Structure Based Design of Cholera Toxin Antagonists

Cholera is an acute enteric infection, with huge pandemic potential, caused by ingestion of food or water contaminated with the bacterium Vibrio cholerae, the gram negative bacteria (Sack, Sack et al. 2004). The etiologic agent responsible for cholera was identified in 1883 when Robert Koch demonstrated that comma(-shaped) bacteria, later designated as V. cholerae, causes cholera infection (Koch 1884). Since Koch's discovery of cholera virulent factor, different specific strain variants of V. cholerae have been identified. For the majority of cholera's disease outbreaks two biotypes of V. cholerae'sserogroup O1 are responsible: Clasical and “El Tor”, as well serogroup O139 that was responsible for a large epidemic in Bangladesh and India (Ramamurthy, Garg et al. 1993). Non-O1 and non-O139 V. cholerae can cause mild diarrhoea but do not generate epidemics (Ramamurthy, Bag et al. 1993). Cholera transmission is closely related to inadequate environmental conditions that can be find in suburban slums where the basic infrastructure is not available, as well as in camps for internally displaced people or refugees, where minimum requirements of clean water and sanitation are not met. A typical example of such non-promising situation has induced an outbreak of Cholera after earthquake in Haiti in January 2010 (Andrews and Basu 2011). The short incubation period of two hours to five days, enhances the potentially explosive pattern of outbreaks. Intensive efforts for the identification of the basis of Cholera disease at a molecular level were done by different research groups during the 1960s, until Finkelstein and co-workers recognized a protein toxin as a major virulent factor that causes the massive fluid release in Cholera infection (Finkelstein, Atthasampunna et al. 1966). The efforts to solve a complete structure of Cholera toxin by X-ray diffraction analysis were concluded during the 1990s (Spangler 1992; Zhang, Scott et al. 1995). Up to date, 27 X-ray structures related to Cholera toxin (CT) are deposited in the Protein Databank. The structure and function of CT at the molecular level will be the subject of our review. In this work we will also show some examples of structure based design of various types of CT inhibitors; we will introduce catechin-like compounds as inhibitors of the enzymatic A unit of CT; mimics of oGM1 as inhibitors of the non-toxic pentamer of B subunits of CT (CTB); as well as multivalent inhibitors that very effectively prevent adhesion of CTB to GM1 receptors at the surface of epithelial cells. At the end, we will also describe a new strategy for developing inhibitors via targeting binding site for blood group antigens in Cholera Toxin.