The role of melanocortin neuronal pathways in circadian biology: a new homeostatic output involving melanocortin-3 receptors?

Pro-Opiomelanocortin and Melanocortin Receptor 3 and 4 Mutations in Genetic Obesity

Genetic obesity results from loss-of-function mutations, including those affecting the leptin-melanocortin system, which regulates body weight. Pro-opiomelanocortin (POMC)-derived neurohormones act as ligands for melanocortin receptors (MCRs), regulating the leptin-melanocortin pathway through protein-protein interactions. Loss-of-function mutations in the genes encoding POMC, MC3R, and MC4R can lead to the dysregulation of energy expenditure and feeding balance, early-onset obesity, and developmental dysregulation. Recent studies have identified new genetic regulatory mechanisms and potential biomarker regions for the POMC gene and MC4R secondary messenger pathway associated with obesity. Recent advances in crystal structure studies have enhanced our understanding of the protein interactions in this pathway. This narrative review focuses on recent developments in two key areas related to POMC regulation and the leptin-melanocortin pathway: (1) genetic variations in and functions of POMC, and (2) MC3R and MC4R variants that lead to genetic obesity in humans. Understanding these novel mutations in POMC and MC4R/MC3R, as well as their structural and intracellular mechanisms, may help identify strategies for the treatment and diagnosis of obesity, particularly childhood obesity.

Taurine Protects against the Fatty Liver Hemorrhagic Syndrome in Laying Hens through the Regulation of Mitochondrial Homeostasis

Metabolic-associated fatty liver disease (MAFLD) is a chronic liver disease caused by fat deposition in the liver of humans and mammals, while fatty liver hemorrhagic syndrome (FLHS) is a fatty liver disease in laying hens which can increase the mortality and cause severe economic losses to the laying industry. Increasing evidence has shown a close relationship between the occurrence of fatty liver disease and the disruption of mitochondrial homeostasis. Studies have proven that taurine can regulate hepatic fat metabolism, reduce hepatic fatty deposition, inhibit oxidative stress, and alleviate mitochondrial dysfunction. However, the mechanisms by which taurine regulates mitochondrial homeostasis in hepatocytes need to be further studied. In this study, we determined the effects and mechanisms of taurine on high-energy low-protein diet-induced FLHS in laying hens and in cultured hepatocytes in free fatty acid (FFA)-induced steatosis. The liver function, lipid metabolism, antioxidant capacity, mitochondrial function, mitochondrial dynamics, autophagy, and biosynthesis were detected. The results showed impaired liver structure and function, mitochondrial damage and dysfunction, lipid accumulation, and imbalance between mitochondrial fusion and fission, mitochondrial autophagy, and biosynthesis in both FLHS hens and steatosis hepatocytes. Taurine administration can significantly inhibit the occurrence of FLHS, protect mitochondria in hepatocytes from disease induced by lipid accumulation and FFA, up-regulate the expression levels of Mfn1, Mfn2, Opa1, LC3I, LC3II, PINK1, PGC-1α, Nrf1, Nrf2, and Tfam, and down-regulate the expression levels of Fis1, Drp1, and p62. In conclusion, taurine can protect laying hens from FLHS through the regulation of mitochondrial homeostasis, including the regulation of mitochondrial dynamics, autophagy, and biosynthesis.

Melanocortin 3 receptor gene polymorphism is associated with polycystic ovary syndrome in Turkish population

Polycystic ovary syndrome (PCOS) is a frequent complex disorder with an ill-defined etiology. Genetic factors seem rather effective at the occurrence of the disease, however, the evidence of established various studies results are unsatisfied. We aimed to make a contribution to the genetic baseline of the disease by investigating melanocortin 3 receptor gene polymorphism in affected patients. 101 PCOS patients and 162 age-matched healthy volunteered control subjects recruited to the study. PCOS patients classified according to their BMI class and insulin resistance situation. Anthropometric measurements, physical examination results, laboratory findings, and hormone levels were recorded for each participant and analysis of two SNPs on the MC3R gene; rs3746619 and rs3827103 were performed. Although no significant difference was observed in rs3827103 polymorphism between PCOS patients and controls; rs3746619 polymorphism was determined associated with PCOS in the heritage of dominant (AA + AC) and co-dominant (AA) genotypes. Two polymorphisms did not found related to obesity and insulin resistance in PCOS subgroups analysis. MC3R gene rs 3746619 polymorphism was found associated with PCOS in the Turkish population and may make a contribution to the genetic baseline of the disease.

MC4R Agonists: Structural Overview on Antiobesity Therapeutics

The melanocortin-4 receptor (MC4R) regulates adipose tissue formation and energy homeostasis, and is believed to be a monogenic target for novel antiobesity therapeutics. Several research efforts targeting this receptor have identified potent and selective agonists. While viable agonists have been characterized in vitro, undesirable side effects frequently appeared during clinical trials. The most promising candidates have diverse structures, including linear peptides, cyclic peptides, and small molecules. Herein, we present a compilation of potent MC4R agonists and discuss the pivotal structural differences within those molecules that resulted in good selectivity for MC4R over other melanocortins. We provide insight on recent progress in the field and reflect on directions for development of new agonists.

Polymorphisms and mutations in the melanocortin-3 receptor and their relation to human obesity

Inactivating mutations in the melanocortin 3 receptor (Mc3r) have been described as causing obesity in mice, but the physiologic effects of MC3R mutations in humans have been less clear. Here we review the MC3R polymorphisms and mutations identified in humans, and the in vitro, murine, and human cohort studies examining their putative effects. Some, but not all, studies suggest that the common human MC3R variant T6K+V81I, as well as several other rare, function-altering mutations, are associated with greater adiposity and hyperleptinemia with altered energy partitioning. In vitro, the T6K+V81I variant appears to decrease MC3R expression and therefore cAMP generation in response to ligand binding. Knockin mouse studies confirm that the T6K+V81I variant increases feeding efficiency and the avidity with which adipocytes derived from bone or adipose tissue stem cells store triglycerides. Other MC3R mutations occur too infrequently in the human population to make definitive conclusions regarding their clinical effects. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Assessing interactions between Ghsr and Mc3r reveals a role for AgRP in the expression of food anticipatory activity in male mice

The stomach hormone ghrelin and hypothalamic melanocortin neurons belong to a gut-brain circuit controlling appetite and metabolic homeostasis. Mice lacking melanocortin-3 receptor (Mc3rKO) or growth hormone secretagogue receptor (GhsrKO) genes exhibit attenuated food anticipatory activity (FAA), a rise in locomotor activity anticipating mealtime, suggesting common circuitry regulating anticipatory responses to nutrient loading. To investigate the interaction between Ghsrs and Mc3rs, we compared food anticipatory responses in GhsrKO, Mc3rKO, and double Ghsr;Mc3r knockout (DKO) mice subjected to a hypocaloric restricted feeding (RF) protocol in constant dark or 12-hour light, 12-hour dark settings. DKO are viable, exhibiting no overt behavioral or metabolic phenotypes in ad libitum or fasting conditions. FAA was initially attenuated in all mutant strains in constant darkness. However, GhsrKO eventually exhibited a robust food anticipatory response, suggesting compensation. Mc3rKO and DKO did not compensate, indicating a continued requirement for Mc3rs in maintaining the expression of FAA in situations of RF. Abnormal regulation of hypothalamic agouti-related peptide/neuropeptide Y (AgRP/Npy) neurons previously observed during fasting may contribute to attenuated FAA in Mc3rKO. AgRP and Npy expression measured 1 hour before food presentation correlated positively with FAA. Absence of Mc3rs (but not Ghsrs) was associated with lower AgRP/Npy expression, suggesting attenuated responses to signals of negative energy balance. These observations support the importance of Mc3rs as modulators of anticipatory responses to feeding, with mice able to compensate for loss of Ghsrs. The behavioral deficits of Mc3rKO displayed during RF may be partially explained by reduced hunger sensations owing to abnormal regulation of orexigenic AgRP/Npy neurons.

Central (mainly) actions of GPCRs in energy homeostasis/balance: view from the Chair

To maintain a constant body weight, energy intake must equal energy expenditure; otherwise, there is a risk of overweight and obesity. The hypothalamus is one of the primary brain regions where multiple nutrient-related signals from peripheral and central sources converge and become integrated to regulate both short- and long-term nutritional states. The aim of the afternoon session of the 15th Annual International Symposium of the Laval University Obesity Research Chair held in Quebec City on 9 November 2012 was to present the most recent insights into the complex molecular mechanisms regulating food intake. The aims were to emphasize on the interaction between central and peripheral actions of some of the key players acting not only at the hypothalamic level but also at the periphery. Presentations were focused on melanocortin-3 receptor (MC3R) and melanin-concentrating hormone (MCH) as anorexigenic and orexigenic components of the hypothalamus, on endocannabinoid receptors, initially as a central neuromodulatory signal, and on glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) as peripheral signals. What becomes clear from these four presentations is that the regulation of food intake and energy homeostasis involves several overlapping pathways, and that we have only touched the tip of the iceberg. From the examples presented in this symposium, it could be expected that in the near future, in addition to a low-fat diet and exercise, a combination of appropriate peptides and small molecules is likely to become available to improve/facilitate the objectives of long-term maintenance of energy balance and body weight.

Neural melanocortin receptors in obesity and related metabolic disorders

Obesity is a global health issue, as it is associated with increased risk of developing chronic conditions associated with disorders of metabolism such as type 2 diabetes and cardiovascular disease. A better understanding of how excessive fat accumulation develops and causes diseases of the metabolic syndrome is urgently needed. The hypothalamic melanocortin system is an important point of convergence connecting signals of metabolic status with the neural circuitry that governs appetite and the autonomic and neuroendocrine system controling metabolism. This system has a critical role in the defense of body weight and maintenance of homeostasis. Two neural melanocortin receptors, melanocortin 3 and 4 receptors (MC3R and MC4R), play crucial roles in the regulation of energy balance. Mutations in the MC4R gene are the most common cause of monogenic obesity in humans, and a large literature indicates a role in regulating both energy intake through the control of satiety and energy expenditure. In contrast, MC3Rs have a more subtle role in energy homeostasis. Results from our lab indicate an important role for MC3Rs in synchronizing rhythms in foraging behavior with caloric cues and maintaining metabolic homeostasis during periods of nutrient scarcity. However, while deletion of the Mc3r gene in mice alters nutrient partitioning to favor accumulation of fat mass no obvious role for MC3R haploinsufficiency in human obesity has been reported. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Metabolism and the circadian clock converge

Circadian rhythms occur in almost all species and control vital aspects of our physiology, from sleeping and waking to neurotransmitter secretion and cellular metabolism. Epidemiological studies from recent decades have supported a unique role for circadian rhythm in metabolism. As evidenced by individuals working night or rotating shifts, but also by rodent models of circadian arrhythmia, disruption of the circadian cycle is strongly associated with metabolic imbalance. Some genetically engineered mouse models of circadian rhythmicity are obese and show hallmark signs of the metabolic syndrome. Whether these phenotypes are due to the loss of distinct circadian clock genes within a specific tissue versus the disruption of rhythmic physiological activities (such as eating and sleeping) remains a cynosure within the fields of chronobiology and metabolism. Becoming more apparent is that from metabolites to transcription factors, the circadian clock interfaces with metabolism in numerous ways that are essential for maintaining metabolic homeostasis.

Metabolic transceivers: in tune with the central melanocortin system

The central melanocortin system plays an essential role in the regulation of energy metabolism. Key to this regulation are the responses of neurons expressing proopiomelanocortin (POMC) and agouti-related protein (AgRP) to blood-borne metabolic signals. Recent evidence has demonstrated that POMC and AgRP neurons are not simply mirror opposites of each other in function and responsiveness to metabolic signals, nor are they exclusively first-order neurons. These neurons act as central transceivers, integrating both hormonal and neural signals, and then transmitting this information to peripheral tissues via the autonomic nervous system to coordinate whole-body energy metabolism. This review focuses on most recent developments obtained from rodent studies on the function, metabolic regulation, and circuitry of the central melanocortin system.

Melanocortin-3 receptors and metabolic homeostasis

Attenuated activity of the central nervous melanocortin system causes obesity and insulin resistance. Obese rodents treated with melanocortins exhibit improvements in obesity and metabolic homeostasis that are not mutually dependent, suggesting metabolic actions that are independent of weight changes. These responses are generally thought to involve G-protein-coupled receptors expressed in the brain. Melanocortin-4 receptors (MC4Rs) regulate satiety and autonomic nervous system and thyroid function. MC3Rs are expressed in hypothalamic and limbic regions involved in controlling ingestive behaviors and autonomic function. Mc3r-/- mice exhibit increased adiposity and an accelerated diet-induced obesity. While this phenotype is not dependent on hyperphagia, data on the regulation of food intake by MC3Rs are inconsistent. Recent investigations by our laboratory suggest a unique combination of behavioral and metabolic disorders in Mc3r-/- mice. MC3Rs are critical for the expression of the anticipatory response and metabolic homeostasis when food intake occurs outside the normal voluntary rhythms driven by photoperiod. Using a Cre-Lox strategy, we can now investigate MC3Rs expressed in different brain regions and organ systems in the periphery. While focusing on the functions of neural MC3Rs, early results suggest an additional layer of complexity with central and peripheral MC3Rs involved in the defense of body weight.

Melanocortin-3 receptor regulates the normal fasting response

The melanocortin-3 receptor-deficient (MC3-R(-/-)) mouse exhibits mild obesity without hyperphagia or hypometabolism. MC3-R deletion is reported to increase adiposity, reduce lean mass and white adipose tissue inflammation, and increase sensitivity to salt-induced hypertension. We show here that the MC3-R(-/-) mouse exhibits defective fasting-induced white adipose tissue lipolysis, fasting-induced liver triglyceride accumulation, fasting-induced refeeding, and fasting-induced regulation of the adipostatic and hypothalamic-adrenal-pituitary axes. Close examination of the hypothalamic-pituitary-adrenal axis showed that MC3-R(-/-) mice exhibit elevated nadir corticosterone as well as a blunted fasting-induced activation of the axis. The previously described phenotypes of this animal and the reduced bone density reported here parallel those of Cushing syndrome. Thus, MC3-R is required for communicating nutritional status to both central and peripheral tissues involved in nutrient partitioning, and this defect explains much of the metabolic phenotype in the model.

Melanocortin-3 receptors are involved in adaptation to restricted feeding

The central nervous melanocortin system forms a neural network that maintains energy homeostasis. Actions involving neural melanocortin-3 receptors (MC3Rs) regulate the expression rhythms in ingestive behaviors and metabolism anticipating nutrient intake. Here, we characterized the response of Mc3r knockout (Mc3r(-/-)) and wild type (WT) mice to a restricted feeding (RF) schedule where food access was limited to a 4-h period mid light cycle using a mechanical barrier. Mc3r(-/-) mice adapted poorly to the food restriction schedule. Anticipatory activity and the initial bout of intense feeding activity associated with granting food access were attenuated in Mc3r(-/-) mice, resulting in increased weight loss relative to controls. To investigate whether activity in specific hypothalamic nuclei contribute to the Mc3r(-/-) phenotype observed, we assessed hypothalamic FOS-immunoreactivity (FOS-IR) associated with food restriction. Food access markedly increased FOS-IR in the dorsomedial hypothalamus (DMH), but not in the suprachiasmatic or ventromedial hypothalamic nuclei (SCN and VMN, respectively) compared to ad libitum fed mice. Mc3r(-/-) mice displayed a significant reduction in FOS-IR in the DMH during feeding. Analysis of MC3R signaling in vitro indicated dose-dependent stimulation of the extracellular signal-regulated kinase (ERK) pathway by the MC3R agonist d-Trp(8)-γMSH. Treatment of WT mice with d-Trp(8)-γMSH administered intracerebroventricularly increased the number of pERK neurons 1.7-fold in the DMH. These observations provide further support for the involvement of the MC3Rs in regulating adaptation to food restriction. Moreover, MC3Rs may modulate the activity of neurons in the DMH, a region previously linked to the expression of the anticipatory response to RF.

Development of melanoma-targeted polymer micelles by conjugation of a melanocortin 1 receptor (MC1R) specific ligand

The incidence of malignant melanoma is rising faster than that of any other cancer in the United States. Because of its high expression on the surface of melanomas, MC1R has been investigated as a target for selective imaging and therapeutic agents against melanoma. Eight ligands were screened against cell lines engineered to overexpress MC1R, MC4R, or MC5R. Of these, compound 1 (4-phenylbutyryl-His-dPhe-Arg-Trp-NH(2)) exhibited high (0.2 nM) binding affinity for MC1R and low (high nanomolar) affinities for MC4R and MC5R. Functionalization of the ligand at the C-terminus with an alkyne for use in Cu-catalyzed click chemistry was shown not to affect the binding affinity. Finally, formation of the targeted polymer, as well as the targeted micelle formulation, also resulted in constructs with low nanomolar binding affinity.

Homeostastic and non-homeostatic functions of melanocortin-3 receptors in the control of energy balance and metabolism

The central nervous melanocortin system is a neural network linking nutrient-sensing systems with hypothalamic, limbic and hindbrain neurons regulating behavior and metabolic homeostasis. Primary melanocortin neurons releasing melanocortin receptor ligands residing in the hypothalamic arcuate nucleus are regulated by nutrient-sensing and metabolic signals. A smaller group of primary neurons releasing melanocortin agonists in the nucleus tractus solitarius in the brainstem are also regulated by signals of metabolic state. Two melanocortin receptors regulate energy homeostasis. Melanocortin-4 receptors regulate satiety and autonomic outputs controlling peripheral metabolism. The functions of melanocortin-3 receptors (MC3R) expressed in hypothalamic and limbic structures are less clear. Here we discuss published data and preliminary observations from our laboratory suggesting that neural MC3R regulate inputs into systems governing the synchronization of rhythms in behavior and metabolism with nutrient intake. Mice subjected to a restricted feeding protocol, where a limited number of calories are presented at a 24h interval, rapidly exhibit bouts of increased wakefulness and activity which anticipate food presentation. The full expression of these responses is dependent on MC3R. Moreover, MC3R knockout mice are unique in exhibiting a dissociation of weight loss from improved glucose homeostasis when subject to a restricted feeding protocol. While mice lacking MC3R fed ad libitum exhibit normal to moderate hyperinsulinemia, when subjected to a restricted protocol they develop hyperglycemia, glucose intolerance, and dyslipidemia. Collectively, our data suggest that the central nervous melanocortin system is a point convergence in the control of energy balance and the expression of rhythms anticipating nutrient intake.

Melanocortin control of energy balance: evidence from rodent models

Regulation of energy balance is extremely complex, and involves multiple systems of hormones, neurotransmitters, receptors, and intracellular signals. As data have accumulated over the last two decades, the CNS melanocortin system is now identified as a prominent integrative network of energy balance controls in the mammalian brain. Here, we will review findings from rat and mouse models, which have provided an important framework in which to study melanocortin function. Perhaps most importantly, this review attempts for the first time to summarize recent advances in our understanding of the intracellular signaling pathways thought to mediate the action of melanocortin neurons and peptides in control of longterm energy balance. Special attention will be paid to the roles of MC4R/MC3R, as well as downstream neurotransmitters within forebrain and hindbrain structures that illustrate the distributed control of melanocortin signaling in energy balance. In addition, distinctions and controversy between rodent species will be discussed.

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.