Metabolic transceivers: in tune with the central melanocortin system

Region-specific transcriptomic responses to obesity and diabetes in macaque hypothalamus

The hypothalamus plays a crucial role in the progression of obesity and diabetes; however, its structural complexity and cellular heterogeneity impede targeted treatments. Here, we profiled the single-cell and spatial transcriptome of the hypothalamus in obese and sporadic type 2 diabetic macaques, revealing primate-specific distributions of clusters and genes as well as spatial region, cell-type-, and gene-feature-specific changes. The infundibular (INF) and paraventricular nuclei (PVN) are most susceptible to metabolic disruption, with the PVN being more sensitive to diabetes. In the INF, obesity results in reduced synaptic plasticity and energy sensing capability, whereas diabetes involves molecular reprogramming associated with impaired tanycytic barriers, activated microglia, and neuronal inflammatory response. In the PVN, cellular metabolism and neural activity are suppressed in diabetic macaques. Spatial transcriptomic data reveal microglia's preference for the parenchyma over the third ventricle in diabetes. Our findings provide a comprehensive view of molecular changes associated with obesity and diabetes.

Effects of Low Energy Availability on Reproductive Functions and Their Underlying Neuroendocrine Mechanisms

It is known that metabolic disturbances suppress reproductive functions in females. The mechanisms underlying metabolic and nutritional effects on reproductive functions have been established based on a large body of clinical and experimental data. From the 1980s to 1990s, it was revealed that disrupted gonadotropin-releasing hormone (GnRH) secretion is the main cause of reproductive impairments in metabolic and nutritional disorders. From the late 1990s to early 2000s, it was demonstrated that, in addition to their primary functions, some appetite- or metabolism-regulating factors affect GnRH secretion. Furthermore, in the early 2000s, kisspeptin, which is a potent positive regulator of GnRH secretion, was newly discovered, and it has been revealed that kisspeptin integrates the effects of metabolic status on GnRH neurons. Recent studies have shown that kisspeptin mediates at least some of the effects of appetite- and metabolism-regulating factors on GnRH neurons. Thus, kisspeptin might be a useful clinical target for treatments aimed at restoring reproductive functions in individuals with metabolic or nutritional disturbances, such as those who exercise excessively, experience marked weight loss, or suffer from eating disorders. This paper presents a review of what is currently known about the effects of metabolic status on reproductive functions and their underlying mechanisms by summarizing the available evidence.

Metabolic Impact on the Hypothalamic Kisspeptin-Kiss1r Signaling Pathway

A large body of data has established the hypothalamic kisspeptin (KP) and its receptor, KISS1R, as major players in the activation of the neuroendocrine reproductive axis at the time of puberty and maintenance of reproductive capacity in the adult. Due to its strategic location, this ligand-receptor pair acts as an integrator of cues from gonadal steroids as well as of circadian and seasonal variation-related information on the reproductive axis. Besides these cues, the activity of the hypothalamic KP signaling is very sensitive to the current metabolic status of the body. In conditions of energy imbalance, either positive or negative, a number of alterations in the hypothalamic KP signaling pathway have been documented in different mammalian models including nonhuman primates and human. Deficiency of metabolic fuels during fasting causes a marked reduction of Kiss1 gene transcript levels in the hypothalamus and, hence, decreases the output of KP-containing neurons. Food intake or exogenous supply of metabolic cues, such as leptin, reverses metabolic insufficiency-related changes in the hypothalamic KP signaling. Likewise, alterations in Kiss1 expression have also been reported in other situations of energy imbalance like diabetes and obesity. Information related to the body's current metabolic status reaches to KP neurons both directly as well as indirectly via a complex network of other neurons. In this review article, we have provided an updated summary of the available literature on the regulation of the hypothalamic KP-Kiss1r signaling by metabolic cues. In particular, the potential mechanisms of metabolic impact on the hypothalamic KP-Kiss1r signaling, in light of available evidence, are discussed.

MC1R variants affect the expression of melanocortin and melanogenic genes and the association between melanocortin genes and coloration

The melanocortin-1 receptor (MC1R) gene influences coloration by altering the expression of genes acting downstream in the melanin synthesis. MC1R belongs to the melanocortin system, a genetic network coding for the ligands that regulate MC1R and other melanocortin receptors controlling different physiological and behavioural traits. The impact of MC1R variants on these regulatory melanocortin genes was never considered, even though MC1R mutations could alter the influence of these genes on coloration (e.g. by decreasing MC1R response to melanocortin ligands). Using barn owl growing feathers, we investigated the differences between MC1R genotypes in the (co)expression of six melanocortin and nine melanogenic-related genes and in the association between melanocortin gene expression and phenotype (feather pheomelanin content). Compared to the MC1R rufous allele, responsible for reddish coloration, the white allele was not only associated with an expected lower expression of melanogenic-related genes (TYR, TYRP1, OCA2, SLC45A2, KIT, DCT) but also with a lower MC1R expression and a higher expression of ASIP, the MC1R antagonist. More importantly, the expression of PCSK2, responsible for the maturation of the MC1R agonist, α-melanocyte-stimulating hormone, was positively related to pheomelanin content in MC1R white homozygotes but not in individuals carrying the MC1R rufous allele. These findings indicate that MC1R mutations not only alter the expression of melanogenic-related genes but also the association between coloration and the expression of melanocortin genes upstream of MC1R. This suggests that MC1R mutations can modulate the regulation of coloration by the pleiotropic melanocortin genes, potentially decoupling the often-observed associations between coloration and other phenotypes.

A distinct brain pathway links viral RNA exposure to sickness behavior

Sickness behaviors and metabolic responses to invading pathogens are common to nearly all types of infection. These responses evolved to provide short-term benefit to the host to ward off infection, but impact on quality of life, and when prolonged lead to neurodegeneration, depression, and cachexia. Among the major infectious agents, viruses most frequently enter the brain, resulting in profound neuroinflammation. We sought to define the unique features of the inflammatory response in the brain to these infections. We demonstrate that the molecular pathway defining the central response to dsRNA is distinct from that found in the periphery. The behavioral and physical response to the dsRNA mimetic poly I:C is dependent on signaling via MyD88 when it is delivered centrally, whereas this response is mediated via the TRIF pathway when delivered peripherally. We also define the likely cellular candidates for this MyD88-dependent step. These findings suggest that symptom management is possible without ameliorating protective antiviral immune responses.

The Obese Brain--Effects of Bariatric Surgery on Energy Balance Neurocircuitry

Obesity is a highly prevalent disease in the world and with a major impact on global health. While genetic components are also involved in its pathogenesis, in recent years, it has shown a critical role of the innate and adaptive immune cell response in many tissues triggered by excess of nutrients such as lipids and glucose. Free fatty acids and other nutrient-related signals induce damage such as insulin resistance in the peripheral tissues but also in the brain. Specifically in the hypothalamus, these metabolic signals can trigger significant changes in the control of energy balance. Recent studies have shown that saturated fat disrupts melanocortin signaling of hypothalamic neuronal subgroups pivotal to energy control. Bariatric surgery is a treatment option for obesity when other tools have failed, because it is more effective than pharmacotherapy concerning of weight loss itself and in improvement of obesity-related comorbidities. Here, we review the mechanisms by which Roux-en Y gastric bypass (RYGB) can change peripheral signals that modulate melanocortin circuits involved in the regulation of energy balance.

A talk between fat tissue, gut, pancreas and brain to control body weight

The incidence of obesity and its related disorders are increasing at a rate of pandemic proportions. Understanding the mechanisms behind the maintenance of energy balance is fundamental in developing treatments for clinical syndromes including obesity and diabetes. A neural network located in the nucleus of the solitary tract-area postrema complex in the hindbrain and the hypothalamus in the forebrain has long been implicated in the control of energy balance. In the hypothalamus this central neuronal network consists of small populations of nuclei with distinct functions such as the arcuate nucleus (ARH), the paraventricular nuclei of the hypothalamus (PVH), the dorsomedial (DMH), the ventromedial (VMH) and the lateral hypothalamus (LH). These hypothalamic areas form interconnected neuronal circuits that respond to fluctuations in energy status by altering the expression of neuropeptides, leading to changes in energy intake and expenditure. Regulation of these hypothalamic nuclei involves the actions of orexigenic peptides (ie ghrelin), which act to stimulate energy intake and decrease energy expenditure, and anorexigenic peptides (ie. leptin and insulin), which act to reduce energy intake and stimulate energy expenditure. Here we review the role of the ARH, DMH and PVH in the control of energy homeostasis and how recent advances in research technologies (Cre-loxP technology, optogenetics and pharmacogenetics) have shed light on the role of these hypothalamic nuclei in the control of energy balance. Such novel findings include the implication of ARH POMC and AgRP neurons in the browning of white adipose tissue to regulate energy expenditure as well as the likely existence of divergent hypothalamic pathways in the DMH and PVH in the control of food intake and energy expenditure.

Effects of chronic growth hormone overexpression on appetite-regulating brain gene expression in coho salmon

Organisms must carefully regulate energy intake and expenditure to balance growth and trade-offs with other physiological processes. This regulation is influenced by key pathways controlling appetite, feeding behaviour and energy homeostasis. Growth hormone (GH) transgenesis provides a model where food intake can be elevated, and is associated with dramatic modifications of growth, metabolism, and feeding behaviour, particularly in fish. RNA-Seq and qPCR analyses were used to compare the expression of multiple genes important in appetite regulation within brain regions and the pituitary gland (PIT) of GH transgenic (fed fully to satiation or restricted to a wild-type ration throughout their lifetime) and wild-type coho salmon (Oncorhynchus kisutch). RNA-Seq results showed that differences in both genotype and ration levels resulted in differentially expressed genes associated with appetite regulation in transgenic fish, including elevated Agrp1 in hypothalamus (HYP) and reduced Mch in PIT. Altered mRNA levels for Agrp1, Npy, Gh, Ghr, Igf1, Mch and Pomc were also assessed using qPCR analysis. Levels of mRNA for Agrp1, Gh, and Ghr were higher in transgenic than wild-type fish in HYP and in the preoptic area (POA), with Agrp1 more than 7-fold higher in POA and 12-fold higher in HYP of transgenic salmon compared to wild-type fish. These data are consistent with the known roles of orexigenic factors on foraging behaviour acting via GH and through MC4R receptor-mediated signalling. Igf1 mRNA was elevated in fully-fed transgenic fish in HYP and POA, but not in ration-restricted fish, yet both of these types of transgenic animals have very pronounced feeding behaviour relative to wild-type fish, suggesting IGF1 is not playing a direct role in appetite stimulation acting via paracrine or autocrine mechanisms. The present findings provide new insights on mechanisms ruling altered appetite regulation in response to chronically elevated GH, and on potential pathways by which elevated feeding response is controlled, independently of food availability and growth.

Long-term hyperphagia and caloric restriction caused by low- or high-density husbandry have differential effects on zebrafish postembryonic development, somatic growth, fat accumulation and reproduction

In recent years, the zebrafish (Danio rerio) has emerged as an alternative vertebrate model for energy homeostasis and metabolic diseases, including obesity and anorexia. It has been shown that diet-induced obesity (DIO) in zebrafish shares multiple pathophysiological features with obesity in mammals. However, a systematic and comprehensive analysis of the different pathways of energy expenditure in obese and starved fish had been missing thus far. Here, we carry out long-term ad libitum feeding (hyperphagia) and caloric restriction studies induced by low- or high-density husbandry, respectively, to investigate the impact of caloric intake on the timing of scale formation, a crucial step of postembryonic development and metamorphosis, and on somatic growth, body weight, fat storage and female reproduction. We show that all of them are positively affected by increased caloric intake, that middle-aged fish develop severe DIO, and that the body mass index (BMI) displays a strict linear correlation with whole-body triglyceride levels in adult zebrafish. Interestingly, juvenile fish are largely resistant to DIO, while BMI and triglyceride values drop in aged fish, pointing to aging-associated anorexic effects. Histological analyses further indicate that increased fat storage in white adipose tissue involves both hyperplasia and hypertrophy of adipocytes. Furthermore, in ovaries, caloric intake primarily affects the rate of oocyte growth, rather than total oocyte numbers. Finally, comparing the different pathways of energy expenditure with each other, we demonstrate that they are differentially affected by caloric restriction / high-density husbandry. In juvenile fish, scale formation is prioritized over somatic growth, while in sexually mature adults, female reproduction is prioritized over somatic growth, and somatic growth over fat storage. Our data will serve as a template for future functional studies to dissect the neuroendocrine regulators of energy homeostasis mediating differential energy allocation.

Fibroblast growth factor 19-targeted therapies for the treatment of metabolic disease

INTRODUCTION

Fibroblast growth factors (FGFs) belong to the FGF superfamily with diverse biological functions, including proliferation, cellular differentiation, wound repair, angiogenesis and tumorigenesis. The ability to reduce liver fat content and concentrations of triglycerides, total cholesterol and plasma glucose, and to improve sensitivity and limit pro-lipogenic properties of insulin, makes FGF19 a promising therapeutic target for the treatment of metabolic syndrome. FGF19 regulates bile acid biosynthesis in the bile duct, glucose metabolism and vitamin D and phosphate homeostasis, raises the metabolic rate, reduces body weight, and ameliorates diabetes in mice. The therapeutic potential of FGF19 to treat metabolic disorders has been widely studied in animal models, but currently there are no reports concerning its use in humans.

AREAS COVERED

The following article highlights the metabolic effects and mechanism of action of FGF19. It also discusses the potential therapies that target FGF19.

EXPERT OPINION

FGF19 is emerging as a new target for the therapy of metabolic disorders, including diabetes. The results obtained from animal models are promising. However, there is still much to be done before the translation of these effects into practice will be possible.

Crosstalk between gastrointestinal neurons and the brain in the control of food intake

Recent data have emphasized that the gastrointestinal nervous system is preponderant in the sensing of nutrients and hormones and its translation in terms of control of food intake by the central nervous system. More specifically, the gastrointestinal neural system participates in the control of hunger via the sensing of at least two major macronutrients, e.g. glucose and protein, which may control hunger sensations from the portal vein. Protein are first sensed by mu-opioid receptors present in the portal vein walls to induce intestinal gluconeogenesis-via a reflex arc and next portal glucose sensing. The gastrointestinal nervous system may also account for the rapid benefits of gastric bypass surgeries on energy homeostasis (hunger and body weight) and glucose homeostasis (insulin sensitivity). This knowledge provides novel mechanisms of control of body weight, which might be useful to envision future approaches of prevention or treatment of obesity.

Adrenocorticotropin hormone 1-39 promotes proliferation and differentiation of oligodendroglial progenitor cells and protects from excitotoxic and inflammation-related damage

Oligodendroglia (OL) are highly susceptible to damage and, like neurons, are terminally differentiated. It is important to protect OL precursors (OPC) because they are reservoirs of differentiating cells capable of myelination following perinatal insult and remyelination in white matter diseases, including multiple sclerosis (MS). Patients with relapsing-remitting MS are commonly treated with high-dose corticosteroids (CS) when experiencing an exacerbation. Adrenocorticotropin hormone (ACTH), a primary component of another approved MS exacerbation treatment, is a melanocortin peptide that stimulates production of CS by the adrenals. Melanocortin receptors are also found in the central nervous system (CNS) and on immune cells. ACTH is produced within the CNS and may have CS-independent effects on glia. We found that ACTH 1-39 stimulated proliferation of OPC, and to a lesser extent astroglia (AS) and microglia (MG), in rat glial cultures. ACTH accelerated differentiation of PDGFRα(+) OPC to a later stage marked by galactolipid expression and caused greater expansion of OL myelin-like sheets compared with untreated cells. Protective effects of ACTH on OPC were assessed by treating cultures with selected toxic agents, with or without ACTH. At 200 nM, ACTH protected OPC from death induced by staurosporine, glutamate, NMDA, AMPA, kainate, quinolinic acid, H2 O2 , and slow NO release, but not against kynurenic acid or rapid NO release. These agents and ACTH were not toxic to AS or MG. Our findings indicate that ACTH 1-39 provides benefits by increasing the number of OPC, accelerating their development into mature OL, and reducing OPC death from toxic insults.

Intestinal glucose metabolism revisited

It is long known that the gut can contribute to the control of glucose homeostasis via its high glucose utilization capacity. Recently, a novel function in intestinal glucose metabolism (gluconeogenesis) was described. The intestine notably contributes to about 20-25% of total endogenous glucose production during fasting. More importantly, intestinal gluconeogenesis is capable of regulating energy homeostasis through a communication with the brain. The periportal neural system senses glucose (produced by intestinal gluconeogenesis) in the portal vein walls, which sends a signal to the brain to modulate hunger sensations and whole body glucose homeostasis. Relating to the mechanism of glucose sensing, the role of the glucose receptor SGLT3 has been strongly suggested. Moreover, dietary proteins mobilize intestinal gluconeogenesis as a mandatory link between their detection in the portal vein and their effect of satiety. In the same manner, dietary soluble fibers exert their anti-obesity and anti-diabetic effects via the induction of intestinal gluconeogenesis. FFAR3 is a key neural receptor involved in the specific sensing of propionate to activate a gut-brain reflex arc triggering the induction of the gut gluconeogenic function. Lastly, intestinal gluconeogenesis might also be involved in the rapid metabolic improvements induced by gastric bypass surgeries of obesity.

Biphasic effect of melanocortin agonists on metabolic rate and body temperature

The melanocortin system regulates metabolic homeostasis and inflammation. Melanocortin agonists have contradictorily been reported to both increase and decrease metabolic rate and body temperature. We find two distinct physiologic responses occurring at similar doses. Intraperitoneal administration of the nonselective melanocortin agonist MTII causes a melanocortin-4 receptor (Mc4r)-mediated hypermetabolism/hyperthermia. This is preceded by a profound, transient hypometabolism/hypothermia that is preserved in mice lacking any one of Mc1r, Mc3r, Mc4r, or Mc5r. Three other melanocortin agonists also caused hypothermia, which is actively achieved via seeking a cool environment, vasodilation, and inhibition of brown adipose tissue thermogenesis. These results suggest that the hypometabolic/hypothermic effect of MTII is not due to a failure of thermoregulation. The hypometabolism/hypothermia was prevented by dopamine antagonists, and MTII selectively activated arcuate nucleus dopaminergic neurons, suggesting that these neurons may contribute to the hypometabolism/hypothermia. We propose that the hypometabolism/hypothermia is a regulated response, potentially beneficial during extreme physiologic stress.

The histone acetyltransferase MOF activates hypothalamic polysialylation to prevent diet-induced obesity in mice

Overfeeding causes rapid synaptic remodeling in hypothalamus feeding circuits. Polysialylation of cell surface molecules is a key step in this neuronal rewiring and allows normalization of food intake. Here we examined the role of hypothalamic polysialylation in the long-term maintenance of body weight, and deciphered the molecular sequence underlying its nutritional regulation. We found that upon high fat diet (HFD), reduced hypothalamic polysialylation exacerbated the diet-induced obese phenotype in mice. Upon HFD, the histone acetyltransferase MOF was rapidly recruited on the St8sia4 polysialyltransferase-encoding gene. Mof silencing in the mediobasal hypothalamus of adult mice prevented activation of the St8sia4 gene transcription, reduced polysialylation, altered the acute homeostatic feeding response to HFD and increased the body weight gain. These findings indicate that impaired hypothalamic polysialylation contribute to the development of obesity, and establish a role for MOF in the brain control of energy balance.

Temporal changes in nutritional state affect hypothalamic POMC peptide levels independently of leptin in adult male mice

Hypothalamic proopiomelanocortin (POMC) neurons constitute a critical anorexigenic node in the central nervous system (CNS) for maintaining energy balance. These neurons directly affect energy expenditure and feeding behavior by releasing bioactive neuropeptides but are also subject to signals directly related to nutritional state such as the adipokine leptin. To further investigate the interaction of diet and leptin on hypothalamic POMC peptide levels, we exposed 8- to 10-wk-old male POMC-Discosoma red fluorescent protein (DsRed) transgenic reporter mice to either 24-48 h (acute) or 2 wk (chronic) food restriction, high-fat diet (HFD), or leptin treatment. Using semiquantitative immunofluorescence and radioimmunoassays, we discovered that acute fasting and chronic food restriction decreased the levels of adrenocorticotropic hormone (ACTH), α-melanocyte-stimulating hormone (α-MSH), and β-endorphin in the hypothalamus, together with decreased DsRed fluorescence, compared with control ad libitum-fed mice. Furthermore, acute but not chronic HFD or leptin administration selectively increased α-MSH levels in POMC fibers and increased DsRed fluorescence in POMC cell bodies. HFD and leptin treatments comparably increased circulating leptin levels at both time points, suggesting that transcription of Pomc and synthesis of POMC peptide products are not modified in direct relation to the concentration of plasma leptin. Our findings indicate that negative energy balance persistently downregulated POMC peptide levels, and this phenomenon may be partially explained by decreased leptin levels, since these changes were blocked in fasted mice treated with leptin. In contrast, sustained elevation of plasma leptin by HFD or hormone supplementation did not significantly alter POMC peptide levels, indicating that enhanced leptin signaling does not chronically increase Pomc transcription and peptide synthesis.

Central action of FGF19 reduces hypothalamic AGRP/NPY neuron activity and improves glucose metabolism

Tight control of glucose excursions has been a long-standing goal of treatment for patients with type 2 diabetes mellitus in order to ameliorate the morbidity and mortality associated with hyperglycemia. Fibroblast growth factor (FGF) 19 is a hormone-like enterokine released postprandially that emerged as a potential therapeutic agent for metabolic disorders, including diabetes and obesity. Remarkably, FGF19 treatment has hypoglycemic actions that remain potent in models of genetic and acquired insulin resistance. Here, we provided evidence that the central nervous system responds to FGF19 administered in the periphery. Then, in two mouse models of insulin resistance, leptin-deficiency and high-fat diet feeding, third intra-cerebro-ventricular infusions of FGF19 improved glycemic status, reduced insulin resistance and potentiated insulin signaling in the periphery. In addition, our study highlights a new mechanism of central FGF19 action, involving the suppression of AGRP/NPY neuronal activity. Overall, our work unveils novel regulatory pathways induced by FGF19 that will be useful in the design of novel strategies to control diabetes in obesity.

Nutrient control of hunger by extrinsic gastrointestinal neurons

The neural sensing of nutrients during food digestion plays a key role in the regulation of hunger. Recent data have emphasized that the extrinsic gastrointestinal nervous system is preponderant in this phenomenon and in its translation to the control of food intake by the central nervous system (CNS). Nutrient sensing by the extrinsic gastrointestinal nervous system may account for the satiation induced by food lipids, the satiety initiated by food protein, and for the rapid benefits of gastric bypass surgeries on both glucose and energy homeostasis. Thus, this recent knowledge provides novel examples of the mechanisms that control food intake and body weight, and this might pave the way for future approaches to the prevention and/or treatment of obesity.