Purification and characterization of Leydig cell luteinizing hormone receptor

Leptin secreted from testicular microenvironment modulates hedgehog signaling to augment the endogenous function of Leydig cells

Although testosterone deficiency (TD) may be present in one out of five men 40 years or older, the factors responsible for TD remain largely unknown. Leydig stem cells (LSCs) differentiate into adult Leydig cells (ALC) and produce testosterone in the testes under the pulsatile control of luteinizing hormone (LH) from the pituitary gland. However, recent studies have suggested that the testicular microenvironment (TME), which is comprised of Sertoli and peritubular myoid cells (PMC), plays an instrumental role in LSC differentiation and testosterone production under the regulation of the desert hedgehog signaling pathway (DHH). It was hypothesized that the TME releases paracrine factors to modulate LSC differentiation. For this purpose, cells (Sertoli, PMCs, LSCs, and ALCs) were extracted from men undergoing testis biopsies for sperm retrieval and were evaluated for the paracrine factors in the presence or absence of the TME (Sertoli and PMC). The results demonstrated that TME secretes leptin, which induces LSC differentiation and increases testosterone production. Leptin's effects on LSC differentiation and testosterone production, however, are inversely concentration-dependent: positive at low doses and negative at higher doses. Mechanistically, leptin binds to the leptin receptor on LSCs and induces DHH signaling to modulate LSC differentiation. Leptin-DHH regulation functions unidirectionally insofar as DHH gain or loss of function has no effect on leptin levels. Taken together, these findings identify leptin as a key paracrine factor released by cells within the TME that modulates LSC differentiation and testosterone release from mature Leydig cells, a finding with important clinical implications for TD.

Expression of the mature luteinizing hormone receptor in rodent urogenital and adrenal tissues is developmentally regulated at a posttranslational level

The LH receptor (LHR) is a G protein-coupled receptor involved in the regulation of ovarian and testicular functions. In this study we demonstrate novel and unexpected patterns of receptor expression and regulation in fetal and adult rodent urogenital and adrenal tissues. Two rat LHR promoter fragments (approximately 2 and 4 kb) were shown to direct expression of the lacZ reporter in transgenic mice to gonads, adrenal glands, and kidneys, starting at 14.5 d post coitum, and to genital tubercles, starting at 11.5 d post coitum. These tissues were also found to express the full-length LHR mRNA and protein during rat fetal development, but, importantly, only immature receptors carrying unprocessed N-linked glycans were detected. After birth, the receptor gene activity ceased, except in the gonads, which started to express the mature receptor carrying fully processed N-linked glycans. Surprisingly, both LHR mRNA and mature protein levels were up-regulated substantially in pregnant female adrenal glands and kidneys at a time that coincides with differentiation of fetal urogenital tissues. Taken together, these results indicate that the LHR protein is expressed constitutively in gonadal and nongonadal urogenital tissues as well in adrenal glands, but its final functional maturation at the posttranslational level appears to be developmentally and physiologically regulated.

Partial characterization of chorionic gonadotropin-like binding sites from the bacteria Xanthomonas maltophilia

The gram-negative bacterium, Xanthomonas maltophilia, has low- and high-affinity luteinizing hormone/chorionic gonadotropin (LH/CG)-binding sites, similar to the LH/CG receptor found in mammals. Although the low-affinity site binds both LH and human CG (hCG), the high-affinity site is specific for hCG. In the current investigation, these two binding sites were independently isolated from X. maltophilia for further characterization. To isolate functional binding sites, we developed a solubilization method using the detergent zwittergent 3,14 and high glycerol concentrations that allowed for the maintenance of ligand-binding integrity. Gel filtration experiments established molecular weights of 170 and 11.5 kDa for the two binding sites, which were supported by data from photoaffinity labeling and ultracentrifugation experiments. Gel filtration data also suggested the presence of a third binding site of 5.4 kDa. The 170-kDa site had a binding affinity of Kd = 12 x 10(-6) and bound both LH and hCG. The small molecular weight site had an affinity of Kd = 9.4 x 10(-8) and was CG specific. Collectively, these data demonstrate the presence of multiple hormone binding sites in X. maltophilia that differ in molecular size, binding affinity, and ligand specificity.

Structure and functional significance of the carbohydrates of the LH/CG receptor

The LH/CG appears to contain 1-4 bi- or multi-antennary complex-type N-linked oligosaccharide side chains, which appear to locate apart from the hormone-binding regions. The exact sites to which the N-linked chains are attached remain to be delineated. The carbohydrates of the mature membrane-inserted receptor do not contribute to either specific high-affinity ligand-binding or signal transduction of the receptor. Thus, the polypeptide core of the receptor is responsible for both high affinity binding and dictating the hormone specificity. Moreover, the deglycosylated receptor, once inserted to the plasma membrane in a functionally mature form, retains its functional conformation or permits the conformational change that is required for coupling of the receptor to effector enzymes. Addition of oligosaccharides to the nascent LH/CG receptor but not their subsequent conversion to complex-type ones appears to be required for acquiring the hormone-binding conformation. On the other hand, neither addition of oligosaccharides to the nascent receptor, nor their further maturation are needed for the transport of the receptor to the plasma membrane. Thus, one function of the N-linked oligosaccharides in the LH/CG receptor appears to be to direct the proper folding of the receptor.

Functional glycosylation sites of the rat luteinizing hormone receptor required for ligand binding

The contribution of N-linked glycosylation to the ligand binding activity of the rat luteinizing hormone receptor (LHR) was studied in wild-type and mutant LHR expressed in mammalian (COS1) cells and overexpressed in insect (Sf9) cells. The binding affinities of the holoreceptor and its truncated splice variant (form B) lacking the transmembrane domain were equivalent in both cell lines. Tunicamycin-treated transfected Sf9 cells expressed a carbohydrate-free LH receptor that lacked hormone binding activity. Functional carbohydrate chains essential for binding activity were localized to glycosylation sites at Asn-173 and Asn-152. Glycosidase treatment of the double mutant N173Q/N152Q revealed the presence of at least one additional carbohydrate chain at Asn-269, Asn-277, or Asn-291 that does not contribute to hormone binding. Asn-77 was not glycosylated, but its mutation to Gln reduced hormone binding. LHR expressed in insect cells contained only high mannose carbohydrate chains, and those located at Asn-173 and Asn-152 were sufficient for high-affinity hormone binding. Enzymatic cleavage of glycosyl chains indicated that only the proximal N-acetylglucosamine residue, which is common to high mannose and complex carbohydrate forms, is necessary for acquisition of the high affinity conformation of the receptor. The carbohydrate chains of the LHR appear to be involved in intramolecular folding of the nascent receptor rather than in its interaction with the hormone.

Enhancement of ganglioside GM3 synthesis in okadaic-acid-treated granulosa cells

Okadaic acid is a potent inhibitor of type-2A (PP2A) and type-1 (PP1) protein phosphatases and has been proved to be a valuable tool for studies on the protein phosphorylation. We have investigated the effects of okadaic acid on rat granulosa cells in order to determine whether the regulation of ganglioside synthesis involves protein phosphorylation via inhibition of PP2A and PP1. Granulosa cells expressed luteinizing hormone (LH) receptors, measured as the binding of 125I-deglycosylated human chorionic gonadotropin (hCG) to intact cells, and synthesized the gangliosides NeuAc alpha 2-->3Gal beta 1-->4Glc beta 1-->1Cer (GM3) and Gal beta 1-->3GalNAc beta 1-->4[NeuAc alpha 2-->3]Gal beta 1-->4Glc beta 1-->1Cer (GM1), demonstrated by metabolic labeling of glycosphingolipids with [3H]galactose, in response to follicle-stimulating hormone (FSH). When FSH-stimulated granulosa cells were treated with 10 nM okadaic acid for 15 h, down-regulation of LH receptors, dissociation of LH receptor-effector coupling and significant decreases of intracellular and extracellular 3',5'-cyclic adenosine monophosphate (cAMP) levels were observed. The okadaic acid-induced desensitization to gonadotropin in granulosa cells was accompanied by increased ganglioside synthesis. The amount of 3H-labeled ganglioside GM3, the major ganglioside (about 95% of the total) synthesized by mature granulosa cells, was enhanced in okadaic acid-desensitized cells (to 215% of the control value) and in those desensitized by hCG (by 354%), forskolin (by 190%) and 12-O-tetradecanoylphorbol 13-acetate (by 143%). The results of this study suggest that an increase in the phosphorylation state of cells is accompanied by enhancement of ganglioside synthesis.

Changes in binding activity of luteinizing hormone receptors by site directed mutagenesis of potential glycosylation sites

Site directed mutagenesis of the rat ovarian luteinizing hormone (LH) receptor cDNA was performed at each of the six potential N-linked glycosylation sites to determine the effect of putative carbohydrate chains on the activity of the membrane receptor. The conversion of Asn173 to Gln resulted in the total loss of hormone binding to the surface of the transfected cell. Mutant receptors synthesized with substitutions at the remaining potential N-linked glycosylation positions of 77, 152, 269, 277 and 291 revealed no significant change in the hormone affinity. However Asn77Gln and Asn152Gln exhibited significant decreases (approximately 80%) in the number of high affinity hormone binding sites. The changes in hormone binding activity upon elimination of the potential glycosylation sites at 77, 152 and 173 indicate the presence of functional carbohydrate chains at these positions in the rat ovarian LH/hCG receptor.

Human choriogonadotropin-induced coupling of receptor and Gs protein and the effect of hormone deglycosylation

The detergent-soluble extract of rat ovary plasma membranes contained a Gs protein of about 100 kDa as shown by its elution behavior on a Bio Gel A-1.5m column. However, the cell membranes exposed to hCG (37 degrees C, 15 min) contained in addition a higher molecular weight Gs protein complex of 300 kDa comprised of human chorionic gonadotropin (hCG) receptor (hCGR) and Gs. The complex bound with an affinity column of GTP-Sepharose and could be released with Gpp(NH)p and GTP inhibited this binding. The presence of the hCGR in the complex was shown by its binding to 125I-hCG. Furthermore, GTP inhibited the binding of hCG to the complex. These results indicate the presence of hCGR and Gs protein complex in the hCG-treated membranes. hCGR and Gs protein were individually purified and reconstituted into phospholipid vesicles. The protein-phospholipid vesicles showed saturation kinetics of binding of 125I-hCG and 3H-Gpp(NH)p. Incubation of phospholipid vesicles with hCG resulted in a 2-3-fold increase in the binding of 3H-Gpp(NH)p and GTPase activity. Activation of Gs protein was dependent on the length of incubation and the hormone concentration. Deglycosylated hCG was about 10 times less potent than hCG suggesting a role of carbohydrates of hCG in inducing hCG-Gs protein interactions. The data with the in vitro reconstitution system rule out the involvement of a carbohydrate-binding lectin in the function of the hormone.

Cloning and sequencing of human LH/hCG receptor cDNA

We have isolated and sequenced a cDNA encoding the human luteinizing hormone--choriogonadotropin (LH/hCG) receptor. The deduced amino acid sequence (699 residues) containing seven putative transmembrane segments displays sequence similarity to G protein-coupled receptors. The receptor consists of 335 residue extracellular domain which contains six N-linked glycosylation sites. While the protein is 85 and 87% identical overall with the previously cloned rat and porcine LH/hCG receptor respectively, the most highly conserved regions are the putative transmembrane segments (91 and 94% similarity, respectively).

Purification and characterization of the human ovarian LH/hCG receptor and comparison of the properties of mammalian LH/hCG receptors

Methods previously published by us [Wimalasena et al., J Biol Chem 260: 10689-10697, 1985; Wimalasena et al., J Biol Chem 261: 9416-9420, 1986] were utilized to solubilize the human corpus luteal leuteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor with 3-[(3-cholamide-propyl)dimethylammonio]-1-propanesulfonate (CHAPS) and to purify the receptor by two steps of hCG-Sepharose affinity chromatography. The specific binding capacity (SBC) of the purified human receptor was 7510 pmol/mg protein, and KA = 2.2 x 10(9) M-1 when iodo hCG was competed by hCG; the yield was 4-7% of starting activity. When hLH was used in competition with hCG, specific binding capacity was 7900 pmol/mg protein and KA 1.0 x 10(9) M-1. Silver staining and autoradiography demonstrated a single protein of Mr 78,000 under reducing and Mr 58-62 x 10(3) under nonreducing conditions. Rat ovarian LH/hCG receptor was purified by similar methods and the KA of 3.5 x 10(10) M-1 for hCG was substantially different from the KA for hLH which was 2.1 x 10(9) M-1. Mr of the rat protein was 78-82 x 10(3) (reduced) and 58-62 x 10(3) (nonreduced) when analyzed by silver staining and autoradiography. For the first time, human LH/hCG receptor has been purified to apparent homogeneity, and its Mr of 78,000 was essentially identical to the Mr values of purified rat and porcine receptors.

Structure of the lutropin/choriogonadotropin receptor

In summary, the LH/CG receptor is a single polypeptide which contains a large hydrophilic domain that is situated extracellularly, attached to a region that spans the plasma membrane seven times, the carboxy-terminal region being intracellular. This topology was predicted by the amino acid sequence and has been confirmed by our immunofluorescence studies. The extracellular domain, which is related to a family of leucine-rich glycoproteins, is presumably involved in binding the large glycoprotein hormones hCG and LH. The carboxy-terminal half of the receptor, which is related to the family of rhodopsinlike receptors, is (by analogy with these receptors) presumably involved in the coupling of the receptor to the G protein. Our transfection studies confirm that this single polypeptide is capable of binding hormone and activating adenylyl cyclase. Therefore, not only is the structure of the LH/CG receptor unique compared to other cell surface receptors characterized to date, but also its structure suggests that the mechanism of the translation of hormone binding to G protein coupling in this receptor is different from other G protein-coupled receptors whose ligands are much smaller and intercalcate among the transmembrane helices. We predict that, due to the homology among the glycoprotein hormones, the structures of the FSH and TSH receptors share extensive amino acid and structural homology with the LH/CG receptor. Last, our newly acquired knowledge about the structure of the LH/CG receptor, and the development of a cDNA and antibodies for this receptor, should enable more detailed studies on the function and regulation of the LH/CG receptor, not previously possible.

Characterization and structure of ovarian and testicular LH/hCG receptors

Ovarian and Leydig cell LH/hCG receptors purified to homogeneity were identified as a single protein of Mr 80,000 and 90,000 respectively. The homogeneity of this protein was confirmed by microsequencing of the first 18 amino acids of the ovarian receptor. The unblocked N-terminal peptide consisted of NH2-R-E-L-S-G-S-R-X-P-E-P-X-D-X-A-P-D-G. These receptors are N-linked sialoglycoproteins which accounts for the size difference between testicular and ovarian receptors and may participate in the interaction with gonadotropin. Crosslinking of pure receptor with hCG with 125I label in either subunit indicated significant interaction of alpha-hCG with the receptor, while beta-hCG seems involved mostly through association and conformational influence on the alpha-subunit. Comparison of Mr derived from SDS with those from FPLC suggested that the native LH receptor are dimers of identical subunits. Autoradiographs of blotted receptors demonstrated that both monomeric and dimeric forms can bind hCG. Receptors from both tissues can be phosphorylated by the catalytic subunit of cAMP-dependent protein kinase and phosphopeptide maps were identical. Receptor occupancy by agonist leads to a conformational change which facilitates its phosphorylation during initial binding and reduces the rate of phosphorylation after more prolonged exposure to gonadotropin. Aggregation or dimerization of the hCG/LH receptors could promote clustering and or crosslinking of receptors in the membrane favouring the initial transduction steps in the action of these hormones.

Lactogen and LH receptors. Rat P-450 17 alpha, structural analysis and hormonal regulation of mRNA levels in the Leydig cell

A complete amino acid sequence for rat testis P-450 17 alpha was deduced from nucleotide analysis of a cDNA clone isolated from a rat Leydig cell library. This cDNA expressed in COS-1 cells both 17 alpha-hydroxylase and 17,20 lyase activities. Rat P-450 17 alpha exhibited significant similarity to the nucleotide and deduced amino acid sequences of the bovine and human P-450 17 alpha, particularly with the highly conserved regions and secondary structure. The rat P-450 17 alpha is anchored to the ER by two transmembrane regions: the N-terminal insertion peptide and the stop-transfer sequence. The C-terminal is associated with the ER by four hydrophobic clefts including the steroid-binding site. We have demonstrated a dual effect of hCG, causing early increases of Leydig cell P-450 17 alpha mRNA levels at low doses, while higher desensitizing doses caused marked subsequent reduction of mRNA levels. Our studies demonstrate that gonadotropin stimulation and desensitization of P-450 17 alpha dependent enzymes (17 alpha-hydroxylase/17,20 desmolase) in the adult rat testis and E2 induced desensitization in fetal Leydig cells are related to levels of P-450 17 alpha mRNA.