The GnRH promoter: target of transcription factors, hormones, and signaling pathways

Social regulation of immediate early gene induction in gonadotropin releasing-hormone 1 neurons and singing behavior in canaries (Serinus canaria)

Social cues modulate the neuroendocrine control of reproduction. However, the neural systems involved in the integration of social cues are not well described. Gonadotropin-releasing hormone 1 (GnRH1) cells in the preoptic area (POA) are the final common node that links the brain with peripheral reproductive physiology. These experiments investigated whether induction of the immediate early gene, EGR1, in anatomically localized GnRH1 cell populations in Border canaries is regulated by the social environment. First, we characterized behavioral modifications in singing behavior and found males paired with a female for 2 weeks significantly reduced many aspects of singing behavior. However, paired males had a significantly higher percentage of GnRH1 cells co-labeled with EGR1. The second experiment manipulated the social environment by pairing males and females in mixed sex dyads, same sex dyads or housed birds in isolation. Only when birds are paired in mixed sex dyads was there a significantly greater percentage of GnRH1 cells expressing EGR1 cells. Increased GnRH1-EGR1 co-expression was localized to the rostral POA. These data reveal that discrete GnRH1 cells are involved in the neural integration of specific social cues and support the hypothesis that the POA exhibits functional topography related to courtship and sexual behaviors.

Molecular cloning, immunological characterization, and expression analysis of gonadotropin-releasing hormone (GnRH) in the brain of the Chinese alligator during different stages of reproductive cycle

The neurohormone gonadotropin-releasing hormone (GnRH) plays an essential role in the control of reproductive functions in vertebrates. However, the full-length complementary DNA (cDNA) encoding the GnRHs precursor and it role in the reproductive cycles regulating has not been illustrated in crocodilian species. In the present study, full-length cDNAs encoding GnRH1 forms, its predominant localization within brain and peripheral tissues, and GnRH1 peptide concentrations in the hypothalamus and pituitary in relation to seasonal gonadal development of Chinese alligator were investigated. The cDNA of GnRH1 is consisted of 282 bp open reading frame encoding 93 amino acids. The deduced amino acid sequence of alligator GnRH1 contains several conserved regions and shows a closer genetic relationship to the avian species than to other reptile species. The GnRH1 immunopositive cells were not only detected widely in cerebrum, diencephalon, medulla oblongata but also observed in peripheral tissues, these widespread distribution characteristics indicated that GnRH1 possibly possess the multi-functionality in Chinese Alligator. GnRH1 peptide concentration within hypothalamus were observed be the highest in RP group (P < 0.05), in association with an peak value in GSI and emerging of late vitellogenic follicles in the ovary. Taken together, our results suggested that GnRH1 was predominantly involved in the vitellogenesis process of seasonal gonadal development of Chinese Alligator.

Genome-Wide Association Study Reveals Candidate Genes for Litter Size Traits in Pelibuey Sheep

The Pelibuey sheep has adaptability to climatic variations, resistance to parasites, and good maternal ability, whereas some ewes present multiple births, which increases the litter size in farm sheep. The litter size in some wool sheep breeds is associated with the presence of mutations, mainly in the family of the transforming growth factor β (TGF-β) genes. To explore genetic mechanisms underlying the variation in litter size, we conducted a genome-wide association study in two groups of Pelibuey sheep (multiparous sheep with two lambs per birth vs. uniparous sheep with a single lamb at birth) using the OvineSNP50 BeadChip. We identified a total of 57 putative SNPs markers (p < 3.0 × 10^-3, Bonferroni correction). The candidate genes that may be associated with litter size in Pelibuey sheep are CLSTN2, MTMR2, DLG1, CGA, ABCG5, TRPM6, and HTR1E. Genomic regions were also identified that contain three quantitative trait loci (QTLs) for aseasonal reproduction (ASREP), milk yield (MY), and body weight (BW). These results allowed us to identify SNPs associated with genes that could be involved in the reproductive process related to prolificacy.

Mathematical modeling of gonadotropin-releasing hormone signaling

Gonadotropin-releasing hormone (GnRH) acts via G-protein coupled receptors on pituitary gonadotropes to control reproduction. These are G_q-coupled receptors that mediate acute effects of GnRH on the exocytotic secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), as well as the chronic regulation of their synthesis. GnRH is secreted in short pulses and GnRH effects on its target cells are dependent upon the dynamics of these pulses. Here we overview GnRH receptors and their signaling network, placing emphasis on pulsatile signaling, and how mechanistic mathematical models and an information theoretic approach have helped further this field.

Induction of Gnrh mRNA expression by the ω-3 polyunsaturated fatty acid docosahexaenoic acid and the saturated fatty acid palmitate in a GnRH-synthesizing neuronal cell model, mHypoA-GnRH/GFP

Gonadotropin-releasing hormone (GnRH) neurons coordinate reproduction. However, whether GnRH neurons directly sense free fatty acids (FFAs) is unknown. We investigated the individual effects of the FFAs docosahexaenoic acid (DHA), palmitate, palmitoleate, and oleate (100 μM each) on Gnrh mRNA expression in the mHypoA-GnRH/GFP neuronal cell model. We report that 2 h exposure to palmitate or DHA increases Gnrh transcription. Using the inhibitors AH7614, K252c, U0126, wortmannin, and LY294002, we demonstrate that the effect of DHA is mediated through GPR120 to downstream PKC/MAPK and PI3K signaling. Our results indicate that the effect of palmitate may depend on palmitoyl-coA synthesis and PI3K signaling. Finally, we demonstrate that both DHA and palmitate increase Gnrh enhancer-derived RNA levels. Overall, these studies provide evidence that GnRH neurons directly sense FFAs. This will advance our understanding of the mechanisms underlying FFA sensing in the brain and provides insight into the links between nutrition and reproductive function.

Social Crowding during Development Causes Changes in GnRH1 DNA Methylation

Gestational and developmental cues have important consequences for long-term health, behavior and adaptation to the environment. In addition, social stressors cause plastic molecular changes in the brain that underlie unique behavioral phenotypes that also modulate fitness. In the adult African cichlid, Astatotilapia burtoni, growth and social status of males are both directly regulated by social interactions in a dynamic social environment, which causes a suite of plastic changes in circuits, cells and gene transcription in the brain. We hypothesized that a possible mechanism underlying some molecular changes might be DNA methylation, a reversible modification made to cytosine nucleotides that is known to regulate gene function. Here we asked whether changes in DNA methylation of the GnRH1 gene, the central regulator of the reproductive axis, were altered during development of A. burtoni. We measured changes in methylation state of the GnRH1 gene during normal development and following the gestational and developmental stress of social crowding. We found differential DNA methylation within developing juveniles between 14-, 28- and 42-day-old. Following gestational crowding of mouth brooding mothers, we saw differential methylation and transcription of GnRH1 in their offspring. Taken together, our data provides evidence for social control of GnRH1 developmental responses to gestational cues through DNA methylation.

Molecular mechanisms of gonadotropin-releasing hormone signaling: integrating cyclic nucleotides into the network

Gonadotropin-releasing hormone (GnRH) is the primary regulator of mammalian reproductive function in both males and females. It acts via G-protein coupled receptors on gonadotropes to stimulate synthesis and secretion of the gonadotropin hormones luteinizing hormone and follicle-stimulating hormone. These receptors couple primarily via G-proteins of the Gq/ll family, driving activation of phospholipases C and mediating GnRH effects on gonadotropin synthesis and secretion. There is also good evidence that GnRH causes activation of other heterotrimeric G-proteins (Gs and Gi) with consequent effects on cyclic AMP production, as well as for effects on the soluble and particulate guanylyl cyclases that generate cGMP. Here we provide an overview of these pathways. We emphasize mechanisms underpinning pulsatile hormone signaling and the possible interplay of GnRH and autocrine or paracrine regulatory mechanisms in control of cyclic nucleotide signaling.

R31C GNRH1 mutation and congenital hypogonadotropic hypogonadism

Normosmic congenital hypogonadotropic hypogonadism (nCHH) is a rare reproductive disease leading to lack of puberty and infertility. Loss-of-function mutations of GNRH1 gene are a very rare cause of autosomal recessive nCHH. R31C GNRH1 is the only missense mutation that affects the conserved GnRH decapeptide sequence. This mutation was identified in a CpG islet in nine nCHH subjects from four unrelated families, giving evidence for a putative “hot spot”. Interestingly, all the nCHH patients carry this mutation in heterozygosis that strikingly contrasts with the recessive inheritance associated with frame shift and non-sense mutations. Therefore, after exclusion of a second genetic event, a comprehensive functional characterization of the mutant R31C GnRH was undertaken. Using different cellular models, we clearly demonstrate a dramatic reduction of the mutant decapeptide capacity to bind GnRH-receptor, to activate MAPK pathway and to trigger inositol phosphate accumulation and intracellular calcium mobilization. In addition it is less able than wild type to induce lh-beta transcription and LH secretion in gonadotrope cells. Finally, the absence of a negative dominance in vitro offers a unique opportunity to discuss the complex in vivo patho-physiology of this form of nCHH.

Neurokinin B causes acute GnRH secretion and repression of GnRH transcription in GT1-7 GnRH neurons

Genetic studies in human patients with idiopathic hypogonadotropic hypogonadism (IHH) identified mutations in the genes that encode neurokinin B (NKB) and the neurokinin 3 receptor (NK3R). However, determining the mechanism whereby NKB regulates gonadotropin secretion has been difficult because of conflicting results from in vivo studies investigating the luteinizing hormone (LH) response to senktide, a NK3R agonist. NK3R is expressed in a subset of GnRH neurons and in kisspeptin neurons that are known to regulate GnRH secretion. Thus, one potential source of inconsistency is that NKB could produce opposing direct and indirect effects on GnRH secretion. Here, we employ the GT1-7 cell model to elucidate the direct effects of NKB on GnRH neuron function. We find that GT1-7 cells express NK3R and respond to acute senktide treatment with c-Fos induction and increased GnRH secretion. In contrast, long-term senktide treatment decreased GnRH secretion. Next, we focus on the examination of the mechanism underlying the long-term decrease in secretion and determine that senktide treatment represses transcription of GnRH. We further show that this repression of GnRH transcription may involve enhanced c-Fos protein binding at novel activator protein-1 (AP-1) half-sites identified in enhancer 1 and the promoter, as well as chromatin remodeling at the promoter of the GnRH gene. These data indicate that NKB could directly regulate secretion from NK3R-expressing GnRH neurons. Furthermore, whether the response is inhibitory or stimulatory toward GnRH secretion could depend on the history or length of exposure to NKB because of a repressive effect on GnRH transcription.

Androgen receptor repression of gonadotropin-releasing hormone gene transcription via enhancer 1

Gonadotropin-releasing hormone (GnRH) plays a major role in the hypothalamic-pituitary-gonadal (HPG) axis, and synthesis and secretion of GnRH are regulated by gonadal steroid hormones. Disruptions in androgen levels are involved in a number of reproductive defects, including hypogonadotropic hypogonadism and polycystic ovarian syndrome. Androgens down-regulate GnRH mRNA synthesis in vivo and in vitro via an androgen receptor (AR)-dependent mechanism. Methyltrienolone (R1881), a synthetic AR agonist, represses GnRH expression through multiple sites in the proximal promoter. In this study, we show AR also represses GnRH transcription via the major enhancer (GnRH-E1). A multimer of the -1800/-1766 region was repressed by R1881 treatment. Mutation of two bases, -1792 and -1791, resulted in decreased basal activity and a loss of AR-mediated repression. AR bound to the -1796/-1791 sequence in electrophoretic mobility shift assays, indicating a direct interaction with DNA or other transcription factors in this region. We conclude that AR repression of GnRH-E1 acts via multiple AR-responsive regions, including the site at -1792/-1791.

Epigenetic changes coincide with in vitro primate GnRH neuronal maturation

Cellular and molecular mechanisms underlying pulsatile GnRH release are not well understood. In the present study, we examined the developmental changes in intracellular calcium dynamics, peptide release, gene expression, and DNA methylation in cultured GnRH neurons derived from the nasal placode of rhesus monkeys. We found that GnRH neurons were functionally immature, exhibiting little fluctuation in intracellular calcium ([Ca(2+)](i)) and sparse pulses of GnRH peptide release in the first 12 d in vitro (div). By 14-18 div, GnRH neurons exhibited periodic [Ca(2+)](i) oscillations, synchronizing at approximately 60-min intervals and GnRH pulses occurred at approximately 60-min intervals. Interestingly, the total GnRH peptide release further increased after 18 div. Measurement of GnRH mRNA and gene CpG methylation status at 0, 14, and 20 div indicated that mRNA levels significantly (P < 0.05) increased between 14 and 20 div, just as maximal decapeptide release was observed. By bisulfite sequencing across a 5' CpG island of the GnRH gene, we further found that methylation at eight of 14 CpG sites significantly (P < 0.05) decreased between 0 and 20 div. These data indicate that epigenetic differentiation occurs during GnRH neuronal development and suggest that increased GnRH gene expression and decreased CpG methylation status are molecular phenotypes of mature GnRH neurons. To our knowledge, this is the first report that developmental DNA demethylation occurs in postmitotic neurons toward a stable neuronal phenotype.

Gonadotrophin-releasing hormone neurons in a photoperiodic songbird express fos and egr-1 protein after a single long day

Birds use a variety of environmental cues, such as day length, temperature and social interactions, to time reproductive efforts. For most seasonally breeding birds, day length is the most important cue and takes precedence over all others. Experimental manipulation of day length has shown that, in a number of galliformes and passeriformes, exposure to a single long day induces a rise in plasma luteinising hormone (LH). The mechanisms underlying this response are only beginning to be understood. In Japanese quail and Zonotrichia sparrows, one long day causes striking up-regulation of the protein products of immediate early genes (IEGs) in the mediobasal hypothalamus, near gonadotrophin-releasing hormone (GnRH) axons and terminals. Photoperiodic induction of the same proteins in the GnRH somata themselves, however, has not been described in these species. In the present study, we used immunohistochemistry to assay the induction of two IEGs, Fos and Egr-1, in the GnRH somata of male and female white-throated sparrows (Zonotrichia albicollis) exposed to a single long day. We found that immunoreactivity for both proteins increased in a subset of the GnRH neurones of the septo-preoptic area by the morning after the long day. Photo-induced expression of Egr-1 or Fos protein in GnRH neurones was limited to a population of cells in the medial preoptic area. Males showed significantly greater induction of both proteins in this population of GnRH neurones than did females, which is consistent with the hypothesis that males may be more sensitive to photic cues. Overall, the results obtained suggest that photostimulation stimulates new protein synthesis in GnRH neurones on a relatively rapid time scale. Further research is required to determine whether the GnRH somata are themselves integrating photic cues, or whether they are responding rapidly to an increased demand for GnRH synthesis.

Regulation of gonadotropin-releasing hormone-1 gene transcription by members of the purine-rich element-binding protein family

Gonadotropin-releasing hormone-1 (GnRH1) controls reproduction by stimulating the release of gonadotropins from the pituitary. To characterize regulatory factors governing GnRH1 gene expression, we employed biochemical and bioinformatics techniques to identify novel GnRH1 promoter-binding proteins from the brain of the cichlid fish, Astatotilapia burtoni (A. burtoni). Using an in vitro DNA-binding assay followed by mass spectrometric peptide mapping, we identified two members of the purine-rich element-binding (Pur) protein family, Puralpha and Purbeta, as candidates for GnRH1 promoter binding and regulation. We found that transcripts for both Puralpha and Purbeta colocalize in GnRH1-expressing neurons in the preoptic area of the hypothalamus in A. burtoni brain. Furthermore, we confirmed in vivo binding of endogenous Puralpha and Purbeta to the upstream region of the GnRH1 gene in A. burtoni brain and mouse neuronal GT1-7 cells. Consistent with the relative promoter occupancy exhibited by endogenous Pur proteins, overexpression of Purbeta, but not Puralpha, significantly downregulated GnRH1 mRNA levels in transiently transfected GT1-7 cells, suggesting that Purbeta acts as a repressor of GnRH1 gene transcription.

Hypothalamic cell lines to investigate neuroendocrine control mechanisms

The hypothalamus is the control center for most physiological processes; yet has been difficult to study due to the inherent heterogeneity of this brain region. For this reason, researchers have turned towards cell models. Primary hypothalamic cultures are difficult to maintain, are heterogeneous neuronal and glial cell populations and often contain a minimal number of viable peptide-secreting neurons. In contrast, immortalized, clonal cell lines represent an unlimited, homogeneous population of neurons that can be manipulated using a number of elegant molecular techniques. Cell line studies and in vivo experimentation are complementary and together provide a powerful tool to drive scientific discovery. This review focuses on three key neuroendocrine systems: energy homeostasis, reproduction, and circadian rhythms; and the use of hypothalamic cell lines to dissect the complex pathways utilized by individual neurons in these systems.

Otx2 induction of the gonadotropin-releasing hormone promoter is modulated by direct interactions with Grg co-repressors

Hormonal communication between the hypothalamus, pituitary, and gonads orchestrates the development and regulation of mammalian reproductive function. In mice, gonadotropin-releasing hormone (GnRH) expression is limited to approximately 1000 neurons that originate in the olfactory placode then migrate to specific positions scattered throughout the hypothalamus. Coordination of the hypothalamic-pituitary-gonadal axis is dependent upon correct migration of GnRH neurons into the hypothalamus followed by the appropriate synthesis and pulsatile secretion of GnRH. Defects in any one of these processes can cause infertility. Recently, substantial progress has been made in identifying transcription factors, and their cofactors, that regulate not only adult expression of GnRH, but also the maturation of GnRH neurons. Here, we show that expression of Otx2, a homeodomain protein required for the formation of the forebrain, is dramatically up-regulated during GnRH neuronal maturation and that overexpression of Otx2 increases GnRH promoter activity in GnRH neuronal cell lines. Furthermore, Otx2 transcriptional activity is modulated by Grg4, a member of the Groucho-related-gene (Grg) family. Using mutational analysis, we show that a WRPW peptide motif within the Otx2 protein is required for physical interaction between Otx2 and Grg4. Without this physical interaction, Grg4 cannot repress Otx2-dependent activation of GnRH gene transcription. Taken together, these data show that Otx2 is important for GnRH expression and that direct interaction between Otx2 and Grg co-repressors regulates GnRH gene expression in hypothalamic neurons.

Heterogeneous nuclear ribonucleoprotein A/B and G inhibits the transcription of gonadotropin-releasing-hormone 1

Gonadotropin-releasing hormone 1 (GnRH1) causes the release of gonadotropins from the pituitary to control reproduction. Here we report that two heterogeneous nuclear ribonucleoproteins (hnRNP-A/B and hnRNP-G) bind to the GnRH-I upstream promoter region in a cichlid fish Astatotilapia burtoni. We identified these binding proteins using a newly developed homology based method of mass spectrometric peptide mapping. We show that both hnRNP-A/B and hnRNP-G co-localize with GnRH1 in the pre-optic area of the hypothalamus in the brain. We also demonstrated that these ribonucleoproteins exhibit similar binding capacity in vivo, using immortalized mouse GT1-7 cells where overexpression of either hnRNP-A/B or hnRNP-G significantly down-regulates GnRH1 mRNA levels in GT1-7 cells, suggesting that both act as repressors in GnRH1 transcriptional regulation.

Synthesis and secretion of GnRH

Comprehensive studies have provided a clear understanding of the effects of gonadal steroids on the secretion of gonadotropin releasing hormone (GnRH), but some inconsistent results exist with regard to effects on synthesis. It is clear that regulation of both synthesis and the secretion of GnRH are effected by neurotransmitter systems in the brain. Thus, steroid regulation of GnRH synthesis and secretion can be direct, but the predominant effects are transmitted through steroid-responsive neuronal systems in various parts of the brain. There is also emerging evidence of direct effects on GnRH cells. Overriding effects on synthesis and secretion of GnRH can be observed during aging, in undernutrition and under stressful situations; these involve various neuronal systems, which may have serial or parallel effects on GnRH cells. The effect of aging is accompanied by changes in GnRH synthesis, but comprehensive studies of synthesis during undernutrition and stress are less well documented. Altered GnRH and gonadotropin secretion that occurs in seasonal breeding animals and during the pubertal transition is not generally accompanied by changes in GnRH synthesis. Secretion of GnRH from the brain is a reflection of the inherent function of GnRH cells and the inputs that integrate all of the central regulatory elements. Ultimately, the pattern of secretion dictates the reproductive status of the organism. In order to fully understand the central mechanisms that control reproduction, more extensive studies are required on the neuronal circuitry that provides input to GnRH cells.

Gonadotropin‐Releasing Hormone: Gene Evolution, Expression, and Regulation

The gonadotropin-releasing hormone (GnRH) gene is a superb example of the diverse regulation that is required to maintain the function of an evolutionarily conserved and fundamental gene. Because reproductive capacity is critical to the survival of the species, physiological homeostasis dictates optimal conditions for reproductive success, and any perturbation from this balance may affect GnRH expression. These disturbances may include alterations in signals dictated by stress, nutritional imbalance, body weight, and neurological problems; therefore, changes in other neuroendocrine systems may directly influence the hypothalamic-pituitary-gonadal axis through direct regulation of GnRH. Thus, to maintain optimal reproductive capacity, the regulation of the GnRH gene is tightly constrained by a number of diverse signaling pathways and neuromodulators. In this review, we summarize what is currently known of GnRH gene structure, the location and function of the two isoforms of the GnRH gene, some of the many hormones and neuromodulators found to affect GnRH expression, and the molecular mechanisms responsible for the regulation of the GnRH gene. We also discuss the latest models used to study the transcriptional regulation of the GnRH gene, from cell models to evolving in vivo technologies. Although we have come a long way in the last two decades toward uncovering the intricacies behind the control of the GnRH neuron, there remain vast distances to cover before direct therapeutic manipulation of the GnRH gene to control reproductive competence is possible.

The biology of gonadotropin hormone-releasing hormone: role in the control of tumor growth and progression in humans

It is now well known that different forms of GnRH coexist in the same vertebrate species. In humans, two forms of GnRH have been identified so far. The first form corresponds to the hypophysiotropic decapeptide, and is now called GnRH-I. The second form has been initially identified in the chicken brain, and it is referred to as GnRH-II. GnRH-I binds to and activates specific receptors, belonging to the 7 transmembrane (7TM) domain superfamily, present on pituitary gonadotropes. These receptors (type I GnRH receptors) are coupled to the Gq/11/PLC intracellular signalling pathway. A receptor specific for GnRH-II (type II GnRH receptor) has been identified in non-mammalian vertebrates as well as in primates, but not yet in humans. In the last 10-15 years experimental evidence has been accumulated indicating that GnRH-I is expressed, together with its receptors, in tumors of the reproductive tract (prostate, breast, ovary, and endometrium). In these hormone-related tumors, activation of type I GnRH receptors consistently decreases cell proliferation, mainly by interfering with the mitogenic activity of stimulatory growth factors (e.g., EGF, IGF). Recent data seem to suggest that GnRH-I might also reduce the migratory and invasive capacity of cancer cells, possibly by affecting the expression and/or activity of cell adhesion molecules and of enzymes involved in the remodelling of the extracellular matrix. These observations point to GnRH-I as an autocrine negative regulatory factor on tumor growth progression and metastatization. Extensive research has been performed to clarify the molecular mechanisms underlying the peculiar antitumor activity of GnRH-I. Type I GnRH receptors in hormone-related tumors correspond to those present at the pituitary level in terms of cDNA nucleotide sequence and protein molecular weight, but do not share the same pharmacological profile in terms of binding affinity for the different synthetic GnRH-I analogs. Moreover, the classical intracellular signalling pathway mediating the stimulatory activity of the decapeptide on gonadotropin synthesis and secretion is not involved in its inhibitory activity on hormone-related tumor growth. In these tumors, type I GnRH receptors are coupled to the Gi-cAMP, rather than the Gq/11-PLC, signal transduction pathway. Recently, we have reported that GnRH-I and type I GnRH receptors are expressed also in tumors not related to the reproductive system, such as melanoma. Also in melanoma cells, GnRH-I behaves as a negative regulator of tumor growth and progression. Interestingly, the biochemical and pharmacological profiles of type I GnRH receptors in melanoma seem to correspond to those of the receptors at pituitary level. The data so far reported on the expression and on the possible functions of GnRH-II in humans are still scanty. The decapeptide has been identified, together with a 'putative' type II GnRH receptor, both in the central nervous system and in peripheral structures, such as tissues of the reproductive tract (both normal and tumoral). The specific biological functions of GnRH-II in humans are presently under investigation.

Regulation of gene promoters of hypothalamic peptides

In order to fulfill their roles in neuroendocrine regulation, specific hypothalamic neurons are devoted to produce and deliver biologically active peptides to the pituitary gland. The biosynthesis and release of peptides are strictly controlled by afferents to these hypothalamic neurons. Cell-specific expression and biosynthetic regulation largely relies on transcription from the gene promoter for which the 5(')-flanking regions of the peptidergic genes contain essential elements. Cell-specific transcription factors employ these regulatory elements to exert their control over the expression of the peptidergic gene. This article explores the properties of regulatory elements of the major hypothalamic peptides, somatostatin, growth hormone-releasing hormone, gonadotropin-releasing hormone, thyrotropin-releasing hormone, corticotropin-releasing hormone, vasopressin and oxytocin, and the transcription factors acting on them. These transcription factors are often endpoints of signal transduction pathways that can be activated by neurotransmitters or steroid hormones. Others are essential to provide cell-specific expression of the peptidergic gene during development and mature regulation.

Molecular and cellular properties of GnRH neurons revealed through transgenics in the mouse

Recent advances in the use of gonadotropin-releasing hormone (GnRH) promoter-driven transgenics in the mouse are beginning to open up the once elusive GnRH neuronal phenotype to detailed molecular and cellular investigation. This review highlights progress in the development of GnRH promoter transgenic constructs and the understanding of murine gene sequences required for the correct temporal and spatial targeting of transgenes to the GnRH phenotype in vivo. Strategies enabling the identification of single, living GnRH neurons in the acute brain slice preparation are allowing gene profiling and electrophysiological experiments to be undertaken. Results so far indicate that, like other neurons, GnRH cells express a variety of sodium, potassium and calcium channels as well as GABAergic and glutamatergic receptors which are responsible for determining the membrane properties and firing characteristics of the GnRH neuron. Many of these receptors and channels appear to be expressed heterogeneously within the GnRH phenotype. Furthermore, several display distinct postnatal developmental expression profiles which are likely to be of consequence to the development of synchronized, pulsatile GnRH secretion in the adult animal.

Transcriptional regulation of the human GnRH II gene is mediated by a putative cAMP response element

Human neuronal medulloblastoma cells (TE-671) were recently demonstrated to express the two forms of GnRH (GnRH-I and GnRH-II). We have used this cell line as a model system to demonstrate regulation of the human GnRH-II gene by cAMP. RT-PCR and Southern hybridization demonstrated that GnRH-II mRNA is strongly up-regulated ( approximately 6-fold) by (Bu)(2)cAMP. The concentration of GnRH-II that was released into the medium of TE-671 cells treated with the cAMP analog was significantly higher than that of the untreated cells. TE-671 cells that were stimulated by (Bu)(2)cAMP demonstrated morphological changes and strong immunoreactive GnRH-II staining in part of the cell population. After screening of the GnRH-II promoter sequence, we identified a putative cAMP response element consensus site. The GnRH-I and GnRH-II promoters were isolated by PCR using human genomic DNA and cloned into the luciferase reporter plasmid. By measuring the basal activity of the promoters that were transfected to TE-671 cells, we found a much stronger basal activity of the GnRH-II promoter compared with that of GnRH-I. Treatment of transfected TE-671 cells with (Bu)(2)cAMP resulted in a strong activation of the GnRH-II promoter compared with a modest activation of the GnRH-I promoter. To determine the functionality of this putative cAMP response element site, we mutated this site. TE-671 cells that were transfected with cAMP response element mutant constructs demonstrated a diminished basal activity of the GnRH-II promoter. Treatment of the transfected cells with the cAMP analog demonstrated a decrease to 0.03% of the activity of the mutated promoter compared with that of the wild type. These results clearly demonstrate the importance of the putative cAMP response element site for the basal activity as well as for induction of the GnRH-II promoter by cAMP.

A functional retinoic acid response element (RARE) is present within the distal promoter of the rat gonadotropin-releasing hormone (GnRH) gene

We previously demonstrated that all-trans-retinoic acid (all-trans-RA) regulates gonadotropin-releasing hormone (GnRH) release and gene expression in rat hypothalamic fragments and GT1-1 neuronal cells. Promoter analysis of rat GnRH gene revealed that the enhancing effect of all-trans-RA on GnRH transcription is mediated by cis-elements localized within --1640/--1438 of the rat GnRH promoter. In the present study, we attempted to localize functional retinoic acid response elements (RAREs) within the all-trans-RA-responsive region of the rat GnRH gene. Sequence analysis showed that there exist three putative repeats of AGGTCA-related sequences (--1637/--1617, --1579/--1562, and --1494/--1470) within this promoter sequence. Among them, only the --1494/--1470 sequence could compete the specific binding of GT1-1 nuclear extracts to the consensus RARE (direct repeat of AGGTCA with a 5-bp spacer, DR-5) and vice versa in electrophoretic mobility shift assays. In addition, like consensus RARE, the --1494/--1470 sequence could confer all-trans-RA responsiveness when inserted into the upstream region of SV40 promoter. Treatment of GT1-1 cells with all-trans- or 9-cis-RA increased the specific bindings of GT1-1 nuclear extracts to the consensus RARE and to the --1494/--1470 sequence while not affecting the specific binding to the cAMP response element (CRE). Both retinoids induced RARbeta gene expression in GT1-1 cells. The --1494/--1470 sequence (5'-TCTTAGGACTCTGTGTGACCTAAGA) is similar to the direct repeat of TGACCT (complementary sequence of AGGTCA) with a spacer of 5 bp (i.e. DR-5 in the reverse orientation). A mutation of the second core recognition motif of the --1494/--1470 sequence to a more divergent one from consensus RARE (from TGACCT to TTACAT) abolished the responsiveness to all-trans-RA, whereas a mutation of first core recognition motif to a more TGACCT-like sequence (from AGGACT to TGAACT) increased the responsiveness to all-trans-RA. These results indicate that the --1494/--1470 sequence is indeed a weak but functional RARE of the modified DR-5 type. Taken together, these data indicate that all-trans-RA enhances GnRH transcription via functional RARE present in the distal region of the GnRH promoter.

9-cis-Retinoic acid represses transcription of the gonadotropin-releasing hormone (GnRH) gene via proximal promoter region that is distinct from all-trans-retinoic acid response element

We previously reported an enhancing effect of all-trans-retinoic acid (all-trans-RA) on gonadotropin-releasing hormone (GnRH) gene transcription via distal promoter elements of the rat GnRH gene. The present study examined the effects of another biologically active retinoid, 9-cis-retinoic acid (9-cis-RA), on GnRH transcription in GT1-1 cells. Similar to the action of all-trans-RA, 9-cis-RA significantly induced the luciferase activity of the strong retinoic acid response element (RARE) reporter construct, 3X beta RARE-Luc, by about 60-fold, indicating that GT1-1 cells are also responsive to 9-cis-RA. In contrast to the stimulatory effect of all-trans-RA on GnRH transcription, 9-cis-RA inhibited the GnRH promoter activity in a dose- and time-dependent manner. Significant inhibition by 9-cis-RA required at least an 18 h treatment and a further decrease of GnRH promoter-driven luciferase activity was observed up to 48 h of incubation. Accordingly, GnRH mRNA levels were decreased by 9-cis-RA treatment in a similar dose- and time-related manner, indicating that mouse GnRH expression is also negatively regulated by 9-cis-RA. Transient transfections of serial deletion constructs of the rat GnRH promoter revealed that the --230/--110 sequence of the rat GnRH promoter is responsible for 9-cis-RA-induced inhibition of GnRH transcription. Within this region, however, no consensus retinoid X receptor response element was found. To gain insights into the role of retinoid X receptors (RXRs) in GnRH expression, we examined the effects of RXR overexpression on GnRH transcriptional activity. Interestingly, co-transfection of RXR overexpression vectors significantly increased the GnRH promoter-driven luciferase activity, while treatment with 9-cis-RA not only nullified the enhancing effect of RXR overexpression but also decreased the basal GnRH promoter-driven luciferase activity by 50% compared to vehicle-treated controls. This implies that RXRs in the absence of its cognate ligand 9-cis-RA contribute to the maintenance of basal GnRH gene transcription. Northern blot analysis revealed that 9-cis-RA, but not all-trans-RA, down-regulated RXR beta expression in GT1-1 cells, suggesting that one possible mechanism of 9-cis-RA-induced repression involves down-regulation of RXR expression. In conclusion, the present study clearly demonstrates that 9-cis-RA is a negative regulator of GnRH gene expression in immortalized GnRH neurons.

Two isoforms of gonadotropin-releasing hormone are coexpressed in neuronal cell lines

GnRH-I serves as the neuropeptide that regulates mammalian reproduction. Recently, several groups have identified in the brain of rodents, monkeys, and humans a second isoform of GnRH (GnRH-II) whose structure is 70% identical to that of GnRH-I. In this study we demonstrate for the first time human and mouse neuronal cell lines that express both GnRH-I and GnRH-II. Following the screening of several human neuronal cell lines by RT-PCR and Southern hybridization, we demonstrated that two cell lines, TE-671 medulloblastoma and LAN-1 neuroblastoma cells, coexpress messenger RNA encoding the two isoforms of GnRH. Nucleotide sequencing indicated that the complementary DNA fragments are identical to those of the known human GnRH-I and GnRH-II sequences. Extracts obtained from the TE-671 and LAN-1 cell lines as well as from the immortalized mouse hypothalamic GT1-7 neuronal cell line were found to contain the two isoforms of GnRH, which exhibited identical chromatographic properties as synthetic GnRH-I and GnRH-II, in HPLC followed by specific RIAs. Furthermore, double immunofluorescence studies demonstrated the two GnRH isoforms in LAN-1, TE-671, and GT1-7 cells. The identification of neuronal cell lines expressing both GnRH-I and GnRH-II provides tools for studying the differential regulation of gene expression and secretion and for studying the interaction between the two isoforms. Such studies may contribute to elucidation of the physiological functions of GnRH-II, which are still unknown.

The negative feedback action of progesterone on luteinizing hormone release is not associated with changes in GnRH mRNA expression in the Ewe

Progesterone is the ovarian hormone that times events in the ovine reproductive cycle. When elevated, this ovarian hormone acts centrally to inhibit both the tonic and surge modes of gonadotrophin releasing hormone (GnRH) release. Two studies were performed to address the underlying neural mechanisms. The first tested the hypothesis that the rapid rise in GnRH release, that results from an acute fall in progesterone concentrations (such as occurs following luteolysis), is temporally associated with a rapid rise in the cellular content of GnRH mRNA. Three groups of ovariectomised (OVX) ewes were treated with exogenous progesterone for 10 days, while one remained steroid free (OVX, n=7). To determine the effects of acute progesterone (P) withdrawal, ewes were killed on day 10 while implants were still in place (OVX+P, n=6) or 4 (OVX-P4, n=7) or 12 h (OVX-P12, n=7) after progesterone removal. Coronal sections through the rostral portion of the medial preoptic area (rPOA) were processed for cellular in-situ hybridization for GnRH mRNA. An increase in progesterone concentrations markedly suppressed luteinizing hormone (LH) release, while removal of the implants caused progesterone concentrations to fall (P<0.01) within 1 h and LH pulse frequency to increase (P<0.05) within 4 h. Despite these progesterone-induced changes in LH/GnRH release there were no differences in the cellular content of GnRH mRNA among the four groups. In the second study, three groups of ovariectomised ewes were used to determined whether the inhibitory actions of early (EL; n=8) and mid-luteal (ML; n=8) phase concentrations of progesterone on LH release are accompanied by a decrease in GnRH mRNA expression. P inhibited the secretion of LH in a dose dependant manner; pulses of LH were virtually absent in the ML group. Despite this marked inhibitory steroid action, there was no significant difference in the cellular content of GnRH mRNA among the OVX, OVX (EL) and OVX (ML) groups. Thus, both the negative feedback actions of physiological concentrations of progesterone on GnRH release and the rapid escape from progesterone-inhibition are independent of changes in the cellular content of GnRH mRNA. These data suggest that the mechanism by which progesterone controls the timing of events in the ovine oestrous cycle is primarily by altering the secretion of GnRH rather than GnRH biosynthesis.

SF-1 (steroidogenic factor-1) and C/EBP beta (CCAAT/enhancer binding protein-beta) cooperate to regulate the murine StAR (steroidogenic acute regulatory) promoter

The steroidogenic acute regulatory (StAR) protein mediates the rate-limiting step of steroidogenesis, which is the transfer of cholesterol to the inner mitochondrial membrane. In steroidogenic tissues, StAR expression is acutely regulated by trophic hormones through a cAMP second messenger pathway, leading to increased StAR mRNA levels within 30 min, reaching maximal levels after 4-6 h of stimulation. The molecular mechanisms underlying such regulation remain unknown. We have examined the StAR promoter for putative transcription factor-binding sites that may regulate transcription in a developmental and/or hormone-induced context. Through sequence analysis, deoxyribonuclease I (DNAse I) footprinting and electrophoretic mobility shift assays (EMSAs), we have identified two putative CCAAT/enhancer binding protein (C/EBP) DNA elements at -113 (C1) and -87 (C2) in the mouse StAR promoter. Characterization of these sites by EMSA indicated that C/EBP beta bound with high affinity to C1 and C2 was a low-affinity C/EBP site. Functional analysis of these sites in the murine StAR promoter showed that mutation of one or both of these binding sites decreases both basal and (Bu)2cAMP-stimulated StAR promoter activity in MA-10 Leydig tumor cells, without affecting the fold activation [(Bu)2cAMP-stimulated/basal] of the promoter. Furthermore, we have demonstrated that these two C/EBP binding sites are required for steroidogenic factor-1 (SF-1)-dependent transactivation of the StAR promoter in a nonsteroidogenic cell line. These data indicate that in addition to SF-1, C/EBP beta is involved in the transcriptional regulation of the StAR gene and may play an important role in developmental and hormone-responsive regulation of steroidogenesis.