miR-155 regulates physiological angiogenesis but an miR-155-rich microenvironment disrupts the process by promoting unproductive endothelial sprouting.

Angiogenesis involves cell specification orchestrated by regulatory interactions between the vascular endothelial growth factor and Notch signaling pathways. However, the role of microRNAs in these regulations remains poorly explored. Here we show that a controlled level of miR-155 is essential for proper angiogenesis. In the mouse retina angiogenesis model, antimiR-155 altered neovascularization. In vitro assays established that endogenous miR-155 is involved in podosome formation, activation of the proteolytic machinery and cell migration but not in morphogenesis. The role of miR-155 was explored using miR-155 mimics. In vivo, exposing the developing vasculature to miR-155 promoted hypersprouting, thus phenocopying defects associated with Notch deficiency. Mechanistically, miR-155 overexpression weakened Notch signaling by reducing Smad1/5 expression, leading to the formation of tip cell-like cells which did not reach full invasive capacity and became unable to undergo morphogenesis. These results identify miR-155 as a novel regulator of physiological angiogenesis and as a novel actor of pathological angiogenesis.