β-Amyloid Precursor Protein Mutants Respond to γ-Secretase Modulators

Pathogenic generation of the 42-amino acid variant of the amyloid beta-peptide (A beta) by beta- and gamma-secretase cleavage of the beta-amyloid precursor protein (APP) is believed to be causative for Alzheimer disease (AD). Lowering of A beta(42) production by gamma-secretase modulators (GSMs) is a hopeful approach toward AD treatment. The mechanism of GSM action is not fully understood. Moreover, whether GSMs target the A beta domain is controversial. To further our understanding of the mode of action of GSMs and the cleavage mechanism of gamma-secretase, we analyzed mutations located at different positions of the APP transmembrane domain around or within the A beta domain regarding their response to GSMs. We found that A beta(42)-increasing familial AD mutations of the gamma-secretase cleavage site domain responded robustly to A beta(42)-lowering GSMs, especially to the potent compound GSM-1, irrespective of the amount of A beta(42) produced. We thus expect that familial AD patients carrying mutations at the gamma-secretase cleavage sites of APP should respond to GSM-based therapeutic approaches. Systematic phenylalanine-scanning mutagenesis of this region revealed a high permissiveness to GSM-1 and demonstrated a complex mechanism of GSM action as other A beta species (A beta(41), A beta(39)) could also be lowered besides A beta(42). Moreover, certain mutations simultaneously increased A beta(42) and the shorter peptide A beta(38), arguing that the proposed precursor-product relationship of these A beta species is not general. Finally, mutations of residues in the proposed GSM-binding site implicated in A beta(42) generation (Gly-29, Gly-33) and potentially in GSM-binding (Lys-28) were also responsive to GSMs, a finding that may question APP substrate targeting of GSMs.