Mitochondria are a direct site of Aβ accumulation in Alzheimer's disease neurons: Implications for free radical generation and oxidative damage in disease progression

Alzheimer's disease (AD) is a complex, neurodegenerative disease characterized by the impairment of cognitive function in elderly individuals. In a recent global gene expression study of APP transgenic mice, we found elevated expression of mitochondrial genes, which we hypothesize represents a compensatory response because of mitochondrial oxidative damage caused by the over-expression of mutant APP and/or amyloid beta (Aβ). We investigated this hypothesis in a series of experiments examining what forms of APP and Aβ localize to the mitochondria, and whether the presence of these species is associated with mitochondrial dysfunction and oxidative damage. Using immunoblotting, digitonin fractionation, immunofluorescence, and electron microscopy techniques, we found a relationship between mutant APP derivatives and mitochondria in brain slices from Tg2576 mice and in mouse neuroblastoma cells expressing mutant human APP. Further, to determine the functional relationship between mutant APP/Aβ and oxidative damage, we quantified Aβ levels, hydrogen peroxide production, cytochrome oxidase activity and carbonyl proteins in Tg2576 mice and age-matched wild-type (WT) littermates. Hydrogen peroxide levels were found to be significantly increased in Tg2576 mice when compared with age-matched WT littermates and directly correlated with levels of soluble Aβ in Tg2576 mice, suggesting that soluble Aβ may be responsible for the production of hydrogen peroxide in AD progression in Tg2576 mice. Cytochrome c oxidase activity was found to be decreased in Tg2576 mice when compared with age-matched WT littermates, suggesting that mutant APP and soluble Aβ impair mitochondrial metabolism in AD development and progression. An increase in hydrogen peroxide and a decrease in cytochrome oxidase activity were found in young Tg2576 mice, prior to the appearance of Aβ plaques. These findings suggest that early mitochondrially targeted therapeutic interventions may be effective in delaying AD progression in elderly individuals and in treating AD patients.