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The Role of Blue Light in the Pathogenesis of AMD
(Information Page for Therapists)

PREFACE: In recent years there has been a growing awareness of the effects blue light entering the eye has on the production of Radical Oxygen Species(ROS) in photoreceptor cells and in the adjacent Retinal Pigment Epithelium (RPE) cells. As well, there is an increased understanding of the cumulative effects that blue light absorption in the outer retina has on increasing the level of oxidative stress and resulting chronic inflamation of the outer retina associated with the development of AMD. This has led a number of researchers to recommend that people reduce their lifetime exposure to blue light, whether from sunlight or artificial sources. The discussion below and the subsequent draft update document the growing understanding of the mechanisms by which blue light exposure may contribute to the development of AMD. The discusssion on the influence of blue light on the pathogenesis of AMD that follows below was written a few years ago. This discussion is still valid. The points made are supported by more recent publications, as can seen in the draft update and in the link to some current studies - see links to the update to Current References at bottom of the discussion below.

Although the pathogenesis of AMD is highly complex and not entirely understood, we propose the understandings described in these discussions together with additional recent research now make it possible to establish that cumulative blue light exposure over a lifetime substantially contributes to the development of AMD. We present our explanation of why it is now possible to demonstrate that increasing exposure to blue light can hasten the onset of age-related vison loss by several years on a seperate page - see following link. However, in order to put the information presented on that page in the context of the growing understanding of the pathogenesis of AMD, we believe it would be helpful to read the discussion below and the draft update before proceeding to the page that demonstrates Increasing Lifetime Exposure to Blue Light Substantially Advances the Onset of Age -Related Blindness.


Age-related Macular Degeneration (AMD) is a condition of advanced degeneration of the macular portion of the retina that leads to progressive blindness in over 35% of persons over the age of 75.1 AMD has been linked to the stress engendered by radical oxygen species in macular photoreceptor cells and proximate retinal pigment epithelium cells (RPE).2 Both of these types of cells are non-replicating (post-mitotic) and must respond to a lifetime of oxidative insult, which includes light-induced oxidative stress.3 While there are numerous mechanisms in the retina for preventing and forestalling oxidative insult, by middle-age many of these anti-oxidative mechanisms have begun to break down, which increases the susceptibility of the retina to accumulated damage with increasing age.4

Visible light absorbed by photoreceptors is a significant factor in the production of reactive oxygen species that induce the molecular damage in retinal tissue which appears to contribute to the formation of AMD.5 To an overwhelming degree, blue wavelengths of light produce the most oxidative stress within the retina and are primarily responsible for exacerbating the extent of oxidative damage that has begun to occur.6 Since blue light wavelengths impart the greatest risk of photochemical damage, the risk of retinal damage from visible light is termed "the blue light hazard", with the greatest hazard peaking at wavelength of 440 nm.7

Because the extent of photic damage induced in photoreceptor cells is proportional to the amount of light absorbed by the photo-pigments, blue light potentiates the retina for a much greater degree of photo-oxidative damage by increasing the photon catch capacity by short circuiting the metabolic visual cycle through a process of photoreversal.8 Normally, when photo-pigments absorb light the photoreceptor cell bleaches and becomes unavailable for light absorption until the photo-pigment is reformed through a lengthy metabolic process called the visual cycle.9 However, if the intermediary formed when the photo-pigment absorbs light then absorbs blue light (λ<470 nm), photoreversal can cause a photoreceptor cell to rapidly become unbleached and increase the level of light absorption by photoreceptor cells by several orders of magnitude, which in turn greatly increases the potential for light induced damage.10

An example of blue light-induced photo-oxidative damage is the formation of A2E, an indigestible toxic compound, which is produced in photoreceptor outer segments when light is absorbed by free retinal (trans).11 A2E itself is a photogenerator of damaging radical oxygen species when it absorbs blue light, with peak production occurring at 440 nm.12 High levels of blue light exposure can generate sufficient ROS to cause photoreceptors to be lost, and can result in permanent visual impairment or blindness.13

Sub-threshold exposure levels of blue light also produce oxidative damage that may contribute to long-term degeneration of the retina.14 In particular, sub-threshold levels of blue light produces oxidative damage that is ingested by proximate retinal pigment epithelium cells (RPEs), which, on a daily basis engulf the tips of photoreceptor cells where light absorption as well as photo-oxidative damage takes place.15 The molecular damage from oxidative stress in photoreceptor outer segments accumulates in the RPE and becomes a major factor in the normal long-term deterioration of the retina as well as in the development of AMD.16 Thus, although the vision loss of AMD results from photoreceptor damage in the central retina, the initial pathogenesis involves degeneration of RPE cells.17

Deterioration of the RPE begins with the accumulation of oxidative debris it has ingested and is unable to degrade, such as the compound A2E which is enzymatically indigestible.18 The RPE accumulates some of this toxic oxidative debris as lipofuscin, the aging pigment.19 When illuminated by blue light, lipofuscin is phototoxic, producing exponentially greater amounts of ROS as blue wavelengths decrease from 490 to 400 nm.20 The ROS generated when blue light is absorbed by lipofuscin can damage cellular DNA, lipoproteins, and other structures within RPE, and may lead to self-induced cell death (apoptosis). 21

This accumulation of high concentrations of lipofuscin in the RPE cells and its ensuing exposure to light have been linked to the subsequent loss of photoreceptor cells and the occurrence of AMD.22 The build-up of lipofuscin impairs the RPE cells' ability to provide nutrients and digest debris, establishing the initial pathogenesis of AMD.23 Furthermore, the absorption of blue light by chromophores in lipofuscin and melanosomes generates high levels of ROS in the RPE membrane, which contributes to increasing levels of oxidative damage in the RPE and adjacent tissues.24

The onset of AMD is considered to occur with the development of drusen, initiated when RPE cells can no longer contain the accumulated lipofuscin and begin to excrete oxidative debris into intracellular space between the RPE and Bruch's membrane.25 The substance drusen is formed when the debris exuded from RPE cells into this intracellular space binds to components drawn from the choroidal vasculature and becomes cross-linked to the Bruch's membrane. 26

This build-up of drusen between the choroidal vasculature in the Bruch's membrane and the RPE cells interferes with the supply of nutrients and oxygen to the RPE cells.27 Subsequently, the photoreceptors overlying areas where drusen has formed are not properly nourished and die off.28 This progressive degeneration of photoreceptor cells in the macula, the centre of the retina where light is focused, leads to the form of blindness known as geographic atrophy, or the "dry" form of AMD.29

Exudative AMD or the "wet" form of AMD is characterized by abnormal blood vessels growing from the choriocapillaris through the retinal pigment epithelium (RPE), typically resulting in hemorrhage, exudation, scarring, and/or serous retinal detachment.30 Wet AMD is also directly related to the accumulation of oxidative damage and possibly the blue light induced production of ROS by chromophores in lipofuscin and drusen.31 These ROS can cause lesions in the blood-retinal barrier formed by the RPE and the Bruch's membrane: small blood vessels then invade the retinal space, producing the conditions for sudden blindness when these vessels leak or break. 32

NOTE: A number of relevant papers on the mechanisms involved in the pathogenesis of AMD were written subsequent to this right up. We have added an addendum to this page which includes the more recent research and and how these developments further implicate blue light exposure in the development of AMD.
This addendum is available at Draft update to blue light and AMD
In the past years researchers have contributed additional understandings of the mechanisms involved in the development of AMD. This research further explains how blue light absorption by cells in the outer retina promotes the development of AMD.
With these developments in the current understanding of the mechanisms involved in the pathogenesis of AMD, we believe it is now possible to conclusively demonstrate that blue light exposure contributes in a substantial manner to the development of Age-related Macular Degeneration.

Current research continues to validate and confirm the propositions outlined on these pages regarding the cumulative exposure of light on the retina and vision over a lifetime, and the role of blue light as a major contributing factor in the development of age-related blindness from macular degeneration. View a few of these recent references here.

Close recent refs

Recent refs

A recent review, Effects of Blue Light on the Circadian System and Eye Physiology. Molecular Vison:(2016) summarizes current understanding of the role of blue light in the development of AMD.

"Experimental evidence indicates that wavelengths in the blue part of the spectrum (400-490 nm) can induce damage in the retina, and although the initial damage following exposure to blue light may be confined to the RPE, a damaged RPE eventually leads to photoreceptor death. Although most studies on the effects of blue light have focused on the mechanisms responsible for the damage to the photoreceptors following an acute exposure to high intensity light, some studies have reported that sub-threshold exposure to blue light can also induce damage in photoreceptors. In addition, several authors have proposed that the amount of blue light received during an individual's entire lifespan can be an important factor in the development of age-related macular degeneration (AMD)."

The paper concludes:

"Because light has a cumulative effect and many different characteristics (e.g., wavelength, intensity, duration of the exposure, time of day), it is important to consider the spectral output of the light source to minimize the danger that may be associated with blue light exposure....Although we are convinced that exposure to blue light from LEDs in the range 470-480 nm for a short to medium period (days to a few weeks) should not significantly increase the risk of development of ocular pathologies, this conclusion cannot be generalized to a long-term exposure (months to years)."

The concern these authors express relates to blue light wavelengths from indirect ambient lighting emitted by LEDs providing white light. As the authors explain:

"The white-light LED (i.e., the most common type of LED) is essentially a bichromatic source that couples the emission from a blue LED (peak of emission around 450-470 nm with a full width at half max of 30-40 nm) with a yellow phosphor (peak of emission around 580 nm with a full width at half max of 160 nm) that appears white to the eye when viewed directly. ...
The specific pump wavelength of the phosphor in the range 450-470 nm depends critically on the absorption properties of the phosphor. ... white-light LEDs degrade over time primarily through bleaching of phosphors so that they no longer efficiently absorb blue light. This shifts the color temperature of the device over time, with a corresponding change in the color-rendering index but, more importantly, an increasing blue emission from the device with time."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4734149/

Several papers in the February 2016 issue of the journal Eye discussed the hazardous effect of blue light exposure on the retina. These papers related to studies on the effect of blue light wavelengths from indoor and outdoor lighting on people with artificial lenses implanted due to cataracts, or concerns about blue light exposure from the increased use of LEDs in general indoor lighting. These concerns relate to exposure to the relatively low intensities of indirect indoor ambient lighting. These intensities are much, much lower than the high intensities of light emitted by white or blue light therapy lamps, which provide direct light exposure of the eye from the therapy lamp.

With regard to blue light exposure of people undergoing cataract surgery, one paper, Ultraviolet or Blue-Filtering Intraocular Lenses: What is the Evidence?, stated:

"With the arrival of blue-filtering intraocular lenses (BFIOLs) in 1990's, a further debate was ignited as to their safety and potential disadvantages. ... The potential disadvantages raised in the literature over the last 25 years since their introduction, regarding compromise of visual function and disruption of the circadian system, have been largely dispelled. The clear benefits of protecting the retina from short-wavelength light make [blue-filtering intraocular lenses] BFIOLs a sensible choice"
www.nature.com/eye/journal/v30/n2/abs/eye2015267a.html

With respect to ambient lighting, another paper Light in Man's Environment by a different research group, stated:

"Many studies have demonstrated the spectral dependence of eye health, with the retinal hazard zone associated with wavelengths in the blue, peaking at 441 nm - many of today's low-energy sources peak in this region. Given the increased longevity and artificial light sources emitting at biologically unfriendly wavelengths, attention has to be directed towards light in man's environment as a risk factor in age-related ocular diseases."
http://www.nature.com/eye/journal/v30/n2/abs/eye2015265a.html

Some studies that discuss the state of retinal damage from blue light exposure examine the protective proprties of the carotinoids lutein, zeaxanthin, and meso-zeaxanthin that make up the layer of blue light filtering macular pigment that forms in the inner retina, and through which light must pass to reach the photoreceptors and RPE cels in the outer retina. These carotinoids are not produced by the body and are obtained though diet or supplements. Several studies have shown that the lower the density of this pigment, that is, the more blue light that is able to pass throgh this layer to reach the outer retina the greater the likelihood of developing AMD.

A few comments from one of these studies, The Photobiology of Lutein and Zeaxanthin in the Eye. Journal of Ophthalmology, Dec 2015, are:

"In adults, the lens absorbs UV-B and all the UV-A (295-400 nm); therefore only visible light ([wavelengths] >400 nm) reaches the retina".
"short-wavelength blue visible light damages the retinas of those over 50 years of age through a photooxidation reaction with an accumulated chromophore, lipofuscin. Lipofuscin...is mainly derived from the chemically modified residues of incompletely digested photoreceptor outer segments. Photoreceptor cells (rods and cones) shed their outer segments (disc shedding) daily to be finally phagocytosed (digested) by RPE cells." ... "In response to [Upon the absorption of] short blue visible light, lipofuscin efficiently produces singlet oxygen and lipid peroxy radicals; there is also some production of superoxide and hydroxyl radicals."
"short-wavelength blue visible light damages the retinas of those over 50 years of age through a photooxidation reaction with an accumulated chromophore, lipofuscin ... a blue light singlet oxygen photosensitizer, leading to damage to RPE cells. Because the rods and cones survival is dependent on healthy RPE, these primary vision cells will eventually die, resulting in a loss of (central) vision (macular degeneration) and other retinopathies"
"Ocular exposure to sunlight, UV, and short blue light-emitting lamps directed at the human eye can lead to the induction of cataracts and retinal degeneration. This process is particularly hazardous after the age of 40 because there is a decrease in naturally protective antioxidant systems and an increase in UV and visible light-absorbing endogenous phototoxic chromophores that efficiently produce singlet oxygen and other reactive oxygen species."
http://www.hindawi.com/journals/joph/2015/687173/

Current studies, including History of Sunlight Exposure is a Risk Factor for Age-Related Macular Degeneration. Retina, 2016 Apr;36(4):787-90. continue to find that increased exposure to sunlight over a lifetime increases the likelihood of developing AMD. Since only the visible light wavelengths from sunlight reach the retina, see previous reference, these studies support the premise that cumulative exposure to visible blue light wavelengths contributes to the development of AMD.

PURPOSE: To evaluate effects of current and past sunlight exposure and iris color on early and late age-related macular degeneration (AMD)
CONCLUSION: Sunlight exposure during working life is an important risk factor for AMD, ... Therefore, preventive measures, for example, wearing sunglasses to minimize sunlight exposure, should start early to prevent development of AMD later in life.
http://www.ncbi.nlm.nih.gov/pubmed/26441265






















More information on risk factors for retinal damage directly related to the use of bright or blue light therapy lamps.
For further validations or for clarification of any of the statements made on this web site regarding the role of blue light in the pathogenesis of AMD, please contact Sunnex Biotechnologies.
Return to Overview of the risk to vision from the use of bright or blue light therapy lamps, for therapists

Endnotes

1. Age-related Macular Degeneration (AMD) is a condition of advanced degeneration of the macular portion of the retina that leads to progressive blindness in over 35% of persons over the age of 75.

"AMD affects a region of the human retina called the macula, which lies in the central axis of vision. The macula is a region 6 mm in diameter... Because the life span of humans continues to increase as a function of improved nutrition and increased awareness of environmental factors, AMD is expected to nearly double in the next 25 years. To place this in perspective, 35% of the human population of 75 years or older has some degree of AMD. Projections by the National Institute of Aging suggest that one in five people in the USA will be 65 or older by 2030. Individuals 85 and older could exceed 10 million at that time."
Bok. New insights and new approaches toward the study of age-related macular degeneration. Proc Natl Acad Sci USA. 2002 Nov 12;99(23):14619-21.
"Age-related macular degeneration (AMD) is a main causes of severe visual impairment in the elderly in industrialized countries .... there appears to exist an unknown switch mechanism from normal ageing to disease. Recent anatomical studies using elegant innovative techniques reveal that there is a 30% rod loss in normal ageing, which is increased in early AMD." Reme CE, Grimm C, Hafezi F, Iseli HP, Wenzel A. Why study rod cell death in retinal degenerations and how? Doc Ophthalmol. 2003 Jan;106(1):25-9.
"An extraordinary fraction of the population is at risk for the development of AMD. For example, the Beaver Dam Eye Study found that nearly 20% of the population between 65 and 75 years of age is affected with either early or late age-related maculopathy and also that >35% of the population >75 years of age is similarly affected. These numbers are especially alarming given that the US Census Bureau has predicted that the number of people in these two age groups will increase by 80% in the next 25 years." Stone EM, Sheffield VC, Hageman GS. Molecular genetics of age-related macular degeneration. Hum Mol Genet. 2001 Oct 1;10(20):2285-92.

2. AMD has been linked to the stress engendered by radical oxygen species in macular photoreceptor cells and proximate retinal pigment epithelium cells (RPE).

"Oxidative stress is thought to be an important contributing factor to the initiation and progression of AMD. The retina is vulnerable to oxidative stress because of its exposure to light and oxygen... In particular, damage to the retinal pigment epithelium by oxidative stress is an early indication of AMD." Activation of Caspase-8 and Caspase-12 Pathways by 7-Ketocholesterol in Human Retinal Pigment Epithelial Cells. Luthra S et al, Investigative Ophthalmology and Visual Science. Dec 2006;47:5569-5575
"Several studies indicate that oxidative stress is one of the causes of age-related macular degeneration (AMD), the leading cause of irreversible vision loss among people age 60 and older in the United States." Chen L, Dentchev T, Wong R, Hahn P, Wen R, Bennett J, Dunaief JL. Increased expression of ceruloplasmin in the retina following photic injury. Mol Vis 2003 Apr 30;9:151-8.
"Evidence that Oxidative Stress is Responsible for ARM
ROIs [Reactive] Oxygen Intermediates] are produced in all cells as a by-product of metabolism, and additionally, in the retina, by photochemical reactions between light and oxygen. ROIs are likely to be particularly abundant in the retina because of its high metabolic rate, and because of the abundance of photosensitisers which increase photochemical production of ROIs in response to incident light energy .... Finally, several studies have demonstrated a link between the pro oxidant effects of light exposure, dietary fatty acids and retinal lipofuscin, and an increased risk for ARM." Macular Carotenoids and Age-related Maculopathy. O'Connell, et al. Ann Acad Med Singapore 2006;35:821-30
Age-related macular degeneration (AMD) is thought to be the result of a lifetime of oxidative insult that results in photoreceptor death within the macula. Increased risk of AMD may result from low levels of lutein and zeaxanthin (macular pigment) in the diet, serum or retina, and excessive exposure to blue light. Through its light-screening capacity and antioxidant activity, macular pigment may reduce photooxidation in the central retina." Lutein and Zeaxanthin Dietary Supplements Raise Macular Pigment Density and Serum Concentrations of these Carotenoids in Humans. Bone RA, Landrum JT, Guerra LH, Ruiz CA. Journal of Nutrition 2003 Apr;133(4):992-8.
"Photoreceptor outer segments live in a potentially toxic environment that includes high oxygen and high photon flux. These conditions are conducive to photooxidative damage, and the production of byproducts of lipid peroxidation such as malondialdehyde, which can cross link proteins." Bok D. New Insights and New Approaches Towards the Study of Age-Related Macular Degeneration (AMD). Proceedings of the National Academy of Sciences U S A. 2002; 99(23):14619-21
"Macular pigment (MP) reduces oxidative damage in the central retina...Reduced levels of MP seem to be associated with a higher risk of development of AMD as they were significantly more often observed in the AMD group." Trieschmann M, Spital G, Lommatzsch A, Van Kuijk E, Fitzke F, Bird AC, Pauleikhoff D Macular pigment: quantitative analysis on autofluorescence images. Arch Clin Exp Ophthalmol. 2003 Dec;241(12):1006-12.
"The retinal pigment epithelium (RPE) is a single cell layer that separates the photoreceptor cells of the retina from their principal blood supply in the choroid. In all vertebrates, the RPE is responsible for vectorial transport of nutrients to rod and cone photoreceptors, removal of waste products to the blood, absorption of scattered light and regeneration of bleached visual pigment." K. A. West, J. Sun, L. Yan, K. Shadrach, A. Hasan, M. Miyagi, J. S. Crabb, J. G. Hollyfield, A. D. Marmorstein, J. W. Crabb. Human Retinal Pigment Epithelium Protein Database. The Association of Bimolecular Research Facilities Annual Meeting 2003 Poster 170.

3.Both of these types of cells are non-replicating (post-mitotic) and must respond to a lifetime of oxidative insult, which includes light-induced oxidative stress.

"Oxidative stress has been suggested to be a major contributing factor for retinal degeneration in AMD. The retina is constantly exposed to light and a relatively high oxygen pressure, which is close to that found in arterial blood, contributes to light-induced oxidative stress in the retina which may result in oxidative damage to biomolecules in these cells. RPE cells are post mitotic and therefore must respond to a life time of oxidative insult. While there are numerous mechanisms for preventing and combating oxidative injuries, by middle-age many of these anti-oxidative mechanisms have begun to break down, which can increase the susceptibility of RPE cells to accumulated damage. LF and MLF granules are thought to result from the accumulation of undegradable material in RPE cells. Modifications, including oxidation, may render the molecules in these granules undegradable by the cell, contributing to their accumulation." Proteomic and Phototoxic Characterization of Melanolipofuscin: Correlation to Disease and Model for its Origin. Warburton S, et al. Molecular Vision March 1, 2007; 13:318-329
"The RPE-choroid complex is known to be exposed to the highest oxidative stress in the eye. RPE cells are postmitotic; they will not divide after they have matured. Thus the exposure of the cells to high oxidative stress and light will last throughout the life of the individual, as may any modifications induced by oxidative stress and the resulting cellular dysfunction." Age-dependent Photoionization Thresholds of Melanosomes and Lipofuscin Isolated from Human Retinal Pigment Epithelium Cells. Hong L. Et al. Photochemistry and Photobiology: Dec 2006; 82(6):1475-1481.
"The retinal pigment epithelium (RPE) is a single layer of post-mitotic cells which functions both as a selective barrier to and a vegetative regulator of the overlying photoreceptor layer, thereby playing a key role in its maintenance." Boulton M, Dayhaw-Barker P. The role of the retinal pigment epithelium: topographical variation and ageing changes.Eye 2001 Jun;15 (Pt 3):384-9.

4. While there are numerous mechanisms in the retina for preventing and forestalling oxidative insult, by middle-age many of these anti-oxidative mechanisms have begun to break down, which increases the susceptibility of the retina to accumulated damage with increasing age.

"Melanin in the human retinal pigment epithelium (RPE) is believed to play an important photoprotective role. However, unlike in skin, melanosomes in the RPE are rather long-lived organelles, which increases their risk of modifications resulting from significant fluxes of light and high oxygen tension....... We have previously shown that a similar loss in the content of the RPE melanin occurs during human lifetime, which may suggest that the normal antioxidant properties of human RPE melanin become compromised with aging." Photobleaching of retinal pigment epithelium melanosomes reduces their ability to inhibit iron-induced peroxidation of lipids. Pigment Cell Research Feb 2007; 20(1): 52-60 Zadlo A et al.
"The integrity of the retinal pigment epithelium, especially that of the macula is essential for the preservation of vision into old age. The chronic exposure to sunlight and peroxidized lipids from phagocytized photoreceptor outer segments imposes a high level of oxidative stress on the retinal tissues, which increases with age as antioxidant protection declines and therefore could accelerate apoptosis." Godley BF, Jin GF, Guo YS, Hurst JS. Bcl-2 Overexpression Increases Survival in Human Retinal Pigment Epithelial Cells Exposed to H2O2. Exp Eye Res 2002 Jun;74(6):663-9.
"Damage to the young and adult eye by intense ambient light is avoided because the eye is protected by a very efficient antioxidant system... 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... Unfortunately, most of these protective enzymes decrease beginning at 40 years of age." Roberts JE. Ocular Phototoxicity. J Photochem Photobiol B. 2001 Nov 15;64(2-3):136-43.
"Excessive light exposure in the elderly may be particularly risky, since the biological repair processes at the cellular level are generally considered to be less effective as one ages." D.H. Sliney. Ocular Injury Due to Light Toxicity. International Ophthalmology Clinics 1988;28(3):246-250.
"There is a close association between the level of oxidative stress in the RPE and its two major pigments, melanin and lipofuscin. Melanin in the form of melanosomes in the RPE is generally considered to be an antioxidant....The accumulation of lipofuscin in the eye is hypothesized to contribute to the age-related increase in aerobic photoreactivity of the RPE and is phototoxic to RPE cells in culture. With age, human RPE cells loses melanosomes and gains lipofuscin."..." These data suggest that thin deposits of lipofuscin on the surface of retinal pigment epithelium melanosomes are common in the aged eye and that this renders the melanosomes more pro-oxidant." Age-dependent Photoionization Thresholds of Melanosomes and Lipofuscin Isolated from Human Retinal Pigment Epithelium Cells. Hong L. Et al. Photochemistry and Photobiology: Dec 2006; 82(6):1475-1481.

5.Visible light absorbed by photoreceptors is a significant factor in the production of radical oxygen species in retinal tissue which appears to induce the molecular damage that accumulates in retinal tissue.

"Retinal oxidative stress caused by light exposure has been implicated in the pathogenesis of age-related macular degeneration (AMD) and other retinal degenerations. Photooxidative stress is exacerbated by an imbalance between light-induced reactive oxygen species (ROS) and antioxidants... Photooxidative stress has been implicated as a mechanism of retinal light damage." Isoprostane F2A-VI, A New Marker of Oxidative Stress, Increases Following Light Damage to the Mouse Retina. Dentchev T, Yao Y, Pratico D, Dunaief J. Molecular Vision Vol 13; February 7, 2007.
"Light is essential for vision but the trade off is the generation of potentially damaging reactive oxygen species within the eye." Boulton M, Rozanowska M, Rozanowski B. Retinal Photodamage. J Photochem Photobiol. B 2001 Nov 15;64(2-3):144-61.
"The photoreceptors of the retina present a puzzling phenomenon: they can be injured or even destroyed by light, the very element they are designed to detect." Reme CE et al. Apoptosis in the Retina: The Silent Death of Vision. News Physiol Sci 15: 120-124, 2000.
"CONCLUSIONS: Oxidative stress to the RPE is believed to contribute to sight threatening diseases such as age-related macular degeneration, ... it is increasingly likely that RPE melanosomes are sensitive to aging changes and that aging changes could be due at least partly to the effects of light exposure. As shown here, experimental treatment of isolated melanosomes with visible light produces changes similar to those that occur with aging of human RPE melanosomes within eyes, including reduced melanin content and an increased capacity to photogenerate superoxide. The consequence of chronic light exposure may therefore be to render RPE melanosomes less competent to perform an antioxidant function and to help protect aged RPE cells from oxidative injury." Effects of Photodegradation on the Physical and Antioxidant Properties of Melanosomes Isolated from Retinal Pigment Epithelium. Zareba M. et al. Photochem Photobiol: 2006; Jul-Aug 82, (4):1024-1029
"Light bombards the retina, and the blood flowing through it teems with oxygen, providing ample opportunity for oxidation. Furthermore, antioxidants sometimes hinder the disease, and smoking, which causes oxidative damage in many tissues, increases risk of AMD." Seydel, The Eyes Have It. Science's SAGE KE (30 October 2002).
"Evidence has accumulated that excessive light exposure may promote age-related and inherited retinal degeneration, in which photoreceptor death by apoptosis leads to loss of vision."Wenzel A, Grimm C, Seeliger MW, Jaissle G, Hafezi F, Kretschmer R, Zrenner E, Reme CE. Prevention of Photoreceptor Apoptosis by Activation of the Glucocorticoid Receptor. Invest Ophthalmol Vis Sci 2001 Jun;42(7):1663-9
"It is now well established that photoretinopathy is a cumulative process, and chronic light damage may be one of the factors contributing to the development of age-related macular degeneration (AMRD)." UP Andley, LT Chylack Jr. Recent Studies on Photodamage to the Eye with Special Reference to Clinical and Therapeutic Procedures. Photodermatology Photoimmunology and Photomedicine 1990; 7:98-105
"It is well established that the retina undergoes several decremental functional and structural changes with age, and it has been suggested that most of these age-related changes may be mainly due to oxidative stress through light-induced mechanisms." Berra A, Ferreira S, Stanga P, Llesuy S. Age-Related Antioxidant Capacity of the Vitreous and its Possible Relationship with Simultaneous Changes in Photoreceptors, Retinal Pigment Epithelium and Bruchs'Membrane in Human Donors' Eyes. Archives of Gerontology and Geriatrics 2002; 34(3):371-7
"Retinal oxidative stress can be caused by light exposure, which has been implicated in the pathogenesis of AMD and other retinal degenerations. Photo-oxidative stress is caused by an imbalance between light-induced reactive oxygen species (ROS) and antioxidants. In the retina, absorption of light by photosensitizers results in electron transition to an unstable excited state, subsequently generating ROS." Chen L, Dentchev T, Wong R, Hahn P, Wen R, Bennett J, Dunaief JL. Increased expression of ceruloplasmin in the retina following photic injury. Mol Vis 2003 Apr 30;9:151-8
"The etiology of age-related macular degeneration (ARMD) is multifactorial, but visible light may play a role in the pathogenesis of this potentially devastating disease. In the Maryland watermen study, advanced ARMD was more common in men exposed to increased levels of blue light (400-500 nm), but not in those with increased levels of ultraviolet exposure. Similarly, the Beaver Dam Eye Study found that exposure to visible light was associated with ARMD in men....Visible light has also been responsible for acute photic retinopathy caused by the operating room microscope." Stenson SM, West CE. Refraction, Spectacles, Contact Lenses, and Visual Rehabilitation. American Academy of Ophthalmologists LEO Clinical Topic Update; April 2003.
"Our data support the hypothesis that exposure to bright visible light may be associated with ARM." Klein R, Klein BE. Sunlight and Age-Related Macular Degeneration. The Beaver Dam Eye Study. K.J. Cruickshanks, Archives of Ophthalmology 1993;111(4):514-518
"The present concepts of the pathogenesis of AMD include cumulative light damage by oxidative processes in the macular photoreceptors as environmental co-factor for the development of AMD." Pauleikhoff D, van Kuijk FJ, Bird AC. Macular Pigment and Age-Related Macular Degeneration [Article in German] .Ophthalmologe 2001 Jun;98(6):511-9.
"In the retina this damage may lead to the development of age-related macular disease. The retina is particularly susceptible to oxidative stress for several reasons: the retina is subject to high levels of radiation, particularly blue light; oxygen consumption by the retina is greater than that of other tissues; Photoreceptor outer segments contain a high proportion of polyunsaturated fatty acids, whose double bonds are a rich source of electrons; the retina contains photosensitisers, which make cells and tissues sensitive to the influence of irradiation; phagocytosis by the RPE generates ROI.." Bartlett H, Eperjesi, F. A randomised controlled trial investigating the effect of nutritional supplementation on visual function in normal, and age-related macular disease affected eyes: design and methodology. Nutrition Journal 2003, 2:12.

6. To an overwhelming degree, blue wavelengths of light produce the most oxidative stress within the retina and are primarily responsible for exacerbating the extent of oxidative damage that has begun to occur.

"We also observed that blue light alone can inflict apoptotic death of visual cells, indicating to us, not only that rhodopsin readily absorbs blue light and is bleached, but also that this same blue light is absorbed by a photochemically active bleaching product that inflicts cellular damage."
"Concerning blue light-induced apoptosis, we have shown that, similar to white light, the photon absorption by rhodopsin plays a critical role. A special feature of blue light, however, comes into play by the fact that rhodopsin bleaching product(s) strongly absorb in the blue (and near UV) range and possibly induce detrimental photochemical lesions in photoreceptors. The determination of such damaging molecules and their absorption characteristics is a logical continuation of studies on blue light-induced damage. Such studies would have practical implications. Manufacturers of therapy lamps, sunglasses, and intraocular lenses would know at which point of the visible spectrum a meaningful cutoff or reduction of blue light should be located." {NOTE: emphasis is ours}
The Dark Side of Light: Rhodopsin and the Silent Death of Vision. The Proctor Lecture. Investigative Ophthalmology and Visual Science. 2005;46:2672-2682 .
"The photoreceptors in the retina, designed to initiate the cascade of events which link the incoming light to the sensation of 'vision', are susceptible to damage by light, particularly blue light. The damage can lead to cell death and diseases." Shaban H, Richter C. A2E and blue light in the retina: the paradigm of age-related macular degeneration. Biol Chem 2002 Mar-Apr;383(3-4):537-45.
"The retina, however, is vulnerable to damage by light, a vulnerability that has long been recognized. Photochemical damage has been widely studied, because it can cause retinal damage within the intensity range of natural light. ...Class II damage is generally confined to the retinal pigment epithelium. The action spectrum peaks in the short wavelength region, providing the basis for the concept of blue light hazard. Several factors can modify the susceptibility of the retina to photochemical damage. Photochemical mechanisms, in particular mechanisms that arise from illumination with blue light, are responsible for solar retinitis and for iatrogenic retinal insult from ophthalmological instruments. Further, blue light may play a role in the pathogenesis of age-related macular degeneration. Laboratory studies have suggested that photochemical damage includes oxidative events. Retinal cells die by apoptosis in response to photic injury, and the process of cell death is operated by diverse damaging mechanisms." Photochemical Damage of the Retina. Wu J. et al. Surv Ophthalmol. 2006 Sep-Oct;51(5):461-81.
"An intriguing question is whether certain wavelengths of the visible spectrum may preferentially induce apoptosis in the retina... We obtained a striking all-or-none response when albino rats were exposed to either monochromatic blue light of 403 nm (3.1 mW/cm2) or monochromatic green light of 550 nm (8.7 mW/cm2) with a light-exposure regimen providing a homogeneous illumination on the anesthetized animal's retina with a ganzfeld device. No apoptosis and no other light-induced lesions could be found in green light-exposed eyes, whereas massive apoptotic cell death occurred after illumination with blue light." Reme et al. Apoptosis in the Retina: The Silent Death of Vision C E. News Physiol Sci 15: 120-124, 2000.
"Young adult rhesus monkeys were anesthetized, and received blue [460 nm] LED exposure from a modified slit-lamp. A 3 mm beam of 0.85 mW was imaged onto the retina through a lens positioned before the cornea and exposure damage was determined at time intervals for 12 to 90 min. ...Two days after 40 min exposure, there was a grey, discolored region, which was over-fluorescent in FAG, and an increase in HRT and S-ERG corresponding to the site which was exposed to LED light. In histological examination at 30 days, the LED had caused produced a marked disruption of the disks of photoreceptor cells, damaged retinal pigment epithelium (RPE) apical villi, and a loss of RPE melanin after 90 min exposure. CONCLUSION: A threshold level was found around 40 min. This morphological damage may impair function and continuous exposure to blue light is potentially dangerous to vision." Koide R, Ueda TN, Dawson WW, Hope GM, Ellis A, Somuelson D, Ueda T, Iwabuchi S, Fukuda S, Matsuishi M, Yasuhara H, Ozawa T, Armstrong D.Nippon. Retinal hazard from blue light emitting diode.Ganka Gakkai Zasshi. 2001 Oct;105(10):687-95.
"Blue-light injury can result from viewing either extremely bright light for a short time or less bright light for a longer time." Sliney DH. Ocular Injury Due to Light Toxicity. International Ophthalmology Clinics 1988;28(3):246-250.
"In the retina, short-wavelength blue light initiates photosensitisation with the consequential generation of ROIs (Reactive Oxygen Intermediates). L[utein] and Z[eaxanthin], by virtue of absorbing blue light enroute to the photoreceptors, may prevent this short-wavelength light from producing ROIs. From this perspective, the blue light filtering property of the macular carotenoids can be considered as a passive, or indirect, antioxidant function. Macular carotenoids are well-suited to act as an optical filter to the potentially damaging blue light for numerous reasons. First, the absorbance spectrum of macular carotenoids peaks at 460 nm, which corresponds to the wavelength of blue light...According to Snodderly et al, it has been estimated that macular carotenoids decrease the incident blue light by approximately 40%. This is particularly important in young individuals with a transparent lens, which allows virtually all the incident visible light to reach the macula. ...The evidence to support a protective role of MP in ARM exists in the form of histopathological, clinical, circumstantial, and epidemiological (observational and interventional) studies." Macular Carotenoids and Age-related Maculopathy. O'Connell, et al. Ann Acad Med Singapore 2006;35:821-30
"Compared with age-matched controls, patients with macular degeneration had significantly higher exposure to blue or visible light in the preceding 20 years but were not different in respect to exposure to UV-A or UV-B. These data suggest that high levels of exposure to blue or visible light may cause ocular damage, especially later in life, and may be related to the development of age-related macular degeneration." Taylor HR, West S, Munoz B, Rosenthal FS, Bressler SB, and Bressler NM. The Long Term Effects of Visible Light on the Eye. Archives of Ophthalmology 1992; 110:99-104
"I think chronic blue light is probably damaging,' says Joshua Dunaief, MD, who has studied photic damage to the retina at the University of Pennsylvania. 'The reason for that belief are the Chesapeake Bay Watermen Study and studies in a variety of animals, from mice to monkeys, that have shown blue light can cause photoreceptor and RPE... The eye seems to have developed a mechanism to protect itself,' Dr. Dunaief says. 'The reason the macula is yellow is because the compound lutein has deposited in front of the photoreceptors. It's very likely that it's deposited there because it's yellow and it helps protect the photoreceptors and RPE by absorbing blue light." Bethke W. Should We Block The Blue. Review of Ophthalmology Oct 15 2003; 10(10)
"By absorbing blue light, the macular pigment protects the underlying photoreceptor cell layer from light damage, possibly initiated by the formation of reactive oxygen species during a photosensitized reaction. There is ample epidemiological evidence that the amount of macular pigment is inversely associated with the incidence of age-related macular degeneration, an irreversible process that is the major cause of blindness in the elderly." Krinsky NI, Landrum JT, Bone RA. Biologic Mechanisms of the Protective Role of Lutein and Zeaxanthin in the Eye. Annu Rev Nutr 2003 Feb 27; [epub ahead of print].
"Age-related macular degeneration (AMD) is thought to be the result of a lifetime of oxidative insult that results in photoreceptor death within the macula. Increased risk of AMD may result from low levels of lutein and zeaxanthin (macular pigment) in the diet, serum or retina, and excessive exposure to blue light." Bone RA, Landrum JT, Guerra LH, Ruiz CA. Lutein and Zeaxanthin Dietary Supplements Raise Macular Pigment Density and Serum Concentrations of these Carotenoids in Humans. Journal of Nutrition 2003 Apr;133(4):992-8.

7. Since blue light wavelengths impart the greatest risk of photochemical damage, the risk of retinal damage from light is termed "the blue light hazard," with the greatest hazard peaking at wavelengths of 440 nm.

"The pathogenesis of age-related maculopathy (ARM), the most common cause of visual loss after the age of 60 years, is indeed a complicated scenario that involves a variety of hereditary and environmental factors. The pathological cellular and molecular events underlying retinal photochemical light damage, including photoreceptor apoptosis, have been analysed in experimental animal models. Studies of age-related alterations of the retina and photoreceptors, the accumulation of lipofuscin in retinal pigment epithelium (RPE) cells, and the formation of drusen have greatly contributed to our knowledge. A new concept of an inflammatory response to drusen has emerged, suggesting immunogenic and systemic reactions in Bruch's membrane and the subretinal space. Oxidative stress and free radical damage also impact on the photoreceptors and RPE cells in the ageing eye. Based on the photoelectric effect, a fundamental concept in quantum physics, the consequences of high-energy irradiation have been analysed in animal models and cell culture. Short-wavelength radiation (rhodopsin spectrum), and the blue light hazard (excitation peak 440 nm), have been shown to have a major impact on photoreceptor and RPE function, inducing photochemical damage and apoptotic cell death." Algvere PV, Marshall J, and Seregard S. Review Article: Age-Related Maculopathy and the Impact of Blue Light Hazard. Acta Ophthalmologica Scandinavica 2006; 84(1):4 -15
"The high-energy segment of the visible region (400-500 nm) is enormously more hazardous than the low energy portion (from 500-700 nm). Because the transition occurs at the border between the perceived colors of green and blue, the phenomenon is known as blue light hazard." Young RW. Solar Radiation and Age Related Macular Degeneration. Survey of Ophthalmology 1988; 32(4): 252-269
"Visible light of short wavelength (blue light) may cause a photochemical injury to the retina, called photoretinitis or blue-light hazard." Okuno T, Saito H, Ojima Evaluation of blue-light hazards from various light sources. J. Dev Ophthalmol. 2002;35:104-12.
[The] Action spectrum for blue-light induced [retinal] damage shows a maximum at 400 nm and 450 nm." Bartlett H, Eperjesi F. A randomised controlled trial investigating the effect of nutritional supplementation on visual function in normal, and age-related macular disease affected eyes: design and methodology. Nutrition Journal 2003, 2:12.
"There is a long-standing controversy about the damaging effects of blue light in the retina and its relevance to human disease ... Visible, non-coherent blue light has a high damage potential. Green light, in sharp contrast, does not induce any lesions ... Because sunlight and many high-intensity artificial light sources contain relatively high proportions of blue, and the retina as well as pigment epithelium contain several types of blue-absorbing molecules, the short-wavelength band of the visible spectrum may contribute to the pathogenesis of age-related macular degeneration and amplify some forms of inherited retinal degeneration [e.g., retinitis pigmentosa (RP)]." Reme CE, Wenzel A, Grimm G, Iseli HP. Mechanisms of Blue Light-Induced Retinal Degeneration and the Potential Relevance for Age-Related Macular Degeneration and Inherited Retinal Diseases. SLTBR Annual Meetings Abstracts 2003.

8. Because the extent of photic damage induced in photroreceptor cells is proportional to the amount of light absorbed by the photo-pigments, blue light potentiates the retina for a much greater degree of photo-oxidative damage by increasing the photon catch capacity by short circuiting the metabolic visual cycle through a process of photoreversal.

"Rhodopsin is essential for the initiation of light-induced rod loss. Following photon absorption, there may be the generation of photochemically active molecules which then induce the apoptotic death cascade... Acute white-light damage to rods depends on the amount of rhodopsin available for bleaching during light exposure... Because photoreversal is faster than metabolic regeneration of rhodopsin by several orders of magnitude, the photon catch capacity of the retina is significantly augmented during blue-light illumination, which may explain the greater susceptibility of the retina to blue light than to green light. However, blue light can also affect function of several blue-light-absorbing enzymes that may lead to the induction of retinal damage." Grimm C, et al. Rhodopsin-Mediated Blue-Light Damage to the rat Retina: Effect of Photoreversal of Bleaching. Invest Ophthalmol Vis Sci 2001 Feb;42(2):497 -505
"Early rod loss is an important denominator of AMD. A fast rhodopsin regeneration rate increased damage susceptibility. Our data indicate that rhodopsin is essential for the initiation of light-induced rod loss. Following photon absorption, there may be the generation of photochemically active molecules which then induce the apoptotic death cascade." Reme CE, Grimm C, Hafezi F, Iseli HP, Wenzel A. Doc Ophthalmol. 2003 Jan;106(1):25-9. Why study rod cell death in retinal degenerations and how?
"When the kinetics of bleaching and regeneration of rhodopsin were analyzed... blue light photoregenerated rhodopsin from a bleaching intermediate and thus provided chromophore during the periods of light exposure. Mice with slow regeneration after exposure to bright light were the least susceptible to light-induced apoptosis, whereas mice with fast regeneration revealed damage after only 10-20 min of light exposure." Reme CE. et al. Apoptosis in the Retina: The Silent Death of Vision. News Physiol Sci 2000 15:120-124, 2000.

9. Normally, when photo-pigments absorb light the photoreceptor cell bleaches and becomes unavailable for light absorption until the photo-pigment is reformed through a lengthy metabolic process called the visual cycle.

"During visual excitation, rhodopsin undergoes photoactivation and bleaches to opsin and all-trans-retinal. To regenerate rhodopsin and maintain normal visual sensitivity, the all-trans isomer must be metabolized and reisomerized to produce the chromophore 11-cis-retinal in biochemical steps that constitute the visual cycle and involve the retinal pigment epithelium. A key step in the visual cycle is isomerization of an all-trans retinoid to 11-cis-retinol in the RPE." Chen P, Hao W, Rife L, Wang XP, Shen D, Chen J, Ogden T, Van Boemel GB, Wu L, Yang M, Fong HK. A photic visual cycle of rhodopsin regeneration is dependent on Rgr. Nature Genet. 2001 Jul;28(3):256-60
"All visual systems detect light in the same way, that is through the photoisomerization of an 11-cis retinoid to a corresponding trans isomer. What is strikingly different between the systems, is the mechanism by which the 11-cis chromophore is reformed and visual pigment regenerated in a process known as the visual cycle." Gonzalez-Fernandez F. Evolution of the visual cycle: the role of retinoid-binding proteins. J Endocrinol. 2002 Oct;175(1):75-88.

10. However, if the intermediary formed when the photo-pigment absorbs light then absorbs blue light (l<470 nm), photoreversal can cause a photoreceptor cell to rapidly become unbleached and increase the level of light absorption by photoreceptor cells by several orders of magnitude, which in turn greatly increases the potential for light induced damage.

"Light induced retinal injury manifests endogenously programmed apoptotic cell death in the photoreceptors and the RPE in addition to toxic bio-oxidation. To elucidate the underlying pathomechanims, effort has been directed to search for the molecular initiator of such apoptotic pathway and accordingly the intrinsic photon receptor of the retina, rhodopsin, has become an obvious target of interest. As the availability of rhodopsin to capture photons depends critically on the rate of visual pigment regeneration, the expression of Rpe65 gene in the RPE, crucial to the re-isomerizaton of all-trans retinal to 11-cis retinal, has also been implicated."
"Rhodopsin recycling can occur in vitro via rapid photoreversal of bleaching by short-wavelength visible light (blue light) distinctive from the slower metabolic pathway via the RPE. With this process rhodopsin is regenerated from retinoid intermediates by rapid photochemical reactions many times faster than the metabolic pathway...these results have provided important clues to the possible mechanisms underlying phototoxicity and highlighted the rationale for avoiding excessive exposure of the retina to high frequency radiation such as ultraviolet and blue light." Toxicology of the Retina: Advances in Understanding the Defence Mechanisms and Pathogenesis of Drug- and Light-Induced Retinopathy. Siu et al. Clinical and Experimental Ophthalmology 2008; 36:176-185
"Blue light can efficiently restore functional rhodopsin from bleaching intermediates through a process termed photoreversal of bleaching. This process does not depend on the visual cycle via the pigment epithelium but nevertheless enables rhodopsin molecules to absorb the critical number of photons required to induce retinal degeneration." Keller C, Grimm C, Wenzel A, Hafezi F, Reme C. Protective effect of halothane anesthesia on retinal light damage: inhibition of metabolic rhodopsin regeneration. Invest Ophthalmol Vis Sci. 2001 Feb;42(2):476-80.
"Acute white-light damage to rods depends on the amount of rhodopsin available for bleaching during light exposure. ... Because photoreversal is faster than metabolic regeneration of rhodopsin by several orders of magnitude, the photon catch capacity of the retina is significantly augmented during blue-light illumination, which may explain the greater susceptibility of the retina to blue light than to green light. ... Blue light can also affect function of several blue-light-absorbing enzymes that may lead to the induction of retinal damage... When a visual pigment molecule is excited by photon absorption, rhodopsin rapidly decays (1 ms) through bleaching intermediates to metarhodopsin I and II (MI and MII). High-energy blue light is absorbed by MII, which is photoreversed back to original chromophore 11-cis retinal attached to its apoprotein opsin. In high-photon fluxes, 1 molecule of rhodopsin can be reversed many times (>50 times within 30 min).
CONCLUSIONS: Short time exposure to blue light has deleterious effects on retinal morphology..... Photoreversal of bleaching, which occurs only in blue but not in green light, increases the photon-catch capacity of the retina and may thus account for the difference in the damage potential between blue and green light." Grimm C, et al. Rhodopsin-Mediated Blue-Light Damage to the Rat Retina: Effect of Photoreversal of Bleaching. Invest Ophthalmol Vis Sci 2001 Feb;42(2):497-50.
"A long-lived bleaching intermediate produced by green light exposure was photoreversed to rhodopsin by exposure to blue light. CONCLUSIONS: Because of the instantaneous, nonmetabolic regeneration of rhodopsin by the process of photoreversal of bleaching, blue light exposure permits the absorption of large numbers of photons by rhodopsin and by a photoreversible intermediate of bleaching in vivo." Grimm C, Reme CE, Rol PO, Williams TP. Blue light's effects on rhodopsin: photoreversal of bleaching in living rat eyes. Invest Ophthalmol Vis Sci. 2000 Nov;41(12):3984-90.

11. An example of blue light-induced photo-oxidative damage is the formation of A2E, an indigestible toxic compound, which is produced in photoreceptor outer segments when blue light is absorbed by free retinal (trans).

"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. ... these compounds have been implicated in the development of various maculopathies such as age-related macular degeneration (AMD)" Oxidation of A2E Results in the Formation of Highly Reactive Aldehydes and Ketones Wang Z et al, Photochemistry and Photobiology: Oct, 2006; 82(5):1251-57.
The turnover of retinal, an essential element of the visual process, is the basis of the events that lead to damage. Free retinal, absorbing in the blue region of the visible spectrum, is phototoxic, and is a precursor of the toxic compound A2E, which specifically targets cytochrome oxidase and thereby induces cell death by apoptosis" Shaban H, Richter C. A2E and blue light in the retina: the paradigm of age-related macular degeneration. Biol Chem 2002 Mar-Apr;383(3-4):537-45.
"The nondegradable pigments that accumulate in retinal pigment epithelial (RPE) cells as lipofuscin constituents ...may also contribute to the etiology of age-related macular degeneration. The best characterized of these fluorophores is A2E,... Evidence indicates that photochemical mechanisms initiated by excitation from the blue region of the spectrum may contribute to the adverse effects of A2E accumulation, with the A2E photooxidation products being damaging intermediates" Characterization of Peroxy-A2E and Furan-A2E Photooxidation Products and Detection in Human and Mouse Retinal Pigment Epithelial Cell Lipofuscin. Jang et al. J Biol Chem. 2005 Dec 2;280(48):39732-9

12. A2E is a strong photogenerator of damaging radical oxygen species when it absorbs blue light, with peak production occurring at 440 nm.

When the pyridinium bisretinoid A2E, an age-related fluorophore in the retinal pigment epithelium (RPE), is irradiated with blue light, photochemical events are initiated that can ultimately provoke cell death." Sparrow JR, Zhou J, Cai B. DNA is a target of the photodynamic effects elicited in A2E-laden RPE by blue-light illumination. Invest Ophthalmol Vis Sci. 2003 May; 44(5): 2245-51.
"The nondegradable pigments that accumulate in retinal pigment epithelial (RPE) cells as lipofuscin constituents ... This autofluorescent material may also contribute to the etiology of age-related macular degeneration. The best characterized of these fluorophores is A2E,... Evidence indicates that photochemical mechanisms initiated by excitation from the blue region of the spectrum may contribute to the adverse effects of A2E accumulation, with the A2E photooxidation products being damaging intermediates" Characterization of peroxy-A2E and furan-A2E photooxidation products and detection in human and mouse retinal pigment epithelial cell lipofuscin. Jang YP et al. J. Biol Chem. 2005 Dec 2;280(48):39732-9.

13. High levels of blue light exposure can generate sufficient ROS to cause photoreceptors to be lost, and can result in permanent visual impairment or blindness.

When amassed to sufficient concentrations, A2E can mediate detergent-like effects on cellular membranes, alter lysosomal function, and release proapoptotic proteins from mitochondria. A2E also confers a susceptibility to blue light-induced RPE cell death, with photooxidative products of A2E being the intermediates that ravage cellular macromolecules." Sparrow JR. Therapy for macular degeneration: Insights from acne. PNAS. April 15, 2003. Vol. 100. no. 8. 4353-4354.

14. Sub-threshold exposure levels of blue light also produce oxidative damage that may contribute to long-term degeneration of the retina.

"It is now well established that photoretinopathy is a cumulative process, and chronic light damage may be one of the factors contributing to the development of age-related macular degeneration (AMRD)." Andley UP, Chylack Jr LT. Recent Studies on Photodamage to the Eye with Special Reference to Clinical and Therapeutic Procedures. Photodermatology Photoimmunology and Photomedicine 1990; 7:98-105.
"Blue visible light damage to retinal pigment epithelial cells occurs through a photooxidative mechanism and the resultant damage is hypothesized to induce or exacerbate age-related macular degeneration... Therefore the inhibition of production of HGF {Hepatocyte Growth Factor] by visible light, especially by blue light, may enhance the phototoxic effects of visible light on the RPE and retinal neurons and plays an important role in the development of damage to RPE and retinal neuron after irradiation with blue light" Blue Light Irradiation Inhibits the Production of HGF by Human Retinal Pigment Epithelium Cells In Vitro. Chu R et al. Photochemistry and Photobiology 2006: 82(5): 1247-1250.

15. In particular sub-threshold levels of blue light produce oxidative damage in the proximate retinal pigment epithelium cells (RPEs) that, on a daily basis, ingest the tips of photoreceptor cells where light absorption as well as photo-oxidative damage takes place.

"A2E and its isomer iso-A2E have been isolated from the eyes of elderly humans ... In vivo, when light strikes the visual pigment rhodopsin, the chromophore all-trans-retinal is liberated from the photoreceptor outer segment membranes, reduced, and transported into the RPE. Additionally, packets of photoreceptor discs are rhythmically shed and phagocytosed by RPE cells." Suter M. et al. Age-Related Macular Degeneration. The Lipofusion Component N-retinyl-N-retinylidene ethanolamine (A2E) Detaches Proapoptotic Proteins from Mitochondria and Induces Apoptosis in Mammalian Retinal Pigment Epithelial Cells Biol Chem 2000 Dec 15; 275(50):39625-30.
"Through the expression and activity of specific proteins, it [the RPE] regulates the transport of nutrients and waste products to and from the retina, it contributes to outer segment renewal by ingesting and degrading the spent tips of photoreceptor outer segments, it protects the outer retina from excessive high-energy light and light-generated oxygen reactive species and maintains retinal homeostasis through the release of diffusible factors." Boulton M, Dayhaw-Barker P. The role of the retinal pigment epithelium: topographical variation and ageing changes. Eye. 2001 Jun;15(Pt 3):384-9.

16. The molecular damage from oxidative stress in photoreceptor outer segments where light absorption takes place accumulates in the RPE and becomes a major factor in the long term deterioration of the retina that occurs in normal aging and in the development of AMD.

"AMD is an advanced stage of a deteriorative process that takes place in all eyes ... Beginning early in life, and continuing throughout the life span, cells of the RPE gradually accumulate sacs of molecular debris. These residual bodies (lipofuscin) are remnants of the incomplete degradation of abnormal molecules which have been damaged within the RPE cells or derived from phagocytized rod and cone membranes. Progressive engorgement of RPE cells with these functionless residues is associated with the extrusion of aberrant materials which accumulate in Bruch's membrane and aggregate in the form of drusen and basal laminar deposits. These excretions contribute to the further deterioration of the RPE. Loss of vision results from death of visual cells due to degeneration of RPE cells, or the effects of leakage from neovascular membranes that invade the region of abnormal extracellular deposits." Young RW. Pathophysiology of age-related macular degeneration. Surv Ophthalmol.1987 Mar-Apr; 31(5):291-306.
"A characteristic of aging of postmitotic cells such as cardiac muscle, neurons, and retinal pigment epithelium (RPE) is the accumulation of lipofuscin. .. In most cell types, lipofuscin is a product of cellular autophagy. In RPE cells, however, a major component of the lipofuscin is derived from the phagocytosis of the rod outer segment (ROS) disc membranes... Thus, blue-light irradiation of A2-E may induce photooxidation, and the resulting epoxides may trigger the apoptosis cascade yielding DNA fragmentation... In conclusion, our data support the hypothesis of direct DNA damage by oxidized A2-E ... Certainly, oxidative processes are responsible for the detrimental effect of A2-E to RPE cells under irradiation." Retinal Pigment Epithelium Cell Damage by A2-E and its Photo-Derivatives. Hammer M., et al. Molecular Vision. Nov 1, 2006 12:1348-54.

17. Thus, although the vision loss of AMD results from photoreceptor damage in the central retina, the initial pathogenesis involves degeneration of RPE cells.

"Oxidative stress is believed to contribute to the pathogenesis of many diseases, including age-related macular degeneration (AMD). Although the vision loss of AMD results from photoreceptor damage in the central retina, the initial pathogenesis involves degeneration of RPE cells. Evidence from a variety of studies suggests that RPE cells are susceptible to oxidative damage." Liang FQ, Godley BF. Oxidative stress-induced mitochondrial DNA damage in human retinal pigment epithelial cells: a possible mechanism for RPE aging and age-related macular degeneration. Exp Eye Res. 2003 Apr;76(4):397-403.
"A precondition for AMD appears to be the accumulation of the age pigment lipofuscin in lysosomes of retinal pigment epithelial (RPE) cells. In AMD, these cells seem to die by apoptosis with subsequent death of photoreceptor cells. A2E, a lipofuscin component, induces apoptosis in RPE and other cells at concentrations found in human retina .... Loss of RPE cell viability ... might constitute a pivotal step toward the progressive degeneration of the central retina." Age-Related Macular Degeneration. The Lipofusion Component N-retinyl-N-retinylidene ethanolamine (A2E) Detaches Proapoptotic Proteins from Mitochondria and Induces Apoptosis in Mammalian Retinal Pigment Epithelial Cells. Suter M. et al Biol Chem 2000 Dec 15;275(50) :39625-30.

18. Deterioration of the RPE begins when the RPE is unable to degrade some components of the oxidative debris it ingests, including the compound A2E, which is enzymatically indigestible.

"The notion that the RPE might encounter problems with digestion of its considerable phagocytic burden is a reasonable one. Photoreceptor outer segments live in a potentially toxic environment that includes high oxygen and high photon flux. These conditions are conducive to photooxidative damage and the production of byproducts of lipid peroxidation such as malondialdehyde, which can cross link proteins." Bok D. New insights and new approaches toward the study of age-related macular degeneration. Proc Natl Acad Sci U S A. 2002 Nov 12;99(23):14619-21.
"Characterization of the composition of RPE lipofuscin has revealed that a major constituent is A2E, a conjugate of vitamin A aldehyde... and once formed it cannot be enzymatically degrade." Therapy for macular degeneration: Insights from acne. Janet R. Sparrow PNAS April 15, 2003;100(8):4353-4354.
"There's some evidence that blue light's damage may be related to the lipofuscin fluorophore A2E, a naturally occurring substance in the retina, and the release of free radicals in the RPE. In one study, researchers allowed RPE cells in culture to accumulate differing amounts of the fluorophore, then exposed the cells to 480 nm (blue) or 545 nm (green) light for 15 seconds to a minute.
They found that nonviable cells were located in blue-light exposed zones of RPE cells that contained A2E, but cells outside those areas were viable. The number of nonviable cells increased with the duration of exposure and as a function of the concentration of the A2E in the cells. Illumination with the green light resulted in more viable cells." Should We Block the Blue. Review of Ophthalmology Oct,2003
"Several retinal diseases including age-related macular degeneration (AMD), Stargardt's disease, and Best's macular dystrophy result from the degeneration of cells in the retina. These diseases have also been associated with the accumulation of autofluorescent lipofuscin granules in the retinal pigment epithelium (RPE). Many tissues accumulate lipofuscin granules, also called age pigments, but the retinal lipofuscin (RLF) that accumulates in RPE cells is probably unique. RPE cells are reported to begin accumulating RLF granules around age 20; and by age 80 these granules may constitute up to 20% of the cell volume. RLF may relate to the onset of these ocular diseases because it has been shown to generate reactive oxygen species via photosensitization with blue light, which may cause damage and death of the RPE and surrounding cells." Examining the Proteins of Functional Retinal Lipofuscin using Proteomic Analysis as a Guide for Understanding its Origin Warburton S. Et al. Molecular Vision 15 Dec 2005; 11:1122-34.

19. The RPE accumulates some of this toxic oxidative debris as lipofuscin, the aging pigment.

"Lipofuscin (LF) is a common morphological result of the aging process and is manifested as a heterogeneous complex of fluorescent, lipid-protein aggregates found in the cytoplasm of postmitotic cells. In the retinal pigment epithelium (RPE) of the human eye, the formation of LF is attributed to the accumulation of indigestible end-products from the phagocytosis of photoreceptor outer segments ... In vitro experiments also show that blue-light excitation of cultured RPE fed LF generates a variety of reactive oxygen intermediates (including hydrogen peroxide, singlet oxygen, and superoxide radical anion), which renders LF phototoxic to cultured RPE cells." Haralampus-Grynaviski NM, Lamb LE, Clancy CM, Skumatz C, Burke JM, Sarna T, Simon JD. Spectroscopic and morphological studies of human retinal lipofuscin granules. Proc Natl Acad Sci U S A. 2003 Mar 18; 100(6): 3179-84.
"Lipofuscin harbors two unusual retinoids, the lipophilic cations, A2E and its isoform, iso-A2E, first isolated from the eyes of old individuals. The molecules can be synthesized from two retinals and one ethanolamine, both components of photoreceptor outer segment membranes, where 11-cis-retinal serves as the chromophore of the visual pigment rhodopsin and phosphatidylethanolamine is an abundant membrane phospholipid." Suter M. et al. Age-Related Macular Degeneration. The Lipofusion Component N-retinyl-N-retinylidene ethanolamine (A2E) Detaches Proapoptotic Proteins from Mitochondria and Induces Apoptosis in Mammalian Retinal Pigment Epithelial Cells. Biol Chem 2000 Dec 15; 275(50):39625-30.
"That RPE cells underlying the macula have the highest accumulations of lipofuscin... From the perspective of the origins of RPE lipofuscin, most of the latter is retinoid-derived. This is certainly the case for A2E. Thus it is not a coincidence that the macula of the retina also has the highest concentration of 11-cis-retinal-containing visual pigment, a feature that reflects, in part, the packing density of cone and rod photoreceptor cells. The heightened capacity for photon absorption conferred by the density of visual pigment in the macula translates into a higher probability that all-trans-retinal will be available for A2E formation. Accordingly, greater amounts of A2E are amassed by RPE cells underlying the macula. Because lifetime accumulations of A2E are also greatest in the macula, critical levels of the fluorophore may be a factor in the onset of age-related macular degeneration. ... A safe and inexpensive approach is to reduce A2E accumulation by avoiding the bright light conditions that accelerate the flux of all-trans-retinal. If, in addition to reducing overall light exposure, ...limit(ing) the amount of blue light reaching the RPE ...protection against A2E photoreactivity may also be gained."Sparrow JR. Therapy for macular degeneration: Insights from acne. PNAS April 15, 2003; 100(8):4353-4354.
"Lipofuscin (also called age pigment) always increases with age. In fact, the time-dependent accumulation of lipofuscin in lysosomes of postmitotic cells and some stable cells is the most consistent and phylogenetically constant morphologic change of aging." Porta EA. Pigments in aging: an overview. Ann N Y Acad Sci. 2002 Apr;959:57-65.
"Oxidative mechanisms are considered to contribute to the aging changes in retinal pigment epithelial (RPE) cells that underlie the pathogenesis of age-related macular degeneration. An important source of oxidative damage is likely to be the photoreactive pigments that progressively accumulate and constitute the lipofuscin of retinal pigment epithelial cells." Indirect Antioxidant Protection against Photooxidative Processes Initiated in Retinal Pigment Epithelial Cells by a Lipofuscin Pigment. Rejuvenation Res. 2006 Summer;9(2):256-63.

20. When illuminated by blue light, lipofuscin is phototoxic, producing exponentially greater amounts of ROS as blue wavelengths decrease from 490 to 400 nm.

"Accumulation of lipofuscin (LF) is a prominent feature of aging in the human retinal pigment epithelium (RPE) cells. This age pigment exhibits substantial photoreactivity, which may increase the risk of retinal photodamage and contribute to age-related maculopathy. In a previous study, we detected singlet oxygen generation by lipofuscin granules excited with blue light. ... The action spectrum of singlet oxygen formation indicated that this process was strongly wavelength-dependent and its efficiency decreased with increasing wavelength by a factor of ten, comparing 420 nm and 520 nm ... The quantum yield of singlet oxygen increased with increasing concentration of oxygen. Using laser flash photolysis we studied the possible mechanism of singlet oxygen formation. The observed transient, with a broad absorption spectrum peaking at around 440 nm, was identified as a triplet with lifetime ca 11 microseconds. It was quenched by both molecular oxygen and beta-carotene with concomitant formation of a beta-carotene triplet state. These results indicate the potential role of hydrophobic components of lipofuscin in blue light-induced damage to the RPE." Rozanowska et al Blue light-induced singlet oxygen generation by retinal lipofuscin in non-polar media. Free Radic Biol Med. 1998 May;24(7-8):1107-12.
"Human RPE lipofuscin exhibits substantial photoreactivity and, under aerobic conditions, is able to form several potentially cytotoxic species. ... The observed photoreactivity of human RPE cells is, to a significant extent, determined by their lipofuscin content. This is evident from the comparison of action spectra of photo-dependent oxygen uptake in RPE cells and in purified lipofuscin granules. Both action spectra exhibit significant similarities in that the efficiency of illuminating light, to induce oxygen uptake and generation of H2O2, falls steeply with the wavelength between 300 and 600 nm. Thus, blue light seems to be the most efficient radiation under physiologically relevant conditions. This is because ultraviolet radiation is completely filtered out by the cornea and lens of the adult human eye and, as a result, virtually no light below 400 nm is transmitted to the retina. ... RPE lipofuscin becomes apparent only during the second decade of life and accumulates with age." Rozanowska M et al. Blue light-induced reactivity of retinal age pigment. In vitro generation of oxygen-reactive species. J Biol Chem. 1995 Aug 11; 270(32): 18825-30.
"[Lipofuscin] action spectra: ... The onset of photoinduced oxygen uptake occurs at 490 nm and then increases with decreasing wavelength. ... Only light of wavelengths longer than 400 nm reaches the adult retina, thus the initial absorbing species relevant to retinal toxicity in vivo must be blue absorbing." Pawlak et al. Action spectra for the photoconsumption of oxygen by human ocular lipofuscin and lipofuscin extracts. Arch Biochem Biophys. 2002 Jul 1;403(1):59-62.
"A2E is a blue-absorbing molecular constituent of human ocular lipofuscin. Lipofuscin photoproduces toxic reactive oxygen intermediates (ROI)." Pawlak A, Wrona M, Rozanowska M, Zareba M, Lamb LE, Roberts JE, Simon JD, Sarna T. Comparison of the aerobic photoreactivity of A2E with its precursor retinal. Photochem Photobiol. 2003 Mar; 77(3):253-8.
"We have previously shown that the accumulation of A2E by RPE cells in a culture model, confers a susceptibility to blue light damage. The wavelength specificity of this effect is consistent with the excitation spectrum of A2E. Moreover, the extent of blue-light induced apoptosis is proportional to the A2E content of the cultures and is not manifest by cells devoid of A2E. In experiments aimed at defining the molecular pathways involved in executing RPE cell death in the context of A2E and blue light, we have detected the release of cytochrome c from mitochondria and the activation of caspase-3. Moreover, we have rescued A2E-loaded blue-light exposed RPE from cell death by tansfection with Bcl-2. In efforts to track the events occurring from the time that blue light is absorbed by intracellular A2E until the cell death program is initiated, we have shown that the death of A2E-laden RPE is blocked in oxygen-depleted media. The enhancement of cell death in the presence of deuterium oxide and the protection afforded by quencher/scavengers of singlet oxygen, are both consistent with singlet oxygen generation being a factor contributing to blue-light mediated death of A2E-containing RPE. Importantly, blue light illumination of A2E also leads to the generation of previously unidentified oxidized derivatives of A2E, which form consequent to singlet oxygen generation. These oxidized derivatives of A2E may contribute to cellular injury." Ow J, Nakanishi K, Blue-Light Induced Death of A2E-Laden RPE: Apoptotic Pathways and Oxidative Mechanisms. Conference on Aging Retina and Early Degeneration, 25-27, 2001 Harvard Medical School Columbia University, New York, NY.
"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." Roberts JE. Ocular Phototoxicity. J Photochem Photobiol B. 2001 Nov 15;64(2-3):136-43.
"The pigment melanin, which is believed to play a photoprotective role...We conclude that the content of melanin in RPE cells undergoes an age-related change to which photo-oxidation may contribute. This observation raises the question of whether age-related changes in melanin reduce the photoprotective role of the pigment in aging RPE cells." Sarna T, Burke JM, Korytowski W, Rozanowska M, Skumatz CM, Zareba A, Zareba M. Loss of melanin from human RPE with aging: possible role of melanin photooxidation. Exp Eye Res. 2003 Jan; 76(1): 89-98.

21. The ROS generated when blue light is absorbed by lipofuscin can damage cellular DNA, lipoproteins, and other structures within RPE, and in some cases can lead to self-induced cell death (apoptosis).

"The presence of the age pigment lipofuscin is associated with numerous age-related diseases. In the retina lipofuscin is located within the pigment epithelium where it is exposed to high oxygen and visible light, a prime environment for the generation of reactive oxygen species. ... We have found that illumination of lipofuscin with visible light is capable of extragranular lipid peroxidation, enzyme inactivation, and protein oxidation. ... Ocular lipofuscin may have a unique role to play in aging of the RPE, a tissue that is continually exposed to visible light (400-700 nm) and high oxygen tensions (~70 mm Hg). Studies have shown this type of lipofuscin to be a photoinducible generator of superoxide ions, singlet oxygen, hydrogen peroxide, and lipid peroxides, all of which are reactive oxygen species implicated in general aging processes. These species can adversely affect cell function by damaging proteins, carbohydrates, DNA... We have confirmed that lipofuscin granules incubated with visible light induce lipid peroxidation and cause enzyme inactivation supporting our hypothesis that lipofuscin contributes to aging of the RPE and is a risk factor for age-related macular degeneration. ... This study shows that lipofuscin can photoinduce the oxidation of lipid membranes and inactivate enzymes and that such effects are mediated by the production of reactive oxygen species. Age-related damage to lipid membranes and cellular proteins by such species has been implicated in the general aging process. This is the first demonstration that lipofuscin-derived reactive oxygen species can induce such effects. ...the light intensity used in this study equates to that which induces photochemical-induced retinal cell loss in animals." Wassell J et al. The Photoreactivity of the Retinal Age Pigment Lipofuscin. Journal of Biological Chemistry 1999;274(34):23828-32.
"Lipofuscin accumulates with age within secondary lysosomes of retinal pigment epithelial (RPE) cells of humans and many animals. The autofluorescent lipofuscin pigment has an excitation maximum within the range of visible blue light...This physico-chemical property of the pigment indicates that it may have a photo-oxidative capacity and, consequently, then should destabilize lysosomal membranes of blue-light-exposed RPE cells... Lipofuscin-loaded blue-light-exposed RPE cells showed a considerably enhanced loss of both lysosomal stability and viability when compared to control cells. It is concluded that the accumulation of lipofuscin within secondary lysosomes of RPE sensitizes these cells to blue light by inducing photo-oxidative alterations of their lysosomal membranes resulting in a presumed leakage of lysosomal contents to the cytosol with ensuing cellular degeneration of apoptotic type." Wihlmark U, Wrigstad A, Roberg K, Nilsson S, Brunk U. Lipofuscin Accumulation in Cultured Retinal Pigment Epithelial Cells Causes Enhanced Sensitivity to Blue Light Irradiation. Free Radical Biol. Med. (1997) 22:1229-1234.
"Photoactivation of lipofuscin yields singlet oxygen, superoxide anion, hydrogen peroxide, and lipid hydroperoxides. These photoinducible ROS have been shown to result in lipid peroxidation, enzyme inactivation, and retinal pigment epithelium (RPE) cell death." Chen L, Dentchev T, Wong R, Hahn P, Wen R, Bennett J, Dunaief JL. Increased expression of ceruloplasmin in the retina following photic injury. Mol Vis 2003 Apr 30;9:151-8.

22. The accumulation of high concentrations of lipofuscin in the RPE cells and its ensuing exposure to light have been linked to the subsequent loss of photoreceptor cells and the occurrence of AMD.

"A precondition for AMD appears to be the accumulation of the age pigment lipofuscin in lysosomes of retinal pigment epithelial (RPE) cells. In AMD, these cells seem to die by apoptosis with subsequent death of photoreceptor cells, and light may accelerate the disease process." Suter M, Reme C, Grimm C, Wenzel A, Jaattela M, Esser P, Kociok N, Leist M, Richter C. Age-Related Macular Degeneration. The Lipofusion Component N-retinyl-N-retinylidene ethanolamine (A2E) Detaches Proapoptotic Proteins from Mitochondria and Induces Apoptosis in Mammalian Retinal Pigment Epithelial Cells. J Biol Chem 2000 Dec 15;275(50):39625-30.
"Damage to the young and adult eye by intense ambient light is avoided because the eye is protected by a very efficient antioxidant system. 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." Roberts JE. Ocular Phototoxicity. J Photochem Photobiol B. 2001 Nov 15;64(2-3):136-43.

23. This build-up of lipofuscin impairs the RPE cells' ability to provide nutrients and digest debris, establishing the initial pathogenesis of AMD.

"The phagocytic capacity of LF-loaded RPE cells was significantly reduced compared to that of unloaded control cells, indicating that LF-loaded RPE cells may be unable to serve the photoreceptors sufficiently regarding phagocytosis of shed outer segment tips. Blue light irradiation destabilized lysosomal membranes in LF-loaded RPE cells and significantly reduced the viability of such cells compared to unloaded, irradiated control cells. These results may be of significance in relation to the development of age-related macular degeneration (AMD)." Nilsson SE, Sundelin SP, Wihlmark U, Brunk UT. Aging of cultured retinal pigment epithelial cells: oxidative reactions, lipofuscin formation and blue light damage. Doc Ophthalmol. 2003 Jan;106(1):13-6.
"Because the RPE is vital to the integrity of the photoreceptor cells, the demise of RPE cells brings about the loss of photoreceptors... Studies performed over the past several years have pointed to the fluorophores that constitute the lipofuscin of RPE cells as being crucial factors in the degeneration of these cells in macular degeneration. Of added importance is the fact that lipofuscin accumulates with age in the RPE cells of all eyes... Much of this indigestible pigment originates in the photoreceptor cell, with deposition in the RPE occurring because it is the responsibility of the RPE to internalize membranous debris discarded daily by the photoreceptor cell." Sparrow JR. Therapy for macular degeneration: Insights from acne. PNAS April 15, 2003; 100(8):4353-4354.

24. Furthermore the absorption of blue light by chromophores in lipofuscin and melanosomes generates high levels of ROS in the RPE membrane, which contributes to increasing levels of oxidative damage in the RPE and adjacent tissues.

"Lipofuscin accumulation in the retinal pigment epithelium is associated with the onset of age-related macular degeneration. Lipofuscin is phototoxic and affects cellular function through the photochemical generation of reactive oxygen intermediates." Retinyl Palmitate and the Blue-Light-Induced Phototoxicity of Human Ocular Lipofuscin. Lamb LE, Zareba M, Plakoudas SN, Sarna T, Simon JD. Arch Biochem Biophys 2001 Sep15;393(2):316-20
"There are an increasing number of indications that an excessive accumulation of RPE lipofuscin can lead to cellular dysfunction and contribute to retinal aging and degeneration. Greater insight into the composition of RPE lipofuscin and an understanding of its genesis may facilitate novel therapeutic approaches to minimize lipofuscin accumulation and thus reduce the progression of retinal conditions such as Stargardt disease and AMD. The propensity for RPE lipofuscin to mediate blue light toxicity also gives credence to the view that blue light filtering may protect against RPE cell injury." RPE Lipofuscin and its role in Retinal Pathobiology.  Sparrow JR and Boulton M. Exp Eye Res. 2005 May;80(5):595-606. Review.
"Blue-light-induced photoreactivity of melanosomes increases with age, perhaps providing a source of reactive oxygen species and leading to depletion of vital cellular reductants, which, together with lipofuscin, may contribute to cellular dysfunction." Invest Ophthalmol Vis Sci. 2002 Jul; 43(7):2088-96. Photoreactivity of Aged Human RPE Melanosomes: A Comparison with Lipofuscin. Rozanowska M, Korytowski W, Rozanowski B, Skumatz C, Boulton ME, Burke JM, Sarna T.
"Melanosomes, melanolipofuscin, and lipofuscin granules were isolated from human RPE donors older than 60. Melanosomes were photodegraded by exposure to blue light. CONCLUSIONS: Human melanosomes act as effective antioxidants by preventing iron ion-induced oxidation. Photodegradation of melanosomes [by absorption of blue light] results in the loss of these antioxidant properties ...Here we have demonstrated that photodegradation of melanosomes with acute blue light decreases their ability to protect from iron-mediated oxidation and, on top of that, the degraded melanosomes become a susceptible target for iron-mediated oxidation themselves." Invest Ophthalmol Vis Sci. 2008 Jul;49(7):2838-47. Human RPE Melanosomes Protect from Photosensitized and Iron-mediated Oxidation but become Pro-Oxidant in the Presence of Iron upon Photodegradation. Rozanowski B, et al.
"Melanosomes of the retinal pigment epithelium (RPE) are long lived organelles that may undergo photobleaching with aging, which can diminish the antioxidant efficiency of melanin. Photobleaching of RPE melanosomes therefore makes cells containing them more sensitive to light-induced cytotoxicity. This observation raises the possibility that aged melanosomes increase RPE cell photic stress in situ, perhaps contributing to reduced tissue function and to degeneration of the adjacent retina that the RPE supports." Photochem Photobiol. 2007 Jul-Aug;83(4):925-30. Photobleaching of Melanosomes from Retinal Pigment Epithelium: II. Effects on the Response of Living Cells to Photic Stress. Janice M. Burke

25. The onset of AMD is considered to occur with the development of drusen, initiated when the RPE cells can no longer contain the accumulated lipofuscin and begin to excrete oxidative debris into intracellular space between the RPE and Bruch's membrane.

"The presence of numerous and/or confluent, soft drusen in the macula is considered a major risk factor for developing AMD. This correlation is so well established that many clinicians refer to individuals with soft drusen in the macula, in the absence of any loss of macular vision, as an early stage of AMD." Crabb JW, et al. Drusen Proteome Analysis: An Approach to the Etiology of Age-Related Macular Degeneration. Proceedings of the National Academy of Sciences U S A. 2002; 99(23):14682-7.
"Drusen are abnormal extracellular matrix deposits characteristic of age-related macular degeneration (AMD), a leading cause of blindness in the aging human population." Leu ST, Batni S, Radeke MJ, Johnson LV, Anderson DH, Clegg DO. Drusen are Cold Spots for Proteolysis: Expression of Matrix Metalloproteinases and Their Tissue Inhibitor Proteins in Age-related Macular Degeneration. Exp Eye Res. 2002 Jan;74(1):141-54.
"The accumulation of numerous or confluent drusen, especially in the macula, is a significant risk factor for the development of age-related macular degeneration (AMD)... The results indicate that cellular remnants and debris derived from degenerate RPE cells become sequestered between the RPE basal lamina and Bruch's membrane." Anderson DH et al. A role for local inflammation in the formation of drusen in the aging eye. Am J Ophthalmol. 2002 Sep;134(3):411-31.
"It is generally believed that the accumulation of the autofluorescent age pigment lipofuscin in RPE cell phagolysosomes constitutes a predicament for the development of the disease. Lipofuscin accumulation and the formation of drusen and other deposits in the region of Bruch's membrane, which separates the pigment epithelium from the underlying choroid are considered initial steps in the pathogenesis of AMD. Drusen formation may be the result of progressive death and/or exocytosis of RPE cells in the central retina." Suter M. et al. Age-Related Macular Degeneration. The Lipofusion Component N-retinyl-N-retinylidene ethanolamine (A2E) Detaches Proapoptotic Proteins from Mitochondria and Induces Apoptosis in Mammalian Retinal Pigment Epithelial Cells. Biol Chem 2000 Dec 15;275(50):39625-30.
"Bruch's membrane is an extra cellular structure that acts like a sieve through which nutrients and waste products exchange between the retinal pigment epithelium and the blood bearing choroid. With age and age-related macular degeneration (AMD), debris accumulates in this membrane, resulting in increased thickness and decreased permeability." X. Gu, K. Shadrach, M. Sun, K. A. West, L. Shan, S. L. Hazen, R. G. Salomon, J. G. Hollyfield, J. W. Crabb. Characterization of Lipid Oxidation Products and Proteins in Bruch's Membrane from Normal and AMD Donor Eyes. The Association of Bimolecular Research Facilities Annual Meeting 2003 Poster P137-T.

26. The substance drusen is formed when the debris exuded from RPE cells into this intracellular space binds to components drawn from the choroidal vasculature and becomes cross-linked to the Bruch's membrane.

"In October 2002, an important contribution to the understanding of drusen formation was made by Crabb et al. in the Proceedings of the National Academy of Science (PNAS 2002:22255), who presented evidence that proteins in drusen originated in the RPE and choroidal vasculature." Lawrence DW. The Lancet October 26, 2002:360(9342):1307.
"Notably, docosahexaenoate is abundant in photoreceptor cell outer segments and is the most oxidizable fatty acid in humans... Carboxyethyl pyrrole adducts are uniquely generated from the oxidation of docosahexaenoate -containing lipids... they were found to be more abundant in AMD than in normal Bruch's membrane and were found associated with drusen proteins. These data strongly support the hypothesis that oxidative injury contributes to the pathogenesis of AMD.
Owing to the high photooxidative stress in the retina, other oxidative modifications are likely to be found in drusen ... The proteins we have identified are found in several tissues, including the RPE, blood, and photoreceptors, and accordingly, support both the RPE and the choroidal vasculature as sources of the components in drusen. ... As oxidative stress defense mechanisms deteriorate with age, oxidative modifications may gradually lock these and other proteins to Bruch's membrane with crosslinks, preventing normal turnover and initiating drusen development. The photooxidative environment in the retina and the lipid-rich photoreceptor outer segments, 10% of which are phagocytosed daily by the RPE, provide an excellent source of reactive oxygen species. Waste products from the RPE and blood components from the choriocapillaris provide a ready source of extracellular material for oxidative modification and drusen formation. Over time, oxidative modifications and subsequent immune-mediated events could cause the expansion of drusen on Bruch's membrane." Crabb JW, et al. Drusen Proteome Analysis: An Approach to the Etiology of Age-Related Macular Degeneration. Proceedings of the National Academy of Sciences U S A. 2002; 99(23):14682-7
"The current work establishes that oxidized products collect in drusen. In addition, the fatty acid that spawned the lipid fragments is most abundant in the retina, which supports the notion that the deposits originate locally, the researchers say. Crabb and colleagues hypothesize that oxidative damage splinters fatty acids and cross-links the pieces to proteins outside cells. The cross-linked molecules attract immune system proteins, which add to the debris." Seydel, The Eyes Have It. Science's SAGE KE (30 October 2002).

27. Build-up of drusen between the choroidal vasculature in the Bruch's membrane and the RPE cells interferes with the supply of nutrients and oxygen to the RPE cells.

"The first stage of AMD develops is with a build-up of extra-cellular debris between the RPE and the adjacent Bruch's membrane. The debris, called drusen, bonds with the Bruch's membrane to form a barrier that prevents the normal flow of nutrients and waste material between the RPE and the blood supply in the choroids." Anderson DH et al A Role for Local Inflammation in the Formation of Drusen in the Aging Eye. Am J Ophthalmol. 2002 Sep;134(3):411-31.

28. Subsequently the photoreceptors overlying areas where drusen has formed are not properly nourished and die off.

"In early AMD, FIAF's colocalization with large, soft drusen and hyperpigmentation is several times greater than chance, suggesting linked disease processes. In advanced atrophic AMD, FIAF is found mostly adjacent to drusen and GA, suggesting that dispersal of drusen-associated lipofuscin is a marker of atrophic disease progression." Autofluorescence Characteristics of Early, Atrophic, and High-Risk Fellow Eyes in Age-Related Macular Degeneration. Smith Rt et al. Invest Ophthalmol Vis Sci. 2006 Dec;47(12):5495-504
"Drusen, pathological deposits that form between the retinal pigmented epithelium (RPE) and Bruch's membrane, are significant risk factors for the development of AMD. As the RPE degenerates and drusen become larger and more numerous there is a concomitant degeneration of photoreceptor cells which may lead to focal serous detachment and gliotic changes in the inner retina. The ensuing cascade of degenerative events eventually manifests itself in the decline in central vision that is characteristic in individuals with AMD." Hageman GS, et al. An integrated hypothesis that considers drusen as biomarkers of immune-mediated processes at the RPE-Bruch's membrane interface in aging and age-related macular degeneration. Prog Retin Eye Res. 2001 Nov;20(6):705-32.
"The purpose of this study was to investigate the impact of drusen on overlying cells of the retina... CONCLUSIONS: Retinal cells overlying both soft and hard drusen exhibit structural and molecular abnormalities indicative of photoreceptor degeneration and Muller glial activation. These abnormalities resemble the degenerative effects common to many forms of retinal degeneration, but are confined to areas directly overlying drusen. This suggests that photoreceptor cell function is compromised as a consequence of drusen formation." Johnson PT, Lewis GP, Talaga KC, Brown MN, Kappel PJ, Fisher SK, Anderson DH, Johnson LV. Drusen-associated degeneration in the retina. Invest Ophthalmol Vis Sci. 2003 Oct;44(10):4481-8.
"Age-related macular degeneration (AMD)... is characterized by abnormal extracellular deposits, known as drusen, that accumulate along the basal surface of the retinal pigmented epithelium... Widespread drusen deposition is associated with retinal pigmented epithelial cell dysfunction and degeneration of the photoreceptor cells of the neural retina." Johnson LV, Leitner WP, Rivest AJ, Staples MK, Radeke MJ, Anderson DH. The Alzheimer's A Beta-Peptide is Deposited at Sites of Complement Activation in Pathologic Deposits Associated with Aging and Age-Related Macular Degeneration. Proc Natl Acad Sci U S A. 2002 Sep 3;99 (18):11830-5.

29. This progressive degeneration of photoreceptor cells in the macula, the centre of the retina where light is focused, leads to the form of blindness known as geographic atrophy or the "dry form" of AMD.

"A2E inhibits hydrolytic activities in lysosomes and mediates blue light-induced damage to RPE cells... Loss of pigment epithelial cells in the retina may result in the so-called geographic atrophy, which is by far the most frequent form of AMD. To date, no cure or prevention of this disease, which affects a large number of elderly people, is available... So far, no efficient AMD therapy or prevention exists. Carotenoids and antioxidants, limiting exposure to light, or targeting of the precursors of A2E may be useful." Age-Related Macular Degeneration. The Lipofusion Component N-retinyl-N-retinylidene ethanolamine (A2E) Detaches Proapoptotic Proteins from Mitochondria and Induces Apoptosis in Mammalian Retinal Pigment Epithelial Cells. Suter M. et al Biol Chem 2000 Dec 15;275(50):39625-30
"Drusen forms between the basal surface of an epithelial monolayer derived from neuroectoderm that is known as the retinal pigmented epithelium (RPE), and a basement membrane complex called Bruch's membrane... numerous and/or confluent drusen are associated clinically with geographic atrophy of the RPE and with a significantly increased risk of developing the exudative (or neovascular) form of AMD. In the absence of a viable RPE, degeneration of adjacent photoreceptor cells and the loss of vision characteristic of AMD ensue." Johnson LV, Leitner WP, Rivest AJ, Staples MK, Radeke MJ, Anderson DH. The Alzheimer's A beta-peptide is deposited at sites of complement activation in pathologic deposits associated with aging and age-related macular degeneration.Proc Natl Acad Sci U S A. 2002 Sep 3;99(18):11830-5.

30. Exudative AMD or the "wet" form of AMD is characterized by abnormal blood vessels growing from the choriocapillaris through the retinal pigment epithelium (RPE), typically resulting in hemorrhage, exudation, scarring, and/or serous retinal detachment.

"Patients with retinal vascular anastomosis to the vascular proliferation of exudative AMD were much more likely to have focal areas of intense autofluorescence... Because the amount of fluorescence is directly related to the amount of lipofuscin, which in turn is related to the cumulative amount of oxidative damage, these findings suggest possible explanations for certain patterns of vessel growth seen in exudative ARMD." Spaide RF. Fundus autofluorescence and age-related macular degeneration. Ophthalmol Feb 2003; 110(2):392-9.
"Choroidal neovascularization (CNV) in age-related macular degeneration (AMD) is the most common cause of severe visual loss in patients over age 60 years in developed countries. ... Angiogenesis is thought to result from the balance between angiogenesis stimulation and inhibition. A potent antiangiogenic factor recently has been identified in the retina and shown to be secreted by RPE cells. The inhibitor, pigment epithelium-derived factor (PEDF) is considered the key factor associated with avascularity of the cornea, vitreous, and outer retinal layer of the eye. We recently demonstrated that an imbalance between PEDF and vascular endothelial growth factor (VEGF) in RPE cells caused by aging and oxidative stress may contribute to the disregulation of endothelial cell proliferation in Choroidal neovascularization." Ohno-Matsui K. Nippon Ganka. Molecular Mechanism for Choroidal Neovascularization in Age-Related Macular Degeneration. Gakkai Zasshi. 2003 Nov;107(11):657-73.
"The wet, exudative, neovascular form of AMD affects only 10% of those with the disease and is characterized by abnormal blood vessels growing from the choriocapillaris through the retinal pigment epithelium (RPE), typically resulting in hemorrhage, exudation, scarring, and/or serous retinal detachment." Crabb JW, et al. Drusen Proteome Analysis: An Approach to the Etiology of Age-Related Macular Degeneration. Proceedings of the National Academy of Sciences U S A. 2002; 99(23):14682-7.

31. Wet AMD is also directly related to the accumulation of oxidative damage and the production of ROS by drusen, which is most likely to occur in the presence of blue light.

"Normal aging is associated with accumulation of lipofuscin pigment in the retinal pigment epithelium (RPE). This may occur as a result of phagocytosis and incomplete degradation of oxidized photoreceptor outer segments (POS)... Ingestion by RPE of oxidized bovine POS stimulates expression of the chemotactic and angiogenic factors IL-8 and MCP-1 that have the capability to promote angiogenesis directly, or indirectly through the accumulation of immune cells such as macrophages, which themselves may release angiogenic promoters and degrade Bruch's membrane. This may be of significance in the development of exudative AMD." Induction of angiogenic cytokine expression in cultured RPE by ingestion of oxidized photoreceptor outer segments. Higgins GT, Wang JH, Dockery P, Cleary PE, Redmond HP. Invest Ophthalmol Vis Sci. 2003 Apr;44(4):1775-82

32. These ROS can cause lesions in the blood-retinal barrier formed by the RPE and the Bruch's membrane: small blood vessels can then invade the retinal space, producing the conditions for sudden blindness when these vessels leak or break, an effect known as exudative AMD or wet AMD.

"A cardinal pathological feature of age-related macular degeneration (AMD) is the deposition of extracellular material between the retinal pigment epithelium (RPE) and Bruch's membrane, pathologically described as sub-RPE deposits.
Our work also provides an insight into the mechanism of CNV (choroidal neovascularization) development. When RPE cells migrate from Bruch's membrane, the RPE-Bruch's membrane complex is dismantled. It seems that the bare Bruch's membrane devoid of RPE is no longer recognized as a barrier and it becomes vulnerable to CNV invasion. Our data therefore emphasize the importance of RPE as a barrier to CNV invasion."
"RPE cells are housekeepers for photoreceptors. They not only are essential for photoreceptor metabolism, but also participate in the RPE-Bruch's membrane complex to form the blood-retinal barrier... In summary, our present work demonstrates that the presence of a subretinal deposit induces RPE cell translocation, which in turn generates pathological features characteristic of AMD, including formation of the sub-RPE deposit and CNV. These findings indicate a subretinal source of sub-RPE deposits and a key role of RPE translocation in the formation of sub-RPE deposits. Our data also provide evidence that the presence of sub-RPE deposits is sufficient to induced CNV to penetrate Bruch's membrane" Translocation of the Retinal Pigment Epithelium and Formation of Sub-Retinal Pigment Epithelium Deposit Induced by Subretinal Deposit. Molecular Vision 2007; 13:873-80 Lian Zhao, Zhenfang Wang, Yun Liu, Ying Song, Yiwen Li, Alan M. Laties, Rong Wen
"Exudative AMD involves pathological angiogenesis that originates from the choroid beneath the retina to form a PNV, called a choroidal neovasculature (CNV), containing abnormal blood vessels that can leak fluid and bleed, hence the name exudative AMD. The fluid and blood released from the CNV can damage the structure and function of the overlying retina, usually in the central macular area, leading to the loss of central vision." Bora PS, Hu Z, Tezel H, Sohn J, Kang SG, Cruz JMC, Bora NS, Garen A, and Kaplan HJ. Immunotherapy for Choroidal Neovascularization in a Laser-Induced Mouse Model Stimulating Exudative (wet) macular degeneration. Proc Natl Acad Sci USA. 2003 March 4; 100 (5):2679-2684
"Lipofuscin results from an incomplete degradation of altered material trapped in lysosomes and the accumulation of lipofuscin is related to an increased risk of choroidal neovascularization (CNV) due to age-related macular degeneration (AMD)."
"RPE lipofuscin is a byproduct of the phagocytosis of lipid-rich photoreceptor outer segments and consists of a complex mixture of pigments. A major fluorophore is A2E... A2E affects normal RPE functions by causing membrane permeabilization inhibiting lysosomal function, inhibiting cytochrome c oxigenase, acting as a detergent inhibiting the ATP-driven proton pump and partly mediating light damage by acting as a photosensitizer, targeting DNA... This study demonstrates that A2E is an endogenous ligand for retinoic acid receptor (RAR). The data suggest that A2E accumulation results in the pro-angiogenic conversion of retinal pigment epithelial cell phenotype predisposing the environment to CNV development via RAR activation. The data in the present study support that the accumulation of A2E induces the enhancement of VEGF expression, suggesting a role of A2E in the progression of exudative AMD." A2E, a Pigment of the Lipofuscin of Retinal Pigment Epithelial Cells, Is an Endogenous Ligand for Retinoic Acid Receptor. J. Biol. Chem. May 2, 2008, 283(18): 11947-11953, Iriyama A et al.
"The neovascular AMD involves proliferation of abnormal choroidal vessels, which penetrate the Bruch's membrane and RPE layer into the subretinal space, thereby forming extensive clots and/or scars. Both environmental and genetic factors are suspected to play a role in AMD." Hamdi HK, Kenney C. Age-related macular degeneration: a new viewpoint. Front Biosci. 2003 May 1; 8:e305-14.

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