Angiogenesis is the main process mediating the expansion of the blood vessel network during development, tissue regeneration or in pathological conditions such as cancer. The formation of new endothelial sprouts, a key step in the angiogenic growth program, involves the selection of endothelial tip cells, which lack a lumen, are highly motile, extend numerous filopodia, and lead new sprouts. Angiogenic sprouting is induced by tissue-derived, pro-angiogenic signals such as vascular endothelial growth factor (VEGF), which activates and triggers signaling by cognate receptor tyrosine kinases in the endothelium. However, this response is strongly modulated by intrinsic signaling interactions between endothelial cells (ECs). For example, expression of the ligand Delta-like 4 (Dll4) in tip cells activates Notch receptors in adjacent (stalk) ECs and is thought to downregulate VEGF receptor expression in these cells. Thus, the tip cell phenotype is suppressed in stalk cells and a balance between sprouting and the necessary preservation of existing endothelial tubes is established. Our work is providing further insight into the regulation of sprouting angiogenesis. The Notch ligand Jagged1 is a potent pro-angiogenic regulator with the opposite role as Dll4. In contrast to current models, we found that Notch controls VEGFR2 only moderately whereas VEGFR3 is strongly regulated. Moreover, blocking of Notch enables angiogenic growth even in mutant animals lacking endothelial VEGFR2 expression. We also found that endothelial sprouting and proliferation extension depend on VEGF receptor endocytosis. Ephrin-B2, a ligand for Eph family receptor tyrosine kinases, is required for endothelial cell motility, VEGF receptor endocytosis and the activation of downstream signal transduction cascades. More recently, we have identified Disabled 2, a clathrin-associated sorting protein, and the cell polarity protein PAR-3 as novel interaction partners of ephrin-B2 and VEGF receptors. These results establish that regional VEGF receptor endocytosis, which is controlled by a complex containing Dab2, PAR-3 and ephrin-B2, play a key role in the spatial organization of angiogenic growth. Finally, we have started to address the role of endothelial cells in the regulation of tissue morphogenesis and homeostasis. In the adult bone marrow, angiogenic signaling by the VEGF and Notch pathways controls sinusoidal endothelial growth as well as endothelial-mesenchymal interactions, the formation of new hematopoietic cell-containing structures (which have termed hemospheres) and hematopoietic stem cell numbers. Our work delineates a first structural and functional framework for bone marrow homeostasis.

Acknowledgement: 

This study has been supported by the Max Planck Society and the German Research Foundation.