Abstract 3660: Signal-Responsive Repression of Notch1 Signaling by the Sirt1 Deacetylase Regulates Sprouting Angiogenesis
The SIRT1 deacetylase is the closest mammalian homologue of yeast Sir2, which regulates life span in response to caloric restriction. A previous study from our laboratory identified SIRT1 as a key regulator of endothelial angiogenic functions during blood vessel growth. To more precisely characterize the molecular nature of the defects in blood vessel formation associated with SIRT1 deficiency, we performed time lapse microscopy in SIRT1-deficient zebrafish embryos focusing on intersegmental vessel development using the tg(fli1:eGFP) line. Compared to the highly organized process of blood vessel development in the control embryos, we observed pathfinding defects and vascular regression in the SIRT1 zebrafish morphants due to dysregulated endothelial tip cell activity and a failure of endothelial stalk cells to maintain vessel growth. Conditional deletion of SIRT1 in the endothelial lineage of mice (Tie2Cretg;SIRT1flox/-) led to a remarkably similar vascular phenotype in the developing retina characterized by defects in filopodial extension and vessel branching and a decrease in vascular density. Likewise, sprouting angiogenesis and branching morphogenesis in vitro were blocked in endothelial cells derived from the endothelial-restricted SIRT1 mutant mice suggesting that SIRT1 modulates endothelial tip/stalk behaviour during sprouting angiogenesis. Since delta-like 4- (Dll4) / Notch1-signaling is essential for tip/stalk cell selection during vascular growth, we investigated whether SIRT1 might interfere with the Notch pathway. Knock down of SIRT1 had no effect on the cleavage of Notch1 or the expression of Notch-regulated genes under basal conditions, but amplified Dll4- and Notch1 intracellular domain (NICD)-induced activation of Notch-regulated genes in endothelial cells. Consistent with these data, overexpression of SIRT1 dose-dependently repressed NICD-mediated activation of Notch1 target genes by specifically associating with NICD, but not the transmembrane Notch1 receptor. In summary, these findings identify SIRT1 as a novel modulator of endothelial tip/stalk formation by acting as a Dll4-dependent repressor of Notch1 signaling within the angiogenic sprout.