Presenilin-1 mutations of leucine 166 equally affect the generation of the Notch and APP intracellular domains independent of their effect on Aβ₄₂ production

The Alzheimer's disease (AD)-associated presenilin (PS) proteins are required for the γ-secretase cleavages of the β-amyloid precursor protein and the site 3 (S3) protease cleavage of Notch. These intramembrane cleavages release amyloid-β peptide (Aβ), including the pathogenic 42-aa variant (Aβ₄₂), as well as the β-amyloid precursor protein and the Notch intracellular domains (AICD, NICD). Whereas Aβ is generated by endoproteolysis in the middle of the transmembrane domain, AICD and NICD are generated by cleavages at analogous positions close to the cytoplasmic border of the transmembrane domain. Numerous mutations causing familial AD (FAD) that all cause increased production of Aβ₄₂ have been found in the PS1 gene. Here we have investigated the previously uncharacterized, very aggressive FAD mutation L166P that causes onset of AD in adolescence. Strikingly, the PS1 L166P mutation not only induces an exceptionally high increase of Aβ₄₂ production but also impairs NICD production and Notch signaling, as well as AICD generation. Thus, FAD-associated PS mutants cannot only affect the generation of NICD, but also that of AICD. Moreover, further analysis with artificial L166 mutants revealed that the γ-secretase cleavage at position 40/42 and the S3-like γ-secretase cleavage at position 49 of the Aβ domain are both differentially affected by PS1 L166 mutants. Finally, we show that PS1 L166 mutants affect the generation of NICD and AICD in a similar manner, supporting the concept that S3 protease and S3-like γ-secretase cleavages are mediated by identical proteolytic activities.