Developmental regulation of p70 S6 kinase by a G protein-coupled receptor dynamically modelized in primary cells

Testicular piRNA Analysis Identified Dysregulated piRNAs in Non-obstructive Azoospermia

Male infertility has remained idiopathic in a remarkable proportion of all cases. Gonadal expression of PIWI-interacting RNAs (piRNAs) has been shown to be vital to normal spermatogenesis, as they are expressed in almost all types of testicular germ cells. These molecules and their related Piwi proteins strictly regulate transposable elements' activity and gene expression. We aimed to identify dysregulated piRNAs in idiopathic non-obstructive azoospermic (NOA) testis by global expression analysis. Testis tissue samples from 18 azoospermic patients (ten NOA and eight OA) were studied by small RNA sequencing. To validate high-throughput sequencing data, quantitative real-time polymerase chain reactions for two differentially altered piRNAs were performed. Bioinformatics analyses were undertaken to identify pathways affected by piRNA dysregulation. In the NOA group, 1328 piRNAs were identified to be differentially expressed, of which 1322 were downregulated and 6 were upregulated. Bioinformatics analysis corroborated the involvement of dysregulated piRNA in spermatogenesis. We also identified 64 clusters of differentially expressed piRNAs, of which 42 clusters had a minimum of ten absolute piRNA hits. Our study suggests that piRNAs show significant dysregulation in infertility. Their target genes play a role in their self-biogenesis, probably by regulating their own production through a feedback mechanism. The downregulated piRNAs may find value as biomarkers for the presence of spermatozoa in the testis of azoospermic individuals, while the upregulated piRNAs are great candidates for further investigation of their precise functions in spermatogenesis.

MAPKs signaling is obligatory for male reproductive function in a development-specific manner

Mitogen-activated protein kinases (MAPKs) represent widely expressed and evolutionarily conserved proteins crucial for governing signaling pathways and playing essential roles in mammalian male reproductive processes. These proteins facilitate the transmission of signals through phosphorylation cascades, regulating diverse intracellular functions encompassing germ cell development in testis, physiological maturation of spermatozoa within the epididymis, and motility regulation at ejaculation in the female reproductive tract. The conservation of these mechanisms appears prevalent across species, including humans, mice, and, to a limited extent, livestock species such as bovines. In Sertoli cells (SCs), MAPK signaling not only regulates the proliferation of immature SCs but also determines the appropriate number of SCs in the testes at puberty, thereby maintaining male fertility by ensuring the capacity for sperm cell production. In germ cells, MAPKs play a crucial role in dynamically regulating testicular cell-cell junctions, supporting germ cell proliferation and differentiation. Throughout spermatogenesis, MAPK signaling ensures the appropriate Sertoli-to-germ cell ratio by regulating apoptosis, controlling the metabolism of developing germ cells, and facilitating the maturation of spermatozoa within the cauda epididymis. During ejaculation in the female reproductive tract, MAPKs regulate two pivotal events-capacitation and the acrosome reaction essential for maintaining the fertility potential of sperm cells. Any disruptions in MAPK pathway signaling possibly may disturb the testicular microenvironment homeostasis, sperm physiology in the male body before ejaculation and in the female reproductive tract during fertilization, ultimately compromising male fertility. Despite decades of research, the physiological function of MAPK pathways in male reproductive health remains inadequately understood. The current review attempts to combine recent findings to elucidate the impact of MAPK signaling on male fertility and proposes future directions to enhance our understanding of male reproductive functions.

Male infertility and gonadotropin treatment: What can we learn from real-world data?

Gonadotropin therapy to treat specific male infertility disorders associated with hypogonadotropic hypogonadism is evidence-based and effective in restoring spermatogenesis and fertility. In contrast, its use to improve fertility in men with idiopathic oligozoospermia or nonobstructive azoospermia remains controversial, despite being widely practiced. The existence of two major inter-related pathways for spermatogenesis, including FSH and intratesticular testosterone, provides a rationale for empiric hormone stimulation therapy in both eugonadal and hypogonadal males with idiopathic oligozoospermia or nonobstructive azoospermia. Real-world data (RWD) on gonadotropin stimulating for these patient subsets, mainly using human chorionic gonadotropin and follicle-stimulating hormone, accumulated gradually, showing a positive therapeutic effect in some patients, translated by increased sperm production, sperm quality, and sperm retrieval rates. Although more evidence is needed, current insights from RWD research indicate that selected male infertility patients might be managed more effectively using gonadotropin therapy, with potential gains for all parties involved.

Follicle-stimulating hormone signaling in Sertoli cells: a licence to the early stages of spermatogenesis

Follicle-stimulating hormone signaling is essential for the initiation and early stages of spermatogenesis. Follicle-stimulating hormone receptor is exclusively expressed in Sertoli cells. As the only type of somatic cell in the seminiferous tubule, Sertoli cells regulate spermatogenesis not only by controlling their own number and function but also through paracrine actions to nourish germ cells surrounded by Sertoli cells. After follicle-stimulating hormone binds to its receptor and activates the follicle-stimulating hormone signaling pathway, follicle-stimulating hormone signaling will establish a normal Sertoli cell number and promote their differentiation. Spermatogonia pool maintenance, spermatogonia differentiation and their entry into meiosis are also positively regulated by follicle-stimulating hormone signaling. In addition, follicle-stimulating hormone signaling regulates germ cell survival and limits their apoptosis. Our review summarizes the aforementioned functions of follicle-stimulating hormone signaling in Sertoli cells. We also describe the clinical potential of follicle-stimulating hormone treatment in male patients with infertility. Furthermore, our review may be helpful for developing better therapies for treating patients with dysfunctional follicle-stimulating hormone signaling in Sertoli cells.

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.

Misfolded G Protein-Coupled Receptors and Endocrine Disease. Molecular Mechanisms and Therapeutic Prospects

Misfolding of G protein-coupled receptors (GPCRs) caused by mutations frequently leads to disease due to intracellular trapping of the conformationally abnormal receptor. Several endocrine diseases due to inactivating mutations in GPCRs have been described, including X-linked nephrogenic diabetes insipidus, thyroid disorders, familial hypocalciuric hypercalcemia, obesity, familial glucocorticoid deficiency [melanocortin-2 receptor, MC2R (also known as adrenocorticotropin receptor, ACTHR), and reproductive disorders. In these mutant receptors, misfolding leads to endoplasmic reticulum retention, increased intracellular degradation, and deficient trafficking of the abnormal receptor to the cell surface plasma membrane, causing inability of the receptor to interact with agonists and trigger intracellular signaling. In this review, we discuss the mechanisms whereby mutations in GPCRs involved in endocrine function in humans lead to misfolding, decreased plasma membrane expression of the receptor protein, and loss-of-function diseases, and also describe several experimental approaches employed to rescue trafficking and function of the misfolded receptors. Special attention is given to misfolded GPCRs that regulate reproductive function, given the key role played by these particular membrane receptors in sexual development and fertility, and recent reports on promising therapeutic interventions targeting trafficking of these defective proteins to rescue completely or partially their normal function.

Actions and Roles of FSH in Germinative Cells

Follicle stimulating hormone (FSH) is produced by the pituitary gland in a coordinated hypothalamic-pituitary-gonadal (HPG) axis event, plays important roles in reproduction and germ cell development during different phases of reproductive development (fetal, neonatal, puberty, and adult life), and is consequently essential for fertility. FSH is a heterodimeric glycoprotein hormone of two dissociable subunits, α and β. The FSH β-subunit (FSHβ) function starts upon coupling to its specific receptor: follicle-stimulating hormone receptor (FSHR). FSHRs are localized mainly on the surface of target cells on the testis and ovary (granulosa and Sertoli cells) and have recently been found in testicular stem cells and extra-gonadal tissue. Several reproduction disorders are associated with absent or low FSH secretion, with mutation of the FSH β-subunit or the FSH receptor, and/or its signaling pathways. However, the influence of FSH on germ cells is still poorly understood; some studies have suggested that this hormone also plays a determinant role in the self-renewal of germinative cells and acts to increase undifferentiated spermatogonia proliferation. In addition, in vitro, together with other factors, it assists the process of differentiation of primordial germ cells (PGCLCs) into gametes (oocyte-like and SSCLCs). In this review, we describe relevant research on the influence of FSH on spermatogenesis and folliculogenesis, mainly in the germ cell of humans and other species. The possible roles of FSH in germ cell generation in vitro are also presented.

Hypo-glycosylated hFSH drives ovarian follicular development more efficiently than fully-glycosylated hFSH: enhanced transcription and PI3K and MAPK signaling

STUDY QUESTION

Does hypo-glycosylated human recombinant FSH (hFSH18/21) have greater in vivo bioactivity that drives follicle development in vivo compared to fully-glycosylated human recombinant FSH (hFSH24)?

SUMMARY ANSWER

Compared with fully-glycosylated hFSH, hypo-glycosylated hFSH has greater bioactivity, enabling greater follicular health and growth in vivo, with enhanced transcriptional activity, greater activation of receptor tyrosine kinases (RTKs) and elevated phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) and Mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling.

WHAT IS KNOWN ALREADY

Glycosylation of FSH is necessary for FSH to effectively activate the FSH receptor (FSHR) and promote preantral follicular growth and formation of antral follicles. In vitro studies demonstrate that compared to fully-glycosylated recombinant human FSH, hypo-glycosylated FSH has greater activity in receptor binding studies, and more effectively stimulates the PKA pathway and steroidogenesis in human granulosa cells.

STUDY DESIGN, SIZE, DURATION

This is a cross-sectional study evaluating the actions of purified recombinant human FSH glycoforms on parameters of follicular development, gene expression and cell signaling in immature postnatal day (PND) 17 female CD-1 mice. To stimulate follicle development in vivo, PND 17 female CD-1 mice (n = 8-10/group) were treated with PBS (150 µl), hFSH18/21 (1 µg/150 µl PBS) or hFSH24 (1 µg/150 µl PBS) by intraperitoneal injection (i.p.) twice daily (8:00 a.m. and 6:00 p.m.) for 2 days. Follicle numbers, serum anti-Müllerian hormone (AMH) and estradiol levels, and follicle health were quantified. PND 17 female CD-1 mice were also treated acutely (2 h) in vivo with PBS, hFSH18/21 (1 µg) or hFSH24 (1 µg) (n = 3-4/group). One ovary from each mouse was processed for RNA sequencing analysis and the other ovary processed for signal transduction analysis. An in vitro ovary culture system was used to confirm the relative signaling pathways.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The purity of different recombinant hFSH glycoforms was analyzed using an automated western blot system. Follicle numbers were determined by counting serial sections of the mouse ovary. Real-time quantitative RT-PCR, western blot and immunofluorescence staining were used to determine growth and apoptosis markers related with follicle health. RNA sequencing and bioinformatics were used to identify pathways and processes associated with gene expression profiles induced by acute FSH glycoform treatment. Analysis of RTKs was used to determine potential FSH downstream signaling pathways in vivo. Western blot and in vitro ovarian culture system were used to validate the relative signaling pathways.

MAIN RESULTS AND THE ROLE OF CHANCE

Our present study shows that both hypo- and fully-glycosylated recombinant human FSH can drive follicular growth in vivo. However, hFSH18/21 promoted development of significantly more large antral follicles compared to hFSH24 (P < 0.01). In addition, compared with hFSH24, hFSH18/21 also promoted greater indices of follicular health, as defined by lower BAX/BCL2 ratios and reduced cleaved Caspase 3. Following acute in vivo treatment with FSH glycoforms RNA-sequencing data revealed that both FSH glycoforms rapidly induced ovarian transcription in vivo, but hypo-glycosylated FSH more robustly stimulated Gαs and cAMP-mediated signaling and members of the AP-1 transcription factor complex. Moreover, hFSH18/21 treatment induced significantly greater activation of RTKs, PI3K/AKT and MAPK/ERK signaling compared to hFSH24. FSH-induced indices of follicle growth in vitro were blocked by inhibition of PI3K and MAPK.

LARGE SCALE DATA

RNA sequencing of mouse ovaries. Data will be shared upon reasonable request to the corresponding author.

LIMITATIONS, REASONS FOR CAUTION

The observations that hFSH glycoforms have different bioactivities in the present study employing a mouse model of follicle development should be verified in nonhuman primates. The gene expression studies reflect transcriptomes of whole ovaries.

WIDER IMPLICATIONS OF THE FINDINGS

Commercially prepared recombinant human FSH used for ovarian stimulation in human ART is fully-glycosylated FSH. Our findings that hypo-glycosylated hFSH has greater bioactivity enabling greater follicular health and growth without exaggerated estradiol production in vivo, demonstrate the potential for its development for application in human ART.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by NIH 1P01 AG029531, NIH 1R01 HD 092263, VA I01 BX004272, and the Olson Center for Women's Health. JSD is the recipient of a VA Senior Research Career Scientist Award (1IK6 BX005797). This work was also partially supported by National Natural Science Foundation of China (No. 31872352). The authors declared there are no conflicts of interest.

The herbicide dinitramine affects the proliferation of murine testicular cells via endoplasmic reticulum stress-induced calcium dysregulation

The hazardous effects of herbicides are well known; however, their effects on the reproductive system remain unclear. In this study, we demonstrated the anti-proliferative effects of dinitramine (DN) on immature murine testicular cell lines (Leydig and Sertoli cells) mediated via endoplasmic reticulum (ER) stress-induced calcium dysregulation in the cytosol and mitochondria. The results demonstrated that the viability and proliferation of DN-treated TM3 and TM4 cells decreased significantly, even in the spheroid state. DN induced the apoptosis of TM3 and TM4 cells and decreased the expression of genes related to cell cycle progression. Treatment with DN increased the cytosolic and intramitochondrial levels of calcium by activating ER stress signals. DN activated the Erk/P38/Jnk Mapk pathway and inactivated the Pi3k/Akt pathway in murine testicular cells. Co-treatment with 2-aminoethoxydiphenyl borate (2-APB) mitigated DN-induced calcium upregulation in both testicular cell lines. Although 2-APB did not antagonize the anti-proliferative effect of DN in TM3 cells, treatment with 2-APB and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid restored the proliferation of DN-treated TM4 cells.

Direct impact of gonadotropins on glucose uptake and storage in preovulatory granulosa cells: Implications in the pathogenesis of polycystic ovary syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS) is often associated with higher levels of LH, and arrested ovarian follicular growth. The direct impact of high LH on FSH mediated metabolic responses in PCOS patients is not clearly understood.

METHOD

In order to investigate the impact of FSH and LH on glucose metabolism in preovulatory granulosa cells (GCs), we used [U¹⁴C]-2 deoxyglucose, D-[U¹⁴C]-glucose or 2-NBD glucose to analyse glucose uptake and its incorporation into glycogen. To reproduce the high androgenic potential in PCOS patients, we administered hCG both in vitro and in vivo. The role of IRS-2/PI3K/Akt2 pathway was studied after knockdown with specific siRNA. Immunoprecipitation and specific assays were used for the assessment of IRS-2, glycogen synthase and protein phosphatase 1. Furthermore, we examined the in vivo effects of hCG on FSH mediated glycogen increase in normal and PCOS rat model. HEK293 cells co-expressing FSHR and LHR were used to demonstrate glucose uptake and BRET change by FSH and hCG.

RESULTS

In normal human and rat granulosa cells, FSH is more potent than hCG in stimulating glucose uptake, however glycogen synthesis was significantly upregulated only by FSH through increase in activity of glycogen synthase via IRS-2/PI3K/Akt2 pathway. On the contrary, an impaired FSH-stimulated glucose uptake and glycogen synthesis in granulosa cells of PCOS-patients indicated a selective defect in FSHR activation. Further, in normal human granulosa cells, and in immature rat model, the impact of hCG on FSH responses was such that it inhibited the FSH-mediated glucose uptake as well as glycogen synthesis through inhibition of FSH-stimulated IRS-2 expression. These findings were further validated in HEK293 cells overexpressing Flag-LHR and HA-FSHR, where high hCG inhibited the FSH-stimulated glucose uptake. Notably, an increased BRET change was observed in HEK293 cells expressing FSHR-Rluc8 and LHR-Venus possibly suggesting increased heteromerization of LHR and FSHR in the presence of both hCG and FSH in comparison to FSH or hCG alone.

CONCLUSION

Our findings confirm a selective attenuation of metabolic responses to FSH such as glucose uptake and glycogen synthesis by high activation level of LHR leading to the inhibition of IRS-2 pathway, resulting in depleted glycogen stores and follicular growth arrest in PCOS patients.

Mathematical modeling approaches of cellular endocrinology within the hypothalamo-pituitary-gonadal axis

The reproductive neuroendocrine axis, or hypothalamo-pituitary-gonadal (HPG) axis, is a paragon of complex biological system involving numerous cell types, spread over several anatomical levels communicating through entangled endocrine feedback loops. The HPG axis exhibits remarkable dynamic behaviors on multiple time and space scales, which are an inexhaustible source of studies for mathematical and computational biology. In this review, we will describe a variety of modeling approaches of the HPG axis from a cellular endocrinology viewpoint. We will in particular investigate the questions raised by some of the most striking features of the HPG axis: (i) the pulsatile secretion of hypothalamic and pituitary hormones, and its counterpart, the cell signaling induced by frequency-encoded hormonal signals, and (ii) the dual, gametogenic and glandular function of the gonads, which relies on the tight control of the somatic cell populations ensuring the proper maturation and timely release of the germ cells.

The role of estrogen receptors in rat Sertoli cells at different stages of development

The aim of the study was to investigate the effects of estrogen receptors (ESR1 and ESR2) on the expression of the proteins involved with proliferation (CCND1) and differentiation (CDKN1B and CTNNB) of Sertoli cells from rat in different stages of development. ESR1-selective agonist PPT, but not ESR2-selective agonist DPN, increased CCND1 expression in Sertoli cells from 5- and 15-day old rats. PPT did not have any effect on CCND1 expression in Sertoli cells from 20- and 30-day-old rats. DPN, but not PPT, increased CDKN1B expression in Sertoli cells from 15-, 20-, 30-day-old rats. DPN did not have any effect on Sertoli cells from 5-day-old rats. 17β-estradiol (E2) and PPT enhanced the [Methyl-³H] thymidine incorporation in Sertoli cells from 15-day-old rats, whereas the treatment did not have any effect in 20-day-old rats. E2 and DPN, but not PPT, increased non-phosphorylated CTNNB expression in Sertoli cells from 20-day-old rats. This upregulation was blocked by ESR2-selective antagonist PHTPP. The activation of ESR1 and ESR2, respectively, plays a role in the proliferation and differentiation of Sertoli cells in a critical period of testicular development. Furthermore, in Sertoli cells from 20-day-old rats, upregulation of non-phosphorylated CTNNB by E2/ESR2, via c-SRC/ERK1/2 and PI3K/AKT, may play a role in the interaction between Sertoli cells and/or in cell-germ cell adhesion and/or in the stabilization and accumulation of CTNNB in the cytosol. CTNNB could be translocated to the nucleus and modulate the transcriptional activity of specific target genes. The present study reinforces the important role of estrogen in normal testis development.

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.

Molecular Mechanisms and Signaling Pathways Involved in Sertoli Cell Proliferation

Sertoli cells are somatic cells present in seminiferous tubules which have essential roles in regulating spermatogenesis. Considering that each Sertoli cell is able to support a limited number of germ cells, the final number of Sertoli cells reached during the proliferative period determines sperm production capacity. Only immature Sertoli cells, which have not established the blood-testis barrier, proliferate. A number of hormonal cues regulate Sertoli cell proliferation. Among them, FSH, the insulin family of growth factors, activin, and cytokines action must be highlighted. It has been demonstrated that cAMP/PKA, ERK1/2, PI3K/Akt, and mTORC1/p70SK6 pathways are the main signal transduction pathways involved in Sertoli cell proliferation. Additionally, c-Myc and hypoxia inducible factor are transcription factors which participate in the induction by FSH of various genes of relevance in cell cycle progression. Cessation of proliferation is a pre-requisite to Sertoli cell maturation accompanied by the establishment of the blood-testis barrier. With respect to this barrier, the participation of androgens, estrogens, thyroid hormones, retinoic acid and opioids has been reported. Additionally, two central enzymes that are involved in sensing cell energy status have been associated with the suppression of Sertoli cell proliferation, namely AMPK and Sirtuin 1 (SIRT1). Among the molecular mechanisms involved in the cessation of proliferation and in the maturation of Sertoli cells, it is worth mentioning the up-regulation of the cell cycle inhibitors p21Cip1, p27Kip, and p19INK4, and of the gap junction protein connexin 43. A decrease in Sertoli cell proliferation due to administration of certain therapeutic drugs and exposure to xenobiotic agents before puberty has been experimentally demonstrated. This review focuses on the hormones, locally produced factors, signal transduction pathways, and molecular mechanisms controlling Sertoli cell proliferation and maturation. The comprehension of how the final number of Sertoli cells in adulthood is established constitutes a pre-requisite to understand the underlying causes responsible for the progressive decrease in sperm production that has been observed during the last 50 years in humans.

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.

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.

G Protein-Coupled Receptors As Regulators of Localized Translation: The Forgotten Pathway?

G protein-coupled receptors (GPCRs) exert their physiological function by transducing a complex signaling network that coordinates gene expression and dictates the phenotype of highly differentiated cells. Much is known about the gene networks they transcriptionally regulate upon ligand exposure in a process that takes hours before a new protein is synthesized. However, far less is known about GPCR impact on the translational machinery and subsequent mRNA translation, although this gene regulation level alters the cell phenotype in a strikingly different timescale. In fact, mRNA translation is an early response kinetically connected to signaling events, hence it leads to the synthesis of a new protein within minutes following receptor activation. By these means, mRNA translation is responsive to subtle variations of the extracellular environment. In addition, when restricted to cell subcellular compartments, local mRNA translation contributes to cell micro-specialization, as observed in synaptic plasticity or in cell migration. The mechanisms that control where in the cell an mRNA is translated are starting to be deciphered. But how an extracellular signal triggers such local translation still deserves extensive investigations. With the advent of high-throughput data acquisition, it now becomes possible to review the current knowledge on the translatome that some GPCRs regulate, and how this information can be used to explore GPCR-controlled local translation of mRNAs.

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.

The Follitropin Receptor: Matching Structure and Function

Follitropin, or follicle-stimulating hormone (FSH) receptor (FSHR), is a G protein-coupled receptor belonging to the glycoprotein hormone receptor family that plays an essential role in reproduction. Although its primary location is the gonad, the FSHR has also been reported in extragonadal tissues including bone, placenta, endometrium, liver, and blood vessels from a number of malignant tumors. The recently resolved crystal structure of FSH bound to the entire FSHR ectodomain has been instrumental in more clearly defining the role of this domain in ligand binding and receptor activation. Biochemical, biophysical, and structural data also indicate that the FSHR exists as a higher order structure and that it may heterodimerize with its closely related receptor, the luteinizing hormone receptor; this association may have physiologic implications during ovarian follicle maturation given that both receptors may simultaneously coexist in the same cell. FSHR heterodimerization is unique to the ovary because in the testes, gonadotropin receptors are expressed in separate compartments. FSHR self-association appears to be required for receptor coupling to multiple effectors and adaptors, for the activation of multiple signaling pathways and the transduction of asymmetric signaling, and for negative and positive receptor cooperativity. It also provides a mechanism through which the glycosylation variants of FSH may exert distinct and differential effects at the target cell level. Given its importance in regulating activation of distinct signaling pathways, functional selectivity at the FSHR is briefly discussed, as well as the potential implications of this particular functional feature on the design of new pharmacological therapies in reproduction.

Crosstalk between FSH and relaxin at the end of the proliferative stage of rat Sertoli cells

Follicle-stimulating hormone (FSH) stimulates the proliferation of immature Sertoli cells through the activation of PI3K/AKT/mTORC1 and MEK/ERK1/2 pathways. Mature Sertoli cells stop proliferating and respond to FSH by stimulating cAMP production. To gain insight into possible mechanisms involved in this switch as well as the impact of paracrine factors that stimulate cell proliferation, we analyzed the effects of FSH and relaxin on intracellular signaling pathways involved with proliferation and differentiation in Sertoli cells from 15-day-old rats, which are close to the transition between the two stages. FSH stimulated ³H-thymidine incorporation and cyclin D1 expression, changes associated with proliferation. In contrast, FSH inhibited AKT and ERK1/2 phosphorylation, activated cAMP production and induced changes in several cell cycle genes that were compatible with differentiation. Relaxin also stimulated ³H-thymidine incorporation but increased phosphorylation of ERK1/2 and AKT. When both hormones were added simultaneously, relaxin attenuated FSH-mediated inhibition of ERK1/2 and AKT phosphorylation and FSH-mediated activation of cAMP production. FSH but not relaxin increased CREB phosphorylation, and relaxin but not FSH shifted NF-κB expression from the cytoplasm to the nucleus. Relaxin did not inhibit the effects of FSH on inhibin α and Bcl2 expression. We propose that at this time of Sertoli cell development, FSH starts to direct cells to differentiation through activation of cAMP/CREB and inhibition of ERK1/2 and AKT pathways. Relaxin counteracts FSH signaling through the inhibition of cAMP and activation of ERK1/2, AKT and NF-κB, but does not block the differentiation process triggered by FSH.

mTOR Regulates Gap Junction Alpha-1 Protein Trafficking in Sertoli Cells and Is Required for the Maintenance of Spermatogenesis in Mice

The mammalian target of rapamycin (Mtor) gene encodes a serine/threonine kinase that acts as a master regulator of processes as diverse as cell growth, protein synthesis, cytoskeleton reorganization, and cell survival. In the testis, physiological roles for Mtor have been proposed in perinatal Sertoli cell proliferation and blood-testis barrier (BTB) remodeling during spermatogenesis, but no in vivo studies of Mtor function have been reported. Here, we used a conditional knockout approach to target Mtor in Sertoli cells. The resulting Mtor(flox/flox); Amhr2(cre/+) mice were characterized by progressive, adult-onset testicular atrophy associated with disorganization of the seminiferous epithelium, loss of Sertoli cell polarity, increased germ cell apoptosis, premature release of germ cells, decreased epididymal sperm counts, increased sperm abnormalities, and infertility. Histopathologic analysis and quantification of the expression of stage-specific markers showed a specific loss of pachytene spermatocytes and spermatids. Although the BTB and the ectoplasmic specializations did not appear to be altered in Mtor(flox/flox);Amhr2(cre/+) mice, a dramatic redistribution of gap junction alpha-1 (GJA1) was detected in their Sertoli cells. Phosphorylation of GJA1 at Ser373, which is associated with its internalization, was increased in the testes of Mtor(flox/flox); Amhr2(cre/+) mice, as was the expression and phosphorylation of AKT, which phosphorylates GJA1 at this site. Together, these results indicate that Mtor expression in Sertoli cells is required for the maintenance of spermatogenesis and the progression of germ cell development through the pachytene spermatocyte stage. One mechanism of mTOR action may be to regulate gap junction dynamics by inhibiting AKT, thereby decreasing GJA1 phosphorylation and internalization. mTOR regulates gap junction alpha-1 protein distribution in Sertoli cells and is necessary for progression through the pachytene spermatocyte stage.

Activation of a GPCR leads to eIF4G phosphorylation at the 5' cap and to IRES-dependent translation

The control of mRNA translation has been mainly explored in response to activated tyrosine kinase receptors. In contrast, mechanistic details on the translational machinery are far less available in the case of ligand-bound G protein-coupled receptors (GPCRs). In this study, using the FSH receptor (FSH-R) as a model receptor, we demonstrate that part of the translational regulations occurs by phosphorylation of the translation pre-initiation complex scaffold protein, eukaryotic initiation factor 4G (eIF4G), in HEK293 cells stably expressing the FSH-R. This phosphorylation event occurred when eIF4G was bound to the mRNA 5' cap, and probably involves mammalian target of rapamycin. This regulation might contribute to cap-dependent translation in response to FSH. The cap-binding protein eIF4E also had its phosphorylation level enhanced upon FSH stimulation. We also show that FSH-induced signaling not only led to cap-dependent translation but also to internal ribosome entry site (IRES)-dependent translation of some mRNA. These data add detailed information on the molecular bases underlying the regulation of selective mRNA translation by a GPCR, and a topological model recapitulating these mechanisms is proposed.

Receptors and signaling pathways involved in proliferation and differentiation of Sertoli cells

The identification of the hormones and other factors regulating Sertoli cell survival, proliferation, and maturation in neonatal, peripubertal, and pubertal life remains one of the most critical questions in testicular biology. The regulation of Sertoli cell proliferation and differentiation is thought to be controlled by cell-cell junctions and a set of circulating and local hormones and growth factors. In this review, we will focus on receptors and intracellular signaling pathways activated by androgen, follicle-stimulating hormone, thyroid hormone, activin, retinoids, insulin, insulin-like growth factor, relaxin, and estrogen, with special emphasis on estrogen receptors. Estrogen receptors activate intracellular signaling pathways that converge on cell cycle and transcription factors and play a role in the regulation of Sertoli cell proliferation and differentiation.

Mutations in G protein-coupled receptors that impact receptor trafficking and reproductive function

G protein coupled receptors (GPCRs) are a large superfamily of integral cell surface plasma membrane proteins that play key roles in transducing extracellular signals, including sensory stimuli, hormones, neurotransmitters, or paracrine factors into the intracellular environment through the activation of one or more heterotrimeric G proteins. Structural alterations provoked by mutations or variations in the genes coding for GPCRs may lead to misfolding, altered plasma membrane expression of the receptor protein and frequently to disease. A number of GPCRs regulate reproductive function at different levels; these receptors include the gonadotropin-releasing hormone receptor (GnRHR) and the gonadotropin receptors (follicle-stimulating hormone receptor and luteinizing hormone receptor), which regulate the function of the pituitary-gonadal axis. Loss-of-function mutations in these receptors may lead to hypogonadotropic or hypergonadotropic hypogonadism, which encompass a broad spectrum of clinical phenotypes. In this review we describe mutations that provoke misfolding and failure of these receptors to traffick from the endoplasmic reticulum to the plasma membrane. We also discuss some aspects related to the therapeutic potential of some target-specific drugs that selectively bind to and rescue function of misfolded mutant GnRHR and gonadotropin receptors, and that represent potentially valuable strategies to treat diseases caused by inactivating mutations of these receptors.

Constitutive activity in gonadotropin receptors

Constitutively active mutants (CAMs) of gonadotropin receptors are, in general, rare conditions. Luteinizing hormone-choriogonadotropin receptor (LHCGR) CAMs provoke the dramatic phenotype of familial gonadotropin-independent isosexual male-limited precocious puberty, whereas in females, there is not yet any identified phenotype. Only one isolated follicle-stimulating hormone receptor (FSHR) CAM (Asp567Gly) has so far been detected in a single male patient, besides other FSHR weak CAMs linked to pregnancy-associated ovarian hyperstimulation syndrome or to impaired desensitization and internalization. Several animal models have been developed for studying enhanced gonadotropin action; in addition to unraveling valuable new information about the possible phenotypes of isolated FSHR and LHCGR CAMs in women, the information obtained from these mouse models has served multiple translational goals, including the development of new diagnostic and therapeutic targets as well as the prediction of phenotypes for mutations not yet identified in humans. Mutagenesis and computational studies have shed important information on the physiopathogenic mechanisms leading to constitutive activity of gonadotropin receptors; a common feature in these receptor CAMs is the release of stabilizing interhelical interactions between transmembrane domains (TMDs) 3 and 6 leading to an increase, with respect to the wild-type receptor, in the solvent accessibility at the cytosolic extension of TMDs 3, 5, and 6, which involves the highly conserved Glu/Asp-Arg-Tyr/Trp sequence. In this chapter, we summarize the structural features, functional consequences, and mechanisms that lead to constitutive activation of gonadotropin receptor CAMs and provide information on pharmacological approaches that might potentially modulate gonadotropin receptor CAM function.

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.

Signal transduction pathways in FSH regulation of rat Sertoli cell proliferation

The final number of Sertoli cells reached during the proliferative periods determines sperm production capacity in adulthood. It is well known that FSH is the major Sertoli cell mitogen; however, little is known about the signal transduction pathways that regulate the proliferation of Sertoli cells. The hypothesis of this investigation was that FSH regulates proliferation through a PI3K/Akt/mTORC1 pathway, and additionally, AMPK-dependent mechanisms counteract FSH proliferative effects. The present study was performed in 8-day-old rat Sertoli cell cultures. The results presented herein show that FSH, in addition to increasing p-Akt, p-mTOR, and p-p70S6K levels, increases p-PRAS40 levels, probably contributing to improving mTORC1 signaling. Furthermore, the decrease in FSH-stimulated p-Akt, p-mTOR, p-p70S6K, and p-PRAS40 levels in the presence of wortmannin emphasizes the participation of PI3K in FSH signaling. Additionally, the inhibition of FSH-stimulated Sertoli cell proliferation by the effect of wortmannin and rapamycin point to the relevance of the PI3K/Akt/mTORC1 signaling pathway in the mitotic activity of FSH. On the other hand, by activating AMPK, several interesting observations were made. Activation of AMPK produced an increase in Raptor phosphorylation, a decrease in p70S6K phosphorylation, and a decrease in FSH-stimulated Sertoli cell proliferation. The decrease in FSH-stimulated cell proliferation was accompanied by an increased expression of the cyclin-dependent kinase inhibitors (CDKIs) p19INK4d, p21Cip1, and p27Kip1. In summary, it is concluded that FSH regulates Sertoli cell proliferation with the participation of a PI3K/Akt/mTORC1 pathway and that AMPK activation may be involved in the detention of proliferation by, at least in part, a decrease in mTORC1 signaling and an increase in CDKI expression.

mRNA-selective translation induced by FSH in primary Sertoli cells

FSH is a key hormonal regulator of Sertoli cell secretory activity, required to optimize sperm production. To fulfil its biological function, FSH binds a G protein-coupled receptor, the FSH-R. The FSH-R-transduced signaling network ultimately leads to the transcription or down-regulation of numerous genes. In addition, recent evidence has suggested that FSH might also regulate protein translation. However, this point has never been demonstrated conclusively yet. Here we have addressed this issue in primary rat Sertoli cells endogenously expressing physiological levels of FSH-R. We observed that, within 90 min of stimulation, FSH not only enhanced overall protein synthesis in a mammalian target of rapamycin-dependent manner but also increased the recruitment of mRNA to polysomes. m(7)GTP pull-down experiments revealed the functional recruitment of mammalian target of rapamycin and p70 S6 kinase to the 5'cap, further supported by the enhanced phosphorylation of one of p70 S6 kinase targets, the eukaryotic initiation factor 4B. Importantly, the scaffolding eukaryotic initiation factor 4G was also recruited, whereas eukaryotic initiation factor 4E-binding protein, the eukaryotic initiation factor 4E generic inhibitor, appeared to play a minor role in translational regulations induced by FSH, in contrast to what is generally observed in response to anabolic factors. This particular regulation of the translational machinery by FSH stimulation might support mRNA-selective translation, as shown here by quantitative RT-PCR amplification of the c-fos and vascular endothelial growth factor mRNA but not of all FSH target mRNA, in polysomal fractions. These findings add a new level of complexity to FSH biological roles in its natural target cells, which has been underappreciated so far.

Proteasome inhibition-induced p38 MAPK/ERK signaling regulates autophagy and apoptosis through the dual phosphorylation of glycogen synthase kinase 3β

Proteasome inhibition is a promising approach for cancer treatment; however, the underlying mechanisms involved have not been fully elucidated. Here, we show that proteasome inhibition-induced p38 mitogen-activated protein kinase regulates autophagy and apoptosis by modulating the phosphorylation status of glycogen synthase kinase 3β (GSK3β) and 70kDa ribosomal S6 kinase (p70S6K). The treatment of MDA-MB-231 cells with MG132 induced endoplasmic reticulum stress through the induction of ATF6a, PERK phosphorylation, and CHOP, and apoptosis through the cleavage of Bax and procaspase-3. MG132 caused the phosphorylation of GSK3β at Ser(9) and, to a lesser extent, Thr(390), the dephosphorylation of p70S6K at Thr(389), and the phosphorylation of p70S6K at Thr(421) and Ser(424). The specific p38 inhibitor SB203080 reduced the p-GSK3β(Ser9) and autophagy through the phosphorylation of p70S6K(Thr389); however, it augmented the levels of p-ERK, p-GSK3β(Thr390), and p-70S6K(Thr421/Ser424) induced by MG132, and increased apoptotic cell death. The GSK inhibitor SB216763, but not lithium, inhibited the MG132-induced phosphorylation of p38, and the downstream signaling pathway was consistent with that in SB203580-treated cells. Taken together, our data show that proteasome inhibition regulates p38/GSK(Ser9)/p70S6K(Thr380) and ERK/GSK3β(Thr390)/p70S6K(Thr421/Ser424) kinase signaling, which is involved in cell survival and cell death.

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.

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.

Effects of heat exposure on Akt/S6K1 signaling and expression of genes related to protein and energy metabolism in chicken (Gallus gallus) pectoralis major muscle

In order to improve understanding of the heat-induced changes in muscle growth, we determined the expression of genes related to protein and energy metabolism in the pectoralis major muscle of chickens. We also explored the protein kinase B (PKB also called Akt)/p70 S6 kinase (S6K1)/S6 pathway that mediates anabolic signals thereby regulating metabolism and hypertrophic/atrophic balance. Four-week-old chickens were exposed to 32 or 22 degrees C for 1 week. Chickens from both groups were then fasted for 16 h or left fed, and submitted to an oral administration of glucose-arginine to induce an anabolic response (30-min treatment) or left untreated. High ambient temperature and the associated decrease in feed intake modified the expression of certain energy-related genes (e.g. -40% for PGC-1alpha) and protein metabolism (e.g. about +80% for atrogin-1), but the expression of several muscle metabolism-related genes considered here was unchanged. The capacity for muscle protein synthesis, i.e. RNA/protein ratio, was reduced in warm conditions (approximately -20%). Slightly lower activation of S6 induced by glucose-arginine treatment was found at 32 degrees C compared to 22 degrees C, which might indicate somewhat lower efficiency of mRNA translation. Analysis of glucose/insulin balance suggested changes in glucose metabolism under heat exposure. However, this remains to be characterized.

FSH-stimulated PTEN activity accounts for the lack of FSH mitogenic effect in prepubertal rat Sertoli cells

Follicle-stimulating hormone (FSH) controls the proliferation and differentiation of Sertoli cells of the testis. FSH binds a G protein-coupled receptor (GPCR) to stimulate downstream effectors of the phosphoinositide-3 kinase (PI3K)-dependent pathway, without enhancing PI3K activity. To clarify this paradox, we explored the activity of phosphatase and tensin homolog deleted in chromosome 10 (PTEN), the PI3K major regulator, in primary cultures of rat Sertoli cells. We show that, within minutes, FSH increases PTEN neo-synthesis, requiring the proteasomal degradation of an unidentified intermediate, as well as PTEN enzymatic activity. Importantly, introducing an antisense cDNA of PTEN into differentiating Sertoli cells restores FSH-dependent cell proliferation. In conclusion, these results provide a new mechanism of PTEN regulation, which could serve to block entry into S phase of Sertoli cells, while they are proceeding through differentiation in prepubertal animals.

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.

GPCR signalling to the translation machinery

G protein-coupled receptors (GPCRs) are involved in most physiological processes, many of them being engaged in fully differentiated cells. These receptors couple to transducers of their own, primarily G proteins and beta-arrestins, which launch intracellular signalling cascades. Some of these signalling events regulate the translational machinery to fine-tune general cell metabolism or to alter protein expression pattern. Though extensively documented for tyrosine kinase receptors, translational regulation by GPCRs is still poorly appreciated. The objective of this review paper is to address the following questions: i) is there a "GPCR signature" impacting on the translational machinery, and ultimately on the type of mRNA translated? ii) are the regulatory networks involved similar as those utilized by tyrosine kinase receptors? In particular, we will discuss the specific features of translational control mediated by GPCRs and highlight the intrinsic properties of GPCRs these mechanisms could rely on.