Mitochondrial ROS

Reactive oxygen species produced by mitochondria

Production of mitochondrial ROS, mitochondrial ROS

Mitochondrial ROS (mtROS or mROS) are reactive oxygen species (ROS) that are produced by mitochondria.[1][2][3] Generation of mitochondrial ROS mainly takes place at the electron transport chain located on the inner mitochondrial membrane during the process of oxidative phosphorylation. Leakage of electrons at complex I and complex III from electron transport chains leads to partial reduction of oxygen to form superoxide. Subsequently, superoxide is quickly dismutated to hydrogen peroxide by two dismutases including superoxide dismutase 2 (SOD2) in mitochondrial matrix and superoxide dismutase 1 (SOD1) in mitochondrial intermembrane space. Collectively, both superoxide and hydrogen peroxide generated in this process are considered as mitochondrial ROS.[1]

Once thought as merely the by-products of cellular metabolism, mitochondrial ROS are increasingly viewed as important signaling molecules,[4] whose levels of generation at 11 currently-identified sites vary depending on cellular energy supply and demand.[5][6] At low levels, mitochondrial ROS are considered to be important for metabolic adaptation as seen in hypoxia.[1] Mitochondrial ROS, stimulated by danger signals such as lysophosphatidylcholine and Toll-like receptor 4 and Toll-like receptor 2 bacterial ligands lipopolysaccharide (LPS) and lipopeptides, are involved in regulating inflammatory response.[7][8] Finally, high levels of mitochondrial ROS activate apoptosis/autophagy pathways capable of inducing cell death.[9]

## COVID-19

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Monocytes/macrophages are the most enriched immune cell types in the lungs of COVID-19 patients and appear to have a central role in the pathogenicity of the disease. These cells adapt their metabolism upon infection and become highly glycolytic, which facilitates SARS-CoV-2 replication. The infection triggers mitochondrial ROS production, which induces stabilization of hypoxia-inducible factor-1α (HIF1A) and consequently promotes glycolysis. HIF1A-induced changes in monocyte metabolism by SARS-CoV-2 infection directly inhibit T cell response and reduce epithelial cell survival. Targeting mitochondrial ROS may have great therapeutic potential for the development of novel drugs to treat patients with coronavirus.[10]

## Aging

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Mitochondrial ROS can promote cellular senescence and aging phenotypes in the skin of mice.[11] Ordinarily mitochondrial SOD2 protects against mitochondrial ROS. Epidermal cells in mutant mice with a genetic SOD2 deficiency undergo cellular senescence, nuclear DNA damage, and irreversible arrest of proliferation in a portion of their keratinocytes.[11][12]

Mutant mice with a conditional deficiency for mitochondrial SOD2 in connective tissue have an accelerated aging phenotype.[13] This aging phenotype includes weight loss, skin atrophy, kyphosis (curvature of the spine), osteoporosis, muscle degeneration and reduced life span.

## DNA damage

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Mitochondrial ROS attack DNA readily, generating a variety of DNA damages such as oxidized bases and strand breaks. The major mechanism that cells use to repair oxidized bases such as 8-hydroxyguanine, formamidopyrimidine and 5-hydroxyuracil is base excision repair (BER).[14] BER occurs in both the cell nucleus and in mitochondria.

## References

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