In all vertebrates, the thyroid axis is an endocrine feedback system that affects growth, differentiation, and reproduction, by sensing and translating central and peripheral signals to maintain homeostasis and a proper thyroidal set-point. Fish, the most diverse group of vertebrates, rely on this system for somatic growth, metamorphosis, reproductive events, and the ability to tolerate changing environments. The vast majority of the research on the thyroid axis pertains to mammals, in particular rodents, and although some progress has been made to understand the role of this endocrine axis in non-mammalian vertebrates, including amphibians and teleost fish, major gaps in our knowledge remain regarding other groups, such as elasmobranchs and cyclostomes. In this review, we discuss the roles of the thyroid axis in fish and its contributions to growth and development, metamorphosis, reproduction, osmoregulation, as well as feeding and nutrient metabolism. We also discuss how thyroid hormones have been/can be used in aquaculture, and potential threats to the thyroid system in this regard.

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The Role of the Thyroid Axis in Fish.

This paper deals with the effects of deltamethrin on endogenous protein phosphorylation of synaptic plasma membrance from hen brain (treated with or without TOCP in vitro ) and compares the effects of deltamethrin at different concentrations. Incubation of deltamethrin with hen brain synaptosomes was followed by SDS PAGE and autoradiography. The total effect of deltamethrin was inhibition of the overall phosphorylation level. Further analysis by two dimensional electrophoresis and autoradiography revealed that the overall phosphorylation level and proteins with molecular weights of 55 and 60 kD could be strongly inhibited by 10 -5 mol/L deltamethrin, but it was stimulated by Ca 2+ (0 25 mol/L) plus Camodulin (0 25 mol/L) .The phosphorylation of proteins with molecular weights of 48 and 45 kD was conspicuously inhibited at the low concentration (10 -6 mol/L) of deltamethrin, which was intensified by Ca 2+ (0 03 mol/L) plus CaM(0 03 mol/L). Moreover, the impact of deltamethrin on the activity of Ca ATPase was also studied. The result showed that deltamethrin at the range 10 -9 ～5×10 -5 mol/L inhibited the Ca ATPase activity.

This work was supported by the National Natural Science Foundation of China(No.39570153)

Ontogeny of the digestive enzymes, thyroid hormones and cortisol in developing embryos and yolk-sac larvae of turbot (Scophthalmus maximus L.)

Revisiting available knowledge on teleostean thyroid hormone receptors.

The toxic effects of thyroxine (T4F), levothyroxine (L-T4), and thyroxine complexed into β-cyclodextrin (β-CD-T4) on the biological parameters of tambaqui (Colossoma macropomum) were evaluated. The animals were exposed to a chronic toxicity test based on concentrations of influent (60 ng/L) for 2 months. Weight, total length, animal behavior, oxygen consumption, photopic electroretinogram (ERG), and the Flicker exam were evaluated. No significant differences were observed (p > 0.05) on the weight and total length measurements between all groups studied. Behavioral observations of the animals exposed to L-T4 and β-CD-T4 complex showed a reduction (p   0.05) in all groups studied. It can be concluded that the complexation of T4 into β-CD and L-T4 modified the toxicity of this hormone, promoting changes in the behavior, oxygen consumption, and electrophysiological responses of the exposed animals, suggesting that inclusion complexes should be submitted to new toxicity tests to ensure higher safety.

Thyroxine, levothyroxine, and thyroxine complexed into cyclodextrin changed animal behavior, oxygen consumption, and photopic electroretinogram of Colossoma macropomum

In vertebrates, thyroid hormone receptors (TRs) and deiodinases are essential for developmental events driven by the thyroid hormones (THs). However, the significance of deiodinases during the metamorphosis of the Japanese flounder (Paralichthys olivaceus) remains unclear. Moreover, regulation and response of the TRs and deiodinases to THs in this fish are poorly understood. Therefore, we detected the expression patterns of THs, deiodinases, and TRs in drug-treated larvae and untreated larvae of P. olivaceus by using enzyme-linked immunosorbent assay and quantitative real-time PCR during P. olivaceus metamorphosis. To further understand the roles of these elements, a rescue assay was performed. Our results show the importance of THs, TRs, and deiodinases in flatfish metamorphosis. Our results also confirm that D1 and D2 activate THs and D3 plays the opposite and complementary role. Moreover, we demonstrated that both TRα and TRβ have important but different roles during P. olivaceus metamorphosis.

Coordinated expression and regulation of deiodinases and thyroid hormone receptors during metamorphosis in the Japanese flounder (Paralichthys olivaceus)

Thyroid hormone (TH) is essential for Paralichthys olivaceus metamorphosis. Exogenous TH treatment induces premature metamorphosis in P. olivaceus larvae and a series of studies have been conducted to identify thyroid hormone-regulated functional genes and microRNAs involved in the metamorphosis of P. olivaceus; however, the proteins involved in this process remain to be fully clarified. In this study, the differential proteomic responses of P. olivaceus larvae to exogenous TH treatment were examined using tandem mass tags (TMT) for quantitation labeling followed by liquid chromatography tandem mass spectrometry (LC-MS/MS). The expression levels of 629 cellular proteins were identified to be significantly affected by TH treatment. The reliability of our TMT-labeled LC-MS/MS analysis was verified by examining the mRNA and protein levels of four selected proteins using quantitative real-time reverse-transcription PCR and western blot analyses. The possible biological significance of these proteins was further investigated by Gene Ontology (GO) enrichment, Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment, and protein-protein interaction analyses. Notably, we identified and described five groups of proteins involved in different important life events that were significantly regulated by exogenous TH treatment. Our study provides an improved understanding of the molecular mechanisms by which TH regulates the metamorphosis of P. olivaceus.

Jie Yu

Papers

Coordinated expression and regulation of deiodinases and thyroid hormone receptors during metamorphosis in the Japanese flounder (Paralichthys olivaceus)

Proteomic variation in metamorphosing Paralichthys olivaceus induced by exogenous thyroid hormone