Characterizing nanoscale morphologic and mechanical properties of α-synuclein amyloid fibrils with atomic force microscopy

We have used atomic force microscopy (AFM) to image wild-type and disease-related mutant α-synuclein (aS) amyloid fibrils deposited on various hard and soft surfaces, ranging from freshly cleaved mica to two different supported lipid bilayers: phosphatidylcholine (POPC) and a mixture of POPC and phosphatidylglycerol (POPG). Quantitative morphologic analyses show that fibrils deposited on freshly cleaved mica, gold and glass substrates appeared to have similar heights and lengths, suggesting that the surface–fibril interaction in these cases does not influence the fibril morphology. When the same fibril sample is deposited on HOPG or quartz, the aS fibrils appear shorter in length and completely distorted, respectively, indicating that the interaction with these surfaces severely affects the fibril morphology. Sequentially recorded AFM images of fibrils on POPC bilayers clearly revealed that the amyloid fibrils were mobile on the bilayer, indicating that the fibrils are in thermal equilibrium on the surface. Persistence lengths of the fibrils on mica and POPC have been determined using two different methods, and reveal no significant differences, indicating that, although the fibrils appear immobile on mica, they do thermally equilibrate in 2D before they finally attach to the substrate. The persistence length of aS fibrils is found to be between 3.3 and 7.1 μm, which is within the expected range for amyloid fibrils. We observe no fibril movement on the mixed POPC/POPG bilayer, suggesting a stronger interaction with the lipids. Furthermore, the reduced heights of the fibrils on top of the mixed bilayer suggest that the fibrils are partly embedded within the bilayer.