Caspase activation increases β-amyloid generation independently of caspase cleavage of the β-amyloid precursor protein (APP)

The amyloid precursor protein (APP) undergoes alternative proteolysis mediated by caspases. Three major caspase recognition sites have been identified in the APP, i.e. one at the C terminus (Asp(720)) and two at the N terminus (Asp(197) and Asp(219)). Caspase cleavage at Asp(720) has been suggested as leading to increased production of Abeta. Thus, we set out to determine which putative caspase sites in APP, if any, are cleaved in Chinese hamster ovary cell lines concurrently with the increased Abeta production that occurs during apoptosis. We found that cleavage at Asp(720) occurred concurrently with caspase 3 activation and the increased production of total secreted Abeta and Abeta(1-42) in association with staurosporine- and etoposide-induced apoptosis. To investigate the contribution of caspase cleavage of APP to Abeta generation, we expressed an APP mutant truncated at Asp(720) that mimics APP caspase cleavage at the C-terminal site. This did not increase Abeta generation but, in contrast, dramatically decreased Abeta production in Chinese hamster ovary cells. Furthermore, the ablation of caspase-dependent cleavage at Asp(720), Asp(197), and Asp(219) (by site-directed mutagenesis) did not prevent enhanced Abeta production following etoposide-induced apoptosis. These findings indicate that the enhanced Abeta generation associated with apoptosis does not require cleavage of APP at its C-terminal (Asp(720)) and/or N-terminal caspase sites.