Investigating the stability of amyloid assemblies towards fragmentation

Amyloid fibrils are densely packed, highly polymorphic protein aggregates typically found in patients with neurodegenerative disorders such as Alzheimer's and Parkinson's disease. The fibrils are characterised by a cross-β core comprising of a succession of contiguous β-strands stacked perpendicularly to the fibril axis. The organisation attributes to the tough and highly stable nature of amyloid structures that allows them to withstand degradation and serve a variety of normal physiological functions in organisms ranging from bacteria to mammals. It is unclear why some amyloid structures are beneficial and others pathogenic. Increasing research into the amyloid lifecycle, the process by which soluble monomeric proteins and peptides misfold and self-assemble into amyloid fibrils, has revealed four main stages: primary nucleation, elongation, secondary nucleation and fibril fragmentation. Interestingly, fragmentation is thought to play a key role in the propagation of the amyloid conformation due to striking effects on the fibril structure, therefore it may provide insight into their stability and pathogenicity. In this study, the mechanical stability of in vitro formed fibrils of Sup35NM, a functional prion, and α-synuclein, a prion-like amyloid associated with Parkinson's disease, were analysed by sonication followed by atomic force microscopy imaging. Results confirm a decrease in the mean α-synuclein particle length with increased sonication, as well as an increase in the number of fibril extension sites for elongation. Both factors would significantly enhance the potential for cytotoxicity due to increased biological availability of short amyloid particles for interaction with other cellular components and membranes, the diffusion across which is further enhanced by the reduction in overall fibril size. The infective potential of amyloid particles is measured by the likelihood of internalisation of particles by cells. Therefore, fragmentation is crucial for the propagation of the amyloid conformation since it successfully increases the load of small cytotoxic amyloid species with highly infectious potentials compared to their longer counterparts.