42 E. B. Pasquale
blood vessel remodeling at least in part by upregulating the expression of angiopoietin-1 and Tie-2 (Sec. 4.2). In turn, however, angiopoietin- 1 has been shown to downregulate ephrin-B2 expression in human dermal microvascular endothelial cells in culture30 and EphB receptor activation by ephrin-B2 Fc counteracts angiopoietin-1-induced cell migration in HUVE cells.26 Furthermore, transgenic co-expression of angiopoietin-2 (another member of the angiopoietin family) and VEGF reduces the number of ephrin-B2-positive blood vessels, resulting in a fraction of blood vessels that express neither ephrin-B2 nor EphB4.52
Interestingly, pulmonary microvascular endothelial cells isolated from EphA2 knockout mice have normal ephrin-A1 expression but increased expression of EphB4 and ephrin-B2, suggesting compensatory mechanisms and crosstalk between Eph receptors and ephrins of the A and B subclasses.56 Although all these findings do not yet provide a cohesive picture, they nevertheless show that there are many forms of crosstalk within the Eph family and between the Eph system and other angiogenic signaling pathways, leading to complex networks of positive and negative feedback loops that coordinately regulate the properties of vascular cells.
3. Endothelial Cell Fate
Eph receptors and ephrins are expressed in stem cells and progenitor cells of different lineages, where they regulate the balance of proliferation and self-renewal versus differentiation, cell fate determination, and even cell death.1,57 Both EphB4 and ephrin-B2 are expressed in mouse undifferentiated embryonic stem cells and in early embryoid bodies derived by culturing those stem cells.58 Interestingly, EphB4 has been implicated in the differentiation of a common precursor cell, the hemangioblast, towards endothelial and hematopoietic cell lineages.58,59 When embryoid bodies are used to generate hemangioblast cells in vitro, mimicking developmental events occurring in the blood islands of the yolk sac, EphB4-deficient embryoid bodies display defects in hemangioblast development.58 EphB4 knockout embryoid bodies also produce reduced numbers of endothelial cells and have impaired ability to develop endothelial cells sprouts in the presence of angiogenic growth