Attempt to map the receptor binding sites of human follicle-stimulating hormone using disulfide peptides of its beta-subunit indicates major involvement of the regions around disulfide bonds Cys28-Cys82 and Cys32-Cys84 in receptor binding of the hormone.

Follicle-stimulating hormone (FSH) is a heterodimeric glycoprotein hormone secreted by the anterior pituitary. It plays a very important role in folliculogenesis in females and is responsible for spermatogenesis in males. The alpha-subunit which is common within a species and the beta-subunit which is hormone-specific are held together by noncovalent association. This association is very essential for the biological activity of the hormone. Each of these subunits are highly cross-linked by disulfide bonds which appear to stabilize the tertiary structures required for the noncovalent association of the subunits to generate hormonal activity. This study was initiated to delineate the role of the disulfide bonds of hFSH beta in receptor binding of the hormone. Five intermolecular and one intramolecular disulfide peptides corresponding to the disulfide bonds found in hFSH beta were synthesized and screened along with their linear counterparts, for their ability to competitively inhibit the radiolabelled [125I]hFSH from binding to the FSH receptor containing membranes from the testis of immature rats. The disulfide peptides Cys28-Cys82 and Cys32-Cys84 were found to be the most potent in inhibiting radiolabelled hFSH from binding to its receptor. The results suggest the involvement of the regions around disulfide bonds Cys28-Cys82 and Cys32-Cys84 in receptor binding of the hormone. The studies also suggest the involvement of beta L2 and beta L3 loop regions in receptor binding of the hormone. This study is the first of its kind to use disulfide peptides rather than linear peptides to map the receptor binding regions of hFSH.     

Disulfide bonds Cys(9)-Cys(57), Cys(34)-Cys(88) and Cys(38)-Cys(90) of the beta-subunit of human chorionic gonadotropin are crucial for heterodimer formation with the alpha-subunit: experimental evidence for the conclusions from the crystal structure of hCG

Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hormone essential for the establishment and maintenance of pregnancy. The alpha- and beta-subunits of hCG are highly cross-linked internally by disulfide bonds that seem to stabilize the tertiary structures required for the noncovalent association of the subunits to generate hormonal activity. This paper describes the results of our studies on the role of the disulfide bonds of hCG-beta in heterodimer formation with the alpha-subunit. Six disulfide peptides incorporating each of the six disulfide bonds of hCG-beta were screened, along with their linear counterparts, for their ability to competitively inhibit the recombination of alpha- and beta-subunits. The disulfide peptides Cys (9-57), Cys (34-88) and Cys (38-90) were found to inhibit the alpha/beta recombination whereas the remaining three disulfide peptides viz. Cys (23-72), Cys (26-110) and Cys (93-100) did not exhibit any inhibition activity. Interestingly, none of the linear peptides could inhibit the alpha/beta recombination. Results clearly demonstrate that the disulfide bonds Cys(9)-Cys(57), Cys(34)-Cys(88) and Cys(38)-Cys(90) of the beta-subunit of hCG are crucial for heterodimer formation with the alpha-subunit thus providing experimental confirmation of the conclusions from the crystal structure of the hormone.     

SDS-PAGE electrophoretic property of human chorionic gonadotropin (hCG) and its beta-subunit

The microheterogeneity property of hCG with regards to its sialic acid contents resulted in variable mobility of the glycoprotein in SDS-PAGE. The intact hCG molecule is composed of two dissimilar subunits, namely alpha- and beta-subunits. The identification of hCG bands in SDS-PAGE was accomplished by the immunoblotting experiment, whereby the antibody directed toward the specific region of beta-subunit of hCG was used. The data shows that the different mobility of intact hCG was attributed to the different degree of desialylation of the glycoprotein. Nevertheless, unlike the intact hCG, the mobility of its beta-subunit was not affected by its variety sialic acid content. This characteristic of beta-hCG is beneficial when semi-quantification of total hCG is required. Quantification of hCG using the HPLC-reversed phase C18 analytical column is not possible as the glycoprotein was eluted in multiple fractions at different retention times. The identification of denatured hCG (HPLC eluted fractions) was carried out by immunoblotting experiment whilst immunoassay technique failed to detect its presence in any fraction.     

Labeling of bovine corpus luteal plasma membrane human chorionic gonadotropin or luteinizing hormone (hCG/LH) receptor and its purification and properties.

A method has been developed for labeling receptors for human chorionic gonadotropin or luteinizing hormone ($\text{hCG}\text{LH}$) present on bovine corpus luteal plasma membranes. It consists of four steps: (a) protection of the receptor by treating the plasma membranes with hCG; (b) iodination of the membranes with KI using glucose, glucose oxidase, and lactoperoxidase; (c) unmasking the receptor with either 2 m NaCl, 1 m guanidine hydrochloride, or rabbit anti-hCG; and (d) reiodination of the membranes using Na¹³¹I. After solubilization by successive treatments with Sepharose-concanavalin A and Sepharose-hCG and finally by preparative disc electrophoresis, the resulting purified receptor after electrophoresis in polyacrylamide gel showed a single radioactive band containing receptor activity. This highly purified receptor is fairly stable and retains its hormonal specificity, binding affinity, and pH optimum. It was observed that the receptor alone or as a complex with the hormone tends to aggregate. The receptorhormone complex does not dissociate during polyacrylamide-gel electrophoresis.     

A low molecular weight agonist signals by binding to the transmembrane domain of thyroid-stimulating hormone receptor (TSHR) and luteinizing hormone/chorionic gonadotropin receptor (LHCGR)

Many cognate low molecular weight (LMW) agonists bind to seven transmembrane-spanning receptors within their transmembrane helices (TMHs). The thienopyrimidine org41841 was identified previously as an agonist for the luteinizing hormone/chorionic gonadotropin receptor (LHCGR) and suggested to bind within its TMHs because it did not compete for LH binding to the LHCGR ectodomain. Because of its high homology with LHCGR, we predicted that thyroid-stimulating hormone receptor (TSHR) might be activated by org41841 also. We show that org41841 is a partial agonist for TSHR but with lower potency than for LHCGR. Analysis of three-dimensional molecular models of TSHR and LHCGR predicted a binding pocket for org41841 in common clefts between TMHs 3, 4, 5, 6, and 7 and extracellular loop 2 in both receptors. Evidence for this binding pocket was obtained in signaling studies with chimeric receptors that exhibited improved responses to org41841. Furthermore, a key receptor-ligand interaction between the highly conserved negatively charged E3.37 and the amino group of org41841 predicted by docking of the ligand into the three-dimensional TSHR model was experimentally confirmed. These findings provide the first evidence that, in contrast to the ectodomain binding of cognate ligands, a LMW agonist can bind to and activate glycoprotein hormone receptors via interaction with their transmembrane domain.     

Search for peptide immunogens of the beta-subunit of human chorionic gonadotropin (hCG) capable of eliciting hormone specific and neutralizing antisera. Identification of an undecapeptide eliciting hCG-specific antisera.

The work reported herein describes our attempts to identify peptide immunogens of the beta-subunit of the pregnancy-specific placental hormone, human chorionic gonadotropin (hCG), capable of eliciting hormone specific and neutralizing antisera. Hydrophilicity profiles of the beta-subunits of hCG and the homologous pituitary hormone, human luteinizing hormone (hLH) were compared and two sequences which are hydrophilic but unique to hCG-beta were identified. They are 6-12 and 109-119 of hCG-beta. Results of the studies with the undecapeptide 109-119 of hCG-beta are reported in this paper. The undecapeptide amide was synthesized using the p-(acyloxy) benzhydrylamine resin and antisera to the peptide were elicited in rabbits using the peptide-diphtheria toxoid conjugate. The peptide is highly immunogenic as both the rabbits responded with high titers of antibodies to the peptide. The antipeptide antibodies bound to hCG but not to hLH showing thereby that the region 109-119 of hCG-beta is a unique determinant of hCG. However, the antibodies were found not to neutralize the biological activity of hCG.     

Effect of desialylated human chorionic gonadotropin (hCG) on the bioactivity of rat Leydig cells

Human chorionic gonadotropin is a glycoprotein hormone that, like LH, stimulates steroidogenesis in gonadal cells. Using a desialylation process, 95 per cent of the sialic acid residues from an intact standard hCG molecule were eliminated and then the electrophoretic properties and the bioactivity of the desialylated hCG were determined. Using rat Leydig cells as a biological model, the binding affinity to LH receptors of Leydig cell membranes, steroidogenic activity and second messenger production were studied. The results indicate that the loss of sialic acid from the hCG molecule slightly increases the binding activity to LH receptors and results in steroidogenic activity with an increased ED₅₀. Cyclic AMP production was significantly reduced however and arachidonic acid release was not observed. Several possible mechanisms that could explain these results are discussed. © 1998 John Wiley & Sons, Ltd.     

促黄体素（LH）和人绒毛膜促性腺激素（hCG）在原索动物神经系统,哈 …

Immunocytochemical localization of the nervous system, Hatschek's pit and gonads in protochordata was carried out using two specific antibodies against human LH and hCG. The results indicate that there are two different kinds of LH- and hCG-like immunopositive cells in the brain vesicle and nerve tuber of amphioxus, and nerve ganglion in Styela plicata showed immuno-negative reaction to LH and hCG antibodies. At the same time, we found for the first time that LH- and hCG-like immuno-positive cells existed in the early gonads (ovaries and testis) in protochordata. Positive substance distributed in the cytoplasm, nucleoplasm and nucleolar membrane of oogonia as well as early spermatogenic cells in testis. Afterwards, this positive substance was still distributed in the cytoplasmic membrane, cytoplasm, nuclear membrane and nucleoplasm of oocyte during the development and maturation of ovary, and spermatid and spermatozoa showed immuno-negative reaction in testis. These results will provide a new mophological proof that LH and hCG may be directly involved in regulation of the gonadal development and maturation of protochordata.     

Site-directed mutagenesis of the human chorionic gonadotropin beta-subunit: bioactivity of a heterologous hormone, bovine alpha-human des-(122-145)beta

Human CG contains an alpha-subunit, common to the pituitary glycoprotein hormones, and a hormone-specific beta-subunit, but unlike the pituitary beta-subunits, hCG beta is characterized by an O-glycosylated carboxy-terminal extension. A mutant beta-subunit, des-(122-145)hCG beta, was prepared using site-directed mutagenesis, and the pRSV expression plasmids were transfected into Chinese hamster ovary cells that produce the bovine alpha-subunit (b alpha). The mutant beta-subunit binds to b alpha, and the heterologous gonadotropin, b alpha-des-(122-145)hCG beta, was capable of stimulating steroidogenesis in cultured Leydig tumor cells (MA-10) to the same extent as standard hCG. When compared with the heterologous gonadotropin, b alpha-hCG beta wild type, the hybrid hormone with the truncated hCG beta exhibited equal potency, within the accuracy of the RIAs used to determine hormone concentrations, and gave a similar time course of steroidogenesis. Interestingly, these transformed Leydig cells do not distinguish between the steroidogenic potencies (as measured by progesterone production) of hCG and human LH (hLH) as do some preparations of normal rodent Leydig cells (as measured by testosterone production). However, the MA-10 cells were able to distinguish hCG from hLH based on their cAMP response; the latter produced a greater response at both maximal and submaximal gonadotropin concentrations. The two expressed heterologous gonadotropins were equipotent in their abilities to stimulate cAMP and gave similar time courses of cAMP accumulation in MA-10 cells. Thus, the carboxy-terminal extension of hCG beta is not required for association with the alpha-subunit nor for functional receptor binding, as judged by cAMP accumulation and progesterone production in MA-10 cells.     

Epitope mapping of the receptor-bound agonistic form of human chorionic gonadotropin (hCG) in comparison to the antagonistic form (deglycosylated hCG).

On the surface of free human chorionic gonadotropin (hCG), we can distinguish with our panel of monoclonal antibodies (MCA) 14 topographically distinct epitopes (designated alpha 1 - alpha 5, beta 1 - beta 5, alpha beta 1 - alpha beta 4, depending on the subunit they are attached to). Only 2, i.e. the adjacent beta 3 and beta 5 epitopes, of these 14 are accessible to 125I-labeled MCA binding, when hCG is first allowed to bind to the rat testis hCG receptor. This result indicates that the agonist hCG assumes a defined orientation in its receptor-bound state and that, except for that small area comprising the beta 3 and beta 5 epitopes, most of its surface is masked by the hCG receptor. We therefore asked whether the competitive antagonist deglycosylated hCG (degly-hCG), which, when free, is antigenically (as to number and topography of epitopes) indistinguishable from native hCG, would interact with the receptor differently, that is, in a way that can be discerned by this epitope accessibility paradigm. Here we describe that on receptor-bound degly-hCG the beta 3 and beta 5 epitopes were concealed as were all other epitopes. This observation, together with finding the receptor affinity of degly-hCG to be 4 times higher than that of native hCG, suggests that degly-hCG assumes a signal transduction-incompetent ligand orientation and at the same time interacts with the receptor more intensively, i.e. establishes additional ("antagonist accessory") protein-protein contacts besides those involved in agonist binding. It thus appears that the carbohydrate moieties function to prevent formation of such accessory contacts.     

Gonadotropin releasing hormone (GnRH) stimulates both secretion and synthesis of human chorionic gonadotropin (hCG) by first trimester human placental minces in vitro

An in vitro system using the minces of placental villi from first trimester human pregnancy (6-10 weeks) has been validated to examine the effect of addition of GnRH and its analogues on hCG secreted into the medium. Addition of low concentration of GnRH or its analogues (1 X 10(-8) M to 1 X 10(-6) M) resulted in an increase in the quantity of hCG in the medium, while addition of high concentrations of GnRH resulted in an inhibitory effect. Of the analogues tested, Buserelin was highly effective in exerting an inhibitory effect. A significant increase in 35S-methionine incorporation into immunoprecipitable hCG was noticed in the presence of GnRH. These results suggests that GnRH stimulates both synthesis and secretion of hCG by first trimester human placenta.     

Effects of human chorionic gonadotropin (hCG) and phenobarbital on the reproductive performance of fat-tailed Ghezel ewes

Increased ewe prolificacy and lambing rates, under certain conditions of sheep husbandry, may be economical and desirable. This paper reports on the effects of human chorionic gonadotropin (hCG) and phenobarbital (PB) on fertility and lambing rate in fat-tailed Ghezel sheep. One-hundred and five cyclic ewes (1.5 to 7.5 years of age) were randomly allotted to seven groups. Estrus was synchronized with two intramuscular injections of a prostaglandin F₂ analogue (PGF₂), eight days apart. Four groups were randomly assigned to the hCG and three groups to PB experiments. Twenty-four hours after the second injection of PGF₂, the ewes in four groups were injected (i.m.), either with 1 ml normal saline (control) or with 125, 250 or 500 IU hCG. Prolificacy of the ewes which conceived at first estrus increased in ewes receiving 500 IU of hCG, as compared with control group (an increase of 0.5 lamb/ewe) but fertility and lambing rate decreased in hCG-treated groups (P<0.05). Nine days after the second injection of PGF₂, ewes in the three remaining groups were orally fed empty gelatin capsules (control), 1.0 or 1.5 g of PB per day until the next estrus. Prolificacy and lambing rate increased (P<0.05) in ewes which received 1.0 g of PB (1.57 and 146.5%, respectively), as compared with the control groups (1.17 and 100.3%, respectively). It is concluded that daily feeding of 1.0 g of PB from day 7 of the estrous cycle until the next estrus period is an effective method of increasing lambing rate. hCG also increases the prolificacy, but it greatly decreases fertility and lambing rates. Since hCG is more convenient to administer on a flock basis, further experiments to improve its efficacy are warranted.     

Relative spatial position of a snake neurotoxin and the reduced disulfide bond alpha (Cys192-Cys193) at the alpha gamma interface of the nicotinic acetylcholine receptor

We determined the distances separating five functionally important residues (Gln(10), Lys(27), Trp(29), Arg(33), and Lys(47)) of a three-fingered snake neurotoxin from the reduced disulfide bond alpha(Cys(192)-Cys(193)) located at the alphagamma interface of the Torpedo nicotinic acetylcholine receptor. Each toxin position was substituted individually for a cysteine, which was then linked to a maleimido moiety through three different spacers, varying in length from 10 to 22 A. We estimated the coupling efficiency between the 15 toxin derivatives and the reduced cystine alpha(192-193) by gel densitometry of Coomassie Blue-stained gels. A nearly quantitative coupling was observed between alphaCys(192) and/or alphaCys(193) and all probes introduced at the tip of the first (position 10) and second (position 33) loops of Naja nigricollis alpha-neurotoxin. These data sufficed to locate the reactive thiolate in a "croissant-shaped" volume comprised between the first two loops of the toxin. The volume was further restrained by taking into account the absence or partial coupling of the other derivatives. Altogether, the data suggest that alphaCys(192) and/or alphaCys(193), at the alphagamma interface of a muscular-type acetylcholine receptor, is (are) located in a volume located between 11.5 and 15.5 A from the alpha-carbons at positions 10 and 33 of the toxin, under the tip of the toxin first loop and close to the second one.     

The carboxy-terminal region of the glycoprotein hormone alpha-subunit: contributions to receptor binding and signaling in human chorionic gonadotropin.

The glycoprotein hormones are heterodimeric and contain a common alpha-subunit, which is noncovalently associated with a hormone-specific beta-subunit. The alpha-subunit has been highly conserved throughout evolution; for example, the five amino acid residues of the carboxy-terminus, Tyr-Tyr-His-Lys-Ser-COOH, are identical in nine of the 10 available amino acid sequences. It has been shown that enzymatic removal of these five amino acid residues, while not affecting holoprotein formation, results in a heterodimer that exhibits very little, if any, binding to the CG/LH receptor. Using site-directed mutagenesis on the human alpha-subunit, we have prepared two deletion mutants, Des-(88-92)alpha and Des-(89-92)alpha, and two point mutants, where each of the two tyrosines, 88 and 89, was replaced with phenylalanine, in order to delineate more specifically the contributions of these aromatic side-chains to receptor binding. The cDNAs for wild-type hCG alpha and mutants were introduced into a pcDNAINEO expression vector, and the cDNA for hCG beta was inserted into a pRSV plasmid; both were transiently cotransfected into DUXB-11 cells. The media were collected, and RIAs showed that all mutants formed heterodimers; moreover, there was no discernable difference in subunit assembly between wild-type hCG alpha and the various mutant alpha-subunits. The gonadotropin mutants were assayed in vitro using a competitive binding assay with [125I]hCG and stimulation of progesterone production in the transformed murine Leydig cell line MA-10.(ABSTRACT TRUNCATED AT 250 WORDS)     

The clinical evaluation of the simultaneous measurements of human chorionic gonadotropin (hCG) and its alpha-subunit in sera of patients with trophoblastic diseases

The concentrations of human chorionic gonadotropin (hCG) and its free immunoreactive alpha-subunit (hCG-alpha) in the sera of patients with trophoblastic diseases were measured by hCG and hCG-alpha radioimmunoassay (RIA), respectively. In the sera of 12 women with hydatidiform mole large amounts of hCG and considerably high level of hCG-alpha were detected in all cases. After the evacuation of mole the serum level of these glycoproteins decreased, the leve of hCG-alpha declined more rapidly than hcg. in the sera of patients with destructive mole the concentration of hCG-alpha was usually lower than that of hCG. After hysterectomy and chemotherapy the levels of hCG-alpha declined practically paralleling that of hCG. However, when hCG had decreased to undetectable level, hCG-alpha could no longer be detected in all cases. Although in the serum of patient with choriocarcinoma involving the uterus and lungs the concentration of hCG-alpha was almost as high as that of hCG, the secretory pattern of hCG and hCG-alpha might not be closely related. The changes in the serum level of free hCG-alpha as well as that of hCG parelled the clinical course of the patients examined in this study. The present results suggest that measurements of the serum free hCG-alpha may be a useful parameter to follow the clinical course and to evaluate the efficacy of treatments of trophoblastic diseases.     

Role of the beta 93-100 determinant loop sequence in receptor binding and biological activity of human luteinizing hormone and chorionic gonadotropin

The intercysteine loop sequence (93-100) in the beta-subunit has been postulated to be important for receptor binding and specificity in the glycoprotein hormones, LH and human CG (hCG). To demonstrate this directly, and to characterize the structural features essential for activity, we prepared a series of synthetic peptides and analogs incorporating this determinant loop region. Peptides were assayed for inhibition of labeled hCG binding to ovarian membrane receptors and stimulation of testosterone production in Leydig cells. Peptides with the native (93-100) sequence from hCG and hLH inhibited hCG binding half maximally at 2.18 and 2.62 x 10(-4) M, respectively, while the sequence from FSH was inactive. Isosteric substitution of Ala for Cys resulted in an inactive peptide, indicating that the (93-100) disulfide bridge is essential for activity. Optimal binding activity requires at least one net positive charge among the side chains, as shown by loss of activity in hybrid analogs with neutral or negative charges conferred by progressive replacement of Arg by Asp at 94 and 95 or by introduction of Asp at 96 and 97. Despite binding to receptors, the native sequence did not promote testosterone production at doses up to 10(-2) M. This contrasts with a second receptor binding sequence, beta (38-57) that activates testosterone production. There are differences between the (93-100) and (38-57) loop sequences in their chemical and physical properties, biological activity and antigenicity. While the cumulative evidence suggests that they associate with counterpart sites in alpha-subunit to form a topographical binding domain in the whole hormone, our results suggest that each sequence may contribute in different ways to activation of postreceptor events.     

A new subclass of the luteinizing hormone/chorionic gonadotropin receptor lacking exon 10 messenger RNA in the New World monkey (Platyrrhini) lineage

The luteinizing hormone receptor (LHR) plays an essential role as a mediator of LH and CG action during embryonic sexual differentiation and in gametogenesis. In a hypogonadal male patient, we recently demonstrated that a genomic deletion of exon 10, located in the hinge region of the extracellular domain, results in discrimination of LH and hCG action. In the common marmoset (Calltithrix jacchus), exon 10 of the LHR is naturally missing at the mRNA level. In order to investigate whether this is an isolated species-specific phenomenon, we performed a phylogenetic screening, searching for the presence of LHR exon 10 mRNA in a number of primate species representative for the major lineages of primate evolution. The expressed LHR region encompassing exon 10 was amplified from testicular tissue by RT-PCR, cloned, and sequenced. In addition, we performed Southern blot analysis of the LHR of selected New World and Old World primates. The results revealed that exon 10 mRNA is lacking in the complete New World monkey (Platyrrhini) lineage but is present in both more primitive and more advanced primates. However, exon 10 seems to be present at the genomic level, arguing for a splicing failure possibly due to a genomic mutation or the lack of appropriate splicing factors. Considering that, in the human, LH is far less active than hCG on the LHR lacking exon 10, we addressed the question whether the existence of such a receptor has any consequences on the dual hormone LH/CG system present in Platyrrhini. Using primers specific for the known marmoset CG beta cDNA, we amplified the CG beta subunit cDNA from male common marmoset pituitaries by RT-PCR, while LH beta could not be amplified, suggesting a possible physiological role of pituitary CG in this species. In conclusion, we demonstrated for the first time that the LH mRNA without exon10 is the natural wild-type LHR in the Platyrrhini lineage. We propose that this LHR represents a new subclass of receptors that should be named LHR type II. In addition, the high expression of CG beta in the marmoset pituitary suggests a physiological role of CG in the reproductive function of these primates beyond pregnancy.