Vitamin A-aldehyde adducts: AMD risk and targeted therapeutics

Chemometric profiling and anti-arthritic activity of aerial parts of Glinus oppositifolius (L.) Aug. DC

ETHNOPHARMACOLOGICAL RELEVANCE

Glinus oppositifolius (L.) Aug. DC. belongs to the family Molluginaceae, an annual prostrate herb traditionally used to treat inflammations, arthritis, malarial, wounds, fevers, diarrhoea, cancer, stomach discomfort, jaundice, and intestinal parasites. However, the anti-arthritic activity of the aerial part has still not been reported.

AIM OF THE STUDY

To investigate the antioxidant and anti-arthritic activity of G. oppositifolius in Complete Freund's Adjuvant (CFA) induced rats.

MATERIALS AND METHODS

The dried aerial parts of this plant material were defatted with n-hexane and extracted by methanol using a soxhlet apparatus. The in vitro anti-arthritic activity of methanolic extract of G. oppositifolius (MEGO) was evaluated in protein denaturation, membrane stabilization, and inhibition of proteinase assay at 25, 50, 100, 200, and 400 μg/ml concentrations. Female Wistar rats were immunized sub-dermally into the right hind paw with 0.1 ml of CFA. Rats were administered with MEGO at doses of 200 and 400 mg/kg once daily for fourteen days after arthritis induction. Assessment of arthritis was performed by measuring paw diameter, arthritic index, arthritic score, body weight, organ weight, and hematological and biochemical parameters, followed by the analysis of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin 6 (IL-6), interleukin-1-beta (IL-1β), cyclooxygenase-2 (COX-2), interleukin 13 (IL-13) and interleukin 10 (IL-10) and histopathological study. In vivo antioxidant effect was investigated in enzymatic assays. The presence of phytoconstituents was analyzed by Gas Chromatography-Mass Spectrometry (GC-MS) and Liquid Chromatography-Mass Spectrometry (LC-MS), respectively. In silico molecular docking study of the compounds was carried out against COX-2, IL-1β, IL-6, and TNF-α using AutoDock 4.2 and BIOVIA-Discovery Studio Visualizer software.

RESULTS

MEGO's in vitro anti-arthritic activity showed dose-dependent inhibition of protein denaturation, membrane stabilization, and proteinase inhibition, followed by significant in vivo anti-arthritic activity. The rats treated with MEGO showed tremendous potential in managing arthritis-like symptoms by restoring hematological, biochemical, and histological changes in CFA-induced rats. MEGO (200 and 400 mg/kg) showed a significant alleviation in the levels of hyper expressed inflammatory mediators (TNF-α, IL-1β, and IL-6) and oxidative stress (SOD, CAT, GSH, and LPO) in CFA-induced rats. Spergulagenin-A as identified by LC-MS analysis, exhibited the highest binding affinity against COX-2 (-8.6), IL-1β (7.2 kcal/mol), IL-6 (-7.4 kcal/mol), and TNF-α (-6.5 kcal/mol).

CONCLUSIONS

Provided with the comprehensive investigation, methanolic extract of G. oppositifolius against arthritic-like condition is a proof of concept that revalidates its ethnic claim. The presence of Spergulagenin-A might be responsible for the anti-arthritic activity.

AI-driven discovery of blood xenobiotic biomarkers in neovascular age-related macular degeneration using iterative random forests

PURPOSE

To investigate the xenobiotic profiles of patients with neovascular age-related macular degeneration (nAMD) undergoing anti-vascular endothelial growth factor (anti-VEGF) intravitreal therapy (IVT) to identify biomarkers indicative of clinical phenotypes through advanced AI methodologies.

METHODS

In this cross-sectional observational study, we analyzed 156 peripheral blood xenobiotic features in a cohort of 46 nAMD patients stratified by choroidal neovascularization (CNV) control under anti-VEGF IVT. We employed Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) for measurement and leveraged an AI-driven iterative Random Forests (iRF) approach for robust pattern recognition and feature selection, aligning molecular profiles with clinical phenotypes.

RESULTS

AI-augmented iRF models effectively refined the metabolite spectrum by discarding non-predictive elements. Perfluorooctanesulfonate (PFOS) and Ethyl β-glucopyranoside were identified as significant biomarkers through this process, associated with various clinically relevant phenotypes. Unlike single metabolite classes, drug metabolites were distinctly correlated with subretinal fluid presence.

CONCLUSIONS

This study underscores the enhanced capability of AI, particularly iRF, in dissecting complex metabolomic data to elucidate the xenobiotic landscape of nAMD and environmental impact on the disease. The preliminary biomarkers discovered offer promising directions for personalized treatment strategies, although further validation in broader cohorts is essential for clinical application.

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.

Does senescence play a role in age-related macular degeneration?

Advanced age is the most established risk factor for developing age-related macular degeneration (AMD), one of the leading causes of visual impairment in the elderly, in Western and developed countries. Similarly, after middle age, there is an exponential increase in pathologic molecular and cellular events that can induce senescence, traditionally defined as an irreversible loss of the cells' ability to divide and most recently reported to also occur in select post-mitotic and terminally differentiated cells, such as neurons. Together these facts raise the question as to whether or not cellular senescence, may play a role in the development of AMD. A number of studies have reported the effect of ocular-relevant inducers of senescence using primarily in vitro models of poorly polarized, actively dividing retinal pigment epithelial (RPE) cell lines. However, in interpretating the data, the fidelity of these culture models to the RPE in vivo, must be considered. Fewer studies have explored the presence and/or impact of senescent cells in in vivo models that present with phenotypic features of AMD, leaving this an open field for further investigation. The goal of this review is to discuss current thoughts on the potential role of senescence in AMD development and progression, with consideration of the model systems used and their relevance to human disease.

Mice Lacking the Systemic Vitamin A Receptor RBPR2 Show Decreased Ocular Retinoids and Loss of Visual Function

Simple Summary

This work represents an initial evaluation of the second RBP4-vitamin A receptor RBPR2 in a mammalian model. We provide evidence that the membrane localized RBPR2 protein, under variable conditions of dietary vitamin A intake, plays an important role for dietary vitamin A transport to the eye for ocular retinoid homeostasis and visual function. These findings are of general interest, as disturbances in blood and ocular vitamin A homeostasis are linked to retinal degenerative diseases, which are blinding diseases. The animal model described here could also serve as an in vivo tool to study mechanisms related to retinal cell degeneration that are associated with vitamin A deficiency.

Abstract

The systemic transport of dietary vitamin A/all-trans retinol bound to RBP4 into peripheral tissues for storage is an essential physiological process that continuously provides visual chromophore precursors to the retina under fasting conditions. This mechanism is critical for phototransduction, photoreceptor cell maintenance and survival, and in the support of visual function. While the membrane receptor STRA6 facilitates the blood transport of lipophilic vitamin A into the eye, it is not expressed in most peripheral organs, which are proposed to express a second membrane receptor for the uptake of vitamin A from circulating RBP4. The discovery of a novel vitamin A receptor, RBPR2, which is expressed in the liver and intestine, but not in the eye, alluded to this long-sort non-ocular membrane receptor for systemic RBP4-ROL uptake and transport. We have previously shown in zebrafish that the retinol-binding protein receptor 2 (Rbpr2) plays an important role in the transport of yolk vitamin A to the eye. Mutant rbpr2 zebrafish lines manifested in decreased ocular retinoid concentrations and retinal phenotypes. To investigate a physiological role for the second vitamin A receptor, RBPR2, in mammals and to analyze the metabolic basis of systemic vitamin A transport for retinoid homeostasis, we established a whole-body Rbpr2 knockout mouse (Rbpr2^−/−) model. These mice were viable on both vitamin A-sufficient and -deficient diets. Rbpr2^−/− mice that were fed a vitamin A-sufficient diet displayed lower ocular retinoid levels, decreased opsins, and manifested in decrease visual function, as measured by electroretinography. Interestingly, when Rbpr2^−/− mice were fed a vitamin A-deficient diet, they additionally showed shorter photoreceptor outer segment phenotypes, altogether manifesting in a significant loss of visual function. Thus, under conditions replicating vitamin A sufficiency and deficiency, our analyses revealed that RBPR2-mediated systemic vitamin A transport is a regulated process that is important for vitamin A delivery to the eye when RBP4-bound ROL is the only transport pathway in the fasting condition or under vitamin A deficiency conditions.

Discovery of phenylpyrrolidine derivatives as a novel class of retinol binding protein 4 (RBP4) reducers

Retinol-binding protein 4 (RBP4) is a potential drug target for metabolic and ophthalmologic diseases. A high-throughput screening of our compound library has identified a small-molecule RBP4 reducer 7a, as a hit compound. Aiming to provide a suitable tool for investigating the pharmacological effects of RBP4 reducers, we conducted a structure-activity relationship study of 7a. Exploration of the aryl head, oxazole core, and propanoic acid tail of 7a resulted in the discovery of novel, potent, and orally available phenylpyrrolidine derivatives 43b and 43c. Compound 43b had a potent and long-lasting blood RBP4-level-reducing effect when orally administered to mice at a dose as low as 0.3 mg/kg.

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.

Potential Therapeutic Candidates for Age-Related Macular Degeneration (AMD)

Aging contributes to the risk of development of ocular diseases including, but not limited to, Age-related Macular Degeneration (AMD) that is a leading cause of blindness in the United States as well as worldwide. Retinal aging, that contributes to AMD pathogenesis, is characterized by accumulation of drusen deposits, alteration in the composition of Bruch’s membrane and extracellular matrix, vascular inflammation and dysregulation, mitochondrial dysfunction, and accumulation of reactive oxygen species (ROS), and subsequent retinal pigment epithelium (RPE) cell senescence. Since there are limited options available for the prophylaxis and treatment of AMD, new therapeutic interventions are constantly being looked into to identify new therapeutic targets for AMD. This review article discusses the potential candidates for AMD therapy and their known mechanisms of cytoprotection in AMD. These target therapeutic candidates include APE/REF-1, MRZ-99030, Ciliary NeuroTrophic Factor (CNTF), RAP1 GTPase, Celecoxib, and SS-31/Elamipretide.

Lutein and zeaxanthin reduce A2E and iso-A2E levels and improve visual performance in Abca4-/-/Bco2-/- double knockout mice

Accumulation of bisretinoids such as A2E and its isomer iso-A2E is thought to mediate blue light-induced oxidative damage associated with age-related macular degeneration (AMD) and autosomal recessive Stargardt disease (STGD1). We hypothesize that increasing dietary intake of the macular carotenoids lutein and zeaxanthin in individuals at risk of AMD and STGD1 can inhibit the formation of bisretinoids A2E and iso-A2E, which can potentially ameliorate macular degenerative diseases. To study the beneficial effect of macular carotenoids in a retinal degenerative diseases model, we used ATP-binding cassette, sub-family A member 4 (Abca4-/-)/β,β-carotene-9',10'-oxygenase 2 (Bco2-/-) double knockout (KO) mice that accumulate elevated levels of A2E and iso-A2E in the retinal pigment epithelium (RPE) and macular carotenoids in the retina. Abca4-/-/Bco2-/- and Abca4-/- mice were fed a lutein-supplemented chow, zeaxanthin-supplemented chow or placebo chow (~2.6 mg of carotenoid/mouse/day) for three months. Visual function and electroretinography (ERG) were measured after one month and three months of carotenoid supplementation. The lutein and zeaxanthin supplemented Abca4-/-/Bco2-/- mice had significantly lower levels of RPE/choroid A2E and iso-A2E compared to control mice fed with placebo chow and improved visual performance. Carotenoid supplementation in Abca4-/- mice minimally raised retinal carotenoid levels and did not show much difference in bisretinoid levels or visual function compared to the control diet group. There was a statistically significant inverse correlation between carotenoid levels in the retina and A2E and iso-A2E levels in the RPE/choroid. Supplementation with retinal carotenoids, especially zeaxanthin, effectively inhibits bisretinoid formation in a mouse model of STGD1 genetically enhanced to accumulate carotenoids in the retina. These results provide further impetus to pursue oral carotenoids as therapeutic interventions for STGD1 and AMD.

Mitochondria-dependent phase separation of disease-relevant proteins drives pathological features of age-related macular degeneration

Age-related macular degeneration (AMD) damages the retinal pigment epithelium (RPE), the tissue that safeguards photoreceptor health, leading to irreversible vision loss. Polymorphisms in cholesterol and complement genes are implicated in AMD, yet mechanisms linking risk variants to RPE injury remain unclear. We sought to determine how allelic variants in the apolipoprotein E cholesterol transporter modulate RPE homeostasis and function. Using live-cell imaging, we show that inefficient cholesterol transport by the AMD risk-associated ApoE2 increases RPE ceramide, leading to autophagic defects and complement-mediated mitochondrial damage. Mitochondrial injury drives redox state–sensitive cysteine-mediated phase separation of ApoE2, forming biomolecular condensates that could nucleate drusen. The protective ApoE4 isoform lacks these cysteines and is resistant to phase separation and condensate formation. In Abca^–/– Stargardt macular degeneration mice, mitochondrial dysfunction induces liquid-liquid phase separation of p62/SQSTM1, a multifunctional protein that regulates autophagy. Drugs that decrease RPE cholesterol or ceramide prevent mitochondrial injury and phase separation in vitro and in vivo. In AMD donor RPE, mitochondrial fragmentation correlates with ApoE and p62 condensates. Our studies demonstrate that major AMD genetic and biological risk pathways converge upon RPE mitochondria, and identify mitochondrial stress-mediated protein phase separation as an important pathogenic mechanism and promising therapeutic target in AMD.

Iron Accumulation and Lipid Peroxidation in the Aging Retina: Implication of Ferroptosis in Age-Related Macular Degeneration

Iron is an essential component in many biological processes in the human body. It is critical for the visual phototransduction cascade in the retina. However, excess iron can be toxic. Iron accumulation and reduced efficiency of intracellular antioxidative defense systems predispose the aging retina to oxidative stress-induced cell death. Age-related macular degeneration (AMD) is characterized by retinal iron accumulation and lipid peroxidation. The mechanisms underlying AMD include oxidative stress-mediated death of retinal pigment epithelium (RPE) cells and subsequent death of retinal photoreceptors. Understanding the mechanism of the disruption of iron and redox homeostasis in the aging retina and AMD is crucial to decipher these mechanisms of cell death and AMD pathogenesis. The mechanisms of retinal cell death in AMD are an area of active investigation; previous studies have proposed several types of cell death as major mechanisms. Ferroptosis, a newly discovered programmed cell death pathway, has been associated with the pathogenesis of several neurodegenerative diseases. Ferroptosis is initiated by lipid peroxidation and is characterized by iron-dependent accumulation. In this review, we provide an overview of the mechanisms of iron accumulation and lipid peroxidation in the aging retina and AMD, with an emphasis on ferroptosis.

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

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

The molecular aspects of absorption and metabolism of carotenoids and retinoids in vertebrates

Vitamin A is an essential nutrient necessary for numerous basic physiological functions, including reproduction and development, immune cell differentiation and communication, as well as the perception of light. To evade the dire consequences of vitamin A deficiency, vertebrates have evolved specialized metabolic pathways that enable the absorption, transport, and storage of vitamin A acquired from dietary sources as preformed retinoids or provitamin A carotenoids. This evolutionary advantage requires a complex interplay between numerous specialized retinoid-transport proteins, receptors, and enzymes. Recent advances in molecular and structural biology resulted in a rapid expansion of our understanding of these processes at the molecular level. This progress opened new avenues for the therapeutic manipulation of retinoid homeostasis. In this review, we summarize current research related to the biochemistry of carotenoid and retinoid-processing proteins with special emphasis on the structural aspects of their physiological actions. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Retinoid analogs and polyphenols as potential therapeutics for age-related macular degeneration

IMPACT STATEMENT

Age-related macular degeneration (AMD) is a devastating retinal degenerative disease. Epidemiological reports showed an expected increasing prevalence of AMD in the near future. The only one existing FDA-approved pharmacological treatment involves an anti-vascular endothelial growth factor (VEGF) therapy with serious disadvantages. This limitation emphasizes an alarming need to develop new therapeutic approaches to prevent and treat AMD. In this review, we summarize scientific data unraveling the therapeutic potential of the specific retinoid and natural compounds. The experimental results reported by us and other research groups demonstrated that retinoid analogs and compounds with natural product scaffolds could serve as lead compounds for the development of new therapeutic agents with potential to prevent or slow down the pathogenesis of AMD.

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.

The retinal pigment epithelium in Sorsby Fundus Dystrophy shows increased sensitivity to oxidative stress-induced degeneration

Sorsby Fundus Dystrophy (SFD) is a rare inherited autosomal dominant macular degeneration caused by specific mutations in TIMP3. Patients with SFD present with pathophysiology similar to the more common Age-related Macular Degeneration (AMD) and loss of vision due to both choroidal neovascularization and geographic atrophy. Previously, it has been shown that RPE degeneration in AMD is due in part to oxidative stress. We hypothesized that similar mechanisms may be at play in SFD. The objective of this study was to evaluate whether mice carrying the S179C-Timp3 mutation, a variant commonly observed in SFD, showed increased sensitivity to oxidative stress. Antioxidant genes are increased at baseline in the RPE in SFD mouse models, but not in the retina. This suggests the presence of a pro-oxidant environment in the RPE in the presence of Timp3 mutations. To determine if the RPE of Timp3 mutant mice is more susceptible to degeneration when exposed to low levels of oxidative stress, mice were injected with low doses of sodium iodate. The RPE and photoreceptors in Timp3 mutant mice degenerated at low doses of sodium iodate, which had no effect in wildtype control mice. These studies suggest that TIMP3 mutations may result in a dysregulation of pro-oxidant-antioxidant homeostasis in the RPE, leading to RPE degeneration in SFD.

Effects of deficiency in the RLBP1-encoded visual cycle protein CRALBP on visual dysfunction in humans and mice

Mutations in retinaldehyde-binding protein 1 (RLBP1), encoding the visual cycle protein cellular retinaldehyde-binding protein (CRALBP), cause an autosomal recessive form of retinal degeneration. By binding to 11-cis-retinoid, CRALBP augments the isomerase activity of retinoid isomerohydrolase RPE65 (RPE65) and facilitates 11-cis-retinol oxidation to 11-cis-retinal. CRALBP also maintains the 11-cis configuration and protects against unwanted retinaldehyde activity. Studying a sibling pair that is compound heterozygous for mutations in RLBP1/CRALBP, here we expand the phenotype of affected individuals, elucidate a previously unreported phenotype in RLBP1/CRALBP carriers, and demonstrate consistencies between the affected individuals and Rlbp1/Cralbp^−/− mice. In the RLBP1/CRALBP-affected individuals, nonrecordable rod-specific electroretinogram traces were recovered after prolonged dark adaptation. In ultrawide-field fundus images, we observed radially arranged puncta typical of RLBP1/CRALBP-associated disease. Spectral domain-optical coherence tomography (SD-OCT) revealed hyperreflective aberrations within photoreceptor-associated bands. In short-wavelength fundus autofluorescence (SW-AF) images, speckled hyperautofluorescence and mottling indicated macular involvement. In both the affected individuals and their asymptomatic carrier parents, reduced SW-AF intensities, measured as quantitative fundus autofluorescence (qAF), indicated chronic impairment in 11-cis-retinal availability and provided information on mutation severity. Hypertransmission of the SD-OCT signal into the choroid together with decreased near-infrared autofluorescence (NIR-AF) provided evidence for retinal pigment epithelial cell (RPE) involvement. In Rlbp1/Cralbp^−/− mice, reduced 11-cis-retinal levels, qAF and NIR-AF intensities, and photoreceptor loss were consistent with the clinical presentation of the affected siblings. These findings indicate that RLBP1 mutations are associated with progressive disease involving RPE atrophy and photoreceptor cell degeneration. In asymptomatic carriers, qAF disclosed previously undetected visual cycle deficiency.

Bisretinoids: More than Meets the Eye

Bisretinoid fluorophores are the major constituents of the lipofuscin of retinal pigment epithelium (RPE) that accumulates with age and contributes to retina disease. Knowledge of the burden placed on the RPE cell by the accumulation of these phototoxic retinaldehyde-adducts depends on the identification and quantitation of the various bisretinoid species that constitute this family of fluorophores. Here we report a previously unidentified fluorescent bisretinoid by UPLC coupled to photodiode array detection, fluorescence, and electrospray ionization mass spectrometry (UPLC-PDA-FLR-ESI-MS) (Kim HJ, Sparrow JR, J Lipid Res 59:1620-1629, 2018). This novel bisretinoid is 1-octadecyl-2-lyso-sn-glycero A2PE (alkyl ether lysoA2PE). The structural assignment was based on molecular weight (m/z 998), UV-visible absorbance maxima (340, 440 nm), and retention time (73 minutes) and was corroborated by biomimetic synthesis using all-trans-retinal and glycerophosphoethanolamine analogues as starting materials. In mechanistic studies, A2PE was hydrolyzed by PLA₂, and plasmalogen lysoA2PE was cleaved under acidic conditions. Unprecedented UPLC detection of the bisretinoid alkyl ether lysoA2PE in human RPE but not in neural retina indicates that the phospholipase A₂ activity that generates the latter bisretinoid resides in RPE.

Retinal Pigment Epithelium Cell Death Is Associated With NLRP3 Inflammasome Activation by All-trans Retinal

Purpose

Visual (retinoid) cycle anomalies induce aberrant build-up of all-trans retinal (atRAL) in the retinal pigment epithelium (RPE), which is a cause of RPE atrophy in Stargardt disease type 1 and age-related macular degeneration. NLR family pyrin domain containing 3 (NLRP3) inflammasome activation is implicated in the etiology of age-related macular degeneration. Here, we elucidated the relationship between NLRP3 inflammasome activation and atRAL-induced death of RPE cells.

Methods

Cellular toxicities were assessed by MTS or MTT assays. Expression levels of mRNAs and proteins were determined by quantitative reverse transcription-polymerase chain reaction, Western blotting, or enzyme-linked immunosorbent assay. Fluorescence microscopy was used to examine intracellular signals. Ultrastructural features of organelles were examined by transmission electron microscope.

Results

Abnormal accumulation of atRAL was associated with a significant increase in the proportion of human ARPE-19 cells exhibiting features of apoptosis and Caspase-3/gasdermin E (GSDME)-mediated pyroptosis. These cells also exhibited elevated expression of NLRP3, ASC, cleaved Caspase-1/poly ADP-ribose polymerase (PARP)/Caspase-3/GSDME, interleukin-1β (IL-1β), and IL-18, as well as NLRP3 inflammasome-related genes (IL1B and IL18). After exposure of human ARPE-19 cells to excess atRAL, reactive oxygen species (ROS) (including mitochondrial ROS) and cathepsins released from lysosomes transmitted signals leading to NLRP3 inflammasome activation. Suppressing the production of ROS, NLRP3 inflammasome, Caspase-1, cathepsin B, or cathepsin D protected ARPE-19 cells against atRAL-associated cytotoxicity. Damage to mitochondria, lysosomes, and endoplasmic reticulum in atRAL-exposed ARPE-19 cells was partially alleviated by treatment with MCC950, a selective NLRP3 inflammasome inhibitor.

Conclusions

Aberrant build-up of atRAL promotes the death of RPE cells via NLRP3 inflammasome activation.

Abnormal Cannabidiol Modulates Vitamin A Metabolism by Acting as a Competitive Inhibitor of CRBP1

Cellular retinol-binding proteins (CRBPs) facilitate the uptake and intracellular transport of vitamin A. They integrate retinoid metabolism, playing an important role in regulating the synthesis of bioactive vitamin A metabolites. Thus, CRBPs constitute potential pharmacological targets to modulate cellular retinoid status that in turn may have applications in the treatment of certain immunological, metabolic, and ocular disorders. Here we identify abnormal cannabidiol (abn-CBD) as a nonretinoid inhibitor of cellular retinol-binding protein 1 (CRBP1). X-ray crystal structures of CRBP1 in complex with abn-CBD and its derivatives revealed a distinctive mode of protein-ligand interaction and provided a molecular basis for the high affinity and selectivity of this compound. We demonstrated that abn-CBD modulates the flux of retinoids via the retinoid cycle in vivo. Furthermore, the biological activity of abn-CBD was evidenced by its ability to protect against light-induced retinal damage in Balb/cJ mice. Altogether, our findings indicate that targeting selected CRBPs with a small-molecule inhibitor can potentially lead to the development of new therapeutic agents to counteract diseases with etiologies involving imbalance in retinoid metabolism or signaling.

Aberrant early endosome biogenesis mediates complement activation in the retinal pigment epithelium in models of macular degeneration

Abnormally enlarged early endosomes (EEs) are pathological features of neurodegenerative diseases, yet insight into the mechanisms and consequences of EE expansion remains elusive. Here, we report swollen apical EEs in the retinal pigment epithelium (RPE) of aged human donors and in the pigmented Abca4^-/- mouse model of Stargardt early-onset macular degeneration. Using high-resolution live-cell imaging, we show that age-related and pathological accumulation of lipofuscin bisretinoids increases ceramide at the apical surface of the RPE, which promotes inward budding and homotypic fusion of EEs. These enlarged endosomes internalize the complement protein C3 into the RPE, resulting in the intracellular generation of C3a fragments. Increased C3a in turn activates the mechanistic target of rapamycin (mTOR), a regulator of critical metabolic processes such as autophagy. The antidepressant desipramine, which decreases ceramide levels by inhibiting acid sphingomyelinase, corrects EE defects in the RPE of Abca4^-/- mice. This prevents C3 internalization and limits the formation of C3a fragments within the RPE. Although uncontrolled complement activation is associated with macular degenerations, how complement contributes to pathology in a progressive disease is not well understood. Our studies link expansion of the EE compartment with intracellular complement generation and aberrant mTOR activation, which could set the stage for chronic metabolic reprogramming in the RPE as a prelude to disease. The pivotal role of ceramide in driving EE biogenesis and fusion in the Abca4^-/- mice RPE suggests that therapeutic targeting of ceramide could be effective in Stargardt disease and other macular degenerations.

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

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

Emixustat Hydrochloride for Geographic Atrophy Secondary to Age-Related Macular Degeneration: A Randomized Clinical Trial

PURPOSE

To determine whether emixustat hydrochloride (emixustat) reduces the rate of enlargement of geographic atrophy (GA) compared with placebo in subjects with age-related macular degeneration (AMD) and to evaluate the safety and tolerability of emixustat over 24 months of treatment.

DESIGN

Multicenter, randomized, double-masked, placebo-controlled, phase 2b/3 clinical trial.

PARTICIPANTS

Patients with GA secondary to AMD, a visual acuity score of at least 35 letters, and GA with a total area of 1.25 to 18 mm² were enrolled.

METHODS

Subjects were randomized (1:1:1:1) to emixustat 2.5 mg, 5 mg, 10 mg, or placebo, administered orally once daily for 24 months. Visits included screening, baseline, and months 1, 2, 3, 6, 9, 12, 15, 18, 21, 24, and 25.

MAIN OUTCOME MEASURES

The primary efficacy end point was the mean annual growth rate of total GA area in the study eye, as measured by a central reading center using fundus autofluorescence (FAF) images. The change from baseline in normal luminance best-corrected visual acuity (NL-BCVA) was a secondary efficacy end point.

RESULTS

Of 508 randomized subjects, 320 completed the study. Demographics and baseline characteristics were comparable between treatment groups. On average, GA lesions in the study eye grew at a similar rate in each group (emixustat: 1.69 to 1.84 mm²/year; placebo: 1.69 mm²/year; P ≥ 0.81). Changes in NL-BCVA were also comparable between groups. Subjects with a larger low luminance deficit (LLD) at baseline (≥20 letters) demonstrated a more rapid growth of GA over 24 months. No relationship was observed between the risk-allele status of the AMD-associated single-nucleotide polymorphisms tested and the growth rate of GA. The most common adverse events in emixustat-treated subjects were delayed dark adaptation (55%), chromatopsia (18%), visual impairment (15%), and erythropsia (15%).

CONCLUSIONS

Emixustat did not reduce the growth rate of GA in AMD. The most common adverse events were ocular in nature and likely related to the drug's mechanism of action. Data gained from this study over a 2-year period add to the understanding of the natural history of GA and the baseline characteristics affecting the growth rate of GA.

Photoreceptor cells as a source of fundus autofluorescence in recessive Stargardt disease

Bisretinoid fluorophores form in photoreceptor outer segments from nonenzymatic reactions of vitamin A aldehyde. The short-wavelength autofluorescence (SW-AF) of fundus flecks in recessive Stargardt disease (STGD1) suggests a connection to these fluorophores. Through multimodal imaging, we sought to elucidate this link. Flecks observed in SW-AF images often colocalized with foci exhibiting reduced or absent near-infrared autofluorescence signal, the source of which is melanin in retinal pigment epithelial (RPE) cells. With serial imaging, changes in near-infrared autofluorescence (NIR-AF) preceded the onset of fleck hyperautofluorescence in SW-AF images and fleck profiles in NIR-AF images tended to be larger. Flecks in SW-AF and NIR-AF images also corresponded to hyperreflective lesions traversing photoreceptor-attributable bands in horizontal SD-OCT scans. The hyperreflective lesions interrupted adjacent OCT reflectivity bands and were associated with thinning of the outer nuclear layer. These SD-OCT findings are attributable to photoreceptor cell degeneration. Progressive increases and decreases in the SW-AF intensity of flecks were evident in color-coded quantitative fundus autofluorescence maps. In some cases, flecks appeared to spread radially from the fovea to approximately 8° of eccentricity, beyond which a circumferential spread characterized the distribution. Since the NIR-AF signal is derived from melanin and loss of this autofluorescence is indicative of RPE atrophy, the SW-AF of flecks cannot be accounted for by bisretinoid lipofuscin in RPE. Instead, we suggest that the bisretinoid serving as the source of the SW-AF signal, resides in photoreceptors, the cell that is also the site of bisretinoid synthesis.

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.

New GABA modulators protect photoreceptor cells from light-induced degeneration in mouse models

No clinically approved therapies are currently available that prevent the onset of photoreceptor death in retinal degeneration. Signaling between retinal neurons is regulated by the release and uptake of neurotransmitters, wherein GABA is the main inhibitory neurotransmitter. In this work, novel 3-chloropropiophenone derivatives and the clinical anticonvulsants tiagabine and vigabatrin were tested to modulate GABA signaling and protect against light-induced retinal degeneration. Abca4^-/-Rdh8^-/- mice, an accelerated model of retinal degeneration, were exposed to intense light after prophylactic injections of one of these compounds. Imaging and functional assessments of the retina indicated that these compounds successfully protected photoreceptor cells from degeneration to maintain a full-visual-field response. Furthermore, these compounds demonstrated a strong safety profile in wild-type mice and did not compromise visual function or damage the retina, despite repeated administration. These results indicate that modulating inhibitory GABA signaling can offer prophylactic protection against light-induced retinal degeneration.-Schur, R. M., Gao, S., Yu, G., Chen, Y., Maeda, A., Palczewski, K., Lu, Z.-R. New GABA modulators protect photoreceptor cells from light-induced degeneration in mouse models.

Saposin B Binds the Lipofuscin Bisretinoid A2E and Prevents its Enzymatic and Photooxidation

Vitamin A based bisretinoid accumulation is a major focus in the study of macular degeneration. Whether specific endogenous lysosomal proteins can bind A2E, a pronounced bisretinoid in lipofuscin granules in retinal pigment epithelial cells, and interfere with enzymatic or photoinduced oxidation of such, has not been explored. Herein, using fluorescence and electronic absorption spectroscopy and mass spectrometry, we demonstrate that Saposin B, a critical protein in the degradation of sulfatides and "flushing" of lipids, can bind A2E, preventing its H₂O₂-dependent enzymatic oxidation by horseradish peroxidase and photooxidation by blue light (λ=450-460 nm).