Clostridial neurotoxins (CNTs)—tetanus and botulinum neurotoxins (BoNTs)—block neurotransmission, causing paralysis and death. Cellular entry of CNTs requires both gangliosides and proteins, and although several binding proteins have been identified, the specific protein associated with CNT internalization has remained unclear. In the September 29 Journal of Cell Biology, Min Dong and colleagues at the University of Wisconsin–Madison demonstrate that the secretory vesicle proteins synaptotagmins (syts) I and II bind and internalize BoNT/B (Journal of Cell Biology, 162:1293-1303 September 29, 2003).

Dong et al. showed that BoNT/Bs, but not related toxins, were able to bind recombinant syt ectodomains in vitro. Gangliosides were essential for syt I but not for syt II binding. The authors then performed reconstitution experiments in PC12 cells, which express syt I but are resistant to BoNT/B entry. When cells were preloaded with gangliosides, they were able to internalize BoNT/B in a syt I–dependent manner. PC12 cells transfected with syt II took up BoNT/B without additional gangliosides, consistent with the in vitro observations. The authors then examined toxin uptake in neurons from diaphragm tissue, the major in vivo target of BoNTs. These express syt II and efficiently internalized BoNT/B after neuronal depolarization. BoNT/B uptake was blocked by addition of a syt II fragment containing the toxin-binding domain. In addition, a whole animal study showed that preinjection of syt fragments and gangliosides could neutralize BoNT/B in mice.

This work may have major applications in medicine and counterbioterrorism. "Receptor ectodomains might provide a novel means to antagonize the action of bacterial toxins in animals," the authors conclude.