Retention of cleaved synaptosome-associated protein of 25 kDa (SNAP-25) inneuromuscular junctions: a new hypothesis to explain persistence of botulinum A poisoning

Botulinum neurotoxins can block neurotransmitter release for several months . The molecular mechanism of these toxins' action is known, but the persist ence of neuromuscular paralysis that they cause is unexplained. At frog neu romuscular junctions, application of botulinum toxin type A caused paralysi s and reduced the C-terminus immunoreactivity of SNAP-25, but not that of t he remaining N-terminus fragment. Botulinum toxin type C caused paralysis a nd reduced syntaxin immunoreactivity without affecting that of SNAP-25. Co- application of botulinum A and C reduced syntaxin immunoreactivity, and tha t of both C and N termini of SNAP-25. Application of hydroxylamine to de-pa lmitoylate SNAP-25 resulted in a slight reduction of the immunoreactivity o f SNAP-25 N terminus, while it had no effect on immunoreactivity of botulin um A cleaved SNAP-25. In contrast, application of hydroxylamine to nerve te rminals where syntaxin had been cleaved by botulinum C caused a considerabl e reduction in SNAP-25 N-terminus immunoreactivity. Hence the retention of immunoreactive SNAP-25 at the neuromuscular junction depends on its interac tions with syntaxin and plasma membrane. Persistence of cleaved SNAP-25 in nerve terminals may prevent insertion of new SNAP-25 molecules, thereby con tributing to the longevity of botulinum A effects.