To investigate molecular mechanisms controlling islet vascularization and revascularization after transplantation, we examined pancreatic expression of three families of angiogenic factors and their receptors in differentiating endocrine cells and adult islets. Using intravital lectin labeling, we demonstrated that development of islet microvasculature and establishment of islet blood flow occur concomitantly with islet morphogenesis. Our genetic data indicate that vascular endothelial growth factor (VEGF)-A is a major regulator of islet vascularization and revascularization of transplanted islets. In spite of normal pancreatic insulin content and ␤-cell mass, mice with ␤-cell-reduced VEGF-A expression had impaired glucose-stimulated insulin secretion. By vascular or diffusion delivery of ␤-cell secretagogues to islets, we showed that reduced insulin output is not a result of ␤-cell dysfunction but rather caused by vascular alterations in islets. Taken together, our data indicate that the microvasculature plays an integral role in islet function. Factors modulating VEGF-A expression may influence islet vascularity and, consequently, the amount of insulin delivered into the systemic circulation. Diabetes

Summary Decreasing glucagon action lowers the blood glucose and may be useful therapeutically for diabetes. However, interrupted glucagon signaling leads to α-cell proliferation. To identify postulated hepatic-derived, circulating factor(s) responsible for α-cell proliferation, we used transcriptomics/proteomics/metabolomics in three models of interrupted glucagon signaling and found that proliferation of mouse, zebrafish, and human α-cells was mTOR- and FoxP transcription factor-dependent. Changes in hepatic amino acid (AA) catabolism gene expression predicted the observed increase in circulating AA. Mimicking these AA levels stimulated α-cell proliferation in a newly developed in vitro assay with L-glutamine being a critical AA. α-cell expression of the AA transporter Slc38a5 was markedly increased in mice with interrupted glucagon signaling and played a role in α-cell proliferation. These results indicate a hepatic-α-islet cell axis where glucagon regulates serum AA availability and AA, especially L-glutamine, regulates α-cell proliferation and mass via mTOR-dependent nutrient sensing.

OBJECTIVE-To characterize differences in whole-body glucose metabolism between commonly used inbred mouse strains.RESEARCH DESIGN AND METHODS-Hyperinsulinemic-euglycemic (ϳ8.5 mmol/l) and -hypoglycemic (ϳ3.0 mmol/l) clamps were done in catheterized, 5-h-fasted mice to assess insulin action and hypoglycemic counter-regulatory responsiveness. Hyperglycemic clamps (ϳ15 mmol/l) were done to assess insulin secretion and compared with results in perifused islets.RESULTS-Insulin action and hypoglycemic counter-regulatory and insulin secretory phenotypes varied considerably in four inbred mouse strains. In vivo insulin secretion was greatest in 129X1/Sv mice, but the counter-regulatory response to hypoglycemia was blunted. FVB/N mice in vivo showed no increase in glucose-stimulated insulin secretion, relative hepatic insulin resistance, and the highest counter-regulatory response to hypoglycemia. In DBA/2 mice, insulin action was lowest among the strains, and islets isolated had the greatest glucose-stimulated insulin secretion in vitro. In C57BL/6 mice, in vivo physiological responses to hyperinsulinemia at euglycemia and hypoglycemia were intermediate relative to other strains. Insulin secretion by C57BL/6 mice was similar to that in other strains in contrast to the blunted glucose-stimulated insulin secretion from isolated islets.CONCLUSIONS-Strain-dependent differences exist in four inbred mouse strains frequently used for genetic manipulation and study of glucose metabolism. These results are important for selecting inbred mice to study glucose metabolism and for interpreting and designing experiments. Diabetes 57:1790-1799, 2008 T he development of new mouse models has allowed investigators to address questions related to glucose metabolism in ways that were not previously possible. Use of inbred mouse strains and proliferation of techniques to produce genetic modifications have been invaluable in defining the role of select genes under physiological conditions. To rigorously examine complex physiological processes in vivo has required the development of new experimental approaches for the mouse and the adaptation of techniques previously used in larger animals. Important technical advancements, including surgical catheterization (1) and miniaturization of clamp techniques (2) for the mouse, have furthered our ability to dissect the physiology underlying insulin action, insulin secretion, and counter-regulation to insulin-induced hypoglycemia under well-controlled physiological conditions.Mouse models produced through genetic modification have been generated in a variety of mouse strains. It is widely recognized that the background mouse strain can influence phenotypes. Several examples have been described where identical genetic mutations in different inbred mouse strains result in different phenotypes (3-5). These findings indicate that the contribution of the inbred strain genetic background to the phenotype is an important factor to consider when designing and interpreting experiments.The goal of the current s...

SUMMARY Pancreatic islet endocrine cell and endothelial cell (EC) interactions mediated by vascular endothelial growth factor-A (VEGF-A) signaling are important for islet differentiation and the formation of highly vascularized islets. To dissect how VEGF-A signaling modulates intra-islet vasculature, islet microenvironment, and β cell mass, we transiently increased VEGF-A production by β cells. VEGF-A induction dramatically increased the number of intra-islet ECs but led to β cell loss. After withdrawal of the VEGF-A stimulus, β cell mass, function, and islet structure normalized as a result of a robust, but transient, burst in proliferation of pre-existing β cells. Bone marrow-derived macrophages (MΦs) recruited to the site of β cell injury were crucial for the β cell proliferation, which was independent of pancreatic location and circulating factors such as glucose. Identification of the signals responsible for the proliferation of adult, terminally differentiated β cells will improve strategies aimed at β cell regeneration and expansion.

Aims/hypothesisOur understanding of the transcription factors that control the development and function of rodent islet beta cells is advancing rapidly, yet less is known of the role they play in similar processes in human islets.MethodsTo characterise the abundance and regulation of key proteins involved in glucose-regulated insulin secretion in human islets, we examined the expression of MAFA, MAFB, GLUT2 (also known as SLC2A2), βGK (also known as GCK) and PDX1 in isolated, highly purified human islets with an intact insulin secretory pattern. We also assessed these features in islets from two different mouse strains (C57BL/6J and FVB).ResultsCompared with mouse islets, human islets secreted more insulin at baseline glucose (5.6 mmol/l), but less upon stimulation with high glucose (16.7 mmol/l) or high glucose plus 3-isobutyl-1-methyl-xanthine. Human islets had relatively more MAFB than PDX1 mRNA, while mouse islets had relatively more Pdx1 than Mafb mRNA. However, v-maf musculoaponeurotic fibrosarcoma oncogene homologue (MAF) B protein was found in human islet alpha and beta cells. This is unusual as this regulator is only produced in islet alpha cells in adult mice. The expression of insulin, MAFA, βGK and PDX1 was not glucose-regulated in human islets with an intact insulin secretory pattern.Conclusions/interpretationOur results suggest that human islets have a distinctive distribution and function of key regulators of the glucose-stimulated insulin secretion pathway, emphasising the urgent need to understand the processes that regulate human islet beta cell function.Electronic supplementary materialThe online version of this article (doi:10.1007/s00125-011-2369-0) contains peer-reviewed but unedited supplementary material, which is available to authorised users.