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

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.

Exposure to nitrate and nitrite disrupts cardiovascular development through estrogen receptor in zebrafish embryos and larvae

Increasing nitrate concentration on surface and groundwater due to anthropogenic activities is an environmental concern. In this study, Tg(fli1: EGFP) zebrafish embryos were exposed to nitrate (NO3-) and nitrite (NO2-), and their cardiovascular development were investigated. Exposure to 10 mg/L NO3-N and 1 and 10 mg/L NO2-N decreased heart rate at 48-96-h post-fertilization (hpf), ventricular volume, and red blood cell flow rate at 96 hpf. Similar concentrations increased the number of embryos and larvae with pericardial edema and missing intersegmental and parachordal vessels in the caudal region at 48-96 hpf. Addition of ICI 182,720 (ICI) reversed the effects of nitrate and nitrite, suggesting estrogen receptors (ER) are involved. 10 mg/L NO3-N and 1 mg/L NO2-N decreased cardiovascular-related genes, gata4,5,6, hand2, nkx2.5, nkx2.7, tbx2a, tbx2b, and fgf1a. Gene expressions of ovarian aromatase and brain aromatase (cyp19a1a and cyp19a1b, respectively) decreased in the exposed groups, whereas ERs (esr1, esr2a, and esr2b) and nitric oxide synthase 2a (nos2a) increased. The effects on gene expression were also reversed by addition of ICI. Taken together, nitrate and nitrite disrupt cardiovascular system through ER in developing zebrafish, implying that environmental nitrate and nitrite contamination may be harmful to aquatic organisms.

Modeling Sex-Bias in Anxiety: Pros and Cons of a Larval Zebrafish Model

Anxiety disorders are the most prevalent psychiatric disorders, exhibiting strong female bias. Clinical studies implicate declining estradiol levels in the exacerbation of anxiety symptoms in the premenstrual phase of the menstrual cycle. This study aimed to simulate estradiol fluctuation-linked anxiety behavior in larval zebrafish, using an estradiol treatment withdrawal model. Contrary to model aims, estradiol treatment withdrawal decreased both basal activity and anxiety-like hyperlocomotion (ANOVA main effect of dose, P < 0.0001 and P < 0.01, respectively) in the light/dark transition test. The accuracy of the estradiol washout model was not improved by longer durations of treatment or withdrawal. Basal activity was slightly altered by supraphysiological concentrations of WAY-200070 in the absence of added estradiol. Estrogen receptor (ER) β expression was not upregulated in larvae exposed to physiologically relevant, low concentrations of estradiol. Longer exposure to low concentrations of estradiol increased antioxidant capacity (P < 0.01). In addition, acute exposure to low concentrations of estradiol increased basal activity. Data suggest that in the current models, estradiol-associated altered activity levels were linked to more favorable redox status, rather than reflecting altered anxiety levels. As such, it is recommended that zebrafish larval behavioral analysis be conducted in parallel with mechanistic studies such as redox indicators, for investigations focused on ER signaling.

Zebrafish as an emerging model to study estrogen receptors in neural development

Estrogens induce several regulatory signals in the nervous system that are mainly mediated through estrogen receptors (ERs). ERs are largely expressed in the nervous system, yet the importance of ERs to neural development has only been elucidated over the last decades. Accumulating evidence shows a fundamental role for estrogens in the development of the central and peripheral nervous systems, hence, the contribution of ERs to neural function is now a growing area of research. The conservation of the structure of the ERs and their response to estrogens make the zebrafish an interesting model to dissect the role of estrogens in the nervous system. In this review, we highlight major findings of ER signaling in embryonic zebrafish neural development and compare the similarities and differences to research in rodents. We also discuss how the recent generation of zebrafish ER mutants, coupled with the availability of several transgenic reporter lines, its amenability to pharmacological studies and in vivo live imaging, could help us explore ER function in embryonic neural development.

Hemato-vascular specification requires arnt1 and arnt2 genes in zebrafish embryos

During embryonic development, a subset of cells in the mesoderm germ layer are specified as hemato-vascular progenitor cells, which then differentiate into endothelial cells and hematopoietic stem and progenitor cells. In zebrafish, the transcription factor npas4l (cloche) is required for the specification of hemato-vascular progenitor cells. However, it is unclear whether npas4l is the sole factor at the top of the hemato-vascular specification cascade. Here, we show that arnt1 and arnt2 genes are required for hemato-vascular specification. We found that arnt1;arnt2 double mutant zebrafish embryos, but not arnt1 or arnt2 single mutants, lack blood cells and most endothelial cells. arnt1/2 mutants have reduced or absent expression of etsrp and tal1, the earliest known endothelial and hematopoietic transcription factor genes. We found that Npas4l binds both Arnt1 and Arnt2 proteins in vitro, consistent with the idea that PAS domain-containing bHLH transcription factors act in a multimeric complex to regulate gene expression. Our results demonstrate that npas4l, arnt1 and arnt2 act together to regulate endothelial and hematopoietic cell fate, where each gene is necessary, but not sufficient, to drive hemato-vascular specification.

Estrogen Signaling Influences Nephron Segmentation of the Zebrafish Embryonic Kidney

Despite significant advances in understanding nephron segment patterning, many questions remain about the underlying genes and signaling pathways that orchestrate renal progenitor cell fate choices and regulate differentiation. In an effort to identify elusive regulators of nephron segmentation, our lab conducted a high-throughput drug screen using a bioactive chemical library and developing zebrafish, which are a conserved vertebrate model and particularly conducive to large-scale screening approaches. 17β-estradiol (E2), which is the dominant form of estrogen in vertebrates, was a particularly interesting hit from this screen. E2 has been extensively studied in the context of gonad development, but roles for E2 in nephron development were unknown. Here, we report that exogenous estrogen treatments affect distal tubule composition, namely, causing an increase in the distal early segment and a decrease in the neighboring distal late. These changes were noted early in development but were not due to changes in cell dynamics. Interestingly, exposure to the xenoestrogens ethinylestradiol and genistein yielded the same changes in distal segments. Further, upon treatment with an estrogen receptor 2 (Esr2) antagonist, PHTPP, we observed the opposite phenotypes. Similarly, genetic deficiency of the Esr2 analog, esr2b, revealed phenotypes consistent with that of PHTPP treatment. Inhibition of E2 signaling also resulted in decreased expression of essential distal transcription factors, irx3b and its target irx1a. These data suggest that estrogenic compounds are essential for distal segment fate during nephrogenesis in the zebrafish pronephros and expand our fundamental understanding of hormone function during kidney organogenesis.

G protein-coupled estrogen receptor 1 regulates renal endothelin-1 signaling system in a sex-specific manner

Demographic studies reveal lower prevalence of hypertension among premenopausal females compared to age-matched males. The kidney plays a central role in the maintenance of sodium (Na⁺) homeostasis and consequently blood pressure. Renal endothelin-1 (ET-1) is a pro-natriuretic peptide that contributes to sex differences in blood pressure regulation and Na⁺ homeostasis. We recently showed that activation of renal medullary G protein-coupled estrogen receptor 1 (GPER1) promotes ET-1-dependent natriuresis in female, but not male, rats. We hypothesized that GPER1 upregulates the renal ET-1 signaling system in females, but not males. To test our hypothesis, we determined the effect of GPER1 deletion on ET-1 and its downstream effectors in the renal cortex, outer and inner medulla obtained from 12-16-week-old female and male mice. GPER1 knockout (KO) mice and wildtype (WT) littermates were implanted with telemetry transmitters for blood pressure assessment, and we used metabolic cages to determine urinary Na⁺ excretion. GPER1 deletion did not significantly affect 24-h mean arterial pressure (MAP) nor urinary Na⁺ excretion. However, GPER1 deletion decreased urinary ET-1 excretion in females but not males. Of note, female WT mice had greater urinary ET-1 excretion than male WT littermates, whereas no sex differences were observed in GPER1 KO mice. GPER1 deletion increased inner medullary ET-1 peptide content in both sexes but increased outer medullary ET-1 content in females only. Cortical ET-1 content increased in response to GPER1 deletion in both sexes. Furthermore, GPER1 deletion notably increased inner medullary ET receptor A (ET_A) and decreased outer medullary ET receptor B (ET_B) mRNA expression in male, but not female, mice. We conclude that GPER1 is required for greater ET-1 excretion in females. Our data suggest that GPER1 is an upstream regulator of renal medullary ET-1 production and ET receptor expression in a sex-specific manner. Overall, our study identifies the role of GPER1 as a sex-specific upstream regulator of the renal ET-1 system.

Comparison of developmental toxicity induced by PFOA, HFPO-DA, and HFPO-TA in zebrafish embryos

Hexafluoropropylene oxide dimer acids (HFPO-DA) and hexafluoropropylene oxide trimer acids (HFPO-TA) are alternatives to perfluorooctanoic acid (PFOA). However, little information on the comparison of their toxicities is available. Here, zebrafish embryos were exposed to PFOA, HFPO-DA, and HFPO-TA with exposure concentrations of 5 and 500 μg/L. Behavioral abnormal, enzyme activities and gene expression profiles in zebrafish embryos were determined. Results showed that exposure to PFOA and its alternatives increased heart rates and inhibited locomotor activity of zebrafish embryos. Further, their exposures changed the enzyme activities (acetylcholinesterase and oxidative stress-related enzymes), ATP content, and expressions of genes related to hypothalamic-pituitary-thyroid (HPT) axis, apoptosis, and lipid metabolism. Comparison analyses found that PFOA, HFPO-TA, and HFPO-DA exposures induced different effects on the embryonic development of zebrafish, which indicates the different modes of action. The HFPO-DA exposure induced specific effects on the disorder of lipid metabolism, HPT axis, and neurodevelopment. The HFPO-TA exposure also induced different effects from the PFOA exposure, which focused on lipid metabolism. The current data shows that the HFPO-DA and HFPO-TA might not be safe alternatives to PFOA. This study provides a new understanding of the biological hazards of PFOA alternatives in aquatic organisms, which can guide their usage.

Transcriptomic sequencing reveals the potential molecular mechanism by which Tetrabromobisphenol A bis (2-hydroxyethyl ether) exposure exerts developmental neurotoxicity in developing zebrafish (Danio rerio)

Tetrabromobisphenol A bis (2-hydroxyethyl ether) (TBBPA-DHEE) is a derivative of Tetrabromobisphenol A (TBBPA) used as an intermediate flame retardant in engineering polymers. The mechanism of neurodevelopmental toxicity of TBBPA-DHEE remains unclear due to limited toxicological data. We performed behavioral and transcriptomic analyses to assess the neurodevelopmental effects of TBBPA-DHEE on developing zebrafish and potential toxicity mechanisms. Our result shows that exposure to TBBPA-DHEE significantly increased mortality, deformity rate, and reduction in hatch rate, hatchability, and body length relative to the DMSO control. The behavior analysis indicates that TBBPA-DHEE significantly reduced the spontaneous movement of larva compared to the control. The TSH and GH levels were significantly reduced in all the exposure groups in a concentration-dependent manner relative to the DMSO control. TBBPA-DHEE exhibited a significant reduction in locomotor activity across all the exposure groups in the light/dark locomotion test. The transcriptomic analysis result shows that 579 genes were differentially expressed. KEGG analysis shows the enrichment of complement cascade, JAK-STAT signaling pathway, cytokine-cytokine interaction, and phototransduction pathway resulting in a change in mRNA expression of their genes. These observed changes in developmental endpoints, hormonal level, and alteration in mRNA expression of component genes involved in neurodevelopmental pathways could be part of the possible mechanism of the observed toxic effects of TBBPA-DHEE exposure on zebrafish. This study could reveal the possible neurodevelopmental toxicity of TBBPA-DHEE to aquatic species, which could help uncover the health implications of emerging environmental contaminants.

Emerging Roles for G Protein-Coupled Estrogen Receptor 1 in Cardio-Renal Health: Implications for Aging

Cardiovascular (CV) and renal diseases are increasingly prevalent in the United States and globally. CV-related mortality is the leading cause of death in the United States, while renal-related mortality is the 8th. Despite advanced therapeutics, both diseases persist, warranting continued exploration of disease mechanisms to develop novel therapeutics and advance clinical outcomes for cardio-renal health. CV and renal diseases increase with age, and there are sex differences evident in both the prevalence and progression of CV and renal disease. These age and sex differences seen in cardio-renal health implicate sex hormones as potentially important regulators to be studied. One such regulator is G protein-coupled estrogen receptor 1 (GPER1). GPER1 has been implicated in estrogen signaling and is expressed in a variety of tissues including the heart, vasculature, and kidney. GPER1 has been shown to be protective against CV and renal diseases in different experimental animal models. GPER1 actions involve multiple signaling pathways: interaction with aldosterone and endothelin-1 signaling, stimulation of the release of nitric oxide, and reduction in oxidative stress, inflammation, and immune infiltration. This review will discuss the current literature regarding GPER1 and cardio-renal health, particularly in the context of aging. Improving our understanding of GPER1-evoked mechanisms may reveal novel therapeutics aimed at improving cardio-renal health and clinical outcomes in the elderly.

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

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

Application of Transgenic Zebrafish Models for Studying the Effects of Estrogenic Endocrine Disrupting Chemicals on Embryonic Brain Development

Endocrine disrupting chemicals (EDCs) are environmental pollutants that mimic hormones and/or disrupt their function. Estrogenic EDCs (eEDCs) interfere with endogenous estrogen signalling pathway(s) and laboratory animal and human epidemiological studies have provided evidence for a causal link between exposure to them during embryonic/early life and neurological impairments. However, our understanding of the molecular and cellular mechanism(s) underlying eEDCs exposure effects on brain development, tissue architecture and function and behaviour are limited. Transgenic (TG) zebrafish models offer new approach methodologies (NAMs) to help identify the modes of action (MoAs) of EDCs and their associated impacts on tissue development and function. Estrogen biosensor TG zebrafish models have been applied to study eEDC interactions and resulting transcriptional activation (via a fluorescent reporter expression) across the entire body of the developing zebrafish embryo, including in real time. These estrogen biosensor TG zebrafish models are starting to deepen our understanding of the spatiotemporal actions of eEDCs and their resulting impacts on neurological development, brain function and behaviour. In this review, we first investigate the links between early life exposure to eEDCs and neurodevelopmental alterations in model organisms (rodents and zebrafish) and humans. We then present examples of the application of estrogen biosensor and other TG zebrafish models for elucidating the mechanism(s) underlying neurodevelopmental toxicities of eEDCs. In particular we illustrate the utility of combining estrogen biosensor zebrafish models with other TG zebrafish models for understanding the effects of eEDCs on the brain, spanning cellular processes, brain circuitry, neurophysiology and behaviour. Finally, we discuss the future prospects of TG zebrafish models as experimental models for studying more complex scenarios for exposure to contaminant mixtures on neurological development and function.

The Interface of Nuclear and Membrane Steroid Signaling

Steroid hormones bind receptors in the cell nucleus and in the cell membrane. The most widely studied class of steroid hormone receptors are the nuclear receptors, named for their function as ligand-dependent transcription factors in the cell nucleus. Nuclear receptors, such as estrogen receptor alpha, can also be anchored to the plasma membrane, where they respond to steroids by activating signaling pathways independent of their function as transcription factors. Steroids can also bind integral membrane proteins, such as the G protein-coupled estrogen receptor. Membrane estrogen and progestin receptors have been cloned and characterized in vitro and influence the development and function of many organ systems. Membrane androgen receptors were cloned and characterized in vitro, but their function as androgen receptors in vivo is unresolved. We review the identity and function of membrane proteins that bind estrogens, progestins, and androgens. We discuss evidence that membrane glucocorticoid and mineralocorticoid receptors exist, and whether glucocorticoid and mineralocorticoid nuclear receptors act at the cell membrane. In many cases, integral membrane steroid receptors act independently of nuclear steroid receptors, even though they may share a ligand.

Inhibition of swim bladder inflation in Japanese medaka (Oryzias latipes) embryos following exposure to select pharmaceuticals alone and in combination

This study leveraged the Japanese medaka fish embryo model for the assessment of effects of select contaminants on early development in fish. Fish embryos were exposed to various pharmaceutical contaminants including synthetic hormones and non-steroidal anti-inflammatory drugs and their effects on development were observed. Initial screening determined that swim bladder inflation failure was the most common endpoint detected. Swim bladder inflation failure was first explored in a study demonstrating that medaka require access to the air-water interphase to inflate their swim bladders in a time-dependent manner, and swim bladder inflation failure was correlated with mortality. Fish embryos were exposed 24-hours post fertilization until hatch to concentration ranges of various pharmaceutical contaminants including: 17β-estradiol, 17α-ethinylestradiol, and levonorgestrel (1 to 1000 µg/L), or diclofenac (0.32 to 100 mg/L). The main effect observed across all four compounds was a significant increase in failure of swim bladder inflation with increasing exposure concentration (24 to 72-hours post-hatch). Following single compound experiments combinatorial exposures using no-observed-effect concentrations were conducted. The main effect observed was a significant decrease in inflation success 24-hours post-hatch following a binary mixture of levonorgestrel and 17α-ethinylestradiol, as well as a significant decrease in swim bladder inflation success at all times following exposure to a quaternary mixture of all four compounds. This study demonstrated that embryonic exposure to pharmaceutical compounds, both alone and in combination, resulted in failure of swim bladder inflation in larval Japanese medaka.

Embryonic Onset of Sexually Dimorphic Heart Rates in the Viviparous Fish, Gambusia holbrooki

In fish, little is known about sex-specific differences in physiology and performance of the heart and whether these differences manifest during development. Here for the first time, the sex-specific heart rates during embryogenesis of Gambusia holbrooki, from the onset of the heart rates (HRs) to just prior to parturition, was investigated using light cardiogram. The genetic sex of the embryos was post-verified using a sex-specific genetic marker. Results reveal that heart rates and resting time significantly increase (p < 0.05) with progressive embryonic development. Furthermore, both ventricular and atrial frequencies of female embryos were significantly higher (p < 0.05) than those of their male sibs at the corresponding developmental stages and remained so at all later developmental stages (p < 0.05). In concurrence, the heart rate and ventricular size of the adult females were also significantly (p < 0.05) higher and larger respectively than those of males. Collectively, the results suggest that the cardiac sex-dimorphism manifests as early as late-organogenesis and persists through adulthood in this species. These findings suggest that the cardiac measurements can be employed to non-invasively sex the developing embryos, well in advance of when their phenotypic sex is discernible. In addition, G. holbrooki could serve as a better model to study comparative vertebrate cardiovascular development as well as to investigate anthropogenic and climatic impacts on heart physiology of this species, that may be sex influenced.

Behavioral change and transcriptomics reveal the effects of 2, 2', 4, 4'-tetrabromodiphenyl ether exposure on neurodevelopmental toxicity to zebrafish (Danio rerio) in early life stage

Polybrominated diphenyl ethers (PBDEs) are a class of widely used flame retardants, and their residue in the environment may threaten the ecosystem and human health. The neurodevelopmental toxic effects of PBDEs have been verified in previous studies, but the mechanisms are still unclear. Behavioral analysis and transcriptomics were performed in this study to assess the neurodevelopmental toxic effects of PBDEs on zebrafish embryos and larvae, and the potential mechanisms. The embryos were collected after fertilization and exposed to control (0.05% DMSO), 10, 50, 100 (ug/L) 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47) for 7 days. The locomotion parameters of larvae were recorded and analyzed by a behavioral analysis system (EthoVision XT, Noldus). Enrichment of functions and signaling pathways of differentially expressed genes (DEGs) were analyzed by GO and DAVID database. The comparison with the control group showed adverse developments such as low hatching rate, high mortality rate, alterative heart rate, and abnormal spontaneous tail coiling frequency of embryos (24hpf). For the zebrafish larvae, behavioral analyses results suggested decreased activities and movements of the treatment in the light-dark period at 120, 144 and 168hpf, especially the 50 and 100μg/L groups. The affected functions included steroid hormone regulation, neuro regulation, circadian regulation, cardioblast differentiation, immune-related regulation. The enrichment of KEGG pathways were Hedgehog signaling (Shh), Toll-like receptor signaling, FoxO signaling, and Steroid biosynthesis pathway. Hedgehog signaling pathway was further verified via RT-qPCR for its major role in the development of neurogenesis. The mRNA levels of Shh pathway indicated the inhibition of Shh signal in our study since shha, patched1, gli1 and gli2 genes were significantly down-regulated. In summary, PBDEs might influence the neurodevelopment of zebrafish in the early life stage by multiple toxic signaling pathways alteration.

Characterization of G protein-coupled estrogen receptors in Japanese medaka, Oryzias latipes

The G protein-coupled estrogen receptor 1 (Gper1) is a membrane-bound estrogen receptor that mediates non-genomic action of estrogens. A Gper1-mediating pathway has been implicated in reproductive activities in fish, including oocyte growth, but Gper1 has been characterized in only a very limited number of fish species. In this study, we cloned and characterized two genes encoding medaka (Oryzias latipes) Gper1s, namely, Gper1a and Gper1b, and phylogenic and synteny analyses suggest that these genes originate through a teleost-specific whole genome duplication event. We found that Gper1a induced phosphorylation of mitogen-activated protein kinase (MAPK) in 293T cells transfected with medaka Gper1s on exposure to the natural estrogen, 17β-estradiol (E2) and a synthetic Gper1 agonist (G-1), and treatment with both E2 and G-1 also decreased the rate of spontaneous maturation in medaka oocytes. These findings show that the processes for oocyte growth and maturation are sensitive to estrogens and are possibly mediated through Gper1a in medaka. We also show that 17α-ethinylestradiol (EE2), one of the most potent estrogenic endocrine-disrupting chemicals, and bisphenol A (BPA, a weak environmental estrogen) augmented phosphorylation of MAPK through medaka Gper1s in 293T cells. Interestingly, however, treatment with EE2 or BPA did not attenuate maturation of medaka oocytes. Our findings support that Gper1-mediated effects on oocytes are conserved among fish species, but effects of estrogenic endocrine-disrupting chemicals on oocytes acting through Gper1 may be divergent among fish species.

Low-dose effects on thyroid disruption in zebrafish by long-term exposure to oxytetracycline

As a feed additive in agriculture, the antibiotic oxytetracycline (OTC) has become widely distributed in the natural environment, leading to the exposure of many organisms to low doses of OTC. Although OTC is clinically contraindicated in children because of its multiple side effects, the effect of exposure to low doses of environmental OTC on children is unknown, particularly during development. In this study, we investigated the effects of OTC on the thyroid endocrine system in zebrafish, through determinations of the whole-body contents of triiodothyronine (T3), thyroxine (T4), and thyroid-stimulating hormone (TSH) by enzyme-linked immunosorbent assay, and analysis of the mRNA expression of regulatory genes involved in the hypothalamus-pituitary-thyroid (HPT) axis using quantitative real-time polymerase chain reaction. Zebrafish embryos were exposed to OTC at environmentally relevant concentrations from 2 h to 120 days post-fertilisation. After exposure to OTC at 1,000 and 5,000 ng/L, T3 contents were significantly enhanced (37.8% and 45.1%, respectively) and TSH contents were reduced (16% and 16.3%, respectively) compared with those in the controls. The OTC-driven increase in the transcription of genes involved in thyroid synthesis (tpo and nis) may be responsible for the altered T3 levels. These data indicate that OTC may cause thyroid dysfunction and lead to reduced TSH secretion owing to enhanced negative feedback control of the HPT axis. Meanwhile, a decrease in body length, weight, and BMI and an increase in heart rate were observed with increasing OTC exposure. In conclusion, our results indicate that long-term exposure to low concentrations of OTC may alter the transcription of key genes involved in the HPT axis, as well as T3 and TSH contents, thereby disrupting the thyroid system and affecting the growth and development of zebrafish.

Effects of environmental steroid mixtures are regulated by individual steroid receptor signaling

Fish are exposed to steroids of different classes in contaminated waters, but their effects are not sufficiently understood. Here we employed an anti-sense technique using morpholino oligonucleotides to knockdown the glucocorticoid receptors (GRs, GRα and GRβ) and androgen receptor (AR) to investigate their role in physiological and transcriptional responses. To this end, zebrafish embryos were exposed to clobetasol propionate (CLO), androstenedione (A4) and mixtures containing different classes of steroids. CLO caused a decrease of spontaneous muscle contraction and increase of heart rate, as well as transcriptional induction of pepck1, fkbp5, sult2st3 and vitellogenin (vtg1) at 24 and/or 48 h post fertilization (hpf). Knockdown of GRs eliminated these effects, while knockdown of AR decreased the ar transcript but caused no expressional changes, except induction of sult2st3 after exposure to A4 at 24 hpf. Exposure to a mixture of 6 steroids comprising progesterone (P4) and three progestins, cyproterone acetate, dienogest, drospirenone, 17β-estradiol (E2) and CLO caused a significant induction of pepck1, sult2st3, vtg1 and per1a. Knockdown of GRs eliminated the physiological effects and the up-regulation of vtg1, sult2st3, pepck1, fkbp5 and per1a. Thus, as with CLO, responses in mixtures were regulated by GRs independently from the presence of other steroids. Exposure to a mixture comprising A4, CLO, E2 and P4 caused induction of vtg1, cyp19b, sult2st3 and fkbp5. Knockdown of AR had no effect, indicating that regulation of these genes occurred by the GRs and estrogen receptor (ER). Our findings show that in early embryos GRs cause vtg1 and sult2st3 induction in addition to known glucocorticoid target genes. Each steroid receptor regulated its own target genes in steroid mixtures independently from other steroids. However, enhanced expressional induction occurred for vtg1 and fkbp5 in steroid mixtures, indicating an interaction/cross-talk between GRs and ER. These findings have importance for the understanding of molecular effects of steroid mixtures.

ahr2, But Not ahr1a or ahr1b, Is Required for Craniofacial and Fin Development and TCDD-dependent Cardiotoxicity in Zebrafish

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that binds environmental toxicants and regulates gene expression. AHR also regulates developmental processes, like craniofacial development and hematopoiesis, in the absence of environmental exposures. Zebrafish have 3 paralogs of AHR: ahr1a, ahr1b, and ahr2. Adult zebrafish with mutations in ahr2 exhibited craniofacial and fin defects. However, the degree to which ahr1a and ahr1b influence ahr2 signaling and contribute to fin and craniofacial development are not known. We compared morphology of adult ahr2 mutants and ahr1a;ahr1b single and double mutant zebrafish. We found that ahr1a;ahr1b single and double mutants were morphologically normal whereas ahr2 mutant zebrafish demonstrated fin and craniofacial malformations. At 5 days post fertilization, both ahr1a;ahr1b and ahr2 mutant larvae were normal, suggesting that adult phenotypes are due to defects in maturation or maintenance. Next, we analyzed the function of zebrafish AHRs activated by environmental ligands. The prototypical AHR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), induces toxicity in humans and rodents via AHR and causes cardiotoxicity in zebrafish embryos. It has been shown that embryos with mutations in ahr2 are resistant to TCDD toxicity, yet it is unclear whether ahr1 receptors are required. Furthermore, though AHR was shown to interact with estrogen receptor alpha following TCDD treatment, it is not known whether this interaction is constitutive or context-dependent. To determine whether estrogen receptors are constitutive cofactors for AHR signaling, we used genetic and pharmacologic techniques to analyze TCDD-dependent toxicity in estrogen receptor and ahr mutant embryos. We found that embryos with mutations in ahr1a;ahr1b or estrogen receptor genes are susceptible to TCDD toxicity whereas ahr2 mutant embryos are TCDD-resistant. Moreover, pharmacologic blockade of nuclear estrogen receptors failed to prevent TCDD toxicity. These findings suggest that ahr1 genes do not have overlapping functions with ahr2 in fin and craniofacial development or TCDD-dependent toxicity, and that estrogen receptors are not constitutive partners of ahr2.

Using Zebrafish to Analyze the Genetic and Environmental Etiologies of Congenital Heart Defects

Congenital heart defects (CHDs) are among the most common human birth defects. However, the etiology of a large proportion of CHDs remains undefined. Studies identifying the molecular and cellular mechanisms that underlie cardiac development have been critical to elucidating the origin of CHDs. Building upon this knowledge to understand the pathogenesis of CHDs requires examining how genetic or environmental stress changes normal cardiac development. Due to strong molecular conservation to humans and unique technical advantages, studies using zebrafish have elucidated both fundamental principles of cardiac development and have been used to create cardiac disease models. In this chapter we examine the unique toolset available to zebrafish researchers and how those tools are used to interrogate the genetic and environmental contributions to CHDs.

Estrogen accelerates heart regeneration by promoting the inflammatory response in zebrafish

Sexual differences have been observed in the onset and prognosis of human cardiovascular diseases, but the underlying mechanisms are not clear. Here, we found that zebrafish heart regeneration is faster in females, can be accelerated by estrogen and is suppressed by the estrogen-antagonist tamoxifen. Injuries to the zebrafish heart, but not other tissues, increased plasma estrogen levels and the expression of estrogen receptors, especially esr2a. The resulting endocrine disruption induces the expression of the female-specific protein vitellogenin in male zebrafish. Transcriptomic analyses suggested heart injuries triggered pronounced immune and inflammatory responses in females. These responses, previously shown to elicit heart regeneration, could be enhanced by estrogen treatment in males and reduced by tamoxifen in females. Furthermore, a prior exposure to estrogen preconditioned the zebrafish heart for an accelerated regeneration. Altogether, this study reveals that heart regeneration is modulated by an estrogen-inducible inflammatory response to cardiac injury. These findings elucidate a previously unknown layer of control in zebrafish heart regeneration and provide a new model system for the study of sexual differences in human cardiac repair.

The Effects of 17α-Ethinylestradiol on the Heart Rate of Embryonic Japanese Medaka (Oryzias latipes)

Estrogen toxicity has been an area of priority in aquatic toxicology over the last 20 yr. Currently, estrogen toxicity is primarily linked to classical estrogen signaling, the interaction of estrogen receptors alpha and beta (ERα and ERβ). Recent evidence has indicated that a rapid, nongenomic, nonclassical estrogen signaling pathway exists via the G protein-coupled estrogen receptor (GPER), which is expressed in many biological systems, with roles in the cardiovascular system. The objective of the present study was to investigate the effect of 17α-ethinylestradiol (EE2) on the heart rate of embryonic Japanese medaka (Oryzias latipes). A significant decrease (bradycardia) in embryonic heart rate was observed at all treatment concentrations (0.1, 1, 10, 100, and 1000 ng/L EE2) at 144, 168, and 192 h postfertilization (hpf; p ≤ 0.05), whereas 120 and 216 hpf embryos experienced a significant decrease from the control at 10, 100, and 1000 ng/L EE2 and 0.1, 100, and 1000 ng/L EE2, respectively (p ≤ 0.05). In addition, using select estrogen receptor modulators, it was demonstrated that estrogen-induced bradycardia appears to be linked to GPER and not ERα and ERβ. The present study highlights GPER as a novel and alternative mode of action for EE2 toxicity at environmentally relevant concentrations. Environ Toxicol Chem 2020;39:904-912. © 2020 SETAC.

Effects of new generation progestins, including as mixtures and in combination with other classes of steroid hormones, on zebrafish early life stages

Fish are exposed to progestins and steroid mixtures in contaminated waters but the ecotoxicological implications are not sufficiently known. Here we analyze effects of the new generation progestin dienogest (DNG) followed by investigating effects of mixtures of new generation progestins containing DNG, cyproterone acetate and drospirenone and the hormone progesterone. Furthermore, effects of this mixture were studied after adding 17β-estradiol (E2) and clobetasol propionate (CLO) in zebrafish embryos and larvae at concentrations between 0.01 and 10 μg/L. DNG showed only very minor transcriptional alterations among the 24 assessed genes with downregulation of the fshb transcript only. The progestin mixture caused weak induction of the lhb, cyp2k22 and sult2st3 transcripts. Addition of E2 to the mixture caused strong induction vtg1, cyp19b, esr1 and lhb, as well as downregulation of fshb from 0.01 μg/L onwards. Besides altering the same transcripts, addition of CLO altered glucocorticoid regulated genes mmp-9, mmp-13, g6pca, fkbp5 and irg1l. While each steroid class exhibited its specific activity independently in the mixture, sult2st3 and cyp2k22 were regulated by both E2 and CLO. Furthermore, CLO alone and in mixtures decreased spontaneous muscle contractions, increased heartrate and induced edema. Our study highlights the prominent effects of E2 and CLO in environmental steroid mixtures, while new generation progestins show relatively low activity.

GPER as a Receptor for Endocrine-Disrupting Chemicals (EDCs)

Endocrine-disrupting chemicals (EDCs) are exogenous chemicals that interfere with endogenous hormonal systems at various levels, resulting in adverse health effects. EDCs belong to diverse chemical families and can accumulate in the environment, diet and body fluids, with different levels of persistence. Their action can be mediated by several receptors, including members of the nuclear receptor family, such as estrogen and androgen receptors. The G protein-coupled estrogen receptor (GPER), a seven-transmembrane domain receptor, has also attracted attention as a potential target of EDCs. This review summarizes our current knowledge concerning GPER as a mediator of EDCs' effects.

Starvation stress affects the maternal development and larval fitness in zebrafish (Danio rerio)

The starvation is a common and severe stress for animal survival and development. In aquatic environment, many fish suffer starvation stress in different extent because of the natural migration or feed limitation. When subjected to starved conditions, organisms will employ various adaptive physiological, biochemical, and behavioral changes to regulate metabolism and maintain homeostasis. In the present study, adult female zebrafish were deprived of feed for 1 to 3 weeks to detect the starved effects on adults and larvae. The results showed that biological indexes, RNA/DNA ratios, and nutritional indexes significantly decreased in the female fish after starvation. The number of mature follicles reduced while the average spawning diameter of oocytes increased. For the larvae, the maternal starvation stress distinctly delayed embryonic hatching, decreased larval body length, disrupted larval swimming ability, and reduced survival rate at early-life stages. Furthermore, we found that DNA methylation might conduce to the downregulated mRNA expression of anti-Müllerian hormone and cytochrome P450 aromatase in retarded ovaries under starved conditions. Significant effects on autophagic transcription were shown in maternal ovary and larvae responded to starvation stress. Taken together, our study systematically revealed the reproductive impairments of starvation stress and would facilitate the investigation of environmental stress in teleost fish.

Morphometric signatures of exposure to endocrine disrupting chemicals in zebrafish eleutheroembryos

Understanding the mode of action of the different pollutants in human and wildlife health is a key step in environmental risk assessment. The aim of this study was to determine signatures that could link morphological phenotypes to the toxicity mechanisms of four Endocrine Disrupting Chemicals (EDCs): bisphenol A (BPA), perfluorooctanesulfonate potassium salt (PFOS), tributyltin chloride (TBT), and 17-ß-estradiol (E2). Zebrafish (Danio rerio) eleutheroembryos were exposed from 2 to 5 dpf to a wide range of BPA, PFOS, TBT and E2 concentrations. At the end of the exposures several morphometric features were assessed. Common and non-specific effects on larvae pigmentation or swim bladder area were observed after exposures to all compounds. BPA specifically induced yolk sac malabsorption syndrome and altered craniofacial parameters, whereas PFOS had specific effects on the notochord formation presenting higher rates of scoliosis and kyphosis. The main effect of E2 was an increase in the body length of the exposed eleutheroembryos. In the case of TBT, main alterations on the morphological traits were related to developmental delays. When integrating all morphometrical parameters, BPA showed the highest rates of malformations in terms of equilethality, followed by PFOS and, distantly, by TBT and E2. In the case of BPA and PFOS, we were able to relate our results with effects on the transcriptome and metabolome, previously reported. We propose that methodized morphometric analyses in zebrafish embryo model can be used as an inexpensive and easy screening tool to predict modes of action of a wide-range number of contaminants.

Differential activity of BPA, BPAF and BPC on zebrafish estrogen receptors in vitro and in vivo

The high volume production compound bisphenol A (BPA) is of environmental concern largely because of its estrogenic activity. Consequently, BPA analogues have been synthesized to be considered as replacement molecules for BPA. These analogues need to be thoroughly evaluated for their estrogenic activity. Here, we combined mechanism zebrafish-based assays to examine estrogenic and anti-estrogenic activities of BPA and two of its analogues, bisphenol AF (BPAF) and bisphenol C (BPC) in vitro and in vivo. In vitro reporter cell lines were used to investigate agonistic and antagonistic effects of the three bisphenols on the three zebrafish estrogen receptors. The transgenic Tg(5 × ERE:GFP) and Cyp19a1b-GFP zebrafish lines were then used to analyze estrogenic and anti-estrogenic responses of the three bisphenols in vivo. BPA, BPAF and BPC were agonists with different potencies for the three zebrafish estrogen receptors in vitro. The potent zfERα-mediated activity of BPA and BPAF in vitro resulted in vivo by activation of GFP expression in zebrafish larvae in the heart (zfERα-dependent) at lower concentrations, and in the liver (zfERβ-dependent) at higher concentrations. BPC induced zfERβ-mediated luciferase expression in vitro, and the zfERβ agonism led to activation of GFP expression in the liver and the brain in vivo. In addition, BPC acted as a full antagonist on zfERα, and completely inhibited estrogen-induced GFP expression in the heart of the zebrafish larvae. To summarize, applying a combination of zebrafish-based in vitro and in vivo methods to evaluate bisphenol analogues for estrogenic activity will facilitate the prioritization of these chemicals for further analysis in higher vertebrates as well as the risk assessment in humans.

The complexation of steroid hormones into cyclodextrin alters the toxic effects on the biological parameters of zebrafish (Danio rerio)

The objective of this study was to evaluate the toxic effects of progesterone (P₄F) and estradiol (E₂F) and the effect of these steroid hormones complexed into cyclodextrins, commercially available drugs, such as micronized progesterone (P₄M) and transdermal estradiol (E₂T), and evaluate them as endocrine disruptors through biological parameters of Danio rerio. An acute toxicity test was performed with hormones using D. rerio embryos according to OECD 236 guidelines. The heart rate, mortality, and teratogenic effects were evaluated. In addition, a chronic toxicity test was assayed with adult animals for evaluation of animal behavior, reproductive capacity, and electrophysiological responses of the retina. Analysis of the results of the acute toxicity test with embryos exposed to progestins and estrogens showed that free hormones caused a higher percentage of teratogenic effects such as pericardial edema, yolk sac edema, and spinal deformation. Behavioral evaluation (30-60 days) of adult animals exposed to P₄M, E₂F, and E₂T demonstrated higher frequencies of aggressive behaviors such as Chase away, Persecution, Escape, and Attack. Analysis of reproductive capacity did not show significant differences in the number of viable eggs, and no significant changes were observed in the electrophysiological responses of the retina. According to these results, there is a higher toxicity effect of hormones in the free form when compared to the commercial forms and inclusion complexes. This indicates that complexation into cyclodextrin reduced the toxicity of the hormones according to the parameters studied.

Role of estrogens in fish immunity with special emphasis on GPER1

It is well accepted that estrogens, the primary female sex hormones, play a key role in modulating different aspects of the immune response. Moreover, estrogens have been linked with the sexual dimorphism observed in some immune disorders, such as chronic inflammatory and autoimmune diseases. Nevertheless, their effects are often controversial and depend on several factors, such as the pool of estrogen receptors (ERs) involved in the response. Their classical mode of action is through nuclear ERs, which act as transcription factors, promoting the regulation of target genes. However, it has long been noted that some of the estrogen-mediated effects cannot be explained by these classical receptors, since they are rapid and mediated by non-genomic signaling pathways. Hence, the interest in membrane ERs, especially in G protein-coupled estrogen receptor 1 (GPER1), has grown in recent years. Although the presence of nuclear ERs, and ER signaling, in immune cells in mammals and fish has been well documented, information on membrane ERs is much scarcer. In this context, the present manuscript aims to review our knowledge concerning the effect of estrogens on fish immunity, with special emphasis on GPER1. For example, the numerous tools developed over recent years allowed us to report for the first time that the regulation of fish granulocyte functions by estrogens through GPER1 predates the split of fish and tetrapods more than 450 million years ago, pointing to the relevance of estrogens as modulators of the immune responses, and the pivotal role of GPER1 in immunity.

G Protein-Coupled Estrogen Receptor Is Not Required for Sex Determination or Ovary Function in Zebrafish

Estrogens regulate vertebrate development and function through binding to nuclear estrogen receptors α and β (ERα and ERβ) and the G protein-coupled estrogen receptor (GPER). Studies in mutant animal models demonstrated that ERα and ERβ are required for normal ovary development and function. However, the degree to which GPER signaling contributes to ovary development and function is less well understood. Previous studies using cultured fish oocytes found that estradiol inhibits oocyte maturation in a GPER-dependent manner, but whether GPER regulates oocyte maturation in vivo is not known. To test the hypothesis that GPER regulates oocyte maturation in vivo, we assayed ovary development and function in gper mutant zebrafish. We found that homozygous mutant gper embryos developed into male and female adults with normal sex ratios. Adult mutant fish exhibited normal secondary sex characteristics and fertility. Additionally, mutant ovaries were histologically normal. We observed no differences in the number of immature versus mature oocytes in mutant versus wild-type ovaries from both young and aged adults. Furthermore, expression of genes associated with sex determination and ovary function was normal in gper mutant ovaries compared with wild type. Our findings suggest that GPER is not required for sex determination, ovary development, or fertility in zebrafish.

The Primodos components Norethisterone acetate and Ethinyl estradiol induce developmental abnormalities in zebrafish embryos

Primodos was a hormone pregnancy test used between 1958-1978 that has been implicated with causing a range of birth defects ever since. Though Primodos is no longer used, it's components, Norethisterone acetate and Ethinyl estradiol, are used in other medications today including treatments for endometriosis and contraceptives. However, whether Primodos caused birth defects or not remains controversial, and has been little investigated. Here we used the developing zebrafish embryo, a human cell-line and mouse retinal explants to investigate the actions of the components of Primodos upon embryonic and tissue development. We show that Norethisterone acetate and Ethinyl estradiol cause embryonic damage in a dose and time responsive manner. The damage occurs rapidly after drug exposure, affecting multiple organ systems. Moreover, we found that the Norethisterone acetate and Ethinyl estradiol mixture can affect nerve outgrowth and blood vessel patterning directly and accumulates in the forming embryo for at least 24 hrs. These data demonstrate that Norethisterone acetate and Ethinyl estradiol are potentially teratogenic, depending on dose and embryonic stage of development in the zebrafish. Further work in mammalian model species are now required to build on these findings and determine if placental embryos also are affected by synthetic sex hormones and their mechanisms of action.

Brain Aromatase Modulates Serotonergic Neuron by Regulating Serotonin Levels in Zebrafish Embryos and Larvae

Teleost fish are known to express two isoforms of P450 aromatase, a key enzyme for estrogen synthesis. One of the isoforms, brain aromatase (AroB), cyp19a1b, is highly expressed during early development of zebrafish, thereby suggesting its role in brain development. On the other hand, early development of serotonergic neuron, one of the major monoamine neurons, is considered to play an important role in neurogenesis. Therefore, in this study, we investigated the role of AroB in development of serotonergic neuron by testing the effects of (1) estradiol (E₂) exposure and (2) morpholino (MO)-mediated AroB knockdown. When embryos were exposed to E₂, the effects were biphasic. The low dose of E₂ (0.005 µM) significantly increased serotonin (5-HT) positive area at 48 hour post-fertilization (hpf) detected by immunohistochemistry and relative mRNA levels of tryptophan hydroxylase isoforms (tph1a, tph1b, and tph2) at 96 hpf measured by semi-quantitative PCR. To test the effects on serotonin transmission, heart rate and thigmotaxis, an indicator of anxiety, were analyzed. The low dose also significantly increased heart rate at 48 hpf and decreased thigmotaxis. The high dose of E₂ (1 µM) exhibited opposite effects in all parameters. The effects of both low and high doses were reversed by addition of estrogen receptor (ER) blocker, ICI 182,780, thereby suggesting that the effects were mediated through ER. When AroB MO was injected to fertilized eggs, 5-HT-positive area was significantly decreased, while the significant decrease in relative tph mRNA levels was found only with tph2 but not with two other isoforms. AroB MO also decreased heart rate and increased thigmotaxis. All the effects were rescued by co-injection with AroB mRNA and by exposure to E₂. Taken together, this study demonstrates the role of brain aromatase in development of serotonergic neuron in zebrafish embryos and larvae, implying that brain-formed estrogen is an important factor to sustain early development of serotonergic neuron.