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

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.

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.

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.

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.

Discovery and Preclinical Development of Orally Active Small Molecules that Exhibit Highly Selective Follicle Stimulating Hormone Receptor Agonism

An orally active follicle stimulating hormone receptor allosteric agonist would provide a preferred treatment for over 16 million infertile women of reproductive age in low complexity methods (ovulation induction-intrauterine insemination) or in high complexity methods (controlled ovarian stimulation-in vitro fertilization). We present two oral follicle stimulating hormone receptor allosteric agonist compounds that have the desired pharmacology, drug metabolism, pharmacokinetics, and safety profile for clinical use. These molecules provide a single agent suitable for ovulation induction-intrauterine insemination or controlled ovarian stimulation-in vitro fertilization that is more convenient for patients and achieves similar preclinical efficacy as rec-hFSH. TOP5668, TOP5300 were evaluated in vitro in Chinese hamster ovary cells transfected with individual glycoprotein receptors measuring cAMP (FSHR, LH/CGR, thyroid stimulating hormone receptor). TOP5668 was found to have solely follicle stimulating hormone receptor allosteric agonist activity while TOP5300 was found to have mixed follicle stimulating hormone receptor allosteric agonist and LHR-AA activity. Both compounds stimulated concentration-dependent increases in estradiol production from cultured rat granulosa cells in the presence or absence of low dose rec-hFSH, while only TOP5300 stimulated testosterone production from rat primary Leydig cells. In pooled human granulosa cells obtained from patients undergoing controlled ovarian stimulation-in vitro fertilization, TOP5300 stimulated 7-fold greater maximal estradiol response than rec-hFSH and TOP5668 was 10-fold more potent than TOP5300. Both TOP5300 and TOP5668 stimulated follicular development in immature rat to the same efficacy as recombinant follicle stimulating hormone. In mice treated with TOP5300, in the presence of low dose of follicle stimulating hormone, there were no differences in oocyte number, fertilization rate, and hatched blastocyst rate in mice with TOP5300 and low dose follicle stimulating hormone vs. reference proteins pregnant mare serum gonadotropin or high dose rec-hFSH. ADME/PK and safety profiles were favorable. In addition, there was no appreciable activity on thyroid hormones by TOP5300 in 14-days toxicological study in rat or dog. The selected lead compound, TOP5300 stimulated a more robust increase in estradiol production from granulosa-lutein cells from women with polycystic ovarian syndrome patient compared to rec-hFSH. Conclusions: Two novel oral FSHR allosteric agonist, TOP5668 and TOP5300, were found to mimic the biological activity of rec hFSH in preclinical studies. Both compounds led to folliculogenesis and superovulation in rat and mice. Specifically, TOP5300 led to a similar number of ovulated oocytes that fertilized and developed into hatched blastocysts in mice when compared to rec-hFSH. The safety profile demonstrated lack of toxicity.

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.

Follicle-stimulating Hormone (FSH) Action on Spermatogenesis: A Focus on Physiological and Therapeutic Roles

BACKGROUND

Human reproduction is regulated by the combined action of the follicle-stimulating hormone (FSH) and the luteinizing hormone (LH) on the gonads. Although FSH is largely used in female reproduction, in particular in women attending assisted reproductive techniques to stimulate multi-follicular growth, its efficacy in men with idiopathic infertility is not clearly demonstrated. Indeed, whether FSH administration improves fertility in patients with hypogonadotropic hypogonadism, the therapeutic benefit in men presenting alterations in sperm production despite normal FSH serum levels is still unclear. In the present review, we evaluate the potential pharmacological benefits of FSH administration in clinical practice.

METHODS

This is a narrative review, describing the FSH physiological role in spermatogenesis and its potential therapeutic action in men.

RESULTS

The FSH role on male fertility is reviewed starting from the physiological control of spermatogenesis, throughout its mechanism of action in Sertoli cells, the genetic regulation of its action on spermatogenesis, until the therapeutic options available to improve sperm production.

CONCLUSION

FSH administration in infertile men has potential benefits, although its action should be considered by evaluating its synergic action with testosterone, and well-controlled, powerful trials are required. Prospective studies and new compounds could be developed in the near future.

FSH for the Treatment of Male Infertility

Follicle-stimulating hormone (FSH) supports spermatogenesis acting via its receptor (FSHR), which activates trophic effects in gonadal Sertoli cells. These pathways are targeted by hormonal drugs used for clinical treatment of infertile men, mainly belonging to sub-groups defined as hypogonadotropic hypogonadism or idiopathic infertility. While, in the first case, fertility may be efficiently restored by specific treatments, such as pulsatile gonadotropin releasing hormone (GnRH) or choriogonadotropin (hCG) alone or in combination with FSH, less is known about the efficacy of FSH in supporting the treatment of male idiopathic infertility. This review focuses on the role of FSH in the clinical approach to male reproduction, addressing the state-of-the-art from the little data available and discussing the pharmacological evidence. New compounds, such as allosteric ligands, dually active, chimeric gonadotropins and immunoglobulins, may represent interesting avenues for future personalized, pharmacological approaches to male infertility.

Gonadotropins and their receptors: coevolution, genetic variants, receptor imaging, and functional antagonists

Gonadotropins belong to the family of dimeric glycoprotein hormones and regulate gonadal physiology mediated by G protein-coupled, seven-transmembrane receptors. These glycoprotein hormones are widely used in the clinic to promote ovarian follicle development and for treating some cases of male infertility. We traced the coevolution of dimeric gonadotropin hormones and their receptors, together with thyrotropin and its receptor. We updated recent findings on human genetic variants of these genes and their association with dizygotic twining, polycystic ovarian syndrome, primary ovarian insufficiency, male-limited precocious puberty, and infertility. In addition to the known physiological roles of gonadotropin-receptor signaling in gonadal tissues, we also discussed emerging understanding of extragonadal functions of gonadotropins in bones and adipose tissues, together with recent advances in in vivo imaging of gonadotropin receptors in live animals. Recent development of gonadotropin receptor agonists and antagonists were summarized with an emphasis on the development of functional antagonists for FSH receptors to alleviate osteoporosis and obesity associated with menopause.

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.

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.

FSH Receptor Signaling: Complexity of Interactions and Signal Diversity

FSH is synthesized in the pituitary by gonadotrope cells. By binding to and interacting with its cognate receptor [FSH receptor (FSHR)] in the gonads, this gonadotropin plays a key role in the control of gonadal function and reproduction. Upon activation, the FSHR undergoes conformational changes leading to transduction of intracellular signals, including dissociation of G protein complexes into components and activation of several associated interacting partners, which concertedly regulate downstream effectors. The canonical Gs/cAMP/protein kinase A pathway, considered for a long time as the sole effector of FSHR-mediated signaling, is now viewed as one of several mechanisms employed by this receptor to transduce intracellular signals in response to the FSH stimulus. This complex network of signaling pathways allows for a fine-tuning regulation of the gonadotropic stimulus, where activation/inhibition of its multiple components vary depending on the cell context, cell developmental stage, and concentration of associated receptors and corresponding ligands. Activation of these multiple signaling modules eventually converge to the hormone-integrated biological response, including survival, proliferation and differentiation of target cells, synthesis and secretion of paracrine/autocrine regulators, and, at the molecular level, functional selectivity and differential gene expression. In this mini-review, we discuss the complexity of FSHR-mediated intracellular signals activated in response to ligand stimulation. A better understanding of the signaling pathways involved in FSH action might potentially influence the development of new therapeutic strategies for reproductive disorders.

β-arrestins and biased signaling in gonadotropin receptors

Gonadotropin receptors include the follicle stimulating hormone receptor (FSHR) and the luteinizing hormone/choriogonadotropin receptor (LHCGR), both belong to the G protein-coupled receptor (GPCR) superfamily and are essential to reproduction. FSHR is activated by follicle stimulating hormone (FSH) while LHCGR is activated by either luteinizing hormone (LH) or choriogonadotropin (CG). Upon ligand binding, gonadotropin receptors undergo conformational changes that lead to the activation of the heterotrimeric G protein, resulting in the production of different second messengers. Gonadotropin receptors can also recruit and bind β-arrestins. This particular class of scaffold proteins were initially identified to mediate GPCRs desensitization and recycling, but it is now well established that β-arrestins can also initiate Gs-independent signaling by assembling signaling modules. Furthermore, new advances in structural biology and biophysical techniques have revealed novel activation mechanisms allowing β-arrestins and G proteins to control signaling in time and space. The ability of different ligands to preferentially elicit G- or β-arrestin-mediated signaling is known as functional selectivity or biased signaling. This new concept has switched the view of pharmacology efficacy from monodimensional to multidimensional. Biased signaling offers the possibility to separate therapeutic benefits of a drug from its adverse effects. The proof of concept that gonadotropin receptors can be subjected to biased signaling is now established. The challenge will now be the design of molecules that can specifically activate beneficial signaling pathway at gonadotropin receptors while reducing or abolishing those leading to side effects. Such strategy could for instance lead to improved treatments for infertility.

Allosteric Regulation of the Follicle-Stimulating Hormone Receptor

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

Biased signalling: from simple switches to allosteric microprocessors

G protein-coupled receptors (GPCRs) are the largest class of receptors in the human genome and some of the most common drug targets. It is now well established that GPCRs can signal through multiple transducers, including heterotrimeric G proteins, GPCR kinases and β-arrestins. While these signalling pathways can be activated or blocked by 'balanced' agonists or antagonists, they can also be selectively activated in a 'biased' response. Biased responses can be induced by biased ligands, biased receptors or system bias, any of which can result in preferential signalling through G proteins or β-arrestins. At many GPCRs, signalling events mediated by G proteins and β-arrestins have been shown to have distinct biochemical and physiological actions from one another, and an accurate evaluation of biased signalling from pharmacology through physiology is crucial for preclinical drug development. Recent structural studies have provided snapshots of GPCR-transducer complexes, which should aid in the structure-based design of novel biased therapies. Our understanding of GPCRs has evolved from that of two-state, on-and-off switches to that of multistate allosteric microprocessors, in which biased ligands transmit distinct structural information that is processed into distinct biological outputs. The development of biased ligands as therapeutics heralds an era of increased drug efficacy with reduced drug side effects.

Small Molecule Follicle-Stimulating Hormone Receptor Agonists and Antagonists

The follicle-stimulating hormone receptor (FSHR) has been targeted therapeutically for decades, due to its pivotal role in reproduction. To date, only purified and recombinant/biosimilar FSH have been used to target FSHR in assisted reproduction, with the exception of corifollitropin alfa; a modified gonadotropin in which the FSH beta subunit is joined to the C-terminal peptide of the human choriogonadotropin beta subunit, to extend serum half-life. Assisted reproduction protocols usually entail the trauma of multiple injections of FSH to initiate and promote folliculogenesis, which has prompted the development of a number of orally-available low molecular weight (LMW) chemical scaffolds targeting the FSHR. Furthermore, the recently documented roles of the FSHR in diverse extragonadal tissues, including cancer, fat metabolism, and bone density regulation, has highlighted the potential utility of LMW modulators of FSHR activity. Despite these chemical scaffolds encompassing a spectrum of in vitro and in vivo activities and pharmacological profiles, none have yet reached the clinic. In this review we discuss the major chemical classes of LMW molecules targeting the FSHR, and document their activity profiles and current status of development, in addition to discussing potential clinical applications.

Whole-cell biopanning with a synthetic phage display library of nanobodies enabled the recovery of follicle-stimulating hormone receptor inhibitors

Antibodies are essential reagents that are increasingly used in diagnostics and therapy. Their specificity and capacity to recognize their native antigen are critical characteristics for their in vivo application. Follicle-stimulating hormone receptor is a GPCR protein regulating ovarian follicular maturation and spermatogenesis. Recently, its potentiality as a cancer biomarker has been demonstrated but no antibody suitable for in vivo tumor targeting and treatment has been characterized so far. In this paper we describe the first successful attempt to recover recombinant antibodies against the FSHR and that: i) are directly panned from a pre-immune library using whole cells expressing the target receptor at their surface; ii) show inhibitory activity towards the FSH-induced cAMP accumulation; iii) do not share the same epitope with the natural binder FSH; iv) can be produced inexpensively as mono- or bivalent functional molecules in the bacterial cytoplasm. We expect that the proposed biopanning strategy will be profitable to identify useful functional antibodies for further members of the GPCR class.

Clinical Applications of Gonadotropins in the Male

The pituitary gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) play a pivotal role in reproduction. The synthesis and secretion of gonadotropins are regulated by complex interactions among several endocrine, paracrine, and autocrine factors of diverse chemical structure. In men, LH regulates the synthesis of androgens by the Leydig cells, whereas FSH promotes Sertoli cell function and thereby influences spermatogenesis. Gonadotropins are complex molecules composed of two subunits, the α- and β-subunit, that are noncovalently associated. Gonadotropins are decorated with glycans that regulate several functions of the protein including folding, heterodimerization, stability, transport, conformational maturation, efficiency of heterodimer secretion, metabolic fate, interaction with their cognate receptor, and selective activation of signaling pathways. A number of congenital and acquired abnormalities lead to gonadotropin deficiency and hypogonadotropic hypogonadism, a condition amenable to treatment with exogenous gonadotropins. Several natural and recombinant preparations of gonadotropins are currently available for therapeutic purposes. The difference between natural and the currently available recombinant preparations (which are massively produced in Chinese hamster ovary cells for commercial purposes) mainly lies in the abundance of some of the carbohydrates that conform the complex glycans attached to the protein core. Whereas administration of exogenous gonadotropins in patients with isolated congenital hypogonadotropic hypogonadism is a well recognized therapeutic approach, their role in treating men with normogonadotropic idiopathic infertility is still controversial. This chapter concentrates on the main structural and functional features of the gonadotropin hormones and how basic concepts have been translated into the clinical arena to guide therapy for gonadotropin deficit in males.

In Vitro Effects of the Endocrine Disruptor p,p'-DDT on Human Follitropin Receptor

BACKGROUND

1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene (p,p'-DDT) is a persistent environmental endocrine disruptor (ED). Several studies have shown an association between p,p'-DDT exposure and reproductive abnormalities.

OBJECTIVES

To investigate the putative effects of p,p'-DDT on the human follitropin receptor (FSHR) function.

METHODS AND RESULTS

We used Chinese hamster ovary (CHO) cells stably expressing human FSHR to investigate the impact of p,p'-DDT on FSHR activity and its interaction with the receptor. At a concentration of 5 μM p,p'-DDT increased the maximum response of the FSHR to follitropin by 32 ± 7.45%. However, 5 μM p,p'-DDT decreased the basal activity and did not influence the maximal response of the closely related LH/hCG receptor to human chorionic gonadotropin (hCG). The potentiating effect of p,p'-DDT was specific for the FSHR. Moreover, in cells that did not express FSHR, p,p'-DDT had no effect on cAMP response. Thus, the potentiating effect of p,p'-DDT was dependent on the FSHR. In addition, p,p'-DDT increased the sensitivity of FSHR to hCG and to a low molecular weight agonist of the FSHR, 3-((5methyl)-2-(4-benzyloxy-phenyl)-5-{[2-[3-ethoxy-4-methoxy-phenyl)-ethylcarbamoyl]-methyl}-4-oxo-thiazolidin-3-yl)-benzamide (16a). Basal activity in response to p,p'-DDT and potentiation of the FSHR response to FSH by p,p'-DDT varied among FSHR mutants with altered transmembrane domains (TMDs), consistent with an effect of p,p'-DDT via TMD binding. This finding was corroborated by the results of simultaneously docking p,p'-DDT and 16a into the FSHR transmembrane bundle.

CONCLUSION

p,p'-DDT acted as a positive allosteric modulator of the FSHR in our experimental model. These findings suggest that G protein-coupled receptors are additional targets of endocrine disruptors.

CITATION

Munier M, Grouleff J, Gourdin L, Fauchard M, Chantreau V, Henrion D, Coutant R, Schiøtt B, Chabbert M, Rodien P. 2016. In vitro effects of the endocrine disruptor p,p'-DDT on human follitropin receptor. Environ Health Perspect 124:991-999; http://dx.doi.org/10.1289/ehp.1510006.

An Investigation on a Novel Anti-tumor Fusion Peptide of FSH33-53-IIKK

A novel fusion peptide FSH_33-53-IIKK was designed and expected to combine the follicle stimulating hormone receptor (FSHR) targeting and tumor toxicity. In vitro and in vivo study showed the anti-tumor activity of FSH_33-53-IIKK was enhanced compared to that of IIKK only. FSH_33-53-IIKK could inhibit the growth of tumor via apoptosis and autophagy pathways. In summary, combining the tumor marker-target peptide and anti-tumor peptide together may be an efficient way to search for better anti-tumor candidates.

Development of Follicle-Stimulating Hormone Receptor Binding Probes to Image Ovarian Xenografts

The Follicle-Stimulating Hormone Receptor (FSHR) is used as an imaging biomarker for the detection of ovarian cancer (OC). FSHR is highly expressed on ovarian tumors and involved with cancer development and metastatic signaling pathways. A decapeptide specific to the FSHR extracellular domain is synthesized and conjugated to fluorescent dyes to image OC cells in vitro and tumors xenograft model in vivo. The in vitro binding curve and the average number of FSHR per cell are obtained for OVCAR-3 cells by a high resolution flow cytometer. For the decapeptide, the measured EC50 was 160 μM and the average number of receptors per cell was 1.7 × 10⁷. The decapeptide molecular imaging probe reached a maximum tumor to muscle ratio five hours after intravenous injection and a dose-dependent plateau after 24-48 hours. These results indicate the potential application of a small molecular weight imaging probe specific to ovarian cancer through binding to FSHR. Based on these results, multimeric constructs are being developed to optimize binding to ovarian cells and tumors.

Structure-function relationships of glycoprotein hormones and their subunits' ancestors

Glycoprotein hormones (GPHs) are the most complex molecules with hormonal activity. They exist only in vertebrates but the genes encoding their subunits' ancestors are found in most vertebrate and invertebrate species although their roles are still unknown. In the present report, we review the available structural and functional data concerning GPHs and their subunits' ancestors.

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.

Evidence for Follicle-stimulating Hormone Receptor as a Functional Trimer

Follicle-stimulating hormone receptor (FSHR), a G-protein coupled receptor, is an important drug target in the development of novel therapeutics for reproductive indications. The FSHR extracellular domains were observed in the crystal structure as a trimer, which enabled us to propose a novel model for the receptor activation mechanism. The model predicts that FSHR binds Asnα⁵²-deglycosylated FSH at a 3-fold higher capacity than fully glycosylated FSH. It also predicts that, upon dissociation of the FSHR trimer into monomers, the binding of glycosylated FSH, but not deglycosylated FSH, would increase 3-fold, and that the dissociated monomers would in turn enhance FSHR binding and signaling activities by 3-fold. This study presents evidence confirming these predictions and provides crystallographic and mutagenesis data supporting the proposed model. The model also provides a mechanistic explanation to the agonist and antagonist activities of thyroid-stimulating hormone receptor autoantibodies. We conclude that FSHR exists as a functional trimer.

Effect of FSH receptor-binding inhibitor-8 on FSH-mediated granulosa cell signaling and proliferation

Follicle-stimulating hormone is important for mammalian reproduction. It acts through specific receptors located on the plasma membrane of granulosa cells in ovaries and Sertoli cells in testes. The binding of follicle-stimulating hormone to its receptor activates intracytoplasmic signaling pathways leading to steroidogenesis. These steroids in turn regulate the follicle-stimulating hormone action from the anterior pituitary through exerting negative feedback effect. In addition to steroids, non-steroidal factors secreted by the ovaries are believed to modulate follicle-stimulating hormone action through autocrine/paracrine mode. One such low molecular weight peptide referred to as follicle-stimulating hormone receptor-binding inhibitor-8 purified from human follicular fluid has been extensively studied. Follicle-stimulating hormone receptor-binding inhibitor-8 has been shown to inhibit binding of follicle-stimulating hormone to its receptor. The present article describes the effect of follicle-stimulating hormone receptor-binding inhibitor-8 on follicle-stimulating hormone-induced signaling in rat granulosa cells. Follicle-stimulating hormone receptor-binding inhibitor-8 inhibited the follicle-stimulating hormone-induced cAMP, and the effect was observed to be mediated through the protein kinase A. Further, an inhibitory effect of follicle-stimulating hormone receptor-binding inhibitor-8 on the granulosa cell proliferation was evaluated using COV434 cell line which is derived from the human granulosa cell tumor. The effect of the peptide on the cell cycle analysis showed an increase in apoptotic population and the arrest of G1 phase. These findings suggest that follicle-stimulating hormone receptor-binding inhibitor-8 acts as a follicle-stimulating hormone antagonist and affects the follicle-stimulating hormone-mediated signaling and proliferation in the granulosa cells.

Biased signalling in follicle stimulating hormone action

Follicle-stimulating hormone (FSH) plays a crucial role in the control of reproduction by specifically binding to and activating a membrane receptor (FSHR) that belongs to the G protein-coupled receptor (GPCR) family. Similar to all GPCRs, FSHR activation mechanisms have generally been viewed as a two-state process connecting a unique FSH-bound active receptor to the Gs/cAMP pathway. Over the last decade, paralleling the breakthroughs that were made in the GPCR field, our understanding of FSH actions at the molecular level has dramatically changed. There are numerous facts indicating that the active FSHR is connected to a complex signalling network rather than the sole Gs/cAMP pathway. Consistently, the FSHR probably exists in equilibrium between multiple conformers, a subset of them being stabilized upon ligand binding. Importantly, the nature of the stabilized conformers of the receptor directly depends on the chemical structure of the ligand bound. This implies that it is possible to selectively control the intracellular signalling pathways activated by using biased ligands. Such biased ligands can be of different nature: small chemical molecules, glycosylation variants of the hormone or antibody/hormone complexes. Likewise, mutations or polymorphisms affecting the FSHR can also lead to stabilization of preferential conformers, hence to selective modulation of signalling pathways. These emerging notions offer a new conceptual framework that could potentially lead to the development of more specific drugs while also improving the way FSHR mutants/variants are functionally characterized.

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.

Gonadotropin therapy in assisted reproduction: an evolutionary perspective from biologics to biotech

Gonadotropin therapy plays an integral role in ovarian stimulation for infertility treatments. Efforts have been made over the last century to improve gonadotropin preparations. Undoubtedly, current gonadotropins have better quality and safety profiles as well as clinical efficacy than earlier ones. A major achievement has been introducing recombinant technology in the manufacturing processes for follicle-stimulating hormone, luteinizing hormone, and human chorionic gonadotropin. Recombinant gonadotropins are purer than urine-derived gonadotropins, and incorporating vial filling by mass virtually eliminated batch-to-batch variations and enabled accurate dosing. Recombinant and fill-by-mass technologies have been the driving forces for launching of prefilled pen devices for more patient-friendly ovarian stimulation. The most recent developments include the fixed combination of follitropin alfa + lutropin alfa, long-acting FSH gonadotropin, and a new family of prefilled pen injector devices for administration of recombinant gonadotropins. The next step would be the production of orally bioactive molecules with selective follicle-stimulating hormone and luteinizing hormone activity.

Signaling of an allosteric, nanomolar potent, low molecular weight agonist for the follicle-stimulating hormone receptor

Follicle-stimulating hormone (FSH) activates FSH receptors (FSHR) in granulosa cells to induce follicle differentiation, growth and estradiol production. FSH is used clinically to treat female infertility and is administered by injection. To increase patient convenience and compliance, compound homogeneity and composition, low molecular weight (LMW), orally bioavailable, FSHR agonists are now being developed to replace FSH. In this study, we present the signaling mechanisms of a newly developed LMW dihydropyridine agonist of the FSHR, Org 214444-0. Org 214444-0 is shown to be a stereoselective, nanomolar potent FSHR agonist and selective over the structurally related LHR and TSHR. Org 214444-0 is an allosteric agonist interacting with the transmembrane region of the FSHR. When co-incubated with FSH, Org 214444-0 augments FSH's potency in binding (6.5-fold) and adenylyl cyclase/cAMP activation (3.5-fold) in a concentration-dependent manner. Like FSH, Org 214444-0 induces FSHR internalization and is only marginally effective in stimulating phospholipase C. Moreover, Org 214444-0 stimulates cAMP and estradiol production in human granulosa cells in culture and supports the follicular phase in mature female rats. We conclude that Org 214444-0 is a bonafide FSHR agonist.

Allosteric modulators of rhodopsin-like G protein-coupled receptors: opportunities in drug development

Rhodopsin-like (class A) G protein-coupled receptors (GPCRs) are one of the most important classes of drug targets. The discovery that these GPCRs can be allosterically modulated by small drug molecules has opened up new opportunities in drug development. It will allow the drugability of "difficult targets", such as GPCRs activated by large (glyco)proteins, or by very polar or highly lipophilic physiological agonists. Receptor subtype selectivity should be more easily achievable with allosteric than with orthosteric ligands. Allosteric modulation will allow a broad spectrum of pharmacological effects largely expanding that of orthosteric ligands. Furthermore, allosteric modulators may show an improved safety profile as compared to orthosteric ligands. Only recently, the explicit search for allosteric modulators has been started for only a few rhodopsin-like GPCRs. The first negative allosteric modulators (allosteric antagonists) of chemokine receptors, maraviroc (CCR5 receptor), used in HIV therapy, and plerixafor (CXCR4 receptor) for stem cell mobilization, have been approved as drugs. The development of allosteric modulators for rhodopsin-like GPCRs as novel drugs is still at an early stage; it appears highly promising.

The follicle-stimulating hormone receptor: a novel target in genitourinary malignancies

Follicle-stimulating hormone (FSH) is a central hormone in mammalian reproductive biology. The FSH receptor (FSHR), which was previously believed to be expressed primarily in the ovary and testis, was recently found to be expressed in the tumor blood vessels of many solid tumor types, including prostate adenocarcinoma, urothelial carcinoma, and renal cell carcinoma. While the biologic significance of FSHR in tumor blood vessels has yet to be elucidated, FSHR may contribute to neoangiogenesis. FSHR has been reported to be expressed by prostate cancer cells and, thus, targeting FSHR in prostate cancer may be of particular utility. In this report, we discuss the finding of FSHR in tumor blood vessels and review the literature concerning FSHR in genitourinary malignancy. We also discuss the features that make FSHR an appealing target for therapeutic and imaging purposes and the potential utility of FSHR as a prognostic and/or predictive biomarker in genitourinary cancers.

Determination of different putative allosteric binding pockets at the lutropin receptor by using diverse drug-like low molecular weight ligands

The lutropin/choriogonadotrophin receptor (LHCGR) is a family A G protein-coupled receptor (GPCR) which binds the endogenous hormone-ligands at the large extracellular domain. In contrast, several drug-like low-molecular-weight ligands (LMWs) have been reported to interact allosterically within the seven transmembrane domain (7TMD) of the LHCGR. Here, we were interested to study the putative allosteric LHCGR binding region with focus on the determination of two pockets for LMW ligands. A library of compounds was screened for their ability to modify the binding of an allosteric radiolabeled LMW agonist [³H]Org 43553. Further experimental and computational studies revealed that the putative binding pocket for a newly identified allosteric enhancer (LUF5419) and a previously described allosteric inhibitor (LUF5771) are overlapping and that this site is different from the Org 43553 binding site. The present study showed that these compounds are useful tools to reveal details on different allosteric binding sites located within the 7TMD of the LHCGR.

Gonadotropin stimulation: past, present and future

Gonadotropin therapy is so central to infertility treatment that it is easy to overlook the considerable discovery and research that preceded production of the effective and safe products available today. The history underpinning this development spans over 300 years and provides a splendid example of how basic animal experimentation and technological advances have progressed to clinical application. Following the discovery of germ cells in 1677 and realizing, in 1870, that fertilization involved the merging of two cell nuclei, one from the egg and one from sperm, it took another 40 years to discover the interplay between hypothalamus, pituitary and gonads. The potential roles of gonadotropin regulation were discovered in 1927. Gonadotropin, such as pregnant mare serum gonadotropin (PMSG), was first introduced for ovarian stimulation in 1930. However, use of PMSG leads to antibody formation, and had to be withdrawn. Following withdrawal of PMSG, human pituitary gonadotropin (HPG) and urinary menopausal gonadotropin (hMG) appeared on the market, and 50 years ago the first child was delivered by our group in 1961 and opened the path to controlled ovarian stimulation. HPG produced good results, but its use came to an end in the late 1980s when it was linked to the development of Creutzfeldt-Jakob disease (CJD). HMG preparations containing a high percentage of unknown urinary proteins, making quality control almost impossible, were then the only gonadotropins remaining on the market. With the availability of hMG, clomiphene citrate, ergot derivatives, GnRH agonists and antagonists, as well as metformin, algorithms were developed for their optimal utilization and were used for the next four decades. Following the first human IVF baby in 1978 and ICSI in 1991, such procedures became standard practice. The main agents for controlled ovarian stimulation for IVF were gonadotropins and GnRH analogues, with batch to batch consistent gonadotropic preparations; methods could be developed to predict and select the correct dose and the optimal protocol for each patient. We are now seeing the appearance of gonadotropin with sustained action and orally active GnRH analogues as well as orally active molecules capable to stimulate follicle growth and inducing ovulation. These new developments may one day remove the need for the classical gonadotropin in clinical work.

Novel pathways in gonadotropin receptor signaling and biased agonism

Gonadotropins play a central role in the control of male and female reproduction. Selective agonists and antagonists of gonadotropin receptors would be of great interest for the treatment of infertility or as non steroidal contraceptive. However, to date, only native hormones are being used in assisted reproduction technologies as there is no pharmacological agent available to manipulate gonadotropin receptors. Over the last decade, there has been a growing perception of the complexity associated with gonadotropin receptors' cellular signaling. It is now clear that the Gs/cAMP/PKA pathway is not the sole mechanism that must be taken into account in order to understand these hormones' biological actions. In parallel, consistent with the emerging paradigm of biased agonism, several examples of ligand-mediated selective signaling pathway activation by gonadotropin receptors have been reported. Small molecule ligands, modulating antibodies interacting with the hormones and glycosylation variants of the native glycoproteins have all demonstrated their potential to trigger such selective signaling. Altogether, the available data and emerging concepts give rise to intriguing opportunities towards a more efficient control of reproductive function and associated disorders.

Contraception by induction of luteinized unruptured follicles with short-acting low molecular weight FSH receptor agonists in female animal models

During recent decades minor innovative drugs have been developed for the female contraceptive market and they all contain steroidal progestagens (and estrogens) that act centrally and have side effects that can be attributed to this central action. In this study, we present an innovative tissue-specific approach for female contraception by low molecular weight (LMW) FSH receptor (FSHR) agonists, which interact with the FSHR that is dominantly expressed in the granulosa cells. The oral administration of LMW FSHR agonists with a short circulation time, induced formation of luteinized unruptured follicles (LUFs) from the Graafian follicles, thereby preventing the release of the oocyte. The short-acting LMW FSHR compounds were fully agonistic to FSHR (EC50=4–5 nM). In an isolated mouse follicle culture, a short incubation period (2 h) resulted in inhibition of follicular rupture, where continuous incubation induced follicle growth. Pharmacokinetics after oral administration showed a surge-like exposure in rats and monkeys. Oral administration of short-acting LMW FSHR agonists inhibited ovulation at 10 mg/kg in rats and guinea pigs by generating LUFs without affecting cyclicity. Also, inhibition of follicular rupture was shown to be reversible within one cycle. Finally, LUFs were induced without affecting the hormonal cyclicity in cynomolgus monkeys, a mono-ovulatory species. In healthy women LUF formation occurs naturally, with a LUF acting as corpus luteum that produces enough progesterone to ensure normal menstrual cyclicity. Together with the presented data this indicates that the innovative approach with short-acting LMW FSHR agonists could lead to oral contraception for females at the ovarian level.

Are circulating gonadotropin isoforms naturally occurring biased agonists? Basic and therapeutic implications

The gonadotropins, luteinizing hormone, human chorionic gonadotropin and follicle-stimulating hormone, are key regulators of reproduction. As a result of this function, they have been the focus of research for many years. Isolated or recombinant proteins have been successfully used therapeutically for the treatment of infertility; and, in the case of compounds that block gonadotropin activity, for their potential utility in contraception. Until recently, selective small molecules modulating gonadotropin receptor activity have proven difficult to identify. The gonadotropins are glycoproteins that are released into the plasma as differently glycosylated isoforms and bind to specific G protein-coupled receptors. The degree of glycosylation on the gonadotropins has been shown to be important for the biological activities of these hormones and is differentially regulated depending on the steroidal status. Recent data from the study of glycosylated variants of LH, hCG and FSH have revealed that these isoforms have distinct signaling properties that allow for gonadotropin pleiotropic signals to be transduced effectively at the level of the receptor. Thus, glycosylated variants of the gonadotropins behave as biased agonists. Recently, newly developed, small molecule, synthetic allosteric compounds have been identified that are capable of mimicking this biased signaling. This opens the door to development of orally available, drug-like therapies for reproductive disorders that offer similar pleiotropic richness as that offered by the complex, endogenous hormones.

A negative allosteric modulator demonstrates biased antagonism of the follicle stimulating hormone receptor

High quality gamete production in males and females requires the pituitary gonadotropin follicle stimulating hormone (FSH). In this report a novel chemical class of small molecule inhibitors of FSH receptor (FSHR) is described. ADX61623, a negative allosteric modulator (NAM), increased the affinity of interaction between (125)I-hFSH and human FSHR (hFSHR) five fold. This form of FSHR occupied simultaneously by FSH and ADX61623 was inactive for cAMP and progesterone production in primary cultures of rat granulosa cells. In contrast, ADX61623 did not block estrogen production. This demonstrates for the first time, biased antagonism at the FSHR. To determine if ADX61623 blocked FSH induction of follicle development in vivo, a bioassay to measure follicular development and oocyte production in immature female rats was validated. ADX61623 was not completely effective in blocking FSH induced follicular development in vivo at doses up to 100mg/kg as oocyte production and ovarian weight gain were only moderately reduced. These data illustrate that FSHR couples to multiple signaling pathways in vivo. Suppression of one pool of FSHR uncouples Gαs and cAMP production, and decreases progesterone production. Occupancy of another pool of FSHR sensitizes granulosa cells to FSH induced estradiol production. Therefore, ADX61623 is a useful tool to investigate further the mechanism of the FSHR signaling dichotomy. This may lead to a greater understanding of the signaling infrastructure which enables estrogen biosynthesis and may prove useful in treating estrogen dependent disease.

[Low-molecular regulators of polypeptide hormones receptors containing LGR repeats]

During the last years the low-molecular non-peptidic regulators of the polypeptide hormones receptors containing LGR-repeats were identified. In the review the data on the structure and the molecular mechanisms of action of these regulators as agonists and antagonists of the luteinizing, follicle-stimulating and thyrotropin hormones are analyzed and systematized. The regulators interact with the serpentine domain of LGR-receptor and trigger the signaling cascades coupled with the receptor. Low-molecular agonists and antagonists of the LGR-receptors are considered as a new generation of the drugs that regulates the functional activity of sensitive to pituitary glycoprotein hormones signaling systems with high efficiency and selectivity. These regulators are more accessible compared to the hormones and can be use orally.

Human TSH receptor ligands as pharmacological probes with potential clinical application

The biologic role of thyroid-stimulating hormone (TSH; thyrotropin) as an activator (agonist) of the TSH receptor (TSHR) in the hypothalamic-pituitary-thyroid axis is well known and activation of TSHR by recombinant human TSH is used clinically in patients with thyroid cancer. TSHR ligands other than TSH could be used to probe TSHR biology in thyroidal and extrathyroidal tissues, and potentially be employed in patients. A number of different TSHR ligands have been reported, including TSH analogs, antibodies and small-molecule, drug-like compounds. In this review, we will provide an update on all these classes of TSHR agonists and antagonists but place emphasis on small-molecule ligands.