β-Amyloid precursor protein mutants respond to γ-secretase modulators

Pathogenic generation of the 42 amino acid variant of the amyloid β-peptide (Abeta42) by β- and γ-secretase cleavage of the β-amyloid precursor protein (APP) is believed to be causative for Alzheimer 's disease (AD). Lowering of Aβ42 production by γ-secretase modulators (GSMs) is a hopeful approach towards AD treatment. The mechanism of GSM action is not fully understood. Moreover, whether GSMs target the Aβ domain is controversial. To further our understanding of the mode of action of GSMs and the cleavage mechanism of γ-secretase, we analyzed mutations located at different positions of the APP TMD around or within the Aβ domain regarding their response to GSMs. We found that Aβ42-increasing familial AD (FAD) mutations of the γ-secretase cleavage site domain responded robustly to Aβ42-lowering GSMs, especially to the potent compound GSM-1, irrespective of the amount of Aβ42 produced. We thus expect that FAD patients carrying mutations at the γ-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 also other Aβ species (Aβ41, Aβ39) could be lowered besides Aβ42. Moreover, certain mutations simultaneously increased Aβ42 and the shorter peptide Aβ38 arguing that the proposed precursor-product relationship of these Aβ species is not general. Finally, mutations of residues in the proposed GSM-binding site implicated in Aβ42 generation (G29, G33) and potentially in GSM-binding (K28) were also responsive to GSMs, a finding that may question APP substrate targeting of GSMs.