The all-trans-retinal dimer series of lipofuscin pigments in retinal pigment epithelial cells in a recessive Stargardt disease model

RPE melanin and its influence on the progression of AMD

OBJECTIVE

The aim of this review article is to summarize the latest findings and current understanding of the origin of melanin in the retinal pigment epithelium (RPE), its function within the RPE, its role in the pathogenesis of age-related macular degeneration (AMD), its effect on retinal development, and its potential therapeutic benefit in the treatment of AMD.

METHODS

A comprehensive search of peer-reviewed journals was conducted using various combinations of key terms such as "melanin," "retinal pigment epithelium" or "RPE," "age-related macular degeneration" or AMD," "lipofuscin," "oxidative stress," and "albinism." Databases searched include PubMed, Scopus, Science Direct, and Google Scholar. 147 papers published between the years of 1957 and 2023 were considered with an emphasis on recent findings.

SUMMARY OF FINDINGS

AMD is thought to result from chronic oxidative stress within the RPE that results in cellular dysfunction, metabolic dysregulation, inflammation, and lipofuscin accumulation. Melanin functions as a photoscreener, free radical scavenger, and metal cation binding reservoir within the RPE. RPE melanin does not regenerate, and it undergoes degradation over time in response to chronic light exposure and oxidative stress. RPE melanin is important for retinal development and RPE function, and in the aging eye, melanin loss is associated with increased lipid peroxidation, inflammation, and the accumulation of toxic oxidized cellular products. Therefore, melanin-based treatments may serve to preserve RPE and retinal function in AMD.

CONCLUSIONS

The pathogenesis of AMD is not fully understood, but RPE dysfunction and melanin loss in response to chronic oxidative stress and inflammation are thought to be primary drivers of the disease. Due to melanin's antioxidative effects, melanin-based nanotechnology represents a promising avenue for the treatment of AMD.

Indenopyrene and Blue-Light Co-Exposure Impairs the Tightly Controlled Activation of Xenobiotic Metabolism in Retinal Pigment Epithelial Cells: A Mechanism for Synergistic Toxicity

High energy visible (HEV) blue light is an increasing source of concern for visual health. Polycyclic aromatic hydrocarbons (PAH), a group of compounds found in high concentrations in smokers and polluted environments, accumulate in the retinal pigment epithelium (RPE). HEV absorption by indeno [1,2,3-cd]pyrene (IcdP), a common PAH, synergizes their toxicities and promotes degenerative changes in RPE cells comparable to the ones observed in age-related macular degeneration. In this study, we decipher the processes underlying IcdP and HEV synergic toxicity in human RPE cells. We found that IcdP-HEV toxicity is caused by the loss of the tight coupling between the two metabolic phases ensuring IcdP efficient detoxification. Indeed, IcdP/HEV co-exposure induces an overactivation of key actors in phase I metabolism. IcdP/HEV interaction is also associated with a downregulation of proteins involved in phase II. Our data thus indicate that phase II is hindered in response to co-exposure and that it is insufficient to sustain the enhanced phase I induction. This is reflected by an accelerated production of endogenous reactive oxygen species (ROS) and an increased accumulation of IcdP-related bulky DNA damage. Our work raises the prospect that lifestyle and environmental pollution may be significant modulators of HEV toxicity in the retina.

Short-Term and Long-Term Effects after Exposure to Ionizing Radiation and Visible Light on Retina and Retinal Pigment Epithelium of Mouse Eye

A comparative in vivo study of the effects of ionizing radiation (accelerated protons) and visible light (400-700 nm) on the retina and retinal pigment epithelium (RPE) of the mouse eye was carried out. Using the methods of fluorescence spectroscopy and high-performance liquid chromatography (HPLC), we analyzed the relative composition of retinoids in chloroform extracts obtained from the retinas and RPEs immediately after exposure of animals to various types of radiation and 4.5 months after they were exposed and maintained under standard conditions throughout the period. The fluorescent properties of chloroform extracts were shown to change upon exposure to various types of radiation. This fact indicates the accumulation of retinoid oxidation and degradation products in the retina and RPE. The data from fluorescence and HPLC analyses of retinoids indicate that when exposed to ionizing radiation, retinoid oxidation processes similar to photooxidation occur. Both ionizing radiation and high-intensity visible light have been shown to be characterized by long-term effects. The action of any type of radiation is assumed to activate the mechanism of enhanced reactive oxygen species production, resulting in a long-term damaging effect.

Retina and RPE lipid profile changes linked with ABCA4 associated Stargardt's maculopathy

Stargardt maculopathy, caused predominantly by mutations in the ABCA4 gene, is characterized by an accumulation of non-degradable visual pigment derivative, lipofuscin, in the retinal pigment epithelium (RPE) - resulting in RPE atrophy. RPE is a monolayer tissue located adjacent to retinal photoreceptors and regulates their health and functioning; RPE atrophy triggers photoreceptor cell death and vision loss in Stargardt patients. Previously, ABCA4 mutations in photoreceptors were thought to be the major contributor to lipid homeostasis defects in the eye. Recently, we demonstrated that ABCA4 loss of function in the RPE leads to cell-autonomous lipid homeostasis defects. Our work underscores that an incomplete understanding of lipid metabolism and lipid-mediated signaling in the retina and RPE are potential causes for lacking treatments for this disease. Here we report altered lipidomic in mouse and human Stargardt models. This work provides the basis for therapeutics that aim to restore lipid homeostasis in the retina and the RPE.

Primary versus Secondary Elevations in Fundus Autofluorescence

The method of quantitative fundus autofluorescence (qAF) can be used to assess the levels of bisretinoids in retinal pigment epithelium (RPE) cells so as to aid the interpretation and management of a variety of retinal conditions. In this review, we focused on seven retinal diseases to highlight the possible pathways to increased fundus autofluorescence. ABCA4- and RDH12-associated diseases benefit from known mechanisms whereby gene malfunctioning leads to elevated bisretinoid levels in RPE cells. On the other hand, peripherin2/RDS-associated disease (PRPH2/RDS), retinitis pigmentosa (RP), central serous chorioretinopathy (CSC), acute zonal occult outer retinopathy (AZOOR), and ceramide kinase like (CERKL)-associated retinal degeneration all express abnormally high fundus autofluorescence levels without a demonstrated pathophysiological pathway for bisretinoid elevation. We suggest that, while a known link from gene mutation to increased production of bisretinoids (as in ABCA4- and RDH12-associated diseases) causes primary elevation in fundus autofluorescence, a secondary autofluorescence elevation also exists, where an impairment and degeneration of photoreceptor cells by various causes leads to an increase in bisretinoid levels in RPE cells.

A High Fat Diet Fosters Elevated Bisretinoids

High dietary fat intake is associated with metabolic dysregulation, but little is known regarding the effects of a high fat diet (HFD) on photoreceptor cell functioning. We explored the intersection of a high fat diet (HFD) and the visual cycle adducts that form in photoreceptor cells by non-enzymatic reactions. In black C57BL/6J mice and albino C57BL/6J^c2j mice raised on a high fat diet until age 3, 6 or 12 months, chromatographically quantified bisretinoids were increased relative to mice on a standard diet. In vivo measurement of fundus autofluorescence, the source of which is bisretinoid, also revealed a significant increase in the HFD-mice. Additionally, mice provided with a diet high in fat presented with elevated retinol-binding protein 4 (RBP4) the protein responsible for transporting retinol in plasma. Vitamin A was elevated in plasma although not in ocular tissue. Bisretinoids form in photoreceptor cell outer segments by random reactions of retinaldehyde with phosphatidylethanolamine. We found that the latter phospholipid was significantly increased in mice fed a HFD versus mice on a control diet. In leptin-deficient ob/ob mice, a genetic model of obesity, plasma levels of Rbp4 protein were higher but bisretinoids in retina were not elevated. Photoreceptor cell viability measured as outer nuclear layer thickness was reduced in the ob/ob mice relative to wild-type. The accelerated formation of bisretinoid we observed in diet induced obese mice is related to the high fat intake and to increased delivery of vitamin A to the visual cycle.

Rapamycin Inhibits Light-Induced Necrosome Activation Occurring in Wild-Type, but not RPE65-Null, Mouse Retina

Purpose

Both photodamage and aberrant visual cycle contribute to disease progress of many retinal degenerative disorders, whereas the signaling pathways causing photoreceptor death remain unclear. Here we investigated the effects of intense photo-stress on (1) necrosome activation in wild-type and RPE65-null mice, (2) interaction of p62/Sequestosome-1 with the necrosome proteins, and (3) the effects of rapamycin on photodamage-induced necrosome activation and retinal degeneration in wild-type mice.

Methods

Dark-adapted rd12 mice and 129S2/Sv mice with or without rapamycin treatment were exposed to 15,000 lux light for different times. Expression levels and subcellular localization of proteins were determined through immunoblot and immunohistochemical analyses. Cone sheaths were stained with peanut agglutinin. Correlation between photoreceptor degeneration and receptor-interacting protein kinase-1 (RIPK1) expression was assessed with Spearman's correlation analysis. Protein-protein interaction was analyzed by immunoprecipitation.

Results

Intense light caused rod and cone degeneration accompanied by a significant increase in RIPK1-RIPK3 expressions, mixed lineage kinase domain-like protein phosphorylation, damage-associated molecular patterns protein release, and inflammatory responses in wild-type mouse retina. The same intense light did not induce the necrosome activation in rd12 retina, but it did in rd12 mice that received 9-cis-retinal supply. RIPK1 expression levels are positively correlated with the degrees of rod and cone degeneration. Photodamage upregulated expression and interaction of the p62 autophagosome cargo protein with the necrosome proteins, whereas rapamycin treatment attenuated the light-induced necrosome activation and photoreceptor degeneration.

Conclusions

Necrosome activation contributed to photodamage-induced rod and cone degeneration. The visual cycle and autophagy are the important therapeutic targets to alleviate light-induced retinal necroptosis.

Lipofuscin-Mediated Photic Stress Induces a Dark Toxic Effect on ARPE-19 Cells

Lipofuscin granules from retinal pigment epithelium (RPE) cells contain bisretinoid fluorophores, which are photosensitizers and are phototoxic to cells. In the presence of oxygen, bisretinoids are oxidized to form various products, containing aldehydes and ketones, which are also potentially cytotoxic. In a prior study, we identified that bisretinoid oxidation and degradation products have both hydrophilic and amphiphilic properties, allowing their diffusion through the lipofuscin granule membrane into the RPE cell cytoplasm, and are thiobarbituric acid (TBA)-active. The purpose of the present study was to determine if these products exhibit a toxic effect to the RPE cell also in the absence of light. The experiments were performed using the lipofuscin-fed ARPE-19 cell culture. The RPE cell viability analysis was performed with the use of flow cytofluorimetry and laser scanning confocal microscopy. The results obtained indicated that the cell viability of the lipofuscin-fed ARPE-19 sample was clearly reduced not immediately after visible light irradiation for 18 h, but after 4 days maintaining in the dark. Consequently, we could conclude that bisretinoid oxidation products have a damaging effect on the RPE cell in the dark and can be considered as an aggravating factor in age-related macular degeneration progression.

A five-year follow-up of ABCA4 carriers showing deterioration of retinal function and increased structural changes

PURPOSE

To investigate whether the reduced retinal function and morphological retinal changes previously demonstrated in ABCA4 carriers had remained stationary or had deteriorated over time at 5-year follow-up to further explore if carriers of an autosomal recessive trait also express a weak phenotype, although this is not expected for an autosomal recessive disorder.

METHODS

Thirteen ABCA4 carriers from a previous study that included parents to patients with well known genetically verified ABCA4-associated retinal degenerations were reexamined 5 years after the initial examination. As novel genes and new variants in already established genes are continuously reported, all subjects underwent renewed genetic testing with a next-generation sequencing (NGS) panel that included 288 genes associated with retinal dystrophies and an analysis of deep intronic mutations and copy number variations in the ABCA4 gene. Moreover, to evaluate any changes in retinal function and/or structure over time, clinical reassessment with Goldmann perimetry, visual acuity testing, fundus photography, fundus autofluorescence (FAF) imaging, optical coherence tomography (OCT), full-field electroretinography (ffERG), and multifocal ERG (mfERG) were performed 5 years after the initial investigation. The values of the ffERG parameters were compared between the two time points (the measurements obtained in the initial study versus the measurements at 5-year follow-up) and with the controls. The mfERG results of the carriers were compared with those of the controls.

RESULTS

The renewed genetic testing confirmed the previously established ABCA4 mutations but also revealed the hypomorph ABCA4 variant c.5603A>T in five ABCA4 carriers. In three of them, the variant was found to be associated with known disease-causing alleles that always carry the c.5603A>T in cis. According to recent publications, the subjects could still be considered ABCA4 carriers because both variants are on the same allele. In the remaining two subjects, c.5603A>T could be in trans with the previously known ABCA4 variant, and the subjects were therefore excluded from the study since they could no longer be considered as carriers only. Statistical comparison of ffERG parameters showed significant reduction of the isolated rod, -as well as the combined rod-cone amplitudes over the five years of follow-up, but not compared with the controls. Concerning macular function, mfERG amplitudes were reduced for all rings in the carriers compared with the controls. Fundus photographs demonstrated morphological changes in 64% of the carriers, and 36% of them had further changes at follow-up. FAF images showed alterations in 55% of the carriers, with increased changes in 36% of them. Abnormalities on OCT were observed in 82% of the carriers, of whom 9% had newly found abnormalities at follow-up.

CONCLUSIONS

At 5-year follow-up, the ABCA4 carriers, who previously demonstrated reduced macular function, presented with deterioration of general retinal function, including reduced isolated rod and mixed rod-cone ffERG responses combined with a slight increase in morphological changes in some subjects. This indicates that carriership of at least some ABCA4 variants may cause a condition similar to a subgroup of dry age-related macular degeneration (AMD). In the long run, this might be of importance concerning the possibilities to also treat this subgroup of AMD patients with future gene-based and pharmacological drugs targeting ABCA4-associated disorders.

Photobiology of lipofuscin granules in the retinal pigment epithelium cells of the eye: norm, pathology, age

Lipofuscin granules (LGs) are accumulated in the retinal pigment epithelium (RPE) cells. The progressive LG accumulation can somehow lead to pathology and accelerate the aging process. The review examines composition, spectral properties and photoactivity of LGs isolated from the human cadaver eyes. By use of atomic force microscopy and near-field microscopy, we have revealed the fluorescent heterogeneity of LGs. We have discovered the generation of reactive oxygen species by LGs, and found that LGs and melanolipofuscin granules are capable of photoinduced oxidation of lipids. It was shown that A2E, as the main fluorophore (bisretinoid) of LGs, is much less active as an oxidation photosensitizer than other fluorophores (bisretinoids) of LGs. Photooxidized products of bisretinoids pose a much greater danger to the cell than non-oxidized one. Our studies of the fluorescent properties of LGs and their fluorophores (bisretinoids) showed for the first time that their spectral characteristics change (shift to the short-wavelength region) in pathology and after exposure to ionizing radiation. By recording the fluorescence spectra and fluorescence decay kinetics of oxidized products of LG fluorophores, it is possible to improve the methods of early diagnosis of degenerative diseases. Lipofuscin ("aging pigment") is not an inert "slag". The photoactivity of LGs can pose a significant danger to the RPE cells. Fluorescence characteristics of LGs are a tool to detect early stages of degeneration in the retina and RPE.

Long-term vitamin A supplementation in a preclinical mouse model for RhoD190N-associated retinitis pigmentosa

Retinitis pigmentosa (RP) is caused by one of many possible gene mutations. The National Institutes of Health recommends high daily doses of vitamin A palmitate for RP patients. There is a critical knowledge gap surrounding the therapeutic applicability of vitamin A to patients with the different subtypes of the disease. Here, we present a case report of a patient with RP caused by a p.D190N mutation in Rhodopsin (RHO) associated with abnormally high quantitative autofluorescence values after long-term vitamin A supplementation. We investigated the effects of vitamin A treatment strategy on RP caused by the p.D190N mutation in RHO by exposing Rhodopsin p.D190N (RhoD190N/+) and wild-type (WT) mice to experimental vitamin A-supplemented and standard control diets. The patient's case suggests that the vitamin A treatment strategy should be further studied to determine its effect on RP caused by p.D190N mutation in RHO and other mutations. Our mouse experiments revealed that RhoD190N/+ mice on the vitamin A diet exhibited higher levels of autofluorescence and lipofuscin metabolites compared to WT mice on the same diet and isogenic controls on the standard control diet. Vitamin A supplementation diminished photoreceptor function in RhoD190N/+ mice while preserving cone response in WT mice. Our findings highlight the importance of more investigations into the efficacy of clinical treatments like vitamin A for patients with certain genetic subtypes of disease and of genotyping in the precision care of inherited retinal degenerations.

Intrinsic differences in rod and cone membrane composition: implications for cone degeneration

PURPOSE

In many retinal pathological conditions, rod and cone degeneration differs. For example, the early-onset maculopathy Stargardts disease type 1 (STGD1) is typified by loss of cones while rods are often less affected. We wanted to examine whether there exist intrinsic membrane differences between rods and cones that might explain such features.

METHODS

Abca4 mRNA and protein levels were quantified in rod- and cone-enriched samples from wild-type and Nrl^-/- mice retinas; rod- and cone-enriched outer segments (ROS and COS respectively) were prepared from pig retinas, and total lipids were analyzed by flame ionization, chromatography, and tandem mass spectrometry. Immunohistochemical staining of cone-rich rodent Arvicanthis ansorgei retinas was conducted, and ultra-high performance liquid chromatography of lipid species in porcine ROS and COS was performed.

RESULTS

Abca4 mRNA and Abca4 protein content was significantly higher (50-300%) in cone compared to rod-enriched samples. ROS and COS displayed dramatic differences in several lipids, including very long chain poly-unsaturated fatty acids (VLC-PUFAs), especially docosahexaenoic acid (DHA, 22:6n-3): ROS 20.6% DHA, COS 3.3% (p < 0.001). VLC-PUFAs (> 50 total carbons) were virtually absent from COS. COS were impoverished (> 6× less) in phosphatidylethanolamine compared to ROS. ELOVL4 ("ELOngation of Very Long chain fatty acids 4") antibody labelled Arvicanthis cones only very weakly compared to rods. Finally, there were large amounts (905 a.u.) of the bisretinoid A2PE in ROS, whereas it was much lower (121 a.u., ~ 7.5-fold less) in COS fractions. In contrast, COS contained fivefold higher amounts of all-trans-retinal dimer (115 a.u. compared to 22 a.u. in rods).

CONCLUSIONS

Compared to rods, cones expressed higher levels of Abca4 mRNA and Abca4 protein, were highly impoverished in PUFA (especially DHA) and phosphatidylethanolamine, and contained significant amounts of all-trans-retinal dimer. Based on these and other data, we propose that in contrast to rods, cones are preferentially vulnerable to stress and may die through direct cellular toxicity in pathologies such as STGD1.

Ionized Radiation-Mediated Retinoid Oxidation in the Retina and Retinal Pigment Epithelium of the Murine Eye

The present study evaluated the effects of proton and gamma-ray ionizing radiation on the mouse eye. The aim of this work was to analyze radiation-mediated retinoid oxidation in the retina and retinal pigment epithelium (RPE). The findings from this analysis can be used to develop a noninvasive method for rapid assessment of the effects of ionizing radiation. Comparative fluorescence and chromatographic analyses of retinoids before and after irradiations were performed. The fluorescent properties of chloroform extracts from irradiated mouse retina and RPE exhibited an increase in fluorescence intensity in the short-wave region of the spectrum (λ < 550 nm). This change is due to increased retinal and RPE retinoid oxidation and degradation products after radiation exposure. Comparative analyses of radiation effects demonstrated that the effect of proton exposure on the retina and RPE was higher than that of gamma-ray exposure. The present study revealed a new approach to assessing the level of radiation exposure in ocular tissues.

Targeting Lipid Metabolism for the Treatment of Age-Related Macular Degeneration: Insights from Preclinical Mouse Models

Age-related macular degeneration (AMD) is a major leading cause of irreversible visual impairment in the world with limited therapeutic interventions. Histological, biochemical, genetic, and epidemiological studies strongly implicate dysregulated lipid metabolism in the retinal pigmented epithelium (RPE) in AMD pathobiology. However, effective therapies targeting lipid metabolism still need to be identified and developed for this blinding disease. To test lipid metabolism-targeting therapies, preclinical AMD mouse models are needed to establish therapeutic efficacy and the role of lipid metabolism in the development of AMD-like pathology. In this review, we provide a comprehensive overview of current AMD mouse models available to researchers that could be used to provide preclinical evidence supporting therapies targeting lipid metabolism for AMD. Based on previous studies of AMD mouse models, we discuss strategies to modulate lipid metabolism as well as examples of studies evaluating lipid-targeting therapeutics to restore lipid processing in the RPE. The use of AMD mouse models may lead to worthy lipid-targeting candidate therapies for clinical trials to prevent the blindness caused by AMD.

Intraocular iron injection induces oxidative stress followed by elements of geographic atrophy and sympathetic ophthalmia

Iron has been implicated in the pathogenesis of age‐related retinal diseases, including age‐related macular degeneration (AMD). Previous work showed that intravitreal (IVT) injection of iron induces acute photoreceptor death, lipid peroxidation, and autofluorescence (AF). Herein, we extend this work, finding surprising chronic features of the model: geographic atrophy and sympathetic ophthalmia. We provide new mechanistic insights derived from focal AF in the photoreceptors, quantification of bisretinoids, and localization of carboxyethyl pyrrole, an oxidized adduct of docosahexaenoic acid associated with AMD. In mice given IVT ferric ammonium citrate (FAC), RPE died in patches that slowly expanded at their borders, like human geographic atrophy. There was green AF in the photoreceptor ellipsoid, a mitochondria‐rich region, 4 h after injection, followed later by gold AF in rod outer segments, RPE and subretinal myeloid cells. The green AF signature is consistent with flavin adenine dinucleotide, while measured increases in the bisretinoid all‐trans‐retinal dimer are consistent with the gold AF. FAC induced formation carboxyethyl pyrrole accumulation first in photoreceptors, then in RPE and myeloid cells. Quantitative PCR on neural retina and RPE indicated antioxidant upregulation and inflammation. Unexpectedly, reminiscent of sympathetic ophthalmia, autofluorescent myeloid cells containing abundant iron infiltrated the saline‐injected fellow eyes only if the contralateral eye had received IVT FAC. These findings provide mechanistic insights into the potential toxicity caused by AMD‐associated retinal iron accumulation. The mouse model will be useful for testing antioxidants, iron chelators, ferroptosis inhibitors, anti‐inflammatory medications, and choroidal neovascularization inhibitors.

Bisretinoids of the Retina: Photo-Oxidation, Iron-Catalyzed Oxidation, and Disease Consequences

The retina and, in particular, retinal pigment epithelial cells are unusual for being encumbered by exposure to visible light, while being oxygen-rich, and also amassing photoreactive molecules. These fluorophores (bisretinoids) are generated as a byproduct of the activity of vitamin A aldehyde-the chromophore necessary for vision. Bisretinoids form in photoreceptor cells due to random reactions of two molecules of vitamin A aldehyde with phosphatidylethanolamine; bisretinoids are subsequently transferred to retinal pigment epithelial (RPE) cells, where they accumulate in the lysosomal compartment with age. Bisretinoids can generate reactive oxygen species by both energy and electron transfer, and they become photo-oxidized and photolyzed in the process. While these fluorescent molecules are accrued by RPE cells of all healthy eyes, they are also implicated in retinal disease.

Phototoxicities Caused by Continuous Light Exposure Were Not Induced in Retinal Ganglion Cells Transduced by an Optogenetic Gene

The death of photoreceptor cells is induced by continuous light exposure. However, it is unclear whether light damage was induced in retinal ganglion cells with photosensitivity by transduction of optogenetic genes. In this study, we evaluated the phototoxicities of continuous light exposure on retinal ganglion cells after transduction of the optogenetic gene mVChR1 using an adeno-associated virus vector. Rats were exposed to continuous light for a week, and visually evoked potentials (VEPs) were recorded. The intensities of continuous light (500, 1000, 3000, and 5000 lx) increased substantially after VEP recordings. After the final recording of VEPs, retinal ganglion cells (RGCs) were retrogradely labeled with a fluorescein tracer, FluoroGold, and the number of retinal ganglion cells was counted under a fluorescent microscope. There was no significant reduction in the amplitudes of VEPs and the number of RGCs after exposure to any light intensity. These results indicated that RGCs were photosensitive after the transduction of optogenetic genes and did not induce any phototoxicity by continuous light exposure.

Polyphenols and Visual Health: Potential Effects on Degenerative Retinal Diseases

Dietary polyphenols are a group of natural compounds that have been proposed to have beneficial effects on human health. They were first known for their antioxidant properties, but several studies over the years have shown that these compounds can exert protective effects against chronic diseases. Nonetheless, the mechanisms underlying these potential benefits are still uncertain and contradictory effects have been reported. In this review, we analyze the potential effects of polyphenol compounds on some visual diseases, with a special focus on retinal degenerative diseases. Current effective therapies for the treatment of such retinal diseases are lacking and new strategies need to be developed. For this reason, there is currently a renewed interest in finding novel ligands (or known ligands with previously unexpected features) that could bind to retinal photoreceptors and modulate their molecular properties. Some polyphenols, especially flavonoids (e.g., quercetin and tannic acid), could attenuate light-induced receptor damage and promote visual health benefits. Recent evidence suggests that certain flavonoids could help stabilize the correctly folded conformation of the visual photoreceptor protein rhodopsin and offset the deleterious effect of retinitis pigmentosa mutations. In this regard, certain polyphenols, like the flavonoids mentioned before, have been shown to improve the stability, expression, regeneration and folding of rhodopsin mutants in experimental in vitro studies. Moreover, these compounds appear to improve the integration of the receptor into the cell membrane while acting against oxidative stress at the same time. We anticipate that polyphenol compounds can be used to target visual photoreceptor proteins, such as rhodopsin, in a way that has only been recently proposed and that these can be used in novel approaches for the treatment of retinal degenerative diseases like retinitis pigmentosa; however, studies in this field are limited and further research is needed in order to properly characterize the effects of these compounds on retinal degenerative diseases through the proposed mechanisms.

Supramolecular organization of rhodopsin in rod photoreceptor cell membranes

Rhodopsin is the light receptor in rod photoreceptor cells that initiates scotopic vision. Studies on the light receptor span well over a century, yet questions about the organization of rhodopsin within the photoreceptor cell membrane still persist and a consensus view on the topic is still elusive. Rhodopsin has been intensely studied for quite some time, and there is a wealth of information to draw from to formulate an organizational picture of the receptor in native membranes. Early experimental evidence in apparent support for a monomeric arrangement of rhodopsin in rod photoreceptor cell membranes is contrasted and reconciled with more recent visual evidence in support of a supramolecular organization of rhodopsin. What is known so far about the determinants of forming a supramolecular structure and possible functional roles for such an organization are also discussed. Many details are still missing on the structural and functional properties of the supramolecular organization of rhodopsin in rod photoreceptor cell membranes. The emerging picture presented here can serve as a springboard towards a more in-depth understanding of the topic.

Bisretinoid phospholipid and vitamin A aldehyde: shining a light

Vitamin A aldehyde covalently bound to opsin protein is embedded in a phospholipid-rich membrane that supports photon absorption and phototransduction in photoreceptor cell outer segments. Following absorption of a photon, the 11-cis-retinal chromophore of visual pigment in photoreceptor cells isomerizes to all-trans-retinal. To maintain photosensitivity 11-cis-retinal must be replaced. At the same time, however, all-trans-retinal has to be handled so as to prevent nonspecific aldehyde activity. Some molecules of retinaldehyde upon release from opsin are efficiently reduced to retinol. Other molecules are released into the lipid phase of the disc membrane where they form a conjugate [N-retinylidene-PE (NRPE)] through a Schiff base linkage with PE. The reversible formation of NRPE serves as a transient sink for retinaldehyde that is intended to return retinaldehyde to the visual cycle. However, if instead of hydrolyzing to PE and retinaldehyde, NRPE reacts with a second molecule of retinaldehyde, a synthetic pathway is initiated that leads to the formation of multiple species of unwanted bisretinoid fluorophores. We report on recently identified members of the bisretinoid family, some of which differ with respect to the acyl chains associated with the glycerol backbone. We discuss processing of the lipid moieties of these fluorophores in lysosomes of retinal pigment epithelial cells, their fluorescence characters, and new findings related to light- and iron-associated oxidation of bisretinoids.

A vicious cycle of bisretinoid formation and oxidation relevant to recessive Stargardt disease

The ability of iron to transfer electrons enables the contribution of this metal to a variety of cellular activities even as the redox properties of iron are also responsible for the generation of hydroxyl radicals (^•OH), the most destructive of the reactive oxygen species. We previously showed that iron can promote the oxidation of bisretinoid by generating highly reactive hydroxyl radical (^•OH). Now we report that preservation of iron regulation in the retina is not sufficient to prevent iron-induced bisretinoid oxidative degradation when blood iron levels are elevated in liver-specific hepcidin knockout mice. We obtained evidence for the perpetuation of Fenton reactions in the presence of the bisretinoid A2E and visible light. On the other hand, iron chelation by deferiprone was not associated with changes in postbleaching recovery of 11-cis-retinal or dark-adapted ERG b-wave amplitudes indicating that the activity of Rpe65, a rate-determining visual cycle protein that carries an iron-binding domain, is not affected. Notably, iron levels were elevated in the neural retina and retinal pigment epithelial (RPE) cells of Abca4^−/− mice. Consistent with higher iron content, ferritin-L immunostaining was elevated in RPE of a patient diagnosed with ABCA4-associated disease and in RPE and photoreceptor cells of Abca4^−/− mice. In neural retina of the mutant mice, reduced Tfrc mRNA was also an indicator of retinal iron overload. Thus iron chelation may defend retina when bisretinoid toxicity is implicated in disease processes.

Protective Effects of Flavonoids in Acute Models of Light-Induced Retinal Degeneration

Degeneration of photoreceptors caused by excessive illumination, inherited mutations, or aging is the principal pathology of blinding diseases. Pharmacological compounds that stabilize the visual receptor rhodopsin and modulate the cellular pathways triggering death of photoreceptors could avert this pathology. Interestingly, flavonoids can modulate the cellular processes, such as oxidative stress, inflammatory responses, and apoptosis, that are activated during retinal degeneration. As we found previously, flavonoids also bind directly to unliganded rod opsin, enhancing its folding, stability, and regeneration. In addition, flavonoids stimulate rhodopsin gene expression. Thus, we evaluated the effect of two main dietary flavonoids, quercetin and myricetin, in ATP-binding cassette subfamily A member 4 ^-/- /retinol dehydrogenase 8 ^-/- and wild-type BALB/c mice susceptible to light-induced photoreceptor degeneration. Using in vivo imaging, such as optical coherence tomography, scanning laser ophthalmoscopy, and histologic assessment of retinal morphology, we found that treatment with these flavonoids prior to light insult remarkably protected retina from deterioration and preserved its function. Using high-performance liquid chromatography-mass spectrometry analysis, we detected these flavonoids in the eye upon their intraperitoneal administration. The molecular events associated with the protective effect of quercetin and myricetin were related to the elevated expression of photoreceptor-specific proteins, rhodopsin and cone opsins, decreased expression of the specific inflammatory markers, and the shift of the equilibrium between cell death regulators BCL2-associated X protein (BAX) and B-cell lymphoma 2 toward an antiapoptotic profile. These results were confirmed in photoreceptor-derived 661W cells treated with either H₂O₂ or all-trans-retinal stressors implicated in the mechanism of retinal degeneration. Altogether, flavonoids could have significant prophylactic value for retinal degenerative diseases. SIGNIFICANCE STATEMENT: Flavonoids commonly present in food exhibit advantageous effects in blinding diseases. They bind to and stabilize unliganded rod opsin, which in excess accelerates degenerative processes in the retina. Additionally, flavonoids enhance the expression of the visual receptors, rod and cone opsins; inhibit the inflammatory reactions; and induce the expression of antiapoptotic markers in the retina, preventing the degeneration in vivo. Thus, flavonoids could have a prophylactic value for retinal degenerative diseases.

Fundus autofluorescence imaging

Fundus autofluorescence (FAF) imaging is an in vivo imaging method that allows for topographic mapping of naturally or pathologically occurring intrinsic fluorophores of the ocular fundus. The dominant sources are fluorophores accumulating as lipofuscin in lysosomal storage bodies in postmitotic retinal pigment epithelium cells as well as other fluorophores that may occur with disease in the outer retina and subretinal space. Photopigments of the photoreceptor outer segments as well as macular pigment and melanin at the fovea and parafovea may act as filters of the excitation light. FAF imaging has been shown to be useful with regard to understanding of pathophysiological mechanisms, diagnostics, phenotype-genotype correlation, identification of prognostic markers for disease progression, and novel outcome parameters to assess efficacy of interventional strategies in chorio-retinal diseases. More recently, the spectrum of FAF imaging has been expanded with increasing use of green in addition to blue FAF, introduction of spectrally-resolved FAF, near-infrared FAF, quantitative FAF imaging and fluorescence life time imaging (FLIO). This article gives an overview of basic principles, FAF findings in various retinal diseases and an update on recent developments.

Preclinical pharmacology of a lipophenol in a mouse model of light-induced retinopathy

Environmental light has deleterious effects on the outer retina in human retinopathies, such as ABCA4-related Stargardt’s disease and dry age-related macular degeneration. These effects involve carbonyl and oxidative stress, which contribute to retinal cell death and vision loss. Here, we used an albino Abca4^−/− mouse model, the outer retina of which shows susceptibility to acute photodamage, to test the protective efficacy of a new polyunsaturated fatty acid lipophenol derivative. Anatomical and functional analyses demonstrated that a single intravenous injection of isopropyl-phloroglucinol-DHA, termed IP-DHA, dose-dependently decreased light-induced photoreceptor degeneration and preserved visual sensitivity. This protective effect persisted for 3 months. IP-DHA did not affect the kinetics of the visual cycle in vivo or the activity of the RPE65 isomerase in vitro. Moreover, IP-DHA administered by oral gavage showed significant protection of photoreceptors against acute light damage. In conclusion, short-term tests in Abca4-deficient mice, following single-dose administration and light exposure, identify IP-DHA as a therapeutic agent for the prevention of retinal degeneration.

Treating retinal damage in both aging and young patients might now be easier, thanks to treatment with a lipophenol, an omega-3 fatty acid linked to an antioxidant. The retina is the part of the eye that senses light, aided by light-sensitive pigments. However, these light-sensitive pigments can be converted by light to toxic byproducts, and in some individuals, these toxic byproducts can accumulate, damaging the retina and leading to vision loss. Philippe Brabet at the Montpellier Institute of Neuroscience in France and co-workers found that lipophenol treatment protected retinal cells from damage in a mouse model of retinal disease, and that a single dose has been effective in preserving vision. These results may help in finding new treatments for retinal diseases such as Stargardt disease and age-related macular degeneration.

The Impact of the Timing of Dosing on the Severity of UNC569-Induced Ultrastructural Changes in the Mouse Retina

Mer proto-oncogene tyrosine kinase (MerTK), expressed in the retinal pigment epithelium (RPE), regulates the phagocytosis of shed photoreceptor outer segments. To investigate the influence of dosing time on MerTK inhibitor UNC569-induced retinal toxicity, UNC569 at 100 mg/kg was orally administered to male mice at 2 different Zeitgeber times (ZT5.5 or ZT22) for 28 days. Electron microscopy was conducted at ZT2 after the final dosing. Additionally, the visual cycle components (11-cis-retinal, all-trans-retinal, all-trans-retinol, and 11-cis-retinol), which play an important role in maintaining retinal homeostasis, were quantified by liquid chromatography/mass spectrometry/mass spectrometry. Under electron microscopic examination, the number of phagosomes and phagolysosomes in the RPE increased in both the ZT5.5 and ZT22 administered groups, while endoplasmic reticulum dilatation in the RPE and chromatin aggregation of photoreceptor nuclei were observed only in the ZT22 administered group. No change was observed in any of the visual cycle components. These results suggest that the timing of the dosing in relation to the physiological MerTK phosphorylation affected the severity of changes in the RPE, leading to the apoptosis of the photoreceptor cells.

Fluorescence characteristics of lipofuscin fluorophores from human retinal pigment epithelium

Lipofuscin granules accumulate in the retinal pigment epithelium (RPE) with age, especially in patients with visual diseases, including progressive age-related macular degeneration (AMD). Bisretinoids and their photooxidation and photodegradation products are major sources of lipofuscin granule fluorescence. The present study focused on examining the fluorescence decay characteristics of bisretinoid photooxidation and photodegradation products to evaluate the connection between fluorescence lifetime and spectral characteristics of target fluorophore groups. The primary objective of the study was to apply experimental spectral analysis results of lipofuscin granule fluorescence properties to interpretation of fluorescence lifetime imaging ophthalmoscopy data. Fluorescence analysis of the lipofuscin granule fluorophores in RPE collected from cadaver eyes was performed. The fluorescence lifetimes were measured by picosecond-resolved time correlated single photon counting technique. A global analytical method was applied to analyze data sets. The photooxidation and photodegradation products of bisretinoids exhibited a longer fluorescence lifetime (average value approximately 6 ns) and a shorter wavelength maximum (530-580 nm). Further, these products significantly contributed (more than 30%), to total fluorescence compared to the other fluorophores in lipofuscin granules. Thus, the contribution of oxidized lipofuscin bisretinoids to autofluorescence decay kinetics is an important characteristic for fluorescence lifetime imaging microscopy data analysis. The higher average fluorescence lifetime in AMD eyes was likely due to the higher abundance of oxidized bisretinoids compared with non-oxidized bisretinoids. Because higher level of oxidized bisretinoids is indicative of pathological processes in the retina and RPE, the present findings have the potential to improve fluorescence lifetime imaging approaches for early diagnosis of degenerative processes in the retina and RPE.

Isopropyl-phloroglucinol-DHA protects outer retinal cells against lethal dose of all-trans-retinal

All‐trans‐retinal (atRAL) is a highly reactive carbonyl specie, known for its reactivity on cellular phosphatidylethanolamine in photoreceptor. It is generated by photoisomerization of 11‐cis‐retinal chromophore linked to opsin by the Schiff's base reaction. In ABCA4‐associated autosomal recessive Stargardt macular dystrophy, atRAL results in carbonyl and oxidative stress, which leads to bisretinoid A2E, accumulation in the retinal pigment epithelium (RPE). This A2E‐accumulation presents as lipofuscin fluorescent pigment, and its photooxidation causes subsequent damage. Here we describe protection against a lethal dose of atRAL in both photoreceptors and RPE in primary cultures by a lipidic polyphenol derivative, an isopropyl‐phloroglucinol linked to DHA, referred to as IP‐DHA. Next, we addressed the cellular and molecular defence mechanisms in commonly used human ARPE‐19 cells. We determined that both polyunsaturated fatty acid and isopropyl substituents bond to phloroglucinol are essential to confer the highest protection. IP‐DHA responds rapidly against the toxicity of atRAL and its protective effect persists. This healthy effect of IP‐DHA applies to the mitochondrial respiration. IP‐DHA also rescues RPE cells subjected to the toxic effects of A2E after blue light exposure. Together, our findings suggest that the beneficial role of IP‐DHA in retinal cells involves both anti‐carbonyl and anti‐oxidative capacities.

Lessons learned from quantitative fundus autofluorescence

Quantitative fundus autofluorescence (qAF) is an approach that is built on a confocal scanning laser platform and used to measure the intensity of the inherent autofluorescence of retina elicited by short-wavelength (488 nm) excitation. Being non-invasive, qAF does not interrupt tissue architecture, thus allowing for structural correlations. The spectral features, cellular origin and topographic distribution of the natural autofluorescence of the fundus indicate that it is emitted from retinaldehyde-adducts that form in photoreceptor cells and accumulate, under most conditions, in retinal pigment epithelial cells. The distributions and intensities of fundus autofluorescence deviate from normal in many retinal disorders and it is widely recognized that these changing patterns can aid in the diagnosis and monitoring of retinal disease. The standardized protocol employed by qAF involves the normalization of fundus grey levels to a fluorescent reference installed in the imaging instrument. Together with corrections for magnification and anterior media absorption, this approach facilitates comparisons with serial images and images acquired within groups of patients. Here we provide a comprehensive summary of the principles and practice of qAF and we highlight recent efforts to elucidate retinal disease processes by combining qAF with multi-modal imaging.

The Y227N mutation affects bestrophin-1 protein stability and impairs sperm function in a mouse model of Best vitelliform macular dystrophy

Human bestrophin-1 (BEST1) is an integral membrane protein known to function as a Ca²⁺-activated and volume-regulated chloride channel. The majority of disease-associated mutations in BEST1 constitute missense mutations and were shown in vitro to lead to a reduction in mutant protein half-life causing Best disease (BD), a rare autosomal dominant macular dystrophy. To further delineate BEST1-associated pathology in vivo and to provide an animal model useful to explore experimental treatment efficacies, we have generated a knock-in mouse line (Best1^Y227N). Heterozygous and homozygous mutants revealed no significant ocular abnormalities up to 2 years of age. In contrast, knock-in animals demonstrated a severe phenotype in the male reproductive tract. In heterozygous Best1^Y227N males, Best1 protein was significantly reduced in testis and almost absent in homozygous mutant mice, although mRNA transcription of wild-type and knock-in allele is present and similar in quantity. Degradation of mutant Best1 protein in testis was associated with adverse effects on sperm motility and the capability to fertilize eggs. Based on these results, we conclude that mice carrying the Best1 Y227N mutation reveal a reproducible pathologic phenotype and thus provide a valuable in vivo tool to evaluate efficacy of drug therapies aimed at restoring Best1 protein stability and function.

Blue light versus green light fundus autofluorescence in normal subjects and in patients with retinochoroidopathy secondary to retinal and uveitic diseases

PURPOSE

The aim of this study is to evaluate the differences in the fundus autofluorescence (FAF) signal between the blue light autofluorescence (BAF) from Spectralis® (Heidelberg, CA) and green light autofluorescence (GAF) 200TxTM (OPTOS, UK, in normal subjects and in patients with retinochoroidopathies (RC).

METHODS

In this prospective study, FAF was performed using BL (λ = 488 nm) and GL (λ = 532 nm) on normal subjects and patients with RC. The corresponding pairs of BAF and GAF images from both groups were analyzed using Photoshop. The strength of the FAF signal was measured on a gray scale, where optic disc was a standard to indicate absence of AF. In addition, gray values obtained from three identical points (foveal center, and points of hypo and hyper autofluorescence) in the corresponding BAF and GAF images of normal and RC subjects were divided by the optic disc value to calculate autofluorescence signal ratio (R). The R values at fovea (R1), hypoautofluorescent point (R2), and hyperautofluorescent point (R3) were compared between BAF and GAF modalities, in normal and in RC subjects separately.

RESULTS

One hundred six pairs (106 eyes) of FAF images analyzed (37 pairs: normal and 69 pairs: RC subjects). In normal subjects, the mean R1, R2, and R3 values for BAF were (1.5 ± 0.88, 1.23 ± 0.58, and 4.73 ± 2.85, respectively) and for GAF were (0.78 ± 0.20, 0.78 ± 0.20, and 1.62 ± 0.39, respectively). Similarly, in subjects with RC, the mean R1, R2, and R3 values for BAF were (1.68 ± 1.02, 1.66 ± 1.15, and 7.75 ± 6.82, respectively) and for GAF were (0.95 ± 0.59, 0.79 ± 0.45, and 2.50 ± 1.65, respectively). The mean difference in the R1, R2, and R3 ratios between BAF and GAF in normal and in RC subjects was statistically significant (p < 0.001). The strength of the correlation (r) between ratios for BAF and GAF was weak or not statistically significant in both normal and RC subjects (p > 0.05).

CONCLUSION

The distribution and intensity of the AF signal differ in BAF and GAF and cannot be used interchangeably. In BAF, optic disc signal is always weaker than in other areas, which was not true for GAF where optic disc signal was stronger than fovea and hypoautofluorescent point in both groups.

Monitoring and Management of the Patient with Stargardt Disease

Stargardt disease (STGD1) represents one of the major common causes of inherited irreversible visual loss. Due to its high phenotypic and genotypic heterogeneity, STGD1 is a complex disease to understand. Non-invasive imaging, biochemical, and genetic advances have led to substantial improvements in unveiling the disease processes and novel promising therapeutic landscapes have been proposed. This review recapitulates the modalities for monitoring patients with STGD1 and the therapeutic options currently under investigation for the different stages of the disease.

Deep Scleral Exposure: A Degenerative Outcome of End-Stage Stargardt Disease

PURPOSE

To describe a distinct phenotypic outcome of outer retinal degeneration in a cohort of genetically confirmed patients with recessive Stargardt disease (STGD1).

DESIGN

Retrospective case series.

METHODS

Twelve patients, who were clinically diagnosed with STGD1 and exhibited a unique degenerative phenotype, were included in the study. Two disease-causing mutations were found in all patients by direct sequencing of the ABCA4 gene. Clinical characterization of patients were defined on fundus photographs, autofluorescence images (488-nm and 532-nm excitation), spectral-domain optical coherence tomography (SD-OCT), and full-field electroretinogram (ffERG) testing.

RESULTS

Mean age at initial presentation was 67.8 years and reported age of symptomatic onset was 14.1 years (mean disease duration = 53.8 years). Best-corrected visual acuity ranged from 20/400 to hand motion. All patients exhibited advanced degeneration across the posterior pole resulting in a reflectively pale, blonde fundus owing to unobstructed exposure of the underlying sclera. SD-OCT revealed complete loss of the outer retinal bands (external limiting membrane, ellipsoid zone, interdigitation zone, and retinal pigment epithelium) and choroidal layers. Scotopic and photopic waveforms on ffERG were nonrecordable or severely attenuated in 8 patients who were tested.

CONCLUSIONS

Widespread scleral exposure is a clinical outcome in a subset of STGD1 following a long duration of disease progression (∼50 years). The blonde fundus in such cases may exhibit phenotypic overlap and shared therapeutic implications with other aggressive chorioretinal dystrophies such as end-stage choroideremia, gyrate atrophy, or RPE65-Leber congenital amaurosis.

Bisretinoids mediate light sensitivity resulting in photoreceptor cell degeneration in mice lacking the receptor tyrosine kinase Mer

The receptor tyrosine kinase Mer is expressed by retinal pigment epithelial (RPE) cells and participates in photoreceptor outer-segment phagocytosis, a process enabling membrane renewal. Mutations in the gene encoding MERTK cause blinding retinitis pigmentosa in humans. Targeted Mertk disruption in mice causes defective RPE-mediated phagocytosis of the outer segments, leading to deposition of autofluorescent debris at the RPE-photoreceptor cell interface, followed by photoreceptor cell degeneration. Here, we show that retinaldehyde adducts (bisretinoid fluorophores) that form in photoreceptor outer segments occupy the unphagocytosed outer-segment debris that accumulates in Mertk ^-/- mice. Bisretinoids measured by HPLC were elevated in Mertk ^-/- mice compared with WT animals. Bisretinoids were also more abundant in albino Mertk ^-/- mice expressing leucine at position 450 of the isomerase RPE65 (Rpe65-Leu450) rather than the variant methionine (Rpe65-450Met) that yields lower bisretinoid levels. In Royal College of Surgeons rats having dysfunctional Mertk, bisretinoids were higher than in WT rats. Intensities of in vivo fundus autofluorescence were higher in Mertk ^-/- mice than in WT mice and peaked earlier in albino Mertk ^-/-/Rpe65-Leu450 mice than in albino Mertk ^-/-/Rpe65-450Met mice. Of note, the rate of photoreceptor cell degeneration was more rapid in albino Mertk ^-/- mice exposed to higher levels of intraocular light (albino versus pigmented mice) and in mice carrying Rpe65-Leu450 than in Rpe65-450Met mice, revealing a link between bisretinoid accumulation and light-mediated acceleration of photoreceptor cell degeneration. In conclusion, the light sensitivity of photoreceptor cell degeneration arising from Mertk deficiency is consistent with the known phototoxicity of bisretinoids.

Novel bisretinoids of human retina are lyso alkyl ether glycerophosphoethanolamine-bearing A2PE species

Bisretinoids are a family of fluorophores that form in photoreceptor cells' outer segments by nonenzymatic reaction of two vitamin A aldehydes (A2) with phosphatidylethanolamine (PE). Bisretinoid fluorophores are the major constituents of the lipofuscin of retinal pigment epithelium (RPE) that accumulate with age and contribute to some retinal diseases. Here, we report the identification of a previously unknown fluorescent bisretinoid. By ultra-performance LC (UPLC) coupled to photodiode array detection, fluorescence (FLR), and ESI-MS, we determined that this novel bisretinoid is 1-octadecyl-2-lyso-sn-glycero A2PE (alkyl ether lysoA2PE). This structural assignment was based on molecular mass (m/z 998), UV-visible absorbance maxima (340 and 440 nm), and retention time (73 min) and was corroborated by biomimetic synthesis using all-trans-retinal and glycerophosphoethanolamine analogs as starting materials. UPLC profiles of ocular extracts acquired from human donor eyes revealed that alkyl ether lysoA2PE was detectable in RPE, but not neural retina. LysoA2PE FLR spectra exhibited a significant hyperchromic shift in hydrophobic environments. The propensity for lysoA2PE to undergo photooxidation/degradation was less pronounced than A2E. In mechanistic studies, A2PE was hydrolyzed by phospholipase A₂ and plasmalogen lysoA2PE was cleaved under acidic conditions. The characterization of these additional members of the bisretinoid family advances our understanding of the mechanisms underlying bisretinoid biogenesis.

Spectral analysis of fundus autofluorescence pattern as a tool to detect early stages of degeneration in the retina and retinal pigment epithelium

PURPOSE

The aim of this work is the determination of quantitative diagnostic criteria based on the spectral characteristics of fundus autofluorescence to detect early stages of degeneration in the retina and retinal pigment epithelium (RPE).

METHODS

RPE cell suspension samples were obtained from the cadaver eyes with and without signs of age-related macular degeneration (AMD). Fluorescence analysis at an excitation wavelength of 488 nm was performed. The fluorescence lifetimes of lipofuscin-granule fluorophores were measured by counting time-correlated photon method.

RESULTS

Comparative analysis of fluorescence spectra of RPE cell suspensions from the cadaver eyes with and without signs of AMD showed a significant difference in fluorescence intensity at 530-580 nm in response to fluorescence excitation at 488 nm. It was notably higher in eyes with visual pathology than in normal eyes regardless of the age of the eye donor. Measurements of fluorescence lifetimes of lipofuscin fluorophores showed that the contribution of photooxidation and photodegradation products of bisretinoids to the total fluorescence at 530-580 nm of RPE cell suspensions was greater in eyes with visual pathology than in normal eyes.

CONCLUSION

Because photooxidation and photodegradation products of bisretinoids are markers of photodestructive processes, which can cause RPE cell death and initiate degenerative processes in the retina, quantitative determination of increases in these bisretinoid products in lipofuscin granules may be used to establish quantitative diagnostic criteria for degenerative processes in the retina and RPE.

Bisretinoid Photodegradation Is Likely Not a Good Thing

Retinaldehyde adducts (bisretinoids) accumulate in retinal pigment epithelial (RPE) cells as lipofuscin. Bisretinoids are implicated in some inherited and age-related forms of macular degeneration that lead to the death of RPE cells and diminished vision. By comparing albino and black-eyed mice and by rearing mice in darkness and in cyclic light, evidence indicates that bisretinoid fluorophores undergo photodegradation in the eye (Ueda et al. Proc Natl Acad Sci 113:6904-6909, 2016). Given that the photodegradation products modify and impair cellular and extracellular molecules, these processes likely impart cumulative damage to retina.

Iron promotes oxidative cell death caused by bisretinoids of retina

Cells are subject to metabolic sources of oxidizing species and to the need to regulate Fe, a redox-active metal. Retinal pigment epithelial (RPE) cells have to contend with an additional, unique source of oxidative stress: photooxidative insult from bisretinoids that accumulate as lipofuscin. Here we report that Fe can interact with bisretinoids in RPE to promote cell damage. These findings inform disease processes in both Fe-related and bisretinoid-associated retinal degeneration. The link between Fe and bisretinoid oxidation also highlights opportunities for repurposed and combination therapies. This could include visual cycle inhibitors as a treatment for maculopathy associated with elevated retinal Fe, and Fe chelation to aid in suppressing the damaging effects of bisretinoids in juvenile and age-related macular degeneration.

Intracellular Fe plays a key role in redox active energy and electron transfer. We sought to understand how Fe levels impact the retina, given that retinal pigment epithelial (RPE) cells are also challenged by accumulations of vitamin A aldehyde adducts (bisretinoid lipofuscin) that photogenerate reactive oxygen species and photodecompose into damaging aldehyde- and dicarbonyl-bearing species. In mice treated with the Fe chelator deferiprone (DFP), intracellular Fe levels, as reflected in transferrin receptor mRNA expression, were reduced. DFP-treated albino Abca4^−/− and agouti wild-type mice exhibited elevated bisretinoid levels as measured by high-performance liquid chromatography or noninvasively by quantitative fundus autofluorescence. Thinning of the outer nuclear layer, a parameter indicative of the loss of photoreceptor cell viability, was also reduced in DFP-treated albino Abca4^−/−. In contrast to the effects of the Fe chelator, mice burdened with increased intracellular Fe in RPE due to deficiency in the Fe export proteins hephaestin and ceruloplasmin, presented with reduced bisretinoid levels. These findings indicate that intracellular Fe promotes bisretinoid oxidation and degradation. This interpretation was supported by experiments showing that DFP decreased the oxidative/degradation of the bisretinoid A2E in the presence of light and reduced cell death in cell-based experiments. Moreover, light-independent oxidation and degradation of A2E by Fenton chemistry products were evidenced by the consumption of A2E, release of dicarbonyls, and generation of oxidized A2E species in cell-free assays.

Full-field ERG as a predictor of the natural course of ABCA4-associated retinal degenerations

PURPOSE

To assess retinal function in combination with the retinal structure in ABCA4-associated retinal degenerations. Moreover, to evaluate the possibility of predicting the natural course of these disorders.

METHODS

34 patients with Stargardt disease or cone rod dystrophy carrying confirmed mutations in ABCA4 were selected from our retinitis pigmentosa (RP) register. Sequence analysis of the entire coding region of the ABCA4 gene was performed. The patients were subdivided into three groups based on their most recent visual fields. Group 1 included ten patients with central scotomas within 10°, group 2 included 19 patients with larger central scotomas of 10-35°, and group 3 included five patients with mere temporal residues. The patients underwent slit-lamp and fundus examinations, visual acuity testing, optical coherence tomography (OCT), fundus photography (color, red-free, and autofluorescence (AF) images), full-field electroretinography (ffERG), and multifocal electroretinography (mERG). FfERG and mERG results were analyzed statistically. Total rod and cone function, as well as macular function, was compared between the three groups and of each group to a normal material. In 23 patients who had undergone ffERG on a previous occasion, the 30 Hz flicker implicit time (IT) from the first visit was also analyzed.

RESULTS

The ffERG statistics revealed significant differences between the groups regarding cone and rod function with group 1 showing the highest amplitudes and the shortest ITs while group 3 demonstrated the lowest amplitudes and the most delayed ITs. When compared to controls, group 1 did not show any significant changes while groups 2 and 3 demonstrated reduced amplitudes and delayed 30 Hz ITs. Regarding estimation of the natural course, identical results of the 30 Hz IT were encountered for the groups also at the first visit early in the course of disease. Comparison of the mERGs showed significant differences with group 1 demonstrating the highest amplitudes and group 3 the lowest for all rings but rings 2 and 3 in the right eye for which the amplitudes were the second highest. The mERGs for each group were also compared to controls showing reduced mERG amplitudes for all rings in all groups, except group 1, left eye. OCT showed macular attenuation in all patients. Evaluation of the inner and outer photoreceptor junction (IS/OS) morphology revealed alterations related to macular function measured with mERG in all eyes. Eight patients in group 1 showed foveal IS/OS junction loss, one had foveal IS/OS junction disorganization, and one had IS/OS loss also beyond the fovea. In group 2, one patient had IS/OS junction loss confined to the fovea, and the rest showed total loss of IS/OS junctions. Group 3 was devoid of IS/OS junctions. Concerning the AF images, group 1 showed small areas of absent AF in the macula, peripapillary sparing, and flecks of increased and reduced AF in the posterior pole. In group 2, the central areas of absent AF were larger. Flecks of reduced AF were the most dominant and reached beyond the posterior pole. Seven of 19 patients had peripapillary sparing. In group 3, large confluent areas of reduced AF were found in the posterior pole and beyond with small areas of increased AF in the far periphery. No peripapillary sparing was seen.

CONCLUSIONS

The current study demonstrates a significant difference in total retinal function, as well as macular function, between patients with ABCA4-associated retinal degeneration and a different degree of visual field defects with gradual deterioration of function along with increased visual field constriction. Likewise, the morphological changes, including the deviant AF pattern and loss of IS/OS junctions, that were related to macular function measured with mERG worsened with the degree of visual field defects. Moreover, in these groups of patients with ABCA4-associated retinal degenerations, full-field cone 30 Hz flicker IT seems to be a predictor of the natural course of the disease also on long-term follow-up.

Acid-Induced Intracellular Dissociation of β-Cyclodextrin-Threaded Polyrotaxanes Directed toward Attenuating Phototoxicity of Bisretinoids through Promoting Excretion

In the retinal pigment epithelium of patients with age-related macular degeneration (AMD), excess N-retinylidene-N-retinylethanolamine (A2E), a dimer of all-trans-retinal, accumulats to induce inflammatory cytokine secretion and phototoxic effects. Therefore, the reduction of intracellular A2E is a promising approach for the prevention and treatment of AMD. In this study, acid-labile β-cyclodextrin (β-CD)-threaded polyrotaxanes (PRXs) were synthesized and investigated their effects on the removal of A2E accumulated in retinal pigment epithelium cells (ARPE-19) in comparison to nonlabile PRXs and 2-hydroxypropyl β-CD (HP-β-CD) were examined. GC-MS and HPLC studies strongly suggest that the acid-labile PRXs dissociated into their constituent molecules in cells by lysosomal acidification and threaded β-CDs were considered to be released from the PRXs. The released β-CDs formed an inclusion complex with A2E, which promoted the excretion of A2E. Indeed, the acid-labile PRXs effectively reduced intracellular A2E level at approximately a 10-fold lower concentration than HP-β-CD. Accompanied with A2E removal, the toxicity and phototoxicity of A2E were attenuated by treatment with acid-labile PRXs. Because the nonlabile PRX failed to reduce intracellular A2E level and attenuate phototoxicity, intracellular release of threaded β-CDs from the acid-labile PRX might contribute to reducing intracellular A2E. We conclude that acid-labile PRXs are promising candidates for the treatment of macular diseases through the removal of toxic metabolites.

Lipofuscin Formation Catalyzed by the Milk Protein β-Lactoglobulin: Lysine Residues in Cycloretinal Synthesis

Lipofuscins are toxic autofluorescent byproducts of the visual cycle. The accumulation of lipofuscins such as cycloretinal in the retina is thought to play a role in the progression of age-related macular degeneration (AMD). Intriguingly, the milk protein β-lactoglobulin (BLG) can promote the cyclodimerization of all-trans-retinal to cycloretinal both in vitro and in vivo. Here, site-directed mutagenesis of BLG and mass spectrometric analysis with substrate analogues demonstrate that lysine residues play a key role in catalysis. It is also shown that catalytic activity necessitates the presence of a physical binding site and cannot be mediated by a peptide chain. These studies provide insight into the mechanism of the cyclodimerization process and provide a model system for biocatalysis and biosynthesis of cycloretinal in vivo. In the long term, these studies may pave the way for drug development and inhibitor design as an early treatment regimen for AMD.

All-trans-retinal dimer formation alleviates the cytotoxicity of all-trans-retinal in human retinal pigment epithelial cells

Effective clearance of all-trans-retinal (atRAL) from retinal pigment epithelial (RPE) cells is important for avoiding its cytotoxicity. However, the metabolism of atRAL in RPE cells is poorly clarified. The present study was designed to analyze metabolic products of atRAL and to compare the cytotoxicity of atRAL versus its derivative all-trans-retinal dimer (atRAL-dimer) in human RPE cells. We found that all-trans-retinol (atROL) and a mixture of atRAL condensation metabolites including atRAL-dimer and A2E were generated after incubating RPE cells with atRAL for 6h, and the amount of atRAL-dimer was significantly higher than that of A2E. In the eyes of Rdh8^-/- Abca4^-/- mice, a mouse model with defects in retinoid cycle that displays some symbolic characteristics of age-related macular degeneration (AMD), the level of atRAL-dimer was increased compared to wild-type mice, and was even much greater than that of A2E & isomers. The cytotoxicity of atRAL-dimer was reduced compared with its precursor atRAL. The latter could provoke intracellular reactive oxygen species (ROS) overproduction, increase the mRNA expression of several oxidative stress related genes (Nrf2, HO-1, and γ-GCSh), and induce ΔΨm loss in RPE cells. By contrast, the abilities of atRAL-dimer to induce intracellular ROS and oxidative stress were much weaker versus that of concentration-matched atRAL, and atRAL-dimer exhibited no toxic effect on mitochondrial function at higher concentrations. In conclusion, the formation of atRAL-dimer during atRAL metabolic process ameliorates the cytotoxicity of atRAL by reducing oxidative stress.

Photo and Collision Induced Isomerization of a Cyclic Retinal Derivative: An Ion Mobility Study

A cationic degradation product, formed in solution from retinal Schiff base (RSB), is examined in the gas phase using ion mobility spectrometry, photoisomerization action spectroscopy, and collision induced dissociation (CID). The degradation product is found to be N-n-butyl-2-(β-ionylidene)-4-methylpyridinium (BIP) produced through 6π electrocyclization of RSB followed by protonation and loss of dihydrogen. Ion mobility measurements show that BIP exists as trans and cis isomers that can be interconverted through buffer gas collisions and by exposure to light, with a maximum response at λ = 420 nm.Graphical Abstract.

Retinoids and Retinal Diseases

Recent progress in molecular understanding of the retinoid cycle in mammalian retina stems from painstaking biochemical reconstitution studies supported by natural or engineered animal models with known genetic lesions and studies of humans with specific genetic blinding diseases. Structural and membrane biology have been used to detect critical retinal enzymes and proteins and their substrates and ligands, placing them in a cellular context. These studies have been supplemented by analytical chemistry methods that have identified small molecules by their spectral characteristics, often in conjunction with the evaluation of models of animal retinal disease. It is from this background that rational therapeutic interventions to correct genetic defects or environmental insults are identified. Thus, most presently accepted modulators of the retinoid cycle already have demonstrated promising results in animal models of retinal degeneration. These encouraging signs indicate that some human blinding diseases can be alleviated by pharmacological interventions.

Photodegradation of retinal bisretinoids in mouse models and implications for macular degeneration

Adducts of retinaldehyde (bisretinoids) form nonenzymatically in photoreceptor cells and accumulate in retinal pigment epithelial (RPE) cells as lipofuscin; these fluorophores are implicated in the pathogenesis of inherited and age-related macular degeneration (AMD). Here we demonstrate that bisretinoid photodegradation is ongoing in the eye. High-performance liquid chromatography (HPLC) analysis of eyes of dark-reared and cyclic light-reared wild-type mice, together with comparisons of pigmented versus albino mice, revealed a relationship between intraocular light and reduced levels of the bisretinoids A2E and A2-glycero-phosphoethanolamine (A2-GPE). Analysis of the bisretinoids A2E, A2-GPE, A2-dihydropyridine-phosphatidylethanolamine (A2-DHP-PE), and all-trans-retinal dimer-phosphatidylethanolamine (all-trans-retinal dimer-PE) also decreases in albino Abca4(-/-) mice reared in cyclic light compared with darkness. In albino Abca4(-/-) mice receiving a diet supplemented with the antioxidant vitamin E, higher levels of RPE bisretinoid were evidenced by HPLC analysis and quantitation of fundus autofluorescence; this effect is consistent with photooxidative processes known to precede bisretinoid degradation. Amelioration of outer nuclear layer thinning indicated that vitamin E treatment protected photoreceptor cells. Conversely, in-cage exposure to short-wavelength light resulted in reduced fundus autofluorescence, decreased HPLC-quantified A2E, outer nuclear layer thinning, and increased methylglyoxal (MG)-adducted protein. MG was also released upon bisretinoid photodegradation in cells. We suggest that the lower levels of these diretinal adducts in cyclic light-reared and albino mice reflect photodegradative loss of bisretinoid. These mechanisms may underlie associations among AMD risk, oxidative mechanisms, and lifetime light exposure.

Spatial and Spectral Characterization of Human Retinal Pigment Epithelium Fluorophore Families by Ex Vivo Hyperspectral Autofluorescence Imaging

Purpose

Discovery of candidate spectra for abundant fluorophore families in human retinal pigment epithelium (RPE) by ex vivo hyperspectral imaging.

Methods

Hyperspectral autofluorescence emission images were captured between 420 and 720 nm (10-nm intervals), at two excitation bands (436–460, 480–510 nm), from three locations (fovea, perifovea, near-periphery) in 20 normal RPE/Bruch's membrane (BrM) flatmounts. Mathematical factorization extracted a BrM spectrum (S0) and abundant lipofuscin/melanolipofuscin (LF/ML) spectra of RPE origin (S1, S2, S3) from each tissue.

Results

Smooth spectra S1 to S3, with perinuclear localization consistent with LF/ML at all three retinal locations and both excitations in 14 eyes (84 datasets), were included in the analysis. The mean peak emissions of S0, S1, and S2 at λ_ex 436 nm were, respectively, 495 ± 14, 535 ± 17, and 576 ± 20 nm. S3 was generally trimodal, with peaks at either 580, 620, or 650 nm (peak mode, 650 nm). At λ_ex 480 nm, S0, S1, and S2 were red-shifted to 526 ± 9, 553 ± 10, and 588 ± 23 nm, and S3 was again trimodal (peak mode, 620 nm). S1 often split into two spectra, S1A and S1B. S3 strongly colocalized with melanin. There were no significant differences across age, sex, or retinal location.

Conclusions

There appear to be at least three families of abundant RPE fluorophores that are ubiquitous across age, retinal location, and sex in this sample of healthy eyes. Further molecular characterization by imaging mass spectrometry and localization via super-resolution microscopy should elucidate normal and abnormal RPE physiology involving fluorophores.

Translational Relevance

Our results help establish hyperspectral autofluorescence imaging of the human retinal pigment epithelium as a useful tool for investigating retinal health and disease.

Quantitative Fundus Autofluorescence and Optical Coherence Tomography in ABCA4 Carriers

PURPOSE

To assess whether carriers of ABCA4 mutations have increased RPE lipofuscin levels based on quantitative fundus autofluorescence (qAF) and whether spectral-domain optical coherence tomography (SD-OCT) reveals structural abnormalities in this cohort.

METHODS

Seventy-five individuals who are heterozygous for ABCA4 mutations (mean age, 47.3 years; range, 9-82 years) were recruited as family members of affected patients from 46 unrelated families. For comparison, 57 affected family members with biallelic ABCA4 mutations (mean age, 23.4 years; range, 6-67 years) and two noncarrier siblings were also enrolled. Autofluorescence images (30°, 488-nm excitation) were acquired with a confocal scanning laser ophthalmoscope equipped with an internal fluorescent reference. The gray levels (GLs) of each image were calibrated to the reference, zero GL, magnification, and normative optical media density to yield qAF. Horizontal SD-OCT scans through the fovea were obtained and the thicknesses of the outer retinal layers were measured.

RESULTS

In 60 of 65 carriers of ABCA4 mutations (age range, 9-60), qAF levels were within normal limits (95% confidence level) observed for healthy noncarrier subjects, while qAF levels of affected family members were significantly increased. Perifoveal fleck-like abnormalities were observed in fundus AF images in four carriers, and corresponding changes were detected in the outer retinal layers in SD-OCT scans. Thicknesses of the outer retinal layers were within the normal range.

CONCLUSIONS

With few exceptions, individuals heterozygous for ABCA4 mutations and between the ages of 9 and 60 years do not present with elevated qAF. In a small number of carriers, perifoveal fleck-like changes were visible.