Brain-derived and in vitro-seeded alpha-synuclein fibrils exhibit distinct biophysical profiles

The alpha-synuclein (Syn) seeding amplification assay (SAA) that allows the generation of disease-specific in vitro seeded fibrils (SAA fibrils) is used as a research tool to study the connection between the structure of Syn fibrils, cellular seeding/spreading, and the clinico-pathological manifestations of different synucleinopathies. However, structural differences between human brain-derived and SAA Syn fibrils have been recently highlighted. Here, we characterize biophysical properties of the human brain-derived Syn fibrils from the brains of patients with Parkinsons disease with and without dementia (PD, PDD), dementia with Lewy bodies (DLB), multiple system atrophy (MSA) and compare them to the model SAA fibrils. We report that the brain-derived Syn fibrils show distinct biochemical profiles, which were not replicated in the corresponding SAA fibrils. Furthermore, the brain-derived Syn fibrils from all synucleinopathies displayed a mixture of- straight and twisted microscopic structures. However, the PD, PDD, and DLB SAA fibrils had a straight structure, whereas MSA SAA fibrils showed a twisted structure. Finally, the brain-derived Syn fibrils from all four synucleinopathies were phosphorylated (S129). Interestingly, phosphorylated Syn were carried over to the PDD and DLB SAA fibrils. Our findings demonstrate the limitation of the SAA fibrils modelling the brain-derived Syn fibrils and pay attention to the necessity of deepening the understanding of the SAA fibrillation methodology.