Functional characterization of melanocortin-4 receptor mutations associated with childhood obesity

Identification and functional characterization of novel mutations in the melanocortin-4 receptor

OBJECTIVE

Melanocortin-4 receptor (MC4R) deficiency is the most common cause of monogenic obesity. In the present study, we screened the MC4R gene for mutations in a population of overweight and obese children and adolescents.

METHOD

Cross-sectional mutation analysis of 112 overweight/obese children and adolescents and 121 lean individuals.

RESULTS

We identified 11 sequence variations, 5 of which were present in our control population or had been previously reported as polymorphisms. The remaining 6 variations are disease-causing mutations including 2 novel ones: a I186V mutation and a F280L mutation. The 4 previously described mutations (D90N, M200V, P260Q, Q307X) were identified in single probands. Using confocal imaging, we demonstrated that F280L and P260Q cause intracellular retention of the mutant receptor. No difference in cell surface expression could be detected for the I186V mutation. Using a cAMP responsive luciferase vector, we demonstrated that the receptor with I186V is unable to activate its intracellular signaling pathway while the P260Q mutation causes reduced activation of the receptor.

CONCLUSION

We detected MC4R deficiency in 6 patients from our cohort, amounting to a prevalence of 5.3%. Two novel mutations were identified. We also confirmed that intracellular retention is a common pathogenic effect of MC4R mutations.

Pharmacological characterization of 30 human melanocortin-4 receptor polymorphisms with the endogenous proopiomelanocortin-derived agonists, synthetic agonists, and the endogenous agouti-related protein antagonist

The melanocortin-4 receptor (MC4R) is a G-protein-coupled receptor (GPCR) that is expressed in the central nervous system and has a role in regulating feeding behavior, obesity, energy homeostasis, male erectile response, and blood pressure. Since the report of the MC4R knockout mouse in 1997, the field has been searching for links between this genetic biomarker and human obesity and type 2 diabetes. More then 80 single nucleotide polymorphisms (SNPs) have been identified from human patients, both obese and nonobese controls. Many significant studies have been performed examining the pharmacological characteristics of these hMC4R SNPs in attempts to identify a molecular defects/insights that might link a genetic factor to the obese phenotype observed in patients possessing these mutations. Our laboratory has previously reported the pharmacological characterization of 40 of these polymorphic hMC4 receptors with multiple endogenous and synthetic ligands. The goal of the current study is to perform a similar comprehensive side-by-side characterization of 30 additional human hMC4R with single nucleotide polymorphisms using multiple endogenous agonists [alpha-, beta-, and gamma(2)-melanocyte stimulating hormones (MSH) and adrenocorticotropin (ACTH)], the antagonist agouti-related protein hAGRP(87-132), and synthetic agonists [NDP-MSH, MTII, and the tetrapeptide Ac-His-dPhe-Arg-Trp-NH(2) (JRH887-9)]. These in vitro data, in some cases, provide a putative molecular link between dysfunctional hMC4R's and human obesity. These 30 hMC4R SNPs include R7H, R18H, R18L, S36Y, P48S, V50M, F51L, E61K, I69T, D90N, S94R, G98R, I121T, A154D, Y157S, W174C, G181D, F202L, A219 V, I226T, G231S, G238D, N240S, C271R, S295P, P299L, E308K, I317V, L325F, and 750DelGA. All but the N240S hMC4R were identified in obese patients. Additionally, we have characterized a double I102T/V103I hMC4R. In addition to the pharmacological characterization, the hMC4R variants were evaluated for cell surface expression by flow cytometry. The F51L, I69T, and A219V hMC4Rs possessed full agonist activity and significantly decreased endogenous agonist ligand potency. At the E61K, D90N, Y157S, and C271R hMC4Rs, all agonist ligands examined were only partially efficacious in generating a maximal signaling response (partial agonists) and possessed significantly decreased endogenous agonist ligand potency. Only the A219V, G238D, and S295P hMC4Rs possessed significantly decreased AGRP(87-132) antagonist potency. These data provide new information for use in GPCR computational development as well as insights into MC4R structure ad function.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

Obesity-linked variants of melanocortin-4 receptor are misfolded in the endoplasmic reticulum and can be rescued to the cell surface by a chemical chaperone

Melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed in the brain where it controls food intake. Many obesity-linked MC4R variants are poorly expressed at the plasma membrane and are retained intracellularly. We have studied the intracellular localization of four obesity-linked MC4R variants, P78L, R165W, I316S, and I317T, in immortalized neurons. We find that these variants are all retained in the endoplasmic reticulum (ER), are ubiquitinated to a greater extent than the wild-type (wt) receptor, and induce ER stress with increased levels of ER chaperones as compared with wt-MC4R and appearance of CCAAT/enhancer-binding protein homologous protein (CHOP). Expression of the X-box-binding-protein-1 (XBP-1) with selective activation of a protective branch of the unfolded protein response did not have any effect on the cell surface expression of MC4R-I316S. Conversely, the pharmacological chaperone 4-phenyl butyric acid (PBA) increased the cell surface expression of wt-MC4R, MC4R-I316S, and I317T by more than 40%. PBA decreased ubiquitination of MC4R-I316S and prevented ER stress induced by expression of the mutant, suggesting that the drug functions to promote MC4R folding. MC4R-I316S rescued to the cell surface is functional, with a 52% increase in agonist-induced cAMP production, as compared with untreated cells. Also direct inhibition of wt-MC4R and MC4R-I316S ubiquitination by a specific inhibitor of the ubiquitin-activating enzyme 1 increased by approximately 40% the expression of the receptors at the cell surface, and the effects of PBA and ubiquitin-activating enzyme 1 were additive. These data offer a cell-based rationale that drugs that improve MC4R folding or decrease ER-associated degradation of the receptor may function to treat some forms of hereditary obesity.

Functions for pro-opiomelanocortin-derived peptides in obesity and diabetes

Melanocortin peptides, derived from POMC (pro-opiomelanocortin) are produced in the ARH (arcuate nucleus of the hypothalamus) neurons and the neurons in the commissural NTS (nucleus of the solitary tract) of the brainstem, in anterior and intermediate lobes of the pituitary, skin and a wide range of peripheral tissues, including reproductive organs. A hypothetical model for functional roles of melanocortin receptors in maintaining energy balance was proposed in 1997. Since this time, there has been an extraordinary amount of knowledge gained about POMC-derived peptides in relation to energy homoeostasis. Development of a Pomc-null mouse provided definitive proof that POMC-derived peptides are critical for the regulation of energy homoeostasis. The melanocortin system consists of endogenous agonists and antagonists, five melanocortin receptor subtypes and receptor accessory proteins. The melanocortin system, as is now known, is far more complex than most of us could have imagined in 1997, and, similarly, the importance of this system for regulating energy homoeostasis in the general human population is much greater than we would have predicted. Of the known factors that can cause human obesity, or protect against it, the melanocortin system is by far the most significant. The present review is a discussion of the current understanding of the roles and mechanism of action of POMC, melanocortin receptors and AgRP (agouti-related peptide) in obesity and Type 2 diabetes and how the central and/or peripheral melanocortin systems mediate nutrient, leptin, insulin, gut hormone and cytokine regulation of energy homoeostasis.

Evidence that multiple genetic variants of MC4R play a functional role in the regulation of energy expenditure and appetite in Hispanic children

BACKGROUND

Melanocortin-4-receptor (MC4R) haploinsufficiency is the most common form of monogenic obesity; however, the frequency of MC4R variants and their functional effects in general populations remain uncertain.

OBJECTIVE

The aim was to identify and characterize the effects of MC4R variants in Hispanic children.

DESIGN

MC4R was resequenced in 376 parents, and the identified single nucleotide polymorphisms (SNPs) were genotyped in 613 parents and 1016 children from the Viva la Familia cohort. Measured genotype analysis (MGA) tested associations between SNPs and phenotypes. Bayesian quantitative trait nucleotide (BQTN) analysis was used to infer the most likely functional polymorphisms influencing obesity-related traits.

RESULTS

Seven rare SNPs in coding and 18 SNPs in flanking regions of MC4R were identified. MGA showed suggestive associations between MC4R variants and body size, adiposity, glucose, insulin, leptin, ghrelin, energy expenditure, physical activity, and food intake. BQTN analysis identified SNP 1704 in a predicted micro-RNA target sequence in the downstream flanking region of MC4R as a strong, probable functional variant influencing total, sedentary, and moderate activities with posterior probabilities of 1.0. SNP 2132 was identified as a variant with a high probability (1.0) of exerting a functional effect on total energy expenditure and sleeping metabolic rate. SNP rs34114122 was selected as having likely functional effects on the appetite hormone ghrelin, with a posterior probability of 0.81.

CONCLUSION

This comprehensive investigation provides strong evidence that MC4R genetic variants are likely to play a functional role in the regulation of weight, not only through energy intake but through energy expenditure.

Constitutive activity of neural melanocortin receptors

The two neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), are G protein-coupled receptors expressed primarily in the brain that regulate different aspects of energy homeostasis. The MCRs are unique in having endogenous antagonists, agouti and agouti-related protein (AgRP). These antagonists were later shown to be inverse agonists. The MC3R has little or no constitutive activity, whereas the MC4R has significant constitutive activity that can easily be detected. We describe herein methods for detecting constitutive activities in these receptors and small molecule ligands as inverse agonists. AgRP is an inverse agonist for both MC3R and MC4R. We also provide models for the constitutively active MC4R mutants.

From monogenic to polygenic obesity: recent advances

The heritability of obesity and body weight in general is high. A small number of confirmed monogenic forms of obesity-the respective mutations are sufficient by themselves to cause the condition in food abundant societies-have been identified by molecular genetic studies. The elucidation of these genes, mostly based on animal and family studies, has led to the identification of important pathways to the disorder and thus to a deeper understanding of the regulation of body weight. The identification of inborn deficiency of the mostly adipocyte-derived satiety hormone leptin in extremely obese children from consanguineous families paved the way to the first pharmacological therapy for obesity based on a molecular genetic finding. The genetic predisposition to obesity for most individuals, however, has a polygenic basis. A polygenic variant by itself has a small effect on the phenotype; only in combination with other predisposing variants does a sizeable phenotypic effect arise. Common variants in the first intron of the 'fat mass and obesity associated' gene (FTO) result in an elevated body mass index (BMI) equivalent to approximately +0.4 kg/m(2) per risk allele. The FTO variants were originally detected in a genome wide association study (GWAS) pertaining to type 2 diabetes mellitus. Large meta-analyses of GWAS have subsequently identified additional polygenic variants. Up to December 2009, polygenic variants have been confirmed in a total of 17 independent genomic regions. Further study of genetic effects on human body weight regulation should detect variants that will explain a larger proportion of the heritability. The development of new strategies for diagnosis, treatment and prevention of obesity can be anticipated.

Follicle stimulating hormone receptor mutations and reproductive disorders

The follicle stimulating hormone receptor (FSHR) plays a critical role in reproductive function. In the males, FSH supports spermatogenesis, whereas in females, FSH is absolutely required for ovarian follicle growth. In females, inactivating mutations in the FSHR result in ovarian dysgenesis with amenorrhea and infertility. The few males reported with severe inactivating mutations exhibited varying spermatogenic defects, but not azoospermia. While these findings may potentially suggest that FSH action is not absolutely required for spermatogenesis, it cannot be ruled out that these individuals have some residual FSHR activity. Gain-of-function mutations in the FSHR cause spontaneous ovarian hyperstimulation syndrome in females due to the inappropriate stimulation of the mutant FSHR by human choriogonadotropin.

Functional characterization and pharmacological rescue of melanocortin-4 receptor mutations identified from obese patients

As the most common monogenic form of human obesity, about 130 naturally occurring melanocortin-4 receptor (MC4R) gene mutations have been identified. In this study, we reported detailed functional characterization of 10 novel human MC4R (hMC4R) mutants including R7C, C84R, S127L, S136F, W174C, A219V, P230L, F261S, I317V and L325F. Flow cytometry experiments showed that six mutants, including R7C, C84R, S127L, W174C, P230L and F261S, have decreased cell surface expression. The other four mutants are expressed at similar levels as the wild-type hMC4R. Binding assays showed that the mutants have similar binding affinities for the agonist and endogenous antagonist agouti-related protein. Signalling assays showed that S136F is defective in signalling. Multiple mutagenesis showed that S136 of hMC4R is required for the normal function of the receptor. To identify potential therapeutic approaches for patients with intracellularly retained MC4R mutants, we tested the effect of an MC4R inverse agonist, ML00253764, on C84R and W174C. We showed that ML00253764 could function as a pharmacological chaperone rescuing the mutant MC4Rs to the cell surface. The rescued mutants are functional with increased cAMP production in response to agonist stimulation. In conclusion, of 10 mutants we studied, 6 had decreased cell surface expression. Pharmacological chaperone is a potential approach for treating obesity caused by MC4R mutations that result in intracellular retention.

A novel melanocortin-4 receptor gene mutation in a female patient with severe childhood obesity

This study targeted the identification of mutations of melanocortin-4 receptor gene (MC4R) in obese children. Fifty-one unrelated probands with early onset severe obesity (body mass index (BMI) > 99th percentile; 21 girls, mean age 10.6 +/- 3.6 years) were analyzed for nucleotide variations in the MC4R coding region, by the polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) method followed by direct DNA sequencing. MC4R variants were detected in three patients: the known I169S variant was found in heterozygote state in two patients and a novel heterozygous Y302F mutation was detected in one 12-year-old girl (BMI = 34 kg/m(2), BMI z-score 2.7) who has been overweight since the second year of life and suffered from hyperinsulinemia (at the age of 12: fasting insulin 45 mU/ml, after oral glucose load max. 300 mU/ml). The mutation also appears in the father, although both parents are obese (BMI father: 30.2 kg/m(2); mother: 31.9 kg/m(2)). This novel mutation is located in the functionally important NPXXY motif of the seventh transmembrane domain of the receptor. Functional characterization revealed reduction in cell surface expression and an alteration in signal transduction properties. These results add to the growing list of loss-of-function MC4R mutations in early onset obese patients and suggest an orexigenic effect of novel Y302F mutation.

Pertussis toxin-sensitive signaling of melanocortin-4 receptors in hypothalamic GT1-7 cells defines agouti-related protein as a biased agonist

Melanocortin-4 receptor (MC4R)-induced anorexigenic signaling in the hypothalamus controls body weight and energy homeostasis. So far, MC4R-induced signaling has been exclusively attributed to its coupling to G(s) proteins. In line with this monogamous G protein coupling profile, most MC4R mutants isolated from obese individuals showed a reduced ability to activate G(s). However, some mutants displayed enhanced G(s) coupling, suggesting that signaling pathways independent of G(s) may be involved in MC4R-mediated anorexigenic signaling. Here we report that the G(s) signaling-deficient MC4R-D90N mutant activates G proteins in a pertussis toxin-sensitive manner, indicating that this mutant is able to selectively interact with G(i/o) proteins. Analyzing a hypothalamic cell line (GT1-7 cells), we observed activation of pertussis toxin-sensitive G proteins by the wild-type MC4R as well, reflecting multiple coupling of the MC4R to G(s) and G(i/o) proteins in an endogenous cell system. Surprisingly, the agouti-related protein, which has been classified as a MC4R antagonist, selectively activates G(i/o) signaling in GT1-7 cells. Thus, the agouti-related protein antagonizes melanocortin-dependent G(s) activation not only by competitive antagonism but additionally by initiating G(i/o) protein-induced signaling as a biased agonist.

Lifestyle intervention in obese children with variations in the melanocortin 4 receptor gene

Because information on weight changes after lifestyle intervention in children with mutations in the melanocortin 4 receptor (MC4R) gene is scarce, we compared weight changes after lifestyle intervention between children with and without MC4R variations. A group of 514 overweight children (aged 5-16 years), who presented to participate in a 1-year lifestyle intervention based on exercise, behavior, and nutrition therapy were screened for MC4R mutations. For comparison, children with MC4R mutations leading to reduced receptor function (group A) were each of them randomly matched with five children of same age and gender without MC4R mutations (group B). Changes of weight status were analyzed as change of BMI standard deviation scores (BMI-SDSs). Furthermore, 16 children (3.1%) harbored MC4R mutations leading to reduced receptor function, and 17 (3.3%) children carried variations not leading to reduced receptor function. Children with and without MC4R mutations reduced their overweight at the end of intervention to a similar degree (P = 0.318 between groups based on an intention-to-treat analysis). The maintenance of weight loss after intervention among children with MC4R mutations leading to reduced receptor function failed in contrast to children without such mutations (P < 0.001 adjusted for BMI-SDS at baseline, age, and gender in an intention-to-treat analysis). In conclusion, children with MC4R mutations leading to reduced receptor function were able to lose weight in a lifestyle intervention but had much greater difficulties to maintain this weight loss supporting the impact of these mutations on weight status.

Polymorphisms in uncoupling protein, melanocortin 3 receptor, melanocortin 4 receptor, and pro-opiomelanocortin genes and association with production traits in a commercial broiler line

Because avian uncoupling protein (avUCP), melanocortin 3 receptor (MC3R), melanocortin 4 receptor (MC4R), and pro-opiomelanocortin (POMC) genes may be associated with production traits [e.g., BW, weight gain (WG), and feed conversion ratio (FCR)], male and female broilers from an elite broiler line were screened for polymorphisms in these genes. The PCR-restriction fragment length polymorphism (RFLP) tests were developed to type the missense polymorphisms UCPAla118Val, MC4RSer76Leu, MC3R-Met54Leu, and Gly104Ser and POMCPro61Leu. Of 39 single nucleotide polymorphisms identified in all 4 genes, 24/39 were transitions with 11 having a C to T change. Of the 23 polymorphisms in UCP, 17 represented at least 7 haplotypes in this pedigreed broiler line. The UCP Ala-118Val allele was associated with a) high feed efficiency (FE; P = 0.03) and WG (P = 0.053) in selected males, and b) high BW in selected females (P = 0.07) and unselected males (P = 0.015). The UCPVal118Val allele was found in approximately 10% of the birds that were screened. Five silent substitutions, 3 in MC3R and 2 in MC4R, were also identified. Thirteen polymorphisms were identified in the POMC gene representing at least 3 different alleles. A missense Pro61Leu heterozygote was associated with greater BW in females. The heterozygote MC3R Gly104Ser polymorphism was associated with greater FE in selected males (P = 0.03) and greater BW in unselected males (P = 0.007). The MC4R Ser76Leu heterozygote polymorphism was associated with greater BW than the Leu76 homozygote in females (P = 0.05). From these findings, we hypothesize that UCP, MC3R, MC4R and POMC genes may play important roles and could be candidate loci for production traits such as feed conversion and BW in commercial broiler breeding stock.

Constitutive activation of G protein-coupled receptors and diseases: insights into mechanisms of activation and therapeutics

The existence of constitutive activity for G protein-coupled receptors (GPCRs) was first described in 1980s. In 1991, the first naturally occurring constitutively active mutations in GPCRs that cause diseases were reported in rhodopsin. Since then, numerous constitutively active mutations that cause human diseases were reported in several additional receptors. More recently, loss of constitutive activity was postulated to also cause diseases. Animal models expressing some of these mutants confirmed the roles of these mutations in the pathogenesis of the diseases. Detailed functional studies of these naturally occurring mutations, combined with homology modeling using rhodopsin crystal structure as the template, lead to important insights into the mechanism of activation in the absence of crystal structure of GPCRs in active state. Search for inverse agonists on these receptors will be critical for correcting the diseases cause by activating mutations in GPCRs. Theoretically, these inverse agonists are better therapeutics than neutral antagonists in treating genetic diseases caused by constitutively activating mutations in GPCRs.

Intracellular retention of human melanocortin-4 receptor: a molecular mechanism underlying early-onset obesity in F261S pedigree of Chinese

OBJECTIVE

To investigate how F261S mutation identified from Chinese obese patients affects the function of melanocortin 4 receptor (MC4R) and to analyze the obesity-related phenotypes in subjects carrying the F261S mutation.

METHODS

F261S mutant of MC4R was generated by site-directed mutagenesis. Plasmids encoding wild-type or F261S mutant of MC4R were transfected into HEK293 and COS-7 cells to examine their functional characteristics. Signaling properties of F261S MC4R were assessed by measuring intracellular cAMP levels in response to alpha-MSH stimulation. Cell surface expression of F261S MC4R was compared with that of wild-type MC4R. Clinical examinations were performed in subjects carrying F261S mutation and in non-mutated controls.

RESULTS

The alpha-MSH-stimulated reporter gene activity was significantly reduced in cells expressing F261S MC4R, with a maximal response equal to 57% of wild-type MC4R. The F261S mutation also led to a significant change in the Es50 value compared with the wild-type receptor (P<0.01). Immunofluorescent assay revealed a marked reduction in plasma membrane localization of the MC4R in cells expressing the F261S mutant receptor. The resting metabolic rate and fat composition of the mutant carriers were not significantly different from those of the non-mutated obese controls.

CONCLUSIONS

The decreased response to alpha-MSH due to the intracellular retention of MC4R may cause early-onset obesity in the F261S pedigree of Chinese.

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.

Pharmacological analyses of two naturally occurring porcine melanocortin-4 receptor mutations in domestic pigs

The melanocortin-4 receptor (MC4R) is critical in regulating mammalian food intake and energy expenditure. Numerous mutations in the MC4R gene have been identified from obese humans. So far two naturally occurring porcine MC4R (pMC4R) mutations, D298N and R236H, have been identified from various strains of pigs and D298N is being utilized as a genetic marker to screen performance traits of pigs. In this study, we performed functional analyses of pMC4R D298N and R236H, including their ligand binding and signaling properties in transiently transfected HEK293T cells. Ligand binding assays showed that both D298N and R236H pMC4Rs had similar binding capacities and affinities for the natural agonist alpha-MSH and the natural antagonist Agouti-related protein as wild-type pMC4R. In signaling assays, both mutants had normal EC50 and maximal signaling to alpha-MSH. In summary, pMC4R mutants D298N and R236H do not have any overt functional defects; therefore we suggest caution using these mutations as selection markers in breeding programs.

Evolutionary aspects in evaluating mutations in the melanocortin 4 receptor

More than 70 missense mutations have been identified in the human melanocortin 4 receptor (MC4R), and many of them have been associated with obesity. In a number of cases, the causal link between mutations in MC4R and obesity is controversially discussed. Here, we mined evolution as an additional source of structural information that may help to evaluate the functional relevance of naturally occurring variations in MC4R. The sequence information of more than 60 MC4R orthologs enabled us to identify residues that are important for maintaining receptor function. More than 90% of all inactivating mutations found in obese patients were located at amino acid positions that are highly conserved during 450 million years of MC4R evolution in vertebrates. However, for a reasonable number of MC4R variants, we found no correlation between structural conservation of the mutated position and the reported functional consequence. By re-evaluating selected mutations in the MC4R, we demonstrate the usefulness of combining functional and evolutionary approaches.

G Protein-Coupled Receptor Trafficking in Health and Disease: Lessons Learned to Prepare for Therapeutic Mutant Rescue in Vivo

G protein-coupled receptors (GPCR) comprise the largest family of drug targets. This is not surprising as many signaling systems rely on this class of receptor to convert external and internal stimuli to intracellular responses. As is the case with other membrane proteins, GPCRs are subjected to a stringent quality control mechanism at the endoplasmic reticulum, which ensures that only correctly folded proteins enter the secretory pathway. Because of this quality control system, point mutations resulting in protein sequence variations may result in the production of misfolded and disease-causing proteins that are unable to reach their functional destinations in the cell. There is now a wealth of information demonstrating the functional rescue of misfolded mutant receptors by small nonpeptide molecules originally designed to serve as receptor antagonists; these small molecules ("pharmacoperones") serve as molecular templates, promoting correct folding and allowing the mutants to pass the scrutiny of the cellular quality control system and be expressed at the cell surface membrane. Two of these systems are especially well characterized: the gonadotropin-releasing hormone and the vasopressin type 2 receptors, which play important roles in regulating reproduction and water homeostasis, respectively. Mutations in these receptors can lead to well defined diseases that are recognized as being caused by receptor misfolding that may potentially be amenable to treatment with pharmacoperones. This review is focused on protein misfolding and misrouting related to various disease states, with special emphasis on these two receptors, which have proved to be of value for development of drugs potentially useful in regulating GPCR trafficking in healthy and disease states.

Molecular mechanism of the constitutive activation of the L250Q human melanocortin-4 receptor polymorphism

The Melanocortin-4 Receptor is a G-protein coupled receptor that has been physiologically linked to participate in the regulation of energy homeostasis. The Melanocortin-4 Receptor is stimulated by endogenous melanocortin agonists derived from the pro-opiomelanocortin gene transcript and antagonized by the endogenous antagonist agouti-related protein. Central administration of melanocortin agonists has been demonstrated to decrease food intake and conversely, treatment with antagonists resulted in increased food intake. Deletion of the Melanocortin-4 Receptor gene from the mouse genome results in an obese and hyperphagic phenotype. Polymorphisms of the human Melanocortin-4-Receptor have been found in severely obese individuals, suggesting that Melanocortin-4 Receptor malfunction might be involved in human obesity and obesity-associated diabetes. Herein, we have performed experiments to understand the molecular mechanisms associated with the L250Q human Melanocortin-4-Receptor polymorphism discovered in an extremely obese woman. This L250Q human Melanocortin-4-Receptor has been pharmacologically characterized to result in a constitutively active receptor. The fact that a constitutively active human Melanocortin-4-Receptor mutation was found in an obese person is a physiologic contradiction, as chronic activation of the human Melanocortin-4-Receptor and subsequently high cyclic adenosine monophosphate levels should theoretically result in a normal or lean phenotype. In this study, we demonstrated that agouti-related protein acts as an inverse agonist at this constitutively active receptor, and we propose a mechanism by which agouti-related protein might contribute to the obese phenotype in the L250Q patient. In addition, using receptor mutagenesis, pharmacology, and computer modeling approaches, we investigated the molecular mechanism by which modification of the L250 residue results in constitutive activation of the human Melanocortin-4-Receptor.

Point mutations in the melanocortin-4 receptor cause variable obesity in mice

Mutations in the melanocortin-4 receptor (MC4R) are associated with early-onset obesity in humans. Furthermore, a null Mc4r allele in mice leads to severe obesity due to hyperphagia and decreased energy expenditure. As part of independent N-ethyl- N-nitrosourea (ENU) mutagenesis screens, two obesity mutants, Fatboy and Southbeach, were isolated. Mapping revealed linkage to the melanocortin-4 receptor (Mc4r) and sequencing found single amino acid changes in Mc4r for each line. Expression of the mutant receptors in HEK 293 cells revealed defects in receptor signaling. The mutated Fatboy receptor (I194T) shows an increase in the effective concentration necessary for 50% of maximal signaling (EC(50)) when stimulated with alpha-MSH. Based on competitive binding, I194T is expressed on the cell surface at lower levels than the nonmutated receptor. In contrast, Southbeach (L300P) displays minimal receptor signaling when stimulated with the natural ligand alpha-MSH or the synthetic agonist NDP-alpha-MSH. Cell surface binding is absent, which usually indicates a lack of cell surface expression. However, antibody binding to Flag-tagged receptors by flow cytometry analysis and immunofluorescence demonstrates that L300P is translocated to the plasma membrane at a level comparable to the wild-type receptor. These results indicate a correlation with remaining receptor activity and the severity of the obesity in the mice homozygous for the mutations. Southbeach has less receptor activity and becomes more obese. These mutants will serve as good models for the variability in phenotype in humans carrying mutations in the MC4R gene.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Identification and functional characterization of three novel human melanocortin-4 receptor gene variants in an obese Chinese population

OBJECTIVE

Mutations in the melanocortin-4 receptor gene (MC4R) are the most common monogenic form of human obesity. However, the contribution of MC4R mutations to obesity in Chinese has not been investigated. We studied the frequency of MC4R mutations in an obese southern Chinese population and the functional consequences of the novel variants identified.

METHODS

We screened for MC4R mutations in 227 obese [body mass index (BMI) 35.29 +/- 5.75 kg/m2] and 100 lean (BMI 21.57 +/- 0.29 kg/m2) southern Chinese subjects using PCR-direct sequencing. In vitro functional studies, including cell surface expression, ligand binding, and cyclic adenosine monophosphate (cAMP) accumulation, were performed to examine the functional properties of three novel missense mutations.

RESULTS

Apart from two previously reported polymorphisms, V103I and -176 A > C, three novel missense heterozygous variants (Y35C, C40R and M218T) were identified. The polymorphisms -176 A > C and Y35C were detected in both obese and normal subjects with similar frequency. C40R was identified only in an obese subject. Pedigree analysis revealed M218T carriers in both lean and obese subjects. The prevalence of V103I carriers in normal-weight controls was significantly higher than that in obese subjects (5.3%vs. 1.3%, P < 0.05). In vitro functional studies showed that all three novel missense variants have normal functions.

CONCLUSIONS

Two known polymorphisms and three novel variants of the MC4R were identified. No overt functional defects were observed for the three novel MC4R variants, suggesting that they might not be the cause of obesity in variant carriers.

Cart overexpression is the only identifiable cause of high bone mass in melanocortin 4 receptor deficiency

The neural regulation of bone remodeling has proven to be increasingly complex at the molecular level because it involves both positive and negative mediators of bone formation and resorption. One of the mediators expressed in hypothalamic neurons that leptin uses to inhibit osteoclast differentiation and thereby bone resorption is cocaine- and amphetamine-regulated transcript (CART). CART expression in the hypothalamus is increased in mice lacking melanocortin 4 receptor (Mc4r(-/-) mice). Moreover, we show here that humans or mice lacking only one allele of Mc4r display a decrease in bone resorption parameters, high bone mass, and an increase in CART serum levels and/or hypothalamic expression. To demonstrate that the Cart overexpression is the only identifiable cause for the high bone mass observed upon Mc4r inactivation, we removed one allele of Cart from mice either heterozygous or homozygous for Mc4r inactivation. This manipulation sufficed to either significantly improve or normalize bone resorption parameters, without improving the energy metabolism disturbance that characterizes Mc4r-deficient mice. These results identify CART signaling as the main if not only molecular pathway accounting for the decrease in bone resorption leading to high bone mass in mice and humans deficient in Mc4r. As importantly, they also indicate that CART regulates bone resorption independently of the role it may exert in energy metabolism, suggesting that the neural control of appetite and bone remodeling are independent of each other.

A two-base deletion -439delGC in the melanocortin-4 receptor promoter associated with early-onset obesity

BACKGROUND/AIMS

Mutations in melanocortin-4 receptor (MC4R) are the most common genetic cause of human obesity. Mutations in MC4R promoter could also underlie obesity, but have so far not been reported. Transcription factor nescient helix-loop-helix 2 (Nhlh2) is a novel obesity candidate gene. We searched for mutations in MC4R promoter and Nhlh2 gene in 152 children with severe early-onset obesity. Lean subjects (n = 447) served as controls.

METHODS

MC4R promoter and Nhlh2 gene were investigated by sequencing. Gel shifts and reporter gene assays were used to investigate a deletion in MC4R promoter. Mutation carriers were carefully characterised. Weight charts from index patients and relatives were analysed.

RESULTS

We identified a deletion, -439delGC, in MC4R promoter in 2 severely obese, unrelated children and their family members, but not in controls. Index patients and mutation-carrying relatives were affected by early-onset obesity, while non-carriers had normal childhood weight development. The deletion is located at a potential Nhlh2-binding site and gel shift assays showed that Nhlh2 binds to this site. No significant differences in mutant compared to wild-type MC4R promoter activities were detected. No mutations were identified in Nhlh2 gene.

CONCLUSION

We report an MC4R promoter mutation, -439delGC, associated with early-onset obesity and show that transcription factor Nhlh2 recognises this site in vitro. Nhlh2 mutations unlikely underlie severe human obesity.

The human obesity gene map: the 2005 update

This paper presents the 12th update of the human obesity gene map, which incorporates published results up to the end of October 2005. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTL) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2005, 176 human obesity cases due to single-gene mutations in 11 different genes have been reported, 50 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 244 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 408. The number of human obesity QTLs derived from genome scans continues to grow, and we now have 253 QTLs for obesity-related phenotypes from 61 genome-wide scans. A total of 52 genomic regions harbor QTLs supported by two or more studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably, with 426 findings of positive associations with 127 candidate genes. A promising observation is that 22 genes are each supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. The electronic version of the map with links to useful publications and relevant sites can be found at http://obesitygene.pbrc.edu.

Obesity-associated mutations in the human melanocortin-4 receptor gene

Mutations in the human melanocortin-4 receptor (MC4R) gene have been associated with severe obesity. Many of the mutations result in partial or complete loss-of-function based on the nature of the mutation or the function of mutated receptors when tested in heterologous expression systems. This review discusses the role of MC4R in the central regulation of body weight, the pathogenic mechanisms of the mutations, and the validity of MC4R as an anti-obesity drug target.

Interactions of human melanocortin 4 receptor with nonpeptide and peptide agonists

Specific interactions of human melanocortin-4 receptor (hMC4R) with its nonpeptide and peptide agonists were studied using alanine-scanning mutagenesis. The binding affinities and potencies of two synthetic, small-molecule agonists (THIQ, MB243) were strongly affected by substitutions in transmembrane alpha-helices (TM) 2, 3, 6, and 7 (residues Glu(100), Asp(122), Asp(126), Phe(261), His(264), Leu(265), and Leu(288)). In addition, a I129A mutation primarily affected the binding and potency of THIQ, while F262A, W258A, Y268A mutations impaired interactions with MB243. By contrast, binding affinity and potency of the linear peptide agonist NDP-MSH were substantially reduced only in D126A and H264A mutants. Three-dimensional models of receptor-ligand complexes with their agonists were generated by distance-geometry using the experimental, homology-based, and other structural constraints, including interhelical H-bonds and two disulfide bridges (Cys(40)-Cys(279), Cys(271)-Cys(277)) of hMC4R. In the models, all pharmacophore elements of small-molecule agonists are spatially overlapped with the corresponding key residues (His(6), d-Phe(7), Arg(8), and Trp(9)) of the linear peptide: their charged amine groups interact with acidic residues from TM2 and TM3, similar to His(6) and Arg(6) of NDP-MSH; their substituted piperidines mimic Trp(9) of the peptide and interact with TM5 and TM6, while the d-Phe aromatic rings of all three agonists contact with Leu(133), Trp(258), and Phe(261) residues.

Gene polymorphisms and their effects in the melanocortin system

In addition to its role in human pigmentation, components of the melanocortin system regulate appetite, energy homeostasis and hormone production. Recent studies have suggested possible roles of this system in immunity, transmission of pain signals, and reproductive potential. A number of polymorphisms have been identified in genes of the melanocortin system and are associated with pigmentation in humans, as well as being causative of disorders of adrenal hormone production and obesity. This review gives an outline of these polymorphisms, their functional significance and possible application to or impact on diagnosis and pharmacotherapy based on melanocortin pathways.

Expression of the human melanocortin-2 receptor in different eukaryotic cells

The human melanocortin-2 receptor (hMC2R) is mainly present in the adrenal cortex and has been difficult to express in heterologous cells. The hMC2R fused to the EGFP at its C-terminus has been stably transfected in the murine M3 melanoma and HEK293 cells. In the M3 cells, the hMC2R-EGFP was well-addressed to the cell membrane and functional whereas in the HEK293 cells, the hMC2R-EGFP was retained intracellularly. These results suggest that some specific factors, missing in cells, which do not express any melanocortin receptor, are involved in the correct addressing of the hMC2R to the cell membrane.

Molecular mechanisms of the neural melanocortin receptor dysfunction in severe early onset obesity

The neural melanocortin receptors, melanocortin-3 and -4 receptors (MC3R and MC4R), have been shown to regulate different aspects of energy homeostasis in rodents. Human genetic studies showed that mutations in the MC4R gene are the most common monogenic form of obesity. Functional analyses of the mutant receptors revealed multiple defects. A classification scheme is presented for cataloguing the ever-increasing array of MC4R mutations. Functional analysis of the only inactivating MC3R mutation is also summarized. Insights from the analyses of the naturally occurring mutations in the MC3R and MC4R on the structure and function of these receptors are highlighted.

Altered cell surface expression of human MC1R variant receptor alleles associated with red hair and skin cancer risk

The human melanocortin-1 receptor gene (MC1R) encodes a G-protein coupled receptor that is primarily expressed on melanocytes, where it plays a key role in pigmentation regulation. Variant alleles are associated with red hair colour and fair skin, known as the RHC phenotype, as well as skin cancer risk. The R151C, R160W and D294H alleles, designated 'R', are strongly associated with the RHC phenotype and have been proposed to result in loss of function receptors due to impaired G-protein coupling. We recently provided evidence that the R151C and R160W variants can efficiently couple to G-proteins in response to alpha-melanocyte stimulating hormone. The possibility that altered cellular localization of the R151C and R160W variant receptors could underlie their association with RHC was therefore considered. Using immunofluorescence and ligand binding studies, we found that melanocytic cells exogenously or endogenously expressing MC1R show strong surface localization of the wild-type and D294H alleles but markedly reduced cell surface expression of the R151C and R160W receptors. In additional exogenous expression studies, the R variant D84E and the rare I155T variant, also demonstrated a significant reduction in plasma membrane receptor numbers. The V60L, V92M and R163Q weakly associated RHC alleles, designated 'r', were expressed with normal or intermediate cell surface receptor levels. These results indicate that reduced receptor coupling activity may not be the only contributing factor to the genetic association between the MC1R variants and the RHC phenotype, with MC1R polymorphisms now linked to a change in receptor localization.

Control of food intake through regulation of cAMP

The 3', 5'-cyclic adenosine monophosphate (cAMP) is a classic second messenger that is intimately involved in the regulation of food intake at the hypothalamus. cAMP can mediate the orexigenic and anorectic effects of various peripheral hormones or neuropeptides in a region-specific and neuron-specific manner. The importance of cAMP is particularly highlighted in a series of findings about cAMP transducing the anorectic signals of leptin and alpha-msh. This chapter provides an overview of several studies on how regulation of food intake takes place with cAMP as the second messenger in the hypothalamus.

Comparison of Gene Expression in Intra-Abdominal and Subcutaneous Fat: A Study of Men with Morbid Obesity and Nonobese Men Using Microarray and Proteomics

Extent of intra-abdominal fat had significant linear relations with six metabolic coronary risk factors: systolic and diastolic blood pressure, fasting blood concentrations of glucose, high density lipoprotein (HDL) cholesterol, triglyceride, and plasminogen activator inhibitor-1. Tumor necrosis factor-alpha and adiponectin can be biological mediators from the intra-abdominal fat to the metabolic coronary risk factors. Complementarily, we describe a new study that will analyze the gene expression in intra-abdominal and subcutaneous fat on mRNA and protein level using high throughput methods. The study will elucidate further whether intra-abdominal obesity is the common denominator for the different components of the metabolic syndrome.

Mutational analysis of the melanocortin-4 receptor (MC4R) gene in children with premature pubarche and adolescent girls with hyperandrogenism

To determine if variants in the melanocortin-4 receptor (MC4R) gene are associated with premature pubarche (PP) in children and hyperandrogenism (HA) in adolescent girls, we performed single-strand conformational polymorphism (SSCP) in 75 children (69 girls/six boys) with PP, 53 adolescent girls with HA, and 95 healthy adult control subjects. DNA sequence analysis of the conformers identified by SSCP revealed variants in six patients (two silent and one missense) and in none of the control subjects.

Melanocortin 4 receptor gene variation is associated with severe obesity in Pima Indians

Heterozygous coding mutations in the melanocortin 4 receptor gene (MC4R) are the most common genetic causes of severe human obesity identified to date. To determine whether MC4R has a role in causing severe obesity in Pima Indians, we sequenced the coding region of this gene in 426 full-heritage, non-first-degree related, adult Pima Indians (300 severely obese and 126 nondiabetic nonobese control subjects). Three coding variations were detected as heterozygotes only in severely obese subjects. One variation, detected in three obese subjects, was a novel single-base insertion (A) at nucleotide 100, and it predicted a frameshift and premature STOP at codon 37. The second variant, detected in 10 obese subjects, predicted a previously identified arginine-to-glutamine substitution at codon 165, and the third variant, detected in one obese subject, predicted a novel glycine-to-serine substitution at codon 231. Three polymorphisms were also identified in the 5' untranslated region, but these variants were detected in both obese and lean subjects and had similar allele frequencies. We conclude that variations in MC4R may account for a small portion of obesity in Pima Indians, but they do not explain the overall high prevalence of obesity in this Native American population.

Inactivation and intracellular retention of the human I183N mutated melanocortin 3 receptor associated with obesity

Melanocortins are known to be involved in the regulation of feeding behavior. These hormones mediate their effects through G protein-coupled receptors (GPCRs) by stimulating adenylate cyclase. The melanocortin 3 receptor (MC3R) in the melanocortin receptor (MCR) family has been identified as a neural receptor subtype mainly expressed in the brain in mammals. Until now, only one heterozygous mutation (I183N) has been identified in the coding region of this receptor in two obese patients of the same family. In this study, we reported the functional characterization of the I183N mutated MC3R compared with that of the wild-type MC3R after transfection in HEK293 cells. Our results showed that the I183N mutation totally abolished the activity of the mutated receptor to generate intracellular cAMP. Furthermore, confocal microscopy observation revealed that the mutation induced an intracellular retention of the mutated receptor. Moreover, we demonstrated for the first time by co-transfection studies that the mutated receptor could reduce the wild-type receptor activity through a dominant negative effect.

Functional characterization of melanocortin-3 receptor variants identify a loss-of-function mutation involving an amino acid critical for G protein-coupled receptor activation

Although melanocortin-4 receptor mutations are the cause of the most common monogenic form of obesity, the involvement of the melanocortin-3 receptor (MC3R) in the pathogenesis of obesity is unknown. Earlier studies failed to identify any mutations in obese patients except for the identification of two variants (K6T and I81V) that likely represent polymorphisms. However, a potential mutation (I183N) was recently reported from patients having high-fat contents. We report here the functional characterization of these variants. We show that K6T and I81V have ligand binding and signaling properties similar to wild-type (wt) MC3R, indicating that they are indeed polymorphisms. However, the other variant, I183N, completely lacks signaling in response to agonist stimulation, although it binds ligand with normal affinity and with only slightly decreased capacity. Coexpression of the wt and I183N MC3Rs showed that I183N does not exert dominant-negative activity on wt MC3R. These results provide supporting evidence for the hypothesis proposed in the original case report that MC3R mutation might be a genetic factor that confers susceptibility to obesity, likely due to haploinsufficiency. Further mutations at I183 revealed a discrete requirement for I183 in agonist-induced MC3R activation. The corresponding residue is also important for agonist-induced human melanocortin-4 receptor and lutropin receptor activation. In summary, we identify a residue that is critical for activation of G protein-coupled receptors.

Deletion of codons 88-92 of the melanocortin-4 receptor gene: a novel deleterious mutation in an obese female

Genetic and pharmacological studies have shown that the melanocortin-4 receptor (MC4R) is an important regulator of food intake and energy homeostasis. Consistent with these studies, several mutations of the MC4R gene have been identified as being associated with early-onset severe obesity. We report here the first in-frame deletion mutation of the MC4R gene (delta88-92) in an obese female patient with onset of obesity at less than 5 yr of age. Functional analysis revealed that the mutant receptor is expressed well on the cell surface but completely devoid of ligand binding and cAMP generation in response to agonist stimulation. We conclude that this novel mutation is the cause of obesity of this patient.