Mummichog (Fundulus heteroclitus) are less sensitive to 17α-ethinylestradiol (EE2) than other common model teleosts: A comparative review of reproductive effects

Estetrol/drospirenone versus 17α-ethinylestradiol/drospirenone: An extended one generation test to evaluate the endocrine disruption potential on zebrafish (Danio rerio)

Combined oral contraceptives, comprising of both an oestrogen and a progestin component, are released in aquatic environments and potentially pose a risk to aquatic wildlife by their capacity to disrupt physiological mechanisms. In this study, the endocrine disruptive potential of two mixtures, 17α-ethinylestradiol (EE2), a synthetic oestrogen, or estetrol (E4), a natural oestrogen, with the progestin drospirenone (DRSP) have been characterised in three generations of zebrafish, according to an adapted Medaka Extended One Generation Reproduction Test. Zebrafish (Danio rerio) were exposed to a range of concentrations of EE2/DRSP and E4/DRSP (∼1×, ∼3×, ∼10× and ∼30× predicted environmental concentration, PEC). Survival, growth, hatching success, fecundity, fertilisation success, vitellogenin (VTG), gonad histopathology, sex differentiation, and transcriptional analysis of genes related to gonadal sex steroid hormones synthesis were assessed. In the F0 generation, exposure to EE2/DRSP at ∼10 and ∼30× PEC decreased fecundity and increased male VTG concentrations. The highest concentration of EE2/DRSP also affected VTG concentrations in female zebrafish and the expression of genes implicated in steroid hormones synthesis. In the F1 generation, sex determination was impaired in fish exposed to EE2/DRSP at concentrations as low as ∼3× PEC. Decreased fecundity and fertility, and abnormal gonadal histopathology were also observed. No effects were observed in the F2 generation. In contrast, E4/DRSP induced only minor histopathological changes and an increase in the proportion of males, at the highest concentration tested (∼30× PEC) in the F1 generation and had no effect on hatching success of F2 generation. Overall, this study suggests that the combination E4/DRSP has a more favourable environmental profile than EE2/DRSP.

Estetrol has a lower impact than 17α-ethinylestradiol on the reproductive capacity of zebrafish (Danio rerio)

Natural and synthetic oestrogens are commonly found in aquatic ecosystems. The synthetic oestrogen 17α-ethinylestradiol (EE2) is widely used in oral contraceptives and its ecotoxicological effects on aquatic organisms have been widely reported. The natural oestrogen estetrol (E4) was recently approved for use in a new combined oral contraceptive and, after therapeutic use, is likely to be found in the aquatic environment. However, its potential effects on non-target species such as fish is unknown. In order to characterize and compare the endocrine disruptive potential of E4 with EE2, zebrafish (Danio rerio) were exposed to E4 or EE2 in a fish short-term reproduction assay conducted according to OECD Test Guideline 229. Sexually mature male and female fish were exposed to a range of concentrations, including environmentally relevant concentrations of E4 and EE2, for 21 days. Endpoints included fecundity, fertilization success, gonad histopathology, head/tail vitellogenin concentrations, as well as transcriptional analysis of genes related to ovarian sex steroid hormones synthesis. Our data confirmed the strong impact of EE2 on several parameters including an inhibition of fecundity, an induction of vitellogenin both in male and female fish, an alteration of gonadal structures and the modulation of genes involved in sex steroid hormone synthesis in female fish. In contrast, only few significant effects were observed with E4 with no impact on fecundity. The results suggest that the natural oestrogen, E4, presents a more favorable environmental profile than EE2 and is less likely to affect fish reproductive capacity.

Chronic estrone exposure affects spermatogenesis and sperm quality in zebrafish (Danio rerio)

Estrone (E1) is a common environmental contaminant found in rivers and streams due to the farming of animals, such as swine and cattle. Our study evaluated the effects of chronic E1 exposure at environmentally relevant concentrations on spermatogenesis and the semen quality of zebrafish (Danio rerio). We exposed the fish to E1 at concentrations of 20, 200, and 2000 ng/L diluted in 0.001% ethanol (v/v) for 49 days. There were two control groups: one was exposed to water only and the other to ethanol at the same concentration used in the E1 groups. Following exposure, we analyzed the proportion of testicular cell types and other components (%), rate of cell proliferation and death, and sex steroid concentrations. Furthermore, we analyzed the expression of insulin-like growth factor 1 (IGF1), IGF2, IGF1 receptor (IGF1R), and inducible nitric oxide synthase and assessed the semen quality. E1 exposure increased spermatogonia, spermatids, Sertoli cells, Leydig cells, and the proportion of inflammatory infiltrate but decreased the spermatozoa amount. These changes were reflected by reductions in the gonadosomatic index and levels of 11-ketotestosterone in the testes. On the other hand, E1 exposure increased testicular estradiol, IGF1R expression, and nitric oxide production. After an evaluation using a computer-assisted sperm analysis (CASA) system, we observed reduced progressive motility, curvilinear velocity, and beat cross frequency of 20 and 2000 ng/L E1 groups. Our findings support that E1 causes deleterious effects on the testicular function and semen quality of D. rerio even at environmental concentrations. Thus, E1 concentrations should be monitored in surface waters for the purposes of fish conservation.

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.

Effects-based monitoring of bioactive compounds associated with municipal wastewater treatment plant effluent discharge to the South Platte River, Colorado, USA

Previous studies have detected numerous organic contaminants and in vitro bioactivities in surface water from the South Platte River near Denver, Colorado, USA. To evaluate the temporal and spatial distribution of selected contaminants of emerging concern, water samples were collected throughout 2018 and 2019 at 11 sites within the S. Platte River and surrounding tributaries with varying proximities to a major wastewater treatment plant (WWTP). Water samples were analyzed for pharmaceuticals, pesticides, steroid hormones, and wastewater indicators and screened for in vitro biological activities. Multiplexed, in vitro assays that simultaneously screen for agonistic activity against 24 human nuclear receptors detected estrogen receptor (ER), peroxisome proliferator activated receptor-gamma (PPARγ), and glucocorticoid receptor (GR) bioactivities in water samples near the WWTP outflow. Targeted in vitro bioassays assessing ER, GR, and PPARγ agonism corroborated bioactivities for ER (up to 55 ± 9.7 ng/L 17β-estradiol equivalents) and GR (up to 156 ± 28 ng/L dexamethasone equivalents), while PPARγ activity was not confirmed. To evaluate the potential in vivo significance of the bioactive contaminants, sexually-mature fathead minnows were caged at six locations upstream and downstream of the WWTP for 5 days after which targeted gene expression analyses were performed. Significant up-regulation of male hepatic vitellogenin was observed at sites with corresponding in vitro ER activity. No site-related differences in GR-related transcript abundance were detected in female adipose or male livers, suggesting observed environmental concentrations of GR-active contaminants do not induce a detectable in vivo response. In line with the lack of detectable targeted in vitro PPARɣ activity, there were no significant effects on PPARɣ-related gene expression. Although the chemicals responsible for GR and PPAR-mediated bioactivities are unknown, results from the present study provide insights into the significance (or lack thereof) of these bioactivities relative to short-term in situ fish exposures.

Development of omics biomarkers for estrogen exposure using mRNA, miRNA and piRNAs

The number of chemicals requiring risk evaluation exceeds our capacity to provide the underlying data using traditional methodology. This has led to an increased focus on the development of novel approach methodologies. This work aimed to expand the panel of gene expression-based biomarkers to include responses to estrogens, to identify training strategies that maximize the range of applicable concentrations, and to evaluate the potential for two classes of small non-coding RNAs (sncRNAs), microRNA (miRNA) and piwi-interacting RNA (piRNA), as biomarkers. To this end larval Pimephales promelas (96 hpf +/- 1h) were exposed to 5 concentrations of 17α- ethinylestradiol (0.12, 1.25, 2.5, 5.0, 10.0 ng/L) for 48 h. For mRNA-based biomarker development, RNA-seq was conducted across all concentrations. For sncRNA biomarkers, small RNA libraries were prepared only for the control and 10.0 ng/L EE2 treatment. In order to develop an mRNA classifier that remained accurate over the range of exposure concentrations, three different training strategies were employed that focused on 10 ng/L, 2.5 ng/L or a combination of both. Classifiers were tested against an independent test set of individuals exposed to the same concentrations used in training and subsequently against concentrations not included in model training. Both random forest (RF) and logistic regression with elastic net regularizations (glmnet) models trained on 10 ng/L EE2 performed poorly when applied to lower concentrations. RF models trained with either the 2.5 ng/L or combination (2.5 + 10 ng/L) treatments were able to accurately discriminate exposed vs. non-exposed across all but the lowest concentrations. glmnet models were unable to accurately classify below 5 ng/L. With the exception of the 10 ng/L treatment, few mRNA differentially expressed genes (DEG) were observed, however, there was marked overlap of DEGs across treatments. Overlapping DEGs have well established linkages to estrogen and several of the 81 DEGs identified in the 10 ng/L treatment have been previously utilized as estrogenic biomarkers (vitellogenin, estrogen receptor-β). Following multiple test correction, no sncRNAs were found to be differentially expressed, however, both miRNA and piRNA classifiers were able to accurately discriminate control and 10 ng/L exposed organisms with AUCs of 0.83 and 1.0 respectively. We have developed a highly discriminative estrogenic mRNA biomarker that is accurate over a range of concentrations likely to be found in real-world exposures. We have demonstrated that both miRNA and piRNA are responsive to estrogenic exposure, suggesting the need to further investigate their regulatory roles in the estrogenic response.