Dramatic Aggregation of Alzheimer Aβ by Cu(II) Is Induced by Conditions Representing Physiological Acidosis*

Abstract

The cortical deposition of Aβ is an event that occurs in Alzheimer’s disease, Down’s syndrome, head injury, and normal aging. Previously, in appraising the effects of different neurochemical factors that impact upon the solubility of Aβ, we observed that Zn²⁺ was the predominant bioessential metal to induce the aggregation of soluble Aβ at pH 7.4 in vitro and that this reaction is totally reversible with chelation. We now report that unlike other biometals tested at maximal biological concentrations, marked Cu²⁺-induced aggregation of Aβ_1–40 emerged as the solution pH was lowered from 7.4 to 6.8 and that the reaction was completely reversible with either chelation or alkalinization. This interaction was comparable to the pH-dependent effect of Cu²⁺ on insulin aggregation but was not seen for aprotinin or albumin. Aβ_1–40 bound three to four Cu²⁺ ions when precipitated at pH 7.0. Rapid, pH-sensitive aggregation occurred at low nanomolar concentrations of both Aβ_1–40 and Aβ_1–42 with submicromolar concentrations of Cu²⁺. Unlike Aβ_1–40, Aβ_1–42was precipitated by submicromolar Cu²⁺ concentrations at pH 7.4. Rat Aβ_1–40 and histidine-modified human Aβ_1–40 were not aggregated by Zn²⁺, Cu²⁺, or Fe³⁺, indicating that histidine residues are essential for metal-mediated Aβ assembly. These results indicate that H⁺-induced conformational changes unmask a metal-binding site on Aβ that mediates reversible assembly of the peptide. Since a mildly acidic environment together with increased Zn²⁺ and Cu²⁺ are common features of inflammation, we propose that Aβ aggregation by these factors may be a response to local injury. Cu²⁺, Zn²⁺, and Fe³⁺ association with Aβ explains the recently reported enrichment of these metal ions in amyloid plaques in Alzheimer’s disease.