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 productio n of the beta amyloid peptide (AP). The exact mechanism whereby mutations i n PSI lead to this effect is not clear. To examine the question of how PS 1 might be involved in amyloid precursor protein (APP) processing, we constr ucted a chimera of human APP695 fused at the C-terminal to enhanced green f luorescent protein (EGFP). This construct was injected into Xenopus laevis oocytes in the presence of wild type PS1 or one of three PS1 mutations asso ciated with AD. The cellular location of the APP695-EGFP construct was exam ined 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 PS 1 were evaluated by Western blot analysis. The results s how that APP695-EGFP is primarily expressed on the cell surface and undergo es limited cleavage. Specifically, APP695 was cleaved in the AP domain to g enerate three distinct C-terminal fragments that correspond in length to st ubs expected after cleavage with alpha-, beta- and -gamma -secretase, respe ctively. 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 PSI mutations, suggesting that PS1 mutations may lead to over-expression of APP not just increased gamma -secretase activity. (C) 2001 Published by Elsevi er Science B.V.