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

Abstract

Microglial interaction with extracellular β-amyloid fibrils (fAβ) is mediated through an ensemble of cell surface receptors, including the B-class scavenger receptor CD36, the α6β1- integrin, and the integrin-associated protein/CD47. The binding of fAβ 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 fAβ. This study reports a direct mechanistic link between the fAβ 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 fAβ peptides as a result of the engagement of the microglia fAβ cell surface receptor complex. Coimmunoprecipitation studies demonstrate an Aβ-dependent association between Vav and both Lyn and Syk kinases. The downstream target of Vav, the small GTPase Rac1, is GTP-loaded in an Aβ-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 fAβ-stimulated intracellular signaling cascades was established using primary microglia obtained from Vav-/- mice. Stimulation of Vav-/- microglia with fAβ failed to generate NADPH oxidase-derived reactive oxygen species and displayed a dramatically attenuated phagocytic response. These findings directly link Vav phosphorylation to the Aβ-receptor complex and demonstrate that Vav activity is required for fAβ-stimulated intracellular signaling events upstream of reactive oxygen species production and phagosome formation. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.