Mutations in the gene encoding the amyloid precursor protein are the only, albeit rare, defined cause of Alzheimer's disease. Recent work has suggested that the amyloid precursor protein (APP) is metabolized through two major pathways. The first, the alpha-secretase pathway involves cleavage of APP in the middle of the beta-amyloid fragment of the molecule and thus precludes beta-amyloid formation. The second, the beta-secretase pathway involves cleavage of APP at the N-terminal of beta-amyloid and is thus thought to lead to beta-amyloid production by all cells. Recently, a pathogenic, Alzheimer-causing mutation in APP at the beta-secretase cleavage site has been shown, in transfected cell lines, to increase the proportion of APP metabolized down the beta- secretase pathway at the expense of that metabolized down the alpha- secretase route. This evidence strongly suggests that the relative rates between the two pathways is key to the pathogenesis of Alzheimer's disease. The site specificity of alpha-secretase has been extensively studied: the major purpose of this study is to determine the site specificity of the beta-secretase enzyme using cell lines transfected with APP sequences with mutations induced at and around the beta- secretase cleavage site. A secondary purpose of this study is to determine whether variant APP sequences which are poor substrates for the alpha-secretase pathway are preferentially metabolized down the beta- secretase pathway. The final purpose of this study is to use both approaches, beta-secretase potentiation and alpha-secretase inhibition, to define an APP sequence which is maximally metabolized to beta-amyloid. Such a sequence will later be used in transgenic experiments.
