Such insight is a significant step toward designing effective drugs against the agent whose potential as a terrorism tool spread fear and anguish across the United States in recent months, the scientists said. Each of the toxins is a legitimate target for drug design, said the authors of the study reported in the British journal Nature.
The investigators detailed the 3-D framework of the so-called edema factor, one of three proteins that turn anthrax into a lethal weapon. Anthrax letters sent through the U.S. Postal Service last fall led to the deaths of five people and shut down postal and govermental facilities.
A killer in its own right, edema factor causes dangerous accumulation of fluids, disrupts the function of the body's disease-fighting immune system and makes the second anthrax toxin, known as lethal factor, 10 to 100 times more potent. The third member of the troublesome threesome, called protective antigen, shields the other two from the body's defenses, transporting them into cells where they can unleash their destructive power.
Knowing the structure, which explains in chemical terms how the toxin works, moves scientists closer to devising ways to block the effects of anthrax, said the leaders of the research team, Wei-Jen Tang of the University of Chicago and Andrew Bohm of the Boston Medical Research Institute.
"Bacteria that cause human diseases have many weapons, and the bacterium that causes anthrax is no exception. The bacteria that cause anthrax carry three major weapons and all three of them together make anthrax deadly," Tang told United Press International. "Our atomic structure of the third anthrax toxin completes the list for the atomic structures of anthrax toxins. This information provides the invaluable tool to search for drugs in fighting against anthrax."
Edema factor, so named because it can trigger a potentially deadly influx of fluid, remains harmless until it meets a crucial signaling molecule called calmodulin. When they meet it generates cAMP, a small molecule with a big regulatory role in a host of cellular processes. In individuals infected with the anthrax bacterium, Bacillus anthracis, the protein manufactures cAMP in quantities massive enough to disrupt cells' normal function.
Bohm, Tang, Chester Drum, a University of Chicago graduate student working at the Boston institute, and colleagues spent three years deciphering the structure of EF and determining its deadly deeds. In comparing the structure of the on and off states of the protein, they observed how it produces cAMP and how it wreaks havoc once inside the cell.
"What we have done is explain precisely how edema factor operates," Bohm told UPI. "With atomic resolution data, we now know exactly which parts of the anthrax protein edema factor interact with the human protein calmodulin -- the interaction which leads to edema factor activation. We also know which parts of edema factor need to move in order to achieve this activation, and precisely which atoms are involved in enzymatic synthesis of cAMP, which is responsible for edema factor pathogenesis."
This understanding is a crucial step toward development of drugs to prevent cAMP formation. Current attacks on anthrax rely primarily on antibiotics. Since some forms of anthrax infection result in few symptoms until the disease is advanced, doctors need more effective solutions to counteract the toxins that remain beyond the reach of these drugs. Based on an entirely different approach, anti-toxin remedies may prove useful in treating anthrax as well as other diseases, such as whooping cough, cystic fibrosis and up to 20 percent of hospital infections in which edema factor-like poisons have a hand.
"Antibiotics are very effective at killing the anthrax bacteria, but do nothing to stop the toxins which have already been secreted. Anti-toxin drugs could provide treatment for the very late stages of the disease, the stage when antibiotics no longer guarantee survival," Bohm, a crystallography expert, told UPI.
The ability to construct new anti-toxin compounds based on known features of the protein rather than by randomly screening large numbers of candidates should hasten the development of novel treatments, scientists said.
The research points to EF as a weak link, as its structure contains a deep, well-defined molecular pocket that could represent an ideal target for future drugs.
"This was a very positive finding from the drug perspective because now we can begin to look for an inhibitor that works in the toxin pocket but not in other pockets so that it can cure the toxin action without making the patient suffer from a lack of enzyme activity," Tang said in an interview.
Of the several types of anthrax, the inhalation variety causes the most concern. Breathing the bacterial spores moves them to the lungs where they germinate, producing a powerful poison.
"If you do not kill the anthrax bacterium soon after infection, the microbe has time to produce potentially fatal levels of toxin, against which current drugs are not likely to be effective," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "These reports greatly increase our understanding of how anthrax toxin destroys cells and offer promising ways to develop treatments for advanced disease by attacking the toxin itself."
The research was done using cutaneous models of anthrax infection, the most common and less deadly form, but the findings apply to all forms of the disease, Bohm said.
Though the research began well before the Sept. 11 terrorist attack on America and the subsequent anthrax-by-mail scare, those tragic events highlight the importance and timeliness of the research, scientists said.
As with any drug, the road to developing new agents against anthrax is a long and difficult one, researchers said.
"Three years ago, when we started this project, Bacillus anthracis was an obscure agricultural pathogen with interesting biological properties," Tang said. "Now anthrax is front and center in every clinician's mind, and within months of the first bioterrorism case we have the structures for all three toxins. We hope this work will quickly lead to new therapies."
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