Familial Alzheimer's disease cells abnormally accumulate beta-amyloid-harbouring peptides preferentially in cytosol but not in extracellular fluid.

The proteolytic processing of beta-amyloid precursor protein (APP) was analyzed and compared for familial Alzheimer's disease and normal lymphoid cells, focusing on beta-amyloid-harbouring peptides and the extracellular fragments released into the medium. Tris/tricine gel electrophoresis of anti-beta A4-(8-17)-immunoprecipitated peptides and subsequent N-terminal amino acid sequencing revealed previously unidentified peptides; the 14-kDa peptide with a beta A4 N-terminus and the 12-kDa peptide with an Esch's-site N-terminus in the cytosols, and the same 12-kDa peptide predominating in the media. Moreover, some early onset familial Alzheimer's disease cells, but not normal cells, express a 4-kDa peptide with a beta-amyloid N-terminus in the cytosol. Two-dimensional gel electrophoresis of the extracellular APP peptides immunoprecipitated with anti-APP-(144-654)-peptide showed that familial Alzheimer's disease cells are deficient in processing, especially the 50-53-kDa peptides with the Kunitz-protease-inhibitor domain. This may reflect their unique expression of a serine protease identified as cleaving APP at the beta-amyloid N-terminus.