Menadione-Induced DNA Damage Leads to Mitochondrial Dysfunction and Fragmentation During Rosette Formation in Fuchs Endothelial Corneal Dystrophy

Regulation of Oxidative Stress in Corneal Endothelial Cells by Prdx6

The inner layer of the cornea, the corneal endothelium, is post-mitotic and unable to regenerate if damaged. The corneal endothelium is one of the most transplanted tissues in the body. Fuchs’ endothelial corneal dystrophy (FECD) is the leading indication for corneal endothelial transplantation. FECD is thought to be an age-dependent disorder, with a major component related to oxidative stress. Prdx6 is an antioxidant with particular affinity for repairing peroxidised cell membranes. To address the role of Prdx6 in corneal endothelial cells, we used a combination of biochemical and functional studies. Our data reveal that Prdx6 is expressed at unusually high levels at the plasma membrane of corneal endothelial cells. RNAi-mediated knockdown of Prdx6 revealed a role for Prdx6 in lipid peroxidation. Furthermore, following induction of oxidative stress with menadione, Prdx6-deficient cells had defective mitochondrial membrane potential and were more sensitive to cell death. These data reveal that Prdx6 is compartmentalised in corneal endothelial cells and has multiple functions to preserve cellular integrity.

Fuchs Endothelial Corneal Dystrophy Through the Prism of Oxidative Stress

The corneal endothelium (CE) is vital for maintaining the water balance and clarity of the cornea. The CE is a cell layer that is particularly susceptible to aging because of its postmitotic arrest, high metabolic activity involving pumping of ions, and lifelong exposure to ultraviolet light. Despite gradual age-related cell loss, a sufficient number of CE cells are preserved during the lifespan of an individual. However, in conditions such as Fuchs endothelial corneal dystrophy (FECD), permanent loss of CE cells leads to corneal edema and loss of vision requiring corneal transplantation. FECD is a genetic and oxidative stress disorder manifested by abnormal cell-matrix interactions and expedited cellular aging culminating in cellular death. Because the endothelium has minimal replicative capacity in vivo and an inability to replace its genome, it is particularly prone to cumulative DNA damage acquired throughout life. In FECD, the underlying genetic defects make the CE genome even more vulnerable to this damage, to the point of causing mitochondrial dysfunction, mitochondrial membrane potential loss, and excessive mitophagy activation. Endogenous and exogenous intracellular stressors alter the synthetic footprint of CE cells, leading to endothelial-mesenchymal transition and secretion of aberrant extracellular matrix (in the form of guttae), resembling scar formation in other organs. In turn, the guttae or endothelial scars contribute to a vicious cycle of FECD pathogenesis and, by further inducing endothelial-mesenchymal transition and oxidant-antioxidant imbalance, perpetuate the molecular changes of the degenerating endothelium.

Sodium Ferulate Attenuates Lidocaine-Induced Corneal Endothelial Impairment

The introduction of intracameral anaesthesia by injection of lidocaine has become popular in cataract surgery for its inherent potency, rapid onset, tissue penetration, and efficiency. However, intracameral lidocaine causes corneal thickening, opacification, and corneal endothelial cell loss. Herein, we investigated the effects of lidocaine combined with sodium ferulate, an antioxidant with antiapoptotic and anti-inflammatory properties, on lidocaine-induced damage of corneal endothelia with in vitro experiment of morphological changes and cell viability of cultured human corneal endothelial cells and in vivo investigation of corneal endothelial cell density and central corneal thickness of cat eyes. Our finding indicates that sodium ferulate from 25 to 200 mg/L significantly reduced 2 g/L lidocaine-induced toxicity to human corneal endothelial cells, and 50 mg/L sodium ferulate recovered the damaged human corneal endothelial cells to normal growth status. Furthermore, 100 mg/L sodium ferulate significantly inhibited lidocaine-induced corneal endothelial cell loss and corneal thickening in cat eyes. In conclusion, sodium ferulate protects human corneal endothelial cells from lidocaine-induced cytotoxicity and attenuates corneal endothelial cell loss and central corneal thickening of cat eyes after intracameral injection with lidocaine. It is likely that the antioxidant effect of sodium ferulate reduces the cytotoxic and inflammatory corneal reaction during intracameral anaesthesia.

Function-Related Protein Expression in Fuchs Endothelial Corneal Dystrophy Cells and Tissue Models

Fuchs endothelial corneal dystrophy (FECD) is a corneal pathology that affects the endothelial cell's ability to maintain deturgescence, resulting in a progressive loss of corneal transparency. In this study, we investigated the expression of function-related proteins in corneal endothelial cells using FECD or healthy corneal endothelial cells, either in a cell culture two-dimensional model or in an engineered corneal endothelium three-dimensional tissue model. No statistically significant difference in gene regulation was observed for the function-related families ATP1, SLC4, SLC16, AQP, TJP, and CDH between the FECD and the healthy cell models. Similarly, no difference in barrier integrity (transendothelial electrical resistance measurements and permeability assays) was observed in vitro between FECD and healthy cultured cells. Protein expression of the key function-related families was decreased for Na⁺/K⁺-ATPase α1 subunit, monocarboxylate transporters 1 and 4 in native ex vivo end-stage FECD specimens, whereas it returned to levels comparable to that of healthy tissues in the engineered FECD model. These results indicate that cell expansion and tissue engineering culture conditions can generate a corneal endothelium from pathologic FECD cells, with levels of function-related proteins similar to that of healthy tissues. Overall, these results explain why it is possible to reform a functional endothelium using corneal endothelial cells isolated from nonfunctional FECD pathologic specimens.

NQO1 downregulation potentiates menadione-induced endothelial-mesenchymal transition during rosette formation in Fuchs endothelial corneal dystrophy

Fuchs endothelial corneal dystrophy (FECD) is a genetic and oxidative stress disorder of post-mitotic human corneal endothelial cells (HCEnCs), which normally exhibit hexagonal shape and form a compact monolayer compatible with normal corneal functioning and clear vision. FECD is associated with increased DNA damage, which in turn leads to HCEnC loss, resulting in the formation rosettes and aberrant extracellular matrix (ECM) deposition in the form of pro-fibrotic guttae. Since the mechanism of ECM deposition in FECD is currently unknown, we aimed to investigate the role of endothelial-mesenchymal transition (EMT) in FECD using a previously established cellular in vitro model that recapitulates the characteristic rosette formation, by employing menadione (MN)-induced oxidative stress. We demonstrate that MN treatment alone, or a combination of MN and TGF-β1 induces reactive oxygen species (ROS), cell death, and EMT in HCEnCs during rosette formation, resulting in upregulation of EMT- and FECD-associated markers such as Snail1, N-cadherin, ZEB1, and transforming growth factor-beta-induced (TGFβI), respectively. Additionally, FECD ex vivo specimens displayed a loss of organized junctional staining of plasma membrane-bound N-cadherin, with corresponding increase in fibronectin and Snail1 compared to ex vivo controls. Addition of N-acetylcysteine (NAC) downregulated all EMT markers and abolished rosette formation. Loss of NQO1, a metabolizing enzyme of MN, led to greater increase in intracellular ROS levels as well as a significant upregulation of Snail1, fibronectin, and N-cadherin compared to normal cells, indicating that NQO1 regulates Snail1-mediated EMT. This study provides first line evidence that MN-induced oxidative stress leads to EMT in corneal endothelial cells, and the effect of which is further potentiated when redox cycling activity of MN is enhanced by the absence of NQO1. Given that NAC inhibits Snail-mediated EMT, this may be a potential therapeutic intervention for FECD.

Activation of mitophagy leads to decline in Mfn2 and loss of mitochondrial mass in Fuchs endothelial corneal dystrophy

Human corneal endothelial cells (HCEnCs) are terminally differentiated cells that have limited regenerative potential. The large numbers of mitochondria in HCEnCs are critical for pump and barrier function required for corneal hydration and transparency. Fuchs Endothelial Corneal Dystrophy (FECD) is a highly prevalent late-onset oxidative stress disorder characterized by progressive loss of HCEnCs. We previously reported increased mitochondrial fragmentation and reduced ATP and mtDNA copy number in FECD. Herein, carbonyl cyanide m-chlorophenyl hydrazone (CCCP)-induced mitochondrial depolarization decreased mitochondrial mass and Mfn2 levels, which were rescued with mitophagy blocker, bafilomycin, in FECD. Moreover, electron transport chain complex (I, V) decrease in FECD indicated deficient mitochondrial bioenergetics. Transmission electron microscopy of FECD tissues displayed an increased number of autophagic vacuoles containing degenerated and swollen mitochondria with cristolysis. An elevation of LC3-II and LAMP1 and downregulation of Mfn2 in mitochondrial fractions suggested that loss of fusion capacity targets fragmented mitochondria to the pre-autophagic pool and upregulates mitophagy. CCCP-induced mitochondrial fragmentation leads to Mfn2 and LC3 co-localization without activation of proteosome, suggesting a novel Mfn2 degradation pathway via mitophagy. These data indicate constitutive activation of mitophagy results in reduction of mitochondrial mass and abrogates cellular bioenergetics during degeneration of post-mitotic cells of ocular tissue.

Mitochondrial dysfunction and oxidative stress in corneal disease

The cornea is the anterior transparent surface and the main refracting structure of the eye. Mitochondrial dysfunction and oxidative stress are implicated in the pathogenesis of inherited (e.g. Kearns Sayre Syndrome) and acquired corneal diseases (e.g. keratoconus and Fuchs endothelial corneal dystrophy). Both antioxidants and reactive oxygen species are found in the healthy cornea. There is increasing evidence of imbalance in the oxidative balance and mitochondrial function in the cornea in disease states. The cornea is vulnerable to mitochondrial dysfunction and oxidative stress due to its highly exposed position to ultraviolet radiation and high oxygen tension. The corneal endothelium is vulnerable to accumulating mitochondrial DNA (mtDNA) damage due to the post- mitotic nature of endothelial cells, yet their mitochondrial genome is continually replicating and mtDNA mutations can develop and accumulate with age. The unique physiology of the cornea predisposes this structure to oxidative damage, and there is interplay between inherited and acquired mitochondrial dysfunction, oxidative damage and a number of corneal diseases. By targeting mitochondrial dysfunction in corneal disease, emerging treatments may prevent or reduce visual loss.

Administration of Menadione, Vitamin K3, Ameliorates Off-Target Effects on Corneal Epithelial Wound Healing Due to Receptor Tyrosine Kinase Inhibition

PURPOSE

The antiangiogenic receptor tyrosine kinase inhibitor (RTKi), 3-[(4-bromo-2,6-difluorophenyl)methoxy]-5-[[[[4-(1-pyrrolidinyl) butyl] amino] carbonyl]amino]-4-isothiazolecarboxamide hydrochloride, targets VEGFR2 (half maximal inhibitory concentration [IC50] = 11 nM); however, off-target inhibition of epidermal growth factor receptor (EGFR) occurs at higher concentrations. (IC50 = 5.8 μM). This study was designed to determine the effect of topical RTKi treatment on EGF-mediated corneal epithelial wound healing and to develop new strategies to minimize off-target EGFR inhibition.

METHODS

In vitro corneal epithelial wound healing was measured in response to EGF using a transformed human cell line (hTCEpi cells). In vivo corneal wound healing was assessed using a murine model. In these complementary assays, wound healing was measured in the presence of varying RTKi concentrations. Immunoblot analysis was used to examine EGFR and VEGFR2 phosphorylation and the kinetics of EGFR degradation. An Alamar Blue assay measured VEGFR2-mediated cell biology.

RESULTS

Receptor tyrosine kinase inhibitor exposure caused dose-dependent inhibition of EGFR-mediated corneal epithelial wound healing in vitro and in vivo. Nanomolar concentrations of menadione, a vitamin K3 analog, when coadministered with the RTKi, slowed EGFR degradation and ameliorated the inhibitory effects on epithelial wound healing both in vitro and in vivo. Menadione did not alter the RTKi's IC50 against VEGFR2 phosphorylation or its inhibition of VEGF-induced retinal endothelial cell proliferation.

CONCLUSIONS

An antiangiogenic RTKi exhibited off-target effects on the corneal epithelium that can be minimized by menadione without deleteriously affecting its on-target VEGFR2 blockade. These data indicate that menadione has potential as a topical supplement for individuals suffering from perturbations in corneal epithelial homeostasis, especially as an untoward side effect of kinase inhibitors.

Existence of Neural Crest-Derived Progenitor Cells in Normal and Fuchs Endothelial Dystrophy Corneal Endothelium

Human corneal endothelial cells are derived from neural crest and because of postmitotic arrest lack competence to repair cell loss from trauma, aging, and degenerative disorders such as Fuchs endothelial corneal dystrophy (FECD). Herein, we identified a rapidly proliferating subpopulation of cells from the corneal endothelium of adult normal and FECD donors that exhibited features of neural crest-derived progenitor (NCDP) cells by showing absence of senescence with passaging, propensity to form spheres, and increased colony forming efficacy compared with the primary cells. The collective expression of stem cell-related genes SOX2, OCT4, LGR5, TP63 (p63), as well as neural crest marker genes PSIP1 (p75(NTR)), PAX3, SOX9, AP2B1 (AP-2β), and NES, generated a phenotypic footprint of endothelial NCDPs. NCDPs displayed multipotency by differentiating into microtubule-associated protein 2, β-III tubulin, and glial fibrillary acidic protein positive neurons and into p75(NTR)-positive human corneal endothelial cells that exhibited transendothelial resistance of functional endothelium. In conclusion, we found that mitotically incompetent ocular tissue cells contain adult NCDPs that exhibit a profile of transcription factors regulating multipotency and neural crest progenitor characteristics. Identification of normal NCDPs in FECD-affected endothelium holds promise for potential autologous cell therapies.

UV-A Irradiation Activates Nrf2-Regulated Antioxidant Defense and Induces p53/Caspase3-Dependent Apoptosis in Corneal Endothelial Cells

Purpose

To examine whether Nrf2-regulated antioxidant defense and p53 are activated in human corneal endothelial cells (CEnCs) by environmental levels of ultraviolet A (UV-A), a known stimulator of oxidative stress.

Methods

Immortalized human CEnCs (HCEnCi) were exposed to UV-A fluences of 2.5, 5, 10, or 25 J/cm², then allowed to recover for 3 to 24 hours. Control HCEnCi did not receive UV-A. Reactive oxygen species (ROS) were measured using H₂DCFDA. Cell cytotoxicity was evaluated by lactate dehydrogenase (LDH) release. Levels of Nrf2, HO-1, NQO-1, p53, and caspase3 were detected by immunnoblotting or real-time PCR. Activated caspase3 was measured by immunoblotting and a fluorescence assay.

Results

Exposure of HCEnCi to 5, 10, and 25 J/cm² UV-A increased ROS levels compared with controls. Nrf2, HO-1, and NQO-1 mRNA increased 1.7- to 3.2-fold at 3 and 6 hours after irradiation with 2.5 and 5 J/cm² UV-A. At 6 hours post irradiation, UV-A (5 J/cm²) enhanced nuclear Nrf2 translocation. At 24 hours post treatment, UV-A (5, 10, and 25 J/cm²) produced a 1.8- to 2.8-fold increase in phospho-p53 and a 2.6- to 6.0-fold increase in activated caspase3 compared with controls, resulting in 20% to 42% cell death.

Conclusions

Lower fluences of UV-A induce Nrf2-regulated antioxidant defense and higher fluences activate p53 and caspase3, indicating that even near-environmental levels of UV-A may affect normal CEnCs. This data suggest that UV-A may especially damage cells deficient in antioxidant defense, and thus may be involved in the etiology of Fuchs' endothelial corneal dystrophy (FECD).