The Role of Venous Endothelial Cells in Developmental and Pathologic Angiogenesis

Background: Angiogenesis is a dynamic process that involves expansion of a pre-existing vascular network that can occur in a number of physiologic and pathologic settings. Despite its importance, the origin of the new angiogenic vasculature is poorly defined. In particular, the primary subtype of endothelial cells (capillary, venous, arterial) driving this process remains undefined. Methods: Endothelial cells were fate-mapped using genetic markers specific to arterial, capillary cells. In addition, we identified a novel venous endothelial marker gene (Gm5127) used it to generate inducible venous endothelial-specific Cre and Dre driver mouse lines. Contributions of these various types of endothelial cells to angiogenesis were examined during normal postnatal development and in disease-specific setting. Results: Using a comprehensive set of endothelial subtype-specific inducible reporter mice, including tip-, arterial- and venous- endothelial reporter lines, we showed that venous endothelial cells are the primary endothelial subtype responsible for the expansion of an angiogenic vascular network. During physiologic angiogenesis, venous endothelial cells proliferate, migrating against the blood flow, and differentiating into tip, capillary and arterial endothelial cells of the new vasculature. Using intravital 2-photon imaging, we observed venous endothelial cells migrating against the blood flow to form new blood vessels. Venous endothelial cell migration also plays a key role in pathologic angiogenesis. This was observed both in formation of arterio-venous malformations in mice with inducible endothelial-specific Smad4 deletion mice and in pathologic vessel growth seen in oxygen-induced retinopathy. Conclusions: Our studies establish venous endothelial cells are primary endothelial subtype responsible for the normal expansion of vascular networks, formation of arterio-venous malformations and pathologic angiogenesis. These observations highlight the central role of the venous endothelium in normal development and disease pathogenesis.