Analysis of zebrafish cyp19 promoters

The expression profiles of cyp19a1, sf-1, esrs and gths in the brain-pituitary during gonadal sex differentiation in juvenile Japanese eels

Eels are gonochoristic species whose gonadal differentiation initiates at the yellow eel stage and is influenced by environmental factors. We revealed some sex-related genes were sex dimorphically expressed in gonads during gonadal sex differentiation of Japanese eel (Anguilla japonica); however, the expression of sex-related genes in the brain-pituitary during gonadal sex differentiation in eels is still unclear. This study aimed to investigate the sex-related gene expressions in the brain-pituitary and tried to clarify their roles in the brain and gonads during gonadal sex differentiation. Based on our previous histological study, the control eels developed as males, and estradiol-17β (E2) was used for feminization. Our results showed that during testicular differentiation, the brain cyp19a1 transcripts and aromatase proteins were increased significantly; moreover, the cyp19a1, sf-1, foxl2s, and esrs (except gperb) transcripts in the midbrain/pituitary also were increased significantly. Forebrain gnrh1 transcripts increased slightly during gonadal differentiation of both sexes, but the gnrhr1b and gnrhr2 transcripts in the midbrain/pituitary were stable during gonadal differentiation. The expression levels of gths and gh in the midbrain/pituitary were significantly increased during testicular differentiation and were much higher in males than in E2-feminized females. These results implied that endogenous estrogens might play essential roles in the brain/pituitary during testicular differentiation, sf-1, foxl2s, and esrs may have roles in cyp19a1 regulation in the midbrain/pituitary of Japanese eels. For the GnRH-GTH axis, gths, especially fshb, may be regulated by esrs and involved in regulating testicular differentiation and development in Japanese eels.

Waterborne Tebuconazole Exposure Induces Male-Biased Sex Differentiation in Zebrafish (Danio rerio) Larvae via Aromatase Inhibition

Tebuconazole is a widely used fungicide for various crops that targets sterol 14-α-demethylase (CYP51) in fungi. However, attention has shifted to aromatase (CYP19) due to limited research indicating its reproductive impact on aquatic organisms. Herein, zebrafish were exposed to 0.5 mg/L tebuconazole at different developmental stages. The proportion of males increased significantly after long-term exposure during the sex differentiation phase (0-60, 5-60, and 19-60 days postfertilization (dpf)). Testosterone levels increased and 17β-estradiol and cyp19a1a expression levels decreased during the 5-60 dpf exposure, while the sex ratio was equally distributed on coexposure with 50 ng/L 17β-estradiol. Chemically activated luciferase gene expression bioassays determined that the male-biased sex differentiation was not caused by tebuconazole directly binding to sex hormone receptors. Protein expression and phosphorylation levels were specifically altered in the vascular endothelial growth factor signaling pathway despite excluding the possibility of tebuconazole directly interacting with kinases. Aromatase was selected for potential target analysis. Molecular docking and aromatase activity assays demonstrated the interactions between tebuconazole and aromatase, highlighting that tebuconazole poses a threat to fish populations by inducing a gender imbalance.

Tissue-specific promoters regulate the transcription of cyp19a1 in the brain-pituitary-gonad axis of Japanese eel (Anguilla japonica)

Aromatase is a key enzyme that catalyzes the biosynthesis of estrogens. Previous study indicated that putative tissue-specific promoters of the one aromatase gene (cyp19a1) may drive the differential regulatory mechanisms of cyp19a1 expression in Anguilla japonica. In the present study, for elucidating the transcription characteristics and the function of putative tissue-specific promoters of cyp19a1 in the brain-pituitary-gonad (BPG) axis during vitellogenesis, we investigated the transcriptional regulation of cyp19a1 by 17β-estrogen (E2), testosterone (T), or human chorionic gonadotropin (HCG) in A. japonica. The expression of estrogen receptor (esra), androgen receptor (ara), or luteinizing hormone receptor (lhr) was up-regulated as cyp19a1 in response to E2, T, or HCG, respectively in the telencephalon, diencephalon, and pituitary. The expression of cyp19a1 was also upregulated in the ovary by HCG or T in a dose-dependent manner. Unlike in the brain and pituitary, the expression of esra and lhr, rather than ara, was upregulated by T in the ovary. Subsequently, four primary subtypes of 5'-untranslated terminal regions of cyp19a1 transcripts and the corresponding two 5' flanking regions (promoter P.I and P.II) were identified. The P.II existed in all BPG axis tissues, whereas the P.I with strong transcriptional activity was brain- and pituitary-specific. Furthermore, the transcriptional activity of promoters, the core promoter region, and the three putative hormone receptor response elements were validated. The transcriptional activity did not change when the HEK291T cells co-transfected with P.II and ar vector were exposed to T. These results suggested that the expression of cyp19a1 was upregulated indirectly through esra and lhr rather than ara by T in the ovary, whereas the expression of cyp19a1 was upregulated directly through androgen receptor and the downstream androgen response element of tissue-specific P.I in the brain and pituitary. The results of the study reveal the regulatory mechanisms of estrogen biosynthesis and provide a reference for optimizing the technology of artificially induced maturation in eels.

Antagonistic mechanisms of bisphenol analogues on the estrogen receptor α in zebrafish embryos: Experimental and computational studies

Bisphenol A (BPA) can disturb the estrogen receptor α (ERα)-mediated signaling pathway, which results in endocrine-disrupting effects and reproductive toxicity. Most BPA analogues as alternatives were evidenced to generate estrogenic activity as agonists or partial agonists of ERα. Recent studies indicated that certain BPA analogues, such as bisphenol M (BPM), bisphenol P (BPP), and bisphenol FL (BPFL), exhibited strong anti-estrogenic effects comparable with the typical antagonist 4-hydroxytamoxifen. However, conflicting findings were also observed for the compounds in different in vitro assays, and whether these BPA analogues can elicit an in vivo effect on ERα at environmentally relevant concentrations remains unknown. The underlying structural basis of estrogenic/anti-estrogenic activity should be further elucidated at the atomic level. To address these issues, we combined zebrafish-based in vivo and in silico methods to assess the effects of the compounds on ERα. The results show that the expressions of ERα-mediated downstream related genes in zebrafish embryos decreased after exposed to the compounds. Further molecular dynamics simulations were used to probe the antagonistic mechanisms of the compounds on ERα. The key H-bonding interactions were identified as important ligand recognition by ERα in the analysis of binding modes and binding free energy calculations. In summary, the current study provides preliminary in vivo evidence of fish species for the anti-estrogenic activity of certain BPA analogues.

cAMP signaling in ovarian physiology in teleosts: A review

Ovarian function in teleosts, like in other vertebrates, is regulated by two distinct gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin effects are mediated by membrane-bound G protein-coupled receptors localized on the surface of follicle cells. Gonadotropin receptor activation results in increased intracellular cAMP, the most important second cellular signaling molecule. FSH stimulation induces the production of 17β-estradiol in the cells of growing follicles to promote vitellogenesis in oocytes. In contrast, in response to LH, fully grown post-vitellogenic follicles gain the ability to synthesize maturation-inducing steroids, which induce meiotic resumption and ovulation. All these events were induced downstream of cAMP. In this review, we summarize studies addressing the role of the cAMP pathway in gonadotropin-induced processes in teleost ovarian follicles. Furthermore, we discuss future problems concerning cAMP signaling in relation to teleost ovarian function and the differences and similarities in the gonadotropin-induced cAMP signaling pathways between mammals and teleosts.

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.

Estrogens regulate early embryonic development of the olfactory sensory system via estrogen-responsive glia

Estrogens are well-known to regulate development of sexual dimorphism of the brain; however, their role in embryonic brain development prior to sex-differentiation is unclear. Using estrogen biosensor zebrafish models, we found that estrogen activity in the embryonic brain occurs from early neurogenesis specifically in a type of glia in the olfactory bulb (OB), which we name estrogen-responsive olfactory bulb (EROB) cells. In response to estrogen, EROB cells overlay the outermost layer of the OB and interact tightly with olfactory sensory neurons at the olfactory glomeruli. Inhibiting estrogen activity using an estrogen receptor antagonist, ICI182,780 (ICI), and/or EROB cell ablation impedes olfactory glomerular development, including the topological organisation of olfactory glomeruli and inhibitory synaptogenesis in the OB. Furthermore, activation of estrogen signalling inhibits both intrinsic and olfaction-dependent neuronal activity in the OB, whereas ICI or EROB cell ablation results in the opposite effect on neuronal excitability. Altering the estrogen signalling disrupts olfaction-mediated behaviour in later larval stage. We propose that estrogens act on glia to regulate development of OB circuits, thereby modulating the local excitability in the OB and olfaction-mediated behaviour.

Profile of gene expression changes during estrodiol-17β-induced feminization in the Takifugu rubripes brain

Background

As the critical tissue of the central nervous system, the brain has been found to be involved in gonad development. Previous studies have suggested that gonadal fate may be affected by the brain. Identifying brain-specific molecular changes that occur during estrodiol-17β (E₂) -induced feminization is crucial to our understanding of the molecular control of sex differentiation by the brains of fish.

Results

In this study, the differential transcriptomic responses of the Takifugu rubripes larvae brain were compared after E₂ treatment for 55 days. Our results showed that 514 genes were differentially expressed between E₂-treated-XX (E-XX) and Control-XX (C-XX) T. rubripes, while 362 genes were differentially expressed between E₂-treated-XY (E-XY) and Control-XY (C-XY). For example, the expression of cyp19a1b, gnrh1 and pgr was significantly up-regulated, while st, sl, tshβ, prl and pit-1, which belong to the growth hormone/prolactin family, were significantly down-regulated after E₂ treatment, in both sexes. The arntl1, bhlbe, nr1d2, per1b, per3, cry1, cipc and ciart genes, which are involved in the circadian rhythm, were also found to be altered. Differentially expressed genes (DEGs), which were identified between E-XX and C-XX, were significantly enriched in neuroactive ligand-receptor interaction, arachidonic acid metabolism, cytokine-cytokine receptor interaction and the calcium signaling pathway. The DEGs that were identified between E-XY and C-XY were significantly enriched in tyrosine metabolism, phenylalanine metabolism, arachidonic acid metabolism and linoleic acid metabolism.

Conclusion

A number of genes and pathways were identified in the brain of E₂-treated T. rubripes larvae by RNA-seq. It provided the opportunity for further study on the possible involvement of networks in the brain-pituitary-gonadal axis in sex differentiation in T. rubripes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08158-0.

Regulation of aromatase expression: Potential therapeutic insight into breast cancer treatment

Estrogen signaling has been implicated in hormone-dependent breast cancer which constitutes >75% of breast cancer diagnosis and other malignancies. Aromatase, the key enzyme involved in the synthesis of estrogen, is often dysregulated in breast cancers. This has led to the administration of aromatase-inhibitors (AIs), commonly used for hormone-dependent breast cancers. Unfortunately, the increasing development of acquired resistance to the current AIs and modulators of estrogen receptors, following initial disease steadiness, has posed a serious clinical challenge in breast cancer treatment. In this review we highlight historical and recent advances on the transcriptional and post-translational regulation of aromatase in both physiological and pathological contexts. We also discuss the different drug combinations targeting various tumor promoting cell signaling pathways currently being developed and tested both in laboratory settings and in the clinic.

In Silico Prediction of Transcription Factor Collaborations Underlying Phenotypic Sexual Dimorphism in Zebrafish (Danio rerio)

The transcriptional regulation of gene expression in higher organisms is essential for different cellular and biological processes. These processes are controlled by transcription factors and their combinatorial interplay, which are crucial for complex genetic programs and transcriptional machinery. The regulation of sex-biased gene expression plays a major role in phenotypic sexual dimorphism in many species, causing dimorphic gene expression patterns between two different sexes. The role of transcription factor (TF) in gene regulatory mechanisms so far has not been studied for sex determination and sex-associated colour patterning in zebrafish with respect to phenotypic sexual dimorphism. To address this open biological issue, we applied bioinformatics approaches for identifying the predicted TF pairs based on their binding sites for sex and colour genes in zebrafish. In this study, we identified 25 (e.g., STAT6-GATA4; JUN-GATA4; SOX9-JUN) and 14 (e.g., IRF-STAT6; SOX9-JUN; STAT6-GATA4) potentially cooperating TFs based on their binding patterns in promoter regions for sex determination and colour pattern genes in zebrafish, respectively. The comparison between identified TFs for sex and colour genes revealed several predicted TF pairs (e.g., STAT6-GATA4; JUN-SOX9) are common for both phenotypes, which may play a pivotal role in phenotypic sexual dimorphism in zebrafish.

Disruption of classical estrogenic targets in brown trout primary hepatocytes by the model androgens testosterone and dihydrotestosterone

Estrogenic effects triggered by androgens have been previously shown in a few studies. Aromatization and direct binding to estrogen receptors (ERs) are the most proposed mechanisms. For example, previously, a modulation of vitellogenin A (VtgA) by testosterone (T), an aromatizable androgen, was reported in brown trout primary hepatocytes. The effect was reversed by an ER antagonist. In this study, using the same model the disruption caused by T and by the non-aromatizable androgen - dihydrotestosterone (DHT), was assessed in selected estrogenic targets. Hepatocytes were exposed (96 h) to six concentrations of each androgen. The estrogenic targets were VtgA, ERα, ERβ1 and two zona pellucida genes, ZP2.5 and ZP3a.2. The aromatase CYP19a1 gene and the androgen receptor (AR) were also included. Modulation of estrogenic targets was studied by quantitative real-time PCR and immunohistochemistry, using an HScore system. VtgA and ERα were up-regulated by DHT (1, 10, 100 μM) and T (10, 100 μM). In contrast, ERβ1 was down-regulated by DHT (10, 100 μM), and T (100 μM). ZP2.5 mRNA levels were increased by DHT and T (1, 10, 100 μM), while ZP3a.2 was up-regulated by DHT (100 μM) and T (10, 100 μM). Positive correlations were found between VtgA and ERα mRNA levels and ZPs and ERα, after exposure to both androgens. The mRNA levels of CYP19a1 were not changed, while AR expression tended to increase after micromolar DHT exposures. HScores for Vtg and ZPs corroborated the molecular findings. Both androgens triggered estrogen signaling through direct binding to ERs, most probably ERα.

Basal teleosts provide new insights into the evolutionary history of teleost-duplicated aromatase

Duplicated cyp19a1 genes (cyp19a1a encoding aromatase a and cyp19a1b encoding aromatase b) have been identified in an increasing number of teleost species. Cyp19a1a is mainly expressed in the gonads, while cyp19a1b is mainly expressed in the brain, specifically in radial glial cells, as largely investigated by Kah and collaborators. The third round of whole-genome duplication that specifically occurred in the teleost lineage (TWGD or 3R) is likely at the origin of the duplicated cyp19a1 paralogs. In contrast to the situation in other teleosts, our previous studies identified a single cyp19a1 in eels (Anguilla), which are representative species of a basal group of teleosts, Elopomorpha. In the present study, using genome data mining and phylogenetic and synteny analyses, we confirmed that the whole aromatase genomic region was duplicated in eels, with most aromatase-neighboring genes being conserved in duplicate in eels, as in other teleosts. These findings suggest that specific gene loss of one of the 3R-duplicated cyp19a1 paralogs occurred in Elopomorpha after TWGD. Similarly, a single cyp19a1 gene was found in the arowana, which is a representative species of another basal group of teleosts, Osteoglossomorpha. In eels, the single cyp19a1 is expressed in both the brain and the gonads, as observed for the single CYP19A1 gene present in other vertebrates. The results of phylogenetic, synteny, closest neighboring gene, and promoter structure analyses showed that the single cyp19a1 of the basal teleosts shared conserved properties with both teleost cyp19a1a and cyp19a1b paralogs, which did not allow us to conclude which of the 3R-duplicated paralogs (cyp19a1a or cyp19a1b) was lost in Elopomorpha. Elopomorpha and Osteoglossomorpha cyp19a1 genes exhibited preserved ancestral functions, including expression in both the gonad and brain. We propose that the subfunctionalization of the 3R-duplicated cyp19a1 paralogs expressed specifically in the gonad or brain occurred in Clupeocephala, after the split of Clupeocephala from Elopomorpha and Osteoglossomorpha, which represented a driving force for the conservation of both 3R-duplicated paralogs in all extant Clupeocephala. In contrast, the functional redundancy of the undifferentiated 3R-duplicated cyp19a1 paralogs in elopomorphs and osteoglossomorphs would have favored the loss of one 3R paralog in basal teleosts.

Refinement of an OECD test guideline for evaluating the effects of endocrine disrupting chemicals on aromatase gene expression and reproduction using novel transgenic cyp19a1a-eGFP zebrafish

Transgenic fish are powerful models that can provide mechanistic information regarding the endocrine activity of test chemicals. In this study, our objective was to use a newly developed transgenic zebrafish line expressing eGFP under the control of the cyp19a1a promoter in the OECD Fish Short Term Reproduction Assay (TG 229) to provide additional mechanistic information on tested substances. For this purpose, we exposed adult transgenic zebrafish to a reference substance of the TG 229, i.e. prochloraz (PCZ; 1.7, 17.2 and 172.6 μg/L). In addition to "classical" endpoints used in the TG 229 (reproductive outputs, vitellogenin), the fluorescence intensity of the ovaries was monitored at 4 different times of exposure using in vivo imaging. Our data revealed that 172.6 μg/L PCZ significantly decreased the number of eggs laid per female per day and the concentrations of vitellogenin in females, reflecting the decreasing E2 synthesis due to the inhibition of the ovarian aromatase activities. At 7 and 14 days, GFP intensities in ovaries were similar over the treatment groups but significantly increased after 21 days at 17.2 and 172.6 μg/L. A similar profile was observed for the endogenous cyp19a1a expression measured by qPCR thereby confirming the reliability of the GFP measurement for assessing aromatase gene expression. The overexpression of the cyp19a1a gene likely reflects a compensatory response to the inhibitory action of PCZ on aromatase enzymatic activities. Overall, this study illustrates the feasibility of using the cyp19a1a-eGFP transgenic line for assessing the effect of PCZ in an OECD test guideline while providing complementary information on the time- and concentration-dependent effects of the compound, without disturbing reproduction of fish. The acquisition of this additional mechanistic information on a key target gene through in vivo fluorescence imaging of the ovaries was realized without increasing the number of individuals.

Androstenedione and 17α-methyltestosterone induce early ovary development of Anguilla japonica

Endocrine effects as 11-ketotestosterone (11-KT), an unaromatizable androgen, regulating the follicles growth in the previtellogenic stage of eel reproduction have been widely elucidated. However, the influence of aromatizable androgens on the brain-pituitary-gonad axis during oogenesis in A. japonica has not been clearly elaborated. In the study, androstenedione (AD) and 17α-methyltestosterone (MT) were employed together to induce ovary development of seven-year-old female Anguilla japonica through feeding or exposure in the migration season. After female A. japonica had been fed with commercial diet containing 5 mg AD and MT kg d^-1 body weight respectively for 45 d in fresh water (Trial I), the development of oocytes still remained at the oil droplet stage, but the GSI and follicle diameter increased significantly. The serum 11-KT level and expression of liver vitellogenin mRNA were significantly elevated. After female fish had been exposed to seawater containing 50 μg L^-1 AD and MT respectively for 45 d (Trial II), the ovaries of A. japonica almost reached midvitellogenic stage and the GSI and follicle diameter increased significantly. Yolk granular layer was observed in the peripheral ooplasm. The serum 11-KT level maintained consistently low, and the serum E2 level declined significantly to a relatively low level. The expression levels of ovarian arα and cyp19a1, brain (with pituitary together) mGnRH and lhβ increased significantly. The results showed that A. japonica in Trial II appeared a higher ovarian development than those in Trial I. These findings indicated that AD and MT increased the oil droplet and enlarged follicle diameter in previtellogenic stage, while the vitellogenesis and gonadotropin release did not occur in Trial I. In Trial II, AD and MT promoted vitellogenesis by stimulating the ovary expression of arα and by up-regulating brain mGnRH and pituitary lhβ expression.

In vitro nuclear receptor activity and in vivo gene expression analysis in Murray-Darling rainbowfish (Melanotaenia fluviatilis) after short-term exposure to fluoxetine

Fluoxetine (FLX) is one of numerous pharmaceuticals found in treated municipal wastewater discharged to the environment. In the present study, we investigated the effects of short-term (96h) waterborne FLX exposure (1μg/L or 100μg/L) on the expression of selected genes in brain, liver, and gonads of female Murray-Darling rainbowfish (Melanotaenia fluviatilis), a small-bodied teleost of ecotoxicological relevance in the Australasia region. Plasma 17β-estradiol (E2) levels were also determined. In the brain, no significant changes in mRNA levels were observed for the selected genes. In ovaries, 100μg/L FLX caused a 10-fold downregulation of aromatase A (cyp19a1a) mRNA and a 4-fold upregulation of estrogen receptor α (esr1) mRNA levels. In liver, mRNA levels for vitellogenin A (vtga) and choriogenin L (chgl) were downregulated by 50-fold and 18-fold compared with controls, respectively, in response to 100μg/L FLX. Concentrations of E2 in plasma were significantly lower than controls in response to 100μg/L FLX. This could be attributable to a decrease in estrogen biosynthesis as a result of the observed downregulation of cyp19a1a mRNA. To establish whether the observed changes in gene expression could be explained by the modulation of selected nuclear receptors by FLX, we employed panel of reporter gene assays in agonistic and antagonistic modes. Apart from minor activation of ERα after exposure to high concentrations (5μM), FLX did not activate or inhibit the nuclear receptors tested. Further study is required to determine whether the observed downregulation of ovarian aromatase expression and liver estrogen-regulated genes also occurs at environmentally relevant FLX concentrations over longer exposure periods.

Mechanistic Evaluation of Benzo[a]pyrene's Developmental Toxicities Mediated by Reduced Cyp19a1b Activity

Benzo[a]pyrene (BaP) is a ubiquitous environmental contaminant that is both an endocrine disruptor and a carcinogen. Aromatase (CYP19) is a key enzyme in steroidogenesis that is responsible for conversion of androgens to estrogens and thus plays a key role in steroid homeostasis. We hypothesized that some of the adverse outcomes of early developmental exposure to BaP are the result of reduced Cyp19a1b activity. Our goal was to investigate the role of estrogen homeostasis during early development and determine the role of aromatase inhibition as a relevant mechanism in BaP's developmental toxicities. One-cell zebrafish embryos were injected with a Cyp19a1b-morpholino (MO) or control-MO. Other non-injected embryos were exposed to waterborne BaP, fadrozole (a Cyp19 inhibitor), estradiol (E2), BaP + E2, Cyp19a1b MO + E2, or fadrozole + E2 for 96 hours post-fertilization (hpf). Adverse outcomes were compared between treatments, and the ability of E2 co-exposure to rescue each observed dysmorphology was assessed. BaP significantly decreased cyp19a1b gene expression in 96 hpf zebrafish larvae homogenates. Concentrations of E2 in 48 hpf larvae were significantly decreased by BaP, fadrozole and Cyp19a1b-MO. Cumulative mortality of zebrafish larvae was significantly increased following BaP or fadrozole exposure or Cyp19a1b knockdown compared to controls. E2 co-treatment rescued mortality caused by 10 μg/L BaP, 10 μg/L fadrozole, or Cyp19a1b-MO. In a treatment-blinded morphological assessment of larvae at 96 hpf, several phenotypes were negatively impacted by BaP, fadrozole, or Cyp19a1b knockdown and rescued by exogenous E2 co-treatment; these included body length, optic vesicle size, swim bladder inflation, pericardial and abdominal edema, and incidence of normal larval tail shape. Abnormal pectoral fins were caused by BaP exposure only. Uninflated swim bladders were caused by all treatments including E2 alone. Our results indicate that certain BaP-mediated adverse developmental outcomes were mechanistically in accordance with BaP-mediated Cyp19a1b inhibition.

Gonadotropins in European sea bass: Endocrine roles and biotechnological applications

Follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) are central endocrine regulators of the gonadal function in vertebrates. They act through specific receptors located in certain cell types found in the gonads. In fish, the differential roles of these hormones are being progressively elucidated due to the development of suitable tools for their study. In European sea bass (Dicentrarchus labrax), isolation of the genes coding for the gonadotropin subunits and receptors allowed in first instance to conduct expression studies. Later, to overcome the limitation of using native hormones, recombinant dimeric gonadotropins, which show different functional characteristics depending on the cell system and DNA construct, were generated. In addition, single gonadotropin beta-subunits have been produced and used as antigens for antibody production. This approach has allowed the development of detection methods for native gonadotropins, with European sea bass being one of the few species where both gonadotropins can be detected in their native form. By administering recombinant gonadotropins to gonad tissues in vitro, we were able to study their effects on steroidogenesis and intracellular pathways. Their administration in vivo has also been tested for use in basic studies and as a biotechnological approach for hormone therapy and assisted reproduction strategies. In addition to the production of recombinant hormones, gene-based therapies using somatic gene transfer have been offered as an alternative. This approach has been tested in sea bass for gonadotropin delivery in vivo. The hormones produced by the genes injected were functional and have allowed studies on the action of gonadotropins in spermatogenesis.

Dopamine D1 receptor activation regulates the expression of the estrogen synthesis gene aromatase B in radial glial cells

Radial glial cells (RGCs) are abundant stem-like non-neuronal progenitors that are important for adult neurogenesis and brain repair, yet little is known about their regulation by neurotransmitters. Here we provide evidence for neuronal-glial interactions via a novel role for dopamine to stimulate RGC function. Goldfish were chosen as the model organism due to the abundance of RGCs and regenerative abilities of the adult central nervous system. A close anatomical relationship was observed between tyrosine hydroxylase-positive catecholaminergic cell bodies and axons and dopamine-D1 receptor expressing RGCs along the ventricular surface of telencephalon, a site of active neurogenesis. A primary cell culture model was established and immunofluorescence analysis indicates that in vitro RGCs from female goldfish retain their major characteristics in vivo, including expression of glial fibrillary acidic protein and brain lipid binding protein. The estrogen synthesis enzyme aromatase B is exclusively found in RGCs, but this is lost as cells differentiate to neurons and other glial types in adult teleost brain. Pharmacological experiments using the cultured RGCs established that specific activation of dopamine D1 receptors up-regulates aromatase B mRNA through a cyclic adenosine monophosphate-dependent molecular mechanism. These data indicate that dopamine enhances the steroidogenic function of this neuronal progenitor cell.

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.

Expression patterns of cytochrome P450 3B and 3C genes in model fish species

Cytochrome P450 (CYP) 3 enzymes are highly expressed in detoxification organs and play an important role in xenobiotic metabolism. In fish, the CYP3 family is diversified and includes several subfamilies (CYP3B, CYP3C, and CYP3D) not found in mammals. The functional role and expression patterns of these novel genes are unknown. In this study, the expression patterns of novel teleost CYP3 genes were determined in medaka(Oryzias latipes; CYP3B4, CYP3B5, CYP3B6) and zebrafish (Danio rerio; CYP3C1, CYP3C2, CYP3C3, CYP3C4), two important model fish species. Expression was quantified with real time PCR in multiple internal organs from adult male and female fish. CYP3C gene expression was determined in zebrafish embryos. Expression in all organs was detected for all genes, except for CYP3B4 in male organs. CYP3C1, CYP3C3, CYP3B4, CYP3B5, and CYP3B6 were more highly expressed in liver and/or intestine from at least one gender, suggesting a role in xenobiotic metabolism. Expression of CYP3C1 and CYP3B5 in olfactory rosette was comparable to liver. CYP3C1, CYP3C4, CYP3B5 and CYP3B6 expression was higher in the female organs; CYP3C2 and CYP3B5 were higher in testis. Estrogen and androgen response elements were found upstream of the start site of many of these genes raising the hypothesis that they are under steroid regulation. CYP3C1-3 were expressed in all developmental stages examined and appear to be maternally deposited. The expression patterns suggest that some of these CYP genes are involved in xenobiotic metabolism.

Genes encoding aromatases in teleosts: evolution and expression regulation

Cytochrome P450 aromatases, encoded by cyp19a1 genes, catalyzes the conversion of androgens to estrogens and plays important roles in the reproduction of vertebrates. Vertebrate cyp19a1 genes showed high synteny in chromosomal locations and conservation in sequences during evolution. However, amphioxus cyp19a1 does not show synteny to vertebrate cyp19a1. Teleost fish possess two copies of the cyp19a1 gene, which were postulated to result from a fish-specific genome duplication. The duplicated copies of fish cyp19a1 genes evolved into the brain and ovarian forms of cytochrome P450 aromatase genes, cyp19a1a and cyp19a1b, respectively, with different regulatory mechanisms of expression, through subfunctionalization under long-term selective pressure. In addition to the estradiol (E2) auto-regulatory loop, there may be other mechanisms responsible for the high expression of aromatase in the teleost brain. The study of the two cyp19a1 copies in teleost fish will shed light on the general evolution, function, and regulation of vertebrate cyp19a1.

Molecular characterization and expression of cyp19a gene in Carassius auratus

A cyp19a gene that contains nine exons and eight introns was identified from Carassius auratus and was mainly expressed in the ovary. The cyp19a mRNA level after hatching was initially low, but began to increase from 25 days after hatching. A number of cis-acting elements, such as the oestrogen receptor, steroidogenic factor 1 and SOX-5 recognition sites, were found in the promoter of the cyp19 gene, which possesses a promoter function confirmed by a recombination green fluorescent protein checking system in vitro.

Mechanisms of crosstalk between endocrine systems: regulation of sex steroid hormone synthesis and action by thyroid hormones

Thyroid hormones (THs) are well-known regulators of development and metabolism in vertebrates. There is increasing evidence that THs are also involved in gonadal differentiation and reproductive function. Changes in TH status affect sex ratios in developing fish and frogs and reproduction (e.g., fertility), hormone levels, and gonad morphology in adults of species of different vertebrates. In this review, we have summarized and compared the evidence for cross-talk between the steroid hormone and thyroid axes and present a comparative model. We gave special attention to TH regulation of sex steroid synthesis and action in both the brain and gonad, since these are important for gonad development and brain sexual differentiation and have been studied in many species. We also reviewed research showing that there is a TH system, including receptors and enzymes, in the brains and gonads in developing and adult vertebrates. Our analysis shows that THs influences sex steroid hormone synthesis in vertebrates, ranging from fish to pigs. This concept of crosstalk and conserved hormone interaction has implications for our understanding of the role of THs in reproduction, and how these processes may be dysregulated by environmental endocrine disruptors.

Molecular players involved in temperature-dependent sex determination and sex differentiation in Teleost fish

The molecular mechanisms that underlie sex determination and differentiation are conserved and diversified. In fish species, temperature-dependent sex determination and differentiation seem to be ubiquitous and molecular players involved in these mechanisms may be conserved. Although how the ambient temperature transduces signals to the undifferentiated gonads remains to be elucidated, the genes downstream in the sex differentiation pathway are shared between sex-determining mechanisms. In this paper, we review recent advances on the molecular players that participate in the sex determination and differentiation in fish species, by putting emphasis on temperature-dependent sex determination and differentiation, which include temperature-dependent sex determination and genetic sex determination plus temperature effects. Application of temperature-dependent sex differentiation in farmed fish and the consequences of temperature-induced sex reversal are discussed.

Exposure to DEHP and MEHP from hatching to adulthood causes reproductive dysfunction and endocrine disruption in marine medaka (Oryzias melastigma)

Concern has increased regarding the adverse effects of di-(2-ethylhexyl)-phthalate (DEHP) on reproduction. However, limited information is available on the effects of DEHP in marine organisms. The aim of the present study was to examine whether long-term exposure to DEHP and its active metabolite mono-(2-ethylhexyl)-phthalate (MEHP) disrupts endocrine function in marine medaka (Oryzias melastigma). Marine medaka larvae were exposed to either DEHP (0.1 and 0.5mg/L) or MEHP (0.1 and 0.5mg/L) for 6 months, and the effects on reproduction, sex steroid hormones, liver vitellogenin (VTG), gonad histology and the expression of genes involved in the hypothalamic-pituitary-gonad (HPG) axis were investigated. Exposure to DEHP, but not MEHP, from hatching to adulthood accelerated the start of spawning and decreased the egg production of exposed females. Moreover, exposure to both DEHP and MEHP resulted in a reduction in the fertilization rate of oocytes spawned by untreated females paired with treated males. A significant increase in plasma 17β-estradiol (E2) along with a significant decrease in testosterone (T)/E2 ratios was observed in males, which was accompanied by the upregulation of ldlr, star, cyp17a1, 17βhsd, and cyp19a transcription in the testis. Increased concentrations of T and E2 were observed in females, which was consistent with the upregulation of ldlr. The expression of brain gnrhr2, fshβ, cyp19b and steroid hormone receptor genes also corresponded well with hormonal and reproductive changes. The liver VTG level was significantly increased after DEHP and MEHP exposure in males. DEHP induced histological changes in the testes and ovaries: the testes displayed a reduced number of spermatozoa, and the ovaries displayed an increased number of atretic follicles. In addition, the tissue concentrations of MEHP, MEHHP and MEOHP in DEHP-exposed groups were much higher than those in MEHP-exposed groups, and there were no dose- or sex-specific effects. Thus, DEHP exerts more obvious toxic effects compared with MEHP. There were some commonalities in the toxic effects and molecular mechanisms of DEHP and MEHP, suggesting that some of the toxic effects of DEHP may be induced by both DEHP itself and DEHP metabolites (including MEHP). Taken together, these results indicate that exposure to DEHP and MEHP from hatching to adulthood causes endocrine disruption with sex-specific effects in marine medaka, with males being more sensitive than females.

Expression and sequence evolution of aromatase cyp19a1 and other sexual development genes in East African cichlid fishes

Sex determination mechanisms are highly variable across teleost fishes and sexual development is often plastic. Nevertheless, downstream factors establishing the two sexes are presumably conserved. Here, we study sequence evolution and gene expression of core genes of sexual development in a prime model system in evolutionary biology, the East African cichlid fishes. Using the available five cichlid genomes, we test for signs of positive selection in 28 genes including duplicates from the teleost whole-genome duplication, and examine the expression of these candidate genes in three cichlid species. We then focus on a particularly striking case, the A- and B-copies of the aromatase cyp19a1, and detect different evolutionary trajectories: cyp19a1A evolved under strong positive selection, whereas cyp19a1B remained conserved at the protein level, yet is subject to regulatory changes at its transcription start sites. Importantly, we find shifts in gene expression in both copies. Cyp19a1 is considered the most conserved ovary-factor in vertebrates, and in all teleosts investigated so far, cyp19a1A and cyp19a1B are expressed in ovaries and the brain, respectively. This is not the case in cichlids, where we find new expression patterns in two derived lineages: the A-copy gained a novel testis-function in the Ectodine lineage, whereas the B-copy is overexpressed in the testis of the speciest-richest cichlid group, the Haplochromini. This suggests that even key factors of sexual development, including the sex steroid pathway, are not conserved in fish, supporting the idea that flexibility in sexual determination and differentiation may be a driving force of speciation.

Epigenetic control of gonadal aromatase (cyp19a1) in temperature-dependent sex determination of red-eared slider turtles

In the red-eared slider turtle (Trachemys scripta), a species with temperature-dependent sex determination (TSD), the expression of the aromatase gene during gonad development is strictly limited to the female-producing temperature. The underlying mechanism remains unknown. In this study, we identified the upstream 5'-flanking region of the aromatase gene, gonad-specific promoter, and the temperature-dependent DNA methylation signatures during gonad development in the red-eared slider turtle. The 5'-flanking region of the slider aromatase exhibited sequence similarities to the aromatase genes of the American alligator, chicken, quail, and zebra finch. A putative TATA box was located 31 bp upstream of the gonad-specific transcription start site. DNA methylation at the CpG sites between the putative binding sites of the fork head domain factor (FOX) and vertebrate steroidogenic factor 1 (SF1) and adjacent TATA box in the promoter region were significantly lower in embryonic gonads at the female-producing temperature compared the male-producing temperature. A shift from male- to female-, but not from female- to male-, producing temperature changed the level of DNA methylation in gonads. Taken together these results indicate that the temperature, particularly female-producing temperature, allows demethylation at the specific CpG sites of the promoter region which leads the temperature-specific expression of aromatase during gonad development.

Neurosteroidogenic enzymes and their regulation in the early brain of the protogynous grouper Epinephelus coioides during gonadal sex differentiation

The regulatory role of neurosteroids in the early brain during gonadal sex differentiation is unclear. The aim of this study was to investigate the expression and cellular localization of key steroidogenic enzymes in the early brain of the protogynous orange-spotted grouper Epinephelus coioides and the temporal expressions has been correlated with gonadal sex differentiation. In this study, we showed that peak neurosteroidogenesis occurs in the early brain during gonadal sex differentiation. The temporal expressions of key enzymes, cyp11a1 (cytochrome P450 side chain cleavage), hsd3b1 (3β-hydroxysteroid dehydrogenase) and cyp17a1 (cytochrome P450c17) were studied at different developmental ages (from 90- to 150-dah: days after hatching) using quantitative real-time PCR (q-PCR). q-PCR analysis indicated that the transcript expressions of cyp11a1, hsd3b1 and cyp17a1 were increased in the brain around the period of gonadal sex differentiation. Further, in situ hybridization (ISH) analysis showed that cyp11a1, hsd3b1 and cyp17a1 transcripts were widely expressed in several discrete brain regions, especially the intense expression in the forebrain, with an overall similar expression pattern. High density in the cyp19a1b/Cyp19a1b expression was detected in radial glial cells. Thus, the expression of grouper cyp19a1b/Cyp19a1b is restricted to radial glial cells, suggesting estrogens can modulate their activity. Next, by combining Cyp19a1b immunohistochemistry (IHC) with florescence ISH (FISH) of cyp11a1, hsd3b1 and cyp17a1, we showed that sub-cellular localization of cyp11a1, hsd3b1 and cyp17a1 transcripts, in partial, appeared to be in Cyp19a1b radial glial cell soma. Further, exogenous estradiol (E(2)) increased the expression of cyp17a1 and cyp19a1b/Cyp19a1b in the brain of grouper. Consequently, our results illustrated that the locally synthesized E(2) upregulated neurosteroidogenic enzymes in the early brain and suggest a role for these enzymes in the neurogenic process during gonadal sex differentiation.

Creating females? Developmental effects of 17α-ethynylestradiol on the mangrove rivulus' ovotestis

Interest in the occurrence and fate of trace organic contaminants in the aquatic environment and their potential effects on all organisms has increased over the past two decades. Researches on contaminants have included both natural and synthetic estrogenic contaminants, neuroactive pharmaceuticals, and other endocrine disrupting chemicals that are mediated by the androgen and progesterone receptors. Exposure to very low concentrations (ng/L or parts per trillion) of compounds such as 17α-ethynylestradiol (EE(2)), a synthetic estrogen, can affect gonadal development, viability and production of eggs, fertilization rate, and sexual differentiation in fishes. Researchers and aquaculturists have used exposures to relatively higher concentrations of androgens and estrogens, for example 17α-methyltestosterone and EE(2), respectively, to direct sexual differentiation in a number of fishes. Rivulus is an androdioecious teleost that in nature exists mostly as selfing, simultaneous hermaphrodites as well as a small number of males that outcross with hermaphrodites. No one has either collected females in the wild or created functional females in the laboratory. This study had two goals: (1) to develop a reliable protocol to produce female rivulus to enable downstream technologies such as embryo injections and (2) to investigate developmental effects of EE(2) on the sexual outcome, reproductive health, and relevant gene expression in rivulus. With these goals in mind, we exposed newly hatched rivulus to nominal concentrations of 0.1, 0.5, or 1.0 parts per million (ppm) EE(2) for 4 weeks, grew them to maturity in control water, and then compared egg production; production and viability of embryos; age of reproductive maturity; and gene expression in the brain, gonad, and liver. Expression levels of seven genes with known relevance to gonadal development and function (cyp19a1b, cyp19a1a, dmrt1, figα, ERα, ERβ, and vtg) were measured using quantitative polymerase chain reaction (PCR). There was a significant decrease in cyp19a1a gene expression in the brain, corresponding to increased exposure to EE(2). Gonadal gene expression for cyp19a1a, ERα, and dmrt1 also decreased in response to EE(2). Vtg expression in the liver was unaffected. Our hypothesis that exposure to EE(2) during gonadal differentiation would direct female development was not supported by the data. However, treated fish exhibited impaired reproductive health that included reduced expression of relevant genes and, importantly, decreased fertility, increased sterility, and delay of age of reproductive maturity. The results of this study suggest that the development and maintenance of a simultaneous hermphrodite ovotestis may be particularly sensitive to its hormonal milieu.

Androgen rather than estrogen up-regulates brain-type cytochrome P450 aromatase (cyp19a1b) gene via tissue-specific promoters in the hermaphrodite teleost ricefield eel Monopterus albus

CYP19A1 in the brain and pituitary of vertebrates is important for reproductive and non-reproductive processes. In teleosts, it is broadly accepted that estradiol (E(2)) up-regulates cyp19a1b gene via a positive autoregulatory loop. Our present study, however, showed that E(2) did not up-regulate ricefield eel cyp19a1b in the hypothalamus and pituitary, whereas dihydrotestosterone (DHT) or testosterone (T) stimulated cyp19a1b expression only in the pituitary. Two tissue-specific promoters, namely promoter I and II directing the expression in the brain and pituitary respectively, were identified. Promoter I contained a non-consensus estrogen response element (ERE), and consequently did not respond to E(2). Promoter II contained an androgen response element (ARE) and consequently responded to DHT. Taken together, these results demonstrated a novel steroidal regulation of cyp19a1b gene expression and an alternative usage of tissue-specific cyp19a1b promoters in the brain and pituitary of a teleost species, the ricefield eel.

Isolation and characterization of cyp19a1a and cyp19a1b promoters in the protogynous hermaphrodite orange-spotted grouper (Epinephelus coioides)

Aromatase (CYP19A1) catalyzes the conversion of androgens to estrogens. In teleosts, duplicated copies of cyp19a1 genes, namely cyp19a1a and cyp19a1b, were identified, however, the transcriptional regulation of these two genes remains poorly understood. In the present study, the 5'-flanking regions of the orange-spotted grouper cyp19a1a (gcyp19a1a) and cyp19a1b (gcyp19a1b) genes were isolated and characterized. The proximal promoter regions of both genes were relatively conserved when compared to those of the other teleosts. Notably, a conserved FOXO transcriptional factor binding site was firstly reported in the proximal promoter of gcyp19a1a, and deletion of the region (-112 to -60) containing this site significantly decreased the promoter activities. The deletion of the region (-246 to -112) containing the two conserved FTZ-F1 sites also dramatically decreased the transcriptional activities of gcyp19a1a promoter, and both two FTZ-F1 sites were shown to be stimulatory cis-acting elements. A FTZ-F1 homologue isolated from ricefield eel (eFTZ-F1) up-regulated gcyp19a1a promoter activities possibly via the FTZ-F1 sites, however, a previously identified orange-spotted grouper FTZ-F1 homologue (gFTZ-F1) did not activate the transcription of gcyp19a1a promoter unexpectedly. As to gcyp19a1b promoter, all the deletion constructs did not show good promoter activities in either TM4 or U251-MG cells. Estradiol (100nM) up-regulated gcyp19a1b promoter activities by about 13- and 36-fold in TM4 and U251-MG cells, respectively, via the conserved ERE motif, but did not stimulate gcyp19a1a promoter activities. These results are helpful to further elucidate the regulatory mechanisms of cyp19a1a and cyp19a1b expression in the orange-spotted grouper as well as other teleosts.

Differential regulation of the expression of cytochrome P450 aromatase, estrogen and androgen receptor subtypes in the brain-pituitary-ovarian axis of the Japanese eel (Anguilla japonica) reveals steroid dependent and independent mechanisms

This study aimed at investigating the role of sexual steroids in the regulation of the expression of the single aromatase gene and steroid receptor subtypes in the brain-pituitary-ovarian axis of the Japanese eel. Unlike other teleosts, which possess duplicated genes for aromatase, cyp19a1a and cyp19a1b, expressed in the gonads and in the brain, respectively, eel species possess a single cyp19a1. Phylogenetic analysis indicated that eel brain/gonadal cyp19a1 branches at the basis of both teleost gonadal cyp19a1a and brain cyp19a1b clades. Female eels treated with catfish pituitary homogenate (CPH) to induce sexual maturation showed an increase in the expression of cyp19a1 and aromatase enzymatic activity in the brain and in the ovaries. Treatments with sex steroids (estradiol-17β, E(2) or testosterone, T) revealed that the increase in cyp19a1 expression in the brain may result from E(2)-specific induction. In contrast, the increase in cyp19a1 expression in the ovaries of CPH-treated eels is a result of steroid-independent control, probably from a direct effect of gonadotropins contained in the pituitary extract. Analysis of the expression of estrogen and androgen receptor subtypes, esr-α, esr-β, ar-α and ar-β, in eels treated with CPH or sex steroids revealed differential regulations. In CPH-treated eels, the expression of esr-α and ar-α was significantly increased in the brain, while the expression of ar-α and ar-β was increased in the ovaries. No change was observed in esr-β in any organ. Steroid treatments induced an upregulation by E(2) of esr-α, but not esr-β expression, in the brain, pituitary and ovaries, while no autoregulation by T of its own receptors could be observed. These results reveal both steroid-dependent and -independent mechanisms in the regulation of cyp19a1 and steroid receptor subtype expression in the eel.

Evolutionary origins of the estrogen signaling system: insights from amphioxus

Classically, the estrogen signaling system has two core components: cytochrome P450 aromatase (CYP19), the enzyme complex that catalyzes the rate limiting step in estrogen biosynthesis; and estrogen receptors (ERs), ligand activated transcription factors that interact with the regulatory region of target genes to mediate the biological effects of estrogen. While the importance of estrogens for regulation of reproduction, development and physiology has been well-documented in gnathostome vertebrates, the evolutionary origins of estrogen as a hormone are still unclear. As invertebrates within the phylum Chordata, cephalochordates (e.g., the amphioxus of the genus Branchiostoma) are among the closest invertebrate relatives of the vertebrates and can provide critical insight into the evolution of vertebrate-specific molecules and pathways. To address this question, this paper briefly reviews relevant earlier studies that help to illuminate the history of the aromatase and ER genes, with a particular emphasis on insights from amphioxus and other invertebrates. We then present new analyses of amphioxus aromatase and ER sequence and function, including an in silico model of the amphioxus aromatase protein, and CYP19 gene analysis. CYP19 shares a conserved gene structure with vertebrates (9 coding exons) and moderate sequence conservation (40% amino acid identity with human CYP19). Modeling of the amphioxus aromatase substrate binding site and simulated docking of androstenedione in comparison to the human aromatase shows that the substrate binding site is conserved and predicts that androstenedione could be a substrate for amphioxus CYP19. The amphioxus ER is structurally similar to vertebrate ERs, but differs in sequence and key residues of the ligand binding domain. Consistent with results from other laboratories, amphioxus ER did not bind radiolabeled estradiol, nor did it modulate gene expression on an estrogen-responsive element (ERE) in the presence of estradiol, 4-hydroxytamoxifen, diethylstilbestrol, bisphenol A or genistein. Interestingly, it has been shown that a related gene, the amphioxus "steroid receptor" (SR), can be activated by estrogens and that amphioxus ER can repress this activation. CYP19, ER and SR are all primarily expressed in gonadal tissue, suggesting an ancient paracrine/autocrine signaling role, but it is not yet known how their expression is regulated and, if estrogen is actually synthesized in amphioxus, whether it has a role in mediating any biological effects. Functional studies are clearly needed to link emerging bioinformatics and in vitro molecular biology results with organismal physiology to develop an understanding of the evolution of estrogen signaling. This article is part of a Special Issue entitled 'Marine organisms'.

A review of studies on androgen and estrogen exposure in fish early life stages: effects on gene and hormonal control of sexual differentiation

Teleost fish are unique among vertebrates in that phenotypic sex or onset of sex inversion can be easily manipulated by hormonal treatments. In recent years, researchers have begun reporting concentrations of synthetic and natural hormones in the environment. Although concentrations are very low (in the parts per trillion to low parts per billion), they are still of concern because of the high potency of synthetic hormones and the enhanced susceptibility of teleost fishes, especially early life stages, to hormonal exposures. In this review, we will focus on sex differentiation in teleost fishes and how these processes in fish early life stages may be impacted by environmental hormones which are known to contaminate aquatic environments. We will start by reviewing information on sources and concentrations of hormones in the environment and continue by summarizing the state of knowledge of sex differentiation in teleost gonochoristic fishes, including information on genes involved (e.g. cyp19, dmrt1, sox9 and foxl2). We will end our review with a summary of studies that have examined the effects of androgens and estrogens on fish sex differentiation after exposure of fish embryos and larvae and with ideas for future research.

5α-Dihydrotestosterone is a potent androgen in the fathead minnow (Pimephales promelas)

Dihydrotestosterone (DHT) is one of the most physiologically important androgens in many male vertebrates, with the exception of teleost fish, in which 11-ketotestosterone (KT) is generally considered the major circulating male androgen. In the present study, we investigated the effects of KT and DHT on fathead minnow juveniles (Pimephales promelas), with the aim to compare the effects of the two androgens on critical physiological processes, such as somatic growth, male secondary sexual characteristics expression, and gonad maturation. Juvenile fish (60 days post-hatch) were exposed to 20 and 200 ng/L of KT and DHT for 45 days. Exposure to both androgens significantly stimulated somatic growth in both males (20 and 200 ng/L) and females (200 ng/L). Nuptial tubercle formation was induced by both KT and DHT, but only the latter, at 200 ng/L, caused the appearance of dorsal fin spot in 92% of males and 75% of females. Circulating plasma T concentrations showed a sex-specific response; a significant increase was recorded in exposed males and a decrease in females. Both androgens induced a significant advancement of the spermatogenic processes in males at 200 ng/L. In contrast, only DHT caused a severe disruption of ovarian physiology and morphology in females, inducing the development of spermatogenic tissue (intersex). These results show that in fathead minnow juveniles, DHT had in vivo androgenic potency comparable to KT in males, and higher than KT in females, suggesting a potential involvement of DHT in the mediation of fathead minnow androgenic responses.

Gene expression profiling of the androgen receptor antagonists flutamide and vinclozolin in zebrafish (Danio rerio) gonads

The studies presented in this manuscript focus on characterization of transcriptomic responses to anti-androgens in zebrafish (Danio rerio). Research on the effects of anti-androgens in fish has been characterized by a heavy reliance on apical endpoints, and molecular mechanisms of action (MOA) of anti-androgens remain poorly elucidated. In the present study, we examined effects of a short term exposure (24-96h) to the androgen receptor antagonists flutamide (FLU) and vinclozolin (VZ) on gene expression in gonads of sexually mature zebrafish, using commercially available zebrafish oligonucleotide microarrays (4×44K platform). We found that VZ and FLU potentially impact reproductive processes via multiple pathways related to steroidogenesis, spermatogenesis, and fertilization. Observed changes in gene expression often were shared by VZ and FLU, as demonstrated by overlap in differentially-expressed genes and enrichment of several common key pathways including: (1) integrin and actin signaling, (2) nuclear receptor 5A1 signaling, (3) fibroblast growth factor receptor signaling, (4) polyamine synthesis, and (5) androgen synthesis. This information should prove useful to elucidating specific mechanisms of reproductive effects of anti-androgens in fish, as well as developing biomarkers for this important class of endocrine-active chemicals.

Neuroendocrine effects of endocrine disruptors in teleost fish

Because a large proportion of potential endocrine disruptors (EDC) end up in surface waters, aquatic species are particularly vulnerable to their potential adverse effects. Recent studies identified a number of brain targets for EDC commonly present in environmentally relevant concentrations in surface waters. Among those neuronal systems disrupted by EDC are the gonadotropin-releasing hormone (GnRH) neurons, the dopaminergic and serotoninergic circuits, and more recently the Kiss/GPR54 system, which regulates gonadotropin release. However, one of the most striking effects of EDC, notably estrogen mimics, is their impact on the cyp19a1b gene that encodes the brain aromatase isoform in fish. Moreover, this is the only example in which the molecular basis of endocrine disruption is fully understood. The aims of this review were to (1) synthesize the most recent discoveries concerning the EDC effects upon neuroendocrine systems of fish and (2) provide, when possible, the underlying molecular basis of disruption for each system concerned. The potential adverse effects of EDC on neurogenesis, puberty, and brain sexualization are also described. It is important to point out the future environmental, social, and economical issues arising from endocrine disruption studies in the context of risk assessment.

Aromatase mRNA expression in the brain of adult Xenopus laevis exposed to Lambro river water and endocrine disrupting compounds

Aromatase P450 (P450 arom; Cyp19) is a key enzyme for vertebrate reproduction and brain development that catalyzes the conversion of androgens to estrogens. The aim of this study was to improve the knowledge on EDC effects by analysing their potential impact on brain P450 arom in adult Xenopus laevis exposed for 4 weeks to an environmental sample, the water of the river Lambro (LAM), the most polluted tributary of the Po river in North Italy. Other groups were exposed to individual compounds 10(-8) M tamoxifen (TAM), ethinylestradiol (EE2), flutamide (FLU) and methyldihydrotestosterone (MDHT) known for their (anti)estrogenic and (anti)androgenic modes of action. Expression of CYP19 was evaluated in brain extracts by quantitative RT-PCR, using a pair of primers located in the open reading frame (ORF) that allowed the simultaneous amplification of all transcripts (Aro-ORF) and a pair of primers specific for brain aromatase (Aro-B). Significant increase in Aro-ORF and Aro-B mRNA levels were observed in both females and males exposed to LAM. Different changes were observed for the model compounds using two pairs of primers. Aro-ORF mRNA expression was significantly increased in EE2 and MDHT exposed males and in FLU-exposed females, while it was significantly decreased in TAM exposed females. Aro-B mRNA was significantly increased in both sexes exposed to FLU and decreased in TAM exposed females. In conclusion, aromatase mRNA in the brain of X. laevis was regulated differentially in a gender specific manner by certain (anti)estrogenic and (anti)androgenic EDCs, supporting previous hypotheses that diverse compounds present in the river Lambro may induce feminization and demasculinization effects.

Developmental expression of steroidogenic factor-1, cyp19a1a and cyp19a1b from common carp (Cyprinus carpio)

Steroidogenic factor-1 (SF-1), cyp19a1a and cyp19a1b play pivotal roles in vertebrate steroidogenesis and reproduction. In this study, a SF-1 cDNA (EU022463) was cloned from common carp (Cyprinus carpio). The transcript contains a 1509 base pair (bp) open reading frame (ORF) encoding a 503 amino acid sequence. Comparisons of deduced amino acid sequences demonstrated that carp SF-1 is highly homologous with those of other vertebrates. Tissue specific expressions of SF-1, cyp19a1a and cyp19a1b mRNA were analyzed in 10-month-old carp. SF-1 was abundant in the hypothalamus, pituitary, gonad, spleen and liver (females only). Cyp19a1b was preferentially expressed in the brain of both sexes but also was present at much lower levels in testis, ovary and kidney (females only). Although cyp19a1a expression was preferentially expressed in ovaries, it was also present at much lower levels in brain, testis, kidney and spleen (males only). Northern blot analysis revealed that testes and brains of both sexes expressed a transcript of about 2.8 kb in size. The expression pattern of SF-1, cyp19a1a and cyp19a1b in carp gonads suggested their involvement in sexual development. In 3-month-old carp, SF-1 and cyp19a1b were expressed highly in testes but were at much lower levels in ovaries, while the opposite pattern was observed with cyp19a1a expression. In 10-month-old carp, SF-1 expression was much higher in testes than in ovaries, while the opposite pattern was observed with cyp19a1a expression. These developmental expression patterns in carp gonads suggest important roles of SF-1 and cyp19a1b in testis development and of cyp19a1a in ovary development.

Effects on guppy brain aromatase activity following short-term steroid and 4-nonylphenol exposures

Brain estrogen production, performed by the enzyme aromatase, can be disrupted/affected in teleost fish exposed to endocrine disruptors found in polluted aquatic environments. The guppy (Poecilia reticulata) was previously studied and confirmed to suffer negative effects on reproductive behaviors following inhibition of the brain aromatase reaction. Here adult guppies (Poecilia reticulata) of both genders were subjected to known endocrine disruptors: the androgen androstenedione (A), the synthetic estrogen 17alpha-ethinylestradiol (EE(2)), and the estrogenic surfactant 4-nonylphenol (NP), at high (50 microg/L) and at environmentally relevant concentrations (10 ng/L EE(2), 5 microg/L NP, and 0.7 microg/L A) for 2 weeks followed by measurements of brain aromatase activity (bAA). In the adult males, bAA was stimulated by A and EE(2) at 50 microg/L. Female activity was also stimulated by the higher estrogenic treatment. At environmentally relevant concentrations only the EE(2) treatment affected bAA, and only in males. The alkylphenolic substance NP produced no effect in either of the experiments, not on males nor females. The results indicate that short-term steroid treatments have stimulatory effects on guppy brain aromatase even at concentrations that can be found in the environment. We thus suggest bAA of adult guppies to be a suitable bioindicator of endocrine disruptors.

Aromatase in the brain of teleost fish: expression, regulation and putative functions

Unlike that of mammals, the brain of teleost fish exhibits an intense aromatase activity due to the strong expression of one of two aromatase genes (aromatase A or cyp19a1a and aromatase B or cyp19a1b) that arose from a gene duplication event. In situ hybridization, immunohistochemistry and expression of GFP (green fluorescent protein) in transgenic tg(cyp19a1b-GFP) fish demonstrate that aromatase B is only expressed in radial glial cells (RGC) of adult fish. These cells persist throughout life and act as progenitors in the brain of both developing and adult fish. Although aromatase B-positive radial glial cells are most abundant in the preoptic area and the hypothalamus, they are observed throughout the entire central nervous system and spinal cord. In agreement with the fact that brain aromatase activity is correlated to sex steroid levels, the high expression of cyp19a1b is due to an auto-regulatory loop through which estrogens and aromatizable androgens up-regulate aromatase expression. This mechanism involves estrogen receptor binding on an estrogen response element located on the cyp19a1b promoter. Cell specificity is achieved by a mandatory cooperation between estrogen receptors and unidentified glial factors. Given the emerging roles of estrogens in neurogenesis, the unique feature of the adult fish brain suggests that, in addition to classical functions on brain sexual differentiation and sexual behaviour, aromatase expression in radial glial cells could be part of the mechanisms authorizing the maintenance of a high proliferative activity in the brain of fish.

Doublesex- and Mab-3-related transcription factor-1 repression of aromatase transcription, a possible mechanism favoring the male pathway in tilapia

Doublesex- and Mab-3-related transcription factor-1 (Dmrt1) is an important transcription factor implicated in early testicular differentiation in vertebrates, but its target genes are largely unknown. In the Nile tilapia, estrogen is the natural inducer of ovarian differentiation. Our recent studies have shown that Forkhead-l2 up-regulated transcription of the Cyp19a1a gene (aromatase) in the gonads in a female-specific manner. However, the upstream factor(s) down-regulating Cyp19a1a expression during testicular differentiation remains unclear. In the present study, we used in vitro (promoter analysis) and in vivo (transgenesis and in situ hybridization) approaches to examine whether Dmrt1 inhibits Cyp19a1a's transcriptional activity. The in vitro analysis using luciferase assays revealed that Dmrt1 repressed basal as well as Ad4BP/SF-1-activated Cyp19a1a transcription in HEK 293 cells. Luciferase assays with various deletions of Dmrt1 also showed that the Doublesex and Mab-3 domain is essential for the repression. In vitro-translated Dmrt1 and the nuclear extract from tilapia testis could directly bind to the palindrome sequence ACATATGT in the Cyp19a1a promoter, as determined by EMSAs. Transgenic overexpression of Dmrt1 in XX fish resulted in decreased aromatase gene expression, reduced serum estradiol-17beta levels, retardation of the ovarian cavity's development, varying degrees of follicular degeneration, and even a partial to complete sex reversal. Our results indicate that aromatase is one of the targets of Dmrt1. Dmrt1 suppresses the female pathway by repressing aromatase gene transcription and estrogen production in the gonads of tilapia and possibly other vertebrates.

Neuroendocrinology of reproduction in teleost fish

This review aims at synthesizing the most relevant information regarding the neuroendocrine circuits controlling reproduction, mainly gonadotropin release, in teleost fish. In teleosts, the pituitary receives a more or less direct innervation by neurons sending projections to the vicinity of the pituitary gonadotrophs. Among the neurotransmitters and neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRH) and dopamine, acting as stimulatory and inhibitory factors (in many but not all fish) on the liberation of LH and to a lesser extent that of FSH. The activity of the corresponding neurons depends on a complex interplay between external and internal factors that will ultimately influence the triggering of puberty and sexual maturation. Among these factors are sex steroids and other peripheral hormones and growth factors, but little is known regarding their targets. However, very recently a new actor has entered the field of reproductive physiology. KiSS1, first known as a tumor suppressor called metastin, and its receptor GPR54, are now central to the regulation of GnRH, and consequently LH and FSH secretion in mammals. The KiSS system is notably viewed as instrumental in integrating both environmental cues and metabolic signals and passing this information onto the reproductive axis. In fish, there are two KiSS genes, KiSS1 and KiSS2, expressed in neurons of the preoptic area and mediobasal hypothalamus. Pionneer studies indicate that KiSS and GPR54 expression seem to be activated at puberty. Although precise information as to the physiological effects of KiSS1 in fish, notably on GnRH neurons and gonadotropin release, is still limited, KiSS neurons may emerge as the "gatekeeper" of puberty and reproduction in fish as in mammals.

Expression pattern, cellular localization and promoter activity analysis of ovarian aromatase (Cyp19a1a) in protogynous hermaphrodite red-spotted grouper

Aromatase plays a key role in sex differentiation of gonads. In this study, we cloned the full-length cDNA of ovarian aromatase from protogynous hermaphrodite red-spotted grouper (Epinephelus akaara), and prepared the corresponding anti-EaCyp19a1a antiserum. Western blot and immunofluorescence studies revealed ovary-specific expression pattern of EaCyp19a1a in adults and its dynamic expression change during artificial sex reversal. EaCyp19a1a was expressed by follicular cells of follicular layer around oocytes because strong EaCyp19a1a immunofluorescence was observed in the cells of ovaries. During artificial sex reversal, EaCyp19a1a expression dropped significantly from female to male, and almost no any positive EaCyp19a1a signal was observed in testicular tissues. Then, we cloned and sequenced a total of 1967 bp 5'-flanking sequence of EaCyp19a1a promoter, and showed a number of potential binding sites for some transcriptional factors, such as SOX5, GATA gene family, CREB, AP1, FOXL1, C/EBP, ARE and SF-1. Moreover, we prepared a series of 5' deletion promoter constructs and performed in vitro luciferase assays of EaCyp19a1a promoter activities. The data indicated that the CREB regulation region from -1010 to -898 might be a major cis-acting element to EaCyp19a1a promoter, whereas the elements GATA and SOX5 in the region from -1216 to -1010 might be suppression elements. Significantly, we found a common conserved sequence region in the fish ovary-type aromatase promoters with identities from 93% to 34%. And, the motifs of TATA box, SF-1, SOX5, and CREB existed in the region and were conserved among the most of fish species.

Polymorphisms within promoter of Japanese flounder (Paralichthys olivaceus) ovary cytochrome P450-c19 (CYP19a) gene associated with reproductive traits

Cytochrome P450 aromatase, which is encoded by the CYP19a gene, converts androgens to estradiol. Considerable evidence suggests that estrogens play an important role in fish reproductive process. Therefore CYP19a is an excellent candidate gene for reproductive traits. Variants in the promoter of the CYP19a gene might also be involved in the control of aromatase expression and affect regulatory mechanism linking cholesterol metabolism to the synthesis of sex steroids. In this study, nine single-nucleotide polymorphisms (SNPs) were detected with polymerase chain reaction-single stranded conformational polymorphism (PCR-SSCP), namely A-680G, G-672A, AGTAGT-649 inserting or deleting, T-623C, C-410A, T7-454A, T-402C, TTTCCAGACTGA-345 inserting or deleting, and G-297C. Nine SNPs within the promoter of the CYP19a gene were tested for association with four reproductive traits [serum testosterone (T), serum 17beta-estradiol (E(2)), hepatosomatic index (HSI), and gonadosomatic index (GSI)] in a population of 50 female Japanese flounder individuals. A locus, P3 (TTTCCAGACTGA-345 inserting or deleting, G-297C), was significantly associated with 17beta-estradiol (E(2)) level (P < 0.05) in female Japanese flounder. In addition, there was significant association between one diplotype based on nine SNPs and reproductive trait. The genetic effect for E(2) level of diplotype D3 was significantly higher than those of other diplotypes (P < 0.05). Results indicate that these genetic effects of those variants on E(2) level may help to explain CYP19a gene status in the reproductive endocrinology of Japanese flounder.

Alternative 5'-untranslated first exons of the mouse Cyp19A1 (aromatase) gene

The human aromatase gene (CYP19A1) has eleven tissue-specific untranslated first exons, while only three have been described in the mouse Cyp19A1 namely brain-, ovary- and testis-specific exons 1. The present study aims to elucidate the complete structure of the mouse Cyp19A1 gene. We detected aromatase transcripts in mouse bone, aorta, hypothalamus, adipose, gonads and placenta, but not nulliparous mammary fat pad. BestFit algorithm analysis against the human CYP19A1 has identified ten putative first exons upstream of mouse Cyp19A1. Based on these putative sequences, we were able to design specific primers for RT-PCR and detected for the first time, the presence of exons I.4 and I.3 in murine fat and gonads, respectively. These are novel 5'UTRs of mouse Cyp19A1. Using RT-PCR and 5' RACE, we confirmed the expression of exon 1f in the hypothalamus and proximal exon P2 in the ovary. The testis-specific exon 1 begins 217bp further upstream than previously reported. Putative exons 2a, I.5, I.7, I.6 and I.2 were not detected in mouse tissues. Therefore, we showed that mouse Cyp19A1 contains more tissue-specific first exons than previously thought and displays a similar genomic organization to human CYP19A1.