Metabolism of the 'Swedish' amyloid precursor protein variant in Madin- Darby canine kidney cells

Abstract

The 4-kDa β-amyloid peptide (Aβ), a major constituent of parenchymal amyloid deposits in Alzheimer's disease, is derived from larger amyloid precursor proteins (APP). We have examined the metabolism of APP in Madin- Darby canine kidney cells stably transfected with cDNA encoding either wild- type human APP-695 or human APP-695 that harbors the Swedish double mutation associated with familial early-onset Alzheimer's disease. Although ~90% of total soluble APP secreted from wild-type cells is secreted basolaterally following cleavage at the α-secretase site, soluble derivatives cleaved near or at the amino terminus of Aβ (presumably the 'β-secretase' site) are preferentially secreted into the apical compartment of SWE cells. Concomitantly, levels of a specific Aβ-containing carboxyl-terminal fragment are elevated in SWE cell lysates. Using domain-specific biotinylation and release assays, we failed to detect a β-secretase-generated soluble derivative (APP(sβ)) released from the surface of SWE cells. However, APP(sβ) can be detected in SWE cell lysates, consistent with 'β-secretase' cleavage occurring in an intracellular compartment. Finally, we demonstrate that Aβ is secreted into the basolateral compartment of SWE cells and that the majority of these Aβ-related species contains an amino-terminal aspartate residue (+1).