Involvement of G protein-coupled receptor kinases and arrestins in desensitization to follicle-stimulating hormone action

An intracellular VHH targeting the Luteinizing Hormone receptor modulates G protein-dependent signaling and steroidogenesis

Luteinizing hormone (LH) is essential for reproduction, controlling ovulation and steroidogenesis. Its receptor (LHR) recruits various transducers leading to the activation of a complex signaling network. We recently identified iPRC1, the first variable fragment from heavy-chain-only antibody (VHH) interacting with intracellular loop 3 (ICL3) of the follicle-stimulating hormone receptor (FSHR). Because of the high sequence similarity of the human FSHR and LHR (LHCGR), here we examined the ability of the iPRC1 intra-VHH to modulate LHCGR activity. In this study, we demonstrated that iPRC1 binds LHCGR, to a greater extent when the receptor was stimulated by the hormone. In addition, it decreased LH-induced cAMP production, cAMP-responsive element-dependent transcription, progesterone and testosterone production. These impairments are not due to Gs nor β-arrestin recruitment to the LHCGR. Consequently, iPRC1 is the first intra-VHH to bind and modulate LHCGR biological activity, including steroidogenesis. It should help further understand signaling mechanisms elicited at this receptor and their outcomes on reproduction.

Combined Multiplexed Phage Display, High-Throughput Sequencing, and Functional Assays as a Platform for Identifying Modulatory VHHs Targeting the FSHR

Developing modulatory antibodies against G protein-coupled receptors is challenging. In this study, we targeted the follicle-stimulating hormone receptor (FSHR), a significant regulator of reproduction, with variable domains of heavy chain-only antibodies (VHHs). We built two immune VHH libraries and submitted them to multiplexed phage display approaches. We used next-generation sequencing to identify 34 clusters of specifically enriched sequences that were functionally assessed in a primary screen based on a cAMP response element (CRE)-dependent reporter gene assay. In this assay, 23 VHHs displayed negative or positive modulation of FSH-induced responses, suggesting a high success rate of the multiplexed strategy. We then focused on the largest cluster identified (i.e., PRC1) that displayed positive modulation of FSH action. We demonstrated that PRC1 specifically binds to the human FSHR and human FSHR/FSH complex while potentiating FSH-induced cAMP production and Gs recruitment. We conclude that the improved selection strategy reported here is effective for rapidly identifying functionally active VHHs and could be adapted to target other challenging membrane receptors. This study also led to the identification of PRC1, the first potential positive modulator VHH reported for the human FSHR.

Overview of follicle stimulating hormone and its receptors in reproduction and in stem cells and cancer stem cells

Follicle stimulating hormone (FSH) and its receptor (FSHR) have been reported to be responsible for several physiological functions and cancers. The responsiveness of stem cells and cancer stem cells towards the FSH-FSHR system make the function of FSH and its receptors more interesting in the context of cancer biology. This review is comprised of comprehensive information on FSH-FSHR signaling in normal physiology, gonadal stem cells, cancer cells, and potential options of utilizing FSH-FSHR system as an anti-cancer therapeutic target.

ERβ Regulation of Gonadotropin Responses during Folliculogenesis

Gonadotropins are essential for regulating ovarian development, steroidogenesis, and gametogenesis. While follicle stimulating hormone (FSH) promotes the development of ovarian follicles, luteinizing hormone (LH) regulates preovulatory maturation of oocytes, ovulation, and formation of corpus luteum. Cognate receptors of FSH and LH are G-protein coupled receptors that predominantly signal through cAMP-dependent and cAMP-independent mechanisms that activate protein kinases. Subsequent vital steps in response to gonadotropins are mediated through activation or inhibition of transcription factors required for follicular gene expression. Estrogen receptors, classical ligand-activated transcriptional regulators, play crucial roles in regulating gonadotropin secretion from the hypothalamic-pituitary axis as well as gonadotropin function in the target organs. In this review, we discuss the role of estrogen receptor β (ERβ) regulating gonadotropin response during folliculogenesis. Ovarian follicles in Erβ knockout (Erβ^KO) mutant female mice and rats cannot develop beyond the antral state, lack oocyte maturation, and fail to ovulate. Theca cells (TCs) in ovarian follicles express LH receptor, whereas granulosa cells (GCs) express both FSH receptor (FSHR) and LH receptor (LHCGR). As oocytes do not express the gonadotropin receptors, the somatic cells play a crucial role during gonadotropin induced oocyte maturation. Somatic cells also express high levels of estrogen receptors; while TCs express ERα and are involved in steroidogenesis, GCs express ERβ and are involved in both steroidogenesis and folliculogenesis. GCs are the primary site of ERβ-regulated gene expression. We observed that a subset of gonadotropin-induced genes in GCs, which are essential for ovarian follicle development, oocyte maturation and ovulation, are dependent on ERβ. Thus, ERβ plays a vital role in regulating the gonadotropin responses in ovary.

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.

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.

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.

Is corifollitropin alfa effective in controlled ovarian stimulation among all poor ovarian responders? A retrospective comparative study

Several studies have compared the effectiveness of corifollitropin alfa versus daily gonadotropins in poor ovarian responders (PORs) undergoing controlled ovarian stimulation (COS), showing conflicting results in terms of IVF outcomes. Given the heterogeneity of patients included in the classification of POR according to 'Bologna criteria', the aim of this study was to evaluate the impact of corifollitropin alfa in two different categories of POR distinguished according to patients' antral follicle count (AFC). We retrospectively evaluated 104 infertile POR, split into two groups according to AFC (Group A ≤ 5; Group B > 5) and subgroups according to the ovarian stimulation regimen (corifollitropin alfa plus daily gonadotropins (Subgroup 1) versus daily gonadotropins alone (Subgroup 2)). Outcome measures were total oocytes, MII oocytes, total embryos, follicular output rate (FORT), implantation rate (IR), clinical pregnancy rate (CPR), miscarriage rate (MR), and live birth rate (LBR). Subgroup A1 experienced a lower number of total oocytes, MII oocytes, total embryos, and FORT (p < .05) in comparison to Subgroup A2, while no difference was found when comparing Subgroups B1 and B2. No difference was found between subgroups even in terms of IR, CPR, MR, and LBR. In conclusion, corifollitropin alfa may be as effective as daily gonadotropins in POR with AFC > 5 undergoing COS, while it might be inferior to daily gonadotropins in POR with AFC ≤ 5.

Molecular Mechanisms of Action of FSH

The glycoprotein follicle-stimulating hormone (FSH) acts on gonadal target cells, hence regulating gametogenesis. The transduction of the hormone-induced signal is mediated by the FSH-specific G protein-coupled receptor (FSHR), of which the action relies on the interaction with a number of intracellular effectors. The stimulatory Gαs protein is a long-time known transducer of FSH signaling, mainly leading to intracellular cAMP increase and protein kinase A (PKA) activation, the latter acting as a master regulator of cell metabolism and sex steroid production. While in vivo data clearly demonstrate the relevance of PKA activation in mediating gametogenesis by triggering proliferative signals, some in vitro data suggest that pro-apoptotic pathways may be awakened as a "dark side" of cAMP/PKA-dependent steroidogenesis, in certain conditions. P38 mitogen-activated protein kinases (MAPK) are players of death signals in steroidogenic cells, involving downstream p53 and caspases. Although it could be hypothesized that pro-apoptotic signals, if relevant, may be required for regulating atresia of non-dominant ovarian follicles, they should be transient and counterbalanced by mitogenic signals upon FSHR interaction with opposing transducers, such as Gαi proteins and β-arrestins. These molecules modulate the steroidogenic pathway via extracellular-regulated kinases (ERK1/2), phosphatidylinositol-4,5-bisphosphate 3-kinases (PI3K)/protein kinase B (AKT), calcium signaling and other intracellular signaling effectors, resulting in a complex and dynamic signaling network characterizing sex- and stage-specific gamete maturation. Even if the FSH-mediated signaling network is not yet entirely deciphered, its full comprehension is of high physiological and clinical relevance due to the crucial role covered by the hormone in regulating human development and reproduction.

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.

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.

Molecular regulation of follicle-stimulating hormone synthesis, secretion and action

Follicle-stimulating hormone (FSH) plays fundamental roles in male and female fertility. FSH is a heterodimeric glycoprotein expressed by gonadotrophs in the anterior pituitary. The hormone-specific FSHβ-subunit is non-covalently associated with the common α-subunit that is also present in the luteinizing hormone (LH), another gonadotrophic hormone secreted by gonadotrophs and thyroid-stimulating hormone (TSH) secreted by thyrotrophs. Several decades of research led to the purification, structural characterization and physiological regulation of FSH in a variety of species including humans. With the advent of molecular tools, availability of immortalized gonadotroph cell lines and genetically modified mouse models, our knowledge on molecular mechanisms of FSH regulation has tremendously expanded. Several key players that regulate FSH synthesis, sorting, secretion and action in gonads and extragonadal tissues have been identified in a physiological setting. Novel post-transcriptional and post-translational regulatory mechanisms have also been identified that provide additional layers of regulation mediating FSH homeostasis. Recombinant human FSH analogs hold promise for a variety of clinical applications, whereas blocking antibodies against FSH may prove efficacious for preventing age-dependent bone loss and adiposity. It is anticipated that several exciting new discoveries uncovering all aspects of FSH biology will soon be forthcoming.

G protein-dependent signaling triggers a β-arrestin-scaffolded p70S6K/ rpS6 module that controls 5'TOP mRNA translation

Many interaction partners of β-arrestins intervene in the control of mRNA translation. However, how β-arrestins regulate this cellular process has been poorly explored. In this study, we show that β-arrestins constitutively assemble a p70S6K/ribosomal protein S6 (rpS6) complex in HEK293 cells and in primary Sertoli cells of the testis. We demonstrate that this interaction is direct, and experimentally validate the interaction interface between β-arrestin 1 and p70S6K predicted by our docking algorithm. Like most GPCRs, the biological function of follicle-stimulating hormone receptor (FSHR) is transduced by G proteins and β-arrestins. Upon follicle-stimulating hormone (FSH) stimulation, activation of G protein-dependent signaling enhances p70S6K activity within the β-arrestin/p70S6K/rpS6 preassembled complex, which is not recruited to the FSHR. In agreement, FSH-induced rpS6 phosphorylation within the β-arrestin scaffold was decreased in cells depleted of Gα_s. Integration of the cooperative action of β-arrestin and G proteins led to the translation of 5' oligopyrimidine track mRNA with high efficacy within minutes of FSH input. Hence, this work highlights new relationships between G proteins and β-arrestins when acting cooperatively on a common signaling pathway, contrasting with their previously shown parallel action on the ERK MAP kinase pathway. In addition, this study provides insights into how GPCR can exert trophic effects in the cell.-Tréfier, A., Musnier, A., Landomiel, F., Bourquard, T., Boulo, T., Ayoub, M. A., León, K., Bruneau, G., Chevalier, M., Durand, G., Blache, M.-C., Inoue, A., Fontaine, J., Gauthier, C., Tesseraud, S., Reiter, E., Poupon, A., Crépieux, P. G protein-dependent signaling triggers a β-arrestin-scaffolded p70S6K/ rpS6 module that controls 5'TOP mRNA translation.

Intracellular Trafficking of Gonadotropin Receptors in Health and Disease

Gonadotropin receptors belong to the highly conserved subfamily of the G protein-coupled receptor (GPCR) superfamily, the so-called Rhodopsin-like family (class A), which is the largest class of GPCRs and currently a major drug target. Both the follicle-stimulating hormone receptor (FSHR) and the luteinizing hormone/chorionic gonadotropin hormone receptor (LHCGR) are mainly located in the gonads where they play key functions associated to essential reproductive functions. As any other protein, gonadotropin receptors must be properly folded into a mature tertiary conformation compatible with quaternary assembly and endoplasmic reticulum export to the cell surface plasma membrane. Several primary and secondary structural features, including presence of particular amino acid residues and short motifs and in addition, posttranslational modifications, regulate intracellular trafficking of gonadotropin receptors to the plasma membrane as well as internalization and recycling of the receptor back to the cell surface after activation by agonist. Inactivating mutations of gonadotropin receptors may derive from receptor misfolding and lead to absent or reduced plasma membrane expression of the altered receptor, thereby manifesting an array of phenotypical abnormalities mostly characterized by reproductive failure and/or abnormal or absence of development of secondary sex characteristics. In this chapter we review the structural requirements necessary for intracellular trafficking of the gonadotropin receptors, and describe how mutations in these receptors may lead to receptor misfolding and disease in humans.

Intracellular Follicle-Stimulating Hormone Receptor Trafficking and Signaling

Models of G protein-coupled receptor (GPCR) signaling have dramatically altered over the past two decades. Indeed, GPCRs such as the follicle-stimulating hormone receptor (FSHR) have contributed to these new emerging models. We now understand that receptor signaling is highly organized at a spatial level, whereby signaling not only occurs from the plasma membrane but distinct intracellular compartments. Recent studies in the role of membrane trafficking and spatial organization of GPCR signaling in regulating gonadotropin hormone receptor activity has identified novel intracellular compartments, which are tightly linked with receptor signaling and reciprocally regulated by the cellular trafficking machinery. Understanding the impact of these cell biological mechanisms to physiology and pathophysiology is emerging for certain GPCRs. However, for FSHR, the potential impact in both health and disease and the therapeutic possibilities of these newly identified systems is currently unknown, but offers the potential to reassess prior strategies, or unveil novel opportunities, in targeting this receptor.

Structure-Function Relationships of the Follicle-Stimulating Hormone Receptor

The follicle-stimulating hormone receptor (FSHR) plays a crucial role in reproduction. This structurally complex receptor is a member of the G-protein coupled receptor (GPCR) superfamily of membrane receptors. As with the other structurally similar glycoprotein hormone receptors (the thyroid-stimulating hormone and luteinizing hormone-chorionic gonadotropin hormone receptors), the FSHR is characterized by an extensive extracellular domain, where binding to FSH occurs, linked to the signal specificity subdomain or hinge region. This region is involved in ligand-stimulated receptor activation whereas the seven transmembrane domain is associated with receptor activation and transmission of the activation process to the intracellular loops comprised of amino acid sequences, which predicate coupling to effectors, interaction with adapter proteins, and triggering of downstream intracellular signaling. In this review, we describe the most important structural features of the FSHR intimately involved in regulation of FSHR function, including trafficking, dimerization, and oligomerization, ligand binding, agonist-stimulated activation, and signal transduction.

Human Luteinizing Hormone and Chorionic Gonadotropin Display Biased Agonism at the LH and LH/CG Receptors

Human luteinizing hormone (LH) and chorionic gonadotropin (hCG) have been considered biologically equivalent because of their structural similarities and their binding to the same receptor; the LH/CGR. However, accumulating evidence suggest that LH/CGR differentially responds to the two hormones triggering differential intracellular signaling and steroidogenesis. The mechanistic basis of such differential responses remains mostly unknown. Here, we compared the abilities of recombinant rhLH and rhCG to elicit cAMP, β-arrestin 2 activation, and steroidogenesis in HEK293 cells and mouse Leydig tumor cells (mLTC-1). For this, BRET and FRET technologies were used allowing quantitative analyses of hormone activities in real-time and in living cells. Our data indicate that rhLH and rhCG differentially promote cell responses mediated by LH/CGR revealing interesting divergences in their potencies, efficacies and kinetics: rhCG was more potent than rhLH in both HEK293 and mLTC-1 cells. Interestingly, partial effects of rhLH were found on β-arrestin recruitment and on progesterone production compared to rhCG. Such a link was further supported by knockdown experiments. These pharmacological differences demonstrate that rhLH and rhCG act as natural biased agonists. The discovery of novel mechanisms associated with gonadotropin-specific action may ultimately help improve and personalize assisted reproduction technologies.

Profiling of FSHR negative allosteric modulators on LH/CGR reveals biased antagonism with implications in steroidogenesis

Biased signaling has recently emerged as an interesting means to modulate the function of many G protein-coupled receptors (GPCRs). Previous studies reported two negative allosteric modulators (NAMs) of follicle-stimulating hormone receptor (FSHR), ADX68692 and ADX68693, with differential effects on FSHR-mediated steroidogenesis and ovulation. In this study, we attempted to pharmacologically profile these NAMs on the closely related luteinizing hormone/chorionic gonadotropin hormone receptor (LH/CGR) with regards to its canonical Gs/cAMP pathway as well as to β-arrestin recruitment in HEK293 cells. The NAMs' effects on cAMP, progesterone and testosterone production were also assessed in murine Leydig tumor cell line (mLTC-1) as well as rat primary Leydig cells. We found that both NAMs strongly antagonized LH/CGR signaling in the different cell models used with ADX68693 being more potent than ADX68692 to inhibit hCG-induced cAMP production in HEK293, mLTC-1 and rat primary Leydig cells as well as β-arrestin 2 recruitment in HEK293 cells. Interestingly, differential antagonism of the two NAMs on hCG-promoted steroidogenesis in mLTC-1 and rat primary Leydig cells was observed. Indeed, a significant inhibition of testosterone production by the two NAMs was observed in both cell types, whereas progesterone production was only inhibited by ADX68693 in rat primary Leydig cells. In addition, while ADX68693 totally abolished testosterone production, ADX68692 had only a partial effect in both mLTC-1 and rat primary Leydig cells. These observations suggest biased effects of the two NAMs on LH/CGR-dependent pathways controlling steroidogenesis. Interestingly, the pharmacological profiles of the two NAMs with respect to steroidogenesis were found to differ from that previously shown on FSHR. This illustrates the complexity of signaling pathways controlling FSHR- and LH/CGR-mediated steroidogenesis, suggesting differential implication of cAMP and β-arrestins mediated by FSHR and LH/CGR. Together, our data demonstrate that ADX68692 and ADX68693 are biased NAMs at the LH/CGR in addition to the FSHR. These pharmacological characteristics are important to consider for potential contraceptive and therapeutic applications based on such compounds.

Ovarian follicle selection and granulosa cell differentiation

The reproductive strategy for avian species that produce a sequence (or clutch) of eggs is dependent upon the maintenance of a small cohort of viable, undifferentiated (prehierarchal) follicles. It is from this cohort that a single follicle is selected on an approximate daily basis to initiate rapid growth and final differentiation before ovulation. This review describes a working model in which follicles within this prehierarchal cohort are maintained in an undifferentiated state by inhibitory cell signaling until the time of selection. Ultimately, follicle selection represents a process in which a single undifferentiated follicle per day is predicted to escape such inhibitory mechanisms to begin rapid growth and final maturation before ovulation. Several processes initiated within the granulosa cell layer at selection are dependent upon G protein-coupled receptors signaling via cyclic adenosine monophosphate (cAMP), and several critical processes are described herein. Finally, reference is made to several practical outcomes that can result from understanding the process of selection, including applications within the poultry industry. Proximal factors and processes that mediate follicle selection can either extend or decrease the length of the laying sequence, and thus directly influence overall egg production. In particular, any aberration that results in the selection of more than one follicle per day will result in decreased egg production. More generally, in wild birds these processes are modified by prevailing environmental conditions and by social interactions to influence clutch size. The elucidation of cellular processes that regulate follicle selection can assist in the development of assisted reproductive technologies for application in threatened and endangered avian species.

The stem cell-expressed receptor Lgr5 possesses canonical and functionally active molecular determinants critical to β-arrestin-2 recruitment

Lgr5 is a membrane protein related to G protein-coupled receptors (GPCR)s whose expression identifies stem cells in multiple tissues and is strongly correlated with cancer. Despite the recent identification of endogenous ligands for Lgr5, its mode of signaling remains enigmatic. The ability to couple to G proteins and βarrestins are classical molecular behaviors of GPCRs that have yet to be observed for Lgr5. Therefore, the goal of this study was to determine if Lgr5 can engage a classical GPCR behavior and elucidate the molecular determinants of this process. Structural analysis of Lgr5 revealed several motifs consistent with its ability to recruit βarr2. Among them, a "SSS" serine cluster located at amino acid position 873-875 within the C-terminal tail (C-tail), is in a region consistent with other GPCRs that bind βarr2 with high-affinity. To test its functionality, a ligand-independent βarr2 translocation assay was implemented. We show that Lgr5 recruits βarr2 and that the "SSS" amino acids (873-875) are absolutely critical to this process. We also demonstrate that for full efficacy, this cluster requires other Lgr5 C-tail serines that were previously shown to be important for constitutive and βarr2 independent internalization of Lgr5. These data are proof of principle that a classical GPCR behavior can be manifested by Lgr5. The existence of alternative ligands or missing effectors of Lgr5 that scaffold this classical GPCR behavior and the downstream signaling pathways engaged should be considered. Characterizing Lgr5 signaling will be invaluable for assessing its role in tissue maintenance, repair, and disease.

GRK-6 mediates FSH action synergistically enhanced by estrogen and the oocyte in rat granulosa cells

Estrogen is known to play a pivotal role in granulosa cell responses to follicle-stimulating hormone (FSH) that is critical for the establishment of dominant follicles and subsequent ovulation in mammals. Thus, elucidating the cellular and molecular mechanisms that regulate FSH activity is important to understand female fertility. We previously discovered that the oocyte is required for estrogen to exert its positive effects on FSH activity in rat granulosa cells. This finding supports the new concept that estrogen action in granulosa cells is mediated by the oocyte. In the current study, we explored the underlying mechanism. In the presence of oocytes, estrogens enhanced FSH-induced increases in aromatase, steroidogenic acute regulatory protein and FSH receptor mRNA expression as well as cAMP production. However, as forskolin did not mimic FSH activity this indicated that coexistence of estrogen/oocytes increases FSH activity at a site upstream of adenylate cyclase in granulosa cells. We therefore sought a possible involvement of the autoregulatory molecules for FSH receptor, G protein-coupled receptor kinases (GRKs) and ß-arrestins in enhancing FSH activity in response to the estrogen/oocyte co-treatment in granulosa cells. Among the seven known GRK and two ß-arrestin molecules, we found that estrogens with oocytes suppressed FSH-induced GRK-6 mRNA expression. Consistent with this finding, transfecting granulosa cells with small interfering RNA of GRK-6 significantly increased FSH induction of aromatase mRNA, suggesting that endogenous GRK-6 plays an inhibitory role in FSH-induced aromatase mRNA expression. Consequently, these findings strongly suggest that GRK-6 is involved in the mechanism by which estrogen and oocytes synergistically augment FSH activity in granulosa cells.

Trafficking of the follitropin receptor

The follitropin or follicle-stimulating hormone receptor (FSHR) belongs to a highly conserved subfamily of the G protein-coupled receptor (GPCR) superfamily and is mainly expressed in specific cells in the gonads. As any other GPCR, the newly synthesized FSHR has to be correctly folded and processed in order to traffic to the cell surface plasma membrane and interact with its cognate ligand. In this chapter, we describe in detail the conditions and procedures used to study outward trafficking of the FSHR from the endoplasmic reticulum to the plasma membrane. We also describe some methods to analyze phosphorylation, β-arrestin recruitment, internalization, and recycling of this particular receptor, which have proved useful in our hands for dissecting its downward trafficking and fate following agonist stimulation.

Normal testicular function without detectable follicle-stimulating hormone. A novel mutation in the follicle-stimulating hormone receptor gene leading to apparent constitutive activity and impaired agonist-induced desensitization and internalization

Activating mutations in the follicle-stimulating hormone (FSH) receptor (FSHR) gene are rarely detected due to the absence of a clearly defined phenotype, particularly in men. We here report the biochemical features of a novel mutation in the first extracellular loop of the FSHR. The mutation (N431I) was detected in an asymptomatic man exhibiting normal spermatogenesis, suppressed serum FSH, and normal or elevated levels of biochemical markers of FSH action. Employing different experimental strategies on HEK-293 cells transiently expressing the N431I FSHR mutant, we found that the mutation led to decreased cell surface plasma membrane expression of the receptor protein, but conferred a low level of constitutive activity associated with markedly altered agonist-stimulated desensitization and internalization. These latter features may contribute and/or amplify the persistent activation of the receptor in both absence and presence of agonist and provide new insights into opportunities for adjuvant therapies based on disruption of these processes.

Decreased degradation of internalized follicle-stimulating hormone caused by mutation of aspartic acid 6.30(550) in a protein kinase-CK2 consensus sequence in the third intracellular loop of human follicle-stimulating hormone receptor

A naturally occurring mutation in follicle-stimulating hormone receptor (FSHR) gene has been reported: an amino acid change to glycine occurs at a conserved aspartic acid 550 (D550, D567, D6.30(567)). This residue is contained in a protein kinase-CK2 consensus site present in human FSHR (hFSHR) intracellular loop 3 (iL3). Because CK2 has been reported to play a role in trafficking of some receptors, the potential roles for CK2 and D550 in FSHR function were evaluated by generating a D550A mutation in the hFSHR. The hFSHR-D550A binds hormone similarly to WT-hFSHR when expressed in HEK293T cells. Western blot analyses showed lower levels of mature hFSHR-D550A. Maximal cAMP production of both hFSHR-D550A as well as the naturally occurring mutation hFSHR-D550G was diminished, but constitutive activity was not observed. Unexpectedly, when (125)I-hFSH bound to hFSHR-D550A or hFSHR-D550G, intracellular accumulation of radiolabeled FSH was observed. Both sucrose and dominant-negative dynamin blocked internalization of radiolabeled FSH and its commensurate intracellular accumulation. Accumulation of radiolabeled FSH in cells transfected with hFSHR-D550A is due to a defect in degradation of hFSH as measured in pulse chase studies, and confocal microscopy imaging revealed that FSH accumulated in large intracellular structures. CK2 kinase activity is not required for proper degradation of internalized FSH because inhibition of CK2 kinase activity in cells expressing hFSHR did not uncouple degradation of internalized radiolabeled FSH. Additionally, the CK2 consensus site in FSHR iL3 is not required for binding because CK2alpha coimmunoprecipitated with hFSHR-D550A. Thus, mutation of D550 uncouples the link between internalization and degradation of hFSH.

Mapping the follicle-stimulating hormone-induced signaling networks

Follicle-stimulating hormone (FSH) is a central regulator of male and female reproductive function. Over the last decade, there has been a growing perception of the complexity associated with FSH-induced cellular signaling. It is now clear that the canonical Gs/cAMP/PKA pathway is not the sole mechanism that must be considered in FSH biological actions. In parallel, consistent with the emerging concept of biased agonism, several examples of ligand-mediated selective signaling pathway activation by gonadotropin receptors have been reported. In this context, it is important to gain an integrative view of the signaling pathways induced by FSH and how they interconnect to form a network. In this review, we propose a first attempt at building topological maps of various pathways known to be involved in the FSH-induced signaling network. We discuss the multiple facets of FSH-induced signaling and how they converge to the hormone integrated biological response. Despite of their incompleteness, these maps of the FSH-induced signaling network represent a first step toward gaining a system-level comprehension of this hormone's actions, which may ultimately facilitate the discovery of novel regulatory processes and therapeutic strategies for infertility and non-steroidal contraception.

Partially deglycosylated equine LH preferentially activates beta-arrestin-dependent signaling at the follicle-stimulating hormone receptor

Deglycosylated FSH is known to trigger poor Galphas coupling while efficiently binding its receptor. In the present study, we tested the possibility that a deglycosylated equine LH (eLHdg) might be able to selectively activate beta-arrestin-dependent signaling. We compared native eLH to an eLH derivative [i.e. truncated eLHbeta (Delta121-149) combined with asparagine56-deglycosylated eLHalpha (eLHdg)] previously reported as an antagonist of cAMP accumulation at the FSH receptor (FSH-R). We confirmed that, when used in conjunction with FSH, eLHdg acted as an antagonist for cAMP accumulation in HEK-293 cells stably expressing the FSH-R. Furthermore, when used alone at concentrations up to 1 nM, eLHdg had no detectable agonistic activity on cAMP accumulation, protein kinase A activity or cAMP-responsive element-dependent transcriptional activity. At higher concentrations, however, a weak agonistic action was observed with eLHdg, whereas eLH led to robust responses whatever the concentration. Both eLH and eLHdg triggered receptor internalization and led to beta-arrestin recruitment. Both eLH and eLHdg triggered ERK and ribosomal protein (rp) S6 phosphorylation at 1 nM. The depletion of endogenous beta-arrestins had only a partial effect on eLH-induced ERK and rpS6 phosphorylation. In contrast, ERK and rpS6 phosphorylation was completely abolished at all time points in beta-arrestin-depleted cells. Together, these results show that eLHdg has the ability to preferentially activate beta-arrestin-dependent signaling at the FSH-R. This finding provides a new conceptual and experimental framework to revisit the physiological meaning of gonadotropin structural heterogeneity. Importantly, it also opens a field of possibilities for the development of selective modulators of gonadotropin receptors.

A phosphorylation cluster of five serine and threonine residues in the C-terminus of the follicle-stimulating hormone receptor is important for desensitization but not for beta-arrestin-mediated ERK activation

Classically, the FSH receptor (FSH-R) mediates its effects through coupling to guanine nucleotide-binding protein alpha S subunit (Galpha(s)) and activation of the cAMP/protein kinase A (PKA) signaling pathway. beta-Arrestins are rapidly recruited to the FSH-activated receptor and play key roles in its desensitization and internalization. Here, we show that the FSH-R expressed in HEK 293 cells activated ERK by two temporally distinct pathways dependent, respectively, on Galpha(s)/PKA and beta-arrestins. Galpha(s)/PKA-dependent ERK activation was rapid, transient, and blocked by H89 (a PKA inhibitor), but it was insensitive to small interfering RNA-mediated depletion of beta-arrestins. beta-Arrestin-dependent ERK activation was slower but more sustained and was insensitive to H89. We identified five Ser/Thr residues in the C terminus of the receptor (638-644) as a major phosphorylation site. Mutation of these residues into Ala (5A FSH-R) significantly reduced the stability of FSH-induced beta-arrestin 1 and 2 interaction when compared with the wild-type receptor. As expected, the 5A FSH-R-mediated cAMP accumulation was enhanced, and its internalization was reduced. In striking contrast, the ability of the 5A FSH-R to activate ERK via the beta-arrestin-dependent pathway was increased. G protein-coupled receptor kinase 5 (GRK5) and GRK6 were required for beta-arrestin-dependent ERK activation by both the wild-type and 5A FSH-R. By contrast, GRK2 depletion enhanced ERK activation by the wild-type FSH-R but not by the 5A FSH-R. In conclusion, we demonstrate the existence of a beta-arrestin-dependent, GRK-regulated mechanism for ERK activation by the FSH-R. A phosphorylation cluster in the C terminus of the FSH-R, identified as a site of beta-arrestin recruitment, positively regulated both desensitization and internalization but negatively regulated beta-arrestin-dependent ERK activation.

Follicle-stimulating hormone (FSH) activates extracellular signal-regulated kinase phosphorylation independently of beta-arrestin- and dynamin-mediated FSH receptor internalization

BACKGROUND

The follicle-stimulating hormone receptor (FSH-R) is a seven transmembrane spanning receptor (7TMR) which plays a crucial role in male and female reproduction. Upon FSH stimulation, the FSH-R activates the extracellular signal-regulated kinases (ERK). However, the mechanisms whereby the agonist-stimulated FSH-R activates ERK are poorly understood. In order to activate ERK, some 7 TMRs require beta-arrestin-and dynamin-dependent internalization to occur, whereas some others do not. In the present study, we examined the ability of the FSH-activated FSH-R to induce ERK phosphorylation, in conditions where its beta-arrestin- and dynamin-mediated internalization was impaired.

METHODS

Human embryonic kidney (HEK) 293 cells were transiently transfected with the rat FSH-R. Internalization of the FSH-R was manipulated by co-expression of either a beta-arrestin (319-418) dominant negative peptide, either an inactive dynamin K44A mutant or of wild-type beta-arrestin 1 or 2. The outcomes on the FSH-R internalization were assayed by measuring 125I-FSH binding at the cell surface when compared to internalized 125I-FSH binding. The resulting ERK phosphorylation level was visualized by Western blot analysis.

RESULTS

In HEK 293 cells, FSH stimulated ERK phosphorylation in a dose-dependent manner. Co-transfection of the beta- arrestin (319-418) construct, or of the dynamin K44A mutant reduced FSH-R internalization in response to FSH, without affecting ERK phosphorylation. Likewise, overexpression of wild-type beta-arrestin 1 or 2 significantly increased the FSH-R internalization level in response to FSH, without altering FSH-induced ERK phosphorylation.

CONCLUSION

From these results, we conclude that the FSH-R does not require beta-arrestin- nor dynamin-mediated internalization to initiate ERK phosphorylation in response to FSH.

Essential role of the oocyte in estrogen amplification of follicle-stimulating hormone signaling in granulosa cells

The establishment of dominant ovarian follicles that are capable of ovulating fertilizable oocytes is a fundamental determinant of female fertility. This process is governed by pituitary gonadotropins as well as local ovarian factors. Within the follicle, estrogen acts in an autocrine/paracrine manner to enhance FSH action in the granulosa cells. These effects include the augmentation of P450aromatase expression and estradiol production. This feed-forward effect of estrogen is believed to play a key role in follicle dominance. Here we found the essential role of the oocyte in this physiological process using primary cultures of rat granulosa cells. In the presence, but not absence, of oocytes, estrogen amplified FSH-stimulated increases in mRNA expression of P450aromatase, FSH receptor, LH receptor, and inhibin alpha-, betaA-, and betaB-subunits as well as cAMP production. Thus, oocytes mediate the estrogen enhancement of FSH action in the granulosa cells. In comparison with FSH, cotreatment with estrogen and oocytes failed to amplify the stimulatory effects of forskolin or 8-bromoadenosine-cAMP on granulosa cell responses including P450aromatase mRNA expression and cAMP production, indicating that estrogen/oocytes amplify FSH action at a site upstream of adenylate cyclase. These findings support the novel conclusion that communication between the oocyte and granulosa cells plays a crucial role in mediating estrogen action during FSH-dependent folliculogenesis.

Gonadotropin receptors: role of post-translational modifications and post-transcriptional regulation

This review focuses on the post-translational modifications of LH and FSH receptors and recent studies on the regulation of LH receptor expression mediated by an RNA binding protein. Both LH and FSH receptors undergo extensive post-translational modifications. N-linked glycosylation occurs co-translationally and plays a role in the maturation and processing of the receptor, while palmitoylation is involved in receptor endocytosis and post-endocytic trafficking. A third type of post-translational modification is phosphorylation and its function has been reviewed. Finally, the regulation of LH receptor at the mRNA level by an RNA binding protein is discussed in the context of ovarian function.

Desensitization to gonadotropic hormones: a model system for the regulation of a G-protein-coupled receptor with 7-transmembrane spanning regions

Gonadotropic hormone, luteinizing hormone, and follicle-stimulating hormone exert their effect via activation of G-coupled receptors, which activate the hormone sensitive adenylyl cyclase, protein kinase A, and cyclic AMP responsive elements. This activation leads to specific de novo synthesis of steroidogenic factors and steroidogenic enzymes. In normal cells and following activation of this signaling pathway, desensitization period will be followed. This down-regulation, which was studied in detail for the last three decays, was found to take place at various steps of these signal transduction pathways as well as at different kinetics. A common and diverse feature of the mechanism of desensitization in other G-coupled-7-transmembrane receptor system is also discussed.

Regulation of follitropin receptor cell surface residency by the ubiquitin-proteasome pathway

Little is known of the normal physiological processes that govern the cell surface residency of the human follitropin receptor (hFSHR), a G protein-coupled receptor expressed in the ovary and testis. In the hFSHR, the third intracellular (3i) loop is considered to be pivotal in attenuation of ligand activation, particularly internalization. To gain a better understanding of these processes, we used a yeast-based interaction trap to identify cytoplasmic proteins in a human ovarian cDNA library that interacted with the hFSHR 3i loop. Among the cDNA identified, four encoded isoforms of ubiquitin. Immunoprecipitated hFSHR probed with an antiubiquitin antibody revealed that the receptor is ubiquitinated, although not exclusively on the 3i loop. Cell-surface hFSHR levels increased when expressed at nonpermissive temperature in a temperature-sensitive, ubiquitination-defective cell line. Similarly, after treatment with proteasome inhibitors, HEK293 cells stably transfected with an hFSHR expression plasmid showed an increase in follitropin binding. Proteasome inhibitors did not affect the rate of FSH internalization when receptors were saturated before internalization was measured. In contrast, internalization decreased when binding experiments were performed under nonequilibrium conditions. A mutant hFSHR-K555R, which removes the only lysine in the 3i loop available for ubiquitination, was still ubiquitinated, illustrating that, although the third loop enables and interaction with ubiquitin, it is not the sole site of ubiquitination. These observations are consistent with a role for ubiquitination in the regulation of hFSHR cell surface residency. Additionally, it can be inferred that a sequence in the 3i loop is involved in regulating receptor ubiquitination and internalization.

Threonine 308 within a putative casein kinase 2 site of the cytoplasmic tail of leukotriene B(4) receptor (BLT1) is crucial for ligand-induced, G-protein-coupled receptor-specific kinase 6-mediated desensitization

Desensitization of G-protein-coupled receptors may involve phosphorylation of serine and threonine residues. The leukotriene B(4) (LTB(4)) receptor (BLT1) contains 14 intracellular serines and threonines, 8 of which are part of consensus target sequences for protein kinase C (PKC) or casein kinase 2. In this study, we investigated the importance of PKC and GPCR-specific kinase (GRK) phosphorylation in BLT1 desensitization. Pretreatment of BLT1-transfected COS-7 cells with PKC activators caused a decrease of LTB(4)-induced inositol phosphate (IP) accumulation. This reduction was prevented with the PKC inhibitor, staurosporine, and not observed in cells expressing a BLT1 deletion mutant (G291stop) lacking the cytoplasmic tail. Moreover LTB(4)-induced IP accumulation was significantly inhibited by overexpression of GRK2, GRK5, and especially GRK6, in cells expressing wild type BLT1 but not in those expressing G291stop. GRK6-mediated desensitization correlated with increased phosphorylation of BLT1. The G319stop truncated BLT1 mutant displayed functional characteristics comparable with wild type BLT1 in terms of desensitization by GRK6, but not by PKC. Substitution of Thr(308) within a putative casein kinase 2 site to proline or alanine in the full-length BLT1 receptor prevented most of GRK6-mediated inhibition of LTB(4)-induced IP production but only partially affected LTB(4)-induced BLT1 phosphorylation. Our findings thus suggest that Thr(308) is a major residue involved in GRK6-mediated desensitization of BLT1 signaling.

Aspartic acid 564 in the third cytoplasmic loop of the luteinizing hormone/choriogonadotropin receptor is crucial for phosphorylation-independent interaction with arrestin2

Arrestin2 binding to the active but unphosphorylated luteinizing hormone/choriogonadotropin receptor (LH/CG R) in ovarian follicles is triggered by activation of ADP-ribosylation factor 6 (ARF6) and leads to uncoupling of this receptor from cAMP signaling. We sought to determine how arrestin2 binds to LH/CG R, if binding is of high affinity, and if the receptor also binds arrestin3. Desensitization of intact LH/CG R was equally sensitive to ectopic constructs of arrestin2 that bind other G protein-coupled receptors (GPCRs) either in a phosphorylation-independent or -dependent manner. Intact LH/CG R was not desensitized by ectopic arrestin3 constructs. Surface plasmon resonance studies showed that arrestin2 bound a synthetic third intracellular (3i) LH/CG R loop peptide with picomolar affinity; arrestin3 bound with millimolar affinity. To determine whether Asp-564 in the 3i loop mimicked the phosphorylated residue of other GPCRs, human embryonic kidney (HEK) cells were transfected with wild-type (WT) and D564G LH/CG R. An agonist-stimulated ARF6-dependent arrestin2 undocking pathway to drive desensitization of WT receptor was recapitulated in HEK cell membranes, and ectopic arrestin2 promoted desensitization of WT LH/CG R. However, D564G LH/CG R in HEK cells was not desensitized, and synthetic 3i D564G peptide did not bind arrestin2. Synthetic 3i loop peptides containing D564E, D564V, or D564N also did not bind arrestin2. We conclude that the ARF6-mediated mechanism to release a pool of membrane-delimited arrestin to bind GPCRs may be a widespread mechanism to deliver arrestin to GPCRs for receptor desensitization. Unlike other GPCRs that additionally require receptor phosphorylation, LH/CG R activation is sufficient to expose a conformation in which Asp-564 in the 3i loop confers high affinity binding selectively to arrestin2.

G protein-coupled receptor kinases and beta arrestins are relocalized and attenuate cyclic 3',5'-adenosine monophosphate response to follicle-stimulating hormone in rat primary Sertoli cells

The FSH receptor (FSH-R) is a member of the rhodopsin-like subfamily of G protein-coupled receptors that undergoes homologous desensitization upon agonist stimulation. In immortalized cell lines overexpressing the FSH-R, G protein-coupled receptor kinases (GRKs) and beta-arrestins are involved in the phosphorylation, uncoupling, and internalization of this receptor. In an effort to appreciate the physiological relevance of GRK/beta-arrestin actions in natural FSH-R-bearing cells, we used primary rat Sertoli cells as a model. GRK2, -3, -5, -6a, and -6b and beta-arrestins 1 and 2 were expressed in primary rat Sertoli cells. Overexpression of these different GRKs and beta-arrestins in primary rat Sertoli cells significantly attenuated the FSH-induced cAMP response, and FSH rapidly triggered a relocalization of endogenously expressed GRK2, -3, -5, and -6 and beta-arrestins 1 and 2 from the cytosol to the membranes. These results highlight the relationship existing between the GRK/beta-arrestin regulatory system and the FSH-R signaling machinery in a physiological model.

Agonist-promoted internalization of a ternary complex between calcitonin receptor-like receptor, receptor activity-modifying protein 1 (RAMP1), and beta-arrestin

The calcitonin receptor-like receptor (CRLR) is a seven-transmembrane domain (7TM) protein that requires the receptor activity-modifying protein 1 (RAMP1) to be expressed at the cell surface as a functional calcitonin gene-related peptide (CGRP) receptor. Although dimerization between the two molecules is well established, very little is known concerning the trafficking of this heterodimer upon receptor activation. Also, the subcellular localization and biochemical state of this ubiquitously expressed protein, in the absence of CRLR, remains poorly characterized. Here we report that when expressed alone RAMP1 is retained inside the cells where it is found in the endoplasmic reticulum and the Golgi predominantly as a disulfide-linked homodimer. In contrast, when expressed with CRLR, it is targeted to the cell surface as a 1:1 heterodimer with the 7TM protein. Although heterodimer formation does not involve intermolecular disulfide bonds, RAMP-CRLR association promotes the formation of intramolecular disulfide bonds within RAMP1. CGRP binding and receptor activation lead to the phosphorylation of CRLR and the internalization of the receptor as a stable complex. The internalization was found to be both dynamin- and beta-arrestin-dependent, indicating that the formation of a ternary complex between CRLR, RAMP1, and beta-arrestin leads to clathrin-coated pit-mediated endocytosis. These results therefore indicate that although atypical by its heterodimeric composition and its targeting to the plasma membrane, the CGRP receptor shares endocytotic mechanisms that are common to most classical 7TM receptors.

Arrestin specificity for G protein-coupled receptors in human airway smooth muscle

Despite a widely accepted role of arrestins as "uncouplers" of G protein-coupled receptor (GPCR) signaling, few studies have demonstrated the ability of arrestins to affect second messenger generation by endogenously expressed receptors in intact cells. In this study we demonstrate arrestin specificity for endogenous GPCRs in primary cultures of human airway smooth muscle (HASM). Expression of arrestin-green fluorescent protein (ARR2-GFP or ARR3-GFP) chimeras in HASM significantly attenuated isoproterenol (beta(2)-adrenergic receptor (beta(2)AR)-mediated)- and 5'-(N-ethylcarboxamido)adenosine (A2b adenosine receptor-mediated)-stimulated cAMP production, with fluorescent microscopy demonstrating agonist-promoted redistribution of cellular ARR2-GFP into a punctate formation. Conversely, prostaglandin E(2) (PGE(2))-mediated cAMP production was unaffected by arrestin-GFP, and PGE(2) had little effect on arrestin-GFP distribution. The pharmacological profile of various selective EP receptor ligands suggested a predominantly EP2 receptor population in HASM. Further analysis in COS-1 cells revealed that ARR2-GFP expression increased agonist-promoted internalization of wild type beta(2)AR and EP4 receptors, whereas EP2 receptors remained resistant to internalization. However, expression of an arrestin whose binding to GPCRs is largely independent of receptor phosphorylation (ARR2(R169E)-GFP) enabled substantial agonist-promoted EP2 receptor internalization, increased beta(2)AR internalization to a greater extent than did ARR2-GFP, yet promoted EP4 receptor internalization to the same degree as did ARR2-GFP. Signaling via endogenous EP4 receptors in CHO-K1 cells was attenuated by ARR2-GFP expression, whereas ARR2(R169E)-GFP expression in HASM inhibited EP2 receptor-mediated cAMP production. These findings demonstrate differential effects of arrestins in altering endogenous GPCR signaling in a physiologically relevant cell type and reveal a variable dependence on receptor phosphorylation in dictating arrestin-receptor interaction.

The ERK-dependent signalling is stage-specifically modulated by FSH, during primary Sertoli cell maturation

Primary cultures of Sertoli cells provide an interesting model to study how signalling pathways induced by a single hormone in a single cell type evolve, depending on the developmental stage. In vivo, follicle-stimulating hormone (FSH) induces proliferation of Sertoli cells in neonate and controls the subsequent differentiation of the entire population. Molecular mechanisms underlying Sertoli cell pleiotropic responses to FSH have long been investigated. But to date, only cAMP-dependent kinase (PKA) activation has been reported to account for most FSH biological activities in male. Here, we demonstrate that FSH activates the ERK MAP kinase pathway following dual coupling of the FSH-R both to Gs and to Gi heterotrimeric proteins, in a PKA- and also Src-dependent manner. This activation is required for FSH-induced proliferation of Sertoli cells isolated 5 days after birth. Consistently, we show that the ERK-mediated FSH mitogenic effect triggers upregulation of cyclin D1. In sharp contrast, at 19 days after birth, as cells proceed through their differentiation program, the ERK pathway is dramatically inhibited by FSH treatment. Taken together, these results show that FSH can exert opposite effects on the ERK signalling cascade during the maturation process of Sertoli cells. Thus, signalling modules triggered by the FSH-R evolve dynamically throughout development of FSH natural target cells.

GRK3 mediates desensitization of CRF1 receptors: a potential mechanism regulating stress adaptation

Potential G protein-coupled receptor kinase (GRK) and protein kinase A (PKA) mediation of homologous desensitization of corticotropin-releasing factor type 1 (CRF1) receptors was investigated in human retinoblastoma Y-79 cells. Inhibition of PKA activity by PKI(5-22) or H-89 failed to attenuate homologous desensitization of CRF1 receptors, and direct activation of PKA by forskolin or dibutyryl cAMP failed to desensitize CRF-induced cAMP accumulation. However, treatment of permeabilized Y-79 cells with heparin, a nonselective GRK inhibitor, reduced homologous desensitization of CRF1 receptors by approximately 35%. Furthermore, Y-79 cell uptake of a GRK3 antisense oligonucleotide (ODN), but not of a random or mismatched ODN, reduced GRK3 mRNA expression by approximately 50% without altering GRK2 mRNA expression and inhibited homologous desensitization of CRF1 receptors by approximately 55%. Finally, Y-79 cells transfected with a GRK3 antisense cDNA construct exhibited an approximately 50% reduction in GRK3 protein expression and an ~65% reduction in homologous desensitization of CRF1 receptors. We conclude that GRK3 contributes importantly to the homologous desensitization of CRF1 receptors in Y-79 cells, a brain-derived cell line.

Kinase-inactive G-protein-coupled receptor kinases are able to attenuate follicle-stimulating hormone-induced signaling

Homologous desensitization of G-protein-coupled receptors (GPCR) is thought to occur in several steps: binding of G-protein-coupled receptor kinases (GRKs) to receptors, receptor phosphorylation, kinase dissociation, and finally binding of beta-arrestin to phosphorylated receptors and functional uncoupling of the associated Galpha protein. It has recently been reported that GRKs can inhibit Galphaq-mediated signaling in the absence of phosphorylation of some GPCRs. Whether or not comparable phosphorylation-independent effects are also possible with Galphas-coupled receptors remains unclear. In the present study, using the tightly Galphas-coupled FSR receptor (FSH-R) as a model, we observed inhibition of the cAMP-dependent signaling pathway using kinase-inactive mutants of GRK2, 5, and 6. These negative effects occur upstream of adenylyl cyclase activation and are likely independent of GRK interaction with G protein alpha or beta/gamma subunits. Moreover, we demonstrated that, when overexpressed in Cos 7 cells, mutated GRK2 associates with the FSH activated FSH-R. We hypothesize that phosphorylation-independent dampening of the FSH-R-associated signaling could be attributable to physical association between GRKs and the receptor, subsequently inhibiting G protein activation.

Cellular distribution of constitutively active mutant parathyroid hormone (PTH)/PTH-related protein receptors and regulation of cyclic adenosine 3',5'-monophosphate signaling by beta-arrestin2

PTH promotes endocytosis of human PTH receptor 1 (PTH1Rc) by activating protein kinase C and recruiting beta-arrestin2. We examined the role of beta-arrestin2 in regulating the cellular distribution and cAMP signaling of two constitutively active PTH1Rc mutants, H223R and T410P. Overexpression of a beta-arrestin2-green fluorescent protein (GFP) conjugate in COS-7 cells inhibited constitutive cAMP accumulation by H223R and T410P in a dose-dependent manner, as well as the response to PTH of both mutant and wild-type PTH1Rcs. The cellular distribution of PTH1Rc-GFP conjugates, fluorescent ligands, and ssarrestin2-GFP was analyzed by fluorescence microscopy in HEK-293T cells. In cells expressing either receptor mutant, a ligand-independent mobilization of beta-arrestin2 to the cell membrane was observed. In the absence of ligand, H223R and wild-type PTH1Rcs were mainly localized on the cell membrane, whereas intracellular trafficking of T410P was also observed. While agonists promoted beta-arrestin2-mediated endocytosis of bot PTH1Rc mutants, antagonists were rapidly internalized only with T410P. The protein kinases inhibitor, staurosporine, significantly decreased internalization of ligand-PTH1Rc mutant complexes, although the recruitment of beta-arrestin2 to the cell membrane was unaffected. Moreover, in cells expressing a truncated wild-type PTH1Rc lacking the C-terminal cytoplasmic domain, agonists stimulated translocation of beta-arrestin2 to the cell membrane followed by ligand-receptor complex internalization without associated beta-arrestin2. In conclusion, cAMP signaling by constitutively active mutant and wild-type PTH1Rcs is inhibited by a receptor interaction with beta-arrestin2 on the cell membrane, possibly leading to uncoupling from G(s)alpha. This phenomenon is independent from protein kinases activity and the receptor C-terminal cytoplasmic domain. In addition, there are differences in the cellular localization and internalization features of constitutively active PTH1Rc mutants H223R and T410P.

Expression of beta-arrestins in toxic and cold thyroid nodules

beta-Arrestins mediate agonist dependent desensitization of G protein-coupled receptors. Somatic TSH receptor mutations were identified in the majority of hot thyroid nodules. When transiently overexpressed in COS 7 cells these mutations resulted in constitutive activation of the cAMP pathway. However, the in vivo mechanisms and the in vivo desensitization of these TSH receptor mutations are unknown. Moreover, constitutively activated beta-adrenergic receptors are known to be constitutively desensitized. Therefore, we investigated the expression of beta-arrestins in toxic thyroid nodules (TTNs) with and without somatic TSH receptor mutation and in cold thyroid nodules (CTNs) by Western blotting and ELISA. Expression of beta-arrestin 2 was increased in all TTNs while beta-arrestin 2 expression was decreased in CTNs compared to their corresponding surrounding tissue. The mean beta-arrestin 1 expression was unchanged in the cytosol of TTNs, in membranes and cytosol of CTNs and decreased in the membranes of TTNs compared to their surrounding tissue. Transient coexpression of beta-arrestins 1 or 2 with the TSH receptor in HEK 293 cells and subsequent determination of cAMP showed that in vitro both beta-arrestins interact with the TSH receptor and are able to desensitize the receptor. The increased beta-arrestin 2 expression in TTNs and the desensitization of the TSH receptor by beta-arrestin 2 in vitro suggest that the beta-arrestin 2 expression is cAMP dependent and that beta-arrestin 2 very likely desensitizes the constitutively activated TSH receptor in toxic thyroid nodules.

Mutations of gonadotropins and gonadotropin receptors: elucidating the physiology and pathophysiology of pituitary-gonadal function

The recent unraveling of structures of genes for the gonadotropin subunits and gonadotropin receptors has provided reproductive endocrinologists with new tools to study normal and pathological functions of the hypothalamic-pituitary-gonadal axis. Rare inactivating mutations that produce distinctive phenotypes of isolated LH or FSH deficiency have been discovered in gonadotropin subunit genes. In addition, there is a common polymorphism in the LHbeta subunit gene with possible clinical significance as a contributing factor to pathologies of LH-dependent gonadal functions. Both activating and inactivating mutations have been detected in the gonadotropin receptor genes, a larger number in the LH receptor gene, but so far only a few in the gene for the FSH receptor. These mutations corroborate and extend our knowledge of clinical consequences of gonadotropin resistance and inappropriate gonadotropin action. The information obtained from human mutations has been complemented by animal models with disrupted or inappropriately activated gonadotropin ligand or receptor genes. These clinical and experimental genetic disease models form a powerful tool for exploring the physiology and pathophysiology of gonadotropin function and provide an excellent example of the power of molecular biological approaches in the study of pathogenesis of diseases.

The interaction of beta-arrestin with the AP-2 adaptor is required for the clustering of beta 2-adrenergic receptor into clathrin-coated pits

Beta-arrestins are cytosolic proteins that regulate the signaling and the internalization of G protein-coupled receptors (GPCRs). Although termination of receptor coupling requires beta-arrestin binding to agonist-activated receptors, GPCR endocytosis involves the coordinate interactions between receptor-beta-arrestin complexes and other endocytic proteins such as adaptor protein 2 (AP-2) and clathrin. Clathrin interacts with a conserved motif in the beta-arrestin C-terminal tail; however, the specific molecular determinants in beta-arrestin that bind AP-2 have not been identified. Moreover, the respective contributions of the interactions of beta-arrestin with AP-2 and clathrin toward the targeting of GPCRs to clathrin-coated vesicles have not been established. Here, we identify specific arginine residues (Arg(394) and Arg(396)) in the beta-arrestin 2 C terminus that mediate beta-arrestin binding to AP-2 and show, in vitro, that these domains in beta-arrestin 1 and 2 interact equally well with AP-2 independently of clathrin binding. We demonstrate in HEK 293 cells by fluorescence microscopy that beta(2)-adrenergic receptor-beta-arrestin complexes lacking the beta-arrestin-clathrin binding motif are still targeted to clathrin-coated pits. In marked contrast, receptor-beta-arrestin complexes lacking the beta-arrestin/AP-2 interactions are not effectively compartmentalized in punctated areas of the plasma membrane. These results reveal that the binding of a receptor-beta-arrestin complex to AP-2, not to clathrin, is necessary for the initial targeting of beta(2)-adrenergic receptor to clathrin-coated pits.