11-cis-Retinal Reduces Constitutive Opsin Phosphorylation and Improves Quantum Catch in Retinoid-deficient Mouse Rod Photoreceptors

Abstract

Rpe65 ؊/؊ mice produce minimal amounts of 11-cis-retinal, the ligand necessary for the formation of photosensitive visual pigments. Therefore, the apoprotein opsin in these animals has not been exposed to its normal ligand. The Rpe65 ؊/؊ mice contain less than 0.1% of wild type levels of rhodopsin. Mass spectrometric analysis of opsin from Rpe65 ؊/؊ mice revealed unusually high levels of phosphorylation in dark-adapted mice but no other structural alterations. Single flash and flicker electroretinograms (ERGs) from 1-month-old animals showed trace rod function but no cone response. B-wave kinetics of the single-flash ERG are comparable with those of dark-adapted wild type mice containing a full compliment of rhodopsin. Application (intraperitoneal injection) of 11-cis-retinal to Rpe65 ؊/؊ mice increased the rod ERG signal, increased levels of rhodopsin, and decreased opsin phosphorylation. Therefore, exogenous 11-cis-retinal improves photoreceptor function by regenerating rhodopsin and removes constitutive opsin phosphorylation. Our results indicate that opsin, which has not been exposed to 11-cis-retinal, does not generate the activity generally associated with the bleached apoprotein.RPE65 is a major protein in the retinal pigment epithelium (RPE) 1 (1) and has also been identified in cone photoreceptors (2, 3). The photoreceptors of Rpe65 Ϫ/Ϫ mice are almost completely depleted of 11-cis-retinal, the native ligand of cone and rod opsins, resulting in minimal levels of rhodopsin and the deterioration of their photosensitivity (4). Photoreceptor function has been shown to be partially restored by supplying exogenous ligand (5, 6). Thus, the Rpe65 Ϫ/Ϫ mouse is an excellent model in which to study the two key factors that control the activity of rhodopsin, the supply of the ligand (7, 8) and the level of rhodopsin/opsin phosphorylation. Therefore, we have analyzed opsin phosphorylation levels of Rpe65 Ϫ/Ϫ mice and correlated those levels with electroretinogram (ERG) responses and rhodopsin levels in the absence and presence of exogenous 11-cis-retinal.Retinal function is controlled by the availability of 11-cisretinal, which can be affected by the absence or dysfunction of any number of participants in the retinal metabolic pathway (e.g. Refs. 9 and 10). RPE65 is essential for production of 11-cis-retinoids (4). RPE65 mutations in humans result in congenital retinal dystrophies ranging from night blindness to loss of vision (11). Although night blindness would suggest loss of rod rather than cone function, the ERG retained in Rpe65 Ϫ/Ϫ mice appears to originate from rod photoreceptors (12).A second key factor that controls visual sensitivity is opsin/ rhodopsin phosphorylation. The C terminus of activated rhodopsin is multiply phosphorylated by rhodopsin kinase (G protein-coupled receptor kinase 1) (13) in vivo (14,15), and this multiple phosphorylation has been shown to be necessary for the rapid return of sensitivity (16). In moderate light levels, it has been proposed that opsin dephosphorylation and its regen...

Lecithin-retinol Acyltransferase Is Essential for Accumulation of All-trans-Retinyl Esters in the Eye and in the Liver

“…This abnormality may result from the constitutive activity of opsin because steady-state activation of phototransduction by the receptor (42)(43)(44) was also proposed as a cause of retinal degeneration in Rpe65Ϫ/Ϫ mice (28,45). Similar observations were made for Rpe65Ϫ/Ϫ mice which also contain large amount of opsin, reduced length of ROS, but minute amounts of active visual pigments (28,46). A similar shortening of ROS can be observed also at the early stages of retinal dystrophies in human (e.g.…”

Light-Dependent Translocation of Arrestin in the Absence of Rhodopsin Phosphorylation and Transducin Signaling

“…Rhodopsin is primarily absent in RPE65 Ϫ/Ϫ mice because of a defect in the visual cycle to regenerate 11-cis-retinal, but the presence of the opsin apoprotein preserves rod outer segment structure (Redmond et al, 1998). Opsin in RPE65 Ϫ/Ϫ mice is constitutively phosphorylated (Ablonczy et al, 2002;Van Hooser et al, 2002), probably as a consequence of the weak constitutive activity of opsin described in vitro (Robinson et al, 1992;Surya et al, 1995;Buczylko et al, 1996). In RPE65 Ϫ/Ϫ mice, opsin appeared to signal constitutively for arrestin translocation to rod outer segments, regardless of the lighting conditions.…”

Why photoreceptors die (and why they don't)

“…As in vitamin A-deprived rats and humans, the sensitivity can be rapidly restored by treatment with supplemental chromophore. (42)(43)(44)(45) Rods in Rpe65 À/À mice behave as if in the presence of a continuous background light. (46) Light responses are only a fraction of their normal amplitude and show an accelerated turn-off, characteristic of light-adapted photoreceptors (see Ref.…”

“…(40) In Rpe65 À/À mice kept in normal room light (12 hours on and 12 hours off), the outer segments of the rods have very little rhodopsin but abundant opsin. (41)(42)(43) The rods continue to show a small response to light even though the retina cannot make 11-cis retinal, because a small amount of another chromophore, 9-cis retinal, probably made in the liver, makes it to the photoreceptors; but the amount of 9-cis retinal is normally so small that less than 1% of the visual pigment is present as rhodopsin, and the rods are greatly desensitized. As in vitamin A-deprived rats and humans, the sensitivity can be rapidly restored by treatment with supplemental chromophore.…”