Increased physical activity cosegregates with higher intake of carbohydrate and total calories in a subcongenic mouse strain

Profound differences in fat versus carbohydrate preferences in CAST/EiJ and C57BL/6J mice: Role of fat taste

In a nutrient self-selection study, CAST/EiJ mice consumed more carbohydrate than fat while C57BL/6J (B6) mice showed the opposite preference. The present study revealed similar strain differences in preferences for isocaloric fat (Intralipid) and carbohydrate (sucrose, maltodextrin) solutions in chow-fed mice. In initial 2-day choice tests, percent fat intakes of CAST and B6 mice were 4-9% and 71-81% respectively. In subsequent nutrient vs. water tests, CAST mice consumed considerably less fat but not carbohydrate compared to B6 mice. Orosensory rather than postoral factors are implicated in the very low fat preference and intake of CAST mice. This is supported by results of a choice test with Intralipid mixed with non-nutritive sweeteners vs. non-sweet maltodextrin. The preference of CAST mice for sweetened fat exceeded that of B6 mice (94 vs. 74%) and absolute fat intakes were similar in the two strains. When given unsweetened Intralipid vs. water tests at ascending fat concentrations CAST mice displayed reduced fat preferences at 0.1-5% and reduced intakes at 0.5-5% concentrations, compared to B6 mice. The differential fat preferences of CAST and B6 mice may reflect differences in fat taste sensing or in central neural processes related to fat selection.

Genetics of metabolic syndrome: potential clues from wild-derived inbred mouse strains

The metabolic syndrome (MetS) is a complex constellation of metabolic abnormalities including obesity, abnormal glucose metabolism, dyslipidemia, and elevated blood pressure that together substantially increase risk for cardiovascular disease and Type 2 diabetes. Both genetic and environmental factors contribute to the development of MetS, but this process is still far from understood. Human studies have revealed only part of the underlying basis. Studies in mice offer many strengths that can complement human studies to help elucidate the etiology and pathophysiology of MetS. Here we review the ways mice can contribute to MetS research. In particular, we focus on the information that can be obtained from studies of the inbred strains, with specific focus on the phenotypes of the wild-derived inbred strains. These are newly derived inbred strains that were created from wild-caught mice. They contain substantial genetic variation that is not present in the classical inbred strains, have phenotypes of relevance for MetS, and various mouse strain resources have been created to facilitate the mining of this new genetic variation. Thus studies using wild-derived inbred strains hold great promise for increasing our understanding of MetS.

Gene expression in salivary glands: effects of diet and mouse chromosome 17 locus regulating macronutrient intake

Dcpp2, Prrt1, and Has1 are plausible candidate genes for the Mnic1 (macronutrient intake-carbohydrate) locus on mouse chromosome 17, based on their map positions and sequence variants, documented expression in salivary glands, and the important role of saliva in oral food processing and taste. We investigated the effects of genotype and diet on gene expression in salivary glands (parotid, submandibular, sublingual) of carbohydrate-preferring, C57BL6J.CAST/EiJ-17.1 subcongenic mice compared to fat-preferring wild-type C57BL/6J. To achieve accurate normalization of real-time quantitative RT-PCR data, we evaluated multiple reference genes to identify the most stably expressed control genes in salivary gland tissues, and then used geometric averaging to produce a reliable normalization factor. Gene expression was measured in mice fed different diets: (1) rodent chow, (2) macronutrient selection diets, (3) high-fat diet, and (4) low-fat diet. In addition, we measured salivary hyaluronan concentrations. All three genes showed strain differences in expression, in at least one major salivary gland, and diet effects were observed in two glands. Dcpp2 expression was limited primarily to sublingual gland, and strongly decreased in B6.CAST-17.1 subcongenic mice compared to wild-type B6, regardless of diet. In contrast, both genotype and diet affected Prrt1 and Has1 expression, in a gland-specific manner, for example, Prrt1 expression in the parotid gland alone was strongly reduced in both mouse strains when fed macronutrient selection diet compared to chow. Notably, we discovered an association between diet composition and salivary hyaluronan content. These results demonstrate robust effects of genetic background and diet composition on candidate gene expression in mouse salivary glands.

High-resolution mapping of a genetic locus regulating preferential carbohydrate intake, total kilocalories, and food volume on mouse chromosome 17

The specific genes regulating the quantitative variation in macronutrient preference and food intake are virtually unknown. We fine mapped a previously identified mouse chromosome 17 region harboring quantitative trait loci (QTL) with large effects on preferential macronutrient intake-carbohydrate (Mnic1), total kilcalories (Kcal2), and total food volume (Tfv1) using interval-specific strains. These loci were isolated in the [C57BL/6J.CAST/EiJ-17.1-(D17Mit19-D17Mit50); B6.CAST-17.1] strain, possessing a ∼ 40.1 Mb region of CAST DNA on the B6 genome. In a macronutrient selection paradigm, the B6.CAST-17.1 subcongenic mice eat 30% more calories from the carbohydrate-rich diet, ∼ 10% more total calories, and ∼ 9% more total food volume per body weight. In the current study, a cross between carbohydrate-preferring B6.CAST-17.1 and fat-preferring, inbred B6 mice was used to generate a subcongenic-derived F2 mapping population; genotypes were determined using a high-density, custom SNP panel. Genetic linkage analysis substantially reduced the 95% confidence interval for Mnic1 (encompassing Kcal2 and Tfv1) from 40.1 to 29.5 Mb and more precisely established its boundaries. Notably, no genetic linkage for self-selected fat intake was detected, underscoring the carbohydrate-specific effect of this locus. A second key finding was the separation of two energy balance QTLs: Mnic1/Kcal2/Tfv1 for food intake and a newly discovered locus regulating short term body weight gain. The Mnic1/Kcal2/Tfv1 QTL was further de-limited to 19.0 Mb, based on the absence of nutrient intake phenotypes in subcongenic HQ17IIa mice. Analyses of available sequence data and gene ontologies, along with comprehensive expression profiling in the hypothalamus of non-recombinant, cast/cast and b6/b6 F2 controls, focused our attention on candidates within the QTL interval. Zfp811, Zfp870, and Btnl6 showed differential expression and also contain stop codons, but have no known biology related to food intake regulation. The genes Decr2, Ppard and Agapt1 are more appealing candidates because of their involvement in lipid metabolism and down-regulation in carbohydrate-preferring animals.

Macronutrient selection by seven inbred mouse strains and three taste-related knockout strains

Many animals thrive when given a choice of separate sources of macronutrients. How they do this is unknown. Here, we report some studies comparing the spontaneous choices between carbohydrate- and fat-containing food sources of seven inbred mouse strains (B6, BTBR, CBA, JF1, NZW, PWD and PWK) and three mouse models with genetic ablation of taste transduction components (T1R3, ITPR3 and CALHM1). For 8days, each mouse could choose between sources of carbohydrate (CHO-P; sucrose-cornstarch) and fat (Fat-P; vegetable shortening) with each source also containing protein (casein). We found that the B6 and PWK strains markedly preferred the CHO-P diet to the Fat-P diet, the BTBR and JF1 strains markedly preferred the Fat-P diet to the CHO-P diet, and the CBA, NZW and PWD strains showed equal intakes of the two diets (by weight). Relative to their WT littermates, ITPR3 and CALHM1 KO mice had elevated Fat-P preferences but T1R3 KO mice did not. There were differences among strains in adaption to the diet choice and there were differences in response between males and females on some days. These results demonstrate the diverse responses to macronutrients of inbred mice and they point to the involvement of chemosensory detectors (but not sweetness) as contributors to macronutrient selection.

Microarray-assisted fine mapping of quantitative trait loci on chromosome 15 for susceptibility to seizure-induced cell death in mice

Prior studies with crosses of the FVB/NJ (FVB; seizure-induced cell death-susceptible) mouse and the C57BL/6J (B6; seizure-induced cell death-resistant) mouse revealed the presence of a quantitative trait locus (QTL) on chromosome 15 that influenced susceptibility to kainic acid-induced cell death (Sicd2). In an earlier study, we confirmed that the Sicd2 interval harbors gene(s) conferring strong protection against seizure-induced cell death through the creation of the FVB.B6-Sicd2 congenic strain, and created three interval-specific congenic lines (ISCLs) that encompass Sicd2 on chromosome 15 to fine-map this locus. To further localise this Sicd2 QTL, an additional congenic line carrying overlapping intervals of the B6 segment was created (ISCL-4), and compared with the previously created ISCL-1-ISCL-3 and assessed for seizure-induced cell death phenotype. Whereas all of the ISCLs showed reduced cell death associated with the B6 phenotype, ISCL-4, showed the most extensive reduction in seizure-induced cell death throughout all hippocampal subfields. In order to characterise the susceptibility loci on Sicd2 by use of this ISCL and identify compelling candidate genes, we undertook an integrative genomic strategy of comparing exon transcript abundance in the hippocampus of this newly developed chromosome 15 subcongenic line (ISCL-4) and FVB-like littermates. We identified 10 putative candidate genes that are alternatively spliced between the strains and may govern strain-dependent differences in susceptibility to seizure-induced excitotoxic cell death. These results illustrate the importance of identifying transcriptomics variants in expression studies, and implicate novel candidate genes conferring susceptibility to seizure-induced cell death.

Macronutrient choice of BTBR.NZW mice congenic for a 21-gene region of chromosome 17

There has been scant work to investigate the mechanisms influencing macronutrient selection by mice. Here, we measured the consumption and choice of carbohydrate- and fat-containing diets by NZW/LacJ (NZW) and BTBR/T+ tf/J (BTBR) strains. We found that NZW mice voluntarily ate more carbohydrate and less fat than did BTBR mice. Mice with a BTBR background and a heterozygous (BTBR/NZW) congenic region on chromosome 17 between 25.7 and 27.5 Mb (N10 generation) or 26.7 and 27.5 Mb (N12 generation) also ate more carbohydrate and less fat than did homozygous (BTBR/BTBR) littermate controls. Of the 21 known and predicted genes in the congenic interval between 26.7 and 27.5 Mb, we raise for consideration as a causative candidate Itpr3, the inositol triphosphate receptor type 3 gene, which is a component of the GPCR-mediated taste transduction cascade. We speculate that a mutation in Itpr3 influences food choice by impairing the detection of nutrients in the macronutrient-containing diets.

Functional genomic architecture of predisposition to voluntary exercise in mice: expression QTL in the brain

The biological basis of voluntary exercise is complex and simultaneously controlled by peripheral (ability) and central (motivation) mechanisms. The accompanying natural reward, potential addiction, and the motivation associated with exercise are hypothesized to be regulated by multiple brain regions, neurotransmitters, peptides, and hormones. We generated a large (n = 815) advanced intercross line of mice (G(4)) derived from a line selectively bred for increased wheel running (high runner) and the C57BL/6J inbred strain. We previously mapped multiple quantitative trait loci (QTL) that contribute to the biological control of voluntary exercise levels, body weight, and composition, as well as changes in body weight and composition in response to short-term exercise. Currently, using a subset of the G(4) population (n = 244), we examined the transcriptional landscape relevant to neurobiological aspects of voluntary exercise by means of global mRNA expression profiles from brain tissue. We identified genome-wide expression quantitative trait loci (eQTL) regulating variation in mRNA abundance and determined the mode of gene action and the cis- and/or trans-acting nature of each eQTL. Subsets of cis-acting eQTL, colocalizing with QTL for exercise or body composition traits, were used to identify candidate genes based on both positional and functional evidence, which were further filtered by correlational and exclusion mapping analyses. Specifically, we discuss six plausible candidate genes (Insig2, Socs2, DBY, Arrdc4, Prcp, IL15) and their potential role in the regulation of voluntary activity, body composition, and their interactions. These results develop a potential initial model of the underlying functional genomic architecture of predisposition to voluntary exercise and its effects on body weight and composition within a neurophysiological framework.

Phosphodiesterases in endocrine physiology and disease

The cAMP-protein kinase A pathway plays a central role in the development and physiology of endocrine tissues. cAMP mediates the intracellular effects of numerous peptide hormones. Various cellular and molecular alterations of the cAMP-signaling pathway have been observed in endocrine diseases. Phosphodiesterases (PDEs) are key regulatory enzymes of intracellular cAMP levels. Indeed, PDEs are the only known mechanism for inactivation of cAMP by catalysis to 5'-AMP. It has been suggested that disruption of PDEs could also have a role in the pathogenesis of many endocrine diseases. This review summarizes the most recent advances concerning the role of the PDEs in the physiopathology of endocrine diseases. The potential significance of this knowledge can be easily envisaged by the development of drugs targeting specific PDEs.

Architecture of energy balance traits in emerging lines of the Collaborative Cross

The potential utility of the Collaborative Cross (CC) mouse resource was evaluated to better understand complex traits related to energy balance. A primary focus was to examine if genetic diversity in emerging CC lines (pre-CC) would translate into equivalent phenotypic diversity. Second, we mapped quantitative trait loci (QTL) for 15 metabolism- and exercise-related phenotypes in this population. We evaluated metabolic and voluntary exercise traits in 176 pre-CC lines, revealing phenotypic variation often exceeding that seen across the eight founder strains from which the pre-CC was derived. Many phenotypic correlations existing within the founder strains were no longer significant in the pre-CC population, potentially representing reduced linkage disequilibrium (LD) of regions harboring multiple genes with effects on energy balance or disruption of genetic structure of extant inbred strains with substantial shared ancestry. QTL mapping revealed five significant and eight suggestive QTL for body weight (Chr 4, 7.54 Mb; CI 3.32-10.34 Mb; Bwq14), body composition, wheel running (Chr 16, 33.2 Mb; CI 32.5-38.3 Mb), body weight change in response to exercise (1: Chr 6, 77.7Mb; CI 72.2-83.4 Mb and 2: Chr 6, 42.8 Mb; CI 39.4-48.1 Mb), and food intake during exercise (Chr 12, 85.1 Mb; CI 82.9-89.0 Mb). Some QTL overlapped with previously mapped QTL for similar traits, whereas other QTL appear to represent novel loci. These results suggest that the CC will be a powerful, high-precision tool for examining the genetic architecture of complex traits such as those involved in regulation of energy balance.

Glyoxalase in diabetes, obesity and related disorders

Diabetes was the first disease state where evidence emerged for increased formation of methylglyoxal. Metabolism of methylglyoxal by the glyoxalase system has been linked to the development of vascular complications of diabetes - nephropathy, retinopathy, neuropathy and cardiovascular disease. Increased formation of methylglyoxal in hyperglycaemia associated with diabetes and down regulation of glyoxalase 1 by inflammatory signalling in vascular cells leads to a marked increased modification of proteins by methylglyoxal to form advanced glycation endproducts at the sites of vascular complications. Hotspot protein targets of methylglyoxal that suffer functional impairment - the dicarbonyl proteome - likely play a key role in the mechanisms underlying the development of vascular complications in diabetes: particularly modification of integrin binding sites in extracellular matrix proteins leading to endothelial cell shedding and anoikis, modification of mitochondrial proteins and increased formation of reaction oxygen species, and modification of apolipoprotein B100 of low density lipoprotein leading to its increased atherogenicity. Some current therapeutic agents counter partially dysfunctional metabolism of methylglyoxal by the glyoxalase system in diabetes - including the recent development of high dose thiamine therapy for early stage diabetic nephropathy. Further pharmacologic strategies are required to overcome the down regulation of glyoxalase1 in diabetes. The glyoxalase system is likely to be a continuing and future focus for research on clinical biomarkers and therapeutic development for respectively assessment of metabolic control and prevention of vascular complications in diabetes and obesity.

The biological control of voluntary exercise, spontaneous physical activity and daily energy expenditure in relation to obesity: human and rodent perspectives

Mammals expend energy in many ways, including basic cellular maintenance and repair, digestion, thermoregulation, locomotion, growth and reproduction. These processes can vary tremendously among species and individuals, potentially leading to large variation in daily energy expenditure (DEE). Locomotor energy costs can be substantial for large-bodied species and those with high-activity lifestyles. For humans in industrialized societies, locomotion necessary for daily activities is often relatively low, so it has been presumed that activity energy expenditure and DEE are lower than in our ancestors. Whether this is true and has contributed to a rise in obesity is controversial. In humans, much attention has centered on spontaneous physical activity (SPA) or non-exercise activity thermogenesis (NEAT), the latter sometimes defined so broadly as to include all energy expended due to activity, exclusive of volitional exercise. Given that most people in Western societies engage in little voluntary exercise, increasing NEAT may be an effective way to maintain DEE and combat overweight and obesity. One way to promote NEAT is to decrease the amount of time spent on sedentary behaviours (e.g. watching television). The effects of voluntary exercise on other components of physical activity are highly variable in humans, partly as a function of age, and have rarely been studied in rodents. However, most rodent studies indicate that food consumption increases in the presence of wheels; therefore, other aspects of physical activity are not reduced enough to compensate for the energetic cost of wheel running. Most rodent studies also show negative effects of wheel access on body fat, especially in males. Sedentary behaviours per se have not been studied in rodents in relation to obesity. Several lines of evidence demonstrate the important role of dopamine, in addition to other neural signaling networks (e.g. the endocannabinoid system), in the control of voluntary exercise. A largely separate literature points to a key role for orexins in SPA and NEAT. Brain reward centers are involved in both types of physical activities and eating behaviours, likely leading to complex interactions. Moreover, voluntary exercise and, possibly, eating can be addictive. A growing body of research considers the relationships between personality traits and physical activity, appetite, obesity and other aspects of physical and mental health. Future studies should explore the neurobiology, endocrinology and genetics of physical activity and sedentary behaviour by examining key brain areas, neurotransmitters and hormones involved in motivation, reward and/or the regulation of energy balance.

Identifying quantitative trait loci (QTLs) and genes (QTGs) for alcohol-related phenotypes in mice

Alcoholism is a complex clinical disorder with genetic and environmental contributions. Although no animal model duplicates alcoholism, models for specific factors, such as the withdrawal syndrome, are useful to identify potential genetic determinants of liability in humans. Murine models have been invaluable to identify quantitative trait loci (QTLs) that influence a variety of alcohol responses. However, the QTL regions are typically large, at least initially, and contain numerous genes, making identification of the causal quantitative trait gene(s) (QTGs) challenging. Here, we present QTG identification strategies currently used in the field of alcohol genetics and discuss relevance to alcoholic human populations.