Nitrated Alpha-Synuclein and Microglial Neuroregulatory Activities

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Abstract
Microglial neuroinflammatory responses affect the onset and progression of Parkinson’s disease (PD). We posit that such neuroinflammatory responses are, in part, mediated by microglial interactions with nitrated and aggregated α-synuclein (α-syn) released from Lewy bodies as a consequence of dopaminergic neuronal degeneration. As disease progresses, secretions from α-syn-activated microglia can engage neighboring glial cells in a cycle of autocrine and paracrine amplification of neurotoxic immune products. Such pathogenic processes affect the balance between a microglial neurotrophic and neurotoxic signature. We now report that microglia secrete both neurotoxic and neuroprotective factors after exposure to nitrated α-syn (N-α-syn). Proteomic (surface enhanced laser desorption–time of flight, 1D sodium dodecyl sulfate electrophoresis, and liquid chromatography-tandem mass spectrometry) and limited metabolomic profiling demonstrated that N-α-syn-activated microglia secrete inflammatory, regulatory, redox-active, enzymatic, and cytoskeletal proteins. Increased extracellular glutamate and cysteine and diminished intracellular glutathione and secreted exosomal proteins were also demonstrated. Increased redox-active proteins suggest regulatory microglial responses to N-α-syn. These were linked to discontinuous cystatin expression, cathepsin activity, and nuclear factor-kappa B activation. Inhibition of cathepsin B attenuated, in part, N-α-syn microglial neurotoxicity. These data support multifaceted microglia functions in PD-associated neurodegeneration.