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

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.

Targeting melanocortin receptors: an approach to treat weight disorders and sexual dysfunction

The melanocortin system has multifaceted roles in the control of body weight homeostasis, sexual behaviour and autonomic functions, and so targeting this pathway has immense promise for drug discovery across multiple therapeutic areas. In this Review, we first outline the physiological roles of the melanocortin system, then discuss the potential of targeting melanocortin receptors by using MC3 and MC4 agonists for treating weight disorders and sexual dysfunction, and MC4 antagonists to treat anorectic and cachectic conditions. Given the complexity of the melanocortin system, we also highlight the challenges and opportunities for future drug discovery in this area.

Sporadic mutations in melanocortin receptor 3 in morbid obese individuals

Several mutations in the melanocortin receptor 4 gene have been identified in humans and account for 3-6% of morbid obesity. In contrast, strong evidence of a causative role for melanocortin receptor 3 (MC3R) mutations are still lacking. In MC3R knockout mice, high feed efficiency rather than hyperphagia seems to contribute to increased fat mass. On the basis of this evidence, the objective of the present study was to investigate the presence of MC3R mutations in a group of 290 obese subjects (mean BMI 44.2+/-5.9 kg/m2). As a control, a group of 215 normal-weight subjects (mean BMI 22.4+/-2.7 kg/m2) was also screened. Three novel mutations in the MC3R gene (A293T, I335S and X361S) were identified among the obese patients. The mutations segregated with obesity in the members of the families studied. In vitro expression studies of each mutation demonstrated a loss of function of the I335S-mutated receptor. These findings suggest that, in humans, MC3R mutations may be a cause of a dominantly inherited form of obesity. However, this association as well as the specific phenotypic characteristics resulting from these mutations need to be further evaluated in larger series of obese subjects.

The role of melanocortin 3 receptor gene in childhood obesity

OBJECTIVE

Melanocortin 3 receptor (MC3R) plays a critical role in weight regulation of rodents, but its role in humans remains unclear. The objective of this study was to identify genetic variants of the MC3R gene and determine its association with childhood obesity.

RESEARCH DESIGN AND METHODS

We screened 201 obese children for MC3R gene mutations with anthropometric measurements, blood tests, feeding behavior, and body composition assessment. We identified three novel heterozygous mutations (Ile183Asn, Ala70Thr, and Met134Ile) in three unrelated subjects, which were not found in 188 control subjects, and two common polymorphisms Thr6Lys and Val81Ile.

RESULTS

In vitro functional studies of the resultant mutant receptors revealed impaired signaling activity but normal ligand binding and cell surface expression. The heterozygotes demonstrated higher leptin levels and adiposity and less hunger compared with obese control subjects, reminiscent of the MC3R knockout mice. Family studies showed that these mutations may be associated with childhood or early-onset obesity. The common variants Thr6Lys and Val81Ile were in complete linkage disequilibrium, and in vitro studies revealed reduced signaling activity compared with wild-type MC3R. Obese subjects with the 6Lys/81Ile haplotype had significantly higher leptin levels, percentage body fat, and insulin sensitivity, and the causative role of the 6Lys/81Ile variants is supported by the presence of an additive effect in which heterozygotes had an intermediate phenotype compared with homozygotes.

CONCLUSIONS

MC3R mutations may not result in autosomal dominant forms of obesity but may contribute as a predisposing factor to childhood obesity and exert an effect on the human phenotype. Our report supports the role of MC3R in human weight regulation.

Design, synthesis and biological evaluation of ligands selective for the melanocortin-3 receptor

The processed products of the proopiomelanocortin gene (ACTH, alpha-MSH, beta-MSH, gamma-MSH, etc.) interact with five melanocortin receptors, the MC1R, MC2R, MC3R, MC4R, and MC5R to modulate and control many important biological functions crucial for good health both peripherally (as hormones) and centrally (as neurotransmitters). Pivotal biological functions include pigmentation, adrenal function, response to stress, fear/flight, energy homeostasis, feeding behavior, sexual function and motivation, pain, immune response, and many others, and are believed to be involved in many disease states including pigmentary disorders, adrenal disorders, obesity, anorexia, prolonged and neuropathic pain, inflammatory response, etc. The melanocortin-3 receptor (MC3R) is found primarily in the brain and spinal cord and also in the periphery, and its biological functions are still not well understood. Here we review some of the biological functions attributed to the MC3R, and then examine in more detail efforts to design and synthesize ligands that are potent and selective for the MC3R, which might help resolve the many questions still remaining about its function. Though some progress has been made, there is still much to be done in this critical area.

Genetics of human obesity

This chapter presents the current state of knowledge in the field of the genetics of human obesity. The molecular approach has proved to be powerful in defining new syndromes associated with obesity. The pivotal role of leptin and melanocortin pathways has been recognized, but only in rare cases of obesity. In the more common form of obesity a multitude of polymorphisms located in genes and candidate regions throughout the genome regulate an individual's susceptibility to weight gain in a permissive environment. The effects are often uncertain and the results not always confirmed. Combining these single nucleotide polymorphisms and defining the associated risks for obesity will be a real challenge in the future. It is now necessary to integrate data of various origins (environment, genotype, expression) to clarify this field.

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.

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.

Co-occurrence of two partially inactivating polymorphisms of MC3R is associated with pediatric-onset obesity

Both human linkage studies and MC3R knockout mouse models suggest that the MC3R may play an important role in energy homeostasis. Here we show that among 355 overweight and nonoverweight children, 8.2% were double homozygous for a pair of missense MC3R sequence variants (Thr6Lys and Val81Ile). Such children were significantly heavier (BMI and BMI SD score: P < 0.0001), had more body fat (body fat mass and percentage fat mass: P < 0.001), and had greater plasma leptin (P < 0.0001) and insulin concentrations (P < 0.001) and greater insulin resistance (P < 0.008) than wild-type or heterozygous children. Both sequence variants were more common in African-American than Caucasian children. In vitro expression studies found the double mutant MC3R was partially inactive, with significantly fewer receptor binding sites, decreased signal transduction, and less protein expression. We conclude that diminished MC3R expression in this double MC3R variant may be a predisposing factor for excessive body weight gain in children.

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.