Copper Redox Cycling Inhibits Aβ Fibre Formation and Promotes Fibre Fragmentation, while Generating a Dityrosine Aβ Dimer

Oxidative stress and the formation of plaques which contain amyloid-beta (A beta) peptides are two key hallmarks of Alzheimer's disease (AD). Dityrosine is found in the plaques of AD patients and A beta dimers have been linked to neurotoxicity. Here we investigate the formation of A beta dityrosine dimers promoted by Cu2+/+ Fenton reactions. Using fluorescence measurements and UV absorbance, we show that dityrosine can be formed aerobically when A beta is incubated with Cu2+ and hydrogen-peroxide, or in a Cu2+ and ascorbate redox mixture. The dityrosine cross-linking can occur for both monomeric and fibrillar forms of A beta. We show that oxidative modification of A beta impedes the ability for A beta monomer to form fibres, as indicated by the amyloid specific dye Thioflavin T (ThT). Transmission electron microscopy (TEM) indicates the limited amyloid assemblies that form have a marked reduction in fibre length for A beta(1-40). Importantly, the addition of Cu2+ and a reductant to preformed A beta(1-40) fibers causes their widespread fragmentation, reducing median fibre lengths from 800 nm to 150 nm upon oxidation. The processes of covalent cross-linking of A beta fibres, dimer formation, and fibre fragmentation within plaques are likely to have a significant impact on A beta clearance and neurotoxicity.