Cyclosporin A Increases Resting Mitochondrial Membrane Potential in SY5Y Cells and Reverses the Depressed Mitochondrial Membrane Potential of Alzheimer's Disease Cybrids

Abstract

Alzheimer's disease (AD) brains exhibit oxidative stress and a biochemical defect of complex IV (cytochrome oxidase, COX) of the mitochondrial electron transport chain (ETC). This defect can be transferred through mitochondrial DNA (mtDNA) into clonal SY5Y cells depleted of their mtDNA. The resulting cytoplasmic hybrids or “cybrids” retain the complex IV defect and exhibit oxidative stress. We measured the mitochondrial membrane potential (Δψm) in AD and control cybrids via H3-tetraphenylphosphonium ion (H3-TPP+) accumulation. AD cybrids exhibited a significant (about 30%) decrease in H3-TPP+accumulation relative to controls. Acute treatment of normal SY5Ys with azide, a COX inhibitor, moderately decreased H3-TPP+retention and strongly inhibited COX activity in a dose-dependent manner. As the mitochondrial transition pore (MTP) can be activated by reactive oxygen species and ETC inhibitors, and its opening causes Δψmdissipation, we tested the effects of the MTP inhibitor cyclosporin A (CsA) on TPP+accumulation. 5mM CsA increased basal H3-TPP+accumulation in SY5Y cells about 10-fold, corresponding to about a 2-fold increase in Δψm. In the AD cybrids, CsA increased the apparent Δψmto the same final levels as it did in controls. These results indicate that low-conductance MTP activity contributes significantly to resting Δψmin SY5Y cells. We propose the novel hypothesis that the COX defect and resulting oxidative stress in AD may pathologically activate the MTP, resulting in lower Δψmand the release of mitochondrial factors involved in apoptosis.