Pathophysiological Actions of the Anthrax Toxins

National Institute Of Allergy And Infectious Diseases

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Abstract

Anthrax toxin protective antigen protein (PA, 83 kDa) binds to receptors on the surface of mammalian cells, is cleaved by the cell surface protease furin, and then captures either of the two other toxin proteins, lethal factor (LF, 90 kDa) or edema factor (EF, 89 kDa). The PA-LF and PA-EF complexes enter cells by endocytosis via lipid rafts and pass through endocytic vesicle populations, and then translocate LF and EF to the cytosol. EF is a calcium and calmodulin-dependent adenylyl cyclase that causes large and unregulated increases in intracellular cAMP concentrations. LF is a metalloprotease that cleaves several mitogen-activated protein kinase kinases (MEKs). The toxin proteins play a central role in the virulence of Bacillus anthracis and the PA protein is the key ingredient in anthrax vaccines. [unreadable] The toxins cause profound effects on cultured cells and in experimental animals. The mechanism underlying the rapid lysis of mouse macrophages by anthrax lethal toxin was shown by others to require the newly described inflammasome and activation of caspase-1. By detailed kinetic and pharmacological studies, we showed that these two factors are late events in the cascade of signaling events that lead to lysis. The proteasome, which we previously showed is also required for macrophage lysis, was shown in new studies to be required upstream of the inflammasome. In other studies, we showed that the heat shock response of cells protects cells from anthrax lethal toxin, and its does so by blocking the inflammasome-dependent caspase-1 activation. Finally, we also showed that the requirement for proteasome activity in macrophage lysis works through the N-end rule, suggesting that an unknown substrate having a destabilizing residue at the N-terminus must be a component of the pathway leading to lysis. [unreadable] Additional studies have been directed to understanding the response of rats and mice to lethal toxin injection. The death of Fischer rats in as little as 38 minutes following lethal toxin injection points to a unique pathological mechanism. With several collaborating groups, we showed that rats exposed to lethal toxin experience pulmonary edema and hemorrhage. Specific effects on the cardiovascular system were observed by continuous monitoring and imaging techniques. This rat model was also used to evaluate and document the efficacy of a monoclonal antibody developed by a commercial entity.[unreadable] The long-standing question of the interaction of the anthrax edema and lethal toxins was explored in DBA/2J mice. It was shown that pretreatment with small doses of edema toxin sensitizes the mice to a dose of lethal toxin given immediately or after some delay. A cooperative action of the two toxins may help to explain why the bacterium has these two toxins which share a common mechanisms of uptake.