Release of iron by human retinal pigment epithelial cells.

Retinal pigment epithelial cells, which form one aspect of the blood-retinal barrier, take up iron in association with transferrin by a typical receptor-mediated mechanism (Hunt et al., 1989. J. Cell Sci. 92:655-666). This iron is dissociated from transferrin in a low pH environment and uptake is sensitive to agents that inhibit endosomal acidification. The dissociated iron enters the cytoplasm as a low molecular weight (less than 10 kD) component and subsequently binds to ferritin. No evidence for recycling of iron in association with transferrin was found. Nevertheless, much of the iron that is taken up is recycled to the extracellular medium, primarily from the low molecular weight pool. This release of iron is not sensitive to inhibitors of energy production or of vesicular acidification but is increased up to a maximum of about 40% of the total 55Fe incorporated when cells are incubated with serum or the medium is changed. When a short loading time for 55Fe from 55Fe-transferrin is used (i.e., when the low molecular weight pool is proportionately larger), a much larger fraction of the cell-associated radiolabel is released than when longer loading times are used. The data suggest that a releasable intracellular iron pool is in equilibrium with the externalized material. The released iron may be separated into a high and a low molecular weight component. The former is similar on polyacrylamide gel electrophoresis to ferritin although it cannot be immune precipitated by anti-ferritin antibodies. The low molecular weight 55Fe which is heterogeneous in nature can be bound by external apo-transferrin and may represent a form that can be taken up by cells beyond the blood-retinal barrier.     

Human retinal pigment epithelial cells

''Human retinal pigment epithelial cells'' is the first set of guidelines on human retinal pigment epithelial cells in China, jointly drafted and agreed upon by experts from the Chinese Society for Stem Cell Research. This standard specifies technical requirements, test methods, inspection rules, instructions for usage, labelling requirements, packaging requirements, storage requirements and transportation requirements and waste disposal requirements for human retinal pigment epithelial cells, which is applicable to quality control during the process of manufacturing and testing of human retinal pigment epithelial cells. It was originally released by the Chinese Society for Cell Biology on 9 January 2021. We hope that publication of these guidelines will promote institutional establishment, acceptance and execution of proper protocols and accelerate the international standardization of human retinal pigment epithelial cells for applications.     

The molecular regulation of organelle transport in mammalian retinal pigment epithelial cells

Retinal pigment epithelial cells contain large numbers of melanosomes that can enter the apical processes extending between the outer segments of the overlying photoreceptors. Every day the distal portion of the photoreceptor outer segment is shed and phagocytosed by the retinal pigment epithelial cell. The phagosome is then transported into the cell body and the contents degraded by lysosomal enzymes. This review focuses on recent progress made in the identification of molecules that regulate the transport of melanosomes into the apical processes and the transport of phagosomes into the cell body. Myosin VIIa is a key player in both processes and, at least in the case of melanosome movement, myosin VIIa is recruited to the melanosome via the GTPase, Rab27a. The possible role played by defects in the transport of melanosomes and phagosomes in the development of retinal degenerative diseases is discussed.     

Erythropoietin protects retinal pigment epithelial cells from oxidative damage

Oxidative damage from reactive oxygen species (ROS) has been implicated in many diseases, including age-related macular degeneration, in which the retinal pigment epithelium (RPE) is considered a primary target. The aim of this study was to determine whether erythropoietin (EPO) protects cultured human RPE cells against oxidative damage and to identify the pathways that may mediate protection. EPO (1 IU/ml) significantly increased the viability of oxidant-treated RPE cells, decreased the release of the inflammatory cytokines tumor necrosis factor-α and interleukin-1β, recovered the RPE cells' barrier integrity disrupted by oxidative stress, prevented oxidant-induced cell DNA fragmentation and membrane phosphatidylserine exposure, and also reduced the levels of oxidant-induced intracellular ROS and restored cellular antioxidant potential, total antioxidant capacity, glutathione peroxidase, and superoxide dismutase and decreased malondialdehyde, the end product of lipid peroxidation. EPO inhibited caspase-3-like activity. Protection by EPO was partly dependent on the activation of Akt1 and the maintenance of the mitochondrial membrane potential. No enhanced or synergistic protection was observed during application of Z-DEVD-FMK (caspase-3 inhibitor) combined with EPO compared with cultures exposed to EPO and H₂O₂ alone. Together, these results suggest that EPO could protect against oxidative injury-induced cell death and mitochondrial dysfunction in RPE cells through modulation of Akt1 phosphorylation, mitochondrial membrane potential, and cysteine protease activity.     

Single-channel recordings from cultured human retinal pigment epithelial cells

We have applied patch-clamp techniques to on-cell and excised-membrane patches from human retinal pigment epithelial cells in tissue culture. Single-channel currents from at least four ion channel types were observed: three or more potassium-selective channels with single-channel slope conductances near 100, 45, and 25 pS as measured in on-cell patches with physiological saline in the pipette, and a relatively nonselective channel with subconductance states, which has a main-state conductance of approximately 300 pS at physiological ion concentrations. The permeability ratios, PK/PNa, measured in excised patches were 21 for the 100-pS channels, 3 for the 25-pS channels, and 0.8 for the 300-pS nonselective channel. The 45-pS channels appeared to be of at least two types, with PK/PNa's of approximately 41 for one type and 3 for the other. The potassium-selective channels were spontaneously active at all potentials examined. The average open time for these channels ranged from a few milliseconds to many tens of milliseconds. No consistent trend relating potassium-selective channel kinetics to membrane potential was apparent, which suggests that channel activity was not regulated by the membrane potential. In contrast to the potassium-selective channels, the activity of the nonselective channel was voltage dependent: the open probability of this channel declined to low values at large positive or negative membrane potentials and was maximal near zero. Single-channel conductances observed at several symmetrical KCl concentrations have been fitted with Michaelis-Menten curves in order to estimate maximum channel conductances and ion-binding constants for the different channel types. The channels we have recorded are probably responsible for the previously observed potassium permeability of the retinal pigment epithelium apical membrane.     

Lipoamide protects retinal pigment epithelial cells from oxidative stress and mitochondrial dysfunction

alpha-Lipoic acid (LA) has been widely studied as an agent for preventing and treating various diseases associated with oxidative disruption of mitochondrial functions. To investigate a related mitochondrial antioxidant, we compared the effects of lipoamide (LM), the neutral amide of LA, with LA for measures of oxidative damage and mitochondrial dysfunction in a human retinal pigment epithelial (RPE) cell line. Acrolein, a major component of cigarette smoke and a product of lipid peroxidation, was used to induce oxidative mitochondrial damage in RPE cells. Overall, using comparable concentrations, LM was more effective than LA at preventing acrolein-induced mitochondrial dysfunction and oxidative stress. Relative to LA, LM improved ATP levels, membrane potentials, and activities of mitochondrial complexes I, II, and V and dehydrogenases that had been decreased by acrolein exposure. LM reduced acrolein-induced oxidant generation, calcium levels, protein oxidation, and DNA damage to a greater degree than LA. And, total antioxidant capacity, glutathione content, glutathione S-transferase, and superoxide dismutase activities and expression of nuclear factor-E2-related factor 2 were increased by LM relative to LA. These results suggest that LM is a more potent mitochondrial-protective agent and antioxidant than LA in protecting RPE from oxidative damage.     

The angiogenic effects of exosomes secreted from retinal pigment epithelial cells on endothelial cells

Exosomes are informative microvesicles associated with intercellular communication via the transfer of many molecular constituents such as proteins, lipids, and nucleic acids; environmental changes and the cellular status around cells greatly affect exosome components. Cells of the retinal pigment epithelium (RPE) are key players in retinal homeostasis. Transforming growth factor (TGF)-β and tumour necrosis factor (TNF)-α are increased in the vitreous and retina in several retinal diseases and activate and undergo epithelial-mesenchymal transition (EMT) in RPE cells. EMT is closely associated with mechanisms of wound healing, including fibrosis and related angiogenesis; however, whether exosome components depend on the cell status, epithelium or mesenchyme and whether these exosomes have pro- or anti-angiogenic roles in the retina are unknown. We performed this study to investigate whether these EMT inducers affect the kinds of components in exosomes secreted from RPE cells and to assess their angiogenic effects. Exosomes were collected from culture media supernatants of a human RPE cell line (ARPE-19) stimulated with or without 10 ng/ml TNF-α and/or 5 ng/ml TGF-β2. NanoSight tracking analysis and immunoblot analysis using exosome markers were used to qualify harvested vesicles. Angiogenic factor microarray analysis revealed that exosomes derived from ARPE-19 cells cultured with TNF-α alone (Exo-TNF) and co-stimulated with TNF-α and TGF-β2 (Exo-CO) contained more angiogenic factors than exosomes derived from control cells (Exo-CTL) or ARPE-19 cells cultured with TGF-β2 alone (Exo-TGF). To assess the effect on angiogenesis, we performed chemotaxis, tube formation, and proliferation assays of human umbilical vein endothelial cells (HUVECs) stimulated with or without exosomes. HUVECs migrated to RPE-derived exosomes, and exosomes derived from ARPE-19 cells accelerated HUVEC tube formation. In contrast, Exo-TNF and Exo-CO reduced HUVEC proliferation. Our findings provide insight into the mechanisms underlying the relation between angiogenesis and exosomes derived from RPE cells.     

Hydroxytyrosol protects retinal pigment epithelial cells from acrolein-induced oxidative stress and mitochondrial dysfunction

Hydroxytyrosol (HTS) is a natural polyphenol abundant in olive oil. Increasing evidence indicates HTS has beneficial effect on human health for preventing various diseases. In the present study, we investigated the protective effects of HTS on acrolein-induced toxicity in human retinal pigment epithelial cell line, ARPE-19, a cellular model of smoking- and age-related macular degeneration. Acrolein, a major component of the gas phase cigarette smoke and also a product of lipid peroxidation in vivo , at 75 μmol/L for 24 h caused significant loss of cell viability, oxidative damage (increase in oxidant generation and oxidative damage to proteins and DNA, decrease in antioxidants and antioxidant enzymes, and also inactivation of the Keap1/Nrf2 pathway), and mitochondrial dysfunction (decrease in membrane potential, activities of mitochondrial complexes, viable mitochondria, oxygen consumption, and factors for mitochondrial biogenesis, and increase in calcium). Pre-treatment with HTS dose dependently and also time dependently protected the ARPE-19 cells from acrolein-induced oxidative damage and mitochondrial dysfunction. A short-term pre-treatment with HTS (48 h) required > 75 μmol/L for showing protection while a long-term pre-treatment (7 days) showed protective effect from 5 μmol/L on. The protective effect of HTS in this model was as potent as that of established mitochondria-targeting antioxidant nutrients. These results suggest that HTS is also a mitochondrial-targeting antioxidant nutrient and that dietary administration of HTS may be an effective measure in reducing and or preventing cigarette smoke-induced or age-related retinal pigment epithelial degeneration, such as age-associated macular degeneration.     

Mesd extrinsically promotes phagocytosis by retinal pigment epithelial cells

Phagocytosis is a critical process to maintain tissue homeostasis. In the retina, photoreceptor cells renew their photoexcitability by shedding photoreceptor outer segments (POSs) in a diurnal rhythm. Shed POSs are phagocytosed by retinal pigment epithelial (RPE) cells to prevent debris accumulation, retinal degeneration, and blindness. Phagocytosis ligands are the key to understanding how RPE recognizes shed POSs. Here, we characterized mesoderm development candidate 2 (Mesd or Mesdc2), an endoplasmic reticulum (ER) chaperon for low-density lipoprotein receptor-related proteins (LRPs), to extrinsically promote RPE phagocytosis. The results showed that Mesd stimulated phagocytosis of fluorescence-labeled POS vesicles by D407 RPE cells. Ingested POSs were partially degraded within 3 h in some RPE cells to dispense undegradable fluorophore throughout the cytoplasm. Internalized POSs were colocalized with phagosome biomarker Rab7, suggesting that Mesd-mediated engulfment is involved in a phagocytosis pathway. Mesd also facilitated phagocytosis of POSs by primary RPE cells. Mesd bound to unknown phagocytic receptor(s) on RPE cells. Mesd was detected in the cytoplasm, but not nuclei, of different retinal layers and is predominantly expressed in the ER-free cellular compartment of POSs. Mesd was not secreted into medium from healthy cells but passively released from apoptotic cells with increased membrane permeability. Released Mesd selectively bound to the surface of POS vesicles and apoptotic cells, but not healthy cells. These results suggest that Mesd may be released from and bind to shed POSs to facilitate their phagocytic clearance.     

A serum-free defined medium for retinal pigment epithelial cells

Human and bovine RPE cells underwent changes in morphology and culture doubling times when passaged in serum-supplemented medium (CM). Furthermore, late passage human RPE cells subcultured in CM medium increased synthesis of three acidic, 43 000–63 000 D proteins. In order to provide a controlled environment for the study of RPE cells in vitro, we have devloped a method for growing human and bovine RPE in a serum-free defined medium (DM). RPE cells grown in DM required a 24 h pretreatment with CM to allow the cells to attach and spread on the substrate. Cells grown in DM retained an epithelioid morphology, a stable culture doubling time, and similar 2-D PAGE patterns through several subculturings.     

Potassium currents in cultured rabbit retinal pigment epithelial cells

Membrane potential and ionic currents were studied in cultured rabbit retinal pigment epithelial (RPE) cells using whole-cell patch clamp and perforated-patch recording techniques. RPE cells exhibited both outward and inward voltage-dependent currents and had a mean membrane capacitance of 26±12 pF (sd, n=92). The resting membrane potential averaged ?31±15 mV (n=37), but it was as high as ?60 mV in some cells. When K⁺ was the principal cation in the recording electrode, depolarization-activated outward currents were apparent in 91% of cells studied. Tail current analysis revealed that the outward currents were primarily K⁺ selective. The most frequently observed outward K⁺ current was a voltage- and time-dependent outward current (I _K) which resembled the delayed rectifier K⁺ current described in other cells. I _K was blocked by tetraethylammonium ions (TEA) and barium (Ba²⁺) and reduced by 4-aminopyridine (4-AP). In a few cells (3–4%), depolarization to ?50 mV or more negative potentials evoked an outwardly rectifying K⁺ current (I _Kt) which showed more rapid inactivation at depolarized potentials. Inwardly rectifying K⁺ current (I _KI) was also present in 41% of cells. I _KI was blocked by extracellular Ba²⁺ or Cs⁺ and exhibited time-dependent decay, due to Na⁺ blockade, at negative potentials. We conclude that cultured rabbit RPE cells exhibit at least three voltage-dependent K⁺ currents. The K⁺ conductances reported here may provide conductive pathways important in maintaining ion and fluid homeostasis in the subretinal space.     

Copper mediates mitochondrial biogenesis in retinal pigment epithelial cells

Age related macular degeneration (AMD) is a multifactorial disease with genetic, biochemical and environmental risk factors. We observed a significant increase in copper levels in choroid-RPE from donor eyeballs with AMD. Adult retinal pigment epithelial cells (ARPE19 cells) exposed to copper in-vitro showed a 2-fold increase in copper influx transporter CTR1 and copper uptake at 50 μM concentration. Further there was 2-fold increase in cytochrome C oxidase activity and a 2-fold increase in the mRNA expression of NRF 2 with copper treatment. There was a significant increase in mitochondrial biogenesis markers PGC1β and TFAM which was confirmed by mitochondrial mass and copy number. On the contrary, in AMD choroid-RPE, the CTR1 mRNA was found to be significantly down-regulated compared to its respective controls. SCO1 and PGC1β mRNA showed an increase in choroid–RPE. Our study proposes copper to play an important role in mitochondrial biogenesis in RPE cells.     

The cytoskeleton of chick retinal pigment epithelial cells in situ

Summary Gelatin-coated slides were used to obtain en face preparations of retinal pigment epithelium (RPE) from 6-to 21-day-old chick embryos in order to study the distribution of F-actin in microfilaments (MF) and the MF-associated proteins, myosin, tropomyosin,-actinin and vinculin in situ at different stages of development by fluorescence microscopy. The epithelial sheets were fixed in formaldehyde and then extracted in a solution containing 0.1% Triton X-100. NBD-Phallacidin was used to visualize the F-actin in MF, and antisera against myosin, tropomyosin,-actinin and vinculin were used to determine the distribution of these four MF-associated proteins. F-actin, myosin, tropomyosin,-actinin and vinculin were present in cortical rings around the apical ends of the RPE cells throughout this period of development. Of these proteins, only F-actin was identified in the apical processes of RPE cells. The increase in the amount of F-actin could be followed as the length and the number of apical processes increased with age and maturation of RPE cells. F-actin was first detected in numerous short apical processes on the surface of each RPE cell on day 12. From day 12 to day 17, they were at an intermediate stage of elongation and from day 17 onward all of the RPE cells had long F-actin-containing apical processes. These results indicate that the F-actin-containing MF assemble much later in the apical processes than in the cortical rings. Also the cortical rings and apical processes of RPE cells resemble those in absorptive intestinal cells in that the cortical rings in both cell types contain MF associated with myosin, tropomyosin,-actinin and vinculin while the MF in the apical processes and microvilli lack these MF associated proteins, and both of these structures lack talin. In addition to apical processes and cortical rings, stained fibers were also observed at a level below the cortical rings. The simple and highly reproducible en face method described is useful for determining changes in the organization of cytoskeletal components and other macromolecules in RPE cells and other epithelial cells in situ.     

Proteome analysis of lipofuscin in human retinal pigment epithelial cells

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

Photocytotoxicity of lipofuscin in human retinal pigment epithelial cells.

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

The unusual microtubule polarity in teleost retinal pigment epithelial cells

In cells of the teleost retinal pigment epithelium (RPE), melanin granules disperse into the RPE cell's long apical projections in response to light onset, and aggregate toward the base of the RPE cell in response to dark onset. The RPE cells possess numerous microtubules, which in the apical projections are aligned longitudinally. Nocodazole studies have shown that pigment granule aggregation is microtubule-dependent (Troutt, L. L., and B. Burnside, 1988b Exp. Eye Res. In press.). To investigate further the mechanism of microtubule participation in RPE pigment granule aggregation, we have used the tubulin hook method to assess the polarity of microtubules in the apical projections of teleost RPE cells. We report here that virtually all microtubules in the RPE apical projections are uniformly oriented with plus ends toward the cell body and minus ends toward the projection tips. This orientation is opposite that found for microtubules of dermal melanophores, neurons, and most other cell types.     

Generation of retinal pigment epithelial cells from human embryonic stem cell-derived spherical neural masses

Dysfunction and loss of retinal pigment epithelium (RPE) are major pathologic changes observed in various retinal degenerative diseases such as aged-related macular degeneration. RPE generated from human pluripotent stem cells can be a good candidate for RPE replacement therapy. Here, we show the differentiation of human embryonic stem cells (hESCs) toward RPE with the generation of spherical neural masses (SNMs), which are pure masses of hESCs-derived neural precursors. During the early passaging of SNMs, cystic structures arising from opened neural tube-like structures showed pigmented epithelial morphology. These pigmented cells were differentiated into functional RPE by neuroectodermal induction and mechanical purification. Most of the differentiated cells showed typical RPE morphologies, such as a polygonal-shaped epithelial monolayer, and transmission electron microscopy revealed apical microvilli, pigment granules, and tight junctions. These cells also expressed molecular markers of RPE, including Mitf, ZO-1, RPE65, CRALBP, and bestrophin. The generated RPE also showed phagocytosis of isolated bovine photoreceptor outer segment and secreting pigment epithelium-derived factor and vascular endothelial growth factor. Functional RPE could be generated from SNM in our method. Because SNMs have several advantages, including the capability of expansion for long periods without loss of differentiation capability, easy storage and thawing, and no need for feeder cells, our method for RPE differentiation may be used as an efficient strategy for generating functional RPE cells for retinal regeneration therapy.