Botulinum neurotoxins: from paralysis to recovery of functional neuromuscular transmission.

The neuromuscular junction is one of the most accessible mammalian synapses which offers a useful model to study long-term synaptic modifications occurring throughout life. It is also the natural target of botulinum neurotoxins (BoNTs) causing a selective blockade of the regulated exocytosis of acetylcholine thereby triggering a profound albeit transitory muscular paralysis. The scope of this review is to describe the principal steps implicated in botulinum toxin intoxication from the early events leading to a paralysis to the cellular response implementing an impressive synaptic remodelling culminating in the functional recovery of neuromuscular transmission. BoNT/A treatment promotes extensive sprouting emanating from intoxicated motor nerve terminals and the distal portion of motor axons. The current view is that sprouts have the ability to form functional synapses as they display a number of key proteins required for exocytosis: SNAP-25, VAMP/synaptobrevin, syntaxin-I, synaptotagmin-II, synaptophysin, and voltage-activated Na+, Ca2+ and Ca2+-activated K+ channels. Exo-endocytosis was demonstrated (using the styryl dye FM1-43) to occur only in the sprouts in vivo, at the time of functional recovery emphasising the direct role of nerve terminal outgrowth in implementing the restoration of functional neurotransmitter release (at a time when nerve stimulation again elicited muscle contraction). Interestingly, sprouts are only transitory since a second distinct phase of the rehabilitation process occurs with a return of synaptic activity to the original nerve terminals. This is accompanied by the elimination of the dispensable sprouts. The growth or elimination of these nerve processes appears to be strongly correlated with the level of synaptic activity at the parent terminal. The BoNT/A-induced extension and later removal of "functional" sprouts indicate their fundamental importance in the rehabilitation of paralysed endplates, a finding with ramifications for the vital process of nerve regeneration.