The transmembrane aspartates in presenilin 1 and 2 are obligatory for gamma-secretase activity and amyloid beta-protein generation

The discovery that a deficiency of presenilin 1 (PS1) decreases the product ion of amyloid beta-protein (A beta) identified the presenilins as importan t mediators of the gamma-secretase cleavage of beta-amyloid precursor prote in (APP). Recently, we found that two conserved transmembrane (TM) aspartat es in PS1 are critical for AP production, providing evidence that PS1 eithe r functions as a required diaspartyl cofactor for gamma-secretase or is its elf gamma-secretase. Presenilin 2 (PS2) shares substantial sequence and pos sibly functional homology with PS1. Here, we show that the two TM aspartate s in PS2 are also critical for gamma-secretase activity, providing further evidence that PS2 is functionally homologous to PS1. Cells stably co-expres sing TM Asp -> Ala mutations in both PS1 and PS2 show further accumulation of the APP-derived gamma-secretase substrates, C83 and C99. The production of A beta is reduced to undetectable levels in the conditioned media of the se cells. Furthermore, endoproteolysis of the exogenous Asp mutant PS2 is a bsent, and endogenous PS1 C-terminal fragments are diminished to undetectab le levels. Therefore, the co-expression of PS1 and PS2 TM Asp -> Ala mutant s suppresses the formation of any detectable PS1 or PS2 heterodimeric fragm ents and essentially abolishes the production of A beta. These results expl ain the residual A beta production seen in PS1-deficient cells and demonstr ate the absolute requirement of functional presenilins for A beta generatio n. We conclude that presenilins, and their TM aspartates in particular, are attractive targets for lowering A beta therapeutically to prevent Alzheime r's disease.