Small Molecule Follicle-Stimulating Hormone Receptor Agonists and Antagonists

Effects and action mechanism of gonadotropins on ovarian follicular cells: A novel role of Sphingosine-1-Phosphate (S1P). A review

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) control antral follicular growth by regulating several processes, such as the synthesis of hormones and signaling molecules, proliferation, survival, apoptosis, luteinization, and ovulation. To exert these effects, gonadotropins bind to their respective Gs protein-coupled receptors, activating the protein kinase A (PKA) pathway or recruiting Gq proteins to activate protein kinase C (PKC) signaling. Although the action mechanism of FSH and LH is clear, recently, it has been shown that both gonadotropins promote the synthesis of sphingosine-1-phosphate (S1P) in granulosa and theca cells through the activation of sphingosine kinase 1. Moreover, the inhibition of SPHKs reduces S1P synthesis, cell viability, and the proliferation of follicular cells in response to gonadotropins, and the addition of S1P to the culture medium increases the proliferation of granulosa and theca cells without apparent effects on sexual steroid synthesis. Therefore, we consider that S1P is a crucial signaling molecule that complements the canonical gonadotropin pathway to promote the proliferation and viability of granulosa and theca cells.

Hormonal and Allosteric Regulation of the Luteinizing Hormone/Chorionic Gonadotropin Receptor

Luteinizing hormone (LH) and human chorionic gonadotropin (CG), like follicle-stimulating hormone, are the most important regulators of the reproductive system. They exert their effect on the cell through the LH/CG receptor (LHCGR), which belongs to the family of G protein-coupled receptors. Binding to gonadotropin induces the interaction of LHCGR with various types of heterotrimeric G proteins (Gs, Gq/11, Gi) and β-arrestins, which leads to stimulation (Gs) or inhibition (Gi) of cyclic adenosine monophosphate-dependent cascades, activation of the phospholipase pathway (Gq/11), and also to the formation of signalosomes that mediate the stimulation of mitogen-activated protein kinases (β-arrestins). The efficiency and selectivity of activation of intracellular cascades by different gonadotropins varies, which is due to differences in their interaction with the ligand-binding site of LHCGR. Gonadotropin signaling largely depends on the status of N- and O-glycosylation of LH and CG, on the formation of homo- and heterodimeric receptor complexes, on the cell-specific microenvironment of LHCGR and the presence of autoantibodies to it, and allosteric mechanisms are important in the implementation of these influences, which is due to the multiplicity of allosteric sites in different loci of the LHCGR. The development of low-molecular-weight allosteric regulators of LHCGR with different profiles of pharmacological activity, which can be used in medicine for the correction of reproductive disorders and in assisted reproductive technologies, is promising. These and other issues regarding the hormonal and allosteric regulation of LHCGR are summarized and discussed in this review.

Bisphenol S causes excessive estrogen synthesis by activating FSHR and the downstream cAMP/PKA signaling pathway

Estrogen excess in females has been linked to a diverse array of chronic and acute diseases. Emerging research shows that exposure to estrogen-like compounds such as bisphenol S leads to increases in 17β-estradiol levels, but the mechanism of action is unclear. The aim of this study was to reveal the underlying signaling pathway-mediated mechanisms, target site and target molecule of action of bisphenol S causing excessive estrogen synthesis. Human ovarian granulosa cells SVOG were exposed to bisphenol S at environmentally relevant concentrations (1 μg/L, 10 μg/L, and 100 μg/L) for 48 h. The results confirms that bisphenol S accumulates mainly on the cell membrane, binds to follicle stimulating hormone receptor (FSHR) located on the cell membrane, and subsequently activates the downstream cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) signaling pathway, leading to enhanced conversion of testosterone to 17β-estradiol. This study deepens our knowledge of the mechanisms of environmental factors in pathogenesis of hyperestrogenism.

Allosteric modulation of gonadotropin receptors

Gonadotropins regulate reproductive functions by binding to G protein-coupled receptors (FSHR and LHCGR) expressed in the gonads. They activate multiple, cell-specific signalling pathways, consisting of ligand-dependent intracellular events. Signalling cascades may be modulated by synthetic compounds which bind allosteric sites of FSHR and LHCGR or by membrane receptor interactions. Despite the hormone binding to the orthosteric site, allosteric ligands, and receptor heteromerizations may reshape intracellular signalling pattern. These molecules act as positive, negative, or neutral allosteric modulators, as well as non-competitive or inverse agonist ligands, providing a set of new compounds of a different nature and with unique pharmacological characteristics. Gonadotropin receptor allosteric modulation is gathering increasing interest from the scientific community and may be potentially exploited for clinical purposes. This review summarizes the current knowledge on gonadotropin receptor allosteric modulation and their potential, clinical use.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

FSHR activation through small molecule modulators: Mechanistic insights from MD simulations

Follicle-stimulating hormone receptor (FSHR) is a glycoprotein hormone receptor that plays a vital role in reproduction, cancer progression and osteoporosis. Owing to its therapeutic importance, several small molecule modulators have been identified by researchers through high throughput studies that usually include virtual screening of chemical libraries followed by in vitro validation through radio-ligand binding assays, cAMP accumulation and luciferase-based luminescence assays. The binding site of these modulators and structural changes that accompany modulator binding remains elusive. Here, we address these aspects through molecular docking and MD simulations on well-studied FSHR modulators and comparing the domain motions between agonist/FSH bound and antagonist bound FSHR structures. It was observed that agonist and antagonist modulators bind to the same site, but interact with distinct residues in transmembrane domain(TMD). FSHR(TMD) residues Ile522, Ala595, Ile602 and Val604 were found to interact only with agonist. Notably, these residues are conserved in the close homolog luteinizing hormone/choriogonadotropin receptor (LHCGR) and participate in interaction with its agonist Org43553. We observed distinctly prominent domain motions and conformational changes in TM helices 3, 4 and 6 for agonist bound FSHR structure. These structural changes have also been reported for LHCGR, and few GPCR members suggesting an important and well conserved mechanism of GPHR activation that could be exploited for design of novel modulators.

Mechanism of hormone and allosteric agonist mediated activation of follicle stimulating hormone receptor

Follicle stimulating hormone (FSH) is an essential glycoprotein hormone for human reproduction, which functions are mediated by a G protein-coupled receptor, FSHR. Aberrant FSH-FSHR signaling causes infertility and ovarian hyperstimulation syndrome. Here we report cryo-EM structures of FSHR in both inactive and active states, with the active structure bound to FSH and an allosteric agonist compound 21 f. The structures of FSHR are similar to other glycoprotein hormone receptors, highlighting a conserved activation mechanism of hormone-induced receptor activation. Compound 21 f formed extensive interactions with the TMD to directly activate FSHR. Importantly, the unique residue H615^7.42 in FSHR plays an essential role in determining FSHR selectivity for various allosteric agonists. Together, our structures provide a molecular basis of FSH and small allosteric agonist-mediated FSHR activation, which could inspire the design of FSHR-targeted drugs for the treatment of infertility and controlled ovarian stimulation for in vitro fertilization.

Current pharmacotherapy and future directions for neuroendocrine causes of female infertility

INTRODUCTION

Infertility is recognized as a major global health issue, often associated with significant psychological distress for affected couples. Causes of female infertility include endocrine conditions leading to oligo/anovulation, in addition to structural causes such as tubal, uterine, or peritoneal disorders. Pharmacological treatments, targeting pathways in the hypothalamic-pituitary-ovarian axis, can improve rates of ovulation, conception, pregnancy, and birth. Some existing therapeutic options are hindered by limited efficacy or by a non-physiological mechanism, which can risk excessive stimulation and treatment-related adverse effects. Therefore, there is a continued need for novel therapies to improve care for patients suffering with infertility.

AREAS COVERED

In this review, the authors focus on endocrine causes of oligo/anovulation in women and on advances in assisted reproductive technology. Current pharmacological treatments and putative future therapeutic avenues in development to aid fertility in women are outlined.

EXPERT OPINION

A deeper understanding of the reproductive neuroendocrine network governing hypothalamic gonadotropin-releasing hormone release can offer novel therapeutic targets for the treatment of female subfertility, leading to improved clinical outcomes, less invasive routes of administration, and decreased treatment-related side-effects. The ultimate aim of development in female subfertility is to offer therapeutic interventions that are effective, reproducible, associated with minimal risks, and have an acceptable route of administration.

The impact of male factors and their correct and early diagnosis in the infertile couple's pathway: 2021 perspectives

PURPOSE

The current clinical practice in reproductive medicine should pose the couple at the centre of the diagnostic-therapeutic management of infertility and requires intense collaboration between the andrologist, the gynaecologist and the embryologist. The andrologist, in particular, to adequately support the infertile couple, must undertake important biological, psychological, economical and ethical task. Thus, this paper aims to provide a comprehensive overview of the multifaceted role of the andrologist in the study of male factor infertility.

METHODS

A comprehensive Medline, Embase and Cochrane search was performed including publications between 1969 and 2021.

RESULTS

Available evidence indicates that a careful medical history and physical examination, followed by semen analysis, always represent the basic starting points of the diagnostic work up in male partner of an infertile couple. Regarding treatment, gonadotropins are an effective treatment in case of hypogonadotropic hypogonadism and FSH may be used in men with idiopathic infertility, while evidence supporting other hormonal and nonhormonal treatments is either limited or conflicting. In the future, pharmacogenomics of FSHR and FSHB as well as innovative compounds may be considered to develop new therapeutic strategies in the management of infertility.

CONCLUSION

To provide a high-level of care, the andrologist must face several critical diagnostical and therapeutical steps. Even though ART may be the final and decisive stage of this decisional network, neglecting to treat the male partner may ultimately increase the risks of negative outcome, as well as costs and psychological burden for the couple itself.

Mapping of FSHR agonists and antagonists binding sites to identify potential peptidomimetic modulators

Owing to the critical role of follicle stimulating hormone receptor (FSHR) signaling in human reproduction, FSHR has been widely explored for development of fertility regulators. Using high-throughput screening approaches, several low molecular weight (LMW) compounds that can modulate FSHR activity have been identified. However, the information about the binding sites of these molecules on FSHR is not known. In the present study, we extracted the structural and functional information of 161 experimentally validated LMW FSHR modulators available in PubMed records. The potential FSHR binding sites for these modulators were identified through molecular docking experiments. The binding sites were further mapped to the agonist or antagonist activity reported for these molecules in literature. MD simulations were performed to evaluate the effect of ligand binding on conformational changes in the receptor, specifically the transmembrane domain. A peptidomimetic library was screened using these binding sites. Six peptidomimetics that interacted with the residues of transmembrane domain and extracellular loops were evaluated for binding activity using in vitro cAMP assay. Two of the six peptidomimetics exhibited positive allosteric modulatory activity and four peptidomimetics exhibited negative allosteric modulatory activity. All six peptidomimetics interacted with Asp521 of hFSHR(TMD). Several of the experimentally known LMW FSHR modulators also participated in H-bond interactions with Asp521, suggesting its important role in FSHR modulatory activity.

Membrane Hormone Receptors and Their Signaling Pathways as Targets for Endocrine Disruptors

The endocrine disruptors are mostly small organic molecules developed for numerous and very diverse industrial applications. They essentially act through nuclear receptors with small and hydrophobic endogenous ligands. Nevertheless, potential adverse effects through membrane hormone receptors cannot be ruled out, and have indeed been observed. The present paper reviews how orthosteric and allosteric binding sites of the different families of membrane receptors can be targets for man-made hydrophobic molecules (components of plastics, paints, flame retardants, herbicides, pesticides, etc.). We also review potential target proteins for such small hydrophobic molecules downstream of membrane receptors at the level of their intracellular signaling pathways. From the currently available information, although endocrine disruptors primarily affect nuclear receptors’ signaling, membrane receptors for hormones, cytokines, neuro-mediators, and growth factors can be affected as well and deserve attention.

Rescue of Cell Surface Expression and Signaling of Mutant Follicle-Stimulating Hormone Receptors

Mutations in G protein-coupled receptors (GPCRs) underlie numerous diseases. Many cause receptor misfolding and failure to reach the cell surface. Pharmacological chaperones are cell-permeant small molecules that engage nascent mutant GPCRs in the endoplasmic reticulum, stabilizing folding and "rescuing" cell surface expression. We previously demonstrated rescue of cell surface expression of luteinizing hormone receptor mutants by an allosteric agonist. Here we demonstrate that a similar approach can be employed to rescue mutant follicle-stimulating hormone receptors (FSHRs) with poor cell surface expression using a small-molecule FSHR agonist, CAN1404. Seventeen FSHR mutations described in patients with reproductive dysfunction were expressed in HEK 293T cells, and cell surface expression was determined by enzyme-linked immunosorbent assay of epitope-tagged FSHRs before/after treatment with CAN1404. Cell surface expression was severely reduced to ≤18% of wild-type (WT) for 11, modestly reduced to 66% to 84% of WT for 4, and not reduced for 2. Of the 11 with severely reduced cell surface expression, restoration to ≥57% of WT levels was achieved for 6 by treatment with 1 µM CAN1404 for 24 h, and a corresponding increase in FSH-induced signaling was observed for 4 of these, indicating restored functionality. Therefore, CAN1404 acts as a pharmacological chaperone and can rescue cell surface expression and function of certain mutant FSHRs with severely reduced cell surface expression. These findings aid in advancing the understanding of the effects of genetic mutations on GPCR function and provide a proof of therapeutic principle for FSHR pharmacological chaperones.

Pharmacological Characterization of Low Molecular Weight Biased Agonists at the Follicle Stimulating Hormone Receptor

Follicle-stimulating hormone receptor (FSHR) plays a key role in reproduction through the activation of multiple signaling pathways. Low molecular weight (LMW) ligands composed of biased agonist properties are highly valuable tools to decipher complex signaling mechanisms as they allow selective activation of discrete signaling cascades. However, available LMW FSHR ligands have not been fully characterized yet. In this context, we explored the pharmacological diversity of three benzamide and two thiazolidinone derivatives compared to FSH. Concentration/activity curves were generated for Gαs, Gαq, Gαi, β-arrestin 2 recruitment, and cAMP production, using BRET assays in living cells. ERK phosphorylation was analyzed by Western blotting, and CRE-dependent transcription was assessed using a luciferase reporter assay. All assays were done in either wild-type, Gαs or β-arrestin 1/2 CRISPR knockout HEK293 cells. Bias factors were calculated for each pair of read-outs by using the operational model. Our results show that each ligand presented a discrete pharmacological efficacy compared to FSH, ranging from super-agonist for β-arrestin 2 recruitment to pure Gαs bias. Interestingly, LMW ligands generated kinetic profiles distinct from FSH (i.e., faster, slower or transient, depending on the ligand) and correlated with CRE-dependent transcription. In addition, clear system biases were observed in cells depleted of either Gαs or β-arrestin genes. Such LMW properties are useful pharmacological tools to better dissect the multiple signaling pathways activated by FSHR and assess their relative contributions at the cellular and physio-pathological levels.

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.

Recent advances in understanding gonadotropin signaling

Gonadotropins are glycoprotein sex hormones regulating development and reproduction and bind to specific G protein–coupled receptors expressed in the gonads. Their effects on multiple signaling cascades and intracellular events have recently been characterized using novel technological and scientific tools. The impact of allosteric modulators on gonadotropin signaling, the role of sugars linked to the hormone backbone, the detection of endosomal compartments supporting signaling modules, and the dissection of different effects mediated by these molecules are areas that have advanced significantly in the last decade. The classic view providing the exclusive activation of the cAMP/protein kinase A (PKA) and the steroidogenic pathway by these hormones has been expanded with the addition of novel signaling cascades as determined by high-resolution imaging techniques. These new findings provided new potential therapeutic applications. Despite these improvements, unanswered issues of gonadotropin physiology, such as the intrinsic pro-apoptotic potential to these hormones, the existence of receptors assembled as heteromers, and their expression in extragonadal tissues, remain to be studied. Elucidating these issues is a challenge for future research.

Pharmacological Programming of Endosomal Signaling Activated by Small Molecule Ligands of the Follicle Stimulating Hormone Receptor

Follicle-stimulating hormone receptor (FSHR) is a G protein-coupled receptor (GPCR) with pivotal roles in reproduction. One key mechanism dictating the signal activity of GPCRs is membrane trafficking. After binding its hormone FSH, FSHR undergoes internalization to very early endosomes (VEEs) for its acute signaling and sorting to a rapid recycling pathway. The VEE is a heterogeneous compartment containing the Adaptor Protein Phosphotyrosine Interacting with Pleckstrin homology Domain and Leucine Zipper 1 (APPL1) with distinct functions in regulating endosomal Gαs/cAMP signaling and rapid recycling. Low molecular weight (LMW) allosteric FSHR ligands were developed for use in assisted reproductive technology yet could also provide novel pharmacological tools to study FSHR. Given the critical nature of receptor internalization and endosomal signaling for FSHR activity, we assessed whether these compounds exhibit differential abilities to alter receptor endosomal trafficking and signaling within the VEE. Two chemically distinct LMW agonists (benzamide, termed B3 and thiazolidinone, termed T1) were employed. T1 was able to induce a greater level of cAMP than FSH and B3. As cAMP signaling drives gonadotrophin hormone receptor recycling, rapid exocytic events were evaluated at single event resolution. Strikingly, T1 was able to induce a 3-fold increase in recycling events compared to FSH and two-fold more compared to B3. As T1-induced internalization was only marginally greater, the dramatic increase in recycling and cAMP signaling may be due to additional mechanisms. All compounds exhibited a similar requirement for receptor internalization to increase cAMP and proportion of FSHR endosomes with active Gαs, suggesting regulation of cAMP signaling induced by T1 may be altered. APPL1 plays a central role for GPCRs targeted to the VEE, and indeed, loss of APPL1 inhibited FSH-induced recycling and increased endosomal cAMP signaling. While T1-induced FSHR recycling was APPL1-dependent, its elevated cAMP signaling was only partially increased following APPL1 knockdown. Unexpectedly, B3 altered the dependence of FSHR to APPL1 in an opposing manner, whereby its endosomal signaling was negatively regulated by APPL1, while B3-induced FSHR recycling was APPL1-independent. Overall, FSHR allosteric compounds have the potential to re-program FSHR activity via altering engagement with VEE machinery and also suggests that these two distinct functions of APPL1 can potentially be selected pharmacologically.

A 5-mer peptide derived from hinge region of hFSHR can function as positive allosteric modulator in vivo

Interaction of follicle stimulating hormone (FSH) with its cognate receptor (FSHR) is critical for maintaining reproductive health. FSHR has a large extracellular domain (ECD), composed of leucine rich repeats (LRRs) and hinge region, a transmembrane domain (TMD) and a short C-terminal domain (CTD). In this study, we have identified a short peptidic stretch in the hinge region (hFSHR(271-275)), through extensive computational modeling, docking and MD simulations, that is capable of independently interacting with the extracellular loops of FSHR(TMD). In vitro studies revealed that FSHR(271-275) peptide increased binding of [¹²⁵I]-FSH to rat Fshr as well as FSH-induced cAMP production. Administration of FSHR(271-275) peptide in immature female rats significantly increased FSH-mediated ovarian weight gain and promoted granulosa cell proliferation. In summary, the results demonstrate that the synthetic peptide corresponding to amino acids 271-275 of hFSHR-hinge region stimulates FSH-FSHR interaction and behaves as positive allosteric modulator of FSHR. The study also lends evidence to the existing proposition that hinge region maintains the receptor in an inactive conformation in the absence of its ligand by engaging in intramolecular interactions with extracellular loops of TMD.

Biased Signaling and Allosteric Modulation at the FSHR

Knowledge on G protein-coupled receptor (GPCRs) structure and mechanism of activation has profoundly evolved over the past years. The way drugs targeting this family of receptors are discovered and used has also changed. Ligands appear to bind a growing number of GPCRs in a competitive or allosteric manner to elicit balanced signaling or biased signaling (i.e., differential efficacy in activating or inhibiting selective signaling pathway(s) compared to the reference ligand). These novel concepts and developments transform our understanding of the follicle-stimulating hormone (FSH) receptor (FSHR) biology and the way it could be pharmacologically modulated in the future. The FSHR is expressed in somatic cells of the gonads and plays a major role in reproduction. When compared to classical GPCRs, the FSHR exhibits intrinsic peculiarities, such as a very large NH2-terminal extracellular domain that binds a naturally heterogeneous, large heterodimeric glycoprotein, namely FSH. Once activated, the FSHR couples to Gαs and, in some instances, to other Gα subunits. G protein-coupled receptor kinases and β-arrestins are also recruited to this receptor and account for its desensitization, trafficking, and intracellular signaling. Different classes of pharmacological tools capable of biasing FSHR signaling have been reported and open promising prospects both in basic research and for therapeutic applications. Here we provide an updated review of the most salient peculiarities of FSHR signaling and its selective modulation.