**4. Mechanisms of Botulinum Neurotoxins (BoNTs)**

BoNT is produced by *Clostridium botulinum*, a Gram-positive anaerobic bacterium. BoNT includes seven kinds of antigenically distinct neurotoxins (A~G) [29,30]. Among them, BoNT-A is the most commonly used serotype in clinics with the most durable effect [13].

BoNT-A is synthesized as an inactive form of 1285 amino acids and becomes activated when it is cleaved into a light chain (50-kDa) and a heavy chain (100-kDa). Unique binding of BoNT to nerve terminals occurs due to their ability to interact with two independent receptors of the presynaptic membrane: a polysialoganglioside (PSG) and a glycosylated luminal domain of a synaptic vesicle protein that mediates BoNT internalization [30]. The mechanism of denervation by BoNT is divided into five steps: (1) binding to nerve terminals, (2) internalization within an endocytic compartment, (3) low pH-driven translocation of the L chain across the vesicle membrane, (4) release of the L chain in the cytosol by reduction of the interchain disulfide bond, and (5) cleavage of SNAREs for ensuing the blockade of neurotransmitter release and consequent neuroparalysis [29–31].
