Egfl7 is differentially expressed in arteries and veins during retinal vascular development

The vasculature of the central nervous system (CNS) is composed of vascular endothelial and mural cells which interact closely with glial cells and neurons. The development of the CNS vascularisation is a unique process which requires the contribution of specific regulators in addition to the classical angiogenic factors. The egfl7 gene is mainly detected in endothelial cells during physiological and pathological angiogenesis. Egfl7 codes for a secreted protein which predominantly accumulates into the extracellular space where it controls vascular elastin deposition or the Notch pathway. Egfl7 is the host gene of the microRNA miR126 which is also expressed in endothelial cells and which plays major functions during blood vessel development. While the expression of egfl7 and that of miR126 were well described in endothelial cells during development, their pattern of expression during the establishment of the CNS vasculature is still unknown. By analysing the expression of egfl7 and miR126 during mouse retina vascularisation, we observed that while expression of miR126 is detected in all endothelia, egfl7 is initially expressed in all endothelial cells and then is progressively restricted to veins and to their neighbouring capillaries. The recruitment of mural cells around retina arteries coincides with the down-regulation of egfl7 in the arterial endothelial cells, suggesting that this recruitment could be involved in the loss of egfl7 expression in arteries. However, the expression pattern of egfl7 is similar when mural cell recruitment is prevented by the injection of a PDGFRβ blocking antibody, suggesting that vessel maturation is not responsible for egfl7 down-regulation in retinal arteries.