Comparison of A2E cytotoxicity and phototoxicity with all-trans-retinal in human retinal pigment epithelial cells

Label-Free Assessment of Key Biological Autofluorophores: Material Characteristics and Opportunities for Clinical Applications

Autofluorophores are endogenous fluorescent compounds that naturally occur in the intra and extracellular spaces of all tissues and organs. Most have vital biological functions - like the metabolic cofactors NAD(P)H and FAD+, as well as the structural protein collagen. Others are considered to be waste products - like lipofuscin and advanced glycation end products - which accumulate with age and are associated with cellular dysfunction. Due to their natural fluorescence, these materials have great utility for enabling non-invasive, label-free assays with direct ties to biological function. Numerous technologies, with different advantages and drawbacks, are applied to their assessment, including fluorescence lifetime imaging microscopy, hyperspectral microscopy, and flow cytometry. Here, the applications of label-free autofluorophore assessment are reviewed for clinical and health-research applications, with specific attention to biomaterials, disease detection, surgical guidance, treatment monitoring, and tissue assessment - fields that greatly benefit from non-invasive methodologies capable of continuous, in vivo characterization.

Autophagy in dry AMD: A promising therapeutic strategy for retinal pigment epithelial cell damage

Dry age-related macular degeneration (AMD) is a prevalent clinical condition that leads to permanent damage to central vision and poses a significant threat to patients' visual health. Although the pathogenesis of dry AMD remains unclear, there is consensus on the role of retinal pigment epithelium (RPE) damage. Oxidative stress and chronic inflammation are major contributors to RPE cell damage, and the NOD-like receptor thermoprotein structural domain-associated protein 3 (NLRP3) inflammasome mediates the inflammatory response leading to apoptosis in RPE cells. Furthermore, lipofuscin accumulation results in oxidative stress, NLRP3 activation, and the development of vitelliform lesions, a hallmark of dry AMD, all of which may contribute to RPE dysfunction. The process of autophagy, involving the encapsulation, recognition, and transport of accumulated proteins and dead cells to the lysosome for degradation, is recognized as a significant pathway for cellular self-protection and homeostasis maintenance. Recently, RPE cell autophagy has been discovered to be closely linked to the development of macular degeneration, positioning autophagy as a cutting-edge research area in the realm of dry AMD. In this review, we present an overview of how lipofuscin, oxidative stress, and the NLRP3 inflammasome damage the RPE through their respective causal mechanisms. We summarized the connection between autophagy, oxidative stress, and NLRP3 inflammatory cytokines. Our findings suggest that targeting autophagy improves RPE function and sustains visual health, offering new perspectives for understanding the pathogenesis and clinical management of dry AMD.

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

Protein-Mediated Carotenoid Delivery Suppresses the Photoinducible Oxidation of Lipofuscin in Retinal Pigment Epithelial Cells

Lipofuscin of retinal pigment epithelium (RPE) cells is a complex heterogeneous system of chromophores which accumulates as granules during the cell's lifespan. Lipofuscin serves as a source of various cytotoxic effects linked with oxidative stress. Several age-related eye diseases such as macular degeneration of the retina, as well as some severe inherited eye pathologies, are accompanied by a significant increase in lipofuscin granule concentration. The accumulation of carotenoids in the RPE could provide an effective antioxidant protection against lipofuscin cytotoxic manifestations. Given the highly lipophilic nature of carotenoids, their targeted delivery to the vulnerable tissues can potentially be assisted by special proteins. In this study, we demonstrate how protein-mediated delivery of zeaxanthin using water-soluble Bombyx mori carotenoid-binding protein (BmCBP-ZEA) suppresses the photoinducible oxidative stress in RPE cells caused by irradiation of lipofuscin with intense white light. We implemented fluorescence lifetime imaging of the RPE cell culture ARPE-19 fed with lipofuscin granules and then irradiated by white light with and without the addition of BmCBP-ZEA. We demonstrate that after irradiation the mean fluorescence lifetime of lipofuscin significantly increases, while the presence of BmCBP-ZEA at 200 nM concentration suppresses the increase in the average lifetime of lipofuscin fluorescence, indicating an approx. 35% inhibition of the oxidative stress. This phenomenon serves as indirect yet important evidence of the efficiency of the protein-mediated carotenoid delivery into pigment epithelium cells.

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.

C20D3-Vitamin A Prevents Retinal Pigment Epithelium Atrophic Changes in a Mouse Model

Purpose

This study aimed to evaluate the contribution of vitamin A dimerization to retinal pigment epithelium (RPE) atrophic changes. Leading causes of irreversible blindness, including Stargardt disease and age-related macular degeneration (AMD), occur as a result of atrophic changes in RPE. The cause of the RPE atrophic changes is not apparent. During the vitamin A cycle, vitamin A dimerizes, leading to vitamin A cycle byproducts, such as vitamin A dimers, in the RPE.

Methods

To study the consequence of vitamin A dimerization to RPE atrophic changes, we used a rodent model with accelerated vitamin A dimerization, Abca4^−/−/Rdh8^−/− mice, and the vitamin A analog C20D₃-vitamin A to selectively ameliorate the accelerated rate of vitamin A dimerization.

Results

We show that ameliorating the rate of vitamin A dimerization with C20D₃-vitamin A mitigates pathological changes observed in the prodromal phase of the most prevalent retinal degenerative diseases, including fundus autofluorescence changes, dark adaptation delays, and signature RPE atrophic changes.

Conclusions

Data demonstrate that the dimerization of vitamin A during the vitamin A cycle is sufficient alone to cause the prerequisite RPE atrophic changes thought to be responsible for the leading causes of irreversible blindness and that correcting the dimerization rate with C20D₃-vitamin A may be sufficient to prevent the RPE atrophic changes.

Translational Relevance

Preventing the dimerization of vitamin A with the vitamin A analog C20D₃-vitamin A may be sufficient to alter the clinical course of the most prevalent forms of blindness, including Stargardt disease and age-related macular degeneration (AMD).

Lipofuscin causes atypical necroptosis through lysosomal membrane permeabilization

Lipofuscin granules enclose mixtures of cross-linked proteins and lipids in proportions that depend on the tissue analyzed. Retinal lipofuscin is unique in that it contains mostly lipids with very little proteins. However, retinal lipofuscin also presents biological and physicochemical characteristics indistinguishable from conventional granules, including indigestibility, tendency to cause lysosome swelling that results in rupture or defective functions, and ability to trigger NLRP3 inflammation, a symptom of low-level disruption of lysosomes. In addition, like conventional lipofuscins, it appears as an autofluorescent pigment, considered toxic waste, and a biomarker of aging. Ocular lipofuscin accumulates in the retinal pigment epithelium (RPE), whereby it interferes with the support of the neuroretina. RPE cell death is the primary cause of blindness in the most prevalent incurable genetic and age-related human disorders, Stargardt disease and age-related macular degeneration (AMD), respectively. Although retinal lipofuscin is directly linked to the cell death of the RPE in Stargardt, the extent to which it contributes to AMD is a matter of debate. Nonetheless, the number of AMD clinical trials that target lipofuscin formation speaks for the potential relevance for AMD as well. Here, we show that retinal lipofuscin triggers an atypical necroptotic cascade, amenable to pharmacological intervention. This pathway is distinct from canonic necroptosis and is instead dependent on the destabilization of lysosomes. We also provide evidence that necroptosis is activated in aged human retinas with AMD. Overall, this cytotoxicity mechanism may offer therapeutic targets and markers for genetic and age-related diseases associated with lipofuscin buildups.

Animal Models of LED-Induced Phototoxicity. Short- and Long-Term In Vivo and Ex Vivo Retinal Alterations

Phototoxicity animal models have been largely studied due to their degenerative communalities with human pathologies, e.g., age-related macular degeneration (AMD). Studies have documented not only the effects of white light exposure, but also other wavelengths using LEDs, such as blue or green light. Recently, a blue LED-induced phototoxicity (LIP) model has been developed that causes focal damage in the outer layers of the superior-temporal region of the retina in rodents. In vivo studies described a progressive reduction in retinal thickness that affected the most extensively the photoreceptor layer. Functionally, a transient reduction in a- and b-wave amplitude of the ERG response was observed. Ex vivo studies showed a progressive reduction of cones and an involvement of retinal pigment epithelium cells in the area of the lesion and, in parallel, an activation of microglial cells that perfectly circumscribe the damage in the outer retinal layer. The use of neuroprotective strategies such as intravitreal administration of trophic factors, e.g., basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) or pigment epithelium-derived factor (PEDF) and topical administration of the selective alpha-2 agonist (Brimonidine) have demonstrated to increase the survival of the cone population after LIP.

Factors Differentiating the Antioxidant Activity of Macular Xanthophylls in the Human Eye Retina

Macular xanthophylls, which are absorbed from the human diet, accumulate in high concentrations in the human retina, where they efficiently protect against oxidative stress that may lead to retinal damage. In addition, macular xanthophylls are uniquely spatially distributed in the retina. The zeaxanthin concentration (including the lutein metabolite meso-zeaxanthin) is ~9-fold greater than lutein concentration in the central fovea. These numbers do not correlate at all with the dietary intake of xanthophylls, for which there is a dietary zeaxanthin-to-lutein molar ratio of 1:12 to 1:5. The unique spatial distributions of macular xanthophylls—lutein, zeaxanthin, and meso-zeaxanthin—in the retina, which developed during evolution, maximize the protection of the retina provided by these xanthophylls. We will correlate the differences in the spatial distributions of macular xanthophylls with their different antioxidant activities in the retina. Can the major protective function of macular xanthophylls in the retina, namely antioxidant actions, explain their evolutionarily determined, unique spatial distributions? In this review, we will address this question.

The Role of Oxidative Stress and Autophagy in Blue-Light-Induced Damage to the Retinal Pigment Epithelium in Zebrafish In Vitro and In Vivo

Age-related macular degeneration (AMD) is the progressive degeneration of the retinal pigment epithelium (RPE), retina, and choriocapillaris among elderly individuals and is the leading cause of blindness worldwide. Thus, a better understanding of the underlying mechanisms in retinal tissue activated by blue light exposure is important for developing novel treatment and intervention strategies. In this study, blue-light-emitting diodes with a wavelength of 440 nm were applied to RPE cells at a dose of 3.7 ± 0.75 mW/cm² for 24 h. ARPE-19 cells were used to investigate the underlying mechanism induced by blue light exposure. A trypan blue exclusion assay was used for the cell viability determination. Flow cytometry was used for apoptosis rate detection and autophagy analysis. An immunofluorescence microscopy analysis was used to investigate cellular oxidative stress and DNA damage using DCFDA fluorescence staining and an anti-γH2AX antibody. Blue light exposure of zebrafish larvae was established to investigate the effect on retinal tissue development in vivo. To further demonstrate the comprehensive effect of blue light on ARPE-19 cells, next-generation sequencing (NGS) was performed for an ingenuity pathway analysis (IPA) to reveal additional related mechanisms. The results showed that blue light exposure caused a decrease in cell proliferation and an increase in apoptosis in ARPE-19 cells in a time-dependent manner. Oxidative stress increased during the early stage of 2 h of exposure and activated DNA damage in ARPE-19 cells after 8 h. Furthermore, autophagy was activated in response to blue light exposure at 24-48 h. The zebrafish larvae model showed the unfavorable effect of blue light in prohibiting retinal tissue development. The RNA-Seq results confirmed that blue light induced cell death and participated in tissue growth inhibition and maturation. The current study reveals the mechanisms by which blue light induces cell death in a time-dependent manner. Moreover, both the in vivo and NGS data uncovered blue light's effect on retinal tissue development, suggesting that exposing children to blue light could be relatively dangerous. These results could benefit the development of preventive strategies utilizing herbal medicine-based treatments for eye diseases or degeneration in the future.

Photoreactivity of Bis-retinoid A2E Complexed with a Model Protein in Selected Model Systems

The bis-retinoid N-retinyl-N-retinylidene ethanolamine (A2E) is formed as a byproduct of visual cycle in retinal pigment epithelium (RPE). It contributes to golden-yellow fluorescence of the age pigment lipofuscin, which accumulates in RPE. Lipofuscin can generate a variety of reactive oxygen species (ROS) upon blue-light excitation. Although in model systems photoreactivity of A2E has been determined to be low, this bis-retinoid exhibited significant phototoxicity in RPE cells in vitro. Although the mechanism of A2E-mediated phototoxicity remains mostly unknown, we hypothesize that formation of A2E-adducts with different biomolecules may play an important role. In this study, we investigated the photochemical reactivity of A2E and its complex with bovine serum albumin (BSA) using UV-Vis absorption and emission spectroscopy, EPR-spin trapping, EPR-oximetry, time-resolved singlet oxygen phosphorescence, and the fluorogenic CBA probe. Our data show that A2E after complexation with this model protein photogenerated an increased level of ROS, particularly singlet oxygen. We also demonstrated the ability of A2E to oxidize BSA upon excitation with blue light in aqueous model systems. The data suggest that pyridinium bis-retinoid could oxidatively modify cellular proteins under physiological conditions.

Cytoprotective effect of crocin and trans-resveratrol on photodamaged primary human retinal pigment epithelial cells

PURPOSE

Light-induced damage to retinal pigment epithelium during pars plana vitrectomy remains a hot topic in ophthalmology. Improvements in technology led to a change of light sources, selective filters, and shorter light exposure time. Currently, there is no satisfying solution to the problem. The aim of the study was to investigate the cytoprotective effects of crocin and resveratrol on light-induced damage to primary human retinal pigment epithelial cells in vitro.

METHODS

Primary human retinal pigment epithelial cells were exposed to light analogous to the illumination during pars plana vitrectomy. To evaluate the cytoprotective effects and potential toxicity of resveratrol and crocin, human retinal pigment epithelial cells were incubated with varying concentrations of both before 3-[4,5-dimethylthiazol-2-yl] tetrazolium bromide (MTT) viability assay. Furthermore, glutathione levels were measured to investigate synergistic antioxidant potential. Apoptosis of human retinal pigment epithelial cells was determined by a nucleosome detection enzyme-linked immunosorbent assay.

RESULTS

Crocin and resveratrol improved cell viability in photodamaged human retinal pigment epithelial cells significantly from 40.65 ± 21.99% in illuminated human retinal pigment epithelial cells and reached a peak viability of 85.64 ± 11.37% in crocin and resveratrol pretreated cells (for all: p < 0.001). In line, the combination of the supplements increased glutathione levels significantly from 39.35 ± 21.96% to 80.74 ± 10.32% (p = 0.017). No toxic effects were detected (p > 0.99). However, no change in apoptosis rates could be observed following pretreatment with crocin and resveratrol (p > 0.99).

CONCLUSION

Crocin and trans-resveratrol revealed cytoprotective effects on human retinal pigment epithelial cells supporting both supplement's development as potential perioperative treatments in light-induced retinal pigment epithelial damage.

Mitochondria as Potential Targets and Initiators of the Blue Light Hazard to the Retina

Commercially available white light-emitting diodes (LEDs) have an intense emission in the range of blue light, which has raised a range of public concerns about their potential risks as retinal hazards. Distinct from other visible light components, blue light is characterized by short wavelength, high energy, and strong penetration that can reach the retina with relatively little loss in damage potential. Mitochondria are abundant in retinal tissues, giving them relatively high access to blue light, and chromophores, which are enriched in the retina, have many mitochondria able to absorb blue light and induce photochemical effects. Therefore, excessive exposure of the retina to blue light tends to cause ROS accumulation and oxidative stress, which affect the structure and function of the retinal mitochondria and trigger mitochondria-involved death signaling pathways. In this review, we highlight the essential roles of mitochondria in blue light-induced photochemical damage and programmed cell death in the retina, indicate directions for future research and preventive targets in terms of the blue light hazard to the retina, and suggest applying LED devices in a rational way to prevent the blue light hazard.

Photoactivation of N-retinylidene-N-retinylethanolamine compromises autophagy in retinal pigmented epithelial cells

As a part of the aging process, N-retinylidene-N-retinylethanolamine (A2E) accumulates in the retina to activate autophagy in retinal pigmented epithelial cells. However, the effect of A2E photoactivation on autophagy, which is more clinically relevant, still remains unclear. Here, we investigated the effect of blue light (BL)-activated A2E on autophagy in human retinal pigmented epithelial cells, ARPE-19. A significant increase in LC3-II protein was observed when BL was irradiated on ARPE-19 cells containing A2E. The mammalian target of rapamycin (mTOR) pathway was examined to verify whether autophagy was activated, but no change in AKT, mTOR, and 4EBP phosphorylation was observed. Transcription factor EB (TFEB) target gene expression, which is another pathway involved in autophagy, was also not altered by A2E and BL. However, intracellular p62 protein levels were significantly increased, which represented the inhibition of autophagic flux. To investigate the mechanism of the suppressed autophagic flux, the lysosomal state was observed. After BL irradiation, lysosomal damage was induced in A2E-treated ARPE-19 cells, and this phenomenon was prevented by treatment with the antioxidant, N-acetylcysteine. Our results suggest that A2E photoactivation compromises autophagy in ARPE-19 cells and that reactive oxygen species (ROS) play an important role in this process.

Conversion of all-trans-retinal into all-trans-retinal dimer reflects an alternative metabolic/antidotal pathway of all-trans-retinal in the retina

Free all-trans-retinal (atRAL) and retinal pigment epithelium (RPE) lipofuscin are both considered to play etiological roles in Stargardt disease and age-related macular degeneration. A2E and all-trans-retinal dimer (atRAL-dimer) are two well characterized bisretinoid constituents of RPE lipofuscin. In this study, we found that, after treatment of primary porcine RPE (pRPE) cells with atRAL, atRAL-dimer readily formed and accumulated in a concentration- and time-dependent manner, but A2E was barely detected. Cell-based assays revealed that atRAL, the precursor of atRAL-dimer, significantly altered the morphology of primary pRPE cells and decreased cell viability at a concentration of 80 μm regardless of light exposure. By contrast, atRAL-dimer was not cytotoxic and phototoxic to primary pRPE cells. Compared with atRAL and A2E, atRAL-dimer was more vulnerable to light, followed by the generation of its photocleaved products. Moreover, we observed the presence of atRAL-dimer in reaction mixtures of atRAL with porcine rod outer segments (ROS), RPE/choroid, or neural retina. Taken together, we here proposed an alternative metabolic/antidotal pathway of atRAL in the retina: atRAL that evades participation of the visual (retinoid) cycle undergoes a condensation reaction to yield atRAL-dimer in both ROS and RPE. Translocation of atRAL, all-trans N-retinylidene-phosphatidylethanolamine (NR-PE), atRAL-dimer, and photocleavage products of atRAL-dimer from ROS into RPE is accomplished by phagocytosing shed ROS on a daily basis. Without causing damage to RPE cells, light breaks up total atRAL-dimer within RPE cells to release low-molecular-weight photocleavage fragments. The latter, together with ROS-atRAL-dimer photocleavage products, may easily move across membranes and thereby be metabolically eliminated.

Blue light excited retinal intercepts cellular signaling

Photoreceptor chromophore, 11-cis retinal (11CR) and the photoproduct, all-trans retinal (ATR), are present in the retina at higher concentrations and interact with the visual cells. Non-visual cells in the body are also exposed to retinal that enters the circulation. Although the cornea and the lens of the eye are transparent to the blue light region where retinal can absorb and undergo excitation, the reported phototoxicity in the eye has been assigned to lipophilic non-degradable materials known as lipofuscins, which also includes retinal condensation products. The possibility of blue light excited retinal interacting with cells; intercepting signaling in the presence or absence of light has not been explored. Using live cell imaging and optogenetic signaling control, we uncovered that blue light-excited ATR and 11CR irreversibly change/distort plasma membrane (PM) bound phospholipid; phosphatidylinositol 4,5 bisphosphate (PIP2) and disrupt its function. This distortion in PIP2 was independent of visual or non-visual G-protein coupled receptor activation. The change in PIP2 was followed by an increase in the cytosolic calcium, excessive cell shape change, and cell death. Blue light alone or retinal alone did not perturb PIP2 or elicit cytosolic calcium increase. Our data also suggest that photoexcited retinal-induced PIP2 distortion and subsequent oxidative damage incur in the core of the PM. These findings suggest that retinal exerts light sensitivity to both photoreceptor and non-photoreceptor cells, and intercepts crucial signaling events, altering the cellular fate.

Microwave assisted synthesis for A2E and development of LC-ESI-MS method for quantification of ocular bisretinoids in human retina

PURPOSE

To develop a microwave assisted method for the rapid synthesis of A2E and also to develop a method to quantify N-retinylidene-N-retinylethanolamine(A2E), all-trans retinal dimer (ATRD), A2-glycerophospho ethanolamine (A2GPE), dihydropyridine phosphatidyl ethanolamine (A2DHPE) and monofuran A2E (MFA2E) in age matched retina.

METHODS

The development of microwave assisted synthesis of A2E, its purification and characterization for its utility in quantification in human retina. The semi-quantitative method development using LC-ESI-MS, LC-ESI-MS/MS and LC-APCI-MS/MS from pooled macula and peripheral retina for the bisretinoid analysis has been done.

RESULTS

Maximum A2E conversion using microwave assisted process took place at 80°C for 45min with a yield of 55.01%. Highly sensitive and specific mass spectrometric method was developed using reverse phase C-18 separation with positive electrospray ionization and positive atmospheric phase chemical ionization of tandom mass spectrometry. A gradient mobile phase separation was achieved using water and methanol with 0.1% TFA. Multiple reaction monitoring acquisition for ESI and APCI was performed at ATRD m/z 551.2/522.2, A2GPE m/z 746.4/729.5, A2DHPEm/z 594.4/576.5, MFA2E m/z 608.2/591.2, A2E m/z 592.4/418.2. Method was validated using LC-ESI-SIM mode to determine selectivity, linearity, sensitivity, precision and accuracy.

CONCLUSION

An attempt towards optimization of the synthetic procedure of A2E was made so as to reduce the lengthy reaction time without compromising the yield. Developed method was capable enough for the detection of low level of bisretinids in retina.

Lipofuscins prepared by modification of photoreceptor cells via glycation or lipid peroxidation show the similar phototoxicity

AIM

To investigate the effect of two ways of lipofuscin production (lipid peroxidation and glycation) on lipofuscin fluorescence characteristics and phototoxicity and to compare them with the properties of natural lipofuscin.

METHODS

Model lipofuscins were prepared on the basis of bovine photoreceptor outer segments (POS) with bisretinoid A2E addition. One set of samples was prepared from POS modified by lipid peroxidation, while another set from POS modified by glycation with fructose. Fluorescent properties and kinetics of photoinduced superoxide generation of model lipofuscins and human retinal pigment epithelium (RPE) lipofuscin were compared. The fluorescence spectra of samples were measured at 365 nm excitation wavelength and 380-650 emission wavelength.

RESULTS

The fluorescence spectra of model lipofuscins are almost the same as the spectrum of natural lipofuscin. Visible light irradiation of both model lipofuscins and natural lipofuscin isolated from RPE cells leads to decrease of a fluorescence maximum at 550 nm and to appearance of a distinct, new maximum at 445-460 nm. The rate of photogeneration of reactive oxygen forms by both model lipofuscins was almost the same and approximately two times less than that of RPE lipofuscin granules.

CONCLUSION

These data suggest that fluorescent characteristics and phototoxicity of lipofuscin granules depend only to an insignificant degree on the oxidative modification of POS proteins and lipids, and generally are defined by the bisretinoid fluorophores contained in them.

Structural Insights into the Drosophila melanogaster Retinol Dehydrogenase, a Member of the Short-Chain Dehydrogenase/Reductase Family

The 11-cis-retinylidene chromophore of visual pigments isomerizes upon interaction with a photon, initiating a downstream cascade of signaling events that ultimately lead to visual perception. 11-cis-Retinylidene is regenerated through enzymatic transformations collectively called the visual cycle. The first and rate-limiting enzymatic reaction within this cycle, i.e., the reduction of all-trans-retinal to all-trans-retinol, is catalyzed by retinol dehydrogenases. Here, we determined the structure of Drosophila melanogaster photoreceptor retinol dehydrogenase (PDH) isoform C that belongs to the short-chain dehydrogenase/reductase (SDR) family. This is the first reported structure of a SDR that possesses this biologically important activity. Two crystal structures of the same enzyme grown under different conditions revealed a novel conformational change of the NAD⁺ cofactor, likely representing a change during catalysis. Amide hydrogen-deuterium exchange of PDH demonstrated changes in the structure of the enzyme upon dinucleotide binding. In D. melanogaster, loss of PDH activity leads to photoreceptor degeneration that can be partially rescued by transgenic expression of human RDH12. Based on the structure of PDH, we analyzed mutations causing Leber congenital amaurosis 13 in a homology model of human RDH12 to obtain insights into the molecular basis of RDH12 disease-causing mutations.

The Photobiology of Lutein and Zeaxanthin in the Eye

Lutein and zeaxanthin are antioxidants found in the human retina and macula. Recent clinical trials have determined that age- and diet-related loss of lutein and zeaxanthin enhances phototoxic damage to the human eye and that supplementation of these carotenoids has a protective effect against photoinduced damage to the lens and the retina. Two of the major mechanisms of protection offered by lutein and zeaxanthin against age-related blue light damage are the quenching of singlet oxygen and other reactive oxygen species and the absorption of blue light. Determining the specific reactive intermediate(s) produced by a particular phototoxic ocular chromophore not only defines the mechanism of toxicity but can also later be used as a tool to prevent damage.

A2E and Lipofuscin

Lipofuscin is highly fluorescent material, formed in several tissues but best studied in the eye. The accumulation of lipofuscin in the retinal pigment epithelium (RPE) is a hallmark of aging in the eye and has been implicated in various retinal degenerations, including age-related macular degeneration. The bis-retinoid N-retinyl-N-retinylidene ethanolamine (A2E), formed from retinal, has been identified as a byproduct of the visual cycle, and numerous in vitro studies have found toxicity associated with this compound. The compound is known to accumulate in the RPE with age and was the first identified compound extracted from lipofuscin. Our studies have correlated the distribution of lipofuscin and A2E across the human and mouse RPE. Lipofuscin fluorescence was imaged in the RPE from human donors of various ages and from assorted mouse models. The spatial distribution of A2E was determined using matrix-assisted laser desorption-ionization imaging mass spectrometry on both flat-mounted and transversally sectioned RPE tissue. Our data support the clinical observations in humans of strong RPE fluorescence, increasing with age, in the central area of the RPE. However, there was no correlation between the distribution of A2E and lipofuscin, as the levels of A2E were highest in the far periphery and decreased toward the central region. Interestingly, in all the mouse models, A2E distribution and lipofuscin fluorescence correlate well. These data demonstrate that the accumulation of A2E is not responsible for the increase in lipofuscin fluorescence observed in the central RPE with aging in humans.

Effects of organic solvents on two retinal pigment epithelial lipofuscin fluorophores, A2E and all-trans-retinal dimer

Gene and drug therapies are being developed to alleviate vision loss in patients with Stargardt's disease and age-related macular degeneration (AMD). To evaluate the therapeutic effects of these treatments, organic solvents are routinely used to extract and quantify bisretinoid lipofuscin constituents, such as N-retinylidene-N-retinyl-ethanolamine (A2E) and all-trans-retinal dimer (ATR-dimer). By high-performance liquid chromatography (HPLC), we found that A2E and ATR-dimer were both altered by tetrahydrofuran (THF) and chloroform, but were stable in dimethyl sulfoxide (DMSO) or methanol (MeOH). In addition, cyclohexane and ethanol (EtOH) did not alter ATR-dimer, whereas an alteration of A2E occurred in EtOH. On the basis of these findings, we designed processes II-IV, generated by modifications of process I, a routine method to measure bisretinoid compounds in vivo. Extra amounts of either ATR-dimer or A2E in mouse eyecups were released by processes II-IV versus process I. Efforts to clarify the effects of organic solvents on lipofuscin pigments are important because such studies can guide the handling of these fluorophores in related experiments.

Photo-damage, photo-protection and age-related macular degeneration

The course of Age-related Macular Degeneration (AMD) is described as the effect of light (400–580 nm) on various molecular targets in photoreceptors and the retinal pigment epithelium (RPE). Photo-damage is followed by inflammation, increasing oxidative stress and, probably, unveiling new photosensitive molecules.

Macular pigment optical density measured by heterochromatic modulation photometry

Purpose

To psychophysically determine macular pigment optical density (MPOD) employing the heterochromatic modulation photometry (HMP) paradigm by estimating 460 nm absorption at central and peripheral retinal locations.

Methods

For the HMP measurements, two lights (B: 460 nm and R: 660 nm) were presented in a test field and were modulated in counterphase at medium or high frequencies. The contrasts of the two lights were varied in tandem to determine flicker detection thresholds. Detection thresholds were measured for different R:B modulation ratios. The modulation ratio with minimal sensitivity (maximal threshold) is the point of equiluminance. Measurements were performed in 25 normal subjects (11 male, 14 female; age: 30±11 years, mean ± sd) using an eight channel LED stimulator with Maxwellian view optics. The results were compared with those from two published techniques – one based on heterochromatic flicker photometry (Macular Densitometer) and the other on fundus reflectometry (MPR).

Results

We were able to estimate MPOD with HMP using a modified theoretical model that was fitted to the HMP data. The resultant MPOD_HMP values correlated significantly with the MPOD_MPR values and with the MPOD_HFP values obtained at 0.25° and 0.5° retinal eccentricity.

Conclusions

HMP is a flicker-based method with measurements taken at a constant mean chromaticity and luminance. The data can be well fit by a model that allows all data points to contribute to the photometric equality estimate. Therefore, we think that HMP may be a useful method for MPOD measurements, in basic and clinical vision experiments.

Vitamin A dimers trigger the protracted death of retinal pigment epithelium cells

Cellular events responsible for the initiation of major neurodegenerative disorders of the eye leading to blindness, including age-related macular degeneration, Stargardt and Best diseases, are poorly understood. Accumulation of vitamin A dimers, such as N-retinylidene-N-retinylethanolamine (A2E) in the retinal pigment epithelium (RPE), is one of the earliest measurable events preceding retinal degeneration. However, the extent to which these dimers contribute to tissue degeneration is not clear. To determine if A2E could trigger morphological changes associated with the degenerating RPE and subsequent cell death, we evaluated its toxicity to cultured human RPE cells (ARPE-19). We show that A2E triggered the accumulation of debris followed by a protracted death. A2E was up to ≈ 14-fold more toxic than its precursor, retinaldehyde. Measurements reveal that the concentration of A2E in the aged human eye could exceed the concentration of all other retinoids, opening the possibility of A2E-triggered cell death by several reported mechanisms. Findings suggest that accumulation of vitamin A dimers such as A2E in the human eye might be responsible for the formation of ubiquitous RPE debris, an early indication of retinal degeneration, and that preventing or reducing the accumulation of vitamin A dimers is a prudent strategy to prevent blindness.

Retinal metabolism in humans induces the formation of an unprecedented lipofuscin fluorophore ‘pdA2E’

We identified a novel by-product of retinal metabolism called pdA2E in the lipofuscin of the retinal pigment epithelium, demonstrated the photoisomerization and oxidation of pdA2E, and proposed a non-enzymatic mechanism for its formation in the retina.

Beta cyclodextrins bind, stabilize, and remove lipofuscin bisretinoids from retinal pigment epithelium

Accumulation of lipofuscin bisretinoids (LBs) in the retinal pigment epithelium (RPE) is the alleged cause of retinal degeneration in genetic blinding diseases (e.g., Stargardt) and a possible etiological agent for age-related macular degeneration. Currently, there are no approved treatments for these diseases; hence, agents that efficiently remove LBs from RPE would be valuable therapeutic candidates. Here, we show that beta cyclodextrins (β-CDs) bind LBs and protect them against oxidation. Computer modeling and biochemical data are consistent with the encapsulation of the retinoid arms of LBs within the hydrophobic cavity of β-CD. Importantly, β-CD treatment reduced by 73% and 48% the LB content of RPE cell cultures and of eyecups obtained from Abca4-Rdh8 double knock-out (DKO) mice, respectively. Furthermore, intravitreal administration of β-CDs reduced significantly the content of bisretinoids in the RPE of DKO animals. Thus, our results demonstrate the effectiveness of β-CDs to complex and remove LB deposits from RPE cells and provide crucial data to develop novel prophylactic approaches for retinal disorders elicited by LBs.

Not just signal shutoff: the protective role of arrestin-1 in rod cells

The retinal rod cell is an exquisitely sensitive single-photon detector that primarily functions in dim light (e.g., moonlight). However, rod cells must routinely survive light intensities more than a billion times greater (e.g., bright daylight). One serious challenge to rod cell survival in daylight is the massive amount of all-trans-retinal that is released by Meta II, the light-activated form of the photoreceptor rhodopsin. All-trans-retinal is toxic, and its condensation products have been implicated in disease. Our recent work has developed the concept that rod arrestin (arrestin-1), which terminates Meta II signaling, has an additional role in protecting rod cells from the consequences of bright light by limiting free all-trans-retinal. In this chapter we will elaborate upon the molecular mechanisms by which arrestin-1 serves as both a single-photon response quencher as well as an instrument of rod cell survival in bright light. This discussion will take place within the framework of three distinct functional modules of vision: signal transduction, the retinoid cycle, and protein translocation.

Cytotoxicity of all-trans-retinal increases upon photodegradation

All-trans-retinal (AtRal) can accumulate in the retina as a result of excessive exposure to light. The purpose of this study was to compare cytotoxicity of AtRal and photodegraded AtRal (dAtRal) on cultured human retinal pigment epithelial cells in dark and upon exposure to visible light. AtRal was degraded by exposure to visible light. Cytotoxicity was monitored by imaging of cell morphology, propidium iodide staining of cells with permeable plasma membrane and measurements of reductive activity of cells. Generation of singlet oxygen photosensitized by AtRal and dAtRal was monitored by time-resolved measurements of characteristic singlet oxygen phosphorescence. Photodegradation of AtRal resulted in a decrease in absorption of visible light and accumulation of the degradation products with absorption maximum at ∼330 nm. Toxicity of dAtRal was concentration-dependent and was greater during irradiation with visible light than in dark. DAtRal was more cytotoxic than AtRal both in dark and during exposure to visible light. Photochemical properties of dAtRal indicate that it may be responsible for the maximum in the action spectra of retinal photodamage recorded in animals. In conclusion, photodegradation products of AtRal may impose a significant threat to the retina and therefore their roles in retinal pathology need to be explored.

Retinal photodamage by endogenous and xenobiotic agents

The human eye is constantly exposed to sunlight and artificial lighting. Light transmission through the eye is fundamental to its unique biological functions of directing vision and circadian rhythm and therefore light absorbed by the eye must be benign. However, exposure to the very intense ambient radiation can pose a hazard particularly if the recipient is over 40 years of age. There are age-related changes in the endogenous (natural) chromophores (lipofuscin, A2E and all-trans-retinal derivatives) in the human retina that makes it more susceptible to visible light damage. Intense visible light sources that do not filter short blue visible light (400-440 nm) used for phototherapy of circadian imbalance (i.e. seasonal affective disorder) increase the risk for age-related light damage to the retina. Moreover, many drugs, dietary supplements, nanoparticles and diagnostic dyes (xenobiotics) absorb ocular light and have the potential to induce photodamage to the retina, leading to transient or permanent blinding disorders. This article will review the underlying reasons why visible light in general and short blue visible light in particular dramatically raises the risk of photodamage to the human retina.

Ultraviolet radiation as a risk factor for cataract and macular degeneration

The human eye is constantly exposed to sunlight and artificial lighting. Light transmission through the eye is fundamental to its unique biological functions of directing vision and circadian rhythm, and therefore, light absorbed by the eye must be benign. However, exposure to the intense ambient radiation can pose a hazard particularly if the recipient is over 40 years of age. This radiation exposure can lead to impaired vision and transient or permanent blindness.Both ultraviolet-A (UV-A) and UV-B induce cataract formation and are not necessary for sight. Ultraviolet radiation is also a risk factor for damage to the retinas of children. The removal of these wavelengths from ocular exposure will greatly reduce the risk of early cataract and retinal damage. One way this may be easily done is by wearing sunglasses that block wavelengths below 400 nm (marked 400 on the glasses). However, because of the geometry of the eye, these glasses must be wraparound sunglasses to prevent reflective UV radiation from reaching the eye. Additional protection may be offered by contact lenses that absorb significant amounts of UV radiation.In addition to UV radiation, short blue visible light (400-440 nm) is a risk factor for the adult human retina. This wavelength of light is not essential for sight and not necessary for a circadian rhythm response. For those over 50 years old, it would be of value to remove these wavelengths of light with specially designed sunglasses or contact lenses to reduce the risk of age-related macular degeneration.

Light-emitting diodes (LED) for domestic lighting: any risks for the eye?

Light-emitting diodes (LEDs) are taking an increasing place in the market of domestic lighting because they produce light with low energy consumption. In the EU, by 2016, no traditional incandescent light sources will be available and LEDs may become the major domestic light sources. Due to specific spectral and energetic characteristics of white LEDs as compared to other domestic light sources, some concerns have been raised regarding their safety for human health and particularly potential harmful risks for the eye. To conduct a health risk assessment on systems using LEDs, the French Agency for Food, Environmental and Occupational Health & Safety (ANSES), a public body reporting to the French Ministers for ecology, for health and for employment, has organized a task group. This group consisted physicists, lighting and metrology specialists, retinal biologist and ophthalmologist who have worked together for a year. Part of this work has comprised the evaluation of group risks of different white LEDs commercialized on the French market, according to the standards and found that some of these lights belonged to the group risk 1 or 2. This paper gives a comprehensive analysis of the potential risks of white LEDs, taking into account pre-clinical knowledge as well as epidemiologic studies and reports the French Agency's recommendations to avoid potential retinal hazards.

C20-D3-vitamin A slows lipofuscin accumulation and electrophysiological retinal degeneration in a mouse model of Stargardt disease

Stargardt disease, also known as juvenile macular degeneration, occurs in approximately one in 10,000 people and results from genetic defects in the ABCA4 gene. The disease is characterized by premature accumulation of lipofuscin in the retinal pigment epithelium (RPE) of the eye and by vision loss. No cure or treatment is available. Although lipofuscin is considered a hallmark of Stargardt disease, its mechanism of formation and its role in disease pathogenesis are poorly understood. In this work we investigated the effects of long-term administration of deuterium-enriched vitamin A, C20-D(3)-vitamin A, on RPE lipofuscin deposition and eye function in a mouse model of Stargardt's disease. Results support the notion that lipofuscin forms partly as a result of the aberrant reactivity of vitamin A through the formation of vitamin A dimers, provide evidence that preventing vitamin A dimerization may slow disease related, retinal physiological changes and perhaps vision loss and suggest that administration of C20-D(3)-vitamin A may be a potential clinical strategy to ameliorate clinical symptoms resulting from ABCA4 genetic defects.

Noninvasive multiphoton fluorescence microscopy resolves retinol and retinal condensation products in mouse eyes

Multi–photon excitation fluorescence microscopy (MPM) can image certain molecular processes in vivo. In the eye, fluorescent retinyl esters in sub–cellular structures called retinosomes mediate regeneration of the visual chromophore, 11–cis–retinal, by the visual cycle. But harmful fluorescent condensation products were also identified previously. We report that in wild type mice, excitation with λ ~730 nm identified retinosomes in the retinal pigment epithelium, whereas excitation with λ ~910 nm revealed at least one additional retinal fluorophore. The latter fluorescence was absent in eyes of genetically modified mice lacking a functional visual cycle, but accentuated in eyes of older WT mice and mice with defective clearance of all–trans–retinal, an intermediate in the visual cycle. MPM, a noninvasive imaging modality that facilitates concurrent monitoring of retinosomes along with potentially harmful products in aging eyes, has the potential to detect early molecular changes due to age–related macular degeneration and other defects in retinoid metabolism.