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

Molecular mechanisms of environmental pollutant-induced cartilage damage: from developmental disorders to osteoarthritis

The objective of the present study was to review the molecular mechanisms of the adverse effects of environmental pollutants on chondrocytes and extracellular matrix (ECM). Existing data demonstrate that both heavy metals, including cadmium (Cd), lead (Pb), and arsenic (As), as well as organic pollutants, including polychlorinated dioxins and furans (PCDD/Fs) and polychlorinated biphenyls (PCB), bisphenol A, phthalates, polycyclic aromatic hydrocarbons (PAH), pesticides, and certain other organic pollutants that target cartilage ontogeny and functioning. Overall, environmental pollutants reduce chondrocyte viability through the induction apoptosis, senescence, and inflammatory response, resulting in cell death and impaired ECM production. The effects of organic pollutants on chondrocyte development and viability were shown to be mediated by binding to the aryl hydrocarbon receptor (AhR) signaling and modulation of non-coding RNA expression. Adverse effects of pollutant exposures were observed in articular and growth plate chondrocytes. These mechanisms also damage chondrocyte precursors and subsequently hinder cartilage development. In addition, pollutant exposure was shown to impair chondrogenesis by inhibiting the expression of Sox9 and other regulators. Along with altered Runx2 signaling, these effects also contribute to impaired chondrocyte hypertrophy and chondrocyte-to-osteoblast trans-differentiation, resulting in altered endochondral ossification. Several organic pollutants including PCDD/Fs, PCBs and PAHs, were shown to induce transgenerational adverse effects on cartilage development and the resulting skeletal deformities. Despite of epidemiological evidence linking human environmental pollutant exposure to osteoarthritis or other cartilage pathologies, the data on the molecular mechanisms of adverse effects of environmental pollutant exposure on cartilage tissue were obtained from studies in laboratory rodents, fish, or cell cultures and should be carefully extrapolated to humans, although they clearly demonstrate that cartilage should be considered a putative target for environmental pollutant toxicity.

Development of the integrated fish endocrine disruptor test-Part B: Implementation of thyroid-related endpoints

Given the vital role of thyroid hormones (THs) in vertebrate development, it is essential to identify chemicals that interfere with the TH system. Whereas, among nonmammalian laboratory animals, fish are the most frequently utilized test species in endocrine disruptor research, for example, in guidelines for the detection of effects on the sex hormone system, there is no test guideline (TG) using fish as models for thyroid-related effects; rather, amphibians are used. Therefore, the objective of the present project was to integrate thyroid-related endpoints for fish into a test protocol combining OECD TGs 229 (Fish Short-Term Reproduction Assay) and 234 (Fish Sexual Development Test). The resulting integrated Fish Endocrine Disruption Test (iFEDT) was designed as a comprehensive approach to covering sexual differentiation, early development, and reproduction and to identifying disruption not only of the sexual and/or reproductive system but also the TH system. Two 85-day exposure tests were performed using different well-studied endocrine disruptors: 6-propyl-2-thiouracil (PTU) and 17α-ethinylestradiol (EE2). Whereas the companion Part A of this study presents the findings on effects by PTU and EE2 on endpoints established in existing TGs, the present Part B discusses effects on novel thyroid-related endpoints such as TH levels, thyroid follicle histopathology, and eye development. 6-Propyl-2-thiouracil induced a massive proliferation of thyroid follicles in any life stage, and histopathological changes in the eyes proved to be highly sensitive for TH system disruption especially in younger life stages. For measurement of THs, further methodological development is required. 17-α-Ethinylestradiol demonstrated not only the well-known disruption of the hypothalamic-pituitary-gonadal axis, but also induced effects on thyroid follicles in adult zebrafish (Danio rerio) exposed to higher EE2 concentrations, suggesting crosstalk between endocrine axes. The novel iFEDT has thus proven capable of simultaneously capturing endocrine disruption of both the steroid and thyroid endocrine systems. Integr Environ Assess Manag 2024;20:830-845. © 2023 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

A realistic mixture of ubiquitous persistent organic pollutants affects bone and cartilage development in zebrafish by interaction with nuclear receptor signaling

"Persistent organic pollutants (POPs)" have a plethora of deleterious effects on humans and the environment due to their bioaccumulative, persistent, and mimicking properties. Individually, each of these chemicals has been tested and its effects measured, however they are rather found as parts of complex mixtures of which we do not fully grasp the extent of their potential consequences. Here we studied the effects of realistic, environmentally relevant mixtures of 29 POPs on cartilage and bone development using zebrafish as a model species. We observed developmental issues in cartilage, in the form of diverse malformations such as micrognathia, reduced size of the Meckel's and other structures. Also, mineralized bone formation was disrupted, hence impacting the overall development of the larvae at later life stages. Assessment of the transcriptome revealed disruption of nuclear receptor pathways, such as androgen, vitamin D, and retinoic acid, that may explain the mechanisms of action of the compounds within the tested mixtures. In addition, clustering of the compounds using their chemical signatures revealed structural similarities with the model chemicals vitamin D and retinoic acid that can explain the effects and/or enhancing the phenotypes we witnessed. Further mechanistic studies will be required to fully understand this kind of molecular interactions and their repercussions in organisms. Our results contribute to the already existing catalogue of deleterious effects caused by exposure to POPs and help to understand the potential consequences in at risk populations.

Transgenerational effects of BDE-47 to zebrafish based on histomorphometry and toxicogenomic analyses

Exposure to 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47) has been found to have an impact on reproductive output and endocrine function in female zebrafish (Danio rerio). However, the transgenerational effects of BDE-47 have not been fully explored in previous reports. In this study, female zebrafish were exposed to BDE-47 for three consecutive weeks. The oogenesis, sex hormones, reproductive histology, and transcriptional profiles of genes along the hypothalamus-pituitary-gonad (HPG) axis were assessed in the exposed-F0 generation. After mating with unexposed males, the transgenerational effects of BDE-47 were evaluated on the basis of histopathology, morphometry and toxicogenome of the unexposed F1 generations at the larval stage. Results indicated that exposure to BDE-47 impaired reproductive capacity, disrupted endocrine system in F0 zebrafish, and compromised craniofacial skeletons and vertebrae development in F1 generations. In addition, through the use of toxicogenomics approach, immune-responsive pathways were found to be significantly enriched, and the transcript expression profiling of immune-related DEGs (IRDs) were dramatically inhibited in F1 generations following maternal BDE-47 exposure, indicating its immunotoxicity to offspring larvae. These findings advance our understanding of the transgenerational toxicity of BDE-47 and advocate for a more comprehensive assessment of other PBDE congeners through histomorphometry and toxicogenomic approaches.

Ciona spp. and ascidians as bioindicator organisms for evaluating effects of endocrine disrupting chemicals: A discussion paper

In context of testing, screening and monitoring of endocrine-disrupting (ED) type of environmental pollutants, tunicates could possibly represent a particularly interesting group of bioindicator organisms. These primitive chordates are already important model organisms within developmental and genomics research due to their central position in evolution and close relationship to vertebrates. The solitary ascidians, such as the genus Ciona spp. (vase tunicates), could possibly be extra feasible as ED bioindicators. They have a free-swimming, tadpole-like larval stage that develops extremely quickly (<20 h under favorable conditions), has a short life cycle (typically 2-3 months), are relatively easy to maintain in laboratory culture, have fully sequenced genomes, and transgenic embryos with 3D course data of the embryo ontogeny are available. In this article, we discuss possible roles of Ciona spp. (and other solitary ascidians) as ecotoxicological bioindicator organisms in general but perhaps especially for effect studies of contaminants with presumed endocrine disrupting modes of action.

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.

Metabolite alterations in zebrafish embryos exposed to hydroxylated polybrominated diphenyl ethers

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) are formed by metabolism from the flame retardants polybrominated diphenyl ethers (PBDEs). In the aquatic environment, they are also produced naturally. OH-PBDEs are known for their potential to disrupt energy metabolism, the endocrine system, and the nervous system. This is the first study focusing on the effects of OH-PBDEs at the metabolite level in vivo. The aim of the current study was to investigate the metabolic effects of exposure to OH-PBDEs using metabolomics, and to identify potential biomarker(s) for energy disruption of OH-PBDEs. Zebrafish (Danio rerio) embryos were exposed to two different concentrations of 6-OH-BDE47 and 6-OH-BDE85 and a mixture of these two compounds. In total, 342 metabolites were annotated and 79 metabolites were affected in at least one exposure. Several affected metabolites, e.g. succinic acid, glutamic acid, glutamine, tyrosine, tryptophan, adenine, and several fatty acids, could be connected to known toxic mechanisms of OH-PBDEs. Several phospholipids were strongly up-regulated with up to a six-fold increase after exposure to 6-OH-BDE47, a scarcely described effect of OH-PBDEs. Based on the observed metabolic effects, a possible connection between disruption of the energy metabolism, neurotoxicity and potential immunotoxicity of OH-PBDEs was suggested. Single compound exposures to 6-OH-BDE47 and 6-OH-BDE85 showed little overlap in the affected metabolites. This shows that compounds of similar chemical structure can induce different metabolic effects, possibly relating to their different toxic mechanisms. There were inter-concentration differences in the metabolic profiles, indicating that the metabolic effects were concentration dependent. After exposure to the mixture of 6-OH-BDE47 and 6-OH-BDE85, a new metabolic profile distinct from the profiles obtained from the single compounds was observed. Succinic acid was up-regulated at the highest, but still environmentally relevant, concentration of 6-OH-BDE47, 6-OH-BDE85, and the mixture. Therefore, succinic acid is suggested as a potential biomarker for energy disruption of OH-PBDEs.

Biotransformation kinetics and pathways of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and its hydroxylated and methoxylated derivatives (6-OH-BDE-47 and 6-MeO-BDE-47) in earthworms (Eisenia fetida)

As a class of persistent organic pollutant, polybrominated diphenyl ethers (PBDEs) and their hydroxylated and methoxylated derivatives (OH-PBDEs and MeO-PBDEs) have been widely detected in soil environments. However, studies on the bioavailability and transformation of PBDEs and their derivatives in soil organisms remain scarce. In this study, a detailed kinetic investigation on the accumulation and biotransformations of BDE-47, 6-MeO-BDE-47 and 6-OH-BDE-47 in earthworms (Eisenia fetida) exposed to artificially contaminated soils was conducted. The uptake and elimination kinetics of BDE-47, 6-MeO-BDE-47 and 6-OH-BDE-47 by earthworms were in accordance with a one-compartment first-order kinetic model. The bioaccumulation factors (BAFs) followed the order 6-MeO-BDE-47 > 6-OH-BDE-47 > BDE-47. All three compounds could undergo step-by-step debromination to produce lower brominated analogs in earthworms. Both BDE-47 and 6-OH-BDE-47 could be transformed to MeO-PBDEs, whereas no transformation from 6-OH-BDE-47 or 6-MeO-BDE-47 to PBDEs or from BDE-47 and 6-MeO-BDE-47 to OH-PBDEs took place in the earthworms. Methoxylation was proposed as a potential metabolic pathway to form MeO-PBDEs in earthworms, with the metabolic rates for the methoxylation of BDE-47 and 6-OH-BDE-47 being 27.7 and 5.1 times greater, respectively, than that of the debromination metabolism. The isomers of 6-MeO-BDE-47 and 6-OH-BDE-47 were formed via the addition of methoxy/hydroxy groups or via bromine shifts on benzene ring in the earthworms. This study provides comprehensive information for a better understanding of the accumulation and biotransformation of PBDEs and their derivatives in earthworms.

Kinetics and toxicity of an environmentally relevant mixture of halogenated organic compounds in zebrafish embryo

Persistent and semi-persistent halogenated compounds cause health problems for the animals occupying the upper level of the food web in the Baltic Sea. Atlantic salmon (Salmo salar), being a top piscivore in the Baltic Sea, has been observed to carry a large body burden of halogenated toxins. Here, a mixture of nine halogenated compounds belonging to different groups was created, based on the observed composition of halogenated toxins in salmon serum. The toxicokinetic properties of the compounds were studied in zebrafish (Danio rerio) embryos to achieve the same proportions between the internal doses of the compounds in the zebrafish as in the salmon. Toxicity was evaluated for the compounds dosed individually as well as in a mixture. Perfluorooctanesulfonic acid (PFOS) was the dominant compound in the salmon and was observed to be the driving force for effects on swimbladder inflation caused by the mixture with a 50% effect concentration of 4.8 µM nominal dose, or 1300 µMD based on the area under the internal concentration-time curve (AUC). The driving compound for other severe effects caused by the mixture, including lethality, spinal deformity, and edemas, was the hydroxylated polybrominated diphenyl ether 6-OH-BDE47, which was observed to have a 50% lethality concentration of 93 nM, corresponding to 94 µMD based on internal dose (AUC). The individual compounds were observed to act additively on most of the documented outcomes when dosed as a mixture.

AOP Report: Thyroperoxidase Inhibition Leading to Altered Visual Function in Fish Via Altered Retinal Layer Structure

Thyroid hormones (THs) are involved in the regulation of many important physiological and developmental processes, including vertebrate eye development. Thyroid hormone system-disrupting chemicals (THSDCs) may have severe consequences, because proper functioning of the visual system is a key factor for survival in wildlife. However, the sequence of events leading from TH system disruption (THSD) to altered eye development in fish has not yet been fully described. The development of this adverse outcome pathway (AOP) was based on an intensive literature review of studies that focused on THSD and impacts on eye development, mainly in fish. In total, approximately 120 studies (up to the end of 2021) were used in the development of this AOP linking inhibition of the key enzyme for TH synthesis, thyroperoxidase (TPO), to effects on retinal layer structure and visual function in fish (AOP-Wiki, AOP 363). In a weight-of-evidence evaluation, the confidence levels were overall moderate, with ample studies showing the link between reduced TH levels and altered retinal layer structure. However, some uncertainties about the underlying mechanism(s) remain. Although the current weight-of-evidence evaluation is based on fish, the AOP is plausibly applicable to other vertebrate classes. Through the re-use of several building blocks, this AOP is connected to the AOPs leading from TPO and deiodinase inhibition to impaired swim bladder inflation in fish (AOPs 155-159), together forming an AOP network describing THSD in fish. This AOP network addresses the lack of thyroid-related endpoints in existing fish test guidelines for the evaluation of THSDCs. Environ Toxicol Chem 2022;41:2632-2648. © 2022 SETAC.

Acetaminophen Disrupts the Development of Pharyngeal Arch-Derived Cartilage and Muscle in Zebrafish

Acetaminophen is a common analgesic, but its potential effects on early embryonic development are not well understood. Previous studies using zebrafish (Danio rerio) have described the effects of acetaminophen on liver development and physiology, and a few have described gross physiological and morphological defects. Using a high but non-embryonic lethal dose of acetaminophen, we probed for defects in zebrafish craniofacial cartilage development. Strikingly, acetaminophen treatment caused severe craniofacial cartilage defects, primarily affecting both the presence and morphology of pharyngeal arch-derived cartilages of the viscerocranium. Delaying acetaminophen treatment restored developing cartilages in an order correlated with their corresponding pharyngeal arches, suggesting that acetaminophen may target pharyngeal arch development. Craniofacial cartilages are derived from cranial neural crest cells; however, many neural crest cells were still seen along their expected migration paths, and most remaining cartilage precursors expressed the neural crest markers sox9a and sox10, then eventually col2a1 (type II collagen). Therefore, the defects are not primarily due to an early breakdown of neural crest or cartilage differentiation. Instead, apoptosis is increased around the developing pharyngeal arches prior to chondrogenesis, further suggesting that acetaminophen may target pharyngeal arch development. Many craniofacial muscles, which develop in close proximity to the affected cartilages, were also absent in treated larvae. Taken together, these results suggest that high amounts of acetaminophen can disrupt multiple aspects of craniofacial development in zebrafish.

The aryl hydrocarbon receptor: A predominant mediator for the toxicity of emerging dioxin-like compounds

The aryl hydrocarbon receptor (AHR) is a member of the basic helix-loop-helix/Per-ARNT-Sim (bHLH-PAS) family of transcription factors and has broad biological functions. Early after the identification of the AHR, most studies focused on its roles in regulating the expression of drug-metabolizing enzymes and mediating the toxicity of dioxins and dioxin-like compounds (DLCs). Currently, more diverse functions of AHR have been identified, indicating that AHR is not just a dioxin receptor. Dioxins and DLCs occur ubiquitously and have diverse health/ecological risks. Additional research is required to identify both shared and compound-specific mechanisms, especially for emerging DLCs such as polyhalogenated carbazoles (PHCZs), polychlorinated diphenyl sulfides (PCDPSs), and others, of which only a few investigations have been performed at present. Many of the toxic effects of emerging DLCs were observed to be predominantly mediated by the AHR because of their structural similarity as dioxins, and the in vitro TCDD-relative potencies of certain emerging DLC congeners are comparable to or even greater than the WHO-TEFs of OctaCDD, OctaCDF, and most coplanar PCBs. Due to the close relationship between AHR biology and environmental science, this review begins by providing novel insights into AHR signaling (canonical and non-canonical), AHR's biochemical properties (AHR structure, AHR-ligand interaction, AHR-DNA binding), and the variations during AHR transactivation. Then, AHR ligand classification and the corresponding mechanisms are discussed, especially the shared and compound-specific, AHR-mediated effects and mechanisms of emerging DLCs. Accordingly, a series of in vivo and in vitro toxicity evaluation methods based on the AHR signaling pathway are reviewed. In light of current advances, future research on traditional and emerging DLCs will enhance our understanding of their mechanisms, toxicity, potency, and ecological impacts.

Impact of environmental chemicals on craniofacial skeletal development: Insights from investigations using zebrafish embryos

Craniofacial skeletal anomalies are among the most common structural birth defects around the world. Various studies using human populations and experimental animals have shown that genetic and environmental factors play significant roles in the causation and progression of these anomalies. Environmental factors, such as teratogens and toxin mixtures, induce craniofacial anomalies are gaining heightened attention. Among experimental investigations, the use of the zebrafish (Danio rerio) has been increasing. A major reason for the increased use is that the zebrafish boast a simple craniofacial structure, and facial morphogenesis is readily observed due to external fertilization and transparent embryo, making it a valuable platform to screen and identify environmental factors involved in the etiology of craniofacial skeletal malformation. This review provides an update on harmful effects from exposure to environmental chemicals, involving metallic elements, nanoparticles, persistent organic pollutants, pesticides and pharmaceutical formulations on craniofacial skeletal development in zebrafish embryos. The collected data provide a better understanding for induction of craniofacial skeletal anomalies and for development of better prevention strategies.

Fish toxicity testing for identification of thyroid disrupting chemicals

Identification of thyroid disrupting chemicals (TDCs), one of the most studied types of endocrine disruptors (EDs), is required according to EU regulations on industrial chemicals, pesticides, and biocides. Following that requirement, the use of fish as a unique non-mammalian model species for identification of EDs may be warranted. This study summarized and evaluated effects of TDCs on fish thyroid sensitive endpoints including thyroid hormones, thyroid related gene expression, immunostaining for thyroid follicles, eye size and pigmentation, swim bladder inflation as well as effects of TDCs on secondary sex characteristics, sex ratio, growth and reproduction. Changes in thyroid sensitive endpoints may reflect the balanced outcome of different processes of the thyroid cascade. Thyroid sensitive endpoints may also be altered by non-thyroid molecular or endocrine pathways as well as non-specific factors such as general toxicity, development, stress, nutrient, and the environmental factors like temperature and pH. Defining chemical specific effects on thyroid sensitive endpoints is important for identification of TDCs. Application of the AOP (adverse outcome pathway) concept could be helpful for defining critical events needed for testing and identification of TDCs in fish.

Emerging environmental pollutants hydroxylated polybrominated diphenyl ethers: From analytical methods to toxicology research

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) are of particular concern due to their ubiquitous distribution and adverse health effects. Significant progress has been made in the characterization of OH-PBDEs by using mass spectrometry (MS). In this review, we summarize applications of MS-based techniques in detection, environmental and biota distribution, and potential health risk effects, hoping to unfold an overall picture on account of current knowledge of OH-PBDEs. The analytical methodologies are discussed from sample pretreatment to MS analysis. The methods including gas chromatography-MS (GC-MS), liquid chromatography-MS (LC-MS), and ion mobility spectrometry-MS (IMS-MS) are discussed. GC-MS is the most frequently adopted method in the analysis of OH-PBDEs due to its excellent chromatographic resolution, high sensitivity, and strong ability for unknown identification. LC-MS has been widely used for its high sensitivity and capability of direct analysis. As a newly developed technique, IMS-MS provides high specificity, which greatly facilitates the identification of isomers. OH-PBDEs pervasively existed in both abiotic and biotic samples, including humans, animals, and environmental matrices. Multiple adverse health effects have been reported, such as thyroid hormone disruption, estrogen effects, and neurotoxicity. The reported potential pathological mechanisms are also reviewed. Additionally, MS-based metabolomics, lipidomics, and proteomics have been shown as promising tools to unveil the molecular mechanisms of the toxicity of OH-PBDEs. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Effect of bisphenol A on craniofacial cartilage development in zebrafish (Danio rerio) embryos: A morphological study

Bisphenol A (BPA), an endocrine-disrupting chemical, is present in everyday-used consumables and common household products. Although the side effects of BPA have been sufficiently explored, little is known the effects of environmentally relevant low levels of BPA on chondrogenesis in skeletal development. Here we used a morphological approach to investigate whether exposure to BPA (0, 0.0038, 0.05, 0.1, 1.0 μM) could affect craniofacial cartilage development of zebrafish embryo. Furthermore, we sought to determine receptor-mediated BPA induced chondrogenesis toxicity by co-exposing developing embryos to BPA and various inhibitors. Low-dose BPA affected heart rate and induced body and head elongation of larvae. Quantitative morphometric and histopathological analysis revealed that BPA exposure changed the angle and length of craniofacial cartilage elements and disrupted chondrocytes. BPA induced pharyngeal cartilage defects via multiple cellular pathways, including estrogen receptor, androgen receptor, and estrogen-related receptors. Our findings demonstrate that BPA alters the normal development of cartilage and craniofacial structures in zebrafish embryos. Furthermore, in this study we find multiple cellular pathways mediating the effects of BPA-induced craniofacial chondrogenesis toxicity. Further experiments will allow for establishing a connection between BPA and increased risk of congenital malformation of the facial cranium in BPA-exposed populations.

Toxicity and accumulation of 6-OH-BDE-47 and newly synthesized 6,6'-diOH-BDE-47 in early life-stages of Zebrafish (Danio rerio)

Dihydroxylated polybrominated diphenyl ethers (diOH-PBDEs) appear to be natural products or metabolites of PBDEs in some marine organisms, yet its toxicity is still largely unknown. With a newly lab-synthesized diOH-PBDE, 6,6'-dihydroxy-2,2',4'4'-tetrabromodiphenyl ether (6,6'-diOH-BDE-47) in hand, the present study has provided the first data set to compare 6-hydroxy-2,2',4'4'- tetrabromodiphenyl ether (6-OH-BDE-47) and 6,6'-diOH-BDE-47 for their acute toxicity and accumulation, and thyroid hormone levels in treated zebrafish larvae. By real time-PCR technique, transcripts of hypothalamic-pituitary-thyroid axis associated genes were also investigated in developing larvae at 96 h post fertilization (96 hpf). Apparently, 6,6'-diOH-BDE-47 was less toxic than that of 6-OH-BDE-47: 1) the 96-h LC₅₀ (96-h median lethal concentration) of 6-OH-BDE-47 and 6,6'-diOH-BDE-47 were 235 nM and 516 nM, respectively; 2) although severe developmental delays and morphological deformities were observed in zebrafish larvae in high exposure doses, at the exposure concentration of 1-50 nM, the accumulated 6-OH-BDE-47 and 6,6'-diOH-BDE-47 is ranged between 226-2279 nmol/g and 123-539 nmol/g in treated larvae; and 3) for 6-OH-BDE-47, its bioconcentration factor (BCF) were 1.83- to 4.30-fold more than that of 6,6'-diOH-BDE-47, suggesting that the lower internal exposure concentration of 6,6'-diOH-BDE-47 may lead to lower toxicity. The increased thyroid hormone levels were recorded for 1 nM of 6-OH-BDE-47 and 20 nM of 6,6'-diOH-BDE-47, and the exposures both significantly increased thyroid gland-specific transcription of thyroglobulin gene, indicating an adverse effect associated with the HPT axis. Therefore, 6,6'-diOH-BDE-47, with lower toxicity compared to that of 6-OH-BDE-47, still possesses hazards and environmental risk.

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.

Developmental abnormalities and epigenetic alterations in medaka (Oryzias latipes) embryos induced by triclosan exposure

Triclosan (TCS), an antibacterial and antifungal agent present in some consumer products, has been detected in the environment at varying concentrations. TCS exposure has been found to cause developmental abnormalities and endocrine disruption in various species of fish. It is not clearly understood whether TCS exposure causes epigenetic alterations in developing embryos and their germ cells. In the present study, we examined the effects of TCS exposure (0, 50, 100 and, 200 μg/L) on embryonic development and primordial germ cells (PGCs), which are precursors of sperm and eggs, in medaka (Oyzias latipes). Developmental TCS exposure from 8 h post-fertilization through 15 days post-fertilization (dpf) resulted in several developmental abnormalities, including enlarged yolk sac, decreased head trunk angle (HTA), and severe edema in the pericardial region. The male ratio increased in the 100 μg/L TCS exposure group, which was negatively correlated with the expression of cyp19ala (a gene encoding aromatase) and arα (androgen receptor alpha). Developmental 50 μg/L TCS exposure resulted in global hypomethylation in the whole body but not in the isolated PGCs. Expression of the gene encoding DNA methyltransferases (dnmt1 and dnmt3aa) was decreased by 50 μg/L TCS exposure both in the whole body and PGCs. TCS altered the expression of genes encoding enzymes involved in DNA methylation and demethylation in PGCs, suggesting epigenetic effects on germ cells. The present results demonstrate that the embryos exposed to the tested concentrations of TCS develop deformities during the early life stages and that the TCS within this range possesses endocrine disrupting properties potential enough to alter sex ratios of developing embryos.

New Insights in the Endocrine Disrupting Effects of Three Primary Metabolites of Organophosphate Flame Retardants

Despite the ubiquity of organophosphate flame retardants (OPFRs) metabolites in the biota, the endocrine disrupting potency has not been well examined. Herein, we chose three primary metabolites of OPFRs (BCIPP, BDCIPP, and DPHP) to investigate their potential endocrine disrupting effects by in vitro, in vivo, and in silico assays. Three metabolites were agonistic to rat estrogenic receptor alpha (ERα) and antagonists to human mineralocorticoid receptor (MR). BCIPP exerted endocrine disrupting effect contrasting to the negative response of its parental compound. It also poses the strongest binding capacity to ERα among the tested compounds. Both BCIPP and BDCIPP upregulated the genes encoded for estrogenic synthesis enzymes in H295R cells, including 17βHSD and CYP19. All three compounds stimulated the transcription of CYP11B1, whereas BCIPP and DPHP also triggered CYP11B2, encoding for corticoid production. BDCIPP inhibits genes for progesterone synthesis including CYP11A1, STAR, and 3-βHSD. The induction of mortality and low hatchability of zebrafish embryo were ranked as BCIPP ≥ BDCIPP > DPHP. All compounds lead to malformation of zebrafish larvae. Both of the hypothalamic-pituitary-adrenocortical and hypothalamic-pituitary-gonadal axes were disrupted, with the highest impact by BCIPP. Altogether, the data clarified OPFRs metabolites may produce comparable or even higher endocrine disrupting effects than OPFRs.

Embryo/larval toxicity and transcriptional effects in zebrafish (Danio rerio) exposed to endocrine active riverbed sediments

Sediment toxicity plays a fundamental role in the health of inland fish communities; however, the assessment of the hazard potential of contaminated sediments is not a common objective in environmental diagnostics or remediation. This study examined the potential of transcriptional endpoints investigated in zebrafish (Danio rerio) exposed to riverbed sediments in ecotoxicity testing. Embryo-larval 10-day tests were conducted on sediment samples collected from five sites (one upstream and four downstream of the city of Milan) along a polluted tributary of the Po River, the Lambro River. Sediment chemistry showed a progressive downstream deterioration in river quality, so that the final sampling site showed up to eight times higher concentrations of, for example, triclosan, galaxolide, PAH, PCB, BPA, Ni, and Pb, compared with the uppermost site. The embryo/larval tests showed widespread toxicity although the middle river sections evidenced worse effects, as evidenced by delayed embryo development, hatching rate, larval survival, and growth. At the mRNA transcript level, the genes encoding biotransformation enzymes (cyp1a, gst, ugt) showed increasing upregulations after exposure to sediment from further downstream sites. The genes involved in antioxidant responses (sod, gpx) suggested that more critical conditions may be present at downstream sites, but even upstream of Milan there seemed to be some level of oxidative stress. Indirect evidences of potential apoptotic activity (bcl2/bax < 1) in turn suggested the possibility of genotoxic effects. The genes encoding for estrogen receptors (erα, erβ1, erβ2) showed exposure to (xeno)estrogens with a progressive increase after exposure to sediments from downstream sites, paralleled by a corresponding downregulation of the ar gene, likely related to antiandrogenic compounds. Multiple levels of thyroid disruption were also evident particularly in downstream zebrafish, as for thyroid growth (nkx2.1), hormone synthesis and transport (tg, ttr, d2), and signal transduction (trα, trβ). The inhibition of the igf2 gene reasonably reflected larval growth inhibitions. Although none of the sediment chemicals could singly explain fish responses, principal component analysis suggested a good correlation between gene transcripts and the overall trend of contamination. Thus, the combined impacts from known and unknown covarying chemicals were proposed as the most probable explanation of fish responses. In summary, transcriptional endpoints applied to zebrafish embryo/larval test can provide sensitive, comprehensive, and timeliness information which may greatly enable the assessment of the hazard potential of sediments to fish, complementing morphological endpoints and being potentially predictive of longer studies.

The organochlorine pesticide toxaphene reduces non-mitochondrial respiration and induces heat shock protein 70 expression in early-staged zebrafish (Danio rerio)

Toxaphene is a restricted-use pesticide produced by reacting chlorine gas with camphene. It was heavily used as a pesticide for agricultural purposes in the 1960-1970s, but despite being banned >30 years ago, it can remain elevated in the soil due to its resistance to metabolic degradation; this has led to longstanding concerns about elevated levels of toxaphene and other organochlorine pesticides (OCPs) in the environment. The objective of this study were to determine the effects of waterborne exposure to toxaphene on early life stages of zebrafish. Based on the LC₅₀, zebrafish embryos were exposed to control (embryo rearing media or DMSO) or to one dose of toxaphene ranging between 0.011 and 111.1 μg/mL from 6 h post fertilization (hpf) up to 120 hpf. Significant mortality and hatch time delays were observed in embryos exposed to toxaphene (at or above 0.11 and 1.11 μg/mL, depending on the assay). Higher prevalence of deformities was noted at higher doses (≥0.011 μg/mL), and these included pericardial edema and skeletal deformities. As energy production is important for normal development, mitochondrial bioenergetics were assessed in embryos following toxaphene exposure. Embryos exposed to 11.1 or 111 μg/mL toxaphene for 24 h showed lower non-mitochondrial respiration (~30%) compared to both solvent and no treatment controls. Expression of transcripts related to oxidative damage responses and apoptosis were measured and heat shock protein 70 was significantly increased with 111 μg/mL toxaphene (14.5 fold), while the expression levels of caspase 3, caspase 9, and superoxide dismutase 1 were not changed. These data demonstrate that developmental deformities induced by toxaphene include pericardial edema and skeletal deformity, and that toxaphene can affect oxidative phosphorylation in early staged zebrafish.

Temporal and spatial surveys of polybromodiphenyl ethers (PBDEs) contamination of soil near a factory using PBDEs in northern Taiwan

Polybrominated diphenyl ethers (PBDEs), previously commonly used as flame retardants, should be monitored in the environment since some are listed as persistent organic pollutants. A contaminated site near a northern Taiwan factory using decabrominated diphenyl ether (deca-BDE) was identified based on a vegetable PBDEs monitoring project in 2013. The subsequent spatial and temporal survey of that contaminated site shows the contamination ingredients in soils were close to ones used by the factory, indicating that contamination was from the factory, possibly through an exhaust vent. The average concentration of deca-BDE in the main contaminated soil was 615 μg/kg d. w. (dry weight) soil in 2015, slightly decreasing to 604 μg/kg d. w. soil in 2016, increasing to 844 μg/kg d. w. soil in 2017, and then slightly decreasing to 670 μg/kg d. w. soil in 2018. The slight change of deca-BDE and the minor change in low brominated congener level indicate a low degradation rate. The contamination of peripheral sites was around 5000 μg/kg d. w. soil for one PBDEs sampling site that was higher than those around or within the main contaminated farm, indicating serious pollution. Concentrations of PBDEs in different soil depths show that depth 2-15 cm accounted for the greatest PBDEs accumulation, indicating that deca-BDE pollution had been present over time and transported into deeper soil. There can be PBDEs uptake by crops consumed by humans, as shown in our previous studies, so continuous monitoring of PBDEs in this site is important and treatments should be established urgently.

Insights from zebrafish deficiency models to understand the impact of local thyroid hormone regulator action on early development

Thyroid hormones (THs) stimulate and coordinate a wide range of processes to ensure normal development, mainly by binding of the most active TH 3,5,3'-triiodothyronine (T₃) to nuclear receptors resulting in changes in gene transcription. Local TH action is monitored at three distinct levels by different types of regulators: transmembrane transporters (TH influx and efflux), deiodinases (TH activation and inactivation) and nuclear receptors (TH signalling). Since TH regulators are strongly conserved among vertebrate species, the externally and rapidly developing zebrafish (Danio rerio) has become one of the favourite models to study their role in TH-dependent development. Most regulators are expressed in zebrafish from early stages in development in a dynamic and tissue-specific pattern. Transient or permanent disruption of a given regulator severely perturbs development of multiple organs. These zebrafish deficiency models help to explain why, next to overall hypo-/hyperthyroidism, inactivating mutations in the genes encoding TH regulators such as MCT8 and THRA/B have irreversible adverse effects on human development. Zebrafish are also increasingly used as a high-throughput model to assess the toxicity of various xenobiotics and their impact on development. While adverse effects on TH metabolism and gene expression have been shown, information on direct interaction with TH regulators is scarce, albeit essential to fully understand their mechanism of action. For the future, the combination of novel gene silencing tools, fluorescent reporter lines and (single-cell) transcriptomics holds promise for new zebrafish models to further elucidate the role of each TH regulator in vertebrate development.

Effects of 25 thyroid hormone disruptors on zebrafish embryos: A literature review of potential biomarkers

It is estimated that many organic compounds found in our environment can interfere with the thyroid system and act as thyroid hormone (TH) disruptor. Despite that, there is a clear lack of assays to identify TH disruptors. Recently zebrafish embryos were suggested as screening tool to identify compounds which impact thyroid synthesis. Effects on hormone level, gene transcript expression, eye development and swim bladder inflation are suggested as potential biomarker for TH disruptors. In order to assess the applicability of these biomarkers we performed a literature review. The effects of 25 known TH disrupting compounds were compared between studies. The studies were limited to exposures with embryos prior 7 days of development. The different study designs and the lack of standardized methods complicated the comparison of the results. The most common responses were morphological alterations and gene transcript expression changes, but no specific biomarker for TH disruption could be identified. In studies addressing TH disruption behavioral effects were more commonly monitored than in studies not mentioning the TH pathway. TH disruption in developing zebrafish embryos might be caused by different modes of action e.g. disruption of follicle development, binding of TH, activation of TH receptors causing different effects. Timing of developmental processes in combination with exposure duration might also play a role. On the other side compound characteristics (uptake, stability, metabolization) could also cause differences between substances. Further studies are necessary to gain better understanding into the mechanisms of TH disruption in early zebrafish development.

Transgenerational thyroid endocrine disruption induced by bisphenol S affects the early development of zebrafish offspring

Maternal thyroid hormones (THs) play an essential role in the embryonic and larval development of fish. Previous studies in fish have reported that parental exposure to thyroid disrupting chemicals (TDCs) changed maternal TH levels in the offspring; however, whether this transgenerational thyroid endocrine disruption can further disturb the early development of the offspring still remains largely unknown. Bisphenol S (BPS), a substitute of bisphenol A, has been reported to be a potential TDC. In this study, zebrafish (F0) were exposed to environmentally relevant concentrations (1, 10, and 100 μg/L) of BPS from 2 h post-fertilization to 120 days post-fertilization and then paired to spawn. Plasma levels of thyroxine (T4) were significantly decreased in F0 females while 3,5,3'-triiodothyronine (T3) plasma levels were significantly increased in F0 females and males; moreover, TH content in eggs (F1) spawned by exposed F0 generation exhibited similar changes as the F0 females, with significant decreases in T4 and increases in T3, demonstrating BPS-induced maternal transfer of thyroid endocrine disruption. Further, excessive levels of maternal T3 in the offspring resulted in delayed embryonic development and hatching, swim bladder inflation defect, reduction in motility, developmental neurotoxicity, and lateral stripe hypopigmentation in non-exposed F1 embryos and larvae. These results highlight the adverse effects on the early development of offspring induced by transgenerational thyroid endocrine disruption, which have been ignored by previous studies. Therefore, these results can further improve our understanding of the ecological risks of TDCs.

Neurobehavioral effects of two metabolites of BDE-47 (6-OH-BDE-47 and 6-MeO-BDE-47) on zebrafish larvae

Two metabolites, OH-BDEs and MeO-BDEs, of polybrominated diphenyl ethers (PBDEs) were ubiquitously detected in animal tissues and environmental samples, drawing a widely public concern to their toxicity. The comparison of toxicity between PBDEs and their metabolites has been a focus in recent years, however, comparisons seldom involve neurobehavioral toxicity of PBDEs metabolites in published works. In this study, zebrafish larvae were exposed to 6-OH-BDE-47 and 6-MeO-BDE-47 and their neurobehavioral traits (including locomotion, path angle, and social activity) were recorded using the instrument Zebrabox; meanwhile, light illumination was used as stimuli in the test duration. The results showed larvae were more active in dark periods than light periods, and preferred turning right (+) to left (-). Effects of the two metabolites varied in different behavioral indicators. They induced different effects on path angle but did not reverse the left-right asymmetry. 6-OH-BDE-47 did not induce the effects on larval locomotion and social activity, but mainly decreased average and routine turn numbers; 6-MeO-BDE-47 promoted larvae responsive turns but inhibited social activity. This study offered new experimental means to the neurobehavioral toxicity of various PBDE metabolites. Further studies may focus on the toxic mechanisms of specific neurobehavioral traits.

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.

Increased coiling frequency linked to apoptosis in the brain and altered thyroid signaling in zebrafish embryos (Danio rerio) exposed to the PBDE metabolite 6-OH-BDE-47

Polybrominated diphenyl ethers (PBDEs) are a group of brominated flame retardants that are ubiquitously detected in the environment and associated with adverse health outcomes. 6-OH-BDE-47 is a metabolite of the flame retardant, 2,2',4,4'-Tetrabromodiphenyl ether (BDE-47), and there is increasing concern regarding its developmental neurotoxicity and endocrine disrupting properties. In this study, we report that early life exposure in zebrafish (Danio rerio) embryos to 6-OH-BDE-47 (50 and 100 nM) resulted in higher coiling frequency and significantly increased apoptotic cells in the brain. These effects were partially rescued by overexpression of thyroid hormone receptor β (THRβ) mRNA. Moreover, exposure to 100 nM 6-OH-BDE-47 significantly reduced the number of hypothalamic 5-hydroxytryptamine (5-HT, serotonin)-immunoreactive (5-HT-ir) neurons and the mRNA expression of tryptophan hydroxylase 2 (TPH2). These results indicate that 6-OH-BDE-47 affected thyroid hormone regulation through THRβ and negatively impacted the nervous system, in turn, affecting coiling behavior. Correlations of these endpoints suggest that coiling frequency could be used as an indicator of neurotoxicity in embryos.

Occurrence and profiles of halogenated phenols, polybrominated diphenyl ethers and hydroxylated polybrominated diphenyl ethers in the effluents of waste water treatment plants around Huang-Bo Sea, North China

Halogenated organic pollutants (HOPs), as ubiquitous environment contaminants, have attracted increasing concerns due to the potential adverse health impacts on organisms and even humans. Waste water treatment plants (WWTPs) are one source of HOPs to the environment through their discharge of treated effluent. In this study, the presence and profiles of 6 halogenated phenols (HP), 17 polybrominated diphenyl ethers (PBDE) and 11 hydroxylated polybrominated diphenyl ethers (OH-PBDE) were investigated in 12 WWTP effluent samples collected near Huang-Bo Sea in Dalian, China. These targeted organohalogen pollutants were found in all the effluent samples with the total concentrations of ΣHPs, ΣPBDEs and ΣOH-PBDEs ranging from 77.2 to 168.5ng/L, from not-detected to 5.3ng/L and from 0.08 to 0.88ng/L, respectively. The most abundant congeners of HPs and PBDEs in the effluents were pentachlorophenol (PCP), BDE-47 and BDE-99, while for OH-PBDEs, 6-OH-BDE-47 and 5-OH-BDE-47 were the most abundant. In addition, the statistical analysis showed that a significant (p<0.05) positive correlation was observed between BDE-47 and its metabolite 6-OH-BDE-47, indicating that PBDEs may be a source of OH-PBDEs detected in the effluents.

Thyroid-disrupting chemicals and brain development: an update

This review covers recent findings on the main categories of thyroid hormone-disrupting chemicals and their effects on brain development. We draw mostly on epidemiological and experimental data published in the last decade. For each chemical class considered, we deal with not only the thyroid hormone-disrupting effects but also briefly mention the main mechanisms by which the same chemicals could modify estrogen and/or androgen signalling, thereby exacerbating adverse effects on endocrine-dependent developmental programmes. Further, we emphasize recent data showing how maternal thyroid hormone signalling during early pregnancy affects not only offspring IQ, but also neurodevelopmental disease risk. These recent findings add to established knowledge on the crucial importance of iodine and thyroid hormone for optimal brain development. We propose that prenatal exposure to mixtures of thyroid hormone-disrupting chemicals provides a plausible biological mechanism contributing to current increases in the incidence of neurodevelopmental disease and IQ loss.

Toxic effects and mechanism of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) on Lemna minor

To investigate the toxic effect and mechanism of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) in aquatic plants, in vivo and in vitro exposure to BDE-47 were conducted. After 14-d exposure to 5-20 μg/L BDE-47, the growth of Lemna minor plants was significantly suppressed, and the chlorophyll and soluble protein contents in fronds markedly decreased. Accordingly, the photosynthetic efficiency (Fv/Fm, PI) decreased. When the thylakoid membranes isolated from healthy fronds was exposed to 5-20 mg/L BDE-47 directly in vitro for 1 h, the photosynthetic efficiency also decreased significantly. In both the in vitro (5-20 μg/L) and in vivo (5-20 mg/L) experiments, BDE-47 led to an increased plasma membrane permeability. Hence, we concluded that BDE-47 had a direct toxicity to photosynthetic membranes and plasma membranes. However, direct effects on the activities of peroxidase (POD), malate dehydrogenase (MDH) and nitroreductase (NR) were not observed by adding 5-20 mg/L BDE-47 into crude enzyme extracts. The malondialdehyde (MDA) and superoxide anion radical (O₂^-) contents in the BDE-47 treated fronds were higher than those in the control fronds, suggesting that L. minor can not effectively relieve reactive oxygen species (ROS). The data above indicates that BDE-47 is toxic to L. minor through acting directly on biomembranes, which induces the production of ROS and thus causes remarkable oxidative damage to cells.

Aroclor 1254 and BDE-47 inhibit dopaminergic function manifesting as changes in locomotion behaviors in zebrafish embryos

Contamination with polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) in the environment is a major concern due to their persistent bioaccumulative toxicity that can disturb neurobehavioral functions including movements. Recently, it was reported that some PBDE including BDE-47 stimulates locomotor activities of zebrafish embryos by unknown mechanism. In this study, motor movements of the zebrafish embryo were used as a model system to evaluate the neuronal toxicity of a non-coplanar PCB-dominant mixture (Aroclor 1254) and BDE-47. Both organohalogens increased tail shaking and rotation of embryos in a concentration-dependent manner. Chemical inhibition and gene knock-down of tyrosine hydroxylase and vesicular monoamine transporter 2 (VMAT2) also induced hyperactivities. Hyperactivities induced by these treatments were all inhibited by supplementation of l-tyrosine and l-dopa, precursors of dopamine synthesis. Both organohalogens reduced dopamine contents and increased the 3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine ratio in whole embryos. The results suggest that functional inhibition of dopaminergic neurons is involved in hyperactivities of zebrafish embryos caused by Aroclor 1254 and BDE-47.

Environmentally relevant levels of λ-cyhalothrin, fenvalerate, and permethrin cause developmental toxicity and disrupt endocrine system in zebrafish (Danio rerio) embryo

Synthetic pyrethroids (SPs) are one of the most widely used pesticides and frequently detected in the aquatic environment. Previous studies have shown that SPs posed high aquatic toxicity, but information on the developmental toxicity and endocrine disruption on zebrafish (Danio rerio) at environmentally relevant concentrations is limited. In this study, zebrafish embryos were employed to examine the adverse effects of λ-cyhalothrin (LCT), fenvalerate (FEN), and permethrin (PM) at 2.5, 10, 25, 125, 500 nM for 96 h. The results showed these 3 SPs caused dose-dependent mortality, malformation rate, and hatching rate. Thyroid hormone triiodothyronine (T₃) levels were significantly decreased after exposure to LCT and FEN. Quantitative real-time PCR analysis was then performed on a series of nuclear receptors (NRs) genes involved in the hypothalamic-pituitary-gonadal (HPG), hypothalamic-pituitary-thyroid (HPT), hypothalamic-pituitary-adrenocortical (HPA) axes, and oxidative-stress-related system. Our results showed that LCT, FEN, and PM downregulated AR expression while upregulated ER1 expression, and caused alteration to ER2a and ER2b expression. As for the expression of TRα and TRβ, they were both decreased following exposure to the 3 SPs. LCT and PM downregulated the MR expression and FEN induced MR expression. In addition, the expression of GR was increased after treating with LCT, while it was suppressed after exposure to FEN and PM. The 3 SPs also caused various alterations to the expression of genes including AhRs, PPARα, and PXR. These findings suggest that these 3 SPs may cause developmental toxicity to zebrafish larvae by disrupting endocrine signaling at environmentally relevant concentrations.

Benzotriazole ultraviolet stabilizers alter the expression of the thyroid hormone pathway in zebrafish (Danio rerio) embryos

Benzotriazole ultraviolet stabilizers (BUVSs) are widely used in industrial products as well as personal-hygiene products to protect the material or skin from harmful UV-radiation. Due to their persistence and bioaccumulation, BUVSs have been ubiquitously detected in aquatic environments. Although the toxicological effects of BUVSs in aquatic organisms have been previously examined, the effects of BUVSs on the thyroid system have not been adequately addressed. In this study, we assessed putative thyroid disrupting effects of BUVSs (UV-234, UV-326, UV-329 and UV-P) in zebrafish embryos at 1, 10 and 100 μg/L for 96 h. The heart rate was assessed in zebrafish and was observed to be decreased by 6.9%-21.4% in exposure of tested BUVSs. We also observed that the transcript levels of HPT axis-related genes were affected by the 4 BUVSs tested in different ways. Specifically, mRNA levels of thyroid hormone receptors (thraa and thrb) in zebrafish embryos were differentially expressed and the direction of change in these transcripts was isoform and BUVSs dependent. Pathway analysis of the targeted genes measured indicated that cellular processes putatively affected by BUVSs included response to organic substance, regulation of transcription from RNA polymerase II promoter, intracellular receptor signaling pathway, and hypothyroidism. Upon expansion of the network, novel genes involved in this predicted gene network may provide insight into the mechanisms of thyroid disrupting mechanisms of BUVSs. Taken together, our results indicate that BUVSs can potentially impact the thyroid system, and that this is dependent upon the type or structure of BUVSs.

Effects of Hydroxylated Polybrominated Diphenyl Ethers in Developing Zebrafish Are Indicative of Disruption of Oxidative Phosphorylation

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) have been detected in humans and wildlife. Using in vitro models, we recently showed that OH-PBDEs disrupt oxidative phosphorylation (OXPHOS), an essential process in energy metabolism. The goal of the current study was to determine the in vivo effects of OH-PBDE reported in marine wildlife. To this end, we exposed zebrafish larvae to 17 OH-PBDEs from fertilisation to 6 days of age, and determined developmental toxicity as well as OXPHOS disruption potential with a newly developed assay of oxygen consumption in living embryos. We show here that all OH-PBDEs tested, both individually and as mixtures, resulted in a concentration-dependant delay in development in zebrafish embryos. The most potent substances were 6-OH-BDE47 and 6'-OH-BDE49 (No-Effect-Concentration: 0.1 and 0.05 µM). The first 24 h of development were the most sensitive, resulting in significant and irreversible developmental delay. All substances increased oxygen consumption, an effect indicative of OXPHOS disruption. Our results suggest that the induced developmental delay may be caused by disruption of OXPHOS. Though further studies are needed, our findings suggest that the environmental concentrations of some OH-PBDEs found in Baltic Sea wildlife in the Baltic Sea may be of toxicological concern.

The legacy pesticide dieldrin acts as a teratogen and alters the expression of dopamine transporter and dopamine receptor 2a in zebrafish (Danio rerio) embryos

Dieldrin (DLD) is a lipophilic pesticide that shows environmental persistence. The objectives were to determine the effects of DLD on GABAergic and dopaminergic systems in developing zebrafish. Both chorionated and dechorionated embryos (~24h post-hatch) were exposed to a single concentration of DLD (0.347-3470μM) for 48h. Following exposure, a subset of larvae was placed into clean water for 6days (i.e. depuration phase). Chorionated embryos showed <15% mortality while dechorionated embryos showed higher mortality (>30%), suggesting that the chorion protected the embryos. Over a 6day depuration phase, there was a dose dependent effect observed in both the "dechorionated and chorionated embryo" treatments for larval mortality (>60%). At the end of depuration, there was no detectable change in neuro-morphological endpoints that included the ratio of notochord length to body length (%) and the ratio of head area to body area (%). However, DLD did induce cardiac edema, skeletal deformities, and tremors. GABA-related transcripts were not affected in abundance by DLD. Conversely, the relative mRNA levels of dopamine transporter (dat1) and dopamine receptor drd2a mRNA were decreased in dechorionated, but not chorionated, embryos. These data suggest that DLD can alter the expression of transcripts related to dopaminergic signaling. Lastly, GABA_A receptor subunits gabrB1 and gabrB2, as well as dopamine receptors drd1 and drd2a, were inherently higher in abundance in dechorionated embryos compared to chorionated embryos. This is an important consideration when incorporating transcriptomics into embryo testing as expression levels can change with removal of the chorion prior to exposure.

Effects of three different embryonic exposure modes of 2, 2', 4, 4'-tetrabromodiphenyl ether on the path angle and social activity of zebrafish larvae

The toxicological research of polybrominated diphenyl ethers (PBDEs) has focused on its neurotoxicity; however, many questions still remain. For example, behavioral effects other than basic locomotion are seldom reported. To further evaluate the neurobehavioral toxicity of 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47), a typical PBDE congener in animal tissues, we employed three different exposure modes, namely, continuous, early pulse, and interval exposure, to investigate the path angle and social activity changes of zebrafish larvae exposed to BDE-47 using automated equipment (Zebrabox). The results showed that different exposure modes might have different effects on the larval path angle and social activity. BDE-47 treatments caused more responsive turns in all exposure modes in the path angle test and more contacts in most of the two-fish social tests, indicating that the neurobehavior of larvae was disturbed by BDE-47. The light condition was also a key impact factor in the effects of BDE-47. The effects of BDE-47 were different during the dark and light conditions. Our study shows a useful neurobehavioral test method for environmental pollutant monitoring and further supports the utility of zebrafish to study neurobehavior, indicating that the path angle has the potential to be a practicable behavioral indicator.

Exposure to a PBDE/OH-BDE mixture alters juvenile zebrafish (Danio rerio) development

Polybrominated diphenyl ethers (PBDEs) and their metabolites (e.g., hydroxylated BDEs [OH-BDEs]) are contaminants frequently detected together in human tissues and are structurally similar to thyroid hormones. Thyroid hormones partially mediate metamorphic transitions between life stages in zebrafish, making this a critical developmental window that may be vulnerable to chemicals disrupting thyroid signaling. In the present study, zebrafish were exposed to 6-OH-BDE-47 (30 nM; 15 μg/L) alone, or to a low-dose (30 μg/L) or high-dose (600 μg/L) mixture of PentaBDEs, 6-OH-BDE-47 (0.5-6 μg/L), and 2,4,6-tribromophenol (5-100 μg/L) during juvenile development (9-23 d postfertilization) and evaluated for developmental endpoints mediated by thyroid hormone signaling. Fish were sampled at 3 time points and examined for developmental and skeletal morphology, apical thyroid and skeletal gene markers, and modifications in swimming behavior (as adults). Exposure to the high-dose mixture resulted in >85% mortality within 1 wk of exposure, despite being below reported acute toxicity thresholds for individual congeners. The low-dose mixture and 6-OH-BDE-47 groups exhibited reductions in body length and delayed maturation, specifically relating to swim bladder, fin, and pigmentation development. Reduced skeletal ossification was also observed in 6-OH-BDE-47-treated fish. Assessment of thyroid and osteochondral gene regulatory networks demonstrated significantly increased expression of genes that regulate skeletal development and thyroid hormones. Overall, these results indicate that exposures to PBDE/OH-BDE mixtures adversely impact zebrafish maturation during metamorphosis. Environ Toxicol Chem 2017;36:36-48. © 2016 SETAC.

Toxicogenomic Assessment of 6-OH-BDE47-Induced Developmental Toxicity in Chicken Embryos

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) are analogs of PBDEs with hundreds of possible structures and are frequently detected in the environment. However, the in vivo evidence on the toxicity of OH-PBDEs is still very limited. Here, the developmental toxicity of 6-OH-BDE47, a predominant congener of OH-PBDEs detected in the environment, in chicken embryos was assessed using a toxicogenomic approach. Fertilized chicken eggs were dosed via in ovo administration of 0.006 to 0.474 nmol 6-OH-BDE47/g egg followed by 18 days of incubation. Significant embryo lethality (LD₅₀ = 1.940 nmol/g egg) and increased hepatic somatic index (HSI) were caused by 6-OH-BDE47 exposure. The functional enrichment of differentially expressed genes (DEGs) was associated with oxidative phosphorylation, generation of precursor metabolites and energy, and electron transport chains, which suggest that 6-OH-BDE47 exposure may disrupt the embryo development by altering the function of energy production in mitochondria. Moreover, aryl hydrocarbon receptor (AhR)-mediated responses including up-regulation of CYP1A4 were observed in the livers of embryos exposed to 6-OH-BDE47. Overall, this study confirmed the embryo lethality by 6-OH-BDE47 and further improved the mechanistic understanding of OH-PBDEs-caused toxicity. Ecological risk assessment via application of both no-observed-effect level (NOEL) and the sensitive NOTEL (transcriptional NOEL) suggested that OH-PBDEs might cause ecological risk to wild birds.

Inhibition of the Human ABC Efflux Transporters P-gp and BCRP by the BDE-47 Hydroxylated Metabolite 6-OH-BDE-47: Considerations for Human Exposure

High body burdens of polybrominated diphenyl ethers (PBDEs) in infants and young children have led to increased concern over their potential impact on human development. PBDE exposure can alter the expression of genes involved in thyroid homeostasis, including those of ATP-binding cassette (ABC) transporters, which mediate cellular xenobiotic efflux. However, little information exists on how PBDEs interact with ABC transporters such as P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP). The purpose of this study was to evaluate the interactions of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and its hydroxylated metabolite 6-OH-BDE-47 with P-gp and BCRP, using human MDR1- and BCRP-expressing membrane vesicles and stably transfected NIH-3T3-MDR1 and MDCK-BCRP cells. In P-gp membranes, BDE-47 did not affect P-gp activity; however, 6-OH-BDE-47 inhibited P-gp activity at low µM concentrations (IC₅₀=11.7 µM). In BCRP membranes, BDE-47 inhibited BCRP activity; however, 6-OH-BDE-47 was a stronger inhibitor [IC₅₀=45.9 µM (BDE-47) vs. IC₅₀=9.4 µM (6-OH-BDE-47)]. Intracellular concentrations of known P-gp and BCRP substrates [(³H)-paclitaxel and (³H)-prazosin, respectively] were significantly higher (indicating less efflux) in NIH-3T3-MDR1 and MDCK-BCRP cells in the presence of 6-OH-BDE-47, but not BDE-47. Collectively, our results indicate that the BDE-47 metabolite 6-OH-BDE-47 is an inhibitor of both P-gp and BCRP efflux activity. These findings suggest that some effects previously attributed to BDE-47 in biological systems may actually be due to 6-OH-BDE-47. Considerations for human exposure are discussed.

Thyroid disruption in zebrafish (Danio rerio) larvae: Different molecular response patterns lead to impaired eye development and visual functions

The vertebrate thyroid system is important for multiple developmental processes, including eye development. Thus, its environmentally induced disruption may impact important fitness-related parameters like visual capacities and behaviour. The present study investigated the relation between molecular effects of thyroid disruption and morphological and physiological changes of eye development in zebrafish (Danio rerio). Two test compounds representing different molecular modes of thyroid disruption were used: propylthiouracil (PTU), which is an enzyme-inhibitor of thyroid hormone synthesis, and tetrabromobisphenol A (TBBPA), which interacts with the thyroid hormone receptors. Both chemicals significantly altered transcript levels of thyroid system-related genes (TRα, TRβ, TPO, TSH, DIO1, DIO2 and DIO3) in a compound-specific way. Despite these different molecular response patterns, both treatments resulted in similar pathological alterations of the eyes such as reduced size, RPE cell diameter and pigmentation, which were concentration-dependent. The morphological changes translated into impaired visual performance of the larvae: the optokinetic response was significantly and concentration-dependently decreased in both treatments, together with a significant increase of light preference of PTU-treated larvae. In addition, swimming activity was impacted. This study provides first evidence that different modes of molecular action of the thyroid disruptors can be associated with uniform apical responses. Furthermore, this study is the first to show that pathological eye development, as it can be induced by exposure to thyroid disruptors, indeed translates into impaired visual capacities of zebrafish early life stages.

Parental dietary seleno-L-methionine exposure and resultant offspring developmental toxicity

Selenium (Se) leaches into water from agricultural soils and from storage sites for coal fly ash. Se toxicity causes population and community level effects in fishes and birds. We used the laboratory aquarium model fish, Japanese medaka (Oryzias latipes), an asynchronous breeder, to determine aspects of uptake in adults and resultant developmental toxicity in their offspring. The superior imaging properties of the model enabled detailed descriptions of phenotypic alterations not commonly reported in the existing Se literature. Adult males and females in treatment groups were exposed, separately and together, to a dry diet spiked with 0, 12.5, 25, or 50 μg/g (dry weight) seleno-L-methionine (SeMet) for 6 days, and their embryo progeny collected for 5 days, maintained under controlled conditions and observed daily for hatchability, mortality and/or developmental toxicity. Sites of alteration included: craniofacial, pericardium and abdomen (Pc/Ab), notochord, gall bladder, spleen, blood, and swim bladder. Next, adult tissue Se concentrations (liver, skeletal muscle, ovary and testis) were determined and compared in treatment groups of bred and unbred individuals. No significant difference was found across treatment groups at the various SeMet concentrations; and, subsequent analysis compared exposed vs. control in each of the treatment groups at 10 dpf. Increased embryo mortality was observed in all treatment groups, compared to controls, and embryos had a decreased hatching rate when both parents were exposed. Exposure resulted in significantly more total altered phenotypes than controls. When altered phenotypes following exposure of both parents were higher than maternal only exposure, a male role was suggested. The comparisons between treatment groups revealed that particular types of phenotypic change may be driven by the sex of the exposed parent. Additionally, breeding reduced Se concentrations in some adult tissues, specifically the liver of exposed females and skeletal muscle of exposed males. Detailed phenotypic analysis of progeny from SeMet exposed parents should inform investigations of later life stages in an effort to determine consequences of early life exposure.