Mutations in Nicastrin Protein Differentially Affect Amyloid β-Peptide Production and Notch Protein Processing

The gamma-secretase complex is responsible for intramembrane processing of over 60 substrates and is involved in Notch signaling as well as in the generation of the amyloid beta-peptide (A beta). Aggregated forms of A beta have a pathogenic role in Alzheimer disease and, thus, reducing the A beta levels by inhibiting gamma-secretase is a possible treatment strategy for Alzheimer disease. Regrettably, clinical trials have shown that inhibition of gamma-secretase results in Notch-related side effects. Therefore, it is of great importance to find ways to inhibit amyloid precursor protein (APP) processing without disturbing vital signaling pathways such as Notch. Nicastrin (Nct) is part of the gamma-secretase complex and has been proposed to be involved in substrate recognition and selection. We have investigated how the four evenly spaced and conserved cysteine residues in the Nct ectodomain affect APP and Notch processing. We mutated these cysteines to serines and analyzed them in cells lacking endogenous Nct. We found that two mutants, C213S (C2) and C230S (C3), differentially affected APP and Notch processing. Both the formation of A beta and the intracellular domain of amyloid precursor protein (AICD) were reduced, whereas the production of Notch intracellular domain (NICD) was maintained on a high level, although C230S (C3) showed impaired complex assembly. Our data demonstrate that single residues in a gamma-secretase component besides presenilin are able to differentially affect APP and Notch processing.