Botulinum neurotoxin C mutants reveal different effects of syntaxin or SNAP-25 proteolysis on neuromuscular transmission

Botulinum neurotoxin serotype c (bont/c) is a neuroparalytic toxin associated with outbreaks of animal botulism,particularly in birds,and is the only bont known to cleave two different snare proteins,snap-25 and syntaxin. bont/c was shown to be a good substitute for bont/a1 in human dystonia therapy because of its long lasting effects and absence of neuromuscular damage. two triple mutants of bont/c,namely bont/c s51t/r52n/n53p (bont/c α-51) and bont/c l200w/m221w/i226w (bont/c α-3w),were recently reported to selectively cleave syntaxin and have been used here to evaluate the individual contribution of snap-25 and syntaxin cleavage to the effect of bont/c in vivo. although bont/c α-51 and bont/c α-3w toxins cleave syntaxin with similar efficiency,we unexpectedly found also cleavage of snap-25,although to a lesser extent than wild type bont/c. interestingly,the bont/c mutants exhibit reduced lethality compared to wild type toxin,a result that correlated with their residual activity against snap-25. in spite of this,a local injection of bont/c α-51 persistently impairs neuromuscular junction activity. this is due to an initial phase in which snap-25 cleavage causes a complete blockade of neurotransmission,and to a second phase of incomplete impairment ascribable to syntaxin cleavage. together,these results indicate that neuroparalysis of bont/c at the neuromuscular junction is due to snap-25 cleavage,while the proteolysis of syntaxin provides a substantial,but incomplete,neuromuscular impairment. in light of this evidence,we discuss a possible clinical use of bont/c α-51 as a botulinum neurotoxin endowed with a wide safety margin and a long lasting effect. © 2017 zanetti et al.