Insights into molecular pathogenesis of type 2 diabetes from knockout mouse models

Glucokinase as a therapeutic target based on findings from the analysis of mouse models

I investigated mouse models to elucidate the pathophysiology and to establish a new treatment strategy for type 2 diabetes, with a particular focus on glucokinase. The decrease in pancreatic beta-cell function and mass are important factors in the pathophysiology of type 2 diabetes. My group have shown that glucokinase plays an important role in high-fat diet-induced and high-starch diet-induced beta-cell expansion. The findings indicated that the mechanism of short-term high-fat diet-induced beta-cell proliferation involved a glucokinase-independent pathway, suggesting that there are different pathways and mechanisms in the proliferation of pancreatic beta-cells during short-term versus long-term high-fat diets. Because enhancement of glucose signals via glucokinase is important for beta-cell proliferation, it was thought that beta-cell mass would be increased and insulin secretion would be maintained by glucokinase activators. However, sub-chronic administration of a glucokinase activator in db/db mice produced an unsustained hypoglycemic effect and promoted hepatic fat accumulation without changes in beta-cell function and mass. In contrast, my group have shown that inactivating glucokinase in beta-cells prevented beta-cell failure and led to an improvement in glucose tolerance in db/db mice. Regulation of glucokinase activity has an influence on the pathophysiology of type 2 diabetes and can be one of the therapeutic targets.

Glucokinase Inactivation Paradoxically Ameliorates Glucose Intolerance by Increasing β-Cell Mass in db/db Mice

Efficacy of glucokinase activation on glycemic control is limited to a short-term period. One reason might be related to excess glucose signaling by glucokinase activation toward β-cells. In this study, we investigated the effect of glucokinase haploinsufficiency on glucose tolerance as well as β-cell function and mass using a mouse model of type 2 diabetes. Our results showed that in db/db mice with glucokinase haploinsufficiency, glucose tolerance was ameliorated by augmented insulin secretion associated with the increase in β-cell mass when compared with db/db mice. Gene expression profiling and immunohistochemical and metabolomic analyses revealed that glucokinase haploinsufficiency in the islets of db/db mice was associated with lower expression of stress-related genes, greater expression of transcription factors involved in the maintenance and maturation of β-cell function, less mitochondrial damage, and a superior metabolic pattern. These effects of glucokinase haploinsufficiency could preserve β-cell mass under diabetic conditions. These findings verified our hypothesis that optimizing excess glucose signaling in β-cells by inhibiting glucokinase could prevent β-cell insufficiency, leading to improving glucose tolerance in diabetes status by preserving β-cell mass. Therefore, glucokinase inactivation in β-cells, paradoxically, could be a potential strategy for the treatment of type 2 diabetes.

The insulin regulatory network in adult hippocampus and pancreatic endocrine system

There is a very strong correlation between the insulin-mediated regulatory system of the central nervous system and the pancreatic endocrine system. There are many examples of the same transcriptional factors being expressed in both regions in their embryonic development stages. Hormonal signals from the pancreatic islets influence the regulation of energy homeostasis by the brain, and the brain in turn influences the secretions of the islets. Diabetes induces neuronal death in different regions of the brain especially hippocampus, causes alterations on the neuronal circuits and therefore impairs learning and memory, for which the hippocampus is responsible. The hippocampus is a region of the brain where steady neurogenesis continues throughout life. Adult neurogenesis from undifferentiated neural stem cells is greatly decreased in diabetic patients, and as a result their learning and memory functions decline. Might it be possible to reactivate stem cells whose functions have deteriorated and that are present in the tissues in which the lesions occur in diabetes, a lifestyle disease, which plagues modern humans and develops as a result of the behavior of insulin-related factor? In this paper we summarize research in regard to these matters based on examples in recent years.

Regenerative medicine using adult neural stem cells: the potential for diabetes therapy and other pharmaceutical applications

Neural stem cells (NSCs), which are responsible for continuous neurogenesis during the adult stage, are present in human adults. The typical neurogenic regions are the hippocampus and the subventricular zone; recent studies have revealed that NSCs also exist in the olfactory bulb. Olfactory bulb-derived neural stem cells (OB NSCs) have the potential to be used in therapeutic applications and can be easily harvested without harm to the patient. Through the combined influence of extrinsic cues and innate programming, adult neurogenesis is a finely regulated process occurring in a specialized cellular environment, a niche. Understanding the regulatory mechanisms of adult NSCs and their cellular niche is not only important to understand the physiological roles of neurogenesis in adulthood, but also to provide the knowledge necessary for developing new therapeutic applications using adult NSCs in other organs with similar regulatory environments. Diabetes is a devastating disease affecting more than 200 million people worldwide. Numerous diabetic patients suffer increased symptom severity after the onset, involving complications such as retinopathy and nephropathy. Therefore, the development of treatments for fundamental diabetes is important. The utilization of autologous cells from patients with diabetes may address challenges regarding the compatibility of donor tissues as well as provide the means to naturally and safely restore function, reducing future risks while also providing a long-term cure. Here, we review recent findings regarding the use of adult OB NSCs as a potential diabetes cure, and discuss the potential of OB NSC-based pharmaceutical applications for neuronal diseases and mental disorders.

Effects of Extract from Solid-State Fermented Cordyceps sinensis on Type 2 Diabetes Mellitus

Diabetes mellitus is the most common chronic disease in the world, and a wide range of drugs, including Chinese herbs, have been evaluated for the treatment of associated metabolic disorders. This study investigated the potential hypoglycemic and renoprotective effects of an extract from the solid-state fermented mycelium of Cordyceps sinensis (CS). We employed the KK/HIJ diabetic mouse model, in which the mice were provided with a high-fat diet for 8 weeks to induce hyperglycemia, followed by the administration of CS or rosiglitazone for 4 consecutive weeks. Several parameters were evaluated, including changes in body weight, plasma lipid profiles, oral glucose tolerance tests, insulin tolerance tests, and plasma insulin concentrations. Our results show that the CS extract significantly elevated HDL/LDL ratios at 4 weeks and decreased body weight gain at 8 weeks. Interestingly, CS treatment did not lead to obvious improvements in hyperglycemia or resistance to insulin, while in vitro MTT assays indicated that CS protects pancreatic beta cells against the toxic effects of STZ. CS also enhanced renal NKA activity and reduced the accumulation of mesangial matrix and collagen deposition. In conclusion, CS extract can potentially preserve β-cell function and offer renoprotection, which may afford a promising therapy for DM.

Transcription factor AP-2beta inhibits glucose-induced insulin secretion in cultured insulin-secreting cell-line

AIM

We previously identified the transcription factor activating enhancer-binding protein-2beta (AP-2beta) gene as a new candidate for conferring susceptibility to type 2 diabetes. To ascertain the possible involvement of AP-2beta in the pathogenesis of type 2 diabetes we examined the effects of AP-2beta on glucose-induced insulin secretion.

METHODS

We measured the insulin secretion stimulated by glucose, tolbutamide, or KCl in the HIT-T15 cells infected with adenovirus vectors encoding AP-2beta or LacZ (control).

RESULTS

We identified clear expression of AP-2beta in isolated rat pancreatic islets and in HIT-T15 cells. Glucose-induced increase in insulin secretion was significantly inhibited in AP-2beta-overexpressing cells (LacZ, 5.0+/-0.8 ng h(-1)mg(-1) protein; AP-2beta, 1.7+/-0.2 ng h(-1)mg(-1) protein; P=0.0015), whereas insulin expression was the same in both types of cells. Tolbutamide-induced insulin secretion was also suppressed in the AP-2beta-overexpressing cells, but KCl-induced insulin secretion was not affected by AP-2beta overexpression. In addition, Kir6.2 and glucokinase expression was significantly decreased in the AP-2beta-overexpressing cells.

CONCLUSION

We identified for the first time that AP-2beta expressed and functioned in insulin-secreting cell-line HIT-T15. These results suggest that AP-2beta contributes to susceptibility to type 2 diabetes by inhibiting glucose-induced insulin secretion in pancreatic beta cells.

OLETF allele of hyperglycemic QTL Nidd3/of is dominant

The OLETF rat is a well-established model for the study of type 2 diabetes associated with obesity and has been shown to possess multiple hyperglycemic alleles in its genome. Here we focused on and carefully characterized one of the previously reported congenic strains, F.O-Nidd3/of that carries the OLETF allele of the Nidd3/of locus (also known as Niddm21 in the Rat Genome Database) in the normoglycemic F344 genetic background. A prominent finding was that the F1 progeny between the congenic and the F344 stain, whose genotype is heterozygote at the Nidd3/of locus, showed mild hyperglycemia equal to the parental congenic rat, suggesting that the OLETF allele is dominant. To our knowledge, this is the first study in which a diabetic QTL has been directly demonstrated to be dominant by using congenic strains.

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.

Correction of insulin resistance and the metabolic syndrome

Insulin resistance is a common phenomenon of the metabolic syndrome, which is clinically characterized by a clustering of various cardiovascular risk factors in a single individual and a higher prevalence of respective complications, such as coronary heart disease and stroke. At the cellular level, insulin resistance is defined as a reduced insulin action, which can affect not only glucose uptake, but also gene regulation. Elucidation of novel signaling networks within the cell which are mediating and affecting insulin action will reveal many new genes and drug targets that are potentially of clinical relevance in the future. In this chapter, we propose that the metabolic syndrome might be a clinical consequence of altered gene regulation. This is illuminated in the context of transcription factors, e.g., sterol regulatory element binding proteins (SREBPs), coupling signals from nutrients, metabolites, and hormones at the gene regulatory level with pathobiochemical features of increased lipid accumulation in lean nonadipose tissues. The phenomenon of ectopic lipid accumulation (lipotoxicity) appears to be a novel link between insulin resistance, obesity, and possibly other features of the metabolic syndrome. Therefore, the investigation of specific gene regulatory networks and their alterations might be a clue to understanding the development and clustering of different cardiovascular risk factors in different individuals. As cellular sensors transcription factors--as common denominators of gene regulatory networks--might thereby also determine the susceptibility of individuals to cardiovascular risk factors and their complications.

Impact of genetic background and ablation of insulin receptor substrate (IRS)-3 on IRS-2 knock-out mice

Although we and others have generated IRS-2 knock-out (IRS-2(-/-)) mice, significant differences were seen between the two lines of IRS-2(-/-) mice in the severity of diabetes and alterations of beta-cell mass. It has been reported that although IRS-1 and IRS-3 knock-out mice showed normal blood glucose levels, IRS-1/IRS-3 double knock-out mice exhibited marked hyperglycemia. Thus, IRS-1 and IRS-3 compensate each other's functions in maintaining glucose homeostasis. To assess the effect of genetic background and also ablation of IRS-3 on IRS-2(-/-), we generated IRS-2/IRS-3 double knock-out (IRS-2(-/-)IRS-3(-/-)) mice by crossing IRS-3(-/-) mice (129/Sv and C57Bl/6 background) with our IRS-2(-/-) mice (CBA and C57Bl/6 background). Intercrosses of IRS-2(+/-)IRS-3(+/-) mice yielded nine genotypes, and all of them including IRS-2(-/-)IRS-3(-/-) mice were apparently healthy and showed normal growth. However, at 10-20 weeks of age, 20-30% mice carrying a null mutation for the IRS-2 gene, irrespective of the IRS-3 genotype, developed diabetes. When mice with diabetes were excluded from the analysis of glucose and insulin tolerance test, IRS-2(-/-)IRS-3(-/-) showed a degree of glucose intolerance and insulin resistance similar to those of IRS-2(-/-) mice. Both IRS-2(-/-) and IRS-2(-/-)IRS-3(-/-) mice had moderately reduced beta-cell mass despite having insulin resistance. Insulin-positive beta-cells were decreased to nearly zero in IRS-2(-/-) mice with diabetes. Although Pdx1 and glucose transporter 2 expressions were essentially unaltered in islets from IRS-2(-/-) mice without diabetes, they were dramatically decreased in IRS-2(-/-) mice with diabetes. Taken together, these observations indicate that IRS-3 does not play a role compensating for the loss of IRS-2 in maintaining glucose homeostasis and that the severity of diabetes in IRS-2(-/-) mice depends upon genetic background, suggesting the existence of modifier gene(s) for diabetes in mice of the 129/Sv genetic strain.