Amyloidogenic function of the Alzheimer's disease-associated presenilin 1 in the absence of endoproteolysis

Alzheimer's disease (AD) is characterized by the invariant accumulation of senile plaques predominantly composed of the pathologically relevant 42-ami no acid amyloid beta-peptide (A beta 42). The presenilin (PS) proteins play a key role in A beta generation. FAD-associated mutations in PS1 and PS2 e nhance the production of A beta 42, and PS1 is required for physiological A beta production, since a gene knockout of PS1 and dominant negative mutati ons of PS1 abolish A beta generation. PS proteins undergo endoproteolytic p rocessing, and current evidence indicates that fragment formation may be re quired for the amyloidogenic function of PS. We have now determined the seq uence requirements for endoproteolysis of PS1. Mutagenizing amino acids at the previously determined major cleavage site (amino acid 298) had no effec t on PS1 endoproteolysis. In contrast, mutations or deletions at the additi onal cleavage site around amino acid 292 blocked endoproteolysis. The uncle avable PS1 derivatives accumulated as full-length proteins and replaced the endogenous PS1 proteins. In contrast to the previously described aspartate mutations within transmembrane domains 6 and 7, the uncleaved PS1 variants do not act as dominant negative inhibitors of A beta production. Moreover, when a FAD-associated mutation (M146L) was combined with a mutation blocki ng endoproteolysis, A beta 42 production still reached pathological levels. These data therefore demonstrate that endoproteolysis of presenilins is no t an absolute prerequisite for the amyloidogenic function of PS1. These dat a also show that accumulation of the PS1 holoprotein is not associated with the pathological activity of PS1 mutations as suggested previously.