The effect of 3,5,3'-triiodothyronine supplementation on zebrafish (Danio rerio) embryonic development and expression of iodothyronine deiodinases and thyroid hormone receptors.

The importance of thyroid hormones (TH) for embryonic development has long been shown in many vertebrates, but is not yet established in pre-hatch teleost models despite the presence of TH, TH receptors and iodothyronine deiodinases. Lack of data about the dynamics of TH metabolism in embryonic stages of fish does not allow to speculate about the involvement and/or role of TH in fish embryonic development. We therefore set up an experiment to examine the effect of 3,5,3'-triiodothyronine (T(3)) on zebrafish (Danio rerio) embryonic development and on the expression of some thyroid hormone-regulated genes as measured by real-time PCR. Maternally derived thyroxine (T(4)) and T(3) were detected throughout embryonic development and total levels remained stable. Thyroid hormone receptor (TR) alpha and beta mRNA were found to be present in early embryos. After an initial fall, TRalpha mRNA levels in the control group increased gradually from 12h post fertilization (HPF) onwards. TRbeta mRNA levels of control embryos were relatively stable during embryonic development, but increased around the hatching period. We also quantified type I (D1) and type II (D2) deiodinase mRNA expression in zebrafish embryos. D1 mRNA levels in the control group gradually increased during development while D2 levels were kept at a low and stable level until hatching. At 75 HPF, a fivefold increase of D2 expression was observed compared to embryonic levels. Exogenous T(3) added to the water (5nM) was taken up by the embryos, causing increased pigmentation and accelerated hatching. T(3) treatment significantly up regulated TRalpha mRNA levels at 48 HPF, while D2 mRNA was significantly down regulated at 75 HPF. Neither TRbeta nor D1 mRNA levels seemed responsive to the treatment. Taken together, these data show that during embryonic development zebrafish already have the necessary regulatory machinery for TH activation and signaling, and that T(3) treatment at that stage indeed influence embryonic development.