Activation of neuronal caspase-3 by intracellular accumulation of wild-type Alzheimer amyloid precursor protein

Forced overexpression of wild-type Alzheimer amyloid precursor protein (APP ) causes postmitotic neurons to degenerate. Caspase-3 (CPP32) is a principa l cell death protease involved in neuronal apoptosis during physiological d evelopment and under pathological conditions. Here, we investigated whether APP overexpression activates caspase-3 in human postmitotic neurons using adenovirus-mediated gene transfer. When a recombinant adenovirus vector exp ressing human wild-type APP695 was infected in vitro into neurally differen tiated embryonal carcinoma NT2 cells, only postmitotic neurons underwent se vere degeneration. Before neurodegeneration, full-length APP- and A beta-im munoreactive peptides were accumulated in infected neurons, and caspase-3-l ike protease activity was markedly elevated. Western blot analysis revealed that activated caspase-3 subunits were generated in APP-accumulating neuro ns. Such neuronal caspase-3 activation was undetectable in NT2 neurons infe cted with beta-galactosidase-expressing adenovirus. Addition of the caspase -3 inhibitor acetyl-Asp-Glu-Val-Asp-aldehyde to the culture medium signific antly reduced the severity of degeneration exhibited by APP-overexpressing neurons. Immunocytochemical analyses revealed that some APP-accumulating ne urons contained activated caspase-3 subunits and exhibited the characterist ics of apoptosis, such as chromatin condensation and DNA fragmentation. Act ivation of caspase-3 was also observed in vivo in rat hippocampal neurons i nfected with the APP-expressing adenovirus. These results suggest that wild -type APP is an intrinsic activator of caspase-3-mediated death machinery i n postmitotic neurons.