Concurrent changes in thermal tolerance thresholds and cellular heat stress response reveals novel molecular signatures and markers of high temperature acclimation in rainbow trout.

The objective of this study was to better understand the molecular mechanisms which regulate acclimatory responses and thermal safety margins of rainbow trout (Oncorhynchus mykiss) at temperatures above physiological optimum. For this, we investigated the time course of changes in critical thermal tolerance thresholds and associated hepatic and renal transcript abundance of molecular markers related to cellular stress response, during high temperature acclimation. The experimental fish were initially acclimated to 17 °C and later exposed to a gradually raised elevated temperature regime (22 °C) for a period of 30 days. CT, CT and mRNA expression of candidate markers were examined before the thermal challenge (T) and over the time-course (days) of high temperature exposure (T, T, T, T and T). With respect to organismal response, CT was significantly elevated at T, but the degree of gain in heat tolerance was not persistent. Contrarily, we observed a gradual loss in cold tolerance with highest CT estimate at T. Based on the time-course of mRNA expression, the studied markers could be categorized into those which were persistently elevated (hsp70a, hsp70b, hspa5, hsp90a, hsp90b, stip1 and serpinh1 in kidney and hsp90b in liver); those which concurred with changes in CT (hspbp1, hsp90b, stip1, gr1, hif1a, hyou1, tnfa and tlr5 in kidney); and those which concurred with changes in CT (hsp90a, serpinh1, tlr5 and lmo2 in liver). Apparently, transcriptional changes in kidney and liver reflected CT and CT trend, respectively. Expression profile of stip1 and tlr5 suggest that they are potential novel markers which could reflect thermal limits in rainbow trout. Hepatic metabolic markers were either initially elevated (alt, glud, g6pase1) or down-regulated at different time-points (ast2, gls1, fas, cpt1b, mtor), linked to gluconeogenesis and metabolic depression, respectively. Whereas, growth-axis markers showed no significant differences. Overall, this time-course analysis has revealed potential associations in organismal and tissue-specific cellular response to high temperature acclimation in a thermally sensitive coldwater ectotherm.