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

Insights into spirotetramat-induced thyroid disruption during zebrafish (Danio rerio) larval development: An integrated approach with in vivo, in vitro, and in silico analyses

Spirotetramat (SPT), a tetronic acid-derived insecticide, is implicated in reproductive and lipid metabolism disorders, as well as developmental toxicity in fish. While these effects are documented, the precise mechanisms underlying its developmental toxicity are not fully elucidated. In this study, zebrafish embryos (2 h post-fertilization, hpf) were exposed to four concentrations of SPT (0, 60, 120, and 240 μg/L) until 21 dpf (days post-fertilization). We delved into the mechanisms by examining its potential disruption of the thyroid endocrine system, employing in vivo, in vitro, and in silico assays. The findings showed notable developmental disturbances, including reduced hatching rates, shortened body lengths, and decelerated heart rates. Additionally, there was an increase in malformations and a decline in locomotor activity. Detailed analyses revealed that SPT exposure led to elevated thyroid hormone levels, perturbed the hypothalamic-pituitary-thyroid (HPT) axis transcript levels, amplified deiodinase type I (Dio1) and deiodinase type II (Dio2) activities, and both transcriptionally and proteomically upregulated thyroid receptor beta (TRβ) in larvae. Techniques like molecular docking and surface plasmon resonance (SPR) confirmed SPT's affinity for TRβ, consistent with in vitro findings suggesting its antagonistic effect on the T3-TR complex. These insights emphasize the need for caution in using tetronic acid-derived insecticides.

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.

Exposure to environmentally relevant concentrations of TnBP results in tissue-specific bio-accumulation and inhibits growth of silver carp (Hypophthalmichthys molitrix)

Tri-n-butyl phosphate (TnBP) is commonly used as flame retardant and rubber plasticizer, and has been widely detected in aquatic organisms and natural waters. However, the potential toxicity of TnBP in fish remains unclear. In the present study, silver carp (Hypophthalmichthys molitrix) larvae were treated with environmentally relevant concentrations (100 or 1000 ng/L) of TnBP for 60 d and then they were depurated in clean water for 15 d, and the accumulation and depuration of the chemical in six tissues of silver carp were measured. Furthermore, effects on growth were evaluated and potential molecular mechanisms were explored. Results indicated that TnBP could be rapidly accumulated and depurated in silver carp tissues. In addition, the bio-accumulation of TnBP displayed tissue-specificity, where intestine contained the greatest and vertebra had the smallest level of TnBP. Furthermore, exposure to environmentally relevant concentrations of TnBP led to time- and concentration-dependent growth inhibition of silver carp, even though TnBP was completely depurated in tissues. Mechanistic studies suggested that exposure to TnBP up- and down-regulated the expression of ghr and igf1 in liver, respectively, and increased GH contents in plasma of silver carp. TnBP exposure also up-regulated the expression of ugt1ab and dio2 in liver, as well as decreased T4 contents in plasma of silver carp. Our findings provide direct evidence of health hazards of TnBP to fish in natural waters, calling for more attention of environmental risks of TnBP in aquatic environment.

Effect of temperature and dietary pesticide exposure on neuroendocrine and olfactory responses in juvenile Chinook salmon (Oncorhynchus tshawytscha)

Projected water temperature increases based on predicted climate change scenarios and concomitant pesticide exposure raises concern about the responses of aquatic organisms. To better understand the effect of pesticide mixtures and influence of water temperature to fish, juvenile Chinook salmon (Oncorhynchus tshawytscha) were dietarily exposed to a mixture of legacy and current use pesticides (p,p'-DDE, bifenthrin, chlorpyrifos, esfenvalerate, and fipronil) at concentrations detected from field-collected prey items in the Sacramento-San Joaquin Delta, California (Delta) and exposed under current and predicted future water temperature scenarios, 11, 14, or 17 °C, for 14 days. The expression of a subset of genes (deiodinase 2-dio2, gonadotropin releasing hormone 2-gnrh2, and catechol-o-methyltransferase-comt) involved in neuroendocrine, dopaminergic, and olfactory function previously shown to be altered by individual pesticide exposures germane to this study were determined and olfactory function assessed using a Y-maze behavioral assay. When total body burdens of pesticides were measured, a significant decrease in dio2 expression was observed in Chinook salmon exposed at 14 °C compared to fish kept at 11 °C. Increases in gnrh2 expression were also observed in fish exposed to 14 °C. Similarly, increases in comt expression was noted at 14 and 17 °C. Additionally, altered expression of all transcripts was observed, showing interactions between temperature and individual pesticide concentrations. Chinook salmon spent significantly more time actively avoiding the odorant arm at baseline conditions of 11 °C in the Y-maze. At higher temperatures, Chinook spent significantly more time not making a choice between the odorant or clean arm following exposure to the low pesticide mixture, relative to 11 °C. These results suggest that dietary exposure to pesticide mixtures can potentially induce neuroendocrine effects and behavior. Impaired olfactory responses exhibited by Chinook salmon could have implications for predator avoidance in the wild under increased temperature scenarios and impact populations in the future.

Maternal exposure to chiral triazole fungicide tebuconazole induces enantioselective thyroid disruption in zebrafish offspring

Pesticides could induce long-term impacts on aquatic ecosystem via transgenerational toxicity. However, for many chiral pesticides, the potential enantioselectivity of transgenerational toxicity has yet to be fully understood. In this study, we used zebrafish as models to evaluate the maternal transfer risk of tebuconazole (TEB), which is a chiral triazole fungicide currently used worldwide and has been frequently detected in surface waters. After 28-day food exposure (20 and 400 ng/g) to the two enantiomers of TEB (S- and R-TEB) in adult female zebrafish (F0), increased malformation rate and decreased swimming speed were found in F1 larvae, with R-TEB showing higher impacts than S-enantiomer. Additionally, enantioselective effects on the secretion of thyroid hormones (THs) and expression of TH-related key genes along the hypothalamic-pituitary-thyroid (HPT) axis were found in both F0 and F1 after maternal exposure. Both the two enantiomers significantly disrupted the triiodothyronine (T3) and thyroxine (T4) contents in F0 with different degrees, whereas in F1, significant effects were only found in R-TEB groups with decreasing of both T3 and T4 contents. Most of the HPT axis related genes in F0 were upregulated by TEB and more sensitive to R-TEB than to S-TEB. In contrast, most of the genes in F1 were downregulated by both R- and S-TEB, especially the genes that are primarily responsible for thyroid development and growth (Nkx2-1), TH synthesis (NIS and TSHꞵ) and metabolism (Deio1). Findings from this study highlight the key role of enantioselectivity in the ecological risk assessment of chiral pesticides through maternal transfer.

The protective role of lycopene against toxic effects induced by the herbicide Harness® and its active ingredient acetochlor on the African catfish Clarias gariepinus (Burchell, 1822)

The effects of Harness® toxicity on fish health are little known. So, current work aimed to study the impact of sub-lethal doses of Harness® (an acetochlor-based herbicide) on the African catfish, Clarias gariepinus, and also investigated the potential role of lycopene (LYCO) administration in alleviating Harness® negative effects. Fish were divided into five groups in triplicates as follows: group 1 (control) received no treatment, group 2 was exposed to 10 μm Harness®/L, group 3 was orally administered 10 mg LYCO/kg body weight and exposed to 10 μm Harness®/L, group 4 was exposed to 100 μm Harness®/L, and group 5 was orally administered 10 mg LYCO/kg body weight and exposed to 100 μm Harness®/L for 2 weeks. Some hemato-biochemical parameters, genotoxicity, and histopathological changes were assessed at the end of this period. Sub-lethal doses of Harness® altered the shape of erythrocytes in contrast to the control sample. Also, hematological parameters of exposed fish exhibited a significant (P < 0.05) reduction in the values of red blood cell count (RBCs), hemoglobin (Hb), hematocrit (HCT), and platelets (PL), as well as an insignificant (P > 0.05) drop in mean corpuscular volume (MCV). Harness® was also found to cause genotoxicity as well as histopathological alterations. LYCO administration decreased hemato-biochemical changes and returned them to near-normal levels. The findings showed that LYCO administration (10 mg LYCO/kg body weight) decreased Harness® toxicity in C. gariepinus and alleviated its destructive effects.

Crosstalk of cholinergic pathway on thyroid disrupting effects of the insecticide chlorpyrifos in zebrafish (Danio rerio)

Chlorpyrifos is a widely used organophosphate insecticide and ubiquitously detected in the environment. However, little attention has been paid to its endocrine disrupting effect to non-target organisms. In the present study, zebrafish was exposed to 13 and 65 μg/L of chlorpyrifos for 7 and 10 days to determine the induced neurotoxicity and the alteration of thyroid metabolism. The 120 h LC₅₀ and LC₁₀ of chlorpyrifos was estimated as 1.35 mg/L and 0.62 mg/L based on the acute embryo toxicity assay, respectively. The acetylcholinesterase (AChE) inhibitory was detected by 13 μg/L chlorpyrifos and could be reversed by the co-exposure of 100 and 1000 μg/L anticholinergic agent atropine. For thyroid hormone level, 13 and 65 μg/L of chlorpyrifos induced increased free T₃ levels in 10 dpf (days post-fertilization). The expression of thyroid related genes in 7 and 10 dpf exposed zebrafish were measured by the quantitative Real-Time PCR (qRT-PCR) assay. The mRNA expression of tshba, thrb, crhb, ttr, tpo, ugt1ab and slc5a5 had significant change. However, the alterations of thyroid hormone and mRNA expression could be partly rescued by the addition of atropine. The molecular docking of chlorpyrifos and T₃ to the thyroid receptor β in zebrafish using homology modelling and CDOCKER procedures shown weaker binding ability of chlorpyrifos compared to T₃. Therefore, we concluded that the disturbance of thyroid signaling in zebrafish might arise from the developmental neurotoxicity induced by chlorpyrifos.

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.

Impact of herbicide pretilachlor on reproductive physiology of walking catfish, Clarias batrachus (Linnaeus)

Herbicide pretilachlor is widely used in paddy fields to control annual weeds. The present study has been carried out in walking catfish, Clarias batrachus, to evaluate the impact of herbicide pretilachlor on reproductive physiology after chronic exposure. Based on the median lethal concentration value (96 h), fish were exposed to three nominal test concentrations of pretilachlor ((SL-I (1/20th LC₅₀), SLII (1/15th LC₅₀), and SL-III (1/10th LC₅₀)) for 30, 45, and 60 days after which plasma sex steroid profile, plasma vitellogenin concentration, and gonadal aromatase activity were analyzed in both sexes. Plasma concentration of testosterone decreases in herbicide-exposed male fish. Significant increase in plasma 17β-estradiol, plasma vitellogenin concentration, and gonadal aromatase activity were observed in herbicide-exposed male fish. All these alterations in reproductive parameters in male fish are dependent on concentration and exposure duration of herbicide. On the other hand, significant decrease in plasma concentration of testosterone was observed in female fish which was also dependent on concentration and exposure duration of herbicide. No significant changes in plasma 17β-estradiol concentrations, plasma vitellogenin concentration, and gonadal aromatase activity were observed in female fish. Above findings clearly suggested that herbicide pretilachlor acts as endocrine disruptor in fish and affects overall reproductive physiology of fish, but its ability to induce reproductive toxicity in male and female differs considerably.

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.

Changes in thyroid hormone levels and related gene expressions in embryo-larval zebrafish exposed to binary combinations of bifenthrin and acetochlor

Bifenthrin (BF) and acetochlor (AT) are widely used as an insecticide and herbicide, respectively, which are introduced to the aquatic environment as a natural result. Although the thyroid active substances may coexist in the environment, their joint effects on fish have not been identified. We examined the joint toxicity of BF and AT in zebrafish (Danio rerio) in this study. An acute lethal toxicity test indicated that the median lethal concentration (LC₅₀) values of BF and AT under 96 h treatment were 0.40 and 4.56 µmol L^-1, respectively. The binary mixture of BF + AT displayed an antagonistic effect on the acute lethal toxicity. After 14 days post fertilization (dpf) with exposure to individual pesticides at sub-lethal concentrations of, no effects were observed on the catalase (CAT) and peroxidase (POD) activities, while the binary mixtures (except for the 7.2 × 10^-3 µmol L^-1 BF + 1.2 × 10^-2 µmol L^-1 AT exposure group) significantly induced the CAT activity. The superoxide dismutase (SOD) activity and triiodothyronine (T3) level were significantly increased in all exposure groups. The thyroxine (T4) level remained unchanged after exposure to individual pesticides, but significantly increased in the 7.2 × 10^-3 µmol L^-1 BF + 1.2 × 10^-2 µmol L^-1 AT group. The expressions of the genes Dio2, TRa, TSHβ and CRH in the thyroid hormone (TH) axis were significantly up-regulated in the 7.2 × 10^-3 µmol L^-1 BF + 0.4 × 10^-2 µmol L^-1 AT group. Our data indicated that the binary mixture of BF + AT significantly altered the antioxidant enzyme activities and gene expressions in the hypothalamic-pituitary-thyroid (HPT) axis and changed the TH levels.

Evaluation and mechanistic study of chlordecone-induced thyroid disruption: Based on in vivo, in vitro and in silico assays

The present study aimed to evaluate the thyroid-disrupting potency of chlordecone, and reveal the underlying mechanism. In the in vivo assays, rare minnow embryos were exposed to 0, 0.01, 0.1, 1 and 10 μg·L^-1 chlordecone until sexually mature. The results showed decreased T4 but increased T3 concentrations in plasma, upregulated mRNA levels of thyrotropin-releasing hormone receptor (trhr) and sodium-iodide symporter (nis) in the brain, and transthyretin (ttr), thyroid hormone receptor α (trα) and deiodinase enzymes (dio1 and dio2) in the liver of adult fish. In the in vitro assays, single chlordecone treatments promoted growth hormone (GH) and prolactin (PRL) secretion in GH3 cells. Transcription of thyroid receptor (trβ) was inhibited, but this is not likely responsible for chlordecone-induced GH secretion and altered transcription. When co-treated with T3, chlordecone acted independently of the effect of T3 on GH secretion; chlordecone-induced GH/PRL secretion and mRNA expression were further promoted when co-treated with E2, but inhibited when co-treated with ICI, indicating an important role for estrogen receptors (ERs) in chlordecone-induced changes in GH3 cells. Furthermore, in silico prediction suggested no stable interactions between chlordecone and thyroid hormone-related proteins, as well as a regulatory role for ERs in thyroid systems. Overall, our results indicated that chlordecone may have adverse effects on thyroid systems upon long-term exposure. However, rather of TRs, ERs may be responsible for thyroid disruption following chlordecone exposure.

Impacts of herbicide pendimethalin on sex steroid level, plasma vitellogenin concentration and aromatase activity in teleost Clarias batrachus (Linnaeus)

Pendimethalin (PM) is a selective herbicide, widely present in aquatic environment. It causes detrimental effects in fishes, but little is known regarding its reproductive toxicity. The present study was carried out in Clarias batrachus exposed to sub lethal concentrations of PM for 30, 45 and 60 days. Male fish showed a significant increase in plasma 17β-estradiol (E2) however plasma E2 in females was not affected. Plasma testosterone levels were significantly decreased in both sexes. In male plasma vitellogenin (VTG) and gonadal aromatase activity was increased irrespective of herbicide concentration and exposure duration. In females concentration and time dependent reduction in plasma VTG but no significant change in the gonadal aromatase activity were observed. Results indicated that PM act as endocrine disruptor but act differentially in male and female fishes and plasma E2, T and VTG levels and aromatase activity can be considered as reliable biomarkers for PM toxicity in fishes.

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.

Exposure to humidifier disinfectants induces developmental effects and disrupts thyroid endocrine systems in zebrafish larvae

Humidifier disinfectants have been widely used in Korea to prevent the growth of microorganisms in humidifier water. However, their use has been banned since 2011 after epidemiological studies reported humidifier disinfectant induced lung injury. In the present study, the developmental effects of exposure to two humidifier disinfectants (Oxy® and Wiselect) and their main component, polyhexamethylene guanidine (PHMG)-phosphate, were investigated in zebrafish embryos/larvae for seven days. The effects on triiodothyronine (T3) and thyroxine (T4) hormones, reactive oxygen species (ROS) generation, antioxidant enzyme activities, and changes in expression of the genes related to the hypothalamus-pituitary-thyroid (HPT) axis and oxidative stress were also investigated. Zebrafish embryos exposed to the highest concentration (amounts recommended for use by the manufacturers) of all tested humidifier disinfectants showed an increase in embryo coagulation, leading to death without hatching. Exposure to Oxy® and Wiselect resulted in significantly decreased body length, increased ROS generation and antioxidant enzyme activities, decreased T4, and up-regulated genes related to the HPT axis (trh, trβ, and tpo) and oxidative damage (sod2 and gpx1b). The humidifier disinfectants and PHMG-phosphate could induce oxidative stress and disrupt thyroid hormone systems in zebrafish, leading to developmental retardation when used at sub-lethal concentrations. Potential effects of long-term exposure to humidifier disinfectants and mixture effects of several major components deserve further investigation.

Establishment of cumulative assessment groups of pesticides for their effects on the thyroid

Cumulative assessment groups of pesticides have been established for two specific effects on the thyroid: firstly hypothyroidism, and secondly parafollicular cell (C-cell) hypertrophy, hyperplasia and neoplasia. Sources of uncertainties resulting from the methodological approach and from the limitations in available data and scientific knowledge have been identified and considered. This report supports the publication of a scientific report on cumulative risk assessment to pesticides affecting the thyroid, in which all uncertainties identified for either the exposure assessment or the establishment of the cumulative assessment groups are incorporated into a consolidated risk characterisation.

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.

[Thyroid disruptors and their consequences on brain development and behavior]

An increase in the prevalence of many diseases affecting the nervous system in both children and adults has been reported. Some of these diseases are related to endocrine dysfunction, notably of the thyroid axis. Examples in children are attention deficit/hyperactivity disorders and Autism Spectrum Disorders, diagnosed but most often affecting the whole life, and multiple sclerosis or Alzheimer's disease in adults. It is becoming increasingly clear that embryonic exposure to thyroid hormone disruptors can lead to short- and long-term consequences, that often escape conventional neonatal diagnosis. Endocrine disruptors comprise a wide range of molecules, plasticizers, some pesticides, surfactants, flame-retardants, etc., many of which can interfere with thyroid hormone synthesis or their actions. We here report briefly the history of endocrine disruptors, their properties and the consequences on neuronal development of embryonic exposure to some of them.

Toxicokinetic patterns, metabolites formation and distribution in various tissues of the Chinese rare minnow (Gobiocypris rarus) exposed to tri(2‑butoxyethyl) phosphate (TBOEP) and tri-n-butyl phosphate (TNBP)

Alkylated organophosphate esters (alkyl-OPEs) are widely used and extensively detected in aquatic organisms. This work investigated the tissue-specific toxicokinetics of two common alkyl-OPEs, tri(2‑butoxyethyl) phosphate (TBOEP) and tri‑n‑butyl phosphate (TNBP) in Chinese rare minnow (Gobiocypris rarus) through a 50 day uptake and depuration experiment. The tissue-specific bioconcentration factor (BCF) values for the two alkyl-OPEs ranged from 1 to 30 L/kg wet weight (ww), with the kidney and ovary as the tissues with the highest accumulation. The tissue BCFs only exhibited a significant correlation with lipid contents only in storage tissues (i.e., muscle, brain, ovary and testis), indicating that lipids might not be the major contributor to tissue distribution of TBOEP and TNBP. However, the contribution of blood perfusion and active transport to tissue-specific OPE accumulation needs to be further investigated. Lower accumulation of metabolites than parent chemicals was observed, with metabolite parent concentration factors (MPCFs) <1. Di-alkyl phosphate (DAP), bis(2‑butoxyethyl) phosphate (BBOEP) and di(n-butyl) phosphate (DNBP) were the most abundantly formed metabolites of TBOEP and TNBP in various tissues, followed by the monohydroxylated OPEs (OH-OPEs). However, bis(2‑butoxyethyl) hydroxyethyl phosphate (BBOEHEP), was detected at much lower levels in the tissues. All the investigated metabolites showed high production rates (k_{prod,metabolites}) in the fish liver, followed by the GI tract and the kidney, indicating the importance of the hepatobiliary and urinary systems in eliminating the metabolites. Our study suggested that metabolism plays an important role in eliminating these two alkyl-OPEs in rare minnow and results in different tissue distribution mechanisms for metabolites and their compounds.

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.

Enantioselective thyroid disruption in zebrafish embryo-larvae via exposure to environmental concentrations of the chloroacetamide herbicide acetochlor

Acetochlor (ACT) is a chiral chloroacetamide pesticide that has been heavily used around the world, resulting in its residues being frequently found in surface waters. It has been reported that ACT is an endocrine disrupting chemical (EDC) with strong thyroid hormone-disrupting activity in aquatic organisms. However, the enantioselectivity underlying thyroid disruption has yet to be understood. In this study, using a zebrafish embryo-larvae model, the enantioselective thyroid disruption of ACT was investigated at a series of environmentally relevant concentrations (1, 2, 10 and 50 μg/L). Our results showed that both racemic ACT and its enantiomers significantly increased the malformation rates of embryos at 72 h postfertilization (hpf). Decreased thyroxine (T₄) contents and increased triiodothyronine (T₃) contents were found in larvae at 120 hpf, with (+)-S-ACT exhibiting a greater effect than (-)-R-enantiomer. Similarly, (+)-S-ACT also showed a stronger effect on the mRNA expressions of thyroid hormone receptors (TRα and TRβ), deiodinase2 (Dio2) and thyroid-stimulating hormone-β (TSHβ) genes. The observed enantioselectivity in TR expressions was consistent with that of in silico binding analysis, which suggested that (+)-S-enantiomer binds more potently to the TRs than (-)-R-enantiomer. In general, ACT enantiomers showed different influences on the secretion of THs, expression of TH-related key genes and binding affinity to TRs. Considering the different toxicity of different enantiomers, our study highlights the importance of enantioselectivity in understanding of thyroid disruption effects of chiral pesticides.

Multiple approaches to assess the effects of F-53B, a Chinese PFOS alternative, on thyroid endocrine disruption at environmentally relevant concentrations

A Chinese perfluorooctane sulfonate (PFOS) substitute frequently detected in the environment, 6:2 chlorinated polyfluorinated ether sulfonate (F-53B), has a similar structure to PFOS and it is proposed to cause thyroid dysfunction. To further confirm this hypothesis, the effects of F-53B on the thyroid endocrine system and underlying mechanisms were investigated in vitro and in vivo using rat pituitary GH3 cells and developing zebrafish, respectively. In GH3 cells, F-53B enhanced cell proliferation in a dose-dependent manner, indicative of thyroid receptor agonistic activity. In zebrafish larvae, F-53B exposure induced significant developmental inhibition and increased thyroxine (T₄) but not 3,5,3'-triiodothyronine (T₃) levels accompanied by a decrease in thyroglobulin (TG) protein and transcript levels of most genes involved in the hypothalamic-pituitary-thyroid (HPT) axis. Interestingly, T₄ levels remained significantly increased while TG protein and gene transcription levels were markedly upregulated after depuration. Molecular docking studies revealed that F-53B binds to transthyretin (TTR) by forming hydrogen bonds with Lys123 and Lys115, thereby interfering with thyroid hormone homeostasis. Our collective in vitro, in vivo and in silico studies provide novel evidence that F-53B disrupts the thyroid endocrine system at environmentally relevant concentrations, which cannot be recovered after depuration. Given the persistence of F-53B in the environment, the long-term consequences of thyroid hormone disruption by this chemical warrant further investigation.

Early life exposure of zebrafish (Danio rerio) to synthetic pyrethroids and their metabolites: a comparison of phenotypic and behavioral indicators and gene expression involved in the HPT axis and innate immune system

Ecotoxicological studies have revealed the association between synthetic pyrethroid (SP) exposure and aquatic toxicity in fish; however, research on the toxic effects of SP metabolites is still limited. In this study, the toxicity of two SPs (permethrin (PM) and β-cypermethrin (β-CP)) and their three metabolites (3-phenoxybenzoic alcohol (PBCOH), 3-phenoxybenzaldehyde (PBCHO), and 3-phenoxybenzoic acid (PBCOOH)) towards zebrafish embryos and larvae was evaluated. Both SPs and their metabolites exhibited significant developmental toxicities, caused abnormal vascular development, and changed locomotor activities in larvae. The alteration of gene expression involved in the thyroid system and the innate immune system indicated that SPs and their three metabolites have the potency to induce thyroid disruption and trigger an immune response. The results from the present study suggest that SP metabolites could induce multiple toxic responses similar to parent compounds, and their toxicity should be considered for improving the understanding of environmental risks of SPs.

Exposure to PFDoA causes disruption of the hypothalamus-pituitary-thyroid axis in zebrafish larvae

Perfluorododecanoic acid (PFDoA), a kind of perfluorinated carboxylic acid (PFCA) with 12 carbon atoms, has an extensive industrial utilization and is widespread in both wildlife and the water environment, and was reported to have the potential to cause a disruption in the thyroid hormone system homeostasis. In this study, zebrafish embryos/larvae were exposed to different concentrations of PFDoA (0, 0.24, 1.2, 6 mg/L) for 96 h post-fertilization (hpf). PFDoA exposure caused obvious growth restriction connected with the reduced thyroid hormones (THs) contents in zebrafish larvae, strengthening the interference effect on the growth of fish larvae. The transcriptional level of genes within the hypothalamic-pituitary-thyroid (HPT) axis was analyzed. The gene expression levels of thyrotropin-releasing hormone (trh) and corticotrophin-releasing hormone (crh) were upregulated upon exposure to 6 mg/L of PFDoA, and iodothyronine deiodinases (dio2) was upregulated in the 1.2 mg/L PFDoA group. The transcription of thyroglobulin (tg) and thyroid receptor (trβ) were significantly downregulated upon exposure to 1.2 mg/L and 6 mg/L of PFDoA. PFDoA could also decrease the levels of sodium/iodide symporter (nis) and transthyretin (ttr) gene expression in a concentration-dependent manner after exposure. A significant decrease in thyroid-stimulating hormoneβ (tshβ), uridinediphosphate-glucuronosyltransferase (ugt1ab) and thyroid receptor (trα) gene expression were observed at 6 mg/L PFDoA exposure. Upregulation and downregulation of iodothyronine deiodinases (dio1) gene expression were observed upon the treatment of 1.2 mg/L and 6 mg/L PFDoA, respectively. All the data demonstrated that gene expression in the HPT axis altered after different PFDoA treatment and the potential mechanisms of the disruption of thyroid status could occur at several steps in the process of synthesis, regulation, and action of thyroid hormones.

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.

Developmental toxicity and thyroid hormone-disrupting effects of 2,4-dichloro-6-nitrophenol in Chinese rare minnow (Gobiocypris rarus)

In the present study, to evaluate embryonic toxicity and the thyroid-disrupting effects of 2,4-dichloro-6-nitrophenol (DCNP), embryos and adults of Chinese rare minnow (Gobiocypris rarus) were exposed to 2, 20, and 200μg/L DCNP. In the embryo-larval assay, increased percentages of mortality and occurrence of malformations, decreased percentage of hatching, and decreased body length and body weight were observed after DCNP treatment. Moreover, the whole-body T3 levels were significantly increased at 20 and 200μg/L treatments, whereas the T4 levels were markedly decreased significantly (p<0.05) for all DCNP concentrations. In the adult fish assay, plasma T3 levels were significantly increased whereas plasma T4 levels were significantly reduced in the fish treated with 20 and 200μg/L (p<0.05). In addition, DCNP exposure significantly changed the transcription levels of thyroid system related genes, including dio1, dio2, me, nis, tr, and ttr. The increased responsiveness of thyroid hormone and mRNA expression levels of thyroid system related genes suggested that DCNP could disrupt the thyroid hormone synthesis and transport pathways. Therefore, our findings provide new insights of DCNP as a thyroid hormone-disrupting chemical.

Effects of environmental chemicals on fish thyroid function: Implications for fisheries and aquaculture in Australia

Numerous environmental stressors exert acute or chronic effects on the fish thyroid cascade. Such effects could be mediated via thyroidal alterations, imbalance of plasma T4 and T3 levels or damage to the structure of the thyroidal tissues (thyroid hypertrophy, hyperplasia). The thyroidal system is intricately linked to other endocrine systems in vertebrates including the control of reproduction. Disruption of fish thyroid function by environmental stressors has the potential to result in deleterious effects including the inhibition of sperm production, reduction in egg production, gonad development, ovarian growth, swimming activity, fertilisation and increase in larval mortality. Thyroid hormones play a major role in the development and growth of fish, particularly during their early life stages, thus, thyroid disruption by environmental stressors could inhibit the growth of fish larvae and juveniles in wild fish and cultured species, limit fish seed production and result in a decline in wild fisheries. This review highlights the effects of several environmental toxicants including PBDE, PCBs, PCDD and PCDF, PAH/oil, phthalates, metals, pesticides, mixed pollutants/chemicals, cyanide; and other stressors including acid (low pH) and ammonia, on fish thyroid function. Environmental sources of chemical stressors and appropriate water quality guidelines to protect the freshwater and marine species for the relevant pollutants are also discussed including (when available) the Australian guidelines (2000) and Canadian water quality guidelines (where Australian guidelines are not available). To date there has been no published research on the effects of anthropogenic environmental pollutants on the thyroid system of any native Australian fish species. However, the detection of high risk chemicals (notably PBDEs, PCBs, PAHs, metals and pesticides) in Australian waterways and Australian fish and shellfish implies that thyroid disruption of Australian wild fish and aquacultured species could occur. It is therefore imperative that the effects of such pollutants on the thyroid system of Australian native fish be investigated.

Benzo[a]pyrene induced p53-mediated cell cycle arrest, DNA repair, and apoptosis pathways in Chinese rare minnow (Gobiocypris rarus)

The p53 pathways play an important role in carcinogenesis. In mammals, p53 and p53 target genes have been extensively studied, but little is known about their functions and regulation in fish. In this study, the cDNA fragments of p53 network genes, including p53, p21, mdm2, gadd45α, gadd45β, igfbp-3, and bax, were cloned from Chinese rare minnow (Gobiocypris rarus). These genes displayed high amino acid sequence identities with their zebrafish orthologs. The mRNA levels of p53 network genes and pathological changes in the liver were determined after adult rare minnow were exposed to 0.4, 2, and 10 µg/L of benzo[a]pyrene (BaP) for 28 days. The results showed that p53, p21, mdm2, gadd45α, and bax mRNA expressions in the livers from males and females were significantly upregulated compared with those of the controls (p < 0.05), but gadd45β and igfbp-3 expression was not significantly changed. Microphotographs revealed enlargement of the cell nuclei and cellular degeneration in males, while atrophy and vacuolization of hepatocytes were observed in females (10 µg/L). These results suggested that BaP induced liver DNA repair and apoptosis pathways and caused adverse pathological changes in rare minnow. The strongly responsive p53 network genes in the livers suggest that rare minnow is suitable as an experimental fish to screen environmental carcinogens. In addition, the p53 network genes in rare minnow could feasibly be used to identify the mechanism of environmental carcinogenesis. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 979-988, 2017.

Refuse leachate exposure causes changes of thyroid hormone level and related gene expression in female goldfish (Carassius auratus)

To elucidate the potential thyroid disrupting effects of refuse leachate on females, female goldfish (Carassius auratus) were exposed to 0.5% diluted leachates from each step of a leachate treatment process (i.e. raw leachate before treatment, after membrane bioreactor treatment, and the final treated leachate) for 21days. Raw leachate exposure caused disturbances in the thyroid cascade of female fish, as evidenced by the elevated plasma 3,3',5-triiodo-l-thyronine (p<0.05) and thyroid-stimulating hormone (p<0.01) levels as well as up-regulated hepatic and gonadal type I deiodinase (p<0.01), type II deiodinase (p<0.01) and thyroid receptor (p<0.05) mRNA levels. Thyroid disrupting potency decreased markedly as raw leachate progressed through the "membrane bioreactor + reverse osmosis" treatment but could still be detected in the treated leachate. As our results indicated, thyroid system in female goldfish was more sensitive to leachate exposure than that of the male fish.

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.

Exposure to butachlor causes thyroid endocrine disruption and promotion of metamorphosis in Xenopus laevis

Butachlor is extensively applied in rice paddy ecosystem in china, and has been widespread contaminant in the aquatic environment. Here, Xenopus laevis was used for the evaluation of teratogenesis developmental toxicity, and disruption of thyroid system when exposure to different concentrations of butachlor by window phase exposure. Acute toxicity investigation shown that 96 h-LC50 value of butachlor was 1.424 mg L(-1) and 0.962 mg L(-1) for tadpoles (starting from stages 46/47) and embryos (starting from stages 8/9), respectively. Exposure to butachlor caused malformation, including abnormal eye, pericardial edema, enlarged proctodaeum and bent tail. Window phase exposure test indicated that butachlor significantly promote the contents of whole-body thyroid hormones (THs, T3 and T4) at higher levels, indicating thyroid endocrine disruption. At 7 days, exposure to butachlor up-regulated the mRNA expression of genes involved in THs synthesis and metabolism (tshα, tg, tpo and dio1) and THs receptors (trα and trβ). At 14 days, up-regulation of the mRNA expression of genes related to THs synthesis and metabolism (tshα, tshβ, tg, tpo, dio1, dio2 and ttr) and THs receptors (trβ) were also observed after the exposure to butachlor. At 21 days, butachlor up-regulated the mRNA expression of tshα, tg, tpo genes and down-regulated the mRNA expression of tshβ, tg, dio1, ttr and trα genes. These results showed that butachlor could change the mRNA expression of genes involved in the HPT axis and increase whole-body thyroid hormones levels of X. laevis tadpoles in a dose- and time-dependent manner, causing thyroid endocrine disruption and developmental toxicity.

Transcriptome profiling analysis of rare minnow (Gobiocypris rarus) gills after waterborne cadmium exposure

Rare minnow (Gobiocypris rarus) is a widely used experimental fish in risk assessments of aquatic pollutants in China. Cadmium (Cd) is one of the most toxic heavy metals in the world; however, few studies have used fish gills, a multi-functional organ. In this study, we characterized the differential expression of adult female rare minnow gills after sub-chronic waterborne Cd (75μg/L CdCl2) exposure for 35d. A total of 452 genes (209 up-regulated and 243 down-regulated) were identified by gene expression profiling using RNA-Seq before and after treatment. Of these differentially expressed genes, 75, 21, and 54 differentially expressed genes are related to ion transport, oxidation-reduction processes, and the immune response, respectively. The results of GO and KEGG enrichment analyses, together with the altered transcript levels of major histocompatibility complex (MHC) class I and class II molecules and the significant increases in the levels of serum tumor necrosis factor α (TNF-α), interleukin 1β (IL1β) and nuclear factor-κB (NF-κB), indicated a disruption of the immune system, particularly the induction of inflammation and autoimmunity. The significant down-regulation of coagulation factor XIII A1 polypeptide (F13A1), tripartite motif-containing protein 21 (TRIM21), and Golgi-associated plant pathogenesis-related protein (GAPr) during both acute (≤96h) and sub-chronic (35d) waterborne Cd exposure, as well as their dosage dependence, suggested that these three genes could be used as sensitive biomarkers for aquatic Cd risk assessment.

Identification and characterization of novel and conserved microRNAs in several tissues of the Chinese rare minnow (Gobiocypris rarus) based on illumina deep sequencing technology

BACKGROUND

MicroRNAs (miRNAs), which comprise a large family of endogenous small non-coding RNA molecules, play important roles in the regulation of gene expression in various biological processes. The Chinese rare minnow (Gobiocypris rarus) is a Chinese native fish species and is used extensively as an experimental fish in China; however, relevant biological data, especially miRNA transcriptome data, have not been well documented. To discover conserved and potential novel miRNAs in Chinese rare minnows, a pool of equal amounts of RNA obtained from 6 different adult rare minnow tissues (brain, eye, gill, liver, muscle and heart) was sequenced using illumina deep sequencing technology.

RESULTS

In the present study, 26,930,553 raw reads, representing 2,118,439 unique high-quality reads, were obtained from the pooled small RNA library. Using bioinformatics analysis, 352 conserved and 112 novel Chinese rare minnow miRNAs were first discovered and characterized in this study. Moreover, we found extensive sequence variations (isomiRs) in rare minnow miRNAs, including internal miRNA isomiRs and terminal isomiRs at both the 5' and 3' ends and nucleotide variants. Six conserved and 4 novel miRNAs were selected and validated in 6 different adult rare minnow tissues using quantitative real-time PCR (qPCR). The results showed that miR-30a, miR-30b, and Novel-37 are ubiquitously expressed in a variety of tissues. miR-16a, miR-9, miR-125b, miR-34a, and Novel-69 were predominantly expressed in the brain. Novel-115 and Novel-7 were highly expressed in gills, but were relatively weakly expressed in other tissues. These results provided the expression patterns of miRNA genes in Chinese rare minnow. Finally, based on bioinformatics predictions, we mainly found that Novel-94 and Novel-1b-5p were simultaneously targeted to the 3'UTR of Dmrt1, which controls sex determination and/or sexual differentiation in a variety of metazoans at different sites. Novel-29b targeted the 3'UTR of Foxl2, which is involved in the maintenance of ovarian function and the transcriptional regulation of gonadal differentiation-related genes. Novel-62 and Novel-53 targeted the 3'UTR of ERbeta1 and ERbeta2 (which regulate the transcription of target genes), respectively.

CONCLUSIONS

Rare minnow is a widely used model for assessing the risk of environmental pollution in China. Identifying and characterizing rare minnow miRNA genes is necessary to discover the biological function of miRNAs and to screen for new molecule biomarkers to assess the risk of environmental pollution in the future.

Acute exposure to synthetic pyrethroids causes bioconcentration and disruption of the hypothalamus-pituitary-thyroid axis in zebrafish embryos

Synthetic pyrethroids (SPs) have the potential to disrupt the thyroid endocrine system in mammals; however, little is known of the effects of SPs and underlying mechanisms in fish. In the current study, embryonic zebrafish were exposed to various concentrations (1, 3 and 10 μg/L) of bifenthrin (BF) or λ-cyhalothrin (λ-CH) until 72 h post fertilization, and body condition, bioaccumulation, thyroid hormone levels and transcription of related genes along the hypothalamus-pituitary-thyroid (HPT) axis examined. Body weight was significantly decreased in the λ-CH exposure groups, but not the BF exposure groups. BF and λ-CH markedly accumulated in the larvae, with concentrations ranging from 90.7 to 596.8 ng/g. In both exposure groups, alterations were observed in thyroxine (T4) and triiodothyronine (T3) levels. In addition, the majority of the HPT axis-related genes examined, including CRH, TSHβ, TTR, UGT1ab, Pax8, Dio2 and TRα, were significantly upregulated in the presence of BF. Compared to BF, λ-CH induced different transcriptional regulation patterns of the tested genes, in particular, significant stimulation of TTR, Pax8, Dio2 and TRα levels along with concomitant downregulation of Dio1. Molecular docking analyses revealed that at the atomic level, BF binds to thyroid hormone receptor (TRα) protein more potently than λ-CH, consequently affecting HPT axis signal transduction. In vitro and in silico experiments disclosed that during the early stages of zebrafish development, BF and λ-CH have the potential to disrupt thyroid endocrine system.

Targeting neurotrophic factors and their receptors, but not cholinesterase or neurotransmitter, in the neurotoxicity of TDCPP in Chinese rare minnow adults (Gobiocypris rarus)

Organophosphate flame retardants (OPFRs) have been detected at high concentrations in various environmental and biotic samples, but little is known about their toxicity. In this study, the potential neurotoxicity of three OPFRs (TCEP, TDCPP, and TPP) and Chlorpyrifos (CPF, an organophosphate pesticide) were compared in Chinese rare minnow using an acute toxicity test and a 21-day fish assay. The acute test demonstrated significant inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) by CPF. Although significant AChE inhibition at high concentration of TPP was also observed, none of the OPFRs had effects similar to CPF on these enzymes, indicating that their acute toxicities to Chinese rare minnow may be unrelated to cholinesterase inhibition. In addition, the 21-day fish assay with TDCPP demonstrated no significant effects on cholinesterase activities or neurotransmitter levels. Nonetheless, this OPFR exhibited widespread effects on the neurotrophic factors and their receptors (e.g., ntf3, ntrk1, ntrk2, ngfr, and fgf2, fgf11, fgf22, fgfr4), indicating that TDCPP or other OPFRs may elicit neurological effects by targeting neurotrophic factors and their receptors in Chinese rare minnow.

Exposure to difenoconazole causes changes of thyroid hormone and gene expression levels in zebrafish larvae

Difenoconazole was believed to induce a large suite of symptoms during zebrafish development, but little is known about the negative invisible effect known as endocrine disruption. In this study, zebrafish (Danio rerio) embryos were exposed to various concentrations of difenoconazole from fertilization to 120 h post-fertilization (hpf), and the whole body content of thyroid hormone and gene transcription in the hypothalamic-pituitary-thyroid (HPT) axis were investigated. Results showed thyroxine (T4) levels were significantly decreased, while triiodothyronine (T3) concentrations were not changed. Moreover, the mRNA transcription of corticotrophin-releasing hormone (crh), thyroid-stimulating hormone (tshβ), transthyretin (ttr), thyronine deiodinase (dio1 and dio2), uridine diphosphate glucuronosyltransferase (ugt1ab) in the HPT axis were significantly up-regulated, but the transcriptions of thyroglobulin (tg), sodium/iodide symporter (nis) and thyroid hormone receptors trβ were not changed. The overall results showed that exposure to difenoconazole could alter thyroid hormone levels and gene transcription in zebrafish larvae, indicating thyroid endocrine disruption.

Thyroid endocrine disruption of acetochlor on zebrafish (Danio rerio) larvae

The herbicide acetochlor is widely used and detected in the environment and biota, and has been suspected to disrupt the thyroid endocrine system, but underlying mechanisms have not yet been clarified. In the present study, zebrafish larvae (7 days post-fertilization) were exposed to a series concentration of acetochlor (0, 1, 3, 10, 30, 100 and 300 µg l(-1) ) within a 14-day window until 21 days post-fertilization. Thyroid hormones and mRNA expression profiles of genes involved in the hypothalamic-pituitary-thyroid (HPT) axis were analyzed. Exposure to the positive control, 3,5,3'-triiodothyronine (T3 ), altered the mRNA expression, suggesting that the HPT axis in the critical window of zebrafish responded to chemical exposure and could be used to evaluate the effects of chemicals on the thyroid endocrine system. The mRNA expressions of genes involved in thyroid hormone synthesis (tshβ, slc5a5 and tpo) were upregulated significantly with acetochlor treatment, which might be responsible for the increased thyroxine concentrations. The downregulation of genes related to thyroid hormone metabolism (dio1 and ugt1ab) and transport (ttr) in zebrafish larvae exposed to acetochlor might further explain the increased thyroxine levels and decreased T3 levels. The mRNA expression of the thyroid hormone receptor (trα) was also upregulated upon acetochlor exposure. Results suggested that acetochlor altered mRNA expression of the HPT axis-related genes and changed the whole body thyroid hormone levels in zebrafish larvae. It demonstrated that acetochlor could cause endocrine disruption of the thyroid system by simulating the biological activity of T3 . Copyright © 2015 John Wiley & Sons, Ltd.

Dietary methimazole-induced hypothyroidism reduces hepatic lipid deposition by down-regulating lipogenesis and up-regulating lipolysis in Pelteobagrus fulvidraco

The present study was conducted to investigate the effects and mechanisms of hypothyroidism, induced by administration of 0.2% methimazole through the food, on lipid metabolism in the liver of juvenile yellow catfish Pelteobagrus fulvidraco. To this end, yellow catfish were fed diets containing either 0 or 2g methimazole per kg of diet for 8weeks, respectively. The results showed that fish fed diet containing methimazole had a significant reduction in growth performance, plasma THs levels and hepatic lipid content. Meanwhile, methimazole treatment inhibited the activities of lipogenic enzymes (6-phosphogluconate dehydrogenase, glucose 6-phosphate dehydrogenase, malic enzyme, isocitrate dehydrogenase and fatty acid synthase) and the mRNA levels of genes involved in lipogenesis (6-phosphogluconate dehydrogenase, glucose 6-phosphate dehydrogenase, fatty acid synthase, acetyl-CoA carboxylase α, sterol-regulator element-binding protein-1 and liver X receptor), but increased lipolytic enzyme (carnitine palmitoyltransferase 1) activity and the expression of genes involved in lipolysis (carnitine palmitoyltransferase 1a, hormone-sensitive lipase and peroxisome proliferators-activated receptor α). Thus, our study indicated that dietary methimazole-induced hypothyroidism could disturb the normal processes of lipid metabolism at the enzymatic and molecular levels in yellow catfish, and the reduced hepatic lipid content by hypothyroidism was attributable to the down-regulation of lipogenesis and up-regulation of lipolysis.

Effect of trifloxystrobin on hatching, survival, and gene expression of endocrine biomarkers in early life stages of medaka (Oryzias latipes)

Trifloxystrobin is a systemic broad-spectrum foliar strobilurin fungicides that enters the aquatic environment during agricultural application. It is highly toxic and poses a potential risk to aquatic organisms, whereas the effect on the development of early life stages of fish are unclear. In this study, hatchability, time to hatching, and larval mortality were measured. Additionally, the expression of biomarker genes, including those involved in sex hormone pathways (er, vtg, cyp17, and cyp19a), thyroid hormone pathways (trα and dio2), and aryl hydrocarbon receptor pathways (ahr and cyp1a), was determined after embryos of medaka (Oryzias latipes) were exposed to different levels of trifloxystrobin (0, 0.1, 1, 10, and 100 μg/L) for 28 days. The results showed that there were significant differences between controls and the 100 μg/L treatment group in both hatchability and time to hatching of fertilized eggs (p<0.05). Larval mortality was significantly increased in the 0.1, 1, and 10 μg/L treatment groups (p<0.05). These results indicate that embryonic and larval development may be affected by trifloxystrobin exposure. Moreover, the mRNA levels of the er gene were significantly up-regulated at levels of trifloxystrobin above 1 μg/L treatment groups. Up-regulation of vtg, cyp17, and cyp19a mRNA levels was observed in the larvae at the lower concentration treatment groups. The mRNA levels of cyp1a genes were significantly up-regulated at all of the treatment groups. These results suggest that trifloxystrobin is a potential endocrine disruptor through effects on the sex hormone pathway and xenobiotic metabolism. The changes in cyp1a expression can be used as a highly sensitive biomarker to assess trifloxystrobin contamination in the early life stages of fish.

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.

Sex specific response in cholesterol level in zebrafish (Danio rerio) after long-term exposure of difenoconazole

Difenoconazole is a widely used triazole fungicide, its extensive application may potentially cause toxic effects on non-target organisms. To investigate the effect of difenoconazole on cholesterol content and related mechanism, adult zebrafish were exposed to environmental related dosage (0.1, 10 and 500 μg/L) difenoconazole. The body weight and hepatic total cholesterol (TCHO) level was tested at 7, 15 and 30 days post exposure (dpe). The expressions of eight cholesterol synthesis genes and one cholesterol metabolism gene were assessed via Quantitative PCR method. The significant decrease of TCHO level in male zebrafish liver was observed at 15 and 30 dpe, which was accompanied by apparent hepatic cholesterol-genesis genes expression decline. In comparison with males, female zebrafish showed different transcription modification of tested genes, and the cholesterol content maintain normal level during the whole exposure.

Subacute microcystin-LR exposure alters the metabolism of thyroid hormones in juvenile zebrafish (Danio Rerio)

Microcystin-LR (MC-LR) has been detected extensively in the aquatic environment and has the potential to disturb the thyroid endocrine system. However, limited information is available on the effects of subacute MC-LR exposure on fish thyroid hormone (TH) metabolism. In the present study, juvenile zebrafish (Danio rerio) were exposed to MC-LR at environmentally relevant concentrations (0, 1, 5, and 25 μg/L) for 28 days. Whole-body TH content and thyroid follicle histology were used as direct endpoints to assess thyroid disruption. The activities of iodothyronine deiodinases (IDs) and the transcription of selected genes associated with TH synthesis were also investigated to study the underlying mechanisms of endocrine disruption. Exposure of zebrafish to MC-LR significantly increased whole-body thyroxine (T₄) content but decreased whole-body triiodothyronine (T₃) content. We also observed hypertrophy and hyperplasia of the thyroid follicle epithelial cells, as well as up-regulation of corticotropin-releasing hormone (CRH), thyroid-stimulating hormone (TSH), thyroid peroxidase (TPO), and transthyretin (TTR) genes. The decreases in ID1 and ID2 activities coupled with an increase in ID3 activity were observed in MC-LR treatment groups. These results demonstrate that exposure to MC-LR at environmental concentrations results in the disturbance of TH homeostasis by disrupting the synthesis and conversion of THs.

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.

Endocrine disrupting effects of benzotriazole in rare minnow (Gobiocypris rarus) in a sex-dependent manner

Benzotriazole (BT), an anticorrosive agent, is widely used in industrial applications and household dishwashing agents. Despite its reported toxicity to aquatic organisms, little is known about its endocrine disrupting effects. In this study, adult Chinese rare minnows (Gobiocypris rarus) were exposed to 0.05, 0.5, and 5 mg L(-1) BT for 28 d. The pathological damage in liver was associated with hypertrophy of the hepatocytes, nuclei pyknosis and vacuolization at 5 mg L(-1) groups. Additionally, the degeneration of the ovary and the stimulation of spermatogenesis were observed at 5 mg L(-1) groups. The plasma 17β-estradiol level was significantly increased in the males but decreased in the females at 5 mg L(-1) (p<0.05). In the brain, the up-regulation of CYP19B, GnRHs, and LHβ mRNA was detected across all doses (p<0.05). In the gonad, the transcriptional levels of StAR, CYP11A, 3βHSD, CYP17, 17βHSD, and CYP19A were generally decreased in the males at 5 mg L(-1) (p<0.05), whereas these genes, except for 3βHSD, were significantly increased in females at all concentrations (p<0.05). Moreover, the expression level of VTG in the livers from all exposure groups was significantly increased compared with controls (p<0.05). Taken together, our results indicate that BT could adversely affect the rare minnows in a sex-dependent manner.

Effects of water temperature on perchlorate toxicity to the thyroid and reproductive system of Oryzias latipes

Water temperature is expected to increase in many parts of the world due to global climate change. The change in water temperature may affect ecosystems through alterations of the chemical properties or by affecting the susceptibility of organisms. Perchlorate can disrupt thyroid function of an organism by inhibiting iodide uptake. In the present study, the effect of water temperature on perchlorate toxicity was evaluated using Japanese medaka (Oryzias latipes). Pairs of adult medaka fish were exposed to a sublethal concentration of sodium perchlorate (100mg/L) and a control, at a 'low' (26°C), 'medium' (29°C) or 'high' water temperature (33°C) for seven days. The effects of the water temperature on reproduction, thyroid hormones and cortisol concentrations were determined. Transcription of several genes related to thyroid function and stress were also investigated. Significant down-regulation of thyroid hormone receptor alpha (THR-α) and beta (THR-β) transcripts and up-regulation of deiodinase 2 (DIO2) transcripts were observed in the fish exposed to perchlorate. Thyroxine (T4) concentrations were decreased, while triiodothyronine (T3) levels remained constant following exposure to perchlorate, and this effect became more pronounced under the high water temperature conditions (33°C). Up-regulation of the DIO2 gene may explain these observations. The total number of spawned eggs decreased slightly as the water temperature increased, and this reduction became significant when fish were exposed to perchlorate. Our observations indicate that exposure to perchlorate could affect thyroid function and overall reproductive fitness, and these effects could be aggravated under high water temperatures.