Mitochondrial respiration reduces exposure of the nucleus to oxygen

The endosymbiotic theory posits that ancient eukaryotic cells engulfed O(2)-consuming prokaryotes, which protected them against O(2) toxicity. Previous studies have shown that cells lacking cytochrome c oxidase (COX), required for respiration, have increased DNA damage and reduced proliferation, which could be improved by reducing O(2) exposure. With recently developed fluorescence lifetime microscopy–based probes demonstrating that the mitochondrion has lower [O(2)] than the cytosol, we hypothesized that the perinuclear distribution of mitochondria in cells may create a barrier for O(2) to access the nuclear core, potentially affecting cellular physiology and maintaining genomic integrity. To test this hypothesis, we utilized myoglobin-mCherry fluorescence lifetime microscopy O(2) sensors without subcellular targeting (“cytosol”) or with targeting to the mitochondrion or nucleus for measuring their localized O(2) homeostasis. Our results showed that, similar to the mitochondria, the nuclear [O(2)] was reduced by ∼20 to 40% compared with the cytosol under imposed O(2) levels of ∼0.5 to 18.6%. Pharmacologically inhibiting respiration increased nuclear O(2) levels, and reconstituting O(2) consumption by COX reversed this increase. Similarly, genetic disruption of respiration by deleting SCO2, a gene essential for COX assembly, or restoring COX activity in SCO2(−/−) cells by transducing with SCO2 cDNA replicated these changes in nuclear O(2) levels. The results were further supported by the expression of genes known to be affected by cellular O(2) availability. Our study reveals the potential for dynamic regulation of nuclear O(2) levels by mitochondrial respiratory activity, which in turn could affect oxidative stress and cellular processes such as neurodegeneration and aging.