Characterization of AHR1 and its functional activity in Atlantic sturgeon and shortnose sturgeon.

Sturgeon species are imperiled world-wide by a variety of anthropogenic stressors including chemical contaminants. Atlantic sturgeon, Acipenser oxyrinchus, and shortnose sturgeon, Acipenser brevirostrum, are largely sympatric acipenserids whose young life-stages are often exposed to high levels of benthic-borne PCBs and PCDD/Fs in large estuaries along the Atlantic Coast of North America. In previous laboratory studies, we demonstrated that both sturgeon species are sensitive to early life-stage toxicities from exposure to environmentally relevant concentrations of coplanar PCBs and TCDD. The sensitivity of young life-stages of fishes to these contaminants varies among species by three orders of magnitude and often is due to variation in the structure and function of the aryl hydrocarbon receptor (AHR) pathway. Unlike mammals, fishes have two forms of AHR (AHR1 and AHR2) with AHR2 usually being more highly expressed across tissues and functional in mediating toxicities. Based on previous studies in white sturgeon, A. transmontanus, we hypothesized that sturgeon taxa are unusually sensitive to these contaminants because of higher levels of expression and functional activity of AHR1 than in other fish taxa. To address this possibility, we characterized AHR1 in both Atlantic Coast sturgeon species, evaluated its' in vivo expression in young life-stages and in multiple tissues of shortnose sturgeon, and tested its ability to drive reporter gene expression in AHR-deficient cells treated with graded doses of PCB126 and TCDD. Similar to white sturgeon and lake sturgeon, AHR1 amino acid sequences in Atlantic sturgeon and shortnose sturgeon were more similar to mammalian AHRs and avian AHR1s than to AHR1 in other fishes, suggesting their greater functionality in sturgeon species than in other fishes. Exposure to graded doses of coplanar PCBs and TCDD usually failed to significantly induce AHR1 expression in young life-stages or most tissues of shortnose sturgeon. However, in reporter gene assays, AHR1 drove higher levels of gene expression than AHR2 alone, but their binary combination failed to drive higher levels of expression than either AHR alone. In total, our results suggest that AHR1 may be more functional in sturgeon species than in other fishes, but probably does not explain their heightened sensitivity to these contaminants.