Ontogeny and mechanisms of action for the stimulatory effect of kisspeptin on gonadotropin-releasing hormone system of the rat

PFAS association with kisspeptin and sex hormones in teenagers of the HBM4EU aligned studies

Exposure to Perfluoroalkyl acids (PFAS) can impair human reproductive function, e.g., by delaying or advancing puberty, although their mechanisms of action are not fully understood. We therefore set out to evaluate the relationship between serum PFAS levels, both individually and as a mixture, on the Hypothalamic-Pituitary-Gonadal (HPG) axis by analyzing serum levels of reproductive hormones and also kisspeptin in European teenagers participating in three of the HBM4EU Aligned Studies. For this purpose, PFAS compounds were measured in 733 teenagers from Belgium (FLEHS IV study), Slovakia (PCB cohort follow-up), and Spain (BEA study) by high performance liquid chromatography-tandem mass spectrometry (HPLC/MS) in laboratories under the HBM4EU quality assurance quality control (QA/QC) program. In the same serum samples, kisspeptin 54 (kiss-54) protein, follicle-stimulating hormone (FSH), total testosterone (TT), estradiol (E2), and sex hormone-binding globulin (SHBG) levels were also measured using immunosorbent assays. Sex-stratified single pollutant linear regression models for separate studies, mixed single pollutant models accounting for random effects for pooled studies, and g-computation and Bayesian kernel machine regression (BKMR) models for the mixture of the three most available (PFNA, PFOA, and PFOS) were fit. PFAS associations with reproductive markers differed according to sex. Each natural log-unit increase of PFOA, PFNA, and PFOS were associated with higher TT [18.41 (6.18; 32.31), 15.60 (7.25; 24.61), 14.68 (6.18; 24.61), respectively] in girls, in the pooled analysis (all studies together). In males, G-computation showed that PFAS mixture was associated with lower FSH levels [-10.51 (-18.81;-1.36)]. The BKMR showed the same patterns observed in G-computation, including a significant increase on male Kiss-54 and SHBG levels. Overall, effect biomarkers may enhance the current epidemiological knowledge regarding the adverse effect of PFAS in human HPG axis, although further research is warranted.

The interplay between kisspeptin and endocannabinoid systems modulates male hypothalamic and gonadic control of reproduction in vivo

Introduction

Male reproduction is under the control of the hypothalamus-pituitary-gonadal (HPG) axis. The endocannabinoid system (ECS) and the kisspeptin system (KS) are two major signaling systems in the central and peripheral control of reproduction, but their possible interaction has been poorly investigated in mammals. This manuscript analyzes their possible reciprocal modulation in the control of the HPG axis.

Materials and methods

Adolescent male rats were treated with kisspeptin-10 (Kp10) and endocannabinoid anandamide (AEA), the latter alone or in combination with the type 1 cannabinoid receptor (CB1) antagonist rimonabant (SR141716A). The hypothalamic KS system and GnRH expression, circulating sex steroids and kisspeptin (Kiss1) levels, and intratesticular KS and ECS were evaluated by immunohistochemical and molecular methods. Non-coding RNAs (i.e., miR145-5p, miR-132-3p, let7a-5p, let7b-5p) were also considered.

Results

Circulating hormonal values were not significantly affected by Kp10 or AEA; in the hypothalamus, Kp10 significantly increased GnRH mRNA and aromatase Cyp19, Kiss1, and Kiss1 receptor (Kiss1R) proteins. By contrast, AEA treatment affected the hypothalamic KS at the protein levels, with opposite effects on the ligand and receptor, and SR141716A was capable of attenuating the AEA effects. Among the considered non-coding RNA, only the expression of miR145-5p was positively affected by AEA but not by Kp10 treatment. Localization of Kiss1+/Kiss1R+ neurons in the arcuate nucleus revealed an increase of Kiss1R-expressing neurons in Kp10- and AEA-treated animals associated with enlargement of the lateral ventricles in Kp10-treated animals. In the brain and testis, the selected non-coding RNA was differently modulated by Kp10 or AEA. Lastly, in the testis, AEA treatment affected the KS at the protein levels, whereas Kp10 affected the intragonadal levels of CB1 and FAAH, the main modulator of the AEA tone. Changes in pubertal transition-related miRNAs and the intratesticular distribution of Kiss1, Kiss1R, CB1, and CB2 following KP and AEA treatment corroborate the KS-ECS crosstalk also showing that the CB1 receptor is involved in this interplay.

Conclusion

For the first time in mammals, we report the modulation of the KS in both the hypothalamus and testis by AEA and revealed the KP-dependent modulation of CB1 and FAAH in the testis. KP involvement in the progression of spermatogenesis is also suggested.

Kisspeptin as potential biomarker of environmental chemical mixture effect on reproductive hormone profile: A pilot study in adolescent males

BACKGROUND

Kisspeptin has been proposed as an effect biomarker to understand the mechanisms by which some environmental chemicals adversely affect the human reproductive system.

OBJECTIVE

To ascertain whether kisspeptin serum protein and DNA methylation levels are associated with exposure to several environmental chemicals (individually and as a mixture) and serum reproductive hormone levels in adolescent males.

METHODS

Three phenols (bisphenol A [BPA], methyl-paraben [MPB], and benzophenone-3 [BP3]); two toxic metals (arsenic and cadmium); and four metabolites of non-persistent pesticides, including insecticides (2-isopropyl-6-methyl-4-pyrimidinol [IMPy], malathion diacid [MDA], and dimethylcyclopropane carboxylic acid [DCCA]) and fungicides (ethylene thiourea [ETU]) were measured in first-morning urine samples of 133 adolescent males aged 15-17 years from the INMA-Granada cohort. In blood samples collected on the same day, KISS1 gene DNA methylation was measured at four CpGs from the Exon IV, as well as serum levels of kiss54 protein, total testosterone (T), estradiol (E₂), sex hormone binding-globulin, dehydroepiandrosterone sulfate, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). Multiple linear regression and mixture (quantile g-computation) models were fit.

RESULTS

Urinary MDA and DCCA concentrations were associated with higher kiss54 levels [% change (95%CI) for each log-unit increase in concentration = 2.90 (0.32;5.56), and 1.93 (0.45,3.43), respectively]; IMPy with lower DNA methylation percentage at CpG1 and total CpGs [% change (95%CI) = -1.15 (-1.96;-0.33): -0.89 (-1.73;-0.01), respectively]; and BP3 and DCCA with lower total CpGs methylation [-0.53 (-1.04;-0.01) and - 0.69 (-1.37;-0.01), respectively]. The pesticide mixture and the whole chemical mixture were associated with higher kiss54 [% change (95%CI) = 9.09 (3.29;15.21) and 11.61 (3.96;19.82), respectively] and lower methylation levels at several CpGs. Additionally, serum kiss54 in the third tertile was associated with higher LH levels [% change (95%CI) = 28.69 (3.75-59.63)], and third-tertile CpG1, CpG2, and total CpG methylation percentages were associated with lower FSH and E₂.

CONCLUSION

The findings of the present study and the negative correlation between serum kiss54 levels and KISS1 DNA methylation percentages suggested that kisspeptin may be a promising effect biomarker.

Developmental sex differences in the peri-pubertal pattern of hypothalamic reproductive gene expression, including Kiss1 and Tac2, may contribute to sex differences in puberty onset

The mechanisms regulating puberty still remain elusive, as do the underlying causes for sex differences in puberty onset (girls before boys) and pubertal disorders. Neuroendocrine puberty onset is signified by increased pulsatile GnRH secretion, yet how and when various upstream reproductive neural circuits change developmentally to govern this process is poorly understood. We previously reported day-by-day peri-pubertal increases (Kiss1, Tac2) or decreases (Rfrp) in hypothalamic gene expression of female mice, with several brain mRNA changes preceding external pubertal markers. However, similar pubertal measures in males were not previously reported. Here, to identify possible neural sex differences underlying sex differences in puberty onset, we analyzed peri-pubertal males and directly compared them with female littermates. Kiss1 expression in male mice increased over the peri-pubertal period in both the AVPV and ARC nuclei but with lower levels than in females at several ages. Likewise, Tac2 expression in the male ARC increased between juvenile and older peri-pubertal stages but with levels lower than females at most ages. By contrast, both DMN Rfrp expressionand Rfrp neuronal activation strongly decreased in males between juvenile and peri-pubertal stages, but with similar levels as females. Neither ARC KNDy neuronal activation nor Kiss1r expression in GnRH neurons differed between males and females or changed with age. These findings delineate several peri-pubertal changes in neural populations in developing males, with notable sex differences in kisspeptin and NKB neuron developmental patterns. Whether these peri-pubertal hypothalamic sex differences underlie sex differences in puberty onset deserves future investigation.

Sheep as a model for neuroendocrinology research

Animal models remain essential to understand the fundamental mechanisms of physiology and pathology. Particularly, the complex and dynamic nature of neuroendocrine cells of the hypothalamus make them difficult to study. The neuroendocrine systems of the hypothalamus are critical for survival and reproduction, and are highly conserved throughout vertebrate evolution. Their roles in controlling body metabolism, growth and body composition, stress, electrolyte balance, and reproduction, have been intensively studied, and have yielded groundbreaking discoveries. Many of these discoveries would not have been feasible without the use of the domestic sheep (Ovis aries). The sheep has been used for decades to study the neuroendocrine systems of the hypothalamus and has become a model for human neuroendocrinology. The aim of this chapter is to review some of the profound biomedical discoveries made possible by the use of sheep. The advantages and limitations of sheep as a neuroendocrine model will be discussed. While no animal model can perfectly recapitulate a human disease or condition, sheep are invaluable for enabling manipulations not possible in human subjects and isolating physiologic variables to garner insight into neuroendocrinology and associated pathologies.

Lower FSH With Normal Fertility in Male Mice Lacking Gonadotroph Kisspeptin Receptor

The kisspeptin receptor, crucial for hypothalamic control of puberty and reproduction, is also present in the pituitary gland. Its role in the pituitary gland is not defined. Kisspeptin signaling via the Kiss1r could potentially regulate reproductive function at the level of pituitary gonadotrope. Using Cre/Lox technology, we deleted the Kiss1r gene in pituitary gonadotropes (PKiRKO). PKiRKO males have normal genital development (anogenital distance WT: 19.1 ± 0.4 vs. PKiRKO: 18.5 ± 0.4 mm), puberty onset, testes cell structure on gross histology, normal testes size, and fertility. PKiRKO males showed significantly decreased serum FSH levels compared to WT males (5.6 ± 1.9 vs. 10.2 ± 1.8 ng/ml) with comparable LH (1.1 ± 0.2 vs. 1.8 ± 0.4 ng/ml) and testosterone levels (351.8 ± 213.0 vs. 342.2 ± 183.0 ng/dl). PKiRKO females have normal puberty onset, cyclicity, LH and FSH levels and fertility. Overall, these findings indicate that absence of pituitary Kiss1r reduces FSH levels in male mice without affecting testis function. PKiRKO mice have normal reproductive function in both males and females.

The electrophysiologic properties of gonadotropin-releasing hormone neurons

For about two decades, recordings of identified gonadotropin-releasing hormone (GnRH) neurons have provided a wealth of information on their properties. We describe areas of consensus and debate the intrinsic electrophysiologic properties of these cells, their response to fast synaptic and neuromodulatory input, Ca²⁺ imaging correlates of action potential firing, and signaling pathways regulating these aspects. How steroid feedback and development change these properties, functions of GnRH neuron subcompartments and local networks, as revealed by chemo- and optogenetic approaches, are also considered.

Connecting nutritional deprivation and pubertal inhibition via GRK2-mediated repression of kisspeptin actions in GnRH neurons

BACKGROUND

Perturbations in the timing of puberty, with potential adverse consequences in later health, are increasingly common. The underlying neurohormonal mechanisms are unfolded, but nutritional alterations are key contributors. Efforts to unveil the basis of normal puberty and its metabolic control have focused on mechanisms controlling expression of Kiss1, the gene encoding the puberty-activating neuropeptide, kisspeptin. However, other regulatory phenomena remain ill-defined. Here, we address the putative role of the G protein-coupled-receptor kinase-2, GRK2, in GnRH neurons, as modulator of pubertal timing via repression of the actions of kisspeptin, in normal maturation and conditions of nutritional deficiency.

METHODS

Hypothalamic RNA and protein expression analyses were conducted in maturing female rats. Pharmacological studies involved central administration of GRK2 inhibitor, βARK1-I, and assessment of gonadotropin responses to kisspeptin or phenotypic and hormonal markers of puberty, under normal nutrition or early subnutrition in female rats. In addition, a mouse line with selective ablation of GRK2 in GnRH neurons, aka G-GRKO, was generated, in which hormonal responses to kisspeptin and puberty onset were monitored, in normal conditions and after nutritional deprivation.

RESULTS

Hypothalamic GRK2 expression increased along postnatal maturation in female rats, especially in the preoptic area, where most GnRH neurons reside, but decreased during the juvenile-to-pubertal transition. Blockade of GRK2 activity enhanced Ca+2 responses to kisspeptin in vitro, while central inhibition of GRK2 in vivo augmented gonadotropin responses to kisspeptin and advanced puberty onset. Postnatal undernutrition increased hypothalamic GRK2 expression and delayed puberty onset, the latter being partially reversed by central GRK2 inhibition. Conditional ablation of GRK2 in GnRH neurons enhanced gonadotropin responses to kisspeptin, accelerated puberty onset, and increased LH pulse frequency, while partially prevented the negative impact of subnutrition on pubertal timing and LH pulsatility in mice.

CONCLUSIONS

Our data disclose a novel pathway whereby GRK2 negatively regulates kisspeptin actions in GnRH neurons, as major regulatory mechanism for tuning pubertal timing in nutritionally-compromised conditions.

Conserved functions of hypothalamic kisspeptin in vertebrates

Hypothalamic kisspeptin encoded by KISS1/Kiss1 gene emerged as a regulator of the reproductive axis in mammals following the discovery of the kisspeptin receptor (Kissr) and its role in reproduction. Kisspeptin-Kissr systems have been investigated in various vertebrates, and a conserved sequence of kisspeptin-Kissr has been identified in most vertebrate species except in the avian linage. In addition, multiple paralogs of kisspeptin sequences have been identified in the non-mammalian vertebrates. The allegedly conserved role of kisspeptin-Kissr in reproduction became debatable when kiss/kissr genes-deficient zebrafish and medaka showed no apparent effect on the onset of puberty, sexual development, maturation and reproductive capacity. Therefore, it is questionable whether the role of kisspeptin in reproduction is conserved among vertebrate species. Here we discuss from a comparative and evolutional aspect the diverse functions of kisspeptin and its receptor in vertebrates. Primarily this review focuses on the role of hypothalamic kisspeptin in reproductive and non-reproductive functions that are conserved in vertebrate species.

Precocious sexual maturation: Unravelling the mechanisms of pubertal onset through clinical observations

Puberty is a crucial biological process normally occurring at a specific time during the lifespan, during which sexual and somatic maturation are completed, and reproductive capacity is reached. Pubertal timing is not only determined by genetics, but also by endogenous and environmental cues, including nutritional and metabolic signals. During the last decade, we have learned much regarding the essential roles of kisspeptins and the neuropeptide pathways that converge on these neurones to modulate kisspeptin signalling, as well as neurokinin B and dynorphin, the co-transmitters of Kiss1 neurones in the arcuate nucleus, and the effects of melanocortins on puberty. Indeed, melanocortins are involved in transmitting the regulatory actions of metabolic cues on pubertal maturation. Intracellular metabolic sensors, such as the AMP-activated protein kinase and the fuel-sensing deacetylase SIRT1, have been shown to contribute to puberty. Further understanding of these signals and regulatory circuits will help uncover the intimacies of the central control of puberty, as well as how alterations in metabolic status, ranging from undernutrition to obesity, affect the pubertal process. Precocious puberty is rare and has a clear female predominance. Central precocious puberty (CPP) is diagnosed when premature activation of the hypothalamic-pituitary axis occurs. Its causes are heterogeneous, with alterations of the central nervous system being of special interest, and with environmental factors also playing a role in some cases. During the last decade, several mutations in different genes (including KISS1, KISS1R, MKRN3 and DLK1) that cause CPP have been discovered. Loss-of-function mutations in MKRN3 are the most common monogenic cause of CPP known to date. Here, we review and update what is known regarding the genotype-phenotype relationship in patients with CPP.

Advances in the Regulation of Mammalian Follicle-Stimulating Hormone Secretion

Simple Summary

The reproduction of mammals is regulated by the hypothalamic-pituitary-gonadal axis. Follicle stimulating hormone, as one of the gonadotropins secreted by the pituitary gland, plays an immeasurable role. This article mainly reviews the molecular basis and classical signaling pathways that regulate the synthesis and secretion of follicle stimulating hormone, and summarizes its internal molecular mechanism, which provides a certain theoretical basis for the research of mammalian reproduction regulation and the application of follicle stimulating hormone in production practice.

Abstract

Mammalian reproduction is mainly driven and regulated by the hypothalamic-pituitary-gonadal (HPG) axis. Follicle-stimulating hormone (FSH), which is synthesized and secreted by the anterior pituitary gland, is a key regulator that ultimately affects animal fertility. As a dimeric glycoprotein hormone, the biological specificity of FSH is mainly determined by the β subunit. As research techniques are being continuously innovated, studies are exploring the underlying molecular mechanism regulating the secretion of mammalian FSH. This article will review the current knowledge on the molecular mechanisms and signaling pathways systematically regulating FSH synthesis and will present the latest hypothesis about the nuclear cross-talk among the various endocrine-induced pathways for transcriptional regulation of the FSH β subunit. This article will provide novel ideas and potential targets for the improved use of FSH in livestock breeding and therapeutic development.

Role of kisspeptin on cell proliferation and steroidogenesis in luteal cells in vitro and in vivo

BACKGROUND

Kisspeptin (KISS1) and kisspeptin receptor (KISS1R) are essential gatekeepers of the reproductive system. The functions of KISS1 and KISS1R in corpus luteal cells remain ambiguous. The objective was to observe normal physiologic functions of corpus luteal cells in vivo and clarify the functions of KISS1 in vitro.

METHODS

We conducted an in vivo observation of cellular patterns as well as the levels of steroidogenic enzymes and KISS1/KISS1R in corpus luteal cells obtained from female crossbred Taiwan native goats in the estrous cycle; the observation was performed using hematoxylin and eosin and immunohistochemistry staining. Subsequently, we used kisspeptin-10 (Kp-10) to stimulate temperature sensitive-caprine luteal cell line (ts-CLC-D) cells to investigate the progesterone (P4) levels, steroidogenic messenger RNA (mRNA)/protein levels, cell survival rate, intracellular Ca2+ concentration, and cell proliferation-related mRNA/protein levels in the mitogen-activated protein kinase pathway in vitro by applying immunofluorescence staining, Western blotting, 2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide assay, and real-time polymerase chain reaction.

RESULTS

We observed the presence of proteins and mRNAs for STAR, CYP11A1, HSD3B, KISS1, and KISS1R in the corpus luteal cells from goats in vivo. In vitro, the addition of Kp-10 reduced the P4 levels (p < 0.01) and increased cell proliferation (p < 0.05) of the ts-CLC-D cells. Furthermore, we found that the levels of proteins and mRNA for STAR, CYP11A1, and HSD3B decreased significantly when Kp-10 was added (p < 0.05). However, adding Kp-10 did not affect the mRNA levels for PLCG2, DAG1, PRKCA, KRAS, RAF1, MAP2K1, MAP2K2, MAPK3, MAPK1, and MAPK14.

CONCLUSION

We determined that KISS1 could affect the P4 levels, steroidogenesis, and cell proliferation in luteal cells. However, further research is required to clarify how KISS1 regulates proliferation and steroid production in luteal cells.

Role of KNDy Neurons Expressing Kisspeptin, Neurokinin B, and Dynorphin A as a GnRH Pulse Generator Controlling Mammalian Reproduction

Increasing evidence accumulated during the past two decades has demonstrated that the then-novel kisspeptin, which was discovered in 2001, the known neuropeptides neurokinin B and dynorphin A, which were discovered in 1983 and 1979, respectively, and their G-protein-coupled receptors, serve as key molecules that control reproduction in mammals. The present review provides a brief historical background and a summary of our recent understanding of the roles of hypothalamic neurons expressing kisspeptin, neurokinin B, and dynorphin A, referred to as KNDy neurons, in the central mechanism underlying gonadotropin-releasing hormone (GnRH) pulse generation and subsequent tonic gonadotropin release that controls mammalian reproduction.

Biased signaling in naturally occurring mutations of G protein-coupled receptors associated with diverse human diseases

G protein-coupled receptors (GPCRs) play critical roles in transmitting a variety of extracellular signals into the cells and regulate diverse physiological functions. Naturally occurring mutations that result in dysfunctions of GPCRs have been known as the causes of numerous diseases. Significant progresses have been made in elucidating the pathophysiology of diseases caused by mutations. The multiple intracellular signaling pathways, such as G protein-dependent and β-arrestin-dependent signaling, in conjunction with recent advances on biased agonism, have broadened the view on the molecular mechanism of disease pathogenesis. This review aims to briefly discuss biased agonism of GPCRs (biased ligands and biased receptors), summarize the naturally occurring GPCR mutations that cause biased signaling, and propose the potential pathophysiological relevance of biased mutant GPCRs associated with various endocrine diseases.

Central Ceramide Signaling Mediates Obesity-Induced Precocious Puberty

Childhood obesity, especially in girls, is frequently bound to earlier puberty, which is linked to higher disease burden later in life. The mechanisms underlying this association remain elusive. Here we show that brain ceramides participate in the control of female puberty and contribute to its alteration in early-onset obesity in rats. Postnatal overweight caused earlier puberty and increased hypothalamic ceramide content, while pharmacological activation of ceramide synthesis mimicked the pubertal advancement caused by obesity, specifically in females. Conversely, central blockade of de novo ceramide synthesis delayed puberty and prevented the effects of the puberty-activating signal, kisspeptin. This phenomenon seemingly involves a circuit encompassing the paraventricular nucleus (PVN) and ovarian sympathetic innervation. Early-onset obesity enhanced PVN expression of SPTLC1, a key enzyme for ceramide synthesis, and advanced the maturation of the ovarian noradrenergic system. In turn, obesity-induced pubertal precocity was reversed by virogenetic suppression of SPTLC1 in the PVN. Our data unveil a pathway, linking kisspeptin, PVN ceramides, and sympathetic ovarian innervation, as key for obesity-induced pubertal precocity.

The kisspeptin receptor: A key G-protein-coupled receptor in the control of the reproductive axis

The kisspeptin receptor, Kiss1R, also known as Gpr54, is a G protein-coupled receptor (GPCR), deorphanized in 2001, when it was recognized as canonical receptor for the Kiss1-derived peptides, kisspeptins. In 2003, inactivating mutations of Kiss1R gene were first associated to lack of pubertal maturation and hypogonadotropic hypogonadism in humans and rodents. These seminal findings pointed out the previously unsuspected, essential role of Kiss1R and its ligands in control of reproductive maturation and function. This contention has been fully substantiated during the last decade by a wealth of clinical and experimental data, which has documented a fundamental function of the so-called Kiss1/Kiss1R system in the regulation of puberty onset, gonadotropin secretion and ovulation, as well as the metabolic and environmental modulation of fertility. In this review, we provide a succinct summary of some of the most salient facets of Kiss1R, as essential GPCR for the proper maturation and function of the reproductive axis.

Continuous Kisspeptin Administration in Postmenopausal Women: Impact of Estradiol on Luteinizing Hormone Secretion

CONTEXT

Kisspeptin stimulates the reproductive endocrine cascade in both men and women. Circulating sex steroids are thought to modulate the ability of kisspeptin to stimulate gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) release.

OBJECTIVE

To probe the effects of sex steroids on kisspeptin-stimulated GnRH-induced LH pulses.

PARTICIPANTS

Eight healthy postmenopausal women.

INTERVENTION

Subjects underwent every-10-minute blood sampling to measure GnRH-induced LH secretion at baseline and in response to a continuous kisspeptin infusion (12.5 µg/kg/h) over 24 hours. A subset of the participants also received kisspeptin (0.313 µg/kg) and GnRH (75 ng/kg) intravenous boluses.

RESULTS

Postmenopausal women are resistant to the stimulatory effect of continuous kisspeptin on LH secretion. Postmenopausal women receiving estradiol replacement therapy are also resistant to kisspeptin initially, but they demonstrate a significant increase in LH pulse amplitude in direct proportion to the circulating estradiol concentration and duration of kisspeptin administration.

CONCLUSIONS

Kisspeptin administration has complex effects on GnRH, and by extension, on LH secretion. The ability of kisspeptin to affect LH secretion can be modulated by the ambient sex-steroid milieu in a time- and dose-dependent manner.

Kisspeptin signalling and its roles in humans

Kisspeptins are a group of peptide fragments encoded by the KISS1 gene in humans. They bind to kisspeptin receptors with equal efficacy. Kisspeptins and their receptors are expressed by neurons in the arcuate and anteroventral periventricular nuclei of the hypothalamus. Oestrogen mediates negative feedback of gonadotrophin-releasing hormone secretion via the arcuate nucleus. Conversely, it exerts positive feedback via the anteroventral periventricular nucleus. The sexual dimorphism of these nuclei accounts for the differential behaviour of the hypothalamic-pituitary-gonadal axis between genders. Kisspeptins are essential for reproductive function. Puberty is regulated by the maturation of kisspeptin neurons and by interactions between kisspeptins and leptin. Hence, kisspeptins have potential diagnostic and therapeutic applications. Kisspeptin agonists may be used to localise lesions in cases of hypothalamic-pituitary-gonadal axis dysfunction and evaluate the gonadotrophic potential of subfertile individuals. Kisspeptin antagonists may be useful as contraceptives in women, through the prevention of premature luteinisation during in vitro fertilisation, and in the treatment of sex steroid-dependent diseases and metastatic cancers.

Interactions between Two Different G Protein-Coupled Receptors in Reproductive Hormone-Producing Cells: The Role of PACAP and Its Receptor PAC1R

Gonadotropin-releasing hormone (GnRH) and gonadotropins are indispensable hormones for maintaining female reproductive functions. In a similar manner to other endocrine hormones, GnRH and gonadotropins are controlled by their principle regulators. Although it has been previously established that GnRH regulates the synthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—both gonadotropins—from pituitary gonadotrophs, it has recently become clear that hypothalamic GnRH is under the control of hypothalamic kisspeptin. Prolactin, which is also known as luteotropic hormone and is released from pituitary lactotrophs, stimulates milk production in mammals. Prolactin is also regulated by hypothalamic factors, and it is thought that prolactin synthesis and release are principally under inhibitory control by dopamine through the dopamine D2 receptor. In addition, although it remains unknown whether it is a physiological regulator, thyrotropin-releasing hormone (TRH) is a strong secretagogue for prolactin. Thus, GnRH, LH and FSH, and prolactin are mainly regulated by hypothalamic kisspeptin, GnRH, and TRH, respectively. However, the synthesis and release of these hormones is also modulated by other neuropeptides in the hypothalamus. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a hypothalamic peptide that was first isolated from sheep hypothalamic extracts based on its ability to stimulate cAMP production in anterior pituitary cells. PACAP acts on GnRH neurons and pituitary gonadotrophs and lactotrophs, resulting in the modulation of their hormone producing/secreting functions. Furthermore, the presence of the PACAP type 1 receptor (PAC1R) has been demonstrated in these cells. We have examined how PACAP and PAC1R affect GnRH- and pituitary hormone-secreting cells and interact with their principle regulators. In this review, we describe our understanding of the role of PACAP and PAC1R in the regulation of GnRH neurons, gonadotrophs, and lactotrophs, which are regulated mainly by kisspeptin, GnRH, and TRH, respectively.

Defining a novel leptin-melanocortin-kisspeptin pathway involved in the metabolic control of puberty

Objective

Puberty is a key developmental phenomenon highly sensitive to metabolic modulation. Worrying trends of changes in the timing of puberty have been reported in humans. These might be linked to the escalating prevalence of childhood obesity and could have deleterious impacts on later (cardio-metabolic) health, but their underlying mechanisms remain unsolved. The neuropeptide α-MSH, made by POMC neurons, plays a key role in energy homeostasis by mediating the actions of leptin and likely participates in the control of reproduction. However, its role in the metabolic regulation of puberty and interplay with kisspeptin, an essential puberty-regulating neuropeptide encoded by Kiss1, remain largely unknown. We aim here to unveil the potential contribution of central α-MSH signaling in the metabolic control of puberty by addressing its role in mediating the pubertal effects of leptin and its potential interaction with kisspeptin.

Methods

Using wild type and genetically modified rodent models, we implemented pharmacological studies, expression analyses, electrophysiological recordings, and virogenetic approaches involving DREADD technology to selectively inhibit Kiss1 neurons, in order to interrogate the physiological role of a putative leptin→α-MSH→kisspeptin pathway in the metabolic control of puberty.

Results

Stimulation of central α-MSH signaling robustly activated the reproductive axis in pubertal rats, whereas chronic inhibition of melanocortin receptors MC3/4R, delayed puberty, and prevented the permissive effect of leptin on puberty onset. Central blockade of MC3/4R or genetic elimination of kisspeptin receptors from POMC neurons did not affect kisspeptin effects. Conversely, congenital ablation of kisspeptin receptors or inducible, DREADD-mediated inhibition of arcuate nucleus (ARC) Kiss1 neurons resulted in markedly attenuated gonadotropic responses to MC3/4R activation. Furthermore, close appositions were observed between POMC fibers and ARC Kiss1 neurons while blockade of α-MSH signaling suppressed Kiss1 expression in the ARC of pubertal rats.

Conclusions

Our physiological, virogenetic, and functional genomic studies document a novel α-MSH→kisspeptin→GnRH neuronal signaling pathway involved in transmitting the permissive effects of leptin on pubertal maturation, which is relevant for the metabolic (and, eventually, pharmacological) regulation of puberty onset.

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Puberty is highly sensitive to metabolic modulation and disturbed by child obesity.

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Altered puberty is linked to adverse metabolic health outcomes via unclear mechanisms.

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The POMC product, α-MSH, transmit leptin-mediated metabolic regulation of puberty.

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A novel α-MSH→kisspeptin→GnRH signaling pathway is involved in the control of puberty

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This pathway is important for the metabolic (and pharmacologic) control of puberty.

Kisspeptin Responsiveness Signals Emergence of Reproductive Endocrine Activity: Implications for Human Puberty

CONTEXT

Some patients with idiopathic hypogonadotropic hypogonadism (IHH) undergo spontaneous activation of their hypothalamic-pituitary-gonadal axis resulting in normalization of steroidogenesis and/or gametogenesis, a phenomenon termed reversal.

OBJECTIVE

To assess the responsiveness of the GnRH neuronal network to exogenous kisspeptin administration in IHH patients who have undergone reversal.

PARTICIPANTS

Six men with congenital IHH and evidence for reversal.

INTERVENTION

Subjects underwent q10 min blood sampling to measure GnRH-induced LH secretion at baseline and in response to iv boluses of kisspeptin (0.24-2.4 nmol/kg) and GnRH (75 ng/kg).

RESULTS

Individuals with sustained reversal of their hypogonadotropism (spontaneous LH pulses) responded to exogenous kisspeptin with a GnRH-induced LH pulse. Individuals who had reversal but then subsequently suffered relapse of their IHH (loss of spontaneous LH pulsatility) did not respond to kisspeptin.

CONCLUSIONS

The ability of kisspeptin to stimulate a GnRH-induced LH pulse correlates with the presence of endogenous LH pulses. These data suggest that reversal of hypogonadotropism, and by extension sexual maturation, may be due to the acquisition of kisspeptin responsiveness.

RF-amide neuropeptides and their receptors in Mammals: Pharmacological properties, drug development and main physiological functions

RF-amide neuropeptides, with their typical Arg-Phe-NH2 signature at their carboxyl C-termini, belong to a lineage of peptides that spans almost the entire life tree. Throughout evolution, RF-amide peptides and their receptors preserved fundamental roles in reproduction and feeding, both in Vertebrates and Invertebrates. The scope of this review is to summarize the current knowledge on the RF-amide systems in Mammals from historical aspects to therapeutic opportunities. Taking advantage of the most recent findings in the field, special focus will be given on molecular and pharmacological properties of RF-amide peptides and their receptors as well as on their implication in the control of different physiological functions including feeding, reproduction and pain. Recent progress on the development of drugs that target RF-amide receptors will also be addressed.

Cell Type-Specific Sexual Dimorphism in Rat Pituitary Gene Expression During Maturation

The most obvious functional differences between mammalian males and females are related to the control of reproductive physiology and include patterns of GnRH and gonadotropin release, the timing of puberty, sexual and social behavior, and the regulation of food intake and body weight. Using the rat as the best-studied mammalian model for maturation, we examined the expression of major anterior pituitary genes in five secretory cell types of developing males and females. Corticotrophs show comparable Pomc profiles in both sexes, with the highest expression occurring during the infantile period. Somatotrophs and lactotrophs also exhibit no difference in Gh1 and Prl profiles during embryonic to juvenile age but show the amplification of Prl expression in females and Gh1 expression in males during peripubertal and postpubertal ages. Gonadotrophs exhibit highly synchronized Lhb, Fshb, Cga, and Gnrhr expression in both sexes, but the peak of expression occurs during the infantile period in females and at the end of the juvenile period in males. Thyrotrophs also show different developmental Tshb profiles, which are synchronized with the expression of gonadotroph genes in males but not in females. These results indicate the lack of influence of sex on Pomc expression and the presence of two patterns of sexual dimorphism in the expression of other pituitary genes: a time shift in the peak expression during postnatal development, most likely reflecting the perinatal sex-specific brain differentiation, and modulation of the amplitude of expression during late development, which is secondary to the establishment of the hypothalamic-pituitary-gonadal and -thyroid axes.

Cold exposure inhibits hypothalamic Kiss-1 gene expression, serum leptin concentration, and delays reproductive development in male Brandt's vole (Lasiopodomys brandtii)

Cold commonly affects growth and reproductive development in small mammals. Here, we test the hypothesis that low ambient temperature will affect growth and puberty onset, associated with altered hypothalamic Kiss-1 gene expression and serum leptin concentration in wild rodents. Male Brandt's voles (Lasiopodomys brandtii) were exposed to cold (4 ± 1 °C) and warm (23 ± 1 °C) conditions from the birth and sacrificed on different developmental stages (day 26, day 40, day 60, and day 90, respectively). Brandt's voles increased the thermogenic capacity of brown adipose tissue, mobilized body fat, decreased serum leptin levels, and delayed the reproductive development especially on day 40 in the cold condition. They increased food intake to compensate for the high energy demands in the cold. The hypothalamic Kiss-1 gene expression on day 26 was decreased, associated with lower wet testis mass and testis testosterone concentration on day 40, in the cold-exposed voles compared to that in the warm. Serum leptin was positively correlated with body fat, testis mass, and testosterone concentration. These data suggested that cold exposure inhibited hypothalamic Kiss-1 gene expression during the early stage of development, decreased serum leptin concentration, and delayed reproductive development in male Brandt's voles.

Daily successive changes in reproductive gene expression and neuronal activation in the brains of pubertal female mice

Puberty is governed by the secretion of gonadotropin releasing hormone (GnRH), but the roles and identities of upstream neuropeptides that control and time puberty remain poorly understood. Indeed, how various reproductive neural gene systems change before and during puberty, and in relation to one another, is not well-characterized. We detailed the daily pubertal profile (from postnatal day [PND] 15 to PND 30) of neural Kiss1 (encoding kisspeptin), Kiss1r (kisspeptin receptor), Tac2 (neurokinin B), and Rfrp (RFRP-3, mammalian GnIH) gene expression and day-to-day c-fos induction in each of these cell types in developing female mice. Kiss1 expression in the AVPV/PeN increased substantially over the pubertal transition, reaching adult levels around vaginal opening (PND 27.5), a pubertal marker. However, AVPV/PeN Kiss1 neurons were not highly activated, as measured by c-fos co-expression, at any pubertal age. In the ARC, Kiss1 and Tac2 cell numbers showed moderate increases across the pubertal period, and neuronal activation of Tac2/Kiss1 cells was moderately elevated at all pubertal ages. Additionally, Kiss1r expression specifically in GnRH neurons was already maximal by PND 15 and did not change with puberty. Conversely, both Rfrp expression and Rfrp/c-fos co-expression in the DMN decreased markedly in the early pre-pubertal stage. This robust decrease of the inhibitory RFRP-3 population may diminish inhibition of GnRH neurons during early puberty. Collectively, our data identify the precise timing of important developmental changes - and in some cases, lack thereof - in gene expression and neuronal activation of key reproductive neuropeptides during puberty, with several changes occurring well before vaginal opening.

A Computational Model of the Dendron of the GnRH Neuron

Gonadotropin-releasing hormone (GnRH) neurons have two major processes that have properties of both dendrites (they receive synaptic input from other neurons) and axons (they actively propagate action potentials to the synaptic terminal). These processes have thus been termed dendrons. We construct a stochastic spatiotemporal model of the dendron of the GnRH neuron, with the goal of studying how stochastic synaptic input along the length of the dendron affects the initiation and propagation of action potentials. We show (1) that synaptic inputs closer to the soma are effective controllers of action potential initiation and electrical bursting and (2) that although the effects on the amplitude and width of propagating action potentials are critically dependent on the timing and location of synaptic input addition, the effects remain small. We conclude that although stochastic synaptic input along the length of the dendron is likely to be a major determinant of action potential initiation, it is an unlikely mechanism for controlling whether or not action potentials reach the synaptic terminal. Thus, the role of synaptic inputs situated along the dendron a long way from the site of action potential initiation remains unclear. We also show that the actions of kisspeptin can result in significant modulation of the amount of calcium released by an action potential at the synaptic terminal. Furthermore, we show that the actions of kisspeptin are greatest when multiple effects operate together and that a kisspeptin-induced increase in firing rate is, by itself, less effective at increasing Ca2+ release than is a combination of an increased firing rate, an increase in Ca2+ influx, and an increase in inositol trisphosphate (IP3) production. We conclude that the inherent synergies in the various actions of kisspeptin make it a likely candidate for the precise control of Ca2+ transients at the synaptic terminal.

KISS1R signals independently of Gαq/11 and triggers LH secretion via the β-arrestin pathway in the male mouse

Hypothalamic GnRH is the master regulator of the neuroendocrine reproductive axis, and its secretion is regulated by many factors. Among these is kisspeptin (Kp), a potent trigger of GnRH secretion. Kp signals via the Kp receptor (KISS1R), a Gαq/11-coupled 7-transmembrane-spanning receptor. Until this study, it was understood that KISS1R mediates GnRH secretion via the Gαq/11-coupled pathway in an ERK1/2-dependent manner. We recently demonstrated that KISS1R also signals independently of Gαq/11 via β-arrestin and that this pathway also mediates ERK1/2 activation. Because GnRH secretion is ERK1/2-dependent, we hypothesized that KISS1R regulates GnRH secretion via both the Gαq/11- and β-arrestin-coupled pathways. To test this hypothesis, we measured LH secretion, a surrogate marker of GnRH secretion, in mice lacking either β-arrestin-1 or β-arrestin-2. Results revealed that Kp-dependent LH secretion was significantly diminished relative to wild-type mice (P < .001), thus supporting that β-arrestin mediates Kp-induced GnRH secretion. Based on this, we hypothesized that Gαq/11-uncoupled KISS1R mutants, like L148S, will display Gαq/11-independent signaling. To test this hypothesis, L148S was expressed in HEK 293 cells. and results confirmed that, although strongly uncoupled from Gαq/11, L148S retained the ability to trigger significant Kp-dependent ERK1/2 phosphorylation (P < .05). Furthermore, using mouse embryonic fibroblasts lacking β-arrestin-1 and -2, we demonstrated that L148S-mediated ERK1/2 phosphorylation is β-arrestin-dependent. Overall, we conclude that KISS1R signals via Gαq/11 and β-arrestin to regulate GnRH secretion. This novel and important finding could explain why patients bearing some types of Gαq/11-uncoupled KISS1R mutants display partial gonadotropic deficiency and even a reversal of the condition, idiopathic hypogonadotropic hypogonadism.

Effect of Peripheral Administration of Kisspeptin-10 on Dynamic LH Secretion in Prepubertal Ewes

The aim of the present study was to clarify the effect of kisspeptin-10 on LH secretion in prepubertal ewes. In experiment 1, prepubertal ewes fitted with indwelling jugular catheters were randomly assigned to receive 0, 0.5, 1 or 2 mg of kisspeptin-10 dissolved in saline, and serial blood samples were collected at 15-min intervals for 180 min to analyze the response curves of LH after injection. In experiment 2, prepubertal ewes fitted with indwelling jugular catheters were injected with 0 or 1 mg of kisspeptin-10 dissolved in saline and the injection was repeated 3 times at 1 h interval and serial blood samples were collected at 15-min intervals for 210 min to analyze the response curves of LH after injection. The results showed that single intravenous administration of 0.5, 1 and 2 mg of kisspeptin-10 all could significantly increased LH secretion in prepubertal ewes, and the effect of 1 and 2 mg of kisspeptin-10 on LH secretion was higher than that of 0.5 mg group. The results also showed that repeated intravenous administration of kisspeptin-10 could effectively increase LH secretion and repeated administration did not influence the effect of kisspeptin-10 on LH secretion in prepubertal ewe. In conclusion, the present study indicated that single or repeated intravenous administration of kisspeptin-10 could effectively increase LH secretion in prepubertal ewes.

A critical point of male gonad development: neuroendocrine correlates of accelerated testicular growth in rats during early life

Testis growth during early life is important for future male fertility and shows acceleration during the first months of life in humans. This acceleration coincides with the peak in gonadotropic hormones in the blood, while the role of hypothalamic factors remains vague. Using neonatal rats to assess this issue, we found that day 9 of life is likely critical for testis development in rats. Before this day, testicular growth was proportional to body weight gain, but after that the testes showed accelerated growth. Hypothalamic kisspeptin and its receptor mRNA levels begin to elevate 2 days later, at day 11. A significant increase in the mRNA levels for gonadotropin-releasing hormone (GnRH) receptors in the hypothalamus between days 5 and 7 was followed by a 3-fold decrease in GnRH mRNA levels in this brain region during the next 2 days. Starting from day 9, hypothalamic GnRH mRNA levels increased significantly and positively correlated with accelerated testicular growth. Triptorelin, an agonist of GnRH, at a dose that had no effect on testicular growth during “proportional” period, increased testis weights during the period of accelerated growth. The insensitivity of testicular growth to GnRH during “proportional” period was supported by inability of a 2.5-fold siRNA knockdown of GnRH expression in the hypothalamus of the 7-day-old animals to produce any effect on their testis weights. GnRH receptor blockade with cetrorelix was also without effect on testis weights during “proportional” period but the same doses of this GnRH antagonist significantly inhibited “accelerated” testicular growth. GnRH receptor mRNA levels in the pituitary as well as plasma LH concentrations were higher during “accelerated” period of testicular growth than during “proportional” period. In general, our data defined two distinct periods in rat testicular development that are primarily characterized by different responses to GnRH signaling.

Cellular mechanisms and integrative timing of neuroendocrine control of GnRH secretion by kisspeptin

The hypothalamus integrates endogenous and exogenous inputs to control the pituitary-gonadal axis. The ultimate hypothalamic influence on reproductive activity is mediated through timely secretion of GnRH in the portal blood, which modulates the release of gonadotropins from the pituitary. In this context neurons expressing the RF-amide neuropeptide kisspeptin present required features to fulfill the role of the long sought-after hypothalamic integrative centre governing the stimulation of GnRH neurons. Here we focus on the intracellular signaling pathways triggered by kisspeptin through its cognate receptor KISS1R and on the potential role of proteins interacting with this receptor. We then review evidence implicating both kisspeptin and RFRP3--another RF-amide neuropeptide--in the temporal orchestration of both the pre-ovulatory LH surge in female rodents and the organization of seasonal breeding in photoperiodic species.

RFamide Peptides in Early Vertebrate Development

RFamides (RFa) are neuropeptides involved in many different physiological processes in vertebrates, such as reproductive behavior, pubertal activation of the reproductive endocrine axis, control of feeding behavior, and pain modulation. As research has focused mostly on their role in adult vertebrates, the possible roles of these peptides during development are poorly understood. However, the few studies that exist show that RFa are expressed early in development in different vertebrate classes, perhaps mostly associated with the central nervous system. Interestingly, the related peptide family of FMRFa has been shown to be important for brain development in invertebrates. In a teleost, the Japanese medaka, knockdown of genes in the Kiss system indicates that Kiss ligands and receptors are vital for brain development, but few other functional studies exist. Here, we review the literature of RFa in early vertebrate development, including the possible functional roles these peptides may play.

Lactational anovulation in mice results from a selective loss of kisspeptin input to GnRH neurons

In mammals, lactation is associated with a period of infertility characterized by the loss of pulsatile secretion of GnRH and cessation of ovulatory cycles. Despite the importance of lactational infertility in determining overall fecundity of a species, the mechanisms by which the suckling stimulus suppresses GnRH secretion remain unclear. Because kisspeptin neurons are critical for fertility, the aim of this study was to test the hypothesis that reduced kisspeptin expression might mediate the lactation-induced suppression of fertility, using mouse models. In the rostral periventricular area of the third ventricle (RP3V), a progressive decrease in RP3V Kiss1 mRNA levels was observed during pregnancy culminating in a 10-fold reduction during lactation compared with diestrous controls. This was associated with approximately 60% reduction in the numbers of kisspeptin-immunoreactive neurons in the RP3V detected during lactation. Similarly, in the arcuate nucleus there was also a significant decrease in Kiss1 mRNA levels during late pregnancy and midlactation, and a notable decrease in kisspeptin fiber density during lactation. The functional characteristics of the RP3V kisspeptin input to GnRH neurons were assessed using electrophysiological approaches in an acute brain slice preparation. Although endogenous RP3V kisspeptin neurons were found to activate GnRH neurons in diestrous mice, this was never observed during lactation. This did not result from an absence of kisspeptin receptors because GnRH neurons responded normally to 100 nM exogenous kisspeptin during lactation. The kisspeptin deficit in lactating mice was selective, because GnRH neurons responded normally to RP3V gamma aminobutryic acid inputs during lactation. These data demonstrate that a selective loss of RP3V kisspeptin inputs to GnRH neurons during lactation is the likely mechanism causing lactational anovulation in the mouse.

Metabolic control of puberty: roles of leptin and kisspeptins

This article is part of a Special Issue "Puberty and Adolescence". Reproduction is an energy-demanding function. Accordingly, puberty is metabolically gated, as a means to prevent fertility in conditions of energy insufficiency. In addition, obesity has been shown to impact the timing of puberty and may be among the causes for the earlier trends of pubertal age reported in various countries. The metabolic control of puberty in such a spectrum of situations, ranging from energy deficit to extreme overweight, is the result of the concerted action of different peripheral hormones and central transmitters that sense the metabolic state of the organism and transmit this information to the various elements of the reproductive axis, mainly the GnRH neurons. Among the peripheral signals involved, the adipose hormone, leptin, is known to play an essential role in the regulation of puberty, especially in females. Yet, although it is clear that the effects of leptin on puberty onset are predominantly permissive and mainly conducted at central (hypothalamic) levels, the primary sites and mechanisms of action of leptin within the reproductive brain remain unsolved. In this context, neurons expressing kisspeptins, the products of the Kiss1 gene that have emerged recently as essential upstream regulators of GnRH neurons, operate as key sensors of the metabolic state and funnel of the reproductive effects of leptin. Yet, much debate has arisen recently on whether the putative actions of leptin on the Kiss1 system are actually indirect and/or may primarily target Kiss1-independent pathways, such as those originating from the ventral premmamilary nucleus. Moreover, evidence has been presented for extra-hypothalamic or peripheral actions of leptin, including direct gonadal effects, which may contribute to the metabolic control of reproduction in extreme body weight conditions. In this work, we will critically review the experimental evidence supporting a role of leptin, kisspeptin and putatively related pathways in the concerted control of puberty by energy balance and metabolism.

Kisspeptin as a link between metabolism and reproduction: evidences from rodent and primate studies

Changes in metabolic status gate reproductive activity by still incompletely deciphered mechanisms. Many neuropeptides have been shown to be involved in restraining hypothalamic gonadotropin releasing hormone (GnRH) release under conditions of negative energy balance. Broadly, on the basis of their effect on feeding, these can be grouped as orexigenic and anorexigenic neuropeptides. Reciprocally correlated, in response to changes in systemic concentrations of metabolic hormones, the secretion of orexigenic neuropeptides increases while that of anorexigenic neuropeptides decreases during conditions of food restriction. Recently, kisspeptin signaling in hypothalamus has appeared as a pivotal regulator of the GnRH pulse generator. Kisspeptin apparently does not affect feeding, but in light of accumulating data, it has emerged as one of the major conduits in relaying body metabolic status information to GnRH neurons. The present review examines such data obtained from rodent and primate models, which suggest kisspeptin-Kiss1r signaling as a possible pathway providing a link between metabolism and reproduction.

Arg-Phe-amide-related peptides influence gonadotropin-releasing hormone neurons

The hypothalamic Arg-Phe-amide-related peptides, gonadotropin-inhibitory hormone and orthologous mammalian peptides of Arg-Phe-amide, may be important regulators of the hypothalamus-pituitary-gonadal reproductive axis. These peptides may modulate the effects of kisspeptins because they are presently recognized as the most potent activators of the hypothalamus-pituitary-gonadal axis. However, their effects on gonadotropin-releasing hormone neurons have not been investigated. In the current study, the GT1-7 cell line-expressing gonadotropin-releasing hormone was used as a model to explore the effects of Arg-Pheamide-related peptides on kisspeptin activation. Intracellular calcium concentration was quantified using the calcium-sensitive dye, fura-2 acetoxymethyl ester. Gonadotropin-releasing hormone released into the medium was detected via enzyme-linked immunosorbent assay. Results showed that 100 nmol/L kisspeptin-10 significantly increased gonadotropin-releasing hormone levels (at 120 minutes of exposure) and intracellular calcium concentrations. Co-treatment of kisspeptin with 1 μmol/L gonadotropin-inhibitory hormone or 1 μmol/L Arg-Phe-amide-related peptide-1 significantly attenuated levels of kisspeptin-induced gonadotropin-releasing hormone but did not affect kisspeptin-induced elevations of intracellular calcium concentration. Overall, the results suggest that gonadotropin-inhibitory hormone and Arg-Phe-amide-related peptide-1 may have inhibitory effects on kisspeptin-activated gonadotropin-releasing hormone neurons independent of the calcium signaling pathway.

PRR repeats in the intracellular domain of KISS1R are important for its export to cell membrane

Inactivating mutations of KISS-1 receptor (KISS1R) have been recently described as a rare cause of isolated hypogonadotropic hypogonadism transmitted as a recessive trait. Few mutations have been described, and the structure-function relationship of KISS1R remains poorly understood. Here, we have taken advantage of the discovery of a novel mutation of KISS1R to characterize the structure and function of an uncommon protein motif composed of 3 proline-arginine-arginine (PRR) repeats located within the intracellular domain. A heterozygous insertion of 1 PRR repeat in-frame with 3 PRR repeats leading to synthesis of a receptor bearing 4 PRR repeats (PRR-KISS1R) was found in the index case. Functional analysis of PRR-KISS1R showed a decrease of the maximal response to kisspeptin stimulation, associated to a lower cell surface expression without modification of total expression. PRR-KISS1R exerts a dominant negative effect on the synthesis of the wild-type (WT)-KISS1R. This effect was due to the nature of inserted residues but also to the difference of the length of the intracellular domain between PRR-KISS1R and WT-KISS1R. A molecular dynamic analysis showed that the additional PRR constrained this arginine-rich region into a polyproline type II helix. Altogether, this study shows that a heterozygous insertion in KISS1R may lead to hypogonadotropic hypogonadism by a dominant negative effect on the WT receptor. An additional PRR repeat into a proline-arginine-rich motif can dramatically changed the conformation of the intracellular domain of KISS1R and its probable interaction with partner proteins.

Effects of estradiol on kisspeptin neurons during puberty

The activation of the gonadotropin-releasing hormone (GnRH) neurons from a state of relative quiescence is critical for initiating puberty in mammals. Kisspeptin and its G-protein coupled receptor Gpr54 are essential for puberty, with disruption to either resulting in failed puberty in humans and mice. Robust data from several species indicate that Kiss1 mRNA and/or kisspeptin peptide expression within the hypothalamus increases during pubertal development. Kisspeptin fiber innervation of GnRH neurons and kisspeptin release within the hypothalamus also increase during pubertal development, indicating that there is increased kisspeptinergic drive to GnRH neurons during pubertal development. It is becoming increasingly apparent that gonadal steroids play important roles in the regulation of kisspeptin expression during pubertal development, and in particular, estradiol signaling through estrogen receptor alpha appears to be necessary for these changes to occur. This review focuses on the role that estradiol plays in the regulation of kisspeptin expression during pubertal development.

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

Kisspeptin receptor (KISS1R) signaling is essential for the hallmark increase in pulsatile GnRH secretion characteristic of the onset of puberty in humans and experimental animals. Loss-of-function mutations in KISS1R are associated with idiopathic hypogonadotropic hypogonadism in humans. Also, mutations with confirmed association with idiopathic central precocious puberty were identified in kisspeptin and KISS1R. These observations underscore the role of KISS1R signaling for normal pubertal development. Moreover, investigation of the mechanisms underlying the gain-of-function mutation in KISS1R indicates that the duration of KISS1R signaling is critical for the role of this receptor in timing the onset of puberty in humans. These findings further endorse the need to uncover the mechanisms, as well as yet-unknown proteins, involved in each step of KISS1R signaling. This knowledge is expected to advance our understanding of normal and abnormal pubertal development, as well as to help uncover the role of KISS1R signaling in non-hypothalamic tissues such as the placenta. This chapter discusses recent advances in the investigation of KISS1R signaling and function, as well as potential pathophysiological implications of naturally occurring mutations in this receptor identified in humans with reproductive disorders.

The effects of kisspeptin on gonadotropin release in non-human mammals

The Kiss1 gene encodes a 145-amino acid pre-peptide, kisspeptin, which is cleaved into smaller peptides of 54, 14, 13, and 10 amino acids. This chapter reviews in detail the effects of kisspeptin on gonadotropin secretion in non-human mammals. Studies of kisspeptin's effects have included both acute and chronic administration regimens via a number of administration routes. Acute kisspeptin stimulates gonadotropin secretion in a wide range of species of non-human mammals, including rats, mice, hamsters, sheep, pigs, goats, cows, horses, and monkeys. In general, the stimulatory effect of kisspeptin treatment is more pronounced for LH than FSH secretion. Kisspeptin is thought to exert its stimulatory effects on LH and FSH release via stimulation of GnRH release from the hypothalamus, since pre--administration of a GnRH antagonist prevents kisspeptin's stimulation of gonadotropin secretion. Although the kisspeptin receptor is also expressed on anterior pituitary cells of some species, and incubation of anterior pituitary cells with high concentrations of kisspeptin can stimulate in vitro LH release, the contribution of direct effects of kisspeptin on the pituitary is thought to be negligible in vivo. Continuous kisspeptin administration results in reduced sensitivity to the effects of kisspeptin, in some species. This desensitization is thought to occur at the level of the kisspeptin receptor, since the response of the pituitary gland to exogenous GnRH is maintained. Overall, the findings discussed in this chapter are invaluable to the understanding of the reproductive role of kisspeptin and the potential therapeutic uses of kisspeptin for the treatment of fertility disorders.

Kisspeptin excitation of GnRH neurons

Kisspeptin binding to its cognate G protein-coupled receptor (GPR54, aka Kiss1R) in gonadotropin-releasing hormone (GnRH) neurons stimulates peptide release and activation of the reproductive axis in mammals. Kisspeptin has pronounced pre- and postsynaptic effects, with the latter dominating the excitability of GnRH neurons. Presynaptically, kisspeptin increases the excitatory drive (both GABA-A and glutamate) to GnRH neurons and postsynaptically, kisspeptin inhibits an A-type and inwardly rectifying K(+) (Kir 6.2 and GIRK) currents and activates nonselective cation (TRPC) currents to cause long-lasting depolarization and increased action potential firing. The signaling cascades and the multiple intracellular targets of kisspeptin actions in native GnRH neurons are continuing to be elucidated. This review summarizes our current state of knowledge about kisspeptin signaling in GnRH neurons.

Development and Aging of the Kisspeptin-GPR54 System in the Mammalian Brain: What are the Impacts on Female Reproductive Function?

The prominent role of the G protein coupled receptor GPR54 and its peptide ligand kisspeptin in the progression of puberty has been extensively documented in many mammalian species including humans. Kisspeptins are very potent gonadotropin-releasing hormone secretagogues produced by two main populations of neurons located in two ventral forebrain regions, the preoptic area and the arcuate nucleus. Within the last 2 years a substantial amount of data has accumulated concerning the development of these neuronal populations and their timely regulation by central and peripheral factors during fetal, neonatal, and peripubertal stages of development. This review focuses on the development of the kisspeptin-GPR54 system in the brain of female mice, rats, sheep, monkeys, and humans. We will also discuss the notion that this system represents a major target through which signals from the environment early in life can reprogram reproductive function.

Organizational and activational effects of sex steroids on kisspeptin neuron development

Kisspeptin, encoded by the Kiss1 gene, is a neuropeptide required for puberty and adult reproductive function. Understanding the regulation and development of the kisspeptin system provides valuable knowledge about the physiology of puberty and adult fertility, and may provide insights into human pubertal or reproductive disorders. Recent studies, particularly in rodent models, have assessed how kisspeptin neurons develop and how hormonal and non-hormonal factors regulate this developmental process. Exposure to sex steroids (testosterone and estradiol) during critical periods of development can induce organizational (permanent) effects on kisspeptin neuron development, with respect to both sexually dimorphic and non-sexually dimorphic aspects of kisspeptin biology. In addition, sex steroids can also impart activational (temporary) effects on kisspeptin neurons and Kiss1 gene expression at various times during neonatal and peripubertal development, as they do in adulthood. Here, we discuss the current knowledge--and in some cases, lack thereof--of the influence of hormones and other factors on kisspeptin neuronal development.