Autophagic failure promotes the exocytosis and intercellular transfer of α-synuclein

The accumulation of abnormal protein aggregates is a major characteristic of many neurodegenerative disorders, including Parkinson's disease (PD). The intracytoplasmic deposition of alpha-synuclein aggregates and Lewy bodies, often found in PD and other alpha-synucleinopathies, is thought to be linked to inefficient cellular clearance mechanisms, such as the proteasome and autophagy/lysosome pathways. The accumulation of alpha-synuclein aggregates in neuronal cytoplasm causes numerous autonomous changes in neurons. However, it can also affect the neighboring cells through transcellular transmission of the aggregates. Indeed, a progressive spreading of Lewy pathology among brain regions has been hypothesized from autopsy studies. We tested whether inhibition of the autophagy/lysosome pathway in alpha-synuclein-expressing cells would increase the secretion of alpha-synuclein, subsequently affecting the alpha-synuclein deposition in and viability of neighboring cells. Our results demonstrated that autophagic inhibition, via both pharmacological and genetic methods, led to increased exocytosis of alpha-synuclein. In a mixed culture of alpha-synuclein-expressing donor cells with recipient cells, autophagic inhibition resulted in elevated transcellular alpha-synuclein transmission. This increase in protein transmission coincided with elevated apoptotic cell death in the recipient cells. These results suggest that the inefficient clearance of alpha-synuclein aggregates, which can be caused by reduced autophagic activity, leads to elevated alpha-synuclein exocytosis, thereby promoting alpha-synuclein deposition and cell death in neighboring neurons. This finding provides a potential link between autophagic dysfunction and the progressive spread of Lewy pathology.