<i>Rickettsia parkeri</i> subverts nicotinamide adenine dinucleotide ( <scp>NAD</scp> +) metabolic pathway for its survival in endothelial cells

Rickettsia parkeri rickettsiosis is an emerging tick-borne disease in humans. The bacterium R. parkeri is primarily transmitted to humans by the Gulf Coast tick, Amblyomma maculatum. Upon transmission to the host, R. parkeri infects and multiplies in host endothelial cells. In this study, we provide evidence that R. parkeri modulates the host nicotinamide adenine dinucleotide (NAD+) pathway for its survival in endothelial cells. Analysis performed with arrays containing antibodies against proteins involved in different metabolic pathways revealed increased levels of BST1 (an NAD+-dependent molecule) in R. parkeri-infected endothelial cells compared to the levels noted in uninfected controls. Quantitative PCR analysis further validated antibody array results. We noted modulation of several NAD+ pathway genes and increased reactive oxygen species (ROS) production upon R. parkeri infection in endothelial cells. Treatment of endothelial cells with NAD+ before R. parkeri infection but not postinfection significantly increased ROS levels that affected the bacterial burden. In contrast, we noted increased R. parkeri loads and reduced ROS levels in endothelial cells when bacteria were pre-incubated with NAD+ before infection. Furthermore, siRNA-mediated silencing of bst1 expression increased bacterial loads, reduced ROS levels, and affected the expression of NAD+ metabolic pathway genes. Collectively, these results not only elucidate the importance of the host NAD+ metabolic pathway in limiting R. parkeri infection by ROS production but also suggest its therapeutic role in preventing this and perhaps other rickettsial infections of medical importance.