A2E-epoxides damage DNA in retinal pigment epithelial cells. Vitamin E and other antioxidants inhibit A2E-epoxide formation

Blue Light-Induced Accelerated Formation of Melanolipofuscin-Like Organelles in Japanese Quail RPE Cells: An Electron Microscopic Study

Purpose

The retinal pigment epithelium (RPE) is a monolayer of epithelial cells essential for photoreceptor function and viability. Quail Coturnix japonica is a convenient experimental animal model for the study of age and pathological retina processes to an accelerated time regime. The three main types of pigment granules present in the RPE are melanin-containing melanosomes, lipofuscin-containing lipofuscin granules, and mixed melanolipofuscin granules containing both melanin and lipofuscin. The purpose of this work was to study the process of melanolipofuscinogenesis during aging and under light exposure.

Methods

We examined melanolipofuscin granules in "macular" areas, the area of the retina containing oxycarotenoids, as a function of the macula in humans, of the quail retina by transmission electron microscopy in young, middle-aged, and old birds, and in middle-aged birds irradiated with blue LED light (450 nm, 4 J/cm2).

Results

It has been shown that during photo-oxidative stress caused by the action of blue light on the quail eye, active fusion of melanosomes and lipofuscin granules occurs with formation of various types, including giant, mixed melanolipofuscin-like granules. Increased accumulation of melanolipofuscin-like granules was also observed in non-irradiated old birds.

Conclusions

It is assumed that the decrease in the number of melanosomes in the RPE during aging and photo-oxidative stress is associated with their fusion with lipofuscin granules and subsequent degradation of melanin by reactive oxygen species formed in melanolipofuscin-like granules. The disappearance of melanin deprives the RPE cells of light-filtering and antioxidant protection, and significantly increases the risk of their oxidative stress.

Phenotyping and genotyping inherited retinal diseases: Molecular genetics, clinical and imaging features, and therapeutics of macular dystrophies, cone and cone-rod dystrophies, rod-cone dystrophies, Leber congenital amaurosis, and cone dysfunction syndromes

Inherited retinal diseases (IRD) are a leading cause of blindness in the working age population and in children. The scope of this review is to familiarise clinicians and scientists with the current landscape of molecular genetics, clinical phenotype, retinal imaging and therapeutic prospects/completed trials in IRD. Herein we present in a comprehensive and concise manner: (i) macular dystrophies (Stargardt disease (ABCA4), X-linked retinoschisis (RS1), Best disease (BEST1), PRPH2-associated pattern dystrophy, Sorsby fundus dystrophy (TIMP3), and autosomal dominant drusen (EFEMP1)), (ii) cone and cone-rod dystrophies (GUCA1A, PRPH2, ABCA4, KCNV2 and RPGR), (iii) predominant rod or rod-cone dystrophies (retinitis pigmentosa, enhanced S-Cone syndrome (NR2E3), Bietti crystalline corneoretinal dystrophy (CYP4V2)), (iv) Leber congenital amaurosis/early-onset severe retinal dystrophy (GUCY2D, CEP290, CRB1, RDH12, RPE65, TULP1, AIPL1 and NMNAT1), (v) cone dysfunction syndromes (achromatopsia (CNGA3, CNGB3, PDE6C, PDE6H, GNAT2, ATF6), X-linked cone dysfunction with myopia and dichromacy (Bornholm Eye disease; OPN1LW/OPN1MW array), oligocone trichromacy, and blue-cone monochromatism (OPN1LW/OPN1MW array)). Whilst we use the aforementioned classical phenotypic groupings, a key feature of IRD is that it is characterised by tremendous heterogeneity and variable expressivity, with several of the above genes associated with a range of phenotypes.

Iron overload and chelation modulates bisretinoid levels in the retina

Aim

Iron dysregulation in conjunction with other disease processes may exacerbate retinal degeneration. We employed models of iron overload and iron chelation to explore the interactions between iron-catalyzed oxidation and photoreactive bisretinoid lipofuscin.

Methods

The mice were injected intravitreally with ferric ammonium citrate (FAC) or were treated using the iron chelator deferiprone (DFP) from birth to 2 months of age. Short-wavelength fundus autofluorescence (SW-AF) and spectral-domain optical coherence tomography (SD-OCT) scans were acquired. The bisretinoid levels were quantified using ultra performance liquid chromatography (UPLC) and in vivo through quantitative fundus autofluorescence (qAF). In histologic sections, the photoreceptor cell viability was assessed by measuring the thickness of the outer nuclear layer (ONL).

Results

The levels of bisretinoids, all-trans-retinal dimers, and A2PE were significantly increased in the FAC-injected eyes of C57BL/6J mice. Seven days after FAC injection, hyperautofluorescent foci were visible in fundus autofluorescence (488 nm) images, and in SD-OCT scans, aberrant hyperreflectivity was present in the outer retina and ONL thinning was observed. In FAC-injected Abca4-/- mice with pronounced RPE bisretinoid lipofuscin accumulation, the hyperautofluorescent puncta were more abundant than in the wild-type mice, and the extent of ONL thinning was greater. Conversely, the intravitreal injection of FAC in Mertk-/- mice led to a more modest increase in A2PE after 2 days. In contrast to the effect of iron accumulation, chelation with DFP resulted in significantly increased levels of A2E and A2-GPE and qAF due to the reduced iron-catalyzed oxidation of bisretinoids. In Mertk-/- mice, the A2E level was significantly lower and the ONL area was smaller than in DFP-treated mice. DFP chelation did not impair the visual cycle in BALB/cJ mice.

Conclusion

Iron accumulation was associated with progressive impairment in photoreceptor cells that was associated with the increased formation of a bisretinoid species known to form in photoreceptor outer segments as a precursor to A2E. Additionally, disease features such as the development of hyperautofluorescence puncta in fundus AF images, hyperreflectivity in the outer retina of SD-OCT scans, and ONL thinning were more pronounced when iron was delivered to Abca4-/- mice with a greater propensity for bisretinoid formation. Higher bisretinoid levels and enhanced qAF are indicative of lesser bisretinoid loss due to oxidation.

Lipofuscin, Its Origin, Properties, and Contribution to Retinal Fluorescence as a Potential Biomarker of Oxidative Damage to the Retina

Lipofuscin accumulates with age as intracellular fluorescent granules originating from incomplete lysosomal digestion of phagocytosed and autophagocytosed material. The purpose of this review is to provide an update on the current understanding of the role of oxidative stress and/or lysosomal dysfunction in lipofuscin accumulation and its consequences, particularly for retinal pigment epithelium (RPE). Next, the fluorescence of lipofuscin, spectral changes induced by oxidation, and its contribution to retinal fluorescence are discussed. This is followed by reviewing recent developments in fluorescence imaging of the retina and the current evidence on the prognostic value of retinal fluorescence for the progression of age-related macular degeneration (AMD), the major blinding disease affecting elderly people in developed countries. The evidence of lipofuscin oxidation in vivo and the evidence of increased oxidative damage in AMD retina ex vivo lead to the conclusion that imaging of spectral characteristics of lipofuscin fluorescence may serve as a useful biomarker of oxidative damage, which can be helpful in assessing the efficacy of potential antioxidant therapies in retinal degenerations associated with accumulation of lipofuscin and increased oxidative stress. Finally, amendments to currently used fluorescence imaging instruments are suggested to be more sensitive and specific for imaging spectral characteristics of lipofuscin fluorescence.

Stages, pathogenesis, clinical management and advancements in therapies of age-related macular degeneration

Age-related macular degeneration (AMD) is a retinal degenerative disorder prevalent in the elderly population, which leads to the loss of central vision. The disease progression can be managed, if not prevented, either by blocking neovascularization ("wet" form of AMD) or by preserving retinal pigment epithelium and photoreceptor cells ("dry" form of AMD). Although current therapeutic modalities are moderately successful in delaying the progression and management of the disease, advances over the past years in regenerative medicine using iPSC, embryonic stem cells, advanced materials (including nanomaterials) and organ bio-printing show great prospects in restoring vision and efficient management of either forms of AMD. This review focuses on the molecular mechanism of the disease, model systems (both cellular and animal) used in studying AMD, the list of various regenerative therapies and the current treatments available. The article also highlights on the recent clinical trials using regenerative therapies and management of the disease.

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.

CRISPR editing demonstrates rs10490924 raised oxidative stress in iPSC-derived retinal cells from patients with ARMS2/HTRA1-related AMD

Genome-wide association studies (GWAS) have identified genetic risk loci for age-related macular degeneration (AMD) on the chromosome 10q26 (Chr10) locus and are tightly linked: the A69S (G>T) rs10490924 single-nucleotide variant (SNV) and the AATAA-rich insertion-deletion (indel, del443/ins54), which are found in the age-related maculopathy susceptibility 2 (ARMS2) gene, and the G512A (G>A) rs11200638 SNV, which is found in the high-temperature requirement A serine peptidase 1 (HTRA1) promoter. The fourth variant is Y402H complement factor H (CFH), which directs CFH signaling. CRISPR manipulation of retinal pigment epithelium (RPE) cells may allow one to isolate the effects of the individual SNV and thus identify SNV-specific effects on cell phenotype. Clustered regularly interspaced short palindromic repeats (CRISPR) editing demonstrates that rs10490924 raised oxidative stress in induced pluripotent stem cell (iPSC)-derived retinal cells from patients with AMD. Sodium phenylbutyrate preferentially reverses the cell death caused by ARMS2 rs10490924 but not HTRA1 rs11200638. This study serves as a proof of concept for the use of patient-specific iPSCs for functional annotation of tightly linked GWAS to study the etiology of a late-onset disease phenotype. More importantly, we demonstrate that antioxidant administration may be useful for reducing reactive oxidative stress in AMD, a prevalent late-onset neurodegenerative disorder.

Blue Light-Ocular and Systemic Damaging Effects: A Narrative Review

Light is a fundamental aspect of our lives, being involved in the regulation of numerous processes in our body. While blue light has always existed in nature, with the ever-growing number of electronic devices that make use of short wavelength (blue) light, the human retina has seen increased exposure to it. Because it is at the high-energy end of the visible spectrum, many authors have investigated the theoretical harmful effects that it poses to the human retina and, more recently, the human body, given the discovery and characterization of the intrinsically photosensitive retinal ganglion cells. Many approaches have been explored, with the focus shifting throughout the years from examining classic ophthalmological parameters, such as visual acuity, and contrast sensitivity to more complex ones seen on electrophysiological assays and optical coherence tomographies. The current study aims to gather the most recent relevant data, reveal encountered pitfalls, and suggest future directions for studies regarding local and/or systemic effects of blue light retinal exposures.

Singlet Oxygen Quenching by Resveratrol Derivatives

We investigated the singlet oxygen quenching ability of several derivatives of trans-resveratrol which have been reported to have significant antioxidant ability, including photoprotective activity. We measured the total rate constants of singlet oxygen removal (kT ) by the methylated resveratrol derivative 1,3-dimethoxy-5-[(E)-2-(4-methoxyphenyl)ethenyl]benzene, and the partially methylated resveratrol derivatives 4-((E)-2-(3,5-dimethoxyphenyl)ethenyl)phenol (pterostilbene), 5-[(E)-2-(4-methoxyphenyl)ethenyl]benzene-1,3-diol and (2R,3R)-3,5,7-trihydroxy-2-(3,4,5-trihydroxyphenyl)-2,3-dihydrochromen-4-one (dihydromyricetin). A protic solvent system results in higher kT values, except for the completely methylated derivative. We also investigated the ability of trans-resveratrol to directly act as a photosensitizer (rather than via secondary photoproducts resulting from other primary photochemical reactions) for the production of singlet oxygen but found that neither resveratrol nor any of its derivatives are able to do so. We then studied the chemical reactions of the methylated derivative with singlet oxygen. The main pathway consists of a [4 + 2] cycloaddition reaction involving the trans-double bond and the para-substituted benzene ring similar to what has been observed for trans-resveratrol. Unlike trans-resveratrol, the primary singlet oxygen product undergoes a second [4 + 2] cycloaddition with singlet oxygen leading to the formation of diendoperoxides. A second reactivity pathway for both trans-resveratrol and the methylated derivative leads to the formation of aldehydes via cleavage of a transient dioxetane.

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.

Photooxidation of A2E by Blue Light Regulates Heme Oxygenase 1 Expression via NF-κB and Lysine Methyltransferase 2A in ARPE-19 Cells

Background: N-retinylidene-N-retinylethanolamine (A2E) is a component of drusen that accumulates in retinal cells and induces oxidative stress through photooxidation, such as blue light (BL). We found that the heme oxygenase 1 (HMOX1) gene responds sensitively to photooxidation by the BL of A2E in retinal pigment epithelial (RPE) cells, and we sought to identify the transcription factors and coactivators involved in the upregulation of HMOX1 by A2E and BL. Methods: A2E-laden human RPE cells (ARPE-19) were exposed to BL (430 nm). RNA sequencing was performed to identify genes responsive to BL exposure. Chromatin immunoprecipitation and RT-qPCR were performed to determine the regulation of HMOX1 transcription. Clinical transcriptome data were used to evaluate HMOX1 expression in patients with age-related macular degeneration (AMD). Results: In ARPE-19 cells, the expression of HMOX1, one of the NF-κB target genes, was significantly increased by A2E and BL. The binding of RELA and RNA polymerase II to the promoter region of HMOX1 was significantly increased by A2E and BL. Lysine methyltransferase 2A (MLL1) plays an important role in H3K4me3 methylation, NF-κB recruitment, chromatin remodeling at the HMOX1 promoter, and, subsequently, HMOX1 expression. The retinal tissues of patients with late-stage AMD showed significantly increased expression of HMOX1 compared to normal retinal tissues. In addition, the expression levels of MLL1 and HMOX1 in retinal tissues were correlated. Conclusions: Taken together, our results suggest that BL induces HMOX1 expression by activating NF-κB and MLL1 in RPE cells.

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.

Secretory autophagy: a turn key for understanding AMD pathology and developing new therapeutic targets?

INTRODUCTION

Age-related macular degeneration (AMD) is an eye disease leading to vision loss with poorly known pathogenesis and limited therapeutic options. Degradative autophagy (DA) is impaired in AMD, but emerging evidence points to secretary autophagy (SA) as a key element in AMD pathogenesis.

AREAS COVERED

SA may cause the release of proteins and protein aggregates, lipofuscin, beta amyloid, faulty mitochondria, pro-inflammatory and pro-angiogenic factors from the retinal pigment epithelium (RPE) that may contribute to drusen formation and choroidal neovascularization. SA may replace DA, when formation of autolysosome is impaired, and then a harmful cargo, instead of being degraded, is extruded from the RPE contributing to drusen and/or angiogenic environment. Therefore, the interplay between DA and SA may be critical for drusen formation and choroidal neovascularization, so it can be a turn key to understand AMD pathogenesis.

EXPERT OPINION

Although SA fulfills some beneficial functions, it is detrimental for the retina in many cases. Therefore, inhibiting SA may be a therapeutic strategy in AMD, but it is challenged by the development of selective SA inhibitors that would not affect DA. The TRIM16, SEC22B and RAB8A proteins, specific for secretory autophagosome, may be primary candidates as therapeutic targets, but their action is not limited to autophagy and therefore requires further studies.

NMDA Receptor Antagonists Degrade Lipofuscin via Autophagy in Human Retinal Pigment Epithelial Cells

Background and Objectives: Age-related macular degeneration is a slow-progressing disease in which lipofuscin accumulates in the retina, causing inflammation and apoptosis of retinal pigment epithelial (RPE) cells. This study aimed to identify N-methyl-D-aspartate (NMDA) signaling as a novel mechanism for scavenging N-retinylidene-N-retinylethanolamine (A2E), a component of ocular lipofuscin, in human RPE cells. Materials and Methods: A2E degradation assays were performed in ARPE-19 cells using fluorescently labeled A2E. The autophagic activity in ARPE-19 cells was measured upon blue light (BL) exposure, after A2E treatment. Autophagy flux was determined by measuring LC3-II formation using immunoblotting and confocal microscopy. To determine whether autophagy via the NMDA receptor is involved in A2E clearance, ATG5-deficient cells were used. Results: Ro 25-6981, an NR2B-selective NMDA receptor antagonist, effectively cleared A2E. Ro 25-6981 reduced A2E accumulation in the lysosomes of ARPE-19 cells at sub-cytotoxic concentrations, while increasing the formation of LC3-II and decreasing p62 protein levels in a concentration-dependent manner. The autophagic flux monitored by RFP-GFP-LC3 and bafilomycin A1 assays was significantly increased by Ro 25-6981. A2E clearance by Ro 25-6981 was abolished in ATG5-depleted ARPE-19 cells, suggesting that A2E degradation by Ro 25-6981 was mediated by autophagy. Furthermore, treatment with other NMDA receptor antagonists, CP-101,606 and AZD6765, showed similar effects on autophagy activation and A2E degradation in ARPE-19 cells. In contrast, glutamate, an NMDA receptor agonist, exhibited a contrasting effect, suggesting that both the activation of autophagy and the degradation of A2E by Ro 25-6981 in ARPE-19 cells occur through inhibition of the NMDA receptor pathway. Conclusions: This study demonstrates that NMDA receptor antagonists degrade lipofuscin via autophagy in human RPE cells and suggests that NMDA receptor antagonists could be promising new therapeutics for retinal degenerative diseases.

Adherence to the Mediterranean-Style Eating Pattern and Macular Degeneration: A Systematic Review of Observational Studies

Age-related macular degeneration (AMD) is a serious degenerative disease affecting the eyes, and is the main cause of severe vision loss among people >55 years of age in developed countries. Its onset and progression have been associated with several genetic and lifestyle factors, with diet appearing to play a pivotal role in the latter. In particular, dietary eating patterns rich in plant foods have been shown to lower the risk of developing the disease, and to decrease the odds of progressing to more advanced stages in individuals already burdened with early AMD. We systematically reviewed the literature to analyse the relationship between the adherence to a Mediterranean diet, a mainly plant-based dietary pattern, and the onset/progression of AMD. Eight human observational studies were analysed. Despite some differences, they consistently indicate that higher adherence to a Mediterranean eating pattern lowers the odds of developing AMD and decreases the risk of progression to more advanced stages of the disease, establishing the way for preventative measures emphasizing dietary patterns rich in plant-foods.

Molecular Mechanisms Underlying the Therapeutic Role of Vitamin E in Age-Related Macular Degeneration

The eye is particularly susceptible to oxidative stress and disruption of the delicate balance between oxygen-derived free radicals and antioxidants leading to many degenerative diseases. Attention has been called to all isoforms of vitamin E, with α-tocopherol being the most common form. Though similar in structure, each is diverse in antioxidant activity. Preclinical reports highlight vitamin E’s influence on cell physiology and survival through several signaling pathways by activating kinases and transcription factors relevant for uptake, transport, metabolism, and cellular action to promote neuroprotective effects. In the clinical setting, population-based studies on vitamin E supplementation have been inconsistent at times and follow-up studies are needed. Nonetheless, vitamin E’s health benefits outweigh the controversies. The goal of this review is to recognize the importance of vitamin E’s role in guarding against gradual central vision loss observed in age-related macular degeneration (AMD). The therapeutic role and molecular mechanisms of vitamin E’s function in the retina, clinical implications, and possible toxicity are collectively described in the present review.

The Age-Related Macular Degeneration (AMD)-Preventing Mechanism of Natural Products

Age-related macular degeneration (AMD) is related to central visual loss in elderly people and, based on the increment in the percentage of the aging population, the number of people suffering from AMD could increase. AMD is initiated by retinal pigment epithelium (RPE) cell death, finally leading to neovascularization in the macula lutea. AMD is an uncurable disease, but the symptom can be suppressed. The current therapy of AMD can be classified into four types: device-based treatment, anti-inflammatory drug treatment, anti-vascular endothelial growth factor treatment, and natural product treatment. All these therapies have adverse effects, however early AMD therapy used with products has several advantages, as it can prevent RPE cell apoptosis in safe doses. Cell death (apoptosis) is caused by various factors, such as oxidative stress, inflammation, carbonyl stress, and a deficiency in essential components for cells, and RPE cell death is related to oxidative stress, inflammation, and carbonyl stress. Some natural products have anti-oxidative effects, anti-inflammation effects, and/or anti-carbonylation effects. The AMD preventive mechanism of natural products varies, with some natural products activating one or more anti-apoptotic pathways, such as the Nrf2/HO-1 anti-oxidative pathway, the anti-inflammasome pathway, and the anti-carbonyl pathway. As AMD drug candidates from natural products effectively inhibit RPE cell death, they have the potential to be developed as drugs for preventing early (dry) AMD.

The Changes of Irisin and Inflammatory Cytokines in the Age-Related Macular Degeneration and Retinal Vein Occlusion

Purpose

Age-related macular degeneration (AMD) and retinal vein occlusion (RVO) are irreversible chorioretinal diseases, which might induce severe damage in visual function. The metabolic factor and inflammatory factors might play important roles in the pathogenesis of AMD and RVO. The levels of irisin and 14 cytokines were analyzed in aqueous humor of AMD and RVO eyes to evaluate the roles of irisin and inflammatory factors.

Methods

We collected aqueous humor samples from patients with AMD (n = 27), RVO (n = 30), and cataract (as control, n = 23) eyes. Samples were assayed using ELISA kit for irisin and a multiplex immunoassay kit for 14 cytokines. The macular thickness (MT) was measured with OCT in all included eyes.

Results

MT in the RVO group is significantly higher than that in the AMD or control group. Irisin levels in the aqueous samples of AMD and RVO eyes were both significantly lower than that in the control. Furthermore, a positive correlation was found between irisin and MT in the RVO. Compared with the controls, AMD eyes had significantly higher levels of BDNF, VEGF-A, VEGF-R1, VEGF-R2, IL-10, TNF-α, VCAM-1, IP-10, and MCP-1. Similarly, RVO eyes had significantly higher levels of BDNF, VEGF-A, VEGF-R1, VEGF-R2, IL-6, IL-8, IL-10, TNF-α, ICAM-1, VCAM-1, IP-10, and MCP-1. However, there was no significant difference between the levels of PDGF-BB or TNF-β in these three groups. A negative correlation was found between VEGF-A and MT in AMD, as well as in control. Furthermore, a positive correlation was found between IL-6 and MT in the 80 included eyes, as well as in RVO. A positive correlation was found between ICAM-1 and MT in the 80 included eyes, as well as in RVO.

Conclusions

The metabolic factor, irisin levels in the aqueous humor are decreased in AMD and RVO eyes and show a positive correlation between irisin and MT in RVO eyes, prompting researchers to explore the relationship between irisin and macular edema. We also identified the higher expression of vascular growth factors (VEGF-A, VEGF-R1, and PDGF-BB), inflammatory cytokines (IL-6, IL-8, IL-10, and TNF-α), and chemokines (ICAM-1, VCAM-1, IP-10, and MCP-1) in AMD and RVO eyes.

Lipofuscin Granule Bisretinoid Oxidation in the Human Retinal Pigment Epithelium forms Cytotoxic Carbonyls

Age-related macular degeneration (AMD) is the primary cause of central blindness among the elderly. AMD is associated with progressive accumulation of lipofuscin granules in retinal pigment epithelium (RPE) cells. Lipofuscin contains bisretinoid fluorophores, which are photosensitizers and are phototoxic to RPE and neuroretinal cells. In the presence of oxygen, bisretinoids are also oxidized, forming various products, consisting primarily of aldehydes and ketones, which are also potentially cytotoxic. In a prior study, we identified that in AMD, bisretinoid oxidation products are increased in RPE lipofuscin granules. The purpose of the present study was to determine if these products were toxic to cellular structures. The physicochemical characteristics of bisretinoid oxidation products in lipofuscin, which were obtained from healthy donor eyes, were studied. Raman spectroscopy and time-of-flight secondary ion mass spectrometry (ToF–SIMS) analysis identified the presence of free-state aldehydes and ketones within the lipofuscin granules. Together, fluorescence spectroscopy, high-performance liquid chromatography, and mass spectrometry revealed that bisretinoid oxidation products have both hydrophilic and amphiphilic properties, allowing their diffusion through lipofuscin granule membrane into the RPE cell cytoplasm. These products contain cytotoxic carbonyls, which can modify cellular proteins and lipids. Therefore, bisretinoid oxidation products are a likely aggravating factor in the pathogenesis of AMD.

Vitamin A cycle byproducts explain retinal damage and molecular changes thought to initiate retinal degeneration

In the most prevalent retinal diseases, including Stargardt disease and age-related macular degeneration (AMD), byproducts of vitamin A form in the retina abnormally during the vitamin A cycle. Despite evidence of their toxicity, whether these vitamin A cycle byproducts contribute to retinal disease, are symptoms, beneficial, or benign has been debated. We delivered a representative vitamin A byproduct, A2E, to the rat's retina and monitored electrophysiological, histological, proteomic, and transcriptomic changes. We show that the vitamin A cycle byproduct is sufficient alone to damage the RPE, photoreceptor inner and outer segments, and the outer plexiform layer, cause the formation of sub-retinal debris, alter transcription and protein synthesis, and diminish retinal function. The presented data are consistent with the theory that the formation of vitamin A byproducts during the vitamin A cycle is neither benign nor beneficial but may be sufficient alone to cause the most prevalent forms of retinal disease. Retarding the formation of vitamin A byproducts could potentially address the root cause of several retinal diseases to eliminate the threat of irreversible blindness for millions of people.

Summary: During the vitamin A cycle, byproducts of vitamin A form in the eye. Using a rat model, we show that the byproducts alone can explain several retinal derangements observed in the prodromal phase of human retinal disease. Retarding the formation of these byproducts may address the root cause of the most prevalent retinal diseases.

Medication Trends for Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is central vision loss with aging, was the fourth main cause of blindness in 2015, and has many risk factors, such as cataract surgery, cigarette smoking, family history, hypertension, obesity, long-term smart device usage, etc. AMD is classified into three categories: normal AMD, early AMD, and late AMD, based on angiogenesis in the retina, and can be determined by bis-retinoid N-retinyl-N-retinylidene ethanolamine (A2E)-epoxides from the reaction of A2E and blue light. During the reaction of A2E and blue light, reactive oxygen species (ROS) are synthesized, which gather inflammatory factors, induce carbonyl stress, and finally stimulate the death of retinal pigment epitheliums (RPEs). There are several medications for AMD, such as device-based therapy, anti-inflammatory drugs, anti-VEGFs, and natural products. For device-based therapy, two methods are used: prophylactic laser therapy (photocoagulation laser therapy) and photodynamic therapy. Anti-inflammatory drugs consist of corticosteroids and non-steroidal anti-inflammatory drugs (NSAIDs). Anti-VEGFs are classified antibodies for VEGF, aptamer, soluble receptor, VEGF receptor-1 and -2 antibody, and VEGF receptor tyrosine kinase inhibitor. Finally, additional AMD drug candidates are derived from natural products. For each medication, there are several and severe adverse effects, but natural products have a potency as AMD drugs, as they have been used as culinary materials and/or traditional medicines for a long time. Their major application route is oral administration, and they can be combined with device-based therapy, anti-inflammatory drugs, and anti-VEGFs. In general, AMD drug candidates from natural products are more effective at treating early and intermediate AMD. However, further study is needed to evaluate their efficacy and to investigate their therapeutic mechanisms.

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.

The Use of Fundus Autofluorescence in Dry Age-Related Macular Degeneration

Fundus autofluorescence (FAF) has been a well-known imaging method for quite some time. However, with developing technologies and novel imaging devices, FAF is being used more often to diagnose and monitor retinal diseases. The density of lipofuscin (LF) and other fluorophores in the retina have a determining role in FAF images. In dry age-related macular degeneration (AMD), hyperautofluorescence is seen in cases of increasing LF in the retina pigment epithelium, whereas hypoautofluorescence is detected in decreasing LF resulting from geographic atrophy. In recent years, studies have shown that FAF images provide prognostic information in patients with AMD. This review aims to highlight the importance of FAF imaging in dry AMD.

Inherited retinal diseases: Therapeutics, clinical trials and end points-A review

Inherited retinal diseases (IRDs) are a clinically and genetically heterogeneous group of disorders characterised by photoreceptor degeneration or dysfunction. These disorders typically present with severe vision loss that can be progressive, with disease onset ranging from congenital to late adulthood. The advances in genetics, retinal imaging and molecular biology, have conspired to create the ideal environment for establishing treatments for IRDs, with the first approved gene therapy and the commencement of multiple clinical trials. The scope of this review is to familiarise clinicians and scientists with the current management and the prospects for novel therapies for: (1) macular dystrophies, (2) cone and cone-rod dystrophies, (3) cone dysfunction syndromes, (4) Leber congenital amaurosis, (5) rod-cone dystrophies, (6) rod dysfunction syndromes and (7) chorioretinal dystrophies. We also briefly summarise the investigated end points for the ongoing trials.

Solanum melongena L. Extract Protects Retinal Pigment Epithelial Cells from Blue Light-Induced Phototoxicity in In Vitro and In Vivo Models

N-retinylidene-N-retinylethanolamine (A2E) accumulation in the retina is a prominent marker of retinal degenerative diseases. Blue light exposure is considered as an important factor contributing to dry age-related macular degeneration (AMD). Eggplant and its constituents have been shown to confer health benefits, but their therapeutic effects on dry AMD remain incompletely understood. In this study, we showed that an extract of Solanum melongena L. (EPX) protected A2E-laden ARPE-19 cells against blue light-induced cell death via attenuating reactive oxygen species. Transcriptomic analysis demonstrated that blue light modulated the expression of genes associated with stress response, inflammation, and cell death, and EPX suppressed the inflammatory pathway induced by blue light in A2E-laden ARPE-19 cells by inhibiting the nuclear translocation of nuclear factor kappa B and transcription of pro-inflammatory genes (CXCL8 and IL1B). The degradation of intracellular A2E was considered the major mechanism underlying the protective effect of EPX. Moreover, chlorogenic acid isolated from EPX exerted protective effects against blue light-induced cell damage in A2E-laden ARPE-19 cells. In vivo, EPX administration in BALB/c mice reduced the fundus damage and degeneration of the retinal layer in a blue light-induced retinal damage model. Collectively, our findings suggest the potential role of Solanum melongena L. extract for AMD treatment.

New lipophenols prevent carbonyl and oxidative stresses involved in macular degeneration

Dry age-related macular degeneration and Stargardt disease undergo a known toxic mechanism caused by carbonyl and oxidative stresses (COS). This is responsible for accumulation in the retinal pigment epithelium (RPE) of A2E, a main toxic pyridinium bis-retinoid lipofuscin component. Previous studies have shown that carbonyl stress in retinal cells could be reduced by an alkyl-phloroglucinol-DHA conjugate (lipophenol). Here, we performed a rational design of different families of lipophenols to conserve anti-carbonyl stress activities and improve antioxidant properties. Five synthetic pathways leading to alkyl-(poly)phenol derivatives, with phloroglucinol, resveratrol, catechin and quercetin as the main backbone, linked to poly-unsaturated fatty acid, are presented. These lipophenols were evaluated in ARPE-19 cell line for their anti-COS properties and a structure-activity relationship study is proposed. Protection of ARPE-19 cells against A2E toxicity was assessed for the four best candidates. Finally, interesting anti-COS properties of the most promising quercetin lipophenol were confirmed in primary RPE cells.

Not All Stressors Are Equal: Mechanism of Stressors on RPE Cell Degeneration

Age-related macular degeneration (AMD) is a major cause of irreversible blindness among the elderly population. Dysfunction and degeneration of the retinal pigment epithelial (RPE) layer in the retina underscore the pathogenesis of both dry and wet AMD. Advanced age, cigarette smoke and genetic factors have been found to be the prominent risk factors for AMD, which point to an important role for oxidative stress and aging in AMD pathogenesis. However, the mechanisms whereby oxidative stress and aging lead to RPE cell degeneration are still unclear. As cell senescence and cell death are the major outcomes from oxidative stress and aging, here we review the mechanisms of RPE cell senescence and different kinds of cell death, including apoptosis, necroptosis, pyroptosis, ferroptosis, with an aim to clarify how RPE cell degeneration could occur in response to AMD-related stresses, including H₂O₂, 4-Hydroxynonenal (4-HNE), N-retinylidene-N-retinyl-ethanolamine (A2E), Alu RNA and amyloid β (Aβ). Besides those, sodium iodate (NaIO₃) induced RPE cell degeneration is also discussed in this review. Although NaIO₃ itself is not related to AMD, this line of study would help understand the mechanism of RPE degeneration.

Complement activation, lipid metabolism, and mitochondrial injury: Converging pathways in age-related macular degeneration

The retinal pigment epithelium (RPE) is the primary site of injury in non-neovascular age-related macular degeneration or dry AMD. Polymorphisms in genes that regulate complement activation and cholesterol metabolism are strongly associated with AMD, but the biology underlying disease-associated variants is not well understood. Here, we highlight recent studies that have used molecular, biochemical, and live-cell imaging methods to elucidate mechanisms by which aging-associated insults conspire with AMD genetic risk variants to tip the balance towards disease. We discuss how critical functions including lipid metabolism, autophagy, complement regulation, and mitochondrial dynamics are compromised in the RPE, and how a deeper understanding of these mechanisms has helped identify promising therapeutic targets to preserve RPE homeostasis in AMD.

Suppressive Effect of Arctium Lappa L. Leaves on Retinal Damage Against A2E-Induced ARPE-19 Cells and Mice

Age-related macular degeneration (AMD) is a major cause of irreversible loss of vision with 80–90% of patients demonstrating dry type AMD. Dry AMD could possibly be prevented by polyphenol-rich medicinal foods by the inhibition of N-retinylidene-N-retinylethanolamine (A2E)-induced oxidative stress and cell damage. Arctium lappa L. (AL) leaves are medicinal and have antioxidant activity. The purpose of this study was to elucidate the protective effects of the extract of AL leaves (ALE) on dry AMD models, including in vitro A2E-induced damage in ARPE-19 cells, a human retinal pigment epithelial cell line, and in vivo light-induced retinal damage in BALB/c mice. According to the total phenolic contents (TPCs), total flavonoid contents (TFCs) and antioxidant activities, ALE was rich in polyphenols and had antioxidant efficacies on 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP), and 2′,7′-dichlorofluorescin diacetate (DCFDA) assays. The effects of ALE on A2E accumulation and A2E-induced cell death were also monitored. Despite continued exposure to A2E (10 μM), ALE attenuated A2E accumulation in APRE-19 cells with levels similar to lutein. A2E-induced cell death at high concentration (25 μM) was also suppressed by ALE by inhibiting the apoptotic signaling pathway. Furthermore, ALE could protect the outer nuclear layer (ONL) in the retina from light-induced AMD in BALB/c mice. In conclusion, ALE could be considered a potentially valuable medicinal food for dry AMD.

Fundus autofluorescence, spectral-domain optical coherence tomography, and histology correlations in a Stargardt disease mouse model

Stargardt disease (STGD1), known as inherited retinal dystrophy, is caused by ABCA4 mutations. The pigmented Abca4-/- mouse strain only reflects the early stage of STGD1 since it is devoid of retinal degeneration. This blue light-illuminated pigmented Abca4-/- mouse model presented retinal pigment epithelium (RPE) and photoreceptor degeneration which was similar to the advanced STGD1 phenotype. In contrast, wild-type mice showed no RPE degeneration after blue light illumination. In Abca4-/- mice, the acute blue light diminished the mean autofluorescence (AF) intensity in both fundus short-wavelength autofluorescence (SW-AF) and near-infrared autofluorescence (NIR-AF) modalities correlating with reduced levels of bisretinoid-fluorophores. Blue light-induced RPE cellular damage preceded the photoreceptors loss. In late-stage STGD1-like patient and blue light-illuminated Abca4-/- mice, lipofuscin and melanolipofuscin granules were found to contribute to NIR-AF, indicated by the colocalization of lipofuscin-AF and NIR-AF under the fluorescence microscope. In this mouse model, the correlation between in vivo and ex vivo assessments revealed histological characteristics of fundus AF abnormalities. The flecks which are hyper AF in both SW-AF and NIR-AF corresponded to the subretinal macrophages fully packed with pigment granules (lipofuscin, melanin, and melanolipofuscin). This mouse model, which has the phenotype of advanced STGD1, is important to understand the histopathology of Stargardt disease.

Senescence in the pathogenesis of age-related macular degeneration

Age-related macular degeneration (AMD) is a complex eye disease underlined by the death of photoreceptors and degeneration of retinal pigment epithelium (RPE) and choriocapillaris (CC). The mechanism(s) responsible for massive and progressive retinal degeneration is not completely known. Senescence, a state of permanent inhibition of cell growth, may be induced by many factors important for AMD pathogenesis and results in senescence-associated secretory phenotype (SASP) that releases growth factors, cytokines, chemokines, proteases and other molecules inducing inflammation and other AMD-related effects. These effects can be induced in the affected cell and neighboring cells, leading to progression of AMD phenotype. Senescent cells also release reactive oxygen species that increase SASP propagation. Many other pathways of senescence-related AMD pathogenesis, including autophagy, the cGAS-STING signaling, degeneration of CC by membrane attack complex, can be considered. A2E, a fluorophore present in lipofuscin, amyloid-beta peptide and humanin, a mitochondria-derived peptide, may link AMD with senescence. Further studies on senescence in AMD pathogenesis to check the possibility of opening a perspective of the use of drugs killing senescent cells (senolytics) and terminating SASP bystander effects (senostatics) might be beneficial for AMD that at present is an incurable disease.

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 value of nutritional supplements in treating Age-Related Macular Degeneration: a review of the literature

PURPOSE

To describe and evaluate the value of nutritional supplements in the management of age-related macular degeneration (AMD) through a review of the current literature.

METHODS

An extensive literature search was performed, and key research articles exploring AREDS and AREDS-2 formulations, genetics, omega fatty acids, calcium and folic acid in high-risk women were reviewed. PubMed and Web of Science databases were used for generating articles to review.

RESULTS

The AREDS and AREDS-2 trials, while difficult to validate, show support for antioxidant supplementation in reducing AMD progression in Caucasian populations. While genetic guided personalized medicine has been studied mainly with complement factor H and age-related maculopathy susceptibility 2 risk alleles, the data have not been reproducible. Women at a higher risk of cardiovascular disease may benefit from antioxidant therapies in preventing AMD. Omega 3 fatty acid supplementation has been widely supported through observational studies; however, randomized controlled trials have not shown benefit in disease progression. Calcium exposure has been linked to increased mechanisms in cell death and may be detrimental to older individuals with AMD.

CONCLUSION

The data regarding nutritional supplements in preventing AMD progression are inconclusive, and therefore recommendations should be based on risk factors and demographic data.

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.

Progressive cone and cone-rod dystrophies: clinical features, molecular genetics and prospects for therapy

Progressive cone and cone-rod dystrophies are a clinically and genetically heterogeneous group of inherited retinal diseases characterised by cone photoreceptor degeneration, which may be followed by subsequent rod photoreceptor loss. These disorders typically present with progressive loss of central vision, colour vision disturbance and photophobia. Considerable progress has been made in elucidating the molecular genetics and genotype-phenotype correlations associated with these dystrophies, with mutations in at least 30 genes implicated in this group of disorders. We discuss the genetics, and clinical, psychophysical, electrophysiological and retinal imaging characteristics of cone and cone-rod dystrophies, focusing particularly on four of the most common disease-associated genes: GUCA1A, PRPH2, ABCA4 and RPGR Additionally, we briefly review the current management of these disorders and the prospects for novel therapies.

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.

Light-induced generation and toxicity of docosahexaenoate-derived oxidation products in retinal pigmented epithelial cells

Oxidative cleavage of docosahexaenoate (DHA) in retinal pigmented epithelial (RPE) cells produces 4-hydroxy-7-oxohept-5-enoic acid (HOHA) esters of 2-lysophosphatidylcholine (PC). HOHA-PC spontaneously releases a membrane-permeant HOHA lactone that modifies primary amino groups of proteins and ethanolamine phospholipids to produce 2-(ω-carboxyethyl)pyrrole (CEP) derivatives. CEPs have significant pathological relevance to age-related macular degeneration (AMD) including activation of CEP-specific T-cells leading to inflammatory M1 polarization of macrophages in the retina involved in "dry AMD" and TLR2-dependent induction of angiogenesis that characterizes "wet AMD". RPE cells accumulate DHA from shed rod photoreceptor outer segments through phagocytosis and from plasma lipoproteins secreted by the liver through active uptake from the choriocapillaris. As a cell model of light-induced oxidative damage of DHA phospholipids in RPE cells, ARPE-19 cells were supplemented with DHA, with or without the lipofuscin fluorophore A2E. In this model, light exposure, in the absence of A2E, promoted the generation HOHA lactone-glutathione (GSH) adducts, depletion of intracellular GSH and a competing generation of CEPs. While DHA-rich RPE cells exhibit an inherent proclivity toward light-induced oxidative damage, photosensitization by A2E nearly doubled the amount of lipid oxidation and expanded the spectral range of photosensitivity to longer wavelengths. Exposure of ARPE-19 cells to 1 μM HOHA lactone for 24 h induced massive (50%) loss of lysosomal membrane integrity and caused loss of mitochondrial membrane potential. Using senescence-associated β-galactosidase (SA β-gal) staining that detects lysosomal β-galactosidase, we determined that exposure to HOHA lactone induces senescence in ARPE-19 cells. The present study shows that products of light-induced oxidative damage of DHA phospholipids in the absence of A2E can lead to RPE cell dysfunction. Therefore, their toxicity may be especially important in the early stages of AMD before RPE cells accumulate lipofuscin fluorophores.

Ultrastructural alterations in the retinal pigment epithelium and photoreceptors of a Stargardt patient and three Stargardt mouse models: indication for the central role of RPE melanin in oxidative stress

Background

Stargardt disease (SD) is characterized by the accumulation of the age-pigment lipofuscin in the retinal pigment epithelium (RPE) and subsequent neuroretinal degeneration. The disease leads to vision loss early in life. Here, we investigate age-dependent ultrastructural changes in three SD mouse models: albino Abca4^-/- and pigmented Abca4^-/- and Abca4^-/-.Rdh8^-/- mice. Since we found indications for oxidative stress primarily in albino SD mice, we tested RPE melanin for its antioxidative capabilities.

Methods

SD mouse eyes were investigated by light, fluorescence and electron microscopy and were compared to the respective albino and pigmented wild type mice and to a human donor SD eye. To confirm the role of RPE melanin in scavenging oxidative stress, melanin from S. officinalis as a standard and porcine RPE were tested for their capability to quench superoxide anions.

Results

Histological alterations indicative of oxidative stress and/or lysosomal dysfunction were present in albino Abca4^-/- and Abca4^-/-.Rdh8^-/- mice. Retinal damage, such as inner segment rupture and pyknotic or free photoreceptor nuclei in the subretinal space and RPE vacuolization were exclusively found in albino Abca4^-/- mice. Shortened and disorganized photoreceptor outer segments and dead RPE cells were found in albino Abca4^-/- and Abca4^-/-.Rdh8^-/- mice, with earlier onset in albino Abca4^-/- mice. Undegraded phagosomes and lipofuscin accumulation were present in the RPE of all three SD strains, but numbers were highest in Abca4^-/-.Rdh8^-/- mice. Lipofuscin morphology differed between SD strains: (melano-)lipofuscin granules in pigmented Abca4^-/- mice had a homogenous electron density and sharp demarcations, while lipofuscin in albino Abca4^-/- mice had a flocculent electron density and often lacked a surrounding membrane, indicating loss of lysosomal integrity. Young Abca4^-/-.Rdh8^-/- mice showed (melano-)lipofuscin granules with homogenous electron density, while in aged animals granules with flocculent electron density predominated. Both strains of pigmented SD mice had melanolipofuscin clusters as found in the human SD eye. Like melanin from S. officinalis, porcine RPE melanin can also quench superoxide anions.

Discussion

The presented pathologies in albino Abca4^-/- and Abca4^-/-.Rdh8^-/- mice suggest oxidative stress and/or lysosomal dysfunction within the RPE. Since albino Abca4^-/- mice have the earliest onset and severest damage and as absence of melanin and also melanin turnover with age are known to diminish RPEs anti-oxidative properties, we assume that RPE melanin plays a role in SD related damages. A lack of pathology in pigmented Abca4^-/- mice due to lower stress levels as compared to the Abca4^-/-.Rdh8^-/- mice underlines this hypothesis. It is also supported by the finding that RPE melanin can quench superoxide anions. We therefore suppose that RPE melanin is important in retinal health and we discuss its role as an oxidative stress scavenger.

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.

Photosensitization of A2E triggers telomere dysfunction and accelerates retinal pigment epithelium senescence

Age-related macular degeneration (AMD) is the leading cause of irreversible vision loss in elderly people. AMD is classified as early, intermediate, advanced non-neovascular, and advanced neovascular forms depending on the clinical features. However, the exact pathogenesis remains unclear. Retinal pigment epithelium (RPE) cells degeneration is a hallmark of AMD. With aging, lipofuscin accumulates in RPE cells. N-retinylidene-N-retinylethanolamine (named A2E), a well-known fluorophore of lipofuscin, may contribute to RPE cells degeneration. In this study, we showed that photosensitization of A2E increased DNA damage, including telomere deprotection and deletion, and triggered cellular senescence. In addition, we found that the antioxidant N-acetyl-cysteine (NAC) partially alleviated this DNA damage. Telomerase overexpression rescued A2E-mediated RPE cell senescence, indicating that telomere dysfunction plays an important role in A2E-based senescence. We further showed that the senescence induced by A2E photosensitization may affect the microenvironment of the retina by expressing several factors of the secretory phenotype (SASP) including IL1B, IL13RA2, and CXCR4 through the NF-κB pathway. We propose that expression of these factors create a pro-inflammatory environment that drives retina degeneration. Moreover, our findings suggest that protecting telomeres is a valuable strategy for treating retinal degeneration diseases, such as AMD.

A2E-associated cell death and inflammation in retinal pigmented epithelial cells from human induced pluripotent stem cells

Accumulation of lipofuscin in the retinal pigmented epithelium (RPE) is observed in retinal degenerative diseases including Stargardt disease and age-related macular degeneration. Bis-retinoid N-retinyl-N-retinylidene ethanolamine (A2E) is a major component of lipofuscin. A2E has been implicated in RPE atrophy and retinal inflammation; however, mice with A2E accumulation display only a mild retinal phenotype. In the current study, human iPSC-RPE (hiPSC-RPE) cells were generated from healthy individuals to examine effects of A2E in human RPE cells. hiPSC-RPE cells displayed RPE-specific features, which include expression of RPE-specific genes, tight junction formation and ability to carry out phagocytosis. hiPSC-RPE cells demonstrated cell death and increased VEGF-A production in a time-dependent manner when they were cocultured with 10 μM of A2E. PCR array analyses revealed upregulation of 26 and 12 pro-inflammatory cytokines upon A2E and H₂O₂ exposure respectively, indicating that A2E and H₂O₂ can cause inflammation in human retinas. Notably, identified gene profiles were different between A2E- and H₂O₂-treated hiPSC-RPE cells. A2E caused inflammatory changes observed in retinal degenerative diseases more closely as compared to H₂O₂. Collectively, these data obtained with hiPSC-RPE cells provide evidence that A2E plays an important role in pathogenesis of retinal degenerative diseases in humans.

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.

Quercetin and cyanidin-3-glucoside protect against photooxidation and photodegradation of A2E in retinal pigment epithelial cells

A family of photoreactive retinaldehyde-derived molecules accumulate in retinal pigment epithelial cells with age; this accumulation is implicated in some retinal diseases. One of these compounds is the diretinal fluorophore A2E. Here we compared polyphenols for their ability to suppress the photooxidation and photodegradation of A2E. In cells that had accumulated A2E and were irradiated with short-wavelength light, quercetin, cyanidin-3-glucoside, ferulic acid and chlorogenic acid diminished cellular levels of reactive oxygen species, but only quercetin and cyanidin-3-glucoside promoted cell viability. By chromatographic quantitation, quercetin and cyanidin-3-glucoside reduced the consumption of A2E by photooxidation in both cell- and cell-free assays. With ultra-high performance liquid chromatography-mass spectrometry, quercetin and cyanidin-3-glucoside also inhibited the formation of photooxidized-A2E species. While photodegradation of A2E is known to result in the release of reactive carbonyls, we demonstrated that quercetin and cyanidin-3-glucoside decreased the formation of methylglyoxal adducts in the cells, and reduced the expression of mRNA encoding receptor for advanced glycation end products. These polyphenols also protected glutathione from reaction with photooxidized A2E. In rod outer segments incubated with all-trans-retinal to generate bisretinoid, followed by irradiation, quercetin and cyanidin-3-glucoside reduced release of the lipid peroxidation product 4-hydroxynonenal. In conclusion, quercetin and cyanidin-3-glucoside can guard against photooxidative processes in retina.