Structural characterization of bisretinoid A2E photocleavage products and implications for age-related macular degeneration

Dietary antioxidants prevent age-related retinal pigment epithelium actin damage and blindness in mice lacking αvβ5 integrin

In the aging human eye, oxidative damage and accumulation of pro-oxidant lysosomal lipofuscin cause functional decline of the retinal pigment epithelium (RPE), which contributes to age-related macular degeneration. In mice with an RPE-specific phagocytosis defect due to lack of αvβ5 integrin receptors, RPE accumulation of lipofuscin suggests that the age-related blindness we previously described in this model may also result from oxidative stress. Cellular and molecular targets of oxidative stress in the eye remain poorly understood. Here we identify actin among 4-hydroxynonenal (HNE) adducts formed specifically in β5(-/-) RPE but not in neural retina with age. HNE modification directly correlated with loss of resistance of actin to detergent extraction, suggesting cytoskeletal damage in aging RPE. Dietary enrichment with natural antioxidants, grapes or marigold extract containing macular pigments lutein/zeaxanthin, was sufficient to prevent HNE-adduct formation, actin solubility, lipofuscin accumulation, and age-related cone and rod photoreceptor dysfunction in β5(-/-) mice. Acute generation of HNE adducts directly destabilized actin but not tubulin cytoskeletal elements of RPE cells. These findings identify destabilization of the actin cytoskeleton as a consequence of a physiological, sublethal oxidative burden of RPE cells in vivo that is associated with age-related blindness and that can be prevented by consuming an antioxidant-rich diet.

Mechanisms for countering oxidative stress and damage in retinal pigment epithelium

Clinical and experimental evidence supports that chronic oxidative stress is a primary contributing factor to numerous retinal degenerative diseases, such as age-related macular degeneration (AMD). Eyes obtained postmortem from AMD patients have extensive free radical damage to the proteins, lipids, DNA, and mitochondria of their retinal pigment epithelial (RPE) cells. In addition, several mouse models of chronic oxidative stress develop many of the pathological hallmarks of AMD. However, the extent to which oxidative stress is an etiologic component versus its involvement in disease progression remains a major unanswered question. Further, whether the primary target of oxidative stress and damage is photoreceptors or RPE cells, or both, is still unclear. In this review, we discuss the major functions of RPE cells with an emphasis on the oxidative challenges these cells encounter and the endogenous antioxidant mechanisms employed to neutralize the deleterious effects that such stresses can elicit if left unchecked.

The susceptibility of the retina to photochemical damage from visible light

The photoreceptor/RPE complex must maintain a delicate balance between maximizing the absorption of photons for vision and retinal image quality while simultaneously minimizing the risk of photodamage when exposed to bright light. We review the recent discovery of two new effects of light exposure on the photoreceptor/RPE complex in the context of current thinking about the causes of retinal phototoxicity. These effects are autofluorescence photobleaching in which exposure to bright light reduces lipofuscin autofluorescence and, at higher light levels, RPE disruption in which the pattern of autofluorescence is permanently altered following light exposure. Both effects occur following exposure to visible light at irradiances that were previously thought to be safe. Photopigment, retinoids involved in the visual cycle, and bisretinoids in lipofuscin have been implicated as possible photosensitizers for photochemical damage. The mechanism of RPE disruption may follow either of these paths. On the other hand, autofluorescence photobleaching is likely an indicator of photooxidation of lipofuscin. The permanent changes inherent in RPE disruption might require modification of the light safety standards. AF photobleaching recovers after several hours although the mechanisms by which this occurs are not yet clear. Understanding the mechanisms of phototoxicity is all the more important given the potential for increased susceptibility in the presence of ocular diseases that affect either the visual cycle and/or lipofuscin accumulation. In addition, knowledge of photochemical mechanisms can improve our understanding of some disease processes that may be influenced by light exposure, such as some forms of Leber's congenital amaurosis, and aid in the development of new therapies. Such treatment prior to intentional light exposures, as in ophthalmic examinations or surgeries, could provide an effective preventative strategy.

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.

Increased cone sensitivity to ABCA4 deficiency provides insight into macular vision loss in Stargardt's dystrophy

Autosomal recessive Stargardt macular dystrophy is caused by mutations in the photoreceptor disc rim protein ABCA4/ABCR. Key clinical features of Stargardt disease include relatively mild rod defects such as delayed dark adaptation, coupled with severe cone defects reflected in macular atrophy and central vision loss. In spite of this clinical divergence, there has been no biochemical study of the effects of ABCA4 deficiency on cones vs. rods. Here we utilize the cone-dominant Abca4(-/-)/Nrl(-/-) double knockout mouse to study this issue. We show that as early as post-natal day (P) 30, Abca4(-/-)/Nrl(-/-) retinas have significantly fewer rosettes than Abca4(+/+)/Nrl(-/-) retinas, a phenotype often associated with accelerated degeneration. Abca4-deficient mice in both the wild-type and cone-dominant background accumulate more of the toxic bisretinoid A2E than their ABCA4-competent counterparts, but Abca4(-/-)/Nrl(-/-) eyes generate significantly more A2E per mole of 11-cis-retinal (11-cisRAL) than Abca4(-/-) eyes. At P120, Abca4(-/-)/Nrl(-/-) produced 340 ± 121 pmoles A2E/nmol 11-cisRAL while Abca4(-/-) produced 50.4 ± 8.05 pmoles A2E/nmol 11-cisRAL. Nevertheless, the retinal pigment epithelium (RPE) of Abca4(-/-)/Nrl(-/-) eyes exhibits fewer lipofuscin granules than the RPE of Abca4(-/-) eyes; at P120: Abca4(-/-)/Nrl(-/-) exhibit 0.045 ± 0.013 lipofuscingranules/μm² of RPE vs. Abca4(-/-) 0.17 ± 0.030 lipofuscingranules/μm² of RPE. These data indicate that ABCA4-deficient cones simultaneously generate more A2E than rods and are less able to effectively clear it, and suggest that primary cone toxicity may contribute to Stargardt's-associated macular vision loss in addition to cone death secondary to RPE atrophy.

Photo-products of retinal pigment epithelial bisretinoids react with cellular thiols

PURPOSE

Bisretinoids such as A2E that accumulate as components of the lipofuscin of retinal pigment epithelial cells are implicated in some retinal disease processes. These compounds undergo light-induced oxidation and cleavage with the latter releasing of a mixture of aldehyde-bearing fragments, including dicarbonyl methylglyoxal. We tested for the reactivity of photooxidation and photodegradation products of A2E with thiol-containing glutathione (GSH).

METHODS

In cell-free assays, we measured the ability of photooxo-A2E to competitively inhibit the GSH-mediated reduction of the thiol reagent 5,5'-dithiobis-(2-nitrobenzoic acid). Cellular GSH was assayed colorimetrically. Products of GSH reduction and GSH-adducts were detected by electrospray ionization mass spectrometry (ESI-MS) and GSH and oxidized GSH (glutathione disulfide [GSSG]) were quantified from chromatographic peak areas.

RESULTS

We found that GSH can donate hydrogen atoms to, and form conjugates with, photooxidized forms of the bisretinoid A2E and with its photocleavage products. Reaction with non-photooxidized A2E was not observed. Chemical reduction by GSH involved the donation of a hydrogen atom from each of two GSHs. The ratio of GSH consumed to GSSG formed was consistent with GSH being used for both reduction and adduct formation. With the aid of synthesized standards, methylglyoxal-GSH adducts were identified within mixtures of GSH and photooxidized A2E; the adducts formed noncatalytically and by glutathione-S-transferase mediation.

CONCLUSIONS

Reduction and adduct formation by GSH likely limits the reactivity of bisretinoid photoproducts and may aid their elimination from the cells. These findings are significant to forms of macular degeneration associated with bisretinoid formation and maculopathy stemming from GSH synthase deficiency.

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.

Enzymatic degradation of A2E, a retinal pigment epithelial lipofuscin bisretinoid

Some forms of blinding macular disease are associated with excessive accumulation of bisretinoid lipofuscin in retinal pigment epithelial (RPE) cells of the eye. This material is refractory to lysosomal enzyme degradation. In addition to gene and drug-based therapies, treatments that reverse the accumulation of bisretinoid would be beneficial. Thus, we have examined the feasibility of degrading the bisretinoids by delivery of exogenous enzyme. As proof of principle we report that horseradish peroxidase (HRP) can cleave the RPE bisretinoid A2E. In both cell-free and cell-based assays, A2E levels were decreased in the presence of HRP. HRP-associated cleavage products were detected by ultraperformance liquid chromatography (UPLC) coupled to electrospray ionization mass spectrometry, and the structures of the aldehyde-bearing cleavage products were elucidated by 18O-labeling and 1H NMR spectroscopy and by recording UV−vis absorbance spectra. These findings indicate that RPE bisretinoids such as A2E can be degraded by appropriate enzyme activities.

The oral iron chelator deferiprone protects against iron overload-induced retinal degeneration

PURPOSE

Iron-induced oxidative stress may exacerbate age-related macular degeneration (AMD). Ceruloplasmin/Hephaestin double-knockout (DKO) mice with age-dependent retinal iron accumulation and some features of AMD were used to test retinal protection by the oral iron chelator deferiprone (DFP).

METHODS

Cultured retinal pigment epithelial (ARPE-19) cells and mice were treated with DFP. Transferrin receptor mRNA (Tfrc), an indicator of iron levels, was quantified by qPCR. In mice, retinal oxidative stress was assessed by mass spectrometry, and degeneration by histology and electroretinography.

RESULTS

DFP at 60 μM decreased labile iron in ARPE-19 cells, increasing Tfrc and protecting 70% of cells against a lethal dose of H(2)O(2). DFP 1 mg/mL in drinking water increased retinal Tfrc mRNA 2.7-fold after 11 days and also increased transferrin receptor protein. In DKOs, DFP over 8 months decreased retinal iron levels to 72% of untreated mice, diminished retinal oxidative stress to 70% of the untreated level, and markedly ameliorated retinal degeneration. DFP was not retina toxic in wild-type (WT) or DKO mice, as assessed by histology and electroretinography.

CONCLUSIONS

Oral DFP was not toxic to the mouse retina. It diminished retinal iron levels and oxidative stress and protected DKO mice against iron overload-induced retinal degeneration. Further testing of DFP for retinal disease involving oxidative stress is warranted.