# The host calcium system contributes to intracellular Rickettsia pathogenesis

## TL;DR

This study shows that calcium channel blockers, commonly used for heart conditions, can inhibit the growth of Rickettsia bacteria inside host cells by disrupting their actin polymerization.

## Contribution

The study identifies calcium channel blockers as potential host-targeted therapeutics against Rickettsia by revealing their impact on rickettsial actin polymerization.

## Key findings

- CCBs inhibit Rickettsia proliferation within endothelial cells.
- Disruption of host Ca2+ equilibrium also disrupts Rickettsia growth.
- CCBs significantly disrupt Rickettsia's ability to polymerize actin.

## Abstract

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.