Identification of a specific α-synuclein peptide (α-Syn 29-40) capable of eliciting microglial superoxide production to damage dopaminergic neurons

Background: Misfolded α-synuclein (α-Syn) aggregates participate in the pathogenesis of synucleinopathies, such as Parkinson's disease. Whereas much is known about how the various domains within full-length α-Syn (FL-α-Syn) contribute to the formation of α-Syn aggregates and therefore to their neurotoxicity, little is known about whether the individual peptides that can be generated from α-syn, possibly as intermediate metabolites during degradation of misfolded α-Syn aggregates, are neurotoxic themselves. Methods: A series of synthesized α-Syn peptides, corresponding to the locus in FL-α-Syn containing alanine 30, substitution of which with a proline causes a familial form of Parkinson's disease, were examined for their capacity of inducing release of microglial superoxide. The neurotoxicity of these peptides was measured according to their influence on the ability of neuroglial cultures deficient in gp91 ^phox , the catalytic unit of NADPH oxidase (Nox2), or wild-type cultures to take up ³H-labeled dopamine and on the number of tyrosine hydroxylase-staining-positive neurons. Western blots and confocal images were utilized to analyze membrane translocation of p47 ^phox and p67 ^phox , phosphorylation of p47 ^phox and Erk1/2 kinase, and binding of α-Syn peptides to gp91 ^phox . Activation of brain microglia in mice injected with α-Syn peptides was demonstrated by immunostaining for major histocompatibility complex (MHC)-II along with qPCR for Iba-1 and MHC-II. Results: We report α-Syn (29-40) as a specific peptide capable of activating microglial Nox2 to produce superoxide and cause dopaminergic neuronal damage. Administered to mice, this peptide also activated brain microglia to increase expression of MHC-II and Iba-1 and stimulated oxidation reaction. Exploring the underlying mechanisms showed that α-Syn (29-40) peptide triggered Nox2 to generate extracellular superoxide and its metabolite H₂O₂ by binding to the catalytic unit gp91 ^phox of Nox2; diffusing into cytosol, H₂O₂ activated Erk1/2 kinase to phosphorylate p47 ^phox and p67 ^phox and further activated Nox2, establishing a positive feedback loop to amplify the Nox2-mediated response. Conclusions: Collectively, our study suggests novel information regarding how α-Syn causes neuronal injury, possibly including mechanisms involving abnormal metabolites of α-Syn aggregates.