Mechanisms of SFG Rickettsia-Host Interactions

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PROJECT SUMMARY/ABSTRACT A subset of emerging, but poorly characterized tickborne diseases in the U.S. are caused by Spotted Fever Group (SFG) Rickettsia. These obligate intracellular bacterial pathogens cause mild-to-life-threatening vascular diseases in humans and have a limited set of diagnostics and therapeutic interventions. To promote widespread disease, SFG Rickettsia species have evolved dynamic strategies to invade host cells, escape into the cytosol, and spread from cell to cell. We hypothesize that SFG Rickettsia coordinate their complex life cycle by delivering an arsenal of secreted bacterial proteins (i.e., effectors) that reprogram host cell processes. Unfortunately, the identity and host targets of these secreted effectors have remained largely unknown. Furthermore, direct analysis of the secretion systems and putative effectors have been hampered due to challenges in growing and genetically manipulating these pathogens in the lab. To overcome this barrier, we recently adapted forward genetics and plasmid-based complementation to the model SFG member Rickettsia parkeri, allowing for powerful functional-genetic studies of SFG Rickettsia pathogenesis. Using these tools, we discovered that the secreted effector Sca4 promotes a late stage of cell-to-cell spread by manipulating host cell-cell adhesion. Additionally, transposon mutagenesis of a secreted effector of unknown function (RARP-1) and a component of the anomalous Rickettsia Type 4 secretion system (VirB6e) impairs distinct stages of R. parkeri infection. This proposal will leverage these key advances to examine how a secretion system and secreted effectors promote different steps of the R. parkeri infectious life cycle. Here, we combine our functional-genetic strategies and expertise in host cell biology, with modern biochemical techniques to reveal critical, mechanistic insights into SFG Rickettsia pathogenesis. In Aim 1, we will examine the function and secretome of the T4SS component VirB6e. In Aim 2, we will determine how the secreted effector RARP-1 promotes R. parkeri infection. In Aim 3, we will elucidate how Sca4 specifically targets host cell-cell adhesion complexes during R. parkeri cell-to-cell spread. Collectively, the proposed research will dramatically improve our fundamental understanding of Rickettsia biology and Rickettsia-host interactions and reveal therapeutic targets to prevent or treat tickborne diseases.