Abstract 3056: Sirt1 Limits Endothelial Notch Signaling During Blood Vessel Growth Through NICD Deacetylation
Expansion of the blood vascular network through sprouting angiogenesis requires the specification of endothelial cells into leading (sprouting) tip and following (non-sprouting) stalk cells. This specification process is tightly controlled by Dll4/Notch signaling, which functions to actively suppress the tip cell phenotype in the stalk via lateral inhibition. Although it is well known that even subtle alterations in Notch activity can profoundly influence endothelial cell behavior and blood vessel formation, relatively little is known about the intrinsic regulation and dynamics of Notch signaling in endothelial cells. Using general and endothelial-restricted gene inactivation strategies, we provide evidence that the NAD-dependent deacetylase SIRT1 acts as a cell-intrinsic negative regulator of Notch signaling. This function of SIRT1 is mediated by its deacetylase activity, and selectively affects the subset of endothelial cells that receive instructive Notch signals, e.g. the stalk cells. Mechanistic analysis in three well-defined models of Dll4/Notch signaling, the mouse retina, the zebrafish model of intersegmental vessel formation and 3D endothelial sprouting assays revealed that endothelial cells lacking SIRT1 deacetylase activity are sensitized to Notch signaling resulting in enhanced Notch target gene expression and reduced endothelial proliferation thereby promoting a non-sprouting, stalk cell-like phenotype. As a molecular mechanism we show that the Notch1 intracellular domain (NICD) is reversibly acetylated and that SIRT1 directly binds NICD to promote its degradation by removing acetyl groups that otherwise competitively inhibit poly-ubiquitination. Our study thereby provides a novel molecular mechanism of how endothelial cells can adapt Notch responses to balance the number of tip and stalk cells by altering the activity of the SIRT1 deacetylase. SIRT1, itself regulated by cell metabolism and nutritional supply, could therefore provide an important adaptation mechanism linking tissue oxygenation and nutritional supply to Notch-dependent control of angiogenic sprouting and branching vessel morphogenesis.