Modulation of Sub-RPE Deposits In Vitro: A Potential Model for Age-Related Macular Degeneration

Dysfunctional Autophagy, Proteostasis, and Mitochondria as a Prelude to Age-Related Macular Degeneration

Retinal pigment epithelial (RPE) cell dysfunction is a key driving force of AMD. RPE cells form a metabolic interface between photoreceptors and choriocapillaris, performing essential functions for retinal homeostasis. Through their multiple functions, RPE cells are constantly exposed to oxidative stress, which leads to the accumulation of damaged proteins, lipids, nucleic acids, and cellular organelles, including mitochondria. As miniature chemical engines of the cell, self-replicating mitochondria are heavily implicated in the aging process through a variety of mechanisms. In the eye, mitochondrial dysfunction is strongly associated with several diseases, including age-related macular degeneration (AMD), which is a leading cause of irreversible vision loss in millions of people globally. Aged mitochondria exhibit decreased rates of oxidative phosphorylation, increased reactive oxygen species (ROS) generation, and increased numbers of mitochondrial DNA mutations. Mitochondrial bioenergetics and autophagy decline during aging because of insufficient free radical scavenger systems, the impairment of DNA repair mechanisms, and reductions in mitochondrial turnover. Recent research has uncovered a much more complex role of mitochondrial function and cytosolic protein translation and proteostasis in AMD pathogenesis. The coupling of autophagy and mitochondrial apoptosis modulates the proteostasis and aging processes. This review aims to summarise and provide a perspective on (i) the current evidence of autophagy, proteostasis, and mitochondrial dysfunction in dry AMD; (ii) current in vitro and in vivo disease models relevant to assessing mitochondrial dysfunction in AMD, and their utility in drug screening; and (iii) ongoing clinical trials targeting mitochondrial dysfunction for AMD therapeutics.

Altered zinc homeostasis in a primary cell culture model of the retinal pigment epithelium

The retinal pigment epithelium (RPE) is progressively degenerated during age-related macular degeneration (AMD), one of the leading causes of irreversible blindness, which clinical hallmark is the buildup of sub-RPE extracellular material. Clinical observations indicate that Zn dyshomeostasis can initiate detrimental intracellular events in the RPE. In this study, we used a primary human fetal RPE cell culture model producing sub-RPE deposits accumulation that recapitulates features of early AMD to study Zn homeostasis and metalloproteins changes. RPE cell derived samples were collected at 10, 21 and 59 days in culture and processed for RNA sequencing, elemental mass spectrometry and the abundance and cellular localization of specific proteins. RPE cells developed processes normal to RPE, including intercellular unions formation and expression of RPE proteins. Punctate deposition of apolipoprotein E, marker of sub-RPE material accumulation, was observed from 3 weeks with profusion after 2 months in culture. Zn cytoplasmic concentrations significantly decreased 0.2 times at 59 days, from 0.264 ± 0.119 ng·μg^-1 at 10 days to 0.062 ± 0.043 ng·μg^-1 at 59 days (p < 0.05). Conversely, increased levels of Cu (1.5-fold in cytoplasm, 5.0-fold in cell nuclei and membranes), Na (3.5-fold in cytoplasm, 14.0-fold in cell nuclei and membranes) and K (6.8-fold in cytoplasm) were detected after 59-days long culture. The Zn-regulating proteins metallothioneins showed significant changes in gene expression over time, with a potent down-regulation at RNA and protein level of the most abundant isoform in primary RPE cells, from 0.141 ± 0.016 ng·mL^-1 at 10 days to 0.056 ± 0.023 ng·mL^-1 at 59 days (0.4-fold change, p < 0.05). Zn influx and efflux transporters were also deregulated, along with an increase in oxidative stress and alterations in the expression of antioxidant enzymes, including superoxide dismutase, catalase and glutathione peroxidase. The RPE cell model producing early accumulation of extracellular deposits provided evidences on an altered Zn homeostasis, exacerbated by changes in cytosolic Zn-binding proteins and Zn transporters, along with variations in other metals and metalloproteins, suggesting a potential role of altered Zn homeostasis during AMD development.

Cell culture models to study retinal pigment epithelium-related pathogenesis in age-related macular degeneration

Age-related macular degeneration (AMD) is a disease that affects the macula - the central part of the retina. It is a leading cause of irreversible vision loss in the elderly. AMD onset is marked by the presence of lipid- and protein-rich extracellular deposits beneath the retinal pigment epithelium (RPE), a monolayer of polarized, pigmented epithelial cells located between the photoreceptors and the choroidal blood supply. Progression of AMD to the late nonexudative "dry" stage of AMD, also called geographic atrophy, is linked to progressive loss of areas of the RPE, photoreceptors, and underlying choriocapillaris leading to a severe decline in patients' vision. Differential susceptibility of macular RPE in AMD and the lack of an anatomical macula in most lab animal models has promoted the use of in vitro models of the RPE. In addition, the need for high throughput platforms to test potential therapies has driven the creation and characterization of in vitro model systems that recapitulate morphologic and functional abnormalities associated with human AMD. These models range from spontaneously formed cell line ARPE19, immortalized cell lines such as hTERT-RPE1, RPE-J, and D407, to primary human (fetal or adult) or animal (mouse and pig) RPE cells, and embryonic and induced pluripotent stem cell (iPSC) derived RPE. Hallmark RPE phenotypes, such as cobblestone morphology, pigmentation, and polarization, vary significantly betweendifferent models and culture conditions used in different labs, which would directly impact their usability for investigating different aspects of AMD biology. Here the AMD Disease Models task group of the Ryan Initiative for Macular Research (RIMR) provides a summary of several currently used in vitro RPE models, historical aspects of their development, RPE phenotypes that are attainable in these models, their ability to model different aspects of AMD pathophysiology, and pros/cons for their use in the RPE and AMD fields. In addition, due to the burgeoning use of iPSC derived RPE cells, the critical need for developing standards for differentiating and rigorously characterizing RPE cell appearance, morphology, and function are discussed.

Looking for In Vitro Models for Retinal Diseases

Retina is a layered structure of the eye, composed of different cellular components working together to produce a complex visual output. Because of its important role in visual function, retinal pathologies commonly represent the main causes of visual injury and blindness in the industrialized world. It is important to develop in vitro models of retinal diseases to use them in first screenings before translating in in vivo experiments and clinics. For this reason, it is important to develop bidimensional (2D) models that are more suitable for drug screening and toxicological studies and tridimensional (3D) models, which can replicate physiological conditions, for investigating pathological mechanisms leading to visual loss. This review provides an overview of the most common retinal diseases, relating to in vivo models, with a specific focus on alternative 2D and 3D in vitro models that can replicate the different cellular and matrix components of retinal layers, as well as injury insults that induce retinal disease and loss of the visual function.

Comparison of Antioxidant Properties of Dehydrolutein with Lutein and Zeaxanthin, and their Effects on Cultured Retinal Pigment Epithelial Cells

Dehydrolutein accumulates in substantial concentrations in the retina. The aim of this study was to compare antioxidant properties of dehydrolutein with other retinal carotenoids, lutein, and zeaxanthin, and their effects on ARPE-19 cells. The time-resolved detection of characteristic singlet oxygen phosphorescence was used to compare the singlet oxygen quenching rate constants of dehydrolutein, lutein, and zeaxanthin. The effects of these carotenoids on photosensitized oxidation were tested in liposomes, where photo-oxidation was induced by light in the presence of photosensitizers, and monitored by oximetry. To compare the uptake of dehydrolutein, lutein, and zeaxanthin, ARPE-19 cells were incubated with carotenoids for up to 19 days, and carotenoid contents were determined by spectrophotometry in cell extracts. To investigate the effects of carotenoids on photocytotoxicity, cells were exposed to light in the presence of rose bengal or all-trans-retinal. The results demonstrate that the rate constants for singlet oxygen quenching are 0.77 × 10¹⁰, 0.55 × 10¹⁰, and 1.23 × 10¹⁰ M^-1s^-1 for dehydrolutein, lutein, and zeaxanthin, respectively. Overall, dehydrolutein is similar to lutein or zeaxanthin in the protection of lipids against photosensitized oxidation. ARPE-19 cells accumulate substantial amounts of both zeaxanthin and lutein, but no detectable amounts of dehydrolutein. Cells pre-incubated with carotenoids are equally susceptible to photosensitized damage as cells without carotenoids. Carotenoids provided to cells together with the extracellular photosensitizers offer partial protection against photodamage. In conclusion, the antioxidant properties of dehydrolutein are similar to lutein and zeaxanthin. The mechanism responsible for its lack of accumulation in ARPE-19 cells deserves further investigation.

A Multi-Omics Approach Identifies Key Regulatory Pathways Induced by Long-Term Zinc Supplementation in Human Primary Retinal Pigment Epithelium

In age-related macular degeneration (AMD), both systemic and local zinc levels decline. Elevation of zinc in clinical studies delayed the progression to end-stage AMD. However, the molecular pathways underpinning this beneficial effect are not yet identified. In this study, we used differentiated primary human fetal retinal pigment epithelium (RPE) cultures and long-term zinc supplementation to carry out a combined transcriptome, proteome and secretome analysis from three genetically different human donors. After combining significant differences, we identified the complex molecular networks using Database for Annotation, Visualization and Integrated Discovery (DAVID) and Ingenuity Pathway Analysis (IPA). The cell cultures from the three donors showed extensive pigmentation, development of microvilli and basal infoldings and responded to zinc supplementation with an increase in transepithelial electrical resistance (TEER) (apical supplementation: 443.2 ± 79.3%, basal supplementation: 424.9 ± 116.8%, compared to control: 317.5 ± 98.2%). Significant changes were observed in the expression of 1044 genes, 151 cellular proteins and 124 secreted proteins. Gene set enrichment analysis revealed changes in specific molecular pathways related to cell adhesion/polarity, extracellular matrix organization, protein processing/transport, and oxidative stress response by zinc and identified a key upstream regulator effect similar to that of TGFB1.

Soft Drusen in Age-Related Macular Degeneration: Biology and Targeting Via the Oil Spill Strategies

AMD is a major cause of legal blindness in older adults approachable through multidisciplinary research involving human tissues and patients. AMD is a vascular-metabolic-inflammatory disease, in which two sets of extracellular deposits, soft drusen/basal linear deposit (BLinD) and subretinal drusenoid deposit (SDD), confer risk for end-stages of atrophy and neovascularization. Understanding how deposits form can lead to insights for new preventions and therapy. The topographic correspondence of BLinD and SDD with cones and rods, respectively, suggest newly realized exchange pathways among outer retinal cells and across Bruch's membrane and the subretinal space, in service of highly evolved, eye-specific physiology. This review focuses on soft drusen/BLinD, summarizing evidence that a major ultrastructural component is large apolipoprotein B,E-containing, cholesterol-rich lipoproteins secreted by the retinal pigment epithelium (RPE) that offload unneeded lipids of dietary and outer segment origin to create an atherosclerosis-like progression in the subRPE-basal lamina space. Clinical observations and an RPE cell culture system combine to suggest that soft drusen/BLinD form when secretions of functional RPE back up in the subRPE-basal lamina space by impaired egress across aged Bruch's membrane-choriocapillary endothelium. The soft drusen lifecycle includes growth, anterior migration of RPE atop drusen, then collapse, and atrophy. Proof-of-concept studies in humans and animal models suggest that targeting the “Oil Spill in Bruch's membrane” offers promise of treating a process in early AMD that underlies progression to both end-stages. A companion article addresses the antecedents of soft drusen within the biology of the macula.

A convenient protocol for establishing a human cell culture model of the outer retina

The retinal pigment epithelium (RPE) plays a key role in the pathogenesis of several blinding retinopathies. Alterations to RPE structure and function are reported in Age-related Macular Degeneration, Stargardt and Best disease as well as pattern dystrophies. However, the precise role of RPE cells in disease aetiology remains incompletely understood. Many studies into RPE pathobiology have utilised animal models, which only recapitulate limited disease features. Some studies are also difficult to carry out in animals as the ocular space remains largely inaccessible to powerful microscopes. In contrast, in-vitro models provide an attractive alternative to investigating pathogenic RPE changes associated with age and disease. In this article we describe the step-by-step approach required to establish an experimentally versatile in-vitro culture model of the outer retina incorporating the RPE monolayer and supportive Bruch's membrane (BrM). We show that confluent monolayers of the spontaneously arisen human ARPE-19 cell-line cultured under optimal conditions reproduce key features of native RPE. These models can be used to study dynamic, intracellular and extracellular pathogenic changes using the latest developments in microscopy and imaging technology. We also discuss how RPE cells from human foetal and stem-cell derived sources can be incorporated alongside sophisticated BrM substitutes to replicate the aged/diseased outer retina in a dish. The work presented here will enable users to rapidly establish a realistic in-vitro model of the outer retina that is amenable to a high degree of experimental manipulation which will also serve as an attractive alternative to using animals. This in-vitro model therefore has the benefit of achieving the 3Rs objective of reducing and replacing the use of animals in research. As well as recapitulating salient structural and physiological features of native RPE, other advantages of this model include its simplicity, rapid set-up time and unlimited scope for detailed single-cell resolution and matrix studies.

Subretinal Pigment Epithelial Deposition of Drusen Components Including Hydroxyapatite in a Primary Cell Culture Model

Purpose

Extracellular deposits containing hydroxyapatite, lipids, proteins, and trace metals that form between the basal lamina of the RPE and the inner collagenous layer of Bruch's membrane are hallmarks of early AMD. We examined whether cultured RPE cells could produce extracellular deposits containing all of these molecular components.

Methods

Retinal pigment epithelium cells isolated from freshly enucleated porcine eyes were cultured on Transwell membranes for up to 6 months. Deposit composition and structure were characterized using light, fluorescence, and electron microscopy; synchrotron x-ray diffraction and x-ray fluorescence; secondary ion mass spectroscopy; and immunohistochemistry.

Results

Apparently functional primary RPE cells, when cultured on 10-μm-thick inserts with 0.4-μm-diameter pores, can produce sub-RPE deposits that contain hydroxyapatite, lipids, proteins, and trace elements, without outer segment supplementation, by 12 weeks.

Conclusions

The data suggest that sub-RPE deposit formation is initiated, and probably regulated, by the RPE, as well as the loss of permeability of the Bruch's membrane and choriocapillaris complex associated with age and early AMD. This cell culture model of early AMD lesions provides a novel system for testing new therapeutic interventions against sub-RPE deposit formation, an event occurring well in advance of the onset of vision loss.

Ex-vivo models of the Retinal Pigment Epithelium (RPE) in long-term culture faithfully recapitulate key structural and physiological features of native RPE

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Damage to the Retinal Pigment Epithelium (RPE) is a key feature of retinopathy.

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We describe 2 substrates which support RPE cultures for long-term studies.

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Substrates were; a polyester transwell membrane and a novel electrospun scaffold.

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Both support RPE cultures with structural and functional features of native RPE.

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Electrospun scaffolds may be better for studying some disease-linked RPE changes.

The Retinal Pigment Epithelium (RPE) forms the primary site of pathology in several blinding retinopathies. RPE cultures are being continuously refined so that dynamic disease processes in this important monolayer can be faithfully studied outside the eye over longer periods. The RPE substrate, which mimics the supportive Bruch’s membrane (BrM), plays a key role in determining how well in-vitro cultures recapitulate native RPE cells. Here, we evaluate how two different types of BrM substrates; (1) a commercially-available polyester transwell membrane, and (2) a novel electrospun scaffold developed in our laboratory, could support the generation of realistic RPE tissues in culture. Our findings reveal that both substrates were capable of supporting long-lasting RPE monolayers with structural and functional specialisations of in-situ RPE cells. These cultures were used to study autofluorescence and barrier formation, as well as activities such as outer-segment internalisation/trafficking and directional secretion of key proteins; the impairment of which underlies retinal disease. Hence, both substrates fulfilled important criteria for generating authentic in-vitro cultures and act as powerful tools to study RPE pathophysiology. However, RPE grown on electrospun scaffolds may be better suited to studying complex RPE-BrM interactions such as the formation of drusen-like deposits associated with early retinal disease.

Cellular models and therapies for age-related macular degeneration

Age-related macular degeneration (AMD) is a complex neurodegenerative visual disorder that causes profound physical and psychosocial effects. Visual impairment in AMD is caused by the loss of retinal pigmented epithelium (RPE) cells and the light-sensitive photoreceptor cells that they support. There is currently no effective treatment for the most common form of this disease (dry AMD). A new approach to treating AMD involves the transplantation of RPE cells derived from either human embryonic or induced pluripotent stem cells. Multiple clinical trials are being initiated using a variety of cell therapies. Although many animal models are available for AMD research, most do not recapitulate all aspects of the disease, hampering progress. However, the use of cultured RPE cells in AMD research is well established and, indeed, some of the more recently described RPE-based models show promise for investigating the molecular mechanisms of AMD and for screening drug candidates. Here, we discuss innovative cell-culture models of AMD and emerging stem-cell-based therapies for the treatment of this vision-robbing disease.

Summary: Here, we discuss the emerging cell-culture models and potential stem-cell-based therapies for AMD, a blinding disorder that affects millions of people worldwide.

A local complement response by RPE causes early-stage macular degeneration

Inherited and age-related macular degenerations (AMDs) are important causes of vision loss. An early hallmark of these disorders is the formation of sub-retinal pigment epithelium (RPE) basal deposits. A role for the complement system in MDs was suggested by genetic association studies, but direct functional connections between alterations in the complement system and the pathogenesis of MD remain to be defined. We used primary RPE cells from a mouse model of inherited MD due to a p.R345W mutation in EGF-containing fibulin-like extracellular matrix protein 1 (EFEMP1) to investigate the role of the RPE in early MD pathogenesis. Efemp1(R345W) RPE cells recapitulate the basal deposit formation observed in vivo by producing sub-RPE deposits in vitro. The deposits share features with basal deposits, and their formation was mediated by EFEMP1(R345W) or complement component 3a (C3a), but not by complement component 5a (C5a). Increased activation of complement appears to occur in response to an abnormal extracellular matrix (ECM), generated by the mutant EFEMP1(R345W) protein and reduced ECM turnover due to inhibition of matrix metalloproteinase 2 by EFEMP1(R345W) and C3a. Increased production of C3a also stimulated the release of cytokines such as interleukin (IL)-6 and IL-1B, which appear to have a role in deposit formation, albeit downstream of C3a. These studies provide the first direct indication that complement components produced locally by the RPE are involved in the formation of basal deposits. Furthermore, these results suggest that C3a generated by RPE is a potential therapeutic target for the treatment of EFEMP1-associated MD as well as AMD.

Cell culture model that mimics drusen formation and triggers complement activation associated with age-related macular degeneration

We introduce a human retinal pigmented epithelial (RPE) cell-culture model that mimics several key aspects of early stage age-related macular degeneration (AMD). These include accumulation of sub-RPE deposits that contain molecular constituents of human drusen, and activation of complement leading to formation of deposit-associated terminal complement complexes. Abundant sub-RPE deposits that are rich in apolipoprotein E (APOE), a prominent drusen constituent, are formed by RPE cells grown on porous supports. Exposure to human serum results in selective, deposit-associated accumulation of additional known drusen components, including vitronectin, clusterin, and serum amyloid P, thus suggesting that specific protein-protein interactions contribute to the accretion of plasma proteins during drusen formation. Serum exposure also leads to complement activation, as evidenced by the generation of C5b-9 immunoreactive terminal complement complexes in association with APOE-containing deposits. Ultrastructural analyses reveal two morphologically distinct forms of deposits: One consisting of membrane-bounded multivesicular material, and the other of nonmembrane-bounded particle conglomerates. Collectively, these results suggest that drusen formation involves the accumulation of sub-RPE material rich in APOE, a prominent biosynthetic product of the RPE, which interacts with a select group of drusen-associated plasma proteins. Activation of the complement cascade appears to be mediated via the classical pathway by the binding of C1q to ligands in APOE-rich deposits, triggering direct activation of complement by C1q, deposition of terminal complement complexes and inflammatory sequelae. This model system will facilitate the analysis of molecular and cellular aspects of AMD pathogenesis, and the testing of new therapeutic agents for its treatment.

Mechanical force enhances MMP-2 activation via p38 signaling pathway in human retinal pigment epithelial cells

BACKGROUND

Rhegmatogenous retinal detachment and proliferative vitreoretinopathy (PVR) are eye diseases that are characterized by mechanical stress involving stretching of the retinal pigment epithelial (RPE) cells by the vitreous or the hyperplastic membranes. Here, we assessed whether mechanical force could change the expression of matrix metalloproteinases (MMPs) in RPE cells via the mitogen-activated protein kinase (MAPK) pathway.

METHODS

Collagen-coated magnetite beads and magnetic fields were used to apply tensile forces to cultured RPE cells at focal adhesions. Activation of the MAPK, including extracellular signal-regulated protein kinase (ERK), c-jun N-terminal kinase (JNK), and p38 were determined over a time course from 5 to 30 min by Western-blot analysis. Activation of p38 was also tested using immunofluorescence staining. The mRNA levels of MMP-2, MMP-9, tissue inhibitor of MMP (TIMP)-2 and fibronectin (FN) were analyzed by RT-PCR. Active MMP-2 and MMP-9 were demonstrated by zymography. MMP-2 secretion was evaluated by enzyme immunoassay.

RESULTS

Stimulation of RPE cells with mechanical stress did not change the total protein expression of the MAPK proteins ERK, JNK, and p38. However, of the three kinases, only active p38 showed an increased protein expression which was also shown by a 2.8-fold increase in immunofluorescence staining at 5 min following mechanical stress stimulation. This increase in active p38 expression was blocked by treating the cells with the p38 inhibitor SB203580. FN mRNA increased 2.4-fold at 15 min and MMP-2 mRNA increased 2.1-fold at 4 h. MMP-2 secretion increased 1.5-fold at 4 h and 1.9-fold at 12 h. The expression of MMP-2 and FN, and the activation and secretion of MMP-2, were inhibited in the presence of SB203580. The mRNA expression of MMP-9 and TIMP-2 did not change throughout.

CONCLUSIONS

This study shows that mechanical stress upregulates MMP-2 and FN expression through activation of the p38 pathway. The increase in MMP-2 levels evoked by mechanical force may contribute to the remodeling of the extracellular matrix around RPE cells, weakening the interlinkage and membrane attachment between RPE cells, and facilitate cellular migration.

Distribution and composition of esterified and unesterified cholesterol in extra-macular drusen

More details about the distribution of esterified and unesterified cholesterol (EC, UC), abundant druse components, would inform models of druse biogenesis and new technologies for ocular imaging. From donors with grossly normal maculas (n=10, 66-86years), whose eyes were preserved in paraformaldehyde within 6h of death, extra-macular drusen encased with retinal pigment epithelium (RPE) were isolated manually. Cryosections of pelleted drusen, stained with filipin for UC and EC, were used to investigate filipin staining patterns within single drusen (n=193) and to quantify fluorescence (n=146). From lipid extracts of other drusen/RPE and RPE samples, total cholesterol (TC) and UC were determined by enzymatic fluorimetry. Drusen contained cores, basally located regions that were intensely bright when stained for UC or deeply dark when stained for EC; many were surrounded by concentric lamellae. Within the same cores, the EC-poor regions were significantly smaller (13.0mum) than UC-rich regions (17.1mum). Drusen with highly fluorescent EC-rich shells lacked UC-rich shells. Small spots representing lakes were visible only in drusen stained for EC. Some drusen had small, refractive spherical inclusions lacking both UC and EC. Of drusen examined, 32% had a UC-rich core, 35% had an EC-poor core, 31% had an EC-rich shell, 25% had EC-rich lakes, and 4-5% had UC-, EC-poor inclusions. Shells and cores occurred in significantly non-overlapping druse populations. The percentage of TC that was esterified ranged from 32-66% for drusen/RPE and 5-21% for RPE. The disposition of cholesterol in cores may reflect the activity of invading cellular process. The greater size of UC-rich cores relative to EC-poor cores may reflect a declining gradient of enzymatic activity with increased radial distance from the putative invaders. The relative sizes of sub-domains defined by cholesterol composition are compared to sub-domains detected in drusen by in vivo imaging methods.

Epigenetic regulation of clusterin/apolipoprotein J expression in retinal pigment epithelial cells

Age-related macular degeneration (AMD) is the leading cause of blindness worldwide. AMD is characterized by the deposition of drusen aggregates under the retinal pigment epithelium (RPE). Clusterin/apo J, a multifunctional secreted chaperone, is one of the major proteins accumulating in drusen deposits. The regulation of clusterin expression is not well characterized but the promoter of clusterin contains a CpG-rich methylation domain. Since aging affects both DNA methylation and histone acetylation status, the epigenetic regulation might have an important role in clusterin/apo J expression. Our purpose was to elucidate whether the induction of DNA hypomethylation with 5-aza-2'-deoxycytidine (AZA) and histone hyperacetylation with trichostatin A (TSA) could affect the clusterin transcription, protein levels, and secretion in retinal pigment epithelial cells. We observed that both TSA and AZA treatments induced a prominent increase in the expression levels of clusterin mRNA and protein in ARPE-19 cells, as well as in the secretion of clusterin protein. Furthermore, valproic acid, an antiepileptic drug and a recently identified inhibitor of histone deacetylases (HDAC), induced a significant increase in clusterin protein expression and secretion in retinal pigment epithelial cells. HDAC inhibitors are characterized as inhibitors of angiogenesis, and clusterin as a complement inhibitor. Our results indicate that epigenetic factors regulate the clusterin expression of RPE cells and thus might affect the pathogenesis of AMD via the inhibition of angiogenesis and inflammation.

Cell culture models of the ocular barriers

The presence of tight barriers, which regulate the environment of ocular tissues in the anterior and posterior part of the eye, is essential for normal visual function. The development of strategies to overcome these barriers for the targeted ocular delivery of drugs, e.g. to the retina, remains a major challenge. During the last years numerous cell culture models of the ocular barriers (cornea, conjunctiva, blood-retinal barrier) have been established. They are considered to be promising tools for studying the drug transport into ocular tissues, and for numerous other purposes, such as the investigation of pathological ocular conditions, and the toxicological screening of compounds as alternative to in vivo toxicity tests. The further development of these in vitro models will require more detailed investigations of the barrier properties of both the cell culture models and the in vivo ocular barriers. It is the aim of this review to describe the current status in the development of cell culture models of the ocular barriers, and to discuss the applicability of these models in pharmaceutical research.

Esterified and unesterified cholesterol in drusen and basal deposits of eyes with age-related maculopathy

To address the potential for an outer segment (OS) contribution to the sub-retinal pigment epithelium (RPE) lesions of age-related maculopathy (ARM), we quantified esterified and unesterified cholesterol (EC, UC) with the sterol-specific fluorescent probe filipin in cryosections of ARM eyes. Twenty six eyes from 20 donors were preserved <5 hr after death in 4% paraformaldehyde (n = 16) or 2.5% glutaraldehyde/1% paraformaldehyde (n = 10). Eyes had exudative late ARM (n = 6), geographic atrophy (n = 15), and drusen > or =125 microm (n = 11). Sections were stained with filipin for UC or were extracted and hydrolysed with cholesterol esterase before filipin staining for EC. Drusen varied in cholesterol content, with a rough correlation between EC and UC. Dome-shaped drusen contained distinctive, loosely packed UC-rich loops. In basal deposits, EC and UC were more prominent near Bruch's membrane than near the RPE. A UC-rich material was localized within the subretinal space (n = 4). Maximum filipin fluorescence due to UC was quantified in 47 lesions (19 drusen, 24 basal deposits, and 4 sub-retinal) from 12 ARM eyes and compared to OS and inner plexiform layer (IPL) of uninvolved retina in the same sections. Relative to IPL, UC fluorescence was higher in lesions (mean+/-s.d: 1.63+/-0.69) and lower in OS (0.64+/-0.18). If only the packing of membranes explained fluorescence intensity, then one would expect much higher intensities in membrane-rich OS than in lesions. Because the converse is true, the membranous material in lesions must be more highly enriched in cholesterol on a per unit area basis. UC in sub-RPE deposits cannot be derived directly from OS without considerable intracellular processing within RPE, additional cholesterol sources, or both.