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Cytochrome oxidase (COX) activity varies between individuals and low activities associate with Alzheimer's disease. Whether genetic heterogeneity influences function of this multimeric enzyme is unknown. To explore this we sequenced three mitochondrial DNA (mtDNA) and ten nuclear COX subunit genes from at least 50 individuals. 20% had non-synonymous mtDNA COX gene polymorphisms, 12% had a COX4I1 non-synonymous G to A transition, and other genes rarely contained non-synonymous polymorphisms. Frequent untranslated region (UTR) polymorphisms were seen in COX6A1, COX6B1, COX6C, and COX7A1; heterogeneity in a COX7A1 5' UTR Sp1 site was extensive. Synonymous polymorphisms were common and less frequent in the more conserved COX1 than the less conserved COX3, suggesting at least in mtDNA synonymous polymorphisms experience selection pressure and are not functionally silent. Compound gene variations occurred within individuals. To test whether variations could have functional consequences, we studied the COX4I1 G to A transition and an AGCCCC deletion in the COX7A1 5' UTR Sp1 site. Cells expressing the COX4I1 polymorphism had reduced COX Vmax activity. In reporter construct-transduced cells where green fluorescent protein expression depended on the COX7A1 Sp1 site, AGCCCC deletion reduced fluorescence. Our findings indicate COX subunit gene heterogeneity is pervasive and may mediate COX functional variation.

Mitochondrial dysfunction has severe cellular consequences, and is linked to aging and neurological disorders in humans. Impaired energy supply or Ca2+ buffering, increased ROS production, or control of apoptosis by mitochondria may contribute to the progressive decline of long-lived postmitotic cells. Mitochondrial biogenesis refers to the process via which cells increase their individual mitochondrial mass. Mitochondrial biogenesis may represent an attempt by cells to increase their aerobic set point, or an attempt to maintain a pre-existing aerobic set point in the face of declining mitochondrial function. Neuronal mitochondrial biogenesis itself has been poorly studied, but investigations from other tissues and model systems suggest a series of transcription factors, transcription co-activators, and signal transduction proteins should function to regulate mitochondrial number and mass within neurons. We review data pertinent to the mitochondrial biogenesis field, and discuss implications for brain aging and neurodegenerative disease research efforts.