Effects of selective modulation of the central melanocortin-3-receptor on food intake and hypothalamic POMC expression

Melanocortin 3 receptor regulates hepatic autophagy and systemic adiposity

Systemic lipid homeostasis requires hepatic autophagy, a major cellular program for intracellular fat recycling. Here, we find melanocortin 3 receptor (MC3R) regulates hepatic autophagy in addition to its previously established CNS role in systemic energy partitioning and puberty. Mice with Mc3r deficiency develop obesity with hepatic triglyceride accumulation and disrupted hepatocellular autophagosome turnover. Mice with partially inactive human MC3R due to obesogenic variants demonstrate similar hepatic autophagic dysfunction. In vitro and in vivo activation of hepatic MC3R upregulates autophagy through LC3II activation, TFEB cytoplasmic-to-nuclear translocation, and subsequent downstream gene activation. MC3R-deficient hepatocytes had blunted autophagosome-lysosome docking and lipid droplet clearance. Finally, the liver-specific rescue of Mc3r was sufficient to restore hepatocellular autophagy, improve hepatocyte mitochondrial function and systemic energy expenditures, reduce adipose tissue lipid accumulation, and partially restore body weight in both male and female mice. We thus report a role for MC3R in regulating hepatic autophagy and systemic adiposity.

Alx3 deficiency disrupts energy homeostasis, alters body composition, and impairs hypothalamic regulation of food intake

The coordination of food intake, energy storage, and expenditure involves complex interactions between hypothalamic neurons and peripheral tissues including pancreatic islets, adipocytes, muscle, and liver. Previous research shows that deficiency of the transcription factor Alx3 alters pancreatic islet-dependent glucose homeostasis. In this study we carried out a comprehensive assessment of metabolic alterations in Alx3 deficiency. We report that Alx3-deficient mice exhibit decreased food intake without changes in body weight, along with reduced energy expenditure and altered respiratory exchange ratio. Magnetic resonance imaging reveals increased adiposity and decreased muscle mass, which was associated with markers of motor and sympathetic denervation. By contrast, Alx3-deficient mice on a high-fat diet show attenuated weight gain and improved insulin sensitivity, compared to control mice. Gene expression analysis demonstrates altered lipogenic and lipolytic gene profiles. In wild type mice Alx3 is expressed in hypothalamic arcuate nucleus neurons, but not in major peripheral metabolic organs. Functional diffusion-weighted magnetic resonance imaging reveals selective hypothalamic responses to fasting in the arcuate nucleus of Alx3-deficient mice. Additionally, altered expression of proopiomelanocortin and melanocortin-3 receptor mRNA in the hypothalamus suggests impaired regulation of feeding behavior. This study highlights the crucial role for Alx3 in governing food intake, energy homeostasis, and metabolic nutrient partitioning, thereby influencing body mass composition.

Chronic Nicotine Consumption and Withdrawal Regulate Melanocortin Receptor, CRF, and CRF Receptor mRNA Levels in the Rat Brain

Alterations in the various neuropeptide systems in the mesocorticolimbic circuitry have been implicated in negative effects associated with drug withdrawal. The corticotropin-releasing factor (CRF) and α-melanocyte-stimulating hormone are two peptides that may be involved. This study investigated the regulatory effects of chronic nicotine exposure and withdrawal on the mRNA levels of melanocortin receptors (MC3R, MC4R), CRF, and CRF receptors (CRFR1 and CRFR2) expressed in the mesocorticolimbic system. Rats were given drinking water with nicotine or without nicotine (control group) for 12 weeks, after which they continued receiving nicotine (chronic exposure) or were withdrawn from nicotine for 24 or 48 h. The animals were decapitated following behavioral testing for withdrawal signs. Quantitative real-time PCR analysis demonstrated that nicotine exposure (with or without withdrawal) increased levels of CRF and CRFR1 mRNA in the amygdala, CRF mRNA in the medial prefrontal cortex, and CRFR1 mRNA in the septum. Nicotine withdrawal also enhanced MC3R and MC4R mRNA levels in different brain regions, while chronic nicotine exposure was associated with increased MC4R mRNA levels in the nucleus accumbens. These results suggest that chronic nicotine exposure and withdrawal regulate CRF and melanocortin signaling in the mesocorticolimbic system, possibly contributing to negative affective state and nicotine addiction.

Type 1 melanocortin receptors in pro-opiomelanocortin-, vasopressin-, and oxytocin-immunopositive neurons in different areas of mouse brain

Information on the localization of the Type 1 melanocortin receptors (MC1Rs) in different regions of the brain is very scarce. As a result, the role of MC1Rs in the functioning of brain neurons and in the central regulation of physiological functions has not been studied. This work aimed to study the expression and distribution of MС1Rs in different brain areas of female C57Bl/6J mice. Using real-time polymerase chain reaction, we demonstrated the Mс1R gene expression in the cerebral cortex, midbrain, hypothalamus, medulla oblongata, and hippocampus. Using an immunohistochemical approach, we showed the MС1R localization in neurons of the hypothalamic arcuate, paraventricular and supraoptic nuclei, nucleus tractus solitarius (NTS), dorsal hippocampus, substantia nigra, and cerebral cortex. Using double immunolabeling, the MC1Rs were visualized on the surface and in the bodies and outgrowths of pro-opiomelanocortin (POMC)-immunopositive neurons in the hypothalamic arcuate nucleus, NTS, hippocampal CA3 and CA1 regions, and cerebral cortex. Co-localization with POMC indicates that MC1R, like MC3R, is able to function as an autoreceptor. In the paraventricular and supraoptic nuclei, MC1Rs were visualized on the surface and in the cell bodies of vasopressin- and oxytocin-immunopositive neurons, indicating a relationship between hypothalamic MC1R signaling and vasopressin and oxytocin production. The data obtained indicate a wide distribution of MC1Rs in different areas of the mouse brain and their localization in POMC-, vasopressin- and oxytocin-immunopositive neurons, which may indicate the participation of MC1Rs in the control of many physiological processes in the central nervous system.

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.

Genetics: A Starting Point for the Prevention and the Treatment of Obesity

Obesity is a common, serious, and costly disease. More than 1 billion people worldwide are obese-650 million adults, 340 million adolescents, and 39 million children. The WHO estimates that, by 2025, approximately 167 million people-adults and children-will become less healthy because they are overweight or obese. Obesity-related conditions include heart disease, stroke, type 2 diabetes, and certain types of cancer. These are among the leading causes of preventable, premature death. The estimated annual medical cost of obesity in the United States was nearly $173 billion in 2019 dollars. Obesity is considered the result of a complex interaction between genes and the environment. Both genes and the environment change in different populations. In fact, the prevalence changes as the result of eating habits, lifestyle, and expression of genes coding for factors involved in the regulation of body weight, food intake, and satiety. Expression of these genes involves different epigenetic processes, such as DNA methylation, histone modification, or non-coding micro-RNA synthesis, as well as variations in the gene sequence, which results in functional alterations. Evolutionary and non-evolutionary (i.e., genetic drift, migration, and founder's effect) factors have shaped the genetic predisposition or protection from obesity in modern human populations. Understanding and knowing the pathogenesis of obesity will lead to prevention and treatment strategies not only for obesity, but also for other related diseases.

The melanocortin-3 receptor is a pharmacological target for the regulation of anorexia

Ablation of hypothalamic AgRP (Agouti-related protein) neurons is known to lead to fatal anorexia, whereas their activation stimulates voracious feeding and suppresses other motivational states including fear and anxiety. Despite the critical role of AgRP neurons in bidirectionally controlling feeding, there are currently no therapeutics available specifically targeting this circuitry. The melanocortin-3 receptor (MC3R) is expressed in multiple brain regions and exhibits sexual dimorphism of expression in some of those regions in both mice and humans. MC3R deletion produced multiple forms of sexually dimorphic anorexia that resembled aspects of human anorexia nervosa. However, there was no sexual dimorphism in the expression of MC3R in AgRP neurons, 97% of which expressed MC3R. Chemogenetic manipulation of arcuate MC3R neurons and pharmacologic manipulation of MC3R each exerted potent bidirectional regulation over feeding behavior in male and female mice, whereas global ablation of MC3R-expressing cells produced fatal anorexia. Pharmacological effects of MC3R compounds on feeding were dependent on intact AgRP circuitry in the mice. Thus, the dominant effect of MC3R appears to be the regulation of the AgRP circuitry in both male and female mice, with sexually dimorphic sites playing specialized and subordinate roles in feeding behavior. Therefore, MC3R is a potential therapeutic target for disorders characterized by anorexia, as well as a potential target for weight loss therapeutics.

The Melanocortin System behind the Dysfunctional Eating Behaviors

The dysfunction of melanocortin signaling has been associated with obesity, given the important role in the regulation of energy homeostasis, food intake, satiety and body weight. In the hypothalamus, the melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) contribute to the stability of these processes, but MC3R and MC4R are also localized in the mesolimbic dopamine system, the region that responds to the reinforcing properties of highly palatable food (HPF) and where these two receptors seem to affect food reward and motivation. Loss of function of the MC4R, resulting from genetic mutations, leads to overeating in humans, but to date, a clear understanding of the underlying mechanisms and behaviors that promote overconsumption of caloric foods remains unknown. Moreover, the MC4R demonstrated to be a crucial modulator of the stress response, factor that is known to be strictly related to binge eating behavior. In this review, we will explore the preclinical and clinical studies, and the controversies regarding the involvement of melanocortin system in altered eating patterns, especially binge eating behavior, food reward and motivation.

Emerging roles of melanocortin receptor accessory proteins (MRAP and MRAP2) in physiology and pathophysiology

Melanocortin-2 receptor accessory protein (MRAP) has an unusual dual topology and influences the expression, localisation, signalling and internalisation of the melanocortin receptor 2 (MC₂); the adrenocorticotropic hormone (ACTH) receptor. Mutations in MRAP are associated with familial glucocorticoid deficiency type-2 and evidence is emerging of the importance of MRAP in adrenal development and ACTH signalling. Human MRAP has two functional splice variants: MRAP-α and MRAP-β, unlike MRAP-β, MRAP-α has little expression in brain but is highly expressed in ovary. MRAP2, identified through whole human genome sequence analysis, has approximately 40% sequence homology to MRAP. MRAP2 facilitates MC2 localisation to the cell surface but not ACTH signalling. MRAP and MRAP2 have been found to regulate the surface expression and signalling of all melanocortin receptors (MC_1-5). Additionally, MRAP2 moderates the signalling of the G-protein coupled receptors (GCPRs): orexin, prokineticin and GHSR1a; the ghrelin receptor. Whilst MRAP appears to be mainly involved in glucocorticoid synthesis, an important role is emerging for MRAP2 in regulating appetite and energy homeostasis. Transgenic models indicate the importance of MRAP in adrenal gland formation. Like MC3R and MC4R knockout mice, MRAP2 knockout mice have an obese phenotype. In vitro studies indicate that MRAP2 enhances the MC3 and MC4 response to the agonist αMSH, which, like ACTH, is produced through precursor polypeptide proopiomelanocortin (POMC) cleavage. Analysis of cohorts of individuals with obesity have revealed several MRAP2 genetic variants with loss of function mutations which are causative of monogenic hyperphagic obesity with hyperglycaemia and hypertension. MRAP2 may also be associated with female infertility. This review summarises current knowledge of MRAP and MRAP2, their influence on GPCR signalling, and focusses on pathophysiology, particularly familial glucocorticoid deficiency type-2 and obesity.

Hypothalamic γ-melanocyte stimulating hormone gene delivery reduces fat mass in male mice

γ-Melanocyte stimulating hormone (γ-MSH) is an endogenous agonist of the melanocortin 3-receptor (MC3R). Genetic disruption of MC3Rs increases adiposity and blunts responses to fasting, suggesting that increased MC3R signaling could be physiologically beneficial in the long term. Interestingly, several studies have concluded that activation of MC3Rs is orexigenic in the short term. Therefore, we aimed to examine the short- and long-term effects of γ-MSH in the hypothalamic arcuate nucleus (ARC) on energy homeostasis and hypothesized that the effect of MC3R agonism is dependent on the state of energy balance and nutrition. Lentiviral gene delivery was used to induce a continuous expression of γ-Msh only in the ARC of male C57Bl/6N mice. Parameters of body energy homeostasis were monitored as food was changed from chow (6 weeks) to Western diet (13 weeks) and back to chow (7 weeks). The γ-MSH treatment decreased the fat mass to lean mass ratio on chow, but the effect was attenuated on Western diet. After the switch back to chow, an enhanced loss in weight (−15% vs −6%) and fat mass (−37% vs −12%) and reduced cumulative food intake were observed in γ-MSH-treated animals. Fasting-induced feeding was increased on chow diet only; however, voluntary running wheel activity on Western diet was increased. The γ-MSH treatment also modulated the expression of key neuropeptides in the ARC favoring weight loss. We have shown that a chronic treatment intended to target ARC MC3Rs modulates energy balance in nutritional state-dependent manner. Enhancement of diet-induced weight loss could be beneficial in treatment of obesity.

Regulation of energy rheostasis by the melanocortin-3 receptor

Like most homeostatic systems, adiposity in mammals is defended between upper and lower boundary conditions. While leptin and melanocortin-4 receptor (MC4R) signaling are required for defending energy set point, mechanisms controlling upper and lower homeostatic boundaries are less well understood. In contrast to the MC4R, deletion of the MC3R does not produce measurable hyperphagia or hypometabolism under normal conditions. However, we demonstrate that MC3R is required bidirectionally for controlling responses to external homeostatic challenges, such as caloric restriction or calorie-rich diet. MC3R is also required for regulated excursion from set point, or rheostasis, during pregnancy. Further, we demonstrate a molecular mechanism: MC3R provides regulatory inputs to melanocortin signaling, acting presynaptically on agouti-related protein neurons to regulate γ-aminobutyric acid release onto anorexigenic MC4R neurons, exerting boundary control on the activity of MC4R neurons. Thus, the MC3R is a critical regulator of boundary controls on melanocortin signaling, providing rheostatic control on energy storage.

Anti-leptin receptor antibodies strengthen leptin biofunction in growing chickens

Antibodies against the extracellular domains of the chicken leptin receptor were used to study the biological function of leptin in growing chickens. Both polyclonal and monoclonal anti-LEPR antibodies were administered intramuscularly to 30-d-old Chinese indigenous Gushi pullets. Both antibody preparations increased feed intake for 6 h after injection and reduced plasma concentrations of glucose, triglycerides, and both high- and low-density lipoproteins. The antibody treatments also upregulated agouti-related peptide and neuropeptide Y in the hypothalamus and downregulated proopiomelanocortin, melanocortin 4 receptor, and leptin receptor. The treatments also upregulated leptin receptor, acetyl CoA carboxylase beta, and acyl-CoA oxidase in the liver, abdominal fat, and breast muscle and downregulated sterol regulatory element-binding protein-1 and fatty acid synthase. Furthermore, even though the anti-leptin receptor antibodies failed to affect leptin receptor signaling transduction when administered alone, they did augment the induction of leptin receptor signaling transduction by leptin. These results demonstrate that antibodies against the extracellular domains of leptin-specific receptor enhance, but do not mimic, the ability of leptin to activate receptors. Furthermore, the enhanced leptin bioactivity observed after the intramuscular injection of anti-LEPR antibodies confirmed the occurrence of de novo leptin in the peripheral tissues and blood of treated chickens.

Clarifying the Ghrelin System's Ability to Regulate Feeding Behaviours Despite Enigmatic Spatial Separation of the GHSR and Its Endogenous Ligand

Ghrelin is a hormone predominantly produced in and secreted from the stomach. Ghrelin is involved in many physiological processes including feeding, the stress response, and in modulating learning, memory and motivational processes. Ghrelin does this by binding to its receptor, the growth hormone secretagogue receptor (GHSR), a receptor found in relatively high concentrations in hypothalamic and mesolimbic brain regions. While the feeding and metabolic effects of ghrelin can be explained by the effects of this hormone on regions of the brain that have a more permeable blood brain barrier (BBB), ghrelin produced within the periphery demonstrates a limited ability to reach extrahypothalamic regions where GHSRs are expressed. Therefore, one of the most pressing unanswered questions plaguing ghrelin research is how GHSRs, distributed in brain regions protected by the BBB, are activated despite ghrelin’s predominant peripheral production and poor ability to transverse the BBB. This manuscript will describe how peripheral ghrelin activates central GHSRs to encourage feeding, and how central ghrelin synthesis and ghrelin independent activation of GHSRs may also contribute to the modulation of feeding behaviours.

Molecular genetics of human obesity: A comprehensive review

Obesity and its related health complications is a major problem worldwide. Hypothalamus and their signalling molecules play a critical role in the intervening and coordination with energy balance and homeostasis. Genetic factors play a crucial role in determining an individual's predisposition to the weight gain and being obese. In the past few years, several genetic variants were identified as monogenic forms of human obesity having success over common polygenic forms. In the context of molecular genetics, genome-wide association studies (GWAS) approach and their findings signified a number of genetic variants predisposing to obesity. However, the last couple of years, it has also been noticed that alterations in the environmental and epigenetic factors are one of the key causes of obesity. Hence, this review might be helpful in the current scenario of molecular genetics of human obesity, obesity-related health complications (ORHC), and energy homeostasis. Future work based on the clinical discoveries may play a role in the molecular dissection of genetic approaches to find more obesity-susceptible gene loci.

Localization of the dopamine receptors of types 1 and 2 on the bodies of POMC-expressing neurons of the arcuate nucleus of the hypothalamus in mice and rats

Using immunofluorescent techniques, we have demonstrated the presence of two main types of dopamine receptors, D1 and D2, on the bodies of neurons of the arcuate nucleus of the hypothalamus expressing the precursor of peptides of the melanocortin family proopiomelanocortin in C57Bl/6J mice and Wistar rats. These data show close functional relationship between the dopamine and melanocortin systems of the brain and involvement of dopamine in the control of synthesis and secretion of peptides of the melanocortin family.

In silico approach to identify non-synonymous SNPs in human obesity related gene, MC3R (melanocortin-3-receptor)

The melanocortin-3-receptor (MC3R) is a novel gene candidate for human obesity, which involved in controlling the energy homeostasis and food intake behavior. The main aim behind this work is to investigate the potentially deleterious non-synonymous single nucleotide polymorphisms (nsSNPs) in obesity related gene MC3R by using six computational tools viz., PolyPhen, I-Mutant, PROVEAN, SIFT, PANTHER and PhD-SNP. In our study, we predicted eight nsSNPs i.e., rs74315393 (Ile146Asn), rs368205448 (Asp121Tyr), rs143321797 (Phe45Ser), rs17847261 (Cys274Ser), rs144166442 (Pro257His), rs370533946 (Leu224Pro), rs371354428 (Pro72Leu) and rs373708098 (Gly249Ser) found to be potentially deleterious. The functional impact of three nsSNPs i.e., rs74315393, rs368205448 and rs143321797 have already been validated experimentally in the context of human obesity. Moreover, Homology modeling and structural analysis were carried out for already experimentally validated nsSNPs i.e., rs74315393, rs368205448 and rs143321797 to check the stability of predicted models. The mutant models showed higher energy and RMSD (Root mean square deviation) values. In addition, FTSite server predicted one nsSNP i.e., rs368205448 (Asp121Tyr) out of eight identified nsSNPs found in the MC3R protein binding site. Thus, the present computational study may suggest that predicted nsSNPs possibly be a better drug target and contribute to the treatment and better understanding of human obesity.

Tankyrases regulate glucoregulatory mechanisms and somatic growth via the central melanocortin system in zebrafish larvae

The central melanocortin system is a key regulator of energy homeostasis. Recent studies indicate that tankyrases (TNKSs), which poly(ADP-ribosyl)ate target proteins and direct them toward proteasomal degradation, affect overall metabolism, but the exact molecular mechanisms remain unclear. We used zebrafish larvae as a model to study the mechanisms by which TNKS1b, the zebrafish ortholog of mammalian TNKS1, regulates glucose homeostasis and somatic growth. In situ hybridization revealed that TNKS1b mRNA is prominently expressed in the hypothalamus and pituitary of the embryonic and larval brain. In the pituitary, TNKS1b is coexpressed with pro-opiomelanocortin a (pomca) gene in corticotropes and melanotropes. Knockdown of TNKS1b reduced the linear growth of the larvae, stimulated insulin gene and glucose transporter 4 protein, and suppressed gluconeogenic phosphoenolpyruvate carboxykinase 1 gene. This result indicates rapid glucose utilization and reduction of gluconeogenesis in TNKS1b-deficient larvae. Knockdown of TNKS1b down-regulated pomca expression and diminished α-melanocyte-stimulating hormone in the pars intermedia. Furthermore, down-regulation of TNKS1b suppressed the expression of melanocortin receptor 3 and increased the expression of melanocortin receptor 4. The collective data suggest that TNKS1b modulates glucoregulatory mechanisms and the somatic growth of zebrafish larvae via the central melanocortin system.

The effects of beta-endorphin: state change modification

Beta-endorphin (β-END) is an opioid neuropeptide which has an important role in the development of hypotheses concerning the non-synaptic or paracrine communication of brain messages. This kind of communication between neurons has been designated volume transmission (VT) to differentiate it clearly from synaptic communication. VT occurs over short as well as long distances via the extracellular space in the brain, as well as via the cerebrospinal fluid (CSF) flowing through the ventricular spaces inside the brain and the arachnoid space surrounding the central nervous system (CNS). To understand how β-END can have specific behavioral effects, we use the notion behavioral state, inspired by the concept of machine state, coming from Turing (Proc London Math Soc, Series 2,42:230-265, 1937). In section 1.4 the sequential organization of male rat behavior is explained showing that an animal is not free to switch into another state at any given moment. Funneling-constraints restrict the number of possible behavioral transitions in specific phases while at other moments in the sequence the transition to other behavioral states is almost completely open. The effects of β-END on behaviors like food intake and sexual behavior, and the mechanisms involved in reward, meditation and pain control are discussed in detail. The effects on the sequential organization of behavior and on state transitions dominate the description of these effects.

The acute salinity changes activate the dual pathways of endocrine responses in the brain and pituitary of tilapia

To analyze and compare the stress and osmoregulatory hormones and receptors in pituitary during acute salinity changes, the expression patterns of corticotropin releasing hormone (crh) in hypothalamus, prolactin (prl) releasing peptide (pRrp) in telencephalon and diencephalon, glucocorticoid receptors 2 (gr2), and mineralocorticoid receptor (mr), crh-r, pro-opiomelanocorticotropin (pomc), pRrp, prl, dopamine 2 receptor (d2-r), growth hormone (gh), gh-receptor (gh-r) and insulin-like growth hormone (igf-1) transcripts in pituitary were characterized in euryhaline tilapia. The results indicate that the crh transcripts increased in the hypothalamus and rostral pars distalis of the pituitary after the transfer of fish to SW. Similarly, the pRrp transcripts were more abundant in SW acclimated tilapia forebrain and hypothalamus. The crh-r, gr2 and mr transcripts were more expressed in rostral pars distalis and pars intermedia of pituitary at SW than FW tilapia. The data indicate that the SW acclimation stimulates these transcripts in the specific regions of the brain and pituitary which may be related to the activation of the hypothalamic-pituitary-interrenal (HPI)-axis. The results of dual in situ hybridization reveal that the transcripts of crh-r, gr2 and mr with pomc are highly co-localized in corticotrophs of pituitary. Furthermore, we demonstrate high expression of pRrp in the brain and low expression of pRrp and prl transcripts in the pituitary of SW fish. No crh-r and corticosteroid receptors were co-localized with prl transcripts in the pituitary. The gh-r and igf-1 mRNA levels were significantly increased in SW acclimated tilapia pituitary whereas there was no difference in the gh mRNA levels. The data suggest that the locally produced pRrp and d2-r may control and regulate the expression of prl mRNA in pituitary. Therefore, the dual roles of pRrp are involved in the stress (via brain-pituitary) and osmoregulatory (via pituitary) pathways in tilapia exposed to acute salinity changes.

Synthesis, biophysical, and pharmacological evaluation of the melanocortin agonist AST3-88: modifications of peptide backbone at Trp 7 position lead to a potent, selective, and stable ligand of the melanocortin 4 receptor (MC4R)

The melanocortin-3 (MC3R) and melanocortin-4 (MC4R) receptors are expressed in the brain and are implicated in the regulation of food intake and energy homeostasis. The endogenous agonist ligands for these receptors (α-, β-, γ-MSH and ACTH) are linear peptides with limited receptor subtype selectivity and metabolic stability, thus minimizing their use as probes to characterize the overlapping pharmacological and physiological functions of the melanocortin receptor subtypes. In the present study, an engineered template, in which the peptide backbone was modified by a heterocyclic reverse turn mimetic at the Trp⁷ residue, was synthesized using solid phase peptide synthesis and characterized by a β-galactosidase cAMP based reporter gene assay. The functional assay identified a ∼5 nM mouse MC4R agonist (AST3-88) with more than 50-fold selectivity over the mMC3R. Biophysical studies (2D ¹H NMR spectroscopy and molecular dynamics) of AST3-88 identified a type VIII β-turn secondary structure spanning the pharmacophore domain stabilized by the intramolecular interactions between the side chains of the His and Trp residues. Enzymatic studies of AST3-88 revealed enhanced stability of AST3-88 over the α-MSH endogenous peptide in rat serum. Upon central administration of AST3-88 into rats, a decreased food intake response was observed. This is the first study to probe the in vivo physiological activity of this engineered peptide-heterocycle template. These findings advance the present knowledge of pharmacophore design for potent, selective, and metabolically stable melanocortin ligands.

α-MSH overexpression in the nucleus tractus solitarius decreases fat mass and elevates heart rate

The POMC pathway is involved in the regulation of energy and cardiovascular homeostasis in the hypothalamus and the brain stem. Although the acute effects of POMC-derived peptides in different brain locations have been elucidated, the chronic site-specific effects of distinct peptides remain to be studied. To this end, we used a lentiviral gene delivery vector to study the long-term effects of α-MSH in the nucleus tractus solitarius (NTS) of the brain stem. The α-MSH vector (LVi-α-MSH-EGFP) based on the N-terminal POMC sequence and a control vector (LVi-EGFP) were delivered into the NTS of C57BL/6N male mice fed on a western diet. Effects on body weight and composition, feeding, glucose metabolism, and hemodynamics by telemetric analyses were studied during the 12-week follow-up. The LVi-α-MSH-EGFP-treated mice had a significantly smaller gain in the fat mass compared with LVi-EGFP-injected mice. There was a small initial decrease in food intake and no differences in the physical activity. Glucose metabolism was not changed compared with the control. LVi-α-MSH-EGFP increased the heart rate (HR), which was attenuated by adrenergic blockade suggesting an increased sympathetic activity. Reduced response to muscarinic blockade suggested a decreased parasympathetic activity. Fitting with sympathetic activation, LVi-α-MSH-EGFP treatment reduced urine secretion. Thus, the results demonstrate that long-term α-MSH overexpression in the NTS attenuates diet-induced obesity. Modulation of autonomic nervous system tone increased the HR and most probably contributed to an anti-obesity effect. The results underline the key role of NTS in the α-MSH-induced long-term effects on adiposity and in regulation of sympathetic and parasympathetic activities.

Targeting central melanocortin receptors: a promising novel approach for treating alcohol abuse disorders

The melanocortin (MC) peptides are produced centrally by propiomelanocortin (POMC) neurons within the arcuate nucleus of the hypothalamus and act through five seven-transmembrane G-protein coupled melanocortin receptor (MCR) subtypes. The MC3R and MC4R subtypes, the most abundant central MCRs, are widely expressed in brain regions known to modulate neurobiological responses to ethanol, including regions of the hypothalamus and extended amygdala. Agouti-related protein (AgRP), also produced in the arcuate nucleus, is secreted in terminals expressing MCRs and functions as an endogenous MCR antagonist. This review highlights recent genetic and pharmacological findings that have implicated roles for the MC and AgRP systems in modulating ethanol consumption. Ethanol consumption is associated with significant alterations in the expression levels of various MC peptides/protein, which suggests that ethanol-induced perturbations of MC/AgRP signaling may modulate excessive ethanol intake. Consistently, MCR agonists decrease, and AgRP increases, ethanol consumption in mice. MCR agonists fail to blunt ethanol intake in mutant mice lacking the MC4R, suggesting that the protective effects of MCR agonists are modulated by the MC4R. Interestingly, recent evidence reveals that MCR agonists are more effective at blunting binge-like ethanol intake in mutant mice lacking the MC3R, suggesting that the MC3R has opposing effects on the MC4R. Finally, mutant mice lacking AgRP exhibit blunted voluntary and binge-like ethanol drinking, consistent with pharmacological studies. Collectively, these preclinical observations provide compelling evidence that compounds that target the MC system may provide therapeutic value for treating alcohol abuse disorders and that the utilization of currently available MC-targeting compounds- such as those being used to treat eating disorders- may be used as effective treatments to this end.

The protective effects of the melanocortin receptor (MCR) agonist, melanotan-II (MTII), against binge-like ethanol drinking are facilitated by deletion of the MC3 receptor in mice

Recent data have implicated the melanocortin (MC) system in modulating voluntary ethanol consumption. Administration of melanotan-II (MTII), a nonselective melanocortin receptor (MCR) agonist, reduces voluntary ethanol consumption in C57BL/6J mice. Previous studies have demonstrated that central infusion of MTII effectively reduced voluntary ethanol drinking in mutant mice lacking normal expression of MC3R (MC3R-/- mice) but failed to alter ethanol drinking in mice lacking expression of MC4R, demonstrating that central MTII administration reduces voluntary ethanol drinking by signaling through the MC4R. However, evidence shows that the neurocircuitry recruited during excessive binge-like ethanol drinking versus moderate ethanol drinking are not identical. Thus the present study sought to investigate the potential role of the MC3R in binge-like ethanol intake. To this end, the "drinking in the dark" (DID) procedure, a commonly used animal model of binge-like ethanol drinking, was employed. Wild-type MC3R+/+ and MC3R-/- mice were given intracerebroventricular (i.c.v.) infusion of MTII (0.0, 0.25, 0.50, or 1.0 μg) prior to the onset of a 4-h testing period in which mice were given access to 20% (v/v) ethanol. Immediately after the 4-h testing period, tail blood samples were collected from each animal in order to assess blood ethanol concentrations (BECs). Consistent with previous findings, central administration of MTII blunted binge-like ethanol drinking in both MC3R+/+ and MC3R-/- mice. Interestingly, all doses of MTII blunted binge-like ethanol drinking in MC3R-/- mice during the first hour of testing, while only the 1.0 μg dose reduced binge-like drinking in MC3R+/+ mice. Thus, MC3R-/- mice were more sensitive to the protective effects of MTII. These data suggest that MC3Rs oppose the protective effects of MTII against binge-like ethanol drinking, and thus selective MC3R antagonists may have potential therapeutic roles in treating excessive ethanol drinking.

β-Endorphin antagonizes the effects of α-MSH on food intake and body weight

Proopiomelanocortin (POMC) is posttranslationally processed to several peptides including α-MSH, a primary regulator of energy balance that inhibits food intake and stimulates energy expenditure. However, another POMC-derived peptide, β-endorphin (β-EP), has been shown to stimulate food intake. In this study we examined the effects of intracerebroventricular (icv) β-EP on food intake and its ability to antagonize the negative effects of α-MSH on energy balance in male rats. A single icv injection of β-EP stimulated food intake over a 2- to 6-h period during both the light and dark cycles. This effect was, however, not sustained with chronic icv β-EP infusion. In the next study, a subthreshold dose of β-EP was injected together with Nle(4), d-Phe(7) (NDP)-MSH after a 16-h fast, and the negative effects of NDP-MSH on refeeding and body weight gain were partially reversed. Finally, peptide interactions were studied in a chronic icv infusion model. Weight gain and food intake were significantly suppressed in the NDP-MSH group during the entire study. A subthreshold dose of β-EP antagonized these suppressive effects on food intake and weight gain for the first 3 d. However on d 4-7, β-EP no longer blocked these effects. Of note, the stimulatory effect of β-EP on feeding and its ability to antagonize MSH were specific for β-EP(1-31) and were not observed with β-EP(1-27). This study highlights the importance of understanding how the balance between α-MSH and β-EP is maintained and the potential role of differential POMC processing in regulating energy balance.

Genetic dissection of the functions of the melanocortin-3 receptor, a seven-transmembrane G-protein-coupled receptor, suggests roles for central and peripheral receptors in energy homeostasis

The melanocortin-3 receptor (MC3R) gene is pleiotropic, influencing body composition, natriuresis, immune function, and entrainment of circadian rhythms to nutrient intake. MC3Rs are expressed in hypothalamic and limbic regions of the brain and in peripheral tissues. To investigate the roles of central MC3Rs, we inserted a "lox-stop-lox" (LoxTB) 5' of the translation initiation codon of the mouse Mc3r gene and reactivated transcription using neuron-specific Cre transgenic mice. As predicted based on earlier observations of Mc3r knock-out mice, Mc3r(TB/TB) mice displayed reduced lean mass, increased fat mass, and accelerated diet-induced obesity. Surprisingly, rescuing Mc3r expression in the nervous system using the Nestin-Cre transgene only partially rescued obesity in chow-fed conditions and had no impact on the accelerated diet-induced obesity phenotype. The ventromedial hypothalamus (VMH), a critical node in the neural networks regulating feeding-related behaviors and metabolic homeostasis, exhibits dense Mc3r expression relative to other brain regions. To target VMH MC3R expression, we used the steroidogenic factor-1 Cre transgenic mouse. Although restoring VMH MC3R signaling also had a modest impact on obesity, marked improvements in metabolic homeostasis were observed. VMH MC3R signaling was not sufficient to rescue the lean mass phenotype or the regulation of behaviors anticipating food anticipation. These results suggest that actions of MC3Rs impacting on energy homeostasis involve both central and peripheral sites of action. The impact of central MC3Rs on behavior and metabolism involves divergent pathways; VMH MC3R signaling improves metabolic homeostasis but does not significantly impact on the expression of behaviors anticipating nutrient availability.

Allelic variants of melanocortin 3 receptor gene (MC3R) and weight loss in obesity: a randomised trial of hypo-energetic high- versus low-fat diets

Introduction

The melanocortin system plays an important role in energy homeostasis. Mice genetically deficient in the melanocortin-3 receptor gene have a normal body weight with increased body fat, mild hypophagia compared to wild-type mice. In humans, Thr6Lys and Val81Ile variants of the melanocortin-3 receptor gene (MC3R) have been associated with childhood obesity, higher BMI Z-score and elevated body fat percentage compared to non-carriers. The aim of this study is to assess the association in adults between allelic variants of MC3R with weight loss induced by energy-restricted diets.

Subjects and Methods

This research is based on the NUGENOB study, a trial conducted to assess weight loss during a 10-week dietary intervention involving two different hypo-energetic (high-fat and low-fat) diets. A total of 760 obese patients were genotyped for 10 single nucleotide polymorphisms covering the single exon of MC3R gene and its flanking regions, including the missense variants Thr6Lys and Val81Ile. Linear mixed models and haplotype-based analysis were carried out to assess the potential association between genetic polymorphisms and differential weight loss, fat mass loss, waist change and resting energy expenditure changes.

Results

No differences in drop-out rate were found by MC3R genotypes. The rs6014646 polymorphism was significantly associated with weight loss using co-dominant (p = 0.04) and dominant models (p = 0.03). These p-values were not statistically significant after strict control for multiple testing. Haplotype-based multivariate analysis using permutations showed that rs3827103–rs1543873 (p = 0.06), rs6014646–rs6024730 (p = 0.05) and rs3746619–rs3827103 (p = 0.10) displayed near-statistical significant results in relation to weight loss. No other significant associations or gene*diet interactions were detected for weight loss, fat mass loss, waist change and resting energy expenditure changes.

Conclusion

The study provided overall sufficient evidence to support that there is no major effect of genetic variants of MC3R and differential weight loss after a 10-week dietary intervention with hypo-energetic diets in obese Europeans.

Implication of the melanocortin-3 receptor in the regulation of food intake

The melanocortin system is well recognized to be involved in the regulation of food intake, body weight, and energy homeostasis. To probe the role of the MC(3) in the regulation of food intake, JRH322-18 a mixed MC(3) partial agonist/antagonist and MC(4) agonist tetrapeptide was examined in wild type (WT) and melanocortin 4 receptor (MC(4)) knockout mice and shown to reduce food intake in both models. In the wild type mice, 2.0 nmol of JRH322-18 statistically reduced food intake 4h post icv treatment into satiated nocturnally feeding wild type mice. The same dose in the MC(4)KO mice significantly reduced cumulative food intake 24h post treatment. Conditioned taste aversion as well as activity studies supports that the decreased food intake was not due to visceral illness. Since these studies resulted in loss-of-function results, the SHU9119 and agouti-related protein (AGRP) melanocortin receptor antagonists were administered to wild type as well as the MC(3) and MC(4) knockout mice in anticipation of gain-of-function results. The SHU9119 ligand produced an increase in food intake in the wild type mice as anticipated, however no effect was observed in the MC(3) and MC(4) knockout mice as compared to the saline control. The AGRP ligand however, produced a significant increase in food intake in the wild type as well as the MC(3) and MC(4) knockout mice and it had a prolonged affect for several days. These data support the hypothesis that the MC(3) plays a subtle role in the regulation of food intake, however the mechanism by which this is occurring remains to be determined.

Physiological roles of the melanocortin MC₃ receptor

The melanocortin MC(3) receptor remains the most enigmatic of the melanocortin receptors with regard to its physiological functions. The receptor is expressed both in the CNS and in multiple tissues in the periphery. It appears to be an inhibitory autoreceptor on proopiomelanocortin neurons, yet global deletion of the receptor causes an obesity syndrome. Knockout of the receptor increases adipose mass without a readily measurable increase in food intake or decrease in energy expenditure. And finally, no melanocortin MC(3) receptor null humans have been identified and associations between variant alleles of the melanocortin MC(3) receptor and diseases remain controversial, so the physiological role of the receptor in humans remains to be determined.

cDNA cloning, pituitary location, and extra-pituitary expression of pro-opiomelanocortin gene in rare minnow (Gobiocypris rarus)

A cDNA encoding pro-opiomelanocortin (POMC) gene was cloned from the pituitary gland of the rare minnow (Gobiocypris rarus), a small freshwater fish endemic to China. This was achieved by reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). Data showed that the predicted rare minnow POMC (rmPOMC) cDNA consisted of 846bps coding for the following sequences, flanked by proteolytic cleavage sites: signal peptide (SP, Met(1)-Ala(28)), N-terminal peptide (Gln(29)-His(105)), ACTH (Ser(108)-Met(146)), α-MSH (Ser(108)-Gal(121)), CLIP (Pro(126)-Met(146)), β-LPH (Glu(149)-His(221)), γ-LPH (Glu1(49)-Ser(186)), β-MSH (Asp(170)-Ser(186)), and β-endorphin (β-EP, Tyr(189)-Gln(221)). Sequence analysis showed no region was homologous to γ-MSH (a tetrapod POMC feature). The amino acid sequence is highly similar to POMC-I and POMC-II of the common carp (92.4%), according to homologous alignment. It was POMCα through the phylogenetic analysis. Pituitary and extra-pituitary expression were studied using RT-PCR and in situ hybridization. The rmPOMC-positive cells were mainly located in the rostral pars distalis (RPD) and pars intermedia (PI). Some rmPOMC-positive cells were detected in the proximal pars distalis (PPD) as well, according to in situ hybridization. In the extra-pituitary tissues, positive signals were observed in the brain, intestines, gonads, hepatopancreas, spleen, and gills by RT-PCR analysis.

Antibodies raised against different extracellular loops of the melanocortin-3 receptor affect energy balance and autonomic function in rats

Melanocortin receptors (MCR) play an important role in the regulation of energy balance and autonomic function. In the present studies, we used active immunization against peptide sequences from the first and the third extracellular loop (EL1 and EL3) of the MC3R to generate selective antibodies (Abs) against this MCR subtype in rats. Immunization with the EL1 peptide resulted in Abs that enhanced the effects of the endogenous ligand α-melanocyte-stimulating hormone (α-MSH), whereas immunization with the EL3 peptide resulted in Abs acting as non-competitive antagonists. The phenotype of immunized rats chronically instrumented with telemetry transducers was studied under four different conditions: a high-fat diet was followed by standard lab chow, by fasting, and finally by an intraperitoneal injection of lipopolysaccharide (LPS). Under high-fat diet, food intake and body weight were higher in the EL3 than in the EL1 or the control group. Blood pressure was increased in EL3 rats and locomotor activity was reduced. Plasma concentrations of triglycerides, insulin, and leptin tended to rise in the EL3 group. After switching to standard lab chow, the EL1 group showed a small significant increase in blood pressure that was more pronounced and associated with an increase in heart rate during food restriction. No differences between the EL1 or the EL3 group were observed after LPS injection. These results show that immunization against the MC3R resulted in the production of Abs with positive or negative allosteric properties. The presence of such Abs induced small changes in metabolic and cardiovascular parameters.

Identification of three novel genetic variants in the melanocortin-3 receptor of obese children

The melanocortin-3 receptor (MC3R), a G-protein-coupled receptor expressed in the hypothalamus, is a key component of the leptin-melanocortin pathway that regulates energy homeostasis. It is suggested that an MC3R defect leads to an increased feed efficiency, by which nutrients are partitioned preferentially into fat. In this study, we hypothesized that early-onset obesity could be induced by mutations in MC3R. To investigate this hypothesis, we screened the entire coding region of the MC3R gene for mutations in obese subjects. A total of 404 overweight and obese children and adolescents, 86 severely obese adults (BMI ≥40 kg/m²), and 150 normal-weight control adults were included. Besides three synonymous coding variations in the MC3R gene (S69S, L95L, I226I), we were able to identify three novel heterozygous, nonsynonymous, coding mutations (N128S, V211I, L299V) in three unrelated obese children. None of these mutations were found in any of the control subjects. Functional studies assessing localization and signaling properties of the mutant receptors provided proof for impaired function of the L299V mutated receptor, whereas no conclusive evidence for functional impairment of the N128S and V211I mutated receptors could be established. First, these results provide supporting evidence for a role of the MC3R gene in the pathogenesis of obesity in a small subset of patients. Second, they show that caution is called for the interpretation of newly discovered mutations in MC3R.

Effect of serotonergic anorectics on food intake and induction of Fos in brain of mice with disruption of melanocortin 3 and/or 4 receptors

Previous studies have indicated that type 3 or 4 melanocortin receptors (MCR) are downstream of the critical anorectic action of drugs that stimulate 5-HT(2C) receptors. To characterize further the receptor types involved, we have studied the effect of serotonergic anorectics in mice with genomic disruption of either MC3R or MC4R, or their combined knockout. In a first experiment, we showed that wild type (WT) and MC4R-/- mice showed comparable inhibition of food intake following acute treatment with dexnorfenfluramine. In a second experiment using WAY-161503, a 5-HT receptor full agonist with selectivity for 2B and 2C subtypes, we found that MC4R-/- responded comparably to WT, while MC3R-/- had reduced sensitivity. Double receptor knockout (DKO) mice responded comparably to WT and MC4R-/-. Surprisingly, brain Fos-ir was not strongly induced in any brain region by WAY-16103 with the exception of the paraventricular nucleus of DKO. These data suggest that MC3Rs may be involved in the response to serotonergic anorectic agents, and more generally in control of food intake.

Organic chemistry and biology: chemical biology through the eyes of collaboration

From a scientific perspective, efforts to understand biology including what constitutes health and disease has become a chemical problem. However, chemists and biologists "see" the problems of understanding biology from different perspectives, and this has retarded progress in solving the problems especially as they relate to health and disease. This suggests that close collaboration between chemists and biologists is not only necessary but essential for progress in both the biology and chemistry that will provide solutions to the global questions of biology. This perspective has directed my scientific efforts for the past 45 years, and in this overview I provide my perspective of how the applications of synthetic chemistry, structural design, and numerous other chemical principles have intersected in my collaborations with biologists to provide new tools, new science, and new insights that were only made possible and fruitful by these collaborations.

Analysis of the therapeutic functions of novel melanocortin receptor agonists in MC3R- and MC4R-deficient C57BL/6J mice

Melanocortin receptor agonists act in the brain to regulate food intake and body weight and, independently of these actions, affect insulin sensitivity. These experiments investigated the function of novel non-selective melanocortin receptor agonists (BIM-22493, BIM-22511) that cross the blood-brain barrier when administered peripherally. Treatment of diet induced obese C57BL/6J (B6) mice with melanocortin agonists administered peripherally improved obesity, hyperinsulinemia (approximately 50%) and fatty liver disease. Specificity of function was determined using B6 melanocortin-3 and melanocortin-4 receptor knockout mice (MC3RKO, MC4RKO). Chow fed MC4RKO but not MC3RKO used for these tests exhibited obesity, hyperinsulinemia and severe hepatosteatosis associated with increased expression of insulin-stimulated genes involved in lipogenesis. Reduced food intake associated with acute BIM-22493 treatment, and weight loss associated with 14 days of treatment with BIM-22511, required functional MC4R but not MC3R. However, while 14 days of treatment with BIM-22511 did not affect body weight and even increased cumulative food intake in MC4RKO, a significant reduction (approximately 50%) in fasting insulin was still observed. Despite lowering insulin, chronic treatment with BIM-22511 did not improve hepatosteatosis in MC4RKO, and did not affect hepatic lipogenic gene expression. Together, these results demonstrate that peripherally administered melanocortin receptor agonists regulate body weight, liver metabolism and glucose homeostasis through independent pathways. MC4R are necessary for melanocortin agonist-induced weight loss and improvements in liver metabolism, but are not required for improvements in hyperinsulinemia. Agonists with activity at MC4R improve glucose homeostasis at least partially by causing weight loss, however other melanocortin receptors may have potential for treating aberrations in glucose homeostasis associated with obesity.

Role of central melanocortin pathways in energy homeostasis

The rise in the global prevalence of human obesity has emphasized the need for a greater understanding of the physiological mechanisms that underlie energy homeostasis. Numerous circulating nutritional cues and central neuromodulatory signals are integrated within the brain to regulate both short- and long-term nutritional state. The central melanocortin system represents a crucial point of convergence for these signals and, thus, has a fundamental role in regulating body weight. The melanocortin ligands, synthesized in discrete neuronal populations within the hypothalamus and brainstem, modulate downstream homeostatic signalling via their action at central melanocortin-3 and -4 receptors. Intimately involved in both ingestive behaviour and energy expenditure, the melanocortin system has garnered much interest as a potential therapeutic target for human obesity.

Substitution of arginine with proline and proline derivatives in melanocyte-stimulating hormones leads to selectivity for human melanocortin 4 receptor

A new series of melanotropin analogues with His or Arg residues in the core pharmacophores of MTII, SHU9119, and Ac-NDP-gamma-MSH-NH(2) replaced by Pro or trans-/cis-4-guanidinyl-Pro derivatives were designed and synthesized to introduce selectivity toward the human melanocortin 4 receptor (hMC4R). Analogues 1, 2, 3, 6, 7, 8 were found to be hMC4R selective. Second messenger studies have demonstrated that analogues 1 and 2 are insurmountable inhibitors of MTII agonist activity at the hMC4R. Molecular modeling studies suggest that the hMC4R selectivity is due to a beta-turn shift induced by the Pro ring that makes the global minimum structures of these analogues resemble the NMR solution structure of the hASIP melanocortin receptor binding motif. Substitution of His in MTII also provided functional selectivity for the hMC3R or the hMC4R. These findings are important for a better understanding of the selectivity mechanism at the hMC3R/hMC4R and the development of therapeutic ligands selectively targeting the hMC4R.

Melanotropins as drugs for the treatment of obesity and other feeding disorders: potential and problems

Current biological and pharmacological evidence suggests that the melanocortin 4 and melanocortin 3 receptors which are seven transmembrane G-protein coupled receptors (GPCRs) are involved in various aspects of energy balance and feeding behaviors in animals including humans. The natural endogenous ligands for these receptors are products of the gene pro-opiomelanocortin (POMC), and include alpha-melanocyte stimulating hormone, gamma-melanocyte stimulating hormone and perhaps other modified products of POMC. Thus well designed agonists and antagonists of these ligands might serve as drugs for the treatment of feeding disorders. However, these melanotropin peptides also can have other biological activities that involve the MC3R and MC4R, and these other biological properties will need to be modulated in ligands that are likely to be useful drugs for feeding disorders. Current progress in these areas with special emphasis on the MC3R will be discussed along with possible new directions that might be fruitful in these important aspects of contemporary biology and medicine.

Functions of acidic transmembrane residues in human melanocortin-3 receptor binding and activation

The melanocortin-3 receptor (MC3R) is an important regulator of energy homeostasis, inflammation, and cardiovascular function. Inactivating mutations in MC3R gene are associated with childhood obesity. How MC3R binds to its ligands has rarely been studied. In the present study, we systematically mutated all ten acidic residues in transmembrane (TM) domains and measured the cell surface expression levels as well as ligand binding and signaling properties of these mutants. Our results showed that of the 19 mutants stably expressed in HEK293 cells, all were expressed on the cell surface, although some mutants had decreased levels of cell surface expression. We showed that with the superpotent analog [Nle(4), D-Phe(7)]-alpha-melanocyte stimulating hormone (MSH), E92, E131, D154, D158, D178, and D332 are important for ligand binding. D121 and D332 are important for binding and signaling. Further experiments using other ligands such as D-Trp(8)-gamma-MSH, alpha-MSH and gamma-MSH showed that different ligands induce or select different conformations. In summary, we showed that acidic residues in TMs 1 and 3 are important for ligand binding whereas the acidic residues in TMs 2 and 7 are important for both ligand binding and signaling.

Y eat?

Feeding behavior is tightly regulated by peptidergic transmission within the hypothalamus. Neuropeptide Y (NPY) is one of the most potent known stimulators of food intake and has robust effects on the hypothalamic feeding neuronal networks. A vast body of literature has documented the substantial effects of NPY on feeding behavior. However, the cellular mechanisms underlying the actions of NPY have only recently begun to be explored. The NPYergic signal, including its expression in hypothalamic neurons, its release into the synaptic space, and its direct or indirect receptor-mediated actions, is highly responsive to decreases in the metabolic state. The orexigenic NPY signal can suppress the anorexigenic drive to restore energy balance homeostasis when energy levels are low, such as after food deprivation. The NPY signal interacts with glucose- and fat-sensitive signals arriving in the hypothalamus and effects changes in anorexigenic pathways, such as those mediated by the melanocortins. Recent applications of electrophysiological methods to examine the neuronal activity and pathways engaged by NPY-mediated signaling have advanced our understanding of this orexigenic system. Furthermore, crucial roles for NPY pathways in the development of hypothalamic feeding circuitry have been identified by these means. Orexigenic NPY signaling is critical during development and its absence is lethal in adults, thus reflecting the essential role of NPY for the regulation of energy homeostasis.