Amyloid β Perturbs Cu(II) Binding to the Prion Protein in a Site-Specific Manner: Insights into Its Potential Neurotoxic Mechanisms

Amyloid beta (A beta) is a Cu-binding peptide that plays a key role in the pathology of Alzheimer's disease. A recent report demonstrated that A beta disrupts the Cu-dependent interaction between cellular prion protein (PrPC) and N-methyl-D-aspartate receptor (NMDAR), inducing overactivation of NMDAR and neurotoxicity. In this context, it has been proposed that A beta competes for Cu with PrPC; however, there is no spectroscopic evidence to support this hypothesis. Prion protein (PrP) can bind up to six Cu(II) ions: from one to four at the octarepeat (OR) region, producing low- and high-occupancy modes, and two at the His96 and His111 sites. Additionally, PrPC is cleaved by alpha-secretases at Lys110/His111, yielding a new Cu(II)-binding site at the alpha-cleaved His111. In this study, the competition for Cu(II) between A beta(1-16) and peptide models for each Cu-binding site of PrP was evaluated using circular dichroism and electron paramagnetic resonance. Our results show that the impact of A beta(1-16) on Cu(II) coordination to PrP is highly site-specific: A beta(1-16) cannot effectively compete with the low-occupancy mode at the OR region, whereas it partially removes the metal ion from the high-occupancy modes and forms a ternary OR-Cu(II)-A beta(1-16) complex. In contrast, A beta(1-16) removes all Cu(II) ions from the His96 and His111 sites without formation of ternary species. Finally, at the alpha-cleaved His111 site, A beta(1-16) yields at least two different ternary complexes depending on the ratio of PrP/ Cu(II)/A beta. Altogether, our spectroscopic results indicate that only the low-occupancy mode at the OR region resists the effect of A beta, while Cu(II) coordination to the high-occupancy modes and all other tested sites of PrP is perturbed, by either removal of the metal ion or formation of ternary complexes. These results provide important insights into the intricate effect of A beta on Cu(II) binding to PrP and the potential neurotoxic mechanisms through which A beta might affect Cu-dependent functions of PrPC, such as NMDAR modulation.