Characterization of clostridium botulinum neurotoxin serotype A (BoNT/A) and fibroblast growth factor receptor interactions using a novel receptor dimerization assay

Clostridium botulinum neurotoxin serotype A (BoNT/A) is a potent neurotoxin that also serves as an effective therapeutic for a variety of neuromuscular and glandular diseases and disorders. The observed pharmacological effect of BoNT/A is due to specific targeting and entry into motor nerve terminals within muscles followed by cleavage of the SNARE protein, SNAP-25, inducing a block in neurotransmission and temporary muscular paralysis. Specific binding and internalization of BoNT/A into neuronal cells are mediated by its binding domain (HC/A), which binds to GT1b ganglioside and protein cell surface receptors. Previously, fibroblast growth factor receptor 3 (FGFR3) was identified as a BoNT/A receptor, in addition to synaptic vesicle protein (SV2). To further study BoNT/A interactions with FGFRs, an FCS & TIRF receptor dimerization assay was developed to measure dimerization of FGFRs in live cells, as FGFR dimerization can be considered an indirect measure for receptor-ligand binding interaction and downstream signaling. The ability of HC/A to facilitate dimerization of three FGFR subtypes (FGFR1-3) was assessed. Recombinant HC/A (rHC/A) was shown to dimerize FGFR subtypes in the rank order FGFR3c > FGFR2b > FGFR1c. With potencies (EC50 values) defined as the concentration of ligand required to dimerize 50% of the receptors, wild type rHC/A dimerized FGFR3c with an EC50 of 24 nM, similar to FGF9, a native FGFR3c ligand, which had an EC50 of 15 nM, while FGFR1c and FGFR2b required higher rHC/A concentrations (≥100 nM and 68 nM, respectively). Furthermore, addition of the GT1b ganglioside to the culture media resulted in increased dimerization, whereas a ganglioside mutant variant of HC/A (rHC/A W1266L;Y1267S) showed decreased dimerization. Interestingly, reduced dimerization was also observed with an SV2 mutant variant of HC/A (rHC/A T1145A;T1146A). These results support a model wherein BoNT/A interacts with FGFRs, gangliosides, and SV2 on the cell surface to facilitate cell uptake.