Researchers at Sanford-Burnham Medical Research Institute and collaborators at the Medical School of Hannover in Germany have discovered how potential bioterrorism agent botulinum neurotoxin survives in the human stomach. A protein made in unison in the same bacterium is the bodyguard that keeps the toxin safe through the stomach, then it releases as the toxin enters the bloodstream. Understanding this releasing mechanism exposes the toxin’s vulnerable point, which can be the target of new therapeutics.
Leading the research was the Rongsheng Jin, Ph.D. (assistant professor at Sanford-Burnham and senior author of the study) and Andreas Rummel, Ph.D. (an expert on clostridial neurotoxins at the Institute of Toxicology at the Medical School of Hannover). The teams used powerful X-ray beams to produce 3D images of proteins at the atomic level, to analyze a genetically inactivated, nontoxic variety of the botulinum neurotoxin, in a technique called x-ray crystallography.
The experiments assisted the team to visualize the atomic structure of all three components of the toxin: the region that recognizes neurons, the enzyme that acts like shears to cut human neural proteins and trigger paralysis, and the needle that punches holes to assist in delivery of the enzyme to the nerve terminal. The researchers also attained the toxin’s interaction with a second bacterial protein, called nontoxic-nonhemagglutinin (NTNHA).
“We were surprised to see that NTNHA, which is not toxic, turned out to be remarkably similar to botulinum neurotoxin. It’s composed of three parts, just like a copy of the toxin itself. These two proteins hug each other and interlock with what looks like a handshake,” said Jin.
Acting as a bodyguard, NTNHA keeps the toxin from being damaged while travelling through the acidic stomach. The vulnerability found by the study is when the toxin/NTNHA complex forces its way out of the small intestine. The change in pH initiates a change in the partnership, breaking up and releasing the now unprotected toxin into the bloodstream.
Jin hopes to exploit the botulinum neurotoxin/NTNHA bonding relationship. “We now hope we might be able to fool the toxin and its bodyguard using a small molecule that sends the wrong signal—mimicking pH change, prematurely breaking up their protective embrace, and leaving the stomach’s digestive enzymes and acid to do their job,” he said. “We envision this type of therapy, either alone or in combination with other therapies currently in development, could be given preventively at a time when botulinum neurotoxin contamination becomes a public health concern.”
This type of treatment could be designed for oral delivery, rather than injection, making it easier to treat large numbers of people during an outbreak. A similar strategy could be used to deliver other protein-based drugs that usually need to be injected.
The research was funded by a start-up fund from Sanford-Burnham, the Alfred P. Sloan Foundation, the German Research Foundation, the Robert-Koch-Institute, the National Institute of Allergy and Infectious Diseases, the U.S. Department of Energy, and the U.S. Department of Health and Human Services.
