Specific mutations in the cholesterol-binding site of APP alter its processing and favor the production of shorter, less toxic Aβ peptides.

Abstract

Excess brain cholesterol is strongly implicated in the pathogenesis of Alzheimer disease (AD). Here we evaluated how the presence of a cholesterol-binding site (CBS) in the transmembrane and juxtamembrane regions of the amyloid precursor protein (APP) regulates its processing.We generated nine point mutations in the APP gene, changing the charge and/or hydrophobicity of the amino-acids which were previously shown as part of the CBS. Most mutations triggered a reduction of amyloid-β peptides Aβ40 and Aβ42 secretion from transfected HEK293T cells. Only the K to A mutation at position 28 of Aβ in the APP sequence (APPK28A) resulted in a concomitant increase in the production of shorter Aβ peptides. Mass Spectrometry (MS) confirmed the predominance of Aβx-33 and Aβx-34. The enzymatic activity of α-, β- and g-secretases remained unchanged in cells expressing the APPK28A and all other mutants. Similarly, subcellular localization of the mutants in early endosomes did not differ from the APPWT protein. A transient increase of plasma membrane cholesterol enhanced the production of Aβ40 and Aβ42 by APPWT, an effect absent in APPK28A mutant. Finally, WT but not CBS mutant Aβ derived peptides bound to cholesterol-rich exosomes. Collectively, the present data revealed a major role of juxtamembrane amino acids of the APP CBS in modulating the production of toxic Aβ species. More generally, they underpin the role of cholesterol in the pathophysiology of AD.