IRS2 signaling in LepR-b neurons suppresses FoxO1 to control energy balance independently of leptin action

Prenatal benzene exposure in mice alters offspring hypothalamic development predisposing to metabolic disease in later life

Maternal exposure to environmental contaminants during pregnancy poses a significant threat to a developing fetus, as these substances can easily cross the placenta and disrupt the neurodevelopment of offspring. Specifically, the hypothalamus is essential in the regulation of metabolism, notably during critical windows of development. An abnormal hormonal and inflammatory milieu during development can trigger persistent changes in the function of hypothalamic circuits, leading to long-lasting effects on the body's energy homeostasis and metabolism. We recently demonstrated that gestational exposure to clinically relevant levels of benzene induces severe metabolic dysregulation in the offspring. Given the central role of the hypothalamus in metabolic control, we hypothesized that prenatal exposure to benzene impacts hypothalamic development, contributing to the adverse metabolic effects in the offspring. C57BL/6JB dams were exposed to benzene at 50 ppm in the inhalation chambers exclusively during pregnancy (from E0.5 to E19). Transcriptomic analysis of the exposed offspring at postnatal day 21 (P21) revealed hypothalamic changes in genes related to metabolic regulation, inflammation, and neurodevelopment exclusively in males. Moreover, the hypothalamus of prenatally benzene-exposed male offspring displayed alterations in orexigenic and anorexigenic projections, impairments in leptin signaling, and increased microgliosis. Additional exposure to benzene during lactation did not promote further microgliosis or astrogliosis in the offspring, while the high-fat diet (HFD) challenge in adulthood exacerbated glucose metabolism and hypothalamic inflammation in benzene-exposed offspring of both sexes. These findings reveal the persistent adverse effects of prenatal benzene exposure on hypothalamic circuits and neuroinflammation, predisposing the offspring to long-lasting metabolic health conditions.

Hepatic follistatin increases basal metabolic rate and attenuates diet-induced obesity during hepatic insulin resistance

OBJECTIVE

Body weight change and obesity follow the variance of excess energy input balanced against tightly controlled EE (energy expenditure). Since insulin resistance can reduce energy storage, we investigated whether genetic disruption of hepatic insulin signaling reduced adipose mass with increased EE.

METHODS

Insulin signaling was disrupted by genetic inactivation of Irs1 (Insulin receptor substrate 1) and Irs2 in hepatocytes of LDKO mice (Irs1L/L·Irs2L/L·CreAlb), creating a state of complete hepatic insulin resistance. We inactivated FoxO1 or the FoxO1-regulated hepatokine Fst (Follistatin) in the liver of LDKO mice by intercrossing LDKO mice with FoxO1L/L or FstL/L mice. We used DEXA (dual-energy X-ray absorptiometry) to assess total lean mass, fat mass and fat percentage, and metabolic cages to measure EE (energy expenditure) and estimate basal metabolic rate (BMR). High-fat diet was used to induce obesity.

RESULTS

Hepatic disruption of Irs1 and Irs2 (LDKO mice) attenuated HFD (high-fat diet)-induced obesity and increased whole-body EE in a FoxO1-dependent manner. Hepatic disruption of the FoxO1-regulated hepatokine Fst normalized EE in LDKO mice and restored adipose mass during HFD consumption; moreover, hepatic Fst disruption alone increased fat mass accumulation, whereas hepatic overexpression of Fst reduced HFD-induced obesity. Excess circulating Fst in overexpressing mice neutralized Mstn (Myostatin), activating mTORC1-promoted pathways of nutrient uptake and EE in skeletal muscle. Similar to Fst overexpression, direct activation of muscle mTORC1 also reduced adipose mass.

CONCLUSIONS

Thus, complete hepatic insulin resistance in LDKO mice fed a HFD revealed Fst-mediated communication between the liver and muscle, which might go unnoticed during ordinary hepatic insulin resistance as a mechanism to increase muscle EE and constrain obesity.

Constitutive and conditional gene knockout mice for the study of intervertebral disc degeneration: Current status, decision considerations, and future possibilities

There have been an increasing number of patients with degenerative disc diseases due to the aging population. In light of this, studies on the pathogenesis of intervertebral disc degeneration have become a hot topic, and gene knockout mice have become a valuable tool in this field of research. With the development of science and technology, constitutive gene knockout mice can be constructed using homologous recombination, zinc finger nuclease, transcription activator-like effector nuclease technology and clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9) system, and conditional gene knockout mice can be constructed using the Cre/LoxP system. The gene-edited mice using these techniques have been widely used in the studies on disc degeneration. This paper reviews the development process and principles of these technologies, functions of the edited genes in disc degeneration, advantages, and disadvantages of different methods and possible targets of the specific Cre recombinase in intervertebral discs. Recommendations for the choice of suitable gene-edited model mice are presented. At the same time, possible technological improvements in the future are also discussed.

Therapeutic Effectiveness of Brain Computer Interfaces in Stroke Patients: A Systematic Review

Background

Brain-computer interfaces (BCIs) are a rapidly advancing field which utilizes brain activity to control external devices for a myriad of functions, including the restoration of motor function. Clinically, BCIs have been especially impactful in patients who suffer from stroke-mediated damage. However, due to the rapid advancement in the field, there is a lack of accepted standards of practice. Therefore, the aim of this systematic review is to summarize the current literature published regarding the efficacy of BCI-based rehabilitation of motor dysfunction in stroke patients.

Methodology

This systematic review was performed in accordance with the guidelines set forth by the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) 2020 statement. PubMed, Embase, and Cochrane Library were queried for relevant articles and screened for inclusion criteria by two authors. All discrepancies were resolved by discussion among both reviewers and subsequent consensus.

Results

11/12 (91.6%) of studies focused on upper extremity outcomes and reported larger initial improvements for participants in the treatment arm (using BCI) as compared to those in the control arm (no BCI). 2/2 studies focused on lower extremity outcomes reported improvements for the treatment arm compared to the control arm.

Discussion/Conclusion

This systematic review illustrates the utility BCI has for the restoration of upper extremity and lower extremity motor function in stroke patients and supports further investigation of BCI for other clinical indications.

Chronic exposure to PPCPs mixture at environmentally relevant concentrations (ERCs) altered carbohydrate and lipid metabolism through gut and liver toxicity in zebrafish

Pharmaceuticals and personal care products (PPCPs) have been widely distributed and posed ecotoxicological risks in the aquatic environment. This study aims to evaluate the toxic effects after chronic exposure to PPCPs mixture at the environment relevant concentrations (ERCs). Our results indicated that PPCPs induced serious metabolic effects by disturbing the carbohydrate and lipid metabolism pathways. Chronic exposure caused a significant reduction in the hepatosomatic index (HSI), the gut weight ratios, and histological alterations in liver and gut tissues. Further, exposure to the combined PPCPs disrupted the carbohydrate metabolism via significant upregulation of hk1, gk, pck1, and insr genes. The lipid metabolism was affected with higher ppars expression levels that increased the fatty acid β-oxidation and ultimately decreased the lipidogenesis. Moreover, the altered responses of the insulin growth factor (IGF) pathway more in male gut tissue than that of female revealed sex-dependent disturbance in the gut homeostasis induced by PPCPs mixture. In conclusion, chronic exposure to PPCPs mixtures at ERCs can induce developmental effects and metabolic dysfunction in both male and female fish. The consumption and environmental disposal of these PPCPs should be regulated to ensure ecological health and environmental safety.

Chronic Timed Sleep Restriction Attenuates LepRb-Mediated Signaling Pathways and Circadian Clock Gene Expression in the Rat Hypothalamus

The sleep duration of adolescents has continued to decline over the past 20 years. Sleep insufficiency is one of the most important risk factors for obesity, but the mechanisms underlying the association are unclear. Therefore, the hypothalamic-regulated mechanisms of appetite and the circadian clock gene expression were examined in sleep-restricted rats. Rats aged 7 weeks were randomly divided into two groups: the control group and sleep restriction group (7 rats/group) rats were sleep-restricted for 4 weeks. Body weight gain and amount of food/water consumption were quantified. The expression of genes or proteins which regulated appetite and energy metabolism via leptin receptor signaling and the circadian clock in the hypothalamus were assessed. Chronic sleep restriction induced increased food intake and weight gain in adolescent and young adult rats from the second week of initiation of sleep restriction. Phosphorylation of Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) was decreased, although levels of circulating leptin or leptin receptor expression were unaltered. Furthermore, insulin receptor substrate (IRS)/phosphoinositide 3-kinase (PI3K)/AKT/mTOR and forkhead box O1 (FoxO1) signaling pathways were also compromised. Moreover, core circadian clock genes were also decreased in the sleep restriction group compared with the control. Chronic timed sleep restriction induced hyperphagic behaviors, attenuated leptin receptor-mediated signaling pathways, and depleted the expression of circadian clock gene in the hypothalamus of adolescent and young adult rats.

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.

Roles of Gangliosides in Hypothalamic Control of Energy Balance: New Insights

Gangliosides are essential components of cell membranes and are involved in a variety of physiological processes, including cell growth, differentiation, and receptor-mediated signal transduction. They regulate functions of proteins in membrane microdomains, notably receptor tyrosine kinases such as insulin receptor (InsR) and epidermal growth factor receptor (EGFR), through lateral association. Studies during the past two decades using knockout (KO) or pharmacologically inhibited cells, or KO mouse models for glucosylceramide synthase (GCS; Ugcg), GM3 synthase (GM3S; St3gal5), and GD3 synthase (GD3S; St8sia1) have revealed essential roles of gangliosides in hypothalamic control of energy balance. The a-series gangliosides GM1 and GD1a interact with leptin receptor (LepR) and promote LepR signaling through activation of the JAK2/STAT3 pathway. Studies of GM3S KO cells have shown that the extracellular signal-regulated kinase (ERK) pathway, downstream of the LepR signaling pathway, is also modulated by gangliosides. Recent studies have revealed crosstalk between the LepR signaling pathway and other receptor signaling pathways (e.g., InsR and EGFR pathways). Gangliosides thus have the ability to modulate the effects of leptin by regulating functions of such receptors, and by direct interaction with LepR to control signaling.

CNS Regulation of Glucose Homeostasis: Role of the Leptin-Melanocortin System

PURPOSE OF REVIEW

In this brief review, we highlight studies that have contributed to our current understanding of glucose homeostasis by the central nervous system (CNS) leptin-melanocortin system, particularly proopiomelanocortin neurons and melanocortin-4 receptors (MC4R).

RECENT FINDINGS

Leptin deficiency is associated with insulin resistance and impaired glucose metabolism whereas leptin administration improves tissue glucose uptake/oxidation and reduces hepatic glucose output. These antidiabetic effects of leptin have been demonstrated in experimental animals and humans, even when circulating insulin levels are barely detectable. Recent evidence suggests that these antidiabetic actions of leptin are mediated, in large part, by stimulation of leptin receptors (LRs) in the CNS and require activation of proopiomelanocortin (POMC) neurons and MC4R. These chronic antidiabetic effects of the CNS leptin-melanocortin system appear to be independent of autonomic nervous system and pituitary-thyroid-adrenal (PTA) axis mechanisms. The powerful antidiabetic actions of the CNS leptin-melanocortin system are capable of normalizing plasma glucose even in the absence of insulin and involve interactions of multiple neuronal populations and intracellular signaling pathways. Although the links between the CNS leptin-melanocortin system and its chronic effects on peripheral tissue glucose metabolism are still uncertain, they are independent of insulin action, activation of the autonomic nervous system, or the PTA axis. Unraveling the pathways that contribute to the powerful antidiabetic effects of the CNS leptin-melanocortin system may provide novel therapeutic approaches for diabetes mellitus.

Leptin receptor-expressing neuron Sh2b1 supports sympathetic nervous system and protects against obesity and metabolic disease

Leptin stimulates the sympathetic nervous system (SNS), energy expenditure, and weight loss; however, the underlying molecular mechanism remains elusive. Here, we uncover Sh2b1 in leptin receptor (LepR) neurons as a critical component of a SNS/brown adipose tissue (BAT)/thermogenesis axis. LepR neuron-specific deletion of Sh2b1 abrogates leptin-stimulated sympathetic nerve activation and impairs BAT thermogenic programs, leading to reduced core body temperature and cold intolerance. The adipose SNS degenerates progressively in mutant mice after 8 weeks of age. Adult-onset ablation of Sh2b1 in the mediobasal hypothalamus also impairs the SNS/BAT/thermogenesis axis; conversely, hypothalamic overexpression of human SH2B1 has the opposite effects. Mice with either LepR neuron-specific or adult-onset, hypothalamus-specific ablation of Sh2b1 develop obesity, insulin resistance, and liver steatosis. In contrast, hypothalamic overexpression of SH2B1 protects against high fat diet-induced obesity and metabolic syndromes. Our results unravel an unrecognized LepR neuron Sh2b1/SNS/BAT/thermogenesis axis that combats obesity and metabolic disease.

Sexual dimorphism in hypothalamic inflammation in the offspring of dams exposed to a diet rich in high fat and branched-chain amino acids

Branched-chain amino acid (BCAAs: leucine, isoleucine, and valine) contribute to the development of obesity-associated insulin resistance in the context of consumption of a high-fat diet (HFD) in humans and rodents. Maternal diet is a major determinant of offspring health, and there is strong evidence that maternal HFD alters hypothalamic developmental programming and disrupts offspring energy homeostasis in rodents. In this study, we exposed pregnant and lactating C57BL/6JB female mice to either HFD, HFD with supplemented BCAA (HFD+BCAA), or standard diet (SC), and we studied offspring metabolic phenotypes. Both maternal HFD and HFD supplemented with BCAA had similar effect rendering the offspring metabolic imbalance and impairing their ability to cope with HFD when challenged during aging. The metabolic effects of HFD challenge were more profound in females, worsening female offspring ability to cope with an HFD challenge by activating hypothalamic inflammation in aging. Moreover, the sex differences in hypothalamic estrogen receptor α (ER-α) expression levels were lost in female offspring upon HFD challenge, supporting a link between ER-α levels and hypothalamic inflammation in offspring and highlighting the programming potential of hypothalamic inflammatory responses and maternal nutrition.

Estrogen Improves Insulin Sensitivity and Suppresses Gluconeogenesis via the Transcription Factor Foxo1

Premenopausal women exhibit enhanced insulin sensitivity and reduced incidence of type 2 diabetes (T2D) compared with age-matched men, but this advantage disappears after menopause with disrupted glucose homeostasis, in part owing to a reduction in circulating 17β-estradiol (E₂). Fasting hyperglycemia is a hallmark of T2D derived largely from dysregulation of hepatic glucose production (HGP), in which Foxo1 plays a central role in the regulation of gluconeogenesis. Here, we investigated the action of E₂ on glucose homeostasis in male and ovariectomized (OVX) female control and liver-specific Foxo1 knockout (L-F1KO) mice and sought to understand the mechanism by which E₂ regulates gluconeogenesis via an interaction with hepatic Foxo1. In both male and OVX female control mice, subcutaneous E₂ implant improved insulin sensitivity and suppressed gluconeogenesis; however, these effects of E₂ were abolished in L-F1KO mice of both sexes. In our use of mouse primary hepatocytes, E₂ suppressed HGP and gluconeogenesis in hepatocytes from control mice but failed in hepatocytes from L-F1KO mice, suggesting that Foxo1 is required for E₂ action on the suppression of gluconeogenesis. We further demonstrated that E₂ suppresses hepatic gluconeogenesis through activation of estrogen receptor (ER)α-phosphoinositide 3-kinase-Akt-Foxo1 signaling, which can be independent of insulin receptor substrates 1 and 2 (Irs1 and Irs2), revealing an important mechanism for E₂ in the regulation of glucose homeostasis. These results may help explain why premenopausal women have lower incidence of T2D than age-matched men and suggest that targeting ERα can be a potential approach to modulate glucose metabolism and prevent diabetes.

Insulin signaling in LepR cells modulates fat and glucose homeostasis independent of leptin

Hypothalamic neurons detect changes in circulating hormones such as leptin and insulin and put forward outputs to sustain energy and glucose homeostasis. Because leptin and insulin receptors colocalize in ~40-60% of neurons in the hypothalamus, we characterized the metabolic phenotype of mice with selective deletion of the insulin receptor (InsR) in LepR cells. LR^ΔInsR mice presented no difference in body weight and insulin levels but increased fat mass. In the light phase, LR^ΔInsR mice exhibited increased food intake, locomotor activity, carbon dioxide production, and respiratory exchange rate. These mice showed reduced fat oxidation and reduced expression of cluster of differentiation 36 and AMP-activated protein kinase-α₁ in the liver, increased glucose oxidation in the light phase, and overall reduced basal glucose levels. To verify the impact of InsR deletion in LepR cells in obesity, we generated ob/ ob InsR^fl, ob/ ob LR^cre, and ob/ ob LR^ΔInsR mice. The ob/ ob LR^ΔInsR mice had higher body weight, fat mass, and expression of genes related to fat metabolism in the liver. No difference in food intake despite increased neuropeptide Y and agouti-related peptide expression, and no difference in energy expenditure, fat, or glucose oxidation was found in ob/ ob LR^ΔInsR compared with LR^cre or LR^ΔInsR controls. Remarkably, basal glucose levels were reduced, and the expression of genes associated with glucose metabolism in the liver was higher. Insulin signaling in LepR cells is required for the proper fat and glucose oxidation. These effects are independent of leptin given that the leptin-deficient ob/ ob LR^ΔInsR mice also presented reduced glycemia and higher adiposity. The mechanisms underlying these responses remain to be unveiled.

Insulin action in the brain: Roles in energy and glucose homeostasis

A growing body of evidence from research in rodents and humans has identified insulin as an important neuoregulatory peptide in the brain, where it coordinates diverse aspects of energy balance and peripheral glucose homeostasis. This review discusses where and how insulin interacts within the brain and evaluates the physiological and pathophysiological consequences of central insulin signalling in metabolism, obesity and type 2 diabetes.

Anti-aging drugs reduce hypothalamic inflammation in a sex-specific manner

Aging leads to hypothalamic inflammation, but does so more slowly in mice whose lifespan has been extended by mutations that affect GH/IGF‐1 signals. Early‐life exposure to GH by injection, or to nutrient restriction in the first 3 weeks of life, also modulate both lifespan and the pace of hypothalamic inflammation. Three drugs extend lifespan of UM‐HET3 mice in a sex‐specific way: acarbose (ACA), 17‐α‐estradiol (17αE2), and nordihydroguaiaretic acid (NDGA), with more dramatic longevity increases in males in each case. In this study, we examined the effect of these anti‐aging drugs on neuro‐inflammation in hypothalamus and hippocampus. We found that age‐associated hypothalamic inflammation is reduced in males but not in females at 12 months of age by ACA and 17αE2 and at 22 months of age in NDGA‐treated mice. The three drugs blocked indices of hypothalamic reactive gliosis associated with aging, such as Iba‐1‐positive microglia and GFAP‐positive astrocytes, as well as age‐associated overproduction of TNF‐α. This effect was not observed in drug‐treated female mice or in the hippocampus of the drug‐treated animals. On the other hand, caloric restriction (CR; an intervention that extends the lifespan in both sexes) significantly reduced hypothalamic microglia and TNF‐α in both sexes at 12 months of age. Together, these results suggest that the extent of drug‐induced changes in hypothalamic inflammatory processes is sexually dimorphic in a pattern that parallels the effects of these agents on mouse longevity and that mimics the changes seen, in both sexes, of long‐lived nutrient restricted or mutant mice.

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.

Imbalanced Insulin Actions in Obesity and Type 2 Diabetes: Key Mouse Models of Insulin Signaling Pathway

Since the discovery of the tyrosine kinase activity of the insulin receptor (IR), researchers have been engaged in intensive efforts to resolve physiological functions of IR and its major downstream targets, insulin receptor substrate 1 (Irs1) and Irs2. Studies conducted using systemic and tissue-specific gene-knockout mice of IR, Irs1, and Irs2 have revealed the physiological roles of these molecules in each tissue and interactions among multiple tissues. In obesity and type 2 diabetes, selective downregulation of Irs2 and its downstream actions to cause reduced insulin actions was associated with increased insulin actions through Irs1 in variety tissues. Thus, we propose the novel concept of "organ- and pathway-specific imbalanced insulin action" in obesity and type 2 diabetes, which includes and extends "selective insulin resistance." This Review focuses on recent progress in understanding insulin signaling and insulin resistance using key mouse models for elucidating pathophysiology of human obesity and type 2 diabetes.

FoxO1 in dopaminergic neurons regulates energy homeostasis and targets tyrosine hydroxylase

Dopaminergic (DA) neurons are involved in the integration of neuronal and hormonal signals to regulate food consumption and energy balance. Forkhead transcriptional factor O1 (FoxO1) in the hypothalamus plays a crucial role in mediation of leptin and insulin function. However, the homoeostatic role of FoxO1 in DA system has not been investigated. Here we report that FoxO1 is highly expressed in DA neurons and mice lacking FoxO1 specifically in the DA neurons (FoxO1 KO^DAT) show markedly increased energy expenditure and interscapular brown adipose tissue (iBAT) thermogenesis accompanied by reduced fat mass and improved glucose/insulin homoeostasis. Moreover, FoxO1 KO^DAT mice exhibit an increased sucrose preference in concomitance with higher dopamine and norepinephrine levels. Finally, we found that FoxO1 directly targets and negatively regulates tyrosine hydroxylase (TH) expression, the rate-limiting enzyme of the catecholamine synthesis, delineating a mechanism for the KO phenotypes. Collectively, these results suggest that FoxO1 in DA neurons is an important transcriptional factor that directs the coordinated control of energy balance, thermogenesis and glucose homoeostasis.

Dopaminergic neurons are important for regulating energy homeostasis. Here, the authors show the transcription factor FoxO1 negatively regulates tyrosine hydroxylase expression in midbrain dopaminergic neurons, and plays an important role in regulation of glucose homeostasis, energy expenditure, and resistance to diet-induced obesity.

Neuroendocrine Regulation of Metabolism

Given the current environment in most developed countries, it is a challenge to maintain a good balance between calories consumed and calories burned, although maintenance of metabolic balance is key to good health. Therefore, understanding how metabolic regulation is achieved and how the dysregulation of metabolism affects health is an area of intense research. Most studies focus on the hypothalamus, which is a brain area that acts as a key regulator of metabolism. Among the nuclei that comprise the hypothalamus, the arcuate nucleus is one of the major mediators in the regulation of food intake. The regulation of energy balance is also a key factor ensuring the maintenance of any species as a result of the dependence of reproduction on energy stores. Adequate levels of energy reserves are necessary for the proper functioning of the hypothalamic-pituitary-gonadal axis. This review discusses valuable data presented in the 2015 edition of the International Workshop of Neuroendocrinology concerning the fundamental nature of the hormonal regulation of the hypothalamus and the impact on energy balance and reproduction.

Leptin Signaling Is Not Required for Anorexigenic Estradiol Effects in Female Mice

Estradiol and leptin are critical hormones in the regulation of body weight. The aim of this study was to determine whether this cross talk between leptin receptor (LepRb) and estrogen receptor-α (ERα) signaling is critical for estradiol's anorexigenic effects. Leprb-Cre mice were crossed with Cre-dependent Tau-green fluorescent protein (GFP) reporter, Stat3-flox or Erα-flox mice to generate female mice with GFP expression, signal transducer and activator of transcription 3 (STAT3) knockout (KO), or ERα KO, specifically in LepRb-expressing cells. The proportion of Leprb-GFP cells colocalizing ERα was high (∼80%) in the preoptic area but low (∼10%) in the mediobasal hypothalamus, suggesting that intracellular cross talk between these receptors is minimal for metabolic regulation. To test whether estradiol enhanced arcuate leptin sensitivity, ovarectomized mice received varying levels of estradiol replacement. Increasing estrogenic states did not increase the degree of leptin-induced STAT3 phosphorylation. LepRb-specific STAT3 KO mice and controls were ovarectomized and given either chronic estradiol or vehicle treatment to test whether STAT3 is required for estrogen-induced body weight suppression. Both groups of estradiol-treated mice showed an equivalent reduction in body weight and fat content compared with vehicle controls. Finally, mice lacking ERα specifically in LepRb-expressing neurons also showed no increase in body weight or impairments in metabolic function compared with controls, indicating that estradiol acts independently of leptin-responsive cells to regulate body weight. However, fecundity was impaired in in Leprb-ERα KO females. Contrary to the current dogma, we report that estradiol has minimal direct actions on LepRb cells in the mediodasal hypothalamus and that its anorexigenic effects can occur entirely independently of LepRb-STAT3 signaling in female mice.

Growth hormone modulates hypothalamic inflammation in long-lived pituitary dwarf mice

Mice in which the genes for growth hormone (GH) or GH receptor (GHR(-/-) ) are disrupted from conception are dwarfs, possess low levels of IGF-1 and insulin, have low rates of cancer and diabetes, and are extremely long-lived. Median longevity is also increased in mice with deletion of hypothalamic GH-releasing hormone (GHRH), which leads to isolated GH deficiency. The remarkable extension of longevity in hypopituitary Ames dwarf mice can be reversed by a 6-week course of GH injections started at the age of 2 weeks. Here, we demonstrate that mutations that interfere with GH production or response, in the Snell dwarf, Ames dwarf, or GHR(-/-) mice lead to reduced formation of both orexigenic agouti-related peptide (AgRP) and anorexigenic proopiomelanocortin (POMC) projections to the main hypothalamic projection areas: the arcuate nucleus (ARH), paraventricular nucleus (PVH), and dorsomedial nucleus (DMH). These mutations also reduce hypothalamic inflammation in 18-month-old mice. GH injections, between 2 and 8 weeks of age, reversed both effects in Ames dwarf mice. Disruption of GHR specifically in liver (LiGHRKO), a mutation that reduces circulating IGF-1 but does not lead to lifespan extension, had no effect on hypothalamic projections or inflammation, suggesting an effect of GH, rather than peripheral IGF-1, on hypothalamic development. Hypothalamic leptin signaling, as monitored by induction of pStat3, is not impaired by GHR deficiency. Together, these results suggest that early-life disruption of GH signaling produces long-term hypothalamic changes that may contribute to the longevity of GH-deficient and GH-resistant mice.

Regulation of Blood Pressure, Appetite, and Glucose by Leptin After Inactivation of Insulin Receptor Substrate 2 Signaling in the Entire Brain or in Proopiomelanocortin Neurons

Insulin receptor substrate 2 (IRS2) is one of the 3 major leptin receptor signaling pathways, but its role in mediating the chronic effects of leptin on blood pressure, food intake, and glucose regulation is unclear. We tested whether genetic inactivation of IRS2 in the entire brain (IRS2/Nestin-cre mice) or specifically in proopiomelanocortin (POMC) neurons (IRS2/POMC-cre mice) attenuates the chronic cardiovascular, metabolic, and antidiabetic effects of leptin. Mice were instrumented with telemetry probes for measurement of blood pressure and heart rate and with venous catheters for intravenous infusions. After a 5-day control period, mice received leptin infusion (2 μg/kg per minute) for 7 days. Compared with control IRS2(flox/flox) mice, IRS2/POMC-cre mice had similar body weight and food intake (33±1 versus 35±1 g and 3.6±0.5 versus 3.8±0.2 g per day) but higher mean arterial pressure (MAP) and heart rate (110±2 versus 102±2 mm Hg and 641±9 versus 616±5 bpm). IRS2/Nestin-cre mice were heavier (38±2 g), slightly hyperphagic (4.5±1.0 g per day), and had higher MAP and heart rate (108±2 mm Hg and 659±9 bpm) compared with control mice. Leptin infusion gradually increased MAP despite decreasing food intake by 31% in IRS2(flox/flox) and in Nestin-cre control mice. In contrast, leptin infusion did not change MAP in IRS2/Nestin-cre or IRS2/POMC-cre mice. The anorexic and antidiabetic effects of leptin, however, were similar in all 3 groups. These results indicate that IRS2 signaling in the central nervous system, and particularly in POMC neurons, is essential for the chronic actions of leptin to raise MAP but not for its anorexic or antidiabetic effects.

Contribution of the hypothalamus and gut to weight gain susceptibility and resistance in mice

Obesity susceptibility in humans and in rodent strains varies in response to the consumption of high-energy density (HED) diets. However, the exact mechanism(s) involved in this susceptibility remain(s) unresolved. The aim of the present study was to gain greater insight into this susceptibility by using C57BL/6J (B6) mice that were separated into obesity-prone (diet-induced obese (DIO)) and obesity-resistant (diet-induced resistant (DR)) groups following an HED diet for 6 weeks. Physiological, biochemical and gene expression assessments of energy balance were performed in the DIO and DR mice on an HED diet and chow-fed mice. The increased weight gain of the DIO mice as compared to the DR mice was associated with increased energy intake and higher plasma leptin and adiponectin levels but not with reduced physical activity or resting energy expenditure. Hypothalamic Pomc gene expression was elevated, but there were no changes in Npy or Agrp expression. Adipose tissue leptin and adiponectin gene expression were significantly reduced in the DIO group as compared to the DR group. Interestingly, ileum expression of G protein-coupled receptor (Gpr) 40 (Gpr40) was significantly increased, whereas Gpr120, Gpr119, Gpr41, and glucagon-like peptide 1 (Glp1) were reduced. Contrastingly, the lower weight gain of the DR group was associated with elevated adipose tissue leptin and adiponectin gene expression, but there were no differences in plasma hormone or hypothalamic gene expression levels as compared to chow-fed mice. Therefore, the present data demonstrate that susceptibility and resistance to diet-induced weight gain in B6 mice appears to be predominantly driven by peripheral rather than hypothalamic modifications, and changes in gut-specific receptors are a potentially important contributor to this variation.

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.

Transient early food restriction leads to hypothalamic changes in the long-lived crowded litter female mice

Transient nutrient restriction in the 3 weeks between birth and weaning (producing "crowded litter" or CL mice) leads to a significant increase in lifespan and is associated with permanent changes in energy homeostasis, leptin, and insulin sensitivity. Here, we show this brief period of early food restriction leads to permanent modulation of the arcuate nucleus of the hypothalamus (ARH), markedly increasing formation of both orexigenic agouti-related peptide (AgRP) and anorexigenic proopiomelanocortin (POMC) projections to the paraventricular nucleus of the hypothalamus (PVH). An additional 4 weeks of caloric restriction, after weaning, does not further intensify the formation of AgRP and POMC projections. Acute leptin stimulation of 12-month-old mice leads to a stronger increase in the levels of hypothalamic pStat3 and cFos activity in CL mice than in controls, suggesting that preweaning food restriction leads to long-lasting enhancement of leptin signaling. In contrast, FoxO1 nuclear exclusion in response to insulin is equivalent in young adult CL and control mice, suggesting that hypothalamic insulin signaling is not modulated by the crowded litter intervention. Markers of hypothalamic reactive gliosis associated with aging, such as Iba1-positive microglia and GFAP-positive astrocytes, are significantly reduced in CL mice as compared to controls at 12 and 22 months of age. Lastly, age-associated overproduction of TNF-α in microglial cells is reduced in CL mice than in age-matched controls. Together, these results suggest that transient early life nutrient deprivation leads to long-term hypothalamic changes which may contribute to the longevity of CL mice.

Differential effects of delayed aging on phenotype and striatal pathology in a murine model of Huntington disease

The common neurodegenerative syndromes exhibit age-related incidence, and many Mendelian neurodegenerative diseases exhibit age-related penetrance. Mutations slowing aging retard age related pathologies. To assess whether delayed aging retards the effects of a mutant allele causing a Huntington's disease (HD)-like syndrome, we generated compound mutant mice, placing a dominant HD knock-in polyglutamine allele onto the slow-aging Snell dwarf genotype. The Snell genotype did not affect mutant huntingtin protein expression. Bigenic and control mice were evaluated prospectively from 10 to 100 weeks of age. Adult HD knock-in allele mice lost weight progressively with weight loss blunted significantly in male bigenic HD knock-in/Snell dwarf mice. Impaired balance beam performance developed significantly more slowly in bigenic HD knock-in/Snell dwarf mice. Striatal dopamine receptor expression was diminished significantly and similarly in all HD-like mice, regardless of the Snell genotype. Striatal neuronal intranuclear inclusion burden was similar between HD knock-in mice with and without the Snell genotype, whereas nigral neuropil aggregates were diminished in bigenic HD knock-in/Snell dwarf mice. Compared with control mice, Snell dwarf mice exhibited differences in regional benzodiazepine and cannabinoid receptor binding site expression. These results indicate that delaying aging delayed behavioral decline with little effect on the development of striatal pathology in this model of HD but may have altered synaptic pathology. These results indicate that mutations prolonging lifespan in mice delay onset of significant phenotypic features of this model and also demonstrate dissociation between striatal pathology and a commonly used behavioral measure of disease burden in HD models.

The Novel Functions of High-Molecular-Mass Complexes Containing Insulin Receptor Substrates in Mediation and Modulation of Insulin-Like Activities: Emerging Concept of Diverse Functions by IRS-Associated Proteins

Insulin-like peptides, such as insulin-like growth factors (IGFs) and insulin, induce a variety of bioactivities, such as growth, differentiation, survival, increased anabolism, and decreased catabolism in many cell types and in vivo. In general, IGFs or insulin bind to IGF-I receptor (IGF-IR) or insulin receptor (IR), activating the receptor tyrosine kinase. Insulin receptor substrates (IRSs) are known to be major substrates of receptor kinases, mediating IGF/insulin signals to direct bioactivities. Recently, we discovered that IRSs form high-molecular-mass complexes (referred to here as IRSomes) even without IGF/insulin stimulation. These complexes contain proteins (referred to here as IRSAPs; IRS-associated proteins), which modulate tyrosine phosphorylation of IRSs by receptor kinases, control IRS stability, and determine intracellular localization of IRSs. In addition, in these complexes, we found not only proteins that are involved in RNA metabolism but also RNAs themselves. Thus, IRSAPs possibly contribute to modulation of IGF/insulin bioactivities. Since it is established that disorder of modulation of insulin-like activities causes various age-related diseases including cancer, we could propose that the IRSome is an important target for treatment of these 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.

Obesity, inflammation, and insulin resistance

White adipose tissue (WAT) is considered an endocrine organ. When present in excess, WAT can influence metabolism via biologically active molecules. Following unregulated production of such molecules, adipose tissue dysfunction results, contributing to complications associated with obesity. Previous studies have implicated pro- and anti-inflammatory substances in the regulation of inflammatory response and in the development of insulin resistance. In obese individuals, pro-inflammatory molecules produced by adipose tissue contribute to the development of insulin resistance and increased risk of cardiovascular disease. On the other hand, the molecules with anti-inflammatory action, that have been associated with the improvement of insulin sensitivity, have your decreased production. Imbalance of these substances contributes significantly to metabolic disorders found in obese individuals. The current review aims to provide updated information regarding the activity of biomolecules produced by WAT.

20 years of leptin: connecting leptin signaling to biological function

Hypothalamic leptin action promotes negative energy balance and modulates glucose homeostasis, as well as serving as a permissive signal to the neuroendocrine axes that control growth and reproduction. Since the initial discovery of leptin 20 years ago, we have learned a great deal about the molecular mechanisms of leptin action. An important aspect of this has been the dissection of the cellular mechanisms of leptin signaling, and how specific leptin signals influence physiology. Leptin acts via the long form of the leptin receptor LepRb. LepRb activation and subsequent tyrosine phosphorylation recruits and activates multiple signaling pathways, including STAT transcription factors, SHP2 and ERK signaling, the IRS-protein/PI3Kinase pathway, and SH2B1. Each of these pathways controls specific aspects of leptin action and physiology. Important inhibitory pathways mediated by suppressor of cytokine signaling proteins and protein tyrosine phosphatases also limit physiologic leptin action. This review summarizes the signaling pathways engaged by LepRb and their effects on energy balance, glucose homeostasis, and reproduction. Particular emphasis is given to the multiple mouse models that have been used to elucidate these functions in vivo.

20 years of leptin: role of leptin in human reproductive disorders

Leptin, as a key hormone in energy homeostasis, regulates neuroendocrine function, including reproduction. It has a permissive role in the initiation of puberty and maintenance of the hypothalamic-pituitary-gonadal axis. This is notable in patients with either congenital or acquired leptin deficiency from a state of chronic energy insufficiency. Hypothalamic amenorrhea is the best-studied, with clinical trials confirming a causative role of leptin in hypogonadotropic hypogonadism. Implications of leptin deficiency have also emerged in the pathophysiology of hypogonadism in type 1 diabetes. At the other end of the spectrum, hyperleptinemia may play a role in hypogonadism associated with obesity, polycystic ovarian syndrome, and type 2 diabetes. In these conditions of energy excess, mechanisms of reproductive dysfunction include central leptin resistance as well as direct effects at the gonadal level. Thus, reproductive dysfunction due to energy imbalance at both ends can be linked to leptin.

IRS2 integrates insulin/IGF1 signalling with metabolism, neurodegeneration and longevity

Understanding how metabolism and nutrient homeostasis integrates with life span and neurodegeneration is a complicated undertaking. Important inconsistencies have emerged recently regarding the role of insulin-like signalling and the progression of neurodegenerative disease. Insulin resistance and type 2 diabetes are associated with clinical Alzheimer's disease, whereas study in lower organisms shows that reduced insulin-like signalling slows the progressive neurodegeneration and increases life span. From a clinical perspective, compensatory hyperinsulinaemia to overcome systemic insulin resistance is thought to be a healthy goal, because it circumvents immediate catastrophic consequences of hyperglycaemia; however, study in flies, nematodes and mice indicate that excess insulin signalling can damage cellular function and accelerate ageing. Maintenance of the central nervous system (CNS) has particular importance for life span and metabolism. A conflict arises because reduced insulin/IGF1 signalling in the CNS is associated with longevity, but can dysregulate glucose and energy homeostasis, and promote overweight. Here, we explore how the genetic manipulation of insulin/IGF1 signalling system can influence systemic metabolism, life span and neurodegeneration.

Leptin and insulin signaling in dopaminergic neurons: relationship between energy balance and reward system

The central actions of leptin and insulin are essential for the regulation of energy and glucose homeostasis. In addition to the crucial effects on the hypothalamus, emerging evidence suggests that the leptin and insulin signaling can act on other brain regions to mediate the reward value of nutrients. Recent studies have indicated the midbrain dopaminergic neurons as a potential site for leptin' and insulin's actions on mediating the feeding behaviors and therefore affecting the energy balance. Although molecular details about the integrative roles of leptin and insulin in this subset of neurons remain to be investigated, substantial body of evidence by far imply that the signaling pathways regulated by leptin and insulin may play an essential role in the regulation of energy balance through the control of food-associated reward. This review therefore describes the convergence of energy regulation and reward system, particularly focusing on leptin and insulin signaling in the midbrain dopaminergic neurons.

Long-lived crowded-litter mice exhibit lasting effects on insulin sensitivity and energy homeostasis

The action of nutrients on early postnatal growth can influence mammalian aging and longevity. Recent work has demonstrated that limiting nutrient availability in the first 3 wk of life [by increasing the number of pups in the crowded-litter (CL) model] leads to extension of mean and maximal lifespan in genetically normal mice. In this study, we aimed to characterize the impact of early-life nutrient intervention on glucose metabolism and energy homeostasis in CL mice. In our study, we used mice from litters supplemented to 12 or 15 pups and compared those to control litters limited to eight pups. At weaning and then throughout adult life, CL mice are significantly leaner and consume more oxygen relative to control mice. At 6 mo of age, CL mice had low fasting leptin concentrations, and low-dose leptin injections reduced body weight and food intake more in CL female mice than in controls. At 22 mo, CL female mice also have smaller adipocytes compared with controls. Glucose and insulin tolerance tests show an increase in insulin sensitivity in 6 mo old CL male mice, and females become more insulin sensitive later in life. Furthermore, β-cell mass was significantly reduced in the CL male mice and was associated with reduction in β-cell proliferation rate in these mice. Together, these data show that early-life nutrient intervention has a significant lifelong effect on metabolic characteristics that may contribute to the increased lifespan of CL mice.

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.

Rictor/mTORC2 facilitates central regulation of energy and glucose homeostasis

Insulin signaling in the central nervous system (CNS) regulates energy balance and peripheral glucose homeostasis. Rictor is a key regulatory/structural subunit of the mTORC2 complex and is required for hydrophobic motif site phosphorylation of Akt at serine 473. To examine the contribution of neuronal Rictor/mTORC2 signaling to CNS regulation of energy and glucose homeostasis, we utilized Cre-LoxP technology to generate mice lacking Rictor in all neurons, or in either POMC or AgRP expressing neurons. Rictor deletion in all neurons led to increased fat mass and adiposity, glucose intolerance and behavioral leptin resistance. Disrupting Rictor in POMC neurons also caused obesity and hyperphagia, fasting hyperglycemia and pronounced glucose intolerance. AgRP neuron specific deletion did not impact energy balance but led to mild glucose intolerance. Collectively, we show that Rictor/mTORC2 signaling, especially in POMC-expressing neurons, is important for central regulation of energy and glucose homeostasis.

Insulin signaling, resistance, and the metabolic syndrome: insights from mouse models into disease mechanisms

Insulin resistance is a major underlying mechanism responsible for the 'metabolic syndrome', which is also known as insulin resistance syndrome. The incidence of the metabolic syndrome is increasing at an alarming rate, becoming a major public and clinical problem worldwide. The metabolic syndrome is represented by a group of interrelated disorders, including obesity, hyperglycemia, hyperlipidemia, and hypertension. It is also a significant risk factor for cardiovascular disease and increased morbidity and mortality. Animal studies have demonstrated that insulin and its signaling cascade normally control cell growth, metabolism, and survival through the activation of MAPKs and activation of phosphatidylinositide-3-kinase (PI3K), in which the activation of PI3K associated with insulin receptor substrate 1 (IRS1) and IRS2 and subsequent Akt→Foxo1 phosphorylation cascade has a central role in the control of nutrient homeostasis and organ survival. The inactivation of Akt and activation of Foxo1, through the suppression IRS1 and IRS2 in different organs following hyperinsulinemia, metabolic inflammation, and overnutrition, may act as the underlying mechanisms for the metabolic syndrome in humans. Targeting the IRS→Akt→Foxo1 signaling cascade will probably provide a strategy for therapeutic intervention in the treatment of type 2 diabetes and its complications. This review discusses the basis of insulin signaling, insulin resistance in different mouse models, and how a deficiency of insulin signaling components in different organs contributes to the features of the metabolic syndrome. Emphasis is placed on the role of IRS1, IRS2, and associated signaling pathways that are coupled to Akt and the forkhead/winged helix transcription factor Foxo1.

Irs2 and Irs4 synergize in non-LepRb neurons to control energy balance and glucose homeostasis

Insulin receptor substrates (Irs1, 2, 3 and Irs4) mediate the actions of insulin/IGF1 signaling. They have similar structure, but distinctly regulate development, growth, and metabolic homeostasis. Irs2 contributes to central metabolic sensing, partially by acting in leptin receptor (LepRb)-expressing neurons. Although Irs4 is largely restricted to the hypothalamus, its contribution to metabolic regulation is unclear because Irs4-null mice barely distinguishable from controls. We postulated that Irs2 and Irs4 synergize and complement each other in the brain. To examine this possibility, we investigated the metabolism of whole body Irs4^−/y mice that lacked Irs2 in the CNS (bIrs2^−/−·Irs4^−/y) or only in LepRb-neurons (Lepr^∆Irs2·Irs4^−/y). bIrs2^−/−·Irs4^−/y mice developed severe obesity and decreased energy expenditure, along with hyperglycemia and insulin resistance. Unexpectedly, the body weight and fed blood glucose levels of Lepr^∆Irs2·Irs4^−/y mice were not different from Lepr^∆Irs2 mice, suggesting that the functions of Irs2 and Irs4 converge upon neurons that are distinct from those expressing LepRb.

Molecular Basis of Insulin Resistance: The Role of IRS and Foxo1 in the Control of Diabetes Mellitus and Its Complications

Insulin/IGF-1 signaling plays a central role in control of cellular metabolism and survival, while insulin receptor substrate (IRS) protein -1 and -2 and downstream PI-3 kinase→Akt→Foxo1 signaling cascade play key roles in many functions of insulin/IGF-1. Dysregulation of this branch of signaling cascades may provide a mechanism for insulin resistance as we observed in cells, animals, and even humans. Targeting this branch of IRS→Foxo1 signaling may provide us with fundamental strategies for drug development in the future.

Leptin signaling and leptin resistance

Leptin is secreted into the bloodstream by adipocytes and is required for the maintenance of energy homeostasis and body weight. Leptin deficiency or genetic defects in the components of the leptin signaling pathways cause obesity. Leptin controls energy balance and body weight mainly through leptin receptor b (LEPRb)-expressing neurons in the brain, particularly in the hypothalamus. These LEPRb-expressing neurons function as the first-order neurons that project to the second-order neurons located within and outside the hypothalamus, forming a neural network that controls the energy homeostasis and body weight. Multiple factors, including inflammation and endoplasmic reticulum (ER) stress, contribute to leptin resistance. Leptin resistance is the key risk factor for obesity. This review is focused on recent advance about leptin action, leptin signaling, and leptin resistance.

Integrating metabolism and longevity through insulin and IGF1 signaling

The insulin pathway coordinates growth, development, metabolic homoeostasis, fertility, and stress resistance, which influence life span. Compensatory hyperinsulinemia to overcome systemic insulin resistance circumvents the immediate consequences of hyperglycemia. Work on flies, nematodes, and mice indicate that excess insulin signaling damages cellular function and accelerates aging. Maintenance of the central nervous system (CNS) has particular importance for life span. Reduced insulin/IGF1 signaling in the CNS can dysregulate peripheral energy homeostasis and metabolism, promote obesity, and extend life span. Genetic manipulations of insulin/IGF1 signaling components are revealing neuronal circuits that might resolve the central regulation of systemic metabolism from organism longevity.

Leptin signaling and circuits in puberty and fertility

Leptin is an adipocyte-derived hormone involved in a myriad of physiological process, including the control of energy balance and several neuroendocrine axes. Leptin-deficient mice and humans are obese, diabetic, and display a series of neuroendocrine and autonomic abnormalities. These individuals are infertile due to a lack of appropriate pubertal development and inadequate synthesis and secretion of gonadotropins and gonadal steroids. Leptin receptors are expressed in many organs and tissues, including those related to the control of reproductive physiology (e.g., the hypothalamus, pituitary gland, and gonads). In the last decade, it has become clear that leptin receptors located in the brain are major players in most leptin actions, including reproduction. Moreover, the recent development of molecular techniques for brain mapping and the use of genetically modified mouse models have generated crucial new findings for understanding leptin physiology and the metabolic influences on reproductive health. In the present review, we will highlight the new advances in the field, discuss the apparent contradictions, and underline the relevance of this complex physiological system to human health. We will focus our review on the hypothalamic circuitry and potential signaling pathways relevant to leptin’s effects in reproductive control, which have been identified with the use of cutting-edge technologies of molecular mapping and conditional knockouts.