The luteinizing hormone receptor

Macrophages in human reproductive tissues contain luteinizing hormone/chorionic gonadotropin receptors

PROBLEM

To determine whether macrophages in human reproductive tissues contain luteinizing hormone (LH)/human chorionic gonadotropin (hCG) receptor mRNA and receptor protein that can bind 125I-hCG.

METHOD OF STUDY

Macrophages isolated from term pregnancy human decidua were used for LH/hCG receptor detection by in situ hybridization for receptor mRNA and immunocytochemistry for a macrophage marker, CD68, performed alone and in combination, reverse transcription-nested polymerase chain reaction, Western and ligand blotting. The LH/hCG receptor presence in macrophages in late luteal phase human endometria and corpora lutea was determined by sequential performance of in situ hybridization and immunocytochemistry.

RESULTS

The macrophages present in term pregnancy human decidua and late luteal phase human endometria and corpora lutea contain LH/hCG receptors.

CONCLUSIONS

This is the first demonstration of macrophages present in human reproductive tissues containing LH/hCG receptors. The receptor presence suggests that LH and hCG may regulate macrophage functions in gonadal as well as in non-gonadal target tissues.

Disorders of pubertal development

Puberty is the period of life during which reproductive capability is acquired. It is characterized clinically by the acquisition of secondary sexual characteristics associated with a growth spurt, and on average takes 3-4 years. Early maturation is defined as the development of sexual characteristics before the age of 8 years in girls and 9 years in boys. Delayed puberty is defined when there are no signs of puberty at the age of 13.4 years in girls and 14 years in boys (2 SD above the mean of chronological age for the onset of puberty). There are many forms of premature sexual maturation: gonadotrophin-dependent (central, or 'idiopathic' or 'true' precocious puberty) and gonadotrophin-independent precocious puberty (McCune-Albright syndrome in girls, testotoxicosis in boys); isolated premature thelarche (in the forms of classical, atypical and variant); premature adrenarche (characterized by the production of significant quantities of androgens between 5 and 8 years of age); premature menarche. The differential diagnosis of delayed puberty is between constitutional delay of growth and puberty, pubertal delay secondary to chronic disease and hypogonadotrophic hypogonadism.

Signal transduction pathways activated by chorionic gonadotropin in the primate endometrial epithelial cells

Successful implantation requires synergism between the developing embryo and the receptive endometrium. In the baboon, infusion of chorionic gonadotropin (CG) modulates both morphology and physiology of the epithelial and stromal cells of the receptive endometrium. This study explored the signal transduction pathways activated by CG in endometrial epithelial cells from baboon (BE) and human (HES). Incubations of BE and HES cells with CG did not significantly alter adenylyl cyclase activity or increase intracellular cAMP when compared with Chinese hamster ovarian cells stably transfected with the full-length human CG/luteinizing hormone (LH) receptor (CHO-LH cells). However, in BE and HES cells, CG induced the phosphorylation of several proteins, among them, extracellular signal-regulated protein kinases 1 and 2 (ERK 1/2). Phosphorylation of ERK 1/2 in uterine epithelial cells was protein kinase A (PKA) independent. This novel signaling pathway is functional because, in response to CG stimulation, prostaglandin E(2) (PGE(2)) was released into the media and increased significantly 2 h following CG stimulation. CG-stimulated PGE(2) synthesis in epithelial cells was inhibited by a specific mitogen-activated protein kinase (MEK 1/2) inhibitor, PD 98059. In conclusion, immediate signal transduction pathways induced by CG in endometrial epithelial cells are cAMP independent and stimulate phosphorylation of ERK 1/2 via a MEK 1/2 pathway, leading to an increase in PGE(2) release as the possible result of cyclooxygenase-2 activation.

Many facets of mammalian lanosterol 14alpha-demethylase from the evolutionarily conserved cytochrome P450 family CYP51

Lanosterol 14alpha-demethylase is a cytochrome P450 enzyme of the cholesterol biosynthetic pathway belonging to the CYP51 gene family which is the most evolutionarily conserved member of the CYP superfamily. Mammalian (human, mouse, rat, pig) CYP51 genes are unique in sharing several common characteristics: highly conserved exon/intron borders and proximal promoter structures, ubiquitous expression at the highest level in the testis, and appearance of testis-specific transcripts that arise from differential polyadenylation site usage. CYP51 protein demethylates lanosterol to form follicular fluid meiosis-activating sterol, FF-MAS, which is, besides being an intermediate of cholesterol biosynthesis, also a signaling sterol that accumulates in ovaries. CYP51 protein resides in the endoplasmatic reticulum of most cells and also in acrosomal membranes of spermatids where transport through the Golgi apparatus is suggested. While sterol regulatory element binding protein (SREBP)-dependent transcriptional regulation of CYP51 contributes to synthesis of cholesterol, the germ-cell-specific cAMP/CREMtau-dependent upregulation might contribute to increased production of MAS.

Yoked complexes of human choriogonadotropin and the lutropin receptor: evidence that monomeric individual subunits are inactive

Human choriogonadotropin (hCG) contains an alpha-subunit, common to other members of the glycoprotein hormone family, and a unique beta-subunit that determines hormone specificity. It is generally thought that heterodimer formation is obligatory for full hormonal activity, although other studies have indicated that individual subunits and homodimeric hCGbeta were capable of low affinity binding to the LH receptor (LHR) and subsequent activation. Previously, we constructed two yoked hormone (hCG)-LHR complexes, where the two hormone subunits and the heptahelical receptor were engineered to form single polypeptide chains, i.e. N-beta-alpha-LHR-C and N-alpha-beta-LHR-C. Expression of both complexes led to constitutive stimulation of cAMP production. In the present study, we investigated whether the human alpha-subunit and hCGbeta can act as functional agonists when covalently attached to or coexpressed with the LH receptor. Our initial results showed that hCGbeta, but not alpha, was able to activate LHR with an increase in intracellular cAMP in human embryonic kidney 293 cells but not in Chinese hamster ovary or COS-7 cells. Further examination of this apparent cell-specific agonist activity of hCGbeta revealed that low levels of endogenous alpha-subunit were expressed in human embryonic kidney 293 cells, thus enabling sufficient amounts of active heterodimer to form with the transfected hCGbeta to activate LHR. The studies in Chinese hamster ovary and COS-7 cells clearly demonstrate that, even under experimental conditions where hormone-receptor interactions are maximized, individual subunits of hCG can not act as functional agonists, at least in their monomeric form.

Human peripheral blood polymorphonuclear leukocytes at the ovulatory period are in an activated state

The ovulatory process has been compared with inflammation. We investigated the state of female peripheral polymorphonuclear leukocytes (PMN) during menstrual cycles, and found that PMN contained high levels of superoxide, hydrogen peroxide and nitric oxide (NO) during at the peri-ovulatory period. Assuming the cause of this elevation to be a luteinizing hormone (LH), the surge of which preceded the ovulation, we examined the responsiveness of PMN to pituitary LH. The results revealed that this hormone elevated dose-dependently the production of reactive oxygen intermediates (ROI). Furthermore, we demonstrated the mRNA expression of LH receptors and their presence on PMN. The data indicated that the LH surge before on the ovulatory day resulted in general activation of PMN, suggesting that this state of PMN may be a necessary step for initiation of ovulation, rather than a defensive role against infection.

Silencing of transcription of the human luteinizing hormone receptor gene by histone deacetylase-mSin3A complex

Modification of chromatin structure by histone acetylases and deacetylases is an important mechanism in modulation of eukaryotic gene transcription. The present study investigated regulation of the human luteinizing hormone receptor (hLHR) gene by histone deacetylases. Inhibition of histone deacetylases (HDACs) by trichostatin A (TSA) increased hLHR promoter activity by 40-fold in JAR cells and markedly elevated endogenous hLHR mRNA levels. Acetylated histones H3 and H4 accumulated in TSA-treated cells and associated predominantly with the hLHR promoter. Furthermore, TSA significantly enhanced the recruitment of RNA polymerase II to the promoter. One of the two Sp1 sites essential for basal promoter activity was identified as critical for the TSA effect, but the binding of Sp1/Sp3 to this site remained unchanged in the absence or presence of TSA. A multiprotein complex was recruited to the hLHR promoter via interaction with Sp1 and Sp3, in which HDAC1 and HDAC2 were docked directly to Sp1-bound DNA and indirectly to Sp3-bound DNA through RbAp48, while mSin3A interacted with both HDACs. HDAC1 and HDAC2 were shown to potently repress the hLHR gene transcription, and mSin3A potentiated the inhibition mediated by HDAC1. Our studies have demonstrated that the HDAC-mSin3A complex has an important role in the regulation of hLHR gene transcription by interaction with Sp1/Sp3 and by region-specific changes in histone acetylation and polymerase II recruitment within the hLHR promoter.

Mutations in theDrosophilaglycoprotein hormone receptor,rickets, eliminate neuropeptide-induced tanning and selectively block a stereotyped behavioral program

Adult insects achieve their final form shortly after adult eclosion by the combined effects of specialized behaviors that generate increased blood pressure, which causes cuticular expansion, and hormones, which plasticize and then tan the cuticle. We examined the molecular mechanisms contributing to these processes in Drosophila by analyzing mutants for the rickets gene. These flies fail to initiate the behavioral and tanning processes that normally follow ecdysis. Sequencing of rickets mutants and STS mapping of deficiencies confirmed that rickets encodes the glycoprotein hormone receptor DLGR2. Although rickets mutants produce and release the insect-tanning hormone bursicon, they do not melanize when injected with extracts containing bursicon. In contrast, mutants do melanize in response to injection of an analog of cyclic AMP, the second messenger for bursicon. Hence, rickets appears to encode a component of the bursicon response pathway, probably the bursicon receptor itself. Mutants also have a behavioral deficit in that they fail to initiate the behavioral program for wing expansion. A set of decapitation experiments utilizing rickets mutants and flies that lack cells containing the neuropeptide eclosion hormone, reveals a multicomponent control to the activation of this behavioral program.

A model for constitutive lutropin receptor activation based on molecular simulation and engineered mutations in transmembrane helices 6 and 7

Many naturally occurring and engineered mutations lead to constitutive activation of the G protein-coupled lutropin receptor (LHR), some of which also result in reduced ligand responsiveness. To elucidate the nature of interhelical interactions in this heptahelical receptor and changes thereof accompanying activation, we have utilized site-directed mutagenesis on transmembrane helices 6 and 7 of rat LHR to prepare and characterize a number of single, double, and triple mutants. The potent constitutively activating mutants, D556(6.44)H and D556(6.44)Q, were combined with weaker activating mutants, N593(7.45)R and N597(7.49)Q, and the loss-of-responsiveness mutant, N593(7.45)A. The engineered mutants have also been simulated using a new receptor model based on the crystal structure of rhodopsin. The results suggest that constitutive LHR activation by mutations at Asp-556(6.44) is triggered by the breakage or weakening of the interaction found in the wild type receptor between Asp-556(6.44) and Asn-593(7.45). Whereas this perturbation is unique to the activating mutations at Asp-556(6.44), common features to all of the most active LHR mutants are the breakage of the charge-reinforced H-bonding interaction between Arg-442(3.50) and Asp-542(6.30) and the increase in solvent accessibility of the cytosolic extensions of helices 3 and 6, which probably participate in the receptor-G protein interface. Asn-593(7.45) and Asn-597(7.49) also seem to be necessary for the high constitutive activities of D556(6.44)H and D556(6.44)Q and for full ligand responsiveness. The new theoretical model provides a foundation for further experimental work on the molecular mechanism(s) of receptor activation.

Structural aspects of luteinizing hormone receptor: information from molecular modeling and mutagenesis

The luteinizing hormone receptor (LHR) is a member of the superfamily of G protein-coupled receptors and, in humans, binds two closely related ligands, members of the heterodimeric glycoprotein hormone family. This receptor is an essential component of the reproductive axis in males and females, and a number of naturally occurring pathophysiologic activating and inactivating mutations have been described. This review deals with the current state of knowledge of the structure of LHR based on molecular modeling and the supporting experimental data from engineered and naturally occurring mutations.

Use of defined-function mutants to access receptor-receptor interactions

This article describes a novel method to access functional interactions of two defective mutant receptors. As a model, luteinizing hormone receptor, a G-protein-coupled receptor, was used by coexpressing two different mutants, one defective in hormone binding and the other defective in signal generation. When these two mutants were coexpressed in a cell, the cell responded to the hormone and induced the hormone action, indicating the interaction of the two receptors and rescue of the activity. The luteinizing hormone receptor consists of a 350-amino-acid extracellular N-terminal domain (exodomain), followed by seven transmembrane domains and connecting loops (endodomain). Hormone binds to the exodomain, whereas hormone signals are generated in the endodomain. Here, we show that binding of hormone to one receptor can activate adenylyl cyclase through its transmembrane bundle, intramolecular activation (cis-activation), as well as intermolecular activation (trans-activation) through the transmembrane bundle of an adjacent receptor, without forming a stable receptor dimer. Our observations provide new insights into the mechanism of receptor activation mechanisms, and have implications for the treatment of inherited disorders of glycoprotein hormone receptors.

Cis- and trans-activation of hormone receptors: the LH receptor

G protein-coupled receptors (GPCRs) accommodate a wide spectrum of activators from ions to glycoprotein hormones. The mechanism of activation for this large and clinically important family of receptors is poorly understood. Although initially thought to function as monomers, there is a growing body of evidence that GPCR dimers form, and in some cases that these dimers are essential for signal transduction. Here we describe a novel mechanism of intermolecular GPCR activation, which we refer to as trans-activation, in the LH receptor, a GPCR that does not form stable dimers. The LH receptor consists of a 350-amino acid amino-terminal domain, which is responsible for high-affinity binding to human CG, followed by seven-transmembrane domains and connecting loops. This seven-transmembrane domain bundle transmits the signal from the extracellular amino terminus to intracellular G proteins and adenylyl cyclase. Here, we show that binding of hormone to one receptor can activate adenylyl cyclase through its transmembrane bundle, intramolecular activation (cis-activation), as well as trans-activation through the transmembrane bundle of an adjacent receptor, without forming a stable receptor dimer. Coexpression of a mutant receptor defective in hormone binding and another mutant defective in signal generation rescues hormone-activated cAMP production. Our observations provide new insights into the mechanism of receptor activation mechanisms and have implications for the treatment of inherited disorders of glycoprotein hormone receptors.

Two defective heterozygous luteinizing hormone receptors can rescue hormone action

Luteinizing hormone receptor is a G protein-coupled receptor and consists of two halves: the N-terminal extracellular half (exodomain) and C-terminal membrane-associated half (endodomain). Hormone binds to the exodomain, and the resulting hormone-exodomain complex modulates the endodomain to generate signals. There are mutations that impair either hormone binding or signal generation. We report that the coexpression of a binding defective mutant and a signal-defective mutant rescues signal generation to produce cAMP. This rescue requires both types of mutant receptors and is dependent on the human chorionic gonadotropin dose, the surface concentration of mutant receptors, and the amino acid position of mutations. Furthermore, random collisions among mutant receptors are not involved in the rescue. Our observations provide new insights into the mechanisms of the functional and structural relationship of the exo- and endodomain, signal transduction, and receptor genetics, in particular for defective heterozygotes.

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.

Consequences of targeted inactivation of LH receptors

The inactivation of luteinizing hormone (LH) receptors was neither lethal nor it had any effect on sex differentiation. However, it dramatically reduced the growth and development of gonads and the reproductive tract. As a result, both female and male animals were infertile. Serum LH levels were dramatically elevated, follicle stimulating hormone (FSH) levels moderately elevated in both sexes, estradiol and progesterone levels partially decreased in females, testosterone levels dramatically decreased and estradiol levels moderately increased in males. The knockout of LH receptors had no effect on gonadal FSH receptors in both sexes, progesterone receptors in females and androgen receptors in males. However, estrogen receptor ERalpha and steroidogenic acute regulatory protein decreased and ERbeta increased in both sexes. cDNA expression array analyses revealed that testes were affected more than ovaries and more genes showed an increase rather than a decrease in testes. The affected genes came from many unexpected families. Both null females and males had a decreased density of femur and became obese with age. The ovarian failure in knockout animals could not be reversed by estradiol/progesterone replacement therapy or by PMSG and human chorionic gonadotropin (hCG) injections. Although, testosterone replacement therapy of 30-60-day old null males partially improved spermatogenesis, the animals still remained infertile. A single testosterone injection on postnatal day 1 followed by 21-45-day testosterone replacement therapy beginning at 30 days of age, however, restored fertility. Studies showed that uterus of null animals could not initiate pregnancy even though the size and morphology were greatly improved by estradiol and progesterone replacement therapy. In general, non-gonadal phenotypes in null females and males were not completely reversed by hormone replacement therapy, suggesting that LH signaling could be important for their function. Heterozygous animals were indistinguishable from wild-type animals at 60 days of age. However, as they grew to about 1 year of age, they began to stop cycling, some became extremely obese, showed a decreased density of femur and all animals developed endometrial tumors with a cancer histology. LH receptor-knockout animals will be useful in advancing our present understanding on the importance of classical as well as non-classical actions of LH in the body, in advancing novel therapeutic uses of hCG, and in better understanding and rationalizing the consequences of inactivating type human LH receptor mutations.

The ectodomain of the luteinizing hormone receptor interacts with exoloop 2 to constrain the transmembrane region: studies using chimeric human and fly receptors

Lutropin (LH) and follitropin (FSH) receptors belong to a group of leucine-rich repeat-containing, G protein-coupled receptors (LGRs) found in vertebrates and flies. We fused the ectodomain of human LH or FSH receptors to the transmembrane region of fly LGR2. The chimeric human/fly receptors, unlike their wild type counterparts, exhibited ligand-independent constitutive activity. Because ectodomains likely interact with exoloops to constrain the receptors, individual exoloops of the chimeric receptor containing the ectodomain of the LH receptor and transmembrane region of fly LGR2 was replaced with LH receptor sequences. Chimeric receptors with the ectodomain and exoloop 2, but not exoloop 1 or 3, from LH receptors showed decreases in constitutive activity, but ligand treatment stimulated cAMP production. Furthermore, substitution of key resides in the hinge region of fly LGR2 with LH receptor sequences led to constitutive receptor activation; however, concomitant substitution of the homologous exoloop 2 of the LH receptor decreased G(s) coupling. These results suggest that the hinge region of the LH receptor interacts with exoloop 2 to constrain the receptor in an inactive conformation whereas ligand binding relieves this constraint, leading to G(s) activation.

EAR2 and EAR3/COUP-TFI regulate transcription of the rat LH receptor

Our previous studies demonstrated regulation of the human LH receptor (hLHR) promoter by nuclear orphan receptors EAR2, EAR3/COUP-TFI (repression), and TR4 (activation) through a direct-repeat motif (hDR). The current studies investigated the differential binding of orphan receptors to rat (rLHR) and hLHR promoters, and their modulation of rLHR gene transcription in rat granulosa cells. The rLHR DR with one nucleotide difference from hDR at its core sequence mediated inhibition of the rLHR transcription, to which EAR2 and EAR3/COUP-TFI but not TR4 bound. The A/C mismatch was responsible for the lack of TR4 binding and function, but had no effect on EAR2 and EAR3/COUP-TFI. EAR2 and EAR3/COUP-TF bound to the rLHR DR with lower affinity than to the hDR, and exhibited lesser inhibitory capacity. This difference resulted from the lack of a guanine in the rDR, which is present 3' next to the hDR core. These studies have identified sequence-specific requirements for the binding of EAR2, EAR3/COUP-TFI, and TR4 to the DRs that explain their differential regulation of rat and human LHR genes. In addition, hCG treatment significantly reduced the inhibition of rLHR gene in granulosa cells and also decreased EAR2 and EAR3/COUP-TFI protein levels. These results indicate that hormonally regulated expression of EAR2 and EAR3/COUP-TFI contributes to gonadotropin-induced derepression of LHR promoter activity in granulosa cells.

Structure and topology of a peptide segment of the 6th transmembrane domain of the Saccharomyces cerevisae alpha-factor receptor in phospholipid bilayers

A detailed analysis of the structure of an 18-residue peptide AQSLLVPSIIFILAYSLK [M6(252-269, C252A)] in 1,2-dimyristoyl-sn-glycero-phosphocholine bilayers was carried out using solid state NMR and attenuated total reflection Fourier transform infrared spectroscopy. The peptide corresponds to a portion of the 6th transmembrane domain of the alpha-factor receptor of Saccharomyces cerevisiae. Ten homologs of M6(252-269, C252A) were synthesized in which individual residues were labeled with (15)N. One- and two-dimensional solid state NMR experiments were used to determine the chemical shifts and (1)H-(15)N dipolar coupling constants for the (15)N-labeled peptides in oriented dimyristoylphosphatidylcholine bilayers on stacked glass plates. These parameters were used to calculate the structure and orientation of M6(252-269, C252A) in the bilayers. The results indicate that the carboxyl terminal residues (9-14) are alpha-helical and oriented with an angle of about 8 degrees with respect to the bilayer normal. Independently, an attenuated total reflection Fourier transform infrared spectroscopy analysis on M6(252-269, C252A) in a 1,2-dimyristoyl-sn-glycero-phosphocholine bilayer concluded that the helix tilt angle was about 12.5 degrees. The results on the structure of M6(252-269, C252A) in bilayers are in good agreement with the structure determined in trifluoroethanol/water solutions (B. Arshava et al. Biopolymers, 1998, Vol. 46, pp. 343-357). The present study shows that solid state NMR spectroscopy can provide high resolution information on the structure of transmembrane domains of a G protein-coupled receptor.

Molecular biology of the channel catfish gonadotropin receptors: 2. Complementary DNA cloning, functional expression, and seasonal gene expression of the follicle-stimulating hormone receptor

Molecular cloning of the channel catfish FSH receptor is reported together with temporal changes in the gene expression throughout a reproductive cycle. A cDNA encoding the receptor was isolated from the testis using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) procedures. The cDNA coded for a 662-amino acid protein that was most identical (51%-59%) to salmon gonadotropin receptor I and the FSH receptors of higher vertebrates, and less identical to LH receptors and thyrotropin receptors (45%-49% and 46%-47%, respectively). In addition, PCR analysis of the genomic DNA showed the absence of the LH receptor-specific intron. Expression of the channel catfish FSH receptor gene was highly restricted to the testis and ovary, except for a low-level expression in the spleen. Transfected COS cells expressed an active recombinant receptor as determined by the ligand-specific activation of a cAMP-responsive reporter gene (luciferase). The recombinant receptor was activated by human FSH and, to a small extent, hCG. Seasonal changes in the ovarian expression of the FSH receptor gene, examined by measuring the transcript abundance by quantitative real-time RT-PCR, showed a rise around the time of onset of ovarian recrudescence and a decrease prior to spawning. This pattern of seasonal expression of FSH receptor differs significantly from that of the LH receptor, which we reported recently. The differential expression of the two gonadotropin receptor genes, in addition to the differential secretion of the gonadotropic hormones, seem to be critical for the regulation of steroidogenesis and other gonadal physiological processes.

Piscine glycoprotein hormone (gonadotropin and thyrotropin) receptors: a review of recent developments

Similar to the higher vertebrates, the pituitary in bony fishes express three glycoprotein hormones: thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In addition to the appropriate secretion of these hormones, the timely and quantitative expression of their specific receptors (TSHR, FSHR and LHR) in the target tissues is an essential requirement for their physiological action. In fishes that constitute a very diverse group of vertebrates, there are only a few published reports of primary structure of these receptors although other examples have been communicated briefly. This review will summarize these reports as well as to describe the insights gained from what is known about the mammalian receptors. The structural organization of the fish receptors (as deduced from the encoding cDNAs) is highly homologous to the higher vertebrate receptors in that there is a 7-pass transmembrane region and an N-terminal extracellular domain, which contributes to ligand specificity. In mammals, the FSHR and the TSHR genes are composed of 10 exons whereas the LHR gene is composed of 11 exons. The position of the 'extra intron' is conserved in the catfish LHR gene. In the mammals, the transmembrane domain of each of the three glycoprotein hormone receptors is encoded by a single exon, however, in the salmon genes and homologous invertebrate genes, this portion of the receptor is encoded by multiple exons. In general, the tissue-specific expression of these receptors is similar to that seen in mammals, however, the gonadal expression of TSHR in the striped bass and sunrise sculpin and the renal expression of LHR in the channel catfish are unique.

Retention of glucose on oligosaccharide chains linked to the LH/hCG receptor prevents cell surface expression

In the first step of asparagine-linked oligosaccharide chain maturation, terminal glucose residues are removed from the high mannose oligosaccharide core by glucosidases I and II. The role that glucose residues play in trafficking the luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor from the endoplasmic reticulum to the cell surface was investigated. Glucosidases I and II were inhibited by incubating 293 T cells transiently transfected with LH/hCG receptor cDNA with 5 mM 1-deoxynojirimycin (DNJ). DNJ treatment resulted in a marked reduction in cell surface [(125)I]hCG binding. Similar results were obtained from glucosidase I-deficient Lec 23 Chinese hamster ovarian (CHO) cells and wild-type CHO cells that were transiently transfected with LH/hCG receptor cDNA. Immunoprecipitation followed by Western blotting of transfected 293 T cells incubated in the presence or absence of 5 mM DNJ revealed that there is substantially less receptor in DNJ-treated cells than in control cells. These results show that the removal of glucose residues is necessary for trafficking the LH/hCG receptor to the cell surface.

Palmitoylation of the luteinizing hormone/human chorionic gonadotropin receptor regulates receptor interaction with the arrestin-mediated internalization pathway

The luteinizing hormone/human chorionic gonadotropin receptor (LH/hCGR) undergoes palmitoylation at cysteine residues 621 and 622 located in the carboxyl terminal tail of the receptor. This study examined the biological function of palmitoylation with respect to its effect on receptor internalization. Coexpression of wild-type (WT) or C621/622G mutant receptors with arrestin-2 increased receptor internalization in 293T cells. Furthermore, measurements of rate enhancement upon overexpression of arrestin indicate that the palmitoylation deficient mutant receptor is more prone to utilizing the arrestin mediated internalization pathway than the WT receptor. Coexpression of G-protein-coupled receptor kinase 4 (GRK4) with wild type receptor resulted in an increase in internalization, while coexpression with the mutant receptor did not result in further enhancement of internalization. Additionally, 293T cells expressing mutant receptor were responsive to hCG with respect to production of inositol phosphates. Taken together, these results suggest that the palmitoylation state of the receptor governs internalization by regulating the accessibility of the receptor to the arrestin-mediated internalization pathway.

Molecular biology of channel catfish gonadotropin receptors: 1. Cloning of a functional luteinizing hormone receptor and preovulatory induction of gene expression

There is little known about the molecular biology of piscine gonadotropin receptors, and information about gene expression during reproductive development is particularly lacking. We have cloned the LH receptor (LHR) in the channel catfish (cc), and examined its gene expression throughout a reproductive cycle. A cDNA encoding the receptor was isolated from the testis using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends procedures. It encoded a 696-amino acid protein that showed the greatest homology (46-50% identity) with the known LHRs and lesser similarity with FSH receptors and thyroid-stimulating hormone receptors (44-47% and 42-44% identity, respectively). In addition, two characteristics unique to the LHRs were conserved in the cloned receptor and the encoding gene: presence of an intron corresponding to intron 10 in mammals and turkey and occurrence of a double cysteine residue in the cytoplasmic tail for potential palmitoylation. The ccLHR gene was well expressed in the gonads and kidney and merely detectable in the gills, muscle, and spleen. The isolated cDNA encoded an active ccLHR protein, as the recombinant receptor expressed in COS7 cells activated a cAMP response element-driven reporter gene (luciferase) upon exposure to hCG in a dose-dependent manner. Seasonal changes in the ovarian expression of the ccLHR gene, as examined by measuring the transcript abundance by quantitative real-time RT-PCR, remained rather low during most of the reproductive cycle but was acutely induced around the time of spawning. This pattern of expression correlates well with the reported expression of its ligand (LH) in fishes and concurs with the notion that LH is a key regulator of the periovulatory maturational events.

Juxtamembrane region of the amino terminus of the corticotropin releasing factor receptor type 1 is important for ligand interaction

The functional properties of the amino terminus (NT) of the corticotropin releasing factor (CRF) receptor type 1 (R1) were studied by use of murine (m) CRFR1 and rat (r) parathyroid hormone (PTH)/parathyroid hormone-related peptide receptor (PTH1R) chimeras. The chimeric receptor CXP, in which the NT of mCRFR1 was annealed to the TMs of PTH1R, and the reciprocal hybrid, PXC, bound radiolabeled analogues of sauvagine and PTH(3--34), respectively. Neither hybrid bound radiolabeled CRF or PTH(1--34). CRF and PTH(1--34) weakly stimulated intracellular cAMP accumulation in COS-7 cells transfected with PXC and CXP, respectively. Thus the NT is required for ligand binding and the TMs are required for agonist-stimulated cAMP accumulation. Replacing individual intercysteine segments of PXC with their mCRFR1 counterparts did not rescue CRF or sauvagine radioligand binding or stimulation of cAMP accumulation. Replacement of residues 1--31 of mCRFR1 with their PTH1R counterparts resulted in a chimeric receptor, PEC, which had normal CRFR1 functional properties. In addition, a series of chimeras (F1PEC--F6PEC) were generated by replacement of the NT intercysteine residues of PEC with their PTH1R counterparts. Only F1PEC, F2PEC, and F3PEC showed detectable CRF and sauvagine radioligand binding. All of the PEC chimeras except F5PEC increased cAMP accumulation. These data indicate that the Cys(68)(-)Glu(109) domain is important for binding and that the Cys(87)(-)Cys(102) region plays an important role in CRFR1 activation.

The role of the hinge region of the luteinizing hormone receptor in hormone interaction and signal generation

Luteinizing hormone receptor, a G protein-coupled receptor, consists of two halves, the N-terminal extracellular hormone binding domain (exodomain) and the C-terminal membrane-associated, signal-generating domain (endodomain). The exodomain has seven to nine Leu-rich repeats, which are generally thought to form a 1/3 donut-like structure and interact with human choriogonadotropin (hCG). The resulting hCG-exodomain complex adjusts the structure and its association with the endodomain, which results in signal generation in the endodomain. It is unclear whether the rigid 1/3 donut structure could provide the agility and versatility of this dynamic action. In addition, there is no clue as to where the endodomain contact point (the signal modulator) in the exodomain is. To address these issues, the exodomain was examined by Ala scan and multiple substitutions, while receptor peptides were used for photoaffinity labeling and affinity cross-linking. Our results show that the C-flanking sequence (hinge region), Thr(250)-Gln(268), of the Leu-rich repeats (LRRs) specifically interacts with hCG, preferentially hCGalpha. This interaction is inhibited by exoloop 2 of the endodomain but not by exoloops 1 and 3, suggesting an intimate relationship between Thr(250)-Gln(268), exoloop 2, and hCG. Taken together, our observations in this article suggest a new paradigm that the LRRs contact the front of hCG, while both flanking regions of the LRRs interact with the sides of hCG. This would trap hCG in the 1/3 donut structure of the LRRs and enhance the binding affinity. In addition, mutations of conserved Ser(255) in the sequence can constitutively activate the receptor. This provides a clue for the signal modulator in the exodomain. In contrast, a phenyl or phenolic group is necessary at conserved Tyr(253) for targeting the receptor to the surface.

Hormone interactions to Leu-rich repeats in the gonadotropin receptors. II. Analysis of Leu-rich repeat 4 of human luteinizing hormone/chorionic gonadotropin receptor

The luteinizing hormone receptor (LHR) consists of an approximately 350-amino acid-long N-terminal extracellular exodomain and a membrane-associated endodomain of similar size. Human chorionic gonadotropin (hCG) binds to the exodomain, and then hCG/exodomain complex is thought to make a secondary contact with the endodomain and generate hormone signals. The sequence alignment of the exodomain shows imperfectly matching eight to nine Leu-rich repeats (LRRs). In the preceding article (Song, Y., Ji, I., Beauchamp, J., Isaacs, N., and Ji, T. (2001) J. Biol. Chem. 276, 3426-3435), we have shown that LRR2 and LRR4 are crucial for hormone binding. In this work, we have examined the residues of LRR4, in particular Leu(103) and Ile(105) in the putative beta strand. Our data show that Leu(103) and Ile(105) are involved in the specific, hydrophobic interaction of the LRR4 loop, likely to form the hydrophobic core. This loop is crucial for the structural integrity of all of the LRRs. In contrast, the downstream sequence consisting of Asn(107), Thr(108), Gly(109), and Ile(110) of LRR4 is crucial for cAMP induction but not for hormone binding, folding, and surface expression. This implicates, for the first time, its involvement in the interaction with the endodomain and signal generation. The evidence for the interaction is presented in the following article.

Hormone interactions to Leu-rich repeats in the gonadotropin receptors. I. Analysis of Leu-rich repeats of human luteinizing hormone/chorionic gonadotropin receptor and follicle-stimulating hormone receptor

The luteinizing hormone receptor (LHR) and follicle-stimulating hormone receptor (FSHR) have an approximately 350-amino acid-long, N-terminal extracellular exodomain. This exodomain binds hormone with high affinity and specificity and contains eight to nine putative Leu-rich repeat (LRR) sequences. LRRs are known to assume the horseshoe structure in ribonuclease inhibitors, and the inner lining of the horseshoe consists of the beta-stranded Leu/Ile-X-Leu/Ile motif. In the case of ribonuclease inhibitors, these beta strands interact with ribonuclease. However, it is unclear whether the putative LRRs of LHR and FSHR play any role in the structure and function. In this work, the beta-stranded Leu/Ile residues in all LRRs of the human LHR and FSHR were Ala-scanned and characterized. In addition, the 23 residues around LRR2 of LHR were Ala-scanned. The results show that beta-stranded Leu and Ile residues in all LRRs are important but not equally. These Leu/Ile-X-Leu/Ile motifs appear to form the hydrophobic core of the LRR loop, crucial for the LRR structure. Interestingly, the hot spots are primarily in the upstream and downstream LRRs of the LHR exodomain, whereas important LRRs spread throughout the FSHR exodomain. This may explain the distinct hormone specificity despite the structural similarity of the two receptors.

Activation of the luteinizing hormone receptor following substitution of Ser-277 with selective hydrophobic residues in the ectodomain hinge region

Glycoprotein hormone receptors are G protein-coupled receptors with ligand-binding ectodomains consisting of leucine-rich repeats. The ectodomain is connected by a conserved cysteine-rich hinge region to the seven transmembrane (TM) region. Gain-of-function mutants of luteinizing hormone (LH) and thyroid-stimulating hormone receptors found in patients allowed identification of residues important for receptor activation. Based on constitutively active mutations at Ser-281 in the hinge region of the thyroid-stimulating hormone receptor, we mutated the conserved serine in the LH (S277I) and follicle-stimulating hormone receptors (S273I) and observed increased basal cAMP production and ligand affinity by mutant receptors. For the LH receptor, conversion of Ser-277 to all natural amino acids led to varying degrees of receptor activation. Hydropathy index analysis indicated that substitution of neutral serine with selective nonpolar hydrophobic residues (Leu>Val>Met>Ile) confers constitutive receptor activation whereas serine deletion or substitution with charged Arg, Lys, or Asp led to defective receptor expression. Furthermore, mutation of the angular proline near Ser-273 to flexible Gly also led to receptor activation. The findings suggest the ectodomain of glycoprotein hormone receptors constrain the TM region. Point mutations in the hinge region of these proteins, or ligand binding to these receptors, could cause conformational changes in the TM region that result in G(s) activation.

Cloning and functional expression of a thyrotropin receptor from the gonads of a vertebrate (bony fish): potential thyroid-independent role for thyrotropin in reproduction

The thyroid stimulating hormone receptor (TSHR) mediates the pituitary control of the development, growth and function of the thyroid. The expression of the gene encoding this receptor is known only in the thyroid, lymphocytes, fibroblasts, retro-orbital tissue and fat cells. We have cloned a TSHR from the gonads of a non-mammalian vertebrate, a bony fish (striped bass, Morone saxatilis) in the course of our search for gonadotropin receptors (follicle stimulating hormone receptor, FSHR and luteinizing hormone receptor, LHR). RT-PCR analysis demonstrated that the striped bass TSHR (stbTSHR) transcripts were abundant in both the thyroid and gonads and detectable in skeletal muscle, heart and brain tissues. The stbTSHR cDNA encoded a 779-amino acid glycoprotein hormone receptor with much higher homology (57-59%) to the mammalian TSH receptors than the mammalian LH receptors (47-49%) and FSH receptors (47%), and salmon and catfish gonadotropin receptors (42-45%). There was a TSHR-specific insertion in the extracellular domain as seen in mammalian receptors. Moreover, PCR analysis of genomic DNA indicated the absence of the LHR-specific intron in the striped bass TSHR gene. Recombinant stbTSHR expressed in COS1 cells activated reporter genes (luciferase) driven by either a cAMP response element or the c-fos promoter in response to bovine TSH, stbLH or hCG, but not human FSH. In situ hybridization studies revealed the presence of stbTSHR transcripts in the gametes but not in the follicular cells. This pattern of expression is unique and suggests a direct, albeit unknown, role for TSH in gamete physiology.

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

A limited spectrum of mutations causes constitutive activation of the yeast alpha-factor receptor

Activation of G protein coupled receptors (GPCRs) by binding of ligand is the initial event in diverse cellular signaling pathways. To examine the frequency and diversity of mutations that cause constitutive activation of one particular GPCR, the yeast alpha-factor receptor, we screened libraries of random mutations for constitutive alleles. In initial screens for mutant receptor alleles that exhibit signaling in the absence of added ligand, 14 different point mutations were isolated. All of these 14 mutants could be further activated by alpha-factor. Ten of the mutants also acquired the ability to signal in response to binding of desTrp(1)¿Ala(3)ălpha-factor, a peptide that acts as an antagonist toward normal alpha-factor receptors. Of these 10 mutants, at least eight alleles residing in the third, fifth, sixth, and seventh transmembrane segments exhibit bona fide constitutive signaling. The remaining alleles are hypersensitive to alpha-factor rather than constitutive. They can be activated by low concentrations of endogenous alpha-factor present in MATa cells. The strongest constitutively active receptor alleles were recovered multiple times from the mutational libraries, and extensive mutagenesis of certain regions of the alpha-factor receptor did not lead to recovery of any additional constitutive alleles. Thus, only a limited number of mutations is capable of causing constitutive activation of this receptor. Constitutive and hypersensitive signaling by the mutant receptors is partially suppressed by coexpression of normal receptors, consistent with preferential association of the G protein with unactivated receptors.

The ADP ribosylation factor nucleotide exchange factor ARNO promotes beta-arrestin release necessary for luteinizing hormone/choriogonadotropin receptor desensitization

Desensitization of guanine nucleotide binding protein-coupled receptors is a ubiquitous phenomenon characterized by declining effector activity upon persistent agonist stimulation. The luteinizing hormone/choriogonadotropin receptor (LH/CGR) in ovarian follicles exhibits desensitization of effector adenylyl cyclase activity in response to the mid-cycle surge of LH. We have previously shown that uncoupling of the agonist-activated LH/CGR from the stimulatory G protein (G(s)) is dependent on GTP and attributable to binding of beta-arrestin present in adenylyl cyclase-rich follicular membrane fraction to the third intracellular (3i) loop of the receptor. Here, we report that LH/CGR-dependent desensitization is mimicked by ADP ribosylation factor nucleotide-binding site opener, a guanine nucleotide exchange factor of the small G proteins ADP ribosylation factors (Arfs) 1 and 6, and blocked by synthetic N-terminal Arf6 peptide, suggesting that the GTP-dependent step of LH/CGR desensitization is receptor-dependent Arf6 activation. Arf activation by GTP and ADP ribosylation factor nucelotide-binding site opener promotes the release of docked beta-arrestin from the membrane, making beta-arrestin available for LH/CGR; Arf6 but not Arf1 peptides block beta-arrestin release from the membrane. Thus, LH/CGR appears to activate two membrane delimited signaling cascades via two types of G proteins: heterotrimeric G(s) and small G protein Arf6. Arf6 activation releases docked beta-arrestin necessary for receptor desensitization, providing a feedback mechanism for receptor self-regulation.

Cloning of partial putative gonadotropin hormone receptor sequence from fish

A search for the presence of mariner-like elements in the Labeo rohita genome by polymerase chain reaction led to the amplification of a partial DNA sequence coding for a putative transmembrane domain of gonadotropin hormone receptor. The amplified DNA sequence shows a high degree of homology to the available turkey and human luteinizing and follicle stimulating hormone receptor coding sequences. This is the first report on cloning such sequences of piscine origin.

Uncovering molecular mechanisms involved in activation of G protein-coupled receptors

G protein-coupled, seven-transmembrane segment receptors (GPCRs or 7TM receptors), with more than 1000 different members, comprise the largest superfamily of proteins in the body. Since the cloning of the first receptors more than a decade ago, extensive experimental work has uncovered multiple aspects of their function and challenged many traditional paradigms. However, it is only recently that we are beginning to gain insight into some of the most fundamental questions in the molecular function of this class of receptors. How can, for example, so many chemically diverse hormones, neurotransmitters, and other signaling molecules activate receptors believed to share a similar overall tertiary structure? What is the nature of the physical changes linking agonist binding to receptor activation and subsequent transduction of the signal to the associated G protein on the cytoplasmic side of the membrane and to other putative signaling pathways? The goal of the present review is to specifically address these questions as well as to depict the current awareness about GPCR structure-function relationships in general.

Nuclear orphan receptors regulate transcription of the gene for the human luteinizing hormone receptor

An imperfect estrogen receptor half-site response element direct-repeat, located within the TATA-less promoter of the human luteinizing hormone receptor (hLHR), was identified as an inhibitory site for Sp1/Sp3-driven basal transcription. Isolation of proteins recognizing this site by yeast one-hybrid screening of a human placenta cDNA library revealed three nuclear orphan receptors, EAR2, EAR3/COUP-TFI, and TR4. Electrophoresis mobility shift assays demonstrated that the in vitro translated nuclear orphan receptors specifically bound the direct-repeat motif of the hLHR promoter. Also, endogenous EAR2 and EAR3/COUP-TFI from JAR cell and human testis and TR4 from testes bound this motif in electrophoresis mobility shift assays. Functional analyses in CV-1 cells showed that EAR2 and EAR3/COUP-TFI repressed the hLHR promoter activity by up to 70% in a dose-dependent and sequence-specific manner. Conversely, TR4 activated the hLHR promoter activity up to 2.5-fold through binding to the same cis-element. The stimulation was reversed by coexpression of EAR2 or EAR3/COUP-TFI, indicating their competitive binding for this site. Such recognition of a common cognate site by the proteins with antagonistic functions implies that a net regulation of the hLHR gene may result from the relative availability of repressors and activator in a physiological state. This also may contribute to the differential expression of the hLHR gene in gonadal and non-gonadal tissues.

Identification and characterization of SorCS, a third member of a novel receptor family

A novel receptor, SorCS, was isolated from murine brain. It shows homology to the mosaic receptor SorLA and the neurotensin receptor sortilin based on a common VPS10 domain which is the hallmark of this new receptor family. In the N-terminus of SorCS two putative cleavage sites for the convertase furin mark the beginning of the VPS10 domain, followed by a module of imperfect leucine-rich repeats and a transmembrane domain. The short intracellular C-terminus contains consensus signals for rapid internalization. The identified putative binding motifs for SH2 and SH3 domains are unique in the family of VPS10 domain receptors. SorCS is predominantly expressed in brain, but also in heart, liver, and kidney. SorCS transcripts detected by in situ hybridization in the murine central nervous system point to a neuronal expression.

Male pseudohermaphroditism due to inactivating luteinizing hormone receptor mutations

Human male sexual development is regulated by chorionic gonadotropin (CG) and luteinizing hormone (LH), the action of both mediated by the LH receptor (LHR). Mutations that inactivate the LHR cause Leydig cell hypoplasia (LCH), an autosomal recessive disorder. In its mild form, LCH patients present with male hypogonadism. In its severe form, patients present with male pseudohermaphroditism, with female external genitalia, and cryptorchid testis. Mullerian derivatives are absent. Histological examination of the testis shows absence of mature Leydig cells. LCH patients have elevated plasma levels of LH, normal-to-elevated levels of follicle stimulating hormone (FSH), and low levels of testosterone that do not respond to CG stimulation. Missense mutations, nonsense mutations, deletion mutations, and in-frame insertion mutation of the LHR have been identified in patients with LCH. These mutations are not localized in any particular region of the gene and cause variable degrees of receptor-activity loss. The clinical manifestation of patients with LCH with homozygous or compound heterozygous mutations can be correlated with the residual activity of their respective mutated LHRs. Homozygous inactivating mutations of the LHR in the female cause hypergonadotrophic hypogonadism with primary amenorrhea or oligoamenorrhea, cystic ovaries, and infertility.

Identification of ligand effector binding sites in transmembrane regions of the human G protein-coupled C3a receptor

The human C3a anaphylatoxin receptor (C3aR) is a G protein-coupled receptor (GPCR) composed of seven transmembrane alpha-helices connected by hydrophilic loops. Previous studies of chimeric C3aR/C5aR and loop deletions in C3aR demonstrated that the large extracellular loop2 plays an important role in noneffector ligand binding; however, the effector binding site for C3a has not been identified. In this study, selected charged residues in the transmembrane regions of C3aR were replaced by Ala using site-directed mutagenesis, and mutant receptors were stably expressed in the RBL-2H3 cell line. Ligand binding studies demonstrated that R161A (helix IV), R340A (helix V), and D417A (helix VII) showed no binding activity, although full expression of these receptors was established by flow cytometric analysis. C3a induced very weak intracellular calcium flux in cells expressing these three mutant receptors. H81A (helix II) and K96A (helix III) showed decreased ligand binding activity. The calcium flux induced by C3a in H81A and K96A cells was also consistently reduced. These findings suggest that the charged transmembrane residues Arg161, Arg340, and Asp417 in C3aR are essential for ligand effector binding and/or signal coupling, and that residues His81 and Lys96 may contribute less directly to the overall free energy of ligand binding. These transmembrane residues in C3aR identify specific molecular contacts for ligand interactions that account for C3a-induced receptor activation.

Identification of ionizable amino acid residues on the extracellular domain of the lutropin receptor involved in ligand binding

The LH receptor (LHR) is a G protein-coupled receptor characterized by a relatively large N-terminal extracellular domain responsible for high affinity ligand binding. Based on a model proposed for a major portion of the extracellular domain that contains a number of leucine-rich repeats, nine ionizable amino acid residues (Glu57, Glu80, Lys158, Glu181, Lys183, Glu184, Glu188, Lys190, and Asp206) were selected for charge reversal mutagenesis based on their locations in the proposed model and their potential to serve as ligand contact sites. Mutant LHR complementary DNAs were transiently transfected into COS-7 cells, and the expressed receptors were characterized by Western blot analysis, competitive ligand (hCG) binding, and ligand-mediated cAMP production. The most interesting mutants were K158E, K183E, E184K, and D206K, which were present on the plasma membrane fraction, as judged by Western blots, but were incapable of binding hCG and, of course, were deficient in hCG-mediated cAMP production. Other replacements at these positions, K158R,Q,G; K183R,Q,G; E184N; and D206E,Q, led to cell surface binding and signaling. The mutants E57K, E189K, and K190E behaved similarly to wild-type LHR; E80K was trapped intracellularly, but bound ligand in solubilized cells; and E181K was not expressed or was rapidly degraded. These results, based on 18 point mutants of LHR, indicate that Lys158, Lys183, Glu184, and Asp206 are involved, either directly or indirectly, in gonadotropin binding and support the general nature of the proposed model.

The human luteinizing hormone receptor gene promoter: activation by Sp1 and Sp3 and inhibitory regulation

To understand the transcriptional mechanism(s) of human LH receptor (LHR) gene expression, we have identified the dominant functional cis-elements that regulate the activity of the promoter domain (-1 to -176 bp from ATG). Mutagenesis demonstrated that the promoter activity was dependent on two Sp1 domains (-79 bp, -120 bp) in a transformed normal placental cell (PLC) and the choriocarcinoma JAR cell. Both elements interacted with endogenous Sp1 and Sp3 factors but not with Sp2 or Sp4. In Drosophila SL2 cells, the promoter was activated by either Sp1 or Sp3. An ERE half-site (EREhs) at -174 bp was inhibitory (by 100%), but was unresponsive to estradiol and did not bind the estrogen receptor or orphan receptors ERR1 and SF-1. The 5' upstream sequence (-177 to -2056 bp) inhibited promoter activity in PLC by 60%, but only minimally in JAR cells. Activation of the human LHR promoter through Sp1/3 factors is negatively regulated through EREhs and upstream sequences to exert control of gene expression.

The luteinizing hormone receptor activates phospholipase C via preferential coupling to Gi2

Binding of lutropin/choriogonadotropin (LH/CG) to its cognate receptor results in the activation of adenylyl cyclase and phospholipase C. This divergent signaling of the LH receptor is based on the independent activation of distinct G protein subfamilies, i.e. , Gs, Gi, and potentially also Gq. To examine the selectivity of LH receptor coupling to phospholipase C beta-activating G proteins, we used an in vivo reconstitution system based on the coexpression of the LH receptor and different G proteins in baculovirus-infected insect cells. In this paper, we describe a refined expression strategy for the LH receptor in insect cells. The receptor protein was inserted into the cell membrane at an expression level of 0.8 pmol/mg of membrane protein. Sf9 cells expressing the LH receptor responded to hCG challenge with a concentration-dependent accumulation of intracellular cAMP (EC50 = 630 nM) but not of inositol phosphates, whereas stimulation of the histamine H1 receptor in Sf9 cells led to increased phospholipase C (PLC) activity. Immunoblotting experiments using G protein-specific antisera revealed the absence of quantitative amounts of alpha i in Sf9 cells, whereas alpha s and alpha q/11 were detected. We therefore attempted to restore the hCG-dependent PLC activation by infection of Sf9 cells with viruses encoding the LH receptor and different G protein alpha subunits. HCG stimulation of cells coexpressing the LH receptor and exogenous alpha i2 resulted in stimulation of PLC activity. In cells coinfected with an alpha i3-baculovirus, hCG challenge led to a minor activation of PLC, whereas no hCG-dependent PLC stimulation was observed in cells coexpressing alpha i1. Most notably, coinfection with baculoviruses encoding alpha q or alpha 11 did not reproduce the PLC activation by the LH receptor. Thus, the murine LH receptor activates adenylyl cyclase via Gs and PLC via selective coupling to Gi2.

Characterization of a region of the lutropin receptor extracellular domain near transmembrane helix 1 that is important in ligand-mediated signaling

The lutropin receptor (LHR), a member of the G protein-coupled receptor family, contains a relatively large N-terminal extracellular domain, accounting for about half of the receptor and responsible for high affinity ligand binding, and a standard heptahelical portion with connecting loops and a C-terminal tail. LHR and the other two glycoprotein hormone receptors, i.e. the follitropin and TSH receptors, contain an invariant 10-amino acid residue sequence, FNPCEDIMGY (residues 328-337 in rat LHR), in the extracellular domain separated by only a few amino acid residues from the beginning of transmembrane helix 1. In view of the invariant nature of this region in the three glycoprotein hormone receptors and preliminary data in the literature on the importance of Glu332 and Asp333 in signal transduction, we undertook a systematic investigation of all 10 amino acid residues because this region may function as a switch or trigger for communicating ligand binding to the extracellular domain with a conformational change of the membrane-embedded C-terminal half of the receptor to activate G proteins, particularly Gs. A total of 36 single, double, and multiple replacements, as well as two deletions, of LHR were prepared and characterized in transiently transfected COS-7 cells. Of these mutants LHRs, 26 expressed on the cell surface in sufficient numbers that quantitative assessments could be made of human choriogonadotropin binding and ligand-mediated cAMP production. Replacements of Cys331 abolished ligand binding to intact cells, although binding could be detected after solubilization of the cells. Replacements of the other nine amino acid residues that did not interfere with receptor folding or trafficking had no significant effect on ligand binding affinity; however, replacements of Pro330, Glu332, and Asp333 resulted in diminished signaling, especially for the two acidic residues. An interesting observation was made in which replacement of Tyr337 with Ala or Asp, while having no profound change on receptor function, could overcome to some extent limited expression of replacements at positions 332 and/or 333, thus permitting a more definitive analysis of signaling. Replacement of the decapeptide sequence with Gly10 prevents expression, whereas deletion of all 10 residues and deletion of Glu332-Asp333 prevents functional expression at the cell surface. Thus, this invariant sequence in the glycoprotein hormones is required for proper folding, trafficking, and ligand-mediated signaling, but not ligand binding, in LHR. Amino acid residues, Glu332, Asp333, and to a limited extent, Pro330, are important in ligand-mediated signaling but not ligand binding.

Luteinizing hormone inhibits conversion of pregnenolone to progesterone in luteal cells from rats on day 19 of pregnancy

We have previously reported that intrabursal ovarian administration of LH at the end of pregnancy in rats induces a decrease in luteal progesterone (P4) synthesis and an increase in P4 metabolism. However, whether this local luteolytic effect of LH is exerted directly on luteal cells or on other structures, such as follicular or stromal cells, to modify luteal function is unknown. The aim of the present study was to determine the effect of LH on isolated luteal cells obtained on Day 19 of pregnancy. Incubation of luteal cells with 1, 10, 100, or 1000 ng/ml of ovine LH (oLH) for 6 h did not modify basal P4 production. The addition to the culture medium of 22(R)-hydroxycholesterol (22R-HC, 10 microgram/ml), a membrane-permeable P4 precursor, or pregnenolone (10(-2) microM) induced a significant increase in P4 accumulation in the medium in relation to the control value. When luteal cells were preincubated for 2 h with oLH, a significant (p < 0.01) reduction in the 22R-HC- or pregnenolone-stimulated P4 accumulation was observed. Incubation of luteal cells with dibutyryl cAMP (1 mM, a cAMP analogue) plus isobutylmethylxanthine (1 mM, a phosphodiesterase inhibitor) also inhibited pregnenolone-stimulated P4 accumulation. Incubation with an inositol triphosphate synthesis inhibitor, neomycin (1 mM), or an inhibitor of intracellular Ca2+ mobilization, (8,9-N, N-diethylamino)octyl-3,4,5-trimethoxybenzoate (1 mM), did not prevent the decrease in pregnenolone-stimulated P4 secretion induced by oLH. It was concluded that the luteolytic action of LH in late pregnancy is due, at least in part, to a direct action on the luteal cells and that an increase in intracellular cAMP level might mediate this effect.

Genomic distribution and gonadal mRNA expression of two human luteinizing hormone receptor exon 1 sequences in random populations

Exon 1 of the human luteinizing hormone receptor (LHR) gene coding region exhibits at least two forms of sequence heterogeneity between 37 and 60 bp, spanning the junction of the signal peptide and the amino terminus of the mature protein. The LHR 1 differs from the LHR 2 by the insertion of 6 bp in exon 1 but is of identical sequence in the 5' flanking region. RFLP analysis of the two haplotypes within a random population of 63 individuals revealed allele frequencies of 0. 37 and 0.63 for LHR 1 and LHR 2, respectively. 94% of the samples contained at least one LHR 2 allele, whereas only 68% contained the LHR 1 allele. No gender differences were observed, and both homozygotes and heterozygotes displayed apparently normal reproduction. Reverse-transcriptase polymerase chain-reaction analyses of LHR mRNA from testes and ovaries revealed that both haplotypes are transcribed in normal individuals, with no difference in tissue specific distribution. Thus, at least two functional polymorphic forms of exon 1 coding region of the same LHR gene are present in a random human population.

Characterization of two LGR genes homologous to gonadotropin and thyrotropin receptors with extracellular leucine-rich repeats and a G protein-coupled, seven-transmembrane region

The receptors for LH, FSH, and TSH belong to the large G protein-coupled, seven-transmembrane (TM) protein family and are unique in having a large N-terminal extracellular (ecto-) domain containing leucine-rich repeats important for interaction with the glycoprotein ligands. We have identified two new leucine-rich repeat-containing, G protein-coupled receptors and named them as LGR4 and LGR5, respectively. The ectodomains of both receptors contain 17 leucine-rich repeats together with N- and C-terminal flanking cysteine-rich sequences, compared with 9 repeats found in known glycoprotein hormone receptors. The leucine-rich repeats in LGR4 and LGR5 are arrays of 24 amino acids showing similarity to repeats found in the acid labile subunit of the insulin-like growth factor (IGF)/IGF binding protein complexes as well as slit, decorin, and Toll proteins. The TM region and the junction between ectodomain and TM 1 are highly conserved in LGR4, LGR5, and seven other LGRs from sea anemone, fly, nematode, mollusk, and mammal, suggesting their common evolutionary origin. In contrast to the restricted tissue expression of gonadotropin and TSH receptors in gonads and thyroid, respectively, LGR4 is expressed in diverse tissues including ovary, testis, adrenal, placenta, thymus, spinal cord, and thyroid, whereas LGR5 is found in muscle, placenta, spinal cord, and brain. Hybridization analysis of genomic DNA indicated that LGR4 and LGR5 genes are conserved in mammals. Comparison of overall amino acid sequences indicated that LGR4 and LGR5 are closely related to each other but diverge, during evolution, from the homologous receptor found in snail and the mammalian glycoprotein hormone receptors. The identification and characterization of new members of the LGR subfamily of receptor genes not only allow future isolation of their ligands and understanding of their physiological roles but also reveal the evolutionary relationship of G protein-coupled receptors with leucine-rich repeats.