Genetic analysis of RPE65: from human disease to mouse model

An inducible amphipathic α-helix mediates subcellular targeting and membrane binding of RPE65

RPE65 retinol isomerase is an indispensable player in the visual cycle between the vertebrate retina and RPE. Although membrane association is critical for RPE65 function, its mechanism is not clear. Residues 107-125 are believed to interact with membranes but are unresolved in all RPE65 crystal structures, whereas palmitoylation at C112 also plays a role. We report the mechanism of membrane recognition and binding by RPE65. Binding of aa107-125 synthetic peptide with membrane-mimicking micellar surfaces induces transition from unstructured loop to amphipathic α-helical (AH) structure but this transition is automatic in the C112-palmitoylated peptide. We demonstrate that the AH significantly affects palmitoylation level, membrane association, and isomerization activity of RPE65. Furthermore, aa107-125 functions as a membrane sensor and the AH as a membrane-targeting motif. Molecular dynamic simulations clearly show AH-membrane insertion, supporting our experimental findings. Collectively, these studies allow us to propose a working model for RPE65-membrane binding, and to provide a novel role for cysteine palmitoylation.

Retinal pigment epithelium 65 kDa protein (RPE65): An update

Vertebrate vision critically depends on an 11-cis-retinoid renewal system known as the visual cycle. At the heart of this metabolic pathway is an enzyme known as retinal pigment epithelium 65 kDa protein (RPE65), which catalyzes an unusual, possibly biochemically unique, reaction consisting of a coupled all-trans-retinyl ester hydrolysis and alkene geometric isomerization to produce 11-cis-retinol. Early work on this isomerohydrolase demonstrated its membership to the carotenoid cleavage dioxygenase superfamily and its essentiality for 11-cis-retinal production in the vertebrate retina. Three independent studies published in 2005 established RPE65 as the actual isomerohydrolase instead of a retinoid-binding protein as previously believed. Since the last devoted review of RPE65 enzymology appeared in this journal, major advances have been made in a number of areas including our understanding of the mechanistic details of RPE65 isomerohydrolase activity, its phylogenetic origins, the relationship of its membrane binding affinity to its catalytic activity, its role in visual chromophore production for rods and cones, its modulation by macromolecules and small molecules, and the involvement of RPE65 mutations in the development of retinal diseases. In this article, I will review these areas of progress with the goal of integrating results from the varied experimental approaches to provide a comprehensive picture of RPE65 biochemistry. Key outstanding questions that may prove to be fruitful future research pursuits will also be highlighted.

Utility of In Vitro Mutagenesis of RPE65 Protein for Verification of Mutational Pathogenicity Before Gene Therapy

Importance

Next-generation sequencing can detect variants of uncertain significance (VUSs), for some of which gene therapy would not be advantageous. Therefore, the pathogenicity of compound heterozygous or homozygous variants should be confirmed before bilateral vitrectomy and administration of voretigene neparvovec-rzyl.

Objective

To describe an in vitro mutagenesis assay for assessing the pathogenicity of variants in the RPE65 gene.

Design, Setting, and Participants

This case series was conducted at 2 tertiary referral centers. Clinical history, imaging, and electrophysiologic testing results were reviewed from September 5, 2008, to December 31, 2019. Participants were 4 pediatric patients with Leber congenital amaurosis who were evaluated for or met the inclusion criteria for phase 1 to 3 clinical trials or were referred for voretigene neparvovec-rzyl treatment.

Main Outcomes and Measures

A functional assay was used to confirm the pathogenicity of novel RPE65 VUSs in 4 patients with Leber congenital amaurosis.

Results

Four patients with Leber congenital amaurosis had VUSs in RPE65. Patients 1 and 2 were siblings with the homozygous VUS c.311G>T p.(G104V). Patient 3 was a compound heterozygote with 1 known pathogenic allele, c.1202_1203insCTGG p.(Glu404AlafsTer4), and 1 VUS, c.311G>T p.(G104V), which segregated to separate alleles. Patient 4 was also a compound heterozygote with 1 pathogenic variant, c.11 + 5G>A, and 1 variant in trans, c.1399C>T p.(P467S). In vitro mutagenesis revealed that the G104V and P467S RPE65 proteins were catalytically inactive (0% isomerase activity). Patients 1 and 2 were excluded from participation in a phase 1 trial owing to high Adeno-associated virus 2 capsid-neutralizing antibodies. Patients 3 (G104V) and 4 (P467S) underwent successful surgical gene therapy with voretigene neparvovec-rzyl, and their response to lower white light intensity and visual field increased in fewer than 30 days after gene therapy intervention.

Conclusions and Relevance

Findings from this study suggest that, in patients with missense mutations in RPE65, functional assays of protein function can be performed to assess the pathogenicity of variants in both compound heterozygous and homozygous cases. Given the potential risks of gene therapy operations, in vitro RPE65 activity testing should be considered to avoid the possibility of treating a false genotype.

Clinical Perspective: Treating RPE65-Associated Retinal Dystrophy

Until recently, there was no approved treatment for a retinal degenerative disease. Subretinal injection of a recombinant adeno-associated virus (AAV) delivering the normal copy of the human RPE65 cDNA led to reversal of blindness first in animal models and then in humans. This led to the first US Food and Drug Administration (FDA)-approved gene therapy product for a genetic disease, voretigene neparvovec-rzyl (Luxturna). Luxturna was then approved by the European Medicines Association and is now available in the US through Spark Therapeutics and worldwide through Novartis. Not only has treatment with Luxturna changed the lives of people previously destined to live a life of blindness, but it has fueled interest in developing additional gene therapy reagents targeting numerous other genetic forms of inherited retinal disease. This review describes many of the considerations for administration of Luxturna and describes how lessons from experience with Luxturna could lead to additional gene-based treatments of blindness.

Voretigene Neparvovec in Retinal Diseases: A Review of the Current Clinical Evidence

Subretinal gene therapy trials began with the discovery of RPE65 variants and their association with Leber congenital amaurosis. The RPE65 protein is critical for the normal functioning of the visual phototransduction cascade. RPE65 gene knockout animal models were developed and showed similar diseased phenotypes to their human counterparts. Proof of concept studies were carried out in these animal models using subretinal RPE65 gene replacement therapy, resulting in improvements in various visual function markers including electroretinograms, pupillary light responses, and object avoidance behaviors. Positive results in animal models led to Phase 1 human studies using adeno-associated viral vectors. Results in these initial human studies also showed positive impact on visual function and acceptable safety. A landmark Phase 3 study was then conducted by Spark Therapeutics using a dose of 1.5 x10¹¹ vector genomes after dose-escalation studies confirmed its efficacy and safety. Multi-luminance mobility testing was used to measure the primary efficacy endpoint due to its excellent reliability in detecting the progression of inherited retinal diseases. After the study met its primary endpoint, the Food and Drug Administration approved voretigene neparvovec (Luxturna^®) for use in RPE65-associated inherited retinal diseases.

Safety of Same-Eye Subretinal Sequential Readministration of AAV2-hRPE65v2 in Non-human Primates

We have demonstrated safe and effective subretinal readministration of recombinant adeno-associated virus serotype (rAAV) to the contralateral eye in large animals and humans even in the setting of preexisting neutralizing antibodies (NAbs). Readministration of AAV to the same retina may be desirable in order to treat additional areas of the retina not targeted initially or to boost transgene expression levels at a later time point. To better understand the immune and structural consequences of subretinal rAAV readministration to the same eye, we administered bilateral subretinal injections of rAAV2-hRPE65v2 to three unaffected non-human primates (NHPs) and repeated the injections in those same eyes 2 months later. Ophthalmic exams and retinal imaging were performed after the first and second injections. Peripheral blood monocytes, serum, and intraocular fluids were collected at baseline and post-injection time points to characterize the cellular and humoral immune responses. Histopathologic and immunohistochemical studies were carried out on the treated retinas. Ipsilateral readministration of AAV2-hRPE65v2 in NHPs did not threaten the ocular or systemic health through the time span of the study. The repeat injections were immunologically and structurally well tolerated, even in the setting of preexisting serum NAbs. Localized structural abnormalities confined to the outer retina and retinal pigmented epithelium (RPE) after readministration of the treatment do not differ from those observed after single or contralateral administration of an AAV carrying a non-therapeutic transgene in NHPs and were not observed in a patient treated with the nearly identical, FDA-approved, AAV2-hRPE65v2 vector (voretigene neparvovec-rzyl), suggesting NHP-specific abnormalities.

Aberrant RNA splicing is the major pathogenic effect in a knock-in mouse model of the dominantly inherited c.1430A>G human RPE65 mutation

Human RPE65 mutations cause a spectrum of retinal dystrophies that result in blindness. While RPE65 mutations have been almost invariably recessively inherited, a c.1430A>G (p.(D477G)) mutation has been reported to cause autosomal dominant retinitis pigmentosa (adRP). To study the pathogenesis of this human mutation, we have replicated the mutation in a knock-in (KI) mouse model using CRISPR/Cas9-mediated genome editing. Significantly, in contrast to human patients, heterozygous KI mice do not exhibit any phenotypes in visual function tests. When raised in regular vivarium conditions, homozygous KI mice display relatively undisturbed visual functions with minimal retinal structural changes. However, KI/KI mouse retinae are more sensitive to light exposure and exhibit signs of degenerative features when subjected to light stress. We find that instead of merely producing a missense mutant protein, the A>G nucleotide substitution greatly affects appropriate splicing of Rpe65 mRNA by generating an ectopic splice site in comparable context to the canonical one, thereby disrupting RPE65 protein expression. Similar splicing defects were also confirmed for the human RPE65 c.1430G mutant in an in vitro Exontrap assay. Our data demonstrate that a splicing defect is associated with c.1430G pathogenesis, and therefore provide insights in the therapeutic strategy for human patients.

Amelioration of Neurosensory Structure and Function in Animal and Cellular Models of a Congenital Blindness

Most genetically distinct inherited retinal degenerations are primary photoreceptor degenerations. We selected a severe early onset form of Leber congenital amaurosis (LCA), caused by mutations in the gene LCA5, in order to test the efficacy of gene augmentation therapy for a ciliopathy. The LCA5-encoded protein, Lebercilin, is essential for the trafficking of proteins and vesicles to the photoreceptor outer segment. Using the AAV serotype AAV7m8 to deliver a human LCA5 cDNA into an Lca5 null mouse model of LCA5, we show partial rescue of retinal structure and visual function. Specifically, we observed restoration of rod-and-cone-driven electroretinograms in about 25% of injected eyes, restoration of pupillary light responses in the majority of treated eyes, an ∼20-fold decrease in target luminance necessary for visually guided behavior, and improved retinal architecture following gene transfer. Using LCA5 patient-derived iPSC-RPEs, we show that delivery of the LCA5 cDNA restores lebercilin protein and rescues cilia quantity. The results presented in this study support a path forward aiming to develop safety and efficacy trials for gene augmentation therapy in human subjects with LCA5 mutations. They also provide the framework for measuring the effects of intervention in ciliopathies and other severe, early-onset blinding conditions.

A subpopulation of activated retinal macrophages selectively migrated to regions of cone photoreceptor stress, but had limited effect on cone death in a mouse model for type 2 Leber congenital amaurosis

BACKGROUND

Studies of antigen presentation in retina using mice that expressed green fluorescent protein (GFP) from a transgenic CD11c promoter found that retinal GFP^hi cells possessed antigen presentation function. Subsequent studies found that these high GFP^hi cells preferentially localized to sites of retinal injury, consistent with their APC function. Interest in the roles of macrophages in degenerative CNS diseases led us to study the GFP^hi cells in a retinal model of neurodegeneration. We asked if apoptotic cone photoreceptor cell death in Rpe65^-/- knockout mice induced the GFP^hi cells, explored their relationship to resident microglia (MG), and tested their role in cone survival.

METHODS

Rpe65^-/- mice were bred to CD11c^GFP mice on the B6/J background. CD11c^GFPRpe65^-/- mice were also backcrossed to CX3CR1^YFP-creERROSA^DTA mice so that CX3CR1⁺ mononuclear cells could be depleted by Tamoxifen. Retinas were analyzed by immunohistochemistry, confocal microscopy, fluorescence fundoscopy and flow cytometry.

RESULTS

Elevated numbers of GFP^hi cells were concentrated in photoreceptor cell layers of CD11c^GFPRpe65^-/- mice coinciding with the peak of cone death at 2 to 4weeks of age, and persisted for at least 14months. After the initial wave of cone loss, a slow progressive loss of cones was found that continued to retain GFP^hi cells in the outer retina. Sustained, four-week Tamoxifen depletions of the GFP^hi cells and MG in Rpe65^-/- mice from day 13 to day 41, and from day 390 to day 420 promoted a small increase in cone survival. We found no evidence that the GFP^hi cells were recruited from the circulation; all data pointed to a MG origin. MG and GFP^hi cells were well segregated in the dystrophic retina; GFP^hi cells were foremost in the photoreceptor cell layer, while MG were concentrated in the inner retina.

CONCLUSIONS

The expression of GFP on a subset of retinal mononuclear cells in CD11c^GFP mice identified a distinct population of cells performing functions previously attributed to MG. Although GFP^hi cells dominated the macrophage response to cone death in the photoreceptor cell layer, their ablation led to only an incremental increase in cone survival. The ability to identify, ablate, and isolate these cells will facilitate analysis of this activated, antigen-presenting subset of MG.

Non-viral therapeutic approaches to ocular diseases: An overview and future directions

Currently there are no viable treatment options for patients with debilitating inherited retinal degeneration. The vast variability in disease-inducing mutations and resulting phenotypes has hampered the development of therapeutic interventions. Gene therapy is a logical approach, and recent work has focused on ways to optimize vector design and packaging to promote optimized expression and phenotypic rescue after intraocular delivery. In this review, we discuss ongoing ocular clinical trials, which currently use viral gene delivery, but focus primarily on new advancements in optimizing the efficacy of non-viral gene delivery for ocular diseases. Non-viral delivery systems are highly customizable, allowing functionalization to improve cellular and nuclear uptake, bypassing cellular degradative machinery, and improving gene expression in the nucleus. Non-viral vectors often yield transgene expression levels lower than viral counterparts, however their favorable safety/immune profiles and large DNA capacity (critical for the delivery of large ocular disease genes) make their further development a research priority. Recent work on particle coating and vector engineering presents exciting ways to overcome limitations of transient/low gene expression levels, but also highlights the fact that further refinements are needed before use in the clinic.

Retinoid uptake, processing, and secretion in human iPS-RPE support the visual cycle

PURPOSE

Retinal pigmented epithelium derived from human induced pluripotent stem (iPS) cells (iPS-RPE) may be a source of cells for transplantation. For this reason, it is essential to determine the functional competence of iPS-RPE. One key role of the RPE is uptake and processing of retinoids via the visual cycle. The purpose of this study is to investigate the expression of visual cycle proteins and the functional ability of the visual cycle in iPS-RPE.

METHODS

iPS-RPE was derived from human iPS cells. Immunocytochemistry, RT-PCR, and Western blot analysis were used to detect expression of RPE genes lecithin-retinol acyl transferase (LRAT), RPE65, cellular retinaldehyde-binding protein (CRALBP), and pigment epithelium-derived factor (PEDF). All-trans retinol was delivered to cultured cells or whole cell homogenate to assess the ability of the iPS-RPE to process retinoids.

RESULTS

Cultured iPS-RPE expresses visual cycle genes LRAT, CRALBP, and RPE65. After incubation with all-trans retinol, iPS-RPE synthesized up to 2942 ± 551 pmol/mg protein all-trans retinyl esters. Inhibition of LRAT with N-ethylmaleimide (NEM) prevented retinyl ester synthesis. Significantly, after incubation with all-trans retinol, iPS-RPE released 188 ± 88 pmol/mg protein 11-cis retinaldehyde into the culture media.

CONCLUSIONS

iPS-RPE develops classic RPE characteristics and maintains expression of visual cycle proteins. The results of this study confirm that iPS-RPE possesses the machinery to process retinoids for support of visual pigment regeneration. Inhibition of all-trans retinyl ester accumulation by NEM confirms LRAT is active in iPS-RPE. Finally, the detection of 11-cis retinaldehyde in the culture medium demonstrates the cells' ability to process retinoids through the visual cycle. This study demonstrates expression of key visual cycle machinery and complete visual cycle activity in iPS-RPE.

Origin and evolution of retinoid isomerization machinery in vertebrate visual cycle: hint from jawless vertebrates

In order to maintain visual sensitivity at all light levels, the vertebrate eye possesses a mechanism to regenerate the visual pigment chromophore 11-cis retinal in the dark enzymatically, unlike in all other taxa, which rely on photoisomerization. This mechanism is termed the visual cycle and is localized to the retinal pigment epithelium (RPE), a support layer of the neural retina. Speculation has long revolved around whether more primitive chordates, such as tunicates and cephalochordates, anticipated this feature. The two key enzymes of the visual cycle are RPE65, the visual cycle all-trans retinyl ester isomerohydrolase, and lecithin:retinol acyltransferase (LRAT), which generates RPE65’s substrate. We hypothesized that the origin of the vertebrate visual cycle is directly connected to an ancestral carotenoid oxygenase acquiring a new retinyl ester isomerohydrolase function. Our phylogenetic analyses of the RPE65/BCMO and N1pC/P60 (LRAT) superfamilies show that neither RPE65 nor LRAT orthologs occur in tunicates (Ciona) or cephalochordates (Branchiostoma), but occur in Petromyzon marinus (Sea Lamprey), a jawless vertebrate. The closest homologs to RPE65 in Ciona and Branchiostoma lacked predicted functionally diverged residues found in all authentic RPE65s, but lamprey RPE65 contained all of them. We cloned RPE65 and LRATb cDNAs from lamprey RPE and demonstrated appropriate enzymatic activities. We show that Ciona ß-carotene monooxygenase a (BCMOa) (previously annotated as an RPE65) has carotenoid oxygenase cleavage activity but not RPE65 activity. We verified the presence of RPE65 in lamprey RPE by immunofluorescence microscopy, immunoblot and mass spectrometry. On the basis of these data we conclude that the crucial transition from the typical carotenoid double bond cleavage functionality (BCMO) to the isomerohydrolase functionality (RPE65), coupled with the origin of LRAT, occurred subsequent to divergence of the more primitive chordates (tunicates, etc.) in the last common ancestor of the jawless and jawed vertebrates.

New insights into retinoid metabolism and cycling within the retina

The retinoid cycle is a series of biochemical reactions within the eye that is responsible for synthesizing the chromophore, 11-cis retinal, for visual function. The chromophore is bound to G-protein coupled receptors, opsins, within rod and cone photoreceptor cells forming the photosensitive visual pigments. Integral to the sustained function of photoreceptors is the continuous generation of chromophore by the retinoid cycle through two separate processes, one that supplies both rods and cones and another that exclusively supplies cones. Recent findings such as RPE65 localization within cones and the pattern of distribution of retinoid metabolites within mouse and human retinas have challenged previous proposed schemes. This review will focus on recent findings regarding the transport of retinoids, the mechanisms by which chromophore is supplied to both rods and cones, and the metabolism of retinoids within the posterior segment of the eye.

RPE65 is present in human green/red cones and promotes photopigment regeneration in an in vitro cone cell model

RPE65 is an abundantly expressed protein within the retinal pigment epithelium (RPE) of the eye that is required for retinoid metabolism to support vision. Its genetic mutations are linked to the congenital disease Leber congenital amaurosis Type 2 (LCA2) characterized by the early onset of central vision loss. Current gene therapy trials have targeted restoration of functional RPE65 within the RPE of these patients with some success. Recent data show that RPE65 is also present within mouse cones to promote function. In this study, we evaluated the presence of RPE65 in human cones and investigated its potential mechanism for supporting cone function in the 661W cone cell line. We found that RPE65 was selectively expressed in human green/red cones but absent from blue cones and mediated ester hydrolysis for photopigment synthesis in vitro. These data suggest that cone RPE65 supports human diurnal vision, potentially enhancing our strategies for treating LCA2.

Regeneration of photopigment is enhanced in mouse cone photoreceptors expressing RPE65 protein

As cone photoreceptors mediate vision in bright light, their photopigments are bleached at a rapid rate and require substantial recycling of the chromophore 11-cis-retinal (RAL) for continued function. The retinal pigment epithelium (RPE) supplies 11-cis-RAL to both rod and cone photoreceptors; however, stringent demands imposed by the function of cones in bright light exceed the output from this source. Recent evidence has suggested that cones may be able to satisfy this demand through privileged access to an additional source of chromophore located within the inner retina. In this study, we demonstrate that the protein RPE65, previously identified in RPE as the isomerohydrolase of the RPE-retinal visual cycle, is found within cones of the rod-dominant mouse retina, and the level of RPE65 in cones is inversely related to the level in the RPE. The light sensitivity of cone ERGs of BALB/c mice, which had an undetectable level of cone RPE65, was enhanced by approximately threefold with administration of exogenous chromophore, indicating that the cones of these animals are chromophore deficient. This enhancement with chromophore administration was not observed in C57BL/6 mice, whose cones contain RPE65. These results demonstrate that RPE65 within cones may be essential for the efficient regeneration of cone photopigments under bright-light conditions.

Deletion of GRK1 causes retina degeneration through a transducin-independent mechanism

Rpe65(-/-) mice are unable to produce 11-cis-retinal, the chromophore of visual pigments. Consequently, the pigment is present as the apoprotein opsin with a minute level of pigment containing 9-cis-retinal as chromophore. Notably, a 10-20% fraction of this opsin is mono-phosphorylated independently of light conditions. To determine the role of rhodopsin kinase (GRK1) in phosphorylating this opsin and to test whether eliminating this phosphorylation would accelerate photoreceptor degeneration, we generated the Rpe65(-/-)Grk1(-/-) mouse. The retinae of Rpe65(-/-)Grk1(-/-) mice had negligible opsin phosphorylation, extensive degeneration with decreased opsin levels, and diminished light-evoked rod responses relative to Rpe65(-/-) mice. These data show that opsin phosphorylation in the Rpe65(-/-) mouse is due to the action of GRK1 and is neuroprotective. However, despite the higher activity of unphosphorylated opsin, the severe loss of opsin in the rapidly degenerating Rpe65(-/-)Grk1(-/-) mice resulted in lower overall opsin activity and in higher rod sensitivity compared with Rpe65(-/-) mice. In Rpe65(-/-)Grk1(-/-)Gnat1(-/-) mice where transduction activation was blocked, degeneration was only partially prevented. Therefore, increased opsin activity in the absence of phosphorylation was not the only mechanism for the accelerated retinal degeneration. Finally, the deletion of GRK1 triggered retinal degeneration in Grk1(-/-) mice after 1 month, even in the absence of apo-opsin. This degeneration was independent of light conditions and occurred even in the absence of transducin in Grk1(-/-)Gnat1(-/-) mice. Taken together, our results demonstrate a light-independent mechanism for retinal degeneration in the absence of GRK1, suggesting a second, not previously recognized role for that kinase.

Recent advances in ocular gene therapy

PURPOSE OF REVIEW

Ocular gene therapy has made significant advances due to improvements in viral vector delivery methods. Recent clinical trials for the treatment of Leber's congenital amaurosis have provided evidence for successful in-vivo gene therapy in humans.

RECENT FINDINGS

Gene therapy for ocular disease has been under investigation just over 15 years. Recently, the first human gene therapy trials for retinal degeneration were undertaken with encouraging preliminary safety and efficacy findings. Building on previous success in reversing blindness in animal models of Leber's congenital amaurosis, several groups proceeded with adeno-associated virus-mediated gene replacement. Many of the humans demonstrated increases in light sensitivity and in visual acuity. Subjective improvements in vision were corroborated in some cases by objective tests such as pupillary light response and nystagmography. Although much of the work in ocular gene therapy has involved retinal applications, significant progress has been seen in other aspects of ophthalmology.

SUMMARY

Ongoing human clinical trials support the safety and efficacy of adeno-associated virus-mediated gene therapy for retinal disease. These and other studies will establish the foundation for methodology to treat additional ocular diseases using gene therapy strategies.

Screening of the RPE65 gene in the Asian Indian patients with leber congenital amaurosis

PURPOSE

To determine the frequency of pathogenic mutations in the gene encoding RPE65 in patients from India with Leber congenital amaurosis (LCA).

METHODS

The coding sequence of all 14 exons and the adjacent flanking intron sequences of the RPE65 gene were directly sequenced in 60 unrelated Indian LCA patients. Bioinformatics tool was used to study the structural changes of the mutant protein.

RESULTS

Three sequence variants were found; two missense and one isocoding change. Of two missense changes, one was a putative polymorphism (N321K) and the other was a novel missense, disease causing change that alters proline to leucine at codon 470 (P470L) in one LCA patient. RPE65 mutations contribute to 1.7% of LCA in our population.

CONCLUSIONS

Mutations in the RPE65 gene are rare in patients with LCA and hence genes other than could be mainly responsible for causing LCA in India.

Reversal of blindness in animal models of leber congenital amaurosis using optimized AAV2-mediated gene transfer

We evaluated the safety and efficacy of an optimized adeno-associated virus (AAV; AAV2.RPE65) in animal models of the RPE65 form of Leber congenital amaurosis (LCA). Protein expression was optimized by addition of a modified Kozak sequence at the translational start site of hRPE65. Modifications in AAV production and delivery included use of a long stuffer sequence to prevent reverse packaging from the AAV inverted-terminal repeats, and co-injection with a surfactant. The latter allows consistent and predictable delivery of a given dose of vector. We observed improved electroretinograms (ERGs) and visual acuity in Rpe65 mutant mice. This has not been reported previously using AAV2 vectors. Subretinal delivery of 8.25 x 10(10) vector genomes in affected dogs was well tolerated both locally and systemically, and treated animals showed improved visual behavior and pupillary responses, and reduced nystagmus within 2 weeks of injection. ERG responses confirmed the reversal of visual deficit. Immunohistochemistry confirmed transduction of retinal pigment epithelium cells and there was minimal toxicity to the retina as judged by histopathologic analysis. The data demonstrate that AAV2.RPE65 delivers the RPE65 transgene efficiently and quickly to the appropriate target cells in vivo in animal models. This vector holds great promise for treatment of LCA due to RPE65 mutations.

Expression of RPE65, a putative receptor for plasma retinol-binding protein, in nonmelanocytic skin tumours

BACKGROUND

In a recent report we described RPE65, a protein originally characterized in retinal pigment epithelium, to be expressed in normal human epidermis. RPE65 is suspected to be involved in cellular uptake of retinol which is transported in the bloodstream complexed with plasma retinol-binding protein.

OBJECTIVES

To evaluate protein and mRNA expression of RPE65 in actinic keratosis (AK), squamous cell carcinoma (SCC) and basal cell carcinoma (BCC) compared with normal skin.

METHODS

RPE65 mRNA expression in skin tumours relative to normal skin of the respective donor was studied by real-time polymerase chain reaction in AK (n = 15), invasive SCC (n = 30) and BCC (n = 18). A peptide-specific anti-RPE65 antibody was used for immunohistochemical staining of formalin-fixed and paraffin-embedded tissue sections of the respective tumours.

RESULTS

RPE65 mRNA expression was reduced in AK. A highly significant reduction of RPE65 mRNA was observed in invasive SCC relative to normal skin of the respective donors. Immunohistochemistry revealed a continuous staining of basal and suprabasal keratinocytes in normal human epidermis. RPE65 in AK shown by immunohistochemical staining was reduced and quite irregular, whereas invasive SCC revealed no staining of tumour cells with the anti-RPE65 antibody. RPE65 mRNA values were elevated, whereas immunohistochemical staining for RPE65 protein was heterogeneous in BCC.

CONCLUSIONS

These results suggest progressive downregulation of RPE65 from AK to invasive SCC.

The retinal G protein-coupled receptor (RGR) enhances isomerohydrolase activity independent of light

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.

RPE65 gene mutation prevents development of autofluorescence in retinal pigment epithelial phagosomes

During senescence, autofluorescent lysosomal storage bodies known as lipofusin or age pigment accumulate in many post-mitotic types of cells. Among these cell types is the retinal pigment epithelium (RPE) of the mammalian eye. The mechanisms of lipofuscin formation and accumulation have been studied more extensively in the RPE than in any other cell type. Substantial evidence indicates that Vitamin A derivatives (retinoids) are required for RPE lipofuscin formation. The RPE and adjacent retina contain retinoids in the forms of retinol, retinyl esters, and retinaldehyde. Previous research has demonstrated that retinaldehydes are directly involved in the formation of one RPE lipofuscin fluorophore. However, RPE lipofuscin contains many other fluorophores. It has not been determined which retinoids are involved in the formation of these fluorescent compounds. Mice with a mutation in the Rpe65 gene contain substantial levels of retinol and retinyl esters in the RPE, but little if any retinaldehydes in either the RPE or retina. Therefore, these mice could be used to determine whether retinaldehydes are required for formation of all of the RPE lipofuscin fluorophores. Normal mice were given intraocular injections of a protease inhibitor, which resulted in the rapid accumulation in the RPE of lipofuscin-like inclusions. These inclusions exhibited fluorescence properties typical of RPE lipofuscin. Rpe65-/- mice treated with the protease inhibitor also accumulated inclusions similar to those observed in the normal mice. However, these inclusions did not fluoresce under the conditions used to visualize lipofuscin fluorescence. These findings indicate that the aldehyde form of Vitamin A is required for the formation of not only one, but all of the RPE lipofuscin fluorophores.

Downregulation of Otx2 in the dedifferentiated RPE cells of regenerating newt retina

Cynops pyrrhogaster (the Japanese common newt) regenerates neural retina from retinal pigmented epithelium (RPE) cells. Otx2 is a transcription factor that is involved in RPE cell differentiation. To understand the role of Otx2 during transdifferentiation of RPE cells, we cloned a Cynops Otx2 cDNA, and explored its expression by RT-PCR, immunohistochemistry and in situ hybridization. The expression of Otx2 was compared with the localization of a proliferating cell marker (PCNA), RPE cell markers (RPE65, CRBP) and an RPE and Muller glial cell marker (CRALBP). At the early stage of regeneration, 2 to 3 cell layered regenerating retina consisting of pigmented cells uniformly expressed Otx2 and other markers. Following this stage, 4-cell layered regenerating retina consisted of two distinct layers, pigmented monolayer (the outer layer) attached to Bruch's membrane and presumptive neural retina (the inner layers). In the outer layer, Otx2 and CRBP expression was maintained and majority of cells lost PCNA expression. Some of cells maintained RPE65. In the inner layers, expression of Otx2, CRBP and RPE65 was downregulated, but a majority of those cells maintained PCNA expression. These results indicate that spatiotemporal regulation of Otx2 expression is consistent with those of RPE markers. Otx2 may play a pivotal role in maintenance and specification of RPE cells during neural retina regeneration. In contrast to RPE cell markers, CRALBP was expressed in both the pigmented and the de-pigmented layers. This observation implicates the appearance of Muller glial cells in an early phase of regenerating retina.

Amniotic membrane maintains the phenotype of rabbit retinal pigment epithelial cells in culture

The success of surgical removal of choroidal neovascularisation followed by transplantation of autologous retinal pigment epithelial cells (RPE) for age-related macular degeneration (ARMD) may be limited by damage in Bruch's membrane. We investigated whether amniotic membrane (AM) might be used as an alternative basement membrane-containing matrix to support RPE growth and differentiation. Primary RPE plastic cultures were established from freshly enucleated Dutch belted rabbit eyes in DMEM/F12 containing 0.1 mM Ca(++) and 10% dialysed FBS. Upon subconfluence, cells were subcultured at 5000-9000 cells cm(-2) in the above-mentioned culture medium on intact AM (iAM), epithelially denuded AM (dAM) or plastic. After confluence, the Ca(++) concentration in the medium was increased to 1.8 mm for 4 weeks. Growth and morphology were monitored by phase contrast microscopy, and the phenotype by immunostaining with antibodies against cytokeratin 18, tight junction protein ZO-1, and RPE65 protein, and by transepithelial resistance (TER) measurement. Immunostaining to cytokeratin 18 confirmed the epithelial origin of isolated cells in both primary culture and subcultures. Compared to plastic cultures, RPE increased pigmentation within 24 hr after seeding on AM, with iAM being more pronounced than dAM. RPE adopted a hexagonal epithelial phenotype with more organised pigmentation, strong expression of ZO-1 and RPE65, and a significantly higher TER 4 weeks after Ca(++) switch on dAM. Our results indicate that AM may be used as a basement membrane-containing matrix to maintain RPE phenotype in vitro, and may facilitate subsequent transplantation to treat ARMD.

Recombinant adeno-associated virus type 2-mediated gene delivery into the Rpe65-/- knockout mouse eye results in limited rescue

BACKGROUND

Leber's congenital amaurosis (LCA) is a severe form of retinal dystrophy. Mutations in the RPE65 gene, which is abundantly expressed in retinal pigment epithelial (RPE) cells, account for approximately 10-15% of LCA cases. In this study we used the high turnover, and rapid breeding and maturation time of the Rpe65^-/- knockout mice to assess the efficacy of using rAAV-mediated gene therapy to replace the disrupted RPE65 gene. The potential for rAAV-mediated gene treatment of LCA was then analyzed by determining the pattern of RPE65 expression, the physiological and histological effects that it produced, and any improvement in visual function.

METHODS

rAAV.RPE65 was injected into the subretinal space of Rpe65^-/- knockout mice and control mice. Histological and immunohistological analyses were performed to evaluate any rescue of photoreceptors and to determine longevity and pattern of transgene expression. Electron microscopy was used to examine ultrastructural changes, and electroretinography was used to measure changes in visual function following rAAV.RPE65 injection.

RESULTS

rAAV-mediated RPE65 expression was detected for up to 18 months post injection. The delivery of rAAV.RPE65 to Rpe65^-/- mouse retinas resulted in a transient improvement in the maximum b-wave amplitude under both scotopic and photopic conditions (76% and 59% increase above uninjected controls, respectively) but no changes were observed in a-wave amplitude. However, this increase in b-wave amplitude was not accompanied by any slow down in photoreceptor degeneration or apoptotic cell death. Delivery of rAAV.RPE65 also resulted in a decrease in retinyl ester lipid droplets and an increase in short wavelength cone opsin-positive cells, suggesting that the recovery of RPE65 expression has long-term benefits for retinal health.

CONCLUSION

This work demonstrated the potential benefits of using the Rpe65^-/- mice to study the effects and mechanism of rAAV.RPE65-mediated gene delivery into the retina. Although the functional recovery in this model was not as robust as in the dog model, these experiments provided important clues about the long-term physiological benefits of restoration of RPE65 expression in the retina.

RPE65 of Retinal Pigment Epithelium, A Putative Receptor Molecule for Plasma Retinol-Binding Protein, is Expressed in Human Keratinocytes

Retinoids are important modulators for cell growth and differentiation of normal skin. In plasma, retinol is transported coupled to plasma retinol-binding protein. In this study, we investigated gene and protein expression of RPE65, a putative receptor for plasma retinol-binding protein in human epidermal keratinocytes. We performed real-time PCR analysis to evaluate expression of RPE65 mRNA in proliferating and differentiating keratinocytes. Immunoblotting with anti-RPE65 antibody shows distinct reactivity to a 61-kDa protein. Indirect immunofluorescence on normal human epidermis reveals cell surface labeling of keratinocytes. Laser scan microscopy exhibits colocalization of plasma retinol-binding protein and RPE65 on cultured keratinocytes. Internalization experiments with [3H]retinoic acid-retinol-binding protein complex in the presence and absence of excess of retinol-binding protein indicates receptor-dependent uptake of retinoids. We further show isolation of RPE65 protein by affinity chromatography from lysates of keratinocytes using a retinol-binding protein-matrix gel column. In summary, we demonstrate mRNA and protein expression of RPE65 in epidermal keratinocytes. Colocalization of plasma retinol-binding protein with RPE65 and affinity binding suggest a direct interaction of RPE65 with plasma retinol-binding protein in cultured human keratinocytes that might be involved in retinoid uptake of keratinocytes.

Isorhodopsin rather than rhodopsin mediates rod function in RPE65 knock-out mice

The chromophore of visual pigments is 11-cis-retinal and, thus, in its absence, opsin is not photosensitive and no visual function exists. However, in the RPE65 knockout (Rpe65-/-) mouse, where synthesis of 11-cis-retinal does not occur, a minimal visual response from rod photoreceptors is obtained. We have examined if an alternative pathway exists for cis-retinoid generation in the absence of RPE65. Cyclic-light-reared, 2-month-old Rpe65-/- mice were placed in complete darkness. No exogenous retinoids were administered. After 4 weeks, enhanced a- and b-wave amplitudes were obtained, increasing >10-fold for the a-wave and >3-fold for the b-wave as compared with cyclic-light-reared Rpe65-/- mice. Visual-pigment levels increased to approximately 10 pmol per retina, compared with no measurable pigment for cyclic-light-reared Rpe65-/- mice. The lambdamax of the isolated pigment was 487 nm, characteristic for isorhodopsin. Retinoid extractions confirmed the presence of 9-cis-retinal and the absence of 11-cis-retinal. Once the Rpe65-/- mice were returned to cyclic light, within 48 h the electroretinogram function returned to levels found in Rpe65-/- mice maintained in cyclic light. This dark-mediated pathway is also operational in older animals, because 13-month-old Rpe65-/- mice kept in prolonged darkness (12 weeks) had increased isorhodopsin levels and electroretinogram a- and b-wave amplitudes. These studies demonstrate that a pathway exists in the eye for the generation of 9-cis-retinal that is independent of RPE65 and light.

Increased light damage susceptibility at night does not correlate with RPE65 levels and rhodopsin regeneration in rats

The susceptibility of rats to light-induced retinal degeneration is increased at night. In mice, an important determinant of light damage susceptibility is the efficacy of rhodopsin regeneration after bleaching. The rate of rhodopsin regeneration is at least partly controlled by RPE65, a protein expressed in the retinal pigment epithelium. We therefore tested a potential involvement of RPE65 and rhodopsin regeneration in the increased light damage susceptibility of rats at night. For this purpose, rats were exposed to visible light at noon or at midnight and extent of light damage was determined by retinal morphology and TUNEL staining. Rpe65 gene expression was analyzed by semiquantitative RT-PCR and levels of RPE65 protein were determined by Western blotting. Rhodopsin regeneration kinetics was determined by measuring rhodopsin content immediately after a strong bleach and after different times of recovery in darkness. Rats were more susceptible to light damage at night as described by Organisciak and collegues [Invest. Ophthalmol. Vis. Sci. 41 (2000) 3694]. Rpe65 gene expression followed a day-night rhythm with highest steady-state mRNA levels at the beginning and lowest levels at the end of the day period. However, RPE65 protein levels remained constant. Rhodopsin regeneration kinetics did not differ during day and night. We conclude that levels of RPE65 protein and rhodopsin regeneration kinetics do not correlate with the increased light damage susceptibility observed in rats at night. Additional genetic or physiologic modifiers may exist in rats that regulate the retinal responsiveness to acute light exposure.

Evaluation of the role of the retinal G protein-coupled receptor (RGR) in the vertebrate retina in vivo

The retinal G protein-coupled receptor (RGR) is a protein that structurally resembles visual pigments and other G protein-coupled receptors. RGR may play a role as a photoisomerase in the production of 11-cis-retinal, the chromophore of the visual pigments. As the proposed function of RGR, in a complex with 11-cis-retinol dehydrogenase (RDH5), is to regenerate 11-cis-retinal under light conditions and RDH5 is expected to function in the light-independent part of the retinoid cycle, we speculated that the simultaneous loss of function of both proteins should more severely affect the rhodopsin regeneration capacity. Here, we evaluated the role of RGR using rgr-/- single and rdh5-/-rgr-/- double knockout mice under a number of light conditions. The most striking phenotype of rgr-/- mice after a single flash of light includes light-dependent formation of 9-cis- and 13-cis-retinoid isomers. These isomers are not formed in wild-type mice because either all-trans-retinal is bound to RGR and protected from isomerization to 9-cis- or 13-cis-retinal or because RGR is able to eliminate these isomers directly or indirectly. After intense bleaching, a transient accumulation of all-trans-retinyl esters and an attenuated recovery of 11-cis-retinal were observed. Finally, even under conditions of prolonged light illumination, as investigated in vitro in biochemical assays or in vivo by electroretinogram (ERG) measurements, no evidence of catalytic-like photoisomerization-driven production of 11-cis-retinal could be attained. These and previous results suggest that RGR and RDH5 are likely to function in the retinoid cycle, although their role is not essential and regeneration of visual pigment is only mildly affected by the absence of both proteins in rod-dominated mice.

Biochemical and physiological properties of rhodopsin regenerated with 11-cis-6-ring- and 7-ring-retinals

Phototransduction is initiated by the photoisomerization of rhodopsin (Rho) chromophore 11-cis-retinylidene to all-trans-retinylidene. Here, using Rho regenerated with retinal analogs with different ring sizes, which prevent isomerization around the C(11)=C(12) double bond, the activation mechanism of this G-protein-coupled receptor was investigated. We demonstrate that 11-cis-7-ring-Rho does not activate G-protein in vivo and in vitro, and that it does not isomerize along other double bonds, suggesting that it fits tightly into the binding site of opsin. In contrast, bleaching 11-cis-6-ring-Rho modestly activates phototransduction in vivo and at low pH in vitro. These results reveal that partial activation is caused by isomerization along other double bonds in more rigid 6-locked retinal isomers and protonation of key residues by lowering pH in 11-cis-6-ring-Rhos. Full activation is not achieved, because isomerization does not induce a complete set of conformational rearrangements of Rho. These results with 6- and 7-ring-constrained retinoids provide new insights into Rho activation and suggest a potential use of locked retinals, particularly 11-cis-7-ring-retinal, to inactivate opsin in some retinal degeneration diseases.

11-cis-retinal reduces constitutive opsin phosphorylation and improves quantum catch in retinoid-deficient mouse rod photoreceptors

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.

Recovery of visual functions in a mouse model of Leber congenital amaurosis

The visual process is initiated by the photoisomerization of 11-cis-retinal to all-trans-retinal. For sustained vision the 11-cis-chromophore must be regenerated from all-trans-retinal. This requires RPE65, a dominant retinal pigment epithelium protein. Disruption of the RPE65 gene results in massive accumulation of all-trans-retinyl esters in the retinal pigment epithelium, lack of 11-cis-retinal and therefore rhodopsin, and ultimately blindness. We reported previously (Van Hooser, J. P., Aleman, T. S., He, Y. G., Cideciyan, A. V., Kuksa, V., Pittler, S. J., Stone, E. M., Jacobson, S. G., and Palczewski, K. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 8623-8628) that in Rpe65-/- mice, oral administration of 9-cis-retinal generated isorhodopsin, a rod photopigment, and restored light sensitivity to the electroretinogram. Here, we provide evidence that early intervention by 9-cis-retinal administration significantly attenuated retinal ester accumulation and supported rod retinal function for more than 6 months post-treatment. In single cell recordings rod light sensitivity was shown to be a function of the amount of regenerated isorhodopsin; high doses restored rod responses with normal sensitivity and kinetics. Highly attenuated residual rod function was observed in untreated Rpe65-/- mice. This rod function is likely a consequence of low efficiency production of 11-cis-retinal by photo-conversion of all-trans-retinal in the retina as demonstrated by retinoid analysis. These studies show that pharmacological intervention produces long lasting preservation of visual function in dark-reared Rpe65-/- mice and may be a useful therapeutic strategy in recovering vision in humans diagnosed with Leber congenital amaurosis caused by mutations in the RPE65 gene, an inherited group of early onset blinding and retinal degenerations.

Distribution and density of medium- and short-wavelength selective cones in the domestic pig retina

The topography of medium (M)- and short (S)-wavelength sensitive cone photoreceptors was studied in the domestic pig retina. Antisera specific for M or S opsin as well as cone photoreceptor proteins arrestin and alpha-transducin were used to label cone types. Retinal wholemounts and their blood vessel patterns were drawn and specific regions removed. The wholemounts were immunocytochemically labelled to detect both M and S cones, and the specific regions labelled to detect S cones. Cones were counted in a 1 mm grid pattern, using the drawings as a guide. Pig retina has a high cone density retinal streak extending across the retina covering the optic disc (OD) and horizontal meridian. Densities in the streak are 20,000-35,000 mm(-2). Two higher peaks occur in the streak, one in temporal retina near the OD (39,000 mm(-2)) and the other in nasal retina 5-7 mm from the OD (40,500 mm(-2)). The lowest cone density is in far peripheral inferior retina (7000 mm(-2)). The total number of cones in pig retina is 17-20 million. Both types of cones are found throughout the retina, with S cone percentages ranging from 7.4 to 17.5% in no consistent topographical pattern. S cones have an irregular local distribution which can vary from a regular hexagonal pattern to small clusters of adjacent S cones to small areas lacking S cones. Double-label immunocytochemistry found that virtually all S cone outer segments (OS) contain some M opsin. M cone OS do not label at detectible levels for S opsin. Domestic pig retina is widely available, large, has a high cone density and has two types of cones. This tissue should be an excellent source for biochemical analysis of cone proteins, and for in vitro approaches to understanding cone survival factors.

Isomerization of 11-cis-retinoids to all-trans-retinoids in vitro and in vivo

The regeneration of 11-cis-retinal, the universal chromophore of the vertebrate retina, is a complex process involving photoreceptors and adjacent retinal pigment epithelial cells (RPE). 11-cis-Retinal is coupled to opsins in both rod and cone photoreceptor cells and is photoisomerized to all-trans-retinal by light. Here, we show that RPE microsomes can catalyze the reverse isomerization of 11-cis-retinol to all-trans-retinol (and 13-cis-retinol), and membrane exposure to UV light further enhances the rate of this reaction. This conversion is inhibited when 11-cis-retinol is in a complex with cellular retinaldehyde-binding protein (CRALBP), providing a clear demonstration of the protective effect of retinoid-binding proteins in retinoid processes in the eye, a function that has been long suspected but never proven. The reverse isomerization is nonenzymatic and specific to alcohol forms of retinoids, and it displays stereospecific preference for 11-cis-retinol and 13-cis-retinol but is much less efficient for 9-cis-retinol. The mechanism of reverse isomerization was investigated using stable isotope-labeled retinoids and radioactive tracers to show that this reaction occurs with the retention of configuration of the C-15 carbon of retinol through a mechanism that does not eliminate the hydroxyl group, in contrast to the enzymatic all-trans-retinol to 11-cis-retinol reaction. The activation energy for the conversion of 11-cis-retinol to all-trans-retinol is 19.5 kcal/mol, and 20.1 kcal/mol for isomerization of 13-cis-retinol to all-trans-retinol. We also demonstrate that the reverse isomerization occurs in vivo using exogenous 11-cis-retinol injected into the intravitreal space of wild type and Rpe65-/- mice, which have defective forward isomerization. This study demonstrates an uncharacterized activity of RPE microsomes that could be important in the normal flow of retinoids in the eye in vivo during dark adaptation.

New views on RPE65 deficiency: the rod system is the source of vision in a mouse model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most serious form of the autosomal recessive childhood-onset retinal dystrophies. Mutations in the gene encoding RPE65, a protein vital for regeneration of the visual pigment rhodopsin in the retinal pigment epithelium, account for 10-15% of LCA cases. Whereas previous studies of RPE65 deficiency in both animal models and patients attributed remaining visual function to cones, we show here that light-evoked retinal responses in fact originate from rods. For this purpose, we selectively impaired either rod or cone function in Rpe65-/- mice by generating double- mutant mice with models of pure cone function (rhodopsin-deficient mice; Rho-/-) and pure rod function (cyclic nucleotide-gated channel alpha3-deficient mice; Cnga3-/-). The electroretinograms (ERGs) of Rpe65-/- and Rpe65-/-Cnga3-/- mice were almost identical, whereas there was no assessable response in Rpe65-/-Rho-/- mice. Thus, we conclude that the rod system is the source of vision in RPE65 deficiency. Furthermore, we found that lack of RPE65 enables rods to mimic cone function by responding under normally cone-isolating lighting conditions. We propose as a mechanism decreased rod sensitivity due to a reduction in rhodopsin content to less than 1%. In general, the dissection of pathophysiological processes in animal models through the introduction of additional, selective mutations is a promising concept in functional genetics.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Gene therapy restores vision in a canine model of childhood blindness

The relationship between the neurosensory photoreceptors and the adjacent retinal pigment epithelium (RPE) controls not only normal retinal function, but also the pathogenesis of hereditary retinal degenerations. The molecular bases for both primary photoreceptor and RPE diseases that cause blindness have been identified. Gene therapy has been used successfully to slow degeneration in rodent models of primary photoreceptor diseases, but efficacy of gene therapy directed at photoreceptors and RPE in a large-animal model of human disease has not been reported. Here we study one of the most clinically severe retinal degenerations, Leber congenital amaurosis (LCA). LCA causes near total blindness in infancy and can result from mutations in RPE65 (LCA, type II; MIM 180069 and 204100). A naturally occurring animal model, the RPE65-/- dog, suffers from early and severe visual impairment similar to that seen in human LCA. We used a recombinant adeno-associated virus (AAV) carrying wild-type RPE65 (AAV-RPE65) to test the efficacy of gene therapy in this model. Our results indicate that visual function was restored in this large animal model of childhood blindness.

The Rpe65 Leu450Met variation increases retinal resistance against light-induced degeneration by slowing rhodopsin regeneration

Excessive light can cause retinal degeneration and may be an environmental cofactor accelerating retinal dystrophies and age-related diseases. In rodent models, the light damage susceptibility (LDS) of the retina is determined genetically. In two mouse strains, with different degrees of LDS, a Leu450Met variation in the pigment epithelial protein RPE65 was shown recently to cosegregate with low LDS. Because light damage is rhodopsin-mediated, and RPE65 is essential for the regeneration of rhodopsin in the visual cycle, we analyzed this variation regarding rhodopsin metabolism and LDS in four mouse strains. We found that, in contrast to previous assertions, LDS does not correlate with the maximal retinal content of rhodopsin present after dark adaptation. Instead, LDS correlated positively with the kinetics of rhodopsin regeneration, which determine rhodopsin availability during light exposure. Light damage occurred after absorption of a threshold dose of photons and thus fast regeneration, as observed in those two strains having Leu at position 450 of RPE65, was correlated with the occurrence of photoreceptor apoptosis after short exposure. In contrast, mice with the Leu450Met variation of Rpe65 regenerated rhodopsin with slow kinetics and showed an increased resistance to light-induced retinal degeneration. In these mice, RPE65 protein levels were reduced by a post-transcriptional mechanism. F(1) hybrid mice, carrying one normal and one variant Rpe65 gene, had intermediate levels of the corresponding protein and showed intermediate rhodopsin regeneration kinetics and an intermediate LDS. Thus, none of the two variants of Rpe65 had a dominant effect.

Rapid restoration of visual pigment and function with oral retinoid in a mouse model of childhood blindness

Mutations in the retinal pigment epithelium gene encoding RPE65 are a cause of the incurable early-onset recessive human retinal degenerations known as Leber congenital amaurosis. Rpe65-deficient mice, a model of Leber congenital amaurosis, have no rod photopigment and severely impaired rod physiology. We analyzed retinoid flow in this model and then intervened by using oral 9-cis-retinal, attempting to bypass the biochemical block caused by the genetic abnormality. Within 48 h, there was formation of rod photopigment and dramatic improvement in rod physiology, thus demonstrating that mechanism-based pharmacological intervention has the potential to restore vision in otherwise incurable genetic retinal degenerations.