Alzheimer's Disease-Linked Mutations in Presenilin-1 Result in a Drastic Loss of Activity in Purified γ-Secretase Complexes

Background: Mutations linked to early onset, familial forms of Alzheimer's disease (FAD) are found most frequently in PSEN1, the gene encoding presenilin-1 (PS1). Together with nicastrin (NCT), anterior pharynx-defective protein 1 (APH1), and presenilin enhancer 2 (PEN2), the catalytic subunit PS1 constitutes the core of the c-secretase complex and contributes to the proteolysis of the amyloid precursor protein (APP) into amyloid-beta (A beta) peptides. Although there is a growing consensus that FAD-linked PS1 mutations affect Ab production by enhancing the A beta 1-42/A beta 1-40 ratio, it remains unclear whether and how they affect the generation of APP intracellular domain (AICD). Moreover, controversy exists as to how PS1 mutations exert their effects in different experimental systems, by either increasing A beta 1-42 production, decreasing A beta 1-40 production, or both. Because it could be explained by the heterogeneity in the composition of gamma-secretase, we purified to homogeneity complexes made of human NCT, APH1aL, PEN2, and the pathogenic PS1 mutants L166P, Delta E9, or P436Q. Methodology/Principal Findings: We took advantage of a mouse embryonic fibroblast cell line lacking PS1 and PS2 to generate different stable cell lines overexpressing human gamma-secretase complexes with different FAD-linked PS1 mutations. A multi-step affinity purification procedure was used to isolate semi-purified or highly purified gamma-secretase complexes. The functional characterization of these complexes revealed that all PS1 FAD-linked mutations caused a loss of gamma-secretase activity phenotype, in terms of A beta 1-40, A beta 1-42 and APP intracellular domain productions in vitro. Conclusion/Significance: Our data support the view that PS1 mutations lead to a strong gamma-secretase loss-of-function phenotype and an increased A beta 1-42/A beta 1-40 ratio, two mechanisms that are potentially involved in the pathogenesis of Alzheimer's disease.