Point mutations in the melanocortin-4 receptor cause variable obesity in mice

Metabolic Syndrome: Lessons from Rodent and Drosophila Models

Overweight and obesity are health conditions tightly related to a number of metabolic complications collectively called “metabolic syndrome” (MetS). Clinical diagnosis of MetS includes the presence of the increased waist circumference or so-called abdominal obesity, reduced high density lipoprotein level, elevated blood pressure, and increased blood glucose and triacylglyceride levels. Different approaches, including diet-induced and genetically induced animal models, have been developed to study MetS pathogenesis and underlying mechanisms. Studies of metabolic disturbances in the fruit fly Drosophila and mammalian models along with humans have demonstrated that fruit flies and small mammalian models like rats and mice have many similarities with humans in basic metabolic functions and share many molecular mechanisms which regulate these metabolic processes. In this paper, we describe diet-induced, chemically and genetically induced animal models of the MetS. The advantages and limitations of rodent and Drosophila models of MetS and obesity are also analyzed.

An ENU-induced splice site mutation of mouse Col1a1 causing recessive osteogenesis imperfecta and revealing a novel splicing rescue

GU-AG consensus sequences are used for intron recognition in the majority of cases of pre-mRNA splicing in eukaryotes. Mutations at splice junctions often cause exon skipping, short deletions, or insertions in the mature mRNA, underlying one common molecular mechanism of genetic diseases. Using N-ethyl-N-nitrosourea, a novel recessive mutation named seal was produced, associated with fragile bones and susceptibility to fractures (spine and limbs). A single nucleotide transversion (T → A) at the second position of intron 36 of the Col1a1 gene, encoding the type I collagen, α1 chain, was responsible for the phenotype. Col1a1 ^seal mRNA expression occurred at greatly reduced levels compared to the wild-type transcript, resulting in reduced and aberrant collagen fibers in tibiae of seal homozygous mice. Unexpectedly, splicing of Col1a1 ^seal mRNA followed the normal pattern despite the presence of the donor splice site mutation, likely due to the action of a putative intronic splicing enhancer present in intron 25, which appeared to function redundantly with the splice donor site of intron 36. Seal mice represent a model of human osteogenesis imperfecta, and reveal a previously unknown mechanism for splicing "rescue."

Innate immunity and the new forward genetics

As it is a hard-wired system for responses to microbes, innate immunity is particularly susceptible to classical genetic analysis. Mutations led the way to the discovery of many of the molecular elements of innate immune sensing and signaling pathways. In turn, the need for a faster way to find the molecular causes of mutation-induced phenotypes triggered a huge transformation in forward genetics. During the 1980s and 1990s, many heritable phenotypes were ascribed to mutations through positional cloning. In mice, this required three steps. First, a genetic mapping step was used to show that a given phenotype emanated from a circumscribed region of the genome. Second, a physical mapping step was undertaken, in which all of the region was cloned and its gene content determined. Finally, a concerted search for the mutation was performed. Such projects usually lasted for several years, but could produce breakthroughs in our understanding of biological processes. Publication of the annotated mouse genome sequence in 2002 made physical mapping unnecessary. More recently we devised a new technology for automated genetic mapping, which eliminated both genetic mapping and the search for mutations among candidate genes. The cause of phenotype can now be determined instantaneously. We have created more than 100,000 coding/splicing mutations. And by screening for defects of innate and adaptive immunity we have discovered many "new" proteins needed for innate immune function.

Identification of mutations through dominant screening for obesity using C57BL/6 substrains

The discovery of leptin substantiated the usefulness of a forward genetic approach in elucidating the molecular network regulating energy metabolism. However, no successful dominant screening for obesity has been reported, which may be due to the influence of quantitative trait loci between the screening and counter strains and the low fertility of obese mice. Here, we performed a dominant screening for obesity using C57BL/6 substrains, C57BL/6J and C57BL/6N, with the routine use of in vitro fertilization. The screening of more than 5000 mutagenized mice established two obese pedigrees in which single nucleotide substitutions in Mc4r and Sim1 genes were identified through whole-exome sequencing. The mutation in the Mc4r gene produces a premature stop codon, and the mutant SIM1 protein lacks transcriptional activity, showing that the haploinsufficiency of SIM1 and MC4R results in obesity. We further examined the hypothalamic neuropeptide expressions in the mutant pedigrees and mice with diet-induced obesity, which showed that each obesity mouse model has distinct neuropeptide expression profiles. This forward genetic screening scheme is useful and applicable to any research field in which mouse models work.

Sex-specific effects of LiCl treatment on preservation of renal function and extended life-span in murine models of SLE: perspective on insights into the potential basis for survivorship in NZB/W female mice

Considerable research effort has been invested in attempting to understand immune dysregulation leading to autoimmunity and target organ damage. In systemic lupus erythematosus (SLE), patients can develop a systemic disease with a number of organs involved. One of the major target organs is the kidney, but patients vary in the progression of the end-organ targeting of this organ. Some patients develop glomerulonephritis only, while others develop rapidly progressive end organ failure. In murine models of SLE, renal involvement can also occur. Studies performed over the past several years have indicated that treatment with LiCl of females, but not males of the NZB/W model, at an early age during the onset of disease, can prevent development of end-stage renal disease in a significant percentage of the animals. While on Li treatment, up to 80 % of the females can exhibit long-term survival with evidence of mild glomerulonephritis which does not progress to renal failure in spite of on-going autoimmunity. Stopping the treatment led to a reactivation of the disease and renal failure. Li treatment of other murine models of SLE was less effective and decreased survivorship in male BxSB mice, exhibited little effect on male MRL-lpr mice, and only modestly improved survivorship in female MRL-lpr mice. This perspective piece discusses the findings of several related studies which support the concept that protecting target organs such as the kidney, even in the face of continued immune insults and some inflammation, can lead to prolonged survival with retention of organ function. Some possible mechanisms for the effectiveness of Li treatment in this context are also discussed. However, the detailed mechanistic basis for the sex-specific effects of LiCl treatment particularly in the NZB/W model remains to be elucidated. Elucidating such details may provide important clues for development of effective treatment for patients with SLE, ~90 % of which are females.

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.

Obesity-induced changes in kidney mitochondria and endoplasmic reticulum in the presence or absence of leptin

We investigated obesity-induced changes in kidney lipid accumulation, mitochondrial function, and endoplasmic reticulum (ER) stress in the absence of hypertension, and the potential role of leptin in modulating these changes. We compared two normotensive genetic mouse models of obesity, leptin-deficient ob/ob mice and hyperleptinemic melanocortin-4 receptor-deficient mice (LoxTB MC4R-/-), with their respective lean controls. Compared with controls, ob/ob and LoxTB MC4R-/- mice exhibit significant albuminuria, increased creatinine clearance, and high renal triglyceride content. Renal ATP levels were decreased in both obesity models, and mitochondria isolated from both models showed alterations that would lower mitochondrial ATP production. Mitochondria from hyperleptinemic LoxTB MC4R-/- mice kidneys respired NADH-generating substrates (including palmitate) at lower rates due to an apparent decrease in complex I activity, and these mitochondria showed oxidative damage. Kidney mitochondria of leptin-deficient ob/ob mice showed normal rates of respiration with no evidence of oxidative damage, but electron transfer was partially uncoupled from ATP synthesis. A fourfold induction of C/EBP homologous protein (CHOP) expression indicated induction of ER stress in kidneys of hyperleptinemic LoxTB MC4R-/- mice. In contrast, ER stress was not induced in kidneys of leptin-deficient ob/ob mice. Our findings show that obesity, in the absence of hypertension, is associated with renal dysfunction in mice but not with major renal injury. Alterations to mitochondria that lower cellular ATP levels may be involved in obesity-induced renal injury. The type and severity of mitochondrial and ER dysfunction differs depending upon the presence or absence of leptin.

Characterization of the melanocortin-4-receptor nonsense mutation W16X in vitro and in vivo

Several genetic diseases are triggered by nonsense mutations leading to the formation of truncated and defective proteins. Aminoglycosides have the capability to mediate a bypass of stop mutations during translation thus resulting in a rescue of protein expression. So far no attention has been directed to obesity-associated stop mutations as targets for nonsense suppression. Herein, we focus on the characterization of the melanocortin-4-receptor (MC4R) nonsense allele W16X identified in obese subjects. Cell culture assays revealed a loss-of-function of Mc4r(X16) characterized by impaired surface expression and defect signaling. The aminoglycoside G-418 restored Mc4r(X16) function in vitro demonstrating that Mc4r(X16) is susceptible to nonsense suppression. For the evaluation of nonsense suppression in vivo, we generated a Mc4r(X16) knock-in mouse line by gene targeting. Mc4r(X16) knock-in mice developed hyperphagia, impaired glucose tolerance, severe obesity and an increased body length demonstrating that this new mouse model resembles typical characteristics of Mc4r deficiency. In a first therapeutic trial, the aminoglycosides gentamicin and amikacin induced no amelioration of obesity. Further experiments with Mc4r(X16) knock-in mice will be instrumental to establish nonsense suppression for Mc4r as an obesity-associated target gene expressed in the central nervous system.

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.

Gene expression in hypothalamus, liver, and adipose tissues and food intake response to melanocortin-4 receptor agonist in pigs expressing melanocortin-4 receptor mutations

Transcriptional profiling was used to identify genes and pathways that responded to intracerebroventricular injection of melanocortin-4 receptor (MC4R) agonist [Nle4, d-Phe7]-α-melanocyte stimulating hormone (NDP-MSH) in pigs homozygous for the missense mutation in the MC4R, D298 allele ( n = 12), N298 allele ( n = 12), or heterozygous ( n = 12). Food intake (FI) was measured at 12 and 24 h after treatment. All pigs were killed at 24 h after treatment, and hypothalamus, liver, and back-fat tissue was collected. NDP-MSH suppressed ( P < 0.004) FI at 12 and 24 h in all animals after treatment. In response to NDP-MSH, 278 genes in hypothalamus ( q ≤ 0.07, P ≤ 0.001), 249 genes in liver ( q ≤ 0.07, P ≤ 0.001), and 5,066 genes in fat ( q ≤ 0.07, P ≤ 0.015) were differentially expressed. Pathway analysis of NDP-MSH-induced differentially expressed genes indicated that genes involved in cell communication, nucleotide metabolism, and signal transduction were prominently downregulated in the hypothalamus. In both liver and adipose tissue, energy-intensive biosynthetic and catabolic processes were downregulated in response to NDP-MSH. This included genes encoding for biosynthetic pathways such as steroid and lipid biosynthesis, fatty acid synthesis, and amino acid synthesis. Genes involved in direct energy-generating processes, such as oxidative phosphorylation, electron transport, and ATP synthesis, were upregulated, whereas TCA-associated genes were prominently downregulated in NDP-MSH-treated pigs. Our data also indicate a metabolic switch toward energy conservation since genes involved in energy-intensive biosynthetic and catabolic processes were downregulated in NDP-MSH-treated pigs.

Metabolic syndrome components in murine models

Animal models have enriched understanding of the physiological basis of metabolic disorders and advanced identification of genetic risk factors underlying the metabolic syndrome (MetS). Murine models are especially appropriate for this type of research, and are an excellent resource not only for identifying candidate genomic regions, but also for illuminating the possible molecular mechanisms or pathways affected in individual components of MetS. In this review, we briefly discuss findings from mouse models of metabolic disorders, particularly in light of issues raised by the recent flood of human genome-wide association studies (GWAS) results. We describe how mouse models are revealing that genotype interacts with environment in important ways, indicating that the underlying genetics of MetS is highly context dependant. Further we show that epistasis, imprinting and maternal effects each contribute to the genetic architecture underlying variation in metabolic traits, and mouse models provide an opportunity to dissect these aspects of the genetic architecture that are difficult if not impossible to ascertain in humans. Finally we discuss how knowledge gained from mouse models can be used in conjunction with comparative genomic methods and bioinformatic resources to inform human MetS research.

A novel melanocortin-4 receptor gene mutation in a female patient with severe childhood obesity

This study targeted the identification of mutations of melanocortin-4 receptor gene (MC4R) in obese children. Fifty-one unrelated probands with early onset severe obesity (body mass index (BMI) > 99th percentile; 21 girls, mean age 10.6 +/- 3.6 years) were analyzed for nucleotide variations in the MC4R coding region, by the polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) method followed by direct DNA sequencing. MC4R variants were detected in three patients: the known I169S variant was found in heterozygote state in two patients and a novel heterozygous Y302F mutation was detected in one 12-year-old girl (BMI = 34 kg/m(2), BMI z-score 2.7) who has been overweight since the second year of life and suffered from hyperinsulinemia (at the age of 12: fasting insulin 45 mU/ml, after oral glucose load max. 300 mU/ml). The mutation also appears in the father, although both parents are obese (BMI father: 30.2 kg/m(2); mother: 31.9 kg/m(2)). This novel mutation is located in the functionally important NPXXY motif of the seventh transmembrane domain of the receptor. Functional characterization revealed reduction in cell surface expression and an alteration in signal transduction properties. These results add to the growing list of loss-of-function MC4R mutations in early onset obese patients and suggest an orexigenic effect of novel Y302F mutation.

Mouse models for the central melanocortin system

Obesity is characterized by an excess storage of body fat and promotes the risk for complex disease traits such as diabetes mellitus and cardiovascular diseases. The obesity prevalence in Europe is rising and meanwhile ranges from 10 to 20% in men and 15-25% in women. Body fat accumulation occurs in states of positive energy balance and is favored by interactions among environmental, psychosocial and genetic factors. Energy balance is regulated by a complex neuronal network of anorexigenic and orexigenic neurons which integrates peripheral and central hormonal and neuronal signals relaying information on the metabolic status of organs and tissues in the body. A key component of this network is the central melanocortin pathway in the hypothalamus that elicits metabolic and behavioral adaptations for the maintenance of energy homeostasis. Genetic defects in this system cause obesity in mice and humans. In this review we emphasize mouse models with spontaneous natural mutations as well as targeted mutations that contributed to our understanding of the central melanocortin system function in the control of energy balance.

Functional characterization and structural modeling of obesity associated mutations in the melanocortin 4 receptor

Mutations in the melanocortin 4 receptor (MC4R) gene are the most common known cause of monogenic human obesity. The MC4R gene was sequenced in 2000 subjects with severe early-onset obesity. We detected seven different nonsense and 19 nonsynonymous mutations in a total of 94 probands, some of which have been reported previously by others. We functionally characterized the 11 novel obesity associated missense mutations. Seven of these mutants (L54P, E61K, I69T, S136P, M161T, T162I, and I269N) showed impaired cell surface trafficking, reduced level of maximal binding of the radioligand [125I]NDP-MSH, and reduced ability to generate cAMP in response to ligand. Four mutant MC4Rs (G55V, G55D, S136F, and A303T) displayed cell surface expression and agonist binding similar to the wild-type receptor but showed impaired cAMP production, suggesting that these residues are likely to be critical for conformational rearrangement essential for receptor activation. Homology modeling of these mutants using a model of MC4R based on the crystal structure of the beta2-adrenoreceptor was used to provide insights into the possible structural basis for receptor dysfunction. Transmembrane (TM) domains 1, 3, 6, 7, and peripheral helix 8 appear to participate in the agonist-induced conformational rearrangement necessary for coupling of ligand binding to signaling. We conclude that G55V, G55D, S136F, and A303T mutations are likely to strengthen helix-helix interactions between TM1 and TM2, TM3 and TM6, and TM7 and helix 8, respectively, preventing relative movement of these helices during receptor activation. The combination of functional studies and structural modeling of naturally occurring pathogenic mutations in MC4R can provide valuable information regarding the molecular mechanism of MC4R activation and its dysfunction in human disease.

Diabetes models by screen for hyperglycemia in phenotype-driven ENU mouse mutagenesis projects

More than 150 million people suffer from diabetes mellitus worldwide, and this number is expected to rise substantially within the next decades. Despite its high prevalence, the pathogenesis of diabetes mellitus is not completely understood. Therefore, appropriate experimental models are essential tools to gain more insight into the genetics and pathogenesis of the disease. Here, we describe the current efforts to establish novel diabetes models derived from unbiased, phenotype-driven, large-scale N-ethyl-N-nitrosourea (ENU) mouse mutagenesis projects started a decade ago using hyperglycemia as a high-throughput screen parameter. Mouse lines were established according to their hyperglycemia phenotype over several generations, thereby revealing a mutation as cause for the aberrant phenotype. Chromosomal assignment of the causative mutation and subsequent candidate gene analysis led to the detection of the mutations that resulted in novel alleles of genes already known to be involved in glucose homeostasis, like glucokinase, insulin 2, and insulin receptor. Additional ENU-induced hyperglycemia lines are under genetic analysis. Improvements in screen for diabetic animals are implemented to detect more subtle phenotypes. Moreover, diet challenge assays are being employed to uncover interactions between genetic and environmental factors in the pathogenesis of diabetes mellitus. The new mouse mutants recovered in phenotype-driven ENU mouse mutagenesis projects complement the available models generated by targeted mutagenesis of candidate genes, all together providing the large resource of models required for a systematic dissection of the pathogenesis of diabetes mellitus.

Comparative genetic analysis: the utility of mouse genetic systems for studying human monogenic disease

One of the long-term goals of mutagenesis programs in the mouse has been to generate mutant lines to facilitate the functional study of every mammalian gene. With a combination of complementary genetic approaches and advances in technology, this aim is slowly becoming a reality. One of the most important features of this strategy is the ability to identify and compare a number of mutations in the same gene, an allelic series. With the advent of gene-driven screening of mutant archives, the search for a specific series of interest is now a practical option. This review focuses on the analysis of multiple mutations from chemical mutagenesis projects in a wide variety of genes and the valuable functional information that has been obtained from these studies. Although gene knockouts and transgenics will continue to be an important resource to ascertain gene function, with a significant proportion of human diseases caused by point mutations, identifying an allelic series is becoming an equally efficient route to generating clinically relevant and functionally important mouse models.