Scientists Decipher Structure Of Toxin Responsible For Botulism

UPTON, NY -- The toxins produced by Clostridium botulinum bacteria are among the deadliest known to humankind. A drop ingested can paralyze the body, including the muscles responsible for breathing, leading to death by asphyxiation. Now, in what could be a first step toward effectively disabling these deadly poisons, scientists at the U.S. Department of Energy's Brookhaven National Laboratory have deciphered the structure of one of the toxins and learned how it binds to the nerve cells it attacks.

"Since binding to the nerve cell is the first step in neurotoxin poisoning, understanding this interaction at the molecular level could help us design a vaccine to prevent the toxin from attaching," says Brookhaven biologist Subramanyam Swaminathan, who leads the research team. Such a vaccine or other therapeutic drugs based on the toxin's molecular structure would be invaluable to people consuming botulinum-tainted food, and to soldiers on the battlefield or others facing the threat of biological weapons. "You have to understand the toxicity and how it works to find preventive measures," Swaminathan says.

To decipher the toxin's molecular structure, the scientists bombard crystalline samples with high-intensity x-rays at Brookhaven's National Synchrotron Light Source (NSLS), one of the most widely used scientific facilities in the world. By studying how the x-rays diffract as they bounce off or pass through parts of the crystal, the scientists work backward to reconstruct the shape and arrangement of the atoms in the molecule.

"Without the intense x-ray beams at the NSLS, it would be very difficult to do this," Swaminathan says.

The current studies were on botulinum neurotoxin B, one of seven types produced by Clostridium botulinum bacteria. Scientists have previously analyzed the structure of botulinum neurotoxin A, but at a much lower resolution. The level of detail in the current analysis is unprecedented.

The scientists have analyzed the part of the toxin molecule that binds to nerve cells, as well as the part that actively blocks the release of neurotransmitters -- the chemical messengers nerve cells use to communicate with one another and with muscle cells. More work is now needed to understand how the toxin moves through the cellular membrane. Finding ways to block any one of these steps could potentially stop the toxin in its tracks.

Such detailed knowledge of the toxin's mechanism of action, in addition to stimulating vaccine development, could also enhance the toxin's use in several therapeutic applications. Botulinum toxins are currently used to treat a variety of involuntary movement disorders. By injecting minute quantities of the toxin into the spasmodic muscles that cause facial twitches, writer's cramp or stuttering, for example, doctors have been able to calm the muscles. Treatments, however, are only temporary and must be repeated. Understanding the structure of the toxin could improve the efficacy of these treatments.

The current research could also lead to better tests for the presence of the toxin in foods and in weapons or weapons factories. One direct aim of the research, which is funded by the Chemical and Biological Non-proliferation Program of the U.S. Department of Energy, is to provide improved means of detecting and limiting the spread of toxins and biological weapons.

The Brookhaven scientists work with minute quantities of purified botulinum toxin, which they purchase from the Food Research Institute in Wisconsin under strict regulation and with the approval of the U.S. Centers for Disease Control and Prevention in Atlanta. Only authorized scientists have access to the laboratory.