Specific Mutations in Aph1 Cause gamma-Secretase Activation

Amyloid beta peptides (A beta s) are generated from amyloid precursor protein (APP) through multiple cleavage steps mediated by gamma-secretase, including endoproteolysis and carboxypeptidase-like trimming. The generation of neurotoxic A beta 42/43 species is enhanced by familial Alzheimer's disease (FAD) mutations within the catalytic subunit of gamma-secretase, presenilin 1 (PS1). FAD mutations of PS1 cause partial loss-of-function and decrease the cleavage activity. Activating mutations, which have the opposite effect of FAD mutations, are important for studying A beta production. Aph1 is a regulatory subunit of gamma-secretase; it is presumed to function as a scaffold of the complex. In this study, we identified Aph1 mutations that are active in the absence of nicastrin (NCT) using a yeast gamma-secretase assay. We analyzed these Aph1 mutations in the presence of NCT; we found that the L30F/T164A mutation is activating. When introduced in mouse embryonic fibroblasts, the mutation enhanced cleavage. The Aph1 mutants produced more short and long A beta s than did the wild-type Aph1, without an apparent modulatory function. The mutants did not change the amount of gamma-secretase complex, suggesting that L30F/T164A enhances catalytic activity. Our results provide insights into the regulatory function of Aph1 in gamma-secretase activity.