The copper-amyloid-beta-peptide complex of Alzheimer’s disease: affinity, structure, fibril formation and toxicity

Abstract

Senile plaques of Alzheimer’s disease (AD) patients are composed primarily of the amyloid-β-peptide peptide (Aβ), and within these plaques Cu2+ ions are found concentrated and directly bound to Aβ. Cu2+ homeostasis is severely impaired in AD patients and recent in vivo studies implicate Cu2+ in the etiology of AD. However the role of Cu2+ ions in AD is currently highly disputed due to the low reported affinity of Aβ for Cu2+ (nM and μM), and Cu2+ binding to Aβ is thought to result in amorphous aggregation rather than fibril formation. These two aspects, along with the coordination geometry, stoichiometry and toxicity of the Cu2+-Aβ complex were investigated in this thesis. In Chapter 3, circular dichroism and fluorescence spectroscopy alongside competitive metal capture show a surprisingly high picomolar affinity for both monomeric and fibrillar Aβ. In Chapter 4 electron paramagnetic resonance was used to study the structure and stoichiometry of the copper-Aβ complex in both monomeric and fibrillar Aβ. Both Aβ forms were able to bind a full stoichiometric complement of Cu2+ ions, with identical square planar coordination geometry. Importantly Cu2+ ion binding did not disrupt fibril structure. In Chapter 5 of this thesis it is shown that, in contrast to the predominant belief in the AD field, stoichiometric and sub-stoichiometric amounts of Cu2+ actually accelerate the rate of fibril formation. Finally, the toxic effects of Aβ and Cu2+ were studied in a PC12 clonal cell line. The presence of Cu2+ ions were found to enhance Aβ cell toxicity, and substoichiometric concentrations of Cu2+ were found to be the most toxic, suggesting that Cu2+ induced fibril formation and Cu2+ induced toxicity may be linked. Therefore this study finds considerable support for an altered amyloid hypothesis where Cu2+ dyshomeostasis has a central role in AD.