Chapter One - Follicle-Stimulating Hormone Receptor: Advances and Remaining Challenges

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Abstract

Follicle-stimulating hormone (FSH) is produced in the pituitary and is essential for reproduction. It specifically binds to a membrane receptor (FSHR) expressed in somatic cells of the gonads. The FSH/FSHR system presents many peculiarities compared to classical G protein-coupled receptors (GPCRs). FSH is a large naturally heterogeneous heterodimeric glycoprotein. The FSHR is characterized by a very large NH2-terminal extracellular domain, which binds FSH and participates to the activation/inactivation switch of the receptor. Once activated, the FSHR couples to Gαs and, in some instances, to other Gα-subunits. GPCR kinases and β-arrestins are also recruited to the FSHR and account for its desensitization, the control of its trafficking and its intracellular signaling. Of note, the FSHR internalization and recycling are very fast and involve very early endosomes (EE) instead of EE. All the transduction mechanisms triggered upon FSH stimulation lead to the activation of a complex signaling network that controls gene expression by acting at multiple levels. The integration of these mechanisms not only leads to context-adapted responses from the target gonadal cells but also indirectly affects the fate of germ cells. Depending on the physiological/developmental stage, FSH elicits proliferation, differentiation, or apoptosis in order to maintain the homeostasis of the reproductive system. Pharmacological tools targeting FSHR recently came to the fore and open promising prospects both for basic research and therapeutic applications. This chapter provides an updated review of the most salient aspects and peculiarities of FSHR biology and pharmacology.

Introduction

Follicle-stimulating hormone (FSH) plays a crucial role in the control of male and female reproduction. FSH is a heterodimeric glycoprotein consisting of an α-subunit, common with the other glycoprotein hormones (i.e., luteinizing hormone—LH, chorionic gonadotropin—CG, and thyroid-stimulating hormone—TSH), which is noncovalently associated with a specific FSHβ-subunit (Pierce and Parsons, 1981; Ryan et al., 1987). FSH is synthesized and secreted by the pituitary. FSH binds to and activates a plasma membrane receptor (FSHR) that belongs to the rhodopsin family of the G protein-coupled receptor (GPCR) superfamily. The FSHR displays a high degree of tissue specificity, being expressed in Sertoli and granulosa cells located in the male and female gonads, respectively (Simoni et al., 1997) (Fig. 1). As the other glycoprotein hormone receptors, the FSHR is characterized by a large NH2-terminal extracellular domain (ECD), where FSH binds specifically.

FSH is required for normal growth and maturation of ovarian follicles in women and for normal spermatogenesis in men (Themmen and Huhtaniemi, 2000). Female mice with FSHβ or FSHR gene knockout are infertile because of an incomplete follicle development, whereas male display oligozoospermia and subfertility (Dierich et al., 1998; Kumar et al., 1997). Consistently, women expressing nonfunctional variants of the FSHR are infertile while men are oligozoospermic, yet fertile (Aittomäki et al., 1995).

Because of its glycosylation, FSH is naturally heterogeneous and must be expressed by mammalian cells (i.e., pituitary or CHO cells) to be fully active in vivo. Because of these characteristics, only native forms of FSH, either purified from urine or recombinant, are being used in reproductive medicine, no other pharmacological agents being currently available in clinic (Lunenfeld, 2004; Macklon et al., 2006). Some women treated with FSH develop an ovarian hyperstimulation syndrome (OHSS), which, in its severe forms, can be life threatening (Vloeberghs et al., 2009). Therefore, pharmacological agents that would induce ovulation without the risk of provoking OHSS would represent a major improvement. It is also well established that, in women, the responsiveness to FSH treatment is heterogeneous and that the dose and sometimes the source of FSH, have to be empirically adjusted for each patient (Loutradis et al., 2003, Loutradis et al., 2004). A larger panel of FSHR agonists with varying pharmacological profiles could certainly help improving the overall efficiency of medically assisted procreation. On the other hand, FSHR blockers could potentially represent a novel nonsteroidal approach for contraception (Naz et al., 2005).

In order to meet these challenges, it is important to gain a better understanding of FSHR biology and the bottlenecks that make the targeting of this receptor particularly difficult.

Section snippets

FSH and FSHR in Pathologies

FSH serum levels vary physiologically during the menstrual cycle in women. Nevertheless, abnormal pituitary FSH secretion can occur in different pathologies such as premature ovarian insufficiency (POI) and polycystic ovarian syndrome (PCOS). POI is a dysfunction of the ovary occurring in about 1% of female population (under 40 years old) (Goswami and Conway, 2005). Patients carrying POI are infertile due to anovulation, amenorrhea, and reduced secretion of oestrogens (Kalantaridou et al., 1998

FSH Role in Assisted Reproduction Technologies (ARTs)

Assisted reproduction techniques (ARTs) are defined as “all treatments or procedures that include the in vitro handling of both human oocytes and sperm or embryos for the purpose of establishing a pregnancy” (Zegers-Hochschild et al., 2009). The procedures involved in ART include in vitro fertilization (IVF), gamete intrafalloppian transfer, and intracytoplasmic sperm injection. Although a universal ART protocol does not exist, the main steps are common to each technique (Casarini et al., 2016a

FSHR Structure and Function

The human FSHR, together with LHR and TSHR, belongs to the glycoprotein hormone receptors subfamily of class A GPCRs. It is encoded by a unique gene constituted of 10 exons and located on chromosome 2p21-p16 (Rousseau-Merck et al., 1993). After releasing of a signal peptide of 17 amino acids, the mature membrane FSHR protein contains 678 amino acids. Its molecular weight varies between 82 and 89 kDa depending on the rate of N-glycosylation (Davis et al., 1995). Several splice variants have been

FSHR Mutations

Although rare in the population, activating and inactivating mutations of FSHR have been reported in both genders (Desai et al., 2013; Riccetti et al., 2017; Ulloa-Aguirre et al., 2014). Both types of mutations can cause alteration of reproductive function, even though the phenotype is often more severe for woman fertility. In most cases, inactivating mutations provoke primary or secondary amenorrhea whereas the activating ones generally lead to OHSS. Some studies described the impact of

FSHR Signaling Through G Proteins

Unlike many GPCRs, in the absence of ligand, the FSHR displays little to no constitutive activity (Ulloa-Aguirre et al., 2014). This functional characteristic correlates with the increased stability of the transmembrane domains in the inactive state compared to other glycoprotein hormone receptors (Ulloa-Aguirre et al., 2014; Zhang et al., 2007). Upon FSH binding, conformational changes in the receptor lead to the transduction of the extracellular signal, hence the activation of several

FSHR Desensitization, Internalization, and Recycling

The FSHR is regulated by the canonical desensitization mechanisms known to operate for most GPCR (Fig. 3). Briefly, agonist-activated FSHR is rapidly phosphorylated on serine/threonine residues located on its carboxyl terminus through the action of GPCR kinases (GRKs), specifically GRKs 2, 3, 5, and 6 (Ayoub et al., 2015; Kara et al., 2006; Lazari et al., 1999; Marion et al., 2006; Moore et al., 2007; Troispoux et al., 1999; Ulloa-Aguirre and Zarinan, 2016). Nonvisual arrestins (β-arrestin 1

FSHR Signaling Through β-Arrestin

Beyond their well-established role in receptor desensitization, internalization, and recycling, β-arrestins have progressively emerged as key players in the control of GPCR-mediated signals in time and space (Fig. 3). Many GPCRs, including FSHR, have been demonstrated to signal independently of heterotrimeric G protein, through ligand-induced β-arrestin 1 and 2 recruitment (Reiter et al., 2012, Reiter et al., 2017; Reiter and Lefkowitz, 2006). Indeed, β-arrestins act as multifunctional

Modeling of FSHR Signaling

FSH signaling acts at different timescales within the hypothalamic–pituitary–gonadal (HPG) axis, encoding and decoding complex signals across several organs and tissues from the pituitary cells to the somatic cells in the gonads. Capturing the mechanisms responsible for such refined controls has proven very challenging. Over the years, this topic has led to the development of numerous mathematical models.

Fluctuations of FSH circulating levels are tightly regulated with respect to those of LH,

Impact on Gene Regulation

FSH directly alters the pattern of genes expressed in somatic cells of the gonads by regulating transcriptional as well as posttranscriptional events at the level of mRNA translation and of the miRNA network. The FSH-induced signaling network also indirectly promotes alterations of chromatin condensation in germ cells. Gaining a comprehensive picture on the FSH-regulated gene expression could provide insights on how gonadal somatic cells communicate with their neighboring germ cells. This could

Impact on Proliferation/Apoptosis/Cell Survival

FSH is an important contributor to the fate of somatic cells of the male and female gonad. Respectively, in Sertoli cells and granulosa cells, the hormone regulates proliferation and commitment to differentiation. In addition, in the ovary, FSH protects granulosa cells from atresia, a degenerative process that leads to the selection of a dominant follicle within a developing cohort. This is the main difference with the role of FSH in Sertoli cells, where apoptosis is negligible. The other

Biased Signaling

It is now well established that GPCRs adopt multiple inactive and active conformations that are connected to distinct transduction mechanisms. The notion of signaling bias is coming from this complexity. Indeed, a given ligand or receptor mutation can modify the stabilized conformation of the receptor–ligand complex, as compared to the wild-type receptor–reference–ligand complex (Galandrin et al., 2007; Granier et al., 2007; Kahsai et al., 2011; Kenakin, 2005; Kobilka, 2011; Nygaard et al., 2013

Conclusions

Many advances have been achieved over the last few years in FSHR research that open intriguing prospects in terms of pharmacological control of this receptor with potential applications in ART and contraception. Now that the proof of concept has been achieved that biased signaling exists for FSHR, the different classes of small-molecule ligands identified for the FSHR will have to be further characterized with respect to their pharmacological profiles. Are they balanced or biased? The same goes

Acknowledgments

This work was funded by “ARD 2020 Biomédicament” grants from Région Centre and with the support from the French National Research Agency under the program “Investissements d’avenir” Grant Agreement LabEx MabImprove: ANR-10-LABX-53. F.D.P. and A.T. are recipients of a Doctoral fellowship from INRA and Région Centre.

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