Host pathways associated with human bacterial infections extend to commensal Wolbachia-Drosophila endosymbiosis

Abstract

Wolbachia bacteria are among the most successful endosymbionts in nature, carried by half of all insect species. Unlike human bacterial pathogens that kill host cells and tissues, Wolbachia endosymbionts are generally carried by insects with little adverse effect. The striking difference in outcome raises a basic question of what aspects of infection mechanisms are conserved across systems. In this study, 37 small molecule inhibitors were used to test whether 14 candidate host processes that affect the abundance of other intracellular bacteria also affect Wolbachia. Compounds that significantly affected the absolute abundance of the Wolbachia surface protein (wsp) gene in D. melanogaster were retested in D. simulans flies. 5 compounds that consistently increased wsp abundance in both systems were associated with the Imd pathway, Calcium signaling, Ras/mTOR signaling, and the Wnt pathway. By contrast, the only compound to suppress wsp abundance was a Ubiquitin-proteasome pathway inhibitor. The implicated host processes were retested for impact on Wolbachia using constitutive and inducible RNAi expression systems in D. melanogaster. These tests corroborated a function for the host target of rapamycin (tor) and armadillo (arm) genes in affecting bodywide wsp abundance. Prior studies have reported that Ras/mTOR and Wnt pathways interact with ATG6 (Beclin-1), representing a possible convergence point for signaling impacts on Wolbachia. ATG6 disruption tests, driven by inducible RNAi expression, also elevated wsp abundance. This work suggests that combined effects of the Wnt pathway, Ras/mTOR signaling, and autophagy normally support Wolbachia containment, moderating the Wolbachia-host endosymbiosis. IMPORTANCEDisease-related microbes have been intensively studied as a model for infection. An intrinsic complication of such studies is bacterial induction of cell stress and cell death. To expand our understanding of cellular infection mechanisms, we studied a bacterial endosymbiont of insects, called Wolbachia, that does not kill the cells it infects. We asked whether cellular processes involved in pathogen infection are also associated with Wolbachia infections. Chemical and genetic tests were used to investigate cellular effects on Wolbachia abundance within fruit flies. We identified a subset of cellular processes with robust, repeatable effects on Wolbachia infection: the Wnt pathway and the Ras/mTor pathway. The results also suggest that multiple cellular pathways act together, which collectively suppresses Wolbachia abundance in vivo. Active host containment may explain in part why Wolbachia is mostly regarded as a neutral endosymbiont, and not as a pathogen.