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

Melanocortin-5 Receptor: Pharmacology and Its Regulation of Energy Metabolism

As the most recent melanocortin receptor (MCR) identified, melanocortin-5 receptor (MC5R) has unique tissue expression patterns, pharmacological properties, and physiological functions. Different from the other four MCR subtypes, MC5R is widely distributed in both the central nervous system and peripheral tissues and is associated with multiple functions. MC5R in sebaceous and preputial glands regulates lipid production and sexual behavior, respectively. MC5R expressed in immune cells is involved in immunomodulation. Among the five MCRs, MC5R is the predominant subtype expressed in skeletal muscle and white adipose tissue, tissues critical for energy metabolism. Activated MC5R triggers lipid mobilization in adipocytes and glucose uptake in skeletal muscle. Therefore, MC5R is a potential target for treating patients with obesity and diabetes mellitus. Melanocortin-2 receptor accessory proteins can modulate the cell surface expression, dimerization, and pharmacology of MC5R. This minireview summarizes the molecular and pharmacological properties of MC5R and highlights the progress made on MC5R in energy metabolism. We poInt. out knowledge gaps that need to be explored in the future.

Mutations in melanocortin-4 receptor: From fish to men

Melanocortin-4 receptor (MC4R), expressed abundantly in the hypothalamus, is a critical regulator of energy homeostasis, including both food intake and energy expenditure. Shortly after the publication in 1997 of the Mc4r knockout phenotypes in mice, including increased food intake and severe obesity, the first mutations in MC4R were reported in humans in 1998. Studies in the subsequent two decades have established MC4R mutation as the most common monogenic form of obesity, especially in early-onset severe obesity. Studies in animals, from fish to mammals, have established the conserved physiological roles of MC4R in all vertebrates in regulating energy balance. Drug targeting MC4R has been recently approved for treating morbid genetic obesity. How the MC4R can be exploited for animal production is highly worthy of active investigation.

Molecular chaperones and G protein-coupled receptor maturation and pharmacology

G protein-coupled receptors (GPCRs) are highly conserved versatile signaling molecules located at the plasma membrane that respond to diverse extracellular signals. They regulate almost all physiological processes in the vertebrates. About 35% of current drugs target these receptors. Mutations in these genes have been identified as causes of numerous diseases. The seven transmembrane domain structure of GPCRs implies that the folding of these transmembrane proteins is extremely complicated and difficult. Indeed, many wild type GPCRs are not folded optimally. The most common defect in genetic diseases caused by GPCR mutations is misfolding and failure to reach the plasma membrane where it functions. General molecular chaperones aid the folding of all proteins, including GPCRs, by preventing aggregation, promoting folding and disaggregating small aggregates. Some GPCRs need additional receptor-specific chaperones to assist their folding. Many of these receptor-specific chaperones interact with additional receptors and alter receptor pharmacology, expanding the understanding of these chaperone proteins.

Pharmacoperones as Novel Therapeutics for Diverse Protein Conformational Diseases

After synthesis, proteins are folded into their native conformations aided by molecular chaperones. Dysfunction in folding caused by genetic mutations in numerous genes causes protein conformational diseases. Membrane proteins are more prone to misfolding due to their more intricate folding than soluble proteins. Misfolded proteins are detected by the cellular quality control systems, especially in the endoplasmic reticulum, and proteins may be retained there for eventual degradation by the ubiquitin-proteasome system or through autophagy. Some misfolded proteins aggregate, leading to pathologies in numerous neurological diseases. In vitro, modulating mutant protein folding by altering molecular chaperone expression can ameliorate some misfolding. Some small molecules known as chemical chaperones also correct mutant protein misfolding in vitro and in vivo. However, due to their lack of specificity, their potential as therapeutics is limited. Another class of compounds, known as pharmacological chaperones (pharmacoperones), binds with high specificity to misfolded proteins, either as enzyme substrates or receptor ligands, leading to decreased folding energy barriers and correction of the misfolding. Because many of the misfolded proteins are misrouted but do not have defects in function per se, pharmacoperones have promising potential in advancing to the clinic as therapeutics, since correcting routing may ameliorate the underlying mechanism of disease. This review will comprehensively summarize this exciting area of research, surveying the literature from in vitro studies in cell lines to transgenic animal models and clinical trials in several protein misfolding diseases.

Pharmacologic analyses of four chicken melanocortin-4 receptor mutations

The melanocortin-4 receptor (MC4R) is a critical regulator of mammalian food intake and energy expenditure, with receptor activation resulting in decreased food intake and increased energy expenditure. Recently, studies on role of MC4R in regulation of food intake have been extended to other species, such as chicken. Functional study of mutant MC4Rs is important in proving the causal link between MC4R mutation and production traits. Herein, we cloned chicken MC4R (cMC4R) complementary DNA and generated 4 mutant cMC4Rs (Q18H, G21R, S76L, and L299P) by site-directed mutagenesis and measured their expression by flow cytometry. Pharmacologic characteristics were analyzed with binding and signaling assays using 3 agonists. We showed that G21R had decreased cell surface and total expression (P < 0.05), whereas the other 3 mutants had similar total and cell surface expression levels as wild-type cMC4R. The 4 mutants had either decreased (Q18H, G21R, S76L; P < 0.05) or no (L299P) binding to radiolabeled [Nle(4), D-Phe(7)]-α-melanocyte-stimulating hormone (MSH). In signaling assays, Q18H was constitutively active. Q18H, G21R, and S76L had decreased responses to α-MSH stimulation (P < 0.05). L299P had decreased basal and ligand-stimulated signaling (P < 0.01). Nle(4), D-Phe(7)-MSH was the most potent agonist for cMC4R and therefore would be better suited for further in vivo studies. We conclude that the cloned cMC4R was a functional receptor and provided detailed functional data for these mutations, contributing to a better understanding of cMC4R variants associated with production traits.

Defect in MAPK signaling as a cause for monogenic obesity caused by inactivating mutations in the melanocortin-4 receptor gene

The melanocortin-4 receptor (MC4R) is a Family A G protein-coupled receptor that plays an essential role in regulating energy homeostasis, including both energy intake and expenditure. Mutations leading to a reduced MC4R function confer a major gene effect for obesity. More than 170 distinct mutations have been identified in humans. In addition to the conventional Gs-stimulated cAMP pathway, the MC4R also activates MAPKs, especially ERK1/2. We also showed there is biased signaling in the two signaling pathways, with inverse agonists in the Gs-cAMP pathway acting as agonists for the ERK1/2 pathway. In the current study, we sought to determine whether defects in basal or agonist-induced ERK1/2 activation in MC4R mutants might potentially contribute to obesity pathogenesis in patients carrying these mutations. The constitutive and ligand-stimulated ERK1/2 activation were measured in wild type and 73 naturally occurring MC4R mutations. We showed that nineteen mutants had significantly decreased basal pERK1/2 level, and five Class V variants (where no functional defects have been identified previously), C40R, V50M, T112M, A154D and S295P, had impaired ligand-stimulated ERK1/2 activation. Our studies demonstrated for the first time that decreased basal or ligand-stimulated ERK1/2 signaling might contribute to obesity pathogenesis caused by mutations in the MC4R gene. We also observed biased signaling in 25 naturally occurring mutations in the Gs-cAMP and ERK1/2 pathways.

A small molecule agonist THIQ as a novel pharmacoperone for intracellularly retained melanocortin-4 receptor mutants

Although mutations in the melanocortin-4 receptor (MC4R) gene cause severe early-onset obesity, we still do not have effective approaches to correct the defects of these mutations. Several antagonists have been identified as pharmacoperones of the MC4R whereas no agonist of the MC4R has been reported. In the present study, we investigated the effect of a small molecule agonist of the MC4R, THIQ, on the cell surface expression and signaling of ten intracellularly retained MC4R mutants using different cell lines. We showed that THIQ increased the cell surface expression of three mutants (N62S, C84R, and C271Y) and two of them (N62S and C84R) had increased signaling in HEK293 cells. Interestingly, THIQ increased the signaling of two other mutants (P78L and P260Q) without increasing their cell surface expression in HEK293 cells. In neuronal cells, THIQ exhibited a more potent effect, correcting the cell surface expression and signaling of seven mutants (N62S, I69R, P78L, C84R, W174C, P260Q, and C271Y). Other mutants were not rescued by THIQ. We also showed that THIQ did not rescue MC4R mutants defective in ligand binding or signaling or one intracellularly retained mutant of the melanocortin-3 receptor. In summary, we demonstrated that a small molecule agonist acted as a pharmacoperone of the MC4R rescuing the cell surface expression and signaling of some intracellularly retained MC4R mutants.

Ipsen 5i is a Novel Potent Pharmacoperone for Intracellularly Retained Melanocortin-4 Receptor Mutants

Inactivating mutations of the melanocortin-4 receptor (MC4R) cause early-onset severe obesity in humans. Comprehensive functional studies show that most of the inactivating mutants of the MC4R are retained intracellularly. In the present study, we investigated whether a small molecule inverse agonist of the MC4R, Ipsen 5i, could act as a pharmacoperone and correct the cell surface expression and function of intracellularly retained mutant MC4Rs using multiple cell lines, including HEK293 and two neuronal cell lines. We showed that Ipsen 5i rescued the cell surface expression of all 11 intracellularly retained mutant MC4Rs studied herein in at least one cell line. Ipsen 5i functionally rescued seven mutants in all cell lines used. One mutant (Y157S) was functionally rescued in HEK293 cells but not in the two neuronal cell lines. Ipsen 5i increased cell surface expression of three mutants (S58C, G98R, and F261S) but did not affect signaling. Ipsen 5i had no effect on mutant MC4Rs with other defects (Δ88-92, D90N, I102S) or no defect (N274S). It also did not affect trafficking of a misrouted MC3R mutant (I335S). Cell impermeable peptide ligands of the MC4R or cell permeable small molecule ligand of δ opioid receptor could not rescue misrouted mutant MC4R. In summary, we demonstrated that Ipsen 5i was a novel potent pharmacoperone of the MC4R, correcting trafficking and signaling of a significant portion (73%) of intracellularly retained mutants. Additional studies are needed to demonstrate its in vivo efficacy.

The Human Obesity Gene Map: The 2004 Update

This paper presents the eleventh update of the human obesity gene map, which incorporates published results up to the end of October 2004. 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 (QTLs) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2004, 173 human obesity cases due to single-gene mutations in 10 different genes have been reported, and 49 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 166 genes which, 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 221. The number of human obesity QTLs derived from genome scans continues to grow, and we have now 204 QTLs for obesity-related phenotypes from 50 genome-wide scans. A total of 38 genomic regions harbor QTLs replicated among two to four studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably with 358 findings of positive associations with 113 candidate genes. Among them, 18 genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, >600 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful publications and genomic and other relevant sites can be found at http://obesitygene.pbrc.edu.

Functional studies on twenty novel naturally occurring melanocortin-4 receptor mutations

The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor critically involved in regulating energy balance. MC4R activation results in decreased food intake and increased energy expenditure. Genetic and pharmacological studies demonstrated that the MC4R regulation of energy balance is conserved from fish to mammals. In humans, more than 150 naturally occurring mutations in the MC4R gene have been identified. Functional study of mutant MC4Rs is an important component in proving the causal link between MC4R mutation and obesity as well as the basis of personalized medicine. In this article, we studied 20 MC4R mutations that were either not characterized or not fully characterized. We showed that 11 mutants had decreased or absent cell surface expression. D126Y was defective in ligand binding. Three mutants were constitutively active but had decreased cell surface expression. Eleven mutants had decreased basal signaling, with two mutants defective only in this parameter, suggesting that impaired basal signaling might also be a cause of obesity. Five mutants had normal functions. In summary, we provided detailed functional data for further studies on identifying therapeutic approaches for personalized medicine to treat patients harboring these mutations.

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.

Association of the MC4R V103I polymorphism with obesity: a Chinese case-control study and meta-analysis in 55,195 individuals

Recently, large-scaled studies suggested a negative association of the infrequent allele of the melanocortin-4 receptor (MC4R) V103I polymorphism with obesity. We conducted a Chinese case-control study, meta-analysis in East Asians and in all populations, in order to further assess the association between the V103I polymorphism and reduced body weight and to explore whether the association varies among different ethnic groups. We conducted a case-control study to analyze this polymorphism in 2,012 children of two independent study groups from Beijing, China, no association was found between the V103I polymorphism and obesity or obesity-related phenotypes (P > 0.10). In the meta-analysis of 3,526 individuals from six East Asian studies, I103 carriers had a 31% lower risk for obesity (odds ratio (OR) = 0.69, 95% confidence interval (CI): 0.50-0.94, P = 0.02). Subsequently, we performed a large meta-analysis of the six East Asian studies and 31 studies of other ethnic groups, involving 55,195 individuals with 19,822 obese cases and 35,373 nonobese controls. In total, the individuals with I103 allele had a 21% lower risk for obesity (OR = 0.79, 95% CI: 0.71-0.88, P < 0.0001). In conclusion, this study confirmed and extended the previous findings, suggesting the MC4R V103I polymorphism is negatively associated with human obesity. It provides significant evidence for the association in East Asian populations. Further large-scaled studies in East Asian populations are needed to validate the association.

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.

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.

Screening for melanocortin-4 receptor mutations in a cohort of Belgian morbidly obese adults and children

OBJECTIVE

To investigate whether pathogenic melanocortin-4 receptor (MC4R) mutations are a common cause of obesity in Belgium.

DESIGN

Cross-sectional mutation analysis.

SUBJECTS

In total, 95 morbidly obese adults (mean age 44.02+/-11.35 years; mean BMI 47.87+/-4.17 kg/m2 and 123 obese children and adolescents were screened for mutations in MC4R (mean age 16.56+/-2.58 years; BMI>95th percentile for age and sex; mean % overweight 170.86 +/- 23.63).

MEASUREMENTS

A series of anthropometric (e.g. weight, height, waist, hip), biochemical and clinical measurements were performed on all subjects. The entire coding region of MC4R was screened using DHPLC, a highly sensitive and specific method for mutation analysis. Direct sequencing was performed when the chromatogram deviated from the WT pattern.

RESULTS

Mutation screening of a cohort of Belgian obese adults and children did not detect any pathogenic mutations as only the previously described polymorphisms Val103Ile, Thr112Met and Ile251Leu were detected.

CONCLUSION

Pathogenic mutations in MC4R are not a common cause of obesity in a Belgian population of obese adults, children and adolescents.

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.

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.

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.

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.

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.

Quantitative trait locus determining dietary macronutrient intakes is located on human chromosome 2p22

BACKGROUND

Obesity is generally accompanied by increased food intake.

OBJECTIVE

We sought to identify the genes influencing variation in dietary macronutrient intakes in Mexican Americans.

DESIGN

We conducted a genome-wide scan by using data derived from food-frequency questionnaires in 816 participants from the San Antonio Family Heart Study. Household effect was simultaneously estimated in a variance component model with the use of SOLAR.

RESULTS

All dietary intake measures (total calories, proteins, fat, saturated fat, monounsaturated fat, polyunsaturated fat, carbohydrates, and sucrose) were moderately heritable. Household effect was insignificant except on total calories and sucrose. Suggestive evidence of linkage with saturated fat intake was found on chromosome 2p22 near marker D2S1346 [logarithm of odds (LOD) = 2.62]. Intakes of total calories, fat, protein, and monounsaturated fat were also suggestively linked to the same marker. A significant linkage signal on chromosome 2p22 for leptin concentrations and fat mass was localized in this population, so we used leptin or fat mass as a covariate. Multipoint LOD scores for saturated fat dropped to 1.27 and 1.90, respectively, which suggested that this region on chromosome 2p contributes to both saturated fat intake and body adiposity. This chromosomal region contains the proopiomelanocortin gene (POMC). However, 2 polymorphisms in exon 3 of the POMC gene showed no association with saturated fat intake.

CONCLUSIONS

The results strengthen the hypothesis that chromosome 2p22 harbors genes that influence a variety of obesity-related phenotypes, including macronutrient intakes.

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.

Neuropeptides in hypothalamic neuronal disorders

A few examples of hypothalamic, peptidergic disorders leading to clinical signs and symptoms are presented in this review. Increased activity of corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus (PVN) and decreased activity of the vasopressin neurons in the biological clock and of the thyroxine-releasing hormone (TRH) neurons in the PVN contribute to the signs and symptoms of depression. In men, the central nucleus of the bed nucleus of the stria terminalis (BSTc) is about twice as large and contains twice as many somatostatin neurons as in women. In transsexuals this sex difference is reversed, pointing to a role of this structure in gender. Luteinizing hormone-releasing hormone (LHRH) neurons are formed in the fetal olfactory placade and migrate along the terminal nerve fibers into the hypothalamus. In Kallmann's syndrome the migration process of the LHRH (gonadotropin-releasing hormone) neurons is aborted, which explains the joint occurrence of hypogonadotropic hypogonadism and anosmia in this syndrome. In postmenopausal women, the neurons of the infundibular nucleus hypertrophy and become hyperactive because of the disappearance of the estrogen feedback and contain hyperactive peptidergic neurons. Climacteric flushes may be caused by hyperactivity of the neurokinin-B or LHRH neurons in this nucleus. The hypocretin (orexin) neurons in the perifornical area are involved in sleep. In narcolepsy with cataplexy, a loss of these neurons, probably due to an autoimmune process, is found. Obese subjects with a mutation in the gene that encodes for leptin, the preproghrelin gene, or the alpha-melanocyte-stimulating hormone (alpha-MSH) gene have been described. Decreased numbers and activity of the oxytocin neurons in the PVN may be responsible for the absence of satiety in Prader-Willi syndrome. Moreover, a glucocorticoid receptor polymorphism is associated with obesitas and dysregulation of the hypothalamus-pituitary-adrenal axis. In contrast, two single nucleotide polymorphisms (SNPs) of the AGRP gene have been associated with anorexia nervosa.