Astrocytic Cathepsin D truncates α-synuclein and promotes Lewy neurite-like aggregation in neurons

Parkinson’s disease (PD) and related synucleinopathies are marked by the accumulation and propagation of α-synuclein (α-syn) aggregates, a process primarily studied in neurons. However, the contribution of astrocytes to this process remains unclear. Here, using a physiologically relevant neuron–astrocyte co-culture system that recapitulates tripartite synapse architecture, we show that astrocytes actively process and propagate α-syn aggregates. Astrocytes internalize α-syn pre-formed fibrils (PFFs) and cleave them into C-terminally truncated, seeding-competent species via the lysosomal protease Cathepsin D (CtsD). These truncated species are subsequently transferred to neurons, where they promote the growth of Lewy neurite (LN)-like aggregates. Notably, α-syn PFF exposure disrupts lysosomal membrane integrity in astrocytes, leading to CtsD upregulation. These findings reveal a feed-forward mechanism in which astrocytic lysosomal dysfunction amplifies α-syn pathogenicity. Our findings identify astrocytes as active contributors to α-syn propagation and highlight the astrocytic lysosomal pathway as a potential therapeutic target in PD.