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

Molecular characterization and expression analysis of thyroid hormone receptors in protogynous rice field eel, Monopterus albus

Thyroid hormones (THs) play important roles in growth, development, morphogenesis, reproduction, and so on. They are mainly meditated by binding to thyroid hormone receptors (TRs) in vertebrates. As important members of the nuclear receptor superfamily, TRs and their ligands are involved in many biological processes. To investigate the potential roles of TRs in the gonadal differentiation and sex change, we cloned and characterized the TRs genes in protogynous rice field eel (Monopterus albus). In this study, three types of TRs were obtained, which were TRαA, TRαB and TRβ, encoding preproproteins of 336-, 409- and 415-amino acids, respectively. Multiple alignments of the three putative TRs protein sequences showed they had a higher similarity. Tissue expression analysis showed that TRαA mainly expressed in the gonad, while TRαB and TRβ in the brain. During female-to-male sex reversal, the expression levels of all the three TRs showed a similar trend of increase followed by a decrease in the gonad. Intraperitoneal injection of triiodothyronine (T3) stimulated the expression of TRαA and TRαB, while it had no significant change on the expression of TRβ in the ovary. Gonadotropin-releasing hormone analogue (GnRHa) injection also significantly upregulated the expression levels of TRαA and TRαB after 6 h, while it had no significant effect on TRβ. These results demonstrated that TRs were involved in the gonadal differentiation and sex reversal, and TRα may play more important roles than TRβ in reproduction by the regulation of GnRHa in rice field eel.

Association between hypothyroidism subtypes and major depression: A two-sample Mendelian randomization study

BACKGROUND

The causal relationship between different hypothyroidism subtypes and the risk of major depression (MD) is yet to be fully elucidated. This study aimed to determine if there's a causal relationship between various hypothyroidism subtypes (and related factors) and the risk of MD.

METHODS

This genetic association study utilized a two-sample Mendelian Randomization (MR) approach to explore the causal relationships between various hypothyroidism subtypes and MD risk. Genome-Wide Association Study (GWAS) summary statistics were obtained from the FinnGen and the UK Biobank. Instrumental variables (IVs) were chosen based on single nucleotide polymorphisms (SNPs).

RESULTS

Among the analyzed hypothyroidism subtypes and related factors, "Hypothyroidism, strict autoimmune" (HTCBSA) and "Hypothyroidism, levothyroxin purchases" (HT/LP) demonstrated a statistically significant positive causal relationship with MD, with odds ratios of 1.020 (95 % CI: 1.004-1.037) and 1.022 (95 % CI: 1.005-1.040), respectively. The sensitivity analysis supported the robustness of these findings, showing no significant horizontal pleiotropy and confirming the stability of results when individual SNPs were removed. "Congenital iodine-deficiency syndrome/hypothyroidism" (CIDS/HT), "Postinfectious hypothyroidism" (PHT), "Hypothyroidism due to medicaments and other exogenous substances" (HDTDM and OES), "Thyroid Stimulating Hormone" (TSH), "Thyrotropin-releasing hormone" (THRH), and "Hypothyroidism, strict autoimmune, 3 medication purchases required" (HTCBSA/3MPR) showed no significant causal relationship with MD.

LIMITATIONS

The study population was limited to individuals of European ancestry, and there may be certain genetic differences between different ethnic groups.

CONCLUSIONS

This MR study suggests a potential causal relationship between certain hypothyroidism subtypes (specifically HTCBSA and HT/LP) and an increased risk of MD.

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.

Stocking density mediated stress modulates growth attributes in cage reared Labeo rohita (Hamilton) using multifarious biomarker approach

The present study was conducted for 240 days to evaluate the effects of stocking density based on growth attributes, digestive enzymes, muscular composition, biochemical and physiological responses of Labeo rohita fingerlings in tropical inland open water cages. L. rohita (30.35 ± 1.08 g) were randomly distributed into three treatments, namely low stocking density, LSD (10 m^-3), medium stocking density, MSD (20 m^-3) and high stocking density, HSD (30 m^-3) in triplicates. Fish were fed twice daily with CIFRI CAGEGROW® floating feed (crude protein-28%, crude fat-4%). Fish growth and feed efficiency were higher (p < 0.05) in LSD, however, MSD registered a higher yield. Amylase and protease activity reduced whereas lipase activity increased with increasing stocking density. Muscle crude protein and crude fat formed an inverse correlation. The fillet quality deteriorated at higher stocking densities based on Muscle pH, drip loss and frozen leakage rate. The stress biomarkers level (glucose, cortisol, superoxide dismutase and catalase) increased in serum under crowding conditions. Glutamate oxaloacetate transaminase and glutamate pyruvate transaminase in serum were significantly increased in HSD. Serum protein levels decreased with the increase in stocking densities. Body ionic imbalance (Na⁺, Cl^- and K⁺) was observed under crowding stress. Based on growth attributes and multiple biomarker responses, L. rohita @ 10 m^-3 was found to be the optimum density for inland open water cage culture.

Regulation of thyroid hormones and branchial iodothyronine deiodinases during freshwater acclimation in tilapia

Euryhaline fishes are capable of maintaining osmotic homeostasis in a wide range of environmental salinities. Several pleiotropic hormones, including prolactin, growth hormone, and thyroid hormones (THs) are mediators of salinity acclimation. It is unclear, however, the extent to which THs and the pituitary-thyroid axis promote the adaptive responses of key osmoregulatory organs to freshwater (FW) environments. In the current study, we characterized circulating thyroxine (T4) and 3-3'-5-triiodothyronine (T3) levels in parallel with the outer ring deiodination (ORD) activities of deiodinases (dios) and mRNA expression of dio1, dio2, and dio3 in gill during the acclimation of Mozambique tilapia (Oreochromis mossambicus) to FW. Tilapia transferred from seawater (SW) to FW exhibited reduced plasma T4 and T3 levels at 6 h. These reductions coincided with an increase in branchial dio2-like activity and decreased branchial dio1 gene expression. To assess whether dios respond to osmotic conditions and/or systemic signals, gill filaments were exposed to osmolalities ranging from 280 to 450 mOsm/kg in an in vitro incubation system. Gene expression of branchial dio1, dio2, and dio3 was not directly affected by extracellular osmotic conditions. Lastly, we observed that dio1 and dio2 expression was stimulated by thyroid-stimulating hormone in hypophysectomized tilapia, suggesting that branchial TH metabolism is regulated by systemic signals. Our collective findings suggest that THs are involved in the FW acclimation of Mozambique tilapia through their interactions with branchial deiodinases that modulate their activities in a key osmoregulatory organ.

Effects of thyroxine and propylthiouracil on feeding behavior and the expression of hypothalamic appetite-regulating peptides and thyroid function in goldfish (Carassius auratus)

There is poor evidence for an association between thyroidal state, feeding and appetite regulation in fish. We assessed how an altered thyroid state influences feeding behavior, food intake and expression of hypothalamic appetite-regulating peptides (Klotho-α and Klotho-β; orexin, OX; cholecystokinin, CCK; agouti-related peptide, AgRP; cannabinoid receptor 1, CB1) in goldfish. We also measured the expressions of hypothalamic, pituitary and liver transcripts that regulate the thyroid [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptor (TRH-R) type 1, thyroid stimulating hormone beta (TSHβ), deiodinases (DIO2, DIO3), UDP-glucuronosyltransferase (UGT1A1), thyroid receptor alpha and beta (TRα, TRβ)], and circulating levels of total thyroxine (tT₄) and total triiodothyronine (tT₃). Goldfish were implanted with propylthiouracil (PTU) or T₄ osmotic pumps for 12 days. T₄- treatment increased feeding behavior but not food intake, increased central TSHβ and DIO2, and hepatic DIO2 transcript expression and increased central DIO3 mRNA. Under hyperthyroid conditions, hypothalamic Klotho and CCK expressions were downregulated, suggesting an increased metabolic state and a hypothalamic response to regulate energy balance. AgRP, OX and CB1 were not affected by T₄ treatment. PTU had no effect on any of the parameters examined, suggesting it is not a sensitive thyroid inhibitor in fish. Overall, we show that unlike in mammals, hyperthyroid conditions in goldfish do not lead to an increased desire or need to consume food, furthering evidence for a weak link between the thyroid and appetite.

Response of the thyroid axis and appetite-regulating peptides to fasting and overfeeding in goldfish (Carassius auratus)

The thyroid axis is a major regulator of metabolism and energy homeostasis in vertebrates. There is conclusive evidence in mammals for the involvement of the thyroid axis in the regulation of food intake, but in fish, this link is unclear. In order to assess the effects of nutritional status on the thyroid axis in goldfish, Carassius auratus, we examined brain and peripheral transcripts of genes associated with the thyroid axis [thyrotropin-releasing hormone (TRH), thyrotropin-releasing hormone receptors (TRH-R type 1 and 2), thyroid stimulating hormone beta (TSHβ), deiodinase enzymes (DIO2, DIO3) and UDP-glucoronsyltransferase (UGT)] and appetite regulators [neuropeptide Y (NPY), proopiomelanocortin (POMC), agouti-related peptide (AgRP) and cholecystokinin (CCK)] in fasted and overfed fish for 7 and 14 day periods. We show that the thyroid axis responds to overfeeding, with an increase of brain TRH and TSHβ mRNA expression after 14 days, suggesting that overfeeding might activate the thyroid axis. In fasted fish, hepatic DIO3 and UGT transcripts were downregulated from 7 to 14 days, suggesting a time-dependent inhibition of thyroid hormone degradation pathways. Nutritional status had no effect on circulating levels of thyroid hormone. Central appetite-regulating peptides exhibited temporal changes in mRNA expression, with decreased expression of the appetite-inhibiting peptide POMC from 7 to 14 days for both fasted and overfed fish, with no change in central NPY or AgRP, or intestinal CCK transcript expression. Compared to control fish, fasting increased AgRP mRNA expression at both 7 and 14 days, and POMC expression was higher than controls only at 7 days. Our results indicate that nutritional status time-dependently affects the thyroid axis and appetite regulators, although no clear correlation between thyroid physiology and appetite regulators could be established. Our study helps to fill a knowledge gap in current fish endocrinological research on the effects of energy balance on thyroid metabolism and function.

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.

8:8 Perfluoroalkyl phosphinic acid affects neurobehavioral development, thyroid disruption, and DNA methylation in developing zebrafish

Recent studies have reported potential neurotoxicity and epigenetic alteration associated with exposure to several per- and polyfluoroalkyl substances (PFASs). However, such information is limited to a few compounds (e.g., perfluorooctane sulfonate), primarily based on rodent experiments, and the underlying toxicological mechanism(s) for many PFAS in the environment remain poorly understood. In the present study, we investigated 8:8 perfluoroalkyl phosphinic acid (8:8 PFPiA), an under-studied PFAS with high persistency in the environment and biota, using the zebrafish model. We exposed zebrafish embryos (<4 hpf) to various concentrations of 8:8 PFPiA (0, 0.0116, 0.112, 0.343, 1.34, 5.79 μM) for 144 h. Although there was no significant change in survival, hatchability and malformations, zebrafish locomotor speed at 120 h significantly decreased in dark photoperiod. At 144 h, several genes related to thyroid hormones that are essential for neurodevelopment, including corticotropin releasing hormone b (crhb), iodothyronine deiodinase 3a (dio3a), thyroid-stimulating hormone receptor (tshr) and nkx2 homeobox1 (nkx 2.1), were up-regulated by 8:8 PFPiA at 5.79 μM. 8:8 PFPiA also significantly down-regulated a neurodevelopmental gene, elav like neuron-specific RNA binding protein (elavl3), at 1.34 and 5.79 μM; in addition, one oxidative stress gene was slightly but significantly up-regulated. Further, global DNA methylation was significantly decreased at higher treatment levels, identifying effects of 8:8 PFPiA on epigenetic regulation. However, promoter DNA methylation of selected genes (dio3, tshr, nkx2.1) were not statistically altered, though dio3 methylation showed a decreasing trend with 8:8 PFPiA exposure. Our results specifically advance an understanding of molecular toxicology of PFPiA and more broadly present an approach to define diverse responses during animal alternative assessments of PFASs.

Expression dynamics of genes in the hypothalamic-pituitary-thyroid (HPT) cascade and their responses to 3,3',5-triiodo-l-thyronine (T3) highlights potential vulnerability to thyroid-disrupting chemicals in zebrafish (Danio rerio) embryo-larvae

Some chemicals in the environment disrupt thyroid hormone (TH) systems leading to alterations in organism development, but their effect mechanisms are poorly understood. In fish, this has been limited by a lack of fundamental knowledge on thyroid gene ontogeny and tissue expression in early life stages. Here we established detailed expression profiles for a suite of genes in the hypothalamic-pituitary-thyroid (HPT) axis of zebrafish (Danio rerio) between 24-120 h post fertilisation (hpf) and quantified their responses following exposure to 3,3',5-triiodo-L-thyronine (T3) using whole mount in situ hybridisation (WISH) and qRT-PCR (using whole-body extracts). All of the selected genes in the HPT axis demonstrated dynamic transcript expression profiles across the developmental stages examined. The expression of thyroid receptor alpha (thraa) was observed in the brain, gastrointestinal tract, craniofacial tissues and pectoral fins, while thyroid receptor beta (thrb) expression occurred in the brain, otic vesicles, liver and lower jaw. The TH deiodinases (dio1, dio2 and dio3b) were expressed in the liver, pronephric ducts and brain and the patterns differed depending on life stage. Both dio1 and dio2 were also expressed in the intestinal bulb (96-120 hpf), and dio2 expression occurred also in the pituitary (48-120 hpf). Exposure of zebrafish embryo-larvae to T3 (30 and 100 μg L^-1) for periods of 48, 96 or 120 hpf resulted in the up-regulation of thraa, thrb, dio3b, thyroid follicle synthesis proteins (pax8) and corticotropin-releasing hormone (crhb) and down-regulation of dio1, dio2, glucuronidation enzymes (ugt1ab) and thyroid stimulating hormone (tshb) (assessed via qRT-PCR) and responses differed across life stage and tissues. T3 induced thraa expression in the pineal gland, pectoral fins, brain, somites, gastrointestinal tract, craniofacial tissues, liver and pronephric ducts. T3 enhanced thrb expression in the brain, jaw cartilage and intestine, while thrb expression was suppressed in the liver. T3 exposure suppressed the transcript levels of dio1 and dio2 in the liver, brain, gastrointestinal tract and craniofacial tissues, while dio2 signalling was also suppressed in the pituitary gland. Dio3b expression was induced by T3 exposure in the jaw cartilage, pectoral fins and brain. The involvement of THs in the development of numerous body tissues and the responsiveness of these tissues to T3 in zebrafish highlights their potential vulnerability to exposure to environmental thyroid-disrupting chemicals.

Effect of triadimefon and its metabolite on adult amphibians Xenopus laevis

The decrease in the population of amphibians all over the world has raised concerns. Adult X. laevis frogs were exposed to 0, 1 and 10 mg/L triadimefon and triadimenol. After 14 or 28 days exposure, we collected male and female specimens to study swimming activity, lactic dehydrogenase (LDH) and antioxidant enzyme activity in blood samples, histopathology of liver and thyroid tissue, thyroid hormone levels and thyroid hormone-related gene expression levels in brains. Our results showed that triadimefon and triadimenol could affect the swimming activity of frogs and that this was distinct at different levels of triadimenol. Moreover, triadimefon and triadimenol exposure produced a greater effect on superoxide dismutase (SOD) in females than in males, which was reverse to the finding for glutathione S-transferase (GST) and catalase (CAT). After 28 days exposure, triadimefon produced more toxic effects on the liver than observed for triadimenol. Besides this, triadimefon and triadimenol exposure exerted a greater effect on liver histology and thyroid hormone levels in male frogs than in the females. Our results also found that the expression of genes related to thyroid hormone in brains depended on the exposure level and time, as well as the sex of the treated individual. This study shed light on the relationships between the toxicity of metabolite products and their parent compounds and provided further understanding of the risk of pesticide use on amphibians.

Hormones, developmental plasticity, and adaptive evolution: Endocrine flexibility as a catalyst for 'plasticity-first' phenotypic divergence

Explaining how populations adapt to environments is among the foremost objectives of evolutionary theory. Over generations, natural selection impels the phenotypic distribution of a population based on individual variation in phenotype and fitness. However, environmental conditions can also shape how individuals develop within their lifetime to influence which phenotypes are expressed in a population. It has been proposed that such environmentally-initiated phenotypic variation - also termed developmental plasticity - may enable adaptive evolution under some scenarios. As dynamic regulators of development and phenotypic expression, hormones are important physiological mediators of developmental plasticity. Patterns of hormone secretion, hormone transport, and the sensitivity of tissues to hormones can each be altered by environmental conditions, and understanding how endocrine regulation shapes phenotypic development in an ecologically-relevant context has much to contribute toward clarifying the role of plasticity in evolutionary adaptation. This article explores how the environmental sensitivity of endocrine regulation may facilitate 'plasticity-first' evolution by generating phenotypic variants that precede adaptation to altered or novel environments. Predictions arising from 'plasticity-first' evolution are examined in the context of thyroid hormone mediation of morphological plasticity in Cyprinodon pupfishes from the Death Valley region of California and Nevada, USA. This clade of extremophile fishes diversified morphologically over the last ~20,000 years, and observations that some populations experienced contemporary phenotypic differentiation under recent habitat change provide evidence that hormone-mediate plasticity preceded genetic assimilation of morphology in one of the region's species: the Devils Hole pupfish, Cyprinodon diabolis. This example illustrates how conceptualizing hormones not only as regulators of homeostasis, but also as developmental intermediaries between environment conditions and phenotypic variation at the individual-, population-, and species-levels can enrich our understanding of endocrine regulation both as a facilitator of phenotypic change under shifting environments, and as important proximate mechanisms that may initiate 'plasticity-first' evolutionary adaptation.

Cadmium exposure affects growth performance, energy metabolism, and neuropeptide expression in Carassius auratus gibelio

Cadmium (Cd) is the most abundant heavy metal in aquatic environments and is easily detected on a global scale. Carassius auratus gibelio is a common aquaculture species. The aim of this study was to explore the toxic effects of 1, 2, and 4 mg/L Cd on the energy metabolism, growth performance, and neurological responses of C. gibelio. After 30 days of exposure, Cd concentrations in the liver and brain were significantly increased in Cd-exposed groups. Low-level Cd exposure (1 mg/L) increased weight and length gains, as well as food intake, in the fish. Acetylcholinesterase activity decreased significantly in the Cd-exposed groups. Energy metabolism levels (as reflected by oxygen consumption, ammonia excretion rate, and swimming activity), as well as serum T3 and T4 levels, increased significantly in the fish exposed to 1 mg/L Cd. However, energy metabolism and serum T3/T4 levels decreased significantly in the 4-mg/L Cd group. Neuropeptide gene expression levels in brain were consistent with the observed changes in food intake. In the Cd-exposed groups, the expression levels of neuropeptide Y (NPY), apelin, and metallothionein (MT) increased significantly, while those of pro-opinmelanocortin (POMC), ghrelin, and corticotrophin-releasing factor (CRF) decreased significantly. Our data suggested that in fish, low doses of Cd might increase food intake, as well as weight and length gains, but high doses of Cd might have the opposite effect. These effects might be a result of neurohumoral regulation. Long-term exposure to low doses of Cd might cause weight gain and affect food intake.

Waterborne exposure to microcystin-LR alters thyroid hormone levels, iodothyronine deiodinase activities, and gene transcriptions in juvenile zebrafish (Danio rerio)

This study investigated the effects of microcystin (MC) on the regulation of thyroid hormone (TH) metabolism in juvenile zebrafish exposed to MC-LR. The results showed that acute MC-LR exposure at concentrations ranging from 50 μg/L to 400 μg/L led to significant reductions in thyroxine (T4) and triiodothyronine (T3) levels in juvenile zebrafish. The transcription levels of genes involved in TH synthesis, such as corticotropin-releasing hormone (crh), thyroid-stimulating hormone (tsh), thyroid peroxidase (tpo) and transthyretin (ttr), were significantly decreased followed by an increase after MC-LR exposure. Transcription of the TH nuclear receptors (tr-α and tr-β) was significantly reduced during the exposure period. Moreover, the activities of iodothyronine deiodinase type Ⅰ (ID1) and iodothyronine deiodinase type Ⅱ (ID2) showed initially decreased and then increased trend, while the activity of iodothyronine deiodinase type Ⅲ (ID3) significantly decreased during MC-LR exposure. In addition, the effect of MC-LR on deiodinase activities and T4 contents were important causes of the decreased T3 at the early exposure stage. These results indicated that acute MC-LR exposure significantly interfered with the transcription of genes related to TH synthesis, transport and metabolism, and affected normal function of the thyroid which leads to decrease of T4 and T3 in juvenile zebrafish. Therefore, the thyroid function is susceptible to interference by MC-LR, and it may cause adverse effects on the growth and development of juvenile zebrafish.

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.

Chronic thiamethoxam exposure impairs the HPG and HPT axes in adult Chinese rare minnow (Gobiocypris rarus): Docking study, hormone levels, histology, and transcriptional responses

Thiamethoxam has emerged as an environmental contaminant detected in aqueous environments, and its endocrine-disrupting effect at chronic exposure in teleosts remains unknown. In the present study, a docking experiment and an in vivo test were integrated to systematically explore the toxic mechanisms of thiamethoxam in fish. Histological analysis, plasma VTG and hormone level (E2, 11-KT, T3 and T4) determinations, and HPG and HPT gene expression quantification were performed after Chinese rare minnow (Gobiocypris rarus) was exposed to thiamethoxam (0, 0.5, 5, and 50 μg/L) for 90 days. According to the docking study, thiamethoxam had different interactions with ERα, AR and TRα via hydrogen bonding. A decrease in body length and plasma T4 was observed in both genders. The histological damage in liver and delayed gonadal development were observed in both genders at 50 μg/L thiamethoxam treatment. In males, the following HPG axis genes were upregulated: gnrh and cyp19b in the brain; vtg and cyp19a in the liver; and cyp17 and cyp19a in the gonad. In females, erɑ in the liver was significantly upregulated with 0.5 μg/L thiamethoxam treatment, and cyp17 in the gonad was upregulated with all treatment. The suppression of cyp19a, gnrh, cyp11a, and ttr was observed at the concentration of 5 μg/L in the female liver. Taken together, the endocrine system of Chinese rare minnow might be disrupted after chronic exposure to thiamethoxam.

Two-generation exposure to 2-ethylhexyl 4-methoxycinnamate (EHMC) in Japanese medaka (Oryzias latipes) and its reproduction and endocrine related effects

2-Ethylhexyl 4-methoxycinnamate (EHMC) is one of the most widely used UV-filters, and hence has been frequently detected in water environment. EHMC has been reported to induce short-term reproductive toxicity in fish, and thyroid disrupting effects in other animal studies. However, limited information is available for its long-term effects on fecundity, and thyroid disrupting effects in fish. In the present study, effects of EHMC on fecundity, measured as number of eggs, were evaluated in Japanese medaka (Oryzias latipes), and its underlying mechanisms on sex and thyroid hormone disruption were explored. For this purpose, a five-month long (154 d) exposure to F0 generation was conducted on fertilized eggs (<24 h post-fertilization (hpf)), with nominal concentration of 0, 0.05, 0.158, 0.5, 1.58, or 5 mg/L EHMC, followed by a 3-8-d exposure of F1 generation. After >3 months exposure, significant decreases in reproductive performances were observed at all test concentrations as low as 0.05 mg/L. Reproduction effects were not accompanied with sex hormone changes, but up-regulation of vitellogenin gene was observed. Thyroid hormones were decreased by EHMC exposure in F1 fish at -38 day post-fertilization (dpf). In addition, down-regulation of type II iodothyronine deiodinase (dio2) and up-regulation of thyrotropin releasing hormone (trh) were observed in both F0 and F1 juvenile fish, suggesting thyroid disruption potential of EHMC. Our observation suggests that EHMC at the levels one to two orders of magnitude higher than those detected in ambient water may affect reproduction and thyroid hormonal balance of fish.

Effects of tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and triphenyl phosphate (TPP) on sex-dependent alterations of thyroid hormones in adult zebrafish

Organophosphate flame retardants (OPFRs) have been widely used as alternatives to polybrominated diphenyl ethers for fire prevention. OPFRs are suspected of causing potential thyroid disruption in humans. In fish, their thyroid hormone modulation is reported but the mechanisms of this modulation are less understood. Thyroid-disturbing effects of OPFRs were evaluated using adult zebrafish (Danio rerio) following 14d exposure to tris(1,3-dichloro-2-propyl) phosphate (TDCPP) or triphenyl phosphate (TPP). Plasma concentrations of thyroid hormones were measured and transcriptions of several genes involved in thyroid function were quantified in brain, thyroid, and liver. Exposure to TDCPP or TPP led to significant decreases in plasma triiodothyronine (T3) and thyroxine (T4) concentrations in the male fish, while the increases were observed in the female fish. Exposure to the OPFRs also altered the transcription of regulatory genes and receptors in hypothalamus, pituitary, and thyroid of the fish in sex-dependent manner. In the male fish, transcriptions of corticotropin-releasing hormone (crh) and thyroid-stimulating hormone (tsh) in the brain were significantly up-regulated, probably as a compensation for hypothyroidism, but thyroglobulin (tg) and deiodinase 2 (dio2) were down-regulated in thyroid or liver. In contrast, in the females, transcriptions of crh and tsh genes were significantly down-regulated. These observations show that TDCPP and TPP exposure can lead to sex-dependent disruptions of thyroid hormone balances in the adult zebrafish through alterations of the hypothalamus-pituitary-thyroid (HPT) axis.

The case for thyroid disruption in early life stage exposures to thiram in zebrafish (Danio rerio)

Thiram, a pesticide in the dithiocarbamate chemical family, is widely used to prevent fungal disease in seeds and crops. Its off-site movement to surface waters occurs and may place aquatic organisms at potential harm. Zebrafish embryos were used for investigation of acute (1 h) thiram exposure (0.001-10 µM) at various developmental stages. Survival decreased at 1 µM and 10 µM and hatching was delayed at 0.1 µM and 1 µM. Notochord curvatures were seen at 0.1 and 1 μM thiram when exposure was initiated at 2 and at 10 hpf. Similar notochord curvatures followed exposure to the known TPO inhibitor, methimazole (MMI). Changes were absent in embryos exposed at later stages, i.e., 12 hpf. In embryos exposed to 0.1 or 1 μM at 10 hpf, levels of the thyroid enzyme, Deiodinase 3, increased by 12 hpf. Thyroid peroxide (TPO), important in T4 synthesis, decreased by 48 hpf in embryos exposed to 1 µM at 10 hpf. Thiram toxicity was stage-dependent and early life stage exposure may be responsible for adverse effects seen later. These effects may be due to impacts on the thyroid via regulation of specific thyroid genes including TPO and Deiodinase 3.

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.

Thyroid hormone actions on male reproductive system of teleost fish

Thyroid hormones (THs) play important roles in the regulation of many biological processes of vertebrates, such as growth, metabolism, morphogenesis and reproduction. An increasing number of studies have been focused on the involvement of THs in the male reproductive system of vertebrates, in particular of fish. Therefore, this mini-review aims to summarize the main findings on THs role in male reproductive system of fish, focusing on sex differentiation, testicular development and spermatogenesis. The existing data in the literature have demonstrated that THs exert their roles at the different levels of the hypothalamic-pituitary-gonadal (HPG) axis. In general a positive correlation has been shown between THs and fish reproductive status; where THs are associated with testicular development, growth and maturation. Recently, the molecular mechanisms underlying the role of THs in spermatogenesis have been unraveled in zebrafish testis. THs promote germ cell proliferation and differentiation by increasing a stimulatory growth factor of spermatogenesis produced by Sertoli cells. In addition, THs enhanced the gonadotropin-induced androgen release in zebrafish testis. Next to their functions in the adult testis, THs are involved in the gonadal sex differentiation through modulating sex-related gene expression, and testicular development via regulation of Sertoli cell proliferation. In conclusion, this mini-review showed that THs modulate the male reproductive system during the different life stages of fish. The physiological and molecular mechanisms showed a link between the thyroid and reproduction, suggesting a possibly co-evolution and interdependence of these two systems.

Detection and localization of the thyroid hormone receptor beta mRNA in the immature olfactory receptor neurons of chum salmon

Thyroid hormone (TH) plays an important role in regulating multiple cellular and metabolic processes, including cell proliferation, cell death, and energy metabolism, in various organs and tissues of vertebrates. It is generally accepted that anadromous Pacific salmon (Oncorhynchus spp.) imprint odorants from their natal stream during their seaward migration, and they then use olfaction to discriminate their natal stream during the spawning migration. Both serum TH levels and the specific binding values of TH in the salmon olfactory epithelium were markedly increased during the seaward migration. However, thyroid hormone receptor (TR) expression in the olfactory epithelium has not been confirmed in vertebrates. We investigated gene expression of TR isoforms in chum salmon (O. keta) by both molecular biological and histochemical techniques. Expression of TRβ mRNA was detected in the olfactory epithelium by reverse transcriptase polymerase chain reaction (RT-PCR). Nucleotide sequencing demonstrated the existence of a remarkable homology between the RT-PCR product and part of the ligand-binding domain of other teleost TRβ isoforms. By in situ hybridization using a digoxygenin-labeled salmon olfactory TRβ cRNA probe, signals for salmon olfactory TRβ mRNA were observed preferentially in the perinuclear regions of immature olfactory receptor neurons (ORNs), as protein gene product 9.5 (PGP9.5)-immunopositive ORNs. Our results provide the first detection of TRβ gene expression in the olfactory epithelium, and suggested the possibility that TRβ may be involved in cell maturation and/or cell differentiation of the ORNs in Pacific salmon.

An AOP-based alternative testing strategy to predict the impact of thyroid hormone disruption on swim bladder inflation in zebrafish

The adverse outcome pathway (AOP) framework can be used to help support the development of alternative testing strategies aimed at predicting adverse outcomes caused by triggering specific toxicity pathways. In this paper, we present a case-study demonstrating the selection of alternative in chemico assays targeting the molecular initiating events of established AOPs, and evaluate use of the resulting data to predict higher level biological endpoints. Based on two AOPs linking inhibition of the deiodinase (DIO) enzymes to impaired posterior swim bladder inflation in fish, we used in chemico enzyme inhibition assays to measure the molecular initiating events for an array of 51 chemicals. Zebrafish embryos were then exposed to 14 compounds with different measured inhibition potentials. Effects on posterior swim bladder inflation, predicted based on the information captured by the AOPs, were evaluated. By linking the two datasets and setting thresholds, we were able to demonstrate that the in chemico dataset can be used to predict biological effects on posterior chamber inflation, with only two outliers out of the 14 tested compounds. Our results show how information organized using the AOP framework can be employed to develop or select alternative assays, and successfully forecast downstream key events along the AOP. In general, such in chemico assays could serve as a first-tier high-throughput system to screen and prioritize chemicals for subsequent acute and chronic fish testing, potentially reducing the need for long-term and costly toxicity tests requiring large numbers of animals.

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.

ChIP-on-chip analysis of thyroid hormone-regulated genes and their physiological significance

Triiodothyronine (T₃) and its receptor (TR) modulate several physiological processes, including cell development, proliferation, differentiation and metabolism. The regulatory mechanism of T₃/TR involves binding to the thyroid hormone response element (TRE) within the target gene promoter. However, the number of target genes directly regulated by TRα1 and the specific pathways of TR-regulated target genes remain largely unknown. Here, we expressed TRα1 in a HepG2 cell line and used chromatin immunoprecipitation coupled with microarray to determine the genes that are directly regulated by TRα1 and also involved in cell metabolism and proliferation. Our analysis identified E74-like factor 2 (ELF2), a transcription factor associated with tumor growth, as a direct target downregulated by T₃/TR. Overexpression of ELF2 enhanced tumor cell proliferation, and conversely, its knockdown suppressed tumor growth. Additionally, ELF2 restored the proliferative ability of hepatoma cells inhibited by T₃/TR. Our findings collectively support a potential role of T₃/TR in tumor growth inhibition through regulation of ELF2.

Structure and Functional Analysis of Promoters from Two Liver Isoforms of CPT I in Grass Carp Ctenopharyngodon idella

Carnitine palmitoyltransferase I (CPT I) is a key enzyme involved in the regulation of lipid metabolism and fatty acid β-oxidation. To understand the transcriptional mechanism of CPT Iα1b and CPT Iα2a genes, we cloned the 2695-bp and 2631-bp regions of CPT Iα1b and CPT Iα2a promoters of grass carp (Ctenopharyngodon idella), respectively, and explored the structure and functional characteristics of these promoters. CPT Iα1b had two transcription start sites (TSSs), while CPT Iα2a had only one TSS. DNase I foot printing showed that the CPT Iα1b promoter was AT-rich and TATA-less, and mediated basal transcription through an initiator (INR)-independent mechanism. Bioinformatics analysis indicated that specificity protein 1 (Sp1) and nuclear factor Y (NF-Y) played potential important roles in driving basal expression of CPT Iα2a gene. In HepG2 and HEK293 cells, progressive deletion analysis indicated that several regions contained cis-elements controlling the transcription of the CPT Iα1b and CPT Iα2a genes. Moreover, some transcription factors, such as thyroid hormone receptor (TR), hepatocyte nuclear factor 4 (HNF4) and peroxisome proliferator-activated receptor (PPAR) family, were all identified on the CPT Iα1b and CPT Iα2a promoters. The TRα binding sites were only identified on CPT Iα1b promoter, while TRβ binding sites were only identified on CPT Iα2a promoter, suggesting that the transcription of CPT Iα1b and CPT Iα2a was regulated by a different mechanism. Site-mutation and electrophoretic mobility-shift assay (EMSA) revealed that fenofibrate-induced PPARα activation did not bind with predicted PPARα binding sites of CPT I promoters. Additionally, PPARα was not the only member of PPAR family regulating CPT I expression, and PPARγ also regulated the CPT I expression. All of these results provided new insights into the mechanisms for transcriptional regulation of CPT I genes in fish.

Impaired swim bladder inflation in early life stage fathead minnows exposed to a deiodinase inhibitor, iopanoic acid

Inflation of the posterior and/or anterior swim bladder is a process previously demonstrated to be regulated by thyroid hormones. We investigated whether inhibition of deiodinases, which convert thyroxine (T4) to the more biologically active form, 3,5,3'-triiodothyronine (T3), would impact swim bladder inflation. Two experiments were conducted using a model deiodinase inhibitor, iopanoic acid (IOP). First, fathead minnow embryos were exposed to 0.6, 1.9, or 6.0 mg/L or control water until 6 d postfertilization (dpf), at which time posterior swim bladder inflation was assessed. To examine anterior swim bladder inflation, a second study was conducted with 6-dpf larvae exposed to the same IOP concentrations until 21 dpf. Fish from both studies were sampled for T4/T3 measurements and gene transcription analyses. Incidence and length of inflated posterior swim bladders were significantly reduced in the 6.0 mg/L treatment at 6 dpf. Incidence of inflation and length of anterior swim bladder were significantly reduced in all IOP treatments at 14 dpf, but inflation recovered by 18 dpf. Throughout the larval study, whole-body T4 concentrations increased and T3 concentrations decreased in all IOP treatments. Consistent with hypothesized compensatory responses, deiodinase-2 messenger ribonucleic acid (mRNA) was up-regulated in the larval study, and thyroperoxidase mRNA was down-regulated in all IOP treatments in both studies. These results support the hypothesized adverse outcome pathways linking inhibition of deiodinase activity to impaired swim bladder inflation. Environ Toxicol Chem 2017;36:2942-2952. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Thyroid hormone related gene transcription in southern sand flathead (Platycephalus bassensis) is associated with environmental mercury and arsenic exposure

Arsenic (As) and mercury (Hg) are ubiquitous elements known to disrupt thyroid function in vertebrates. To explore the underlying mechanisms of Hg and As on the fish thyroid system, we investigated the associations between muscle concentrations of Hg and As with thyroid-related gene transcription in flathead (Platycephalus bassensis) from a contaminated estuary. We sampled fish at several sites to determine the hepatic expression of genes including deiodinases (D1 and D2), transthyretin (TTR), thyroid hormone receptors (TRα and TRβ) and related them to Hg and As levels in the same individuals. Negative correlations were observed between Hg levels and D2, TTR, TRα and TRβ, whereas positive associations were found between As concentrations and TTR and TRβ. These results suggest that Hg and As exposures from environmental pollution affect the regulation of genes important for normal thyroid function in fish. These thyroid-related genes could be used as biomarkers for monitoring environmental thyroid-hormone disrupting chemicals.

Thyroid hormones and learning-associated neuroplasticity

Thyroid hormones (THs) are crucial for brain development and maturation in all vertebrates. Especially during pre- and perinatal development, disruption of TH signaling leads to a multitude of neurological deficits. Many animal models provided insight in the role of THs in brain development, but specific data on how they affect the brain's ability to learn and adapt depending on environmental stimuli are rather limited. In this review, we focus on a number of learning processes like spatial learning, fear conditioning, vocal learning and imprinting behavior and on how abnormal TH signaling during development shapes subsequent performance. It is clear from multiple studies that TH deprivation leads to defects in learning on all fronts, and interestingly, changes in local expression of the TH activator deiodinase type 2 seem to have an important role. Taking into account that THs are regulated in a very space-specific manner, there is thus increasing pressure to investigate more local TH regulators as potential factors involved in neuroplasticity. As these learning processes are also important for proper adult human functioning, further elucidating the role of THs in developmental neuroplasticity in various animal models is an important field for advancing both fundamental and applied knowledge on human brain function.

Negative feedback regulation of thyrotropin subunits and pituitary deiodinases in red drum, Sciaenops ocellatus

Thyroxine (T₄) undergoes dynamic daily cycles in the perciform fish the red drum, Sciaenops ocellatus, that are inversely timed to cycles of thyrotropin (TSH) subunit mRNA expression in the pituitary gland. We have proposed that these daily cycles are regulated by negative feedback of circulating T₄ on expression of pituitary thyroid hormone deiodinase type 3 (Dio3), such that elevated circulating T₄ results in diminished pituitary thyroid hormone catabolism and consequent increased negative feedback on expression of TSH subunits during the day. To determine whether thyroid hormones function to modulate expression of pituitary deiodinase enzymes we developed an immersion technique to administer physiological doses of T₃ and T₄in vivo. Immersion in T₄ or T₃ significantly inhibited the mRNA expression of the TSH α and β subunits from 4 to 66h of immersion. Pituitary Dio3 expression was significantly diminished by T₃ and T₄ at 22h. These results indicate that both T₄ and T₃ are capable of negative feedback regulation of TSH subunit expression in red drum at physiological concentrations and on a time scale consistent with the T₄ daily cycle. Furthermore, thyroid hormones negatively regulate Dio3 expression in the pituitary in a manner suggesting that negative thyroxine feedback on Dio3 promotes the release of TSH subunits from TH inhibition and may be an important mechanism for generating daily thyroid hormone cycles. These results highlight a potentially important role for D3 in mediating thyroid hormone feedback on TSH expression, not previously described in other species.

Effects of exposure to microcystin-LR at environmentally relevant concentrations on the metabolism of thyroid hormones in adult zebrafish (Danio rerio)

Microcystin-LR (MC-LR) has the potential to disturb thyroid hormone homeostasis. However, the effects of MC-LR at environmentally relevant concentrations on the thyroid system in adult fish are still unclear. In this study, adult zebrafish were exposed to 0, 1, 5, and 25 μg/L MC-LR for 7, 14, 21, and 28 days. Whole-body thyroid hormones (THs) levels and thyroid follicle histology were used to assess thyroid function. The transcription of corticotropin-releasing hormone (crh), thyroid-stimulating hormone (tsh), transthyretin (ttr), thyroid hormone receptors (trs) genes, and the activities of iodothyronine deiodinases (IDs) were investigated to study the process of TH metabolism disruption. No differences in the histopathology of thyroid follicles and unchanged T₄ levels were observed in adult zebrafish. A significant decline in T₃ levels associated with a decrease in ID2 activity in male zebrafish was observed at 21 days exposure. Moreover, the mRNA expression of tsh, ttr and trs appeared to be a dynamic process as expression first decreased and then increased with continued exposure. These results indicated that exposure to MC-LR did not inhibit the production of TH. The decrease in ID2 activity may be an important factor in the decline of T₃ levels. Furthermore, it seems that the fish triggered a compensatory mechanism to maintain TH homeostasis in respond to environmental concentrations of MC-LR which induced TH disruption.

Endocrine and metabolic impacts of warming aquatic habitats: differential responses between recently isolated populations of a eurythermal desert pupfish

Temperatures of inland aquatic habitats are increasing with climate change, and understanding how fishes respond physiologically to thermal stress will be crucial for identifying species most susceptible to these changes. Desert fishes may be particularly vulnerable to rising temperatures because many species occupy only a fraction of their historical range and occur in habitats with already high temperatures. Here, we examined endocrine and metabolic responses to elevated temperature in Amargosa pupfish, Cyprinodon nevadensis amargosae. We studied C. n. amargosae from two habitats with distinct thermal conditions: the Amargosa River, which experiences diurnally and seasonally variable temperatures (0.2-40°C); and Tecopa Bore, a spring and marsh fed by hot groundwater (47.5°C) from an artesian borehole. These allopatric populations differ in morphology, and prior evidence suggests that temperature might contribute to these differences via altered thyroid hormone (TH) regulation of morphological development. Here, we document variation in hepatic iodothyronine deiodinase type 2 (dio2) and type 3 (dio3) and TH receptor β (trβ) gene transcript abundance between the Amargosa River and Tecopa Bore wild populations. Fish from these populations acclimated to 24 or 34°C retained differences in hepatic dio2, dio3 and trβ mRNAs and also varied in transcripts encoding the TH membrane transporters monocarboxylate transporter 8 (mct8) and organic anion-transporting protein 1c1 (oatp1c1). Tecopa Bore pupfish also exhibited higher dio2 and trβ mRNA levels in skeletal muscle relative to Amargosa River fish. Muscle citrate synthase activity was lower at 34°C for both populations, whereas lactate dehydrogenase activity and lactate dehydrogenase A-chain (ldhA) transcripts were both higher and 3,5,3'-triiodothryonine responsive in Tecopa Bore pupfish only. These findings reveal that local population variation and thermal experience interact to shape how pupfish respond to elevated temperatures, and point to the need to consider such interactions in management actions for desert fishes under a changing climate.

Tributyltin disrupts feeding and energy metabolism in the goldfish (Carassius auratus)

Tributyltin (TBT) can induce obesogen response. However, little is known about the adverse effects of TBT on food intake and energy metabolism. The present study was designed to investigate the effects of TBT, at environmental concentrations of 2.44 and 24.4 ng/L (1 and 10 ng/L as Sn), on feeding and energy metabolism in goldfish (Carassius auratus). After exposure for 54 d, TBT increased the weight gain and food intake in fish. The patterns of brain neuropeptide genes expression were in line with potential orexigenic effects, with increased expression of neuropeptide Y and apelin, and decreased expression of pro-opiomelanocortin, ghrelin, cocaine and amphetamine-regulated transcript, and corticotropin-releasing factor. Interestingly, the energy metabolism indicators (oxygen consumption, ammonia exertion and swimming activity) and the serum thyroid hormones were all significantly increased at the 2.44 ng/L TBT group in fish. However, no changes of energy metabolism indicators or a decrease of thyroid hormones was found at the 24.4 ng/L TBT group, which indicated a complex disrupting effect on metabolism of TBT. In short, TBT can alter feeding and energy metabolism in fish, which might promote the obesogenic responses.

Thyroid active agents T3 and PTU differentially affect immune gene transcripts in the head kidney of rainbow trout (Oncorynchus mykiss)

In mammals, numerous reports describe an immunomodulating effect of thyroid-active compounds. In contrast, only few reports have been published on this subject in fish. We previously demonstrated that immune cells of rainbow trout (Oncorhynchus mykiss) possess thyroid hormone receptors (THRs) and that exposure of trout to the thyroid hormone 3,3',5-triiodo-l-thyronine (T3) or the antithyroid drug propylthiouracil (PTU) alters immune cell transcript levels of THR and several immune genes. The present study aims to further characterize the immunomodulating action of thyroid-active compounds in trout immune cells. We report here the use of a custom-designed 60-mer oligo immune-targeted microarray for rainbow trout to analyze the gene expression profiles induced in the head kidney by T3 and PTU. Morphometric analyses of the thyroid showed that PTU exposure increased the size of the epithelial cells, whereas T3 induced no significant effects. Both T3 and PTU had diverse and partly contrasting effects on immune transcript profiles. The strongest differential effects of T3 and PTU on gene expressions were those targeting the Mitogen Associated Protein Kinase (MAPK), NFkB, Natural Killer (NK) and Toll-Like Receptor (TLR) pathways, a number of multipath genes (MPG) such as those encoding pleiotropic transcription factors (atf1, junb, myc), as well as important pro-inflammatory genes (tnfa, tnf6, il1b) and interferon-related genes (ifng, irf10). With these results we show for the first time in a fish species that the in vivo thyroidal status modulates a diversity of immune genes and pathways. This knowledge provides the basis to investigate both mechanisms and consequences of thyroid hormone- and thyroid disruptor-mediated immunomodulation for the immunocompetence of fish.

A review on the effects of PBDEs on thyroid and reproduction systems in fish

The objective of this review was to summarize and discuss the effects of Polybrominated diphenyl ethers (PBDEs) on thyroid and reproduction systems in fish. We reviewed the evidences and mechanisms for PBDEs-induced thyroid and reproduction disruption, as well as the cross-talk between the two systems in fish. In thyroid disruption, we mainly paid attention to the effects of PBDEs on hypothalamic-pituitary-thyroid (HPT) axis, thyroid hormones (THs) transport and metabolism, thyroid receptors (TRs) and thyroid follicle histology. In reproduction disruption, we focused on the effects of PBDEs on steroid hormone production, expression of genes involved in steroidogenesis, and gonadal development. Despite that there is an interaction between thyroid and reproductive systems in fish, it is still remains unclear that PBDE-induced reproductive impairments are caused by direct effects on hypothalamic-pituitary-gonadal (HPG) functioning or by indirect action through cross-talk between the two systems. Future studies are needed to explore the relationships between reproductive toxicity and thyroid system disruption after PBDEs exposure.

Microcystin-LR Alters the Gene Transcription and Activities of Iodothyronine Deiodinases in the Hepatic Cells of Grass Carp (Ctenopharyngodon Idella)

This study investigated the gene transcription and activities of iodothyronine deiodinases in the hepatic cell line of grass carp (Ctenopharyngodon idella) exposed in vitro to 1, 10, 100, and 1000 μg /L microcystin-LR (MC-LR) for either 24 or 48 h. The cell viabilities were not significantly affected by MC-LR exposure. The mRNA expressions of type I iodothyronine deiodinase (ID1) and type Ⅱ iodothyronine deiodinase (ID2) reduced after the exposure to MC-LR. However, MC-LR exposure led to the increase in the mRNA expression of type Ⅲ iodothyronine deiodinase (ID3). Moreover, significant ID1 and ID2 activities decline were also observed in the hepatic cell line of grass carp exposed to MC-LR, and the activity of ID3 increased significantly in the MC-LR treated groups. The results suggested that MC-LR could alter the gene transcription or activities of IDs in the hepatic cell line of grass carp.

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.