Early pancreas development and endocrine induction: Ptf1a and VEGF

Abstract

The transcription factor Pdx1 is expressed throughout the pancreas early, but selectively in insulin-producing cells postnatally. Previous studies showed that conserved regulatory regions, Areas I and II (Pdx1PB), directed pancreatic endocrine cell expression, while an adjacent region (Pdx1XB) containing the conserved Area III directed transient ƒÒ cell expression. We generated transgenic mice in order to lineage trace cells in which these promoter fragments were activated: Pdx1PBCre mediated only endocrine cell recombination, while Pdx1XBCre directed broad and early recombination throughout the developing pancreas. An Areas I-II-III reporter transgene was expressed throughout the E10.5 pancreas, gradually becoming ƒÒ cell selective, similar to endogenous Pdx1. These data suggested that sequences within Area III mediate early pancreas-wide Pdx1 expression. Area III contains a binding site for PTF1, a transcription factor complex essential for development of all pancreatic cell types. This site contributed to Area III-dependent reporter gene expression in an acinar cell line, while PTF1a specifically trans-activated an Area III-containing reporter in a non-pancreatic cell line. Importantly, Ptf1a occupied Area III of the endogenous Pdx1 promoter in E11.5 pancreatic buds. These data strongly suggest that PTF1 is an early activator of Pdx1 in pancreatic progenitor cells. Ptf1a expression in the early pancreatic primordium requires signaling from the adjacent vascular endothelial cells, which are reciprocally sensitive to VEGF signaling from the pancreas. To elucidate the role of the VEGF signaling pathway in pancreas differentiation, we analyzed transgenic mice that express agonists selective for VEGF receptor isoforms (VEGFR1 or VEGFR2) throughout the pancreas. Morphometric analysis of antibody labeled cross-sections revealed that at postnatal day one, mice expressing the VEGFR2 agonist had a 49% increase in endothelial to endocrine ratio, but islet size was similar to wild type. In contrast, VEGFR1 agonist expression caused a decrease in endothelial to endocrine ratio, but surprisingly, islets were smaller than in wild type. Taken together, these results suggest that VEGFR1 and VEGFR2 signaling have distinct roles in pancreatic islet development, and the expansion of endocrine mass mediated by increased VEGF may require increased signaling through both receptors.