Differential roles of exoloop 1 of the human follicle-stimulating hormone receptor in hormone binding and receptor activation

A Novel Mutation in the FSH Receptor (I423T) Affecting Receptor Activation and Leading to Primary Ovarian Failure

CONTEXT

Follicle-stimulating hormone (FSH) plays an essential role in gonadal function. Loss-of-function mutations in the follicle-stimulating hormone receptor (FSHR) are an infrequent cause of primary ovarian failure.

OBJECTIVE

To analyze the molecular physiopathogenesis of a novel mutation in the FSHR identified in a woman with primary ovarian failure, employing in vitro and in silico approaches, and to compare the features of this dysfunctional receptor with those shown by the trafficking-defective D408Y FSHR mutant.

METHODS

Sanger sequencing of the FSHR cDNA was applied to identify the novel mutation. FSH-stimulated cyclic adenosine monophosphate (cAMP) production, ERK1/2 phosphorylation, and desensitization were tested in HEK293 cells. Receptor expression was analyzed by immunoblotting, receptor-binding assays, and flow cytometry. Molecular dynamics simulations were performed to determine the in silico behavior of the mutant FSHRs.

RESULTS

A novel missense mutation (I423T) in the second transmembrane domain of the FSHR was identified in a woman with normal pubertal development but primary amenorrhea. The I423T mutation slightly impaired plasma membrane expression of the mature form of the receptor and severely impacted on cAMP/protein kinase A signaling but much less on β-arrestin-dependent ERK1/2 phosphorylation. Meanwhile, the D408Y mutation severely affected membrane expression, with most of the FSH receptor located intracellularly, and both signal readouts tested. Molecular dynamics simulations revealed important functional disruptions in both mutant FSHRs, mainly the loss of interhelical connectivity in the D408Y FSHR.

CONCLUSIONS

Concurrently, these data indicate that conformational differences during the inactive and active states account for the distinct expression levels, differential signaling, and phenotypic expression of the I423T and D408Y mutant FSHRs.

Constitutive Activation and Inactivation of Mutations Inducing Cell Surface Loss of Receptor and Impairing of Signal Transduction of Agonist-Stimulated Eel Follicle-Stimulating Hormone Receptor

In the present study, we investigated the signal transduction of mutants of the eel follicle-stimulating hormone receptor (eelFSHR). Specifically, we examined the constitutively activating mutant D540G in the third intracellular loop, and four inactivating mutants (A193V, N195I, R546C, and A548V). To directly assess functional effects, we conducted site-directed mutagenesis to generate mutant receptors. We measured cyclic adenosine monophosphate (cAMP) accumulation via homogeneous time-resolved fluorescence assays in Chinese hamster ovary (CHO-K1) cells and investigated cell surface receptor loss using an enzyme-linked immunosorbent assay in human embryonic kidney (HEK) 293 cells. The cells expressing eelFSHR-D540G exhibited a 23-fold increase in the basal cAMP response without agonist treatment. The cells expressing A193V, N195I, and A548V mutants had completely impaired signal transduction, whereas those expressing the R546C mutant exhibited little increase in cAMP responsiveness and a small increase in signal transduction. Cell surface receptor loss in the cells expressing inactivating mutants A193V, R546C, and A548V was clearly slower than in the cell expressing the wild-type eelFSHR. However, cell surface receptor loss in the cells expressing inactivating mutant N195I decreased in a similar manner to that of the cells expressing the wild-type eelFSHR or the activating mutant D540G, despite the completely impaired cAMP response. These results provide important information regarding the structure–function relationships of G protein-coupled receptors during signal transduction.

Allosteric Regulation of the Follicle-Stimulating Hormone Receptor

Follicle-stimulating hormone receptor (FSHR) belongs to the leucine-rich repeat family of the G protein-coupled receptor (LGR), which includes the glycoprotein hormone receptors luteinizing hormone receptor, thyrotropin receptor, and other LGRs 4, 5, 6, and 7. FSH is the key regulator of folliculogenesis in females and spermatogenesis in males. FSH elicits its physiological response through its cognate receptor on the cell surface. Binding of the hormone FSH to its receptor FSHR brings about conformational changes in the receptor that are transduced through the transmembrane domain to the intracellular region, where the downstream effector interaction takes place, leading to activation of the downstream signaling cascade. Identification of small molecules that could activate or antagonize FSHR provided interesting tools to study the signal transduction mechanism of the receptor. However, because of the nature of the ligand-receptor interaction of FSH-FSHR, which contains multiple sites in the extracellular binding domain, most of the small-molecule modulators of FSHR are unable to bind to the orthosteric site of the receptors. Rather they modulate receptor activation through allosteric sites in the transmembrane region. This review will discuss allosteric modulation of FSHR primarily through the discovery of small-molecule modulators, focusing on current data on the status of development and the utility of these as tools to better understand signaling mechanisms.

FSH receptor-specific residues L501 and I505 in extracellular loop 2 are essential for its function

The extracellular loop 2 (EL2) of FSH receptor (FSHR) plays a pivotal role in various events downstream of FSH stimulation. Because swapping the six FSHR-specific residues in EL2 (chimeric EL2M) with those from LH/choriogonadotropin receptor resulted in impaired internalization of FSH-FSHR complex and low FSH-induced cAMP production, six substitution mutants of EL2 were generated to ascertain the contribution of individual amino acids to the effects shown by chimeric EL2M. Results revealed that L(501)F mainly and I(505)V to a lesser extent contribute to the diminished receptor function in chimeric EL2M. HEK293 cells stably expressing WT and chimeric EL2M FSHR were generated to track the fate of the receptors post FSH induction. The chimeric EL2M FSHR stable clone showed weak internalization and cAMP response similar to transiently transfected cells. Furthermore, reduced FSH-induced ERK phosphorylation was also observed. The interaction of activated chimeric EL2M and L(501)F FSHR with β-arrestins was weak compared with WT FSHR, thus explaining the impaired internalization of chimeric EL2M and corroborating the indispensable role of EL2 in receptor function.

Role of the Extracellular and Intracellular Loops of Follicle-Stimulating Hormone Receptor in Its Function

Follicle-stimulating hormone receptor (FSHR) is a leucine-rich repeat containing class A G-protein coupled receptor belonging to the subfamily of glycoprotein hormone receptors (GPHRs), which includes luteinizing hormone/choriogonadotropin receptor (LH/CGR) and thyroid-stimulating hormone receptor. Its cognate ligand, follicle-stimulating hormone binds to, and activates FSHR expressed on the surface of granulosa cells of the ovary, in females, and Sertoli cells of the testis, in males, to bring about folliculogenesis and spermatogenesis, respectively. FSHR contains a large extracellular domain (ECD) consisting of leucine-rich repeats at the N-terminal end and a hinge region at the C-terminus that connects the ECD to the membrane spanning transmembrane domain (TMD). The TMD consists of seven α-helices that are connected to each other by means of three extracellular loops (ELs) and three intracellular loops (ILs) and ends in a short-cytoplasmic tail. It is well established that the ECD is the primary hormone binding domain, whereas the TMD is the signal transducing domain. However, several studies on the ELs and ILs employing site directed mutagenesis, generation of chimeric receptors and in vitro characterization of naturally occurring mutations have proven their indispensable role in FSHR function. Their role in every phase of the life cycle of the receptor like post translational modifications, cell surface trafficking, hormone binding, activation of downstream signaling, receptor phosphorylation, hormone-receptor internalization, and recycling of hormone-receptor complex have been documented. Mutations in the loops causing dysregulation of these processes lead to pathophysiological conditions. In other GPHRs as well, the loops have been convincingly shown to contribute to various aspects of receptor function. This review article attempts to summarize the extensive contributions of FSHR loops and C-terminal tail to its function.

Normal testicular function without detectable follicle-stimulating hormone. A novel mutation in the follicle-stimulating hormone receptor gene leading to apparent constitutive activity and impaired agonist-induced desensitization and internalization

Activating mutations in the follicle-stimulating hormone (FSH) receptor (FSHR) gene are rarely detected due to the absence of a clearly defined phenotype, particularly in men. We here report the biochemical features of a novel mutation in the first extracellular loop of the FSHR. The mutation (N431I) was detected in an asymptomatic man exhibiting normal spermatogenesis, suppressed serum FSH, and normal or elevated levels of biochemical markers of FSH action. Employing different experimental strategies on HEK-293 cells transiently expressing the N431I FSHR mutant, we found that the mutation led to decreased cell surface plasma membrane expression of the receptor protein, but conferred a low level of constitutive activity associated with markedly altered agonist-stimulated desensitization and internalization. These latter features may contribute and/or amplify the persistent activation of the receptor in both absence and presence of agonist and provide new insights into opportunities for adjuvant therapies based on disruption of these processes.

Extracellular loop 2 in the FSH receptor is crucial for ligand mediated receptor activation

The present study aims to determine the role of the specific residues of the extracellular loops (ELs) of the FSH receptor (FSHR) in hormone binding and receptor activation. By substituting the sequences of each of the ELs of human FSHR with those of the luteinizing hormone/choriogonadotropin receptor (LH/CGR), we generated three mutant constructs where the three ELs were individually replaced. A fourth construct had all the three substituted ELs. The receptor expression and hormone binding ability of the mutants were comparable to that of the wild type. Hormone-induced signaling and internalization were lower in the EL2 substitution mutant (EL2M). In this mutant, the EL2 of FSHR was substituted with the corresponding loop of LH/CGR. Interestingly, homology modeling revealed a change in the orientation of EL2 in the mutant receptor. Thus, disruption of EL2 affected overall receptor function, suggesting the role of FSHR specific residues of the loop in ligand mediated signaling.

Thyrotropin and homologous glycoprotein hormone receptors: structural and functional aspects of extracellular signaling mechanisms

The TSH receptor (TSHR) together with the homologous lutropin/choriogonadotropin receptor and the follitropin receptor are glycoprotein hormone receptors (GPHRs). They constitute a subfamily of the rhodopsin-like G protein-coupled receptors with seven transmembrane helices. GPHRs and their corresponding hormones are pivotal proteins with respect to a variety of physiological functions. The identification and characterization of intra- and intermolecular signaling determinants as well as signaling mechanisms are prerequisites to gaining molecular insights into functions and (pathogenic) dysfunctions of GPHRs. Knowledge about activation mechanisms is fragmentary, and the specific aspects have still not been understood in their entirety. Therefore, here we critically review the data available for these receptors and bring together structural and functional findings with a focus on the important large extracellular portion of the TSHR. One main focus is the particular function of structural determinants in the initial steps of the activation such as: 1) hormone binding at the extracellular site; 2) hormone interaction at a second binding site in the hinge region; 3) signal regulation via sequence motifs in the hinge region; and 4) synergistic signal amplification by cooperative effects of the extracellular loops toward the transmembrane region. Comparison and consolidation of data from the homologous glycoprotein hormone receptors TSHR, follitropin receptor, and lutropin/choriogonadotropin receptor provide an overview of extracellular mechanisms of signal initiation, conduction, and regulation at the TSHR and homologous receptors. Finally, we address the issue of structural implications and suggest a refined scenario for the initial signaling process on GPHRs.

Multiple facets of follicle-stimulating hormone receptor function

Follicle-stimulating hormone (FSH) is a glycoprotein hormone produced by the anterior pituitary gland. This gonadotropin plays an essential role in reproduction. Its receptor (FSHR) belongs to the superfamily of G protein-coupled receptors (GPCR), specifically the family of rhodopsin-like receptors. Agonist binding to the FSHR triggers the rapid activation of multiple signaling cascades, mainly the cAMP-adenylyl cyclase-protein kinase A cascade, that impact diverse biological effects of FSH in the gonads. As in other G protein-coupled receptors, the several cytoplasmic domains of the FSHR are involved in signal transduction and termination of the FSH signal. Here we summarize some recent information on the signaling cascades activated by FSH as well as on the role of the intracytoplasmic domains of the FSHR in coupling to membrane and cytosolic proteins linked to key biological functions regulated by the FSH-FSHR system.

Differential actions of FSH and LH during folliculogenesis

In the gonadotrophin-dependent stage of follicular development, FSH- and LH-signalling pathways play an obligatory role in follicle differentiation, selection and survival. Under the effect of LH the theca-interstitial cell layer acts as an androgen producer. Thus, androgen diffusing into the mural granulosa cell layer represents the substrate for FSH-induced aromatase for follicular oestradiol synthesis. This is the landmark 'two cell-two gonadotrophin' concept in the physiology of ovarian function in mammals. The increase in plasma FSH during luteo-follicular transition is the basis for follicle selection. The rise of FSH to the threshold concentration represents a critical condition for the growth of the most sensitive follicle in a given time frame of the last 14 days of the dominant follicle odyssey. The gonadotrophin-induced follicular oestradiol secretion inhibits pituitary secretion of FSH, which in turn causes the concentration of FSH in the developing cohort follicles to drop below threshold concentrations and the arrest of the development of the less FSH-sensitive follicle (FSH threshold and window concept). In the gonadotrophin-dependent phase of follicular development, LH also seems to acts within a critical window of the hormone concentration framed between the minimal threshold and a ceiling for the normal functions of the follicle unit.

Trans-activation, cis-activation and signal selection of gonadotropin receptors

It has been thought that when a hormone binds to a receptor, the liganded receptor activates itself and generates hormone signals, such as the cAMP signal and the inositol phosphate signal (cis-activation). We describe that a liganded LH receptor or FSH receptor molecule is capable of intermolecularly activating nonliganded receptors (trans-activation). Particularly, intriguing is the possibility that a pair of compound heterozygous mutants, one defective in binding and the other defective in signaling, may cooperate and rescue signaling. Furthermore, trans-activation of the binding deficient receptors examined in our studies generates either the cAMP signal or the IP signal, but not both. Trans-activation and selective signal generation have broad implications on signal generation mechanisms, and suggest new therapeutic approaches.

Allosteric activation of the follicle-stimulating hormone (FSH) receptor by selective, nonpeptide agonists

The pituitary glycoprotein hormones, luteinizing hormone and follicle-stimulating hormone (FSH), act through their cognate receptors to initiate a series of coordinated physiological events that results in germ cell maturation. Given the importance of FSH in regulating folliculogenesis and fertility, the development of FSH mimetics has been sought to treat infertility. Currently, purified and recombinant human FSH are the only FSH receptor (FSH-R) agonists available for infertility treatment. By screening unbiased combinatorial chemistry libraries, using a cAMP-responsive luciferase reporter assay, we discovered thiazolidinone agonists (EC50's = 20 microm) of the human FSH-R. Subsequent analog library screening and parallel synthesis optimization resulted in the identification of a potent agonist (EC50 = 2 nm) with full efficacy compared with FSH that was FSH-R-selective and -dependent. The compound mediated progesterone production in Y1 cells transfected with the human FSH-R (EC50 = 980 nm) and estradiol production from primary rat ovarian granulosa cells (EC50 = 10.5 nm). This and related compounds did not compete with FSH for binding to the FSH-R. Use of human FSH/thyroid-stimulating hormone (TSH) receptor chimeras suggested a novel mechanism for receptor activation through a binding site independent of the natural hormone binding site. This study is the first report of a high affinity small molecule agonist that activates a glycoprotein hormone receptor through an allosteric mechanism. The small molecule FSH receptor agonists described here could lead to an oral alternative to the current parenteral FSH treatments used clinically to induce ovarian stimulation for both in vivo and in vitro fertilization therapy.

Interactions between the extracellular domain and the extracellular loops as well as the 6th transmembrane domain are necessary for TSH receptor activation

OBJECTIVE

The molecular mechanisms of TSH receptor (TSHR) activation and intramolecular signal transduction are largely unknown. Deletion of the extracellular domain (ECD) of the TSHR results in increased constitutive activity, which suggests a self-inhibitory interaction between the ECD and the extracellular loops (ECLs) or the transmembrane domains (TMDs). To investigate these potential interactions and to pursue the idea that mutations in the ECD affect the constitutive activity of mutants in the ECLs or TMDs we generated double mutants between position 281 in the ECD and mutants in all three ECLs as well as the 6th TMD.

DESIGN

We combined mutation S281D, characterized by an impaired TSH-stimulated cAMP response, with the constitutively activating in vivo mutations I486F (1st ECL), I568T (2nd ECL), V656F (3rd ECL) and D633F (6th TMD). Further, we constructed double mutants containing the constitutively activating mutation S281N and one of the inactivating mutations D474E, T477I (1st ECL) and D633K (6th TMD).

RESULTS

The cAMP level of the double mutants with S281N and the inactive mutants in the 1st ECL was decreased below the level of the inactive single mutants, demonstrating that a constitutively activating mutation in the ECD cannot bypass disruption of signal transduction in the serpentine domain. In double mutants with S281D, basal and TSH-induced cAMP and inositol phosphate production of constitutively active mutants was reduced to the level of S281D.

CONCLUSION

The dominance of S281D and the dependence of constitutively activating mutations in the ECLs on the functionally intact ECD strongly suggest that interactions between these receptor domains are required for TSHR activation and intramolecular signal transduction.

Model of Glycoprotein Hormone Receptor Ligand Binding and Signaling

Studies described here were initiated to develop a model of glycoprotein hormone receptor structure and function. We found that the region that links the lutropin receptor leucine-rich repeat domain (LRD) to its transmembrane domain (TMD) has substantial roles in ligand binding and signaling, hence we term it the signaling specificity domain (SSD). Theoretical considerations indicated the short SSDs in marmoset lutropin and salmon follitropin receptors have KH domain folds. We assembled models of lutropin, follitropin, and thyrotropin receptors by aligning models of their LRD, TMD, and shortened SSD in a manner that explains how substitutions in follitropin and thyrotropin receptors distant from their apparent ligand binding sites enable them to recognize lutropins. In these models, the SSD is parallel to the concave surface of the LRD and makes extensive contacts with TMD outer loops 1 and 2. The LRD appears to contact TMD outer loop 3 and a few residues in helices 1, 5, 6, and 7. We propose that signaling results from contacts of the ligands with the SSD and LRD that alter the LRD, which then moves TMD helices 6 and 7. The positions of the LRD and SSD support the notion that the receptor can be activated by hormones that dock with these domains in either of two different orientations. This would account for the abilities of some ligands and ligand chimeras to bind multiple receptors and for some receptors to bind multiple ligands. This property of the receptor may have contributed significantly to ligand-receptor co-evolution.

Trans-activation of mutant follicle-stimulating hormone receptors selectively generates only one of two hormone signals

Previously, we reported that a liganded LH receptor (LHR) is capable of activating itself (cis-activation) and other nonliganded LHRs to induce cAMP (trans-activation). Trans-activation of the LHR raises two crucial questions. Is trans-activation unique to LHR or common to other G protein-coupled receptors? Does trans-activation stimulate phospholipase Cbeta as it does adenylyl cyclase? To address these questions, two types of novel FSH receptors (FSHRs) were constructed, one defective in hormone binding and the other defective in signal generation. The FSHR, a G protein-coupled receptor, comprises two major domains, the N-terminal extracellular exodomain that binds the hormone and the membrane-associated endodomain that generates the hormone signals. For signal defective receptors, the exodomain was attached to glycosyl phosphatidylinositol (ExoGPI) or the transmembrane domain of CD8 immune receptor (ExoCD). ExoGPI and ExoCD can trans-activate another nonliganded FSH. Surprisingly, the trans-activation generates a signal to activate either adenylyl cyclase or phospholipase Cbeta, but not both. These results indicate that trans-activation in these mutant receptors is selective and limited in signal generation, thus providing new approaches to investigating the generation of different hormone signals and a novel means to selectively generate a particular hormone signal. Our data also suggest that the FSHR's exodomain could not trans-activate LHR.

Follicle-stimulating hormone interacts with exoloop 3 of the receptor

The human follicle-stimulating hormone (FSH) receptor consists of two distinct domains of approximately 330 amino acids, the N-terminal extracellular exodomain and membrane-associated endodomain including three exoloops and seven transmembrane helices. The exodomain binds the hormone with high affinity, and the resulting hormone/exodomain complex modulates the endodomain where receptor activation occurs. It has been an enigma whether the hormone interacts with the endodomain. In a step to address the question, exoloop 3 of (580)KVPLITVSKAK(590) was examined by Ala scan, multiple substitution, assays for hormone binding, cAMP and inositol phosphate (IP) induction, and photoaffinity labeling. We present the evidence for the interaction of FSH and exoloop 3. A peptide mimic of exoloop 3 specifically and saturably photoaffinity-labels FSH alpha but not FSH beta. This is in contrast to photoaffinity labeling of FSH beta by the peptide mimic of the N-terminal region of the receptor. Leu(583) and Ile(584) are crucial for the interaction of FSH and exoloop 3. Substitutions of these two residues enhanced the hormone binding affinity. This is due to the loss of the original side chains but not the introduction of new side chains. The Leu(583) and Ile(584) side chains appear to project in opposite directions. Ile(584) appears to be so specific and to require flexibility and stereo specificity so that no other amino acids can fit into its place. Leu(583) is less specific. The improvement in hormone binding by substitutions was offset by the severe impairment of signal generation of cAMP and/or inositol phosphate. For example, the Phe or Tyr substitution of Leu(583) improved the hormone binding and cAMP induction but impaired IP induction. On the other hand, the substitutions for Ile(584) and Lys(590) abolished the cAMP and IP induction. Our results open a logical question whether Leu(583), Ile(584), and Lys(590) interact with the exodomain and/or the hormone. The answers will provide new insights into the mechanisms of hormone binding and signal generation.

Identification and characterization of a selective, nonpeptide follicle-stimulating hormone receptor antagonist

The glycoprotein hormones (LH, FSH, and TSH) are critical to the maintenance of physiological homeostasis and control of reproduction. However, despite an obvious utility for synthetic pharmacological agents, there are few reports of selective, nonpeptide agonists or antagonists to receptors for these hormones. We have identified and characterized a novel synthetic molecule capable of inhibiting the action of FSH. This compound, 7-[4-[Bis-(2-carbamoyl-ethyl)-amino]-6-chloro-(1,3,5)-triazin-2-ylamino)-4-hydroxy-3-(4-methoxy-phenylazo)-naphthalene]-2-sulfonic acid, sodium salt (compound 1), is a selective, noncompetitive inhibitor of the human (h) and rat (r) FSH receptors (FSHRs). Compound 1 selectively inhibited binding of [(125)I]hFSH with an IC(50) value of 5.4 +/- 2.3 micro M. Radioligand-binding assays were performed using the baculovirus expressed extracellular domain of hFSHR (BV-tFSHR) to demonstrate site-specific interaction. Compound 1 competed for [(125)I]hFSH binding to BV-tFSHR with an IC(50) value of 10 +/- 2.8 micro M. Functionally, compound 1 inhibited hFSH-induced cAMP accumulation and steroidogenesis in vitro with an IC(50) value of 3 +/- 0.6 micro M. Competition of compound 1 for binding to other glycoprotein hormone receptors and other G protein-coupled receptors demonstrated select activity for FHSRs. Compound 1 inhibited ovulation in immature and cycling adult rats. These data provide proof of concept that selective, small molecule antagonists can be designed for glycoprotein hormone receptors.

Harmonic mean relationship between affinity for wild-type receptors and alanine-scan mutants

Alanine-scanning mutagenesis involves systematic substitution of a single alanine for other amino acid constituents of a ligand or receptor protein. The sequential process generates a discrete set of "mutant" proteins, each member of which differs from the native 'wild-type' receptor (R(wt)) or ligand by the single amino acid (alpha) substitution with alanine (R(Ala)). The Ala mapsto alpha substitutions are made for amino acids suspected of being important for binding of ligand to receptor. Binding assays are then used to measure the affinity for R(wt) and mutants, quantified by the ligand-receptor equilibrium constant (K(eq)) or its reciprocal, the dissociation constant (K(d)=1/K(eq)). However, the relationship betweenR(wt)K(d) and R(Ala)K(d) values is not obvious. Neither the arithmetic mean nor the geometric mean (based on a simplified relationship between K(d) and the thermodynamic standard free energy change) of the R(Ala)K(d) values turns out to be adequate. The harmonic mean was found to be a good estimator when applied to the published data from 14 alanine-scan studies. This result is consonant with the present understanding of ligand-receptor interactions.

Rescue of intracellularly trapped lutropin receptor exodomain by endodomain and reconstitution of a functional membrane receptor: interaction between exo- and endodomains

The lutropin receptor consists of an extracellular N-terminal half and a membrane-associated C-terminal half. hCG initially binds the exodomain with a high affinity and the resulting complex is thought to interact with the endodomain through a secondary contact generating a hormonal signal. Therefore, the exodomain and endodomain are likely to associate directly or indirectly with each other, but lack of fruitful materials and technology has hampered knowledge about their physical relationship and contact sites. In this work, we engineered a double-recombinant (separate exodomain and endodomain) baculovirus system successfully expressing on the surface of insect cells high levels of split LH receptor, binding the hormone with high affinity and inducing cAMP synthesis. In contrast, the exodomain and endodomain expressed separately were mostly trapped in cells. Our data indicate that the exodomain and endodomain are disulfide linked in the split receptor. When the disulfide links were reduced, the split receptor still induced cAMP up to 60%, which raises the intriguing possibility of a residual induction activity of the endodomain in the absence of high-affinity ligand binding. Our results also underscore that the targeting and transport of the LH receptor to plasma membrane require both domains, whereas each domain is independently sufficient for folding. The expression level of functional lutropin receptors is the highest ever reported. Our system may also be useful for future studies requiring a high amount of soluble secreted exodomain.

Molecular, structural, and cellular biology of follitropin and follitropin receptor

Follitropin and the follitropin receptor are essential for normal gamete development in males and females. This review discusses the molecular genetics and structural and cellular biology of the follitropin/follitropin receptor system. Emphasis is placed on the human molecules when possible. The structure and regulation of the genes for the follitropin beta subunit and the follitropin receptor is discussed. Control of systemic and cellular protein levels is explained. The structural biology of each protein is described, including protein structure, motifs, and activity relationships. Finally, the follitropin/follitropin receptor signal transduction system is discussed.

Pleiotropic effects of substitutions of a highly conserved leucine in transmembrane helix III of the human lutropin/choriogonadotropin receptor with respect to constitutive activation and hormone responsiveness

It has been shown previously that a naturally occurring mutation of the human LH/CG receptor (hLHR), which replaces L457 in helix III with arginine, results in a receptor that constitutively elevates basal cAMP but does not respond to human CG (hCG) with further cAMP production. In the present study, substitutions of L457 with several amino acids were examined. The constitutive activation of cAMP production was observed only when L457 was replaced with a positively charged residue. Although constitutive activation of the inositol phosphate pathway could not be detected when measuring inositol phosphate production, the use of a more sensitive reporter gene assay for protein kinase C activation revealed the constitutive activation of this pathway by the R- and K-substituted mutants. Therefore, L457 of the hLHR plays a key role in stabilizing the receptor in an inactive conformation. Molecular modeling shows that the insertion of R, K, or H at position 457 triggers the receptor transition toward an active state due to the proximity of an anionic amino acid, D578, in helix VI. These substitutions cause perturbations in helix III-helix VI and helix III-helix VII interactions that culminate in the opening of a solvent-accessible site in the cytosolic domains potentially involved in Gs recognition. Interestingly, L457R was completely unresponsive and the K- and H-substituted L457 hLHR mutants were significantly blunted in their cAMP responses to hCG stimulation. Cells expressing L457R were also unresponsive to hCG with regards to increased inositol phosphate production. Other substitutions of L457 were identified, though, that selectively permit the hormonal stimulation of only one of the two signaling pathways. These results suggest a pivotal role for L457 in hormone-stimulated signal transduction by the hLHR.

The role of the hinge region of the luteinizing hormone receptor in hormone interaction and signal generation

Luteinizing hormone receptor, a G protein-coupled receptor, consists of two halves, the N-terminal extracellular hormone binding domain (exodomain) and the C-terminal membrane-associated, signal-generating domain (endodomain). The exodomain has seven to nine Leu-rich repeats, which are generally thought to form a 1/3 donut-like structure and interact with human choriogonadotropin (hCG). The resulting hCG-exodomain complex adjusts the structure and its association with the endodomain, which results in signal generation in the endodomain. It is unclear whether the rigid 1/3 donut structure could provide the agility and versatility of this dynamic action. In addition, there is no clue as to where the endodomain contact point (the signal modulator) in the exodomain is. To address these issues, the exodomain was examined by Ala scan and multiple substitutions, while receptor peptides were used for photoaffinity labeling and affinity cross-linking. Our results show that the C-flanking sequence (hinge region), Thr(250)-Gln(268), of the Leu-rich repeats (LRRs) specifically interacts with hCG, preferentially hCGalpha. This interaction is inhibited by exoloop 2 of the endodomain but not by exoloops 1 and 3, suggesting an intimate relationship between Thr(250)-Gln(268), exoloop 2, and hCG. Taken together, our observations in this article suggest a new paradigm that the LRRs contact the front of hCG, while both flanking regions of the LRRs interact with the sides of hCG. This would trap hCG in the 1/3 donut structure of the LRRs and enhance the binding affinity. In addition, mutations of conserved Ser(255) in the sequence can constitutively activate the receptor. This provides a clue for the signal modulator in the exodomain. In contrast, a phenyl or phenolic group is necessary at conserved Tyr(253) for targeting the receptor to the surface.

Hormone interactions to Leu-rich repeats in the gonadotropin receptors. II. Analysis of Leu-rich repeat 4 of human luteinizing hormone/chorionic gonadotropin receptor

The luteinizing hormone receptor (LHR) consists of an approximately 350-amino acid-long N-terminal extracellular exodomain and a membrane-associated endodomain of similar size. Human chorionic gonadotropin (hCG) binds to the exodomain, and then hCG/exodomain complex is thought to make a secondary contact with the endodomain and generate hormone signals. The sequence alignment of the exodomain shows imperfectly matching eight to nine Leu-rich repeats (LRRs). In the preceding article (Song, Y., Ji, I., Beauchamp, J., Isaacs, N., and Ji, T. (2001) J. Biol. Chem. 276, 3426-3435), we have shown that LRR2 and LRR4 are crucial for hormone binding. In this work, we have examined the residues of LRR4, in particular Leu(103) and Ile(105) in the putative beta strand. Our data show that Leu(103) and Ile(105) are involved in the specific, hydrophobic interaction of the LRR4 loop, likely to form the hydrophobic core. This loop is crucial for the structural integrity of all of the LRRs. In contrast, the downstream sequence consisting of Asn(107), Thr(108), Gly(109), and Ile(110) of LRR4 is crucial for cAMP induction but not for hormone binding, folding, and surface expression. This implicates, for the first time, its involvement in the interaction with the endodomain and signal generation. The evidence for the interaction is presented in the following article.

Hormone interactions to Leu-rich repeats in the gonadotropin receptors. I. Analysis of Leu-rich repeats of human luteinizing hormone/chorionic gonadotropin receptor and follicle-stimulating hormone receptor

The luteinizing hormone receptor (LHR) and follicle-stimulating hormone receptor (FSHR) have an approximately 350-amino acid-long, N-terminal extracellular exodomain. This exodomain binds hormone with high affinity and specificity and contains eight to nine putative Leu-rich repeat (LRR) sequences. LRRs are known to assume the horseshoe structure in ribonuclease inhibitors, and the inner lining of the horseshoe consists of the beta-stranded Leu/Ile-X-Leu/Ile motif. In the case of ribonuclease inhibitors, these beta strands interact with ribonuclease. However, it is unclear whether the putative LRRs of LHR and FSHR play any role in the structure and function. In this work, the beta-stranded Leu/Ile residues in all LRRs of the human LHR and FSHR were Ala-scanned and characterized. In addition, the 23 residues around LRR2 of LHR were Ala-scanned. The results show that beta-stranded Leu and Ile residues in all LRRs are important but not equally. These Leu/Ile-X-Leu/Ile motifs appear to form the hydrophobic core of the LRR loop, crucial for the LRR structure. Interestingly, the hot spots are primarily in the upstream and downstream LRRs of the LHR exodomain, whereas important LRRs spread throughout the FSHR exodomain. This may explain the distinct hormone specificity despite the structural similarity of the two receptors.

Reversible immunoneutralization of human follitropin receptor

Investigations of the human Follitropin receptor (hFSHR) have failed to identify the tertiary structure that forms the active hormone-receptor interaction site which is essential to develop an immunocontraceptive based upon the receptor. To identify such a domain of hFSHR, an immunoneutralizing monoclonal antibody (mAb) 106-105 (IgG2b) was generated. Flow cytometry tested whether mAb 106-105 recognized native hFSHR. The epitope of mAb 106-105 was mapped by Western blot and by peptide ELISA. Inhibition of hFSH binding and bioactivity was determined by radioreceptor assay and by cAMP production, respectively. MAb 106-105 bound native hFSHR through an epitope including residues 300-315. MAb 106-105 completely blocked hormone binding to receptor and cAMP production by Y1-R cells expressing hFSHR. These effects were completely reversible by increasing the concentration of hFSH. Coincubation of this antibody with peptide D300-F315 blocked antibody activity. These data demonstrate that a discrete linear hFSHR epitope is a target for interference with hormone activity. These results further demonstrate that antibody binding to the extracellular domain (ECD) of hFSHR and subsequent bioactivation can be modulated through a domain specific hindrance, offering a reversible immunoneutralizing target.

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Deletion of follicle-stimulating hormone (FSH) receptor residues encoded by exon one decreases FSH binding and signaling in the rat

The rat FSH receptor (rFSHR) shares considerable homology with the rat LH receptor (rLHR), yet binds human FSH (hFSH) with high fidelity, suggesting that the binding determinant encoded by the rFSHR gene shares no homology with the analogous rLHR primary sequence, thereby affording specificity of ligand binding. Two such regions of primary sequence have been previously identified and studied by peptide challenge tests and immunoneutralization studies. We therefore implemented site-directed mutagenesis to delete the regions S9-N30 and D300-F315 of the mature rFSHR sequence. The mutant receptor (DeltarFSHR) cDNAs were expressed in insect cells. The large deletion DeltarFSHRS9-N30 and a smaller deletion, DeltarFSHRS9-S18, did not bind (125)I-hFSH. However, DeltarFSHRK19-R29 and DeltarFSHRD300-F315 bound (125)I-hFSH with an affinity indistinguishable from wild-type rFSHR. The deletion mutants DeltarFSHR S9-N30 or DeltarFSHRS9-S18 were not detectable on the cell surface by flow cytometry unless cells were sheared. Although (125)I-hFSH binding to DeltarFSHRK19-R29 was normal, this form of the receptor was defective for signal transduction whereas DeltarFSHRD300-F315 was not. Furthermore, neither region seems to be a specificity determinant, since their removal did not result in high-affinity binding of hCG to DeltarFSHR.

Structure-activity relationships of G protein-coupled receptors

The primary function of cell-surface receptors is to discriminate the specific signaling molecule or ligand from a large array of chemically diverse extracellular substances and to activate an effector signaling cascade that triggers an intracellular response and eventually a biological effect. G protein-coupled cell-surface receptors (GPCRs) mediate their intracellular actions through the activation of guanine nucleotide-binding signal-transducing proteins (G proteins), which form a diverse family of regulatory GTPases that, in the GTP-bound state, bind and activate downstream membrane-localized effectors. Hundreds of GPCRs signal through one or more of these G proteins in response to a large variety of stimuli including photons, neurotransmitters, and hormones of variable molecular structure. The mechanisms by which these ligands provoke activation of the receptor/G-protein system are highly complex and multifactorial. Knowledge and mapping of the structural determinants and requirements for optimal GPCR function are of paramount importance, not only for a better and more detailed understanding of the molecular basis of ligand action and receptor function in normal and abnormal conditions, but also for a rational design of early diagnostic and therapeutic tools that may allow exogenous regulation of receptor and G protein function in disease processes.

Molecular tinkering of G protein-coupled receptors: an evolutionary success

Among membrane-bound receptors, the G protein-coupled receptors (GPCRs) are certainly the most diverse. They have been very successful during evolution, being capable of transducing messages as different as photons, organic odorants, nucleotides, nucleosides, peptides, lipids and proteins. Indirect studies, as well as two-dimensional crystallization of rhodopsin, have led to a useful model of a common 'central core', composed of seven transmembrane helical domains, and its structural modifications during activation. There are at least six families of GPCRs showing no sequence similarity. They use an amazing number of different domains both to bind their ligands and to activate G proteins. The fine-tuning of their coupling to G proteins is regulated by splicing, RNA editing and phosphorylation. Some GPCRs have been found to form either homo- or heterodimers with a structurally different GPCR, but also with membrane-bound proteins having one transmembrane domain such as nina-A, odr-4 or RAMP, the latter being involved in their targeting, function and pharmacology. Finally, some GPCRs are unfaithful to G proteins and interact directly, via their C-terminal domain, with proteins containing PDZ and Enabled/VASP homology (EVH)-like domains.

High affinity hormone binding to the extracellular N-terminal exodomain of the follicle-stimulating hormone receptor is critically modulated by exoloop 3

The human follicle-stimulating hormone receptor (FSH-R) consists of two distinct domains of >330 amino acids, the N-terminal extracellular exodomain and membrane-associated endodomain. The exodomain alone binds hormone with high affinity, whereas the endodomain is the site of receptor activation. Coordination of these two domains is essential for successful hormone action but little is known about their functional and structural relationship. In this communication, we report that exoloop 3 of FSH-R constrains follicle-stimulating hormone binding to the exodomain. When the FSH-R exodomain was prepared by truncating its endodomain, the hormone binding affinity of the exodomain was slightly improved, compared with the wild type receptor. The binding affinity was further improved by >3-fold when the exodomain was attached to the membrane-associated domain of CD8. These results suggest that the FSH-R endodomain attenuates hormone binding at the exodomain. As a first step to test this hypothesis, the 11 amino acids except Ala589 of exoloop 3 were individually substituted with Ala. Ala substitution for Leu583 or Ile584 improved the hormone binding affinity by 4-6-fold while totally abolishing cAMP induction, indicating an inverse relationship. The Ala substitution for Lys580 or Pro582 had a similar trend but to a lesser extent. This significant improvement in the binding affinity suggests that the four residues at the N-terminal region of exoloop 3 interact with the exodomain and constrain the hormone binding in the wild type receptor. This effect is specific since substitutions for other than the 4 residues did not improve the hormone binding affinity. Computer modeling shows that the 4 residues can be positioned on one side of exoloop 3. This result and the apparent inverse relationship of hormone binding and cAMP induction suggest that these two essential functions may work against each other. Therefore, hormone binding might be compromised to preserve cAMP inducibility while maintaining a reasonably high, but below maximum, binding affinity.

[Use of a G-protein-coupled receptor to communicate. An evolutionary success]

Among membrane-bound receptors, the seven transmembrane receptors are the most abundant (several thousand, 1% of the genome). They were the most successful during evolution. They are capable of transducing messages as different as photons, organic odorants, nucleotides, nucleosides, peptides, lipids, proteins, etc. They are catalysts of the GDP/GTP nucleotide exchange on heterotrimeric G proteins. They are therefore also called 'G-protein-coupled receptors' (GPCR). G proteins are composed of three subunits, G alpha and two undissociable subunits, G beta gamma. There are at least three families of GPCR showing no sequence similarity. Among G proteins, some have been crystallized (including under the heterotrimeric form) and their structure as well as their activation mechanisms are well known. The structures of GPCR are less known owing to the difficulty in crystallizing membrane-bound proteins. Indirect studies (mutations, 2D crystallization of rhodopsine, molecular modelling, etc.) lead to a useful model of the 'central core' composed of the seven transmembrane domains and of its structural modifications during activation. The intimate contact zones between GPCR and G proteins include, on the GPCR side, domains of intracellular loops and C-terminal, which are specific for each family and on the G protein side, essentially the N- et C-terminal domains plus the alpha 4-beta 6 loop. GPCR can adopt several 'active' conformations some of them being found in mutated receptors responsible for pathologies.

Gonadotropin receptors and the control of gonadal steroidogenesis: physiology and pathology

Over the past few years, knowledge of the structure of gonadotropin receptors and their mode of action has rapidly advanced. The cDNA corresponding to the luteinizeng hormone (LH) receptor (LHR) has been cloned, leading to the identification of a novel family of G-protein-coupled receptors. The follicle stimulating hormone (FSH) receptor (FSHR) was thereafter cloned by cross-hybridization with the LHR. Structure-function relationships have been studied by mutagenesis experiments in several laboratories. The cloning and chromosomal localization to chromosome 2p21 of the two human gonadotropin receptor genes has provided insights into their evolutionary relationships. The LHR and FSHR genes are very large and contain 10 and 11 exons respectively. The obtention of monoclonal antibodies against the receptors resulted in the characterization of the receptor proteins. These antibodies also allowed the study of receptor expression in target cells in physiological and pathological conditions. The internalization of the LHR has been studied by electron microscopy. A mechanism of receptor-mediated transcytosis through the endothelial cells of the testes has been described for the LHR. The polarized expression of receptors has been studied. The cloning of gonadotropin receptor genes has opened the field of genetic study of the receptors. Inactivating mutations of the LHR have been described in Leydig cell agenesis or hypoplasia. Different phenotypes, including complete pseudohermaphroditism, ambiguous genitalia and male phenotype, have been described. In the case of the FSHR, only one mutation has been reported in familial ovarian dysgenesis with primary amenorrhea. Related males have variable alterations of spermatogenesis and fertility. Constitutive mutations of the LHR have been reported in familial testotoxicosis. One similar mutation has also been described for the FSHR. Such mutations may lead to the development of a model of receptor activation.

Strategies for positioning fluorescent probes and crosslinkers on formyl peptide ligands

Chemoattractant receptors represent a major subset of the G-protein coupled receptor (GPCR) family. One of the best characterized, the N-formyl peptide receptor (FPR), participates in host defense responses of neutrophils. The features of the ligand which regulate its interaction with the FPR are well-known. By manipulating these features we have developed new ligands to probe structural and mechanistic aspects of the peptide-receptor interaction. Three ligand groups have been developed: 1) ligands containing a Lys residue located in positions 2 through 7 that can be conjugated to FITC (N-formyl-Met1-Lys2-Phe3-Phe4, N-formyl-Met1-Leu2-Lys3-Phe4, N-formyl-Met1-Leu2-Phe3-Lys4, N-formyl-Met1-Leu2-Phe3-Phe4-Lys5, N-formyl-nLeu1-Leu2-Phe3-nLeu4-Tyr5-Lys6 and N-formyl-Met1-Leu2-Phe3-Phe4-Gly5-Gly6-Lys7; 2) fluorescent pentapeptide ligands (N-formyl-Met-X-Phe-Phe-Lys(FITC) where X = Leu, Ala, Val or Gly); and 3) small crosslinking ligands where the photoaffinity crosslinker 4-azidosalicylic acid (ASA) was conjugated to Lys in positions 3 and 4 and p-benzoyl-phenylalanine (Bpa) was located in position 2 in N-formyl-Met1-Bpa2-Phe3-Tyr4. The peptides were characterized according to activity and affinity in human neutrophils and cell lines transfected with FPR. All of the peptides were agonists, with parallel affinity and activity. In the first group, the peptide activity decreases as Lys is placed closer to the N-formyl group and the activity is improved by 1-3 orders of magnitude by conjugation with FITC. In the second group, the dissociation rate of the peptide from the receptor increases as position 2 is replaced by aliphatic amino acids with smaller alkyl groups. In the third group, crosslinking ligands remain biologically active, display nM affinity and covalently label the FPR.

Aspartate-474 in the first exoplasmic loop of the thyrotropin receptor is crucial for receptor activation

Asp-474 in the first exoplasmic loop of the thyrotropin receptor (TSHR), which is conserved among all glycoprotein hormone receptors, was mutated to Glu which is similarly charged but is longer by one methylene group and expressed in Cos-7 cells. Cells expressing this mutant receptor showed markedly impaired TSH- and TSAb (thyroid stimulating antibody)-stimulated cAMP responses with no effect on TSH binding affinity when compared with cells expressing a similar number of wild-type receptors. These results suggest the importance of Asp-474 in TSHR in receptor activation as demonstrated for LHR (lutropin receptor), but this, unlike LHR, is not due to the electrostatic interaction of this Asp residue with the alpha-subunit Lys-91 of the hormone.

A defined epitope on the human choriogonadotropin alpha-subunit interacts with the second extracellular loop of the transmembrane domain of the lutropin/choriogonadotropin receptor

The monoclonal antibody, HT13 recognizes human choriogonadotropin (CG) bound to the extracellular domain of its receptor, but not to the full-length receptor. The HT13 epitope is located in the regions of residues 15-17 and 73-75 of the human CG alpha-subunit. Only one synthetic peptide, lutropin (LH)/CG-receptor-(481-497)-peptide (EL2 peptide), which spans the second putative extracellular loop of the LH/CG-receptor endodomain, prevents recognition of human CG by HT13 mAb. EL2 peptide decreases hormone-induced cAMP production, but not high-affinity binding. An anti-EL2 serum also displays the capacity to inhibit human CG-stimulated cAMP production. These results suggest that the second extracellular loop of the receptor is in contact with the HT13 epitope of human CG alpha-subunit and is involved in signal transduction. A relative orientation of the hormone versus the endodomain is proposed.