Distinct Effects of Zn2+, Cu2+, Fe3+, and Al3+ on Amyloid-β Stability, Oligomerization, and Aggregation

Abnormally high concentrations of Zn2+, Cu2+, and Fe3+ are present along with amyloid-beta (A beta) in the senile plaques in Alzheimer disease, where Al3+ is also detected. A beta aggregation is the key pathogenic event in Alzheimer disease, where A beta oligomers are the major culprits. The fundamental mechanism of these metal ions on A beta remains elusive. Here, we employ 4,4'-Bis(1-anilinonaphthalene 8-sulfonate) and tyrosine fluorescence, CD, stopped flow fluorescence, guanidine hydrochloride denaturation, and photo-induced cross-linking to elucidate the effect of Zn2+, Cu2+, Fe3+, and Al3+ on A beta at the early stage of the aggregation. Furthermore, thioflavin T assay, dot blotting, and transmission electron microscopy are utilized to examine A beta aggregation. Our results show that Al3+ and Zn2+, but not Cu2+ and Fe3+, induce larger hydrophobic exposures of A beta conformation, resulting in its significant destabilization at the early stage. The metal ion binding induces A beta conformational changes with micromolar binding affinities and millisecond binding kinetics. Cu2+ and Zn2+ induce similar assembly of transiently appearing A beta oligomers at the early state. During the aggregation, we found that Zn2+ exclusively promotes the annular protofibril formation without undergoing a nucleation process, whereas Cu2+ and Fe3+ inhibit fibril formation by prolonging the nucleation phases. Al3+ also inhibits fibril formation; however, the annular oligomers co-exist in the aggregation pathway. In conclusion, Zn2+, Cu2+, Fe3+, and Al3+ adopt distinct folding and aggregation mechanisms to affect A beta, where A beta destabilization promotes annular protofibril formation. Our study facilitates the understanding of annular A beta oligomer formation upon metal ion binding.