Botulinum neurotoxin type A is internalized and translocated from small synaptic vesicles at the neuromuscular junction

Mol Neurobiol. 2013 Aug;48(1):120-7. doi: 10.1007/s12035-013-8423-9. Epub 2013 Mar 8.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Axons / drug effects
  • Axons / metabolism
  • Botulinum Toxins, Type A / metabolism*
  • Botulinum Toxins, Type A / pharmacology
  • Endocytosis* / drug effects
  • Humans
  • Mice
  • Neuromuscular Junction / drug effects
  • Neuromuscular Junction / metabolism*
  • Neuromuscular Junction / ultrastructure
  • Protein Transport / drug effects
  • Rats
  • Synaptic Vesicles / drug effects
  • Synaptic Vesicles / metabolism*
  • Synaptic Vesicles / ultrastructure
  • Time Factors


  • Botulinum Toxins, Type A