First, using a well-characterised human recombinant α-Syn oligomers-induced microglial activation model, we showed that exosomes released from these microglia contained CTSL, which is a key enzyme involved in the pathogenesis of PD, and that CTSL was released from α-Syn oligomers-treated microglia into the extracellular fluid primarily via exosomes. Here, CTSL is linked to Parkinson disease.