Reverse engineering Lewy bodies suggests α-synuclein pathology as condensatopathy

Understanding how soluble monomeric α-synuclein transitions into pathological α-synuclein (Lewy bodies) has remained a central challenge in neurodegeneration. Existing models, largely based on treating cells and animals with pre-formed α-synuclein fibrils cannot account for the emergence of the first inclusion without pre-existing seeds. Here, we combine in vitro reconstitutions, computational modeling and a minimal cellular system to show that phase separation of α-synuclein with an interacting scaffold can recapitulate Lewy body-like structures. An unbiased proteomics screen, live-cell imaging, correlative light-electron microscopy, and soft X-ray tomography show that Lewy body-like structures are associated with redistribution of actin and membrane-bound organelles, with mitochondria accumulating at their periphery in an actin-dependent manner. Structural analyses further indicate that α-synuclein condensates provide local environments in which the insoluble (-sheets can form. Together, these findings support a seed-independent trajectory from soluble α-synuclein through condensation and progressive immobilization to Lewy body-like assemblies and, subsequently, insoluble (-sheet fibrils. This framework offers a new mechanistic route for understanding the disease progression and promises to guide the design of innovative therapeutic interventions.