Project Summary The family of bacterial toxins, botulinum neurotoxins (BoNTs), produced by spore- forming Gram-positive Clostridium bacteria, are the most potent toxins known. They cause the deadly paralytic disease botulism in humans and animals. These toxins are one of the six most dangerous potential bioterrorism agents (Category A and Tier 1). They target motor neurons with extreme specificity, enter the cytosol of neurons, and block neuronal activity, causing muscle paralysis. A major contributor to the threat of BoNTs is their extremely long half-life within the cytosol of motor neurons – the toxin resides in the cytosol for 4-6 months in humans and causes persistent paralysis for months. To date, no therapeutics can inhibit toxins within the cytosol of neurons. Here we propose to create an effective post-exposure treatment for this top-priority bioterrorism agent and deadly bacterial toxins. This treatment is based on modifying a recently discovered new bacterial toxin BoNT/X and utilizing this engineered protein to deliver a fused neutralizing antibody against BoNTs into the cytosol of motor neurons. Our extensive preliminary studies have provided working prototypes, which completely rescue mice from systemic toxicity of BoNTs in multiple post-exposure models. Here we propose three aims to further validate our hypothesis by (1) refining the chimeric toxin platform; (2) optimizing the nanobody cargo against BoNTs; and (3) establishing pharmacokinetic parameters and validating therapeutic efficacy in both rodent and guinea pig models. Success of these aims will create an effective post-exposure treatment for a top-priority bioterrorism agent, and also develop a novel protein-based delivery platform for targeting motor neurons, thus offering new tools for targeting cytosolic processes and “undruggable” proteins in neurons.