Toxins serotypes

The authors of this chapter have studied the effect of C and D toxin serotypes, as well as A, B, and E, on human intercostal muscle (Hihnas, unpublished data). All serotypes showed a similar ability to produce complete muscular paralysis in ex vivo human intercostal muscle. Intercostal muscle was excised from patients receiving a thoracotomy and intercostal muscle flap procedure. The muscle was removed tendon to tendon by surgical excision without electrocautery and dissected into multiple bundles with their associated intercostal nerves. The nerve-muscle units were placed in a vertical twitch bath and stimulated at 0.03 Hz (0.2 ms pulses of supramaximal strength) using a novel nerve clamp electrode to illicit an indirect muscle twitch. Potent toxins (1 nM) from various serotypes were added to the bath after confirming the stability of control muscle responses. In each case, twitch tensions declined to negligible amplitudes by 1 h after direct toxin application to the tissue bath. 

In addition to nonhuman primates, most other animal species that show some sensitivity to botulinum intoxication are in fact susceptible to toxin serotypes Cl and D. Several rodent species are susceptible to oral intoxication with most botulinum toxins, including types Cl and D (Matveev, 1959 ... 

Two recent reports revealed additional layers of complexity regarding the mechanisms involved in the distinct durations of action associated with the different toxin serotypes. Femandez-Salas et al. (2004) investigated the subcellular localization of BoNT/A, /B, and /E LC-GFP fusion proteins following overexpression in several different mammalian cell lines. The LC/A fusion protein was shown to localize within discrete plasma membrane... 

The current Centers for Disease Control and Prevention (CDC) therapy for the public is an FDA-approved, bivalent, botulinum equine antitoxin against serotypes A and B. The trivalent antitoxin against types A, B, and E is no longer available. In cases of exposure to any of the other botulinum toxin serotypes, the US Army can provide an investigational heptavalent (ABCDEFG) equine antitoxin, but the time required for typing a toxin subtype would limit its effectiveness in such cases as an outbreak. A parenteral vaccine against the toxin is currently available, but the need exists for newer nonparenteral vaccines that could be administered orally or via inhalation. 

The major advantage of the lectins is that they are effective against all clostridial toxin serotypes (Bakry et ah, 1991). The disadvantage of using lectins to protect against BoNT is that they must be administered as pretreatments. This limitation is inherent in the basic mechanism of BoNT action thus no antagonist of surface receptor binding would be expected to be protective once BoNT is internalized and symptoms are manifested. 

The family of structural genes for the clostridial neurotoxins is unrestricted in its location, being both chromosomal and extrachromosomal. The structural gene for tetanus toxin is on a plasmid, as is probably the structural gene for botulinum toxin serotype G.15 The structural genes for botulinum toxin serotypes C and D are found on bacteriophages.1017 The remainder are believed to be chromosomal in location, but this is not definitely proven. 

As this is written (1996), it is fair to say that science does not know the structure of the receptors for either botulinum or tetanus toxins. A report published in 199426 suggesting that a synaptic vesicle-associated protein called synaptotagmin may be the receptor for botulinum toxin serotype B. This intriguing work has yet to be confirmed, and... 

Mouse lethality. To determine lethality of microbial toxins (e.g., botulinal). Injected mice are observed for up to 72 h for lethality. Toxin serotypes can be determined by use of specific antiserum. The LD50 of Listeria monocytogenes for normal adult mice is 10 -10 cells, and as few as 50 cells for infant mice... 

The intracellular proteins SNAP-25, syntaxin, and synaptobrevin (or VAMPs) normally interact with each other in mediating neurotransmitter release from cholinergic and other nerve terminals (see Figme 28.4). Toxin types B, D, F, and G cleave the VAMPs, while types A, Cl, and E act on SNAP-25 type Cl toxin also cleaves syntaxin (Dong et al., 2003). The functions of the various nemotoxins are even more specialized, in that one toxin type can cleave its substrate at a different site than that targeted by other toxin serotypes. For example, BoNT/A... 

---