| Summary: | ?-Synuclein is central in Parkinson's disease pathogenesis. Although initially ?-synuclein was considered a purely intracellular protein, recent data suggest that it can be detected in the plasma and CSF of humans and in the culture media of neuronal cells. To address a role of secreted ?-synuclein in neuronal homeostasis, we have generated wild-type ?-synuclein and ?-galactosidase inducible SH-SY5Y cells. Soluble oligomeric and monomeric species of ?-synuclein are readily detected in the conditioned media (CM) of these cells at concentrations similar to those observed in human CSF. We have found that, in this model, ?-synuclein is secreted by externalized vesicles in a calcium-dependent manner. Electron microscopy and liquid chromatography-mass spectrometry proteomic analysis demonstrate that these vesicles have the characteristic hallmarks of exosomes, secreted intraluminar vesicles of multivesicular bodies. Application of CM containing secreted ?-synuclein causes cell death of recipient neuronal cells, which can be reversed after ?-synuclein immunodepletion from the CM. High- and low-molecular-weight ?-synuclein species, isolated from this CM, significantly decrease cell viability. Importantly, treatment of the CM with oligomer-interfering compounds before application rescues the recipient neuronal cells from the observed toxicity. Our results show for the first time that cell-produced ?-synuclein is secreted via an exosomal, calcium-dependent mechanism and suggest that ?-synuclein secretion serves to amplify and propagate Parkinson's disease-related pathology.
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