Zinc and Copper Differentially Modulate Amyloid Precursor Protein Processing by {gamma}-Secretase and Amyloid-{beta} Peptide Production [Enzymology]

Recent evidence suggests involvement of biometal homeostasis in the pathological mechanisms in Alzheimer's disease (AD). For example, increased intracellular copper or zinc has been linked to a reduction in secreted levels of the AD-causing amyloid-β peptide (Aβ). However, little is known about whether these biometals modulate the generation of Aβ. In the present study, we demonstrate in both cell-free and cell-based assays that zinc and copper regulate Aβ production by distinct molecular mechanisms affecting the processing by γ-secretase of its Aβ precursor protein substrate APP-C99. We found that Zn2+ induces APP-C99 dimerization, which prevents its cleavage by γ-secretase and Aβ production, with an IC50 value of 15 μM. Importantly, at this concentration, Zn2+ also drastically raised the production of the aggregation-prone Aβ43 found in the senile plaques of AD brains and elevated the Aβ43:Aβ40 ratio, a promising biomarker for neurotoxicity and AD. We further demonstrate that the APP-C99 histidine residues H6, H13 and H14 control the Zn2+-dependent APP-C99 dimerization and inhibition of Aβ production, while the increased Aβ43:Aβ40 ratio is substrate-dimerization independent and involves the known Zn2+-binding lysine K28 residue that orientates the APP-C99 transmembrane domain within the lipid bilayer. Unlike zinc, copper inhibited Aβ production by directly targeting the subunits presenilin and nicastrin in the γ-secretase complex. Altogether, our data demonstrate that zinc and copper differentially modulate Aβ production. They further suggest that dimerization of APP-C99, or the specific targeting of individual residues regulating the production of the long, toxic Aβ species, may offer two therapeutic strategies for preventing AD.

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