The role of vascular endothelial growth factor-A (VEGF-A) in pancreatic islet function in adults

Abstract

MOLECULAR PHYSIOLOGY AND BIOPHYSICS The Role of Vascular Endothelial Growth Factor-A (VEGF-A) in Pancreatic Islet Function in Adults JEANNELLE A. KANTZ Thesis under the direction of Professor Dr. Alvin Powers Pancreatic islets are highly vascularized and this is important in insulin secretion in response to nutrients like glucose. The purpose of this research was to investigate the importance of vascular endothelial growth factor-A (VEGF-A) in adult islet function as well as the function and morphology of intraislet vasculature. To accomplish this I used a tamoxifen-inducible version of the Cre-loxP system (CreER). Two CreER transgenic lines, (RIP-CreER or Pdx1PB-CreER) allowed for β-cell or islet-specific inactivation, respectively, of VEGF-A following tamoxifen administration. RIP-CreER (Rat Insulin Promoter) is expressed in β-cells of the islet while Pdx1PB-CreER is expressed in all islet cell types. Cre-expressing mice were bred with Rosa26R (R26R) and Z/AP reporter mice in order to test (A) the administration method of tamoxifen and (B) the recombination efficiency. During these characterization studies we observed Cre recombination in the absence of tamoxifen treatment in islets of RIP-CreER;R26R mice, but not RIP-CreER;Z/AP mice. We found that Cre expression in RIP-CreER;R26R islets was ~ 4X higher than Cre expression observed in Pdx1PB-CreER;R26R. These findings indicate that Cre-mediated recombination of loxP sites depends both on the robustness of the promoter driving CreER expression and the susceptibility of the targeted allele to the Cre-mediated recombination. Based on these findings, we crossed Pdx1PB-CreER transgenic mice with VEGFfl/fl mice and measured the expression of VEGF-A in islets and the effects of VEGF-A reduction on islet function and intraislet vasculature. We found that administration of Tamoxifen effectively reduced VEGF-A levels in Pdx1PB-CreER; VEGFfl/fl mice. This reduction in islet VEGF-A expression reduced islet vasculature, and transiently impaired islet function. These findings are particularly important for experiments utilizing inducible systems. The results from our VEGF-A inactivation studies suggest that VEGF-A plays a role in the maintenance of intra-islet vasculature in adults, and that reductions in islet vascularization impact islet function.