Differential Localization of Laminin Subunits and MMP-9 in Mouse Retinal Angiogenesis

Endothelial basement membrane (BM) is a sheet-like extracellular matrix lining the abluminal surface of endothelium and plays critical roles in vascular function. During angiogenesis, the BM is digested locally when endothelial cells sprout, and reassembled when a new branch is formed. However, the detailed changes of BMs in developing blood vessels remain to be clarified. We used neonatal mouse retina as a model of angiogenesis and observed BM structure and composition. Whole-mount immunohistochemistry revealed co-localization of major BM proteins including laminins, type IV collagen, nidogen, and perlecan around the endothelial tip cells, which suggested the presence of endothelial BM even at the tip of growing vessels. The BMs of the tip cells contained only laminin α4 as a major laminin subunit, while that of “root” vessels contained both laminins α4 and α5, indicating that BM components change along with vessel maturation. MMP-9 immunoreactivities were found in association with endothelial BMs, suggesting that MMP-9 plays a role in remodeling of endothelial BM in developing retinal vascular network. These observations suggested that the BM is a constitutive structure for the vascular development and its components are regulated by vascular maturation and environmental factors.