Recent studies have implicated local inflammation and activation of complement amongst the processes involved in the pathogenesis of age-related macular degeneration (AMD). Several lines of investigation also indicate that bis-retinoid pigments, such as A2E, that accumulate as lipofuscin in retinal pigment epithelial (RPE) cells, contribute to the disease process. In an investigation of a potential trigger for complement activation in AMD, we explored the notion that the complex mixture of products resulting from photooxidation of A2E might include a range of fragments that could be recognized by the complement system as ''foreign'' and that could serve to activate the complement system, leading to low-grade inflammation. To this end, we established an in vitro assay by using human serum as a source of complement, and we measured products of C3 activation by enzyme immunoassay. Accordingly, we found that the C3 split products inactivated C3b (iC3b) and C3a were elevated in serum, overlying ARPE-19 cells that had accumulated A2E and were irradiated to induce A2E photooxidation. Precoating of microtiter plates with two species of oxidized A2E, peroxy-A2E, and furano-A2E, followed by incubation with serum, also activated complement. We suggest that products of the photooxidation of bis-retinoid lipofuscin pigments in RPE cells could serve as a trigger for the complement system, a trigger than would predispose the macula to disease and that, over time, could contribute to chronic inflammation. These findings link four factors that have been posited as being associated with AMD: inflammation, oxidative damage, drusen, and RPE lipofuscin.inflammation ͉ macular degeneration

A substantial portion of the lipofuscin that accumulates with age and in some retinal disorders in retinal pigment epithelial (RPE) cells, forms as a consequence of light-related vitamin A recycling. Major constituents of RPE lipofuscin are the di-retinal conjugate A2E and its photoisomers. That the accretion of A2E has consequences for the cell, with the adverse effects of A2E being attributable to its amphiphilic structure and its photoreactivity, is consistent with evidence of an association between atrophic age-related macular degeneration (AMD) and excessive lipofuscin accumulation.

The bis-retinoid pigments that accumulate in retinal pigment epithelial cells as lipofuscin are associated with inherited and age-related retinal disease. In addition to A2E and related cis isomers, we previously showed that condensation of two molecules of all-trans-retinal leads to the formation of a protonated Schiff base conjugate, all-trans-retinal dimer-phosphatidylethanolamine. Here we report the characterization of the related pigments, all-trans-retinal dimer-ethanolamine and unconjugated all-transretinal dimer, in human and mouse retinal pigment epithelium. In eyecups of Abcr ؊/؊ mice, a model of recessive Stargardt macular degeneration, all-trans-retinal dimer-phosphatidylethanolamine was increased relative to wild type and was more abundant than A2E. Total pigment of the all-trans-retinal dimer series (sum of all-trans-retinal dimer-phosphatidylethanolamine, all-trans-retinal dimer-ethanolamine, and all-trans-retinal dimer) increased with age in Abcr ؊/؊ mice and was modulated by amino acid variants in Rpe65. In in vitro assays, enzyme-mediated hydrolysis of all-transretinal dimer-phosphatidylethanolamine generated all-transretinal dimer-ethanolamine, and protonation/deprotonation of the Schiff base nitrogen of all-trans-retinal dimer-ethanolamine was pH-dependent. Unconjugated all-trans-retinal dimer was a more efficient generator of singlet oxygen than A2E, and the all-trans-retinal dimer series was more reactive with singlet oxygen than was A2E. By analyzing chromatographic properties and UV-visible spectra together with mass spectrometry, mono-and bis-oxygenated all-trans-retinal dimer photoproducts were detected in Abcr ؊/؊ mice. The latter findings are significant to an understanding of the adverse effects of retinal pigment epithelial cell lipofuscin. macular degeneration ͉ retinal pigment epithelium ͉ bis-retinoid T he lipofuscin fluorophores that accumulate with age in retinal pigment epithelial (RPE) cells of the eye originate, for the most part, in photoreceptor cells and are deposited in the RPE when these cells phagocytose shed outer segment membrane (1). A number of observations over the years have also indicated that the deposition of lipofuscin fluorophores in RPE depends on vitamin A availability and on visual cycle function. For instance, RPE lipofuscin accumulation is reduced by dietary deficiency in vitamin A (1), by pharmacological agents that reduce serum vitamin A (2), and by genetic variants and small molecules that limit visual cycle kinetics (3, 4).The first of the vitamin A aldehyde derivatives to be identified in RPE lipofuscin was the bisretinoid A2E (1, 5, 6), followed by a C13-C14 Z-isomer of A2E (isoA2E) (6) and other minor isomers (7). A2E and its cis-isomers absorb at Ϸ440 nm and form in photoreceptor outer segments by a multistep biosynthetic pathway involving reactions between phosphatidylethanolamine (PE) and all-trans-retinal (atRAL) and the formation of the precursors dihydro-A2PE (8) and A2PE (9). We recently also identified an RPE pigment with Ϸ510-nm absorban...

The nondegradable pigments that accumulate in retinal pigment epithelial (RPE) cells as lipofuscin constituents are considered to be responsible for the loss of RPE cells in recessive Stargardt disease, a blindness macular disorder of juvenile onset. This autofluorescent material may also contribute to the etiology of age-related macular degeneration. The best characterized of these fluorophores is A2E, a compound consisting of two retinoid-derived side arms extending from a pyridinium ring. 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. By studying the oxidation products (oxo-A2E) generated using oxidizing agents that add one or two oxygens at a time, together with structural analysis by heteronuclear single quantum correlation-NMR spectroscopy, we demonstrated that the oxygen-containing moieties generated within photooxidized A2E include a 5,8-monofuranoid and a cyclic 5,8-monoperoxide. We have shown that the oxidation sites can be assigned to the shorter arm of A2E, to the longer arm, or to both arms by analyzing changes in the UV-visible spectrum of A2E, and we have observed a preference for oxidation on the shorter arm. By liquid chromatography-mass spectrometry, we have also detected both monofuran-A2E and monoperoxy-A2E in aged human RPE and in eye cups of Abca4/Abcr ؊/؊ mice, a model of Stargardt disease. Because the cytotoxicity of endoperoxide moieties is well known, the production of endoperoxide-containing oxo-A2E may account, at least in part, for cellular damage ensuing from A2E photooxidation.The bisretinoid fluorophores that accumulate in retinal pigment epithelial (RPE) 3 cells as lipofuscin constituents are considered to be responsible for the loss of RPE cells in recessive Stargardt disease (1-3), an early onset form of macular degeneration, and may also be involved in the etiology of age-related macular degeneration (4). The RPE lipofuscin fluorophores isolated thus far include A2E (5-7), iso-A2E (7), less abundant double bond isomers of A2E (8), and all-trans-retinal dimer conjugates (9, 10) (Fig. 1). The most intensely studied of the RPE lipofuscin constituents are A2E and related photoisomers, pigments that, when accumulated by RPE cells in culture, have been shown to bestow a sensitivity to light damage (11-13). Blue light produces the most pronounced effect (11). The augmentation of cell death under conditions that prolong the lifetime of singlet oxygen together with the protection provided by quenchers and scavengers of singlet oxygen has implicated singlet oxygen as having a role in the events leading to the death of the cells (14).The propensity for A2E to undergo photooxidation was initially revealed by a tendency for fluorescence quenching of intracellular A2E upon blue light illumination (13). HPLC analysis later confirmed this observation, the absorbance of the A2E peak after 430 nm irradiation,...