New Evidence on a Distinction between A beta 40 and A beta 42 Amyloids: Thioflavin T Binding Modes, Clustering Tendency, Degradation Resistance, and Cross-Seeding

The relative abundance of two main Abeta-peptide types with different lengths, A beta 40 and A beta 42, determines the severity of the Alzheimer's disease progression. However, the factors responsible for different behavior patterns of these peptides in the amyloidogenesis process remain unknown. In this comprehensive study, new evidence on A beta 40 and A beta 42 amyloid polymorphism was obtained using a wide range of experimental approaches, including custom-designed approaches. We have for the first time determined the number of modes of thioflavin T (ThT) binding to A beta 40 and A beta 42 fibrils and their binding parameters using a specially developed approach based on the use of equilibrium microdialysis, which makes it possible to distinguish between the concentration of the injected dye and the concentration of dye bound to fibrils. The binding sites of one of these modes located at the junction of adjacent fibrillar filaments were predicted by molecular modeling techniques. We assumed that the sites of the additional mode of ThT-A beta 42 amyloid binding observed experimentally (which are not found in the case of A beta 40 fibrils) are localized in amyloid clots, and the number of these sites could be used for estimation of the level of fiber clustering. We have shown the high tendency of A beta 42 fibers to form large clots compared to A beta 40 fibrils. It is probable that this largely determines the high resistance of A beta 42 amyloids to destabilizing effects (denaturants, ionic detergents, ultrasonication) and their explicit cytotoxic effect, which we have shown. Remarkably, cross-seeding of A beta 40 fibrillogenesis using the preformed A beta 42 fibrils changes the morphology and increases the stability and cytotoxicity of A beta 40 fibrils. The differences in the tendency to cluster and resistance to external factors of A beta 40 and A beta 42 fibrils revealed here may be related to the distinct role they play in the deposition of amyloids and, therefore, differences in pathogenicity in Alzheimer's disease.