Functional significance of a truncated thyroid receptor subtype lacking a hormone-binding domain in goldfish

Truncated variants of thyroid hormone receptor beta display disease-inflicting malfunctioning at cellular level

Thyroid hormone receptor β (THRβ) is a member of the nuclear receptor superfamily of ligand-modulated transcription factors. Upon ligand binding, THRβ sequentially recruits the components of transcriptional machinery to modulate target gene expression. In addition to regulating diverse physiological processes, THRβ plays a crucial role in hypothalamus-pituitary-thyroid axis feedback regulation. Anomalies in THRβ gene/protein structure are associated with onset of diverse disease states. In this study, we investigated disease-inflicting truncated variants of THRβ using in-silico analysis and cell-based assays. We examined the THRβ truncated variants on multiple test parameters, including subcellular localization, ligand-receptor interactions, transcriptional functions, interaction with heterodimeric partner RXR, and receptor-chromatin interactions. Moreover, molecular dynamic simulation approaches predicted that shortened THRβ-LBD due to point mutations contributes proportionally to the loss of structural integrity and receptor stability. Deviant subcellular localization and compromised transcriptional function were apparent with these truncated variants. Present study shows that 'mitotic bookmarking' property of some THRβ variants is also affected. The study highlights that structural and conformational attributes of THRβ are necessary for normal receptor functioning, and any deviations may contribute to the underlying cause of the inflicted diseases. We anticipate that insights derived herein may contribute to improved mechanistic understanding to assess disease predisposition.

Preself-Feeding Medaka Fry Provides a Suitable Screening System for in Vivo Assessment of Thyroid Hormone-Disrupting Potential

Endocrine-disrupting chemicals are assessed based on their physiological potential and their potential associated adverse effects. However, suitable end points for detection of chemicals that interfere with the thyroid hormone (TH) system have not been established in nonmammals, with the exception of amphibian metamorphosis. The aims of the current study were to develop an in vivo screening system using preself-feeding medaka fry (Oryzias latipes) for the detection of TH-disrupting chemicals and elucidate the underlying molecular mechanism. 17α-Ethinylestradiol (EE2: <100 ng/L) did not induce mRNA expression of estrogen-responsive genes, vitellogenins (vtgs) mRNA. Meanwhile, coexposure with thyroxin (T4) induced an increase of vtg expression. TH-disrupting chemicals (thiourea (TU), perfluorooctanoic acid (PFOA), and tetrabromobisphenol A (TBBPA)) significantly suppressed EE2 (1,000 ng/L)-induced vtg1 expression, while T4 rescued their expression as well as that of thyroid hormone receptor α (tRα) and estrogen receptors (esrs). These results were supported by in silico analysis of the 5'-transcriptional regulatory region of these genes. Furthermore, the esr1 null mutant revealed that EE2-induced vtg1 expression requires mainly esr2a and esr2b in a TH-dependent manner in preself-feeding fry. Application of preself-feeding medaka fry as a screening system might help decipher the in vivo mechanisms of action of TH-disrupting molecules, while providing an alternative to the traditional animal model.

Small Heterodimer Partner Regulates Dichotomous T Cell Expansion by Macrophages

The involvement of small heterodimer partner (SHP) in the inhibition of hepatic bile acid synthesis from cholesterol has been established. However, extrahepatic expression of SHP implies that SHP may have regulatory functions other than those in the liver. Here, we find that SHP mRNA expression is high in murine bone marrow cells, suggesting a physiological role within macrophages. Indeed, expression of SHP in macrophages decreases the transcriptional activity and nuclear localization of nuclear factor κB, whereas downregulation of SHP has the opposite effects. Expression of genes associated with macrophage-T cell crosstalk were altered by overexpression or downregulation of SHP. Intriguingly, increasing SHP expression in macrophages resulted in decreased T cell expansion, a hallmark of T cell activation, whereas knockdown of SHP resulted in increased expansion. Analyses of the expanded T cells revealed a dichotomous skewing between effector T cells and regulatory T cells (Tregs), with SHP overexpression reducing Tregs and downregulation of SHP increasing their expansion. The expanded Tregs were confirmed to be suppressive via adoptive transfers. IL-2 and TGF-β, known inducers of Treg differentiation, were found to be regulated by SHP. Furthermore, SHP occupancy at the promoter region of IL-2 was increased after macrophages were challenged with lipopolysaccharide. Neutralizing antibodies to IL-2 and TGF-β inhibited the expansion of Tregs mediated by downregulation of SHP. This study demonstrates that expression and activity of SHP within macrophages can alter T cell fate and identifies SHP as a potential therapeutic target for autoimmune diseases or solid cancers.

Thyroid hormones and androgens differentially regulate gene expression in testes and ovaries of sexually mature Silurana tropicalis

A series of ex vivo exposures using testicular and ovarian tissues of sexually mature Western clawed frogs (Silurana tropicalis) were designed to examine molecular mechanisms of thyroid hormone (TH) and androgen crosstalk sans hypophyseal feedback as well as investigate potential sex-specific differences. Tissues were exposed ex vivo to either triiodothyronine (T3), iopanoic acid (IOP), one co-treatment of IOP + 5α-dihydrotestosterone (5α-DHT), 5α-DHT, 5β-dihydrotestosterone (5β-DHT), or testosterone (T). Direct exposure to different androgens led to androgen specific increases in thyroid receptor and deiodinase transcripts in testes (trβ and dio1) but a decrease in expression in ovaries (trβ and dio3), suggesting that male and female frogs can be differently affected by androgenic compounds. Moreover, exposure to select androgens differentially increased estrogen-related transcription (estrogen receptor alpha (erα) and aromatase (cyp19)) and production (estradiol) in ovaries and testes indicating the activation of alternate metabolic pathways yielding estrogenic metabolites. Sex-steroid-related transcription (i.e., steroid 5α-reductase type 2 (srd5α2) and erα) and production (i.e., 5α-DHT) were also differentially regulated by THs. The presence and frequency of transcription factor binding sites in the putative promoter regions of TH- and sex steroid-related genes were also examined in S. tropicalis, rodent, and fish models using in silico analysis. In summary, this study provides an improved mechanistic understanding of TH- and androgen-mediated actions and reveals differential transcriptional effects as a function of sex in frogs.

Revisiting available knowledge on teleostean thyroid hormone receptors

Teleosts are the most numerous class of living vertebrates. They exhibit great diversity in terms of morphology, developmental strategies, ecology and adaptation. In spite of this diversity, teleosts conserve similarities at molecular, cellular and endocrine levels. In the context of thyroidal systems, and as in the rest of vertebrates, thyroid hormones in fish regulate development, growth and metabolism by actively entering the nucleus and interacting with thyroid hormone receptors, the final sensors of this endocrine signal, to regulate gene expression. In general terms, vertebrates express the functional thyroid hormone receptors alpha and beta, encoded by two distinct genes (thra and thrb, respectively). However, different species of teleosts express thyroid hormone receptor isoforms with particular structural characteristics that confer singular functional traits to these receptors. For example, teleosts contain two thra genes and in some species also two thrb; some of the expressed isoforms can bind alternative ligands. Also, some identified isoforms contain deletions or large insertions that have not been described in other vertebrates and that have not yet been functionally characterized. As in amphibians, the regulation of some of these teleost isoforms coincides with the climax of metamorphosis and/or life transitions during development and growth. In this review, we aimed to gain further insights into thyroid signaling from a comparative perspective by proposing a systematic nomenclature for teleost thyroid hormone receptor isoforms and summarize their particular functional features when the information was available.

Thyroid hormone receptor localization in target tissues

The thyroid hormone receptors, TRα1, TRβ1 and other subtypes, are members of the nuclear receptor superfamily that mediate the action of thyroid hormone signaling in numerous tissues to regulate important physiological and developmental processes. Their most well-characterized role is as ligand-dependent transcription factors; TRs bind thyroid hormone response elements in the presence or absence of thyroid hormone to facilitate the expression of target genes. Although primarily residing in the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. We have identified multiple nuclear localization signals and nuclear export signals within TRα1 and TRβ1 that interact with importins and exportins, respectively, to mediate translocation across the nuclear envelope. More recently, enigmatic cytoplasmic functions have been ascribed to other TR subtypes, expanding the diversity of the cellular response to thyroid hormone. By integrating data on localization signal motifs, this review provides an overview of the complex interplay between TR's dynamic transport pathways and thyroid hormone signaling activities. We examine the variation in TR subtype response to thyroid hormone signaling, and what is currently known about regulation of the variety of tissue-specific localization patterns, including targeting to the nucleus, the mitochondria and the inner surface of the plasma membrane.

Thyroid hormone regulates vitellogenin by inducing estrogen receptor alpha in the goldfish liver

Vitellogenin (Vtg) is an egg-yolk precursor protein that is synthesized in the liver of oviparous species and taken up from the circulation by the ovary. It is well known that Vtg is induced by circulating estrogens. However, other endocrine factors that regulate the expression of Vtg are less well characterized; factors that might play significant roles, especially in seasonal spawners such as the goldfish which require increased quantities of Vtg for the development of hundreds of follicles. In this regard, thyroid hormones have been shown to cycle with the reproductive season. Therefore, we hypothesized that the thyroid hormones might influence the synthesis of Vtg. Treatment of female goldfish with triiodothyronine (T3) resulted in increased Vtg, an observation that was absent in males. Furthermore, T3 failed to induce Vtg in cultured hepatocytes of either sex. Interestingly however, T3 consistently up-regulated the expression of the estrogen receptor alpha (ERα). The T3 mediated upregulation of ERα requires the presence of both thyroid receptor (TR) α-1 and TRβ. When goldfish or cultured hepatocytes were treated with T3 followed by estradiol, there was a synergistic increase in Vtg, a response which is dependent on the presence of ERα. Therefore, by upregulating ERα, T3 serves to prime the liver to subsequent stimuli from estradiol. This cross-talk likely reveals an important physiologic mechanism by which thyroid hormones, whose circulating levels are high during early gonadal recrudescence, facilitate the production of large amounts of Vtg required for egg development.

Adverse morphological development in embryonic zebrafish exposed to environmental concentrations of contaminants individually and in mixture

Exposure to environmental contaminants has been linked to developmental and reproductive abnormalities leading to infertility, spontaneous abortion, reduced number of offspring, and metabolic disorders. In addition, there is evidence linking environmental contaminants and endocrine disruption to abnormal developmental rate, defects in heart and eye morphology, and alterations in behavior. Notably, these effects could not be explained by interaction with a single hormone receptor. Here, using a whole-organism approach, we investigated morphological changes to developing zebrafish caused by exposure to a number of environmental contaminants, including bisphenol A (BPA), di(2-ethylhexyl)phthalate (DEHP), nonylphenol, and fucosterol at concentrations measured in a local water body (Oldman River, AB), individually and in mixture. Exposure to nanomolar contaminant concentrations resulted in abnormal morphological development, including changes to body length, pericardia (heart), and the head. We also characterize the spatiotemporal expression profiles of estrogen, androgen, and thyroid hormone receptors to demonstrate that localization of these receptors might be mediating contaminant effects on development. Finally, we examined the effects of contaminants singly and in mixture. Combined, our results support the hypothesis that adverse effects of contaminants are not mediated by single hormone receptor signaling, and adversity of contaminants in mixture could not be predicted by simple additive effect of contaminants. The findings provide a framework for better understanding of developmental toxicity of environmental contaminants in zebrafish and other vertebrate species.

Detection of Endogenous Selective Estrogen Receptor Modulators such as 27-Hydroxycholesterol

The estrogen receptors (ERs) belong to the nuclear receptor superfamily, and as such act as ligand inducible transcription factors, mediating the effects of estrogens. However, their pharmacology is complex, having the ability to be differentially activated by ligands. Such ligands possess the ability to behave as either ER-agonists or ER-antagonists, depending on the cellular and tissue context, and have been termed Selective Estrogen Receptor Modulators (SERMs). Several SERMs have been identified with clinical relevance such as tamoxifen and raloxifene. Recently, 27-hydroxycholesterol has been characterized as the first identified endogenous SERM leading to the notion that other endogenous SERMs may exist, each having potential pathophysiological functions. This, coupled with the historic pharmaceutical interest as well as growing concern over chemicals in the environment with the ability to behave like SERMs, has increased the demand for assays to detect SERM-like activity. Here, we describe a common, straightforward in vitro assay investigating the induction of classic ER-target genes in MCF7 breast cancer cells, allowing one to identify ligands with SERM-like activity.

Using whole mount in situ hybridization to examine thyroid hormone deiodinase expression in embryonic and larval zebrafish: a tool for examining OH-BDE toxicity to early life stages

Polybrominated diphenyl ethers (PBDEs) and their oxidative metabolites (hydroxylated PBDEs; OH-BDEs) are known endocrine disrupting contaminants that have been shown to disrupt thyroid hormone regulation both in mammals and in fish. The purpose of this study was to determine the precise organ and tissue locations that express genes critical to thyroid hormone regulation in developing zebrafish (Danio rerio), and to determine the effects of an OH-BDE on their expression. While RT-PCR can provide quantitative data on gene expression, it lacks spatial sensitivity to examine localized gene expression; and, isolation of organs from zebrafish embryos is technically difficult, if not impossible. For this reason, the present study used whole mount in situ hybridization to simultaneously localize and quantify gene expression in vivo. While PBDEs and OH-BDEs have been shown to inhibit the activity and expression of deiodionases, a family of enzymes that regulate thyroid hormone concentrations intracellularly, it is unclear whether or not they can affect regional expression of the different isoforms during early development. In this study we investigated deiodinase 1 (Dio1), deiodinase 2 (Dio2), and deiodinase 3 (Dio3) mRNA expression at the following life stages (2, 8, and 1k-cells; 50%-epiboly, 6 and 18-somites, 22, 24, 48, 72 hpf and/or 10 dpf) in zebrafish and found life stage specific expression of these genes that were highly localized. To demonstrate the use of this technique for investigating potential endocrine disrupting effects, zebrafish embryos were exposed to 1, 10 and 100nM 6-OH-BDE-47. Significant increases in mean intensity of Dio1 and Dio3 expression in the periventricular zone of brain and pronephric duct, respectively (quantified by measuring intensity of coloration using ImageJ analysis software) were observed, suggesting localized response at the HPT axis with the possibility of impacting neurodevelopment. Our results demonstrate effects of OH-BDEs on thyroid regulating gene expression and provide more insight into potential sites of injury during early life stages.

Estradiol and triiodothyronine differentially modulate reproductive and thyroidal genes in male goldfish

While the reproductive and thyroidal systems are extensively studied in fish, they are largely studied in isolation from one another, but there is evidence supporting cross-regulation between these two systems. To better understand hormone action and the potential cross-regulation between estrogen and thyroid hormones, we examined gene expression changes in estrogen receptor (ER) and thyroid receptor (TR) subtypes and key enzymes responsible for the local synthesis and availability of estrogen and thyroid hormones (aromatase B and deiodinase, respectively) in sexually regressed, adult, male goldfish in response to 3 days waterborne exposures to 17β-estradiol (E2; 1 nM), triiodothyronine (T3; 20 and 100 nM), and co-treatments thereof. Treatments with E2 alone did not effect ER subtype transcripts in the liver, telencephalon, or testis; however, in the testis, 1 nM T3 decreased ERα and ERβ1 and co-treatments of T3 and E2 decreased ERβ1 levels. TRα-1 and TRβ transcripts were not auto-regulated by T3 or cross-regulated by E2. Although deiodinase type I levels were also unaffected, deiodinase type II decreased in response to T3 treatments. Liver deiodinase type III transcripts increased in response to T3 treatments, while E2 exhibited antagonistic effects on this T3-mediated induction. These results provide novel evidence of cross-talk between the reproductive and thyroid endocrine axes in a model teleost.

New insights into thyroid hormone function and modulation of reproduction in goldfish

A number of studies have provided evidence for a link between thyroid hormones and physiological or pathophysiological conditions associated with reproduction. Most of the information available is based on clinical observations in human or research in mammals. There are also a number of studies in non-mammalian species, primarily investigating thyroid and reproductive endocrinology in isolation. The findings demonstrate that hyperthyroidism or hypothyroidism are associated with altered fertility due to changes in the levels and activities of hormones of the brain-pituitary-gonadal axis. There appears to be a consistent pattern based on a number of studies in mammalian and non-mammalian species, linking thyroid with reproduction. Results obtained in goldfish suggest that increased levels of thyroid hormones may reduce overall reproductive function. Since thyroid hormones influence metabolism and are known to stimulate growth in most species, it is likely that increased thyroid hormone levels may divert energy from reproduction and promote somatotropic functions. This is particularly important in oviparous species such as fish since energy investment in females during reproductive season is very significant, and increasing thyroid hormone levels after ovulation may be a contributing factor in promoting growth response. Thyroid hormones will likely work in concert with other hormones to influence reproduction in fish and other vertebrates.

Tissue-specific thyroid hormone regulation of gene transcripts encoding iodothyronine deiodinases and thyroid hormone receptors in striped parrotfish (Scarus iseri)

In fish as in other vertebrates, the diverse functions of thyroid hormones are mediated at the peripheral tissue level through iodothyronine deiodinase (dio) enzymes and thyroid hormone receptor (tr) proteins. In this study, we examined thyroid hormone regulation of mRNAs encoding the three deiodinases dio1, dio2 and dio3 - as well as three thyroid hormone receptors trαA, trαB and trβ - in initial phase striped parrotfish (Scarus iseri). Parrotfish were treated with dissolved phase T(3) (20 nM) or methimazole (3 mM) for 3 days. Treatment with exogenous T(3) elevated circulating T(3), while the methimazole treatment depressed plasma T(4). Experimentally-induced hyperthyroidism increased the relative abundance of transcripts encoding trαA and trβ in the liver and brain, but did not affect trαB mRNA levels in either tissue. In both sexes, methimazole-treated fish exhibited elevated dio2 transcripts in the liver and brain, suggesting enhanced outer-ring deiodination activity in these tissues. Accordingly, systemic hyperthyroidism elevated relative dio3 transcript levels in these same tissues. In the gonad, however, patterns of transcript regulation were distinctly different with elevated T(3) increasing mRNAs encoding dio2 in testicular and ovarian tissues and dio3, trαA and trαB in the testes only. Thyroid hormone status did not affect dio1 transcript abundance in the liver, brain or gonads. Taken as a whole, these results demonstrate that thyroidal status influences relative transcript abundance for dio2 and dio3 in the liver, provide new evidence for similar patterns of dio2 and dio3 mRNA regulation in the brain, and make evident that fish exhibit tr subtype-specific transcript abundance changes to altered thyroid status.

Auto-regulation of thyroid hormone receptors in the goldfish ovary and testis

Thyroid hormones, acting via their cognate thyroid receptors (TRs) act as mediators and modulators of several physiological processes and homeostasis. A clear role for the TRs in reproduction has not yet been established although several lines of recent evidence suggest that they are involved in the regulation of reproduction. To further study the role of TRs in control of reproduction, we investigated homologous regulation of TR subtypes in the gonads of goldfish, in vivo and in vitro. It was found that tri-iodothyronine (T(3)) down-regulates the traditional TRs (TRα-1 and TRβ) and up-regulates a dominant-negative form, TRα-t. This indicates a 'feedback' mechanism whereby an acute treatment with T(3) down regulates further T(3) mediated response. The results provide novel information on auto-regulation of TRs in the goldfish ovary and testis, and support the hypothesis that thyroid hormones are involved in the control of reproduction.

Hormonal and ion regulatory response in three freshwater fish species following waterborne copper exposure

We evaluated effects of sublethal copper exposure in 3 different freshwater fish: rainbow trout (Oncorhynchus mykiss), common carp (Cyprinus carpio) and gibel carp (Carassius auratus gibelio). In a first experiment we exposed these fishes to an equally toxic Cu dose, a Cu level 10 times lower than their 96 h LC50 value: 20, 65, and 150 microg/L Cu. In a second series we exposed them to the same Cu concentration (50 microg/L). Na+/K+-ATPase activity in gill tissue was disturbed differently in rainbow trout then in common and gibel carp. Rainbow trout showed a thorough disruption of plasma ion levels at the beginning of both exposures, whereas common carp and gibel carp displayed effects only after 3 days. Rainbow trout and common carp thyroid hormones experienced adverse effects in the beginning of the exposure. The involvement of prolactin in handling metal stress was reflected in changes of mRNA prolactin receptor concentrations in gill tissue, with an up regulation of this mRNA in rainbow trout and a down regulation in gibel carp, which was more pronounced in the latter. Overall, rainbow trout appeared more sensitive in the beginning of the exposure, however, when it overcame this first challenge, it handled copper exposure in a better manner then common and gibel carp as they showed more long term impacts of Cu exposure.

Functional significance of nuclear estrogen receptor subtypes in the liver of goldfish

Estrogens work by binding to and activating specific estrogen receptors (ERs). Although mammals have two major nuclear ER subtypes (ERalpha and ERbeta), three subtypes have been shown in teleost fish (ERalpha, ERbeta-I, and ERbeta-II). 17beta-Estradiol stimulates the production of an egg yolk precursor protein (vitellogenin) in the liver of oviparous species, including the goldfish. However, the functional involvement of the ER subtypes in this process is not fully understood. Here, using primary goldfish hepatocytes, we test the hypothesis that all three ER subtypes are functionally involved in the liver of goldfish by using RNA interference to specifically knock-down the different ER subtypes. The results suggest that ERalpha is induced by estradiol through activation of the ERbeta subtypes. This induction serves to sensitize the liver to further stimulation by estradiol. The knock-down results were supported by use of ER subtype specific antagonists. Sensitization by up-regulation of ERalpha is likely to be important for seasonal spawners such as goldfish, to bring about a change from somatic growth to reproductive development, and vitellogenesis. The novel data presented in this study provide strong support for the hypothesis that the goldfish ER subtypes play functional roles in the regulation of vitellogenin and ERalpha and provide a framework for the better understanding of ER signaling in fish and other vertebrates.

Gender-related expression of TRalpha and TRbeta in the protandrous black porgy, Acanthopagrus schlegeli, during sex change processes

We cloned the thyroid hormone receptor alpha (TRalpha) and beta (TRbeta) cDNAs from the ovaries of the protandrous black porgy and compared the expression levels of TRalpha and TRbeta mRNA during the sex change in black porgy. We observed that the TRalpha mRNA by quantitative real-time PCR and protein levels by Western blot were highest in the mature ovaries. Additionally, TRbeta mRNA levels were only expressed highly in the mature ovaries when compared to any other gonadal stages. Then, we injected gonadotropin-releasing hormone analogue (GnRHa) to know the effects on TRs mRNA in immature black porgy. Injection with GnRHa resulted in a significant increase in TRalpha level while significantly reducing TRbeta level after 12h. We concluded that TRalpha was related in testicular development as well as ovarian development and TRbeta was only affect to ovarian development in black porgy. These results will provide a framework for better understanding of the role of TRs during sex change processes in this fish.

Changes in thyroid hormone reception precede SWS1 opsin downregulation in trout retina

Rainbow trout undergo natural cone degeneration and thus are interesting models for examining mechanisms of neural degeneration. They have ultraviolet-sensitive (UVS) cones that are lost over most of the retina during development; only a small functional population remains in the dorsal retina. How this spatial distribution of UVS cones is maintained is unclear. Thyroxine (T4) induces UVS cone loss, and local thyroid hormone regulation was hypothesized to control UVS cone distribution. Thyroid hormone receptor alpha (TRalpha), thyroid hormone receptor beta (TRbeta) and Type 2 deiodinase (D2) regulate thyroid hormone exposure to target cells. Regional retinal expression of these genes was investigated during exogenous T4 treatment and natural smoltification of rainbow trout. Each retina from dark-adapted parr, T4-treated parr and natural smolts was divided into four quadrants, and total RNA was isolated. Quantitative real-time RT-PCR analysis demonstrated that all retinal quadrants had increased accumulation of TRbeta transcripts 2 days post-T4 treatment, corresponding to initiation of SWS1 opsin downregulation. Smolts exhibited decreased accumulation of TRalpha and TRbeta transcripts in all quadrants, but this effect was most pronounced in the dorso-temporal (DT) retinal quadrant where UVS cones persist. By contrast, in 2 day T4-treated parr, the DT quadrant showed increased expression of TRalpha and TRbeta. Furthermore, D2 transcripts decreased in the DT quadrant of T4-treated parr but increased in the DT quadrant of smolts. These results suggest that T4 upregulates TRbeta expression to initiate SWS1 opsin downregulation, while TRalpha and TRbeta downregulation occurs to prevent natural loss of UVS cones from the DT retina.

Thyroid receptor subtypes: structure and function in fish

Thyroid hormones are important regulators of vertebrate growth and development, and are under the control of the hypothalamic-pituitary-thyroid axis. Nuclear thyroid receptors (TRs), which act as inducible transcription factors, mediate cellular functions of thyroid hormones. The molecular structure of several subtypes of TRs have been elucidated in vertebrate species, including N-terminal truncations as well as C-terminal variations in the domain responsible for binding hormone. In this paper, we review current information on the thyroid receptors studied in the vertebrate species with emphasis on recent findings in goldfish concerning functional significance of the thyroid receptor subtypes.