Type II iodothyronine deiodinase is preferentially expressed in rainbow trout (Oncorhynchus mykiss) liver and gonads

Molecular biological, physiological, cytological, and epigenetic mechanisms of environmental sex differentiation in teleosts: A systematic review

Human activities have been exerting widespread stress and environmental risks in aquatic ecosystems. Environmental stress, including temperature rise, acidification, hypoxia, light pollution, and crowding, had a considerable negative impact on the life histology of aquatic animals, especially on sex differentiation (SDi) and the resulting sex ratios. Understanding how the sex of fish responds to stressful environments is of great importance for understanding the origin and maintenance of sex, the dynamics of the natural population in the changing world, and the precise application of sex control in aquaculture. This review conducted an exhaustive search of the available literature on the influence of environmental stress (ES) on SDi. Evidence has shown that all types of ES can affect SDi and universally result in an increase in males or masculinization, which has been reported in 100 fish species and 121 cases. Then, this comprehensive review aimed to summarize the molecular biology, physiology, cytology, and epigenetic mechanisms through which ES contributes to male development or masculinization. The relationship between ES and fish SDi from multiple aspects was analyzed, and it was found that environmental sex differentiation (ESDi) is the result of the combined effects of genetic and epigenetic factors, self-physiological regulation, and response to environmental signals, which involves a sophisticated network of various hormones and numerous genes at multiple levels and multiple gradations in bipotential gonads. In both normal male differentiation and ES-induced masculinization, the stress pathway and epigenetic regulation play important roles; however, how they co-regulate SDi is unclear. Evidence suggests that the universal emergence or increase in males in aquatic animals is an adaptation to moderate ES. ES-induced sex reversal should be fully investigated in more fish species and extensively in the wild. The potential aquaculture applications and difficulties associated with ESDi have also been addressed. Finally, the knowledge gaps in the ESDi are presented, which will guide the priorities of future research.

A multi-level assessment of biological effects associated with mercury concentrations in smallmouth bass, Micropterus dolomieu

Total mercury (THg) was measured in muscle (fillet) and liver tissue of adult smallmouth bass Micropterus dolomieu collected at multiple sites in the Potomac and Susquehanna River drainages within the Chesapeake Bay watershed. Smallmouth bass in these drainages have experienced episodic mortality events, a high prevalence of skin lesions and reproductive endocrine disruption (intersex or testicular oocytes and plasma vitellogenin in males). A multi-level assessment of general and reproductive health including indicators at the organismal, organ, cellular and molecular levels was conducted on adult smallmouth bass during the spring (prespawn) season. Concentrations of THg were correlated with increased visible abnormalities, increased macrophage aggregates and tissue parasite burdens. In male bass positive correlations of THg were observed with plasma vitellogenin and hepatic transcript abundance of estrogen receptor β1 and androgen receptor α, while there was a negative association with estrogen receptors α and β2 and androgen receptors β. In female bass there was a negative correlation between THg and plasma vitellogenin as well as hepatic transcript abundance of vitellogenin, choriogenin, estrogen receptor β2 and 17β hydroxysteroid dehydrogenase. Associations of THg concentrations with various biological indicators suggest mercury may be an important environmental stressor contributing to the observed adverse effects in smallmouth bass populations.

Reproductive health and endocrine disruption in smallmouth bass (Micropterus dolomieu) from the Lake Erie drainage, Pennsylvania, USA

Smallmouth bass Micropterus dolomieu were sampled from three sites within the Lake Erie drainage (Elk Creek, Twentymile Creek, and Misery Bay, an embayment in Presque Isle Bay). Plasma, tissues for histopathological analyses, and liver and testes preserved in RNALater® were sampled from 30 smallmouth bass (of both sexes) at each site. Liver and testes samples were analyzed for transcript abundance with Nanostring nCounter® technology. Evidence of estrogenic endocrine disruption was assessed by the presence and severity of intersex (testicular oocytes; TO) and concentrations of plasma vitellogenin in male fish. Abundance of 17 liver transcripts associated with reproductive function, endocrine activity, and contaminant detoxification pathways and 40 testes transcripts associated with male and female reproductive function, germ cell development, and steroid biosynthesis were also measured. Males with a high rate of TO (87-100%) and plasma vitellogenin were noted at all sites; however, TO severity was greatest at the site with the highest agricultural land cover. Numerous transcripts were differentially regulated among the sites and patterns of transcript abundance were used to better understand potential risk factors for estrogenic endocrine disruption. The results of this study suggest endocrine disruption is prevalent in this region and further research would benefit to identify the types of contaminants that may be associated with the observed biological effects.

The effects of temperature and salinity on the endocrinology in two life stages of juvenile rainbow/steelhead trout (Oncorhynchus mykiss)

The San Francisco Bay Delta is experiencing seasonally warmer waters and saltwater intrusion into historically freshwater ecosystems due to climate change. Steelhead/rainbow trout (Oncorhynchus mykiss) are resident in the Bay-Delta from juvenile development through the smoltification process. Due to increases in sea level, premature seawater (SW) acclimation may co-occur with increased temperatures on pre-smolt juveniles. To evaluate the interactive effects of salinity and temperature on juvenile life stages of salmonids, rainbow trout alevin (3 days post-hatching) were exposed to 13, 16.4 and 19°C for 10 days and then challenged for 24 h to 18 parts per thousand SW. Similarly, fry (4 weeks post-hatching) were exposed to 13, 16.4 and 19°C for 2 weeks (14 days) and then challenged to SW. Estradiol-17β (E₂ ), cortisol, triiodothyronine (T₃ ) and thyroxine (T₄ ) were measured in whole animal homogenates and muscle tissue using enzyme-linked immunosorbent assays. Transcripts of gill Na⁺ /K⁺ ATPase β (NKAα1b), brain growth hormone I (gh1) and brain gonadotropin-releasing hormone receptor 2 (gnrh2) were also measured. Alevin exhibited a significant temperature-dependent decrease in survival, and fry showed a temperature-dependent decrease in condition factor. The gene expression of NKAα1b, gh1 and gnrh2 significantly decreased in all SW-challenged alevin, and a significant decrease in gnrh2 expression was observed in fry with temperature. Alevin T₃ and T₄ concentrations significantly increased with increasing temperature. There was a temperature-dependent increase in E₂ of fry but not of alevin. The results of this study demonstrate that increasing temperature and SW exposure may adversely affect the survival and SW acclimation of alevin and fry stages of salmonids and that the tolerances of younger juvenile stages should be considered when assessing the response of salmonid populations to climate change stressors.

Effects of the Thyroid Endocrine System on Gonadal Sex Ratios and Sex-Related Gene Expression in the Pufferfish Takifugu rubripes

To examine the effect and mechanism of thyroid hormone on gonadal sex differentiation, Takifugu rubripes larvae were treated with goitrogen (methimazole, MET, 1000 g/g), and thyroxine (T4, 2nM) from 25 to 80 days after hatching (dah). Gonadal histology and sex ratios of fish were then determined at 80 dah. MET treatment induced masculinization, but T4 treatment did not induce feminization in T. rubripes larvae. Transcriptomic analysis of gonads at 80 dah was then conducted. Among the large number of differentially expressed genes between the groups, the expression of foxl2, cyp19a1a, and dmrt1 was altered. The expression of foxl2, cyp19a1a, dmrt1 and gsdf at 25, 40, 55 days after treatment (dat) was further analyzed by qPCR. MET treatment suppressed the expression of foxl2 and cyp19a1a, and induced the expression of dmrt1 in genetic females (p < 0.05). Additionally, T4 treatment induced an increase in the expression of cyp19a1a in genetic XY gonads only at 25 dat. However, the increase in cyp19a1a expression did not continue to 40 and 55 dat. This may explain why feminization of larvae was not found in the T4-treated group. Thus, the present study provides the first evidence that MET treatment causes masculinization in teleost fish. The effects of MET-induced masculinization in T. rubripes may act primarily via suppression of the expression of foxl2 and cyp19a1a, and stimulation of the expression of dmrt1. Moreover, the effects of higher concentrations of T4 or different concentrations of T3, on sex differentiation require further testing.

The Role of the Thyroid Axis in Fish

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.

Expression profiles of types 2 and 3 iodothyronine deiodinase genes in relation to vitellogenesis in a tropical damselfish, Chrysiptera cyanea

Thyroid hormone (TH) is involved in regulating the reproduction of vertebrates. Its physiological action in the target tissues is due to the conversion of TH by iodothyronine deiodinases. In this study, we aimed to clone and characterize type 2 (sdDio2) and type 3 (sdDio3) of the sapphire devil Chrysiptera cyanea, a tropical damselfish that undergoes active reproduction under long-day conditions, and to study the involvement of THs in the ovarian development of this species. When the cDNAs of sdDio2 and sdDio3 were partially cloned, they had deduced amino acid sequences of lengths 271 and 267, respectively, both of which were characterized by one selenocysteine residue. Real-time quantitative PCR (qPCR) revealed that both genes are highly expressed in the whole brain, and sdDio2 and sdDio3 are highly transcribed in the liver and ovary, respectively. In situ hybridization analyses showed positive signals of sdDio2 and sdDio3 transcripts in the hypothalamic area of the brain. Little change in mRNA abundance of sdDio2 and sdDio3 in the brain was observed during the vitellogenic phases. It is assumed that simultaneous activation and inactivation of THs occur in this area because oral administration of triiodothyronine (T3), but not of thyroxine (T4), upregulated mRNA abundance of both genes in the brain. The transcript levels of sdDio2 in the liver and sdDio3 in the ovary increased as vitellogenesis progressed, suggesting that, through the metabolism of THs, sdDio2 and sdDio3 play a role in vitellogenin synthesis in the liver and yolk accumulation/E2 synthesis in the ovary. Taken together, these results suggest that iodothyronine deiodinases act as a driver for vitellogenesis in tropical damselfish by conversion of THs in certain peripheral tissues.

Octylphenol influence growth and development of Rana chensinensis tadpoles via disrupting thyroid function

Octylphenol (OP), a class of endocrine disrupting chemicals (EDCs), could produce adverse effects on developmental process of animals. Thyroid hormone is one of the important hormones involved in animal development. To determine whether OP affect the growth and development of amphibian larvae via interfering the thyroid function, Rana chensinensis larvae at Gosner stage 29 were exposed to 10^-8, 10^-7 and 10^-6 mol/L OP in the present study. Results demonstrated that OP could decrease the body length and mass and retard the development of tadpoles. The histologic evaluation showed microscopic structures of thyroid gland were changed in 10^-7 and 10^-6 mol/L OP treated groups on day 40. The expression levels of Dio2, Dio3, TRα and TRβ mRNA in the liver, brain, skin and tail of tadpoles were detected by qRT-PCR, when treated with OP for 20, 30, 40 and 50 day, respectively. The results of qRT-PCR showed OP could affect the expressions of Dio2, Dio3, TRα and TRβ mRNA in the four tissues, and then influence the activity and function of THs, further affecting the growth and development of the tadpoles.

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.

Lifecycle exposure to perchlorate differentially alters morphology, biochemistry, and transcription as well as sperm motility in Silurana tropicalis frogs

Perchlorate (ClO₄^-) contamination has been reported in ground and surface waters across North America. However, few studies have examined the effects of prolonged exposure to this thyroid hormone disrupting chemical, particularly at environmentally relevant concentrations in lower vertebrates, such as amphibians. The aim of this study was to examine the effects of a yearlong chronic exposure to ClO₄^- in adult male and female Western clawed frogs (Silurana tropicalis). Frogs were spawned and raised from fertilized embryo until sexual maturity in potassium perchlorate (KClO₄)-treated water at different concentrations (0, 20, 53, and 107 μg/L). Developmental and reproductive indices - including adult morphology, androgen plasma levels, gonadal thyroid hormone- and sex steroid-related transcript levels, and sperm motility - were evaluated in male and female adult frogs. Female growth (e.g., body mass, snout-vent length, and hind limb length) was significantly reduced following chronic exposure to environmentally relevant concentrations of KClO₄ resulting in females with morphometric indices similar to those of control males - indicating potential sex-specific sensitivities to KClO₄. Changes to reproductive indices (i.e., plasma androgen levels, gonadal thyroid hormone- and sex steroid-related transcript levels, and sperm motility) were also observed in both sexes and suggest that KClO₄ exposure may also have indirect secondary effects on the reproductive axes in male and female adult frogs. These effects were observed at concentrations at or below those reported in surface waters contaminated with ClO₄^- suggesting that this contaminant may have developmental and reproductive effects post-metamorphosis in natural amphibian populations.

Effects of ZnO nanoparticles on perfluorooctane sulfonate induced thyroid-disrupting on zebrafish larvae

Perfluorooctane sulfonate (PFOS) and ZnO nanoparticles (nano-ZnO) are widely distributed in the environment. However, the potential toxicity of co-exposure to PFOS and nano-ZnO remains to be fully elucidated. The test investigated the effects of co-exposure to PFOS and nano-ZnO on the hypothalamic-pituitary-thyroid (HPT) axis in zebrafish. Zebrafish embryos were exposed to a combination of PFOS (0.2, 0.4, 0.8mg/L) and nano-ZnO (50mg/L) from their early stages of life (0-14days). The whole-body content of TH and the expression of genes and proteins related to the HPT axis were analyzed. The co-exposure decreased the body length and increased the malformation rates compared with exposure to PFOS alone. Co-exposure also increased the triiodothyronine (T3) levels, whereas the thyroxine (T4) content remained unchanged. Compared with the exposure to PFOS alone, exposure to both PFOS (0.8mg/L) and nano-ZnO (50mg/L) significantly up-regulated the expression of corticotropin-releasing factor, sodium/iodidesymporter, iodothyronine deiodinases and thyroid receptors and significantly down-regulated the expression of thyroid-stimulating hormone, thyroglobulin (TG), transthyretin (TTR) and thyroid receptors. The protein expression levels of TG and TTR were also significantly down-regulated in the co-exposure groups. In addition, the expression of the thyroid peroxidase gene was unchanged in all groups. The results demonstrated that PFOS and nano-ZnO co-exposure could cause more serious thyroid-disrupting effects in zebrafish than exposure to PFOS alone. Our results also provide insight into the mechanism of disruption of the thyroid status by PFOS and nano-ZnO.

Waterborne exposure to microcystin-LR causes thyroid hormone metabolism disturbances in juvenile Chinese rare minnow (Gobiocypris rarus)

Microcystin-LR (MC-LR) has the potential to disturb thyroid hormone homeostasis, but little is known about the underlying mechanisms of MC-LR in fish. In the present study, juvenile Chinese rare minnows (Gobiocypris rarus) were exposed to various concentrations of MC-LR (0 µg/L, 50 µg/L, 100 µg/L, and 500 µg/L) for 7 d. The whole-body thyroid hormone content, the histology of thyroid follicle epithelial cells, the activities of hepatic iodothyronine deiodinases, and the transcription of selected genes associated with thyroid hormone synthesis, transport, and metabolism were analyzed. Following exposure to MC-LR, whole-body concentrations of both thyroxine (T4 ) and triiodothyronine (T3 ) were significantly decreased. The levels of messenger RNA for sodium/iodide symporter, transthyretin, thyroid hormone receptor-α, iodothyronine deiodinase2, and iodothyronine deiodinase3 were significantly down-regulated after exposure to 500 µg/L MC-LR. A significant decrease in ID2 activity was also observed in the 500-µg/L MC-LR exposure group. Moreover, hypertrophy of thyroid follicle epithelial cells was observed after exposure to MC-LR. The results indicate that acute MC-LR exposure has the potential to disturb the homeostasis of thyroid hormone metabolism, leading to a hypothyroidism state in the juvenile Chinese rare minnow.

Deiodinases and thyroid metabolism disruption in teleost fish

Many xenobiotic compounds with endocrine disrupting activity have been described since the late eighties. These compounds are able to interact with natural hormone systems and potentially induce deleterious effects in wildlife, notably piscine species. However, while the characterization of endocrine disruptors with "dioxin-like", estrogenic or androgenic activities is relatively well established, little is known about environmentally relevant pollutants that may act at thyroid system level. Iodothyronine deiodinases, the key enzymes in the activation and inactivation of thyroid hormones, have been suggested as suitable biomarkers for thyroid metabolism disruption. The present article reviews the biotic and abiotic factors that are able to modulate deiodinases in teleosts, a representative model organism for vertebrates. Data show that deiodinases are highly sensitive to several physiological and physical variables, so they should be taken into account to establish natural basal deiodination patterns to further understand responses under chemical exposure. Among xenobiotic compounds, brominated flame retardants are postulated as chemicals of major concern because of their similar structure shared with thyroid hormones. More ambiguous results are shown for the rest of compounds, i.e. polychlorinated biphenyls, perfluorinated chemicals, pesticides, metals and synthetic drugs, in part due to the limited information available. The different mechanisms of action still remain unknown for most of those compounds, although several hypothesis based on observed effects are discussed. Future tasks are also suggested with the aim of moving forward in the full characterization of chemical compounds with thyroid disrupting activity.

Crossover of the hypothalamic pituitary-adrenal/interrenal, -thyroid, and -gonadal axes in testicular development

Besides the well-known function of thyroid hormones (THs) for regulating metabolism, it has recently been discovered that THs are also involved in testicular development in mammalian and non-mammalian species. THs, in combination with follicle stimulating hormone, lead to androgen synthesis in Danio rerio, which results in the onset of spermatogenesis in the testis, potentially relating the hypothalamic-pituitary-thyroid (HPT) gland to the hypothalamic-pituitary-gonadal (HPG) axes. Furthermore, studies in non-mammalian species have suggested that by stimulating the thyroid-stimulating hormone (TSH), THs can be induced by corticotropin-releasing hormone. This suggests that the hypothalamic-pituitary-adrenal/interrenal gland (HPA) axis might influence the HPT axis. Additionally, it was shown that hormones pertaining to both HPT and HPA could also influence the HPG endocrine axis. For example, high levels of androgens were observed in the testis in Odonthestes bonariensis during a period of stress-induced sex-determination, which suggests that stress hormones influence the gonadal fate toward masculinization. Thus, this review highlights the hormonal interactions observed between the HPT, HPA, and HPG axes using a comparative approach in order to better understand how these endocrine systems could interact with each other to influence the development of testes.

Crosstalk between the thyroid hormone and androgen axes during reproductive development in Silurana tropicalis

The objectives of this study were to examine the effects of potassium perchlorate (KClO4) treatment on androgen- and thyroid hormone (TH)-related transcript levels during gonadogenesis in the frog Silurana tropicalis. Androgen- and TH-related gene expression was examined in gonad-mesonephros complex (GMC) and liver tissues at stage NF 56 and stage NF 60. These stages of development coincide with the period of sexual differentiation. Real-time RT-PCR analyses revealed that androgen- and TH-related transcript levels in the GMC and liver of stage NF 56 and NF 60 frogs are responsive to KClO4 exposure during prometamorphosis. An increase in srd5α2 mRNA levels in hepatic tissues of KClO4-treated NF 56 tadpoles suggests an important role for hepatic tissues in androgen metabolism. Gene transcript differences highlight possible stage- and tissue-specific sensitivities to KClO4. A greater number of TH- and androgen-related transcriptional changes were discerned in the hepatic tissues compared to the gonads, and overall fewer transcriptional changes were observed in stage NF 60 tadpoles compared to stage NF 56 larvae. Perchlorate suppressed somatic and hind-limb development during the 96-d exposure period. Treatment with KClO4 had no significant effect on sex ratios, however a notable reduction in the percentage of males (33.3% M: 66.7% F) in the highest KClO4 concentration (107 μg/L) was observed. Overall, these findings suggest that KClO4 has secondary androgenic disrupting properties in addition to its known primary thyroid hormone-disrupting role.

Mechanisms of crosstalk between endocrine systems: regulation of sex steroid hormone synthesis and action by thyroid hormones

Thyroid hormones (THs) are well-known regulators of development and metabolism in vertebrates. There is increasing evidence that THs are also involved in gonadal differentiation and reproductive function. Changes in TH status affect sex ratios in developing fish and frogs and reproduction (e.g., fertility), hormone levels, and gonad morphology in adults of species of different vertebrates. In this review, we have summarized and compared the evidence for cross-talk between the steroid hormone and thyroid axes and present a comparative model. We gave special attention to TH regulation of sex steroid synthesis and action in both the brain and gonad, since these are important for gonad development and brain sexual differentiation and have been studied in many species. We also reviewed research showing that there is a TH system, including receptors and enzymes, in the brains and gonads in developing and adult vertebrates. Our analysis shows that THs influences sex steroid hormone synthesis in vertebrates, ranging from fish to pigs. This concept of crosstalk and conserved hormone interaction has implications for our understanding of the role of THs in reproduction, and how these processes may be dysregulated by environmental endocrine disruptors.

Thyroid signaling in immune organs and cells of the teleost fish rainbow trout (Oncorhynchus mykiss)

Thyroid hormones are involved in modulating the immune system in mammals. In contrast, there is no information on the role played by these hormones in the immune system of teleost fish. Here we provide initial evidence for the presence of active thyroid signaling in immune organs and cells of teleosts. We demonstrate that immune organs (head kidney and spleen) and isolated leukocytes (from head kidney and peripheral blood) of the rainbow trout (Oncorhynchus mykiss) express both thyroid receptor α (THRA) and β (THRB). Absolute mRNA levels of THRA were significantly higher than those of THRB. THRA showed higher expression in immune organs and isolated immune cells compared to the reference organ, liver, while THRB showed the opposite. In vivo exposure of trout to triiodothryronine (T3) or the anti-thyroid agent propylthiouracil (PTU) altered THR expression in immune organs and cells. Effect of T3 and PTU over the relative expression of selected marker genes of immune cell subpopulations was also studied. Treatments changed the relative expression of markers of cytotoxic, helper and total T cells (cd4, cd8a, trb), B lymphocytes (mIgM) and macrophages (csf1r). These findings suggest that the immune system of rainbow trout is responsive to thyroid hormones.

Thyroid hormones in male reproductive development: evidence for direct crosstalk between the androgen and thyroid hormone axes

Thyroid hormones (THs) exert a broad range of effects on development in vertebrate species, demonstrating connections in nearly every biological endocrine system. In particular, studies have shown that THs play a role in sexual differentiation and gonadal development in mammalian and non-mammalian species. There is considerable evidence that the effects of THs on reproductive development are mediated through the female hormonal axis; however, recent findings suggest a more direct crosstalk between THs and the androgen axis. These findings demonstrate that THs have considerable influence in the sexual ontogeny of male vertebrates, through direct interactions with select sex-determining-genes and regulation of gonadotropin production in the hypothalamus-pituitary-gonad axis. THs also regulate androgen biosynthesis and signaling through direct and indirect regulation of steroidogenic enzyme expression and activity. Novel promoter analysis presented in this work demonstrates the potential for direct and vertebrate wide crosstalk at the transcriptional level in mice (Mus musculus), Western clawed frogs (Silurana tropicalis) and medaka (Oryzias latipes). Cumulative evidence from previous studies; coupled with novel promoter analysis suggests mechanisms for a more direct crosstalk between the TH and male reproductive axes across vertebrate species.

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.

Iodothyronine deiodinase structure and function: from ascidians to humans

Iodothyronine deiodinases are important mediators of thyroid hormone (TH) action. They are present in tissues throughout the body where they catalyse 3,5,3'-triiodothyronine (T(3)) production and degradation via, respectively, outer and inner ring deiodination. Three different types of iodothyronine deiodinases (D1, D2 and D3) have been identified in vertebrates from fish to mammals. They share several common characteristics, including a selenocysteine residue in their catalytic centre, but show also some type-specific differences. These specific characteristics seem very well conserved for D2 and D3, while D1 shows more evolutionary diversity related to its Km, 6-n-propyl-2-thiouracil sensitivity and dependence on dithiothreitol as a cofactor in vitro. The three deiodinase types have an impact on systemic T(3) levels and they all contribute directly or indirectly to intracellular T(3) availability in different tissues. The relative contribution of each of them, however, varies amongst species, developmental stages and tissues. This is especially true for amphibians, where the impact of D1 may be minimal. D2 and D3 expression and activity respond to thyroid status in an opposite and conserved way, while the response of D1 is variable, especially in fish. Recently, a number of deiodinases have been cloned from lower chordates. Both urochordates and cephalochordates possess selenodeiodinases, although they cannot be classified in one of the three vertebrate types. In addition, the cephalochordate amphioxus also expresses a non-selenodeiodinase. Finally, deiodinase-like sequences have been identified in the genome of non-deuterostome organisms, suggesting that deiodination of externally derived THs may even be functionally relevant in a wide variety of invertebrates.

Expression profiles of metamorphosis-related genes during natural transformations in tadpoles of wild Wood Frogs (Lithobates sylvaticus)

Numerous studies using laboratory-reared tadpoles have shown the importance of thyroid hormones (TH), thyroid receptors (TR), and deiodinase (Dio) enzymes during anuran metamorphosis. Our study focuses on the analysis of thyroid-related genes in tadpoles of wild Wood Frogs ( Lithobates sylvaticus (LeConte, 1825); also known as Rana sylvatica (Cope, 1889)) during metamorphosis. Results showed that, in concordance with laboratory-reared studies, thyroid receptor beta (trb) gene expression profiles presented the most marked changes. At climax and compared with premetamorphic stages, brains, tails, and gonad–mesonephros complex (GMC) tissues increased trb expression levels 5-, 21-, and 41-fold, respectively (p < 0.05). In addition, gene expression levels of brain deiodinase type II and III showed opposite trends, where 3-fold decrease and 10-fold increase were, respectively, found. This finding supports the idea that thyroid hormone, as it has been demonstrated in laboratory-reared tadpoles, is also involved in natural metamorphosis in wild tadpoles. Interestingly, and contrary to our predictions, we observed that whole brain corticotropin-releasing factor (crf) and crf receptor 1 (crfr1) gene expression levels significantly decrease through metamorphosis in wild L. sylvaticus tadpoles. Further analyses are required to determine if a role of TH in the timing of anuran gonadal development exists, as well as the importance of cell-specific and tissue-specific expression of crf and crfr1 to metamorphosis.

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.

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.

Waterborne exposure to PFOS causes disruption of the hypothalamus-pituitary-thyroid axis in zebrafish larvae

Thyroid hormones (THs) play an important role in the normal development and physiological functions in fish. Environmental chemicals may adversely affect thyroid function by disturbing gene transcription. Perfluorooctane sulfonate (PFOS), a persistent compound, is widely distributed in the aquatic environment and wildlife. In the present study, we investigated whether PFOS could disrupt the hypothalamic-pituitary-thyroid (HPT) axis. Zebrafish embryos were exposed to various concentrations of PFOS (0, 100, 200 and 400 microgL(-1)) and gene expression patterns were examined 15d post-fertilization. The expression of several genes in the HPT system, i.e., corticotropin-releasing factor (CRF), thyroid-stimulating hormone (TSH), sodium/iodide symporter (NIS), thyroglobulin (TG), thyroid peroxidase (TPO), transthyretin (TTR), iodothyronine deiodinases (Dio1 and Dio2) and thyroid receptor (TRalpha and TRbeta), was quantitatively measured using real-time PCR. The gene expression levels of CRF and TSH were significantly up-regulated and down-regulated, respectively, upon exposure to 200 and 400 microg L(-1) PFOS. A significant increase in NIS and Dio1 gene expression was observed at 200 microg L(-1) PFOS exposure, while TG gene expression was down-regulated at 200 and 400 microg L(-1) PFOS exposure. TTR gene expression was down-regulated in a concentration-dependent manner. Up-regulation and down-regulation of TRalpha and TRbeta gene expression, respectively, was observed upon exposure to PFOS. The whole body thyroxine (T(4)) content remained unchanged, whereas triiodothyronine (T(3)) levels were significantly increased, which could directly reflect disrupted thyroid hormone status after PFOS exposure. The overall results indicated that PFOS exposure could alter gene expression in the HPT axis and that mechanisms of disruption of thyroid status by PFOS could occur at several steps in the synthesis, regulation, and action of thyroid hormones.

Seasonal variations in testicular connexin levels and their regulation in the brook trout, Salvelinus fontinalis

Spermatogenesis requires coordinated intercellular communication mediated by gap junctions. Gap junctions are composed of connexons that are themselves composed of connexins (Cxs). The present objective was to determine the regulation of testicular Cxs in a seasonal breeder, the brook trout. To assess seasonal variations in testicular Cxs, trout were sampled monthly throughout spermatogenesis (June-November). Circulating levels of testosterone (T) and 11-ketotestosterone (11-KT) as well as mRNA levels for testicular androgen receptors (ar-alpha, ar-beta), thyroid hormone receptors (tr-alpha, tr-beta) and gonadotropin I receptor (rgthI) were measured. Plasma T levels peaked in October, one month prior to spawning, while 11-KT levels peaked at spawning. ar-alpha and ar-beta mRNA levels increased during spermatogenesis and peaked in November while tr-alpha, tr-beta mRNA levels stayed constant throughout spermatogenesis and increased dramatically in November. rgthI mRNA levels decreased progressively during spermatogenesis. Cx43 and Cx30 levels were constant during spermatogenesis and decreased in November. Cx31 levels were also constant during spermatogenesis but decreased dramatically in October and November. Cx43.4 levels peaked in July then decreased in September and levels were undetectable thereafter. Using in vitro cultures of testicular fragments we demonstrated that cx43 mRNA levels were regulated in a dose-response manner by 3,5,3'-triiodo-l-thyronine (0-370 nM) and cAMP (0-100 ng/ml) but levels were not regulated by 11-KT. These results indicate that testicular Cxs vary as a function of spermatogenesis and that the expression of cx43 in the trout testis is regulated by both cAMP and TH.

Characterization of goldfish fin cells in culture: some evidence of an epithelial cell profile

Comprehensive characterization of cultured cells in fish was little explored and cell origin is often deduced from morphological analogies with either epithelial of fibroblastic cells. This study aims to characterize cell origin in goldfish fin culture using morphological, immunochemical, and molecular approaches. Time lapse analysis revealed that cultured cell morphology changed within minutes. Therefore, cell morphology cannot predict whether cells are from fibroblastic or epithelial origin. The labeling pattern of heterologous anti-cytokeratin and anti-vimentin antibodies against goldfish epithelial cells and fibroblasts was first tested on skin sections and the corresponding labeling of the cultured cells was analyzed. No cell origin specificity could be obtained with the chosen antibodies. In the molecular approach, detection levels of three cytokeratin (CauK8-IIS, CauK49-IE and CauK50-Ie) and one vimentin transcripts were assessed on skin and fin samples. Specificity for epithelial cells of the most abundant mRNA, CauK49-Ie, was thereafter validated on skin sections by in situ hybridization. The selected markers were used afterwards to characterize fin cultures. CauK49-IE riboprobe labeled every cell in young cultures whereas no labeling was observed in older cultures. Accordingly, CauK49-IE transcript levels decreased after 15 days culture while CauK8-IIS ones increased. The use of homologous marker gave evidence that young cultured cells from goldfish fin are homogeneously of epithelial type and that cell characteristics may change over culture time.

Thyroid hormone-dependent gene expression as a biomarker of short-term 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) exposure in European common frog (Rana temporaria) tadpoles

The effects on thyroid hormone-dependent gene biomarker responses of the persistent organochlorine pesticide metabolite 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) were investigated after exposure of 4-week-old European common frog (Rana temporaria) (stage 36) tadpoles to two (0.001 and 0.01 ppm) DDE concentrations. Total body weight, total length, and tail length and width increased after 3-day exposure to DDE. Expression patterns of genes encoding for growth hormone, thyroid-stimulating hormone (TSHbeta) and thyroid hormone receptor (TRalpha and TRbeta) isoforms were evaluated in the head, body and tail regions using a validated real-time polymerase chain reaction (PCR) method. The mRNA expression of growth hormone in the body, and TSHbeta in the head showed significant DDE concentration-dependent decreases. While DDE caused variable effects on TRalpha mRNA steady-state, the expression of TRbeta was significantly decreased in the tail by DDE in a concentration-specific manner. The effect of DDE exposure on TRbeta mRNA expression showed a negative correlation with tail length and width during the exposure period. The unique pattern of a DDE-induced decrease of tail TRbeta expression probably reflects the significant role of this thyroid hormone receptor isoform in tail re-absorption and overall metamorphosis in anuran species. Therefore, the present study shows that the evaluation of thyroid hormone-dependent genes may represent quantitative biomarkers of acute exposure to organochlorine pesticides in anuran species during critical developmental periods such as metamorphosis. Given the widespread environmental levels of DDT and its metabolites, these pollutants will remain a subject of concern and their effects on anuran species should be studied in more detail.

Epididymis expresses the highest 5'-deiodinase activity in the male reproductive system: kinetic characterization, distribution, and hormonal regulation

We characterized the enzymes that catalyze the deiodination of T(4) to T(3) in the male reproductive tract. Testis, epididymis (EPI), seminal vesicles, prostate, bulbourethral glands, spermatozoa, and semen were taken from sexually mature rats (300 g). Iodothyronine 5'-deiodinase (5'-D) activity was quantified by the radiolabeled-iodide-release method. 5'-D activity was 10-fold higher in EPI and semen than in the rest of the tissues. In EPI, semen, and prostate, the enzymatic activity was completely inhibited by 1 mm 6-n-propyl-2-thiouracil, whereas in the other tissues the inhibition was partial (50%). The high susceptibility to 6-n-propyl-2-thiouracil inhibition, a ping-pong kinetic pattern, and low cofactor (Michaelis Menten constant for dithiothreitol=0.7 mm) and high substrate (Michaelis Menten constant for reverse T(3)=0.4 microm) requirements indicate that EPI 5'-D corresponds to type 1 deiodinase (D1). Real-time RT-PCR amplification of D1 mRNA in this tissue confirms this conclusion. The highest EPI D1 expression occurred at the onset of puberty and sexual maturity, and in the adult, this activity was more abundant in corpus and caput than in the caudal region. EPI D1 expression was elevated under conditions of hyperthyroidism and with addition of 17beta-estradiol. Our data also showed a direct association between D1 and a functional epididymis marker, the neutral alpha-glucosidase enzyme, suggesting that local generation of T(3) could be associated with the development and function of EPI and/or spermatozoa maturation. Further studies are necessary to analyze the possible physiological relevance of 5'-D in the male reproductive system.

Thyroid disruption in walleye (Sander vitreus) exposed to environmental contaminants: cloning and use of iodothyronine deiodinases as molecular biomarkers

Thyroid hormones play a role in the initiation of ovarian maturation in fish. Thus, reports of delayed sexual maturation in female walleye (Sander vitreus) exposed to contaminants in the Ottawa River suggest the presence of endocrine disrupting chemicals. The objectives of this study were to assess the effects of environmental contaminants in the Ottawa River on thyroid hormones of immature walleye and to develop a molecular biomarker of thyroid status. Walleye were sampled in the Ottawa River at Deep River (reference site), at Rivière Blanche (downstream from the Ottawa and Gatineau municipal wastewater treatment plants outflows), and at Plaisance (downstream from a pulp and paper mill). Plasma thyroid hormone levels were measured by radioimmunoassay. Walleye at Plaisance had two-fold elevated levels of thyroxine (T(4)) and 1.5-fold elevated levels triiodothyronine (T(3)), whereas the molar ratio of T(3):T(4) was reduced by over 50% compared to Deep River. Plasma T(3) levels were also elevated by approximately 1.5-fold at Rivière Blanche. Three iodothyronine deiodinases, a family of enzymes responsible for converting the prohormone T(4) to biologically active T(3), as well as for inactivating these two hormones, were partially cloned in walleye. A real-time PCR assay of deiodinase expression indicated that hepatic mRNA levels of type I and type III deiodinase were not modified between sites, whereas they were increased for type II deiodinase at Rivière Blanche as compared to the other sites. The response of this novel molecular transcript indicates a divergence with that expected based on the effects of experimentally induced hyperthyroidism on fish deiodinase expression; additional endpoints are therefore necessary to interpret changes in thyroid hormones levels in fish exposed to environmental contaminants.

Low salinity acclimation and thyroid hormone metabolizing enzymes in gilthead seabream (Sparus auratus)

We investigated the effect of acclimation to low salinity water of gilthead seabream (Sparus auratus), a euryhaline seawater teleost, on the activities of thyroid hormone-metabolizing enzymes in gills, kidney, and liver. Following acclimation to low salinity water, the plasma free thyroxine (T(4)) concentration increases 2.5-fold, and outer ring deiodination activities towards T(4), 3,5,3'-triiodothyronine (T(3)) and 3,3',5'-triiodothyronine (reverse T(3), rT(3)) in the gills are reduced by 20-32%. Conjugation (catalyzed by sulfotransferase and UDP-glucuronyltransferase) and deconjugation pathways (arylsulfatase, beta-glucuronidase) play a role in the biological activity of native and conjugated thyroid hormones. Branchial, renal, and hepatic activities of the enzymes involved in these metabolic pathways respond differentially to low salinity conditions. The results substantiate that thyroid hormones are involved in S. auratus osmoregulation, and that the gills are well equipped to play an important role in the modulation of plasma hormone titers.

In vivo and in vitro debromination of decabromodiphenyl ether (BDE 209) by juvenile rainbow trout and common carp

Decabromodiphenyl ether (BDE 209), the major congener in the high volume industrial flame retardant mixture "DecaBDE", has recently been shown to be metabolized by carp. To further explore this phenomenon, juvenile rainbow trout were exposed to BDE 209 via the diet for a five month period. Analysis of the whole body homogenate, liver, serum, and intestinal tissues revealed that BDE 209 accumulated in rainbow trout tissues and was most concentrated in the liver. In addition to BDE 209, several hepta-, octa-, and nonaBDE congeners also accumulated in rainbow trout tissues over the same period as a result of BDE 209 debromination. Based on the total body burden of the hepta- through decaBDE congeners, uptake of BDE 209 was estimated at 3.2%. Congener profiles were different among whole body homogenate, liver, and serum, with the whole body homogenates having a greater contribution of the debrominated biotransformation products. Extracts of the rainbow trout whole body homogenates were compared with extracts from a previous experiment with common carp. This comparison revealed that BDE 202 (2,2',3,3',5,5',6,6'-octabromodiphenyl ether) was a dominant debromination product in both studies. To determine whether the observed debromination was metabolically driven, liver microsomal fractions were prepared from both common carp and rainbow trout. Analysis of the microsomal fractions following incubation with BDE 209 revealed that rainbow trout biotransformed as much as 22% of the BDE 209 mass, primarily to octa- and nonaBDE congeners. In contrast, carp liver microsomes biotransformed up to 65% of the BDE 209 mass, primarily down to hexaBDE congeners. These microsomal incubations confirm a metabolic pathway for BDE 209 debromination.

Two unrelated putative membrane-bound progestin receptors, progesterone membrane receptor component 1 (PGMRC1) and membrane progestin receptor (mPR) beta, are expressed in the rainbow trout oocyte and exhibit similar ovarian expression patterns

BACKGROUND

In lower vertebrates, steroid-induced oocyte maturation is considered to involve membrane-bound progestin receptors. Two totally distinct classes of putative membrane-bound progestin receptors have been reported in vertebrates. A first class of receptors, now termed progesterone membrane receptor component (PGMRC; subtypes 1 and 2) has been studied since 1996 but never studied in a fish species nor in the oocyte of any animal species. A second class of receptors, termed membrane progestin receptors (mPR; subtypes alpha, beta and gamma), was recently described in vertebrates and implicated in the progestin-initiated induction of oocyte maturation in fish.

METHODS

In the present study, we report the characterization of the full coding sequence of rainbow trout PGMRC1 and mPR beta cDNAs, their tissue distribution, their ovarian expression profiles during oogenesis, their hormonal regulation in the full grown ovary and the in situ localization of PGMRC1 mRNA in the ovary.

RESULTS

Our results clearly show, for the first time in any animal species, that rainbow trout PGMRC1 mRNA is present in the oocyte and has a strong expression in ovarian tissue. In addition, we show that both mPR beta and PGMRC1, two members of distinct membrane-bound progestin receptor classes, exhibit highly similar ovarian expression profiles during the reproductive cycle with maximum levels during vitellogenesis and a down-expression during late vitellogenesis. In addition, the mRNA abundance of both genes is not increased after in vitro hormonal stimulation of full grown follicles by maturation inducing hormones.

CONCLUSION

Together, our findings suggest that PGMRC1 is a new possible participant in the progestin-induced oocyte maturation in fish. However, its participation in the process of oocyte maturation, which remains to be confirmed, would occur at post-transcriptional levels.

A cDNA for a putative type III deiodinase in the trout (Oncorhynchus mykiss): influence of holding conditions and thyroid hormone treatment on its hepatic expression

A putative rainbow trout type III deiodinase (D3) cDNA was amplified by PCR, using primers to evolutionarily conserved sequences. The RACE-derived complete cDNA was then identified by sequence comparison to that in tilapia and other vertebrates. The cDNA coded for a predicted 31,500 kDa protein of 278 amino acids, with a hydrophobic trans-membrane segment and with 80% similarity to tilapia D3 and 39% similarity to rainbow trout type II deiodinase (D2). It also showed a selenocysteine codon at position 141 and a putative SECIS element in the 3' untranslated end. In the liver, a second form of D3 was found that differed only at this 3' untranslated region; the coding region was identical in both forms. The D3 mRNA, measured by RT-PCR using primers located within the common, translated portion of the cDNA, was expressed in the brain and, depending on thyroidal status, in liver and kidney. Holding trout for 7 days in static water as opposed to flowing water caused increased plasma T4 levels, decreased hepatic D2 mRNA levels and T4 outer-ring deiodination (ORD) activity and increased D3 mRNA levels and T3 inner-ring (IRD) activity. Trout held in flowing water and fed T3 for 7 days showed increased plasma T3 levels and hepatic D3 mRNA levels and T3 IRD activity but decreased D2 mRNA levels and T4ORD activity. Trout held in static water and exposed to ambient T4 for 7 days showed increased plasma T4 levels and hepatic T3IRD activity but with no significant change in D2 or D3 mRNA levels. We conclude that hepatic D3 mRNA levels and T3IRD activity are enhanced and D2 mRNA levels and T4ORD activity are suppressed by adverse holding conditions or T3 treatment suggesting that the putative D3 cDNA and D2 cDNA represent respectively the genes determining T3IRD and T4ORD activities. However, there were changes in the ratios of mRNA levels to enzyme activity, raising the potential for post-transcriptional regulation and showing that mRNA levels alone may be unreliable indices of deiodinase activity. Post-transcriptional regulation of D3 enzyme activity may be influenced by the observed alternative 3' and 5' untranslated regions of the D3 mRNA.

Differential organ expression patterns of thyroid hormone receptor isoform genes in p,p'-DDE-treated adult male common frog, Rana temporaria

Using the European common frog, Rana temporaria, as a model, we have studied the organ-specific gene expression patterns of thyroid hormone receptor isoforms after exposure to an organochlorine (OC) compound, p,p'-DDE. Four groups of frogs were subcutaneously injected with p,p'-DDE at 0.01, 0.1, 1 and 10mg/kg body weight, respectively. In addition, one group, serving as the control group, was injected with pure corn oil. TH receptor isoforms (TRα and TRβ) gene expressions were evaluated in the brain, kidney, testis and liver using real-time PCR with gene-specific primers. Our results show that p,p'-DDE doses induced slight elevations of TRα and TRβ mRNA in the brain. In the testis, p,p'-DDE induced an initial significant 3-fold increase of TRα mRNA at 0.01mg/kg and thereafter clear dose-dependent decreases of TRα mRNA levels were observed. For testicular TRβ mRNA levels, p,p'-DDE induced a slight elevation at 0.01mg/kg and thereafter significant decreases in TRβ mRNA levels were observed. p,p'-DDE induced significant 2-4-fold elevations of both TR isoforms in frog kidney. The strongest transcriptional effect of p,p'-DDE on TR isoforms was observed in the kidney. While TRα mRNA was not measurable in the liver, p,p'-DDE induced an initial 1.7-fold increase at 0.01mg/kg of TRβ mRNA and thereafter an apparent dose-dependent decrease was observed. The relative abundance of TRα and TRβ gene expression in different organs are in the order: kidney>testis>brain>liver. While the induction TRα and TRβ might result to hypersensitivity and subsequent gain of biological functions, the inhibition might result to loss of biological function. Given the high persistency in the environment and continued use in developing countries coupled with the tendency for global atmospheric transport, DDT and its metabolites such as p,p'-DDE will remain a focus of concern both for scientific and societal reasons.

Thyroid hormone transporters in health and disease

Cellular entry is required for conversion of thyroid hormone by the intracellular deiodinases and for binding of 3,3',5-triiodothyronine (T(3)) to its nuclear receptors. Recently, several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have demonstrated that organic anion transporters (e.g., OATPs), and L-type amino acid transporters (LATs) facilitate thyroid hormone uptake. Among these, OATP1C1 has a high affinity and specificity for thyroxine (T(4)). OATP1C1 is expressed in capillaries throughout the brain, suggesting it is critical for transport of T(4) over the blood-brain barrier. We have also characterized a member of the monocarboxylate transporter family, MCT8, as a very active and specific thyroid hormone transporter. Human MCT8 shows preference for T(3) as the ligand. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. The MCT8 gene is located on the X chromosome. Recently, mutations in MCT8 have been found to be associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels.

Thyroid hormone deiodination in fish

We review the experimental evidence accumulated within the past decade regarding the physiologic, biochemical, and molecular characterization of iodothyronine deiodinases (IDs) in piscine species. Agnathans, chondrichthyes, and teleosts express the three isotypes of IDs: ID1, ID2, and ID3, which are responsible for the peripheral fine-tuning of thyroid hormone (TH) bioactivity. At the molecular and operational level, fish IDs share properties with their corresponding vertebrate counterparts. However, fish IDs also exhibit discrete features that seem to be distinctive for piscine species. Indeed, teleostean ID1 is conspicuously resistant to propylthiouracil (PTU) inhibition, and its response to thyroidal status differs from that exhibited by other ID1s. Moreover, both the high level of ID2 activity and its expression in the liver of teleosts are unique among vertebrates. The physiologic role of iodothyronine deiodination in functions regulated by TH in fish is not entirely clear. Nevertheless, current experimental evidence suggests that IDs may coordinate and facilitate, in a tissue-specific fashion, the action of iodothyronines and other hormones involved in such processes.

Thyroid hormone transporters

Thyroid hormone is essential for the development of the brain and the nervous system. Cellular entry is required for conversion of thyroid hormones by the intracellular deiodinases and for binding of T(3) to its nuclear receptors. Several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have so far identified two categories of transporters involved in thyroid hormone uptake (i.e., organic anion transporters and amino acid transporters). Among the organic anion transporters, both Na(+) taurocholate cotransporting polypeptide (NTCP) and various members of the organic anion transporting polypeptide (OATP) family mediate transport of iodothyronines. Because iodothyronines are a particular class of amino acids derived from tyrosine residues, it is no surprise that some amino acid transporters have been shown to be involved in thyroid hormone transport. We have characterized monocarboxylate transporter 8 (MCT8) as a very active and specific thyroid hormone transporter, the gene of which is located on the X chromosome. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. MCT8 shows 50% amino acid identity with a system T amino acid transporter 1 (TAT1). TAT1, also called MCT10, has been characterized to transport aromatic amino acids but no iodothyronines. We have also found that mutations in MCT8 are associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels in young boys.

Halometabolites and cellular dehalogenase systems: an evolutionary perspective

We review the role of iodothyronine deiodinases (IDs) in the evolution of vertebrate thyroidal systems within the larger context of biological metabolism of halogens. Since the beginning of life, the ubiquity of organohalogens in the biosphere has provided a major selective pressure for the evolution and conservation of cellular mechanisms specialized in halogen metabolism. Among naturally available halogens, iodine emerged as a critical component of unique developmental and metabolic messengers. Metabolism of iodinated compounds occurs in the three major domains of life, and invertebrate deuterostomes possess several biochemical traits and molecular homologs of vertebrate thyroidal systems, including ancestral homologs of IDs identified in urochordates. The finely tuned cellular regulation of iodometabolite uptake and disposal is a remarkable event in evolution and might have been decisive for the explosive diversification of ontogenetic strategies in vertebrates.

Differential expression of the two GH genes during embryonic development of rainbow trout Oncorhynchus mykiss in relation with the IGFs system

The Growth hormone (GH)/insulin-like growth factor (IGF) system promotes embryonic growth in higher vertebrates. Such a system exists in salmonids, but exhibits an additional level of complexity resulting from a recent whole genome tetraploidisation. Thus, two nonallelic GH genes are present in the trout genome. Although the two GH genes are similar, the possibility remains that the two genes have evolved separately, acquiring a distinct expression pattern. In this study, using whole mounted in situ hybridisation, we observed a one stage delay between the appearance of GH-2 (Stage 22) and GH-1 (Stage 23) soon after pituitary formation (Stage 21). In addition, by double in situ hybridisation, we clearly evidenced two types of somatotroph, one expressing only GH-2 and the other type both GH-1 and GH-2 at Stage 24. Consequently, at this stage more cells expressed GH-2 than GH-1 as confirmed by quantitative RT-PCR. However at hatching, as in adult, the difference between the expression of the two GH genes was no longer observed. In addition, our immunohistochemical studies did not show any delay between the expression of the mRNA and its translation as a protein at Stage 24. A comparison of the expression pattern of the IGF system components (IGF-1, IGF-2, and the receptor type I) determined by real time RT-PCR, have shown an IGF-1 mRNA increase concomitantly to the appearance of GH expression. On the whole, our results demonstrate a differential regulation of GH-1 and GH-2 genes in rainbow trout embryo. The relationship observed between the expression of different component of the GH/IGF system seems to indicate that this system could be functional early on during embryonic development.

Substitution of cysteine for a conserved alanine residue in the catalytic center of type II iodothyronine deiodinase alters interaction with reducing cofactor

Human type II iodothyronine deiodinase (D2) catalyzes the activation of T(4) to T(3). The D2 enzyme, like the type I (D1) and type III (D3) deiodinases, contains a selenocysteine (SeC) residue (residue 133 in D2) in the highly conserved catalytic center. Remarkably, all of the D2 proteins cloned so far have an alanine two residue-amino terminal to the SeC, whereas all D1 and D3 proteins contain a cysteine at this position. A cysteine residue in the catalytic center could assist in enzymatic action by providing a nucleophilic sulfide or by participating in redox reactions with a cofactor or enzyme residues. We have investigated whether D2 mutants with a cysteine (A131C) or serine (A131S) two-residue amino terminal to the SeC are enzymatically active and have characterized these mutants with regard to substrate affinity, reducing cofactor interaction and inhibitor profile. COS cells were transfected with expression vectors encoding wild-type (wt) D2, D2 A131C, or D2 A131S proteins. Kinetic analysis was performed on homogenates with dithiothreitol (DTT) as reducing cofactor. The D2 A131C and A131S mutants displayed similar Michaelis-Menten constant values for T(4) (5 nM) and reverse T(3) (9 nM) as the wt D2 enzyme. The limiting Michaelis-Menten constant for DTT of the D2 A131C enzyme was 3-fold lower than that of the wt D2 enzyme. The wt and mutant D2 enzymes are essentially insensitive to propylthiouracil [concentration inhibiting 50% of activity (IC(50)) > 2 mM] in the presence of 20 mM DTT, but when tested in the presence of 0.2 mM DTT the IC(50) value for propylthiouracil is reduced to about 0.1 mM. During incubations of intact COS cells expressing wt D2, D2 A131C, or D2 A131S, addition of increasing amounts of unlabeled T(4) resulted in the saturation of [(125)I]T(4) deiodination, as reflected in a decrease of [(125)I]T(3) release into the medium. Saturation first appeared at medium T(4) concentrations between 1 and 10 nM.

IN CONCLUSION

substitution of cysteine for a conserved alanine residue in the catalytic center of the D2 protein does not inactivate the enzyme in vitro and in situ, but rather improves the interaction with the reducing cofactor DTT in vitro.