Parkinson's disease is thought to be caused by the aggregation of alpha-synuclein (αS) within brain nerve cells. Astrocytes are the most abundant type of glial cell in the brain and function in a neuroprotective manner. However, in the disease stage, astrocytes are reported to exert neurotoxic effects. The precise manner in which astrocytes influence αS aggregation and propagation remains uncertain. In this study, we employ an in vitro co-culture system and elucidate the role of astrocytes in αS aggregation and propagation.