Functional Characterization of RGR-opsin in Retinal Muller Cells

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 DESCRIPTION (provided by applicant): Capture of a photon by a rhodopsin or cone-opsin pigment in a photoreceptor isomerizes the 11-cis-retinaldehyde (11-cis-RAL) chromophore to all-trans-retinaldehyde (all-trans-RAL), which activates the pigment. Shortly afterwards, the all-trans-RAL dissociates, leaving behind light-insensitive apo-opsin. To regenerate the opsin pigment, the released all-trans-RAL must be converted back to 11-cis-RAL, which then recombines with apo-opsin. Synthesis of visual chromophore takes place in the retinal pigment epithelium (RPE) and Müller cells of the retina, both adjacent to photoreceptors. The biochemical pathways that mediate this conversion are different in RPE and Müller cells. RPE cells produce 11-cis-RAL at a slow rate, and are thought to provide chromophore for both rods and cones under dim light. Müller cells mainly produce the chromophore- precursor, 11-cis-retinol (11-cis-ROL), which is utilized by cones, but not rods, to synthesize 11-cis-RAL. In bright light, Müller cells turn-over retinoids at a much higher rate than do RPE cells. RGR-opsin is a non-visual opsin located in both RPE and Müller cells. Mice with a knock-out mutation in the rgr gene exhibit slow regeneration of visual pigments and accumulate retinyl esters in the RPE and retina. Retinyl esters are lipid-soluble storage forms of retinol (vitamin A). Preliminary studies suggest that RGR-opsin effects light-dependent mobilization of retinyl esters in the RPE. Nothing is known about the function of RGR-opsin in Müller cells, which is the subject of the current study. This project tests the broad hypothesis that RGR-opsin regulates the flow of visual retinoids between RPE and Müller cells in a light-dependent fashion. For example, by stimulating hydrolysis of retinyl esters in the RPE, RGR-opsin depletes substrate for the isomerase in RPE cells (Rpe65) while providing all-trans-ROL substrate for the isomerase in Müller cells (DES1). This study will be performed on cultured mammalian cells expressing selected retinoid-processing proteins, and ocular tissues from genetically modified mice. Experimental methods include liquid chromatographic analysis of retinoids, single- and two-photon confocal microscopy of animal tissues and cultured cells, and ex vivo electroretinography of live mouse eyecups and retina explants.