title: Independent roles of methionine sulfoxide reductase A in mitochondrial ATP synthesis and as antioxidant in retinal pigment epithelial cells.
abstract: The antioxidant enzyme methionine sulfoxide reductase A (MsrA) is highly expressed in the retinal pigment epithelium (RPE), a support tissue for neighboring photoreceptors. MsrA protein levels correlate with sensitivity of RPE in culture to experimental oxidative stress. To investigate whether and how MsrA affects RPE functionality regardless of oxidative stress, we tested the effects of acute silencing or overexpression of MsrA on the phagocytosis of photoreceptor outer segment fragments (POS), a demanding, daily function of the RPE that is essential for vision. Endogenous MsrA localized to mitochondria and cytosol of rat RPE in culture. RPE cells manipulated to express higher or lower levels of MsrA than control cells showed no signs of cell death but increased or decreased, respectively, POS binding as well as engulfment. These effects of altered MsrA protein concentration on phagocytosis were independent of the levels of oxidative stress. However, altering MsrA expression had no effect on phagocytosis when mitochondrial respiration was inhibited. Furthermore, ATP content directly correlated with MsrA protein levels in RPE cells that used mitochondrial oxidative phosphorylation for ATP synthesis but not in RPE cells that relied on glycolysis alone. Overexpressing MsrA was sufficient to increase specifically the activity of complex IV of the respiratory chain, whereas activity of complex II and mitochondrial content were unaffected. Thus, MsrA probably enhances ATP synthesis by increasing complex IV activity. Such contribution of MsrA to energy metabolism is independent of its function in protection from elevated oxidative stress but contributes to routine but vital photoreceptor support by RPE cells.
authors.affiliation: Department of Biological Sciences, Center for Cancer, Genetic Diseases, and Gene Regulation, Fordham University, Bronx, NY 10458, USA.
authors.forename: Ying
authors.initials: Y
authors.lastname: Dun
authors.affiliation: Department of Biological Sciences, Center for Cancer, Genetic Diseases, and Gene Regulation, Fordham University, Bronx, NY 10458, USA.
authors.forename: Jade
authors.initials: J
authors.lastname: Vargas
authors.affiliation: Center for Molecular Biology and Biotechnology, Florida Atlantic University, Boca Raton, FL 33431, USA; Department of Microbiology and Immunology, Weill Medical College of Cornell University, New York, NY 10065, USA.
authors.forename: Nathan
authors.initials: N
authors.lastname: Brot
authors.affiliation: Department of Biological Sciences, Center for Cancer, Genetic Diseases, and Gene Regulation, Fordham University, Bronx, NY 10458, USA. Electronic address: finnemann@fordham.edu.
authors.forename: Silvia C
authors.initials: SC
authors.lastname: Finnemann
mesh_terms: D000255:Adenosine Triphosphate / Q000096:biosynthesis*; D000818:Animals; D000975:Antioxidants; D002460:Cell Line; D042963:Electron Transport Complex II / Q000235:genetics; D003576:Electron Transport Complex IV / Q000235:genetics; D004734:Energy Metabolism / Q000235:genetics / Q000276:immunology; D004847:Epithelial Cells / Q000166:cytology; D006019:Glycolysis; D008928:Mitochondria / Q000378:metabolism*; D010085:Oxidative Phosphorylation; D018384:Oxidative Stress / Q000276:immunology; D010088:Oxidoreductases / Q000096:biosynthesis / Q000235:genetics / Q000276:immunology*; D010587:Phagocytosis / Q000235:genetics / Q000276:immunology*; D011485:Protein Binding / Q000276:immunology; D034622:RNA Interference; D034741:RNA, Small Interfering; D051381:Rats; D055214:Retinal Photoreceptor Cell Outer Segment / Q000276:immunology; D055213:Retinal Pigment Epithelium / Q000166:cytology / Q000378:metabolism*
chemical_list: D000975:Antioxidants; D034741:RNA, Small Interfering; D000255:Adenosine Triphosphate; D010088:Oxidoreductases; D042963:Electron Transport Complex II; C488525:MsrA protein, rat; D003576:Electron Transport Complex IV
keywords: AMD; Free radicals; Mitochondria; MsrA; Oxidative stress; PBS; PFA; POS; Phagocytosis; RPE; Respiratory chain; Retinal pigment epithelium; age-related macular degeneration; methionine sulfoxide reductase A; paraformaldehyde; phosphate-buffered saline; retinal pigment epithelium; shed photoreceptor outer segment tips; siRNA; silencing RNA; β-gal; β-galactosidase