Toxicity Clostridial neurotoxins

Classical bacterial exotoxins, such as diphtheria toxin, cholera toxin, clostridial neurotoxins, and the anthrax toxins are enzymes that modify their substrates within the cytosol of mammalian cells. To reach the cytosol, these toxins must first bind to different cell-surface receptors and become subsequently internalized by the cells. To this end, many bacterial exotoxins contain two functionally different domains. The binding (B-) domain binds to a cellular receptor and mediates uptake of the enzymatically active (A-) domain into the cytosol, where the A-domain modifies its specific substrate (see Figure 1). Thus, three important properties characterize the mode of action for any AB-type toxin selectivity, specificity, and potency. Because of their selectivity toward certain cell types and their specificity for cellular substrate molecules, most of the individual exotoxins are associated with a distinct disease. Because of their enzymatic nature, placement of very few A-domain molecules in the cytosol will normally cause a cytopathic effect. Therefore, bacterial AB-type exotoxins which include the potent neurotoxins from Clostridium tetani and C. botulinum are the most toxic substances known today. However, the individual AB-type toxins can greatly vary in terms of subunit composition and enzyme activity (see Table 2). 

The extreme toxicity of clostridial neurotoxins (CNTs) derives from their absolute neurospecificity as well as from catalytic activity. TeTx and BoNTs bind specifically to the neuromuscular junction (NMJ) of motor neurons. The identity of the receptor(s) on the presynaptic membrane is unknown, but their extreme toxicity suggests that the binding affinity to the cognate receptor must be very high. The receptor-bound toxin is internalized at the presynaptic membrane of the NMJ and gains access to the neuronal cytosol. Here it blocks the release of acetylcholine (ACh), causing a flaccid paralysis (Simpson,... 

Clostridial neurotoxins are very toxic. However they are ineffective in individuals immunized with the corresponding toxoids. In most countries children are vaccinated with tetanus toxoid and this is sufficient to provide full protection against tetanus for decades. A booster injection of tetanus toxoid (available from health authorities) before starting research with TeTx is advisable. On the other hand, the vaccine for BoNT/A, B, C, D and E is not commercially available, but can be obtained from the Center for Disease Control (CDC, Atlanta, GA). Due to the rather low efficacy of the BoNTs vaccine, a protective serum anti-BoNT titre is generally, but not always, achieved. Human anti-TeTx antibodies and horse anti-BoNT antibodies are also available from health authorities, and their injection immediately after accidental penetration of the toxin into the circulatory system is sufficient to prevent the disease. 

The clostridial neurotoxins are the most toxic substances known to science. The neurotoxin produced from Clostridium tetani (tetanus toxin) is encountered by humans as a result of wounds and remains a serious public health problem in developing countries around the world. However, nearly everyone reared in the western world is protected from tetanus toxin as a result of the ordinary course of childhood immunizations. Humans are usually exposed to the neurotoxins produced by Clostridium botulinum (ie, the botu-linum toxins, of which there are seven in all) by means of food poisoning, although there are rare incidents of wound botulism and a colonizing infection of neonates known as infant botulism.1 Since the incidence of botulinum poisoning by all routes is very rare,... 

Demonstration of this activity was initially difficult because the toxins are very specific for their substrates. However, it is now clear that (a) the clostridial neurotoxins express proteolytic activity and (b) this activity is absolutely required for toxicity.30 The substrate proteins for this action appear to be part of a hetero-oligomeric assembly associated with the synaptic vesicles. Interestingly, the specific target site for cleavage seems to be different for each serotype of the botulinum toxins in some cases, different locations on the same protein in others, different proteins of the assembly.30 The basis of this marked specificity is not yet clear and remains fascinating to scientists interested in neurosecretion. 

The toxin BoTx, produced by the anaerobe Clostridium botulinum, has the reputation of being the most toxic substance by weight known to humans, being at least 5000 times more toxic than sarin. Botulism is a disease of both humans and animals. Seven dilferent functionally related neurotoxins are produced by various strains (A-G). Botulism is essentially an intoxication, brought on by ingestion of the toxin produced by clostridial infection of food, usually incorrectly canned meats. Primary botulism, a direct infection, is rare and only affects infants in the human species. Botulinum intoxication can, however, be treated, and this modifies the toxicity considerably. It is estimated that less than 10 % of natural cases receiving ventilatory and antitoxin support are fatal. 

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