Functional Genomics of Tick Selenoproteins: An Examination of How the Dynamics of Reactive Oxygen Species Affect Tick Feeding and Pathogen Movement

The Gulf-Coast tick, Amblyomma maculatum, possesses an elaborate set of selenoproteins, which prevent the deleterious effects from oxidative stress that occur during feeding.  Selenocysteine is the 21st naturally-occurring amino acid. Selenoproteins have diverse functions and many remain uncharacterized, but they are typically associated with antioxidant activity. In our current work, we examined the role of selenoproteins by bioinformatics, transcriptional gene expression, RNA interference, antioxidant assays, and RNA-seq. Knockdown of Selenoprotein K or Selenoprotein M did not reveal substantial differences in feeding or egg viability, though oviposition was reduced.  Similarly, we targeted an essential co-factor (selenocysteine-specific elongation factor (SEF)) in selenoprotein synthesis for RNAi and found no significant impact on feeding or oviposition.  Surprisingly, we detected no antioxidant activity in the saliva collected from SEF knockdowns, but the activity was higher in selenoprotein knockouts compared to controls, which is likely due to compensatory transcriptional expression of genes involved in combating reactive oxygen species.  These data indicate that SelK and SelM are salivary antioxidants but are not essential for tick survival or reproduction and are compensated by other antioxidant systems.  The transcriptional profile of selenoproteins in R. parkeri-infected Amblyomma maculatum revealed declining activity of selenoprotein M and catalase and increased activity of selenoprotein O, selenoprotein S, and selenoprotein T.  Furthermore, the pathogen burden was significantly altered in SEF-knockdowns.  We then determined the global impact of SEF-knockdown by RNA-seq, and mapped huge shifts in secretory gene expression that could be the result of down-regulation of the Sin3 histone deacetylase corepressor complex.