Multiple system atrophy (MSA) is a neurodegenerative disease in which oligodendrocytes and neurons are affected in the central nervous system. MSA is pathologically characterized by abnormal accumulation of α-synuclein inclusions in oligodendrocytes, which are diagnostic of MSA. Accumulation of α-synuclein compromises neuronal function and viability. To clearify how oligodendrocytic α-synuclein inclusions cause neuronal degeneration in MSA, we established primary culture cells derived from transgenic (Tg) mice in which human wild-type α-synuclein was overexpressed selectively in oligodendrocytes. We identified the protein microtubule β-III tubulin, which interacts with α-synuclein to form an insoluble protein complex that progressively accumulates in neurons. The binding of neuronal α-synuclein to β-III tubulin is a key process in the development of neuronal degeneration in the MSA mouse model. We investigated how oligodendrocytic accumulation of α-synuclein causes neuronal accumulation of insoluble α-synuclein. We showed that the expression of α-synuclein was induced by treatment of the conditioned media derived from Tg mouse cell cultures, and neuronal accumulation of α-synuclein was developed in non-Tg mouse cells. Our data suggested that oligodendrocyte-drived signals are induced neuronal α-synuclein accumulation in an MSA model. In the present study, we identify cystatin C that triggers α-synuclein upregulation and insoluble α-synuclein accumulation in neurons. Cystatin C is released by oligodendrocytes derived from Tg mice and extracellular cystatin C increases the expression of the endogenous α-synuclein gene in non-Tg mouse neurons. These neurons then accumulate insoluble α-synuclein, leading to the degeneration. Cystatin C is a pathogenic signal triggering neurodegeneration in MSA.