The diabetes-prone NZO/Hl strain. II. Pancreatic immunopathology

Novel regulators of islet function identified from genetic variation in mouse islet Ca2+ oscillations

Insufficient insulin secretion to meet metabolic demand results in diabetes. The intracellular flux of Ca2+ into β-cells triggers insulin release. Since genetics strongly influences variation in islet secretory responses, we surveyed islet Ca2+ dynamics in eight genetically diverse mouse strains. We found high strain variation in response to four conditions: (1) 8 mM glucose; (2) 8 mM glucose plus amino acids; (3) 8 mM glucose, amino acids, plus 10 nM glucose-dependent insulinotropic polypeptide (GIP); and (4) 2 mM glucose. These stimuli interrogate β-cell function, α- to β-cell signaling, and incretin responses. We then correlated components of the Ca2+ waveforms to islet protein abundances in the same strains used for the Ca2+ measurements. To focus on proteins relevant to human islet function, we identified human orthologues of correlated mouse proteins that are proximal to glycemic-associated single-nucleotide polymorphisms in human genome-wide association studies. Several orthologues have previously been shown to regulate insulin secretion (e.g. ABCC8, PCSK1, and GCK), supporting our mouse-to-human integration as a discovery platform. By integrating these data, we nominate novel regulators of islet Ca2+ oscillations and insulin secretion with potential relevance for human islet function. We also provide a resource for identifying appropriate mouse strains in which to study these regulators.

Alternative exon splicing and differential expression in pancreatic islets reveals candidate genes and pathways implicated in early diabetes development

Type 2 diabetes (T2D) has a strong genetic component. Most of the gene variants driving the pathogenesis of T2D seem to target pancreatic β-cell function. To identify novel gene variants acting at early stage of the disease, we analyzed whole transcriptome data to identify differential expression (DE) and alternative exon splicing (AS) transcripts in pancreatic islets collected from two metabolically diverse mouse strains at 6 weeks of age after three weeks of high-fat-diet intervention. Our analysis revealed 1218 DE and 436 AS genes in islets from NZO/Hl vs C3HeB/FeJ. Whereas some of the revealed genes present well-established markers for β-cell failure, such as Cd36 or Aldh1a3, we identified numerous DE/AS genes that have not been described in context with β-cell function before. The gene Lgals2, previously associated with human T2D development, was DE as well as AS and localizes in a quantitative trait locus (QTL) for blood glucose on Chr.15 that we reported recently in our N₂(NZOxC3H) population. In addition, pathway enrichment analysis of DE and AS genes showed an overlap of only half of the revealed pathways, indicating that DE and AS in large parts influence different pathways in T2D development. PPARG and adipogenesis pathways, two well-established metabolic pathways, were overrepresented for both DE and AS genes, probably as an adaptive mechanism to cope for increased cellular stress. Our results provide guidance for the identification of novel T2D candidate genes and demonstrate the presence of numerous AS transcripts possibly involved in islet function and maintenance of glucose homeostasis.

Acylcarnitine Profiles in Plasma and Tissues of Hyperglycemic NZO Mice Correlate with Metabolite Changes of Human Diabetes

The New Zealand obese (NZO) mouse is a polygenic model for obesity and diabetes with obese females and obese, diabetes-prone males, used to study traits of the metabolic syndrome like type 2 diabetes mellitus (T2DM), obesity, and dyslipidaemia. By using LC-MS/MS, we here examine the suitability of this model to mirror tissue-specific changes in acylcarnitine (AC) and amino acid (AA) species preceding T2DM which may reflect patterns investigated in human metabolism. We observed high concentrations of fatty acid-derived ACs in 11 female mice, high abundance of branched-chain amino acid- (BCAA-) derived ACs in 6 male mice, and slight increases in BCAA-derived ACs in the remaining 6 males. Principal component analysis (PCA) including all ACs and AAs confirmed our hypothesis especially in plasma samples by clustering females, males with high BCAA-derived ACs, and males with slight increases in BCAA-derived ACs. Concentrations of insulin, blood glucose, NEFAs, and triacylglycerols (TAGs) further supported the hypothesis of high BCAA-derived ACs being able to mirror the onset of diabetic traits in male individuals. In conclusion, alterations in AC and AA profiles overlap with observations from human studies indicating the suitability of NZO mice to study metabolic changes preceding human T2DM.

Pathological and gene expression analysis of a polygenic diabetes model, NONcNZO10/LtJ mice

The NONcNZO10/LtJ mouse is a polygenic model of type-2 diabetes (T2D) that shows moderate obesity and diabetes, and is regarded as a good model reflective of the conditions of human T2D. In this study, we analyzed pathological changes of pancreases with the progress of time by using histopathology and gene expression analysis, including microRNA. A number of gene expression changes associated with decreased insulin secretion (possibly regulated by miR-29a/b) were observed, and zinc homeostasis (Slc30a8, Mt1 and Mt2) or glucose metabolism (Slc2a2) was suggested as being the candidate mechanism of pancreas failure in NONcNZO10/LtJ mice. These results demonstrate NONcNZO10/LtJ mice have a complex pathogenic mechanism of diabetes, and moreover, this fundamental information of NONcNZO10/LtJ mice would offer the opportunity for research and development of a novel antidiabetic drug.

Insights into obesity and diabetes at the intersection of mouse and human genetics

Many of our insights into obesity and diabetes come from studies in mice carrying natural or induced mutations. In parallel, genome-wide association studies (GWAS) in humans have identified numerous genes that are causally associated with obesity and diabetes, but discovering the underlying mechanisms required in-depth studies in mice. We discuss the advantages of studying natural variation in mice and summarize several examples where the combination of human and mouse genetics opened windows into fundamental physiological pathways. A noteworthy example is the melanocortin-4 receptor (MC4R) and its role in energy balance. The pathway was delineated by discovering the gene responsible for the Agouti mutation in mice. With more targeted phenotyping, we predict that additional pathways relevant to human pathophysiology will be discovered.

Chickens from lines artificially selected for juvenile low and high body weight differ in glucose homeostasis and pancreas physiology

Artificial selection of White Plymouth Rock chickens for juvenile (day 56) body weight resulted in two divergent genetic lines: hypophagic low weight (LWS) chickens and hyperphagic obese high weight (HWS) chickens, with the latter more than 10-fold heavier than the former at selection age. A study was designed to investigate glucose regulation and pancreas physiology at selection age in LWS chickens and HWS chickens. Oral glucose tolerance and insulin sensitivity tests revealed differences in threshold sensitivity to insulin and glucose clearance rate between the lines. Results from real-time PCR showed greater pancreatic mRNA expression of four glucose regulatory genes (preproinsulin, PPI; preproglucagon, PPG; glucose transporter 2, GLUT2; and pancreatic duodenal homeobox 1, Pdx1) in LWS chickens, than HWS chickens. Histological analysis of the pancreas revealed that HWS chickens have larger pancreatic islets, less pancreatic islet mass, and more pancreatic inflammation than LWS chickens, all of which presumably contribute to impaired glucose metabolism.

Insulin therapy for pre-hyperglycemic beta-cell endoplasmic reticulum crowding

Insulin therapy improves β-cell function in early stages of diabetes by mechanisms that may exceed alleviation of glucotoxicity. In advance type 2 diabetes, hyperglycemia causes β-cell damage and ultimately β-cell loss. At such an advanced stage, therapeutic modalities are often inadequate. Growing evidence indicates that in early stages of type-2 diabetes and some types of monogenic diabetes linked with malfunctioning endoplasmic-reticulum (ER), the β-cell ER fails to process sufficient proinsulin once it becomes overloaded. These changes manifest with ER distention (ER-crowding) and deficiency of secretory granules. We hypothesize that insulin therapy may improves β-cell function by alleviating ER-crowding. To support this hypothesis, we investigated pre-diabetic β-cell changes in hProC(A7)Y-CpepGFP transgenic mice that develop prolonged pre-diabetes due to proinsulin dysmaturation and ER-crowding. We attenuated the β-cell ER proinsulin synthesis with a treat-to-target insulin therapy while avoiding hypoglycemia and weight gain. Alleviation of ER-crowding resulted in temporary improvement in proinsulin maturation, insulin secretion and glucose tolerance. Our observations suggest that alleviation of pre-diabetic ER-crowding using a treat-to-target insulin therapy may improve β-cell function and may prevent further metabolic deterioration.

The use of animal models in diabetes research

Diabetes is a disease characterized by a relative or absolute lack of insulin, leading to hyperglycaemia. There are two main types of diabetes: type 1 diabetes and type 2 diabetes. Type 1 diabetes is due to an autoimmune destruction of the insulin-producing pancreatic beta cells, and type 2 diabetes is caused by insulin resistance coupled by a failure of the beta cell to compensate. Animal models for type 1 diabetes range from animals with spontaneously developing autoimmune diabetes to chemical ablation of the pancreatic beta cells. Type 2 diabetes is modelled in both obese and non-obese animal models with varying degrees of insulin resistance and beta cell failure. This review outlines some of the models currently used in diabetes research. In addition, the use of transgenic and knock-out mouse models is discussed. Ideally, more than one animal model should be used to represent the diversity seen in human diabetic patients.

Pathophysiology and genetics of obesity and diabetes in the New Zealand obese mouse: a model of the human metabolic syndrome

The New Zealand Obese (NZO) mouse is one of the most thoroughly investigated polygenic models for the human metabolic syndrome and type 2 diabetes. It presents the main characteristics of the disease complex, including early-onset obesity, insulin resistance, dyslipidemia, and hypertension. As a consequence of this syndrome, a combination of lipotoxicity and glucotoxicity produces beta-cell failure and apoptosis resulting in hypoinsulinemia and diabetic hyperglycemia. With NZO as a breeding partner, several adipogenic and diabetogenic gene variants have been identified by hypothesis-free positional cloning (Tbc1d1, Zfp69) or by combining genetic screens and candidate gene approaches (Pctp, Abcg1, Nmur2, Lepr). This chapter summarizes the present knowledge of the NZO strain and describes its pathophysiology as well as the known underlying genetic defects.

High-fat, carbohydrate-free diet markedly aggravates obesity but prevents beta-cell loss and diabetes in the obese, diabetes-susceptible db/db strain

OBJECTIVE

We have previously reported that a high-fat, carbohydrate-free diet prevents diabetes and beta-cell destruction in the New Zealand Obese (NZO) mouse strain. Here we investigated the effect of diets with and without carbohydrates on obesity and development of beta-cell failure in a second mouse model of type 2 diabetes, the db/db mouse.

RESULTS

When kept on a carbohydrate-containing standard (SD; with (w/w) 5.1, 58.3, and 17.6% fat, carbohydrates and protein, respectively) or high-fat diet (HFD; 14.6, 46.7 and 17.1%), db/db mice developed severe diabetes (blood glucose >20 mmol/l, weight loss, polydipsia and polyurea) associated with a selective loss of pancreatic beta-cells, reduced GLUT2 expression in the remaining beta-cells, and reduced plasma insulin levels. In contrast, db/db mice kept on a high-fat, carbohydrate-free diet (CFD; with 30.2 and 26.4% (w/w) fat or protein) did not develop diabetes and exhibited near-normal, hyperplastic islets in spite of a morbid obesity (fat content >60%) associated with hyperinsulinaemia.

CONCLUSION

These data indicate that in genetically different mouse models of obesity-associated diabetes, obesity and dietary fat are not sufficient, and dietary carbohydrates are required, for beta-cell destruction.

The endoplasmic reticulum in pancreatic beta cells of type 2 diabetes patients

AIMS/HYPOTHESIS

Pancreatic beta cells have highly developed endoplasmic reticulum (ER) due to their role in insulin secretion. Since ER stress has been associated with beta cell dysfunction, we studied several features of beta cell ER in human type 2 diabetes.

METHODS

Pancreatic samples and/or isolated islets from non-diabetic controls (ND) and type 2 diabetes patients were evaluated for insulin secretion, apoptosis (electron microscopy and ELISA), morphometric ER assessment (electron microscopy), and expression of ER stress markers in beta cell prepared by laser capture microdissection and in isolated islets.

RESULTS

Insulin release was lower and beta cell apoptosis higher in type 2 diabetes than ND islets. ER density volume was significantly increased in type 2 diabetes beta cells. Expression of alpha-mannosidase (also known as mannosidase, alpha, class 1A, member 1) and UDP-glucose glycoprotein glucosyl transferase like 2 (UGCGL2), assessed by microarray and/or real-time reverse transcriptase polymerase chain reaction (RT-PCR), differed between ND and type 2 diabetes beta cells. Expression of immunoglobulin heavy chain binding protein (BiP, also known as heat shock 70 kDa protein 5 [glucose-regulated protein, 78 kDa] [HSPA5]), X-box binding protein 1 (XBP-1, also known as XBP1) and C/EBP homologous protein (CHOP, also known as damage-inducible transcript 3 [DDIT3]) was not higher in type 2 diabetes beta cell or isolated islets cultured at 5.5 mmol/l glucose (microarray and real-time RT-PCR) than in ND samples. When islets were cultured for 24 h at 11.1 mmol/l glucose, there was induction of BiP and XBP-1 in type 2 diabetes islets but not in ND islets.

CONCLUSIONS/INTERPRETATION

Beta cell in type 2 diabetes showed modest signs of ER stress when studied in pancreatic samples or isolated islets maintained at physiological glucose concentration. However, exposure to increased glucose levels induced ER stress markers in type 2 diabetes islet cells, which therefore may be more susceptible to ER stress induced by metabolic perturbations.

Development of diabetes in obese, insulin-resistant mice: essential role of dietary carbohydrate in beta cell destruction

AIMS/HYPOTHESIS

The role of dietary carbohydrate in the pathogenesis of type 2 diabetes is still a subject of controversial debate. Here we analysed the effects of diets with and without carbohydrate on obesity, insulin resistance and development of beta cell failure in the obese, diabetes-prone New Zealand Obese (NZO) mouse.

MATERIALS AND METHODS

NZO mice were kept on a standard diet (4% [w/w] fat, 51% carbohydrate, 19% protein), a high-fat diet (15, 47 and 17%, respectively) and a carbohydrate-free diet in which carbohydrate was exchanged for fat (68 and 20%, respectively). Body composition and blood glucose were measured over a period of 22 weeks. Glucose tolerance tests and euglycaemic-hyperinsulinaemic clamps were performed to analyse insulin sensitivity. Islet morphology was assessed by immunohistochemistry.

RESULTS

Mice on carbohydrate-containing standard or high-fat diets developed severe diabetes (blood glucose >16.6 mmol/l, glucosuria) due to selective destruction of pancreatic beta cells associated with severe loss of immunoreactivity of insulin, glucose transporter 2 (GLUT2) and musculoaponeurotic fibrosarcoma oncogene homologue A (MafA). In contrast, mice on the carbohydrate-free diet remained normoglycaemic and exhibited hyperplastic islets in spite of a morbid obesity associated with severe insulin resistance and a massive accumulation of macrophages in adipose tissue.

CONCLUSIONS/INTERPRETATION

These data indicate that the combination of obesity, insulin resistance and the inflammatory response of adipose tissue are insufficient to cause beta cell destruction in the absence of dietary carbohydrate.

The genetic landscape of type 2 diabetes in mice

Inbred mouse strains provide genetic diversity comparable to that of the human population. Like humans, mice have a wide range of diabetes-related phenotypes. The inbred mouse strains differ in the response of their critical physiological functions, such as insulin sensitivity, insulin secretion, beta-cell proliferation and survival, and fuel partitioning, to diet and obesity. Most of the critical genes underlying these differences have not been identified, although many loci have been mapped. The dramatic improvements in genomic and bioinformatics resources are accelerating the pace of gene discovery. This review describes how mouse genetics can be used to discover diabetes-related genes, summarizes how the mouse strains differ in their diabetes-related phenotypes, and describes several examples of how loci identified in the mouse may directly relate to human diabetes.

The diabetes-prone NZO/Hl strain. Proliferation capacity of beta cells in hyperinsulinemia and hyperglycemia

New Zealand Obese (NZO) male mice develop a polygenic juvenile-onset obesity and maturity onset hyperinsulinemia. Approximately 50% transit to chronic hyperglycemia. Here we report on the proliferation of beta cells in relation to both the individual's metabolic status and structural parameters of the endocrine pancreas. Proliferating beta cells were quantified in pancreas sections by immunoenzymatic double staining of Ki-67 protein, as a marker for proliferating cells, and endocrine non-beta cells in order to distinguish them from beta cells. In normoglycemic NZO/Hl males Ki-67 labelling indices (IKi-67) of beta cells varied between 0.14 and 1.5%, and correlated significantly with both serum insulin levels and beta cell size. There was no correlation with the glycemic status. In diabetic males, beta cell size was increased. IKi-67 varied between 1 and 3%. The data suggest that the secretory activity of beta cells triggered by glucose, entailed changes in both beta cell hypertrophy and proliferation. As shown by morphometric measurements, beta cell expansion in diabetic mice was limited, in spite of high IKi-67 values. This suggested increased death rates of beta cells.

Mouse models and the genetics of diabetes: is there evidence for genetic overlap between type 1 and type 2 diabetes?

In humans, both type 1 and type 2 diabetes exemplify genetically heterogeneous complex diseases in which epigenetic factors contribute to underlying genetic susceptibility. Extended human pedigrees often show inheritance of both diabetes types. A common pathophysiological denominator in both disease forms is pancreatic beta-cell exposure to proinflammatory cytokines. Hence, it is intuitive that systemically expressed genes regulating beta-cell ability to withstand chronic diabetogenic stress may represent a component of shared susceptibility to both major disease forms. In this review, the authors assemble evidence from genetic experiments using animal models developing clearly distinct diabetes syndromes to inquire whether some degree of overlap in genes contributing susceptibility can be demonstrated. The conclusion is that although overlap exists in the pathophysiological insults leading to beta-cell destruction in the currently studied rodent models, the genetic bases seem quite distinct.

Contributions of dysregulated energy metabolism to type 2 diabetes development in NZO/H1Lt mice with polygenic obesity

New Zealand Obese (NZO) male mice develop a polygenic juvenile-onset obesity and maturity-onset hyperinsulinemia and hyperglycemia (diabesity). Here we report on metabolic and molecular changes associated with the antidiabesity action of CL316,243 (CL), a beta(3)-adrenergic receptor agonist. Dietary CL treatment initiated at weaning reduced the peripubertal rise in body weight and adiposity while promoting growth without suppressing hyperphagia. The changes in adiposity, in turn, suppressed development of hyperinsulinemia, hyperleptinemia, hyperlipidemia, and hyperglycemia. These CL-induced alterations were reflected by decreased adipose tissue mass, increased expression of transcripts for uncoupling protein-1 (UCP-1), peroxisome proliferator-activated receptor alpha (PPARalpha), peroxisome proliferater-activated receptor coactivator-1 (PGC-1), and robust development of brown adipocyte function in white fat. Increased drug-mediated energy dissipation elicited a 1.5 degrees C increase in whole body temperature under conditions of increased food intake but with no change in physical activity. Indirect calorimetry of mice treated with CL showed both increased energy expenditure and a restoration of a prominent diurnal pattern in the respiratory exchange ratio suggesting improved nutrient sensing. Our data suggest that CL promotes increased energy dissipation in white and brown fat depots by augmenting thermogenesis and by metabolic re-partitioning of energy in a diabesity-protective fashion. This is the first report demonstrating the effects of dietary beta(3)-agonist in preventing the onset of diabesity in a polygenic rodent model of type 2 diabetes.

Differential levels of diabetogenic stress in two new mouse models of obesity and type 2 diabetes

The genetic basis for the more common forms of human obesity predisposing to insulin resistance and development of type 2 diabetes is multigenic rather than monogenic in origin. New mouse "diabesity" models have been created by combining independent diabetes risk-conferring quantitative trait loci from two unrelated parental strains: New Zealand Obese (NZO/HlLt) and Nonobese Nondiabetic (NON/Lt). F1 hybrid males, heterozygous at all polymorphic autosomal loci distinguishing the two parental strains, are driven to obesity-induced diabetes (diabesity) at high frequencies. This review focuses on two new recombinant congenic strains (RCSs) developed by introgressing multiple NZO/HlLt chromosomal segments into the nominally diabesity-resistant NON/Lt strain background. Both RCSs gain more weight than NON animals. Although exhibiting comparable weight gain and adiposity, only one of the two RCSs develops diabetes. Hence, these two RCSs will be instructive in elucidating genetic and pathophysiological differences underlying uncomplicated obesity syndromes versus diabetogenic obesity (diabesity) syndromes. Unlike mice with null mutations in a single gene producing morbid obesity, the new models develop a more moderate obesity produced by the interaction of numerous genes with relatively small effects. These RCSs are differentially sensitive to adverse side effects of thiazolidinediones and thus should be particularly useful for pharmacogenetic analyses.

The diabetes-prone NZO/HlLt strain. I. Immunophenotypic comparison to the related NZB/BlNJ and NZW/LacJ strains

New Zealand Obese (NZO)/HlLt male mice exhibit a polygenic obesity and approximately 50% develop type 2 diabetes. This strain is known to produce a variety of autoantibodies, including autoantibodies to the insulin receptor. Because of their relatedness to the autoimmune-predisposed New Zealand Black (NZB) and New Zealand White (NZW) inbred strains, we compared NZO to its two related strains for shared hematologic and immunologic characteristics. Comparison of the three strains by serotyping and genotyping methods indicated that NZO shared with NZW the rare (recombinant) H2(z) haplotype at the major histocompatibility complex. Similar to the NZB and NZW strains, spleens from NZO mice contained increased numbers of CD19(+)CD43(+) IgM(+) B-1 B cells, a phenotype associated with natural autoantibody production. NZO mice developed a progressive microcytic anemia that was distinguished from NZB hemolytic anemia by absence of demonstrable antierythrocyte antibodies in the former. Outcross of NZO females with NZB males accelerated development of obesity and diabetes in F1 males. NZO males made B-lymphocyte-deficient by a disrupted immunoglobulin heavy chain gene did not become diabetic. These results suggest that NZO mice should be useful to investigators interested in studying the genetic contributions to autoimmunity made by the related NZW and NZB strains. Further, these results, combined with the pancreatic histopathology contained in the companion manuscript, suggest that B lymphocytes may be important contributors to diabetes pathogenesis in the NZO mouse.