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

Detrimental consequences of micropolymers associated plasticizers on endocrinal disruption

The prevalence of polymer usage in everyday activities has emerged as a detriment to both human life and the environment. A large number of studies describe severe impacts of micropolymers (MP) and nanopolymers (NP) on various organ systems, including the endocrine system. Additionally, plasticizers utilized as additives have been identified as endocrine-disrupting chemicals (EDCs). MP/NP, along with associated plasticizers, affect principal signalling pathways of endocrine glands such as the pituitary, thyroid, adrenal, and gonads, thereby disrupting hormone function and metabolic processes crucial for maintaining homeostasis, fertility, neural development, and fetal growth. This review delves into the sources, distribution, and effects of micropolymers, nanopolymers, and associated plasticizers acting as EDCs. Furthermore, it provides a detailed review of the mechanisms underlying endocrine disruption in relation to different types of MP/NP.

Comparative study of the effects of different surface-coated silver nanoparticles on thyroid disruption and bioaccumulation in zebrafish early life

The widespread use of silver nanoparticles (AgNPs) in commercial and industrial applications has led to their increased presence in the environment, raising concerns about their ecological and health impacts. This study pioneers an investigation into the chronic versus short-term acute toxicological impacts of differently coated AgNPs on zebrafish, with a novel focus on the thyroid-disrupting effects previously unexplored. The results showed that acute toxicity ranked from highest to lowest as AgNO3 (0.128 mg/L), PVP-AgNPs (1.294 mg/L), Citrate-AgNPs (6.984 mg/L), Uncoated-AgNPs (8.269 mg/L). For bioaccumulation, initial peaks were observed at 2 days, followed by fluctuations over time, with the eventual highest enrichment seen in Uncoated-AgNPs and Citrate-AgNPs at concentrations of 13 and 130 μg/L. Additionally, the four exposure groups showed a significant increase in T3 levels, which was 1.28-2.11 times higher than controls, and significant changes in thyroid peroxidase (TPO) and thyroglobulin (TG) content, indicating thyroid disruption. Gene expression analysis revealed distinct changes in the HPT axis-related genes, providing potential mechanisms underlying the thyroid toxicity induced by different AgNPs. The higher the Ag concentration in zebrafish, the stronger the thyroid disrupting effects, which in turn affected growth and development, in the order of Citrate-AgNPs, Uncoated-AgNPs > AgNO3, PVP-AgNPs. This research underscores the importance of considering nanoparticle coatings in risk assessments and offers insights into the mechanisms by which AgNPs affect aquatic organisms' endocrine systems, highlighting the need for careful nanotechnology use and the relevance of these findings for understanding environmental pollutants' role in thyroid disease.

Exposure to thiourea during the early stages of development impedes the formation of the swim bladder in zebrafish larvae

Thiourea, a widely used agrochemical, is known to inhibit the activity of thyroid peroxidase, a key enzyme in the biosynthetic pathway of thyroid hormones. Thyroid insufficiency compromises the basal metabolic rate in warm-blooded organisms and embryonic development in vertebrates. In this study, we looked for developmental defects by exposing the zebrafish embryos to an environmentally relevant dose of thiourea (3 mg/mL). Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was performed to validate thiourea's presence in the treated zebrafish embryos. Structural anomalies like bent tail and pericardial edema were noticed in 96-h post-fertilization (hpf) larvae. On histological examination, underdeveloped swim bladder was noticed in 96 hpf larvae exposed to 3 mg/mL thiourea. The treated larvae also failed to follow the characteristic swimming behavior in response to stimuli due to defective swim bladder. Swim bladder being homologous to the lung of tetrapod, the role of Bmp4, a major regulator of lung development, was studied along with the associated regulatory genes. Gene expression analysis revealed that thiourea administration led to the downregulation of bmp4, shh, pcna, anxa5, acta2, and the downstream effector snail3 but the upregulation of caspase3. The protein expression showed a similar trend, wherein Bmp4, Shh, and Pcna were downregulated, but Cleaved Caspase3 showed an increased expression in the treated group. Therefore, it is prudent to presume that exposure to thiourea significantly reduces the expression of Bmp4 and other key regulators; hence, the larvae fail to develop a swim bladder, a vital organ that regulates buoyancy.

Aggravated visual toxicity in zebrafish larvae upon co-exposure to titanium dioxide nanoparticles and bis(2-ethylhexyl)-2,3,4,5-tetrabromophthalate

The bioavailability and toxicity of organic pollutants in aquatic organisms can be largely affected by the co-existed nanoparticles. However, the impacts of such combined exposure on the visual system remain largely unknown. Here, we systematically investigated the visual toxicity in zebrafish larvae after single or joint exposure to titanium dioxide nanoparticles (n-TiO2) and bis(2-ethylhexyl)-2,3,4,5-tetrabromophthalate (TBPH) at environmentally relevant levels. Molecular dynamics simulations revealed the enhanced transmembrane capability of the complex than the individual, which accounted for the increased bioavailability of both TBPH and n-TiO2 when combined exposure to zebrafish. Transcriptome analysis showed that co-exposure to n-TiO2 and TBPH interfered with molecular pathways related to eye lens structure and sensory perception of zebrafish. Particularly, n-TiO2 or TBPH significantly suppressed the expression of βB1-crystallin and rhodopsin in zebrafish retina and lens, which was further enhanced after co-exposure. Moreover, we detected disorganized retinal histology, stunted lens development and significant visual behavioral changes of zebrafish under co-exposure condition. The overall results suggest that combined exposure to water borne n-TiO2 and TBPH increased their bioavailability, resulted in severer damage to optic nerve development and ultimately abnormal visual behavior patterns, highlighting the higher potential health risks of co-exposure to aquatic vertebrates.

Combined effects of ammonia-N exposure and salinity changes on hematological and serum biochemical factors and thyroid hormones in Nile tilapia (Oreochromis niloticus)

The aim of this research was to evaluate the interaction effects of ammonia-N levels and salinity on hematological and serum biochemical parameters in Nile tilapia (Oreochromis niloticus). The fish were randomly divided into 12 treatments including the levels of salinity (0, 4, 8 and 12 ppt) and 0, 50% of LC50-96 h of ammonia-N and 30% of LC50-96 h of ammonia-N in a factorial design (4 salinity levels x 3 ammonia levels). Hemoglobin value in all treatments, except for salinity treatments, namely 2, 3, 4, showed a significant decrease than the control (0 ppt and no poisoning). Also, red blood cells in treatment ammonia-N levels were significantly less than the control. Serum protein concentration, in treatments 9 (50% of LC50-96 h of ammonia-N) and 5 and also with increasing salinity (treatments 2, 3 and 4) had a significant decrease compared to the control. There is a significant increase in serum glucose, cortisol, ammonia and urea levels in 50% and 30% of LC50-96 h of ammonia-N treatments compared to the control, meanwhile these parameters were significantly increased with increasing salinity. Serum thyroid stimulating hormone (TSH), T3 and T4 levels in acute and sub-acute ammonia-N treatments were significantly lower than the control. Moreover, with increasing salinity in 50% and 30% of LC50-96 h of ammonia-N treatments, TSH showed a decreasing pattern. According to the results, fluctuations in blood biochemical factors, increase of stress and decrease of thyroid hormones show that the salinity, ammonia, and their interaction caused adverse effects on fish health during the 96 h of testing.

Bioaccumulation and thyroid endcrione disruption of 2-ethylhexyl diphenyl phosphate at environmental concentration in zebrafish larvae

2-ethylhexyl diphenyl phosphate (EHDPP) strongly binds to transthyretin (TTR) and affects the expression of genes involved in the thyroid hormone (TH) pathway in vitro. However, it is still unknown whether EHDPP induces endocrine disruption of THs in vivo. In this study, zebrafish (Danio rerio) embryos (< 2 h post-fertilization (hpf)) were exposed to environmentally relevant concentrations of EHDPP (0, 0.1, 1, 10, and 100 μg·L-1) for 120 h. EHDPP was detected in 120 hpf larvae at concentrations of 0.06, 0.15, 3.71, and 59.77 μg·g-1 dry weight in the 0.1, 1, 10, and 100 μg·L-1 exposure groups, respectively. Zebrafish development and growth were inhibited by EHDPP, as indicated by the increased malformation rate, decreased survival rate, and shortened body length. Exposure to lower concentrations of EHDPP (0.1 and 1 μg·L-1) significantly decreased the whole-body thyroxine (T4) and triiodothyronine (T3) levels and altered the expressions of genes and proteins involved in the hypothalamic-pituitary-thyroid axis. Downregulation of genes related to TH synthesis (nis and tg) and TH metabolism (dio1 and dio2) may be partially responsible for the decreased T4 and T3 levels, respectively. EHDPP exposure also significantly increased the transcription of genes involved in thyroid development (nkx2.1 and pax8), which may stimulate the growth of thyroid primordium to compensate for hypothyroidism. Moreover, EHDPP exposure significantly decreased the gene and protein expression of the transport protein transthyretin (TTR) in a concentration-dependent manner, suggesting a significant inhibitory effect of EHDPP on TTR. Molecular docking results showed that EHDPP and T4 partly share the same mode of action of binding to the TTR protein, which might result in decreased T4 transport due to the binding of EHDPP to the TTR protein. Taken together, our findings indicate that EHDPP can cause TH disruption in zebrafish and help elucidate the mechanisms underlying EHDPP toxicity.

Differential gene expression and developmental pathologies associated with persistent organic pollutants in sentinel fish in Troutman Lake, Sivuqaq, Alaska

Persistent organic pollutants (POPs) are lipophilic compounds that bioaccumulate in animals and biomagnify within food webs. Many POPs are endocrine disrupting compounds that impact vertebrate development. POPs accumulate in the Arctic via global distillation and thereby impact high trophic level vertebrates as well as people who live a subsistence lifestyle. The Arctic also contains thousands of point sources of pollution, such as formerly used defense (FUD) sites. Sivuqaq (St. Lawrence Island), Alaska was used by the U.S. military during the Cold War and FUD sites on the island remain point sources of POP contamination. We examined the effects of POP exposure on ninespine stickleback (Pungitius pungitius) collected from Troutman Lake in the village of Gambell as a model for human exposure and disease. During the Cold War, Troutman Lake was used as a dump site by the U.S. military. We found that PCB concentrations in stickleback exceeded the U.S. Environmental Protection Agency's guideline for unlimited consumption despite these fish being low trophic level organisms. We examined effects at three levels of biological organization: gene expression, endocrinology, and histomorphology. We found that ninespine stickleback from Troutman Lake exhibited suppressed gonadal development compared to threespine stickleback (Gasterosteus aculeatus) studied elsewhere. Troutman Lake stickleback also displayed two distinct hepatic phenotypes, one with lipid accumulation and one with glycogen-type vacuolation. We compared the transcriptomic profiles of these liver phenotypes using RNA sequencing and found significant upregulation of genes involved in ribosomal and metabolic pathways in the lipid accumulation group. Additionally, stickleback displaying liver lipid accumulation had significantly fewer thyroid follicles than the vacuolated phenotype. Our study and previous work highlight health concerns for people and wildlife due to pollution hotspots in the Arctic, and the need for health-protective remediation.

BDE-209-induced genotoxicity, intestinal damage and intestinal microbiota dysbiosis in zebrafish (Danio Rerio)

The environmental presence of polybrominated diphenyl ethers (PBDEs) is ubiquitous due to their wide use as brominated flame retardants in industrial products. As a common congener of PBDEs, decabromodiphenyl ether (BDE-209) can pose a health risk to animals as well as humans. However, to date, few studies have explored BDE-209's toxic effects on the intestinal tract, and its relevant mechanism of toxicity has not been elucidated. In this study, adult male zebrafish were exposed to BDE-209 at 6 μg/L, 60 μg/L and 600 μg/L for 28 days, and intestinal tissue and microbial samples were collected for analysis to reveal the underlying toxic mechanisms. Transcriptome sequencing results demonstrated a dose-dependent pattern of substantial gene differential expression in the group exposed to BDE-209, and the differentially expressed genes were mainly concentrated in pathways related to protein synthesis and processing, redox reaction, and steroid and lipid metabolism. In addition, BDE-209 exposure caused damage to intestinal structure and barrier function, and promoted intestinal oxidative stress, inflammatory response, apoptosis and steroid and lipid metabolism disorders. Mechanistically, BDE-209 induced intestinal inflammation by increasing the levels of TNF-α and IL-1β and activating the NFκB signaling pathway, and might induce apoptosis through the p53-Bax/Bcl2-Caspase3 pathway. BDE-209 also significantly inhibited the gene expression of rate-limiting enzymes such as Sqle and 3βhsd (p < 0.05) to inhibit cholesterol synthesis. In addition, BDE-209 induced lipid metabolism disorders through the mTOR/PPARγ/RXRα pathway. 16S rRNA sequencing results showed that BDE-209 stress reduced the richness and diversity of intestinal microbiota, and reduced the abundance of probiotics (e.g., Bifidobacterium and Faecalibacterium). Overall, the results of this study help to clarify the intestinal response mechanism of BDE-209 exposure, and provide a basis for evaluating the health risks of BDE-209 in animals.

Environmentally relevant concentrations of Mn2+ disrupts the endocrine regulation of growth in juvenile Yunlong groupers (Epinephelus moara♀×Epinephelus lanceolatus♂)

Even though manganese is a bioelement essential for metabolism, excessive manganese levels in water can be detrimental to fish development and growth. Therefore, the aim of this study was to evaluate the effects of Mn2+ (0, 0.5,1, 2, and 4 mg·L-1) exposure for 30 d on the growth performance, growth hormone/insulin-like growth factor (GH/IGF) axis, hypothalamic-pituitary-thyroid (HPT) axis, and monoaminergic neurotransmitters of Epinephelus moara♀×Epinephelus lanceolatus♂(Yunlong grouper). Compared with the control and low Mn2+concentration groups of (0.5 and 1 mg·L-1), the high concentration of Mn2+ (4 mg·L-1) significantly reduced body weight (BW), body length (BL), weight gain rate (WGR), and specific growth rate (SGR), increased the feed coefficient rate (FCR) and mortality of Yunlong groupers (P < 0.05). Further, the levels of GH and IGF, along with the expression of ghra and ghrb were significantly reduced after exposure to 2 and 4 mg·L-1 Mn2+for 30 d, whereas the expression of sst5 was significantly up-regulated after exposure to 2 and 4 mg·L-1 Mn2+for 20 and 30 days. Moreover, Mn2+exposure increased thyroid hormone (T3) and thyroid stimulating hormone (TSH) contents, accompanied by increased mRNA levels of dio1 and dio2, however, the T4 level was decreased. Finally, dopamine (DA) and serotonin (5-HT) levels significantly decreased after long-term exposure to higher concentrations of Mn2+, and the levels their metabolites changed as well, suggesting that the synthesis and metabolism of DA and 5-HT were affected. Accordingly, changes in the GH/IGF and HPT axes-related parameters may be the cause of growth inhibition in juvenile groupers under Mn2+ exposure, indicating that the relationship between endocrine disorder and growth inhibition should not be ignored.

Toxic Effects and Mechanisms of Polybrominated Diphenyl Ethers

Polybrominated diphenyl ethers (PBDEs) are a group of flame retardants used in plastics, textiles, polyurethane foam, and other materials. They contain two halogenated aromatic rings bonded by an ester bond and are classified according to the number and position of bromine atoms. Due to their widespread use, PBDEs have been detected in soil, air, water, dust, and animal tissues. Besides, PBDEs have been found in various tissues, including liver, kidney, adipose, brain, breast milk and plasma. The continued accumulation of PBDEs has raised concerns about their potential toxicity, including hepatotoxicity, kidney toxicity, gut toxicity, thyroid toxicity, embryotoxicity, reproductive toxicity, neurotoxicity, and immunotoxicity. Previous studies have suggested that there may be various mechanisms contributing to PBDEs toxicity. The present study aimed to outline PBDEs' toxic effects and mechanisms on different organ systems. Given PBDEs' bioaccumulation and adverse impacts on human health and other living organisms, we summarize PBDEs' effects and potential toxicity mechanisms and tend to broaden the horizons to facilitate the design of new prevention strategies for PBDEs-induced toxicity.

Environmental occurrence and ecotoxicity of aquaculture-derived plastic leachates

Plastic products such as fishing nets and foam buoys have been widely used in aquaculture. To enhance the desirable characteristics of the final equipment, plastic gear for aquaculture is mixed with a wide range of additives. Recent studies have shown that additives could be leached out to the environment with a long-term use of aquaculture plastics, forming aquaculture-derived plastic leachates. It should be emphasized that some leachates such as phthalic acid esters (PAEs) and organophosphate esters (OPEs) are endocrine disruptors, which could increase the exposure risk of aquatic products and subsequently display potential threats to human health via food chain. However, systematic studies on the release, occurrence, bioaccumulation, and toxic effects of aquaculture-derived plastic leachates are missing, overlooking their potential sources and ecotoxicological risks in aquatic environments. We have reviewed and compared the concentrations of major plastic leachates in the water environment and organisms of global aquaculture and non-farmed areas, confirming that aquaculture leachate is an important source of contaminants in the environment. Moreover, the toxic effects of aquaculture-derived plastic additives and the related mechanisms are summarized with fish as a representative, revealing their potential health risk. In addition, we proposed current challenges and future research needs, which provides scientific guidance for the use and management of plastic products in aquaculture industries.

Tris(1,3-dichloro-2-propyl) Phosphate Inhibits Early Embryonic Development by Binding to Gsk-3β Protein in Zebrafish

Recently, several studies have reported that exposure to tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) results in abnormal development of zebrafish embryos in blastocyst and gastrula stages, but molecular mechanisms are still not clear. This lacking strongly affects the interspecific extrapolation of embryonic toxicity induced by TDCIPP and hazard evaluation. In this study, zebrafish embryos were exposed to 100, 500 or 1000 μg/L TDCIPP, and 6-bromoindirubin-3'-oxime (BIO, 35.62 μg/L) was used as a positive control. Results demonstrated that treatment with TDCIPP or BIO caused an abnormal stacking of blastomere cells in mid blastula transition (MBT) stage, and subsequently resulted in epiboly delay of zebrafish embryos. TDCIPP and BIO up-regulated the expression of β-catenin protein and increased its accumulation in nuclei of embryonic cells. This accumulation was considered as a driver for early embryonic developmental toxicity of TDCIPP. Furthermore, TDCIPP and BIO partly shared the same modes of action, and both of them could bind to Gsk-3β protein, and then decreased the phosphorylation level of Gsk-3β in TYR·216 site and lastly inhibited the activity of Gsk-3β kinase, which was responsible for the increased concentrations of β-catenin protein in embryonic cells and accumulation in nuclei. Our findings provide new mechanisms for clarifying the early embryonic developmental toxicity of TDCIPP in zebrafish.

Zebrafish-based platform for emerging bio-contaminants and virus inactivation research

Emerging bio-contaminants such as viruses have affected health and environment settings of every country. Viruses are the minuscule entities resulting in severe contagious diseases like SARS, MERS, Ebola, and avian influenza. Recent epidemic like the SARS-CoV-2, the virus has undergone mutations strengthen them and allowing to escape from the remedies. Comprehensive knowledge of viruses is essential for the development of targeted therapeutic and vaccination treatments. Animal models mimicking human biology like non-human primates, rats, mice, and rabbits offer competitive advantage to assess risk of viral infections, chemical toxins, nanoparticles, and microbes. However, their economic maintenance has always been an issue. Furthermore, the redundancy of experimental results due to aforementioned aspects is also in examine. Hence, exploration for the alternative animal models is crucial for risk assessments. The current review examines zebrafish traits and explores the possibilities to monitor emerging bio-contaminants. Additionally, a comprehensive picture of the bio contaminant and virus particle invasion and abatement mechanisms in zebrafish and human cells is presented. Moreover, a zebrafish model to investigate the emerging viruses such as coronaviridae and poxviridae has been suggested.

Adverse thyroid hormone and behavioral alterations induced by three frequently used synthetic musk compounds in embryo-larval zebrafish (Danio rerio)

Synthetic musk compounds (SMCs) have been extensively used in numerous consumer products, such as perfumes, cosmetics, soap, and fabric softener. Due to their bioaccumulative nature, these compounds have often been detected in the aquatic ecosystem. However, their effects on endocrine and behavioral effects in freshwater fish have rarely been investigated. In the present study, thyroid disruption and neurobehavioral toxicity of SMCs were investigated using embryo-larval zebrafish (Danio rerio). Three frequently used SMCs, i.e., musk ketone (MK), 1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethyl-cyclopenta [g]- benzopyran (HHCB), and 6-acetyl-1,1,2,4,4,7-hexamethyltetralin (AHTN), were chosen. Experimental concentrations for HHCB and AHTN were selected to include the maximum levels reported in the ambient water. The 5-day exposure to either MK or HHCB led to significant decrease of T4 concentration in the larval fish at the levels as low as 0.13 μg/L, even though compensatory transcriptional changes, e.g., up-regulation of hypothalamic crhβ gene and/or down-regulation of ugt1ab gene, were taken place. In contrast, AHTN exposure resulted in up-regulation of crhβ, nis, ugt1ab, and dio2 genes but did not alter T4 level, suggesting its lesser thyroid disrupting potential. All tested SMCs caused hypoactivity of the larval fish. Several genes related to neurogenesis or development, e.g., mbp and syn2a, were down-regulated, but the patterns of transcriptional changes were different among the tested SMCs. The present observations demonstrate that MK and HHCB can decrease T4 levels and cause hypoactivity of the larval zebrafish. It requires attention that HHCB and AHTN could influence thyroid hormone or behavior of the larval fish even at the levels close to those observed in the ambient environment. Further studies on potential ecological consequences of these SMCs in freshwater environment are warranted.

Thyroid endocrine disruption induced by [C8mim]Br: An integrated in vivo, in vitro, and in silico study

Conventional thyroid-disrupting chemicals (TDCs) such as polybrominated diphenyl ethers, polychlorinated biphenyls, and bisphenols perturb animal's thyroid endocrine system by mimicking the action of endogenous thyroid hormones (THs), since they share a similar backbone structure of coupled benzene rings with THs. 1-methyl-3-octylimidazolium bromide ([C₈mim]Br), a commonly used ionic liquid (IL), has no structural similarity to THs. Whether it interferes with thyroid function and how its mode of action differs from conventional TDCs is largely unknown. Herein, zebrafish embryo-larvae experiments (in vivo), GH3 cell line studies (in vitro), and molecular simulation analyses (in silico) were carried out to explore the effect of [C₈mim]Br on thyroid homeostasis and its underlying mechanism. Molecular docking results suggested that [C₈mim]⁺ likely bound to retinoid X receptors (RXRs), which may compromise the formation of TH receptor/RXR heterodimers. This then perturbed the negative regulation of thyroid-stimulating hormone β (tshβ) transcription by T3 in GH3 cell line. The resulting enhancement of tshβ expression further caused hyperthyroidism and developmental toxicity in larval zebrafish. These findings provided a crucial aspect of the ecological risks of ILs, and presented a new insight into the thyroid-disrupting mechanisms for emerging pollutants that do not have structural similarity to THs.

Parental and transgenerational impairments of thyroid endocrine system in zebrafish by 2,4,6-tribromophenol

Many environmental contaminants could be transmitted from parents and generate impairments to their progeny. The 2,4,6-tribromophenol (TBP), a novel brominated flame retardant which has been frequently detected in various organisms, was supposed to be bioaccumulated and intergenerational transmitted in human beings. Previous studies revealed that TBP could disrupt thyroid endocrine system in zebrafish larvae. However, there is no available data regarding the parental and transgenerational toxicity of this contaminant. Thus, in this study adult zebrafish were exposed to environmental contaminated levels of TBP for 60 days to investigate the parental and transgenerational impairments on thyroid endocrine system. Chemical analysis verified the bioaccumulation of TBP in tested organs of parents (concentration: liver>gonads>brain) and its transmission into eggs. For adults, increased thyroid hormones, disturbed transcriptions of related genes and histopathological changes in thyroid follicles indicate obvious thyroid endocrine disruptions. Transgenerational effects are indicated by the increased thyroid hormones both in eggs (maternal source) and in developed larvae (newly synthesized), as well as disrupted transcriptional profiles of key genes in HPT axis. The overall results suggest that the accumulated TBP could be transmitted from parent to offspring and generate thyroid endocrine disruptions in both generations.

Sex-specific thyroid disruption caused by phenanthrene in adult zebrafish (Danio rerio)

Polycyclic aromatic hydrocarbons (PAHs) are well-known contaminants with widespread distribution in environment and food. Phenanthrene is one of the most abundant PAHs in food and aquatic environment and generates reproductive and developmental toxicity in zebrafish. Nonetheless, whether phenanthrene caused sex-specific thyroid disruption in adult zebrafish is unclear. To determine this, adult zebrafish (male and female) were treated with phenanthrene (0, 0.85, 8.5, and 85 μg/L) for 60 days. After the treatment period, we assessed the concentrations of thyroid hormones (THs) and expression levels of genes in the hypothalamic-pituitary-thyroid (HPT) axis. The results showed that phenanthrene exposure can lead to thyroid disruption in both male and female zebrafish. Exposure to phenanthrene dramatically reduced the levels of L-thyroxine (T4) and L-triiodothyronine (T3) in both male and female zebrafish, with a similar trend in both. However, the genes expression profiles of hypothalamic-pituitary-thyroid (HPT) axis were sex-specific. In all, the present study demonstrated that phenanthrene exposure could result in sex-specific thyroid disruption in adult zebrafish.

Environmentally relevant concentrations of selenite trigger reproductive toxicity by affecting oocyte development and promoting larval apoptosis

As a trace element, selenium (Se) has been widely added to food to maintain the physiological homeostasis of the organism. The adverse effects of Se on the reproduction of zebrafish have been investigated, however, the effects of Se on the maturation and apoptosis of zebrafish oocytes remain unclear. In this study, zebrafish embryos (2 h post fertilization, hpf) were exposed to 0, 12.5, 25, 50, and 100 μg Se/L for 120 days. The results demonstrated that exposure to selenite decreased the gonad-somatic index (GSI) and cumulative production of eggs, inhibited oocyte maturation (OM), and increased oocyte apoptosis in females. Exposure to selenite decreased the contents of sex hormones (E2) in the serum and increased the levels of reactive oxygen species (ROS) and cyclic adenosine monophosphate (cAMP) in the ovary. Furthermore, exposure to selenite downregulated the transcription level of genes on the HPG axis, decreased the phosphorylation level of CyclinB and the protein content of cAMP-dependent protein kinase (Pka), and upregulated the expression of genes (eif2s1a and chop) and proteins (Grp78, Chop) related to endoplasmic reticulum stress (ERS) and apoptosis. Moreover, maternal exposure to selenite resulted in the apoptosis of offspring and upregulated the content of ROS and the transcription level of genes related to ERS and apoptosis.

Parental whole life-cycle exposure to tris (2-chloroethyl) phosphate (TCEP) disrupts embryonic development and thyroid system in zebrafish offspring

Tris (2-chloroethyl) phosphate (TCEP), an emerging environmental pollutant, has been frequently detected in natural waters. The objective of this study was to investigate possible parental transfer of TCEP and transgenerational effects on the early development and thyroid hormone homeostasis in F1 larvae following parental whole life-cycle exposure to TCEP. To this end, zebrafish (Danio rerio) embryos were exposed to environmentally relevant concentrations (0.8, 4, 20 and 100 μg/L) of TCEP for 120 days until sexual maturation. Parental exposure to TCEP resulted in significant levels of TCEP, developmental toxicity including decreased survival and final hatching rates, accelerated heart rate and elevated malformation rate, as well as induction of oxidative stress and cell apoptosis in F1 offspring. In F1 eggs, declined thyroxin (T4) levels were observed, consistent with those in plasma of F0 adult females, indicating the maternal transfer of thyroid endocrine disruption to the offspring. In addition, mRNA levels of several genes along the hypothalamic-pituitary-thyroid (HPT) axis were significantly modified in F1 larvae, which could be linked to transgenerational developmental toxicity and thyroid hormone disruption. For the first time, we revealed that the parental exposure to environmentally relevant levels of TCEP could cause developmental toxicity and thyroid endocrine disruption in subsequent unexposed generation.

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

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

Sex-specific effects of fluoride and lead on thyroid endocrine function in zebrafish (Danio rerio)

Fluoride (F) and lead (Pb) are widespread pollutants in the environment. F and Pb affect the thyroid endocrine system, but the mechanism of action between F and Pb is still unclear. In this study, in order to evaluate the effects of F or/and Pb on histopathological changes, antioxidant indices, the levels of thyroid hormones (THs), and the expression of endocrine-related genes in zebrafish thyroid. One thousand and two hundred zebrafish (female:male = 1:1) were randomly divided into four groups: control group (C group), 80 mg/L F group (F group), 60 mg/L Pb group (Pb group), and 80 mg/L F + 60 mg/L Pb group (F + Pb group) for 45 d and 90 d. Histopathological sections showed a loss of glia and follicular epithelial hyperplasia in the thyroid gland after exposure to F and Pb. Oxidative stress in the thyroid was induced after F and Pb exposure. And each oxidation index was increased after F + Pb exposure. Combined F and Pb exposure aggravated the downregulation of thyroid hormones T3 and T4 compared to exposure alone. Furthermore, F and Pb exposure altered the expression of thyroid endocrine-related genes in a time-dependent manner. These results indicate that F and Pb can affect the endocrine system of thyroid by changing the tissue structure, antioxidant capacity, thyroid hormone secretion and the levels of endocrine-related genes in thyroid. F and Pb can also produce toxic effects on thyroid, but the degree of poisoning is different in different indicators, mainly for the additive effect between them. Additionally, males are more sensitive than females to F or Pb toxicity. However, the specific molecular mechanism of the effects of F and Pb on thyroid endocrine system needs to be further studied.

Parental exposure to waterborne selenite induces transgenerational development toxicity in zebrafish offspring

Excessive selenium (Se), especially selenite form exerts great toxicity to fish. Most studies have attached considerable attention to the adverse effects of Se on parental fish. However, the transgenerational toxicity of Se on fish has been rarely reported. In the present study, zebrafish embryos were exposed to environmentally relevant concentrations of Na₂SeO₃ (0, 12.5, 25, 50, and 100 μg/L) for 120 days. And the exposed zebrafish (F0) were allowed to spawn with normal zebrafish after sexual maturity. Subsequently, the offspring (F1) were cultured in clean water for 5 days. In the F0 generation, exposure to 100 μg/L Na₂SeO₃ significantly increased the Se content in the tissues (liver, brain and gonad) and decreased the body length and weight. After parental exposure to 100 μg/L Na₂SeO₃, the increased mortality, elevated malformation rate and reduced body length were measured in F1 zebrafish. The Se content was only significantly increased in F1 larvae derived from exposed females in the 100 μg/L exposure group. The contents of thyroid hormones (THs), growth hormone (GH) and insulin-like growth factor (IGF) significantly decreased in F0 and F1 zebrafish. The transcriptional levels of genes along the hypothalamic-pituitary-thyroid (HPT) axis and growth hormone/insulin-like growth factor (GH/IGF) axis were detected to further explore the possible mechanisms of Se-induced thyroid and growth hormone disruption. The results suggest that the toxicity of Se in zebrafish can be markedly transmitted to offspring. And the transgenerational development toxicity might be different due to the differences in gender of exposed parents.

Exposure to diclofenac alters thyroid hormone levels and transcription of genes involved in the hypothalamic-pituitary-thyroid axis in zebrafish embryos/larvae

Diclofenac (DCF), one of typical non-steroidal anti-inflammatory drugs (NSAIDs), has been frequently detected in various environmental media. Nevertheless，the potential endocrine disrupting effects of DCF on fish were poorly understood. In the present study, zebrafish embryos/larvae were used as a model to evaluate the adverse effects of DCF on development and thyroid system. The results demonstrated that DCF only significantly decreased the heart rate at 72 h post-fertilization (hpf), exhibiting limited influence on the embryonic development of zebrafish. Treatment with DCF significantly reduced whole-body thyroxine (T4) levels, and changed transcriptional levels of several genes related to the hypothalamic-pituitary-thyroid (HPT) axis. These findings provide important information regarding to the mechanisms of DCF-induced developmental toxicity and thyroid disruption in fish.

Effects of Polybrominated Diphenyl Ethers on Hormonal and Reproductive Health in E-Waste-Exposed Population: A Systematic Review

Electronic waste management is a global rising concern that is primarily being handled by informal recycling practices. These release a mix of potentially hazardous chemicals, which is an important public health concern. These chemicals include polybrominated diphenyl ethers (PBDEs), used as flame retardants in electronic parts, which are persistent in nature and show bioaccumulative characteristics. Although PBDEs are suspected endocrine disruptors, particularly targeting thyroid and reproductive hormone functions, the relationship of PBDEs with these health effects are not well established. We used the Navigation Guide methodology to conduct a systematic review of studies in populations exposed to e-waste to better understand the relationships of these persistent flame retardants with hormonal and reproductive health. We assessed nineteen studies that fit our pre-determined inclusion criteria for risk of bias, indirectness, inconsistency, imprecision, and other criteria that helped rate the overall evidence for its quality and strength of evidence. The studies suggest PBDEs may have an adverse effect on thyroid hormones, reproductive hormones, semen quality, and neonatal health. However, more research is required to establish a relationship of these effects in the e-waste-exposed population. We identified the limitations of the data available and made recommendations for future scientific work.

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.

Enantioselective acute toxicity, oxidative stress effects, neurotoxicity, and thyroid disruption of uniconazole in zebrafish (Danio rerio)

Uniconazole is a widely used plant growth retardant in the agricultural field. However, toxicological effects of uniconazole in aquatic ecosystem at chiral level are still unclear. Herein, acute toxicity, oxidative stress effects, neurotoxicity, and thyroid disruption of uniconazole enantiomers were investigated through using zebrafish as a model. (R)-Uniconazole possessed 1.16-fold greater acute toxicity to zebrafish than (S)-enantiomer. Then, integrated biomarker response values of oxidative stress parameters in zebrafish exposed to (R)-uniconazole were about 1.27~1.53 times greater than those treated by (S)-uniconazole, revealing that (R)-uniconazole could result in more significant adverse effects than (S)-uniconazole. Subsequently, the results of acetylcholinesterase activity of experimental fish demonstrated a state of inhibition-activation-inhibition after 14-day exposure to uniconazole, and a significant enantioselective neurotoxicity of uniconazole was observed in zebrafish after exposure for 4 and 7 days (p < 0.05). Moreover, thyroxine and triiodothyronine contents in (R)-uniconazole-exposed zebrafish were 0.89-fold (p=0.007) and 0.80-fold (p=0.007) than those in (S)-enantiomer-treated group, respectively. Furthermore, molecular docking results between uniconazole enantiomers and thyroid hormone receptors revealed that (R)-uniconazole was more tightly bound than (S)-uniconazole to the receptors. Briefly, our findings provide favorable information for ecological risk assessments of chiral agrochemicals in the environment and health of aquatic organisms.

Effect of Acute Exposure to the Ionic Liquid 1-Methyl-3-octylimidazolium Chloride on the Embryonic Development and Larval Thyroid System of Zebrafish

Simple Summary

In this study, we aimed to evaluate the effect of acute exposure to the ionic liquid 1-methyl-3-octylimidazolium chloride on the embryonic development and larval thyroid system of zebrafish. The results showed that the fish embryonic development, thyroid hormone level, and expression of HPTs-related genes were altered, suggesting that the ionic liquid [C8mim]Cl might pose an aquatic environmental threat to fish.

Abstract

Previous studies have shown that ILs can induce toxicity in animals, plants, and cells. However, the effect of imidazolium-based ILs on the hypothalamus–pituitary–thyroid (HPT) axis of fish remains unknown. The present study aimed to evaluate the acute effect of [C8mim]Cl on the embryonic development and thyroid-controlled internal secretion system of zebrafish by determining the thyroid hormone level and the expression of HPT-related genes. The results obtained for embryonic developmental toxicity showed the survival rate, heart beats, and body length of fish had decreased 96 h after exposure to [C8mim]Cl, but the hatching rate had increased by the 48 h time point. The transcription levels of HTP-related genes showed that the genes dio3, tg, ttr, tsh, trhrα, trhrβ, trhr2, and tpo were up-regulated, while the expression levels of dio1, trh, tshr, and nis were significantly suppressed. Furthermore, we found that exposure to [C8mim]Cl induced an alteration in the levels of thyroid hormones that increased the T3 but decreased the T4 content. In conclusion, our study indicated that acute exposure to [C8mim]Cl altered the expression of HTP-related genes and disturbed the thyroid hormone level, suggesting that the ionic liquid [C8mim]Cl might pose an aquatic environmental threat to fish.

Environmental Co-Exposure to Potassium Perchlorate and Cd Caused Toxicity and Thyroid Endocrine Disruption in Zebrafish Embryos and Larvae (Danio rerio)

The increasing pollution of aquatic habitats with anthropogenic compounds has led to various test strategies to detect hazardous chemicals. However, information on the effects of pollutants on the thyroid system in fish, which is essential for growth, development, and parts of reproduction, is still scarce. Modified early life-stage tests were carried out with zebrafish exposed to the known thyroid inhibitor potassium perchlorate (0.1, 1, 1.5, 2, 2.5, and 5 mM) to identify adverse effects on embryo development. The endogenous antioxidant defense mechanism is one of the key functions of the thyroid gland; in this regard, we examined the co-exposure to potassium perchlorate (KClO₄), which could disrupt thyroid function, with cadmium (Cd), a known pro-oxidant compound. Zebrafish embryos were exposed to control KClO₄ 1 mM and Cd 0.5 μM for 96 h after fertilization (hpf) individually and in combination. The morphological alteration, body length, and messenger RNA (mRNA) expression related to thyroid function and oxidative stress, thyroid hormone levels, and malondialdehyde were measured. Significant down-regulation of mRNAs related to thyroid function (thyroid hormone receptor-alpha (THRα), thyroid hormone receptor-beta (THRβ), haematopoietically expressed homeobox (hhex)) and decreased thyroxin (T4) levels were observed after co-exposure to KClO₄ and Cd, but this was not observed in the individually treated groups. These results suggest that co-exposure to KClO₄ and Cd could affect antioxidant defense mechanisms and potentially normally increase Cd toxicity on mRNA expression, altering the thyroid functions important in zebrafish embryonic developmental stages.

Developmental toxicity of bromoacetamide via the thyroid hormone receptors-mediated disruption of thyroid hormone homeostasis in zebrafish embryos

Bromoacetamide (BAcAm) is a nitrogenous disinfection by-product. We previously found that BAcAm induced developmental toxicity in zebrafish embryos, but the underlying mechanisms remain to be elucidated. Since thyroid hormones (THs) homeostasis is crucial to development, we hypothesized that disruption of THs homeostasis may play a role in the developmental toxicity of BAcAm. In this study, we found BAcAm exposure significantly increased mortality and malformation rate, decreased hatching rate and body length, inhibited the locomotor capacity in zebrafish embryos. BAcAm elevated TSH, T₃ and T₄ levels, down-regulated T₃/T₄ ratios, and up-regulated mRNA expression changes of THs related genes (trh, tsh, tg, nis, tpo, dio1, dio2, ugt1ab，klf9 and rho), but down-regulated mRNA expression changes of TH receptors (tr α and tr β). Up-regulated tr α and tr β mRNAs by rescue treatment confirmed that both tr α and tr β were involved in the developmental toxicity of BAcAm. In conclusion, our study indicates disruption of THs homeostasis via the thyroid hormone receptors was responsible for the developmental toxicity of BAcAm.

Effects of long‐term exposure to TDCPP in zebrafish ( Danio rerio ) – Alternations of hormone balance and gene transcriptions along hypothalamus‐pituitary axes

Background

TDCPP is one of the major chemical of organophosphate flame retardants (OPFRs) that has been detected ubiquitously in both the environment and biota. Previously we observed that it influenced the concentrations of sex and thyroid hormones in a sex‐dependent pattern, leading to reproductive impairments after short‐term exposure in zebrafish. Here we investigate the consequences of longer‐term exposure to TDCPP on the hypothalamic‐pituitary‐gonad (HPG), hypothalamic‐pituitary‐interrenal (HPI), and hypothalamic‐pituitary‐thyroid (HPT) axes of zebrafish (Danio rerio).

Methods

A 120‐day exposure test to 0.005, 0.05 and 0.5 mg/L TDCPP was initiated with fertilized eggs. Sex steroid hormones in the treated fishes were measured and transcriptional changes were analyzed.

Results

In female fish, exposure to TDCPP resulted in increases in plasma cortisol, follicle stimulating hormone (FSH), luteinizing hormone (LH), 17β‐estradiol (E2), cortisol, thyroxine (T4), and triiodothyronine (T3). Transcription of most target genes along HPG, HPI and HPT axes were increased by the exposure. While in male fish the exposure led to decreases in cortisol, FSH, LH, T4, T3, testosterone (T), and 11‐ketotestosterone (11‐KT). Transcription of genes along HPG, HPI and HPT axes, especially steroidogenic genes, were inhibited in male zebrafish. While, E2/T or E2/11‐KT ratio was increased in both female and females. The sex‐dependent changes in hormones might be due to differential responses to TDCPP induced stresses. An increase in cortisol level coincided with increases in E2 and THs in female fish, while in males decreases in cortisol as well as T, 11‐KT and THs were observed. Long‐term exposure to TDCPP at very low (μg/L) concentrations could disrupt hormone balances in a sex dependent way.

Conclusion

This study revealed that TDCPP could affect endocrine axes – HPG, HPI and HPT – in zebrafish, and impair zebrafish development.

Phenanthrene disrupting effects on the thyroid system of Arabian seabream, Acanthopagrus arabicus: In situ and in vivo study

Phenanthrene, a polycyclic aromatic hydrocarbon (PAH), is one of the endocrine disrupting chemicals (EDCs). The present study aimed to evaluate the effects of phenanthrene on histophysiology of thyroid in Arabian seabream (Acanthopagrus arabicus). In this regards, different concentrations of phenanthrene (2, 20 and 40 pg/gbw) were injected to Acanthopagrus arabicus and changes in thyroid tissue structure and the serum levels of triiodothyronine (T3) and Thyroxine (T4) were assessed. The experiment lasted 21 days. Alterations in thyroid tissue structure and T3 and T4 serum levels also were assessed in Acanthopagrus arabicus caught from different stations of the Persian Gulf (Jafari, Samail, Arvand, Zangi, Bahrakan). In addition, the concentration of phenanthrene was measured in the fish muscle and sediment samples from the stations. Phenanthrene concentration reached the maximum level in the muscle of all injected fish after 4 days and then decreased by the end of the experiment. The highest and lowest concentrations of phenanthrene were recorded in the fish muscle and sediment samples collected from Jafari and Bahrakan, respectively. The levels of T3 and T4 decreased dose dependently in phenanthrene-injected fish up to day 7 and then increased by the end of the experiment. The serum level of T3 and T4 in fish collected from different stations was as follows: Jafari<Samail<Arvand<Zangi≤Bahrakan. Some tissue changes observed in fish included decrease in the thickness of thyroid follicle epithelium and increase in follicle diameter. In conclusion, according to the results, phenanthrene significantly affected thyroid function in fish.

The interactive effects of ethinylestradiol and progesterone on transcriptional expression of genes along the hypothalamus-pituitary-thyroid axis in embryonic zebrafish (Danio rerio)

Progestins and estrogens are widespread in various aquatic environments and their potential endocrine disruption effects to aquatic organisms have drawn growing concern. However, their combined effects in aquatic organisms remain elusive. The aim of the present study was to assess the effects of the binary mixtures of gestodene (GES) and 17α-ethinylestradiol (EE2) on the hypothalamic-pituitary-thyroid (HPT) axis of zebrafish (Danio rerio) using the eleuthero-embryos. Embryos were exposed to GES and EE2 alone or in combination at concentrations ranging from 41 to 5329 ng L^-1 (nominal ones from 50 to 5000 ng L^-1) for 48 h, 96 h and 144 h post fertilization (hpf). The results showed that the transcripts of the genes along the HPT axis displayed pronounced alterations. There was no clear pattern in the change of the transcripts of these genes over time and with concentrations. However, in general, the transcripts of the genes were inversely affected by EE2 (increase 0.5 to 4.2-folds) and GES (inhibition 0.4 to 4.9-folds), and their mixtures showed interactive effects in embryonic zebrafish. In addition, physiological data (mortality, malformation, body length and heart rate etc.) denoted higher toxicity of the two chemicals in combination than alone based on the developmental toxicity and neurotoxicity (locomotor behavior). These results indicated that the interactive effects of these two chemicals might be different between at the transcriptional level and at the whole organismal level. In summary, GES and EE2 affect the HPT axis (related genes expression and thyroid hormones (THs) levels) and exhibit developmental toxicity and neurotoxicity.

A review of the endocrine disrupting effects of micro and nano plastic and their associated chemicals in mammals

Over the years, the vaste expansion of plastic manufacturing has dramatically increased the environmental impact of microplastics [MPs] and nanoplastics [NPs], making them a threat to marine and terrestrial biota because they contain endocrine disrupting chemicals [EDCs] and other harmful compounds. MPs and NPs have deleteriouse impacts on mammalian endocrine components such as hypothalamus, pituitary, thyroid, adrenal, testes, and ovaries. MPs and NPs absorb and act as a transport medium for harmful chemicals such as bisphenols, phthalates, polybrominated diphenyl ether, polychlorinated biphenyl ether, organotin, perfluorinated compounds, dioxins, polycyclic aromatic hydrocarbons, organic contaminants, and heavy metals, which are commonly used as additives in plastic production. As the EDCs are not covalently bonded to plastics, they can easily leach into milk, water, and other liquids affecting the endocrine system of mammals upon exposure. The toxicity induced by MPs and NPs is size-dependent, as smaller particles have better absorption capacity and larger surface area, releasing more EDC and toxic chemicals. Various EDCs contained or carried by MPs and NPs share structural similarities with specific hormone receptors; hence they interfere with normal hormone receptors, altering the hormonal action of the endocrine glands. This review demonstrates size-dependent MPs' bioaccumulation, distribution, and translocation with potential hazards to the endocrine gland. We reviewed that MPs and NPs disrupt hypothalamic-pituitary axes, including the hypothalamic-pituitary-thyroid/adrenal/testicular/ovarian axis leading to oxidative stress, reproductive toxicity, neurotoxicity, cytotoxicity, developmental abnormalities, decreased sperm quality, and immunotoxicity. The direct consequences of MPs and NPs on the thyroid, testis, and ovaries are documented. Still, studies need to be carried out to identify the direct effects of MPs and NPs on the hypothalamus, pituitary, and adrenal glands.

Porous microplastics enhance polychlorinated biphenyls-induced thyroid disruption in juvenile Japanese flounder (Paralichthys olivaceus)

Microplastics and polychlorinated biphenyls are ubiquitous in the marine environments. To illuminate their combined biological impacts, juvenile Japanese flounder (Paralichthys olivaceus) were exposed to 500 ng/L PCBs alone or 500 ng/L PCBs plus 2, 20, and 200 μg/L 10-μm porous MPs for 21 days. Compared to PCBs alone, co-exposure to PCBs and 20, 200 μg/L MPs reduced fish body length and body weight, and the concurrence of MPs aggravated PCBs-induced thyroid-disrupting effects, including significantly decreased L-thyroxine and L-triiodothyronine levels, more severe damage to the thyroid tissue and gill morphology, and disturbance on the expression of hypothalamus-pituitary-thyroid axis genes. The PCBs concentrations in the seawater were decreased dramatically with the increase of MPs concentrations, confirming that MPs absorbed PCBs from the seawater. Our results demonstrated that MPs enhanced the thyroid disruption of PCBs, suggesting that the risk of MPs and thyroid-disrupting chemicals on marine organisms should be paid more attention.

Microplastics habituated with biofilm change decabrominated diphenyl ether degradation products and thyroid endocrine toxicity

Microplastics (MPs) are rapidly colonized by microbial biofilms in a natural aquatic environment, and the nature of the microbial community and type of MP can result in different degradation products of organic pollutants. Here, we quantified the degradation products of a ubiquitously detected pollutant, decabrominated diphenyl ether (BDE-209), under both light-only and biota conditions and in the absence or presence of three kinds of MPs, styrofoam polystyrene, hard polyamide, and polypropylene film. The results showed that the BDE-209 concentration increased by 0.7-2.8 fold in the presence of MPs, probably due to the "sustained release" desorption effect. Under light-only conditions, the penta- and hexa-BDE concentrations in the presence of styrofoam or hard MPs were significantly reduced, which can be deemed a beneficial effect. However, when biota were present, the debromination products increased with the addition of MPs, particularly in the presence of styrofoam MPs. These products caused a 1.7-fold upregulation in triiodothyronine content and a 5.9-fold upregulation of thyroid stimulating hormone β expression in zebrafish larvae. The increase in debromination products could be attributed to the distinct high abundance of the bacteria Chloroflexi, Proteobacteria, and Basidiomycotina on styrofoam MPs that can participate in pollutant degradation. Collectively, our results indicate that MPs can alter the degradation pathways of BDE-209 and increase the toxicity to the endocrine system and the thyroid in aquatic organisms.

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.

Parental transfer of nanopolystyrene-enhanced tris(1,3-dichloro-2-propyl) phosphate induces transgenerational thyroid disruption in zebrafish

Plastic is a globally recognized superwaste that can affect human health and wildlife when it accumulates and is amplified in the food chain. Microplastics (plastic particles < 5 mm) and nanoplastics (plastic particles < 100 nm) can interact with organic pollutants already present in the aquatic environment, potentially acting as carriers for pollutants entering organisms and thus influencing the bioavailability and toxicity of those pollutants. In this study, we investigated the transfer kinetics and transgenerational effects of exposure to tris(1,3-dichloro-2-propyl) phosphate (TDCIPP) and polystyrene nanoplastics (PS-NPs) in F1 offspring. At 90 days postfertilization, zebrafish (Danio rerio) strain AB was exposed to either TDCIPP (0, 0.47, 2.64, or 12.78 μg/L) or PS-NPs (10 mg/L) or their combination for 120 days. The results showed that TDCIPP and PS-NPs accumulated in the gut, gill, head, and liver of the zebrafish in a sex-dependent manner. The presence of PS-NPs promoted the bioaccumulation of TDCIPP in the adult fish and increased the parental transfer of TDCIPP to their offspring. We demonstrate that parental exposure to TDCIPP alone or in combination with PS-NPs induces thyroid disruption in adults, and then leads to thyroid endocrine disruption in their larval offspring. Reduced thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels contributed to the observed transgenerational thyroid dysfunction, which inhibited developmental growth and disturbed the transcription of genes and expression of proteins involved in the hypothalamic-pituitary-thyroid (HPT) axis in the F1 larvae. The increased transfer of TDCIPP to the offspring in the presence of PS-NPs also enhanced transgenerational thyroid endocrine disruption, demonstrated by a further reduction in T4 and the upregulation of thyroglobulin (tg), uridine diphosphate-glucuronosyltransferase (ugt1ab), thyroid-stimulating hormone (tshβ), and thyroid hormone receptor (trα) expression in the F1 larvae compared with the effects of parental TDCIPP exposure alone. Overall, our results indicate that the presence of PS-NPs modifies the bioavailability of TDCIPP and aggravates transgenerational thyroid disruption in zebrafish.

The possible hormetic effects of fluorene-9-bisphenol on regulating hypothalamic-pituitary-thyroid axis in zebrafish

Fluorene-9-bisphenol (BHPF) is a bisphenol A substitute, which has been introduced for the production of so-called 'bisphenol A (BPA)-free' plastics. However, it has been reported that BHPF can enter living organisms through using commercial plastic bottles and cause adverse effects. To date, the majority of the toxicologic study of BHPF focused on investigating its doses above the toxicological threshold. Here, we studied the effects of BHPF on development, locomotion, neuron differentiation of the central nervous system (CNS), and the expression of genes in the hypothalamic-pituitary-thyroid (HPT) axis in zebrafish exposed to different doses of BHPF ranging from 1/5 of LD1 to LD50 (300, 500, 750, 1500, 3000, and 4500 nM). As a result, the possible hormetic effects of BHPF on regulating the HPT axis were revealed, in which low-dose BHPF positively affected the HPT axis while this regulation was inhibited as the dose increased. Underlying mechanism investigation suggested that BHPF disrupted myelination through affecting HPT axis including related genes expression and TH levels, thus causing neurotoxic characteristics. Collectively, this study provides the full understanding of the environmental impact of BHPF and its toxicity on living organisms, highlighting a substantial and generalized ongoing dose-response relationship with great implications for the usage and risk assessment of BHPF.

Mechanisms of parental co-exposure to polystyrene nanoplastics and microcystin-LR aggravated hatching inhibition of zebrafish offspring

The combined toxicity effects of microcystins-LR (MCLR) and polystyrene nanoplastics (PSNPs) on the hatching of F1 zebrafish (Danio rerio) embryos were investigated in this study due to the increasing concerns of both plastic pollution and eutrophication in aquatic environments. Three-month-old zebrafish were used to explore the molecular mechanisms underlying the combined effect of MCLR (0, 0.9, 4.5, and 22.5 μg/L) on egg hatching in the existence of PSNPs (100 μg/L). The results demonstrated the existence of PSNPs further increased the accumulation of MCLR in F1 embryos. The hatching rates of F1 embryos were inhibited after exposure to 22.5 μg/L MCLR, and the presence of PSNPs aggravated the hatching inhibition induced by MCLR. The decrease of hatching enzyme activity and the abnormality of spontaneous movement were observed. We examined the altered expression levels of the genes associated with the hatching enzyme (tox16, foxp1, ctslb, xpb1, klf4, cap1, bmp4, cd63, He1.2, zhe1, and prl), cholinergic system (ache and chrnα7), and muscle development (Wnt, MyoD, Myf5, Myogenin, and MRF4). The results suggested the existence of PSNPs exacerbated the hatching inhibition of F1 embryos through decreasing the activity of enzyme, interfering with the cholinergic system, and affecting the muscle development.

Sub-chronic exposure to ammonia inhibits the growth of juvenile Wuchang bream (Megalobrama amblycephala) mainly by downregulation of growth hormone/insulin-like growth factor axis

In this study, healthy Wuchang bream (Megalobrama amblycephala) juveniles were exposed to 0, 5, 10, 20 and 30 mg/L total ammonia nitrogen for 30 days to elucidate toxic effects and mechanisms of ammonia on growth performance involved with the regulation of growth hormone/insulin-like growth factor (GH/IGF) and hypothalamic-pituitary-thyroid (HPT) axes. Our results showed that the increasing total ammonia nitrogen concentrations caused dose-depend decreases in the weight gain and specific growth rate but increases in the food conversion ratio and mortality in juvenile bream, indicating growth inhibitory effects induced by ammonia. Concurrently, GH, IGF-1 at protein and mRNA levels were significantly decreased in ammonia exposure groups (p < .05), while serum thyroid stimulating hormone, free thyroxine, free triiodothyronine levels were significantly reduced only in fish exposed to higher concentrations of 20 and 30 mg/L ammonia (p < .05), suggesting that ammonia exposure could perturb both GH/IGF-axis and HPT-axis functions. Furthermore, transcriptional levels of extracellular regulated protein kinases 2 (erk2), phosphatidylinositol 3-kinase (pi3k), protein kinase B (akt), target of rapamycin (tom) and ribosomal protein S6 kinase-polypeptide 1(s6k1) in the dorsal muscle were significantly down-regulated in the fish exposed to ammonia (p < .05). This fact indicated that MAPK/ERK pathway and PI3K/AKT pathway should be responsible for the growth inhibition. Combining the results of spearman correlation coefficient, it should be noted that the GH/IGF axis played a more important role in regulating the growth than the HPT axis in Wuchang bream under persistent ammonia stress.

The joint effect of parental exposure to microcystin-LR and polystyrene nanoplastics on the growth of zebrafish offspring

The coexistence of nanoplastics (NPs) and various pollutants in the environment has become a problem that cannot be ignored. In order to identify the microcystin-LR (MCLR) bioaccumulation and the potential impacts on the early growth of F1 zebrafish (Danio rerio) offspring in the presence of polystyrene nanoplastics (PSNPs), PSNPs and MCLR were used to expose adult zebrafish for 21days. The exposure groups divided into MCLR (0, 0.9, 4.5 and 22.5μgL^-1) alone groups and PSNP (100μgL^-1) and MCLR co-exposure groups. F1 embryos were collected and developed to 120 h post-fertilization (hpf) in clear water. Compared with the exposure to MCLR only, the combined exposure increased the parental transfer of MCLR to the offspring and subsequently exacerbated the growth inhibition of F1 larvae. Further research clarified that combined exposure of PSNPs and MCLR could reduce the levels of thyroxine (T4) and 3, 5, 3'-triiodothyronine (T3) by altering the expression of hypothalamus-pituitary-thyroid (HPT) axis-related genes, eventually leading to growth inhibition of F1 larvae. Our results also exhibited combined exposure of PSNPs and MCLR could change the transcription of key genes of the GH/IGF axis compared with MCLR single exposure, suggesting the GH/IGF axis was a potential target for the growth inhibition of F1 larvae in PSNPs and MCLR co-exposure groups. The present study highlights the potential risks of coexistence of MCLR and PSNPs on development of fish offspring, and the environmental risks to aquatic ecosystems.

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.

An environmentally relevant mixture of polychlorinated biphenyls (PCBs) and polybrominated diphenylethers (PBDEs) disrupts mitochondrial function, lipid metabolism and neurotransmission in the brain of exposed zebrafish and their unexposed F2 offspring

Polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) are persistent organic pollutants still present in aquatic environments despite their total or partial ban. Previously, we observed that an environmentally realistic mixture of these compounds affects energy balance, growth, and reproduction in exposed zebrafish (F0), and behavior in their unexposed offspring (F1-F4). In the present work, we performed lipidomic and transcriptomic analyses on brains of zebrafish (F0-F2) from exposed and control lineages to identify molecular changes that could explain the observed phenotypes. The use of both technologies highlighted that F0 zebrafish displayed impaired mitochondrial function and lipid metabolism regulation (depletion in triacylglycerols and phospholipids) which can explain disruption of energy homeostasis. A subset of the regulated biological pathways related to energetic metabolism and neurotransmission were inherited in F2. In addition, there were increasing effects on epigenetic pathways from the F0 to the F2 generation. Altogether, we show that the effects of an environmental exposure to PCBs and PBDEs on energetic metabolism as well as neurotransmission extend over 2 generations of zebrafish, possibly due to transgenerational epigenetic inheritance.

Effects of SiO2 nanoparticles on the uptake of tetrabromobisphenol A and its impact on the thyroid endocrine system in zebrafish larvae

The coexistence of nanoparticles and organic toxicants in the environment modifies pollutant bioavailability and toxicity. This study investigated the influence of silicon dioxide nanoparticles (n-SiO₂) on the uptake of tetrabromobisphenol A (TBBPA) and its impact on the thyroid endocrine system in zebrafish larvae. Zebrafish (Danio rerio) embryos were exposed to TBBPA at different concentrations (50, 100, and 200 μg/L) alone or in combination with n-SiO₂ (25 mg/L) until 120 h post-fertilization (hpf). Chemical measurements showed that both TBBPA and n-SiO₂ were bioconcentrated in zebrafish larvae, and the uptake of TBBPA was enhanced by n-SiO₂. Furthermore, zebrafish larvae exposed to 200 μg/L TBBPA alone exhibited significantly increased T4 contents and decreased T3 contents, whereas n-SiO₂ treatment alone did not have a detectable effect. Furthermore, the thyroid hormone levels changed more upon treatment with 200 μg/L TBBPA combined with 25 mg/L n-SiO₂ than upon TBBPA treatment alone. Alterations in gene transcription along the related hypothalamic-pituitary-thyroid (HPT) axis were observed, and expression of the binding and transport protein transthyretin (TTR) was significantly decreased for both TBBPA alone and co-exposure with n-SiO₂. Thus, the current study demonstrates that n-SiO₂, even at the nontoxic concentrations, increases thyroid hormone disruption in zebrafish larvae co-exposed to TBBPA by promoting its bioaccumulation and bioavailability.

Inhibition in growth and cardiotoxicity of tris (2-butoxyethyl) phosphate through down-regulating Wnt signaling pathway in early developmental stage of zebrafish (Danio rerio)

As a common organophosphorus flame retardant, tris (2-butoxyethyl) phosphate (TBOEP) is detected in water environment and aquatic animals extensively. Despite previous researches have reported the developmental toxicity of TBOEP in zebrafish (Danio rerio) larvae, few research focused on its underlying mechanisms. In this study, zebrafish embryos were exposed to 0, 20, 200, 1000 and 2000 µg/L TBOEP from 2 until 120 h post-fertilization (hpf) to determine potential mechanisms of developmental toxicity of this compound. Early developmental stage parameters such as body length, survival rate, hatching rate and heart rate were decreased, while malformation rate was ascended. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) assay was carried out at 12, 24, 72 and 120 hpf to demonstrate alterations in expression of genes of Wnt signaling pathway. The results indicated that axin1 was significantly up-regulated, while β-catenin, pkc and wnt11 were down-regulated. Correlation analysis indicated that expression of these genes was significantly correlated with body length. Furthermore, apoptosis was detected in heart region by acridine orange (AO) staining and terminal deoxynucleotide transferase-mediated deoxy-UTP nick end labeling (TUNEL) assay. In addition, at 120 hpf, occurrence of oxidative stress was observed in zebrafish larvae. Moreover, 6-Bromoindirubin-3'-oxime (BIO)， an activator of Wnt pathway, was found to alleviate the inhibiting effects of TBOEP on zebrafish growth. The overall outcomes offered novel viewpoints in toxic effects of TBOEP, and down-regulating Wnt signaling pathway were able to reveal some potential mechanisms of developmental toxicity of TBOEP in zebrafish larvae.

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.

Deregulation of whole-transcriptome gene expression in zebrafish (Danio rerio) after chronic exposure to low doses of imatinib mesylate in a complete life cycle study

Imatinib mesylate (IM) is an anticancer drug that belongs to tyrosine kinase inhibitors. We report the results of the first investigation of the chronic exposure of zebrafish (Danio rerio) to IM. The exposure to IM (0.01, 1 and 100 μg/L) was initiated in adult fish and continued through hatching and the offspring generation for seven months. In addition to standard toxicological endpoints, induction of genotoxic effects and whole-genome transcriptome of liver samples of offspring generation of zebrafish were analysed. Exposure to IM did not affect the survival and growth of zebrafish, did not cause any histopathological changes, but it induced a marginal increase in the chromosomal damage in blood cells. The whole-genome transcriptome analyses demonstrated dose-dependent increase in the number of differentially expressed genes with a significantly higher number of deregulated genes in female fish compared to male. Differentially expressed genes included genes involved in response to DNA damage, cell cycle control and regulation of circadian rhythm. Based on the low genotoxic activity and the pattern of the changes in DNA damage responsive genes we consider that at current environmental exposure levels, IM represents low risk for genotoxic effects in aquatic organisms. Exposure to IM also induced deregulation of the expression of genes associated with steroidogenesis and hormone metabolism and function, which indicates hormone-disrupting activity of IM that has not been studied so far. The study provide new information on the potential consequences of chronic exposure to the residues of tyrosine kinase inhibitors, which remain to be further explored.

Effects of 2-ethylhexyl-4-methoxycinnamate (EHMC) on thyroid hormones and genes associated with thyroid, neurotoxic, and nephrotoxic responses in adult and larval zebrafish (Danio rerio)

One of the most widely used UV filters, 2-ethylhexyl-4-methoxycinnamate (EHMC), has been widely detected in the environment. While its endocrine disruption potential has often been reported, toxicological information on EHMC is limited. This study was conducted to determine the thyroid, neurological and renal toxicity potentials of EHMC in adult male and embryo-larval zebrafish (Danio rerio). Following 21 d of exposure, plasma T3 concentration decreased in a concentration-dependent manner in adult zebrafish. Several genes related to thyroid hormone regulation were also downregulated in the brain, thyroid, and liver of the adult fish. In addition, upregulation of syn2a in the brain and downregulation of podocin and wt1a in the kidney were observed following the exposure in adult fish. In zebrafish larvae, following 120 h exposure to EHMC, whole-body T3 and T4 contents decreased, and thyroid hormone-related genes were downregulated. However, several genes showed different patterns of transcription in the larvae; for example, mbp and etv1 genes were downregulated and podocin was upregulated. Unlike adult fish, the larval fish showed significant genetic changes related to neurotoxicity. The hypothyroidism induced in the larval fish by the exposure might be potentially associated with the neurotoxic potential of EHMC. The implications of the observed hormonal and transcriptional-level changes in zebrafish at different life stages following long-term exposure warrant further investigation.

Bioconcentration of 2,4,6-tribromophenol (TBP) and thyroid endocrine disruption in zebrafish larvae

2,4,6-tribromophenol (TBP) is generally used as a brominated flame retardant but is produced in the degradation of tetrabromobisphenol-A. Although TBP is frequently detected in the environment and in various biota, including fish species, we still know little about its toxicity and environmental health risk. Here we investigated the bioconcentration and effects of TBP on the thyroid endocrine system by using zebrafish as a model. Zebrafish embryos (2 h post-fertilization, hpf) were exposed to five concentrations of TBP (0, 0.3, 1, 10, and 100 μg/L) until 144 hpf. According to our chemical analysis, TBP underwent bioconcentration in zebrafish larvae. However, acute exposure to TBP did not affect the hatching of embryos or their risk of malformation, nor the growth and survival of larvae, indicating low developmental toxicity of TBP. The whole-body thyroxine (T4) contents were significantly increased in zebrafish larvae after exposure to TBP, indicating thyroid endocrine disruption occurred. Gene transcription levels in the hypothalamic-pituitary-thyroid (HPT) axis were also examined in larvae; these results revealed that the transcription of corticotrophin-releasing hormone (crh), thyrotropin-releasing hormone (trh), and thyroid-stimulating hormone (tshβ) were all significantly downregulated by exposure to TBP. Likewise, genes encoding thyronine deiodinases (dio1, dio2, and dio3a/b) and thyroid hormone receptors (trα and trβ) also had their transcription downregulated in zebrafish. Further, the gene transcription and protein expression of binding and transport protein transthyretin (TTR) were significantly increased after TBP exposure. Taken together, our results suggest the bioavailability of and potential thyroid endocrine disruption by TBP in fish.