Studies on Environmental Regulation of Iodothyronine Deiodinase Genes in Tropical Fish

Abstract

Iodothyronine deiodinases play a role in regulating thyroid hormone (TH) dependant processes in vertebrates. The aim of this study was to clone and characterize the cDNA of type II (D2) and type III (D3) iodothyronine deiodinases of the goldlined spinefoot Siganus guttatus, and to examine involvement of environmental factors in mRNA expression of D2 and D3 in the goldlined spinefoot and the sapphire devil Chrysiptera cyanea. D2 of the goldlined spinefoot (SgD2) had 1013 bp that encoded a protein with 270 amino acids (aa) without the SECIS element. D3 of this fish (SgD3) yielded 1492 bp (269 aa) and a SECIS element between nucleotides 1267–1367. The deduced amino acid sequences of SgD2 and SgD3 showed high homology with those of fish in D2 and D3 clusters. Abundance of SgD2 and SgD3 mRNA in several tissues were expressed with an increase at 12.00 h and a decreased at 24.00 h. High expressions of these genes were observed in the brain and liver. Food deprivation suppressed the expression of SgD2, but not SgD3. The mRNA levels of SgD2 and SgD3 in the liver at 25 °C were higher than those reared at 20 and 30 °C. These results suggest that exogenous factors influence the mRNA levels of these genes in the liver and that transcription of the genes in certain tissues is partially regulated in a circadian manner. The abundance of hypothalamic D2 mRNA was higher at 12.00 h than at 06.00 h or 24.00 h. Rearing fish under constant dark conditions resulted in a decrease in SgD2 mRNA abundance during the subjective night. A single injection of melatonin lowered SgD2 mRNA abundance within 3 h. Collectively, it appears that hypothalamic SgD2 mRNA abundance is regulated by the circadian system and/or melatonin. No differences in D2 mRNA abundance were observed, when fish were reared at 20, 25, and 30 °C. However, food deprivation stimulated SgD2 mRNA expression during the daytime. Effects of food availability and photoperiod on mRNA abundance of type-II (CcD2) and type-III (CcD3) iodothyronine deiodinase in the sapphire devil’s brain were also examined. Fish out of breeding season were acclimated to LD=12:12 and then transferred to long (LD14:10; LP) and short (LD=10:14; SP) photoperiods with or without feeding for 1 week at 27 °C. The abundance of CcD2 and CcD3 mRNA in the brain was measured using quantitative real-time polymerase chain reaction (qPCR). Under LD=12:12, no day/night change occurred in CcD2 and CcD3 mRNA abundance. Both genes’ abundance in the fed and unfed fish was higher at 12.00 h than at 24.00 h under LP. Under SP, there was no day-night difference in CcD2 mRNA for the fed and unfed fish, although its expression was higher in the unfed fish than the fed fish. On the other hand, abundance of CcD3 mRNA in the fed fish, but not unfed ones, was higher at 12.00 h than at 24.00 h under SP. These results are indicative of an interaction between photic stimuli and nutritional status on expressions of D2 and D3 mRNA in the hypothalamus and liver of these two fish. Since these are possible environmental factors affecting reproductive performance in fish, it is suggested that local concentrations of thyroid hormones change in accordance with aquatic environments which fish live in.