Microscopic Dynamics of Amyloid β Fibrillization

Amyloid fibrils are β-sheet-rich protein assemblies, which could trigger various diseases, including Alzheimer’s. One crucial step in searching for cures is to understand the aggregation mechanism; yet despite ever-developing techniques and tools, it remains poorly understood. Common experimental methods bear certain limitations. In this study, we employed time-resolved in situ scanning probe microscopy to directly probe the fibrillization pathway of amyloid-β 40, a short peptide believed to be responsible for Alzheimer’s. Amyloid plaque deposition is the hallmark of Alzheimer’s. The fibrillization pathway is extraordinarily complex. Numerous fundamental questions remain unanswered. To elucidate the nature of the intermediate state for Aβ monomer incorporation into fibril, we employed time-resolved in situ atomic force microscopy to monitor the growth of single fibrils at various peptide and urea concentrations. We proved that AFM is a reliable tool for direct measurement of individual fibril growth rates. The growth rates do not correlate with the fibril thickness, indicating lack of cooperativity between adjacent protofilaments during growth. The opposite ends on a fibril grow at similar rates and are steady, in contrast to the “stop-and-go” mechanism. Most importantly, the bimolecular rate constant for monomer incorporation into fibril is significantly smaller than the diffusion limit, indicating an intermediate state with relatively high free energy. With urea in the system, we discovered that both the Aβ peptide solubility and the fibril growth rate constant increase. We attribute this behavior to the presence of a frustrated complex supported by nonnative contacts in the equilibrium structure of the fibril tip. Further investigation of the role of frustrated complex in intermediate state focused on the “Dutch variant” E22QAβ, which is responsible for early-onset Alzheimer’s. The E22QAβ peptide exhibits a lower solubility and a significantly higher growth rate constant, confirming the role of E22 residue in the frustrated contacts that impedes fibrillization Fibrils nucleated on supported phospholipid bilayers were also investigated, for their distinct polymorphism and toxic nature, and possible relationship to a modified frustrated fibrillization intermediate state. We found that lipid bilayers interact with A oligomers and fibrils, and different curvatures induce different polymorphisms and kinetics of fibrillization.