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

The discovery that a deficiency of presenilin 1 (PS1) decreases the production of amyloid β-protein (Aβ) identified the presenilins as important mediators of the γ-secretase cleavage of β-amyloid precursor protein (APP). Recently, we found that two conserved transmembrane (TM) aspartates in PS1 are critical for Aβ production, providing evidence that PS1 either functions as a required diaspartyl cofactor for γ-secretase or is itself γ-secretase. Presenilin 2 (PS2) shares substantial sequence and possibly functional homology with PS1. Here, we show that the two TM aspartates in PS2 are also critical for γ-secretase activity, providing further evidence that PS2 is functionally homologous to PS1. Cells stably co- expressing TM Asp → Ala mutations in both PS1 and PS2 show further accumulation of the APP-derived γ-secretase substrates, C83 and C99. The production of Aβ is reduced to undetectable levels in the conditioned media of these cells. Furthermore, endoproteolysis of the exogenous Asp mutant PS2 is absent, and endogenous PS1 C-terminal fragments are diminished to undetectable levels. Therefore, the co-expression of PS1 and PS2 TM ASp → Ala mutants suppresses the formation of any detectable PS1 or PS2 heterodimeric fragments and essentially abolishes the production of Aβ. These results explain the residual Aβ production seen in PS1-deficient cells and demonstrate the absolute requirement of functional presenilins for Aβ generation. We conclude that presenilins, and their TM aspartates in particular, are attractive targets for lowering Aβ therapeutically to prevent Alzheimer's disease.