Coordination of deiodinase and thyroid hormone receptor expression during the larval to juvenile transition in sea bream (Sparus aurata, Linnaeus)

To test the hypothesis that THs play an important role in the larval to juvenile transition in the marine teleost model, sea bream (Sparus auratus), key elements of the thyroid axis were analysed during development. Specific RT-PCR and Taqman quantitative RT-PCR were established and used to measure sea bream iodothyronine deiodinases and thyroid hormone receptor (TR) genes, respectively. Expression of deiodinases genes (D1 and D2) which encode enzymes producing T3, TRs and T4 levels start to increase at 20-30 days post-hatch (dph; beginning of metamorphosis), peak at about 45 dph (climax) and decline to early larval levels after 90-100 dph (end of metamorphosis) when fish are fully formed juveniles. The profile of these different TH elements during sea bream development is strikingly similar to that observed during the TH driven metamorphosis of flatfish and suggests that THs play an analogous role in the larval to juvenile transition in this species and probably also in other pelagic teleosts. However, the effect of T3 treatment on deiodinases and TR transcript abundance in sea bream is not as clear cut as in larval flatfish and tadpoles indicating divergence in the responsiveness of TH axis elements and highlighting the need for further studies of this axis during development of fish.

Type 2 iodothyronine deiodinase is essential for thyroid hormone-dependent embryonic development and pigmentation in zebrafish

Despite the known importance of thyroid hormones (THs) in vertebrate growth and development, the role of tissue-specific TH activation in early embryogenesis remains unclear. We therefore examined the function of type 2 iodothyronine deiodinase (D2), one of the two tissue-specific enzymes catalyzing the conversion of T4 to T3, in developing zebrafish embryos (Danio rerio). Microinjection of early embryos with antisense oligonucleotides targeting either the D2 translation start site or the splice junction between the first exon and intron induced delays in development and pigmentation, as determined through the measurement of otic vesicle length, head-trunk angle, and pigmentation index at 31 h after fertilization. The antisense-induced delays in developmental progression and pigmentation were reversible through treatment with T3, suggesting that these phenotypic effects may be due to the depletion of intracellular T3 levels. Additional evidence for this hypothesis was provided by quantitative RT-PCR analysis of TH receptor-beta expression in D2 knockdown embryos, revealing a significant down-regulation of this T3-induced transcript that could be reversed by T3 treatment. Tyrosinase expression was also down-regulated in D2 knockdown embryos to a greater degree than could be predicted by the observed delay in developmental progression, suggesting that reduced D2 activity and resultant low intracellular T3 availability may directly influence pigmentation in zebrafish. These data indicate that TH activation by D2 is essential for embryonic development and pigmentation in zebrafish.

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.

Age-dependent changes in the response of the stomach to thyroidal signaling in developmentally arrested larval summer flounder

Thyroid-dependent stomach development occurs between approximately 35 and 50 days post-hatch (dph) in laboratory-reared summer flounder larvae. The process can be blocked by thiourea (TU), and TU effects are reversed by exogenous thyroxine (T4). To establish whether a window of sensitivity exists for T4-dependent gastric development, we arrested development of larvae with 0.39 mM TU given from 26 to 61dph, and measured weekly the developmental response of the stomach to 13 nM T4 for 0, 2, or 7 days. We examined cell proliferation in surface epithelium, gastric glands, and connective tissue, pepsinogen immunoreactivity in gastric glands, and Ulex Europaeus I (UEA I) lectin staining of gastric mucous neck cells, indicative of mucous content. In 26-47dph larvae, cell proliferation was increased 5- to 10-fold in all cell types after 2 days in T4, and returned to pretreatment values by 7 days of treatment. In 54dph fish, however, the proliferative response of gastric gland and surface epithelial cells decreased significantly from that of younger fish, while that of connective tissue was unchanged. For the differentiation markers, T4-induced mucous content increased at 54dph, but not in older fish, while pepsinogen induction was not different at any of the ages tested. The age interval between 47 and 54dph corresponds with the completion of gastric development in spontaneously metamorphosing larvae. The findings suggest that a critical window exists for the mitogenic actions of T4 in epithelial cells, but not for connective tissue cells, whereas no critical period was found for markers of differentiation.

Metamorphosis of summer flounder, Paralichthys dentatus: cell proliferation and differentiation of the gastric mucosa and developmental effects of altered thyroidal status

Summer flounder, like most marine fishes studied to date, are stomachless at first feeding, and subsequently acquire gastric function during the process of metamorphosis. Stomach formation is controlled largely by thyroxine (T4). In the present work we sought to understand gastric organogenesis in terms of cell proliferation and its relationship to histological differentiation. The objectives of the study were (1) to obtain a developmental pattern of cell proliferation in the gastric mucosa and to relate that pattern to the progress of gastric differentiation; and (2) to understand the regulatory role of T4 on cell proliferation and histological differentiation by altering the thyroidal status of the developing larvae. We observed that (1) in normally developing larvae, cell proliferation increased by early metamorphic climax (MC), remained high until mid-MC, and decreased to basal levels by late MC; concomitantly, the gastric glands appeared and differentiated in the fundic mucosa, and were complete by late MC; (2) T4 accelerated the differentiation of gastric glands and mucus neck cells, while inhibiting the concomitant increase in cell proliferation observed in controls; and (3) the goitrogen thiourea inhibited both cell proliferation and gastric differentiation compared to controls. These results indicate that T(4) is necessary for the three-fold increase in cell proliferation that occurs in early metamorphic climax, but that high T4 levels promote differentiation at the expense of proliferation. The observed effects would be consistent with the normal, metamorphosis-related increase in whole body T4.