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Familial Alzheimer disease mutations of presenilin 1 (PS-1) enhance the generation of Aβ1–42, indicating that PS-1 is involved in amyloidogenesis. However, PS-1 transgenic mice have failed to show amyloid plaques in their brains. Because PS-1 mutations facilitate apoptotic neuronal death in vitro, we did careful quantitative studies in PS-1 transgenic mice and found that neurodegeneration was significantly accelerated in mice older than 13 months (aged mice) with familial Alzheimer disease mutant PS-1, without amyloid plaque formation. However, there were significantly more neurons containing intracellularly deposited Aβ42 in aged mutant transgenic mice. Our data indicate that the pathogenic role of the PS-1 mutation is upstream of the amyloid cascade.