Redox reactivities of membrane-bound amyloid-β-Cu complexes and their targeting by metallothionein-3

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-β peptide (Aβ_1-40/42) in the central nervous system (CNS). Copper coordination to Aβ triggers Aβ_1-40/42 aggregation and promotes the catalytic generation of reactive oxygen species (ROS). Due to its amphiphilic nature, Aβ_1-40/42 can interact with cell membranes and compromise their integrity. In this work, we characterized the insertion of Aβ_1-42 into an artificial lipid bilayer system mimicking cell membranes and demonstrate that the Aβ_1-42-lipid interaction does not prevent the Cu²⁺ coordination to Aβ_1-42. We performed a comparative analysis of the redox reactivities of membrane-bound Aβ_1-42 (memAβ_1-42-Cu²⁺) with soluble Aβ_1-42-Cu²⁺ establishing that membrane insertion leads to memAβ_1-42-Cu²⁺ complexes featuring an enhanced detrimental catechol oxidase activity towards the neurotransmitter dopamine. Moreover, memAβ_1-42-Cu²⁺ efficiently catalyzes Aβ di-tyrosine crosslinking and hydroxyl radical production in the presence of ascorbate. In addition, we establish that memAβ_1-42-Cu²⁺ redox reactivity catalyzes polyunsaturated fatty acids (PUFAs) lipid peroxidation, leading to the generation of malondialdehyde (MDA) toxic end-product. This reactivity compromises the structural integrity of the lipid bilayers resulting in membrane leakage.

Metallothioneins (MTs) are cysteine-rich metalloproteins central to neuronal and astrocytic metal homeostasis. MTs bind d¹⁰ metals (Cu⁺ and Zn²⁺) forming two metal thiolate clusters in their structure. In the CNS, the metallothionein-3 (MT-3) isoform possess a neuroprotective role, but it is downregulated in AD patients. MT-3 controls aberrant protein-Cu²⁺ interactions and redox reactivities of amyloidogenic protein-Cu²⁺ complexes, including soluble Aβ_1-40. In this work, we unravel that the detrimental memAβ_1-42-Cu²⁺ redox reactivities can also be efficiently silenced by MT-3 via metal swap reactions, by scavenging and reducing Cu²⁺ to Cu⁺ in its β-domain using thiolates as electron source, forming the redox-inert Cu⁺₄Zn²⁺₄MT-3 species. Consequently, MT-3 efficiently prevents lipid peroxidation and protects membrane structural integrity. New strategies targeting membrane-bound Aβ_1-42-Cu²⁺ complexes as key players in AD etiology could be envisioned.