Mitochondrial proteomics of the retinal pigment epithelium at progressive stages of age-related macular degeneration

DJ-1-dependent regulation of oxidative stress in the retinal pigment epithelium (RPE)

BACKGROUND

DJ-1 is found in many tissues, including the brain, where it has been extensively studied due to its association with Parkinson's disease. DJ-1 functions as a redox-sensitive molecular chaperone and transcription regulator that robustly protects cells from oxidative stress.

METHODOLOGY

Retinal pigment epithelial (RPE) cultures were treated with H2O2 for various times followed by biochemical and immunohistological analysis. Cells were transfected with adenoviruses carrying the full-length human DJ-1 cDNA and a mutant construct, which has the cysteine residues at amino acid 46, 53 and 106 mutated to serine (C to S) prior to stress experiments. DJ-1 localization, levels of expression and reactive oxygen species (ROS) generation were also analyzed in cells expressing exogenous DJ-1 under baseline and oxidative stress conditions. The presence of DJ-1 and oxidized DJ-1 was evaluated in human RPE total lysates. The distribution of DJ-1 was assessed in AMD and non-AMD cryosectionss and in isolated human Bruch's membrane (BM)/choroid from AMD eyes.

PRINCIPAL FINDINGS

DJ-1 in RPE cells under baseline conditions, displays a diffuse cytoplasmic and nuclear staining. After oxidative challenge, more DJ-1 was associated with mitochondria. Increasing concentrations of H2O2 resulted in a dose-dependent increase in DJ-1. Overexpression of DJ-1 but not the C to S mutant prior to exposure to oxidative stress led to significant decrease in the generation of ROS. DJ-1 and oxDJ-1 intensity of immunoreactivity was significantly higher in the RPE lysates from AMD eyes. More DJ-1 was localized to RPE cells from AMD donors with geographic atrophy and DJ-1 was also present in isolated human BM/choroid from AMD eyes.

CONCLUSIONS/SIGNIFICANCE

DJ-1 regulates RPE responses to oxidative stress. Most importantly, increased DJ-1 expression prior to oxidative stress leads to decreased generation of ROS, which will be relevant for future studies of AMD since oxidative stress is a known factor affecting this disease.

The cell stress machinery and retinal degeneration

Retinal degenerations are a group of clinically and genetically heterogeneous disorders characterised by progressive loss of vision due to neurodegeneration. The retina is a highly specialised tissue with a unique architecture and maintaining homeostasis in all the different retinal cell types is crucial for healthy vision. The retina can be exposed to a variety of environmental insults and stress, including light-induced damage, oxidative stress and inherited mutations that can lead to protein misfolding. Within retinal cells there are different mechanisms to cope with disturbances in proteostasis, such as the heat shock response, the unfolded protein response and autophagy. In this review, we discuss the multiple responses of the retina to different types of stress involved in retinal degenerations, such as retinitis pigmentosa, age-related macular degeneration and glaucoma. Understanding the mechanisms that maintain and re-establish proteostasis in the retina is important for developing new therapeutic approaches to fight blindness.

Autophagy and heterophagy dysregulation leads to retinal pigment epithelium dysfunction and development of age-related macular degeneration

Age-related macular degeneration (AMD) is a complex, degenerative and progressive eye disease that usually does not lead to complete blindness, but can result in severe loss of central vision. Risk factors for AMD include age, genetics, diet, smoking, oxidative stress and many cardiovascular-associated risk factors. Autophagy is a cellular housekeeping process that removes damaged organelles and protein aggregates, whereas heterophagy, in the case of the retinal pigment epithelium (RPE), is the phagocytosis of exogenous photoreceptor outer segments. Numerous studies have demonstrated that both autophagy and heterophagy are highly active in the RPE. To date, there is increasing evidence that constant oxidative stress impairs autophagy and heterophagy, as well as increases protein aggregation and causes inflammasome activation leading to the pathological phenotype of AMD. This review ties together these crucial pathological topics and reflects upon autophagy as a potential therapeutic target in AMD.

Mitochondrial DNA haplogroups confer differences in risk for age-related macular degeneration: a case control study

BACKGROUND

Age-related macular degeneration (AMD) is the leading cause of vision loss in elderly, Caucasian populations. There is strong evidence that mitochondrial dysfunction and oxidative stress play a role in the cell death found in AMD retinas. The purpose of this study was to examine the association of the Caucasian mitochondrial JTU haplogroup cluster with AMD. We also assessed for gender bias and additive risk with known high risk nuclear gene SNPs, ARMS2/LOC387715 (G > T; Ala69Ser, rs10490924) and CFH (T > C; Try402His, rs1061170).

METHODS

Total DNA was isolated from 162 AMD subjects and 164 age-matched control subjects located in Los Angeles, California, USA. Polymerase chain reaction (PCR) and restriction enzyme digestion were used to identify the J, U, T, and H mitochondrial haplogroups and the ARMS2-rs10490924 and CFH-rs1061170 SNPs. PCR amplified products were sequenced to verify the nucleotide substitutions for the haplogroups and ARMS2 gene.

RESULTS

The JTU haplogroup cluster occurred in 34% (55/162) of AMD subjects versus 15% (24/164) of normal (OR = 2.99; p = 0.0001). This association was slightly greater in males (OR = 3.98, p = 0.005) than the female population (OR = 3.02, p = 0.001). Assuming a dominant effect, the risk alleles for the ARMS2 (rs10490924; p = 0.00001) and CFH (rs1061170; p = 0.027) SNPs were significantly associated with total AMD populations. We found there was no additive risk for the ARMS2 (rs10490924) or CFH (rs1061170) SNPs on the JTU haplogroup background.

CONCLUSIONS

There is a strong association of the JTU haplogroup cluster with AMD. In our Southern California population, the ARMS2 (rs10490924) and CFH (rs1061170) genes were significantly but independently associated with AMD. SNPs defining the JTU mitochondrial haplogroup cluster may change the retinal bioenergetics and play a significant role in the pathogenesis of AMD.

Small-molecule inducers of Aβ-42 peptide production share a common mechanism of action

The pathways leading specifically to the toxic Aβ42 peptide production, a key event in Alzheimer's disease (AD), are unknown. While searching for pathways that mediate pathological increases of Aβ42, we identified Aftin-4, a new compound that selectively and potently increases Aβ42 compared to DMSO (N2a cells: 7-fold; primary neurons: 4-fold; brain lysates: 2-fold) with an EC(50) of 30 μM. These results were confirmed by ELISA and IP-WB. Using affinity chromatography and mass spectrometry, we identified 3 proteins (VDAC1, prohibitin, and mitofilin) relevant to AD that interact with Aftin-4, but not with a structurally similar but inactive molecule. Electron microscopy studies demonstrated that Aftin-4 induces a reversible mitochondrial phenotype reminiscent of the one observed in AD brains. Sucrose gradient fractionation showed that Aftin-4 perturbs the subcellular localization of γ-secretase components and could, therefore, modify γ-secretase specificity by locally altering its membrane environment. Remarkably, Aftin-4 shares all these properties with two other "AD accelerator" compounds. In summary, treatment with three Aβ42 raising agents induced similar biochemical alterations that lead to comparable cellular phenotypes in vitro, suggesting a common mechanism of action involving three structural cellular targets.

Genetics in Age-Related Macular Degeneration: Current Research and Implications for Future Treatment

Age-related macular degeneration (AMD) remains a leading cause of visual loss in the developed world, and genetic factors are known risks for disease development, as well as affecting response to therapy. Here, we review genetic factors associated with AMD and the influence of genetics on the understanding of AMD pathogenesis. The potential role of genetics in assisting in the management of AMD and future research avenues are also discussed.

Clinical and differential utility of VEGF inhibitors in wet age-related macular degeneration: focus on aflibercept

Age-related macular degeneration (AMD) has become a major public health problem and a leading cause of blindness in industrialized nations. AMD results from the ageing eye's inability to metabolize and dispose completely of photoreceptor outer segments and other waste products. As a result, lipids, particularly apolipoproteins, accumulate within Bruch's membrane, leading to chronic ischemia and inflammation. The subsequent upregulation of inflammatory cytokines and growth factors, including vascular endothelial growth factor (VEGF), induces the growth of neovascular membranes from the choriocapillaris into the subretinal or subretinal pigment epithelium spaces. To counter this, intravitreally administered drugs (pegaptanib, bevacizumab, ranibizumab) that specifically target VEGF have become the standard treatment for exudative AMD. Aflibercept, a recently approved fusion protein, binds to all isoforms of both VEGF-A and placental growth factor with high affinity. Phase III trials showed that monthly or every other month injections of aflibercept prevent vision loss (fewer than 15 letters) in 95% of patients. Additionally, aflibercept injections every 4 or 8 weeks produce average vision gains of 6.9 letters to 10.9 letters, comparable with those achieved with monthly ranibizumab. After one year of regularly administered aflibercept injections, patients required an average of only 4.2 injections during the second year. Aflibercept promises to decrease the injection frequency required for many patients and appears to serve as an effective "salvage" therapy for patients who respond poorly to other anti-VEGF drugs.

Noninvasive imaging of mitochondrial dysfunction in dry age-related macular degeneration

BACKGROUND AND OBJECTIVE

Oxidative stress and mitochondrial dysfunction are implicated in the pathogenesis of age-related macular degeneration (AMD). Because increased flavoprotein fluorescence (FPF) is indicative of mitochondrial dysfunction, the authors attempted to detect mitochondrial dysfunction in eyes with AMD using FPF.

PATIENTS AND METHODS

Six nonexudative eyes with AMD, including three with geographic atrophy (GA), and age-matched control eyes were imaged with a FPF device. Qualitative and quantitative analyses were conducted on the FPF images.

RESULTS

Five eyes with AMD, including all three eyes with GA, showed qualitative and/or quantitative FPF heterogeneity that was not present in control eyes. Mean FPF average intensities of eyes with AMD with (P = .044) and without (P = .00060) GA were significantly greater than those of control eyes. The standard deviations of FPF images were greater in eyes with AMD (P = .020).

CONCLUSION

In this small cluster of patients with AMD, retinal FPF is increased, suggesting elevated mitochondrial dysfunction. FPF heterogeneity indicates that an increased variability in mitochondrial dysfunction seems to be present in eyes with advanced disease.

Mitochondrial proteomic analysis of isopsoralen protection against oxidative damage in human lens epithelial cells

OBJECTIVE

To investigate the protective effects of the natural medicinal monomer isopsoralen (ISR) with estrogenic activity against oxidative damage in human lens epithelial cells B3 (HLE-B3) caused by hydrogen peroxide (H(2)O(2)) and to pursue the possible mitochondrial proteomic regularity of the protective effects.

METHODS

HLE-B3 cells were treated with H(2)O(2) (300 μ mol/L), β-estradiol (E(2): 10(-8) mol/L) and H(2)O(2), ISR (10(-5) mol/L) and H(2)O(2), or left untreated. Altered expressions of all mitochondrial proteins were analyzed by protein array and surfaceenhanced laser desorption ionization time of flight mass spectrometry (SELDI-TOF-MS). The mass/charge (m/z) ratios of each peak were tested by the Kruskal-Wallis rank sum test, and the protein peak value of the m/z ratio for each treatment by pair comparison was analyzed with the Nemenyi test.

RESULTS

H(2)O(2) up-regulated the expressions of two protein spots (with m/z of 6532 and 6809). E(2) mitigated the oxidative damage, and the expression of one protein spot (m/z 6532) was down-regulated. In contrast, ISR down-regulated both of protein spots (m/z 6532 and 6809).

CONCLUSIONS

ISR could effectively inhibit H(2)O(2)-induced oxidative damage in HLE-B3 cells. The protein spot at m/z of 6532 might be the target spot of ISR against oxidative damage induced by H(2)O(2).

Molecular mechanisms of retinal pigment epithelium damage and development of age-related macular degeneration

Age-related macular degeneration (AMD) is attributed to a complex interaction of genetic and environmental factors. It is characterized by degeneration involving the retinal photoreceptors, retinal pigment epithelium (RPE) and Bruch's membrane, as well as alterations in choroidal capillaries. AMD pathogenesis is strongly associated with chronic oxidative stress and inflammation that ultimately lead to protein damage, aggregation and degeneration of RPE. Specific degenerative findings for AMD are accumulation of intracellular lysosomal lipofuscin and extracellular drusens. In this review, we discuss thoroughly RPE-derived mechanisms in AMD pathology.

DICER1 loss and Alu RNA induce age-related macular degeneration via the NLRP3 inflammasome and MyD88

Alu RNA accumulation due to DICER1 deficiency in the retinal pigmented epithelium (RPE) is implicated in geographic atrophy (GA), an advanced form of age-related macular degeneration that causes blindness in millions of individuals. The mechanism of Alu RNA-induced cytotoxicity is unknown. Here we show that DICER1 deficit or Alu RNA exposure activates the NLRP3 inflammasome and triggers TLR-independent MyD88 signaling via IL18 in the RPE. Genetic or pharmacological inhibition of inflammasome components (NLRP3, Pycard, Caspase-1), MyD88, or IL18 prevents RPE degeneration induced by DICER1 loss or Alu RNA exposure. These findings, coupled with our observation that human GA RPE contains elevated amounts of NLRP3, PYCARD, and IL18 and evidence of increased Caspase-1 and MyD88 activation, provide a rationale for targeting this pathway in GA. Our findings also reveal a function of the inflammasome outside the immune system and an immunomodulatory action of mobile elements.

Systems-level analysis of age-related macular degeneration reveals global biomarkers and phenotype-specific functional networks

Please see related commentary: http://www.biomedcentral.com/1741-7015/10/21/abstract

BACKGROUND

Age-related macular degeneration (AMD) is a leading cause of blindness that affects the central region of the retinal pigmented epithelium (RPE), choroid, and neural retina. Initially characterized by an accumulation of sub-RPE deposits, AMD leads to progressive retinal degeneration, and in advanced cases, irreversible vision loss. Although genetic analysis, animal models, and cell culture systems have yielded important insights into AMD, the molecular pathways underlying AMD's onset and progression remain poorly delineated. We sought to better understand the molecular underpinnings of this devastating disease by performing the first comparative transcriptome analysis of AMD and normal human donor eyes.

METHODS

RPE-choroid and retina tissue samples were obtained from a common cohort of 31 normal, 26 AMD, and 11 potential pre-AMD human donor eyes. Transcriptome profiles were generated for macular and extramacular regions, and statistical and bioinformatic methods were employed to identify disease-associated gene signatures and functionally enriched protein association networks. Selected genes of high significance were validated using an independent donor cohort.

RESULTS

We identified over 50 annotated genes enriched in cell-mediated immune responses that are globally over-expressed in RPE-choroid AMD phenotypes. Using a machine learning model and a second donor cohort, we show that the top 20 global genes are predictive of AMD clinical diagnosis. We also discovered functionally enriched gene sets in the RPE-choroid that delineate the advanced AMD phenotypes, neovascular AMD and geographic atrophy. Moreover, we identified a graded increase of transcript levels in the retina related to wound response, complement cascade, and neurogenesis that strongly correlates with decreased levels of phototransduction transcripts and increased AMD severity. Based on our findings, we assembled protein-protein interactomes that highlight functional networks likely to be involved in AMD pathogenesis.

CONCLUSIONS

We discovered new global biomarkers and gene expression signatures of AMD. These results are consistent with a model whereby cell-based inflammatory responses represent a central feature of AMD etiology, and depending on genetics, environment, or stochastic factors, may give rise to the advanced AMD phenotypes characterized by angiogenesis and/or cell death. Genes regulating these immunological activities, along with numerous other genes identified here, represent promising new targets for AMD-directed therapeutics and diagnostics.

Mechanisms for countering oxidative stress and damage in retinal pigment epithelium

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

Increase in mitochondrial DNA mutations impairs retinal function and renders the retina vulnerable to injury

Mouse models that accumulate high levels of mitochondrial DNA (mtDNA) mutations owing to impairments in mitochondrial polymerase γ (PolG) proofreading function have been shown to develop phenotypes consistent with accelerated aging. As increase in mtDNA mutations and aging are risk factors for neurodegenerative diseases, we sought to determine whether increase in mtDNA mutations renders neurons more vulnerable to injury. We therefore examined the in vivo functional activity of retinal neurons and their ability to cope with stress in transgenic mice harboring a neural-targeted mutant PolG gene with an impaired proofreading capability (Kasahara, et al. (2006) Mol Psychiatry11(6):577-93, 523). We confirmed that the retina of these transgenic mice have increased mtDNA deletions and point mutations and decreased expression of mitochondrial oxidative phosphorylation enzymes. Associated with these changes, the PolG transgenic mice demonstrated accelerated age-related loss in retinal function as measured by dark-adapted electroretinogram, particularly in the inner and middle retina. Furthermore, the retinal ganglion cell-dominant inner retinal function in PolG transgenic mice showed greater vulnerability to injury induced by raised intraocular pressure, an insult known to produce mechanical, metabolic, and oxidative stress in the retina. These findings indicate that an accumulation of mtDNA mutations is associated with impairment in neural function and reduced capacity of neurons to resist external stress in vivo, suggesting a potential mechanism whereby aging central nervous system can become more vulnerable to neurodegeneration.

Mitochondria impairment correlates with increased sensitivity of aging RPE cells to oxidative stress

Impairment of mitochondria function and cellular antioxidant systems are linked to aging and neurodegenerative diseases. In the eye, the retinal pigment epithelium (RPE) is exposed to a highly oxidative environment that contributes to age-related visual dysfunction. Here, we examined changes in mitochondrial function in human RPE cells and sensitivity to oxidative stress with increased chronological age. Primary RPE cells from young (9-20)-, mid-age (48-60)-, and >60 (62-76)-year-old donors were grown to confluency and examined by electron microscopy and flow cytometry using several mitochondrial functional assessment tools. Susceptibility of RPE cells to H(2)O(2) toxicity was determined by lactate dehydrogenase and cytochrome c release, as well as propidium iodide staining. Reactive oxygen species, cytoplasmic Ca(2+) [Ca(2+)](c), and mitochondrial Ca(2+) [Ca(2+)](m) levels were measured using 2',7'-dichlorodihydrofluorescein diacetate, fluo-3/AM, and Rhod-2/AM, respectively, adenosine triphosphate (ATP) levels were measured by a luciferin/luciferase-based assay and mitochondrial membrane potential (ΔΨm) estimated using 5,5',6,6'-tetrachloro 1,1'3,3'-tetraethylbenzimid azolocarbocyanine iodide. Expression of mitochondrial and antioxidant genes was determined by real-time polymerase chain reaction. RPE cells show greater sensitivity to oxidative stress, reduction in expression of mitochondrial heat shock protein 70, uncoupling protein 2, and superoxide dismutase 3, and greater expression of superoxide dismutase 2 levels with increased chronological age. Changes in mitochondrial number, size, shape, matrix density, cristae architecture, and membrane integrity were more prominent in samples obtained from >60 years old compared to mid-age and younger donors. These mitochondria abnormalities correlated with lower ATP levels, reduced ΔΨm, decreased [Ca(2+)](c), and increased sequestration of [Ca(2+)](m) in cells with advanced aging. Our study provides evidence for mitochondrial decay, bioenergetic deficiency, weakened antioxidant defenses, and increased sensitivity of RPE cells to oxidative stress with advanced aging. Our findings suggest that with increased severity of mitochondrial decay and oxidative stress, RPE function may be altered in some individuals in a way that makes the retina more susceptible to age-related injury.

Mitochondrial DNA damage and repair in RPE associated with aging and age-related macular degeneration

PURPOSE

Mitochondrial DNA (mtDNA) damage may be associated with age-related diseases, such as age-related macular degeneration (AMD). The present study was designed to test whether the frequency of mtDNA damage, heteroplasmic mtDNA mutations, and repair capacity correlate with progression of AMD.

METHODS

Macular and peripheral RPE cells were isolated and cultured from human donor eyes with and without AMD. The stages of AMD were graded according to the Minnesota Grading System. Confluent primary RPE cells were used to test the frequency of endogenous mtDNA damage by quantitative PCR. Mutation detection kits were used to detect heteroplasmic mtDNA mutation. To test the mtDNA repair capacity, cultured RPE cells were allowed to recover for 3 and 6 hours after exposure to H(2)O(2), and repair was assessed by quantitative PCR. The levels of human OGG1 protein, which is associated with mtDNA repair, were analyzed by Western blot.

RESULTS

This study showed that mtDNA damage increased with aging and that more lesions occurred in RPE cells from the macular region than the periphery. Furthermore, mtDNA repair capacity decreased with aging, with less mtDNA repair capacity in the macular region compared with the periphery in samples from aged subjects. Most interestingly, the mtDNA damage was positively correlated with the grading level of AMD, whereas repair capacity was negatively correlated. In addition, more mitochondrial heteroplasmic mutations were detected in eyes with AMD.

CONCLUSIONS

These data show macula-specific increases in mtDNA damage, heteroplasmic mutations, and diminished repair that are associated with aging and AMD severity.

Genetics of age-related macular degeneration: current concepts, future directions

Age-related macular degeneration (AMD) is a progressive degenerative disease which leads to blindness, affecting the quality of life of millions of Americans. More than 1.75 million individuals in the United States are affected by the advanced form of AMD. The etiological pathway of AMD is not yet fully understood, but there is a clear genetic influence on disease risk. To date, the 1q32 (CFH) and 10q26 (PLEKHA1/ARMS2/HTRA1) loci are the most strongly associated with disease; however, the variation in these genomic regions alone is unable to predict disease development with high accuracy. Therefore, current genetic studies are aimed at identifying new genes associated with AMD and their modifiers, with the goal of discovering diagnostic or prognostic biomarkers. Moreover, these studies provide the foundation for further investigation into the pathophysiology of AMD by utilizing a systems-biology-based approach to elucidate underlying mechanistic pathways.

Theoretic, experimental, clinical bases of the water oscillator hypothesis in near-infrared photobiomodulation

The objective of this review is to propose and document a role for the water oscillator in near-infrared (NIR) photobiomodulation. Greater understanding of the role of the water oscillator may add to a more-coherent description of central effects of NIR light on redox centers and key transmembrane enzymes such as cytochrome c oxidase (CcO). In addition, water provides a complementary pathway for absorption and transportation of NIR energy in photobiomodulation. Because of its unexpected potential, we propose terming it the "water oscillator paradox." Photobiologic mechanisms involved in the treatment of complex diseases are discussed in light of the present state of the art.

Early biosignature of oxidative stress in the retinal pigment epithelium

The retinal pigment epithelium (RPE) is essential for retinoid recycling and phagocytosis of photoreceptors. Understanding of proteome changes that mediate oxidative stress-induced degeneration of RPE cells may provide further insight into the molecular mechanisms of retinal diseases. In the current study, comparative proteomics has been applied to investigate global changes of RPE proteins under oxidative stress. Proteomic techniques, including 2D SDS-PAGE, differential gel electrophoresis (DIGE), and tandem time-of-flight (TOF-TOF) mass spectrometry, were used to identify early protein markers of oxidative stress in the RPE. Two biological models of RPE cells revealed several differentially expressed proteins that are involved in key cellular processes such as energy metabolism, protein folding, redox homeostasis, cell differentiation, and retinoid metabolism. Our results provide a new perspective on early signaling molecules of redox imbalance in the RPE and putative therapeutic target proteins of RPE diseases caused by oxidative stress.

The importance of mitochondria in age-related and inherited eye disorders

Mitochondria are critical for ocular function as they represent the major source of a cell's supply of energy and play an important role in cell differentiation and survival. Mitochondrial dysfunction can occur as a result of inherited mitochondrial mutations (e.g. Leber's hereditary optic neuropathy and chronic progressive external ophthalmoplegia) or stochastic oxidative damage which leads to cumulative mitochondrial damage and is an important factor in age-related disorders (e.g. age-related macular degeneration, cataract and diabetic retinopathy). Mitochondrial DNA (mtDNA) instability is an important factor in mitochondrial impairment culminating in age-related changes and pathology, and in all regions of the eye mtDNA damage is increased as a consequence of aging and age-related disease. It is now apparent that the mitochondrial genome is a weak link in the defenses of ocular cells since it is susceptible to oxidative damage and it lacks some of the systems that protect the nuclear genome, such as nucleotide excision repair. Accumulation of mitochondrial mutations leads to cellular dysfunction and increased susceptibility to adverse events which contribute to the pathogenesis of numerous sporadic and chronic disorders in the eye.

Quantitative proteomics: comparison of the macular Bruch membrane/choroid complex from age-related macular degeneration and normal eyes

A quantitative proteomics analysis of the macular Bruch membrane/choroid complex was pursued for insights into the molecular mechanisms of age-related macular degeneration (AMD). Protein in trephine samples from the macular region of 10 early/mid-stage dry AMD, six advanced dry AMD, eight wet AMD, and 25 normal control post-mortem eyes was analyzed by LC MS/MS iTRAQ (isobaric tags for relative and absolute quantitation) technology. A total of 901 proteins was quantified, including 556 proteins from > or =3 AMD samples. Most proteins differed little in amount between AMD and control samples and therefore reflect the proteome of normal macular tissues of average age 81. A total of 56 proteins were found to be elevated and 43 were found to be reduced in AMD tissues relative to controls. Analysis by category of disease progression revealed up to 16 proteins elevated or decreased in each category. About 60% of the elevated proteins are involved in immune response and host defense, including many complement proteins and damage-associated molecular pattern proteins such as alpha-defensins 1-3, protein S100s, crystallins, histones, and galectin-3. Four retinoid processing proteins were elevated only in early/mid-stage AMD, supporting a role for retinoids in AMD initiation. Proteins uniquely decreased in early/mid-stage AMD implicate hematologic malfunctions and weakened extracellular matrix integrity and cellular interactions. Galectin-3, a receptor for advanced glycation end products, was the most significantly elevated protein in advanced dry AMD, supporting a role for advanced glycation end products in dry AMD progression. The results endorse inflammatory processes in both early and advanced AMD pathology, implicate different pathways of progression to advanced dry and wet AMD, and provide a new database for hypothesis-driven and discovery-based studies of AMD.

Mitochondrial DNA damage as a potential mechanism for age-related macular degeneration

PURPOSE

Increasing evidence suggests a central role for mitochondrial (mt) dysfunction in age-related macular degeneration (AMD). Previous proteomic data from the retinal pigment epithelium (RPE) revealed significant changes to mt proteins, suggesting potential functional defects and damage to mitochondrial DNA (mtDNA) with AMD progression. The present study tests the hypothesis that mtDNA damage increases with aging and AMD.

METHODS

Genomic DNA was isolated from the macular region of human donor RPE graded for stages of AMD (Minnesota Grading System [MGS] 1-4). Region-specific mtDNA damage with normal aging was evaluated in 45 control subjects (ages 34-88 years, MGS 1) and AMD-associated damage in diseased subjects (n = 46), compared with that in age-matched control subjects (n = 26). Lesions per 10 kb per genome in the mtDNA and nuclear DNA were measured with long-extension polymerase chain reaction (LX PCR). The level of deleted mtDNA in each donor was measured with quantitative real-time PCR (qPCR).

RESULTS

With aging, an increase in mtDNA damage was observed only in the common deletion region of the mt genome. In contrast, with AMD, mtDNA lesions increased significantly in all regions of the mt genome beyond levels found in age-matched control subjects. mtDNA accumulated more lesions than did two nuclear genes, with total damage of the mt genome estimated to be eight times higher.

CONCLUSIONS

Collectively, the data indicate that mtDNA is preferentially damaged with AMD progression. These results suggest a potential link between mt dysfunction due to increased mtDNA lesions and AMD.

Abundant lipid and protein components of drusen

Background

Drusen are extracellular lesions characteristic of aging and age-related maculopathy, a major retinal disease of the elderly. We determined the relative proportions of lipids and proteins in drusen capped with retinal pigment epithelium (RPE) and in RPE isolated from non-macular regions of 36 human retinas with grossly normal maculas obtained <6 hr after death.

Methodology/Principal Findings

Druse pellets were examined by light and electron microscopy. Component proteins were extracted using novel methods for preserved tissues, separated, subjected to tryptic digestion and LC-MS(MS)² analysis using an ion trap mass spectrometer, and identified with reference to databases. Lipid classes were separated using thin layer chromatography and quantified by densitometry. Major druse components were esterified cholesterol (EC), phosphatidylcholine (PC), and protein (37.5±13.7, 36.9±12.9, and 43.0±11.5 ng/druse, respectively). Lipid-containing particles (median diameter, 77 nm) occupied 37–44% of druse volume. Major proteins include vitronectin, complement component 9, apoE, and clusterin, previously seen in drusen, and ATP synthase subunit β, scavenger receptor B2, and retinol dehydrogenase 5, previously seen in RPE. Drusen and RPE had similar protein profiles, with higher intensities and greater variability in drusen. C8, part of the complement membrane attack complex, was localized in drusen by immunofluorescence.

Conclusions/Significance

At least 40% of druse content is comprised by lipids dominated by EC and PC, 2 components that are potentially accounted for by just one pathway, the secretion of lipoproteins by RPE. Manipulating genes encoding apolipoprotein pathways would be a fruitful approach to producing drusen with high EC content in laboratory animals. Therapies that directly mitigate drusen should prepare for the substantial volume of neutral lipids. The catalog of major druse proteins is nearing completion.

Mitochondrial decay and impairment of antioxidant defenses in aging RPE cells

In the eye, the retinal pigment epithelium (RPE) is exposed to a highly oxidative environment, partly due to elevated oxygen partial pressure from the choriocapillaris and to digestion of polyunsaturated fatty acid laden photoreceptor outer segments. Here we examined the vulnerability of RPE cells to stress and changes in their mitochondria with increased chronological aging and showed that there is greater sensitivity of the cells to oxidative stress, alterations in their mitochondrial number, size, shape, matrix density, cristae architecture, and membrane integrity as a function of age. These features correlate with reduced cellular levels of ATP, ROS, and [Ca(2+)](c), lower Deltapsim, increased [Ca(2+)](m) sequestration and decreased expression of mtHsp70, UCP2, and SOD3. Mitochondrial decay, bioenergetic deficiencies, and weakened antioxidant defenses in RPE cells occur as early as age 62. With increased severity, these conditions may significantly reduce RPE function in the retina and contribute to age related retinal anomalies.

Near-infrared light protect the photoreceptor from light-induced damage in rats

BACKGROUND

A project originally developed for NASA plant growth experiments in space demonstrating the Light-Emitting Diode (LED) could promote the wound healing. Further study showed that the LED's could protect cells by stimulating the basic energy processes in the mitochondria of each cell.

OBJECTIVE

The purpose of this study was to assess the effects of 670 nm LED to protect the photoreceptor from the light-induced damage in a rodent model.

METHODS

SD rats were randomly assigned to one of eight groups: untreated control group, the LED-treated control group, three light-induced damage groups, and three LED-protected groups. The rats were exposed to constant light for 3 h of different illuminations of 900, 1,800 and 2,700 lux, respectively. The LED treatment (50 mW) were done for 30 min, 3 h before the light damage and 0, 24 and 48 h after the light damage. Using the electroretinogram as a sensitive indicator of retinal function, and the histopathologic change was showed as a proof of the protective effect of LED treatment.

RESULTS

The 900 lux illumination for 3 h did not cause damage to the retina of rats, however, the 1,800 lux illumination for 3 h caused significant damage to ONL of an approximate half retina, which caused the swing of ERG b wave to be 431 muV. With the LED protection: the damage of ONL was near 1/6 of retina, which was significantly reduced than the ones without LED protection (P < 0.01); and the swing of ERG b wave was recorded to be 1,011 muV, which was increased significantly than the ones without LED protection (P < 0.01). The illumination of 2,700 lux for 3 h caused severe damage to the rats' retinas and the LED could not protect them significantly in both of morphology and function (P > 0.05, P > 0.05).

CONCLUSIONS

670 nm LED treatment has an evident protective effect on retinal cells against light-induced damage, which may be an innovative and non-invasive therapeutic approach to prevent or to delay age-related macular degeneration.

Ocular proteomics with emphasis on two-dimensional gel electrophoresis and mass spectrometry

The intention of this review is to provide an overview of current methodologies employed in the rapidly developing field of ocular proteomics with emphasis on sample preparation, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and mass spectrometry (MS). Appropriate sample preparation for the diverse range of cells and tissues of the eye is essential to ensure reliable results. Current methods of protein staining for 2D-PAGE, protein labelling for two-dimensional difference gel electrophoresis, gel-based expression analysis and protein identification by MS are summarised. The uses of gel-free MS-based strategies (MuDPIT, iTRAQ, ICAT and SILAC) are also discussed. Proteomic technologies promise to shed new light onto ocular disease processes that could lead to the discovery of strong novel biomarkers and therapeutic targets useful in many ophthalmic conditions.

DNA damage/repair and polymorphism of the hOGG1 gene in lymphocytes of AMD patients

Oxidative stress is thought to play a role in the pathogenesis of age-related macular degeneration (AMD). We determined the extent of oxidative DNA damage and the kinetics of its removal as well as the genotypes of the Ser326Cys polymorphism of the hOGG1 gene in lymphocytes of 30 wet AMD patients and 30 controls. Oxidative DNA damage induced by hydrogen peroxide and its repair were evaluated by the comet assay and DNA repair enzymes. We observed a higher extent of endogenous oxidative DNA damage and a lower efficacy of its repair in AMD patients as compared with the controls. We did not find any correlation between the extent of DNA damage and efficacy of DNA repair with genotypes of the Ser326Cys polymorphism. The results obtained suggest that oxidative DNA damage and inefficient DNA repair can be associated with AMD and the variability of the hOOG1 gene may not contribute to this association.

Regulatory role of HIF-1alpha in the pathogenesis of age-related macular degeneration (AMD)

Age-related macular degeneration (AMD) is a leading cause of irreversible blindness in the elderly throughout the world. AMD is attributed to a complex interaction of genetic and environmental factors. It is characterized by degeneration involving the retinal photoreceptors, retinal pigment epithelium (RPE), and Bruch's membrane, as well as alterations in choroidal capillaries. Aging and age-associated degenerative diseases, such as AMD, are intimately associated with decreased levels of tissue oxygenation and hypoxia that may induce accumulation of detrimental RPE-associated deposits, inflammation and neovascularization processes in retina. Hypoxia-inducible factor (HIF) is the master regulator for hypoxia-induced cellular adaptation that is involved in NF-kappaB signaling and the autophagic protein clearance system. In this review, we discuss role of HIF in AMD pathology and as a possible therapeutic target.

Heat shock proteins as gatekeepers of proteolytic pathways-Implications for age-related macular degeneration (AMD)

Age-related macular degeneration (AMD) is the major diagnosis for severe and irreversible central loss of vision in elderly people in the developed countries. The loss of vision involves primarily a progressive degeneration and cell death of postmitotic retinal pigment epithelial cells (RPE), which secondarily evokes adverse effects on photoreceptor cells. The RPE cells are exposed to chronic oxidative stress from three sources: their high levels of oxygen consumption, their exposure to the high levels of lipid peroxidation derived from the photoreceptor outer segments and their exposure to constant light stimuli. Cells increase the expression of heat shock proteins (HSPs) in order to normalize their growth conditions in response to various environmental stress factors, e.g. oxidative stress. The HSPs function as molecular chaperones by preventing the accumulation of cellular cytotoxic protein aggregates and assisting in correct folding of both nascent and misfolded proteins. Increased HSPs levels are observed in the retina of AMD patients, evidence of stressed tissue. A hallmark of RPE cell aging is lysosomal lipofuscin accumulation reflecting a weakened capacity to degrade proteins in lysosomes. The presence of lipofuscin increases the misfolding of intracellular proteins, which evokes additional stress in the RPE cells. If the capacity of HSPs to repair protein damages is overwhelmed, then the proteins are mainly cleared in proteasomes or in lysosomes. In this review, we discuss the role of heat shock proteins, proteasomes, and lysosomes and autophagic processes in RPE cell proteolysis and how these might be involved in development of AMD. In addition to classical lysosomal proteolysis, we focus on the increasing evidence that, HSPs, proteasomes and autophagy regulate protein turnover in the RPE cells and thus have important roles in AMD disease.

Autophagy and exosomes in the aged retinal pigment epithelium: possible relevance to drusen formation and age-related macular degeneration

Age-related macular degeneration (AMD) is a major cause of loss of central vision in the elderly. The formation of drusen, an extracellular, amorphous deposit of material on Bruch's membrane in the macula of the retina, occurs early in the course of the disease. Although some of the molecular components of drusen are known, there is no understanding of the cell biology that leads to the formation of drusen. We have previously demonstrated increased mitochondrial DNA (mtDNA) damage and decreased DNA repair enzyme capabilities in the rodent RPE/choroid with age. In this study, we found that drusen in AMD donor eyes contain markers for autophagy and exosomes. Furthermore, these markers are also found in the region of Bruch's membrane in old mice. By in vitro modeling increased mtDNA damage induced by rotenone, an inhibitor of mitochondrial complex I, in the RPE, we found that the phagocytic activity was not altered but that there were: 1) increased autophagic markers, 2) decreased lysosomal activity, 3) increased exocytotic activity and 4) release of chemoattractants. Exosomes released by the stressed RPE are coated with complement and can bind complement factor H, mutations of which are associated with AMD. We speculate that increased autophagy and the release of intracellular proteins via exosomes by the aged RPE may contribute to the formation of drusen. Molecular and cellular changes in the old RPE may underlie susceptibility to genetic mutations that are found in AMD patients and may be associated with the pathogenesis of AMD in the elderly.

Unraveling a multifactorial late-onset disease: from genetic susceptibility to disease mechanisms for age-related macular degeneration

Aging-associated neurodegenerative diseases significantly influence the quality of life of affected individuals. Genetic approaches, combined with genomic technology, have provided powerful insights into common late-onset diseases, such as age-related macular degeneration (AMD). Here, we discuss current findings on the genetics of AMD to highlight areas of rapid progress and new challenges. We also attempt to integrate available genetic and biochemical data with cellular pathways involved in aging to formulate an integrated model of AMD pathogenesis.

Multifactor effects and evidence of potential interaction between complement factor H Y402H and LOC387715 A69S in age-related macular degeneration

Background

Variants in the complement cascade genes and the LOC387715/HTRA1, have been widely reported to associate with age-related macular degeneration (AMD), the most common cause of visual impairment in industrialized countries.

Methods/Principal Findings

We investigated the association between the LOC387715 A69S and complement component C3 R102G risk alleles in the Finnish case-control material and found a significant association with both variants (OR 2.98, p = 3.75×10⁻⁹; non-AMD controls and OR 2.79, p = 2.78×10⁻¹⁹, blood donor controls and OR 1.83, p = 0.008; non-AMD controls and OR 1.39, p = 0.039; blood donor controls), respectively. Previously, we have shown a strong association between complement factor H (CFH) Y402H and AMD in the Finnish population. A carrier of at least one risk allele in each of the three susceptibility loci (LOC387715, C3, CFH) had an 18-fold risk of AMD when compared to a non-carrier homozygote in all three loci. A tentative gene-gene interaction between the two major AMD-associated loci, LOC387715 and CFH, was found in this study using a multiplicative (logistic regression) model, a synergy index (departure-from-additivity model) and the mutual information method (MI), suggesting that a common causative pathway may exist for these genes. Smoking (ever vs. never) exerted an extra risk for AMD, but somewhat surprisingly, only in connection with other factors such as sex and the C3 genotype. Population attributable risks (PAR) for the CFH, LOC387715 and C3 variants were 58.2%, 51.4% and 5.8%, respectively, the summary PAR for the three variants being 65.4%.

Conclusions/Significance

Evidence for gene-gene interaction between two major AMD associated loci CFH and LOC387715 was obtained using three methods, logistic regression, a synergy index and the mutual information (MI) index.

Molecular pathology of age-related macular degeneration

Age-related macular degeneration (AMD) is a leading cause of irreversible blindness in the world. Although the etiology and pathogenesis of AMD remain largely unclear, a complex interaction of genetic and environmental factors is thought to exist. AMD pathology is characterized by degeneration involving the retinal photoreceptors, retinal pigment epithelium, and Bruch's membrane, as well as, in some cases, alterations in choroidal capillaries. Recent research on the genetic and molecular underpinnings of AMD brings to light several basic molecular pathways and pathophysiological processes that might mediate AMD risk, progression, and/or response to therapy. This review summarizes, in detail, the molecular pathological findings in both humans and animal models, including genetic variations in CFH, CX3CR1, and ARMS2/HtrA1, as well as the role of numerous molecules implicated in inflammation, apoptosis, cholesterol trafficking, angiogenesis, and oxidative stress.