Host-cell specificity and transcytosis of nontoxic nonhemagglutinin protein of botulinum neurotoxin serotype D

Serotype D botulinum toxin (BoNT) complex (TC), a causative agent of food-borne botulism in animals, traverses the gastrointestinal tract and circulation, eventually becoming localized in neuromuscular junctions, where the serotype D BoNT cleaves SNARE substrate synaptobrevin II involved in neurotransmitter release. During this process, BoNT must pass through cells, thus from the intestinal lumen to the cells of the intestinal tract and blood vessels. The botulinum TC is formed by association of the BoNT with at least one nontoxic protein, which may be a nontoxic nonhemagglutinin (NTNHA). In this work, we examined the binding and transcytosis of serotype D NTNHA protein in epithelial and endothelial cells to clarify the role played by the protein in toxin delivery. Our studies showed that NTNHA bound to and transcytosed across rat intestinal epithelial (IEC-6) and bovine aortic endothelial (BAEC) cells. While NTNHA also bound to canine renal (MDCK) or human colon carcinoma (Caco-2) cells, but it did not traverse across MDCK or Caco-2 cells. Such specificity of NTNHA protein transcytosis may explain why only some animals are sensitive to botulinum toxin. The sensitivity depends on the toxin serotype in play, and the route of toxin delivery.