Study: Botulinum attacks like a ‘smart bomb’

A pilot without a map can locate an airport by first finding a nearby landmark, like a big river, and then searching for the airport.

New research from the University of Wisconsin School of Medicine and Public Health (SMPH) and Scripps Research Institute shows how the astonishingly powerful botulinum toxin uses a similar strategy to latch onto nerve cells, the first step in inactivating them.

The research helps explain how the toxin first attaches to a receptor on the surface of a nerve cell, then looks for a second type of receptor that is nearby. Once the toxin links to this second receptor, it can enter the nerve cell and break a protein needed to deliver molecules that can signal other nerve cells.

By blocking this signaling molecule, tiny amounts of botulinum toxin can cause paralysis and even death through respiratory failure. The bacteria that makes this toxin grows in soil, and can be found inside cans of food that were improperly processed. Botulinum toxin is the reason for the extreme danger from bulging cans of food.

Researchers have been working on the unique nerve-blocking ability of the seven individual botulinum toxins for decades, says botulinum expert Edwin Chapman, UW–Madison professor of physiology and a Howard Hughes Medical Institute investigator. “A major question is how the toxin enters neurons,” he says.

The research was a close collaboration with Ray Stevens of Scripps, who crystallized the structure that forms when botulinum toxin links to the protein receptor on a nerve cell. The research is supported by the National Institute of Allergy and Infectious Diseases, a division of the National Institutes of Health.

“This is the first paper to show in atomic detail the structure of botulinum neurotoxin touching the receptor on the surface of the neuron,” Chapman says. “The toxin has to bind to the neuron it wants to poison. This is a snapshot of the first stage of that poisoning.”

The report on the work, in the journal Nature this week, identified a short section on the protein receptor as the exact spot where botulinum toxin grips the cell immediately before entering it.

UW–Madison has long been a center of botulism research. In 2003, Min Dong, a post-doctoral fellow in Chapman’s lab, showed that a known protein receptor for one botulinum toxin was a key point of entry into the nerve cell. Dong shares first authorship on the current study along with Qing Chai and Joseph Arndt of Scripps.

The Nature paper is an elaboration on that 2003 discovery, which was published in The Journal of Cell Biology. Stevens’s lab bombarded a crystal of the toxin bound to a small sub-region of the primary receptor with X-rays, then measured the reflections to portray the toxin and the receptor bound in deadly embrace.

The research could have several practical applications. Botulinum toxin is a potential biological weapon, so the U.S. military is interested in finding anti-toxins to protect soldiers – molecules that attach to the binding site on the toxin or on the cell. The search for such a blocking molecule becomes easier now that the exact structure of the link between the toxin and the nerve cell are known.

Better knowledge of botulinum toxin’s structure could also enhance the growing number of treatments that use the toxin to block nerve signals. The medical treatments “are not just for wrinkles,” Chapman says. “People with paralysis get spasms in the muscles that are shut off, and this could solve that. In a wide variety of dystonias, where spasms can cause really severe pain, this can relax the muscles.”

A third potential benefit is further down the line. After the researchers found the binding site on the protein receptor, they varied it until the toxin could no longer bind to it. If a mutated toxin was made to attach to the mutated receptor, the combination might target botulinum toxin against over-active cells in the body, Chapman suggests.

Using genetic engineering, “you might be able to sensitize whatever cell you want to the toxin,” he says. Theoretically, such a treatment could be used to slow mucus production in the lungs of cystic fibrosis patients, or to attack hyperactive cells in a wide range of other disorders.

Overall, the research improves our knowledge of a devilishly clever toxin, says Dong. Botulinum is an enzyme – a biological catalyst – that can move through a cell, breaking one protein molecule and quickly attacking another. Botulinum toxin attacks communications between nerve cells, “one of the most sensitive parts of the animal physiology,” Dong says. “That provides an efficient way to immobilize an animal, far easier than targeting muscles directly.”

Another reason for botulinum toxin’s extraordinary power becomes clear from this study, Dong says. The toxin is only able to attach to a nerve cell that is working. “If the synapse between two nerve cells is not active, all the receptors will be hiding inside the cells. But a synapse that controls a very important muscle must be firing all the time, and it will be exposing more receptors, and the toxin will therefore target them.”

Botulinum toxin, he says, “Only goes where it can be effective. It’s like a smart bomb.”