Astrocytic cathepsin D promotes α-synuclein processing and Lewy neurite-like aggregate maturation.

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. Whether astrocytes actively contribute to α-syn processing and intercellular transfer remains unclear. Here, using a physiologically relevant neuron-astrocyte co-culture system that recapitulates tripartite synapse architecture, we show that astrocytes process α-syn and influence the morphological maturation of neuronal aggregates. Astrocytes internalize α-syn pre-formed fibrils (PFFs) and generate C-terminally truncated α-syn species via a Cathepsin D (CtsD)-dependent process. PFF-loaded astrocytes transfer α-syn-containing material to neurons and promote the formation and maturation of pS129-α-syn-positive Lewy neurite (LN)-like aggregates. Notably, α-syn PFF exposure induces lysosomal membrane damage, a senescence-like reactive state, and CtsD upregulation in astrocytes. Although PFF-containing lysosomes undergo autophagic engulfment, they persist within astrocytes, and a subset is detected beyond the astrocytic membrane boundary. Together, our findings support a model in which astrocytic lysosomal remodeling of internalized α-syn contributes to the morphological maturation and intercellular propagation of neuronal α-syn pathology in this co-culture system.