Specific Functions of Melanocortin 3 Receptor (MC3R)

Melanocortin 3 receptor (MC3R) is a G-protein coupled receptor which has been defined mostly as a regulator of the appetite/hunger balance mechanisms to date. In addition to its function regarding the weight gain and appetite control mechanisms of MC3R, recent studies have shown that MC3R controls growth, puberty, and circadian rhythms as well. Despite the drastic effects of MC3R deficiency in humans and other mammals, its cellular mechanisms are still under investigation. In this review paper, we aimed to point out the importance of MC3R regulations in three main areas: 1) its impact on weight and appetite control, 2) its role in the control of growth, puberty, and the circadian rhythm, and, 3) its protein-protein interactions and cellular mechanisms.

Loss-of-function mutations in the melanocortin-3 receptor gene confer risk for human obesity: A systematic review and meta-analysis

The association between rare coding loss-of-function (LOF) mutations in the melanocortin receptor 3 (MC3R) gene and human obesity is controversial. To fill this gap of knowledge, we performed a systematic review and meta-analysis of genetic association studies in all ages and ethnicities. Two reviewers independently performed risk of bias assessment and extracted data. We searched Medline, Embase, Web of Science Core Collection, BIOSIS Preview, CINAHL, ProQuest Dissertations & Theses, and reference lists of relevant studies. All case-control, cross-sectional, prospective, and retrospective studies that evaluated prevalence of rare (less than 1% frequency) coding partial/complete LOF mutations in MC3R among individuals with obesity and normal weight were included. Our systematic search identified 1925 references relevant to the present review. Six studies were deemed eligible. Meta-analysis of 2969 individuals with obesity and 2572 with normal weight showed a positive association between rare heterozygous coding partial/complete LOF mutations in MC3R and obesity in children and adults of European, North African, and Asian ancestries (odds ratio = 3.07; 95% CI, 1.48-7.00; P = 4.2 × 10^-3 ). Our data demonstrates that rare partial/complete LOF mutations in the coding region of MC3R confer three-time increased risk of obesity in humans, and implies that rare genetic variants with intermediate effects contribute to the missing heritability of obesity.

Associations of the melanocortin 3 receptor C17A + G241A haplotype with body composition and inflammation in African-American adults

BACKGROUND

The MC3R haplotype C17A + G241A, which encodes a partially inactivated receptor, has high prevalence in individuals of predominately African ancestry. In pediatric cohorts, homozygosity for this common variant has been associated with obesity, reduced lean mass, and greater fasting insulin. However, metabolic and body composition measures have not been well studied in adults with this haplotype.

METHODS

A convenience sample of 237 healthy African-American adult volunteers was studied. TaqMan assays were used to genotype MC3R variants. Labs were drawn in the morning in the fasted state. Body composition data was obtained via dual-energy X-ray absorptiometry. An analysis of covariance was used to examine the associations of genotype with metabolic and body composition measures controlling for age and sex.

RESULTS

Individuals homozygous for the MC3R C17A + G241A haplotype had significantly greater body mass index, fat mass, fat mass percentage, and C-reactive protein, with reduced lean mass percentage as compared to heterozygous and wild-type participants (all ps < 0.05); fasting insulin was marginally nonsignificant between groups (p = 0.053). After adjusting for fat mass, laboratory differences no longer remained significant.

CONCLUSIONS

Homozygosity for MC3R C17A + G241A is associated with increased adiposity in African-American adults. Further studies are needed to elucidate the mechanisms behind these associations.

Functions of the DRY motif and intracellular loop 2 of human melanocortin 3 receptor

The melanocortin 3 receptor (MC3R) regulates several physiological functions, including feed efficiency, nutrient partitioning, fasting response, natriuresis, and immune reactions. Naturally occurring mutations in the MC3R gene have been shown to be associated with increased adiposity and lung diseases such as tuberculosis and cystic fibrosis. The DRY motif at the cytoplasmic end of transmembrane domain 3 (TM3) and the second intracellular loop 2 (ICL2) are known to be important for receptor function in several G protein-coupled receptors (GPCRs). To gain a better understanding of the functions of this domain in MC3R, we performed alanine-scanning mutagenesis on 18 residues. We showed that alanine mutation of 11 residues reduced the maximal binding and maximal cAMP production stimulated by agonists. Mutation of two residues did not change maximal binding but resulted in impaired signaling in the Gs-cAMP pathway. Mutation of five residues impaired signaling in the ERK1/2 pathway. We have also shown that alanine mutants of seven residues that were defective in the cAMP pathway were not defective in the ERK1/2 pathway, demonstrating biased signaling. In summary, we demonstrated that the cytoplasmic end of TM3 and the ICL2 were critical for MC3R function. We also reported for the first time biased signaling in MC3R.

Genomic structure and genetic drift in C57BL/6 congenic metabolic mutant mice

We used a genome-wide single nucleotide polymorphism (SNP) approach to characterize the genomic structures of four representative C57BL/6 (B6) congenic mutant mouse lines to include the A) long-chain acyl-CoA dehydrogenase (Acadl), B) melanocortin 3 receptor (Mc3r), C) endothelial nitric oxide synthase (Nos3), and D) a replacement of mouse apolipoprotein E (Apoe) by human apolipoprotein E-2 (APOE2). We wanted to evaluate the size and flanking genes of the 129 strain origin mutant allele intervals on the B6 background. Additionally, we wanted to evaluate genetic drift among not only the four mutant lines and their respective B6 origin substrains, but also the drift between two commonly used B6 lines obtained from Jackson Laboratory and Taconic. Overall, we found a range of 129 origin interval sizes in the congenic mutant lines analyzed that ranged from a ~2.8 kb human sequence for APOE2 embedded in a 129S6 interval to the largest being a ~16 Mb fragment containing the targeted Nos3 (eNos) gene. Given the range of 129 strain interval sizes, we found 129 strain flanking genes via annotation in genome data bases ranging from one gene both upstream and downstream of the APOE2 allele to seven genes-upstream and five genes-downstream at the Nos3 locus. Furthermore, we found fourteen SNP differences between the Jackson Laboratory and Taconic B6 mice. These genetic differences were associated with marked adiposity differences between the two B6 substrains. This study demonstrates both the fidelity and the caveats of using congenic gene targeted mouse models and recognizing the importance of selecting the appropriately matched wild-type control mouse line.

Prolyl carboxypeptidase mRNA expression in the mouse brain

Prolyl carboxypeptidase (PRCP), a serine protease, is widely expressed in the body including liver, lung, kidney and brain, with a variety of known substrates such as plasma prekallikrein, bradykinin, angiotensins II and III, and α-MSH, suggesting its role in the processing of tissue-specific substrates. In the brain, PRCP has been shown to inactivate hypothalamic α-MSH, thus modulating melanocortin signaling in the control of energy metabolism. While its expression pattern has been reported in the hypothalamus, little is known on the distribution of PRCP throughout the mouse brain. This study was undertaken to determine PRCP expression in the mouse brain. Radioactive in situ hybridization was performed to determine endogenous PRCP mRNA expression. In addition, using a gene-trap mouse model for PRCP deletion, X-gal staining was performed to further determine PRCP distribution. Results from both approaches showed that PRCP gene is broadly expressed in the brain.