Truncation of the C-terminal tail of the follitropin receptor does not impair the agonist- or phorbol ester-induced receptor phosphorylation and uncoupling

Association of FSHR gene polymorphisms with poor ovarian response in patients undergoing IVF: A systematic review and meta-analysis

BACKGROUND

The results of studies on the association between polymorphisms in the FSHR gene and the risk of POR undergoing IVF have been inconsistent with each other, so we conducted a meta-analysis of all the available studies to explore the association between polymorphisms in the FSHR gene and the risk of POR.

METHODS

Literature that met the inclusion criteria was collected by searching six electronic databases and basic data from included studies were extracted. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to assess the strength of association between follicle-stimulating hormone receptor (FSHR) gene polymorphism and poor ovarian response (POR) risk. Begg's and Egger's tests were used to determine whether there was publication bias, and sensitivity analysis and TSA analysis were used to verify the stability and reliability of the results.

RESULTS

We included 24 articles, 22 of which explored rs6166, including 2,206 cases and 3,897 controls. 6 articles explored rs6165, including 444 cases and 875 controls. Under additive, heterozygote, and dominant models, rs6166 was significantly associated with POR (S vs. N: OR = 1.29, 95 % CI = 1.05-1.59, P = 0.017; NS vs. NN: OR = 1.33, 95 % CI = 1.02-1.74, P = 0.038; NS + SS vs. NN: OR = 1.38, 95 % CI = 1.04-1.84, P = 0.025). In ethnicity-based subgroup analyses, the additive, homozygote, heterozygote, and dominant models increased Asian POR risk. Among the five genetic models, rs6165 was significantly associated with POR (T vs. C: OR = 1.64, 95 % CI = 1.25-2.16, P = 0.000; TT vs. CC: OR = 2.76, 95 % CI = 1.43-5.32, P = 0.003; CT vs. CC: OR = 1.58, 95 % CI = 1.19-2.10, P = 0.001; TT vs. CC + CT: OR = 2.32, 95 % CI = 1.67-3.23, P = 0.000; CT + TT vs. CC: OR = 1.80, 95 % CI = 1.22-2.65, P = 0.003). In ethnicity-based subgroup analyses, all five genetic models increased the risk of POR in Caucasians.

CONCLUSION

According to the current meta-analysis, the rs6166 S allele was significantly associated with an increased risk of POR, especially in Asian populations. The rs6165 T allele was significantly associated with an increased risk of POR, especially in Caucasian populations.

Luteinizing hormone receptor promotes angiogenesis in ovarian endothelial cells of Macaca fascicularis and Homo sapiens†

Angiogenesis within the ovarian follicle is an important component of ovulation. New capillary growth is initiated by the ovulatory surge of luteinizing hormone (LH), and angiogenesis is well underway at the time of follicle rupture. LH-stimulated follicular production of vascular growth factors has been shown to promote new capillary formation in the ovulatory follicle. The possibility that LH acts directly on ovarian endothelial cells to promote ovulatory angiogenesis has not been addressed. For these studies, ovaries containing ovulatory follicles were obtained from cynomolgus macaques and used for histological examination of ovarian vascular endothelial cells, and monkey ovarian microvascular endothelial cells (mOMECs) were enriched from ovulatory follicles for in vitro studies. mOMECs expressed LHCGR mRNA and protein, and immunostaining confirmed LHCGR protein in endothelial cells of ovulatory follicles in vivo. Human chorionic gonadotropin (hCG), a ligand for LHCGR, increased mOMEC proliferation, migration and capillary-like sprout formation in vitro. Treatment of mOMECs with hCG increased cAMP, a common intracellular signal generated by LHCGR activation. The cAMP analog dibutyryl cAMP increased mOMEC proliferation in the absence of hCG. Both the protein kinase A (PKA) inhibitor H89 and the phospholipase C (PLC) inhibitor U73122 blocked hCG-stimulated mOMEC proliferation, suggesting that multiple G-proteins may mediate LHCGR action. Human ovarian microvascular endothelial cells (hOMECs) enriched from ovarian aspirates obtained from healthy oocyte donors also expressed LHCGR. hOMECs also migrated and proliferated in response to hCG. Overall, these findings indicate that the LH surge may directly activate ovarian endothelial cells to stimulate angiogenesis of the ovulatory follicle.

The N-Linked Glycosylation Site N191 Is Necessary for PKA Signal Transduction in Eel Follicle-Stimulating Hormone Receptor

The follicle-stimulating hormone receptor (FSHR) contains several N-linked glycosylation sites in its extracellular region. We conducted the present study to determine whether conserved glycosylated sites in eel FSHR are necessary for cyclic adenosine monophosphate (cAMP) signal transduction. We used site-directed mutagenesis to induce four mutations (N120Q, N191Q, N272Q, and N288Q) in the N-linked glycosylation sites of eel FSHR. In the eel FSHR wild-type (wt), the cAMP response was gradually increased in a dose-dependent manner (0.01–1500 ng/mL), displaying a high response (approximately 57.5 nM/10⁴ cells) at the Rmax level. Three mutants (N120Q, N272Q, and N288Q) showed a considerably decreased signal transduction as a result of high-ligand treatment, whereas one mutant (N191Q) exhibited a completely impaired signal transduction. The expression level of the N191Q mutant was only 9.2% relative to that of the eel FSHR-wt, indicating a negligible expression level. The expression levels of the N120Q and N272Q mutants were approximately 35.9% and 24% of the FSHG-wt, respectively. The N288Q mutant had an expression level similar to that of the eel FSHR-wt, despite the mostly impaired cAMP responsiveness. The loss of the cell surface agonist-receptor complexes was very rapid in the cells expressing eel FSHR-wt and FSHR-N288Q mutants. Specifically, the N191Q mutant was completely impaired by the loss of cell surface receptors, despite treatment with a high concentration of the agonist. Therefore, we suggest that the N191 site is necessary for cAMP signal transduction. This finding implies that the cAMP response, mediated by G proteins, is directly related to the loss of cell surface receptors as a result of high-agonist treatment.

Signal Transduction of C-Terminal Phosphorylation Regions for Equine Luteinizing Hormone/Chorionic Gonadotropin Receptor (eLH/CGR)

This study aimed to investigate the signal transduction of phosphorylation sites at the carboxyl (C)-terminal region of equine luteinizing hormone/chorionic gonadotropin receptor (eLH/ CGR). The eLH/CGR has a large extracellular domain of glycoprotein hormone receptors within the G protein-coupled receptors. We constructed a mutant (eLH/CGR-t656) of eLH/ CGR, in which the C-terminal cytoplasmic tail was truncated at the Phe656 residue, through polymerase chain reaction. The eLH/CGR-t656 removed 14 potential phosphorylation sites in the intracellular C-terminal region. The plasmids were transfected into Chinese hamster ovary (CHO)-K1 and PathHunter Parental cells expressing β-arrestin, and agonist-induced cAMP responsiveness was analyzed. In CHO-K1 cells, those expressing eLH/CGR-t656 were lower than those expressing eLH/CGR wild-type (eLH/CGR-wt). The EC₅₀ of the eLH/ CGR-t656 mutant was approximately 72.2% of the expression observed in eLH/CGR-wt. The maximal response in eLH/CGR-t656 also decreased to approximately 43% of that observed in eLH/CGR-wt. However, in PathHunter Parental cells, cAMP activity and maximal response of the eLH/CGR-t656 mutant were approximately 173.5% and 100.8%, respectively, of that of eLH/CGR-wt. These results provide evidence that the signal transduction of C-terminal phosphorylation in eLH/CGR plays a pivotal role in CHO-K1 cells. The cAMP level was recovered in PathHunter Parental cells expressing β-arrestin. We suggest that the signal transduction of the C-terminal region phosphorylation sites is remarkably different depending on the cells expressing β-arrestin in CHO-K1 cells.

The C-terminal Phosphorylation Sites of eel Follicle-Stimulating Hormone Receptor are Important Role in the Signal Transduction

The large extracellular domain of glycoprotein hormone receptors is a unique feature within the G protein-coupled receptors (GPCRs) family. After interaction with the hormone, the receptor becomes coupled to Gs, which, in turn stimulates adenylyl cyclase and the production of cAMP. Potential phosphorylation sites exist in the C-terminal region of GPCRs. The experiments described herein represent attempts to determine the functions of the eel follicle-stimulating hormone receptor (eelFSHR). We constructed a mutant of eelFSHR, in which the C-terminal cytoplasmic tail was truncated at residue 614 (eelFSHR-t614). The eelFSHR-t614 lacked all potential phosphorylation sites present in the C-terminal region of eelFSHR. In order to obtain the eelFSHR ligand, we produced recombinant follicle-stimulating hormone (rec-eelFSHβ/α) in the CHO-suspension cells. The expression level was 2-3 times higher than that of the transient expression of eelFSH in attached CHO-K1 cells. The molecular weight of the rec-eelFSHβ/α protein was identified to be approximately 34 kDa. The cells expressing eelFSHR-t614 showed an increase in agonist-induced cAMP responsiveness. The maximal cAMP responses of cells expressing eelFSHR-t614 were lower than those of cells expressing eelFSHR-wild type (eelFSHR-WT). The EC₅₀ following C-terminal deletion in CHO-K1 cells was approximately 60.4% of that of eelFSHR-WT. The maximal response in eelFSHR-t614 cells was also drastically lower than that of eelFSHR-WT. We also found similar results in PathHunter Parental cells expressing β-arrestin. Thus, these data provide evidence that the truncation of the C-terminal cytoplasmic tail phosphorylation sites in the eelFSHR greatly decreased cAMP responsiveness and maximal response in both CHO-K1 cells and PathHunter Parental cells expressing β-arrestin.

Gq/11-Dependent Changes in the Murine Ovarian Transcriptome at the End of Gestation

Parturition in rodents is highly dependent on the engagement of the luteal prostaglandin F2 alpha receptor, which, through activation of the G_q/11 family of G proteins, increases the expression of Akr1c18, leading to an increase in progesterone catabolism. To further understand the involvement of G_q/11 on luteolysis and parturition, we used microarray analysis to compare the ovarian transcriptome of mice with a granulosa/luteal cell-specific deletion of Galpha_q/11 with their control littermates on Day 18 of pregnancy, when mice from both genotypes are pregnant, and on Day 22, when mice with a granulosa/luteal cell-specific deletion of Galpha_q/11 are still pregnant but their control littermates are 1–2 days postpartum. Ovarian genes up-regulated at the end of gestation in a Galpha_q/11 -dependent fashion include genes involved in focal adhesion and extracellular matrix interactions. Genes down-regulated at the end of gestation in a Galpha_q/11-dependent manner include Serpina6 (which encodes corticosteroid-binding globulin); Enpp2 (which encodes autotaxin, the enzyme responsible for the synthesis of lysophosphatidic acid); genes involved in protein processing and export; reproductive genes, such as Lhcgr; the three genes needed to convert progesterone to estradiol (Cyp17a1, Hsd17b7, and Cyp19a1); and Inha. Activation of ovarian G_q/11 by engagement of the prostaglandin F2 alpha receptor on Day 18 of pregnancy recapitulated the regulation of many but not all of these genes. Thus, although the ovarian transcriptome at the end of gestation is highly dependent on the activation of G_q/11, not all of these changes are dependent on the actions of prostaglandin F2 alpha.

The Asn680Ser polymorphism of the follicle stimulating hormone receptor gene and ovarian cancer risk: a meta-analysis

PURPOSE

The purpose of this study was to conduct a meta-analysis to assess the association between FSHR Asn680Ser polymorphism and ovarian cancer susceptibility.

METHODS

A literature search was conducted in PubMed, Embase and the China National Knowledge Infrastructure (CNKI) for all relevant studies published up to September 2013. The pooled odds ratios (ORs) with the corresponding 95 % confidence intervals (95 % CIs) were calculated to evaluate the association.

RESULTS

Four case-control studies including 474 ovarian cancer cases and 659 controls met the inclusion criteria. The pooled analyses showed that FSHR Asn680Ser polymorphism was associated with the risk of ovarian cancer (Ser vs Asn: OR=1.295, 95 % CI 1.057-1.498, P=0.01; Ser/Ser + Asn/Ser vs Asn/Asn: OR=1.611, 95 % CI 1.027-2.528, P=0.038). Subgroup analyses by ethnicity (Caucasian and Asian) further revealed significant associations among Asians (Ser vs Asn: OR=1.386, 95 % CI 1.066-1.802, P=0.015; Ser/Ser + Asn/Ser vs Asn/Asn: OR=1.893, 95 % CI 1.329-2.689, P=0.000) but not Caucasians. There was no obvious risk of publication bias.

CONCLUSIONS

The meta-analysis suggests that FSHR Asn680Ser polymorphism may be a risk factor for ovarian cancer in Asians. Due to the limited quantity of the included studies, further studies are needed to validate the above conclusions.

Structure, function and regulation of gonadotropin receptors - a perspective

Luteinizing hormone receptor and follicle stimulating hormone receptor play a crucial role in female and male reproduction. Significant new information has emerged about the structure, mechanism of activation, and regulation of expression of these receptors. Here we provide an overview of the current information on those aspects with an in-depth discussion of the recent developments in the post-transcriptional mechanism of LH receptor expression mediated by a specific LH receptor mRNA binding protein, designated as LRBP. LRBP was identified by electrophoretic gel mobility shift assay using cytosolic fractions from ovaries in the down regulated state. LRBP was purified, its binding site on LH receptor mRNA was identified and characterized. During ligand-induced down regulation, LRBP expression is increased through the cAMP/PKA and ERK signaling pathway, is translocated to translating ribosomes, binds LH receptor mRNA and forms an untranslatable ribonucleoprotein complex. This complex is then routed to the mRNA degradation machinery resulting in diminished levels of both LHR mRNA and cell surface expression of LH receptor. The studies leading to these conclusions are presented.

Somatostatin receptor-3 mediated intracellular signaling and apoptosis is regulated by its cytoplasmic terminal

In the present study, we describe the role of cytoplasmic terminal (C-tail) domain in regulating coupling to adenylyl cyclase, signaling, and apoptosis in human embryonic kidney (HEK-293) cells transfected with wild type (wt)-hSSTR3 and C-tail deleted mutants. Cells transfected with wt-hSSTR3 and C-tail mutants show comparable membrane expression; however, display decreased expression in presence of agonist. wt-hSSTR3 exists as preformed homodimer at cell surface in basal conditions and decreases in response to agonist. Cells expressing C-tail mutants also show evidence of homodimerization with the same intensity as wt-hSSTR3. The agonist-dependent inhibition of cyclic adenosine monophosphate (cAMP) was lost in cells expressing C-tail mutants. Agonist treatment in cells expressing wt-hSSTR3 resulted in inhibition of cell proliferation, increased expression of PARP-1, and TUNEL positivity in proliferating cell nuclear antigen (PCNA)-positive cells. The agonist mediated increase in membrane expression of protein tyrosine phosphatase (PTP) seen with wt-hSSTR3 was diminished in C-tail mutants, which was accompanied with the loss of receptor's ability to induce apoptosis. Taken together, our data provide new insights into C-tail-dependent regulation of cell signaling and apoptosis by hSSTR3.

Follicle stimulating hormone receptor mutations and reproductive disorders

The follicle stimulating hormone receptor (FSHR) plays a critical role in reproductive function. In the males, FSH supports spermatogenesis, whereas in females, FSH is absolutely required for ovarian follicle growth. In females, inactivating mutations in the FSHR result in ovarian dysgenesis with amenorrhea and infertility. The few males reported with severe inactivating mutations exhibited varying spermatogenic defects, but not azoospermia. While these findings may potentially suggest that FSH action is not absolutely required for spermatogenesis, it cannot be ruled out that these individuals have some residual FSHR activity. Gain-of-function mutations in the FSHR cause spontaneous ovarian hyperstimulation syndrome in females due to the inappropriate stimulation of the mutant FSHR by human choriogonadotropin.

Genetic polymorphisms of FSHR, CYP17, CYP1A1, CAPN10, INSR, SERPINE1 genes in adolescent girls with polycystic ovary syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS), whose genetic basis is not completely well understood, is the most common endocrine disorder in women and it typically develops during adolescence. The aim of this study is to investigate the possible association between single nucleotide polymorphisms (SNPs) of FSHR, CYP17, CYP1A1, CAPN10, INSR, SERPINE1 genes and PCOS in adolescent girls.

METHODS

DNA samples from forty-four adolescent girls with PCOS and 50 healthy controls were analyzed by PCR-RFLP and direct DNA sequencing to determine the genotypic frequency of 17 different polymorphic loci on the FSHR (A307T, N680S), CYP17 (-34 T/C), CYP1A1 (T6235C), CAPN10 (44, 43, 19, 63), INSR (exon 17 C/T), SERPINE1 (4G/5G) genes. Genotyping of exon 12 (six polymorphisms) and intron 12 (one polymorphism) of INSR gene by direct DNA sequencing was performed for the first time in this study.

RESULTS

No significant differences were observed in the genotype and allele distributions of above mentioned polymorphisms between cases and control groups.

CONCLUSION

Our data does not support an association between SNPs of FSHR, CYP17, CYP1A1, CAPN10, INSR, SERPINE1 genes and susceptibility to PCOS or related traits in Turkish adolescent girls.

Mutations of the lutropin/choriogonadotropin receptor that do not activate the phosphoinositide cascade allow hCG to induce aromatase expression in immature rat granulosa cells

Using primary cultures of immature rat granulosa cells and adenoviral infections we expressed two mutants of the human lutropin receptor (hLHR) that do not activate the phosphoinositide cascade. One mutant (hLFF) has the extracellular domain of the hLHR and the transmembrane and intracellular domains of the hFSHR. The other (hLHR-L457D) has a leucine to aspartate mutation in residue 457 of transmembrane helix 3. When expressed in immature rat granulosa cells the hLHR stimulates cAMP and inositol phosphate accumulation, transactivates the epidermal growth factor receptor (EGFR), elicits a transient increase in Akt phosphorylation, and a sustained increase in ERK1/2 phosphorylation but aromatase expression is not enhanced. When expressed at comparable densities, hLFF and hLHR-L457D support cAMP accumulation and transient Akt phosphorylation but do not support inositol phosphate accumulation, EGFR transactivation or a sustained phosphorylation of ERK1/2. Cells expressing either of these two mutants respond to hCG with increased aromatase expression. We also show that addition of hCG to cells expressing the hLHR antagonizes the effects of hFSH on aromatase expression whereas addition of hCG to cells expressing the hLHR-L457D mutant does not. These results show that activation of the phosphoinositide cascade is upstream of EGFR transactivation and ERK1/2 phosphorylation and that this pathway is a negative regulator of aromatase expression in granulosa cells.

The effect of splice variant of the human luteinizing hormone (LH) receptor on the expression of gonadotropin receptor

A splice variant of human lutropin/choriogonadotropin-receptor [hLHR (exon 9)] that lacks exon 9 was previously cloned in the corpus luteum of a woman with a regular menstrual-cycle. Supported by detergent soluble binding assay and receptor biotinylation experiment, the receptor binding assay shows hLHR (exon 9) is neither expressed at the cell surface nor have the capability of binding to hCG. Interactions between hLHR (exon 9) with the immature bands of gonadotropin receptors not with the mature bands were seen. This phenomenon is specific among gonadotropin receptors since human thyrotropin-receptor (hTSHR) failed to be coimmunoprecipitated. Furthermore, this receptor complex attenuated the receptor protein level within the cells. To elucidate the mechanism underlying the decrease in receptor protein by this receptor complex, we performed a Percoll-fractionation experiment, which indicated the receptor complex drove hLHR to the lysosome instead of the plasma-membrane. Moreover, the expression of hLHR (exon 9) mRNA was seen at all phases of the menstrual cycle and relatively increased as the luteal phase progressed. These results reveal a novel mechanism of regulation for gonadotropin receptor expression.

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.

Single nucleotide polymorphisms of follicle-stimulating hormone receptor are associated with ovarian cancer susceptibility

Epidemiological studies suggested that ovulation was associated with ovarian carcinogenesis. Follicle-stimulating hormone (FSH) played an important role in follicular development and was recently found to affect growth of ovarian epithelial cells. Single nucleotide polymorphisms (SNPs) Thr307Ala and Asn680Ser were two non-synonymous variations in the coding region of the FSH receptor (FSHR) gene. This hitherto first case-control study investigating the association between these two FSHR SNPs and the risk of ovarian cancer involved 202 histopathologically confirmed ovarian cancer patients and 266 age-matched cancer-free control subjects using restriction fragment length polymorphism assay and direct sequencing. Our results demonstrated that the 307Ala and 680Ser carriers were associated with significantly increased risk of developing serous and mucinous types of ovarian cancers (P < 0.0005, OR = 2.60, 95% CI = 1.56-4.34; and P < 0.0005, OR = 2.89, 95% CI = 1.73-4.84, adjusted for age, respectively) but not endometrioid and clear cell types. The two SNPs were found to be in modest linkage disequilibrium, D' = 0.804 and 0.701, r2 = 0.581 and 0.406 for the cancer and control groups, respectively. The major haplotype of 307Ala-680Ser was also associated with higher cancer risk (P = 0.033, OR = 1.39, 95% CI = 1.03-1.88), especially for the serous and mucinous carcinomas (P = 0.001, OR = 1.82, 95% CI = 1.27-2.60). Our results suggested that the two FSHR SNPs might affect the susceptibility of women to specific subtypes of ovarian cancer. Different types of ovarian cancer might adopt distinct carcinogenetic pathways. Such understanding may be important in selecting patients for ovulation induction therapy.

The differential effects of the gonadotropin receptors on aromatase expression in primary cultures of immature rat granulosa cells are highly dependent on the density of receptors expressed and the activation of the inositol phosphate cascade

Signaling pathways mediating the divergent effects of FSH and LH on aromatase in immature rat granulosa cells were studied by infecting cells with increasing amounts of adenoviral vectors for the human LH receptor (hLHR) or FSH receptor (hFSHR). Increasing amounts of Ad-hLHR, used at a multiplicity of infection (MOI) of 20 or 200 viable viral particles/cell, increased human chorionic gonadotropin (hCG) binding and hCG-induced cAMP and Akt phosphorylation, but inositol phosphates only increased in response to hCG in cells infected with 200 MOI Ad-hLHR. In contrast, hCG increased aromatase expression in cells infected with 20, but not in cells infected with 200, MOI Ad-hLHR. Cells infected with 20 or 200 MOI Ad-hFSHR showed increased hFSH binding and hFSH-induced Akt phosphorylation, but the hFSH-induced cAMP response was unchanged relative to control cells. However, hFSH was able to stimulate the inositol phosphate cascade in the Ad-hFSHR-infected cells, and the hFSH induction of aromatase was abolished. We also found that activation of C kinase or expression of a constitutively active form of Galphaq inhibited the induction of aromatase by hFSH or 8Br-cAMP. We conclude that the differential effects of FSH and LH on aromatase in immature granulosa cells are highly dependent on gonadotropin receptor density and on the signaling pathways activated. We propose that aromatase is induced by common signals generated by activation of the FSHR and LHR (possibly cAMP and Akt) and that the activation of the inositol phosphate cascade in cells expressing a high density of LHR or FSHR antagonizes this induction.

The delta e13 isoform of the calcitonin receptor forms a six-transmembrane domain receptor with dominant-negative effects on receptor surface expression and signaling

The CTRdelta e13 splice variant of the rabbit calcitonin receptor, which lacks the 14 amino acids of the seventh transmembrane domain (TMD) that are encoded by exon 13, is poorly expressed on the cell surface, fails to mobilize intracellular calcium or activate Erk, and inhibits the cell surface expression of the full-length C1a isoform. Nuclear magnetic resonance- and fluorescence-activated cell sorter-based experiments showed that the residual seventh TMD of CTRdelta e13 fails to partition into the lipid bilayer, resulting in an extracellular C terminus. Truncating the receptor after residue 397 to delete the cytoplasmic tail resulted in reduced cell surface expression and an inability to mobilize intracellular calcium or activate Erk, but the truncated receptor did not inhibit C1a cell surface expression. In contrast, when the receptor was truncated after residue 374 to eliminate the entire seventh TMD domain and the C-terminal domain, the resulting receptor reduced the cell surface expression of C1a in a manner similar to that of CTRdelta e13. Thus, normal cell surface expression, mobilization of intracellular calcium, and Erk activation requires the cytoplasmic C-terminal tail of the CTR, whereas the absence of the seventh TMD in the transmembrane helical bundle causes the dominant-negative effect on the surface expression of C1a.

Human follitropin receptor (FSHR) interacts with the adapter protein 14-3-3tau

The human follitropin (follicle stimulating hormone, FSH) receptor (FSHR) is a G protein-coupled receptor (GPCR). To identify cytoplasmic proteins that may regulate FSHR function, a yeast-based interaction trap was performed. A linked construct of the first and second intracellular loops (iL1-iL2 bait) of FSHR was used as bait and a human ovarian cDNA library was used as prey. Among the proteins identified that interacted with the bait was 14-3-3tau, a member of a family of homodimeric cytoplasmic adapter proteins. Human granulosa cells, the site of FSHR expression in the ovary, were found to contain 14-3-3tau. Importantly, 14-3-3tau co-immunoprecipitated with FSHR stably expressed in HEK 293 cells. Its association with FSHR was follitropin-dependent. Over-expression of 14-3-3tau resulted in a modest decrease of follitropin-induced cAMP accumulation. Collectively, these data support a role for 14-3-3tau in follitropin action. The finding that 14-3-3tau interacts with FSHR is novel and should lead to new insights into the regulation of GPCR in general and FSHR specifically.

Galpha protein selectivity determinant specified by a viral chemokine receptor-conserved region in the C tail of the human herpesvirus 8 g protein-coupled receptor

The viral G-protein coupled receptor (vGPCR) specified by human herpesvirus 8 (HHV-8) open reading frame 74 (ORF74) is a ligand-independent chemokine receptor that has structural and functional homologues among other characterized gammaherpesviruses and related receptors in the betaherpesviruses. Sequence comparisons of the gammaherpesvirus vGPCRs revealed a highly conserved region in the C tail, just distal to the seventh transmembrane domain. Mutagenesis of the corresponding codons of HHV-8 ORF74 was carried out to provide C-tail-altered proteins for functional analyses. By measuring receptor-activated vascular endothelial growth factor promoter induction and NF-kappaB, mitogen-activated protein kinase, and Ca(2+) signaling, we found that while some altered receptors showed general signaling deficiencies, others had distinguishable activation profiles, suggestive of selective Galpha protein coupling. This was supported by the finding that vGPCR and representative functionally altered variants, vGPCR.8 (R322W) and vGPCR.15 (M325S), were affected differently by inhibitors of Galpha(i) (pertussis toxin), protein kinase C (GF109203X), and phosphatidylinositol 3-kinase (wortmannin). Consistent with the signaling data, [(35)S]GTPgammaS incorporation assays revealed preferential coupling of vGPCR.15 to Galpha(q) and an inability of vGPCR.8 to couple functionally to Galpha(q). However, both variants, wild-type vGPCR, and a C-tail deletion version of the receptor were equally able to associate physically with Galpha(q). Combined, our data demonstrate that HHV-8 vGPCR contains discrete sites of Galpha interaction and that receptor residues in the proximal region of the cytoplasmic tail are determinants of Galpha protein coupling specificity.

The association of arrestin-3 with the follitropin receptor depends on receptor activation and phosphorylation

We have recently shown that the binding of arrestin-3 to the lutropin receptor (LHR) is dependent mostly on receptor activation rather than on phosphorylation. The experiments presented here were designed to test the involvement of these two events in the association of arrestin-3 with the closely related follitropin receptor (FSHR). Activation of the FSHR leads to the phosphorylation of residues in the first and third intracellular loops. Mutation of the phosphorylation sites in the third intracellular loop of the rat (r) FSHR partially reduces phosphorylation but has no effect on arrestin-3 association. Mutation of the phosphorylation sites in the first intracellular loop abolishes phosphorylation and arrestin-3 association. Dominant-negative mutants of G protein-coupled receptor kinase (GRKs) 2 and 6 inhibit rFSHR phosphorylation to the same extent but only the dominant-negative mutant of GRK2 inhibits arrestin-3 association. Two mutations of the rFSHR (D389N and Y530F) that impair activation and abolish phosphorylation also impair arrestin-3 binding. GRK2 restores the phosphorylation of both mutants but it restores arrestin-3 association only to the D389N mutant. We conclude that, in contrast to the data obtained with the LHR, the association of arrestin-3 with the FSHR is dependent on receptor phosphorylation. The phosphorylation of the third intracellular loop residues is not needed for arrestin-3 association, however.

Identification of a short linear sequence present in the C-terminal tail of the rat follitropin receptor that modulates arrestin-3 binding in a phosphorylation-independent fashion

The rat follitropin receptor (rFSHR) is an unusual G protein-coupled receptor in that agonist-induced activation leads to the phosphorylation of the first and third intracellular loops instead of the C-terminal tail. To determine regions of G protein-coupled receptors that affect internalization independently of phosphorylation we examined the effects of truncations of the C-terminal tail of the rFSHR on agonist-induced internalization. Our studies show that progressive truncations of a region flanked by residues 642 and 651 enhance the internalization of human follicle-stimulating hormone (hFSH). Further characterization of a mutant truncated at residue 649 (designated rFSHR-t649) and another mutant in which the 642-651 region was deleted in the context of the full-length rFSHR, designated rFSHR(Delta642-651), showed that both of them internalized hFSH at rates that were 2-3 times faster than rFSHR-wild type (wt). Like rFSHR-wt, however, the internalization of hFSH mediated by rFSHR-t649 and rFSHR(Delta642-651) can be inhibited with dominant-negative mutants of the non-visual arrestins or dynamin. Alanine-scanning mutagenesis of the 642-651 region suggests that the effects on internalization are not mediated by a single residue, however. In an attempt to understand the molecular basis of the enhanced internalization of hFSH mediated by these mutants we used an assay that can be readily used to assess the association of the rFSHR with the arrestin-3 in co-transfected cells. Using this assay we were able to show that, when compared with rFSHR-wt, rFSHR(Delta642-651) displays an approximately 4-fold enhancement in binding affinity for arrestin-3 and an approximately 1.7-fold reduction in maximal arrestin-3 binding capacity. We conclude that a short linear sequence present in the C-terminal tail of the rFSHR (642SATHNFHARK651) that is not phosphorylated limits internalization by lowering the affinity of the rFSHR for the endogenous non-visual arrestins.

Molecular, structural, and cellular biology of follitropin and follitropin receptor

Follitropin and the follitropin receptor are essential for normal gamete development in males and females. This review discusses the molecular genetics and structural and cellular biology of the follitropin/follitropin receptor system. Emphasis is placed on the human molecules when possible. The structure and regulation of the genes for the follitropin beta subunit and the follitropin receptor is discussed. Control of systemic and cellular protein levels is explained. The structural biology of each protein is described, including protein structure, motifs, and activity relationships. Finally, the follitropin/follitropin receptor signal transduction system is discussed.

Acute agonist-mediated desensitization of the human alpha 1a-adrenergic receptor is primarily independent of carboxyl terminus regulation: implications for regulation of alpha 1aAR splice variants

Despite important roles in myocardial hypertrophy and benign prostatic hyperplasia, little is known about acute effects of agonist stimulation on alpha(1a)-adrenergic receptor (alpha(1a)AR) signaling and function. Regulatory mechanisms are likely complex since 12 distinct human alpha(1a)AR carboxyl-terminal splice variants have been isolated. After determining the predominance of the alpha(1a-1)AR isoform in human heart and prostate, we stably expressed an epitope-tagged alpha(1a-1)AR cDNA in rat-1 fibroblasts and subsequently examined regulation of signaling, phosphorylation, and internalization of the receptor. Human alpha(1a)AR-mediated inositol phosphate signaling is acutely desensitized in response to both agonist and phorbol 12-myristate 13-acetate (PMA) exposure. Concurrent with desensitization, alpha(1a)ARs in (32)P(i)-labeled cells are rapidly phosphorylated in response to both NE and PMA stimulation. Despite the ability of PKC to desensitize alpha(1a)ARs when directly activated with PMA, inhibitors of PKC have no effect on agonist-mediated desensitization. In contrast, involvement of GRK kinases is suggested by the ability of GRK2 to desensitize alpha(1a)ARs. Internalization of cell surface alpha(1a)ARs also occurs in response to agonist stimulation (but not PKC activation), but is initiated more slowly than receptor desensitization. Significantly, deletion of the alpha(1a)AR carboxyl terminus has no effect on receptor internalization or either agonist-induced or GRK-mediated receptor desensitization. Because mechanisms underlying acute agonist-mediated regulation of human alpha(1a)ARs are primarily independent of the carboxyl terminus, they may be common to all functional alpha(1a)AR isoforms.

Mechanisms of gonadotropin desensitization

The gonadotropic hormones, FSH and LH exert a major effect on ovarian and testicular function through interaction with specific seven-transmembrane domain glycoprotein receptors. Desensitization to the hormones, which can occur both in vivo and in vitro, is essential for prevention of overstimulation of the gonadal cells. The long-term process of desensitization to the gonadotropic hormones is probably mediated, in part, by extensive clustering and internalization of the hormone-receptor complex. Short-term desensitization may occur as a result of phosphorylation of serine or threonine residues on the receptor molecules, although a specific receptor kinase has not yet been identified. Recently, we have discovered a novel mechanism of gonadotropin desensitization, which is exerted by down-regulation of StAR expression and steroidogenesis mediated by MAPK activation as a result of hormone-receptor interaction, cAMP accumulation and PKA activation. Thus, PKA not only mediates gonadotropin-induced steroidogenesis, it also activates the down-regulation mechanism that can silence steroidogenesis under certain conditions. Moreover, our findings raise the possibility that activation or inhibition of ERK by other pathways could be an important mechanism for diminution or amplification of gonadotropin-stimulated steroidogenesis. This could contribute to functional luteolysis, a process in which luteinized granulosa cells show reduced sensitivity to LH despite maintenance of LH receptors, or to up-regulation of the steroidogenic machinery during luteinization of granulosa cells.

Distinct regions of C-terminus of the high affinity neurotensin receptor mediate the functional coupling with pertussis toxin sensitive and insensitive G-proteins

The functional coupling of C-terminally truncated mutants of the high affinity rat neurotensin (NT) receptor (NTS1) was characterized in transfected Chinese hamster ovary cells. On cells expressing NTRDelta372 (truncated NTS1 lacking the entire 52 amino acid C-terminus), NT failed to promote [(35)S]guanosine 5'-[gamma-(35)S]triphosphate binding whereas a robust pertussis toxin (PTx) sensitive response was observed in cells expressing a partially truncated receptor (NTRDelta401 lacking the last 23 residues). Similar results were obtained when measuring the ability of NT to induce the production of arachidonic acid. Since neither deletions impaired the NT-induced phosphoinositide hydrolysis, these results indicate that the membrane proximal region of the C-terminus is specifically involved in the functional coupling of the receptor with PTx sensitive G-proteins. This region was also found to be involved in the control of receptor internalization. However, PTx failed to impair internalization, indicating that these two properties are not directly related.

Mechanisms involved in the homologous down-regulation of transcription of the follicle-stimulating hormone receptor gene in Sertoli cells

The action of follicle-stimulating hormone (FSH) in spermatogenesis is regulated at a fundamental level by controlling the number of competent receptors present at the surface of Sertoli cells. By controlling the number of receptors, the cell is able to modulate the timing and magnitude of subsequent signal transduction in response to FSH. One mechanism of control is the down-regulation of the steady state levels of the FSH receptor gene after exposure to FSH or agents that stimulate or prolong the cAMP signal transduction cascade (homologous down-regulation) in Sertoli cells. The goals of this study were to examine possible mechanisms involved in the down-regulation of mRNA levels of this gene. Analysis of transcription and processing by a PCR-based assay showed that treatment of Sertoli cells with FSH caused at least a 50% reduction of hnRNA for the FSH receptor gene. Reporter genes controlled by 5' flanking sequences of the FSH receptor gene that were transiently transfected into Sertoli cells were not down-regulated. In electrophoretic mobility shift assays (EMSA), cAMP-inducible nuclear protein complex containing c-Fos formed on the activator protein-1/cAMP responsive element-like site located at -216 to -210 in the promoter of the rat FSH receptor gene. We concluded from this study that there was no evidence for the putative role of ICER in the down-regulation of the FSH receptor promoter. In addition, the FSH-induced down-regulation of the transcription of the FSH receptor gene in Sertoli cells was prevented by the treatment of Sertoli cells with trichostatin A prior to the addition of FSH. This experiment coupled with other observations suggested that the down-regulation may be mediated by changes in chromatin structure.

Multiple distant amino acid residues present in the serpentine region of the follitropin receptor modulate the rate of agonist-induced internalization

The amino acid sequences of the human (h) and rat (r) follitropin receptors (FSHR) are approximately 89% identical, but the half-time of internalization of agonist mediated by the rFSHR is approximately 3 times faster than that of the hFSHR. Chimeras of the hFSHR and the rFSHR showed that this difference in rate is dictated mostly by the serpentine domain. Further analysis identified six residues, two non-contiguous residues in the transmembrane helix 4 (Leu/Thr in the rFSHR and Met/Ile in the hFSHR), three non-contiguous residues in the third intracellular loop (Thr/Thr/Lys in the rFSHR and Ile/Asn/Arg in the hFSHR), and one in transmembrane helix 7 (Tyr in the rFSHR and His in the hFSHR) that are fully responsible for the difference in the rates of internalization of the hFSHR and the rFSHR.

Receptor density and recycling affect the rate of agonist-induced desensitization of mu-opioid receptor

Previously, we reported that the time course for the rapid phosphorylation rate of mu-opioid receptor expressed in human embryonic kidney (HEK)293 cells did not correlate with the slow receptor desensitization rate induced by [D-Ala(2),N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO). However, others have suggested that receptor phosphorylation is the trigger for mu-opioid receptor desensitization. In this study, we demonstrated the relatively slow rate of receptor desensitization could be attributed partially to the recycling of internalized receptor as determined by fluorescence-activated cell-sorting analysis. However, the blockade of the endocytic and Golgi transport events in HEK293 cells with monensin and brefeldin A did not increase the initial rate of receptor desensitization. But the desensitization rate was increased by reduction of the mu-opioid receptor level with beta-furnaltrexamine (betaFNA). The reduction of the receptor level with 1 microM betaFNA significantly increased the rate of etorphine-induced receptor desensitization. By blocking the ability of receptor to internalize with 0.4 M sucrose, a significant degree of receptor being rapidly desensitized was observed in HEK293 cells pretreated with betaFNA. Hence, mu-opioid receptor is being resensitized during chronic agonist treatment. The significance of resensitization of the internalized receptor in affecting receptor desensitization was demonstrated further with human neuroblastoma SHSY5Y cells that expressed a low level of mu-opioid receptor. Although DAMGO could not induce a rapid desensitization in these cells, in the presence of monensin and brefeldin A, DAMGO desensitized the mu-opioid receptor's ability to regulate adenylyl cyclase with a t(1/2) = 9.9 +/- 2.1 min and a maximal desensitized level at 70 +/- 4.7%. Furthermore, blockade of receptor internalization with 0.4 M sucrose enhanced the DAMGO-induced receptor desensitization, and the inclusion of monensin prevented the resensitization of the mu-opioid receptor after chronic agonist treatment in SHSY5Y cells. Thus, the ability of the mu-opioid receptor to resensitize and to recycle, and the relative efficiency of the receptor to regulate adenylyl cyclase activity, contributed to the observed slow rate of mu-opioid receptor desensitization in HEK293 cells.

Human follicle stimulating hormone receptor variants lacking transmembrane domains display altered post-translational conformations

Variant splicing of gonadotropin receptor mRNA commonly occurs, however expression of receptor protein variants and their trafficking has yet to be studied in detail. To determine receptor variant trafficking and intracellular processing in mammalian cells, the intracellular fate of intentionally truncated variants of human follicle stimulating hormone receptor (hFSH-R) expressed in CHO cells was examined. Monoclonal antibodies (mAbs) were made against the hFSH-R's extracellular domain (ECD) expressed in insect cells. Four mAbs 106.156, 106.290, 106.318, and 106.263 were chosen as probes. Epitope mapping using synthetic peptides, and truncated hFSH-R variants revealed that mAb 106.156 bound to ECD residues 183-220, while mAbs 106.318, 106.290, 106.263 bound ECD residues 300-331. Immunofluorescence microscopy showed that mAbs 106.318 and 106.156 stained the surface of fixed, intact CHO cells expressing wild type hFSH-R. However, following cell permeabilization all four antibodies stained hFSH-R in Golgi and endoplasmic reticulum. Permeabilized cells expressing truncated variants ECD213 and ECD254 showed staining accumulated in the endoplasmic reticulum/nuclear envelope continuum. ECD335/His was found to accumulate in extended endoplasmic reticulum (ER). The ER location of ECD335/His was confirmed by double labeling experiments with concanavalin A and ECD mAb. Glycosidase digestion followed by Western blot analysis show ECD213 and ECD335/His to be glycosylated, but not ECD254. Both glycosylated truncated hFSH-R variants were sensitive to peptide-N-glycanase F and endoglycosidase H but insensitive to neuraminidase indicating that these variants possess high mannose type oligosaccharides. Thus truncated hFSH-R variants do not reach the medial or trans Golgi where high mannose oligosaccharides are trimmed and sialic acid is added. These data suggest that the conformation the ECD of the wild type receptor is different from the ECD alone expressed in the endoplasmic reticulum. This information suggests that the ECD serves two distinct roles; the first is to bind FSH and the other is likely to contact the endodomain of the receptor, which presumably leads to activation of the endodomain for signal transduction.

Follicle-stimulating hormone ligand and receptor mutations, and gonadal dysfunction

In contrast to the general contention, infertility can be an inherited condition. Some of the genetic causes of male and female infertility have turned out to be due to inactivating mutations in the gonadotropin and gonadotropin receptor genes. The topic of the present text is to review current knowledge on mutations affecting the function of follicle-stimulating hormone (FSH). This gonadotropin, by binding to its specific G protein-coupled cell membrane receptor (FSHR), is important for normal gonadal function. Mutations affecting gonadotropin genes are extremely rare, but recent genetic studies have revealed that the pathogenesis of subfertility or infertility can be due to mutations in the FSH receptor (FSHR) gene. While mutations affecting FSHR are sporadic, polymorphism of the FSHR gene seems to be a common phenomenon. To date, six inactivating and only one activating mutation have been detected in the FSHR gene. In contrast to LHR gene, the majority of these mutations affect the extracellular domain of the receptor. Together with animal models using the transgenic and knock-out approaches, systematic analysis of alterations in the FSHR gene increases our knowledge on the structure and function of the FSHR and demonstrates that the integrity of each FSHR segment is required for proper expression of the fully active protein and for normal gonadal function. Mutations in the FSHR gene have different consequences in the reproductive function depending on the sex of the patient: while normal ovarian function is critically dependent on FSH, male fertility is possible with minimal or absent FSH action.

The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

As the oxytocin receptor plays a key role in parturition and lactation, there is considerable interest in defining its structure/functional relationships. We previously showed that the rat oxytocin receptor transfected into Chinese hamster ovary cells was coupled to both G(q/11) and G(i/o), and that oxytocin stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis via protein kinase C activity. In this study, we show that deletion of 51 amino acid residues from the carboxyl terminus resulted in reduced affinity for oxytocin and a corresponding rightward shift in the dose-response curve for oxytocin-stimulated [Ca(2+)](i). However, oxytocin-stimulated ERK-2 phosphorylation and prostaglandin E(2) synthesis did not occur in cells expressing the truncated receptor. Oxytocin also failed to increase phospholipase A activity or activate protein kinase C, indicating that the mutant receptor is uncoupled from G(q)-mediated pathways. The Delta51 receptor is coupled to G(i), as oxytocin-stimulated Ca(2+) transients were inhibited by pertussis toxin, and a Gbetagamma sequestrant. Preincubation of Delta51 cells with the tyrosine kinase inhibitor, genistein, also blocked the oxytocin effect. A Delta39 mutant had all the activities of the wild type oxytocin receptor. These results show that the portion between 39 and 51 residues from the COOH terminus of the rat oxytocin receptor is required for interaction with G(q/11), but not G(i/o). Furthermore, an increase in intracellular calcium was generated via a G(i)betagamma-tyrosine kinase pathway from intracellular stores that are distinct from G(q)-mediated inositol trisphosphate-regulated stores.

Role of G protein-coupled receptor kinases on the agonist-induced phosphorylation and internalization of the follitropin receptor

The experiments presented herein were designed to identify members of the G protein-coupled receptor kinase (GRK) family that participate in the agonist-induced phosphorylation and internalization of the rat FSH receptor (rFSHR). Western blots of human kidney 293 cells (the cell line used in transfection experiments) and MSC-1 cells (a cell line derived from Sertoli cells that displays many of the differentiated functions of their normal counterparts) reveal the presence of GRK2 and GRK6 in both cell lines as well as GRK4 in MSC-1 cells. Cotransfection of 293 cells with the rFSHR and GRK2, GRK4alpha, or GRK6 resulted in an increase in the agonist-induced phosphorylation of the rFSHR. Cotransfections of the rFSHR with GRKs or arrestin-3 enhanced the agonist-induced internalization of the rFHSR, and combinations of GRKs and arrestin-3 were more effective than the individual components. To characterize the involvement of endogenous GRKs on phosphorylation and internalization, we inhibited endogenous GRK2 by overexpression of a kinase-deficient mutant of GRK2 or G alpha t, a scavenger of G betagamma. We also inhibited endogenous GRK6 by overexpression of a kinase-deficient mutant of GKR6. All three constructs were effective inhibitors of phosphorylation, but only the kinase-deficient mutant of GRK2 and G alpha t inhibited internalization. The inhibition of internalization induced by these two constructs was less pronounced than that induced by a dominant-negative mutant of the nonvisual arrrestins, however. The finding that inhibitors of GRK2 and GRK6 impair phosphorylation, but only the inhibitors of GRK2 impair internalization, suggests that different GRKs have differential effects on receptor internalization.

Structural basis of G protein-coupled receptor function

The vast majority of extracellular signaling molecules, like hormones and neurotransmitters, interact with a class of membranous receptors characterized by a uniform molecular architecture of seven transmembrane alpha-helices linked by extra- and intracelluar peptide loops. In a reversible manner, binding of diverse agonists to heptahelical receptors leads to activation of a limited repertoire of heterotrimeric guanine nucleotide-binding proteins (G proteins) forwarding the signal to intracellular effectors such as enzymes and ion channels. Proper functioning of a G protein-coupled receptor is based on a complex interplay of structural determinants which are ultimately responsible for receptor folding, trafficking and transmembrane signaling. Applying novel biochemical and molecular biological methods interesting insights into receptor structure/function relationships became available. These studies have a significant impact on our understanding of the molecular basis of human diseases and may eventually lead to novel therapeutic strategies.

Molecular basis of receptor/G-protein-coupling selectivity

Molecular cloning studies have shown that G-protein-coupled receptors form one of the largest protein families found in nature, and it is estimated that approximately 1000 different such receptors exist in mammals. Characteristically, when activated by the appropriate ligand, an individual receptor can recognize and activate only a limited set of the many structurally closely related heterotrimeric G-proteins expressed within a cell. To understand how this selectivity is achieved at a molecular level has become the focus of an ever increasing number of laboratories. This review provides an overview of recent structural, molecular genetic, biochemical, and biophysical studies that have led to novel insights into the molecular mechanisms governing receptor-mediated G-protein activation and receptor/G-protein coupling selectivity.

Mechanisms of desensitization of follicle-stimulating hormone (FSH) action in a murine granulosa cell line stably transfected with the human FSH receptor complementary deoxyribonucleic acid

The desensitization of follicle-stimulating hormone (FSH)-evoked cAMP synthesis occurs upon continuous or repeated hormonal stimulation, and it involves the hormone-receptor interaction and post-receptor events. These mechanisms were studied in a murine granulosa cell line (KK-1) stably transfected with the human FSH receptor (hFSHR) complementary deoxyribonucleic acid (cDNA) under a powerful viral promoter. Hence, the FSHR transcriptional regulation was eliminated from the experimental model. Stimulation of the cells with recombinant human FSH (rhFSH) or a phorbol ester, 12-O-tetradecanoylphorbol-13 acetate (TPA), resulted in clear desensitization, i.e. subsequent rhFSH-stimulated cAMP formation was 73.4 +/-2.2%, (P < 0.001) and 66.3 +/-3.4%, (P < 0.0001), respectively, of that of cells preincubated in medium. TPA prestimulation evoked also clear inhibition (65-74% of control) of rhFSH or forskolin (a non-specific activator of adenylate cyclase) induced progesterone production. The suppression by TPA preincubation of the rhFSH-induced cAMP synthesis was completely abolished by the protein kinase C (PKC) inhibitor staurosporine (STR). Preincubation with STR exhibited a significant (P < 0.0001) increasing effect on the rhFSH-stimulated cAMP accumulation. The specific involvement of PKC was further evidenced by other inhibitors, all of them exerted significant elevation of cAMP synthesis following rhFSH restimulation. Furthermore, only the PKC beta isoform appeared to be constitutively expressed in these cells during desensitization. Prestimulation of the G-protein activity by sodium fluoride (NaF) or cholera toxin (CT), followed by rhFSH challenge, accounted for a decrease in the cAMP-mediated responsiveness, down to 69.4 +/- 2.8 or 74.2 +/- 1.9%, of control (P < 0.001), respectively, indicating that the post-receptor events are critical for desensitization. [125I]iodo-rhFSH binding to the cells did not change significantly during desensitization and the different stimulations. In contrast, approximately 50% increase (P < 0.001) occurred in the steady-state levels of FSHR mRNA in the cells stimulated with FSH. This was apparently due to prolonged half-time of mRNA, and not to altered transcription, since the FSHR cDNA was driven by a powerful viral promoter. In accordance, the cells transfected with Simian Virus (SV40) promoter-driven luciferase gene did not display alterations in luciferase activity following stimulatory treatments. The effects of the post-receptor stimulations (NaF or CT) on [125I]iodo-rhFSH binding were minor (8-12% reduction). Taken together, these data provide evidence that the agonist-responsive hFSHR desensitization appears through a PKC-beta isoform-mediated modulation of cAMP production. The desensitization of FSH action involves modifications of functional properties of the existing components of the FSH signal transduction complex, and does not require concomitant suppression of transcription or translation of the FSHR gene.

Signaling and phosphorylation-impaired mutants of the rat follitropin receptor reveal an activation- and phosphorylation-independent but arrestin-dependent pathway for internalization

We have previously shown that the rat follitropin receptor (rFSHR) expressed in transfected cells becomes phosphorylated upon stimulation of the cells with agonist or a phorbol ester. Peptide mapping and mutagenesis studies have also shown that the agonist- or phorbol ester-induced phosphorylation of the rFSHR maps to Ser/Thr residues present in the first and third intracellular loops. The experiments presented herein were initially designed to test for the presence of additional phosphorylation sites on the second intracellular loop of the rFSHR. Analysis of two new mutants in which the two threonines in the second intracellular loop (rFSHR-2L) or the two threonines in the second intracellular loop and the seven Ser/Thr residues in the third intracellular loop (rFSHR-2L + 3L) were mutated showed that one or more of the two threonines in the second intracellular loop are phosphorylated in response to phorbol ester, but not in response to agonist stimulation. Since rFSHR-2L and rFSHR-2L + 3L displayed a reduction in agonist-induced signaling, two additional mutants (rFSHR-D389N and rFSHR-Y530F) were constructed in an attempt to better understand the relationship between the agonist-induced activation, phosphorylation, and internalization of the rFSHR. These point mutations impaired agonist-stimulated signal transduction and abolished agonist-induced phosphorylation. Co-transfection studies revealed that the phosphorylation of these mutants can be rescued by overexpression of G protein-coupled receptor kinase 2, but this increased phosphorylation only rescues the internalization of rFSHR-D389N. The internalization of both mutants could be rescued by overexpression of arrestin-3, however. Taken together, these results argue that the agonist-induced activation and phosphorylation of the rFSHR are not essential for internalization. while the interaction of the rFSHR with a nonvisual arrestin is essential for internalization.

Gonadotropin receptors and the control of gonadal steroidogenesis: physiology and pathology

Over the past few years, knowledge of the structure of gonadotropin receptors and their mode of action has rapidly advanced. The cDNA corresponding to the luteinizeng hormone (LH) receptor (LHR) has been cloned, leading to the identification of a novel family of G-protein-coupled receptors. The follicle stimulating hormone (FSH) receptor (FSHR) was thereafter cloned by cross-hybridization with the LHR. Structure-function relationships have been studied by mutagenesis experiments in several laboratories. The cloning and chromosomal localization to chromosome 2p21 of the two human gonadotropin receptor genes has provided insights into their evolutionary relationships. The LHR and FSHR genes are very large and contain 10 and 11 exons respectively. The obtention of monoclonal antibodies against the receptors resulted in the characterization of the receptor proteins. These antibodies also allowed the study of receptor expression in target cells in physiological and pathological conditions. The internalization of the LHR has been studied by electron microscopy. A mechanism of receptor-mediated transcytosis through the endothelial cells of the testes has been described for the LHR. The polarized expression of receptors has been studied. The cloning of gonadotropin receptor genes has opened the field of genetic study of the receptors. Inactivating mutations of the LHR have been described in Leydig cell agenesis or hypoplasia. Different phenotypes, including complete pseudohermaphroditism, ambiguous genitalia and male phenotype, have been described. In the case of the FSHR, only one mutation has been reported in familial ovarian dysgenesis with primary amenorrhea. Related males have variable alterations of spermatogenesis and fertility. Constitutive mutations of the LHR have been reported in familial testotoxicosis. One similar mutation has also been described for the FSHR. Such mutations may lead to the development of a model of receptor activation.

The agonist-induced phosphorylation of the rat follitropin receptor maps to the first and third intracellular loops

Previous results from this laboratory have shown that the rat FSH receptor (rFSHR) becomes phosphorylated on S/T residues upon stimulation of transfected cells with human (h)FSH and that a truncation of the C-terminal tail that removes 12 of the 25 intracellular S/T residues does not affect phosphorylation. Based on the results of phosphopeptide-mapping experiments we analyzed three new mutants. rFSHR-1L and rFSHR-3L were constructed by mutating the S/T residues in the first intracellular loop or the third intracellular loop, respectively. rFSHR-(3L+CT) was constructed by mutating all the S/T residues in the third loop as well as S624, the only C-terminal tail residue that was not previously eliminated as a potential phosphorylation site. All mutants were biologically active. The agonist-induced phosphorylation of rFSHR-3L and rFSHR-(3L+CT) were partially reduced, while that of rFSHR-1L was almost completely lost. The agonist-induced uncoupling of rFSHR-1L and rFSHR-3L are retarded to about the same extent, while the agonist-induced internalization is retarded only in rFSHR-1L. Four major conclusions can be made from the present studies: 1) the phosphorylated rFSHR is a common molecular intermediate in agonist-induced uncoupling and internalization; 2) agonist-induced phosphorylation of the rFSHR maps to the first and third intracellular loops; 3) the phosphorylation of the third intracellular loop facilitates agonist-induced uncoupling but is not necessary for agonist-induced internalization; 4) agonist-induced internalization is facilitated by phosphorylation but it is not known if only the first loop, only the third loop, or both the first and third loops need to be phosphorylated for this response.

Structure-function relationship of different domains of the rat corticotropin-releasing factor receptor

The significance of different domains of corticotropin-releasing factor receptor, type 1, (CRFR1) for ligand binding and cAMP accumulation was investigated with C-terminally truncated forms of rat CRFR1 (rCRFR1) tagged by a sequence of six histidine residues (His-tag) to facilitate protein purification and identification. These different forms of the receptor were N-glycosylated and transported properly to the membranes of transfected mammalian cells as indicated by Western blot analysis and immunocytochemical staining with two polyclonal antibodies developed against the N- and C-terminus of rCRFR1. The N-terminal fragment, rCRFR1(23-121), expressed in Escherichia coli bound oCRF specifically, but with low affinity. Several mutants lacking transmembrane domain (TM) 7 and the C-terminus exhibited similarly low affinities to oCRF after expression in transfected mammalian cells. None of these cells produced significant amounts of cAMP after exposure to oCRF. Only mutants containing the N-terminus, all loops and TMs bound oCRF and produced cAMP with high affinity (Kd = 62 nM) and efficacy (EC50 = 0.8 nM). The additional presence of the C-terminus provided similar characteristics (Kd = 5 nM, EC50 = 0.3 nM) as known for the native receptor. It is suggested on the basis of these data that the last extracellular loop is involved in ligand binding.

Cloning and functional expression of the luteinizing hormone receptor complementary deoxyribonucleic acid from the marmoset monkey testis: absence of sequences encoding exon 10 in other species

Based on sequence homologies among the human, porcine, rat, and mouse genes for the LH receptor (LHR), overlapping partial fragments of LHR complementary DNAs (cDNAs) were multiplied from marmoset monkey testicular RNA using reverse transcription-PCR. Ligations of the individual cDNA fragments generated a full-length monkey LHR cDNA (2031 bp) containing the complete amino acid-coding sequence (676 amino acids). Northern hybridization analysis of monkey testicular RNA, using a complementary RNA probe corresponding to the full-length cDNA, demonstrated major transcripts of 5.5 and 1.4 kilobases and minor ones of 4.0, 2.7, and 1.9 kilobases. Sequence analysis of the monkey LHR cDNA revealed a striking feature, i.e. the absence of an 81-bp nucleotide sequence corresponding to exon 10, present in the LHR cDNAs of all other species studied to date. The monkey LHR cDNA displayed 83-94% overall sequence homology with the other mammalian LHR cDNAs. Reverse transcription-PCR with human exon 10-specific primers demonstrated the total absence of this sequence from the monkey LHR messenger RNA. Southern hybridization of monkey genomic DNA using a human exon 10 probe demonstrated its presence in the monkey gene and that it is totally spliced out from the primary transcript. COS cells transfected with the monkey LHR cDNA showed similar high affinity (Kd = 0.25 nmol/liter) of [125I]iodo-hCG binding as those transfected with human LHR cDNA (Kd = 0.20 nmol/liter). The cells expressing the recombinant monkey and human LHR displayed similar responses of extracellular cAMP and inositol trisphosphate to hCG. In conclusion, marmoset monkey LHR seems to lack the sequence corresponding to exon 10 of the LHR gene in other mammalian species. The truncation does not alter LHR function, as the monkey receptor protein bound hCG and evoked cAMP and inositol trisphosphate responses comparable to those of the human LHR containing the exon 10-encoded structure. As the sequence homologous to exon 10 is missing in the other two glycoprotein receptors, i.e. those of FSH and TSH, this extra exon is apparently inserted into the LHR messenger RNA of some species during evolution from intronic sequences by a change in alternative splicing.

Modulation of FSH receptor phosphorylation correlates with hormone-induced coupling to the adenylate cyclase system

The authors have recently demonstrated that an inhibitor of protein phosphorylation, staurosporine (SSP), can dramatically enhance follicle-stimulating hormone (FSH) stimulated cyclic adenosine monophosphate (cAMP) accumulation in rat granulosa cell line (GFSHR-17) overexpressing about 20-fold FSH receptor than primary granulosa cells. Moreover, incubation with SSP can partially release the cells from FSH-induced desensitization. In this work, it was examined whether coupling of FSH receptor to the adenylate cyclase is correlated with the degree of receptor phosphorylation. Immunoprecipitation of FSH receptor after metabolic labeling of the cells with 32P-orthophosphate revealed that preincubation of the cells with SSP resulted in pronounced reduction in FSH receptor phosphorylation compared to control cells, concomitantly with a dramatic increase in FSH-stimulated cAMP accumulation. In contrast, incubation of the cells with saturating dose of FSH, which leads to uncoupling between the receptor and the adenylate cyclase, resulted in enhanced receptor phosphorylation. Moreover, cells preincubated with FSH could be released from desensitization by further incubation with SSP and a significant reduction in FSH receptor phosphorylation. Immunostaining of the cells with FSH receptor antibody reveal a homogenous distribution of the receptor on the surface of SSP-treated cells. Some aggregation of the receptor was evident in control cells that were not treated with SSP. In contrast, massive clustering and capping of the receptor molecules were observed on the surface of FSH-stimulated cells. The current data suggest that phosphorylation-dephosphorylation of the receptor molecules play an important role in the degree of coupling between the receptor and the adenylate cyclase system. Moreover, desensitization to FSH stimulation that is implicated with high degree of receptor phosphorylation may lead to aggregation of the receptor molecules on the cell surface.

A mutation in the follicle-stimulating hormone receptor occurs frequently in human ovarian sex cord tumors

A subset of ovarian tumors, referred to as sex cord-stromal tumors, produce endocrine manifestations due to the secretion of estrogens or androgens. Because gonadotropins induce the growth, differentiation, and function of the steroid-producing cells of the ovary, we hypothesized that mutations in the FSH receptor (FSH-R) might occur in this group of tumors. Ovarian sex cord tumors (n = 13), small cell carcinomas of the ovary (n = 3), and control DNA specimens (n = 116) were screened for mutations in the transmembrane domains of the FSH-R. A heterozygous T-->C mutation was found at nucleotide 1777 that converts codon 591 from phenylalanine to serine (F591S). This sixth transmembrane domain mutation was found in 9 of 13 (69%) sex cord tumors and 2 of 3 ovarian small cell carcinomas, but it was not present in control specimens, including 5 normal ovaries, 5 nonsex cord ovarian tumors, 16 thyroid tumors, or 90 specimens of peripheral blood leukocyte DNA, suggesting that this nucleotide change is not a polymorphism. The functional effects of identified mutations were assessed by expression of the wild-type or the F591S mutant FSH-R in COS-7 cells. The F591S mutation eliminated FSH-stimulated cAMP production, and a similar effect was observed when this mutation was introduced into the homologous location of the LH receptor. The high prevalence of the F591S mutation in the FSH-R suggests that it plays a role in the development of ovarian sex cord tumors.

Functional consequences of the phosphorylation of the gonadotropin receptors

When target cells are exposed to a hormone, their responsiveness wanes with time, in spite of the continuous presence of the hormone. This phenomenon, referred to as desensitization, is due to regulatory steps that occur at the level of the hormone receptor as well as at post-receptor steps. While post-receptor events may be specific for the metabolic pathways activated in a given target cell, receptor events are more general in nature and conserved within a given family of receptors. There are at least two categories of regulatory events that occur at the level of the receptor and contribute to the process of desensitization. One of them, referred to as receptor uncoupling, will be used here to denote a change in the functional properties of a constant number of receptors resulting in a reduction in the ability of the receptor to activate its most proximal effector system. The other, referred to as receptor down-regulation, will be used here to denote a reduction in the density of cell surface receptors. Uncoupling is generally faster than down-regulation, and is believed to be due to post-translational modifications of the receptor. Down-regulation, on the other hand, is slower, and could be due to an increase in the rate of receptor internalization, sequestration or degradation, and/or to a decrease in the rate of receptor externalization, processing or synthesis. In this paper, recent studies from my laboratory designed to directly address the potential involvement of gonadotropin receptor phosphorylation in the process of uncoupling have been reviewed.