the spores in 10% of tested store-bought honey. But it was concluded that most cases in infants are caused by the ingestion of spores with dust particles.

The Toxin and Its Effects

The site of action of the botulinum toxin is the neuromuscular junction, where the toxin attaches to the nerve ending, then is taken into the nerve cell where it interferes with the release of the neurotramsmitter acetylcholine from presynaptic vesicles into the neuromuscular synapse. This results in paralysis of the innervated muscle. Due to its ability to paralyze the muscles of the diaphragm, the most severe cases of botulism affect breathing, and victims may have to spend months in a respirator. For lack of proper hospital-assisted breathing apparatus, in previous times the fatality rate was very high, though today fatalities are rare.

Botulinum toxin is comprised of the known serotypes A-G. There are structural and activity similarities with the toxin from C. tetanus. The light chains of botulinum neurotoxin (BoNT) and tetanus neurotoxin (TeNT) show low homology except in the central portion which contains a His-Glu-Leu-Iso-His motif, which is commonly found in the pocket of all Zn-endopeptidases. The different serotypes have structural activity similarities and differences. For example, BoNT type A, B, E, and F infect/affect man, while BoNT/C infects birds and BoNT/D infects cattle.

These toxins are zinc-endopeptidases that selectively cleave the SNARE (synaptic vesicle docking and fusion complex) proteins VAMP (vesicle associated membrane protein), SNAP-25 (synaptosome associated protein, 25 kDa) and syntaxin. All of these proteins are implicated in neurotransmitter exocytosis, explaining botulinum toxin’s role in neuromuscular paralysis.

The toxins are translated as single chain proteins, then subsequently cleaved within a disulfide loop and resulting in a 50 kDa light chain and a 100 kDa heavy chain, still connected by a disulfide linkage as well as non-covalent interactions. Reduction of this interchain disulfide bridge occurs during intoxication and is a necessary step for the toxin’s intracellular action. This is because the N-terminal portion of the heavy chain masks the catalytic cleft of the light chain when the bridge is intact in BoNT/A. Although the active site is not occluded in BoNT/B, conformational changes occur in the L chain upon separation from the H chain that likely regulate catalytic activity.

Crystal Structure Of Clostridium Botulinum Neurotoxin B. Notice the complexed trisaccharide and the bound zinc.

Because the botulinum toxin paralyses muscles, it is the first bacterial toxin to be used as a therapeutic agent for conditions involving muscle spasms. It has been approved by the U.S. Food and Drug Administration for treatment of strabismus (misalignment of the eyes), blepharospasm (forceful eyelid closure resulting in visual disability), hemifacial spasm (unilateral contraction of muscles innervated by the facial nerve), cervical dystonia (muscle spasms in the neck causing the head to tilt and tremor), oromandibular dystonia (spasms of the muscles of the face, jaw, neck, pharynx, tongue and, in several cases, larynx and respiratory system), focal and segmental limb dystonias (involuntary spasms of distal limb muscles; includes writer’s cramp and musician’s cramp), anismus (dyssynergia of the external anal sphincter and puborectalis muscles, causing profound constipation), urinary detrusor-sphincter spasm with bladder contraction (causes urinary obstruction). It has also been effective in treating voice and speech disorders, including stuttering. In most of these cases highly trained physicians and teams of others (psychiatrists, otolaryngologists, etc.) must be involved in the treatments which include the injection of small amounts of the toxin into affected muscles. The injections must be repeated at intervals of a few months because the affected nerves grow functional offshoots and the spastic muscle activity returns.

Other uses of the toxin have yet to be approved but are being tried for the following conditions: Cosmetic correction of wrinkles (glabellar wrinkles often associated with contractions of the corrugator muscles in the forehead, and crow’s feet at the eye exterior), anal fissure (painful ulceration of the anal mucuous membrane, associated with spasms of the anal sphincter), sphincter of Oddi dysfunction (abnormal patterns of bile flow, with abdominal pain), focal hyperhidrosis (excessive localized sweat production), nystagmus (constant involuntary cyclical movement of the eyeball), cerebral palsy, stroke, and tremors. Obviously BTX is being used to treat a number of different contractile problems, and trials of these treatments have met with varied success.

Conclusion

Obviously botulinum toxin is providing much information about exocytosis, toxin effects at the neuromuscular junction, and practical therapeutic benefits. Knowledge of its action is also providing new ways to treat botulism, and to develop antitoxin treatments to counteract the use of botulinum toxin in biological warfare.

Botulism Web Site Links

The Botulinum Web - everything you wanted to know

FDA - Botulinum Toxin: A Poison That Can Heal

Annals of Internal Medicine - Botuism review

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