Similar molecules spatially correlate with lipofuscin and N-retinylidene-N-retinylethanolamine in the mouse but not in the human retinal pigment epithelium

The accumulation of lipofuscin in the retinal pigment epithelium (RPE) has been implicated in the development of age-related macular degeneration (AMD) in humans. The exact composition of lipofuscin is not known but its best characterized component is N-retinylidene-N-retinylethanolamine (A2E), a byproduct of the retinoid visual cycle. Utilizing our recently developed matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI–IMS)-based technique to determine the spatial distribution of A2E, this study compares the relationships of lipofuscin fluorescence and A2E in the murine and human RPE on representative normal tissue. To identify molecules with similar spatial patterns, the images of A2E and lipofuscin were correlated with all the individual images in the MALDI–IMS dataset. In the murine RPE, there was a remarkable correlation between A2E and lipofuscin. In the human RPE, however, minimal correlation was detected. These results were reflected in the marked distinctions between the molecules that spatially correlated with the images of lipofuscin and A2E in the human RPE. While the distribution of murine lipofuscin showed highest similarities with some of the known A2E-adducts, the composition of human lipofuscin was significantly different. These results indicate that A2E metabolism may be altered in the human compared to the murine RPE.     

Retinal pigment epithelium lipofuscin proteomics

Lipofuscin accumulates with age in the retinal pigment epithelium (RPE) in discrete granular organelles and may contribute to age-related macular degeneration. Because previous studies suggest that lipofuscin contains protein that may impact pathogenic mechanisms, we pursued proteomics analysis of lipofuscin. The composition of RPE lipofuscin and its mechanisms of pathogenesis are poorly understood in part because of the heterogeneity of isolated preparations. We purified RPE lipofuscin granules by treatment with proteinase K or SDS and showed by light, confocal, and transmission electron microscopy that the purified granules are free of extragranular material and associated membranes. Crude and purified lipofuscin preparations were quantitatively compared by (i) LC MS/MS proteomics analyses, (ii) immunoanalyses of oxidative protein modifications, (iii) amino acid analysis, (iv) HPLC of bisretinoids, and (v) assaying phototoxicity to RPE cells. From crude lipofuscin preparations 186 proteins were identified, many of which appeared to be modified. In contrast, very little protein ( approximately 2% (w/w) by amino acid analysis) and no identifiable protein were found in the purified granules, which retained full phototoxicity to cultured RPE cells. Our analyses showed that granules in purified and crude lipofuscin preparations exhibit no statistically significant differences in diameter or circularity or in the content of the bisretinoids A2E, isoA2E, and all-trans-retinal dimer-phosphatidylethanolamine. The finding that the purified granules contain minimal protein yet retain phototoxic activity suggests that RPE lipofuscin pathogenesis is largely independent of associated protein. The purified granules also exhibited oxidative protein modifications, including nitrotyrosine generated from reactive nitrogen oxide species and carboxyethylpyrrole and iso[4]levuglandin E(2) adducts generated from reactive lipid fragments. This finding is consistent with previous studies demonstrating RPE lipofuscin to be a potent generator of reactive oxygen species and supports the hypothesis that such species, including reactive fragments from lipids and retinoids, contribute to the mechanisms of RPE lipofuscin pathogenesis.     

Small molecule RPE65 antagonists limit the visual cycle and prevent lipofuscin formation

The accumulation of the lipofuscin fluorophores in retinal pigment epithelial (RPE) cells leads to the blinding degeneration characteristic of Stargardt disease and related forms of macular degeneration. RPE lipofuscin, including the fluorophore A2E, forms in large part as a byproduct of the visual cycle. Inhibiting visual cycle function with small molecules is required to prevent the formation of the retinotoxic lipofuscins. This in turn requires identification of rate-limiting steps in the operation of the visual cycle. Specific, non-retinoid isoprenoid compounds are described here, and shown through in both in vitro and in vivo experiments, to serve as antagonists of RPE65, a protein that is essential for the operation of the visual cycle. These RPE65 antagonists block regeneration of 11-cis-retinal, the chromophore of rhodopsin, thereby demonstrating that RPE65 is at least partly rate-limiting in the visual cycle. Furthermore, chronic treatment of a mouse model of Stargardt disease with the RPE65 antagonists abolishes the formation of A2E. Thus, RPE65 is also on the rate-limiting pathway to A2E formation. These nontoxic isoprenoid RPE65 antagonists are candidates for the treatment of forms of macular degeneration wherein lipofuscin accumulation is an important risk factor. These antagonists will also be used to probe the molecular function of RPE65 in vision.     

Action spectra for the photoconsumption of oxygen by human ocular lipofuscin and lipofuscin extracts

The action spectra for the photoconsumption of oxygen by lipofuscin isolated from human retinal pigment epithelium cells and liposomal suspensions containing extracts of lipofuscin are reported. The lipofuscin and lipofuscin extract action spectra are similar, demonstrating the phototoxic constituents of lipofuscin are present in the lipofuscin solvent extract. 2-[2,6-Dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-octatetraenyl]-1-(2-hydroxyethyl)-4-[4-methyl-6-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E-hexatrienyl]-pyridinium (A2E), present in both intact granules and the solvent extract, has been invoked as an important contributor to the phototoxicity of lipofuscin. The action spectrum for oxygen photoconsumption by A2E follows its absorption spectrum but does not resemble the action spectrum for photoconsumption of oxygen by lipofuscin granules or lipofuscin extract. These results combined with recently reported experimental studies on the aerobic photoreactivity of A2E indicate that it is not a major contributor to the phototoxicity of lipofuscin.     

A2E,a component of lipofuscin,is pro‐angiogenic in vivo

A recent study in vitro demonstrated that a major lipofuscin component, A2E, serves as a retinoic acid receptor ligand. The current study investigated the effects of A2E on retinal pigment epithelial (RPE) cells in vivo and was performed to extend the understanding of the effects of A2E. Firstly, subretinal injection of A2E was performed and 3 weeks after the injection, and it was demonstrated that subretinal injection of A2E induced RPE cell death, and concomitant upregulation of vascular endothelial growth factor (VEGF) in the RPE and choroid. The upregulation of VEGF was attenuated by an RARα antagonist. Next we performed laser photocoagulation in mice that accumulated A2E either after subretinal injection, by Ccl2 gene knockout or by aging demonstrated that mice that accumulated A2E in the RPE, which showed higher rates of choroidal neobascularization (CNV) formation after weak laser injury than the controls and the formation of CNV was inhibited by an RARα antagonist in all models tested. The data suggest that A2E accumulation induces RPE cell death, and concomitant increase of VEGF. Accumulation of A2E alone is not sufficient to induce CNV in vivo, but induces the expression of VEGF in RPE and choroid. The mice that accumulated A2E in RPE cells are vulnerable to CNV development via RAR activation, at least in part. J. Cell. Physiol. 220: 469–475, 2009. © 2009 Wiley‐Liss, Inc.     

Lipofuscin and N-retinylidene-N-retinylethanolamine (A2E) accumulate in retinal pigment epithelium in absence of light exposure: their origin is 11-cis-retinal

The age-dependent accumulation of lipofuscin in the retinal pigment epithelium (RPE) has been associated with the development of retinal diseases, particularly age-related macular degeneration and Stargardt disease. A major component of lipofuscin is the bis-retinoid N-retinylidene-N-retinylethanolamine (A2E). The current model for the formation of A2E requires photoactivation of rhodopsin and subsequent release of all-trans-retinal. To understand the role of light exposure in the accumulation of lipofuscin and A2E, we analyzed RPEs and isolated rod photoreceptors from mice of different ages and strains, reared either in darkness or cyclic light. Lipofuscin levels were determined by fluorescence imaging, whereas A2E levels were quantified by HPLC and UV-visible absorption spectroscopy. The identity of A2E was confirmed by tandem mass spectrometry. Lipofuscin and A2E levels in the RPE increased with age and more so in the Stargardt model Abca4(-/-) than in the wild type strains 129/sv and C57Bl/6. For each strain, the levels of lipofuscin precursor fluorophores in dark-adapted rods and the levels and rates of increase of RPE lipofuscin and A2E were not different between dark-reared and cyclic light-reared animals. Both 11-cis- and all-trans-retinal generated lipofuscin-like fluorophores when added to metabolically compromised rod outer segments; however, it was only 11-cis-retinal that generated such fluorophores when added to metabolically intact rods. The results suggest that lipofuscin originates from the free 11-cis-retinal that is continuously supplied to the rod for rhodopsin regeneration and outer segment renewal. The physiological role of Abca4 may include the translocation of 11-cis-retinal complexes across the disk membrane.     

Age-dependent accumulation of lipofuscin in perivascular and subretinal microglia in experimental mice

Fundus autofluorescence (AF) imaging by confocal scanning laser ophthalmoscopy has been widely used by ophthalmologists in the diagnosis/monitoring of various retinal disorders. It is believed that fundus AF is derived from lipofuscin in retinal pigment epithelial (RPE) cells; however, direct clinicopathological correlation has not been possible in humans. We examined fundus AF by confocal scanning laser ophthalmoscopy and confocal microscopy in normal C57BL/6 mice of different ages. Increasingly strong AF signals were observed with age in the neuroretina and subretinal/RPE layer by confocal scanning laser ophthalmoscopy. Unlike fundus AF detected in normal human subjects, mouse fundus AF appeared as discrete foci distributed throughout the retina. Most of the AF signals in the neuroretina were distributed around retinal vessels. Confocal microscopy of retinal and choroid/RPE flat mounts demonstrated that most of the AF signals were derived from Iba-1+ perivascular and subretinal microglia. An age-dependent accumulation of Iba-1+ microglia at the subretinal space was observed. Lipofuscin granules were detected in large numbers in subretinal microglia by electron microscopy. The number of AF+ microglia and the amount of AF granules/cell increased with age. AF granules/lipofuscin were also observed in RPE cells in mice older than 12 months, but the number of AF+ RPE cells was very low (1.48 mm(-2) and 5.02 mm(-2) for 12 and 24 months, respectively) compared to the number of AF+ microglial cells (20.63 mm(-2) and 76.36 mm(-2) for 6 and 24 months, respectively). The fluorescence emission fingerprints of AF granules in subretinal microglia were the same as those in RPE cells. Our observation suggests that perivascular and subretinal microglia are the main cells producing lipofuscin in normal aged mouse retina and are responsible for in vivo fundus AF. Microglia may play an important role in retinal aging and age-related retinal diseases.     

Characterization of peroxy-A2E and furan-A2E photooxidation products and detection in human and mouse retinal pigment epithelial cell lipofuscin

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.     

C20-D3-vitamin A slows lipofuscin accumulation and electrophysiological retinal degeneration in a mouse model of Stargardt disease

Stargardt disease, also known as juvenile macular degeneration, occurs in approximately one in 10,000 people and results from genetic defects in the ABCA4 gene. The disease is characterized by premature accumulation of lipofuscin in the retinal pigment epithelium (RPE) of the eye and by vision loss. No cure or treatment is available. Although lipofuscin is considered a hallmark of Stargardt disease, its mechanism of formation and its role in disease pathogenesis are poorly understood. In this work we investigated the effects of long-term administration of deuterium-enriched vitamin A, C20-D(3)-vitamin A, on RPE lipofuscin deposition and eye function in a mouse model of Stargardt's disease. Results support the notion that lipofuscin forms partly as a result of the aberrant reactivity of vitamin A through the formation of vitamin A dimers, provide evidence that preventing vitamin A dimerization may slow disease related, retinal physiological changes and perhaps vision loss and suggest that administration of C20-D(3)-vitamin A may be a potential clinical strategy to ameliorate clinical symptoms resulting from ABCA4 genetic defects.     

Proteome analysis of lipofuscin in human retinal pigment epithelial cells

Excessive accumulation of lipofuscin in postmitotic retinal pigment epithelial cells is a common pathogenetic pathway in various blinding retinal diseases including age-related macular degeneration, which is now the most common cause of registerable blindness in the industrialized nations. To better understand the role of lipofuscin accumulation and to manipulate the pathogenetic mechanisms on both experimental and therapeutic levels we analyzed the proteome of isolated human ocular lipofuscin granules from human RPE cells. After homogenization and fractionation by gradient ultracentrifugation of the RPE/choroid complex from 10 pairs of human donors, protein compounds were separated by 2D gel electrophoresis and analyzed using matrix-assisted laser desorption/ionization mass spectrometry and HPLC-coupled electrospray tandem mass spectrometry. Besides a better understanding of downstream pathways, this approach may provide new targets for therapeutic interventions in a currently untreatable disease.     

不同浓度A2E对离体猪视网膜色素上皮细胞的影响

目的评价体外合成的A2E对猪视网膜色素上皮（RPE）细胞的细胞活力和生物学特性影响,为进一步研究A2E在RPE细胞相关疾病中的作用提供细胞模型。方法利用全反式视黄醛和乙醇胺体外合成脂褐质荧光基团A2E。不同浓度的A2E（0,50,75,100μmol／L）作用第3代体外培养的猪RPE细胞30,45,60,90min,换10％FBS DMEM-F12培养液孵育24h后,倒置荧光显微镜观察荧光强度,IPP6．0软件灰度扫描定量荧光强度。采用MTT法检测A2E作用细胞各个时间段的吸光度值,应用SPSS11．0软件包对数据进行统计学分析,评价A2E的细胞毒性及RPE细胞活性。结果A2E被RPE细胞摄取后主要分布于细胞核周围,具有自发荧光。MTT实验及荧光灰度扫描结果显示,不同浓度的A2E被细胞摄取后细胞活力和荧光灰度扫描结果不同,以50μmol／L浓度A2E作用RPE细胞60min时,细胞内荧光强度高同时细胞活力强。结论体外培养的猪RPE细胞摄取体外合成的50μmol／L A2E 60min后细胞对A2E的摄取较多,A2E对细胞的毒性相对较低,该条件下进行A2E对离体猪RPE细胞的研究较好。     

Zinc deficiency leads to lipofuscin accumulation in the retinal pigment epithelium of pigmented rats

Background

Age-related macular degeneration (AMD) is associated with lipofuscin accumulation whereas the content of melanosomes decreases. Melanosomes are the main storage of zinc in the pigmented tissues. Since the elderly population, as the most affected group for AMD, is prone to zinc deficit, we investigated the chemical and ultrastructural effects of zinc deficiency in pigmented rat eyes after a six-month zinc penury diet.

Methodology/Principal Findings

Adult Long Evans (LE) rats were investigated. The control animals were fed with a normal alimentation whereas the zinc-deficiency rats (ZD-LE) were fed with a zinc deficient diet for six months. Quantitative Energy Dispersive X-ray (EDX) microanalysis yielded the zinc mole fractions of melanosomes in the retinal pigment epithelium (RPE). The lateral resolution of the analysis was 100 nm. The zinc mole fractions of melanosomes were significantly smaller in the RPE of ZD-LE rats as compared to the LE control rats. Light, fluorescence and electron microscopy, as well as immunohistochemistry were performed. The numbers of lipofuscin granules in the RPE and of infiltrated cells (Ø>3 µm) found in the choroid were quantified. The number of lipofuscin granules significantly increased in ZD-LE as compared to control rats. Infiltrated cells bigger than 3 µm were only detected in the choroid of ZD-LE animals. Moreover, the thickness of the Bruch''s membrane of ZD-LE rats varied between 0.4–3 µm and thin, rangy ED1 positive macrophages were found attached at these sites of Bruch''s membrane or even inside it.

Conclusions/Significance

In pigmented rats, zinc deficiency yielded an accumulation of lipofuscin in the RPE and of large pigmented macrophages in the choroids as well as the appearance of thin, rangy macrophages at Bruch''s membrane. Moreover, we showed that a zinc diet reduced the zinc mole fraction of melanosomes in the RPE and modulated the thickness of the Bruch''s membrane.     

Photocytotoxicity of lipofuscin in human retinal pigment epithelial cells.

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

The 5HT1a receptor agonist 8-Oh DPAT induces protection from lipofuscin accumulation and oxidative stress in the retinal pigment epithelium

Age-related macular degeneration (AMD), a major cause of blindness in the elderly, is associated with oxidative stress, lipofuscin accumulation and retinal degeneration. The aim of this study was to determine if a 5-HT(1A) receptor agonist can reduce lipofuscin accumulation, reduce oxidative damage and prevent retinal cell loss both in vitro and in vivo. Autophagy-derived and photoreceptor outer segment (POS)-derived lipofuscin formation was assessed using FACS analysis and confocal microscopy in cultured retinal pigment epithelial (RPE) cells in the presence or absence of the 5-HT(1A) receptor agonist, 8-OH DPAT. 8-OH DPAT treatment resulted in a dose-dependent reduction in both autophagy- and POS-derived lipofuscin compared to control. Reduction in autophagy-induced lipofuscin was sustained for 4 weeks following removal of the drug. The ability of 8-OH DPAT to reduce oxidative damage following exposure to 200 μM H(2)O(2) was assessed. 8-OH DPAT reduced superoxide generation and increased mitochondrial superoxide dismutase (MnSOD) levels and the ratio of reduced glutathione to the oxidized form of glutathione in H(2)O(2)-treated cells compared to controls and protected against H(2)O(2)-initiated lipid peroxidation, nitrotyrosine levels and mitochondrial damage. SOD2 knockdown mice, which have an AMD-like phenotype, received daily subcutaneous injections of either saline, 0.5 or 5.0 mg/kg 8-OH DPAT and were evaluated at monthly intervals. Systemic administration of 8-OH DPAT improved the electroretinogram response in SOD2 knockdown eyes of mice compared to knockdown eyes receiving vehicle control. There was a significant increase in the ONL thickness in mice treated with 8-OH DPAT at 4 months past the time of MnSOD knockdown compared to untreated controls together with a 60% reduction in RPE lipofuscin. The data indicate that 5-HT(1A) agonists can reduce lipofuscin accumulation and protect the retina from oxidative damage and mitochondrial dysfunction. 5-HT(1A) receptor agonists may have potential as therapeutic agents in the treatment of retinal degenerative disease.     

Protective effect of autophagy on human retinal pigment epithelial cells against lipofuscin fluorophore A2E: implications for age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of central vision loss in the elderly. Degeneration of retinal pigment epithelial (RPE) cells is a crucial causative factor responsible for the onset and progression of AMD. A2E, a major component of toxic lipofuscin implicated in AMD, is deposited in RPE cells with age. However, the mechanism whereby A2E may contribute to the pathogenesis of AMD remains unclear. We demonstrated that A2E was a danger signal of RPE cells, which induced autophagy and decreased cell viability in a concentration- and time-dependent manner. Within 15 min after the treatment of RPE with 25 μM A2E, the induction of autophagosome was detected by transmission electron microscopy. After continuous incubating RPE cells with A2E, intense punctate staining of LC3 and increased expression of LC3-II and Beclin-1 were identified. Meanwhile, the levels of intercellular adhesion molecule (ICAM), interleukin (IL)1β, IL2, IL-6, IL-8, IL-17A, IL-22, macrophage cationic peptide (MCP)-1, stromal cell-derived factor (SDF)-1, and vascular endothelial growth factor A (VEGFA) were elevated. The autophagic inhibitor 3-methyladenine (3-MA) and activator rapamycin were also used to verify the effect of autophagy on RPE cells against A2E. Our results revealed that 3-MA decreased the autophagosomes and LC3 puncta induced by A2E, increased inflammation-associated protein expression including ICAM, IL1β, IL2, IL-6, IL-8, IL-17A, IL-22, and SDF-1, and upregulated VEGFA expression. Whereas rapamycin augmented the A2E-mediated autophagy, attenuated protein expression of inflammation-associated and angiogenic factors, and blocked the Akt/mTOR pathway. Taken together, A2E induces autophagy in RPE cells at the early stage of incubation, and this autophagic response can be inhibited by 3-MA or augmented by rapamycin via the mTOR pathway. The enhancement of autophagy has a protective role in RPE cells against the adverse effects of A2E by reducing the secretion of inflammatory cytokines and VEGFA.Age-related macular degeneration (AMD) is the leading cause of irreversible blindness among elderly people and is becoming a major public health issue.^{1, 2, 3} The pathological change in AMD is located in the macula, which is the central and posterior portion of the retina containing the retinal pigment epithelium (RPE) and photoreceptors. Central visual impairment caused by AMD results from the loss or damage of RPE cells and the photoreceptors.⁴ Currently, the etiology and pathogenesis of AMD is not fully understood and there is no effective treatment.^{5, 6} A chronic aberrant inflammatory response in RPE cells is considered to be one of the major factors contributing to the pathogenesis of AMD.^{7, 8}Lipofuscin is a complex aggregate of fluorescent material, formed in a variety of tissues but best studied in the eye.⁹ The buildup of lipofuscin in RPE cells has been identified as a byproduct of the visual cycle, and is derived from the ingestion of photoreceptor outer segments, which has been implicated in several retinal degenerations, including AMD.^{10, 11} As revealed by spectroscopic analyses, the bis-retinoid N-retinyl-N-retinylidene ethanolamine (A2E) is the first isolated, major fluorophore from RPE lipofuscin. Numerous in vitro and in vivo studies have found that toxicity effects associated with this compound, and A2E is involved in the pathological pathways of AMD, especially the inflammatory response.^{12, 13} Although several studies have suggested that A2E may induce cytokine production, activate inflammasomes or the complement system in RPE cells, and contribute to chronic inflammation in AMD,^{14, 15, 16} the exact mechanisms by which A2E exerts an effect on RPE cells remains unclear.Autophagy is an evolutionarily conserved cellular housekeeping process that removes damaged organelles and protein aggregates that are unnecessary or dysfunctional to the cells by delivering cytoplasmic substrates to lysosomes for degeneration.¹⁷ In addition to turnover of cellular components, autophagy is involved in development, differentiation, and tissue remodeling in various organisms.¹⁸ The failure of autophagy in aged postmitotic cells, including RPE cells, can result in the accumulation of aggregation-prone proteins, cellular degeneration, and finally the induction of cell death.^{19, 20} Currently, a large amount of evidence indicates that autophagy is associated with RPE damage and AMD pathology.^{21, 22, 23} In RPE cells, the preservation of autophagic activity, together with functional lysosomal enzymes, is a prerequisite to prevent detrimental intracellular accumulation of damaged molecules.²¹ A well-functioning proteolytic machine guarantees that there is sufficient capacity to handle damaged proteins and organelles.²⁴ In addition, Saadat KA et al.²⁵ have shown that RPE cell death is induced in the presence of A2E and the autophagic inhibitor 3-methyladenine (3-MA). Nevertheless, whether the autophagic pathway has effects on A2E-induced cell damage through the production of chemokines and cytokines remains unclear. Furthermore, the relationship between A2E and autophagy and how this interaction influences RPE cells'' inflammatory response requires further clarification.Therefore, the protective effect of autophagy on human RPE cells against lipofuscin fluorophore A2E-induced cell death and the inflammatory response were studied in the present article. This work facilitates our understanding of the role of autophagy in the survival and death of RPE cells accumulating excess lipofuscin and provides a new strategy in the treatment of AMD.     

The age lipid A2E and mitochondrial dysfunction synergistically impair phagocytosis by retinal pigment epithelial cells

Accumulation of indigestible lipofuscin and decreased mitochondrial energy production are characteristic age-related changes of post-mitotic retinal pigment epithelial (RPE) cells in the human eye. To test whether these two forms of age-related impairment have interdependent effects, we quantified the ATP-dependent phagocytic function of RPE cells loaded or not with the lipofuscin component A2E and inhibiting or not mitochondrial ATP synthesis either pharmacologically or genetically. We found that physiological levels of lysosomal A2E reduced mitochondrial membrane potential and inhibited oxidative phosphorylation (OXPHOS) of RPE cells. Furthermore, in media with physiological concentrations of glucose or pyruvate, A2E significantly inhibited phagocytosis. Antioxidants reversed these effects of A2E, suggesting that A2E damage is mediated by oxidative processes. Because mitochondrial mutations accumulate with aging, we generated novel genetic cellular models of RPE carrying mitochondrial DNA point mutations causing either moderate or severe mitochondrial dysfunction. Exploring these mutant RPE cells we found that, by itself, only the severe but not the moderate OXPHOS defect reduces phagocytosis. However, sub-toxic levels of lysosomal A2E are sufficient to reduce phagocytic activity of RPE with moderate OXPHOS defect and cause cell death of RPE with severe OXPHOS defect. Taken together, RPE cells rely on OXPHOS for phagocytosis when the carbon energy source is limited. Our results demonstrate that A2E accumulation exacerbates the effects of moderate mitochondrial dysfunction. They suggest that synergy of sub-toxic lysosomal and mitochondrial changes in RPE cells with age may cause RPE dysfunction that is known to contribute to human retinal diseases like age-related macular degeneration.     

Isolation of melanin from human plasma lipofuscin

Human plasma lipofuscin and its melanin component were isolated and quantified. Electron paramagnetic resonance, infrared, ultraviolet and visible spectra of this melanin exhibited absorption characteristics very similar to those of known melanins. The human plasma lipofuscin contained approximately 85% protein, 3% melanin, 0.4% lipid and 0.25% mucoprotein constituents and emitted yellow-green fluorescence in 366-nm light. The ethanol-ether lipid extract obtained after acid hydrolysis from the lipid-melanin fraction of this lipofuscin was also found to fluoresce in yellow-green color in 366-nm light and produced similar fluorescence excitation and emission spectra as those of the human plasma lipofuscin in water solution. The isolated melanin component was not fluorescent.     

Complement system dysregulation and inflammation in the retinal pigment epithelium of a mouse model for Stargardt macular degeneration

Accumulation of vitamin A-derived lipofuscin fluorophores in the retinal pigment epithelium (RPE) is a pathologic feature of recessive Stargardt macular dystrophy, a blinding disease caused by dysfunction or loss of the ABCA4 transporter in rods and cones. Age-related macular degeneration, a prevalent blinding disease of the elderly, is strongly associated with mutations in the genes for complement regulatory proteins (CRP), causing chronic inflammation of the RPE. Here we explore the possible relationship between lipofuscin accumulation and complement activation in vivo. Using the abca4(-/-) mouse model for recessive Stargardt, we investigated the role of lipofuscin fluorophores (A2E-lipofuscin) on oxidative stress and complement activation. We observed higher expression of oxidative-stress genes and elevated products of lipid peroxidation in eyes from abca4(-/-) versus wild-type mice. We also observed higher levels of complement-activation products in abca4(-/-) RPE cells. Unexpectedly, expression of multiple CRPs, which protect cells from attack by the complement system, were lower in abca4(-/-) versus wild-type RPE. To test whether acute exposure of healthy RPE cells to A2E-lipofuscin affects oxidative stress and expression of CRPs, we fed cultured fetal-derived human RPE cells with rod outer segments from wild-type or abca4(-/-) retinas. In contrast to RPE cells in abca4(-/-) mice, human RPE cells exposed to abca4(-/-) rod outer segments adaptively increased expression of both oxidative-stress and CRP genes. These results suggest that A2E accumulation causes oxidative stress, complement activation, and down-regulation of protective CRP in the Stargardt mouse model. Thus, Stargardt disease and age-related macular degeneration may both be caused by chronic inflammation of the RPE.     

Soluble lipofuscin in commercially-available human serum albumin solutions

Two studied commercial human serum albumin solutions had developed yellow colors during storage. These yellow materials were isolated and shown to be soluble lipofuscin. Aqueous solutions of this lipofuscin exhibited fluorescence spectra with 355 nm excitation and 432 nm emission maxima. After acid hydrolysis of this lipofuscin a nonhydrolysable lipid-melanin fraction was obtained. Ethanol-ether extraction yielded a lipid-containing solution. When evaporated and mixed with water, a solution-suspension was obtained that produced very similar fluorescence spectra to those described above, with 368 nm excitation and 432 nm emission maxima. The separated melanin component was not fluorescent. The isolated lipofuscin exhibited a weak electron paramagnetic resonance spectrum and its g-value has been found to be 2.0069 and its line width 9.8 G. The albumin solution contained approximately 0.23 g of melanin precipitate per 9.31 g of soluble lipofuscin isolated from 25 g of albumin. The deleterious cardiac, pulmonary, renal and clotting changes associated with the use of albumin solution might be due to this lipofuscin.     

The autofluorescent products of lipid peroxidation may not be lipofuscin-like

The fluorescent molecules of cellular age pigment granules (lipofuscin) are commonly thought to be end products of membrane lipid autoxidation. Lipofuscin fluorophores of the retinal pigment epithelium (RPE) appear to be derived from photoreceptor outer segment membranes. Experiments were therefore conducted to determine whether the in vitro oxidation of retinal homogenates would generate fluorophores similar to the naturally occurring lipofuscin fluorophores of the RPE. Neural retina and RPE-choroid homogenates from young (2-3 month old) albino rats were subjected to an iron-ascorbate-air pro-oxidant reaction medium, and compared to unoxidized control samples from young age-matched animals as well as senescent (24 month old) rats. In addition, neural retina and RPE-choroid homogenates from 3 month old albino rats were subjected to a 100% oxygen atmosphere to test whether the fluorescent products of autoxidation differ substantially from those generated in the pro-oxidant medium. The chloroform-soluble fluorophores of chloroform-methanol sample extracts were analyzed by corrected fluorescence spectroscopy and thin-layer chromatography (TLC). In vitro pro-oxidation of both the neural retina and the RPE from young rats produced blue-emitting fluorophores which differed from the orange- and yellow-emitting fluorophores extracted from the RPE of senescent rats. Corrected fluorescence spectroscopy of aged tissue extracts revealed vitamin A-related fluorescence (330 nm excitation maximum; 515 nm emission maximum) and a spectrally resolvable age-related fluorescence (420 nm excitation maximum; 600 nm emission maximum). Only the vitamin A-related fluorescence could be measured in the control of young samples.(ABSTRACT TRUNCATED AT 250 WORDS)