Design and Characterization of Recombinant and Chimeric BoNT/A Neurotoxins with Receptor-Binding Domain Grafting

Botulinum neurotoxins (BoNTs) act on peripheral cholinergic nerve terminals, inducing reversible muscle paralysis and profound therapeutic effects. However, their limited cell-type specificity and narrow therapeutic window have motivated the development of engineered variants. Here, a modular strategy was employed to construct full-length chimeric BoNTs, grafting receptor-binding segments from BoNT/B or BoNT/F onto the BoNT/A framework. The novel chimeras AAAF and AAFF efficiently cleaved rSNAP-25 in cell-free assays. Firstly, both toxins showed effective cellular uptake and cleaved endogenous SNAP-25 in Neuro-2a cells, with cleavage efficiencies of approximately 46% for AAAF and 73% for AAFF, highlighting the enhanced activity of AAFF. Secondly, AAAF induced faster recovery from reversible muscle paralysis compared to rBoNT/A-WT, whereas AAFF produced more sustained paralysis, with both exhibiting reduced systemic toxicity. Despite these altered pharmacological profiles, the chimeras required higher doses than rBoNT/A-WT to induce neuromuscular effects. Collectively, this study presents the design of novel chimeric BoNT/A-F proteins, characterizes their functional activities, and provides a preliminary exploration of how domain grafting affects cellular uptake, enzymatic activity, and neuromuscular pharmacodynamics.