Unexpected Features Of Anthrax Toxin May Lead To New Types Of Therapies

The anthrax toxin is believed to play a critical role in causingdisease symptoms, in many cases leading to death even when antibioticshave been administered to stop bacterial growth. Consequently, there isa great deal of interest in better understanding how the toxinfunctions so that effective inhibitors with known mechanisms of actioncan be developed.

The findings, published in the online early edition of theProceedings of the National Academy of Sciences, come from a jointstudy by scientists at the Salk Institute for Biological Studies andHarvard Medical School.

"This research tells us that we need to revise the model ofanthrax toxin entry so that more effective drugs can be developed,"says the study's principal investigator, John A. T. Young, Ph.D., aprofessor in the Infectious Disease Laboratory at Salk. In previousresearch on the mechanisms used by the anthrax toxin to invade cells,Young's group discovered that the toxin targets two different types ofreceptors known as TEM8 and CMG2, on the cell's surface.

In the new study, Young and his colleagues found that toxinentry occurred at near neutral pH conditions when it was bound to theTEM8 receptor, but at strongly acidic conditions when bound to CMG2.The researchers in Young's lab at Salk revealed the two different pHlevels when they created cells that had only TEM8 receptors or CMG2receptors, but not both. They then used a drug that neutralizes the pHinside cells to test how it affected toxin entry in both cell types.

To their great surprise, the toxin behaved differently in thetwo cell types. For cells with the TEM8 receptor, a near-neutral pH wassufficient for toxin entry, but the cells with CMG2 required much moreacidic conditions. Although the types of receptors found on differentcells in the body have not yet been well defined, the investigatorswent on to show that different cultured cell lines display either ofthese two behaviors.

"Pore formation and translocation of the toxin occurred understrikingly different conditions," Young says. "The finding thatreceptor type dictates different pH thresholds was completelyunexpected."

The lack of a uniform pH threshold suggests that the anthraxtoxin may take two alternative pathways to reach different regionsinside the cell, and that "drugs that target a single pathway may beineffective," says Jonah Rainey, Ph.D., a research associate in Young'slab and the lead author of the PNAS paper.

Pharmaceutical agents that are designed to block theacid-dependent route of toxin entry may fail to block the neutralpH-dependent pathway, the researchers say.

In addition, the researchers made a finding that couldpotentially lead to a new approach for blocking toxin entry into acell. The toxin forms a syringe-like pore through which the activeparts of the toxin can enter the cell.

The scientists found that pore formation is associated withrelease of the receptor from the toxin. "Prior to this work it wasthought that the receptor was only partially released during toxin poreformation but our results suggest that that complete receptor releaseis required," says Rainey

"This is a key new finding, because blocking receptor release might disarm the toxin," says Young.

Rainey adds, "A pharmaceutical agent that keeps the receptorslocked in their original position may render the anthrax toxinharmless."

Anthrax is an acute infectious disease caused by thespore-forming bacterium Bacillus anthracis. It is most commonly foundin such animals as cattle, sheep and goats, but it can also occur inhumans as a result of exposure to infected animals or to anthrax sporesused as a bioterrorist weapon, as was the case in 2001 when letterscontaining spores were mailed through the postal system.

Co-authors on the study, which was funded by the NationalInstitutes of Health, include Salk researchers Patricia L. Ryan andHeather Scobie, and Darran J. Wigelsworth, Ph.D., and John Collier,Ph.D. of Harvard Medical School.

The Salk Institute for Biological Studies in La Jolla, California,is an independent nonprofit organization dedicated to fundamentaldiscoveries in the life sciences, the improvement of human health andthe training of future generations of researchers. Jonas Salk, M.D.,whose polio vaccine all but eradicated the crippling diseasepoliomyelitis in 1955, founded the Institute in 1960 on land donated bythe City of San Diego and with the financial support of the March ofDimes. For more information: www.salk.edu.