Transcriptome alterations in zebrafish embryos after exposure to environmental estrogens and anti-androgens can reveal endocrine disruption

Exploring estrogen antagonism using CRISPR/Cas9 to generate specific mutants for each of the receptors

Endocrine disruptors are chemicals that have been in the spotlight for some time now. Their modulating action on endocrine signaling pathways made them a particularly interesting topic of research within the field of ecotoxicology. Traditionally, endocrine disrupting properties are studied using exposure to suspected chemicals. In recent years, a major breakthrough in biology has been the advent of targeted gene editing tools to directly assess the function of specific genes. Among these, the CRISPR/Cas9 method has accelerated progress across many disciplines in biology. This versatile tool allows to address antagonism differently, by directly inactivating the receptors targeted by endocrine disruptors. Here, we used the CRISPR/Cas9 method to knock out the different estrogen receptors in zebrafish and we assessed the potential effects this generates during development. We used a panel of biological tests generally used in zebrafish larvae to investigate exposure to compounds deemed as endocrine disrupting chemicals. We demonstrate that the absence of individual functional estrogen receptors (Esr1, Esr2b, or Gper1) does affect behavior, heart rate and overall development. Each mutant line was viable and could be grown to adulthood, the larvae tended to be morphologically grossly normal. A substantial fraction (70%) of the esr1 mutants presented severe craniofacial deformations, while the remaining 30% of esr1 mutants also had changes in behavior. esr2b mutants had significantly increased heart rate and significant impacts on craniofacial morphometrics. Finally, mutation of gper1 affected behavior, decreased standard length, and decreased bone mineralization as assessed in the opercle. Although the exact molecular mechanisms underlying these effects will require further investigations in the future, we added a new concept and new tools to explore and better understand the actions of the large group of endocrine disrupting chemicals found in our environment.

Effects of primary leachates of conventional and alternative plastics in Cyprinodon variegatus fish larvae: Endocrine disruption and toxicological responses

The inclusion of hazardous substances in the formulation of plastics raises significant concerns, particularly, if those substances are released as primary leachates during plastic degradation and/or fragmentation. In this sense, the production of degradable plastics holding deleterious additives can increase the release of harmful substances into the environment. Additionally, the effects of primary leachates of "eco-friendly" materials remain unexplored. To address this, we performed exposures to primary leachates of alternative polymers, and commercial bags to verify possible responses associated with endocrine disruption and/or activation of the detoxification pathway in larvae of the marine fish model Cyprinodon variegatus. The chemical characterization evidenced a great number of additives in the formulation of the materials analyzed in this study. Those include, except for the PLA sample, relevant levels of the hazardous phthalates DEHP and DiBP. Regarding the effects on marine fish larvae, exposure to leachates from alternative polymers (10 g/L) PHB and PHBV produced remarkable mortality (100%). While the exposure to bag leachates of all tested materials (1 and 10 g/L) produced alterations in biomarkers for steroidogenic and detoxification pathways. To a lesser extent (10 g/L), three materials produced significant alterations in estrogenic biomarkers (Home-compostable bag 1, LDPE and Recycled PE bags). Although the alterations in gene expression were not directly correlated to the amount of DEHP or DiBP, we can conclude that primary leachates of "eco-friendly" bags are harmful to marine vertebrates.

Endocrine disrupting effects of parabens in zebrafish (Danio rerio): New insights from transcriptomics, metabolomics, and molecular dynamics simulation

Parabens (PBs), a group of widely used synthetic preservatives with potential endocrine disrupting activity, have been detected with increasing frequency in organisms and environmental matrices. This study assessed the hormone interference effects of four typical PBs, namely methylparaben (MeP), ethylparaben (EtP), propylparaben (PrP), and butylparaben (BuP), in zebrafish and elucidated the probable underlying mechanisms. Transcriptomic and metabolomic analyses showed that the differentially expressed genes and metabolites were associated with the tyrosine metabolism, arachidonate metabolism, and glycerophospholipid metabolism, indicating they were essential precursors of steroid hormone biosynthesis and metabolism. Histopathological analysis revealed impaired gonad development in the zebrafish exposed to PBs, as evidenced by the significantly increased vitellogenin (VTG) and estradiol (E2) levels. Furthermore, molecular dynamics simulation suggested that the four PBs could preferentially activate the zebrafish estrogen receptor, zfERβ2, to regulate the downstream pathways. Disruption of the amino acid metabolism and lipid metabolism, and activation of zfERβ2 signaling pathway were found to be the key mechanisms for the endocrine disrupting effects of PBs. The hormone interference effects of PBs were apparently dependent on the shared oxybenzene on their structures, with the degree of interference determined largely by the length of their alkyl chains. These findings provide new insights into the endocrine disrupting effects of PBs and could help better assess their risk to human health.

Are New Phthalate Ester Substitutes Safer than Traditional DBP and DiBP? Comparative Endocrine-Disrupting Analyses on Zebrafish Using In Vivo, Transcriptome, and In Silico Approaches

Although previous studies have confirmed the association between phthalate esters (PAEs) exposure and endocrine disorders in humans, few studies to date have systematically assessed the threats of new PAE alternatives to endocrine disruptions. Herein, zebrafish embryos were continuously exposed to two PAEs [di-n-butyl phthalate (DBP) and diisobutyl phthalate (DiBP)], two structurally related alternatives [diiononyl phthalate (DINP) and diisononyl hexahydrophthalate (DINCH)], and two non-PAE substitutes [dipropylene glycol dibenzoate (DGD) and glyceryl triacetate (GTA)], and the endocrine-disrupting effects were investigated during the early stages (8-48 hpf). For five endogenous hormones, including progesterone, testosterone, 17β-estradiol, triiodothyronine (T3), and cortisol, the tested chemicals disturbed the contents of at least one hormone at environmentally relevant concentrations (≤3.9 μM), except DINCH and GTA. Then, the concentration-dependent reduced zebrafish transcriptome analysis was performed. Thyroid hormone (TH)- and androgen/estrogen-regulated adverse outcome pathways (AOPs) were the two types of biological pathways most sensitive to PAE exposure. Notably, six compounds disrupted four TH-mediated AOPs, from the inhibition of deiodinases (molecular initiating event, MIE), a decrease in T3 levels (key event, KE), to mortality (adverse outcome, AO) with the quantitatively linear relationships between MIE-KE (|r| = 0.96, p = 0.002), KE-AO (|r| = 0.88, p = 0.02), and MIE-AO (|r| = 0.89, p = 0.02). Multiple structural analyses showed that benzoic acid is the critical toxicogenic fragment. Our data will facilitate the screening and development of green alternatives.

Comprehensive identification of gene expression fingerprints and biomarkers of sexual endocrine disruption in zebrafish embryo

Endocrine disruptors (EDs), capable of modulating the sex hormone system of an organism, can exert long-lasting negative effects on reproduction in both humans and the environment. For these reasons, the properties of EDs prevent a substance from being approved for marketing. However, regulatory testing to evaluate endocrine disruption is time-consuming, costly, and animal-intensive. Here, we combined sublethal zebrafish embryo assays with transcriptomics and proteomics for well-characterized endocrine disrupting reference compounds to identify predictive biomarkers for sexual endocrine disruption in this model. Using RNA and protein gene expression fingerprints from two different sublethal exposure concentrations, we identified specific signatures and impaired biological processes induced by ethinylestradiol, tamoxifen, methyltestosterone and flutamide 96 h post fertilization (hpf). Our study promotes vtg1 as well as cyp19a1b, fam20cl, lhb, lpin1, nr1d1, fbp1b, and agxtb as promising biomarker candidates for identifying and differentiating estrogen and androgen receptor agonism and antagonism. Evaluation of these biomarkers for pre-regulatory zebrafish embryo-based bioassays will help identify endocrine disrupting hazards of compounds at the molecular level. Such approaches additionally provide weight-of-evidence for the identification of putative EDs and may contribute significantly to a reduction in animal testing in higher tier studies.

Reproductive toxicity induced by benzo[a]pyrene exposure: first exploration highlighting the multi-stage molecular mechanism in female scallop Chlamys farreri

Reproductive toxicity induced by benzo[a]pyrene (B[a]P) exposure has received great ecotoxicological concerns. However, huge gaps on the molecular mechanism still exist in bivalves. In this study, reproduction-related indicators were investigated in female scallops Chlamys farreri during life cycle of proliferative, growth, mature, and spawn stages, under gradient concentrations of B[a]P at 0, 0.04, 0.4, and 4 μg/L. Meanwhile, a multi-stage ovarian transcriptome analysis under 4 μg/L B[a]P exposure was also conducted to elucidate the potential molecular mechanisms. The results indicated that life-cycle exposure to 0.4 and 4 μg/L B[a]P significantly decreased GSI and sex steroid levels. Even 0.04 μg/L B[a]P could play the adverse role in DNA integrity at the mature and spawn stages. Ovarian histological sections showed that B[a]P inhibited the maturation and release of oocytes. Through the functional enrichment analysis of differentially expressed genes (DEGs) from transcriptome data, 18 genes involved in endocrine disruption effects, DNA damage and repair, and oogenesis were selected and further determined by qRT-PCR. The downregulation of genes involved in steroidogenic and estrogen signaling pathways indicated that B[a]P could cause endocrine disruption through both receptor-dependent and receptor-independent pathways. The variations of gene expressions involved in DNA single-strand break and repair implied the presence of toxic mechanisms similar with vertebrates. Additionally, the changes of gene expressions of cell cycle, apoptosis, and cell adhesion suggested that exposure to B[a]P possibly caused the reproductive toxicity effects by affecting oogenesis. Taken together, this study was a pioneer in combining genome-wide transcriptomic analysis with its corresponding reproductive indicators (GSI, sex steroid levels, DNA single-strand break, and histological sections) to explore the bivalves' toxic mechanisms under B[a]P exposure. Meanwhile, some genes involved in estrogen signaling pathway and DNA damage were firstly analyzed in bivalves, and the expression data might be useful in establishing new hypotheses and discovering new biomarkers for marine biomonitoring.

Effects of 17α-Ethinylestradiol (EE2) exposure during early life development on the gonadotropic axis ontogenesis of the European sea bass, Dicentrarchus labrax

Exposure of young organisms to oestrogenic endocrine disrupting chemicals (EDCs) can elicit adverse effects, particularly on the reproductive function. In fish, as in other vertebrates, reproduction is controlled by the neuroendocrine gonadotropic axis, whose components are mainly regulated by sex steroids and may then be targets for EDCs. In the present study, we investigated the effects of a xenoestrogen exposure on the ontogenesis of the gonadotropic axis in European sea bass. After exposure of hatching larvae for 8 days to 17α-ethinylestradiol (EE2) (0.5 nM and 50 nM), gene expression for kisspeptins (kiss1, kiss2), gonadotropin-releasing hormones (gnrh1, gnrh2, gnrh3), gonadotropin beta subunits (lhβ and fshβ) and brain type aromatase (cyp19a1b) were measured using quantitative real-time PCR. Our results demonstrate that EE2 strongly stimulated the expression of brain type aromatase (cyp19a1b) in sea bass larvae. In addition, EE2 exposure also affected the mRNA levels of kiss1, gnrh1 and gnrh3 by inducing a downregulation of these genes during the early developmental stages, while no effect was seen in gnrh2, lhβ and fshβ. These results reinforce the idea that the larval development is a sensitive critical period in regard to endocrine disruption and that the gonadotropic axis in the developing sea bass is sensitive to xenoestrogen exposure.

The fish early-life stage sublethal toxicity syndrome - A high-dose baseline toxicity response

A large number of toxicity studies report abnormalities in early life-stage (ELS) fish that are described here as a sublethal toxicity syndrome (TxSn_FELS) and generally include a reduced heart rate, edemas (yolk sac and cardiac), and a variety of morphological abnormalities. The TxSn_FELS is very common and not diagnostic for any chemical or class of chemicals. This sublethal toxicity syndrome is mostly observed at high exposure concentrations and appears to be a baseline, non-specific toxicity response; however, it can also occur at low doses by specific action. Toxicity metrics for this syndrome generally occur at concentrations just below those causing mortality and have been reported for a large number of diverse chemicals. Predictions based on tissue concentrations or quantitative-structure activity relationship (QSAR) models support the designation of baseline toxicity for many of the tested chemicals, which is confirmed by observed values. Given the sheer number of disparate chemicals causing the TxSn_FELS and correlation with QSAR derived partitioning; the only logical conclusion for these high-dose responses is baseline toxicity by nonspecific action and not a lock and key type receptor response. It is important to recognize that many chemicals can act both as baseline toxicants and specific acting toxicants likely via receptor interaction and it is not possible to predict those threshold doses from baseline toxicity. We should search out these specific low-dose responses for ecological risk assessment and not rely on high-concentration toxicity responses to guide environmental protection. The goal for toxicity assessment should not be to characterize toxic responses at baseline toxicity concentrations, but to evaluate chemicals for their most toxic potential. Additional aspects of this review evaluated the fish ELS teratogenic responses in relation to mammalian oral LD50s and explored potential key events responsible for baseline toxicity.

Parental exposure to the synthetic estrogen 17α-ethinylestradiol (EE2) affects offspring development in the Sydney rock oyster, Saccostrea glomerata

Very little is currently known regarding the effects of estrogenic endocrine disrupting chemicals on embryonic and larval development in molluscs, nor the potential effects of parental (F₀) exposure on resultant F₁ offspring. In this study, we assessed the embryotoxic impacts of exposure to environmentally relevant concentrations of the synthetic estrogen, 17α-ethinylestradiol (EE2), to male and female parents (50 ng/L) and their offspring (5 and 50 ng/L) in the native Australian Sydney rock oyster, Saccostrea glomerata. There were no detectable effects of parental exposure on fertilisation success, proportions of early larval (F₁) morphs and unfertilised eggs. Offspring impacts were evidenced in terms of developmental delays, with decreased percentages of D-veligers retained by 45 μm mesh, along with a reduction of swimming capabilities of larvae at 2 days post-fertilisation (dpf) when both parents had been exposed to 50 ng/L EE2. Although no significant parental effects were found on the survival of F₁ larvae at 9 dpf, retardation of shell growth was observed on F₁ larvae in treatments where both parents had been exposed to 50 ng/L EE2. Subsequent larval exposure from 2 to 9 dpf caused declines in survival and reduction of shell length in F₁ larvae at both 5 and 50 ng/L EE2 across all parental exposure treatments. Collectively, parental EE2 imparts effects on offspring in terms of retardation of larval development, and subsequent offspring exposure to EE2 further exacerbates impacts to development. Future research should aim to understand the potential mechanisms of EE2 induced toxicity and its transmission resulting in altered phenotypes of the F₁ generation.

Tebuconazole induces ROS-dependent cardiac cell toxicity by activating DNA damage and mitochondrial apoptotic pathway

Tebuconazole (TEB) is a common triazole fungicide that is widely used throughout the world in agriculture applications. We previously reported that TEB induces cardiac toxicity in rats. The aim of this study was to investigate the underlying mechanism of the toxicity induced by TEB in cardiac cells. TEB induced dose-dependent cell death in H9c2 cardiomyoblasts and in adult rat ventricular myocytes (ARVM). The comet assay and western blot analysis showed a concentration-dependent increase in DNA damage and in p53 and p21 protein levels 24 h after TEB treatment. Our findings also showed that TEB triggered the mitochondrial pathway of apoptosis as evidenced by a loss of mitochondrial transmembrane potential (ΔΨm), an increase in Bax/Bcl-2 ratio, an activation of caspase-9 and caspase-3, a cleavage of poly (ADP-ribose) polymerase (PARP) and an increase in the proportion of cells in the sub-G1 phase. In addition, TEB promoted ROS production in cardiac cells and consequently increased the amounts of MDA, the end product of lipid peroxidation. Treatment of cardiomyocytes with the ROS scavenger N-acetylcysteine reduced TEB-induced DNA damage and activation of the mitochondrial pathway of apoptosis. These results indicate that the genotoxic and cytotoxic effects of TEB are mediated through a ROS-dependent pathway in cardiac cells.

Evidence-based assessment on environmental mixture using a concentration-dependent transcriptomics approach

Development of new approach methodologies is urgently needed to characterize the likelihood that complex mixtures of chemicals affect water quality. Omics advances in ecotoxicology allow assessment on a broadest coverage of disrupted biological pathway by mixtures. Here the usefulness of transcriptomic analyses for evaluation of combined effects and identification of main effect components are explored. Two artificial mixtures (Mix 1 and Mix 2) were tested by a concentration-dependent reduced zebrafish transcriptome (CRZT) approach and toxicity bioassays using zebrafish embryos. Then, the toxicities and transcriptomic effects of 12 component chemicals on embryos were incorporated into additivity models to characterize the combined effects of chemicals in mixtures and to identify the main bioactive compounds. Mix 1 and Mix 2 displayed similar embryo toxicities (LD₅₀: 6.6 μM and 8.7 μM, respectively), however, Mix 2 elicited broader biological process perturbations and 5-fold higher transcriptome potency (point of departure eliciting a 20% pathway response, POD_path20) than Mix 1. The predicted mixture toxicities derived from additivity expectations deviated by 2-fold or less from the measured embryo toxicities except for the Jaw defect endpoint; most biological processes deviated by 3-fold or less. Finally, diclofenac (DFC) and propiconazole (PCZ) were identified as the main contributing components (≥80% explanation) to the embryo toxicity and biological process perturbations by Mix 1. While DFC and chlorophene (CLP) explained up to 80% of the embryo toxicities and biological effects of Mix 2 associated with development and Metabolism processes. The CRZT approach provides a powerful tool for assessment of biological pathways perturbed by chemicals in mixtures and for identification of main bioactive compounds.

Sub-lethal toxicity assessment of the phenylurea herbicide linuron in developing zebrafish (Danio rerio) embryo/larvae

Due to run-off and rain events, agrochemicals can enter water catchments, exerting endocrine disruption effects and toxicity to aquatic organisms. Linuron is a phenylurea herbicide used to control a wide variety of vegetative weeds in agriculture in addition to residential applications. However, there are few studies that quantify its toxicity to early developmental stages of fish. The objectives of this study were to assess the acute toxicity of linuron to zebrafish embryos/larvae by measuring mortality, morphological deformities, oxidative respiration, gene expression, and locomotor activity via the Visual Motor Response test. Zebrafish embryos at ~6-h post-fertilization (hpf) were exposed to either embryo rearing medium (ERM), or one dose of 0.625, 1.25, 2.5, 5, and 10 μM linuron for up to 7 days post-fertilization (dpf) depending on the assay. Zebrafish larvae exposed to linuron displayed pericardial edema, yolk sac edema, and spinal curvature. Oxidative respiration assessments in embryos using the Agilent XF^e24 Flux Analyzer revealed that linuron decreased mean basal respiration and oligomycin-induced ATP-linked respiration in 30 hpf embryos at 20 μM after a 24-hour exposure. In 7 dpf larvae, transcript abundance was determined for 6 transcripts that have a role in oxidative respiration (atp06, cox1, cox4-1, cox5a1, cytb, and nd1); the relative abundance of these transcripts was not altered with linuron treatment. A Visual Motor Response test was conducted on 7 dpf larvae to determine whether linuron (0.625 to 5 μM) impaired locomotor activity. Larval activity in the dark period decreased in a dose dependent manner and there were indications of hypoactivity as low as 1.25 μM. Transcript abundance was thus determined for tyrosine hydroxylase (th1) and glutamic acid decarboxylase 67 (gad1b), two rate limiting enzymes that control the production of dopamine and gamma-aminobutyric acid respectively. The mRNA levels of gad1b (p = 0.019) were reduced with increasing concentrations of linuron while th1 (p = 0.056) showed a similar decreasing trend, suggesting that neurotransmitter biosynthesis may be altered with exposure to linuron. This study improves knowledge related to the toxicity mechanisms for linuron and is the first to demonstrate that this anti-androgenic chemical impairs oxidative respiration and exerts neurotoxic effects associated with neurotransmitter biosynthesis during early development. These data are significant for environmental risk assessment of agrochemicals.

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.

Concentration-dependent transcriptome of zebrafish embryo for environmental chemical assessment

Mechanistic information is essential to screen and predict the adverse effects of a large number of chemicals during early-life exposure. Concentration-dependent omics can capture the extent of perturbations of biological pathways or processes and provide information on the mechanism of toxicity. However, the application of concentration-dependent transcriptome to assess the developmental toxicity of environmental chemicals is still limited. Here, twelve chemicals representing five different modes of action (MOAs) were tested by the concentration-dependent reduced zebrafish transcriptome approach (CRZT) in combination with a phenotype-based high content screen (PHCS). The responsiveness, sensitivity and mechanistic differentiation of CRZT were validated in comparison with PHCS. First, PHCS identified 10 chemicals with obvious embryotoxicity (LD₅₀ range: 2.11-70.68 μM), while the potencies of the biological pathways perturbed by 12 chemicals (POD_path20 range: 0.002-2.1 μM) were demonstrated by CRZT. Second, although the potency of the transcriptome perturbations was positively correlated with lethality (LD₅₀) (R² = 0.64, P-value < 0.05) for most tested chemicals, BbF was non-embryotoxic but was the most potent on the perturbance of biological pathways. Finally, the profiles of the perturbed biological processes and the transcriptome potency (POD_path20) captured by CRZT could effectively classify most chemicals corresponding to their known MOAs. In summary, CRZT could significantly improve testing the developmental toxicity of environmental chemicals.

Twenty years of transcriptomics, 17alpha-ethinylestradiol, and fish

In aquatic toxicology, perhaps no pharmaceutical has been investigated more intensely than 17alpha-ethinylestradiol (EE2), the active ingredient of the birth control pill. At the turn of the century, the fields of comparative endocrinology and endocrine disruption research witnessed the emergence of omics technologies, which were rapidly adapted to characterize potential hazards associated with exposures to environmental estrogens, such as EE2. Since then, significant advances have been made by the scientific community, and as a result, much has been learned about estrogen receptor signaling in fish from environmental xenoestrogens. Vitellogenin, the egg yolk precursor protein, was identified as a major estrogen-responsive gene, establishing itself as the premier biomarker for estrogenic exposures. Omics studies have identified a plethora of estrogen responsive genes, contributing to a wealth of knowledge on estrogen-mediated regulatory networks in teleosts. There have been ~40 studies that report on transcriptome responses to EE2 in a variety of fish species (e.g., zebrafish, fathead minnows, rainbow trout, pipefish, mummichog, stickleback, cod, and others). Data on the liver and testis transcriptomes dominate in the literature and have been the subject of many EE2 studies, yet there remain knowledge gaps for other tissues, such as the spleen, kidney, and pituitary. Inter-laboratory genomics studies have revealed transcriptional networks altered by EE2 treatment in the liver; networks related to amino acid activation and protein folding are increased by EE2 while those related to xenobiotic metabolism, immune system, circulation, and triglyceride storage are suppressed. EE2-responsive networks in other tissues are not as comprehensively defined which is a knowledge gap as regulated networks are expected to be tissue-specific. On the horizon, omics studies for estrogen-mediated effects in fish include: (1) Establishing conceptual frameworks for incorporating estrogen-responsive networks into environmental monitoring programs; (2) Leveraging in vitro and computational toxicology approaches to identify chemicals associated with estrogen receptor-mediated effects in fish (e.g., male vitellogenin production); (3) Discovering new tissue-specific estrogen receptor signaling pathways in fish; and (4) Developing quantitative adverse outcome pathway predictive models for estrogen signaling. As we look ahead, research into EE2 over the past several decades can serve as a template for the array of hormones and endocrine active substances yet to be fully characterized or discovered.

Use of Zebrafish in Drug Discovery Toxicology

Unpredicted human safety events in clinical trials for new drugs are costly in terms of human health and money. The drug discovery industry attempts to minimize those events with diligent preclinical safety testing. Current standard practices are good at preventing toxic compounds from being tested in the clinic; however, false negative preclinical toxicity results are still a reality. Continual improvement must be pursued in the preclinical realm. Higher-quality therapies can be brought forward with more information about potential toxicities and associated mechanisms. The zebrafish model is a bridge between in vitro assays and mammalian in vivo studies. This model is powerful in its breadth of application and tractability for research. In the past two decades, our understanding of disease biology and drug toxicity has grown significantly owing to thousands of studies on this tiny vertebrate. This Review summarizes challenges and strengths of the model, discusses the 3Rs value that it can deliver, highlights translatable and untranslatable biology, and brings together reports from recent studies with zebrafish focusing on new drug discovery toxicology.

Bis(2-ethylhexyl) phthalate induces DNA strand breaks and gene expression alterations in larval zebrafish Danio rerio

Contamination of the aquatic environment by plastic industrial products and their by-products is remarkable. Because of their physical, chemical, and biological degradation resistance, plasticizers can enter the food chain of living organisms, accumulate in the body and generate toxic effects. Here we determined the potential toxic effects of bis(2-ethylhexyl) phthalate (DEHP) plasticizer to larval (72 h post fertilization) zebrafish ( Danio rerio) by analyzing changes in expression levels of stress-related genes ( p53, rad51, and xrcc5) by the quantitative real-time polymerase chain reaction. Also, possible DNA damage by DEHP in larvae was determined. The concentration of DEHP (0–160 mg/l) that killed 50% of the larval zebrafish within 96 h was 54.02 mg/l. There was a concentration-related increase in DNA damage in cells from larvae exposed (96 h) to DEHP. DNA damage of 31.13% (mean ± standard error of the mean) was observed in larvae at the highest sublethal DEHP concentration (10 mg/l). Some significant differences in the induction of stress-related genes were also observed in larvae exposed to DEHP relative to control ( p < 0.05). The conclusion drawn from this ecotoxicological risk assessment is that, under present use and exposure patterns, DEHP presents a small hazard to zebrafish larvae.

Effects of 17α‑ethinylestradiol on caudal fin regeneration in zebrafish larvae

The ability to restore tissue function and morphology after injury is a key advantage of many fish for a greater chance of survival. The tissue regeneration process is regulated by multiple pathways, and it can therefore be hypothesized that environmental contaminants targeting components of these signaling pathways, may disrupt the fish's capability to repair or regenerate. This could lead to higher mortality and eventually even to a decline in populations. In this study, the effects of 17α‑ethinylestradiol (EE2), a synthetic estrogen, were assessed on the regenerative capacity of larval zebrafish. Zebrafish aged 2 hour post fertilization (hpf) were exposed to 1, 10, or 100 ng/L EE2, and the caudal fins were amputated at 72 hpf. It was found that EE2 exposure significantly inhibited fin regeneration and changed locomotor behavior. The transcription levels for most of the genes involved in the signaling networks regulating the fin regeneration, such as axin2, fgfr1, bmp2b and igf2b, were down-regulated in the amputated fish in response to EE2 exposure, which was in contrast to their increased patterns in the vehicle-exposed control fish. Additionally, the mRNA levels of several immune-related genes, such as il-1β, il-6, il-10 and nf-κb2, were significantly decreased after EE2 exposure, accompanied by a lower density of neutrophils migrated into the wound site. In conclusion, the present study indicated for the first time that estrogenic endocrine disrupting chemicals (EEDCs) could inhibit the regenerative capacity of zebrafish, and this effect was speculated to be mediated through the alteration in regeneration-related signaling pathways and immune competence. This work expands our knowledge of the potential effects of EEDCs on injured aquatic organisms, and highlights the ecotoxicological significance of relationships between regenerative process and endocrine system. This study also implies the potential application of fin regeneration assay for assessing immunotoxicity in ecotoxicological risk assessment.

Toxicity and non-harmful effects of the soya isoflavones, genistein and daidzein, in embryos of the zebrafish, Danio rerio

Based on the assumed oestrogenic and apoptotic properties of soya isoflavones (genistein, daidzein), and following the current OECD test-guidelines and principle of 3Rs, we have studied the potential toxicity of phytochemicals on the zebrafish embryos test (ZFET). For this purpose, zebrafish embryos at 2-3 h post-fertilisation (hpf) were exposed to both soya isoflavones (from 1.25 mg/L to 20 mg/L) and assayed until 96 hpf. Lethal and sub-lethal endpoints (mortality, hatching rates and malformations) were estimated in the ZFET, which was expanded to potential gene expression markers, determining the lowest observed effect (and transcriptional) concentrations (LOEC, LOTEC), and the no-observable effect (and transcriptional) concentrations (NOEC, NOTEC). The results revealed that genistein is more toxic (LC50-96 hpf: 4.41 mg/L) than daidzein (over 65.15 mg/L). Both isoflavones up-regulated the oestrogen (esrrb) and death receptors (fas) and cyp1a transcript levels. Most thyroid transcript signals were up-regulated by genistein (except for thyroid peroxidase/tpo), and the hatching enzyme (he1a1) was exclusively up-regulated by daidzein (from 1.25 mg/L onwards). The ZFET proved suitable for assessing toxicant effects of both isoflavones and potential disruptions (i.e. oestrogenic, apoptotic, thyroid, enzymatic) during the embryogenesis and the endotrophic larval period.

Reduced Zebrafish Transcriptome Atlas toward Understanding Environmental Neurotoxicants

Transcriptomic approaches monitoring gene responses at genome-scale are increasingly used in toxicological research and help to clarify the molecular mechanisms of adverse effects caused by environmental toxicants. However, their applications for chemical assessment are hampered due to high expenses required and more importantly the lack of in-depth data mining and mechanistic perspectives. Here, we described a reduced transcriptome atlas (RTA) approach which integrates transcriptomic data sets and a comprehensive panel of genes generated to represent neurogenesis and the early neuronal development of zebrafish, to determine the potential neurodevelopmental toxicities of environmental chemicals. Transcriptomic data sets of 74 chemicals and 736 related gene expression profiles were integrated resulting in 135 exposure signatures. Chemical prioritization demonstrated four sets of hits to be neurotoxic: neuro-active chemicals (representatively, Valproic acid, VPA and Carbamazepine, CAR), xenoestrogens (Bisphenol A, BPA; Genistein, GEN; 17-α ethinylestradiol, EE2), microcystins (cyanopeptolin, CP1020; microcystin-LR, MCLR) and heavy metals (AgNO₃, AgNPs). The enriched biological pathways and processes were distinct among the four sets, while the overlapping functional enrichments were observed within each set, for example, over 25% differentially expressed genes and four of top five KEGG pathways were shared between VPA and CAR. Furthermore, gene expression index (GEI) analysis demonstrated that a gene panel with 300 genes was sufficient to effectively characterize and cluster chemicals and therefore offer an efficient and cost-effective tool for the prioritization of neurotoxicants. Thus, the RTA approach provides novel insights into the understanding of the in-depth molecular mechanisms of environmental neurotoxicants and can be used as an indication for potential adverse outcomes.

Transcriptomic and phenotypic profiling in developing zebrafish exposed to thyroid hormone receptor agonists

There continues to be a need to develop in vivo high-throughput screening (HTS) and computational methods to screen chemicals for interaction with the estrogen, androgen, and thyroid pathways and as complements to in vitro HTS assays. This study explored the utility of an embryonic zebrafish HTS approach to identify and classify endocrine bioactivity using phenotypically-anchored transcriptome profiling. Transcriptome analysis was conducted on zebrafish embryos exposed to 25 estrogen-, androgen-, or thyroid-active chemicals at concentrations that elicited adverse malformations or mortality at 120 h post-fertilization in 80% of animals exposed. Analysis of the top 1000 significant differentially expressed transcripts and developmental toxicity profiles across all treatments identified a unique transcriptional and phenotypic signature for thyroid hormone receptor agonists. This unique signature has the potential to be used as a tiered in vivo HTS and may aid in identifying chemicals that interact with the thyroid hormone receptor.

The Transcriptome of the Zebrafish Embryo After Chemical Exposure: A Meta-Analysis

Numerous studies have been published in the past years investigating the transcriptome of the zebrafish embryo (ZFE) upon being subjected to chemical stress. Aiming at a more mechanistic understanding of the results of such studies, knowledge about commonalities of transcript regulation in response to chemical stress is needed. Thus, our goal in this study was to identify and interpret genes and gene sets constituting a general response to chemical exposure. Therefore, we aggregated and reanalyzed published toxicogenomics data obtained with the ZFE. We found that overlap of differentially transcribed genes in response to chemical stress across independent studies is generally low and the most commonly differentially transcribed genes appear in less than 50% of all treatments across studies. However, effect size analysis revealed several genes showing a common trend of differential expression, among which genes related to calcium homeostasis emerged as key, especially in exposure settings up to 24 h post-fertilization. Additionally, we found that these and other downregulated genes are often linked to anatomical regions developing during the respective exposure period. Genes showing a trend of increased expression were, among others, linked to signaling pathways (e.g., Wnt, Fgf) as well as lysosomal structures and apoptosis. The findings of this study help to increase the understanding of chemical stress responses in the developing zebrafish embryo and provide a starting point to improve experimental designs for this model system. In future, improved time- and concentration-resolved experiments should offer better understanding of stress response patterns and access to mechanistic information.

Effects of the isoflavone genistein in early life stages of the Senegalese sole, Solea senegalensis: role of the Survivin and proliferation versus apoptosis pathways

BACKGROUND

Phytochemical flavonoids are widely distributed in the environment and are derived from many anthropogenic activities. The isoflavone genistein is a naturally occurring compound found in soya products that are habitual constituents of the aquafeeds. This isoflavone possesses oestrogenic biological activity and also apoptotic properties. The present study has been performed to determine the effects of the genistein in the early life stages of the flatfish Senegalese sole during the first month of larval life, and it is focused especially at the metamorphosis, analysing the expression transcript levels and the immunohistochemical protein patterns implicated in the cell proliferation and apoptosis pathways (proliferation cellular/PCNA, anti-apoptosis Survivin/BIRC-5, death receptors/Fas, and Caspases).

RESULTS

The isoflavone genistein induced some temporal disrupting effects in several pro-apoptotic signalling pathways (Fas, CASP-6) at both genistein doses (3 mg/L and 10 mg/L), with increased Fas transcripts and also decreasing CASP-6 mRNA expression levels during metamorphic and post-metamorphic stages of the Senegalese sole. On the other hand, the anti-apoptotic BIRC-5 expression levels were weakly down-regulated with both the highest and lowest doses, but all of these imbalances were stabilised to the baseline levels. In early life stages of the controls, the constitutive basal transcript levels were temporarily and differentially expressed, reaching the highest levels at the pre-metamorphosis phase, as especially in endotrophic larvae (i.e. BIRC-5 mRNA), as well as in the metamorphic (i.e. CASP-6 mRNA) and post-metamorphic stages (i.e. Fas mRNA). In general, through development, continuous and progressive increases in the protein patterns of cell proliferation-PCNA (e.g. mitotic nuclei), anti-apoptotic Survivin (e.g. haematopoietic system, brain, digestive system, gills) and CASP-2 and -6 (e.g. brain, gills, kidney, digestive system, vascular systems, among others) have been immunohistochemically detected. Besides, both the controls and genistein exposed larvae displayed parallel immunostaining protein patterns in the different organ-systems and tissues.

CONCLUSIONS

The transcriptional imbalances observed in the studied genes (BIRC-5, CASP-6, Fas) were only temporarily induced, and apparently no changes in the immunohistochemical protein patterns were detected. Thus, the isoflavone genistein caused not harmful effects in the development and metamorphosis of the Senegalese sole exposed to chronic environmentally relevant concentrations (3 and 10 mg/L).

A Reduced Transcriptome Approach to Assess Environmental Toxicants Using Zebrafish Embryo Test

Omics approaches can monitor responses and alterations of biological pathways at genome-scale, which are useful to predict potential adverse effects by environmental toxicants. However, high throughput application of transcriptomics in chemical assessment is limited due to the high cost and lack of "standardized" toxicogenomic methods. Here, a reduced zebrafish transcriptome (RZT) approach was developed to represent the whole transcriptome and to profile bioactivity of chemical and environmental mixtures in zebrafish embryo. RZT gene set of 1637 zebrafish Entrez genes was designed to cover a wide range of biological processes, and to faithfully capture gene-level and pathway-level changes by toxicants compared with the whole transcriptome. Concentration-response modeling was used to calculate the effect concentrations (ECs) of DEGs and corresponding molecular pathways. To validate the RZT approach, quantitative analysis of gene expression by RNA-ampliseq technology was used to identify differentially expressed genes (DEGs) at 32 hpf following exposure to seven serial dilutions of reference chemical BPA (10-10E^-5μM) or each of four water samples ranging from wastewater to drinking water (relative enrichment factors 10-6.4 × 10^-4). The RZT-ampliseq-embryo approach was both sensitive and able to identify a wide spectrum of biological activities associated with BPA exposure. Water quality was benchmarked based on the sensitivity distribution curve of biological pathways detected using RZT-ampliseq-embryo. Finally, the most sensitive biological pathways were identified, including those linked with adverse reproductive outcomes, genotoxicity and development outcomes. RZT-ampliseq-embryo approach provides an efficient and cost-effective tool to prioritize toxicants based on responsiveness of biological pathways.

The Plasticizer Bisphenol A Perturbs the Hepatic Epigenome: A Systems Level Analysis of the miRNome

Ubiquitous exposure to bisphenol A (BPA), an endocrine disruptor (ED), has raised concerns for both human and ecosystem health. Epigenetic factors, including microRNAs (miRNAs), are key regulators of gene expression during cancer. The effect of BPA exposure on the zebrafish epigenome remains poorly characterized. Zebrafish represents an excellent model to study cancer as the organism develops a disease that resembles human cancer. Using zebrafish as a systems toxicology model, we hypothesized that chronic BPA-exposure impacts the miRNome in adult zebrafish and establishes an epigenome more susceptible to cancer development. After a 3 week exposure to 100 nM BPA, RNA from the liver was extracted to perform high throughput mRNA and miRNA sequencing. Differential expression (DE) analyses comparing BPA-exposed to control specimens were performed using established bioinformatics pipelines. In the BPA-exposed liver, 6188 mRNAs and 15 miRNAs were differently expressed (q ≤ 0.1). By analyzing human orthologs of the DE zebrafish genes, signatures associated with non-alcoholic fatty liver disease (NAFLD), oxidative phosphorylation, mitochondrial dysfunction and cell cycle were uncovered. Chronic exposure to BPA has a significant impact on the liver miRNome and transcriptome in adult zebrafish with the potential to cause adverse health outcomes including cancer.

Effects of the soya isoflavone genistein in early life stages of the Senegalese sole, Solea senegalensis: Thyroid, estrogenic and metabolic biomarkers

This study examines the effects induced by environmentally relevant concentrations of the isoflavone genistein (3mg/L and 10mg/L) during early life stages of the Senegalese sole. Throughout the hypothalamus-pituitary-thyroid (HPT) axis, several neurohormonal regulatory thyroid signalling patterns (thyroglobulin/Tg, thyroid peroxidase/TPO, transthyretin/TTR, thyroid receptors/TRβ, and iodothrynonine deiodinases, Dio2 and Dio3) were analysed. Furthermore, the expression patterns of estrogen receptor ERβ and haemoprotein Cyp1a were also evaluated. In the control larvae, progressive increases of constitutive hormonal signalling pathways have been evidenced from the pre-metamorphosis phase onwards, reaching the highest expression basal levels at the metamorphosis (Tg, TPO, Dio2) and/or during post-metamorphosis (TTR, TRβ, ERβ). When the early larvae were exposed to both genistein concentrations (3mg/L and 10mg/L), a statistically significant down-regulation of TPO, TTR and Tg mRNA levels was clearly detected at the metamorphic stages. In addition, the Dio2 and Dio3 transcript expression levels were also down and up-regulated when exposed to both genistein concentrations. In the larvae exposed to genistein, no statistically significant responses were recorded for the TRβ expression patterns. Nevertheless, the ERβ and Cyp1a transcript levels were up-regulated at the middle metamorphic stage (S2, at 16 dph) in the larvae exposed to high genistein concentrations and, only the ERβ was down-regulated (S1, at 12dph) at the lower doses. Finally, all these pointed out imbalances were only temporarily disrupted by exposure to genistein, since most of the modulated transcriptional signals (i.e. up or down-regulation) were quickly restored to the baseline levels. Additionally, the control and genistein-exposed Senegalese sole specimens showed characteristic ontogenetic patterns and completely suitable for an optimal development, metamorphosis, and growth.

Endocrine and physiological effects of linuron and S-metolachlor in zebrafish developing embryos

Evaluation of the effects of linuron and S-metolachlor on apical, biochemical and transcriptional endpoints in zebrafish (Danio rerio) early life stages was the main purpose of this work. Embryos were exposed for 96h to a range of concentrations of each herbicide to determine lethal and sub-lethal effects on apical (e.g. malformations, hatching) and biochemical parameters (cholinesterase, ChE; catalase, CAT; glutathione S-transferase, GST; lipid peroxidation, LPO and lactate dehydrogenase, LDH). To evaluate endocrine disruption effects, embryos were exposed during 96h to 0.88mg/L linuron and 9.66mg/L S-metolachlor, isolated or in binary mixture. Expression of a suite of genes involved in HPT, HPG and HPA-axis was then assessed. Highest concentration of linuron (5.0mg/L) decreased hatching rate to 5% and 70.0mg/L S-metolachlor completely inhibited hatching, about 100%. Both herbicides impaired development by inducing several malformations (100% in 5.0mg/L linuron and 70.0mg/L S-metolachlor). Linuron only affected GST and CAT at concentrations of 0.25 and 0.0025mg/L, respectively. S-metolachlor induced GST (to 256%), inhibited ChE (to 61%) and LDH (to 60%) and reduced LPO levels (to 63%). Linuron isolated treatment seems to have an estrogenic mode of action due to the observed induction of vtg1. Exposure to S-metolachlor seems to interfere with steroidogenesis and with HPT and HPA-axis, since it has inhibited cyp19a2, TSHβ and CRH gene expression. In addition to vtg1 induction and CRH inhibition, herbicide combination also induced sox9b that has a role in regulation of sexual development in zebrafish. This study pointed out adverse effects of linuron and S-metolachlor, namely impairment of neurotransmission and energy production, induction of steroidogenesis, and interference with HPT and HPA-axis. These results contributed to elucidate modes of action of linuron and S-metolachlor in zebrafish embryo model. Furthermore, gene expression patterns obtained are indicative of endocrine disruption action of these herbicides.

Persistent Effects of Developmental Exposure to 17α-Ethinylestradiol on the Zebrafish (Danio rerio) Brain Transcriptome and Behavior

The synthetic estrogen 17α-ethinylestradiol (EE₂) is an endocrine disrupting compound of concern due to its persistence and widespread presence in the aquatic environment. Effects of developmental exposure to low concentrations of EE₂ in fish on reproduction and behavior not only persisted to adulthood, but have also been observed to be transmitted to several generations of unexposed progeny. To investigate the possible biological mechanisms of the persistent anxiogenic phenotype, we exposed zebrafish embryos for 80 days post fertilization to 0, 3, and 10 ng/L EE₂ (measured concentrations 2.14 and 7.34 ng/L). After discontinued exposure, the animals were allowed to recover for 120 days in clean water. Adult males and females were later tested for changes in stress response and shoal cohesion, and whole-brain gene expression was analyzed with RNA sequencing. The results show increased anxiety in the novel tank and scototaxis tests, and increased shoal cohesion in fish exposed during development to EE₂. RNA sequencing revealed 34 coding genes differentially expressed in male brains and 62 in female brains as a result of EE₂ exposure. Several differences were observed between males and females in differential gene expression, with only one gene, sv2b, coding for a synaptic vesicle protein, that was affected by EE₂ in both sexes. Functional analyses showed that in female brains, EE₂ had significant effects on pathways connected to the circadian rhythm, cytoskeleton and motor proteins and synaptic proteins. A large number of non-coding sequences including 19 novel miRNAs were also differentially expressed in the female brain. The largest treatment effect in male brains was observed in pathways related to cholesterol biosynthesis and synaptic proteins. Circadian rhythm and cholesterol biosynthesis, previously implicated in anxiety behavior, might represent possible candidate pathways connecting the transcriptome changes to the alterations to behavior. Further the observed alteration in expression of genes involved in synaptogenesis and synaptic function may be important for the developmental modulations resulting in an anxiety phenotype. This study represents an initial survey of the fish brain transcriptome by RNA sequencing after long-term recovery from developmental exposure to an estrogenic compound.

ZEBRAFISH AS AN IN VIVO MODEL FOR SUSTAINABLE CHEMICAL DESIGN

Heightened public awareness about the many thousands of chemicals in use and present as persistent contaminants in the environment has increased the demand for safer chemicals and more rigorous toxicity testing. There is a growing recognition that the use of traditional test models and empirical approaches is impractical for screening for toxicity the many thousands of chemicals in the environment and the hundreds of new chemistries introduced each year. These realities coupled with the green chemistry movement have prompted efforts to implement more predictive-based approaches to evaluate chemical toxicity early in product development. While used for many years in environmental toxicology and biomedicine, zebrafish use has accelerated more recently in genetic toxicology, high throughput screening (HTS), and behavioral testing. This review describes major advances in these testing methods that have positioned the zebrafish as a highly applicable model in chemical safety evaluations and sustainable chemistry efforts. Many toxic responses have been shown to be shared among fish and mammals owing to their generally well-conserved development, cellular networks, and organ systems. These shared responses have been observed for chemicals that impair endocrine functioning, development, and reproduction, as well as those that elicit cardiotoxicity and carcinogenicity, among other diseases. HTS technologies with zebrafish enable screening large chemical libraries for bioactivity that provide opportunities for testing early in product development. A compelling attribute of the zebrafish centers on being able to characterize toxicity mechanisms across multiple levels of biological organization from the genome to receptor interactions and cellular processes leading to phenotypic changes such as developmental malformations. Finally, there is a growing recognition of the links between human and wildlife health and the need for approaches that allow for assessment of real world multi-chemical exposures. The zebrafish is poised to be an important model in bridging these two conventionally separate areas of toxicology and characterizing the biological effects of chemical mixtures that could augment its role in sustainable chemistry.

Transcriptional responses in male Japanese medaka exposed to antiandrogens and antiandrogen/androgen mixtures

The occurrence of androgenic endocrine disrupting chemicals (EDCs) in water is thought to be linked to deviation from normal male developmental and reproductive functions in exposed aquatic organisms. Because aquatic environments represent a chemically complex medium, the combined effects of androgenic EDCs require urgent attention. In the present study, the effects of two model androgen receptor (AR) antagonists, flutamide (FLU), and vinclozolin (VIN), were first determined individually in male Japanese medaka using the transcriptional response for genes associated with the hypothalamic-pituitary-gonadal axis. The fish were further exposed to binary mixtures of VIN and 17β-trenbolone (TRE, AR agonist) to confirm the theoretical opposing effects of the AR antagonist and agonist. The results showed that exposure to FLU or VIN alone induced very similar transcriptional responses, demonstrating that gene transcription analysis could be successfully employed in identifying the action of single chemicals. For example, both exposures increased the transcription of cyp17b but decreased that of cyp19b in the gonad, demonstrating the compensatory response for AR blockage. However, in the case of exposure to mixtures, although the joint antagonistic action of TRE and VIN affected the most genes, the transcription profiles after exposure to mixtures were not consistent with expectations based on the results for individual chemicals, such as hepatic vtg, and star or cyp19a in gonads. Therefore, the limitation of gene transcription analyses in exposures to mixtures, as well as the potential for the extrapolation of single chemicals, should be considered in future studies. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1591-1599, 2016.

Phenotypically anchored transcriptome profiling of developmental exposure to the antimicrobial agent, triclosan, reveals hepatotoxicity in embryonic zebrafish

Triclosan (TCS) is an antimicrobial agent commonly found in a variety of personal care products and cosmetics. TCS readily enters the environment through wastewater and is detected in human plasma, urine, and breast milk due to its widespread use. Studies have implicated TCS as a disruptor of thyroid and estrogen signaling; therefore, research examining the developmental effects of TCS is warranted. In this study, we used embryonic zebrafish to investigate the developmental toxicity and potential mechanism of action of TCS. Embryos were exposed to graded concentrations of TCS from 6 to 120hours post-fertilization (hpf) and the concentration where 80% of the animals had mortality or morbidity at 120hpf (EC80) was calculated. Transcriptomic profiling was conducted on embryos exposed to the EC80 (7.37μM). We identified a total of 922 significant differentially expressed transcripts (FDR adjusted P-value≤0.05; fold change ≥2). Pathway and gene ontology enrichment analyses identified biological networks and transcriptional hubs involving normal liver functioning, suggesting TCS may be hepatotoxic in zebrafish. Tissue-specific gene enrichment analysis further supported the role of the liver as a target organ for TCS toxicity. We also examined the in vitro bioactivity profile of TCS reported by the ToxCast screening program. TCS had a diverse bioactivity profile and was a hit in 217 of the 385 assay endpoints we identified. We observed similarities in gene expression and hepatic steatosis assays; however, hit data for TCS were more concordant with the hypothesized CAR/PXR activity of TCS from rodent and human in vitro studies.

Linking the response of endocrine regulated genes to adverse effects on sex differentiation improves comprehension of aromatase inhibition in a Fish Sexual Development Test

The Fish Sexual Development Test (FSDT) is a non-reproductive test to assess adverse effects of endocrine disrupting chemicals. With the present study it was intended to evaluate whether gene expression endpoints would serve as predictive markers of endocrine disruption in a FSDT. For proof-of-concept, a FSDT according to the OECD TG 234 was conducted with the non-steroidal aromatase inhibitor fadrozole (test concentrations: 10μg/L, 32μg/L, 100μg/L) using zebrafish (Danio rerio). Gene expression analyses using quantitative RT-PCR were included at 48h, 96h, 28days and 63days post fertilization (hpf, dpf). The selection of genes aimed at finding molecular endpoints which could be directly linked to the adverse apical effects of aromatase inhibition. The most prominent effects of fadrozole exposure on the sexual development of zebrafish were a complete sex ratio shift towards males and an acceleration of gonad maturation already at low fadrozole concentrations (10μg/L). Due to the specific inhibition of the aromatase enzyme (Cyp19) by fadrozole and thus, the conversion of C19-androgens to C18-estrogens, the steroid hormone balance controlling the sex ratio of zebrafish was altered. The resulting key event is the regulation of directly estrogen-responsive genes. Subsequently, gene expression of vitellogenin 1 (vtg1) and of the aromatase cyp19a1b isoform (cyp19a1b), were down-regulated upon fadrozole treatment compared to controls. For example, mRNA levels of vtg1 were down-regulated compared to the controls as early as 48 hpf and 96 hpf. Further regulated genes cumulated in pathways suggested to be controlled by endocrine mechanisms, like the steroid and terpenoid synthesis pathway (e.g. mevalonate (diphospho) decarboxylase (mvd), lanosterol synthase (2,3-oxidosqualene-lanosterol cyclase; lss), methylsterol monooxygenase 1 (sc4mol)) and in lipid transport/metabolic processes (steroidogenic acute regulatory protein (star), apolipoprotein Eb (apoEb)). Taken together, this study demonstrated that the existing Adverse Outcome Pathway (AOP) for aromatase inhibition in fish can be translated to the life-stage of sexual differentiation. We were further able to identify MoA-specific marker gene expression which can be instrumental in defining new measurable key events (KE) of existing or new AOPs related to endocrine disruption.

Activity of binary mixtures of drospirenone with progesterone and 17α-ethinylestradiol in vitro and in vivo

Despite potential exposure of aquatic organisms to mixtures of steroid hormones, very little is known on their joint activity in fish. Drospirenone (DRS) is a new synthetic progestin used in contraceptive pills in combination with 17α-ethinylestradiol (EE2). Here we systematically analyzed effects of DRS in binary mixtures with progesterone (P4) and EE2. First, we determined the in vitro activity of single compounds in recombinant yeast assays that express the human progesterone, androgen, or estrogen receptor, followed by determination of mixture activities of DRS and P4, DRS and EE2, as well as medroxyprogesterone acetate (MPA) and dydrogesterone (DDG). Mixtures of DRS and P4, as well as of DRS and EE2 showed additive progestogenic and androgenic activities. However, DDG and MPA showed non-additive progestogenic and androgenic activities. We then analyzed the in vivo activity of single compounds and mixtures of DRS and P4, as well as DRS and EE2, by assessing transcriptional changes of up to 14 selected target genes in zebrafish embryos at 48h post fertilization (hpf), and in eleuthero-embryos at 96hpf and 144hpf. DRS, P4, and EE2 led to significant transcriptional alteration of genes, including those encoding hormone receptors (pgr, esr1), a steroidogenic enzyme (hsd17b3), and estrogenic markers (vtg1, cyp19b), in particular at 144 hpf. In general, DRS showed stronger transcriptional changes than P4. In mixtures of DRS and P4, they were mainly non-additive (antagonistic interaction). In mixtures of DRS and EE2, transcriptional responses of esr1, vtg1 and cyp19b were dominated by EE2, suggesting an antagonistic interaction or independent action. Equi-effective mixtures of DRS and EE2, based on progesterone receptor transcripts, showed antagonistic interactions. Our data suggest that interactions in mixtures assessed in vitro in recombinant yeast cannot be translated to the in vivo situation. The receptor-based responses did not correspond well to the transcriptional responses in embryos which are much more complex due to the interplay between hormonal pathways, receptor crosstalk, and hormonal feedback loops.

Investigation of the effects of estrogen on skeletal gene expression during zebrafish larval head development

The development of craniofacial skeletal structures requires well-orchestrated tissue interactions controlled by distinct molecular signals. Disruptions in normal function of these molecular signals have been associated with a wide range of craniofacial malformations. A pathway mediated by estrogens is one of those molecular signals that plays role in formation of bone and cartilage including craniofacial skeletogenesis. Studies in zebrafish have shown that while higher concentrations of 17-β estradiol (E 2) cause severe craniofacial defects, treatment with lower concentrations result in subtle changes in head morphology characterized with shorter snouts and flatter faces. The molecular basis for these morphological changes, particularly the subtle skeletal effects mediated by lower E 2 concentrations, remains unexplored. In the present study we address these effects at a molecular level by quantitative expression analysis of sets of candidate genes in developing heads of zebrafish larvae treated with two different E 2 concentrations. To this end, we first validated three suitable reference genes, ppia2, rpl8 and tbp, to permit sensitive quantitative real-time PCR analysis. Next, we profiled the expression of 28 skeletogenesis-associated genes that potentially respond to estrogen signals and play role in craniofacial development. We found E 2 mediated differential expression of genes involved in extracellular matrix (ECM) remodelling, mmp2/9/13, sparc and timp2a, as well as components of skeletogenic pathways, bmp2a, erf, ptch1/2, rankl, rarab and sfrp1a. Furthermore, we identified a co-expressed network of genes, including cpn1, dnajc3, esr1, lman1, rrbp1a, ssr1 and tram1 with a stronger inductive response to a lower dose of E 2 during larval head development.

Zebrafish dives into food research: effectiveness assessment of bioactive compounds

Zebrafish ease of use and characteristics reveal it to be an interesting and underused model in food and nutrition research.

Exposing native cyprinid (Barbus plebejus) juveniles to river sediments leads to gonadal alterations, genotoxic effects and thyroid disruption

Juveniles (50 days post hatch) of a native cyprinid fish (Barbus plebejus) were exposed for 7 months to sediments from the River Lambro, a polluted tributary impairing the quality of the River Po for tens of kilometers from their confluence. Sediments were collected upstream of the city of Milan and downstream at the closure of the drainage basin of the River Lambro. Chemical analyses revealed the presence of a complex mixture of bioavailable endocrine-active chemicals, with higher exposure levels in the downstream section of the tributary. Mainly characterized by brominated flame retardants, alkylphenols, polychlorinated biphenyls, and minor co-occurring personal care products and natural hormones, the sediment contamination induced reproductive disorders, as well as other forms of endocrine disruption and toxicity. In particular, exposed male barbel exhibited higher biliary PAH-like metabolites, overexpression of the cyp1a gene, vitellogenin production in all specimens, the presence of oocytes (up to 22% intersex), degenerative alterations in their testis, liver fat vacuolization, a marked depression of total thyroxine (T4) and triiodothyronine (T3) plasma levels, and genotoxic damages determined as hepatic DNA adducts. These results clearly demonstrate that Lambro sediments alone are responsible for recognizable changes in the structure and function of the reproductive and, in general, the endocrine system of a native fish species. In the real environment, exposure to waterborne and food-web sources of chemicals are responsible for additional toxic loads, and the present findings thus provide evidence for a causal role of this tributary in the severe decline observed in barbel in recent decades and raise concern that the fish community of the River Po is exposed to endocrine-mediated health effects along tens of kilometres of its course.

Transcriptional and morphological effects of tamoxifen on the early development of zebrafish (Danio rerio)

Tamoxifen is a widely used anticancer drug with both an estrogen agonist and antagonist effect. This study focused on its endocrine disrupting effect, and overall environmental significance. Zebrafish embryos were exposed to different concentrations (0.5, 5, 50 and 500 µg l(-1) ) of tamoxifen for 96 h. The results showed a complex effect of tamoxifen on zebrafish embryo development. For the 500 µg l(-1) exposure group, the heart rate was decreased by 20% and mild defects in caudal fin and skin were observed. Expressions of a series of genes related to endocrine and morphological changes were subsequently tested through quantitative real-time polymerase chain reaction. Bisphenol A as a known estrogen was also tested as an endocrine-related comparison. Among the expression of endocrine-related genes, esr1, ar, cyp19a1b, hsd3b1 and ugt1a1 were all increased by tamoxifen exposure, similar to bisphenol A. The cyp19a1b is a key gene that controls estrogen synthesis. Exposure to 0.5, 5, 50 and 500 µg l(-1) of tamoxifen caused upregulation of cyp19a1b expression to 152%, 568%, 953% and 2024% compared to controls, higher than the effects from the same concentrations of bisphenol A treatment, yet vtg1 was suppressed by 24% from exposure to 500 µg l(-1) tamoxifen. The expression of metabolic-related genes such as cyp1a, cyp1c2, cyp3a65, gpx1a, gstp1, gsr and genes related to observed morphological changes such as krt17 were also found to be upregulated by high concentrations of tamoxifen. These findings indicated the potential environmental effect of tamoxifen on teleost early development. Copyright © 2015 John Wiley & Sons, Ltd.

Estrogen receptor 2b deficiency impairs the antiviral response of zebrafish

Although several studies have demonstrated the ability of some endocrine disruptive chemicals (EDCs) to alter the physiology of zebrafish, the immune-reproductive interaction has received little attention in this species. In this study, we used a homozygous line carrying an insertion of 8 amino acids in the ligand-binding domain of the estrogen receptor 2b gene (esr2b) to further understand the role of estrogen signaling on innate immunity. Adult mutant fish showed distorted sexual ratios related with alterations in testicular morphology and supraphysiological testosterone and 17β-estradiol (E2) levels. Immunity-wise, although esr2b mutant fish showed unaltered antibacterial responses, they were unable to mount an effective antiviral response upon viral challenge. RT-qPCR analysis demonstrated that mutant fish were able to induce the genes encoding major antiviral molecules, including Ifnphi1, Ifnphi2, Infphi3, Mxb and Mxc, and the negative feedback regulator of cytokine signaling Socs1. Notably, although esr2b mutant larvae showed a similar resistance to SVCV infection to their wild type siblings, waterborne E2 increased their viral susceptibility. Similarly, the exposure of adult wild type zebrafish to E2 also resulted in increased susceptibility to SVCV infection. Finally, the administration of recombinant Ifnphi1 hardly reversed the higher viral susceptibility of esr2b mutant zebrafish, suggesting that elevated socs1 levels impair Ifn signaling. All together, these results uncover an important role for E2 and Esr signaling in the fine-tuning of sexual hormone balance and the antiviral response of vertebrates.

The fish embryo test (FET): origin, applications, and future

Originally designed as an alternative for the acute fish toxicity test according to, e.g., OECD TG 203, the fish embryo test (FET) with the zebrafish (Danio rerio) has been optimized, standardized, and validated during an OECD validation study and adopted as OECD TG 236 as a test to assess toxicity of embryonic forms of fish. Given its excellent correlation with the acute fish toxicity test and the fact that non-feeding developmental stages of fish are not categorized as protected stages according to the new European Directive 2010/63/EU on the protection of animals used for scientific purposes, the FET is ready for use not only for range-finding but also as a true alternative for the acute fish toxicity test, as required for a multitude of national and international regulations. If-for ethical reasons-not accepted as a full alternative, the FET represents at least a refinement in the sense of the 3Rs principle. Objections to the use of the FET have mainly been based on the putative lack of biotransformation capacity and the assumption that highly lipophilic and/or high molecular weight substances might not have access to the embryo due to the protective role of the chorion. With respect to bioactivation, the only substance identified so far as not being activated in the zebrafish embryo is allyl alcohol; all other biotransformation processes that have been studied in more detail so far were found to be present, albeit, in some cases, at lower levels than in adult fish. With respect to larger molecules, the extension of the test duration to 96 h (i.e., beyond hatch) has-at least for the substances tested so far-compensated for the reduced access to the embryo; however, more research is necessary to fully explore the applicability of the FET to substances with a molecular weight >3 kDa as well as substances with a neurotoxic mode of action. An extension of the endpoints to also cover sublethal endpoints makes the FET a powerful tool for the detection of teratogenicity, dioxin-like activity, genotoxicity and mutagenicity, neurotoxicity, as well as various forms of endocrine disruption.

Identification and Characterization of Androgen-Responsive Genes in Zebrafish Embryos

Responsive genes for fish embryos have been identified so far for some endocrine pathways but not for androgens. Using transcriptome analysis and multiple concentration-response modeling, we identified putative androgen-responsive genes in zebrafish embryos exposed to 0.05-5000 nM 11-ketotestosterone for 24 h. Four selected genes with sigmoidal concentration-dependent expression profiles (EC50 = 6.5-30.0 nM) were characterized in detail. The expression of cyp2k22 and slco1f4 was demonstrated in the pronephros; lipca was detected in the liver, and sult2st3 was found in the olfactory organs and choroid plexus. Their expression domains, the function of human orthologs, and a pathway analysis suggested a role of these genes in the metabolism of hormones. Hence, it was hypothesized that they were induced to compensate for elevated hormone levels. The induction of sult2st3 and cyp2k22 by 11-ketotestosterone was repressed by co-exposure to the androgen receptor antagonist nilutamide supporting a potential androgen receptor mediated regulation. Sensitivity (expressed as EC50 values) of sult2st3 and cyp2k22 gene expression induction after exposure to other steroidal hormones (11-ketotestosterone ∼ testosterone > progesterone > cortisol > ethinylestradiol) correlated with their known binding affinities to zebrafish androgen receptor. Hence, these genes might represent potential markers for screening of androgenic compounds in the zebrafish embryo.

Long-term effects of bisphenol AF (BPAF) on hormonal balance and genes of hypothalamus-pituitary-gonad axis and liver of zebrafish (Danio rerio), and the impact on offspring

Bisphenol AF (BPAF) is one of the analogues of bisphenol A (BPA) and is widely used as a raw material in the plastics industry. The potential toxicity to fish from exposure to BPAF in the aquatic environment is largely unknown. In this study, zebrafish (Danio rerio) were exposed to BPAF at 5, 25 and 125 μg L(-1), from 4 hour-post-fertilization (hpf) to 120 day-post-fertilization (dpf), representing the period from embryo to adult. The levels of plasma hormones were measured and the expression of selected representative genes along the hypothalamus-pituitary-gonad (HPG) axis and liver were examined. The concentration of 17β-estradiol (E2) was significantly increased in male and female fish and a significant decrease of testosterone (T) was observed in male fish. The mRNA expression of genes along the HPG axis and in liver tissues in F0 generation fish demonstrated that the steroid hormonal balances of zebrafish were modulated through the alteration of steroidgenesis. The significant decrease of egg fertilization among offspring indicates the possibility of sperm deterioration of parent following exposure to BPAF. The higher occurrence of malformation and lower survival rate in the offspring from the exposure group suggested a possibility of maternal transfer of BPAF, which could be responsible for the increased prevalence of adverse health signs in the offspring. The hatching delay in 5 μg L(-1) BPAF indicated that parental exposure to environmentally relevant concentration of BPAF would result in delayed hatching of the offspring. A potential consequence of adverse effects in the offspring by BPAF deserves further investigation.

The herbicide linuron inhibits cholesterol biosynthesis and induces cellular stress responses in brown trout

The herbicide linuron is used worldwide, and has been detected in surface waters as well as in food and drinking water. Toxicological studies have reported that linuron acts as an antiandrogen in vitro and in vivo and disrupts mammalian male reproductive function. However, global mechanisms of linuron toxicity are poorly documented. We used RNA-seq to characterize the hepatic transcriptional response of mature male brown trout exposed for 4 days to 1.7, 15.3, and 225.9 μg/L linuron. We identified a striking decrease in the expression of transcripts encoding the majority of enzymes forming the cholesterol biosynthesis pathway. We also measured a very significant decrease in total hepatic cholesterol in fish exposed to 225.9 μg/L linuron and a negative correlation between total cholesterol and linuron treatment concentration. We hypothesize that inhibition of cholesterol biosynthesis may result from the disruption of androgen signaling by linuron. Additionally, there was increased expression of a number of transcripts involved in cellular stress responses, including cyp1a (up to 560-fold), molecular chaperones, and antioxidant enzymes. We found some evidence of similar patterns of transcriptional change in fish exposed to an environmentally relevant concentration of linuron, and further research should investigate the potential for adverse effects to occur following chronic environmental exposure.

Low-dose exposure to bisphenol A and replacement bisphenol S induces precocious hypothalamic neurogenesis in embryonic zebrafish

Bisphenol A (BPA), a ubiquitous endocrine disruptor that is present in many household products, has been linked to obesity, cancer, and, most relevant here, childhood neurological disorders such as anxiety and hyperactivity. However, how BPA exposure translates into these neurodevelopmental disorders remains poorly understood. Here, we used zebrafish to link BPA mechanistically to disease etiology. Strikingly, treatment of embryonic zebrafish with very low-dose BPA (0.0068 μM, 1,000-fold lower than the accepted human daily exposure) and bisphenol S (BPS), a common analog used in BPA-free products, resulted in 180% and 240% increases, respectively, in neuronal birth (neurogenesis) within the hypothalamus, a highly conserved brain region involved in hyperactivity. Furthermore, restricted BPA/BPS exposure specifically during the neurogenic window caused later hyperactive behaviors in zebrafish larvae. Unexpectedly, we show that BPA-mediated precocious neurogenesis and the concomitant behavioral phenotype were not dependent on predicted estrogen receptors but relied on androgen receptor-mediated up-regulation of aromatase. Although human epidemiological results are still emerging, an association between high maternal urinary BPA during gestation and hyperactivity and other behavioral disturbances in the child has been suggested. Our studies here provide mechanistic support that the neurogenic period indeed may be a window of vulnerability and uncovers previously unexplored avenues of research into how endocrine disruptors might perturb early brain development. Furthermore, our results show that BPA-free products are not necessarily safer and support the removal of all bisphenols from consumer merchandise.

Persistence of endocrine disruption in zebrafish (Danio rerio) after discontinued exposure to the androgen 17β-trenbolone

The aim of the present study was to investigate the effects of the androgenic endocrine disruptor 17β-trenbolone on the sexual development of zebrafish (Danio rerio) with special emphasis on the question of whether adverse outcomes of developmental exposure are reversible or persistent. An exposure scenario including a recovery phase was chosen to assess the potential reversibility of androgenic effects. Zebrafish were exposed to environmentally relevant concentrations of 17β-trenbolone (1 ng/L-30 ng/L) from fertilization until completion of gonad sexual differentiation (60 d posthatch). Thereafter, exposure was either followed by 40 d of recovery in clean water or continued until 100 d posthatch, the age when zebrafish start being able to reproduce. Fish exposed for 100 d to 10 ng/L or 30 ng/L 17β-trenbolone were masculinized at different biological effect levels, as evidenced from a concentration-dependent shift of the sex ratio toward males as well as a significantly increased maturity of testes. Gonad morphological masculinization occurred in parallel with decreased vitellogenin concentrations in both sexes. Changes of brain aromatase (cyp19b) mRNA expression showed no consistent trend with respect to either exposure duration or concentration. Gonad morphological masculinization as well as the decrease of vitellogenin persisted after depuration over 40 d in clean water. This lack of recovery suggests that androgenic effects on sexual development of zebrafish are irreversible.

Effects of changes in food supply at the time of sex differentiation on the gonadal transcriptome of juvenile fish. Implications for natural and farmed populations

Background

Food supply is a major factor influencing growth rates in animals. This has important implications for both natural and farmed fish populations, since food restriction may difficult reproduction. However, a study on the effects of food supply on the development of juvenile gonads has never been transcriptionally described in fish.

Methods and Findings

This study investigated the consequences of growth on gonadal transcriptome of European sea bass in: 1) 4-month-old sexually undifferentiated fish, comparing the gonads of fish with the highest vs. the lowest growth, to explore a possible link between transcriptome and future sex, and 2) testis from 11-month-old juveniles where growth had been manipulated through changes in food supply. The four groups used were: i) sustained fast growth, ii) sustained slow growth, iii) accelerated growth, iv) decelerated growth. The transcriptome of undifferentiated gonads was not drastically affected by initial natural differences in growth. Further, changes in the expression of genes associated with protein turnover were seen, favoring catabolism in slow-growing fish and anabolism in fast-growing fish. Moreover, while fast-growing fish took energy from glucose, as deduced from the pathways affected and the analysis of protein-protein interactions examined, in slow-growing fish lipid metabolism and gluconeogenesis was favored. Interestingly, the highest transcriptomic differences were found when forcing initially fast-growing fish to decelerate their growth, while accelerating growth of initially slow-growing fish resulted in full transcriptomic convergence with sustained fast-growing fish.

Conclusions

Food availability during sex differentiation shapes the juvenile testis transcriptome, as evidenced by adaptations to different energy balances. Remarkably, this occurs in absence of major histological changes in the testis. Thus, fish are able to recover transcriptionally their testes if they are provided with enough food supply during sex differentiation; however, an initial fast growth does not represent any advantage in terms of transcriptional fitness if later food becomes scarce.