A novel source of methylglyoxal and glyoxal in retina: implications for age-related macular degeneration

Vitamin A-aldehyde adducts: AMD risk and targeted therapeutics

Although currently available treatment options for age-related macular degeneration (AMD) are limited, particularly for atrophic AMD, the identification of predisposing genetic variations has informed clinical studies addressing therapeutic options such as complement inhibitors and anti-inflammatory agents. To lower risk of early AMD, recommended lifestyle interventions such as the avoidance of smoking and the intake of low glycemic antioxidant-rich diets have largely followed from the identification of nongenetic modifiable factors. On the other hand, the challenge of understanding the complex relationship between aging and cumulative damage leading to AMD has fueled investigations of the visual cycle adducts that accumulate in retinal pigment epithelial (RPE) cells and are a hallmark of aging retina. These studies have revealed properties of these compounds that provide insights into processes that may compromise RPE and could contribute to disease mechanisms in AMD. This work has also led to the design of targeted therapeutics that are currently under investigation.

The Rate of Vitamin A Dimerization in Lipofuscinogenesis, Fundus Autofluorescence, Retinal Senescence and Degeneration

One of the earliest events preceding several forms of retinal degeneration is the formation and accumulation of vitamin A dimers in the retinal pigment epithelium (RPE) and underlying Bruch's membrane (BM). Such degenerations include Stargardt disease, Best disease, forms of retinitis pigmentosa, and age-related macular degeneration (AMD). Since their discovery in the 1990's, dimers of vitamin A, have been postulated as chemical triggers driving retinal senescence and degeneration. There is evidence to suggest that the rate at which vitamin A dimerizes and the eye's response to the dimerization products may dictate the retina's lifespan. Here, we present outstanding questions, finding the answers to which may help to elucidate the role of vitamin A dimerization in retinal degeneration.

Phloroglucinol protects retinal pigment epithelium and photoreceptor against all-trans-retinal-induced toxicity and inhibits A2E formation

Among retinal macular diseases, the juvenile recessive Stargardt disease and the age‐related degenerative disease arise from carbonyl and oxidative stresses (COS). Both stresses originate from an accumulation of all‐trans‐retinal (atRAL) and are involved in bisretinoid formation by condensation of atRAL with phosphatidylethanolamine (carbonyl stress) in the photoreceptor and its transformation into lipofuscin bisretinoids (oxidative stress) in the retinal pigment epithelium (RPE). As atRAL and bisretinoid accumulation contribute to RPE and photoreceptor cell death, our goal is to select powerful chemical inhibitors of COS. Here, we describe that phloroglucinol, a natural phenolic compound having anti‐COS properties, protects both rat RPE and mouse photoreceptor primary cultures from atRAL‐induced cell death and reduces hydrogen peroxide (H₂O₂)‐induced damage in RPE in a dose‐dependent manner. Mechanistic analyses demonstrate that the protective effect encompasses decrease in atRAL‐induced intracellular reactive oxygen species and free atRAL levels. Moreover, we show that phloroglucinol reacts with atRAL to form a chromene adduct which prevents bisretinoid A2E synthesis in vitro. Taken together, these data show that the protective effect of phloroglucinol correlates with its ability to trap atRAL and to prevent its further transformation into deleterious bisretinoids. Phloroglucinol might be a good basis to develop efficient therapeutic derivatives in the treatment of retinal macular diseases.

One-step isolation of sappanol and brazilin from Caesalpinia sappan and their effects on oxidative stress-induced retinal death

Caesalpinia sappan is a well-distributed plant that is cultivated in Southeast Asia, Africa, and the Americas. C. sappan has been used in Asian folk medicine and its extract has been shown to have pharmacological effects. Two homoisoflavonoids, sappanol and brazilin, were isolated from C. sappan by using centrifugal partition chromatography (CPC), and tested for protective effects against retinal cell death. The isolated homoisoflavonoids produced approximately 20-fold inhibition of N-retinylidene-N-retinyl-ethanolamine (A2E) photooxidation in a dose-dependent manner. Of the 2 compounds, brazilin showed better inhibition (197.93 ± 1.59 μM of IC50). Cell viability tests and PI/Hoechst 33342 double staining method indicated that compared to the negative control, sappanol significantly attenuated H2O2-induced retinal death. The compounds significantly blunted the up-regulation of intracellular reactive oxygen species (ROS), and sappanol inhibited lipid peroxidation in a concentration-dependent manner. Thus, both compounds represent potential antioxidant treatments for retinal diseases.

Correlations between Photodegradation of Bisretinoid Constituents of Retina and Dicarbonyl Adduct Deposition

Non-enzymatic collagen cross-linking and carbonyl adduct deposition are features of Bruch's membrane aging in the eye, and disturbances in extracellular matrix turnover are considered to contribute to Bruch's membrane thickening. Because bisretinoid constituents of the lipofuscin of retinal pigment epithelial (RPE) cells are known to photodegrade to mixtures of aldehyde-bearing fragments and small dicarbonyls (glyoxal (GO) and methylglyoxal (MG)), we investigated RPE lipofuscin as a source of the reactive species that covalently modify protein side chains. Abca4(-/-) and Rdh8(-/-)/Abca4(-/-) mice that are models of accelerated bisretinoid formation were studied and pre-exposure of mice to 430 nm light enriched for dicarbonyl release by bisretinoid photodegradation. MG protein adducts were elevated in posterior eyecups of mutant mice, whereas carbonylation of an RPE-specific protein was observed in Abca4(-/-) but not in wild-type mice under the same conditions. Immunolabeling of cryostat-sectioned eyes harvested from Abca4(-/-) mice revealed that carbonyl adduct deposition in Bruch's membrane was accentuated. Cell-based assays corroborated these findings in mice. Moreover, the receptor for advanced glycation end products that recognizes MG and GO adducts and glyoxylase 1 that metabolizes MG and GO were up-regulated in Abca4(-/-) mice. Additionally, in acellular assays, peptides were cross-linked in the presence of A2E (adduct of two vitamin A aldehyde and ethanolamine) photodegradation products, and in a zymography assay, reaction of collagen IV with products of A2E photodegradation resulted in reduced cleavage by the matrix metalloproteinases MMP2 and MMP9. In conclusion, these mechanistic studies demonstrate a link between the photodegradation of RPE bisretinoid fluorophores and aging changes in underlying Bruch's membrane that can confer risk of age-related macular degeneration.

Photobleaching and Fluorescence Recovery of RPE Bisretinoids

The autofluorescence of the retina that originates primarily from lipofuscin fluorophores in retinal pigment epithelial cells, is observed to undergo photobleaching during the acquisition of fundus autofluorescence images. Bisretinoid fluorophores isolated from retinal pigment epithelial cells have the spectral characteristics consistent with their being the source of fundus autofluorescence. Clinically relevant experiments were designed to better understand conditions in the micromilieu of bisretinoid fluorophores that can influence fluorescence efficiencies, photobleaching, and subsequent fluorescence recovery of this fluorophore. The consumption of the bisretinoid A2E due to photooxidation-induced degradation was quantified in solvent systems of variable relative permittivity (formerly called dielectric constant), in micelles, and in phospholipid vesicles of varying composition. Reorganization within biphasic systems was also examined. A2E content was measured by high performance liquid chromatography (HPLC) and fluorescence intensity was quantified spectroscopically. As solvent polarity was increased, A2E fluorescent spectra exhibited red-shifted maxima and reduced intensity. A2E was depleted by light irradiation and the loss was more pronounced in less polar solvents, lower concentrations of anionic surfactant, and in gel- versus fluid-ordered phospholipid liposomes. Conditions that permit A2E aggregation promoted photooxidation/photodegradation, while movement of A2E between bisphasic systems was associated with fluorescence recovery after photobleaching. The fluorescence characteristics of A2E are subject to environmental modulation. Photooxidation and photodegradation of bisretinoid can account for fundus autofluorescence photobleaching. Return of fluorescence intensity after photobleaching likely occurs due to redistribution of A2E fractions amongst co-existing heterogeneous microdomains of the lysosomal compartment.

Fundus Autofluorescence and RPE Lipofuscin in Age-Related Macular Degeneration

Genes that increase susceptibility to age-related macular degeneration (AMD) have been identified; however, since many individuals carrying these risk alleles do not develop disease, other contributors are involved. One additional factor, long implicated in the pathogenesis of AMD, is the lipofuscin of retinal pigment epithelium (RPE). The fluorophores that constitute RPE lipofuscin also serve as a source of autofluorescence (AF) that can be imaged by confocal laser ophthalmoscopy. The AF originating from lipofuscin is excited by the delivery of short wavelength (SW) light. A second autofluorescence is emitted from the melanin of RPE (and choroid) upon near-infrared (NIR-AF) excitation. SW-AF imaging is currently used in the clinical management of retinal disorders and the advantages of NIR-AF are increasingly recognized. Here we visit the damaging properties of RPE lipofuscin that could be significant when expressed on a background of genetic susceptibility. To advance interpretations of disease-related patterns of fundus AF in AMD, we also consider the photochemical and spectrophotometric features of the lipofuscin compounds responsible for generating the fluorescence emission.

Methylglyoxal, a reactive glucose metabolite, enhances autophagy flux and suppresses proliferation of human retinal pigment epithelial ARPE-19 cells

Methylglyoxal (MGO), a glycolytic metabolite, induces oxidative injury and apoptotic cell death that play a pathogenetic role in age-related macular degeneration (AMD). This study examined the impact of MGO on cell proliferation and autophagy flux in retinal pigment epithelium (RPE) ARPE-19 cells and elucidated the underlying mechanism. Short-term MGO exposure suppressed cell proliferation without induction of apoptotic cell death, increased production of reactive oxygen species, and potentiated H2O2-exhibited cytotoxicity in ARPE-19 cells. Conversely, pretreatment with N-acetylcysteine, a ROS scavenger, and aminoguanidine, an MGO blocker, prevented MGO-induced growth retardation. MGO significantly enhanced autophagy flux and increased intracellular accumulation of autophagosomes, which was functionally confirmed by addition of autophagy enhancer or inhibitors. Signaling kinetic observation indicated that MGO remarkably triggered phosphorylation of Akt, ERK1/2, p38 MAPK, and JNK1/2. Blockade of kinase activity demonstrated that the hyperphosphorylation of Akt, ERK1/2, JNK, and p38 MAPK were all involved in the MGO-enhanced autophagy and growth-arresting effect in ARPE-19 cells. Moreover, pretreatment with autophagic flux inhibitors including 3-methyladenine, bafilomycin A, and chloroquine effectively ameliorated MGO- but not H2O2-mediated ARPE-19 cytotoxicity. In conclusion, modulation of autophagy flux activity by using autophagic or kinase inhibitors may be an applicable modality to treat AMD.

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

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

Morphological and physiological retinal degeneration induced by intravenous delivery of vitamin A dimers in rabbits

The eye uses vitamin A as a cofactor to sense light and, during this process, some vitamin A molecules dimerize, forming vitamin A dimers. A striking chemical signature of retinas undergoing degeneration in major eye diseases such as age-related macular degeneration (AMD) and Stargardt disease is the accumulation of these dimers in the retinal pigment epithelium (RPE) and Bruch's membrane (BM). However, it is not known whether dimers of vitamin A are secondary symptoms or primary insults that drive degeneration. Here, we present a chromatography-free method to prepare gram quantities of the vitamin A dimer, A2E, and show that intravenous administration of A2E to the rabbit results in retinal degeneration. A2E-damaged photoreceptors and RPE cells triggered inflammation, induced remolding of the choroidal vasculature and triggered a decline in the retina's response to light. Data suggest that vitamin A dimers are not bystanders, but can be primary drivers of retinal degeneration. Thus, preventing dimer formation could be a preemptive strategy to address serious forms of blindness.

Changes in spectral properties and composition of lipofuscin fluorophores from human-retinal-pigment epithelium with age and pathology

Fundus autofluorescence mostly originates from bisretinoid fluorophores in lipofuscin granules, which accumulate in retinal-pigment-epithelium cells with age. The dynamics of accumulation, photo-oxidation, and photodegradation of bisretinoids during aging or in the presence of pathology have been insufficiently investigated. Changes in spectral properties and composition of human lipofuscin-granule fluorophores with age and pathology have now been investigated by a high-performance liquid chromatography method using spectrophotometric and fluorescent detectors connected in series. It was found that: (i) N-retinylidene-N-retinylethanolamine (A2E) fluorescence intensity is not predominant in the chloroform extract of human-cadaver-eye retinal pigment epithelium studied; bisretinoid photo-oxidation and photodegradation products have much higher fluorescent properties; (ii) the relative emission maximum in the fluorescence spectrum of suspended retinal-pigment-epithelium cells obtained from an individual human-cadaver eye without pathology is irrespective of donor age and falls within the range 575 ± 15 nm; in two cadaver eyes with signs of age-related macular degeneration, emission maxima were shifted by 23-36 nm towards the shortwave region; and (iii) the ratio of bisretinoid photo-oxidation and photodegradation products to unoxidized bisretinoids in the chloroform extract of cadaver-eye retinal pigment epithelium increases with donor age, from 0.69 ± 0.03 to 1.32 ± 0.04. The differences in fluorescence properties between chloroform extracts obtained from cadaver eyes with and without signs of age-related macular degeneration could be used to increase the potential of fundus autofluorescence imaging as a noninvasive diagnostic method.

Studying melanin and lipofuscin in RPE cell culture models

The retinal pigment epithelium contains three major types of pigment granules; melanosomes, lipofuscin and melanolipofuscin. Melanosomes in the retinal pigment epithelium (RPE) are formed during embryogenesis and mature during early postnatal life while lipofuscin and melanolipofuscin granules accumulate as a function of age. The difficulty in studying the formation and consequences of melanosomes and lipofuscin granules in RPE cell culture is compounded by the fact that these pigment granules do not normally occur in established RPE cell lines and pigment granules are rapidly lost in adult human primary culture. This review will consider options available for overcoming these limitations and permitting the study of melanosomes and lipofuscin in cell culture and will briefly evaluate the advantages and disadvantages of the different protocols.

Site-specific AGE modifications in the extracellular matrix: a role for glyoxal in protein damage in diabetes

Non-enzymatic modification of proteins in hyperglycemia is a major proposed mechanism of diabetic complications. Specifically, advanced glycation end products (AGEs) derived from hyperglycemia-induced reactive carbonyl species (RCS) can have pathogenic consequences when they target functionally critical protein residues. Modification of a small number of these critical residues, often undetectable by the methodologies relying on measurements of total AGE levels, can cause significant functional damage. Therefore, detection of specific sites of protein damage in diabetes is central to understanding the molecular basis of diabetic complications and for identification of biomarkers which are mechanistically linked to the disease. The current paradigm of RCS-derived protein damage places a major focus on methylglyoxal (MGO), an intermediate of cellular glycolysis. We propose that glyoxal (GO) is a major contributor to extracellular matrix (ECM) damage in diabetes. Here, we review the current knowledge and provide new data about GO-derived site-specific ECM modification in experimental diabetes.

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

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

Emerging role of advanced glycation-end products (AGEs) in the pathobiology of eye diseases

Advanced glycation end products (AGEs) have been implicated in vision loss associated with macula degeneration, cataract formation, diabetic retinopathy and glaucoma. This pathogenic potential is mainly attributed to their accumulation in ocular tissues where they mediate aberrant crosslinking of extracellular matrix proteins and disruption of endothelial junctional complexes that affects cell permeability, mediates angiogenesis and breakdown of the inner blood-retinal barrier. Furthermore, AGEs severely affect cellular metabolism by disrupting ATP production, enhancing oxidative stress and modulating gene expression of anti-angiogenic and anti-inflammatory genes. Elucidation of AGE-induced mechanisms of action in different eye compartments will help in the understanding of the complex cellular and molecular processes associated with eye diseases. Several pharmaceutical agents with anti-glycating and anti-oxidant properties as well as AGE crosslink 'breakers' have been currently applied to eye diseases. The role of diet and the beneficial effects of certain nutriceuticals provide an alternative way to manage chronic visual disorders that affect the quality of life of millions of people.

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

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

A2E-mediated photochemical modification to fibronectin and its implications to age-related changes in Bruch's membrane

Lipofuscin accumulates normally with age and is more pronounced in retinal dystrophies such as age-related macular degeneration. The major bis-retinoid component of lipofuscin is A2E. In addition to cell damage effects by A2E, we have previously demonstrated that blue-light-mediated A2E leads to modifications in the basement membrane protein laminin. Therefore, the purpose of this study was to advance the understanding of A2E photooxidation effects on fibronectin, the major glycoprotein of Bruch's membrane. In this study, A2E was irradiated with blue light in the presence of a fibronectin peptide consisting of amino acids from the integrin binding region. The modification sites were identified via LC/MS. Our research indicated that blue light irradiation caused cleavage throughout the A2E molecule closest to the pyridinium ring, and attached to the fibronectin peptide preferentially at lysine and arginine residues. All of these reactions are similar to the Maillard reaction. Altogether this study suggests that blue-light-irradiated A2E modifies peptides and forms advance glycation endproducts. Furthermore, these results can be used to identify modifications that occur in Bruch's membrane in vivo.

Aging is not a disease: distinguishing age-related macular degeneration from aging

Age-related macular degeneration (AMD) is a disease of the outer retina, characterized most significantly by atrophy of photoreceptors and retinal pigment epithelium accompanied with or without choroidal neovascularization. Development of AMD has been recognized as contingent on environmental and genetic risk factors, the strongest being advanced age. In this review, we highlight pathogenic changes that destabilize ocular homeostasis and promote AMD development. With normal aging, photoreceptors are steadily lost, Bruch's membrane thickens, the choroid thins, and hard drusen may form in the periphery. In AMD, many of these changes are exacerbated in addition to the development of disease-specific factors such as soft macular drusen. Para-inflammation, which can be thought of as an intermediate between basal and robust levels of inflammation, develops within the retina in an attempt to maintain ocular homeostasis, reflected by increased expression of the anti-inflammatory cytokine IL-10 coupled with shifts in macrophage plasticity from the pro-inflammatory M1 to the anti-inflammatory M2 polarization. In AMD, imbalances in the M1 and M2 populations together with activation of retinal microglia are observed and potentially contribute to tissue degeneration. Nonetheless, the retina persists in a state of chronic inflammation and increased expression of certain cytokines and inflammasomes is observed. Since not everyone develops AMD, the vital question to ask is how the body establishes a balance between normal age-related changes and the pathological phenotypes in AMD.

Quantitative fundus autofluorescence in healthy eyes

PURPOSE

Fundus autofluorescence was quantified (qAF) in subjects with healthy retinae using a standardized approach. The objective was to establish normative data and identify factors that influence the accumulation of RPE lipofuscin and/or modulate the observed AF signal in fundus images.

METHODS

AF images were acquired from 277 healthy subjects (age range: 5-60 years) by employing a Spectralis confocal scanning laser ophthalmoscope (cSLO; 488-nm excitation; 30°) equipped with an internal fluorescent reference. For each image, mean gray level was calculated as the average of eight preset regions, and was calibrated to the reference, zero-laser light, magnification, and optical media density from normative data on lens transmission spectra. Relationships between qAF and age, sex, race/ethnicity, eye color, refraction/axial length, and smoking status were evaluated as was measurement repeatability and the qAF spatial distribution.

RESULTS

qAF levels exhibited a significant increase with age. qAF increased with increasing eccentricity up to 10° to 15° from the fovea and was highest superotemporally. qAF values were significantly greater in females, and, compared with Hispanics, qAF was significantly higher in whites and lower in blacks and Asians. No associations with axial length and smoking were observed. For two operators, between-session repeatability was ± 9% and ± 12%. Agreement between the operators was ± 13%.

CONCLUSIONS

Normative qAF data are a reference tool essential to the interpretation of qAF measurements in ocular disease.

Cellular and molecular mechanisms of age-related macular degeneration: from impaired autophagy to neovascularization

Age-related macular degeneration (AMD) is a complex, degenerative and progressive disease involving multiple genetic and environmental factors. It can result in severe visual loss e.g. AMD is the leading cause of blindness in the elderly in the western countries. Although age, genetics, diet, smoking, and many cardiovascular factors are known to be linked with this disease there is increasing evidence that long-term oxidative stress, impaired autophagy clearance and inflammasome mediated inflammation are involved in the pathogenesis. Under certain conditions these may trigger detrimental processes e.g. release of vascular endothelial growth factor (VEGF), causing choroidal neovascularization e.g. in wet AMD. This review ties together these crucial pathological threads in AMD.