Impaired insulin secretion in vivo but enhanced insulin secretion from isolated islets in pancreatic beta cell-specific vascular endothelial growth factor-A knock-out mice

Vascular niche of pancreatic islets

Pancreatic islets are highly vascularized micro-organs. Approximately 10% of an islet consists of blood vessels. The induction and maintenance of the islet vascular system depend on VEGF secreted from β-cells. VEGF is also critical for the phenotype of the islet vasculature by induction of a vast number of fenestrae. The islet vasculature serves the role of supplying the endocrine cells with oxygen and nutrients, but may also be important for proper glucose sensing of the cells, for paracrine support of endocrine function and growth, and for drainage of metabolites and secreted islet hormones into the systemic circulation. Emerging evidence suggests an important role of islet endothelial cells to maintain β-cell function and growth by secretion of molecules such as hepatocyte growth factor, thrombospondin-1 and laminins, thereby forming a vascular niche for the endocrine cells.

Disruption of G protein-coupled receptor 39 impairs insulin secretion in vivo

GPR39 is a G protein-coupled receptor expressed in liver, gastrointestinal tract, adipose tissue, and pancreas. We have recently shown that young GPR39(-/-) mice have normal body weight, food intake, and fasting glucose and insulin levels. In this study, we examined the role of GPR39 in aging and diet-induced obese mice. Body weight and food intake were similar in wild-type and GPR39(-/-) mice as they aged from 12 to 52 wk or when fed a low-fat/high-sucrose or high-fat/high-sucrose diet. Fifty-two-week-old GPR39(-/-) mice showed a trend toward decreased insulin levels after oral glucose challenge. When fed either a low-fat/high-sucrose or high-fat/high-sucrose diet, GPR39(-/-) mice had increased fed glucose levels and showed decreased serum insulin levels during an oral glucose tolerance test in the face of unchanged insulin tolerance. Pancreas morphology and glucose-stimulated insulin secretion in isolated islets from wild-type and GPR39(-/-) mice were comparable, suggesting that GPR39 is not required for pancreas development or ex vivo insulin secretion. Small interfering RNA-mediated knockdown of GPR39 in clonal NIT-1 beta-cells revealed that GPR39 regulates the expression of insulin receptor substrate-2 and pancreatic and duodenal homeobox-1 in a cell-autonomous manner; insulin receptor substrate-2 mRNA was also significantly decreased in the pancreas of GPR39(-/-) mice. Taken together, our data indicate that GPR39 is required for the increased insulin secretion in vivo under conditions of increased demand, i.e. on development of age-dependent and diet-induced insulin resistance. Thus, GPR39 agonists may have potential for the treatment of type 2 diabetes.

Normal islet vascularization is dispensable for expansion of beta-cell mass in response to high-fat diet induced insulin resistance

The inability to increase of islet mass adequately to compensate for the demand of insulin due to insulin resistance is an important pathophysiological feature of type 2 diabetes. Previous studies suggested a relationship between pancreatic beta-cell mass and islet vascularization, although no evidence has confirmed this association in response to insulin resistance. Vascular endothelial growth factor-A (VEGF-A) in islets is essential for maintaining normal islet blood vessels. Here, insulin resistance was induced in mice carrying a beta-cell-specific VEGF-A gene mutation (RIP-Cre:Vegf(fl/fl)) by 20-week feeding of high-fat diet as a model of impaired islet vascularization. These mice showed only a modest decrease in glucose tolerance, compared with control mice. In addition, although the endothelial cell area in the islets of high-fat-fed RIP-Cre:Vegf(fl/fl) mice remained diminished, the pancreatic beta-cell area was modestly more than in high-fat-fed control mice. Thus, normal islet vascularization does not seem to be essential for expansion of beta cell mass in response to insulin resistance.

Impact of whole body irradiation and vascular endothelial growth factor-A on increased beta cell mass after bone marrow transplantation in a mouse model of diabetes induced by streptozotocin

AIMS/HYPOTHESIS

Recent studies have shown that bone marrow transplantation reduces hyperglycaemia in a mouse model of diabetes induced by streptozotocin. However, the essential factors for the improvement of hyperglycaemia by bone marrow transplantation have not been fully elucidated. The aim of this study was to search for such factors.

METHODS

We investigated the effect of irradiation to whole body, to abdomen alone or to whole body excluding abdomen, followed by infusion or no infusion of bone marrow cells. We also investigated the effect of bone marrow transplantation on beta cell-specific vascular endothelial growth factor-A gene (Vegfa) knockout mice.

RESULTS

Bone marrow transplantation improved streptozotocin-induced hyperglycaemia and partially restored islet mass. This change was associated with increased islet vascularisation. Among the other methods investigated, low-dose irradiation of the whole body without infusion of bone marrow cells also improved blood glucose level. In streptozotocin-treated beta cell-specific Vegfa knockout mice, which exhibit impaired islet vascularisation, bone marrow transplantation neither improved hyperglycaemia, relative beta cell mass nor islet vascularisation.

CONCLUSION/INTERPRETATION

Our results indicate that whole body irradiation is essential and sufficient for restoration of beta cell mass after streptozotocin treatment independent of infusion of bone marrow cells. Vascular endothelial growth factor-A produced in beta cells is also essential for this phenomenon.

Dose-dependent requirement of patched homologue 1 in mouse pancreatic beta cell mass

AIMS/HYPOTHESIS

Ectopic activation of hedgehog (HH) signalling in pancreas induces various abnormal morphogenetic events in the pancreas. This study analysed the dose-dependent requirement of patched homologue 1 (PTCH1), a negative regulator of HH signalling on pancreatic development.

METHODS

We used a recessive spontaneous mutant mouse denoted as mes which carries a mutated Ptch1 resulting in deletion of the most carboxy-terminal cytoplasmic domain of the PTCH1 protein. In this study, we analysed pancreatic morphology in Ptch1 ( +/+ ), Ptch1 ( +/mes ), Ptch1 (+/-), Ptch1 ( mes/me ) (s) and Ptch1 (-/mes ) mouse embryos, as well as the islet mass in adult Ptch1 (+/+), Ptch1 (+/mes ) and Ptch1 (+/-) mice.

RESULTS

Until embryonic day (E) 12.5, no obvious abnormality of pancreas was observed in any of the Ptch1 mutants. The levels of PDX1 and glucagon were also not evidently different among the mice genotypes studied. Thereafter, morphological abnormalities appeared in the Ptch1 mutant mice. The beta, alpha and exocrine cell masses decreased at E18.5 in parallel with increased HH signalling, with beta cell mass showing the highest sensitivity to HH signalling with a significant decrease even in Ptch1 (+/mes ) mice. Adult Ptch1 (+/-) mice also showed a significant decrease in beta cell mass compared with wild-type mice.

CONCLUSIONS/INTERPRETATION

Our findings indicate that the carboxy-terminal domain of Ptch1 is essential for pancreatic development. In addition, the loss of Ptch1 function decreases both the endocrine and exocrine cell mass in a dose-dependent manner, with beta cells particularly sensitive to changes in HH signalling.