Exploring transcriptional and post-transcriptional epigenetic regulation of crf and 11fihsd2 in rainbow trout brain during chronic social stress

Using dominance hierarchies in juvenile rainbow trout (Oncorhynchus mykiss) as a model of chronic social stress in fish, we explored whether epigenetic transcriptional and post-transcriptional mechanisms are involved in the gene expression of corticotropin-releasing factor (crf) and 11 beta-hydroxysteroid dehydrogenase (11fihsd2), key factors involved in the regulation of the endocrine stress axis response. In juvenile rainbow trout pairs, subordinate individuals display sustained elevation of circulating cortisol concentrations. Cortisol production is controlled by the hypothalamic-pituitary-interrenal (HPI) axis in fish and initiated by CRF release from the preoptic area (POA). Given that crf is modulated during chronic social stress, and that such stress has been implicated in the epigenetic regulation of crf in other taxa, we probed a role for epigenetic regulation of crf transcript abundance in chronically stressed rainbow trout. We also investigated the regulation of the cortisolmetabolising enzyme 11fihsd2 in the POA, which is upregulated in subordinates. The potential involvement of DNA methylation and microRNAs (miRNAs) in the regulation of crf transcript abundance was investigated during social stress in the POA of fish, as was the potential involvement of miRNAs in 11fihsd2 regulation. Although transcript abundances of crf were elevated in subordinate fish after 4 days, DNA methylation profiles within putative promoter sequences upstream of the crf gene were not significantly affected by chronic stress. An inverse relationship between crf and its predicted posttranscriptional regulator miR-103a-3p in the POA suggests that miRNAs may be involved in mediating the effects of chronic social stress on key components of the endocrine stress axis.