Pathogenic bacteria causing human
rickettsioses, transmitted in nature by arthropod vectors, primarily infect vascular endothelial cells lining the blood vessels, resulting in 'endothelial activation' and onset of innate immune responses.
Nucleotide second messengers are long presumed to be the stimulators of
type I interferons, of which bacterial
cyclic-di-GMP (
c-di-GMP) has been implicated in multiple signaling pathways governing communication with other bacteria and host cells, yet its importance in the context of rickettsial interactions with the host has not been investigated. Here, we report that all rickettsial genomes encode a putative
diguanylate cyclase pleD, responsible for the synthesis of
c-di-GMP. In silico analysis suggests that although the domain architecture of
PleD is apparently well-conserved among different rickettsiae, the
protein composition and sequences likely vary. Interestingly, cloning and sequencing of the
pleD gene from virulent (Sheila Smith) and avirulent (Iowa) strains of R. rickettsii reveals a nonsynonymous substitution, resulting in an
amino acid change (
methionine to
isoleucine) at position 236. Additionally, a previously reported 5-bp insertion in the genomic sequence coding for
pleD (NCBI accession: NC_009882) was not present in the sequence of our cloned
pleD from R. rickettsii strain Sheila Smith. In vitro
infection of HMECs with R. rickettsii (Sheila Smith), but not R. rickettsii (Iowa), resulted in dynamic changes in the levels of
pleD up to 24 h post-
infection. These findings thus provide the first evidence for the potentially important role(s) of
c-di-GMP in the determination of host-cell responses to pathogenic rickettsiae. Further studies into molecular mechanisms through which rickettsial
c-di-GMP might regulate pathogen virulence and host responses should uncover the contributions of this versatile bacterial second messenger in disease pathogenesis and immunity to human
rickettsioses.