Lipidic α-synuclein fibrils catalyze the formation of pathological mature fibrils
The aggregation of -synuclein into amyloid fibrils is a hallmark of a collection of neurodegenerative diseases termed synucleinopathies, including Parkinson's disease (PD). Interactions between -synuclein and lipid membranes modulate -synuclein aggregation and can produce protofibril structures consisting of both protein and lipid, termed lipidic fibrils. These lipidic fibrils can mature into mature fibrils, which resemble ex vivo fibrils extracted from Lewy bodies found in post-mortem brain samples of PD patients. Here, through the combination of experiments and kinetic theory, we study the influence of lipids and lipidic fibrils on the formation of mature fibrils. We find that lipidic fibrils catalyze mature fibril formation via a heterogeneous primary nucleation mechanism, which lowers the activation energy barrier of mature fibril formation relative to lipid-independent primary nucleation. The final stage of the reaction is governed by mature fibril elongation becoming limited by the rate at which protein monomers dissociate from the metastable lipidic fibrils, confirming that fibrils form in order of stability following Ostwald's Rule of Stages. This mechanism is validated using the small molecule inhibitor ambroxol, which selectively suppresses the formation of lipidic fibrils and in this manner shifts the aggregation kinetics to the slow, lipid-independent pathway. Taken together, these findings reveal that lipidic fibrils act as on-pathway intermediates and accelerate the aggregation of mature fibrils, which could provide a quantitative framework to design novel therapeutic strategies to inhibit pathological -synuclein aggregation.