Stimulation of growth hormone by kisspeptin antagonists in ewes

Kisspeptin signalling is indispensable for fertility, stimulating gonadotropin-releasing hormone (GnRH) secretion and mediating gonadal steroid feedback on GnRH neurons. Moreover, kisspeptin neurons have been implicated in other non-reproductive neuroendocrine roles. Kisspeptin appears to also regulate growth hormone secretion but much of the data appear contradictory. We sought to clarify a potential role of kisspeptin in growth hormone (GH) regulation by examining the effect of kisspeptin antagonists on GH secretion in ewes under various physiological conditions. Our data show clear and robust increases in GH secretion following lateral ventricle or third ventricle infusion of kisspeptin antagonists p-234 and p-271 in either ovariectomized or anestrous ewes. Central infusion of kisspeptin-10 had no effect on GH secretion. To determine the level at which kisspeptin may influence GH secretion, we examined expression of the cognate kisspeptin receptor, GPR54, in pituitary cells and showed by immunocytochemistry that the majority of somatotropes express GPR54 while expression was largely negative in other pituitary cells. Overall, we have demonstrated that blocking kisspeptin signalling by antagonists stimulates GH secretion in ewes and that this is likely mediated by inhibiting endogenous kisspeptin activation of GPR54 expressed on somatotropes. The findings suggest that endogenous kisspeptin inhibits GH secretion through GPR54 expressed on somatotropes.

97 Social Dominance does not Affect Semen Quality in African Wild Dogs (Lycaon pictus)

Sperm banking and AI could benefit conservation of endangered African wild dogs (AWD). However, it is not clear whether their strict dominance hierarchy causes subfertility in subdominant males that typically do not breed. Our study investigated the effect of dominance on male reproductive parameters, including faecal glucocorticoids (fGCM) and androgens (fAM), testis and prostate volume, preputial gland size, semen collection success, and the number, motility, morphology, viability, acrosome integrity (PSA-FITC), and DNA integrity (TUNEL) of spermatozoa collected by electroejaculation. Samples were obtained from n = 12 captive AWD (4 US packs) in the pre-breeding season and n = 28 captive AWD (n = 11 from 4 US packs; n = 17 from 3 Namibian packs) in the breeding season. Male hierarchy was clearly determined by behavioural observations in all but 1 Namibian pack. Data were grouped by dominance status and means were compared by ANOVA or t-test; P ≤ 0.05 was significant. In the pre-breeding season, there was no significant difference in body weight, fGCM, fAM, or prostate and testis volume between dominance groups. Semen was successfully collected from all alphas but only half the subdominants; urine contamination was negatively associated with dominance. Sperm quality was low (17.3 ± 10.2% total motility, 12.8 ± 8.5% progressive motility, 27.4 ± 11.5 × 106 ejaculated spermatozoa, 40.6 ± 9.8% normal morphology, 63.1 ± 5.1% viability, 72.6 ± 5.2% acrosome integrity) with no difference observed in any parameter except progressive motility and normal sperm morphology, which were significantly lower in subdominants (27.7 ± 16.8% v. 0.0 ± 0.0% and 59.8 ± 13.0% v. 21.4 ± 5.7%). From pre-breeding to breeding season, testis and prostate volume increased significantly, particularly in beta and gamma males respectively. Prostate volume was higher in alpha than beta males (16.0 ± 6.4 cm3 v. 5.7 ± 1.4 cm3), but testis volume, body weight, fAM, and fGCM did not differ between dominance groups (12.0 ± 0.9 cm3, 28.5 ± 0.8 kg, 0.51 ± 0.07 µg g−1, and 30.6 ± 2.3 ng/g of dry weight). Semen was successfully collected from 75% of males with reduced urine contamination. Collection success, urine contamination, and preputial gland size were not associated with dominance. Sperm quality improved with significantly greater number, viability, and total motility. However, sperm quality did not differ between dominance groups (47.4 ± 6.7% total motility, 30.5 ± 5.8% progressive motility, 32.3 ± 9.2 × 106 ejaculated spermatozoa, 50.9 ± 5.2% normal morphology, 74.4 ± 4.2% viability, 85.6 ± 3.0% acrosome integrity, and 99.7 ± 0.1% DNA integrity). In conclusion, subdominant males are at higher risk of urine contamination and have lower sperm motility and normal morphology when semen is collected in the pre-breeding season. However, their semen is of similar quality to dominant males in the breeding season, indicating that reproductive suppression of subdominant males is only behavioural. Thus, AWD males of all social ranks in the breeding season are suitable candidates for sperm banking.

Effect of gonadotropin-inhibitory hormone on luteinizing hormone secretion in humans

BACKGROUND

Gonadotropin-inhibitory hormone (GnIH, human homologue of RFRP-3) suppresses gonadotropin secretion in animal models, but its effects have not been studied in the human.

OBJECTIVE

We tested the hypotheses that exogenous GnIH inhibits LH secretion (i) in postmenopausal women and (ii) in men concurrently administered exogenous kisspeptin.

DESIGN

Following in vitro and in vivo preclinical studies to functionally characterize the GnIH peptide, a dose-finding study (human GnIH: 1·5-150 μg/kg/h, iv for 3 h) was undertaken, and 50 μg/kg/h selected for further evaluation. Five postmenopausal women were administered 50 μg/kg/h iv infusion for 3 h or vehicle on two separate days. Four men were administered kisspeptin-10 (0·3 μg/kg iv bolus) with simultaneous infusion of GnIH (50 μg/kg/h, iv for 3 h) or vehicle.

PARTICIPANTS

Healthy postmenopausal women (mean age 58 ± 2 years, LH: 30·8 ± 2·9 IU/l, FSH: 78·7 ± 6·4 IU/l, oestradiol: <50 pmol/l) and men (39·8 ± 2·1 years, mean total testosterone 12·1 ± 1·8 nmol/l, LH 2·2 ± 0·2 IU/l).

PRIMARY OUTCOME

Change in area under curve (AUC) of LH during GnIHvs vehicle.

RESULTS

During GnIH administration in postmenopausal women, LH secretion decreased (ΔAUC: -9·9 ± 1·8 IU/3 h) vs vehicle (ΔAUC: -0·5 ± 1·7 IU/3 h; P = 0·02). Kisspeptin-10-stimulated LH responses in men were not affected by GnIH co-administration (60-min AUC of LH 6·2 ± 0·8 IU/h with kisspeptin-10 alone, 6·3 ± 1·0 IU/h, kisspeptin-10 with GnIH, P = 0·72). Exogenous GnIH was well tolerated, with no adverse events reported.

CONCLUSIONS

Gonadotropin-inhibitory hormone decreased LH secretion in postmenopausal women in this first-in-human study. Kisspeptin-stimulated LH secretion in men was not inhibited during concomitant administration of GnIH.

A Direct Neurokinin B Projection from the Arcuate Nucleus Regulates Magnocellular Vasopressin Cells of the Supraoptic Nucleus

Central administration of neurokinin B (NKB) agonists stimulates immediate early gene expression in the hypothalamus and increases the secretion of vasopressin from the posterior pituitary through a mechanism that depends on the activation of neurokinin receptor 3 receptors (NK3R). The present study reports that, in the rat, immunoreactivity for NK3R is expressed in magnocellular vasopressin and oxytocin neurones in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus, and that NKB immunoreactivity is expressed in fibres in close juxtaposition with vasopressin neurones at both of these sites. Retrograde tracing in the rat shows that some NKB-expressing neurones in the arcuate nucleus project to the SON and, in mice, using an anterograde tracing approach, it is found that kisspeptin-expressing neurones of the arcuate nucleus, which are known to co-express NKB, project to the SON and PVN. Finally, i.c.v. injection of the NK3R agonist senktide is shown to potently increase the electrical activity of vasopressin neurones in the SON in vivo with no significant effect detected on oxytocin neurones. The results suggest that NKB-containing neurones in the arcuate nucleus regulate the secretion of vasopressin from magnocellular neurones in rodents, and the possible significance of this is discussed.

Neurokinin B signaling in the female rat: a novel link between stress and reproduction

Acute systemic stress disrupts reproductive function by inhibiting pulsatile gonadotropin secretion. The underlying mechanism involves stress-induced suppression of the GnRH pulse generator, the functional unit of which is considered to be the hypothalamic arcuate nucleus kisspeptin/neurokinin B/dynorphin A neurons. Agonists of the neurokinin B (NKB) receptor (NK3R) have been shown to suppress the GnRH pulse generator, in a dynorphin A (Dyn)-dependent fashion, under hypoestrogenic conditions, and Dyn has been well documented to mediate several stress-related central regulatory functions. We hypothesized that the NKB/Dyn signaling cascade is required for stress-induced suppression of the GnRH pulse generator. To investigate this ovariectomized rats, iv administered with Escherichia coli lipopolysaccharide (LPS) following intracerebroventricular pretreatment with NK3R or κ-opioid receptor (Dyn receptor) antagonists, were subjected to frequent blood sampling for hormone analysis. Antagonism of NK3R, but not κ-opioid receptor, blocked the suppressive effect of LPS challenge on LH pulse frequency. Neither antagonist affected LPS-induced corticosterone secretion. Hypothalamic arcuate nucleus NKB neurons project to the paraventricular nucleus, the major hypothalamic source of the stress-related neuropeptides CRH and arginine vasopressin (AVP), which have been implicated in the stress-induced suppression of the hypothalamic-pituitary-gonadal axis. A separate group of ovariectomized rats was, therefore, used to address the potential involvement of central CRH and/or AVP signaling in the suppression of LH pulsatility induced by intracerebroventricular administration of a selective NK3R agonist, senktide. Neither AVP nor CRH receptor antagonists affected the senktide-induced suppression of the LH pulse; however, antagonism of type 2 CRH receptors attenuated the accompanying elevation of corticosterone levels. These data indicate that the suppression of the GnRH pulse generator by acute systemic stress requires hypothalamic NKB/NK3R signaling and that any involvement of CRH therewith is functionally upstream of NKB.

The role of neurokinin B signalling in reproductive neuroendocrinology

The KNDy neuropeptides, kisspeptin, neurokinin B (NKB) and dynorphin A (Dyn), have been implicated in regulating pulsatile luteinising hormone (LH) secretion. Studies of the interactions between KNDy signalling systems, however, are currently few. Although the stimulatory effect of kisspeptin and the inhibitory effect of Dyn on the gonadotropin-releasing hormone pulse generator are widely accepted, the effects of NKB in rodents are variable and sometimes controversial. Literature describing increased LH secretion in response to NKB receptor agonism predominates and is in line with human physiology, as well as the pathophysiology of pubertal failure associated with disruption of NKB signalling. However, the robust suppression of the LH pulse, induced by the same treatment under hypoestrogenic conditions, may hold clues as to the mechanisms of reproductive inhibition under pathological conditions. This review discusses the recent evidence for this paradox and outlines a revised working model incorporating the mechanisms by which KNDy neuropeptides modulate the reproductive axis.

Suppression of the GnRH pulse generator by neurokinin B involves a κ-opioid receptor-dependent mechanism

Neurokinin B (NKB) and its receptor (NK3R) are coexpressed with kisspeptin, Dynorphin A (Dyn), and their receptors [G-protein-coupled receptor-54 (GPR54)] and κ-opioid receptor (KOR), respectively] within kisspeptin/NKB/Dyn (KNDy) neurons in the hypothalamic arcuate nucleus (ARC), the proposed site of the GnRH pulse generator. Much previous research has employed intracerebroventricular (icv) administration of KNDy agonists and antagonists to address the functions of KNDy neurons. We performed a series of in vivo neuropharmacological experiments aiming to determine the role of NKB/NK3R signaling in modulating the GnRH pulse generator and elucidate the interaction between KNDy neuropeptide signaling systems, targeting our interventions to ARC KNDy neurons. First, we investigated the effect of intra-ARC administration of the selective NK3R agonist, senktide, on pulsatile LH secretion using a frequent automated serial sampling method to obtain blood samples from freely moving ovariectomized 17β-estradiol-replaced rats. Our results show that senktide suppresses LH pulses in a dose-dependent manner. Intra-ARC administration of U50488, a selective KOR agonist, also caused a dose-dependent, albeit more modest, decrease in LH pulse frequency. Thus we tested the hypothesis that Dyn/KOR signaling localized to the ARC mediates the senktide-induced suppression of the LH pulse by profiling pulsatile LH secretion in response to senktide in rats pretreated with nor-binaltorphimine, a selective KOR antagonist. We show that nor-binaltorphimine blocks the senktide-induced suppression of pulsatile LH secretion but does not affect LH pulse frequency per se. In order to address the effects of acute activation of ARC NK3R, we quantified (using quantitative RT-PCR) changes in mRNA levels of KNDy-associated genes in hypothalamic micropunches following intra-ARC administration of senktide. Senktide down-regulated expression of genes encoding GnRH and GPR54 (GNRH1 and Kiss1r, respectively), but did not affect the expression of Kiss1 (which encodes kisspeptin). We conclude that NKB suppresses the GnRH pulse generator in a KOR-dependent fashion and regulates gene expression in GnRH neurons.

Contrast imaging ultrasound detects abnormalities in the marmoset ovary

The development of a functional vascular tree within the primate ovary is critical for reproductive health. To determine the efficacy of contrast agents to image the microvascular environment within the primate ovary, contrast ultrasonography was performed in six reproductive-aged female common marmosets (Callithrix jacchus) during the late luteal phase of the cycle, following injection of Sonovue™. Regions of interest (ROIs), representing the corpus luteum (CL) and noncorpus luteum ovarian tissue (NCLOT), were selected during gray-scale B-mode ultrasound imaging. The magnitude of backscatter intensity of CL and NCLOT ROIs were calculated in XnView, post hoc: subsequent gamma-variate modeling was implemented in Matlab to determine perfusion parameters. Histological analysis of these ovaries revealed a total of 11 CL, nine of which were identified during contrast ultrasonography. The median enhancement ratio was significantly increased in the CL (5.54AU; 95% CI -2.21-68.71) compared to the NCLOT (2.82AU; 95% CI 2.73-15.06; P < 0.05). There was no difference in time parameters between the CL and NCLOT. An additional avascular ROI was identified in the ovary of Animal 5, both histologically and by ultrasonography. This cystic ROI displayed a markedly lower enhancement ratio (0.79AU) and higher time parameters than mean CL and NCLOT, including time to peak and time to wash out. These data demonstrate, for the first time, the ability of commercially available contrast agents, to differentiate structures within the nonhuman primate ovary. Contrast-enhanced ultrasonography has a promising future in reproductive medicine.

Kisspeptin-10 inhibits cell migration in vitro via a receptor-GSK3 beta-FAK feedback loop in HTR8SVneo cells

Kisspeptin inhibits cancer cell metastasis and placental trophoblast cell migration. Kisspeptin gene expression in the placenta and circulating kisspeptin levels change during normal pregnancy and they are altered in preeclampsia. We therefore assessed the effect of kisspeptin-10 on the in vitro migration of a human placental cell line derived from first trimester extravillious trophoblasts (HTR8SVneo). HTR8SVneo cells specifically bound 125I-Kisspeptin-10 but kisspeptin-10 did not induce inositol phosphate production. Cell migration was inhibited by kisspeptin-10 with a maximal inhibition at 100nM. The signaling pathways involved in inhibition of cell migration were examined. Treatment with kisspeptin-10 elicited phosphorylation of GSK3 beta at Ser9 (which inhibits activity), with a 3-fold increase at 5 min. Transient phosphorylation of ERK1/2 and p38MAPK peaked at 10min. Phosphorylation of focal adhesion kinase (FAK) at Tyr925 increased 3-fold at 10 min. Inhibition of GSK3 beta correlated with release of beta-catenin into the cytoplasm. These signaling events were differentially blocked by inhibitors of G(q/11), Src, EGFR, PI(3)K, PKC and MEK. The data suggest that kisspeptin/GPR54 EGF-receptor transactivation leads to phosphorylation of ERK1/2, causing activation of p90rsk which in turn inhibits GSK3 beta via Ser9 phosphorylation. Inactivation of GSK3 beta results in release of beta-catenin into the cytoplasm, affecting cell-cell adhesion and Tyr925 phosphorylation of FAK, which increases phosphorylation of ERK1/2 via RAS/Raf-1 creating a feedback loop to enhance the effects on migration. These findings indicate that kisspeptin-10 inhibits the migration of human placental trophoblast-derived HTR8SVneo cells by stimulating complex ERK1/2-p90rsk-GSK3 beta-FAK feedback interactions.

Kisspeptin-10 is a potent stimulator of LH and increases pulse frequency in men

CONTEXT

Kisspeptins stimulate GnRH and thus gonadotropin secretion. Kisspeptin-10 is the minimal kisspeptin sequence with full intrinsic bioactivity, but it has not been studied in man.

OBJECTIVE

We investigated our hypothesis that kisspeptin-10 increases GnRH and thus LH pulse frequency.

DESIGN AND PARTICIPANTS

The dose response of kisspeptin-10 was investigated by administering iv bolus doses (0.01-3.0 μg/kg) and vehicle to healthy men. Effects on LH pulse frequency and size were determined by deconvolution analysis during infusion of kisspeptin-10 for up to 22.5 h.

RESULTS

Intravenous bolus kisspeptin-10 resulted in a rapid and dose-dependent rise in serum LH concentration, with maximal stimulation at 1 μg/kg (4.1 ± 0.4 to 12.4 ± 1.7 IU/liter at 30 min, P < 0.001, n = 6). Administration of 3 μg/kg elicited a reduced response vs. 1 μg/kg (P < 0.05). Infusion of kisspeptin-10 at 4 μg/kg · h for 22.5 h elicited an increase in LH from a mean of 5.4 ± 0.7 to 20.8 ± 4.9 IU/liter (n = 4; P < 0.05) and serum testosterone increased from 16.6 ± 2.4 to 24.0 ± 2.5 nmol/liter (P < 0.001). LH pulses were obscured at this high rate of secretion, but a lower dose infusion of kisspeptin-10 (1.5 μg/kg · h) increased mean LH from 5.2 ± 0.8 to 14.1 ± 1.7 IU/liter (n = 4; P < 0.01) and increased LH pulse frequency from 0.7 ± 0.1 to 1.0 ± 0.2 pulses/h (P < 0.05) and secretory burst mass from 3.9 ± 0.4 to 12.8 ± 2.6 IU/liter (P < 0.05).

CONCLUSIONS

Kisspeptin-10 boluses potently evoke LH secretion in men, and continuous infusion increases testosterone, LH pulse frequency, and pulse size. Kisspeptin analogues have therapeutic potential as regulators of LH and thus testosterone secretion.

Critical roles of kisspeptins in female puberty and preovulatory gonadotropin surges as revealed by a novel antagonist

Kisspeptins (Kp) have recently emerged as master regulators of the reproductive axis and among the most potent elicitors of GnRH-gonadotropin secretion. Despite their paramount importance in reproductive physiology and their potential therapeutic implications, development of Kp antagonists has remained elusive, and only recently has the first compound with the ability to block Kp actions in vitro and in vivo, namely p234, been reported. However, previous in vivo studies all used acute central injections, whereas characterization of the effects of the antagonist after continuous or systemic administration, which poses pharmacological challenges, is still pending. We report herein a comprehensive series of analyses on the impact of continuous intracerebroventricular infusion of p234 on puberty onset and the preovulatory surge of gonadotropins in the female rat. In addition, the effects of systemic (ip) administration of a tagged p234-penetratin, with a predicted higher permeability at the blood-brain barrier, on Kp-10 induced gonadotropin secretion were evaluated. Central infusion of p234 to pubertal females delayed vaginal opening and decreased uterine and ovarian weights at the expected time of puberty, without affecting body weight. Likewise, chronic intracerebroventricular administration of p234 for 4 d prevented the preovulatory surges of LH and FSH. In addition, systemic (ip) administration of p234-penetratin significantly attenuated acute LH and FSH responses to Kp-10, either after intracerebroventricular or ip injection of Kp. Our data document the validity of p234 for antagonizing Kp actions in vivo and provide direct experimental evidence for the important role of Kp signaling in the key events of female reproduction, such as puberty onset and the preovulatory surge of gonadotropins.

KiSS-1 in the mammalian ovary: distribution of kisspeptin in human and marmoset and alterations in KiSS-1 mRNA levels in a rat model of ovulatory dysfunction

Kisspeptins, the products of the KiSS-1 gene acting via G protein-coupled receptor 54 (GPR54), have recently emerged as pivotal signals in the hypothalamic network triggering the preovulatory surge of gonadotropins and, hence, ovulation. Additional actions of kisspeptins at other levels of the hypothalamic-pituitary-ovarian axis have been suggested but remain to date scarcely studied. We report herein the pattern of expression of KiSS-1 and GPR54 in the human and nonhuman primate ovary and evaluate changes in ovarian KiSS-1 expression in a rat model of ovulatory dysfunction. KiSS-1 and GPR54 mRNAs were detected in human ovarian tissue and cultured granulosa-lutein cells. In good agreement, kisspeptin immunoreactivity was observed in cyclic human and marmoset ovaries, with prominent signals in the theca layer of growing follicles, corpora lutea, interstitial gland, and ovarian surface epithelium. GPR54 immunoreactivity was also found in human theca and luteal cells. Administration of indomethacin to cyclic female rats disturbed ovulation and resulted in a dramatic drop in ovarian KiSS-1, but not GPR54, cyclooxygenase-2 (COX-2), or progesterone receptor, mRNA levels at the time of ovulation; an effect mimicked by the selective COX-2 inhibitor NS398 and rescued by coadministration of PGE(2). Likewise, the stimulatory effect of human choriogonadotropin on ovarian KiSS-1 expression was partially blunted by indomethacin. In contrast, KiSS-1 mRNA levels remained unaltered in another model of ovulatory failure, i.e., the RU486-treated rat. In summary, we document for the first time the expression of KiSS-1/kisspeptin and GPR54 in the human and nonhuman primate ovary. In addition, we provide evidence for the ability of inhibitors of COX-2, known to disturb follicular rupture and ovulation, to selectively alter the expression of KiSS-1 gene in rat ovary. Altogether, our results are suggestive of a conserved role of local KiSS-1 in the direct control of ovarian functions in mammals.

The role of kisspeptin in the control of gonadotrophin secretion

BACKGROUND

Kisspeptins, and their cognate receptor gpr-54, were first found to regulate the hypothalamic-pituitary-gonadal (HPG) axis in 2003, when two groups demonstrated that mutations in gpr-54 cause idiopathic hypogonadotropic hypogonadism characterized by delayed or absent puberty. This review aims to highlight discoveries in the KiSS-1/gpr-54 system, focusing on their regulation of the HPG axis in male and female reproductive systems of both mammalian and non-mammalian vertebrates.

METHODS

A search of PUBMED and the authors' files was done without limitations by language or species for citations relevant to kisspeptin, reproduction and signal transduction.

RESULTS

Kisspeptins and gpr-54 are critical for puberty and the regulation of reproduction. Kisspeptins have been implicated in mediating many of the important signals relayed to the gonadotrophin-releasing hormone (GnRH) neuron such as positive and negative feedback, metabolic input and photoperiod. The ability of kisspeptin neurons to co-ordinate different signals impinging on the HPG axis makes it one of the most important regulators of GnRH and the reproductive axis.

CONCLUSIONS

Kisspeptins are pivotal regulators of the HPG axis and reproduction, with the ability to integrate signals from both internal and external sources. Knowledge about the signalling mechanisms involved in kisspeptin stimulation of GnRH would help improve the understanding of the importance of this critical pathway in reproduction.

Kisspeptin is present in ovine hypophysial portal blood but does not increase during the preovulatory luteinizing hormone surge: evidence that gonadotropes are not direct targets of kisspeptin in vivo

There is strong evidence that kisspeptin acts to regulate GnRH secretion, but whether there is also a component of action on the gonadotropes is not clear. Using quantitative RT-PCR, we found that G protein-coupled receptor-54 mRNA is expressed in ovine pituitary cell fractions enriched for gonadotropes as well as in somatotropes and lactotropes. To test whether kisspeptin acts directly on the pituitary gonadotropes, we first examined LH release from primary ovine pituitary cell cultures treated with kisspeptin. We found that kisspeptin treatment increased the concentration of LH in culture media by 80%, compared with control, but only in pituitary cultures from ewes during the follicular phase of the estrous cycle. After this, we determined whether kisspeptin acts on the pituitary gland in vivo. Using GnRH-replaced ovariectomized hypothalamo-pituitary-disconnected ewes, we were not able to achieve any effect of kisspeptin on LH under steady-state conditions or during the period of an estrogen-induced LH surge. Finally, we collected hypophysial portal blood samples from ovariectomized ewes and measured kisspeptin levels. Low but detectable amounts of kisspeptin were found in portal plasma, but levels were similar in ovariectomized ewes that were untreated or given estrogen to elicit an LH surge. Thus, although we observed an effect of kisspeptin on LH release in vitro in some situations, similar findings were not obtained in vivo. Moreover, the low concentrations of kisspeptin in hypophysial portal blood and the lack of any change during the period of an estrogen-induced GnRH/LH surge suggest that action on the pituitary gland is not of major consequence in terms of LH release.

Hypothalamic pro-GnRH-GAP, GnRH-I and GnRH-II during the onset of photorefractoriness in the white-crowned sparrow (Zonotrichia leucophrys gambelii)

Gambel's white-crowned sparrow is a long distance migrant that undergoes spontaneous gonadal regression as a result of long day exposure. This termination of breeding is caused by the development of photorefractoriness and the birds become insensitive to long days, including continuous light. The present study investigated its possible mechanisms by examining the activity of the gonadotrophin-releasing hormone (GnRH) system under different photoperiodic regimes. We investigated the localisation and distribution of GnRH-I, its precursor pro-GnRH-GAP and GnRH-II in Gambel's white-crowned sparrow brain using immunocytochemistry with specific antibodies during photostimulation and the development of photorefractoriness. The study revealed that photoperiodic treatment, including the onset of photorefractoriness, had no significant effect on the size or number of GnRH-I, pro-GnRH-GAP or GnRH II immunoreactive cells, or the density of the GnRH-I, pro-GnRH-GAP immunoreactive fibres at the median eminence. GnRH-II was not found in the median eminence, suggesting that it does not regulate pituitary gonadotrophin secretion. GnRH-I measurement in hypothalamic extracts by radioimmunoassay did not reveal any significant difference between birds that were photostimulated or in the early stages of photorefractoriness. Furthermore, the action of the excitatory amino acid glutamate agonist N-methyl-D-aspartate on GnRH neurones in photorefractory birds was demonstrated by the significant blockade of luteinising hormone release with a specific GnRH antagonist. Taken together, these results suggest that, in Gambel's white-crowned sparrow, a decrease in GnRH-I secretion is the initial step for the onset of photorefractoriness and not a decrease in GnRH-I biosynthesis.

Evidence that the type-2 gonadotrophin-releasing hormone (GnRH) receptor mediates the behavioural effects of GnRH-II on feeding and reproduction in musk shrews

Gonadotrophin-releasing hormone (GnRH) is a regulatory neuropeptide of which there are multiple structural variants. In mammals, a hypothalamic form (GnRH-I) controls gonadotrophin secretion whereas a midbrain form (GnRH-II) appears to have a neuromodulatory role affecting feeding and reproduction. In female musk shrews and mice, central administration of GnRH-II reinstates mating behaviour previously inhibited by food restriction. In addition, GnRH-II treatment also decreases short-term food intake in musk shrews. GnRH-II can bind two different mammalian GnRH receptors (type-1 and type-2), and thus it is unclear which receptor subtype mediates the behavioural effects of this peptide. Adult female musk shrews implanted with i.c.v. cannula were food restricted or fed ad lib and then tested for sexual behaviour or food intake. One hour before testing, animals were pretreated with vehicle or Antide, a potent type-1 GnRH receptor antagonist (at a dose that blocks GnRH-I or -II mediated ovulation). Twenty minutes before testing, females were infused a second time with either GnRH-II or vehicle. Additional females were tested after an infusion of 135-18, a type-1 receptor antagonist that displays agonist actions at the primate type-2 receptor. GnRH-II treatment increased sexual behaviour in underfed female shrews; pretreatment with Antide did not block this action, suggesting that the effects of GnRH-II are not mediated via the type-1 receptor. Similarly, the inhibitory effects of GnRH-II on short-term food intake were not prevented by pretreatment with Antide. The behavioural effects of the type-2 receptor agonist 135-18 were similar to those seen in GnRH-II-treated females, with 135-18 promoting sexual behaviour and decreasing food intake. Collectively, these results indicate that GnRH-II does not act via the type-1 GnRH receptor to regulate mammalian behaviour but likely activates the type-2 GnRH receptor.

Expression and regulation of the prokineticins (endocrine gland-derived vascular endothelial growth factor and Bv8) and their receptors in the human endometrium across the menstrual cycle

This study investigated the possible role of the newly discovered endocrine gland-derived vascular endothelial growth factors and their cognate receptors in the human endometrium during the menstrual cycle. Endocrine gland-derived vascular endothelial growth factors are also known as prokineticin (PK) 1 and PK2 and their receptors as PKR1 and PKR2. Expression of PK1 was elevated in the secretory compared with the proliferative phase of the menstrual cycle (P < 0.05). There was no temporal variation in expression of PK2, PKR1, or PKR2. PK1 and PK2 and their receptors were localized to multiple cellular compartments, including glandular epithelial, stromal, and endothelial cells in the endometrium and endothelial and smooth muscle cells in the myometrium. The elevation in PK1 expression in the secretory phase of the menstrual cycle indicated potential regulation of PK1 by progesterone. To investigate this, endometrial tissue was treated with 1 microM (micromol/liter(-1)) progesterone for 24 h, and PK1 expression was assessed by quantitative RT-PCR. Treatment with 1 microM ( micromol/liter(-1)) progesterone resulted in 2.91 +/- 0.75-fold elevation in PK1 expression, compared with controls (P < 0.05). These data identify a paracrine role for the PKs and their receptors in endometrial vascular function.

Involvement of thyrotropin-releasing hormone receptor, somatostatin receptor subtype 2 and corticotropin-releasing hormone receptor type 1 in the control of chicken thyrotropin secretion

Thyrotropin or thyroid-stimulating hormone (TSH) secretion in the chicken is controlled by several hypothalamic hormones. It is stimulated by thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH), whereas somatostatin (SRIH) exerts an inhibitory effect. In order to determine the mechanism by which these hypothalamic hormones modulate chicken TSH release, we examined the cellular localization of TRH receptors (TRH-R), CRH receptors type 1 (CRH-R1) and somatostatin subtype 2 receptors (SSTR2) in the chicken pars distalis by in situ hybridization (ISH), combined with immunological staining of thyrotropes. We show that thyrotropes express TRH-Rs and SSTR2s, allowing a direct action of TRH and SRIH at the level of the thyrotropes. CRH-R1 expression is virtually confined to corticotropes, suggesting that CRH-induced adrenocorticotropin release is the result of a direct stimulation of corticotropes, whereas CRH-stimulated TSH release is not directly mediated by the known chicken CRH-R1. Possibly CRH-induced TSH secretion is mediated by a yet unknown type of CRH-R in the chicken. Alternatively, a pro-opiomelanocortin (POMC)-derived peptide, secreted by the corticotropes following CRH stimulation, could act as an activator of TSH secretion in a paracrine way.

Spatio-temporal expression of gonadotropin-releasing hormone receptor subtypes in gonadotropes, somatotropes and lactotropes in the cichlid fish

The description of two or more forms of gonadotropin-releasing hormone (GnRH) in most vertebrates suggests multiple roles for this family of peptide hormones. In order to verify these functions, we analysed the anatomical location, time of initial expression and ontogenic changes in three distinct GnRH receptors (GnRH-Rs) in developing and sexually mature tilapia, using antisera raised against the extracellular loop three of the receptor, which is a determinant in ligand-selectivity and receptor coupling to signalling pathways. In all age groups, including males and females, using in situ hybridization and double-label immunological methods, GnRH-R type IA was colocalized in cells containing luteinizing hormone (LH) beta-subunit in the pituitary. GnRH-R type IB was visualized in prolactin cells and LH cells. The type III GnRH-R was expressed in growth hormone cells. On day 8 after fertilization, GnRH-R type III was first seen in growth hormone cells and, subsequently, on day 15, GnRH-Rs type IA and type IB were first seen in LH and prolactin cells, respectively. On day 25, the receptor occupied area per pituitary and the staining intensity of GnRH-R type IA increased significantly, consistent with the hypothesis that differentiation of GnRH neurones and their inputs to the pituitary coincide precisely with gonadal sex differentiation and steroidogenesis in tilapia. The differential distribution of GnRH-Rs in the pituitary provides the first clear evidence that the three native GnRH variants in tilapia have cognate receptors, each capable of regulating different pituitary endocrine cells.

Role of aspartate7.32(302) of the human gonadotropin-releasing hormone receptor in stabilizing a high-affinity ligand conformation

Mammalian gonadotropin-releasing hormone (GnRH) receptors preferentially bind mammalian GnRH, which has Arg in position eight. The Glu(7.32(301)) residue, which determines selectivity of the mouse GnRH receptor for Arg(8)-containing GnRH, is Asp(7.32(302)) in the human GnRH receptor. We have confirmed that Asp(7.32(302)) confers selectivity of the human GnRH receptor for Arg(8) of GnRH and investigated the mechanism of this specificity using site-directed mutagenesis and ligand modification. We find that although Arg(8) and Asp(7.32(302)) are required for high-affinity binding of GnRH, conformationally constrained peptides, with D-amino acid substitutions in position six or with a 6,7 gamma-lactam, bind the human GnRH receptor with high affinity, which is independent of the presence of Asp(7.32(302)) in the receptor or Arg(8) in the ligand. The ability of the ligand constraints to compensate for the absence of both Arg(8) and Asp(7.32(302)) indicates that these residues both have roles in stabilizing a high affinity ligand conformation and that their roles are complementary. This suggests that the Arg(8) and Asp(7.32(302)) side chains interact to induce a high affinity conformation of native GnRH. Thus, Asp(7.32(302)) of the human GnRH receptor determines selectivity for mammalian GnRH by its ability to induce a high affinity conformation of its native ligand. However, this initial interaction seems not to contribute to the final ligand-receptor complex. We propose that Arg(8) interacts transiently with Asp(7.32(302)) to induce a high-affinity ligand conformation of GnRH, which then interacts with a binding pocket that is common for both constrained and unconstrained analogs of GnRH.

Signaling and antiproliferative effects mediated by GnRH receptors after expression in breast cancer cells using recombinant adenovirus

GnRH receptors (GnRH-Rs) are found in human cancers, including those of the breast, and GnRH can inhibit the growth of cell lines derived from such cancers. Although pituitary and extrapituitary GnRH-R transcripts appear identical, their functional characteristics may differ. Most extrapituitary GnRH-Rs have low affinity for GnRH analogs and may not activate PLC or discriminate between agonists and antagonists in the same way as pituitary GnRH-Rs. Here we have assessed whether GnRH-Rs expressed exogenously in breast cancer cells differ from those in gonadotropes. We found no evidence for endogenous GnRH-Rs in MCF7 cells, but after infection with adenovirus expressing the GnRH-R (Ad GnRH-R) at a multiplicity of infection of 10 or greater, at least 80% expressed GnRH-Rs. These had high affinity (K(d) for [(125)I]buserelin, 1.4 nM) and specificity (rank order of potency, buserelin>GnRH>>chicken GnRH-II) and mediated stimulation of [(3)H]IP accumulation. Increasing viral titer [from multiplicity of infection, 3-300] increased receptor number (10,000-225,000 sites/cell) and [(3)H]IP responses. GnRH stimulated ERK2 phosphorylation in Ad GnRH-R-infected cells, and this effect, like stimulation of [(3)H]IP accumulation, was blocked by GnRH-R antagonists. GnRH also inhibited [(3)H]thymidine incorporation into Ad GnRH-R-infected cells (but not control cells). This effect was mimicked by agonist analogs and inhibited by two antagonists. Thus, when exogenous GnRH-Rs are expressed at density comparable to that in gonadotropes, they are functionally indistinguishable from the endogenous GnRH-Rs in gonadotropes, and increasing expression of high affinity GnRH-Rs can dramatically enhance the direct antiproliferative effect of GnRH agonists on breast cancer cells.

The genes encoding the type II gonadotropin-releasing hormone receptor and the ribonucleoprotein RBM8A in humans overlap in two genomic loci

We have cloned and characterized two genomic loci encoding the human type II gonadotropin-releasing hormone (GnRH) receptor and RNA-binding motif protein-8 (RBM8A). In both loci the genes overlap and are in antisense orientation to each other. The locus on chromosome 1 encompasses the type II GnRH receptor gene (GNRHR2), which is composed of three exons. We found transcripts from this gene in a wide range of tissues, but they lacked a methionine initiation codon and had a stop codon in exon 2. In the antisense orientation, this locus contains RBM8A, which consists of six exons and directs the synthesis of an RBM8A protein of 173 or 174 amino acids by alternative splicing. A second locus on chromosome 14 contains pseudogenes of RBM8A and the type II GnRH receptor and probably originated from the chromosome 1 locus by retrotransposition.

Presence of luteinizing hormone-releasing hormone fragments in the rhesus monkey forebrain

Previously, we have shown that two types of luteinizing hormone-releasing hormone (LHRH) -like neurons, "early" and "late" cells, were discernible in the forebrain of rhesus monkey fetuses by using antiserum GF-6, which cross-reacts with several forms of LHRH. The "late" cells that arose from the olfactory placode of monkey fetuses at embryonic days (E) 32-E36, are bona fide LHRH neurons. The "early" cells were found in the forebrain at E32-E34 and settled in the extrahypothalamic area. The molecular form of LHRH in "early" cells differs from "late" cells, because "early" cells were not immunopositive with any specific antisera against known forms of LHRH. In this study, we investigated the molecular form of LHRH in the "early" cells in the nasal regions and brains of 13 monkey fetuses at E35 to E78. In situ hybridization studies suggested that both "early" and "late" LHRH cells expressed mammalian LHRH mRNA. Furthermore, "early" cells predominantly contain LHRH1-5-like peptide and its cleavage enzyme, metalloendopeptidase E.C.3.4.24.15 (EP24.15), which cleaves LHRH at the Tyr5-Gly6 position. This conclusion was based on immunocytochemical labeling with various antisera, including those against LHRH1-5, LHRH4-10, or EP24.15, and on preabsorption tests. Therefore, in primates, a group of neurons containing mammalian LHRH mRNA arises at an early embryonic stage before the migration of bona fide LHRH neurons, and is ultimately distributed in the extrahypothalamic region. These extrahypothalamic neurons contain LHRH fragments, rather than fully mature mammalian LHRH. The origin and function of these neurons remain to be determined.

Differential internalization of mammalian and non-mammalian gonadotropin-releasing hormone receptors. Uncoupling of dynamin-dependent internalization from mitogen-activated protein kinase signaling

Desensitization and internalization of G-protein-coupled receptors can reflect receptor phosphorylation-dependent binding of beta-arrestin, which prevents G-protein activation and targets receptors for internalization via clathrin-coated vesicles. These can be pinched off by a dynamin collar, and proteins controlling receptor internalization can also mediate mitogen-activated protein kinase signaling. Gonadotropin-releasing hormone (GnRH) stimulates internalization of its receptors via clathrin-coated vesicles. Mammalian GnRH receptors (GnRH-Rs) are unique in that they lack C-terminal tails and do not rapidly desensitize, whereas non-mammalian GnRH-R have C-terminal tails and, where investigated, do rapidly desensitize and internalize. Using recombinant adenovirus expressing human and Xenopus GnRH-Rs we have explored the relationship between receptor internalization and mitogen-activated protein kinase signaling in HeLa cells with regulated tetracycline-controlled expression of wild-type or a dominant negative mutant (K44A) of dynamin. These receptors were phospholipase C-coupled and had appropriate ligand affinity and specificity. K44A dynamin expression did not alter human GnRH-R internalization but dramatically reduced internalization of Xenopus GnRH-R (and epidermal growth factor (EGF) receptor). Blockade of clathrin-mediated internalization (sucrose) abolished internalization of all three receptors. Both GnRH-Rs also mediated phosphorylation of ERK 2 and for both receptors, this was inhibited by K44A dynamin. The same was true for EGF- and protein kinase C-mediated ERK 2 phosphorylation. ERK 2 phosphorylation was also inhibited by a protein kinase C inhibitor but not affected by an EGF receptor tyrosine kinase inhibitor. We conclude that a) desensitizing and non-desensitizing GnRH-Rs are targeted for clathrin-coated vesicle-mediated internalization by functionally distinct mechanisms, b) GnRH-R signaling to ERK 2 is dynamin-dependent and c) this does not reflect a dependence on dynamin-dependent GnRH-R internalization.

Gonadotropin-releasing hormone receptor in the teleost Haplochromis burtoni: structure, location, and function

GnRH acts via GnRH receptors (GnRH-R) in the pituitary to cause the release of gonadotropins that regulate vertebrate reproduction. In the teleost fish, Haplochromis burtoni, reproduction is socially regulated through the hypothalamus-pituitary-gonadal axis, making the pituitary GnRH-R a likely site of action for this control. As a first step toward understanding the role of GnRH-R in the social control of reproduction, we cloned and sequenced candidate GnRH-R complementary DNAs from H. burtoni tissue. We isolated a complementary DNA that predicts a peptide encoding a G protein-coupled receptor that shows highest overall identity to other fish type I GnRH-R (goldfish IA and IB and African catfish). Functional testing of the expressed protein in vitro confirmed high affinity binding of multiple forms of GNRH: Localization of GnRH-R messenger RNA using RT-PCR revealed that it is widely distributed in the brain and retina as well as elsewhere in the body. Taken together, these data suggest that this H. burtoni GnRH receptor probably interacts in vivo with all three forms of GNRH:

Cyclooxygenase-2 expression and prostaglandin E(2) synthesis are up-regulated in carcinomas of the cervix: a possible autocrine/paracrine regulation of neoplastic cell function via EP2/EP4 receptors

The prevalence of cervical cancer in South African women is reported as being the highest in the world, occurring, on the average, in 60 of every 100,000 women. Cervical cancer is thus considered an important clinical problem in sub-Saharan AFRICA: Recent studies have suggested that epithelial tumors may be regulated by cyclooxygenase (COX) enzyme products. The purpose of this study was to determine whether cyclooxygenase-2 (COX-2) expression and PGE(2) synthesis are up-regulated in cervical cancers. Real-time quantitative RT-PCR and Western blot analysis confirmed COX-2 ribonucleic acid and protein expression in all cases of squamous cell carcinoma (n = 8) and adenocarcinoma (n = 2) investigated. In contrast, minimal expression of COX-2 was detected in histologically normal cervix (n = 5). Immunohistochemical analyses localized COX-2 expression and PGE(2) synthesis to neoplastic epithelial cells of all squamous cell (n = 10) and adenocarcinomas (n = 10) studied. Immunoreactive COX-2 and PGE(2) were also colocalized to endothelial cells lining the microvasculature. Minimal COX-2 and PGE(2) immunoreactivity were detected in normal cervix (n = 5). To establish whether PGE(2) has an autocrine/paracrine effect in cervical carcinomas, we investigated the expression of two subtypes of PGE(2) receptors, namely EP2 and EP4, by real-time quantitative RT-PCR. Expression of EP2 and EP4 receptors was significantly higher in carcinoma tissue (n = 8) than in histologically normal cervix (n = 5; P < 0.01). Finally, the functionality of the EP2/EP4 receptors was assessed by investigating cAMP generation after in vitro culture of cervical cancer biopsies and normal cervix in the presence or absence of 300 nmol/L PGE(2). cAMP production was detected in all carcinoma tissue after treatment with exogenous PGE(2) and was significantly higher in carcinoma tissue (n = 7) than in normal cervix (n = 5; P < 0.05). The fold induction of cAMP in response to PGE(2) was 51.1 +/- 12.3 in cervical carcinoma tissue compared with 5.8 +/- 2.74 in normal cervix. These results confirm that COX-2, EP2, and EP4 expression and PGE(2) synthesis are up-regulated in cervical cancer tissue and suggest that PGE(2) may regulate neoplastic cell function in cervical carcinoma in an autocrine/paracrine manner via the EP2/EP4 receptors.

A chicken gonadotropin-releasing hormone receptor that confers agonist activity to mammalian antagonists. Identification of D-Lys(6) in the ligand and extracellular loop two of the receptor as determinants

Mammalian receptors for gonadotropin-releasing hormone (GnRH) have over 85% sequence homology and similar ligand selectivity. Biological studies indicated that the chicken GnRH receptor has a distinct pharmacology, and certain antagonists of mammalian GnRH receptors function as agonists. To explore the structural determinants of this, we have cloned a chicken pituitary GnRH receptor and demonstrated that it has marked differences in primary amino acid sequence (59% homology) and in its interactions with GnRH analogs. The chicken GnRH receptor had high affinity for mammalian GnRH (K(i) 4.1 +/- 1.2 nM), similar to the human receptor (K(i) 4.8 +/- 1.2 nM). But, in contrast to the human receptor, it also had high affinity for chicken GnRH ([Gln(8)]GnRH) and GnRH II ([His(5),Trp(7),Tyr(8)]GnRH) (K(i) 5.3 +/- 0.5 and 0.6 +/- 0.01 nM). Three mammalian receptor antagonists were also pure antagonists in the chicken GnRH receptor. Another three, characterized by D-Lys(6) or D-isopropyl-Lys(6) moieties, functioned as pure antagonists in the human receptor but were full or partial agonists in the chicken receptor. This suggests that the Lys side chain interacts with functional groups of the chicken GnRH receptor to stabilize it in the active conformation and that these groups are not available in the activated human GnRH receptor. Substitution of the human receptor extracellular loop two with the chicken extracellular loop two identified this domain as capable of conferring agonist activity to mammalian antagonists. Although functioning of antagonists as agonists has been shown to be species-dependent for several GPCRs, the dependence of this on an extracellular domain has not been described.

Contribution of growth hormone-releasing hormone and somatostatin to decreased growth hormone secretion in elderly men

OBJECTIVE

The pathophysiology of the decline in circulating growth hormone (GH) concentrations that may occur with ageing remains elusive. We have investigated the potential contributions of decreased endogenous GH-releasing hormone (GHRH) and increased somatostatin secretion to this phenomenon.

DESIGN AND METHODS

The strategy used was to stimulate GH secretion in 8 young (20-24 years old, body mass index (BMI) 22.8 +/- 2.8 kg/m2) and 8 elderly (68-82 years old, BMI 23.4 +/- 1.6 kg/m2) male subjects on separate occasions by means of: (i) intravenous bolus 0.5 microgram/kg D-Ala2 GHRH(1-29)-NH2 alone; (ii) 0.5 microgram/kg GHRH after pre-treatment with two oral doses of 50 mg atenolol (to inhibit somatostatin secretion); (iii) 1.25 mg oral bromocriptine alone (to increase endogenous GHRH and/or inhibit somatostatin); (iv) 50 mg oral atenolol plus 1.25 mg oral bromocriptine; and (v) 0.5 microgram/kg GHRH after pre-treatment with 1.25 mg oral bromocriptine.

RESULTS

The elderly men had a significantly lower peak and area under curve (AUC) GH response to intravenous GHRH when compared with 8 young men (peak 3.1 +/- 1.0 ng/ml v. 21.6 +/- 5.0 ng/ml, AUC 205 +/- 56 ng/ml/min v. 1,315 +/- 295 ng/ml/min, P < 0.05). Pre-treatment with atenolol before GHRH administration produced no significant increase in peak and AUC GH response in both groups, which remained lower in the elderly men than in their young counterparts (peak 5.5 +/- 1.8 ng/ml v. 29.3 +/- 7.0 ng/ml, AUC 327 +/- 90 ng/ml/min v. 2,017 +/- 590 ng/ml/min, P < 0.05). Bromocriptine alone did not cause a significant rise in GH concentration in either elderly or young subjects (peak 3.1 +/- 1.1 v. 8.8 +/- 3.2 ng/ml, P > 0.05). When atenolol was administered before bromocriptine, both groups responded but the elderly subjects had a significantly greater peak and AUC response (peak 3.6 +/- 0.7 v. 10.7 +/- 2.1 ng/ml; AUC 191 +/- 39 v. 533 +/- 125 ng/ml/min, P < 0.05). Bromocriptine given before GHRH failed to potentiate GHRH action on GH release in either group. Of 5 elderly men who underwent further evaluation of GH secretory ability, 2 subjects had GH levels > 10 ng/ml, either basally or after intravenous GHRH. The remaining 3 had an initially impaired GH response to bolus intravenous GHRH. After 100 micrograms GHRH subcutaneously twice daily for up to 2 weeks the GH responses to intravenous bolus GHRH (0.5 microgram/kg) were reassessed. One exhibited a normal response (> 10 ng/ml) after 1 week of daily GHRH treatment, another had a near-normal response after 2 weeks (9.7 ng/ml), while the third still had an impaired response by the end of the 2-week treatment period (3.2 ng/ml).

CONCLUSIONS

The restoration of endogenous GH secretion in these elderly subjects by means of GHRH priming, and the failure of manipulation of somatostatinergic tone to restore a normal GH response to GHRH suggests that somatotroph atrophy due to a reduction in endogenous GHRH secretion is the principal cause of the diminished GH secretion with ageing.

Three distinct types of GnRH receptor characterized in the bullfrog

It has been proposed recently that two types of GnRH receptors (GnRHR) exist in a particular species. Here we present data demonstrating that at least three types of GnRHR are expressed in a single diploid species, the bullfrog. Three different cDNAs, encoding distinct types of bullfrog GnRHR (bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3), were isolated from pituitary and hindbrain of the bullfrog. BfGnRHR-1 mRNA was expressed predominantly in pituitary, whereas bfGnRHR-2 and -3 mRNAs were expressed in brain. The bfGnRHR-1, bfGnRHR-2, and bfGnRHR-3 proteins have an amino acid identity of approximately 30% to approximately 35% with mammalian GnRHRs and approximately 40% to approximately 50% with nonmammalian GnRHRs. Interestingly, bfGnRHR-2 has an 85% amino acid homology with Xenopus GnRHR. Less than 53% amino acid identity was observed among the three bfGnRHRs. All isolated cDNAs encode functional receptors because their transient expression in COS-7 cells resulted in a ligand-dependent increase in inositol phosphate production. Notably, all three receptors exhibited a differential ligand selectivity. For all receptors, cGnRH-II has a higher potency than mGnRH. In addition, salmon GnRH also has a strikingly high potency to stimulate all three receptors. In conclusion, we demonstrated the presence of three GnRHRs in the bullfrog. Their expression in pituitary and brain suggests that bfGnRHRs play an important role in the regulation of reproductive functions in the bullfrog.

Distribution and regulation by oestrogen of fully processed and variant transcripts of gonadotropin releasing hormone I and gonadotropin releasing hormone receptor mRNAs in the male chicken

The aim of this study was to increase understanding of the occurrence and regulation of chicken gonadotropin releasing hormone I (cGnRH I) and chicken gonadotropin releasing hormone receptor (cGnRH-R) mRNA variants in the hypothalamic-pituitary-testicular axis (HPTA). The study was carried out in the cockerel. Fully processed cGnRH I mRNA (cGnRH Ia) and a variant transcript (cGnRH Ib) with a retained intron 1 were observed in the preoptic/anterior hypothalamus (POA), the basal hypothalamus, anterior pituitary gland, and testes. Fully processed cGnRH-R mRNA (cGnRH-Ra) and a variant transcript (cGnRH-Rb) with a deletion were detected in the same tissues. In juvenile cockerels, concentrations of cGnRH Ia and b in the POA increased after castration, and this was prevented by oestrogen treatment. In the anterior pituitary gland, the concentration of cGnRH-Ra increased after castration and this was reversed by oestrogen treatment. In intact adult cockerels, oestrogen treatment depressed plasma luteinizing hormone but did not affect concentrations of cGnRH I and cGnRH-R mRNAs in the POA, basal hypothalamus, and anterior pituitary gland, suggesting that locally produced oestrogen, by aromatization, may exert maximal suppression on cGnRH I and GnRH-R mRNAs. In intact adult cockerels, the concentrations of cGnRH Ia and b in the testis, but not cGnRH-Ra and b, were depressed by oestrogen treatment. It was concluded that fully processed and variant cGnRH I and cGnRH-R mRNAs occur in all components of the HPTA. Oestrogen appears to play a role in the regulation of cGnRH Ia and b in the POA and testes, and of cGnRH-Ra in the POA and anterior pituitary gland.

Desensitization and internalization of human and xenopus gonadotropin-releasing hormone receptors expressed in alphaT4 pituitary cells using recombinant adenovirus

Nonmammalian vertebrates express at least two forms of GnRH and distinct forms of GnRH receptor (GnRH-R) have coevolved with their ligands. Mammalian and nonmammalian GnRH-R have key structural differences (notably the lack of C-terminal tails in mammalian GnRH-R) and comparative studies are beginning to reveal their functional relevance. However, cellular context and receptor density influence G protein-coupled receptor function and may be important variables in such work using heterologous expression systems. Here we report a comparative study using alphaT4 cells (gonadotrope progenitors that lack endogenous GnRH-R) transfected with a mammalian (human) or nonmammalian (Xenopus laevis type I) GnRH-R. Because conventional transfection strategies proved inefficient, recombinant adenovirus expressing these receptors were constructed, enabling controlled and efficient GnRH-R expression. When expressed in alphaT4 cells at physiological density, these GnRH-Rs retain the pharmacology of their endogenous counterparts (as judged by ligand specificity in radioligand binding and inositol phosphate accumulation assays) but do not activate adenylyl cyclase and are not constitutively active. Moreover, the Xenopus GnRH-R rapidly desensitizes and internalizes in these cells, whereas the human GnRH-R does not, and the internalization rates are not dependent upon receptor number. These data extend studies in COS, HEK, and GH3 cells showing that other GnRH-R with C-terminal tails desensitize and internalize rapidly, whereas tail-less mammalian GnRH-R do not. Retention of these distinctions at physiological receptor density in gonadotrope lineage cells, supports the argument that the evolution of nondesensitizing mammalian GnRH-Rs is functionally relevant and related to the development of mammalian reproductive strategies.

Cloning and gene expression of a novel human ribonucleoprotein

This paper reports on the cloning and characterization of a novel human ribonucleoprotein, RBM8, containing a single RNA binding domain comprising the two RNP-CS and RNP-2 consensus motifs. The protein has 55% identity to a segment of a C. elegans ribonucleoprotein of unknown function. The RBM8 gene shows ubiquitous tissue expression, predominantly as a 0.9 kb transcript. An interesting feature of the RBM8 transcript is an homology of 42% in the 3' untranslated region, in the antisense orientation, to the human gonadotropin-releasing hormone receptor polypeptide. RBM8 maps to human chromosome 14 in the 14q21-q23 region.

Multiple interactions of the Asp(2.61(98)) side chain of the gonadotropin-releasing hormone receptor contribute differentially to ligand interaction

Mutation of Asp(2.61(98)) at the extracellular boundary of transmembrane helix 2 of the gonadotropin-releasing hormone (GnRH) receptor decreased the affinity for GnRH. Using site-directed mutagenesis, ligand modification, and computational modeling, different side chain interactions of Asp(2.61(98)) that contribute to high-affinity binding were investigated. The conservative Asp(2. 61(98))Glu mutation markedly decreased the affinity for a series of GnRH analogues containing the native His(2) residue. This mutant showed smaller decreases in affinity for His(2)-substituted ligands. The loss of preference for His(2)-containing ligands in the mutant receptor shows that Asp(2.61(98)) determines the specificity for His(2). Analysis of the affinities of a series of position 2-substituted ligands suggests that a hydrogen bond forms between Asp(2.61(98)) and the delta NH group of His(2) and that Asp(2. 61(98)) forms a second hydrogen bond with the ligand. Substitution of Asp(2.61(98)) with an uncharged residue further decreased the affinity for all ligands and also decreased receptor expression. Computational modeling indicates an intramolecular ionic interaction of Asp(2.61(98)) with Lys(3.32(121)) in transmembrane helix 3. The uncharged, Lys(3.32(121))Gln mutation also markedly decreased agonist affinity. The modeling and the similar phenotypes of mutants with uncharged substitutions for Asp(2.61(98)) or Lys(3.32(121)) are consistent with the presence of this helix 2-helix 3 interaction. These studies support a dual role for Asp(2.61(98)): formation of an interhelical interaction with Lys(3.32(121)) that contributes to the structure of the agonist binding pocket and an interaction with His(2) of GnRH that helps stabilize agonist complexing.

Molecular cloning, distribution and pharmacological characterization of a novel gonadotropin-releasing hormone ([Trp8] GnRH) in frog brain

To date nine structural variants of GnRH have been identified in vertebrates and two additional forms have been isolated from a tunicate. In amphibians only mammalian GnRH ([Arg8] GnRH) and type II GnRH (chicken GnRH II, [His5, Trp7, Tyr8] GnRH) have been identified. In the present study, a full-length cDNA encoding a novel type of GnRH was isolated from pituitary of Rana dybowskii. The GnRH gene encodes a GnRH peptide ([Trp8] GnRH) in which tryptophan is substituted for arginine of mammalian GnRH Northern blot analysis revealed the presence of a single 500 bp transcript for the [Trp8] GnRH precursor in forebrain but its absence in testis, ovary, kidney and liver. Restriction digests of genomic DNA demonstrated a single copy of the gene. The [Trp8] GnRH immunoreactive cells were identified in the preoptic area of the frog brain. Synthetic [Trp8] GnRH was tested for its ability to stimulate inositol phosphate production by COS-1 cells transfected with the cloned Xenopus pituitary GnRH receptor and the cloned human GnRH receptor. [Trp8] GnRH had a potency of about 60% compared with mammalian GnRH ([Arg8] GnRH) for the Xenopus receptor, whereas the potency of [Trp8] GnRH was approximately 5% compared with mammalian GnRH for the human receptor. Both mammalian GnRH and [Trp8] GnRH were 1000-fold less potent than type II GnRH for the Xenopus GnRH receptor. The similar potency of [Arg8] GnRH and the novel [Trp8] GnRH for the Xenopus pituitary receptor indicates that, unlike the human receptor, the Xenopus receptor does not discriminate between these amino acids in position eight thereby allowing substitution of the arginine in the mammalian GnRH.

Complementary deoxyribonucleic acid cloning, gene expression, and ligand selectivity of a novel gonadotropin-releasing hormone receptor expressed in the pituitary and midbrain of Xenopus laevis

We have cloned the full-length complementary DNA (cDNA) for a GnRH receptor from Xenopus laevis pituitary cDNA and determined its gene structure. The cDNA encodes a 368-amino acid protein that has a 46% amino acid identity to the human GnRH receptor. The X laevis GnRH receptor has all of the amino acids identified in the mammalian GnRH receptors as sites of interaction with the GnRH ligand. However, this receptor cDNA shares the same distinguishing structural features of the GnRH receptor that have been characterized from other nonmammalian vertebrates. These include the pair of aspartate residues in the transmembrane domains II and VII compared with the aspartate/asparagine arrangement in mammalian receptors, the amino acid PEY motif in extracellular loop III (SEP in mammals), and the presence of a carboxyl-terminal tail. Previous studies have reported that mammalian GnRH was equipotent to other naturally occurring GnRH subtypes in stimulating LH release from the amphibian pituitary. However, in this study we show that the X. laevis GnRH receptor has ligand selectivity for the naturally occurring GnRHs similar to other nonmammalian GnRH receptors. The order of potency of the GnRHs in stimulating inositol phosphate production in COS-1 cells transiently transfected with the X. laevis GnRH receptor cDNA was chicken GnRH II>salmon GnRH>mammalian GnRH. Transcripts of this GnRH receptor are expressed in the pituitary and midbrain of X. laevis.

Cloning and expression, pharmacological characterization, and internalization kinetics of the pituitary GnRH receptor in a metatherian species of mammal

Gonadotropin-releasing hormone receptors (GnRH-Rs) expressed in the pituitary of eutherian species of mammal are unique in lacking the cytoplasmic C-terminal tail characteristic of GnRH-Rs of nonmammalian vertebrates and other G protein-coupled receptors. To further investigate evolutionary relationships among vertebrate GnRH-Rs, a full-coding region cDNA of the pituitary GnRH-R was cloned from a metatherian marsupial mammal, the Australian brushtail possum (Trichosurus vulpecula). We have determined the pharmacological characteristics and internalization kinetics of this GnRH-R from an early evolved, metatherian species of mammal and compared it with the corresponding receptors in eutherian species of mammal and nonmammalian vertebrates. The predicted GnRH-R protein from the possum pituitary has high homology with the other mammalian GnRH-Rs (80% identity) and, in common with other mammals, lacks an intracellular C-terminal tail. The ligand selectivity of the possum GnRH-R transfected into COS-1 cells, assessed using inositol phosphate assays and radioreceptor binding assays, was similar to that of the other mammalian GnRH-Rs, and distinct from those of the nonmammalian GnRH-Rs. The pharmacological characteristics of the possum GnRH-R were similar to those of other mammalian GnRH-Rs, for a selection of agonists (including naturally occurring GnRH ligands and superagonists) and antagonists. Receptor-mediated internalization of GnRH agonist by the possum GnRH-R was slightly more rapid than that of the human GnRH-R, while the internalization kinetics of the chicken GnRH-R, in which a cytoplasmic C-terminal tail is present, was considerably more rapid. In terms of the evolution of the GnRH-R in vertebrates, the possum (a metatherian mammal) GnRH-R has a striking resemblance, in both structure and pharmacological characteristics, to GnRH-Rs in eutherian mammals, which are quite distinct from the nonmammalian vertebrate GnRH-Rs, and are unique among G protein-coupled receptors in lacking an intracellular C-terminal tail. The distinct structure of the pituitary GnRH-R in mammalian vertebrates is likely to have important functional consequences in the reproductive physiology of mammals.

Progress towards the development of non-peptide orally-active gonadotropin-releasing hormone (GnRH) antagonists: therapeutic implications

Gonadotropin-releasing hormone (GnRH) is a decapeptide (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly.NH2) which is produced from a precursor polypeptide in hypothalamic neurons and secreted in a pulsatile fashion to stimulate the secretion of LH and FSH via its interaction with a cognate receptor on gonadotropes. Low doses of the native peptide delivered in a pulsatile manner to mimic that found in the hypothalamic portal vessels restore fertility in hypogonadal patients, and are also effective in treating cryptorchidism and delayed puberty. Administration of high doses of GnRH, or agonist analogues, causes desensitization of the gonadotrope with consequent decline in gonadal gametogenesis and steroid and peptide hormone synthesis. This phenomenon finds extensive therapeutic application in clinical medicine in a wide spectrum of disease (Table 1). In addition, GnRH analogues have promise as new generation male and female contraceptives in conjunction with steroid hormone replacement. GnRH antagonists inhibit the reproductive system through competition with endogenous GnRH for the receptor and, in view of their rapid effects, are being increasingly used for the above mentioned applications. The peptide agonists and antagonists currently available require parenteral administration, typically in the form of long-acting depots. A new generation of non-peptide GnRH antagonists are beginning to emerge which should allow oral administration and, therefore, may provide greater flexibility of dosing, lower costs and increased patient acceptance.

A new photoreactive antagonist cross-links to the N-terminal domain of the gonadotropin-releasing hormone receptor

A new photoreactive gonadotropin-releasing hormone (GnRH) antagonist [Ac-(4-azidobenzoyl)-D-Lys1, D-4-Cl-Phe2, D-Trp3, D-Arg6, D-Ala10]GnRH (PAnt-1) was synthesized and shown to bind covalently to mouse and human GnRH receptors after ultraviolet irradiation. PAnt-1 exhibited high binding affinity (Ki = 3.1 +/- 0.8 nM), and high crosslinking efficiency as shown by loss of 78% of binding sites following crosslinking at saturating concentration. Crosslinking resulted in irreversible receptor blockade as shown by inhibition of GnRH-stimulated inositol phosphate production. PAnt-1 has a photoreactive group at residue 1 of the peptide, a region believed to be critical in determining antagonist versus agonist properties of GnRH analogues. The attachment site of PAnt- to the receptor was localized between residues 11 and 19 of the extracellular N-terminal domain of the receptor by peptide mapping studies using natural sequence differences between human, mouse and sheep GnRH receptors, as well as a panel of GnRH receptor constructs with a series of engineered protease cleavage sites. A disulphide bridge between Cys14 and Cys200 was cleaved during crosslinking, suggesting that Cys14 is the crosslinked residue. These results suggest that peptide GnRH antagonists bind to the receptor with the N-terminal end of the peptide positioned in a site comprising the constrained regions of the N-terminal domain and second extracellular loop in the vicinity of the Cys14-Cys200 disulphide bridge.

The functional microdomain in transmembrane helices 2 and 7 regulates expression, activation, and coupling pathways of the gonadotropin-releasing hormone receptor

Structural microdomains of G protein-coupled receptors (GPCRs) consist of spatially related side chains that mediate discrete functions. The conserved helix 2/helix 7 microdomain was identified because the gonadotropin-releasing hormone (GnRH) receptor appears to have interchanged the Asp(2.50) and Asn(7.49) residues which are conserved in transmembrane helices 2 and 7 of rhodopsin-like GPCRs. We now demonstrate that different side chains of this microdomain contribute specifically to receptor expression, heterotrimeric G protein-, and small G protein-mediated signaling. An Asn residue is required in position 2.50(87) for expression of the GnRH receptor at the cell surface, most likely through an interaction with the conserved Asn(1.50(53)) residue, which we also find is required for receptor expression. Most GPCRs require an Asp side chain at either the helix 2 or helix 7 locus of the microdomain for coupling to heterotrimeric G proteins, but the GnRH receptor has transferred the requirement for an acidic residue from helix 2 to 7. However, the presence of Asp at the helix 7 locus precludes small G protein-dependent coupling to phospholipase D. These results implicate specific components of the helix 2/helix 7 microdomain in receptor expression and in determining the ability of the receptor to adopt distinct activated conformations that are optimal for interaction with heterotrimeric and small G proteins.