Characterization and expression of the nuclear progestin receptor in zebrafish gonads and brain

Nuclear progestin receptor (pgr) knockouts in zebrafish demonstrate role for pgr in ovulation but not in rapid non-genomic steroid mediated meiosis resumption

Progestins, progesterone derivatives, are the most critical signaling steroid for initiating final oocyte maturation (FOM) and ovulation, in order to advance fully-grown immature oocytes to become fertilizable eggs in basal vertebrates. It is well-established that progestin induces FOM at least partly through a membrane receptor and a non-genomic steroid signaling process, which precedes progestin triggered ovulation that is mediated through a nuclear progestin receptor (Pgr) and genomic signaling pathway. To determine whether Pgr plays a role in a non-genomic signaling mechanism during FOM, we knocked out Pgr in zebrafish using transcription activator-like effector nucleases (TALENs) and studied the oocyte maturation phenotypes of Pgr knockouts (Pgr-KOs). Three TALENs-induced mutant lines with different frame shift mutations were generated. Homozygous Pgr-KO female fish were all infertile while no fertility effects were evident in homozygous Pgr-KO males. Oocytes developed and underwent FOM normally in vivo in homozygous Pgr-KO female compared to the wild-type controls, but these mature oocytes were trapped within the follicular cells and failed to ovulate from the ovaries. These oocytes also underwent normal germinal vesicle breakdown (GVBD) and FOM in vitro, but failed to ovulate even after treatment with human chronic gonadotropin (HCG) or progestin (17α,20β-dihydroxyprogesterone or DHP), which typically induce FOM and ovulation in wild-type oocytes. The results indicate that anovulation and infertility in homozygous Pgr-KO female fish was, at least in part, due to a lack of functional Pgr-mediated genomic progestin signaling in the follicular cells adjacent to the oocytes. Our study of Pgr-KO supports previous results that demonstrate a role for Pgr in steroid-dependent genomic signaling pathways leading to ovulation, and the first convincing evidence that Pgr is not essential for initiating non-genomic progestin signaling and triggering of meiosis resumption.

Fsh and Lh direct conserved and specific pathways during flatfish semicystic spermatogenesis

The current view of the control of spermatogenesis by Fsh and Lh in non-mammalian vertebrates is largely based on studies carried out in teleosts with cystic and cyclic spermatogenesis. Much less is known concerning the specific actions of gonadotropins during semicystic germ cell development, a type of spermatogenesis in which germ cells are released into the tubular lumen where they transform into spermatozoa. In this study, using homologous gonadotropins and a candidate gene approach, for which the genes' testicular cell-type-specific expression was established, we investigated the regulatory effects of Fsh and Lh on gene expression during spermatogenesis in Senegalese sole (Solea senegalensis), a flatfish with asynchronous and semicystic germ cell development. During early spermatogenesis, Fsh and Lh upregulated steroidogenesis-related genes and nuclear steroid receptors, expressed in both somatic and germ cells, through steroid-dependent pathways, although Lh preferentially stimulated the expression of downstream genes involved in androgen and progestin syntheses. In addition, Lh specifically promoted the expression of spermatid-specific genes encoding spermatozoan flagellar proteins through direct interaction with the Lh receptor in these cells. Interestingly, at this spermatogenic stage, Fsh primarily regulated genes encoding Sertoli cell growth factors with potentially antagonistic effects on germ cell proliferation and differentiation through steroid mediation. During late spermatogenesis, fewer genes were regulated by Fsh or Lh, which was associated with a translational and posttranslational downregulation of the Fsh receptor in different testicular compartments. These results reveal that conserved and specialized gonadotropic pathways regulate semicystic spermatogenesis in flatfish, which may spatially adjust cell germ development to maintain a continuous reservoir of spermatids in the testis.

Blocking of progestin action disrupts spermatogenesis in Nile tilapia (Oreochromis niloticus)

In vitro studies have indicated that the maturation-inducing hormone 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DP, DHP), probably through nuclear progestin receptor (Pgr), might be involved in the proliferation of spermatogonial cells and the initiation of meiosis in several fish species. However, further in vivo evidence is required to elucidate the role of DHP in spermatogenesis during sexual differentiation in teleosts. In this study, we cloned pgr and analyzed its expression in Nile tilapia (Oreochromis niloticus) and treated XY fish with RU486 (a synthetic Pgr antagonist) from 5 days after hatching (dah) to determine the role of DHP in spermatogenesis. Sequence and phylogenetic analyses revealed that the Pgr identified in tilapia is a genuine Pgr. Pgr was found to be expressed in the Sertoli cells surrounding spermatogonia and spermatids in the testis of tilapia. Real-time PCR analysis revealed that the expression of pgr in the testis was significantly upregulated from 10 dah, further increased at 50 dah, and persisted until adulthood in fish. In the testis of RU486-treated fish, the transcript levels of germ cell markers and a meiotic marker were substantially reduced. However, the expression of markers in Sertoli cells remained unchanged. Moreover, the production of 11-ketotestosterone and the expression of genes encoding various steroidogenic enzymes were also not altered. In contrast, the expression of cyp17a2, encoding one of the critical steroidogenic enzymes involved in DHP biosynthesis, declined significantly, possibly indicating the inhibition of DHP production by RU486. RU486 treatment given for 2 months did not affect spermatogenesis; however, treatment given for more than 3 months resulted in a decrease in spermatogonial cell numbers and depletion of later-phase spermatogenic cells. Simultaneous excessive DHP supplementation restored spermatogenesis in RU486-treated XY fish. Taken together, our data further indicated that DHP, possibly through Pgr, might be essential for spermatogonial cell proliferation and spermatogenesis in fish.

Environmental gestagens activate fathead minnow (Pimephales promelas) nuclear progesterone and androgen receptors in vitro

Gestagen is a collective term for endogenous and synthetic progesterone receptor (PR) ligands. In teleost fishes, 17α,20β-dihydroxy-4-pregnen-3-one (DHP) and 17α,20β,21-trihydroxy-4-pregnen-3-one (20β-S) are the predominant progestogens, whereas in other vertebrates the major progestogen is progesterone (P4). Progestins are components of human contraceptives and hormone replacement pharmaceuticals and, with P4, can enter the environment and alter fish and amphibian reproductive health. In this study, our primary objectives were to clone the fathead minnow (FHM) nuclear PR (nPR), to develop an in vitro assay for FHM nPR transactivation, and to screen eight gestagens for their ability to transactivate FHM nPR. We also investigated the ability of these gestagens to transactivate FHM androgen receptor (AR). Fish progestogens activated FHM nPR, with DHP being more potent than 20β-S. The progestin drospirenone and P4 transactivated the FHM nPR, whereas five progestins and P4 transactivated FHM AR, all at environmentally relevant concentrations. Progestins are designed to activate human PR, but older generation progestins have unwanted androgenic side effects in humans. In FHMs, several progestins proved to be strong agonists of AR. Here, we present the first mechanistic evidence that environmental gestagens can activate FHM nPR and AR, suggesting that gestagens may affect phenotype through nPR- and AR-mediated pathways.

A living biosensor model to dynamically trace glucocorticoid transcriptional activity during development and adult life in zebrafish

Glucocorticoids (GCs) modulate many cellular processes through the binding of the glucocorticoid receptor (GR) to specific responsive elements located upstream of the transcription starting site or within an intron of GC target genes. Here we describe a transgenic fish line harboring a construct with nine GC-responsive elements (GREs) upstream of a reporter (EGFP) coding sequence. Transgenic fish exhibit strong fluorescence in many known GC-responsive organs. Moreover, its enhanced sensitivity allowed the discovery of novel GC-responsive tissue compartments, such as fin, eyes, and otic vesicles. Long-term persistence of transgene expression is seen during adult stages in several organs. Pharmacological and genetic analysis demonstrates that the transgenic line is highly responsive to drug administration and molecular manipulation. Moreover, reporter expression is sensitively and dynamically modulated by the photoperiod, thus proving that these fish are an in vivo valuable platform to explore GC responsiveness to both endogenous and exogenous stimuli.

The synthetic progestin megestrol acetate adversely affects zebrafish reproduction

Synthetic progestins contaminate the aquatic ecosystem, and may cause adverse health effects on aquatic organisms. Megestrol acetate (MTA) is present in the aquatic environment, but its possible effects on fish reproduction are unknown. In the present study, we investigated the endocrine disruption and impact of MTA on fish reproduction. After a pre-exposure period of 14 days, reproductively mature zebrafish (Danio rerio) (F0) were exposed to MTA at environmental concentrations (33, 100, 333, and 666 ng/L) for 21 days. Egg production was decreased in F0 fish exposed to MTA, with a significant decrease at 666 ng/L. The exposure significantly decreased the circulating concentrations of estradiol (E2) and testosterone (T) in female fish or 11-keto testosterone (11-KT) in male fish. MTA exposure significantly downregulated the transcription of certain genes along the hypothalamic-pituitary-gonadal (HPG) axis. MTA did not affect early embryonic development or hatching success in the F1 generation. The present study showed that MTA is a potent endocrine disruptor in fish, and short-term exposure to MTA could significantly affect reproduction in fish and negatively impact the fish population.

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

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

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

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

Transcriptional signature of progesterone in the fathead minnow ovary (Pimephales promelas)

A growing number of studies have examined transcriptional responses to sex steroids along the hypothalamic-pituitary-gonadal axis in teleost fishes. However, data are lacking on the molecular cascades that underlie progesterone signaling. The objective of this study was to characterize the transcriptional response in the ovary of fathead minnows (Pimephales promelas) in response to progesterone (P4). Fathead minnow ovaries were exposed in vitro to 500 ng P4/L. Germinal vesicle migration and breakdown (GVBD) was observed and microarrays were used to identify gene cascades affected by P4. Microarray analysis identified 1702 differentially expressed transcripts after P4 treatment. Functional enrichment analysis revealed that transcripts involved in the molecular functions of protein serine/threonine kinase activity, ATP binding, and activity of calcium channels were increased after P4 treatment. There was an overwhelming decrease in levels of transcripts of genes that are structural constituents of ribosomes with P4 treatment. There was also evidence for gene expression changes in steroid and maturation-related transcripts. Pathway analyses identified cell cycle regulation, insulin action, hedgehog, and B cell activation as pathways containing an over-representation of highly regulated transcripts. Significant regulatory sub-networks of P4-mediated transcripts included genes regulated by tumor protein p53 and E2F transcription factor 1. These data provide novel insight into the molecular signaling cascades that underlie P4-signaling in the ovary and identify genes and processes that may indicate premature GVBD due to environmental pollutants that mimic progestins.

Transcriptomic profiling of progesterone in the male fathead minnow (Pimephales promelas) testis

P4 is a hormone with diverse functions that include roles in reproduction, growth, and development. The objectives of this study were to examine the effects of P4 on androgen production in the mature teleost testis and to identify molecular signaling cascades regulated by P4 to improve understanding of its role in male reproduction. Fathead minnow (FHM) testis explants were treated in vitro with two concentrations of P4 (10(-8) and 10(-6) M) for 6 and 12 h. P4 significantly increased testosterone (T) production in the FHM testis but did not affect 11-ketotestosterone. Gene network analysis revealed that insulin growth factor (Igf1) and tumor necrosis factor receptor (Tnfr) signaling was significantly depressed with P4 treatment after 12h. There was also a 20% increase in a gene network for follicle-stimulating hormone secretion and an 18% decrease in genes involved in vasopressin signaling. Genes in steroid metabolism (e.g. star, cyp19a, 11bhsd) were not significantly affected by P4 treatments in this study, and it is hypothesized that pre-existing molecular machinery may be more involved in the increased production of T rather than the de novo expression of steroid-related transcripts and receptors. There was a significant decrease in prostaglandin E synthase 3b (cytosolic) (ptges3b) after treatment with P4, suggesting that there is cross talk between P4 and prostaglandin pathways in the reproductive testis. P4 has a role in regulating steroid production in the male testis and may do so by modulating gene networks related to endocrine pathways, such as Igf1, Tnfr, and vasopressin.

Response to fish specific reproductive hormones and endocrine disrupting chemicals of a Sertoli cell line expressing endogenous receptors from an endemic cyprinid Gnathopogon caerulescens

Fish Sertoli cells play a critical role in spermatogenesis by mediating androgen and progestogen signaling. Their hormonal response, however, considerably differ among species. Therefore it would be ideal to use Sertoli cells originated from the fish of interest to investigate the effects of hormones as well as endocrine disrupting chemicals (EDCs). The aim of this study was to investigate the responses to reproductive hormones and EDCs of a Sertoli cell line that we established from an endemic cyprinid Gnathopogon caerulescens. As the Sertoli cell line expressed endogenous androgen and progestogen receptors, we were able to detect hormone responses by transfecting only a reporter vector (pGL4.36) expressing luciferase under the control of the mouse mammary tumor virus-long terminal repeat (MMTV-LTR) promoter into the cell line. Unlike previous reporter gene assays using fish steroid hormone receptors expressed in mammalian cell lines, luciferase activities were induced by the fish specific androgen (11-ketotestosterone) and progestogen (17α,20β-dihydroxy-4-pregnen-3-one), but not by testosterone and progesterone, at physiologically relevant concentrations. Furthermore, we found 4-nonylphenol (NP) but not bisphenol A showed strong anti-androgenic effects, implying that NP may have direct anti-androgenic effects on fish Sertoli cells in vivo. This is the first evidence, to the best of our knowledge, of anti-androgenic effects of NP in a fish Sertoli cell line. In addition, neither NP nor BPA showed anti-progestogenic effects. These results suggest that the Sertoli cell line established from the fish of interest can be a useful in vitro tool for investigating the mechanisms of reproductive hormones and EDCs in the specific fish.

Zebrafish and steroids: what do we know and what do we need to know?

Zebrafish, Danio rerio, has long been used as a model organism in developmental biology. Nowadays, due to their advantages compared to other model animals, the fish gain popularity and are also increasingly used in endocrinology. This review focuses on an important aspect of endocrinology in zebrafish by summarizing the progress in steroid hormone related research. We present the state of the art of research on steroidogenesis, the action of steroid hormones, and steroid catabolism and cover the incremental usage of zebrafish as a test animal in endocrine disruption research. By this approach, we demonstrate that some aspects of steroid hormone research are well characterized (e.g., expression patterns of the genes involved), while other aspects such as functional analyses of enzymes, steroid hormone elimination, or the impact of steroid hormones on embryonic development or sex differentiation have not been extensively studied and are poorly understood. This article is part of a Special Issue entitled 'CSR 2013'.

Out-of-season production of 17,20β-dihydroxypregn-4-en-3-one in the roach Rutilus rutilus

In this study, although the highest production of two physiologically significant progestins in teleosts [17,20β-dihydroxypregn-4-en-3-one (17,20β-P) and 17,20β,21-trihydroxypregn-4-en-3-one (17,20β,21-P)] was observed in the period just prior to spawning in both male and female roach Rutilus rutilus, there was also a substantial production (mean levels of 5-10 ng ml(-1) in blood; and a rate of release of 5-20 ng fish(-1)  h(-1) into the water) in males and females in the late summer and early autumn (at least 7 months prior to spawning). During this period, the ovaries were increasing rapidly in size and histological sections were dominated by oocytes in the secondary growth phase [i.e. incorporation of vitellogenin (VTG)]. At the same time, the testes were also increasing rapidly in size and histological sections were dominated by cysts containing mainly spermatogonia type B. Measurements were also made of 11-ketotestosterone (11-KT) in males and 17β-oestradiol and VTG in females. The 3 months with the highest production of 11-KT coincided with the period that spermatozoa were present in the testes. In females, the first sign of a rise in 17β-oestradiol concentrations coincided with the time of the first appearance of yolk globules in the oocytes (in August). The role of the progestins during the late summer and autumn has not been established.

A progestin (17α,20β-dihydroxy-4-pregnen-3-one) stimulates early stages of spermatogenesis in zebrafish

Recently, evidence has been provided for multiple regulatory functions of progestins during the late mitotic and meiotic phases of spermatogenesis in teleost fish. For example, our previous studies suggested that 17α,20β-dihydroxy-4-pregnen-3-one (DHP), potentially via Sertoli cells that express the progesterone receptor (pgr) gene, can contribute to the regulation of zebrafish spermatogenesis. To further our understanding of the function of DHP at early spermatogenetic stages, we investigated in the present study the expression of genes reflecting Sertoli cell function and spermatogenic development in adult zebrafish testis after DHP treatment in tissue culture. Moreover, using an in vivo model of estrogen-mediated down-regulation of androgen production to interrupt adult spermatogenesis, we studied the effects of DHP on estrogen-interrupted spermatogenesis. In this model, DHP treatment doubled the testis weight, and all differentiating germ cell types, such as type B spermatogonia and primary spermatocytes, were abundantly present and incorporated the DNA-synthesis marker (BrdU). Accordingly, transcript levels of germ cell marker genes were up-regulated. Moreover, transcripts of two Sertoli cell-derived genes anti-müllerian hormone (amh) and gonadal soma-derived growth factor (gsdf) were up-regulated, as were three genes of the insulin-like growth factor signaling system, insulin-like growth factor 2b (igf2b), insulin-like growth factor 3 (igf3) and insulin-like growth factor 1b receptor (igf1rb). We further analyzed the relationship between these genes and DHP treatment using a primary zebrafish testis tissue culture system. In the presence of DHP, only igf1rb mRNA levels showed a significant increase among the somatic genes tested, and germ cell marker transcripts were again up-regulated. Taken together, our results show that DHP treatment induced the proliferation of early spermatogonia, their differentiation into late spermatogonia and spermatocytes as well as expression of marker genes for these germ cell stages. DHP-mediated stimulation of spermatogenesis and hence growth of spermatogenic cysts and the associated increase in Sertoli cell number may in part explain the elevated expression of Sertoli cell genes, but our data also suggest an up-regulation of the activity of the Igf signaling system.

Primary oocyte transcriptional activation of aqp1ab by the nuclear progestin receptor determines the pelagic egg phenotype of marine teleosts

In marine teleosts, the aqp1ab water channel plays a vital role in the development of the pelagic egg phenotype. However, the developmental control of aqp1ab activation during oogenesis remains to be established. Here, we report the isolation of the 5'-flanking region of the teleost gilthead seabream aqp1ab gene, in which we identify conserved cis-regulatory elements for the binding of the nuclear progestin receptor (Pgr) and members of the Sox family of transcription factors. Subcellular localization studies indicated that the Pgr, as well as sox3 and -8b transcripts, are co-expressed in seabream oogonia, whereas in meiosis-arrested primary growth (pre-vitellogenic) oocytes, when aqp1ab mRNA and protein are first synthesized, the Pgr appears to be completely translocated from the ooplasm into the nucleus. By contrast, sox9b is highly expressed in more advanced oocytes, coinciding with a strong depletion of aqp1ab transcripts in the oocyte. Functional characterization of wild-type and mutated aqp1ab promoter constructs, using mammalian cells and Xenopus laevis oocytes, demonstrated that aqp1ab transcription is initiated by the Pgr, which is activated by the progestin 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P), the natural ligand of the seabream Pgr. In vitro incubation of seabream primary ovarian explants with the follicle-stimulating hormone or 17,20β-P confirmed that progestin-activated Pgr enhanced Aqp1ab synthesis via the aqp1ab promoter. However, transactivation assays in heterologous systems showed that Sox transcription factors can potentially modulate this mechanism. These data uncover the existence of an endocrine pathway involved in the early activation of a water channel necessary for egg formation in marine teleosts.

Water homeostasis in the fish oocyte: new insights into the role and molecular regulation of a teleost-specific aquaporin

The discovery of the role of a teleost-specific aquaporin (Aqp1ab) during the process of oocyte hydration in marine fish producing pelagic (floating) eggs, recently confirmed by molecular approaches, has revealed that this mechanism is more sophisticated than initially thought. Recent phylogenetic and genomic studies suggest that Aqp1ab likely evolved by tandem duplication from a common ancestor and further neofunctionalized in oocytes for water transport. Investigations into the regulation of Aqp1ab during oogenesis indicate that the mRNA and protein product are highly accumulated during early oocyte growth, possibly through the transcriptional activation of the aqp1ab promoter by the classical nuclear progesterone receptor and perhaps by Sry-related high mobility group [HMG]-box (Sox) transcription factors. During oocyte growth and maturation, Aqp1ab intracellular trafficking may be regulated by phosphorylation and/or dephosphorylation of specific C-terminal residues in Aqp1ab, as well as by signal-mediated sorting processes. These mechanisms possibly regulate the temporal insertion of Aqp1ab into the oocyte plasma membrane during oocyte hydration, although the intracellular signaling pathways involved are yet unknown. Interestingly, in some freshwater species that spawn partially hydrated eggs, high accumulation of transcripts encoding functional Aqp1ab channels have also been found in the ovary. These findings suggest that the Aqp1ab-mediated mechanism for oocyte hydration is likely conserved in teleosts. The tight regulation of Aqp1ab during oogenesis, at both the transcriptional and posttranslational levels, highlights the essential physiological role of this water channel and opens new research avenues for understanding the molecular basis of egg formation in fish.

Alternative splicing of the nuclear progestin receptor in a perciform teleost generates novel mechanisms of dominant-negative transcriptional regulation

In mammals, downstream function of the nuclear progestin receptor (PGR) can be differentially regulated in each target tissue by altering the expression levels of PGR mRNA variants. Such PGR isoforms have also been identified in birds and reptiles, but not in non-amniote vertebrates. Based upon extensive phylogenetic, syntenic and functional analyses, here we show that higher orders of Teleostei retain a single pgr gene, and that four different pgr transcript variants of the extant gene are expressed in the ovary of an evolutionary advanced perciform teleost, the gilthead seabream (Sparus aurata). Three of the isoforms (pgr_tv2, pgr_tv3 and pgr_tv4) arise from alternative pre-mRNA splicing resulting in different N-terminally truncated receptors, whereas one isoform (pgr_tv1) is a deletion variant. Seabream wild-type Pgr shows the highest transactivational response to native euteleostean progestins, 17α,20β-dihydroxy-4-pregnen-3-one and 17α,20β,21-trihydroxy-4-pregnen-3-one, whereas the Pgr_tv3 and Pgr_tv4 isoforms independently regulate novel nuclear and cytosolic mechanisms of dominant-negative repression of Pgr-mediated transcription. In the seabream ovary, the wild-type Pgr protein is localized in oogonia, in the nuclei of primary (previtellogenic) oocytes, as well as in follicular (granulosa) cells and the oocyte cytoplasm of early and late vitellogenic ovarian follicles. Expression of wild-type pgr, pgr_tv3 and pgr_tv4 was the highest in seabream primary ovaries, while expression of both inhibitory receptor isoforms, but not of pgr, decreased during vitellogenesis. Stimulation of primary ovarian explants in vitro with recombinant piscine follicle-stimulating hormone and estrogen differentially regulated the temporal expression of pgr, pgr_tv3 and pgr_tv4. These findings suggest that, as in mammals, ovarian progestin responsiveness in the seabream, particularly during early oogenesis, may be regulated through alternative splicing of the nuclear pgr mRNA. Thus, the dominant-negative mechanism of PGR transcriptional regulation likely evolved prior to the separation of Actinopterygii (ray-finned fishes) from Sarcopterygii (lobe-finned fishes).

Designing Endocrine Disruption Out of the Next Generation of Chemicals

A central goal of green chemistry is to avoid hazard in the design of new chemicals. This objective is best achieved when information about a chemical's potential hazardous effects is obtained as early in the design process as feasible. Endocrine disruption is a type of hazard that to date has been inadequately addressed by both industrial and regulatory science. To aid chemists in avoiding this hazard, we propose an endocrine disruption testing protocol for use by chemists in the design of new chemicals. The Tiered Protocol for Endocrine Disruption (TiPED) has been created under the oversight of a scientific advisory committee composed of leading representatives from both green chemistry and the environmental health sciences. TiPED is conceived as a tool for new chemical design, thus it starts with a chemist theoretically at "the drawing board." It consists of five testing tiers ranging from broad in silico evaluation up through specific cell- and whole organism-based assays. To be effective at detecting endocrine disruption, a testing protocol must be able to measure potential hormone-like or hormone-inhibiting effects of chemicals, as well as the many possible interactions and signaling sequellae such chemicals may have with cell-based receptors. Accordingly, we have designed this protocol to broadly interrogate the endocrine system. The proposed protocol will not detect all possible mechanisms of endocrine disruption, because scientific understanding of these phenomena is advancing rapidly. To ensure that the protocol remains current, we have established a plan for incorporating new assays into the protocol as the science advances. In this paper we present the principles that should guide the science of testing new chemicals for endocrine disruption, as well as principles by which to evaluate individual assays for applicability, and laboratories for reliability. In a 'proof-of-principle' test, we ran 6 endocrine disrupting chemicals (EDCs) that act via different endocrinological mechanisms through the protocol using published literature. Each was identified as endocrine active by one or more tiers. We believe that this voluntary testing protocol will be a dynamic tool to facilitate efficient and early identification of potentially problematic chemicals, while ultimately reducing the risks to public health.

New evidence for the involvement of prostaglandin receptor EP4b in ovulation of the medaka, Oryzias latipes

A cDNA for a prostaglandin E(2) (PGE(2)) receptor subtype 4, EP4b (Ptger4b), was cloned from the medaka ovary. The effect of PGE(2) was examined using COS-7 cells expressing the recombinant Ptger4b protein. An increase in intracellular cAMP levels was observed when the cells were incubated with PGE(2), but the increase in cAMP levels was nullified by the addition of the EP4 antagonist GW627368X. The expression of ptger4b mRNA was drastically induced by the addition of pregnant mare serum gonadotropin to the in vitro culture of large preovulatory follicles. In in vitro ovulation studies of the effect of GW627368X addition on follicle ovulation, the critical timing of the PGE(2)/Ptger4b interaction was suggested to be between -1 and 0 h of ovulation. These results further substantiate that PGE(2)/Ptger4b signaling is involved in follicle rupture during ovulation in the medaka ovary.

Cloning, pharmacological characterization and expression analysis of Atlantic cod (Gadus morhua, L.) nuclear progesterone receptor

To better understand the role(s) of progesterone in fish spermatogenesis, we cloned the nuclear progesterone receptor (Pgr) of Atlantic cod. The open-reading frame of the cod pgr consists of 2076 bp, coding for a 691-amino acids-long protein that shows the highest similarity with other piscine Pgr proteins. Functional characterization of the receptor expressed in mammalian cells revealed that the cod Pgr exhibited progesterone-specific, dose-dependent induction of reporter gene expression, with 17α,20β-dihydroxy-4-pregnen-3-one (DHP), a typical piscine progesterone, showing the highest potency in activating the receptor. During ontogenesis, the pgr mRNA was undetectable in embryo's 24 h after fertilization, but became detectable 4 days after fertilization. During the larval stage, the expression levels increased steadily with the development of the larvae. In adult fish, pgr was predominantly expressed in gonads of both sexes. During the onset of puberty, testicular pgr transcript levels started to increase during rapid spermatogonial proliferation, and peaked when spermiation started. In situ hybridization studies using testis tissue during the rapid growth phase containing all germ cell stages indicated that in cod, pgr mRNA is predominantly located in Sertoli cells that are in contact with proliferating spermatogonia. Taken together, our data suggests that the Pgr is involved in mediating progestagen stimulation of the mitotic expansion of spermatogonia, and in processes associated with the spermiation/spawning period in Atlantic cod.

Evolution of a vertebrate social decision-making network

Animals evaluate and respond to their social environment with adaptive decisions. Revealing the neural mechanisms of such decisions is a major goal in biology. We analyzed expression profiles for 10 neurochemical genes across 12 brain regions important for decision-making in 88 species representing five vertebrate lineages. We found that behaviorally relevant brain regions are remarkably conserved over 450 million years of evolution. We also find evidence that different brain regions have experienced different selection pressures, because spatial distribution of neuroendocrine ligands are more flexible than their receptors across vertebrates. Our analysis suggests that the diversity of social behavior in vertebrates can be explained, in part, by variations on a theme of conserved neural and gene expression networks.

The brain of teleost fish, a source, and a target of sexual steroids

Neurosteroids are defined as steroids de novo synthesized in the central nervous system. While the production of neurosteroids is well documented in mammals and amphibians, there is less information about teleosts, the largest group of fish. Teleosts have long been known for their high brain aromatase and 5α-reductase activities, but recent data now document the capacity of the fish brain to produce a large variety of sex steroids. This article aims at reviewing the available information regarding expression and/or activity of the main steroidogenic enzymes in the brain of fish. In addition, the distribution of estrogen, androgen, and progesterone nuclear receptors is documented in relation with the potential sites of production of neurosteroids. Interestingly, radial glial cells acting as neuronal progenitors, appear to be a potential source of neurosteroids, but also a target for centrally and/or peripherally produced steroids.

Nuclear progesterone receptors are up-regulated by estrogens in neurons and radial glial progenitors in the brain of zebrafish

In rodents, there is increasing evidence that nuclear progesterone receptors are transiently expressed in many regions of the developing brain, notably outside the hypothalamus. This suggests that progesterone and/or its metabolites could be involved in functions not related to reproduction, particularly in neurodevelopment. In this context, the adult fish brain is of particular interest, as it exhibits constant growth and high neurogenic activity that is supported by radial glia progenitors. However, although synthesis of neuroprogestagens has been documented recently in the brain of zebrafish, information on the presence of progesterone receptors is very limited. In zebrafish, a single nuclear progesterone receptor (pgr) has been cloned and characterized. Here, we demonstrate that this pgr is widely distributed in all regions of the zebrafish brain. Interestingly, we show that Pgr is strongly expressed in radial glial cells and more weakly in neurons. Finally, we present evidence, based on quantitative PCR and immunohistochemistry, that nuclear progesterone receptor mRNA and proteins are upregulated by estrogens in the brain of adult zebrafish. These data document for the first time the finding that radial glial cells are preferential targets for peripheral progestagens and/or neuroprogestagens. Given the crucial roles of radial glial cells in adult neurogenesis, the potential effects of progestagens on their activity and the fate of daughter cells require thorough investigation.

Role of Aquaporins during Teleost Gametogenesis and Early Embryogenesis

Aquaporins are believed to be involved in homeosmotic mechanisms of marine teleosts. Increasing data suggest that these molecular water channels play critical roles associated with the adaptation of gametes and early embryos to the external spawning environment. In this mini-review, we discuss recent studies suggesting the function of aquaporin-mediated fluid homeostasis during spermatozoa activation and egg formation in teleosts. In addition, we address the potential role of water channels in osmosensing and cell migration during early embryonic development.

Progesterone signals through membrane progesterone receptors (mPRs) in MDA-MB-468 and mPR-transfected MDA-MB-231 breast cancer cells which lack full-length and N-terminally truncated isoforms of the nuclear progesterone receptor

The functional characteristics of membrane progesterone receptors (mPRs) have been investigated using recombinant mPR proteins over-expressed in MDA-MB-231 breast cancer cells. Although these cells do not express the full-length progesterone receptor (PR), it is not known whether they express N-terminally truncated PR isoforms which could possibly account for some progesterone receptor functions attributed to mPRs. In the present study, the presence of N-terminally truncated PR isoforms was investigated in untransfected and mPR-transfected MDA-MB-231 cells, and in MDA-MB-468 breast cancer cells. PCR products were detected in PR-positive T47D Yb breast cancer cells using two sets of C-terminus PR primers, but not in untransfected and mPR-transfected MDA-MB-231 cells, nor in MDA-MB-468 cells. Western blot analysis using a C-terminal PR antibody, 2C11F1, showed the same distribution pattern for PR in these cell lines. Another C-terminal PR antibody, C-19, detected immunoreactive bands in all the cell lines, but also recognized α-actinin, indicating that the antibody is not specific for PR. High affinity progesterone receptor binding was identified on plasma membranes of MDA-MB-468 cells which was significantly decreased after treatment with siRNAs for mPRα and mPRβ. Plasma membranes of MDA-MB-468 cells showed very low binding affinity for the PR agonist, R5020, ≤1% that of progesterone, which is characteristic of mPRs. Progesterone treatment caused G protein activation and decreased production of cAMP in MDA-MB-468 cells, which is also characteristic of mPRs. The results indicate that the progestin receptor functions in these cell lines are mediated through mPRs and do not involve any N-terminally truncated PR isoforms.

Visualization of estrogen receptor transcriptional activation in zebrafish

Estrogens regulate a diverse range of physiological processes and affect multiple tissues. Estrogen receptors (ERs) regulate transcription by binding to DNA at conserved estrogen response elements, and such elements have been used to report ER activity in cultured cells and in transgenic mice. We generated stable, transgenic zebrafish containing five consecutive elements upstream of a c-fos minimal promoter and green fluorescent protein (GFP) to visualize and quantify transcriptional activation in live larvae. Transgenic larvae show robust, dose-dependent estrogen-dependent fluorescent labeling in the liver, consistent with er gene expression, whereas ER antagonists inhibit GFP expression. The nonestrogenic steroids dexamethasone and progesterone fail to activate GFP, confirming ER selectivity. Natural and synthetic estrogens activated the transgene with varying potency, and two chemicals, genistein and bisphenol A, preferentially induce GFP expression in the heart. In adult fish, fluorescence was observed in estrogenic tissues such as the liver, ovary, pituitary gland, and brain. Individual estrogen-responsive neurons and their projections were visualized in the adult brain, and GFP-positive neurons increased in number after 17β-estradiol exposure. The transgenic estrogen-responsive zebrafish allow ER signaling to be monitored visually and serve as in vivo sentinels for detection of estrogenic compounds.

Neural and hormonal mechanisms of reproductive-related arousal in fishes

The major classes of chemicals and brain pathways involved in sexual arousal in mammals are well studied and are thought to be of an ancient, evolutionarily conserved origin. Here we discuss what is known of these neurochemicals and brain circuits in fishes, the oldest and most species-rich group of vertebrates from which tetrapods arose over 350 million years ago. Highlighted are case studies in vocal species where well-delineated sensory and motor pathways underlying reproductive-related behaviors illustrate the diversity and evolution of brain mechanisms driving sexual motivation between (and within) sexes. Also discussed are evolutionary insights from the neurobiology and reproductive behavior of elasmobranch fishes, the most ancient lineage of jawed vertebrates, which are remarkably similar in their reproductive biology to terrestrial mammals.

Controls of meiotic signaling by membrane or nuclear progestin receptor in zebrafish follicle-enclosed oocytes

Both membrane progestin receptors (mPRs) and the nuclear progestin receptor (nPR or Pgr) decode the non-genomic progestin signaling (NGPS) in vertebrates. However, the receptor for deciphering extracellular NGPS and initiating meiosis resumption in vertebrate oocytes is still contested hotly. We studied the roles of nPR and mPRs by determining their localization, changes of expression, and activation of NGPS during final oocyte maturation (FOM) in zebrafish. The nPR transcript and protein were expressed abundantly in follicular cells that were surrounding stage IV oocytes, but nPR transcript appeared absent within stage IV oocytes. The most significant daily changes of nPR transcript were observed in stage IV follicular cells, with the highest level observed just prior to ovulation. In contrast, the expressions of mPRα and mPRβ transcripts and proteins were abundant and increased significantly in late stage denuded oocytes prior to oocyte maturation, consistent with the purported role of mPRs in interpreting NGPS. Moreover, over-expression of mPRα in follicle-enclosed oocytes significantly increased the activity of MAPK, the production of cyclin B protein, and the number of oocytes that underwent FOM without exogenous progestin, while over-expression of mPRβ or nPR alone had no such effect. Intriguingly, significant acceleration of FOM was observed when follicle-enclosed oocytes were incubated with the maturation inducing steroid, 4-pregnen-17, 20β-diol-3-one (DHP) following over-expression of nPR or mPRα. Interestingly, this acceleration in oocyte maturation was observed approximately 1h later in oocytes over-expressing nPR compared to those over-expressing mPRα. Importantly, the acceleration of maturation in the nPR injected group was blocked by treatment with the transcription inhibitor actinomycin D, implying a requirement of the genomic signaling pathway, while the same treatment did not affect the accelerated rate of maturation in mPRα injected oocytes. Taken together, these results imply that nPR and mPRβ are unlikely receptors for inducing FOM, while mPRα is the long-sought-after nongenomic progestin receptor that deciphers extracellular NGPS to initiate meiosis resumption in follicle-enclosed zebrafish oocytes.

Cloning, pharmacological characterization, and expression analysis of Atlantic salmon (Salmo salar L.) nuclear progesterone receptor

To better understand the role(s) of progestogens during early stages of spermatogenesis, we carried out studies on the nuclear progesterone receptor (Pgr) of the Atlantic salmon. Its open-reading frame shows the highest similarity with other piscine Pgr proteins. When expressed in mammalian cells, salmon Pgr exhibited progestogen-specific, dose-dependent induction of reporter gene expression, with 17α,20β-dihydroxy-4-pregnen-3-one (DHP) showing the highest potency. We then analyzed testicular pgr mRNA and DHP plasma levels in animals during the onset of spermatogenesis, which were exposed to natural light or to constant light, to induce significant differences in testis growth. Grouping of the animals according to their progress through spermatogenesis showed that testicular pgr mRNA levels as well as DHP plasma levels first increased when germ cells had reached the stage of late type B spermatogonia and further increased when entered meiosis, i.e. when spermatocytes were present. However, in situ hybridization studies revealed that pgr mRNA expression was restricted to Sertoli cells, with a strong signal in Sertoli cells contacting type A/early type B spermatogonia, while Sertoli cells contacting larger germ cell clones with further differentiated stages (e.g. late type B spermatogonia) were less intensely/not stained. We conclude that the increase in pgr mRNA levels per pair of testis reflects, at least in part, the increased number of Sertoli cells enveloping type A and early type B spermatogonia. We propose that Sertoli cell-expressed Pgr may mediate DHP-stimulated early steps in spermatogenesis in Atlantic salmon, such as an increase in the number of new spermatogonial cysts.

Molecular characterization and brain distribution of the progesterone receptor in whiptail lizards

Progesterone and its nuclear receptor are critical in modulating reproductive physiology and behavior in female and male vertebrates. Whiptail lizards (genus Cnemidophorus) are an excellent model system in which to study the evolution of sexual behavior, as both the ancestral and descendent species exist. Male-typical sexual behavior is mediated by progesterone in both the ancestral species and the descendant all-female species, although the molecular characterization and distribution of the progesterone receptor protein throughout the reptilian brain is not well understood. To better understand the gene targets and ligand binding properties of the progesterone receptor in whiptails, we cloned the promoter and coding sequence of the progesterone receptor and analyzed the predicted protein structure. We next determined the distribution of the progesterone receptor protein and mRNA throughout the brain of Cnemidophorus inornatus and Cnemidophorus uniparens by immunohistochemistry and in situ hybridization. We found the progesterone receptor to be present in many brain regions known to regulate social behavior and processing of stimulus salience across many vertebrates, including the ventral tegmental area, amygdala, nucleus accumbens and several hypothalamic nuclei. Additionally, we quantified immunoreactive cells in the preoptic area and ventromedial hypothalamus in females of both species and males of the ancestral species. We found differences between both species and across ovarian states. Our results significantly extend our understanding of progesterone modulation in the reptilian brain and support the important role of the nuclear progesterone receptor in modulating sexual behavior in reptiles and across vertebrates.

Distribution of sex steroid hormone receptors in the brain of an African cichlid fish, Astatotilapia burtoni

Sex steroid hormones released from the gonads play an important role in mediating social behavior across all vertebrates. Many effects of these gonadal hormones are mediated by nuclear steroid hormone receptors, which are crucial for integration in the brain of external (e.g., social) signals with internal physiological cues to produce an appropriate behavioral output. The African cichlid fish Astatotilapia burtoni presents an attractive model system for the study of how internal cues and external social signals are integrated in the brain as males display robust plasticity in the form of two distinct, yet reversible, behavioral and physiological phenotypes depending on the social environment. In order to better understand where sex steroid hormones act to regulate social behavior in this species, we have determined the distribution of the androgen receptor, estrogen receptor alpha, estrogen receptor beta, and progesterone receptor mRNA and protein throughout the telencephalon and diencephalon and some mesencephalic structures of A. burtoni. All steroid hormone receptors were found in key brain regions known to modulate social behavior in other vertebrates including the proposed teleost homologs of the mammalian amygdalar complex, hippocampus, striatum, preoptic area, anterior hypothalamus, ventromedial hypothalamus, and ventral tegmental area. Overall, there is high concordance of mRNA and protein labeling. Our results significantly extend our understanding of sex steroid pathways in the cichlid brain and support the important role of nuclear sex steroid hormone receptors in modulating social behaviors in teleosts and across vertebrates.