Role of Borrelia burgdorferi host adaptation Regulator (BADR) in the pathophysiology of the agent of lyme disease
Lyme disease, caused by the spirochete bacterium Borrelia burgdorferi (Bb), is the most common tick-borne disease in the US and no vaccine is currently available. The RpoS transcription factor of Bb is a "gatekeeper" because it activates genes required for the spirochetes to transition from tick to vertebrate hosts. However, it remains unknown how RpoS becomes repressed to allow the spirochetes to transition back from the vertebrate host to the tick vector. This study is the first to show that a putative carbohydrate-responsive transcriptional regulator, designated BadR (Borrelia host adaptation Regulator), is a transcriptional repressor of rpoS. BadR levels are elevated in Bb cultures grown under in vitro conditions mimicking unfed-ticks and badR-deficient strains are defective for growth under these same conditions. Microarray and immunoblot analyses of badR-deficient strains showed up-regulation of rpoS and other factors important for virulence in vertebrate hosts, as well as down-regulation of putative tick-specific determinants (e.g. chitobiose utilization and linear plasmid 28-4 genes). Approximately two-thirds of differentially expressed genes, both up- and down-regulated, in badR-deficient Bb are annotated as hypothetical proteins of unknown function. DNA-binding assays revealed BadR binds to upstream regions of rpoS. Site-directed mutations in BadR and the presence of phosphorylated sugars affected BadR's binding to the rpoS promoters. badR-deficient Bb were unable to colonize both immuno-competent and -compromised mice. We show that BadR is a key component in the lifecycle of Bb because it most likely couples nutrient availability to niche-specific gene expression. This study has revealed novel targets to reduce the transmission and incidence of Lyme disease.
Proteins Bb uses to interface with the tick vector represent logical targets as vaccines because they would less likely result in autoimmunity in vaccinated individuals. Therefore, we have also begun to study three putative tick-specific determinants, lp28-4 encoded proteins (BB41, BBI29, and BI20) that were highly repressed in badR-deficient Bb. Since the chitinous tick integument may serve as a barrier and/or nutrient source for Bb and is involved in the transmission kinetics of other arthropod-borne pathogens, this study has also begun to characterize three previously predicted putative chitinases (BB0068, BB0168, and BB0600) for potential vaccines against Bb.