In vivo induction of oocyte maturation and ovulation in zebrafish

In vivo induction of male sexual behavior in zebrafish by adding agents to water

Successful fertilization in fish mating occurs when egg maturation in the ovary of the female, ovulation, sperm maturation in the testis of the male, and reproductive behaviors in both sexes are triggered in synchrony. The male sexual behavior of fish is induced by hormones and pheromones. In a previous study, we demonstrated that externally applied hormones added to the water can induce oocyte maturation and ovulation in female zebrafish. Here, we attempted to establish a similar method to induce the sexual behavior of male zebrafish. The male sex steroid testosterone (Tes) triggered sexual behavior within several hours in vivo when administered directly into the surrounding water. A selective agonist for membrane progesterone receptor (mPR), Org OD-02 (Org), also induced sexual behavior. Through trials of various combinations of compounds, we found that the most effective conditions were achieved by treatment with a mixture of testosterone (Tes) and Org. The effect of treatment was evaluated by the number of fertilized eggs obtained by pairing with females with induced ovulation in vivo. The period necessary for the induction of male sexual behavior was evaluated by time course experiments. The success rate of mating and the number of fertilized eggs reached the maximum level at 3-4 hours of treatment. The duration of hormonal treatment was confirmed by counting the number of hooking occurrences, which is the final cue to induce spawning by females. In summary, we have established a method to induce male sexual behavior in zebrafish in vivo. The method can be used to obtain fertilized eggs in zebrafish by simply adding agents into the water.

Establishment of a graphene quantum dot (GQD) based steroid binding assay for the nuclear progesterone receptor (pgr)

Previously, we established a homogeneous assay for membrane progesterone receptor alpha (mPRα) ligands by conjugating semiconductor nanoparticles known as graphene quantum dots (GQDs) to mPRα. When mixed with a progesterone-BSA-fluorescein isothiocyanate conjugate (P4-BSA-FITC), fluorescence occurred by fluorescence resonance energy transfer (FRET) but was reduced by the ligand-receptor binding activity. The established way showed ligand specificity as mPRα protein. In this study, we tried to establish the same way for nuclear progesterone receptor (Pgr). The ligand-binding domain (LBD) of zebrafish Pgr (zPgrLBD) was expressed as a fusion protein with glutathione S-transferase (GST) (GST-zPgrLBD). The recombinant protein was then purified and coupled with GQDs to produce GQD-conjugated GST-zPgrLBD (GQD-GST-zPgrLBD). When mixed with a P4-BSA-FITC and activated by 370 nm light, fluorescence at 520 nm appeared by FRET mechanism. Fluorescence at 520 nm was reduced by adding free progesterone to the reaction mixture. Reduction of fluorescence was induced by zPgr ligands but not by steroids or chemicals that do not interact with zPgr. The results showed the formation of a complex of GQD-GST-zPgrLBD and P4-BSA-FITC with ligand-receptor binding. The binding of the compounds was further confirmed by a radiolabeled steroid binding assay. A homogenous ligand-binding assay for nuclear progesterone receptor has been established.

Evidence of binding between diethylstilbestrol (DES) and the goldfish (Carassius auratus) membrane progesterone receptor α

BACKGROUND

In a previous study, diethylstilbestrol (DES) was shown to induce oocyte maturation in fish. In the present study, the interaction of DES on goldfish membrane progesterone receptor α (GmPRα) was investigated using a competitive binding assay with radiolabeled steroids. The results indicate that DES exerts its effects on membrane progesterone receptor alpha (mPRα) and induces oocyte maturation through nongenomic steroid mechanisms. This study provides empirical data that demonstrate the binding between DES and GmPRα.

METHODS

Binding of DES to GmPRα was achieved by using radiolabeled DES and recombinant GmPRα expressed in culture cells or purified GmPRα proteins that coupled to graphene quantum dots (GQDs). Additionally, the competitive binding of fluorescently labeled progesterone to GmPRα-expressing cells was evaluated.

RESULTS

Although significant nonspecific binding of radiolabeled DES to the cell membrane that expresses GmPRα has been observed, specific binding of DES to GmPRα has been successfully identified in the presence of digitonin. Furthermore, the specific binding of DES to GmPRα was confirmed by a binding assay using GQD-GmPRα. The radiolabeled DES was shown to bind to GQD-GmPRα. Additionally, the competition for the binding of fluorescently labeled progesterone to GmPRα-expressing cells was achieved with the DES.

CONCLUSIONS

The results of the experiments revealed that DES binds to GmPRα. Thus, it can be concluded that DES induces goldfish oocyte maturation by binding to GmPRα.

Chronic exposure to gestodene impairs reproductive system in adult female zebrafish (Daniarerio)

Gestodene (GES) is widely used in human therapy and animal husbandry and is frequently detected in aquatic environments. Although GES adversely affects aquatic organisms at trace levels, its effects on the reproductive biology of fish remain inconclusive. In this study, female zebrafish (Danio rerio) were exposed to environmentally relevant levels of GES for the evaluation of the effects of GES on the reproductive system by using endpoints including gene expression, plasma steroid concentrations, histological and morphological analyses, copulatory behavior, and reproductive output. Adult female zebrafish exposed to environmentally relevant concentrations of GES (4.0, 40.2, and 372.7 ng/L) for 60 d demonstrated stagnant ovarian oocyte development, evidenced by an increase in the percentage of perinuclear and atretic oocytes and a decrease in the percentage of late vitellogenic oocytes. GES-exposed females were less attractive to males and had lower copulatory intimacy than females in control. Consequently, spawning (44.3-49.2 %) and egg fertilization rates (27.9-32.0 %) were decreased. The decreased survival of fertilized eggs and hatching rates were accompanied by increased malformations. These negative effects were associated with abnormal transcriptional levels of gonadal steroid hormones, which were regulated by genes (Hsd17β3, Hsd11β2, Hsd20β, Cyp19a1a, and Cyp11b). Overall, our findings suggest that GES impairs the reproductive system of zebrafish, which may threaten population stability.

Zebrafish prss59.1 is involved in chorion development

The prss59.1 gene was identified as one of 11 genes that were highly upregulated during the induction of ovulation in zebrafish by using an in vivo ovulation assay. Previously, we conducted biochemical characterization of Prss59.1 and revealed it to be a trypsin-like proteolytic enzyme. In this study, we established a prss59.1 gene knockout strain using the CRISPR/Cas9 system. Phenotypic analysis of prss59.1 knockout fish showed that prss59.1 is associated with chorion elevation, a prominent event in egg activation during fertilization. The chorions of heterozygous and homozygous prss59.1 mutant zebrafish were smaller than those of the wild type. The results suggested that Prss59.1 is necessary for chorion expansion. The homozygous prss59.1 mutant strain, with a small chorion, showed an extremely low survival rate. Fiber-supported knob-like structures (KS) on the chorion showed an abnormal structure in prss59.1 mutants. Prss59.1 was detected in the KS on the chorion. The pores on the chorion were smaller in the prss59.1 mutants than in the wild type. Transmission electron microscopy (TEM) observations of the cross sections of the chorions showed abnormalities in the chorion structure in prss59.1 mutants. These results demonstrated that Prss59.1 is involved in chorion elevation and in proper formation of the chorion, which is necessary for embryo development.

Deleterious effect of gestagens from wastewater effluent on fish reproduction in aquatic environment: A review

Gestagens are common pollutants accumulated in the aquatic ecosystem. Gestagens are comprised of natural gestagens (i.e. progesterone) and synthetic gestagens (i.e. progestins). The major contributors of gestagens in the environment are paper plant mill effluent, wastewater treatment plants, discharge from pharmaceutical manufacturing, and livestock farming. Gestagens present in the aquatic environment interact with progesterone receptors and other steroid hormone receptors, negatively influencing fish reproduction, development, and behavior. In fish, the gonadotropin induces 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) production, an important steroid hormone involved in gametogenesis. DHP interacts with the membrane progestin receptor (mPR), which regulates sperm motility and oocyte maturation. Gestagens also interfere with the hypothalamic-pituitary-gonadal (HPG) axis, which results in altered hormone levels in fish. Moreover, recent studies showed that even at low concentrations exposure to gestagens can have detrimental effects on fish reproduction, including reduced egg production, masculinization, feminization in males, and altered sex ratio, raising concerns about their impact on the fish population. This review highlights the hormonal regulation of sperm motility, oocyte maturation, the concentration of environmental gestagens in the aquatic environment, and their detrimental effects on fish reproduction. However, the long-term and combined impacts of multiple gestagens, including their interactions with other pollutants on fish populations and ecosystems are not well understood. The lack of standardized regulations and monitoring protocols for gestagens pollution in wastewater effluent hampers effective control and management. Nonetheless, advancements in analytical techniques and biomonitoring methods provide potential solutions by enabling better detection and quantification of gestagens in aquatic ecosystems.

The induction of oocyte maturation and ovulation in the European eel (Anguilla anguilla): in vitro and in vivo comparison of progesterone with 17α,20β-dihydroxy-4-pregnen-3-one

Ovulation in European eel is induced by injection of 17α,20β-dihydroxy-4-pregnen-3-one (DHP) as the maturation-inducing hormone (MIH). Female eels need to ovulate within 18 h after injection to release good quality eggs. Progesterone (P), as an upstream precursor of DHP, may promote endogenous DHP production and improve egg quality. The purpose of this study was therefore to compare treatment of P with DHP on batch level, in vitro, to determine dose-response effects, and in vivo, at a single dose. For the in vitro experiment, ovarian tissue was extracted and placed in culture plates containing hormone-free medium and media supplemented with the treatment: DHP at 1, 10 and 100 ng mL-1, or P at 10, 100 and 1,000 ng mL-1. At the start of incubation, the folliculated oocytes were sampled for histology, microscopy and qPCR. After incubation for 12 and 18 h, the oocytes were sampled for microscopy and qPCR analysis. For the in vivo experiment, females were either injected with DHP or P at a dose of 2 mg kg-1 to assess their effects on ovulation and reproductive success. At the moment of release, eggs were sampled for RNA sequencing to compare effects of DHP and P on the expression of genes involved in egg quality aspects. Remaining eggs were fertilized and larval viability was recorded. Both DHP and P were able to induce GVBD (DHP at 10 and 100 ng mL-1, P at 100 and 1,000 ng mL-1) in vitro. Expression of genes involved in oocyte maturation and ovulation was similar in vitro for both DHP and P treatments. Regarding the in vivo results, RNAseq results reflected similar DHP and P effects on the expression of genes involved in egg quality aspects. Females injected with either DHP or P ovulated, released eggs, and were equally able to produce larvae without any differences in reproductive success. Our results support the conclusion that DHP and P work equally well in vitro and in vivo. P is more attractive to apply as the price is 3,000 times lower than the price of DHP.

Functional evaluation of a novel kisspeptin analogue on the reproduction of female goldfish

Kisspeptin (kp) is a key regulator of reproduction, which stimulates sexual maturation and gametogenesis in mammals, amphibians, and teleosts. In the present study, to enhance the biological activity of kp10, a novel analog (referred to as M-kp10) was designed based on the endogenous goldfish variant, in which phenylalanine 6 was substituted by tryptophan and the N-terminus was acetylated. Compared with the native kp-10 and salmon gonadotropin-releasing hormone (GnRH3), the effect of M-kp10 on sexual hormones and reproductive indices as well as the expression of kiss1, cyp19a1, and kiss1ra genes in goldfish (Carassius auratus) was investigated. In practice, peptides were synthesized based on the standard Fmoc-solid-phase peptide synthesis and purified by employing RP-HPLC, followed by approving their structure using ESI-MS. The results showed that M-kp10 increased significantly 17,20β-DHP, LH, FSH and E2 as well as fecundity, hatching and fertilization percentages than the other peptides. Histological studies revealed that M-kp10 led to the faster growth of ovarian follicles compared to the kp-10 and GnRH3. The genes of cyp19a1, kiss1ra, and kiss1 were remarkably more expressed after treatment with M-kp10. In conclusion, the results indicated the superiority of M-kp10 over kp-10 in inducing sexual maturation and accelerating the percentage of fecundity, suggesting that M-kp10 could be a promising candidate for application in the artificial breeding of fish.

The antagonistic activity of Padina arborescens extracts on mPRα

The current study attempted to evaluate the antagonistic activity of compounds isolated and purified from the marine algae Padina arborescens during cultivation. The compounds were collected on a filter, concentrated on ODS columns and separated by HPLC. Two peaks that showed competitive progesterone binding activity with membrane progesterone receptor α (mPRα) were purified. Their physiological activity was further uncovered by in vitro and in vivo oocyte maturation and ovulation-inducing assays using zebrafish. The compounds inhibited the induction of oocyte maturation and ovulation. Moreover, the results showed that the compounds have antagonistic activity against mPRα. The purified compounds with antagonistic activity against mPRα would be considered as new pharmaceutical candidate.

Inhibition of Oocyte Maturation by Malathion and Structurally Related Chemicals in Zebrafish (Danio rerio) After In Vitro and In Vivo Exposure

Oogenesis is the process by which a primary oocyte develops into a fertilizable oocyte, making it critical to successful reproduction in fish. In zebrafish (Danio rerio), there are five stages of oogenesis. During the final step (oocyte maturation), the maturation-inducing hormone 17α,20β-dihydroxy-4-pregnen-3-one (MIH) activates the membrane progestin receptor, inducing germinal vesicle breakdown. Using in vitro assays, it has been shown that anthropogenic stressors can dysregulate MIH-induced oocyte maturation. However, it is unknown whether the in vitro assay is predictive of reproductive performance after in vivo exposure. We demonstrate that a known inhibitor of oocyte maturation, malathion, and a structurally related chemical, dimethoate, inhibit oocyte maturation. However, malaoxon and omethoate, which are metabolites of malathion and dimethoate, did not inhibit oocyte maturation. Malathion and dimethoate inhibited maturation to a similar magnitude when oocytes were exposed for 4 h in vitro or 10 days in vivo, suggesting that the in vitro zebrafish oocyte maturation assay might be predictive of alterations to reproductive performance. However, when adult zebrafish were exposed to malathion for 21 days, there was no alteration in fecundity or fertility in comparison with control fish. Our study supports the oocyte maturation assay as being predictive of the success of in vitro oocyte maturation after in vivo exposure, but it remains unclear whether inhibition of MIH-induced oocyte maturation in vitro correlates to decreases in reproductive performance. Environ Toxicol Chem 2022;41:1381-1389. © 2022 SETAC.

Signal pathway of LH-induced expression of nuclear progestin receptor in vertebrate ovulation

Nuclear progestin receptor (PGR), which is induced in the follicles destined to undergo ovulation, is believed to be obligatory for rupture of the follicles during ovulation in vertebrates. Studies in some mammals and teleost medaka have revealed the outline of the central signaling pathway that leads to the PGR expression in the preovulatory follicles at ovulation. In this review, we summarize the current knowledge on what signaling mediators are involved in the LH-induced follicular expression of PGR at ovulation in these animals. LH-inducibility of follicular PGR expression is conserved. In both group of animals, activation of the LH receptor on the granulosa cell surface with LH commonly results in the increase of intracellular cAMP levels, while the downstream signaling cascades activated by high level of cAMP are totally different between mice and medaka. PGR is currently presumed to be induced via PKA/CREB-mediated transactivation and ERK1/2-dependent signaling in mice, but the receptor is induced via EPAC/RAP and AKT/CREB pathways in the teleost medaka. The differences and similarities in the signaling pathways for PGR expression between them is discussed from comparative and evolutionary aspects. We also discussed questions concerning PGR expression and its regulation needed to be investigated in future.

Zebrafish stm is involved in the development of otoliths and of the fertilization envelope

Using an in vivo assay, we selected 11 genes that were highly upregulated during the induction of ovulation in zebrafish using microarray analysis and RNA sequencing. The starmaker gene (stm) was one of these genes. Although stm has been previously reported to be involved in otolith formation during the early development of zebrafish, we detected its expression in eggs and showed that stm was related to fertilization by establishing an stm gene knockout strain using the CRISPR/Cas9 system. Further phenotypic analysis of stm knockout fish was conducted in this study. With a higher nonfertilization rate, the stm mutant strain showed an extremely low survival rate. Otoliths of stm homozygous mutant zebrafish showed abnormal morphology in embryos and adult fish. However, fish did not show any abnormalities in swimming behaviour in either embryos or adults. Stm proteins were detected on the chorion of ovulated eggs before spawning. Fibre-supported knob-like structures on the fertilization envelope (FE) also showed abnormal structures in stm mutants. The Stm protein is necessary for otolith formation, and a lack of Stm causes abnormal otolith formation. The partial defect of otolith formation does not cause defects in swimming behaviour. The Stm protein is expressed in the chorion and is responsible for the formation of fibre-supported knob-like structures on the FE. It was suggested that a lack of Stm caused a lower fertilization rate due to inadequate formation of the FE.

Lay summary

In zebrafish, the protein Starmaker (Stm) was identified as having a role in ovulation. Stm is also known to be required for the formation of ear stones (otoliths) which are needed to keep the body in balance. Zebrafish lacking Stm were produced by genome editing. As expected, Stm-deficient fish formed abnormal otoliths. To investigate the role of Stm in ovulation, fertilization and early development, we tried mating of Stm mutants and observed their juveniles. Although no problem found in ovulation, we found low fertilization rate and abnormal structure of knob-like structure (small pit) on the egg membrane. Survival rate of embryos with abnormal egg membrane was extremely low. It was demonstrated that Stm protein is necessary to form the functional egg membrane to protect embryos from the outside environment.

In vitro induction of catfish, Clarias batrachus, oocyte maturation by conspecific vitellogenin 1 (CFVg1)

Present study demonstrates that conspecific vitellogenin1 (CFVg1) induces oocyte maturation in the catfish, Clarias batrachus. CFVg1 is able to develop fertilizable eggs in the Clarias batrachus. Therefore, different in vitro oocyte culture experiments were designed to see whether CFVg1 has efficacy of oocyte maturation and its pathway. In in vitro oocyte culture experiment, CFVg1 showed a dose- and time-dependent response and 64% maturation was obtained at the dose level of 10 µg/ml or more. CFVg1 induction of oocyte maturation was confirmed by co-incubating CFVg1 with CFVg1-antiserum (a-CFVg1), which inhibited the CFVg1-induced oocyte maturation. To answer issues lead to the understanding of the mechanism of vitellogenin (Vg) on oocyte maturation, trypsin digested CFVg1 and Indian major carp Cirhinus mrigala Vg HAI (Hydroxy appetite peak I) also showed significant level of maturation. Actinomycin-D and cycloheximide blocked the effect of CFVg1, indicating that CFVg1 acts through transcription and translation. Theophylline, the phosphodiesterase inhibitor, and cAMP also inhibited the stimulatory effect of CFVg1 on oocyte maturation, indicating indirectly that CFVg1-induced oocyte maturation by decreasing the intracellular cAMP possibly by activating the phosphodiesterase enzyme. Trilostane, the 3β-HSD-blocker, did not inhibit the CFVg1-induced oocyte maturation but wortmannin and Ly294002 two mechanistically different specific inhibitors of PI3 kinase blocked the oocyte maturation. The results thus indicate that oocyte maturation in catfish by Vg may be regulated by two pathways: (1) through decreasing the intraoocyte cAMP level by activating the cAMP-PKA pathway and (2) by cAMP-dependent PI3K/Akt pathway. Therefore, there might be role of vitellogenin itself in initiation of oocyte maturation.

Establishment of a steroid binding assay for membrane progesterone receptor alpha (PAQR7) by using graphene quantum dots (GQDs)

Currently, semiconductor nanoparticles known as quantum dots (QDs) have attracted interest in various application fields such as those requiring sensing properties, binding assays, and cellular imaging and are the very important in the acceleration of drug discovery due to their unique photophysical properties. Here, we applied graphene quantum dots (GQDs) for the binding assay of membrane progesterone receptor alpha (mPRα), one of the probable membrane receptors that have potential in drug discovery applications. By coupling the amino groups of mPRα with GQDs, we prepared fluorogenic GQD-conjugated mPRα (GQD-mPRα). When mixed with a progesterone-BSA-fluorescein isothiocyanate conjugate (P4-BSA-FITC) to check the ligand receptor binding activity of GQD-mPRα, fluorescence at 520 nm appeared. The fluorescence at 520 nm was reduced by the addition of free progesterone into the reaction mixture. GQD-coupled BSA (GQD-BSA) did not show a reduction in fluorescence at 520 nm. The results demonstrated the formation of a complex of GQD-mPRα and P4-BSA-FITC with ligand receptor binding. We established a ligand binding assay for membrane steroid receptors that is applicable for high-throughput assays.

Evidence-based hormonal, mutational, and endocrine-disrupting chemical-induced zebrafish as an alternative model to study PCOS condition similar to mammalian PCOS model

Polycystic ovarian syndrome (PCOS) causes swollen ovaries in women at reproductive age due to hormonal disorder with small cysts on the outer edges. The cause of the disorder is still yet to be found. Multiple factors have increased PCOS prevalence, hyperandrogenism, oxidative stress, inflammation, and insulin resistance. Various animal PCOS models have been developed to imitate the pathophysiology of PCOS in humans. Zebrafish is one of the most versatile animal experimental models because of the transparency of the embryos, small size, and rapid growth. The zebrafish similarity to higher vertebrates made it a useful non-mammalian model for PCOS drug testing and screening. This review provides an insight into the usage of zebrafish, a non-mammalian model for PCOS, as an opportunity for evaluating future initiatives in such a research domain.

ADAMTS1 is regulated by the EP4 receptor in the zebrafish ovary

Prostaglandins (PGs) are a class of fatty-acid derived hormones that are essential in ovulation of teleosts, but their exact role remains unknown. One putative target of PGs in ovulation is regulation of the expression of members of the A Disintegrin and Metalloproteinase with Thrombospondin motifs (ADAMTS) family, which are implicated in follicular rupture. This study investigated the regulation of ADAMTS, other proteases, and their inhibitors in response to treatment with PGE₂ or PGF_2α. Four members of the ADAMTS family, ADAMTS1, ADAMTS5, ADAMTS9, and ADAMTS16 were shown to be expressed in the ovary of zebrafish, but only adamts1 was upregulated in full-grown follicles following treatment with PGE₂. Inhibitors of the PG receptors EP1 and EP2 had no effect on PGE₂-stimulated adamts1 expression, while treatment of full-grown follicles with both PGE₂ and GW627368x, an inhibitor of EP4 function, prevented the PGE₂-induced increase in adamts1 expression. Treatment of full-grown follicles with the maturation-inducing hormone 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P) in vitro had no effect on the expression of adamts1 mRNA. These findings suggest that expression of ADAMTS1 in zebrafish ovarian follicles is regulated by the prostaglandin PGE₂ via the EP4 series prostaglandin receptor.

Nuclear progesterone receptor regulates ptger4b and PLA2G4A expression in zebrafish (Danio rerio) ovulation

Previous studies have implicated the nuclear progesterone receptor (Pgr or nPR) as being critical to ovulation in fishes. This study investigated the expression of Pgr in zebrafish ovarian follicles throughout development as well as putative downstream targets of Pgr by searching the promoter regions of selected genes for specific DNA sequences to which Pgr binds and acts as a transcription factor. Expression of Pgr mRNA increases dramatically as follicles grow and mature. In silico analysis of selected genes linked to ovulation showed that the prostaglandin receptors ptger4a and ptger4b contained the progesterone responsive element (PRE) GRCCGGA in their promoter regions. Studies using full-grown follicles incubated in vitro revealed that ptger4b was upregulated in response to 17,20β-P. Our studies also showed that the expression of phospholipase A2 (PLA2G4A) mRNA and protein, a key enzyme in prostaglandin synthesis, was upregulated in response to 17,20β-P treatment. pla2g4a was not found to contain a PRE, indicating that it is regulated indirectly by 17,20β-P or that it may contain an as-of-yet unidentified PRE in its promoter region. Collectively, these studies provide further evidence of the importance of Pgr during the periovulatory periods through its involvement in prostaglandin production and function by controlling expression of PLA2G4A and the receptor EP4b and that these genes appear to be regulated through the actions of 17,20β-P.

An agonist for membrane progestin receptor (mPR) induces oocyte maturation and ovulation in zebrafish in vivo

The maturation and ovulation of fish oocytes are well-characterized biological processes induced by progestins via coordination of nongenomic actions and genomic actions. Previously, we established a procedure that enables the induction of oocyte maturation and ovulation in live zebrafish by simple administration of the natural teleost maturation-inducing hormone 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20β-DHP) into the surrounding water. By this in vivo assay, the potencies of chemicals in inducing or preventing oocyte maturation and ovulation can be evaluated. The potencies of compounds in inducing ovulation of zebrafish oocytes also can be evaluated in vivo with improved in vitro assays. Here, we attempted to evaluate the effect of Org OD 02-0 (Org OD 02), a selective agonist for membrane progestin receptor (mPR), on fish oocyte maturation and ovulation with in vitro and in vivo assays. As reported previously, Org OD 02 triggered oocyte maturation in vitro. The same Org OD 02 triggered oocyte maturation within several hours in vivo. Surprisingly, Org OD 02 even induced ovulation both in in vivo and in vitro. Eggs from Org OD 02-induced ovulation could be fertilized by artificial insemination. The juveniles developed normally. These results indicated that Org OD 02 triggered physiological ovulation in live zebrafish. In summary, we have demonstrated the effect of Org OD 02 on fish oocyte maturation and ovulation in vitro and in vivo. The results suggested that Org OD 02 acted as an agonist not only of mPR but also of nuclear progesterone receptor (nPR).

Rapid Induction of Female-to-Male Sex Change in Adult Zebrafish by Injection of an Aromatase Inhibitor

Previously, we examined whether aromatase inhibitor (AI) treatment induces a sex change in adult female zebrafish. A 5-month AI treatment regime resulted in the retraction of the ovaries and testis formation. Eight weeks after changing the diet to AI-free food, a large number of normal sperm were obtained. Artificial fertilization using sperm from the sex-changed females was successful. These results demonstrated that sex plasticity remains in the mature ovaries of zebrafish. However, >7 months of treatment was necessary; thus, pairing was unsuccessful. In this study, we tried to induce sex change through the injection of an AI to shorten the time course of sex change. When the AI solution was directly injected into the abdomen of zebrafish, retraction of the ovary was induced within 2 months. The natural mating of sex-changed females with normal females was successful at 3 months. Although the fertilization rate was low, juveniles resulting from these matings developed normally. We succeeded in establishing a method for inducing sex changes in adult zebrafish within 3 months. The procedure will support the study of how sexual plasticity persists in adult zebrafish following sex differentiation and the identification of undifferentiated stem cells.

Puberty visualized: sexual maturation in the transparent Casper zebrafish

Transparent Casper zebrafish allow studies of vertebrate sexual maturation and gonad development in vivo. Casper gonad dynamics can be observed longitudinally over time and non-invasively. Gonad maturation and reproduction are complex processes subject to disruption by endocrine-disrupting chemicals (EDCs), such as diethylstilbestrol (DES). DES was used as a 'proof of principle' to ascertain the usefulness of the Casper model to determine EDC effects on gonad maturation. Puberty onset in control juvenile Casper zebrafish (N = 43) averaged 13.2 weeks post fertilization (WPF) for females and included increased vent size, while in males puberty occurred at 11.7 WPF along with maintenance of small vents. DES treatment for 6 days in early juveniles (N = 20) induced an average delay in puberty of 5 weeks in females and 10 weeks in males. DES induced loss of breeding tubercles and vent enlargement in post-pubescent males. Puberty in control fish was correlated with an average body length of 1.7 cm for males and 1.8 cm for females. Increased testes opacity, small vent and breeding tubercles denoted male puberty. Puberty in females was defined as ovarian follicle diameters reaching 400 µm with increasingly opaque follicles and by an increased vent size. These results are like those for wild-type zebrafish and indicate that the Casper model is a useful system for studying gonad dynamics in vivo. Future use of transgenic reporter lines in Casper will allow new avenues of investigation into the reproductive biology of this vertebrate model.

Investigating the role of prostaglandin receptor isoform EP4b in zebrafish ovulation

Prostaglandins (PGs) are a class of fatty acid-derived hormones that play an essential role in the regulation of ovulation of teleosts. This study investigated the various isoforms of ovarian PG receptors in the zebrafish ovary and their role in ovulation. Using real time qPCR, six PG receptor isoforms (ptger1a, ptger1b, ptger2a, ptger4a, ptger4b, and ptgfr) were shown to be expressed in the ovary. Only the PG receptor isoform ptger4b was upregulated at the time of ovulation in vivo, or following treatment in vivo with Ovaprim, which contains a gonadotropin releasing hormone analogue and a dopamine receptor antagonist and stimulates ovulation. Treatment of full-grown follicles with the maturation-inducing hormone 17α,20β-dihydroxy-4-pregnen-3-one (17,20βP) in vitro also induced expression of EP4b mRNA. Females ovulate in vivo after injection with Ovaprim, or injection with Ovaprim and inhibitors of EP1 (ONO-8130) or EP2 (TG4-155) function; they do not ovulate when injected with Ovaprim and an EP4 inhibitor (GW237368x). These findings suggest that the EP4 receptor, in particular the EP4b isoform, is essential for ovulation.

In vivo assessment of gonad status, secondary sex characteristics and spawning in transparent Casper zebrafish

Important aspects of vertebrate reproduction, such as gametogenesis, involve changes in organs found deep internally and thus not easily studied directly in most living vertebrates due to obscuring pigment and overlying tissues. Transparent lines of zebrafish, especially the Casper double mutant, allow direct observation and analysis of reproductive events in the gonads in vivo. The natural production of fertilized eggs in zebrafish is a complex process involving oogenesis, spermatogenesis, mating behavior, endocrine and neurological processes with inputs from the environment including light, temperature and nutrition. While these factors play important roles, the hypothalamic-pituitary-gonadal axis (HPGA) is central in the regulation of embryo output. Endocrine disrupting compounds (EDCs) include a variety of pollutants often present in the environment. EDCs may have direct effects on the HPGA or indirect effects through toxic action on supporting organs such as the liver or kidney. Estrogenic compounds such as diethylstilbestrol (DES) have been reported to affect reproduction in a variety of species including man. In this study, the effects of DES on reproduction were determined in a novel way by using transparent Casper zebrafish that allow direct visualization of gonad status over time. Changes in gonad status with DES treatment were correlated with effects on secondary sex characteristics (i.e., genital vent size and breeding tubercles) spawning and embryo production. The results suggest that the Casper zebrafish is a useful model for studying dynamics of reproductive events in vertebrate gonads in vivo and for determining effects of EDCs on zebrafish reproduction.

Microcystin-LR influences the in vitro oocyte maturation of zebrafish by activating the MAPK pathway

Harmful cyanobacteria and their production of microcystins (MCs) exert significant toxicity on reproduction of fish, especially the process of oogenesis. Our previous studies demonstrated that MCs have negative impacts on the quantity and quality of mature oocytes in female zebrafish. However, the underlying mechanisms of MCs disrupting oocyte maturation (OM) have been rarely reported. In the present study, in vitro oocytes (immature) were separated from zebrafish and treated with 1, 10, 100 μg/L MC-LR. The serine/threonine protein phosphatase 2A (PP2A) activity was downregulated significantly in oocytes exposed to 10 and 100 μg/L MC-LR for both 2 and 4 h. The phosphorylation levels of mitogen-activated protein kinase (MAPK) were detected without noticeable change in all oocytes treated with MC-LR for 2 h, whereas the activated levels of MAPK subtypes (ERK, p38 and JNK) increased remarkably in the 100 μg/L MC-LR treatment of 4 h. In the oocytes exposed to 100 μg/L MC-LR for 4 h, germinal vesicle breakdown (GVBD) rates changed abnormally and maturation-promoting factor (MPF) activity increased significantly, in accordance with the upregulation of Cyclin B protein levels. Moreover, the MAPK inhibitors (10 μM) were applied to explore the role of MAPK subtypes during MC-LR influencing OM and results showed that ERK inhibitor U0126 and p38 inhibitor SB203580 mitigated the effects of 100 μg/L MC-LR-induced MAPK hyper-phosphorylation and elevated GVBD in the oocytes. In conclusion, the present study indicates that microcystins disrupt the meiotic maturation by the pathway of MC-PP2A-MAPK-OM due to the phosphorylation disorder in oocytes.

Dydrogesterone exposure induces zebrafish ovulation but leads to oocytes over-ripening: An integrated histological and metabolomics study

Dydrogesterone (DDG) is a synthetic progestin widely used in numerous gynecological diseases. DDG has been shown to disturb fish reproduction, however, the mechanism is still unclear. Here we studied the histological changes and differences of metabolome between exposed and control fish gonads after exposure of zebrafish (Danio rerio) embryos to 2.8, 27.6, and 289.8 ng/L DDG until sexual maturity for a total of 140 days. Dydrogesterone exposure led to male-biased zebrafish sex ratios. Histological examination revealed that DDG induced postovulatory follicles and atretic follicles in the ovary of the female fish. Postovulatory follicles indicated the occurrence of ovulation. DDG also increased spermatids and spermatozoa in the male fish testis, suggesting promotion of spermatogenesis. Ovarian metabolome showed that DDG increased the concentrations of free amino acids, urea, putrescine, free fatty acids, acylcarnitines, lysophospholipids, and other metabolites catabolized from phospholipids. Most of these metabolites are biodegradation products of proteins and lipids, suggesting the existence of ovulated oocytes over-ripening. Further, DDG upregulated arachidonic acid (AA) and its 5‑lipoxygenase (5-LOX) metabolites 5‑oxo‑6,8,11,14‑eicosatetraenoic acid (5-oxo-ETE) in the ovary, which could lead to suppression of AA cyclooxygenase (COX) metabolite prostaglandin F2α (PGF2α). It is believed that AA induced oocyte maturation, while 5-oxo-ETE and related metabolites in purinergic signaling promoted ovulation. Whereas, the suppression of PGF2α production might block spawning and damaged follicular tissue digestion, which explained the oocytes over-ripening and atretic follicles in the treated ovary. Overall, our results suggested that DDG exposure induced zebrafish oocyte maturation and ovulation but led to oocytes over-ripening via the AA metabolic pathway and purinergic signaling.

Maternal Ybx1 safeguards zebrafish oocyte maturation and maternal-to-zygotic transition by repressing global translation

Maternal mRNAs and proteins dictate early embryonic development before zygotic genome activation. In the absence of transcription, elaborate control of maternal mRNA translation is of particular importance for oocyte maturation and early embryogenesis. By analyzing zebrafish ybx1 mutants with a null allele, we demonstrate an essential role of maternal ybx1 in repressing global translation in oocytes and embryos. Loss of maternal Ybx1 leads to impaired oocyte maturation and egg activation. Maternal ybx1 (Mybx1) mutant embryos fail to undergo normal cleavage and the maternal-to-zygotic transition (MZT). Morpholino knockdown of ybx1 also results in MZT loss and epiboly failure, suggesting the postfertilization requirement of Ybx1. In addition, elevated global translation level and the unfolded protein response were found in Ybx1-depleted embryos. Supplementing translational repression by eIF4E inhibition markedly rescues the Mybx1 phenotype. Mechanistically, Ybx1 in embryos may associate with processing body components and repress translation when tethered to target mRNAs. Collectively, our results identify maternal Ybx1 as a global translational repressor required for oocyte maturation and early embryogenesis.

Ovulation is associated with the LH-dependent induction of pla2g4aa in zebrafish

The effects of the preovulatory luteinizing hormone (LH) surge on the ovulatory process are mediated by prostaglandins (PGs), the synthesis of which involves prostaglandin synthetase and cytosolic phospholipase A2 (cPLA2). In our previous study, we systematically investigated the function of prostaglandin endoperoxide synthase (ptgs) genes on ovulation in zebrafish. However, the role of cPLA2 in ovulation was not determined in zebrafish. In this study, we investigated the function of cpla2α in PGs production and ovulation in periovulatory follicles. Our data showed that the expression of pla2g4aa increased during zebrafish folliculogenesis and the follicular layer was the primary region with expression of pla2g4aa. In addition, the expression of pla2g4aa was regulated by LH in vitro and in vivo. Furthermore, injection of AACOCF3, a specific inhibitor of cPLA2, significantly reduced ovarian PGs level and blocked hCG-induced ovulation. Collectively, these findings suggest that pla2g4aa is related to the ovulation process in zebrafish.

Roles of progesterone receptor membrane component 1 and membrane progestin receptor alpha in regulation of zebrafish oocyte maturation

Although previous studies suggest membrane progesterone receptor alpha (mPRα/Paqr7) mediates 17, 20β-dihydroxy-4-pregnen-3-one (DHP) induction of oocyte maturation (OM) in zebrafish, critical information needed to establish mPRα as the receptor mediating OM is lacking. The relative potencies of progestins and specific mPRα agonists in inducing OM matched their relative binding affinities for zebrafish mPRα, supporting its role in OM. Microinjection of pertussis toxin blocked DHP induction of OM and the progestin-induced decrease in cyclic AMP levels, suggesting mPRα activates an inhibitory G protein (Gi). Microinjection of morpholino antisense oligonucleotides to zebrafish pgrmc1 blocked induction of OM by DHP which was accompanied by decreased levels of Pgrmc1 and mPRα on the oocyte plasma membranes. Similarly, treatment of denuded oocytes with a PGRMC1 inhibitor, AG205, blocked the gonadotropin-induced increase in plasma membrane mPRα levels and attenuated DHP induction of OM. Co-incubation with two inhibitors of epidermal growth factor Erbb2, ErbB2 inhibitor II and AG 879, prevented induction of OM by DHP, indicating the likely involvement of Erbb2 in mPRα-mediated signaling. Treatment with AG205 reversed the inhibitory effects of the Erbb2 inhibitors on OM and also inhibited insulin-like growth factor-1 induction of OM. Close associations between Pgrmc1 and mPRα, and between Pgrmc1 and Erbb2 were detected in zebrafish oocytes with in situ proximity ligation assays. The results suggest progestin induction of OM in zebrafish is mediated through an mPRα/Gi/Erbb2 signaling pathway that requires Pgrmc1 for expression of mPRα on oocyte membranes and that Pgrmc1 also is required for induction of OM through Erbb2.

Candidate gene identification of ovulation-inducing genes by RNA sequencing with an in vivo assay in zebrafish

We previously reported the microarray-based selection of three ovulation-related genes in zebrafish. We used a different selection method in this study, RNA sequencing analysis. An additional eight up-regulated candidates were found as specifically up-regulated genes in ovulation-induced samples. Changes in gene expression were confirmed by qPCR analysis. Furthermore, up-regulation prior to ovulation during natural spawning was verified in samples from natural pairing. Gene knock-out zebrafish strains of one of the candidates, the starmaker gene (stm), were established by CRISPR genome editing techniques. Unexpectedly, homozygous mutants were fertile and could spawn eggs. However, a high percentage of unfertilized eggs and abnormal embryos were produced from these homozygous females. The results suggest that the stm gene is necessary for fertilization. In this study, we selected additional ovulation-inducing candidate genes, and a novel function of the stm gene was investigated.

Diethylstilbestrol impaired oogenesis of yellow catfish juveniles through disrupting hypothalamic-pituitary-gonadal axis and germ cell development

Diethylstilbestrol (DES), a non-steroidal estrogen, has been found to cause altered germ cell development and disordered ovarian development in fish females. However, the mechanisms that might be involved are poorly understood. In this study, female juveniles of yellow catfish (Pelteobagrus fulvidraco) (120 days post-hatching) were exposed to two doses (10 and 100 ng l^-1 ) of DES for 28 days. After the endpoint of exposure, decreased ovary weight and gonadosomatic index, as well as various ovarian impairments were observed in response to DES. Besides, DES elevated the mRNA levels of vitellogenin 1 (vtg 1) and estrogen receptor 1 (esr 1) in liver and decreased 17β-estradiol level in plasma. Correspondingly, suppressed mRNA levels of the key genes in the hypothalamic-pituitary-gonadal axis (such as cyp19a1b, gnrh-II, fshβ and lhβ in brain and fshr, lhr and cyp19a1a in ovary) after DES exposure were also observed. The declined level of plasma 17β-estradiol and altered gene expressions of genes in the hypothalamic-pituitary-gonadal axis were thus supposed to be closely related to the disrupted oogenesis in DES-treated fish. Analyses further demonstrated that, higher concentration of DES elevated the expression ratio of bax/bcl-2, indicating the enhanced apoptosis occurred in ovary. Moreover, DES upregulated the expressions of genes involved in proliferation (cyclin d1 and pcna), meiotic entry (cyp26a1 and scp3) and meiotic maintenance (dmc1), resulting in arrested oogenesis in catfish. The present study greatly extended our understanding on the mechanisms underlying of reproductive toxicity of DES on fish oogenesis.

Oocyte viability and cortical activation under different salt solutions in Prochilodus lineatus (Teleostei: Prochilodontidae)

This study aimed to evaluate the effect of five salt solutions in the maintenance of morphological features of cortical alveolus, hydration and fertilization capacity of Prochilodus lineatus oocytes. For this purpose, five saline solutions were tested: Ringer's solution, Ringer's lactate solution, Hank's balanced salt solution (HBSS), Hank's balanced salt solution without calcium (HBSS without calcium) and solution for salmonid eggs. Oocytes were maintained for 2 hr in saline solution with controlled temperature subsequently evaluated for hydration, cortical activation and fertilization ability. In the evaluation of the fertilization ability, two controls were used: C1-fertilized oocytes after extrusion-and C2-oocytes kept in ovarian fluid and fertilized after 2 hr. There was a significant reduction in the viability of oocytes C2 (28.8% ± 12.9%) compared to C1 (65.3% ± 26.7%), and no significant differences were found between treatments HBSS and HBSS without calcium and C2. Only HBSS and HBSS without calcium maintained the non-activated state of the gametes, with a fertilization rate of 16.4% ± 6.7% and 5.6% ± 2.3%, respectively; however, they did not extend the viability of oocytes, such that they continued to undergo degradation during the storage period, similar to oocytes retained only in ovarian fluid.

Derivation and Evaluation of Putative Adverse Outcome Pathways for the Effects of Cyclooxygenase Inhibitors on Reproductive Processes in Female Fish

Cyclooxygenase (COX) inhibitors are ubiquitous in aquatic systems and have been detected in fish tissues. The exposure of fish to these pharmaceuticals is concerning because COX inhibitors disrupt the synthesis of prostaglandins (PGs), which modulate a variety of essential biological functions, including reproduction. In this study, we investigated the effects of well-characterized mammalian COX inhibitors on female fathead minnow reproductive health. Fish (n = 8) were exposed for 96 h to water containing indomethacin (IN; 100 µg/l), ibuprofen (IB; 200 µg/l) or celecoxib (CX; 20 µg/l), and evaluated for effects on liver metabolome and ovarian gene expression. Metabolomic profiles of IN, IB and CX were not significantly different from control or one another. Exposure to IB and CX resulted in differential expression of comparable numbers of genes (IB = 433, CX = 545). In contrast, 2558 genes were differentially expressed in IN-treated fish. Functional analyses (canonical pathway and gene set enrichment) indicated extensive effects of IN on PG synthesis pathway, oocyte meiosis, and several other processes consistent with physiological roles of PGs. Transcriptomic data were congruent with PG data; IN-reduced plasma PG F2α concentration, whereas IB and CX did not. Five putative AOPs were developed linking the assumed molecular initiating event of COX inhibition, with PG reduction and the adverse outcome of reproductive failure via reduction of: (1) ovulation, (2) reproductive behaviors mediated by exogenous or endogenous PGs, and (3) oocyte maturation in fish. These pathways were developed using, in part, empirical data from the present study and other publicly available data.

Fine selection of up-regulated genes during ovulation by in vivo induction of oocyte maturation and ovulation in zebrafish

BACKGROUND

Two essential processes, oocyte maturation and ovulation, are independently induced, but proceed cooperatively as the final step in oogenesis before oocytes become fertilizable. Although these two processes are induced by the same maturation-inducing steroid, 17α, 20β-dihydroxy-4-pregnen-3-one (17, 20β-DHP), in zebrafish, it has been suggested that the receptor, and thus the signal transduction pathway is different for each process. Although much progress has been made in understanding the molecular mechanisms underlying the induction of oocyte maturation, the mechanisms for inducing ovulation remain under investigation. In the present study, in vivo induction techniques that permit the induction of oocyte maturation and ovulation in living zebrafish (in vivo assays) were used to select highly up-regulated genes (genes associated with ovulation). Using an in vivo assay, ovarian tissues that induced only oocyte maturation could be obtained. This made it possible for the first time to distinguish maturation-inducing genes from ovulation-inducing genes. Using a genome-wide microarray of zebrafish sequences, the gene expression levels were compared among an ethanol (EtOH)-treated group (non-activated group), a diethylstilbestrol (DES)- or testosterone (Tes)-treated group (maturation-induced group), and a 17, 20β-DHP-treated group (maturation- and ovulation-induced group). Ovulation-specific up-regulated genes were selected. The mRNA expression levels of the selected genes were measured by quantitative polymerase chain reaction (qPCR).

RESULTS

Among 34 genes identified, three that showed ovulation-specific increases were selected as candidates potentially associated with ovulation. The ovulation-specific up-regulation of three candidates, slc37a4a, zgc:65811 and zgc:92184 was confirmed by qPCR.

CONCLUSION

Our in vivo assay provides a new approach to precisely select genes associated with ovulation.

Methods for the analysis of early oogenesis in Zebrafish

Oocyte differentiation is a highly dynamic and intricate developmental process whose mechanistic understanding advances female reproduction, fertility, and ovarian cancer biology. Despite the many attributes of the zebrafish model, it has yet to be fully exploited for the investigation of early oocyte differentiation and ovarian development. This is partly because the properties of the adult zebrafish ovary make it technically challenging to access early stage oocytes. As a result, characterization of these stages has been lacking and tools for their analysis have been insufficient. To overcome these technical hurdles, we took advantage of the juvenile zebrafish ovary, where early stage oocytes can readily be found in high numbers and progress in a predictable manner. We characterized the earliest stages of oocyte differentiation and ovarian development and defined accurate staging criteria. We further developed protocols for quantitative microscopy, live time-lapse imaging, ovarian culture, and isolation of stage-specific oocytes for biochemical analysis. These methods have recently provided us with an unprecedented view of early oogenesis, allowing us to study formation of the Balbiani body, a universal oocyte granule that is associated with oocyte survival in mice and required for oocyte and egg polarity in fish and frogs. Despite its tremendous developmental significance, the Bb has been little investigated and how it forms was unknown in any species for over two centuries. We were able to trace Balbiani body formation and oocyte symmetry breaking to the onset of meiosis. Through this investigation we revealed novel cytoskeletal structures in oocytes and the contribution of specialized cellular organization to differentiation. Overall, the juvenile zebrafish ovary arises as an exciting model for studies of cell and developmental biology. We review these and other recent advances in vertebrate oogenesis in an accompanying manuscript in this issue of Developmental Biology. Here, we describe the protocols for ovarian investigation that we developed in the zebrafish, including all experimental steps that will easily allow others to reproduce such analysis. This juvenile ovary toolbox also contributes to establishing the zebrafish as a model for post-larval developmental stages.

Deep Brain Photoreceptor (val-opsin) Gene Knockout Using CRISPR/Cas Affects Chorion Formation and Embryonic Hatching in the Zebrafish

Non-rod non-cone photopigments in the eyes and the brain can directly mediate non-visual functions of light in non-mammals. This was supported by our recent findings on vertebrate ancient long (VAL)-opsin photopigments encoded by the val-opsinA (valopa) and val-opsinB (valopb) genes in zebrafish. However, the physiological functions of valop isoforms remain unknown. Here, we generated valop-mutant zebrafish using CRISPR/Cas genome editing, and examined the phenotypes of loss-of-function mutants. F0 mosaic mutations and germline transmission were confirmed via targeted insertions and/or deletions in the valopa or valopb gene in F1 mutants. Based on in silico analysis, frameshift mutations converted VAL-opsin proteins to non-functional truncated forms with pre-mature stop codons. Most F1 eggs or embryos from F0 female valopa/b mutants showed either no or only partial chorion elevation, and the eggs or embryos died within 26 hour-post-fertilization. However, most F1 embryos from F0 male valopa mutant developed but hatched late compared to wild-type embryos, which hatched at 4 day-post-fertilization. Late-hatched F1 offspring included wild-type and mutants, indicating the parental effects of valop knockout. This study shows valop gene knockout affects chorion formation and embryonic hatching in the zebrafish.

Establishment of procedures for studying mPR-interacting agents and physiological roles of mPR

More than 10years have passed since the discovery of membrane progestin receptors (mPRs). Although the identification of mPR genes in various organisms and mPR expression patterns have been described since then, the precise physiological roles of mPRs are still unclear, except their function as a receptor for maturation-inducing steroid in fish. The wide distribution of mPRs suggests variable actions for progestins through mPRs in the tissues. Information about the physiological roles of mPRs, such as roles in the progression of breast cancer and T-cell proliferation, has gradually accumulated recently. These results suggest that mPRs are possible targets for new pharmaceuticals. We established a cell line that was transformed with cDNAs for mPRα and a recombinant luciferase gene named GloSensor. The cells can be used for monitoring the effects of ligands on mPRα based on intracellular cyclic adenosine monophosphate (cAMP) levels. Studies using these cell lines indicated that the cAMP concentration is decreased by ligands for mPRα. The results provide support for previous results suggesting that mPRα is coupled to inhibitory G protein (Gi). We also established screening methods that make it possible to screen ligands for mPR. Recently, we succeeded in expressing and purifying recombinant mPR protein in the yeast Pichia pastoris. Relatively large amounts of mPR protein with hormonal binding activity can be purified by our method. The recombinant protein will be applicable to establishing a molecular probe to detect mPR-interacting agents. To obtain decisive evidence for the roles of mPRs, we are establishing strains of medaka fish that are deficient in mPRs. In medaka, four subtypes of mPR genes (α, β, γ, and α2) have been identified. By reverse genetic screening, we have selected three to four strains in which a point mutation has been induced in the coding sequence of the mPR subtypes. However, homozygous mutants of each mPR gene showed no phenotype. The results suggested that mPR genes share redundancy. We are currently producing double and triple mutants of the mPR subtypes. The physiological roles of mPRs will be demonstrated using the mutant medaka strains.

Progestins as endocrine disrupters in aquatic ecosystems: Concentrations, effects and risk assessment

In aquatic ecosystems, progesterone (P4) and synthetic progestins (gestagens) originate from excretion by humans and livestock. Synthetic progestins are used for contraception and as P4 for medical treatments as well. Despite significant use, their ecotoxicological implications are poorly understood. Only about 50% of the progestins in use have been analyzed for their environmental occurrence and effects in aquatic organisms. Here we critically summarize concentrations and effects of progestins in aquatic systems. P4 and progestins were mostly detected when analyzed for, and they occurred in the low ng/L range in wastewater and surface water. In animal farm waste and runoff, they reached up to several μg/L. P4 and synthetic progestins act through progesterone receptors but they also interact with other steroid hormone receptors. They act on the hypothalamus-pituitary-gonad axis, lead to oocyte maturation in female and sperm motility in male fish. Additionally, other pathways are affected as well, including the circadian rhythm. Effects of P4, mifepristone and eleven synthetic progestins have been studied in fish and a few compounds in frogs and mussels. Environmental risks may be associated with P4, dydrogesterone and medroxyprogesterone acetate, where transcriptional effects were found at highest environmental levels. Reproductive effects occurred at higher levels. However, norethindrone, levonorgestrel and norgestrel compromised reproduction at environmental (ng/L) concentrations. Thus, some of the progestins are very active endocrine disrupters. This review summarizes the current state of the art and highlights risks for fish. Further research is needed into environmental concentrations and effects of non-investigated progestins, unexplored modes of action, and the activity of mixtures of progestins and other steroids to fully assess their environmental risks.

Estrogen receptor 2b deficiency impairs the antiviral response of zebrafish

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

The role of eicosanoids in 17α, 20β-dihydroxy-4-pregnen-3-one-induced ovulation and spawning in Danio rerio

This study employed a hormone bioassay to characterize the eicosanoids involved in zebrafish ovulation and spawning, in particular the prostaglandin (PG) products of cyclooxygenase (COX) metabolism and the leukotriene (LT) products of lipoxygenase (LOX) metabolism. Exposure to the teleost progestogen 17α, 20β-dihydroxy-4-pregnen-3-one (17,20βP) induced ovulation, but not spawning, in solitary females and both ovulation and spawning in male-female pairs. Transcription of the eicosanoid-synthesizing enzymes cytosolic phospholipase A2 (cPLA(2)) and COX-2 increased and LTC(4) synthase decreased in peri-ovulatory ovaries of 17,20βP-exposed fish. Ovarian PGF(2α) levels increased post-spawning in 17,20βP-exposed fish, but there was no difference in LTB(4) or LTC(4). Pre-exposure to cPLA(2) or LOX inhibitors reduced 17,20βP-induced ovulation rates, while a COX inhibitor had no effect on ovulation or spawning. Collectively, these findings suggest that eicosanoids, in particular LOX metabolites, mediate 17,20βP-induced ovulation in zebrafish. COX metabolites also appear to be involved in ovulation and spawning but their role remains undefined.

Impact of endocrine-disrupting chemicals on reproductive function in zebrafish (Danio rerio)

The prevalence of endocrine-disrupting chemicals (EDCs) in the aquatic environment has been associated with the wide detection of alterations in the development and physiology of vertebrates. Zebrafish, as a model species, has been extensively used in toxicological research. In this review, we focus on recent published evidence of the harmful effects of EDCs on reproductive function in zebrafish, including skewed sex ratio, immature gonads, diminished sexual behaviour, decreased sperm count, reduced spawning and fertilization. These impairments mostly result from disruption to sex-steroid hormones induced by endocrine disruptors. We also discuss other effects of exposure to EDCs. In EDC exposure research, despite incomplete assessments of altered gonad histopathology and sexual behaviour, these present potential effective biomarkers or pathways for evaluating the reproductive function in zebrafish on EDC exposure. To date, the pernicious effects of some EDCs on the reproductive performance in laboratory zebrafish are well understood; however, similar alterations remain for further determination in wild-type fish and more kinds of EDCs. More studies should be performed under established scientific regulatory criteria to investigate the impact of EDCs on reproduction in zebrafish. Moreover, further research is required to explain the definite mechanism of sexual differentiation, which helps in understanding the shift of sexual phenotype with EDC exposure.

Progesterone increases ex vivo testosterone production and decreases the expression of progestin receptors and steroidogenic enzymes in the fathead minnow (Pimephales promelas) ovary

Progesterone (P4) is a metabolic precursor for a number of steroids, including estrogens and androgens. P4 also has diverse roles within the vertebrate ovary that include oocyte growth and development. The objectives of this study were to measure the effects of P4 on testosterone (T) and 17β-estradiol (E2) production in the fathead minnow (FHM) ovary and on the mRNA abundance of transcripts involved in steroidogenesis and steroid receptor signaling. Ovary explants were treated with P4 (10(-6)M) for 6 and 12h. P4 administration significantly increased T production ∼3-fold at both 6 and 12h, whereas E2 production was not affected, consistent with the hypothesis that excess P4 is not converted to terminal estrogens in the mature ovary. Nuclear progesterone receptor mRNA was decreased at 6h and membrane progesterone receptor gamma-2 mRNA was significantly down-regulated at both 6 and 12h; however there was no change in membrane progesterone receptor alpha or beta mRNA levels. Androgen receptor (ar) and estrogen receptor 2a (esr2a) mRNA were significantly reduced at 6h with P4 treatment, but there was no change in esr2b mRNA at either time point. Transcripts for enzymes in the steroid pathway (star, hsd11b2) were significantly lower at 6h compared to controls, whereas cyp17a and cyp19a mRNA abundance did not change with treatments at either time point. These data suggest that P4 incubation can lead to increased T production in the FHM ovary without a concomitant change in E2, and that the membrane bound progestin receptors are differentially regulated by P4 in the teleost ovary. As environmental progestins have received increased attention due to their suspected role as endocrine disruptors, mechanistic data on the role of exogenous P4 treatments in the male and female gonad is warranted.

Induction of female-to-male sex change in adult zebrafish by aromatase inhibitor treatment

This study investigated whether undifferentiated germ and/or somatic stem cells remain in the differentiated ovary of a species that does not undergo sex changes under natural conditions and retain their sexual plasticity. The effect of aromatase inhibitor (AI)-treatment on sexually mature female zebrafish was examined. A 5-month AI treatment caused retraction of the ovaries after which testes-like organs appeared, and cyst structures filled with spermatozoa-like cells were observed in sections of these tissues. Electron microscopic observations revealed that these cells appeared as large sperm heads without tails. Sperm formation was re-examined after changing the diet to an AI-free food. A large number of normal sperm were obtained after eight weeks, and no formation of ovarian tissue was observed. Artificial fertilization using sperm from the sex-changed females was successful. These results demonstrated that sex plasticity remains in the mature ovaries of this species.

Progesterone alters global transcription profiles at environmental concentrations in brain and ovary of female zebrafish (Danio rerio)

Progesterone (P4) is a natural steroid hormone excreted by humans and animals. Noncomplete degradation in treatment plants result in levels in the ng/L range in surface waters. Very little is known of the effects on fish at such concentrations. Here we determine the global expression profile in the brain and ovary of female zebrafish exposed for 14 days to 3.5, 33 and 306 ng/L P4 to elucidate molecular effects. For validation selected transcripts were determined by RT-qPCR. In the brain, 54 and 255 transcripts were altered at 3.5 and 306 ng/L, respectively. Genes related to circadian rhythm (nr1d2b, per1b), cell cycle and reproduction (cdc20, ccnb1) were down-regulated. In the ovary, transcriptional changes occurred in 200, 84 and 196 genes at 3.5, 33 and 306 ng/L, respectively. The genes belong to different pathways including cardiac hypertrophy, cell cycle and its regulation. P4 slightly influenced oocyte maturation as revealed by histology of the ovaries. In the liver, vtg1 was down-regulated at all concentrations and VTG protein at 306 ng/L in the blood. The data show molecular effects and the modes of action of P4 at environmental concentrations. Ultimately they may translate to adverse effects on reproduction.

A novel method for induction of pairing in Xenopus by addition of steroids into the water

The African clawed frog, Xenopus laevis, is widely used in biological studies. Ovulation of Xenopus is normally induced by the injection of human chorionic gonadotropin (hCG) into the dorsal lymph sac of fully-grown female frogs. Previously, we reported a novel method for inducing Xenopus ovulation by adding a mixture of steroids into the surrounding water. In the present study, we demonstrate how to induce reproductive behavior in male frogs using the same methodology. The types and concentrations of steroids were evaluated, and the efficiency of the selected steroid for the induction of ejaculation was examined. New procedures were also examined for inducing mating by mixing both females and males activated by steroids. In male frogs, testosterone was effective for the induction of physiological changes, accumulation of melanin in the hands and induction of amplexus. Time course experiments revealed that eight hours were sufficient to induce male reproductive behavior and ovulation in females. Finally, we established an efficient means of inducing pairing in frogs that involved pre-treatment of frogs with salt solution followed by testosterone for males and a mixture of estradiol and progesterone for females. Although the numbers of oocytes obtained were relatively fewer than those resulting from hCG injection, the fertilization rate of eggs ovulated using the new treatment method was similar to that with eggs obtained by hCG-injection, and juveniles developed normally. In conclusion, we have developed a novel method to induce pairing in frogs without the need for injections.