Leptin receptor null mice with reexpression of LepR in GnRHR expressing cells display elevated FSH levels but remain in a prepubertal state

Conditional knockout of leptin receptor in the female reproductive tract reduces fertility due to parturition defects in mice

Leptin is required for fertility, including initiation of estrous cycles. It is therefore challenging to assess the role of leptin signaling during pregnancy. While neuron-specific transgene approaches suggest that leptin signaling in the central nervous system is most important, experiments with pharmacologic inhibition of leptin in the uterus or global replacement of leptin during pregnancy suggest leptin signaling in the reproductive tract may be required. Here, conditional leptin receptor knockout (Lepr cKO) with a progesterone receptor-driven Cre recombinase was used to examine the importance of leptin signaling in pregnancy. Lepr cKO mice have almost no leptin receptor in uterus or cervix, and slightly reduced leptin receptor levels in corpus luteum. Estrous cycles and progesterone concentrations were not affected by Lepr cKO. Numbers of viable embryos did not differ between primiparous control and Lepr cKO dams on days 6.5 and 17.5 of pregnancy, despite a slight reduction in the ratio of embryos to corpora lutea, showing that uterine leptin receptor signaling is not required for embryo implantation. Placentas of Lepr cKO dams had normal weight and structure. However, over four parities, Lepr cKO mice produced 22% fewer live pups than controls, and took more time from pairing to delivery by their fourth parity. Abnormal birth outcomes of either dystocia or dead pups occurred in 33% of Lepr cKO deliveries but zero control deliveries, and the average time to deliver each pup after crouching was significantly increased. Thus, leptin receptor signaling in the reproductive tract is required for normal labor and delivery. Summary sentence. Mice lacking leptin receptor in the reproductive tract produce fewer live pups and have more adverse labor outcomes than controls, but normal numbers of embryos near term, showing that leptin receptor signaling is required for normal parturition.

Mouse Cre drivers: tools for studying disorders of the human female neuroendocrine-reproductive axis†

Benign disorders of the human female reproductive system, such primary ovarian insufficiency and polycystic ovary syndrome are associated with infertility and recurrent miscarriage, as well as increased risk of adverse health outcomes, including cardiovascular disease and type 2 diabetes. For many of these conditions, the contributing molecular and cellular processes are poorly understood. The overarching similarities between mice and humans have rendered mouse models irreplaceable in understanding normal physiology and elucidating pathological processes that underlie disorders of the female reproductive system. The utilization of Cre-LoxP recombination technology, which allows for spatial and temporal control of gene expression, has identified the role of numerous genes in development of the female reproductive system and in processes, such as ovulation and endometrial decidualization, that are required for the establishment and maintenance of pregnancy in mammals. In this comprehensive review, we provide a detailed overview of Cre drivers with activity in the neuroendocrine-reproductive axis that have been used to study disruptions in key intracellular signaling pathways. We first summarize normal development of the hypothalamus, pituitary, ovary, and uterus, highlighting similarities and differences between mice and humans. We then describe human conditions resulting from abnormal development and/or function of the organ. Finally, we describe loss-of-function models for each Cre driver that elegantly recapitulate some key features of the human condition and are associated with impaired fertility. The examples we provide illustrate use of each Cre driver as a tool for elucidating genetic and molecular underpinnings of reproductive dysfunction.

Gonadotrope-specific deletion of the BMP type 2 receptor does not affect reproductive physiology in mice†‡

Activins selectively stimulate follicle-stimulating hormone (FSH) secretion by pituitary gonadotrope cells. More recently, other members of the TGFbeta superfamily, the bone morphogenetic proteins (BMPs), were reported to regulate FSH synthesis. Activins and BMPs independently and synergistically stimulate transcription of the FSHbeta subunit (Fshb) gene in immortalized gonadotrope-like cells. Both ligands can signal via the activin receptor type IIA (ACVR2A) to regulate FSH synthesis in vitro. In vivo, global Acvr2a knockout mice exhibit a 60% reduction in circulating FSH relative to wild-type animals, suggesting that activins, BMPs, or related ligands might signal through additional type II receptors to regulate FSH in vivo. Although the leading candidates are ACVR2B and the BMP type II receptor (BMPR2), only the latter mediates activin or BMP2 induction of Fshb transcription in vitro. Here, we generated mice carrying a loss of function mutation in Bmpr2 specifically in gonadotropes. Puberty onset, estrous cyclicity, and reproductive organ weights were similar between control and conditional knockout females. Serum FSH and luteinizing hormone (LH) and pituitary expression of Fshb and the LHbeta subunit (Lhb) were similarly unaffected by the gene deletion in both sexes. These results suggest that BMPR2 might not play a necessary role in FSH synthesis or secretion in vivo or that another type II receptor, such as ACVR2A, can fully compensate for its absence. These data also further contribute to the emerging concept that BMPs may not be physiological regulators of FSH in vivo.

Leptin regulates exon-specific transcription of the Bdnf gene via epigenetic modifications mediated by an AKT/p300 HAT cascade

Leptin is an adipocyte-derived hormone with pleiotropic functions affecting appetite and mood. While leptin's role in the regulation of appetite has been extensively studied in hypothalamic neurons, its function in the hippocampus, where it regulates mood-related behaviors, is poorly understood. Here, we show that the leptin receptor (LepRb) colocalizes with brain-derived neurotrophic factor (BDNF), a key player in the pathophysiology of major depression and the action of antidepressants, in the dentate gyrus of the hippocampus. Leptin treatment increases, whereas deficiency of leptin or leptin receptors decreases, total Bdnf mRNA levels, with distinct expression profiles of specific exons, in the hippocampus. Epigenetic analyses reveal that histone modifications, but not DNA methylation, underlie exon-specific transcription of the Bdnf gene induced by leptin. This is mediated by stimulation of AKT signaling, which in turn activates histone acetyltransferase p300 (p300 HAT), leading to changes in histone H3 acetylation and methylation at specific Bdnf promoters. Furthermore, deletion of Bdnf in the dentate gyrus, or specifically in LepRb-expressing neurons, abolishes the antidepressant-like effects of leptin. These findings indicate that leptin, acting via an AKT-p300 HAT epigenetic cascade, induces exon-specific Bdnf expression, which in turn is indispensable for leptin-induced antidepressant-like effects.

The Importance of Leptin to Reproduction

A healthy nutritional state is required for all aspects of reproduction and is signaled by the adipokine leptin. Leptin acts in a relatively narrow concentration range: too much or too little will compromise fertility. The leptin signal timing is important to prepubertal development in both sexes. In the brain, leptin acts on ventral premammillary neurons which signal kisspeptin (Kiss1) neurons to stimulate gonadotropin releasing hormone (GnRH) neurons. Suppression of Kiss1 neurons occurs when agouti-related peptide neurons are activated by reduced leptin, because leptin normally suppresses these orexigenic neurons. In the pituitary, leptin stimulates production of GnRH receptors (GnRHRs) and follicle-stimulating hormone at midcycle, by activating pathways that derepress actions of the messenger ribonucleic acid translational regulatory protein Musashi. In females, rising estrogen stimulates a rise in serum leptin, which peaks at midcycle, synchronizing with nocturnal luteinizing hormone pulses. The normal range of serum leptin levels (10-20 ng/mL) along with gonadotropins and growth factors promote ovarian granulosa and theca cell functions and oocyte maturation. In males, the prepubertal rise in leptin promotes testicular development. However, a decline in leptin levels in prepubertal boys reflects inhibition of leptin secretion by rising androgens. In adult males, leptin levels are 10% to 50% of those in females, and high leptin inhibits testicular function. The obesity epidemic has elucidated leptin resistance pathways, with too much leptin in either sex leading to infertility. Under conditions of balanced nutrition, however, the secretion of leptin is timed and regulated within a narrow level range that optimizes its trophic effects.

Lack of AR in LepRb Cells Disrupts Ambulatory Activity and Neuroendocrine Axes in a Sex-Specific Manner in Mice

Disorders of androgen imbalance, such as hyperandrogenism in females or hypoandrogenism in males, increase risk of visceral adiposity, type 2 diabetes, and infertility. Androgens act upon androgen receptors (AR) which are expressed in many tissues. In the brain, AR are abundant in hypothalamic nuclei involved in regulation of reproduction and energy homeostasis, yet the role of androgens acting via AR in specific neuronal populations has not been fully elucidated. Leptin receptor (LepRb)-expressing neurons coexpress AR predominantly in hypothalamic arcuate and ventral premammillary nuclei (ARH and PMv, respectively), with low colocalization in other LepRb neuronal populations, and very low colocalization in the pituitary gland and gonads. Deletion of AR from LepRb-expressing cells (LepRbΔAR) has no effect on body weight, energy expenditure, and glucose homeostasis in male and female mice. However, LepRbΔAR female mice show increased body length later in life, whereas male LepRbΔAR mice show an increase in spontaneous ambulatory activity. LepRbΔAR mice display typical pubertal timing, estrous cycles, and fertility, but increased testosterone levels in males. Removal of sex steroid negative feedback action induced an exaggerated rise in luteinizing hormone in LepRbΔAR males and follicle-stimulating hormone in LepRbΔAR females. Our findings show that AR can directly affect a subset of ARH and PMv neurons in a sex-specific manner and demonstrate specific androgenic actions in the neuroendocrine hypothalamus.

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.

Obesity and High-Fat Diet Induce Distinct Changes in Placental Gene Expression and Pregnancy Outcome

Obese women are at high risk of pregnancy complications, including preeclampsia, miscarriage, preterm birth, stillbirth, and neonatal death. In the current study, we aimed to determine the effects of obesity on pregnancy outcome and placental gene expression in preclinical mouse models of genetic and nutritional obesity. The leptin receptor (LepR) null-reactivatable (LepRloxTB), LepR-deficient (Leprdb/+), and high-fat diet (HFD)-fed mice were assessed for fertility, pregnancy outcome, placental morphology, and placental transcriptome using standard quantitative polymerase chain reaction (qPCR) and qPCR arrays. The restoration of fertility of LepRloxTB was performed by stereotaxic delivery of adeno-associated virus-Cre into the hypothalamic ventral premammillary nucleus. Fertile LepRloxTB females were morbidly obese, whereas the wild-type mice-fed HFD showed only a mild increase in body weight. Approximately 80% of the LepRloxTB females had embryo resorptions (∼40% of the embryos). In HFD mice, the number of resorptions was not different from controls fed a regular diet. Placentas of resorbed embryos from obese mice displayed necrosis and inflammatory infiltrate in the labyrinth and changes in the expression of genes associated with angiogenesis and inflammation (e.g., Vegfa, Hif1a, Nfkbia, Tlr3, Tlr4). In contrast, placentas from embryos of females on HFD showed changes in a different set of genes, mostly associated with cellular growth and response to stress (e.g., Plg, Ang, Igf1, Igfbp1, Fgf2, Tgfb2, Serpinf1). Sexual dimorphism in gene expression was only apparent in placentas from obese LepRloxTB mice. Our findings indicate that an obese environment and HFD have distinct effects on pregnancy outcome and the placental transcriptome.

Association of Gnrhr mRNA With the Stem Cell Determinant Musashi: A Mechanism for Leptin-Mediated Modulation of GnRHR Expression

The cyclic expression of pituitary gonadotropin-releasing hormone receptors (GnRHRs) may be an important checkpoint for leptin regulatory signals. Gonadotrope Lepr-null mice have reduced GnRHR levels, suggesting these receptors may be leptin targets. To determine if leptin stimulated GnRHR directly, primary pituitary cultures or pieces were exposed to 1 to 100 nM leptin. Leptin increased GnRHR protein levels and the percentages of gonadotropes that bound biotinylated analogs of gonadotropin-releasing hormone (bio-GnRH) but had no effect on Gnrhr messenger RNA (mRNA). An in silico analysis revealed three consensus Musashi (MSI) binding elements (MBEs) for this translational control protein in the 3' untranslated region (UTR) of Gnrhr mRNA. Several experiments determined that these Gnrhr mRNA MBE were active: (1) RNA electrophoretic mobility shift assay analyses showed that MSI1 specifically bound Gnrhr mRNA 3'-UTR; (2) RNA immunoprecipitation of pituitary fractions with MSI1 antibody pulled down a complex enriched in endogenous MSI protein and endogenous Gnrhr mRNA; and (3) fluorescence reporter assays showed that MSI1 repressed translation of the reporter coupled to the Gnrhr 3'-UTR. In vitro, leptin stimulation of pituitary pieces reduced Msi1 mRNA in female pituitaries, and leptin stimulation of pituitary cultures reduced MSI1 proteins selectively in gonadotropes identified by binding to bio-GnRH. These findings show that leptin's direct stimulatory actions on gonadotrope GnRHR correlate with a direct inhibition of expression of the posttranscriptional regulator MSI1. We also show MSI1 interaction with the 3'-UTR of Gnrhr mRNA. These findings now open the door to future studies of leptin-modulated posttranscriptional pathways.

Timing of spermarche and menarche among urban students in Guangzhou, China: trends from 2005 to 2012 and association with Obesity

In Guangzhou, China, whether the trend of a decreasing pubertal age has continued in recent years remained unknown, and the association between obesity and early puberty was still controversial. Herein, we conducted a serial cross-sectional study using data from physical fitness surveillance (2005–2012), to determine the recent trends in age at spermarche and menarche among students in Guangzhou, and to investigate whether elevated BMI modified timing of spermarche and menarche. This study included 1,278,258 urban students. In boys, no significant differences were observed in median ages of spermarche (MAS) from 2005 to 2012, with overlapping 95% CIs. Similar results were observed for median ages of menarche (MAM) in girls. The Cox-Stuart trend test showed neither upward nor downward shift in MAS and MAM over time (P = 0.625; 1.000). Each year, both MAS and MAM decreased with increasing BMI. Furthermore, a higher BMI was associated with early age at spermarche and menarche, with ORs of 1.052 (95% CI = 1.045–1.059) and 1.233 (95% CI = 1.220–1.247) in 2012 for boys and girls, respectively. In conclusion, the pubertal timing has been stable in urban students from 2005 to 2012. Furthermore, obesity was associated with early timing of spermarche and menarche.

Leptin Regulation of Gonadotrope Gonadotropin-Releasing Hormone Receptors As a Metabolic Checkpoint and Gateway to Reproductive Competence

The adipokine leptin signals the body's nutritional status to the brain, and particularly, the hypothalamus. However, leptin receptors (LEPRs) can be found all throughout the body and brain, including the pituitary. It is known that leptin is permissive for reproduction, and mice that cannot produce leptin (Lep/Lep) are infertile. Many studies have pinpointed leptin's regulation of reproduction to the hypothalamus. However, LEPRs exist at all levels of the hypothalamic-pituitary-gonadal axis. We have previously shown that deleting the signaling portion of the LEPR specifically in gonadotropes impairs fertility in female mice. Our recent studies have targeted this regulation to the control of gonadotropin releasing hormone receptor (GnRHR) expression. The hypotheses presented here are twofold: (1) cyclic regulation of pituitary GnRHR levels sets up a target metabolic checkpoint for control of the reproductive axis and (2) multiple checkpoints are required for the metabolic signaling that regulates the reproductive axis. Here, we emphasize and explore the relationship between the hypothalamus and the pituitary with regard to the regulation of GnRHR. The original data we present strengthen these hypotheses and build on our previous studies. We show that we can cause infertility in 70% of female mice by deleting all isoforms of LEPR specifically in gonadotropes. Our findings implicate activin subunit (InhBa) mRNA as a potential leptin target in gonadotropes. We further show gonadotrope-specific upregulation of GnRHR protein (but not mRNA levels) following leptin stimulation. In order to try and understand this post-transcriptional regulation, we tested candidate miRNAs (identified with in silico analysis) that may be binding the Gnrhr mRNA. We show significant upregulation of one of these miRNAs in our gonadotrope-Lepr-null females. The evidence provided here, combined with our previous work, lay the foundation for metabolically regulated post-transcriptional control of the gonadotrope. We discuss possible mechanisms, including miRNA regulation and the involvement of the RNA binding protein, Musashi. We also demonstrate how this regulation may be vital for the dynamic remodeling of gonadotropes in the cycling female. Finally, we propose that the leptin receptivity of both the hypothalamus and the pituitary are vital for the body's ability to delay or slow reproduction during periods of low nutrition.

Metabolic state can define the ovarian response to environmental contaminants and medicinal plants

Environmental contaminants and medicinal plants can affect reproductive processes. The aim of this study was to investigate the effect of maternal metabolic status on the response of mouse ovaries to the environmental contaminants benzene and xylene, as well as to extracts of the medicinal plant yucca. Ovaries isolated from normal-lean and slightly obese mice were cultured with or without 0.1% benzene or xylene for 24 h. Similarly, ovaries isolated from normal-lean, slightly obese, and significantly obese mice were cultured for 24 h with or without an extract of Yucca shidigera (YS, 10 μ g/mL). We found that the metabolic status did not influence the release of basal progesterone (P4), testosterone (T), or insulin-like growth factor I (IGF-I), but obesity influenced the effects of the environmental contaminants and YS. Benzene reduced P4 output in ovaries from obese but not normal-lean mice; it also reduced IGF-I (but not T) release from ovaries irrespective of the metabolic status. Xylene dramatically increased P4 and T (but not IGF-I) release by ovaries from normal-lean mice, but there were no changes in P4 and only small increases in T output in obese mice. YS increased P4 (but not T or IGF-I) release in normal-lean and slightly obese animal ovaries, whilst significant obesity was associated with a lack of P4 response to YS. Obesity might affect the basal ovarian release of T or IGF-I and increases the sensitivity of ovaries to the action of benzene but decreases their responsiveness to xylene and YS.

Sexually dimorphic distribution of Prokr2 neurons revealed by the Prokr2-Cre mouse model

Prokineticin receptor 2 (PROKR2) is predominantly expressed in the mammalian central nervous system. Loss-of-function mutations of PROKR2 in humans are associated with Kallmann syndrome due to the disruption of gonadotropin releasing hormone neuronal migration and deficient olfactory bulb morphogenesis. PROKR2 has been also implicated in the neuroendocrine control of GnRH neurons post-migration and other physiological systems. However, the brain circuitry and mechanisms associated with these actions have been difficult to investigate mainly due to the widespread distribution of Prokr2-expressing cells, and the lack of animal models and molecular tools. Here, we describe the generation, validation and characterization of a new mouse model that expresses Cre recombinase driven by the Prokr2 promoter, using CRISPR-Cas9 technology. Cre expression was visualized using reporter genes, tdTomato and GFP, in males and females. Expression of Cre-induced reporter genes was found in brain sites previously described to express Prokr2, e.g., the paraventricular and the suprachiasmatic nuclei, and the area postrema. The Prokr2-Cre mouse model was further validated by colocalization of Cre-induced GFP and Prokr2 mRNA. No disruption of Prokr2 expression, GnRH neuronal migration or fertility was observed. Comparative analysis of Prokr2-Cre expression in male and female brains revealed a sexually dimorphic distribution confirmed by in situ hybridization. In females, higher Cre activity was found in the medial preoptic area, ventromedial nucleus of the hypothalamus, arcuate nucleus, medial amygdala and lateral parabrachial nucleus. In males, Cre was higher in the amygdalo-hippocampal area. The sexually dimorphic pattern of Prokr2 expression indicates differential roles in reproductive function and, potentially, in other physiological systems.

Adipokines (Leptin, Adiponectin, Resistin) Differentially Regulate All Hormonal Cell Types in Primary Anterior Pituitary Cell Cultures from Two Primate Species

Adipose-tissue (AT) is an endocrine organ that dynamically secretes multiple hormones, the adipokines, which regulate key physiological processes. However, adipokines and their receptors are also expressed and regulated in other tissues, including the pituitary, suggesting that locally- and AT-produced adipokines might comprise a regulatory circuit that relevantly modulate pituitary cell-function. Here, we used primary pituitary cell-cultures from two normal nonhuman-primate species [Papio-anubis/Macaca-fascicularis] to determine the impact of different adipokines on the functioning of all anterior-pituitary cell-types. Leptin and resistin stimulated GH-release, a response that was blocked by somatostatin. Conversely, adiponectin decreased GH-release, and inhibited GHRH-, but not ghrelin-stimulated GH-secretion. Furthermore: 1) Leptin stimulated PRL/ACTH/FSH- but not LH/TSH-release; 2) adiponectin stimulated PRL-, inhibited ACTH- and did not alter LH/FSH/TSH-release; and 3) resistin increased ACTH-release and did not alter PRL/LH/FSH/TSH-secretion. These effects were mediated through the activation of common (AC/PKA) and distinct (PLC/PKC, intra-/extra-cellular calcium, PI3K/MAPK/mTOR) signaling-pathways, and by the gene-expression regulation of key receptors/transcriptional-factors involved in the functioning of these pituitary cell-types (e.g. GHRH/ghrelin/somatostatin/insulin/IGF-I-receptors/Pit-1). Finally, we found that primate pituitaries expressed leptin/adiponectin/resistin. Altogether, these and previous data suggest that local-production of adipokines/receptors, in conjunction with circulating adipokine-levels, might comprise a relevant regulatory circuit that contribute to the fine-regulation of pituitary functions.