Abstract 327: ETS Factors Regulate the VEGF-Dependent, Arterial-Specific Expression of Dll4

Development of the vascular system begins with vasculogenesis, followed by the specification of endothelial cells into arterial and venous fates. Establishing and maintaining these separate fates is critical for the proper functioning of circulatory networks in the embryo as well as the mature adult. In fact, the congenital condition, Arteriovenous Malformation (AVM), arises due to defects during arterial-venous (AV) specification of endothelial cells, resulting in abnormal AV shunts that may burst leading to vascular lesions and stroke. To date, the molecular processes that control early AV specification remain incompletely understood. In the mouse, Delta-like 4 (Dll4) is a critical mediator of AV identity as loss of only one copy of Dll4 produces AV specification defects and embryonic lethality. Although previous studies have shown that Dll4 is the first Notch ligand expressed in the arterial endothelium and that its expression is induced by Vascular endothelial growth factor, the transcriptional pathways that direct its arterial-specific expression are unknown. Here we report the identification of an arterial-specific enhancer of the Dll4 gene. We show through deletion analyses as well as gain-of-function and loss-of-function studies that Vegf signaling activates ETS factors in the arterial endothelium to drive expression of Dll4. Surprisingly, Notch signaling is not required for the initiation of Dll4 expression but rather functions as a maintenance factor in the arterial endothelium. These studies provide novel mechanistic insight into the transcriptional program that mediates arterial specification and may provide insight into the underlying defects that occur during the development of AVMs.