Melatonin protects human retinal pigment epithelial (RPE) cells against oxidative stress

The Role and Therapeutic Potential of Melatonin in Degenerative Fundus Diseases: Diabetes Retinopathy and Age-Related Macular Degeneration

Degenerative fundus disease encompasses a spectrum of ocular diseases, including diabetic retinopathy (DR) and age-related macular degeneration (AMD), which are major contributors to visual impairment and blindness worldwide. The development and implementation of effective strategies for managing and preventing the onset and progression of these diseases are crucial for preserving patients' visual acuity. Melatonin, a neurohormone primarily produced by the pineal gland, exhibits properties such as circadian rhythm modulation, antioxidant activity, anti-inflammatory effects, and neuroprotection within the ocular environment. Furthermore, melatonin has been shown to suppress neovascularization and reduce vascular leakage, both of which are critical in the pathogenesis of degenerative fundus lesions. Consequently, melatonin emerges as a promising therapeutic candidate for degenerative ocular diseases. This review provides a comprehensive overview of melatonin synthesis, its localization within ocular tissues, and its mechanisms of action, particularly in regulating melatonin production, thereby underscoring its potential as a therapeutic agent for degenerative fundus diseases.

Melatonin in the mammalian retina: Synthesis, mechanisms of action and neuroprotection

Melatonin is an important player in the regulation of many physiological functions within the body and in the retina. Melatonin synthesis in the retina primarily occurs during the night and its levels are low during the day. Retinal melatonin is primarily synthesized by the photoreceptors, but whether the synthesis occurs in the rods and/or cones is still unclear. Melatonin exerts its influence by binding to G protein-coupled receptors named melatonin receptor type 1 (MT1) and type 2 (MT2). MT1 and MT2 receptors activate a wide variety of signaling pathways and both receptors are present in the vertebrate photoreceptors where they may form MT1/MT2 heteromers (MT1/2h). Studies in rodents have shown that melatonin signaling plays an important role in the regulation of retinal dopamine levels, rod/cone coupling as well as the photopic and scotopic electroretinogram. In addition, melatonin may play an important role in protecting photoreceptors from oxidative stress and can protect photoreceptors from apoptosis. Critically, melatonin signaling is involved in the modulation of photoreceptor viability during aging and other studies have implicated melatonin in the pathogenesis of age-related macular degeneration. Hence melatonin may represent a useful tool in the fight to protect photoreceptors-and other retinal cells-against degeneration due to aging or diseases.

Influence of different LED wavelengths on retinal melatonin levels - A rodent study

BACKGROUND

Retinal melatonin is crucial for neuroprotection. Exposure to light-emitting diodes (LEDs) affects retinal neurons, possibly influencing retinal melatonin levels. Hence, we aimed to quantify the retinal melatonin level with different LED wavelengths.

METHOD

A total of 24 Sprague Dawley (SD) male rats were divided into four groups (n = 6 in each group) as normal controls (NC), blue light (BL), white light (WL), and yellow light (YL). The rats in the experimental groups were exposed to different wavelengths of LEDs for 28 days (12:12 h light-dark cycle) with uniform illumination of 450-500 lx. Following exposure, the rats were subjected to behavioral tests such as passive avoidance and elevated plus maze tests. Following the behavior tests, the rats were sacrificed, eyes were enucleated, and retinal tissue was stored at -80 °C. The homogenized retina was used for reactive oxygen species (ROS) and melatonin quantification using an enzyme-linked immunosorbent assay (ELISA) kit.

RESULTS

Passive avoidance test revealed a significant difference across the groups (p < 0.0004). The BL exposure group demonstrated increased latency to enter the dark compartment (DC) and impaired motor memory. The elevated plus maze test revealed a significant difference across all the groups (p < 0.012), where the time spent in the closed arm was greater in the BL exposure group. Comparison of ROS levels revealed a significant difference across the groups (p < 0.0001), with increased nitric oxide concentrations in the experimental groups. Melatonin levels were significantly decreased in the light exposure groups (p < 0.0001) compared to the NC group.

CONCLUSION

Cumulative exposure to different LED wavelengths resulted in increased anxiety with impaired motor activity. This was also complemented by the addition of oxidative stress leading to decreased melatonin levels in the retina, which might trigger retinal neuronal damage.

Ultraviolet Radiation-Induced Mitochondrial Disturbances Are Attenuated by Metabolites of Melatonin in Human Epidermal Keratinocytes

Melatonin (N-acetyl-5-methoxytryptamine) is recognized as an effective antioxidant produced by the pineal gland, brain and peripheral organs, which also has anti-inflammatory, immunomodulatory, and anti-tumour capacities. Melatonin has been reported as a substance that counteracts ultraviolet radiation B (UVB)-induced intracellular disturbances. Nevertheless, the mechanistic actions of related molecules including its kynurenic derivatives (N1-acetyl-N2-formyl-5-methoxykynurenine (AFMK)), its indolic derivatives (6-hydroxymelatonin (6(OH)MEL) and 5-methoxytryptamine (5-MT)) and its precursor N-acetylserotonin (NAS) are only poorly understood. Herein, we treated human epidermal keratinocytes with UVB and assessed the protective effect of the studied substances in terms of the maintenance of mitochondrial function or their radical scavenging capacity. Our results show that UVB caused the significant elevation of catalase (CAT) and superoxide dismutase (Mn-SOD), the dissipation of mitochondrial transmembrane potential (mtΔΨ), a reduction in ATP synthesis, and the enhanced release of cytochrome c into cytosol, leading subsequently to UVB-mediated activation of the caspases and apoptosis (appearance of sub-G1 population). Our findings, combined with data reported so far, indicate the counteracting and beneficial actions of melatonin and its molecular derivatives against these deleterious changes within mitochondria. Therefore, they define a path to the development of novel strategies delaying mitochondrial aging and promoting the well-being of human skin.

A Novel Donepezil–Caffeic Acid Hybrid: Synthesis, Biological Evaluation, and Molecular Docking Studies

A novel donepezil-caffeic acid (DP-CA) hybrid molecule was designed, synthesis, and investigated by molecular modeling. Its biological activity and protective effect were investigated by the IR spectroscopy, ¹H and ¹³C NMR spectroscopy, and mass spectrometry. DP-CA was highly active against acetylcholine esterase and inhibited it at the micromolar concentrations. Fluorescence and UV-Vis spectroscopy studies showed strong binding of DP-CA to DNA. Moreover, DP-CA exhibited protective effects against H2O2-induced toxicity in U-118 MG glioblastoma cells. Finally, molecular docking showed a high affinity of DP-CA in all concentrations, and the active 4EY7 site exhibited essential residues with polar and apolar contacts. Taken together, these findings indicate that DP-CA could be a prospective multifunctional agent for the treatment of neurodegenerative diseases.

Melatonin loaded hybrid nanomedicine: DoE approach, optimization and in vitro study on diabetic retinopathy model

Melatonin (MEL) is a pleiotropic neurohormone of increasing interest as a neuroprotective agent in ocular diseases. Improving the mucoadhesiveness is a proposed strategy to increase the bioavailability of topical formulations. Herein, the design and optimization of MEL-loaded lipid-polymer hybrid nanoparticles (mel-LPHNs) using Design of Experiment (DoE) was performed. LPHNs consisted of PLGA-PEG polymer nanoparticles coated with a cationic lipid-shell. The optimized nanomedicine showed suitable size for ophthalmic administration (189.4 nm; PDI 0.260) with a positive surface charge (+39.8 mV), high encapsulation efficiency (79.8 %), suitable pH and osmolarity values, good mucoadhesive properties and a controlled release profile. Differential Scanning Calorimetry and Fourier-Transform Infrared Spectroscopy confirmed the encapsulation of melatonin in the systems and the interaction between lipids and polymer matrix. Biological evaluation in an in vitro model of diabetic retinopathy demonstrated enhanced neuroprotective and antioxidant activities of mel-LPHNs, compared to melatonin aqueous solution at the same concentration (0.1 and 1 μM). A modified Draize test was performed to assess the ocular tolerability of the formulation showing no signs of irritation. To the best our knowledge, this study reported for the first time the development of mel-LPHNs, a novel and safe hybrid platform suitable for the topical management of retinal diseases.

Melatonin Type 2 Receptor Activation Regulates Blue Light Exposure-Induced Mouse Corneal Epithelial Damage by Modulating Impaired Autophagy and Apoptosis

The MT1/2 receptors, members of the melatonin receptor, belong to G protein-coupled receptors and mainly regulate circadian rhythms and sleep in the brain. Previous studies have shown that in many other cells and tissues, such as HEK293T cells and the retina, MT1/2 receptors can be involved in mitochondrial homeostasis, antioxidant, and anti-inflammatory responses. In our study, we aimed to investigate the effects of blue light (BL) exposure on the expression of melatonin and its receptors in the mouse cornea and to evaluate their functional role in corneal epithelial damage. After exposing 8-week-old C57BL/6 mice to BL at 25 and 100 J/cm² twice a day for 14 days, a significant increase in the expression of 4-HNE and MT2 was observed in the cornea. MT2 antagonist-treated mice exposed to BL showed an increased expression of p62 and decreased expression of BAX and cleaved caspase 3 compared with mice exposed only to BL. In addition, MT2 antagonist-treated mice showed more enhanced MDA and corneal damage. In conclusion, BL exposure can induce MT2 expression in the mouse cornea. MT2 activation can modulate impaired autophagy and apoptosis by increasing the expression of BAX, an apoptosis activator, thereby regulating the progression of corneal epithelial damage induced by BL exposure.

A critical review on air pollutant exposure and age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of visual impairments and blindness worldwide in the elderly and its incidence strongly increases with ages. The etiology of AMD is complex and attributed to the genetic modifiers, environmental factors and gene-environment interactions. Recently, the impacts of air pollution on the development of eye diseases have become the new area of focus, and disordered air exposure combined with inadequate health management has caused problems for the eye health, such as dry eye, glaucoma, and retinopathy, while its specific role in the occurrence of AMD is still not well understood. In order to summarize the progress of this research field, we performed a critical review to summarize the epidemiological and mechanism evidence on the association between air pollutants exposure and AMD. This review documented that exposure to air pollutants will accelerate or worsen the morbidity and prevalence of AMD. Air pollutants exposure may change the homeostasis, interfere with the inflammatory response, and take direct action on the lipid metabolism and oxidative stress in the macula. More attention should be given to understanding the impact of ambient air pollution on AMD worldwide.

Role of Mitochondria in Retinal Pigment Epithelial Aging and Degeneration

Retinal pigment epithelial (RPE) cells form a monolayer between the neuroretina and choroid. It has multiple important functions, including acting as outer blood-retina barrier, maintaining the function of neuroretina and photoreceptors, participating in the visual cycle and regulating retinal immune response. Due to high oxidative stress environment, RPE cells are vulnerable to dysfunction, cellular senescence, and cell death, which underlies RPE aging and age-related diseases, including age-related macular degeneration (AMD). Mitochondria are the powerhouse of cells and a major source of cellular reactive oxygen species (ROS) that contribute to mitochondrial DNA damage, cell death, senescence, and age-related diseases. Mitochondria also undergo dynamic changes including fission/fusion, biogenesis and mitophagy for quality control in response to stresses. The role of mitochondria, especially mitochondrial dynamics, in RPE aging and age-related diseases, is still unclear. In this review, we summarize the current understanding of mitochondrial function, biogenesis and especially dynamics such as morphological changes and mitophagy in RPE aging and age-related RPE diseases, as well as in the biological processes of RPE cellular senescence and cell death. We also discuss the current preclinical and clinical research efforts to prevent or treat RPE degeneration by restoring mitochondrial function and dynamics.

Relationship between shift work and age-related macular degeneration: a cross-sectional analysis of data from the 5th Korea National Health and Nutrition Examination Survey (2010-2012)

Background

Age-related macular degeneration (AMD) is the leading cause of blindness. Shift work has well-known adverse effects on health. However, few studies have investigated the relationship between shift work and AMD. This study was conducted to investigate the relationship between shift work and AMD.

Methods

This study used aggregated data from the 2010-2012 cycles of the Korea National Health and Nutrition Examination Survey. The work schedules were classified into 2 types: day work and shift work. AMD was determined using fundus photographs. The χ² test and multiple logistic regression analysis were used to assess sex-stratified relationship between shift work and AMD.

Results

The odds ratio (OR) of AMD in male shift workers was higher (1.54 [95% confidence interval, CI: 1.01-2.36]) than that in male day workers after adjusting for covariates. After dividing into subgroups of the shift work pattern, the OR of AMD in male night shift workers was higher (1.75 [95% CI: 1.07-2.85]) than that in male day workers after adjusting for covariates. However, results of the female worker group were not significant.

Conclusions

The results of this study provide limited support for the hypothesis that shift work is related to AMD. Further prospective studies are needed to define the relationship between shift work and AMD.

Mitochondrial Dysfunction and Endoplasmic Reticulum Stress in Age Related Macular Degeneration, Role in Pathophysiology, and Possible New Therapeutic Strategies

Age related macular degeneration (AMD) is the main cause of legal blindness in developed countries. It is a multifactorial disease in which a combination of genetic and environmental factors contributes to increased risk of developing this vision-incapacitating condition. Oxidative stress plays a central role in the pathophysiology of AMD and recent publications have highlighted the importance of mitochondrial dysfunction and endoplasmic reticulum stress in this disease. Although treatment with vascular endothelium growth factor inhibitors have decreased the risk of blindness in patients with the exudative form of AMD, the search for new therapeutic options continues to prevent the loss of photoreceptors and retinal pigment epithelium cells, characteristic of late stage AMD. In this review, we explain how mitochondrial dysfunction and endoplasmic reticulum stress participate in AMD pathogenesis. We also discuss a role of several antioxidants (bile acids, resveratrol, melatonin, humanin, and coenzyme Q10) in amelioration of AMD pathology.

Influence of Circadian Rhythm in the Eye: Significance of Melatonin in Glaucoma

Circadian rhythm and the molecules involved in it, such as melanopsin and melatonin, play an important role in the eye to regulate the homeostasis and even to treat some ocular conditions. As a result, many ocular pathologies like dry eye, corneal wound healing, cataracts, myopia, retinal diseases, and glaucoma are affected by this cycle. This review will summarize the current scientific literature about the influence of circadian patterns on the eye, focusing on its relationship with increased intraocular pressure (IOP) fluctuations and glaucoma. Regarding treatments, two ways should be studied: the first one, to analyze if some treatments could improve their effect on the ocular disease when their posology is established in function of circadian patterns, and the second one, to evaluate new drugs to treat eye pathologies related to the circadian rhythm, as it has been stated with melatonin or its analogs, that not only could be used as the main treatment but as coadjutant, improving the circadian pattern or its antioxidant and antiangiogenic properties.

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

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

Melatonin protects the retina from experimental nonexudative age-related macular degeneration in mice

Nonexudative age-related macular degeneration (NE-AMD) represents the leading cause of blindness in the elderly. Currently, there are no available treatments for NE-AMD. We have developed a NE-AMD model induced by superior cervical ganglionectomy (SCGx) in C57BL/6J mice, which reproduces the disease hallmarks. Several lines of evidence strongly support the involvement of oxidative stress in NE-AMD-induced retinal pigment epithelium (RPE) and outer retina damage. Melatonin is a proven and safe antioxidant. Our aim was analysing the effect of melatonin in the RPE/outer retina damage within experimental NE-AMD. The treatment with melatonin starting 48 h after SCGx, which had no effect on the ubiquitous choriocapillaris widening, protected visual functions and avoided Bruch´s membrane thickening, RPE melanin content, melanosome number loss, retinoid isomerohydrolase (RPE65)-immunoreactivity decrease, and RPE and hotoreceptor ultrastructural damage induced within experimental NE-AMD exclusively located at the central temporal (but not nasal) region. Melatonin also prevented the increase in outer retina/RPE oxidative stress markers and a decrease in mitochondrial mass at 6 weeks post-SCGx. Moreover, when the treatment with melatonin started at 4 weeks post-SCGx, it restored visual functions and reversed the decrease in RPE melanin content and RPE65-immunoreactivity. These findings suggest that melatonin could become a promising safe therapeutic strategy for NE-AMD.

Mitochondrial dysfunction in age-related macular degeneration: melatonin as a potential treatment

Introduction: Age-related Macular Degeneration (AMD), a retinal neurodegenerative disease is the most common cause of blindness among the elderly in developed countries. The impairment of mitochondrial biogenesis has been reported in human retinal pigment epithelium (RPE) cells affected by AMD. Oxidative/nitrosative stress plays an important role in AMD development. The mitochondrial respiratory system is considered a major site of reactive oxygen species (ROS) generation. During aging, insufficient free radical scavenger systems, impairment of DNA repair mechanisms and reduction of mitochondrial degradation and turnover contribute to the massive accumulation of ROS disrupting mitochondrial function. Impaired mitochondrial function leads to the decline in the autophagic capacity and induction of inflammation and apoptosis in human RPE cells affected by AMD.Areas covered: This article evaluates the ameliorative effect of melatonin on AMD and examines AMD pathogenesis with an emphasis on mitochondrial dysfunction. It also considers the potential effects of melatonin on mitochondrial function.Expert opinion: The effect of melatonin on mitochondrial function results in the reduction of oxidative stress, inflammation and apoptosis in the retina; these findings demonstrate that melatonin has the potential to prevent and treat AMD.

The association between clinically diagnosed insomnia and age-related macular degeneration: a population-based cohort study

PURPOSE

The decreased level of melatonin, the substance involved in the control of the sleep-wake cycle, has been reported among the patients with age-related macular degeneration (AMD). However, knowledge about the relationship between sleep disturbance and AMD is still limited. This longitudinal case-control study aims to investigate the risk of incident AMD among the patients with clinically diagnosed insomnia using the Taiwan National Health Insurance Research Database.

METHODS

The insomnia cohort (n = 15 465) consisted of newly diagnosed insomnia cases aged ≥55 years between 2000 and 2009. Subjects without insomnia, matched for age, gender and enrolment time, were randomly sampled as the control cohort (n = 92 790). Cox proportional hazard regressions were performed to calculate the hazard ratios (HR) of incident AMD for the two cohorts after adjusting for potential confounders.

RESULTS

Of the 108 255 sampled subjects, 2094 (1.9%) were diagnosed with AMD, including 214 (0.2%) with neovascular AMD, during a mean follow-up period of 5.1 ± 2.8 years. Insomnia patients were more likely to have subsequent AMD than those without insomnia (2.5% versus 1.8%, p < 0.001). Further, the incidence of exudative AMD was also higher in the insomnia cohort than the control cohort (0.3% versus 0.2%, p = 0.002). The adjusted HR was 1.33 (95% confidence interval [CI], 1.18-1.48, p < 0.001) for AMD and 1.67 (95% CI, 1.20-2.33, p = 0.002) for exudative AMD.

CONCLUSIONS

Clinically diagnosed insomnia is an independent indicator for the increased risk of subsequent AMD development.

Intravitreal Delivery of Melatonin Is Protective Against the Photoreceptor Loss in Mice: A Potential Therapeutic Strategy for Degenerative Retinopathy

Melatonin is a circadian hormone with potent cytoprotective effects. Retinitis pigmentosa (RP) comprises a heterogeneous group of inherent retinopathies that characterized by the photoreceptor death in bilateral eyes. The N-methyl-N-nitrosourea (MNU) administered mouse is a type of chemically induced RP model with rapid progressive rate. We intend to study the melatonin mediated effects on the MNU administered mice. Melatonin was delivered into the vitreous body of the MNU administered mice. Subsequently, the melatonin treated mice were subjected to histological analysis, optokinetic behavior tests, spectral-domain optical coherence tomography (SD-OCT), and electroretinogram (ERG) examination. Multi-electrodes array (MEA) was used to analyze the status of visual signal transmission within retinal circuits. Biochemical analysis was performed to quantify the expression levels of antioxidative enzymes, oxidative stress markers, and apoptotic factors in the retinas. The intravitreal injection of melatonin ameliorated effectively the MNU induced photoreceptor degeneration. Melatonin therapy mitigated the spontaneous firing response, and preserved the basic configurations of visual signal pathway in MNU administered mice. MEA is effective to evaluate the pharmacological effects on retina. Of note, the cone photoreceptors in degenerative retinas were rescued efficiently by melatonin therapy. Melatonin afforded these protective effects by modulating the apoptotic cascades and alleviating the oxidative stress. These findings suggest that melatonin could act as an alternative treatment for degenerative retinopathy. Melatonin might be used in combination with other therapeutic approaches to alleviate the photoreceptor loss and preserve the visual function of RP patients.

Melatonin attenuates epidermal growth factor-induced cathepsin S expression in ARPE-19 cells: Implications for proliferative vitreoretinopathy

Abnormal proliferation and motility of retinal pigment epithelial cells leads to proliferative vitreoretinopathy (PVR). Melatonin is a known effective antitumour and anti-invasive agent, but whether it affects the formation and underlying mechanisms of PVR remains unclear. In this study, the results of the MTT assay, colony formation and propidium iodide (PI) staining with flow cytometry revealed that melatonin dose dependently inhibited epidermal growth factor (EGF)-induced proliferation of human ARPE-19 cells. Furthermore, melatonin reduced EGF-induced motility by suppressing cathepsin S (CTSS) expression. Pretreatment with ZFL (a CTSS inhibitor) or overexpression of CTSS (pCMV-CTSS) significantly inhibited EGF-induced cell motility when combined with melatonin. Epidermal growth factor induced the phosphorylation of AKT(S473)/mTOR (S2448) and transcription factor (c-Jun/Sp1) signaling pathways. Pretreatment of LY294002 (a PI3K inhibitor) or rapamycin (an mTOR inhibitor) markedly reduced EGF-induced motility and p-AKT/p-mTOR/c-Jun/Sp1 expression when combined with melatonin. Taken together, these data indicate that melatonin inhibited EGF-induced proliferation and motility of human ARPE-19 cells by activating the AKT/mTOR pathway, which is dependent on CTSS modulation of c-Jun/Sp1 signalling. Melatonin may be a promising therapeutic drug against PVR.

Melatonin as the Possible Link Between Age-Related Retinal Regeneration and the Disrupted Circadian Rhythm in Elderly

The association between age-related macular degeneration (AMD) and biological rhythms has been insufficiently studied; however there are several reasons to believe that impairment in circadian rhythm may affect incidence and pathogenesis of AMD. The current understanding of AMD pathology is based on age-related, cumulative oxidative damage to the retinal pigmented epithelium (RPE) partially due to impaired clearance of phagocytosed photoreceptor outer segments. In higher vertebrates, phagocytosis of the outer segments is synchronized by circadian rhythms and occurs shortly after dawn, followed by lysosomal-mediated clearance. Aging has been shown to be associated with the changes in circadian rhythmicity of melatonin production, which can be a major factor contributing to the impaired balance between phagocytosis and clearance and increased levels of reactive oxygen species resulting in degenerative changes in the retina. This minireview summarizes studies linking AMD with melatonin production and discusses challenges and perspectives of this area of research.

Risk factors for Age-related Macular Degeneration in a Greek population: The Thessaloniki Eye Study

PURPOSE

To assess the association of potential risk factors with early and late age-related macular degeneration (AMD) in the Thessaloniki Eye Study (TES) population Design: Population-based, cross-sectional study of subjects over age of 60 living in Thessaloniki, Greece Methods: Subjects without any AMD features and subjects with early and late AMD (neovascular AMD or geographic atrophy) were identified in the TES cohort using standardized procedures and masked grading of stereo color fundus photos. Demographic, lifestyle, systemic and other ophthalmic covariates were also collected during a detailed examination process. Their association with AMD was investigated using univariate and multivariate adjusted logistic regression models.

RESULTS

Among the 2108 participants with gradable photos, the grading process identified 1204 subjects with no AMD, 848 subjects with early AMD, and 56 subjects with late AMD (24 with geographic atrophy and 32 with neovascular AMD). In multivariate analysis, compared to no AMD, late AMD was positively associated with older age (OR:1.16; 95%CI:1.10-1.22 per year of age), current smoking (smoking vs. never smoking, OR:2.34; 95%CI:1.12-4.90), prior cataract surgery (cataract surgery vs. no cataract surgery OR:2.06; 95%CI:0.96-4.40), marital status (divorced/separated vs. married, OR:3.10; 95%CI:1.08-8.93) and with 60% lower odds when sleeping in the afternoon (yes vs. no, OR:0.40; 95%CI:0.22-0.72). Early AMD was positively associated with older age (OR: 1.03; 95%CI:1.01-1.05 per year of age) and negatively with higher pulse pressure (OR:0.99; 95%CI:0.98-0.99 per mmHg).

CONCLUSIONS

In TES, apart for well-known risk factors for AMD like age, smoking, and cataract surgery, two novel behavioral risk factors for prevalent late AMD were suggested. Sleeping in the afternoon was associated with 60% decreased odds for late AMD and 67% decreased odds for neovascular AMD. Being divorced/separated compared to married was associated with 3-fold higher odds for late AMD. Large longitudinal population-based studies will be necessary to further establish the potential late AMD risk effects of these two novel factors, to demonstrate potential implications of underlying pathogenetic mechanisms, and to explore preventive measures and therapeutic targets.

Diplomats' Mystery Illness and Pulsed Radiofrequency/Microwave Radiation

Importance: A mystery illness striking U.S. and Canadian diplomats to Cuba (and now China) "has confounded the FBI, the State Department and US intelligence agencies" (Lederman, Weissenstein, & Lee, 2017). Sonic explanations for the so-called health attacks have long dominated media reports, propelled by peculiar sounds heard and auditory symptoms experienced. Sonic mediation was justly rejected by experts. We assessed whether pulsed radiofrequency/microwave radiation (RF/MW) exposure can accommodate reported facts in diplomats, including unusual ones. Observations: (1) Noises: Many diplomats heard chirping, ringing or grinding noises at night during episodes reportedly triggering health problems. Some reported that noises were localized with laser-like precision or said the sounds seemed to follow them (within the territory in which they were perceived). Pulsed RF/MW engenders just these apparent "sounds" via the Frey effect. Perceived "sounds" differ by head dimensions and pulse characteristics and can be perceived as located behind in or above the head. Ability to hear the "sounds" depends on high-frequency hearing and low ambient noise. (2) Signs/symptoms: Hearing loss and tinnitus are prominent in affected diplomats and in RF/MW-affected individuals. Each of the protean symptoms that diplomats report also affect persons reporting symptoms from RF/MW: sleep problems, headaches, and cognitive problems dominate in both groups. Sensations of pressure or vibration figure in each. Both encompass vision, balance, and speech problems and nosebleeds. Brain injury and brain swelling are reported in both. (3) Mechanisms: Oxidative stress provides a documented mechanism of RF/MW injury compatible with reported signs and symptoms; sequelae of endothelial dysfunction (yielding blood flow compromise), membrane damage, blood-brain barrier disruption, mitochondrial injury, apoptosis, and autoimmune triggering afford downstream mechanisms, of varying persistence, that merit investigation. (4) Of note, microwaving of the U.S. embassy in Moscow is historically documented. Conclusions and relevance: Reported facts appear consistent with pulsed RF/MW as the source of injury in affected diplomats. Nondiplomats citing symptoms from RF/MW, often with an inciting pulsed-RF/MW exposure, report compatible health conditions. Under the RF/MW hypothesis, lessons learned for diplomats and for RF/MW-affected civilians may each aid the other.

Protective Effect of Melatonin against Oxidative Stress-Induced Apoptosis and Enhanced Autophagy in Human Retinal Pigment Epithelium Cells

Age-related macular degeneration (AMD) affects the retinal macula and results in loss of vision, and AMD is the primary cause of blindness and severe visual impairment among elderly people worldwide. AMD is characterized by the accumulation of drusen in the Bruch's membrane and dysfunction of retinal pigment epithelial (RPE) cells and photoreceptors. The pathogenesis of AMD remains unclear, and no effective treatment exists. Accumulating evidence indicates that oxidative stress plays a critical role in RPE cell degeneration and AMD. Melatonin is an antioxidant that scavenges free radicals, and it has anti-inflammatory, antitumor, and antiangiogenic effects. This study investigated the antioxidative, antiapoptotic, and autophagic effects of melatonin on oxidative damage to RPE cells. We used hydrogen peroxide (H₂O₂) to stimulate reactive oxygen species production to cause cell apoptosis in ARPE-19 cell lines. Our findings revealed that treatment with melatonin significantly inhibited H₂O₂-induced RPE cell damage, decreased the apoptotic rate, increased the mitochondrial membrane potential, and increased the autophagy effect. Furthermore, melatonin reduced the Bax/Bcl-2 ratio and the expression levels of the apoptosis-associated proteins cytochrome c and caspase 7. Additionally, melatonin upregulated the expression of the autophagy-related proteins LC3-II and Beclin-1 and downregulated the expression of p62. Thus, melatonin's effects on autophagy and apoptosis can protect against H₂O₂-induced oxidative damage in human RPE cells. Melatonin may have multiple protective effects on human RPE cells against H₂O₂-induced oxidative damage.

Stem Cell Derived Retinal Pigment Epithelium: The Role of Pigmentation as Maturation Marker and Gene Expression Profile Comparison with Human Endogenous Retinal Pigment Epithelium

In age-related macular degeneration (AMD) the retinal pigment epithelium (RPE) deteriorates, leading to photoreceptor decay and severe vision loss. New therapeutic strategies aim at RPE replacement by transplantation of pluripotent stem cell (PSC)-derived RPE. Several protocols to generate RPE have been developed where appearance of pigmentation is commonly used as indicator of RPE differentiation and maturation. It is, however, unclear how different pigmentation stages reflect developmental stages and functionality of PSC-derived RPE cells. We generated human embryonic stem cell-derived RPE (hESC-RPE) cells and investigated their gene expression profiles at early pigmentation (EP) and late pigmentation (LP) stages. In addition, we compared the hESC-RPE samples with human endogenous RPE. We used a common reference design microarray (44 K). Our analysis showed that maturing hESC-RPE, upon acquiring pigmentation, expresses markers specific for human RPE. Interestingly, our analysis revealed that EP and LP hESC-RPE do not differ much in gene expression. Our data further showed that pigmented hESC-RPE has a significant lower expression than human endogenous RPE in the visual cycle and oxidative stress pathways. In contrast, we observed a significantly higher expression of pathways related to the process adhesion-to-polarity model that is typical of developing epithelial cells. We conclude that, in vitro, the first appearance of pigmentation hallmarks differentiated RPE. However, further increase in pigmentation does not result in much significant gene expression changes and does not add important RPE functionalities. Consequently, our results suggest that the time span for obtaining differentiated hESC-RPE cells, that are suitable for transplantation, may be greatly reduced.

Nobiletin protects human retinal pigment epithelial cells from hydrogen peroxide-induced oxidative damage

Nobiletin (3',4',5,6,7,8-hexamethoxyflavone), a dietary polymethoxylated flavonoid found in Citrus fruits, has been reported to have antioxidant effect. However, the effect of nobiletin on human retinal pigment epithelium (RPE) cells induced by hydrogen peroxide (H₂ O₂ ) is still unclear. Therefore, we investigated the protective effect of nobiletin against H₂ O₂ -induced cell death in RPE cells. Our results demonstrated that nobiletin significantly increased cell viability from oxidative stress. Nobiletin inhibited H₂ O₂ -induced ROS production and caspase-3/7 activity in ARPE-19 cells. Furthermore, nobiletin significantly increased Akt phosphorylation in ARPE-19 cells exposed to H₂ O₂ . Meanwhile, LY294002, an inhibitor of PI3K/Akt, abolished the protective effect of nobiletin against H₂ O₂ -induced decreased cell viability and increased caspase-3/7 activity in ARPE-19 cells. In summary, these data show that nobiletin protects RPE cells against oxidative stress through activation of the Akt-signaling pathway. Thus, nobiletin should be an oxidant that attenuates the development of age-related macular degeneration.

Curcumin regulates intracellular calcium release and inhibits oxidative stress parameters, VEGF, and caspase-3/-9 levels in human retinal pigment epithelium cells

In this study, we aimed to observe whether curcumin (cur), a polyphenolic compound derived from the dietary spice turmeric, a yellow substance obtained from the root of the plant Curcuma longa Linn, has any protective effect against blue light irradiation in human retinal pigment epithelium (ARPE-19) cells. For this purpose, we evaluated the intracellular calcium release mechanism, poly ADP ribose polymerase (PARP), procaspase-3/-9 protein expression levels, caspase activation, and reactive oxygen species levels. ARPE-19 cells were divided into four main groups, such as control, cur, blue light, and cur + blue light. Results were evaluated by Kruskal-Wallis and Mann-Whitney U tests as post hoc tests. The cells in cur and cur + blue light samples were incubated with 20 μM cur. Blue light exposure was performed for 24 h in an incubator. Lipid peroxidation and cytosolic-free Ca²⁺ [Ca²⁺]_i concentrations were higher in the blue light exposure samples than in the control samples; however, their levels were determined as significantly lower in the cur and cur + blue light exposure samples than in the blue light samples alone. PARP and procaspase-3 levels were significantly higher in blue light samples. Cur administration significantly decreased PARP and procaspase-3 expression levels. Reduced glutathione and glutathione peroxidase values were lower in the blue light exposure samples, although they were higher in the cur and cur + blue light exposure samples. Caspase-3 and -9 activities were lower in the cur samples than in the blue light samples. Moreover, vascular endothelial growth factor (VEGF) levels were significantly higher in the blue light exposure samples. In conclusion, cur strongly induced regulatory effects on oxidative stress, intracellular Ca²⁺ levels, VEGF levels, PARP expression levels, and caspase-3 and -9 values in an experimental oxidative stress model in ARPE-19 cells.

Activation of Nrf2 by Ginsenoside Rh3 protects retinal pigment epithelium cells and retinal ganglion cells from UV

Excessive Ultra-violet (UV) radiation shall induce damages to resident retinal pigment epithelium (RPE) cells (RPEs) and retinal ganglion cells (RGCs). Here we tested the potential activity of Ginsenoside Rh3 ("Rh3") against the process. In cultured human RPEs and RGCs, pretreatment with Rh3 inhibited UV-induced reactive oxygen species (ROS) production and following apoptotic/non-apoptotic cell death. Rh3 treatment in retinal cells induced nuclear-factor-E2-related factor 2 (Nrf2) activation, which was evidenced by Nrf2 protein stabilization and its nuclear translocation, along with transcription of antioxidant responsive element (ARE)-dependent genes (HO1, NOQ1 and GCLC). Nrf2 knockdown by targeted-shRNA almost abolished Rh3-induced retinal cell protection against UV. Further studies found that Rh3 induced microRNA-141 ("miR-141") expression, causing downregulation of its targeted gene Keap1 in RPEs and RGCs. On the other hand, Rh3-induced Nrf2 activation and retinal cell protection were largely attenuated by the miR-141's inhibitor, antagomiR-141. In vivo, intravitreal injection of Rh3 inhibited retinal dysfunction by light damage in mice. Rh3 intravitreal injection also induced miR-141 expression, Keap1 downregulation and Nrf2 activation in mouse retinas. We conclude that Rh3 protects retinal cells from UV via activating Nrf2 signaling.

Protective effects of 6-ureido/thioureido-2,4,5-trimethylpyridin-3-ols against 4-hydroxynonenal-induced cell death in adult retinal pigment epithelial-19 cells

Dysfunction or progressive degeneration of retinal pigment epithelium (RPE) contributes in the initial pathogenesis of age-related macular degeneration (AMD) causing irreversible vision loss, which makes RPE the prime target of the disease. The present study aimed to identify compounds to protect 4-hydroxynonenal (4-HNE)-induced RPE cell death by inhibiting NADPH oxidase 4 (NOX4) activity, not just as free radical scavengers, using ARPE-19, a human adult retinal pigment epithelial cell line, as a RPE representative. Novel thirty-two 6-ureido/thioureido-2,4,5-trimethylpyridin-3-ol derivatives 17 were synthesized and tested. We found that there was a strong correlation between level of protective effect of compounds 17 against 4-HNE-induced APRE-19 cell death and that of inhibitory activity against 4-HNE-induced superoxide production, and that most of the compounds 17 showed minimal DPPH radical scavenging activity. Compound 17-28 showed the best protective activity against 4-HNE-induced superoxide production (79.5% inhibition) and cell death (85.1% recovery) at 10 μM concentration, which was better than that of VAS2870, a NOX2/4 inhibitor. In addition, compound 17-28 blocked 4-HNE-induced apoptosis of ARPE-19 cells in a concentration-dependent manner. The results indicate that compound 17-28 may be a lead compound to develop AMD therapeutics.

Neuroprotection in Glaucoma: Old and New Promising Treatments

Glaucoma is a major global cause of blindness, but the molecular mechanisms responsible for the neurodegenerative damage are not clear. Undoubtedly, the high intraocular pressure (IOP) and the secondary ischemic and mechanical damage of the optic nerve have a crucial role in retinal ganglion cell (RGC) death. Several studies specifically analyzed the events that lead to nerve fiber layer thinning, showing the importance of both intra- and extracellular factors. In parallel, many neuroprotective substances have been tested for their efficacy and safety in hindering the negative effects that lead to RGC death. New formulations of these compounds, also suitable for chronic oral administration, are likely to be used in clinical practice in the future along with conventional therapies, in order to control the progression of the visual impairment due to primary open-angle glaucoma (POAG). This review illustrates some of these old and new promising agents for the adjuvant treatment of POAG, with particular emphasis on forskolin and melatonin.

Antiglycation activity of melatonin

For the first time, it was found that the hormone melatonin exhibited antiglycation activity in vitro. It was shown that melatonin significantly slowed down the accumulation of fluorescent Schiff adducts formed as a result of BSA modification in the presence of high concentration of fructose. It was noted that, unlike the fructosylation reaction, melatonin did not affect the process of modification of BSA by methylglyoxal. We assume that melatonin is able to inhibit the development of the Maillard reaction but does not affect the process of BSA modification by reactive carbonyls.

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.

Melatonin in Retinal Physiology and Pathology: The Case of Age-Related Macular Degeneration

Melatonin, an indoleamine, is synthesized mainly in the pineal gland in a circadian fashion, but it is produced in many other organs, including the retina, which seems to be especially important as the eye is a primary recipient of circadian signals. Melatonin displays strong antioxidative properties, which predispose it to play a protective role in many human pathologies associated with oxidative stress, including premature aging and degenerative disease. Therefore, melatonin may play a role in age-related macular degeneration (AMD), a disease affecting photoreceptors, and retinal pigment epithelium (RPE) with an established role of oxidative stress in its pathogenesis. Several studies have shown that melatonin could exert the protective effect against damage to RPE cells evoked by reactive oxygen species (ROS), but it has also been reported to increase ROS-induced damage to photoreceptors and RPE. Melatonin behaves like synthetic mitochondria-targeted antioxidants, which concentrate in mitochondria at relatively high levels; thus, melatonin may prevent mitochondrial damage in AMD. The retina contains telomerase, an enzyme implicated in maintaining the length of telomeres, and oxidative stress inhibits telomere synthesis, while melatonin overcomes this effect. These features support considering melatonin as a preventive and therapeutic agent in the treatment of AMD.

Self-Reported Sleep Duration in Patients with Neovascular Age-Related Macular Degeneration

PURPOSE

To examine the relationship between self-reported sleep duration and neovascular age-related macular degeneration (nAMD).

METHODS

This case-control study comprised 165 subjects (57 patients with nAMD and 108 controls). Controls were matched to cases by age and sex. Participants completed a questionnaire that included questions about sleep duration and quality. Four categories of sleep duration were established; <6 hours, 6-7 hours, 7-8 hours and >8 hours. Association of sleep duration and nAMD was assessed by logistic regression analysis. Multiple logistic regression models were performed to control for possible confounders.

RESULTS

We found a significant association between short sleep duration and nAMD (for <6 hours, odds ratio, OR, 3.29, 95% confidence interval, CI, 1.32-8.27; for 6-7 hours, OR 2.25, 95% CI 0.80-6.32; and for >8 hours, OR 1.39, 95% CI 0.53-3.73) compared with the reference category of 7-8 hours. This association remained significant after adjustment for confounders (<6 hours, OR 3.09, 95% CI 1.20-7.97). In addition, a borderline significant association was observed between self-reported very bad sleep quality and nAMD (OR 2.84, 95% CI 1.02-7.88). The highest rate of sleep medication use was found in the nAMD group (p < 0.001).

CONCLUSION

Our findings provide evidence to support an association between short sleep duration and nAMD. Considering strategies to improve sleep in these patients may prevent the negative effects of sleep deficiency.

Relationship between Oxidative Stress, Circadian Rhythms, and AMD

This work reviews concepts regarding oxidative stress and the mechanisms by which endogenous and exogenous factors produce reactive oxygen species (ROS). It also surveys the relationships between oxidative stress, circadian rhythms, and retinal damage in humans, particularly those related to light and photodamage. In the first section, the production of ROS by different cell organelles and biomolecules and the antioxidant mechanisms that antagonize this damage are reviewed. The second section includes a brief review of circadian clocks and their relationship with the cellular redox state. In the third part of this work, the relationship between retinal damage and ROS is described. The last part of this work focuses on retinal degenerative pathology, age-related macular degeneration, and the relationships between this pathology, ROS, and light. Finally, the possible interactions between the retinal pigment epithelium (RPE), circadian rhythms, and this pathology are discussed.

Melatonin Entrains PER2::LUC Bioluminescence Circadian Rhythm in the Mouse Cornea

PURPOSE

Previous studies have reported the presence of a circadian rhythm in PERIOD2::LUCIFERASE (PER2::LUC) bioluminescence in mouse photoreceptors, retina, RPE, and cornea. Melatonin (MLT) modulates many physiological functions in the eye and it is believed to be one of the key circadian signals within the eye. The aim of the present study was to investigate the regulation of the PER2::LUC circadian rhythm in mouse cornea and to determine the role played by MLT.

METHODS

Corneas were obtained from PER2::LUC mice and cultured to measure bioluminescence rhythmicity in isolated tissue using a Lumicycle or CCD camera. To determine the time-dependent resetting of the corneal circadian clocks in response to MLT or IIK7 (a melatonin type 2 receptor, MT2, agonist) was added to the cultured corneas at different times of the day. We also defined the location of the MT2 receptor within different corneal layers using immunohistochemistry.

RESULTS

A long-lasting bioluminescence rhythm was recorded from cultured PER2::LUC cornea and PER2::LUC signal was localized to the corneal epithelium and endothelium. MLT administration in the early night delayed the cornea rhythm, whereas administration of MLT at late night to early morning advanced the cornea rhythm. Treatment with IIK7 mimicked the MLT phase-shifting effect. Consistent with these results, MT2 immunoreactivity was localized to the corneal epithelium and endothelium.

CONCLUSIONS

Our work demonstrates that MLT entrains the PER2::LUC bioluminescence rhythm in the cornea. Our data indicate that the cornea may represent a model to study the molecular mechanisms by which MLT affects the circadian clock.

Topical nutraceutical Optixcare EH ameliorates experimental ocular oxidative stress in rats

PURPOSE

Based on the hypothesis that oral nutraceuticals do not adequately reach all ocular tissues in the anterior segment, we evaluated the ability of a 3% concentration of the ingredients in a topical nutraceutical antioxidant formulation called Optixcare Eye Health (Optixcare EH) to ameliorate oxidative stress in rat models of age-related ocular diseases.

METHODS

Diabetes was induced by tail-vein injection of streptozotocin, and the development of cataracts was monitored by slit lamp. Young rats were exposed to ultraviolet (UV) light, and the reduction in lens glutathione (GSH) levels and increase in 4-hydroxynonenol (4-HNE) were measured. Oxidative stress in the neural retina was generated by exposure of dark-adapted rats to 1,000 lx of light, and oxidative stress markers were measured. Dry eye was induced in rats by twice daily (b.i.d.) subcutaneous scopolamine injections. Topical Optixcare EH was administered b.i.d. and compared in select experiments to the multifunctional antioxidant JHX-4, the topical aldose reductase inhibitor (ARI) Kinostat™, oral Ocu-GLO™, and the topical ocular comfort agents Optixcare Eye Lube, Optixcare Eye Lube + Hyaluron, and Idrop Vet Plus hyaluronic acid.

RESULTS

In diabetic rats, topical ARI treatment prevented cataract formation while the nutraceuticals delayed their development with Optixcare EH>Ocu-GLO. In UV-exposed rats, the reduction of GSH and increase in 4-HNE in the lens were normalized in order JHX-4>Optixcare EH>Ocu-GLO. In the retina, oxidative stress markers were reduced better by oral JHX-4 compared with topical Optixcare EH. In the scopolamine-induced dry-eye rats, tear flow was maintained by Optixcare EH treatment, while none of the comfort agents examined altered tear flow.

CONCLUSIONS

Topical administration of a 3% concentration of the ingredients in Optixcare EH reduces experimentally induced reactive oxygen species in rats exposed to several sources of ocular oxidative stress. In addition, Optixcare EH maintains tear volume in scopolamine-induced dry eye. This suggests that in the anterior segment, the ingredients in Optixcare EH may have clinical potential against ocular oxidative stress.

Melatonin and amfenac modulate calcium entry, apoptosis, and oxidative stress in ARPE-19 cell culture exposed to blue light irradiation (405 nm)

PURPOSE

Under conditions of oxidative stress, cell apoptosis is triggered through the mitochondrial intrinsic pathway. Increased levels of reactive oxygen species (ROS) are linked to excess cell loss and mediate the initiation of apoptosis in a diverse range of cell types. The aims of this study were to assess intracellular Ca(2+) release, ROS production, and caspase-3, and -9 activation in ARPE-19 cells during the blue light-mediated cell death, and to examine a potential protective effect of melatonin and amfenac, in the apoptotic cascade.

METHODS

ARPE-19 cells were cultured in their medium. First, MTT tests were performed to determine the protective effects of amfenac and melatonin. Cells were then exposed to blue light irradiation in an incubator. Intracellular Ca(2+) release experiments, mitochondrial membrane depolarization, apoptosis assay, glutathione (GSH), glutathione peroxidase (GSH-Px), and ROS experiments were done according to the method stated in the Materials and methods section.

RESULTS

Cell death was clearly associated with increased levels of ROS production, as measured by 2',7'-dichlorofluorescein fluorescence, and associated increase in Ca(2+) levels, as measured by Fura-2-AM. Blue light-induced cell death was associated with an increased level of caspase-3 and 9, suggesting mediation via the apoptotic pathway. Cell death was also associated with mitochondrial depolarization. Melatonin was shown to delay these three steps.

CONCLUSION

Melatonin, amfenac, and their combination protect ARPE-19 cells against blue light-triggered ROS accumulation and caspase-3 and -9 activation. The antiapoptotic effect of melatonin and amfenac at doses inhibiting caspase synthesis modified Ca(2+) release and prevented excessive ROS production, suggesting a new therapeutic approach to age-related macular degeneration.

670 nm light mitigates oxygen-induced degeneration in C57BL/6J mouse retina

Background

Irradiation with light wavelengths from the far red (FR) to the near infrared (NIR) spectrum (600 nm -1000 nm) has been shown to have beneficial effects in several disease models. In this study, we aim to examine whether 670 nm red light pretreatment can provide protection against hyperoxia-induced damage in the C57BL/6J mouse retina. Adult mice (90–110 days) were pretreated with 9 J/cm² of 670 nm light once daily for 5 consecutive days prior to being placed in hyperoxic environment (75% oxygen). Control groups were exposed to hyperoxia, but received no 670 nm light pretreatment. Retinas were collected after 0, 3, 7, 10 or 14 days of hyperoxia exposure (n = 12/group) and prepared either for histological analysis, or RNA extraction and quantitative polymerase chain reaction (qPCR). Photoreceptor damage and loss were quantified by counting photoreceptors undergoing cell death and measuring photoreceptor layer thickness. Localization of acrolein, and cytochrome c oxidase subunit Va (Cox Va) were identified through immunohistochemistry. Expression of heme oxygenase-1 (Hmox-1), complement component 3 (C3) and fibroblast growth factor 2 (Fgf-2) genes were quantified using qPCR.

Results

The hyperoxia-induced photoreceptor loss was accompanied by reduction of metabolic marker, Cox Va, and increased expression of oxidative stress indicator, acrolein and Hmox-1. Pretreatment with 670 nm red light reduced expression of markers of oxidative stress and C3, and slowed, but did not prevent, photoreceptor loss over the time course of hyperoxia exposure.

Conclusion

The damaging effects of hyperoxia on photoreceptors were ameliorated following pretreatment with 670 nm light in hyperoxic mouse retinas. These results suggest that pretreatment with 670 nm light may provide stability to photoreceptors in conditions of oxidative stress.

Assessment of mitochondrial damage in retinal cells and tissues using quantitative polymerase chain reaction for mitochondrial DNA damage and extracellular flux assay for mitochondrial respiration activity

Mitochondrial dysfunction and genomic instability are associated with a number of retinal pathologies including age-related macular degeneration, diabetic retinopathy, and glaucoma. Consequences of mitochondrial dysfunction within cells include elevation of the rate of ROS production due to damage of electron transport chain proteins, mitochondrial DNA (mtDNA) damage, and loss of metabolic capacity. Here we introduce the quantitative polymerase chain reaction assay (QPCR) and extracellular flux assay (XF) as powerful techniques to study mitochondrial behavior. The QPCR technique is a gene-specific assay developed to analyze the DNA damage repair response in mitochondrial and nuclear genomes. QPCR has proved particularly valuable for the measurement of oxidative-induced mtDNA damage and kinetics of mtDNA repair. To assess the functional consequence of mitochondrial oxidative damage, real-time changes in cellular bioenergetics of cell monolayers can be measured with a Seahorse Biosciences XF24 analyzer. The advantages and limitations of these procedures will be discussed and detailed methodologies provided with particular emphasis on retinal oxidative stress.

Melatonin: an underappreciated player in retinal physiology and pathophysiology

In the vertebrate retina, melatonin is synthesized by the photoreceptors with high levels of melatonin at night and lower levels during the day. Melatonin exerts its influence by interacting with a family of G-protein-coupled receptors that are negatively coupled with adenylyl cyclase. Melatonin receptors belonging to the subtypes MT(1) and MT(2) have been identified in the mammalian retina. MT(1) and MT(2) receptors are found in all layers of the neural retina and in the retinal pigmented epithelium. Melatonin in the eye is believed to be involved in the modulation of many important retinal functions; it can modulate the electroretinogram (ERG), and administration of exogenous melatonin increases light-induced photoreceptor degeneration. Melatonin may also have protective effects on retinal pigment epithelial cells, photoreceptors and ganglion cells. A series of studies have implicated melatonin in the pathogenesis of age-related macular degeneration, and melatonin administration may represent a useful approach to prevent and treat glaucoma. Melatonin is used by millions of people around the world to retard aging, improve sleep performance, mitigate jet lag symptoms, and treat depression. Administration of exogenous melatonin at night may also be beneficial for ocular health, but additional investigation is needed to establish its potential.

Ocular disorders and the utility of animal models in the discovery of melatoninergic drugs with therapeutic potential

INTRODUCTION

  The pineal indole-derived hormone melatonin is a modulator of circadian and seasonal rhythms with an important role in ocular health and disease. This could be due to specific melatonin receptors that have been identified in structures such as cornea, lens, ciliary body, retina, choroid and sclera. In addition, a local synthesis of melatonin occurs in several of these ocular tissues.

AREAS COVERED

The authors review existing literature on the most common animal models where ocular melatonin actions have been tested. The therapeutic potential of melatonin in diabetic keratopathy and retinopathy, keratitis, cataracts, glaucoma, uveitis, age-related macular degeneration and retinitis pigmentosa is discussed. Furthermore, the authors comment on the usefulness of different animal models for the development of melatoninergic drugs with therapeutic potential.

EXPERT OPINION

The use of animals for the study of ocular diseases and the potentiality of melatonin and its analogs, as future therapeutic drugs, should be performed on the basis of a rationale study. It is important to note that melatonin receptors seem to be widespread all over the eye. This strongly suggests that, in order to modify the physiology and biochemistry of malfunctioning ocular tissue, the melatonin receptors which are present in that tissue must be first identified. Second there is the need to confirm that those receptors targeted perform the desirable responses, and as a third measure, to use selective agonists (or antagonists) instead of melatonin. However, although some animals mimic ocular pathologies relatively well, and these can be used in melatonin studies, there is still a long way to go till some of the results obtained in animal models could be used for human therapy.

Consequences of oxidative stress in age-related macular degeneration

The retina resides in an environment that is primed for the generation of reactive oxygen species (ROS) and resultant oxidative damage. The retina is one of the highest oxygen-consuming tissues in the human body. The highest oxygen levels are found in the choroid, but this falls dramatically across the outermost retina, creating a large gradient of oxygen towards the retina and inner segments of the photoreceptors which contain high levels of polyunsaturated fatty acids. This micro-environment together with abundant photosensitizers, visible light exposure and a high energy demand supports a highly oxidative milieu. However, oxidative damage is normally minimized by the presence of a range of antioxidant and efficient repair systems. Unfortunately, as we age oxidative damage increases, antioxidant capacity decreases and the efficiency of reparative systems become impaired. The result is retinal dysfunction and cell loss leading to visual impairment. It appears that these age-related oxidative changes are a hallmark of early age-related macular degeneration (AMD) which, in combination with hereditary susceptibility and other retinal modifiers, can progress to the pathology and visual morbidity associated with advanced AMD. This review reassesses the consequences of oxidative stress in AMD and strategies for preventing or reversing oxidative damage in retinal tissues.

Mitochondrial dysfunction in retinal diseases

The mitochondrion is a vital intracellular organelle for retinal cell function and survival. There is growing confirmation to support an association between mitochondrial dysfunction and a number of retinal degenerations. Investigations have also unveiled mitochondrial genomic instability as one of the contributing factors for age-related retinal pathophysiology. This review highlights the role of mitochondrial dysfunction originating from oxidative stress in the etiology of retinal diseases including diabetic retinopathy, glaucoma and age-related macular degeneration (AMD). Moreover, mitochondrial DNA (mtDNA) damage associated with AMD due to susceptibility of mtDNA to oxidative damage and failure of mtDNA repair pathways is also highlighted in this review. The susceptibility of neural retina and retinal pigment epithelium (RPE) mitochondria to oxidative damage with ageing appears to be a major factor in retinal degeneration. It thus appears that the mitochondrion is a weak link in the antioxidant defenses of retinal cells. In addition, failure of mtDNA repair pathways can also specifically contribute towards pathogenesis of AMD. This review will further summarize the prospective role of mitochondria targeting therapeutic agents for the treatment of retinal disease. Mitochondria based drug targeting to diminish oxidative stress or promote repair of mtDNA damage may offer potential alternatives for the treatment of various retinal degenerative diseases.

Prevention of Oxidative Stress-Induced Retinal Pigment Epithelial Cell Death by the PPARgamma Agonists, 15-Deoxy-Delta 12, 14-Prostaglandin J(2)

Cellular oxidative stress plays an important role in retinal pigment epithelial (RPE) cell death during aging and the development of age-related macular degeneration. Early reports indicate that during phagocytosis of rod outer segments, there is an increase of RPE oxidative stress and an upregulation of PPARγ mRNA in these cells. These studies suggest that activation of PPARγ may modulate cellular oxidative stress. This paper presents a brief review of recent studies that investigate RPE oxidative stress under various experimental conditions. This is followed by a detailed review on those reports that examine the protective effect of the natural PPARγ ligand, 15d-PGJ₂, against RPE oxidative stress. This agent can upregulate glutathione and prevent oxidant-induced intracellular reactive oxygen species accumulation, mitochondrial depolarization, and apoptosis. The cytoprotective effect of this agent, however, is not shared by other PPARγ agonists. Nonetheless, this property of 15d-PGJ₂ may be useful in future development of pharmacological tools against retinal diseases caused by oxidative stress.

Circadian dysfunction in P23H rhodopsin transgenic rats: effects of exogenous melatonin

This study focuses on the effects of retinal degeneration on the circadian patterns of P23H rats, as well as on the effect of exogenous melatonin administration. To this end, the body temperature of P23H and Sprague-Dawley rats was continuously monitored and their retinas examined at different stages of degeneration, by means of histological labeling and electroretinogram recordings. Melatonin (2 mg/kg BW/day) was supplied ad libitum throughout the experiment to a subset of animals. The body temperature recordings from wild-type and mutant animals showed no differences in the periodogram and the pattern of the mean waveform. However, a progressive decrease in the relative amplitude of the rhythm (RA), a decline in the coupling strength of the rhythm to environmental zeitgebers (interdaily stability, IS) and increased rhythm fragmentation (intradaily variability, IV) were observed in P23H rats, when compared to wild-type animals. The P23H animals showed a progressive decrease in light-induced retinal responses until reaching 18 months of age. By this age, all photoreceptors had already disappeared, and no responses were found in the EGRs. Exogenous administration of melatonin improved the visual response of P23H rats. In fact, the maximum b-wave recorded at 14 months of age was significantly higher in melatonin-treated P23H rats than in the control animals. Furthermore, the maximum b-wave recorded for P23H rats at the age of 14 months significantly correlated with RA, IS, and IV. This leads us to conclude that vision loss in P23H rats is correlated with a progressive fragmentation of their circadian patterns. Both effects are partially reversed by melatonin administration.