Alanine-261 in intracellular loop III of the human gonadotropin-releasing hormone receptor is crucial for G-protein coupling and receptor internalization

Functional Characterization of Gonadotropin-Releasing Hormone and Corazonin Signaling Systems in Pacific Abalone: Toward Reclassification of Invertebrate Neuropeptides

INTRODUCTION

The proposed evolutionary origins and corresponding nomenclature of bilaterian gonadotropin-releasing hormone (GnRH)-related neuropeptides have changed tremendously with the aid of receptor deorphanization. However, the reclassification of the GnRH and corazonin (CRZ) signaling systems in Lophotrochozoa remains unclear.

METHODS

We characterized GnRH and CRZ receptors in the mollusk Pacific abalone, Haliotis discus hannai (Hdh), by phylogenetic and gene expression analyses, bioluminescence-based reporter, Western blotting, substitution of peptide amino acids, in vivo neuropeptide injection, and RNA interference assays.

RESULTS

Two Hdh CRZ-like receptors (Hdh-CRZR-A and Hdh-CRZR-B) and three Hdh GnRH-like receptors (Hdh-GnRHR1-A, Hdh-GnRHR1-B, and Hdh-GnRHR2) were identified. In phylogenetic analysis, Hdh-CRZR-A and -B grouped within the CRZ-type receptors, whereas Hdh-GnRHR1-A/-B and Hdh-GnRHR2 clustered within the GnRH/adipokinetic hormone (AKH)/CRZ-related peptide-type receptors. Hdh-CRZR-A/-B and Hdh-GnRHR1-A were activated by Hdh-CRZ (pQNYHFSNGWHA-NH2) and Hdh-GnRH (pQISFSPNWGT-NH2), respectively. Hdh-CRZR-A/-B dually coupled with the Gαq and Gαs signaling pathways, whereas Hdh-GnRHR1-A was linked only with Gαq signaling. Analysis of substituted peptides, [I2S3]Hdh-CRZ and [N2Y3H4]Hdh-GnRH, and in silico docking models revealed that the N-terminal amino acids of the peptides are critical for the selectivity of Hdh-CRZR and Hdh-GnRHR. Two precursor transcripts for Hdh-CRZ and Hdh-GnRH peptides and their receptors were mainly expressed in the neural ganglia, and their levels increased in starved abalones. Injection of Hdh-CRZ peptide into abalones decreased food consumption, whereas Hdh-CRZR knockdown increased food consumption. Moreover, Hdh-CRZ induced germinal vesicle breakdown in mature oocytes.

CONCLUSION

Characterization of Hdh-CRZRs and Hdh-GnRHRs and their cognate peptides provides new insight into the evolutionary route of GnRH-related signaling systems in bilaterians.

Structural features of the G-protein/GPCR interactions

BACKGROUND

The details of the functional interaction between G proteins and the G protein coupled receptors (GPCRs) have long been subjected to extensive investigations with structural and functional assays and a large number of computational studies.

SCOPE OF REVIEW

The nature and sites of interaction in the G-protein/GPCR complexes, and the specificities of these interactions selecting coupling partners among the large number of families of GPCRs and G protein forms, are still poorly defined.

MAJOR CONCLUSIONS

Many of the contact sites between the two proteins in specific complexes have been identified, but the three dimensional molecular architecture of a receptor-Gα interface is only known for one pair. Consequently, many fundamental questions regarding this macromolecular assembly and its mechanism remain unanswered.

GENERAL SIGNIFICANCE

In the context of current structural data we review the structural details of the interfaces and recognition sites in complexes of sub-family A GPCRs with cognate G-proteins, with special emphasis on the consequences of activation on GPCR structure, the prevalence of preassembled GPCR/G-protein complexes, the key structural determinants for selective coupling and the possible involvement of GPCR oligomerization in this process.

Molecular dynamics simulations of the adenosine A2a receptor: structural stability, sampling, and convergence

Molecular dynamics (MD) simulations of membrane-embedded G-protein coupled receptors (GPCRs) have rapidly gained popularity among the molecular simulation community in recent years, a trend which has an obvious link to the tremendous pharmaceutical importance of this group of receptors and the increasing availability of crystal structures. In view of the widespread use of this technique, it is of fundamental importance to ensure the reliability and robustness of the methodologies so they yield valid results and enable sufficiently accurate predictions to be made. In this work, 200 ns simulations of the A2a adenosine receptor (A2a AR) have been produced and evaluated in the light of these requirements. The conformational dynamics of the target protein, as obtained from replicate simulations in both the presence and absence of an inverse agonist ligand (ZM241385), have been investigated and compared using principal component analysis (PCA). Results show that, on this time scale, convergence of the replicates is not readily evident and dependent on the types of the protein motions considered. Thus rates of inter- as opposed to intrahelical relaxation and sampling can be different. When studied individually, we find that helices III and IV have noticeably greater stability than helices I, II, V, VI, and VII in the apo form. The addition of the inverse agonist ligand greatly improves the stability of all helices.

SET protein interacts with intracellular domains of the gonadotropin-releasing hormone receptor and differentially regulates receptor signaling to cAMP and calcium in gonadotrope cells

In mammals, the receptor of the neuropeptide gonadotropin-releasing hormone (GnRHR) is unique among the G protein-coupled receptor (GPCR) family because it lacks the carboxyl-terminal tail involved in GPCR desensitization. Therefore, mechanisms involved in the regulation of GnRHR signaling are currently poorly known. Here, using immunoprecipitation and GST pull-down experiments, we demonstrated that SET interacts with GnRHR and targets the first and third intracellular loops. We delineated, by site-directed mutagenesis, SET binding sites to the basic amino acids (66)KRKK(69) and (246)RK(247), located next to sequences required for receptor signaling. The impact of SET on GnRHR signaling was assessed by decreasing endogenous expression of SET with siRNA in gonadotrope cells. Using cAMP and calcium biosensors in gonadotrope living cells, we showed that SET knockdown specifically decreases GnRHR-mediated mobilization of intracellular cAMP, whereas it increases its intracellular calcium signaling. This suggests that SET influences signal transfer between GnRHR and G proteins to enhance GnRHR signaling to cAMP. Accordingly, complexing endogenous SET by introduction of the first intracellular loop of GnRHR in αT3-1 cells significantly reduced GnRHR activation of the cAMP pathway. Furthermore, decreasing SET expression prevented cAMP-mediated GnRH stimulation of Gnrhr promoter activity, highlighting a role of SET in gonadotropin-releasing hormone regulation of gene expression. In conclusion, we identified SET as the first direct interacting partner of mammalian GnRHR and showed that SET contributes to a switch of GnRHR signaling toward the cAMP pathway.

Molecular cloning and pharmacological characterization of two novel GnRH receptors in the lamprey (Petromyzon marinus)

This paper reports the identification, expression, binding kinetics, and functional studies of two novel type III lamprey GnRH receptors (lGnRH-R-2 and lGnRH-R-3) in the sea lamprey, a basal vertebrate. These novel GnRH receptors share the structural features and amino acid motifs common to other known gnathostome GnRH receptors. The ligand specificity and activation of intracellular signaling studies showed ligands lGnRH-II and -III induced an inositol phosphate (IP) response at lGnRH-R-2 and lGnRH-R-3, whereas the ligand lGnRH-I did not stimulate an IP response. lGnRH-II was a more potent activator of lGnRH-R-3 than lGnRH-III. Stimulation of lGnRH-R-2 and lGnRH-R-3 testing all three lGnRH ligands did not elicit a cAMP response. lGnRH-R-2 has a higher binding affinity in response to lGnRH-III than lGnRH-II, whereas lGnRH-R-3 has a higher binding affinity in response to lGnRH-II than IGnRH-III. lGnRH-R-2 precursor transcript was detected in a wide variety of tissues including the pituitary whereas lGnRH-R-3 precursor transcript was not as widely expressed and primarily expressed in the brain and eye of male and female lampreys. From our phylogenetic analysis, we propose that lGnRH-R-1 evolved from a common ancestor of all vertebrate GnRH receptors and lGnRH-R-2 and lGnRH-R-3 likely occurred due to a gene duplication within the lamprey lineage. In summary, we propose from our findings of receptor subtypes in the sea lamprey that the evolutionary recruitment of specific pituitary GnRH receptor subtypes for particular physiological functions seen in later evolved vertebrates was an ancestral character that first arose in a basal vertebrate.

Evolution of vertebrate GnRH receptors from the perspective of a Basal vertebrate

This minireview provides the current status on gonadotropin-releasing hormone receptors (GnRH-R) in vertebrates, from the perspective of a basal vertebrate, the sea lamprey, and provides an evolutionary scheme based on the recent advance of whole genome sequencing. In addition, we provide a perspective on the functional divergence and evolution of the receptors. In this review we use the phylogenetic classification of vertebrate GnRH receptors that groups them into three clusters: type I (mammalian and non-mammalian), type II, and type III GnRH receptors. New findings show that the sea lamprey has two type III-like GnRH receptors and an ancestral type GnRH receptor that is more closely related to the type II-like receptors. These two novel GnRH receptors along with lGnRH-R-1 share similar structural features and amino acid motifs common to other known gnathostome type II/III receptors. Recent data analyses of the lamprey genome provide strong evidence that two whole rounds of genome duplication (2R) occurred prior to the gnathostome-agnathan split. Based on our current knowledge, it is proposed that lGnRH-R-1 evolved from an ancestor of the type II receptor following a vertebrate-shared genome duplication and that the two type III receptors resulted from a duplication within lamprey of a gene derived from a lineage shared by many vertebrates.

Emerging gonadotropin-releasing hormone agonists

INTRODUCTION

Gonadotropin-releasing hormone agonist analogs (GnRHa) are peptides that mimic the action of gonadotropin-releasing hormone (GnRH) and are used to suppress subsequent sex steroid production. Although the analogs are a rather defined group of drugs, there have been developments in the past decades and there is still ample room for improvement. New therapeutic strategies in the use of GnRHs are discussed.

AREAS COVERED

Major points of discussion include: i) the use of concomitant treatment of early breast cancer in premenopausal estrogen-positive and -negative patients, ii) the use of GnRHa for fertility preservation in young female patients with malignant diseases and iii) the use of GnRH analogs in assisted reproduction. The manuscript provides a better understanding of GnRH agonists as well as an explanation of their major indications, biochemical pathways and concluding therapeutic strategies. Recent results from international meetings and debates are described to explain current controversies.

EXPERT OPINION

This paper highlights the need for more complex GnRH analogs. In the next few years, there will be longer acting GnRHas that may improve adherence. New therapeutic targets in oncological concepts may go beyond fertility preservation and focus on the antiproliferative effects of GnRH analogs.

Using automated imaging to interrogate gonadotrophin-releasing hormone receptor trafficking and function

Gonadotrophin-releasing hormone (GnRH) acts via seven transmembrane receptors on gonadotrophs to stimulate gonadotrophin synthesis and secretion, and thereby mediates central control of reproduction. Type I mammalian GnRHR are unique, in that they lack C-terminal tails. This is thought to underlie their resistance to rapid homologous desensitisation as well as their slow rate of internalisation and inability to provoke G-protein-independent (arrestin-mediated) signalling. More recently it has been discovered that the vast majority of human GnRHR are actually intracellular, in spite of the fact that they are activated at the cell surface by a membrane impermeant peptide hormone. This apparently reflects inefficient exit from the endoplasmic reticulum and again, the absence of the C-tail likely contributes to their intracellular localisation. This review is intended to cover some of these novel aspects of GnRHR biology, focusing on ways that we have used automated fluorescence microscopy (high content imaging) to explore GnRHR localisation and trafficking as well as spatial and temporal aspects of GnRH signalling via the Ca(2+)/calmodulin/calcineurin/NFAT and Raf/MEK/ERK pathways.

Plasma membrane expression of gonadotropin-releasing hormone receptors: regulation by peptide and nonpeptide antagonists

Gonadotropin-releasing hormone acts via cell surface receptors but most human (h) GnRH receptors (GnRHRs) are intracellular. A membrane-permeant nonpeptide antagonist [(2S)-2-[5-[2-(2-axabicyclo[2.2.2]oct-2-yl)-1,1-dimethy-2-oxoethyl]-2-(3,5-dimethylphenyl)-1H-indol-3-yl]-N-(2-pyridin-4-ylethyl)propan-1-amine (IN3)] increases hGnRHR expression at the surface, apparently by facilitating its exit from the endoplasmic reticulum. Here we have quantified GnRHR by automated imaging in HeLa cells transduced with adenovirus expressing hemagglutinin-tagged GnRHR. Consistent with an intracellular site of action, IN3 increases cell surface hGnRHR, and this effect is not blocked or mimicked by membrane-impermeant peptide antagonists [Ac-D2Nal-D4Cpa-D3Pal-Ser-Tyr-d-Cit-Leu-Arg-Pro-d-Ala-NH(2) (cetrorelix) and antide]. However, when the C-terminal tail of a Xenopus (X) GnRHR was added (h.XGnRHR) to increase expression, both peptides further increased cell surface GnRHR. Cetrorelix also synergized with IN3 to increase expression of hGnRHR and a G-protein coupling-deficient mutant (A261K-hGnRHR). Cetrorelix also increased cell surface expression of hGnRHR, h.XGnRHR, and mouse GnRHR in gonadotrope-lineage LbetaT2 cells, and in HeLa cells it slowed h.XGnRHR internalization (measured by receptor-mediated antihemagglutinin uptake). Thus cetrorelix has effects other than GnRHR blockade; it acts as an inverse agonist in internalization assays, supporting the potential importance of ligand-biased efficacy at GnRHR. We also developed an imaging assay for GnRH function based on Ca(2+)-dependent nuclear translocation of a nuclear factor of activated T cells reporter. Using this in HeLa and LbetaT2 cells, IN3 and cetrorelix behaved as competitive antagonists when coincubated with GnRH, and long-term pretreatment (16 h) with IN3 reduced its effectiveness as an inhibitor whereas pretreatment with cetrorelix increased its inhibitory effect. This distinction between peptide and nonpeptide antagonists may prove important for therapeutic applications of GnRH antagonists.

Trafficking and signalling of gonadotrophin-releasing hormone receptors: an automated imaging approach

Gonadotrophin-releasing hormone (GnRH) is a neuropeptide that mediates central control of reproduction by stimulating gonadotrophin secretion from the pituitary. It acts via 7 transmembrane region (7TM) receptors that lack C-terminal tails, regions that for many 7TM receptors, are necessary for agonist-induced phosphorylation and arrestin binding as well as arrestin-dependent desensitization, internalization and signalling. Recent work has revealed that human GnRH receptors (GnRHR) are poorly expressed at the cell surface. This apparently reflects inefficient exit from the endoplasmic reticulum, which is thought to be increased by pharmacological chaperones (non-peptide GnRHR antagonists that increase cell surface GnRHR expression) or reduced by point mutations that further impair GnRHR trafficking and thereby cause infertility. Here, we review recent work in this field, with emphasis on the use of semi-automated imaging to interrogate compartmentalization and trafficking of these unique 7TM receptors.

Agonist-induced internalization and downregulation of gonadotropin-releasing hormone receptors

Gonadotropin-releasing hormone (GnRH) acts via seven transmembrane receptors to stimulate gonadotropin secretion. Sustained stimulation desensitizes GnRH receptor (GnRHR)-mediated gonadotropin secretion, and this underlies agonist use in hormone-dependent cancers. Since type I mammalian GnRHR do not desensitize, agonist-induced internalization and downregulation may underlie desensitization of GnRH-stimulated gonadotropin secretion; however, research focus has recently shifted to anterograde trafficking, with the finding that human (h)GnRHR are mostly intracellular. Moreover, there is little direct evidence for agonist-induced trafficking of hGnRHR, and whether or not type I mammalian GnRHR show agonist-induced internalization is controversial. Here we use automated imaging to monitor expression and internalization of hemagglutinin (HA)-tagged hGnRHRs, mouse (m) GnRHR, Xenopus (X) GnRHRs, and chimeric receptors (hGnRHR with added XGnRHR COOH tails, h.XGnRHR) expressed by adenoviral transduction in HeLa cells. We find that agonists stimulate downregulation and/or internalization of mGnRHR and XGnRHR, that GnRH stimulates trafficking of hGnRHR and can stimulate internalization or downregulation of hGnRHR when steps are taken to increase cell surface expression (addition of the XGnRHR COOH tail or pretreatment with pharmacological chaperone). Agonist effects on internalization (of h.XGnRHR) and downregulation (of hGnRHR and h.XGnRHR) were not mimicked by a peptide antagonist and were prevented by a mutation that prevents GnRHR signaling, demonstrating dependence on receptor signaling as well as agonist occupancy. Thus agonist-induced internalization and downregulation of type I mammalian GnRHR occurs in HeLa cells, and we suggest that the high throughput imaging systems described here will facilitate study of the molecular mechanisms involved.

The chicken type III GnRH receptor homologue is predominantly expressed in the pituitary, and exhibits similar ligand selectivity to the type I receptor

Two GnRH isoforms (cGnRH-I and GnRH-II) and two GnRH receptor subtypes (cGnRH-R-I and cGnRH-R-III) occur in chickens. Differential roles for these molecules in regulating gonadotrophin secretion or other functions are unclear. To investigate this we cloned cGnRH-R-III from a broiler chicken and compared its structure, expression and pharmacological properties with cGnRH-R-I. The broiler cGnRH-R-III cDNA was 100% identical to the sequence reported in the red jungle fowl and white leghorn breed. Pituitary cGnRH-R-III mRNA was approximately 1400-fold more abundant than cGnRH-R-I mRNA. Northern analysis indicated a single cGnRH-R-III transcript. A pronounced sex and age difference existed, with higher pituitary transcript levels in sexually mature females versus juvenile females. In contrast, higher expression levels occurred in juvenile males versus sexually mature males. Functional studies in COS-7 cells indicated that cGnRH-R-III has a higher binding affinity for GnRH-II than cGnRH-I (K(d): 0.57 vs 19.8 nM) with more potent stimulation of inositol phosphate production (ED(50): 0.8 vs 4.38 nM). Similar results were found for cGnRH-R-I, (K(d): 0.51 vs 10.8 nM) and (ED(50): 0.7 vs 2.8 nM). The initial rate of internalisation was faster for cGnRH-R-III than cGnRH-R-I (26 vs 15.8%/min). Effects of GnRH antagonists were compared at the two receptors. Antagonist #27 distinguished between cGnRH-R-I and cGnRH-R-III (IC(50): 2.3 vs 351 nM). These results suggest that cGnRH-R-III is probably the major mediator of pituitary gonadotroph function, that antagonist #27 may allow delineation of receptor subtype function in vitro and in vivo and that tissue-specific recruitment of cGnRH-R isoforms has occurred during evolution.

GnRH signaling in intrauterine tissues

Type I GnRH (GnRH-I, GNRH1) and type II GnRH (GnRH-II, GNRH2), each encoded by separate genes, have been identified in humans. The tissue distribution and functional regulation of GnRH-I and GnRH-II clearly differ despite their comparable cDNA and genomic structures. These hormones exert their effects by binding to cell surface transmembrane G protein coupled receptors and stimulating the Gq/11 subfamily of G proteins. The hypothalamus and pituitary are the main origin and target sites of GnRH, but numerous studies have demonstrated that extra-hypothalamic GnRH and extra-pituitary GnRH receptors exist in different reproductive tissues such as the ovary, endometrium, placenta, and endometrial cancer cells. In addition to endocrine regulation, GnRH is also known to act in an autocrine and paracrine manner to suppress cell proliferation and activate apoptosis in the endometrium and endometrial cancer cells through several mechanisms. Both GnRH-I and GnRH-II exhibit regulatory roles in tissue remodelling during embryo implantation and placentation, which suggests that these hormones may have important roles in embryo implantation and early pregnancy. The presence of varied GnRH and GnRH receptor systems demonstrate their different roles in distinct tissues using dissimilar mechanisms. These may result in the generation of new GnRH analogues used for several hormone-related diseases.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

Pituitary gonadotropin-releasing hormone (GnRH) receptor: structure, distribution and regulation of expression

Reproduction in mammals is controlled by interactions between the hypothalamus, anterior pituitary and gonads. Interaction of GnRH with its cognate receptor is essential to regulating reproduction. Characterization of the structure, distribution and expression of GnRH receptors (GnRH-R) has furthered our understanding of the physiological consequences of GnRH stimulation of pituitary gonadotropes. Based on the putative topology of the amino acid sequence of the GnRH-R and point mutation studies, key elements of the GnRH-R have been identified to play a role in ligand recognition and binding, G-protein activation and internalization. Normally, reproductive function is mediated by GnRH-R expressed only on the membranes of pituitary gonadotropes. The density of GnRH-R on gonadotropes determines their ability to respond to GnRH. This density is highest just prior to ovulation and likely is important for complete expression of the pre-ovulatory surge of LH. Therefore, knowledge regarding what regulates the density of GnRH-R is essential to understanding changes in pituitary sensitivity to GnRH and ultimately, to expression of the LH surge. Regulation of GnRH-R gene expression is influenced by a multitude of factors including gonadal steroid hormones, inhibin, activin and perhaps most importantly GnRH itself.

Antagonist and agonist binding models of the human gonadotropin-releasing hormone receptor

G-protein-coupled receptors (GPCRs) constitute one of the most important classes of drug targets. Since the first high-resolution structure of a GPCR was determined by Palczewski and co-workers [K. Palczewski, T. Kumasaka, T. Hori, C.A. Behnke, H. Motoshima, B.A. Fox, I. Le Trong, D.C. Teller, T. Okada, R.E. Stenkamp, M. Yamamoto, M. Miyano, Crystal structure of rhodopsin: a G-protein-coupled receptor, Science 289 (2000) 739-745], development of in silico models of rhodopsin-like GPCRs could be rationally founded. In this work, we present a model of the human gonadotropin-releasing hormone receptor based on the rhodopsin structure. The transmembrane helices are modeled by homology, while the extra- and intra-cellular loops are modeled in such a way that experimentally determined interactions and microdomains (e.g., hydrophobic cores) are retained. We conclude that specifically tailored models, compared to more automatic approaches, have the benefit that known interactions are easily introduced early in the homology modeling. Furthermore, tailored models, although more tedious to construct, are better suited for drug lead finding and for compound optimization. To test the stability of the receptor, we performed a 1 ns molecular dynamics simulation. Moreover, we docked two agonists (native GnRH and Triptorelin, [dTrp(6)]-GnRH) and two antagonists (Cetrorelix, dNal(1)-dCpa(2)-dPal(3)-Ser(4)-Tyr(5)-dCit(6)-Leu(7)-Arg(8)-Pro(9)-dAla(10)), and the covalently constrained dicyclic decapeptide dicyclo(1,1'-5/4-10)[Ac-Glu(1)(Gly(1)')-dCpa(2)-dTrp(3)-Asp(4)-dbu(5)-dNal(6)-Leu(7)-Arg(8)-Pro(9)-dpr(10)-NH(2)] into the putative receptor binding site. The docked ligand conformations result in ligand-receptor interactions that are generally in good agreement with site-directed mutagenesis and ligand-binding studies presented in the literature. Our results indicate that the binding conformation of the antagonists differs from that of the agonists. This difference can be linked to the activation or inhibition of the receptor.

GnRHs and GnRH receptors

GnRH is the pivotal hypothalamic hormone regulating reproduction. Over 20 forms of the decapeptide have been identified in which the NH2- and COOH-terminal sequences, which are essential for receptor binding and activation, are conserved. In mammals, there are two forms, GnRH I which regulates gonadotropin and GnRH II which appears to be a neuromodulator and stimulates sexual behaviour. GnRHs also occur in reproductive tissues and tumours in which a paracrine/autocrine role is postulated. GnRH agonists and antagonists are now extensively used to treat hormone-dependent diseases, in assisted conception and have promise as novel contraceptives. Non-peptide orally-active GnRH antagonists have been recently developed and may increase the flexibility and range of utility. As with GnRH, GnRH receptors have undergone co-ordinated gene duplications such that cognate receptor subtypes for respective ligands exist in most vertebrates. Interestingly, in man and some other mammals (e.g. chimp, sheep and bovine) the Type II GnRH receptor has been silenced. However, GnRH I and GnRH II still appear to have distinct roles in signalling differentially through the Type I receptor (ligand-selective-signalling) to have different downstream effects. The ligand-receptor interactions and receptor conformational changes involved in receptor activation have been partly delineated. Together, these findings are setting the scene for generating novel selective GnRH analogues with potential for wider and more specific application.

Sequence analysis, endocrine regulation, and signal transduction of GnRH receptors in teleost fish

Three gonadotropin-releasing hormones (GnRHs) and three cognate receptors have been identified in vertebrates, with distinct distributions and functions. According to their sequences, the receptors can be grouped into distinct classes: types I, II, and III. One branch contains all type-I GnRH receptors (GnRH-R-I) from mammals and fish; another branch clusters mainly amphibian and human type-II GnRH receptors; and a third branch includes evolved fish, mainly perciform species, type-III GnRH receptors. Taken tilapia GnRH receptors as a model, the present study summarizes the information regarding the amino-acid residues assumed to be involved in the receptors' structure, binding, activation, and intracellular signal transduction, including arrangement of the disulfide bonds, glycosylation sites, coupling to G proteins, and protein kinase A or protein kinase C phosphorylation sites.

Molecular biology of gonadotropin-releasing hormone (GnRH)-I, GnRH-II, and their receptors in humans

In human beings, two forms of GnRH, termed GnRH-I and GnRH-II, encoded by separate genes have been identified. Although these hormones share comparable cDNA and genomic structures, their tissue distribution and regulation of gene expression are significantly dissimilar. The actions of GnRH are mediated by the GnRH receptor, which belongs to a member of the rhodopsin-like G protein-coupled receptor superfamily. However, to date, only one conventional GnRH receptor subtype (type I GnRH receptor) uniquely lacking a carboxyl-terminal tail has been found in the human body. Studies on the transcriptional regulation of the human GnRH receptor gene have indicated that tissue-specific gene expression is mediated by differential promoter usage in various cell types. Functionally, there is growing evidence showing that both GnRH-I and GnRH-II are potentially important autocrine and/or paracrine regulators in some extrapituitary compartments. Recent cloning of a second GnRH receptor subtype (type II GnRH receptor) in nonhuman primates revealed that it is structurally and functionally distinct from the mammalian type I receptor. However, the human type II receptor gene homolog carries a frameshift and a premature stop codon, suggesting that a full-length type II receptor does not exist in humans.

Receptors for hypothalamic releasing hormones TRH and GnRH: oligomerization and interactions with intracellular proteins

Studies of TRH and GnRH receptors have revealed much information about the roles of G-proteins and beta-arrestins, as well as receptor residues important for signaling, desensitization and internalization. However, the proteins involved are only just beginning to be identified and characterized. Additional complexity now exists with the observation that these receptors form oligomers in live cells. Indeed, hetero-oligomerization of TRH receptor subtypes 1 and 2 potentially alters interactions with intracellular regulatory proteins. Knowledge of proteins that interact with TRH or GnRH receptors will increase our understanding of receptor function and provide potential drug targets for a range of receptor-associated conditions.

Gonadotropin-releasing hormone receptors

GnRH and its analogs are used extensively for the treatment of hormone-dependent diseases and assisted reproductive techniques. They also have potential as novel contraceptives in men and women. A thorough delineation of the molecular mechanisms involved in ligand binding, receptor activation, and intracellular signal transduction is kernel to understanding disease processes and the development of specific interventions. Twenty-three structural variants of GnRH have been identified in protochordates and vertebrates. In many vertebrates, three GnRHs and three cognate receptors have been identified with distinct distributions and functions. In man, the hypothalamic GnRH regulates gonadotropin secretion through the pituitary GnRH type I receptor via activation of G(q). In-depth studies have identified amino acid residues in both the ligand and receptor involved in binding, receptor activation, and translation into intracellular signal transduction. Although the predominant coupling of the type I GnRH receptor in the gonadotrope is through productive G(q) stimulation, signal transduction can occur via other G proteins and potentially by G protein-independent means. The eventual selection of intracellular signaling may be specifically directed by variations in ligand structure. A second form of GnRH, GnRH II, conserved in all higher vertebrates, including man, is present in extrahypothalamic brain and many reproductive tissues. Its cognate receptor has been cloned from various vertebrate species, including New and Old World primates. The human gene homolog of this receptor, however, has a frame-shift and stop codon, and it appears that GnRH II signaling occurs through the type I GnRH receptor. There has been considerable plasticity in the use of different GnRHs, receptors, and signaling pathways for diverse functions. Delineation of the structural elements in GnRH and the receptor, which facilitate differential signaling, will contribute to the development of novel interventive GnRH analogs.

Developmental expression and subcellular localization of wallaby gonadotropin-releasing hormone receptor and its splice variants

The developmental expression of gonadotropin-releasing hormone receptor (GnRH-R) and its splice variants was examined in the gonads of tammar wallaby pouch young in order to elucidate the functional role of GnRH-R in the developing testis and ovary. Wallaby GnRH-R, like eutherian GnRH-Rs, contains three exons and two introns. In the present study, the transcripts of two splice variants (GnRH-R Delta 1 and GnRH-R Delta 2) were cloned from the pituitary. GnRH-R Delta 1 contained a 291 bp deletion from nucleotide positions 232 to 522 within exon 1. This transcript appears to be distinctive in the wallaby and has not been reported in other species. GnRH-R Delta 2 contained a 220 bp deletion from nucleotide positions 523 to 742, corresponding to exon 2. We examined the subcellular localization of the wild type GnRH-R and its splice variants with confocal microscopy, showing that both the wild type receptor and the splice variants were membrane-associated molecules. The different pattern of expression of the wild type receptor and the variants transcripts found in adult and neonatal tissues suggests a specific developmental regulation of the GnRH-R Delta 2 transcript. In addition, the developmental expression of the GnRH-R and GnRH-R Delta 1 transcripts showed a possible association with key physiological events during gonadal development in the wallaby pouch young, suggesting that GnRH-R may be involved in the regulation of early development in the testis and ovary.

Mechanism of GnRH receptor signaling on gonadotropin release and gene expression in pituitary gonadotrophs

Gonadotropin releasing hormone (GnRH), the first key hormone of reproduction, is synthesized and secreted from the hypothalamus in a pulsatile manner and stimulates pituitary gonadotrophs (5-10% of the pituitary cells) to synthesize and release gonadotropin luteinizing hormone (LH) and follicle stimulating hormone (FSH). Gonadotrophs consist of 60% multihormonal cells (LH+FSH) and 18% LH- and 22% FSH-containing cells. LH and FSH, members of the glycoprotein hormone family, stimulate spermatogenesis, folliculogenesis, and ovulation. Although GnRH plays a pivotal role in gonadotropin synthesis and release, other factors such as gonadal steroids and gonadal peptides exert positive and negative feedback mechanisms, which affect GnRH actions. GnRH actions include activation of phosphoinositide turnover as well as phospholipase D and A2, mobilization and influx of Ca2+, activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). A complex crosstalk between the above messenger molecules mediates the diverse actions of GnRH. Understanding the signaling mechanisms involved in GnRH actions is the basis for our understanding of basic reproductive functions in general and gonadotropin synthesis and release in particular.

A chicken gonadotropin-releasing hormone receptor that confers agonist activity to mammalian antagonists. Identification of D-Lys(6) in the ligand and extracellular loop two of the receptor as determinants

Mammalian receptors for gonadotropin-releasing hormone (GnRH) have over 85% sequence homology and similar ligand selectivity. Biological studies indicated that the chicken GnRH receptor has a distinct pharmacology, and certain antagonists of mammalian GnRH receptors function as agonists. To explore the structural determinants of this, we have cloned a chicken pituitary GnRH receptor and demonstrated that it has marked differences in primary amino acid sequence (59% homology) and in its interactions with GnRH analogs. The chicken GnRH receptor had high affinity for mammalian GnRH (K(i) 4.1 +/- 1.2 nM), similar to the human receptor (K(i) 4.8 +/- 1.2 nM). But, in contrast to the human receptor, it also had high affinity for chicken GnRH ([Gln(8)]GnRH) and GnRH II ([His(5),Trp(7),Tyr(8)]GnRH) (K(i) 5.3 +/- 0.5 and 0.6 +/- 0.01 nM). Three mammalian receptor antagonists were also pure antagonists in the chicken GnRH receptor. Another three, characterized by D-Lys(6) or D-isopropyl-Lys(6) moieties, functioned as pure antagonists in the human receptor but were full or partial agonists in the chicken receptor. This suggests that the Lys side chain interacts with functional groups of the chicken GnRH receptor to stabilize it in the active conformation and that these groups are not available in the activated human GnRH receptor. Substitution of the human receptor extracellular loop two with the chicken extracellular loop two identified this domain as capable of conferring agonist activity to mammalian antagonists. Although functioning of antagonists as agonists has been shown to be species-dependent for several GPCRs, the dependence of this on an extracellular domain has not been described.

Internalization rates of murine and ovine gonadotropin-releasing hormone receptors

Rates of internalization of the murine GnRH receptor fused via its C-terminus to green fluorescent protein (GnRH-R-GFP) were examined in Chinese hamster ovary cells (CHO cells) and compared to those of native murine GnRH-R in a clonal murine gonadotroph cell line (LbetaT2 cells). The resulting rates of internalization of murine receptors were then compared with those of sheep GnRH-R in ovine gonadotrophs. Cells were incubated with radioiodinated [D-Ala6]GnRH on ice for 4 h to allow binding of the ligand to GnRH-R, then cells were warmed to 37 degrees C to permit internalization. Surface-bound radioligand began to decrease as soon as the cells were warmed and had decreased significantly within 20 min. A steady-state level of surface-bound radioligand was achieved after 60 min in both CHO cells and LbetaT2 cells (38% and 41%, respectively, of initial value; P < 0.05). Internalization of radioligand began immediately after warming the cells to 37 degrees C, and a significant proportion of surface ligand had been internalized by 20 min. A steady-state maximum of internalization was reached after 60 min in both CHO cells and LbetaT2 cells (29% and 28%, respectively, of total cell-associated ligand; P < 0.05). Changes in surface-bound radioligand and internalized radioligand in sheep pituitary cells were similar to those in CHO cells and LbetaT2 cells, but the amount of radioligand internalized after 60 min (40% of total cell-associated ligand) was 1.4 times higher than in CHO cells and LbetaT2 cells (P < 0.05). In a separate experiment, the effect of estradiol on the rate of internalization of GnRH-R in ovine pituitary cells was examined. Although treatment of ovine pituitary cells with estradiol approximately doubled the number of GnRH receptors, it did not alter either the rate or extent of receptor internalization. These results show that rates of internalization of recombinant murine GnRH-R-GFP in CHO cells and native murine and ovine GnRH-R in LbetaT2 cells and in sheep pituitary cells, respectively, are similar, but amounts of ovine GnRH-R internalized are greater than those for murine GnRH-R. Further, the rate of internalization of occupied receptor is similar in gonadotroph and nongonadotroph cells, and the addition of GFP to the C-terminus of the murine GnRH-R does not alter the rate of internalization.

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.

A limited spectrum of mutations causes constitutive activation of the yeast alpha-factor receptor

Activation of G protein coupled receptors (GPCRs) by binding of ligand is the initial event in diverse cellular signaling pathways. To examine the frequency and diversity of mutations that cause constitutive activation of one particular GPCR, the yeast alpha-factor receptor, we screened libraries of random mutations for constitutive alleles. In initial screens for mutant receptor alleles that exhibit signaling in the absence of added ligand, 14 different point mutations were isolated. All of these 14 mutants could be further activated by alpha-factor. Ten of the mutants also acquired the ability to signal in response to binding of desTrp(1)¿Ala(3)ălpha-factor, a peptide that acts as an antagonist toward normal alpha-factor receptors. Of these 10 mutants, at least eight alleles residing in the third, fifth, sixth, and seventh transmembrane segments exhibit bona fide constitutive signaling. The remaining alleles are hypersensitive to alpha-factor rather than constitutive. They can be activated by low concentrations of endogenous alpha-factor present in MATa cells. The strongest constitutively active receptor alleles were recovered multiple times from the mutational libraries, and extensive mutagenesis of certain regions of the alpha-factor receptor did not lead to recovery of any additional constitutive alleles. Thus, only a limited number of mutations is capable of causing constitutive activation of this receptor. Constitutive and hypersensitive signaling by the mutant receptors is partially suppressed by coexpression of normal receptors, consistent with preferential association of the G protein with unactivated receptors.

Complementary deoxyribonucleic acid cloning, gene expression, and ligand selectivity of a novel gonadotropin-releasing hormone receptor expressed in the pituitary and midbrain of Xenopus laevis

We have cloned the full-length complementary DNA (cDNA) for a GnRH receptor from Xenopus laevis pituitary cDNA and determined its gene structure. The cDNA encodes a 368-amino acid protein that has a 46% amino acid identity to the human GnRH receptor. The X laevis GnRH receptor has all of the amino acids identified in the mammalian GnRH receptors as sites of interaction with the GnRH ligand. However, this receptor cDNA shares the same distinguishing structural features of the GnRH receptor that have been characterized from other nonmammalian vertebrates. These include the pair of aspartate residues in the transmembrane domains II and VII compared with the aspartate/asparagine arrangement in mammalian receptors, the amino acid PEY motif in extracellular loop III (SEP in mammals), and the presence of a carboxyl-terminal tail. Previous studies have reported that mammalian GnRH was equipotent to other naturally occurring GnRH subtypes in stimulating LH release from the amphibian pituitary. However, in this study we show that the X. laevis GnRH receptor has ligand selectivity for the naturally occurring GnRHs similar to other nonmammalian GnRH receptors. The order of potency of the GnRHs in stimulating inositol phosphate production in COS-1 cells transiently transfected with the X. laevis GnRH receptor cDNA was chicken GnRH II>salmon GnRH>mammalian GnRH. Transcripts of this GnRH receptor are expressed in the pituitary and midbrain of X. laevis.

Consensus bioactive conformation of cyclic GnRH antagonists defined by NMR and molecular modeling

Little is known of the conformation of peptide hormones as they interact with their receptors for a number of reasons: peptide hormones are notoriously flexible in solution, their receptors are particularly complex, and there is strong evidence that receptor-ligand interaction leading to activation is a dynamic process. Insights into the active conformation of the decapeptide gonadotropin releasing hormone (GnRH) have been obtained previously from the solution structures of four constrained GnRH antagonists ¿cyclo(1-10)[Ac-Delta(3)-Pro(1),DCpa(2),DTrp(3,6),NMeLeu+ ++(7), betaAla(10)]GnRH (1), cyclo(4-10)[Ac-Delta(3)Pro(1),DFpa(2),DTrp(3), Asp(4),DNal(6),Dpr(10)]GnRH (2), dicyclo(4-10/5-8)[Ac-DNal(1), DCpa(2),DTrp(3),Asp(4),Glu(5),DArg(6),Lys(8),Dpr (10)]GnRH (3), and dicyclo(4-10/5-5'-8)[Ac-DNal(1),DCpa(2),DPal(3), Asp(4),Glu(5)(Gly), DArg(6),Dbu(8),Dpr(10)]GnRH (4)¿. However, the precise location of the N-terminal tripeptide in the highly potent (K(i) < 0.4 nM) 2-4 remained unclear due to the lack of constraints in this region. The NMR structure of the newly discovered and potent (K(i) = 0.24 nM) dicyclo(1-1'-5/4-10)[Ac-Glu(1)(Gly),DCpa(2),DTrp(3),As p(4),Dbu(5), DNal(6),Dpr(10)]GnRH (5) now allows the definition of the conformation of this region. A combined computational analysis (consensus forcing) of compounds 2-5, designed to explore the common conformations available to them that are simultaneously consistent with the NMR data corresponding to each compound, leads to a consensus structural model for the GnRH pharmacophore. This model shares some common features with the structure of the nonpeptidic GnRH mimetic T-98475. In the course of that comparative study, two additional contact points to those proposed by the authors are identified, suggesting that this model has predictive value.

Expression and function of the gonadotropin-releasing hormone receptor are dependent on a conserved apolar amino acid in the third intracellular loop

The coupling of agonist-activated heptahelical receptors to their cognate G proteins is often dependent on the amino-terminal region of the third intracellular loop. Like many G protein-coupled receptors, the gonadotropin-releasing hormone (GnRH) receptor contains an apolar amino acid in this region at a constant distance from conserved Pro and Tyr/Asn residues in the fifth transmembrane domain (TM V). An analysis of the role of this conserved residue (Leu(237)) in GnRH receptor function revealed that the binding affinities of the L237I and L237V mutant receptors were unchanged, but their abilities to mediate GnRH-induced inositol phosphate signaling, G protein coupling, and agonist-induced internalization were significantly impaired. Receptor expression at the cell surface was reduced by replacement of Leu(237) with Val, and abolished by replacement with Ala, Arg, or Asp residues. These results are consistent with molecular modeling of the TM V and VI regions of the GnRH receptor, which predicts that Leu(237) is caged by several apolar amino acids (Ile(233), Ile(234), and Val(240) in TM V, and Leu(262), Leu(265), and Val(269) in TM VI) to form a tight hydrophobic cluster. These findings indicate that the conserved apolar residue (Leu(237)) in the third intracellular loop is an important determinant of GnRH receptor expression and activation, and possibly that of other G protein-coupled receptors.

Resistance of hypogonadic patients with mutated GnRH receptor genes to pulsatile GnRH administration

We have studied a kindred with three siblings with isolated hypogonadotropic hypogonadism caused by compound heterozygote mutations in the GnRH receptor gene. The disorder was transmitted as an autosomal recessive trait. The R262Q mutation in intracellular loop 3 of the receptor was associated with a mutation in the third transmembrane domain of the receptor, A129D, that has never been described before. This A129D mutation results in a complete loss of function, indicated by the lack of inositol triphosphate (TP3) 3 production by transfected Chinese hamster ovary (CHO) cells after GnRH stimulation. The two brothers had microphallus and bilateral cryptorchidism and were referred for lack of puberty, whereas their sister had primary amenorrhea and a complete lack of puberty. Their basal gonadotropin concentrations were below the reference range, and their endogenous LH secretory patterns were abnormal, with a low-normal frequency of small pulses or no apparent LH pulse. Pulsatile GnRH administration (10 microg/pulse every 90 min for 40 h) resulted in increased mean LH without any significant changes in testosterone levels in the two brothers, whereas the LH secretory profile of their sister remained apulsatile. Larger pulses of exogenous GnRH (20 microg every 90 min for 24 h) caused the sister to produce recognizable low amplitude LH pulses. The concentrations of free alpha-subunit significantly increased in all patients during the pulsatile GnRH administration. Thus, these hypogonadal patients are partially resistant to pulsatile GnRH administration, suggesting that they should be treated with gonadotropins to induce spermatogenesis or ovulation rather than with pulsatile GnRH.

Mutations at the consensus phosphorylation sites in the third intracellular loop of the rat gonadotropin-releasing hormone receptor: effects on receptor ligand binding and signal transduction

In this study, site-directed mutagenesis of potential phosphorylation sites (Thr238, Ser253, and Thr264) for protein kinase C and C-terminal portion (Ala260-Leu265) of the third intracellular loop of the rat GnRH receptor (rGnRHR) was performed to assess the significance of these regions in the function of the GnRHR. Mutation at one or all of the three potential phosphorylation sites had differential effects on receptor ligand binding. Mutation of Ser253 or Thr264 to Ala did not significantly affect the receptor-binding affinity but decreased the number of measurable binding sites. Mutation of Thr238 to Ala or triple mutation of Thr238, Ser253, and Thr264 impaired or abolished receptor-binding affinity. Mutations of the potential phosphorylation sites affected receptor-mediated inositol phospholipid (IP) production and correlated with alterations in receptor binding after mutation, but they did not significantly affect receptor-mediated cAMP production or cAMP-mediated prolactin release. In addition, mutation of Ser253 or Thr264 to Ala did not affect the GnRH-provoked desensitization in terms of GnRH agonist-stimulated IP production. Deletion of the C-terminal portion (Ala260-Leu265) of the third intracellular loop of the rGnRHR, including a potential phosphorylation site (Thr264), abolished the receptor-binding affinity and receptor-mediated signal transduction. Replacement of the deleted C-terminal portion with a C-terminal portion (Ala-Ala-Arg-Thr-Leu-Ser) of the third intracellular loop of the Gq/11-coupled rat M1 muscarinic acetylcholine receptor did not restore receptor function. These results suggest that the potential phosphorylation sites or the region around the phosphorylation site of the third intracellular loop of the GnRHR is important for the structural integrity and expression of the receptor but that phosphorylation at these sites is not required for desensitization.