Progress and approaches in mapping the surfaces of human follicle-stimulating hormone: comparison with the other human pituitary glycoprotein hormones

Effect of human follicle-stimulating hormone on immunomodulatory function of decidual mesenchymal stem cells by reducing interleukin-6 levels

Objective

Women with an elevated basal FSH indicate diminished ovarian reserve and reduced oocyte and embryo numbers. DMSCs are likely to be involved in immune tolerance of pregnancy maintenance. We investigate the effect of follicle-stimulating hormones on the immunomodulatory functions of DMSCs.

Methods

DMSCs were primary cultured from decidual tissue. Pretreated DMSCs with mitomycin C, combined with CD4⁺ T lymphocytes, DMSCs + CD4⁺T co-culture system was established. Different physiological dose FSH (3 ng/ml,10 ng/ml,30 ng/ml,100 ng/ml) were used to co-culture system. Cytokines (IFN-γ, IL-2, IL-4, IL-6, IL-10, TNF-α) and other proteins (FSHR, MyD88) were measured.

Results

Compared with the control group (FSH (0 ng/mL) + CD4⁺T + DMSCs), the FSH concentration was 10, 30, and 100 ng/ml, IL-6 levels were significantly reduced (P < 0.05). IL-6, MyD88 protein expression was remarkably decreased (P < 0.05).

Conclusion

FSH/FSHR could negatively regulate the immunosuppressive function of DMSCs by reducing secretion of IL-6 levels through MyD88 pathways, but upstream and downstream signalling pathways require further validation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13048-022-00993-3.

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.

An immunoreactive peptide of the FSH involved in autoimmune infertility

The purpose of this study was to identify autoantigens contained in human ovary extracts. Serum samples from 36 infertile women with anti-ovary antibodies as detected with an ELISA technique were tested in Western blot against human ovary extracts. A reactive protein with a molecular mass matching that of the FSH was detected in 34 cases. These serum samples also reacted strongly in Western blot and ELISA with purified FSH and, in immunofluorescence, with pituitary cells. Using the Pepscan approach, with overlapping peptides matching the amino acid sequence of the human FSH beta-chain, several immunoreactive regions were evidenced. The 78-93 amino acid sequence of the human FSH beta-chain appeared as one of the major epitopes. Synthetic peptides of this region were prepared and demonstrated to react with human serum samples from women with anti-ovary antibodies. These data demonstrate that FSH can be an autoantigen, recognized by autoantibodies associated with infertility.

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.

Fertility-disrupting potential of synthetic peptides derived from the beta-subunit of follicle-stimulating hormone

PROBLEM

Hormone immunoneutralization is hampered by immunologic cross-reactivity caused by close-sequence homology between related molecules. One solution is to use smaller fragments to induce antibodies of greater specificity.

METHOD OF STUDY

A number of peptides selected from beta-follicle-stimulating hormone (FSH) were conjugated to tetanus toxoid and were used to immunize female rats. The antisera were examined for FSH cross-reactivity by immunoassays and in an in vitro bioassay.

RESULTS

In the immunoassays, the antisera did not react with FSH but did react with their respective peptides. In the bioassay, sera from VYKDPARPC- and CDSLYTYP-immunized animals inhibited FSH-receptor interaction by 73% and 68%, respectively. These animals also showed reduced estradiol levels. Sequences were synthesized around VYKDPARPC and were tested on a FSH-receptor-bearing Chinese hamster ovary cell line. LVYKDPARPC, VYKDPARPC, YKDPARPIC, CLVYKDPARP, and LVYKDPARP inhibited FSH-receptor interaction by greater than 50%. In female mice, TRDLVYKDPARPKI and LVYKDPARP disrupted estrous cycling in all animals; LVYKDPARPC and CLVYKDPARP disrupted cycling in three of five animals, whereas VYKDPARPC disrupted cycling in one of four animals.

CONCLUSIONS

Peptides from two areas of beta-FSH (VYKDPARP and DSLYTYP) were shown to raise FSH-neutralizing antibodies, which were able to suppress estradiol levels. An additional leucine residue to VYKDPARP greatly enhanced the peptide's ability to inhibit FSH-receptor binding and caused fertility disruption in vivo.

Identification of bioneutralization epitopes of human follicle stimulating hormone in the regions 31-52 and 66-75 of its beta-subunit

The crucial role played by follicle stimulating hormone (FSH) in regulating both male and female reproduction and the possibilities of developing contraceptive methods for males by blocking the function of the hormone, makes it important to delineate the hormone-specific bioneutralization epitopes of human follicle stimulating hormone (hFSH) on its beta-subunit. Predictive methods were used to identify the potential surface-oriented regions of hFSH-beta. Peptides corresponding to these regions, i.e. 31-52, 66-75 and 86-95 hFSH-beta, were synthesized, anti-peptide antibodies were elicited in rabbits and the properties of these antisera to bind hFSH and neutralize its biological activity were assessed. Anti-31-52 hFSH-beta antisera bound hFSH specifically, whereas anti-66-75 and anti-86-95 hFSH-beta antisera did not show any detectable binding, proving the region 31-52 hFSH-beta to be a specific antigenic determinant of hFSH. The bioneutralizing abilities of the anti-peptide antibodies were assessed by measuring the hFSH-induced progesterone secretion by rat granulosa cells in vitro. Antibodies to 31-52 and 66-75 hFSH-beta neutralized the bioactivity of hFSH, but anti-86-95 hFSH-beta antibodies did not. Furthermore, the three linear peptides and two disulphide looped peptides of 31-52 hFSH-beta and 86-95 hFSH-beta were also subjected to the in-vitro granulosa cell assay. The linear peptides 31-52 hFSH-beta and 66-75 hFSH-beta and the cyclic 31-52 hFSH-beta disulphide loop peptide significantly inhibited the hFSH-induced progesterone secretion by rat granulosa cells, but the linear 86-95 hFSH-beta peptide and the corresponding cyclic disulphide loop peptide did not. The results clearly show that the regions 31-52 and 66-75 of hFSH-beta harbor bioneutralization epitopes of the hormone. The studies also indicate that cyclization of the linear 31-52 hFSH-beta peptide greatly enhances receptor recognition and that the region 66-75 hFSH-beta may also be involved in hormone-receptor interaction.

hCG-receptor binding and transmembrane signaling

The technique of site-directed mutagenesis has proven to be quite powerful in elucidating contact sites involved in the interaction of the heterodimeric glycoprotein hormones and their respective seven transmembrane (TM) G protein-coupled receptors. Our laboratory has focused on identification of the minimum core sequences of the alpha and beta subunits required for bioactivity, the minimum length of a conjoined (yoked) single-chain hCG, the amino acid residues on hCG and the LH/CG-receptor (LH/CG-R) responsible for high-affinity binding, and the regions of the receptor that are involved in TM signaling. A number of amino acid residues have been mapped on the alpha and beta subunits of hCG that appear important in receptor binding. When projected onto the crystal structure of HF-treated hCG, these residues, by and large, cluster on one side of the molecule and cover a sizeable surface area, indicating that the hormone-receptor binding interface is rather extensive. Based on mutagenesis studies of several conserved ionizable amino acid residues in the extracellular domain (ECD) of LH/CG-R and a model that we, in collaboration with Drs Lapthorn and Isaacs, have developed for this region based on the crystal structure of porcine ribonuclease inhibitor, a charged region that appears to play an important role in hormone-receptor recognition has been identified. We have also delineated several regions of LH/CG-R that do not appear to participate in hCG binding but are involved in hCG-mediated signaling. These regions are located in the ECD and extracellular loop III just prior to entry into the membrane via TM helices I and VII, respectively, and in TM helices VI and VII. Similarly, a homologous region in the ECD of the FSH receptor, located with ten residues of TM helix I, is important in signaling but not hormone binding. These results suggest that ligand binding and ligand-mediated receptor activation are quasi-distinct, albeit sequential phenomena. Collectively, our mutagenesis and modeling studies, coupled with results from other laboratories, argue for a ligand-induced conformational change of the receptor that may involve a relative reorientation of the TM helices.

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.

Structure-function studies of human TSH: new advances in design of glycoprotein hormone analogs

Recent progress in structure-function studies of glycoprotein hormones has provided new insights into the molecular mechanisms of action of these hormones and has further supported the concept that physiological modulation of assembly, bioactivity, and clearance of these hormones is dependent on specific structural components. This review emphasizes current advances in the structure-function relationships of human TSH, which have contributed to further elucidation of common and hormone specific features within the glycoprotein hormones family. Novel strategies are now being applied to investigate the role of individual structural elements. The principks discovered in such studies are essential to understand the physiological regulation of hormone bioactivity and allow for the rational design of novel analogs with potential therapeutic applications.

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.

Scanning-alanine mutagenesis of long loop residues 33-53 in follicle stimulating hormone beta subunit

Follicle stimulating hormone (FSH) is a gonadotropin and member of the pituitary/placental glycoprotein hormone family which bind to G-protein-coupled receptors. These hormones are heterodimers composed of a common alpha and distinct beta -subunits. Previous experimental evidence suggested that the FSH beta -subunit long loop comprised of amino acids Tyr33 to Phe53 is involved in receptor binding and activation and in subunit interaction. According to recently reported crystal structures of human chorionic gonadotropin (hCG), the homologous long loop of the beta -subunit of hCG associates with the alpha -subunit and is partially exposed to solvent. This report describes the results of scanning alanine mutagenesis used to determine if amino acid side chains in this region of the molecule are required for receptor binding and/or subunit contact. Five mutations were made which spanned this loop and the mutant FSH beta-subunits were co-expressed with alpha-subunit in a Baculovirus-infected insect-cell expression system. Mutation of 48QKTCT52 to 48AAACA52 produced a FSH beta-subunit that failed to form heterodimer, consistent with the crystal structure of hCG which shows these amino acids are buried at the subunit interface. The four remaining mutants produced heterodimer and were assayed for binding to and activation of human FSH receptors. Mutation of 37LVY39 to 37AAA39 caused a 20-fold reduction binding (ID50 of 7.0 nM compared with 0.3 nM for wildtype). Mutation of 34TRDL37 to 34AAAA37 or 44RPKI47 to 44APAA47 caused lesser but measurable effects with ID50 values of 1.1 nM and 1.9 nM, respectively. The (40)KDPA(43) to 40KDPA43 to 40AAPA43 mutation had little effect on receptor binding (ID50 = 0.5 nM).

European collaborative study of luteinizing hormone assay: 1. Epitope specificity of luteinizing hormone monoclonal antibodies and surface mapping of pituitary and urinary luteinizing hormone

This report describes the results of the first part of the collaborative study organized by a working group sponsored by the Community Bureau of Reference of the European Community Commission. The whole study was designed to understand the causes of discrepancy among LH immunoassay methods. In the parent work, we studied the characteristics of 55 monoclonal antibodies to LH which allowed us to establish a detailed map of the antigenic surface of the hormone. In the present report we used this information to interpret the discrepancy in LH concentrations assayed with 12 different methods in 300 sera from subjects with various clinical conditions. The 55 monoclonal antibodies provided by 11 commercial companies were tested in various experiments: the apparent affinity of the antibodies was, generally but not always, higher for LH presented by a second antibody coated to plastic than for LH directly coated to plastic; 26% of the antibodies recognized the alpha subunit, 26% the beta subunit and 48% reacted only with the holomolecule (anti-alpha beta); only 28% of the antibodies were strictly specific for LH. Criss-cross experiments allowed us to distinguish 13 antigenic regions on the surface of LH: 6 were located on the alpha subunit, 3 on the beta subunit and 4 on the holomolecule. The monoclonal antibodies to the alpha beta regions further separated into 12 clusters of reactivity. Accordingly, LH appeared to exhibit at least 21 epitopes. Comparison of the immunoreactivity of various LH preparations indicated that highly purified pituitary LH and immunoaffinity purified urinary LH reacted similarly with the monoclonal antibodies and strongly differed from crude urinary LH. These data indicated that the immunoreactivity of an LH preparation depends mainly upon the degree of purification and not that much upon the origin of the preparation. The epitope specificity of the monoclonal antibodies used in 11 commercially available LH assay kits was also determined: 10 kits used at least one anti-alpha beta monoclonal antibody associated with an anti-beta monoclonal antibody in 7 cases or another anti-alpha beta monoclonal antibody in 3 cases; one kit used an anti-alpha monoclonal antibody associated with an anti-beta monoclonal antibody. None of the kits were strictly identical with regards to the epitope specificity of the monoclonal antibodies used.

Primary amenorrhoea and infertility due to a mutation in the beta-subunit of follicle-stimulating hormone

We report a woman with primary amenorrhoea and infertility associated with an isolated deficiency of pituitary follicle-stimulating hormone (FSH), but normal luteinizing hormone (LH) secretion. Ovulation was induced by administration of exogenous FSH and resulted in a successful pregnancy. Sequence analysis of the FSH beta-subunit gene indicated that she is homozygous for a two nucleotide frameshift deletion in the coding sequence. Her mother and son are heterozygous for this mutation. This deletion results in an alteration of amino acid codons 61-86 followed by a premature termination codon. The predicted truncated beta-subunit peptide lacks regions which are important for association with the alpha subunit and for binding to and activation of the FSH receptor. Abnormalities of FSH structure or function might be an under recognised but treatable cause of infertility.

Immunoneutralization of heterodimeric FSH using FSH beta subunit as the immunogen

PROBLEM

Immunization with beta subunit of gonadotropin may elicit antibody formation to endogenous heterodimeric gonadotropin and result in reproductive unresponsiveness. The objectives of this study were to determine if antibodies produced in rats following immunization with human follicle stimulating hormone beta-subunit (hFSH-beta) could bind to and immunoneutralize heterodimeric FSH, and to elucidate the immunoneutralizing epitope.

METHOD

Mature female Sprague-Dawley rats received subcutaneous injections of 10 micrograms of hFSH-beta emulsified in complete Freund's adjuvant, while control animals received only adjuvant. Animals received 10 micrograms hFSH-beta booster injections emulsified in incomplete Freund's adjuvant 2, 4, 11, and 21.5 wk after the initial immunization.

RESULTS

Immunization with hFSH-beta produced appreciable antibody titers against human FSH (hFSH) as measured in enzyme-linked immunosorbent assays (ELISA) of immunized rat sera. A more modest titer to rat (rFSH) and no antibody response to rat Luteinizing Hormone (rLH) was observed, thus confirming the specificity of the immune response. Titers against hFSH increased throughout the study. Rat anti-hFSH-beta sera was tested to determine its ability to inhibit binding (immunoneutralizing) of 125I-hFSH to the FSH receptor. Continued immunization resulted in all animals producing immunoneutralizing antibodies. Immunization of rats also resulted in disrupted estrous cycles, but only animals with subsequent titers high enough to completely block binding of FSH to its receptor failed to conceive. In order to assess the immunoneutralizing epitope, antisera were tested in a peptide ELISA. Peptides used in the ELISA corresponded to amino acids that spanned the entire hFSH beta sequence. It was found that antibodies from all rats immunized with hFSH beta bound to amino acids within hFSH-beta 33-53.

CONCLUSIONS

Collectively, these data suggest that amino acids within hFSH-beta 33-53 are necessary but not sufficient to confer immunocontraception. Amino acids within this linear sequence appear in a variety of epitopes of hFSH-beta and hFSH, only some of which are immunoneutralizing. (Am J Reprod Immunol. 1993; 29:000-000.)

Identification of assembled epitopes on the alpha-subunit of human follicle stimulating hormone

In order to characterize further the antigenic structure of human follitropin (hFSH), BALB/c mice were immunized with hFSH and anti-hFSH monoclonal antibodies (mAbs) were generated. The hFSH subunit specificity of the mAbs was assessed by a solid-phase enzyme-linked immunosorbent assay (ELISA) and a solution-phase radioimmunoassay (RIA), each using hFSH, hFSH alpha, and hFSH beta. Five mAbs bound hFSH and hFSH alpha in the ELISA and the RIA. In addition, some mAbs recognized hFSH beta, albeit to a much lower degree, as demonstrated by displacement of [125I]hFSH binding to the mAbs by hFSH beta, in the solution-phase RIAs. Next, synthetic peptides corresponding to the hFSH alpha-subunit sequence were used to identify sequences specific to the epitopes of each of the five mAbs. Using this epitope mapping strategy, two assembled epitopes were identified. mAbs 3A and 4B distinguish one discontinuous epitope comprised minimally of sequences alpha-16-21 and alpha-66-92, whereas mAbs 5F and 2E distinguish a second discontinuous epitope comprised minimally of sequences alpha-40-50 and alpha-66-72.