The photochemistry of human retinal lipofuscin as studied by EPR

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.

Polyunsaturated Lipids in the Light-Exposed and Prooxidant Retinal Environment

The retina is an oxidative stress-prone tissue due to high content of polyunsaturated lipids, exposure to visible light stimuli in the 400–480 nm range, and high oxygen availability provided by choroidal capillaries to support oxidative metabolism. Indeed, lipids’ peroxidation and their conversion into reactive species promoting inflammation have been reported and connected to retinal degenerations. Here, we review recent evidence showing how retinal polyunsaturated lipids, in addition to oxidative stress and damage, may counteract the inflammatory response triggered by blue light-activated carotenoid derivatives, enabling long-term retina operation despite its prooxidant environment. These two aspects of retinal polyunsaturated lipids require tight control over their synthesis to avoid overcoming their protective actions by an increase in lipid peroxidation due to oxidative stress. We review emerging evidence on different transcriptional control mechanisms operating in retinal cells to modulate polyunsaturated lipid synthesis over the life span, from the immature to the ageing retina. Finally, we discuss the antioxidant role of food nutrients such as xanthophylls and carotenoids that have been shown to empower retinal cells’ antioxidant responses and counteract the adverse impact of prooxidant stimuli on sight.

Photodegradation of Lipofuscin in Suspension and in ARPE-19 Cells and the Similarity of Fluorescence of the Photodegradation Product with Oxidized Docosahexaenoate

Retinal lipofuscin accumulates with age in the retinal pigment epithelium (RPE), where its fluorescence properties are used to assess retinal health. It was observed that there is a decrease in lipofuscin fluorescence above the age of 75 years and in the early stages of age-related macular degeneration (AMD). The purpose of this study was to investigate the response of lipofuscin isolated from human RPE and lipofuscin-laden cells to visible light, and to determine whether an abundant component of lipofuscin, docosahexaenoate (DHA), can contribute to lipofuscin fluorescence upon oxidation. Exposure of lipofuscin to visible light leads to a decrease in its long-wavelength fluorescence at about 610 nm, with a concomitant increase in the short-wavelength fluorescence. The emission spectrum of photodegraded lipofuscin exhibits similarity with that of oxidized DHA. Exposure of lipofuscin-laden cells to light leads to a loss of lipofuscin granules from cells, while retaining cell viability. The spectral changes in fluorescence in lipofuscin-laden cells resemble those seen during photodegradation of isolated lipofuscin. Our results demonstrate that fluorescence emission spectra, together with quantitation of the intensity of long-wavelength fluorescence, can serve as a marker useful for lipofuscin quantification and for monitoring its oxidation, and hence useful for screening the retina for increased oxidative damage and early AMD-related changes.

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.

Products of Docosahexaenoate Oxidation as Contributors to Photosensitising Properties of Retinal Lipofuscin

Retinal lipofuscin which accumulates with age in the retinal pigment epithelium (RPE) is subjected to daily exposures to high fluxes of visible light and exhibits potent photosensitising properties; however, the molecules responsible for its photoreactivity remain unknown. Here, we demonstrate that autooxidation of docosahexaenoate (DHE) leads to the formation of products absorbing, in addition to UVB and UVA light, also visible light. The products of DHE oxidation exhibit potent photosensitising properties similar to photosensitising properties of lipofuscin, including generation of an excited triplet state with similar characteristics as the lipofuscin triplet state, and photosensitised formation of singlet oxygen and superoxide. The quantum yields of singlet oxygen and superoxide generation by oxidised DHE photoexcited with visible light are 2.4- and 3.6-fold higher, respectively, than for lipofuscin, which is consistent with the fact that lipofuscin contains some chromophores which do contribute to the absorption of light but not so much to its photosensitising properties. Importantly, the wavelength dependence of photooxidation induced by DHE oxidation products normalised to equal numbers of incident photons is also similar to that of lipofuscin—it steeply increases with decreasing wavelength. Altogether, our results demonstrate that products of DHE oxidation include potent photosensitiser(s) which are likely to contribute to lipofuscin photoreactivity.

Lipofuscin in keratinocytes: Production, properties, and consequences of the photosensitization with visible light

A dysfunction in the mitochondrial-lysosomal axis of cellular homeostasis is proposed to cause cells to age quicker and to accumulate lipofuscin. Typical protocols to mediate lipofuscinogenesis are based on the induction of the senescent phenotype either by allowing many consecutive cycles of cell division or by treating cells with physical/chemical agents such as ultraviolet (UV) light or hydrogen peroxide. Due to a direct connection with the physiopathology of age-related macular degeneration, lipofuscin that accumulates in retinal pigment epithelium (RPE) cells have been extensively studied, and the photochemical properties of RPE lipofuscin are considered as standard for this pigment. Yet, many other tissues such as the brain and the skin may prompt lipofuscinogenesis, and the properties of lipofuscin granules accumulated in these tissues are not necessarily the same as those of RPE lipofuscin. Here, we present a light-induced protocol that accelerates cell aging as judged by the maximization of lipofuscinogenesis. Photosensitization of cells previously incubated with nanomolar concentrations of 1,9-dimethyl methylene blue (DMMB), severely and specifically damages mitochondria and lysosomes, leading to a lipofuscin-related senescent phenotype. By applying this protocol in human immortalized non-malignant keratinocytes (HaCaT) cells, we observed a 2.5-fold higher level of lipofuscin accumulation compared to the level of lipofuscin accumulation in cells treated with a typical UV protocol. Lipofuscin accumulated in keratinocytes exhibited the typical red light emission, with excitation maximum in the blue wavelength region (~450 nm). Fluorescence lifetime image microscopy data showed that the keratinocyte lipofuscin has an emission lifetime of ~1.7 ns. Lipofuscin-loaded cells (but not control cells) generated a substantial amount of singlet oxygen (¹O₂) when irradiated with blue light (420 nm), but there was no ¹O₂ generation when excitation was performed with a green light (532 nm). These characteristics were compared with those of RPE cells, considering that keratinocyte lipofuscin lacks the bisretinoids derivatives present in RPE lipofuscin. Additionally, we showed that lipofuscin-loaded keratinocytes irradiated with visible light presented critical DNA damages, such as double-strand breaks and Fpg-sensitive sites. We propose that the DMMB protocol is an efficient way to disturb the mitochondrial-lysosomal axis of cellular homeostasis, and consequently, it can be used to accelerate aging and to induce lipofuscinogenesis. We also discuss the consequences of the lipofuscin-induced genotoxicity of visible light in keratinocytes.

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.

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.

Effect of ultraviolet irradiation on free radical scavenging activity of immunosuppressants used in lung transplantation and comparative electron paramagnetic resonance study of kinetics of their interactions with model free radicals

WHAT IS KNOWN AND OBJECTIVE

The immunosuppressive drugs used in solid organ transplantation or autoimmunological processes were studied by electron paramagnetic resonance (EPR) spectroscopy to estimate their free radical scavenging activity. The interactions of immunosuppressants with free radicals were examined by an X-band (9.3 GHz) EPR spectroscopy and a model of DPPH free radicals.

METHODS

The EPR spectra of DPPH and DPPH interacting with individual drugs were compared. Kinetic studies were performed, and the effect of ultraviolet (UV) irradiation on the free radical scavenging activity of the tested drugs was determined.

RESULTS AND DISCUSSION

The free radical scavenging activity of non-irradiated drugs decreased in the order: rapamycin > mycophenolate mofetil > ciclosporin > tacrolimus. UV irradiation increased the free radical scavenging activity of all the tested immunosuppressive drugs, and the effect was highest for tacrolimus. For the non-irradiated samples, the speed of free radical interactions decreased in the order: ciclosporin > tacrolimus > mycophenolate mofetil > rapamycin. UV irradiation only slightly affected the speed of interactions of the immunosuppressive drugs with the model DPPH free radicals.

WHAT IS NEW AND CONCLUSION

Electron paramagnetic resonance spectroscopy is useful for obtaining information on interactions of immunosuppressive drugs with free radicals. We hypothesized that the long-term immunosuppressive effects of these drugs after transplantation or during autoimmune disorders may be mediated by anti-inflammatory action in addition to the known receptor/cell cycle inhibition.

Application of EPR spectroscopy to examine free radicals evolution during storage of the thermally sterilized Ungentum ophthalmicum

Free radicals formed during thermal sterilization of the Ungentum ophthalmicum were examined by an X-band EPR spectroscopy. The influence of storage time (15 min; 1, 2 and 3 days after heating) on free radical properties and concentrations in this sample was determined. Thermal sterilization was done according to the pharmaceutical norms. The first-derivative EPR spectra with g-values about 2 were measured with magnetic modulation of 100 kHz in the range of microwave power 2.2-70 mW. The changes of amplitudes (A) and linewidths (ΔB_pp) with microwave powers were evaluated. Free radicals in concentration ∼10¹⁷ spin/g were formed during heating of the tested Ungentum. Free radical concentration decreased with increase in storage time, and reached values ∼10¹⁷ spin/g after 3 days from sterilization. The tested U. ophthalmicum should not be sterilized at a temperature of 160 °C because of the free radicals formation, or it should be used 3 days after heating, when free radicals were considerably quenched. Free radical properties remain unchanged during storage of the Ungentum. The EPR lines of the U. ophthalmicum were homogeneously broadened and their linewidths (ΔB_pp) increased with increase in microwave power. EPR spectroscopy is useful to examine free radicals to optimize sterilization process and storage conditions of ophthalmologic samples.

The Photobiology of Lutein and Zeaxanthin in the Eye

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

Defects in retinal pigment epithelial cell proteolysis and the pathology associated with age-related macular degeneration

Maintenance of protein homeostasis, also referred to as "Proteostasis", integrates multiple pathways that regulate protein synthesis, folding, translocation, and degradation. Failure in proteostasis may be one of the underlying mechanisms responsible for the cascade of events leading to age-related macular degeneration (AMD). This review covers the major degradative pathways (ubiquitin-proteasome and lysosomal involvement in phagocytosis and autophagy) in the retinal pigment epithelium (RPE) and summarizes evidence of their involvement in AMD. Degradation of damaged and misfolded proteins via the proteasome occurs in coordination with heat shock proteins. Evidence of increased content of proteasome and heat shock proteins in retinas from human donors with AMD is consistent with increased oxidative stress and extensive protein damage with AMD. Phagocytosis and autophagy share key molecules in phagosome maturation as well as degradation of their cargo following fusion with lysosomes. Phagocytosis and degradation of photoreceptor outer segments ensures functional integrity of the neural retina. Autophagy rids the cell of toxic protein aggregates and defective mitochondria. Evidence suggesting a decline in autophagic flux includes the accumulation of autophagic substrates and damaged mitochondria in RPE from AMD donors. An age-related decrease in lysosomal enzymatic activity inhibits autophagic clearance of outer segments, mitochondria, and protein aggregates, thereby accelerating the accumulation of lipofuscin. This cumulative damage over a person's lifetime tips the balance in RPE from a state of para-inflammation, which strives to restore cell homeostasis, to the chronic inflammation associated with AMD.

Photic injury to cultured RPE varies among individual cells in proportion to their endogenous lipofuscin content as modulated by their melanosome content

PURPOSE

We determined whether photic stress differentially impairs organelle motility of RPE lipofuscin and melanin granules, whether lethal photic stress kills cells in proportion to lipofuscin abundance, and whether killing is modulated by melanosome content.

METHODS

Motility of endogenous lipofuscin and melanosome granules within the same human RPE cells in primary culture was quantified by real-time imaging during sublethal blue light irradiation. Cell death during lethal irradiation was quantified by dynamic imaging of the onset of nuclear propidium iodide fluorescence. Analyzed were individual cells containing different amounts of autofluorescent lipofuscin, or similar amounts of lipofuscin and a varying content of phagocytized porcine melanosomes, or phagocytized black latex beads (control for light absorbance).

RESULTS

Lipofuscin granules and melanosomes showed motility slowing with mild irradiation, but slowing was greater for lipofuscin. On lethal irradiation, cell death was earlier in cells with higher lipofuscin content, but delayed by the copresence of melanosomes. Delayed death did not occur with black beads, suggesting that melanosome protection was due to properties of the biological granule, not simple screening.

CONCLUSIONS

Greater organelle motility slowing of the more photoreactive lipofuscin granule compared to melanosomes suggests that lipofuscin mediates mild photic injury within RPE cells. With lethal light stress endogenous lipofuscin mediates killing, but the effect is cell autonomous and modulated by coincident melanosome content. Developing methods to quantify the frequency of individual cells with combined high lipofuscin and low melanosome content may have value for predicting the photic stress susceptibility of the RPE monolayer in situ.

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.

A2E induces IL-1ß production in retinal pigment epithelial cells via the NLRP3 inflammasome

AIMS

With ageing extracellular material is deposited in Bruch's membrane, as drusen. Lipofuscin is deposited in retinal pigment epithelial cells. Both of these changes are associated with age related macular degeneration, a disease now believed to involve chronic inflammation at the retinal-choroidal interface. We hypothesise that these molecules may act as danger signals, causing the production of inflammatory chemokines and cytokines by the retinal pigment epithelium, via activation of pattern recognition receptors.

METHODS

ARPE-19 cells were stimulated in vitro with the following reported components of drusen: amyloid-ß (1-42), Carboxyethylpyrrole (CEP) modified proteins (CEP-HSA), Nε-(Carboxymethyl)lysine (CML) modified proteins and aggregated vitronectin. The cells were also stimulated with the major fluorophore of lipofuscin: N-retinylidene-N-retinylethanolamine (A2E). Inflammatory chemokine and cytokine production was assessed using Multiplex assays and ELISA. The mechanistic evaluation of the NLRP3 inflammasome pathway was assessed in a stepwise fashion.

RESULTS

Of all the molecules tested only A2E induced inflammatory chemokine and cytokine production. 25 µM A2E induced the production of significantly increased levels of the chemokines IL-8, MCP-1, MCG and MIP-1α, the cytokines IL-1ß, IL-2, IL-6, and TNF-α, and the protein VEGF-A. The release of IL-1ß was studied further, and was determined to be due to NLRP3 inflammasome activation. The pathway of activation involved endocytosis of A2E, and the three inflammasome components NLRP3, ASC and activated caspase-1. Immunohistochemical staining of ABCA4 knockout mice, which show progressive accumulation of A2E levels with age, showed increased amounts of IL-1ß proximal to the retinal pigment epithelium.

CONCLUSIONS

A2E has the ability to stimulate inflammatory chemokine and cytokine production by RPE cells. The pattern recognition receptor NLRP3 is involved in this process. This provides further evidence for the link between A2E, inflammation, and the pathogenesis of AMD. It also supports the recent discovery of NLRP3 inflammasome activation in AMD.

Retinal photodamage by endogenous and xenobiotic agents

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

WITHDRAWN: Complement dysregulation in AMD: RPE-Bruch's membrane-choroid

The Publisher regrets that this article is an accidental duplication of an article that has already been published, doi:10.1016/j.mam.2012.03.011. The duplicate article has therefore been withdrawn.

Complement dysregulation in AMD: RPE-Bruch's membrane-choroid

The question as to why the macula of the retina is prone to an aging disease (age-related macular degeneration) remains unanswered. This unmet challenge has implications since AMD accounts for approximately 54% of blindness in the USA (Swaroop, Chew, Bowes Rickman and Abecasis, 2009). While AMD has onset in the elder years, it likely develops over time. Genetic discovery to date has accounted for approximately 50% of the inheritable component of AMD. The polymorphism that has been most widely studied is the Y402H allele in the complement factor H gene. The implication of this genetic association is that in a subset of AMD cases, unregulated complement activation is permissive for AMD. Given that this gene variant results in an amino acid substitution, it is assumed that this change will have functional consequences although the precise mechanisms are still unknown. Genetic predisposition is not the only factor however, since in this complex disease there is substantial evidence that lifestyle factors such as diet and smoking contribute to risk. Here we provide an overview of current knowledge with respect to factors involved in AMD pathogenesis. Interwoven with these issues is a discussion of the significant role played by aging processes, some of which are unique to the retina and retinal pigment epithelium. One recurring theme is the potential for disease promotion by diverse types of oxidation products.

The bisretinoids of retinal pigment epithelium

The retina exhibits an inherent autofluorescence that is imaged ophthalmoscopically as fundus autofluorescence. In clinical settings, fundus autofluorescence examination aids in the diagnosis and follow-up of many retinal disorders. Fundus autofluorescence originates from the complex mixture of bisretinoid fluorophores that are amassed by retinal pigment epithelial (RPE) cells as lipofuscin. Unlike the lipofuscin found in other cell-types, this material does not form as a result of oxidative stress. Rather, the formation is attributable to non-enzymatic reactions of vitamin A aldehyde in photoreceptor cells; transfer to RPE occurs upon phagocytosis of photoreceptor outer segments. These fluorescent pigments accumulate even in healthy photoreceptor cells and are generated as a consequence of the light capturing function of the cells. Nevertheless, the formation of this material is accelerated in some retinal disorders including recessive Stargardt disease and ELOVL4-related retinal degeneration. As such, these bisretinoid side-products are implicated in the disease processes that threaten vision. In this article, we review our current understanding of the composition of RPE lipofuscin, the structural characteristics of the various bisretinoids, their related spectroscopic features and the biosynthetic pathways by which they form. We will revisit factors known to influence the extent of the accumulation and therapeutic strategies being used to limit bisretinoid formation. Given their origin from vitamin A aldehyde, an isomer of the visual pigment chromophore, it is not surprising that the bisretinoids of retina are light sensitive molecules. Accordingly, we will discuss recent findings that implicate the photodegradation of bisretinoid in the etiology of age-related macular degeneration.

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

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

Compositional studies of human RPE lipofuscin: mechanisms of molecular modifications

The accumulation of lipofuscin has previously been implicated in several retinal diseases including Best's macular dystrophy, Stargardt's disease and age-related macular degeneration (AMD). Previously one of the major fluorophores of lipofuscin was identified as a bis-retinoid pyridinium salt called A2E, which is known to photochemically cause damage. In addition to A2E, there are numerous components in RPE lipofuscin that are unidentified. These compounds were determined to be structurally related to A2E by their fragmentation pattern with losses of 106, 190, 174 and/or 150 amu from the parent ion and the formation of fragments of ca 592 amu. The vast majority consists of relatively hydrophobic components corresponding to derivatized A2E with molecular weights in discrete groups of 800-900, 970-1080 and > 1200 m/z regions. In order to determine the mechanism of these modifications, A2E was chemically modified by; (1) the formation of specific esters, (2) reaction with specific aldehydes and (3) spontaneous auto-oxidation. The contribution of ester formation to the naturally occurring components of lipofuscin was discounted since their fragmentation patterns were different to those found in vivo. Alternatively, reactions with specific aldehydes result in nearly identical products as those found in vivo. Artificial aging of RPE lipofuscin gives a complex mixture of structurally related components. This results from the auto- and/or photooxidation of A2E to form aldehydes, which then back react with A2E giving a series of higher molecular weight products. The majority of these modifications result in compounds that are much more hydrophobic than A2E. These higher molecular weight materials have increased values of log P compared to A2E. This increase in hydrophobicity most likely aids in the sequestering of A2E into granules with the concomitant diminution of its reactivity. Therefore, these processes may serve as protective mechanisms for the RPE.

Intraocular lens short wavelength light filtering

There is increasing interest in the effects of reactive oxygen species ('free radicals') in ageing, both in the body overall and specifically in the eye. Cataract and age-related macular degeneration (AMD) are two major causes of blindness, with cataract accounting for 48 per cent of world blindness and AMD accounting for 8.7 per cent. Both cataract and AMD affect an older population (over 50 years of age) and while cataract is largely treatable provided resources are available, AMD is a common cause of untreatable, progressive visual loss. There is evidence that AMD is linked to exposure to short wavelength electromagnetic radiation, which includes ultraviolet, blue and violet wavelengths. The ageing crystalline lens provides some protection to the posterior pole because, as it yellows with age, its spectral absorption increasingly blocks the shorter wavelengths of light. Ultraviolet blocking intraocular lenses (IOLs) have been the standard of care for many years but a more recent trend is to include blue-blocking filters based on theoretical benefits. As these filters absorb part of the visible spectrum, they may affect visual function. This review looks at the risks and the benefits of filtering out short wavelength light in pseudophakic patients.

Compositional studies of human RPE lipofuscin

Age-related macular degeneration (AMD) is an ocular disease that causes visual loss and legal blindness in the elderly population. The etiology of AMD is complex and may include genetic predispositions, accumulation of lipofuscin and drusen, local inflammation and neovascularization. The accumulation of lipofuscin has been shown to precede the death of photoreceptor cells and the deterioration of the RPE. As a result, the determination of the photosensitive components of lipofuscin has been of major interest. One of these components, previously identified as a bis-retinoid pyridinium compound, is referred to as A2E. A2E has been characterized by mass spectrometry and is known to have a mass of 592 Da. Most remaining chromophores in RPE lipofuscin are structurally related to A2E as determined by their fragmentation pattern with losses of M ± 190, 174 and/or 150 Da. Analysis of lipofuscin from various donors indicated that the extracts consist of as many as 15 of these hydrophobic components, which are also observed to form spontaneously in vitro over extended periods of time. These consist of ca 90% of the A2E-like components in RPE lipofuscin and correspond to derivatized A2E with discrete molecular weights of 800-900 m/z, 970-1080 m/z and above 1200 m/z regions. It was determined that these species are formed from self-reaction of A2E oxidation products or their reaction with A2E itself to form higher molecular weight products. The majority of modifications are much more hydrophobic than A2E and exhibit increasingly higher values of log P. This acts as a driving force for the sequestering of A2E into granules resulting in a concomitant diminution of its reactivity in vivo.

Electron Paramagnetic Resonance - A Powerful Tool of Medical Biochemistry in Discovering Mechanisms of Disease and Treatment Prospects

In pathophysiological conditions related to oxidative stress, the application of selected antioxidants could have beneficial effects on human health. Electron paramagnetic resonance (EPR) spectroscopy is a technique that provides unique insight into the redox biochemistry, due to its ability to: (i) distinguish and quantify different reactive species, such as hydroxyl radical, superoxide, carbon centered radicals, hydrogen atom, nitric oxide, ascorbyl radical, melanin, and others; (ii) evaluate the antioxidative capacity of various compounds, extracts and foods; (iii) provide information on other important parameters of biological systems. A combination of EPR spectroscopy and traditional biochemical methods represents an efficient tool in the studies of disease mechanisms and antioxidative therapy prospects, providing a more complete view into the redox processes in the human organism.

Nitrite-Modified Extracellular Matrix Proteins Deleteriously Affect Retinal Pigment Epithelial Cell Function and Viability: A Comparison Study with Nonenzymatic Glycation Mechanisms

PURPOSE

Extracellular matrix (ECM) plays an important role in the regulation of cell function. The aging process may involve chemical modifications to ECM proteins, which may contribute to the aging of the Bruch membrane and pathogenesis of age-related macular degeneration (AMD). The purpose of this study is to investigate nitrite modification of basement membrane-like proteins on RPE cell behavior as a model for the aging of the Bruch membrane in age-related eye diseases. As a comparison, retinal pigment epithelium (RPE) cell behavior on glycolaldehyde-modified matrices (GMM) was also studied.

METHODS

Growth factor reduced Matrigel was reacted with nitrite or glycolaldehyde for 1 week or 12 hr, respectively. Calf RPE cells were plated on the modified matrices and examined in several ways. Attachment rates, proliferation rates, apoptosis, and necrosis were determined. Cell morphology and cell susceptibility to A2E-mediated damage was also monitored.

RESULTS

Nitrite-modified matrices (NMMs) inhibited cell attachment by 65% and proliferation by 33.7% compared to 69.6% and 21.7%, respectively, by GMM. Proliferation inhibition was not significant when cells were plated at high density on GMM (3.47%) but significant on NMM (20.9%). NMM induced cell apoptosis and necrosis, but GMM induced cell apoptosis only. Both modifications inhibited RPE differentiation. RPE cells on both matrices were more susceptible to blue light mediated damage by A2E, but damage was greater on NMM.

CONCLUSIONS

NMM has significant damaging effects on RPE cell function and viability that is similar to the damaging effects of GMM. These studies may have relevance to the RPE dysfunction observed during the progression of AMD.

Mechanisms involved in A2E oxidation

A2E is one of the bis-retinoid pyridinium compounds that accumulate as lipofuscin pigments in retinal pigment epithelial (RPE) cells in association with aging and in some inherited forms of retinal degeneration. Here we observed that 430nm irradiation of A2E in the presence of the spin trap DMPO, led to the appearance of a superoxide dismutase-inhibitable electron paramagnetic resonance (EPR) spectrum characteristic of DMPO-OH; this finding was indicative of hydroxyl radical (OH) formation following initial spin trapping of superoxide anion by DMPO. We also observed an increase in dihydroethidium (HEt) fluorescence and luminol-based chemiluminescence that on the basis of inhibition by superoxide dismutase, was indicative of superoxide anion generation when A2E was irradiated at 430nm in cell-free systems. Nevertheless, while A2E was readily oxidized in the presence of a singlet oxygen generator, superoxide anion did not serve to oxidize A2E. Specifically, by HPLC quantitation and FAB-mass spectroscopy, there was no evidence of A2E oxidation when A2E was incubated with a superoxide anion generator (xanthine/xanthine oxidase) in a variety of solvents (100% PBS, 30% DMSO in PBS, 100% MeOH and CHCl3) or in the presence of detergent. On the other hand, however, peroxy-A2E, an oxidized form of A2E with an endoperoxide moiety on the short-arm of the molecule, readily underwent further oxygen addition when incubated with xanthine/xanthine oxidase. Superoxide anion may be generated by irradiation of A2E but is not involved in the early events that oxidize A2E. Superoxide can contribute to the further oxidation of already-oxidized A2E.

UV-A induced oxidative stress is more prominent in naturally pigmented aged human RPE cells compared to non-pigmented human RPE cells independent of zinc treatment

To investigate the effects of zinc supplementation on human amelanotic (ARPE-19) and native pigmented retinal pigment epithelial cells (hRPE) under normal light conditions and after ultraviolet A light exposure. hRPE cells, containing both melanin and lipofuscin granules, were prepared from human donor eyes of 60-70 year old patients. Cells of the amelanotic ARPE-19 cell line and pigmented hRPE cells were treated with zinc chloride and subjected to oxidative stress by UV-A irradiation. Intracellular H(2)O(2) formation was measured using a fluorescence oxidation assay. Additionally, apoptosis and viability assays were performed. Control cells were treated identically except for irradiation and zinc supplementation. Under normal light conditions, zinc treated hRPE cells produced less H(2)O(2) than unsupplemented hRPE cells. Viability and apoptosis events did not change. After UV-A irradiation, ARPE and hRPE cells were greatly impaired in all tests performed compared to the non-irradiated controls. No differences were found after zinc supplementation. hRPE cells showed a higher apoptosis and mortality rate than non-pigmented cells when stressed by UV-A light. ARPE cells never showed any zinc related effects. In contrast, without irradiation, zinc supplementation reduced H(2)O(2) production in pigmented hRPE cells slightly. We did not find any zinc effect in irradiated hRPE cells. After UV light exposure, pigmented cells showed a higher apoptosis and mortality than cells lacking any pigmentation. We conclude that cells with pigmentation consisting of melanin and lipofuscin granules have more prooxidative than antioxidative capacity when stressed by UV light exposure compared to cells lacking any pigmentation.

Light-induced damage to the retina: role of rhodopsin chromophore revisited

The presence of the regenerable visual pigment rhodopsin has been shown to be primarily responsible for the acute photodamage to the retina. The photoexcitation of rhodopsin leads to isomerization of its chromophore 11-cis-retinal to all-trans-retinal (ATR). ATR is a potent photosensitizer and its role in mediating photodamage has been suspected for over two decades. However, there was lack of experimental evidence that free ATR exists in the retina in sufficient concentrations to impose a risk of photosensitized damage. Identification in the retina of a retinal dimer and a pyridinium bisretinoid, so called A2E, and determination of its biosynthetic pathway indicate that substantial amounts of ATR do accumulate in the retina. Both light damage and A2E accumulation are facilitated under conditions where efficient retinoid cycle operates. Efficient retinoid cycle leads to rapid regeneration of rhodopsin, which may result in ATR release from the opsin "exit site" before its enzymatic reduction to all-trans-retinol. Here we discuss photodamage to the retina where ATR could play a role as the main toxic and/or phototoxic agent. Moreover, we discuss secondary products of (photo)toxic properties accumulating within retinal lipofuscin as a result of ATR accumulation.

Oxidation of A2E Results in the Formation of Highly Reactive Aldehydes and Ketones

It has been reported that the photo-oxidation of A2E, a component of human retinal lipofuscin, leads to products that are toxic to cells via dark reactions. Because these compounds have been implicated in the development of various maculopathies such as age-related macular degeneration (AMD), it is important to determine the structures of those deleterious compounds. Both the photo-oxidation and auto-oxidation of A2E lead to the same complex mixture of products, some of which have lower molecular weights than the staring material. Because A2E is homologous to beta-carotene, it was hypothesized that its oxidation would lead to products analogous to those found in oxidized beta-carotene, namely, a series of cleavage products along the acyclic chain with the concomitant formation of aldehydes. This was found to be the case based upon 1) the formation of all of the aldehydes predicted from the oxidation of beta-carotene, 2) the loss of 28 amu (carbonyl moiety) from the molecular ion, 3) the facile reaction of the aldehydes with nitrophenylhydrazines to form nitrophenylhydrazones and 4) the subsequent MS/MS cleavage of those derivatives at the N-N bond. If formed in vivo, these aldehydes would have toxic effects on any cell. Finally, the similarity in product mixtures from both the photo-oxidation and auto-oxidation strongly suggests that the intermolecular photo-oxidation of A2E results primarily from a radical process without the involvement of singlet oxygen. Any formation of singlet oxygen most likely arises from sensitization by the aldehyde oxidation products, as this process is well known for aldehydes, in general, and retinal, specifically.

Spectroscopy and Photoreactivity of Trichochromes: Molecular Components of Pheomelanins†

The trichochromes are a class of small molecules present in pheomelanin (the red melanin) and absent in eumelanin (the black melanin). Herein trichochrome F (TF) and decarboxytrichochrome C (dTC) are examined. Both trichochromes are characterized by a visible absorption band, which is shown to be the result of overlapping transitions of the cis and trans isomers. The temperature dependence of the absorption spectrum of dTC suggests the additional presence of equilibrium between the enol and keto forms of the molecule. These conclusions are supported by ground-state energies of these isomers obtained using a continuum solvation model. Near-infrared emission measurements were not able to detect photoproduction of 1O2, and spin-trapping experiments revealed formation of O2*-. DNA nicking assays also revealed a low level of light-induced aerobic activity of dTC, suggesting a quantum efficiency of at most 5 x 10(-6) for the photogeneration of O2*-. These results are consistent with pump-probe optical experiments, which reveal efficient and nearly complete ground-state recovery within a few picoseconds of excitation. Both trichochromes are efficient quenchers of 1O2, exhibiting a bimolecular rate constant comparable with vitamin C. These results suggest that trichochromes could serve a protective role in pheomelanin pigments.

Spectroscopic properties and reactivity of free radical forms of A2E

A pyridinium bisretinoid (A2E) is the only identified blue-absorbing chromophore of retinal lipofuscin that has been linked to its aerobic photoreactivity and phototoxicity. Pulse radiolysis has been used to study both the one-electron oxidation and the one-electron reduction of A2E in aqueous micellar solutions. The reduction to the semireduced A2E (lambda(max) broad and between 500 and 540 nm) was achieved with formate radicals and the subsequent decay of A2E* was slow (over hundreds of milliseconds) via complex kinetics. The long lifetime of the A2E* should facilitate its reactions with other biomolecules. For example, with oxygen, the A2E* produced the superoxide radical anion with a rate constant of 3 x 10(8) M(-1) s(-1). The A2E was also reduced by the NAD radical, the corresponding rate constant being 2.3 x 10(8) M(-1) s(-1). Other experiments showed that the one-electron reduction potential of A2E lies in the range -640 to -940 mV. The semioxidized form of A2E (lambda(max) 590 nm) was formed via oxidation with the Br2*- radical and had a much shorter lifetime than the semireduced form. With strongly oxidizing peroxyl radicals (CCl3O2*) our kinetic data suggest the formation of a radical adduct followed by dissociation to the semioxidized A2E. With milder oxidizing peroxyl radicals such as that from methanol, our results were inconclusive. In benzene we observed an efficient oxidation of zeaxanthin to its radical cation by the A2E radical cation; this may be relevant to a detrimental effect of A2E in vision.

A New Approach to Measuring the Action Spectrum for Singlet Oxygen Production by Human Retinal Lipofuscin

The human retinal pigment epithelial (RPE) layer contains a complex mixture of components called lipofuscin; this mixture forms with age and with various genetic disorders such as Stargardt's disease. Its presence may contribute to retinal deterioration via several mechanisms including photochemical processes. In the lipofuscin mixture, both type I and II mechanisms have been identified, with the latter consisting of the generation of singlet oxygen. Several components of that mixture have been identified, most notably a bis-retinoid pyridinium compound called A2E and its derivatives. Photooxidative studies on the compound A2E have revealed that its dominant photochemical mechanism is via free radical or type I processes. Because singlet oxygen is an important photooxidative intermediate in tissue, its generation in the RPE may contribute to retinal maculopathies. It is therefore necessary to determine which specific component(s) in the lipofuscin mixture produce singlet oxygen upon excitation with light. This was ascertained by evaluating the action spectrum for singlet oxygen production for the whole lipofuscin mixture using time-resolved spectroscopy. Singlet oxygen was generated by excitation of the sample at different wavelengths while maintaining a constant beam energy, and was directly detected by its phosphorescence decay at 1270 nm using a Ge photodiode. The action spectrum for singlet oxygen sensitization by the organic soluble portion of lipofuscin had an absorption maximum at ca 380 nm, which is to the blue of A2E (maximum at 430 nm). Compounds with a similar absorption maximum eluted in the HPLC earlier than A2E and were detected in human lipofuscin. The concentration of this component apparently increased in concentration in human RPE lipofuscin mixture as a function of age up to 90 years old.

Transmission of light to the aging human retina: possible implications for age related macular degeneration

The purpose of this study is to determine the transmission properties of the anterior segment of the human eye as a function of age and relate those changes to possible consequences for retinal disorders. For this a new method has been developed. This consists of a probe which is inserted into the posterior sclera and detects light passing through the anterior segment. The probe is connected to a CCD spectrophotometer via a fibre optic bundle. Using this, the transmission properties of human cadaver eyes were determined. A young primate anterior segment has a maximum absorption of 365 nm due to the O-beta-glucoside of 3-hydroxykynurenine (3-HKG) in the lens. There is a steep increase in transmission of the human anterior segment at wavelengths longer than 400 nm. With aging there is an increase in absorption throughout the visible such that by the sixth decade only 20% of blue light is transmitted to the retina compared to the young primate eye. The rate of decrease of blue light was similar to the age related change of the ratio of absorbance at 365/320 nm of the lens. (IOVS 41:1454;1999). The age related rate of decrease in the transmission of blue light to the retina was similar to the rate of increase of lipofuscin formation in the retina, and the amount of light absorbed by A2E in the RPE is constant from the second to seventh decade. Although this yellowing is thought to be detrimental to the lens, it would appear to be beneficial to the retina. It was determined that the implantation of a standard IOL after cataract surgery increased the amount of light absorbed by A2-E by approximately a factor of five.

The photochemical oxidation of A2E results in the formation of a 5,8,5′,8′-bis-furanoid oxide

With age and certain diseases, the retinal pigment epithelium (RPE) accumulates a complex mixture of fluorophores generally called lipofuscin. This mixture has been implicated in those diseases via several possible mechanisms including photochemically mediated damage and detergent effects. Recently, a component of that mixture has been identified, a bis-retinoid pyridinium compound called A2E. Furthermore, the photolysis of this compound has been shown to lead to a series of photooxidation products each differing by 16 amu; with the addition of up to a total of nine oxygen atoms to A2E. It has been suggested that these products consist of a series of epoxides forming along the acyclic side chains. The present study reinvestigates this hypothesis and demonstrates that the bis-oxygenated product is in fact a relatively stable 5-8 5'-8'-bis-furanoid oxide. This is in agreement with the oxidation products observed for a homologue of A2E, beta-carotene. In addition, it was found that the bis-oxygenated photoproduct of A2E contained a mixture of symmetrical and unsymmetrical oxidized products where the second oxygen was present in the cyclohexenyl ring structure in the unsymmetrical product. Most importantly, both of these oxidation products of A2E were detected in lipofuscin extracts from human RPE cells, suggesting that these processes occur in vivo.

A mechanistic study of the photooxidation of A2E, a component of human retinal lipofuscin

A major constituent of human retinal lipofuscin is A2E (2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-hexatrienyl]-pyridinium). Light transmitted by the lens is absorbed by A2E and the processes initiated by this absorption has been implicated in several maculopothies. The purpose of this study was to evaluate the dominant photochemical mechanisms involved in these reactions, whether through free radical or singlet oxygen intermediacy. The photodestruction of A2E occurs faster in water vs. chloroform and hydrogenated vs. perdeuterated methanol. Both results suggest a free radical mechanism. Product distributions indicate sequential oxygen addition rather than the addition of two oxygen atoms which would be expected if singlet oxygen was an intermediate. Finally, EPR trapping studies lead to the detection of superoxide as the primary intermediate in the photochemical reactions. It is concluded that if singlet oxygen is involved in these photochemical processes it is of minor importance.

Observation of A2E oxidation products in human retinal lipofuscin

Several retinal dystrophies are associated with the accumulation of lipofuscin in the retinal pigment epithelium (RPE). The only structurally characterized component of human retinal lipofuscin is the bis-retinoid pyridinium compound A2E. We report here on the observation of a monooxygenated product of A2E in the organic soluble portion of human retinal lipofuscin and in the organic extract of bovine RPE cells that have been fed A2E and irradiated. Liquid chromatography mass spectrometry confirms that the products are identical. This is the first observation of a photoproduct of A2E in human retinal lipofuscin.

Screening for ocular phototoxicity

Normally light transmission through the eye is benign and serves to direct vision and circadian rhythm. However, with very intense light exposure, or with ambient light exposure to the aged eye and/or young or adult eye in the presence of light-activated (photosensitizing) drugs or dietary supplements, cosmetics, or diagnostic dyes, light can be hazardous, leading to blinding disorders. Light damage to the human eye is avoided because the eye is protected by a very efficient antioxidant system and the chromophores present absorb light and dissipate its energy. After middle age, there is a decrease in the production of antioxidants and antioxidant enzymes and an accumulation of endogenous chromophores that are phototoxic. The extent to which a particular exogenous photosensitizing substance is capable of producing phototoxic side effects in the eye depends on several parameters, including (1) the chemical structure; (2) the absorption spectra of the drug; (3) binding of the drug to ocular tissue (lens proteins, melanin, DNA); and (4) the ability to cross blood-ocular barriers (amphiphilic or lipophilic). For instance, compounds that have either a tricyclic, heterocyclic, or porphyrin ring structure and are incorporated into ocular tissues are potentially phototoxic agents in the eye. The extent to which these substances might damage the eye (photoefficiency) can be predicted using in vitro and photophysical techniques. With simple, inexpensive testing, compounds can be screened for their potential ocular phototoxicity at the developmental stage. It may be that a portion of the molecule can be modified to reduce phototoxicity while leaving the primary drug effect intact. Preclinical safety testing may prevent ocular side effects that can range from mild, reversible blurred vision to permanent blindness.

The role of A2E in prevention or enhancement of light damage in human retinal pigment epithelial cells

The process of sight (photostasis) produces, as a by-product, a chromophore called 2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E, 5E,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E, 3E, 5E-hexatrienyl]-pyridinium (A2E), whose function in the eye has not been defined as yet. In youth and adulthood, A2E is removed from human retinal pigment epithelial (h-RPE) cells as it is made, and so it is present in very low concentrations, but with advanced age, it accumulates to concentrations reaching 20 microM. In the present study we have used photophysical techniques and in vitro cellular measurements to explore the role of A2E in h-RPE cells. We have found that A2E has both pro- and antioxidant properties. It generated singlet oxygen (phiso = 0.004) much less efficiently than its precursor trans-retinal (phiso = 0.24). It also quenched singlet oxygen at a rate (10(8) M(-1) s(-1)) equivalent to two other endogenous quenchers of reactive oxygen species in the eye: alpha-tocopherol (vitamin E) and ascorbic acid (vitamin C). The endogenous singlet oxygen quencher lutein, whose quenching rate is two orders of magnitude greater than that of A2E, completely prevented light damage in vitro, suggesting that singlet oxygen does indeed play a role in light-induced damage to aged human retinas. We have used multiphoton confocal microscopy and the comet assay to measure the toxic, phototoxic and protective capacity of A2E in h-RPE cells. At 1-5 microM, A2E protected these cells from UV-induced breaks in DNA; at 20 microM, A2E no longer exerted this protective effect. These results imply that the role of A2E is not simple and may change over the course of a lifetime. A2E itself may play a protective role in the young eye but a toxic role in older eyes.

Ocular phototoxicity

The human eye is constantly exposed to sunlight and artificial lighting. Therefore the eye is exposed to UV-B (295-320 nm), UV-A (320-400 nm), and visible light (400-700 nm). Light is transmitted through the eye and then signals the brain directing both sight and circadian rhythm. Therefore light absorbed by the eye must be benign. Damage to the young and adult eye by intense ambient light is avoided because the eye is protected by a very efficient antioxidant system. In addition, there are protective pigments such as the kynurenines, located in the human lens, and melanin, in the uvea and retina, which absorb ambient radiation and dissipate its energy without causing damage. After middle age there is a decrease in the production of antioxidants and antioxidant enzymes. At the same time, the protective pigments are chemically modified (lenticular 3-hydroxy kynurenine pigment is enzymatically converted into the phototoxic chromophore xanthurenic acid; melanin is altered from an antioxidant to pro-oxidant) and fluorescent chromophores (lipofuscin) accumulate to concentrations high enough to produce reactive oxygen species. We have known for some time that exposure to intense artificial light and sunlight either causes or exacerbates age-related ocular diseases. We now know many of the reasons for these effects, and with this knowledge methods are being developed to interfere with these damaging processes.

Photochemical studies of A2-E

Lipofuscin is thought to be involved in age-related macular degeneration as is one of its proposed components, an amphiphillic pyridinium-based bis-retinoid with a quaternary nitrogen atom, known as A2-E. We report the triplet state spectra obtained from photosensitisation using anthracene and 1-nitronaphthalene in benzene and methanol. The triplet state of A2-E has lambda(max) at 550 nm and a lifetime of approximately 30 micros, it is efficiently quenched by molecular oxygen with a second-order quenching rate constant of approximately 1 x 10(9) dm(3) mol(-1) s(-1). There is no significant triplet state formation from direct laser excitation of A2-E and hence its quantum yield of triplet state formation must be <0.01.

Environmental effects on the photochemistry of A2-E, a component of human retinal lipofuscin

Several retinal dystrophies are associated with the accumulation of lipofuscin, a pigment mixture, in the retinal pigment epithelium (RPE). One of the major fluorophores of this mixture has been identified as the bis-retinoid pyridinium compound, A2-E. Because this compound absorbs incident radiation that is transmitted by the anterior segment of the human eye, photophysical and photochemical studies were performed to determine if A2-E could photosensitize potentially damaging reactions. Steady-state fluorescence measurements indicate that the fluorescence emission maximum and quantum yield are very sensitive to the chemical environment and a correlation between these two parameters and the solvent dielectric constant is observed. Time-resolved absorption experiments of A2-E in pure organic solvents showed no formation of transient species on the timescale of our experiments. However, when these measurements were repeated for A2-E in Triton X-100 micelles, a short-lived (tau approximately 14 microseconds), weak absorption was observed. This species is quenched by oxygen (k = 2 x 10(9) M-1 s-1) and by the addition of the antioxidants, cysteine and N,N,N',N'-tetramethylphenylenediamine. Quenching of this species by 2,3,5-trimethylhydroquinone results in the formation of the 2,3,5-trimethylsemiquinone free radical and an increase in yield of the A2-E-derived species. Sensitization of the A2-E triplet excited state indicates that the species observed in micelles upon direct excitation is not consistent with the triplet excited state. Based on these data we tentatively assign this absorption to a free radical. In the RPE these initial processes can ultimately lead to damage to the tissue through the formation of peroxides and other oxidized species.

Photocytotoxicity of lipofuscin in human retinal pigment epithelial cells

Lipofuscin accumulates with age in a variety of highly metabolically active cells, including the retinal pigment epithelium (RPE) of the eye, where its photoreactivity has the potential for cellular damage. The aim of this study was to assess the phototoxic potential of lipofuscin in the retina. RPE cell cultures were fed isolated lipofuscin granules and maintained in basal medium for 7 d. Control cells lacking granules were cultured in an identical manner. Cultures were either maintained in the dark or exposed to visible light (2.8 mWcm2) at 37 degrees C for up to 48 h. Cells were subsequently assessed for alterations in cell morphology, cell viability, lysosomal stability, lipid peroxidation, and protein oxidation. Exposure of lipofuscin-fed cells to short wavelength visible light (390-550 nm) caused lipid peroxidation (increased levels of malondialdehyde and 4-hydroxy-nonenal), protein oxidation (protein carbonyl formation), loss of lysosomal integrity, cytoplasmic vacuolation, and membrane blebbing culminating in cell death. This effect was wavelength-dependent because light exposure at 550 to 800 nm had no adverse effect on lipofuscin-loaded cells. These results confirm the photoxicity of lipofuscin in a cellular system and implicate it in cell dysfunction such as occurs in ageing and retinal diseases.