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 Mller cells of the retina, both adjacent to photoreceptors. The biochemical pathways that mediate this conversion are different in RPE and Mller 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. Mller 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, Mller 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 Mller 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 Mller 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 Mller 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 Mller 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.
This project is to study the function of RGR-opsin, a light-sensitive opsin protein that works in cells of the eye to regulate the metabolism of vitamin A and related retinoids. Relatively little is currently known about this protein, although mutations in is gene cause the inherited blinding disease, retinitis pigmentosa. This proposal will extend our understanding of light-dependent regulation of retinoid metabolism, and may explain why photoreceptors die when RGR-opsin is missing in the human retina.
|Cook, Jeremy D; Ng, Sze Yin; Lloyd, Marcia et al. (2017) Peropsin modulates transit of vitamin A from retina to retinal pigment epithelium. J Biol Chem 292:21407-21416|
|Kaylor, Joanna J; Xu, Tongzhou; Ingram, Norianne T et al. (2017) Blue light regenerates functional visual pigments in mammals through a retinyl-phospholipid intermediate. Nat Commun 8:16|