Receptor binding and functional properties of chimeric human follitropin prepared by an exchange between a small hydrophilic intercysteine loop of human follitropin and human lutropin

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.

Central residues of FSHβ (89-97) peptide are not critical for FSHR binding: Implications for peptidomimetic design

In our previous study, we had identified a 9-mer peptide (FSHβ (89-97)) derived from seat belt loop of human FSHβ and demonstrated its ability to function as FSHR antagonist in vivo. Structure analysis revealed that the four central residues ⁹¹STDC⁹⁴ within this peptide may not be critical for receptor binding. In the present study, ⁹¹STDC⁹⁴ residues were substituted with alanine to generate ΔFSHβ 89-97(⁹¹STDC⁹⁴/AAAA) peptide. Analogous to the parent peptide, ΔFSHβ 89-97(⁹¹STDC⁹⁴/AAAA) peptide inhibited binding of iodinated FSH to rat FSHR and reduced FSH-induced cAMP production. The peptide could impede granulosa cell proliferation leading to reduction in FSH-mediated ovarian weight gain in immature female rats. In these rats, peptide administration further downregulated androgen receptor and estrogen receptor-alpha expression and upregulated estrogen receptor-beta expression. The results indicate that substitution of ⁹¹STDC⁹⁴ with alanine did not significantly alter FSHR antagonist activity of FSHβ (89-97) peptide implying that these residues are not critical for FSH-FSHR interaction and can be replaced with non-peptidic moieties for development of more potent peptidomimetics.

Identification and in vivo validation of a 9-mer peptide derived from FSHβ with FSHR antagonist activity

FSH-FSHR interaction is critical for folliculogenesis, spermatogenesis and progression of several cancers. Therefore, FSHR is an attractive target for fertility regulation and cancer therapeutics. Based on homology and structural analysis of hFSH-FSHR(ECD) complex, a minimal continuous stretch within FSHβ seat-belt loop (FSHβ (89-97)) was identified to be crucial for FSHR interaction. The ability of FSHβ (89-97) peptide to neutralize FSHR activity was evaluated by a panel of in vitro and in vivo experiments. The synthetic peptide significantly inhibited binding of [¹²⁵I]-FSH to rat Fshr as well as FSH-induced cAMP production. In immature rats, FSHβ (89-97) peptide administration reduced FSH-mediated increase in ovarian weight. The peptide inhibited transition of follicles from pre-antral to antral stage and hindered the cell cycle progression of granulosa cells beyond G0/G1 phase. In adult rats, administration of the peptide inhibited estradiol synthesis and significantly perturbed folliculogenesis.

Molecular cloning and characterization of pirarucu (Arapaima gigas) follicle-stimulating hormone and luteinizing hormone β-subunit cDNAs

The common gonadotrophic hormone α-subunit (GTHα) has been previously isolated by our research group from A. gigas pituitaries; in the present work the cDNA sequences encoding FSHβ and LHβ subunits have also been isolated from the same species of fish. The FSH β-subunit consists of 126 amino acids with a putative 18 amino acid signal peptide and a 108 amino acid mature peptide, while the LH β-subunit consists of 141 amino acids with a putative 24 amino acid amino acid signal peptide and a 117 amino acid mature peptide. The highest identity, based on the amino acid sequences, was found with the order of Anguilliformes (61%) for FSHβ and of Cypriniformes (76%) for LHβ, followed by Siluriformes, 53% for FSHβ and 75% for LHβ. Interestingly, the identity with the corresponding human amino acid sequences was still remarkable: 45.1% for FSHβ and 51.4% for LHβ. Three dimensional models of ag-FSH and ag-LH, generated by using the crystal structures of h-FSH and h-LH as the respective templates and carried out via comparative modeling and molecular dynamics simulations, suggested the presence of the so-called "seat-belt", favored by a disulfide bond formed between the 3rd and 12th cysteine in both β-subunits. The sequences found will be used for the biotechnological synthesis of A. gigas gonadotrophic hormones (ag-FSH and ag-LH). In a first approach, to ascertain that the cloned transcripts allow the expression of the heterodimeric hormones, ag-FSH has been synthesized in human embryonic kidney 293 (HEK293) cells, preliminarily purified and characterized.

Discovery and Development of Small Molecule Allosteric Modulators of Glycoprotein Hormone Receptors

Glycoprotein hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH) are heterodimeric proteins with a common α-subunit and hormone-specific β-subunit. These hormones are dominant regulators of reproduction and metabolic processes. Receptors for the glycoprotein hormones belong to the family of G protein-coupled receptors. FSH receptor (FSHR) and LH receptor are primarily expressed in somatic cells in ovary and testis to promote egg and sperm production in women and men, respectively. TSH receptor is expressed in thyroid cells and regulates the secretion of T3 and T4. Glycoprotein hormones bind to the large extracellular domain of the receptor and cause a conformational change in the receptor that leads to activation of more than one intracellular signaling pathway. Several small molecules have been described to activate/inhibit glycoprotein hormone receptors through allosteric sites of the receptor. Small molecule allosteric modulators have the potential to be administered orally to patients, thus improving the convenience of treatment. It has been a challenge to develop a small molecule allosteric agonist for glycoprotein hormones that can mimic the agonistic effects of the large natural ligand to activate similar signaling pathways. However, in the past few years, there have been several promising reports describing distinct chemical series with improved potency in preclinical models. In parallel, proposal of new structural model for FSHR and in silico docking studies of small molecule ligands to glycoprotein hormone receptors provide a giant leap on the understanding of the mechanism of action of the natural ligands and new chemical entities on the receptors. This review will focus on the current status of small molecule allosteric modulators of glycoprotein hormone receptors, their effects on common signaling pathways in cells, their utility for clinical application as demonstrated in preclinical models, and use of these molecules as novel tools to dissect the molecular signaling pathways of these receptors.

Genome analysis reveals insights into physiology and longevity of the Brandt's bat Myotis brandtii

Bats account for one-fifth of mammalian species, are the only mammals with powered flight, and are among the few animals that echolocate. The insect-eating Brandt’s bat (Myotis brandtii) is the longest-lived bat species known to date (lifespan exceeds 40 years) and, at 4–8 g adult body weight, is the most extreme mammal with regard to disparity between body mass and longevity. Here we report sequencing and analysis of the Brandt’s bat genome and transcriptome, which suggest adaptations consistent with echolocation and hibernation, as well as altered metabolism, reproduction and visual function. Unique sequence changes in growth hormone and insulin-like growth factor 1 receptors are also observed. The data suggest that an altered growth hormone/insulin-like growth factor 1 axis, which may be common to other long-lived bat species, together with adaptations such as hibernation and low reproductive rate, contribute to the exceptional lifespan of the Brandt’s bat.

Bats account for 20 per cent of all mammals, these are the only mammals with powered flight, and are among the few animals that echolocate. Here, Seim et al. sequence the genome of the long-lived (>40 years) Brandt’s bat, Myotis brandtii and provide clues to its evolution, longevity and other traits.

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.

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.

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.

Follitropin receptors contain cryptic ligand binding sites

Human choriogonadotropin (hCG) and follitropin (hFSH) have been shown to contact different regions of the extracellular domains of G-protein coupled lutropin (LHR) and follitropin (FSHR) receptors. We report here that hCG and hFSH analogs interact with different regions of an FSHR/LHR chimera having only two unique LHR residues and that binds both hormones with high affinity. hCG and hFSH analogs dock with this receptor chimera in a manner similar to that in which they bind LHR and FSHR, respectively. This shows that although the FSHR does not normally bind hCG, it contains a cryptic lutropin binding site that has the potential to recognize hCG in a manner similar to the LHR. The presence of this cryptic site may explain why equine lutropins bind many mammalian FSHR and why mutations in the transmembrane domain distant from the extracellular domain enable the FSHR to bind hCG. The leucine-rich repeat domain (LRD) of the FSHR also appears to contain a cryptic FSH binding site that is obscured by other parts of the extracellular domain. This will explain why contacts seen in crystals of hFSH complexed with an LRD fragment of the human FSHR are hard to reconcile with the abilities of FSH analogs to interact with membrane G-protein coupled FSHR. We speculate that cryptic lutropin binding sites in the FSHR, which are also likely to be present in thyrotropin receptors (TSHR), permit the physiological regulation of ligand binding specificity. Cryptic FSH binding sites in the LRD may enable alternate spliced forms of the FSHR to interact with FSH.

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.

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.

Molecular cloning of follicle-stimulating hormone (FSH)-beta subunit cDNA from duck pituitary

We have cloned FSH-beta cDNA from duck pituitary gland by reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA end (RACE) methods. The cloned duck FSH-beta cDNA contains 1909-bp nucleotides including 396-bp of open-reading frame and 1491-bp of 3'-untranslational region. The open-reading frame encodes a 131-amino acid protein with a putative 20-amino acid signal peptide and a putative 111-amino acid mature protein. The deduced amino acid sequence shows a remarkable similarity (94-98%) to those of other avian FSH-beta subunits; while it exhibits lower similarities with those of turtles (82-84%), mammals (63-71%), and amphibians (53-57%). The structural model analysis of duck FSH suggests that the cysteine-knot and beta-strands for maintaining the specific structural frame, and the "seat-belt" loop for specific binding to FSH receptor have been conserved in tetrapodian FSH-betas.

Biological properties of a novel follicle-stimulating hormone/human chorionic gonadotropin chimeric gonadotropin

A chimeric recombinant human gonadotropin, termed C3, demonstrates both follitropic and lutropic bioactivities. The alpha-subunit construct for C3 is comprised of the recombinant wild-type human glycoprotein hormone alpha-subunit. The beta-subunit DNA construct for C3 encodes residues 1-145 from human chorionic gonadotropin (hCG)-beta with the exceptions that FSH beta amino acid 88 (D) is substituted for hCG beta amino acid 94 (R) and FSH beta amino acids 95-108 (TVRGLGPSYCSFGE) are substituted for hCG beta amino acids 101-114 (GGPKDHPLTCDDPR). C3 is a potent FSH and LH agonist able to bind and to signal through FSH and LH receptors in vitro. In in vivo bioassays optimized to quantify each type of activity, C3 was found to have lutropin and follitropin potencies at levels similar to those of recombinant human LH and recombinant human FSH, respectively. In immature rats, C3 was sufficient to support the maturation of normal ovarian follicles. Moreover, a significant portion of follicles matured by C3 ruptured in response to an ovulatory hCG stimulus and gave rise to morphologically normal oocytes. Furthermore, a low dose of C3 promoted weight gain in the rodent uterus, suggesting it also supported preparation for implantation without histological evidence of excessive luteinization of the ovary. In summary, the biological properties of C3 indicate that its chimeric nature has resulted in a fully functional, dual-acting human gonadotropin.

Prediction of orthologous relationship by functionally important sites

Making accurate functional predictions plays an important role in the era of proteomics. Reliable functional information can be extracted from orthologs in other species when annotating an unknown gene. Here a site-based approach called PORFIS is proposed to predict orthologous relationship. When applied to the bacterial transcription factor PurR/LacI family and the protein kinase AGC family, our method was able to identify, with few false positives, the important sites that agree with those verified by biological experiments. We also tested it on the alpha-proteasome family, the glycoprotein hormone family and the growth hormone family to demonstrate its ability to predict orthologous relationship. Compared with other prediction methods based on phylogenetic analysis or hidden Markov models, PORFIS not only has competitive prediction accuracy, but also provides valuable biological information of functionally important sites associated with orthologs which can be further studied in biological experiments.

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).

Analysis of glycoprotein hormone receptor extracellular domain interactions using a solid-phase capture assay

The receptors for the glycoprotein hormones are unique in having a large extracellular domain that is responsible for mediating ligand binding. We describe the characterization, validation, and application of a solid-phase radioligand binding assay that can be used to assess the interaction of peptides and small molecules at the extracellular domain (ECD) of the follicle-stimulating hormone receptor (FSHR). The assay utilizes a C-terminal tag on the FSHR-ECD, which is used to capture the ECD and position it in a sterically favorable orientation on a solid-phase platform. Competition experiments with the cognate ligand, FSH, indicated that the interaction at the FSHR-ECD using the solid-phase assay was comparable to the full-length receptor assayed using a standard filtration assay. The utility of the assay was evaluated by competing several peptides and a small molecule for both the full-length FSHR and the FSHR-ECD. The solid-phase capture format allowed for the establishment of an assay to specifically evaluate compounds that interact at the ECD or require the full-length receptor, thereby facilitating structure-activity studies. This assay format should be applicable to the other receptors of this family.

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.

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.

Receptor-selective determinants in catfish gonadotropin seat-belt loops

Mammalian gonadotropins are highly selective. Charge differences between the Cys(10-11) sequence of FSHbeta and LHbeta/CGbeta seat-belt loops determine the ability of these hormones to interact with the LH-R. Selective FSH-R binding is mainly dependent on the presence of an FSHbeta-specific sequence between Cys(11-12) of the seat-belt loop. Intriguingly, African catfish LHbeta (cfLHbeta) lacks a positively charged Cys(10-11) region and stimulates both catfish LH-R and FSH-R with comparable potencies. Our studies on the promiscuous behaviour of cfLH using chimeric gonadotropins revealed that the Cys(10-11) region of cfLHbeta contains cfLH-R-selective determinants, whereas the Cys(11-12) region of cfLHbeta confers FSH-R-stimulating activity to cfLH. Hence, the location of receptor-selective determinants appeared to be fairly well conserved throughout evolution, despite the low sequence identity between mammalian and catfish seat-belt loops. Moreover, various structure-function differences between gonadotropins are discussed in the context of the different (female) reproductive strategies between mammalian and non-mammalian species that required the divergence to a more specific LH-R-stimulating activity of one of the gonadotropins in mammals.

Glycoprotein Hormone Assembly in the Endoplasmic Reticulum

Vertebrate glycoprotein hormone heterodimers are stabilized by a strand of their beta-subunits known as the "seatbelt" that is wrapped around loop 2 of their alpha-subunits (alpha2). The cysteine that terminates the seatbelt is "latched" by a disulfide to a cysteine in beta-subunit loop 1 (beta1) of all vertebrate hormones except some teleost follitropins (teFSH), wherein it is latched to a cysteine in the beta-subunit NH(2) terminus. As reported here, teFSH analogs of human choriogonadotropin (hCG) are assembled by a pathway in which the subunits dock before the seatbelt is latched; assembly is completed by wrapping the seatbelt around loop alpha2 and latching it to the NH(2) terminus. This differs from hCG assembly, which occurs by threading the glycosylated end of loop alpha2 beneath the latched seatbelt through a hole in the beta-subunit. The seatbelt is the part of the beta-subunit that has the greatest influence on biological function. Changes in its sequence during the divergence of lutropins, follitropins, and thyrotropins and the speciation of teleost fish may have impeded heterodimer assembly by a threading mechanism, as observed when the hCG seatbelt was replaced with its salmon FSH counterpart. Whereas wrapping is less efficient than threading, it may have facilitated natural experimentation with the composition of the seatbelt during the co-evolution of glycoprotein hormones and their receptors. Migration of the seatbelt latch site to the NH(2)-terminal end of the beta-subunit would have facilitated teFSH assembly by a wraparound mechanism and may have contributed also to its ability to distinguish lutropin and follitropin receptors.

Glycoprotein Hormone Assembly in the Endoplasmic Reticulum

Glycoprotein hormone heterodimers are stabilized by their unusual structures in which a glycosylated loop of the alpha-subunit straddles a hole in the beta-subunit. This hole is formed when a cysteine at the end of a beta-subunit strand known as the "seatbelt" becomes "latched" by a disulfide to a cysteine in the beta-subunit core. The heterodimer is stabilized in part by the difficulty of threading the glycosylated end of the alpha-subunit loop 2 through this hole, a phenomenon required for subunit dissociation. Subunit combination in vitro, which occurs by the reverse process, can be accelerated by removing the alpha-subunit oligosaccharide. In cells, heterodimer assembly was thought to occur primarily by a mechanism in which the seatbelt is wrapped around the alpha-subunit after the subunits dock. Here we show that this "wraparound" process can be used to assemble disulfide cross-linked human choriogonadotropin analogs that contain an additional alpha-subunit cysteine, but only if the normal beta-subunit latch site has been removed. Normally, the seatbelt is latched before the subunits dock and assembly is completed when the glycosylated end of alpha-subunit loop 2 is threaded beneath the seatbelt. The unexpected finding that most assembly of human choriogonadotropin, human follitropin, and human thyrotropin heterodimers occurs in this fashion, indicates that threading may be an important phenomenon during protein folding and macromolecule assembly in the endoplasmic reticulum. We suggest that the unusual structures of the glycoprotein hormones makes them useful for identifying factors that influence this process in living cells.

Probing receptor structure/function with chimeric G-protein-coupled receptors

Owing its name to an image borrowed from Greek mythology, a chimera is seen to represent a new entity created as a composite from existing creatures or, in this case, molecules. Making use of various combinations of three basic domains of the receptors (i.e., exofacial, transmembrane, and cytoplasmic segments) that couple agonist binding into activation of effectors through heterotrimeric G-proteins, molecular pharmacology has probed the basic organization, structure/function relationships of this superfamily of heptahelical receptors. Chimeric G-protein-coupled receptors obviate the need for a particular agonist ligand when the ligand is resistant to purification or, in the case of orphan receptors, is not known. Chimeric receptors created from distant members of the heptahelical receptors enable new strategies in understanding how these receptors transduce agonist binding into receptor activation and may be able to offer insights into the evolution of G-protein-coupled receptors from yeast to humans.

Glycoprotein hormone assembly in the endoplasmic reticulum: II. Multiple roles of a redox sensitive beta-subunit disulfide switch

All three human glycoprotein hormone heterodimers are assembled in the endoplasmic reticulum by threading the glycosylated end of alpha-subunit loop two (alpha2) beneath a disulfide "latched" strand of the beta-subunit known as the "seatbelt." This remarkable event occurs efficiently even though the seatbelt effectively blocks the reverse process, thereby stabilizing each heterodimer. Studies described here show that assembly is facilitated by the formation, disruption, and reformation of a loop within the seatbelt that is stabilized by the most easily reduced disulfide in the free beta-subunit. We refer to this disulfide as the "tensor" because it shortens the seatbelt, thereby securing the heterodimer. Formation of the tensor disulfide appears to precede and facilitate seatbelt latching in most human choriogonadotropin beta-subunit molecules. Subsequent disruption of the tensor disulfide elongates the seatbelt, thereby increasing the space beneath the seatbelt and the beta-subunit core. This permits the formation of hydrogen bonds between backbone atoms of the beta-subunit cystine knot and the tensor loop with backbone atoms in loop alpha2, a process that causes the glycosylated end of loop alpha2 to be threaded between the seatbelt and the beta-subunit core. Contacts between the tensor loop and loop alpha2 promote reformation of the tensor disulfide, which explains why it is more stable in the heterodimer than in the uncombined beta-subunit. These findings unravel the puzzling nature of how a threading mechanism can be used in the endoplasmic reticulum to assemble glycoprotein hormones that have essential roles in vertebrate reproduction and thyroid function.

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

The nine leucine-rich repeat-containing exodomains of the human FSH receptor (hFSH-R) and the human LH/chorionic gonadotropin receptor (hLH-R) harbor molecular determinants that allow the mutually exclusive binding of human FSH (hFSH) and human LH (hLH)/human chorionic gonadotropin (hCG) when these hormones are present in physiological concentrations. Previously, we have shown that the beta-strands of hLH-R leucine-rich repeats 3 and 6 can confer full hCG/hLH responsiveness and binding when simultaneously introduced into a hFSH-R background without affecting the receptor's responsiveness to hFSH. In the present study, we have determined the nature of contribution of each of these two beta-strands in conferring hCG/hLH responsiveness to this mutant hFSH-R. Human LH-R beta-strand 3 appeared to function as a positive hCG/hLH determinant by increasing the hCG/hLH responsiveness of the hFSH-R. In contrast, mutagenesis of hFSH-R beta-strand 6, rather than the introduction of its corresponding hLH-R beta-strand, appeared to allow the interaction of hCG/hLH with the hFSH-R. Hence, hFSH-R beta-strand 6 functions as a negative determinant and, as such, restrains binding of hCG/hLH to the hFSH-R. Detailed mutagenic analysis revealed that the ability of the hFSH-R to interact with hCG/hLH depends primarily on the identity of two amino acids (Asn104, a positive LH-R determinant, and Lys179 a negative FSH-R determinant) that are situated on the C-terminal ends of beta-strands 3 and 6, respectively.

Glycosylation of an N-terminal extension prolongs the half-life and increases the in vivo activity of follicle stimulating hormone

FSH is a key component in assisted reproductive technologies. Because of rapid clearance of the hormone, patients have to be treated with daily injections. To address this problem, a long-acting FSH mutein was created by introduction of additional N-linked glycosylation into the molecule. New glycosylation sites were introduced by two different approaches: structure-aided, site-directed introduction of sites within the FSH molecule and addition of N-terminal extensions. A mutein with the extension sequence ANITVNITV at the N terminus of the alpha-chain (FSH1208) was efficiently glycosylated at both new sites. This resulted in a molecule with increased size and charge, factors known to reduce renal clearance of proteins. FSH1208 was found to have a 3- to 4-fold increased serum half-life, compared with wild-type recombinant FSH. Furthermore, in spite of a lower in vitro activity, FSH1208 had a markedly increased in vivo potency, as shown by increased ability to augment the ovarian weight and stimulate the serum estradiol levels in rats. These characteristics make FSH1208 a possible candidate for improved infertility treatment.

Recombinant human follicle-stimulating hormone: a scientific step to clinical improvement

Urinary-derived follicle-stimulating hormone (FSH) preparations have been used clinically for many years. Although effective, these have a number of disadvantages, not least of which is their variable composition. The availability of recombinant human FSH (r-hFSH), produced from CHO cells, with its constant composition and exceptionally high purity, has, therefore, aroused great interest. This review focuses on the use of r-hFSH for ovarian stimulation in assisted reproduction technology protocols and the treatment of World Health Organization Group I and II anovulation. The use of r-hFSH has been shown to lead to improvements in efficacy over urinary-derived preparations, particularly in assisted reproductive treatment, and a recent meta-analysis has shown higher ongoing pregnancy rates with the recombinant product. Although the two available recombinant products from CHO cells (follitropin alpha [Gonal-F((R))] and beta [Puregon((R))]) are similar from a physicochemical perspective, some minor advantages have been reported for follitropin alpha in relation to pregnancy rates and better local tolerance to injections. The apparent higher bioactivity of r-hFSH has led to reduced total FSH consumption over shorter treatment periods compared with conventional preparations, thus reducing overall exposure for patients. This is likely to confer not only safety benefits, but also cost-effectiveness as demonstrated through pharmaco-economic modelling.

Specificity of cognate ligand-receptor interactions: fusion proteins of human chorionic gonadotropin and the heptahelical receptors for human luteinizing hormone, thyroid-stimulating hormone, and follicle-stimulating hormone

The family of glycoprotein hormones and their homologous heptahelical receptors represent an excellent system for comparative structure-function studies. We have engineered single chain molecules of human chorionic gonadotropin (hCG) fused to its cognate receptor, LH receptor (LHR), and to the noncognate receptors, TSH receptor (TSHR) and FSH receptor (FSHR; N-beta-alpha-receptor-C), to create the yoked (Y) complexes YCG/LHR, YCG/TSHR, and YCG/FSHR. The expression and bioactivity of these fusion proteins were examined in transiently transfected HEK 293 cells. Western blot analysis and antibody binding assays demonstrated that each of the proteins was expressed. In the case of YCG/LHR, minimal binding of exogenous hormone was observed due to the continued occupation of receptor by the fused ligand. The presence of hCG in the YCG/TSHR and YCG/FSHR, however, did not prevent binding of exogenous cognate ligand, presumably due to the lower affinity of hCG. The basal cAMP levels in cells expressing the YCG/LHR complex was approximately 20-fold higher than that in cells expressing LHR. Increases in basal cAMP production were also observed with YCG/TSHR and YCG/FSHR, e.g. 13- and 4-fold increases, respectively. Whereas the affinity and specificity of hCG for LHR are extraordinarily high, the hormone is capable of binding to and activating both TSHR and FSHR under these conditions that mimic high ligand concentrations. These findings were confirmed by adding high concentrations of hCG to cells expressing TSHR and FSHR. Although the functional interaction of hCG and TSHR has been recognized in gestational hyperthyroidism, there are no reports linking hCG to FSHR activation. This study, however, suggests that such a functional interaction is capable of occurring under conditions of high circulating levels of hCG, e.g. the first trimester of pregnancy and in patients with hCG-secreting tumors.

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.

Lysine 183 and glutamic acid 157 of the TSH receptor: two interacting residues with a key role in determining specificity toward TSH and human CG

A naturally occurring mutation in the ectodomain of the TSH receptor (TSHr), K183R, has been described recently in a familial case of gestational hyperthyroidism. Hyperthyroidism was explained by the widening of the specificity of the mutant receptor toward human CG (hCG). In the present study, we attempted to understand in molecular terms the structure-phenotype relationships of this mutant in light of the available structural model of TSHr ectodomain established on the template of the atomic structure of the porcine ribonuclease inhibitor. To this aim, we studied by site-directed mutagenesis and functional assays in transfected COS cells the effects of substituting amino acids with different physicochemical properties for lysine 183. Unexpectedly, all TSHr mutants displayed widening of their specificity toward hCG. Molecular dynamics simulations suggested that the gain of function would be secondary to the release of a nearby glutamate residue (E157) from a salt bridge with K183. This hypothesis was supported by further site-directed mutagenesis experiments showing that the presence of an acidic residue in position 157, or in its vicinity, was required to observe the increase in sensitivity to hCG (an acidic residue in position 183 can partially fulfill the role of a free acidic residue in position 157 when tested on the background of a E157A mutant). Our results suggest also that additional natural mutations (especially K183M, N, or Q) in position 183 of TSHr are expected to be found in gestational hyperthyroidism.

Cloning and gene expression of a cDNA for the chicken follicle-stimulating hormone (FSH)-beta-subunit

Follicle-stimulating hormone (FSH) is a member of pituitary glycoprotein hormones that are composed of two dissimilar subunits, alpha and beta. Very little information is available regarding the nucleotide and amino acid sequence of FSH-beta in avian species. For better understanding of the phylogenic diversity and evolution of FSH molecule, we have isolated and sequenced the complete complementary DNA (cDNA) encoding chicken FSH-beta precursor molecule by reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA end (RACE) methods. The cloned chicken FSH-beta cDNA consists of 2457-bp nucleotides, including 44-bp nucleotides of the 5'-untranslated region (UTR), 396 bp of the open reading frame, and an extraordinarily long 3'-UTR of 2001-bp nucleotides followed by a poly(A)((16)) tail. It encodes a 131-amino-acid precursor molecule of FSH-beta-subunit with a signal peptide of 20 amino acids followed by a mature protein of 111 amino acids. Twelve cysteine residues, forming six disulfide bonds within beta-subunit and two putative asparagine-linked glycosylation sites, are also conserved in the chicken FSH-beta-subunit. Four proline residues, presumably responsible for changing the backbone direction of protein structure, are conserved in chicken FSH-beta-subunit as well. The nucleotide sequence of chicken FSH-beta cDNA shows high homology with quail FSH-beta cDNA, 97% homology in the open reading frame, and 85% homology in the 3'-UTR. The deduced amino acid sequence of chicken FSH-beta-subunit shows a remarkable similarity to other avian FSH-beta-subunits, 98% homology with quail, and 93% homology with ostrich, whereas a lower similarity (66 to 70%) is noted when compared with mammalian FSH-beta-subunits. By contrast, when comparing with the beta-subunits of chicken luteinizing hormone and thyroid-stimulating hormone, the homologies are as low as 37 and 40%, respectively. FSH-beta mRNA was only expressed in pituitary gland out of various tissues examined and can be up-regulated by gonadotropin-releasing hormone in pituitary tissue culture as estimated by real-time quantitative PCR.

Hormone-induced conformational change of the purified soluble hormone binding domain of follitropin receptor complexed with single chain follitropin

Human follicle-stimulating hormone receptor (hFSHR) belongs to family I of G protein-coupled receptors. FSHR extracellular domain (ECD) is predicted to have 8-9 alphabeta or leucine-rich repeat motif elements. The objective of this study was to identify elements of the FSHR ECD involved in ligand binding. Preincubation of recombinant hFSHR ECD with rabbit antisera raised against synthetic peptides of hFSHR ECD primary sequence abolished follitropin binding primarily in the region of amino acids 150-254. Accessibility of hFSHR ECD after hormone binding, captured by monoclonal antibodies against either ECD or FSH, was decreased for the region of amino acids 150-220 but additionally for amino acids 15-100. Thus, when hFSH bound first, accessibility of antibody binding was decreased to a much larger extent than if antibody was bound first. This suggestion of a conformational change upon binding was examined further. Circular dichroism spectra were recorded for purified single chain hFSH, hFSHR ECD, and hFSHR ECD-single chain hFSH complex. A spectral change indicated a small but consistent conformational change in the ECD.FSH complex after hormone binding. Taken together, these data demonstrate that FSH binding requires elements within the leucine-rich repeat motifs that form a central region of hFSHR ECD, and a conformational change occurs upon hormone binding.

Three-dimensional structure of human follicle-stimulating hormone

The crystal structure of a betaThr26Ala mutant of human follicle-stimulating hormone (hFSH) has been determined to 3.0 A resolution. The hFSH mutant was expressed in baculovirus-infected Hi5 insect cells and purified by affinity chromatography, using a betahFSH-specific monoclonal antibody. The betaThr26Ala mutation results in elimination of the betaAsn24 glycosylation site, yielding protein more suitable for crystallization without affecting the receptor binding and signal transduction activity of the glycohormone. The crystal structure has two independent hFSH molecules in the asymmetric unit and a solvent content of about 80%. The alpha- and betasubunits of hFSH have similar folds, consisting of central cystine-knot motifs from which three beta-hairpins extend. The two subunits associate very tightly in a head-to-tail arrangement, forming an elongated, slightly curved structure, similar to that of human chorionic gonadotropin (hCG). The hFSH heterodimers differ only in the conformations of the amino and carboxy termini and the second loop of the beta-subunit (L2beta). Detailed comparison of the structures of hFSH and hCG reveals several differences in the beta-subunits that may be important with respect to receptor binding specificity or signal transduction. These differences include conformational changes and/or differential distributions of polar or charged residues in loops L3beta (hFSH residues 62-73), the cystine noose, or determinant loop (residues 87-94), and the carboxy-terminal loop (residues 94-104). An additional interesting feature of the hFSH structure is an extensive hydrophobic patch in the area formed by loops alphaL1, alphaL3, and betaL2. Glycosylation at alphaAsn52 is well known to be required for full signal transduction activity and heterodimer stability. The structure reveals an intersubunit hydrogen bonding interaction between this carbohydrate and betaTyr58, an indication of a mechanism by which the carbohydrate may stabilize the heterodimer.

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.

Substitution of the seat-belt region of the thyroid-stimulating hormone (TSH) beta-subunit with the corresponding regions of choriogonadotropin or follitropin confers luteotropic but not follitropic activity to chimeric TSH

The region between the 10th and 12th cysteine (Cys88-Cys105 in human thyroid-stimulating hormone beta-subunit (hTSHbeta)) of the glycoprotein hormone beta-subunits corresponds to the disulfide-linked seat-belt region. It wraps around the common alpha-subunit and has been implicated in regulating specificity between human choriogonadotropin (hCG) and human follicle-stimulating hormone (hFSH), but determinants of hTSH specificity are unknown. To characterize the role of this region for hTSH, we constructed hTSH chimeras in which the entire seat-belt region Cys88-Cys105 or individual intercysteine segments Cys88-Cys95 and Cys95-Cys105 were replaced with the corresponding sequences of hCG and hFSH or alanine cassettes. Alanine cassette mutagenesis of hTSH showed that the Cys95-Cys105 segment of the seat-belt was more important for TSH receptor binding and signal transduction than the Cys88-Cys95 determinant loop region. Replacing the entire seat-belt of hTSHbeta with the hCG sequence conferred full hCG receptor binding and activation to the hTSH chimera, whereas TSH receptor binding and activation were abolished. Conversely, introduction of the hTSHbeta seat-belt sequence into hCGbeta generated an hCG chimera that bound to and activated the TSH receptor but not the CG/lutropin (LH) receptor. In contrast, an hTSH chimera bearing hFSH seat-belt residues did not possess any follitropic activity, and its thyrotropic activity was only slightly reduced. This may in part be due to the fact that the net charge of the seat-belt is similar in hTSH and hFSH but different from hCG. However, exchanging other regions of charge heterogeneity between hTSHbeta and hFSHbeta did not confer follitropic activity to hTSH. Thus, exchanging the seat-belt region between hTSH and hCG switches hormonal specificity in a mutually exclusive fashion. In contrast, the seat-belt appears not to discriminate between the TSH and the FSH receptors, indicating for the first time that domains outside the seat-belt region contribute to glycoprotein hormone specificity.

Chimeric proteins can exceed the sum of their parts: implications for evolution and protein design

Chimeric analogs derived from pairs of homologous proteins routinely exhibit activities found in one or both parents. We describe chimeras of two glycoprotein hormones, human chorionic gonadotropin (hCG) and human follitropin (hFSH), that exhibit activity unique to a third family member, human thyrotropin (hTSH). The results show that biological activity can be separated from hormone-specific amino acid residues. This is consistent with a model for the evolution of homologous ligand-receptor pairs involving gene duplication and the creation of inhibitory determinants that restrict binding. Disruption of these determinants can unmask activities characteristic of other members of a protein family. Combining portions of two ligands to create analogs with properties of a third family member can facilitate identifying key determinants of protein-protein interaction and may be a useful strategy for creating novel therapeutics. In the case of the glycoprotein hormones, this showed that two different hormone regions (i.e., the seat-belt and the intersubunit groove) appear to limit inappropriate contacts with receptors for other members of this family. These observations also have important caveats for chimera-based protein design because an unexpected gain of function may limit the therapeutic usefulness of some chimeras.

Human follitropin heterodimerization and receptor binding structural motifs: identification and analysis by a combination of synthetic peptide and mutagenesis approaches

The family of human glycoprotein hormones, including follitropin (FSH), are heterodimeric proteins, each composed of single alpha- and beta-subunits that are tightly associated but non-covalently linked. To study structure and function relationships of FSH, synthetic peptides were used to inhibit subunit association, to map epitopes of FSH antibodies and as antigens to generate site specific antipeptide antibodies which could be used for topographic analysis. Interpretation of such results are generally more straightforward than when peptides are used with radioreceptor assays or in cell cultures which are complex systems. The data we collected using the synthetic peptide approach suggested that FSH residues homologous to human chorionic gonadotropin (hCG) loops L3 beta and L2 alpha are involved in subunit contact. FSH residues homologous to hCG loops L2 beta and L3 alpha seemed involved in receptor binding. Loop L2 beta also seemed involved in subunit contact. Those data provided a rationale for extensive mutagenesis of the four regions of hFSH. Mutagenesis data provided additional information and higher resolution of function when combined with the three dimensional structure of hCG. In the aggregate, this information has provided a reasonable model of the receptor binding site of hFSH. Our current model of the FSH receptor site is that of a discontinuous functional epitope including L3 beta, L2 alpha and L3 alpha. The juxtaposition of residues beta D93, alpha K5 1, alpha Y88 and of alpha Y89 in the 'binding-facet' of hFSH suggest the feasibility of designing a synthetic peptide mimetic of FSH. Additional residues of the alpha-subunit are involved, along this facet of the molecule. The data collected studying hFSH therefore demonstrates that the alpha-subunit features prominently in the mechanism of FSH binding to and stabilizing the interaction with its receptor. In contrast, the beta-subunit determinant loop serves as discriminator in addition to stabilizing the binding interaction whereas mutagenesis data indicates that L2 beta does neither. Instead, L2 beta appears to stabilize FSH conformation, possibly, the alpha-subunit, required for competent binding. In this regard, synthetic peptides provided data which were a useful guide to plan mutagenesis studies and which contributed to the process of understanding the structure and function of the gonadotropins.

hCG beta residues 94-96 alter LH activity without appearing to make key receptor contacts

The ability of human chorionic gonadotropin (hCG) to distinguish lutropin (LHR) and follitropin (FSHR) receptors is controlled principally by beta-subunit residues 94-117. To learn how residues 94-96 (Arg-Arg-Ser) influence LHR binding, we studied the effects of replacing them on the LH and FSH activities of a bifunctional hCG analog in which residues 101-109 were derived from FSH. Analogs containing 1-3 arginines and no aspartates at residues 94-96 bound LHR with 25-400% the potency of hCG. When residues 94-96 were neutral or contained 1-3 aspartates, LHR binding was reduced 6-100 fold but remained at least ten-fold greater than the negative control analog containing residues 94-117 derived from FSH. Residues 94-96 had little influence on FSHR binding. These observations support a model [Moyle et al. (1995) J. Biol. Chem. 270:20,020] in which residues 94-96 influence LHR binding specificity primarily through an effect on hormone conformation rather than by direct participation in essential high affinity receptor contacts.

Engineering human glycoprotein hormone superactive analogues

We report the generation of superactive analogues of human glycoprotein hormones, with potential applications in thyroid and reproductive disorders. Current biological and structural data were used to rationalize mutagenesis. The 11-20 region in the alpha-subunit with a cluster of lysine residues forms a previously unrecognized domain critical for receptor binding and signal transduction, as well as an important motif in the evolution of glycoprotein hormone activities. The gradual elimination of basic residues in the alpha-subunit coincided with the evolutionary divergence of the hominids from the Old World monkeys. By selective reconstitution of certain critical residues present in homologous nonhuman hormones we have developed human thyroid stimulating hormone and chorionic gonadotropin analogues with substantial increases in receptor binding affinity and bioactivity, thus providing a paradigm for the design of novel therapeutic protein analogues.

Follitropin conformational stability mediated by loop 2 beta effects follitropin-receptor interaction

Follicle-stimulating hormone (FSH) is in the family of pituitary/placental glycoprotein hormones which also includes luteinizing hormone (LH), chorionic gonadotropin (hCG), and thyroid-stimulating hormone. These hormones are heterodimers composed of common alpha- and similar but unique beta-subunits. The 21 amino acid loop between Y33 and F53 of the FSH beta-subunit (L2 beta) can be switched into L2 beta of hCG beta without a loss of receptor binding, yet mutation of hFSH beta 37LVY39 to 37AAA39 was antecendent to a 20-fold reduction in receptor binding (based on ID50). A mutation in the LH beta gene, which causes Q54 to be R, causes hypogonadism. This residue is conserved in the glycoprotein hormones and corresponds to Q48 in hFSH beta. Mutation of hFSH beta 48QKTCT52 to 48AAACA52 resulted in a failure of heterodimer formation. In the current study single mutations were made to pinpoint which of the seven hFSH beta residues in the 37LVY39 to 37AAA39 and the 48QKTCT52 to 48AAACA52 mutants were responsible for the observed phenotypes. A single mutation of T52 to alanine was sufficient to cause a reduction in expression of heterodimeric hormone. Single mutants Q48A, T50A, V38A, Y39A, and, to a lesser extent, T52A formed heterodimer. However, these hFSH mutants were markedly unstable at pH 2.0. Thus, acid dissociation can be used to reveal metastable forms of this protein. Mutant hFSH beta Q48A was also 8-fold less active than wild-type hFSH when assayed for binding to hFSH receptors. hFSH beta V38A and Y39A mutants affected receptor binding; however, neither mutation alone caused greater than a 2-fold decrease in receptor binding activity. In summary, these results identify single important residues in the long loop (between Y33 and F53) of the hFSH beta-subunit which are required for proper subunit interactions that provide conformational stability which in turn is necessary for FSH-receptor interaction.