The Retinal G Protein-coupled Receptor (RGR) Enhances Isomerohydrolase Activity Independent of Light

Abstract

Rod and cone visual pigments use 11-cis-retinal, a vitamin A derivative, as their chromophore. Light isomerizes 11-cis- into all-trans-retinal, triggering a conformational transition of the opsin molecule that initiates phototransduction. After bleaching all-trans-retinal leaves the opsin, and light sensitivity must be restored by regeneration of 11-cis-retinal. Under bright light conditions the retinal G protein-coupled receptor (RGR) was reported to support this regeneration by acting as a photoisomerase in a proposed photic visual cycle. We analyzed the contribution of RGR to rhodopsin regeneration under different light regimes and show that regeneration, during light exposure and in darkness, is slowed about 3-fold in Rgr-/- mice. These findings are not in line with the proposed function of RGR as a photoisomerase. Instead, RGR, independent of light, accelerates the conversion of retinyl esters to 11-cis-retinal by positively modulating isomerohydrolase activity, a key step in the “classical” visual cycle. Furthermore, we find that light accelerates rhodopsin regeneration, independent of RGR. Rod and cone visual pigments use 11-cis-retinal, a vitamin A derivative, as their chromophore. Light isomerizes 11-cis- into all-trans-retinal, triggering a conformational transition of the opsin molecule that initiates phototransduction. After bleaching all-trans-retinal leaves the opsin, and light sensitivity must be restored by regeneration of 11-cis-retinal. Under bright light conditions the retinal G protein-coupled receptor (RGR) was reported to support this regeneration by acting as a photoisomerase in a proposed photic visual cycle. We analyzed the contribution of RGR to rhodopsin regeneration under different light regimes and show that regeneration, during light exposure and in darkness, is slowed about 3-fold in Rgr-/- mice. These findings are not in line with the proposed function of RGR as a photoisomerase. Instead, RGR, independent of light, accelerates the conversion of retinyl esters to 11-cis-retinal by positively modulating isomerohydrolase activity, a key step in the “classical” visual cycle. Furthermore, we find that light accelerates rhodopsin regeneration, independent of RGR. Rhodopsin is the light-sensing molecule of rod photoreceptors. It comprises a protein component, opsin, and a vitamin A-derived chromophore 11-cis-retinal. Absorption of a photon by rhodopsin isomerizes the retinal from its 11-cis-isomer to its all-trans-retinal isomer, leading rapidly to a conformational change of the opsin molecule. In its activated form rhodopsin interacts with transducin thereby activating the phototransduction cascade that leads to closure of cGMP-activated channels in the plasma membrane. The resulting hyperpolarization leads to a reduction of synaptic glutamate release, completing the conversion of a light stimulus into a neurochemical signal (reviewed in Ref. 1Burns M.E. Baylor D.A. Annu. Rev. Neurosci. 2001; 24: 779-805Crossref PubMed Scopus (337) Google Scholar). Light-induced activity of rhodopsin is quickly terminated by phosphorylation and capping of its transducin interaction site. The chromophore, now in the all-trans form, is hydrolyzed and released. To become photosensitive again, the bleached opsin molecule requires the re-introduction of a new molecule of 11-cis-retinal. The source of 11-cis-retinal for rod photoreceptors is the retinal pigment epithelium (RPE), 1The abbreviations used are: RPE, retinal pigment epithelium; sER, smooth endoplasmic reticulum; RDH5, 11-cis-retinol dehydrogenase; REST, retinyl ester storage particle; sRPE65, soluble RPE65; CRALBP, cellular retinaldehyde-binding protein; WT, wild type; HPLC, high performance liquid chromatography; ERG, electroretinogram recordings; mRPE65, membrane-bound RPE65. where 11-cis- retinal is regenerated from all-trans-retinoids. This “classical” visual cycle involves formation of all-trans-retinol in the photoreceptor, transport to the RPE, several steps of metabolic conversion, and transport of 11-cis-retinal back to the photoreceptor (reviewed in Refs. 2McBee J.K. Palczewski K. Baehr W. Pepperberg D.R. Prog. Retin. Eye Res. 2001; 20: 469-529Crossref PubMed Scopus (320) Google Scholar and 3Lamb T.D. Pugh Jr., E.N. Prog. Retin. Eye Res. 2004; 23: 307-380Crossref PubMed Scopus (550) Google Scholar). The retinal pigment epithelial protein 65 (RPE65, on mouse chromosome 3 (4Hamel C.P. Jenkins N.A. Gilbert D.J. Copeland N.G. Redmond T.M. Genomics. 1994; 20: 509-512Crossref PubMed Scopus (58) Google Scholar)) is essential for this classical visual cycle: absence of RPE65 leads to undetectable levels of 11-cis-retinal (5Redmond T.M. Yu S. Lee E. Bok D. Hamasaki D. Chen N. K. 20: PubMed Scopus Google and photoreceptor function E. Redmond T.M. 2001; PubMed Scopus Google Scholar). the of 11-cis-retinal under photic conditions was on of in the retinal G protein-coupled receptor on mouse chromosome W. D. Chen 2001; PubMed Scopus Google In RGR