Neuroprotection in relation to retinal ischemia and relevance to glaucoma

Neuroprotection: A Valuable Goal in Glaucoma Management?

Glaucoma is characterized by an accelerated loss of retinal ganglion cells, as a result of damage to optic nerve axons. One factor involved in the disease process is elevated intraocular pressure (IOP) and this is the current focus of therapies. However, up to 45% of patients experience glaucoma progression despite good IOP control and partly as a result, the treatment principle of direct neuroprotection has been developed, which consists of treating optic nerve degeneration in glaucoma independently of IOP lowering. Animal models have shown the potential of this approach but there are limited clinical trial data. Brimonidine and memantine currently show promise, in terms of efficacy and side effects, among the compounds entering clinical trials. Brimonidine has been shown to have a neuroprotective effect independent of IOP lowering in humans with glaucoma, and data from a large clinical trial are being analyzed. Memantine has shown neuroprotective effects in animal models of glaucoma, and data from a clinical trial in humans are awaited.

Brimonidine's Neuroprotective Effects against Transient Ischaemia-Induced Retinal Ganglion Cell Death

Purpose

Brimonidine is a lowering pressure agent currently used in glaucoma. This chronic degenerative condition is characterised by neuronal death, and an agent which offers neuroprotection may slow down or impede the progression of neuronal cell death.

Methods

The effects of brimonidine (BMD) on the short- and long-term survival of retinal ganglion cells (RGCs) after transient retinal ischaemia are reported here using a rat model. The fluorescent tracer Fluorogold (FG) was applied to both superior colliculi to retrogradely label RGCs. A ninety-minute period of ischaemia was induced and densities of surviving RGCs were estimated over time by counting FG-labelled RGCs in 12 standard regions of each retina.

Results

Seven days after inducing transient ischaemia, there was loss of approximately half of the RGC population. Topical pre-treatment with 0.1% or 0.5% BMD prevented ischaemia-induced RGC death.

Conclusions

These results indicate that optimal neuroprotective effects against the early loss of RGCs are seen with 0.1% or 0.5% BMD. Ischaemia-induced RGC loss continued between day 7 and day 21 in the vehicle treated groups and amounted to approximately 25% of the RGC population. Topical pre-treatment with 0.1% or 0.5% BMD was also effective in reducing the slow loss of RGCs.

Growth hormone protects against kainate excitotoxicity and induces BDNF and NT3 expression in chicken neuroretinal cells

There is increasing evidence to suggest a beneficial neuroprotective effect of growth hormone (GH) in the nervous system. While our previous studies have largely focused on retinal ganglion cells (RGCs), we have also found conclusive evidence of a pro-survival effect of GH in cells of the inner nuclear layer (INL) as well as a protective effect on the dendritic trees of the inner plexiform layer (IPL) in the retina. The administration of GH in primary neuroretinal cell cultures protected and induced neural outgrowths. Our results, both in vitro (embryo) and in vivo (postnatal), showed neuroprotective actions of GH against kainic acid (KA)-induced excitotoxicity in the chicken neuroretina. Intravitreal injections of GH restored brain derived neurotrophic factor (BDNF) expression in retinas treated with KA. In addition, we demonstrated that GH over-expression and exogenous administration increased BDNF and neurotrophin-3 (NT3) gene expression in embryonic neuroretinal cells. Thus, GH neuroprotective actions in neural tissues may be mediated by a complex cascade of neurotrophins and growth factors which have been classically related to damage prevention and neuroretinal tissue repair.

Poly(ε-caprolactone) nanocapsule carriers with sustained drug release: single dose for long-term glaucoma treatment

Glaucoma is an eye-related disease accompanied by highly elevated intraocular pressure (IOP), which causes damage to the optic nerve and results in vision loss and even blindness. Although the treatment of glaucoma with eye drops may reduce the IOP, eye drops have some limitations, such as poor patient compliance and short duration. To develop drug carriers that facilitate the sustained and long-term release of drugs for glaucoma therapy, we synthesized poly(ε-caprolactone) nanoparticles (PCL NPs) capable of loading pilocarpine, a widely used drug for the treatment of dry eye and glaucoma. We prepared two types of PCL NPs, namely, nanospheres (NSs), which are solid spheres capable of harboring the drug in their solid mass, and nanocapsules (NCs), which are hollow spherical structures for encapsulating the drug. The influence of the vesicular structure of PCL NPs on the drug loading efficiencies and release was investigated. The loading of pilocarpine in the PCL NCs was approximately 3 times higher than that in the PCL NSs. In addition, pilocarpine-loaded PCL NCs (PILO-PCL NCs) exhibited a sustained drug release profile. Effective pharmacological responses (i.e., IOP reduction and pupillary constriction) were observed in rabbits intracamerally treated with pilocarpine-loaded PCL NPs. Moreover, the PILO-PCL NCs show long-term therapeutic ability in alleviating ocular hypertension-induced corneal and retinal injuries under physiological conditions, even after 42 days. The results of in vivo studies also reveal that the PCL NCs are advantageous for the treatment of chronic ocular hypertension in glaucomatous eyes.

In vivo Pharmacological Evaluations of Pilocarpine-Loaded Antioxidant-Functionalized Biodegradable Thermogels in Glaucomatous Rabbits

To alleviate oxidative stress-induced ocular hypertension, grafting of antioxidant molecules to drug carriers enables a dual-function mechanism to effectively treat glaucomatous intraocular pressure (IOP) dysregulation. Providing potential application for intracameral administration of antiglaucoma medications, this study, for the first time, aims to examine in vivo pharmacological efficacy of pilocarpine-loaded antioxidant-functionalized biodegradable thermogels in glaucomatous rabbits. A series of gallic acid (GA)-grafted gelatin-g-poly(N-isopropylacrylamide) (GN) polymers were synthesized via redox reactions at 20-50 °C. Our results showed that raising redox radical initiation reaction temperature maximizes GA grafting level, antioxidant activity, and water content at 40 °C. Meanwhile, increase in overall hydrophilicity of GNGA carriers leads to fast polymer degradation and early pilocarpine depletion in vivo, which is disadvantageous to offer necessary pharmacological performance at prolonged time. By contrast, sustained therapeutic drug concentrations in aqueous humor can be achieved for long-term (i.e., 28 days) protection against corneal aberration and retinal injury after pilocarpine delivery using dual-function optimized carriers synthesized at 30 °C. The GA-functionalized injectable hydrogels are also found to contribute significantly to enhancement of retinal antioxidant defense system and preservation of histological structure and electrophysiological function, thereby supporting the benefits of drug-containing antioxidant biodegradable thermogels to prevent glaucoma development.

The role of alkyl chain length of monothiol-terminated alkyl carboxylic acid in the synthesis, characterization, and application of gelatin-g-poly(N-isopropylacrylamide) carriers for antiglaucoma drug delivery

To improve ocular bioavailability and extend pharmacological response, this study aims to investigate the role of alkyl chain length of monothiol-terminated alkyl carboxylic acids in the synthesis, characterization, and application of gelatin-g-poly(N-isopropylacrylamide) (GN) biodegradable in situ gelling carriers for antiglaucoma drug delivery. In the presence of mercaptoacetic acid (MAA), mercaptopropionic acid (MPA), mercaptobutyric acid (MBA), or mercaptohexanoic acid (MHA) as a chain transfer agent, the carboxylic end-capped poly(N-isopropylacrylamide) samples were prepared by free radical polymerization technique. Our results showed that with increasing alkyl chain length, the hydrophobicity of thermo-responsive polymer segments significantly increased, mainly due to an increase in CH stretching frequencies. In addition, the greater hydrophobic association favored the decrease in both phase transition temperature and weight loss of GN copolymers, thereby accelerating their temperature-triggered gelation process and retarding the degradation progress under physiological conditions. The benefits from these features allowed the pilocarpine carriers to increase drug payload and extend drug release. Irrespective of carbon number of monothiol-terminated alkyl carboxylic acid, the synthesized GN materials exhibited high tolerance to corneal endothelial cells without any evidence of inhibited proliferation, viability loss, inflammatory stimulation, and functional abnormality, indicating good biocompatibility. Results of clinical observations and histological examinations demonstrated that the therapeutic efficacies in treating glaucomatous damage are in response to in vivo drug release profiles from various intracamerally injected GN carriers. The research findings suggest the influence of alkyl chain length of chain transfer agent-mediated polymer hydrophobicity and degradability on pharmacological bioavailability and action of pilocarpine in a glaucomatous rabbit model.

STATEMENT OF SIGNIFICANCE

Considering that glaucoma is a chronic disease that requires long-term medical therapy to preserve vision in patients, it is highly desirable to augment pharmacological bioavailability and govern release profile by tuning the properties of drug delivery carriers. For the first time, the present study provide striking evidence that the alkyl chain length of monothiol-terminated alkyl carboxylic acid related to the synthesis of biodegradable in situ gelling copolymers plays a key role in molecular functionalization of intracameral delivery systems for ocular administration and controlled release of antiglaucoma medications. The therapeutic efficacies in treating glaucomatous damage are in response to in vivo pilocarpine release profiles modulated by the carbon number of thermo-responsive polymer segment-mediated carrier hydrophobicity and degradability.

Apelin protects against NMDA-induced retinal neuronal death via an APJ receptor by activating Akt and ERK1/2, and suppressing TNF-α expression in mice

Glutamate excitotoxicity mediated by N-methyl-d-aspartate (NMDA) receptors is an important cause of retinal ganglion cell death in glaucoma. To elucidate whether apelin protects against retinal neuronal cell death, we examined protective effects of exogenous and endogenous apelin on neuronal cell death induced by intravitreal injection of NMDA in the retinas of mice. An intravitreal injection of NMDA induced neuronal cell death in both the retinal ganglion cell layer and inner nuclear layer, and reduced the amplitudes of scotopic threshold response (STR) in electroretinography studies. Both cell death and STR amplitudes decrease induced by NMDA were prevented by a co-injection of [Pyr¹]-apelin-13, and were facilitated by apelin deficiency. The neuroprotective effects of [Pyr¹]-apelin-13 were blocked by an apelin receptor APJ antagonist, and by inhibitors of Akt and extracellular signal-regulated kinase 1/2 signaling pathways. Additionally, an intravitreal injection of tumor necrosis factor-α (TNF-α) neutralizing antibody prevented NMDA-induced retinal neuronal cell death, and exogenous and endogenous apelin suppressed NMDA-induced upregulation of TNF-α in the retina. These results suggest that apelin protects neuronal cells against NMDA-induced death via an APJ receptor in the retina, and that apelin may have beneficial effects in the treatment of glaucoma.

Cellular Stress Response and Immune Signaling in Retinal Ischemia-Reperfusion Injury

Ischemia–reperfusion injury is a well-known pathological hallmark associated with diabetic retinopathy, glaucoma, and other related retinopathies that ultimately can lead to visual impairment and vision loss. Retinal ischemia pathogenesis involves a cascade of detrimental events that include energy failure, excitotoxic damage, calcium imbalance, oxidative stress, and eventually cell death. Retina for a long time has been known to be an immune privileged site; however, recent investigations reveal that retina, as well as the central nervous system, elicits immunological responses during various stress cues. Stress condition, such as reperfusion of blood supply post-ischemia results in the sequestration of different immune cells, inflammatory mediators including cytokines, chemokines, etc., to the ischemic region, which in turn facilitates induction of inflammatory conditions in these tissues. The immunological activation during injury or stress per se is beneficial for repair and maintenance of cellular homeostasis, but whether the associated inflammation is good or bad, during ischemia–reperfusion injury, hitherto remains to be explored. Keeping all these notions in mind, the current review tries to address the immune response and host stress response mechanisms involved in ischemia–reperfusion injury with the focus on the retina.

The protection of rat retinal ganglion cells from ischemia/reperfusion injury by the inhibitory peptide of mitochondrial μ-calpain

Intracellular Ca(2+)-dependent cysteine proteases such as calpains have been suggested as critical factors in retinal ganglion cell (RGC) death. However, it is unknown whether mitochondrial calpains are involved in RGC death. The purpose of the present study was to determine whether the inhibition of mitochondrial μ-calpain activity protects against RGC death during ischemia/reperfusion (I/R) injury. This study used a well-established rat model of experimental acute glaucoma involving I/R injury. A specific peptide inhibitor of mitochondrial μ-calpain, Tat-μCL, was topically applied to rats via eye drops three times a day for 5 days after I/R. RGC death was determined by the terminal deoxynucleotidyl transferase dUTP nick end labeling assay. The truncation of apoptosis-inducing factor (AIF) was determined by western blot analyses. Retinal morphology was determined after staining with hematoxyline and eosin. In addition, the number of Fluoro Gold-labeled RGCs in flat-mounted retinas was used to determine the percentage of surviving RGCs after I/R injury. After 1 day of I/R, RGC death was observed in the ganglion cell layer. Treatment with Tat-μCL eye drops significantly prevented the death of RGCs and the truncation of AIF. After 5 days of I/R, RGC death decreased by approximately 40%. However, Tat-μCL significantly inhibited the decrease in the retinal sections and flat-mounted retinas. The results suggested that mitochondrial μ-calpain is associated with RGC death during I/R injury via truncation of AIF. In addition, the inhibition of mitochondrial μ-calpain activity by Tat-μCL had a neuroprotective effect against I/R-induced RGC death.

Neuroprotection in Glaucoma

Glaucoma is a degenerative optic neuropathy characterized by retinal ganglion cell (RGC) loss and visual field defects. It is known that in some glaucoma patients, death of RGCs continues despite intraocular pressure (IOP) reduction. Neuroprotection in the field of glaucoma is defined as any treatment, independent of IOP reduction, which prevents RGC death. Glutamate antagonists, ginkgo biloba extract, neurotrophic factors, antioxidants, calcium channel blockers, brimonidine, glaucoma medications with blood regulatory effect and nitric oxide synthase inhibitors are among compounds with possible neuroprotective activity in preclinical studies. A few agents (such as brimonidine or memantine) with neuroprotective effects in experimental studies have advanced to clinical trials; however the results of clinical trials for these agents have not been conclusive. Nevertheless, lack of compelling clinical evidence has not prevented the off-label use of some of these compounds in glaucoma practice. Stem cell transplantation has been reported to halt experimental neurodegenerative disease processes in the absence of cell replacement. It has been hypothesized that transplantation of some types of stem cells activates multiple neuroprotective pathways via secretion of various factors. The advantage of this approach is a prolonged and targeted effect. Important concerns in this field include the secretion of unwanted harmful mediators, graft survival issues and tumorigenesis. Neuroprotection in glaucoma, pharmacologically or by stem cell transplantation, is an interesting subject waiting for broad and multidisciplinary collaborative studies to better clarify its role in clinical practice.

Real-time imaging of RGC death with a cell-impermeable nucleic acid dyeing compound after optic nerve crush in a murine model

The retinal ganglion cells (RGCs) are the main source of therapeutic targets for neuroprotective glaucoma treatment, and evaluating RGCs is key for effective glaucoma care. Thus, we developed a minimally invasive, quick, real-time method to evaluate RGC death in mice. In this article we describe the details of our method, report new results obtained from C57BL/6J mice, and report that our method was usable in wild type (WT) and knockout (KO) mice lacking an RGC-death-suppressing gene. It used a non-invasive confocal scanning laser ophthalmoscope (cSLO) and a low molecular weight, photo-switching, cell-impermeant, fluorescent nucleic acid dyeing compound, SYTOX orange (SO). The RGCs were retrogradely labeled with Fluorogold (FG), the optic nerve was crushed (ONC), and SO was injected into the vitreous. After ten minutes, RGC death was visualized with cSLO in vivo. The retinas were then extracted and flat mounted for histological observation. SO-labeled RGCs were counted in vivo and FG-labeled RGCs were counted in retinal flat mounts. The time course of RGC death was examined in Calpastatin KO mice and wild type (WT) mice. Our in vivo imaging method revealed that SO-positive dead RGCs were mainly present from 4 to 6 days after ONC, and the peak of RGC death was after 5 days. Moreover, the number of SO-positive dead RGCs after 5 days differed significantly in the Calpastatin KO mice and the WT mice. Counting FG-labeled RGCs in isolated retinas confirmed these results. Thus, real-time imaging with SO was able to quickly quantify ONC-induced RGC death. This technique may aid research into RGC death and the development of new neuroprotective therapies for glaucoma.

Involvement of microRNA-181a and Bim in a rat model of retinal ischemia-reperfusion injury

AIM

To investigate the changes in the expression of microRNA-181a (miR-181a) and Bim in a rat model of retinal ischemia-reperfusion (RIR), to explore their target relationship in RIR and their involvement in regulating apoptosis of retinal ganglion cells (RGCs).

METHODS

Target gene prediction for miR-181a was performed with the aid of bioinformatics and Bim was identified as a potential target gene of miR-181a. A rat model of RIR was created by increasing the intraocular pressure. RGCs in the flatmounted retinas were labeled with Brn3, a marker for alive RGCs, by immunofluorescent staining. The changes in the number of RGCs after RIR were recorded. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) was used to determine the expression level of miR-181a in the retina. Bim/Brn3 double immunofluorescence was used to detect the localization of Bim. The expression of Bim in the retina was determined with the aids of Western blot and qRT-PCR.

RESULTS

Compared with the negative control group, the density of RGCs was significantly lower in the ischemia/reperfusion (I/R)-24h and I/R-72h groups (P<0.001). The expression level of miR-181a started to decrease at 0h after RIR, and further decreased at 24h and 72h compared with the negative control group (P<0.001). Bim was significantly upregulated at 12h after RIR (P<0.05) and reached peak at 24, 72h compared with the negative control group (P<0.01). Pearson correlation analysis showed that the expression level of Bim was negatively correlated with the expression level of miR-181a and the density of RGCs.

CONCLUSION

Bim may be a potential target gene of miR-181a. Both miR-181a and Bim are involved in RGCs death in RIR. RIR may promote RGCs apoptosis in the retina via downregulation of miR-181a and its inhibition on Bim expression.

Retinal Cell Degeneration in Animal Models

The aim of this review is to provide an overview of various retinal cell degeneration models in animal induced by chemicals (N-methyl-d-aspartate- and CoCl₂-induced), autoimmune (experimental autoimmune encephalomyelitis), mechanical stress (optic nerve crush-induced, light-induced) and ischemia (transient retinal ischemia-induced). The target regions, pathology and proposed mechanism of each model are described in a comparative fashion. Animal models of retinal cell degeneration provide insight into the underlying mechanisms of the disease, and will facilitate the development of novel effective therapeutic drugs to treat retinal cell damage.

Endogenous and Synthetic Cannabinoids as Therapeutics in Retinal Disease

The functional significance of cannabinoids in ocular physiology and disease has been reported some decades ago. In the early 1970s, subjects who smoked Cannabis sativa developed lower intraocular pressure (IOP). This led to the isolation of phytocannabinoids from this plant and the study of their therapeutic effects in glaucoma. The main treatment of this disease to date involves the administration of drugs mediating either the decrease of aqueous humour synthesis or the increase of its outflow and thus reduces IOP. However, the reduction of IOP is not sufficient to prevent visual field loss. Retinal diseases, such as glaucoma and diabetic retinopathy, have been defined as neurodegenerative diseases and characterized by ischemia-induced excitotoxicity and loss of retinal neurons. Therefore, new therapeutic strategies must be applied in order to target retinal cell death, reduction of visual acuity, and blindness. The aim of the present review is to address the neuroprotective and therapeutic potential of cannabinoids in retinal disease.

Current perspective of neuroprotection and glaucoma

Glaucoma is the second leading cause of blindness worldwide and is most notably characterized by progressive optic nerve atrophy and advancing loss of retinal ganglion cells (RGCs). The main concomitant factor is the elevated intraocular pressure (IOP). Existing treatments are focused generally on lowering IOP. However, both RGC loss and optic nerve atrophy can independently occur with IOP at normal levels. In recent years, there has been substantial progress in the development of neuroprotective therapies for glaucoma in order to restore vital visual function. The present review intends to offer a brief insight into conventional glaucoma treatments and discuss exciting current developments of mostly preclinical data in novel neuroprotective strategies for glaucoma that include recent advances in noninvasive diagnostics going beyond IOP maintenance for an enhanced global view. Such strategies now target RGC loss and optic nerve damage, opening a critical therapeutic window for preventative monitoring and treatment.

Decreased TNF Levels and Improved Retinal Ganglion Cell Survival in MMP-2 Null Mice Suggest a Role for MMP-2 as TNF Sheddase

Matrix metalloproteinases (MMPs) have been designated as both friend and foe in the central nervous system (CNS): while being involved in many neurodegenerative and neuroinflammatory diseases, their actions appear to be indispensable to a healthy CNS. Pathological conditions in the CNS are therefore often related to imbalanced MMP activities and disturbances of the complex MMP-dependent protease network. Likewise, in the retina, various studies in animal models and human patients suggested MMPs to be involved in glaucoma. In this study, we sought to determine the spatiotemporal expression profile of MMP-2 in the excitotoxic retina and to unravel its role during glaucoma pathogenesis. We reveal that intravitreal NMDA injection induces MMP-2 expression to be upregulated in the Müller glia. Moreover, MMP-2 null mice display attenuated retinal ganglion cell death upon excitotoxic insult to the retina, which is accompanied by normal glial reactivity, yet reduced TNF levels. Hence, we propose a novel in vivo function for MMP-2, as an activating sheddase of tumor necrosis factor (TNF). Given the pivotal role of TNF as a proinflammatory cytokine and neurodegeneration-exacerbating mediator, these findings generate important novel insights into the pathological processes contributing to glaucomatous neurodegeneration and into the interplay of neuroinflammation and neurodegeneration in the CNS.

Mangiferin Protects Retinal Ganglion Cells in Ischemic Mouse Retina via SIRT1

PURPOSE

To investigate whether mangiferin can increase the viability of retinal ganglion cells (RGCs) in ischemic mouse retina, and to determine the possible mechanism of neuroprotection.

METHODS

C57BL/6J mice underwent constant elevation of intraocular pressure for 60 min and received saline or mangiferin (30 mg/kg) intraperitoneally once daily until sacrifice. HIF-1α, GFAP and SIRT1 expression was assessed at 1, 4, and 7 days after retinal ischemia. Bax and Bcl-2 expression was also analyzed at 1 and 4 days. RGC survival was assessed by labeling flat-mounted retinas with Brn3a at 2 weeks after retinal ischemia. The effect of co-treatment with mangiferin and sirtinol (SIRT1 inhibitor) was also evaluated.

RESULTS

The expression of HIF-1α and GFAP was upregulated in saline-treated retinas within 7 days after ischemia. Mangiferin treatment suppressed this upregulation. The expression of SIRT1 was downregulated in saline-treated ischemic retinas. This downregulation was reversed by mangiferin treatment, resulting in a significant difference from saline-treated ischemic retinas. In mangiferin-treated ischemic retinas, Bax expression was downregulated, whereas Bcl-2 expression was upregulated in comparison with saline-treated ischemic retinas. Mangiferin treatment protected ischemic retinas against RGC loss. Treatment of sirtinol decreased the neuroprotective effect of mangiferin.

CONCLUSIONS

Our findings suggest that mangiferin has a neuroprotective effect on RGC through downregulation of HIF-1a and GFAP, and upregulation of SIRT1 in ischemic mouse retinas. We suggest that mangiferin might be a potential neuroprotective agent against RGC loss under oxidative stress.

Analysis of the expression and polymorphism of APOE, HSP, BDNF, and GRIN2B genes associated with the neurodegeneration process in the pathogenesis of primary open angle glaucoma

Glaucoma is characterized by optic neuropathy of the RGC or retinal nerve fiber. The aim of this study was to evaluate a relationship between the neurodegenerative genes' polymorphisms of the APOE (rs449647), BDNF (rs2030324), GRIN2B (rs3764028), and HSP70-1 (rs1043618) and the occurrence risk of POAG and to investigate its effect on allele-specific gene expression. Genomic DNA was extracted from peripheral blood. Analysis of the genes' polymorphisms was performed using PCR-RFLP. The level of mRNA expression was determined by QRT-PCR. We showed a statistically significant association of BDNF and APOE genes' polymorphisms with a risk of POAG occurrence. There was a statistically significant association of the rs2030324 polymorphism with progression of POAG based on cup disc ratio value and rs1043618 polymorphism based on nerve fiber index and rim area. Furthermore, we found that mean HSP70-1 mRNA expression was significantly lower in the case of individuals with the G/G genotype than in the case of minor allele carriers, that is, G/C and C/C. We also found that BDNF and HSP70-1 expression level are associated with the progression of POAG based on rim area value. In conclusion, our results suggest that BDNF, APOE, and HSP70-1 genes might be associated with a risk of POAG occurrence in the Polish population.

Neuroprotection in the treatment of glaucoma--A focus on connexin43 gap junction channel blockers

Glaucoma is a form of optic neuropathy and a common cause of blindness, affecting over 60 million people worldwide with an expected rise to 80 million by 2020. Successful treatment is challenging due to the various causes of glaucoma, undetectable symptoms at an early stage and inefficient delivery of drugs to the back of the eye. Conventional glaucoma treatments focus on the reduction of elevated intraocular pressure (IOP) using topical eye drops. However, their efficacy is limited to patients who suffer from high IOP glaucoma and do not address the underlying susceptibility of retinal ganglion cells (RGC) to degeneration. Glaucoma is known as a neurodegenerative disease which starts with RGC death and eventually results in damage of the optic nerve. Neuroprotective strategies therefore offer a novel treatment option for glaucoma by not only preventing neuronal loss but also disease progression. This review firstly gives an overview of the pathophysiology of glaucoma as well as current treatment options including conventional and novel delivery strategies. It then summarizes the rational for neuroprotection as a novel therapy for glaucomatous neuropathies and reviews current potential neuroprotective strategies to preserve RGC, with a focus on connexin43 (Cx43) gap junction channel blockers.

Visual improvement following glaucoma surgery: a case report

Background

Glaucoma is a progressive optic neuropathy and a leading cause of blindness. Neural losses from glaucoma are irreversible, and so the aim of glaucoma treatment is to slow progression and minimize the risk of further damage. Functional improvement with treatment is not expected. We report the case of a patient who experienced a significant improvement in vision following glaucoma surgery and review the literature regarding this phenomenon.

Case presentation

A 64-year old male presented with a 13-month history of gradual vision loss in the right eye to the extent that he could only perceive hand movements. His intraocular pressure (IOP) measured 50 mmHg and he was found to have advanced primary open angle glaucoma. Medical treatment was commenced and he underwent a successful right Mitomycin C-augmented trabeculectomy. Unexpectedly he experienced marked improvement in vision post-operatively, with improvements maintained through six months of follow-up. At his most recent visit visual acuity was 6/18 in the affected eye. Although the mechanism of improved vision cannot be proven it is likely that successful lowering of IOP resulted in some reversal of retinal ganglion cell dysfunction. Important factors may have included his relatively young age, high IOP and short duration of symptoms.

Conclusion

Although rare, functional improvements may occur following trabeculectomy. Glaucoma surgery should be offered early to those with advanced disease, and considered even in those with reduced visual acuity.

Nestin, A New Marker, Expressed in Müller Cells Following Retinal Injury

Purpose:

To investigate whether nestin would be a useful marker for retinal injury and also to ascertain a better understanding of the roles of Müller cells in the injured retina by the use of damaged rat retina.

Methods:

A total of 33 adult female Wistar rats were used in this study. Three were used as controls and the remaining as retinal injury modes (6 for hypoxia; 15 for experimental glaucoma and 9 for optic nerve transection). Double immunofluorescence labeling was carried out between nestin and glutamine synthetase (GS), and between glial fibrillary acidic protein (GFAP) and GS antisera in normal and pathological retinae.

Results:

The results showed that there were no nestin nor GFAP staining in mature Müller cells of the normal retina. A major finding was that nestin expression was induced in Müller cells subjected to hypoxia, glaucoma and optic nerve transection.

Conclusions:

These results suggest that nestin as well as GFAP (even more sensitive than GFAP) are useful and reliable biomarkers for retinal damage. The more intense expression of nestin, GFAP and GS in the end-feet of Müller cells suggest that they may help to maintain the retinal structural integrity and to enhance functional recovery in various retinal diseases.

Role of C/EBP homologous protein in retinal ganglion cell death after ischemia/reperfusion injury

PURPOSE

To investigate the role of C/EBP homologous protein (CHOP), a proapoptotic protein, and the unfolded protein response (UPR) marker that is involved in endoplasmic reticulum (ER) stress-mediated apoptosis in mouse retinal ganglion cell (RGC) death following ischemia/reperfusion (I/R) injury.

METHODS

Retinal I/R injury was induced in adult C57BL/6J wild-type (WT) and CHOP knockout (Chop(-/-)) mice by raising IOP to 120 mm Hg for 60 minutes. Expression of CHOP and other UPR markers was studied by Western blot and immunohistochemistry. Retinal ganglion cell counts were performed in retinal flat mounts stained with an RGC marker. Retinal ganglion cell function was evaluated by scotopic threshold response (STR) electroretinography.

RESULTS

In WT mice, retinal CHOP was upregulated by 30% in I/R-injured eyes compared to uninjured eyes 3 days after injury (P < 0.05). Immunohistochemistry confirmed CHOP upregulation specifically in RGCs. CHOP knockout did not affect baseline RGC density or STR amplitude. Ischemia/reperfusion injury decreased RGC densities and STR amplitudes in both WT and Chop(-/-) mice. However, survival of RGCs in I/R-injured Chop(-/-) mouse was 48% higher (P < 0.05) than that in I/R-injured WT mouse 3 days after I/R injury. Similarly, RGC density was significantly higher in Chop(-/-) eyes at 7, 14, and 28 days after I/R injury. Scotopic threshold response amplitudes of Chop(-/-) mice were significantly higher at 3 and 7 days after I/R than those of WT mice.

CONCLUSIONS

Absence of CHOP partially protects against RGC loss and reduction in retinal function after I/R injury, indicating that CHOP and, thus, ER stress play an important role in RGC apoptosis in retinal I/R injury.

Deletion of thioredoxin-interacting protein preserves retinal neuronal function by preventing inflammation and vascular injury

BACKGROUND AND PURPOSE

Retinal neurodegeneration is an early and critical event in several diseases associated with blindness. Clinically, therapies that target neurodegeneration fail. We aimed to elucidate the multiple roles by which thioredoxin-interacting protein (TXNIP) contributes to initial and sustained retinal neurodegeneration.

EXPERIMENTAL APPROACH

Neurotoxicity was induced by intravitreal injection of NMDA into wild-type (WT) and TXNIP-knockout (TKO) mice. The expression of apoptotic and inflammatory markers was assessed by immunohistochemistry, elisa and Western blot. Microvascular degeneration was assessed by periodic acid-Schiff and haematoxylin staining and retinal function by electroretinogram.

KEY RESULTS

NMDA induced early (1 day) and significant retinal PARP activation, a threefold increase in TUNEL-positive nuclei and 40% neuronal loss in ganglion cell layer (GCL); and vascular permeability in WT but not TKO mice. NMDA induced glial activation, expression of TNF-α and IL-1β that co-localized with Müller cells in WT but not TKO mice. In parallel, NMDA triggered the expression of NOD-like receptor protein (NLRP3), activation of caspase-1, and release of IL-1β and TNF-α in primary WT but not TKO Müller cultures. After 14 days, NMDA induced 1.9-fold microvascular degeneration, 60% neuronal loss in GCL and increased TUNEL-labelled cells in the GCL and inner nuclear layer in WT but not TKO mice. Electroretinogram analysis showed more significant reductions in b-wave amplitudes in WT than in TKO mice.

CONCLUSION AND IMPLICATIONS

Targeting TXNIP expression prevented early retinal ganglion cell death, glial activation, retinal inflammation and secondary neuro/microvascular degeneration and preserved retinal function. TXNIP is a promising new therapeutic target for retinal neurodegenerative diseases.

Time-dependent Gene Profiling Indicates the Presence of Different Phases for Ischemia/Reperfusion Injury in Retina

Ischemia/reperfusion (IR) injury has been associated with several retinal pathologies, and a few genes/gene products have been linked to IR injury. However, the big picture of temporal changes, regarding the affected gene networks, pathways, and processes remains to be determined. The purpose of the present study was to investigate initial, intermediate, and later stages to characterize the etiology of IR injury in terms of the pathways affected over time. Analyses indicated that at the initial stage, 0-hour reperfusion following the ischemic period, the ischemia-associated genes were related to changes in metabolism. In contrast, at the 24-hour time point, the signature events in reperfusion injury include enhanced inflammatory and immune responses as well as cell death indicating that this would be a critical period for the development of any interventional therapeutic strategies. Genes in the signal transduction pathways, particularly transmitter receptors, are downregulated at this time. Activation of the complement system pathway clearly plays an important role in the later stages of reperfusion injury. Together, these results demonstrate that the etiology of injury related to IR is characterized by the appearance of specific patterns of gene expression at any given time point during retinal IR injury. These results indicate that evaluation of treatment strategies with respect to time is very critical.

S-Allyl L-Cysteine Protects the Retina Against Kainate Excitotoxicity in the Rat

Excitotoxicity has been proposed to play a pivotal role in retinal ischemia. Retinal ischemia-associated ocular disorders are vision threatening. The aim was to also examine whether and how S-allyl L-cysteine (SAC) can protect the retina against kainate excitotoxicity. In vivo retinal excitotoxicity was induced by an intravitreous injection of 100 μM kainate into a Wistar rat eye for 1 day. The management and mechanisms involved in the processes were evaluated by electrophysiology, immunohistochemistry, histopathology, and various biochemical approaches. In the present study, the cultured retinal cells were shown to possess kainate receptors. The defined retinal excitotoxic changes were characterized by a decrease in electroretinogram (ERG) b-wave amplitudes, a loss of the fluorogold retrograde labeled retinal ganglion cells (RGCs), an increase in the apoptotic cells in the RGC layer, and an increase in vimentin or glial fibrillary acidic protein (GFAP) immunoreactivity, a marker for Müller cells. An up-regulation in the mRNA levels of inducible nitric oxide synthase (iNOS) and matrix metalloproteinases-9 (MMPs-9) was also detected in the retina subjected to kainate excitoxicity. Importantly, the excitotoxicity-induced alterations were significantly blunted when 100 μM SAC and/or the kainate receptor antagonist CNQX was applied. Conclusively, SAC would seem to protect the retina against kainate excitotoxicity via an inhibition of the up-regulation of iNOS and MMP-9 as well as a modulation of glial activation and apoptosis.

Retinal ganglion cell (RGC) programmed necrosis contributes to ischemia-reperfusion-induced retinal damage

Retinal ischemia-reperfusion (IR) injury remains a common cause of blindness and has a final pathway of retinal ganglion cell (RGC) death by apoptosis and necrosis. RGC apoptosis was intensively studied in IR injury, while RGC necrosis did not receive nearly enough consideration since it was viewed as an accidental and unregulated cellular event. However, there is evidence that necrosis, like apoptosis, can be implemented by a programmed mechanism. In this study, we tested the role of RGC programmed necrosis (necroptosis) in IR-induced retinal injury. We employed the mouse model of retinal IR injury for in vivo experiments. The oxygen and glucose deprivation (OGD) model was used as an IR model in vitro. Primary RGCs were isolated by an immunopanning technique. Necrostatin 1 (Nec1) was used to inhibit necroptosis in in vitro and in vivo experiments. The changes in gene expression were assessed by quantitative RT-PCR. The distribution of proteins in the retina and in RGC cultures was evaluated by immunohistochemistry and immunocytochemistry, respectively. Our data suggest that proteins (Ripk1 and Ripk3), which initiate necroptosis, were present in normal and ischemic RGCs. Treatment with Nec1 significantly reduced retinal damage after IR. Increased RGC survival and reduced RGC necrosis following OGD were observed in Nec1-treated cultures. We found significantly reduced expression of genes coding pro-inflammatory markers Il1b, Ccl5, Cxcl10, Nos2 and Cybb in Nec1-treated ischemic retinas. Thus, our findings suggest that RGC necroptosis contributes to retinal damage after IR through direct loss of cells and induction of associated inflammatory responses.

Activation of muscarinic receptors protects against retinal neurons damage and optic nerve degeneration in vitro and in vivo models

AIMS

Muscarinic acetylcholine receptor agonist pilocarpine reduces intraocular pressure (IOP) of glaucoma mainly by stimulating ciliary muscle contraction and then increasing aqueous outflow. It is of our great interest to know whether pilocarpine has the additional properties of retinal neuroprotection independent of IOP lowering in vitro and in vivo models.

METHODS

In rat primary retinal cultures, cell viability was measured using an MTT assay and the trypan blue exclusion method, respectively. Retinal ganglion cells (RGCs) were identified by immunofluorescence and quantified by flow cytometry. For the in vivo study, the retinal damage after retinal ischemia/reperfusion injury in rats was evaluated by histopathological study using hematoxylin and eosin staining, transmission electron microscopy, and immunohistochemical study on cleaved caspase-3, caspase-3, and ChAT.

RESULTS

Pretreatment of pilocarpine attenuated glutamate-induced neurotoxicity of primary retinal neurons in a dose-dependent manner. Protection of pilocarpine in both retinal neurons and RGCs was largely abolished by the nonselective muscarinic receptor antagonist atropine and the M1-selective muscarinic receptor antagonist pirenzepine. After ischemia/reperfusion injury in retina, the inner retinal degeneration occurred including ganglion cell layer thinning and neuron lost, and the optic nerve underwent vacuolar changes. These degenerative changes were significantly lessened by topical application of 2% pilocarpine. In addition, the protective effect of pilocarpine on the ischemic rat retina was favorably reflected by downregulating the expression of activated apoptosis marker cleaved caspase-3 and caspase-3 and upregulating the expression of cholinergic cell marker ChAT.

CONCLUSIONS

Taken together, this highlights pilocarpine through the activation of muscarinic receptors appear to afford significant protection against retinal neurons damage and optic nerve degeneration at clinically relevant concentrations. These data also further support muscarinic receptors as potential therapeutic neuroprotective targets in glaucoma.

Secondary neuroprotective effects of hypotensive drugs and potential mechanisms of action

Primary open-angle glaucoma, a long-term degenerative ocular neuropathy, remains a significant cause of vision impairment worldwide. While many risk factors have been correlated with increased risk for primary open-angle glaucoma, intraocular pressure (IOP) remains the only modifiable risk factor and primary therapeutic target. Pharmacologic therapies are administered topically; these include α(2)-agonists, β-antagonists, prostaglandin analogs and carbonic anhydrase inhibitors. Some of these topical medications exhibit secondary neuroprotective effects independent of their effect on IOP. This review covers the possible mechanisms of neuroprotection stimulated by drugs currently marketed for the lowering of IOP, based on known literature. While the neuroprotective properties of many glaucoma pharmaceuticals are promising from an experimental standpoint, key challenges for the development of new clinical practices include unknown systemic side effects, limited methods of drug delivery to the retina and optic nerve, and development of extended-release formulations.

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.

Lipid peroxidation: pathophysiological and pharmacological implications in the eye

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

Ocular disposition of the hemiglutarate ester prodrug of ∆⁹-Tetrahydrocannabinol from various ophthalmic formulations

PURPOSE

The overall goal of this project is to enhance ocular delivery of ∆(9)-Tetrahydrocannabinol (THC) through the topical route.

METHODS

Solubility, stability and in vitro transcorneal permeability of the relatively hydrophilic hemiglutarate ester derivative, THC-HG, was studied in the presence of surfactants. The solutions were characterized with respect to micelle size, zeta potential and solution viscosity. In vivo studies were carried out in New Zealand albino rabbits. A previously reported promising THC-HG ion-pair formulation was also studied in vivo.

RESULTS

Aqueous solubility and stability and in vitro transcorneal permeability of THC-HG was enhanced significantly in the presence of surfactants. THC levels in the ocular tissues (except cornea) were found to be below detection limits from mineral oil, surfactant or emulsion based formulations containing THC. In contrast, micellar and ion pair based THC-HG formulations produced significantly higher total THC concentrations in the anterior ocular chamber.

CONCLUSION

In this study, although delivery of THC to the anterior chamber ocular tissues could be significantly increased through the prodrug and formulation approaches tested, further studies are needed to increase penetration to the back-of-the eye.

Acid-sensing ion channels under hypoxia

Hypoxia represents the lack of oxygen below the basic level, and the range of known channels related to hypoxia is continually increasing. Since abnormal hypoxia initiates pathological processes in numerous diseases via, to a great degree, producing acidic microenvironment, the significance of these channels in this environment has, until now, remained completely unknown. However, recent discovery of acid-sensing ion channels (ASICs) have enhanced our understanding of the hypoxic channelome. They belong to the degenerin/epithelial Na (+) channel family and function once extracellular pH decreases to a certain level. So does the ratiocination emerge that ASICs participate in many hypoxia-induced pathological processes, including pain, apoptosis, malignancy, which all appear to involve them. Since evidence suggests that activity of ASICs is altered under pathological hypoxia, future studies are needed to deeply explore the relationship between ASICs and hypoxia, which may provide a progressive understanding of hypoxic effects in cancer, arthritis, intervertebral disc degeneration, ischemic brain injury and so on.

The effect of melatonin on retinal ganglion cell survival in ischemic retina

Our objective was to determine whether melatonin increases retinal ganglion cell (RGC) survival in ischemic mouse retina. Transient retinal ischemia was induced by an acute elevation of intraocular pressure in C57BL/6 mice. To evaluate the effect of melatonin on retinal ischemia, an equal amount of either melatonin or vehicle was intraperitoneally injected into the mice 1 hour before ischemia, at the time of ischemia, and 1 hour after ischemia. Hypoxia inducible factor 1α (HIF-1α) and glial fibrillary acidic protein (GFAP) expression were assessed 6, 12, and 24 hours after ischemia-reperfusion by Western blot. RGC survival was measured 2 weeks after ischemia-reperfusion. The expression of HIF-1α and GFAP peaked 24 hours after ischemia-reperfusion in ischemic retina. The treatment of ischemic retina with melatonin resulted in the inhibition of increased expression of HIF-1α and GFAP. RGC survival was greater in retinas treated with melatonin than in retinas treated with vehicle 2 weeks after ischemia-reperfusion. On the basis of our results, we suggest that melatonin treatment increased RGC survival in ischemic mouse retina. The neuroprotective effect of melatonin is mediated by the inhibition of HIF-1α stabilization and reduced activity of glial cells in ischemic mouse retina.

Edible seaweed, Eisenia bicyclis, protects retinal ganglion cells death caused by oxidative stress

The purpose of the present study was to determine whether edible seaweed, Eisenia bicyclis, is effective in blunting the negative influence of N-methyl-D-aspartate (NMDA) on rat retinas and of oxidative stress-induced transformed retinal ganglion cell (RGC-5 cell line) death. The ethanol extract of E. bicyclis (EEEB) significantly attenuated the negative insult of L: -buthionine-(S,R)-sulfoximine plus glutamate on RGC-5 cells. Treatment of the RGC-5 cells with EEEB reduced the reactive oxygen species and recovered the reduced glutathione level caused by various radical species such as H(2)O(2), OH·, or O(2)·(-). Moreover, EEEB inhibited lipid peroxidation on rat brain homogenates caused by sodium nitroprusside. Applying NMDA to the retina affected the thickness of the inner plexiform layer (IPL) and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) produced a positive effect on ganglion cells. Importantly, EEEB protected the thinning of IPL and increased TUNEL positive cells in the ganglion cell layer (GCL). Five phlorotannin derivatives were isolated using chromatographic methods and liquid chromatography-mass spectroscopy analysis which has been known as an antioxidant. In conclusion, EEEB has a neuroprotective effect in vitro and in vivo. Furthermore, the major constituents of this extract, phlorotannins, could possibly be active compounds due to their antioxidative potency.

Expression of the Na+-K+-2Cl(-)-cotransporter 2 in the normal and pressure-induced ischemic rat retina

Purpose

To evaluate the expression of the Na⁺-K⁺-2Cl^--cotransporter 2 (NKCC2) in the ischemic rat retina.

Methods

Retinal ischemia was induced by pressures 90 to 120 mmHg, above systemic systolic pressure. Immunohistochemistry and western blot analysis were performed.

Results

NKCC2 is expressed in the normal retina and its expression is increased by ischemia caused by intraocular pressure elevation. NKCC2 immunoreactivity was observed mainly in axon bundles of ganglion cells and horizontal cell processes in the retina. NKCC2 expression continuously increased with a peak value 3 days (to 415% of normal levels) after ischemic injury, and then gradually decreased to 314% of controls until 2 weeks post injury. The mean density of NKCC2-labeled ganglion cells per mm² changed from 1,255 ± 109 in normal retinas to 391 ± 49 and 185 ± 37 at 3 days and 2 weeks after ischemia, respectively (p < 0.05), implying cell death of ganglion cells labeled with NKCC2.

Conclusions

Taken together, these results suggest that NKCC2, which is expressed in retinal ganglion and horizontal cells, may contribute to cell death by ischemic injury in the retina, although the molecular mechanisms involved remain to be clarified.

Diosmin protects rat retina from ischemia/reperfusion injury

OBJECTIVE

Diosmin, a natural flavone glycoside, possesses antioxidant activity and has been used to alleviate ischemia/reperfusion (I/R) injury. The aim of this study was to clarify whether the administration of diosmin has a protective effect against I/R injury induced using the high intraocular pressure (IOP) model in rat retina, and to determine the possible antioxidant mechanisms involved.

METHODS

Retinal I/R injury was induced in the rats by elevating the IOP to 110 mmHg for 60 min. Diosmin (100 mg/kg) or vehicle solution was administered intragastrically 30 min before the onset of ischemia and then daily after I/R injury until the animals were sacrificed. The levels of malondialdehyde (MDA) and the activities of total-superoxide dismutase (T-SOD), glutathione peroxidase (GSH-Px), and catalase (CAT) in the retinal tissues were determined 24 h after I/R injury. At 7 days post-I/R injury, electroretinograms (ERGs) were recorded, and the density of surviving retinal ganglion cells (RGCs) was estimated by counting retrograde tracer-labeled cells in whole-mounted retinas. Retinal histological changes were also examined and quantified using light microscopy.

RESULTS

Diosmin significantly decreased the MDA levels and increased the activities of T-SOD, GSH-Px, and CAT in the retina of rats compared with the ischemia group (P<0.05), and suppressed the I/R-induced reduction in the a- and b-wave amplitudes of the ERG (P<0.05). The thickness of the entire retina, inner nuclear layer, inner plexiform layer, and outer retinal layer and the number of cells in the ganglion cell layer were significantly less after I/R injury (P<0.05), and diosmin remarkably ameliorated these changes on retinal morphology. Diosmin also attenuated the I/R-induced loss of RGCs of the rat retina (P<0.05).

CONCLUSION

Diosmin protected the retina from I/R injury, possibly via a mechanism involving the regulation of oxidative parameters.

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.

Deleterious role of anti-high mobility group box 1 monoclonal antibody in retinal ischemia-reperfusion injury

PURPOSE

To investigate the effect of anti-high mobility group box 1 (HMGB1) monoclonal antibody (mAb) against ischemia-reperfusion injury in the rat retina.

MATERIALS AND METHODS

Retinal ischemia was induced by increasing and then maintaining intraocular pressure at 130 mmHg for 45 min. An intraperitoneal injection of anti-HMGB1 mAb was administered 30 min before ischemia. Retinal damage was evaluated at 7 days after the ischemia. Immunohistochemistry and image analysis were used to measure changes in the levels of reactive oxygen species (ROS) and the localization of anti-HMGB1 mAb. Dark-adapted full-field electroretinography (ERG) was also performed.

RESULTS

Pretreatment with anti-HMGB1 mAb significantly enhanced the ischemic injury of the retina. HMGB1 expression increased at 6-12 h after ischemia in the retina. After the ischemia, production of ROS was detected in retinal cells. However, pretreatment with anti-HMGB1 mAb increased the production of ROS. On the seventh postoperative day, the amplitudes of both the ERG a- and b-waves were significantly higher in the vehicle group than in the groups pretreated with anti-HMGB1 mAb.

CONCLUSIONS

The current in vivo model of retinal injury demonstrated that anti-HMGB1 mAb plays a large deleterious role in ischemia-reperfusion injury. In order to develop neuroprotective therapeutic strategies for acute retinal ischemic disorders, further studies on anti-HMGB1 mAb function are needed.

Thioredoxin interacting protein is a novel mediator of retinal inflammation and neurotoxicity

BACKGROUND AND PURPOSE

Up-regulation of thioredoxin interacting protein (TXNIP), an endogenous inhibitor of thioredoxin (Trx), compromises cellular antioxidant and anti-apoptotic defences and stimulates pro-inflammatory cytokines expression, implying a role for TXNIP in apoptosis. Here we have examined the causal role of TXNIP expression in mediating retinal neurotoxicity and assessed the neuroprotective actions of verapamil, a calcium channel blocker and an inhibitor of TXNIP expression.

EXPERIMENTAL APPROACH

Retinal neurotoxicity was induced by intravitreal injection of NMDA in Sprague-Dawley rats, which received verapamil (10 mg·kg(-1), p.o.) or vehicle. Neurotoxicity was examined by terminal dUTP nick-end labelling assay and ganglion cell count. Expression of TXNIP, apoptosis signal-regulating kinase 1 (ASK-1), NF-κB, p38 MAPK, JNK, cleaved poly-ADP-ribose polymerase (PARP), caspase-3, nitrotyrosine and 4-hydroxy-nonenal were examined by Western and slot-blot analysis. Release of TNF-α and IL-1β was examined by elisa.

KEY RESULTS

NMDA injection enhanced TXNIP expression, decreased Trx activity, causing increased oxidative stress, glial activation and release of TNF-α and IL-1β. Enhanced TXNIP expression disrupted Trx/ASK-1 inhibitory complex leading to release of ASK-1 and activation of the pro-apoptotic p38 MAPK/JNK pathway, as indicated by cleaved PARP and caspase-3 expression. Treatment with verapamil blocked these effects.

CONCLUSION AND IMPLICATIONS

Elevated TXNIP expression contributed to retinal neurotoxicity by three different mechanisms, inducing release of inflammatory mediators such as TNF-α and IL-1β, altering antioxidant status and disrupting the Trx-ASK-1 inhibitory complex leading to activation of the p38 MAPK/JNK apoptotic pathway. Targeting TXNIP expression is a potential therapeutic target for retinal neurodegenerative disease.

From oxygen to erythropoietin: relevance of hypoxia for retinal development, health and disease

Photoreceptors and other cells of the retina consume large quantities of energy to efficiently convert light information into a neuronal signal understandable by the brain. The necessary energy is mainly provided by the oxygen-dependent generation of ATP in the numerous mitochondria of retinal cells. To secure the availability of sufficient oxygen for this process, the retina requires constant blood flow through the vasculature of the retina and the choroid. Inefficient supply of oxygen and nutrients, as it may occur in conditions of disturbed hemodynamics or vascular defects, results in tissue ischemia or hypoxia. This has profound consequences on retinal function and cell survival, requiring an adaptational response by cells to cope with the reduced oxygen tension. Central to this response are hypoxia inducible factors, transcription factors that accumulate under hypoxic conditions and drive the expression of a large variety of target genes involved in angiogenesis, cell survival and metabolism. Prominent among these factors are vascular endothelial growth factor and erythropoietin, which may contribute to normal angiogenesis during development, but may also cause neovascularization and vascular leakage under pathologically reduced oxygen levels. Since ischemia and hypoxia may have a role in various retinal diseases such as diabetic retinopathy and retinopathy of prematurity, studying the cellular and molecular response to reduced tissue oxygenation is of high relevance. In addition, the concept of preconditioning with ischemia or hypoxia demonstrates the capacity of the retina to activate endogenous survival mechanisms, which may protect cells against a following noxious insult. Part of these mechanisms is the local production of protective factors such as erythropoietin. Due to its plethora of effects in the retina including neuro- and vaso-protective activities, erythropoietin has gained strong interest as potential therapeutic factor for retinal degenerative diseases.

The effects and underlying mechanisms of S-allyl l-cysteine treatment of the retina after ischemia/reperfusion

PURPOSE

Retinal ischemia-associated ocular disorders are vision-threatening. The aim of the present study was to examine whether S-allyl l-cysteine (SAC) is able to protect against retina ischemia/reperfusion injury.

METHODS

In vivo, retinal ischemia in the rat was induced by raising intraocular pressure (IOP) to 120 mmHg for 60 min. In vitro, an ischemic-like insult, namely oxidative stress, was established by incubating retinal ganglion cell-5 (RGC-5) with 500 μM H(2)O(2) for 24 h. The mechanisms involved in these processes were evaluated by electrophysiology, immunohistochemistry, and molecular biological approaches.

RESULTS

The retinal changes caused by the high IOP were characterized by a decrease in electroretinogram b-wave amplitudes, a loss of choline acetyltransferase immunolabeling amacrine cell bodies/neuronal processes, and an upregulation of the mRNA levels of hypoxia-inducible factor-1α (HIF-1α), vascular endothelium growth factor (VEGF), and matrix metalloproteinase-9 (MMP-9). The increased protein levels of HIF-1α, VEGF, and MMP-9 were also seen in RGC-5 cells subjected to defined oxidative stress. Of clinical importance, the ischemic/ischemic-like detrimental effects were concentration-dependently (least effect at 25 μM) and/or significantly (50 and/or 100 μM) blunted when SAC was applied 15 min before retinal ischemia or ischemic-like insult, respectively.

CONCLUSION

SAC would seem to protect against retinal ischemia by acting as an antioxidant and inhibiting the upregulation of HIF-1α, VEGF, and MMP-9.

The neuroprotective effects of lidocaine and methylprednisolone in a rat model of retinal ischemia-reperfusion injury

Retinal ischemia is a common cause of visual impairment for humans and animals. The neuroprotective effects of lidocaine (LDC) and methylprednisolone (MP) upon retinal ischemic injury were investigated in a rat model. Sprague-Dawley rats were divided into 3 groups, the IR control, LDC and MP. A very high intraocular pressure (HIOP) and retinal ischemia were induced. In LDC group, LDC bolus (1.5 mg/kg) was i.v. injected 30 min before ischemia and then a constant rate infusion (CRI) with 2 mg/kg/hr was given until 60 min after reperfusion. In MP group, MP bolus (30 mg/kg) was i.v. administered twice at 2 min before and immediately after ischemia, respectively. The HIOP damage to retina was evaluated by electroretinogram (ERG) and morphometrical histology. The functional analysis of the retina by ERG revealed a 35.2% reduction of a-wave in the IR group, 49.7% reduction in the LDC group but no significant change in the MP group compared to normal controls. An 81.0% reduction of b-wave was observed in the IR group, 80.7% reduction in the LDC group and 17.6% reduction in the MP group. In the morphometrical histology, the retinal inner plexiform layer/outer nuclear layer (IPL/ONL) ratio was reduced to 48.8% in the IR group, 80.1% in the LDC group and 96.2% in MP group. In conclusion, the MP showed significantly good neuroprotective effects on retinal IR injury, and the LDC showed moderate neuroprotective effects demonstrated in retinal structure but not in retinal function.