Proteomic analysis of microglial contribution to mouse strain-dependent dopaminergic neurotoxicity

Although the pathogenesis of Parkinson's disease (PD) remains unknown, it appears that microglial activation is associated with enhanced neurodegeneration in animal models of PD as well as in PD patients. Experimentally, C57BL/6 and SWR/J mice demonstrate striking differences in the extent of dopaminergic (DAergic) neurodegeneration induced by a parkinsonian toxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The purpose of this study was to determine whether differences in microglial activation between these two strains of mice could provide insight into the variability seen in toxicant induced neuronal death, and subsequently to use a high-throughput proteomic method, combining stable isotope labeling with amino acids in cell culture (SILAC) with liquid chromatography and tandem mass spectrometry, to compare the microglial proteomes of C57BL/6 and SWR/J mice after stimulation with a classical microglial activator, lipopolysaccharide (LPS). We found that DAergic neurotoxicity induced by LPS in a primary neuron-microglia coculture was twofold greater with microglia isolated from the brains of C57BL/6 mice compared with that of SWR/J mice. Upon proteomic analysis we found that, out of over 1,000 proteins identified and quantified, 400 displayed a significant difference in their relative abundance between these two murine strains. Several proteins, which had relatively higher levels in C57BL/6 mice, have previously been implicated in LPS-mediated microglial activation, including those involved in the COX-2 pathway and in prostaglandin E-2 (PGE(2)) production. To validate our proteomic results we confirmed the increased expression level of iNOS in C57BL/6 vs. SWR/J microglia with semiquantitative Western blot. Further analysis of our proteomic discovery data will likely reveal numerous novel proteins involved in inflammation-mediated neurotoxicity in PD.