Spectral photoprotection of chronic macular photochemical injury

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The associations of autofluorescence changes in age-related macular degeneration (AMD) and Stargardts Dystrophy along with new understanding of role of different photochemistries in creation and modification of the fluorescent bisretinoids support the hypothesis that rising steady-state levels of bisretinoids within the RPE and photoreceptors induce chronic stress and cellular injury. Our biophysical model of bisretinoid pathways with age in humans predicts that spectral sunglasses can remove chronic imbalances between the distinct photochemical processes 1)that create bisretinoids in the photoreceptor outer segments and 2) those that detoxify bisretinoids within the RPE lipofuscin. Since rates of bisretinoid production in the human retina are expected to increase with the square of rate of activation of rhodopsin, cumulative exposures to bright daylight should dominate their production. Therefore we proposed that vermilion, rhodopsin-protecting spectral sunglasses should dramatically reduce bisetinoid production while allowing shorter wavelengths to detoxify accumulated lipofsucin. A common alternative hypothesis is that blue-violet light photochemistries within the retina in the presence of higher levels of retinaldehydes and accumulated bisretinoids drive chronic injury to RPE and photoreceptors through the generation and reaction of reactive oxygen species (ROS). To test these alternative hypotheses, we designed bicolored sunglasses in which one eye is provided a vermilion green-blocking filter that specifically protects rhodopsin and the other eye a yellow blue-violet-blocking filter that specifically protects from short-wavelength photochemical injury. When worn whenever in bright sunlight, such sunglasses allow us to compare changes in both eyes in which only spectral irradiance is changed while genetics, physiology and environmental exposures are otherwise the same. We hypothesize that segregation of A2E into lipofuscin granules and prevention of its redistribution into critical membranes is required for RPE health. To make quantitative predictions, we developed a biophysical model using normal values of pupil size, lens transmission, and rod dark adaptation time constant (trh) to determine average retinal spectral irradiance, steady-state concentration of all-trans-retinal, all-trans-retinal photosensitization of oxidative damage, all-trans-retinal reactions to form A2E-related species in the ROS, and A2E photo-oxidation within RPE lipofuscin granules as a function of age and ambient light intensity. Our model predicts a decline of about one third in the action spectra-weighted short-wavelength macular irradiance with each decade and a nearly constant production rate of A2E-related fluorophores in the RPE during the first 60 years (falling significantly thereafter). A similar age dependence of total lipofuscin granule volume and total fluorescence per RPE cell was reported recently in human cadaver eyes. Since the rates of lipofuscin increase with age are slower than the rate of decrease in short-wavelength macular irradiance in the phakic eye with age, ROI photosensitization in the RPE should also fall with increasing age. Photo-oxidative stress in the outer retina might arise from the smaller amounts of A2E-related fluorophores in critical membranes outside of safer lipofuscin stores. However, if the RPE/BM complex were the site of photo-oxidative injury driving AMD progression, the magnitude and rate of this oxidative injury would be expected to increase dramatically (not observed) following cataract removal and intraocular lens (IOL) implantation. To explain the relationship between bisretinoids and macular degenerations, we propose a novel hypothesis that singlet oxygen generation by RPE lipofuscin allows the chemical alteration of accumulating A2E, thereby limiting the steady-state levels of A2E (A2Ess) in the RPE, the redistribution of A2E into retinal membranes, and associated A2E chemical toxicity. Singlet oxygen generated photochemically within the lipofuscin granule reacts with its A2E to form A2E epoxides which then chemically react to form the increasingly complex cross-linked molecules within the lipofuscin that have not been characterized. As short-wavelength macular irradiance falls with age, the rate of A2E photo-oxidation falls approximately up to 20-fold, causing A2Ess in the normal phakic eye to increase even as rod bleaching and A2E production decrease. Our theoretical model of macular aging reproduces the normal age dependence of lipofuscin and A2E and provides a primary cytotoxic mechanism in which, once A2E exceeds a cytotoxic threshold concentration in the RPE cell, A2E redistribution into critical membranes causes damage and loss of RPE function with or without additional photo-activation. In our model, the yellowing of the lens with age that distorts the spectral balance between rate of production and rate of photo-oxidation found in normal youth, and allows the A2Ess to rise with age. In Stargardts patients mutations in the ABCA4 flippase leads to >5-fold increased production rates of bisretinoids relative to the rates of our proposed RPE photochemical detoxification that results in cytotoxic levels and visual loss at younger ages. In these patients the higher bisretinoid fluorescence and lower contributions of lens autofluorescence, should make identification of spectrally-induced changes within the bisretinoid pathways easier.