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) and its aggregation in senile amyloid plaques. Copper coordination to Aβ triggers Aβ _1-40/42 aggregation and, in the presence of biological reducing agents, it promotes the catalytic generation of reactive oxygen species (ROS) via Fenton-type and Haber-Weiss reactions. Due to its amphiphilic nature, Aβ _1-40/42 can interact with cell membranes and compromise their integrity by thinning the lipid bilayer and forming channel-like structures potentially leading to cell death. In this work, by applying biophysical and biochemical approaches, 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 Cu ²⁺ coordination to Aβ _1-42 . We performed a comparative analysis of the redox reactivities of membrane-bound Aβ _1-42 (memAβ _1-42 -Cu ²⁺ ) species with soluble Aβ _1-42 -Cu ²⁺ establishing that membrane insertion leads to memAβ _1-42 -Cu ²⁺ complexes featuring an enhanced detrimental catechol oxidase activity towards the neurotransmitter dopamine. Moreover, memAβ _1-42 -Cu ²⁺ efficiently catalyzes Aβ di-tyrosine crosslinking and hydroxyl radical production in the presence of ascorbate. In addition, we establish that memAβ _1-42 -Cu ²⁺ redox reactivity catalyze lipid peroxidation in membranes containing polyunsaturated fatty acids (PUFAs), such as arachidonic acid (AA), leading to the generation of malondialdehyde (MDA) toxic end-products. This reactivity compromises the structural integrity of the lipid bilayers resulting in membrane leakage, further substantiating how important is to control aberrant Aβ _1-40/42 -Cu ²⁺ interactions in AD.

Metallothioneins (MTs) are key metalloproteins central to neuronal and astrocytic transition metal homeostasis and buffering. These cysteine-rich proteins bind with high affinity d ¹⁰ metals (Cu ⁺ and Zn ²⁺ ) forming two metal thiolate clusters in their N-terminal β-domain and C-terminal α-domain. The metallothionein-3 (MT-3) isoform is central to metal homeostasis in the CNS, but it is downregulated in AD patients, possessing a neuroprotective role in AD. MT-3 can control aberrant protein-Cu ²⁺ interactions and the Cu-centered redox reactivities of amyloidogenic protein-Cu ²⁺ complexes such as α-synuclein (Parkinson’s disease), PrP (prion disease), and soluble and aggregated Aβ _1-40 (AD). In this work, we unravel that the detrimental memAβ _1-42 -Cu ²⁺ catechol oxidase and redox reactivities can be efficiently silenced by MT-3 via metal swap reactions, effectively scavenging and reducing Cu ²⁺ to Cu ⁺ in its β-domain using thiolates as electron source, forming the redox-inert species Cu ⁺ ₄ Zn ²⁺ ₄ MT-3. Consequently, MT-3 can efficiently prevent lipid peroxidation and protect membrane structural integrity. New strategies targeting membrane-bound Aβ _1-42 -Cu ²⁺ complexes as key players of the AD etiology could be envisioned.