Anti-apoptotic activity of hemagglutinin-33 and botulinum neurotoxin and its implications to therapeutic and countermeasure issues.

Botulinum neurotoxins (BoNTs), produced by Clostridium botulinum are the most toxic substances known to the mankind. BoNTs (seven serotypes, A-G) are produced along with a group of neurotoxin associated proteins (NAPs) in a physiologically coordinated manner, regulated by a common transcription factor for the gene cluster that encodes for the BoNT and NAPs. Hemagglutinin-33 (Hn-33) is a 33 kDa subcomponent of NAPs, which is resistant to protease digestion, and accounts for about half of the NAPs molecules in the BoNT/A complex. Natural exposures to BoNT in food poisoning cases as well as in the medical applications of BoNT as a therapeutic agent, humans are exposed to the BoNT/A complex. The toxin itself is known to block neurotransmitter release from presynaptic nerves, but the effect of NAPs is unexplored. In this article, we report an important observation of the anti-apoptotic effect of Hn-33 in Hn-33-preincubated human neuroblastoma SH-SY5Y cells. Activity of caspases, which are the central executioners of apoptosis, was substantially (78%) reduced by Hn-33. Degradation of chromosomal DNA, another biochemical hallmark of apoptosis, was blocked in Hn-33 incubated SH-SY5Y cells. Interestingly, purified BoNT/A also showed substantial anti-apoptotic activity. These findings may have significant implications to the use of BoNT as a therapeutic agent, and to devise counter measures to botulinum poisoning.