Environmental salinity selectively modifies the outer-ring deiodinating activity of liver, kidney and gill in the rainbow trout

Reproductive and endocrine-disrupting toxicity of pyrogallol in catfish (Clariasgariepinus)

Endocrine disruptors are synthetic or natural chemicals that can agonize/antagonize hormone receptors or can interfere with the production and secretion of hormones, leading to altered tissue histology and physiology. Pyrogallol is a contaminant widely distributed in aquatic environments that presents health risks to both humans and animals. However, the potential for endocrine disruption by pyrogallol, particularly in fish, are lacking. The purpose of this study was to shed light on how pyrogallol may affect hormone signalling, histopathology, and reproductive outcomes in African catfish Clarias gariepinus. To investigate this, African catfish were exposed to one sublethal concentration of pyrogallol at either 0, 1, 5 or 10 mg/L for 15 days. We then assessed the effects of pyrogallol on the thyroid gland as well as the reproductive system by measuring sex hormone, seminal quality, gonadal histopathology, and histochemistry. Thyroid stimulating hormone and thyroxine showed notable decreases in catfish, and triiodothyronine was decreased with 10 mg/L pyrogallol. Unlike luteinizing hormone, follicle-stimulating hormone was significantly reduced in fish following exposure to pyrogallol relative to controls. Testosterone was also decreased in fish following pyrogallol exposure, whereas 17β-estradiol increased in catfish exposed to pyrogallol. Additionally, in response to pyrogallol toxicity, sperm quality indices, including count, spermatocrit, motility, and sperm viability were adversely affected in a concentration-dependent manner. Pyrogallol exposure also induced several changes in the gonad following exposure to 1, 5, or 10 mg/L. Deformed tubular structures, vacuolation, thickening of the basement membrane, hypertrophy of the seminiferous tubules, intense melanomacrophage localization, spermatozoa loss, and necrosis were all observed in the testes. In the ovary, atretic follicles, deteriorated mature oocytes, degenerated yolk globules, and an increase in perinucleolar oocytes were observed in catfish exposed to pyrogallol. These findings suggest that pyrogallol may act as endocrine disrupting substance in aquatic environments. Further research on the mechanisms by which pyrogallol impairs endocrine systems, particularly in fish, is recommended.

Maternal exposure to dietary uranium causes oxidative stress and thyroid disruption in zebrafish offspring

The contamination of uranium in aquatic ecosystems has raised growing global concern. However, the understanding of its chronic effects on aquatic organisms is limited, particularly with regards to transgenerational toxicity. In this study, we evaluated the maternal transfer risk of uranium using zebrafish. Sexually mature female zebrafish were exposed to 2 and 20 ng/g of uranium-spiked food for 28 days. The induced bioconcentration, thyroid disruption, and oxidative stress in both the adults (F0) and their embryos (F1) were further investigated. Element analysis showed that uranium was present in both F0 and F1, with higher concentrations observed in F1, indicating significant maternal offloading to the offspring. Meanwhile, an increased malformation and decreased swim speed were observed in the F1. Thyroid hormone analysis revealed significant decreases in the levels of triiodothyronine (T3) in both the F0 adults and F1 embryos, but thyroxine (T4) was not significantly affected. Additionally, the activities of antioxidant defenses, including catalase (CAT) and superoxide dismutase (SOD), and the expression of glutathione (GSH) and malondialdehyde (MDA) were significantly altered in the F0 and F1 larvae at 120 hpf. The hypothalamic-pituitary-thyroid (HPT) axis, oxidative stress, and apoptosis-related gene transcription expression were also significantly affected in both generations. Taken together, these findings highlight the importance of considering maternal transfer in uranium risk assessments.

Lycopene supplementation: effects on oxidative stress, sex hormones, gonads and thyroid tissue in tilapia Oreochromis niloticus during Harness® exposure

Harness® is a commercial herbicide that contains acetochlor at a concentration of 84% as an active ingredient. Ubiquitous, persistent, and substantial uses of Harness® in agricultural processes have resulted in the pollution of nearby water sources, posing a threat to various aquatic biotas, including fish. The effects of Harness® toxicity on fish health are little known. So, this study aimed to describe the impact of herbicide Harness® on the oxidative stress and reproductive and thyroid performance of male and female tilapia (Oreochromis niloticus) and also investigate the prospective role of the natural antioxidant lycopene supplementation in dismissing the adverse properties of Harness®. Antioxidant enzyme (catalase, superoxide dismutase, and total antioxidant capacity) and hormone measurements (T, E2, T3, and T4) were carried out, and gonadal and thyroid follicle histological sections were examined as a method to investigate the effects of Harness® toxicity on fish. Male and female tilapia were exposed to 10 μmol/L and 100 μmol/L of Harness® and treated with 10 mg lycopene/kg for 15 days of exposure. Our results demonstrated that the antioxidant enzyme activity was altered by Harness exposure and serum T for both males and females dropped; also, female E2 levels decreased, but male E2 increased. Exposure to higher dose of Harness® induced elevation in both T3 and T4 levels, although the low exposure dose stimulated T4 levels. Harness® exposure prompted histological variations and degenerative changes in testicular, ovarian, and thyroid follicle tissues. Lycopene supplement administration diminished oxidative stress induced by Harness®, alleviating its endocrine disparaging effects by neutralizing T3, T4, T, and E2 and ameliorating the histological structure of gonadal and thyroid tissues. In conclusion, lycopene supplementation was preformed to normalize the alterations and oxidative damage caused by Harness® in Nile tilapia, suggesting that lycopene-supplemented diet functioned as potent antioxidants and had the ability to alleviate oxidative stress and thyroid and reproductive toxicity caused by herbicide Harness®. Moreover, it is crucial to take appropriate care when consuming herbicides to defend the aquatic environment.

Potential Endocrine Disruption of Cyanobacterial Toxins, Microcystins and Cylindrospermopsin: A Review

Microcystins (MCs) and cylindrospermopsin (CYN), although classified as hepatotoxins and cytotoxins, respectively, have been shown to also induce toxic effects in many other systems and organs. Among them, their potential endocrine disruption (ED) activity has been scarcely investigated. Considering the increasing relevance of ED on humans, mammals, and aquatic organisms, this work aimed to review the state-of-the-art regarding the toxic effects of MCs and CYN at this level. It has been evidenced that MCs have been more extensively investigated than CYN. Reported results are contradictory, with the presence or absence of effects, but experimental conditions also vary to a great extent. In general, both toxins have shown ED activity mediated by very different mechanisms, such as estrogenic responses via a binding estrogen receptor (ER), pathological changes in several organs and cells (testis, ovarian cells), and a decreased gonad-somatic index. Moreover, toxic effects mediated by reactive oxygen species (ROS), changes in transcriptional responses on several endocrine axes and steroidogenesis-related genes, and changes in hormone levels have also been reported. Further research is required in a risk assessment frame because official protocols for assessment of endocrine disrupters have not been used. Moreover, the use of advanced techniques would aid in deciphering cyanotoxins dose-response relationships in relation to their ED potential.

Toxicity in Takifugu rubripes exposed to acute ammonia: Effects on immune responses, brain neurotransmitter levels, and thyroid endocrine hormones

Exposure to ammonia can cause convulsions, coma, and death. In this study, we investigate the effects of ammonia exposure on immunoregulatory and neuroendocrine changes in Takifugu rubripes. Fish were sampled at 0, 12, 24, 48, and 96 h following exposure to different ammonia concentrations (0, 5, 50, 100, and 150 mg/L). Our results showed that exposure to ammonia significantly reduced the concentrations of C3, C4, IgM, and LZM whereas the heat shock protein 70 and 90 levels significantly increased. In addition, the transcription levels of Mn-SOD, CAT, GRx, and GR in the liver were significantly upregulated following exposure to low ammonia concertation, however, downregulated with increased exposure time. These findings suggest that ammonia poisoning causes oxidative damage and suppresses plasma immunity. Ammonia exposure also resulted in the elevation and depletion of the T3 and T4 levels, respectively. Furthermore, ammonia stress induced an increase in the corticotrophin-releasing hormone, adrenocorticotropic hormone, and cortisol levels, and a decrease in dopamine, noradrenaline, and 5-hydroxytryptamine levels in the brain, illustrating that ammonia poisoning can disrupt the endocrine and neurotransmitter systems. Our results provide insights into the mechanisms underlying the neurotoxic effects of ammonia exposure, which helps to assess the ecological and environmental health risks of this contaminant in marine fish.

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.

Waterborne uranium causes toxic effect and thyroid disruption in zebrafish larvae

Uranium is a radioactive element that is widely present in aquatic environment. However, limited knowledge is available about the effect of uranium on thyroid system, which plays a key role in the development of animals. In this study, zebrafish embryos were exposed to different environmentally relevant concentrations of uranium (2, 20 and 100 μg/L) for 120 h. The bioaccumulation, developmental toxicities, changes of thyroid hormones (THs) and key genes related to the hypothalamic-pituitary-thyroid (HPT) axis in larvae were analyzed after exposure. Results showed that uranium could bioaccumulate in zebrafish larvae, with the bioconcentration factors ranging from 49.6 to 523. Consequently, significant developmental toxicities and changes in locomotor activities were observed with a concentration-dependent manner. The levels of triiodothyronine (T3) levels in larvae were substantially decreased, whereas those of thyroxine (T4) were increased in fish bodies. The levels of THs were regulated by the negative feedback loops through HPT axis related genes, most of which (NIS, Deio1, Deio2, TRα, TSHβ and UGT1ab) were significantly depressed after exposure to uranium. Our results suggest the potential toxicities and thyroid disruption of uranium on zebrafish, which would provide baseline data set for better understanding the impact of waterborne uranium on aquatic organisms and the associated mechanisms. This study also highlights the key role of thyroid disruption in the ecological risk assessment of uranium pollution.

Toxicokinetics and persistent thyroid hormone disrupting effects of chronic developmental exposure to chlorinated polyfluorinated ether sulfonate in Chinese rare minnow

The abnormality in thyroid hormone modulation in developmental fish, vulnerable to per- and polyfluorinated substances, is of particular concerns for the alternative substances. Juvenile rare minnows, were exposed to chlorinated polyfluoroalkyl ether sulfonates (Cl-PFESAs), the novel alternatives to perfluorooctane sulfonate (PFOS), for 4 weeks followed by 12 weeks of depuration. Half lives were determined to be 33 d, 29 d, and 47 d for total Cl-PFESAs, C8 Cl-PFESA and C10 Cl-PFESA, respectively. Preliminary toxicity test suggested that Cl-PFESAs are moderately toxic to Rare minnow with a LC50 of 20.8 mg/L (nominal concentration） after 96 h of exposure. In the chronic toxicity test, fishes were exposed to Cl-PFESAs at geometric mean measured concentrations of 86.5 μg/L, 162 μg/L and 329 μg/L. In juvenile fishes exposed to Cl-PFESAs for 4 weeks, gene profile sequencing analysis identified 3313 differentially expressed genes, based on which pathways regulating thyroid hormone synthesis and steroid synthesis were enriched. Both whole body total and free 3,5,3'-triiodothyronine (T3) levels were significantly increased. mRNA expression of genes regulating thyroid hormone synthesis (corticotropin-releasing hormone (CRH), thyroid-stimulating hormone (THS), sodium/iodide symporter (NIS), thyroglobulin (TG), and thyroid peroxidase (TPO), transport (transthyretin,TTR), deiodinase (Dio1, Dio2) and receptor (TRα and TRβ) were decreased. Uridinediphosphate glucoronosyl-transferases (UGT1A) gene, regulating THs metabolism, was also decreased. In adult fish, thyroid hormone and genes expression in hypothalamic-pituitary-thyroid axis remained at disturbed levels after 12 weeks of depuration without exposure. Chronic developmental exposure to Cl-PFESAs caused persistent thyroid hormone disrupting effects in fish, highlighting a necessity of comprehensive ecological risk assessment.

Lethal toxicity and gene expression changes in embryonic zebrafish upon exposure to individual and mixture of malathion, chlorpyrifos and lambda-cyhalothrin

Pesticides are usually present as mixtures in water environments. Evaluating the toxic effects of individual pesticide may not be enough for protecting ecological environment due to interactions among substances. In this study, we aimed to examine the lethal doses and gene expression changes in zebrafish (Danio rerio) upon exposure to individual and mixture pesticides [malathion (MAL), chlorpyrifos (CHL) and lambda-cyhalothrin (LCY)]. Individual pesticide toxicity evaluation manifested that the toxicity of the three pesticides to D. rerio at various developmental stages (embryonic, larval, juvenile and adult stages) followed the order of LCY > CHL > MAL. On the contrary, the least toxicity to the animals was discovered from MAL. Most of the tested pesticides displayed lower toxicities to the embryonic stage compared with other life stages of zebrafish. Synergistic effects were monitored from two binary mixtures of LCY in combination with MAL or CHL and ternary mixture of MAL + CHL + LCY. The expressions of 16 genes involved in oxidative stress, immunity system, cell apoptosis and endocrine disruption at the mRNA level revealed that embryonic zebrafish were influenced by the individual or mixture pesticides. The expressions of Tnf, P53, TRα, Crh and Cyp19a exerted greater variations upon exposure to pesticide mixtures compared with their individual compounds. Collectively, the transcriptional responses of these genes might afford early warning biomarkers for identifying pollutant exposure, and the data acquired from this study provided valuable insights into the comprehensive toxicity of pesticide mixtures to zebrafish.

Exposure to nitrite alters thyroid hormone levels and morphology in Takifugu rubripes

Nitrite (NO^2-) can act as a toxic nitrogenous compound with the potential to disrupt endocrine systems in fish. The aim of the present study was to investigate the effects of nitrite on the thyroid endocrine system of Takifugu rubripes. Fish were exposed to 0, 0.5, 1, 3, and 6 mM nitrite concentrations. Blood was collected to assay the concentrations of thyroid-stimulating hormone (TSH), thyroxine (T₄), triiodothyronine (T₃), free thyroxine (FT₄), free triiodothyronine (FT₃), and 3,3,5'-triiodothyronine (rT₃), as well as the activity of iodothyronine deiodinases (Dio1, Dio2, and Dio3,) after 0, 12, 24, 48, and 96 h of exposure to nitrite. The first branchial arch to the third branchial arch of T. rubripes were sampled and fixed, and thyroid morphology was observed. The results showed that exposure to nitrite significantly increased the concentrations of TSH, T₃, FT₃, and reduced the concentrations of T₄, FT₄, and rT₃. The activity of Dio1 and Dio2 increased significantly, whereas Dio3 activity decreased significantly. Additionally, thyroid follicles degenerated and became blurred and most colloid material disappeared 96 h after exposure to high nitrite concentrations. Based on these results, high nitrite concentration exposure can disturb thyroid hormone homeostasis, alter thyroid follicle morphology, and result in thyroid endocrine toxicity.

Salinity reduction benefits European eel larvae: Insights at the morphological and molecular level

European eel (Anguilla anguilla) is a euryhaline species, that has adapted to cope with both, hyper- and hypo-osmotic environments. This study investigates the effect of salinity, from a morphological and molecular point of view on European eel larvae reared from 0 to 12 days post hatch (dph). Offspring reared in 36 practical salinity units (psu; control), were compared with larvae reared in six scenarios, where salinity was decreased on 0 or 3 dph and in rates of 1, 2 or 4 psu/day, towards iso-osmotic conditions. Results showed that several genes relating to osmoregulation (nkcc2α, nkcc2β, aqp1dup, aqpe), stress response (hsp70, hsp90), and thyroid metabolism (thrαA, thrαB, thrβB, dio1, dio2, dio3) were differentially expressed throughout larval development, while nkcc1α, nkcc2β, aqp3, aqp1dup, aqpe, hsp90, thrαA and dio3 showed lower expression in response to the salinity reduction. Moreover, larvae were able to keep energy metabolism related gene expression (atp6, cox1) at stable levels, irrespective of the salinity reduction. As such, when reducing salinity, an energy surplus associated to reduced osmoregulation demands and stress (lower nkcc, aqp and hsp expression), likely facilitated the observed increased survival, improved biometry and enhanced growth efficiency. Additionally, the salinity reduction decreased the amount of severe deformities such as spinal curvature and emaciation but also induced an edematous state of the larval heart, resulting in the most balanced mortality/deformity ratio when salinity was decreased on 3 dph and at 2 psu/day. However, the persistency of the pericardial edema and if or how it represents an obstacle in further larval development needs to be further clarified. In conclusion, this study clearly showed that salinity reduction regimes towards iso-osmotic conditions facilitated the European eel pre-leptocephalus development and revealed the existence of highly sensitive and regulated osmoregulation processes at such early life stage of this species.

Temperature induced variation in gene expression of thyroid hormone receptors and deiodinases of European eel (Anguilla anguilla) larvae

Thyroid hormones (THs) are key regulators of growth, development, and metabolism in vertebrates and influence early life development of fish. TH is produced in the thyroid gland (or thyroid follicles) mainly as T4 (thyroxine), which is metabolized to T3 (3,5,3'-triiodothyronine) and T2 (3,5-diiodothyronine) by deiodinase (DIO) enzymes in peripheral tissues. The action of these hormones is mostly exerted by binding to a specific nuclear thyroid hormone receptor (THR). In this study, we i) cloned and characterized thr sequences, ii) investigated the expression pattern of the different subtypes of thrs and dios, and iii) studied how temperature affects the expression of those genes in artificially produced early life history stages of European eel (Anguilla anguilla), reared in different thermal regimes (16, 18, 20 and 22 °C) from hatch until first-feeding. We identified 2 subtypes of thr (thrα and thrβ) with 2 isoforms each (thrαA, thrαB, thrβA, thrβB) and 3 subtypes of deiodinases (dio1, dio2, dio3). All thr genes identified showed high similarity to the closely related Japanese eel (Anguilla japonica). We found that all genes investigated in this study were affected by larval age (in real time or at specific developmental stages), temperature, and/or their interaction. More specifically, the warmer the temperature the earlier the expression response of a specific target gene. In real time, the expression profiles appeared very similar and only shifted with temperature. In developmental time, gene expression of all genes differed across selected developmental stages, such as at hatch, during teeth formation or at first-feeding. Thus, we demonstrate that thrs and dios show sensitivity to temperature and are involved in and during early life development of European eel.

Acute nitrite exposure alters the metabolism of thyroid hormones in grass carp (Ctenopharyngodon idellus)

Nitrite has the potential to disturb thyroid hormone homeostasis, but little is known about the underlying mechanisms. In the present study, juvenile grass carp (Ctenopharyngodon idellus) were exposed to various concentrations of nitrite (0, 0.5, 1, 4, and 16 mg/L, respectively). Serum concentrations of triiodothyronine (T3), thyroxine (T4), free triiodothyronine (FT3), free thyroxine (FT4), 3,3,5'-triiodothyronine (rT3), thyroid-stimulating hormone (TSH), and the activity of iodothyronine deiodinases were assayed at 0, 12, 24, 48, and 96 h after exposure. It was found that acute nitrite exposure significantly altered the TH levels and iodothyronine deiodinase activities. The rT3 levels were significantly increased in the treatment groups, whereas the concentrations of T3, FT3, FT4, and TSH decreased significantly. The concentration of T4 was elevated in the lower-dose exposure group, but was reduced in the higher-dose exposure group. Increases in type I iodothyronine deiodinase (ID1) and type III iodothyronine deiodinase (ID3) activities were observed in the exposure groups. The activity of type II iodothyronine deiodinase (ID2) decreased at 12 and 24 h after exposure. A decrease of colloid in the thyroid follicles was observed in the exposure group. The results indicate that acute nitrite exposure has the potential to disturb the homeostasis of thyroid hormone metabolism, leading to a hypothyroidism state in the juvenile grass carp.

NFAT5 genes are part of the osmotic regulatory system in Atlantic salmon (Salmo salar)

The anadromous Atlantic salmon utilizes both fresh and salt water (FW and SW) habitats during its life cycle. The parr-smolt transformation (PST) is an important developmental transition from a FW adapted juvenile parr to a SW adapted smolt. Physiological changes in osmoregulatory tissues, particularly the gill, are key in maintaining effective ion regulation during PST. Changes are initiated prior to SW exposure (preparative phase), and are completed when smolts enter the sea (activational phase) where osmotic stress may directly stimulate changes in gene expression. In this paper we identify 4 nuclear factor of activated T cells (NFAT5, an osmotic stress transcription factor) paralogues in Atlantic salmon, which showed strong homology in characterized functional domains with those identified in other vertebrates. Two of the identified paralogues (NFAT5b1 and NFAT5b2) showed increased expression following transfer from FW to SW. This effect was largest in parr that were maintained under short day photoperiod, and showed the highest increases in chloride ion levels in response to SW exposure. The results of this study suggest that NFAT5 is involved in the osmotic stress response of Atlantic salmon.

Effect of acetochlor on transcription of genes associated with oxidative stress, apoptosis, immunotoxicity and endocrine disruption in the early life stage of zebrafish

The study presented here aimed to characterize the effects of acetochlor on expression of genes related to endocrine disruption, oxidative stress, apoptosis and immune system in zebrafish during its embryo development. Different trends in gene expression were observed after exposure to 50, 100, 200μg/L acetochlor for 96h. Results demonstrated that the transcription patterns of many key genes involved in the hypothalamic-pituitary-gonadal/thyroid (HPG/HPT) axis (e.g., VTG1, ERβ1, CYP19a and TRα), cell apoptosis pathway (e.g., Bcl2, Bax, P53 and Cas8), as well as innate immunity (e.g., CXCL-C1C, IL-1β and TNFα) were affected in newly hatched zebrafish after exposure to acetochlor. In addition, the up-regulation of CAT, GPX, GPX1a, Cu/Zn-SOD and Ogg1 suggested acetochlor might trigger oxidative stress in zebrafish. These finding indicated that acetochlor could simultaneously induce multiple responses during zebrafish embryonic development, and bidirectional interactions among oxidative stress, apoptosis pathway, immune and endocrine systems might be present.

Microcystin-RR exposure results in growth impairment by disrupting thyroid endocrine in zebrafish larvae

Recent studies have shown that cyanobacteria-derived microcystins (MCs) have the potential to disrupt endocrine systems. However, the effects of microcystin-RR (MC-RR) and their underlying mechanisms are poorly resolved in fish. In this study, MC-RR exposure through submersion caused serious developmental toxicity, such as growth delay and depressed heart rates in zebrafish larvae. We also detected decreased levels of thyroid hormones (THs), suggesting that MC-RR-triggered thyroid endocrine disruption might contribute to the growth impairment observed in developing zebrafish. To further our understanding of mechanisms of MC-RR-induced endocrine toxicity, quantitative real-time PCR (QPCR) analysis was performed on hypothalamic-pituitary-thyroid (HPT) axis related genes, i.e., corticotropin-releasing factor (CRF), thyroid-stimulating hormone (TSH), sodium/iodide symporter (NIS), thyroglobulin (TG), thyroid receptors (TRα and TRβ) and iodothyronine deiodinases (Dio1 and Dio2), of developing zebrafish embryos exposed to 0, 0.3, 1.0 or 3.0mgL(-1) MC-RR until 96h post-fertilization. Our results showed that transcription pattern of HPT axis related genes were greatly changed by MC-RR exposure, except TG gene. Furthermore, western blot was used to validate the results of gene expression. The results showed protein synthesis of TG was not affected, while that of NIS was significantly up-regulated, which are in accordance with gene expression. The overall results indicated that exposure to MC-RR can induce developmental toxicity, which might be associated with thyroid endocrine disruption in developing zebrafish larvae.

Relative developmental toxicities of pentachloroanisole and pentachlorophenol in a zebrafish model (Danio rerio)

Pentachloroanisole (PCA) and pentachlorophenol (PCP) are chlorinated aromatic compounds that have been found in the environment and in human populations. The objective of this study is to characterize the effects of PCA in comparison to those of PCP on development at environmental relevant levels using a fish model. Zebrafish embryos were exposed to 0.1, 1, 10, 100, 500, 1000 μg/L PCA and PCP respectively for 96 h. Malformation observation, LC50 testing for survival rate at 96 hours post fertilization (hpf) and EC50 testing for hatching rate at 72 hpf indicated that the developmental toxicity of PCP was about 15 times higher than that of PCA. PCP exposure at 10 μg/L resulted in elevated 3, 3', 5-triiodothyronine (T3) levels and decreased thyroxine (T4) levels, whereas PCA had no effects on T3 or T4 levels. PCP and PCA exposure at 1 and 10 μg/L showed possible hyperthyroid effects similar to that of T3, due to increased relative mRNA expression of synapsin I (SYN), iodothyronine deiodinase type III (Dio3), thyroid hormone receptor alpha a (THRαa) and thyroid hormone receptor beta (THRβ), and decreased expression of iodothyronine deiodinase type II (Dio2). The results indicate that both PCA and PCP exposure can cause morphological deformities, possibly affect the timing and coordination of development in the central nervous system, and alter thyroid hormone levels by disrupting thyroid hormone regulating pathways. However, the developmental toxicity of PCA is at least ten times lower than that of PCP. Our results on the relative developmental toxicities of PCA and PCP and the possible underlying mechanisms will be useful to support interpretation of envrionmental concentrations and body burden levels observed in human populations.

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.

Effects of acclimation salinity on the expression of selenoproteins in the tilapia, Oreochromis mossambicus

Selenoproteins are ubiquitously expressed, act on a variety of physiological redox-related processes, and are mostly regulated by selenium levels in animals. To date, the expression of most selenoproteins has not been verified in euryhaline fish models. The Mozambique tilapia, Oreochromis mossambicus, a euryhaline cichlid fish, has a high tolerance for changes in salinity and survives in fresh water (FW) and seawater (SW) environments which differ greatly in selenium availability. In the present study, we searched EST databases for cichlid selenoprotein mRNAs and screened for their differential expression in FW and SW-acclimated tilapia. The expression of mRNAs encoding iodothyronine deiodinases 1, 2 and 3 (Dio1, Dio2, Dio3), Fep15, glutathione peroxidase 2, selenoproteins J, K, L, M, P, S, and W, was measured in the brain, eye, gill, kidney, liver, pituitary, muscle, and intraperitoneal white adipose tissue. Gene expression of selenophosphate synthetase 1, Secp43, and selenocysteine lyase, factors involved in selenoprotein synthesis or in selenium metabolism, were also measured. The highest variation in selenoprotein and synthesis factor mRNA expression between FW- and SW-acclimated fish was found in gill and kidney. While the branchial expression of Dio3 was increased upon transferring tilapia from SW to FW, the inverse effect was observed when fish were transferred from FW to SW. Protein content of Dio3 was higher in fish acclimated to FW than in those acclimated to SW. Together, these results outline tissue distribution of selenoproteins in FW and SW-acclimated tilapia, and indicate that at least Dio3 expression is regulated by environmental salinity.

Effects on development, growth responses and thyroid-hormone systems in eyed-eggs and yolk-sac larvae of Atlantic salmon (Salmo salar) continuously exposed to 3,3',4,4'-tetrachlorobiphenyl (PCB-77)

Thyroid hormones (triiodothyronine, T3; and thyroxine, T4) play significant roles in development, metamorphosis, metabolism, homeostasis, cellular proliferation, and differentiation, for which the effects are mediated through thyroid hormone receptors (TRα and TRβ). Similarly, the insulin-like growth factor (IGF) is involved in growth and development through regulation of somatic growth. This study was designed to examine the effects of the dioxin-like 3,3',4,4'-tetrachlorobiphenyl (PCB-77) on responses related to growth and thyroid hormone system in eyed eggs and yolk-sac larvae of Atlantic salmon. Salmon eggs were continuously exposed to two waterborne concentrations of PCB-77 (1 or 10 ng/L) over a period of 50 d covering hatching and through yolk-sac absorption stages. Sampling was performed regularly throughout the exposure period and at different time intervals. Gene expression patterns were performed on whole-body homogenate at age 500, 548, 632, 674, and 716 dd (dd: day degrees) using quantitative polymerase chain reaction (PCR). Total T3 (TT3) and total T4 (TT4) were measured using radioimmunoassay (RIA). Data showed that 10 ng PCB-77 increased dioiodinase 2 (Dio2) at 500 dd and both PCB-77 concentrations decreased dio2 expression at 548 dd. PCB-77 elevated cellular TT3 at 500 dd and was lowered at 548 dd only at 10 ng. Otherwise, time-related reduction was not affected by PCB-77 exposure as observed for the rest of the exposure period. For TT4, 1 ng PCB-77 produced a rise at 500 dd, and an apparent concentration decrease at 548 dd, before a total inhibition at 632 dd. The IGF-1 and IGF-1R were variably affected by PCB-77. For IGF-2, PCB-77 produced a concentration-dependent increase at 548 dd, and thereafter an elevation (1 ng) and fall (10 ng) at 632 dd. TRβ mRNA demonstrated PCB-77 related increases during the exposure period, and this effect returned to control levels at 716 dd. For TRα, a rise was noted only after exposure to 10 ng PCB-77 at 500 dd. Overall, the present study demonstrates some possible growth and developmental consequences following exposure to PCB-77 during early life stages of Atlantic salmon.

Endocrine and developmental effects in Atlantic salmon (Salmo salar) exposed to perfluorooctane sulfonic or perfluorooctane carboxylic acids

In this study, we have investigated the effect of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) on endocrine signalling, growth and development in Atlantic salmon (Salmo salar) embryos and larvae. Expression of genes related to the hypothalamic-pituitary-thyroid (HPT) axis, growth-hormone/insulin-like growth factor (GH/IGF) axis and the steroid hormone axis were used as indicators of endocrine disruption. We also studied bone development in larvae, both by observing skeletal structure formation and by investigating expression of genes involved in ossification process. Atlantic salmon embryos, kept in plastic tanks at 5-7°C, were exposed to 100 μg/L PFOA or PFOS from egg stage for a period of 52 days, followed by one-week recovery period. Sampling was performed at day 21, 35, 49 and 56 representing age 549, 597, 679 and 721 dd (dd or day degrees = number of days × temperature in degree Celsius:°C). Note that day 56 or 721 dd is the end of the 1-week recovery period. Larvae were divided into designated head and body regions for the purpose of gene expression analysis, except for genes that regulate ossification that were analyzed in whole larvae. Expression of thyroid receptor α and β (TRα and TRβ), thyroid-stimulating hormone β (TSHβ), T(4) outer-ring deiodinase (T(4)ORD), growth hormone (GH), insulin-like growth factor-I and II (IGF-I and II), insulin-like growth factor I receptor (IGF-IR), and estrogen receptor α and β (ERα and ERβ) were investigated using quantitative PCR. Both PFOS and PFOA exposure produced non-significant alterations in larvae weight (except after the recovery period when a decrease was observed), while larvae length was unaffected. PFOS and PFOA exposure produced body- and head region-specific alterations in expression of all the investigated gene transcripts. Expression of IGF-I and IGF-IR paralleled that of GH, indicating that perturbation of GH expression is a possible end point for disruption of the GH-IGF axis. We did not observe developmental changes related to angiogenesis, ossification and chondrogenesis after exposure to PFOS and PFOA. Transcriptional abnormalities may serve as indicators of chronic exposure, although the concrete mechanisms causing the observed effects remain ambiguous. The implications of these findings for the complete lifecycle, including other developmental and/or reproductive damage, are areas of future study.

Kinetics and thiol requirements of iodothyronine 5'-deiodination are tissue-specific in common carp (Cyprinus carpio L.)

Iodothyronine deiodinases determine the biological activity of thyroid hormones. Despite the homology of the catalytic sites of mammalian and teleostean deiodinases, in-vitro requirements for the putative thiol co-substrate dithiothreitol (DTT) vary considerably between vertebrate species. To further our insights in the interactions between the deiodinase protein and its substrates: thyroid hormone and DTT, we measured enzymatic iodothyronine 5'-deiodination, Dio1 and Dio2 mRNA expression, and Dio1 affinity probe binding in liver and kidney preparations from a freshwater teleost, the common carp (Cyprinus carpio L.). Deiodination rates, using reverse T3 (rT3, 3,3',5'-triiodothyronine) as the substrate, were analysed as a function of the iodothyronine and DTT concentrations. In kidney rT3 5'-deiodinase activity measured at rT3 concentrations up to 10 μM and in the absence of DTT does not saturate appreciably. In the presence of 1mM DTT, renal rT3 deiodination rates are 20-fold lower. In contrast, rT3 5'-deiodination in liver is potently stimulated by 1mM DTT. The marked biochemical differences between 5'-deiodination in liver and kidney are not associated with the expression of either Dio1 or Dio2 mRNA since both organs express both deiodinase types. In liver and kidney, DTT stimulates the incorporation of N-bromoacetylated affinity labels in proteins with estimated molecular masses of 57 and 55, and 31 and 28 kDa, respectively. Although primary structures are highly homologous, the biochemistry of carp deiodinases differs markedly from their mammalian counterparts.

The role of thyroid hormones in stress response of fish

Thyroxine (T(4)) and triiodothyronine (T(3)), the principal thyroid hormones (THs) secreted from the hypothalamic-pituitary-thyroid (HPT) axis, produce a plethora of physiologic actions in fish. The diverse actions of THs in fishes are primarily due to the sensitivity of thyroid axis to many physical, chemical and biological factors of both intrinsic and extrinsic origins. The regulation of THs homeostasis becomes more complex due to extrathyroidal deiodination pathways by which the delivery of biologically active T(3) to target cells has been controlled. As primary stress hormones and the end products of hypothalamic-pituitary-interrenal (HPI) and brain-sympathetic-chromaffin (BSC) axes, cortisol and adrenaline exert its actions on its target tissues where it promote and integrate osmotic and metabolic competence. Despite possessing specific osmoregulatory and metabolic actions at cellular and whole-body levels, THs may fine-tune these processes in accordance with the actions of hormones like cortisol and adrenaline. Evidences are presented that THs can modify the pattern and magnitude of stress response in fishes as it modifies either its own actions or the actions of stress hormones. In addition, multiple lines of evidence indicate that hypothalamic and pituitary hormones of thyroid and interrenal axes can interact with each other which in turn may regulate THs/cortisol-mediated actions. Even though it is hard to define these interactions, the magnitude of stress response in fish has been shown to be modified by the changes in the status of THs, pointing to its functional relationship with endocrine stress axes particularly with the interrenal axis. The fine-tuned mechanism that operates in fish during stressor-challenge drives the THs to play both fundamental and modulator roles in stress response by controlling osmoregulation and metabolic regulation. A major role of THs in stress response is thus evident in fish.

Effects of metolachlor on transcription of thyroid system-related genes in juvenile and adult Japanese medaka (Oryzias latipes)

Metolachlor (MT) is one of the most important pesticides applied to corn and other crops for controlling broadleaf and grass weeds. However, the effects of MT on the thyroid system in fish remain to be elucidated. In the present experiment, transcription of genes related to the thyroid system, including thyrotropin releasing hormone (Trh), deiodinase 2 (Dio2), thyroid hormone receptor α (Thrα), and thyroid hormone receptor β (Thrβ), were induced by MT in a sex-, developmental stage-, and tissue- specific manner when medaka were exposed to various concentrations of MT for 14 days. The transcriptional levels of the genes were only significantly altered in both juvenile and adult female medaka in response to MT exposure. And the lowest concentrations able to significantly induce transcription of the selected genes were 10 and 100 μg/L in juvenile and adult female medaka, respectively. In adult female medaka, a significant up-regulation of these genes was detected only in the brain, with little or no effect in the liver. Furthermore, MT-induced (100 μg/L) transcription of thyroid system-related genes was enhanced significantly in male juvenile medaka in the presence of estrogen (E2) (50 and 100 ng/L). Moreover, the mRNA levels of Thrα and Thrβ in males increase with the combined treatments of 100 μg/L MT and 100 ng/L E2. Dio2 increased when exposed to 100 μg/L MT and 50 or 100 ng/L E2. The information obtained in the present study suggests that MT has the potential to influence several steps of the hypothalamus-pituitary-thyroid (HPT) axis homeostasis and to disrupt the thyroid system in medaka.

Regulation of thyroid hormone related genes mRNA expression by exogenous T₃ in larvae and adult Chinese rare minnow (Gobiocypris rarus)

In this study, the expression time and profiles of thyroid hormone receptor alpha (trα), type I and II deiodinase enzymes (d1 and d2), transthyretin (ttr), sodium iodide symporter (nis), and thyrotropin-releasing hormone receptor (trhr) genes in Chinese rare minnow (Gobiocypris rarus) were determined using real-time PCR. Meanwhile, the changes of these genes were investigated by exogenous T(3) (3.8 nM) in larvae and adult fish. The retardation of swim bladder development and growth inhibition were observed for larvae, and the transcription of trα, d1, d2, nis, and trhr was significantly down-regulated at the end of exposure (21 d). In adults, a down-regulation of trα, d1, nis, and trhr mRNA levels occurred at 7th or 14th day of exposure, but returned back to their normal levels similar to control at the end of exposure. The down-regulation of gene mRNA expression could serve as a compensatory mechanism for the activation of thyroid system.

Exposure to DE-71 alters thyroid hormone levels and gene transcription in the hypothalamic-pituitary-thyroid axis of zebrafish larvae

Polybrominated diphenyl ethers (PBDEs) have the potential to disrupt thyroid hormone homeostasis, but the molecular mechanisms underlying this process have not yet been clarified. In the present study, zebrafish (Danio rerio) embryos were exposed to a low concentration (0, 1, 3, and 10microg/L) of DE-71 from fertilization to 14 days thereafter. The whole-body content of thyroid hormone and transcription of genes in the hypothalamic-pituitary-thyroid (HPT) axis were analyzed. Exposure to up to 10microg/L of DE-71 significantly reduced thyroxine (T4) levels and significantly upregulated the transcription of corticotrophin-releasing hormone (CRH) and thyroid-stimulating hormone (TSHbeta) genes in a concentration-dependent manner. The transcription of genes involved in the synthesis of TH proteins, sodium/iodide symporter (Slc5a5), and thyroglobulin (TG) and the transcription of marker genes associated with early thyroid development (Pax8 and Nkx2.1) were significantly upregulated upon DE-71 exposure. The expression of thyronine deiodinase (Deio1 and Deio2) mRNAs was also significantly upregulated, possibly as a compensatory response to the decreased T4 levels. However, DE-71 exposure resulted in the downregulation of transthyretin (TTR) gene transcription and did not affect the transcription of thyroid hormone receptors (TRs). Exposure to DE-71 significantly induced the transcription of the uridinediphosphate-glucuronosyltransferase (UGT1ab) gene. The results of our study confirmed the reliability of the zebrafish larvae as models for assessment of the developmental toxicity of PBDEs and transcription of genes of the HPT axis can evaluate the potential mechanisms of thyroid disruption.

Co-ordination of osmotic stress responses through osmosensing and signal transduction events in fishes

This review centres upon the molecular regulation of osmotic stress responses in fishes, focusing on how osmosensing and signal transduction events co-ordinate changes in the activity and abundance of effector proteins during osmotic stress and how these events integrate into osmotic stress responses of varying magnitude. The concluding sections discuss the relevance of osmosensory signal transduction to the evolution of euryhalinity and present experimental approaches that may best stimulate future research. Iterating the importance of osmosensing and signal transduction during fish osmoregulation may be pertinent amidst the increased use of genomic technologies that typically focus solely on changes in the abundances of gene products, and may limit insight into critical upstream events that occur mainly through post-translational mechanisms.

Effects of exposure to acetochlor on the expression of thyroid hormone related genes in larval and adult rare minnow (Gobiocypris rarus)

Acetochlor is a commonly used herbicide in agricultural environments, which can accelerate T(3)-induced metamorphosis in amphibians. However, effects of acetochlor on the thyroid system in fish are still unclear at environmentally relevant concentrations. In this study, expression profiles of thyroid hormone receptor alpha (tralpha), deiodinase (d1 and d2), malic enzyme (me), and sodium iodide symporter (nis) genes were determined in larval and adult rare minnow (Gobiocypris rarus) after exposure to different levels of acetochlor (20, 200, and 2000ng/l) for 21 days, respectively. Furthermore, plasma thyroid hormones (THs) levels in adults were evaluated simultaneously. The results showed that d1, d2, me, and nis mRNA levels were significantly down-regulated in the larvae. Similar expression changes of these genes in female brains were found. However, in adult livers, these gene expressions had no significantly effects, except the tralpha mRNA level significantly up-regulated. These results indicated that exposure to acetochlor could result in tissue-specific alternative expression of TH-related genes in adults. Moreover, the expression of d2 and me showed a positive correlation with plasma T(4) levels in female brains. Therefore, larval development and adult brain of rare minnow could be affected by acetochlor at environmentally relevant concentrations.

Changes of thyroid hormone levels and related gene expression in Chinese rare minnow (Gobiocypris rarus) during 3-amino-1,2,4-triazole exposure and recovery

Thyroid hormones (THs) play an important role in the development and metabolism of fish through their influences on genetic transcription and are targets for endocrine disruptive agents in the aquatic environment. Amitrole is a pesticide potentially interfering with thyroid hormone regulation. In this study, the rare minnow (Gobiocypris rarus) was exposed to different levels of 3-amino-1,2,4-triazole (amitrole) and allowed to recover in clean water. Plasma TH levels and the expression of TH-related genes, including transthyretin (ttr), deiodinases (d1 and d2), and the thyroid hormone receptor (tralpha) from the livers and brains were evaluated. After exposure, the plasma TH levels did not change. Histopathological observations showed that livers were degenerated at 10,000 ng/l and these damages could be recovered by the withdrawal of amitrole. However, the ttr, d1, and d2 mRNA levels in the livers of males were significantly up-regulated in all exposure groups (p<0.05). The ttr and d2 mRNA levels were significantly up-regulated at 10,000 ng/l and 10, 100, and 1000 ng/l in the livers of females, respectively (p<0.05). In the brains of males, a twofold increase of d2 mRNA levels at > or = 100 ng/l and a fivefold decrease of tralpha mRNA levels at > or = 10 ng/l were observed (p<0.05), whereas no significant differences were observed in the expression of d2 and tralpha in the brains of females. After a recovery period, the ttr, d1, and d2 mRNA levels in the livers of males returned to control levels, but the tralpha mRNA levels were irreversibly decreased at all treatments (p<0.05). In addition, the d2 mRNA levels in the livers of females were significantly induced at > or = 100 ng/l. Moreover, the d2 mRNA levels in the brains of males and females were up-regulated at 10,000 ng/l. These results indicated that amitrole exposure could result in alternations of ttr, d1, d2, and tralpha gene expression in different tissues of the rare minnow. The expression of these TH-related genes in males was more sensitive to amitrole than those of females. Recovery in clean water was associated with the selective regulation of TH-related gene transcription in the rare minnow. Therefore, these TH-related genes can serve as biomarkers to screen the effects of thyroid disruption chemicals in rare minnow.

The involvement of thyroid hormones and cortisol in the osmotic acclimation of Solea senegalensis

The peripheral conversion of the prohormone 3,5,3',5'-tetraiodothyronine (T4) to the biologically active 3,5,3'-triiodothyronine (T3), via enzymatic deiodination by deiodinases, is an important pathway in thyroid hormone metabolism. The aim of this study was to test if thyroid hormones and cortisol, as well as the outer ring deiodination (ORD) metabolic pathway, are involved in the osmoregulatory response of Senegalese sole (Solea senegalensis, Kaup 1858). We measured osmoregulatory and endocrine parameters in immature juveniles S. senegalensis acclimated to seawater (SW, 38 per thousand) and that were transferred and allowed to acclimate to different salinities (5 per thousand, 15 per thousand, 38 per thousand and 55 per thousand) for 17 days. An adjustment and a chronic regulatory period were identified following acclimation. The adjustment period immediately follows the transfer, and is characterized by altered plasma osmolalities. During this period, plasma cortisol levels increased while plasma free T4 (fT4) levels decreased. Both hormones levels returned to normal values on day 3 post-transfer. In the adjustment period, renal and hepatic ORD activities had increased concomitantly with the decrease in plasma fT4 levels in fishes transferred to extreme salinities (5 per thousand and 55 per thousand). In the chronic regulatory period, where plasma osmolality returned to normal values, plasma cortisol had increased, whereas plasma fT4 levels decreased in animals that were transferred to salinities other than SW. No major changes were observed in branchial ORD activity throughout the experiment. The inverse relationship between plasma cortisol and fT4 suggests an interaction between these hormones during both osmoregulatory periods while ORD pathway can be important in the short-term adjustment period.

Functional identification of an osmotic response element (ORE) in the promoter region of the killifish deiodinase 2 gene (FhDio2)

The physiological role played by thyroid hormones (TH) in hydro-osmotic homeostasis in fish remains a controversial issue. Previous studies have shown that in Fundulus heteroclitus (killifish) hypo-osmotic stress increases liver iodothyronine deiodinase type 2 (D2) mRNA and D2 activity. In this study we identified two conserved osmotic response element (ORE) motifs in the promoter region of the killifish D2 gene (FhDio2) and examined their possible role in the transcriptional regulation of FhDio2 during hypo-osmotic stress. As assessed by the electrophoretic mobility shift assay, results from in vivo and in vitro experiments demonstrate that exposure to an abrupt hyposmotic challenge triggers in the liver of killifish a strong nuclear recruitment of a putative osmotic response element binding protein (OREBP). This protein-DNA binding is time-dependent, attains a maximum within 2-8 h after the osmotic stress, and is followed by a significant increase in D2 activity. Furthermore, protein-DNA binding and the subsequent elevation in enzyme activity were blocked by the tyrosine kinase inhibitor genistein. Thus, during hypo-osmotic stress, a putative OREBP kinase-activated pathway stimulates FhDio2 transcription and enzymatic activity. These data and the fact that D2 is the major enzyme providing local intracellular T(3) suggest that TH plays a direct role in osmoregulation in fish, possibly by participating in hepatic ammonia metabolism. This study provides important insight into the physiological role of TH in hydro-osmotic homeostasis in fish.

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.

The effect of 3,5,3'-triiodothyronine supplementation on zebrafish (Danio rerio) embryonic development and expression of iodothyronine deiodinases and thyroid hormone receptors

The importance of thyroid hormones (TH) for embryonic development has long been shown in many vertebrates, but is not yet established in pre-hatch teleost models despite the presence of TH, TH receptors and iodothyronine deiodinases. Lack of data about the dynamics of TH metabolism in embryonic stages of fish does not allow to speculate about the involvement and/or role of TH in fish embryonic development. We therefore set up an experiment to examine the effect of 3,5,3'-triiodothyronine (T(3)) on zebrafish (Danio rerio) embryonic development and on the expression of some thyroid hormone-regulated genes as measured by real-time PCR. Maternally derived thyroxine (T(4)) and T(3) were detected throughout embryonic development and total levels remained stable. Thyroid hormone receptor (TR) alpha and beta mRNA were found to be present in early embryos. After an initial fall, TRalpha mRNA levels in the control group increased gradually from 12h post fertilization (HPF) onwards. TRbeta mRNA levels of control embryos were relatively stable during embryonic development, but increased around the hatching period. We also quantified type I (D1) and type II (D2) deiodinase mRNA expression in zebrafish embryos. D1 mRNA levels in the control group gradually increased during development while D2 levels were kept at a low and stable level until hatching. At 75 HPF, a fivefold increase of D2 expression was observed compared to embryonic levels. Exogenous T(3) added to the water (5nM) was taken up by the embryos, causing increased pigmentation and accelerated hatching. T(3) treatment significantly up regulated TRalpha mRNA levels at 48 HPF, while D2 mRNA was significantly down regulated at 75 HPF. Neither TRbeta nor D1 mRNA levels seemed responsive to the treatment. Taken together, these data show that during embryonic development zebrafish already have the necessary regulatory machinery for TH activation and signaling, and that T(3) treatment at that stage indeed influence embryonic development.

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.

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.

Effects of iodothyronines on the hepatic outer-ring deiodinating pathway in killifish

Substrate availability has been thought to be a major regulator of the outer-ring deiodinating pathway (ORD) in fish. However, current information strongly suggests that while fish iodothyronine deiodinase type 2 (D2) responds to iodothyronines in the same manner as its mammalian counterpart, fish deiodinase type 1 (D1) exhibits a distinct response. Furthermore, 3,5-T2, generally considered to be an inactive product of iodothyronine metabolism, has recently been described as bioactive, but its effects upon D1 and D2 are not yet known. We examined the effect that short-term immersion in T4, T3, and 3,5-T2 (0.1 microM; 12 or 24 h) exerts on both D1 and D2 activities and on the levels of expression of D1 and D2 mRNAs in killifish liver. In agreement with previous reports in teleosts, no iodothyronine exerted a significant effect on D1 enzymatic activity. However, all three iodothyronines significantly decreased D2 activity. Furthermore, at 24 h post-immersion T4, T3, and 3,5-T2 inhibited both D1 and D2 transcription. Together, the present results confirm the differential effect of iodothyronines upon the hepatic ORD pathway in fish and show that this effect can occur at a transcriptional level. Furthermore, we provide the first evidence that 3,5-T2 can affect both activity and transcription of hepatic deiodinases in teleosts.

The liver of Fundulus heteroclitus expresses deiodinase type 1 mRNA

The presence of a type 1 deiodinase (D1) in the liver of teleosts has been a controversial issue. Recently we characterized the deiodinase activity in rainbow trout and killifish liver and found that the liver of both species co-expresses the two enzymes (D1 and D2) that catalyze the outer ring-deiodinating pathway. We here report the cloning and characterization of an mRNA from the liver of the killifish Fundulus heteroclitus that encodes a D1 (FhD1). The cDNA amplified by RT-PCR from F. heteroclitus liver is 1314 nt long and encodes a protein of 248 aa. It contains a TGA codon in its open reading frame and a selenocysteine insertion sequence in its 3(') untranslated region, consistent with the structure of a selenoenzyme mRNA. The deduced peptide sequence is 73% identical to that encoded by the tilapia D1 cDNA cloned from kidney and 46% identical to the D1s reported in other vertebrates. Northern blot analysis shows that FhD1 mRNA is expressed in F. heteroclitus liver, consistent with prior biochemical evidence for hepatic D1 activity. Furthermore, heterologous expression of the FhD1 cDNA resulted in a protein with properties similar to the D1-like activity in F. heteroclitus liver. The cloned enzyme, like the native species, is relatively insensitive to inhibition by PTU, but mutation of Ser-159 in FhD1 to the Pro residue found in D2 and D3 isoforms increased the sensitivity to PTU. Our results show that, under basal conditions, killifish liver indeed expresses a D1 enzyme that is homologous to mammalian D1s, establishing this as a useful model in which to study the regulation of D1 and D2 concurrently.