Therapeutic effectiveness of botulinum neurotoxin A: potent blockade of autonomic transmission by targeted cleavage of only the pertinent SNAP-25

Abstract

In search of a basis for the impressive potency of an endoprotease that cleaves SNAP-25, botulinum neurotoxin type A (BoNT/A), in treating numerous diseases due to hyper-active autonomic nerves, truncation of its target and inhibition of neurotransmission were studied in rat sympathetic neurons. Tetrodotoxin-sensitive spontaneous cholinergic neurotransmission was blocked >80% by 1 pM BoNT/A despite cleaving <20% of the SNAP-25. A maximum cleavage of ∼60% SNAP-25 could be achieved with >1 nM BoNT/A, despite an absence of non-cleavable SNAP-25 in the detergent-solubilised neurons. In contrast, BoNT/E (100 nM) truncated nearly all the SNAP-25 in the intact cells, but was unable to block neurotransmission at low concentrations like BoNT/A. Chimeras created by inserting the acceptor-binding HC domain of BoNT/A into BoNT/E still cleaved all the SNAP-25, indicating ubiquitous expression of BoNT/A acceptors. Accordingly, SV2 and SNAP-25 were found to be co-expressed and broadly co-localised in neurons, but absent from non-neuronal cells. On the other hand, partial cleavage by the BoNT/A protease persisted upon replacing its HC with counterparts from BoNT/E or BoNT/B. Moreover, limited cleavage of SNAP-25 was conferred onto the protease from BoNT/E when fused to the N-terminus of BoNT/A. Thus, the BoNT/A protease is uniquely well-adapted for selectively inactivating the SNAP-25 directly involved in neurotransmission; this together with the toxin's acceptor and its target being localised on the peri-somatic boutons likely contribute to its exceptional therapeutic utility in the clinic.