Clinical spectrum of obesity and mutations in the melanocortin 4 receptor gene

Proopiomelanocortin and energy balance: insights from human and murine genetics

Proopiomelanocortin (POMC) undergoes extensive and tissue-specific posttranslational processing to yield a range of biologically active peptides. Historically, the most clearly defined roles of these peptides are in the control of adrenal steroidogenesis by corticotroph-derived ACTH and skin pigmentation by alphaMSH. However, a rapidly expanding body of work has established that POMC-derived peptides synthesized in neurons of the hypothalamus play a central role in the control of energy homeostasis. We review how inherited abnormalities in POMC synthesis and processing and defects in the action of POMC-derived peptides in both humans and mice have helped shape our current understanding of the importance of the melanocortin system in human energy balance.

Insulin resistance syndrome in children

The insulin resistance syndrome (syndrome X, metabolic syndrome) has become the major health problem of our times. Associated obesity, dyslipidemia, atherosclerosis, hypertension, and type 2 diabetes conspire to shorten life spans, while hyperandrogenism with polycystic ovarian syndrome affect the quality of life and fertility of increasing numbers of women. Whereas a growing number of single genetic diseases affecting satiety or energy metabolism have been found to produce the clinical phenotype, strong familial occurrences, especially in racially prone groups such as those from the Indian subcontinent, or individuals of African, Hispanic, and American Indian descents, together with emerging genetic findings, are revealing the polygenetic nature of the syndrome. However, the strong lifestyle factors of excessive carbohydrate and fat consumption and lack of exercise are important keys to the phenotypic expression of the syndrome. The natural history includes small for gestational age birth weight, excessive weight gains during childhood, premature pubarche, an allergic diathesis, acanthosis nigricans, striae compounded by gynecomastia, hypertriglyceridemia, hepatic steatosis, premature atherosclerosis, hypertension, polycystic ovarian syndrome, and focal glomerulonephritis appearing increasingly through adolescence into adulthood. Type 2 diabetes, which develops because of an inherent and/or an acquired failure of an insulin compensatory response, is increasingly seen from early puberty onward, as is atheromatous disease leading to coronary heart disease and stroke. A predisposition to certain cancers and Alzheimer's disease is also now recognized. The looming tragedy from growing numbers of individuals affected by obesity/insulin resistance syndrome requires urgent public health approaches directed at their early identification and intervention during childhood. Such measures include educating the public on the topic, limiting the consumption of sucrose-containing drinks and foods with high carbohydrate and fat contents, and promoting exercise programs in our nation's homes and schools.

Hypothalamic gene expression is altered in underweight but obese juvenile male Sprague-Dawley rats fed a high-energy diet

The incidence of obesity, with its associated health risks, is on the increase throughout the western world affecting all age groups, including children. The typical western diet is high in fat and sugar and low in complex carbohydrates. This study looks at the effects of feeding an equivalent high-energy (HE) diet to growing rats. Juvenile male Sprague-Dawley rats that were fed an HE (18.9 kJ/g) diet starting approximately 10 d after weaning gained less weight than littermates fed a nonpurified (14 kJ/g) diet. Despite an initial hyperphagia following the change in diet, HE rats also consumed less energy. Although they exhibited reduced weight gain, HE rats were relatively obese; fat pad weights were elevated for all 4 dissected depots. HE-fed rats exhibited symptoms of developing metabolic syndrome with elevated plasma concentrations of glucose, triglycerides, nonesterified fatty acids, insulin, and leptin. In addition, leptin receptor gene expression in the hypothalamic arcuate nucleus (ARC) and ventromedial nucleus of HE rats was reduced. Consistent with the elevated serum leptin and other peripheral signals in HE rats, hypothalamic gene expression for the orexigenic neuropeptides, neuropeptide Y (ARC and dorsomedial nucleus), and agouti-related peptide (AgRP), was reduced. This reduction in orexigenic signaling and decline in energy intake is consistent with an apparent attempt to counter the further development of an obese state in rats consuming an energy-dense diet. The juvenile Sprague-Dawley rat has potential in the development of a model of childhood diet-induced obesity.

Mutational analysis of the serotonin receptor 5HT2c in severe early-onset human obesity

Deletion of the serotonin receptor 5HT2c in mice results in increased food intake and obesity. We screened 95 individuals with severe early-onset obesity for mutations in the coding sequence of this gene. We found a novel missense variant c.1255A > G (Thr419Ala) in a single Caucasian subject that was not found in 192 Caucasian control subjects. In transiently-transfected COS cells, the Thr419Ala variant was indistinguishable from the wild-type receptor in its ability to generate inositol phosphate, although differences in coupling to other pathways were not excluded. Three previously unreported silent variants: IVS3 + 30G > A, IVS3 + 80C > G and IVS4 – 31A > G were found with prevalences of 11.5%, 0.5% and 17.9%, respectively. In conclusion, mutations in 5HT2c are unlikely to be a common cause of severe early-onset human obesity. The identification of several novel polymorphisms at this locus may aid future genetic epidemiological studies.Key words: G-protein coupled receptor, hyperphagia, obesity, serotonin, X-linked.

The intricate interface between immune system and metabolism

Increasing experimental evidence indicates that several factors that influence metabolism also play a role in the regulation of immune responses. Dissection of the interface connecting the metabolic and immune systems has recently gained wide interest. Particular focus has been on certain cytokines [interleukin-1 (IL-1), IL-6, tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma)], hormones (leptin and insulin), neuropeptides (corticotropin-releasing hormone and alpha-melanocyte-stimulating hormone), immune-related proteins (zinc-alpha2-glycoprotein and attractin and/or mahogany), transcription factors (peroxisome-proliferator-activated receptors) and glucose metabolism. A better knowledge of the intricate network of interactions among energy regulation, immune surveillance and vital organ functions could in the near future lead to valuable strategies for therapeutic intervention in several immune-mediated diseases.

Evidence for a gene on chromosome 13 influencing postural systolic blood pressure change and body mass index

Previous analysis in the Hypertension Genetic Epidemiology Network (HyperGEN) of the National Heart Lung and Blood Institute (NHLBI) Family Blood Pressure Program, a multicenter study of genetic and environmental factors related to hypertension, indicated regions of linkage for blood pressure traits together with several coincident regions for phenotypically correlated traits, including systolic blood pressure (SBP) response to a postural challenge and body mass index (BMI). Motivated by these findings and by our desire to better understand the physiology of these traits, we conducted bivariate linkage analysis of postural SBP change and BMI. Sibships in HyperGEN were recruited from 5 field centers in Massachusetts, North Carolina, Minnesota, Utah, and Alabama. All available affected siblings, their parents, and selected nonmedicated offspring were recruited. Among 1636 whites and 1747 blacks, we performed a maximum likelihood bivariate genome scan for quantitative trait loci influencing postural SBP change and BMI, similarly adjusted for race, study center, sex, age, and age-by-sex interactions. Genome scans were performed using SOLAR (version 2.0) and race-specific marker allele frequencies derived from founders. The maximum genome-wide logarithm of odds (LOD) score of 3.2 was detected on chromosome 13 at 24 cM. This marker (D13S493) lies within 20 cM of a marker previously linked to BMI in the Family Heart Study and is substantially higher than the univariate linkage for each trait (LOD scores for BMI and postural SBP change were 2.4 and 0.9, respectively). These findings suggest that a gene(s) on chromosome 13q jointly regulates the SBP response to postural change and BMI.

Loss-of-function polymorphic variants of the human angiotensin II type 1 receptor

The angiotensin II type 1 (AT1) receptor is the primary effector for angiotensin II (Ang II), a key peptide regulator of blood pressure and fluid homeostasis. AT1 receptors are involved in the pathogenesis of several cardiovascular diseases, including hypertension, cardiac hypertrophy, and congestive heart failure, which are characterized by significant interindividual variation in disease risk, progression, and response to pharmacotherapy. Such variation could arise from genomic polymorphisms in the AT1 receptor. To pursue this notion, we have pharmacologically characterized seven known and putative nonsynonymous AT1 receptor variants. Functional analysis using the cell-based assay receptor selection and amplification technology (R-SAT) revealed that three variants (AT1-G45R, AT1-F204S, and AT1-C289W) displayed altered responses to Ang II and other AT1 receptor agonists and antagonists. Agonist responses to Ang II were absent for AT1-G45R and significantly reduced in potency for AT1-C289W (11-fold) and AT1-F204S (57-fold) compared with the wild-type (WT) receptor. AT1-F204S also displayed reduced relative efficacy (57%). Quantitatively similar results were obtained in two additional functional assays, phosphatidyl inositol hydrolysis and extracellular signal-regulated kinase activation. Radioligand binding studies revealed that AT1-G45R failed to bind Ang II, whereas cell surface staining clearly showed that it trafficked to the cell surface. AT1-C289W and AT1-F204S displayed reduced binding affinities of 3- and 5-fold and reduced cell surface expression of 43 and 60% of that observed for the WT receptor, respectively. These data demonstrate that polymorphic variation in the human AT1 receptor induces loss of functional phenotypes, which may constitute the molecular basis of variability of AT1 receptor-mediated physiological responses.

Gamma-MSH, sodium metabolism, and salt-sensitive hypertension

Alpha-, beta-, and gamma-melanocyte stimulating hormones (MSHs) are melanotropin peptides that are derived from the ACTH/beta-endorphin prohormone proopiomelanocortin (POMC). They have been highly conserved through evolutionary development, although their functions in mammals have remained obscure. The identification in the last decade of a family of five membrane-spanning melanocortin receptors (MC-Rs), for which the melanotropins are the natural ligands, has permitted the characterization of a number of important actions of these peptides, although the physiological function(s) of gamma-MSH have remained elusive. Much evidence indicates that gamma-MSH stimulates sympathetic outflow and raises blood pressure through a central mechanism. However, this review focuses on newer cardiovascular and renal actions of the peptide, acting in most cases through the MC3-R. In rodents, a high-sodium diet (HSD) increases the pituitary abundance of POMC mRNA and of gamma-MSH content and results in a doubling of plasma gamma-MSH concentration. The peptide is natriuretic and acts through renal MC3-Rs, which are also upregulated by the HSD. Thus the system appears designed to participate in the integrated response to dietary sodium excess. Genetic or pharmacologic induction of gamma-MSH deficiency results in marked salt-sensitive hypertension that is corrected by the administration of the peptide, probably through a central site of action. Deletion of the MC3-R also produces salt-sensitive hypertension, which, however, is not corrected by infusion of the hormone. These observations in aggregate suggest the operation of a hormonal system important in blood pressure control and in the regulation of sodium excretion. The relationship of these two actions to each other and the significance of this system in humans are important questions for future research.

Cholecystokinin-mediated suppression of feeding involves the brainstem melanocortin system

Hypothalamic pro-opiomelanocortin (POMC) neurons help regulate long-term energy stores. POMC neurons are also found in the nucleus tractus solitarius (NTS), a region regulating satiety. We show here that mouse brainstem NTS POMC neurons are activated by cholecystokinin (CCK) and feeding-induced satiety and that activation of the neuronal melanocortin-4 receptor (MC4-R) is required for CCK-induced suppression of feeding; the melanocortin system thus provides a potential substrate for integration of long-term adipostatic and short-term satiety signals.

Melanocortin-4 receptor gene variant I103 is negatively associated with obesity

Several rare mutations in the melanocortin-4 receptor gene (MC4R) predispose to obesity. For the most common missense variant V103I (rs2229616), however, the previously reported similar carrier frequencies in obese and nonobese individuals are in line with in vitro studies, which have not shown a functional implication of this variant. In the present study, we initially performed a transmission/disequilibrium test on 520 trios with obesity, and we observed a lower transmission rate of the I103 allele (P=.017), which was an unexpected finding. Therefore, we initiated two large case-control studies (N=2,334 and N=661) and combined the data with those from 12 published studies, for a total of 7,713 individuals. The resulting meta-analysis provides evidence for a negative association of the I103 allele with obesity (odds ratio 0.69; 95% confidence interval 0.50-0.96; P=.03), mainly comprising samples of European origin. Additional screening of four other ethnic groups showed comparable I103 carrier frequencies well below 10%. Genomic sequencing of the MC4R gene revealed three polymorphisms in the noncoding region that displayed strong linkage disequilibrium with V103I. In our functional in vitro assays, the variant was indistinguishable from the wild-type allele, as was the result in previous studies. This report on an SNP/haplotype that is negatively associated with obesity expands the successful application of meta-analysis of modest effects in common diseases to a variant with a carrier frequency well below 10%. The respective protective effect against obesity implies that variation in the MC4R gene entails both loss and gain of function.

Inherited Diseases Involving G Proteins and G Protein–Coupled Receptors

Heterotrimeric G proteins couple seven-transmembrane receptors for diverse extracellular signals to effectors that generate intracellular signals altering cell function. Mutations in the gene encoding the alpha subunit of the G protein-coupling receptors to stimulation of adenylyl cyclase cause developmental abnormalities of bone, as well as hormone resistance (pseudohypoparathyroidism caused by loss-of-function mutations) and hormone hypersecretion (McCune-Albright syndrome caused by gain-of-function mutations). Loss- and gain-of-function mutations in genes encoding G protein-coupled receptors (GPCRs) have been identified as the cause of an increasing number of retinal, endocrine, metabolic, and developmental disorders. GPCRs comprise an evolutionarily conserved gene superfamily ( 1 ). By coupling to heterotrimeric G proteins, GPCRs transduce a wide variety of extracellular signals including monoamine, amino acid, and nucleoside neurotransmitters, as well as photons, chemical odorants, divalent cations, hormones, lipids, peptides and proteins. Following a brief overview of G protein-coupled signal transduction, we review the growing body of evidence that mutations in genes encoding GPCRs and G proteins are an important cause of human disease.

Impaired coordination of nutrient intake and substrate oxidation in melanocortin-4 receptor knockout mice

Mutations in the melanocortin-4 receptor (MC4R) are associated with obesity. The obesity syndrome observed in humans with MC4R haploinsufficiency is similar to that observed in MC4R knockout mice, including increased longitudinal growth, hyperphagia, and fasting hyperinsulinemia. For comparison with other commonly investigated models of obesity and insulin resistance, we have backcrossed Mc4r-/- mice into the C57BL/6J (B6) background. Female obese Mc4r-/- mice exhibit reduced energy expenditure and an attenuated increase in fatty acid (FA) oxidation after exposure to high-fat diets compared with obese Lepob/Lepob mice. The reduced energy expenditure and FA oxidation correlates with changes in hepatic gene expression. The expression of genes involved in FA oxidation increased in obese Lepob/Lepob mice compared with wild-type and obese Mc4r-/- mice. In contrast, a key lipogenic enzyme, FA synthase (FAS), is increased in obese Mc4r-/- mice compared with obese Lepob/Lepob mice. Hyperinsulinemia, increased FAS mRNA expression and hepatic steatosis appear to be secondary to obesity in B6 Mc4r-/- mice. However, Mc4r-/- mice in a mixed genetic background develop severe hepatic steatosis at an early age. This might suggest an important role of the MC4R in regulating liver FA metabolism that is masked on the B6 background. Interestingly, the 10- to 20-fold increase in liver triglyceride in the outbred strain of Mc4r-/- mice is not always associated with fasting hyperinsulinemia or increased FAS mRNA expression. This observation suggests that changes in liver secondary to triglyceride accumulation lead to hyperinsulinemia and increased hepatic FAS expression in Mc4r-/- mice.

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.

The human MC4R promoter: characterization and role in obesity

Heterozygous mutations in the coding sequence of the serpentine melanocortin 4 receptor (MC4R) are the most frequent genetic cause of severe human obesity. Since haploinsufficiency has been proposed as a causal mechanism of obesity associated with these mutations, reduction in gene transcription caused by mutations in the transcriptionally essential regions of the MC4R promoter may also be a cause of severe obesity in humans. To test this hypothesis we defined the minimal promoter region of the human MC4R and evaluated the extent of genetic variation in this region compared with the coding region in two cohorts of severely obese subjects. 5'RACE followed by functional promoter analysis in multiple cell lines indicates that an 80-bp region is essential for the transcriptional activity of the MC4R promoter. Systematic screening of 431 obese children and adults for mutations in the coding sequence and the minimal core promoter of MC4R reveals that genetic variation in the transcriptionally essential region of the MC4R promoter is not a significant cause of severe obesity in humans.

The genetics of obesity

Obesity prevalence has increased markedly over the past few decades. The obesity pandemic has huge implications for public health and our society. Although multiple studies show that the genetic contribution to obesity is significant, our genes have not changed appreciably over this time period. It was hypothesized that natural selection favors genotypes that result in a thrifty metabolism because individuals who carry these genotypes would be more likely to survive times of nutrient scarcity and to pass these genotypes to successive generations. Now that most of the world has adopted an increasingly "obesigenic" lifestyle of excess caloric intake and decreased physical activity, these same genes contribute to obesity and poor health. With the exception of the rare mutations that cause severe morbid obesity, it seems that numerous genes, each with modest effect, contribute to an individual's predisposition toward the more common forms of obesity. Variants in several candidate genes have been identified: association analyses and functional studies show that they contribute to modest obesity and related phenotypes. More recently, insights regarding gene-gene interactions have begun to emerge. Genome-wide scans for obesity phenotypes have led to the identification of several chromosome regions that are likely to harbor obesity susceptibility genes. Because of the increasing number of genome scans, several regions of replication have emerged. Positional cloning of these genes will undoubtedly unveil new insights into the molecular and pathophysiologic mechanisms of energy homeostasis and obesity.

Mutationally Induced Disulfide Bond Formation within the Third Extracellular Loop Causes Melanocortin 4 Receptor Inactivation in Patients with Obesity

By screening patients with severe early onset obesity for mutations within the melanocortin 4 receptor (MC4R) gene, we have identified a missense mutation (C271R) that occurs homozygous in two siblings with obesity. In-depth functional characterization of C271R revealed a right-shifted concentration response curve due to lower affinity to natural and synthetic MC4R agonists and a reduced cell surface expression. Cys-271 is located in the third extracellular loop. Here, we provide evidence that Cys-271 forms an intra-loop disulfide bond with Cys-277. Unexpectedly, we found that loss of receptor function is not only caused by the disruption of this disulfide bridge. Our data strongly support a new mechanism in which the receptor malfunction in the C271R mutant is induced by formation of a functionally disastrous disulfide bridge between Cys-277 and a third Cys residue at position 279. Mutational and chemical disruption of this improper disulfide bond was able to restore normal receptor potency. By demonstrating that a loss of a disulfide bond-participating Cys residue can favor a functionally disastrous disulfide bond, we now add a new mechanism of how Cys residues can be involved in G-protein-coupled receptor malfunction.

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.

Studies of natural allele effects in mice can be used to identify genes causing common human obesity

Although genes causing rare Mendelian forms of human obesity have provided much useful information about underlying causes of obesity, these genes do not explain significant proportions of common obesity. This review presents evidence that animal models can be used to uncover subtle genetic effects on obesity and can provide a powerful rigorous compliment to human association studies. We discuss the advantages of animal models of obesity, various approaches to discovering obesity genes, and the future of mapping and isolating naturally occurring alleles of obesity genes. We review evidence that it is important to map naturally occurring obesity genes using quantitative trait locus (QTL) mapping, instead of mutagenesis and knockout models because the latter do not allow study of interactions and because naturally occurring obesity alleles can interfere with cloning from mutagenesis projects. Because a substantial percentage of human obesity results from complex interactions, the underlying genes can only be identified by direct studies in humans, which are still very difficult, or by studies in mice that begin with QTL mapping. Finally, we emphasize that animal model studies can be used to prove that a specific gene, only associated with obesity in humans, can indeed be the underlying cause of obesity in mammals.

Neural systems controlling food intake and energy balance in the modern world

PURPOSE OF REVIEW

Obesity continues to increase around the globe, and creates a major health problem because of its comorbidities such as diabetes and cardiovascular disease. Considering that the major cause of most human obesities is the modern lifestyle in a rapidly changing environment, this suggests that the battle is between brain areas controlling internal metabolic homeostasis and those dealing with cognitive and emotional processing of external information. Thus, this review is intended to bridge this apparent neural dichotomy and highlight possible ways of interactions between these homeostatic and nonhomeostatic systems.

RECENT FINDINGS

Rapid progress has been made in identifying the molecular mechanisms underlying the neural circuits regulating food intake and energy balance. Specific populations of peptidergic neurons in the medial hypothalamus can be considered metabolic integrators sensing both short and long-term availability of fuels. These cell groups in turn connect with various other brain regions to orchestrate adaptive responses through changes in food intake, as well as endocrine and autonomic responses. Alternatively, the impact of the environmental changes on ingestive behavior have been documented, and many of the underlying cortico-limbic pathways are starting to be identified. It has been proposed that a switch from instinctual control of energy balance to a more cognitive control will be necessary to stop the obesity epidemic.

SUMMARY

The realization that there is nothing wrong with the homeostatic regulatory system in common obesity suggests that a systematic investigation of pathways that link the 'cognitive' with the 'metabolic' brain might be a more promising area of research. Such an investigation would reveal new molecular and cellular mechanisms providing new drug targets, leading to behavioral therapies that are directed more towards the causes of common obesity.

Obesity--is it a genetic disorder?

Obesity is one of the most pressing problems in the industrialized world. Twin, adoption and family studies have shown that genetic factors play a significant role in the pathogenesis of obesity. Rare mutations in humans and model organisms have provided insights into the pathways involved in body weight regulation. Studies of candidate genes indicate that some of the genes involved in pathways regulating energy expenditure and food intake may play a role in the predisposition to obesity. Amongst these genes, sequence variations in the adrenergic receptors, uncoupling proteins, peroxisome proliferator-activated receptor, and the leptin receptor genes are of particular relevance. Results that have been replicated in at least three genome-wide scans suggest that key genes are located on chromosomes 2p, 3q, 5p, 6p, 7q, 10p, 11q, 17p and 20q. We conclude that the currently available evidence suggests four levels of genetic determination of obesity: genetic obesity, strong genetic predisposition, slight genetic predisposition, and genetically resistant. This growing body of research may help in the development of anti-obesity agents and perhaps genetic tests to predict the risk for obesity.

Impact of interrupted leptin pathways on ventilatory control

Leptin deficiency in ob/ob mice produces marked depression of the hypercapnic ventilatory response, particularly during sleep. We now extend our previous findings to determine whether 1) leptin deficiency affects the hypoxic ventilatory response and 2) blockade of the downstream excitatory actions of leptin on melanocortin 4 receptors or inhibitory actions on neuropeptide Y (NPY) pathways has an impact on hypercapnic and hypoxic sensitivity. We have found that leptin-deficient ob/ob mice have the same hypoxic ventilatory response as weight-matched wild-type obese mice. There were no differences in the hypoxic sensitivity between agouti yellow mice and weight-matched controls, or NPY-deficient mice and wild-type littermates. Agouti yellow mice, with blocked melanocortin pathways, exhibited a significant depression of the hypercapnic sensitivity compared with weight-matched wild-type controls during non-rapid eye movement sleep (5.8 +/- 0.7 vs. 8.9 +/- 0.7 ml x min(-1) x %CO(2)(-1), P < 0.01), but not during wakefulness. NPY-deficient transgenic mice exhibited a small increase in the hypercapnic ventilatory response compared with wild-type littermates, but this was only present during wakefulness. We conclude that interruption of leptin pathways does not affect hypoxic sensitivity during sleep and wakefulness but that melanocortin 4 blockade is associated with depressed hypercapnic sensitivity in non-rapid eye movement sleep.

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

The melanocortin-4 receptor (MC4R) is a member of the rhodopsin-like G protein-coupled receptor family. The binding of alpha-MSH to the MC4R leads to increased cAMP production. Recent pharmacological and genetic studies have provided compelling evidence that MC4R is an important regulator of food intake and energy homeostasis. Allelic variants of MC4R were reported in some children with early-onset severe obesity. However, few studies have been performed to confirm that these allelic variants result in an impairment of the receptor's function. In this study, we expressed wild-type and variant MC4Rs in HEK293 cells and systematically studied ligand binding, agonist-stimulated cAMP, and cell surface expression. Six of the 11 mutants examined had either decreased (S58C, N62S, Y157S, C271Y) or no (P78L, G98R) ligand binding, with proportional impairments in [Nle4, d-Phe7]-alpha-MSH-stimulated cAMP production. Confocal microscopy confirmed that the observed decreases in hormone binding by these mutants are associated with decreased cell surface expression due to intracellular retention of the mutants. The other five allelic variants (D37V, P48S, V50M, I170V, N274S) were found to be expressed at the cell surface and to bind agonist and respond with increased cAMP production normally. The data on these latter five variants raise the question as to whether they are indeed causative of the obesity or not and, if so, by what mechanism. Our data, therefore, stress the importance of characterizing the properties of MC4R variants associated with early-onset severe obesity. We further propose a classification scheme for mutant MC4Rs based upon their properties.

Human recombinant resistin protein displays a tendency to aggregate by forming intermolecular disulfide linkages

Resistin, a small cysteine rich protein secreted by adipocytes, has been proposed to be a link between obesity and type II diabetes by modulating the insulin signaling pathway and thus inducing insulin resistance. Resistin protein, with 11 cysteine residues, was not significantly homologous at the amino acid level to any other known cysteine rich proteins. Resistin cDNA derived from human subcutaneous adipose tissue was expressed in Escherichia coli as an N-terminal six-His-tag fusion protein. The overexpressed recombinant resistin was purified to homogeneity from inclusion bodies, after solubilization in 8 M urea, using a metal affinity column. While MALDI-TOF mass spectrometric analysis of the purified protein generated a single peak corresponding to the estimated size of 11.3 kDa, the protein exhibited a concentration-dependent oligomerization which is evident from size exclusion chromatography. The oligomeric structure was SDS-insensitive but beta-mercaptoethanol-sensitive, pointing to the importance of disulfide linkages in resistin oligomerization. Estimation of free cysteine residues using the NBD-Cl assay revealed a concentration- and time-dependent increase in the extent of formation of disulfide linkages. The presence of intermolecular disulfide bond(s), crucial in maintaining the global conformation of resistin, was further evident from fluorescence emission spectra. Circular dichroism spectra revealed that recombinant resistin has a tendency to reversibly convert from alpha-helical to beta-sheet structure as a direct function of protein concentration. Our novel observations on the biophysical and biochemical features of human resistin, particularly those shared with prion proteins, may have a bearing on its likely physiological function.

Minireview: human obesity-lessons from monogenic disorders

Genetic influences on the determination of human fat mass are profound and powerful, a statement that does not conflict with the obvious influence of environmental factors that drive recent changes in the prevalence of obesity. The assertion of the importance of genetic factors has, until recently, largely been based on twin and adoption studies. However, in the last 6 yr, a number of human genes have been identified in which major missense or nonsense mutations are sufficient in themselves to result in severe early-onset obesity, usually associated with disruption of normal appetite control mechanisms. Progress in the identification of more common, subtler genetic variants that influence fat mass in larger numbers of people has been slower, but discernible. Human genetics will continue to make an invaluable contribution to the study of human obesity by identifying critical molecular components of the human energy balance regulatory systems, pointing the way toward more targeted and effective therapies and assisting the prediction of individual responses to environmental manipulations.

Controle neuroendócrino do peso corporal: implicações na gênese da obesidade

O peso corporal é regulado por uma interação complexa entre hormônios e neuropeptídeos, sob o controle principal de núcleos hipotalâmicos. Mutações nos genes de hormônios e neuropeptídeos, de seus receptores ou de elementos regulatórios, têm sido descritas na espécie humana, mas são tidas como raras, não explicando as formas mais comuns de obesidade. No entanto, o estudo destas mutações tem propiciado um grande avanço nos conhecimentos sobre a base genética e a fisiopatologia da obesidade, possibilitando o estudo e abrindo perspectivas para o desenvolvimento de novas modalidades terapêuticas. Recentemente, demonstrou-se que mutações no receptor 4 da melanocortina podiam ser encontradas em até 5% dos casos de obesidade severa, representando até o presente momento a forma mais prevalente de obesidade monogênica na espécie humana. Nesta revisão, são discutidas as diversas mutações descritas nos seres humanos de elementos da rede neuroendócrina de controle do peso corporal, bem como as implicações dos mesmos na gênese da obesidade.