Neuroinflammation contributes to neurodegeneration. In the neurodegenerative brain regions of patients with Parkinson’s disease, proinflammatory microglia colocalize with extracellular aggregates of the neuronal protein α-synuclein. Activation of microglia by α-synuclein involves the integrin subunit CD11b. Using a Boyden chamber migration assay, Wang et al. found that rat or mouse primary microglia migrated toward rat primary neurons that overexpressed α-synuclein, suggesting that this soluble protein acts as a chemoattractant. Either adding blocking antibodies to CD11b or knocking out CD11b prevented the migration of cultured microglia toward either neurons overexpressing α-synuclein or medium containing recombinant oligomerized α-synuclein (aggregates). Recombinant human α-synuclein aggregates interacted with purified Myc-tagged human CD11b in vitro and endogenous CD11b in wild-type microglia. Flow cytometry analysis of microglia exposed to α-synuclein detected both α-synuclein and CD11b on the surface of microglia. Preincubating microglia with either α-synuclein or the CD11b-blocking antibody prevented binding of the other, suggesting that α-synuclein is a ligand for CD11b. Wild-type microglia cultured with α-synuclein aggregates had increased activity of the enzyme NOX2 [nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2] and consequently increased production and secretion of superoxide anions. Either inhibiting NOX2 or knocking out the catalytic subunit of NOX2 in microglia inhibited the chemoattractant effect of α-synuclein, either when added as recombinant protein aggregates to the culture medium or produced by cocultured neurons. Superoxide anions are extracellularly converted to H2O2, and the extracellular concentration of H2O2 increased in microglia cultures exposed to α-synuclein. In a gel migration assay, mouse microglia extended lamellipodia and migrated toward H2O2. Addition of α-synuclein stimulated phosphorylation of the kinase Lyn in cell lysates from wild-type but not CD11b-/-microglia. Inhibiting Lyn pharmacologically or with RNA interference revealed that Lyn mediates the phosphorylation of cortactin and rearrangement of the cytoskeleton to enable microglial migration in response to H2O2 or α-synuclein. The findings provide a molecular mechanism for how α-synuclein aggregates promote neuroinflammation.
S. Wang, C. -H. Chu, T. Stewart, C. Ginghina, Y. Wang, H. Nie, M. Guo, B. Wilson, J. -S. Hong, J. Zhang, α-Synuclein, a chemoattractant, directs microglial migration via H2O2-dependent Lyn phosphorylation. Proc. Natl. Acad. Sci. U.S.A. 112, E1926–E1935 (2015). [Abstract] [Full Text]