Clearance phagocytosis by the retinal pigment epithelial during photoreceptor outer segment renewal: Molecular mechanisms and relation to retinal inflammation

Mammalian photoreceptor outer segment renewal is a highly coordinated process that hinges on timed cell signaling between photoreceptor neurons and the adjacent retinal pigment epithelial (RPE). It is a strictly rhythmic, synchronized process that underlies in part circadian regulation. We highlight findings from recently developed methods that quantify distinct phases of outer segment renewal in retinal tissue. At light onset, outer segments expose the conserved "eat-me" signal phosphatidylserine exclusively at their distal, most aged tip. A coordinated two-receptor efferocytosis process follows, in which ligands bridge outer segment phosphatidylserine with the RPE receptors αvβ5 integrin, inducing cytosolic signaling toward Rac1 and focal adhesion kinase/MERTK, and with MERTK directly, additionally inhibiting RhoA/ROCK and thus enabling F-actin dynamics favoring outer segment fragment engulfment. Photoreceptors and RPE persist for life with each RPE cell in the eye servicing dozens of overlying photoreceptors. Thus, RPE cells phagocytose more often and process more material than any other cell type. Mutant mice with impaired outer segment renewal largely retain functional photoreceptors and retinal integrity. However, when anti-inflammatory signaling in the RPE via MERTK or the related TYRO3 is lacking, catastrophic inflammation leads to immune cell infiltration that swiftly destroys the retina causing blindness.

Astragaloside-IV alleviates high glucose-induced ferroptosis in retinal pigment epithelial cells by disrupting the expression of miR-138-5p/Sirt1/Nrf2

Astragaloside-IV (AS-IV) (C₄₁H₆₈O₁₄) is a high-purity natural product extracted from Astragalus, which has demonstrated biological activities. However, the effect of AS-IV on retinal pigment epithelial (RPE) cells in diabetic retinopathy (DR) remains unclear. In this study, high glucose (HG) was shown to promote ARPE-19 RPE cell death, increase the contents of reactive oxygen species (ROS) and oxidized glutathione (GSSG), and enhance lipid peroxidation density of mitochondrial membrane. In contrast, AS-IV decreased glutathione (GSH) content, mitochondria size and ridge. Addition of iron death inhibitor Ferrostatin-1 (Fer-1) to RPE cells decreased cell dead rate, thus indicating that HG-induced mitochondrial damage occurred due to ferroptosis. AS-IV alleviated HG-induced RPE cell damage. Furthermore, HG decreased levels of silent information regulator 1 (Sirt1) and nuclear factor (erythroid-derived 2)-like 2 (Nrf2) in the nucleus of RPE cells; AS-IV could alleviate these effects and increased expression of glutathione peroxidase 4 (GPX4), glutamate cysteine ligase (GCLM) and glutamate cysteine ligase catalytic subunit (GCLC), which are Nrf2 downstream genes. Mechanistically, AS-IV was shown to alleviate the effects of HG by increasing mir-138-5p expression in RPE cells and promoting expression of Sirt1 and Nrf2 in the nucleus. Transfection of mir-138-5p agonist inhibited the regulatory effects of AS-IV on Sirt1 and Nrf2, accompanied by decreased GPX4, GCLM and GCLC levels, and restoration of ferroptosis-related changes. Collectively, HG increased ferroptosis rate in RPE cells. In addition, AS-IV inhibited miR-138-5p expression, subsequently increasing Sirt1/Nrf2 activity and cellular antioxidant capacity to alleviate ferroptosis, resulting decreased cell death, which potentially inhibits the DR pathological process.

Oxidative stress induces Z-DNA-binding protein 1-dependent activation of microglia via mtDNA released from retinal pigment epithelial cells

Oxidative stress, inflammation, and aberrant activation of microglia in the retina are commonly observed in ocular pathologies. In glaucoma or age-related macular degeneration, the chronic activation of microglia affects retinal ganglion cells and photoreceptors, respectively, contributing to gradual vision loss. However, the molecular mechanisms that cause activation of microglia in the retina are not fully understood. Here we show that exposure of retinal pigment epithelial (RPE) cells to chronic low-level oxidative stress induces mitochondrial DNA (mtDNA)-specific damage, and the subsequent translocation of damaged mtDNA to the cytoplasm results in the binding and activation of intracellular DNA receptor Z-DNA-binding protein 1 (ZBP1). Activation of the mtDNA/ZBP1 pathway triggers the expression of proinflammatory markers in RPE cells. In addition, we show that the enhanced release of extracellular vesicles (EVs) containing fragments of mtDNA derived from the apical site of RPE cells induces a proinflammatory phenotype of microglia via activation of ZBP1 signaling. Collectively, our report establishes oxidatively damaged mtDNA as an important signaling molecule with ZBP1 as its intracellular receptor in the development of an inflammatory response in the retina. We propose that this novel mtDNA-mediated autocrine and paracrine mechanism for triggering and maintaining inflammation in the retina may play an important role in ocular pathologies. Therefore, the molecular mechanisms identified in this report are potentially suitable therapeutic targets to ameliorate development of ocular pathologies.

Improvement of human embryonic stem cell-derived retinal pigment epithelium cell adhesion, maturation, and function through coating with truncated recombinant human vitronectin

AIM

To explore an xeno-free and defined coating substrate suitable for the culture of H9 human embryonic stem cell-derived retinal pigment epithelial (hES-RPE) cells in vitro, and compare the behaviors and functions of hES-RPE cells on two culture substrates, laminin521 (LN-521) and truncated recombinant human vitronectin (VTN-N).

METHODS

hES-RPE cells were used in the experiment. The abilities of LN-521 and VTN-N at different concentrations to adhere to hES-RPE cells were compared with a high-content imaging system. Quantitative real-time polymerase chain reaction was used to evaluate RPE-specific gene expression levels midway (day 10) and at the end (day 20) of the time course. Cell polarity was observed by immunofluorescent staining for apical and basal markers of the RPE. The phagocytic ability of hES-RPE cells was identified by flow cytometry and immunofluorescence.

RESULTS

The cell adhesion assay showed that the ability of LN-521 to adhere to hES-RPE cells was dose-dependent. With increasing coating concentration, an increasing number of cells attached to the surface of LN-521-coated wells. In contrast, VTN-N presented a strong adhesive ability even at a low concentration. The optimal concentration of LN-521 and VTN-N required to coat and adhesion to hES-RPE cells were 2 and 0.25 µg/cm², respectively. Furthermore, both LN-521 and VTN-N could facilitate adoption of the desired cobblestone cellular morphology with tight junction and showed polarity by the hES-RPE cells. However, hES-RPE cells cultivated in VTN-N had a greater phagocytic ability, and it took less time for these hES-RPE cells to mature.

CONCLUSION

VTN-N is a more suitable coating substrate for cultivating hES-RPE cells.

Silenced SNHG1 Inhibited Epithelial-Mesenchymal Transition and Inflammatory Response of ARPE-19 Cells Induced by High Glucose

PURPOSE

The lncRNA small nucleolar RNA host gene 1 (SNHG1) is a cerebral infarction-associated gene, its biological role and mechanism in diabetic retinopathy remain to be illuminated. The present study was designed to investigate the role of SNHG1 in high glucose induced human retinal pigment epithelial cells (ARPE-19).

METHODS

ARPE-19 cells were cultured and exposed to 60 mM high glucose for 48h, and 5.5mM glucose-exposed ARPE-19 cells were used as the control. The levels of the epithelial-mesenchymal transition (EMT) markers E-cadherin, ZO-1, vimentin and α-SMA were measured, and the Cell inflammatory response was evaluated by detecting IL-6 and IL-1β levels. Then, cell migration, proliferation and apoptosis were detected. The expression of the lncRNA SNHG1 in ARPE-19 cells was detected by quantitative real-time PCR. SNHG1 was knocked down by small interfering RNA (siRNA) transfection. The effects of SNHG1 inhibition on inflammation, EMT, migration, proliferation and apoptosis were observed.

RESULTS

The results showed that the expression of SNHG1 was significantly increased in ARPE-19 cells exposed to high glucose. Silencing SNHG1 reduced the expression of vimentin, α-SMA, and the expression of inflammatory chemokines IL-6 and IL-1β, inhibited migration and proliferation, elevated the expression of E-cadherin and ZO-1, and promoted apoptosis in ARPE-19 cells.

CONCLUSION

The lncRNA SNHG1 is involved in hyperglycemia-induced EMT and the inflammatory response of ARPE-19 cells and provides a new understanding of the pathogenesis of DR.

Computational Model-Based Estimation of Mouse Eyeball Structure From Two-Dimensional Flatmount Microscopy Images

Purpose

Retinal pigment epithelial (RPE) cells serve as a supporter for the metabolism and visual function of photoreceptors and a barrier for photoreceptor protection. Morphology dynamics, spatial organization, distribution density, and growth patterns of RPE cells are important for further research on these RPE main functions. To enable such investigations within the authentic eyeball structure, a new method for estimating the three-dimensional (3D) eyeball sphere from two-dimensional tissue flatmount microscopy images was investigated.

Methods

An error-correction term was formulated to compensate for the reconstruction error as a result of tissue distortions. The effect of the tissue-distortion error was evaluated by excluding partial data points from the low- and high-latitude zones. The error-correction parameter was learned automatically using a set of samples with the ground truth eyeball diameters measured with noncontact light-emitting diode micrometry at submicron accuracy and precision.

Results

The analysis showed that the error-correction term in the reconstruction model is a valid method for modeling tissue distortions in the tissue flatmount preparation steps. With the error-correction model, the average relative error of the estimated eyeball diameter was reduced from 14% to 5%, and the absolute error was reduced from 0.22 to 0.03 mm.

Conclusions

A new method for enabling RPE morphometry analysis with respect to locations on an eyeball sphere was created, an important step in increasing RPE research and eye disease diagnosis.

Translational Relevance

This method enables one to derive RPE cell information from the 3D eyeball surface and helps characterize eyeball volume growth patterns under diseased conditions.

METTL3 attenuates proliferative vitreoretinopathy and epithelial-mesenchymal transition of retinal pigment epithelial cells via wnt/β-catenin pathway

Proliferative vitreoretinopathy (PVR) is a refractory vitreoretinal fibrosis disease, and epithelial-mesenchymal transition (EMT) of retinal pigment epithelial (RPE) cells is the key pathological mechanism of PVR. However, few studies focused on the role of METTL3, the dominating methyltransferase for m6A RNA modification in PVR pathogenesis. Immunofluorescence staining and qRT-PCR were used to determine the expression of METTL3 in human tissues. Lentiviral transfection was used to stably overexpress and knockdown METTL3 in ARPE-19 cells. MTT assay was employed to study the effects of METTL3 on cell proliferation. The impact of METTL3 on the EMT of ARPE-19 cells was assessed by migratory assay, morphological observation and expression of EMT markers. Intravitreal injection of cells overexpressing METTL3 was used to assess the impact of METTL3 on the establishment of the PVR model. We found that METTL3 expression was less in human PVR membranes than in the normal RPE layers. In ARPE-19 cells, total m6A abundance and the METTL3 expression were down-regulated after EMT. Additionally, METTL3 overexpression inhibited cell proliferation through inducing cell cycle arrest at G0/G1 phase. Furthermore, METTL3 overexpression weakened the capacity of TGFβ1 to trigger EMT by regulating wnt/β -catenin pathway. Oppositely, knockdown of METTL3 facilitated proliferation and EMT of ARPE-19 cells. In vivo, intravitreal injection of METTL3-overexpressing cells delayed the development of PVR compared with injection of control cells. In summary, this study suggested that METTL3 is involved in the PVR process, and METTL3 overexpression inhibits the EMT of ARPE-19 cells in vitro and suppresses the PVR process in vivo.

Effect of ambient temperature and intracellular pigmentation on photothermal damage rate kinetics

Computational models predicting cell damage responses to transient temperature rises generated by exposure to lasers have implemented the damage integral (Ω), which time integrates the chemical reaction rate constant described by Arrhenius. However, few published reports of empirical temperature histories (thermal profiles) correlated with damage outcomes at the cellular level are available to validate the breadth of applicability of the damage integral. In our study, an analysis of photothermal damage rate processes in cultured retinal pigment epithelium cells indicated good agreement between temperature rise, exposure duration (τ), and threshold cellular damage. Full-frame thermograms recorded at high magnification during laser exposures were overlaid with fluorescence damage images taken 1 h postexposure. From the image overlays, pixels of the thermogram correlated with the boundary of cell death were used to extract threshold thermal profiles. Assessing photothermal responses at these boundaries standardized all data points, irrespective of laser irradiance, damage size, or optical and thermal properties of the cells. These results support the hypothesis that data from boundaries of cell death were equivalent to a minimum visible lesion, where the damage integral approached unity (Ω  =  1) at the end of the exposure duration. Empirically resolved Arrhenius coefficients for use in the damage integral determined from exposures at wavelengths of 2  μm and 532 nm and durations of 0.05-20 s were consistent with literature values. Varying ambient temperature (Tamb) between 20°C and 40°C during laser exposure did not change the τ-dependent threshold peak temperature (Tp). We also show that, although threshold laser irradiance varied due to pigmentation differences, threshold temperatures were irradiance independent.

LYTAK1 attenuates proliferation of retinal pigment epithelial cells through TGF-β-mediated epithelial-mesenchymal transition via the ERK/AKT signaling pathway

Retinal pigment epithelial (RPE) cells have crucial roles in the initiation and development of human ophthalmic diseases. Our previous study suggested that transforming growth factor-β (TGF-β)-activated kinase 1 (TAK1) is a potential target in the progression and pathogenesis of human proliferative vitreoretinopathy disease. The present study further analyzed the role of TAK1 inhibitor, LYTAK1, in human RPE cells and explored the potential molecular mechanism of LYTAK1-mediated proliferation of human RPE cells. Proliferation of human RPE cells was investigated following treatment with LYTAK1 and knockdown of TGF-β. TGF-β-mediated epithelial-mesenchymal transition (EMT) through regulation of the extracellular signal-regulated kinase (ERK)/protein kinase B (AKT) signaling pathway was also explored to analyze the LYTAK1-mediated mechanism of proliferation in human RPE cells. The present results demonstrated that LYTAK1 administration suppressed TAK1 gene and protein expression in human RPE cells. LYTAK1 administration also inhibited proliferation and migration of human RPE cells in vitro. Outcomes indicated that LYTAK1 treatment downregulated expression levels of TGF-β1 and EMT markers, including cadherin, fibronectin and α-smooth muscle actin in human RPE cells. Notably, results demonstrated that the ERK/AKT signal pathway was blocked by LYTAK1 in human RPE cells. Knockdown of TGF-β markedly inhibited phosphorylation and activity of TAK1 and suppressed the LYTAK1-mediated ERK/AKT signaling pathway in RPE cells, which further canceled inhibition of RPE cell proliferation by LYTAK1. In conclusion, these findings indicated that LYTAK1 may inhibit RPE cell proliferation through the TGF-β-mediated EMT/ERK/AKT signaling pathway, suggesting that TAK1 may be a potential target for the treatment of RPE diseases.

DKK1 inhibits proliferation and migration in human retinal pigment epithelial cells via the Wnt/β-catenin signaling pathway

Retinal pigment epithelial (RPE) cells play important roles in diabetic retinopathy (DR). Dickkopf 1 (DKK1) has been reported to be important in the regulation of cell proliferation and migration. However, there are few previous studies regarding DKK1 in RPE cells. Therefore, in the present study, we investigated the effect of DKK1 on the proliferation and migration of human RPE cells, and the signaling mechanisms underlying these effects. The results showed that the overexpression of DKK1 significantly inhibited the proliferation and migration of ARPE-19 cells. In addition, overexpression of DKK1 markedly inhibited the expression of β-catenin and cyclin D1 in ARPE-19 cells. Collectively, the present findings suggest that the overexpression of DKK1 inhibited the proliferation and migration of RPE cells by suppressing the Wnt/β-catenin signaling pathway. Therefore, DKK1 are able to augment the growth of human RPE, and further studies are warranted to investigate the effects of DKK1 effects on DR.

Stem cells in clinical trials for treatment of retinal degeneration

INTRODUCTION

After decades of basic science research involving the testing of regenerative strategies in animal models of retinal degenerative diseases, a number of clinical trials are now underway, with additional trials set to begin shortly. These efforts will evaluate the safety and preliminary efficacy of cell-based products in the eyes of patients with a number of retinal conditions, notably including age-related macular degeneration, retinitis pigmentosa and Stargardt's disease.

AREAS COVERED

This review considers the scientific work and early trials with fetal cells and tissues that set the stage for the current clinical investigatory work, as well the trials themselves, specifically those either now completed, underway or close to initiation. The cells of interest include retinal pigment epithelial cells derived from embryonic stem or induced pluripotent stem cells, undifferentiated neural or retinal progenitors or cells from the vascular/bone marrow compartment or umbilical cord tissue.

EXPERT OPINION

Degenerative diseases of the retina represent a popular target for emerging cell-based therapeutics and initial data from early stage clinical trials suggest that short-term safety objectives can be met in at least some cases. The question of efficacy will require additional time and testing to be adequately resolved.

Spontaneous or secondary to intravitreal injections of anti-angiogenic agents retinal pigment epithelial tears in age-related macular degeneration

AIM

To evaluate the visual function evolution of retinal pigment epithelial (RPE) tears in patients with age-related macular degeneration (AMD) according to type of occurrence [spontaneous or secondary to anti-vascular endothelial growth factor (anti-VEGF) injection] and the topographic location of the tear after a two-year follow-up period.

METHODS

A total of 15 eyes of 14 patients with RPE tears in exudative AMD were analyzed retrospectively at the University Eye Clinic of Trieste. Inclusion criteria were: patient age of 50 or older with AMD and RPE tears both spontaneous occurring or post anti-VEGF treatment. Screening included: careful medical history, complete ophthalmological examination, fluorescein angiography (FA), indocyanine green angiography (ICG), autofluorescence and infrared imaging and optical coherence tomography (OCT). Patients were evaluated every month for visual acuity (VA), fundus examination and OCT. Other data reported were: presence of PED, number of injections before the tear, location of the lesion.

RESULTS

Mean follow-up was 24wk (SD±4wk). A total of 15 eyes were studied for RPE tear. In 6 cases (40%), the RPE tears occurred within two years of anti-VEGF injections the others occurred spontaneously. In 13 cases (86.6%), the RPE tear was associated with pigment epithelial detachment (PED). In 7 cases (46.6%), the RPE tear occurred in the central area of the retina and involved the fovea. Two lesions were found in the parafoveal region, six in the extra-macular area. In all cases visual acuity decreased at the end of the follow-up period (P<0.01) independently of the type or the topographical location of the lesion.

CONCLUSION

RPE tear occurs in exudative AMD as a spontaneous complication or in relation to anti-VEGF injections. Visual acuity decreased significantly and gradually in the follow-up period in all cases. No correlation was found between visual loss and the type of onset or the topographic location of the tears.

The generation of induced pluripotent stem cells for macular degeneration as a drug screening platform: identification of curcumin as a protective agent for retinal pigment epithelial cells against oxidative stress

Age-related macular degeneration (AMD) is one retinal aging process that may lead to irreversible vision loss in the elderly. Its pathogenesis remains unclear, but oxidative stress inducing retinal pigment epithelial (RPE) cells damage is perhaps responsible for the aging sequence of retina and may play an important role in macular degeneration. In this study, we have reprogrammed T cells from patients with dry type AMD into induced pluripotent stem cells (iPSCs) via integration-free episomal vectors and differentiated them into RPE cells that were used as an expandable platform for investigating pathogenesis of the AMD and in-vitro drug screening. These patient-derived RPEs with the AMD-associated background (AMD-RPEs) exhibited reduced antioxidant ability, compared with normal RPE cells. Among several screened candidate drugs, curcumin caused most significant reduction of ROS in AMD-RPEs. Pre-treatment of curcumin protected these AMD-RPEs from H2O2-induced cell death and also increased the cytoprotective effect against the oxidative stress of H2O2 through the reduction of ROS levels. In addition, curcumin with its versatile activities modulated the expression of many oxidative stress-regulating genes such as PDGF, VEGF, IGFBP-2, HO1, SOD2, and GPX1. Our findings indicated that the RPE cells derived from AMD patients have decreased antioxidative defense, making RPE cells more susceptible to oxidative damage and thereby leading to AMD formation. Curcumin represented an ideal drug that can effectively restore the neuronal functions in AMD patient-derived RPE cells, rendering this drug an effective option for macular degeneration therapy and an agent against aging-associated oxidative stress.

TGF-β2 promotes RPE cell invasion into a collagen gel by mediating urokinase-type plasminogen activator (uPA) expression

Transforming growth factor-beta (TGF-β) is one of the main epithelial-mesenchymal transition (EMT)-inducing factors. In general, TGF-β-induced EMT promotes cell migration and invasion. TGF-β also acts as a potent regulator of pericellular proteolysis by regulating the expression and secretion of plasminogen activators. Urokinase-type plasminogen activator (uPA) is a serine protease that binds to its cell surface receptor (uPAR) with high affinity. uPA binding to uPAR stimulates uPAR's interaction with transmembrane proteins, such as integrins, to regulate cytoskeletal reorganization and cell migration, differentiation and proliferation. However, the influence of TGF-β and the uPA/uPAR system on EMT in retinal pigment epithelial (RPE) cells is still unclear. The purpose of this study was to determine the effect of TGF-β2, which is the predominant isoform in the retina, and the uPA/uPAR system on RPE cells. In this study, we first examined the effect of TGF-β2 and/or the inhibitor of uPA (u-PA-STOP(®)) on the proliferation of a human retinal pigment epithelial cell line (ARPE-19 cells). Treatment with TGF-β2 or u-PA-STOP(®) suppressed cell proliferation. Combination treatment of TGF-β2 and u-PA-STOP(®) enhanced cell growth suppression. Furthermore, western blot analysis, fibrin zymography and real-time reverse transcription PCR showed that that TGF-β2 induced EMT in ARPE-19 cells and that the expression of uPA and uPAR expression was up-regulated during EMT. The TGF-β inhibitor SB431542 suppressed TGF-β2-stimulated uPA expression and secretion but did not suppress uPAR expression. Furthermore, we seeded ARPE-19 cells onto Transwell chambers and allowed them to invade the collagen matrix in the presence of TGF-β2 alone or with TGF-β2 and u-PA-STOP(®). TGF-β2 treatment induced ARPE-19 cell invasion into the collagen gel. Treatment with a combination of TGF-β2 and the uPA inhibitor strongly inhibited ARPE-19 cell invasion compared with treatment with TGF-β2 alone. Furthermore, the interaction between uPA and ARPE-19 cells was analyzed using a surface plasmon biosensor system. The binding of uPA to ARPE-19 cells was observed. In addition, TGF-β2 significantly promoted the binding activity of uPA to ARPE-19 cells in a time-dependent or cell-number-dependent fashion. These results indicate that TGF-β-induced EMT-associated phenotype changes in ARPE-19 cells and the invasiveness of ARPE-19 cells into a collagen gel matrix are mediated, at least in part, by uPA.

The role of SIRT1/AKT/ERK pathway in ultraviolet B induced damage on human retinal pigment epithelial cells

Ultraviolet (UV)-induced damage plays a major role in ocular diseases, such as cataracts and retinal degeneration. UVB may also cause retinal phototoxicity and photic retinopathy. In this study, we explored the effects of UVB on the cell cycle and the role of silent mating type information regulation 2 homolog 1 (SIRT1) in the UVB-induced damage. UVB dose-dependently suppressed the growth of retinal pigment epithelial (RPE) cells by activating the phosphatidylinositol 3-kinase (PI3K) pathway and triggering cell cycle arrest at the S phase. SIRT1, an NAD-dependent histone deacetylase, is involved in multiple biological processes, such as the stress response and the regulation of the cell cycle. However, its role in the effects of UVB on RPE cells is unclear. We showed that UVB down-regulates SIRT1 expression in a dose-dependent manner. Resveratrol, an SIRT1 activator, prevented the UVB-induced damage by inhibiting AKT and ERK phosphorylation. A specific PI3K inhibitor attenuated the UVB-induced ERK1/2 and p53 phosphorylation. Finally, UVB activated the PI3K/AKT/ERK pathway by reducing the expression of SIRT1 in ARPE-19 cells. Our study, therefore, illustrated the molecular mechanisms of UVB-induced phototoxicity and damage of RPE cells. SIRT1 and resveratrol may be significant regulators, protecting against UVB-induced injury.