The thyroid hormone response profile of olfactory epithelium and its potential toward molecular bioindication of endocrine disruption in aquatic systems




Jackman, Kevin William

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Rana (Lithobates) catesbeiana is a sentinel species for xenoendocrine disruption in aquatic and semi-aquatic environments. These anurans equip an olfactory system that requires extensive and dramatic restructuring to allow for successful transition from aqueous to semi-aqueous environments, and for a dietary lifestyle that transitions from herbivorous to carnivorous. This transformation is complex and driven principally by the action of the thyroid hormones (THs), L-thyroxine (T4) and 3,5,3’-triiodothyronine (T3). Little is known about the genes involved in this change in the olfactory system, nor about how endocrine disrupting chemicals (EDCs) in the environment may interfere molecularly or behaviorally. R. catesbeiana tadpoles were exposed to either physiologically relevant concentrations of T4, T3, or 17-beta-estradiol (E2), or environmentally relevant concentrations of treated municipal wastewater effluent from two different contemporary treatment systems for 48 h. Effluent was prepared from either anaerobic membrane bioreactor (AnMBR) or membrane enhanced biological phosphorous removal (MEBPR), where municipal wastewater feed stocks for each reactor were split into two separate treatment trains per system, with each feed stock receiving either a cocktail of personal care products (PPCP) or a vehicle control. Tadpoles were evaluated for olfactory-mediated avoidance responses following these exposures. Significant disruptions to typical avoidance behavior were observed among the tadpoles exposed to T3 and treated effluent, but not to T4 or E2. qPCR analysis of the olfactory epithelium (OE) and the olfactory bulb (OB) was performed on the animals involved in the behavioral assays, as well as on parallel groups exposed to the same conditions. Transcript abundance in thra, thrb, and thibz was significantly greater in the T3-exposed behavioral group than that of the T4. E2 exposures exhibited no transcript response of these genes, whereas thibz exhibited an increase in transcript levels when tadpoles were exposed to either type of municipal wastewater effluent, regardless of addition of the PPCP cocktail, indicating presence of TH-mimic activity. A transcriptomic analysis was performed on the OE from T3-, T4-, and E2-exposed tadpoles compared to their respective controls using RNA-seq. While the overall number of contigs identified were comparable between hormone treatments, the OE was ~100X more sensitive to TH than E2. While many contigs were in common between T3- and T4-treated tadpoles, the T3-exposed hormone group contained over 20% more significant contigs than the T4-exposed group relative to their respective controls. Gene ontology (GO) analysis showed a stronger response in the T3-exposed tissue toward detection of chemical stimulus involved in sensory perception compared to T4-exposed tissue; a finding that is consistent with the difference in behavioural response to an avoidance cue. Using the results of the transcriptomic analysis, new qPCR tools were designed to additional TH-responsive transcripts and applied to OE from tadpoles exposed to effluent. Substantial removal of known and suspected EDCs was observed in both treatment systems, but molecular evidence of EDC activity remained. Further analysis of the OE as a means for bioindication of endocrine disruption is justified.



olfactory system, avoidance behavior, quantitative real-time polymerase chain reaction, endocrine disruption, amphibian, frog, thyroid hormone, municipal wastewater treatment, bioindicators, environmental toxicology, transcriptomics, functional genomics, adverse outcome pathway