Examining the Role of Cone-expressed RPE65 in Mouse Cone Function

Retinal Proteome Profiling of Inherited Retinal Degeneration Across Three Different Mouse Models Suggests Common Drug Targets in Retinitis Pigmentosa

Inherited retinal degenerations (IRDs) are a leading cause of blindness among the population of young people in the developed world. Approximately half of IRDs initially manifest as gradual loss of night vision and visual fields, characteristic of retinitis pigmentosa (RP). Due to challenges in genetic testing, and the large heterogeneity of mutations underlying RP, targeted gene therapies are an impractical largescale solution in the foreseeable future. For this reason, identifying key pathophysiological pathways in IRDs that could be targets for mutation-agnostic and disease-modifying therapies (DMTs) is warranted. In this study, we investigated the retinal proteome of three distinct IRD mouse models, in comparison to sex- and age-matched wild-type mice. Specifically, we used the Pde6βRd10 (rd10) and RhoP23H/WT (P23H) mouse models of autosomal recessive and autosomal dominant RP, respectively, as well as the Rpe65-/- mouse model of Leber's congenital amaurosis type 2 (LCA2). The mice were housed at two distinct institutions and analyzed using LC-MS in three separate facilities/instruments following data-dependent and data-independent acquisition modes. This cross-institutional and multi-methodological approach signifies the reliability and reproducibility of the results. The large-scale profiling of the retinal proteome, coupled with in vivo electroretinography recordings, provided us with a reliable basis for comparing the disease phenotypes and severity. Despite evident inflammation, cellular stress, and downscaled phototransduction observed consistently across all three models, the underlying pathologies of RP and LCA2 displayed many differences, sharing only four general KEGG pathways. The opposite is true for the two RP models in which we identify remarkable convergence in proteomic phenotype even though the mechanism of primary rod death in rd10 and P23H mice is different. Our data highlights the cAMP and cGMP second-messenger signaling pathways as potential targets for therapeutic intervention. The proteomic data is curated and made publicly available, facilitating the discovery of universal therapeutic targets for RP.

The multifaceted roles of retinoids in eye development, vision, and retinal degenerative diseases

Vitamin A (all-trans-retinol; at-Rol) and its derivatives, known as retinoids, have been adopted by vertebrates to serve as visual chromophores and signaling molecules, particularly in the eye/retina. Few tissues rely on retinoids as heavily as the retina, and the study of genetically modified mouse models with deficiencies in specific retinoid-metabolizing proteins has allowed us to gain insight into the unique or redundant roles of these proteins in at-Rol uptake and storage, or their downstream roles in retinal development and function. These processes occur during embryogenesis and continue throughout life. This review delves into the role of these genes in supporting retinal function and maps the impact that genetically modified mouse models have had in studying retinoid-related genes. These models display distinct perturbations in retinoid biochemistry, physiology, and metabolic flux, mirroring human ocular diseases.

The Retina-Based Visual Cycle

The continuous function of vertebrate photoreceptors requires regeneration of their visual pigment following its destruction upon activation by light (photobleaching). For rods, the chromophore required for the regeneration of rhodopsin is derived from the adjacent retinal pigmented epithelium (RPE) cells through a series of reactions collectively known as the RPE visual cycle. Mounting biochemical and functional evidence demonstrates that, for cones, pigment regeneration is supported by the parallel supply with chromophore by two pathways-the canonical RPE visual cycle and a second, cone-specific retina visual cycle that involves the Müller glial cells in the neural retina. In this article, we review historical information that led to the discovery of the retina visual cycle and discuss what is currently known about the reactions and molecular components of this pathway and its functional role in supporting cone-mediated vision.

A combination treatment based on drug repurposing demonstrates mutation-agnostic efficacy in pre-clinical retinopathy models

Inherited retinopathies are devastating diseases that in most cases lack treatment options. Disease-modifying therapies that mitigate pathophysiology regardless of the underlying genetic lesion are desirable due to the diversity of mutations found in such diseases. We tested a systems pharmacology-based strategy that suppresses intracellular cAMP and Ca2+ activity via G protein-coupled receptor (GPCR) modulation using tamsulosin, metoprolol, and bromocriptine coadministration. The treatment improves cone photoreceptor function and slows degeneration in Pde6βrd10 and RhoP23H/WT retinitis pigmentosa mice. Cone degeneration is modestly mitigated after a 7-month-long drug infusion in PDE6A-/- dogs. The treatment also improves rod pathway function in an Rpe65-/- mouse model of Leber congenital amaurosis but does not protect from cone degeneration. RNA-sequencing analyses indicate improved metabolic function in drug-treated Rpe65-/- and rd10 mice. Our data show that catecholaminergic GPCR drug combinations that modify second messenger levels via multiple receptor actions provide a potential disease-modifying therapy against retinal degeneration.

Single Center Experience with Voretigene Neparvovec Gene Augmentation Therapy in RPE65 Mutation-Associated Inherited Retinal Degeneration in a Clinical Setting

PURPOSE

To assess the impact of baseline data on psychophysical and morphological outcomes of subretinal voretigene neparvovec (VN) (Luxturna, Spark Therapeutics, Inc.) treatment.

DESIGN

Single-center, retrospective, longitudinal, consecutive case series.

PARTICIPANTS

Patients with RPE65-biallelic mutation-associated inherited retinal degeneration (RPE65-IRD) treated between February 2020 and March 2022 with VN and oral immunosuppression according to the manufacturer's recommendation by one surgeon (F.G.H.).

METHODS

Retrospective analysis of surgical and clinical records, ancillary testing, and retinal imaging after VN therapy for RPE65-IRD. Descriptive statistics compared data at baseline up to 32 months post-treatment.

MAIN OUTCOME MEASURES

Best-corrected visual acuity (BCVA), low-luminance VA (LLVA), Goldmann visual fields (GVFs), chromatic full-field stimulus threshold (FST) testing (FST), scotopic and photopic 2-color threshold perimetry (2CTP), and multimodal retinal imaging.

RESULTS

Thirty eyes of 19 patients were analyzed (10 pediatric patients < 20 years; 20 adult patients > 20 years of age; overall range: 8-40 years) with a median follow-up of 15 months (range, 1-32). The fovea was completely or partially detached in 16 eyes, attached in 12 eyes, and not assessable in 2 eyes on intraoperative imaging. Median BCVA at baseline was better in the pediatric group (P < 0.05) and did not change significantly independent of age. Meaningful loss of BCVA (≥ 0.3 logarithm of the minimal angle of resolution [logMAR]) occurred in 5 of 18 adult eyes, and a meaningful gain (≥-0.3 logMAR) occurred in 2 of 18 adult and 2 of 8 pediatric eyes. The LLVA and scotopic 2CTP improved considerably in pediatric patients. Scotopic blue FST improved at all ages but more in pediatric patients (8/8 eyes gained ≥ 10 decibels [dB]; P < 0.05). In pediatric patients, median GVF improved by 20% for target V4e and by 50% for target III4e (target I4e not detected). Novel atrophy developed in 13 of 26 eyes at the site of the bleb or peripheral of vascular arcades. Improvements in FST did not correlate with development of chorioretinal atrophy at 12 months. Mean central retinal thickness was 165.87 μm (± 26.26) at baseline (30 eyes) and 157.69 μm (± 30.3) at 12 months (26 eyes). Eight adult patients were treated unilaterally. The untreated eyes did not show meaningful changes during follow-up.

CONCLUSIONS

These data in a clinical setting show the effectiveness of VN therapy with stable median BCVA and mean retinal thickness and improvements of LLVA, FST, and 2CTP up to 32 months. Treatment effects were superior in the pediatric group. We observed new chorioretinal atrophy in 50% of the treated eyes.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Proteomic Analysis of Retinal Mitochondria-Associated ER Membranes Identified Novel Proteins of Retinal Degeneration in Long-Term Diabetes

The mitochondria-associated endoplasmic reticulum (ER) membrane (MAM) is the physical contact site between the ER and the mitochondria and plays a vital role in the regulation of calcium signaling, bioenergetics, and inflammation. Disturbances in these processes and dysregulation of the ER and mitochondrial homeostasis contribute to the pathogenesis of diabetic retinopathy (DR). However, few studies have examined the impact of diabetes on the retinal MAM and its implication in DR pathogenesis. In the present study, we investigated the proteomic changes in retinal MAM from Long Evans rats with streptozotocin-induced long-term Type 1 diabetes. Furthermore, we performed in-depth bioinformatic analysis to identify key MAM proteins and pathways that are potentially implicated in retinal inflammation, angiogenesis, and neurodegeneration. A total of 2664 unique proteins were quantified using IonStar proteomics-pipeline in rat retinal MAM, among which 179 proteins showed significant changes in diabetes. Functional annotation revealed that the 179 proteins are involved in important biological processes such as cell survival, inflammatory response, and cellular maintenance, as well as multiple disease-relevant signaling pathways, e.g., integrin signaling, leukocyte extravasation, PPAR, PTEN, and RhoGDI signaling. Our study provides comprehensive information on MAM protein changes in diabetic retinas, which is helpful for understanding the mechanisms of metabolic dysfunction and retinal cell injury in DR.

Longitudinal Photoreceptor Phenotype Observation and Therapeutic Evaluation of a Carbonic Anhydrase Inhibitor in a X-Linked Retinoschisis Mouse Model

Purpose

To study the long-term photoreceptor changes and to evaluate the effects of topical application of a carbonic anhydrase inhibitor (CAI) in a mouse model of X-linked retinoschisis (XLRS).

Methods

Conventional electroretinograms (ERGs) and dark-adapted 10-Hz flicker ERGs were recorded in control and Rs1^−/Y mice generated with CRISPR/Cas9. ON-pathway blocker 2-amino-4-phosphobutyric acid (APB) was injected intravitreally. Morphology was evaluated with histology and optical coherence tomography (OCT). Mice were treated with a CAI inhibitor brinzolamide eye drops (10 mg/ml) three times a day for 3 months. OCT and ERG findings at 1, 4, and 10 months were analyzed.

Results

Negative ERGs and retinal cavities were evident in Rs1^−/Y mice. Both a-wave and b-wave amplitudes decreased with age when compared with age-matched controls. The APB-isolated a-wave (a′) amplitudes of Rs1^−/Y mice were reduced in all age groups. In dark-adapted 10-Hz flicker ERG, the amplitude-intensity curve of Rs1^−/Y mice shifted down. The thickness of ONL and IS/OS decreased in Rs1^−/Y mice. CAI reduced the splitting retinal cavities but didn't affect the ERG.

Conclusions

In addition to post receptoral impairments, photoreceptor cells underwent progressive dysfunction since early age in Rs1^−/Y mice. Long-term CAI treatment improved the shrinkage of the splitting retinal cavity, while no functional improvement was observed.

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.

An inducible Cre mouse for studying roles of the RPE in retinal physiology and disease

The retinal pigment epithelium (RPE) provides vital metabolic support for retinal photoreceptor cells and is an important player in numerous retinal diseases. Gene manipulation in mice using the Cre-LoxP system is an invaluable tool for studying the genetic basis of these retinal diseases. However, existing RPE-targeted Cre mouse lines have critical limitations that restrict their reliability for studies of disease pathogenesis and treatment, including mosaic Cre expression, inducer-independent activity, off-target Cre expression, and intrinsic toxicity. Here, we report the generation and characterization of a knockin mouse line in which a P2A-CreER^T2 coding sequence is fused with the native RPE-specific 65 kDa protein (Rpe65) gene for cotranslational expression of CreER^T2. Cre^+/– mice were able to recombine a stringent Cre reporter allele with more than 99% efficiency and absolute RPE specificity upon tamoxifen induction at both postnatal days (PD) 21 and 50. Tamoxifen-independent Cre activity was negligible at PD64. Moreover, tamoxifen-treated Cre^+/– mice displayed no signs of structural or functional retinal pathology up to 4 months of age. Despite weak RPE65 expression from the knockin allele, visual cycle function was normal in Cre^+/– mice. These data indicate that Rpe65^CreERT2 mice are well suited for studies of gene function and pathophysiology in the RPE.

Redox-Responsive Hyaluronic Acid-Based Nanogels for the Topical Delivery of the Visual Chromophore to Retinal Photoreceptors

Delivering therapeutics to the posterior segment of the eye is challenging due to various anatomical and physical barriers. While significant improvements have been realized by introducing direct injections to diseased sites, these approaches come with potential side effects that can range from simple inflammation to severe retinal damage. The topical instillation of drugs remains a safer and preferred alternative for patients' compliance. Here, we report the synthesis of penetratin-complexed, redox-responsive hyaluronic acid-based nanogels for the triggered release and delivery of therapeutics to the posterior part of the eye via topical application. The synthesized nanogels were shown to release their load only when exposed to a reducing environment, similar to the cytoplasm. As a model drug, visual chromophore analog, 9-cis-retinal, was loaded into nanogels and efficiently delivered to the mouse retina's photoreceptors when applied topically. Electroretinogram measurements showed a partial recovery of photoreceptor function in all treated eyes versus untreated controls. To the best of our knowledge, this report constitutes the first attempt to use a topically applied triggered-release drug delivery system to target the pigmented layer of the retina, in addition to the first attempt to deliver the visual chromophore topically.

Pathways and disease-causing alterations in visual chromophore production for vertebrate vision

All that we view of the world begins with an ultrafast cis to trans photoisomerization of the retinylidene chromophore associated with the visual pigments of rod and cone photoreceptors. The continual responsiveness of these photoreceptors is then sustained by regeneration processes that convert the trans-retinoid back to an 11-cis configuration. Recent biochemical and electrophysiological analyses of the retinal G-protein-coupled receptor (RGR) suggest that it could sustain the responsiveness of photoreceptor cells, particularly cones, even under bright light conditions. Thus, two mechanisms have evolved to accomplish the reisomerization: one involving the well-studied retinoid isomerase (RPE65) and a second photoisomerase reaction mediated by the RGR. Impairments to the pathways that transform all-trans-retinal back to 11-cis-retinal are associated with mild to severe forms of retinal dystrophy. Moreover, with age there also is a decline in the rate of chromophore regeneration. Both pharmacological and genetic approaches are being used to bypass visual cycle defects and consequently mitigate blinding diseases. Rapid progress in the use of genome editing also is paving the way for the treatment of disparate retinal diseases. In this review, we provide an update on visual cycle biochemistry and then discuss visual-cycle-related diseases and emerging therapeutics for these disorders. There is hope that these advances will be helpful in treating more complex diseases of the eye, including age-related macular degeneration (AMD).

Properties and Therapeutic Implications of an Enigmatic D477G RPE65 Variant Associated with Autosomal Dominant Retinitis Pigmentosa

RPE65 isomerase, expressed in the retinal pigmented epithelium (RPE), is an enzymatic component of the retinoid cycle, converting all-trans retinyl ester into 11-cis retinol, and it is essential for vision, because it replenishes the photon capturing 11-cis retinal. To date, almost 200 loss-of-function mutations have been identified within the RPE65 gene causing inherited retinal dystrophies, most notably Leber congenital amaurosis (LCA) and autosomal recessive retinitis pigmentosa (arRP), which are both severe and early onset disease entities. We previously reported a mutation, D477G, co-segregating with the disease in a late-onset form of autosomal dominant RP (adRP) with choroidal involvement; uniquely, it is the only RPE65 variant to be described with a dominant component. Families or individuals with this variant have been encountered in five countries, and a number of subsequent studies have been reported in which the molecular biological and physiological properties of the variant have been studied in further detail, including observations of possible novel functions in addition to reduced RPE65 enzymatic activity. With regard to the latter, a human phase 1b proof-of-concept study has recently been reported in which aspects of remaining vision were improved for up to one year in four of five patients with advanced disease receiving a single one-week oral dose of 9-cis retinaldehyde, which is the first report showing efficacy and safety of an oral therapy for a dominant form of RP. Here, we review data accrued from published studies investigating molecular mechanisms of this unique variant and include hitherto unpublished material on the clinical spectrum of disease encountered in patients with the D477G variant, which, in many cases bears striking similarities to choroideremia.

Efficient Coding by Midget and Parasol Ganglion Cells in the Human Retina

In humans, midget and parasol ganglion cells account for most of the input from the eyes to the brain. Yet, how they encode visual information is unknown. Here, we perform large-scale multi-electrode array recordings from retinas of treatment-naive patients who underwent enucleation surgery for choroidal malignant melanomas. We identify robust differences in the function of midget and parasol ganglion cells, consistent asymmetries between their ON and OFF types (that signal light increments and decrements, respectively) and divergence in the function of human versus non-human primate retinas. Our computational analyses reveal that the receptive fields of human midget and parasol ganglion cells divide naturalistic movies into adjacent spatiotemporal frequency domains with equal stimulus power, while the asymmetric response functions of their ON and OFF types simultaneously maximize stimulus coverage and information transmission and minimize metabolic cost. Thus, midget and parasol ganglion cells in the human retina efficiently encode our visual environment.

Non-photopic and photopic visual cycles differentially regulate immediate, early, and late phases of cone photoreceptor-mediated vision

Cone photoreceptors in the retina enable vision over a wide range of light intensities. However, the processes enabling cone vision in bright light (i.e. photopic vision) are not adequately understood. Chromophore regeneration of cone photopigments may require the retinal pigment epithelium (RPE) and/or retinal Müller glia. In the RPE, isomerization of all-trans-retinyl esters to 11-cis-retinol is mediated by the retinoid isomerohydrolase Rpe65. A putative alternative retinoid isomerase, dihydroceramide desaturase-1 (DES1), is expressed in RPE and Müller cells. The retinol-isomerase activities of Rpe65 and Des1 are inhibited by emixustat and fenretinide, respectively. Here, we tested the effects of these visual cycle inhibitors on immediate, early, and late phases of cone photopic vision. In zebrafish larvae raised under cyclic light conditions, fenretinide impaired late cone photopic vision, while the emixustat-treated zebrafish unexpectedly had normal vision. In contrast, emixustat-treated larvae raised under extensive dark-adaptation displayed significantly attenuated immediate photopic vision concomitant with significantly reduced 11-cis-retinaldehyde (11cRAL). Following 30 min of light, early photopic vision was recovered, despite 11cRAL levels remaining significantly reduced. Defects in immediate cone photopic vision were rescued in emixustat- or fenretinide-treated larvae following exogenous 9-cis-retinaldehyde supplementation. Genetic knockout of Des1 (degs1) or retinaldehyde-binding protein 1b (rlbp1b) did not eliminate photopic vision in zebrafish. Our findings define molecular and temporal requirements of the nonphotopic or photopic visual cycles for mediating vision in bright light.

Dydrogesterone affects the transcription of genes in visual cycle and circadian rhythm network in the eye of zebrafish

Dydrogesterone (DDG) is a synthetic progestin used in contraception and hormone replacement therapy. Our previous transcriptome data showed that the response to light stimulus, photoperiodism and rhythm related gene ontology (GO) terms were significantly enriched in the brain of zebrafish after chronic exposure to DDG. Here we investigated the effects of DDG on the eye of zebrafish. Zebrafish were exposed to DDG at three concentration levels (3.39, 33.1, and 329 ng L^-1) for 120 days. Based on our previous transcriptome data, the transcription of genes involved in visual cycle and circadian rhythm network was examined by qPCR analysis. In the visual cycle network, exposure to all concentrations of DDG significantly decreased transcription of grk7a, aar3a and guca1d, while increased the transcription of opn1mw4 and opn1sw2 at the low concentration. Importantly, exposure to all concentrations of DDG down-regulated the transcription of rep65a that encodes a critical enzyme to catalyze the conversion from all-trans-retinal to 11-cis-retinal in the eye of male zebrafish. In the circadian rhythm network, DDG enhanced the transcription of clocka, arntl2 and nifil3-5 at all three concentrations, while it decreased the transcription of cry5, per1b, nr1d2b and si:ch211.132b12.7. In addition, DDG decreased the transcription of tefa in both males and females. Moreover, histological analysis showed the exposure to 329 ng L^-1 of DDG decreased the thickness of retinal ganglion cell in the eye of male zebrafish. These results indicated that DDG exposure could affect the transcription of genes in visual cycle and circadian rhythm network in the eyes of zebrafish. This suggests that DDG has potential negative impact on the normal eye function.