Caspase Activation Increases β-Amyloid Generation Independently of Caspase Cleavage of the β-Amyloid Precursor Protein (APP)

Abstract

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 (Asp720) and two at the N terminus (Asp197 and Asp219). Caspase cleavage at Asp720 has been suggested as leading to increased production of Aβ. 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 Aβ production that occurs during apoptosis. We found that cleavage at Asp720 occurred concurrently with caspase 3 activation and the increased production of total secreted Aβ and Aβ1-42 in association with staurosporine- and etoposide-induced apoptosis. To investigate the contribution of caspase cleavage of APP to Aβ generation, we expressed an APP mutant truncated at Asp720 that mimics APP caspase cleavage at the C-terminal site. This did not increase Aβ generation but, in contrast, dramatically decreased Aβ production in Chinese hamster ovary cells. Furthermore, the ablation of caspase-dependent cleavage at Asp720, Asp197, and Asp219 (by site-directed mutagenesis) did not prevent enhanced Aβ production following etoposide-induced apoptosis. These findings indicate that the enhanced Aβ generation associated with apoptosis does not require cleavage of APP at its C-terminal (Asp720) and/or N-terminal caspase sites.