Presenilin 1 mutations increase amyloid precursor protein production and proteolysis in Xenopus laevis oocytes

Recent findings suggest that Presenilin 1 (PS1) mutations play a major role in the development of Alzheimer's disease (AD) by increasing the production of the beta amyloid peptide (Aβ). The exact mechanism whereby mutations in PS1 lead to this effect is not clear. To examine the question of how PS1 might be involved in amyloid precursor protein (APP) processing, we constructed a chimera of human APP695 fused at the C-terminal to enhanced green fluorescent protein (EGFP). This construct was injected into Xenopus laevis oocytes in the presence of wild type PS1 or one of three PS1 mutations associated with AD. The cellular location of the APP695-EGFP construct was examined by fluorescent confocal microscopy. In addition, membrane fractions of oocytes expressing APP695-EGFP in the presence or absence of wild type or mutant forms of PS1 were evaluated by Western blot analysis. The results show that APP695-EGFP is primarily expressed on the cell surface and undergoes limited cleavage. Specifically, APP695 was cleaved in the Aβ domain to generate three distinct C-terminal fragments that correspond in length to stubs expected after cleavage with α-, β- and γ-secretase, respectively. The presence of wild type PS1 not only increased the production of proteolytic C-terminal fragments of APP, but the production of APP itself. These findings were even more pronounced in the presence of all three PS1 mutations, suggesting that PS1 mutations may lead to over-expression of APP not just increased γ-secretase activity.