A Review of Pathogenic Drivers of Age-Related Macular Degeneration, Beyond Complement, with a Focus on Potential Endpoints for Testing Therapeutic Interventions in Preclinical Studies

Identification of the CDH18 gene associated with age-related macular degeneration using weighted gene co-expression network analysis

Purpose: Age-related macular degeneration (AMD) is a chronic and progressive macular degenerative disease that culminates in a gradual deterioration of central vision. Despite its prevalence, the key biomarkers for AMD have not yet been fully elucidated. In this study, we aimed to efficiently identify biomarkers crucial for diagnosing AMD. Methods: Three datasets pertaining to retinal pigment epithelium (RPE)/choroid tissues associated with AMD were selected from the GEO database. The GSE50195 dataset was utilized to conduct weighted gene co-expression network analysis (WGCNA) for identifying module genes linked to AMD. KEGG and GO enrichment analyses were subsequently conducted on these module genes. GSE29801 and GSE135092 datasets were subjected to differential expression analysis to pinpoint the DEGs intersecting with the module genes. Subsequently, wet AMD (wAMD) and dry AMD (dAMD) mouse models were developed, from which RPE/choroid tissues were harvested to validate the hub genes via RT-qPCR and Western blot. Results: Using the WGCNA, we selected the "antiquewhite4" module (r = 0.91 and p = 7e-07), which contains a total of 325 genes. Through the intersection of module genes with DEGs, nine hub genes were identified. Pathways involved in complement and coagulation cascades, ECM-receptor interactions, unsaturated fatty acid biosynthesis, and fatty acid elongation play important roles in AMD. Notably, CDH18 demonstrated notable variance across all three datasets. Post validation using RT-qPCR experiments revealed a significant downregulation of CDH18 in both dAMD and wAMD. EGLN3 was expressed at low levels in wAMD. In dAMD, EYA2, LTB, and PODXL were significantly downregulated, whereas APOC1 was notably upregulated. Western blot confirmed that CDH18 was lowly expressed in dAMD and wAMD mouse models. Conclusion: CDH18 was identified as the key gene involved in the pathogenesis of AMD. An imbalance of the complement and coagulation cascades is a potential mechanism of AMD. This study provides a novel idea for diagnosing and treating AMD in the future.

Metformin protects retinal pigment epithelium cells against H2O2-induced oxidative stress and inflammation via the Nrf2 signaling cascade

PURPOSE

Dysfunctions of retinal pigment epithelium (RPE) attributed to oxidative stress and inflammation are implicated with age-related macular degeneration (AMD). A debate on the curative role of metformin in AMD has been raised, though several recent clinical studies support the lower odds by using metformin. This study aimed to determine whether metformin could exert cytoprotection against RPE oxidative damages and the potential mechanisms.

METHODS

A cellular AMD model was established by treating ARPE-19 cells with hydrogen peroxide (H2O2) for 24 h. The reactive oxygen species (ROS) generation, expression of antioxidant enzymes, and levels of pro-inflammatory cytokines were monitored under administrations with H2O2 with/without metformin. The expression and DNA-binding activity of transcription factor erythroid-related factor 2 (Nrf2) were determined by western blot, immunofluorescence, and electrophoretic mobility shift assay. Knockout of Nrf2 was conducted by CRISPR/Cas9 gene deletion system.

RESULTS

Metformin pretreatment significantly improved the H2O2-induced low viability of ARPE-19 cells, reduced ROS production, and increased contents of antioxidative molecules. Concurrently, metformin also suppressed levels of pro-inflammatory cytokines caused by H2O2. The metformin-augmented nuclear translocation and DNA-binding activity of Nrf2 were further verified by the increased expression of its downstream targets. Genetic deletion of Nrf2 blocked the cytoprotective role of metformin.

CONCLUSION

Metformin possesses antioxidative and anti-inflammatory properties in ARPE-19 cells by activating the Nrf2 signaling. It supports the potential use for the control and prevention of AMD.

Association of lipid-lowering drugs and antidiabetic drugs with age-related macular degeneration: a meta-analysis in Europeans

BACKGROUND/AIMS

To investigate the association of commonly used systemic medications with prevalent age-related macular degeneration (AMD) in the general population.

METHODS

We included 38 694 adults from 14 population-based and hospital-based studies from the European Eye Epidemiology consortium. We examined associations between the use of systemic medications and any prevalent AMD as well as any late AMD using multivariable logistic regression modelling per study and pooled results using random effects meta-analysis.

RESULTS

Between studies, mean age ranged from 61.5±7.1 to 82.6±3.8 years and prevalence ranged from 12.1% to 64.5% and from 0.5% to 35.5% for any and late AMD, respectively. In the meta-analysis of fully adjusted multivariable models, lipid-lowering drugs (LLD) and antidiabetic drugs were associated with lower prevalent any AMD (OR 0.85, 95% CI=0.79 to 0.91 and OR 0.78, 95% CI=0.66 to 0.91). We found no association with late AMD or with any other medication.

CONCLUSION

Our study indicates a potential beneficial effect of LLD and antidiabetic drug use on prevalence of AMD across multiple European cohorts. Our findings support the importance of metabolic processes in the multifactorial aetiology of AMD.

Beyond VEGF: Targeting Inflammation and Other Pathways for Treatment of Retinal Disease

Neovascular eye diseases include conditions such as retinopathy of prematurity, proliferative diabetic retinopathy, and neovascular age-related macular degeneration. Together, they are a major cause of vision loss and blindness worldwide. The current therapeutic mainstay for these diseases is intravitreal injections of biologics targeting vascular endothelial growth factor (VEGF) signaling. Lack of universal response to these anti-VEGF agents coupled with the challenging delivery method underscore a need for new therapeutic targets and agents. In particular, proteins that mediate both inflammatory and proangiogenic signaling are appealing targets for new therapeutic development. Here, we review agents currently in clinical trials and highlight some promising targets in preclinical and early clinical development, focusing on the redox-regulatory transcriptional activator APE1/Ref-1, the bioactive lipid modulator soluble epoxide hydrolase, the transcription factor RUNX1, and others. Small molecules targeting each of these proteins show promise for blocking neovascularization and inflammation. The affected signaling pathways illustrate the potential of new antiangiogenic strategies for posterior ocular disease. SIGNIFICANCE STATEMENT: Discovery and therapeutic targeting of new angiogenesis mediators is necessary to improve treatment of blinding eye diseases like retinopathy of prematurity, diabetic retinopathy, and neovascular age-related macular degeneration. Novel targets undergoing evaluation and drug discovery work include proteins important for both angiogenesis and inflammation signaling, including APE1/Ref-1, soluble epoxide hydrolase, RUNX1, and others.

Decreased Expression of Soluble Epoxide Hydrolase Suppresses Murine Choroidal Neovascularization

Neovascular or "wet" age-related macular degeneration (nAMD) is a leading cause of blindness among older adults. Choroidal neovascularization (CNV) is a major pathological feature of nAMD, in which abnormal new blood vessel growth from the choroid leads to irreversible vision loss. There is a critical need to develop novel therapeutic strategies to address limitations of the current anti-vascular endothelial growth factor biologics. Previously, we identified soluble epoxide hydrolase (sEH) as a possible therapeutic target for CNV through a forward chemical genetic approach. The purpose of this study was to validate sEH as a target by examining retinal expression of sEH protein and mRNA by immunohistochemistry and RNAscope in situ hybridization, respectively, and to assess the efficacy of an adeno-associated virus (AAV) vector designed to knock down the sEH gene, Ephx2, in the murine laser-induced (L-) CNV model. nAMD patient postmortem eye tissue and murine L-CNV showed overexpression of sEH in photoreceptors and retinal pigment epithelial cells. Ephx2 knockdown significantly reduced CNV and normalized mRNA expression levels of CNV-related inflammatory markers. Thus, this study further establishes sEH as a promising therapeutic target against CNV associated with nAMD.

Characterization and identification of measurable endpoints in a mouse model featuring age-related retinal pathologies: a platform to test therapies

Apolipoprotein B100 (apoB100) is the structural protein of cholesterol carriers including low-density lipoproteins. It is a constituent of sub-retinal pigment epithelial (sub-RPE) deposits and pro-atherogenic plaques, hallmarks of early dry age-related macular degeneration (AMD), an ocular neurodegenerative blinding disease, and cardiovascular disease, respectively. Herein, we characterized the retinal pathology of transgenic mice expressing mouse apoB100 in order to catalog their functional and morphological ocular phenotypes as a function of age and establish measurable endpoints for their use as a mouse model to test potential therapies. ApoB100 mice were found to exhibit an age-related decline in retinal function, as measured by electroretinogram (ERG) recordings of their scotopic a-wave, scotopic b-wave; and c-wave amplitudes. ApoB100 mice also displayed a buildup of the cholesterol carrier, apolipoprotein E (apoE) within and below the supporting extracellular matrix, Bruch's membrane (BrM), along with BrM thickening, and accumulation of thin diffuse electron-dense sub-RPE deposits, the severity of which increased with age. Moreover, the combination of apoB100 and advanced age were found to be associated with RPE morphological changes and the presence of sub-retinal immune cells as visualized in RPE-choroid flatmounts. Finally, aged apoB100 mice showed higher levels of circulating and ocular pro-inflammatory cytokines, supporting a link between age and increased local and systemic inflammation. Collectively, the data support the use of aged apoB100 mice as a platform to evaluate potential therapies for retinal degeneration, specifically drugs intended to target removal of lipids from Bruch's membrane and/or alleviate ocular inflammation.

TGF-β Superfamily Signaling in the Eye: Implications for Ocular Pathologies

The TGF-β signaling pathway plays a crucial role in several key aspects of development and tissue homeostasis. TGF-β ligands and their mediators have been shown to be important regulators of ocular physiology and their dysregulation has been described in several eye pathologies. TGF-β signaling participates in regulating several key developmental processes in the eye, including angiogenesis and neurogenesis. Inadequate TGF-β signaling has been associated with defective angiogenesis, vascular barrier function, unfavorable inflammatory responses, and tissue fibrosis. In addition, experimental models of corneal neovascularization, diabetic retinopathy, proliferative vitreoretinopathy, glaucoma, or corneal injury suggest that aberrant TGF-β signaling may contribute to the pathological features of these conditions, showing the potential of modulating TGF-β signaling to treat eye diseases. This review highlights the key roles of TGF-β family members in ocular physiology and in eye diseases, and reviews approaches targeting the TGF-β signaling as potential treatment options.

Potential participation of CTRP6, a complement regulator, in the pathology of age related macular degeneration

PURPOSE

To investigate the localized expression of C1q/tumor necrosis factor related protein (CTRP) 6 in human age-related macular degeneration (AMD) retinal tissues.

EXPERIMENTAL STUDY DESIGN

4 AMD and 3 non-AMD whole eyes of Caucasian donors were used. Eyecups were excised at Eye Bank CorneaGen, Inc.

METHODS

To elucidate the effects of CTRP6, C3b was measured by an enzyme-linked immunosorbent-like assay. CFB versus CTRP6 competitive binding assay was applied to clarify the inhibition by CTRP6 of C3bBb complex formation. The cornea, iris, lens, and vitreous were removed and the eyes were cut into a posterior eye-cup including the retina, choroid, and sclera. Six-µm-thick serial sections of frozen samples underwent hematoxylin-eosin (HE) staining and indirect immunohistochemical staining using primary antibodies, anti-CTRP6, -CTRP5, -CTRP10, -Complement factor H (CFH) and -Clusterin (CLU). Results The two in vitro studies confirmed that CTRP6 has an inhibitory effect on alternative pathways of complement (APC) function and that the molecular target of CTRP6 is the inhibition of the formation of C3bBb. Localized expression for CTRP6 and CFH was found in the drusen of the AMD eyes, both associated with APC inhibition, CLU associated with membrane-attack complex (MAC) inhibition, and CTRP5 associated with retinal degeneration.

CONCLUSION

The localized expression of CTRP6 in the drusen of AMD eyes may open a new insight into the possible involvement of APC regulatory factors in the pathogenesis of AMD, together with the known CFH so far analyzed solely as an APC inhibitor.

Age-Related Macular Degeneration and Cardiovascular Diseases: Revisiting the Common Soil Theory

ABSTRACT

Age-related macular degeneration (AMD), a complex disease associated with aging, remains one of the leading causes of visual loss in high-income countries and its prevalence is expected to increase over the next decades. Polypoidal choroidal vasculopathy has been considered a variant of neovascular AMD and is highly prevalent in Asian populations. Similarly, cardiovascular disease (CVD)-another complex disease associated with aging-is a leading cause of morbidity and mortality in high-income countries and its prevalence is also expected to increase due to population aging. Previous studies reported an increased risk for CVD in AMD patients, indicating an underlying "common soil." Reviewing the current literature, consistent evidence for common risk factors and mutual comorbidity was identified for both diseases. Cardiovascular risk factors include smoking, diet, and low levels of physical activity, which also play a role in AMD pathogenesis. Several studies demonstrated AMD patients to be at higher risk for CVD compared to the general older population. The complexity of both diseases, however, complicates research on their relation, and thus studies ought to be interpreted with caution. Herein we present an overview of selected studies and their main "take-home messages" on this topic, and hypothesize on the patho-etiologic "common ground" of these 2 diseases.

Kinins and Their Receptors as Potential Therapeutic Targets in Retinal Pathologies

The kallikrein-kinin system (KKS) contributes to retinal inflammation and neovascularization, notably in diabetic retinopathy (DR) and neovascular age-related macular degeneration (AMD). Bradykinin type 1 (B1R) and type 2 (B2R) receptors are G-protein-coupled receptors that sense and mediate the effects of kinins. While B2R is constitutively expressed and regulates a plethora of physiological processes, B1R is almost undetectable under physiological conditions and contributes to pathological inflammation. Several KKS components (kininogens, tissue and plasma kallikreins, and kinin receptors) are overexpressed in human and animal models of retinal diseases, and their inhibition, particularly B1R, reduces inflammation and pathological neovascularization. In this review, we provide an overview of the KKS with emphasis on kinin receptors in the healthy retina and their detrimental roles in DR and AMD. We highlight the crosstalk between the KKS and the renin-angiotensin system (RAS), which is known to be detrimental in ocular pathologies. Targeting the KKS, particularly the B1R, is a promising therapy in retinal diseases, and B1R may represent an effector of the detrimental effects of RAS (Ang II-AT1R).

New insights into oxidative stress and immune mechanisms involved in age-related macular degeneration tackled by novel therapies

The prevalence of age-related macular degeneration (AMD) has increased in the last years. Although anti-VEGF agents have improved the prognosis of exudative AMD, dry AMD has still devastating effects on elderly people vision. Oxidative stress and inflammation are mechanisms involved in AMD pathogenesis and its progression. Molecular pathways involving epidermal growth factor receptor (EGFR), bone morphogenetic protein (BMP4) and the nuclear erythroid related factor 2 (Nrf2) are behind oxidative stress in AMD due to their participation in antioxidant cellular pathways. As a consequence of the disbalance produced in the antioxidant mechanisms, there is an activation of innate and adaptative immune response with cell recruitment, changes in complement factors expression, and modification of cellular milieu. Different therapies are being studied to treat dry AMD based on the possible effects on antioxidant molecular pathways or their action on the immune response. There is a wide range of treatments presented in this review, from natural antioxidant compounds to cell and gene therapy, based on their mechanisms. Finally, we hypothesize that alpha-1-antitrypsin (AAT), an anti-inflammatory and immunomodulatory molecule that can also modulate antioxidant cellular defenses, could be a good candidate for testing in AMD. This article is part of the special ssue on 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Complement-mediated release of fibroblast growth factor 2 from human RPE cells

PURPOSE

Complement activation is associated with choroidal neovascularization (CNV) in age-related macular degeneration (AMD). Fibroblast growth factor 2 (FGF2) and membrane attack complex (MAC) are present in eyes of patients with CNV. Herein, we investigated the effect of complement activation on FGF2 release in human retinal pigment epithelial (RPE) cells.

METHODS

Cultured human RPE cells were primed with an anti-RPE antibody and then treated with C1q-depleted human serum in the presence or absence of Tec kinases inhibitor (LFM-A13). 38 cytokines/chemokines levels were measured by Luminex technology. Secretion of FGF2 and interleukin (IL)-6 was assessed by ELISA. Tec protein was measured by Western blot. mRNA expression of FGF2, chemokine (C-X-C motif) ligand 1 (CXCL-1), and family members of Tec kinases was evaluated by qPCR. Cell viability and MAC deposition were determined by WST-1 assay and flow cytometry, respectively.

RESULTS

Complement activation caused increased FGF2 and IL-6 release. FGF2 was released when C6-depleted human serum was reconstituted with C6. Anti-C5 antibody significantly attenuated complement-mediated FGF2 release, but not IL-6. FGF2 mRNA levels were not affected, while CXCL-1 mRNA levels were increased by complement activation. FGF2-containing extracellular vesicles were detected in response to complement challenge. Tec mRNA and protein were expressed in RPE cells. In the presence of LFM-A13, secretion of FGF2, but not IL-6, and MAC deposition were significantly decreased and cell viability was significantly increased in complement-treated cells when compared to controls.

CONCLUSIONS

Complement plays an important role to release FGF2 from RPE cells. Tec kinase is involved in MAC formation and complement-mediated FGF2 release. This information suggests a role for complement activation to mediate neovascularization in conditions such as AMD, and may elucidate potential therapeutic targets.

The Tie2 signaling pathway in retinal vascular diseases: a novel therapeutic target in the eye

Background

Retinal vascular diseases such as neovascular age-related macular degeneration, diabetic retinopathy and/or diabetic macular edema, and retinal vein occlusion with macular edema—share several key pathophysiologic aspects including neovascularization, vascular permeability, and inflammation. The role of vascular endothelial growth factor (VEGF) in these processes, and the therapeutic benefits of VEGF inhibition, have been well characterized. Anti-VEGF therapy is highly effective for many patients but is not uniformly effective in all patients and imposes a significant treatment burden. More recently, the role of the Tie2 signaling pathway in the pathophysiology of retinal vascular diseases has been investigated, and the Tie2 pathway represents a novel therapeutic target for these conditions.

Areas covered

The index review describes the Tie2 pathway and its complementary role to the VEGF pathway in the angiogenesis cascade and will summarize studies of molecules in development to therapeutically modulate the Tie2 pathway in retinal vascular diseases.

Conclusions

Activation of the Tie2 pathway leads to downstream signaling that promotes vascular health and stability and decreases vascular permeability and inflammation. AXT107 is a collagen IV–derived synthetic peptide with a dual mechanism of action that involves suppression of VEGF signaling and activation of the Tie2 pathway; these actions are accomplished by AXT107 binding to and disrupting different integrin, leading to blockade of the VEGF receptor and rearrangement of cellular Tie2 rendering it susceptible to Ang2 agonism. Other Tie2 agonist compounds are also in development, including faricimab and razuprotafib. Tie2 activation only modestly impacts angiogenesis on its own but significantly potentiates VEGF suppression. Co-regulation of the VEGF and Tie2 signaling pathways has the potential to improve functional and structural outcomes in eyes with retinal vascular diseases.