(49) REGULATION OF MITOCHONDRIAL GENE EXPRESSION IN DIFFERENTIATED PC12 CELLS

Li-Ing Liu (1), K. Hatanp (1), S.I. Rapoport (1) and K. Chandrasekaran (2)
(1) Lab of Neurosciences, National Institute on Aging, NIH, Bethesda, MD 20892
(2) Department of Anesthesiology, University of Maryland School of Medicine, Baltimore, MD

In Alzheimer's disease (AD) brain, levels of mitochondrial DNA (mtDNA)-encoded genes of oxidative phosphorylation system (OXPHOS), such as cytochrome oxidase subunit I-III (COX I-III), are decreased in vulnerable brain regions, whereas there are no changes in mtDNA-encoded ribosomal RNA and mtDNA. These results suggest regulation occurring at the level of mtDNA transcription or/and turnover of mRNA. We propose that ions may play a role in these regulatory mechanisms, because ATP in the brain is used primarily for ionic pumping during repolarization. To examine the mechanism(s), we developed a cell culture system in which levels of mtDNA-encoded mRNA can be reduced by inhibiting ionic pumping. After differentiated PC12 cells were treated with drugs for various time, total RNA was isolated and levels of COX III mRNA were measured (normalized to ?-actin mRNA). Ouabain (1 mM), a Na+/K+-ATPase inhibitor, caused a 50% decrease in COX III mRNA within 6 h. Addition of monensin (0.1 uM), a sodium ionophore, almost totally abolished COX III mRNA within 6 h. The decreases in COX III mRNA were observed in the absence of significant changes in the levels of mtDNA-encoded ribosomal RNA (12S rRNA) and mtDNA. Measurement of the ADP/ATP ratio in ouabain and monensin treated cells showed that the decrease in COX III mRNA was unrelated to the energetic status of the cell. These data indicate that intracellular sodium is involved in the regulation of mtDNA gene expression. The roles of calcium and turnover of COX III mRNA are being studied to further understand regulation.