Epigenetic changes coincide with in vitro primate GnRH neuronal maturation

Methylome analysis in girls with idiopathic central precocious puberty

BACKGROUND

Genetic and environmental factors are implicated in many developmental processes. Recent evidence, however, has suggested that epigenetic changes may also influence the onset of puberty or the susceptibility to a wide range of diseases later in life. The present study aims to investigate changes in genomic DNA methylation profiles associated with pubertal onset analyzing human peripheral blood leukocytes from three different groups of subjects: 19 girls with central precocious puberty (CPP), 14 healthy prepubertal girls matched by age and 13 healthy pubertal girls matched by pubertal stage. For this purpose, the comparisons were performed between pre- and pubertal controls to identify changes in normal pubertal transition and CPP versus pre- and pubertal controls.

RESULTS

Analysis of methylation changes associated with normal pubertal transition identified 1006 differentially methylated CpG sites, 86% of them were found to be hypermethylated in prepubertal controls. Some of these CpG sites reside in genes associated with the age of menarche or transcription factors involved in the process of pubertal development. Analysis of methylome profiles in CPP patients showed 65% and 55% hypomethylated CpG sites compared with prepubertal and pubertal controls, respectively. In addition, interestingly, our results revealed the presence of 43 differentially methylated genes coding for zinc finger (ZNF) proteins. Gene ontology and IPA analysis performed in the three groups studied revealed significant enrichment of them in some pathways related to neuronal communication (semaphorin and gustation pathways), estrogens action, some cancers (particularly breast and ovarian) or metabolism (particularly sirtuin).

CONCLUSIONS

The different methylation profiles of girls with normal and precocious puberty indicate that regulation of the pubertal process in humans is associated with specific epigenetic changes. Differentially methylated genes include ZNF genes that may play a role in developmental control. In addition, our data highlight changes in the methylation status of genes involved in signaling pathways that determine the migration and function of GnRH neurons and the onset of metabolic and neoplastic diseases that may be associated with CPP in later life.

Social regulation of immediate early gene induction in gonadotropin releasing-hormone 1 neurons and singing behavior in canaries (Serinus canaria)

Social cues modulate the neuroendocrine control of reproduction. However, the neural systems involved in the integration of social cues are not well described. Gonadotropin-releasing hormone 1 (GnRH1) cells in the preoptic area (POA) are the final common node that links the brain with peripheral reproductive physiology. These experiments investigated whether induction of the immediate early gene, EGR1, in anatomically localized GnRH1 cell populations in Border canaries is regulated by the social environment. First, we characterized behavioral modifications in singing behavior and found males paired with a female for 2 weeks significantly reduced many aspects of singing behavior. However, paired males had a significantly higher percentage of GnRH1 cells co-labeled with EGR1. The second experiment manipulated the social environment by pairing males and females in mixed sex dyads, same sex dyads or housed birds in isolation. Only when birds are paired in mixed sex dyads was there a significantly greater percentage of GnRH1 cells expressing EGR1 cells. Increased GnRH1-EGR1 co-expression was localized to the rostral POA. These data reveal that discrete GnRH1 cells are involved in the neural integration of specific social cues and support the hypothesis that the POA exhibits functional topography related to courtship and sexual behaviors.

The mechanism underlying the pubertal increase in pulsatile GnRH release in primates

In primates, the gonatotropin-releasing hormone (GnRH) neurosecretory system, consisting of GnRH, kisspeptin, and neurokinin B neurons, is active during the neonatal/early infantile period. During the late infantile period, however, activity of the GnRH neurosecretory system becomes minimal as a result of gonadal steroid independent central inhibition, and this suppressed GnRH neurosecretory state continues throughout the prepubertal period. At the initiation of puberty, the GnRH neurosecretory system becomes active again because of the decrease in central inhibition. During the progress of puberty, kisspeptin and neurokinin B signaling to GnRH neurons further increases, resulting in the release of gonadotropins and subsequent gonadal maturation, and hence puberty. This review further discusses potential substrates of central inhibition and subsequent pubertal modification of the GnRH neurosecretory system by the pubertal increase in steroid hormones, which ensures the regulation of adult reproductive function.

MKRN3 role in regulating pubertal onset: the state of art of functional studies

Puberty is a critical process characterized by several physical and psychological changes that culminate in the achievement of sexual maturation and fertility. The onset of puberty depends on several incompletely understood mechanisms that certainly involve gonadotropin-releasing hormone (GnRH) and its effects on the pituitary gland. The role of makorin ring finger protein 3 (MKRN3) in the regulation of pubertal timing was revealed when loss-of-function mutations were identified in patients with central precocious puberty (CPP), which to date, represent the most commonly known genetic cause of this condition. The MKRN3 gene showed ubiquitous expression in tissues from a broad spectrum of species, suggesting an important cellular role. Its involvement in the initiation of puberty and endocrine functions has just begun to be studied. This review discusses some of the recent approaches developed to predict MKRN3 functions and its involvement in pubertal development.

Methylation status of hypothalamic Mkrn3 promoter across puberty

Makorin RING finger protein 3 (MKRN3) is an important factor located on chromosome 15 in the imprinting region associated with Prader-Willi syndrome. Imprinted MKRN3 is expressed in hypothalamic regions essential for the onset of puberty and mutations in the gene have been found in patients with central precocious puberty. The pubertal process is largely controlled by epigenetic mechanisms that include, among other things, DNA methylation at CpG dinucleotides of puberty-related genes. In the present study, we investigated the methylation status of the Mkrn3 promoter in the hypothalamus of the female mouse before, during and after puberty. Initially, we mapped the 32 CpG dinucleotides in the promoter, the 5'UTR and the first 50 nucleotides of the coding region of the Mkrn3 gene. Moreover, we identified a short CpG island region (CpG islet) located within the promoter. Methylation analysis using bisulfite sequencing revealed that CpG dinucleotides were methylated regardless of developmental stage, with the lowest levels of methylation being found within the CpG islet region. In addition, the CpG islet region showed significantly lower methylation levels at the pre-pubertal stage when compared with the pubertal or post-pubertal stage. Finally, in silico analysis of transcription factor binding sites on the Mkrn3 CpG islet identified the recruitment of 29 transcriptional regulators of which 14 were transcriptional repressors. Our findings demonstrate the characterization and differential methylation of the CpG dinucleotides located in the Mkrn3 promoter that could influence the transcriptional activity in pre-pubertal compared to pubertal or post-pubertal period. Further studies are needed to clarify the possible mechanisms and effects of differential methylation of the Mkrn3 promoter.

Emerging Roles of Epigenetics in the Control of Reproductive Function: Focus on Central Neuroendocrine Mechanisms

Reproduction is an essential function for perpetuation of the species. As such, it is controlled by sophisticated regulatory mechanisms that allow a perfect match between environmental conditions and internal cues to ensure adequate pubertal maturation and achievement of reproductive capacity. Besides classical genetic regulatory events, mounting evidence has documented that different epigenetic mechanisms operate at different levels of the reproductive axis to finely tune the development and function of this complex neuroendocrine system along the lifespan. In this mini-review, we summarize recent evidence on the role of epigenetics in the control of reproduction, with special focus on the modulation of the central components of this axis. Particular attention will be paid to the epigenetic control of puberty and Kiss1 neurons because major developments have taken place in this domain recently. In addition, the putative role of central epigenetic mechanisms in mediating the influence of nutritional and environmental cues on reproductive function will be discussed.

Genetic and Epigenetic Control of Puberty

Puberty is a complex transitional phase in which reproductive capacity is achieved. There is a very wide variation in the age range of the onset of puberty, which follows a familial, ethnic, and sex pattern. The hypothalamic-pituitary-gonadal axis and several genetic, environmental, and nutritional factors play an important role in the onset of and throughout puberty. Recently, there has been significant progress in identifying factors that affect normal pubertal timing. Different studies have identified single nucleotide polymorphisms (SNPs) that affect pubertal timing in both sexes and across ethnic groups. Single genes are implicated in both precocious and delayed puberty, and epigenetic mechanisms have been suggested to affect the development and function of the GnRH neuronal network and responsiveness of end organs. All these factors can influence normal puberty timing, precocious puberty, and delayed puberty. The objective of this review is to describe recent findings related to the genetic and epigenetic control of puberty and highlight the need to deepen the knowledge of the regulatory mechanisms of this process in the normal and abnormal context.

Endocrine-disrupting chemicals and their effects on puberty

Sexual maturation in humans is characterized by a unique individual variability. Pubertal onset is a highly heritable polygenic trait but it is also affected by environmental factors such as obesity or endocrine disrupting chemicals. The last 30 years have been marked by a constant secular trend toward earlier age at onset of puberty in girls and boys around the world. More recent data, although more disputed, suggest an increased incidence in idiopathic central precocious puberty. Such trends point to a role for environmental factors in pubertal changes. Animal data suggest that the GnRH-neuronal network is highly sensitive to endocrine disruption during development. This review focuses on the most recent data regarding secular trend in pubertal timing as well as potential new epigenetic mechanisms explaining the developmental and transgenerational effects of endocrine disrupting chemicals on pubertal timing.

Biological clock and heredity in pubertal timing: what is new?

Puberty represents a milestone during a person's life and is characterized by several physical and psychological changes which end with the achievement of sexual maturation and of fertility. Puberty onset depends on a series of sophisticated, not completely understood, mechanisms certainly involving Gonadotropin-Releasing Hormone (GnRH) and its effects on pituitary gonadotropins. As recent evidence has demonstrated that pubertal timing deeply affects future adult health life, much efforts have been performed in order to clarify the exact actors involved in the onset and progression of puberty. Genetic factors are undoubtedly essential players in the regulation of pubertal development, accounting for approximately 50-80% of its variability. Mutations in genes such as KISS1, MKRN3 and DLK1 have been associated with central precocious puberty. Interestingly, a possible involvement of epigenetic mechanisms has been proposed as additional element able to affect pubertal phase. Environmental factors have recently attracted much attention. Indeed, an overall decrease in the age of puberty has been observed in the last decades. As genetic factors require long time to exert their effect, other players, such as environmental ones, may be involved. Special focus has been posed on nutritional status, endocrine-disrupting chemicals with non-conclusive results. Pubertal timing deeply affects future life, suggesting the need to clarify mechanisms driving pubertal onset and progression, in order to identify tailored therapeutic strategies and targets.

Epigenetics of the developing and aging brain: Mechanisms that regulate onset and outcomes of brain reorganization

Brain development is a life-long process that encompasses several critical periods of transition, during which significant cognitive changes occur. Embryonic development, puberty, and reproductive senescence are all periods of transition that are hypersensitive to environmental factors. Rather than isolated episodes, each transition builds upon the last and is influenced by consequential changes that occur in the transition before it. Epigenetic marks, such as DNA methylation and histone modifications, provide mechanisms by which early events can influence development, cognition, and health outcomes. For example, parental environment influences imprinting patterns in gamete cells, which ultimately impacts gene expression in the embryo which may result in hypersensitivity to poor maternal nutrition during pregnancy, raising the risks for cognitive impairment later in life. This review explores how epigenetics induce and regulate critical periods, and also discusses how early environmental interactions prime a system towards a particular health outcome and influence susceptibility to disease or cognitive impairment throughout life.

Unravelling the role of epigenetics in reproductive adaptations to early-life environment

Reproductive function adjusts in response to environmental conditions in order to optimize success. In humans, this plasticity includes age of pubertal onset, hormone levels and age at menopause. These reproductive characteristics vary across populations with distinct lifestyles and following specific childhood events, and point to a role for the early-life environment in shaping adult reproductive trajectories. Epigenetic mechanisms respond to external signals, exert long-term effects on gene expression and have been shown in animal and cellular studies to regulate normal reproductive function, strongly implicating their role in these adaptations. Moreover, human cohort data have revealed differential DNA methylation signatures in proxy tissues that are associated with reproductive phenotypic variation, although the cause-effect relationships are difficult to discern, calling for additional complementary approaches to establish functionality. In this Review, we summarize how adult reproductive function can be shaped by childhood events. We discuss why the influence of the childhood environment on adult reproductive function is an important consideration in understanding how reproduction is regulated and necessitates consideration by clinicians treating women with diverse life histories. The resolution of the molecular mechanisms responsible for human reproductive plasticity could also lead to new approaches for intervention by targeting these epigenetic modifications.

Epigenomic control of gonadotrophin-releasing hormone neurone development and hypogonadotrophic hypogonadism

Mammalian reproductive success depends on gonadotrophin-releasing hormone (GnRH) neurones to stimulate gonadotrophin secretion from the anterior pituitary and activate gonadal steroidogenesis and gametogenesis. Genetic screening studies in patients diagnosed with Kallmann syndrome (KS), a congenital form of hypogonadotrophic hypogonadism (CHH), identified several causal mutations, including those in the fibroblast growth factor (FGF) system. This signalling pathway regulates neuroendocrine progenitor cell proliferation, fate specification and cell survival. Indeed, the GnRH neurone system was absent or abrogated in transgenic mice with reduced (ie, hypomorphic) Fgf8 and/or Fgf receptor (Fgfr) 1 expression, respectively. Moreover, we found that GnRH neurones were absent in the embryonic olfactory placode of Fgf8 hypomorphic mice, the putative birthplace of GnRH neurones. These observations, together with those made in human KS/CHH patients, indicate that the FGF8/FGFR1 signalling system is a requirement for the ontogenesis of the GnRH neuronal system and function. In this review, we discuss how epigenetic factors control the expression of genes such as Fgf8 that are known to be critical for GnRH neurone ontogenesis, fate specification, and the pathogenesis of KS/CHH.

Mechanism of pulsatile GnRH release in primates: Unresolved questions

The pulsatility of GnRH release is essential for reproductive function. The key events in reproductive function, such as puberty onset and ovulatory cycles, are regulated by the frequency and amplitude modulation of pulsatile GnRH release. Abnormal patterns of GnRH pulsatility are seen in association with disease states, such as polycystic ovarian syndrome and anorexia nervosa. Recent studies with physiological, track-tracing, optogenetic and electrophysiological recording experiments indicate that a group of kisspeptin neurons in the arcuate nucleus (ARC) of the hypothalamus are responsible for pulsatile GnRH release. Thus, the kisspeptin neuron in the ARC has been called the "GnRH pulse-generator." However, a few pieces of evidence do not quite fit into this concept. This article reviews some old works and discusses unresolved issues on the mechanism of GnRH pulse generation.

Minireview: The Epigenetic Modulation of KISS1 in Reproduction and Cancer

Epigenetics describes how both lifestyle and environment may affect human health through the modulation of genome functions and without any change to the DNA nucleotide sequence. The discovery of several epigenetic mechanisms and the possibility to deliver epigenetic marks in cells, gametes, and biological fluids has opened up new perspectives in the prevention, diagnosis, and treatment of human diseases. In this respect, the depth of knowledge of epigenetic mechanisms is fundamental to preserving health status and to developing targeted interventions. In this minireview, we summarize the epigenetic modulation of the KISS1 gene in order to provide an example of epigenetic regulation in health and disease.

Emerging Genetic and Epigenetic Mechanisms Underlying Pubertal Maturation in Adolescence

The adolescent transition begins with the onset of puberty which, upstream in the brain, is initiated by the gonadotropin-releasing hormone (GnRH) pulse generator that activates the release of peripheral sex hormones. Substantial research in human and animal models has revealed a myriad of cellular networks and heritable genes that control the GnRH pulse generator allowing the individual to begin the process of reproductive competence and sexual maturation. Here, we review the latest knowledge in neuroendocrine pubertal research with emphasis on genetic and epigenetic mechanisms underlying the pubertal transition.

The Emerging Role of Chromatin Remodeling Factors in Female Pubertal Development

To attain sexual competence, all mammalian species go through puberty, a maturational period during which body growth and development of secondary sexual characteristics occur. Puberty begins when the diurnal pulsatile gonadotropin-releasing hormone (GnRH) release from the hypothalamus increases for a prolonged period of time, driving the adenohypophysis to increase the pulsatile release of luteinizing hormone with diurnal periodicity. Increased pubertal GnRH secretion does not appear to be driven by inherent changes in GnRH neuronal activity; rather, it is induced by changes in transsynaptic and glial inputs to GnRH neurons. We now know that these changes involve a reduction in inhibitory transsynaptic inputs combined with increased transsynaptic and glial excitatory inputs to the GnRH neuronal network. Although the pubertal process is known to have a strong genetic component, during the last several years, epigenetics has been implicated as a significant regulatory mechanism through which GnRH release is first repressed before puberty and is involved later on during the increase in GnRH secretion that brings about the pubertal process. According to this concept, a central target of epigenetic regulation is the transcriptional machinery of neurons implicated in stimulating GnRH release. Here, we will briefly review the hormonal changes associated with the advent of female puberty and the role that excitatory transsynaptic inputs have in this process. In addition, we will examine the 3 major groups of epigenetic modifying enzymes expressed in the neuroendocrine hypothalamus, which was recently shown to be involved in pubertal development and progression.

Molecular and Environmental Mechanisms Regulating Puberty Initiation: An Integrated Approach

The mechanisms underlying the initiation of puberty, one of the cornerstones of human evolution, have not been fully elucidated as yet. However, recently, an accumulating body of evidence has helped unravel several critical aspects of the process. It is clear that a change in the pattern of pituitary gonadotropin secretion serves as a hormonal trigger for puberty induction. This change is directly guided by the hypothalamic GnRH pulse generation, a phenomenon regulated by the Kisspeptin-Neurokinin-Dynorphin (KNDy) system also in the hypothalamus. This represents the kisspeptin molecule, which is crucial in augmenting GnRH secretion at puberty, whose secretion is fine-tuned by the opposing signals neurokinin B and dynorphin. Recently, the novel kisspeptin inhibitory signal MKRN3 was described, whose role in puberty initiation provided further insight into the mechanistic aspects of pubertal onset. Furthermore, the description of higher inhibitory and stimulatory signals acting upstream of the KNDy neurons suggested that the trigger point of puberty is located upstream of the KNDy system and the GnRH pulse generator. However, the mechanism of pubertal onset should not be considered as an isolated closed loop system. On the contrary, it is influenced by such factors as adipose tissue, gastrointestinal function, adrenal androgen production, energy sensing, and physical and psychosocial stress. Also, fetal and early life stressful events, as well as exposure to endocrine disruptors, may play important roles in pubertal initiation, the latter primarily through epigenetic modifications. Here we present the available data in the field and attempt to provide an integrated view of this unique and crucial phenomenon.

The interaction between DNA methylation and long non-coding RNA during the onset of puberty in goats

Epigenetics plays an important role in controlling female puberty. Both DNA methylation and long non-coding RNAs (lncRNA) regulate the initiation of puberty by affecting the expression of genes related to puberty. While recent studies have indicated that DNA methylation of lncRNA represses the expression of lncRNA, its role in regulating puberty remains unclear. To explore the mechanism between DNA methylation and lncRNAs during puberty onset, we performed whole-genome bisulphite sequencing (WGBS) and RNA-sequencing (RNA-seq). We found that DNA methylation was inversely correlated to gene expression levels during puberty. Methylation levels gradually decreased near the transcription initiation site and were present at high levels in the exon, intron and 3' untranslated regions. In the promoter, lncRNA expression was negatively related to DNA methylation. We reported hypermethylation in the gene body and downstream of the lncRNA compared with upstream regions. In GO and KEGG analyses, we found enriched target genes of lncRNA, XLOC_960044 and XLOC_767346. During puberty, methylation of these genes increased while expression decreased. Our study indicates that DNA methylation of the promoter is negatively correlated with lncRNA during puberty onset, and methylation regulates the initiation of puberty via lncRNA, which provides new insight into the epigenetic mechanism of puberty onset.

Hypothalamic epigenetics driving female puberty

Puberty involves a series of morphological, physiological and behavioural changes during the last part of the juvenile period that culminates in the attainment of fertility. The activation of the pituitary-gonadal axis by increased hypothalamic secretion of gonadotrophin-releasing hormone (GnRH) is an essential step in the process. The current hypothesis postulates that a loss of transsynaptic inhibition and a rise in excitatory inputs are responsible for the activation of GnRH release. Similarly, a shift in the balance in the expression of puberty activating and puberty inhibitory genes exists during the pubertal transition. In addition, recent evidence suggests that the epigenetic machinery controls this genetic balance, giving rise to the tantalising possibility that epigenetics serves as a relay of environmental signals known for many years to modulate pubertal development. Here, we review the contribution of epigenetics as a regulatory mechanism in the hypothalamic control of female puberty.

Disentangling puberty: novel neuroendocrine pathways and mechanisms for the control of mammalian puberty

BACKGROUND

Puberty is a complex developmental event, controlled by sophisticated regulatory networks that integrate peripheral and internal cues and impinge at the brain centers driving the reproductive axis. The tempo of puberty is genetically determined but is also sensitive to numerous modifiers, from metabolic and sex steroid signals to environmental factors. Recent epidemiological evidence suggests that the onset of puberty is advancing in humans, through as yet unknown mechanisms. In fact, while much knowledge has been gleaned recently on the mechanisms responsible for the control of mammalian puberty, fundamental questions regarding the intimate molecular and neuroendocrine pathways responsible for the precise timing of puberty and its deviations remain unsolved.

OBJECTIVE AND RATIONALE

By combining data from suitable model species and humans, we aim to provide a comprehensive summary of our current understanding of the neuroendocrine mechanisms governing puberty, with particular focus on its central regulatory pathways, underlying molecular basis and mechanisms for metabolic control.

SEARCH METHODS

A comprehensive MEDLINE search of articles published mostly from 2003 to 2017 has been carried out. Data from cellular and animal models (including our own results) as well as clinical studies focusing on the pathophysiology of puberty in mammals were considered and cross-referenced with terms related with central neuroendocrine mechanisms, metabolic control and epigenetic/miRNA regulation.

OUTCOMES

Studies conducted during the last decade have revealed the essential role of novel central neuroendocrine pathways in the control of puberty, with a prominent role of kisspeptins in the precise regulation of the pubertal activation of GnRH neurosecretory activity. In addition, different transmitters, including neurokinin-B (NKB) and, possibly, melanocortins, have been shown to interplay with kisspeptins in tuning puberty onset. Alike, recent studies have documented the role of epigenetic mechanisms, involving mainly modulation of repressors that target kisspeptins and NKB pathways, as well as microRNAs and the related binding protein, Lin28B, in the central control of puberty. These novel pathways provide the molecular and neuroendocrine basis for the modulation of puberty by different endogenous and environmental cues, including nutritional and metabolic factors, such as leptin, ghrelin and insulin, which are known to play an important role in pubertal timing.

WIDER IMPLICATIONS

Despite recent advancements, our understanding of the basis of mammalian puberty remains incomplete. Complete elucidation of the novel neuropeptidergic and molecular mechanisms summarized in this review will not only expand our knowledge of the intimate mechanisms responsible for puberty onset in humans, but might also provide new tools and targets for better prevention and management of pubertal deviations in the clinical setting.

Molecular cloning and epigenetic change detection of Kiss1 during seasonal reproduction in Chinese indigenous sheep

Like most seasonal domesticated species, sheep are short-day breeders, which means that the reproduction axis is activated by short days. The annual photoperiodic cycle affects the amount of daylength information that is transmitted to the hypothalamic-pituitary-gonadal (HPG) axis by regulating pulsatile secretion of gonadotrophin-releasing hormone from the hypothalamus. Kisspeptin, which is encoded by Kiss1, plays a major role in reproductive seasonality. Based on results from our previous Solexa sequencing data obtained from Tan (T) and Small Tail Han (STH) sheep during anoestrus and the breeding season, full-length mRNA information for ovine Kiss1 was obtained; 894bp in T sheep and 1145bp in STH sheep. Both encode 135 amino acids. Additionally, T and STH sheep have different transcription start sites of Kiss1. Kiss1 expression during oestrus was significantly higher than that during dioestrus, both in T and STH sheep (P<0.01). We also found a strong relationship between Kiss1 mRNA levels and histone H3 acetylation status in the 5' promoter region of ovine Kiss1. These data indicated that epigenetic modification occurs during reproduction in sheep, and this is the first report that histone H3 deacetylation occurs in the hypothalamus of seasonal sheep breeders during the transition from dioestrus to oestrus.

How is GnRH regulated in GnRH-producing neurons? Studies using GT1-7 cells as a GnRH-producing cell model

Hypothalamic secretion of gonadotropin-releasing hormone (GnRH) has been established as a principle pathway for initiating and integrating female reproductive function. GnRH stimulates the release of two gonadotropins-luteinizing hormone and follicle-stimulating hormone-from the anterior pituitary, which eventually stimulate the synthesis of sex steroids in association with follicular growth and ovulation. This reproductive control of the hypothalamic-pituitary-gonadal (HPG) axis also mediates gonadal feedback mechanisms. Although GnRH neurons certainly play a pivotal role in the HPG axis, the detailed mechanisms of their functional network, including regulatory systems, remain unknown. After the discovery of the indispensable role of kisspeptin in the development of human reproductive functions, our understanding of the neuroendocrine regulation of the HPG axis was revolutionized, and it is now recognized that kisspeptin acts upstream of GnRH and is responsible for sex steroid feedback mechanisms. Kisspeptin can stimulate gonadotropin release from the pituitary gland by stimulating GnRH release and GnRH antagonists prevent kisspeptin-induced gonadotropin release. Furthermore, it has been shown that GnRH neurons express kisspeptin receptors. Nevertheless, the detailed mechanisms underlying the regulation of homogeneous populations of GnRH neurons are still largely unknown because of the limitations of experimental models used for investigation. The hypothalamus consists of a complex network of distinct neuronal cells, and it is difficult to isolate single-cell populations of GnRH neurons. The establishment of GnRH-expressing cell lines has allowed us to examine the events happening at the single-cell level. In this review, we describe in vitro studies using a GnRH-producing cell model, GT1-7 cells, which have been used to examine how GnRH-producing cells respond to hypothalamic factors and how they are involved in GnRH synthesis.

DNA Methylation Patterns in the Hypothalamus of Female Pubertal Goats

Female pubertal development is tightly controlled by complex mechanisms, including neuroendocrine and epigenetic regulatory pathways. Specific gene expression patterns can be influenced by DNA methylation changes in the hypothalamus, which can in turn regulate timing of puberty onset. In order to understand the relationship between DNA methylation changes and gene expression patterns in the hypothalamus of pubertal goats, whole-genome bisulfite sequencing and RNA-sequencing analyses were carried out. There was a decline in DNA methylation levels in the hypothalamus during puberty and 268 differentially methylated regions (DMR) in the genome, with differential patterns in different gene regions. There were 1049 genes identified with distinct expression patterns. High levels of DNA methylation were detected in promoters, introns and 3′-untranslated regions (UTRs). Levels of methylation decreased gradually from promoters to 5′-UTRs and increased from 5′-UTRs to introns. Methylation density analysis demonstrated that methylation level variation was consistent with the density in the promoter, exon, intron, 5′-UTRs and 3′-UTRs. Analyses of CpG island (CGI) sites showed that the enriched gene contents were gene bodies, intergenic regions and introns, and these CGI sites were hypermethylated. Our study demonstrated that DNA methylation changes may influence gene expression profiles in the hypothalamus of goats during the onset of puberty, which may provide new insights into the mechanisms involved in pubertal onset.

The Methylcytosine Dioxygenase Ten-Eleven Translocase-2 (tet2) Enables Elevated GnRH Gene Expression and Maintenance of Male Reproductive Function

Reproduction depends on the establishment and maintenance of elevated GnRH neurosecretion. The elevation of primate GnRH release is accompanied by epigenetic changes. Specifically, cytosine residues within the GnRH gene promoter are actively demethylated, whereas GnRH mRNA levels and peptide release rise. Whether active DNA demethylation has an impact on GnRH neuron development and consequently reproductive function remains unknown. In this study, we investigated whether ten-eleven translocation (tet) enzymes, which initiate the process of active DNA demethylation, influence neuronal function and reproduction. We found that tet2 expression increases with age in the developing mouse preoptic area-hypothalamus and is substantially higher in a mature (GT1-7) than an immature (GN11) GnRH cell line. GnRH mRNA levels and mean GnRH peptide release elevated after overexpression of tet2 in GN11 cells, whereas CRISPR/cas9-mediated knockdown of tet2 in GT1-7 cells led to a significant decline in GnRH expression. Manipulations of tet2 expression altered tet2 genome binding and histone 3 lysine 4 trimethylation abundance at the GnRH promoter. Mice with selective disruption of tet2 in GnRH neurons (GnRH-specific tet2 knockout mice) exhibited no sign of altered pubertal timing in either sex, although plasma LH levels were significantly lower, and fecundity was altered specifically in adult male GnRH-specific tet2 knockout animals, indicating that tet2 may participate in the maintenance GnRH neuronal function. Exposure to bisphenol A, an environmental contaminant that alters GnRH neuron activity, caused a shift in tet2 subcellular localization and a decrease in histone 3 lysine 4 trimethylation abundance at the GnRH promoter. Finally, evaluation of tet2 protein interactions in GT1-7 cells suggests that the influence of tet2 on neuronal function are not limited to nuclear mechanisms but could depend on mitochondrial function, and RNA metabolism. Together, these studies implicate tet2 in the maintenance of GnRH neuronal function and neuroendocrine control of male reproduction.

Fibroblast Growth Factor 8 Expression in GT1-7 GnRH-Secreting Neurons Is Androgen-Independent, but Can Be Upregulated by the Inhibition of DNA Methyltransferases

Fibroblast growth factor 8 (FGF8) is a potent morphogen that regulates the embryonic development of hypothalamic neuroendocrine cells. Indeed, using Fgf8 hypomorphic mice, we showed that reduced Fgf8 mRNA expression completely eliminated the presence of gonadotropin-releasing hormone (GnRH) neurons. These findings suggest that FGF8 signaling is required during the embryonic development of mouse GnRH neurons. Additionally, in situ hybridization studies showed that the embryonic primordial birth place of GnRH neurons, the olfactory placode, is highly enriched for Fgf8 mRNA expression. Taken together these data underscore the importance of FGF8 signaling for GnRH emergence. However, an important question remains unanswered: How is Fgf8 gene expression regulated in the developing embryonic mouse brain? One major candidate is the androgen receptor (AR), which has been shown to upregulate Fgf8 mRNA in 60-70% of newly diagnosed prostate cancers. Therefore, we hypothesized that ARs may be involved in the regulation of Fgf8 transcription in the developing mouse brain. To test this hypothesis, we used chromatin-immunoprecipitation (ChIP) assays to elucidate whether ARs interact with the 5'UTR region upstream of the translational start site of the Fgf8 gene in immortalized mouse GnRH neurons (GT1-7) and nasal explants. Our data showed that while AR interacts with the Fgf8 promoter region, this interaction was androgen-independent, and that androgen treatment did not affect Fgf8 mRNA levels, indicating that androgen signaling does not induce Fgf8 transcription. In contrast, inhibition of DNA methyltransferases (DNMT) significantly upregulated Fgf8 mRNA levels indicating that Fgf8 transcriptional activity may be dependent on DNA methylation status.

The Emerging Role of Epigenetics in the Regulation of Female Puberty

In recent years the pace of discovering the molecular and genetic underpinnings of the pubertal process has accelerated considerably. Genes required for human puberty to occur have been identified and evidence has been provided suggesting that the initiation of puberty requires coordinated changes in the output of a multiplicity of genes organized into functional networks. Recent evidence suggests that a dual mechanism of epigenetic regulation affecting the transcriptional activity of neurons involved in stimulating gonadotropin-releasing hormone release plays a fundamental role in the timing of puberty. The Polycomb group (PcG) of transcriptional silencers appears to be a major component of the repressive arm of this mechanism. PcG proteins prevent the premature initiation of female puberty by silencing the Kiss1 gene in kisspeptin neurons of the arcuate nucleus (ARC) of the hypothalamus. Because the abundance of histone marks either catalyzed by--or associated with--the Trithorax group (TrxG) of transcriptional activators increases at the time when PcG control subsides, it appears that the TrxG complex is the counteracting partner of PcG-mediated gene silencing. In this chapter, we discuss the concept that a switch from epigenetic repression to activation within ARC kisspeptin neurons is a core mechanism underlying the initiation of female puberty.

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.

Kisspeptin Induces Dynamic Chromatin Modifications to Control GnRH Gene Expression

In vitro studies have demonstrated an increase in GnRH gene expression associated with an elevated secretory response to kisspeptin administration, suggesting that kisspeptin mediates GnRH expression at both the secretory and pretranslational levels. However, the kisspeptin-mediated intracellular mechanisms associated with the dynamic chromatin modifications modulating GnRH gene expression are unclear. The studies in this manuscript describe specific histone modifications on the enhancer and promoter of the mouse GnRH (mGnRH) gene induced by kisspeptin in GnRH neuronal cell lines (GT1-7 cells). ChIP assays followed by quantitative real-time PCR (qPCR) demonstrate that 15 and 45 min of 10(-9) M kisspeptin significantly increased histone 3 acetylation (H3Ac) at the kisspeptin response element (KsRE) contained between -3446 and -2806 bp of the mGnRH enhancer (GnRHen) in GT1-7 cells, while no changes were observed in the downstream neuron-specific element (NSE). Moreover, kisspeptin specifically induced acetylation of H3AcK14 and K27 and trimethylation of H3 lysine 4 at the KsRE (markers of active chromatin) and no changes in dimethylation of H3K9 (a marker associated with gene repression). Occupancy of RNA Pol II (RNAPII) and a differential carboxyl-terminal domain (CTD) phosphorylation pattern was observed. An interaction between the NSE and the KsRE via a chromatin loop in the mGnRH gene by kisspeptin was detected by the chromosome conformation capture assay (3C). In conclusion, these results demonstrate that kisspeptin induces histone acetylation/methylation and consequently enhances the formation of a chromatin loop in the mGnRH gene which results in known increase in kisspeptin-dependent mGnRH expression.

Epigenetic regulation of female puberty

Substantial progress has been made in recent years toward deciphering the molecular and genetic underpinnings of the pubertal process. The availability of powerful new methods to interrogate the human genome has led to the identification of genes that are essential for puberty to occur. Evidence has also emerged suggesting that the initiation of puberty requires the coordinated activity of gene sets organized into functional networks. At a cellular level, it is currently thought that loss of transsynaptic inhibition, accompanied by an increase in excitatory inputs, results in the pubertal activation of GnRH release. This concept notwithstanding, a mechanism of epigenetic repression targeting genes required for the pubertal activation of GnRH neurons was recently identified as a core component of the molecular machinery underlying the central restraint of puberty. In this chapter we will discuss the potential contribution of various mechanisms of epigenetic regulation to the hypothalamic control of female puberty.

Real-Time GnRH Gene Transcription in GnRH Promoter-Driven Luciferase-Expressing Transgenic Mice: Effect of Kisspeptin

Pulsatile secretion of hypothalamic gonadotropin-releasing hormone (GnRH) is indispensable for controlling proper pituitary gonadotrope functions; however, the mechanism underlying GnRH pulse generation remains largely unknown. It is important to understand the cellular oscillator in individual GnRH neurons and temporal synchronization among GnRH neurons. In this brief review, we summarize our recent findings on episodic GnRH gene transcription at the single GnRH neuron level and in synchronized multicellular burst in relation to the temporal pattern of GnRH secretion. We also detail the effects of kisspeptin on ultradian rhythmic GnRH gene transcription and secretion. We extend our discussion to the hierarchical interaction between circadian and ultradian rhythms. Taken together, the current review elucidates the genomic control of GnRH pulse generation in hypothalamic neurons.

Epigenetics of schizophrenia: an open and shut case

During the last decade and a half, there has been an explosion of data regarding epigenetic changes in schizophrenia. Most initial studies have suggested that schizophrenia is characterized by an overly restrictive chromatin state based on increases in transcription silencing histone modifications and DNA methylation at schizophrenia candidate gene promoters and increases in the expression of enzymes that catalyze their formation. However, recent studies indicate that the pathology is more complex. This complexity may greatly impact pharmacological approaches directed at targeting epigenetic abnormalities in schizophrenia. The current review explores epigenetic studies of schizophrenia and what this can tell us about the underlying pathophysiology. We hypothesize based on recent studies that it is also plausible that drugs that further restrict chromatin may be efficacious.

Maternal exposure to dioxin imprints sexual immaturity of the pups through fixing the status of the reduced expression of hypothalamic gonadotropin-releasing hormone

Our previous studies have shown that treatment of pregnant rats with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 1 μg/kg) at gestational day (GD) 15 reduces the pituitary synthesis of luteinizing hormone (LH) during the late fetal and early postnatal period, leading to the imprinting of defects in sexual behaviors at adulthood. However, it remains unclear how the attenuation of pituitary LH is linked to sexual immaturity. To address this issue, we performed a DNA microarray analysis to identify the gene(s) responsible for dioxin-induced sexual immaturity on the pituitary and hypothalamus of male pups, born of TCDD-treated dams, at the age of postnatal day (PND) 70. Among the reduced genes, we focused on gonadotropin-releasing hormone (GnRH) in the hypothalamus because of published evidence that it has a role in sexual behaviors. An attenuation by TCDD of GnRH expression emerged at PND4, and no subsequent return to the control level was seen. A change in neither DNA methylation nor histone acetylation accounted for the reduced expression of GnRH. Intracerebroventricular infusion of GnRH to the TCDD-exposed pups after reaching maturity restored the impairment of sexual behaviors. Supplying equine chorionic gonadotropin, an LH-mimicking hormone, to the TCDD-exposed fetuses at GD15 resulted in a recovery from the reduced expression of GnRH, as well as from the defects in sexual behavior. These results strongly suggest that maternal exposure to TCDD fixes the status of the lowered expression of GnRH in the offspring by reducing the LH-assisted steroidogenesis at the perinatal stage, and this mechanism imprints defects in sexual behaviors at adulthood.

Permanent and plastic epigenesis in neuroendocrine systems

The emerging area of neuroepigenetics has been linked to numerous mental health illnesses. Importantly, a large portion of what we know about early gene×environment interactions comes from examining epigenetic modifications of neuroendocrine systems. This review will highlight how neuroepigenetic mechanisms during brain development program lasting differences in neuroendocrine systems and how other neuroepigenetic processes remain plastic, even within the adult brain. As epigenetic mechanisms can either be stable or plastic, elucidating the mechanisms involved in reversing these processes could aid in understanding how to reverse pathological epigenetic programming.

A system biology approach to identify regulatory pathways underlying the neuroendocrine control of female puberty in rats and nonhuman primates

This article is part of a Special Issue "Puberty and Adolescence". Puberty is a major developmental milestone controlled by the interaction of genetic factors and environmental cues of mostly metabolic and circadian nature. An increased pulsatile release of the decapeptide gonadotropin releasing hormone (GnRH) from hypothalamic neurosecretory neurons is required for both the initiation and progression of the pubertal process. This increase is brought about by coordinated changes that occur in neuronal and glial networks associated with GnRH neurons. These changes ultimately result in increased neuronal and glial stimulatory inputs to the GnRH neuronal network and a reduction of transsynaptic inhibitory influences. While some of the major players controlling pubertal GnRH secretion have been identified using gene-centric approaches, much less is known about the system-wide control of the overall process. Because the pubertal activation of GnRH release involves a diversity of cellular phenotypes, and a myriad of intracellular and cell-to-cell signaling molecules, it appears that the overall process is controlled by a highly coordinated and interactive regulatory system involving hundreds, if not thousands, of gene products. In this article we will discuss emerging evidence suggesting that these genes are arranged as functionally connected networks organized, both internally and across sub-networks, in a hierarchical fashion. According to this concept, the core of these networks is composed of transcriptional regulators that, by directing expression of downstream subordinate genes, provide both stability and coordination to the cellular networks involved in initiating the pubertal process. The integrative response of these gene networks to external inputs is postulated to be coordinated by epigenetic mechanisms.

Epigenetic control of female puberty

The timing of puberty is controlled by many genes. The elements coordinating this process have not, however, been identified. Here we show that an epigenetic mechanism of transcriptional repression times the initiation of female puberty in rats. We identify silencers of the Polycomb group (PcG) as principal contributors to this mechanism and show that PcG proteins repress Kiss1, a puberty-activating gene. Hypothalamic expression of two key PcG genes, Eed and Cbx7, decreased and methylation of their promoters increased before puberty. Inhibiting DNA methylation blocked both events and resulted in pubertal failure. The pubertal increase in Kiss1 expression was accompanied by EED loss from the Kiss1 promoter and enrichment of histone H3 modifications associated with gene activation. Preventing the eviction of EED from the Kiss1 promoter disrupted pulsatile gonadotropin-releasing hormone release, delayed puberty and compromised fecundity. Our results identify epigenetic silencing as a mechanism underlying the neuroendocrine control of female puberty.

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.

Epigenetic control of gonadotropin releasing hormone neurons

Epigenetic modifications to the genome, including DNA methylation and histone modifications, occur in response to external stimuli. Reproductive function is highly sensitive to environmental conditions including season, diet, hormonal changes, and exposure to chemical contaminants. GnRH neurons, which play a key role in reproduction, are particularly sensitive to various environmental stimuli. We recently reported that the rhesus monkey GnRH gene exhibits distinct epigenetic changes during embryonic development. More recently, we further found that a similar epigenetic phenomenon occurs across puberty. In this article we highlight recent findings, including those of afferent inputs, to describe the epigenetic control of GnRH circuit development as a link between the environment and reproductive function.

Molecular profiling of postnatal development of the hypothalamus in female and male rats

Reproductive function is highly dynamic during postnatal developmental. Here, we performed molecular profiling of gene expression patterns in the hypothalamus of developing male and female rats to identify which genes are sexually dimorphic, to gain insight into a more complex network of hypothalamic genes, and to ascertain dynamic changes in their relationships with one another and with sex steroid hormones during development. Using a low-density PCR platform, we quantified mRNA levels in the preoptic area (POA) and medial basal hypothalamus (MBH), and assayed circulating estradiol, testosterone, and progesterone at six ages from birth through adulthood. Numerous genes underwent developmental change, particularly postnatal increases, decreases, or peaks/plateaus at puberty. Surprisingly, there were few sex differences; only Esr1, Kiss1, and Tac2 were dimorphic (higher in females). Cluster analysis of gene expression revealed sexually dimorphic correlations in the POA but not the MBH from P30 (Postnatal Day 30) to P60. Hormone measurements showed few sex differences in developmental profiles of estradiol; higher levels of progesterone in females only after P30; and a developmental pattern of testosterone with a nadir at P30 followed by a dramatic increase through P60 (males). Furthermore, bionetwork analysis revealed that hypothalamic gene expression profiles and their relationships to hormones undergo dynamic developmental changes that differ considerably from adults. These data underscore the importance of developmental stage in considering the effects of hormones on the regulation of neuroendocrine genes in the hypothalamus. Moreover, the finding that few neuroendocrine genes are sexually dimorphic highlights the need to consider postnatal development from a network approach that allows assessment of interactions and patterns of expression.

Neuroestrogen, rapid action of estradiol, and GnRH neurons

Estradiol plays a pivotal role in the control of GnRH neuronal function, hence female reproduction. A series of recent studies in our laboratory indicate that rapid excitatory actions of estradiol directly modify GnRH neuronal activity in primate GnRH neurons through GPR30 and STX-sensitive receptors. Similar rapid direct actions of estradiol through estrogen receptor beta are also described in mouse GnRH neurons. In this review, we propose two novel hypotheses as a possible physiological role of estradiol in primates. First, while ovarian estradiol initiates the preovulatory GnRH surge through interneurons expressing estrogen receptor alpha, rapid direct membrane-initiated action of estradiol may play a role in sustaining GnRH surge release for many hours. Second, locally produced neuroestrogens may contribute to pulsatile GnRH release. Either way, estradiol synthesized in interneurons in the hypothalamus may play a significant role in the control of the GnRH surge and/or pulsatility of GnRH release.

Rapid direct action of estradiol in GnRH neurons: findings and implications

Estradiol plays a pivotal role in the control of gonadotropin-releasing hormone (GnRH) neuronal function and female reproduction. While positive and negative feedback actions of estradiol that enhance and suppress release of GnRH and LH are primarily mediated through estrogen receptor alpha located in interneurons, a series of recent studies in our laboratory indicate that rapid excitatory actions of estradiol also directly modify GnRH neuronal activity. We observed this phenomenon in cultured primate GnRH neurons, but similar rapid direct actions of estradiol are also described in cultured GnRH neurons and green fluorescent protein-labeled GnRH neurons of mice. Importantly, rapid direct action of estradiol in GnRH neurons is mediated through membrane or membrane associated receptors, such as GPR30, STX-sensitive receptors, and ERβ. In this review, possible implications of this rapid estradiol action in GnRH neurons are discussed.

Colocalization of FM1-43, Bassoon, and GnRH-1: GnRH-1 release from cell bodies and their neuroprocesses

Pulsatile release of GnRH-1 is critical for reproductive function. However, the cellular mechanism of GnRH-1 neurosecretion is still elusive. In this study, we examined the neurosecretory process of GnRH-1 neurons using time-lapse image acquisition followed by immunocytochemistry with confocal microscopy. To monitor exocytotic processes, cultured GnRH-1 neurons derived from monkey embryos were labeled with the lipophilic dye, FM1-43, or its fixable form FM1-43Fx, in the presence or absence of depolarization signals, and changes in vesicles labeled with FM1-43 were analyzed. The results show FM1-43 was taken up into the cell and labeled puncta in the soma and neuroprocesses in the absence of depolarization signals, indicating that GnRH-1 neurons were spontaneously active. Depolarization of GnRH-1 neurons with high K+ or veratridine challenge increased the intensity and size of puncta in both soma and neuroprocesses, and the veratridine-induced changes in puncta were blocked by tetrodotoxin, indicating that changes in the puncta intensity and size reflect neurosecretory activity. Subsequent double immunocytochemistry for GnRH-1 and the synaptic vesicle marker, vesicle-associated membrane protein, demonstrated that the FM1-43Fx-labeled puncta were synaptic vesicles with the GnRH-1 peptide. Additional double immunocytochemistry for GnRH-1 and the marker of the neurosecretory active zone, Bassoon, indicated that the FM1-43Fx-labeled puncta were located at the sites of neurosecretory active zones in GnRH-1 neurons. These results suggest that GnRH-1 neurons have the capacity to release the peptide from the soma and dendrites. Collectively, we hypothesize that soma-dendritic release of the peptide may be a mechanism of synchronized activity among GnRH-1 neurons.

Dynamic chromatin modifications control GnRH gene expression during neuronal differentiation and protein kinase C signal transduction

GnRH, a neuropeptide produced by rare, specialized hypothalamic secretory neurons, is critical for reproduction. During development, GnRH gene expression increases as neurons migrate from the olfactory placode to the hypothalamus, with highest levels in the mature, postmitotic state. While neuronal differentiation is known to be controlled by chromatin modulations, the role of chromatin dynamics in GnRH gene regulation has not been studied. Here, we use mature and immature GnRH neuronal cell models to show that both neuron-specific and protein kinase C regulation of GnRH expression are mediated by chromatin structure and histone modifications. Only in GT1-7 mature GnRH neuronal cells did GnRH regulatory elements display high sensitivity to DNase and enrichment of active histone markers histone-H3 acetylation and H3 lysine 4 trimethylation (H3K4-Me3), as well as RNA polymerase II (RNAPII) binding and enhancer RNA transcription. In contrast, H3K9-Me2, a marker of inactive chromatin, was highest in nonneuronal cells, low in GT1-7 cells, and intermediate in immature GnRH neuronal cells. The chromatin of the GnRH gene was therefore active in mature GnRH neuronal cells, inactive in nonneuronal cells, but not fully inactive in immature GnRH neuronal cells. Activation of protein kinase C (PKC) potently represses GnRH expression. PKC activation caused closing of the chromatin and decreased RNAPII occupancy at the GnRH minimal promoter (-278/-97). At GnRH-Enhancer-1 (-2404/-2100), PKC activation decreased phosphorylated-RNAPII binding, enhancer RNA transcription, and H3 acetylation, and reciprocally increased H3K9-Me2. Chromatin modifications therefore participate in the dynamic regulation and specification of GnRH expression to differentiated hypothalamic neurons.