Fatty liver and obesity: phenotypically correlated but genetically distinct traits in a mouse model of type 2 diabetes

Novel loci for hyperglycemia identified by QTL mapping of longitudinal phenotypes and congenic analysis

We previously reported that four hyperglycemia loci are located on three chromosomes in the Nagoya-Shibata-Yasuda (NSY) mouse model, commonly used to study type 2 diabetes. However, we did not search for hyperglycemia loci across all chromosomes. In this study, we performed quantitative trait loci (QTLs) mapping of longitudinal phenotypes from crosses between NSY (hyperglycemic) and C3H (normoglycemic) mice. We identified four new QTLs for hyperglycemia, namely Nidd5nsy, Nidd6nsy, Nidd1c3h, and Nidd2c3h, on Chromosome 1, 4, 10, and 13, respectively. These QTLs were associated with hyperglycemia in young mice and had attenuated effects in older mice. Nidd5nsy and Nidd6nsy were hyperglycemic with NSY alleles, and Nidd1c3h and Nidd2c3h were hyperglycemic with C3H alleles. We further bred Nidd5nsy congenic mice and demonstrated that Nidd5nsy has a strong effect on hyperglycemia when young, accompanied by insulin resistance and visceral fat accumulation. These results showed that the effects of individual QTLs strengthened or weakened with age, and that the sum of the effects of QTLs captured the age-related deterioration of glucose tolerance in individuals. Our results support the importance of longitudinal phenotypes in the genetic analysis of polygenic traits and have implications for the genetic basis and pathogenesis of type 2 diabetes in humans.

Genetic connection of carotid atherosclerosis with coat color and body weight in an intercross between hyperlipidemic mouse strains

Atherosclerosis in the carotid artery is a major cause of ischemic stroke and has a strong genetic component. The aim of this study was to identify genetic factors contributing to carotid atherosclerosis. 154 female F2mice were generated from an intercross between LP/J and BALB/cJ Apoe-null (Apoe^-/-) mice and fed 12 weeks of Western diet. Atherosclerotic lesions, body weight, and coat color were measured and genotyping was performed using miniMUGA arrays. A significant QTL on chromosome (Chr) 7, named Cath20, and 5 suggestive QTL on Chr 6, 12, 13, 15, and X were identified for carotid lesions. Three significant QTL, Bwfq2, Bw1n, Bwtq6, on Chr 2, 7, and 15 were identified for body weight. Two significant QTL, Chop2 and Albc2, on Chr 4 and 7 were identified for coat color, with Tyr, encoding tyrosinase, being the causal gene of Albc2. Cath20 coincided precisely with QTL Bw1n and Albc2 on Chr7. Carotid lesion sizes were significantly correlated with body weight and graded coat color in F2 mice. Cath20 on Chr7 disappeared after adjustment for coat color but remained after adjustment for body weight. These results demonstrate genetic connections of carotid atherosclerosis with body weight and coat color in hyperlipidemic mice and suggest a potential role for Tyr in carotid atherosclerosis.

Selectively Bred Diabetes Models: GK Rats, NSY Mice, and ON Mice

The polygenic background of selectively bred diabetes models mimics the etiology of type 2 diabetes. So far, three different rodent models (Goto-Kakizaki rats, Nagoya-Shibata-Yasuda mice, and Oikawa-Nagao mice) have been established in the diabetes research field by continuous selective breeding for glucose tolerance from outbred rodent stocks. The origin of hyperglycemia in these rodents is mainly insulin secretion deficiency from the pancreatic β-cells and mild insulin resistance in insulin target organs. In this chapter, we summarize backgrounds and phenotypes of these rodent models to highlight their importance in diabetes research. Then, we introduce experimental methodologies to evaluate β-cell exocytosis as a putative common defect observed in these rodent models.

Determination of peripheral neuropathy in high-fat diet fed low-dose streptozotocin-treated female C57Bl/6J mice and Sprague-Dawley rats

AIMS/INTRODUCTION

Peripheral neuropathy is a common complication of diabetes and also occurs in 30% of human obese individuals with impaired glucose tolerance. Even though peripheral neuropathy affects both sexes, most pre-clinical studies have been carried out using male rodents. The aim of the present study was to create diet-induced obesity and type 2 diabetes in female rats and mice in order to examine the development of peripheral neuropathy.

MATERIALS AND METHODS

At 12 weeks-of-age, rats and mice were separated into three groups. Two groups or rats and mice were fed a 60-kcal% high-fat diet for 12 weeks (rats) or 8 weeks (mice). To induce type 2 diabetes, one group of high-fat diet-fed rats and mice were treated with a low dose of streptozotocin. Analyses of multiple neural end-points were carried out 12 weeks later.

RESULTS

Glucose utilization was impaired in diet-induced obese female rats and mice, as was a number of neurological end-points including nerve conduction velocity, intraepidermal and subepithelial corneal nerve fiber densities, and thermal and mechanical sensitivity. When female diet-induced obese rats or mice were made hyperglycemic, glucose utilization and sensory nerve density of the skin and cornea, as well as thermal and mechanical sensitivity, were more significantly impaired compared with diet-induced obese female rodents.

CONCLUSIONS

These studies show that diet-induced obese and type 2 diabetic female rodents develop peripheral neuropathy that is similar to that occurring in male rodents. However, for female rats, more aggressive treatment is required to induce dietary obesity.

Verification That Mouse Chromosome 14 Is Responsible for Susceptibility to Streptozotocin in NSY Mice

INTRODUCTION

Streptozotocin- (STZ-) induced diabetes is under polygenic control, and the genetic loci for STZ susceptibility are mapped to chromosome (Chr) 11 in Nagoya-Shibata-Yasuda (NSY) mice. In addition to Chr11, other genes on different chromosomes may contribute to STZ susceptibility in NSY mice. The aim of this study was to determine whether NSY-Chr14 contributes to STZ susceptibility and contains the STZ-susceptible region.

MATERIALS AND METHODS

A consomic C3H-14^NSY strain (R0: homozygous for NSY-derived whole Chr14 on the control C3H background), two congenic strains (R1: the region retained proximal and middle segments of NSY-Chr14 and R2: the region retained a proximal segment of NSY-Chr14), and parental NSY and C3H mice were intraperitoneally injected with a single injection of STZ at a dose of 175 mg/kg body weight at 12 weeks of age. Blood glucose levels and body weights were measured at days 0, 1, 2, 4, 5, 7, 8, and 14 after STZ injection. At day 14 after STZ injection, pancreata were dissected and fixed.

RESULTS

After STZ injection, blood glucose levels were significantly higher in R0 mice than in C3H mice. However, blood glucose levels in R0 mice were not as severely affected as those in NSY mice. In R1 and R2 mice, blood glucose levels were similar to those in C3H mice and were significantly lower than those in R0 mice. Body weights were decreased in NSY and R0 mice; however, this change was not observed in R1, R2, and C3H mice. Although islet tissues in all strains exhibited degeneration and cellular infiltration, histological changes in NSY and R0 mice were more severe than those in R1, R2, and C3H mice.

CONCLUSIONS

These data demonstrated that NSY-Chr14 was a STZ-susceptible chromosome and that STZ susceptibility was mapped to the distal segment of NSY-Chr14.

Genetic dissection of the fatty liver QTL Fl1sa by using congenic mice and identification of candidate genes in the liver and epididymal fat

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a multifactorial disease caused by interactions between environmental and genetic factors. The SMXA-5 mouse is a high-fat diet-induced fatty liver model established from SM/J and A/J strains. We have previously identified Fl1sa, a quantitative trait locus (QTL) for fatty liver on chromosome 12 (centromere-53.06 Mb) of SMXA-5 mice. However, the chromosomal region containing Fl1sa was too broad. The aim of this study was to narrow the Fl1sa region by genetic dissection using novel congenic mice and to identify candidate genes within the narrowed Fl1sa region.

RESULTS

We established two congenic strains, R2 and R3, from parental A/J-12^SM and A/J strains. R2 and R3 strains have genomic intervals of centromere-29.20 Mb and 29.20-46.75 Mb of chromosome 12 derived from SM/J, respectively. Liver triglyceride content in R2 and R3 mice was significantly lower than that in A/J mice fed with a high-fat diet for 7 weeks. This result suggests that at least one of the genes responsible for fatty liver exists within the two chromosomal regions centromere-29.20 Mb (R2) and 29.20-46.75 Mb (R3). We found that liver triglyceride accumulation is inversely correlated with epididymal fat weight among the parental and congenic strains. Therefore, the ectopic fat accumulation in the liver may be due to organ-organ interactions between the liver and epididymal fat. To identify candidate genes in Fl1sa, we performed a DNA microarray analysis using the liver and epididymal fat in A/J and A/J-12^SM mice fed with a high-fat diet for 7 weeks. In epididymal fat, mRNA levels of Zfp125 (in R2) and Nrcam (in R3) were significantly different in A/J-12^SM mice from those in A/J mice. In the liver, mRNA levels of Iah1 (in R2) and Rrm2 (in R2) were significantly different in A/J-12^SM mice from those in A/J mice.

CONCLUSIONS

In this study, using congenic mice analysis, we narrowed the chromosomal region containing Fl1sa to two regions of mouse chromosome 12. We then identified 4 candidate genes in Fl1sa: Iah1 and Rrm2 from the liver and Zfp125 and Nrcam from epididymal fat.

Genetic dissection of susceptibility genes for diabetes and related phenotypes on mouse chromosome 14 by means of congenic strains

Background

A susceptibility locus, Nidd2n, for type 2 diabetes has been mapped to mouse chromosome 14 (Chr 14) and confirmed using the consomic strain (C3H-Chr 14^NSY) of the Nagoya-Shibata-Yasuda (NSY) mouse, an animal model of spontaneous type 2 diabetes. The aim of this study was to localize and characterize Nidd2n.

Results

We constructed two novel congenic strains homozygous for different segments of NSY-Chr 14 on the control C3H/HeNcrj (C3H) background: R1 (C3H.NSY-(D14Mit206-D14Mit5)) possesses the proximal and middle segment, and R2 (C3H.NSY-(D14Mit206-D14Mit186)) possesses the most proximal segment of NSY-Chr 14. Diabetes-related phenotypes were studied in comparison with those of consomic C3H-Chr 14^NSY (R0) and parental NSY and C3H strains. Congenic R1 and R2 showed significantly higher post-challenge glucose than that in C3H mice. Fasting glucose, in contrast, was significantly lower in R1 and R2 than in C3H mice. Insulin sensitivity was significantly impaired in R1 and R2 compared to C3H mice. R2 showed significantly higher body weight and fat-pad weight than those in C3H and R1. Leptin level was significantly higher in R0, R1 and R2 than in C3H mice, with R2 showing the highest level, similar to that in NSY mice. Serum adiponectin level was significantly lower in R0, R1 and R2 than in C3H mice, while it was significantly higher in NSY than in C3H mice.

Conclusions

These data indicate that Chr 14 harbors multiple genes for diabetes-related phenotypes. The original Nidd2n, which is located in the middle region of Chr 14, was divided into two segments; Nidd2.1n in proximal Chr 14 and Nidd2.2n in distal Chr 14. Nidd2.1n contributes to post-challenge hyperglycemia, insulin resistance and adiposity. Nidd2.2n contributes to fasting as well as post-challenge hyperglycemia and insulin resistance. Adp1n, which contributes to decreased adiposity and increased insulin sensitivity, rather than a diabetogenic gene, was mapped in the middle segment.

Bhlhe40, a potential diabetic modifier gene on Dbm1 locus, negatively controls myocyte fatty acid oxidation

We have previously identified significant quantitative trait loci (QTL) Dbm1 (diabetic modifier QTL 1) on chromosome 6, affecting plasma glucose and insulin concentrations and body weight on F(2) progeny of hypoinsulinemic diabetic Akita mice, with the heterozygous Ins2 gene Cys96Tyr mutation, and non-diabetic A/J mice. To discover diabetic modifier genes on Dbm1, we constructed congenic strain for Dbm1 using the Akita allele as donor in A/J allele genetic background, and compared diabetes-related phenotypes to control mice. The homozygote for Akita allele of Dbm1 was associated with lower plasma glucose concentrations in glucose tolerance test (GTT) in the hypoinsulinemic condition derived from the Ins2 mutation and lower plasma insulin concentrations and body weight in the normoinsulinemic condition without the Ins2 mutation than the homozygote for A/J allele, as we performed QTL analysis on F(2) intercross mice. The Akita allele also decreased the epididymal white adipose tissue (EWAT) weight. According to the analysis of sub-congenic strains, we narrowed down the responsible diabetic modifier region to 9 Mb. As fourteen candidate genes exist in this region, we analyzed genomic variants of these genes and gene expression in the muscle, liver, and EWAT and identified that Bhlhe40 gene expression in muscle is decreased in congenic mice. According to the in vitro functional analyses, Bhlhe40 was shown to negatively control fatty acid oxidation in cultured myocyte. Based on these, we conclude that Bhlhe40 is a possible candidate diabetic modifier gene responsible for Dbm1 locus affecting diabetes and/or obesity through negatively controlling fatty acid oxidation in muscle.

Analysis of hepatic gene expression profile in a spontaneous mouse model of type 2 diabetes under a high sucrose diet

Both genetic factors and diabetogenic environmental factors, such as a high-sucrose diet (HSD), are involved in the development of type 2 diabetes. In this study, the Nagoya-Shibata-Yasuda (NSY) mouse, an animal model of type 2 diabetes and C3H mice used as controls, were fed a HSD, a high-fat diet (HFD) or a regular diet (RD) from weaning. In C3H mice, HFD significantly increased body weight gain, but maintained glucose tolerance. In contrast, in NSY mice, HSD resulted in increased body weight gain and liver steatosis and increased glucose intolerance to a greater extent than HFD. Furthermore, we performed DNA microarray analysis to detect differences in hepatic gene expression levels in both strains under HSD. We then performed RT-PCR analysis on selected genes to evaluate basal expression level under RD and changes under HSD conditions. HSD-fed NSY, but not C3H mice, exhibited increased hepatic expression levels of Pparg2, an isoform of Pparg as well as G0s2, a target of Pparg, which are known to be adipocyte-specific genes. Compared to RD-fed C3H mice, hepatic expression levels of Kat2b (transcriptional regulation), Hsd3b5 (steroid hormone metabolism) and Cyp7b1 (bile acid metabolism) were initially lower in RD-fed NSY mice, and were further decreased in HSD-fed NSY mice. Expression of Metallothionein (Mt1) and Metallothionein 2 (Mt2) was significantly lower in NSY mice compared to C3H mice, irrespective of dietary condition. These data suggest that elucidation of this heterogeneity in response to HSD might contribute to further understanding of the gene-environment interactions leading to diabetes in humans.

Dose effect and mode of inheritance of diabetogenic gene on mouse chromosome 11

The quantitative trait locus (QTL) mapping in segregating crosses of NSY (Nagoya-Shibata-Yasuda) mice, an animal model of type 2 diabetes, with nondiabetic strain C3H/He mice has identified diabetogenic QTLs on multiple chromosomes. The QTL on chromosome 11 (Chr11) (Nidd1n) showing the largest effect on hyperglycemia was confirmed by our previous studies with homozygous consomic mice, C3H-11(NSY), in which the NSY-derived whole Chr11 was introgressed onto control C3H background genes. C3H-11(NSY) mice also showed a streptozotocin (STZ) sensitivity. In the present study, we constructed heterozygous C3H-11(NSY) mice and the phenotypes were analyzed in detail in comparison with those of homozygous C3H-11(NSY) and C3H mice. Heterozygous C3H-11(NSY) mice had significantly higher blood glucose levels and STZ sensitivity than those in C3H mice. Hyperglycemia and STZ sensitivity in heterozygous C3H-11(NSY) mice, however, were not as severe as in homozygous C3H-11(NSY) mice. The body weight and fat pad weight in heterozygous C3H-11(NSY) mice were similar to those in C3H and homozygous C3H-11(NSY) mice. These data indicated that the introgression of Chr11 of the diabetes-susceptible NSY strain onto diabetes-resistant C3H caused marked changes in the glucose tolerance and STZ susceptibility even in a heterozygous state, and suggested that the mode of inheritance of a gene or genes on Chr11 for hyperglycemia and STZ sensitivity is additive.

Wx/ae double-mutant brown rice prevents the rise in plasma lipid and glucose levels in mice

A wx/ae double-mutant rice is generated by crossing waxy mutant and amylose-extender mutant in rice. Wx/ae brown rice contains highly beneficial nutrients for lipid and glucose metabolism, including resistant starch, dietary fiber, and γ-oryzanol, when compared to Koshihikari brown rice, the non-waxy japonica cultivar. To examine the effects of wx/ae brown rice on glucose and lipid metabolism, type 2 diabetic NSY/Hos mice were fed a high-fat diet containing 25% of wx/ae brown rice or Koshihikari brown rice for 10 weeks. The plasma total cholesterol, non-high-density lipoprotein cholesterol, triglyceride, and non-esterified fatty acid levels of the wx/ae group were significantly lower than those of the Koshihikari group. Moreover, the fasting blood glucose level and pathological score of glycosuria of the wx/ae group were also significantly lower than those of the Koshihikari group. These results indicate that wx/ae brown rice has the potential to prevent the rise in plasma lipid and glucose levels.

Quantitative trait loci affecting liver fat content in mice

Nonalcoholic fatty liver disease, a condition in which excess fat accumulates in the liver, is strongly associated with the metabolic syndrome, including obesity and other related conditions. This disease has the potential to progress from steatosis to steatohepatitis, fibrosis, and cirrhosis. The recent increase in the prevalence of the metabolic syndrome is largely driven by changes in diet and activity levels. Individual variation in the response to this obesogenic environment, however, is attributable in part to genetic variation between individuals, but very few mammalian genetic loci have been identified with effects on fat accumulation in the liver. To study the genetic basis for variation in liver fat content in response to dietary fat, liver fat proportion was determined using quantitative magnetic resonance imaging in 478 mice from 16 LG/J X SM/J recombinant inbred strains fed either a high-fat (42% kcal from fat) or low-fat (15% kcal from fat) diet. An analysis of variance confirmed that there is a genetic basis for variation in liver fat content within the population with significant effects of sex and diet. Three quantitative trail loci that contribute to liver fat content also were mapped.

Direct evidence for susceptibility genes for type 2 diabetes on mouse chromosomes 11 and 14

AIMS/HYPOTHESIS

Diabetogenic loci for type 2 diabetes have been mapped to mouse chromosome (Chr) 11 and 14 in the Nagoya-Shibata-Yasuda (NSY) mouse, an animal model of type 2 diabetes. We aimed to obtain direct evidence of these genes on each chromosome and to clarify their function and interaction in conferring susceptibility to type 2 diabetes.

METHODS

We established three consomic strains homozygous for diabetogenic NSY-Chr11, NSY-Chr14 or both on the control C3H background (C3H-11(NSY), C3H-14(NSY) and C3H-11(NSY)14(NSY), respectively), and monitored diabetes-related phenotypes longitudinally. The glucokinase gene was sequenced as a positional candidate gene on Chr11.

RESULTS

C3H-11(NSY) mice showed hyperglycaemia associated with impaired insulin secretion and age-dependent insulin resistance without obesity. C3H-14(NSY) mice exhibited hyperglycaemia mainly due to insulin resistance, with a slight increase in percentage body fat. C3H-11(NSY)14(NSY) double consomic mice showed marked hyperglycaemia and obesity, which was not observed in single consomic strains. Sequences of the glucokinase gene were allelically variant between NSY and C3H mice.

CONCLUSIONS/INTERPRETATION

These data provide direct evidence that Chr11 and Chr14 harbour major susceptibility genes for type 2 diabetes. These two chromosomes interact to cause more severe hyperglycaemia and obesity, which was not observed with the presence of either single chromosome, indicating different modes of gene-gene interaction depending on the phenotype. Marked changes in the phenotypes retained in the consomic strains will facilitate fine mapping and the identification of the responsible genes and their interaction with each other, other genes and environmental factors.

Quantitative trait loci for impaired glucose tolerance in nondiabetic SM/J and A/J mice

The SMXA-5 recombinant inbred strain, which was established from nondiabetic parental SM/J and A/J mice, develops diabetic phenotypes such as impaired glucose tolerance. The combination of diabetogenic genes in the SM/J and A/J genomes impairs glucose tolerance in SMXA-5 mice. Using (SM/J x SMXA-5)F2 mice fed a high-fat diet, we previously detected a diabetogenic locus, T2dm2sa, on chromosome (Chr) 2. The A/J allele at this locus is diabetogenic. The SM.A-T2dm2sa congenic mouse, in which the Chr 2 region of A/J including T2dm2sa was introgressed into SM/J, showed obviously impaired glucose tolerance. These results indicate that SM.A-T2dm2sa mice develop diabetogenic traits due to T2dm2sa with the A/J allele and unknown diabetogenic loci with the SM/J allele. The aim of this study was to dissect these unknown loci, using quantitative trait locus (QTL) analysis in the (A/J x SM.A-T2dm2sa) F2 intercross fed a high-fat diet. The results revealed a highly significant QTL, T2dm4sa, for glucose tolerance on Chr 6 and a significant QTL, T2dm5sa, for glucose tolerance on Chr 11. These loci with the SM/J allele were diabetogenic. The diabetogenic effect of T2dm4sa or T2dm5sa was verified by the impairment of glucose tolerance in the A/J-6(SM) or A/J-11(SM) consomic strain, in which Chr 6 or Chr 11 of SM/J is introgressed into A/J, respectively. These results demonstrate that diabetogenic loci exist in the genomes of nondiabetic A/J and SM/J mice and suggest that T2dm2sa with the A/J allele and T2dm4sa and/or T2dm5sa with the SM/J allele elicit impaired glucose tolerance in SM.A-T2dm2sa mice.

Adrenal gland tumorigenesis after gonadectomy in mice is a complex genetic trait driven by epistatic loci

Postgonadectomy adrenocortical tumorigenesis is a strain-specific phenomenon in inbred mice, assumed to be caused by elevated LH secretion and subsequent ectopic LH receptor (LHR) overexpression in adrenal gland. However, the molecular mechanisms of this cascade of events remain unknown. In this study, we took advantage of the mouse strain dependency of the phenotype to unravel its genetic basis. Our results present the first genome-wide screening related to this pathology in two independent F2 and backcross populations generated between the neoplastic DBA/2J and the nonsusceptible C57BL/6J strains. Surprisingly, the postgonadectomy elevation of serum LH was followed by similar up-regulation of adrenal LHR expression in both parental strains and their crosses, irrespective of their tumor status, indicating that it is not the immediate cause of the tumorigenesis. Linkage analysis revealed one major significant locus for the tumorigenesis on chromosome 8, modulated by epistasis with another quantitative trait locus on chromosome 18. Weight gain, a secondary phenotype after gonadectomy, showed a significant but separate quantitative trait locus on chromosome 7. Altogether, postgonadectomy adrenocortical tumorigenesis in DBA/2J mice is a dominant trait that is not a direct consequence of adrenal LHR expression but is driven by a complex genetic architecture. Analysis of candidate genes in the tumorigenesis linkage region showed that Sfrp1 (secreted frizzled-related protein 1), a tumor suppressor gene, is differentially expressed in the neoplastic areas. These findings may have relevance to the human pathogenesis of macronodular adrenal hyperplasia and adrenocortical tumors in postmenopausal women and why some of them develop obesity.