Toll-like receptor 3 activation in retinal pigment epithelium cells - Mitogen-activated protein kinase pathways of cell death and vascular endothelial growth factor secretion

Effects of Fucoidans on Activated Retinal Microglia

Sulfated marine polysaccharides, so-called fucoidans, have been shown to exhibit anti-inflammatory and immunomodulatory activities in retinal pigment epithelium (RPE). In this study, we tested the effects of different fucoidans (and of fucoidan-treated RPE cells) on retinal microglia to investigate whether its anti-inflammatory effect can be extrapolated to the innate immune cells of the retina. In addition, we tested whether fucoidan treatment influenced the anti-inflammatory effect of RPE cells on retinal microglia. Three fucoidans were tested (FVs from Fucus vesiculosus, Fuc1 and FucBB04 from Laminaria hyperborea) as well as the supernatant of primary porcine RPE treated with fucoidans for their effects on inflammatory activated (using lipopolysaccharide, LPS) microglia cell line SIM-A9 and primary porcine retinal microglia. Cell viability was detected with a tetrazolium assay (MTT), and morphology by Coomassie staining. Secretion of tumor necrosis factor alpha (TNFα), interleukin 1 beta (IL1β) and interleukin 8 (IL8) was detected with ELISA, gene expression (NOS2 (Nitric oxide synthase 2), and CXCL8 (IL8)) with qPCR. Phagocytosis was detected with a fluorescence assay. FucBB04 and FVs slightly reduced the viability of SIM-A9 and primary microglia, respectively. Treatment with RPE supernatants increased the viability of LPS-treated primary microglia. FVs and FucBB04 reduced the size of LPS-activated primary microglia, indicating an anti-inflammatory phenotype. RPE supernatant reduced the size of LPS-activated SIM-A9 cells. Proinflammatory cytokine secretion and gene expression in SIM-A9, as well as primary microglia, were not significantly affected by fucoidans, but RPE supernatants reduced the secretion of LPS-induced proinflammatory cytokine secretion in SIM-A9 and primary microglia. The phagocytosis ability of primary microglia was reduced by FucBB04. In conclusion, fucoidans exhibited only modest effects on inflammatorily activated microglia by maintaining their cell size under stimulation, while the anti-inflammatory effect of RPE cells on microglia irrespective of fucoidan treatment could be confirmed, stressing the role of RPE in regulating innate immunity in the retina.

Co-cultivation of primary porcine RPE cells and neuroretina induces inflammation: a potential inflammatory AMD-model

One common aspect in the pathology of many retinal diseases like age-related macular degeneration (AMD) is the death of retinal pigment epithelium (RPE) cells. RPE cells are essential for photoreceptor survival as they recycle and remove compounds of the visual cycle and secrete protective cytokines. Studying RPE cells is crucial to improve our understanding of retinal pathologies, yet only a few retinal ex vivo models include them or do so only indirectly. Besides the positive effects in indirect co-cultivation models, also a slight inflammation was observed. In this study we developed an ex vivo model consisting of a primary porcine RPE monolayer directly co-cultured with porcine retinal organ cultures, to investigate and simulate inflammatory retinal diseases, such as (dry) AMD. The direct co-cultivation resulted in immune reactivity (enhanced expression of pro-inflammatory cytokines e.g., IL-1β, IL-6, IL-8) and cell death. These effects were evaluated for the retinal explant as well as for the RPE-monolayer to further understand the complex interactions between these two compartments. Taken together, this ex vivo model can be used to study inflammatory retinal diseases like AMD as well as the rejection observed after RPE-transplantation.

Suppressing Inflammation for the Treatment of Diabetic Retinopathy and Age-Related Macular Degeneration: Dazdotuftide as a Potential New Multitarget Therapeutic Candidate

Diabetic retinopathy (DR) and age-related macular degeneration (AMD) are major causes of blindness globally. The primary treatment option for DME and neovascular AMD (nAMD) is anti-vascular endothelial growth factor (VEGF) compounds, but this treatment modality often yields insufficient results, and monthly injections can place a burden on the health system and patients. Although various inflammatory pathways and mediators have been recognized as key players in the development of DR and AMD, there are limited treatment options targeting these pathways. Molecular pathways that are interlinked, or triggers of multiple inflammatory pathways, could be promising targets for drug development. This review focuses on the role of inflammation in the pathogenesis of DME and AMD and presents current anti-inflammatory compounds, as well as a potential multitarget anti-inflammatory compound (dazdotuftide) that could be a candidate treatment option for the management of DME and AMD.

Comparison of Fucoidans from Saccharina latissima Regarding Age-Related Macular Degeneration Relevant Pathomechanisms in Retinal Pigment Epithelium

Fucoidans from brown algae are described as anti-inflammatory, antioxidative, and antiangiogenic. We tested two Saccharina latissima fucoidans (SL-FRO and SL-NOR) regarding their potential biological effects against age-related macular degeneration (AMD). Primary porcine retinal pigment epithelium (RPE), human RPE cell line ARPE-19, and human uveal melanoma cell line OMM-1 were used. Cell survival was assessed in tetrazolium assay (MTT). Oxidative stress assays were induced with erastin or H2O2. Supernatants were harvested to assess secreted vascular endothelial growth factor A (VEGF-A) in ELISA. Barrier function was assessed by measurement of trans-epithelial electrical resistance (TEER). Protectin (CD59) and retinal pigment epithelium-specific 65 kDa protein (RPE65) were evaluated in western blot. Polymorphonuclear elastase and complement inhibition assays were performed. Phagocytosis of photoreceptor outer segments was tested in a fluorescence assay. Secretion and expression of proinflammatory cytokines were assessed with ELISA and real-time PCR. Fucoidans were chemically analyzed. Neither toxic nor antioxidative effects were detected in ARPE-19 or OMM-1. Interleukin 8 gene expression was slightly reduced by SL-NOR but induced by SL-FRO in RPE. VEGF secretion was reduced in ARPE-19 by SL-FRO and in RPE by both fucoidans. Polyinosinic:polycytidylic acid induced interleukin 6 and interleukin 8 secretion was reduced by both fucoidans in RPE. CD59 expression was positively influenced by fucoidans, and they exhibited a complement and elastase inhibitory effect in cell-free assay. RPE65 expression was reduced by SL-NOR in RPE. Barrier function of RPE was transiently reduced. Phagocytosis ability was slightly reduced by both fucoidans in primary RPE but not in ARPE-19. Fucoidans from Saccharina latissima, especially SL-FRO, are promising agents against AMD, as they reduce angiogenic cytokines and show anti-inflammatory and complement inhibiting properties; however, potential effects on gene expression and RPE functions need to be considered for further research.

Anti-inflammatory properties of antiangiogenic fucoidan in retinal pigment epithelium cells

Age-related macular degeneration (AMD) is a multifactorial disease in which angiogenesis, oxidative stress and inflammation are important contributing factors. In this study, we investigated the anti-inflammatory effects of a fucoidan from the brown algae Fucus vesiculosus (FV) in primary porcine RPE cells. Inflammation was induced by lipopolysaccharide (LPS), polyinosinic:polycytidylic acid (Poly I:C), Pam2CSK4 (Pam), or tumor necrosis factor alpha (TNF-α). Cell viability was tested with thiazolyl blue tetrazolium bromide (MTT) test, barrier function by measuring transepithelial electric resistance (TEER), interleukin 6 (IL-6) and interleukin 8 (IL-8) secretion in ELISA, retinal pigment epithelium-specific 65 kDa protein (RPE65) and protectin (CD59) expression in Western blot, gene expression with quantitative polymerase chain reaction (qPCR) (IL6, IL8, MERTK, PIK3CA), and phagocytotic activity in a microscopic assay. FV fucoidan did not influence RPE cell viability. FV fucoidan reduced the Poly I:C proinflammatory cytokine secretion of IL-6 and IL-8. In addition, it decreased the expression of IL-6 and IL-8 in RT-PCR. LPS and TNF-α reduced the expression of CD59 in Western blot, this reduction was lost under FV fucoidan treatment. Also, LPS and TNF-α reduced the expression of visual cycle protein RPE65, this reduction was again lost under FV fucoidan treatment. Furthermore, the significant reduction of barrier function after Poly I:C stimulation is ameliorated by FV fucoidan. Concerning phagocytosis, however, the inflammation-induced reduction was not improved by FV fucoidan. FV and proinflammatory milieu did not relevantly influence phagocytosis relevant gene expression either. In conclusion, we show that fucoidan from FV can reduce proinflammatory stimulation in RPE induced by toll-like receptor 3 (TLR-3) activation and is of high interest as a potential compound for early AMD treatment.

Properties of Cephalopod Skin Ommochromes to Inhibit Free Radicals, and the Maillard Reaction and Retino-Protective Mechanisms in Cellular Models Concerning Oxidative Stress, Angiogenesis, and Inflammation

Ommochromes are pigments of invertebrates that exhibit oxidative stress protection. The aim of this study was to investigate ommochromes extracted from cephalopod’s skin for their ability to inhibit age-related-macular degeneration (AMD)-related factors such as H₂O₂-induced and iron-dependent oxidative stress (ferroptosis and erastin), accumulation of advanced glycation end-products (AGEs), as well as vascular endothelial growth factor (VEGF), and inflammatory cytokines (interleukin 6 and interleukin 8) secretion. As cell systems, we used primary porcine retinal pigment epithelium (RPE), human retinal pigment epithelium cell line ARPE-19 and uveal melanoma cell line OMM-1. In vitro, ommochromes produced an antiglycation effect by the inhibition of fructosylation reaction. The ommochromes showed protective effects against erastin- induced cell death in ARPE-19. In addition, in long-term stimulation (7 days) ommochromes decreased constitutively secreted VEGF, as well as interleukin 6 and interleukin 8 induced by Poly I:C in primary RPE. No relevant effects were detected in OMM-1 cells. The effects are dependent on the cell system, time of exposition, and concentration. This substance is of interest for further research concerning age-related macular degeneration.

Toll-Like Receptor Signalling Pathways and the Pathogenesis of Retinal Diseases

There is growing evidence that the pathogenesis of retinal diseases such as diabetic retinopathy (DR) and age-related macular degeneration (AMD) have a significant chronic inflammatory component. A vital part of the inflammatory cascade is through the activation of pattern recognition receptors (PRR) such as toll-like receptors (TLR). Here, we reviewed the past and current literature to ascertain the cumulative knowledge regarding the effect of TLRs on the development and progression of retinal diseases. There is burgeoning research demonstrating the relationship between TLRs and risk of developing retinal diseases, utilising a range of relevant disease models and a few large clinical investigations. The literature confirms that TLRs are involved in the development and progression of retinal diseases such as DR, AMD, and ischaemic retinopathy. Genetic polymorphisms in TLRs appear to contribute to the risk of developing AMD and DR. However, there are some inconsistencies in the published reports which require further elucidation. The evidence regarding TLR associations in retinal dystrophies including retinitis pigmentosa is limited. Based on the current evidence relating to the role of TLRs, combining anti-VEGF therapies with TLR inhibition may provide a longer-lasting treatment in some retinal vascular diseases.

Pro-inflammatory activation changes intracellular transport of bevacizumab in the retinal pigment epithelium in vitro

Purpose

Bevacizumab is taken up and transported through the retinal pigment epithelium. Inflammatory signaling may influence this interaction. In the present study, we have investigated the effect of pro-inflammatory stimuli on the uptake, intracellular localization, and transepithelial transport of bevacizumab.

Methods

ARPE-19 cell line or primary porcine RPE cells were treated with clinical relevant concentrations of bevacizumab (250 µg/ml). Pro-inflammatory signaling was induced by TLR-3 agonist polyinosinic:polycytidylic acid (Poly I:C). Viability was investigated with MTT and trypan-blue exclusion assay, and cell number, uptake, and intracellular localization were investigated with immunofluorescence, investigating also actin filaments, the motor protein myosin 7a and lysosomes. Immunofluorescence signals were quantified. Intracellular bevacizumab was additionally detected in Western blot. Barrier function was investigated with transepithelial resistant measurements (TER). The transepithelial transport of bevacizumab and its influence on cytokine (IL-6, IL-8, IL-1β, TNFα) secretion was investigated with ELISA.

Results

Poly I:C in combination with bevacizumab reduced the viability of the cells. Treatment with Poly I:C reduced the uptake of bevacizumab, changed the intensity of the actin filaments, and reduced the colocalization with myosin 7a. In addition, Poly I:C reduced the capacity of RPE cells to transport bevacizumab over the barrier. In addition, bevacizumab reduced the secretion of IL-8 and TNFα after Poly I:C stimulation at selected time points.

Conclusions

Pro-inflammatory activation of RPE cells with TLR-3 agonist Poly I:C changes the interaction of RPE cells with the anti-VEGF compound bevacizumab, reducing its uptake and transport. On the other hand, bevacizumab might influence pro-inflammatory cytokine release. Our data indicate that inflammation may influence the pharmacokinetic of bevacizumab in the retina.

Retinal Pigment Epithelium Expressed Toll-like Receptors and Their Potential Role in Age-Related Macular Degeneration

(1) Background: Inflammation is a major pathomechanism in the development and progression of age-related macular degeneration (AMD). The retinal pigment epithelium (RPE) may contribute to retinal inflammation via activation of its Toll-like receptors (TLR). TLR are pattern recognition receptors that detect the pathogen- or danger-associated molecular pattern. The involvement of TLR activation in AMD is so far not understood. (2) Methods: We performed a systematic literature research, consulting the National Library of Medicine (PubMed). (3) Results: We identified 106 studies, of which 54 were included in this review. Based on these studies, the current status of TLR in AMD, the effects of TLR in RPE activation and of the interaction of TLR activated RPE with monocytic cells are given, and the potential of TLR activation in RPE as part of the AMD development is discussed. (4) Conclusion: The activation of TLR2, -3, and -4 induces a profound pro-inflammatory response in the RPE that may contribute to (long-term) inflammation by induction of pro-inflammatory cytokines, reducing RPE function and causing RPE cell degeneration, thereby potentially constantly providing new TLR ligands, which could perpetuate and, in the long run, exacerbate the inflammatory response, which may contribute to AMD development. Furthermore, the combined activation of RPE and microglia may exacerbate neurotoxic effects.

The potential of marine resources for retinal diseases: a systematic review of the molecular mechanisms

We rely on vision more than on any other sense to obtain information about our environment. Hence, the loss or even impairment of vision profoundly affects our quality of life. Diet or food components have already demonstrated beneficial effects on the development of retinal diseases. Recently, there has been a growing interest in resources from marine animals and plants for the prevention of retinal diseases through nutrition. Especially fish intake and omega-3 fatty acids have already led to promising results, including associations with a reduced incidence of retinal diseases. However, the underlying molecular mechanisms are insufficiently explained. The aim of this review was to summarize the known mechanistic effects of marine resources on the pathophysiological processes in retinal diseases. We performed a systematic literature review following the PRISMA guidelines and identified 107 studies investigating marine resources in the context of retinal diseases. Of these, 46 studies described the underlying mechanisms including anti-inflammatory, antioxidant, antiangiogenic/vasoprotective, cytoprotective, metabolic, and retinal function effects, which we critically summarize. We further discuss perspectives on the use of marine resources for human nutrition to prevent retinal diseases with a particular focus on regulatory aspects, health claims, safety, and bioavailability.

The Influence of Melatonin and Light on VEGF Secretion in Primary RPE Cells

(1) Background: Retinal pigment epithelial cells (RPE) cells constitutively secrete vascular endothelial growth factor (VEGF) in the retina, protecting the neuronal cells and the choroid. Increased VEGF secretion, however, can result in neovascularization and edema. Many factors regulate VEGF secretion. In this study, we investigated the effect of external stimuli in relation to diurnal rhythm on constitutive VEGF secretion. (2) Methods: Single-eye RPE cell culture was prepared from porcine eyes. RPE cells were cultured in darkness, treated with daylight or room light, and treated with melatonin at different time frames, either respectively or in combination. Supernatants were collected and VEGF content evaluated using ELISA. Expression of the clock protein BMAL1 was evaluated with Western blot. (3) Results: VEGF secretion of the RPE shows a diurnal rhythm. While the rhythm is not influenced by either light or melatonin, the amount of secreted VEGF can be increased by nocturnal melatonin, especially in combination with morning daylight. These findings disclose another layer of VEGF regulation in the retina.

Basolateral activation with TLR agonists induces polarized cytokine release and reduces barrier function in RPE in vitro

PURPOSE

Systemic inflammation may be of importance in the development of AMD. RPE cells can recognize danger signals with toll-like receptors (TLR) and may react in a pro-inflammatory manner. In this study, we evaluated the basal and apical secretions of TNFα, IL-6, and IL-1β in primary RPE cells and RPE/choroid explant cells under basolateral stimulation of TLR2, 3, and 4; the effects on barrier function; and their influence on neuronal cell viability.

METHODS

RPE/choroid tissue explants were prepared from porcine eyes and cultivated in modified Ussing chambers; primary porcine RPE cells on transwell plates. Cells were basally stimulated with agonists Pam2CSK4 (Pam; TLR2), polyinosinic/polycytidylic acid (Poly I:C; TLR3), and lipopolysaccharide (LPS; TLR4) for 24 h. Supernatants were evaluated with ELISA for cytokines TNFα, IL-6, and IL-1β. Apical supernatants were applied to SHSY-5Y cells, and cell viability was evaluated in MTT assay. Barrier function was tested by measuring transepithelial electrical resistance (TER) and occludin immunostaining.

RESULTS

None of the tested TLR agonists was toxic on RPE cells after 24 h of exposure. Unstimulated RPE cells secreted hardly any cytokines. Pam induced IL-6, IL-1ß, and TNFα on the basal and apical sides at all concentrations tested. Poly I:C induced IL-6 and TNFα primarily at the basal side at lower but on both sides at higher concentrations. LPS induced IL-6, IL-1ß, and TNFα apically and basally at all concentrations tested. In the RPE/choroid, a strong difference between apical and basal secretions could be found. IL-6 was constitutively secreted basally, but not apically, but was induced by all agonists on both sides. IL-1ß and TNFα alpha were strongly induced on the basal side by all agonists. TER was reduced by all agonists, with Pam and LPS being effective in all concentrations tested. Occludin expression was unaltered, but the distribution was influenced by the agonists, with a less distinct localization at the cell borders after treatment. None of the agonists or supernatants of treated RPE and RPE/choroid organ cultures exerted any effect on viability of SHSY-5Y cells.

CONCLUSIONS

Danger signals activating TLRs can induce polarized cytokine expression and contribute to the loss of barrier function in the RPE.

Fucoidans as Potential Therapeutics for Age-Related Macular Degeneration-Current Evidence from In Vitro Research

Age-related macular degeneration (AMD) is the major reason for blindness in the industrialized world with limited treatment options. Important pathogenic pathways in AMD include oxidative stress and vascular endothelial growth factor (VEGF) secretion. Due to their bioactivities, fucoidans have recently been suggested as potential therapeutics. This review gives an overview of the recent developments in this field. Recent studies have characterized several fucoidans from different species, with different molecular characteristics and different extraction methods, in regard to their ability to reduce oxidative stress and inhibit VEGF in AMD-relevant in vitro systems. As shown in these studies, fucoidans exhibit a species dependency in their bioactivity. Additionally, molecular properties such as molecular weight and fucose content are important issues. Fucoidans from Saccharina latissima and Laminaria hyperborea were identified as the most promising candidates for further development. Further research is warranted to establish fucoidans as potential therapeutics for AMD.

Effect of long-term inflammation on viability and function of RPE cells

PURPOSE

Degenerative ocular disorders like age-related macular degeneration (AMD) are associated with long-term pro-inflammatory signals on retinal pigment epithelial (RPE) cells. In this study, we investigated the effect of long term treatment of RPE cells with agonists of toll-like receptor (TLR) -3 (Polyinosinic:polycytidylic acid, Poly I:C), TLR-4 (lipopolysaccharide, LPS) and the pro-inflammatory cytokine TNFα.

METHODS

All tests were conducted with primary porcine RPE. Cells were stimulated with Poly I:C (1, 10, 100 μg/ml), LPS (0.1, 1, 10 μg/ml) or TNFα (12.5, 25 or 50 ng/ml) for 1 day, 7 days or 4 weeks. Cell viability tests (MTT) were additionally tested in ARPE-19 cells. Cytokine secretion (IL-6, IL-1β, IL-8, TNFα, TGF-β) was tested in ELISA, phagocytosis in a microscopic assay, and expression of RPE65 in Western blot. Barrier function was tested in transwell-cultured cells by measuring transepithelial resistance for up to 3 days.

RESULTS

LPS and TNFα significantly reduce cell viability after 1 day and 7 days, Poly I:C after 7 days and 4 weeks. LPS, Poly I:C and TNFα significantly induce the secretion of IL-6 and IL-8 at all tested time points. IL-1β is increased by LPS and Poly I:C after 1 day, but not by TNFα. TNFα secretion is increased by Poly I:C and LPS after 1 day but not at later time points. TGF-β secretion is not influenced by any stimulus. Concerning RPE function, LPS decreased phagocytosis after 7 days, while Poly I:C and TNFα showed no effect. RPE65 expression was strongly reduced by TNFα and LPS after 4 weeks. Wound healing capacity was reduced by Poly I:C but induced by LPS after 7 d and 4 w. Barrier function was not affected by Poly I:C or LPS, while TNFα reduced barrier function after 1 h, 4 h and 3 days.

CONCLUSION

Long term pro-inflammatory stimuli reduce RPE viability, barrier properties and cellular function and induce pro-inflammatory cytokines and therefore may contribute directly to atrophic changes in AMD.

Association study of toll-like receptors 4 polymorphisms and the risk of age-related macular degeneration: a meta-analysis

BACKGROUND

Toll-like receptor 4 (rs4986790, rs4986791) single nucleotide polymorphisms (SNPs) have been shown to be associated with age-related macular degeneration (AMD), but the results are still inconclusive. The present meta-analysis was conducted to evaluate the association between SNPs of TLR4 gene and AMD susceptibility.

METHODS

Relevant articles were obtained through computer retrieval of Pubmed, Embase, Chinese National Knowledge Infrastructure and China wanfang database. Eligible articles were selected according to the inclusion and exclusion criteria, and quality scores were made for them by NOS-scale. Relevant data were extracted for meta-analysis. The combined OR value and 95% confidence interval were used to evaluate the strength of the correlation. Funnel plot and Egger's regression were used to evaluate publication bias. All analyses were performed using STATA 11.0 software.

RESULTS

A total of nine case-control studies were included in this meta-analysis. After combination, an significant association was found between rs4986790 polymorphism and AMD susceptibility in heterozygote model (AG vs. AA, OR = 1.400, 95%CI = 1.049-1.867, P = .022) and dominant model (GG+AG vs. AA, OR = 1.365, 95%CI = 1.028-1.813, P = .032). There was no association found between rs4986791 polymorphism and AMD susceptibility in all genetic models (all P > .05). Funnel plot and Egger's regression analysis showed no publication bias existed in this study.

CONCLUSIONS

Meta-analysis suggested that there is an association between TLR4 gene rs4986790 polymorphism and AMD susceptibility, while no association between rs4986791 polymorphism and AMD susceptibility.

Interaction of inflammatorily activated retinal pigment epithelium with retinal microglia and neuronal cells

In age-related macular degeneration, inflammatory events are presumed to contribute to disease development. A primary suspect of this contribution is the microglia, the innate immune cell of the retina. In addition, retinal pigment epithelium (RPE) cells can be inflammatorily activated. In this study, we investigate the effect of activated RPE cells on retinal microglia and on neuronal cells. RPE cells and microglia were harvested from porcine eyes. In addition, a neuronal cell line (SHSY-5Y) of human origin was used. For inflammatory activation, agonists of toll-like receptors in different concentrations were used: Pam2CSK4 (Pam; TLR-2), Polyinosinic:polycytidylic acid (Poly I:C; TLR-3) and lipopolysaccharid (LPS; TLR-4). Cell viability was investigated with an MTT assay. The secretion of cytokines was assessed in an ELISA and their expression in real-time PCR. There was no effect of the agonists on cell viability in RPE cells. All agonists induced the secretion of IL-6 and IL-8 in RPE cells with the strongest effect induced by LPS. In microglia, pro-inflammatory stimulation increased the metabolic activity. All agonists induced the secretion of IL-1ß, IL-8, and TNFα in microglia cells while in real-time PCR, LPS and Pam induced the expression of IL-6, IL-1ß and iNOS. Direct stimulation of SHSY-5Y with the agonists induced only minor alterations of viability. Stimulated RPE cell supernatant reduced the secretion of TNFα and IL-8 irrespective of the inducing agent in microglia cells. Additionally a slight induction of IL-1ß was found in microglia treated with supernatant of RPE cells treated with Pam. In real time PCR, the supernatant of RPE cells stimulated with LPS significantly reduced the expression of iNOS and IL-6, but not of IL-1ß. Of note, the expression of iNOS was also reduced by naive RPE cells. The treatment of the SHSY-5Y with supernatant of microglia previously treated with RPE conditioned medium significantly decreased SHSY-5Y viability with and without pro-inflammatory treatment. In conclusion, inflammatory activated RPE cells have a regulatory effect on the pro-inflammatory activation of microglia, stressing the importance of the interaction between these two retinal cell types. Microglia treated with RPE supernatant reduced viability of a neuronal cell line, indicating a neurotoxic effect.

Retina in a dish: Cell cultures, retinal explants and animal models for common diseases of the retina

For many retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR), the exact pathogenesis is still unclear. Moreover, the currently available therapeutic options are often unsatisfactory. Research designed to remedy this situation heavily relies on experimental animals. However, animal models often do not faithfully reproduce human disease and, currently, there is strong pressure from society to reduce animal research. Overall, this creates a need for improved disease models to understand pathologies and develop treatment options that, at the same time, require fewer or no experimental animals. Here, we review recent advances in the field of in vitro and ex vivo models for AMD, glaucoma, and DR. We highlight the difficulties associated with studies on complex diseases, in which both the initial trigger and the ensuing pathomechanisms are unclear, and then delineate which model systems are optimal for disease modelling. To this end, we present a variety of model systems, ranging from primary cell cultures, over organotypic cultures and whole eye cultures, to animal models. Specific advantages and disadvantages of such models are discussed, with a special focus on their relevance to putative in vivo disease mechanisms. In many cases, a replacement of in vivo research will mean that several different in vitro models are used in conjunction, for instance to analyze and validate causative molecular pathways. Finally, we argue that the analytical decomposition into appropriate cell and tissue model systems will allow making significant progress in our understanding of complex retinal diseases and may furthermore advance the treatment testing.

The RPE Cell and the Immune System

The RPE cell plays a pivotal role in retinal immunity. In fact, the RPE cell orchestrates both innate and adaptive immunity and contains a plethora of factors to regulate the immune response. Many immunoregulatory activities of the RPE cell are accomplished through cytokine production, toll-like receptor (TLR) activation, complement regulation and antigen presentation. The RPE cells immune regulatory network plays an essential role in retinal immunity and autoimmunity, retinal infections and in a variety of retinal degenerative disorders. Understanding the immune regulatory properties of this cell may provide additional clues to disease mechanisms that may lead to future treatments for many human retinal diseases.

Grass carp (Ctenopharyngodon idella) RSK2 protects cells anti-apoptosis by up-regulating BCL-2

In mammals, toll-like receptor 3 (TLR3) is capable of recognizing double-stranded RNA and then initiates transcription of IFN-β. TLR3 can activate the innate immune system by phosphorylating extracellular signal-regulated kinase 1 (ERK1) in the mitogen-activated protein kinase (MAPK) pathway. As a downstream signaling protein of ERK1, ribosomal protein S6 kinase alpha 3 (RSK2) is activated through the "classical" MAPK pathway. So RSK2 plays a critical role in response to innate immune system induced by TRL3. However, the innate immune mechanism of RSK2 remains indistinct in fish. In this study, we cloned and characterized a full length cDNA sequence of RSK2 from Ctenopharyngodon idella (named CiRSK2, MH844551). The full length cDNA of CiRSK2 is 3930 bp with a coding sequence of 2202 bp encoding a polypeptide of 734 amino acids. The expression of CiRSK2 was ubiquitous and significantly up-regulated under the stimulation of poly (I:C) in eight different tissues of C. idella and C. idella kidney cells (CIK). In addition, poly (I:C) stimulation also up-regulated the expression of CiERK1 mRNA in CIK cells and the phosphorylation of CiERK1. We also demonstrated that the activated CiERK1 interacted with CiRSK2 by CO-IP assay and immunofluorescence assay. To further investigate the relationship between CiRSK2 and CiERK1, we performed subcellular localization of CiRSK2 at different periods of CiERK1 stimulation. The result showed that CiERK1 can make CiRSK2 enter the nucleus. Subsequently, we found that CiRSK2 increased the transcriptional level of CiBCL-2 and protein level of CiBCL-2 significantly. Then cell apoptosis was inhibited to a certain extent. Overall, our results suggested that CiRSK2 plays important roles in fish innate immunity and is able to inhibit cell apoptosis by up-regulating CiBCL-2.

Activation of Toll-like receptor 3 promotes pathological corneal neovascularization by enhancement of SDF-1-mediated endothelial progenitor cell recruitment

Toll-like receptors (TLRs) play an important role in inflammatory and immunological responses, which are intimately related to neovascularization. However, the precise mode of action of TLR3 in neovascularization still remains ambiguous. In this study, we sought to investigate the role of TLR3 in pathological corneal neovascularization (CNV) using a mouse model of alkali-induced CNV. CNV was attenuated in TLR3-deficient mice, and the absence of TLR3 led to decreased production of stromal cell-derived factor 1 (SDF-1), a well-characterized cytokine that regulates the recruitment of endothelial progenitor cells (EPCs) to the sites of neo-angiogenic niches in the injured tissues. Topical administration of polyinosinic-polycytidylic acid [poly (I:C)], a synthetic ligand for TLR3, to the injured cornea promoted CNV in wild type (WT) mice but not in TLR3-deficient mice. In addition, the effect of poly (I:C) on WT mice was abolished by addition of SDF-1 receptor antagonist AMD 3100. Furthermore, poly (I:C) treatment in vitro enhanced the migration of EPCs, whereas the enhanced migration was abolished by AMD 3100. These results indicate an essential role of TLR3 signalling in CNV that involves upregulating SDF-1 production and recruiting EPCs to the sites of injury for neovascularization. Thus, targeting the TLR3 signalling cascade may constitute a novel therapeutic approach for treating neovascularization-related diseases.

Pharmacology of the retinal pigment epithelium, the interface between retina and body system

The retinal pigment epithelium (RPE) is a close, interactive partner to the photoreceptors as well as an interface with the endothelium of the choroid and thus with the body's circulatory system. To fulfill these roles, the RPE communicates with neighboring tissue by secretion of a large variety of factors and is able to react to secreted factors via a plethora of transmembrane receptors. Clinically relevant local pharmacological effects are caused by anti-VEGF-A treatment in choroidal neovascularization or by carboanhydrase inhibitors reducing fluid accumulation in the macula. Being exposed to the bloodstream, the RPE reacts to systemic disease, such as diabetes or hypertension, but also to systemic pharmacological intervention, for example to hypotensive drugs acting on the renin-angiotensin-system. Sustained pharmacological treatments, in particular, cause side effects at the RPE with consequences for both RPE function and photoreceptor survival. Among these are systemic inhibition of angiotensin-converting enzyme, insulin treatment in diabetes and anti-VEGF-A therapy. Given the special anatomical and functional relationships of the RPE, pharmacological intervention targeting either the eye or the body systemically should take potential alteration of RPE and subsequently photoreceptor function into account.

Retinal pigment epithelium cells alter the pro-inflammatory response of retinal microglia to TLR-3 stimulation

PURPOSE

Microglia are the local cells of the innate immunity in the retina. Toll-like receptor (TLR) 3 is a receptor of the innate immune system, recognizing viral double-stranded RNA. Retinal pigment epithelium (RPE) cells express TLR-3 and react to TLR-3 stimulation. In this study, we investigated the effect of TLR-3-activated RPE on microglia.

METHODS

Primary porcine RPE cells were prepared from freshly prepared pigs' eyes. Retinal microglia were prepared from porcine retina. Expression of the microglia marker Iba1 was evaluated using immunocytochemistry. RPE cells were treated with polyinosinic/polycytidylic acid (Poly I:C; 100 ng/ml, 1 μg/ml, 10 μg/ml, 100 μg/ml) for 24 hr. Either the supernatant was applied to microglia for 6 or 24 hr or microglia cells were directly treated with Poly I:C for 6 and 24 hr. Expression of interleukin (IL)-1ß, IL-6, tumour necrosis factor (TNF)α, IL-10, Cox2 and iNOS was evaluated in quantitative PCR. Phagocytosis was evaluated with a microscope-based and a fluoroscan-based phagocytosis assay.

RESULTS

Retinal pigment epithelium (RPE) cells induce the expression of IL-6, IL-1ß and IL-10 in microglia cells. Microglia cells respond to Poly I:C stimulation in a concentration-dependent manner with the induction of IL-1ß, IL-6, TNFα, Cox2, iNOS and, to a lesser degree, IL-10. Stimulation of microglia cells with supernatant of Poly I:C-treated RPE cells further elevated IL-6, IL-1ß and Cox2 expression, while it reduced the expression of iNOS. No changes in phagocytosis could be detected.

CONCLUSIONS

TLR-3-activated RPE exacerbates inflammatory response of microglia in a differentiated manner. This indicates that viral infections in the RPE may have a proinflammatory influence on retinal microglia.

The retinal pigment epithelium (RPE) induces FasL and reduces iNOS and Cox2 in primary monocytes

PURPOSE

Retinal pigment epithelium (RPE) cells may alter the phenotype of monocytes by soluble factors that may be influenced by stimulation of the RPE. Since RPE cells carry the toll-like receptor-3 (TLR3) that detects and reacts to viral infection through binding of dsRNA we investigated the effects of RPE cells with or without TLR3 stimulation on blood-derived monocytes with respect to regulation of pro-/anti-inflammatory cytokines, anti-angiogenic factors and migratory properties.

METHODS

Primary RPE cells were prepared from porcine eyes; monocytes were prepared from porcine blood. TLR3 activation was induced by polyinosinic:polycytidylic acid (Poly I:C). RPE cells were stimulated with Poly I:C in different concentrations for 24 hours and a cell culture supernatant was applied to the monocytes. Expression of CD14 and Fas ligand (FasL) was determined via flow cytometry. The expression of IL-6, IL-1ß, TNFα, Cox2, iNOS and IL-10 was determined via quantitative RT-PCR. Migration was determined using Boyden chamber experiments.

RESULTS

The supernatant of RPE cells, irrespective of TLR3 activation, induced FasL expression in the monocytes. Expression of iNOS and Cox2 was reduced by RPE cells and the reduction of Cox2 but not if iNOS was lost under TLR3 activation. No induction of IL-6, IL-1ß, IL-10 or TNFα by the RPE was seen. TLR3-activated RPE cells induced monocyte migration.

CONCLUSION

RPE cells induce an upregulation of FasL and a downregulation of iNOS and Cox2 without upregulating inflammatory cytokines, possibly inducing an anti-angiogenic phenotype in the monocytes. This phenotype is still upheld after challenging RPE cells with dsRNA, mimicking a viral infection.

Comparison of the efficacy of aflibercept, ranibizumab, and bevacizumab in an RPE/choroid organ culture

PURPOSE

Anti-VEGF treatment is the therapy of choice in age-related macular degeneration and is also applied in diabetic macular edema or retinal vein occlusion. Recently, aflibercept has been approved for therapeutic use. In this study, we investigate the efficacy of aflibercept in comparison with the VEGF-antagonists ranibizumab and bevacizumab in RPE/choroid organ cultures.

METHODS

RPE/choroid organ cultures were prepared from freshly slaughtered pigs' eyes. Organ cultures were treated with 125 μg/ml aflibercept, ranibizumab, or bevacizumab, and the VEGF content of the supernatant was evaluated over the course of 7 days. Additionally, the minimal concentration of VEGF inhibition was evaluated in organ cultures, measured after 6 h of application.

RESULTS

Aflibercept was able to completely inhibit VEGF detection for 6 h at a minimal concentration of 0.031 μg/ml, in contrast to bevacizumab (3.9 μg/ml) and ranibizumab (0.244 μg/ml). A statistically significant VEGF inhibition compared to control could be found for aflibercept and ranibizumab down to and including 0.031 μg/ml, while bevacizumab was significantly reduced compared to control down to a concentration of 0.244 μg/ml and again at 0.061 μg/ml. Inhibition of VEGF after a single aflibercept application of 125 μg/ml could be found over the course of 7 days, with some VEGF detectable at the 7th day. In contrast, VEGF was detectable after 72 h of ranibizumab treatment and some VEGF could already be found 12 h after bevacizumab treatment.

CONCLUSIONS

In conclusion, aflibercept displays a prolonged VEGF inhibition, confirming its effectiveness but also raising concerns about possible side effects of long-term usage.

Hyperthermia-induced upregulation of vascular endothelial growth factor in retinal pigment epithelial cells is regulated by mitogen-activated protein kinases

PURPOSE

Localized application of hyperthermia is a potential treatment for retinal diseases. Vascular endothelial growth factor (VEGF) derived from the retinal pigment epithelium (RPE) is implicated in a variety of retinal pathologies. As it has been recently shown that hyperthermia may induce VEGF in the RPE, the aim of this study was to investigate hyperthermia-induced VEGF secretion and the pathways of hyperthermal VEGF upregulation in the RPE.

MATERIAL AND METHODS

The human RPE cell line (Arpe-19) was exposed to 40°, 42°, 45° and 50 °C for one, five and 15 min. Cell viability was evaluated using a trypan blue exclusion assay, VEGF secretion was evaluated by an enzyme-linked immunosorbent assay ELISA) and VEGF expression was investigated using a Western blot. Involvement of mitogen-activated protein kinase (MAPK) pathways (ERK1/2, JNK, p38) and transient receptor potential vanilloid (TRPV) channels on VEGF induction was investigated using commercially available inhibitors (U0126, SB203580, SP600125, ruthenium red). Expression and phosphorylation of MAPKs was investigated using a Western blot.

RESULTS

Hyperthermia induces time- and temperature-dependent cell death in human RPE cells. VEGF expression and secretion is induced by hyperthermia in a time- and temperature-dependent manner mediated by p38 and to a lesser degree by JNK. TRPV channels seem to play a minor role in regulation of hyperthermia-induced VEGF secretion.

CONCLUSIONS

Hyperthermia induces temperature-dependent secretion of VEGF in the RPE, which is mediated by p38 and, to a lesser extent, JNK. This may lead to undesired effects from hyperthermal treatment of retinal diseases.

CD73 expression in RPE cells is associated with the suppression of conventional CD4 cell proliferation

CD73 is intensively involved in the regulation of immune responses through the conversion of pro-inflammatory ATP to immunosuppressive adenosine. Herein, we clarified whether cells in the retina express CD73 and participate in the regulation of inflammatory eye diseases such as experimental autoimmune uveitis (EAU). First, immunofluorescence staining was performed to compare the distribution of CD73(+) cells in the retinas of EAU-induced and normal B10RIII mice. The results revealed that a layer of cells in the normal retina that was consistent with the location of retinal pigment epithelial (RPE) cells strongly expressing CD73, and the expression was markedly reduced in the presence of EAU. Thereafter, EAU was also induced in C57BL/6 mice by active immunization or adoptive transfer. CD73 expression in isolated RPE cells was assessed by real-time RT-PCR and western blotting, and the catalytic abilities of the cells to convert AMP to adenosine were determined using HPLC analyses. Compared to the normal control, significantly decreased CD73 expression and AMP catalytic ability were found in the RPE cells isolated from inflamed eyes. CD73 expression and activity were also studied in cultured RPE cells treated with different stimuli, such as Toll-like receptor ligands and cytokines. Highly varied functional CD73 expression was observed in RPE cells through cytokines or Toll-like receptor agonist treatments. Finally, whether RPE cells could regulate the immune response, particularly the proliferation of CD4 cells, through surface-expressed CD73 was determined using a two-chamber assay. The robust inhibition of conventional T-cell proliferation was uniquely observed when CD73(+) RPE cells in the upper chamber were in the presence of AMP. To further confirm the function of CD73 in RPE cells, Cd73(-/-) RPE cells were isolated, and CD73-rescued control cells were constructed. CD73(+)Cd73(-/-) RPE, not Cd73(-/-) RPE, significantly suppressed interacted CD4 cells proliferation and cytokine production. Taken together, these data suggest that naive RPE cells suppressed the immune response through their high expression of CD73. The expression of CD73 in RPE cells could be regulated through many factors, and down-regulated CD73 expression attenuated the suppressive effect of RPE on the proliferation of conventional CD4 cells.

MUC18 Differentially Regulates Pro-Inflammatory and Anti-Viral Responses in Human Airway Epithelial Cells

OBJECTIVE

MUC18 or CD146, a transmembrane glycoprotein, is mainly expressed by endothelial cells and smooth muscle cells where it serves as a cell-cell adhesion molecule. We have found MUC18 up-regulation in airway epithelial cells of patients with asthma and chronic obstructive pulmonary disease (COPD). However, the function of MUC18 in airway epithelial cells remains unclear. In the present study, we tested the hypothesis that MUC18 exerts a pro-inflammatory function during stimulation with a viral mimic polyI:C or human rhinovirus infection.

METHODS

Normal human primary airway epithelial cells were transduced with lentivirus encoding MUC18 cDNA to over-express MUC18 or with GFP (control), and treated with polyI:C or HRV for detection of pro-inflammatory cytokine IL-8 and anti-viral gene IFN-β. Additionally, we performed cell culture of human lung epithelial cell line NCIH292 cells to determine the mechanisms of MUC18 function.

RESULTS

We found that MUC18 over-expression promoted IL-8 production, while it inhibited IFN-β expression following polyI:C stimulation or HRV infection. Increased phosphorylation of MUC18 serines was observed in MUC18 over-expressing cells. Reduction of MUC18 serine phosphorylation by inhibiting ERK activity was associated with less production of IL-8 following polyI:C stimulation.

CONCLUSIONS

Our results for the first time demonstrate MUC18's pro-inflammatory and anti-viral function in human airway epithelial cells.