Phenotypic heterogeneity of retinal pigment epithelial cells in vitro and in situ

Continuum description of confluent tissues with spatial heterogeneous activity

A continuum description is built to characterize the stationary and transient deformations of confluent tissues subject to heterogeneous activities. By defining a coarse-grained texture matrix field to represent the shape and size of cells, we derive the coarse-grained stress tensor for the vertex model. Activity in the tissue takes the form of inhomogeneous apical contractions, which can be modeled as reductions of the vertex model reference areas or perimeters representing activity in the medial and perimeter regions of the cells, respectively. For medial activity, the extra stress is just an isotropic pressure, while for perimeter activity, it also has a deviatoric component, which is aligned with the texture matrix. The predictions of the continuum description are compared with the average spatiotemporal deformations obtained in simulations of the vertex model subject to localized apical contractions, showing an excellent agreement, even if the active patch is as small as one cell. The fluctuations around the average are more prominent when the activity is in the medial region due to the lack of negative active shape feedback, which, coupled with the confluent property, increases cellular shape and size variations.

Zinc Supplementation Induced Transcriptional Changes in Primary Human Retinal Pigment Epithelium: A Single-Cell RNA Sequencing Study to Understand Age-Related Macular Degeneration

Zinc supplementation has been shown to be beneficial to slow the progression of age-related macular degeneration (AMD). However, the molecular mechanism underpinning this benefit is not well understood. This study used single-cell RNA sequencing to identify transcriptomic changes induced by zinc supplementation. Human primary retinal pigment epithelial (RPE) cells could mature for up to 19 weeks. After 1 or 18 weeks in culture, we supplemented the culture medium with 125 µM added zinc for one week. RPE cells developed high transepithelial electrical resistance, extensive, but variable pigmentation, and deposited sub-RPE material similar to the hallmark lesions of AMD. Unsupervised cluster analysis of the combined transcriptome of the cells isolated after 2, 9, and 19 weeks in culture showed considerable heterogeneity. Clustering based on 234 pre-selected RPE-specific genes divided the cells into two distinct clusters, we defined as more and less differentiated cells. The proportion of more differentiated cells increased with time in culture, but appreciable numbers of cells remained less differentiated even at 19 weeks. Pseudotemporal ordering identified 537 genes that could be implicated in the dynamics of RPE cell differentiation (FDR < 0.05). Zinc treatment resulted in the differential expression of 281 of these genes (FDR < 0.05). These genes were associated with several biological pathways with modulation of ID1/ID3 transcriptional regulation. Overall, zinc had a multitude of effects on the RPE transcriptome, including several genes involved in pigmentation, complement regulation, mineralization, and cholesterol metabolism processes associated with AMD.

Gap junctions and connexin hemichannels both contribute to the electrical properties of retinal pigment epithelium

Gap junctions are intercellular channels that permit the transfer of ions and small molecules between adjacent cells. These cellular junctions are particularly dense in the retinal pigment epithelium (RPE), and their contribution to many retinal diseases has been recognized. While gap junctions have been implicated in several aspects of RPE physiology, their role in shaping the electrical properties of these cells has not been characterized in mammals. The role of gap junctions in the electrical properties of the RPE is particularly important considering the growing appreciation of RPE as excitable cells containing various voltage-gated channels. We used a whole-cell patch clamp to measure the electrical characteristics and connectivity between RPE cells, both in cultures derived from human embryonic stem cells and in the intact RPE monolayers from mouse eyes. We found that the pharmacological blockade of gap junctions eliminated electrical coupling between RPE cells, and that the blockade of gap junctions or Cx43 hemichannels significantly increased their input resistance. These results demonstrate that gap junctions function in the RPE not only as a means of molecular transport but also as a regulator of electrical excitability.

The Effect of Antioxidants on Photoreactivity and Phototoxic Potential of RPE Melanolipofuscin Granules from Human Donors of Different Age

One of the most prominent age-related changes of retinal pigment epithelium (RPE) is the accumulation of melanolipofuscin granules, which could contribute to oxidative stress in the retina. The purpose of this study was to determine the ability of melanolipofuscin granules from younger and older donors to photogenerate reactive oxygen species, and to examine if natural antioxidants could modify the phototoxic potential of this age pigment. Electron paramagnetic resonance (EPR) oximetry, EPR-spin trapping, and time-resolved detection of near-infrared phosphorescence were employed for measuring photogeneration of superoxide anion and singlet oxygen by melanolipofuscin isolated from younger and older human donors. Phototoxicity mediated by internalized melanolipofuscin granules with and without supplementation with zeaxanthin and α-tocopherol was analyzed in ARPE-19 cells by determining cell survival, oxidation of cellular proteins, organization of the cell cytoskeleton, and the cell specific phagocytic activity. Supplementation with antioxidants reduced aerobic photoreactivity and phototoxicity of melanolipofuscin granules. The effect was particularly noticeable for melanolipofuscin mediated inhibition of the cell phagocytic activity. Antioxidants decreased the extent of melanolipofuscin-dependent oxidation of cellular proteins and disruption of the cell cytoskeleton. Although melanolipofuscin might be involved in chronic phototoxicity of the aging RPE, natural antioxidants could partially ameliorate these harmful effects.

The Petri Dish-N2B27 Culture Condition Maintains RPE Phenotype by Inhibiting Cell Proliferation and mTOR Activation

Objective

To develop a method for the rapid isolation of rat RPE cells with high yield and maintain its epithelial state in modified culture system.

Methods

The eyeballs were incubated with dispase. The retina was isolated with RPE attached and cut into several pieces. Following a brief incubation in growth medium, large RPE sheets can be harvested rapidly. RPE cells were divided into four groups and cultured for several weeks, that is, (1) in cell culture dishes with 10% FBS containing medium (CC dish-FBS), (2) in petri dishes with 10% FBS containing medium (Petri dish-FBS), (3) in cell culture dishes with N2 and B27 containing medium (CC dish-N2B27), and (4) in petri dishes with N2 and B27 containing medium (Petri dish-N2B27). Morphological and biological characteristics were investigated using light microscopy, Q-PCR, and western blot.

Results

The retina would curl inwardly during the growth medium incubation period, releasing RPE sheets in the medium. Compared with low density group (5,000 cells/cm²), RPE cells plated at high density (15,000 cells/cm²) can maintain RPE morphology for a more extended period. Meanwhile, plating RPE cells at low density significantly reduced the expression of RPE cell type-specific genes (RPE65, CRALBP, and bestrophin) and increased the expression of EMT-related genes (N-cadherin, fibronectin, and α-SMA), in comparison with the samples from the high density group. The petri dish culture condition reduced cell adhesion and thus inhibited RPE cell proliferation. As compared with other culture conditions, RPE cells in the petri dish-N2B27 condition could maintain RPE phenotype with increased expression of RPE-specific genes and decreased expression of EMT-related genes. The AKT/mTOR pathway was also decreased in petri dish-N2B27 condition.

Conclusion

The current study provided an alternative method for easy isolation of RPE cells with high yield and maintenance of its epithelial morphology in the petri dish-N2B27 condition.

Nanoscopic Approach to Study the Early Stages of Epithelial to Mesenchymal Transition (EMT) of Human Retinal Pigment Epithelial (RPE) Cells In Vitro

The maintenance of visual function is supported by the proper functioning of the retinal pigment epithelium (RPE), representing a mosaic of polarized cuboidal postmitotic cells. Damage factors such as inflammation, aging, or injury can initiate the migration and proliferation of RPE cells, whereas they undergo a pseudo-metastatic transformation or an epithelial to mesenchymal transition (EMT) from cuboidal epithelioid into fibroblast-like or macrophage-like cells. This process is recognized as a key feature in several severe ocular pathologies, and is mimicked by placing RPE cells in culture, which provides a reasonable and well-characterized in vitro model for a type 2 EMT. The most obvious characteristic of EMT is the cell phenotype switching, accompanied by the cytoskeletal reorganization with changes in size, shape, and geometry. Atomic force microscopy (AFM) has the salient ability to label-free explore these characteristics. Based on our AFM results supported by the genetic analysis of specific RPE differentiation markers, we elucidate a scheme for gradual transformation from the cobblestone to fibroblast-like phenotype. Structural changes in the actin cytoskeletal reorganization at the early stages of EMT lead to the development of characteristic geodomes, a finding that may reflect an increased propensity of RPE cells to undergo further EMT and thus become of diagnostic significance.

Characterization of a tissue-engineered choroid

The choroid of the eye is a vascularized and pigmented connective tissue lying between the retina and the sclera. Increasing evidence demonstrates that, beyond supplying nutrients to the outer retina, the different choroidal cells contribute to the retina's homeostasis, especially by paracrine signaling. However, the precise role of each cell type is currently unclear. Here, we developed a choroidal substitute using the self-assembly approach of tissue engineering. Retinal pigment epithelial (RPE) cells, as well as choroidal stromal fibroblasts, vascular endothelial cells and melanocytes, were isolated from human eye bank donor eyes. Fibroblasts were cultured in a medium containing serum and ascorbic acid. After six weeks, cells formed sheets of extracellular matrix (ECM), which were stacked to produce a tissue-engineered choroidal stroma (TECS). These stromal substitutes were then characterized and compared to the native choroid. Their ECM composition (collagens and proteoglycans) and biomechanical properties (ultimate tensile strength, strain and elasticity) were similar. Furthermore, RPE cells, human umbilical vein endothelial cells and choroidal melanocytes successfully repopulated the stromas. Physiological structures were established, such as a confluent monolayer of RPE cells, vascular-like structures and a pigmentation of the stroma. Our TECS thus recaptured the biophysical environment of the native choroid, and can serve as study models to understand the normal interactions between the RPE and choroidal cells, as well as their reciprocal exchanges with the ECM. This will consequently pave the way to derive accurate insight in the pathophysiological mechanisms of diseases affecting the choroid. STATEMENT OF SIGNIFICANCE: The choroid is traditionally known for supplying blood to the avascular outer retina. There has been a renewed attention directed towards the choroid partly due to its implication in the development of age-related macular degeneration (AMD), the leading cause of blindness in industrialized countries. Since AMD involves the dysfunction of the choroid/retinal pigment epithelium (RPE) complex, a three-dimensional (3D) model of RPE comprising the choroid layer is warranted. We used human choroidal cells to engineer a choroidal substitute. Our approach takes advantage of the ability of cells to recreate their own environment, without exogenous materials. Our model could help to better understand the role of each choroidal cell type as well as to advance the development of new therapeutics for AMD.

Lipofuscin-mediated photodynamic stress induces adverse changes in nanomechanical properties of retinal pigment epithelium cells

Retinal pigment epithelium (RPE) is an important part of the blood-retina barrier (BRB) that separates the retina from the choroid. Although melanin granules contribute to the mechanical stability of the BRB complex, it is unknown if the age pigment lipofuscin affects mechanical properties of the tissue. To address this issue the effect of sub-lethal photic stress mediated by phagocytized lipofuscin granules, isolated from RPE of human donors, on morphology and mechanical properties of ARPE-19 cells was investigated. Nanomechanical analysis using atomic force spectroscopy revealed that irradiation of cells containing lipofuscin granules with blue light induced significant softening of the cells, which was accompanied by substantial reorganization of the cell cytoskeleton due to peroxidation of cellular proteins. Our results indicate that lipofuscin-mediated photic stress can cause significant modification of the RPE cells with the potential to disturb biological function of the BRB complex.

Rapid differentiation of the human RPE cell line, ARPE-19, induced by nicotinamide

Human RPE cell lines, especially the ARPE-19 cell line, are widely-used in eye research, as well as general epithelial cell studies. In comparison with primary RPE cells, they offer relative convenience and consistency, but cultures derived from these lines are typically not well differentiated. We describe a simple, rapid method to establish cultures from ARPE-19 cells, with significantly improved epithelial cell morphology and cytoskeletal organization, and RPE-related functions. We identify the presence of nicotinamide, a member of the vitamin B family, as an essential factor in promoting the observed differentiation, indicating the importance of metabolism in RPE cell differentiation.

A cell culture condition that induces the mesenchymal-epithelial transition of dedifferentiated porcine retinal pigment epithelial cells

The pathological change of retinal pigment epithelial (RPE) cells is one of the main reasons for the development of age-related macular degeneration (AMD). Thus, cultured RPE cells are a proper cell model for studying the etiology of AMD in vitro. However, such cultured RPE cells easily undergo epithelial-mesenchymal transition (EMT) that results in changes of cellular morphology and functions of the cells. To restore and maintain the mesenchymal-epithelial transition (MET) of the cultured RPE cells, we cultivated dedifferentiated porcine RPE (pRPE) cells and compared their behaviors in four conditions: 1) in cell culture dishes with DMEM/F12 containing FBS (CC dish-FBS), 2) in petri dishes with DMEM/F12 containing FBS (Petri dish-FBS), 3) in cell culture dishes with DMEM/F12 containing N2 and B27 supplements (CC dish-N2B27), and 4) in petri dishes with DMEM/F12 containing N2 and B27 (Petri dish-N2B27). In addition to observing the cell morphology and behavior, RPE specific markers, as well as EMT-related genes and proteins, were examined by immunostaining, quantitative real-time PCR and Western blotting. The results showed that dedifferentiated pRPE cells maintained EMT in CC dish-FBS, Petri dish-FBS and CC dish-N2B27 groups, whereas MET was induced when the dedifferentiated pRPE cells were cultured in Petri dish-N2B27. Such induced pRPE cells showed polygonal morphology with increased expression of RPE-specific markers and decreased EMT-associated markers. Similar results were observed in induced pluripotent stem cell-derived RPE cells. Furthermore, during the re-differentiation of those dedifferentiated pRPE cells, Petri dish-N2B27 reduced the activity of RhoA and induced F-actin rearrangement, which promoted the nuclear exclusion of transcriptional co-activator with PDZ-binding motif (TAZ) and TAZ target molecule zinc finger E-box binding protein (ZEB1), both of which are EMT inducing factors. This study provides a simple and reliable method to reverse dedifferentiated phenotype of pRPE cells into epithelialized phenotype, which is more appropriate for studying AMD in vitro, and suggests that MET of other cell types might be induced by a similar approach.

Visualizing melanosomes, lipofuscin, and melanolipofuscin in human retinal pigment epithelium using serial block face scanning electron microscopy

To assess serial section block-face scanning electron microscopy (SBFSEM) for retinal pigment epithelium (RPE) ultrastructure, we determined the number and distribution within RPE cell bodies of melanosomes (M), lipofuscin (L), and melanolipofuscin (ML). Eyes of 4 Caucasian donors (16M, 32F, 76F, 84M) with unremarkable maculas were sectioned and imaged using an SEM fitted with an in-chamber automated ultramicrotome. Aligned image stacks were generated by alternately imaging an epoxy resin block face using backscattered electrons, then removing a 125 nm-thick layer. Series of 249-499 sections containing 5-24 nuclei were examined per eye. Trained readers manually assigned boundaries of individual cells and x,y,z locations of M, L, and ML. A Density Recovery Profile was computed in three dimensions for M, L, and ML. The number of granules per RPE cell body in 16M, 32F, 76F, and 84M eyes, respectively, was 465 ± 127 (mean ± SD), 305 ± 92, 79 ± 40, and 333 ± 134 for L; 13 ± 9; 6 ± 7, 131 ± 55, and 184 ± 66 for ML; and 29 ± 19, 24 ± 12, 12 ± 7, and 7 ± 3 for M. Granule types were spatially organized, with M near apical processes. The effective radius, a sphere of decreased probability for granule occurrence, was 1 μm for L, ML, and M combined. In conclusion, SBFEM reveals that adult human RPE has hundreds of L, LF, and M and that granule spacing is regulated by granule size alone. When obtained for a larger sample, this information will enable hypothesis testing about organelle turnover and regulation in health, aging, and disease, and elucidate how RPE-specific signals are generated in clinical optical coherence tomography and autofluorescence imaging.

Differential behavioral outcomes following neonatal versus fetal human retinal pigment epithelial cell striatal implants in parkinsonian rats

Following the failure of a Phase II clinical study evaluating human retinal pigment epithelial (hRPE) cell implants as a potential treatment option for Parkinson's disease, speculation has centered on implant function and survival as possible contributors to the therapeutic outcomes. We recently reported that neonatal hRPE cells, similar to hRPE cells used in the Phase II clinical study, produced short-lived in vitro and limited in vivo trophic factors, which supports that assumption. We hypothesize that the switch from fetal to neonatal hRPE cells, between the Phase I and the Phase II clinical trial may be partly responsible for the later negative outcomes. To investigate this hypothesis, we used two neonatal hRPE cell lots, prepared in a similar manner to neonatal hRPE cells used in the Phase II clinical study, and compared them to previously evaluated fetal hRPE cells for behavioral changes following unilateral striatal implantation in 6-hydroxydopamine-lesioned rats. The results showed that only fetal, not neonatal, hRPE cell implants, were able to improve behavioral outcomes following striatal implantation in the lesioned rats. These data suggest that fetal hRPE cells may be preferential to neonatal hRPE cells in restoring behavioral deficits.

The nanomechanical role of melanin granules in the retinal pigment epithelium

Nanomechanical properties of cells and tissues, in particular their elasticity, play an important role in different physiological and pathological processes. Recently, we have demonstrated that melanin granules dramatically modify nanomechanical properties of melanoma cells making them very stiff and, as a result, less aggressive. Although the mechanical effect of melanin granules was demonstrated in pathological cells, it was never studied in the case of normal cells. In this work, we analyzed the impact of melanin granules on nanomechanical properties of primary retinal pigment epithelium tissue fragments isolated from porcine eyes. The obtained results clearly show that melanin granules are responsible for the exceptional nanomechanical properties of the tissue. Our findings suggest that melanin granules in the retinal pigment epithelium may play an important role in sustaining the stiffness of this single cell layer, which functions as a natural mechanical barrier separating the retina from the choroid.

Texture Descriptors Ensembles Enable Image-Based Classification of Maturation of Human Stem Cell-Derived Retinal Pigmented Epithelium

Aims

A fast, non-invasive and observer-independent method to analyze the homogeneity and maturity of human pluripotent stem cell (hPSC) derived retinal pigment epithelial (RPE) cells is warranted to assess the suitability of hPSC-RPE cells for implantation or in vitro use. The aim of this work was to develop and validate methods to create ensembles of state-of-the-art texture descriptors and to provide a robust classification tool to separate three different maturation stages of RPE cells by using phase contrast microscopy images. The same methods were also validated on a wide variety of biological image classification problems, such as histological or virus image classification.

Methods

For image classification we used different texture descriptors, descriptor ensembles and preprocessing techniques. Also, three new methods were tested. The first approach was an ensemble of preprocessing methods, to create an additional set of images. The second was the region-based approach, where saliency detection and wavelet decomposition divide each image in two different regions, from which features were extracted through different descriptors. The third method was an ensemble of Binarized Statistical Image Features, based on different sizes and thresholds. A Support Vector Machine (SVM) was trained for each descriptor histogram and the set of SVMs combined by sum rule. The accuracy of the computer vision tool was verified in classifying the hPSC-RPE cell maturation level.

Dataset and Results

The RPE dataset contains 1862 subwindows from 195 phase contrast images. The final descriptor ensemble outperformed the most recent stand-alone texture descriptors, obtaining, for the RPE dataset, an area under ROC curve (AUC) of 86.49% with the 10-fold cross validation and 91.98% with the leave-one-image-out protocol. The generality of the three proposed approaches was ascertained with 10 more biological image datasets, obtaining an average AUC greater than 97%.

Conclusions

Here we showed that the developed ensembles of texture descriptors are able to classify the RPE cell maturation stage. Moreover, we proved that preprocessing and region-based decomposition improves many descriptors’ accuracy in biological dataset classification. Finally, we built the first public dataset of stem cell-derived RPE cells, which is publicly available to the scientific community for classification studies. The proposed tool is available at https://www.dei.unipd.it/node/2357 and the RPE dataset at http://www.biomeditech.fi/data/RPE_dataset/. Both are available at https://figshare.com/s/d6fb591f1beb4f8efa6f.

Cell culture of retinal pigment epithelium: Special Issue

This series of review articles highlights how in vitro models of RPE can be effectively used to understand essential functions of the RPE that are not only fundamental to epithelial biology, but also have direct relevance to the visual system. The issue contains reviews from experts in the field covering aspects of basic cell and epithelial biology, namely: the barrier properties of the RPE (Rizzolo, 2014), epithelial polarity (Lehmann et al., 2014), cytoskeleton (Bonilha, 2014), and lysosomes (Guha et al., 2014), as well as properties more unique to the RPE, e.g., vitamin A metabolism (Hu and Bok, 2014), bioenergetics (Adijanto and Philp, 2014), phagocytosis (Mazzoni et al., 2014), ion transport (Reichhart and Strauß, 2014), and melanin/lipofuscin (Boulton, 2014).

Cell models to study regulation of cell transformation in pathologies of retinal pigment epithelium

The retinal pigment epithelium (RPE) plays a key role in the development of many eye diseases leading to visual impairment and even blindness. Cell culture models of pathological changes in the RPE make it possible to study factors responsible for these changes and signaling pathways coordinating cellular and molecular mechanisms of cell interactions under pathological conditions. Moreover, they give an opportunity to reveal target cells and develop effective specific treatment for degenerative and dystrophic diseases of the retina. In this review, data are presented on RPE cell sources for culture models, approaches to RPE cell culturing, phenotypic changes of RPE cells in vitro, the role of signal pathways, and possibilities for their regulation in pathological processes.

Photic injury to cultured RPE varies among individual cells in proportion to their endogenous lipofuscin content as modulated by their melanosome content

PURPOSE

We determined whether photic stress differentially impairs organelle motility of RPE lipofuscin and melanin granules, whether lethal photic stress kills cells in proportion to lipofuscin abundance, and whether killing is modulated by melanosome content.

METHODS

Motility of endogenous lipofuscin and melanosome granules within the same human RPE cells in primary culture was quantified by real-time imaging during sublethal blue light irradiation. Cell death during lethal irradiation was quantified by dynamic imaging of the onset of nuclear propidium iodide fluorescence. Analyzed were individual cells containing different amounts of autofluorescent lipofuscin, or similar amounts of lipofuscin and a varying content of phagocytized porcine melanosomes, or phagocytized black latex beads (control for light absorbance).

RESULTS

Lipofuscin granules and melanosomes showed motility slowing with mild irradiation, but slowing was greater for lipofuscin. On lethal irradiation, cell death was earlier in cells with higher lipofuscin content, but delayed by the copresence of melanosomes. Delayed death did not occur with black beads, suggesting that melanosome protection was due to properties of the biological granule, not simple screening.

CONCLUSIONS

Greater organelle motility slowing of the more photoreactive lipofuscin granule compared to melanosomes suggests that lipofuscin mediates mild photic injury within RPE cells. With lethal light stress endogenous lipofuscin mediates killing, but the effect is cell autonomous and modulated by coincident melanosome content. Developing methods to quantify the frequency of individual cells with combined high lipofuscin and low melanosome content may have value for predicting the photic stress susceptibility of the RPE monolayer in situ.

Retinal pigment epithelium (RPE) cytoskeleton in vivo and in vitro

The retinal pigment epithelium (RPE) constitutes a monolayer of cuboidal cells that interact apically with the interphotoreceptor matrix (IPM) and outer segments of the photoreceptor cells and basally with the subjacent Bruch's membrane. This highly polarized structure is maintained by the cytoskeleton of individual cells and their interactions at the basolateral junctional complexes that stabilize this epithelial structure. This RPE complex network of filaments, tubules and associated proteins is modeled by the cellular environment, the RPE intercellular interactions, and by its interactions with the extracellular matrix. This is a review of the key features of the RPE cytoskeleton in vivo and in vitro.

Montmorillonite clay based polyurethane nanocomposite as substrate for retinal pigment epithelial cell growth

The subretinal transplantation of retinal pigment epithelial cells (RPE cells) grown on polymeric supports may have interest in retinal diseases affecting RPE cells. In this study, montmorillonite based polyurethane nanocomposite (PU-NC) was investigated as substrate for human RPE cell growth (ARPE-19 cells). The ARPE-19 cells were seeded on the PU-NC, and cell viability, proliferation and differentiation were investigated. The results indicated that ARPE-19 cells attached, proliferated onto the PU-NC, and expressed occludin. The in vivo ocular biocompatibility of the PU-NC was assessed by using the HET-CAM; and through its implantation under the retina. The direct application of the nanocomposite onto the CAM did not compromise the vascular tissue in the CAM surface, suggesting no ocular irritancy of the PU-NC film. The nanocomposite did not elicit any inflammatory response when implanted into the eye of rats. The PU-NC may have potential application as a substrate for RPE cell transplantation.

The culture and maintenance of functional retinal pigment epithelial monolayers from adult human eye

The retinal pigment epithelium (RPE) is implicated in many eye diseases, including age-related macular degeneration, and therefore isolating and culturing these cells from recently deceased adult human donors is the ideal source for disease studies. Adult RPE could also be used as a cell source for transplantation therapy for RPE degenerative disease, likely requiring first in vitro expansion of the cells obtained from a patient. Previous protocols have successfully extracted RPE from adult donors; however improvements in yield, cell survival, and functionality are needed. We describe here a protocol optimized for adult human tissue that yields expanded cultures of RPE with morphological, phenotypic, and functional characteristics similar to freshly isolated RPE. These cells can be expanded and cultured for several months without senescence, gross cell death, or undergoing morphological changes. The protocol takes around a month to obtain functional RPE monolayers with accurate morphological characteristics and normal protein expression, as shown through immunohistochemistry analysis, RNA expression profiles via quantitative PCR (qPCR), and transepithelial resistance (TER) measurements. Included in this chapter are steps used to extract RPE from human adult globes, cell culture, cell splitting, cell bleaching, immunohistochemistry, and qPCR for RPE markers, and TER measurements as functional test.

Adult human RPE can be activated into a multipotent stem cell that produces mesenchymal derivatives

The retinal pigment epithelium (RPE) is a monolayer of cells underlying and supporting the neural retina. It begins as a plastic tissue, capable, in some species, of generating lens and retina, but differentiates early in development and remains normally nonproliferative throughout life. Here we show that a subpopulation of adult human RPE cells can be activated in vitro to a self-renewing cell, the retinal pigment epithelial stem cell (RPESC) that loses RPE markers, proliferates extensively, and can redifferentiate into stable cobblestone RPE monolayers. Clonal studies demonstrate that RPESCs are multipotent and in defined conditions can generate both neural and mesenchymal progeny. This plasticity may explain human pathologies in which mesenchymal fates are seen in the eye, for example in proliferative vitroretinopathy (PVR) and phthisis bulbi. This study establishes the RPESC as an accessible, human CNS-derived multipotent stem cell, useful for the study of fate choice, replacement therapy, and disease modeling.

Polyurethanes as supports for human retinal pigment epithelium cell growth

PURPOSE

The transplant of retinal pigment epithelium (RPE) cells on supports may well be an effective therapeutic approach to improve the visual results of patients with age-related macular degeneration. In this study, two biodegradable polyurethanes were investigated as supports for human RPE cells (ARPE-19).

METHODS

Polyurethane aqueous dispersions based on poly(caprolactone) and/or poly(ethylene glycol) as soft segments, and isophorone diisocyanate and hydrazine as hard segments were prepared. Polyurethane films were produced by casting the dispersions and allowing them to dry at room temperature for one week. The ARPE-19 cells were seeded onto the polyurethane films and they were investigated as supports for in vitro adhesion, proliferation, and uniform distribution of differentiated ARPE-19 cells. Additionally, the in vivo ocular biocompatibility of the polyurethane films was evaluated.

RESULTS

The RPE adhered to and proliferated onto the polyurethane supports, thus establishing cell-PUD surface interactions. Upon confluence, the cells formed an organized monolayer, exhibited a polygonal appearance, and displayed actin filaments which ran along the upper cytoplasm. At 15 days of seeding, the occluding expression was confirmed between adjacent cells, representing the barrier functionality of epithelial cells on polymeric surfaces and the establishment of cell-cell interactions. Results from the in vivo study indicated that polyurethanes exhibited a high degree of short-term intraocular biocompatibility.

CONCLUSIONS

Biodegradable polyurethane films display the proper mechanical properties for an easy transscleral-driven subretinal implantation and can be considered as biocompatible supports for a functional ARPE-19 monolayer.

Integration of tight junctions and claudins with the barrier functions of the retinal pigment epithelium

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier by regulating the movement of solutes between the fenestrated capillaries of the choroid and the photoreceptor layer of the retina. Blood-tissue barriers use various mechanisms to accomplish their tasks including membrane pumps, transporters, and channels, transcytosis, metabolic alteration of solutes in transit, and passive but selective diffusion. The last category includes tight junctions, which regulate transepithelial diffusion through the spaces between neighboring cells of the monolayer. Tight junctions are extraordinarily complex structures that are dynamically regulated. Claudins are a family of tight junctional proteins that lend tissue specificity and selectivity to tight junctions. This review discusses how the claudins and tight junctions of the RPE differ from other epithelia and how its functions are modulated by the neural retina. Studies of RPE-retinal interactions during development lend insight into this modulation. Notably, the characteristics of RPE junctions, such as claudin composition, vary among species, which suggests the physiology of the outer retina may also vary. Comparative studies of barrier functions among species should deepen our understanding of how homeostasis is maintained in the outer retina. Stem cells provide a way to extend these studies of RPE-retinal interactions to human RPE.

Nicotine increases the VEGF/PEDF ratio in retinal pigment epithelium: a possible mechanism for CNV in passive smokers with AMD

PURPOSE

Cigarette smoking is the strongest environmental risk factor for wet age-related macular degeneration (AMD). Inappropriate expression of proangiogenic vascular endothelial growth factor (VEGF) and antiangiogenic pigment epithelium derived factor (PEDF) may cause choroidal neovascularization (CNV), a key event in wet AMD, resulting in vision loss. Nicotine (NT), a potent angiogenic agent abundant in second-hand smoke, may play a major role in the pathogenesis of wet AMD. The purpose of this study was to evaluate the expression of nicotinic acetylcholine receptors (nAchR) in retinal pigment epithelium (RPE) and determine the effects of NT on RPE-derived VEGF and PEDF expression in the context of passive smoking.

METHODS

Human RPE cells were treated with NT (10(-8) M), with or without the nAchR-nonspecific antagonist hexamethonium (HXM) (10(-5) M) for 72 hours. RPE sheets were microdissected from rats exposed to NT in drinking water (100 μg/mL), with or without HXM (40 mg/kg/d, intraperitoneally), for 72 hours. Cell death was determined by cell count and proliferation by Western blot for proliferating cell nuclear antigen (PCNA). nAchR expression was examined by real-time PCR and Western blot. ERK activation was evaluated by Western blot analysis. VEGF and PEDF expression was assessed by ELISA, Western blot, and real-time PCR.

RESULTS

Cultured RPE cells constitutively expressed the nAchR α3, α10, and β1 subunits, with β1 being the most prevalent. The nAchR α4, α5, α7, and β2 subunits were detected in RPE sheets from rats, among which α4 is the predominant subtype. NT, which did not result in either cell death or proliferation, induced β1 nAchR, upregulated VEGF, and downregulated PEDF expression through nAChR in ARPE-19 cells. Transcriptional activation of the nAchR α4 subunit and nAChR-mediated upregulation of VEGF and PEDF were observed in RPE from rats exposed to NT.

CONCLUSIONS

NT increased the VEGF-to-PEDF ratio in the RPE through nAchR in vitro and in vivo. This alteration in the ratio may play a key role in the progression to wet AMD in passive smokers.

Equivalence of conventionally-derived and parthenote-derived human embryonic stem cells

Background

As human embryonic stem cell (hESC) lines can be derived via multiple means, it is important to determine particular characteristics of individual lines that may dictate the applications to which they are best suited. The objective of this work was to determine points of equivalence and differences between conventionally-derived hESC and parthenote-derived hESC lines (phESC) in the undifferentiated state and during neural differentiation.

Methodology/Principal Findings

hESC and phESC were exposed to the same expansion conditions and subsequent neural and retinal pigmented epithelium (RPE) differentiation protocols. Growth rates and gross morphology were recorded during expansion. RTPCR for developmentally relevant genes and global DNA methylation profiling were used to compare gene expression and epigenetic characteristics. Parthenote lines proliferated more slowly than conventional hESC lines and yielded lower quantities of less mature differentiated cells in a neural progenitor cell (NPC) differentiation protocol. However, the cell lines performed similarly in a RPE differentiation protocol. The DNA methylation analysis showed similar general profiles, but the two cell types differed in methylation of imprinted genes. There were no major differences in gene expression between the lines before differentiation, but when differentiated into NPCs, the two cell types differed in expression of extracellular matrix (ECM) genes.

Conclusions/Significance

These data show that hESC and phESC are similar in the undifferentiated state, and both cell types are capable of differentiation along neural lineages. The differences between the cell types, in proliferation and extent of differentiation, may be linked, in part, to the observed differences in ECM synthesis and methylation of imprinted genes.

Sublethal photic stress and the motility of RPE phagosomes and melanosomes

PURPOSE

To determine whether sublethal oxidative stress to the retinal pigment epithelium by visible light treatment affects the translocation of organelles, notably phagosomes and melanosomes.

METHODS

Isolated porcine melanosomes were phagocytized by ARPE-19 cells, then cultures were treated with blue light to generate reactive oxygen intermediates (ROIs) by endogenous retinal pigment epithelial (RPE) chromophores throughout the cytoplasm. Other melanosomes were preloaded with a photosensitizer before phagocytosis, and cells were light treated to generate ROIs specifically at the granule surface. Phagosome movement was analyzed by live cell imaging. Also analyzed were phagocytized black latex beads, phagocytized melanosomes pretreated to simulate age-related melanin photobleaching, and endogenous RPE melanosomes in primary cultures of porcine retinal pigment epithelium.

RESULTS

Sublethal blue light treatment slowed the movement of some, but not all, phagocytized melanosomes. All phagosomes slowed when ROIs were generated near the organelles through a photosensitized reaction. Melanosome photobleaching, which makes granules more photoreactive, increased the effects of blue light. Blue light treatment also slowed the motility of phagosomes containing latex beads and endogenous pigment granules.

CONCLUSIONS

Blue light-induced stress impairs phagosome motility in RPE cells but affects individual organelles differently, suggesting that the effects of mild oxidative injury vary with subcellular location. The mechanisms underlying slowed motility are at least partially local because slowing can be induced by a photosensitized reaction in the subdomain of the organelle and the magnitude of the slowing is greater when the phagosome contents are photoreactive. Photic stress may impair the movement and positioning of RPE organelles, which would have widespread consequences for maintaining a functionally efficient subcellular organization.

Epithelial phenotype and the RPE: is the answer blowing in the Wnt?

Cells of the human retinal pigment epithelium (RPE) have a regular epithelial cell shape within the tissue in situ, but for reasons that remain elusive the RPE shows an incomplete and variable ability to re-develop an epithelial phenotype after propagation in vitro. In other epithelial cell cultures, formation of an adherens junction (AJ) composed of E-cadherin plays an important early inductive role in epithelial morphogenesis, but E-cadherin is largely absent from the RPE. In this review, the contribution of cadherins, both minor (E-cadherin) and major (N-cadherin), to RPE phenotype development is discussed. Emphasis is placed on the importance for future studies of actin cytoskeletal remodeling during assembly of the AJ, which in epithelial cells results in an actin organization that is characteristically zonular. Other markers of RPE phenotype that are used to gauge the maturation state of RPE cultures including tissue-specific protein expression, protein polarity, and pigmentation are described. An argument is made that RPE epithelial phenotype, cadherin-based cell-cell adhesion and melanization are linked by a common signaling pathway: the Wnt/beta-catenin pathway. Analyzing this pathway and its intersecting signaling networks is suggested as a useful framework for dissecting the steps in RPE morphogenesis. Also discussed is the effect of aging on RPE phenotype. Preliminary evidence is provided to suggest that light-induced sub-lethal oxidative stress to cultured ARPE-19 cells impairs organelle motility. Organelle translocation, which is mediated by stress-susceptible cytoskeletal scaffolds, is an essential process in cell phenotype development and retention. The observation of impaired organelle motility therefore raises the possibility that low levels of stress, which are believed to accompany RPE aging, may produce subtle disruptions of cell phenotype. Over time these would be expected to diminish the support functions performed by the RPE on behalf of photoreceptors, theoretically contributing to aging retinal disease such as age-related macular degeneration (AMD). Analyzing sub-lethal stress that produces declines in RPE functional efficiency rather than overt cell death is suggested as a useful future direction for understanding the effects of age on RPE organization and physiology. As for phenotype and pigmentation, a role for the Wnt/beta-catenin pathway is also suggested in regulating the RPE response to oxidative stress. Exploration of this pathway in the RPE therefore may provide a unifying strategy for advancing our understanding of both RPE phenotype and the consequences of mild oxidative stress on RPE structure and function.

Transplantation of the RPE in AMD

The retinal pigment epithelium (RPE) maintains retinal function as the metabolic gatekeeper between photoreceptors (PRs) and the choriocapillaries. The RPE and Bruch's membrane (BM) suffer cumulative damage over lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals. Unlike palliative pharmacologic treatments, replacement of the RPE has a curative potential for AMD. This article reviews mechanisms leading to RPE dysfunction in aging and AMD, laboratory studies on RPE transplantation, and surgical techniques used in AMD patients. Future strategies using ex vivo steps prior to transplantation, BM prosthetics, and stem cell applications are discussed. The functional peculiarity of the macular region, epigenetic phenomena leading to an age-related shift in protein expression, along with the accumulation of lipofuscin may affect the metabolism in the central RPE. Thickening of BM with age decreases its hydraulic conductivity. Drusen are deposits of extracellular material and formed in part by activation of the alternative complement pathway in individuals carrying a mutant allele of complement factor H. AMD likely represents an umbrella term for a disease entity with multifactorial etiology and manifestations. Presently, a slow progressing (dry) non-neovascular atrophic form and a rapidly blinding neovascular (wet) form are discerned. No therapy is currently available for the former, while RPE transplantation and promising (albeit non-causal) anti-angiogenic therapies are available for the latter. The potential of RPE transplantation was demonstrated in animal models. Rejection of allogeneic homologous transplants in patients focused further studies on autologous sources. In vitro studies elucidated cell adhesion and wound healing mechanisms on aged human BM. Currently, autologous RPE, harvested from the midperiphery, is being transplanted as a cell suspension or a patch of RPE and choroid in AMD patients. These techniques have been evaluated from several groups. Autologous RPE transplants may have the disadvantage of carrying the same genetic information that may have led to AMD manifestation. An intermittent culturing step would allow for in vitro therapy of the RPE, its rejuvenation and prosthesis of BM to improve the success RPE transplants. Recent advances in stem cell biology when combined with lessons learned from studies of RPE transplantation are intriguing future therapeutic modalities for AMD patients.

Characterization of barrier properties and inducible VEGF expression of several types of retinal pigment epithelium in medium-term culture

PURPOSE

To investigate and compare the characteristics of four different types of retinal pigment epithelium (RPE) cells cultured for 2 to 5 weeks to provide guidance when choosing RPE cells for experimentation.

METHODS

Human cell lines ARPE-19 (ARPE) and D407, primary RPE cells from C57Bl/6 mouse (mRPE), and primary human fetal RPE (hfRPE) cells were grown in respective media previously reported to be optimal for each cell type. Two methods to obtain hfRPE were used: one isolated outside and transported to our laboratory, and one isolated primarily within our laboratory from donor human fetal eyes. Barrier function was determined by transepithelial electrical resistance (TER) and permeability and structure by localization of Na+,K+-ATPase alpha-1, ZO-1, and actin. VEGF expression, determined by real-time polymerase chain reaction (PCR) for mRNA and ELISA for protein, was determined after exposure to 24 h of 1% oxygen. Madin-Darby canine kidney (MDCK) cells were compared as a non-RPE epithelial cell line.

RESULTS

ARPE at passage 15, but not passage 32, maintained steady low TER measurements (up to 30 ohms x cm(2)) despite forming a monolayer with apical Na+,K+-ATPase alpha-1 labeling after 35 days. mRPE developed and maintained a TER of 30 ohms x cm(2) for 2 weeks but did not localize ATPase. hfRPE showed two phenotypes. hfRPE isolated remotely and sent to us appeared more mesenchymal and undifferentiated (hfRPE-U) and had a slow but steady increase in measured TER to approximately 25 ohms x cm(2), whereas hfRPE isolated from donor eyes in our laboratory showed well-differentiated monolayers (hfRPE-D) with TER measurements > 500 ohms x cm(2) within 1 month of culture. TER measurements reflected permeability determined by the measurement of paracellular movement of sodium fluorescein. All human RPE cell types showed expression of VEGF mRNA and protein, and expression was upregulated by hypoxia in hfRPE and D407, but not in ARPE, which had constitutively high expression. ARPE expressed high levels of VEGF protein in media and cell lysates (777.2; 54.4 pg/mg protein, respectively), whereas hfRPE and D407 produced significantly less (media: 5.7 [p = 0.001], 323.6 pg/mg protein [p = 0.01]; lysate: 0 [p < 0.001], 3.5 pg/mg protein [p < 0.001], respectively).

CONCLUSIONS

Primary RPE cells and those from cell lines had different responses to medium-term culture or hypoxic stress. Primary isolation of hfRPE cells with careful control of culture conditions to assure adequate differentiation is recommended when using this cell as an example of a highly polarized epithelium. For disease, use of RPE cells that do not require long-term culture are more efficient and may be more relevant to study certain pathologies.

Effect of photodynamic therapy on the function of the outer blood-retinal barrier in an in vitro model

BACKGROUND

Photodynamic therapy (PDT) is a well established clinical treatment for age-related macular degeneration (AMD), and comprises intravenous injection of verteporfin and subsequent application of a non-thermal laser beam to the area of AMD to induce selective vascular occlusion. Since there is evidence that PDT may cause outer blood-retinal barrier (BRB) breakdown and possibly RPE cell alteration, we investigated the effect of PDT on the BRB function of the RPE in an in vitro model.

METHODS

Twenty-one monolayers of human RPE cells were cultured on semipermeable membranes until a stable barrier function was achieved as determined by transepithelial electrical resistance (TER) and sodium fluorescein permeability. To test the effect of PDT on the outer BRB function, non-thermal laser (692 nm), verteporfin or a combination of both were applied. TER assessment prior to and after PDT was utilized to identify changes in barrier function of the RPE in this in vitro model. Finally, monolayers of RPE cells were evaluated by transmission electron microscopy (TEM).

RESULTS

No significant TER decrease was observed after application of non-thermal laser alone or after administration of verteporfin in therapeutic concentrations, but combination of these modalities resulted in significantly decreased TER within 4 h. Except for intercellular blisters, no damage to the RPE was evident in TEM. Verteporfin added at concentrations higher than therapeutic doses (2 mg/ml) resulted in an immediate decrease in TER and damage to the RPE cells.

CONCLUSION

The combination of a therapeutic concentration of verteporfin and application of non-thermal laser resulted in a morphologically and functionally detectable breakdown of the outer BRB function of the RPE without any damage to the RPE cells themselves in vitro. However, increasing the concentration of verteporfin can result in RPE cell damage.

Potentiation of ATP-induced Ca2+ mobilisation in human retinal pigment epithelial cells

Interaction of signalling pathways directs the functional output of many cells. This study investigated the consequences of activating adenosine and adrenergic receptors on ATP-induced Ca2+ responses in human retinal pigment epithelial (RPE) cells. Intracellular Ca2+ concentration ([Ca2+]i) of human RPE cells in primary culture was monitored using Fura-2. Cyclic adenosine monophosphate (cAMP) concentration was measured using an enzyme-linked immunosorbent assay. Both ATP and UTP (10 microM) increased [Ca2+]i in human RPE cells. Adenosine (10 nM-10 microM) had no effect on resting [Ca2+]i, but potentiated a sub-threshold response to ATP (100 nM) when ATP was added in the presence of adenosine. The potentiation occurred with other G-protein receptor agonists such as acetylcholine. Potentiation persisted in Ca-free medium, but was blocked by prior application of thapsigargin. The A1 and A2 adenosine receptor antagonists, DPCPX and MRS1706 (100 nM) respectively, inhibited potentiation in 76+/-7 and 23+/-12% of cells, respectively, but the A3 antagonist MRS1191 had no effect. Conversely, agents that activate the cAMP pathway, including isoproterenol (10 microM), forskolin (10 microM), and the protein kinase A (PKA) activator Sp-cBIMPS (1 microM), potentiated the ATP-induced response in the RPE cells. Agents that are known to inhibit the production of cAMP in other systems also caused potentiation, including clonidine (10 microM) and the Gi-activator mastoparan (10 microM). Under resting conditions, cAMP concentration in RPE cells was 7.1+/-0.5 pmol mg(-1) protein. Isoproterenol (10 microM) and forskolin (10 microM) increased levels to 104.6+/-5.2 and 113.7+/-4.2 pmol mg(-1) protein, respectively, while adenosine, clonidine, and mastoparan (all 10 microM) had no significant effect on cAMP levels. These data indicate that whilst activation of A1 and A2 adenosine receptors and alpha2 and beta adrenergic receptors does not influence basal Ca2+ levels, stimulation of these receptors can potentiate Ca2+ signalling by cAMP dependent and independent mechanisms in human RPE cells.

Impaired RPE survival on aged submacular human Bruch's membrane

Resurfacing of diseased or iatrogenically damaged Bruch's membrane with healthy retinal pigment epithelium (RPE) has been proposed as adjunctive treatment for age-related macular degeneration (AMD). The purpose of this study was to determine whether cultured fetal human RPE cells can attach and differentiate on aged submacular human Bruch's membrane. Bruch's membrane was debrided to expose native RPE basement membrane, the superficial inner collagenous layer directly below the RPE basement membrane, or the deep inner collagenous layer. These are three surfaces that transplanted RPE cells will encounter in situ. Approximately 3146 cultured fetal RPE cells mm(-2) were seeded onto these three surfaces and grown in organ culture for 1, 7, or 14 days. Explants were bisected and examined histologically or analyzed with a scanning electron microscope. RPE nuclear density was measured on stained sections. Morphology and cell density were compared to cells seeded onto bovine corneal endothelial cell-extracellular matrix (BCE-ECM). In situ submacular RPE nuclear density was also measured in tissue sections of donor eyes ranging from 18 weeks gestation to 88 years of age to determine the effect of age on RPE density. Compared to cells seeded onto BCE-ECM at similar density, RPE cell coverage and cellular morphology on aged submacular human Bruch's membrane was poor at all time points. In contrast to cells on BCE-ECM, RPE cell density on Bruch's membrane decreased with time. In general, cell morphology on all three Bruch's membrane surfaces worsened by day-7 compared to day-1. Although some cells were more pigmented on RPE basement membrane and the deep inner collagenous layer at day-7, poor cellular morphology indicated the remaining cells were not well differentiated. At day-14, the cells were uniform and cuboidal on BCE-ECM, with cell density similar to that at day-7 and similar to in situ density of young donors (<age 30 years). The morphology of cells on Bruch's membrane was variable, and the nuclear density declined over time. A Bruch's membrane explant from a donor with large soft drusen showed the poorest resurfacing at day-7 in organ culture. These data indicate that aged submacular human Bruch's membrane does not support transplanted RPE survival and differentiation readily. The formation of localized RPE defects, cell death, and worsening cellular morphology on aged Bruch's membrane indicates that modification of Bruch's membrane may be necessary to prevent graft failure in AMD patients receiving RPE transplants.

Integrin α5 expression by the ARPE-19 cell line: comparison with primary RPE cultures and effect of growth medium on the α5 gene promoter strength

Primary cultures of human retinal pigment epithelium (RPE) requires young human donors with short post-mortem time and no known retinal diseases. The use of an established human RPE cell line, like ARPE-19, would be a welcomed alternative to primary cultures. This cell line retains many of the characteristics of RPE cells, including cell morphology, functional tight junctions and expression of CRALBP and RPE65. This study was conducted in order to investigate integrin alpha5 expression at both the gene and protein level in the ARPE-19 cell line and compare the results with those obtained with primary cultures of RPE cells. The potential use of this cell line as a substitute for primary cultures of RPE cells was also considered. Integrin alpha5 protein was detected on RPE and ARPE-19 cultures at different confluencies by immunofluorescence and immunoprecipitation analyses. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used to study alpha5 mRNA levels. Transient transfections were performed in order to compare alpha5 promoter strength in both types of cells. Immunofluorescence studies showed that both primary RPE and ARPE-19 cells yielded similar alpha5 staining patterns at all cell confluencies. Both immunoprecipitation and RT-PCR analyses provided evidence that sub-confluent and confluent RPE and ARPE-19 cells have similar cell surface alpha5 protein and mRNA levels whereas post-confluent cells had a marked decrease in both protein and transcript levels. ARPE-19 cells show a large increase in promoter strength compared to primary cultures. When compared to primary cultures, the cell line exhibited major differences in the way the alpha5 promoter is regulated, even if both cell types are cultured under identical conditions. This study demonstrates that primary cultures of human RPE and ARPE-19 cells show reductions in both the alpha5 protein and the mRNA when cells reach post-confluency. However, major differences have been observed in the strength of the alpha5 promoter between both cell types. We also show that culturing ARPE-19 cells in a different growth medium alters the transcriptional activity directed by the alpha5 promoter.

Products of lipid peroxidation induce missorting of the principal lysosomal protease in retinal pigment epithelium

Phagocytosis of photoreceptor outer segments (OS) by retinal pigment epithelium (RPE) is essential for OS renewal and survival of photoreceptors. Internalized, oxidatively modified macromolecules perturb the lysosomal function of the RPE and can lead to impaired processing of photoreceptor outer segments. In this study, we sought to investigate the impact of intracellular accumulation of oxidatively damaged lipid-protein complexes on maturation and distribution of cathepsin D, the major lysosomal protease in the RPE. Primary cultures of human RPE cells were treated with copper-oxidized low density lipoprotein (LDL) and then challenged with serum-coated latex beads to stimulate phagocytosis. Three observations were noted to occur in this experimental system. First, immature forms of cathepsin D (52 and 46 kDa) were exclusively associated with latex-containing phagosomes. Second, maturation of cathepsin D was severely impaired in RPE cells loaded with oxidized LDL (oxLDL) prior to the phagocytic challenge. Third, pre-treatment with oxLDL caused sustained secretion of pro-cathepsin D and the latent form of gelatinase A into the extracellular space in a dose-dependent manner. These data stimulate the hypothesis that intracellular accumulation of poorly degradable, oxidized lipid-protein cross-links, may alter the turnover of cathepsin D, causing its mistargeting into the extracellular space together with the enhanced secretion of a gelatinase.

Retinal pigment epithelium and endothelial cell interaction causes retinal pigment epithelial barrier dysfunction via a soluble VEGF-dependent mechanism

PURPOSE

To investigate the effect of endothelial cells (EC) on the barrier function of the retinal pigment epithelium (RPE).

METHODS

Primary bovine RPE were grown in solo culture or in coculture with bovine EC. Culture media of RPE were varied to develop a monolayer with stable barrier properties determined by transepithelial electrical resistance (TER) and permeability to sodium fluorescein. The effect of EC on the barrier properties of RPE was tested in contacting and non-contacting cocultures of RPE and EC. The conditioned media of cocultures were analysed for soluble vascular endothelial growth factor (VEGF) by ELISA. A neutralizing antibody to VEGF(165) was added to cocultures of RPE and EC and the TER was measured.

RESULTS

RPE had maximal barrier properties (high TER, low permeability, positive staining for barrier proteins) at day 10 that persisted until day 20. Compared to solo RPE culture, cocultivation of RPE with EC reduced RPE barrier function significantly and led to a greater release of soluble VEGF into the conditioned media (p<0.05). Neutralizing VEGF with antibody led to partial recovery of barrier properties in the coculture conditions (p<0.03).

CONCLUSIONS

Coculture of RPE with EC reduces RPE barrier properties and the reduction is, in part, mediated by soluble VEGF. EC-RPE contact-induced disruption of barrier properties occurs in ocular pathologies such as choroidal neovascularization, where EC move through Bruch's membrane and contact the RPE, leading to further exacerbation of the already compromised blood-retinal barrier.

Alginate as a new biomaterial for the growth of porcine retinal pigment epithelium

OBJECTIVE

Determine the effect of a 3-dimensional alginate matrix on the growth and differentiation of cells isolated from porcine retinal pigment epithelium (RPE).

PROCEDURES

Porcine RPE cells were harvested from enucleated eyecups, isolated by differential gravity sedimentation and cultured in either alginate alone (Group 1) or on plastic tissue culture plates followed by alginate (Group 2). Group 1 cells were cultured in alginate to evaluate the efficacy of the matrix as a culture medium. Group 2 cells were initially cultured on plastic to induce dedifferentiation. The cells were then harvested, suspended in alginate beads, and incubated for a second culture period to determine if the induced dedifferentiation was reversible.

RESULTS

The number of Group 1 cells was significantly greater (P < or = 0.01) at the end of the culture period. The amount of pigment and cell morphology of Group 1 cells at the end of the culture period was similar to that seen at initial cell isolation. The initial culture of Group 2 cells on plastic showed characteristic features of dedifferentiation marked by the loss of pigment and alterations in microscopic appearance. Secondary culture of dedifferentiated Group 2 cells in alginate beads resulted in a return to pigmentation and characteristic morphology for a majority of the cultured cells.

CONCLUSIONS

Porcine RPE cells can be propagated in alginate culture with a significant increase in cell numbers while maintaining normal morphology. Under the conditions described in the present study, the dedifferentiation of porcine RPE induced by standard in vitro culture methods is reversible.

Glutamate uptake is inhibited by tamoxifen and toremifene in cultured retinal pigment epithelial cells

The systemic drugs chloroquine and tamoxifen have caused retinal defects in human eye. The aim of our study was to investigate the effects of the amphiphilic drug tamoxifen, of its homologue toremifene, and of chloroquine on the glutamate uptake in retinal pigment epithelial (RPE) cells. Cultured human RPE cell line D407 and pig RPE cells were used in the study. Glutamate uptake was characterised and the glutamate transporters of pig RPE cells and the human RPE cell line D407 were compared to each other. The uptake of glutamate was studied using L-[3H]glutamate as a tracer. The radioactivity in the solubilised RPE was measured with a liquid scintillation counter. In the uptake experiments, the cells were exposed to the test drugs, to the selected glutamate receptor antagonists, and to the glutamate transporter inhibitors. Both RPE cell types exhibited a high-affinity transport system for glutamate. The glutamate transporter in RPE exhibited features characteristic of the uptake systems of neurotransmitters. The transport was Na+-dependent, and L- and D-aspartate were transported into the cell by the same transporter. Chloroquine had no effect on glutamate uptake, but tamoxifen and toremifene decreased the glutamate uptake of RPE cells dose-dependently both in pig RPE cells and in human RPE cell line. The IC50 values of tamoxifen and toremifene were lower for pig RPE cells, compared to the human RPE cell line D407. The glutamate uptake was a sensitive target for the effects of tamoxifen and toremifene, and disturbances in this function could be considered as one of the possible mechanisms of retinal defects.