Phosphorylation of the lutropin/choriogonadotropin receptor facilitates uncoupling of the receptor from adenylyl cyclase and endocytosis of the bound hormone

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.

Constitutively Activating Mutants of Equine LH/CGR Constitutively Induce Signal Transduction and Inactivating Mutations Impair Biological Activity and Cell-Surface Receptor Loss In Vitro

The signal transduction of the equine lutropin/choriogonadotropin receptor (eLH/CGR) is unclear in naturally occurring activating/inactivating mutants of this receptor, which plays an important role in reproductive physiology. We undertook the present study to determine whether conserved structurally related mutations in eLH/CGR exhibit similar mechanisms of signal transduction. We constructed four constitutively activating mutants (M398T, L457R, D564G, and D578Y) and three inactivating mutants (D405N, R464H, and Y546F); measured cyclic adenosine monophosphate (cAMP) accumulation via homogeneous time-resolved fluorescence assays in Chinese hamster ovary cells; and investigated cell-surface receptor loss using an enzyme-linked immunosorbent assay in human embryonic kidney 293 cells. The eLH/CGR-L457R-, -D564G-, and -D578Y-expressing cells exhibited 16.9-, 16.4-, and 11.2-fold increases in basal cAMP response, respectively. The eLH/CGR-D405N- and R464H-expressing cells presented a completely impaired signal transduction, whereas the Y546F-expressing cells exhibited a small increase in cAMP response. The cell-surface receptor loss was 1.4- to 2.4-fold greater in the activating-mutant-expressing cells than in wild-type eLH/CGR-expressing cells, but was completely impaired in the D405N- and Y546F-expressing cells, despite treatment with a high concentration of agonist. In summary, the state of activation of eLH/CGR influenced agonist-induced cell-surface receptor loss, which was directly related to the signal transduction of constitutively activating mutants.

Constitutive Activation and Inactivation of Mutations Inducing Cell Surface Loss of Receptor and Impairing of Signal Transduction of Agonist-Stimulated Eel Follicle-Stimulating Hormone Receptor

In the present study, we investigated the signal transduction of mutants of the eel follicle-stimulating hormone receptor (eelFSHR). Specifically, we examined the constitutively activating mutant D540G in the third intracellular loop, and four inactivating mutants (A193V, N195I, R546C, and A548V). To directly assess functional effects, we conducted site-directed mutagenesis to generate mutant receptors. We measured cyclic adenosine monophosphate (cAMP) accumulation via homogeneous time-resolved fluorescence assays in Chinese hamster ovary (CHO-K1) cells and investigated cell surface receptor loss using an enzyme-linked immunosorbent assay in human embryonic kidney (HEK) 293 cells. The cells expressing eelFSHR-D540G exhibited a 23-fold increase in the basal cAMP response without agonist treatment. The cells expressing A193V, N195I, and A548V mutants had completely impaired signal transduction, whereas those expressing the R546C mutant exhibited little increase in cAMP responsiveness and a small increase in signal transduction. Cell surface receptor loss in the cells expressing inactivating mutants A193V, R546C, and A548V was clearly slower than in the cell expressing the wild-type eelFSHR. However, cell surface receptor loss in the cells expressing inactivating mutant N195I decreased in a similar manner to that of the cells expressing the wild-type eelFSHR or the activating mutant D540G, despite the completely impaired cAMP response. These results provide important information regarding the structure–function relationships of G protein-coupled receptors during signal transduction.

Molecular cloning of two gonadotropin receptors in mummichog Fundulus heteroclitus and their gene expression during follicular development and maturation

Two cDNAs encoding gonadotropin receptors, follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR) were cloned from mummichog (Fundulus heteroclitus) ovary. Deduced amino acid sequences of the mummichog FSHR (fhFSHR) and LHR (fhLHR) showed high homologies to teleost FSHRs (77-53%) and teleost LHRs (76-62%), respectively. Both the fhFSHR and fhLHR are composed of a typical structural architecture of glycoprotein hormone receptors consisting of the large N-terminal extracellular domain, the transmembrane domain containing seven cell surface membrane-spanning regions, and the intracellular domain. Functional analysis using HEK293 cells stably expressing the fhFSHR or fhLHR demonstrated that both the receptors are specifically activated by mummichog FSH or LH, respectively. Reverse transcription-polymerase chain reaction revealed that both the fhFSHR and fhLHR were expressed in the ovary, testis, and pituitary, and the fhLHR was also expressed in several extra-gonadal tissues. Real-time quantitative-PCR analysis revealed that the fhFSHR gene was abundantly expressed in developing follicles whereas expression of the fhLHR gene markedly increased in follicles of the final maturational stage. These results indicate that gonadotropin stimulation on follicles is regulated by the two distinct pathways via their cognate receptors.

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.

Location, location, location...site-specific GPCR phosphorylation offers a mechanism for cell-type-specific signalling

It is now established that most of the approximately 800 G-protein-coupled receptors (GPCRs) are regulated by phosphorylation in a process that results in the recruitment of arrestins, leading to receptor desensitization and the activation of arrestin-dependent processes. This generalized view of GPCR regulation, however, does not provide an adequate mechanism for the control of tissue-specific GPCR signalling. Here, we review the evidence that GPCR phosphorylation is, in fact, a flexible and dynamic regulatory process in which GPCRs are phosphorylated in a unique manner that is associated with the cell type in which the receptor is expressed. In this scenario, phosphorylation offers a mechanism of regulating the signalling outcome of GPCRs that can be tailored to meet a specific physiological role.

Regulation of luteinizing hormone/human chorionic gonadotropin receptor expression: a perspective

The LH/hCG receptor, a member of the G protein coupled receptor family mediates the cellular actions of LH in the ovary. A considerable amount of information regarding its structure, mechanism of activation, and regulation of expression has emerged in recent years. Here we provide a brief overview of the current information on the structural organization of the receptor and the mechanism of receptor mediated signaling as well as an in-depth discussion on recent developments pertaining to the regulation of receptor expression. Specifically, we describe studies from our laboratory showing that the posttranscriptional regulation of the receptor involves an LH/hCG receptor mRNA-binding protein. We also propose a model to explain the loss of steady-state LH/hCG receptor mRNA levels during receptor down-regulation.

Membrane organization of luteinizing hormone receptors differs between actively signaling and desensitized receptors

Signaling by the luteinizing hormone/choriogonadotropin receptor (LHR) is of considerable interest because of its requirement for successful reproduction. Time-resolved phosphorescence anisotropy and fluorescence resonance energy transfer were used to investigate the organization of endogenous LHRs in porcine follicular membranes in two distinct signaling states, active and desensitized. Desensitized LHRs exhibited approximately 3-fold slower rotational correlation times compared with active LHRs (59 +/- 4 and 21 +/- 9 mus, respectively), suggesting that with agonist-dependent desensitization the receptors are organized into larger protein complexes. Incubation of membranes with inhibitors of LHR desensitization, such as neutralizing anti-arrestin antibodies, a synthetic peptide corresponding to the third intracellular loop of the LHR but not the corresponding scrambled peptide, or catalytically inactive ARNO, resulted in faster rotational diffusion times equivalent to those of actively signaling LHRs. Furthermore, desensitized LHRs exhibited a 2.4-fold increase in fluorescence resonance energy transfer between LHRs suggesting that the larger protein aggregates formed during desensitization contain more self-associated LHRs. These results indicate that agonist-dependent LHR desensitization precedes organization of LHRs at the cells surface into larger protein aggregates.

Identification of a transferable two-amino-acid motif (GT) present in the C-terminal tail of the human lutropin receptor that redirects internalized G protein-coupled receptors from a degradation to a recycling pathway

Although highly homologous in amino acid sequence, the agonist-receptor complexes formed by the human lutropin receptor (hLHR) and rat (r) LHR follow different intracellular routes. The agonist-rLHR complex is routed mostly to a lysosomal degradation pathway whereas a substantial portion of the agonist-hLHR complex is routed to a recycling pathway. In a previous study, we showed that grafting a five-residue sequence (GTALL) present in the C-terminal tail of the hLHR into the equivalent position of the rLHR redirects a substantial portion of the internalized agonist-rLHR complex to a recycling pathway. Using a number of mutations of the GTALL motif, we now show that only the first two residues (GT) of this motif are necessary and sufficient to induce recycling of the internalized agonist-rLHR complex. Phosphoamino acid analysis and mutations of the GT motif show that phosphorylation of the threonine residue is not necessary for recycling. Lastly, we show that addition of portions of the C-terminal tail of the hLHR that include the GT motif to the C-terminal tails of the rat follitropin or murine delta-opioid receptors promotes the post-endocytotic recycling of these G protein-coupled receptors.We conclude that the GT motif present in the C-terminal tail of the hLHR is a transferable motif that promotes the postendocytotic recycling of several G protein-coupled receptors and that the GT-induced recycling does not require the phosphorylation of the threonine residue.

Specificity of the cyclic adenosine 3',5'-monophosphate signal in granulosa cell function

Cyclic AMP signaling is involved in most aspects of differentiation and maturation of the granulosa cells in the ovarian follicle. As the genetic programs activated at different stages of follicle growth maturation are being elucidated, it is becoming increasingly difficult to reconcile the simplicity of the cAMP cascade with the complexity and the divergent patterns of gene expression activated in these cells. To account for these divergent outcomes of the cAMP signal, three aspects of this signaling cascade in granulosa cells will be reviewed. We will discuss the evidence for gonadotropin receptors coupling to different G proteins and effectors. Next, we will explore the possibility that the temporal and spatial dimensions of the cAMP signal itself may contribute to the diverse outcomes. Finally, we will summarize available data showing that the cAMP signal is distributed through several cascades of kinase activation. It is hoped this compendium will provide a framework with which to understand how the initial signals activated by gonadotropins control the complex patterns of gene expression that are required for follicle maturation and ovulation.

ARF6: a newly appreciated player in G protein-coupled receptor desensitization

The luteinizing hormone/choriogonadotropin hormone receptor (LH/CG R) signals to regulate ovulation, corpus luteum formation, and fetal survival during pregnancy. Agonist binding to the LH/CG R is poorly reversible, emphasizing the importance of a cellular mechanism to temper signaling by a potentially persistently active receptor. Like other G protein-coupled receptors (GPCRs), signaling by this receptor is modulated by its binding of an arrestin. We have identified ADP ribosylation factor 6 (ARF6) as a protein whose activation state is regulated by the LH/CG R and which functions to regulate the availability of plasma membrane-docked arrestin 2 to this receptor. We hypothesize that ARF6 might also serve GPCRs other than the LH/CG R to regulate the availability of arrestin 2 for receptor desensitization.

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

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

The lutropin/choriogonadotropin receptor, a 2002 perspective

Reproduction cannot take place without the proper functioning of the lutropin/choriogonadotropin receptor (LHR). When the LHR does not work properly, ovulation does not occur in females and Leydig cells do not develop normally in the male. Also, because the LHR is essential for sustaining the elevated levels of progesterone needed to maintain pregnancy during the first trimester, disruptions in the functions of the LHR during pregnancy have catastrophic consequences. As such, a full understanding of the biology of the LHR is essential to the survival of our species. In this review we summarize our current knowledge of the structure, functions, and regulation of this important receptor.

MA-10 cells transfected with the human lutropin/choriogonadotropin receptor (hLHR): a novel experimental paradigm to study the functional properties of the hLHR

MA-10 cells are a clonal strain of mouse Leydig tumor cells that retain many of the properties of Leydig cells including expression of the endogenous lutropin/choriogonadotropin receptor (LHR) and the ability to respond to LH/CG with increased steroidogenesis. Recently we noted a dramatic decrease in expression of the endogenous LHR. Although we do not have an explanation for this decline, we took advantage of it to devise a method that allows for the expression of the recombinant human LHR (hLHR) in a Leydig cell model that is now practically devoid of endogenous LHR. We show that the recombinant hLHR can be expressed at variable densities in MA-10 cells and that it can stimulate cAMP and steroid synthesis as well as activate the inositol phosphate and MAPK cascades. We also show that two naturally occurring mutants of the hLHR associated with Leydig cell hyperplasia and one mutant associated with Leydig cell adenomas are constitutively active when assayed for activation of cAMP, inositol phosphate, progesterone, and MAPK. Our ability to express the hLHR in MA-10 cells (now practically devoid of endogenous LHR) provides a novel paradigm to study the cellular and molecular basis of the functions of the LHR in Leydig cells.

Identification of two distinct structural motifs that, when added to the C-terminal tail of the rat LH receptor, redirect the internalized hormone-receptor complex from a degradation to a recycling pathway

We show that most of the internalized rat LH receptor is routed to a lysosomal degradation pathway whereas a substantial portion of the human LH receptor is routed to a recycling pathway. Chimeras of these two receptors identified a linear amino acid sequence (GTALL) present near the C terminus of the human LH receptor that, when grafted onto the rat LH receptor, redirects most of the rat LH receptor to a recycling pathway. Removal of the GTALL sequence from the human LH receptor failed to affect its routing, however. The GTALL sequence shows homology with the C-terminal tetrapeptide (DSLL) of the beta2-adrenergic receptor, a motif that has been reported to mediate the recycling of the internalized beta2-adrenergic receptor by binding to ezrin-radixin-moesin-binding phosphoprotein-50. Addition of the DSLL tetrapeptide to the C terminus of the rat LH receptor also redirects most of the internalized rat LH receptor to a recycling pathway but, like the recycling of the human LH receptor, this rerouting is not mediated by ezrin-radixin-moesin-binding phosphoprotein-50. We conclude that most of the internalized rat LH receptor is degraded because its C-terminal tail lacks motifs that promote recycling and that two distinct, but homologous, motifs (DSLL at the C terminus or GTALL near the C terminus) can reroute the internalized rat LH receptor to a recycling pathway that is independent of ezrin-radixin-moesin-binding phosphoprotein-50.

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

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

The C-terminal tail of the rat lutropin/choriogonadotropin (CG) receptor independently modulates human (h)CG-induced internalization of the cell surface receptor and the lysosomal targeting of the internalized hCG-receptor complex

The analysis of 21 progressive truncations of the C-terminal tail of the rat LH/CG receptor (rLHR) revealed the presence of a region delineated by residues 628-649 that, when removed, enhanced the degradation of the internalized human (h)CG. The analysis of these truncations also revealed the presence of a region delineated by residues 624-631 that, when removed, enhanced the rate of internalization of hCG. Since there is little overlap between these two regions, we conclude that the structural features of the rLHR that mediate internalization and degradation of the internalized hormone are different. Detailed analyses of cells expressing a truncation at Y637 (designated rLHR-t637) showed that the enhanced degradation of hCG observed in the these cells is due to an increase in the rate of transfer of the internalized hCG-rLHR complex from the endosomes to the lysosomes rather than to the enhanced dissociation of the hCG-rLHR complex in the lysosomes.

Seven non-contiguous intracellular residues of the lutropin/choriogonadotropin receptor dictate the rate of agonist-induced internalization and its sensitivity to non-visual arrestins

The amino acid sequences of the human (h) and rat (r) lutropin/choriogonadotropin receptors (LHR) are 87% identical, but the rate of agonist-induced internalization of the hLHR is approximately 7 times faster than that of the rLHR. Chimeras of the hLHR and the rLHR showed that this rate is dictated by the serpentine domain and the cytoplasmic tail. Further mutational analysis identified seven residues, two adjacent residues in the second intracellular loop (Val/Gln in the rLHR and Ile/His in the hLHR), four non-contiguous residues in the third intracellular loop (Arg/Gln/Thr/Pro in the rLHR and Lys/Arg/Met/Thr in the hLHR), and one in the C-terminal tail (Leu in the rLHR and Phe in the hLHR), that are necessary and sufficient to impart the slow rate of internalization of the rLHR and the fast rate of internalization of the hLHR. The internalization of the rLHR and the hLHR display different sensitivities to the non-visual arrestins. Therefore, we also tested if the simultaneous exchange of these seven residues resulted in the exchange of this property. Since this was found to be the case, we propose that these seven residues identified here form a non-visual arrestin-binding site.

The rate of internalization of the gonadotropin receptors is greatly affected by the origin of the extracellular domain

Previous results from this laboratory have shown that human kidney (293) cells transfected with the rat follitropin receptor (rFSHR) internalize agonist (i.e. human follitropin, hFSH) at a rate similar to that of other agonist-G protein-coupled receptor complexes while 293 cells transfected with the rat lutropin/choriogonadotropin receptor (rLHR) internalize agonist (human choriogonadotropin, hCG) at a rate that is about 1 order of magnitude slower. Taking advantage of this difference and the high degree of homology between the rLHR and rFSHR, we have now used chimeras of these two receptors to begin to delineate structural features that influence their internalization. Analysis of six chimeras that exchanged only the transmembrane domains (designated FLF and LFL), only the COOH-terminal domains (FFL or LLF) or both domains (FLL or LFF) show that the origin of the extracellular domain is at least as important, if not more, than the origin of the transmembrane and COOH-terminal domains in determining the rate of internalization of the gonadotropin receptors. Thus, the rates of internalization of agonist internalization mediated by FFL, FLF, and FLL more closely resemble rFSHR than rLHR, while the rates of agonist internalization mediated by LLF, LFL, and LFF more closely resemble rLHR than rFSHR. The importance of the extracellular domain was also evident even upon overexpression of arrestin-3, a protein that enhances the rate of internalization of the wild-type receptors and chimeras by binding to their intracellular regions.

Inhibition of rat mammary tumorigenesis by human chorionic gonadotropin associated with increased expression of inhibin

This work was designed to test whether the suppression of 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary carcinomas by human chorionic gonadotropin (hCG) is associated with the synthesis of inhibin. For this purpose, virgin rats received 8 mg DMBA/100 g body weight; 20 d later they were injected daily with 100 IU of hCG for 40 d (DMBA+hCG group). Age-matched untreated (control), hCG-, and DMBA+saline-treated rats were used for comparisons. Mammary tissues were collected for histopathological and mRNA analyses after 5, 10, 20, and 40 d of hCG injection and 20 d after treatment. None of the animals in the control and hCG-treated groups developed mammary tumors. DMBA-treated rats developed a high incidence of both microscopic lesions, i.e., intraductal proliferations and ductal carcinomas in situ, and palpable tumors. In DMBA+hCG-treated rats, the incidence of microscopic and palpable tumors was markedly reduced. In these animals, alpha- and beta-inhibin immunoreactivity was elevated in the non-tumoral mammary glands in association with lobule formation and in the tumors. Inhibin A and B mRNAs were also elevated in the mammary tissue, and c-myc and c-jun were induced by the hormonal treatment. DMBA alone did not modify the expression of these genes. Our findings indicate that inhibin production and gene activation are associated with both mammary gland differentiation and tumor regression.

Role of the rate of internalization of the agonist-receptor complex on the agonist-induced down-regulation of the lutropin/choriogonadotropin receptor

The extent of agonist-induced down-regulation of the LH/CG receptor (LHR) in human kidney 293 cells transfected with the rat LHR (rLHR) is much lower than in two Leydig tumor cell lines (MA-10 and R2C) that express the rodent LHR endogenously. This difference can not be attributed to differences in the recycling of internalized receptors, or in the replenishment of new receptors at the cell surface. It can be correlated, however, with the half-life of internalization of the bound agonist, which is approximately 60 min in Leydig tumor cells and about 100 min in transfected 293 cells. To determine whether the rate of internalization of the bound agonist affects down-regulation, we compared these two parameters in 293 cells expressing four rLHR mutants that enhance internalization and three mutants that impair internalization. We show that all four mutations of the rLHR that enhanced internalization enhanced down-regulation, while only one of the three mutations that impaired internalization impaired down-regulation. In addition, cotransfections of 293 cells with the rLHR-wt and three constructs that enhanced internalization (G protein-coupled receptor kinase 2, beta-arrestin, and arrestin-3) increased down-regulation, while a related construct (visual arrestin) that had no effect on internalization also had no effect on down-regulation. We conclude that the rate of internalization of the agonist-LHR complex is the main determinant of the extent of down-regulation of the LHR.

G-protein coupled receptor kinases as modulators of G-protein signalling

G-protein coupled receptors (GPCRs) comprise one of the largest classes of signalling molecules. A wide diversity of activating ligands induce the active conformation of GPCRs and lead to signalling via heterotrimeric G-proteins and downstream effectors. In addition, a complex series of reactions participate in the 'turn-off' of GPCRs in both physiological and pharmacological settings. Some key players in the inactivation or 'desensitization' of GPCRs have been identified, whereas others remain the target of ongoing studies. G-protein coupled receptor kinases (GRKs) specifically phosphorylate activated GPCRs and initiate homologous desensitization. Uncoupling proteins, such as members of the arrestin family, bind to the phosphorylated and activated GPCRs and cause desensitization by precluding further interactions of the GPCRs and G-proteins. Adaptor proteins, including arrestins, and endocytic machinery participate in the internalization of GPCRs away from their normal signalling milieu. In this review we discuss the roles of these regulatory molecules as modulators of GPCR signalling.

beta-arrestin-dependent desensitization of luteinizing hormone/choriogonadotropin receptor is prevented by a synthetic peptide corresponding to the third intracellular loop of the receptor

Desensitization is a ubiquitous response of guanine nucleotide-binding protein-coupled receptors (GPCRs) characterized by the waning of effector activity despite continued presence of agonist. Binding of an arrestin to the activated, often phosphorylated GPCR triggers desensitization. We reported for the luteinizing hormone/choriogonadotropin receptor (LH/CG R) that beta-arrestin tightly bound to porcine ovarian follicular membranes mediates agonist-dependent desensitization of LH/CG R-stimulated adenylyl cyclase (AC) activity (Mukherjee, S., Palczewski, K., Gurevich, V. V., Benovic, J. L., Banga, J. P., and Hunzicker-Dunn, M. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 493-498). We now show that addition of a synthetic peptide corresponding to the entire third intracellular loop (3i) of the LH/CG R completely and specifically reverses desensitization of AC activity, with an ED50 of 10 microM but does not modulate basal, hCG-stimulated, or forskolin-stimulated AC activities. beta-Arrestin binds selectively to the 3i peptide coupled to activated Sepharose. Desensitization of LH/CG R-stimulated AC activity is rescued when the 3i peptide is preincubated with exogenous beta-arrestin. These results show that endogenous beta-arrestin participates in cell-free desensitization of agonist-dependent LH/CG R-stimulated AC activity in follicular membranes by interacting directly with the 3i loop of the receptor, thereby preventing Gs activation.

Roles of Gi and Gq/11 in mediating desensitization of the luteinizing hormone/choriogonadotropin receptor in porcine ovarian follicular membranes

Although desensitization of most guanine nucleotide-binding (G) protein receptors is triggered by phosphorylation of the receptor, desensitization of the LH/CG receptor (-R) in porcine follicular ovarian membranes appears to be independent of LH/CG-R phosphorylation. We therefore evaluated whether desensitization of the LH/CG-R reflected a direct inhibition of adenylyl cyclase (AC) activity by either the alpha-subunit of Gi or betagamma-subunits derived from any of the membrane G proteins activated in response to LH/CG-R activation or whether desensitization reflected a competition between Gs and a G protein that activated phospholipase C for binding sites on the LH/CG-R. The results showed that follicular membrane AC activity was not inhibited upon activation of the LH/CG-R despite evidence that the ACs in follicular membranes, when maximally activated by forskolin, could be inhibited when membrane G proteins were activated by guanyl-5'-yl imidodiphosphate, and that pertussis toxin pretreatment of membranes raised forskolin-stimulated AC activity, consistent with a tonic inhibition of follicular membrane AC activity. Similarly, agonist-stimulated desensitization of LH/CG-R-stimulated AC activity was not inhibited by pertussis toxin. Therefore, desensitization is not the result of inhibition of AC mediated by an inhibitory Gi subunit. Follicular membrane AC was also not inhibited by Gbetagamma subunits freed with activation of Gs Gq/11, or G13, based on the inabilities of exogenous Gbetagamma to promote desensitization and of a protein that sequesters Gbetagamma to inhibit desensitization. Desensitization was also not inhibited by a Gq/11 C-terminal peptide or antiserum directed toward the C-terminus of Gq/11, nor was it reversed with the addition of Gbetagamma to membranes exhibiting desensitized LH/CG-R, suggesting that desensitization is independent of coupling of the LH/CG-R to Gq/11. These results indicate that agonist-dependent desensitization of LH/CG-R-stimulated AC activity is mediated by a unique mechanism.

Epidermal growth factor-induced heterologous desensitization of the luteinizing hormone/choriogonadotropin receptor in a cell-free membrane preparation is associated with the tyrosine phosphorylation of the epidermal growth factor receptor

Epidermal growth factor (EGF) attenuated hCG-stimulated adenylyl cyclase activity in rat luteal and follicular membranes. H7, an equipotent serine/threonine protein kinase inhibitor of cAMP-dependent protein kinases, cGMP-dependent protein kinases, and lipid-dependent protein kinase C, did not effect the ability of EGF to decrease hCG-responsive adenylyl cyclase activity, suggesting that a serine/threonine phosphorylation event catalyzed by these kinases was not critically involved in EGF-induced desensitization. Likewise, pertussis toxin-catalyzed ADP-ribosylation of a 40-kDa luteal membrane protein, which exhibited immunoreactivity with an antibody against Gi alpha, did not hinder the ability of EGF to attenuate hCG-stimulated adenylyl cyclase activity, indicating that Gi did not mediate EGF-induced desensitization. Rather, EGF-induced heterologous desensitization of LH/CG receptor in ovarian membranes was closely associated with the specific and prominent tyrosine phosphorylation of the 170-kDa EGF receptor. Both EGF-stimulated autophosphorylation of EGF receptor and EGF-induced LH/CG receptor desensitization were attenuated by genistein, a tyrosine kinase inhibitor. These results suggest that tyrosine phosphorylation of the 170-kDa EGF receptor is a necessary component of the signaling pathway in EGF-induced heterologous desensitization of the LH/CG receptor.

Mutations that induce constitutive activation and mutations that impair signal transduction modulate the basal and/or agonist-stimulated internalization of the Lutropin/Choriogonadotropin receptor

Previous results from this laboratory suggested that the same active conformation of the lutropin/choriogonadotropin receptor (LHR) is involved in the stimulation of G proteins and in triggering the internalization of the bound agonist. We have now analyzed two naturally occurring, constitutively active mutants of the human LHR. These mutations were introduced into the rat LHR (rLHR) and are designated L435R and D556Y. Cells expressing rLHR-D556Y bind human choriogonadotropin (hCG) with normal affinity, exhibit a 25-fold increase in basal cAMP and respond to hCG with a normal increase in cAMP accumulation. This mutation does not affect the internalization of the free receptor, but it enhances the internalization of the agonist-occupied receptors approximately 3-fold. Cells expressing rLHR-L435R also bind hCG with normal affinity, exhibit a 47-fold increase in basal cAMP, and do not respond to hCG with a further increase in cAMP accumulation. This mutation enhances the internalization of the free and agonist-occupied receptors approximately 2- and approximately 17-fold, respectively. We conclude that the state of activation of the rLHR can modulate its basal and/or agonist-stimulated internalization. Since the internalization of hCG is involved in the termination of hCG actions, we suggest that the lack of responsiveness detected in cells expressing rLHR-L435R is due to the fast rate of internalization of the bound hCG. The finding that membranes expressing rLHR-L435R (a system where internalization does not occur) respond to hCG with an increase in adenylyl cyclase activity supports this suggestion.

Surface retention of an inactivating lutropin receptor mutant in exoloop 3

The heptahelical lutropin receptor (LHR) signals primarily via the Gs-adenylyl cyclase pathway and undergoes ligand-mediated receptor desensitization and internalization. A loss-of-function rat LHR mutant was recently described in which a single amino acid residue replacement in exoloop 3, K583E, had no effect on human choriogonadotropin (hCG) binding but essentially abolished signaling. This LHR mutant is a prime candidate for which to study hCG-mediated receptor internalization since it is highly unlikely that an amino acid residue in exoloop 3 , i.e. an extracellular portion of LHR connecting transmembrane helices 6 and 7, could have any direct interaction with Galpha(s), which is located on the cytoplasmic face of the plasma membrane. A method to study endocytosis was adapted that involves concanavalin A binding to the glycoproteins on the cell surface, thus facilitating separation of the plasma membrane fraction from other cellular membrane fractions by sucrose gradient centrifugation. Conditions were used such that a single round of endocytosis could be determined with [125I]hCG. Endocytic rate constants of 0.03 and O min(-1) were obtained for LHR and the mutant, respectively, in transfected human embryonic kidney 293 cells; moreover, internalization of the mutant could not be restored by the addition of 8-Br-cAMP. Thus, the presence of the second messenger cAMP is not sufficient for internalization of ligand-occupied LHR. Rather, it appears that ligand-mediated activation and subsequent internalization of LHR results from an altered conformational state or a conformation-dependent post-ligand binding modification such as phosphorylation.

Mutation of individual serine residues in the C-terminal tail of the lutropin/choriogonadotropin receptor reveal distinct structural requirements for agonist-induced uncoupling and agonist-induced internalization

We have previously mapped the agonist-induced phosphorylation of the rat lutropin/choriogonadotropin receptor (rLHR) to a locus of four serines (Ser635, Ser639, Ser649, and Ser652) located in the C-terminal tail. The removal or mutation of this locus delays the time course of agonist-induced uncoupling of the rLHR from its effector system without affecting the overall magnitude of uncoupling, and it retards the endocytosis of the agonist-receptor complex. We have now prepared and analyzed four new rLHR mutants in which each of these serines were individually mutated to alanines. The data presented show that each mutation reduces agonist-promoted rLHR phosphorylation by 20-40%. Mutation of Ser635 or Ser639 delayed the time course of agonist-induced uncoupling to about the same extent as the simultaneous mutation of all four serines. Mutation of Ser635 or Ser639 also retarded agonist-induced internalization, but the magnitude of this decrease was less than that induced by the simultaneous mutation of all four serines. Mutation of Ser649 had no effect on agonist-induced uncoupling but retarded agonist-induced internalization to the same extent as the simultaneous mutation of all four serines. Mutation of Ser652 has little or no effect on either of these two parameters. Co-transfection studies with dominant-negative arrestins and dominant-negative dynamin reveal that, despite differences in their rates of internalization, rLHR-wild-type, rLHR-S639A, and rLHR-S649A are internalized by an arrestin- and dynamin-dependent pathway. These data show that the structural requirements needed for the agonist-induced uncoupling and internalization of the rLHR are distinct.

The luteinizing hormone receptor

The luteinizing hormone receptor (LHR) is a member of the subfamily of glycoprotein hormone receptors within the superfamily of G protein-coupled receptor (GPCR)/seven-transmembrane domain receptors. Over the past eight years, major advances have been made in determining the structure and function of the LHR and its gene. The hormone-binding domain has been localized to exons 1-7 in the extracellular (EC) domain/region of the receptor, which contains several leucine-rich repeats. High-affinity binding of LH and human chorionic gonadotrophin (hCG) causes secondary hormone or receptor contacts to be established with regions of the EC loop/transmembrane module that initiate signal transduction. Models of hormone-receptor interaction have been derived from the crystal structures of hCG and of the ribonuclease inhibitor, which also contains leucine-rich repeats. Such models provide a framework for the interpretation of mutational studies and for further experiments. The extracellular domain of the receptor has been overexpressed in vitro, which will facilitate crystallographic resolution of the structure of the receptor-binding site. The transmembrane domain/loop/cytoplasmic module transduces the signal for coupling to G proteins. Several constitutive, activating mutations that cause human disease have been found in helix VI and adjacent structures. These mutations have provided valuable information about mechanisms of signal transfer and G protein coupling. The structure of the LHR gene has been elucidated, and the regulation of its transcription is beginning to be understood. Valuable insights into receptor evolution have been derived from analysis of sequence homologies, the gene structure of glycoprotein hormone receptors and other members of the GPCR family, and the glycoprotein hormone receptor-like precursors identified in several invertebrate species.

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.