Ascorbate Enhances Photogeneration of Hydrogen Peroxide Mediated by the Iris Melanin

Modeling complex age-related eye disease

Modeling complex eye diseases like age-related macular degeneration (AMD) and glaucoma poses significant challenges, since these conditions depend highly on age-related changes that occur over several decades, with many contributing factors remaining unknown. Although both diseases exhibit a relatively high heritability of >50%, a large proportion of individuals carrying AMD- or glaucoma-associated genetic risk variants will never develop these diseases. Furthermore, several environmental and lifestyle factors contribute to and modulate the pathogenesis and progression of AMD and glaucoma. Several strategies replicate the impact of genetic risk variants, pathobiological pathways and environmental and lifestyle factors in AMD and glaucoma in mice and other species. In this review we will primarily discuss the most commonly available mouse models, which have and will likely continue to improve our understanding of the pathobiology of age-related eye diseases. Uncertainties persist whether small animal models can truly recapitulate disease progression and vision loss in patients, raising doubts regarding their usefulness when testing novel gene or drug therapies. We will elaborate on concerns that relate to shorter lifespan, body size and allometries, lack of macula and a true lamina cribrosa, as well as absence and sequence disparities of certain genes and differences in their chromosomal location in mice. Since biological, rather than chronological, age likely predisposes an organism for both glaucoma and AMD, more rapidly aging organisms like small rodents may open up possibilities that will make research of these diseases more timely and financially feasible. On the other hand, due to the above-mentioned anatomical and physiological features, as well as pharmacokinetic and -dynamic differences small animal models are not ideal to study the natural progression of vision loss or the efficacy and safety of novel therapies. In this context, we will also discuss the advantages and pitfalls of alternative models that include larger species, such as non-human primates and rabbits, patient-derived retinal organoids, and human organ donor eyes.

Relationship between expression levels of TDRD7 and CRYBB3 and development of age-related cortico-nuclear cataracts

Background

The human lens develops age-related cataracts (ARCs) because of the complicated effects of aging and stressful conditions. Under conditions involving oxidative stress, cells form stress granules (SGs). TDRD7 has been identified as an RNA granule component and an important component of SGs. TDRD7 plays a role in the post-transcriptional expression of genes, such as the crystallin gene CRYBB3. Therefore, the present study investigated TDRD7 and CRYBB3 mRNA expressions in relation to age-related cortico-nuclear cataracts.

Methods

Quantitative real-time PCR was used to determine the expression levels of TDRD7 and CRYBB3 in 52 patients with ARC and 52 healthy controls. Anterior lens capsules and peripheral blood samples from patients with ARC were included in the patient group, and peripheral blood samples from healthy subjects and human lens epithelial cells (HLE-B3) were included in the control group. Gene expression levels in the different age groups were compared. Correlation analysis was used to assess the gene expression levels and age.

Results

The expression of TDRD7 and CRYBB3 was significantly up-regulated (P < 0.0001) in anterior lens capsules compared to that in HLE-B3 cells. Similarly, the expression of TDRD7 (P = 0.0004) and CRYBB3 (P < 0.0001) was higher in the peripheral blood samples of patients with ARC than in those of healthy subjects. Significant upregulation (P < 0.05) was observed in the 71–81-year age group of patients. No correlation was found between gene expression levels and age.

Conclusion

Significantly higher expression levels of TDRD7 and CRYBB3 in patients with ARC than in controls suggest that TDRD7 and CRYBB3 are associated with the development of age-related cortico-nuclear cataracts and the aging process under chronic stress.

Antioxidant Defense and Pseudoexfoliation Syndrome: An Updated Review

Oxidative stress (OS) affects the anterior ocular tissues, rendering them susceptible to several eye diseases. On the other hand, protection of the eye from harmful factors is achieved by unique defense mechanisms, including enzymatic and non-enzymatic antioxidants. The imbalance between oxidants and antioxidants could be the cause of pseudoexfoliation syndrome (PEXS), a condition of defective extracellular matrix (ECM) remodeling. A systematic English-language literature review was conducted from May 2022 to June 2022. The main antioxidant enzymes protecting the eye from reactive oxygen species (ROS) are superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx), which catalyze the reduction of specific types of ROS. Similarly, non-enzymatic antioxidants such as vitamins A, E and C, carotenoids and glutathione (GSH) are involved in removing ROS from the cells. PEXS is a genetic disease, however, environmental and dietary factors also influence its development. Additionally, many OS products disrupting the ECM remodeling process and modifying the antioxidative defense status could lead to PEXS. This review discusses the antioxidative defense of the eye in association with PEXS, and the intricate link between OS and PEXS. Understanding the pathways of PEXS evolution, and developing new methods to reduce OS, are crucial to control and treat this disease. However, further studies are required to elucidate the molecular pathogenesis of PEXS.

Emerging roles of oxidative stress in the pathogenesis of pseudoexfoliation syndrome (Review)

Pseudoexfoliation syndrome (PEXS) is a systemic disease caused by defects in the extracellular matrix (ECM) remodelling process leading to the chronic deposition of extracellular, fibrillary, white flaky pseudoexfoliation material (PEXM) throughout the body. Specifically, PEXM deposits on the lens capsule cause open-angle glaucoma, cataracts and blindness in patients with PEXS. Several gene single nucleotide polymorphisms are linked to the development of PEXS in humans, including lysyl oxidase-like 1 gene, clusterin and fibulin-5. The exact reason for the PEXM generation and its resulting pathogenesis is not well understood. However, defective ECM remodelling and oxidative stress (OS) have been hypothesized as significant events leading to the PEXM. Specifically, the link between OS and PEXS has been well studied, although the investigation is still ongoing. The present review explored recent advances in various aspects of PEXS and the involvement of OS in the eye for PEXS development.

Hydrogen Peroxide Effects on Natural-Sourced Polysacchrides: Free Radical Formation/Production, Degradation Process, and Reaction Mechanism-A Critical Synopsis

Numerous reactive oxygen species (ROS) entities exist, and hydrogen peroxide (H2O2) is very key among them as it is well known to possess a stable but poor reactivity capable of generating free radicals. Considered among reactive atoms, molecules, and compounds with electron-rich sites, free radicals emerging from metabolic reactions during cellular respirations can induce oxidative stress and cause cellular structure damage, resulting in diverse life-threatening diseases when produced in excess. Therefore, an antioxidant is needed to curb the overproduction of free radicals especially in biological systems (in vivo and in vitro). Despite the inherent properties limiting its bioactivities, polysaccharides from natural sources increasingly gain research attention given their position as a functional ingredient. Improving the functionality and bioactivity of polysaccharides have been established through degradation of their molecular integrity. In this critical synopsis; we articulate the effects of H2O2 on the degradation of polysaccharides from natural sources. Specifically, the synopsis focused on free radical formation/production, polysaccharide degradation processes with H2O2, the effects of polysaccharide degradation on the structural characteristics; physicochemical properties; and bioactivities; in addition to the antioxidant capability. The degradation mechanisms involving polysaccharide's antioxidative property; with some examples and their respective sources are briefly summarised.

Oxidative stress in the trabecular meshwork (Review)

Glaucoma is the second leading cause of blindness worldwide and elevated intraocular pressure (IOP) is the most important risk factor. High IOP usually occurs as a result of an increase in aqueous humor outflow resistance at the trabecular meshwork (TM). An abnormal TM contributes to the development of glaucoma. Oxidative stress and vascular damage are considered two major cellular factors that lead to alterations in the TM. In this review, we discuss the findings related to oxidative damage to the TM, including the sources of oxidative stress in the TM such as the mitochondria, peroxisomes, endoplasmic reticulum, membrane, cytosol and exogenous factors. We also discuss antioxidants and clinical studies related to protection against oxidative stress in the TM. Although many questions remain unanswered, it is becoming increasingly clear that oxidative stress-induced damage to the TM is related to glaucoma. This may inspire new studies to find better and more stable antioxidants, and better models with which to elucidate the mechanisms involved, and to determine whether in vitro findings translate into in vivo observations. The regulation of the oxidative/redox balance may be the ultimate target for protecting the TM from oxidative stress and preventing glaucoma.

Lipid peroxidation: pathophysiological and pharmacological implications in the eye

Oxygen-derived free radicals such as hydroxyl and hydroperoxyl species have been shown to oxidize phospholipids and other membrane lipid components leading to lipid peroxidation. In the eye, lipid peroxidation has been reported to play an important role in degenerative ocular diseases (age-related macular degeneration, cataract, glaucoma, diabetic retinopathy). Indeed, ocular tissues are prone to damage from reactive oxygen species due to stress from constant exposure of the eye to sunlight, atmospheric oxygen and environmental chemicals. Furthermore, free radical catalyzed peroxidation of long chain polyunsaturated acids (LCPUFAs) such as arachidonic acid and docosahexaenoic acid leads to generation of LCPUFA metabolites including isoprostanes and neuroprostanes that may further exert pharmacological/toxicological actions in ocular tissues. Evidence from literature supports the presence of endogenous defense mechanisms against reactive oxygen species in the eye, thereby presenting new avenues for the prevention and treatment of ocular degeneration. Hydrogen peroxide (H2O2) and synthetic peroxides can exert pharmacological and toxicological effects on tissues of the anterior uvea of several mammalian species. There is evidence suggesting that the retina, especially retinal ganglion cells can exhibit unique characteristics of antioxidant defense mechanisms. In the posterior segment of the eye, H2O2 and synthetic peroxides produce an inhibitory action on glutamate release (using [(3)H]-D-aspartate as a marker), in vitro and on the endogenous glutamate and glycine concentrations in vivo. In addition to peroxides, isoprostanes can elicit both excitatory and inhibitory effects on norepinephrine (NE) release from sympathetic nerves in isolated mammalian iris ciliary bodies. Whereas isoprostanes attenuate dopamine release from mammalian neural retina, in vitro, these novel arachidonic acid metabolites exhibit a biphasic regulatory effect on glutamate release from retina and can regulate amino acid neurotransmitter metabolism without inducing cell death in the retina. Furthermore, there appears to be an inhibitory role for neuroprostanes in the release of excitatory amino acid neurotransmitters in mammalian retina. The ability of peroxides and metabolites of LCPUFA to alter the integrity of neurotransmitter pools provides new potential target sites and pathways for the treatment of degenerative ocular diseases.

Antioxidants Protect Trabecular Meshwork Cells From Hydrogen Peroxide-Induced Cell Death

PURPOSE

To examine the protective effects of antioxidants in cultured trabecular meshwork (TM) cells exposed to oxidative stress.

METHODS

Primary porcine TM cells were pretreated with 50 μM resveratrol, 0.2 mM urate, 1 mM ascorbate, 1 mM reduced glutathione (rGSH), or 1 mM ρ-coumarate followed by exposure to hydrogen peroxide (0.5-4 mM). Cell metabolism was determined by mitochondrial enzyme activity and cell viability by uptake of the vital dye calcein, a fluorescent calcium binding dye. Reactive oxygen species (ROS), which may reflex oxidative damage, were determined by 2'7'-dichlorofluorescein diacetate.

RESULTS

Trabecular meshwork cell metabolism was reduced to 72 ± 5% of control levels with 1 mM hydrogen peroxide (H₂O₂) treatment. TM cells that co-incubated with ascorbate (85% ± 5%), ρ-coumarate (98 ± 11%) or rGSH (103 ± 17%) had significantly increased metabolism compared to 1 mM H₂O₂ treatment. Resveratrol significantly increased TM cell metabolism at both 2 mM (102 ± 14% live) and 4 mM H₂O₂ (27 ± 12% live), with H₂O₂-treated cultures containing mostly metabolically inactive cells (3% at 2 mM; 2% at 4 mM). Similar results were obtained in cell viability assays. Ascorbate and resveratrol, but not ρ-coumarate or rGSH, decreased ROS levels in TM cells exposed to a sublethal dose of H₂O₂ (0.5 mM). Urate had no protective effect against H₂O₂ damage in any of the assays.

CONCLUSIONS

Increased oxidative damage was demonstrated in the TM of patients with primary open angle glaucoma. The antioxidants (resveratrol, ascorbate, ρ-coumarate) and the antioxidant enzyme cofactor (rGSH) protected TM cells from H₂O₂-induced damage.

TRANSLATIONAL RELEVANCE

Future experiments are needed to determine whether addition of antioxidants may maintain TM cell viability in vivo. Antioxidants could be applied either topically or coupled with extended-release vehicles for intraocular injection to reduce free radical formation leading to enhanced therapeutic outcomes. Ultimately, studies using animal models could determine whether application of antioxidants can ameliorate progression in diseases such as glaucoma and macular degeneration.

Impact of iris pigment and pupil size in ultraviolet radiation cataract in rat

PURPOSE

To investigate the effect of iris pigment and pupil size in ultraviolet radiation (UVR)-induced cataract.

METHODS

Brown-Norway rats (pigmented) and Fischer-344 rats (non-pigmented) were unilaterally exposed in vivo to 5 kJ/m(2) UVR. Each strain was split into two groups, each receiving either mydriatic (tropicamide) or miotic (pilocarpine) eye-drops. One week after exposure, the degree of ocular inflammation and damage in the anterior segment was determined. The lenses were extracted, photographed and the degree of forward light scattering (cataract) was quantified.

RESULTS

The cataract types differed between the two strains. All Fischer rats developed macroscopically identifiable UVR cataract while only 41% of Brown-Norway rats did so. All groups except the miotic Brown-Norway developed significant light scattering. The Fischer rats developed 3-4-fold more lens light scattering than the Brown-Norway rats. The miotic Fischer group exhibited significantly more light scattering than the mydriatic Fischer group. There was no significant difference in light scattering between the two Brown-Norway groups. There was a correlation between ocular inflammation and degree of light scattering, with Brown-Norway rats exhibiting less inflammation and lens light scattering.

CONCLUSIONS

Pigmented rats develop less UVR cataract and less ocular inflammation than non-pigmented rats. Pupil size plays a smaller role in UVR cataract development in pigmented rats than in non-pigmented. The role of UVR-induced ocular inflammation in cataract development is still ambiguous.

Ultraviolet light-induced cyclobutane pyrimidine dimers in rabbit eyes

Sunlight exposure of the eye leads to pathologies including photokeratitis, cortical cataracts, pterygium, actinic conjunctivitis and age-related macular degeneration. It is well established that exposure to ultraviolet (UV) radiations leads to DNA damage, mainly cyclobutane pyrimidine dimers (CPDs). CPD formation is the principal factor involved in skin cancer. However, the exact mechanism by which sunlight induces ocular pathologies is not well understood. To shed light on this issue, we quantified the CPD formation onto DNA of rabbit ocular cells following UVB exposure. We found that CPDs were induced only in the structures of the ocular anterior chamber (cornea, iris and lens) and were more concentrated in the corneal epithelium. Residual UVB that pass through the cornea are completely absorbed by the anterior layers of the iris. CPDs were also detected in the central portion of the lens that is not protected by the iris (pupil). By determining the UV-induced DNA damage formation in eyes, we showed that anterior ocular structures are a reliable physical barrier that protects the subjacent structures from the toxic effects of UV. Although the corneal epithelium is the structure where most of the CPDs were detected, no cancer is related to solar exposure.

Real-time measurements of nicotinamide adenine dinucleotide in live human trabecular meshwork cells: effects of acute oxidative stress

The trabecular meshwork (TM) region of the eye is exposed to a constant low-level of oxidative insult. The cumulative damage may be the reason behind age-dependent risk for developing primary open angle glaucoma. Chronic and acute effects of hydrogen peroxide (H(2)O(2)) on TM endothelial cells include changes in viability, protein synthesis, and cellular adhesion. However, little if anything is known about the immediate effect of H(2)O(2) on the biochemistry of the TM cells and the initial response to oxidative stress. In this report, we have used two-photon excitation autofluorescence (2PAF) to monitor changes to TM cell nicotinamide adenine dinucleotide (NADPH). 2PAF allows non-destructive, real-time analysis of concentration of intracellular NADPH. Coupled to reduced glutathione, NADPH, is a major component in the anti-oxidant defense of TM cells. Cultured human TM cells were monitored for over 30 min in control and H(2)O(2)-containing solutions. Peroxide caused both a dose- and time-dependent decrease in NADPH signal. NADPH fluorescence in control and in 4 mM H(2)O(2) solutions showed little attenuation of NADPH signal (4% and 9% respectively). TM cell NADPH fluorescence showed a linear decrease with exposure to 20 mM H(2)O(2) (-29%) and 100 mM H(2)O(2) (37%) after a 30 min exposure. Exposure of TM cells to 500 mM H(2)O(2) caused an exponential decrease in NADPH fluorescence to a final attenuation of 46% of starting intensity. Analysis of individual TM cells indicates that cells with higher initial NADPH fluorescence are more refractive to the apparent loss of viability caused by H(2)O(2) than weakly fluorescing TM cells. We conclude that 2PAF of intracellular NADPH is a valuable tool for studying TM cell metabolism in response to oxidative insult.