Rat gonadotropin-releasing hormone receptor expressed in insect cells induces activation of adenylyl cyclase

Structure prediction of the second extracellular loop in G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) play key roles in living organisms. Therefore, it is important to determine their functional structures. The second extracellular loop (ECL2) is a functionally important region of GPCRs, which poses significant challenge for computational structure prediction methods. In this work, we evaluated CABS, a well-established protein modeling tool for predicting ECL2 structure in 13 GPCRs. The ECL2s (with between 13 and 34 residues) are predicted in an environment of other extracellular loops being fully flexible and the transmembrane domain fixed in its x-ray conformation. The modeling procedure used theoretical predictions of ECL2 secondary structure and experimental constraints on disulfide bridges. Our approach yielded ensembles of low-energy conformers and the most populated conformers that contained models close to the available x-ray structures. The level of similarity between the predicted models and x-ray structures is comparable to that of other state-of-the-art computational methods. Our results extend other studies by including newly crystallized GPCRs.

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.

Gonadotropin-releasing hormone modulates voltage-activated sodium current and odor responses in Necturus maculosus olfactory sensory neurons

The terminal nerve (nervus terminalis) extends from the basal forebrain to the nasal cavity and has been shown to contain gonadotropin-releasing hormone (GnRH). The specific function of the terminal nerve is unknown, but it has been hypothesized that it modulates the function of olfactory neurons. To examine the effects of GnRH on isolated Necturus maculosus olfactory sensory neurons (OSNs), we used the perforated configuration of the patch clamp technique to record current responses. GnRH had no effect on the membrane current at any holding potential but did modulate voltage-activated TTX-sensitive sodium current (INa). Within 1 min of applying GnRH, approximately 60% of the OSNs showed a decrease in the magnitude of INa. Initial responses to GnRH were inhibitory, although in one group of cells the initial inhibitory response was followed by a potentiation of INa with continual application (approximately 5 min). The time course of the GnRH response suggested that a second messenger pathway mediated the response. Inhibitors of PKC, tyrosine kinase, and PI3K were all able to inhibit the INa, but none of them could prevent the GnRH response. Application of a cAMP analog mimicked the effects of GnRH, and only inhibitors of PKA and PKG could prevent GnRH-induced inhibition of INa. This suggests that the modulation of voltage-activated sodium currents by GnRH involve a cyclic nucleotide pathway. In addition, GnRH modulated the odor responses of OSNs. Our data suggest the release of GnRH, presumably from the terminal nerve, can serve to modulate olfactory sensory neurons.

Structure of the GnRH receptor-stimulated signaling network: insights from genomics

The GnRH receptor influences gene expression in the gonadotrope through activating signaling cascades that modulate transcription factor expression and activity. A longstanding question in neuroendocrinology is how instructions received at the membrane in the form of the pattern of receptor stimulation are processed into specific biosynthetic changes at each gonadotropin promoter. Signal transduction from the membrane to preformed transcription factors relies on recognition of altered conformations. Signal transduction through the layers of the gene network also requires the biosynthesis of new transcription factors. The signal processing of this system depends on its molecular connectivity map and its feedback and feed-forward loops. Review of signal transduction, gene control, and genomic studies provide evidence of key loops that cross between cellular and nuclear compartments. Genomic studies suggest that the signal transduction and gene network form a continuum. We propose that information transfer in the gonadotrope depends on robust signaling modules that serve to integrate events at different time scales across cytoplasmic and nuclear compartments.

G protein-coupled receptor overexpression with the baculovirus-insect cell system: a tool for structural and functional studies

G protein-coupled receptors, whose topology shows seven transmembrane domains, form the largest known family of receptors involved in higher organism signal transduction. These receptors are generally of low natural abundance and overexpression is usually a prerequisite to their structural or functional characterisation. The baculovirus-insect cell system constitutes a versatile tool for the maximal production of receptors. This heterologous expression system also provides interesting alternatives for receptor functional studies in a well-controlled cellular context.

Intracellular signaling pathways mediated by the gonadotropin-releasing hormone (GnRH) receptor

The hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator of the reproductive system, triggering the synthesis and release of LH and FSH in the pituitary. GnRH transmits its signal via two specific serpentine receptors that belong to the large group of G-protein coupled receptors (GPCRs). Here we review the intracellular signaling pathways mediated by the GnRH receptor (GnRHR). In pituitary-derived alpha T3-1 cells, a widely used model for GnRH action, GnRHR signaling includes activation of mitogen-activated protein kinase (MAPK) cascades, which provide an important link for the transmission of signals from the cell surface to the nucleus and play a role in the regulation of gonadotropin transcription. Activation of ERK--one of the MAPK cascades--by GnRH in these cells depends mainly on the phosphorylation of Raf1 by PKC, supported by a pathway involving c-Src, dynamin, and Ras. On the other hand, the activation of JNK, another MAPK cascade, involves PKC, c-Src, CDC42/Rac1, and probably MEKK1. The GnRHR is also expressed in non-pituitary cells and was found to be involved in the inhibition of cell proliferation in certain cells. Therefore, GnRHR represents a potential target for GnRH-analogs used for cancer treatment. Interestingly, the signaling mechanism of the GnRHR in other cell types significantly differs from that in pituitary cells. Studies conducted in GnRHR-expressing COS7 cells have shown that GnRHR transmits its signals mainly via Gi, EGF receptor, c-Src, and is not dependent on PKC. Understanding the signaling mechanisms elicited by GnRHR can shed light on the mechanism of action of GnRH in pituitary and extra-pituitary tissues.

Critical implication of transmembrane Phe310, possibly in conjunction with Trp279, in the rat gonadotropin-releasing hormone receptor activation

The GnRH-R belongs to the superfamily of heptahelical GPCRs. A three-dimensional model of GnRH binding to its receptor predicted that Trp3 was the most deeply buried residue, potentially allowing it to interact with both Trp279, a highly conserved residue in the TMH 6 of GPCRs, and Phe310, present essentially in TMH 7 of GnRH-Rs. Replacement of Phe310 with Leu, the most common positional residue in GPCRs, induced a slightly decreased Bmax (1.6-fold) and affinity (3.8-fold); in addition, IP production was completely abolished. Similarly, replacement of Trp279 with Ser depressed the Bmax by 5.2-fold, the affinity by 2.3-fold, and totally abrogated IP production. The effect of the double mutation was not additive on binding, since the Bmax was reduced to the level of the Phe310Leu mutant, although the Kd was restored to a value not significantly different from that of the wild-type. The double mutant was also unable to induce IP production. Unexpectedly, no influence of any single or double substitution was noted on receptor internalization. These data provide evidence for the crucial role of Phe310, possibly in conjunction with Trp279, on GnRH transduction and suggest that the conformation for phospholipase C activation may not be required for GnRH-R internalization.

Human gonadotropin-releasing hormone receptor gene transcription: up-regulation by 3',5'-cyclic adenosine monophosphate/protein kinase A pathway

Transient transfection of mouse gonadotrope-derived (alphaT3-1) cells with a 2297 bp human GnRHR promoter-luciferase construct (p2300-LucF) showed a dose- and time-dependent increase in the human gonodotropin-releasing hormone receptor (GnRHR) promoter activity after forskolin treatment. An average of 4.8-fold increase in promoter activity was observed after 12 h of 10 microM forskolin treatment. This effect was mimicked by administration of cholera toxin, cAMP analog or pituitary adenylate cyclase activating polypeptide 38 (PACAP). A specific adenylate cyclase (AC) inhibitor (ACI) or protein kinase A (PKA) inhibitor (PKAI) pretreatment reversed the forskolin- and PACAP-induced increase in the human GnRHR promoter activity. These results not only confirm the stimulatory effect of Cyclic adenosine monophosphate (cAMP) in human GnRHR promoter activation, but also suggest that hormones or neurotransmitters that activate adenylate cyclase in pituitary gonadotropes may increase the expression of human GnRHR gene in transcriptional level. Progressive 5' deletion assays identified a 412 bp fragment (-577 to 167) in the human GnRHR 5'-flanking region that is essential in maintaining the basal responsiveness to cAMP. Mutagenesis coupled with functional studies have identified two putative AP-1/CREB binding sites, namely hGR-AP/CRE-1 and hGR-AP/CRE-2 that participated in mediating the cAMP-stimulatory effect. Mutation of the putative hGR-AP/CRE-1 and hGR-CRE-2 resulted in a 38 and 32% decrease in the forskolin-induced stimulation. However, mutation of both binding sites did not completely abolish the cAMP-stimulatory effect, suggesting that multiple transcription factor binding sites were involved in full response in cAMP stimulation. The binding of CREB to these motifs was confirmed by gel mobility shift assay and antibody supershift assay.

Recombinant perciform GnRH-R activates different signaling pathways in fish and mammalian heterologous cell lines

Perciforms have three forms of gonadotropin-releasing hormone (GnRH) in their brain. All three GnRHs are potent secretogogues for luteinizing hormone (LH) from the pituitary. The pivotal role of GnRH-R-GnRH interactions in reproductive homeostasis is well established; however, there is a paucity of information on how a GnRH-R responds to the three endogenous GnRH forms in a perciform species. In this study, a recombinant pituitary GnRH-R from striped bass (stb) was expressed in a mammalian cell line (COS-7) and a fish cell line (CHSE-214). Activation of the signaling pathways was monitored by reporter gene (luciferase) based assays, which were specific for cAMP-PKA or Ca 2+/calmodulin kinase (activated via c-fos promoter) signaling pathways. The stbGnRH-R expressed in two different cell lines triggered different downstream signaling in response to the treatments with chicken (c) GnRH II. Interestingly, when endogenous GnRHs were used in combinations, the luciferase activity was significantly attenuated in transfected CHSE-214 cells.

Stimulation of mitogen-activated protein kinase by gonadotropin-releasing hormone in human granulosa-luteal cells

The present study investigated the activation of mitogen-activated protein kinases (MAPKs) by a GnRH agonist (GnRHa) in human granulosa-luteal cells (hGLCs). The phosphorylation state of p44 and p42 MAPK was examined using antibodies that distinguish phospho-p44/42 MAPK (Thr(202)/Tyr(204)) from total p44/42 MAPK (activated plus inactivated). Activation of MAPK by GnRHa was observed within 5 min and was sustained for 60 min after treatment. GnRHa stimulated MAPK activation in a dose-dependent manner, with maximum stimulation (6.7-fold over basal levels) at 10(-7) M. Pretreatment with a protein kinase C (PKC) inhibitor, GF109203X, completely blocked GnRHa-induced MAPK activation. In addition, pretreatment with a PKC activator, phorbol-12-myristate 13-acetate, potentiated GnRH-induced MAPK activation. These results indicate that GnRHa stimulates MAPK activation through a PKC-dependent pathway in hGLCs, possibly coupled to G(q)alpha protein. MAPK activation was also observed in response to 8-bromo-cAMP or cholera toxin, but not pertussis toxin. Forskolin (50 microM) substantially stimulated a rapid cAMP accumulation, whereas GnRHa (10(-7) M) or pertussis toxin (100 mg/ml) did not affect basal intracellular cAMP levels. Cotreatment of GnRHa (10(-7) M) did not attenuate forskolin- or hCG-stimulated cAMP accumulation. These results suggest that the GnRH receptor is probably not coupled to G(s)alpha or G(i)alpha in hGLCs. Finally, GnRHa (10(-7) M) stimulated a significant increase in Elk-1 phosphorylation and c-fos messenger RNA expression, as revealed by an in vitro kinase assay and Northern blot analysis, respectively. These results clearly demonstrate that GnRH activates the MAPK cascade through a PKC-dependent pathway in the human ovary.

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.

GnRH stimulates LH release directly via inositol phosphate and indirectly via cAMP in African catfish

In African catfish, two gonadotropin-releasing hormone (GnRH) peptides have been identified: chicken GnRH (cGnRH)-II and catfish GnRH (cfGnRH). The GnRH receptors on pituitary cells producing gonadotropic hormone signal through inositol phosphate (IP) elevation followed by increases in intracellular calcium concentration (¿Ca(2+)(i)). In primary pituitary cell cultures of male African catfish, both cGnRH-II and cfGnRH dose dependently elevated IP accumulation, ¿Ca(2+)(i), and the release of the luteinizing hormone (LH)-like gonadotropin. In all cases, cGnRH-II was more potent than cfGnRH. The GnRH-stimulated LH release was not associated with elevated cAMP levels, and forskolin-induced cAMP elevation had no effect on LH release. With the use of pituitary tissue fragments, however, cAMP was elevated by GnRH, and forskolin was able to stimulate LH secretion. Incubating these fragments with antibodies against cfGnRH abolished the forskolin-induced LH release but did not compromise the forskolin-induced cAMP elevation. This suggests that cfGnRH-containing nerve terminals are present in pituitary tissue fragments and release cfGnRH via cAMP signaling on GnRH stimulation, whereas the GnRH receptors on gonadotrophs use IP/¿Ca(2+)(i) to stimulate the release of LH.

Activation of MAPK cascades by G-protein-coupled receptors: the case of gonadotropin-releasing hormone receptor

G-protein-coupled receptors (GPCRs) are a large group of integral membrane receptors that transmit signals from a diverse array of external stimuli, including neurotransmitters, hormones, phospholipids, photons, odorants and taste ligands. In response to ligand binding, the GPCRs initiate diverse downstream signaling pathways through four groups of G proteins and other interacting proteins. Key components in GPCR-induced intracellular signaling are four groups of mitogen-activated protein kinase (MAPK) cascades: extracellular signal-related kinase (ERK), Jun N-terminal kinase (JNK), p38MAPK and big MAPK (BMK). The hallmark of MAPK signaling is the stimulation-dependent nuclear translocation of the involved kinases, which regulate gene expression and the cytoplasmic acute response to mitogenic, stress-related, apoptotic and survival stimuli. A special type of GPCR is the gonadotropin-releasing hormone (GnRH) receptor, which uses primarily the Gq protein for its downstream signaling. GnRH activates all four MAPK cascades by a PKC-dependent mechanism. Common signaling molecules, including the tyrosine kinase c-SRC and the small GTPases CDC42, RAC and RAS, are implicated in various aspects of the GnRH-MAPK pathways. Thus, the activation of MAPK cascades by GnRH opens a new vista in the understanding of the transcriptional regulation of genes encoding gonadotropins. However, additional studies on cell lines and whole animals are required to understand GnRH signaling in the context of other hormones during the reproductive cycle of mouse and human.

Gonadotropin-releasing hormone receptor initiates multiple signaling pathways by exclusively coupling to G(q/11) proteins

The agonist-bound gonadotropin-releasing hormone (GnRH) receptor engages several distinct signaling cascades, and it has recently been proposed that coupling of a single type of receptor to multiple G proteins (G(q), G(s), and G(i)) is responsible for this behavior. GnRH-dependent signaling was studied in gonadotropic alphaT3-1 cells endogenously expressing the murine receptor and in CHO-K1 (CHO#3) and COS-7 cells transfected with the human GnRH receptor cDNA. In all cell systems studied, GnRH-induced phospholipase C activation and Ca(2+) mobilization was pertussis toxin-insensitive, as was GnRH-mediated extracellular signal-regulated kinase activation. Whereas the G(i)-coupled m2 muscarinic receptor interacted with a chimeric G(s) protein (G(s)i5) containing the C-terminal five amino acids of Galpha(i2), the human GnRH receptor was unable to activate the G protein chimera. GnRH challenge of alphaT3-1, CHO#3 and of GnRH receptor-expressing COS-7 cells did not result in agonist-dependent cAMP formation. GnRH challenge of CHO#3 cells expressing a cAMP-responsive element-driven firefly luciferase did not result in increased reporter gene expression. However, coexpression of the human GnRH receptor and adenylyl cyclase I in COS-7 cells led to clearly discernible GnRH-dependent cAMP formation subsequent to GnRH-elicited rises in [Ca(2+)](i). In alphaT3-1 and CHO#3 cell membranes, addition of [alpha-(32)P]GTP azidoanilide resulted in GnRH receptor-dependent labeling of Galpha(q/11) but not of Galpha(i), Galpha(s) or Galpha(12/13) proteins. Thus, the murine and human GnRH receptors exclusively couple to G proteins of the G(q/11) family. Multiple GnRH-dependent signaling pathways are therefore initiated downstream of the receptor/G protein interface and are not indicative of a multiple G protein coupling potential of the GnRH receptor.

The role of protein kinases A and C pathways in the regulation of mitogen-activated protein kinase activation in response to gonadotropin-releasing hormone receptor activation

There is convincing evidence that mitogen-activated protein kinase (MAPK) activation is coupled to both receptor tyrosine kinase and G protein-coupled receptors. The presence of the epidermal growth factor (EGF) receptor and the GnRH receptor on the surface of GGH(3)1' cells makes this cell line a good model for the assessment of MAPK activation by receptor tyrosine kinases and G protein-coupled receptors. In this study, to assess the activated and total (i.e. activated plus inactivated) MAPK, the phosphorylation state of p44 and p42 MAPKs was examined using antisera that distinguish phospho-p44/42 MAPK (Thr202/Tyr204) from p44/42 MAPK (phosphorylation state independent). The data show that both EGF (200 ng/ml) and Buserelin (a GnRH agonist; 10 ng/ml) provoke rapid activation of MAPK (within 5 and 15 min, respectively) after binding to their receptors. The role of protein kinase A (PKA) and protein kinase C (PKC) signal transduction pathways in mediating MAPK activation was also assessed. Both phorbol ester (phorbol 12-myristate 13-acetate; 10 ng/ml) and (Bu)2cAMP (1 mM) trigger the phosphorylation of MAPK, suggesting potential roles for PKC and PKA signaling events in MAPK activation in GGH(3)1' cells. Treatment of PKC-depleted cells with Buserelin activated MAPK, suggesting involvement of PKC-independent signal transduction pathways in MAPK activation in response to GnRH. Similarly, treatment of PKC-depleted cells with forskolin (50 microM) or cholera toxin (100 ng/ml) stimulated MAPK activation, whereas pertussis toxin (100 ng/ml) had no measurable effect. To further assess the role of PKA in response to EGF and Buserelin, cells were treated with EGF (200 ng/ml) for 3 min or with Buserelin (10 ng/ml) for 10 min after pretreatment with 3-isobutyl-1-methylxanthine (0.5 mM), forskolin (50 microM), or (Bu)2cAMP (1 mM) for 15 min. The results show that MAPK can be activated in a PKA-dependent manner in GGH(3)1' cells. Consistent with previous reports, the current data support the view that MAPK activation can be achieved via both PKC- and PKA-dependent signaling pathways triggered by the GnRH receptor that couples to G(q/11) and Gs alpha-subunit proteins. In contrast, G(i/o)alpha does not appear to participate in MAPK activation in GGH(3)1' cells.