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Laura S. Frost, Monika Damek-Poprawa, Kathleen Boesze-Battaglia; Role of Melanoregulin in Photoreceptor Outer Segment Phagocytosis and Lysosomal Maturation. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2691.
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© ARVO (1962-2015); The Authors (2016-present)
Our previous studies document an essential role for melanoregulin (MREG) in lysosome-dependent degradation of photoreceptor outer segments (POS) by the retinal pigment epithelium (RPE), J.Biol Chem (2009) 284:10877-10889. Loss of MREG function results in delayed phagosome maturation leading to the accumulation of toxic debris associated with retinal degenerative disease. In these studies we investigate the role of MREG in the phagocytosis of POSs and test the hypothesis that MREG regulates lysosomal maturation through a MerTK dependent pathway.
We use a combination of biochemistry and immunocytochemistry to analyze the localization of MREG. To investigate the function of MREG an ARPE-19 cell line with reduced MREG expression was developed. MREG gene expression in response to prolonged phagocytic challenge was monitored by real-time PCR, protein expression by Western blotting and intracellular localization by immunocytochemistry.
MREG, a 28 kDa peripheral membrane protein localizes to cholesterol rich RPE plasma membrane microdomains. Within intracellular punctate structures it colocalizes with late endosomal/lysosomal markers but not early endosomes. Knockdown of MREG expression by shRNA leads to fewer LAMP1 positive puncta suggesting a role in lysosomal formation. MREG expression increases in RPE cells fed POS over a prolonged period, with a greater increase seen in response to UV-oxidized POS, suggestive of a stress response. MREG expression in RPE cells decreases in response to stimulation with Protein S, the ligand for MerTK, and increases at later time points. Using reciprocal co-IPs and immunofluorescence analysis MREG was shown to interact with MerTK.
Collectively, our results suggest that MREG regulates the production of lysosomes in response to excessive phagocytic challenge. Possibly via a MerTK dependent pathway.
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