title: Botulinum neurotoxin type A is internalized and translocated from small synaptic vesicles at the neuromuscular junction.
abstract: Botulinum neurotoxin type A (BoNT/A) is the most frequent cause of human botulism and, at the same time, is largely used in human therapy. Some evidence indicates that it enters inside nerve terminals via endocytosis of synaptic vesicles, though this has not been directly proven. The metalloprotease L chain of the neurotoxin then reaches the cytosol in a process driven by low pH, but the acidic compartment wherefrom it translocates has not been identified. Using immunoelectron microscope, we show that BoNT/A does indeed enter inside synaptic vesicles and that each vesicle contains either one or two toxin molecules. This finding indicates that it is the BoNT/A protein receptor synaptic vesicle protein 2, and not its polysialoganglioside receptor that determines the number of toxin molecules taken up by a single vesicle. In addition, by rapid quenching the vesicle trans-membrane pH gradient, we show that the neurotoxin translocation into the cytosol is a fast process. Taken together, these results strongly indicate that translocation of BoNT/A takes place from synaptic vesicles, and not from endosomal compartments, and that the translocation machinery is operated by no more than two neurotoxin molecules.
authors.affiliation: Laboratoire de Neurobiologie et Développement, Institut de Neurobiologie Alfred Fessard, CNRS-UPR 3294, 91198, Gif-sur-Yvette Cedex, France.
authors.forename: Cesare
authors.initials: C
authors.lastname: Colasante
authors.forename: Ornella
authors.initials: O
authors.lastname: Rossetto
authors.forename: Laura
authors.initials: L
authors.lastname: Morbiato
authors.forename: Marco
authors.initials: M
authors.lastname: Pirazzini
authors.forename: Jordi
authors.initials: J
authors.lastname: Molgó
authors.forename: Cesare
authors.initials: C
authors.lastname: Montecucco
mesh_terms: D000818:Animals; D001369:Axons / Q000187:drug effects / Q000378:metabolism; D019274:Botulinum Toxins, Type A / Q000378:metabolism* / Q000494:pharmacology; D004705:Endocytosis / Q000187:drug effects; D006801:Humans; D051379:Mice; D009469:Neuromuscular Junction / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D021381:Protein Transport / Q000187:drug effects; D051381:Rats; D013572:Synaptic Vesicles / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D013997:Time Factors
chemical_list: D019274:Botulinum Toxins, Type A