Although mice lacking rod and cone photoreceptors are blind, they retain many eye-mediated responses to light, possibly through photosensitive retinal ganglion cells. These cells express melanopsin, a photopigment that confers this photosensitivity. Mice lacking melanopsin still retain nonvisual photoreception, suggesting that rods and cones could operate in this capacity. We observed that mice with both outer-retinal degeneration and a deficiency in melanopsin exhibited complete loss of photoentrainment of the circadian oscillator, pupillary light responses, photic suppression of arylalkylamine-N-acetyltransferase transcript, and acute suppression of locomotor activity by light. This indicates the importance of both nonvisual and classical visual photoreceptor systems for nonvisual photic responses in mammals.

Lecithin-retinol acyltransferase (LRAT), an enzyme present mainly in the retinal pigmented epithelial cells and liver, converts all-trans-retinol into all-trans-retinyl esters. In the retinal pigmented epithelium, LRAT plays a key role in the retinoid cycle, a two-cell recycling system that replenishes the 11-cis-retinal chromophore of rhodopsin and cone pigments. We disrupted mouse Lrat gene expression by targeted recombination and generated a homozygous Lrat knock-out (Lrat؊/؊) mouse. Despite the expression of LRAT in multiple tissues, the Lrat؊/؊ mouse develops normally. The histological analysis and electron microscopy of the retina for 6 -8-week-old Lrat؊/؊ mice revealed that the rod outer segments are ϳ35% shorter than those of Lrat؉/؉ mice, whereas other neuronal layers appear normal. Lrat؊/؊ mice have trace levels of all-trans-retinyl esters in the liver, lung, eye, and blood, whereas the circulating all-trans-retinol is reduced only slightly. Scotopic and photopic electroretinograms as well as pupillary constriction analyses revealed that rod and cone visual functions are severely attenuated at an early age. We conclude that Lrat؊/؊ mice may serve as an animal model with early onset severe retinal dystrophy and severe retinyl ester deprivation.Lecithin-retinol acyltransferase (LRAT) 1 converts all-transretinol (vitamin A) to all-trans-retinyl esters in several tissues, including the liver, lung, pancreas, intestine, testis, and the retinal pigmented epithelium (RPE) (1-5). LRAT activity in the RPE has been studied for more than 60 years (6), but the enzyme was only recently identified on the molecular level as a 25-kDa integral membrane protein (7). All-trans-retinyl esters are intermediate compounds in a metabolic pathway ("visual cycle" or "retinoid cycle") that recycles 11-cis-retinal, the chromophore of rhodopsin and cone pigments (for review, see Refs. 8 -10). In this cycle, all-trans-retinal dissociates from rhodopsin and cone pigments after photobleaching. In the photoreceptors, all-trans-retinal is reduced to all-trans-retinol and subsequently exported to the adjacent RPE. In the RPE, alltrans-retinol is esterified by LRAT and stored. All-trans-retinyl esters have been suggested to be the substrate for a putative isomerohydrolase in the RPE (11) and for a retinyl ester hydrolase that produces all-trans-retinol, a substrate for the putative isomerase (for review, see Ref. 12). Ultimately, 11-cisretinol is produced, oxidized to 11-cis-retinal, and exported to the photoreceptors. In the rod and cone photoreceptor outer segments, 11-cis-retinal recombines with opsins to form rhodopsin and cone pigments (for review, see Ref. 8).Human LRAT cDNA was cloned from a retinal-RPE cDNA library (7) and rodent Lrat cDNA from liver and other tissues (13-15). Lrat mRNA was shown to be a 5.0-kb species expressed in the RPE, and the multiple transcripts based on differential polyadenylation were detected in several other tissues known for the highest LRAT activity (13). The human LRAT polypeptide consisted of 230 ...

Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate numerous nonvisual phenomena, including entrainment of the circadian clock to light-dark cycles, pupillary light responsiveness, and light-regulated hormone release. We have applied multielectrode array recording to characterize murine ipRGCs. We find that all ipRGC photosensitivity is melanopsin dependent. At least three populations of ipRGCs are present in the postnatal day 8 (P8) murine retina: slow onset, sensitive, fast off (type I); slow onset, insensitive, slow off (type II); and rapid onset, sensitive, very slow off (type III). Recordings from adult rd/rd retinas reveal cells comparable to postnatal types II and III. Recordings from early postnatal retinas demonstrate intrinsic light responses from P0. Early light responses are transient and insensitive but by P6 show increased photosensitivity and persistence. These results demonstrate that ipRGCs are the first light-sensitive cells in the retina and suggest previously unappreciated diversity in this cell population.

BackgroundLeber congenital amaurosis (LCA), a heterogeneous early-onset retinal dystrophy, accounts for ~15% of inherited congenital blindness. One cause of LCA is loss of the enzyme lecithin:retinol acyl transferase (LRAT), which is required for regeneration of the visual photopigment in the retina.Methods and FindingsAn animal model of LCA, the Lrat −/− mouse, recapitulates clinical features of the human disease. Here, we report that two interventions—intraocular gene therapy and oral pharmacologic treatment with novel retinoid compounds—each restore retinal function to Lrat −/− mice. Gene therapy using intraocular injection of recombinant adeno-associated virus carrying the Lrat gene successfully restored electroretinographic responses to ~50% of wild-type levels (p < 0.05 versus wild-type and knockout controls), and pupillary light responses (PLRs) of Lrat −/− mice increased ~2.5 log units (p < 0.05). Pharmacological intervention with orally administered pro-drugs 9-cis-retinyl acetate and 9-cis-retinyl succinate (which chemically bypass the LRAT-catalyzed step in chromophore regeneration) also caused long-lasting restoration of retinal function in LRAT-deficient mice and increased ERG response from ~5% of wild-type levels in Lrat −/− mice to ~50% of wild-type levels in treated Lrat −/− mice (p < 0.05 versus wild-type and knockout controls). The interventions produced markedly increased levels of visual pigment from undetectable levels to 600 pmoles per eye in retinoid treated mice, and ~1,000-fold improvements in PLR and electroretinogram sensitivity. The techniques were complementary when combined.ConclusionIntraocular gene therapy and pharmacologic bypass provide highly effective and complementary means for restoring retinal function in this animal model of human hereditary blindness. These complementary methods offer hope of developing treatment to restore vision in humans with certain forms of hereditary congenital blindness.