Agouti-related protein: more than a melanocortin-4 receptor antagonist?

Targeting the central melanocortin system for the treatment of metabolic disorders

A large body of preclinical and clinical data shows that the central melanocortin system is a promising therapeutic target for treating various metabolic disorders such as obesity and cachexia, as well as anorexia nervosa. Setmelanotide, which functions by engaging the central melanocortin circuitry, was approved by the FDA in 2020 for use in certain forms of syndromic obesity. Furthermore, the FDA approvals in 2019 of two peptide drugs targeting melanocortin receptors for the treatment of generalized hypoactive sexual desire disorder (bremelanotide) and erythropoietic protoporphyria-associated phototoxicity (afamelanotide) demonstrate the safety of this class of peptides. These approvals have also renewed excitement in the development of therapeutics targeting the melanocortin system. Here, we review the anatomy and function of the melanocortin system, discuss progress and challenges in developing melanocortin receptor-based therapeutics, and outline potential metabolic and behavioural disorders that could be addressed using pharmacological agents targeting these receptors.

Gut hormones and obesity: physiology and therapies

Over the past 30 years, it has been established that hormones produced by the gut, pancreas, and adipose tissue are key players in the control of body weight. These hormones act through a complex neuroendocrine system, including the hypothalamus, to regulate metabolism and energy homeostasis. In obesity, this homeostatic balance is disrupted, either through alterations in the levels of these hormones or through resistance to their actions. Alterations in gut hormone secretion following gastric bypass surgery are likely to underlie the dramatic and persistent loss of weight following this procedure, as well as the observed amelioration in type 2 diabetes mellitus. Medications based on the gut hormone GLP-1 are currently in clinical use to treat type 2 diabetes mellitus and have been shown to produce weight loss. Further therapies for obesity based on other gut hormones are currently in development.

Role of GnRH Neurons and Their Neuronal Afferents as Key Integrators between Food Intake Regulatory Signals and the Control of Reproduction

Reproductive function is regulated by a plethora of signals that integrate physiological and environmental information. Among others, metabolic factors are key components of this circuit since they inform about the propitious timing for reproduction depending on energy availability. This information is processed mainly at the hypothalamus that, in turn, modulates gonadotropin release from the pituitary and, thereby, gonadal activity. Metabolic hormones, such as leptin, insulin, and ghrelin, act as indicators of the energy status and convey this information to the reproductive axis regulating its activity. In this review, we will analyse the central mechanisms involved in the integration of this metabolic information and their contribution to the control of the reproductive function. Particular attention will be paid to summarize the participation of GnRH, Kiss1, NPY, and POMC neurons in this process and their possible interactions to contribute to the metabolic control of reproduction.

Direct regulation of GnRH neuron excitability by arcuate nucleus POMC and NPY neuron neuropeptides in female mice

Hypothalamic neuropeptide Y (NPY) and proopiomelanocortin (POMC) neurons act to sense and coordinate the brain's responses to metabolic cues. One neuronal network that is very sensitive to metabolic status is that controlling fertility. In this study, we investigated the impact of neuropeptides released by NPY and POMC neurons on the cellular excitability of GnRH neurons, the final output cells of the brain controlling fertility. The majority (∼70%) of GnRH neurons were activated by α-melanocyte-stimulating hormone, and this resulted from the direct postsynaptic activation of melanocortin receptor 3 and melanocortin receptor 4. A small population of GnRH neurons (∼15%) was excited by cocaine and amphetamine-regulated transcript or inhibited by β-endorphin. Agouti-related peptide, released by NPY neurons, was found to have variable inhibitory (∼10%) and stimulatory (∼25%) effects upon subpopulations of GnRH neurons. A variety of NPY and pancreatic polypeptide analogs was used to examine potential NPY interactions with GnRH neurons. Although porcine NPY (Y1/Y2/Y5 agonist) directly inhibited the firing of approximately 45% of GnRH neurons, [Leu(31),Pro(34)]-NPY (Y1/Y4/Y5 agonist) could excite (56%) or inhibit (19%). Experiments with further agonists indicated that Y1 receptors were responsible for suppressing GnRH neuron activity, whereas postsynaptic Y4 receptors were stimulatory. These results show that the activity of GnRH neurons is regulated in a complex manner by neuropeptides released by POMC and NPY neurons. This provides a direct route through which different metabolic cues can regulate fertility.

Molecular characterization of CART, AgRP, and MC4R genes and their expression with fasting and re-feeding in common carp (Cyprinus carpio)

Cocaine and amphetamine-regulated transcript (CART), agouti-related proteins (AgRP) and Melanocortin 4 Receptor (MC4R) involves in the control of appetite. The genes were cloned and characterized, and their regulation was studied in common carp. The CARTI and CARTII genes encode 117- and 120-amino acids, respectively. The AgRP-1 and AgRP-2 genes encode 128- and 136-amino acids, respectively. CARTI was principally expressed in the brain, eye and ovary, while CARTII was highly expressed in the brain. AgRP-1 was strongly expressed in the brain, intestine, testis and eye, while AgRP-2 was highly expressed only in the gill and eye. The MC4R gene, encoding 326-amino acids, was mainly expressed in the brain testis, pituitary and eye. Phylogenetic analysis had been conducted which implied that both CARTI/CARTII and AgRP-1/AgRP-2 might derived from gene duplication events during genome evolution of common carp. CART, AgRP and MC4R gene expression in brain were decreased after fasting treatment and increased sharply after refeeding comparing with normal fed controls, which suggested that CART, AgRP and MC4R are involved in appetite regulation in common carp.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Effects of weaning and syndyphalin-33 on expression of melanocortinergic appetite-regulating genes in swine

Syndyphalin-33 (SD-33) increases feed intake in sheep and recently weaned pigs. To assess the effects of SD-33 on hypothalamic gene expression, hypothalami were collected from unweaned pigs (n=19; 21±3 d of age) on day 0. Remaining pigs received an intramuscular injection of 0.5 μmole/kg SD-33 (SD) or saline (VEH) and weaned into individual pens. On days 1, 4, and 7 after weaning, hypothalami were collected from subsets of pigs (n=8 or 9) within each treatment group. Expression of μ-opioid receptor (MOR) was less in SD pigs than in VEH pigs on day 1 and day 4, suggesting down-regulation of the receptor by SD-33. Expression of hypothalamic melanocortin 4 receptor (MC4R) at 1 d after weaning was increased in VEH pigs (but not SD pigs) relative to levels before weaning. Expression of AGRP was not significantly altered by weaning or treatment at 1 d after weaning. At 4 d after weaning, expression of AGRP was greater in SD pigs than in VEH pigs, but at day 7 expression was less in SD pigs than in VEH pigs. A strong positive correlation was noted between expression levels of MOR and MC4R across treatment and time. Treatment with SD-33 appeared to partially abrogate the effects of weaning on expression of two key appetite-regulating genes within 24 h. Effects of SD-33 appear to be mediated at least in part by the μ-opioid receptor and include actions on the melanocortinergic pathway.

Epigenetic changes in the hypothalamic proopiomelanocortin and glucocorticoid receptor genes in the ovine fetus after periconceptional undernutrition

Maternal food restriction is associated with the development of obesity in offspring. This study examined how maternal undernutrition in sheep affects the fetal hypothalamic glucocorticoid receptor (GR) and the appetite-regulating neuropeptides, proopiomelanocortin (POMC) and neuropeptide Y, which it regulates. In fetuses from ewes undernourished from -60 to +30 d around conception, there was increased histone H3K9 acetylation (1.63-fold) and marked hypomethylation (62% decrease) of the POMC gene promoter but no change in POMC expression. In the same group, acetylation of histone H3K9 associated with the hypothalamic GR gene was increased 1.60-fold and the GR promoter region was hypomethylated (53% decrease). In addition, there was a 4.7-fold increase in hypothalamic GR expression but no change in methylation of GR gene expression in the anterior pituitary or hippocampus. Interestingly, hypomethylation of both POMC and GR promoter markers in fetal hypothalami was also identified after maternal undernutrition from -60 to 0 d and -2 to +30 d. In comparison, the Oct4 gene, was hypermethylated in both control and underfed groups. Periconceptional undernutrition is therefore associated with marked epigenetic changes in hypothalamic genes. Increase in GR expression in the undernourished group may contribute to fetal programming of a predisposition to obesity, via altered GR regulation of POMC and neuropeptide Y. These epigenetic changes in GR and POMC in the hypothalamus may also predispose the offspring to altered regulation of food intake, energy expenditure, and glucose homeostasis later in life.

New ligands for melanocortin receptors

Named originally for their effects on peripheral end organs, the melanocortin system controls a diverse set of physiological processes through a series of five G-protein-coupled receptors and several sets of small peptide ligands. The central melanocortin system plays an essential role in homeostatic regulation of body weight, in which two alternative ligands, alpha-melanocyte-stimulating hormone and agouti-related protein, stimulate and inhibit receptor signaling in several key brain regions that ultimately affect food intake and energy expenditure. Much of what we know about the relationship between central melanocortin signaling and body weight regulation stems from genetic studies. Comparative genomic studies indicate that melanocortin receptors used for controlling pigmentation and body weight regulation existed more than 500 million years ago in primitive vertebrates, but that fine-grained control of melanocortin receptors through neuropeptides and endogenous antagonists developed more recently. Recent studies based on dog coat-color genetics revealed a new class of melanocortin ligands, the beta-defensins, which reveal the potential for cross talk between the melanocortin and the immune systems.

Administration of IL-1beta to the 4th ventricle causes anorexia that is blocked by agouti-related peptide and that coincides with activation of tyrosine-hydroxylase neurons in the nucleus of the solitary tract

Inflammation-associated cachexia is associated with multiple chronic diseases and involves activation of appetite regulating centers in the arcuate nucleus of the hypothalamus (ARH). The nucleus of the solitary tract (NTS) in the brainstem has also been implicated as an important nucleus involved in appetite regulation. We set out to determine whether the NTS may be involved in inflammation-associated anorexia by injecting IL-1 beta into the 4th ventricle and assessing food intake and NTS neuronal activation. Injection of IL-1 beta produced a decrease in food intake at 3 and 12h after injection which was ameliorated at the 12h time point by a sub-threshold dose of agouti-related peptide (AgRP). Investigation into neuron types in the NTS revealed that IL-1 beta injection was associated with an increase in c-Fos activity in NTS neurons expressing tyrosine hydroxylase (TH). Additionally, injection of IL-1 beta into the 4th ventricle did not produce c-Fos activation of neurons expressing pro-opiomelanocortin (POMC) in the ARH, cells known to be involved in producing anorexia in response to systemic inflammation. Double-label in situ hybridization revealed that TH neurons did not express IL-1 receptor I (IL1-RI) transcript, demonstrating that c-Fos activation of TH neurons in this setting was not via direct stimulation of IL-1 beta on TH neurons themselves. We conclude that IL-1 beta injection into the 4th ventricle produces anorexia and is accompanied by an increase in activation in TH neurons in the NTS. This provides evidence that the brainstem may be an important mediator of anorexia in the setting of inflammation.

Agouti-related peptide and MC3/4 receptor agonists both inhibit excitatory hypothalamic ventromedial nucleus neurons

Anorexigenic melanocortins decrease food intake by activating MC3/MC4 receptors (MC3/4R); the prevailing view is that the orexigenic neuropeptide agouti-related peptide (AgRP) exerts the opposite action by acting as an antagonist at MC3/MC4 receptors. A total of 370 hypothalamic ventromedial nucleus (VMH) glutamatergic neurons was studied using whole-cell recording in hypothalamic slices from a novel mouse expressing green fluorescent protein (GFP) under control of the vesicular glutamate transporter 2 (vGluT2) promoter. Massive numbers of GFP-expressing VMH dendrites extended out of the core of the nucleus into the surrounding cell-poor shell. VMH dendrites received frequent appositions from AgRP-immunoreactive axons in the shell of the nucleus, but not the core, suggesting that AgRP may influence target VMH neurons. alpha-MSH, melanotan II (MTII), and selective MC3R or MC4R agonists were all inhibitory, reducing the spontaneous firing rate and hyperpolarizing vGluT2 neurons. The MC3/4R antagonist SHU9119 was excitatory. Unexpectedly, AgRP did not attenuate MTII actions on these neurons; instead, these two compounds showed an additive inhibitory effect. In the absence of synaptic activity, no hyperpolarization or change in input resistance was evoked by either MTII or AgRP, suggesting indirect actions. Consistent with this view, MTII increased the frequency of spontaneous and miniature IPSCs. In contrast, the mechanism of AgRP inhibition was dependent on presynaptic inhibition of EPSCs mediated by G(i)/G(o)-proteins, and was attenuated by pertussis toxin and NF023, inconsistent with mediation by G(s)-proteins associated with MC receptors. Together, our data suggest that the mechanism of AgRP actions on these excitatory VMH cells appears to be independent of the actions of melanocortins on MC receptors.

Feeding induced by cannabinoids is mediated independently of the melanocortin system

Background

Cannabinoids, the active components of marijuana, stimulate appetite, and cannabinoid receptor-1 (CB1-R) antagonists suppress appetite and promote weight loss. Little is known about how CB1-R antagonists affect the central neurocircuitry, specifically the melanocortin system that regulates energy balance.

Methodology/Principal Findings

Here, we show that peripherally administered CB1-R antagonist (AM251) or agonist equally suppressed or stimulated feeding respectively in A^y , which lack a functional melanocortin system, and wildtype mice, demonstrating that cannabinoid effects on feeding do not require melanocortin circuitry. CB1-R antagonist or agonist administered into the ventral tegmental area (VTA) equally suppressed or stimulated feeding respectively, in both genotypes. In addition, peripheral and central cannabinoid administration similarly induced c-Fos activation in brain sites suggesting mediation via motivational dopaminergic circuitry. Amperometry-detected increases in evoked dopamine (DA) release by the CB1-R antagonist in nucleus accumbens slices indicates that AM251 modulates DA release from VTA terminals.

Conclusions/Significance

Our results demonstrate that the effects of cannabinoids on energy balance are independent of hypothalamic melanocortin circuitry and is primarily driven by the reward system.

Central neural and endocrine mechanisms of non-exercise activity thermogenesis and their potential impact on obesity

The rise in obesity is associated with a decline in the amount of physical activity in which people engage. The energy expended through everyday non-exercise activity, called non-exercise activity thermogenesis (NEAT), has a considerable potential impact on energy balance and weight gain. Comparatively little attention has been paid to the central mechanisms of energy expenditure and how decreases in NEAT might contribute to obesity. In this review, we first examine the sensory and endocrine mechanisms through which energy availability and energy balance are detected that may influence NEAT. Second, we describe the neural pathways that integrate these signals. Lastly, we consider the effector mechanisms that modulate NEAT through the alteration of activity levels as well as through changes in the energy efficiency of movement. Systems that regulate NEAT according to energy balance may be linked to neural circuits that modulate sleep, addiction and the stress response. The neural and endocrine systems that control NEAT are potential targets for the treatment of obesity.

The role of proopiomelanocortin (POMC) neurones in feeding behaviour

The precursor protein, proopiomelanocortin (POMC), produces many biologically active peptides via a series of enzymatic steps in a tissue-specific manner, yielding the melanocyte-stimulating hormones (MSHs), corticotrophin (ACTH) and β-endorphin. The MSHs and ACTH bind to the extracellular G-protein coupled melanocortin receptors (MCRs) of which there are five subtypes. The MC3R and MC4R show widespread expression in the central nervous system (CNS), whilst there is low level expression of MC1R and MC5R. In the CNS, cell bodies for POMC are mainly located in the arcuate nucleus of the hypothalamus and the nucleus tractus solitarius of the brainstem. Both of these areas have well defined functions relating to appetite and food intake. Mouse knockouts (ko) for pomc, mc4r and mc3r all show an obese phenotype, as do humans expressing mutations of POMC and MC4R. Recently, human subjects with specific mutations in β-MSH have been found to be obese too, as have mice with engineered β-endorphin deficiency. The CNS POMC system has other functions, including regulation of sexual behaviour, lactation, the reproductive cycle and possibly central cardiovascular control. However, this review will focus on feeding behaviour and link it in with the neuroanatomy of the POMC neurones in the hypothalamus and brainstem.

Structural and molecular evolutionary analysis of Agouti and Agouti-related proteins

Agouti (ASIP) and Agouti-related protein (AgRP) are endogenous antagonists of melanocortin receptors that play critical roles in the regulation of pigmentation and energy balance, respectively, and which arose from a common ancestral gene early in vertebrate evolution. The N-terminal domain of ASIP facilitates antagonism by binding to an accessory receptor, but here we show that the N-terminal domain of AgRP has the opposite effect and acts as a prodomain that negatively regulates antagonist function. Computational analysis reveals similar patterns of evolutionary constraint in the ASIP and AgRP C-terminal domains, but fundamental differences between the N-terminal domains. These studies shed light on the relationships between regulation of pigmentation and body weight, and they illustrate how evolutionary structure function analysis can reveal both unique and common mechanisms of action for paralogous gene products.