Mouse models and type 2 diabetes: translational opportunities

Type 2 diabetes prevalence is increasing worldwide. Treatments are available, but glycaemic control is not always effective in many patients. Better models are needed to create new and improved therapies and to expand our understanding of how type 2 diabetes begins and progresses. Translational research involves the transformation of knowledge from basic scientific discoveries to impacting on public health. This can allow identification of novel molecular mechanisms underlying the disease which can lead to preventative measures, biomarkers for diagnosis, or future therapies. Generation of genetically modified mice has allowed us to investigate the function of genes and develop reproducible models in which the phenotype of the animal can be tested. Mouse models have already given us insight into glucose metabolism and insulin secretion, identified novel pathways, and have been used to confirm genome-wide association studies. In this review we discuss the use of the mouse to clarify human genome-wide association study loci, understand genes and pathways involved in type 2 diabetes, and uncover novel targets for drug discovery.     

Abnormal glucose homeostasis and pancreatic islet function in mice with inactivation of the Fem1b gene

Type 2 diabetes mellitus is a disorder of glucose homeostasis involving complex gene and environmental interactions that are incompletely understood. Mammalian homologs of nematode sex determination genes have recently been implicated in glucose homeostasis and type 2 diabetes mellitus. These are the Hedgehog receptor Patched and Calpain-10, which have homology to the nematode tra-2 and tra-3 sex determination genes, respectively. Here, we have developed Fem1b knockout (Fem1b-KO) mice, with targeted inactivation of Fem1b, a homolog of the nematode fem-1 sex determination gene. We show that the Fem1b-KO mice display abnormal glucose tolerance and that this is due predominantly to defective glucose-stimulated insulin secretion. Arginine-stimulated insulin secretion is also affected. The Fem1b gene is expressed in pancreatic islets, within both beta cells and non-beta cells, and is highly expressed in INS-1E cells, a pancreatic beta-cell line. In conclusion, these data implicate Fem1b in pancreatic islet function and insulin secretion, strengthening evidence that a genetic pathway homologous to nematode sex determination may be involved in glucose homeostasis and suggesting novel genes and processes as potential candidates in the pathogenesis of diabetes mellitus.     

The emerging genetic architecture of type 2 diabetes

Type 2 diabetes is a genetically heterogeneous disease, with several relatively rare monogenic forms and a number of more common forms resulting from a complex interaction of genetic and environmental factors. Previous studies using a candidate gene approach, family linkage studies, and gene expression profiling uncovered a number of type 2 genes, but the genetic basis of common type 2 diabetes remained unknown. Recently, a new window has opened on defining potential type 2 diabetes genes through genome-wide SNP association studies of very large populations of individuals with diabetes. This review explores the pathway leading to discovery of these genetic effects, the impact of these genetic loci on diabetes risk, the potential mechanisms of action of the genes to alter glucose homeostasis, and the limitations of these studies in defining the role of genetics in this important disease.     

Type 2 diabetes – An autoinflammatory disease driven by metabolic stress

Type 2 diabetes has traditionally been viewed as a metabolic disorder characterised by chronic high glucose levels, insulin resistance, and declining insulin secretion from the pancreas. Modern lifestyle, with abundant nutrient supply and reduced physical activity, has resulted in dramatic increases in the rates of obesity-associated disease conditions, including diabetes. The associated excess of nutrients induces a state of systemic low-grade chronic inflammation that results from production and secretion of inflammatory mediators from the expanded pool of activated adipocytes. Here, we review the mechanisms by which obesity induces adipose tissue dysregulation, detailing the roles of adipose tissue secreted factors and their action upon other cells and tissues central to glucose homeostasis and type 2 diabetes. Furthermore, given the emerging importance of adipokines, cytokines and chemokines in disease progression, we suggest that type 2 diabetes should now be viewed as an autoinflammatory disease, albeit one that is driven by metabolic dysregulation.     

Beta-cell ABCA1 influences insulin secretion, glucose homeostasis and response to thiazolidinedione treatment

Type 2 diabetes is characterized by both peripheral insulin resistance and reduced insulin secretion by beta-cells. The reasons for beta-cell dysfunction in this disease are incompletely understood but may include the accumulation of toxic lipids within this cell type. We examined the role of Abca1, a cellular cholesterol transporter, in cholesterol homeostasis and insulin secretion in beta-cells. Mice with specific inactivation of Abca1 in beta-cells had markedly impaired glucose tolerance and defective insulin secretion but normal insulin sensitivity. Islets isolated from these mice showed altered cholesterol homeostasis and impaired insulin secretion in vitro. We found that rosiglitazone, an activator of the peroxisome proliferator-activated receptor-gamma, which upregulates Abca1 in beta-cells, requires beta-cell Abca1 for its beneficial effects on glucose tolerance. These experiments establish a new role for Abca1 in beta-cell cholesterol homeostasis and insulin secretion, and suggest that cholesterol accumulation may contribute to beta-cell dysfunction in type 2 diabetes.     

A novel insulin sensitizer (S15511) enhances insulin-stimulated glucose uptake in rat skeletal muscles.

Type 2 diabetes is preceded by the presence of skeletal muscle insulin resistance, and drugs that increase insulin sensitivity in skeletal muscle prevent the disease. S15511 is an original compound with demonstrated effects on insulin sensitivity in animal models of insulin resistance. However, the mechanisms behind the insulin-sensitizing effect of S15511 are unknown. The aim of our study was to explore whether S15511 improves insulin sensitivity in skeletal muscles. Insulin sensitivity was assessed in skeletal muscles from S15511-treated rats by measuring intracellular insulin-signaling activity and insulin-stimulated glucose transport in isolated muscles. In addition, GLUT4 expression and glycogen levels were assessed after treatment. S15511 treatment was associated with an increase in insulin-stimulated glucose transport in type IIb fibers, while type I fibers were unaffected. The enhanced glucose transport was mirrored by a fiber type-specific increase in GLUT4 expression, while no improvement in insulin-signaling activity was observed. S15511 is a novel insulin sensitizer that is capable of improving glucose homeostasis in nondiabetic rats. The compound enhances skeletal muscle insulin sensitivity and specifically targets type IIb muscle fibers by increasing GLUT4 expression. Together these data show S15511 to be a potentially promising new drug in the treatment and prevention of type 2 diabetes.     

Regulating insulin signaling and beta-cell function through IRS proteins

Diabetes mellitus is a complex disorder that arises from various causes, including dysregulated glucose sensing and impaired insulin secretion (maturity onset diabetes of youth, MODY), autoimmune-mediated beta-cell destruction (type 1), or insufficient compensation for peripheral insulin resistance (type 2). Type 2 diabetes is the most prevalent form that usually occurs at middle age; it afflicts more than 30 million people over the age of 65, but is appearing with greater frequency in children and adolescents. Dysregulated insulin signaling exacerbated by chronic hyperglycemia promotes a cohort of systemic disorders--including dyslipidemia, hypertension, cardiovascular disease, and female infertility. Understanding the molecular basis of insulin resistance can prevent these disorders and their inevitable progression to type 2 diabetes.     

Signal integration and the specificity of insulin action

Insulin is a potent metabolic hormone essential for the maintenance of normal circulating blood glucose level in mammals. The physiologic control of glucose homeostasis results from a balance between hepatic glucose release (glycogenolysis and gluconeogenesis) and dietary glucose absorption versus skeletal muscle and adipose tissue glucose uptake and disposal. Disruption of this delicate balance either through defects in insulin secretion, liver glucose output, or peripheral tissue glucose uptake results in pathophysiological states of insulin resistance and diabetes. In particular, glucose transport into skeletal muscle and adipose tissue is the rate-limiting step in glucose metabolism and reduction in the efficiency of this process (insulin resistance) is one of the earliest predictors for the development of Type II diabetes. Importantly, recent studies have directly implicated an impairment in insulin receptor signal transduction as the prime mechanism for peripheral tissue insulin resistance. In this review, we have focused on recent developments in our understanding of the molecular mechanisms and signal transduction pathways that insulin utilizes to specifically regulate glucose uptake. The detailed understanding of these events will provide a conceptual framework for the development of new therapeutic targets to treat this chronic and debilitating disease process.     

Transmitted beta-cell dysfunction as a cause for type 2-diabetes

The pathways that control insulin release and regulate pancreatic beta-cell mass are crucial on the development of type 2 diabetes mellitus. Maturity-onset diabetes of the young comprises a number of single-gene disorders affecting beta-cell development and/or function. A genetic basis for the more common forms of type 2 diabetes which affect adults in developed as well as many developing countries is less clear cut. It is also characterized by abnormal beta-cell function. Appropriate inbred rodent models are an essential tool for the identification of genes and environmental factors that increase the risk of type 2 diabetes. The informations available from studies in the Goto-Kakizaki (GK) rat are here reviewed in such a perspective. This model was obtained by selective breeding of individuals with mild glucose intolerance from a non-diabetic Wistar rat colony. Heritability of defective beta-mass and beta-cell function in GK model is proposed to reflect the complex interactions of three pathogenic players: (1) three independent loci containing genes causating impaired insulin secretion; (2) gestational metabolic (hyperglycaemic) impairment inducing a programming of endocrine pancreas (decreased beta-cell mass) which is transmitted to the next generation; (3) secondary (acquired) loss of beta-cell differentiation due to chronic exposure to hyperglycaemia (glucotoxicity). A better understanding of the mechanisms involved in the failure of beta-cell function in the GK model will lead to identification of new therapeutic targets for both the prevention and treatment of type 2 diabetes.     

Molecular and cellular bases of diabetes: Focus on type 2 diabetes mouse model-TallyHo

Diabetes is a chronic lifestyle disorder that affects millions of people worldwide. Diabetes is a condition where the body does not produce sufficient insulin or does not use it efficiently. Insulin resistance in diabetes or obesity causes the pancreatic β-cells to increase the insulin output. Diabetes occurs in multiple forms, including type 1, type 2, type 3 and gestational. Type 2 diabetes accounts for ～90–95% of total affected population and is associated with both impaired insulin production by the β-cells of the pancreas and impaired insulin release in response to high blood glucose levels. Diabetes is tightly linked with genetic mutations and genetic and lifestyle activities, including diet and exercise. Recent epidemiological studies established a close link between the diabetes and progression to Alzheimer's disease. This article summarizes various molecular mechanisms involved in the developments of diabetes, including biochemical characteristics, genetic and molecular links with Alzheimer's disease, β-cell function, and factors associated with diabetes. This will help us in the development of novel therapeutic strategies targeting AD in future.     

Computational Modeling of Glucose Transport in Pancreatic β-Cells Identifies Metabolic Thresholds and Therapeutic Targets in Diabetes

Pancreatic β-cell dysfunction is a diagnostic criterion of Type 2 diabetes and includes defects in glucose transport and insulin secretion. In healthy individuals, β-cells maintain plasma glucose concentrations within a narrow range in concert with insulin action among multiple tissues. Postprandial elevations in blood glucose facilitate glucose uptake into β-cells by diffusion through glucose transporters residing at the plasma membrane. Glucose transport is essential for glycolysis and glucose-stimulated insulin secretion. In human Type 2 diabetes and in the mouse model of obesity-associated diabetes, a marked deficiency of β-cell glucose transporters and glucose uptake occurs with the loss of glucose-stimulated insulin secretion. Recent studies have shown that the preservation of glucose transport in β-cells maintains normal insulin secretion and blocks the development of obesity-associated diabetes. To further elucidate the underlying mechanisms, we have constructed a computational model of human β-cell glucose transport in health and in Type 2 diabetes, and present a systems analysis based on experimental results from human and animal studies. Our findings identify a metabolic threshold or “tipping point” whereby diminished glucose transport across the plasma membrane of β-cells limits intracellular glucose-6-phosphate production by glucokinase. This metabolic threshold is crossed in Type 2 diabetes and results in β-cell dysfunction including the loss of glucose stimulated insulin secretion. Our model further discriminates among molecular control points in this pathway wherein maximal therapeutic intervention is achieved.     

Mechanisms of insulin secretion in malnutrition: modulation by amino acids in rodent models

Protein restriction at early stages of life reduces β-cell volume, number of insulin-containing granules, insulin content and release by pancreatic islets in response to glucose and other secretagogues, abnormalities similar to those seen in type 2 diabetes. Amino acids are capable to directly modulate insulin secretion and/or contribute to the maintenance of β-cell function, resulting in an improvement of insulin release. Animal models of protein malnutrition have provided important insights into the adaptive mechanisms involved in insulin secretion in malnutrition. In this review, we discuss studies focusing on the modulation of insulin secretion by amino acids, specially leucine and taurine, in rodent models of protein malnutrition. Leucine supplementation increases insulin secretion by pancreatic islets in malnourished mice. This effect is at least in part due to increase in the expression of proteins involved in the secretion process, and the activation of the PI3K/PKB/mTOR pathway seems also to contribute. Mice supplemented with taurine have increased insulin content and secretion as well as increased expression of genes essential for β-cell functionality. The knowledge of the mechanisms through which amino acids act on pancreatic β-cells to stimulate insulin secretion is of interest for clinical medicine. It can reveal new targets for the development of drugs toward the treatment of endocrine diseases, in special type 2 diabetes.     

High passage MIN6 cells have impaired insulin secretion with impaired glucose and lipid oxidation

Type 2 diabetes is a metabolic disorder characterized by the inability of beta-cells to secrete enough insulin to maintain glucose homeostasis. MIN6 cells secrete insulin in response to glucose and other secretagogues, but high passage (HP) MIN6 cells lose their ability to secrete insulin in response to glucose. We hypothesized that metabolism of glucose and lipids were defective in HP MIN6 cells causing impaired glucose stimulated insulin secretion (GSIS). HP MIN6 cells had no first phase and impaired second phase GSIS indicative of global functional impairment. This was coupled with a markedly reduced ATP content at basal and glucose stimulated states. Glucose uptake and oxidation were higher at basal glucose but ATP content failed to increase with glucose. HP MIN6 cells had decreased basal lipid oxidation. This was accompanied by reduced expressions of Glut1, Gck, Pfk, Srebp1c, Ucp2, Sirt3, Nampt. MIN6 cells represent an important model of beta cells which, as passage numbers increased lost first phase but retained partial second phase GSIS, similar to patients early in type 2 diabetes onset. We believe a number of gene expression changes occurred to produce this defect, with emphasis on Sirt3 and Nampt, two genes that have been implicated in maintenance of glucose homeostasis.     

Genetics of type 2 diabetes: insight from targeted mouse mutants

Diabetes affects millions of people worldwide, and its chronic complications are a leading cause of death in many industrialized countries. In a minority of patients, diabetes is brought about by the auto-immune destruction of insulin-producing pancreatic beta cells (Type 1 diabetes). In the vast majority of patients, diabetes is brought about by a combination of genetic and environmental factors that affect the organism's ability to respond to insulin (Type 2 diabetes). This impairment is due to a complex abnormality involving insulin action at the periphery and insulin production in the beta cell. Genetic factors play a key role in the development of type 2 diabetes. However, the inheritance of diabetes is non-Mendelian in nature, due to genetic heterogeneity, polygenic pathogenesis and incomplete penetrance. For these reasons, many laboratories have developed "designer" mice bearing targeted mutations in genes of the insulin action and insulin secretion pathways in order to develop a better model for the inheritance and pathogenesis of type 2 diabetes. These mutant mice are beginning to challenge established paradigms in the pathogenesis of type 2 diabetes and to shed light onto the genetic interactions underlying its complex inheritance. Here we review recent progress in the field and assess its impact on human studies of the genetics, prevention and treatment of type 2 diabetes.     

Development of Type 2 Diabetes Mellitus in the Obese Adolescent:A Growing Challenge

ObjectiveTo describe the metabolic phenotype of type 2 diabetes mellitus in youth and possible metabolic defects leading to its development with particular emphasis on fatty liver.MethodsWe present data gathered from studies performed in obese adolescents across the spectrum of glucose tolerance to assess both alterations in insulin sensitivity and secretion. Discussion regarding treatment options is presented using the data from the Treatment Options for Type 2 Diabetes in Adolescents and Youth (TODAY) study.ResultsAs the number of children with obesity continues to grow, the health implications of the condition are becoming increasingly evident. An unprecedented phenomenon rarely seen before has emerged: type 2 diabetes mellitus. At the time of diabetes diagnosis, cardiovascular disease may already be present, even in young adults. The progression from normal glucose tolerance to type 2 diabetes in adults occurs through an intermediate phase of altered glucose metabolism known as impaired glucose tolerance or prediabetes. Previous studies from our group and others reported a high prevalence of impaired glucose tolerance among children and adolescents with marked obesity. Cross-sectional studies demonstrate that impaired glucose tolerance in obese youth is associated with severe insulin resistance, β-cell dysfunction, and altered abdominal and muscle fat partitioning. We end briefly by discussing the current data available on treatment of this condition from the TODAY study, the largest clinical trial ever performed in youth with type 2 diabetes.ConclusionThe observed rapid progression of the glucose homeostasis alterations in adolescents underlines the importance of focusing attention on the earliest stages of the disease before the onset of any alterations in glucose tolerance. (Endocr Pract. 2012;18:791-795)     

Unraveling pancreatic islet biology by quantitative proteomics

The pancreatic islets of Langerhans play a critical role in maintaining blood glucose homeostasis by secreting insulin and several other important peptide hormones. Impaired insulin secretion due to islet dysfunction is linked to the pathogenesis underlying both Type 1 and Type 2 diabetes. Over the past 5 years, emerging proteomic technologies have been applied to dissect the signaling pathways that regulate islet functions and gain an understanding of the mechanisms of islet dysfunction relevant to diabetes. Herein, we briefly review some of the recent quantitative proteomic studies involving pancreatic islets geared towards gaining a better understanding of islet biology relevant to metabolic diseases.     

GIP and GLP-1 as incretin hormones: lessons from single and double incretin receptor knockout mice

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gut-derived incretins secreted in response to nutrient ingestion. Both incretins potentiate glucose-dependent insulin secretion and enhance beta-cell mass through regulation of beta-cell proliferation, neogenesis and apoptosis. In contrast, GLP-1, but not GIP, inhibits gastric emptying, glucagon secretion, and food intake. Furthermore, human subjects with Type 2 diabetes exhibit relative resistance to the actions of GIP, but not GLP-1R agonists. The physiological importance of both incretins has been investigated through generation and analysis of incretin receptor knockout mice. Elimination of incretin receptor action in GIPR-/- or GLP-1R-/- mice produces only modest impairment in glucose homeostasis. Similarly, double incretin receptor knockout (DIRKO) mice exhibit normal body weight and normal levels of plasma glucagon and hypoglycemic responses to exogenous insulin. However, glucose-stimulated insulin secretion is significantly decreased following oral but not intraperitoneal glucose challenge in DIRKO mice and the glucose lowering actions of dipeptidyl peptidase-IV (DPP-IV) inhibitors are extinguished in DIRKO mice. Hence, incretin receptor signaling exerts physiologically relevant actions critical for glucose homeostasis, and represents a pharmacologically attractive target for development of agents for the treatment of Type 2 diabetes.