Multiple elements in the distal part of the 1.2 kb 5'-flanking region of the rat GnRH receptor gene regulate gonadotrope-specific expression conferred by proximal domain

Intrinsic and Regulated Gonadotropin-Releasing Hormone Receptor Gene Transcription in Mammalian Pituitary Gonadotrophs

The hypothalamic decapeptide gonadotropin-releasing hormone (GnRH), acting via its receptors (GnRHRs) expressed in pituitary gonadotrophs, represents a critical molecule in control of reproductive functions in all vertebrate species. GnRH-activated receptors regulate synthesis of gonadotropins in a frequency-dependent manner. The number of GnRHRs on the plasma membrane determines the responsiveness of gonadotrophs to GnRH and varies in relation to age, sex, and physiological status. This is achieved by a complex control that operates at transcriptional, translational, and posttranslational levels. This review aims to overview the mechanisms of GnRHR gene (Gnrhr) transcription in mammalian gonadotrophs. In general, Gnrhr exhibits basal and regulated transcription activities. Basal Gnrhr transcription appears to be an intrinsic property of native and immortalized gonadotrophs that secures the presence of a sufficient number GnRHRs to preserve their functionality independently of the status of regulated transcription. On the other hand, regulated transcription modulates GnRHR expression during development, reproductive cycle, and aging. GnRH is crucial for regulated Gnrhr transcription in native gonadotrophs but is ineffective in immortalized gonadotrophs. In rat and mouse, both basal and GnRH-induced Gnrhr transcription rely primarily on the protein kinase C signaling pathway, with subsequent activation of mitogen-activated protein kinases. Continuous GnRH application, after a transient stimulation, shuts off regulated but not basal transcription, suggesting that different branches of this signaling pathway control transcription. Pituitary adenylate cyclase-activating polypeptide, but not activins, contributes to the regulated transcription utilizing the protein kinase A signaling pathway, whereas a mechanisms by which steroid hormones modulate Gnrhr transcription has not been well characterized.

Divergent activity of the gonadotropin-releasing hormone receptor gene promoter among genetic lines of pigs is partially conferred by nuclear factor (NF)-B, specificity protein (SP)1-like and GATA-4 binding sites

BACKGROUND

Binding of gonadotropin-releasing hormone (GnRH) to its receptor (GnRHR) on gonadotropes within the anterior pituitary gland is essential to reproduction. In pigs, the GnRHR gene is also located near a genetic marker for ovulation rate, a primary determinant of prolificacy. We hypothesized that pituitary expression of the GnRHR gene is alternatively regulated in genetic strains with elevated ovulation rates (Chinese Meishan and Nebraska Index) vs. standard white crossbred swine (Control).

METHODS

Luciferase reporter vectors containing 5118 bp of GnRHR gene promoter from either the Control, Index or Meishan swine lines were generated. Transient transfection of line-specific, full length, deletion and mutation constructs into gonadotrope-derived αT3-1 cells were performed to compare promoter activity and identify regions necessary for divergent regulation of the porcine GnRHR gene. Additionally, transcription factors that bind the GnRHR promoter from each line were identified with electrophoretic mobility shift assays (EMSA).

RESULTS

Dramatic differences in luciferase activity among Control, Index and Meishan promoters (19-, 27- and 49-fold over promoterless control, respectively; P < 0.05) were established. A single bp substitution (-1690) within a previously identified upstream enhancer (-1779/-1667) bound GATA-4 in the Meishan promoter and the p52/p65 subunits of nuclear factor (NF)-κB in the homologous Control/Index promoters. Transient transfection of vectors containing block replacement mutations of either the GATA-4 or NF-κB binding sites within the context of their native promoters resulted in a 50 and 60 % reduction of luciferase activity, respectively (P < 0.05). Furthermore, two single-bp substitutions in the Meishan compared to Control/Index promoters resulted in binding of the p52 and p65 subunits of NF-κB and a specificity protein 1 (SP1)-like factor (-1235) as well as GATA-4 (-845). Vectors containing the full-length Meishan promoter harboring individual mutations spanning these regions reduced luciferase activity by 25 and 20 %, respectively, compared to native sequence (P < 0.05).

CONCLUSIONS

Elevated activity of the Meishan GnRHR gene promoter over Control/Index promoters in αT3-1 cells is partially due to three single nucleotide polymorphisms resulting in the unique binding of GATA-4 (-1690), the p52/p65 subunits of NF-kB in combination with a SP1-like factor (-1235), and GATA-4 (-845).

Inside and outside the pituitary: comparative analysis of Gnrhr expression provides insight into the mechanisms underlying the evolution of gene expression

DNA cis-acting elements involved in gene regulation may actively contribute to adaptation processes because they are submitted to lower evolutionary constraints than coding DNA. In this regard, comparisons of the mechanisms underlying basal and regulated Gnrhr expression have revealed some features that promote stable and consistent Gnrhr expression in pituitary gonadotroph cells in different species. The presence of two divergent SF1 (NR5A1) response elements in all analysed mammalian Gnrhr promoters probably comprises one of the features that ensures reliable expression in the pituitary. By contrast, in other tissues, such as the hippocampus and testis, our analyses revealed dissimilar levels of Gnrhr expression among species. Indeed, Gnrhr was consistently expressed after birth in the rat but not the mouse hippocampus. Similar discrepancies were observed in foetal and adult testes. The ability of the rat promoter to drive reporter gene expression in the hippocampus and testis of transgenic mice just as it naturally directs the expression of the endogenous Gnrhr in rats strongly suggests that regulatory DNA sequences contained species-specific instructions prevailing over other controls. The major conclusion emerging from these studies is that Gnrhr promoter sequences are mainly responsible for directing transcriptional programmes and play a predominant role over the species-specific cell environment.

Rat Gnrhr promoter directs species-specific gene expression in the pituitary and testes of transgenic mice

The GnRH receptor (GnRHR) is expressed in several non-pituitary tissues, notably in gonads. However, mechanisms underlying the gonad-specific expression of Gnrhr are not well understood. Here, Gnrhr expression was analysed in the developing testes and pituitaries of rats and transgenic mice bearing the human placental alkaline phosphatase reporter gene (ALPP) under the control of the rat Gnrhr promoter. We showed that the 3.3 kb, but not the pituitary-specific 1.1 kb promoter, directs ALPP expression exclusively to testis Leydig cells from embryonic day 12 onwards. Real-time PCR analysis revealed that promoter activity displayed the same biphasic profile as marker genes in Leydig cells, i.e. abrupt declines after birth followed by progressive rises after a latency phase, in coherence with the differentiation and evolution of foetal and adult Leydig cell lineages. Interestingly, the developmental profile of transgene expression showed high similarity with the endogenous Gnrhr profile in the rat testis, while mouse Gnrhr was only poorly expressed in the mouse testis. In the pituitary, both transgene and Gnrhr were co-expressed at measurable levels with similar ontogenetic profiles, which were markedly distinct from those in the testis. Castration that induced pituitary Gnrhr up-regulation in rats did not affect the mouse Gnrhr. However, it duly up-regulated the transgene. In addition, in LβT2 cells, the rat, but not mouse, Gnrhr promoter was sensitive to GnRH agonist stimulation. Collectively, our data highlight inter-species variations in the expression and regulation of Gnrhr in two different organs and reveal that the rat promoter sequence contains relevant genetic information that dictates rat-specific gene expression in the mouse context.

GATA2-induced silencing and LIM-homeodomain protein-induced activation are mediated by a bi-functional response element in the rat GnRH receptor gene

GATA2 transcription factor and LIM homeodomain proteins Islet1 (ISL1) and LIM homeobox 3 (LHX3) are suspected to be involved in gonadotrope cell fate and maintenance. The GnRH receptor gene (Gnrhr), crucial for gonadotrope function, is expressed in the pituitary gland from embryonic day 13.5 onward, well before LH and FSH β-subunits. This expression pattern together with the presence of WGATAR and TAAT motifs in Gnrhr promoter sequences suggests the involvement of early transcription factors in promoter activation. In this study, using a well-characterized transgenic mouse model, GATA2 was found colocalized with Gnrhr promoter activity in the pituitary. Transient transfection of Gnrhr promoter luciferase fusion constructs together with either GATA2 expression vectors or small interfering RNA in gonadotrope cell lines indicated that GATA2, which typically acts as a trans-activator, unexpectedly repressed Gnrhr promoter activity. Using DNA chromatography affinity and EMSA, we demonstrated that GATA2 operates via a response element containing a peculiar palindromic GATA motif that overlaps a critical TAAT motif involved in LHX3/ISL1 trans-activation. Indeed, despite the inhibitory action of GATA2, this element displayed a clear-cut enhancer activity in gonadotrope cells. Chromatin immunoprecipitation assays indicated that GATA2, LHX3, and ISL1 interact with a Gnrhr promoter fragment encompassing this element. The trans-repressive action of GATA2 on Gnrhr promoter activity is likely balanced or even hindered by trans-activating effects of LIM homeodomain proteins via this novel bifunctional LIM/GATA response element. Such a hierarchical interplay may contribute to finely adjust Gnrhr gene expression in gonadotrope cell lineage during pituitary development as well as in the adult animal.

Mechanisms underlying the tissue-specific and regulated activity of the Gnrhr promoter in mammals

The GnRH receptor (GnRHR) plays a central role in the development and maintenance of reproductive function in mammals. Following stimulation by GnRH originating from the hypothalamus, GnRHR triggers multiple signaling events that ultimately stimulate the synthesis and the periodic release of the gonadotropins, luteinizing-stimulating hormone (LH) and follicle-stimulating hormones (FSH) which, in turn, regulate gonadal functions including steroidogenesis and gametogenesis. The concentration of GnRHR at the cell surface is essential for the amplitude and the specificity of gonadotrope responsiveness. The number of GnRHR is submitted to strong regulatory control during pituitary development, estrous cycle, pregnancy, lactation, or after gonadectomy. These modulations take place, at least in part, at the transcriptional level. To analyze this facet of the reproductive function, the 5' regulatory sequences of the gene encoding the GnRHR have been isolated and characterized through in vitro and in vivo approaches. This review summarizes results obtained with the mouse, rat, human, and ovine promoters either by transient transfection assays or by means of transgenic mice.

The LIM-homeodomain proteins Isl-1 and Lhx3 act with steroidogenic factor 1 to enhance gonadotrope-specific activity of the gonadotropin-releasing hormone receptor gene promoter

The GnRH receptor (GnRH-R) plays a central role in mammalian reproductive function throughout adulthood. It also appears as an early marker gene of the presumptive gonadotrope lineage in developing pituitary. Here, using transient transfections combined with DNA/protein interaction assays, we have delineated cis-acting elements within the rat GnRH-R gene promoter that represent targets for the LIM-homeodomain (LIM-HD) proteins, Isl-1 and Lhx3. These factors, critical in early pituitary development, are thus also crucial for gonadotrope-specific expression of the GnRH-R gene. In heterologous cells, the expression of Isl-1 and Lhx3, together with steroidogenic factor 1 (SF-1), culminates in the activation of both the rat as well as human GnRH-R promoter, suggesting that this combination is evolutionarily conserved among mammals. The specificity of these LIM-HD factors is attested by the inefficiency of related proteins, including Lhx5 and Lhx9, to activate the GnRH-R gene promoter, as well as by the repressive capacity of a dominant-negative derivative of Lhx3. Accordingly, targeted deletion of the LIM response element decreases promoter activity. In addition, experiments with Gal4-SF-1 fusion proteins suggest that LIM-HD protein activity in gonadotrope cells is dependent upon SF-1 binding. Finally, using a transgenic model that allows monitoring of in vivo promoter activity, we show that the overlapping expression of Isl-1 and Lhx3 in the developing pituitary correlates with promoter activity. Collectively, these data suggest the occurrence of a specific LIM-HD pituitary code and designate the GnRH-R gene as the first identified transcriptional target of Isl-1 in the anterior pituitary.

Regulation of expression of mammalian gonadotrophin-releasing hormone receptor genes

Gonadotrophin-releasing hormone (GnRH), acting via its cognate GnRH receptor (GnRHR), is the primary regulator of mammalian reproductive function, and hence GnRH analogues are extensively used in the treatment of hormone-dependent diseases, as well as for assisted reproductive techniques. In addition to its established endocrine role in gonadotrophin regulation in the pituitary, evidence is rapidly accumulating to support the expression and functional roles for two forms of GnRHR (GnRHR I and GnRHR II) in multiple and diverse extra-pituitary mammalian tissues and cells. These findings, together with findings indicating that mutations of the GnRHR are linked to the disease hypogonadotrophic hypogonadism and that GnRHRs play a direct role in neuronal migration and reproductive cancers, have presented new therapeutic targets and intensified research into the structure, function and mechanisms of regulation of expression of GnRHR genes. The present review focuses on the current knowledge on tissue-specific and hormonal regulation of transcription of mammalian GnRH receptor genes. Emerging insights, such as the discovery of diverse regulatory mechanisms in pituitary and extra-pituitary cell types, nonclassical mechanisms of steroid regulation, the use of composite elements for cell-specific expression, the increasing profile of hormones involved in regulation, the complexity of kinase pathways that target the GnRHR I gene, as well as species-differences, are highlighted. Although further research is necessary to understand the mechanisms of regulation of expression of GnRHR I and GnRHR II genes, the GnRHR is emerging as a potential target gene for facilitating cross-talk between neuroendocrine, immune and stress-response systems in multiple tissues via autocrine, paracrine and endocrine signalling.

The promoter of the rat gonadotropin-releasing hormone receptor gene directs the expression of the human placental alkaline phosphatase reporter gene in gonadotrope cells in the anterior pituitary gland as well as in multiple extrapituitary tissues

Previous studies dealing with the mechanisms underlying the tissue-specific and regulated expression of the GnRH receptor (GnRH-R) gene led us to define several cis-acting regulatory sequences in the rat GnRH-R gene promoter. These include functional sites for steroidogenic factor 1, activator protein 1, and motifs related to GATA and LIM homeodomain response elements as demonstrated primarily in transient transfection assays in mouse gonadotrope-derived cell lines. To understand these mechanisms in more depth, we generated transgenic mice bearing the 3.3-kb rat GnRH-R promoter linked to the human placental alkaline phosphatase reporter gene. Here we show that the rat GnRH-R promoter drives the expression of the reporter gene in pituitary cells expressing the LHbeta and/or FSHbeta subunit but not in TSHbeta- or GH-positive cells. Furthermore, the spatial and temporal pattern of the transgene expression during the development of the pituitary was compatible with that characterizing the emergence of the gonadotrope lineage. In particular, transgene expression is colocalized with the expression of the glycoprotein hormone alpha-subunit at embryonic day 13.5 and with that of steroidogenic factor 1 at later stages of pituitary development. Transgene expression was also found in specific brain areas, such as the lateral septum and the hippocampus. A single promoter is thus capable of directing transcription in highly diverse tissues, raising the question of the different combinations of transcription factors that lead to such a multiple, but nevertheless cell-specific, expressions of the GnRH-R gene.

Gonadotropin-releasing hormone receptor: cloning, expression and transcriptional regulation

In summary, isolation of GnRH receptor cDNA, its gene, and identification of regulatory elements in the flanking region of the gene have added to our knowledge regarding the tissue-specific expression of the GnRH receptor gene, and the mechanisms that mediate and influence its transcriptional regulation. However, the interactions of the different regulatory factors (nuclear factors) and the effects of these interactions on the regulation of the GnRH receptor gene remain unclear. Due to existence of multiple promoters and transcriptional start sites in human GnRH receptor gene and the lack of a human gonadotrope cell line, the precise promoter and transcriptional start sites in human pituitary, extra-pituitary tissues and tumors have not yet been identified.

Two messenger RNA isoforms of the gonadotrophin-releasing hormone receptor, generated by alternative splicing and/or promoter usage, are differentially expressed in rainbow trout gonads during gametogenesis

The recent cloning of a gonadotrophin-releasing hormone receptor (GnRH-R) cDNA from rainbow trout showed that it contains several in-frame ATG codons, one of which, ATG2, corresponds to that found in other species. However, an upstream codon, ATG1, could give rise to a protein with a larger extracellular domain. Using S1 nuclease assay and a method combining primer extension and RACE-PCR, we characterized a second population of mRNA, termed mRNA-2, with a distinct 5'untranslated region and lacking ATG1. The genomic origin of the two mRNAs was determined by establishing the complete gene structure, which shows, for the first time in a vertebrate species that an alternative splicing and promoter usage generate two GnRH-R mRNA variants whose 5' extremities are encoded by two different exons. The analysis of the tissue distribution indicated that mRNA-2 presents a broader pattern of expression and is detected at higher levels than mRNA-1. Interestingly, it was found that those two mRNAs are differentially expressed in male and female gonads during gametogenesis. In particular, the variations of mRNA-1 levels parallel those of sGnRH expression during spermatogenesis, indicating that tissue-specific processing of the GnRH-R mRNA may underlie the effects of GnRH as a paracrine/autocrine regulator of gonadal functions.

Pituitary adenylate cyclase-activating polypeptide and cyclic adenosine 3',5'-monophosphate stimulate the promoter activity of the rat gonadotropin-releasing hormone receptor gene via a bipartite response element in gonadotrope-derived cells

Specific type I receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are present in gonadotrope cells of the anterior pituitary gland. By transient transfection of mouse gonadotrope-derived alphaT3-1 cells, which are direct targets for PACAP and express gonadotropin-releasing hormone receptor (GnRH-R), a marker of the gonadotrope lineage, we provide the first evidence that PACAP stimulates rat GnRH-R gene promoter activity. The EC(50) of this stimulation is compatible with a mediation via activation of the cyclic AMP-dependent signaling pathway and, consistently, co-transfection of an expression vector expressing the protein kinase A inhibitor causes reduction in PACAP as well as cholera toxin-stimulated promoter activity. Deletion and mutational analyses indicate that PACAP activation necessitates a bipartite response element that consists of a first region (-272/-237) termed PACAP response element (PARE) I that includes a steroidogenic factor-1 (SF-1)-binding site and a second region (-136/-101) referred to as PARE II that contains an imperfect cyclic AMP response element. Gel shift experiments indicate the specific binding of the SF-1 and a potential SF-1-interacting factor to PARE I while a protein immunologically related to the cyclic AMP response element-binding protein interacts with PARE II. These findings suggest that PACAP might regulate the GnRH-R gene at the transcriptional level, providing novel insights into the regulation of pituitary-specific genes by hypothalamic hypophysiotropic signals.

Proximal cis-acting elements, including steroidogenic factor 1, mediate the efficiency of a distal enhancer in the promoter of the rat gonadotropin-releasing hormone receptor gene

The gonadotrope-specific and regulated expression of the GnRH receptor (GnRH-R) gene is dependent on multiple transcription factors that interact with the noncanonical GnRH-R activating sequence (GRAS), the activator protein-1 (AP-1) element, and the steroidogenic factor-1 (SF-1) binding site. However, these three elements are not sufficient to mediate the complete cell-specific expression of the rat GnRH-R gene. In the present study, we demonstrate, by transient transfection in gonadotrope-derived alphaT3-1 and LssT2 cell lines, the existence of a distal enhancer [GnRH-R- specific enhancer (GnSE)] that is highly active in the context of the GnRH-R gene promoter. We show that the GnSE activity (-1,135/-753) is mediated through a functional interaction with a proximal region (-275/-226) that includes the SF-1 response element. Regions of similar length containing either the AP-1 or GRAS elements are less active or inactive. Transfection assays using an artificial promoter containing two SF-1 elements fused to a minimal PRL promoter indicate that SF-1 is crucial in this interaction. In addition, by altering the promoter with deletion and block- replacement mutations, we have identified the active elements of GnSE within two distinct sequences at positions -983/-962 and -871/-862. Sequence analysis and electrophoretic mobility shift experiments suggest that GnSE response elements interact, in these two regions, with GATA- and LIM-related factors, respectively. Altogether, these data establish the importance of the GnSE in the GnRH-R gene expression and reveal a novel role for SF-1 as a mediator of enhancer activity, a mechanism that might regulate other SF-1 target genes.

The expression, regulation and signal transduction pathways of the mammalian gonadotropin-releasing hormone receptor

Normal mammalian sexual maturation and reproductive functions require the integration and precise coordination of hormones at the hypothalamic, pituitary, and gonadal levels. Hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator in this system; after binding to its receptor (GnRHR), it stimulates de novo synthesis and release of gonadotropins in anterior pituitary gonadotropes. Since the isolation of the GnRHR cDNA, the expression of GnRHR mRNA has been detected not only in the pituitary, but also in extrapituitary tissues, including the ovary and placenta. It has been shown that change in GnRHR mRNA is one of the mechanisms for regulating the expression of the GnRHR. To help understand the molecular mechanism(s) involved in transcriptional regulation of the GnRHR gene, the 5' flanking region of the GnRHR gene has recently been isolated. Initial characterization studies have identified several DNA regions in the GnRHR 5' flanking region which are responsible for both basal expression and GnRH-mediated homologous regulation of this gene in pituitary cells. The mammalian GnRHR lacks a C-terminus and possesses a relatively short third intracellular loop; both features are important in desensitization of many others G-protein coupled receptors (GPCRs), Homologous desensitization of GnRHR has been shown to be regulated by various serine-threonine protein kinases including protein kinase A (PKA) and protein kinase C (PKC), as well as by G-protein coupled receptor kinases (GRKs). Furthermore, GnRHR was demonstrated to couple with multiple G proteins (Gq/11, Gs, and Gi), and to activate cascades that involved the PKC, PKA, and mitogen-activator protein kinases. These results suggest the diversity of GnRHR-G protein coupling and signal transduction systems. The identification of second form of GnRH (GnRH-II) in mammals adds to the complexity of the GnRH-GnRHR system. This review summaries our recent progress in understanding the regulation of GnRHR gene expression and the GnRHR signal transduction pathways.Key words: gonadotropin-releasing hormone receptor, transcriptional regulation, desensitization, signal transduction.