Agouti-related peptide plays a critical role in leptin's effects on female puberty and reproduction

Loss of Agrp1 in zebrafish: Effects on the growth and reproductive axis

Loss of agouti related neuropeptide (AgRP) does not lead to overt phenotypes in mammals unless AgRP neurons are ablated. In contrast, in zebrafish it has been shown that Agrp1 loss of function (LOF) leads to reduced growth in Agrp1 morphant as well as Agrp1 mutant larvae. Further, it has been shown that multiple endocrine axes are dysregulated upon Agrp1 LOF in Agrp1 morphant larvae. Here we show that adult Agrp1 LOF zebrafish show normal growth and reproductive behavior in spite of a significant reduction in multiple related endocrine axes namely reduced expression in pituitary growth hormone (gh) follicle stimulating hormone (fshb) as well as luteinizing hormone (lhb). We looked for compensatory changes in candidate gene expression but found no changes in growth hormone and gonadotropin hormone receptors that would explain the lack of phenotype. We further looked at expression in the hepatic and muscular insulin-like growth factor (Igf) axis which appears to be normal. Fecundity as well as ovarian histology also appear largely normal while we do see an increase in mating efficiency specifically in fed but not fasted AgRP1 LOF animals. This data shows that zebrafish can grow and reproduce normally in spite of significant central hormone changes and suggests a peripheral compensatory mechanism additional to previously reported central compensatory mechanisms in other zebrafish neuropeptide LOF lines.

The gut signals to AGRP-expressing cells of the pituitary to control glucose homeostasis

Glucose homeostasis can be improved after bariatric surgery, which alters bile flow and stimulates gut hormone secretion, particularly FGF15/19. FGFR1 expression in AGRP-expressing cells is required for bile acids' ability to improve glucose control. We show that the mouse Agrp gene has 3 promoter/enhancer regions that direct transcription of each of their own AGRP transcripts. One of these Agrp promoters/enhancers, Agrp-B, is regulated by bile acids. We generated an Agrp-B knockin FLP/knockout allele. AGRP-B-expressing cells are found in endocrine cells of the pars tuberalis and coexpress diacylglycerol lipase B - an endocannabinoid biosynthetic enzyme - distinct from pars tuberalis thyrotropes. AGRP-B expression is also found in the folliculostellate cells of the pituitary's anterior lobe. Mice without AGRP-B were protected from glucose intolerance induced by high-fat feeding but not from excess weight gain. Chemogenetic inhibition of AGRP-B cells improved glucose tolerance by enhancing glucose-stimulated insulin secretion. Inhibition of the AGRP-B cells also caused weight loss. The improved glucose tolerance and reduced body weight persisted up to 6 weeks after cessation of the DREADD-mediated inhibition, suggesting the presence of a biological switch for glucose homeostasis that is regulated by long-term stability of food availability.

Fasting and prolonged food restriction differentially affect GH secretion independently of GH receptor signaling in AgRP neurons

Growth hormone (GH) receptor (GHR) is abundantly expressed in neurons that co-release the agouti-related protein (AgRP) and neuropeptide Y (NPY) in the arcuate nucleus of the hypothalamus (ARH). Since ARH^AgRP/NPY neurons regulate several hypothalamic-pituitary-endocrine axes, this neuronal population possibly modulates GH secretion via a negative feedback loop, particularly during food restriction, when ARH^AgRP/NPY neurons are highly active. The present study aims to determine the importance of GHR signaling in ARH^AgRP/NPY neurons on the pattern of GH secretion in fed and food-deprived male mice. Additionally, we compared the effect of two distinct situations of food deprivation: 16 h of fasting or four days of food restriction (40% of usual food intake). Overnight fasting strongly suppressed both basal and pulsatile GH secretion. Animals lacking GHR in ARH^AgRP/NPY neurons (AgRP^∆GHR mice) did not exhibit differences in GH secretion either in the fed or fasted state, compared to control mice. In contrast, four days of food restriction increased GH pulse frequency, basal GH secretion, and pulse irregularity/complexity (measured by sample entropy), whereas pulsatile GH secretion was not affected in both control and AgRP^∆GHR mice. Hypothalamic Ghrh mRNA levels were unaffected by fasting or food restriction, but Sst expression increased in acutely fasted mice, but decreased after prolonged food restriction in both control and AgRP^∆GHR mice. Our findings indicate that short-term fasting and prolonged food restriction differentially affect the pattern of GH secretion, independently of GHR signaling in ARH^AgRP/NPY neurons.

Physiology of the Weight-Reduced State and Its Impact on Weight Regain

Obesity is a chronic disease characterized by long duration, slow progression, and periods of remission and relapses. Despite the development of effective medical and surgical interventions and millions of people conducting tremendous personal efforts to manage their weight every year, recidivism remains a significant barrier to attaining long-term weight maintenance. This review aimed to explain the underlying physiology of the weight-reduced state including changes in energy balance, adipose tissue, genetic, environmental, and behavioral factors that may predispose individuals to weight regain following weight loss.

Agouti-related peptide neuronal silencing overcomes delayed puberty in neonatally underfed male mice

Agouti-related peptide (AgRP) neurons are thought to indirectly regulate the activity of hypothalamic gonadotrophin-releasing hormone neurons which control fertility. AgRP neurons also drive caloric intake and are modulated by metabolically-relevant hormones, providing a link to the hypothalamic-pituitary-gonadal axis. In mice expressing Cre-dependant designer receptors (DREADDs) in AgRP neurons, we activated or silenced these neurons in vivo using the synthetic ligand clozapine-N-oxide (CNO) to observe the effect of AgRP neuron activity on timing of puberty. To validate these animals, we chronically treated both stimulatory (hM3Dq) and inhibitory (hM4Di) DREADD × AgRP-Cre mice with CNO, observing a pronounced increase and decrease of food intake, respectively, consistent with the known orexigenic effects of these neurons. RNAscope was performed to visually confirm the activation of AgRP neurons. Puberty onset was assessed in males and females. There was no effect on preputial separation in males or vaginal opening and first oestrus in females after CNO treatment from day 26 to 30 to chronically modulate AgRP neurons. Next, to determine whether the delay in puberty onset occurring in response to neonatal underfeeding could be overcome by inhibiting AgRP neuronal activity, mice were raised in large (neonatally underfed) or normal litter sizes. The delay in puberty from underfeeding was completely reversed in CNO-treated AgRP-hM4Di male mice. These data highlight the inhibitory role of AgRP neurons to delay puberty onset when undernutrition occurs during the neonatal period, at least in male mice. TRAIL REGISTRATION NUMBER: JNE-22-0081-OA.R2.

Hypothalamic mechanisms of obesity-associated disturbance of hypothalamic-pituitary-ovarian axis

Ovulatory disorders are the most common clinical feature exhibited among obese women. Initiation of ovulation physiologically requires a surge of gonadotropin-releasing hormone (GnRH) released from GnRH neurons located in the hypothalamus. These GnRH neurons receive metabolic signals from circulation and vicinal neurons to regulate GnRH release. Leptin acts indirectly on GnRH via adjacent leptin receptor (LEPR)-expressing neurons such as proopiomelanocortin (POMC), neuropeptide Y (NPY)/agouti-related peptide (AgRP), and neuronal nitric oxide (NO) synthase (nNOS) neurons to affect GnRH neuronal activities. Additionally, hypothalamic inflammation also affects ovulation independent of obesity. Therefore, this review focuses on hypothalamic mechanisms that underlie the disturbance of hypothalamic-pituitary-ovarian (HPO) axis during obesity with an attempt to promote future studies and/or novel therapeutic strategies for ovulatory disorders in obesity.

Leptin Receptor Expression in GABAergic Cells is Not Sufficient to Normalize Metabolism and Reproduction in Mice

Previous studies indicate that leptin receptor (LepR) expression in GABAergic neurons is necessary for the biological effects of leptin. However, it is not clear whether LepR expression only in GABAergic neurons is sufficient to prevent the metabolic and neuroendocrine imbalances caused by LepR deficiency. In the present study, we produced mice that express the LepR exclusively in GABAergic cells (LepRVGAT mice) and compared them with wild-type (LepR+/+) and LepR-deficient (LepRNull/Null) mice. Although LepRVGAT mice showed a pronounced reduction in body weight and fat mass, as compared with LepRNull/Null mice, male and female LepRVGAT mice exhibited an obese phenotype relative to LepR+/+ mice. Food intake was normalized in LepRVGAT mice; however, LepRVGAT mice still exhibited lower energy expenditure in both sexes and reduced ambulatory activity in the females, compared with LepR+/+ mice. The acute anorexigenic effect of leptin and hedonic feeding were normalized in LepRVGAT mice despite the hyperleptinemia they present. Although LepRVGAT mice showed improved glucose homeostasis compared with LepRNull/Null mice, both male and female LepRVGAT mice exhibited insulin resistance. In contrast, LepR expression only in GABAergic cells was sufficient to normalize the density of agouti-related peptide (AgRP) and α-MSH immunoreactive fibers in the paraventricular nucleus of the hypothalamus. However, LepRVGAT mice exhibited reproductive dysfunctions, including subfertility in males and alterations in the estrous cycle of females. Taken together, our findings indicate that LepR expression in GABAergic cells, although critical to the physiology of leptin, is insufficient to normalize several metabolic aspects and the reproductive function in mice.

Recent Advances in Hypertension: Intersection of Metabolic and Blood Pressure Regulatory Circuits in the Central Nervous System

Obesity represents the single greatest ongoing roadblock to improving cardiovascular health. Prolonged obesity is associated with fundamental changes in the integrative control of energy balance, including the development of selective leptin resistance, which is thought to contribute to obesity-associated hypertension, and adaptation of resting metabolic rate (RMR) when excess weight is reduced. Leptin and the melanocortin system within the hypothalamus contribute to the control of both energy balance and blood pressure. While the development of drugs to stimulate RMR and thereby reverse obesity through activation of the melanocortin system has been pursued, most of the resulting compounds simultaneously cause hypertension. Evidence supports the concept that although feeding behaviors, RMR, and blood pressure are controlled through mechanisms that utilize similar molecular mediators, these mechanisms exist in anatomically dissociable networks. New evidence supports a major change in molecular signaling within AgRP (Agouti-related peptide) neurons of the arcuate nucleus of the hypothalamus during prolonged obesity and the existence of multiple distinct subtypes of AgRP neurons that individually contribute to control of feeding, RMR, or blood pressure. Finally, ongoing work by our laboratory and others support a unique role for AT₁ (angiotensin II type 1 receptor) within one specific subtype of AgRP neuron for the control of RMR. We propose that understanding the unique biology of the AT₁-expressing, RMR-controlling subtype of AgRP neurons will help to resolve the selective dysfunctions in RMR control that develop during prolonged obesity and potentially point toward novel druggable antiobesity targets that will not simultaneously cause hypertension.

Reporting The Effects of Exposure to Monosodium Glutamate on The Regulatory Peptides of The Hypothalamic-Pituitary-Gonadal Axis

Monosodium glutamate (MSG) is a flavour enhancer that is used as a food additive (E621) in many parts of the world, especially in East Asian countries. However, in recent studies, it has been used as a neurotoxin because MSG is reported to cause neural degeneration in the hypothalamic arcuate of neonatal animals. The results of several studies show the negative effects of MSG injections on different parts of the hypothalamic-pituitary-gonadal (HPG) axis, in addition to its ability to inhibit secretion many reproductive neuropeptides, neurotrophic factors, and hormones, all of which play vital roles in the regulation of reproductive function. Oral administration or injection of large quantities of MSG into newborn animals results in a decrease in or overabundance of the production of many regulatory peptides of the male and female reproductive systems. In this review, we summarize the results of the most important studies that have examined the effect of oral consumption or injection of MSG on regulatory peptides of the HPG axis.

Novel actions of kisspeptin signaling outside of GnRH-mediated fertility: a potential role in energy balance

Kisspeptin, encoded by Kiss1 gene expressing neurons in the hypothalamus, is a requisite for fertility and now appears critical in the regulation of energy balance. Kisspeptin neurons, particularly those in the arcuate nucleus (ARC), receive information directly and indirectly from a diverse array of brain regions including the bed nucleus of the stria terminalis, amygdala, interpeduncular nucleus, hippocampus, and cortex. On the other hand, kisspeptin neuron projections clearly extend to GnRH neuron cell bodies in rodents, sheep, and primates and beyond to other-non-GnRH-brain areas. Kiss1r, the kisspeptin receptor, is expressed on GnRH neurons and also in additional brain areas and peripheral tissues, indicating a nonreproductive role. Kisspeptin neurons clearly receive signals pertinent to deviations in energy balance but are now recognized as a novel neuroendocrine player in the fine balance of energy intake and expenditure. Mice that have a dysfunctional gene for Kiss1r develop an obese and diabetic phenotype. The mechanism behind this altered metabolic state is still mostly unknown; however, Kiss1r expression in the pancreas and brown adipose tissue is clearly functional and required for normal glucose tolerance and energy expenditure, respectively. Kisspeptin neurons in the ARC also participate in the generation of circadian rhythms, specifically those concerning food intake and metabolism, offering a potential explanation for the obesity in Kiss1r knockout mice. Overall, the discoveries of new mechanistic roles for kisspeptin in both normal and pathophysiologic states of energy balance may lead to further understating of obesity prevalence and novel therapeutic targets and interventions.

Sirtuin (SIRT)-1: At the crossroads of puberty and metabolism

In the arcuate nucleus (ARC) of the hypothalamus reside two neuronal systems in charge of regulating feeding control and reproductive development. The melanocortin system responds to metabolic fluctuations adjusting food intake, whereas kisspeptin neurons are in charge of the excitatory control of Gonadotropin Hormone Releasing Hormone (GnRH) neurons. While it is known that the melanocortin system regulates GnRH neuronal activity, it was recently demonstrated that kisspeptin neurons not only innervate melanocortin neurons, but also play an active role in the control of metabolism. These two neuronal systems are intricately interconnected forming loops of stimulation and inhibition according to metabolic status. Furthermore, intracellular and epigenetic pathways respond to external environmental signals by changing DNA conformation and gene expression. Here we review the role of Silent mating type Information Regulation 2 homologue 1 (Sirt1), a class III NAD+ dependent protein deacetylase, in the ARC control of pubertal development and feeding behavior.

The Peripubertal Decline in Makorin Ring Finger Protein 3 Expression is Independent of Leptin Action

A critical body weight is necessary for pubertal development, an effect mediated in part by leptin. The potential regulation by leptin of Makorin Ring Finger Protein 3 (MKRN3), in which loss-of-function mutations are the most common genetic cause of central precocious puberty, has not been previously explored. In mice, expression of Mkrn3 in the hypothalamic arcuate nucleus is high early in life and declines before the onset of puberty. Therefore, we aimed to explore if leptin contributes to the decrease in hypothalamic Mkrn3 mRNA levels observed in mice during pubertal development. We first used a leptin-deficient (ob/ob) mouse model. Mkrn3 mRNA levels in the mediobasal hypothalamus (MBH), which includes the arcuate nucleus, and in the preoptic area (POA), both showed a significant decrease with age from postnatal day (PND) 12 to PND30 in ob/ob mice in both males and females, similar to that observed in wild-type mice. To further explore the effects of leptin on Mkrn3 expression, we exposed prepubertal wild-type mice to high levels of leptin from age PND9-12, which did not result in any significant difference in Mkrn3 expression levels in either the MBH or POA. In summary, regulation of Mkrn3 expression by leptin was not observed in either the MBH or the POA, 2 hypothalamic sites important for pubertal maturation. These data suggest that the decline in Mkrn3 at the onset of puberty may occur independently of leptin and support our hypothesis that MKRN3 is a bona fide controller of puberty initiation.

Single-Nucleus RNA Sequencing of the Hypothalamic Arcuate Nucleus of C57BL/6J Mice After Prolonged Diet-Induced Obesity

Prolonged obesity is associated with blunted feeding and thermogenic autonomic responses to leptin, but cardiovascular responses to leptin are maintained. This state of selective leptin resistance is, therefore, proposed to contribute to the pathogenesis and maintenance of obesity-associated hypertension. Cells of the arcuate nucleus of the hypothalamus detect leptin, and although the cellular and molecular mechanisms remain unclear, altered arcuate nucleus biology is hypothesized to contribute to selective leptin resistance. Male C57BL/6J mice were fed a high-fat diet (HFD) or chow from 8 to 18 weeks of age, as this paradigm models selective leptin resistance. Nuclei were then isolated from arcuate nucleus for single-nucleus RNA sequencing. HFD caused expected gains in adiposity and circulating leptin. Twenty-three unique cell-type clusters were identified, and Ingenuity Pathway Analysis was used to explore changes in gene expression patterns due to chronic HFD within each cluster. Notably, gene expression signatures related to leptin signaling exhibited suppression predominantly in neurons identified as the Agouti-related peptide (Agrp) subtype. Ingenuity Pathway Analysis results were also consistent with alterations in CREB (cAMP response element-binding protein) signaling in Agrp neurons after HFD, and reduced phosphorylated CREB was confirmed in arcuate nucleus after prolonged HFD by capillary electrophoresis-based Western blotting. These findings support the concept that prolonged HFD-induced obesity is associated with selective changes in Agrp neuron biology, possibly secondary to altered CREB signaling.

Activation of a Classic Hunger Circuit Slows Luteinizing Hormone Pulsatility

INTRODUCTION

The central regulation of fertility is carefully coordinated with energy homeostasis, and infertility is frequently the outcome of energy imbalance. Neurons in the hypothalamus expressing neuropeptide Y and agouti-related peptide (NPY/AgRP neurons) are strongly implicated in linking metabolic cues with fertility regulation.

OBJECTIVE

We aimed here to determine the impact of selectively activating NPY/AgRP neurons, critical regulators of metabolism, on the activity of luteinizing hormone (LH) pulse generation.

METHODS

We employed a suite of in vivo optogenetic and chemogenetic approaches with serial measurements of LH to determine the impact of selectively activating NPY/AgRP neurons on dynamic LH secretion. In addition, electrophysiological studies in ex vivo brain slices were employed to ascertain the functional impact of activating NPY/AgRP neurons on gonadotropin-releasing hormone (GnRH) neurons.

RESULTS

Selective activation of NPY/AgRP neurons significantly decreased post-castration LH secretion. This was observed in males and females, as well as in prenatally androgenized females that recapitulate the persistently elevated LH pulse frequency characteristic of polycystic ovary syndrome (PCOS). Reduced LH pulse frequency was also observed when optogenetic stimulation was restricted to NPY/AgRP fiber projections surrounding GnRH neuron cell bodies in the rostral preoptic area. However, electrophysiological studies in ex vivo brain slices indicated these effects were likely to be indirect.

CONCLUSIONS

These data demonstrate the ability of NPY/AgRP neuronal signaling to modulate and, specifically, reduce GnRH/LH pulse generation. The findings suggest a mechanism by which increased activity of this hunger circuit, in response to negative energy balance, mediates impaired fertility in otherwise reproductively fit states, and highlight a potential mechanism to slow LH pulsatility in female infertility disorders, such as PCOS, that are associated with hyperactive LH secretion.

PI3K signalling in leptin receptor cells: Role in growth and reproduction

Nutrition and growth are important signals for pubertal development, although how they are perceived and integrated in brain circuits has not been well defined. Growth hormones and metabolic cues both recruit phosphatidylinositol 3-kinase (PI3K) signalling in hypothalamic sites, although whether they converge into the same neuronal population(s) is also not known. In this review, we discuss recent findings from our laboratory showing the role of PI3K subunits in cells directly responsive to the adipocyte-derived hormone leptin in the coordination of growth, pubertal development and fertility. Mice with deletion of PI3K p110α and p110β catalytic subunits in leptin receptor cells (LR^Δα+β ) have a lean phenotype associated with increased energy expenditure, locomotor activity and thermogenesis. The LR^Δα+β mice also show deficient growth and delayed puberty. Deletion of a single subunit (ie, p110α) in LR cells (LR^Δα ) causes a similar phenotype of increased energy expenditure, deficient growth and delayed pubertal development, indicating that these functions are preferably controlled by p110α. The LR^Δα mice show enhanced leptin sensitivity in metabolic regulation but, remarkably, these mice are unresponsive to the effects of leptin on growth and puberty. PI3K is also recruited by insulin and a subpopulation of LR neurones is responsive to i.c.v. insulin administration. Deletion of insulin receptor in LR cells causes no changes in body weight or linear growth and induces only a mild delay in pubertal completion. Our findings demonstrate that PI3K in LR cells plays an essential role in growth and reproduction. We will also discuss the potential neural pathways underlying these effects.

Neuroanatomical Framework of the Metabolic Control of Reproduction

A minimum amount of energy is required for basic physiological processes, such as protein biosynthesis, thermoregulation, locomotion, cardiovascular function, and digestion. However, for reproductive function and survival of the species, extra energy stores are necessary. Production of sex hormones and gametes, pubertal development, pregnancy, lactation, and parental care all require energy reserves. Thus the physiological systems that control energy homeostasis and reproductive function coevolved in mammals to support both individual health and species subsistence. In this review, we aim to gather scientific knowledge produced by laboratories around the world on the role of the brain in integrating metabolism and reproduction. We describe essential neuronal networks, highlighting key nodes and potential downstream targets. Novel animal models and genetic tools have produced substantial advances, but critical gaps remain. In times of soaring worldwide obesity and metabolic dysfunction, understanding the mechanisms by which metabolic stress alters reproductive physiology has become crucial for human health.

Hypothalamic Dysfunction in Obesity and Metabolic Disorders

The hypothalamus is the brain region responsible for the maintenance of energetic homeostasis. The regulation of this process arises from the ability of the hypothalamus to orchestrate complex physiological responses such as food intake and energy expenditure, circadian rhythm, stress response, and fertility. Metabolic alterations such as obesity can compromise these hypothalamic regulatory functions. Alterations in circadian rhythm, stress response, and fertility further contribute to aggravate the metabolic dysfunction of obesity and contribute to the development of chronic disorders such as depression and infertility.At cellular level, obesity caused by overnutrition can damage the hypothalamus promoting inflammation and impairing hypothalamic neurogenesis. Furthermore, hypothalamic neurons suffer apoptosis and impairment in synaptic plasticity that can compromise the proper functioning of the hypothalamus. Several factors contribute to these phenomena such as ER stress, oxidative stress, and impairments in autophagy. All these observations occur at the same time and it is still difficult to discern whether inflammatory processes are the main drivers of these cellular dysfunctions or if the hypothalamic hormone resistance (insulin, leptin, and ghrelin) can be pinpointed as the source of several of these events.Understanding the mechanisms that underlie the pathophysiology of obesity in the hypothalamus is crucial for the development of strategies that can prevent or attenuate the deleterious effects of obesity.

Regulation of Hepatic Lipid Accumulation and Distribution by Agouti-Related Protein in Male Mice

Proper regulation of energy metabolism requires neurons in the central nervous system to respond dynamically to signals that reflect the body's energy reserve, and one such signal is leptin. Agouti-related protein (AgRP) is a hypothalamic neuropeptide that is markedly upregulated in leptin deficiency, a condition that is associated with severe obesity, diabetes, and hepatic steatosis. Because deleting AgRP in mice does not alter energy balance, we sought to determine whether AgRP plays an indispensable role in regulating energy and hepatic lipid metabolism in the sensitized background of leptin deficiency. We generated male mice that are deficient for both leptin and AgRP [double-knockout (DKO)]. DKO mice and ob/ob littermates had similar body weights, food intake, energy expenditure, and plasma insulin levels, although DKO mice surprisingly developed heightened hyperglycemia with advancing age. Overall hepatic lipid content was reduced in young prediabetic DKO mice, but not in the older diabetic counterparts. Intriguingly, however, both young and older DKO mice had an altered zonal distribution of hepatic lipids with reduced periportal lipid deposition. Moreover, leptin stimulated, whereas AgRP inhibited, hepatic sympathetic activity. Ablating sympathetic nerves to the liver, which primarily innervate the portal regions, produced periportal lipid accumulation in wild-type mice. Collectively, our results highlight AgRP as a regulator of hepatic sympathetic activity and metabolic zonation.

Long-term consequences of obesity on female fertility and the health of the offspring

PURPOSE OF REVIEW

Obesity has reached near epidemic levels among reproductive age women with a myriad of consequences. Obesity adversely affects the maternal milieu by creating conditions that decrease fertility and increase the risk of gestational diabetes, hypertensive disease in pregnancy, fetal growth abnormalities and congenital anomalies. The effects of obesity are not limited to pregnancy. Indeed, beyond the immediate postpartum period, obese women maintain a higher prevalence of insulin resistance and cardiovascular disease. In this article, we will review the pathophysiology underlying the effects of obesity on fertility, pregnancy outcome and health status of offspring. The purpose of this review is to outline proposed models responsible for the short-term and long-term consequences of obesity on fertility and offspring development, and identify knowledge gaps where additional research is needed.

RECENT FINDINGS

Maternal over or under nutrition adversely affect maternal reproductive capacity and pregnancy success. Separate from effects on maternal reproductive function, maternal over or under nutrition may also 'program' fetal pathophysiology through inheritance mechanisms that suggest epigenetic modification of DNA, differential RNA translation and protein expression, or modification of the fetal hypothalamic-pituitary axis function through programmed adverse effects on the developing hypothalamic circuitry. The concept of maternal health modifying the risk of developing noncommunicable diseases in the offspring is based on Developmental Origins of Health and Disease hypothesis.

SUMMARY

Of importance, the long-term effects of obesity are not limited to maternal health, but also programs pathophysiology in their offspring. Children of obese gravida are at increased risk for the development of cardiometabolic disease in childhood and throughout adulthood. Future studies directly interrogating mechanisms underlying the risks associated with obesity will allow us to develop interventions and therapies to decrease short-term and long-term morbidities associated with maternal obesity.

Connecting metabolism and gonadal function: Novel central neuropeptide pathways involved in the metabolic control of puberty and fertility

Albeit essential for perpetuation of species, reproduction is an energy-demanding function that can be adjusted to body metabolic status. Reproductive maturation and function can be suppressed in conditions of energy deficit, but can be altered also in situations of persistent energy excess, e.g., morbid obesity. This metabolic-reproductive integration, of considerable pathophysiological relevance to explain different forms of perturbed puberty and sub/infertility, is implemented by the concerted action of numerous central and peripheral regulators, which impinge at different levels of the hypothalamic-pituitary-gonadal (HPG) axis, permitting a tight fit between nutritional/energy status and gonadal function. We summarize here the major physiological mechanisms whereby nutritional and metabolic cues modulate the maturation and function of the HPG axis. We will focus on recent progress on the major central neuropeptide pathways, including kisspeptins, neurokinin B and the products of POMC and NPY neurons, which convey metabolic information to GnRH neurons, as major hierarchical hub of our reproductive brain.

Kisspeptin and Metabolism: The Brain and Beyond

Apart from the well-established role of kisspeptin (Kp) in the regulation of reproductive functions, recent data described its action in the control of metabolism. Of particular interest for the review is the population of Kp neurons localized in the arcuate nucleus (ARC) of the hypothalamus, the site of the brain where reproductive and metabolic cross talk occurs. However, within the hypothalamus Kp does not work alone, but rather interacts with other neuropeptides, e.g., neurokinin B, dynorphin A, proopiomelanocortin, the cocaine- and amphetamine-regulated transcript, agouti-related peptide, and neuropeptide Y. Beyond the brain, Kp is expressed in peripheral tissues involved in metabolic functions. In this review, we will mainly focus on the local action of this peptide in peripheral organs such as the pancreas, liver, and the adipose tissue. We will concentrate on dysregulation of the Kp system in cases of metabolic imbalance, e.g., obesity and diabetes. Importantly, these patients besides metabolic health problems often suffer from disruptions of the reproductive system, manifested by abnormalities in menstrual cycles, premature child birth, miscarriages in women, decreased testosterone levels and spermatogenesis in men, hypogonadism, and infertility. We will review the evidence from animal models and clinical data indicating that Kp could serve as a promising agent with clinical applications in regulation of reproductive problems in individuals with obesity and diabetes. Finally, emerging data indicate a role of Kp in regulation of insulin secretion, potentially leading to development of further therapeutic uses of this peptide to treat metabolic problems in patients with these lifestyle diseases.

PI3Kα inactivation in leptin receptor cells increases leptin sensitivity but disrupts growth and reproduction

The role of PI3K in leptin physiology has been difficult to determine due to its actions downstream of several metabolic cues, including insulin. Here, we used a series of mouse models to dissociate the roles of specific PI3K catalytic subunits and of insulin receptor (InsR) downstream of leptin signaling. We show that disruption of p110α and p110β subunits in leptin receptor cells (LRΔα+β) produces a lean phenotype associated with increased energy expenditure, locomotor activity, and thermogenesis. LRΔα+β mice have deficient growth and delayed puberty. Single subunit deletion (i.e., p110α in LRΔα) resulted in similarly increased energy expenditure, deficient growth, and pubertal development, but LRΔα mice have normal locomotor activity and thermogenesis. Blunted PI3K in leptin receptor (LR) cells enhanced leptin sensitivity in metabolic regulation due to increased basal hypothalamic pAKT, leptin-induced pSTAT3, and decreased PTEN levels. However, these mice are unresponsive to leptin's effects on growth and puberty. We further assessed if these phenotypes were associated with disruption of insulin signaling. LRΔInsR mice have no metabolic or growth deficit and show only mild delay in pubertal completion. Our findings demonstrate that PI3K in LR cells plays an essential role in energy expenditure, growth, and reproduction. These actions are independent from insulin signaling.

The role of leptin in health and disease

Leptin is a master regulator of energy balance and body adiposity. Additionally, leptin exerts important control on glucose homeostasis, thermogenesis, autonomic nervous system and neuroendocrine axes. In metabolic diseases, such as obesity and diabetes mellitus, leptin signaling may be compromised, indicating the important role of this hormone in the etiology and pathophysiological manifestations of these conditions. In the present manuscript, we reviewed important concepts of leptin signaling, as well as about the effects of leptin on several biologic functions. We also discussed the possible therapeutic use of leptin administration and how our current obesogenic environment contributes to the development of leptin resistance. Our objective was to provide a comprehensive and state-of-the-art review about the importance of leptin to maintain the homeostasis and during pathological conditions.

Intracerebroventricular injection of RFRP-3 delays puberty onset and stimulates growth hormone secretion in female rats

BACKGROUND

Puberty onset is a complex, organized biological process with multilevel regulation, and its physiopathological mechanisms are yet to be elucidated. RFRP-3, the mammalian ortholog to gonadotropin-inhibitory hormone, is implicated in inhibiting the synthesis and release of gonadotropin in mammals. However, it is unclear whether RFRP-3 participates in regulating pubertal development.

METHODS

This study investigated the functional significance and regulatory mechanism of hypothalamic RFRP-3 neuropeptide in the onset of puberty in young female rats. On postnatal day 22, we implanted cannulas into the lateral ventricles of female rat pups. From postnatal day 28 to postnatal day 36, the intracerebroventricular injection of RFRP-3, or vehicle, was conducted twice a day. To investigate whether puberty onset was affected, we examined the body weight, age of vaginal opening, serum hormone levels, uterus and ovary development, and hypothalamic Kiss-1 mRNA expression.

RESULTS

Intracerebroventricular injection of RFRP-3 significantly decreased the serum concentrations of luteinizing hormone and estradiol, delayed uterine maturation, and postponed the time of vaginal opening. This study suggests that RFRP-3 can delay the onset of puberty in young female rats; the expression of Kiss-1 mRNA is potently inhibited in the RFRP-3 group. Moreover, our data show that RFRP-3 elevates serum growth hormone levels.

CONCLUSIONS

These data suggest that intracerebroventricular injection of RFRP-3 significantly delays the onset of puberty in female rats. Additionally, RFRP-3 may be associated with prepubertal rise in the secretion of growth hormone.

Leptin Signaling in AgRP Neurons Modulates Puberty Onset and Adult Fertility in Mice

The hormone leptin indirectly communicates metabolic information to brain neurons that control reproduction, using GABAergic circuitry. Agouti-related peptide (AgRP) neurons in the arcuate nucleus are GABAergic, express leptin receptors (LepR), and are known to influence reproduction. This study tested whether leptin actions on AgRP neurons are required and sufficient for puberty onset and subsequent fertility. First, Agrp-Cre and Lepr-flox mice were used to target deletion of LepR to AgRP neurons. AgRP-LepR knock-out female mice exhibited mild obesity and adiposity as described previously, as well as a significant delay in the pubertal onset of estrous cycles compared with control animals. No significant differences in male puberty onset or adult fecundity in either sex were observed. Next, mice with a floxed polyadenylation signal causing premature transcriptional termination of the Lepr gene were crossed with AgRP-Cre mice to generate mice with AgRP neuron-specific rescue of LepR. Lepr-null control males and females were morbidly obese and exhibited delayed puberty onset, no evidence of estrous cycles, and minimal fecundity. Remarkably, AgRP-LepR rescue partially or fully restored all of these reproductive attributes to levels similar to those of LepR-intact controls despite minimal rescue of metabolic function. These results indicate that leptin signaling in AgRP neurons is sufficient for puberty onset and normal adult fecundity in both sexes when leptin signaling is absent in all other cells and that in females, the absence of AgRP neuron leptin signaling delays puberty. These actions appear to be independent of leptin's metabolic effects.SIGNIFICANCE STATEMENT Sexual maturation and fertility are dispensable at the individual level but critical for species survival. Conditions such as nutritional imbalance may therefore suppress puberty onset and fertility in an individual. In societies characterized by widespread obesity, the sensitivity of reproduction to metabolic imbalance has significant public health implications. Deficient leptin signaling attributable to diet-induced leptin resistance is associated with infertility in humans and rodents, and treatments for human infertility show a decreased success rate with increasing body mass index. Here we show that the transmission of metabolic information to the hypothalamo-pituitary-gonadal axis is mediated by leptin receptors on AgRP neurons. These results provide conclusive new insights into the mechanisms that cause infertility attributable to malnourishment.

Neuroendocrine integration of nutritional signals on reproduction

Reproductive function in mammals is energetically costly and therefore tightly regulated by nutritional status. To enable this integration of metabolic and reproductive function, information regarding peripheral nutritional status must be relayed centrally to the gonadotropin-releasing hormone (GNRH) neurons that drive reproductive function. The metabolically relevant hormones leptin, insulin and ghrelin have been identified as key mediators of this 'metabolic control of fertility'. However, the neural circuitry through which they act to exert their control over GNRH drive remains incompletely understood. With the advent of Cre-LoxP technology, it has become possible to perform targeted gene-deletion and gene-rescue experiments and thus test the functional requirement and sufficiency, respectively, of discrete hormone-neuron signaling pathways in the metabolic control of reproductive function. This review discusses the findings from these investigations, and attempts to put them in context with what is known from clinical situations and wild-type animal models. What emerges from this discussion is clear evidence that the integration of nutritional signals on reproduction is complex and highly redundant, and therefore, surprisingly difficult to perturb. Consequently, the deletion of individual hormone-neuron signaling pathways often fails to cause reproductive phenotypes, despite strong evidence that the targeted pathway plays a role under normal physiological conditions. Although transgenic studies rarely reveal a critical role for discrete signaling pathways, they nevertheless prove to be a good strategy for identifying whether a targeted pathway is absolutely required, critically involved, sufficient or dispensable in the metabolic control of fertility.

Effects of Orchidectomy and Testosterone Replacement on Numbers of Kisspeptin-, Neurokinin B-, and Dynorphin A-Immunoreactive Neurones in the Arcuate Nucleus of the Hypothalamus in Obese and Diabetic Rats

Neurones expressing kisspeptin, neurokinin B and dynorphin A, located in the arcuate nucleus of the hypothalamus (ARC), are important regulators of reproduction. Their functions depend on metabolic and hormonal status. We hypothesised that male rats with high-fat diet-induced obesity (DIO) and/or streptozotocin-induced diabetes mellitus type 1 (DM1) and type 2 (DM2) will have alterations in numbers of immunoreactive (-IR) cells: kisspeptin-IR and/or neurokinin B-IR and dynorphin A-IR neurones in the ARC in the sham condition. In addition, orchidectomy alone (ORX) and with testosterone treatment (ORX+T) will unmask possible deficits in the response of these neurones in DIO, and/or DM1 and DM2 rats. Rats were assigned to four groups: a control (C) and one diabetic group (DM1) were fed a regular chow diet, whereas the obese group (DIO) and the other diabetic group (DM2) were fed a high-fat diet. To induce diabetes, streptozotocin was injected. After 6 weeks, each group was divided into three subgroups: ORX, ORX+T and sham. After another 2 weeks, metabolic and hormonal profiles were assessed and immunocytochemistry was performed. We found that: (1) under sham conditions: (i) DM1 and DM2 animals had higher numbers of kisspeptin-IR cells than controls and (ii) DM2 rats had increased numbers of neurokinin B-IR and dynorphin A-IR cells compared to C animals; (2) ORX and ORX+T treatments unmasked deficits of the studied neurones in DM1 and DM2 but not in DIO animals; and (3) DIO, DM1 and DM2 rats had altered metabolic and hormonal profiles, in particular decreased levels of testosterone. We concluded that alterations in numbers of kisspeptin-IR and neurokinin B-IR neurones in the ARC and their response to ORX and ORX+T may account for disruptions of metabolic and reproductive functions in diabetic but not in obese rats.

Obese Neuronal PPARγ Knockout Mice Are Leptin Sensitive but Show Impaired Glucose Tolerance and Fertility

The peroxisome-proliferator activated receptor γ (PPARγ) is expressed in the hypothalamus in areas involved in energy homeostasis and glucose metabolism. In this study, we created a deletion of PPARγ brain-knockout (BKO) in mature neurons in female mice to investigate its involvement in metabolism and reproduction. We observed that there was no difference in age at puberty onset between female BKOs and littermate controls, but the BKOs gave smaller litters when mated and fewer oocytes when ovulated. The female BKO mice had regular cycles but showed an increase in the number of cycles with prolonged estrus. The mice also had increased luteinizing hormone (LH) levels during the LH surge and histological examination showed hemorrhagic corpora lutea. The mice were challenged with a 60% high-fat diet (HFD). Metabolically, the female BKO mice showed normal body weight, glucose and insulin tolerance, and leptin levels but were protected from obesity-induced leptin resistance. The neuronal knockout also prevented the reduction in estrous cycles due to the HFD. Examination of ovarian histology showed a decrease in the number of primary and secondary follicles in both genotypes due to the HFD, but the BKO ovaries showed an increase in the number of hemorrhagic follicles. In summary, our results show that neuronal PPARγ is required for optimal female fertility but is also involved in the adverse effects of diet-induced obesity by creating leptin resistance potentially through induction of the repressor Socs3.

Central leptin action on euglycemia restoration in type 1 diabetes: Restraining responses normally induced by fasting?

Leptin monotherapy is sufficient to restore euglycemia in insulinopenic type 1 diabetes (T1D), yet the underlying mechanism remains poorly understood. Accumulating evidence demonstrates that the brain mediates the leptin action on euglycemia restoration. Here, we first review evidence supporting that symptoms in T1D resemble an uncontrolled response to fasting. Then, we discuss recent research progress on brain neurons and their neurotransmitters that potentially mediate the leptin action. Finally, peripheral effective pathways, which are normally involved in fasting responses and associated with leptin action on euglycemia restoration in T1D, will also be discussed. This summary complements several previous excellent reviews on this topic (Meek and Morton, 2016; Perry et al., 2016; Fujikawa and Coppari, 2015). A deep understanding of neurocircuitry and the peripheral effective pathways that mediate the leptin action on euglycemia restoration will likely lead to novel targets for an insulin-independent therapeutics against T1D.

Hypothesis: Irisin is a metabolic trigger for the activation of the neurohormonal axis governing puberty onset

A large body of data suggests that body weight influences puberty onset and adult reproduction. However, the underlying mechanism of how body weight influences puberty onset and fertility is not completely understood. The hypothalamic neuronal circuit regulating reproduction is restrained by inhibitory signals during childhood. At the time of puberty, these inhibitory signals are weakened and supplanted by stimulatory signals that, in turn, stimulate the release of gonadotropin-releasing hormone (GnRH) - a hypothalamic neuropeptide governing reproduction. A number of studies, however, suggest that puberty commencement occurs when body (fat) weight reaches a certain threshold, which is critical for the initiation of puberty and for support of the adult reproductive function. Previously, various signals have been studied which might link body (fat) weight-related information to the hypothalamic neuronal network regulating reproduction. However, the nature of the signal(s) that may link body fat and/or muscle mass with the hypothalamic neuronal network governing reproduction is still unclear. It has been intuitively speculated that augmentation of such signal(s) will cause a restriction of inhibitory input and activation of stimulatory input to GnRH secreting neurons at the time of puberty onset. Therefore, the unveiling of such signal(s) will greatly help in understanding the mechanism of puberty onset. Recently, it has been shown that expression of fibronectin type III domain containing-5 (FNDC5) mRNA in central and peripheral tissues upsurges during postnatal development, especially around the time of puberty onset. Moreover, the systemic level of irisin - one of the protein products of the FNDC5 gene that is secreted as myokine and adipokine - also rises during postnatal development and correlates with the timing of puberty onset. Therefore, we propose here that irisin might serve as a possible signal for linking body fat/muscle mass with the hypothalamic center governing reproductive function. We hypothesize that irisin acts as a trigger for the activation of the hypothalamic neuronal network monitoring the onset of puberty.

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.

New concepts of the central control of reproduction, integrating influence of stress, metabolic state, and season

Gonadotropin releasing hormone is the primary driver of reproductive function and pulsatile GnRH secretion from the brain causes the synthesis and secretion of LH and FSH from the pituitary gland. Recent work has revealed that the secretion of GnRH is controlled at the level of the GnRH secretory terminals in the median eminence. At this level, projections of kisspeptin cells from the arcuate nucleus of the hypothalamus are seen to be closely associated with fibers and terminals of GnRH cells. Direct application of kisspeptin into the median eminence causes release of GnRH. The kisspeptin cells are activated at the time of a natural "pulse" secretion of GnRH, as reflected in the secretion of LH. This appears to be due to input to the kisspeptin cells from glutamatergic cells in the basal hypothalamus, indicating that more than 1 neural element is involved in the secretion of GnRH. Because the GnRH secretory terminals are outside the blood-brain barrier, factors such as kisspeptin may be administered systemically to cause GnRH secretion; this offers opportunities for manipulation of the reproductive axis using factors that do not cross the blood-brain barrier. In particular, kisspeptin or analogs of the same may be used to activate reproduction in the nonbreeding season of domestic animals. Another brain peptide that influences reproductive function is gonadotropin inhibitory hormone (GnIH). Work in sheep shows that this peptide acts on GnRH neuronal perikarya, but projections to the median eminence also allow secretion into the hypophysial portal blood and action of GnIH on pituitary gonadotropes. GnIH cells are upregulated in anestrus, and infusion of GnIH can block the ovulatory surge in GnRH and/or LH secretion. Metabolic status may also affect the secretion of reproduction, and this could involve action of gut peptides and leptin. Neuropeptide Y and Y-receptor ligands have a negative impact on reproduction, and Neuropeptide Y production is markedly increased in negative energy balance; this may be the cause of lowered GnRH and gonadotropin secretion in this state. There is a complex interaction between appetite-regulating peptide neurons and kisspeptin neurons that enables the former to regulate the latter both positively and negatively. In terms of how GnRH secretion is reduced during stress, recent data indicate that GnIH cells are integrally involved, with increased input to the GnRH cells. The secretion of GnIH into the portal blood is not increased during stress, so the negative effect is most likely effected at the level of GnRH neuronal cell bodies.

Regulation of arcuate genes by developmental exposures to endocrine-disrupting compounds in female rats

Developmental exposure to endocrine-disrupting compounds (EDCs) alters reproduction and energy homeostasis, both of which are regulated by the arcuate nucleus (ARC). Little is known about the effects of EDC on ARC gene expression. In Experiment #1, pregnant dams were treated with either two doses of bisphenol A (BPA) or oil from embryonic day (E)18-21. Neonates were injected from postnatal day (PND)0-7. Vaginal opening, body weights, and ARC gene expression were measured. Chrm3 (muscarinic receptor 3) and Adipor1 (adiponectin receptor 1) were decreased by BPA. Bdnf (brain-derived neurotropic factor), Igf1 (insulin-like growth factor 1), Htr2c (5-hydroxytryptamine receptor), and Cck2r (cholescystokinin 2 receptor) were impacted. In Experiment #2, females were exposed to BPA, diethylstilbestrol (DES), di(2-ethylhexyl)phthalate, or methoxychlor (MXC) during E11-PND7. MXC and DES advanced the age of vaginal opening and ARC gene expression was impacted. These data indicate that EDCs alter ARC genes involved in reproduction and energy homeostasis in females.

Fatness rather than leptin sensitivity determines the timing of puberty in female mice

Leptin is a permissive factor for the onset of puberty. However, changes in adiposity frequently influence leptin sensitivity. Thus, the objective of the present study was to investigate how changes in body weight, fatness, leptin levels and leptin sensitivity interact to control the timing of puberty in female mice. Pre-pubertal obesity, induced by raising C57BL/6 mice in small litters, led to an early puberty onset. Inactivation of Socs3 gene in the brain or exclusively in leptin receptor-expressing cells reduced the body weight and leptin levels at pubertal onset, and increased leptin sensitivity. Notably, these female mice exhibited significant delays in vaginal opening, first estrus and onset of estrus cyclicity. In conclusion, our findings suggest that increased leptin sensitivity did not play an important role in favoring pubertal onset in female mice. Rather, changes in pubertal body weight, fatness and/or leptin levels were more important in influencing the timing of puberty.

Differential gene regulation of GHSR signaling pathway in the arcuate nucleus and NPY neurons by fasting, diet-induced obesity, and 17β-estradiol

Ghrelin's receptor, growth hormone secretagogue receptor (GHSR), is highly expressed in the arcuate nucleus (ARC) and in neuropeptide Y (NPY) neurons. Fasting, diet-induced obesity (DIO), and 17β-estradiol (E2) influence ARC Ghsr expression. It is unknown if these effects occur in NPY neurons. Therefore, we examined the expression of Npy, Agrp, and GHSR signaling pathway genes after fasting, DIO, and E2 replacement in ARC and pools of NPY neurons. In males, fasting increased ARC Ghsr and NPY Foxo1 but decreased NPY Ucp2. In males, DIO decreased ARC and NPY Ghsr and Cpt1c. In fed females, E2 increased Agrp, Ghsr, Cpt1c, and Foxo1 in ARC. In NPY pools, E2 decreased Foxo1 in fed females but increased Foxo1 in fasted females. DIO in females suppressed Agrp and augmented Cpt1c in NPY neurons. In summary, genes involved in GHSR signaling are differentially regulated between the ARC and NPY neurons in a sex-dependent manner.

Altered Gene Expression Profiles of the Hypothalamic Arcuate Nucleus of Male Mice Suggest Profound Developmental Changes in Peptidergic Signaling

Neuropeptides of the hypothalamic arcuate nucleus (ARC) regulate important homeostatic and endocrine functions and also play critical roles in pubertal development. The altered peptidergic and aminoacidergic neurotransmission accompanying pubertal maturation of the ARC is not fully understood. Here we studied the developmental shift in the gene expression profile of the ARC of male mice. RNA samples for quantitative RT-PCR studies were isolated from the ARC of 14-day-old infantile and 60-day-old adult male mice with laser capture microdissection. The expression of 18 neuropeptide, 15 neuropeptide receptor, 4 sex steroid receptor and 6 classic neurotransmitter marker mRNAs was compared between the two time points. The adult animals showed increased mRNA levels encoding cocaine- and amphetamine-regulated transcripts, galanin-like peptide, dynorphin, kisspeptin, proopiomelanocortin, proenkephalin and galanin and a reduced expression of mRNAs for pituitary adenylate cyclase-activating peptide, calcitonin gene-related peptide, neuropeptide Y, substance P, agouti-related protein, neurotensin and growth hormone-releasing hormone. From the neuropeptide receptors tested, melanocortin receptor-4 showed the most striking increase (5-fold). Melanocortin receptor-3 and the Y1 and Y5 neuropeptide Y receptors increased 1.5- to 1.8-fold, whereas δ-opioid receptor and neurotensin receptor-1 transcripts were reduced by 27 and 21%, respectively. Androgen receptor, progesterone receptor and α-estrogen receptor transcripts increased by 54-72%. The mRNAs of glutamic acid decarboxylases-65 and -67, vesicular GABA transporter and choline acetyltransferase remained unchanged. Tyrosine hydroxylase mRNA increased by 44%, whereas type-2 vesicular glutamate transporter mRNA decreased by 43% by adulthood. Many of the developmental changes we revealed in this study suggest a reduced inhibitory and/or enhanced excitatory neuropeptidergic drive on fertility in adult animals.

Peptides: Basic determinants of reproductive functions

Mammalian reproduction is a costly process in terms of energy consumption. The critical information regarding metabolic status is signaled to the hypothalamus mainly through peripheral peptides from the adipose tissue and gastrointestinal tract. Changes in energy stores produce fluctuations in leptin, insulin, ghrelin and glucose signals that feedback mainly to the hypothalamus to regulate metabolism and fertility. In near future, possible effects of the nutritional status on GnRH regulation can be evaluated by measuring serum or tissue levels of leptin and ghrelin in patiens suffering from infertility. The fact that leptin and ghrelin are antagonistic in their effects on GnRH neurons, their respective agonistic and antagonistic roles make them ideal candidates to use instead of GnRH agonist and antagonist. Similarly, kisspeptin expressing neurons are likely to mediate the well-established link between energy balance and reproductive functions. Exogenous kisspeptin can be used for physiological ovarian hyperstimulation for in-vitro fertilization. Moreover, kisspeptin antagonist therapy can be used for the treatment of postmenapousal women, precocious puberty, PCOS, endometriosis and uterine fibroids. In this review, we will analyze the central mechanisms involved in the integration of metabolic information and their contribution to the control of the reproductive function. Particular attention will be paid to summarize the participation of leptin, kisspeptin, ghrelin, NPY, orexin, urocortin, VIP, insulin, galanin, galanin like peptide, oxytocin, agouti gene-related peptide, and POMC neurons in this process and their possible interactions to contribute to the metabolic control of reproduction.

Understanding how discrete populations of hypothalamic neurons orchestrate complicated behavioral states

A major question in systems neuroscience is how a single population of neurons can interact with the rest of the brain to orchestrate complex behavioral states. The hypothalamus contains many such discrete neuronal populations that individually regulate arousal, feeding, and drinking. For example, hypothalamic neurons that express hypocretin (Hcrt) neuropeptides can sense homeostatic and metabolic factors affecting wakefulness and orchestrate organismal arousal. Neurons that express agouti-related protein (AgRP) can sense the metabolic needs of the body and orchestrate a state of hunger. The organum vasculosum of the lamina terminalis (OVLT) can detect the hypertonicity of blood and orchestrate a state of thirst. Each hypothalamic population is sufficient to generate complicated behavioral states through the combined efforts of distinct efferent projections. The principal challenge to understanding these brain systems is therefore to determine the individual roles of each downstream projection for each behavioral state. In recent years, the development and application of temporally precise, genetically encoded tools has greatly improved our understanding of the structure and function of these neural systems. This review will survey recent advances in our understanding of how these individual hypothalamic populations can orchestrate complicated behavioral states due to the combined efforts of individual downstream projections.

Role of the adipocyte-derived hormone leptin in reproductive control

Achievement of sexual maturation and maintenance of fertility in adulthood are functions that are sensitive to the metabolic status of the organism, particularly the magnitude of fat reserves. In this sense, the adipocyte-derived hormone, leptin, plays a major role in linking metabolic cues and the control of multiple neuroendocrine axes. The hypothalamus is a key site mediating leptin actions, including those involved in the modulation of the hypothalamus-pituitary-gonads (HPG) axis at different stages of development and in different environmental conditions. In the present review, we provide an update of the role of leptin in reproduction and discuss its interactions with neurons, neurotransmitters and downstream targets of the reproductive axis, with a special emphasis on the actions of leptin in the central nervous system. We hope this review will contribute to the understanding of the mechanisms whereby metabolic signals, especially leptin, influence the reproductive neuroendocrine axis modulating its activity in different nutritional states. Special attention will be given to recent advances in the identification of key hypothalamic sites and signaling pathways relevant to leptin's action in reproductive control.

Neuronal basis of reproductive dysfunctions associated with diet and alcohol: From the womb to adulthood

The theory that individuals are born as tabula rasa and that their knowledge comes from experience and perception is no longer true. Studies suggest that experience is gained as early as in the mother's womb. Moreover, environmental stressors like alcohol or inadequate diet can affect physiological systems such as the hypothalmic-pituitary-gonadal (HPG) axis. The effects of these stressors can manifest as alterations in sexual development and adult reproductive functions. In this review, we consider and compare evidence from animal models and human studies demonstrating the role of environmental stressors (alcohol and under- or overnutrition) on the HPG axis. We review the role of alcohol and inadequate diet in prenatal reproductive system programming and consider specific candidate neurons in the adult hypothalamus through which reproductive function is being regulated. Finally, we review evidence from animal studies on the role that alcohol and diet play in fertility and reproductive disorders. We conclude that in order to better understand reproductive failure in animals and humans we need to consider in utero development and pay more attention to early life experience when searching for the origins of reproductive diseases.

Kisspeptin cell-specific PI3K signaling regulates hypothalamic kisspeptin expression and participates in the regulation of female fertility

Hypothalamic kisspeptin neurons integrate and translate cues from the internal and external environments that regulate gonadotropin-releasing hormone (GnRH) secretion and maintain fertility in mammals. However, the intracellular signaling pathways utilized to translate such information into changes in kisspeptin expression, release, and ultimately activation of the kisspeptin-receptive GnRH network have not yet been identified. PI3K is an important signaling node common to many peripheral factors known to regulate kisspeptin expression and GnRH release. We investigated whether PI3K signaling regulates hypothalamic kisspeptin expression, pubertal development, and adult fertility in mice. We generated mice with a kisspeptin cell-specific deletion of the PI3K catalytic subunits p110α and p110β (kiss-p110α/β-KO). Using in situ hybridization, we examined Kiss1 mRNA expression in gonad-intact, gonadectomized (Gdx), and Gdx + steroid-replaced mice. Kiss1 cell number in the anteroventral periventricular hypothalamus (AVPV) was significantly reduced in intact females but not in males. In contrast, compared with WT and regardless of steroid hormone status, Kiss1 cell number was lower in the arcuate (ARC) of kiss-p110α/β-KO males, but it was unaffected in females. Both intact Kiss-p110α/β-KO males and females had reduced ARC kisspeptin-immunoreactive (IR) fibers compared with WT animals. Adult kiss-p110α/β-KO males had significantly lower circulating luteinizing hormone (LH) levels, whereas pubertal development and fertility were unaffected in males. Kiss-p110α/β-KO females exhibited a reduction in fertility despite normal pubertal development, LH levels, and estrous cyclicity. Our data show that PI3K signaling is important for the regulation of hypothalamic kisspeptin expression and contributes to normal fertility in females.

20 years of leptin: leptin and reproduction: past milestones, present undertakings, and future endeavors

The association between leptin and reproduction originated with the leptin-mediated correction of sterility in ob/ob mice and initiation of reproductive function in normal female mice. The uncovering of a central leptin pathway regulating food intake prompted the dissection of neuroendocrine mechanisms involving leptin in the metabolic control of reproduction. The absence of leptin receptors on GnRH neurons incited a search for intermediary neurons situated between leptin-responsive and GnRH neurons. This review addresses the most significant findings that have furthered our understanding of recent progress in this new field. The role of leptin in puberty was impacted by the discovery of neurons that co-express kisspeptin, neurokinin B, and dynorphin and these could act as leptin intermediates. Furthermore, the identification of first-order leptin-responsive neurons in the premammilary ventral nucleus and other brain regions opens new avenues to explore their relationship to GnRH neurons. Central to these advances is the unveiling that agouti-related protein/neuropeptide Y neurons project onto GnRH and kisspeptin neurons, allowing for a crosstalk between food intake and reproduction. Finally, while puberty is a state of leptin sensitivity, mid-gestation represents a state of leptin resistance aimed at building energy stores to sustain pregnancy and lactation. The mechanisms underlying leptin resistance in pregnancy have lagged; however, the establishment of this natural state is significant. Reproduction and energy balance are tightly controlled and backed up by redundant mechanisms that are critical for the survival of our species. It will be the goal of the following decade to shed new light on these complex and essential pathways.

Exogenous leptin administered intramuscularly induces sex hormone disorder and Ca loss via downregulation of Gnrh and PI3K expression

Obesity is a public health problem that increases the risk of metabolic disease, infertility, and other chronic health problems. The present study aimed to develop a new rat model for sex hormone disorder with overweight and Ca loss by intramuscular injection of exogenous leptin (LEP). Thirty female Sprague-Dawley (SD) rats (40 days old) were injected thrice intramuscularly with LEP or keyhole limpet hemocyanin immunogen. The following analyses were performed to determine the development of appetite, overweight, reproductive related-hormones, and calcium (Ca)/phosphorus (Pi) in SD rats: measurement of Lee’s index, body weight, food intake; serum Ca, Pi, and hormone tests by enzyme-linked immunosorbent analysis; histological analysis of abdominal fat; real-time polymerase chain reaction analysis of neuropeptide Y, pro-opiomelanocortin, gonadotropin-releasing hormone (Gnrh) mRNA, and gonadotropin-releasing hormone receptor (Gnrhr) mRNA expression; and western blotting analysis of enzyme phosphatidylinositol-3-kinase (PI3K). Rats injected with LEP immunogen displayed significantly increased body weight, food intake, Lee’s index, serum LEP, serum cortisol, fat deposition in the abdomen, and decreased hormones including follicle stimulating hormone, luteinizing hormone, estradiol, cholecystokinin, and Ca. Exogenous LEP administered intramuscularly also downregulate Gnrh and PI3K. In conclusion, exogenous LEP administered intramuscularly is a novel animal model for sex hormones disorder with overweight and Ca loss in SD rats. The downregulation of PI3K and Gnrh may be involved in the development of this animal model.

Chemical identity of hypothalamic neurons engaged by leptin in reproductive control

The adipocyte-derived hormone leptin plays a critical role as a metabolic cue for the reproductive system. Conditions of low leptin levels observed in negative energy balance and loss-of-function mutations of leptin or leptin receptor genes are characterized by decreased fertility. In recent years, advances have been made for identifying possible hypothalamic neurons relaying leptin's neuroendocrine control of reproductive function. Studies from different laboratories have demonstrated that leptin action in the hypothalamo-pituitary-gonadal (HPG) axis is exerted via hypothalamic interneurons regulating gonadotropin-releasing hormone (GnRH) cells, oppose to direct action on GnRH neurons. Following this observation, studies focused on identifying leptin responsive interneurons. Using a Cre-loxP system to re-express or delete the leptin receptor long form (LepRb) from kisspeptin neurons, our laboratory found that leptin's action on kiss1 cells is neither required nor sufficient for leptin's role in reproductive function. Endogenous re-expression of LepRb however, in glutamatergic neurons of the ventral premammilary nucleus (PMV) or ablation of agouti-related protein (AgRP) neurons from leptin signaling-deficient mice are both sufficient to induce puberty and improve fertility. Recent studies have also shown that leptin action in first order GABAergic neurons is required for fertility. Together, these studies begin to delineate key neuronal populations involved in leptin's action in reproduction. In this review, we discuss recent advances made in the field and highlight the questions yet to be answered.

Minireview: Metabolic control of the reproductive physiology: insights from genetic mouse models

This article is part of a Special Issue Energy Balance. Over the past two decades, and in particular over the past 5-7 years, there has been a tremendous advancement in the understanding of the metabolic control of reproductive physiology. This has been in large part due to the advancement and refinement of gene targeting tools and techniques for molecular mapping. Yet despite the emergence of exciting and often times thought-provoking data through the use of new mouse models, the heavy reliance on gene targeting strategies has become fundamental in this process and thus caution must be exercised when interpreting results. This minireview article will explore the generation of new mouse models using genetic manipulation, such as viral vector delivery and the use of the Cre/loxP system, to investigate the role of circulating metabolic hormones in the coordination of reproductive physiology. In addition, we will also highlight some of the pitfalls in the use of genetic manipulation in the current paradigms. However, it has become clear that metabolic cues employ integrated and plastic neural circuits in order to modulate the neuroendocrine reproductive axis, and despite recent advances much remains to be elucidated about this circuitry.

Hypothalamic agouti-related peptide neurons and the central melanocortin system are crucial mediators of leptin's antidiabetic actions

Leptin has beneficial effects on glucose metabolism via actions in the hypothalamus, but the roles of specific subgroups of neurons responsible for these antidiabetic effects remain unresolved. We generated diabetic Lep(ob/ob) or Lepr(db/db) mice lacking or re-expressing leptin receptors (LepRb) in subgroups of neurons to explore their contributions to leptin's glucose-lowering actions. We show that agouti-related peptide (AgRP)-expressing neurons are both required and sufficient to correct hyperglycemia by leptin. LepRb in pro-opiomelanocortin (POMC) neurons or steroidogenic factor-1 (SF1) neurons are not required. Furthermore, normalization of blood glucose by leptin is blunted in Lep(ob/ob)/MC4R-null mice, but not in Lep(ob/ob) mice lacking neuropeptide Y (NPY) or gamma-aminobutyric acid (GABA) in AgRP neurons. Leptin's ability to improve glucose balance is accompanied by a reduction in circulating glucagon. We conclude that AgRP neurons play a crucial role in glucose-lowering actions by leptin and that this requires the melanocortin system, but not NPY and GABA.

Thyroid hormone and leptin in the testis

Leptin is primarily expressed in white adipose tissue; however, it is expressed in the hypothalamus and reproductive tissues as well. Leptin acts by activating the leptin receptors (Ob-Rs). Additionally, the regulation of several neuroendocrine and reproductive functions, including the inhibition of glucocorticoids and enhancement of thyroxine and sex hormone concentrations in human beings and mice are leptin functions. It has been suggested that thyroid hormones (TH) could directly regulate leptin expression. Additionally, hypothyroidism compromises the intracellular integration of leptin signaling specifically in the arcuate nucleus. Two TH receptor isoforms are expressed in the testis, TRa and TRb, with TRa being the predominant one that is present in all stages of development. The effects of TH involve the proliferation and differentiation of Sertoli and Leydig cells during development, spermatogenesis, and steroidogenesis. In this context, TH disorders are associated with sexual dysfunction. An endocrine and/or direct paracrine effect of leptin on the gonads inhibits testosterone production in Leydig cells. Further studies are necessary to clarify the effects of both hormones in the testis during hypothyroidism. The goal of this review is to highlight the current knowledge regarding leptin and TH in the testis.