Thyroid hormone (TH) is an ancestral signal linked to seasonal life history transitions throughout vertebrates. TH action depends upon tissue-localized regulation of levels of active TH (triiodothyronine, T3), through spatiotemporal expression of thyroid hormone deiodinase (dio) genes. We investigated the dio gene family in juvenile Atlantic salmon (Salmo salar) parr, which prepare for seaward migration in the spring (smoltification) through TH-dependent changes in physiology. We identified two type 2 deiodinase paralogs, dio2a and dio2b, responsible for conversion of thyroxine (T4) to T3. During smoltification, dio2b was induced in the brain and gills in zones of cell proliferation following increasing day length. Contrastingly, dio2a expression was induced in the gills by transfer to salt water (SW), with the magnitude of the response proportional to the plasma chloride level. This response reflected a selective enrichment for osmotic response elements (OREs) in the dio2a promoter region. Transcriptomic profiling of gill tissue from fish transferred to SW plus or minus the deiodinase inhibitor, iopanoic acid, revealed SW-induced increases in cellular respiration as the principal consequence of gill dio2 activity. Divergent evolution of dio2 paralogs supports organ-specific timing of the TH-dependent events governing the phenotypic plasticity required for migration to sea.

It is becoming clear that epigenetic modifications such as DNA methylation can be dynamic and, in many cases, reversible. Here we investigated the photoperiod and hormone regulation of DNA methylation in testes, ovaries, and uterine tissue across multiple time scales. We hypothesized that DNA methyltransferase 3a (dnmt3a) is driven by photoperiodic treatment and exhibits natural variation across the female reproductive cycle and that melatonin increases whereas estrogen reduces DNA methylation. We used Siberian hamsters (Phodopus sungorus) due to their robust changes in reproductive physiology across seasonal and estrus time scales. Our findings indicate that short-day (SD) winter-like conditions significantly increased global DNA methylation and dnmt3a expression in the testes. Using immunohistochemistry, we confirm that increased dnmt3a expression was primarily localized to spermatogonium. Conversely, the ovaries did not exhibit variation in DNA methylation or dnmt3a/3b expression. However, exposure to SD significantly increased uterine dnmt3a expression. We then determined that dnmt3a was significantly decreased during the estrus stage. Next, we ovariectomized females and subsequently identified that a single estrogen+progesterone injection was sufficient to rapidly inhibit dnmt3a and dnmt3b expression. Finally, we demonstrate that treatment of human embryonic kidney-293 cells with melatonin significantly increased both dnmt3a and dnmt3b expression, suggesting that long-duration nocturnal signaling in SD may be involved in the regulation of DNA methylation in both sexes. Overall, our data indicate that dnmt3a shows marked photoperiod and estrus plasticity that likely has broad downstream effects on the timing of the genomic control of reproductive function.

The anadromous Atlantic salmon utilizes both fresh and salt water (FW and SW) habitats during its life cycle. The parr-smolt transformation (PST) is an important developmental transition from a FW adapted juvenile parr to a SW adapted smolt. Physiological changes in osmoregulatory tissues, particularly the gill, are key in maintaining effective ion regulation during PST. Changes are initiated prior to SW exposure (preparative phase), and are completed when smolts enter the sea (activational phase) where osmotic stress may directly stimulate changes in gene expression. In this paper we identify 4 nuclear factor of activated T cells (NFAT5, an osmotic stress transcription factor) paralogues in Atlantic salmon, which showed strong homology in characterized functional domains with those identified in other vertebrates. Two of the identified paralogues (NFAT5b1 and NFAT5b2) showed increased expression following transfer from FW to SW. This effect was largest in parr that were maintained under short day photoperiod, and showed the highest increases in chloride ion levels in response to SW exposure. The results of this study suggest that NFAT5 is involved in the osmotic stress response of Atlantic salmon.