Free radicals in retinal ischemia

Inhibition of retinal ischemia-reperfusion injury in rats by inhalation of low-concentration hydrogen gas

PURPOSE

To investigate the inhibitory effect of hydrogen gas inhalation on retinal ischemia reperfusion (I/R) injury using a rat model.

METHODS

Six-week-old male Sprague-Dawley rats were used. A 27G needle connected by a tube to a saline bottle placed 200 cm above the eye was inserted into the anterior eye chamber to create a rat retinal I/R model. In the ischemia-plus-hydrogen-gas group (H2( +) group), the ischemia time was set to 90 min, and 1.8% hydrogen was added to the air delivered by the anesthesia mask simultaneously with the start of ischemia. In the non-hydrogen-treatment ischemia group (H2( -) group), I/R injury was created similarly, but only air was inhaled. ERGs were measured; after removal of the eyes, the retina was examined for histological, immunostaining, and molecular biological analyses.

RESULTS

The mean thickness of the inner retinal layer in the H2( +) group was 107.2 ± 16.0 μm (n = 5), significantly greater than that in the H2( -) group (60.8 ± 6.7 μm). Immunostaining for Iba1 in the H2( -) group showed increased numbers of microglia and microglial infiltration into the subretinal space, while there was no increase in microglia in the H2( +) group. B-wave amplitudes in the H2( +) group were significantly higher than in the H2( -) group. In the membrane antibody array, levels of interleukin-6, monocyte chemotactic protein 1, and tumor necrosis factor alpha were significantly lower in the H2( +) group than in the H2( -) group.

CONCLUSION

Inhalation of 1.8% hydrogen gas inhibited the induction of inflammation, morphological/structural changes, and glial cell increase caused by retinal I/R injury.

Metformin protects against retinal ischemia/reperfusion injury through AMPK-mediated mitochondrial fusion

Retinal ischemia/reperfusion (I/R) injury is a common pathological process responsible for cellular damage in glaucoma, diabetic retinopathy and hypertensive retinopathy. Metformin is a biguanide drug that exerts strong effects on multiple diseases. This study aims to evaluate the protective effect of metformin against retinal I/R injury and its underlying mechanism. I/R induced reduction in retina thickness and cell number in ganglion cell layer, and metformin alleviated I/R-induced retinal injury. Both retinal I/R and simulated ischemia/reperfusion (SIR) in R28 cells down-regulated expression of mitochondrial fusion protein Mfn2 and OPA1, which led to mitochondrial fission. Metformin also alleviated damage in R28 cells, and reversed the alteration in Mfn2 and OPA1, mitochondrial fission and mitochondrial membrane potential (MMP) disruption-induced by I/R or SIR as well. Intriguingly, inhibition of AMPK by compound C or siRNA prevented metformin-mediated up-regulation of Mfn2 and OPA1. Compound C and knockdown of Mfn2 or OPA1 dramatically alleviated the protective effect of metformin against intracellular ROS generation, MMP disruption, mitochondrial fission and loss of RGCs in ganglion cell layer induced by SIR or I/R. Moreover, scavenging mitochondrial ROS (mito-ROS) by mito-TEMPO exerted the similar protection against I/R-induced retinal injury or SIR-induced damage in R28 cells as metformin. Our data show for the first time that metformin protects against retinal I/R injury through AMPK-mediated mitochondrial fusion and the decreased mito-ROS generation. These findings might also repurpose metformin as a therapeutic agent for retinal I/R injury.

Inhibition of mitochondrial VDAC1 oligomerization alleviates apoptosis and necroptosis of retinal neurons following OGD/R injury

Ischemia-reperfusion (I/R) injury is a common pathological mechanism in many retinal diseases, which can lead to cell death via mitochondrial dysfunction. Voltage-dependent anion channel 1 (VDAC1), which is mainly located in the outer mitochondrial membrane, is the gatekeeper of mitochondria. The permeability of mitochondrial membrane can be regulated by controlling the oligomerization of VDAC1. However, the functional mechanism of VDAC1 in retinal I/R injury was unclear. Our results demonstrate that oxygen-glucose deprivation and re-oxygenation (OGD/R) injury leads to apoptosis, necroptosis, and mitochondrial dysfunction of R28 cells. The OGD/R injury increases the levels of VDAC1 oligomerization. Inhibition of VDAC1 oligomerization by VBIT-12 rescued mitochondrial dysfunction by OGD/R and also reduced apoptosis/necroptosis of R28 cells. In vivo, the use of VBIT-12 significantly reduced aHIOP-induced neuronal death (apoptosis/necroptosis) in the rat retina. Our findings indicate that VDAC1 oligomers may open and enlarge mitochondrial membrane pores during OGD/R injury, leading to the release of death-related factors in mitochondria, resulting in apoptosis and necroptosis. This study provides a potential therapeutic strategy against ocular diseases caused by I/R injury.

GPR3 expression in retinal ganglion cells contributes to neuron survival and accelerates axonal regeneration after optic nerve crush in mice

Glaucoma is an optic neuropathy and is currently one of the most common diseases that leads to irreversible blindness. The axonal degeneration that occurs before retinal ganglion neuronal loss is suggested to be involved in the pathogenesis of glaucoma. G protein-coupled receptor 3 (GPR3) belongs to the class A rhodopsin-type GPCR family and is highly expressed in various neurons. GPR3 is unique in its ability to constitutively activate the Gαs protein without a ligand, which elevates the basal intracellular cAMP level. Our earlier reports suggested that GPR3 enhances both neurite outgrowth and neuronal survival. However, the potential role of GPR3 in axonal regeneration after neuronal injury has not been elucidated. Herein, we investigated retinal GPR3 expression and its possible involvement in axonal regeneration after retinal injury in mice. GPR3 was relatively highly expressed in retinal ganglion cells (RGCs). Surprisingly, RGCs in GPR3 knockout mice were vulnerable to neural death during aging without affecting high intraocular pressure (IOP) and under ischemic conditions. Primary cultured neurons from the retina showed that GPR3 expression was correlated with neurite outgrowth and neuronal survival. Evaluation of the effect of GPR3 on axonal regeneration using GPR3 knockout mice revealed that GPR3 in RGCs participates in axonal regeneration after optic nerve crush (ONC) under zymosan stimulation. In addition, regenerating axons were further stimulated when GPR3 was upregulated in RGCs, and the effect was further augmented when combined with zymosan treatment. These results suggest that GPR3 expression in RGCs helps maintain neuronal survival and accelerates axonal regeneration after ONC in mice.

Histological Assessment of Rat Retinas with Ischemia-Reperfusion Injury

INTRODUCTION

Retinal ischemia-reperfusion (IR) injury occurs in pathological situations that interrupt the blood flow to the retina, such as is the case during central retinal artery occlusion (CRAO). The animal models described in the literature are based on the pressure produced by the weight of a given quantity of saline elevated to a certain height; however, to establish these parameters it is necessary to perform mathematical calculations that cannot be easily redone in the case of punctual variations of intraocular pressure (IOP). The aim of this study was to present a new system that allows us to reproduce the conditions of retinal IR and thereby properly assess the level of injury in retinal histological samples.

METHODS

We developed a retinal IR model in WAG/RijHsd rats based on CRAO through increasing IOP. To develop this model, we produced ischemia for 1 h using a hydrostatic pressure system that maintained a constant high IOP and then allowed reperfusion for 1 h. The injury attributable to IR was assessed by histological examination of retinal samples, determining whether there was histological damage and/or dendritic swelling and counting the outer nuclear layer cells showing cytoplasmic swelling.

RESULTS

The increase in IOP to 150 mm Hg produced CRAO, in turn causing observable histological damage and dendritic swelling in all retinas subjected to IR. Counting the number of cells showing cytoplasmic swelling yielded a mean of 102.5 ± 35 cells/field. The contralateral retinas were healthy, showing no significant changes.

CONCLUSION

The retinal IR model proposed is simple, reproducible, and allows variable durations of ischemia and reperfusion, and most importantly, it allows easy correction by adjusting the pressure of the sphygmomanometer, of any change in IOP to keep the ischemia stable, without having to recalculate the elevation height of the ischemia induction system. Moreover, the damage caused by IR can be effectively assessed by the type of histopathological assessment performed. For these reasons, it can be considered a reliable method for studying drugs that may prevent retinal IR injury.

Lentiviral vector-mediated expression of C3 transferase attenuates retinal ischemia and reperfusion injury in rats

AIMS

Our previous study showed that intravitreal delivery of self-complementary AAV2 (scAAV2)-mediated exoenzyme C3 transferase (C3) can attenuate retinal ischemia/reperfusion (I/R) injury. The current study investigated the neuroprotective effects of lentivirus (LV)-mediated C3 transgene expression on rat retinal I/R injury.

MAIN METHODS

The LV encoding C3 and green fluorescent protein (GFP) together (LV-C3-GFP) or GFP only (LV-GFP) was intravitreally injected to SPRAGUE-DAWLEY rats. On day 5 post-intravitreal injection, eyes were evaluated by slit-lamp examination. The GFP expression on retina was confirmed by in vivo and ex vivo assessments. RhoA GTPase expression in retina was examined by western blot. Retinal I/R injury was generated by transiently increasing intraocular pressure (110 mmHg, 90 min). Eyes were then enucleated, and retinas processed for morphological analysis and TdT-dUTP terminal nick-end labeling (TUNEL) assay.

KEY FINDINGS

No obvious inflammatory reactions or surgical complications were observed after intravitreal injection of LV vectors. There was a significant decrease of total RhoA GTPase level in the retina treated with LV-C3-GFP. Compared to the blank control group, LV-C3-GFP and LV-GFP did not affect the retinal thickness, cell density in ganglion cell layer (GCL), or numbers of apoptotic cells in retinal flat-mounts. In the LV-GFP-treated retinas, I/R decreased the retinal thickness and GCL cell density and increased apoptotic retinal cell numbers. LV-C3-GFP significantly protected against all these degenerative effects of I/R.

SIGNIFICANCE

This study indicated that LV-mediated C3 transgene expression exhibits neuroprotective effects on the retinal I/R injury and holds potential as a novel neuroprotective approach targeting certain retinopathies.

[Current Status of the Pharmacological Treatment of Glaucoma and Its Prospects]

Glaucoma, the leading cause of blindness in adults, is a progressive neurodegenerative disease characterized by retinal ganglion cell (RGC) death. Currently, many intraocular pressure (IOP)-lowering drugs known to affect this disease progression have been developed as therapeutic agents. However, there are many cases of disease progression, even with sufficient IOP reduction. Therefore, newer therapeutic approaches other than IOP-lowering drugs are needed. To elucidate the pathogenesis of glaucoma and to develop therapeutic agents, the evaluation of RGCs is imperative, as their degeneration is the main cause of this disease. However, it is difficult to obtain RGCs from healthy individuals, let alone glaucoma patients. Therefore, research on the pathophysiology of glaucoma and drug discovery has not progressed sufficiently. Recent developments have made it possible to generate induced pluripotent stem (iPS) cells from the blood or skin of glaucoma patients and induce them to differentiate into RGCs to study the pathogenesis of glaucoma. In addition, drug repositioning for ophthalmological diseases such as glaucoma is one of the most active fields. Many of these repositioned drugs have found therapeutic applications in ophthalmology. Here, we introduce the current status of the pharmacological treatment of glaucoma and its prospects.

Oral administration of NSP-116, a free radical scavenger, suppresses the symptoms of retinal vein occlusion in the murine model

Retinal vein occlusion (RVO) is an intractable eye disease that results in reduced visual acuity, associated with retinal ischemia, hemorrhage, and edema. RVO results in excessive ROS production in the retina, causing inflammation and retinal edema. A free radical scavenger, 4-(4-acetylpiperazin-1-yl)-2-(1H-imidazole-1-yl) aniline (NSP-116), has been reported to demonstrate antioxidative effects and prevent ROS production in the retina. Therefore, NSP-116 may represent a useful drug for treating the pathological symptoms of RVO, such as retinal edema and ischemic symptoms. This study aimed to investigate the effects of NSP-116 in a murine model of RVO. We evaluated the thickness of the retinal layer and the size of the non-perfused area following the oral administration of NSP-116. Moreover, we used western blot analysis to examine the expression levels of vascular endothelial growth factor (VEGF) and tumor necrosis factor (TNF)-α, after NSP-116 administration, and examined the localization of 8-hydroxy-2'-deoxyguanosine (8-OHdG), by immunostaining. The findings indicate that NSP-116 suppressed retinal edema and expansion the non-perfused area by suppressing the increased expression of VEGF, TNF-α, and 8-OHdG in the murine RVO model. In conclusion, the oral administration of NSP-116 may serve as an effective pharmacological treatment for the pathological symptoms of RVO.

Protective effect of metformin on rat diabetic retinopathy involves suppression of toll-like receptor 4/nuclear factor-k B expression and glutamate excitotoxicity

Microvascular complications of diabetes mellitus are progressively significant reasons for mortality. Metformin (MET) is considered as the first-line therapy for type 2 diabetes patients, and may be especially beneficial in cases of diabetic retinopathy although the precise mechanisms of MET action are not fully elucidated. The current study was designed to inspect the antioxidant and modulatory actions of MET on DRET in streptozotocin-induced diabetic rats. The effect of MET on the toll-like receptor 4/nuclear factor kappa B (TLR4/NFkB), inflammatory burden and glutamate excitotoxicity was assessed. Twenty-four male rats were assigned to four experimental groups: (1) Vehicle group, (2) Diabetic control: developed diabetes by injection of streptozotocin (60 mg/kg, i.p.). (3&4) Diabetic + MET group: diabetic rats were left for 9 weeks without treatment and then received oral MET 100 and 200 mg/kg for 6 weeks. Retinal samples were utilized in biochemical, histological, immunohistochemical and electron microscopic studies. MET administration significantly decreased retinal level of insulin growth factor and significantly suppressed the diabetic induced increase of malondialdehyde, glutamate, tumor necrosis factor-α and vascular endothelial growth factor (VEGF). Further, MET decreased the retinal mRNA expression of NFkB, tumor necrosis factor-α and TLR4 in diabetic rats. The current findings shed the light on MET's efficacy as an adjuvant therapy to hinder the development of diabetic retinopathy, at least partly, via inhibition of oxidative stress-induced NFkB/TLR4 pathway and suppression of glutamate excitotoxicity.

Emodin protected against retinal ischemia insulted neurons through the downregulation of protein overexpression of β-catenin and vascular endothelium factor

Background

Emodin has been proved to have an anti-ischemic effect on the brain, however little research has been done on its effect on vision-threatening retinal ischemia. Thus, an investigation was carried out into the hypothetical efficacy of emodin against retinal ischemia and the role of β-catenin/VEGF in its therapeutic mechanism.

Methods

Retinal ischemia, followed by reperfusion (IR), was inducted by raising the intraocular pressure of a Wistar rat’s eye to 120 mmHg for 60 min. Additionally, pre-ischemic/post-ischemic intravitreous injections of emodin (4, 10 and 20 μM) or vehicle were carried out on the eye with retinal ischemia. MTT assay, electroretinograms, cresyl violet staining retinal thickness measurements, and fluorogold retrograde labelling of retinal ganglion cells (RGCs) as well as Western blotting were carried out.

Results

Cultured RGC-5 cells subjected to oxygen glucose deprivation (OGD) were used to confirm the effective concentrations of emodin (administered 1 h pre-OGD, pre-OGD emodin). The most effective and significant (P = 0.04) dose of pre-OGD emodin was observed at 0.5 μM (cell viability: 47.52 ± 3.99%) as compared to pre-OGD vehicle treatment group (38.30 ± 2.51%). Furthermore, pre-ischemic intravitreous injection of 20 μM emodin (Emo20 + IR = 0.99 ± 0.18, P < 0.001) significantly attenuated the ischemia induced reduction in ERG b-wave amplitude, as compared to pre-ischemic intravitreous vehicle (Vehicle+IR = 0.04 ± 0.02). Post-ischemic intravitreous 20 μM emodin also significantly (P < 0.001) attenuated the ischemia associated b-wave reduction (IR + Em20 = 0.24 ± 0.09). Compared with pre-ischemic intravitreous vehicle (Vehicle+IR; whole retina thickness = 71.80 ± 1.08 μm; inner retina thickness = 20.97 ± 0.85 μm; RGC =2069.12 ± 212.82/0.17mm²), the significant (P < 0.001) protective effect was also present with pre-ischemic administration of emodin. This was shown by observing cresyl violet stained retinal thickness (Emo20 + IR: whole retina = 170.10 ± 0.10 μm; inner retina = 70.65 ± 2.06 μm) and retrograde fluorogold immunolabeled RGC density (4623.53 ± 179.48/0.17mm²). As compared to the normal control (the ratio of β-catenin/VEGF to β-actin was set as 1 in the Sham group), the β-catenin/VEGF protein level significantly (P < 0.001) increased after retinal ischemia and when pre-ischemic intravitreous vehicle (Vehicle+IR = 1.64 ± 0.14/7.67 ± 2.57) was carried out. However, these elevations were significantly (P = 0.02) attenuated by treatment with emodin 20 μM (Emo20 + IR = 1.00 ± 0.19/1.23 ± 0.44).

Conclusions

The present results suggest that emodin might protect against retinal ischemia insulted neurons such as RGCs by significantly downregulating the upregulation of β-catenin/VEGF protein that occurs during ischemia.

Supplementary Information

Supplementary information accompanies this paper at 10.1186/s12906-020-03136-7.

Fortified S-Allyl L-Cysteine: Animal Safety, Effect on Retinal Ischemia, and Role of Wnt in the Underlying Therapeutic Mechanism

Purpose

Retinal ischemia is a medical condition associated with numerous retinal vascular disorders, such as age-related macular degeneration, glaucoma, and diabetic retinopathy. This in vitro cell and in vivo animal study investigated not only the protective effect of S-allyl L-cysteine (SAC, an active component of garlic) against retinal ischemia but also its associated protective mechanisms.

Methods

Retinal ischemia was mimicked by raising the intraocular pressure to 120 mmHg for 1 hour in one eye. The effects of pre-/postischemic administration of vehicle vs. SAC 0.18 mg vs. SAC 0.018 mg vs. SAC 0.0018 mg treatments on retina cells were evaluated through cellular viability (MTT assay), flash electroretinograms (ERGs), and fluorogold retrograde labelling (retinal ganglion cell (RGC) counting). Also, protein immunoblot was utilized to assess the role of Wnt, hypoxia inducible factor (HIF)-1α, and vascular endothelium factor (VEGF) in the proposed anti-ischemic mechanism. Lastly, the safety of drug consumption was investigated for changes in the animal's body weight, ERG waves, and blood biochemical parameters (e.g., glucose levels).

Results

The characteristic ischemic changes including significant reduction in ERG b-wave ratio and RGC number were significantly counteracted by pre- and postischemic low dose of SAC. Additionally, ischemia-induced overexpression of Wnt/HIF-1α/VEGF protein was ameliorated significantly by preischemic low dose of SAC. In terms of the animal safety, no significant body weight and electrophysiological differences were observed among defined different concentrations of SAC without following ischemia. In low SAC dosage and vehicle groups, various blood biochemical parameters were normal; however, high and medium concentrations of SAC significantly lowered the levels of uric acid, Hb, and MCHC.

Conclusion

This study shows that preischemic administration of low SAC dosage has been proved to be safe and most effective against rat retinal ischemia electrophysiologically and/or histopathologically. Moreover, counteracting the ischemia-induced overexpression of Wnt/HIF-1α/VEGF might presently explain SAC's anti-ischemic mechanism.

Distribution of Carotenoids and Protective Effects of Zeaxanthin on Retina of Ayu Sweetfish (Plecoglossus altivelis)

Ayu sweetfish (Plecoglossus altivelis) is a diurnal freshwater fish that are surface swimmers and active under broad and short wavelength-dominated light. Biochemical analyses have shown that the ayu fish have abundant carotenoids including zeaxanthin in their integuments. Although zeaxanthin plays an important role in the physiological function of the retina, the amount and location of zeaxanthin in the ayu eye have not been accurately determined. In this study, circular dichroism spectral data and chiral high-performance liquid chromatography analysis showed that zeaxanthin was the primary carotenoid in the ayu eye, and the eye had the highest carotenoid content compared to those in the integuments, subcutaneous fat, and digestive tract. Interestingly, zeaxanthin in the ayu eyeball was expressed in the photoreceptor layer and near the retinal pigmented epithelium. In vitro assays showed that zeaxanthin could protect photoreceptors and retinal pigmented epithelial cell lines against the oxidative stress induced by exposure to L-buthionine-(S,R)-sulfoximine/glutamate. These findings indicate that zeaxanthin plays protective roles against oxidative stress in the vision of wild ayu.

Puerarin ameliorates retinal ganglion cell damage induced by retinal ischemia/reperfusion through inhibiting the activation of TLR4/NLRP3 inflammasome

AIMS

Retinal ischemia/reperfusion (I/R) injury is common in the development of ophthalmic diseases and potentially causes blindness. In present study, the aim is to investigate the possible protective effects of puerarin on retinal I/R.

MAIN METHODS

Retinal I/R injury was conducted on the left eyes of male Sprague Dawley rats, which were subsequently received treatment with puerarin. After administration, retinal I/R-induced apoptosis, oxidative stress and inflammatory responses were detected. Meanwhile, we purified retinal ganglion cells (RGCs) from 7-day-old rats. After subjected RGCs to oxygen and glucose deprivation/reoxygenation (OGD/R), apoptosis and TLR4/NLRP3 inflammasome activation in RGCs were detected.

KEY FINDINGS

Puerarin prominently suppressed apoptosis, alleviated oxidative stress and suppressed TLR4/NLRP3 inflammasome activation in rats with retinal I/R injury. Consistent with our in vivo study, we found puerarin ameliorated retinal I/R injury through suppressing apoptosis and TLR4/NLRP3 inflammasome activation in RGCs.

SIGNIFICANCE

Our findings reveal that puerarin plays a protective role against retinal I/R injury by alleviating RGC damage, and is beneficial for the treatment of I/R injury-caused ophthalmic diseases.

Nuclear Factor (Erythroid-Derived 2)-Like 2 (Nrf2) Contributes to the Neuroprotective Effects of Histone Deacetylase Inhibitors In Retinal Ischemia-Reperfusion Injury

Histone deacetylase inhibitors (HDACis) have displayed neuroprotective effects in animal models of retinal ischemia/reperfusion (I/R) injury. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a redox-sensitive transcription factor responds to oxidative damage. We investigated the role of Nrf2 in retinal I/R injury, and further explored the mechanisms underlying Nrf2-mediated neuroprotection exerted by HDACi. High intraocular pressure was used to establish retinal I/R model in wild type (WT) and Nrf2 knockout (KO) mice. Nrf2 KO mice displayed more severe retinal damage after I/R. Trichostatin A (TSA) was administered to both WT and Nrf2 KO mice with retinal I/R damage. TSA significantly diminished the retinal ganglion cell degeneration in WT mice but offered no notable protection in Nrf2 KO mice. TSA markedly promoted Nrf2 nuclear translocation and its acetylation. In addition, TSA upregulated Nrf2 downstream proteins, such as Ho-1 and Nqo1, in retinal tissues. In the retinal neuronal cell line 661W, TSA reduced the expression of proinflammatory cytokines, Il-1β, Il-6, Tnf-α and Mmp-9, and it upregulated Bdnf under oxidative stress. However, this trend was not continued after silencing Nrf2. Chromatin immunoprecipitation assay demonstrated that Nrf2 at the Ho-1 promoter significantly increased transcriptional activity after oxidative stress induction. Nrf2, which is dispensable in HDACi-mediated neuroprotection, plays a major neuroprotective role in retinal I/R injury.

Methyl 3,4-dihydroxybenzoate protects retina in a mouse model of acute ocular hypertension through multiple pathways

Methyl 3,4 dihydroxybenzoate (MDHB) is a small molecule that shows neuroprotective effects in vitro and in a photoreceptor-degenerative mouse model. Here we investigated whether MDHB protects retina in a mouse model of acute ocular hypertension (AOH) and explores the underlying mechanisms. AOH was induced in mice by increasing intraocular pressure to approximately 90 mmHg for 60 min, then MDHB or vehicle was intraperitoneally injected daily up to 7 days. Immunostaining and multi-electrode array recordings were performed to examine the structure and function of retinas receiving the treatments. Western-blotting was applied to test the expression of several proteins related to oxidative stress and brain-derived neurotrophic factor (BDNF)-initiated signaling. Results showed that AOH injury reduced the number of Brn3a-stained retinal ganglion cells (RGCs) and ChAT-amacrine cells; thinned the inner retinal layers and induced apoptosis. Physiologically, AOH decreased the response of OFF and ON-OFF RGCs. All of these changes were reversed by MDHB-treatment. Mechanistically, MDHB appeared to work on three parallel pathways: (1) MDHB decreased the production of reactive oxygen species, the expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and cytosol heme oxygenase 1 (HO-1); (2) It upregulated the expression of BDNF and its receptor tropomyosin-related kinase B (TrkB), and activated the downstream AKT pathways; (3) It inhibited reactive gliosis by reducing GFAP and Iba-1 expression. Thus our results suggest that MDHB protects retina against AOH injury by inhibiting oxidative stress, activating the BDNF/AKT signaling and inhibiting inflammatory pathways. Therefore, MDHB may serve as a promising candidate to treat retinal ischemia.

Review of clinical approaches in fluorescence lifetime imaging ophthalmoscopy

Autofluorescence-based imaging techniques have become very important in the ophthalmological field. Being noninvasive and very sensitive, they are broadly used in clinical routines. Conventional autofluorescence intensity imaging is largely influenced by the strong fluorescence of lipofuscin, a fluorophore that can be found at the level of the retinal pigment epithelium. However, different endogenous retinal fluorophores can be altered in various diseases. Fluorescence lifetime imaging ophthalmoscopy (FLIO) is an imaging modality to investigate the autofluorescence of the human fundus in vivo. It expands the level of information, as an addition to investigating the fluorescence intensity, and autofluorescence lifetimes are captured. The Heidelberg Engineering Spectralis-based fluorescence lifetime imaging ophthalmoscope is used to investigate a 30-deg retinal field centered at the fovea. It detects FAF decays in short [498 to 560 nm, short spectral channel (SSC) and long (560 to 720 nm, long spectral channel (LSC)] spectral channels, the mean fluorescence lifetimes (τm) are calculated using bi- or triexponential approaches. These are meant to be relatively independent of the fluorophore's intensity; therefore, fluorophores with less intense fluorescence can be detected. As an example, FLIO detects the fluorescence of macular pigment, retinal carotenoids that help protect the human fundus from light damages. Furthermore, FLIO is able to detect changes related to various retinal diseases, such as age-related macular degeneration, albinism, Alzheimer's disease, diabetic retinopathy, macular telangiectasia type 2, retinitis pigmentosa, and Stargardt disease. Some of these changes can already be found in healthy eyes and may indicate a risk to developing such diseases. Other changes in already affected eyes seem to indicate disease progression. This review article focuses on providing detailed information on the clinical findings of FLIO. This technique detects not only structural changes at very early stages but also metabolic and disease-related alterations. Therefore, it is a very promising tool that might soon be used for early diagnostics.

Taurine protects against retinal and optic nerve damage induced by endothelin-1 in rats via antioxidant effects

Endothelin-1 (ET-1), a potent vasoconstrictor, is involved in retinal vascular dysregulation and oxidative stress in glaucomatous eyes. Taurine (TAU), a naturally occurring free amino acid, is known for its neuroprotective and antioxidant properties. Hence, we evaluated its neuroprotective properties against ET-1 induced retinal and optic nerve damage. ET-1 was administered intravitreally to Sprague-Dawley rats and TAU was injected as pre-, co- or post-treatment. Animals were euthanized seven days post TAU injection. Retinae and optic nerve were examined for morphology, and were also processed for caspase-3 immunostaining. Retinal redox status was estimated by measuring retinal superoxide dismutase, catalase, glutathione, and malondialdehyde levels using enzyme-linked immuosorbent assay. Histopathological examination showed significantly improved retinal and optic nerve morphology in TAU-treated groups. Morphometric examination showed that TAU pre-treatment provided marked protection against ET-1 induced damage to retina and optic nerve. In accordance with the morphological observations, immunostaining for caspase showed a significantly lesser number of apoptotic retinal cells in the TAU pre-treatment group. The retinal oxidative stress was reduced in all TAU-treated groups, and particularly in the pre-treatment group. The findings suggest that treatment with TAU, particularly pre-treatment, prevents apoptosis of retinal cells induced by ET-1 and hence prevents the changes in the morphology of retina and optic nerve. The protective effect of TAU against ET-1 induced retinal and optic nerve damage is associated with reduced retinal oxidative stress.

Oxidant-Antioxidant Balance in the Aqueous Humor of Patients with Retinal Vein Occlusion

PURPOSE

To analyze oxidative stress parameters in the aqueous humor with retinal vein occlusion.

MATERIALS AND METHODS

A total of 56 patients who underwent cataract surgery were enrolled in this study. The eyes with retinal vein occlusion were included in the study group. Aqueous humor was sampled from all patients and analyzed by an automated measurement method. Total oxidant status (TOS), total antioxidant status (TAS), and oxidative stres index (OSI) were calculated.

RESULTS

OSI was significantly higher in the study group (p = 0.04). TAS was significantly lower (p = 0.01) and OSI was significantly higher (p = 0.01) in cases with both central retinal vein occlusion and macular edema. Previous laser photocoalugation to the retina and the duration of disease had no significant effect on the levels of oxidative stress parameters.

CONCLUSION

OSI was found higher in patients with retinal vein occlusion. This increase is more significant in patients with macular edema and central retinal vein occlusion; in which TAS was also found to decrease.

Fluorescence lifetime imaging ophthalmoscopy

Imaging techniques based on retinal autofluorescence have found broad applications in ophthalmology because they are extremely sensitive and noninvasive. Conventional fundus autofluorescence imaging measures fluorescence intensity of endogenous retinal fluorophores. It mainly derives its signal from lipofuscin at the level of the retinal pigment epithelium. Fundus autofluorescence, however, can not only be characterized by the spatial distribution of the fluorescence intensity or emission spectrum, but also by a characteristic fluorescence lifetime function. The fluorescence lifetime is the average amount of time a fluorophore remains in the excited state following excitation. Fluorescence lifetime imaging ophthalmoscopy (FLIO) is an emerging imaging modality for in vivo measurement of lifetimes of endogenous retinal fluorophores. Recent reports in this field have contributed to our understanding of the pathophysiology of various macular and retinal diseases. Within this review, the basic concept of fluorescence lifetime imaging is provided. It includes technical background information and correlation with in vitro measurements of individual retinal metabolites. In a second part, clinical applications of fluorescence lifetime imaging and fluorescence lifetime features of selected retinal diseases such as Stargardt disease, age-related macular degeneration, choroideremia, central serous chorioretinopathy, macular holes, diabetic retinopathy, and retinal artery occlusion are discussed. Potential areas of use for fluorescence lifetime imaging ophthalmoscopy will be outlined at the end of this review.

A novel porcine ex vivo retina culture model for oxidative stress induced by H₂O₂

Oxidative stress is a key player in many ophthalmic diseases. However, the role of oxidative stress in most degenerative processes is not yet known. Therefore, accurate and practical models are required to efficiently screen for therapeutics. Porcine eyes are closely related to the human eye, and can be obtained from the abattoir as a by-product of the food industry. Therefore, they offer excellent opportunities for the development of culture models with which to pre-screen potential therapies, while reducing the use of laboratory animals. To induce oxidative stress, organotypic cultures of porcine retina were treated with different doses of hydrogen peroxide (H₂O₂; 100, 300 and 500μM) for three hours. On days 3 and 8, the retinas were conserved for histological and Western blotting analyses and for evaluation of gene expression, which determined the number of retinal ganglion cells (RGCs), the activation state of glial cells, and the expression levels of several oxidative stress markers. H₂O₂ treatment led to a reduction in the number of RGCs and to an increase in apoptotic RGCs. In addition, a dose-dependent increase of microglia and an elevation of CD11b expression was observed. On day 3, a reduction of IL-1β, and an increase of iNOS, as well as of HSP70 mRNA were found. On day 8, an increase in TNF-α and IL-1β mRNA expression was detected. In conclusion, this ex vivo model offers an opportunity to study the molecular mechanisms underlying certain eye disorders and to test new therapeutic approaches to diminish the effects of oxidative stress.

Rapid repeatable in vivo detection of retinal reactive oxygen species

Oxidative injuries, such as those related to reactive oxygen species (ROS), have been implicated in various retinal and optic nerve disorders. Many ROS detection methods have been developed. Although widely utilized, many of these methods are useful only in post mortem tissues, or require relatively expensive equipment, or involve intraocular injection. In the present study, we demonstrated and characterized a chemiluminescent probe L-012 as a noninvasive, in vivo ROS detection agent in the mouse retina. Using optic nerve crush (ONC) and retinal ischemia/reperfusion (I/R) as injury models, we show that L-012 produced intensive luminescent signals specifically in the injured eyes. Histological examination showed that L-012 administration was safe to the retina. Additionally, compounds that reduce tissue superoxide levels, apocynin and TEMPOL, decreased injury-induced L-012 chemiluminescence. The decrease in L-012 signals correlated with their protective effects against retinal I/R-induced morphological and functional changes in the retina. Together, these data demonstrate the feasibility of a fast, simple, reproducible, and non-invasive detection method to monitor in vivo ROS in the retina. Furthermore, the results also show that reduction of ROS is a potential therapeutic approach for protection from these retinal injuries.

Ischemic optic neuropathy as a model of neurodegenerative disorder: A review of pathogenic mechanism of axonal degeneration and the role of neuroprotection

Optic neuropathy is a neurodegenerative disease which involves optic nerve injury. It is caused by acute or intermittent insults leading to visual dysfunction. There are number of factors, responsible for optic neuropathy, and the optic nerve axon is affected in all type which causes the loss of retinal ganglion cells. In this review we will highlight various mechanisms involved in the cell loss cascades during axonal degeneration as well as ischemic optic neuropathy. These mechanisms include oxidative stress, excitotoxicity, angiogenesis, neuroinflammation and apoptosis following retinal ischemia. We will also discuss the effect of neuroprotective agents in attenuation of the negative effect of factors involve in the disease occurrence and progression.

Madecassic Acid protects against hypoxia-induced oxidative stress in retinal microvascular endothelial cells via ROS-mediated endoplasmic reticulum stress

Madecassic acid (MA) is an abundant triterpenoid in Centella asiatica (L.) Urban. (Apiaceae) that has been used as a wound-healing, anti-inflammatory and anti-cancer agent. Up to now, the effects of MA against oxidative stress remain unclear. In this study, we investigated the effect of MA and its mechanisms on hypoxia-induced human Retinal Microvascular Endothelial Cells (hRMECs). hRMECs were pre-treated with different concentrations of MA (0-50μM) for 30min before being incubated under hypoxia condition (37°C, 5% CO₂ and 95% N₂). Cell apoptosis was evaluated with MTT assay and TUNEL staining, and the expression of apoptosis- and endoplasmic reticulum (ER) stress-related molecules was assessed with western blotting and RT-PCR analysis. Intracellular ROS level was evaluated using DCFH-DA. Intracellular malondialdehyde (MDA), dehydrogenase (LDH), glutathione peroxidase (GSH-PX) and superoxide dismutase (SOD) were evaluated using related Kits. Activating transcription factor 4 (ATF4) nuclear translocation was assessed with western blotting analysis and immunofluorescence staining. MA significantly reduced oxidative stress in hypoxia-induced hRMECs, as shown by increased cell viability, SOD and GSH-PX leakage, decreased TUNEL- and ROS-positive cell ratio, LDH and MDA leakage, caspase-3 and -9 activity, and Bax/Bcl-2 ratio. In addition, MA also attenuated hypoxia-induced ER stress in hRMECs, as shown by reduced mRNA levels of glucose-regulated protein 78 (GRP78), C/EBP homologous transcription factor (CHOP), protein levels of cleaved activating transcription factor 6 (ATF6) and inositol-requiring kinase/endonuclease 1 alpha (IRE1α), phosphorylation of pancreatic ER stress kinase (PERK) and eukaryotic initiation factor 2 alpha (eIF2α), cleaved caspase-12 and ATF4 translocation to nucleus. The current study indicated that the regulation of oxidative stress and ER stress by MA would be a promising therapy to reverse the process and development of hypoxia-induced hRMECs dysfunction.

A Mouse Model of Retinal Ischemia-Reperfusion Injury Through Elevation of Intraocular Pressure

Retinal ischemia-reperfusion (I/R) is a pathophysiological process contributing to cellular damage in multiple ocular conditions, including glaucoma, diabetic retinopathy, and retinal vascular occlusions. Rodent models of I/R injury are providing significant insights into mechanisms and treatment strategies for human I/R injury, especially with regard to neurodegenerative damage in the retinal neurovascular unit. Presented here is a protocol for inducing retinal I/R injury in mice through elevation of intraocular pressure (IOP). In this protocol, the ocular anterior chamber is cannulated with a needle, through which flows the drip of an elevated saline reservoir. Using this drip to raise IOP above systolic arterial blood pressure, a practitioner temporarily halts inner retinal blood flow (ischemia). When circulation is reinstated (reperfusion) by removal of the cannula, severe cellular damage ensues, resulting ultimately in retinal neurodegeneration. Recent studies demonstrate inflammation, vascular permeability, and capillary degeneration as additional elements of this model. Compared to alternative retinal I/R methodologies, such as retinal arterial ligation, retinal I/R injury by elevated IOP offers advantages in its anatomical specificity, experimental tractability, and technical accessibility, presenting itself as a valuable tool for examining neuronal pathogenesis and therapy in the retinal neurovascular unit.

Modeling transient retinal ischemia in mouse by ligation of pterygopalatine artery

Background

Retinal ischemia is a major cause of visual impairment and blindness worldwide. The available therapeutic strategies have limited potential.

Purpose

In order to understand the pathophysiology and validating therapies for retinal ischemia, establishment of reproducible animal models is necessary.

Methods

In the model discussed in this article, the pterygopalatine artery (PPA) is ligated along with the external carotid artery for 3.5 hours and thereafter allowed to reperfuse. Because PPA supplies the blood to the ophthalmic artery, the ligation of this artery causes retinal ischemia.

Results

This article describes the validation of retinal ischemia-reperfusion model in mouse through PPA ligation and its validation through fluorescein fundus angiography (FFA) and immunofluorescence staining for glial fibrillary acidic protein (GFAP), a glial injury marker.

Conclusions

In conclusion this article describes the creation of mouse model of retinal ischemia-reperfusion injury which can be reproduced in a shorter time duration resulting in reduced mortality.

Neuroprotective effect of water-dispersible hesperetin in retinal ischemia reperfusion injury

PURPOSE

To determine whether water-dispersible hesperetin (WD-Hpt) can prevent degeneration of ganglion cell neurons in the ischemic retina.

METHODS

Ischemia reperfusion (I/R) injury was induced by increasing the intraocular pressure of mice to 110 mmHg for 40 min. Mice received daily intraperitoneal injections with either normal saline (NS, 0.3 ml/day) or WD-Hpt (0.3 ml, 200 mg/kg/day). Reactive oxygen species (ROS) was assessed by dihydroethidium and nitrotyrosine formation. Inflammation was estimated by microglial morphology in the retina. Lipopolysaccharide (LPS)-stimulated BV-2 cells were used to explore the anti-inflammatory effect of WD-Hpt on activated microglia by quantifying the expression of IL-1β using real-time quantitative reverse transcription-polymerase chain reaction. Ganglion cell loss was assessed by immunohistochemistry of NeuN. Glial activation was quantified with glial fibrillary acidic protein (GFAP) immunoreactivity. Apoptosis was evaluated with a terminal deoxynucleotidyl transferase (TUNEL) assay and immunohistochemistry of cleaved caspase-3. Phosphorylation of extracellular signal-regulated kinase (p-ERK) was surveyed by western blotting.

RESULTS

WD-Hpt decreased I/R-induced ROS formation. WD-Hpt alleviated microglial activation induced by I/R and reduced mRNA levels of IL-1β in LPS-stimulated BV-2. I/R resulted in a 37% reduction in the number of ganglion cells in the NS-treated mice, whereas the reduction was only 5% in the WD-Hpt-treated mice. In addition, WD-Hpt mitigated the immunoreactivity of GFAP, increased expression of cleaved caspase-3, increased number of TUNEL positive cells and p-ERK after I/R.

CONCLUSIONS

WD-Hpt protected ganglion cells from I/R injury by inhibiting oxidative stress and modulating cell death signaling. Moreover, WD-Hpt had an anti-inflammatory effect through the suppression of activated microglia.

The progress in understanding and treatment of diabetic retinopathy

Diabetic retinopathy is the most frequently occurring complication of diabetes mellitus and remains a leading cause of vision loss globally. Its aetiology and pathology have been extensively studied for half a century, yet there are disappointingly few therapeutic options. Although some new treatments have been introduced for diabetic macular oedema (DMO) (e.g. intravitreal vascular endothelial growth factor inhibitors ('anti-VEGFs') and new steroids), up to 50% of patients fail to respond. Furthermore, for people with proliferative diabetic retinopathy (PDR), laser photocoagulation remains a mainstay therapy, even though it is an inherently destructive procedure. This review summarises the clinical features of diabetic retinopathy and its risk factors. It describes details of retinal pathology and how advances in our understanding of pathogenesis have led to identification of new therapeutic targets. We emphasise that although there have been significant advances, there is still a pressing need for a better understanding basic mechanisms enable development of reliable and robust means to identify patients at highest risk, and to intervene effectively before vision loss occurs.

Identification of p58IPK as a novel neuroprotective factor for retinal neurons

PURPOSE

Endoplasmic reticulum (ER)-resident chaperone protein p58(IPK) plays a vital role in regulation of protein folding and biosynthesis. The goal of this study was to examine the role of p58(IPK) in retinal neuronal cells under normal and stressed conditions.

METHODS

Retinal expression of p58(IPK), retinal morphology, apoptosis, ER stress, and apoptotic gene expression were examined in p58(IPK) knockout (KO) and/or wild-type (WT) mice with or without intravitreal injection of N-methyl-D-aspartic acid (NMDA). In in vitro experiments, differentiated R28 retinal neuronal cells transduced with adenovirus encoding p58(IPK) (Ad-p58(IPK)) or control virus (Ad-LacZ) were exposed to tunicamycin (TM) or hydrogen peroxide (H2O2). Levels of ER stress, apoptosis, and cell survival were evaluated.

RESULTS

Chaperone protein p58(IPK) is expressed predominantly in retinal ganglion cells (RGC), inner retinal neurons, and the photoreceptor inner segments. Mice lacking p58(IPK) exhibited increased CHOP expression and loss of RGCs with aging (8-10 months). Intravitreal injection of NMDA induced retinal ER stress and increased p58(IPK) expression in WT mice; this resulted in greater ER stress and enhanced RGC apoptosis in p58(IPK) KO mice. In cultured R28 cells, overexpression of p58(IPK) significantly reduced eIF2α phosphorylation, decreased CHOP expression, and alleviated the activation of caspase-3 and PARP. Overexpression of p58(IPK) also protected against oxidative and ER stress-induced cell apoptosis. Furthermore, p58(IPK) downregulated the proapoptotic gene Bax and upregulated the antiapoptotic gene Bcl-2 expression in stressed R28 cells.

CONCLUSIONS

Our study has demonstrated a protective role of p58(IPK) in retinal neurons, which may act in part through a mechanism involving modulation of ER homeostasis and apoptosis, particularly under conditions of cellular stresses.

Identification of protein network alterations upon retinal ischemia-reperfusion injury by quantitative proteomics using a Rattus norvegicus model

Retinal ischemia is a common feature associated with several ocular diseases, including diabetic retinopathy. In this study, we investigated the effect of a retinal ischemia and reperfusion (I/R) injury on protein levels via a quantitative shotgun strategy using stable isotope dimethyl labeling combined with LC-MS/MS analysis. Based on the relative quantitation data of 1088 proteins, 234 proteins showed a greater than 1.5-fold change following I/R injury, 194 of which were up-regulated and 40 were down-regulated. Gene ontology analysis revealed that after I/R injury, there was an increase in the metabolic-process related proteins but a decline in cell communication, system process and transport-related proteins. A ribosome protein network and a secreted protein network consisting of many protease inhibitors were identified among the up-regulated proteins, despite a suppression of the mammalian target of rapamycin (mTOR) pathway following the I/R injury. A synaptic-related protein network was found to be significantly down-regulated, implicating a functional reduction of neurons following a retinal I/R injury. Our results provide new systems-biology clues for the study of retinal ischemia.

Diabetic retinopathy: could the alpha-1 antitrypsin be a therapeutic option?

Diabetic retinopathy is one of the most important causes of blindness. The underlying mechanisms of this disease include inflammatory changes and remodeling processes of the extracellular-matrix (ECM) leading to pericyte and vascular endothelial cell damage that affects the retinal circulation. In turn, this causes hypoxia leading to release of vascular endothelial growth factor (VEGF) to induce the angiogenesis process. Alpha-1 antitrypsin (AAT) is the most important circulating inhibitor of serine proteases (SERPIN). Its targets include elastase, plasmin, thrombin, trypsin, chymotrypsin, proteinase 3 (PR-3) and plasminogen activator (PAI). AAT modulates the effect of protease-activated receptors (PARs) during inflammatory responses. Plasma levels of AAT can increase 4-fold during acute inflammation then is so-called acute phase protein (APPs). Individuals with low serum levels of AAT could develop disease in lung, liver and pancreas. AAT is involved in extracellular matrix remodeling and inflammation, particularly migration and chemotaxis of neutrophils. It can also suppress nitric oxide (NO) by nitric oxide sintase (NOS) inhibition. AAT binds their targets in an irreversible way resulting in product degradation. The aim of this review is to focus on the points of contact between multiple factors involved in diabetic retinopathy and AAT resembling pleiotropic effects that might be beneficial.

Effect of retinal ischemia on the non-image forming visual system

Retinal ischemic injury is an important cause of visual impairment. The loss of retinal ganglion cells (RGCs) is a key sign of retinal ischemic damage. A subset of RGCs expressing the photopigment melanopsin (mRGCs) regulates non-image-forming visual functions such as the pupillary light reflex (PLR), and circadian rhythms. We studied the effect of retinal ischemia on mRGCs and the non-image-forming visual system function. For this purpose, transient ischemia was induced by raising intraocular pressure to 120 mm Hg for 40 min followed by retinal reperfusion by restoring normal pressure. At 4 weeks post-treatment, animals were subjected to electroretinography and histological analysis. Ischemia induced a significant retinal dysfunction and histological alterations. At this time point, a significant decrease in the number of Brn3a(+) RGCs and in the anterograde transport from the retina to the superior colliculus and lateral geniculate nucleus was observed, whereas no differences in the number of mRGCs, melanopsin levels, and retinal projections to the suprachiasmatic nuclei and the olivary pretectal nucleus were detected. At low light intensity, a decrease in pupil constriction was observed in intact eyes contralateral to ischemic eyes, whereas at high light intensity, retinal ischemia did not affect the consensual PLR. Animals with ischemia in both eyes showed a conserved locomotor activity rhythm and a photoentrainment rate which did not differ from control animals. These results suggest that the non-image forming visual system was protected against retinal ischemic damage.

Down-regulation of GRP78 enhances apoptosis via CHOP pathway in retinal ischemia-reperfusion injury

Ischemia/reperfusion (I/R) injury is the main cause of retinal apoptosis. But the mechanism remains elusive. During I/R injury, the intracellular calcium levels increase, resulting in the generation of reactive oxygen species, which have been shown to cause endoplasmic reticulum (ER) stress. However, little is known about the correlation between apoptosis and ER stress in retinal I/R injury. In the present study, we demonstrated that ER stress was activated in the retina of rat I/R models. The transcriptional expression of ER stress-associated molecules, glucose-regulated protein-78 (GRP78) and C/EBP-homologous protein (CHOP) were significantly increased in I/R retinas in a time-dependent manner. Partial inhibition of the endogenous expression of GRP78 with antisense oligonucleotide resulted in significant retinal damage and apoptosis in I/R injury rats. Also, the transcriptional expression of CHOP was persistently increased. Our findings indicate that ER stress may play a critical role in I/R injury induced retinal damage, and GRP78 may exert anti-apoptotic actions in I/R retina. Importantly, the persistent high expression of CHOP might serve as a possible mechanism that contributes to the enhanced the I/R-induced apoptosis after GRP78 down-regulation. These results may provide insight into the pathology of retinal I/R injury.

Activation of the Nrf2/HO-1 antioxidant pathway contributes to the protective effects of Lycium barbarum polysaccharides in the rodent retina after ischemia-reperfusion-induced damage

Lycium barbarum polysaccharides (LBP), extracts from the wolfberries, are protective to retina after ischemia-reperfusion (I/R). The antioxidant response element (ARE)–mediated antioxidant pathway plays an important role in maintaining the redox status of the retina. Heme oxygenase-1 (HO-1), combined with potent AREs in its promoter, is a highly effective therapeutic target for the protection against neurodegenerative diseases, including I/R-induced retinal damage. The aim of our present study was to investigate whether the protective effect of LBP after I/R damage was mediated via activation of the Nrf2/HO-1-antioxidant pathway in the retina. Retinal I/R was induced by an increase in intraocular pressure to 130 mm Hg for 60 minutes. Prior to the induction of ischemia, rats were orally treated with either vehicle (PBS) or LBP (1 mg/kg) once a day for 1 week. For specific experiments, zinc protoporphyrin (ZnPP, 20 mg/kg), an HO-1 inhibitor, was intraperitoneally administered at 24 h prior to ischemia. The protective effects of LBP were evaluated by quantifying ganglion cell and amacrine cell survival, and by measuring cell apoptosis in the retinal layers. In addition, HO-1 expression was examined using Western blotting and immunofluorescence analyses. Cytosolic and nuclear Nrf2 was measured using immunofluorescent staining. LBP treatment significantly increased Nrf2 nuclear accumulation and HO-1 expression in the retina after I/R injury. Increased apoptosis and a decrease in the number of viable cells were observed in the ganglion cell layer (GCL) and inner nuclear layer (INL) in the I/R retina, which were reversed by LBP treatment. The HO-1 inhibitor, ZnPP, diminished the LBP treatment-induced protective effects in the retina after I/R. Taken together, these results suggested that LBP partially exerted its beneficial neuroprotective effects via the activation of Nrf2 and an increase in HO-1 protein expression.

Protective effects of placental growth factor on retinal neuronal cell damage

Placental growth factor (PlGF) is a member of the vascular endothelial growth factor family. Although it has been reported that PlGF protects against neuronal damage in the brain, little is known about the effects of PlGF in the retina. Therefore, we investigated the effects of PlGF on retinal neuronal cells. To evaluate the effects of PlGF against L-buthionine-(S,R)-sulfoximine (BSO)/glutamate cell death, oxygen-glucose deprivation (OGD)-induced cell death, and light-induced cell death, RGC-5 and 661W cells were used. We evaluated the mechanism responsible for the protective effects of PlGF against retinal neuronal cell death by performing the examinations with U1026, which is a mitogen-activated protein kinase (MEK) inhibitor, and LY294002, which is a phosphoinositide 3-kinase (PI3K) inhibitor. In addition, we measured caspase-3/7 activity in RGC-5 cells and 661W cells. PlGF protected against RGC-5 cell death induced by BSO/glutamate and OGD and against 661W cell death induced by light irradiation. Moreover, an anti-PlGF antibody negated these protective effects. The protective effects of PlGF against OGD-induced RGC-5 cell death and light-induced 661W cell death were suppressed by using an anti-PlGF antibody, U1026, and LY294002. Treatment with PlGF suppressed caspase-3/7 activity in both cell lines. We demonstrated for the first time that PlGF exerts a protective effect by inhibiting the activation of caspase-3/7 through the MEK and PI3K pathway in retinal neuronal cells. These data suggest that PlGF may be an important protective factor in the retina.

Annatto prevents retinal degeneration induced by endoplasmic reticulum stress in vitro and in vivo

SCOPE

Annatto (Bixa orellana) seeds have been used as a colorant in butter and in a variety of other foods. In this study, we investigated the amelioration of retinal damage by an acetone extract of annatto (A-ext.), bixin (a main component of annatto), and four bixin derivatives (Bx-1, Bx-2, Bx-3, and Bx-4) that we have synthesized.

METHODS AND RESULTS

We used cultured retinal ganglion cells (RGC-5) to examine in vitro effects of A-ext. on stress pathways, focusing on intracellular oxidation induced by reactive oxygen species, expression of endoplasmic reticulum (ER) stress-related proteins, caspase-3 activation, and cell membrane damage. In vivo retinal damage in mice following intravitreous injection of tunicamycin was evaluated by counting the cell numbers in the ganglion cell layer (GCL) and measuring the thickness of outer nuclear layer (ONL). A-ext., bixin, and Bx-1 treatment inhibited both tunicamycin- and H₂O₂-induced cell death. Bixin derivatives also inhibited tunicamycin-induced cell death. Treatment with A-ext., bixin, and Bx-1 reduced tunicamycin-induced caspase-3 activity and inhibited the inversion of phosphatidylserine, an early apoptotic event without antioxidant effect or reduction of ER stress itself. A-ext., bixin, and Bx-1 significantly inhibited the tunicamycin-induced loss of cells from the GCL, and these materials also suppressed the tunicamycin-induced thinning of ONL.

CONCLUSION

A-ext., its main component bixin, and bixin derivatives may therefore be useful for preventive and therapeutic treatment of retinal-related diseases.

Challenges in neuroprotective nanomedicine development: progress towards noninvasive gene therapy of glaucoma

Over the past decade the application of gene therapy of retinal diseases such as glaucoma has produced promising results. However, optic nerve regeneration and restoration of vision in patients with glaucoma is still far from reality. Neuroprotective approaches in the form of gene therapy may provide significant advantages, but are still limited by many factors both at the organ and cellular levels. In general, gene delivery systems for eye diseases range from simple eye drops and ointments to more advanced bio- and nanotechnology-based systems such as muco-adhesive systems, polymers, liposomes and ocular inserts. Most of these technologies were developed for front-of-the-eye ophthalmic therapies and are not applicable as back-of-the-eye delivery systems. Currently, only the invasive intravitreal injections are capable of successfully delivering genes to the retina. Here we review the challenges and possible strategies for the noninvasive gene therapy of glaucoma including the barriers in the eye and in neural cells, and present a cross-sectional view of gene delivery as it pertains to the prevention and treatment of glaucoma.

Involvement of endoplasmic reticulum stress in optic nerve degeneration following N-methyl-D-aspartate-induced retinal damage in mice

We evaluated time-dependent optic nerve degeneration and the role of endoplasmic reticulum (ER) stress in this process following retinal ganglion cell death in mice. Retinal damage was induced by intravitreal injection of N-methyl-D-aspartate (NMDA). Neurofilament heavy (NFH)- and phosphorylated NFH (pNFH)-positive axons were time-dependently decreased in optic nerves at 1, 3, 7, 14, and 28 days after NMDA injection. Expression of glial fibrillary acidic protein (GFAP)-positive astroglial cells and ionized calcium-binding adaptor molecule 1 (Iba1)-positive microglial cells showed a significant increase in the optic nerve at 7, 14, and 28 days after NMDA injection. In contrast, expression of myelin basic protein (MBP)-positive oligodendrocytes showed a significant decrease in the optic nerve at 7, 14, and 28 days after NMDA injection. In quantitative RT-PCR analysis, expressions of glucose-regulated protein 78 (Grp78)/BiP, Grp94, Calreticulin, C/EBP homologous protein (Chop), and the ER degradation enhancer mannosidase alpha-like 1 (Edem1) genes were increased in the optic nerve at 14 days after NMDA injection. In addition, the Grp94 gene was increased at 7 days after NMDA injection, and the Edem1 gene was increased at 3, 7, and 28 days after NMDA injection. GRP78 and CHOP proteins were colocalized with MBP in the optic nerve after NMDA injection. These findings suggest that the axonal degeneration is dramatic until 7 days after NMDA injection and that glial cells may play some role in the degeneration of the optic nerve. Furthermore, ER stress may play a pivotal role in the decrease of MBP-positive oligodendrocytes after NMDA-induced retinal damage.

Valproic acid regulates antioxidant enzymes and prevents ischemia/reperfusion injury in the rat retina

PURPOSES

To investigate whether valproic acid (VPA) has a neuroprotective effect against ischemia/reperfusion (I/R) injury in the rat retina, and to elucidate the potential antioxidant mechanisms involved.

METHODS

Adult male Wistar rats were randomly divided into four groups: sham (group A), sham plus VPA (group B), I/R plus vehicle (group C), and I/R plus VPA (group D). Retinal I/R injury was produced by inducing an exceedingly high intraocular pressure (IOP). Prior to insult, VPA was administered subcutaneously (300 mg/kg twice daily) for 7 days, after which the animal was sacrificed. Levels of retinal malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) were determined. Protein expressions of retinal poly(ADP-ribose) (PAR) and nitrotyrosine (NT) were analyzed by Western blotting 24 h after injury. Apoptosis of retinal cells was evaluated 24 h after I/R injury by immunofluorescence of activated caspase-3 in histological sections of retina. Seven days after reperfusion, electroretinography (ERG) was performed, and retinal histological changes were examined by light microscopy.

RESULTS

Following ischemia, the thickness of the entire retina, including the inner nuclear layer (INL) and inner plexiform layer (IPL), as well as the number of cells in the ganglion cell layer (GCL) were significantly greater in group D than in group C (p < 0.05). VPA suppressed I/R-induced reductions in ERG a- and b-wave amplitudes (p < 0.05). VPA attenuated I/R-induced activation of caspase-3 in ganglion cells and INL cells (p < 0.001). VPA significantly decreased MDA levels and increased activities of SOD, GSH-Px, and CAT in group D (p < 0.05). VPA attenuated activation of PAR and accumulation of NT in the retina after I/R (p < 0.01).

CONCLUSIONS

VPA protects the retina from I/R injury by enhancing anti-oxidative effects and inhibiting apoptosis of retinal cells.

Effects of antioxidant gene therapy on retinal neurons and oxidative stress in a model of retinal ischemia/reperfusion

Retinal ischemia/reperfusion (I/R) results in neuronal death and generation of reactive oxygen species. The aim of this study was to investigate the neuroprotective effect of manganese superoxide dismutase (SOD2) on retinal ganglion cells (RGCs) in an I/R-induced retinal injury model. One eye of each Wistar rat was pretreated with recombinant adeno-associated virus containing the SOD2 gene (AAV-SOD2) or recombinant AAV containing the GFP gene (AAV-GFP) by intravitreal injection 21 days before initiation of I/R injury. Retinal I/R injury was induced by elevating intraocular pressure for 1h, and reperfusion was established immediately afterward. The number of RGCs and the inner plexiform layer (IPL) thickness were measured by Fluorogold retrograde labeling and hematoxylin and eosin staining at 6 h, 24 h, 72 h, and 5 days after injury. Superoxide anion, the number of RGCs, IPL thickness, malondialdehyde (MDA) level, 8-hydroxy-2-deoxyguanosine (8-OHdG) level, MnSOD (manganese superoxide dismutase) activity, and nitrotyrosine level were measured by fluorescence staining, immunohistochemistry, and enzyme-linked immunosorbent analysis at 5 days after I/R injury. Severe RGC loss, reduced IPL thickness, reduced MnSOD activity, and increased superoxide ion, MDA, 8-OHdG, and nitrotyrosine production were observed after I/R injury. Administration of AAV-SOD2 significantly reduced the levels of superoxide ion, MDA, 8-OHdG, and nitrotyrosine and prevented the damage to RGCs and IPL. Delivery of the antioxidant gene inhibited I/R-induced RGC and IPL damage by reducing oxidative stress and nitrative stress, suggesting that MnSOD may be relevant for the neuroprotection of the inner retina from I/R-related diseases.

Neuroglobin protection in retinal ischemia

PURPOSE

Neuroglobin (Ngb) is a vertebrate globin that is predominantly expressed in the retina and brain. To explore the role of Ngb in retinal neuroprotection during ischemia reperfusion (IR), the authors examined the effect of Ngb overexpression in the retina in vivo by using Ngb-transgenic (Ngb-Tg) mice.

METHODS

Retinal IR was induced in Ngb overexpressing Ngb-Tg mice and wild type (WT) mice by cannulating the anterior chamber and transiently elevating the IOP for 60 minutes. After Day 7 of reperfusion, the authors evaluated Ngb mRNA and protein expression in nonischemic control as well as ischemic mice and its effect on retinal histology, mitochondrial oxidative stress, and apoptosis, using morphometry and immunohistochemistry, quantitative PCR analysis and Western blot techniques.

RESULTS

Ngb-Tg mice without ischemia overexpress Ngb mRNA 11.3-fold (SE ± 0.457, P < 0.05) higher than WT control mice, and this overexpression of Ngb protein was localized to the mitochondria of the ganglion cells, outer and inner plexiform layers, and photoreceptor inner segments. This overexpression of Ngb is associated with decreased mitochondrial DNA damage in Ngb-Tg mice with IR in comparison with WT. Ngb-Tg mice with IR also revealed significant preservation of retinal thickness, significantly less activated caspase 3 protein expression, and apoptosis in comparison with WT mice.

CONCLUSIONS

Neuroglobin overexpression plays a neuroprotective role against retinal ischemia reperfusion injury due to decreasing of mitochondrial oxidative stress-mediated apoptosis.

Acute retinal ischemia inhibits endothelium-dependent nitric oxide-mediated dilation of retinal arterioles via enhanced superoxide production

PURPOSE

Because retinal vascular disease is associated with ischemia and increased oxidative stress, the vasodilator function of retinal arterioles was examined after retinal ischemia induced by elevated intraocular pressure (IOP). The role of superoxide anions in the development of vascular dysfunction was assessed.

METHODS

IOP was increased and maintained at 80 to 90 mm Hg for 30, 60, or 90 minutes by infusing saline into the anterior chamber of a porcine eye. The fellow eye with normal IOP (10-20 mm Hg) served as control. In some pigs, superoxide dismutase mimetic TEMPOL (1 mM) or vehicle (saline) was injected intravitreally before IOP elevation. After enucleation, retinal arterioles were isolated and pressurized without flow for functional analysis by recording diameter changes using videomicroscopic techniques. Dihydroethidium (DHE) was used to detect superoxide production in isolated retinal arterioles.

RESULTS

Isolated retinal arterioles developed stable basal tone and the vasodilations to endothelium-dependent nitric oxide (NO)-mediated agonists bradykinin and L-lactate were significantly reduced only by 90 minutes of ischemia. However, vasodilation to endothelium-independent NO donor sodium nitroprusside was unaffected after all time periods of ischemia. DHE staining showed that 90 minutes of ischemia significantly increased superoxide levels in retinal arterioles. Intravitreal injection of membrane-permeable radical scavenger but not vehicle before ischemia prevented elevation of vascular superoxide and preserved bradykinin-induced dilation.

CONCLUSIONS

Endothelium-dependent NO-mediated dilation of retinal arterioles is impaired by 90 minutes of ischemia induced by elevated IOP. The inhibitory effect appears to be mediated by the alteration of NO signaling via vascular superoxide.

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.

Protective effects of catalase on retinal ischemia/reperfusion injury in rats

Retinal ischemia/reperfusion (I/R) injury causes profound tissue damage, especially retinal ganglion cell (RGC) death. The aims of the study were to investigate whether catalase (CAT) has a neuroprotective effect on RGC after I/R injury in rats, and to determine the possible antioxidant mechanism. Wistar female rats were randonmized into four groups: normal control group (Control group), retinal I/R with vehicle group (I/R with vehicle group), retinal I/R with AAV-CAT group (I/R with AAV-CAT group), and normal retina with AAV-CAT group (normal with AAV-CAT group). One eye of each rat was pretreated with recombinant adeno-associated virus containing catalase gene (I/R with AAV-CAT group or normal with AAV-CAT group) and recombinant adeno-associated virus containing GFP gene (I/R with vehicle group) by intravitreal injection 21 days before initiation of I/R injury. Retinal I/R injury was induced by elevating intraocular pressure to 100mmHg for 1h. The number of RGC and inner plexiform layer (IPL) thickness were measured by fluorogold retrograde labeling and hematoxylin and eosin staining at 6h, 24h, 72 h and 5d after injury. Hydrogen peroxide (H(2)O(2)), the number of RGC, IPL thickness, malondialdehyde(MDA), 8-hydroxy-2-deoxyguanosine (8-OHdG), CAT activity and nitrotyrosine were measured by fluorescence staining, immunohistochemistry and enzyme-linked immunosorbent assay analysis at 5 days after injury. Electroretinographic (ERG) evaluation was also used. Pretreatment of AAV-CAT significantly decreased the levels of H(2)O(2), MDA, 8-OHdG and nitrotyrosine, increased the catalase activity, and prevented the reduction of a- and b- waves in the I/R with AAV-CAT group compare with the I/R with vehicle group (p<0.01). Catalase attenuated the I/R-induced damage of RGC and IPL and retinal function. Therefore, catalase can protect the rat retina from I/R-induced injury by enhancing the antioxidative ability and reducing oxidative stress, which suggests that catalase may be relevant for the neuroprotection of inner retina from I/R-related diseases.

Edaravone, an ROS scavenger, ameliorates photoreceptor cell death after experimental retinal detachment

PURPOSE

To investigate whether edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one), a free radical scavenger, would be neuroprotective against photoreceptor cell death in a rat model of retinal detachment (RD).

METHODS

RD was induced in adult Brown Norway rats by subretinal injection of sodium hyaluronate. Edaravone (3, 5, or 10 mg/kg) or physiologic saline was administered intraperitoneally once a day until death on day 3 or 5. Oxidative stress in the retina was assessed by 4-hydroxynonenal staining or ELISA for protein carbonyl content. Photoreceptor death was assessed by TUNEL and measurement of the outer nuclear layer thickness. Western blot analysis and caspase activity assays were performed. Inflammatory cytokine secretion and inflammatory cell infiltration were evaluated by ELISA and immunostaining, respectively.

RESULTS

RD resulted in increased generation of ROS. Treatment with 5 mg/kg edaravone significantly reduced the ROS level, along with a decrease in TUNEL-positive cells in the photoreceptor layer. A caspase assay also confirmed decreased activation of caspase-3, -8, and -9 in RD treated with edaravone. The level of the antiapoptotic Bcl-2 was increased in detached retinas after edaravone treatment, whereas the levels of the stress-activated p-ERK1/2 were decreased. In addition, edaravone treatment resulted in a significant decrease in the levels of TNF-α, MCP-1, and macrophage infiltration.

CONCLUSIONS

Oxidative stress plays an important role in photoreceptor cell death after RD. Edaravone treatment may aid in preventing photoreceptor cell death after RD by suppressing ROS-induced photoreceptor damage.