Molecular biology of the kisspeptin receptor: signaling, function, and mutations

New Aspects of Corpus Luteum Regulation in Physiological and Pathological Conditions: Involvement of Adipokines and Neuropeptides

The corpus luteum is a small gland of great importance because its proper functioning determines not only the appropriate course of the estrous/menstrual cycle and embryo implantation, but also the subsequent maintenance of pregnancy. Among the well-known regulators of luteal tissue functions, increasing attention is focused on the role of neuropeptides and adipose tissue hormones-adipokines. Growing evidence points to the expression of these factors in the corpus luteum of women and different animal species, and their involvement in corpus luteum formation, endocrine function, angiogenesis, cells proliferation, apoptosis, and finally, regression. In the present review, we summarize the current knowledge about the expression and role of adipokines, such as adiponectin, leptin, apelin, vaspin, visfatin, chemerin, and neuropeptides like ghrelin, orexins, kisspeptin, and phoenixin in the physiological regulation of the corpus luteum function, as well as their potential involvement in pathologies affecting the luteal cells that disrupt the estrous cycle.

Effect of hypothyroidism on the hypothalamic-pituitary-ovarian axis and reproductive function of pregnant rats

BACKGROUND

This study aimed to detect changes in hormone levels in the hypothalamic-pituitary-ovarian axis in Sprague-Dawley (SD) rats with hypothyroidism, and identify differences in the pregnancy and abortion rates of female adult rats. The potential role of gonadotropin releasing hormone (GnRH) as the link between the hypothalamic-pituitary-ovarian axis and reproductive function regulated by thyroid hormones was also investigated.

METHODS

Female SD rats (n = 136) were causally classified into two groups: the normal-drinking-water group (n = 60) and the 0.05% propylthiouracil-drinking-water group (PTU 2 mg/kg/day, n = 76) to establish an adult rat model of hypothyroidism (6 weeks). Female and male rats at a ratio of 1:2 were used to establish a hypothyroidism pregnancy model. GnRH mRNA and GnRH receptor (GnRHR) expression in rats was detected using real time quantitative PCR(qRT-PCR) and immunohistochemistry, respectively.

RESULTS

The abortion rate differed significantly between the hypothyroidism pregnancy group and the normal pregnancy group (P < 0.05). No significant differences were found in the distribution of the GnRHR among the five nuclei (hypothalamic arcuate nucleus, hypothalamic ventromedial nucleus, hypothalamic anterior nucleus, paraventricular nucleus of the hypothalamus, and ventral premammillary nucleus) of the hypothalamus and ovary (P > 0.05). Hypothyroidism had no significant effect on GnRH mRNA expression in the hypothalamic-pituitary-ovarian axis in the four groups (normal control group, normal pregnancy group, hypothyroidism pregnancy group, and hypothyroidism group) (P > 0.05).

CONCLUSIONS

Hypothyroidism had an adverse impact on pregnancy in rats and may affect the distribution of pituitary GnRHR, whereas it did not obviously affect the distribution of GnRHR in the nuclei of the hypothalamus and ovary. Hypothyroidism had no effect on GnRH mRNA expression.

The Emerging Role(s) for Kisspeptin in Metabolism in Mammals

Kisspeptin was initially identified as a metastasis suppressor. Shortly after the initial discovery, a key physiologic role for kisspeptin emerged in the regulation of fertility, with kisspeptin acting as a neurotransmitter via the kisspeptin receptor, its cognate receptor, to regulate hypothalamic GnRH neurons, thereby affecting pituitary-gonadal function. Recent work has demonstrated a more expansive role for kisspeptin signaling in a variety of organ systems. Kisspeptin has been revealed as a significant player in regulating glucose homeostasis, feeding behavior, body composition as well as cardiac function. The direct impact of kisspeptin on peripheral metabolic tissues has only recently been recognized. Here, we review the emerging endocrine role of kisspeptin in regulating metabolic function. Controversies and current limitations in the field as well as areas of future studies toward kisspeptin's diverse array of functions will be highlighted.

Towards new strategies to manage livestock reproduction using kisspeptin analogs

The discovery of the hypothalamic neuropeptide kisspeptin and its receptor (KISS1R) have dramatically improved our knowledge about the central mechanisms controlling reproduction. Kisspeptin neurons could be considered the hub where internal and external information controlling reproduction converge. The information is here elaborated and the command dispatched to GnRH neurons, the final output of the brain system controlling reproduction. Several studies have shown that in mammals administration of kisspeptin could finely modulate many aspects of reproduction from puberty to ovulation. For example in ewes kisspeptin infusion triggered ovulation during the non-breeding season and in prepubertal rat repeated injections advanced puberty onset. However, especially in livestock, the suboptimal pharmacological properties of endogenous kisspeptin, notably it short half-life and consequently its poor pharmacodynamics, fetters its use to experimental setting. To overcome this issue synthetic KISS1R agonists, mainly based on kisspeptin backbone, were created. Their more favorable pharmacological profile, longer half-life and duration of action, allowed to perform promising initial experiments for controlling ovulation and puberty. Additional experiments and further refinement of analogs would still be necessary to exploit fully the potential of targeting the kisspeptin system. Nevertheless, it is already clear that this new strategy may represent a breakthrough in the field of reproduction control.

Development of an excitatory kisspeptin projection to the oxytocin system in late pregnancy

KEY POINTS

Oxytocin release from the posterior pituitary gland stimulates uterine contraction during birth but the central mechanisms that activate oxytocin neurones for birth are not well characterized. We found that that kisspeptin fibre density around oxytocin neurones increases in late-pregnant rats. These kisspeptin fibres originated from hypothalamic periventricular nucleus neurones that upregulated kisspeptin expression in late pregnancy. Oxytocin neurones were excited by central kisspeptin administration in late-pregnant rats but not in non-pregnant rats or early- to mid-pregnant rats. Our results reveal the emergence of a new excitatory kisspeptin projection to the oxytocin system in late pregnancy that might contribute to oxytocin neurone activation for birth.

ABSTRACT

The hormone oxytocin promotes uterine contraction during parturition. Oxytocin is synthesized by magnocellular neurones in the hypothalamic supraoptic and paraventricular nuclei and is released into the circulation from the posterior pituitary gland in response to action potential firing. Systemic kisspeptin administration increases oxytocin neurone activity to elevate plasma oxytocin levels. Here, immunohistochemistry revealed that rats on the expected day of parturition (day 21 of gestation) had a higher density of kisspeptin-positive fibres in the perinuclear zone surrounding the supraoptic nucleus (which provides dense glutamatergic and GABAergic innervation to the supraoptic nucleus) than was evident in non-pregnant rats. Retrograde tracing showed the kisspeptin projections to the perinuclear zone originated from the hypothalamic periventricular nucleus. Quantitative RT-PCR showed that kisspeptin receptor mRNA, Kiss1R mRNA, was expressed in the perinuclear zone-supraoptic nucleus and that the relative Kiss1R mRNA expression does not change over the course of pregnancy. Finally, intracerebroventricular administration of kisspeptin increased the firing rate of oxytocin neurones in anaesthetized late-pregnant rats (days 18-21 of gestation) but not in non-pregnant rats, or in early- or mid-pregnant rats. Taken together, these results suggest that kisspeptin expression is upregulated in the periventricular nucleus projection to the perinuclear zone of the supraoptic nucleus towards the end of pregnancy. Hence, this input might activate oxytocin neurones during parturition.

Differential regulation of GnRH secretion in the preoptic area (POA) and the median eminence (ME) in male mice

GnRH release in the median eminence (ME) is the central output for control of reproduction. GnRH processes in the preoptic area (POA) also release GnRH. We examined region-specific regulation of GnRH secretion using fast-scan cyclic voltammetry to detect GnRH release in brain slices from adult male mice. Blocking endoplasmic reticulum calcium reuptake to elevate intracellular calcium evokes GnRH release in both the ME and POA. This release is action potential dependent in the ME but not the POA. Locally applied kisspeptin induced GnRH secretion in both the ME and POA. Local blockade of inositol triphospate-mediated calcium release inhibited kisspeptin-induced GnRH release in the ME, but broad blockade was required in the POA. In contrast, kisspeptin-evoked secretion in the POA was blocked by local gonadotropin-inhibitory hormone, but broad gonadotropin-inhibitory hormone application was required in the ME. Although action potentials are required for GnRH release induced by pharmacologically-increased intracellular calcium in the ME and kisspeptin-evoked release requires inositol triphosphate-mediated calcium release, blocking action potentials did not inhibit kisspeptin-induced GnRH release in the ME. Kisspeptin-induced GnRH release was suppressed after blocking both action potentials and plasma membrane Ca(2+) channels. This suggests that kisspeptin action in the ME requires both increased intracellular calcium and influx from the outside of the cell but not action potentials. Local interactions among kisspeptin and GnRH processes in the ME could thus stimulate GnRH release without involving perisomatic regions of GnRH neurons. Coupling between action potential generation and hormone release in GnRH neurons is thus likely physiologically labile and may vary with region.

DS1, a delta subunit-containing GABA(A) receptor agonist, increases gonadotropin subunit gene expression in mouse pituitary gonadotrophs

4-Chloro-N-[6,8-dibromo-2-(2-thienyl)imidazo[1,2-alpyridine-3-yl] (DS1) is a GABA(A) receptor agonist that selectively binds to delta subunit-containing GABA(A) alpha4beta3delta receptors. In the present study, we examined the effect of DS1 on pituitary gonadotropin subunit gene expression using the mouse pituitary gonadotroph cell line LbetaT2. DS1 increased the promoter activity of the gonadotropin subunits luteinizing hormone beta (LHbeta), follicle-stimulating hormone beta (FSHbeta), and alpha. Gonadotropin-releasing hormone (GnRH) receptor promoters were also activated by DS1. The effects of DS1 on gonadotropin subunit promoters were obvious, but they were less than those induced by stimulation with GnRH. GnRH-stimulated gonadotropin subunit promoters were enhanced in the presence of DS1. A prototypic specific agonist for GABAA receptors, muscimol, failed to increase LHbeta and FSHbeta subunit promoter activity and had no effect on GnRH-increased LHbeta and FSHbeta promoter activity. In addition, SKF97541, a specific agonist for GABAB receptors, did not modulate basal or GnRH-induced LHbeta and FSHbeta promoter activity. A natural GABA compound failed to increase gonadotropin promoter activity and potentiated the effect of GnRH on the FSHbeta promoter. DS1 increased the activity of serum response element (SRE) and cAMP response element (CRE) promoters, which reflect the activity of the extracellular signal-regulated kinase and cAMP/protein kinase A (PKA) pathways, and GnRH-increased SRE and CRE promoter activity was enhanced in the presence of DS1. A specific inhibitor of the ERK signaling pathway, U0126, prevented DS1-induced LHbeta and FSHbeta promoter activity almost completely; however, H89, a PKA inhibitor, did not modulate the effect of DS1. Our current observations demonstrate that the GABAA alpha4beta3delta receptor agonist DS1 can stimulate gonadotropin subunit gene expression in association with the ERK signaling pathway.

Disrupted kisspeptin signaling in GnRH neurons leads to hypogonadotrophic hypogonadism

Landmark studies have shown that mutations in kisspeptin and the kisspeptin receptor (Kiss1r) result in reproductive dysfunction in humans and genetically altered mouse models. However, because kisspeptin and its receptor are present in target cells of the central and peripheral reproductive axis, the precise location(s) for the pathogenic signal is unknown. The study described herein shows that the kisspeptin-Kiss1r signaling pathway in the GnRH neuron is singularly critical for both the onset of puberty as well as the attainment of normal reproductive function. In this study, we directly test the hypothesis that kisspeptin neurons regulate GnRH secretion through the activation of Kiss1r on the plasma membrane of GnRH neurons. A GnRH neuron-specific Kiss1r knockout mouse model (GKirKO) was generated, and reproductive development and phenotype were assessed. Both female and male GKirKO mice were infertile, having low serum LH and FSH levels. External abnormalities such as microphallus and decreased anogenital distance associated with failure of preputial gland separation were present in GKirKO males. A delay in pubertal onset and abnormal estrous cyclicity were observed in female GKirKO mice. Taken together, these data provide in vivo evidence that Kiss1r in GnRH neurons is critical for reproductive development and fertility.