Enhancement of Reactive Oxygen Species Production and Chlamydial Infection by the Mitochondrial Nod-like Family Member NLRX1

Abstract

Chlamydia trachomatis infections cause severe and irreversible damage that can lead to infertility and blindness in both males and females. Following infection of epithelial cells, Chlamydia induces production of reactive oxygen species (ROS). Unconventionally, Chlamydiae use ROS to their advantage by activating caspase-1, which contributes to chlamydial growth. NLRX1, a member of the Nod-like receptor family that translocates to the mitochondria, can augment ROS production from the mitochondria following Shigella flexneri infections. However, in general, ROS can also be produced by membrane-bound NADPH oxidases. Given the importance of ROS-induced caspase-1 activation in growth of the chlamydial vacuole, we investigated the sources of ROS production in epithelial cells following infection with C. trachomatis. In this study, we provide evidence that basal levels of ROS are generated during chlamydial infection by NADPH oxidase, but ROS levels, regardless of their source, are enhanced by an NLRX1-dependent mechanism. Significantly, the presence of NLRX1 is required for optimal chlamydial growth.During electron transfer reactions, eukaryotic cells generate highly reactive O 2 metabolites collectively known as reactive oxygen species (ROS).2 Given their high cellular toxicity, cells have developed mechanisms to maintain the levels of ROS in a homeostatic state and utilize them in cellular functions, including signaling pathways, gene expression regulation, and host defense mechanisms (1-3). Intracellular ROS generation is mediated by several enzymatic reactions, but the bulk of ROS is generated from two sources: the mitochondrial electron transport chain complex and membrane-bound NADPH oxidase (NOX and DUOX) (4).During oxidative phosphorylation in the mitochondria, O 2 is reduced to O 2 . , which in turn is converted by superoxide dismutase into H 2 O 2 , which then diffuses across the mitochondrial membrane to the cytoplasm (4). Notably, NLRX1, a member of the intracellular Nod-like receptor (NLR) family that is localized in mitochondria, enhances ROS production induced by poly(I⅐C), TNF␣, and Shigella flexneri infection (5). Although there is evidence that NLRX1 is the only NLR member that is found in mitochondria, its exact dynamic localization remains to be determined. Results from one study suggested that NLRX1 localizes to the outer mitochondrial membrane (6), but subsequent characterization suggests that NLRX1 is transported to the mitochondrial matrix, where it interacts with UQCRC2, a matrix-facing protein of respiratory chain complex III (bc 1 complex). The bc 1 complex is known to play an essential role in ROS generation from the mitochondria (5, 7). Chlamydia trachomatis is the first cause of bacterial sexually transmitted disease in the United States, and the incidence of infection has been increasing for the past 20 years (8 -10). The bacteria infect their preferred target cells, cervical epithelial cells, through entry vacuoles that avoid fusion with host cell lysosomes and then grow by subverting c...

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