NLRP3 Upregulation in Retinal Pigment Epithelium in Age-Related Macular Degeneration

Natural plant medications for the treatment of retinal diseases: The blood-retinal barrier as a clue

BACKGROUND

Retinal diseases significantly contribute to the global burden of visual impairment and blindness. The occurrence of retinal diseases is often accompanied by destruction of the blood‒retinal barrier, a vital physiological structure responsible for maintaining the stability of the retinal microenvironment. However, detailed summaries of the factors damage the blood‒retinal barrier and treatment methods involving natural plant medications are lacking.

PURPOSE

To comprehensively summarize and analyze the protective effects of active substances in natural plant medications on damage to the blood-retina barrier that occurs when retinal illnesses, particularly diabetic retinopathy, and examine their medicinal value and future development prospects.

METHODS

In this study, we searched for studies published in the ScienceDirect, PubMed, and Web of Science databases. The keywords used included natural plant medications, plants, natural herbs, blood retinal barrier, retinal diseases, diabetic retinopathy, age-related macular degeneration, and uveitis. Chinese herbal compound articles, non-English articles, warning journals, and duplicates were excluded from the analysis.

RESULTS

The blood‒retinal barrier is susceptible to high glucose, aging, immune responses, and other factors that destroy retinal homeostasis, resulting in pathological changes such as apoptosis and increased vascular permeability. Existing studies have shown that the active compounds or extracts of many natural plants have the effect of repairing blood-retinal barrier dysfunction. Notably, berberine, puerarin, and Lycium barbarum polysaccharides exhibited remarkable therapeutic effects. Additionally, curcumin, astragaloside IV, hesperidin, resveratrol, ginsenoside Rb1, luteolin, and Panax notoginseng saponins can effectively protect the blood‒retinal barrier by interfering with distinct pathways. The active ingredients found in natural plant medications primarily repair the blood‒retinal barrier by modulating pathological factors such as oxidative stress, inflammation, pyroptosis, and autophagy, thereby alleviating retinal diseases.

CONCLUSION

This review summarizes a series of plant extracts and plant active compounds that can treat retinal diseases by preventing and treating blood‒retinal barrier damage and provides reference for the research of new drugs for treating retinal diseases.

Genetic factors associated with age-related macular degeneration modulating plasma inflammatory biomarker levels in patients with AIDS

INTRODUCTION

Patients with the acquired immunodeficiency syndrome (AIDS) have an increased prevalence and incidence of intermediate-stage age-related macular degeneration (AMD). Several elevated plasma inflammatory biomarkers are associated with increased incidence of intermediate-stage AMD in this population. We evaluated the association between AMD risk alleles and plasma inflammatory biomarker levels in persons with AIDS.

MATERIALS AND METHODS

Cryopreserved plasma specimens of 229 non-Hispanic White and 252 non-Hispanic blacks from the Longitudinal Study of the Ocular Complications of AIDS cohort were assayed for plasma levels of soluble tumor necrosis factor receptor (sTNFR) 2, interleukin (IL)-18, C × 3motif chemokine ligand 1 (CX3CL1), C-reactive protein (CRP), and soluble CD14 (sCD14). Genotyping included AMD-associated variants rs10801553 and rs800292 for complement factor H (CFH) rs9332739 and rs547154 for complement factor 2 (C2), rs2230199 for C3, rs2285714 for CFI, and rs3732379 and rs3732378 for C × 3motif chemokine receptor 1 (CX3CR1).

RESULTS

In Whites, AMD low-risk CX3CR1 variants (V249I and T280M) were associated with reduced plasma levels of IL-18. In Blacks, AMD low-risk C3 R102G and low-risk CX3CR1 T280M variants were associated with reduced CRP levels.

CONCLUSIONS

Genetic variants in AMD-associated immune genes may influence AMD-associated systemic plasma inflammatory biomarker levels in patients with AIDS.

Early onset drusen and RPE dysfunction in a patient with NLRP3-AID

Retinal pigment epithelium (RPE) dysfunction, manifested as drusen formation and RPE mottling, is a characteristic lesion of aging. The mechanism of RPE dysfunction remains unknown. Previous animal studies have proven that the activation of NLRP3 inflammasome in RPE leads to apoptosis and pyroptosis, which may play a very important role in the development of age-related macular degeneration (AMD). However, there is a lack of clinical evidence to support the above hypothesis. Herein, we report a 38-year-old Chinese Han woman who had NLRP3-associated autoinflammatory disease (NLRP3-AID) with widely scattered drusen at the posterior pole in both eyes. NLRP3-AID is a rare disease caused by mutations of the NLRP3 gene, leading to NLRP3 inflammasome activation. This report of early-onset drusen provides clinical evidence that the NLRP3 inflammasome might contribute to the occurrence of RPE dysfunction and is a potential cause of age-related macular degeneration (AMD).

Revealing Academic Evolution and Frontier Pattern in the Field of Uveitis Using Bibliometric Analysis, Natural Language Processing, and Machine Learning

PURPOSE

Numerous uveitis articles were published in this century, underneath which hides valuable intelligence. We aimed to characterize the evolution and patterns in this field.

METHODS

We divided the 15,994 uveitis papers into four consecutive time periods for bibliometric analysis, and applied latent Dirichlet allocation topic modeling and machine learning techniques to the latest period. .

RESULTS

The yearly publication pattern fitted the curve: 1.21335x2 - 4,848.95282x + 4,844,935.58876 (R2 = 0.98311). The USA, the most productive country/region, focused on topics like ankylosing spondylitis and biologic therapy, whereas China (mainland) focused on topics like OCT and Behcet disease. The logistic regression showed the highest accuracy (71.6%) in the test set.

CONCLUSION

In this century, a growing number of countries/regions/authors/journals are involved in the uveitis study, promoting the scientific output and thematic evolution. Our pioneering study uncovers the evolving academic trends and frontier patterns in this field using bibliometric analysis and AI algorithms.

The Role of Retinal Pigment Epithelial Cells in Age-Related Macular Degeneration: Phagocytosis and Autophagy

Age-related macular degeneration (AMD) causes vision loss in the elderly population. Dry AMD leads to the formation of Drusen, while wet AMD is characterized by cell proliferation and choroidal angiogenesis. The retinal pigment epithelium (RPE) plays a key role in AMD pathogenesis. In particular, helioreceptor renewal depends on outer segment phagocytosis of RPE cells, while RPE autophagy can protect cells from oxidative stress damage. However, when the oxidative stress burden is too high and homeostasis is disturbed, the phagocytosis and autophagy functions of RPE become damaged, leading to AMD development and progression. Hence, characterizing the roles of RPE cell phagocytosis and autophagy in the pathogenesis of AMD can inform the development of potential therapeutic targets to prevent irreversible RPE and photoreceptor cell death, thus protecting against AMD.

Role of inflammasome activation in neovascular age-related macular degeneration

Current anti-VEGF-A therapies inhibit choroidal neovascularization (CNV) in a subset of patients with neovascular age-related macular degeneration (NV-AMD). However, long-term treatment with such anti-VEGF-A therapies may impair physiological functions of the choriocapillaris and retina for which VEGF-A is needed. Moreover, disease progression can occur despite continuous anti-VEGF-A treatment. Thus, novel therapies for NV-AMD are urgently needed that target specifically disease-associated mechanisms without impairing growth factors and cellular pathways that are required for homeostatic functions of the retina and choroid. Inhibiting the inflammatory pathways that promote CNV would be such a promising novel approach that would likely not interfere with the normal functions of healthy retinal and choroidal cells. In this context, the inflammasome, a proinflammatory protein complex that promotes pathologic angiogenesis largely through generation of IL-1β and which has been reported to be activated in AMD, has become an area of much interest in the AMD field. However, most studies have focused mainly on the NLRP3 inflammasome in retinal pigment epithelial cells (RPE), and conflicting findings have resulted in an unclear picture of the role of the inflammasome for AMD pathogenesis. Recent data suggest that inflammasome activation in activated macrophages and retinal microglia but not in RPE cells promotes CNV. Furthermore, inflammasome activation can occur in CNV macrophages and microglia despite lack of NLRP3. Thus, activation of both NLRP3 inflammasomes as well as non-NLRP3 inflammasomes in macrophages/microglia at sites of CNV formation likely promote NV-AMD.

Exploring the pathogenesis of age-related macular degeneration: A review of the interplay between retinal pigment epithelium dysfunction and the innate immune system

Age-related macular degeneration (AMD) is a leading cause of irreversible vision loss in the older population. Classical hallmarks of early and intermediate AMD are accumulation of drusen, a waste deposit formed under the retina, and pigmentary abnormalities in the retinal pigment epithelium (RPE). When the disease progresses into late AMD, vision is affected due to death of the RPE and the light-sensitive photoreceptors. The RPE is essential to the health of the retina as it forms the outer blood retinal barrier, which establishes ocular immune regulation, and provides support for the photoreceptors. Due to its unique anatomical position, the RPE can communicate with the retinal environment and the systemic immune environment. In AMD, RPE dysfunction and the accumulation of drusen drive the infiltration of retinal and systemic innate immune cells into the outer retina. While recruited endogenous or systemic mononuclear phagocytes (MPs) contribute to the removal of noxious debris, the accumulation of MPs can also result in chronic inflammation and contribute to AMD progression. In addition, direct communication and indirect molecular signaling between MPs and the RPE may promote RPE cell death, choroidal neovascularization and fibrotic scarring that occur in late AMD. In this review, we explore how the RPE and innate immune cells maintain retinal homeostasis, and detail how RPE dysfunction and aberrant immune cell recruitment contribute to AMD pathogenesis. Evidence from AMD patients will be discussed in conjunction with data from preclinical models, to shed light on future therapeutic targets for the treatment of AMD.

Spotlight on pyroptosis: role in pathogenesis and therapeutic potential of ocular diseases

Pyroptosis is a programmed cell death characterized by swift plasma membrane disruption and subsequent release of cellular contents and pro-inflammatory mediators (cytokines), including IL‐1β and IL‐18. It differs from other types of programmed cell death such as apoptosis, autophagy, necroptosis, ferroptosis, and NETosis in terms of its morphology and mechanism. As a recently discovered form of cell death, pyroptosis has been demonstrated to be involved in the progression of multiple diseases. Recent studies have also suggested that pyroptosis is linked to various ocular diseases. In this review, we systematically summarized and discussed recent scientific discoveries of the involvement of pyroptosis in common ocular diseases, including diabetic retinopathy, age-related macular degeneration, AIDS-related human cytomegalovirus retinitis, glaucoma, dry eye disease, keratitis, uveitis, and cataract. We also organized new and emerging evidence suggesting that pyroptosis signaling pathways may be potential therapeutic targets in ocular diseases, hoping to provide a summary of overall intervention strategies and relevant multi-dimensional evaluations for various ocular diseases, as well as offer valuable ideas for further research and development from the perspective of pyroptosis.

Inflammasome Activation in Retinal Pigment Epithelium from Human Donors with Age-Related Macular Degeneration

Age-related macular degeneration (AMD), the leading cause of blindness in the elderly, is characterized by the death of retinal pigment epithelium (RPE) and photoreceptors. One of the risk factors associated with developing AMD is the single nucleotide polymorphism (SNP) found within the gene encoding complement factor H (CFH). Part of the innate immune system, CFH inhibits alternative complement pathway activation. Multi-protein complexes called inflammasomes also play a role in the innate immune response. Previous studies reported that inflammasome activation may contribute to AMD pathology. In this study, we used primary human adult RPE cell cultures from multiple donors, with and without AMD, that were genotyped for the Y402H CFH risk allele. We found complement and inflammasome-related genes and proteins at basal levels in RPE tissue and cell cultures. Additionally, treatment with rotenone, bafilomycin A, and ATP led to inflammasome activation. Overall, the response to priming and activation was similar, irrespective of disease state or CFH genotype. While these data show that the inflammasome is present and active in RPE, our results suggest that inflammasome activation may not contribute to early AMD pathology.

HMGB2 causes photoreceptor death via down-regulating Nrf2/HO-1 and up-regulating NF-κB/NLRP3 signaling pathways in light-induced retinal degeneration model

In the pathogenesis of retinal degenerative diseases, oxidative stress is a key driver leading to photoreceptor death and eventually vision loss. Currently, there are no effective therapies available to rescue photoreceptors in these diseases. High-mobility group box 2 (HMGB2), a pro-inflammatory factor and damage-associated molecular patterns (DAMPs), has been proven to mediate various inflammatory diseases, but its role in retinal degenerative diseases, especially in retinal inflammation and photoreceptor degeneration, still remains unknown. In this study, we assessed the localization and function of HMGB2 under oxidative stress and explored the underlying mechanisms in a mouse model of light-induced retinal damage (LIRD). The results showed that increased oxidative stress, the photoreceptors death, as well as the pyroptosis-related proteins were evidenced in mice retina after light exposure. HMGB2 protein was predominantly expressed in the outer nuclear layer (ONL), which was translocated to the cytoplasm and released after injury. The mechanistic effect of HMGB2 was studied in the 661w cell line treated with H₂O₂, showing that exogenous recombinant HMGB2 protein reduced the expressions of the antioxidant protein nuclear erythroid factor 2-related factor 2 (Nrf2) and its downstream target heme oxygenase-1 (HO-1), and induced NF-κB/NLRP3 signaling pathway. HMGB2 knockdown increased cell viability, up-regulated the expressions of Nrf2 and HO-1, down-regulated the expressions of pyroptosis-related proteins in H₂O₂-treated 661w cells; and also prevented photoreceptors loss and maintained ONL in mice model of LIRD. The present study proposed HMGB2 as a potential therapeutic target for treatment of retinal degenerative diseases.

PKA and Epac1 Reduce Nek7 to Block the NLRP3 Inflammasome Proteins in the Retinal Vasculature

Purpose

To determine whether protein kinase a (PKA) and exchange protein for cAMP 1 (Epac1) inhibit NIMA-related kinase 7 (Nek7) to block the NOD-like receptor family pyrin domain-containing family member 3 (NLRP3) signaling pathway.

Methods

Retinal endothelial cells (RECs) were grown in normal (5 mM) or high (25 mM) glucose. Some cells were treated with a Nek7 cDNA plasmid, Nek7 siRNA; an Epac1 agonist, forskolin; a PKA agonist; or an empty vector. Epac1 floxed and Cdh5-cre Epac1 mice and Nek7 floxed and Cdh5-cre Nek7 mice were also used. Western blot analyses were done on cell culture or whole retinal lysates for NLRP3, cleaved caspase 1, interleukin-1-beta (IL-1β). A PKA activity assay was also done.

Results

Nek7 cDNA increased NLRP3 signaling proteins, but Nek7 siRNA inhibited high-glucose induction of these proteins in retinal endothelial cells. Epac1 and forskolin both reduced Nek7 and NLRP3 pathway proteins, even when given in combination with Nek7 cDNA. Elimination of Nek7 in endothelial cells reduced NLRP3 signaling proteins in whole retinal lysates from mice.

Conclusions

Nek7 regulated NLRP3 inflammasome protein levels both in vitro and in vivo. Both Epac1 and PKA lie upstream of Nek7 and NLRP3 and can overcome excessive Nek7 levels. These studies establish that cAMP proteins can inhibit Nek7 and block activation of the NLRP3 inflammasome proteins.

Plasma Rich in Growth Factors Promotes Autophagy in ARPE19 Cells in Response to Oxidative Stress Induced by Blue Light

Age-related macular degeneration (AMD) causes the degeneration of photoreceptors and retinal cells leading to vision loss in older subjects. Among possible exogenous risk factors, it has been recently proposed that long-term exposure to blue light could aggravate the course of AMD. In the search for therapeutic options, plasma rich in growth factors (PRGF) has been shown to enhance cell antioxidant pathways and protect photoreceptors against the harm produced by blue light, although its mechanism of action remains unknown. One possible mechanism, autophagy, is one of the most conservative cell renewal systems used in eukaryotes to destroy cellular components that have been damaged by some kind of insult. The oxidative stress of exposure to blue light is known to induce cell autophagy. In this study, we examined the combined effects on autophagy of blue light and PRGF in a retinal cell line, ARPE19. In response to treatment with both PRGF and blue light, we detected the modulated expression of autophagy markers such as NF-kB, p62/sqstm1, Atg5, LC3 and Beclin1, and inflammatory markers such as IL1B and IL18. Our findings suggest that PRGF promotes cell autophagy in response to exposure to blue light.

lncRNA MEG3, Acting as a ceRNA, Modulates RPE Differentiation Through the miR-7-5p/Pax6 Axis

Accumulated evidence indicated that long non-coding RNAs (lncRNAs) involves in numerous biological and pathological processes, including age-related macular degeneration (AMD). Dysfunction and dedifferentiation of retinal pigment epithelium (RPE) cells have been demonstrated to be one of the crucial factor in AMD etiology. Herein, we aim to investigate the essential role of lncRNA maternally expressed gene 3 (MEG3) in AMD progression. Expression patterns of MEG3 were measured in dysfunctional REP cells exposed with H₂O₂ or TNF-α using qRT-PCR assay. Specifically, the intercellular distribution of MEG3 in REP cells was further explored using the subcellular fraction detection. Relative expression of RPE markers or RPE dedifferentiation-related markers was determined using qRT-PCR and western blot analysis, respectively. Immunofluorescence staining was performed to examine the expressions of RPE markers ZO-1 and β-catenin. Concentration of vascular endothelial growth factor (VEGFA) in the supernatant was detected using ELISA kit. Luciferase reporter assay was performed to verify the MEG3/miR-7-5p/Pax6 regulatory network, which was further determined in in vitro studies. MEG3 expression was significantly decreased in H₂O₂ or TNF-α-treated REP cells, and it was upregulated along with RPE differentiation. Reduced MEG3 expression resulted in RPE dedifferentiation, which was indicated by decreased expressions of RPE markers, accumulated mitochondrial reactive oxygen species, and reduced VEGFA. Mechanistically, MEG3 functioned as a sponge for miR-7-5p to restore the expression of Pax6. Our study demonstrated that MEG3 exerts a protective role against AMD by maintaining RPE differentiation via miR-7-5p/Pax6 axis, suggesting a protective therapeutic target in AMD treatment.

Anti-inflammatory activities of Gardenia jasminoides extracts in retinal pigment epithelial cells and zebrafish embryos

Age-related macular degeneration (AMD) is the most common cause of visual impairment in developed countries. Inflammation serves a critical role in the pathogenesis of AMD. Gardenia jasminoides is found in several regions of China and is traditionally used as an organic yellow dye but has also been widely used as a therapeutic agent in numerous diseases, including inflammation, depression, hepatic and vascular disorders, which may reflect the variability of functional compounds that are present in Gardenia jasminoides extracts (GJE). To investigate the therapeutic potential of GJE for AMD, ARPE-19 cells were treated with lipopolysaccharide (LPS) or LPS plus GJE. GJE significantly decreased LPS-induced expression of proinflammatory cytokines, including IL-1β, IL-6 and TNF-α. In the in vivo study, GJE inhibited CuSO4-induced migration of primitive macrophages to the lateral line in zebrafish embryos. GJE also attenuated expression of cytokines (IL-1β, IL-6 and TNF-α), NFKB activating protein (nkap) and TLR4 in ARPE-19 cells. The results of the present study demonstrated the anti-inflammatory potential of GJE in vitro and in vivo, and suggested GJE as a therapeutic candidate for AMD.

Nutraceutical Strategies for Suppressing NLRP3 Inflammasome Activation: Pertinence to the Management of COVID-19 and Beyond

Inflammasomes are intracellular protein complexes that form in response to a variety of stress signals and that serve to catalyze the proteolytic conversion of pro-interleukin-1β and pro-interleukin-18 to active interleukin-1β and interleukin-18, central mediators of the inflammatory response; inflammasomes can also promote a type of cell death known as pyroptosis. The NLRP3 inflammasome has received the most study and plays an important pathogenic role in a vast range of pathologies associated with inflammation-including atherosclerosis, myocardial infarction, the complications of diabetes, neurological and autoimmune disorders, dry macular degeneration, gout, and the cytokine storm phase of COVID-19. A consideration of the molecular biology underlying inflammasome priming and activation enables the prediction that a range of nutraceuticals may have clinical potential for suppressing inflammasome activity-antioxidants including phycocyanobilin, phase 2 inducers, melatonin, and N-acetylcysteine, the AMPK activator berberine, glucosamine, zinc, and various nutraceuticals that support generation of hydrogen sulfide. Complex nutraceuticals or functional foods featuring a number of these agents may find utility in the prevention and control of a wide range of medical disorders.

Mechanisms of mitochondrial dysfunction and their impact on age-related macular degeneration

Oxidative stress-induced damage to the retinal pigment epithelium (RPE) is considered to be a key factor in age-related macular degeneration (AMD) pathology. RPE cells are constantly exposed to oxidative stress that may lead to the accumulation of damaged cellular proteins, lipids, nucleic acids, and cellular organelles, including mitochondria. The ubiquitin-proteasome and the lysosomal/autophagy pathways are the two major proteolytic systems to remove damaged proteins and organelles. There is increasing evidence that proteostasis is disturbed in RPE as evidenced by lysosomal lipofuscin and extracellular drusen accumulation in AMD. Nuclear factor-erythroid 2-related factor-2 (NFE2L2) and peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) are master transcription factors in the regulation of antioxidant enzymes, clearance systems, and biogenesis of mitochondria. The precise cause of RPE degeneration and the onset and progression of AMD are not fully understood. However, mitochondria dysfunction, increased reactive oxygen species (ROS) production, and mitochondrial DNA (mtDNA) damage are observed together with increased protein aggregation and inflammation in AMD. In contrast, functional mitochondria prevent RPE cells damage and suppress inflammation. Here, we will discuss the role of mitochondria in RPE degeneration and AMD pathology focused on mtDNA damage and repair, autophagy/mitophagy signaling, and regulation of inflammation. Mitochondria are putative therapeutic targets to prevent or treat AMD.

Learning from Fifteen Years of Genome-Wide Association Studies in Age-Related Macular Degeneration

Over the last 15 years, genome-wide association studies (GWAS) have greatly advanced our understanding of the genetic landscape of complex phenotypes. Nevertheless, causal interpretations of GWAS data are challenging but crucial to understand underlying mechanisms and pathologies. In this review, we explore to what extend the research community follows up on GWAS data. We have traced the scientific activities responding to the two largest GWAS conducted on age-related macular degeneration (AMD) so far. Altogether 703 articles were manually categorized according to their study type. This demonstrates that follow-up studies mainly involve "Review articles" (33%) or "Genetic association studies" (33%), while 19% of publications report on findings from experimental work. It is striking to note that only three of 16 AMD-associated loci described de novo in 2016 were examined in the four-year follow-up period after publication. A comparative analysis of five studies on gene expression regulation in AMD-associated loci revealed consistent gene candidates for 15 of these loci. Our random survey highlights the fact that functional follow-up studies on GWAS results are still in its early stages hampering a significant refinement of the vast association data and thus a more accurate insight into mechanisms and pathways.

The role of oxidized phospholipids in the development of disease

Oxidized phospholipids (OxPLs), complex mixtures of phospholipid oxidation products generated during normal or pathological processes, are increasingly recognized to show bioactive effects on many cellular signalling pathways. There is a growing body of evidence showing that OxPLs play an important role in many diseases, so it is essential to define the specific role of OxPLs in different diseases for the design of disease therapies. In vastly diverse pathological processes, OxPLs act as pro-inflammatory agents and contribute to the progression of many diseases; in addition, they play a role in anti-inflammatory processes, promoting the dissipation of inflammation and inhibiting the progression of some diseases. In addition to participating in the regulation of inflammatory responses, OxPLs affect the occurrence and development of diseases through other pathways, such as apoptosis promotion. In this review, the different and even opposite effects of different OxPL molecular species are discussed. Furthermore, the specific effects of OxPLs in various diseases, as well as the receptor and cellular mechanisms involved, are summarized.

The Role of Inflammation in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a blinding eye disease which incidence gradually increases with age. Inflammation participates in AMD pathogenesis, including choroidal neovascularization and geographic atrophy. It is also a kind of self-protective regulation from injury for the eyes. In this review, we described inflammation in AMD pathogenesis, summarized the roles played by inflammation-related cytokines, including pro-inflammatory and anti-inflammatory cytokines, as well as leukocytes (macrophages, dendritic cells, neutrophils, T lymphocytes and B lymphocytes) in the innate or adaptive immunity in AMD. Possible clinical applications such as potential diagnostic biomarkers and anti-inflammatory therapies were also discussed. This review overviews the inflammation as a target of novel effective therapies in treating AMD.

Properdin Modulates Complement Component Production in Stressed Human Primary Retinal Pigment Epithelium Cells

The retinal pigment epithelium (RPE) maintains visual function and preserves structural integrity of the retina. Chronic dysfunction of the RPE is associated with retinal degeneration, including age-related macular degeneration (AMD). The AMD pathogenesis includes both increased oxidative stress and complement dysregulation. Physiological sources of oxidative stress in the retina are well known, while complement sources and regulation are still under debate. Using human primary RPE (hpRPE) cells, we have established a model to investigate complement component expression on transcript and protein level in AMD-risk and non-risk hpRPE cells. We evaluated the effect of properdin, a complement stabilizer, on the hpRPE cell-dependent complement profile exposed to oxidative stress. hpRPE cells expressed complement components, receptors and regulators. Complement proteins were also stored and secreted by hpRPE cells. We associated AMD-risk single nucleotide polymorphisms with an increased secretion of complement factors D (CFD) and I (CFI). Furthermore, we detected hpRPE cell-associated complement activation products (C3a, C5a) independent of any extracellularly added complement system. Exogenous properdin increased the mRNA expression of CFI and CFD, but decreased levels of complement components (C1Q, C3), receptors (C3AR, C5AR1, CD11B) and inflammation-associated transcripts (NLRP3, IL1B) in hpRPE cells exposed to oxidative stress. This properdin effect was time-dependently counter regulated. In conclusion, our data unveiled a local, genotype-associated complement component production in hpRPE cells, regulated by exogenous properdin. The local complement production and activation via blood-independent mechanisms can be a new therapeutic target for AMD.

Detrimental Effects of UVB on Retinal Pigment Epithelial Cells and Its Role in Age-Related Macular Degeneration

Retinal pigment epithelial (RPE) cells are an essential part of the human eye because they not only mediate and control the transfer of fluids and solutes but also protect the retina against photooxidative damage and renew photoreceptor cells through phagocytosis. However, their function necessitates cumulative exposure to the sun resulting in UV damage, which may lead to the development of age-related macular degeneration (AMD). Several studies have shown that UVB induces direct DNA damage and oxidative stress in RPE cells by increasing ROS and dysregulating endogenous antioxidants. Activation of different signaling pathways connected to inflammation, cell cycle arrest, and intrinsic apoptosis was reported as well. Besides that, essential functions like phagocytosis, osmoregulation, and water permeability of RPE cells were also affected. Although the melanin within RPE cells can act as a photoprotectant, this photoprotection decreases with age. Nevertheless, the changes in lens epithelium-derived growth factor (LEDGF) and autophagic activity or application of bioactive compounds from natural products can reverse the detrimental effect of UVB. Additionally, in vivo studies on the whole retina demonstrated that UVB irradiation induces gene and protein level dysregulation, indicating cellular stress and aberrations in the chromosome level. Morphological changes like retinal depigmentation and drusen formation were noted as well which is similar to the etiology of AMD, suggesting the connection of UVB damage with AMD. Therefore, future studies, which include mechanism studies via in vitro or in vivo and other potential bioactive compounds, should be pursued for a better understanding of the involvement of UVB in AMD.

Autophagy in Age-Related Macular Degeneration: A Regulatory Mechanism of Oxidative Stress

Age-related macular degeneration (AMD) is a leading cause of severe visual loss and irreversible blindness in the elderly population worldwide. Retinal pigment epithelial (RPE) cells are the major site of pathological alterations in AMD. They are responsible for the phagocytosis of shed photoreceptor outer segments (POSs) and clearance of cellular waste under physiological conditions. Age-related, cumulative oxidative stimuli contribute to the pathogenesis of AMD. Excessive oxidative stress induces RPE cell degeneration and incomplete digestion of POSs, leading to the continuous accumulation of cellular waste (such as lipofuscin). Autophagy is a major system of degradation of damaged or unnecessary proteins. However, degenerative RPE cells in AMD patients cannot perform autophagy sufficiently to resist oxidative damage. Increasing evidence supports the idea that enhancing the autophagic process can properly alleviate oxidative injury in AMD and protect RPE and photoreceptor cells from degeneration and death, although overactivated autophagy may lead to cell death at early stages of retinal degenerative diseases. The crosstalk among the NFE2L2, PGC-1, p62, AMPK, and PI3K/Akt/mTOR pathways may play a crucial role in improving disturbed autophagy and mitigating the progression of AMD. In this review, we discuss how autophagy prevents oxidative damage in AMD, summarize potential neuroprotective strategies for therapeutic interventions, and provide an overview of these neuroprotective mechanisms.

A Comprehensive Proteomic and Phosphoproteomic Analysis of Retinal Pigment Epithelium Reveals Multiple Pathway Alterations in Response to the Inflammatory Stimuli

Overwhelming and persistent inflammation of retinal pigment epithelium (RPE) induces destructive changes in the retinal environment. However, the precise mechanisms remain unclear. In this study, we aimed to investigate RPE-specific biological and metabolic responses against intense inflammation and identify the molecular characteristics determining pathological progression. We performed quantitative analyses of the proteome and phosphoproteome of the human-derived RPE cell line ARPE-19 after treatment with lipopolysaccharide (LPS) for 45 min or 24 h using the latest isobaric tandem-mass tags (TMTs) labeling approach. This approach led to the identification of 8984 proteins, of which 261 showed a 1.5-fold change in abundance after 24 h of treatment with LPS. A parallel phosphoproteome analysis identified 20,632 unique phosphopeptides from 3207 phosphoproteins with 3103 phosphorylation sites. Integrated proteomic and phosphoproteomic analyses showed significant downregulation of proteins related to mitochondrial respiration and cell cycle checkpoint, while proteins related to lipid metabolism, amino acid metabolism, cell-matrix adhesion, and endoplasmic reticulum (ER) stress were upregulated after LPS stimulation. Further, phosphorylation events in multiple pathways, including MAPKK and Wnt/β-catenin signalings, were identified as involved in LPS-triggered pathobiology. In essence, our findings reveal multiple integrated signals exerted by RPE under inflammation and are expected to give insight into the development of therapeutic interventions for RPE disorders.

Epigenetics in age-related macular degeneration: new discoveries and future perspectives

The study of epigenetics has explained some of the 'missing heritability' of age-related macular degeneration (AMD). The epigenome also provides a substantial contribution to the organisation of the functional retina. There is emerging evidence of specific epigenetic mechanisms associated with AMD. This 'AMD epigenome' may offer the chance to develop novel AMD treatments.

Caspase-1-dependent inflammasomes mediate photoreceptor cell death in photo-oxidative damage-induced retinal degeneration

Activation of the inflammasome is involved in the progression of retinal degenerative diseases, in particular, in the pathogenesis of Age-Related Macular Degeneration (AMD), with NLRP3 activation the focus of many investigations. In this study, we used genetic and pharmacological approaches to explore the role of the inflammasome in a mouse model of retinal degeneration. We identify that Casp1/11^−/− mice have better-preserved retinal function, reduced inflammation and increased photoreceptor survivability. While Nlrp3^−/− mice display some level of preservation of retinal function compared to controls, pharmacological inhibition of NLRP3 did not protect against photoreceptor cell death. Further, Aim2^−/−, Nlrc4^−/−, Asc^−/−, and Casp11^−/− mice show no substantial retinal protection. We propose that CASP-1-associated photoreceptor cell death occurs largely independently of NLRP3 and other established inflammasome sensor proteins, or that inhibition of a single sensor is not sufficient to repress the inflammatory cascade. Therapeutic targeting of CASP-1 may offer a more promising avenue to delay the progression of retinal degenerations.

NOD-like Receptors in the Eye: Uncovering Its Role in Diabetic Retinopathy

Diabetic retinopathy (DR) is an ocular complication of diabetes mellitus (DM). International Diabetic Federations (IDF) estimates up to 629 million people with DM by the year 2045 worldwide. Nearly 50% of DM patients will show evidence of diabetic-related eye problems. Therapeutic interventions for DR are limited and mostly involve surgical intervention at the late-stages of the disease. The lack of early-stage diagnostic tools and therapies, especially in DR, demands a better understanding of the biological processes involved in the etiology of disease progression. The recent surge in literature associated with NOD-like receptors (NLRs) has gained massive attraction due to their involvement in mediating the innate immune response and perpetuating inflammatory pathways, a central phenomenon found in the pathogenesis of ocular diseases including DR. The NLR family of receptors are expressed in different eye tissues during pathological conditions suggesting their potential roles in dry eye, ocular infection, retinal ischemia, cataract, glaucoma, age-related macular degeneration (AMD), diabetic macular edema (DME) and DR. Our group is interested in studying the critical early components involved in the immune cell infiltration and inflammatory pathways involved in the progression of DR. Recently, we reported that NLRP3 inflammasome might play a pivotal role in the pathogenesis of DR. This comprehensive review summarizes the findings of NLRs expression in the ocular tissues with special emphasis on its presence in the retinal microglia and DR pathogenesis.

Expression of a CARD Slows the Retinal Degeneration of a Geographic Atrophy Mouse Model

Age-related macular degeneration (AMD) has been linked to oxidative damage and para-inflammation, an activation of inflammasome signaling in the retinal pigment epithelium (RPE) and the underlying choriocapillaris. Herein, we tested the efficacy of a gene-delivered caspase-1 inhibitor in controlling the retinal degeneration observed in two models of RPE-choroid oxidative damage. In an acute model of oxidative stress (NaIO3 injection), eyes pre-treated with the sGFP-TatCARD (trans-activator of transcription; caspase activation and recruitment domain) vector demonstrated a recovery of retinal function and partial protection of RPE structure 1 month after damage, in contrast with control-treated eyes. In a model of chronic oxidative stress (RPE-specific deletion of Sod2), eyes treated with the sGFP-TatCARD vector after the onset of degeneration had a significantly slower decline in retinal function when compared to control-treated eyes. Earlier treatment of this model with the same adeno-associated virus (AAV) vector resulted in a greater protection of RPE function in eyes treated with the TatCARD when compared to control-treated eyes. Our results demonstrate that intravitreal delivery of sGFP-TatCARD reduces inflammation and can protect the retina from both acute and sustained oxidative damage within the RPE and choroid. Therefore, gene therapy with a cell-penetrating inflammasome inhibitor such as CARD may stem the progression of AMD.

Epac1 inhibits PKR to reduce NLRP3 inflammasome proteins in retinal endothelial cells

Purpose: Inflammation has been strongly associated with retinal damage in diseases such as diabetic retinopathy. Several studies have reported that high glucose exposure induces damage to the retinal vasculature. We and others have shown that high glucose can activate the NOD-like receptor family, pyrin domain containing family member 3 (NLRP3) pathway, leading to increased levels of cleaved caspase 1 and IL-1β to activate a number of inflammatory pathways in the retina.

Methods: We used retinal endothelial cells grown in normal (5 mM) or high (25 mM) glucose or retinal lysates from endothelial cell-specific knockout mice for exchange protein activated by cAMP 1 (Epac1). Human recombinant protein kinase R (PKR) or C16, a PKR inhibitor, was used on the cells to dissect PKR and NLRP3 signaling.

Results: Using retinal endothelial cells (REC) in high glucose and whole retinal lysates from endothelial cell-specific knockout of Epac1, we demonstrate that Epac1 regulates PKR phosphorylation. Using an Epac1 agonist or PKR inhibition with C16, we demonstrated that loss of PKR resulted in reduced NLRP3, cleaved caspase 1, and IL-1β levels. Furthermore, despite the addition of recombinant human PKR, Epac1 was still able to significantly reduce NLRP3 signaling.

Conclusion: Overall, these studies demonstrated that PKR regulates the NLRP3 inflammasome in REC, and that Epac1 inhibition of PKR can reduce activation of the NLRP3 inflammasome.

Modulation of three key innate immune pathways for the most common retinal degenerative diseases

This review highlights the role of three key immune pathways in the pathophysiology of major retinal degenerative diseases including diabetic retinopathy, age‐related macular degeneration, and rare retinal dystrophies. We first discuss the mechanisms how loss of retinal homeostasis evokes an unbalanced retinal immune reaction involving responses of local microglia and recruited macrophages, activity of the alternative complement system, and inflammasome assembly in the retinal pigment epithelium. Presenting these key mechanisms as complementary targets, we specifically emphasize the concept of immunomodulation as potential treatment strategy to prevent or delay vision loss. Promising molecules are ligands for phagocyte receptors, specific inhibitors of complement activation products, and inflammasome inhibitors. We comprehensively summarize the scientific evidence for this strategy from preclinical animal models, human ocular tissue analyses, and clinical trials evolving in the last few years.

Autophagy Dysfunction, Cellular Senescence, and Abnormal Immune-Inflammatory Responses in AMD: From Mechanisms to Therapeutic Potential

Age-related macular degeneration (AMD) is a blinding disease caused by multiple factors and is the primary cause of vision loss in the elderly. The morbidity of AMD increases every year. Currently, there is no effective treatment option for AMD. Intravitreal injection of antivascular endothelial growth factor (anti-VEGF) is currently the most widely used therapy, but it only aims at neovascularization, which is an intermediate pathological phenomenon of wet AMD, not at the etiological treatment. Anti-VEGF therapy can only temporarily delay the degeneration process of wet AMD, and AMD is easy to relapse after drug withdrawal. Therefore, it is urgent to deepen our understanding of the pathophysiological processes underlying AMD and to identify integrated or new strategies for AMD prevention and treatment. Recent studies have found that autophagy dysfunction in retinal pigment epithelial (RPE) cells, cellular senescence, and abnormal immune-inflammatory responses play key roles in the pathogenesis of AMD. For many age-related diseases, the main focus is currently the clearing of senescent cells (SNCs) as an antiaging treatment, thereby delaying diseases. However, in AMD, there is no relevant antiaging application. This review will discuss the pathogenesis of AMD and how interactions among RPE autophagy dysfunction, cellular senescence, and abnormal immune-inflammatory responses are involved in AMD, and it will summarize the three antiaging strategies that have been developed, with the aim of providing important information for the integrated prevention and treatment of AMD and laying the ground work for the application of antiaging strategies in AMD treatment.

Lipopolysaccharide-Induced Autophagy Mediates Retinal Pigment Epithelium Cells Survival. Modulation by the Phospholipase D Pathway

Inflammation and oxidative stress are common factors involved in the pathogenesis of retinal diseases, such as aged-related macular degeneration (AMD) and diabetic retinopathy (DR). Autophagy is a catabolic process essential to cell survival in response to stress. This process is highly active in retinal pigment epithelium (RPE) cells. Our previous findings demonstrated that lipopolysaccharide (LPS) induces an inflammatory response of RPE cells that implies classical phospholipases D (PLD1 and 2) activation, cyclooxygenase-2 (COX-2) expression, prostaglandin E₂ (PGE₂) production and reduced cell viability. In this work, we studied the autophagic process and its modulation by the PLD pathway in D407 and ARPE-19 RPE cells exposed to LPS. LPS (10 μg/ml or 25 μg/ml) exposure for 24 h increased light chain 3B-II (LC3B-II) content (an autophagy marker) and LC3B-positive punctate structures in both RPE cell lines studied. Next, the drug bafilomycin A₁ (BAF, 50 nM) was used to block the autophagic flux. In cells pre-incubated with BAF, LC3B-II and sequestosome 1 (SQSTM1/p62) levels and autophagosome-like structures were increased by LPS, demonstrating that the inflammatory injury increases the autophagic process in RPE cells. To study the role of the PLD pathway, cells were pre-incubated for 1 h with selective PLD1 (VU0359595) or PLD2 (VU0285655-1) inhibitors prior to LPS addition. Under control condition, LC3B-positive punctate structures were increased in cells pre-incubated with PLD2 inhibitor while with PLD1 inhibitor were increased in cells exposed to LPS. MTT reduction assays showed that early autophagy inhibitors, 3-methyladenin (3-MA) or LY294002, enhanced the loss in cell viability induced by LPS exposure for 48 h. On the contrary, the inhibition of PLD1 and PLD2 prevented the loss in cell viability induced by LPS. In conclusion, our results show that even though LPS treatment promotes an inflammatory response in RPE cells, it also triggers the activation of the autophagic process which in turn may serve as a protective mechanism for the cells. In addition, we demonstrate that the PLD pathway modulates the autophagic process in RPE cells. Our findings contribute to the knowledge of the molecular basis of retinal inflammatory and degenerative diseases and open new avenues for potential therapeutic exploration.

Oxidative stress, autophagy and pyroptosis in the neovascularization of oxygen‑induced retinopathy in mice

Retinal neovascularization (RNV) is a principal cause of visual impairment and blindness worldwide. The present study aimed to investigate how oxidative stress, autophagy and pyroptosis alter in RNV. The oxygen‑induced retinopathy (OIR) model was established in C57BL/6J mice by exposing them to a high concentration of oxygen. RNV was clearly visible in the fundus images and was qualitatively analyzed by counting the number of neovascular endothelial cell nuclei at postnatal day 17. Subsequently, the expression of vascular endothelial growth factor (VEGF)‑A and hypoxia‑inducible factor‑1α (HIF‑1α) at the protein level were measured. Furthermore, oxidative stress was examined using dihydroethidium (DHE) staining, and NADPH oxidase (NOX) 1 and 4 in the retinas were detected using reverse transcription‑quantitative polymerase chain reaction analysis. Additionally, immunostaining of microtubule associated protein 1 light chain 3α (LC3) was performed and the expression levels of the LC3, p62, autophagy protein (Atg)5, Atg7, Atg12, Beclin1, NOD‑like receptor family pyrin domain‑containing 3 (NLRP3), caspase‑1, interleukin (IL)‑1β, pro‑caspase‑1 and pro‑IL‑1β proteins were determined using western blotting in order to detect pyroptosis and autophagic flux. Autophagosomes were also detected using transmission electron microscopy. The results revealed that VEGF‑A and HIF‑1α protein expression levels, the DHE‑positive area, and NOX1 and NOX4 mRNA expression levels were significantly increased in the OIR mice. Furthermore, increased levels of NLRP3, caspase‑1, IL‑1β, pro‑caspase‑1 and pro‑IL‑1β proteins demonstrated that pyroptosis was activated. However, an accumulation of p62 and a reduction in the levels of LC3II/I and autophagosomes indicated that autophagic flux was compromised. Therefore, elevated levels of reactive oxygen species and pyroptosis along with attenuated autophagy were demonstrated in the OIR mice. The combination of oxidative stress, pyroptosis and impaired autophagy may serve an important role in the pathophysiology of RNV and may be a potential target to prevent RNV.

Expression of NLPR3 in Psoriasis Is Associated with Enhancement of Interleukin-1β and Caspase-1

Background

NLPR3 is an important gene that belongs to the family of NOD-like receptors and is thought to play an important role in psoriasis. The aim of the present study was to investigate the expression of NLRP3 in psoriasis biopsy samples and to assess the possible correlation of its expression with that of interleukin IL-1β and Caspase-1.

Material/Methods

The mRNA expression was checked by qRT-PCR. The expression of the proteins was checked by Western blotting. The distribution of the proteins was determined by immunohistochemistry.

Results

The results of our study indicate that the expression of NLRP3 was significantly upregulated in all the psoriatic biopsy samples as indicated by qRT-PCR and Western blotting. The expression of NLRP3 in psoriatic samples was 3.5 to 4.3 times higher than the expression of NLRP3 in normal skin biopsy samples. Moreover, our results indicated that the expression levels of IL-1β were higher as compared to the normal skin biopsy samples. Relative to the expression of IL-1β in normal skin biopsy samples, the expression of IL-1β was about 2.7–4.6 times higher. Additionally, the expression of caspase-1 was considerably upregulated in the psoriatic samples. Caspase-1 gene expression was 2.2–3.4 times higher than in normal skin biopsy samples.

Conclusions

NLPR3 may prove to be an important therapeutic target for psoriasis.

Blockade of microglial adenosine A2A receptor impacts inflammatory mechanisms, reduces ARPE-19 cell dysfunction and prevents photoreceptor loss in vitro

Age-related macular degeneration (AMD) is characterized by pathological changes in the retinal pigment epithelium (RPE) and loss of photoreceptors. Growing evidence has demonstrated that reactive microglial cells trigger RPE dysfunction and loss of photoreceptors, and inflammasome pathways and complement activation contribute to AMD pathogenesis. We and others have previously shown that adenosine A_2A receptor (A_2AR) blockade prevents microglia-mediated neuroinflammatory processes and mediates protection to the retina. However, it is still unknown whether blocking A_2AR in microglia protects against the pathological features of AMD. Herein, we show that an A_2AR antagonist, SCH58261, prevents the upregulation of the expression of pro-inflammatory mediators and the alterations in the complement system triggered by an inflammatory challenge in human microglial cells. Furthermore, blockade of A_2AR in microglia decreases the inflammatory response, as well as complement and inflammasome activation, in ARPE-19 cells exposed to conditioned medium of activated microglia. Finally, we also show that blocking A_2AR in human microglia increases the clearance of apoptotic photoreceptors. This study opens the possibility of using selective A_2AR antagonists in therapy for AMD, by modulating the interplay between microglia, RPE and photoreceptors.

Issues with the Specificity of Immunological Reagents for NLRP3: Implications for Age-related Macular Degeneration

Contradictory data have been presented regarding the implication of the NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome in age-related macular degeneration (AMD), the leading cause of vision loss in the Western world. Recognizing that antibody specificity may explain this discrepancy and in line with recent National Institutes of Health (NIH) guidelines requiring authentication of key biological resources, the specificity of anti-NLRP3 antibodies was assessed to elucidate whether non-immune RPE cells express NLRP3. Using validated resources, NLRP3 was not detected in human primary or human established RPE cell lines under multiple inflammasome-priming conditions, including purported NLRP3 stimuli in RPE such as DICER1 deletion and Alu RNA transfection. Furthermore, NLRP3 was below detection limits in ex vivo macular RPE from AMD patients, as well as in human induced pluripotent stem cell (hiPSC)-derived RPE from patients with overactive NLRP3 syndrome (Chronic infantile neurologic cutaneous and articulate, CINCA syndrome). Evidence presented in this study provides new data regarding the interpretation of published results reporting NLRP3 expression and upregulation in RPE and addresses the role that this inflammasome plays in AMD pathogenesis.

Involvement of Innate Immune System in Late Stages of Inherited Photoreceptor Degeneration

Retinitis pigmentosa (RP) is a group of inherited retinal degenerations that lead to progressive vision loss. Many mutations in 60 different genes have been shown to cause RP. Given the diversity of genes and mutations that cause RP, corrective gene therapy approaches currently in development may prove both time-consuming and cost-prohibitive for treatment of all forms of RP. An alternative approach is to find common biological pathways that cause retinal degeneration in various forms of RP, and identify new molecular targets. With this goal, we analyzed the retinal transcriptome of two non-allelic forms of RP in dogs, rcd1 and xlpra2, at clinically relevant advanced stages of the two diseases. Both diseases showed very similar trends in changes in gene expression compared to control normal dogs. Pathway analysis revealed upregulation of various components of the innate immune system in both diseases, including inflammasome and complement pathways. Our results show that the retinal transcriptome at advanced stages of RP is very similar to that of other retinal degenerative diseases such as age-related macular degeneration and diabetic retinopathy. Thus, drugs and therapeutics already in development for targeting these retinopathies may also prove useful for the treatment of many forms of RP.

The role and therapeutic potential of melatonin in age-related ocular diseases

The eye is continuously exposed to solar UV radiation and pollutants, making it prone to oxidative attacks. In fact, oxidative damage is a major cause of age-related ocular diseases including cataract, glaucoma, age-related macular degeneration, and diabetic retinopathy. As the nature of lens cells, trabecular meshwork cells, retinal ganglion cells, retinal pigment epithelial cells, and photoreceptors is postmitotic, autophagy plays a critical role in their cellular homeostasis. In age-related ocular diseases, this process is impaired, and thus, oxidative damage becomes irreversible. Other conditions such as low-grade chronic inflammation and angiogenesis also contribute to the development of retinal diseases (glaucoma, age-related macular degeneration and diabetic retinopathy). As melatonin is known to have remarkable qualities such as antioxidant/antinitridergic, mitochondrial protector, autophagy modulator, anti-inflammatory, and anti-angiogenic, it can represent a powerful tool to counteract all these diseases. The present review analyzes the role and therapeutic potential of melatonin in age-related ocular diseases, focusing on nitro-oxidative stress, autophagy, inflammation, and angiogenesis mechanisms.

Oxidized Lipoprotein Uptake Through the CD36 Receptor Activates the NLRP3 Inflammasome in Human Retinal Pigment Epithelial Cells

Purpose

Accumulation of oxidized phospholipids/lipoproteins with age is suggested to contribute to the pathogenesis of AMD. We investigated the effect of oxidized LDL (ox-LDL) on human RPE cells.

Methods

Primary human fetal RPE (hf-RPE) and ARPE-19 cells were treated with different doses of LDL or ox-LDL. Assessment of cell death was measured by lactate dehydrogenase release into the conditioned media. Barrier function of RPE was assayed by measuring transepithelial resistance. Lysosomal accumulation of ox-LDL was determined by immunostaining. Expression of CD36 was determined by RT-PCR; protein blot and function was examined by receptor blocking. NLRP3 inflammasome activation was assessed by RT-PCR, protein blot, caspase-1 fluorescent probe assay, and inhibitor assays.

Results

Treatment with ox-LDL, but not LDL, for 48 hours caused significant increase in hf-RPE and ARPE-19 (P < 0.001) cell death. Oxidized LDL treatment of hf-RPE cells resulted in a significant decrease in transepithelial resistance (P < 0.001 at 24 hours and P < 0.01 at 48 hours) relative to LDL-treated and control cells. Internalized ox-LDL was targeted to RPE lysosomes. Uptake of ox-LDL but not LDL significantly increased CD36 protein and mRNA levels by more than 2-fold. Reverse transcription PCR, protein blot, and caspase-1 fluorescent probe assay revealed that ox-LDL treatment induced NLRP3 inflammasome when compared with LDL treatment and control. Inhibition of NLRP3 activation using 10 μM isoliquiritigenin significantly (P < 0.001) inhibited ox-LDL induced cytotoxicity.

Conclusions

These data are consistent with the concept that ox-LDL play a role in the pathogenesis of AMD by NLRP3 inflammasome activation. Suppression of NLRP3 inflammasome activation could attenuate RPE degeneration and AMD progression.

Inhibition of NLRP3 Inflammasome Pathway by Butyrate Improves Corneal Wound Healing in Corneal Alkali Burn

Epithelial cells are involved in the regulation of innate and adaptive immunity in response to different stresses. The purpose of this study was to investigate if alkali-injured corneal epithelia activate innate immunity through the nucleotide-binding oligomerization domain-containing protein (NOD)-like receptor family pyrin domain containing 3 (NLRP3) inflammasome pathway. A unilateral alkali burn (AB) was created in the central cornea of C57BL/6 mice. Mice received either no topical treatment or topical treatment with sodium butyrate (NaB), β-hydroxybutyric acid (HBA), dexamethasone (Dex), or vehicle (balanced salt solution, BSS) quater in die (QID) for two or five days (d). We evaluated the expression of inflammasome components including NLRP3, apoptosis-associated speck-like protein (ASC), and caspase-1, as well as the downstream cytokine interleukin (IL)-1β. We found elevation of NLRP3 and IL-1β messenger RNA (mRNA) transcripts, as well as levels of inflammasome component proteins in the alkali-injured corneas compared to naïve corneas. Treatment with NLRP3 inhibitors using NaB and HBA preserved corneal clarity and decreased NLRP3, caspase-1, and IL-1β mRNA transcripts, as well as NLRP3 protein expression on post-injury compared to BSS-treated corneas. These findings identified a novel innate immune signaling pathway activated by AB. Blocking the NLRP3 pathway in AB mouse model decreases inflammation, resulting in greater corneal clarity. These results provide a mechanistic basis for optimizing therapeutic intervention in alkali injured eyes.

Epac1 Blocks NLRP3 Inflammasome to Reduce IL-1β in Retinal Endothelial Cells and Mouse Retinal Vasculature

Inflammation is an important component of diabetic retinal damage. We previously reported that a novel β-adrenergic receptor agonist, Compound 49b, reduced Toll-like receptor 4 (TLR4) signaling in retinal endothelial cells (REC) grown in high glucose. Others reported that TLR4 activates high-mobility group box 1 (HMGB1), which has been associated with the NOD-like receptor 3 (NLRP3) inflammasome. Thus, we hypothesized that Epac1, a downstream mediator of β-adrenergic receptors, would block TLR4/HMGB1-mediated stimulation of the NLRP3 inflammasome, leading to reduced cleavage of caspase-1 and interleukin-1 beta (IL-1β). We generated vascular specific conditional knockout mice for Epac1 and used REC grown in normal and high glucose treated with an Epac1 agonist and/or NLRP3 siRNA. Protein analyses were done for Epac1, TLR4, HMGB1, NLRP3, cleaved caspase-1, and IL-1β. Loss of Epac1 in the mouse retinal vasculature significantly increased all of the inflammatory proteins. Epac1 effectively reduced high glucose-induced increases in TLR4, HMGB1, cleaved caspase-1, and IL-1β in REC. Taken together, the data suggest that Epac1 reduces formation of the NLRP3 inflammasome to reduce inflammatory responses in the retinal vasculature.

Autophagy regulates death of retinal pigment epithelium cells in age-related macular degeneration

Age-related macular degeneration (AMD) is an eye disease underlined by the degradation of retinal pigment epithelium (RPE) cells, photoreceptors, and choriocapillares, but the exact mechanism of cell death in AMD is not completely clear. This mechanism is important for prevention of and therapeutic intervention in AMD, which is a hardly curable disease. Present reports suggest that both apoptosis and pyroptosis (cell death dependent on caspase-1) as well as necroptosis (regulated necrosis dependent on the proteins RIPK3 and MLKL, caspase-independent) can be involved in the AMD-related death of RPE cells. Autophagy, a cellular clearing system, plays an important role in AMD pathogenesis, and this role is closely associated with the activation of the NLRP3 inflammasome, a central event for advanced AMD. Autophagy can play a role in apoptosis, pyroptosis, and necroptosis, but its contribution to AMD-specific cell death is not completely clear. Autophagy can be involved in the regulation of proteins important for cellular antioxidative defense, including Nrf2, which can interact with p62/SQSTM, a protein essential for autophagy. As oxidative stress is implicated in AMD pathogenesis, autophagy can contribute to this disease by deregulation of cellular defense against the stress. However, these and other interactions do not explain the mechanisms of RPE cell death in AMD. In this review, we present basic mechanisms of autophagy and its involvement in AMD pathogenesis and try to show a regulatory role of autophagy in RPE cell death. This can result in considering the genes and proteins of autophagy as molecular targets in AMD prevention and therapy.

Mitochondria and the NLRP3 inflammasome: physiological and pathological relevance

The NLRP3 inflammasome is assembled and activated in certain types of myeloid cells upon sensing microbe-derived toxins or host-derived danger signals. Activation of the NLRP3 inflammasome by endogenous ligands has been discovered in various disorders, including metabolic syndrome, type 2 diabetes, atherosclerosis, gout, reperfusion injury of the heart, neurodegeneration, such as Alzheimer's disease, chronic kidney diseases, and macular degeneration of the eyes. Despite the potential significance of the NLRP3 inflammasome in the pathogenesis of several diseases, details on the activation mechanism of the NLRP3 inflammasome by a variety of stimulators have yet to be reported. Emerging evidence suggests that mitochondrial events are associated with NLRP3 activation in disease conditions. Mitochondrial dysfunction acts upstream of NLRP3 activation by providing reactive oxygen species (ROS) to trigger NLRP3 oligomerization or by inducing α-tubulin acetylation to relocate mitochondria to the proximity of NLRP3. In addition, mitochondria work as a platform for inflammasome assembly. Mitochondrial events may also lie downstream of NLRP3 activation. While the molecular mechanisms of mitochondrial dysfunction associated with NLRP3 activation are still unclear, they may involve the perturbation of mitochondria by K⁺ efflux and subsequent intracellular disequilibrium. Thus, mitochondria and NLRP3 machinery appear to be closely interwoven at multiple levels.

A Revised Hemodynamic Theory of Age-Related Macular Degeneration

Age-related macular degeneration (AMD) afflicts one out of every 40 individuals worldwide, causing irreversible central blindness in millions. The transformation of various tissue layers within the macula in the retina has led to competing conceptual models of the molecular pathways, cell types, and tissues responsible for the onset and progression of AMD. A model that has persisted for over 6 decades is the hemodynamic, or vascular theory of AMD progression, which states that vascular dysfunction of the choroid underlies AMD pathogenesis. Here, we re-evaluate this hypothesis in light of recent advances on molecular, anatomic, and hemodynamic changes underlying choroidal dysfunction in AMD. We propose an updated, detailed model of hemodynamic dysfunction as a mechanism of AMD development and progression.

Retinal pigment epithelial cell necroptosis in response to sodium iodate

Age-related macular degeneration (AMD) is a degenerative disease of the retina and the leading cause of blindness in the elderly in developed countries. The late stage of dry AMD, or geographic atrophy (GA), is characterized by extensive retinal pigment epithelium (RPE) degeneration. The underlying molecular mechanism for RPE cell death in GA remains unclear. Our previous study has established that RPE cells die predominantly from necroptosis in response to oxidative stress in vitro. Here, we extend our study and aim to characterize the nature of RPE cell death in response to sodium iodate (NaIO₃) in vitro and in a NaIO₃-induced retina degeneration mouse model. We found that NaIO₃ induces RPE necroptosis in vitro by using a combination of molecular hallmarks. By using TUNEL assays, active caspase-3 and HMGB1 immunostaining, we confirmed that photoreceptor cells die mainly from apoptosis and RPE cells die mainly from necroptosis in response to NaIO₃in vivo. RPE necroptosis in this model is also supported by use of the RIPK1 inhibitor, Necrostatin-1. Furthermore, using novel RIPK3-GFP transgenic mouse lines, we detected RIPK3 aggregation, a hallmark of necroptosis, in the RPE cells in vivo after NaIO₃ injection. Our findings suggest the necessity of re-evaluating RPE cell death mechanism in AMD models and have the potential to influence therapeutic development for dry AMD, especially GA.

Dry Age-Related Macular Degeneration Pharmacology

Age-related macular degeneration (AMD), the most common form of irreversible blindness in the industrially developed world, can present years before a patient begins to lose vision. For most of these patients, AMD never progresses past its early stages to the advanced forms that are principally responsible for the vast majority of vision loss. Advanced AMD can manifest as either an advanced avascular form known as geographic atrophy (GA) marked by regional retinal pigment epithelium (RPE) cell death or as an advanced form known as neovascular AMD marked by the intrusion of fragile new blood vessels into the normally avascular retina. Physicians have several therapeutic interventions available to combat neovascular AMD, but GA has no approved effective therapies as of yet. In this chapter, we will discuss the current strategies for limiting dry AMD in patients. We will also discuss previous attempts at pharmacological intervention that were tested in a clinical setting and consider reasons why these putative therapeutics did not perform successfully in large-scale trials. Despite the number of unsuccessful past trials, new pharmacological interventions may succeed. These future therapies may aid millions of AMD patients worldwide.