Conversion of human choriogonadotropin into a follitropin by protein engineering

Development of a model for studying the developmental consequences of oxidative sperm DNA damage by targeting redox-cycling naphthoquinones to the Sertoli cell population

Oxidative stress can be induced in the testes by a wide range of factors, including scrotal hyperthermia, varicocele, environmental toxicants, obesity and infection. The clinical consequences of such stress include the induction of genetic damage in the male germ line which may, in turn, have serious implications for the health and wellbeing of the progeny. In order to confirm the transgenerational impact of oxidative stress in the testes, we sought to develop an animal model in which this process could be analysed. Our primary approach to this problem was to induce Sertoli cells (robust, terminally differentiated, tissue-specific testicular cells whose radioresistance indicates significant resistance to oxidative stress) to generate high levels of reactive oxygen species (ROS) within the testes. To achieve this aim, six follicle-stimulating hormone (FSH) peptides were developed and compared for selective targeting to Sertoli cells both in vitro and in vivo. Menadione, a redox-cycling agent, was then conjugated to the most promising FSH candidate using a linker that had been optimised to enable maximum production of ROS in the targeted cells. A TM4 Sertoli cell line co-incubated with the FSH-menadione conjugate in vitro exhibited significantly higher levels of mitochondrial ROS generation (10-fold), lipid peroxidation (2-fold) and oxidative DNA damage (2-fold) than the vehicle control. Additionally, in a proof-of-concept study, ten weeks after a single injection of the FSH-menadione conjugate in vivo, injected male mice were found to exhibit a 1.6 fold increase in DNA double strand breaks and 13-fold increase in oxidative DNA damage to their spermatozoa while still retaining their ability to initiate a pregnancy. We suggest this model could now be used to study the influence of chronic oxidative stress on testicular function with emphasis on the impact of DNA damage in the male germ line on the mutational profile and health of future generations.

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.

Characterization of gonadotropin receptors Fshr and Lhr in Japanese medaka, Oryzias latipes

Reproduction in vertebrates is controlled by the brain-pituitary-gonad axis, where the two gonadotropins follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) play vital parts by activating their cognate receptors in the gonads. The main purpose of this work was to study intra- and interspecies ligand promiscuity of teleost gonadotropin receptors, since teleost receptor specificity is unclear, in contrast to mammalian receptors. Receptor activation was investigated by transfecting COS-7 cells with either Fsh receptor (mdFshr, tiFshr) or Lh receptor (mdLhr, tiLhr), and tested for activation by recombinant homologous and heterologous ligands (mdFshβα, mdLhβα, tiFshβα, tiLhβα) from two representative fish orders, Japanese medaka (Oryzias latipes, Beloniformes) and Nile tilapia (Oreochromis niloticus, Cichliformes). Results showed that each gonadotropin preferentially activates its own cognate receptor. Cross-reactivity was detected to some extent as mdFshβα was able to activate the mdLhr, and mdLhβα the mdFshr. Medaka pituitary extract (MPE) stimulated CRE-LUC activity in COS-7 cells expressing mdlhr, but could not stimulate cells expressing mdfshr. Recombinant tiLhβα, tiFshβα and tilapia pituitary extract (TPE) could activate the mdLhr, suggesting cross-species reactivity for mdLhr. Cross-species reactivity was also detected for mdFshr due to activation by tiFshβα, tiLhβα, and TPE, as well as for tiFshr and tiLhr due to stimulation by mdFshβα, mdLhβα, and MPE. Tissue distribution analysis of gene expression revealed that medaka receptors, fshr and lhr, are highly expressed in both ovary and testis. High expression levels were found for lhr also in brain, while fshr was expressed at low levels. Both fshr and lhr mRNA levels increased significantly during testis development. Amino acid sequence alignment and three-dimensional modelling of ligands and receptors highlighted conserved beta sheet domains of both Fsh and Lh between Japanese medaka and Nile tilapia. It also showed a higher structural homology and similarity of transmembrane regions of Lhr between both species, in contrast to Fshr, possibly related to the substitution of the conserved cysteine residue in the transmembrane domain 6 in medaka Fshr with glycine. Taken together, this is the first characterization of medaka Fshr and Lhr using homologous ligands, enabling to better understand teleost hormone-receptor interactions and specificities. The data suggest partial ligand promiscuity and cross-species reactivity between gonadotropins and their receptors in medaka and tilapia.

Gonadotropins and Their Analogs: Current and Potential Clinical Applications

The gonadotropin receptors LH receptor and FSH receptor play a central role in governing reproductive competency/fertility. Gonadotropin hormone analogs have been used clinically for decades in assisted reproductive therapies and in the treatment of various infertility disorders. Though these treatments are effective, the clinical protocols demand multiple injections, and the hormone preparations can lack uniformity and stability. The past two decades have seen a drive to develop chimeric and modified peptide analogs with more desirable pharmacokinetic profiles, with some displaying clinical efficacy, such as corifollitropin alfa, which is now in clinical use. More recently, low-molecular-weight, orally active molecules with activity at gonadotropin receptors have been developed. Some have excellent characteristics in animals and in human studies but have not reached the market-largely as a result of acquisitions by large pharma. Nonetheless, such molecules have the potential to mitigate risks currently associated with gonadotropin-based fertility treatments, such as ovarian hyperstimulation syndrome and the demands of injection-based therapies. There is also scope for novel use beyond the current remit of gonadotropin analogs in fertility treatments, including application as novel contraceptives; in the treatment of polycystic ovary syndrome; in the restoration of function to inactivating mutations of gonadotropin receptors; in the treatment of ovarian and prostate cancers; and in the prevention of bone loss and weight gain in postmenopausal women. Here we review the properties and clinical application of current gonadotropin preparations and their analogs, as well as the development of novel orally active, small-molecule nonpeptide analogs.

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.

Human lutropin (hLH) and choriogonadotropin (CG) are assembled by different pathways: a model of hLH assembly

The glycoprotein hormones are all structurally related heterodimers consisting of an α-subunit and a ligand-specific β-subunit that confers their unique biological activity. Crystal structures showed how the β-subunit surrounds a part of the α-subunit, and we showed the existence of the two mechanisms responsible for that assembly. In human choriogonadotropin, the β-subunit is folded before the subunits dock, and the α-subunit becomes incorporated into the dimer by a mechanism we termed "threading," passing between parts of the preassembled β-subunit. Here, we show that the human lutropin β-subunit is not folded completely prior to its interaction with the α-subunit and show that docking of the subunits enables the α-subunit to serve as a chaperone to the β-subunit. Based on data described here, we propose that the α-subunit facilitates formation of the human lutropin β-subunit by two mechanisms. First, the cystine knot of the α-subunit potentiates formation of the β-subunit cystine knot, and second, contacts between α-subunit loop 2 and a hydrophobic tail in the β-subunit facilitate formation of the seatbelt latch disulfide, which stabilizes the heterodimer. The primary influence of the α-subunit was seen when the hydrophobic tail was present or absent, but the secondary mechanism was required only when the hydrophobic tail of the β-subunit was present. During the evolution of human choriogonadotropin, neither of these α-subunit roles was necessary for folding of the β-subunit. The complex mechanism for lutropin assembly may be required to provide an additional control on its positive feedback function in vertebrate reproduction.

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.

Experimental and computational study of inter- and intra- species specificity of gonadotropins for various gonadotropin receptors

The gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and their receptors play critical roles in vertebrate reproduction. In order to study intra- and interspecies ligand promiscuity of gonadotropins, COS-7 cells were transiently transfected with one of the gonadotropin receptor genes, FSHR or LHR, and tested for activation by gonadotropins from representative fish orders: Aquilliformes (eel; e), Salmoniformes (trout; tr), and Perciformes (tilapia; ta), and of mammalian origin: porcine (p), bovine (b) and human (h). The study reveals complex relations between the gonadotropin hormones and their receptors. Each gonadotropin activated its own cognate receptor. However, taLHR was also activated by hCG and eLHR was activated by hFSH, hCG, and trFSH. For FSHR, the only cross-reactivity detected was for hFSHR, which was activated by pFSH and bFSH. These findings are of great interest and applicability in the context of activation of various GTHRs by their ligands and by ligands from other vertebrates. Analysis of the three-dimensional models of the structures highlights the importance of residues outside of the currently established hormone-receptor interface region. In addition, the interface residues in taFSHR and the effect of exon duplication, which causes an insert in the LRR domain, are suggested to affect the interaction and binding of taFSH.

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.

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.

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

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

Ligand-selective determinants in gonadotropin receptors

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

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

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

Biological 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.

Expression of a recombinant bifunctional protein from a chimera of human lutropin receptor and human chorionic gonadotropin beta-subunit

Human lutropin (hLH) and human chorionic gonadotropin (hCG) are structurally and functionally similar and play important roles in reproduction via a common gonadal receptor (LH-R). However, hormone specific hCG-beta subunit contains 24 additional amino acid carboxyl terminal peptide (CTP), which produce specific antibodies to hCG-beta with little cross-reaction with LH. A chimeric protein containing both hLH-R and hCG-beta would provide a unique bifunctional antigen for immunocontraception. In this study is described the synthesis of a chimeric DNA construct of full-length of hLH-receptor and hCG-beta and its expression in Sf9 cells to produce a bifunctional protein. Recombinant protein was recognized by antibodies to LH-R as well as anti-hCG-beta in Western Blots, thus indicating the preservation of immunological epitopes for both hLH-R and hCG-beta in the chimera. Specific ligand binding of recombinant hLH-R component was demonstrated by the displacement of bound labeled hCG at increasing concentrations of unlabeled hCG, indicating that, the presence of hCG-beta component of the chimera did not interfere with the binding of hCG to LH-R. hCG-beta was also present in the recombinant chimeric protein as shown by a specific hCG-beta chemiluminescence assay. Treatment of transfected Sf9 cells with hCG induced dose-dependent increase in the stimulation of intracellular cAMP production, which showed that the ligand binding had functional activity. These results demonstrate that the chimeric DNA construct of hLHR-hCG-beta expressed a bifunctional protein containing both hLH-R and hCG-beta activities, which could provide a unique potential antigen for immunocontraception in vertebrates.

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).

Molecular pharmacology of gonadotropins

Gonadotropins have been studied in biological systems for decades and many of their properties are well defined. These include pharmacological properties such as affinity, stability, and pharmacokinetics also used to characterize drugs. Technologies applied to research on gonadotropins have led to the creation of hormone analogs with alterations to one or more of these proper-ties. Some of these analogs have potential therapeutic applications. A challenge to realizing this potential is the accurate prediction of how these compounds will perform in humans. This could be facilitated by advances in biological models and the understanding of specific effects of the hormones on their receptors.

Structural biology of glycoprotein hormones and their receptors

Glycoprotein hormones regulate reproduction in vertebrates and exert their actions through specific G protein-coupled receptors on target cell surfaces. Structural information is now available for human chorionic gonadotropin (CG), follicle-stimulating hormone (FSH), and FSH bound to the extracellular binding domain of its receptor (FSHR(HB)). The recently determined structure of a human FSH-FSHR(HB) complex provides an explanation for the specificity of glycoprotein hormones binding to their receptors, and it suggests hypotheses concerning the mechanism of transmembrane signal transduction.

Models of glycoprotein hormone receptor interaction

The glycoprotein hormones regulate reproduction and development through their interactions with receptors in ovarian, testicular, and thyroid tissues. Efforts to design hormone agonists and antagonists useful for treat-ing infertility and hyperthyroidism would benefit from a molecular understanding of hormone-receptor interaction. The structure of a complex containing FSH bound to a fragment of its receptor has been determined at 2.9 Angstroms resolution, but this does not explain several observations made with cell-surface G protein receptors and may reflect the manner in which FSH binds a short alternate spliced receptor form. We discuss observations that must be explained by any model of the cell-surface G protein-coupled glycoprotein hormone receptors and suggest structures for these receptors that satisfy these requirements. Glycoprotein hormones appear to contact two distinct sites in the extracellular domains of their receptors, not just the leucine-rich repeat domain. These dual contacts contribute to ligand binding specificity and appear to be essential for signal transduction. As outlined in this minireview, differences in the manners in which these ligands contact their receptors explain why some ligands and ligand analogs interact with more than one class of receptor and why some receptors and receptor analogs bind more than one ligand. The unique manner in which these ligands appear to interact with their receptors may have facilitated hormone and receptor co-evolution during early vertebrate speciation.

Only a Portion of the Small Seatbelt Loop in Human Choriogonadotropin Appears Capable of Contacting the Lutropin Receptor

Twenty residues of the human choriogonadotropin (hCG) beta-subunit that are wrapped around alpha-subunit loop 2 like a "seatbelt" stabilize the heterodimer and enable the hormone to distinguish lutropin (LHR), follitropin, and thyrotropin receptors. The N-terminal portion of the seatbelt contains a small disulfide-stabilized loop needed for heterodimer assembly and is thought to mediate hCG-LHR interactions. To test the latter notion, we compared the LHR binding and signal transduction activities of hCG analogs in which the alpha-subunit C terminus (alphaCT) was cross-linked to residues in the small seatbelt loop. Analogs having an intersubunit disulfide between a cysteine in place of alphaCT residue alphaSer-92 and cysteines substituted for loop residues betaArg-94, betaArg-95, or betaSer-96 had high activities in LHR binding and signaling assays despite the fact that both portions of the hormone are thought to be essential for hCG activity. Use of a larger probe blocked hormone activity when the alphaCT was cross-linked to cysteines in place of residues betaArg-95 and betaAsp-99, but not to cysteines in place of residues betaArg-94, betaSer-96, or betaThr-97. This suggested that the side chains of residues betaArg-95 and betaAsp-99, which face in the same outward direction from the heterodimer, are nearer than the others to the LHR interface. The finding that residue 95 can be cross-linked to small alphaCT probes without eliminating hormone activity indicates its side chain does not participate in essential LHR contacts. We suggest that contacts between the small seatbelt loop and the LHR, if any, involve its backbone atoms and possibly the side chain of residue betaAsp-99.

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.

Use of protein knobs to characterize the position of conserved alpha-subunit regions in lutropin receptor complexes

Efforts to identify the manner in which human choriogonadotropin (hCG) contacts lutropin receptors (LHR) have been stymied by the complex structure of the hormone and the likelihood that it contacts the receptor at multiple sites. During studies of hCG assembly in mammalian cells, we found that addition of a cysteine to the long disordered beta-subunit COOH terminus (betaCT) enabled it to become cross-linked by a disulfide to cysteines that are substituted for residues in loop alpha2 or in the alpha-subunit COOH terminus (alphaCT). This created a "knob" on the alpha-subunit at the location of the cysteine. Knobs of various sizes and charges were useful for probing surfaces of the alpha-subunit thought previously to contact the LHR. Attachment of the betaCT to residues in loop alpha2 facing loops beta1 and beta3 reduced hormone activity only a few fold revealing that this surface does not participate in essential high affinity receptor contacts, a finding inconsistent with our earlier view of the hCG-LHR complex. In contrast, this approach showed that the opposite surface of loop alpha2 appeared to be nearer the receptor interface. Although attachment of knobs to portions of the alphaCT reduced hormone activity substantially, this finding was difficult to interpret. As discussed, this procedure should be adapted readily to other proteins and may facilitate the introduction of fluorophores, enzymes, or other reagents at specific sites on protein surfaces. It may also permit one to cross-link proteins or to obscure specific protein surfaces during the development of "Trojan Horse" therapeutics.

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.

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.

Tight attachment of chitin-binding-domain-tagged proteins to surfaces coated with acetylated chitosan

Several excellent procedures for trapping tagged proteins have been devised, but many of these are expensive, cannot be used outside a limited pH range, fail to work in the presence of chaotropic agents, or are difficult to use. The chitin binding domain (CBD) of Bacillus circulans chitinase, which binds to chitin matrices prepared from inexpensive reagents isolated from crab shells, is an alternative tag that can be used under a variety of pH and denaturing conditions. Kits based on the interaction between the CBD and the chitin beads are available commercially. Here, we show that simultaneous treatment of microtiter plates with chitosan, a deacetylated form of chitin, and acetic anhydride produces a surface-bound film of chitin that also interacts tightly with the CBD. Chitin-coated microtiter well plates captured a CBD-tagged heterodimeric human glycoprotein hormone analog directly from mammalian cell culture media, even when present in trace amounts. Binding to the surface was stable in sodium dodecylsulfate and reversed only partially at low pH or in 8M urea at 37 degrees C. This technique appears well suited to surface attachment and permits biochemical or other analyses of molecules that can be tagged with a CBD.

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.

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.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.

Alternatively folded choriogonadotropin analogs. Implications for hormone folding and biological activity

Most heterodimeric proteins are stabilized by intersubunit contacts or disulfide bonds. In contrast, human chorionic gonadotropin (hCG) and other glycoprotein hormones are secured by a strand of their beta-subunits that is wrapped around alpha-subunit loop 2 "like a seatbelt." During studies of hCG synthesis in COS-7 cells, we found that, when the seatbelt was prevented from forming the disulfide that normally "latches" it to the beta-subunit, its carboxyl-terminal end can "scan" the surface of the heterodimer and become latched by a disulfide to cysteines substituted for residues in the alpha-subunit. Analogs in which the seatbelt was latched to residues 35, 37, 41-43, and 56 of alpha-subunit loop 2 had similar lutropin activities to those of hCG; that in which it was latched to residue 92 at the carboxyl terminus had 10-20% the activity of hCG. Attachment of the seatbelt to alpha-subunit residues 45-51, 86, 88, 90, and 91 reduced lutropin activity substantially. These findings show that the heterodimer can form before the beta-subunit has folded completely and support the notions that the carboxyl-terminal end of the seatbelt, portions of alpha-subunit loop 2, and the end of the alpha-subunit carboxyl terminus do not participate in lutropin receptor interactions. They suggest also that several different architectures could have been sampled without disrupting hormone activity as the glycoprotein hormones diverged from other cysteine knot proteins.

Mapping the receptor binding regions of human chorionic gonadotropin (hCG) using disulfide peptides of its beta-subunit: possible involvement of the disulfide bonds Cys(9)-Cys(57) and Cys(23)-Cys(72) in receptor binding of the hormone

Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone essential for the establishment and maintenance of pregnancy. The alpha- and beta-subunits of hCG are highly cross-linked internally by disulfide bonds which seem to stabilize the tertiary structures required for the noncovalent association of the subunits to generate hormonal activity. The purpose of this study was to delineate the role of the disulfide bonds of hCGbeta in receptor binding of the hormone. Six disulfide peptides incorporating each of the six disulfide bonds of hCGbeta were synthesized and screened, along with their linear counterparts, for their ability to competitively inhibit the binding of [125I] hCG to sheep ovarian corpora luteal LH/CG receptor. Disulfide peptide Cys (9-57) was found to be approximately 4-fold more potent than the most active of its linear counterparts in inhibiting radiolabeled hCG from binding to its receptor. Similarly, disulfide peptide Cys (23-72) exhibited receptor binding inhibition activity, whereas the constituent linear peptides were found to be inactive. The results suggest the involvement of the disulfide bonds Cys(9)-Cys(57) and Cys(23)-Cys(72) of the beta-subunit of hCG in receptor binding of the hormone. This study is the first of its kind to use disulfide peptides rather than linear peptides to map the receptor binding regions of hCG.

Partial restoration of lutropin activity by an intersubunit disulfide bond: implications for structure/function studies

Gonadal function is controlled by lutropins and follitropins, heterodimeric cystine knot proteins that have nearly identical alpha-subunits. These heterodimeric proteins are stabilized by a portion of the hormone-specific beta-subunit termed the "seatbelt" that is wrapped around alpha-subunit loop 2 (alpha 2). Here we show that replacing human chorionic gonadotropin (hCG) alpha 2 residue Lys51 with cysteine or alanine nearly abolished its lutropin activity, an observation that implies that alpha Lys51 has a key role in hormone activity. The activity of the heterodimer containing alpha K51C, but not that containing alpha K51A, was increased substantially when beta-subunit seatbelt residue beta Asp99 was converted to cysteine. As had been reported by others, heterodimers containing alpha K51C and beta D99C were crosslinked by a disulfide. The finding that an intersubunit disulfide restored some of the activity lost by replacing alpha Lys51 suggests that this residue is not crucial for receptor binding or signaling and also that hCG and related hormones may be particularly sensitive to mutations that alter interactions between their subunits. We propose the unique structures of hCG and related family members may permit some subunit movement in the heterodimer, making it difficult to deduce key residues involved in receptor contacts simply by correlating the activities of hormone analogs with their amino acid sequences.

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.

The surface of alpha-subunit loop 1 distant from the subunit interface is exposed in the hCG lutropin receptor complex

Interactions of the placental glycoprotein hormone human choriogonadotropin (hCG) with lutropin receptors (LHR) are required for maintenance of early pregnancy. Knowledge of how hCG interacts with LHR is useful for understanding the mechanism of receptor function, an issue of considerable debate. A large surface of hCG remains exposed after the hormone binds the LHR and can be readily detected with monoclonal antibodies. Here we show that the surface of hCG alpha-subunit loop 1 furthest from the beta-subunit interface can also be recognized by a monoclonal antibody when hCG is bound to the LHR. This extends the area of hCG known to be exposed in the hormone receptor complex, an observation that further restricts models of hCG-LHR interaction.

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.

Definition and measurement of follicle stimulating hormone

FSH has a key role in the development and function of the reproductive system and is widely used both diagnostically and therapeutically in developmental and reproductive medicine. The accurate measurement of FSH levels, in patients for diagnosis and monitoring and in therapeutic preparations for clinical use, is essential for safe and successful treatment. Historically, FSH was defined on the basis of classical in vivo endocrine activity, and early therapeutic preparations were calibrated using in vivo bioassays. There was early recognition that reference preparations were required for calibration if the results from different laboratories were to be comparable. In response to the perceived need, the World Health Organization established the first standard for such preparations in 1959. Subsequent developments in biotechnology have led to recognition that there is no single molecule that can be uniquely defined as FSH, and that FSH can induce a range of biological activities. Several highly purified standards for FSH are now available, but discontinuity and heterogeneity of estimates of FSH activity in terms of these standards made using in vitro assays and binding assays have been noted. It is thus essential that any measurement of FSH include specification both of the standard with which the measured FSH is compared and the assay method used for that comparison.

Role of glycosylation in function of follicle-stimulating hormone

The oligosaccharide structures of heterodimeric glycoprotein hormones, such as follicle-stimulating hormone (FSH), have been shown to play an important role in the biosynthesis, secretion, metabolic fate, and regulation of potency of the hormone. The oligosaccharide structures attached to each subunit of the protein seem to exhibit distinct roles in some of these functions. Glycans attached to the alpha-subunit are critical for dimer assembly, integrity, and secretion, as well as for signal transduction; although beta-subunit glycans are also important for dimer assembly and secretion, they play a crucial role in clearance of the dimer from the circulation. Alternative glycosylation on FSH and other glycoprotein hormones not only may affect the metabolic clearance and net in vivo biopotency of the hormone, but also offers the interesting possibility that some glycosylation variants of the hormone may provoke differential or even unique effects at the target cell level. Glycosylation of FSH is regulated by hypothalamic and/or end products from the glands under the control of this hormone. In particular, estrogens regulate terminal sialylation and thus some functional properties of the gonadotropin influenced by sialic acid. Through these extrapituitary inputs, the gonadotroph may regulate not only the amount but also the intensity of the gonadotropin signal to be secreted by the pituitary in a given physiological condition.

Addition of an N-terminal dimerization domain promotes assembly of hCG analogs: implications for subunit combination and structure-function analysis

Human chorionic gonadotropin (hCG) is a heterodimeric placental glycoprotein hormone that acts through ovarian lutropin receptors (LHR) to maintain early pregnancy. Its ability to distinguish LHR and follitropin receptors (FSHR) is controlled by 20 beta-subunit 'seatbelt' residues that surround alpha-subunit loop 2. Positively charged amino acids between residues 93-100, a small loop within the seatbelt, have been postulated to make essential LH receptor contacts. Previous studies showed that analogs containing negatively charged amino acids in this small loop had 5-10% the activity of hCG and 1-10% the lutropin activities of hCG/hFSH chimeric analogs capable of binding LHR and FSHR. These effects might be due to the influence of these residues on receptor contacts or on hormone conformation. During efforts to distinguish these possibilities, we increased and decreased the number of residues in this loop, mutations we anticipated would distort its conformation. Consistent with this supposition, these changes inhibited dimer formation, precluding assessment of these mutations on hormone activity. Addition of Fos and Jun dimerization domains to the N-termini of hCGalpha- and hCG/hFSHbeta-subunit chimeras overcame the effects of the seatbelt mutations on subunit combination and enabled preparation of heterodimers containing six, seven, or nine residues in their seatbelt loops. These had 0.1-10% the lutropin and 3-60% the follitropin activities of bifunctional chimeras containing 8 residues derived from hCG in the seatbelt loop. The abilities of N-terminal dimerization domains to promote subunit combination may permit structure/function analysis of other residues that influence heterodimer formation.

Lutropins appear to contact two independent sites in the extracellular domain of their receptors

Human chorionic gonadotropin (hCG) and bovine lutropin (bLH), a hormone chemically more similar to most mammalian lutropins than hCG, interact with the extracellular domains of their gonadal lutropin receptors (LHRs). These portions of the rat and human LHRs are 85% identical and both receptors bind hCG with high, albeit not identical, affinity. However, at least 1000-fold more bLH is required to inhibit binding of radiolabelled hCG to the human LHR than to the rat LHR, a phenomenon that proved useful for identifying regions of the extracellular domain that contact lutropins. Previous studies using truncated receptors and lutropin/follitropin receptor chimaeras localized most, if not all, high-affinity ligand contacts to the N-terminal three-fifths of the rat LHR extracellular domain. We report here that 10-fold more bLH was needed to inhibit binding of labelled hCG to rat/human LHR chimaeras containing the N-terminal three-fifths of the human LHR extracellular domain than to the rat LHR. Unexpectedly, 100-fold more bLH was required to inhibit binding of labelled hCG to chimaeras containing the C-terminal one-fifth of the human LHR extracellular domain than to the rat LHR. The ability of the C-terminal portion of the human LHR extracellular domain to inhibit bLH binding suggests this region of the receptor also contacts the ligand even though it is not needed for ligand binding. The extracellular domains of all the glycoprotein hormone receptors are thought to be horseshoe-shaped, a consequence of their leucine-rich repeat motifs. Portions of the ligand that become located within the cavity created by the concave surface of the horseshoe would have the opportunity to contact residues in the C-terminal portion of the extracellular domain. Changes to the ligand or receptor that influence this interaction would be expected to alter binding and confound efforts to identify residues in key ligand-receptor contacts.

The luteinizing hormone receptor

The luteinizing hormone receptor (LHR) is a member of the subfamily of glycoprotein hormone receptors within the superfamily of G protein-coupled receptor (GPCR)/seven-transmembrane domain receptors. Over the past eight years, major advances have been made in determining the structure and function of the LHR and its gene. The hormone-binding domain has been localized to exons 1-7 in the extracellular (EC) domain/region of the receptor, which contains several leucine-rich repeats. High-affinity binding of LH and human chorionic gonadotrophin (hCG) causes secondary hormone or receptor contacts to be established with regions of the EC loop/transmembrane module that initiate signal transduction. Models of hormone-receptor interaction have been derived from the crystal structures of hCG and of the ribonuclease inhibitor, which also contains leucine-rich repeats. Such models provide a framework for the interpretation of mutational studies and for further experiments. The extracellular domain of the receptor has been overexpressed in vitro, which will facilitate crystallographic resolution of the structure of the receptor-binding site. The transmembrane domain/loop/cytoplasmic module transduces the signal for coupling to G proteins. Several constitutive, activating mutations that cause human disease have been found in helix VI and adjacent structures. These mutations have provided valuable information about mechanisms of signal transfer and G protein coupling. The structure of the LHR gene has been elucidated, and the regulation of its transcription is beginning to be understood. Valuable insights into receptor evolution have been derived from analysis of sequence homologies, the gene structure of glycoprotein hormone receptors and other members of the GPCR family, and the glycoprotein hormone receptor-like precursors identified in several invertebrate species.

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.