Depletion of Tick Thioredoxin Reductase Attenuates the Native Tick Microbiota

The gulf-coast tick (Amblyomma maculatum) is a competent vector for a variety of pathogenic microbes, including Rickettsia parkeri, a causative agent of Spotted Fever Rickettsiosis.  Ticks experience a variety of oxidative stress condition while on and off the vertebrate host.  To counter-act the deleterious effects of reactive oxygen species, ticks have numerous antioxidant molecules in their repertoire, such as the Thioredoxin-Thioredoxin Reductase (Trx-TrxR) system (Karim et al., 2011).  Tick Thioredoxin Reductase has barely been investigated.  Our long-term goal is to reduce or block the spreading of vector-borne pathogens by interfering with vector proteins.  In this study, we tested our hypothesis that tick TrxR facilitates the colonization of microbes in tick tissues by mitigating the reactive oxygen species.  Transcriptional gene expression studies examining the level of TrxR during the prolonged blood-meal in both midguts and salivary glands indicates a potential need of this system during unfed stage.  In order to evaluate the functional significance of this highly conserved system, we utilized RNA interference to selectively deplete TrxR transcripts in vivo. Both transcriptional gene expression and enzymatic activity studies confirmed the successful depletion of TrxR transcript and activity. However, no significant effect was observed on total tick engorgement likely due to high redundancies or compensatory mechanism in ticks but, the tick salivary glands super oxide dismutase (SOD) was found similarly down regulated with the TrxR depletion.  Disruption of TrxR reduces the microbial load in the salivary glands examined by using bacterial universal 16s rRNA gene primers. Our results support the potential role of TrxR in preserving bacterial communities in tick tissues by alleviating the deleterious effect of reactive oxygen species.  This work opens new avenues of research in oxidative stress within tick vectors and vector-borne pathogens.