Copper Abolishes the β-Sheet Secondary Structure of Preformed Amyloid Fibrils of Amyloid-β 42

Abstract

The observation of the co-deposition of metals and amyloid-β₄₂ (Aβ₄₂) in brain tissue in Alzheimer's disease prompted myriad investigations into the role played by metals in the precipitation of this peptide. Copper is bound by monomeric Aβ₄₂ and upon precipitation of the copper-peptide complex thereby prevents Aβ₄₂ from adopting a β-sheet secondary structure. Copper is also bound by β-sheet conformers of Aβ₄₂, and herein we have investigated how this interaction affects the conformation of the precipitated peptide. Copper significantly reduced the thioflavin T fluorescence of aged, fibrillar Aβ₄₂ with, for example, a 20-fold excess of the metal resulting in a ca 90% reduction in thioflavin T fluorescence. Transmission electron microscopy showed that copper significantly reduced the quantities of amyloid fibrils while Congo red staining and polarized light demonstrated a copper-induced abolition of apple-green birefringence. Microscopy under cross-polarized light also revealed the first observation of spherulites of Aβ₄₂. The size and appearance of these amyloid structures were found to be very similar to spherulites identified in Alzheimer's disease tissue. The combined results of these complementary methods strongly suggested that copper abolished the β-sheet secondary structure of pre-formed, aged amyloid fibrils of Aβ₄₂. Copper may protect against the presence of β-sheets of Aβ₄₂ in vivo, and its binding by fibrillar Aβ₄₂ could have implications for Alzheimer's disease therapy.