APP substitutions V715F and L720P alter PS1 conformation and differentially affect Aβ and AICD generation

The 37-43 amino acid Aβ peptide is the principal component of β-amyloid deposits in Alzheimer's disease (AD) brain, and is derived by serial proteolysis of the amyloid precursor protein (APP) by β- and γ-secretase. γ-Secretase also cleaves APP at Val50 in the Aβ numbering (ε cleavage), resulting in the release of a fragment called APP intracellular domain (AICD). The aim of this study was to determine whether amino acid substitutions in the APP transmembrane domain differentially affect Aβ and AICD generation. We found that the APPV715F substitution, which has been previously shown to dramatically decrease Aβ₄₀ and Aβ₄₂ while increasing Aβ₃₈ levels, does not affect in vitro generation of AICD. Furthermore, we found that the APPL720P substitution, which has been previously shown to prevent in vitro generation of AICD, completely prevents Aβ generation. Using a fluorescence resonance energy transfer (FRET) method, we next found that both the APPV715F and APPL720P substitutions significantly increase the distance between the N- and C-terminus of presenilin 1 (PS1), which has been proposed to contain the catalytic site of γ-secretase. In conclusion, both APPV715F and APPL720P change PS1 conformation with differential effects on Aβ and AICD production.