Abnormal interactions of Cu and Zn ions with the amyloid β (Aβ) peptide are proposed to play an important role in the pathogenesis of Alzheimer’s disease (AD). Disruption of these metal–peptide interactions using chemical agents holds considerable promise as a therapeutic strategy to combat this incurable disease. Reported herein are two bifunctional compounds (BFCs) L1 and L2 that contain both amyloid-binding and metal-chelating molecular motifs. Both L1 and L2 exhibit high stability constants for Cu2+ and Zn2+ and thus are good chelators for these metal ions. In addition, L1 and L2 show strong affinity toward Aβ species. Both compounds are efficient inhibitors of the metal–mediated aggregation of the Aβ42 peptide and promote disaggregation of amyloid fibrils, as observed by ThT fluorescence, native gel electrophoresis/Western blotting, and transmission electron microscopy (TEM). Interestingly, the formation of soluble Aβ42 oligomers in presence of metal ions and BFCs leads to an increased cellular toxicity. These results suggest that for the Aβ42 peptide – in contrast to the Aβ40 peptide, the previously employed strategy of inhibiting Aβ aggregation and promoting amyloid fibril dissagregation may not be optimal for the development of potential AD therapeutics, due to formation of neurotoxic soluble Aβ42 oligomers.

The coordination chemistry of Cu and Zn metal ions with the amyloid β (Aβ) peptides has attracted a lot of attention in recent years due to its implications in Alzheimer’s disease. A number of reports indicate that Cu and Zn have profound effects on Aβ aggregation. However, the impact of these metal ions on Aβ oligomerization and fibrillization is still not well understood, especially for the more rapidly aggregating and more neurotoxic Aβ42 peptide. Here we report the effect of Cu2+ and Zn2+ on Aβ42 oligomerization and aggregation using a series of methods such as Thioflavin T (ThT) fluorescence, native gel and Western blotting, transmission electron microscopy (TEM), and cellular toxicity studies. Our studies suggest that both Cu2+ and Zn2+ ions inhibit Aβ42 fibrillization. While presence of Cu2+ stabilizes Aβ42 oligomers, Zn2+ leads to formation of amorphous, non-fibrillar aggregates. The effects of temperature, buffer, and metal ion concentration and stoichiometry were also studied. Interestingly, while Cu2+ increases the Aβ42-induced cell toxicity, Zn2+ causes a significant decrease in Aβ42 neurotoxicity. While previous reports have indicated that Cu2+ can disrupt β-sheets and lead to nonfibrillar Aβ aggregates, the neurotoxic consequences were not investigated in detail. The data presented herein including cellular toxicity studies strongly suggest that Cu2+ increases the neurotoxicity of Aβ42 due to stabilization of soluble Aβ42 oligomers.

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