Opposite contribution of two ligand-selective determinants in the N-terminal hormone-binding exodomain of human gonadotropin receptors

Autoantibody mimicry of hormone action at the thyrotropin receptor

Thyroid hormones are vital in metabolism, growth and development1. Thyroid hormone synthesis is controlled by thyrotropin (TSH), which acts at the thyrotropin receptor (TSHR)2. In patients with Graves' disease, autoantibodies that activate the TSHR pathologically increase thyroid hormone activity3. How autoantibodies mimic thyrotropin function remains unclear. Here we determined cryo-electron microscopy structures of active and inactive TSHR. In inactive TSHR, the extracellular domain lies close to the membrane bilayer. Thyrotropin selects an upright orientation of the extracellular domain owing to steric clashes between a conserved hormone glycan and the membrane bilayer. An activating autoantibody from a patient with Graves' disease selects a similar upright orientation of the extracellular domain. Reorientation of the extracellular domain transduces a conformational change in the seven-transmembrane-segment domain via a conserved hinge domain, a tethered peptide agonist and a phospholipid that binds within the seven-transmembrane-segment domain. Rotation of the TSHR extracellular domain relative to the membrane bilayer is sufficient for receptor activation, revealing a shared mechanism for other glycoprotein hormone receptors that may also extend to other G-protein-coupled receptors with large extracellular domains.

Insulin-like 3 affects zebrafish spermatogenic cells directly and via Sertoli cells

Pituitary hormones can use local signaling molecules to regulate target tissue functions. In adult zebrafish testes, follicle-stimulating hormone (Fsh) strongly increases the production of insulin-like 3 (Insl3), a Leydig cell-derived growth factor found in all vertebrates. Little information is available regarding Insl3 function in adult spermatogenesis. The Insl3 receptors Rxfp2a and 2b were expressed by type A spermatogonia and Sertoli and myoid cells, respectively, in zebrafish testis tissue. Loss of insl3 increased germ cell apoptosis in males starting at 9 months of age, but spermatogenesis appeared normal in fully fertile, younger adults. Insl3 changed the expression of 409 testicular genes. Among others, retinoic acid (RA) signaling was up- and peroxisome proliferator-activated receptor gamma (Pparg) signaling was down-regulated. Follow-up studies showed that RA and Pparg signaling mediated Insl3 effects, resulting in the increased production of differentiating spermatogonia. This suggests that Insl3 recruits two locally active nuclear receptor pathways to implement pituitary (Fsh) stimulation of spermatogenesis.

FSHR Trans-Activation and Oligomerization

Follicle stimulating hormone (FSH) plays a key role in human reproduction through, among others, induction of spermatogenesis in men and production of estrogen in women. The function FSH is performed upon binding to its cognate receptor-follicle-stimulating hormone receptor (FSHR) expressed on the surface of target cells (granulosa and Sertoli cells). FSHR belongs to the family of G protein-coupled receptors (GPCRs), a family of receptors distinguished by the presence of various signaling pathway activation as well as formation of cross-talking aggregates. Until recently, it was claimed that the FSHR occurred naturally as a monomer, however, the crystal structure as well as experimental evidence have shown that FSHR both self-associates and forms heterodimers with the luteinizing hormone/chorionic gonadotropin receptor-LHCGR. The tremendous gain of knowledge is also visible on the subject of receptor activation. It was once thought that activation occurs only as a result of ligand binding to a particular receptor, however there is mounting evidence of trans-activation as well as biased signaling between GPCRs. Herein, we describe the mechanisms of aforementioned phenomena as well as briefly describe important experiments that contributed to their better understanding.

Gonadal response after a single-dose stimulation test with recombinant human chorionic gonadotropin (rhCG) in patients with isolated prepubertal cryptorchidism

Background

The evaluation of prepubertal gonadal Leydig cells secretion requires gonadotropin stimulation. Urinary hCG (human chorionic gonadotropin) is currently unavailable in many countries, however, recombinant hCG (rhCG) can be used. Our aim was to evaluate rhCG-stimulated testicular hormones in a group of patients with cryptorchidism.

Methods

We evaluated 31 prepubertal boys (age range, 0.75–9.0 years) presenting with unilateral (n = 24) or bilateral (n = 7) cryptorchidism. Patients with other genital abnormalities, previous use of hCG or testosterone or previous surgeries were excluded. Blood samples were obtained at baseline and 7 days after a single subcutaneous dose of rhCG (Ovidrel® 250 mcg) to measure the testosterone, DHT (dihydrotestosterone), AMH (anti-Mullerian hormone), and inhibin B levels.

Results

rhCG stimulation significantly increased testosterone levels from 10 ng/dl to 247.8 ± 135.8 ng/dl, increased DHT levels from 4.6 ± 0.8 to 32.3 ± 18.0 ng/dl, and increased the T/DHT ratio from 2.2 ± 0.4 to 8.0 ± 3.5. There was also a significant increase in inhibin B (from 105.8 ± 65.2 to 132.4 ± 56.1 pg/ml; p < 0.05) and AMH levels (from 109.4 ± 52.6 to 152.9 ± 65.2 ng/ml; p < 0.01) after the rhCG stimulation.

Conclusions

In this cohort, hormonal responses can be elicited after the rhCG stimulation test, suggesting that rhCG is a promising stimulation test to replace the urinary hCG test during the evaluation of gonadal Leydig cells function. The clinical applicability and adequate performance of rhCG testing must be investigated in future studies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12610-016-0039-2) contains supplementary material, which is available to authorized users.

G Protein-Coupled Receptor Multimers: A Question Still Open Despite the Use of Novel Approaches

Heteromerization of G protein-coupled receptors (GPCRs) can significantly change the functional properties of involved receptors. Various biochemical and biophysical methodologies have been developed in the last two decades to identify and functionally evaluate GPCR heteromers in heterologous cells, with recent approaches focusing on GPCR complex stoichiometry and stability. Yet validation of these observations in native tissues is still lagging behind for the majority of GPCR heteromers. Remarkably, recent studies, particularly some involving advanced fluorescence microscopy techniques, are contributing to our current knowledge of aspects that were not well known until now, such as GPCR complex stoichiometry and stability. In parallel, a growing effort is being applied to move the field forward into native systems. This short review will highlight recent developments to study the stoichiometry and stability of GPCR complexes and methodologies to detect native GPCR dimers.

Structural biology of glycoprotein hormones and their receptors: insights to signaling

This article reviews the progress made in the field of glycoprotein hormones (GPH) and their receptors (GPHR) by several groups of structural biologists including ourselves aiming to gain insight into GPH signaling mechanisms. The GPH family consists of four members, with follicle-stimulating hormone (FSH) being the prototypic member. GPH members belong to the cystine-knot growth factor superfamily, and their receptors (GPHR), possessing unusually large N-terminal ectodomains, belong to the G-protein coupled receptor Family A. GPHR ectodomains can be divided into two subdomains: a high-affinity hormone binding subdomain primarily centered on the N-terminus, and a second subdomain that is located on the C-terminal region of the ectodomain that is involved in signal specificity. The two subdomains unexpectedly form an integral structure comprised of leucine-rich repeats (LRRs). Following the structure determination of hCG in 1994, the field of FSH structural biology has progressively advanced. Initially, the FSH structure was determined in partially glycosylated free form in 2001, followed by a structure of FSH bound to a truncated FSHR ectodomain in 2005, and the structure of FSH bound to the entire ectodomain in 2012. Comparisons of the structures in three forms led a proposal of a two-step monomeric receptor activation mechanism. First, binding of FSH to the FSHR high-affinity hormone-binding subdomain induces a conformational change in the hormone to form a binding pocket that is specific for a sulfated-tyrosine found as sTyr 335 in FSHR. Subsequently, the sTyr is drawn into the newly formed binding pocket, producing a lever effect on a helical pivot whereby the docking sTyr provides as the 'pull & lift' force. The pivot helix is flanked by rigid LRRs and locked by two disulfide bonds on both sides: the hormone-binding subdomain on one side and the last short loop before the first transmembrane helix on the other side. The lift of the sTyr loop frees the tethered extracellular loops of the 7TM domain, thereby releasing a putative inhibitory influence of the ectodomain, ultimately leading to the activating conformation of the 7TM domain. Moreover, the data lead us to propose that FSHR exists as a trimer and to present an FSHR activation mechanism consistent with the observed trimeric crystal form. A trimeric receptor provides resolution of the enigmatic, but important, biological roles played by GPH residues that are removed from the primary FSH-binding site, as well as several important GPCR phenomena, including negative cooperativity and asymmetric activation. Further reflection pursuant to this review process revealed additional novel structural characteristics such as the identification of a 'seat' sequence in GPH. Together with the 'seatbelt', the 'seat' enables a common heteodimeric mode of association of the common α subunit non-covalently and non-specifically with each of the three different β subunits. Moreover, it was possible to establish a dimensional order that can be used to estimate LRR curvatures. A potential binding pocket for small molecular allosteric modulators in the FSHR 7TM domain has also been identified.

In silico study on binding specificity of gonadotropins and their receptors: design of a novel and selective peptidomimetic for human follicle stimulating hormone receptor

Gonadotropins bind to specific receptors in spite of sharing a high level of sequence and structural similarity. This specific binding is crucial for maintaining the reproductive health of an organism. In this study, residues that dictate the receptor binding specificity of the gonadotropins (FSH and LH) have been identified using combination of in silico methods. Docking studies (ZDOCK), based on the systematic replacement of these residues, confirmed its importance in receptor binding. An interesting observation is that the relative positioning of the residues conferring binding specificity varied for the gonadotropin-receptor complexes. This spatial difference of the key residues could be exploited for design of specific modulators. Based on the identified residues, we have rationally designed a peptidomimetic (FSHP) that displays good binding affinity and specificity for hFSHR. FSHP was developed by screening 3.9 million compounds using pharmacophore-shape similarity followed by fragment-based approach. It was observed that FSHP and hFSHâ can share the same receptor binding site thereby mimicking the native hFSHR-FSH interactions. FSHP also displayed higher binding affinity to hFSHR as compared to two reported hFSHR antagonists. MD simulation studies on hFSHR-FSHP complex revealed that FSHP is conformationally rigid and the intermolecular interactions are maintained during the course of simulation.

The luteinizing hormone receptor: insights into structure-function relationships and hormone-receptor-mediated changes in gene expression in ovarian cancer cells

The luteinizing hormone receptor (LHR), one of the three glycoprotein hormone receptors, is necessary for critical reproductive processes, including gonadal steroidogenesis, oocyte maturation and ovulation, and male sex differentiation. Moreover, it has been postulated to contribute to certain neoplasms, particularly ovarian cancer. A member of the G protein-coupled receptor family, LHR contains a relatively large extracellular domain responsible for high affinity hormone binding; transmembrane activation then leads to G protein coupling and subsequent second messenger production. This review deals with recent advances in our understanding of LHR structure and structure-function relationships, as well as hormone-mediated changes in gene expression in ovarian cancer cells expressing LHR. Suggestions are also made for critical gaps that need to be filled as the field advances, including determination of the three-dimensional structure of inactive and active receptor, elucidation of the mechanism by which hormone binding to the extracellular domain triggers the activation of Gs, clarification of the putative roles of LHR in non-gonadal tissues, and the role, if any, of activated receptor in the development or progression of ovarian cancer.

Functional differences of invariant and highly conserved residues in the extracellular domain of the glycoprotein hormone receptors

Multiple interactions exist between human follicle-stimulating hormone (FSH) and the N-terminal hormone-binding fragment of the human FSH receptor (FSHR) extracellular domain (ECD). Binding of the other human glycoprotein hormones to their cognate human receptors (luteinizing hormone receptor (LHR) and thyroid-stimulating hormone receptor (TSHR)) was expected to be similar. This study focuses on amino acid residues in β-strands 2 (Lys(74)), 4 (Tyr(124), Asn(129), and Thr(130)), and 5 (Asp(150) and Asp(153)) of the FSHR ECD identified in the human FSH·FSHR ECD crystal structure as contact sites with the common glycoprotein hormone α-subunit, and on noncontact residues in β-strands 2 (Ser(78)) and 8 (Asp(224) and Ser(226)) as controls. These nine residues are either invariant or highly conserved in LHR and TSHR. Mutagenesis and functional characterization of these residues in all three human receptors allowed an assessment of their contribution to binding and receptor activation. Surprisingly, the six reported α-subunit contact residues of the FSHR ECD could be replaced without significant loss of FSH binding, while cAMP signaling potency was diminished significantly with several replacements. Comparative studies of the homologous residues in LHR and TSHR revealed both similarities and differences. The results for FSH/FSHR were analyzed on the basis of the crystal structure of the FSH·FSHR ECD complex, and comparative modeling was used to generate structures for domains, proteins, and complexes for which no structures were available. Although structural information of hormone-receptor interaction allowed the identification of hormone-receptor contact sites, functional analysis of each contact site was necessary to assess its contribution to hormone binding and receptor activation.

Perspectives on fish gonadotropins and their receptors

Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve fibers from the preoptic region to the pituitary. The various cell types in the teleost pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH) similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist of a common alpha subunit non-covalently associated with a hormone-specific beta subunit. In recent years the availability of molecular cloning techniques allowed the isolation of the genes coding for the GtH subunits in 56 fish species representing at least 14 teleost orders. Advanced molecular engineering provides the technology to produce recombinant GtHs from isolated cDNAs. Various expression systems have been used for the production of recombinant proteins. Recombinant fish GtHs were produced for carp, seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA, PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal steroids and peptides exert their effects on the gonadotropins either directly or via the hypothalamus. All these are discussed in detail in this review. In mammals, the biological activities of FSH and LH are directed to different gonadal target cells through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR), respectively, and the interaction between each gonadotropin-receptor couple is highly selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific as a result of promiscuous hormone-receptor interactions, while FSHR expression in Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.

The cloned equine thyrotropin receptor is hypersensitive to human chorionic gonadotropin; identification of three residues in the extracellular domain involved in ligand specificity

The receptors for TSH, LH/chorionic gonadotropin (CG), and FSH belong to the same subfamily of G protein-coupled receptors. The specificity of recognition of their cognate hormone involves a limited number of residues in the leucine-rich repeats present in the N-terminal ectodomain of the receptor. It is admitted that receptors of this subfamily coevoluted with their respective ligands. The secretion of CG is restricted to gestation of primates and Equidae. We hypothesized that, facing the challenge of a new hormone, the glycoprotein hormone receptors would have evolved differently in Equidae and human so that distinct residues are involved in hormone specificity. In particular, it is known that equine CG has a dual (FSH and LH) activity when administered to other species. In the present work, we cloned and characterized functionally the equine TSH receptor (TSHR), which shares 89% homology with the human TSHR. The equine TSHR is not responsive to equine CG but is more sensitive to human CG than the human TSHR. Three residues, at positions 60, 229, and 235 of the ectodomain, are responsible for this difference in sensitivity as shown by modelization and targeted mutagenesis, followed by in vitro functional characterization. The phylogenetic approach is a suitable approach to identify determinants of specificity of receptors.

Identification of a luteinizing hormone-selective determinant in the exodomain of a follicle-stimulating hormone receptor

Mammalian glycoprotein hormone receptors (GpHRs) display a stringent selectivity for their cognate hormones. In contrast, the follicle-stimulating hormone receptor of the African catfish (cfFSHR) is promiscuously activated by catfish luteinizing hormone (cfLH). Glycoprotein hormones bind to the concave site of the cusp-shaped N-terminal GpHR exodomain, which is formed by 9-10 parallel beta-strands. Hence, hormone selectivity of each GpHR for its cognate ligand is defined by amino acid sequence divergence in these beta-strands between different GpHRs. To identify the molecular determinants that allow promiscuous activation of the cfFSHR by cfLH, beta-strands were systematically exchanged between the cfFSHR and the human FSHR. Both gain-of-function and loss-of-function mutational approaches revealed that beta-strand 2 of the cfFSHR contains determinants that contribute to the receptor's responsiveness to cfLH.

The gonadotropin receptors FSH-R and LH-R of Atlantic halibut (Hippoglossus hippoglossus), 1: isolation of multiple transcripts encoding full-length and truncated variants of FSH-R

As a first step towards understanding the regulatory mechanisms underlying the asynchronous oogenesis in repetitive spawning fish, full-length cDNAs encoding the receptors for follicle stimulating hormone (FSH-R) and luteinizing hormone (LH-R) were isolated from the gonads of the flatfish Atlantic halibut (Hippoglossus hippoglossus). The predicted halibut FSH-R and LH-R of 664 and 698 amino acids, respectively, both contain the characteristic features of a large extracellular (EC) domain, a hepta-helical transmembrane (TM) domain, and a short cytoplasmic C-terminal tail. Halibut FSH-R and LH-R share only 42% overall sequence identity mostly due to low homology in the ligand-binding EC domain. Both receptors show high sequence identity to their orthologs of Nile tilapia, but seem to be more remotely related to the receptors in catfish, zebrafish and salmonids. In contrast to the intron-less TM domain of almost all vertebrate gonadotropin receptors, three introns were identified in this domain of halibut FSH-R, thus resembling the gene structure of Drosophila glycoprotein hormone receptor type I. The FSH-R pre-mRNA was shown to be processed in alternative ways by isolating two different transcripts encoding the complete receptor and four alternative spliced transcripts encoding different truncated receptor variants. Based on the DNA sequence variation and chromosomal organization of the gonadotropin receptors in several teleosts, we propose that the encoding genes have been duplicated in the fish lineage.

Identification of the first germline mutation in the extracellular domain of the follitropin receptor responsible for spontaneous ovarian hyperstimulation syndrome

The receptors for follitropin (FSHR), thyrotropin (TSHR), and lutropin/chorionic gonadotropin (LHCGR) are the members of the glycoprotein hormone (GPH) receptors (GPHR) family. They present a bipartite structure with a large extracellular amino-terminal domain (ECD), responsible for high-affinity hormone binding, and a carboxyl-terminal serpentine region, implicated in transduction of the activation signal. Spontaneous ovarian hyperstimulation syndrome (sOHSS) is a rare genetic condition in which human chorionic gonadotropin (hCG) promiscuously stimulates the FSHR during the first trimester of pregnancy. Surprisingly, germline FSHR mutations responsible for the disease have so far been found only in the transmembrane helices of the serpentine region of the FSHR, outside the hormone binding domain. When tested functionally, all mutants were abnormally sensitive to both hCG and thyrotropin (TSH) while displaying constitutive activity. This loss of ligand specificity was attributed to the lowering of an intramolecular barrier of activation rather than to an increase of binding affinity. Here we report the first germline mutation responsible for sOHSS (c.383C>A, p.Ser128Tyr), located in the ECD of the FSHR. Contrary to the mutations described previously, the p.Ser128Tyr FSHR mutant displayed increase in affinity and sensitivity toward hCG and did not show any constitutive activity, nor promiscuous activation by TSH. Thus, sOHSS can be achieved from different molecular mechanisms involving each functional domains of the FSHR. Based on the structure of the FSHR/FSH complex and site-directed mutagenesis studies, we provide robust molecular models for the GPH/GPHR complexes and we propose a molecular explanation to the binding characteristics of the p.Ser128Tyr mutant.

Defining the LGR8 Residues Involved in Binding Insulin-Like Peptide 3

The peptide hormone insulin-like peptide 3 (INSL3) is essential for testicular descent and has been implicated in the control of adult fertility in both sexes. The human INSL3 receptor leucine-rich repeat-containing G protein-coupled receptor 8 (LGR8) binds INSL3 and relaxin with high affinity, whereas the relaxin receptor LGR7 only binds relaxin. LGR7 and LGR8 bind their ligands within the 10 leucine-rich repeats (LRRs) that comprise the majority of their ectodomains. To define the primary INSL3 binding site in LGR8, its LRRs were first modeled on the crystal structure of the Nogo receptor (NgR) and the most likely binding surface identified. Multiple sequence alignment of this surface revealed the presence of seven of the nine residues implicated in relaxin binding to LGR7. Replacement of these residues with alanine caused reduced [125I]INSL3 binding, and a specific peptide/receptor interaction point was revealed using competition binding assays with mutant INSL3 peptides. This point was used to crudely dock the solution structure of INSL3 onto the LRR model of LGR8, allowing the prediction of the INSL3 Trp-B27 binding site. This prediction was then validated using mutant INSL3 peptide competition binding assays on LGR8 mutants. Our results indicated that LGR8 Asp-227 was crucial for binding INSL3 Arg-B16, whereas LGR8 Phe-131 and Gln-133 were involved in INSL3 Trp-B27 binding. From these two defined interactions, we predicted the complete INSL3/LGR8 primary binding site, including interactions between INSL3 His-B12 and LGR8 Trp-177, INSL3 Val-B19 and LGR8 Ile-179, and INSL3 Arg-B20 with LGR8 Asp-181 and Glu-229.

Molecular characterization of two sea bass gonadotropin receptors: cDNA cloning, expression analysis, and functional activity

The follicle-stimulating hormone (FSH) and the luteinizing hormone (LH) play central roles in vertebrate reproduction. They act through their cognate receptors to stimulate testicular and ovarian functions. The present study reports the cloning and characterization of two sea bass (Dicentrarchus labrax) cDNAs encoding a FSH receptor (sbsFSHR) and a LH receptor (sbsLHR). The mature proteins display typical features of the glycoprotein hormone receptor family members, but the sbsFSHR also contains some remarkable differences when compared with other fish or mammalian FSHRs. Among them, a distinct extracellular N-terminal cysteine domain as regards to its length and cysteine number, and the presence of an extra leucine-rich repeat. Expression analysis revealed that the sbsFSHR is exclusively expressed in gonadal tissues, specifically in the follicular wall of previtellogenic and early-vitellogenic follicles. On the contrary, sbsLHR mRNA was found to be widely distributed in sea bass somatic tissues. When stably expressed in mammalian cell lines, sbsFSHR was specifically stimulated by bovine FSH, while sbsLHR was activated by both bovine LH and FSH. Nevertheless, specific stimulation of the sbsLHR was observed when recombinant sea bass gonadotropins were used. The isolation of a FSHR and a LHR in sea bass opens new ways to study gonadotropin action in this species.

A functional transmembrane complex: the luteinizing hormone receptor with bound ligand and G protein

The luteinizing hormone receptor (LHR) is one of eight members in a cluster of the rhodopsin family of the large G protein-coupled receptor (GPCR) superfamily that contains some 800-900 genes in the human genome. LHR, along with its paralogons, follicle stimulating hormone receptor (FSHR) and thyroid stimulating hormone receptor, form one of the three classes in this cluster; the two other classes contain the relaxin-binding GPCRs and orphan GPCRs. These GPCRs are characterized by a relatively large ectodomain (ECD) containing leucine-rich-repeats (LRRs); in the class of glycoprotein hormone receptors, the LRR region is capped by N-terminal and C-terminal cysteine-rich regions. Binding of human chorionic gonadotropin (hCG) or luteinizing hormone to the LHR-ECD triggers a conformational change of the transmembrane region of the receptor facilitating binding and activation of Gs, followed by effector enzyme activation and subsequent intracellular signaling. Viewing LHR as a transmembrane anchoring protein that sequentially binds hCG and Gs to give the hCG-LHR-Gs complex, numerous interactions and conformational changes must be considered. There is, unfortunately, a paucity of structural data on LHR, but crystal structures exist for hCG, the homologous FSH-FSHR-ECD (N-terminal fragment) complex, rhodopsin (in the inactive state), an active form of Galphas (transducin), and the betagamma heterodimer. Using a combined experimental (site-directed mutagenesis followed by characterization in transfected cells) and computational (homology modeling and molecular dynamics simulations) approach, good working models are being developed for the protein-protein interaction faces and, in some cases, the ensuing conformational changes induced by complex formation. hCG binding to the LHR-ECD appears to involve several LRRs; LHR activation can be described in terms of disrupting a network of H-bonds in the cytosolic halves of helices 1-3, 6, and 7; and binding of LHR to Gs involves, in large part, intracellular loop 2 binding, presumably to Gsalpha at its C-terminus. Major gaps exist in our understanding at the molecular level of the six-polypeptide chain complex, hCG-LHR-Gs, but considerable progress has been made in the past few years.

Assembly and structural characterization of an authentic complex between human follicle stimulating hormone and a hormone-binding ectodomain of its receptor

Follicle stimulating hormone (FSH) is secreted from the pituitary gland to regulate reproduction in vertebrates. FSH signals through a G-protein coupled receptor (FSHR) on the target cell surface. We describe here the strategy to produce a soluble FSH-FSHR complex that involves the co-secretion of a truncated FSHR ectodomain (FSHR(HB)) and a covalently linked FSHalphabeta heterodimer from baculovirus-infected insect cells. FSH binds to FSHR(HB) with a high affinity comparable to that for the full-length receptor. The crystal structure of the FSH-FSHR(HB) complex provides explanations for the high affinity and specificity of FSH interaction with FSHR, and it shows an unexpected dimerization of these complexes. Here we also compare the crystal structure with theoretical models of the FSH-FSHR-binding mode. We conclude that the FSH-FSHR(HB) structure gives an authentic representation of FSH binding to intact FSHR.

Ligand-selective determinants in gonadotropin receptors

In mammals, the interactions between gonadotropins and their cognate receptors are highly specific; unintended cross-reactivity under normal physiological conditions has not been observed. This paper summarizes the comparative structure-function studies that aim at elucidating the molecular basis of the ligand selectivity.

The antigen binding sites of various hCG monoclonal antibodies show homology to different domains of LH receptor

The common feature of receptors and antibodies against the ligand is that both display very specific, high affinity binding towards the ligand. Therefore, it can be hypothesized that the paratope of antibodies may exhibit homology with distinct domains of the receptor. By locating the hormone epitopes and determining the structure of the paratopes, it should be possible to identify the contact points between the ligand and the receptor. This hypothesis has been tested using hCG monoclonal antibodies (MAbs) recognizing different epitopes and having different effects on hormone binding and response. The beta subunit and heterodimer specific antibodies inhibited both hormone binding and response, while the alpha subunit specific antibodies inhibited response without affecting binding. The single chain fragment variables (ScFvs) produced from these antibodies also retained the properties of the parent antibodies. The amino acid sequences of the ScFvs exhibited homology to different regions of the receptor; the beta subunit specific antibody being homologous to the concave surface of the leucine rich repeats (LRR) of the receptor, particularly the concave surface of the LRRs, while the heterodimer specific antibody showed homology to the hinge region. The alpha subunit specific antibody showed homology to the transmembrane domain of the receptor. The exact locations of the epitopes of the monoclonal antibodies in the hormone molecule have also been identified. The data presented here also support the model of glycoprotein hormone-receptor interaction in which the hormone binds to the extracellular domain through the beta subunit and then the alpha subunit is brought in contact with the transmembrane domain leading to signal transduction.

Biological effect of a novel mutation in the third leucine-rich repeat of human luteinizing hormone receptor

A novel heterozygous mutation A340T leading to the substitution of Phe for the conserved amino acid Ile114 was identified by nucleotide sequencing of the human LH/chorionic gonadotropin receptor (hLHR) of a patient with Leydig cell hypoplasia. This mutation is located in the third leucine-rich repeat in the ectodomain of the hLHR. In vitro expression studies demonstrated that this mutation results in reduced ligand binding and signal transduction of the receptor. Studies of hLHR constructs in which various amino acids were substituted for the conserved Ile114 showed that receptor activity is sensitive to changes in size, shape, and charge of the side chain. A homology model of the wild-type hLHR ectodomain was made, illustrating the packing of conserved hydrophobic side chains in the protein core. Substitution of Ile114 by Phe might disrupt intermolecular contacts between hormone and receptor. This mutation might also affect an LHR-dimer interaction. Thus, the I114F mutation reduces ligand binding and signal transduction by the hLHR, and it is partially responsible for Leydig cell hypoplasia in the patient.

Identification of follicle-stimulating hormone-selective beta-strands in the N-terminal hormone-binding exodomain of human gonadotropin receptors

Glycoprotein hormone receptors contain large N-terminal extracellular domains (ECDs) that distinguish these receptors from most other G protein-coupled receptors. Each glycoprotein hormone receptor ECD consists of a curved leucine-rich repeat domain flanked by N- and C-terminal cysteine-rich regions. Selectivity of the different glycoprotein hormone receptors for their cognate hormones is exclusively determined by their ECDs and, in particular, their leucine-rich repeat domain. To identify human (h)FSH-selective determinants we used a gain-of-function mutagenesis strategy in which beta-strands of the hLH receptor (hLH-R) were substituted with their hFSH receptor (hFSH-R) counterparts. Introduction of hFSH-R beta-strand 1 into hLH-R conferred responsiveness to hFSH, whereas hLH-R mutants harboring one of the other hFSH-R beta-strands displayed none or very limited sensitivity to hFSH. However, combined substitution of hFSH-R beta-strand 1 and some of the other hFSH-R beta-strands further increased the sensitivity of the mutant hLH-R to hFSH. The apparent contribution of multiple hFSH-R beta-strands in providing a selective hormone binding interface corresponds well with their position in relation to hFSH as recently determined in the crystal structure of hFSH in complex with part of the hFSH-R ECD.

GRIS: glycoprotein-hormone receptor information system

The glycoprotein-hormone receptor information system (GRIS) presents a comprehensive view on all available molecular data for the lutropin/choriogonadotropin receptor, follitropin receptor, and thyrotropin receptor G protein-coupled receptors. It features a mutation database presently containing 696 point mutations, combined with all sequences and the associated homology models. The mutation information was automatically extracted from the literature and manually augmented with respect to constitutivity, surface expression, sensitivity to hormones, and binding affinity. All information in this integrated system is presented in a G protein-coupled receptor specialist-friendly way. A series of interactive tools such as rotamer analysis, mutation prediction, or cavity visualization aids with the design and interpretation of experiments. A universal residue numbering system has been introduced to ease database searches as well as the use of the information in conjunction with literature data from diverse origins. Users can upload new mutations. GRIS is freely accessible at http://gris.ulb.ac.be/.

Fish FSH receptors bind LH: how to make the human FSH receptor to be more fishy?

In mammals, the interactions between glycoprotein hormones and their cognate receptors are highly specific; unintended cross-reactivity under normal physiological conditions has not been observed. The interactions between fish gonadotropins and their receptors, on the other hand, appeared to be less discriminatory. For example, the catfish follicle-stimulating hormone (FSH) receptor was highly responsive to both catfish luteinizing hormone (LH) and catfish FSH. Similarly, the FSH receptor of coho salmon bound both salmon FSH and LH. In contrast, LH receptors of both species were found to be rather specific for their cognate LH. This paper intends to summarize the current situation with special emphasis to our comparative structure-function studies that aim at elucidating the molecular basis of ligand selectivity (in mammals) and ligand promiscuity (in fish).

Understanding ovarian hyperstimulation syndrome

The ovarian hyperstimulation syndrome (OHSS) is a potentially life-threatening complication of ovarian stimulation treatments. Severe forms are characterized by a massive ovarian enlargement with the formation of multiple ovarian cysts associated with extravascular fluid shifts resulting in the development of ascites, pleural and/or pericardial effusion. The pathophysiology of the syndrome has not been completely elucidated yet. The vascular fluid leakage is thought to result from an increased capillary permeability of mesothelial surfaces under the action of one or several vasoactive ovarian factor(s) produced by the multiple corpora lutea. The paper focuses on the recent identification of mutations in the FSH receptor gene that display an increased sensitivity to hCG and are responsible for the development of spontaneous OHSS occurring during pregnancy. These findings have shed light for the first time on the molecular basis of the pathophysiology of the spontaneous form of the syndrome. As spontaneous and iatrogenic OHSS share similar pathophysiological sequences including massive recruitment and growth of ovarian follicles, extensive luteinization provoked by hCG, and oversecretion of vasogenic molecules by the corpora lutea, they have also opened new research perspectives for the understanding of the much more frequent iatrogenic OHSS.

Selective ligand-binding determinants in catfish and human gonadotropin receptors

In mammals, the specificity of FSH-FSH receptor (FSHR), LH-LH receptor (LHR) and TSH-TSH receptor couples is such that no cross-activation occurs under normal physiological conditions. The interactions between fish gonadotropins and their receptors, however, appear to be less discriminatory. For example, the catfish FSHR is highly responsive to both catfish LH and catfish FSH, while the catfish LHR is specific for its cognate LH. Comparative structure-function studies aimed at elucidating the molecular basis of ligand promiscuity (in fish) and ligand selectivity (in mammals) are described in this paper.

Structure of human follicle-stimulating hormone in complex with its receptor

Follicle-stimulating hormone (FSH) is central to reproduction in mammals. It acts through a G-protein-coupled receptor on the surface of target cells to stimulate testicular and ovarian functions. We present here the 2.9-A-resolution structure of a partially deglycosylated complex of human FSH bound to the extracellular hormone-binding domain of its receptor (FSHR(HB)). The hormone is bound in a hand-clasp fashion to an elongated, curved receptor. The buried interface of the complex is large (2,600 A2) and has a high charge density. Our analysis suggests that all glycoprotein hormones bind to their receptors in this mode and that binding specificity is mediated by key interaction sites involving both the common alpha- and hormone-specific beta-subunits. On binding, FSH undergoes a concerted conformational change that affects protruding loops implicated in receptor activation. The FSH-FSHR(HB) complexes form dimers in the crystal and at high concentrations in solution. Such dimers may participate in transmembrane signal transduction.

The differential binding affinities of the luteinizing hormone (LH)/choriogonadotropin receptor for LH and choriogonadotropin are dictated by different extracellular domain residues

The high degree of amino acid sequence homology and the divergent ligand binding affinities of the rat (r) and human (h) LH receptors (LHRs) allowed us to identify amino acid residues of their extracellular domain that are responsible for the different binding affinities of bovine (b) and hLH, and human choriogonadotropin (hCG) to the hLHR and rLHR. Because of the proposed importance of the beta-sheets of the leucine-rich repeats (LRRs) of the extracellular domain of the LHR on hormone binding, we examined 10 divergent residues present in these regions by analyzing two complementary sets of mutants in which hLHR residues were substituted with the corresponding rLHR residues and vice versa. These experiments resulted in the identification of a single residue (a Ile or Ser in the C-terminal end of LRR2 of the hLHR or rLHR, respectively) that is important for hLH binding affinity. Surprisingly, however, this residue does not affect hCG or for bLH binding affinity. In fact, the results obtained with bLH and hCG show that several of the divergent residues in the beta-sheets of LRR1-9 affect bLH binding affinity, but none of them affect hCG binding affinity. Importantly, our results also emphasize the involvement of residues outside of the beta-sheets of the LRRs of the LHR in ligand binding affinity. This finding has to be considered in future models of the interaction of LH/CG with the LHR.

Modulation of ligand selectivity associated with activation of the transmembrane region of the human follitropin receptor

Recently, three naturally occurring mutations in the serpentine region of the FSH receptor (FSHr) (D567N and T449I/A) have been identified in three families with spontaneous ovarian hyperstimulation syndrome (OHSS). All mutant receptors displayed abnormally high sensitivity to human chorionic gonadotropin and, in addition, D567N and T449A displayed concomitant increase in sensitivity to TSH and detectable constitutive activity. In the present study, we have used a combination of site-directed mutagenesis experiments and molecular modeling to explore the mechanisms responsible for the phenotype of the three OHSS FSHr mutants. Our results suggest that all mutations lead to weakening of interhelical locks between transmembrane helix (TM)-VI and TM-III, or TM-VI and TM-VII, which contributes to maintaining the receptor in the inactive state. They also indicate that broadening of the functional specificity of the mutant FSHr constructs is correlated to their increase in constitutive activity. This relation between basal activity and functional specificity is a characteristic of the FSHr, which is not shared by the other glycoprotein hormone receptors. It leads to the interesting suggestion that different pathways have been followed during primate evolution to avoid promiscuous stimulation of the TSHr and FSHr by human chorionic gonadotropin. In the hFSHr, specificity would be exerted both by the ectodomain and the serpentine portion.

Evolutionary divergence of thyrotropin receptor structure

The availability of 18 thyrotropin receptor (TSHR) sequences, including two recent entries for primates and seven from fish, have allowed us to investigate diversification of residues or domains during evolution. We used a likelihood ratio test for evolutionary rate shifts [Proc. Natl. Acad. Sci. 98 (2001) 14512] using LH/CGR sequences as an out-group. At each residue in the alignment, a statistical test was performed for a rate shift at the divergence between mammals and fish. Eighty-two rate shift sites were found, significantly more than was expected (p < 0.0001). The occurrence of rate shifts was highest in the intracellular tail, lowest in the transmembrane serpentine and intermediate in the ectodomain. In 52 mammalian sites, the rates were significantly faster than for the corresponding sites in fish. We have identified rate shift in sites important to TSHR function or in intimate proximity to such regions. The former category includes residues 53 and 55 (of LLR1 beta strand) and 253 and 255 (of LLR9 beta strand), crucial to TSH thyrotropic activity, residue 113, the site of N-linked glycosylation limited to humans, residue 310, an important switch in the hinge region for receptor binding and constitutive activity and residue 382 which centres a motif important for TSH-mediated receptor activation. The rate shifts positions close to functional region include a site proximal to a TSHR-specific motif on LLR3 beta strand, sites important in TM helix structure and homodimerization as well as, in the case of the third intracellular loop, to TSHR/G protein coupling. Rate shift analyses have identified residues whose manipulation in the human TSHR may lead to better understanding of receptor functions and help in the creation of designer analogues.