The host calcium system contributes to intracellular Rickettsia pathogenesis

Bacteria in the genus Rickettsia are obligate intracellular parasites of the eukaryotic cytoplasm. Pathogenic Rickettsia species are exquisitely evolved to only proliferate within eukaryotic host cells, particularly within endothelial cells of the mammalian vasculature. Through evolution in this very specific niche, Rickettsia have developed an inextricable dependence on multiple host functions. This absolute dependence on host cell biology offers a potential strategy for antibacterial development called host-targeted therapeutics. A previous screen of compounds that specifically target mammalian cell biology indicated that host-targeted calcium channel blockers (CCBs) inhibit Rickettsia conorii proliferation within human cells. CCBs are routinely prescribed to human patients as antihypertensives or antianginals that function by disrupting the calcium ion equilibrium in vesicula/cardiac smooth muscle cells. To further investigate the potential anti-Rickettsia activities of CCBs, we sought to define the interaction between pathogenic Rickettsia and the host Ca²⁺ system. Achieved data demonstrate that CCBs inhibit Rickettsia proliferation within endothelial cells, and that physical disruption of the host Ca²⁺ ion gradient also disrupts Rickettsia growth. Additional analyses reveal that Rickettsia infection leads to a rapid and persistent disruption of the host Ca²⁺ equilibrium. By querying Rickettsia pathogenesis, we demonstrate that some CCBs marginally disrupt rickettsial adherence to the host cell or induce apoptosis. However, all tested CCBs universally and significantly disrupt the ability of Rickettsia to polymerize actin. Together, these data demonstrate that CCBs possess anti-Rickettsia properties that function by disrupting rickettsial actin polymerization, and these results highlight the complex interdependence of Rickettsia and host cell biology.