Relationships between aromatase and estrogen receptors in the brain of teleost fish

Arginine vasopressin injection rescues delayed oviposition in cyp19a1b-/- mutant female zebrafish

In zebrafish, estrogens produced in the ovaries via Cyp19a1a activity are required for both sexual differentiation of the ovary during early development as well as maintenance of the ovarian state during adulthood. The importance of Cyp19a1b that is highly expressed in the brain for female reproduction is still under study. We previously reported that female cyp19a1b -/- mutant zebrafish have significantly lower brain estradiol levels and impaired spawning behavior characterized by an increased latency to oviposition during dyadic sexual behavior encounters. In the current study, we provide evidence that the delayed oviposition in female cyp19a1b -/- mutants is linked to impaired arginine vasopressin (Avp) signaling. Droplet digital PCR experiments revealed that levels of the estrogen receptors, avp, and oxytocin (oxt) are lower in the hypothalamus of mutant females compared to wildtype fish. We then used acute intraperitoneal injections of Avp and Oxt, along with mixtures of their respective receptor antagonists, to determine that Avp can uniquely rescue the delayed oviposition in female cyp19a1b -/- mutants. Using immunohistochemistry, we demonstrated that Cyp19a1b-expressing radial glial cell (RGC) fibers surround and are in contact with Avp-immunopositive neurons in the preoptic areas of the brain. This could provide the neuroanatomical proximity for RGC-derived estrogens to diffuse to and activate estrogen receptors and regulate avp expression levels. Together these findings identify a positive link between Cyp19a1b and Avp for female zebrafish sexual behavior. They also suggest that the female cyp19a1b -/- mutant behavioral phenotype is likely a consequence of impaired processing of Avp-dependent social cues important for mate identification and assessment.

Zebrafish as an emerging model to study estrogen receptors in neural development

Estrogens induce several regulatory signals in the nervous system that are mainly mediated through estrogen receptors (ERs). ERs are largely expressed in the nervous system, yet the importance of ERs to neural development has only been elucidated over the last decades. Accumulating evidence shows a fundamental role for estrogens in the development of the central and peripheral nervous systems, hence, the contribution of ERs to neural function is now a growing area of research. The conservation of the structure of the ERs and their response to estrogens make the zebrafish an interesting model to dissect the role of estrogens in the nervous system. In this review, we highlight major findings of ER signaling in embryonic zebrafish neural development and compare the similarities and differences to research in rodents. We also discuss how the recent generation of zebrafish ER mutants, coupled with the availability of several transgenic reporter lines, its amenability to pharmacological studies and in vivo live imaging, could help us explore ER function in embryonic neural development.

Gonad recrudescence and annual sex steroid patterns in female goldspot mullet, Liza parsia reared in brackishwater pond

Goldspot mullet, Liza parsia is a commercially important fish of South East Asia, where its farming depends on wild seed resources due to unavailability of hatchery technology. It, therefore, is important to understand the annual reproductive cycle of female L. parsia in captivity. In this study, adult male and female L. parsia (body weight ranges: 45-90 g; total length ranges: 100-125 mm, age >1 year) were collected from the wild and reared in a brackishwater pond. Thereafter, fish were randomly sampled at monthly intervals to measure the hepatosomatic index (HSI), gonadosomatic index (GSI), levels of serum steroids (testosterone, T; 17β-estradiol, E2 and 17α-hydroxyprogesterone, 17-OHP), and oocyte growth. Results exhibited that female L. parsia undergoes six different maturation stages, namely I (oocyte diameter, OD: <100 µm), II (OD: 100-350 µm), III (OD: 350-400 µm), IV (vitellogenic oocyte, OD: 400-450 µm), V (ripe oocyte, OD: 450-550 µm) and VI (atretic oocyte, OD: 60-150 µm), with synchronous oocyte development. The highest (P < 0.05) HSI (1.96 ± 0.24) and GSI (12.01 ± 0.73) were recorded in December and January, respectively. Concentration of E2 gradually increased from August and reached its peak (807.67 ± 25.98 pg mL^-1, P < 0.05) in December. The level of 17-OHP (85.87 ± 0.91 pg mL^-1) was at its peak during the normal spawning month (January) (P < 0.05). Overall, the results indicated that L. parsia attains maturity in brackishwater pond, which is consistent with previous observations, and altogether provide the basis to develop a breeding technology in captivity through hormonal and environmental manipulations.

The Role of Estrogen and Thyroid Hormones in Zebrafish Visual System Function

Visual system development is a highly complex process involving coordination of environmental cues, cell pathways, and integration of functional circuits. Consequently, a change to any step, due to a mutation or chemical exposure, can lead to deleterious consequences. One class of chemicals known to have both overt and subtle effects on the visual system is endocrine disrupting compounds (EDCs). EDCs are environmental contaminants which alter hormonal signaling by either preventing compound synthesis or binding to postsynaptic receptors. Interestingly, recent work has identified neuronal and sensory systems, particularly vision, as targets for EDCs. In particular, estrogenic and thyroidogenic signaling have been identified as critical modulators of proper visual system development and function. Here, we summarize and review this work, from our lab and others, focusing on behavioral, physiological, and molecular data collected in zebrafish. We also discuss different exposure regimes used, including long-lasting effects of developmental exposure. Overall, zebrafish are a model of choice to examine the impact of EDCs and other compounds targeting estrogen and thyroid signaling and the consequences of exposure in visual system development and function.

mdka Expression Is Associated with Quiescent Neural Stem Cells during Constitutive and Reactive Neurogenesis in the Adult Zebrafish Telencephalon

In contrast to mammals, adult zebrafish display an extraordinary capacity to heal injuries and repair damage in the central nervous system. Pivotal for the regenerative capacity of the zebrafish brain at adult stages is the precise control of neural stem cell (NSC) behavior and the maintenance of the stem cell pool. The gene mdka, a member of a small family of heparin binding growth factors, was previously shown to be involved in regeneration in the zebrafish retina, heart, and fin. Here, we investigated the expression pattern of the gene mdka and its paralogue mdkb in the zebrafish adult telencephalon under constitutive and regenerative conditions. Our findings show that only mdka expression is specifically restricted to the telencephalic ventricle, a stem cell niche of the zebrafish telencephalon. In this brain region, mdka is particularly expressed in the quiescent stem cells. Interestingly, after brain injury, mdka expression remains restricted to the resting stem cell, which might suggest a role of mdka in regulating stem cell quiescence.

Effects of Low Concentration Benzophenone-3 Exposure on the Sex Ratio and Offspring Development of Zebrafish (Danio rerio)

Benzophenone-3 (BP-3) is an important ultraviolet (UV)-screening agent using in cosmetics, however, the associated environmental pollution and the toxicity to organisms, particularly aquatic organisms, cannot be neglected. In this study, the potential risks posed to zebrafish when exposed to environmental residual concentrations of BP-3 were evaluated. Zebrafish embryos (F0) were exposed to 0, 0.056, 2.3, and 38 μg/L BP-3 until 42 days' post-fertilization (dpf). The effects of BP-3 on the sex ratio and gene expression of F0 zebrafish were investigated. In the F1 embryos, cumulative hatching rate, body length, and heartbeats were observed. The result showed that F0 and F1 exposure to concentrations of 0.056 and 38 μg/L BP-3 elicited stronger toxicity at 96 hpf than single generation exposures. Overall, our results provide a new understanding on the effects of low BP-3 concentration chronic exposure on sex ratio and offspring developmental toxicity of the F0 zebrafish.

HDL biodistribution and brain receptors in zebrafish, using HDLs as vectors for targeting endothelial cells and neural progenitors

High density lipoproteins (HDLs) display pleiotropic functions such as anti-inflammatory, antioxidant, anti-protease, and anti-apoptotic properties. These effects are mediated by four main receptors: SCARB1 (SR-BI), ABCA1, ABCG1, and CD36. Recently, HDLs have emerged for their potential involvement in brain functions, considering their epidemiological links with cognition, depression, and brain plasticity. However, their role in the brain is not well understood. Given that the zebrafish is a well-recognized model for studying brain plasticity, metabolic disorders, and apolipoproteins, it could represent a good model for investigating the role of HDLs in brain homeostasis. By analyzing RNA sequencing data sets and performing in situ hybridization, we demonstrated the wide expression of scarb1, abca1a, abca1b, abcg1, and cd36 in the brain of adult zebrafish. Scarb1 gene expression was detected in neural stem cells (NSCs), suggesting a possible role of HDLs in NSC activity. Accordingly, intracerebroventricular injection of HDLs leads to their uptake by NSCs without modulating their proliferation. Next, we studied the biodistribution of HDLs in the zebrafish body. In homeostatic conditions, intraperitoneal injection of HDLs led to their accumulation in the liver, kidneys, and cerebral endothelial cells in zebrafish, similar to that observed in mice. After telencephalic injury, HDLs were diffused within the damaged parenchyma and were taken up by ventricular cells, including NSCs. However, they failed to modulate the recruitment of microglia cells at the injury site and the injury-induced proliferation of NSCs. In conclusion, our results clearly show a functional HDL uptake process involving several receptors that may impact brain homeostasis and suggest the use of HDLs as delivery vectors to target NSCs for drug delivery to boost their neurogenic activity.

Direct and Indirect Effects of Sex Steroids on Gonadotrope Cell Plasticity in the Teleost Fish Pituitary

The pituitary gland controls many important physiological processes in vertebrates, including growth, homeostasis, and reproduction. As in mammals, the teleost pituitary exhibits a high degree of plasticity. This plasticity permits changes in hormone production and secretion necessary to meet the fluctuating demands over the life of an animal. Pituitary plasticity is achieved at both cellular and population levels. At the cellular level, hormone synthesis and release can be regulated via changes in cell composition to modulate both sensitivity and response to different signals. At the cell population level, the number of cells producing a given hormone can change due to proliferation, differentiation of progenitor cells, or transdifferentiation of specific cell types. Gonadotropes, which play an important role in the control of reproduction, have been intensively investigated during the last decades and found to display plasticity. To ensure appropriate endocrine function, gonadotropes rely on external and internal signals integrated at the brain level or by the gonadotropes themselves. One important group of internal signals is the sex steroids, produced mainly by the gonadal steroidogenic cells. Sex steroids have been shown to exert complex effects on the teleost pituitary, with differential effects depending on the species investigated, physiological status or sex of the animal, and dose or method of administration. This review summarizes current knowledge of the effects of sex steroids (androgens and estrogens) on gonadotrope cell plasticity in teleost anterior pituitary, discriminating direct from indirect effects.

Diving into the streams and waves of constitutive and regenerative olfactory neurogenesis: insights from zebrafish

The olfactory system is renowned for its functional and structural plasticity, with both peripheral and central structures displaying persistent neurogenesis throughout life and exhibiting remarkable capacity for regenerative neurogenesis after damage. In general, fish are known for their extensive neurogenic ability, and the zebrafish in particular presents an attractive model to study plasticity and adult neurogenesis in the olfactory system because of its conserved structure, relative simplicity, rapid cell turnover, and preponderance of neurogenic niches. In this review, we present an overview of the anatomy of zebrafish olfactory structures, with a focus on the neurogenic niches in the olfactory epithelium, olfactory bulb, and ventral telencephalon. Constitutive and regenerative neurogenesis in both the peripheral olfactory organ and central olfactory bulb of zebrafish is reviewed in detail, and a summary of current knowledge about the cellular origin and molecular signals involved in regulating these processes is presented. While some features of physiologic and injury-induced neurogenic responses are similar, there are differences that indicate that regeneration is not simply a reiteration of the constitutive proliferation process. We provide comparisons to mammalian neurogenesis that reveal similarities and differences between species. Finally, we present a number of open questions that remain to be answered.

Common and Distinct Features of Adult Neurogenesis and Regeneration in the Telencephalon of Zebrafish and Mammals

In contrast to mammals, the adult zebrafish brain shows neurogenic activity in a multitude of niches present in almost all brain subdivisions. Irrespectively, constitutive neurogenesis in the adult zebrafish and mouse telencephalon share many similarities at the cellular and molecular level. However, upon injury during tissue repair, the situation is entirely different. In zebrafish, inflammation caused by traumatic brain injury or by induced neurodegeneration initiates specific and distinct neurogenic programs that, in combination with signaling pathways implicated in constitutive neurogenesis, quickly, and efficiently overcome the loss of neurons. In the mouse brain, injury-induced inflammation promotes gliosis leading to glial scar formation and inhibition of regeneration. A better understanding of the regenerative mechanisms occurring in the zebrafish brain could help to develop new therapies to combat the debilitating consequences of brain injury, stroke, and neurodegeneration. The aim of this review is to compare the properties of neural progenitors and the signaling pathways, which control adult neurogenesis and regeneration in the zebrafish and mammalian telencephalon.

Bone morphogenetic protein signaling regulates Id1-mediated neural stem cell quiescence in the adult zebrafish brain via a phylogenetically conserved enhancer module

In the telencephalon of adult zebrafish, the inhibitor of DNA binding 1 (id1) gene is expressed in radial glial cells (RGCs), behaving as neural stem cells (NSCs), during constitutive and regenerative neurogenesis. Id1 controls the balance between resting and proliferating states of RGCs by promoting quiescence. Here, we identified a phylogenetically conserved cis-regulatory module (CRM) mediating the specific expression of id1 in RGCs. Systematic deletion mapping and mutation of conserved transcription factor binding sites in stable transgenic zebrafish lines reveal that this CRM operates via conserved smad1/5 and 4 binding motifs under both homeostatic and regenerative conditions. Transcriptome analysis of injured and uninjured telencephala as well as pharmacological inhibition experiments identify a crucial role of bone morphogenetic protein (BMP) signaling for the function of the CRM. Our data highlight that BMP signals control id1 expression and thus NSC proliferation during constitutive and induced neurogenesis.

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.

Prediction of the combined effects of multiple estrogenic chemicals on MCF-7 human breast cancer cells and a preliminary molecular exploration of the estrogenic proliferative effects and related gene expression

The environmental risks of environmental estrogens (EEs) are often assessed via the same mode of action in the concentration addition (CA) model, neglecting the complex combined mechanisms at the genetic level. In this study, the cell proliferation effects of estrone, 17α-ethinylestradiol, 17β-estradiol, estriol, diethylstilbestrol, estradiol valerate, bisphenol A, 4-tert-octylphenol and 4-nonylphenol were determined individually using the CCK-8 method, and the proliferation effects of a multicomponent mixture of estrogenic chemicals mixed at equipotent concentrations using a fixed-ratio design were studied using estrogen-sensitive MCF-7 cells. Furthermore, transcription factors related to cell proliferation were analyzed using RT-PCR assays to explore the potential molecular mechanisms related to the estrogenic proliferative effects. The results showed that the estrogenic chemicals act together in an additive mode, and the combined proliferative effects could be predicted more accurately by the response addition model than the CA model with regard to their adverse outcomes. Furthermore, different signaling pathways were involved depending on the different mixtures. The RT-PCR analyses showed that different estrogens have distinct avidities and preferences for different estrogen receptors at the gene level. Furthermore, the results indicated that estrogenic mixtures increased ERα, PIK3CA, GPER, and PTEN levels and reduced Akt1 level to display combined estrogenicity. These findings indicated that the potential combined environmental risks were greater than those found in some specific assessment procedures based on a similar mode of action due to the diversity of environmental pollutions and their multiple unknown modes of action. Thus, more efforts are needed for mode-of-action-driven analyses at the molecular level. Furthermore, to more accurately predict and assess the individual responses in vivo from the cellular effects in vitro, more parameters and correction factors should be taken into consideration in the addition model.

A comparative review on estrogen receptors in the reproductive male tract of non mammalian vertebrates

Estrogen receptors alpha (ERα) and beta (ERβ) are transcription factors known to be involved in the regulation of many complex physiological processes in mammals. They are expressed primarily in the reproductive tract of all vertebrates females, thus indicating important and conserved functions in female reproductive success. ERs are also present in physiological different tissues as bone, brain, liver, skin and adipose tissues, in both females and males. In the latter, ERs have been found also in the genital tract, supporting the findings of a complex role for estrogen in spermatogenesis and, more generally, in male reproduction. This review provides an overview and update on ERα and ERβ expression and synthesis in male reproductive tract of non-mammalian vertebrates, with focus on their role in germ cells proliferation, maturation and survival. Data from studies on fish, amphibians, reptiles and birds were collated and common or species-specific distribution highlighted. The widespread distribution of estrogen receptors in testicular cells and ducts of all vertebrates so far investigated suggests that whatever are the roles that estrogens may exert on these structures, they are phylogenetically conserved and are possibly related to the physiological support given to achieve male reproductive success.

Steroid Transport, Local Synthesis, and Signaling within the Brain: Roles in Neurogenesis, Neuroprotection, and Sexual Behaviors

Sex steroid hormones are synthesized from cholesterol and exert pleiotropic effects notably in the central nervous system. Pioneering studies from Baulieu and colleagues have suggested that steroids are also locally-synthesized in the brain. Such steroids, called neurosteroids, can rapidly modulate neuronal excitability and functions, brain plasticity, and behavior. Accumulating data obtained on a wide variety of species demonstrate that neurosteroidogenesis is an evolutionary conserved feature across fish, birds, and mammals. In this review, we will first document neurosteroidogenesis and steroid signaling for estrogens, progestagens, and androgens in the brain of teleost fish, birds, and mammals. We will next consider the effects of sex steroids in homeostatic and regenerative neurogenesis, in neuroprotection, and in sexual behaviors. In a last part, we will discuss the transport of steroids and lipoproteins from the periphery within the brain (and vice-versa) and document their effects on the blood-brain barrier (BBB) permeability and on neuroprotection. We will emphasize the potential interaction between lipoproteins and sex steroids, addressing the beneficial effects of steroids and lipoproteins, particularly HDL-cholesterol, against the breakdown of the BBB reported to occur during brain ischemic stroke. We will consequently highlight the potential anti-inflammatory, anti-oxidant, and neuroprotective properties of sex steroid and lipoproteins, these latest improving cholesterol and steroid ester transport within the brain after insults.

Knotting nets: Molecular junctions of interconnecting endocrine axes identified by application of the adverse outcome pathway concept

To be defined as an endocrine disruptor, a substance has to meet several criteria, including the induction of specific adverse effects, a specific endocrine mode of action, and a plausible link between both. The latter criterion in particular might not always be unequivocally determined, especially because the endocrine system consists of diverse endocrine axes. The axes closely interact with each other, and manipulation of one triggers effects on the other. The present review aimed to identify some of the many interconnections between these axes. The focus was on fish, but data obtained in studies on amphibians and mammals were considered if they assisted in closing data gaps, because most of the endocrine mechanisms are evolutionarily conserved. The review includes data both from ecotoxicological studies and on physiological processes and gives information on hormone/hormone receptor interactions or gene transcription regulation. The key events and key event relationships identified provide explanations for unexpected effects on one axis, exerted by substances suspected to act specifically on another axis. Based on these data, several adverse outcome pathway (AOP) segments are identified, describing connections between the hypothalamic-pituitary-gonadal (HPG) and hypothalamic-pituitary-thyroid (HPT) axes, the HPG and hypothalamic-pituitary-adrenal/interrenal (HPA/I) axes, and the HPT and HPA/I axes. Central key events identified across axes were altered aromatase activity as well as altered expression and function of the proteins 11β-hydroxysteroid dehydrogenase (11β-HSD) and steroidogenic acute regulatory (StAR) protein. Substance classes that act on more than one endocrine axis were, for example, goitrogens or aromatase inhibitors. Despite the wealth of information gathered, the present review only provides a few insights into the molecular nets of endocrine axes, demonstrating the complexity of their interconnections. Environ Toxicol Chem 2018;37:318-328. © 2017 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Estrogen-regulated expression of P450arom genes in the brain and ovaries of adult female Indian climbing perch, Anabas testudineus

Cytochrome P450arom (CYP19), a product of cyp19a1 gene, catalyzes the conversion of androgens to estrogens and is essential for regulation of reproductive function in vertebrates. In the present study, we isolated partial cDNA encoding the ovarian (cyp19a1a) and brain (cyp19a1b) P450arom genes from adult female perch, Anabas testudineus and investigated their regulation by estrogen in vivo. Results demonstrated that cyp19a1a and cyp19a1b predominate in ovary and brain respectively, with quantity of both attuned to reproductive cycle. To elucidate estrogen-regulated expression of cyp19a1b in brain and cyp19a1a in ovary, dose- and time-dependent studies were conducted with estrogen in vitellogenic-stage fish in the presence or absence of specific aromatase inhibitor fadrozole. Results demonstrated that treatment of fish with 17β-estradiol (E2; 1.0 μM)) for 6 days caused significant upregulation of cyp19a1b transcripts, aromatase B protein, and aromatase activity in brain in a dose- and time-dependent manner. Ovarian cyp19a1a mRNA, aromatase protein, and aromatase activity, however, was less responsive to E2 than brain. Treatment of fish with an aromatase inhibitor fadrozole for 6 days attenuated both brain and ovarian cyp19a1 mRNAs expression and stimulatory effects of E2 was also significantly reduced. These results indicate that expression of cyp19a1b in brain and cyp19a1a in ovary of adult female A. testudineus was closely associated to plasma E2 levels and seasonal reproductive cycle. Results further show apparent differential regulation of cyp19a1a and cyp19a1b expression by E2/fadrozole manipulation.

Secretoneurin A Directly Regulates the Proteome of Goldfish Radial Glial Cells In Vitro

Radial glial cells (RGCs) are the main macroglia in the teleost brain and have established roles in neurogenesis and neurosteroidogenesis. They are the only brain cell type expressing aromatase B (cyp19a1b), the enzyme that synthesizes estrogens from androgen precursors. There are few studies on the regulation of RGC functions, but our previous investigations demonstrated that dopamine stimulates cyp19a1b expression in goldfish RGCs, while secretoneurin A (SNa) inhibits the expression of this enzyme. Here, we determine the range of proteins and cellular processes responsive to SNa treatments in these steroidogenic cells. The focus here is on SNa, because this peptide is derived from selective processing of secretogranin II in magnocellular cells embedded within the RGC-rich preoptic nucleus. Primary cultures of RGCs were treated (24 h) with 10, 100, or 1,000 nM SNa. By using isobaric tagging for relative and absolute quantitation and a Hybrid Quadrupole Obritrap Mass Spectrometry system, a total of 1,363 unique proteins were identified in RGCs, and 609 proteins were significantly regulated by SNa at one or more concentrations. Proteins that showed differential expression with all three concentrations of SNa included H1 histone, glutamyl-prolyl-tRNA synthetase, Rho GDP dissociation inhibitor γ, vimentin A2, and small nuclear ribonucleoprotein-associated protein. At 10, 100, and 1,000 nM SNa, there were 5, 195, and 489 proteins that were downregulated, respectively, whereas the number of upregulated proteins were 72, 44, and 51, respectively. Subnetwork enrichment analysis of differentially regulated proteins revealed that processes such as actin organization, cytoskeleton organization and biogenesis, apoptosis, mRNA processing, RNA splicing, translation, cell growth, and proliferation are regulated by SNa based on the proteomic response. Moreover, we observed that, at the low concentration of SNa, there was an increase in the abundance of proteins involved in cell growth, proliferation, and migration, whereas higher concentration of SNa appeared to downregulate proteins involved in these processes, indicating a dose-dependent proteome response. At the highest concentration of SNa, proteins linked to the etiology of diseases of the central nervous system (brain injuries, Alzheimer disease, Parkinson's disease, cerebral infraction, brain ischemia) were also differentially regulated. These data implicate SNa in the control of cell proliferation and neurogenesis.

Pattern of aromatase mRNA expression in the brain of a weakly electric fish, Apteronotus leptorhynchus

Aromatase is a steroidogenic enzyme involved in the conversion of testosterone into estradiol. Teleosts are unique among vertebrates in possessing two distinct aromatase genes that show different expression patterns within the body. Since the brain is the essential organ underlying the control of behavior, an understanding of the expression pattern of aromatase in the brain can help to identify neural circuits and behaviors that are most likely to be affected by aromatase activity. In addition, identifying species differences in aromatase expression in the brain can further our understanding of divergence in behaviors regulated by local estradiol production and estrogen signaling. Apteronotus leptorhynchus is a species of weakly electric fish in which little is known about sex steroid expression within the brain and its role in electric signaling behavior. The goal of this study was to identify the mRNA expression pattern of aromatase in the brain of A. leptorhynchus. Aromatase mRNA was detected in several parts of the forebrain and in the pituitary gland; however, no aromatase expression was detected in the midbrain or hindbrain. These findings in A. leptorhynchus support a role for aromatase activity in reproduction, but no direct role in electric signaling behavior in non-breeding fish. The findings of this study help to broaden the basis for making phylogenetic comparisons of aromatase expression across teleost lineages as well as different signaling systems, and provide information on behaviors and neural circuits that are potentially affected by local estradiol production in A. leptorhynchus.

Paternal exposure to environmental 17-alpha-ethinylestradiol concentrations modifies testicular transcription, affecting the sperm transcript content and the offspring performance in zebrafish

The synthetic estrogen 17-α-ethinylestradiol (EE2), a major constituent in contraceptive pills, is an endocrine disrupting chemical (EDC) present in the aquatic environment at concentrations of ng/L. Developmental exposure to these low concentrations in fish can induce several disorders. Zebrafish (Danio rerio) is a perfect organism for monitoring the effects of environmental contaminants. Our hypothesis is that changes promoted by EE2 in the germ line of male adults could be transmitted to the unexposed progeny. We exposed male zebrafish to 2.5, 5 and 10ng/L of EE2 during spermatogenesis and mated them with untreated females. Detailed progeny development was studied concentrating to survival, hatching and malformations. Due to the high incidence of lymphedemas within larvae, we performed qPCR analysis of genes involved in lymphatic development (vegfc and vegfr3) and endothelial cell migration guidance (cxcr4a and cxcl12b). Estrogen receptor (ER) transcript presence was also evaluated in sperm, testis and embryos. Progenies showed a range of disorders although at a low incidence: skeletal distortions, uninflated swimbladder, lymphedema formation, cartilage deformities and otolith tethering. Swimming evaluation revealed less active locomotion. All these processes are related to pathways involving ERs (esr1, esr2a and esr2b). mRNA analysis revealed that environmental EE2 causes the up-regulation of esr1 an esr2b in testis and the increase of esr2b transcripts in sperm pointing to a link between lymphedema in embryos and ER expression impairment. We demonstrate that the effects induced by environmental toxicants can be paternally inherited and point to the changes on the sperm transcriptome as the responsible mechanism.

Secretoneurin A regulates neurogenic and inflammatory transcriptional networks in goldfish (Carassius auratus) radial glia

Radial glial cells (RGCs) are the most abundant macroglia in the teleost brain and have established roles in neurogenesis and neurosteroidogenesis; however, their transcriptome remains uncharacterized, which limits functional understanding of this important cell type. Using cultured goldfish RGCs, RNA sequencing and de novo transcriptome assembly were performed, generating the first reference transcriptome for fish RGCs with 17,620 unique genes identified. These data revealed that RGCs express a diverse repertoire of receptors and signaling molecules, suggesting that RGCs may respond to and synthesize an array of hormones, peptides, cytokines, and growth factors. Building upon neuroanatomical data and studies investigating direct neuronal regulation of RGC physiology, differential gene expression analysis was conducted to identify transcriptional networks that are responsive to the conserved secretogranin II-derived neuropeptide secretoneurin A (SNa). Pathway analysis of the transcriptome indicated that cellular processes related to the central nervous system (e.g., neurogenesis, synaptic plasticity, glial cell development) and immune functions (e.g., immune system activation, leukocyte function, macrophage response) were preferentially modulated by SNa. These data reveal an array of new functions that are proposed to be critical to neuronal-glial interactions through the mediator SNa.

Stress differentially affects the systemic and leukocyte estrogen network in common carp

Both systemic and locally released steroid hormones, such as cortisol and estrogens, show immunomodulatory actions. This research gives evidence that circulating and leukocyte-derived estrogens can be involved in the regulation of the immune response in common carp, during homeostasis and upon restraining stress. It was found that stress reduced level of blood 17β-estradiol (E2) and down-regulated the gene expression of components of the "classical" estrogen system: the nuclear estrogen receptors and the aromatase CYP19, in the hypothalamus, the pituitary and in the ovaries. In contrast, higher gene expression of the nuclear estrogen receptors and cyp19a was found in the head kidney of stressed animals. Moreover, stress induced changes in the E2 level and in the estrogen sensitivity at local/leukocyte level. For the first time in fish, we showed the presence of physiologically relevant amounts of E2 and the substrates for its conversion (estrone - E1 and testosterone - T) in head kidney monocytes/macrophages and found that its production is modulated upon stress. Moreover, stress reduced the sensitivity of leukocytes towards estrogens, by down-regulation the expression of the erb and cyp19 genes in carp phagocytes. In contrast, era expression was up-regulated in the head kidney monocytes/macrophages and in PBLs derived from stressed animals. We hypothesize that, the increased expression of ERα, that was observed during stress, can be important for the regulation of leukocyte differentiation, maturation and migration. In conclusion, these results indicate that, in fish, the estrogen network can be actively involved in the regulation of the systemic and local stress response and the immune response.

Immunocytochemical characterisation of neural stem-progenitor cells from green terror cichlid Aequidens rivulatus

In this study, cultures of neural stem-progenitor cells (NSPC) from the brain of green terror cichlid Aequidens rivulatus were established and various NSPCs were demonstrated using immunocytochemistry. All of the NSPCs expressed brain lipid-binding protein, dopamine- and cAMP-regulated neuronal phosphoprotein 32 (DARPP-32), oligodendrocyte transcription factor 2, paired box 6 and sex determining region Y-box 2. The intensity and localisation of these proteins, however, varied among the different NSPCs. Despite being intermediate cells, NSPCs can be divided into radial glial cells, oligodendrocyte progenitor cells (OPC) and neuroblasts by expressing the astrocyte marker glial fibrillary acidic protein (GFAP), OPC marker A2B5 and neuronal markers, including acetyl-tubulin, βIII-tubulin, microtubule-associated protein 2 and neurofilament protein. Nevertheless, astrocytes were polymorphic and were the most dominant cells in the NSPC cultures. By using Matrigel, radial glia exhibiting a long GFAP⁺ or DARPP-32⁺ fibre and neurons exhibiting a significant acetyl-tubulin⁺ process were obtained. The results confirmed that NSPCs obtained from A. rivulatus brains can proliferate and differentiate into neurons in vitro. Clonal culture can be useful for further studying the distinct NSPCs.

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.

Comparative aspects of neurosteroidogenesis: From fish to mammals

It is now clearly established that the central and peripheral nervous systems have the ability to synthesize de novo steroids referred to as neurosteroids. The major evidence for biosynthesis of neuroactive steroids by nervous tissues is based on the expression of enzymes implicated in the formation of steroids in neural cells. The aim of the present review is to summarize the current knowledge regarding the presence of steroidogenic enzymes in the brain of vertebrates and to highlight the very considerable contribution of Professor Kazuyoshi Tsutsui in this domain. The data indicate that expression of steroid-producing enzymes in the brain appeared early during vertebrate evolution and has been preserved from fish to mammals.

Direct Regulation of Aromatase B Expression by 17β-Estradiol and Dopamine D1 Receptor Agonist in Adult Radial Glial Cells

Aromatase cytochrome P450arom (cyp19) is the only enzyme that has the ability to convert androgens into estrogens. Estrogens, which are produced locally in the vertebrate brain play many fundamental roles in neuroendocrine functions, reproductive functions, socio-sexual behaviors, and neurogenesis. Radial glial cells (RGCs) are neuronal progenitor cells that are abundant in fish brains and are the exclusive site of aromatase B expression and neuroestrogen synthesis. Using a novel in vitro RGC culture preparation we studied the regulation of aromatase B by 17β-estradiol (E2) and dopamine (DA). We have established that activation of the dopamine D1 receptor (D1R) by SKF 38393 up-regulates aromatase B gene expression most likely through the phosphorylation of cyclic AMP response element binding protein (CREB). This up-regulation can be enhanced by low concentration of E2 (100 nM) through increasing the expression of D1R and the level of p-CREB protein. However, a high concentration of E2 (1 μM) and D1R agonist together failed to up-regulate aromatase B, potentially due to attenuation of esr2b expression and p-CREB levels. Furthermore, we found the up-regulation of aromatase B by E2 and DA both requires the involvement of esr1 and esr2a. The combined effect of E2 and DA agonist indicates that aromatase B in the adult teleost brain is under tight control by both steroids and neurotransmitters to precisely regulate neuroestrogen levels.

Differential expression of three estrogen receptors mRNAs in tissues, growth development, embryogenesis and gametogenesis from large yellow croaker, Larimichthys crocea

The biological activity of estrogens in target organs is mainly mediated by estrogen receptors (ERs). Herein, we addressed the isolation and expression analysis of three nuclear estrogen receptors, namely LcERα, LcERβ1, and LcERβ2 from Larimichthys crocea by means of SMART-RACE, qRT-PCR, and in situ hybridization. Results in different tissues showed that both LcERα and LcERβ2 had the highest expression levels in female liver, followed by testis, but LcERβ1 expression level was significantly higher in testis and ovary than in other tissues. Expression of LcERα and LcERβ2 was significantly higher than LcERβ1 in female liver, and LcERβ2 was significantly higher than LcERα and LcERβ1 in male liver. Moreover, we analyzed the expression of LcERs in gonad and liver at three different growth stages during the same breeding season. Significant up-regulated expression of LcERα and LcERβ2 were found in female liver at 1000dph compared with at 270dph. The expression of LcERβ2 was prominently higher in male liver than LcERα, LcERβ1 and LcAR, while LcERβ1 was lower than other receptors in male and female liver at all the three stages. In ovary, LcERα at 270dph was lower than at 635dph and 1000dph, but had no significant change in testis. The two LcERβ subtypes and LcAR highly expressed in the early testis, and gradually decrease with the development of testis. In embryogenesis, a significant increase in the expression of LcERα and LcERβ2 were observed after appearance of optic vesicles phase (11.8hpf). LcERβ1 gradually decrease with the embryogenesis but increased dramatically at 1dph. Results of in situ hybridization showed that signals of LcERα and LcERβ1 mRNA were mainly detected in Stage I-Stage IV oocytes, as well as in follicle cells around the Stage II-Stage IV and degenerated oocytes. Signals of LcERβ2 were detected in the cytoplasm of Stage I and Stage II oocytes but not in the follicle cells of all oocytes stages. In parallel, LcERα and LcERβ1 were detected in all cell types of spermatogenesis, but in terms of LcERβ2, little or no signals were detected during spermatogenesis. Based on these results, we deduced that both LcERα and LcERβ2 play a major role in mediating the physiological effects of estrogen in female liver, and LcERβ2 maybe also play an important role in regulation of vitellogenesis in male liver. Differential expression of LcERs and LcAR imply their physiological functions during development and differentiation of gonad. The signals for LcERα and LcERβ1 in follicle cells suggested that the follicle cell maybe an important site of estrogen action, by which estrogens exert influences on the maturation oocytes and ovulation. Furthermore, the steroid hormones produced by follicle cells may be related to the differential distributions among ER subtypes. Besides, we deduced that LcERα and LcERβ1 rather than LcERβ2 may play a major role in spermatogenesis of croaker. However, the differential expression of LcERβ2 during gametogenesis also implicates its certain functions in mediating physiological process of estrogen action.

Influence of short-term exposure to low levels of 17α-ethynylestradiol on expression of genes involved in immunity and on immune parameters in rainbow trout, Oncorhynchus mykiss

Fish are exposed to endocrine-disrupting chemicals (EDC), which are well known to disturb not only the reproductive system but also the immune system in vertebrates. However, the mechanisms by which these compounds are able to modify fish immunity are not well understood. In order to test the EE2 effects on immunity in selected organs, we exposed rainbow trout male juveniles for 3 weeks to EE2 concentrations ranging from 0.01 to 1 μg/L. The results of this study suggest that EE2 affects the immunity of rainbow trout in a tissue dependent manner. This molecule affects both cellular and humoral immune systems. Indeed, blood leukocyte populations, as well as hepatic and plasma lysozyme, plasma MPO and renal complement activities, are modulated by EE2. Moreover, EE2 alters the gene expression of some mucus compounds, hepatic expression of complement sub-unit and lysozyme, or genes involved in the hepatic phagocytosis and transport of immunoglobulin across the liver.

Effects of benzophenone-3 exposure on endocrine disruption and reproduction of Japanese medaka (Oryzias latipes)--a two generation exposure study

Benzophenone-3 (BP-3) has been widely used in sunscreens and cosmetics to protect human skin from the harmful effects of UV irradiation. While BP-3 has been frequently detected in surface waters, sediments and biota, only limited information is available on its in vivo toxicity, particularly in fish. In the present study the endocrine disrupting capacity of BP-3 and its underlying mechanisms were investigated using Japanese medaka (Oryzias latipes). Adult Japanese medaka pairs (F0) were exposed to 0, 4.7, 8.4, 26, or 90 μg/L (or 0, 15, 50, 150, or 500 μg/L of BP-3 based on nominal concentration) for 14 d and its effects on sex steroid hormones, and transcription of various associated genes were determined. Following additional 14 d of exposure, the F1 eggs reproduced were counted and were further exposed to 0, 5.4, 12, or 30 μg/L of BP-3 (or 0, 15, 50, or 150 μg/L based on nominal concentrations) until 30 d after hatching. Chemical analysis of the exposed media confirmed transformation of BP-3 to benzophenone-1 (BP-1), a more potent estrogen agonist. After 14 d of the adult fish exposure, plasma concentrations of testosterone (T) significantly increased in male fish. The 17β-estradiol (E2) to T (E2/T) ratio showed significant decreases in both male and female fish. Overall down-regulation of gonadal steroidogenic genes such as star, cyp11a, cyp17, hsd3b, hsd17b3, and cyp19a was also observed. After 28 d of exposure, the daily average egg reproduction per female was significantly reduced at 26 μg/L of BP-3. However, hatchability of F1 eggs was not affected by continuous exposure. After continued exposure until 30 dph, juvenile fish showed concentration-dependent decrease of condition factor, but mortality was not affected. Our observation clearly indicates that endocrine balance and reproduction performance in fish could be affected by μg/L level exposure to BP-3. Consequences of longer term exposure over multi-generations warrant further investigation.

Intersex occurrence in rainbow trout (Oncorhynchus mykiss) male fry chronically exposed to ethynylestradiol

This study aimed to investigate the male-to-female morphological and physiological transdifferentiation process in rainbow trout (Oncorhynchus mykiss) exposed to exogenous estrogens. The first objective was to elucidate whether trout develop intersex gonads under exposure to low levels of estrogen. To this end, the gonads of an all-male population of fry exposed chronically (from 60 to 136 days post fertilization – dpf) to several doses (from environmentally relevant 0.01 µg/L to supra-environmental levels: 0.1, 1 and 10 µg/L) of the potent synthetic estrogen ethynylestradiol (EE2) were examined histologically. The morphological evaluations were underpinned by the analysis of gonad steroid (testosterone, estradiol and 11-ketotestosterone) levels and of brain and gonad gene expression, including estrogen-responsive genes and genes involved in sex differentiation in (gonads: cyp19a1a, ER isoforms, vtg, dmrt1, sox9a2; sdY; cyp11b; brain: cyp19a1b, ER isoforms). Intersex gonads were observed from the first concentration used (0.01 µg EE2/L) and sexual inversion could be detected from 0.1 µg EE2/L. This was accompanied by a linear decrease in 11-KT levels, whereas no effect on E2 and T levels was observed. Q-PCR results from the gonads showed downregulation of testicular markers (dmrt1, sox9a2; sdY; cyp11b) with increasing EE2 exposure concentrations, and upregulation of the female vtg gene. No evidence was found for a direct involvement of aromatase in the sex conversion process. The results from this study provide evidence that gonads of male trout respond to estrogen exposure by intersex formation and, with increasing concentration, by morphological and physiological conversion to phenotypic ovaries. However, supra-environmental estrogen concentrations are needed to induce these changes.

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.

Estrogen receptor signaling during vertebrate development

Estrogen receptors are expressed and their cognate ligands produced in all vertebrates, indicative of important and conserved functions. Through evolution estrogen has been involved in controlling reproduction, affecting both the development of reproductive organs and reproductive behavior. This review broadly describes the synthesis of estrogens and the expression patterns of aromatase and the estrogen receptors, in relation to estrogen functions in the developing fetus and child. We focus on the role of estrogens for the development of reproductive tissues, as well as non-reproductive effects on the developing brain. We collate data from human, rodent, bird and fish studies and highlight common and species-specific effects of estrogen signaling on fetal development. Morphological malformations originating from perturbed estrogen signaling in estrogen receptor and aromatase knockout mice are discussed, as well as the clinical manifestations of rare estrogen receptor alpha and aromatase gene mutations in humans. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Di-n-butyl phthalate causes estrogenic effects in adult male Murray rainbowfish (Melanotaenia fluviatilis)

Phthalic acid esters (PAEs) are a class of synthetic industrial chemicals commonly found in the aquatic environment worldwide. PAEs have been recognised as anti-androgens in male mammals but little is known about their endocrine disrupting effects in fish. This study investigated the effects of 7-day exposures to nominal (measured) concentrations of 125 (62), 250 (140), 500 (230) and 1,000 (383) μg/L of di-n-butyl phthalate (DnBP) on the biomarkers of reproduction in adult male Murray River rainbowfish (Melanotaenia fluviatilis) using molecular, biochemical and histological endpoints. None of the tested concentrations of DnBP had any effect on survival or the vital body indices of the fish. The sizes of spermatogonia, Type A and B spermatocytes and spermatids were significantly smaller relative to the controls after treatment with DnBP. This was accompanied by a significant increase in the proportion of spermatogonia in fish treated with 250-1,000 μg/L of DnBP in comparison to the unexposed fish. At the end of the exposure period, the expressions of the transcripts for the androgen receptors α and β were significantly elevated in the livers of the fish treated with 500 and 1,000 μg/L of DnBP. In addition, there was also an increase in the circulating concentrations of vitellogenin in the plasma in the higher treatment groups. An induction in the activity of aromatase was noted in the brains of 1,000 μg/L DnBP-treated fish. This was accompanied by an increase in the hepatic expression of the genes (here and later, whenever the phrase gene expression is used as a synonym for gene transcription although it is acknowledged that it is also regulated, e.g., by translation, mRNA stability and protein stability) encoding for the oestrogen receptors α and β and choriogenin L. Collectively, an increase in the proportion of spermatogonia in the testes, the upregulation of the genes for the oestrogen receptors and choriogenin in the liver, an induction in the brain aromatase activity and the increase in the circulating levels of plasma vitellogenin suggest that continuous exposures for 7 days to sub-acute concentrations of DnBP can adversely affect the reproductive health of the male Murray rainbowfish by an estrogenic mode of action.

Characterization of aromatase expression in the adult male and female mouse brain. I. Coexistence with oestrogen receptors α and β, and androgen receptors

Aromatase catalyses the last step of oestrogen synthesis. There is growing evidence that local oestrogens influence many brain regions to modulate brain development and behaviour. We examined, by immunohistochemistry, the expression of aromatase in the adult male and female mouse brain, using mice in which enhanced green fluorescent protein (EGFP) is transcribed following the physiological activation of the Cyp19A1 gene. EGFP-immunoreactive processes were distributed in many brain regions, including the bed nucleus of the stria terminalis, olfactory tubercle, medial amygdaloid nucleus and medial preoptic area, with the densest distributions of EGFP-positive cell bodies in the bed nucleus and medial amygdala. Differences between male and female mice were apparent, with the density of EGFP-positive cell bodies and fibres being lower in some brain regions of female mice, including the bed nucleus and medial amygdala. EGFP-positive cell bodies in the bed nucleus, lateral septum, medial amygdala and hypothalamus co-expressed oestrogen receptor (ER) α and β, or the androgen receptor (AR), although single-labelled EGFP-positive cells were also identified. Additionally, single-labelled ERα-, ERβ- or AR-positive cell bodies often appeared to be surrounded by EGFP-immunoreactive nerve fibres/terminals. The widespread distribution of EGFP-positive cell bodies and fibres suggests that aromatase signalling is common in the mouse brain, and that locally synthesised brain oestrogens could mediate biological effects by activating pre- and post-synaptic oestrogen α and β receptors, and androgen receptors. The higher number of EGFP-positive cells in male mice may indicate that the autocrine and paracrine effects of oestrogens are more prominent in males than females.

Identification of estrogen target genes during zebrafish embryonic development through transcriptomic analysis

Estrogen signaling is important for vertebrate embryonic development. Here we have used zebrafish (Danio rerio) as a vertebrate model to analyze estrogen signaling during development. Zebrafish embryos were exposed to 1 µM 17β-estradiol (E2) or vehicle from 3 hours to 4 days post fertilization (dpf), harvested at 1, 2, 3 and 4 dpf, and subjected to RNA extraction for transcriptome analysis using microarrays. Differentially expressed genes by E2-treatment were analyzed with hierarchical clustering followed by biological process and tissue enrichment analysis. Markedly distinct sets of genes were up and down-regulated by E2 at the four different time points. Among these genes, only the well-known estrogenic marker vtg1 was co-regulated at all time points. Despite this, the biological functional categories targeted by E2 were relatively similar throughout zebrafish development. According to knowledge-based tissue enrichment, estrogen responsive genes were clustered mainly in the liver, pancreas and brain. This was in line with the developmental dynamics of estrogen-target tissues that were visualized using transgenic zebrafish containing estrogen responsive elements driving the expression of GFP (Tg(5xERE:GFP)). Finally, the identified embryonic estrogen-responsive genes were compared to already published estrogen-responsive genes identified in male adult zebrafish (Gene Expression Omnibus database). The expressions of a few genes were co-regulated by E2 in both embryonic and adult zebrafish. These could potentially be used as estrogenic biomarkers for exposure to estrogens or estrogenic endocrine disruptors in zebrafish. In conclusion, our data suggests that estrogen effects on early embryonic zebrafish development are stage- and tissue- specific.

Morpholino-mediated knockdown of ERα, ERβa, and ERβb mRNAs in zebrafish (Danio rerio) embryos reveals differential regulation of estrogen-inducible genes

Genetically distinct estrogen receptor (ER) subtypes (ERα and ERβ) play a major role in mediating estrogen actions in vertebrates, but their unique and overlapping functions are not entirely clear. Although mammals have 1 gene of each subtype (ESR1 and ESR2), teleost fish have a single esr1 (ERα) and 2 esr2 (ERβa and ERβb) genes. To determine the in vivo role of different ER isoforms in regulating estrogen-inducible transcription targets, zebrafish (Danio rerio) embryos were microinjected with esr-specific morpholino (MO) oligonucleotides to disrupt splicing of the exon III/intron III junction in the DNA-binding domain. Each MO knocked down its respective normal transcript and increased production of variants with a retained intron III (esr1 MO) or a deleted or mis-spliced exon III (esr2a and esr2b MOs). Both esr1 and esr2b MOs blocked estradiol induction of vitellogenin and ERα mRNAs, predominant hepatic genes, but esr2b was the only MO that blocked induction of cytochrome P450 aromatase B mRNA, a predominant brain gene. Knockdown of ERβa with the esr2a MO had no effect on estrogen induction of the 3 mRNAs but, when coinjected with esr1 MO, attenuated the effect of ERα knockdown. Results indicate that ERα and ERβb, acting separately or cooperatively on specific gene targets, are positive transcriptional regulators of estrogen action, but the role of ERβa, if any, is unclear. We conclude that MO technology in zebrafish embryos is an advantageous approach for investigating the interplay of ER subtypes in a true physiological context.

Plasma sex steroid hormonal profile and gonad histology during the annual reproductive cycle of river catfish Hemibagrus nemurus (Valenciennes, 1840) in captivity

Plasma sex steroid hormonal profile and gonad histology were correlated to study the annual reproductive cycle of Hemibagrus nemurus. Hormones were measured by Enzyme Linked Immunosorbent Assay. Gonad tissues were observed by using light microscopy. The highest testosterone (T) value for male was observed in November and that of female was in October. 11-ketotestosterone (11-KT) and 17β-estradiol (E2) levels were highest in June and November, respectively. Hormonal profiles of T, 11-KT and E2 showed several peaks which indicated a non-seasonal pattern. There were significant differences (p < 0.05) in the monthly levels of T, 11-KT and E2. Gonadosomatic index of H. nemurus ranged from 1.14 ± 0.02 % to 7.06 ± 1.40 %, and high gonadosomatic indices were recorded in May, August and November. Gonad histology revealed that spermatozoa were always present in the testes which implied continuous spermatogenesis and asynchronous ovarian development pattern was observed in the ovaries. The annual reproductive cycle of H. nemurus did not show a seasonal pattern and this indicate that H. nemurus is a non-seasonal breeder with several spawning cycles and can be referred to as indeterminate batch spawner. The major significances of this study are annual sex steroid hormonal profile and asynchronous ovarian development of H. nemurus. This information will contribute to our knowledge of reproductive biology of H. nemurus.

Ketamine attenuates cytochrome p450 aromatase gene expression and estradiol-17β levels in zebrafish early life stages

Ketamine, a dissociative anesthetic, is a noncompetitive antagonist of N-methyl-D-aspartate-type glutamate receptors. In rodents and non-human primates as well as in zebrafish embryos, ketamine has been shown to be neurotoxic. In cyclic female rats, ketamine has been shown to decrease serum estradiol-17β (E2) levels. E2 plays critical roles in neurodevelopment and neuroprotection. Cytochrome p450 (CYP) aromatase catalyzes E2 synthesis from androgens. Although ketamine down-regulates a number of CYP enzymes in rodents, its effect on the CYP aromatase (CYP19) is not known. Zebrafish have been used as a model system for examining mechanisms underlying drug effects. Here, using wild-type (WT) zebrafish (Danio rerio) embryos, we demonstrate that ketamine significantly reduced E2 levels compared with the control. However, the testosterone level was elevated in ketamine-treated embryos. These results are concordant with data from mammalian studies. Ketamine also attenuated the expression of the ovary form of CYP aromatase (cyp19a1a) at the transcriptional level but not the brain form of aromatase, cyp19a1b. Exogenous E2 potently induced the expression of cyp19a1b and vtg 1, both validated biomarkers of estrogenicity and endocrine disruption, but not cyp19a1a expression. Attenuation of activated ERK/MAPK levels, reportedly responsible for reduced human cyp19 transcription, was also observed in ketamine-treated embryos. These results suggest that reduced E2 levels in ketamine-treated embryos may have resulted from the suppression of cyp19a1a transcription.

Interaction of 17β-estradiol and ketoconazole on endocrine function in goldfish (Carassius auratus)

An understanding of the effects of toxic mixtures of endocrine disrupting chemicals (EDCs) on aquatic organisms is challenging as these organisms are exposed to multiple classes of contaminants in their natural habitat. The aim of the present study was to evaluate the interactions of two classes of EDCs, 17β-estradiol (E2) and ketoconazole (KTC), on endocrine function in male goldfish (Carassius auratus), including vitellogenesis, metabolic capability and serum steroid synthesis. Changes in vitellogenin (VTG) concentration, liver 7-ethoxyresorufin-O-deethylase (EROD) activity and circulating serum E2 level were examined. The expression of related genes was also determined using quantitative real-time polymerase chain reaction. Exposure to E2 caused a significant increase in VTG concentrations which corresponded with the gene expression of VTG and estrogen receptor (ER) in males, which were further elevated after combined exposure to E2 and KTC, indicative of a synergetic relationship. Exposure to E2 also resulted in a distinct increase in serum steroid biosynthesis and associated cytochrome P450 (CYP) aromatase expression after 10 days. However, these changes were inhibited by the presence of KTC, which acted as a steroidogenic inhibitor in fish. Moreover, KTC significantly decreased liver EROD activity and increased the related gene expression of CYP1A. However, these KTC-mediated metabolic reactions in goldfish were up-regulated following exposure to KTC in combination with E2. These findings reveal complex interactions on endocrine functions in male goldfish when exposed to multiple contaminations and may provide a better understanding of the effects of toxic mixtures.

Effects of tris(1,3-dichloro-2-propyl) phosphate and triphenyl phosphate on receptor-associated mRNA expression in zebrafish embryos/larvae

Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and triphenyl phosphate (TPP) are frequently detected in biota, including fish. However, knowledge of the toxicological and molecular effects of these currently used flame retardants is limited. In the present study, an in vivo screening approach was developed to evaluate effects of TDCPP and TPP on developmental endpoints and receptor-associated expression of mRNA in zebrafish embryos/larvae. Exposure to TDCPP or TPP resulted in significantly smaller rates of hatching and survival, in dose- and time-dependent manners. The median lethal concentration (LC(50)) was 7.0 mg/L for TDCPP and 29.6 mg/L for TPP at 120 hour post-fertilization (hpf). Real-time PCR revealed alterations in expression of mRNAs involved in aryl hydrocarbon receptors (AhRs)-, peroxisome proliferator-activated receptor alpha (PPARα)-, estrogenic receptors (ERs)-, thyroid hormone receptor alpha (TRα)-, glucocorticoid receptor (GR)-, and mineralocorticoid receptor (MR)-centered gene networks. Exposure to positive control chemicals significantly altered abundances of mRNA in corresponding receptor-centered gene networks, a result that suggests that it is feasible to use zebrafish embryos/larvae to evaluate effects of chemicals on mRNA expression in these gene networks. Exposure to TDCPP altered transcriptional profiles in all six receptor-centered gene networks, thus exerting multiple toxic effects. TPP was easily metabolized and its potency to change expression of mRNA involved in receptor-centered gene networks was weaker than that of TDCPP. The PPARα- and TRα-centered gene networks might be the primary pathways affected by TPP. Taken together, these results demonstrated that TDCPP and TPP could alter mRNA expression of genes involved in the six receptor-centered gene networks in zebrafish embryos/larvae, and TDCPP seemed to have higher potency in changing the mRNA expression of these genes.

Brain-specific promoter/exon I.f of the cyp19a1 (aromatase) gene in Xenopus laevis

Aromatase, encoded by the cyp19a1 gene, is the key enzyme for estrogen biosynthesis. Exon I.f of aromatase transcripts in the Xenopus brain is driven in a brain-specific manner. In this study, we cloned brain aromatase with a 5'-end of various lengths by 5'-RACE and detected the expression pattern of the aromatase mRNA. In Xenopus at the larval stage, the brain aromatase mRNA expression was five-fold higher than those in the gonad and liver, and was upregulated from stage 42 to stage 50. After isolating the brain-specific promoter I.f, which was located ∼6.5 kb upstream from gonad-specific exon PII, we observed this promoter in a potential cis-elements for several transcriptional factors, such as Oct-1, c-Myc, the GATA gene family, C/EBPalpha, Sox5, p300, XFD-1, AP1, the STAT gene family, FOXD3, and the Smad gene family. In addition, the core promoter elements of two initiators and an atypical TATA box were found around the 5'-RACE products. In the 5'-flanking region of exon I.f, the binding sites for nuclear extracts suggested that the followings are important: the STAT gene family, a 38-bp conserved region among five species, FOXD3, and the Smad gene family within the region 200 bp upstream from the transcription initiation site. Real-time RT-PCR analysis showed that the foxd3, smad2 and smad4.1/4.2 mRNAs are specifically expressed in the brain. Furthermore, the expression change of foxd3, which has been reported as a repressor, indicated that expression decreased to stage 50 from stage 42, contrary to that of aromatase mRNA. These results may imply that foxd3 expression decreases and aromatase expression increases as a result of the contribution to promoter I.f by transcriptional activators such as smads. However, since these putative cis-elements and transcription initiation sites are not conserved in the brain-specific promoter of other species, this transcriptional regulatory mechanism of exon I.f may be characteristic of Xenopus.

Rapid control of male typical behaviors by brain-derived estrogens

Beside their genomic mode of action, estrogens also activate a variety of cellular signaling pathways through non-genomic mechanisms. Until recently, little was known regarding the functional significance of such actions in males and the mechanisms that control local estrogen concentration with a spatial and time resolution compatible with these non-genomic actions had rarely been examined. Here, we review evidence that estrogens rapidly modulate a variety of behaviors in male vertebrates. Then, we present in vitro work supporting the existence of a control mechanism of local brain estrogen synthesis by aromatase along with in vivo evidence that rapid changes in aromatase activity also occur in a region-specific manner in response to changes in the social or environmental context. Finally, we suggest that the brain estrogen provision may also play a significant role in females. Together these data bolster the hypothesis that brain-derived estrogens should be considered as neuromodulators.