Chronic exposure to sub-lethal beta-amyloid (Aβ) inhibits the import of nuclear-encoded proteins to mitochondria in differentiated PC12 cells

Studies on amyloid beta (Aβ|), the peptide thought to play a crucial role in the pathogenesis of Alzheimer's disease, have implicated mitochondria in Aβ-mediated neurotoxicity. We used differentiated PC12 cells stably transfected with an inducible green fluorescent protein (GFP) fusion protein containing an N'-terminal mitochondrial targeting sequence (mtGFP), to examine the effects of sub-lethal Aβ on the import of nuclear-encoded proteins to mitochondria. Exposure to sub-lethal Aβ₂₅₋₃₅ (10 μmol/L) for 48 h inhibited mtGFP import to mitochondria; average rates decreased by 20 ± 4%. Concomitant with the decline in mtGFP, cytoplasmic mtGFP increased significantly while mtGFP expression and intramitochondrial mtGFP turnover were unchanged. Sub-lethal Aβ₁₋₄₂ inhibited mtGFP import and increased cytoplasmic mtGFP but only after 96 h. The import of two endogenous nuclear-encoded mitochondrial proteins, mortalin/mtHsp70 and Tom20 also declined. Prior to the decline in import, mitochondrial membrane potential (mmp), and reactive oxygen species levels were unchanged in Aβ-treated cells versus reverse phase controls. Sustained periods of decreased import were associated with decreased mmp, increased reactive oxygen species, increased vulnerability to oxygen-glucose deprivation and altered mitochondrial morphology. These findings suggest that an Aβ-mediated inhibition of mitochondrial protein import, and the consequent mitochondrial impairment, may contribute to Alzheimer's disease.