Molecular analysis of apoptosis inhibition by Chlamydia trachomatis
Final Report Abstract
Chlamydia trachomatis is an obligate intracellular bacterium that multiplies inside a cytoplasmic vacuole in human epithelial cells. Apoptosis is a form of regulated cell death, which can be triggered by many external stimuli as well as cell-intrinsic alterations. With its obligate intracellular lifestyle, C. trachomatis is sensitive to apoptosis induction in the host cell, and it is generally believed that it has to be able to inhibit apoptosis in the host cell to develop. An anti-apoptotic activity of C. trachomatis-infection has been functionally described over 20 years ago, but its molecular nature is still unclear and contentious. We have in this project first mapped the antiapoptotic activity with high precision onto the apoptotic signalling pathway and have been able to rule out host cell components in the inhibition of apoptosis. Rather, the data suggested that a chlamydial protein localized to mitochondria and inhibited the cellular effectors of mitochondrial apoptosis, Bax and Bak. We isolated the chlamydial outer membrane porin, OmpA, as an interactor of Bak on human mitochondria. A number of experimental approaches identified OmpA on mitochondria, where it was found membrane-integrated in the proximity of and interacting with Bak. Ectopic expression of OmpA in non-infected human cells also inhibited apoptosis and precisely pheno-copied the anti-apoptotic targeting of Bak. At mitochondria, OmpA was inserted into membranes; the available evidence makes it most likely that OmpA is transported from the bacterial outer membrane into the mitochondrial outer membrane by trafficking in outer membrane vesicles (OMVs). The results further suggest that mitochondria-inserted OmpA has a similar function to the mitochondrial outer membrane porin, VDAC2, an established inhibitor of Bak. Both gram-negative bacteria and mitochondria have porins, that is solute-flux regulating, membraneinserted beta-barrel-proteins in their outer membrane. Mitochondria very likely have evolved from gram-negative bacteria. It thus appears that Chlamydia is able to exploit this evolutionary relationship to inhibit apoptosis in infected human cells, by delivering a porin from its outer membrane to the mitochondrial outer membrane. The goals of this project have been fully reached. Ways of OmpA-delivery, precise mechanism of apoptosis inhibition and details of the evolutionary relationship within the Chlamydiales order are now intriguing questions that will have to be addressed in future studies.
