Temporal expression and T3 induction of thyroid hormone receptors alpha1 and beta1 during early embryonic and larval development in zebrafish, Danio rerio

Estrogenic and growth inhibitory responses to organophosphorus flame retardant metabolites in zebrafish embryos

Recent evidence has revealed that organophosphorus flame retardants (OPFRs) elicit a variety of toxic effects, including endocrine disruption. The present study examined estrogenic and growth inhibitory responses to OPFR metabolites in comparison to their parent compounds using zebrafish eleutheroembryos.¹ Exposure to 4-hydroxylphenyl diphenyl phosphate (HO-p-TPHP) but not its parent compound triphenyl phosphate (TPHP) elicited upregulation of a marker gene of estrogenic responses, cytochrome P450 19A1b (CYP19A1b), and this upregulation was reversed by co-exposure to an estrogen receptor antagonist. Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) and bis(1,3-dichloro-2-propyl) phosphate (BDCIPP), as well as 3-hydroxylphenyl diphenyl phosphate (HO-m-TPHP) and diphenyl phosphate (DPHP), did not elicit significant changes in the CYP19A1b expression. Reduction in body length was induced by TPHP and to a lesser extent by its hydroxylated metabolites. Altered expression of genes involved in the synthesis and action of thyroid hormones, including iodothyronine deiodinases 1 and 2, thyroid hormone receptor alpha, and transthyretin, were commonly observed for TPHP and its hydroxylated metabolites. Reduction in the body length was also seen in embryos exposed to TDCIPP but not BDCIPP. The transcriptional effect of TDCIPP was largely different from that of TPHP, with decreased expression of growth hormone and prolactin observed only in TDCIPP-exposed embryos. Considering the concentration-response relationships for the growth retardation and gene expression changes, together with existing evidence from other researchers, it is likely that prolactin is in part involved in the growth inhibition caused by TDCIPP. The present study showed similarities and differences in the endocrine disruptive effects of OPFRs and their metabolites.

The Role of Estrogen and Thyroid Hormones in Zebrafish Visual System Function

Visual system development is a highly complex process involving coordination of environmental cues, cell pathways, and integration of functional circuits. Consequently, a change to any step, due to a mutation or chemical exposure, can lead to deleterious consequences. One class of chemicals known to have both overt and subtle effects on the visual system is endocrine disrupting compounds (EDCs). EDCs are environmental contaminants which alter hormonal signaling by either preventing compound synthesis or binding to postsynaptic receptors. Interestingly, recent work has identified neuronal and sensory systems, particularly vision, as targets for EDCs. In particular, estrogenic and thyroidogenic signaling have been identified as critical modulators of proper visual system development and function. Here, we summarize and review this work, from our lab and others, focusing on behavioral, physiological, and molecular data collected in zebrafish. We also discuss different exposure regimes used, including long-lasting effects of developmental exposure. Overall, zebrafish are a model of choice to examine the impact of EDCs and other compounds targeting estrogen and thyroid signaling and the consequences of exposure in visual system development and function.

Thyroid disruption and growth inhibition of zebrafish embryos/larvae by phenanthrene treatment at environmentally relevant concentrations

Phenanthrene induces reproductive and developmental toxicity in fish, but whether it can disrupt the thyroid hormone balance and inhibit growth had not been determined to date. In this study, zebrafish embryos were exposed to phenanthrene (0, 0.1, 1, 10 and 100 μg/L) for 7 days. The results of this experiment demonstrated that phenanthrene induced thyroid disruption and growth inhibition in zebrafish larvae. Phenanthrene significantly decreased the concentration of l-thyroxine (T4) but increased that of 3,5,3'-l-triiodothyronine (T3). The expression of genes related to the hypothalamic-pituitary-thyroid (HPT) axis was altered in zebrafish larvae exposed to phenanthrene. Moreover, phenanthrene exposure significantly increased the malformation rate and significantly reduced the survival rate and the body length of zebrafish larvae. Furthermore, phenanthrene significantly decreased the concentrations of growth hormone (GH) and insulin-like growth factor-1 (IGF-1). Changes observed in gene expression patterns further support the hypothesis that these effects may be related to alterations along the GH/IGF-1 axis. In conclusion, our study indicated that exposure to phenanthrene at concentrations as low as 0.1 μg/L resulted in thyroid disruption and growth inhibition in zebrafish larvae. Therefore, the estimation of phenanthrene levels in the aquatic environment needs to be revisited.

Single and binary-combined toxic effects of acetochlor and Cu2+ on goldfish (Carassius auratus) larvae

Acetochlor and copper are common freshwater pollutants and pose a severe threat to aquatic animals. The toxicity of acetochlor (Ac) and Cu²⁺ toward goldfish larvae was investigated by subjecting the larvae to different concentrations of acetochlor, Cu²⁺, and mixed solutions for 1, 3, and 7 days. The length of goldfish larvae exposed to the 100 μg/L Ac + 100 μg/L Cu²⁺ mixed solution was considerably lower than that of the control on day 3, but there were no significant differences among the other groups. The heart rates of the larvae in 100 μg/L Ac + 100 μg/L Cu²⁺ mixed solution were higher than those of the control group on days 3 and 7. Acetochlor and Cu²⁺ also caused severe damage to the liver and intestine of the larvae, especially in the 100 μg/L Ac + 100 μg/L Cu²⁺ mixed solution group. Indicators related to oxidative stress (hydrogen peroxide, catalase, glutathione peroxidase, and total superoxide dismutase) that could potentially be induced by acetochlor or Cu²⁺ began to increase on day 7, and the enzyme activities of the larvae in the mixed groups were significantly lower than those in the control group. In contrast, the expression levels of the genes related to antioxidant stress were rapidly down-regulated in all groups on the 7th day after exposure. Briefly, the combined toxicity of acetochlor and Cu²⁺ was stronger than that of the single toxicity treatments. Furthermore, toxicity toward larvae in the mixed solution group (100 μg/L Ac + 100 μg/L Cu²⁺) was more obvious.

Physiological and transcriptomic changes of zebrafish (Danio rerio) embryos-larvae in response to 2-MIB exposure

2-Methylisoborneol (2-MIB), a natural odorous substance, is widely distributed in water environment, but there is a paucity of information concerning its systemic toxicity. Herein, we investigated the effects of 2-MIB exposure on developmental parameters, locomotive behavior, oxidative stress, apoptosis and transcriptome of zebrafish. Zebrafish embryos exposed to different concentrations (0, 0.5, 5 and 42.8 μg/L) of 2-MIB showed no changes in mortality, hatchability, and malformation rate, but the body length of zebrafish larvae was significantly increased in a dose-dependent manner, and accompanied by the changes of growth hormone/insulin-like growth factor (GH/IGF) axis and the hypothalamic-pituitary-thyroid (HPT) axis genes. Moreover, the swimming activity of zebrafish larvae increased, which may be due to the increase of acetylcholinesterase (AChE) activity. Meanwhile, 2-MIB caused oxidative stress and apoptosis in zebrafish larvae by altering the NF-E2-related factor 2 (Nrf2) and mitochondrial signaling pathways, respectively. Transcriptome sequencing assay showed that the phototransduction signaling pathway was significantly enriched, and most of the genes in this pathway exhibited enhanced expression after exposure to 2-MIB. These findings provide an important reference for risk assessment and early warning to 2-MIB exposure.

Dietary selenium promotes the growth performance through growth hormone-insulin-like growth factor and hypothalamic-pituitary-thyroid axes in grass carp (Ctenopharyngodon idella)

Selenium (Se), an essential component of deiodinases (DIOs), regulates the contents of thyroid hormones and thus improves animal growth. To explore the influences of selenium supplementation on fish growth metabolism, a total of 270 healthy grass carp (Ctenopharyngodon idella) were divided into three groups and feed three graded dietary selenium (0.141, 0.562, and 1.044 mg Se/kg) levels. The results showed that after 60-day feeding, dietary selenium improved the final body weight and specific growth rate (SGR) of grass carp. The hepatic DIO activities in selenium-supplemented groups were higher than those in control group. A significant increase in triiodothyronine (T3), free triiodothyronine (FT3), and thyroid-stimulating hormone (TSH) levels was accompanied by a decrease in the contents of thyroxine (T4) and free thyroxine (FT4) in selenium-supplemented groups. The histopathological observation of thyroid suggested that selenium deficiency resulted in hypertrophy of follicular epithelial cells. Moreover, the gene relative expression levels of dio1, dio2, and dio3 showed an increasing trend with the rising concentration of dietary selenium. The transcription levels of HPT axis-related genes (crh, tsh-β, ttr, tr-s, tpo, nis) and GH/IGF1-related genes (gh, ghr, igf1, igf1r) were significantly upregulated in selenium-supplemented groups. No significant differences in the above indicators were observed between 0.562 and 1.044 mg Se/kg diet group except T3 content and dio1 relative expression ratio. These results indicate that dietary selenium supplementation improves the hepatic DIO activities and thyroid hormone metabolism and regulates the transcription levels of HPT and GH/IGF axis-related genes, which may be responsible for the growth promotion in grass carp.

Effects of environmentally relevant concentrations of tris (2-chloroethyl) phosphate (TCEP) on early life stages of zebrafish (Danio rerio)

Tris (2-chloroethyl) phosphate (TCEP) has been received great concerns because of its increasing presence in various environmental compartments and toxicity. In the present study, zebrafish embryos were exposed to environmentally relevant concentrations of TCEP (0.2, 2, 20, 200 μg/L) from 3 to 120 h post-fertilization (hpf). The results showed that TCEP exposure (20, 200 μg/L) led to developmental toxicity including decreased body length and delay of hatching. Treatment with TCEP significantly decreased whole-body thyroxine (T4) levels and mRNA level of thyroglobulin (tg), and enhanced transcriptions of genes sodium/iodide symporter (nis), thyroid hormone receptor α (trα) and ugt1ab involved in thyroid synthesis and metabolism, respectively. Additionally, TCEP altered the transcription of α1-tubulin, gap43 and mbp related to nervous system development, even at relatively low concentrations. Overall, our results revealed that TCEP exposure can lead to developmental toxicity, thyroid endocrine disruption and neurotoxicity on early developmental stages of zebrafish.

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.

Tetrabromobisphenol A Perturbs Erythropoiesis and Impairs Blood Circulation in Zebrafish Embryos

Tetrabromobisphenol A (TBBPA), a ubiquitous environmental pollutant, has been implicated in developmental toxicity of aquatic animals. However, the impact of TBBPA on development and the related mechanism have not been fully elucidated. In this study, using a live imaging technique and transgenic labeling of zebrafish embryos, we described the toxic effects of TBBPA on hematopoietic development in zebrafish. We demonstrated that TBBPA induced erythroid precursor expansion in the intermediate cell mass (ICM), which perturbed the onset of blood circulation at 24-26 hours postfertilization (hpf). Consequently, excessive blood cells accumulated in the posterior blood island (PBI) and vascular cells formed defective caudal veins (CVs), preventing blood cell flow to the heart at 32-34 hpf. We found that the one-cell to 50% epiboly stage was the most sensitive period to TBBPA exposure during hematopoietic development. Furthermore, our results demonstrated that PBI malformation induced by TBBPA resulted from effects on erythroid precursor cells, which might involve THR signaling in complex ways. These findings will improve the understanding of TBBPA-induced developmental toxicity in teleost.

Thyroid disruption and developmental toxicity caused by Cd2+ in Schizopygopsis younghusbandi larvae

In recent years, the adverse effects of cadmium (Cd²⁺) on aquatic systems have attracted much attention because Cd²⁺ can induce endocrine disorders and toxicity in aquatic organisms at low levels. However, its effects on the thyroid system in native fish in Lhasa are still unclear. In the present study, Schizopygopsis younghusbandi larvae were exposed to Cd²⁺ (0.25, 2.5, 25 or 250 μg/L) for 7 or 14 days to determine its toxic effects on thyroid function. The results showed that whole-body total T4 and T3 levels were significantly decreased, which was accompanied by the significant upregulation of the expression of the dio1 and dio2 genes after exposure to Cd²⁺ for 7 or 14 days. Genes related to thyroid hormone synthesis (crh and tshβ) were upregulated after both 7 and 14 days of Cd²⁺ exposure, possibly due to the negative feedback regulation of the hypothalamic-pituitary-thyroid (HPT) axis caused by a decrease in thyroid hormone. In addition, survival rates and body lengths were reduced after treatment with Cd²⁺. This suggests that Cd²⁺ caused developmental toxicity in Schizopygopsis younghusbandi larvae. An integrated assessment of biomarker response (IBR) showed that there were dose-dependent and time-dependent effects of Cd²⁺ exposure on Schizopygopsis younghusbandi larvae. Schizopygopsis younghusbandi larvae were sensitive to Cd²⁺, which caused adverse effects at a concentration as low as 2.5 μg/L. In summary, the results indicated that Cd²⁺ causes thyroid disruption and developmental toxicity in Schizopygopsis younghusbandi larvae and that wild Schizopygopsis younghusbandi larvae living in the Lhasa River are at potential ecological risk.

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

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

Thyroid hormone receptor beta mutations alter photoreceptor development and function in Danio rerio (zebrafish)

We investigate mutations in trβ2, a splice variant of thrb, identifying changes in function, structure, and behavior in larval and adult zebrafish retinas. Two N-terminus CRISPR mutants were identified. The first is a 6BP+1 insertion deletion frameshift resulting in a truncated protein. The second is a 3BP in frame deletion with intact binding domains. ERG recordings of isolated cone signals showed that the 6BP+1 mutants did not respond to red wavelengths of light while the 3BP mutants did respond. 6BP+1 mutants lacked optomotor and optokinetic responses to red/black and green/black contrasts. Both larval and adult 6BP+1 mutants exhibit a loss of red-cone contribution to the ERG and an increase in UV-cone contribution. Transgenic reporters show loss of cone trβ2 activation in the 6BP+1 mutant but increase in the density of cones with active blue, green, and UV opsin genes. Antibody reactivity for red-cone LWS1 and LWS2 opsin was absent in the 6BP+1 mutant, as was reactivity for arrestin3a. Our results confirm a critical role for trβ2 in long-wavelength cone development.

There are four cone photoreceptors responsible for color vision in zebrafish: red, green, blue, and UV. The thyroid hormone receptor trβ2 is localized in the vertebrate retina. We know that it is necessary for the development of cones expressing long-wavelength-sensitive opsins (red cones), but here we investigate the functional alterations that accompany a loss of trβ2. Our work contributes to the ongoing investigations of retinal development and the involvement of thyroid hormone receptors. As suggested by previous morphological findings, fish became red colorblind when trβ2 was knocked out, and the contributions of the other three cone types shifted. Our work highlights the plasticity of photoreceptor patterning as we see changes in opsin peaks and cone sensitivity, increases in contributions of UV cones, and an attempt at a mosaic pattern in the adult retina, all in the absence of trβ2 and red cones. We now have an increased understanding of mechanisms underlying retinal development.

Integrated thyroid endocrine disrupting effect on zebrafish (Danio rario) larvae via simultaneously repressing type II iodothyronine deiodinase and activating thyroid receptor-mediated signaling following waterborne exposure to trace azocyclotin

As a widely used organotin acaricide nowadays, azocyclotin (ACT) could induce thyroidal endocrine disruption in fishes and amphibians, but its dominant disrupting mode remains unknown. In this study, zebrafish were firstly exposed to ACT (0.18-0.36 ng/mL) from 2 hpf (hours post fertilization) to 30 dpf (days post fertilization), and a series of developmental toxicological endpoints and thyroid hormones were measured. Result showed that no developmental toxicity to zebrafish was found in 0.18 and 0.24 ng/mL groups except decreased body weight (30 dpf, 0.24 ng/mL). However, exposed to 0.36 ng/mL ACT led to reductions in heartbeat (48 hpf), hatching rate (72 hpf) and bodyweight (30 dpf). General tendencies of decreases in free T3 but increases in free T4 and reductions in ratio of free T3/T4 were also found, inferring that type II deiodinase (Dio2) was repressed. This inference was confirmed by Western analysis that Dio2 expression reduced by 42.7% after 0.36 ng/mL ACT treatment. Moreover, RNA-Seq analysis implied that exposed to 0.36 ng/mL ACT altered the genome-wide gene expression profiles of zebrafish. Totally 5660 genes (involving 3154 down-regulated and 2596 up-regulated genes) were differentially expressed, and 13 deferentially expressed genes including down-regulated dio2 were significantly enriched in thyroid hormone signaling pathway. Subsequently, an in vitro thyroid receptor-reporter gene assay using GH3 cells was performed to further explore the potential disrupting mechanism. Result showed that luciferase activity slightly increased after exposure to ACT alone or ACT combined with low level T3, but was suppressed when combined with high level T3. It indicted there probably existed a competitive relationship in some extent between ACT and T3 in vivo. Overall, the present study provided preliminary evidences that long-term exposure to trace ACT repressed Dio2 expression, declined T3 and then activated thyroid receptor-mediated signaling, thereby leading to integrated thyroid endocrine disruption in zebrafish larvae.

Comparative thyroid disruption by o,p'-DDT and p,p'-DDE in zebrafish embryos/larvae

1,1-Trichloro-2-(p-chlorophenyl)-2-(o-chlorophenyl) ethane (o,p'-DDT) and 1,1-dichloro-2,2-bis (p-chlorophenyl)-ethylene (p,p'-DDE) cause thyroid disruption, but the underlying mechanisms of these disturbances in fish remain unclear. To explore the potential mechanisms of thyroid dysfunction caused by o,p'-DDT and p,p'-DDE, thyroid hormone and gene expression levels in the hypothalamic-pituitary-thyroid (HPT) axis were measured, and the developmental toxicity were recorded in zebrafish larvae. Zebrafish embryos/larvae were exposed to o,p'-DDT (0, 0.28, 2.8, and 28 nM; or 0, 0.1, 1, and 10 μg/L) and p,p'-DDE (0, 1.57, 15.7, and 157 nM; or 0, 0.5, 5, and 50 μg/L) for 7 days. The genes related to thyroid hormone synthesis (crh, tshβ, tg, nis and tpo) and thyroid development (nkx2.1 and pax8) were up-regulated in both the o,p'-DDT and p,p'-DDE exposure groups. Zebrafish embryos/larvae exposed to o,p'-DDT showed significantly increased total whole-body T4 and T3 levels, with the expression of ugt1ab and dio3 being significantly down-regulated. However, the p,p'-DDE exposure groups showed significantly lowered whole-body total T4 and T3 levels, which were associated with up-regulation and down-regulation expression of the expression of dio2 and ugt1ab, respectively. Interestingly, the ratio of T3 to T4 was significantly decreased in the o,p'-DDT (28 nM) and p,p'-DDE (157 nM) exposure groups, suggesting an impairment of thyroid function. In addition, reduced survival rates and body lengths and increased malformation rates were recorded after treatment with either o,p'-DDT or p,p'-DDE. In summary, our study indicates that the disruption of thyroid states was different in response to o,p'-DDT and p,p'-DDE exposure in zebrafish larvae.

Effects of hyperthyroidism in the development of the appendicular skeleton and muscles of zebrafish, with notes on evolutionary developmental pathology (Evo-Devo-Path)

The hypothalamus-pituitary-thyroid (HPT) axis plays a crucial role in the metabolism, homeostasis, somatic growth and development of teleostean fishes. Thyroid hormones regulate essential biological functions such as growth and development, regulation of stress, energy expenditure, tissue compound, and psychological processes. Teleost thyroid follicles produce the same thyroid hormones as in other vertebrates: thyroxin (T4) and triiodothyronine (T3), making the zebrafish a very useful model to study hypo- and hyperthyroidism in other vertebrate taxa, including humans. Here we investigate morphological changes in T3 hyperthyroid cases in the zebrafish to better understand malformations provoked by alterations of T3 levels. In particular, we describe musculoskeletal abnormalities during the development of the zebrafish appendicular skeleton and muscles, compare our observations with those recently done by us on the normal developmental of the zebrafish, and discuss these comparisons within the context of evolutionary developmental pathology (Evo-Devo-Path), including human pathologies.

Parental exposure to tris(1,3-dichloro-2-propyl) phosphate results in thyroid endocrine disruption and inhibition of growth in zebrafish offspring

Tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) is a re-emerging environmental contaminant used as a suitable substitute for brominated flame retardants. The objective of this study was to evaluate the effects of TDCIPP on thyroid disruption and growth inhibition in zebrafish (Danio rerio) offspring after chronic parental exposure, and to examine the possible molecular mechanisms involved. When adult zebrafish (4 months old) were exposed to 5.66, 25.55, or 92.8 μg TDCIPP/L for 90 days, bioconcentration of TDCIPP and its metabolic product [bis(1,3-dichloro-2-propyl) phosphate, BDCIPP] was observed in 7-day postfertilization (dpf) F1 larvae, which suggests the transfer of this compound from adult fish to their offspring. Our results demonstrated that parental exposure to TDCIPP induced thyroid disruption in the offspring, demonstrated by significantly decreased thyroxine (T4) and increased 3,5,3'-triiodothyronine (T3) levels, and disruption of the transcription of several genes and expression of proteins involved in the hypothalamic-pituitary-thyroid (HPT) axis in F1 larvae. Parental exposure to TDCIPP resulted in developmental abnormalities in offspring; the smaller body length that was recorded might be partly the result of the perturbation of the HPT axis. In addition, the results revealed that growth inhibition also resulted from the downregulation of the transcription of genes and expression of proteins involved in the growth hormone/insulin-like growth factor (GH/IGF) axis. Our study provides a new set of evidence showing that parental exposure to TDCIPP can induce thyroid disruption and inhibition of growth in offspring, and that perturbation of the HPT axis and GH/IGF axis contribute to these adverse effects.

Blood Transcriptomics Analysis of Fish Exposed to Perfluoro Alkyls Substances: Assessment of a Non-Lethal Sampling Technique for Advancing Aquatic Toxicology Research

In contrast to mammals, the blood from other vertebrates such as fish contains nucleated red cells. Using a fathead minnow ( Pimephales promelas) oligonucleotide microarray, we compared altered transcripts in the liver and whole blood after exposure to environmentally relevant concentrations of perfluorooctanesulfonic acid (PFOS) and a mixture of seven types of perfluoro alkyl substances (PFAS), including perfluorooctanoic acid (PFOA). We used quantitative polymerase chain reactions and cell-based assays to confirm the main effects and found that blood responded with a greater number of altered genes than the liver. The exposure to PFAS altered similar genes with central roles in a cellular pathway in both tissues, including estrogen receptor α and peroxisome proliferator activator β and γ, indicating that the genes previously associated with PFAS exposure are differentially expressed in blood and liver. The altered transcripts are involved with cholesterol metabolism and mitochondrial function. Our data confirmed that PFAS are weak xenoestrogens and exert effects on DNA integrity. Gene expression profiling from blood samples not related with the immune system, including very-low-density lipoprotein, vitellogenin, estrogen receptor, and thyroid hormone receptor, demonstrated that blood is a useful tissue for assessing endocrine disruption in non-mammalian vertebrates. We conclude that the use of blood for non-lethal sampling in genomics studies is informative and particularly useful for assessing the effects of pollution in endangered species. Further, using blood will reduce animal use and widen the experimental design options for studying the effects of contaminant exposure on wildlife.

Effects of thyroid hormone disruption on the ontogenetic expression of thyroid hormone signaling genes in developing zebrafish (Danio rerio)

Thyroid hormones (THs) regulate neurodevelopment, thus TH disruption is widely posited as a mechanism of developmental neurotoxicity for diverse environmental chemicals. Zebrafish have been proposed as an alternative model for studying the role of TH in developmental neurotoxicity. To realize this goal, it is critical to characterize the normal ontogenetic expression profile of TH signaling molecules in the developing zebrafish and determine the sensitivity of these molecules to perturbations in TH levels. To address these gaps in the existing database, we characterized the transcriptional profiles of TH transporters, deiodinases (DIOs), receptors (TRs), nuclear coactivators (NCOAs), nuclear corepressors (NCORs), and retinoid X receptors (RXRs) in parallel with measurements of endogenous TH concentrations and tshβ mRNA expression throughout the first five days of zebrafish development. Transcripts encoding these TH signaling components were identified and observed to be upregulated around 48-72 h post fertilization (hpf) concurrent with the onset of larval production of T4. Exposure to exogenous T4 and T3 upregulated mct8, dio3-b, trα-a, trβ, and mbp-a levels, and downregulated expression of oatp1c1. Morpholino knockdown of TH transporter mct8 and treatment with 6-propyl-2-thiouracil (PTU) was used to reduce cellular uptake and production of TH, an effect that was associated with downregulation of dio3-b at 120 hpf. Collectively, these data confirm that larval zebrafish express orthologs of TH signaling molecules important in mammalian development and suggest that there may be species differences with respect to impacts of TH disruption on gene transcription.

Effects of bisphenol analogs on thyroid endocrine system and possible interaction with 17β-estradiol using GH3 cells

This study was conducted using a rat pituitary (GH3) cell line to understand the effects of bisphenol analogs (BPs) on the thyroid endocrine system, in the presence of 17β-estradiol (E2). In the first series of experiments, changes in cell proliferation were examined after exposure to each of ten BPs, in the absence or presence of a median effective concentration (6.4 × 10-10 M) of triiodothyronine (T3). All tested BPs significantly increased cell proliferation, suggesting thyroid hormone (TH) agonistic effects of BPs. BPs did not potentiate the T3-induced cell proliferation at 48 h exposure, while several tested BPs including BPA, BPAF, BPB, BPF, BPS, and BPZ elicited a potentiating effect on the T3-induced cell proliferation at 96 h exposure. These results indicate that TH-antagonistic effects of BPs depend on the tested dose and exposure time. In the second set of experiments, one of the most potent BPs, i.e., BPAF, was selected, and its possible interaction with E2 on the thyroid endocrine system was evaluated. Co-exposure of GH3 cells to 10-12 M E2 showed an additive-like effect. The extent of increase in cell proliferation was more pronounced with a combination of BPAF and E2 than with that of BPA and E2. Significant down-regulation of Trα, Trβ, and Dio2 genes and up-regulation of the Tshβ gene were observed in GH3 cells following co-exposure to BPAF and E2. Our results showed that some BP analogs might influence the thyroid endocrine system, and such perturbation appeared to be enhanced in the presence of E2.

Revisiting available knowledge on teleostean thyroid hormone receptors

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

Evaluation of joint effects of cyprodinil and kresoxim-methyl on zebrafish, Danio rerio

Aquatic organisms are usually exposed to a mixture of pesticides instead of individual chemicals. However, risk assessment of pesticides is traditionally based on toxicity data of individual compounds. In this study, we aimed to examine the joint toxicity of two fungicides cyprodinil (CYP) and kresoxim-methyl (KRM) to zebrafish (Danio rerio) using a systematic experimental approach. Results from 96-h semi-static test indicated that the LC₅₀ values of KRM to D. rerio at multiple life stages (embryonic, larval, juvenile and adult stages) ranged from 0.034 (0.015-0.073) to 0.61 (0.39-0.83) mg a.i. L^-1, which were higher than those of CYP ranging from 1.05 (0.88-1.52) to 4.42 (3.24-6.02) mg a.i. L^-1. Pesticide mixtures of CYP and KRM exhibited synergistic effect on embryonic zebrafish. The activities of carboxylesterase (CarE) and cytochrome P450 (Cyp450) were significantly altered in most of the individual and combined exposures compared with the control group. The expressions of seven genes (Mnsod, cyp17, crhr 2, crh, gnrhr 4, gnrhr 1 and hmgrb) were significantly altered upon exposure to combined pesticides compared with their individual pesticides. Collectively, these findings suggested joint effects should be considered in the risk assessment of pesticides and development of water quality criteria for the protection of aquatic environment.

The Affinity of Brominated Phenolic Compounds for Human and Zebrafish Thyroid Receptor β: Influence of Chemical Structure

Brominated phenolic compounds (BPCs) are found in the environment, and in human and wildlife tissues, and some are considered to have endocrine disrupting activities. The goal of this study was to determine how structural differences of 3 BPC classes impact binding affinities for the thyroid receptor beta (TRβ) in humans and zebrafish. BPC classes included halogenated bisphenol A derivatives, halogenated oxidative transformation products of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), and brominated phenols. Affinities were assessed using recombinant TRβ protein in competitive binding assays with 125I-triiodothyronine (125I-T3) as the radioligand. Zebrafish and human TRβ displayed similar binding affinities for T3 (Ki = 0.40 and 0.49 nM) and thyroxine (T4, Ki = 6.7 and 6.8 nM). TRβ affinity increased with increasing halogen mass and atomic radius for both species, with the iodinated compounds having the highest affinity within their compound classes. Increasing halogen mass and radius increases the molecular weight, volume, and hydrophobicity of a compound, which are all highly correlated with increasing affinity. TRβ affinity also increased with the degree of halogenation for both species. Human TRβ displayed higher binding affinities for the halogenate bisphenol A compounds, whereas zebrafish TRβ displayed higher affinities for 2,4,6-trichlorophenol and 2,4,6-trifluorophenol. Observed species differences may be related to amino acid differences within the ligand binding domains. Overall, structural variations impact TRβ affinities in a similar manner, supporting the use of zebrafish as a model for TRβ disruption. Further studies are necessary to investigate how the identified structural modifications impact downstream receptor activities and potential in vivo effects.

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.

Heterochronic development of lateral plates in the three-spined stickleback induced by thyroid hormone level alterations

The three-spined stickleback Gasterosteus aculeatus is an important model for studying microevolution and parallel adaptation to freshwater environments. Marine and freshwater forms differ markedly in their phenotype, especially in the number of lateral plates, which are serially repeated elements of the exoskeleton. In fishes, thyroid hormones are involved in adaptation to salinity, as well as the developmental regulation of serially repeated elements. To study how thyroid hormones influence lateral plate development, we manipulated levels of triiodothyronine and thiourea during early ontogeny in a marine and freshwater population with complete and low plate phenotypes, respectively. The development of lateral plates along the body and keel was heterochronic among experimental groups. Fish with a low dosage of exogenous triiodothyronine and those treated with thiourea exhibited retarded development of bony plates compared to both control fish and those treated with higher a triiodothyronine dosage. Several triiodothyronine-treated individuals of the marine form expressed the partial lateral plate phenotype. Some individuals with delayed development of lateral plates manifested 1–2 extra bony plates located above the main row of lateral plates.

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

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

G protein-coupled estrogen receptor regulates embryonic heart rate in zebrafish

Estrogens act by binding to estrogen receptors alpha and beta (ERα, ERβ), ligand-dependent transcription factors that play crucial roles in sex differentiation, tumor growth and cardiovascular physiology. Estrogens also activate the G protein-coupled estrogen receptor (GPER), however the function of GPER in vivo is less well understood. Here we find that GPER is required for normal heart rate in zebrafish embryos. Acute exposure to estrogens increased heart rate in wildtype and in ERα and ERβ mutant embryos but not in GPER mutants. GPER mutant embryos exhibited reduced basal heart rate, while heart rate was normal in ERα and ERβ mutants. We detected gper transcript in discrete regions of the brain and pituitary but not in the heart, suggesting that GPER acts centrally to regulate heart rate. In the pituitary, we observed gper expression in cells that regulate levels of thyroid hormone triiodothyronine (T3), a hormone known to increase heart rate. Compared to wild type, GPER mutants had reduced levels of T3 and estrogens, suggesting pituitary abnormalities. Exposure to exogenous T3, but not estradiol, rescued the reduced heart rate phenotype in gper mutant embryos, demonstrating that T3 acts downstream of GPER to regulate heart rate. Using genetic and mass spectrometry approaches, we find that GPER regulates maternal estrogen levels, which are required for normal embryonic heart rate. Our results demonstrate that estradiol plays a previously unappreciated role in the acute modulation of heart rate during zebrafish embryonic development and suggest that GPER regulates embryonic heart rate by altering maternal estrogen levels and embryonic T3 levels.

Estrogen hormones are important for the formation and function of the nervous, reproductive and cardiovascular systems. Here we report that acute exposure to estrogens increases heart rate, a previously unappreciated function of estrogens. Using zebrafish with mutations in genes that respond to estrogens, we found that heart rate is regulated not by the typical molecules that respond to estrogens–the nuclear estrogen receptors–but rather by a different molecule, the G protein-coupled estrogen receptor. We also show that estrogens increase heart rate by increasing levels of thyroid hormone. Our results reveal a new function for the G protein-coupled estrogen receptor and a new connection between estrogens and thyroid hormone. Environmental compounds that mimic estrogens can be harmful because they can influence gonad function. Our results suggest that endocrine disrupting compounds may also influence cardiac function.

Low Thyroid Hormone Levels Disrupt Thyrotrope Development

Low thyroid hormone (TH) conditions caused by a variety of prenatal and perinatal problems have been shown to alter postnatal regulatory thyrotropin (TSH) responsiveness to TH in humans and rodents. The mechanisms underlying this pituitary TH resistance remain unknown. Here we use the evolutionarily conserved zebrafish model to examine the effects of low TH on thyrotrope development and function. Zebrafish were exposed to the goitrogen 6-propyl-2-thiouracil (PTU) to block TH synthesis, and this led to an approximately 50% increase in thyrotrope numbers and an 8- to 10-fold increase in tshb mRNA abundance in 2-week-old larvae and 1-month-old juveniles. Thyrotrope numbers returned to normal 3 weeks after cessation of PTU treatment, demonstrating that these effects were reversible and revealing substantial plasticity in pituitary-thyroid axis regulation. Using a T4 challenge assay, we found that development under low-TH conditions did not affect the ability of T4 to suppress tshb mRNA levels despite the thyrotrope hyperplasia that resulted from temporary low-TH conditions. Together, these studies show that low developmental TH levels can lead to changes in thyrotrope number and function, providing a possible cellular mechanism underlying elevated TSH levels seen in neonates with either permanent or transient congenital hypothyroidism.

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.

Semicarbazide-induced thyroid disruption in Japanese flounder (Paralichthys olivaceus) and its potential mechanisms

Semicarbazide (SMC) is a carcinogenic and genotoxic substance that has been found in aquatic systems. SMC may also cause thyroid follicular epithelial cell injury in rats, but the thyroid-disrupting properties of SMC and its potential mechanisms remain unclear. In this study, we exposed fertilized eggs of Japanese flounder (Paralichthys olivaceus) to 1, 10, 100, and 1000μg/L SMC for 55 d to assess the impact of SMC exposure on the thyroid system. The number of larvae in each metamorphic stage was counted, the concentrations of whole-body thyroid hormones (THs) 3,5,3'-triiodothyronine (T3) and thyroxine (T4) were measured, and the transcription levels of genes involved in the hypothalamic-pituitary-thyroid (HPT) axis and gamma-aminobutyric acid (GABA) synthesis were quantified. The results showed that 10μg/L SMC significantly increased whole-body T3 levels, and 100 and 1000μg/L SMC markedly enhanced whole-body T4 and T3 levels. Furthermore, 100μg/L SMC exposure shortened the time it took for flounder larvae to complete metamorphosis by 2 d as compared to the control group. Thus, this study demonstrated that SMC exerted thyroid-disrupting effects on Japanese flounder. SMC-mediated stimulation of TH levels was primarily related to transcriptional alterations of pituitary-derived thyroid stimulating hormone β-subunit (tshβ) and hepatic deiodinase (id). In the 10 and 100μg/L SMC exposure groups, the increased TH levels may have resulted from inhibition of TH metabolism caused by down-regulation of id3 mRNA expression, while at 1000μg/L SMC-exposed group, up-regulation of tshβ and id1 transcripts was expected to enhance the synthesis of T4 and the conversion of T4 to T3 and, consequently, result in higher T4 and T3 levels. In addition, 1000μg/L SMC-induced down-regulation in glutamic acid decarboxylase gad65 and gad67 transcription may have also contributed to the increased TH levels. The thyroid-disrupting effects of 10 and 100μg/L SMC indicated that environmentally relevant concentrations of SMC posed potential environmental risks to aquatic organisms. Overall, our results demonstrated for the first time that SMC exhibited thyroid-disrupting properties by affecting the HPT axis and GABA synthesis, providing theoretical support for environmental risk assessment.

Hepatic gene expression profiles of a non-model cyprinid (Barbus plebejus) chronically exposed to river sediments

In this study, we characterized the gene expression responses of the Padanian barbel (Barbus plebejus), a native benthivorous cyprinid with a very compromised presence within the fish community of the River Po. Barbel juveniles were exposed in the laboratory to two river sediments reflecting an upstream/downstream gradient of increasing contamination and collected from one of the most anthropized tributaries of the River Po. After 7months of exposure, hepatic transcriptional changes that were diagnostic of sediment exposure were assessed. We investigated a set of 24 genes involved in xenobiotic biotransformation (cyp1a, gstα, ugt), antioxidant defense (gpx, sod, cat, hsp70), trace metal exposure (mt-I, mt-II), DNA repair (xpa, xpc), apoptosis (bax, casp3), growth (igf2), and steroid (erα, erβ1, erβ2, ar, vtg) and thyroid (dio1, dio2, trα, trβ, nis) hormone signaling pathways. In a consistent overall picture, the results showed that long-term sediment exposure mainly increased the levels of mRNAs encoding proteins involved in xenobiotic metabolism, oxidative stress defense, repair of DNA damage and activation of the apoptotic process. Transcript up-regulation of three receptor genes (erβ2, ar, trβ), likely representing compensatory responses to antagonistic/toxic effects, was also observed, confirming the exposure to disruptors of the reproductive and thyroidal axes. In contrast to expectations, a few genes showed no response (e.g., casp3) or even downregulation (vtg), further suggesting that the timing of exposure/assessment, potential compensatory effects or post-transcriptional modifications interact to modify the gene expression profiles, particularly during exposure to mixtures of contaminants.

Adverse effects of BDE-47 on in vivo developmental parameters, thyroid hormones, and expression of hypothalamus-pituitary-thyroid (HPT) axis genes in larvae of the self-fertilizing fish Kryptolebias marmoratus

2,2',4,4'-tetrabromodiphenylether (BDE-47) is known to have the potential to disrupt the thyroid endocrine system in fishes due to its structural similarity to the thyroid hormones triiodothyronine (T3) and thyroxine (T4). However, the effects of BDE-47 on thyroid function in fishes remain unclear. In this study, abnormal development (e.g. deformity, hemorrhaging) and an imbalance in thyroid hormone (TH) homeostasis was shown in the early developmental stages of the mangrove killifish Kryptolebias marmoratus in response to BDE-47 exposure. To examine the thyroid endocrinal effect of BDE-47 exposure in mangrove killifish K. marmoratus larvae, transcript levels of genes involved in TH homeostasis and hypothalamus-pituitary-thyroid (HPT) axis-related genes were measured. The expression of thyroid hormone metabolism-related genes (e.g. deiodinases, UGT1ab) and HPT axis-related genes was up-regulated and there were significant changes in TH levels (P < 0.05) in response to BDE-47 exposure. This study provides insights into the regulation of TH homeostasis at the transcriptional level and provides a better understanding on the potential impacts of BDE-47 on the thyroid endocrine system of fishes.

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.

Coordinated expression and regulation of deiodinases and thyroid hormone receptors during metamorphosis in the Japanese flounder (Paralichthys olivaceus)

In vertebrates, thyroid hormone receptors (TRs) and deiodinases are essential for developmental events driven by the thyroid hormones (THs). However, the significance of deiodinases during the metamorphosis of the Japanese flounder (Paralichthys olivaceus) remains unclear. Moreover, regulation and response of the TRs and deiodinases to THs in this fish are poorly understood. Therefore, we detected the expression patterns of THs, deiodinases, and TRs in drug-treated larvae and untreated larvae of P. olivaceus by using enzyme-linked immunosorbent assay and quantitative real-time PCR during P. olivaceus metamorphosis. To further understand the roles of these elements, a rescue assay was performed. Our results show the importance of THs, TRs, and deiodinases in flatfish metamorphosis. Our results also confirm that D1 and D2 activate THs and D3 plays the opposite and complementary role. Moreover, we demonstrated that both TRα and TRβ have important but different roles during P. olivaceus metamorphosis.

Thyroid endocrine disruption in male zebrafish following exposure to binary mixture of bisphenol AF and sulfamethoxazole

Thyroid endocrine disruption by bisphenol AF (BPAF) alone or in combination with sulfamethoxazole (SMX) exposure was evaluated in adult male zebrafish. Changes in thyroid gene transcription were examined using microarrays and were linked to effects on thyroxine hormone production and transcription of genes related to the hypothalamic-pituitary-thyroid axis. BPAF alone or in combination with SMX affected genes related to thyroid hormone production and receptor activity, thyroid gland development, and deiodinase activity. Increases in thyroxine levels, and gene transcription were more pronounced in the BPAF and SMX mixture group than in the BPAF group. Significant down-regulation of trh and tshβ genes in the brain suggested a negative feedback response resulting in increased thyroxine levels. The present study indicated that BPAF exposure alone alters transcription of genes associated with the thyroid endocrine system, and combination with SMX could increase the endocrine disrupting effect of BPAF.

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

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

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.

Developmental toxicity of the PBDE metabolite 6-OH-BDE-47 in zebrafish and the potential role of thyroid receptor β

6-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (6-OH-BDE-47) is both a polybrominated diphenyl ether (PBDE) flame retardant metabolite and a marine natural product. It has been identified both as a neurotoxicant in cell-based studies and as a developmental toxicant in zebrafish. However, hydroxylated PBDE metabolites are also considered thyroid hormone disruptors due to their structural similarity to endogenous thyroid hormones. The purpose of this study was to evaluate the effects of 6-OH-BDE-47 on a developmental pathway regulated by thyroid hormones in zebrafish. Morphological measurements of development (head trunk angle, otic vesicle length, and eye pigmentation) were recorded in embryos at 30h post fertilization (hpf) and detailed craniofacial morphology was examined in 4 day old larvae using cartilage staining. Exposure to 6-OH-BDE-47 resulted in severe developmental delays. A 100nM concentration resulted in a 26% decrease in head trunk angle, a 54% increase in otic vesicle length, and a 42% decrease in eye pigmentation. Similarly, altered developmental morphology was observed following thyroid receptor β morpholino knockdown, exposure to the thyroid hormone triiodothyronine (T3) or to thyroid disrupting chemicals (TDC; iopanoic acid and propylthiouracil). The threshold for lower jaw deformities and craniofacial cartilage malformations was at doses greater than 50nM. Of interest, these developmental delays and effects were rescued by microinjection of TRβ mRNA during the 1-2 cell stage. These data indicate that OH-BDEs can adversely affect early life development of zebrafish and suggest they may be impacting thyroid hormone regulation in vivo through downregulation of the thyroid hormone receptor.

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 Disruption in Zebrafish Larvae by Short-Term Exposure to Bisphenol AF

Bisphenol AF (BPAF) is extensively used as a raw material in industry, resulting in its widespread distribution in the aqueous environment. However, the effect of BPAF on the hypothalamic-pituitary-thyroidal (HPT) axis remains unknown. For elucidating the disruptive effects of BPAF on thyroid function and expression of the representative genes along the HPT axis in zebrafish (Danio rerio) embryos, whole-body total 3,3',5-triiodothyronine (TT3), total 3,5,3',5'-tetraiodothyronine (TT4), free 3,3',5-triiodothyronine (FT3) and free 3,5,3',5'-tetraiodothyronine (FT4) levels were examined following 168 h post-fertilization exposure to different BPAF concentrations (0, 5, 50 and 500 μg/L). The results showed that whole-body TT3, TT4, FT3 and FT4 contents decreased significantly with the BPAF treatment, indicating an endocrine disruption of thyroid. The expression of thyroid-stimulating hormone-β and thyroglobulin genes increased after exposing to 50 μg/L BPAF in seven-day-old larvae. The expressions of thyronine deiodinases type 1, type 2 and transthyretin mRNAs were also significantly up-regulated, which were possibly associated with a deterioration of thyroid function. However, slc5a5 gene transcription was significantly down-regulated at 50 μg/L and 500 μg/L BPAF exposure. Furthermore, trα and trβ genes were down-regulated transcriptionally after BPAF exposure. It demonstrates that BPAF exposure triggered thyroid endocrine toxicity by altering the whole-body contents of thyroid hormones and changing the transcription of the genes involved in the HPT axis in zebrafish larvae.

Short-term exposure of arsenite disrupted thyroid endocrine system and altered gene transcription in the HPT axis in zebrafish

Arsenic (As) pollution in aquatic environment may adversely impact fish health by disrupting their thyroid hormone homeostasis. In this study, we explored the effect of short-term exposure of arsenite (AsIII) on thyroid endocrine system in zebrafish. We measured As concentrations, As speciation, and thyroid hormone thyroxine levels in whole zebrafish, oxidative stress (H2O2) and damage (MDA) in the liver, and gene transcription in hypothalamic-pituitary-thyroid (HPT) axis in the brain and liver tissues of zebrafish after exposing to different AsIII concentrations for 48 h. Result indicated that exposure to AsIII increased inorganic As in zebrafish to 0.46-0.72 mg kg(-1), induced oxidative stress with H2O2 being increased by 1.4-2.5 times and caused oxidative damage with MDA being augmented by 1.6 times. AsIII exposure increased thyroxine levels by 1.3-1.4 times and modulated gene transcription in HPT axis. Our study showed AsIII caused oxidative damage, affected thyroid endocrine system and altered gene transcription in HPT axis in zebrafish.

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.

Thyroid disruption by triphenyl phosphate, an organophosphate flame retardant, in zebrafish (Danio rerio) embryos/larvae, and in GH3 and FRTL-5 cell lines

Triphenyl phosphate (TPP), one of the most widely used organophosphate flame retardants (OPFRs), has frequently been detected in the environment and biota. However, knowledge of its toxicological effects is limited. The present study was conducted to determine the adverse effects of TPP on the thyroid endocrine system of embryonic/larval zebrafish, and the underlying mechanisms for these effects were studied using rat pituitary (GH3) and thyroid follicular (FRTL-5) cell lines. In the GH3 cells, TPP up-regulated the expression of the tshβ, trα, and trβ genes, while T3, a positive control, down-regulated the expression of these genes. In the FRTL-5 cells, the expression of the nis and tpo genes was significantly up-regulated, suggesting that TPP stimulates thyroid hormone synthesis in the thyroid gland. In zebrafish larvae at 7 days post-fertilization (dpf), TPP exposure led to significant increases in both T3 and T4 concentrations and expression of the genes involved in thyroid hormone synthesis. Exposure to TPP also significantly up-regulated the expression of the genes related to the metabolism (dio1), transport (ttr), and elimination (ugt1ab) of thyroid hormones. The down-regulation of the crh and tshβ genes in the zebrafish larvae suggests the activation of a central regulatory feedback mechanism induced by the increased T3 levels in vivo. Taken together, our observations show that TPP could increase the thyroid hormone concentrations in the early life stages of zebrafish by disrupting the central regulation and hormone synthesis pathways.

The PBDE metabolite 6-OH-BDE 47 affects melanin pigmentation and THRβ MRNA expression in the eye of zebrafish embryos

Polybrominated diphenyl ethers and their hydroxyl-metabolites (OH-BDEs) are commonly detected contaminants in human serum in the US population. They are also considered to be endocrine disruptors, and are specifically known to affect thyroid hormone regulation. In this study, we investigated and compared the effects of a PBDE and its OH-BDE metabolite on developmental pathways regulated by thyroid hormones using zebrafish as a model. Exposure to 6-OHBDE 47 (10–100 nM), but not BDE 47 (1–50 μM), led to decreased melanin pigmentation and increased apoptosis in the retina of zebrafish embryos in a concentration-dependent manner in short-term exposures (4 – 30 hours). Six-OH-BDE 47 exposure also significantly decreased thyroid hormone receptor β (THRβ) mRNA expression, which was confirmed using both RT-PCR and in situ hybridization (whole mount and paraffin- section). Interestingly, exposure to the native thyroid hormone, triiodothyronine (T3) also led to similar responses: decreased THRβ mRNA expression, decreased melanin pigmentation and increased apoptosis, suggesting that 6-OH-BDE 47 may be acting as a T3 mimic. To further investigate short-term effects that may be regulated by THRβ, experiments using a morpholino gene knock down and THRβ mRNA over expression were conducted. Knock down of THRβ led to decreases in melanin pigmentation and increases in apoptotic cells in the eye of zebrafish embryos, similar to exposure to T3 and 6-OH-BDE 47, but THRβ mRNA overexpression rescued these effects. Histological analysis of eyes at 22 hpf from each group revealed that exposure to T3 or to 6-OH-BDE 47 was associated with a decrease of melanin and diminished proliferation of cells in layers of retina near the choroid. This study suggests that 6-OH-BDE 47 disrupts the activity of THRβ in early life stages of zebrafish, and warrants further studies on effects in developing humans.

Embryonic exposure to carbendazim induces the transcription of genes related to apoptosis, immunotoxicity and endocrine disruption in zebrafish (Danio rerio)

Carbendazim is one of the most widespread environmental contaminant that can cause major concern to human and animal reproductive system. To date, very few studies have been conducted on the toxic effect of carbendazim in the non-target organism zebrafish (Danio rerio). The study presented here aimed to assess how carbendazim triggers apoptosis, immunotoxicity and endocrine disruption pathways in zebrafish during its embryo development. Our results demonstrated that the expression patterns of many key genes involved in cell apoptosis pathway (e.g. P53, Mdm2, Bbc3 and Cas8) were significantly up-regulated upon the exposure to carbendazim at the concentration of 500 μg/L, while the Bcl2 and Cas3 were down-regulated at the same concentration, interestingly, the expression level of Ogg1 decreased at all the exposure concentrations. It was also observed that the mRNA levels of CXCL-C1C, CCL1, IL-1b and TNFα which were closely related to the innate immune system, were affected in newly hatched zebrafish after exposed to different concentrations of carbendazim. Moreover, the expression of genes that are involved in the hypothalamic-pituitary-gonadal/thyroid (HPG/HPT) axis including VTG, ERα, ERβ2, Dio1, Dio2, Thraa and Thrb were all down-regulated significantly after the exposure to carbendazim. The expression levels of two cytochrome P450 aromatases CYP19a and CYP19b were increased significantly after 20 and 100 μg/L carbendazim exposure, respectively. Taken together, our results indicated that carbendazim had the potential to induce cell apoptosis and cause immune toxicity as well as endocrine disruption in zebrafish during the embryo developmental stage. The information presented here also help to elucidate the environmental risks caused by the carbendazim-induced toxicity in aquatic organisms.

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.