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A. S. Kazi, R. Dierova, J. Brancato, A. Bragin, K. Boesze-Battaglia; Loss of Melanoregulin (MREG) May Function as a Possible Risk Factor for Retinal Degenerative Diseases. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5977.
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Genetic and biochemical evidence suggests a role for melanoregulin (MREG) in organelle biogenesis and phagosome maturation. Retinal pigmented epithelial (RPE) cells phagocytose photoreceptor outer segments (POS) and have a physiological requirement for efficient phagosome formation, maturation, and correct processing of several phagosomal enzymes for degradation of engulfed material. Age related accumulation of POS derived cellular debris and pigmentary changes due to accumulation of lipofuscin (A2E) cause RPE dysfunction leading to retinal degenerative diseases such as age-related macular degeneration (AMD). Choroidal neovascularization (CNV) is the most common clinical manifestation seen in AMD. In the current study we investigated whether decreased phagosomal function in RPE due to loss of MREG can lead to aberrant vasculogenesis.
In MREG-/- mice the enzymatic activity and maturation of cathepsin-D (Cat-D), the principal endosomal protease in RPE cells was inhibited. ARPE-19 cells stably expressing MREG shRNA demonstrated enhanced levels of secreted Pro-Cat-D (42-48 kDa) in the growth medium as compared to control shRNA cells indicating that incorrectly processed Cat-D is missorted and secreted into extracellular space by RPE cells. Loss of MREG resulted in of lysososmal pH alkalization suggesting decreased activities of many other pH sensitive lysosomal enzymes. Immunofluorescence analysis using CD31 antibody demonstrated enhanced vascular staining in the choroidal regions of MREG -/- mice as compared to age matched control animals.
MREG expression is necessary for efficient phagosome functions and degradation of OS. We hypothesize that the 42-48 kDa pro-Cat-D is a paracrine communicator between RPE and choroid endothelial cells that may trigger CNV leading to AMD/ blindness. Our data suggests that mutations/ polymorphisms in MREG may be an important risk factor for retinal degenerative diseases.
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