Lack of the antioxidant enzyme methionine sulfoxide reductase A in mice impairs RPE phagocytosis and causes photoreceptor cone dysfunction

Methionine sulfoxide reductase A (MsrA) is a widely expressed antioxidant enzyme that counteracts oxidative protein damage and contributes to protein regulation by reversing oxidation of protein methionine residues. In retinal pigment epithelial (RPE) cells in culture, MsrA overexpression increases phagocytic capacity by supporting mitochondrial ATP production. Here, we show elevated retinal protein carbonylation indicative of oxidation, decreased RPE mitochondrial membrane potential, and attenuated RPE phagocytosis in msra^-/- mice. Moreover, electroretinogram recordings reveal decreased light responses specifically of cone photoreceptors despite normal expression and localization of cone opsins. Impairment in msra^-/- cone-driven responses is similar from 6 weeks to 13 months of age. These functional changes match dramatic decreases in lectin-labeled cone sheaths and reduction in cone arrestin in msra^-/- mice. Strikingly, cone defects in light response and in lectin-labeled cone sheath are completely prevented by dark rearing. Together, our data show that msra^-/- mice provide a novel small animal model of preventable cone-specific photoreceptor dysfunction that may have future utility in analysis of cone dystrophy disease mechanisms and testing therapeutic approaches aiming to alleviate cone defects.