Molecular cloning and characterization of Japanese eel ovarian P450c17 (CYP17) cDNA

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.

Establishment of cell-lines stably expressing recombinant Japanese eel follicle-stimulating hormone and luteinizing hormone using CHO-DG44 cells: fully induced ovarian development at different modes

The gonadotropins (Gth), follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), play central roles in gametogenesis in vertebrates. However, available information on their differential actions in teleost, especially in vivo, is insufficient. In this study, we established stable CHO-DG44 cell lines expressing long-lasting recombinant Japanese eel Fsh and Lh with extra O-glycosylation sites (Fsh-hCTP and Lh-hCTP), which were produced in abundance. Immature female eels received weekly intraperitoneal injections of Gths. Fsh-hCTP induced the entire ovarian development by 8 weeks from the beginning of injection; thus, the ovaries of most fish were at the migratory nucleus stage while the same stage was observed in eels after 4 weeks in the Lh-hCTP-treated group. In contrast, all pretreated and saline-injected eels were in the pre-vitellogenic stage. Gonadosomatic indices in the Fsh-hCTP-treated group were significantly higher than those in the Lh-hCTP group at the migratory nucleus stage because of the significantly higher frequency of advanced ovarian follicles. Ovarian mRNA levels of genes related to E2 production (cyp11a1, cyp17a1, cyp19a1, hsd3b, fshr, and lhr) were measured using real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). All genes were induced by both Fsh-hCTP and Lh-hCTP, with a peak at either the mid- or late vitellogenic stages. Transcript abundance of cyp19a1 and fshr in the Lh-hCTP group were significantly higher than those in the Fsh-hCTP group, whereas no difference in the expression of other genes was observed between the groups. Fluctuations in serum levels of sex steroid hormones (estradiol-17β, 11-ketotestosterone, and testosterone) in female eels were comparable in the Fsh-hCTP and Lh-hCTP groups, thus increasing toward the maturational phase. Furthermore, the fecundity of the eels induced to mature by Fsh-hCTP was significantly higher than that induced by Lh-hCTP. These findings indicate that Fsh and Lh can induce ovarian development in distinctively different modes in the Japanese eel.

Stimulatory Effects of Androgens on Eel Primary Ovarian Development - from Phenotypes to Genotypes

Androgens stimulate primary ovarian development in Vertebrate. Japanese eels underwent operation to sample the pre- and post-treated ovarian tissues from the same individual. Ovarian phenotypic or genotypic data were mined in a pair. A correlation between the initial ovarian status (determined by kernel density estimation (KDE), presented as a probability density of oocyte size) and the consequence of androgen (17MT) treatment (change in ovary) has been showed. The initial ovarian status appeared to be important to influence ovarian androgenic sensitivity. The initial ovary was important to the outcomes of androgen treatments, and ePAV (expression presence-absence variation) is existing in Japanese eel by analyze DEGs; core, unique, or accessory genes were identified, the sensitivities of initial ovaries were correlated with their gene expression profiles. We speculated the importance of genetic differential expression on the variations of phenotypes by 17MT, and transcriptomic approach seems to allow extracting multiple layers of genomic data.

Regulation of 17α-Hydroxyprogesterone Production during Induced Oocyte Maturation and Ovulation in Amur Sturgeon (Acipenser schrenckii)

In several teleosts, 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) has been identified as a maturation-inducing steroid. DHP is synthesized from 17α-hydroxyprogesterone (17OHP) by 17β-hydroxysteroid dehydrogenase type 12-like (hsd17b12L). Along with 3β-hydroxysteroid dehydrogenase/Δ5-4 isomerase (3β-HSD), 17α-hydroxylase and C17-20 lyase are associated with 17OHP production. This study aimed to determine the roles of Amur sturgeon hsd3b, P450c17-I (cyp17a1), and P450c17-II (cyp17a2) in 17OHP production and to examine their enzyme activity and mRNA expression pattern during oocyte maturation. In the sturgeons used in this study, hsd3b encoded 3β-HSD, cyp17a1 catalyzed 17α-hydroxylase production with C17-20 lyase activity, and cyp17a2 processed 17α-hydroxylase activity alone. In the ovarian follicles of individuals that underwent induced ovulation, hsd3b mRNA levels increased rapidly, cyp17a1 expression was downregulated, and cyp17a2 expression was upregulated during oocyte maturation. Finally, an in vitro study revealed that salmon pituitary extract (SPE) stimulation rapidly induced hsd3b expression, whereas cyp17a1 expression was downregulated. In vitro, cyp17a2 expression did not rapidly increase with SPE stimulation. This rapid upregulation of hsd3b during oocyte maturation was first observed in teleosts. It was suggested that hsd17b12L expression is upregulated after 17OHP production, which is regulated by hsd3b, cyp17a1, and cyp17a2, resulting in DHP production.

Dynamic transcriptome analysis of ovarian follicles in artificial maturing Japanese eel (Anguilla japonica)

Inducing maturation of the ovaries to enable the production of good-quality eggs is critical for the successful artificial breeding of Anguilla japonica. During the spawning season, however, the ovaries of A. japonica have been found to develop into asynchronous clutches, impeding the success of artificial breeding on a commercial scale. The dynamic molecular regulation of follicular development in the same individual was assessed by transcriptome analysis of the five stages of follicles, the pre-vitellogenic, early vitellogenic, midvitellogenic, late vitellogenic, and migratory nucleus stages in artificial maturing A. japonica. Comparisons across these developmental stages identified a total of 19,298 differentially expressed transcripts (DETs). Short time-series expression miner analysis across these DETs revealed four significant expression profiles. Gene Ontology function and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses found that some of the significantly enriched biological processes and metabolic pathways included those related to steroid hormone biosynthesis (cyp11a1, cyp17a1, cyp17a2, hsd17b1, and hsd17b12), cargo receptor activity (vtgr and vldlr), meiosis and ovulation (pgrs and mPRγ), hydration (cts and aqp1), and egg coat formation (zp). These genes and pathways were associated with serum 17β-estradiol concentrations and morphological changes. The levels of hsd17b12 and mPRγ mRNAs were much higher during the migratory nucleus stage, suggesting their respective involvement in the biosynthesis and functional pathway of the maturation-inducing steroid 17α,20β-dihydroxy-4-pregnen-3-one. The gene subtypes aqp1b and ctsd may regulate water influx into oocytes and yolk protein proteolysis, respectively. To our knowledge, the present study is the first to describe combined transcriptome profiling of asynchronously developing follicles in the same individual. The findings suggest that steroid hormone synthesis and nutrient absorption in follicular somatic cells play important roles during follicular development and maturation, despite the same external physiological surroundings.

Japanese eel retinol dehydrogenases 11/12-like are 17-ketosteroid reductases involved in sex steroid synthesis

The synthesis of 11-ketotestosterone (11KT) and estradiol-17β (E2), which play important roles in the regulation of gametogenesis in teleost fishes, is catalyzed by several steroidogenic enzymes. In particular, 17β-hydroxysteroid dehydrogenases (Hsd17bs) with 17-ketosteroid reducing activity (17KSR activity) are essential enzymes in the formation of these sex steroid hormones in the gonads and other tissues. Retinol dehydrogenase 11 (RDH11) has been suggested to be a novel tentative HSD17B (HSD17B15) in humans for a decade, however no definitive proof has been provided yet. In this study, three cDNAs related to human RDH11 were isolated from Japanese eel testis and characterized. Sequence similarity and phylogenetic analyses revealed their close relationship to human rdh11 and rdh12 gene products and they were designated as rdh11/12-like 1, rdh11/12-like 2, and rdh11/12-like 3. Three recombinant Rdh11/12-like proteins expressed in HEK293T cells catalyzed the transformation of estrone into E2 and androstenedione into testosterone. Only Rdh11/12-like 1 catalyzed the conversion of 11-ketoandrostenedione into 11KT. Tissue-distribution analysis by quantitative real-time polymerase chain reaction revealed, in immature male Japanese eel, that rdh11/12-like 1 and rdh11/12-like 2 are predominantly expressed in testis and brain, while rdh11/12-like 3 is expressed ubiquitously. Moreover, we analyzed the effects of gonadotropins and 11KT on the expression of the three rdh11/12-like mRNAs in the immature testis. In vitro incubation of immature testes with various doses of recombinant Japanese eel follicle stimulating hormone, luteinizing hormone, and 11KT indicated that the expression of rdh11/12-like 1 mRNA, rdh11/12-like 2, and rdh11/12-like 3 did not change. These findings suggest that the three Rdh11/12-like proteins metabolize sex steroids. Rdh11/12-like 1 may be one of the enzymes with 17KSR activity involved in the production of 11KT in the testis.

Correlation between the ovarian status and the androgen sensibility in the cultured Japanese eel, Anguilla japonica

Different responsive abilities of different types of eels (cultured, cultured feminized, and wild silver eels) during artificial maturation are recognized, and maturity status at the beginning of artificial maturation might be important. Maturity may represented by the distribution pattern of oocyte diameters. Androgens have been demonstrated to stimulate ovarian development in eels. To determine the initial status, operations were performed on eels to identify sex and to sample ovarian tissue. The recovered eels were then treated with 17α-methyltestosterone (17MT), and the responses of individual eels to 17MT were determined by the fold change in the mean oocyte diameter before and after treatment. Sampled ovarian tissues were fixed in Bouin's solution, oocytes were isolated, and the diameter of isolated oocytes was measured. The ovarian status, determined by kernel density estimation (KDE), was presented by the probability density of measured oocyte diameters; compared with histograms, a description method, KDE, provided more subtle information on the investigated ovary. Our data indicated a correlation between the initial ovarian status (density pattern) and the consequence of treatment (change of ovary); we also argued the semelparity of the Japanese eel. Our results supported the hypothesis that the initial ovarian status is an important factor affecting artificial maturation and that androgens could ameliorate the initial status of the eel ovary.

Steroidogenesis and its regulation in teleost-a review

Steroid hormones modulate several important biological processes like metabolism, stress response, and reproduction. Steroidogenesis drives reproductive function wherein development and differentiation of undifferentiated gonads into testis or ovary, and their growth and maturation, are regulated. Steroidogenesis occurs in gonadal and non-gonadal tissues like head kidney, liver, intestine, and adipose tissue in teleosts. This process is regulated differently through multi-level modulation of promoter motif transcription factor regulation of steroidogenic enzyme genes to ultimately control enzyme activity and turnover. In view of this, understanding teleostean steroidogenesis provides major inputs for technological innovation of pisciculture. Unlike higher vertebrates, steroidal intermediates and shift in steroidogenesis is critical for gamete maturation in teleosts, more essentially oogenesis. Considering these characteristics, this review highlights the promoter regulation of steroidogenic enzyme genes by several transcription factors that are involved in teleostean steroidogenesis. It also addresses different methodologies involved in promoter regulation studies together with glucocorticoids and androgen relationship with reference to teleosts.

Functional analysis of recombinant single-chain Japanese eel Fsh and Lh produced in FreeStyle 293-F cell lines: Binding specificities to their receptors and differential efficacy on testicular steroidogenesis

Pituitary gonadotropins, follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), play central roles in the control of gonadal development of vertebrates. In mammals, Fsh and Lh exclusively activate their respective cognate receptors: Fsh receptor (Fshr) in the Sertoli cell and Lh/choriogonadotropin receptor (Lhcgr) in the Leydig cell. In teleosts, the distinct functions of Fsh and Lh and information on cellular localization of their receptors are still poorly understood. Recently we established FreeStyle 293-F cell lines producing recombinant Japanese eel Fsh and Lh (reFsh and reLh), which form a single chain consisting of a common α-subunit and β-subunits. In this study, we conducted functional analyses of reFsh and reLh, focusing on the binding specificities to their receptors and effects on testicular steroidogenesis in vitro. Assays with gonadotropin receptors-expressing COS-7 cells indicated reFsh stimulated its cognate receptor, meanwhile reLh activated both receptors. Although results of in vitro incubations showed that reFsh and reLh induced testicular 11-ketotestosterone production in a dose and time-dependent manner by upregulating expression of steroidogenic enzymes, the effective doses of reLh were apparently lower and the effects of reLh emerged faster in comparison with reFsh. Results of quantitative real-time PCR using testicular cell fractions showed that fshr and lhcgr1 mRNA were detected both in Sertoli and Leydig cells. These analyses revealed that reFsh and reLh were biologically active and hence will be useful for future studies. Moreover, our data showed that both eel Fsh and Lh acted as steroidogenic hormones through their receptors in testicular somatic cells; however, Lh was more potent on androgen production, implying differential functions on spermatogenesis.

Changes in Sex Steroids and Ovarian Steroidogenic Enzyme mRNA Levels in Artificially Maturing Japanese Eel (Anguilla japonica) and Naturally Maturing New Zealand Longfin Eel (Anguilla dieffenbachii) during Vitellogenesis

Repeated hormone injections are routinely used to induce advanced stages of oogenesis in freshwater eels, but this approach may result in aberrant germ cell development. To investigate the underlying causes, levels of sex steroids (testosterone, T; estradiol-17β, E2) and ovarian steroidogenic enzyme mRNAs were compared between artificially maturing Japanese eels and wild-caught, spontaneously maturing New Zealand longfin eels. The latter were employed as reference, as wild Japanese eels in advanced stages of oogenesis are near-impossible to catch. Serum T levels in artificially maturing Japanese eel changed with stage in a pattern that was comparable to that in longfin eels. Likewise, ovarian mRNA levels of most steroidogenic enzyme genes were not qualitatively dissimilar between both eel species when taking developmental stage into account. However, aromatase (cyp19a) mRNA levels, together with serum E2 levels, rapidly increased in artificially maturing Japanese eels in mid-late stages of oogenesis (gonadosomatic index, GSI = 13.8%), whereas no such increase was evident in longfin eels (GSI ~ 6.9%). In addition, sex steroid and target gene mRNA levels fluctuated drastically with each hormone injection. We contend that expression of most target genes, possibly even that of cyp19a, during induced oogenesis could be “normal”, with the drastic fluctuations due most likely to hormone delivery through repeated injections. The effects of these fluctuations on gamete quality remain unknown and resolving this issue may prove fruitful in the future to further artificial propagation of anguillid eels.

Two cyp17 genes perform different functions in the sex hormone biosynthesis and gonadal differentiation in Japanese flounder (Paralichthys olivaceus)

P450c17, a key enzyme in the steroid generation pathway, plays an important role in the production of sex steroid and cortisol. In this study, two cyp17 gene isoforms, Pocyp17-I and Pocyp17-II were isolated from Paralichthys olivaceus gonads. Domain architecture analysis of Pocyp17-I and Pocyp17-II revealed that they had three regions important to enzymatic function. Structural analysis showed that Pocyp17-I and Pocyp17-II had 8 and 9 exons respectively, and the difference was caused by the insertion of an extra intron (intron1) in the latter. Quantitative real-time polymerase chain reaction results indicated that the expression of these two genes showed sexually dimorphism that Pocyp17-I and Pocyp17-II were highest expressed in testis and ovary, respectively. The in situ hybridization analysis of gonads indicated that Pocyp17-I and Pocyp17-II mRNA were both detected in oocytes, spermatocytes and Sertoli cells. After injection of androgen and estrogen (17α-methyltestosterone, 17β-estradiol) of different concentrations, the expression level of Pocyp17-I decreased significantly (P < 0.01), whereas estrogen had no influence on Pocyp17-II, but androgen upregulated the expression of Pocyp17-II (P < 0.05). Moreover, Pocyp17-I expression level was down-regulated significantly by NR0b1 but up-regulated by NR5a2 (P < 0.05), whereas Pocyp17-II expression level was down-regulated significantly by NR0b1 and NR5a2 (P < 0.05). All these results demonstrated that there were differences in expression patterns, feedback actions of sex hormones and transcriptional regulations between cyp17-I and cyp17-II, which revealed that cyp17-I and cyp17-II might perform different functions in sex hormones biosynthesis and gonadal differentiation in Japanese flounder.

Molecular identification of steroidogenesis-related genes in scallops and their potential roles in gametogenesis

Sex steroids are crucial for controlling gametogenesis and germ cell maturation in vertebrates. It has been proposed that Yesso scallop (Mizuhopecten yessoensis) has the same sex steroids as those animals, but the scallop biosynthetic pathway is unclear. In this study, we characterized several steroidogenesis-related genes in M. yessoensis and proposed a putative biosynthetic pathway for sex steroids that is similar to that of vertebrates. Specifically, we identified several steroidogenesis-related gene sequences that encode steroid metabolizing enzymes: StAR-related lipid transfer (START) protein, 17α-hydroxylase, 17,20-lyase (cyp17a), 17β-hydroxysteroid dehydrogenase (hsd17b), and 3β-hydroxysteroid dehydrogenase (hsd3b). We sampled adult scallops throughout their reproductive phase to compare their degree of maturation with their intensity of mRNA expression. Semi-quantitative RT-PCR analysis revealed a ubiquitous expression of transcripts for steroid metabolizing enzymes (i.e., star, cyp17a, hsd17b, and hsd3b) in peripheral and gonadal tissues. Real-time PCR analysis revealed a high level of expression of star3 and cyp17a genes in gonadal tissues at the early stage of cell differentiation in scallops. Interestingly, mRNA expression of hsd3b and hsd17b genes showed a synchronous pattern related to degree of gonad maturity. These results indicate that both hsd3b and hsd17b genes are likely involved in steroidogenesis in scallops. We therefore believe that these steroid-metabolizing enzymes allow scallops to endogenously produce sex steroids to regulate reproductive events.

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.

Changes in ovarian gene expression profiles and plasma hormone levels in maturing European eel (Anguilla anguilla); Biomarkers for broodstock selection

Complete sexual maturation of European eels (Anguilla anguilla) in captivity can only be achieved via injections with gonadotropins. For female eels this procedure takes 4-6months and the response ranges from "unresponsive" to final maturation and ovulation. Reproductive success could be significantly increased via early selection of responders based on predictive markers and minimally invasive sampling methods. To get a better understanding of the genetic background of ovarian maturation of the European eel we performed a pilot deep-sequencing transcriptome analysis of ovarian tissue derived from a yellow eel, a prepubertal silver eel and a post-spawning matured eel. Two key players in steroidogenesis were strongly correlated with advanced sexual maturation, namely P450c17 and liver receptor homolog-1, suggesting that blood plasma steroids might qualify as minimally invasive markers for early detection of responders. Since the predictive value of plasma sex steroid levels for final maturation of the European eel had not yet been carefully examined, we performed an extensive artificial maturation trial. Farmed silver eels were treated with pituitary extracts and sampled at multiple time intervals. Expression of steroidogenesis-related genes in ovarian tissue of responding and non-responding eels after four weekly injections with pituitary extract was compared using a custom-built microarray and RNAseq. Increased expression of 17β-hsd1 was strongly linked to sexual maturation. Blood plasma levels of sex steroids were measured using ELISAs. We show that a 2.5-fold increase in blood-plasma estradiol level after 4 weekly pituitary extract injections is a strong predictor of final sexual maturation of female European eel.

Gonadal Transcriptome Analysis in Sterile Double Haploid Japanese Flounder

Sterility is a serious problem that can affect all bionts. In teleosts, double haploids (DHs) induced by mitogynogenesis are often sterile. This sterility severely restricts the further application of DHs for production of clones, genetic analysis, and breeding. However, sterile DH individuals are good source materials for investigation of the molecular mechanisms of gonad development, especially for studies into the role of genes that are indispensable for fish reproduction. Here, we used the Illumina sequencing platform to analyze the transcriptome of sterile female DH Japanese flounder in order to identify major genes that cause sterility and to provide a molecular basis for an intensive study of gonadal development in teleosts. Through sequencing, assembly, and annotation, we obtained 52,474 contigs and found that 60.7% of these shared homologies with existing sequences. A total of 1225 differentially expressed unigenes were found, including 492 upregulated and 733 downregulated genes. Gene Ontology and KEGG analyses showed that genes showing significant upregulation, such as CYP11A1, CYP11B2, CYP17, CYP21, HSD3β, bcl2l1, and PRLR, principally correlated with sterol metabolic process, steroid biosynthetic process, and the Jak-stat signaling pathway. The significantly downregulated genes were primarily associated with immune response, antigen processing and presentation, cytokine–cytokine receptor interaction, and protein digestion and absorption. Using a co-expression network analysis, we conducted a comprehensive comparison of gene expression in the gonads of fertile and sterile female DH Japanese flounder. Identification of genes showing significantly different expression will provide further insights into DH reproductive dysfunction and oocyte maturation processes in teleosts.

Steroids in teleost fishes: A functional point of view

Steroid hormones are involved in the regulation of a variety of processes like embryonic development, sex differentiation, metabolism, immune responses, circadian rhythms, stress response, and reproduction in vertebrates. Teleost fishes and humans show a remarkable conservation in many developmental and physiological aspects, including the endocrine system in general and the steroid hormone related processes in particular. This review provides an overview of the current knowledge about steroid hormone biosynthesis and the steroid hormone receptors in teleost fishes and compares the findings to the human system. The impact of the duplicated genome in teleost fishes on steroid hormone biosynthesis and perception is addressed. Additionally, important processes in fish physiology regulated by steroid hormones, which are most dissimilar to humans, are described. We also give a short overview on the influence of anthropogenic endocrine disrupting compounds on steroid hormone signaling and the resulting adverse physiological effects for teleost fishes. By this approach, we show that the steroidogenesis, hormone receptors, and function of the steroid hormones are reasonably well understood when summarizing the available data of all teleost species analyzed to date. However, on the level of a single species or a certain fish-specific aspect of physiology, further research is needed.

Characterization and expression of cDNAs encoding P450c17-II (cyp17a2) in Japanese eel during induced ovarian development

Estradiol-17β (E2) and maturation-inducing hormone (MIH) are two steroid hormones produced in the teleost ovary that are required for vitellogenic growth and final oocyte maturation and ovulation. During this transition, the main steroid hormone produced in the ovary shifts from estrogens to progestogens. In the commercially important Japanese eel (Anguilla japonica), the MIH 17α,20β-dihydroxy-4-pregnen-3-one (DHP) is generated from its precursor by P450c17, which has both 17α-hydroxylase and C17-20 lyase activities. In order to elucidate the regulatory mechanism underlying the steroidogenic shift from E2 to DHP and the mechanistic basis for the failure of this shift in artificially matured eels, the cDNA for cyp17a2-which encodes P450c17-II-was isolated from the ovary of wild, mature Japanese eel and characterized, and the expression patterns of cyp17a1 and cyp17a2 during induced ovarian development were investigated in cultured eel ovaries. Five cDNAs (types I-V) encoding P450c17-II were identified that had minor sequence variations. HEK293T cells transfected with all but type II P450c17-II converted exogenous progesterone to 17α-hydroxyprogesterone (17α-P), providing evidence for 17α-hydroxylase activity; however, a failure to convert 17α-P to androstenedione indicated that C17-20 lyase activity was absent. Cyp17a2 mRNA was expressed mainly in the head kidney, ovary, and testis, and quantitative PCR analysis demonstrated that expression in the ovary increased during induced vitellogenesis and oocyte maturation/ovulation. In contrast, P450c17-I showed both 17α-hydroxylase and C17-20 lyase activities, and cyp17a1 expression increased until the mid-vitellogenic stage and remained high thereafter. Considering the high level of cyp17a2 transcript in the eel ovary at the migratory nucleus stage together with our previous report demonstrating that eel ovaries have strong 17α-P-to-DHP conversion activity, the failure of artificially maturing eels to produce the maturation-inducing DHP may be explained by a deficiency in 17α-P production due to the persistence of cyp17a1 expression after the completion of vitellogenesis.

Polymorphysims of CYP17-I Gene in the Exons Were Associated with the Reproductive Endocrine of Japanese Flounder (Paralichthys olivaceus)

The cytochrome P450c17-I (CYP17-I) is one of the enzymes critical to gonadal development and the synthesis of androgens. Two single nucleotide polymorphisms (SNPs) were detected within the coding region of the CYP17-I gene in a population of 75 male Japanese flounder (Paralichthys olivaceus). They were SNP1 (c.C445T) located in exon2 and SNP2 (c.T980C (p.Phe307Leu)) located in exon5. Four physiological indices, which were serum testosterone (T), serum 17β-estradiol (E₂), Hepatosomatic index (HSI), and Gonadosomatic index (GSI), were studied to examine the effect of the two SNPs on the reproductive endocrines of Japanese flounder. Multiple comparisons revealed that CT genotype of SNP1 had a much lower T level than CC genotype (p<0.05) and the GSI of individuals with CC genotype of SNP2 was higher than those with TT genotype (p<0.05). Four diplotypes were constructed based on the two SNPs and the diplotype D3 had a significantly lower T level and GSI. In conclusion, the two SNPs were significantly associated with reproductive traits of Japanese flounder.

Molecular cloning, characterization expression of P450c17-I and P450c17-II and their functions analysis during the reproductive cycle in males of barfin flounder (Verasper moseri)

P450c17, a key steroidogenic enzyme, plays important roles in the production of sex steroid and cortisol. In teleost, there are two types of P450c17, P450c17-I possessing 17α-hydroxylase and 17, 20-lyase activities, and P450c17-II only possessing 17α-hydroxylase activity. This work describes the molecular cloning of the cDNA encoding the barfin flounder (Verasper moseri) P450c17-I and P450c17-II by means of RT-PCR and 5' and 3' rapid amplification of cDNA ends (RACE) analyses and mRNA expression profiles analyzing by semiquantitative RT-PCR. Respectively, P450c17-I and P450c17-II mRNA levels in the testes correlated with serum testosterone (T) level, as well as gonadosomatic index (GSI) of males during specific stages of spermatogenesis. P450c17-I and P450c17-II mRNA were expressed in the testis and ovary, suggesting that both of them participate in the production of sex steroid in barfin flounder gonads. P450c17-I mRNA was undetectable; in contrast, P450c17-II mRNA was detected at the highest level in the head kidney, meaning that only P450c17-II is involved in the production of cortisol in barfin flounder. The results demonstrated that both of P450c17-I and P450c17-II participate in the production of sex steroid in male barfin flounder gonads.

Ovarian steroidogenesis and the role of sex steroid hormones on ovarian growth and maturation of the Japanese eel

Three sex steroid hormones, estradiol-17β (E2), 11-ketotestosterone (11-KT), and 17α,20β-dihydroxy-4-pregnen-3-one (DHP), are well established as primary estrogen, androgen, and progestin, respectively, in teleost fish. Japanese eel, Anguilla japonica, would be a suitable candidate to study ovarian steroid physiology of fish because the ovarian growth and steroidogenesis is dormant under laboratory condition but can be induced by administration of exogenous gonadotropic reagents. In this review, we summarized our work on the function and production of sex steroid hormones in the ovary of the Japanese eel during ovarian growth and oocyte maturation artificially induced by treatment with extract of salmon pituitary. In vitro and in vivo assays suggest that 11-KT and E2 play primary roles in previtellogenic and vitellogenic growth of oocytes, respectively, whereas DHP is essential for induction of final oocyte maturation. We also reviewed the correlation between ovarian steroidogenesis to produce these sex steroid hormones, serum titers and gene expression.

Isolation and characterization of the CYP17A1 gene and its processed pseudogene in Rana rugosa

CYP17A1 expression is up-regulated in the gonad in Rana (Glandirana) rugosa tadpoles treated with androgens to induce female-to-male sex-reversal. In this study, we isolated the CYP17A1 gene and its processed pseudogene from R. rugosa. The former was found to consist of 8 exons, and the latter a single-exon gene, designated CYP17A1P. The sequence of the promoter region of CYP17A1 differed from that of CYP17A1P. We found several consensus binding-sites for candidate transcription factors including androgen receptor (AR), Sox and FoxL2 in the CYP17A1 promoter region, but an AR-binding site was absent from CYP17A1P. When AR was over-expressed in Xenopus A6 cells, it did not increase CYP17A1 transcription in luciferase assays. CYP17A1 was strongly expressed in indifferent male gonads during sex determination and exclusively in testis, among eight adult tissues of R. rugosa. By contrast, CYP17A1P was expressed at very low, and similar levels in the adult tissues of both sexes. Fluorescent In-Situ Hybridization (FISH) analysis showed that CYP17A1P is localized to chromosome 4, while CYP17A1 is on chromosome 9. These results collectively suggest that CYP17A1, but not CYP17A1P is involved in male sex-determination in R. rugosa, and that androgens may not have a direct effect on the CYP17A1 transcription.

Cloning and expression of P450c17-I (17α-hydroxylase/17,20-lyase) in brain and ovary during gonad development in Cynoglossus semilaevis

Cytochrome P450c17 (CYP17, 17a-hydroxylase/17,20-lyase) is a critical enzyme in the production of androgens and estrogens in vertebrates. A 2,469 bp full length cDNA of P450c17-I (CYP17A1) has been isolated from the ovary of half-smooth tongue sole, Cynoglossus semilaevis which encodes 509 amino acids. Additionally, a relatively shorter cDNA (1,742 bp), a likely result of polyadenylation, was also found. The putative P450c17-I enzyme shares high sequence identity with that of the fathead minnow (73%), zebrafish (71%), the Japanese eel (70%), catfish (70%), tilapia (79%), three-spined stickleback (81%), medaka (79%), dogfish (60%), chicken (65%), rat (47%), and human (49%). Semi-quantitative RT-PCR analysis of spatial expression showed the enzyme was predominantly expressed in the ovaries and the brain. P450c17-I was also detected in the stomach, intestine, gill, spleen, kidney, and head kidney, albeit weakly. Further examination of temporal expression pattern of P450c17-I in ovary and brain revealed developmental stage-dependency. In addition to this our data on T and E2 levels further endorse the critical role of P450c17-I during shift in steroidogenesis. Based on the present study we indicate an important role for P450c17-I during ovarian development. However, further studies are needed at transcriptional regulation level for deeper insights into the physiological functions of P450c17-I.

Expression of androgen receptor mRNA in the ovary of Japanese eel, Anguilla japonica, during artificially induced ovarian development

In order to elucidate how androgens may mediate their effects on ovarian growth, we investigated the mRNA levels of two subtypes of androgen receptor (ara and arb) in the ovary of feminized Japanese eel (Anguilla japonica) during artificially induced ovarian development by quantitative real-time reverse transcriptase polymerase chain reaction and in situ hybridization. Ara mRNA levels were high from the late oil droplet stage to the late vitellogenic stage, whereas arb mRNA levels were high from the late oil droplet stage to the midvitellogenic stage. Both ar mRNAs were predominantly observed in the follicle cells and the epithelial cells of the ovigerous lamellae in all stages. In the oil droplet stage, oogonia exhibited intense signals for ar mRNAs. There was no obvious difference in localization pattern between ara and arb in all ovaries examined, irrespective of maturational stage. It was difficult to identify the follicle cell types that were positive for ar mRNA during ovarian development. Only in post-ovulatory follicles could theca and granulosa cells be clearly identified, and ar signals were observed in both layers. The predominant localization of ar mRNA in the follicle cells suggests that androgens play important roles in oocyte growth by acting on these cells in this species. We have shown the expression profile and localization of ar mRNA during ovarian development for the first time in an oviparous vertebrate.

Comparative transcriptomic analysis of follicle-enclosed oocyte maturational and developmental competence acquisition in two non-mammalian vertebrates

BACKGROUND

In vertebrates, late oogenesis is a key period during which the oocyte acquires its ability to resume meiosis (i.e. maturational competence) and to develop, once fertilized, into a normal embryo (i.e. developmental competence). However, the molecular mechanisms involved in these key biological processes are far from being fully understood. In order to identify key mechanisms conserved among teleosts and amphibians, we performed a comparative analysis using ovarian tissue sampled at successive steps of the maturational competence acquisition process in the rainbow trout (Oncorhynchus mykiss) and in the clawed toad (Xenopus laevis). Our study aimed at identifying common differentially expressed genes during late oogenesis in both species. Using an existing transcriptomic analysis that had previously been carried out in rainbow trout, candidate genes were selected for subsequent quantitative PCR-based comparative analysis.

RESULTS

Among the 1200 differentially expressed clones in rainbow trout, twenty-six candidate genes were selected for further analysis by real-time PCR in both species during late oogenesis. Among these genes, eight had similar expression profiles in trout and Xenopus. Six genes were down-regulated during oocyte maturation (cyp19a1, cyp17a1, tescalcin, tfr1, cmah, hsd11b3) while two genes exhibited an opposite pattern (apoc1, star). In order to document possibly conserved molecular mechanisms, four genes (star, cyp19a1, cyp17a1 and hsd11b3) were further studied due to their known or suspected role in steroidogenesis after characterization of the orthology relationships between rainbow trout and Xenopus genes. Apoc1 was also selected for further analysis because of its reported function in cholesterol transport, which may modulate steroidogenesis by regulating cholesterol bioavailability in the steroidogenic cells.

CONCLUSIONS

We have successfully identified orthologous genes exhibiting conserved expression profiles in the ovarian follicle during late oogenesis in both trout and Xenopus. While some identified genes were previously uncharacterized during Xenopus late oogenesis, the nature of these genes has pointed out molecular mechanisms possibly conserved in amphibians and teleosts. It should also be stressed that in addition to the already suspected importance of steroidogenesis in maturational competence acquisition, our approach has shed light on other regulatory pathways which may be involved in maturational and developmental competence acquisitions that will require further studies.

A role for cytochrome P450 17alpha-hydroxylase/c17-20 lyase during shift in steroidogenesis occurring in ovarian follicles prior to oocyte maturation

Cytochrome P450 17alpha-hydroxylase/c17-20 lyase (P450c17) is regarded as one of the key enzymes involved in the steroidogenic shift that occurs prior to oocyte maturation in teleosts. Role of P450c17 in the shift in steroidogenesis during oocyte maturation is a contentious issue even after identification of a novel type of P450c17 that lacks lyase activity. To understand the role of P450c17 in steroidogenic shift explicitly, a full length cDNA encoding p450c17 from ovary of air-breathing catfish, Clarias gariepinus was cloned. p450c17 transiently expressed in COS-7 cells converted progesterone to androstenedione through 17alpha-hydroxyprogesterone and catfish p450c17 was found to be expressed ubiquitously with relatively higher levels in gonads, brain, kidney and gills. Immunocytochemical analysis revealed the presence of P450C17 in follicular layer of ovarian follicle, interstitial cells and spermatocytes of testis. p450c17 expression and ratio of lyase to hydroxylase was high in preparatory and pre-spawning phases of ovary and low in spawning phase. Expression of p450c17 correlated well with testicular recrudescence with maximum expression in preparatory and spawning phases. Neither protein expression nor lyase/hydroxylase activity changed significantly during hCG-induced oocyte maturation, in vitro and in vivo though mRNA levels increased. These results tend to suggest that the ovarian follicles attains capacity to produce maximum precursor steroid levels before spawning that might contribute to the shift in steroidogenesis.

Ca(2+)-Calmodulin regulation of testicular androgen production in Mozambique tilapia (Oreochromis mossambicus)

The Ca(2+)-Calmodulin (CaM) signaling pathway has previously been shown to be involved in the regulation of teleost fish ovarian steroidogenesis. However, a putative role of CaM in testicular steroidogenesis and potential targets has not been examined. To examine whether basal steroidogenesis is modulated by Ca(2+) and CaM levels in the testis of Mozambique tilapia (Oreochromis mossambicus) we have incubated testicular fragments in vitro under different conditions and analyzed steroid output. Calcium-free medium with or without EGTA did not affect testicular basal 11-ketotestosterone (11-KT) and testosterone (T) secretion. However, addition of 80microM the CaM inhibitor W7 significantly reduced basal 11-KT, T and androstenedione secretion. Interestingly, the decreased androgen production by 80microM of W7 was accompanied by increased 11-desoxicortisol output and by the activation of cortisol synthesis in the testis, the latter undetected in untreated tissues. However, production of 17,20alpha-dihydroxy-4-pregnen-3-one was unaltered by W7. This suggests that C17,20 desmolase, 21-hydroxylase and possibly 11beta-hydroxysteroid dehydrogenase are targets for CaM. In addition, androgen production was also found to be regulated by the level of cAMP since incubations with forskolin (FK) significantly increased 11-KT and T output. A cross-talk between the cAMP and Ca(2+)-CaM signaling pathways was detected since W7 administration also decreased FK stimulated androgen production. Altogether, these data show that both basal and cAMP stimulated androgen levels were modulated by intracellular Ca(2+)-dependent CaM and that possibly Ca(2+)-CaM determines the shift in steroidogenesis from C21 steroids to androgens.

Immunohistochemical detection and biological activities of CYP17 (P450c17) in the indifferent gonad of the frog Rana rugosa

Sex steroids play a crucial role in the gonad differentiation in various species of vertebrates. However, little is known regarding the localization and biological activity of steroid-metabolizing enzymes during gonadal sex differentiation in amphibians. In the present study, we showed by real-time RT-PCR analysis that the expression of CYP17, one of the key steroidogenic enzymes, was higher in the indifferent gonad during sex differentiation in male than in female tadpoles of Rana rugosa but that there was no difference detected in the 3betaHSD mRNA level between the male and female gonads. We next examined the localization of CYP17, 3betaHSD and 17betaHSD in the indifferent and differentiating gonads by using three kinds of antibodies specific for CYP17, 3betaHSD and 17betaHSD, respectively. Positive signals for CYP17, 3betaHSD and 17betaHSD were observed in somatic cells of the indifferent gonad of males and in the interstitial cell of the testis. The enzymatic activity of CYP17 was also examined in the gonad during sex differentiation in this species. [(3)H]Progesterone (Prog) was converted to [(3)H]androstenedione (AE) in the indifferent gonad in males and females, but the rate of its conversion was higher in males than in females. Moreover, fluorescence in situ hybridization (FISH) analysis revealed that the CYP17 gene was located on the q arm of chromosome 9, indicating that CYP17 was autosomal in R. rugosa. Taken together, the results demonstrate that the CYP17 protein is synthesized in somatic cells of the indifferent gonad during gonadal sex differentiation in R. rugosa and that it is more active in converting Prog to AE in males than in females. The data suggest that CYP17 may be involved in testicular formation during sex differentiation in this species.

Japanese eel follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh): production of biologically active recombinant Fsh and Lh by Drosophila S2 cells and their differential actions on the reproductive biology

Two gonadotropins (Gths), follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), control gonadal steroidogenesis and gametogenesis in vertebrates, including teleost fish. Here, we report on the production of biologically active recombinant Fsh (rec-Fsh) and Lh (rec-Lh) in Japanese eel using Drosophila S2 cells. The three subunits composing Gths, i.e., glycoprotein hormone, alpha polypeptide (Cga), follicle-stimulating hormone, beta polypeptide (Fshb), and luteinizing hormone, beta polypeptide (Lhb), were at first independently produced and were proven to be glycosylated and secreted as the mature peptides. Each beta subunit, along with its Cga, was simultaneously coexpressed to produce heterodimeric rec-Fsh and rec-Lh that were subsequently highly purified. The biological activity of rec-Gths was demonstrated in various in vitro assays. The rec-Gths differentially activated their receptors, which resulted in an increase in 11-ketotestosterone (11KT) secretion, a differential alteration of gene expression of steroidogenic enzymes in immature testis, and the induction of the complete process of spermatogenesis in vitro. The data strongly suggest that Fsh and Lh differentially play important roles in the reproductive physiology of the Japanese eel. By contrast, these rec-Gths exhibited little activity in the gonad when administered in vivo. This difference between in vitro and in vivo bioactivity is probably due to the qualitative nature of glycosylation in S2 cells, which resulted in degradation of the recombinant protein in vivo. These differences in the carbohydrate moieties need to be elucidated and ameliorated.

Characterization, expression and transcriptional regulation of P450c17-I and -II in the medaka, Oryzias latipes

Recently we reported the isolation of two types of cytochrome P450c17s (steroid 17alpha-hydroxylase/C17, 20 lyase) encoded by two different genes, from genomes of teleost fish. In this study, we characterized the expression profile, enzymatic activity and transcriptional regulation of P450c17-I and -II in the medaka ovary. Similar to tilapia, medaka P450c17-I possessed both hydroxylase and lyase activities, while P450c17-II possessed only the hydroxylase activity. In situ hybridization and gene expression profiles during 48h prior to spawning indicated that P450c17-I is responsible for the production of estradiol-17beta during oocyte growth, while P450c17-II for the production of 17alpha, 20beta-dihydroxy-4-pregnen-3-one during oocyte maturation and cortisol production in the head kidney. Luciferase assays and expression profiles of transcription factors as revealed by real time PCR suggested that P450c17-I and -II expression are tightly controlled by Ad4BP/SF-1, Lrh-1, Foxl2, and Dax1 during the 48 h prior to spawning.

A novel type of P450c17 lacking the lyase activity is responsible for C21-steroid biosynthesis in the fish ovary and head kidney

Cytochrome P450c17 is the single enzyme that mediates the 17 alpha-hydroxylase and 17, 20 lyase activities during the biosynthesis of steroid hormones in the gonads and adrenal gland. However, the mechanism underlying its dual action continues to be a controversy in the field of steroidogenesis in fish. In an attempt to resolve this issue, we identified a novel type of P450c17 (P450c17-II) by an in silico analysis from the genomes of six fish species. We cloned P450c17-II from tilapia and medaka, and comparison with the conventional P450c17-I revealed that they differ in gene structure and enzymatic activity. Enzymatic assays by thin-layer chromatography revealed that P450c17-II possesses only the 17 alpha-hydroxylase activity without any 17, 20 lyase activity, unlike P450c17-I, which has both these activities. In testis, both P450c17-I and -II express in the interstitial cells. Remarkable differences, revealed by in situ hybridization, in the expression patterns of the P450c17-I and -II in the ovary and head kidney of tilapia during various stages of development strongly suggest that P450c17-I is responsible for the synthesis of estradiol-17beta in the ovary, whereas P450c17-II is required for the production of C21 steroids such as cortisol in the head kidney. More interestingly, a temporally controlled switching is observable in the expression of these two genes during the steroidogenic shift from estradiol-17beta to the C21 steroid, 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (maturation-inducing hormone of fish oocytes) in the fish ovary, revealing a role for P450c17-II in the production of hormones that induce oocyte maturation in fish.

Cloning and characterization of a cDNA encoding cholesterol side-chain cleavage cytochrome P450 (CYP11A1): tissue-distribution and changes in the transcript abundance in ovarian tissue of Japanese eel, Anguilla japonica, during artificially induced sexual development

Cholesterol side-chain cleavage cytochrome P450 (CYP11A1: P450scc) is a crucial steroidogenic enzyme that catalyzes an initial step in the production of all classes of steroids. A cDNA encoding Japanese eel P450scc was cloned and characterized. The cDNA putatively encoded 521 amino acid residues with high homology to those of other vertebrate forms. The recombinant P450scc produced in COS-7 cells efficiently catalyzed the conversion of 25-hydroxycholesterol into pregnenolone. By northern blot, a single P450scc transcript of approximately 3.3 kb was detected in both ovary and head kidney. Transcript levels of this enzyme significantly increased throughout ovarian development artificially induced by salmon pituitary homogenate, which suggests that gonadotropic stimuli can induce ovarian expression of the P450scc gene in teleosts, as has been reported in mammals. Furthermore, RT-PCR analysis revealed that gene expression of three steroidogenic enzymes, P450scc, P450c17 and 3beta-hydroxysteroid dehydrogenase (3beta-HSD) show distinctly different tissue-specific patterns of expression in the Japanese eel. The P450scc gene was expressed in ovary and head kidney while the sole source of the P450c17 transcript was ovary. In contrast, 3beta-HSD transcript was detected in all tissues examined, brain, liver, spleen and trunk kidney, etc. These suggest that some steroidogenic enzymes are also expressed in non-endocrine tissues and could potentially regulate the local and/or circulating steroid levels in teleosts, as they do in mammals.

Changes in localization of cytochrome P450 cholesterol side-chain cleavage (P450scc) in Japanese eel testis and ovary during gonadal development

In this study, we generated and characterized a polyclonal antiserum against eel P450 cholesterol side-chain cleavage (P450scc) using a recombinant protein as the antigen. We examined the localization and abundance of P450scc by immunohistochemistry in Japanese eel testes and ovaries during artificially induced gonadal development. P450scc mRNA localization was also examined by in situ hybridization. In male eels, testicular development was induced by a single injection of human chorionic gonadotropin (HCG). In females, ovarian development was induced by weekly injections of salmon pituitary homogenate (SPH). Before HCG injection, the testis contained germ cells that were primarily type A spermatogonia. Additionally, several clusters of immunoreactive cells for P450scc were localized in the interstitial Leydig cells, but no P450scc mRNA signals were detected. This suggests that P450scc is either a relatively stable protein or it is produced by a mRNA that is present at too low a level to detect. Shortly after a single injection of HCG, expression of P450scc mRNA was stimulated and the number of immunoreactive clusters and their staining intensity were both increased. P450scc mRNA fell to an undetectable level 3 days after hormonal stimulation. Although the P450scc protein also decreased at the same time as the mRNA, it remained at a detectable level throughout this period. P450scc mRNA, but not the P450scc protein, was also detected in the spermatids and spermatozoa. The biological significance of P450scc mRNA expression at this stage is unknown. Prior to experimentation, the ovary contained oocytes that were developed to the oil-droplet stage, with several clusters of immunoreactive cells localized in the thecal layer and ovigerous lamella epithelium. Expression of P450scc mRNA was also stimulated by SPH injections in the ovary. In contrast to the testis, P450scc mRNA was continuously detected in the thecal cell layer throughout artificially induced maturation, possibly due to a repeated stimulus by the SPH injection every week. Clusters of immunoreactive cells in the thecal cell layer increased in number as ovarian development progressed. This increase in P450scc mRNA and protein may explain, at least in part, the increase in serum steroid hormones in female eels. The P450scc antiserum clearly immunostained interrenal steroidogenic cells in the head kidney of not only eel but also goldfish, indicating that this antibody could also be used in other teleost species.

Cloning of zebrafish ovarian P450c17 (CYP17, 17alpha-hydroxylase/17, 20-lyase) and characterization of its expression in gonadal and extra-gonadal tissues

Cytochrome P450c17 (CYP17, 17alpha-hydroxylase/17, 20-lyase) has been viewed as a critical enzyme for the biosynthesis of gonadal and adrenal steroids in vertebrates. A full-length cDNA coding for P450c17 has been cloned from the zebrafish ovary. It encodes 519 amino acids and shares high sequence identity with that of fathead minnow (92%), rainbow trout (74%), chicken (64%), rat (47%), and human (48%). Northern blot analysis demonstrated that it is predominantly expressed in the ovary and testis; however, a weak hybridization signal was also detected in the whole kidney of the zebrafish, suggesting a role for P450c17 in the steroidogenesis of both gonads and renal/interrenal tissues. We further examined the expression of P450c17 in the developing ovarian follicles. Northern analysis and RT-PCR revealed abundant expression of the enzyme in the follicles of all major stages including pre-vitellogenic, vitellogenic and post-vitellogenic full-grown follicles; however, unlike P450c17 in the rainbow trout and eel, the expression of zebrafish P450c17 did not show significant temporal difference among different developmental stages, similar to the finding in the fathead minnow. The constitutive nature of P450c17 expression during zebrafish follicle development may partly explain the poor transcriptional response of the enzyme to the treatments with gonadotropin or activin, both of which enhance oocyte maturation in this species. It is also of interest to note that when assayed by RT-PCR, the enzyme P450c17 also seemed to be expressed in several other non-gonadal tissues including the brain, gill, liver and intestine, and its expression did not show significant sexual dimorphism in the brain of adult zebrafish. The physiological relevance of these findings remains to be further elucidated.

Alternative splicing and differential expression of P450c17 (CYP17) in gonads during sex transformation in the rice field eel

Several mechanisms were used in determination of the development of the male or female of vertebrates. The genes for determination of sequential hermaphrodite sex are unknown. Here, we reported cloning, alternative splicing, and expression patterns of the CYP17 gene of the rice field eel, a teleost fish with a characteristic of nature sex reversal. The CYP17 gene of the rice field eel was clustered into the CYP17 gene group of all the other vertebrates, especially into the fish subgroup. Four isoforms of the CYP17 were generated in gonads by alternative splicing and polyadenylation. Alternative splicing events of all these isoforms occurred in 3(') regions, which encoded three different sizes (517, 512, and 159aa) of proteins. RT-PCR results indicate specific expression in gonads of these isoforms. Northern blot analysis shows that expression patterns of the CYP17 (dominantly expressed in testis, less in ovary, and the least in ovotestis) are consistent with the sex reversal process of the rice field eel. In situ hybridization further shows its specific expression in germinal lamellae, the gonadal epithelium of the gonads. These findings indicate that CYP17 is differentially regulated in a sex- and developmentally specific manner, suggesting that the CYP17 potentially has important roles in gonad differentiation during sex reversal of the rice field eel.

Molecular cloning and characterization of 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase cDNAs from Japanese eel ovary

3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase (3beta-HSD) is a crucial steroidogenic enzyme which catalyzes an essential step in the biosynthesis of all classes of steroid hormones. Two closely related cDNAs, encoding Japanese eel ovarian types I and II 3beta-HSD, were cloned and characterized. Both cDNAs putatively encoded 375 amino acid residues sharing high sequence homology with those of rainbow trout (71%) and mammalian (approximately 45-50%) 3beta-HSD. Transient expression of types I and II 3beta-HSD in COS-7 cells revealed that both proteins possess 3beta-hydroxysteroid dehydrogenase as well as Delta(5)-Delta(4) isomerase activity for both pregnenolone and dehydroepiandrosterone, with the preference of pregnenolone over dehydroepiandrosterone as substrate, although the type I protein is more active than the type II. By northern blot analysis, a single band of the 3beta-HSD transcript of approximately 1.5kb in length was observed in ovarian tissue and the total transcript abundance of both 3beta-HSDs remained constant throughout ovarian development artificially induced by gonadotropin-rich salmon pituitary homogenate. This lack of change in 3beta-HSD transcript abundance during ovarian development did not correlate with the fluctuation of its enzymatic activity reported previously, which may suggest that changes in 3beta-HSD activity during ovarian development may be, in part, post-transcriptionally regulated in the Japanese eel ovary.

Xenopus laevis ovarian CYP17 is a highly potent enzyme expressed exclusively in oocytes. Evidence that oocytes play a critical role in Xenopus ovarian androgen production

Progesterone has long been considered the primary mediator of Xenopus oocyte maturation. We have recently shown, however, that androgens, which are equal or more potent promoters of maturation and are present at higher levels in ovulating frogs, may also be playing an important physiologic role in mediating maturation. Here, we examined the role of CYP17, a key enzyme mediating sex steroid synthesis, in Xenopus ovarian androgen production. We found that the 17,20-lyase activities of Xenopus CYP17 exceeded the 17alpha-hydroxylase activities in both the Delta4 and Delta5 pathways; thus, Xenopus CYP17 rapidly converted pregnenolone and progesterone to dehydroepiandrosterone (DHEA) and androstenedione, respectively. This remarkably robust activity exceeds that of CYP17 from most higher vertebrates, and likely explains why virtually no progesterone is detected in ovulating frogs. Additionally, ovarian CYP17 activity was present exclusively in oocytes, although all other enzymes involved in sex steroid production were expressed almost entirely in surrounding follicular cells. This compartmentalization suggests a "two-cell" model whereby Xenopus ovarian androgen production requires both follicular cells and oocytes themselves. The requirement of oocytes for ovarian androgen production further introduces the unusual paradigm whereby germ cells may be responsible for producing important steroids used to mediate their own maturation.

Cloning and gene expression of P450 17alpha-hydroxylase,17,20-lyase cDNA in the gonads and brain of the fathead minnow Pimephales promelas

P450 17alpha-hydroxylase,17,20-lyase (P450c17) is a key steroidogenic enzyme in the production of androgens and, therefore, is also indispensable for the production of oestrogens (that are produced from the aromatisation of androgens). In this study, P450c17 cDNA was cloned from the ovary of the fathead minnow (FHM) and its gene expression was examined in the gonads and brains of male and female FHM at different stages of gonadal development with a view to developing an understanding of its involvement in the reproductive physiology in this species. The FHM-P450c17 cDNA sequence cloned was 1812 bp in length, with an open reading frame of 1554 nucleotides encoding a protein of 518 amino acids. Amino acid identity of FHM-P450c17 with P450c17s in other animals was up to 81.8% in other teleosts (channel catfish), 62% in elasmobranches (spiny dogfish), 64% in birds (chicken), and up to 48.8% in mammals (human). FHM-P450c17 gene expression occurred in the ovary, testis, and also in the brain (both male and female) at all stages of sexual development studied. Expression in the brain was at least 30-fold lower than in the gonads, but consistent in all fish life stages studied. In the testis, FHM-P450c17 gene expression was negatively correlated with gonadal development, but there was no obvious association between P450c17 gene expression and sexual development in the ovary, or brain (in both males and females). The results from this study demonstrate the expression of P450c17 in the brain for the first time in fish. Enzymatic studies are now needed to investigate the possible role of P450c17 in neurosteroid production in teleosts.

Steroidogenic acute regulatory protein in eels: cDNA cloning and effects of ACTH and seawater transfer on its mRNA expression

Steroidogenic acute regulatory protein (StAR) is a key molecule for steroid production by translocating cholesterol from the outer to inner mitochondrial membrane. Two cDNAs of different length encoding StAR was cloned from the head kidney of the eel (Anguilla japonica). In the 3'-untranslated region (UTR) of the longer cDNA, two putative polyadenylation signals were found. The shorter one differed from the longer one solely by the lack of middle of 3'-UTR including the first polyadenylation signal. Reverse transcription-polymerase chain reaction (RT-PCR) that differentiates the two mRNAs showed that the ratio of the two was highly variable among individuals, and no preferential expression was detected between freshwater and seawater eels. The predicted protein consists of 285 amino acid residues with 64-83% identity to other StARs thus far obtained. RT-PCR analyses revealed that eel StAR mRNA was expressed abundantly in the head kidney and gonad, and faintly in the brain; but no expression was detected in the gill, heart, liver, intestine, kidney and skeletal muscle. Plasma cortisol concentration increased, but StAR mRNA content in the head kidney did not change, 3 and 24 h after transfer of freshwater eels to seawater, indicating that the transcriptional regulation of StAR may not be involved in cortisol production after seawater transfer. However, ACTH elevated both plasma cortisol and StAR mRNA levels in the head kidney 1.5 and 4.5 h after injection. Thus, the steroidogenic effect of ACTH is mediated by increased StAR production as observed in mammals.

Zebrafish as a model for vertebrate reproduction: characterization of the first functional zebrafish (Danio rerio) gonadotropin receptor

Vertebrate reproduction is tightly regulated by conserved glycoprotein hormones produced by the pituitary gland. Follicle-stimulating hormone (FSH) in tetrapods and gonadotropic hormone I (GTH-I) in fishes are orthologous glycoprotein hormones that control the timing of egg production and the number of eggs produced. Zebrafish, a well-established genetic model for developmental biology, also offers potential advantages for studies of reproductive toxicology, especially for modeling the impact of pollutants on fish reproductive processes. To facilitate these studies we have identified, expressed, and characterized the zebrafish GTH-I receptor. This receptor (zfGTHR-I)exhibits strong sequence similarity to the tetrapod FSH receptors and to GTHR-I from salmon and catfish. Human 293 cells transfected with zfGTHR-I exhibit increased cAMP levels after treatment with carp pituitary extracts or human FSH, but not when treated with a ligand to a related receptor (human chorionic gonadotropin). Northern blotting and RT-PCR analyses indicate that zfGTHR is expressed in ovaries from sexually mature fish, but not in immature fish. Several alternative splice variants of the receptor affecting putative exons 2-4 that encode dramatically shortened receptor fragments lacking the transmembrane domain as well as regions previously implicated in ligand binding were identified by RT-PCR. The zfGTHR-I sequence opens the way to study effects of genetic mutations or chemicals on ovarian zfGTHR-I expression and function in zebrafish.

20beta-Hydroxysteroid dehydrogenase of the Japanese eel ovary: its cellular localization and changes in the enzymatic activity during sexual maturation

In many species of teleost, 20beta-hydroxysteroid dehydrogenase (20beta-HSD) is a key steroidogenic enzyme in the production of the oocyte maturation-inducing steroid (MIS), 17alpha, 20beta-dihydroxy-4-pregnen-3-one. In this study, 20beta-HSD in the ovary of the Japanese eel was biochemically characterized using a cell-free system. 20beta-HSD activity was located mainly in the membrane-bound fractions of the mitochondria and microsome, with lower levels detected in the cytosolic fraction. The enzymatic activity of membrane-bound 20beta-HSD was strikingly enhanced by treatment of eels with gonadotropin-rich salmon pituitary homogenate. The activity of eel ovarian mitochondrial 20beta-HSD in the presence of different solubilizing detergents was then assessed. Mitochondrial fractions solubilized by sodium deoxycholate and octhylthioglucoside retained approximately 30% of 20beta-HSD activity when compared to those of nontreated mitochondria. These results suggest that Japanese eel ovarian 20beta-HSD is composed of membrane-bound and soluble activities, and that the membrane-bound component is stimulated by gonadotropin.

Cloning of 17beta-hydroxysteroid dehydrogenase-I cDNAs from Japanese eel ovary

17beta-hydroxysteroid dehydrogenase-I (17beta-HSD-I) is a key steroidogenic enzyme for estradiol-17beta (E(2)) production. cDNAs encoding 17beta-HSD-I were cloned for the first time in lower vertebrates from the ovary of a teleost, the Japanese eel. The deduced amino acid sequence from these cDNAs was approximately 50% identical to mammalian 17beta-HSD-Is. 17beta-HSD-I mRNA was not detected in previtellogenic ovaries by Northern blotting. However, transcript abundance increased in early vitellogenic ovaries obtained from fish artificially matured by gonadotropic treatment, but thereafter did not appear to change further. Recombinant 17beta-HSD-I expressed in human kidney 293 cells selectively converted estrone to E(2), but androstenedione, testosterone, or E(2) were not converted to any other steroids. Although it is widely accepted that E(2) is produced from testosterone in other species of teleosts, the substrate specificity of eel 17beta-HSD-I suggests that a steroidogenic pathway for production of E(2) from androstenedione via estrone exists in the Japanese eel ovary.