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

Interactions between lysophosphatidylinositol receptor GPR55 and sphingosine-1-phosphate receptor S1P5 in live cells

Lysophosphatidylinositol (LPI) and sphingosine-1-phosphate (S1P) are bioactive lipids implicated in various cellular events including proliferation, migration, and cancer progression. LPI and S1P act as ligands for G-protein coupled GPR55 and S1P receptors, respectively, and activate specific signaling pathways. Both receptors are highly expressed in various cancer tissues and associated with tumor progression. However, physical and functional crosstalk between the two receptors has not been elucidated to date. Bioluminescence resonance energy transfer (BRET) experiments in the current study showed that S1P₅ strongly and specifically interacts with GPR55. We observed co-internalization of both receptors upon agonist stimulation. Notably, activation of one receptor induced co-internalization of the partner receptor. Next, we examined functional crosstalk of the two receptors. Interestingly, while activation of the individual receptors augmented cell proliferation, ERK phosphorylation and cancer-associated gene expression in HCT116 cells, co-activation of both receptors inhibited these stimulatory effects. Our collective findings indicate that GPR55 and S1P₅ form a heterodimer and their co-activation attenuates the stimulatory activity of each receptor on colon cancer progression.

Glycoprotein G-protein Coupled Receptors in Disease: Luteinizing Hormone Receptors and Follicle Stimulating Hormone Receptors

Signal transduction by luteinizing hormone receptors (LHRs) and follicle-stimulating hormone receptors (FSHRs) is essential for the successful reproduction of human beings. Both receptors and the thyroid-stimulating hormone receptor are members of a subset of G-protein coupled receptors (GPCRs) described as the glycoprotein hormone receptors. Their ligands, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and a structurally related hormone produced in pregnancy, human chorionic gonadotropin (hCG), are large protein hormones that are extensively glycosylated. Although the primary physiologic functions of these receptors are in ovarian function and maintenance of pregnancy in human females and spermatogenesis in males, there are reports of LHRs or FSHRs involvement in disease processes both in the reproductive system and elsewhere. In this review, we evaluate the aggregation state of the structure of actively signaling LHRs or FSHRs, their functions in reproduction as well as summarizing disease processes related to receptor mutations affecting receptor function or expression in reproductive and non-reproductive tissues. We will also present novel strategies for either increasing or reducing the activity of LHRs signaling. Such approaches to modify signaling by glycoprotein receptors may prove advantageous in treating diseases relating to LHRs or FSHRs function in addition to furthering the identification of new strategies for modulating GPCR signaling.

Effects of vanadium(IV) compounds on plasma membrane lipids lead to G protein-coupled receptor signal transduction

Luteinizing hormone receptors (LHR), expressed at physiological numbers <30,000 receptors per cell, translocate to and signal within membrane rafts following binding of human chorionic gonadotropin (hCG). Similarly LHR signal in cells when treated with bis(maltolato)oxovanadium(IV) (BMOV), bis(ethylmaltolato)oxovanadium(IV) (BEOV) or VOSO₄, which decrease membrane lipid packing. Overexpressed LHR (>85,000 receptors per cell) are found in larger clusters in polarized homo-transfer fluorescence resonance energy transfer (homo-FRET) studies that were not affected by either hCG or vanadium compounds. Intracellular cyclic adenylate monophosphate (cAMP) levels indicate that only clustered LHR are active and produce the intracellular second messenger, cAMP. When LHR are over-expressed, cell signaling is unaffected by binding of hCG or vanadium compounds. To confirm the existence of intact complex, the EPR spectra of vanadium compounds in cell media were obtained using 1 mM BMOV, BEOV or VOSO₄. These data were used to determine intact complex in a 10 μM solution and verified by speciation calculations. Effects of BMOV and BEOV samples were about two-fold greater than those of aqueous vanadium(IV) making it likely that intact vanadium complex are responsible for effects of LHR function. This represents a new mechanism for activation of a G protein-coupled receptor; perturbations in the lipid bilayer by vanadium compounds lead to aggregation and accumulation of physiological numbers of LHR in membrane raft domains where they initiate signal transduction and production of cAMP, a second messenger involved in signaling.

Diseases caused by mutations in luteinizing hormone/chorionic gonadotropin receptor

Accumulating evidence showed that the luteinizing hormone/chorionic gonadotropin receptor (LHCGR) is an essential regulator of sexual development and reproduction from zebrafish to human. Activating and inactivating mutations of LHCGR gene have been identified from patients of different phenotypes. Familial male-limited precocious puberty, Leydig cell hypoplasia, and empty follicle syndrome are caused by LHCGR mutations. More than 50 mutations have been reported from subjects of different ethnic backgrounds. Functional analyses of the mutant LHCGR revealed multiple defects, including cell surface expression, ligand binding, and signaling. The difference of the two native ligands and signaling pathway activated by LHCGR are illustrated. Potential therapeutic implications from the analyses of the naturally occurring LHCGR mutations, such as pharmacological chaperones, are highlighted.

Two Hormones for One Receptor: Evolution, Biochemistry, Actions, and Pathophysiology of LH and hCG

LH and chorionic gonadotropin (CG) are glycoproteins fundamental to sexual development and reproduction. Because they act on the same receptor (LHCGR), the general consensus has been that LH and human CG (hCG) are equivalent. However, separate evolution of LHβ and hCGβ subunits occurred in primates, resulting in two molecules sharing ~85% identity and regulating different physiological events. Pituitary, pulsatile LH production results in an ~90-minute half-life molecule targeting the gonads to regulate gametogenesis and androgen synthesis. Trophoblast hCG, the "pregnancy hormone," exists in several isoforms and glycosylation variants with long half-lives (hours) and angiogenic potential and acts on luteinized ovarian cells as progestational. The different molecular features of LH and hCG lead to hormone-specific LHCGR binding and intracellular signaling cascades. In ovarian cells, LH action is preferentially exerted through kinases, phosphorylated extracellular-regulated kinase 1/2 (pERK1/2) and phosphorylated AKT (also known as protein kinase B), resulting in irreplaceable proliferative/antiapoptotic signals and partial agonism on progesterone production in vitro. In contrast, hCG displays notable cAMP/protein kinase A (PKA)-mediated steroidogenic and proapoptotic potential, which is masked by estrogen action in vivo. In vitro data have been confirmed by a large data set from assisted reproduction, because the steroidogenic potential of hCG positively affects the number of retrieved oocytes, and LH affects the pregnancy rate (per oocyte number). Leydig cell in vitro exposure to hCG results in qualitatively similar cAMP/PKA and pERK1/2 activation compared with LH and testosterone. The supposed equivalence of LH and hCG has been disproved by such data, highlighting their sex-specific functions and thus deeming it an oversight caused by incomplete understanding of clinical data.

Prolactin Is a Strong Candidate for the Regulation of Luteal Steroidogenesis in Vizcachas (Lagostomus maximus)

Prolactin (PRL) is essential for the maintenance of the corpora lutea and the production of progesterone (P4) during gestation of mice and rats, which makes it a key factor for their successful reproduction. Unlike these rodents and the vast majority of mammals, female vizcachas (Lagostomus maximus) have a peculiar reproductive biology characterized by an ovulatory event during pregnancy that generates secondary corpora lutea with a consequent increment of the circulating P4. We found that, although the expression of pituitary PRL increased steadily during pregnancy, its ovarian receptor (PRLR) reached its maximum in midpregnancy and drastically decreased at term pregnancy. The luteinizing hormone receptor (LHR) exhibited a similar profile than PRLR. Maximum P4 and LH blood levels were recorded at midpregnancy as well. Remarkably, the P4-sinthesizing enzyme 3β-HSD accompanied the expression pattern of PRLR/LHR throughout gestation. Instead, the luteolytic enzyme 20α-HSD showed low expression at early and midpregnancy, but reached its maximum at the end of gestation, when PRLR/LHR/3ß-HSD expressions and circulating P4 were minimal. In conclusion, both the PRLR and LHR expressions in the ovary would define the success of gestation in vizcachas by modulating the levels of 20α-HSD and 3ß-HSD, which ultimately determine the level of serum P4 throughout gestation.

The LH/CG receptor activates canonical signaling pathway when expressed in Drosophila

G-protein coupled receptors (GPCRs) and their ligands provide precise tissue regulation and are therefore often restricted to specific animal phyla. For example, the gonadotropins and their receptors are crucial for vertebrate reproduction but absent from invertebrates. In mammals, LHR mainly couples to the PKA signaling pathway, and CREB is the major transcription factor of this pathway. Here we present the results of expressing elements of the human gonadotropin system in Drosophila. Specifically, we generated transgenic Drosophila expressing the human LH/CG receptor (denoted as LHR), a constitutively active form of LHR, and an hCG analog. We demonstrate activation-dependent signaling by LHR to direct Drosophila phenotypes including lethality and specific midline defects; these phenotypes were due to LHR activation of PKA/CREB pathway activity. That the LHR can act in an invertebrate demonstrates the conservation of factors required for GPCR function among phylogenetically distant organisms. This novel gonadotropin model may assist the identification of new modulators of mammalian fertility by exploiting the powerful genetic and pharmacological tools available in Drosophila.

Luteinizing hormone receptors are confined in mesoscale plasma membrane microdomains throughout recovery from receptor desensitization

We examined the involvement of membrane microdomains during human luteinizing hormone (LH) receptor recovery from receptor desensitization after removal of bound hormone. Lateral motions of individual desensitized LH receptors expressed on the surface of Chinese hamster ovary cells and transient association of these receptors with detergent-resistant membrane (DRM) microdomains isolated using isopycnic sucrose gradient ultracentrifugation were assessed. Single particle tracking experiments showed untreated individual LH receptors to be confined within cell-surface membrane compartments with an average diameter of 199 ± 17 nm and associated with membrane fractions characteristic of bulk plasma membrane. After brief exposure to human chorionic gonadotropin (hCG), LH receptors remained for several hours desensitized to hCG challenge. Throughout this period, significantly increased numbers of LH receptors were confined within smaller diameter (<120 nm) membrane compartments and associated with DRM fragments of characteristically low density. By 5 h, when cells again produced cAMP in response to hCG, unoccupied LH receptors were found in larger 169 ± 22 nm diameter cell-surface membrane compartments and >90 % of LH receptors were again found in high-density membrane fragments characteristic of bulk plasma membrane. Taken together, these results suggest that, during recovery from LH receptor desensitization, LH receptors are both located with DRM lipid environments and confined within small, mesoscale (80-160 nm) cell-surface compartments. This may reflect hormone-driven translocation of receptors into DRM and formation there of protein aggregates too large or too rigid to permit effective signaling. Once bound hormone is removed, receptor structures would have to dissociate before receptors can again signal effectively in response to hormone challenge. Moreover, such larger protein complexes would be more easily constrained laterally by membrane structural elements and so appear resident in smaller cell-surface compartments.

Regulatory processes governing the cell surface expression of LH and FSH receptors

The LH receptor (LHR) and FSH receptor (FSHR), collectively termed the gonadotropin receptors, are members of the Family A of GPCRs. The gonadotropin receptors each contain N-linked carbohydrates that are not directly involved in hormone binding, but contribute to the proper folding, and therefore, cell surface expression of the receptor. Loss-of-function mutations of an LHR or FSHR results in decreased target cell responsiveness. Most inactivating mutations cause receptor misfolding, resulting in the retention of the mutant in its immature form in the endoplasmic reticulum. A membrane-permeable allosteric agonist of the LHR has been shown to serve as a pharmacological chaperone for misfolded and intracellularly retained LHRs by promoting their cell surface expression. Wild-type LHR and FSHR each form homodimers and heterodimers while in the ER. Therefore, when wild-type receptor is co-expressed with a misfolded mutant, the misfolded receptor dimerizes with immature wild-type receptor in the ER, causing a dominant-negative effect on cell surface expression of the mature wild-type receptor. Notably, the propensity for homodimerization is not affected by the activation status of the receptor. However, within a receptor dimer, the activity of one protomer may allosterically regulate the other protomer. Therefore, the dimerization of the gonadotropin receptors appears to be an obligate process that is part of the normal itinerary for trafficking to the cell surface and, once there, the dimerized receptors allow for additional modulations of cell signaling.

Restricted lateral diffusion of luteinizing hormone receptors in membrane microdomains

Single particle tracking was used to evaluate lateral motions of individual FLAG-tagged human luteinizing hormone (LH) receptors expressed on CHO cells and native LH receptors on both KGN human granulosa-derived tumor cells and M17 human neuroblastoma cells before and after exposure to human chorionic gonadotropin (hCG). Compared with LH receptors on untreated cells, LH receptors on cells treated with 100 nm hCG exhibit restricted lateral diffusion and are confined in small, nanometer-scale, membrane compartments. Similar to LH receptors labeled with Au-hCG, LH receptors labeled with gold-deglycosylated hCG, an hCG antagonist, also exhibit restricted lateral diffusion and are confined in nanoscale membrane compartments on KGN cells treated with 100 nm hCG. LH receptor point mutants lacking potential palmitoylation sites remain in large compartments despite treatment with 100 nm hCG as do LH receptors on cells treated with cytochalasin D. Finally, both polarization homotransfer fluorescence resonance energy transfer imaging and photon counting histogram analysis indicate that treatment with hCG induces aggregation of YFP-coupled LH receptors stably expressed on CHO cells. Taken together, our results demonstrate that binding of hCG induces aggregation of LH receptors within nanoscale, cell surface membrane compartments, that hCG binding also affects the lateral motions of antagonist binding LH receptors, and that receptor surface densities must be considered in evaluating the extent of hormone-dependent receptor aggregation.

Leydig cell aging and the mechanisms of reduced testosterone synthesis

In males, serum testosterone levels decline with advancing age. Though part of a complex process, this age-related decline in testosterone appears to occur, in part, due to a significant decline in the ability of aged Leydig cells to produce testosterone maximally in response to luteinizing hormone (LH). The structure of the molecular machinery responsible for the synthesis of testosterone is described, and placed in the context of Leydig cell biology. Multiple parameters related to the synthesis of testosterone by the Leydig cell have been observed to change with age. Relationships among these changes are reviewed. A discussion of potential causes of the age-related decline in Leydig cell steroidogenic capacity presents a model in which the inability of aged cells to adequately respond to hormonal stimulation results in cellular regression with concomitant decline in maximal testosterone output.

Bioluminescence resonance energy transfer studies reveal constitutive dimerization of the human lutropin receptor and a lack of correlation between receptor activation and the propensity for dimerization

Previous studies from our laboratory using co-immunoprecipitation techniques suggested that the human lutropin receptor (hLHR) constitutively self-associates into dimers/oligomers and that agonist treatment of cells either increased hLHR dimerization/oligomerization and/or stabilized hLHR dimers/oligomers to detergent solubilization (Tao, Y. X., Johnson, N. B., and Segaloff, D. L. (2004) J. Biol. Chem. 279, 5904-5914). In this study, bioluminescence resonance energy transfer (BRET(2)) analyses confirmed that the hLHR constitutively self-associates in living cells. After subcellular fractionation, hLHR dimers/oligomers were detected in both the plasma membrane and endoplasmic reticulum (ER). Further evidence supporting the constitutive formation of hLHR dimer/oligomers in the ER is provided by data showing homodimerization of misfolded hLHR mutants that are retained in the ER. These mutants, when co-expressed with wild-type receptor, are shown by BRET(2) to heterodimerize, accounting for their dominant-negative effects on cell surface receptor expression. Hormone desorption assays using intact cells demonstrate allosterism between hLHR protomers, indicating functional cell surface hLHR dimers. However, quantitative BRET(2) analyses in intact cells indicate a lack of effect of agonist on the propensity of the hLHR to dimerize. Using purified plasma membranes, human chorionic gonadotropin was similarly observed to have no effect on the BRET(2) signal. An examination of the propensity for constitutively active and signaling inactive hLHR mutants to dimerize further showed no correlation between dimerization and the activation state of the hLHR. Taken altogether, our data suggest that hLHR dimers/oligomers are formed early in the biosynthetic pathway in the ER, are constitutively expressed on the plasma membrane, and are not affected by the activation state of the hLHR.

Hormonal regulation of MicroRNA expression in periovulatory mouse mural granulosa cells

MicroRNAs (miRNAs) mediate posttranscriptional gene regulation by binding to the 3' untranslated region of messenger RNAs to either inhibit or enhance translation. The extent and hormonal regulation of miRNA expression by ovarian granulosa cells and their role in ovulation and luteinization is unknown. In the present study, miRNA array analysis was used to identify 212 mature miRNAs as expressed and 13 as differentially expressed in periovulatory granulosa cells collected before and after an ovulatory dose of hCG. Two miRNAs, Mirn132 and Mirn212 (also known as miR-132 and miR-212), were found to be highly upregulated following LH/hCG induction and were further analyzed. In vivo and in vitro temporal expression analysis by quantitative RT-PCR confirmed that LH/hCG and cAMP, respectively, increased transcription of the precursor transcript as well as the mature miRNAs. Locked nucleic acid oligonucleotides complementary to Mirn132 and Mirn212 were shown to block cAMP-mediated mature miRNA expression and function. Computational analyses indicated that 77 putative mRNA targets of Mirn132 and Mirn212 were expressed in ovarian granulosa cells. Furthermore, upon knockdown of Mirn132 and Mirn212, a known target of Mirn132, C-terminal binding protein 1, showed decreased protein levels but no change in mRNA levels. The following studies are the first to describe the extent of miRNA expression within ovarian granulosa cells and the first to demonstrate that LH/hCG regulates the expression of select miRNAs, which affect posttranscriptional gene regulation within these cells.

Regulation of human luteinizing hormone receptor in the ovary

The luteinizing hormone receptor (LHR) is essential for elevated levels of progesterone to maintain pregnancy during the first trimester; the maintenance of the expression of LHR is a key factor controlling the duration of luteal function. Therefore, as the expression of LHR is most likely to be regulated by the stability of the receptor mRNA at the luteal phase of the human menstrual cycle, we focused on studies examining the stability of mRNA rather than the production of mRNA. In addition, LHR (exon 9), one of the splice variants of human LHR (hLHR), was cloned in the corpus luteum of a patient with a regular menstrual cycle. The results of Western blots using Percoll gradient fractionation indicated that hLHR formed complexes with hLHR (exon 9), which are transferred to the lysosome, where they are eventually degraded, instead of being translocated from the endoplasmic reticulum to the transducing organelle. These results showed that hLHR (exon 9) caused a reduction in the expression of functional receptor number and affected the signaling condition of wild-type hLHR. As the luteal phase progressed hLHR (exon 9) increased relative to hLHR, demonstrating that hLHR (exon 9) was expressed more than hLHR in the late luteal phase. This work reveals the essential function of the regulatory and structural elements involved in human LH receptor splicing, and that hLHR (exon 9) can negatively control the function of wild-type receptors. Moreover, this finding presented a novel mechanism of regulation of LHR in the human corpus luteum. (Reprod Med Biol 2008; 7: 11-16).

Lipid rafts are triage centers for multimeric and monomeric thyrotropin receptor regulation

The TSH receptor (TSHR), a heptahelical G protein-coupled receptor on the surface of thyrocytes, is a major autoantigen and physiological regulator of the thyroid gland. Unlike other G protein-coupled receptors, the TSHR undergoes posttranslational cleavage of its ectodomain, leading to the existence of several forms of the receptor on the plasma membrane. We previously hypothesized that to achieve high fidelity and specificity of TSH ligand or TSHR autoantibody signaling, the TSHR may compartmentalize into microdomains within the plasma membrane. In support of this hypothesis we have shown previously that TSHRs reside in GM1 ganglioside-enriched lipid rafts in the plasma membrane of TSHR-expressing cells. In this study, we further explored the different forms of TSHRs that reside in lipid rafts. We studied both TSHR-transfected cells and rat thyrocytes, using both nondetergent biochemical analyses and receptor-lipid raft colocalization. Using the biochemical approach, we observed that monomeric receptors existed in both raft and nonraft fractions of the cell surface in the steady state. We also demonstrated that the multimeric forms of the receptor were preferentially partitioned into the lipid microdomains. Different TSHR forms, including multimers, were dynamically regulated both by receptor-specific and postreceptor-specific modulators. TSH ligand and TSHR antibody of the stimulating variety induced a decrease of multimeric forms in the raft fractions. In addition, multimeric and monomeric forms of the receptor were both associated with Gsalpha within and without the rafts. Although failure to achieve total lipid raft disruption prevented a conclusion regarding the relative power of TSHR signaling within and without the raft domains, these data showed clearly that not only were a significant proportion of TSHRs residing within lipid microdomains but that constitutive multimerization of TSHRs was actually regulated within the lipid rafts.

Follice-stimulating hormone receptor forms oligomers and shows evidence of carboxyl-terminal proteolytic processing

FSH receptor (FSHR), a member of the G protein-coupled receptor superfamily, is present in the plasma membrane of ovarian granulosa cells and testicular Sertoli cells. FSH regulates normal ovarian follicle development and spermatogenesis through FSHR. The extracellular domain of FSHR is a weakly associated homodimer in the recently solved crystal structure of FSH in complex with the extracellular domain of FSHR. However, there is currently no biochemical data that demonstrate that FSHR exists as a dimer or higher-order oligomer in cell membranes. A fluorescence resonance energy transfer assay was used to determine whether full-length native FSHR is an oligomer. FSHR-specific monoclonal antibody or Fab fragments, labeled with two different fluorophores, allowed the study of nontagged receptor in situ. Unoccupied FSHR exhibited strong fluorescence resonance energy transfer profiles in situ. Complementary coimmunoprecipitation experiments of myc- or FLAG-tagged FSHR indicated that FSHR forms oligomers early in receptor biosynthesis. No effect of FSH treatment was observed. Thus, immature forms of FSHR, not yet fully processed, were observed to coimmunoprecipitate. An unexpected observation was made that the C-terminal epitope tags are removed from FSHR before arrival at the cell surface. These results provide the first evidence for oligomers of full-length FSHR in situ and for C-terminal proteolytic processing of FSHR and that both events take place during biosynthesis. This may explain how heterozygous mutations in the FSHR gene that affect receptor trafficking may be ameliorated by oligomer formation.

The molecular control of corpus luteum formation, function, and regression

The corpus luteum (CL) is one of the few endocrine glands that forms from the remains of another organ and whose function and survival are limited in scope and time. The CL is the site of rapid remodeling, growth, differentiation, and death of cells originating from granulosa, theca, capillaries, and fibroblasts. The apparent raison d'etre of the CL is the production of progesterone, and all the structural and functional features of this gland are geared toward this end. Because of its unique importance for successful pregnancies, the mammals have evolved a complex series of checks and balances that maintains progesterone at appropriate levels throughout gestation. The formation, maintenance, regression, and steroidogenesis of the CL are among the most significant and closely regulated events in mammalian reproduction. During pregnancy, the fate of the CL depends on the interplay of ovarian, pituitary, and placental regulators. At the end of its life span, the CL undergoes a process of regression leading to its disappearance from the ovary and allowing the initiation of a new cycle. The generation of transgenic, knockout and knockin mice and the development of innovative technologies have revealed a novel role of several molecules in the reprogramming of granulosa cells into luteal cells and in the hormonal and molecular control of the function and demise of the CL. The current review highlights our knowledge on these key molecular events in rodents.

Dimerization of the lutropin receptor: insights from computational modeling

A computational approach based upon rigid-body docking, ad hoc filtering, and cluster analysis has been carried out to predict likely interfaces in LHR homodimers. Quaternary structure predictions emphasize the role of helices 4, 5 and 6, with prominence to helix 4, in mediating inter-monomer interactions. Intermolecular interactions essentially involve the transmembrane domains rather than the hydrophilic loops and do not implicate disulfide bridges.Collectively, molecular dynamics simulations on the isolated receptor and computational modeling of LHR homodimerization suggest that mutation-induced LHR activation favors H4-H4 contacts involving the highly conserved W491 from both the receptors monomers.

Trans-activation, cis-activation and signal selection of gonadotropin receptors

It has been thought that when a hormone binds to a receptor, the liganded receptor activates itself and generates hormone signals, such as the cAMP signal and the inositol phosphate signal (cis-activation). We describe that a liganded LH receptor or FSH receptor molecule is capable of intermolecularly activating nonliganded receptors (trans-activation). Particularly, intriguing is the possibility that a pair of compound heterozygous mutants, one defective in binding and the other defective in signaling, may cooperate and rescue signaling. Furthermore, trans-activation of the binding deficient receptors examined in our studies generates either the cAMP signal or the IP signal, but not both. Trans-activation and selective signal generation have broad implications on signal generation mechanisms, and suggest new therapeutic approaches.

Constitutively-active human LH receptors are self-associated and located in rafts

Several naturally occurring mutations in human luteinizing hormone receptors (LHR) at position 578 are associated with constitutive activation of the receptor. To determine whether human LHRs that signal in the absence of ligand are self-associated, fluorescence resonance energy transfer (FRET) between receptors was evaluated. Values for FRET between wild type LHR in the absence of ligand were less than 1% and increased significantly to over 11% after exposure to hCG. Constitutively active receptors exhibited 11-15% FRET efficiency in the absence of hormone and these values did not change with hCG treatment. A large fraction of constitutively active LHR-D578H receptors were also associated with so-called plasma membrane rafts. Disruption of these membrane microdomains reduced FRET efficiency but did not affect signalling through cAMP. Thus, in the absence of ligand, constitutively active receptors are self-associated and located in high buoyancy membrane fractions, both characteristics of the hormone-treated wild type receptor.

The active metabolite of Clopidogrel disrupts P2Y12 receptor oligomers and partitions them out of lipid rafts

P2Y12, a G protein-coupled receptor that plays a central role in platelet activation has been recently identified as the receptor targeted by the antithrombotic drug, clopidogrel. In this study, we further deciphered the mechanism of action of clopidogrel and of its active metabolite (Act-Met) on P2Y12 receptors. Using biochemical approaches, we demonstrated the existence of homooligomeric complexes of P2Y12 receptors at the surface of mammalian cells and in freshly isolated platelets. In vitro treatment with Act-Met or in vivo oral administration to rats with clopidogrel induced the breakdown of these oligomers into dimeric and monomeric entities in P2Y12 expressing HEK293 and platelets respectively. In addition, we showed the predominant association of P2Y12 oligomers to cell membrane lipid rafts and the partitioning of P2Y12 out of rafts in response to clopidogrel and Act-Met. The raft-associated P2Y12 oligomers represented the functional form of the receptor, as demonstrated by binding and signal transduction studies. Finally, using a series of receptors individually mutated at each cysteine residue and a chimeric P2Y12/P2Y13 receptor, we pointed out the involvement of cysteine 97 within the first extracellular loop of P2Y12 in the mechanism of action of Act-Met.

Induction of alpha-caveolin-1 (αCAV1) expression in bovine granulosa cells in response to an ovulatory dose of human chorionic gonadotropin

Caveolins are implicated in endocytosis, cholesterol trafficking and signal transduction. A cDNA fragment corresponding to caveolin-1 (CAV1) was identified in a mRNA profiling expression study in bovine granulosa cells (GC) following human chorionic gonadotropin (hCG)-induced ovulation. Thus, we have characterized CAV1 cDNA and studied its spatio-temporal expression pattern in bovine ovarian follicles. The full-length bovine alphaCAV1 cDNA was cloned and encodes a putative 22 kDa protein. Expression of alphaCAV1 was studied in bovine GC obtained from follicles at different developmental stages: small follicles (SF: 2-4 mm), dominant follicles (DF), ovulatory follicles (OF: 24 hr post-hCG), and corpus luteum (CL). Semiquantitative RT-PCR analysis showed a 6.5-fold increase in alphaCAV1 mRNA in GC of OF versus DF (P < 0.0001), whereas CAV2 mRNA was increased by only twofold (P < 0.0007). Temporal expression of alphaCAV1 mRNA from OF recovered at 0, 6, 12, 18, and 24 hr after hCG injection showed an 8.5-fold increase of alphaCAV1 mRNA after 24 hr compared to 0 hr (P < 0.0018) whereas no significant variation was detected for CAV2. Immunoblot demonstrated an initial increase in alphaCAV1 protein level 12 hr post-hCG, reaching a maximum at 24 hr. Immunohistochemical localization of CAV1 was observed in GC of OF isolated 18 and 24 hr after hCG injection, whereas no signal was detected in GC of DF and SF. The induction of alphaCAV1 in GC of OF suggests that alphaCAV1 likely contributes to control the increase in membrane signaling that occurs at the time of ovulation and luteinization.

Heterotrimeric G protein-independent signaling of a G protein-coupled receptor. Direct binding of ARNO/cytohesin-2 to the carboxyl terminus of the A2A adenosine receptor is necessary for sustained activation of the ERK/MAP kinase pathway

The A2A adenosine receptor is a prototypical G(s)-coupled receptor, but it also signals, e.g. to mitogen-activated protein (MAP) kinase, via a pathway that is independent of heterotrimeric G proteins. Truncation of the carboxyl terminus affects the strength of the signal through these alternative pathways. In a yeast two-hybrid interaction hunt, we screened a human brain library for proteins that bound to the juxtamembrane portion of the carboxyl terminus of the A2A receptor. This approach identified ARNO/cytohesin-2, a nucleotide exchange factor for the small (monomeric) G proteins of the Arf (ADP-ribosylation factor) family, as a potential interaction partner. We confirmed a direct interaction by mutual pull down (of fusion proteins expressed in bacteria) and by immunoprecipitation of the proteins expressed in mammalian cells. To circumvent the long term toxicity associated with overexpression of ARNO/cytohesin-2, we created stable cell lines that stably expressed the A2A receptor and where ARNO/cytohesin-2 or the dominant negative version E156K-ARNO/cytohesin-2 was inducible by mifepristone. Cyclic AMP accumulation induced by an A2A-specific agonist was neither altered by ARNO/cytohesin-2 nor by the dominant negative version. This was also true for agonist-induced desensitization. In contrast, expression of dominant negative E156K-ARNO/cytohesin-2 and of dominant negative T27N-Arf6 abrogated the sustained phase of MAP kinase stimulation induced by the A2A receptor. We therefore conclude that ARNO/cytohesin-2 is required to support the alternative, heterotrimeric G protein-independent, signaling pathway of A2A receptor, which is stimulation of MAP kinase.

Roles of G-protein-coupled receptor dimerization

The classical idea that G-protein-coupled receptors (GPCRs) function as monomeric entities has been unsettled by the emerging concept of GPCR dimerization. Recent findings have indicated not only that many GPCRs exist as homodimers and heterodimers, but also that their oligomeric assembly could have important functional roles. Several studies have shown that dimerization occurs early after biosynthesis, suggesting that it has a primary role in receptor maturation. G-protein coupling, downstream signalling and regulatory processes such as internalization have also been shown to be influenced by the dimeric nature of the receptors. In addition to raising fundamental questions about GPCR function, the concept of dimerization could be important in the development and screening of drugs that act through this receptor class. In particular, the changes in ligand-binding and signalling properties that accompany heterodimerization could give rise to an unexpected pharmacological diversity that would need to be considered.

Constitutive and Agonist-dependent Self-association of the Cell Surface Human Lutropin Receptor

The human lutropin receptor (hLHR) is a G protein-coupled receptor (GPCR) that plays an essential role in reproductive physiology. The present studies were undertaken to determine whether the hLHR self-associates. We show that high molecular weight complexes of the hLHR can be co-immunoprecipitated from 293 cells transfected with differentially tagged hLHRs. These complexes are detected only in extracts from cells that have been co-transfected and not in extracts combined from cells expressing only one form of tagged hLHR, confirming the in vivo self-association of the receptor. In transiently transfected cells, in which a small percentage of cells overexpress hLHR and most of the hLHR is located intracellularly in the ER, the self-associated hLHR is composed predominantly of immature hLHR. When cells were transiently co-transfected with wild-type hLHR and a misfolded mutant of the hLHR, a physical association of the ER-localized misfolded mutant with the immature hLHR was observed, resulting in a decreased cell surface expression of the wild-type receptor. In contrast, in stably transfected cells, where the majority of cells express receptor and there is much less intracellular accumulation of hLHR, the self-associated forms of the hLHR are composed predominantly of cell surface receptor. The abundance of cell surface hLHR dimers and oligomers, as detected on SDS gels, is increased further upon human choriogonadotropin treatment of the stably transfected cells. In addition to documenting the self-association of cell surface hLHR, our results underscore the importance of the cellular distribution of recombinant GPCR as it relates to the nature of the GPCR dimerization and oligomerization.

Trans-activation of mutant follicle-stimulating hormone receptors selectively generates only one of two hormone signals

Previously, we reported that a liganded LH receptor (LHR) is capable of activating itself (cis-activation) and other nonliganded LHRs to induce cAMP (trans-activation). Trans-activation of the LHR raises two crucial questions. Is trans-activation unique to LHR or common to other G protein-coupled receptors? Does trans-activation stimulate phospholipase Cbeta as it does adenylyl cyclase? To address these questions, two types of novel FSH receptors (FSHRs) were constructed, one defective in hormone binding and the other defective in signal generation. The FSHR, a G protein-coupled receptor, comprises two major domains, the N-terminal extracellular exodomain that binds the hormone and the membrane-associated endodomain that generates the hormone signals. For signal defective receptors, the exodomain was attached to glycosyl phosphatidylinositol (ExoGPI) or the transmembrane domain of CD8 immune receptor (ExoCD). ExoGPI and ExoCD can trans-activate another nonliganded FSH. Surprisingly, the trans-activation generates a signal to activate either adenylyl cyclase or phospholipase Cbeta, but not both. These results indicate that trans-activation in these mutant receptors is selective and limited in signal generation, thus providing new approaches to investigating the generation of different hormone signals and a novel means to selectively generate a particular hormone signal. Our data also suggest that the FSHR's exodomain could not trans-activate LHR.