Involvement of exosomes in dopaminergic neurodegeneration by microglial activation in midbrain slice cultures

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive degeneration of dopamine neurons in the substantia nigra. Microglial activation is frequently observed in the brains of patients with PD and animal models. Interferon-γ (IFN-γ)/lipopolysaccharide (LPS) treatment triggers microglial activation and the reduction of dopamine neurons in midbrain slice cultures. We have previously reported that nitric oxide (NO) is mainly involved in this dopaminergic degeneration. However, this degeneration was not completely suppressed by the inhibition of NO synthesis, suggesting that factors other than NO also contribute to dopaminergic neurodegeneration. Exosomes are extracellular vesicles with diameters of 40-200 nm that contain various proteins and micro RNAs and are regarded as a novel factor that mediates cell-to-cell interactions. Previous studies have demonstrated that exosome release is enhanced by microglial stimulation and that microglia-derived exosomes increases neuronal apoptosis. In the present study, we investigated whether exosomes are involved in dopaminergic neurodegeneration triggered by microglial activation in midbrain slice cultures. IFN-γ/LPS treatment to the midbrain slice cultures activated microglia, increased exosomal release, and decreased dopamine neurons. GW4869, an inhibitor of a neutral sphingomyelinase 2, decreased exosomal release and significantly prevented dopaminergic neurodegeneration by IFN-γ/LPS without affecting NO production. In contrast, D609, an inhibitor of sphingomyelin synthase and NO synthase, did not affect dopaminergic neurodegeneration, although it strongly inhibited NO production. The protective effect mediated by inhibition of NO synthase would be counteracted by enhanced exosomal release caused by D609 treatment. In addition, dopaminergic neurodegeneration is triggered by the treatment of exosomes isolated from culture media of IFN-γ/LPS-treated slices. These results suggest that exosomes are involved in dopaminergic neurodegeneration by microglial activation.