Unique biological activity of botulinum D/C mosaic neurotoxin in murine species

Clostridium botulinum types C and D cause animal botulism by the production of serotype-specific or mosaic botulinum neurotoxin (BoNT). The D/C mosaic BoNT (BoNT/DC), which is produced by the isolate from bovine botulism in Japan, exhibits the highest toxicity to mice among all BoNTs. In contrast, rats appeared to be very resistant to BoNT/DC in type C and D BoNTs and their mosaic BoNTs. We attempted to characterize the enzymatic and receptor-binding activities of BoNT/DC by comparison with those of type C and D BoNTs (BoNT/C and BoNT/D). BoNT/DC and D showed similar toxic effects on cerebellar granule cells (CGCs) derived from the mouse, but the former showed less toxicity to rat CGCs. In recombinant murine-derived vesicleassociated membrane protein (VAMP), the enzymatic activities of both BoNTs to rat isoform 1 VAMP (VAMP1) were lower than those to the other VAMP homologues. We then examined the physiological significance of gangliosides as the binding components for types C and D, and mosaic BoNTs. BoNT/DC and C were found to cleave an intracellular substrate of PC12 cells upon the exogenous addition of GM1a and GT1b gangliosides, respectively, suggesting that each BoNT recognizes a different ganglioside moiety. The effect of BoNT/DC on glutamate release from CGCs was prevented by cholera toxin B-subunit (CTB) but not by a site-directed mutant of CTB that did not bind to GM1a. Bovine adrenal chromaffin cells appeared to be more sensitive to BoNT/DC than to BoNT/C and D. These results suggest that a unique mechanism of receptor binding of BoNT/DC may differentially regulate its biological activities in animals.