Yellow mice, red hair, and childhood obesity: the melanocortinergic pathway in energy homeostasis

Solution structure of the transmembrane 2 domain of the human melanocortin-4 receptor in sodium dodecyl sulfate (SDS) micelles and the functional implication of the D90N mutant

The melanocortin receptors (MCRs) are members of the G protein-coupled receptor (GPCR) 1 superfamily with seven transmembrane (TM) domains. Among them, the melanocortin-4 receptor (MC4R) subtype has been highlighted recently by genetic studies in obese humans. In particular, in a patient with severe early-onset obesity, a novel heterozygous mutation in the MC4R gene was found in an exchange of Asp to Asn in the 90th amino acid residue located in the TM 2 domain (MC4RD90N). Mutations in the MC4R gene are the most frequent monogenic causes of severe obesity and are described as heterozygous with loss of function. We determine solution structures of the TM 2 domain of MC4R (MC4RTM2) and compared secondary structure of Asp90 mutant (MC4RTM2-D90N) in a micelle environment by nuclear magnetic resonance (NMR) spectroscopy. NMR structure shows that MC4RTM2 forms a long α-helix with a kink at Gly98. Interestingly, the structure of MC4RTM2-D90N is similar to that of MC4RTM2 based on data from CD and NMR spectrum. However, the thermal stability and homogeneity of MC4RD90N is quite different from those of MC4R. The structure from molecular modeling suggests that Asp90(2.50) plays a key role in allosteric sodium ion binding. Our data suggest that the sodium ion interaction of Asp90(2.50) in the allosteric pocket of MC4R is essential to its function, explaining the loss of function of the MC4RD90N mutant.

Energy requirement and food intake behaviour in young adult intact male cats with and without predisposition to overweight

Obesity is a common problem in cats. In the experimental cat family of the institute of animal nutrition besides a “normal” lean phenotype, cats with predisposition to an overweight phenotype are present. To investigate energy requirements and food intake behaviour of intact male cats of different phenotypes, six “normal” lean cats (GL) and six cats disposed to overweight (GO) were used. At the beginning of the experiment, all cats had an ideal body condition score of 5. To reach this the GO cats had to pass a weight-loss program. Energy requirements of the cats were determined using respiration chambers, whereas the amount and frequency of food intake was measured with a feeding station recording the data automatically. Energy requirement at weight constancy of the GO cats was even on fat-free mass (FFM) significantly (P = 0.02) lower (162.6 kJ/kg FFM/d) than that of the “normal” lean cats (246 kJ/kg FFM/d). The GO cats also showed a higher food intake 34.5 ± 1.5 g dry matter/kg body weight^0.67 compared to the GL cats (24.0 ± 2.1 g dry matter/kg body weight^0.67)(P = 0.001). In conclusion quantifiable differences in food intake and behaviour in cats predisposed to overweight compared to “normal” lean cats were found.

Obesity: the integrated roles of environment and genetics

Obesity represents one of the most serious global health issues with approximately 310 million people presently affected. It develops because of a mismatch between energy intake and expenditure that results from behavior (feeding behavior and time spent active) and physiology (resting metabolism and expenditure when active). Both of these traits are affected by environmental and genetic factors. The dramatic increase in the numbers of obese people in Western societies reflects mostly changing environmental factors and is linked to reduced activity and perhaps also increased food intake. However, in all societies and subpopulations, there are both obese and nonobese subjects. These differences are primarily a consequence of genetic factors as is revealed by the high heritability for body mass index. Most researchers agree that energy balance and, hence, body weight are regulated phenomena. There is some disagreement about exactly how this regulation occurs. However, a common model is the "lipostatic" regulation system, whereby our energy stores generate signals that are compared with targets encoded in the brain, and differences between these drive our food intake levels, activity patterns, and resting and active metabolisms. Considerable advances were made in the last decade in understanding the molecular basis of this lipostatic system. Some obese people have high body weight because they have broken lipostats, but these are a rare minority. This suggests that for the majority of obese people, the lipostat is set at an inappropriately high level. When combined with exposure to an environment where there is ready availability of food at low energy costs to obtain it, obesity develops. The evolutionary background to how such a system might have evolved involves the evolution of social behavior, the harnessing of fire, and the development of weapons that effectively freed humans from the risks of predation. The lipostatic model not only explains why some people become obese whereas others do not, but also allows us to understand why energy-controlled diets do not work. Drug-based solutions to the obesity problem that work with the lipostat, rather than against it, are presently under development and will probably be in regular use within 5-10 y. However, several lines of evidence including genetic mapping studies of quantitative trait loci associated with obesity suggest that our present understanding of the regulatory system is still rudimentary. In particular, we know nothing about how the target body weight in the brain is encoded. As our understanding in this field advances, new drug targets are likely to emerge and allow us to treat this crippling disorder.

Autosomal-dominant mode of inheritance of a melanocortin-4 receptor mutation in a patient with severe early-onset obesity is due to a dominant-negative effect caused by receptor dimerization

Mutations in the melanocortin-4 receptor (MC4R) gene are the most frequent monogenic causes of severe obesity. Most mutations have been described as heterozygous with loss of function, suggesting that haploinsufficiency is the most likely mechanism of dominant inheritance. We detected a heterozygous mutation, D90N, in a patient with severe early-onset obesity. Functional characterization of the mutant receptor revealed normal cell-surface expression and binding properties but loss of signal transduction activity. In coexpression studies of wild-type (WT)-MC4R and D90N, the mutant receptor had a dominant-negative effect on WT-receptor function. Further investigation of this effect with sandwich enzyme-linked immunosorbent assays and fluorescence resonance energy transfer studies showed that the WT-MC4R and the D90N mutant form homodimers and heterodimers. We hypothesize that the dominant-negative effect of the D90N mutation is caused by a functionally altered WT-MC4R/D90N receptor heterodimer. These findings necessitate the reinvestigation of other heterozygous MC4R missense mutations to discriminate between haploinsufficiency and a dominant-negative effect. The finding of receptor dimerization highlights a more complex hypothalamic signaling network governing the regulation of body weight.