Drosophila melanogaster as a model for studying Ehrlichia chaffeensis infections

Abstract

Ehrlichia chaffeensis is an obligate, intracellular bacterium that causes human monocytic ehrlichiosis (HME). The bacteria are vectored by the Lone Star tick (Amblyomma americanum), which is found primarily in the Midwestern and Southeastern United States E. chaffeensis was first reported in 1986 and HME was designated a nationally reportable disease by the United States Centers for Disease Control in 1999. Ehrlichia grows in several mammalian cell lines, but NO consensus model for pathogenesis exists for arthropods or vertebrates. Moreover, the host genes required for intracellular growth of this bacteria are unknown. We first established that the bacteria could infect and replicate both in vitro and in vivo in Drosophila melanogaster S2 cells and adult flies, respectively. We performed microarrays on S2 cells, comparing host gene expression between permissive or non-permissive conditions for E. chaffeensis growth. A total of 210 permissive, exclusive and 83 non-permissive, exclusive genes were up-regulated greater than 1.5-fold above uninfected cells. We screened flies mutant for genes identified in our microarrays for their ability to support Ehrlichia replication. Five mutant stocks were resistant to infection with Ehrlichia (genes CG6479, separation anxiety, CG3044, CG6364, and CG6543). qRT-PCR confirmed that bacterial load was decreased in mutant flies compared to wild-type controls. In particular, gene CG6364 is predicted to have uridine kinase activity. Thus, the in vivo mutation of this gene putatively disrupts the nucleotide salvage pathway, causing a decrease in bacterial replication. To further test the function of gene CG6364 in bacterial replication, we obtained cyclopentenyl cytosine (CPEC) from the National Cancer Institute. CPEC is a cytidine triphosphate (CTP) inhibitor known to deplete CTP pools in various cancers and to exhibit antiviral activity. Consequently, it inhibits de novo nucleotide synthesis, but doesn’t affect the nucleotide salvage pathway. When S2 cells were treated with CPEC and infected with Ehrlichia, an increase in bacterial replication was confirmed by qRT-PCR. Furthermore, addition of cytosine to S2 cells also resulted in increased bacterial replication. Therefore the nucleotide salvage pathway through cytidine appears necessary for bacterial replication. Our approach has successfully identified host genes that contribute to the pathogenicity of E. chaffeensis in Drosophila.