The Influence of Copper and Zinc on the Self-assembly of Amyloid-β from Alzheimer's Disease.

Abstract

Alzheimer’s disease is characterised by the misfolding and aggregation of a native peptide, Aβ, for which there are several isoforms, Aβ(1-40) being the most common, and Aβ(1-42) being most closely associated with Alzheimer’s disease. Upon misfolding, Aβ self-associates to form a number of aggregate species. What triggers this process of misfolding-aggregation, and determines which aggregate species forms, is not known. One possible determinant is metal homeostasis, which in Alzheimer’s patients is deregulated. Chapter 3 characterises how physiologically relevant levels of Cu2+ influence the misfolding pathway of Aβ. A ThT fluorescence assay found that Cu2+ is able to accelerate formation of Aβ(1-40) amyloid fibres; however, for Aβ(1-42), Cu2+ abolished fibre formation. Electron microscopy revealed that this is because Cu2+ stabilised Aβ(1-42) oligomers. These oligomers more readily disrupted lipid membranes than mature amyloid fibres, suggesting that the elevated levels of Cu2+ and the greater Aβ(1-42) synaptotoxicity in Alzheimer’s disease may be related. Chapter 4 investigates the effect of Zn2+ on Aβ misfolding. Trace levels of Zn2+ are demonstrated to entirely abolish fibre growth, for both Aβ(1-40) and Aβ(1-42). It is found that that Zn2+ likely exerts such a dramatic effect through a rapid exchange of Zn2+ between Aβ molecules. Chapter 5 found that Cu2+ accelerated Aβ(1-40) fibre growth regardless of growth conditions, despite growth conditions influencing fibril morphology. It was also found that Cu2+ generated Aβ(1-40) fibres did not exhibit an altered stability, further suggesting that the effect of Cu2+ upon Aβ(1-40) is limited to fibril growth kinetics, in contrast to the effect of Cu2+ on Aβ(1-42), as well as the effect of Zn2+ upon either peptide. The present research has identified a diversity of significant interactions between Aβ, and Cu2+ and Zn2+, highlighting a potential role for these metal ions in Alzheimer’s disease.