The role of thyroid hormones during the development of eye pigmentation in the Pacific bluefin tuna (Thunnus orientalis)

Sensitivity of Anguilliformes leptocephali to metamorphosis stimulated by thyroid hormone depends on larval size and metamorphic stage

Metamorphosis of teleosts including Anguilliformes is well known to be induced by thyroid hormone (TH), although the underlying mechanism is not fully understood. In this study, we investigated the experimental conditions needed to induce normal metamorphosis in artificially spawned Japanese eel (Anguilla japonica), including initial larval size, TH concentration, and timing of TH immersion. Around 37 mm TL was found to be the minimum size of larvae that underwent successful metamorphosis induced by l-thyroxine (T₄); notably, smaller larvae did not show increased expression of TH receptors in response to T₄, suggesting that small leptocephali are not sufficiently responsive to TH. Furthermore, successful completion of metamorphosis depended on sensitivity to TH, which changed with metamorphic stage; for example, prolonged exposure to higher TH concentrations led to morphological defects. Collectively, these results reveal that the induction of metamorphosis by TH is dependent on larval size, and that the concentration of TH must be adjusted in line with metamorphic stage to achieve successful progression of metamorphosis. Our findings will contribute to improving production technology in the aquaculture of Japanese eels by facilitating the earlier induction of metamorphosis in artificial leptocephali.

Changes in thyroid hormone levels and related gene expressions in embryo-larval zebrafish exposed to binary combinations of bifenthrin and acetochlor

Bifenthrin (BF) and acetochlor (AT) are widely used as an insecticide and herbicide, respectively, which are introduced to the aquatic environment as a natural result. Although the thyroid active substances may coexist in the environment, their joint effects on fish have not been identified. We examined the joint toxicity of BF and AT in zebrafish (Danio rerio) in this study. An acute lethal toxicity test indicated that the median lethal concentration (LC₅₀) values of BF and AT under 96 h treatment were 0.40 and 4.56 µmol L^-1, respectively. The binary mixture of BF + AT displayed an antagonistic effect on the acute lethal toxicity. After 14 days post fertilization (dpf) with exposure to individual pesticides at sub-lethal concentrations of, no effects were observed on the catalase (CAT) and peroxidase (POD) activities, while the binary mixtures (except for the 7.2 × 10^-3 µmol L^-1 BF + 1.2 × 10^-2 µmol L^-1 AT exposure group) significantly induced the CAT activity. The superoxide dismutase (SOD) activity and triiodothyronine (T3) level were significantly increased in all exposure groups. The thyroxine (T4) level remained unchanged after exposure to individual pesticides, but significantly increased in the 7.2 × 10^-3 µmol L^-1 BF + 1.2 × 10^-2 µmol L^-1 AT group. The expressions of the genes Dio2, TRa, TSHβ and CRH in the thyroid hormone (TH) axis were significantly up-regulated in the 7.2 × 10^-3 µmol L^-1 BF + 0.4 × 10^-2 µmol L^-1 AT group. Our data indicated that the binary mixture of BF + AT significantly altered the antioxidant enzyme activities and gene expressions in the hypothalamic-pituitary-thyroid (HPT) axis and changed the TH levels.

Impact of co-exposure with butachlor and triadimefon on thyroid endocrine system in larval zebrafish

INTRODUCTION

Butachlor (BTL) and triadimefon (TDF), the widely used herbicide and fungicide, are unavoidable enter into the aquatic environment. However, there were limited study regarding to the joint toxicity of these two pesticides on fish at present.

AIM

To evaluate the potential thyroid-disrupting toxicity and exposed to different concentrations of BTL mixed with TDF.

MATERIALS AND METHODS

Zebrafish embryo (n=3) were exposed to 0.01 and 0.05 fold of LC50 from the acute joint toxicity test, of which 0.32mg/L (BTL) and 9.41mg/L (TDF) for single or mixture agents (BTL: 0.0064mg/L, 0.032mg/L; TDF: 0.1882mg/L, 0.9410mg/L; co-exposure: 0.0032mg/L BTL+0.0941mg/L TDF, 0.016mg/l BTL+0.4705mg/L TDF) after 10-day post-fertilization. Hatching, malformation, survival rates and thyroid hormones (THs), genes expression involved in HPT-axis of embryos were measured and detected in control and separately/co-exposure treatments. THs contents were evaluated by ELISA kit and the expression levels of genes were determined by RT-PCR.

RESULTS

Hatching, malformation and survival rates of embryos exposed to single BTL exhibited no statistically significant difference from the control besides decreased of high concentration in survival rates. Exposure to TDF reduced hatching, survival rate and increased malformation. The combined exposure to BTL and TDF resulted in greater adverse effects on embryonic development. BTL exposure significantly increased free T3 and T4 contents. Elevated free T3 content was also observed in the larvae exposed with single BTL. Co-exposure of the two pesticides caused greater enhanced of T3 and T4 levels. Furthermore, gene data showed BTL up-regulated the mRNA expression of tpo, tshβ, tg, ttr, dio2, TDF up-regulated the mRNA expression of tpo, trα, ttr, dio2 and down-regulated trβ gene. The mixture of the two pesticides caused up-regulation mRNA expression of trα, trβ, tg, ttr, dio2.

CONCLUSION

BTL and TDF resulted in adverse effects on zebrafish embryonic development and caused thyroid endocrine disruption, BTL and TDF have a synergistic effect on development and thyroid endocrine by enhanced level of thyroid hormone.

Dietary methimazole-induced hypothyroidism reduces hepatic lipid deposition by down-regulating lipogenesis and up-regulating lipolysis in Pelteobagrus fulvidraco

The present study was conducted to investigate the effects and mechanisms of hypothyroidism, induced by administration of 0.2% methimazole through the food, on lipid metabolism in the liver of juvenile yellow catfish Pelteobagrus fulvidraco. To this end, yellow catfish were fed diets containing either 0 or 2g methimazole per kg of diet for 8weeks, respectively. The results showed that fish fed diet containing methimazole had a significant reduction in growth performance, plasma THs levels and hepatic lipid content. Meanwhile, methimazole treatment inhibited the activities of lipogenic enzymes (6-phosphogluconate dehydrogenase, glucose 6-phosphate dehydrogenase, malic enzyme, isocitrate dehydrogenase and fatty acid synthase) and the mRNA levels of genes involved in lipogenesis (6-phosphogluconate dehydrogenase, glucose 6-phosphate dehydrogenase, fatty acid synthase, acetyl-CoA carboxylase α, sterol-regulator element-binding protein-1 and liver X receptor), but increased lipolytic enzyme (carnitine palmitoyltransferase 1) activity and the expression of genes involved in lipolysis (carnitine palmitoyltransferase 1a, hormone-sensitive lipase and peroxisome proliferators-activated receptor α). Thus, our study indicated that dietary methimazole-induced hypothyroidism could disturb the normal processes of lipid metabolism at the enzymatic and molecular levels in yellow catfish, and the reduced hepatic lipid content by hypothyroidism was attributable to the down-regulation of lipogenesis and up-regulation of lipolysis.

Characterization of thyroid hormone receptors during early development of the Japanese eel (Anguilla japonica)

We studied the profiles of thyroid hormone receptors (TRs) in Japanese eels (Anguilla japonica) during development from hatched larvae to juveniles. Two TRαs (TRαA and TRαB) and one TRβ (TRβA) cDNA clones were generated by RACE. The TRαA, TRαB and TRβA cDNAs encoded 416, 407 and 397 amino acid proteins with much higher homologies to the Japanese conger eel (Conger myriaster) TRs than to other fish TRs. In a transiently transfected Japanese eel cell line, Hepa-E1, the TRs showed thyroid hormone (TH)-dependent activation of transcription from the TH-responsive promoter. Four TR cDNA clones, including TRβB reported in a previous study, were analyzed by real-time RT-PCR. The TR mRNA levels in hatched larvae were determined. The two TRβ mRNAs were present at low levels but there was a peak in the TRαs during the larval stage before metamorphosis. During metamorphosis, the two TRαs both exhibited peaks and expression of the two TRβs was higher than during the early growth stage. This expression pattern is similar to that of the Japanese conger eel. It is possible that thyroid hormones control the early development of Japanese eels and Japanese conger eels through TRs. This is the first analysis of the expression sequence of TRs during early larval stages of Anguilliformes.

Metamorphosis in teleosts

Teleosts are the largest and most diverse group of vertebrates, and many species undergo morphological, physiological, and behavioral transitions, "metamorphoses," as they progress between morphologically divergent life stages. The larval metamorphosis that generally occurs as teleosts mature from larva to juvenile involves the loss of embryo-specific features, the development of new adult features, major remodeling of different organ systems, and changes in physical proportions and overall phenotype. Yet, in contrast to anuran amphibians, for example, teleost metamorphosis can entail morphological change that is either sudden and profound, or relatively gradual and subtle. Here, we review the definition of metamorphosis in teleosts, the diversity of teleost metamorphic strategies and the transitions they involve, and what is known of their underlying endocrine and genetic bases. We suggest that teleost metamorphosis offers an outstanding opportunity for integrating our understanding of endocrine mechanisms, cellular processes of morphogenesis and differentiation, and the evolution of diverse morphologies and life histories.