Sex differences in aromatase gene expression in the medaka brain

Estrogenic influences on agonistic behavior in teleost fishes

Teleost fishes show an extraordinary diversity of sexual patterns, social structures, and sociosexual behaviors. Sex steroid hormones are key modulators of social behaviors in teleosts as in other vertebrates and act on sex steroid receptor-containing brain nuclei that form the evolutionarily conserved vertebrate social behavior network (SBN). Fishes also display important differences relative to tetrapod vertebrates that make them particularly well-suited to study the physiological mechanisms modulating social behavior. Specifically, fishes exhibit high levels of brain aromatization and have what has been proposed to be a lifelong, steroid hormone dependent plasticity in the neural substrates mediating sociosexual behavior. In this review, we examine how estrogenic signaling modulates sociosexual behaviors in teleosts with a particular focus on agonistic behavior. Estrogens have been shown to mediate agonistic behaviors in a broad range of fishes, from sexually monomorphic gonochoristic species to highly dimorphic sex changers with alternate reproductive phenotypes. These similarities across such diverse taxa contribute to a growing body of evidence that estrogens play a crucial role in the modulation of aggression in vertebrates. As analytical techniques and genomic tools rapidly advance, methods such as LC-MS/MS, snRNAseq, and CRISPR-based mutagenesis show great promise to further elucidate the mechanistic basis of estrogenic effects on social behavior in the diverse teleost lineage.

Let's talk about sex: Mechanisms of neural sexual differentiation in Bilateria

In multicellular organisms, sexed gonads have evolved that facilitate release of sperm versus eggs, and bilaterian animals purposefully combine their gametes via mating behaviors. Distinct neural circuits have evolved that control these physically different mating events for animals producing eggs from ovaries versus sperm from testis. In this review, we will describe the developmental mechanisms that sexually differentiate neural circuits across three major clades of bilaterian animals-Ecdysozoa, Deuterosomia, and Lophotrochozoa. While many of the mechanisms inducing somatic and neuronal sex differentiation across these diverse organisms are clade-specific rather than evolutionarily conserved, we develop a common framework for considering the developmental logic of these events and the types of neuronal differences that produce sex-differentiated behaviors. This article is categorized under: Congenital Diseases > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

Adult sex change leads to extensive forebrain reorganization in clownfish

Sexual differentiation of the brain occurs in all major vertebrate lineages but is not well understood at a molecular and cellular level. Unlike most vertebrates, sex-changing fishes have the remarkable ability to change reproductive sex during adulthood in response to social stimuli, offering a unique opportunity to understand mechanisms by which the nervous system can initiate and coordinate sexual differentiation. This study explores sexual differentiation of the forebrain using single nucleus RNA-sequencing in the anemonefish Amphiprion ocellaris, producing the first cellular atlas of a sex-changing brain. We uncover extensive sex differences in cell type-specific gene expression, relative proportions of cells, baseline neuronal excitation, and predicted inter-neuronal communication. Additionally, we identify the cholecystokinin, galanin, and estrogen systems as central molecular axes of sexual differentiation. Supported by these findings, we propose a model of neurosexual differentiation in the conserved vertebrate social decision-making network spanning multiple subtypes of neurons and glia, including neuronal subpopulations within the preoptic area that are positioned to regulate gonadal differentiation. This work deepens our understanding of sexual differentiation in the vertebrate brain and defines a rich suite of molecular and cellular pathways that differentiate during adult sex change in anemonefish.

Estrogen-dependent expression and function of secretogranin 2a in female-specific peptidergic neurons

Secretogranin 2 (Scg2) is a member of the secretogranin/chromogranin family of proteins that is involved in neuropeptide and hormone packaging to secretory granules and serves as a precursor for several secreted pleiotropic peptides. A recent study in zebrafish showed that the teleost Scg2 orthologs, scg2a and scg2b, play an important role in mating behavior, but its modes of action and regulatory mechanisms remain unclear. In this study, we identify scg2a in another teleost species, medaka, by transcriptomic analysis as a gene that is expressed in an ovarian secretion-dependent manner in a group of neurons relevant to female sexual receptivity, termed FeSP neurons. Investigation of scg2a expression in the FeSP neurons of estrogen receptor (Esr)-deficient medaka revealed that it is dependent on estrogen signaling through Esr2b, the major determinant of female-typical mating behavior. Generation and characterization of scg2a-deficient medaka showed no overt changes in secretory granule packaging in FeSP neurons. This, along with the observation that Scg2a and neuropeptide B, a major neuropeptide produced by FeSP neurons, colocalize in a majority of secretory granules, suggests that Scg2a mainly serves as a precursor for secreted peptides that act in conjunction with neuropeptide B. Further, scg2a showed sexually biased expression in several brain nuclei implicated in mating behavior. However, we found no significant impact of scg2a deficiency on the performance of mating behavior in either sex. Collectively, our results indicate that, although perhaps not essential for mating behavior, scg2a acts in an estrogen/Esr2b signaling-dependent manner in neurons that are relevant to female sexual receptivity.

Evaluating the alterations of the estrogen-responsive genes in Cyprinodon variegatus larvae for biomonitoring the impacts of estrogenic endocrine disruptors (EEDs)

Currently, endocrine disruptors (EDs) can be found in all the environmental compartments. To understand the effects of estrogenic EDs (EEDs), adults of Cyprinodon variegatus have been classically used as a marine model. However, it is during development that exposure to contaminants may generate permanent consequences. Thus, the aim of this study was to verify the effects produced by acute exposure to 17α-ethinylestradiol (EE2) in C. variegatus larvae. Quantitative PCR (qPCR) results revealed the induction of vtg and zp gene expression on exposure to 1000 ng/L EE2 and the induction of vtgc, zp2, zp3 and cyp19a2, and inhibition of vtgab, wap and cyp1a1 on exposure to 100 ng/L EE2. Lower concentrations inhibited the gene expression of vtgab and wap (50 ng/L), cyp1a1 (25 ng/L) and zp2 (12.5 ng/L). These alterations in gene expression allow us to affirm that larvae of C. variegatus are an efficient and sensitive model for biomonitoring EEDs.

Developmental changes in gene expression and gonad morphology during sex differentiation in Atlantic salmon (Salmo salar)

The Atlantic salmon (Salmo salar) is a globally important species for its value in fisheries and aquaculture, and as a research model. In order to characterise aspects of sex differentiation at the morphological and mRNA level in this species, the present study examined developmental changes in gonad morphology and gene expression in males and females between 0 and 79 days post hatch (dph). Morphological differentiation of the ovary (indicated by the formation of germ cell cysts) became apparent from 52 dph. By 79 dph, ovarian phenotype was evident in 100% of genotypic females. Testes remained in an undifferentiated-like state throughout the experiment, containing germ cells dispersed singularly within the gonadal region distal to the mesentery. There were no significant sex-related differences in gonad cross-section size, germ cell number or germ cell diameter during the experiment. The expression of genes involved in teleost sex differentiation (anti-müllerian hormone (amh), cytochrome P450, family 19, subfamily A, polypeptide 1a (cyp19a1a), forkhead box L2a (foxl2a), gonadal soma-derived factor (gsdf), r-spondin 1 (rspo1), sexually dimorphic on the Y chromosome (sdY)), retinoic acid-signalling (aldehyde dehydrogenase 1a2 (aldh1a2), cytochrome P450 family 26 a1 (cyp26a1), cytochrome P450 family 26 b1 (cyp26b1), t-box transcription factor 1 (tbx1a)) and neuroestrogen production (cytochrome P450, family 19, subfamily A, polypeptide 1b (cyp19a1b)) was investigated. Significant sex-related differences were observed only for the expression of amh, cyp19a1a, gsdf and sdY. In males, amh, gsdf and sdY were upregulated from 34, 59 and 44 dph respectively. In females, cyp19a1a was upregulated from 66 dph. Independent of sex, foxl2a expression was highest at 0 dph and had reduced ∼ 47-fold by the time of morphological sex differentiation at 52 dph. This study provides new insights into the timing and sequence of some physiological changes associated with sex differentiation in Atlantic salmon. These findings also reveal that some aspects of the mRNA sex differentiation pathways in Atlantic salmon are unique compared to other teleost fishes, including other salmonids.

Molecular Characterization and Expression of Cytochrome P450 Aromatase in Atlantic Croaker Brain: Regulation by Antioxidant Status and Nitric Oxide Synthase During Hypoxia Stress

We have previously shown that nitric oxide synthase (NOS, an enzyme) is significantly increased during hypoxic stress in Atlantic croaker brains and modulated by an antioxidant (AOX). However, the influence of NOS and AOX on cytochrome P450 aromatase (AROM, CYP19a1, an enzyme) activity on vertebrate brains during hypoxic stress is largely unknown. In this study, we characterized brain AROM (bAROM, CYP19a1b) cDNA in croaker and examined the interactive effects of hypoxia and a NOS-inhibitor or AOX on AROM activity. The amino acid sequence of croaker bAROM cDNA is highly homologous (76–80%) to other marine teleost bAROM cDNAs. Both real-time PCR and Northern blot analyses showed that bAROM transcript (size: ∼2.8 kb) is highly expressed in the preoptic-anterior hypothalamus (POAH). Hypoxia exposure (dissolved oxygen, DO: 1.7 mg/L for 4 weeks) caused significant decreases in hypothalamic AROM activity, bAROM mRNA and protein expressions. Hypothalamic AROM activity and mRNA levels were also decreased by pharmacological treatment with N-ethylmaleimide (NEM, an alkylating drug that modifies sulfhydryl groups) of fish exposed to normoxic (DO: ∼6.5 mg/L) conditions. On the other hand, treatments with Nω-nitro-L-arginine methyl ester (NAME, a competitive NOS-inhibitor) or vitamin-E (Vit-E, a powerful AOX) prevented the downregulation of hypothalamic AROM activity and mRNA levels in hypoxic fish. Moreover, NAME and Vit-E treatments also restored gonadal growth in hypoxic fish. Double-labeled immunohistochemistry results showed that AROM and NOS proteins are co-expressed with NADPH oxidase (generates superoxide anion) in the POAH. Collectively, these results suggest that the hypoxia-induced downregulation of AROM activity in teleost brains is influenced by neuronal NOS activity and AOX status. The present study provides, to the best of our knowledge, the first evidence of restoration of AROM levels in vertebrate brains by a competitive NOS-inhibitor and potent AOX during hypoxic stress.

Epigenetic Regulation and Environmental Sex Determination in Cichlid Fishes

Studying environmental sex determination (ESD) in cichlids provides a phylogenetic and comparative approach to understand the evolution of the underlying mechanisms, their impact on the evolution of the overlying systems, and the neuroethology of life history strategies. Natural selection normally favors parents who invest equally in the development of male and female offspring, but evolution may favor deviations from this 50:50 ratio when environmental conditions produce an advantage for doing so. Many species of cichlids demonstrate ESD in response to water chemistry (temperature, pH, and oxygen concentration). The relative strengths of and the exact interactions between these factors vary between congeners, demonstrating genetic variation in sensitivity. The presence of sizable proportions of the less common sex towards the environmental extremes in most species strongly suggests the presence of some genetic sex-determining loci acting in parallel with the ESD factors. Sex determination and differentiation in these species does not seem to result in the organization of a final and irreversible sexual fate, so much as a life-long ongoing battle between competing male- and female-determining genetic and hormonal networks governed by epigenetic factors. We discuss what is and is not known about the epigenetic mechanism behind the differentiation of both gonads and sex differences in the brain. Beyond the well-studied tilapia species, the 2 best-studied dwarf cichlid systems showing ESD are the South American genus Apistogramma and the West African genus Pelvicachromis. Both species demonstrate male morphs with alternative reproductive tactics. We discuss the further neuroethology opportunities such systems provide to the study of epigenetics of alternative life history strategies and other behavioral variation.

Estrogen mediates sex differences in preoptic neuropeptide and pituitary hormone production in medaka

The preoptic area (POA) is one of the most evolutionarily conserved regions of the vertebrate brain and contains subsets of neuropeptide-expressing neurons. Here we found in the teleost medaka that two neuropeptides belonging to the secretin family, pituitary adenylate cyclase-activating polypeptide (Pacap) and vasoactive intestinal peptide (Vip), exhibit opposite patterns of sexually dimorphic expression in the same population of POA neurons that project to the anterior pituitary: Pacap is male-biased, whereas Vip is female-biased. Estrogen secreted by the ovary in adulthood was found to attenuate Pacap expression and, conversely, stimulate Vip expression in the female POA, thereby establishing and maintaining their opposite sexual dimorphism. Pituitary organ culture experiments demonstrated that both Pacap and Vip can markedly alter the expression of various anterior pituitary hormones. Collectively, these findings show that males and females use alternative preoptic neuropeptides to regulate anterior pituitary hormones as a result of their different estrogen milieu.

Epigenetic regulation of gonadal and brain aromatase expression in a cichlid fish with environmental sex determination

A fit animal must develop testes or ovaries, with brain and physiology to match. In species with alternative male morphs this coordination of development across tissues operates within sexes as well as between. For Pelvicachromis pulcher, an African cichlid in which early pH exposure influences both sex and alternative male morph, we sequence both copies of aromatase (cyp19a1), a key gene for sex determination. We analyze gene expression and epigenetic state, comparing gonad and brain tissue from females, alternative male morphs, and fry. Relative to brain, we find elevated expression of the A-copy in the ovaries but not testes. Methylation analysis suggests strong epigenetic regulation, with one region specifying sex and another specifying tissue. We find elevated brain expression of the B-copy with no sex or male morph differences. B-copy methylation follows that of the A-copy rather than corresponding to B-copy expression. In 30-day old fry, we see elevated B-copy expression in the head, but we do not see the expected elevated A-copy expression in the trunk that would reflect ovarian development. Interestingly, the A-copy epialleles that distinguish ovaries from testes are among the most explanatory patterns for variation among fry, suggesting epigenetic marking of sex prior to differentiation and thus laying the groundwork for mechanistic studies of epigenetic regulation of sex and morph differentiation.

Male-predominant galanin mediates androgen-dependent aggressive chases in medaka

Recent studies in mice demonstrate that a subset of neurons in the medial preoptic area (MPOA) that express galanin play crucial roles in regulating parental behavior in both sexes. However, little information is available on the function of galanin in social behaviors in other species. Here, we report that, in medaka, a subset of MPOA galanin neurons occurred nearly exclusively in males, resulting from testicular androgen stimulation. Galanin-deficient medaka showed a greatly reduced incidence of male-male aggressive chases. Furthermore, while treatment of female medaka with androgen induced male-typical aggressive acts, galanin deficiency in these females attenuated the effect of androgen on chases. Given their male-biased and androgen-dependent nature, the subset of MPOA galanin neurons most likely mediate androgen-dependent male-male chases. Histological studies further suggested that variability in the projection targets of the MPOA galanin neurons may account for the species-dependent functional differences in these evolutionarily conserved neural substrates.

Estrogen Receptor β2 Oversees Germ Cell Maintenance and Gonadal Sex Differentiation in Medaka, Oryzias latipes

In vertebrates, estrogen receptors are essential for estrogen-associated early gonadal sex development. Our previous studies revealed sexual dimorphic expression of estrogen receptor β2 (ERβ2) during embryogenesis of medaka, and here we investigated the functional importance of ERβ2 in female gonad development and maintenance using a transgenerational ERβ2-knockdown (ERβ2-KD) line and ERβ2-null mutants. We found that ERβ2 reduction favored male-biased gene transcription, suppressed female-responsive gene expression, and affected SDF1a and CXCR4b co-assisted chemotactic primordial germ cell (PGC) migration. Co-overexpression of SDF1a and CXXR4b restored the ERβ2-KD/KO associated PGC mismigration. Further analysis confirmed that curtailment of ERβ2 increased intracellular Ca²⁺ concentration, disrupted intra- and extracellular calcium homeostasis, and instigated autophagic germ cell degradation and germ cell loss, which in some cases ultimately affected the XX female sexual development. This study is expected improve our understanding of germ cell maintenance and sex spectrum, and hence open new avenues for reproductive disorder management.

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ERβ2 has a multifaceted role in early gonadal sex differentiation

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ERβ2 directly influences SDF1a/CXCR4b PGC chemotaxis and germ cell migration

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ERβ2-KD impairs intra- and extracellular calcium homeostasis and triggers cell death

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In some cases, ERβ2-KD and KO alters sexual development in female gonad

Plasticity in medaka gonadotropes via cell proliferation and phenotypic conversion

Follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) produced by the gonadotropes play a major role in control of reproduction. Contrary to mammals and birds, Lh and Fsh are mostly produced by two separate cell types in teleost. Here, we investigated gonadotrope plasticity, using transgenic lines of medaka (Oryzias latipes) where DsRed2 and hrGfpII are under the control of the fshb and lhb promotors respectively. We found that Fsh cells appear in the pituitary at 8 dpf, while Lh cells were previously shown to appear at 14 dpf. Similar to Lh cells, Fsh cells show hyperplasia from juvenile to adult stages. Hyperplasia is stimulated by estradiol. Both Fsh and Lh cells show hypertrophy during puberty with similar morphology. They also share similar behavior, using their cellular extensions to make networks. We observed bi-hormonal gonadotropes in juveniles and adults but not in larvae where only mono-hormonal cells are observed, suggesting the existence of phenotypic conversion between Fsh and Lh in later stages. This is demonstrated in cell culture, where some Fsh cells start to produce Lhβ, a phenomenon enhanced by gonadotropin-releasing hormone (Gnrh) stimulation. We have previously shown that medaka Fsh cells lack Gnrh receptors, but here we show that with time in culture, some Fsh cells start responding to Gnrh, while fshb mRNA levels are significantly reduced, both suggestive of phenotypic change. All together, these results reveal high plasticity of gonadotropes due to both estradiol-sensitive proliferation and Gnrh promoted phenotypic conversion, and moreover, show that gonadotropes lose part of their identity when kept in cell culture.

Transcriptome of tambaqui Colossoma macropomum during gonad differentiation: Different molecular signals leading to sex identity

Tambaqui (Colossoma macropomum) is the major native species in Brazilian aquaculture, and we have shown that females exhibit a higher growth compared to males, opening up the possibility for the production of all-female population. To date, there is no information on the sex determination and differentiation molecular mechanisms of tambaqui. In the present study, transcriptome sequencing of juvenile trunks was performed to understand the molecular network involved in the gonadal sex differentiation. The results showed that before differentiation, components of the Wnt/β-catenin pathway, fox and fst genes imprint female sex development, whereas antagonistic pathways (gsk3b, wt1 and fgfr2), sox9 and genes for androgen synthesis indicate male differentiation. Hence, in undifferentiated tambaqui, the Wnt/β-catenin exerts a role on sex differentiation, either upregulated in female-like individuals, or antagonized in male-like individuals.

A conceptual framework for understanding sexual differentiation of the teleost brain

Vertebrate brains are sexually differentiated, giving rise to differences in various physiological and behavioral phenotypes between the sexes. In developing mammals and birds, the neural substrate underlying sex-dependent physiology and behavior undergoes an irreversible process of sexual differentiation due to the effects of perinatal gonadal steroids and sex chromosome complement. The differentiated neural substrate is then activated in the adult by the sex-specific steroid milieu to facilitate the expression of sex-typical phenotypes. However, this well-established concept does not hold for teleost fish, whose sexual phenotypes (behavioral or otherwise) are highly labile throughout life and can be reversed even in adulthood. Indeed, the available evidence suggests that, in teleosts, neither gonadal steroids early in development nor the sex chromosome complement contribute much to brain sexual differentiation; instead, steroids in adulthood serve to both differentiate the neural substrate and activate it to elicit sex-typical phenotypes in a transient and reversible manner. Evidence further suggests that marked sexual dimorphisms and adult steroid-dependent lability in the neural expression of sex steroid receptors constitute the primary molecular basis for sexual differentiation and lability of the teleost brain. The consequent sexually dimorphic but reversible steroid sensitivity in response to the adult steroid milieu may enable the teleost brain to maintain lifelong sexual lability and to undergo phenotypic sex reversal.

Identification of sex differences in zebrafish (Danio rerio) brains during early sexual differentiation and masculinization using 17α-methyltestoterone

Sexual behavior in teleost fish is highly plastic. It can be attributed to the relatively few sex differences found in adult brain transcriptomes. Environmental and hormonal factors can influence sex-specific behavior. Androgen treatment stimulates behavioral masculinization. Sex dimorphic gene expression in developing teleost brains and the molecular basis for androgen-induced behavioral masculinization are poorly understood. In this study, juvenile zebrafish (Danio rerio) were treated with 100 ng/L of 17 alpha-methyltestosterone (MT) during sexual development from 20 days post fertilization to 40 days and 60 days post fertilization. We compared brain gene expression patterns in MT-treated zebrafish with control males and females using RNA-Seq to shed light on the dynamic changes in brain gene expression during sexual development and how androgens affect brain gene expression leading to behavior masculinization. We found modest differences in gene expression between juvenile male and female zebrafish brains. Brain aromatase (cyp19a1b), prostaglandin 3a synthase (ptges3a), and prostaglandin reductase 1 (ptgr1) were among the genes with sexually dimorphic expression patterns. MT treatment significantly altered gene expression relative to both male and female brains. Fewer differences were found among MT-treated brains and male brains compared to female brains, particularly at 60 dpf. MT treatment upregulated the expression of hydroxysteroid 11-beta dehydrogenase 2 (hsd11b2), deiodinase, iodothyronine, type II (dio2), and gonadotrophin releasing hormones (GnRH) 2 and 3 (gnrh2 and gnrh3) suggesting local synthesis of 11-ketotestosterone, triiodothyronine, and GnRHs in zebrafish brains which are influenced by androgens. Androgen, estrogen, prostaglandin, thyroid hormone, and GnRH signaling pathways likely interact to modulate teleost sexual behavior.

Neuropeptide B mediates female sexual receptivity in medaka fish, acting in a female-specific but reversible manner

Male and female animals display innate sex-specific mating behaviors. In teleost fish, altering the adult sex steroid milieu can effectively reverse sex-typical mating behaviors, suggesting remarkable sexual lability of their brains as adults. In the teleost medaka, neuropeptide B (NPB) is expressed female-specifically in the brain nuclei implicated in mating behavior. Here, we demonstrate that NPB is a direct mediator of estrogen action on female mating behavior, acting in a female-specific but reversible manner. Analysis of regulatory mechanisms revealed that the female-specific expression of NPB is dependent on direct transcriptional activation by estrogen via an estrogen-responsive element and is reversed in response to changes in the adult sex steroid milieu. Behavioral studies of NPB knockouts revealed that female-specific NBP mediates female receptivity to male courtship. The female-specific NPB signaling identified herein is presumably a critical element of the neural circuitry underlying sexual dimorphism and lability of mating behaviors in teleosts.

Sexually Dimorphic Neuropeptide B Neurons in Medaka Exhibit Activated Cellular Phenotypes Dependent on Estrogen

Brain and behavior of teleosts are highly sexually plastic throughout life, yet the underlying neural mechanisms are largely unknown. On examining brain morphology in the teleost medaka (Oryzias latipes), we identified distinctively large neurons in the magnocellular preoptic nucleus that occurred much more abundantly in females than in males. Examination of sex-reversed medaka showed that the sexually dimorphic abundance of these neurons is dependent on gonadal phenotype, but independent of sex chromosome complement. Most of these neurons in females, but none in males, produced neuropeptide B (Npb), whose expression is known to be estrogen-dependent and associated with female sexual receptivity. In phenotypic analysis, the female-specific Npb neurons had a large euchromatic nucleus with an abundant cytoplasm containing plentiful rough endoplasmic reticulum, exhibited increased overall transcriptional activity, and typically displayed a spontaneous regular firing pattern. These phenotypes, which are probably indicative of cellular activation, were attenuated by ovariectomy and restored by estrogen replacement. Furthermore, the population of Npb-expressing neurons emerged in adult males treated with estrogen, not through frequently occurring neurogenesis in the adult teleost brain, but through the activation of preexisting, quiescent male counterpart neurons. Collectively, our results demonstrate that the morphological, transcriptional, and electrophysiological phenotypes of sexually dimorphic preoptic Npb neurons are highly dependent on estrogen and can be switched between female and male patterns. These properties of the preoptic Npb neurons presumably underpin the neural mechanism for sexual differentiation and plasticity of brain and behavior in teleosts.

Plasticity of Lh cells caused by cell proliferation and recruitment of existing cells

Luteinizing hormone (Lh) and follicle-stimulating hormone (Fsh) control reproduction in vertebrates. Using a transgenic line of medaka, in which green fluorescent protein expression is controlled by the endogenous lhb promotor, we studied development and plasticity of Lh cells, comparing juveniles and adults of both genders. Confocal imaging and 3D reconstruction revealed hypertrophy and hyperplasia of Lh cells in both genders from juvenile to adult stages. We show that Lh cell hyperplasia may be caused by recruitment of existing pituitary cells that start to produce lhb, as evidenced by time lapse recordings of primary pituitary cell cultures, and/or through Lh cell proliferation, demonstrated through a combination of 5-bromo-2'-deoxyuridine incubation experiments and proliferating cell nuclear antigen staining. Proliferating Lh cells do not belong to the classical type of multipotent stem cells, as they do not stain with anti-sox2. Estradiol exposure in vivo increased pituitary cell proliferation, particularly Lh cells, whereas pituitary lhb and gpa expression levels decreased. RNA-seq and in situ hybridization showed that Lh cells express two estrogen receptors, esr1 and esr2b, and the aromatase gene cyp19a1b, suggesting a direct effect of estradiol, and possibly androgens, on Lh cell proliferation. In conclusion, our study reveals a high degree of plasticity in the medaka Lh cell population, resulting from a combination of recruitment and cell proliferation.

Expression of isotocin is male-specifically up-regulated by gonadal androgen in the medaka brain

Oxytocin, a mammalian neuropeptide primarily synthesised in the supraoptic and paraventricular nuclei of the hypothalamus, mediates a variety of physiological and behavioural processes, ranging from parturition and lactation to affiliation and prosociality. Multiple studies in rodents have shown that the expression of the oxytocin gene (Oxt) is stimulated by oestrogen, whereas androgen has no apparent effect. However, this finding is not consistent across all studies, and no study has examined sex steroid regulation of Oxt or its orthologues in other animals. In the present study, we show that, in the teleost fish, medaka (Oryzias latipes), the expression of the isotocin gene (it), the teleost orthologue of Oxt, in the parvocellular preoptic nuclei (homologous to the mammalian supraoptic nucleus) is male-specifically up-regulated by gonadal androgen, whereas it expression in the magnocellular/gigantocellular preoptic nuclei (homologous to the mammalian paraventricular nucleus) is independent of sex steroids in both sexes. None of the it-expressing neurones appear to co-express androgen receptors, suggesting that the effect of androgen on it expression is indirect. We found that the expression of a kisspeptin gene, kiss2, in the male brain is dependent on gonadal androgen, raising the possibility that the androgen-dependent expression of it may be mediated by kiss2 neurones. Our data also show that the isotocin peptide synthesised in response to androgen is axonally transported to the posterior pituitary to act peripherally. Given that levels of it expression are higher in females than in males, androgen may serve to compensate for the female-biased it expression to ensure a role for isotocin that is equally important for both sexes. These results are unexpectedly quite different from those reported in rodents, indicating that the regulatory role of sex steroids in Oxt/it expression has diverged during evolution, possibly with accompanying changes in the role of oxytocin/isotocin.

Three urocortins in medaka: identification and spatial expression in the central nervous system

The urocortin (UCN) group of neuropeptides includes urocortin 1/sauvagine/urotensin 1 (UTS1), urocortin 2 (UCN2) and urocortin 3 (UCN3). In recent years, evidence has accumulated showing that UCNs play pivotal roles in mediating stress response and anxiety in mammals. Evidence has also emerged regarding the evolutionary conservation of UCNs in vertebrates, but very little information is available about UCNs in non-mammalian vertebrates. Indeed, at present, there are no reports of the empirical identification of ucn2 in non-mammalian vertebrates or of the distribution of ucn2 and ucn3 expression in the adult central nervous system (CNS) of these animals. To gain insight into the evolutionary nature of UCNs in vertebrates, we cloned uts1, ucn2 and ucn3 in a teleost fish, medaka and examined the spatial expression of these genes in the adult brain and spinal cord. Although all known UCN2 genes except those in rodents have been reported to likely lack the necessary structural features to produce a functional pre-pro-protein, all three UCN genes in medaka, including ucn2, displayed all of these features, suggesting their functionality. The three UCN genes exhibited distinct spatial expression patterns in the medaka brain: uts1 was primarily expressed in broad regions of the dorsal telencephalon, ucn2 was expressed in restricted regions of the thalamus and brainstem and ucn3 was expressed in discrete nuclei throughout many regions of the brain. We also found that these genes were all expressed throughout the medaka spinal cord, each with a distinct spatial pattern. Given that many of these regions have been implicated in stress responses and anxiety, the three UCNs may serve distinct physiological roles in the medaka CNS, including those involved in stress and anxiety, as shown in the mammalian CNS.

Steroid modulation of neurogenesis: Focus on radial glial cells in zebrafish

Estrogens are known as steroid hormones affecting the brain in many different ways and a wealth of data now document effects on neurogenesis. Estrogens are provided by the periphery but can also be locally produced within the brain itself due to local aromatization of circulating androgens. Adult neurogenesis is described in all vertebrate species examined so far, but comparative investigations have brought to light differences between vertebrate groups. In teleost fishes, the neurogenic activity is spectacular and adult stem cells maintain their mitogenic activity in many proliferative areas within the brain. Fish are also quite unique because brain aromatase expression is limited to radial glia cells, the progenitor cells of adult fish brain. The zebrafish has emerged as an interesting vertebrate model to elucidate the cellular and molecular mechanisms of adult neurogenesis, and notably its modulation by steroids. The main objective of this review is to summarize data related to the functional link between estrogens production in the brain and neurogenesis in fish. First, we will demonstrate that the brain of zebrafish is an endogenous source of steroids and is directly targeted by local and/or peripheral steroids. Then, we will present data demonstrating the progenitor nature of radial glial cells in the brain of adult fish. Next, we will emphasize the role of estrogens in constitutive neurogenesis and its potential contribution to the regenerative neurogenesis. Finally, the negative impacts on neurogenesis of synthetic hormones used in contraceptive pills production and released in the aquatic environment will be discussed.

Rspo1-activated signalling molecules are sufficient to induce ovarian differentiation in XY medaka (Oryzias latipes)

In contrast to our understanding of testicular differentiation, ovarian differentiation is less well understood in vertebrates. In mammals, R-spondin1 (Rspo1), an activator of Wnt/β-catenin signaling pathway, is located upstream of the female sex determination pathway. However, the functions of Rspo1 in ovarian differentiation remain unclear in non-mammalian species. In order to elucidate the detailed functions of Rspo/Wnt signaling pathway in fish sex determination/differentiation, the ectopic expression of the Rspo1 gene was performed in XY medaka (Oryzias latipes). The results obtained demonstrated that the gain of Rspo1 function induced femininity in XY fish. The overexpression of Rspo1 enhanced Wnt4b and β-catenin transcription, and completely suppressed the expression of male-biased genes (Dmy, Gsdf, Sox9a2 and Dmrt1) as well as testicular differentiation. Gonadal reprograming of Rspo1-over-expressed-XY (Rspo1-OV-XY) fish, induced the production of female-biased genes (Cyp19a1a and Foxl2), estradiol-17β production and further female type secondary sexuality. Moreover, Rspo1-OV-XY females were fertile and produced successive generations. Promoter analyses showed that Rspo1 transcription was directly regulated by DM domain genes (Dmy, the sex-determining gene, and Dmrt1) and remained unresponsive to Foxl2. Taken together, our results strongly suggest that Rspo1 is sufficient to activate ovarian development and plays a decisive role in the ovarian differentiation in medaka.

Molecular cloning and characterization of brain and ovarian cytochrome P450 aromatase genes in the catfish Heteropneustes fossilis: Sex, tissue and seasonal variation in, and effects of gonadotropin on gene expression

Cytochrome P450 aromatase (Cyp19arom) is the rate-limiting enzyme controlling estrogen biosynthesis, coded by Cyp19a1 in most gnathostomes. Most teleosts have two forms expressed differentially in ovary (cyp19a1a) and neural tissue (cyp19a1b). In this study, full length cDNAs of 2006 bp and 1913 bp with ORFs of 1575 bp and 1488 bp were isolated from the brain and ovary, respectively, of the catfish Heteropneustes fossilis, an air-breathing species with high aquaculture potential. The ORFs encode predicted proteins of 495 and 524 amino acid residues, respectively. The proteins show 62% identity with each other and cluster in two distinct clades (the brain type and ovary type) in the teleost taxon, separated from the tetrapod type. In the in situ localization study, both cyp19a1a and cyp19a1b transcripts were localized in the brain but the signal intensity was higher for the brain type paralog. The transcript signals were observed in the radial glial cells and in neuronal populations of the dorso-lateral region of the telencephalon, pre-tectum, hypothalamus and medulla oblongata. In the ovary, both paralogs were expressed in the follicular layer with a high signal intensity of the ovarian type (cyp19a1a). The differential expression of the gene paralogs was evident from qPCR analysis. Cyp19a1b has relatively a high abundance in the female brain, followed by other peripheral tissues (gonads, liver, gill, kidney and muscle). On the other hand, cyp19a1a has relatively a high transcript abundance in the ovary and female brain, followed by the testis and male brain, and female liver and muscle. The expression was low in male liver and muscle, and the lowest in the gill and kidney. The expression of the two paralogs exhibit brain regional differences; both types have relatively a high transcript abundance in telencephalon-preoptic area with the cyp19a1b expression higher in females than males. In hypothalamus, the expression of both types is higher in males than females. In medulla, the expression of the cyp19a1b is higher than cyp19a1a, and the transcript abundance of the ovarian type is higher in females than males. The expression of the gene paralogs elicits significant seasonal variations in the ovary and brain. In both tissues, the expression increases from the resting to preparatory phases, and decreases through the prespawning phase to low levels in spawning phase. In vivo and/or in vitro treatments with human chorionic gonadotropin (hCG) stimulated the expression of the gene paralogs in the brain and ovary, time-dependently. In conclusion, both paralogs have an overlapping distribution at different levels of the brain-pituitary-gonad axis and may function as a single functional unit as far as the estrogen synthesis is concerned.

"Brain sex differentiation" in teleosts: Emerging concepts with potential biomarkers

"Brain sex differentiation" in teleosts is a contentious topic of research as most of the earlier reports tend to suggest that gonadal sex differentiation drives brain sex differentiation. However, identification of sex-specific marker genes in the developing brain of teleosts signifies brain-gonadal interaction during early sexual development in lower vertebrates. In this context, the influence of gonadotropin-releasing hormone (GnRH)-gonadotropin (GTH) axis on gonadal sex differentiation, if any requires in depth analysis. Presence of seabream (sb) GnRH immunoreactivity (ir-) in the brain of XY Nile tilapia was found as early as 5days post hatch (dph) followed by qualitative reduction in the preoptic area-hypothalamus region. In contrast, in the XX female brain a steady ir- of sbGnRH was evident from 15dph. Earlier studies using sea bass already implied the importance of hypothalamic gonadotropic axis completion during sex differentiation period. Such biphasic pattern of localization was also seen in pituitary GTHs using heterologous antisera in tilapia. However, more recent analysis in the same species could not detect any sexually dimorphic pattern using homologous antisera for pituitary GTHs. Detailed studies on the development of hypothalamo-hypophyseal-gonadal axis in teleosts focusing on hypothalamic monoamines (MA) and MA-related enzymes demonstrated sex-specific differential expression of tryptophan hydroxylase (Tph) in the early stages of developing male and female brains of tilapia and catfish. The changes in Tph expression was in agreement with the levels of serotonin (5-HT) and 5-hydroxytryptophan in the preoptic area-hypothalamus. Considering the stimulatory influence of 5-HT on GnRH and GTH release, it is possible to propose a network association between these correlates during early development, which may bring about brain sex dimorphism in males. A recent study from our laboratory during female brain sex development demonstrated high expression of tyrosine hydroxylase in correlation with catecholamine levels, brain aromatase and its related transcription factors such as fushi tarazu factor 1, Ftz-f1 and fork head box protein L2, foxl2. Taken together, gender differences in the levels of various transcripts provide new perspectives on brain sex differentiation in lower vertebrates. Sexually dimorphic or differentially expressing genes may play an essential role at the level of brain in response to gonadal differentiation, which might consequentially or causatively respond to gonadal sex.

Genetics of sexual development: an evolutionary playground for fish

Teleost fishes are the most species-rich clade of vertebrates and feature an overwhelming diversity of sex-determining mechanisms, classically grouped into environmental and genetic systems. Here, we review the recent findings in the field of sex determination in fish. In the past few years, several new master regulators of sex determination and other factors involved in sexual development have been discovered in teleosts. These data point toward a greater genetic plasticity in generating the male and female sex than previously appreciated and implicate novel gene pathways in the initial regulation of the sexual fate. Overall, it seems that sex determination in fish does not resort to a single genetic cascade but is rather regulated along a continuum of environmental and heritable factors.

Limited sex-biased neural gene expression patterns across strains in Zebrafish (Danio rerio)

Background

Male and female vertebrates typically differ in a range of characteristics, from morphology to physiology to behavior, which are influenced by factors such as the social environment and the internal hormonal and genetic milieu. However, sex differences in gene expression profiles in the brains of vertebrates are only beginning to be understood. Fishes provide a unique complement to studies of sex differences in mammals and birds given that fish show extreme plasticity and lability of sexually dimorphic characters and behaviors during development and even adulthood. Hence, teleost models can give additional insight into sexual differentiation. The goal of this study is to identify neurotranscriptomic mechanisms for sex differences in the brain.

Results

In this study we examined whole-brain sex-biased gene expression through RNA-sequencing across four strains of zebrafish. We subsequently conducted systems level analyses by examining gene network dynamics between the sexes using weighted gene coexpression network analysis. Surprisingly, only 61 genes (approximately 0.4% of genes analyzed) showed a significant sex effect across all four strains, and 48 of these differences were male-biased. Several of these genes are associated with steroid hormone biosynthesis. Despite sex differences in a display of stress-related behaviors, basal transcript levels did not predict the intensity of the behavioral display. WGCNA revealed only one module that was significantly associated with sex. Intriguingly, comparing intermodule dynamics between the sexes revealed only moderate preservation. Further we identify sex-specific gene modules.

Conclusions

Despite differences in morphology, physiology, and behavior, there is limited sex-biased neural gene expression in zebrafish. Further, genes found to be sex-biased are associated with hormone biosynthesis, suggesting that sex steroid hormones may be key contributors to sexual behavioral plasticity seen in teleosts. A possible mechanism is through regulating specific brain gene networks.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-905) contains supplementary material, which is available to authorized users.

Aromatase, estrogen receptors and brain development in fish and amphibians

Estrogens affect brain development of vertebrates, not only by impacting activity and morphology of existing circuits, but also by modulating embryonic and adult neurogenesis. The issue is complex as estrogens can not only originate from peripheral tissues, but also be locally produced within the brain itself due to local aromatization of androgens. In this respect, teleost fishes are quite unique because aromatase is expressed exclusively in radial glial cells, which represent pluripotent cells in the brain of all vertebrates. Expression of aromatase in the brain of fish is also strongly stimulated by estrogens and some androgens. This creates a very intriguing positive auto-regulatory loop leading to dramatic aromatase expression in sexually mature fish with elevated levels of circulating steroids. Looking at the effects of estrogens or anti-estrogens in the brain of adult zebrafish showed that estrogens inhibit rather than stimulate cell proliferation and newborn cell migration. The functional meaning of these observations is still unclear, but these data suggest that the brain of fish is experiencing constant remodeling under the influence of circulating steroids and brain-derived neurosteroids, possibly permitting a diversification of sexual strategies, notably hermaphroditism. Recent data in frogs indicate that aromatase expression is limited to neurons and do not concern radial glial cells. Thus, until now, there is no other example of vertebrates in which radial progenitors express aromatase. This raises the question of when and why these new features were gained and what are their adaptive benefits. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Estradiol treatment decreases cell proliferation in the neurogenic zones of adult female zebrafish (Danio rerio) brain

While estrogens are known to play a crucial role in the neurogenesis of the mammalian and avian brain, their role in teleost adult proliferation pattern is not yet fully understood. The present study aimed to determine the estrogen effects in adult brain proliferation zones, using zebrafish, as a model organism. Indeed, teleost fish brain provides a unique adult neurogenesis model, based on its extensive proliferation, contrasting the restricted adult telencephalic neurogenesis observed in birds and mammals. To determine the effect of estrogens, 17-β estradiol was administrated for 7 days in adult female zebrafish, followed by bromodeoxyuridine (BrdU)-immunohistochemistry and double immunofluorescence. Stereological analyses of the BrdU-positive cells within the neurogenic zones, showed region-specific decreases of actively proliferating cells in the estrogen-treated animals, compared to matched controls. Interestingly, the most prominent estradiol effects were found in the number of cycling cells of the ventral nucleus of ventral telencephalic area (Vv) and cerebellar areas. Significant decreases were also determined in the dorso-lateral telencephalic, preoptic and dorsal hypothalamic areas. In contrast, medial dorsal telencephalic, caudal (Hc) and ventral (Hv) hypothalamic areas were unaffected by estrogen treatment. The majority of the BrdU-labeled cells were found to co-express PCNA proliferating marker in Hc, Hv and Vv. Additionally, a population of proliferating cells co-expressed the early neuronal marker TOAD in all areas studied. These results provide significant evidence on the 17-β estradiol impact on adult neurogenesis, down-regulating the fast-cycling and post-mitotic cells within the female zebrafish brain neurogenetic zones.

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

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

Neuropeptide B is female-specifically expressed in the telencephalic and preoptic nuclei of the medaka brain

In the brain of medaka (Oryzias latipes), a teleost fish, we recently found that the supracommissural/posterior nuclei of the ventral telencephalic area (Vs/Vp) and the magnocellular/gigantocellular portions of the magnocellular preoptic nucleus (PMm/PMg) express estrogen receptor (ER) and androgen receptor (AR) specifically in females. This finding led us to postulate that sex steroid hormones might induce gene expression unique to females in these nuclei. In the present study, we searched for genes differentially expressed between the sexes in the medaka brain and identified the gene encoding neuropeptide B (npb) as being female-specifically expressed in Vs/Vp and PMm/PMg. As expected, the neurons expressing npb female-specifically constituted a significant proportion of the ER/AR-expressing neurons in these nuclei. Subsequent analyses provided evidence that the female-specific expression of npb in Vs/Vp and PMm/PMg results from the reversible and transient action of estrogens secreted from the ovary and that this estrogenic action is most likely mediated by the direct transcriptional activation of npb through an estrogen-responsive element in its proximal promoter region. Vs/Vp and PMm/PMg are generally recognized in teleost fish as the sites where neurons expressing 2 other neuropeptides, isotocin and vasotocin, are present, but the female-specific npb/ER/AR-expressing neurons were distinct from, although adjacent to, isotocin and vasotocin neurons. Taken together, these data demonstrate that npb is female-specifically expressed in novel, as-yet undefined populations of Vs/Vp and PMm/PMg neurons, resulting from the direct stimulatory action of ovarian estrogens via female-specific ER in these neurons.

Post-proliferative immature radial glial cells female-specifically express aromatase in the medaka optic tectum

Aromatase, the key enzyme responsible for estrogen biosynthesis, is present in the brain of all vertebrates. Much evidence has accumulated that aromatase is highly and exclusively expressed in proliferating mature radial glial cells in the brain of teleost fish even in adulthood, unlike in other vertebrates. However, the physiological significance of this expression remains unknown. We recently found that aromatase is female-specifically expressed in the optic tectum of adult medaka fish. In the present study, we demonstrated that, contrary to the accepted view of the teleost brain, female-specific aromatase-expressing cells in the medaka optic tectum represent a transient subset of post-proliferative immature radial glial cells in the neural stem cell lineage. This finding led us to hypothesize that female-specific aromatase expression and consequent estrogen production causes some sex difference in the life cycle of tectal cells. As expected, the female tectum exhibited higher expression of genes indicative of cell proliferation and radial glial maturation and lower expression of an anti-apoptotic gene than did the male tectum, suggesting a female-biased acceleration of the cell life cycle. Complicating the interpretation of this result, however, is the additional observation that estrogen administration masculinized the expression of these genes in the optic tectum, while simultaneously stimulating aromatase expression. Taken together, these results provide evidence that a unique subpopulation of neural stem cells female-specifically express aromatase in the optic tectum and suggest that this aromatase expression and resultant estrogen synthesis have an impact on the life cycle of tectal cells, whether stimulatory or inhibitory.

hebp3, a novel member of the heme-binding protein gene family, is expressed in the medaka meninges with higher abundance in females due to a direct stimulating action of ovarian estrogens

The brains of teleost fish exhibit remarkable sexual plasticity throughout their life span. To dissect the molecular basis for the development and reversal of sex differences in the teleost brain, we screened for genes differentially expressed between sexes in the brain of medaka (Oryzias latipes). One of the genes identified in the screen as being preferentially expressed in females was found to be a new member of the heme-binding protein gene family that includes hebp1 and hebp2 and was designated here as hebp3. The medaka hebp3 is expressed in the meninges with higher abundance in females, whereas there is no expression within the brain parenchyma. This female-biased expression of hebp3 is not attributable to the direct action of sex chromosome genes but results from the transient and reversible action of estrogens derived from the ovary. Moreover, estrogens directly activate the transcription of hebp3 via a palindromic estrogen-responsive element in the hebp3 promoter. Taken together, our findings demonstrate that hebp3 is a novel transcriptional target of estrogens, with female-biased expression in the meninges. The definite but reversible sexual dimorphism of the meningeal hebp3 expression may contribute to the development and reversal of sex differences in the teleost brain.

Increase in estrogen signaling in the early brain of orange-spotted grouper Epinephelus coioides: a mini-review

Despite neurosteroidogenic enzymes are playing important roles in the regulation of brain development and function, the potential link between brain and gonad by the action of steroid hormones during gonadal sex differentiation is still not clear in teleosts. In this mini-review, we summarized our understanding on the early brain development related to the synthesis of neurosteroids and receptor signaling during gonadal sex differentiation in protogynous orange-spotted grouper, Epinephelus coioides (functional females for the first 6 years of life and start to sex change around the age of 7 years) and protandrous black porgy (functional males for the first 2 years of life but begin to change sex during the third year). We found a similar profile in the increased expression of brain aromatase gene (aromatatse B or cyp19a1b), aromatase activity, estradiol (E(2)), and estrogen signaling in the brain of both grouper and black porgy fish during gonadal sex differentiation. In contrast to mammals, teleost fish Cyp19a1b expressed in a unique cell type, a radial glial cell, which is acted as progenitors in the brain of developing and adult fish. In agreement with these pioneer studies, we demonstrated that the grouper cyp19a1b/Cyp19a1b was expressed in radial glial cells. Further, in vivo data in the grouper brain showed that exogenous E(2) upregulated Cyp19a1b immunoreactivity (ir) in radial glial cells. These data suggest the possible roles of Cyp19a1b and E(2) in early brain development which is presumably related to gonadal sex differentiation.

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

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

Female-specific target sites for both oestrogen and androgen in the teleost brain

To dissect the molecular and cellular basis of sexual differentiation of the teleost brain, which maintains marked sexual plasticity throughout life, we examined sex differences in neural expression of all subtypes of nuclear oestrogen and androgen receptors (ER and AR) in medaka. All receptors were differentially expressed between the sexes in specific nuclei in the forebrain. The most pronounced sex differences were found in several nuclei in the ventral telencephalic and preoptic areas, where ER and AR expression were prominent in females but almost completely absent in males, indicating that these nuclei represent female-specific target sites for both oestrogen and androgen in the brain. Subsequent analyses revealed that the female-specific expression of ER and AR is not under the direct control of sex-linked genes but is instead regulated positively by oestrogen and negatively by androgen in a transient and reversible manner. Taken together, the present study demonstrates that sex-specific target sites for both oestrogen and androgen occur in the brain as a result of the activational effects of gonadal steroids. The consequent sex-specific but reversible steroid sensitivity of the adult brain probably contributes substantially to the process of sexual differentiation and the persistent sexual plasticity of the teleost brain.

Expression of aromatase in radial glial cells in the brain of the Japanese eel provides insight into the evolution of the cyp191a gene in Actinopterygians

The cyp19a1 gene that encodes aromatase, the only enzyme permitting conversion of C19 aromatizable androgens into estrogens, is present as a single copy in the genome of most vertebrate species, except in teleosts in which it has been duplicated. This study aimed at investigating the brain expression of a cyp19a1 gene expressed in both gonad and brain of Japanese eel, a basal teleost. By means of immunohistochemistry and in situ hybridization, we show that cyp19a1 is expressed only in radial glial cells of the brain and in pituitary cells. Treatments with salmon pituitary homogenates (female) or human chorionic gonadotrophin (male), known to turn on steroid production in immature eels, strongly stimulated cyp19a1 messenger and protein expression in radial glial cells and pituitary cells. Using double staining studies, we also showed that aromatase-expressing radial glial cells exhibit proliferative activity in both the brain and the pituitary. Altogether, these data indicate that brain and pituitary expression of Japanese eel cyp19a1 exhibits characteristics similar to those reported for the brain specific cyp19a1b gene in teleosts having duplicated cyp19a1 genes. This supports the hypothesis that, despite the fact that eels also underwent the teleost specific genome duplication, they have a single cyp19a1 expressed in both brain and gonad. Such data also suggest that the intriguing features of brain aromatase expression in teleost fishes were not gained after the whole genome duplication and may reflect properties of the cyp19a1 gene of ancestral Actinopterygians.

Sex differences in the expression of vasotocin/isotocin, gonadotropin-releasing hormone, and tyrosine and tryptophan hydroxylase family genes in the medaka brain

In teleost fish, sex differences in several behavioral and physiological traits have been assumed to reflect underlying sex differences in the central expression of neurotransmitter/neuromodulator-related molecules, including vasotocin (VT)/isotocin (IT), gonadotropin-releasing hormone (GnRH), and tyrosine and tryptophan hydroxylases (TH and TPH). However, the sex-dependent expression patterns of these molecules have not been fully characterized in the teleost brain. In the present study, we therefore systematically evaluated sex differences in their expression in the medaka (Oryzias latipes) brain. The most prominent sex difference was observed in vt expression in the nucleus posterior tuberis (NPT) and the posterior part of the nucleus ventral tuberis (NVT) in the hypothalamus, where the expression was completely male-specific. Male-biased expression of gnrh1, tph1, and tph2 was also evident in the supracommissural and posterior nuclei of the ventral telencephalic area (Vs/Vp), medial nucleus of the dorsal telencephalic area (Dm), and thalamic dorsal posterior nucleus (DP), respectively. In contrast, the overall expression levels of it and gnrh3 were higher in the female brain than in the male brain. Equally importantly, no conspicuous sex differences were observed in the expression of gnrh2, th1, and th2, despite several previous reports of their sex-biased expression in the brains of other teleost species. Taken together, these data have uncovered previously unidentified sex differences in the expression of VT/IT, GnRH, and TPH in the teleost brain, which may possibly be relevant to sexual dimorphism in some behavioral and/or physiological traits, and have simultaneously highlighted potential species differences in the roles of these molecules.