Laminin-Dependent Interaction between Astrocytes and Microglia: A Role in Retinal Angiogenesis

Retinal vascular diseases are among the leading causes of acquired blindness. In recent years, retinal microglia have been shown to influence vascular branching density and endothelial cell proliferation. However, how microglial recruitment and activation are regulated during development remains unclear. We hypothesized that microglial recruitment, activation, and down-stream signaling are modulated by components of the mural basement membrane. We used a reverse genetic approach to disrupt laminin expression in the vascular basement membrane and demonstrate that microglia respond to the mural basement membrane in an isoform-specific manner. Microglial density is significantly increased in the laminin γ3-null (Lamc3^-/-) retinal superficial vascular plexus and consequently the vascular branching density is increased. Microglia also respond to astrocyte-derived matrices and become hyperactivated in the Lamc3^-/- retina or when tested in vitro with cell-derived matrix. Pharmacological activation of microglia in the wild-type retina produced an Lamc3^-/--like vascular phenotype, whereas pharmacological blocking of microglial activation in the Lamc3^-/- retina rescued the wild-type vascular phenotype. On the molecular level, microglial transforming growth factor-β1 expression is down-regulated in the Lamc3^-/- retina, and SMAD signaling decreased in endothelial cells with a consequent increase in endothelial proliferation. The reverse effects were seen in the Lamb2^-/- retina. Together, our results demonstrate a novel mechanism by which laminins modulate vascular branching and endothelial cell proliferation during retinal angiogenesis.