Basic aspects of the control of GnRH and LH secretions by kisspeptin: potential applications for better control of fertility in females

Effect of kisspeptin on in vitro maturation of sheep oocytes

Aim:

The aim of this study was to investigate the effect of kisspeptin (KP) on in vitro maturation (IVM) of sheep oocytes aspirated from the ovaries collected from slaughterhouse.

Materials and Methods:

Two different experiments were conducted to investigate the effect of KP (5, 10 and 15 µg/ml) alone (experiment 1) or in combination with follicle-stimulating hormone (FSH), luteinizing hormone (LH), and Estradiol (E₂) (experiment 2) on IVM of sheep oocytes. Tissue culture medium 199 supplemented with Gentamicin was used as control medium. Good quality oocytes were randomly allocated into different IVM media and cultured at 38.5°C in 5% CO₂ under humidified atmosphere for 24 h. The oocytes were evaluated for their cumulus cell expansion (CCE) and extrusion of the 1^st polar body (PB) at the end of maturation.

Results:

The proportion of oocytes showing CCE and extrusion of PB was highest when the oocytes were matured in the medium supplemented with 10 µg/ml of KP. In experiment 2, oocytes were matured in 12 different maturation media (G₁-G₁₂: G₁: Control, G₂: KP alone, G₃: FSH, G₄: FSH+KP, G₅: LH, G₆: LH+KP, G₇: E₂, G₈: E₂+KP, G₉: FSH+LH+E₂, G₁₀: FSH+LH+E₂+KP, G₁₁: FSH+LH+E2+fetal bovine serum (FBS), G₁₂: FSH+LH+E₂+FBS+KP). The proportion of oocytes showing cumulus expansion and PB extrusion was highest (98.33±1.05 and 89.17±2.38) when they were matured in FSH+LH+E₂+FBS+KP (G₁₂) and was significantly higher than other groups. The proportion of CCE and extrusion of PB was significantly increased when KP was supplemented to FSH and E₂, but no effect was observed with LH. The maturation rates were significantly increased when FSH, LH, and E₂ (G₉) containing media were additionally supplemented with KP (G₁₀).

Conclusion:

This study demonstrated that the addition of KP (10 µg/ml) to the FSH, LH, and E₂ supplemented media would enhance the sheep oocyte maturation in vitro.

Trichostatin A reduces GnRH mRNA expression with a concomitant increase in retinaldehyde dehydrogenase in GnRH-producing neurons

Trichostatin A (TSA) is a selective inhibitor of mammalian histone deacetylase and is widely used to modify the ability of DNA transcription factors to bind DNA within chromatin by interfering with histone deacetylation. In the GnRH-producing neuronal cell line GT1-7, TSA significantly reduced expression of GnRH mRNA. Kisspeptin, a known regulator of GnRH release, failed to increase GnRH mRNA expression and did not modify TSA-induced reduction of GnRH expression. TSA, but not kisspeptin, increased histone acetylation in whole-cell lysates and significantly stimulated the expression of retinaldehyde dehydrogenase (RALDH), a retinoic acid (RA)-synthesizing enzyme that is known to be involved in cell differentiation. In addition, treatment of the GT1-7 cells with RA dose-dependently inhibited the expression of GnRH mRNA. Whereas, TSA-induced reduction of GnRH mRNA was not modulated by treatment with the pan-RA receptor inverse agonist BMS493 or the RA metabolism inhibitor liarozole. Our current results suggest that the RALDH and RA might not be directly involved in the reduction of GnRH expression induced by TSA, however these substances could be a novel regulator of GnRH.

Vaginal fold histology reduces the variability introduced by vaginal exfoliative cytology in the classification of mouse estrous cycle stages

Vaginal exfoliative cytology is commonly used in biomedical and toxicological research to classify the stages of the rodent estrous cycle. However, mouse vaginal exfoliative cytology is commonly used as a stand-alone tool and has not been evaluated in reference to vaginal histology and serum sex hormone levels. In this study, the direct and Giemsa-stained methods of vaginal exfoliative cytology were compared in reference to vaginal fold histology and serum sex hormone levels. Both methods predicted the estrous stages similarly with mean discordance rates of 55%, 77%, 46%, and 31%, for diestrus, proestrus, estrus, and metestrus, respectively. From these results, we conclude that vaginal exfoliative cytology may be used as a general guide to determine the desired estrous stage end point and that a definitive confirmation of the estrous stage should be obtained from evaluation of vaginal fold histology. Confirmation of the stage of the estrous cycle by vaginal fold histology will decrease the variability otherwise introduced by misclassification of estrous cycle stages with vaginal exfoliative cytology.

Genetic mechanisms mediating kisspeptin regulation of GnRH gene expression

Kisspeptins (Kiss) have been shown to be key components in the regulation of gonadotropin-releasing hormone (GnRH) secretion. In vitro studies have demonstrated an increase in GnRH gene expression by Kiss suggesting regulation of GnRH at both the secretory and pretranslational levels. Here, we define genetic mechanisms that mediate Kiss action on target gene expression. In vitro, sequential deletions of the mouse GnRH (mGnRH) gene promoter fused to the luciferase (LUC) reporter gene localized at kisspeptin-response element (KsRE) between -3446 and -2806 bp of the mGnRH gene. In vivo, transgenic mice bearing sequential deletions of the mGnRH gene promoter linked to the LUC reporter localized an identical KsRE. To define the mechanism of regulation, Kiss was first shown to induce nucleosome-depleted DNA within the KsRE, and a potential binding site for the transcription factor, Otx-2, was revealed. Furthermore, increased Otx-2 mRNA, protein, and binding to the KsRE after Kiss treatment were demonstrated. In conclusion, this work identified elements in GnRH-neuronal cell lines and in transgenic mice that mediate positive regulation of GnRH by Kiss. In addition, we show for the first time that Otx-2 is regulated by Kiss, and plays a role in mediating the transcriptional response of mGnRH gene.

Stimulatory effect of RFRP-3 on the gonadotrophic axis in the male Syrian hamster: the exception proves the rule

In seasonal mammals, a distinct photoneuroendocrine circuit that involves the pineal hormone melatonin tightly synchronizes reproduction with seasons. In the Syrian hamster, a seasonal model in which sexual activity is inhibited by short days, we have previously shown that the potent GnRH stimulator, kisspeptin, is crucial to convey melatonin's message; however, the precise mechanisms through which melatonin affects kisspeptin remain unclear. Interestingly, rfrp gene expression in the neurons of the dorsomedial hypothalamic nucleus, a brain region in which melatonin receptors are present in the Syrian hamster, is strongly down-regulated by melatonin in short days. Because a large body of evidence now indicates that RFamide-related peptide (RFRP)-3, the product of the rfrp gene, is an inhibitor of gonadotropin secretion in various mammalian species, we sought to investigate its effect on the gonadotrophic axis in the Syrian hamster. We show that acute central injection of RFRP-3 induces c-Fos expression in GnRH neurons and increases LH, FSH, and testosterone secretion. Moreover, chronic central administration of RFRP-3 restores testicular activity and Kiss1 levels in the arcuate nucleus of hamsters despite persisting photoinhibitory conditions. By contrast RFRP-3 does not have a hypophysiotrophic effect. Overall, these findings demonstrate that, in the male Syrian hamster, RFRP-3 exerts a stimulatory effect on the reproductive axis, most likely via hypothalamic targets. This places RFRP-3 in a decisive position between the melatonergic message and Kiss1 seasonal regulation. Additionally, our data suggest for the first time that the function of this peptide depends on the species and the physiological status of the animal model.

Kisspeptin-10 stimulates the release of luteinizing hormone and testosterone in pre- and post-pubertal male goats

The aims of the present study were to clarify the effect of kisspeptin-10 (Kp10) on the secretion of luteinizing hormone (LH) and testosterone (T) in pre-pubertal and post-pubertal male ruminants. Four male goats (Shiba goats) were given an intravenous (i.v.) injection of Kp10 (5 µg/kg body weight (b.w.)), gonadotoropin-releasing hormone (GnRH, 1 µg/kg b.w.), or 2 mL of saline as a control at the ages of 3 (pre-pubertal) and 6 (post-pubertal) months. A single i.v. injection of Kp10 significantly stimulated the release of LH and T in both groups. The area under the response curve (AUC) of LH for a 60-min period after the i.v. injection of Kp10 was significantly greater in the pre-pubertal goats (P < 0.05). The AUC of T for a 120 min period post-injection did not differ between the two age groups. A single i.v. injection of GnRH also significantly stimulated the release of LH and T in both groups (P < 0.05). The secretory pattern of LH and T in response to GnRH resembled that in response to Kp10. These results show that the LH-releasing response to Kp10 is greater in pre-pubertal than post-pubertal male goats. They also show that Kp10, as well as GnRH, is able to stimulate the release of T in male goats.

Kisspeptin activates the hypothalamic–adenohypophyseal–gonadal axis in prepubertal ewe lambs

The onset of puberty in mammals involves an increase in the pulsatile release of GNRH and LH. The KISS1 gene is essential for pubertal development, and its product, kisspeptin, stimulates the release of LH. The objective of this study was to determine the effects of kisspeptin in the hypothalamic–adenohypophyseal–gonadal axis of prepubertal ewe lambs. Ewe lambs (28 weeks of age) were treated intravenously with saline (control, n=6) or kisspeptin (20 μg kisspeptin; n=6) every hour for 24 h. Kisspeptin stimulated pulse-like release of LH within 15 min following injections, and increased the frequency and amplitude of LH pulses, and mean circulating concentrations of LH and estradiol. A surge-like release of LH was observed in four kisspeptin-treated lambs beginning 17 h after the onset of treatment, and all four lambs had elevated circulating concentrations of progesterone within 5 days post-treatment. However, circulating concentrations of progesterone decreased within 2 days after the initial rise in three of the four ewe lambs, indicating that induced luteal activity was of short duration. The proportion of lambs that were pubertal (defined by circulating concentrations of progesterone above 1 ng/ml for at least 7 days) by 35 weeks of age (8/11) and the mean age at puberty (32±1 weeks) for those reaching puberty within the experimental period did not differ between treatments. Results support a role for kisspeptin in the activation of the hypothalamic–adenohypophyseal axis leading to the onset of puberty in ewe lambs.

Insights into the mechanism by which kisspeptin stimulates a preovulatory LH surge and ovulation in seasonally acyclic ewes: potential role of estradiol

We have previously demonstrated that a constant intravenous infusion of kisspeptin (Kp) for 48 h in anestrous ewes induces a preovulatory luteinizing hormone (LH) surge followed by ovulation in approximately 75% of animals. The mechanisms underlying this effect are unknown. In this study, we investigated whether Kp-induced preovulatory LH surges in anestrous ewes were the result of the general activation of the whole gonadotropic axis or of the direct activation of central GnRH neurons required for the GnRH/LH surge. In the first experiment, a constant iv infusion of ovine kisspeptin 10 (Kp; 15.2 nmol/h) was given to 11 seasonally acyclic ewes over 43 h. Blood samples were taken every 10 min for 15 h, starting 5h before the infusion, and then hourly until the end of the infusion. We found that the infusion of Kp induced a well-synchronized LH surge (around 22 h after the start of the Kp infusion) in 82% of the animals. In all ewes with an LH surge, there was an immediate but transient increase in the plasma concentrations of LH, follicle-stimulating hormone (FSH), and growth hormone (GH) at the start of the Kp infusion. Mean (+/- SEM) concentrations for the 5-h periods preceding and following the start of the Kp infusion were, respectively, 0.33 +/- 0.09 vs 2.83 +/- 0.49 ng/mL (P = 0.004) for LH, 0.43 +/- 0.05 vs 0.55 +/- 0.03 ng/mL (P = 0.015) for FSH, and 9.34 +/- 1.01 vs 11.51 +/- 0.92 ng/mL (P = 0.004) for GH. In the first experiment, surges of LH were observed only in ewes that also had a sustained rise in plasma concentrations of estradiol (E(2)) in response to Kp. Therefore, a second experiment was undertaken to determine the minimum duration of Kp infusion necessary to induce such a pronounced and prolonged increase in plasma E(2) concentration. Kisspeptin (15.2 nmol/h) was infused for 6, 12, or 24h in seasonally acyclic ewes (N = 8), and blood samples were collected hourly for 28 h (beginning 5h before the start of infusion), then every 2h for the following 22 h. Kisspeptin infused for 24h induced LH surges in 75% of animals, and this percentage decreased with the duration of the infusion (12h = 50%; 6h = 12.5%). The plasma concentration of E(2) was greater in ewes with an LH surge compared to those without LH surges; mean (+/- SEM) concentrations for the 5-h period following the Kp infusion were, respectively, 2.23 +/- 0.16 vs 1.27 +/- 0.13 pg/mL (P < 0.001). Collectively, our results strongly suggest that the systemic delivery of Kp induced LH surges by activating E(2)-positive feedback on gonadotropin secretion in acyclic ewes.

Kisspeptin increases GnRH mRNA expression and secretion in GnRH secreting neuronal cell lines

Kisspeptins, and their G-protein coupled receptor 54 (GPR54), are key components in the regulation of gonadotropin-releasing hormone (GnRH) secretion in humans and other mammals. Several studies demonstrate that the central or systemic administration of kisspeptin increases GnRH and gonadotropin secretion in both prepubertal and adult animals; however, the cellular targets and intracellular mechanisms of action in the central reproductive axis are unclear. In this study, we documented the presence of GPR54 in two GnRH secreting neuronal cell lines (GT1-7 and GN11). Kisspeptin treatment increases GnRH secretion and GnRH mRNA levels in a dose and time dependent manner. 10(-9)M kisspeptin maximally stimulated GnRH secretion by 2-fold and GnRH mRNA levels up to 4-fold after 4h of treatment in both cell lines. Negative regulation by 17beta-estradiol of GnRH secretion and GnRH mRNA was antagonized by kisspeptin. Co-treatment with kisspeptin and 17beta-estradiol increased GnRH secretion by 2-fold and GnRH mRNA by 4-fold over estradiol alone in both cell lines. Intracellular signaling pathway studies showed that an ERK1/2 MAPK inhibitor (PD98059) and a PI3K inhibitor, LY29402, attenuated the effects of kisspeptin on GnRH mRNA modulation. Furthermore, Western blot analysis showed that phosphorylation of both MAPK and Akt substrates increased with kisspeptin treatment. This work demonstrates that the kisspeptin-GPR54 system plays a significant role stimulating GnRH secretion and positive regulation of GnRH mRNA levels in GnRH neurons in culture, and also, demonstrates the activation of MAPK and Akt signaling pathways by kisspeptin in GT1-7 and GN11 cell lines.

Postnatal development of an estradiol-kisspeptin positive feedback mechanism implicated in puberty onset

The regulation of GnRH neurons by kisspeptin is critical for normal puberty onset in mammals. In the rodent the kisspeptin neurons innervating GnRH neurons are thought to reside in the rostral periventricular area of the third ventricle (RP3V). Using kisspeptin immunocytochemistry we show that kisspeptin peptide expression in the RP3V of female mice begins around postnatal d 15 (P15) and rapidly increases to achieve adult-like levels by P30, the time of puberty onset. Ovariectomy of female pups at P15 resulted in a 70-90% reduction (P < 0.01) in kisspeptin peptide expression within the RP3V of P30 or P60 mice. Replacement of 17-beta-estradiol (E2) in P15-ovariectomized mice from P15-30 or P22-30 resulted in a complete restoration of kisspeptin peptide expression in the RP3V (P < 0.01). Kisspeptin-immunoreactive fibers throughout the hypothalamus, including the arcuate nucleus, followed the same pattern of estrogen-dependent expression. To test the absolute necessity of estrogen for kisspeptin expression in the RP3V, aromatase knockout mice were examined. Kisspeptin-immunoreactive cells were detected in the arcuate nucleus, but there was a complete absence of kisspeptin peptide in RP3V neurons of aromatase knockout adult females. These results demonstrate that E2 is essential for the prepubertal development of kisspeptin peptide within RP3V neurons and suggest that an E2-kisspeptin positive feedback mechanism exists before puberty. This implies that RP3V kisspeptin neurons are E2-dependent amplifiers of GnRH neuron activity in the prepubertal period.