Fibrillar β-amyloid-stimulated intracellular signaling cascades require Vav for induction of respiratory burst and phagocytosis in monocytes and microglia

Microglial interaction with extracellular beta-amyloid fibrils ( fA beta) is mediated through an ensemble of cell surface receptors, including the B-class scavenger receptor CD36, the alpha(6)beta(1)-integrin, and the integrin-associated protein/CD47. The binding of fA beta to this receptor complex has been shown to drive a tyrosine kinase-based signaling cascade leading to production of reactive oxygen species and stimulation of phagocytic activity; however, little is known about the intracellular signaling cascades governing the microglial response to fA beta. This study reports a direct mechanistic link between the fA beta cell surface receptor complex and downstream signaling events responsible for NADPH oxidase activation and phagosome formation. The Vav guanine nucleotide exchange factor is tyrosine-phosphorylated in response to fA beta peptides as a result of the engagement of the microglia fA beta cell surface receptor complex. Co-immunoprecipitation studies demonstrate an A beta-dependent association between Vav and both Lyn and Syk kinases. The downstream target of Vav, the small GTPase Rac1, is GTP-loaded in an A beta-dependent manner. Rac1 is both an essential component of the NADPH oxidase and a critical regulator of microglial phagocytosis. The direct role of Vav in fA beta-stimulated intracellular signaling cascades was established using primary microglia obtained from Vav(-/-) mice. Stimulation of Vav(-/-) microglia with fA beta failed to generate NADPH oxidase-derived reactive oxygen species and displayed a dramatically attenuated phagocytic response. These findings directly link Vav phosphorylation to the A beta-receptor complex and demonstrate that Vav activity is required for fA beta-stimulated intracellular signaling events upstream of reactive oxygen species production and phagosome formation.