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Silvia Finnemann, Jade Vargas, Ying Dun; Independent roles of methionine sulfoxide reductase A in mitochondrial ATP synthesis and in protection from oxidative stress of the retinal pigment epithelium. Invest. Ophthalmol. Vis. Sci. 2013;54(15):6108.
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The antioxidant enzyme methionine sulfoxide reductase A (MsrA) is highly expressed in the retinal pigment epithelium (RPE), the primary support tissue for neighboring photoreceptors. MsrA protein levels correlate with sensitivity of RPE in culture to experimental oxidative stress. Here, we investigated whether and how MsrA affects functionality of RPE cells that were not compromised by elevated oxidative stress.
We used laser scanning confocal microscopy to compare the subcellular distribution of endogenous MsrA with the distribution of mitotracker or mitochondrial respiratory chain protein in unpassaged primary rat RPE and the stable RPE-J cell line. We tested the effects of acute silencing or overexpression of MsrA in primary and immortalized RPE cells on the demanding task of phagocytosis of photoreceptor outer segment fragments (POS), on sensitivity to hydrogen peroxide, on ATP content, on mitochondrial protein expression, and on mitochondrial respiratory chain activity. We also tested if antioxidants or mitochondrial ATP synthesis inhibitors altered the effects of manipulating MsrA.
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, while activity of complex-II and mitochondrial content were unaffected.
Our data indicate that MsrA enhances ATP synthesis by increasing specifically the activity of complex-IV of the respiratory chain. Such contribution of MsrA to energy metabolism is independent of its additional function in protection from elevated oxidative stress and contributes to routine but vital photoreceptor support by RPE cells.
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