Two scientific teams under Drs. Axel Brunger at Stanford University and Edwin Chapman at the University of Wisconsin at Madison have independently described new working details of the mechanism behind botulinum neurotoxin’s ability to hijack synaptic vesicle recycling at neuromuscular junctions.
Howard Hughes Medical Institute reports:
Botulinum neurotoxins are among the most deadly natural toxins in the world. They act by first attaching themselves to receptors on the surface of neurons. The toxins then insinuate an enzyme into the neuron that degrades key proteins required for neurons to communicate with one another. The toxins principally affect muscle-controlling motor neurons activated by the neurotransmitter acetylcholine. They kill by paralyzing the respiratory muscles. There are seven structurally and functionally related botulinum neurotoxins (BoNTs), called serotypes A through G, with each acting in a slightly different manner. In 2004, Brunger’s group published an article in Nature detailing how the toxins that cause botulism and tetanus can recognize and attack particular nerve cell proteins at the neuromuscular junction.
Researchers knew that the toxins simultaneously bind to two distinct neuronal receptors – one a protein and one a sugar-containing lipid called ganglioside – but the details of that binding had not been established prior to these studies.
Both research groups began by crystallizing the BoNT/B serotype toxin in complex with its protein receptor, called synaptotagmin II…
Both groups discovered that the toxin holds its receptor in an intimate molecular embrace. The toxin induces a helix in the synaptotagmin protein that fits precisely into a groove in the toxin molecule. Both teams showed that they could disrupt this binding by introducing mutations that would subtly alter the shape of the synaptotagmin receptor.
Brunger and his colleagues found that altering the toxin at the binding site by single amino acid changes (obtained from the high resolution crystal structure) drastically reduced its toxicity. Specifically, when they incubated the altered neurotoxin with mouse diaphragm, it produced far less muscular paralysis than the natural toxin.
“This tells us that it is possible to design a small-molecule inhibitor that could powerfully disrupt the interaction between the toxin and the receptor,” said Brunger. “Such inhibitors would act as powerful, specific anti-toxins, with fewer side effects than current drugs.” Also, he said, detailed knowledge of toxin-receptor binding could help in designing botulism vaccines. Such vaccines could consist of fragments of protein corresponding to the toxin’s binding region, which could be used to trigger antibody production against the toxin that would block its action.
Picture caption: Botulinum neurotoxin hijacks synaptic vesicle recycling at neuromuscular junctions. The toxin first docks to the active zone (blue) by binding to two membrane-anchored receptors, synaptotagmin (red) and ganglioside (yellow). The toxin-receptor complexes are then internalized by endocytosis.