Effect of geranylgeranylacetone on the protection of retinal ganglion cells in a mouse model of normal tension glaucoma

Drug Treatment Attenuates Retinal Ganglion Cell Death by Inhibiting Collapsin Response Mediator Protein 2 Phosphorylation in Mouse Models of Normal Tension Glaucoma

Normal tension glaucoma (NTG) is a progressive neurodegenerative disease in glaucoma families. Typical glaucoma develops because of increased intraocular pressure (IOP), whereas NTG develops despite normal IOP. As a subtype of open-angle glaucoma, NTG is characterized by retinal ganglion cell (RGC) degeneration, gradual loss of axons, and injury to the optic nerve. The relationship between glutamate excitotoxicity and oxidative stress has elicited great interest in NTG studies. We recently reported that suppressing collapsin response mediator protein 2 (CRMP2) phosphorylation in S522A CRMP2 mutant (CRMP2 KIKI) mice inhibited RGC death in NTG mouse models. This study evaluated the impact of the natural compounds huperzine A (HupA) and naringenin (NAR), which have therapeutic effects against glutamate excitotoxicity and oxidative stress, on inhibiting CMRP2 phosphorylation in mice intravitreally injected with N-methyl-D-aspartate (NMDA) and GLAST mutant mice. Results of the study demonstrated that HupA and NAR significantly reduced RGC degeneration and thinning of the inner retinal layer, and inhibited the elevated CRMP2 phosphorylation. These treatments protected against glutamate excitotoxicity and suppressed oxidative stress, which could provide insight into developing new effective therapeutic strategies for NTG.

Rodent genetically modified models of glaucoma

Glaucoma, one of the leading causes of irreversible blindness worldwide, is a complex and heterogenous disease. While environmental factors are important, it is well-recognized that the disease has a strong heritable component. With the advent of large-cohort genome wide association studies, a myriad of genetic risk loci has been linked to different forms of glaucoma. Animal models have been an indispensable tool in characterizing these loci, especially if they lie within coding regions in the genome. Not only do these models connect genotype to phenotype, advancing our understanding of glaucoma pathogenesis in the process, they also have valuable utility as a platform for the pre-clinical testing of potential therapies. In this review, we will outline genetic models used for studying the major forms of glaucoma, including primary open angle glaucoma, normal tension glaucoma, primary angle closure glaucoma, pigmentary glaucoma, pseudoexfoliation glaucoma, and early onset glaucoma, including congenital and developmental glaucoma, and how studying these models have helped shed light on human glaucoma.

Novel Role of Molecular Hydrogen: The End of Ophthalmic Diseases?

Molecular hydrogen (H2) is a colorless, odorless, and tasteless gas which displays non-toxic features at high concentrations. H2 can alleviate oxidative damage, reduce inflammatory reactions and inhibit apoptosis cascades, thereby inducing protective and repairing effects on cells. H2 can be transported into the body in the form of H2 gas, hydrogen-rich water (HRW), hydrogen-rich saline (HRS) or H2 produced by intestinal bacteria. Accumulating evidence suggest that H2 is protective against multiple ophthalmic diseases, including cataracts, dry eye disease, diabetic retinopathy (DR) and other fields. In particular, H2 has been tested in the treatment of dry eye disease and corneal endothelial injury in clinical practice. This medical gas has brought hope to patients suffering from blindness. Although H2 has demonstrated promising therapeutic potentials and broad application prospects, further large-scale studies involving more patients are still needed to determine its optimal application mode and dosage. In this paper, we have reviewed the basic characteristics of H2, and its therapeutic effects in ophthalmic diseases. We also focus on the latest progress in the administration approaches and mechanisms underlying these benefits.

Immunomodulatory and Antioxidant Drugs in Glaucoma Treatment

Glaucoma, a group of diseases characterized by progressive retinal ganglion cell loss, cupping of the optic disc, and a typical pattern of visual field defects, is a leading cause of severe visual impairment and blindness worldwide. Elevated intraocular pressure (IOP) is the leading risk factor for glaucoma development. However, glaucoma can also develop at normal pressure levels. An increased susceptibility of retinal ganglion cells to IOP, systemic vascular dysregulation, endothelial dysfunction, and autoimmune imbalances have been suggested as playing a role in the pathophysiology of normal-tension glaucoma. Since inflammation and oxidative stress play a role in all forms of glaucoma, the goal of this review article is to present an overview of the inflammatory and pro-oxidant mechanisms in the pathophysiology of glaucoma and to discuss immunomodulatory and antioxidant treatment approaches.

Oxidative Stress: A Suitable Therapeutic Target for Optic Nerve Diseases?

Optic nerve disorders encompass a wide spectrum of conditions characterized by the loss of retinal ganglion cells (RGCs) and subsequent degeneration of the optic nerve. The etiology of these disorders can vary significantly, but emerging research highlights the crucial role of oxidative stress, an imbalance in the redox status characterized by an excess of reactive oxygen species (ROS), in driving cell death through apoptosis, autophagy, and inflammation. This review provides an overview of ROS-related processes underlying four extensively studied optic nerve diseases: glaucoma, Leber's hereditary optic neuropathy (LHON), anterior ischemic optic neuropathy (AION), and optic neuritis (ON). Furthermore, we present preclinical findings on antioxidants, with the objective of evaluating the potential therapeutic benefits of targeting oxidative stress in the treatment of optic neuropathies.

Regulatory mechanisms of retinal ganglion cell death in normal tension glaucoma and potential therapies

Normal tension glaucoma (NTG) is a multifactorial optic neuropathy characterized by normal intraocular pressure, progressive retinal ganglion cell (RGC) death, and glaucomatous visual field loss. Recent studies have described the mechanisms underlying the pathogenesis of NTG. In addition to controlling intraocular pressure, neuroprotection and reduction of RGC degeneration may be beneficial therapies for NTG. In this review, we summarized the main regulatory mechanisms of RGC death in NTG, including autophagy, glutamate neurotoxicity, oxidative stress, neuroinflammation, immunity, and vasoconstriction. Autophagy can be induced by retinal hypoxia and axonal damage. In this process, ischemia can cause mutations of optineurin and activate the nuclear factor-kappa B pathway. Glutamate neurotoxicity is induced by the over-stimulation of N-methyl-D-aspartate membrane receptors by glutamate, which occurs in RGCs and induces progressive glaucomatous optic neuropathy. Oxidative stress also participates in NTG-related glaucomatous optic neuropathy. It impairs the mitochondrial and DNA function of RGCs through the apoptosis signal-regulating kinase-JUN N-terminal kinase pathway. Moreover, it increases inflammation and the immune response of RGCs. Endothelin 1 causes endothelial dysfunction and impairment of ocular blood flow, promoting vasospasm and glaucomatous optic neuropathy, as a result of NTG. In conclusion, we discussed research progress on potential options for the protection of RGCs, including TANK binding kinase 1 inhibitors regulating autophagy, N-methyl-D-aspartate receptor antagonists inhibiting glutamate toxicity, ASK1 inhibitors regulating mitochondrial function, and antioxidants inhibiting oxidative stress. In NTG, RGC death is regulated by a network of mechanisms, while various potential targets protect RGCs. Collectively, these findings provide insight into the pathogenesis of NTG and potential therapeutic strategies.

Genetic inhibition of collapsin response mediator protein-2 phosphorylation ameliorates retinal ganglion cell death in normal-tension glaucoma models

Glaucoma is a neurodegenerative disorder caused by the death of retinal ganglion cells (RGCs). Elevated intraocular pressure (IOP) is a cause of glaucoma. However, glaucoma often develops with normal IOP and is known as normal-tension glaucoma (NTG). Glutamate neurotoxicity is considered as one of the significant causes of NTG, resulting in excessive stimulation of retinal neurons via the N-methyl-D-aspartate (NMDA) receptors. The present study examined the phosphorylation of collapsin response mediator protein-2 (CRMP2), a protein that is abundantly expressed in neurons and involved in their development. In two mouse models, NMDA-injection and glutamate/aspartate transporter (GLAST) mutant, CRMP2 phosphorylation at the cyclin-dependent kinase-5 (Cdk5) site was elevated in RGCs. We confirmed that the decrease in the number of RGCs and thickness of the inner retinal layer (IRL) could be suppressed after NMDA administration in CRMP2KI/KI mice with genetically inhibited CRMP2 phosphorylation. Next, we investigated GLAST heterozygotes (GLAST+/-) with CRMP2KI/KI (GLAST+/-;CRMP2KI/KI) and GLAST knockout (GLAST-/-) mice with CRMP2KI/KI (GLAST-/-;CRMP2KI/KI) mice and compared them with GLAST+/- and GLAST-/- mice. pCRMP2 (S522) inhibition significantly reduced RGC loss and IRL thinning. These results suggest that the inhibition of CRMP2 phosphorylation could be a novel strategy for treating NTG.

Cytoprotective Effect of Astaxanthin in a Model of Normal Intraocular Pressure Glaucoma

Glaucoma is characterized by axonal degeneration of retinal ganglion cells (RGCs) and apoptotic death of their cell bodies. Lowering intraocular pressure is currently the only way to treat glaucoma, but it is often insufficient to inhibit the progression of the disease. Glaucoma is a multifactorial disease, and the involvement of oxidative stress has recently received much attention. In the present study, we investigated the cytoprotective effect of astaxanthin (AST) on RGC degeneration using a normal-tension glaucoma (NTG) mouse model, which lacks the glutamate/aspartate transporter (Glast) and demonstrates spontaneous RGC and optic nerve degeneration without elevated intraocular pressure. Three-week-old Glast^± mice were given intraperitoneal injections of AST at 10, 30, or 60 mg/kg/day or vehicle alone, and littermate control mice were given vehicle alone for 14 days, respectively. Five weeks after birth, the number of RGCs was counted in paraffin sections of retinal tissues stained with hematoxylin and eosin. We also used a retrograde labeling technique to quantify the number of RGCs. Additionally, the phosphorylated (p) IκB/total IκB ratio and the 4-hydroxynonenal (HNE) were measured in retinal tissues. The number of RGCs in Glast^± mice was significantly decreased compared with that in control mice. RGC loss was suppressed by the administration of AST at 60 mg/kg/day, compared with vehicle alone. Following AST administration, the concentration of 4-HNE in the retina was also suppressed, but the pIκB/IκB ratio did not change. Our study revealed that the antioxidative stress effects of AST inhibit RGC degeneration in the retina and may be useful in the treatment of NTG.

Suppression of Oxidative Stress as Potential Therapeutic Approach for Normal Tension Glaucoma

Glaucoma is a neurodegenerative disease of the eye, which involves degeneration of retinal ganglion cells (RGCs): the output neurons of the retina to the brain, which with their axons comprise the optic nerve. Recent studies have shown the possible involvement of oxidative stress in the pathogenesis of glaucoma, especially in the subtype of normal tension glaucoma. Basic experiments utilizing rodent and primate models of glaucoma revealed that antioxidants protect RGCs under various pathological conditions including glutamate neurotoxicity and optic nerve injury. These results suggested that existing drugs and food factors may be useful for prevention and hence therapy of glaucoma. In this review, we highlight some therapeutic candidates, particularly those with antioxidant properties, and discuss the therapeutic potential of RGC protection by modulating gene expressions that prevent and ameliorate glaucoma.

The protective effect of astaxanthin on the ganglion cell complex in glutamate/aspartate transporter deficient mice, a model of normal tension glaucoma, analyzed by spectral domain-optical coherence tomography

Astaxanthin (AST), a natural marine carotenoid, possess a wide variety of biological functions. In particular, as a strong antioxidant, AST effectively scavenges oxygen free radicals and reduces oxidative stress. In addition, recent in vitro studies have suggested that AST attenuates glutamate-induced apoptosis and cytotoxicity. The glutamate/aspartate transporter (GLAST) deficient (GLAST-/-) mouse is a mouse model of normal tension glaucoma (NTG) caused by both the glutamate neurotoxicity and oxidative stress in the retina. In the present study, we investigated the effects of AST on the ganglion cell complex, indicator of glaucomatous structural damage, using spectral domain-optical coherence tomography. As a result, AST significantly attenuated the thinning of ganglion cell complex in GLAST-/- mice in comparison to an AST-free control group. Our results suggest the possibility that AST has protective effects against glutamate neurotoxicity and oxidative stress in the retina. At present, the only treatment for NTG that is available in the clinical setting is to reduce the IOP as much as possible. Thus, our results suggest that AST supplementation may be effective for some types of NTG in which glutamate neurotoxicity and oxidative stress are involved.

The role of commensal microflora-induced T cell responses in glaucoma neurodegeneration

Over the last decade, new evidence has become increasingly more compelling that commensal microflora profoundly influences the maturation and function of resident immune cells in host physiology. The concept of gut-retina axis is actively being explored. Studies have revealed a critical role of commensal microbes linked with neuronal stress, immune responses, and neurodegeneration in the retina. Microbial dysbiosis changes the blood-retina barrier permeability and modulates T cell-mediated autoimmunity to contribute to the pathogenesis of retinal diseases, such as glaucoma. Heat shock proteins (HSPs), which are evolutionarily conserved, are thought to function both as neuroprotectant and pathogenic antigens of T cells contributing to cell protection and tissue damage, respectively. Activated microglia recruit and interact with T cells during this process. Glaucoma, characterized by the progressive loss of retinal ganglion cells, is the leading cause of irreversible blindness. With nearly 70 million people suffering glaucoma worldwide, which doubles the number of patients with Alzheimer's disease, it represents the most frequent neurodegenerative disease of the central nervous system (CNS). Thus, understanding the mechanism of neurodegeneration in glaucoma and its association with the function of commensal microflora may help unveil the secrets of many neurodegenerative disorders in the CNS and develop novel therapeutic interventions.

Inducible rodent models of glaucoma

Glaucoma is one of the leading causes of vision impairment worldwide. In order to further understand the molecular pathobiology of this disease and to develop better therapies, clinically relevant animal models are necessary. In recent years, both the rat and mouse have become popular models in glaucoma research. Key reasons are: many important biological similarities shared among rodent eyes and the human eye; development of improved methods to induce glaucoma and to evaluate glaucomatous damage; availability of genetic tools in the mouse; as well as the relatively low cost of rodent studies. Commonly studied rat and mouse glaucoma models include intraocular pressure (IOP)-dependent and pressure-independent models. The pressure-dependent models address the most important risk factor of elevated IOP, whereas the pressure-independent models assess "normal tension" glaucoma and other "non-IOP" related factors associated with glaucomatous damage. The current article provides descriptions of these models, their characterizations, specific techniques to induce glaucoma, mechanisms of injury, advantages, and limitations.

Retinal Cell Protection in Ocular Excitotoxicity Diseases. Possible Alternatives Offered by Microparticulate Drug Delivery Systems and Future Prospects

Excitotoxicity seems to play a critical role in ocular neurodegeneration. Excess-glutamate-mediated retinal ganglion cells death is the principal cause of cell loss. Uncontrolled glutamate in the synapsis has significant implications in the pathogenesis of neurodegenerative disorders. The exploitation of various approaches of controlled release systems enhances the pharmacokinetic and pharmacodynamic activity of drugs. In particular, microparticles are secure, can maintain therapeutic drug concentrations in the eye for prolonged periods, and make intimate contact by improving drug bioavailability. According to the promising results reported, possible new investigations will focus intense attention on microparticulate formulations and can be expected to open the field to new alternatives for doctors, as currently required by patients.

Excess Glutamate May Cause Dilation of Retinal Blood Vessels in Glutamate/Aspartate Transporter-Deficient Mice

Purpose

To investigate the longitudinal findings of fundus features and spectral-domain optical coherence tomography (SD-OCT) to characterize the morphologic features in a mouse model of defective glutamate/aspartate transporter (GLAST^−/− mice).

Materials and Methods

The fundus findings and SD-OCT images were longitudinally recorded at five time points from postnatal (P) 22 to P156 in GLAST^−/− mice. As a control wild type, age-matched C57BL/6J mice were employed. The mouse retina was subdivided into five layers, and the thickness of each layer was longitudinally measured by InSight® using SD-OCT pictures. The SD-OCT findings were compared with the histologic appearances. The diameter of the retinal blood vessels was measured by the ImageJ® software program using SD-OCT images. The data were statistically compared between both age-matched mouse groups.

Results

The retinal blood vessels appeared more dilated in GLAST^−/− mice than in wild-type mice. This tendency was statistically significant at all time points after P44 by analyses using SD-OCT images. The ganglion cell complex (GCC) and outer nuclear layer (ONL) were significantly thinner in GLAST^−/− mice at all time points after P80 than in the wild-type mice. This tendency was more clearly indicated by SD-OCT than histologic sections.

Discussion

In the present study, we found for the first time the dilation of the retinal blood vessels and the thinning of the ONL in GLAST^−/− mice, in addition to the thinning of the GCC.

Mitochondria-targeted antioxidant peptide SS-31 mediates neuroprotection in a rat experimental glaucoma model

To investigate the neuroprotective effects of the mitochondria-targeted antioxidant Szeto-Schiller peptide 31 (SS-31) in a rat experimental glaucoma model, SS-31 was intraperitoneally (IP) injected into Sprague-Dawley rats, followed by intracameral injection of polystyrene microspheres to induce elevated intraocular pressure (IOP). After 6 weeks, electroretinography (ERG) and flash visual-evoked potentials (F-VEPs) were recorded to assess retinal function. Hematoxylin-eosin staining was performed on retinal cross-sections to measure ganglion cell complex (GCC) thickness. Apoptotic retinal cells were assessed by TUNEL staining. Brn3a-positive retinal ganglion cells (RGCs) were counted in retinal flat mounts via immunofluorescence. The retinal total SOD, SOD2, and MDA expression levels were assessed in retinal tissue homogenates. The cyt c, Bax, and Bcl-2 protein levels in rat retinas were detected by western blot analysis. Bax and Bcl-2 expressions were also evaluated using immunohistochemistry in paraffinized sections. Our results showed that the rats that received microsphere injection developed elevated IOP. SS-31 ameliorated the reductions in the a- and b-wave amplitudes on ERG and the F-VEP amplitude in glaucomatous eyes. GCC thickness was preserved, TUNEL-positive cells were decreased in the retina, and Brn3a-positive RGCs were increased in the SS-31-treated glaucoma group compared with those in the non-treated glaucoma group. SS-31 significantly reduced MDA levels and increased SOD2 levels after glaucoma induction. Significant suppression of cyt c release, upregulation of Bcl-2, and downregulation of Bax were observed following SS-31 administration. In summary, SS-31 exerts neuroprotective effects in this experimental glaucoma model by inhibiting mitochondrial dysfunction and therefore represents a promising therapeutic agent for glaucoma.

Reactive Oxygen Species-Mediated Damage of Retinal Neurons: Drug Development Targets for Therapies of Chronic Neurodegeneration of the Retina

The significance of oxidative stress in the development of chronic neurodegenerative diseases of the retina has become increasingly apparent in recent years. Reactive oxygen species (ROS) are free radicals produced at low levels as a result of normal cellular metabolism that are ultimately metabolized and detoxified by endogenous and exogenous mechanisms. In the presence of oxidative cellular stress, ROS are produced in excess, resulting in cellular injury and death and ultimately leading to tissue and organ dysfunction. Recent studies have investigated the role of excess ROS in the pathogenesis and development of chronic neurodegenerative diseases of the retina including glaucoma, diabetic retinopathy, and age-related macular degeneration. Findings from these studies are promising insofar as they provide clear rationales for innovative treatment and prevention strategies of these prevalent and disabling diseases where currently therapeutic options are limited. Here, we briefly outline recent developments that have contributed to our understanding of the role of ROS in the pathogenesis of chronic neurodegenerative diseases of the retina. We then examine and analyze the peer-reviewed evidence in support of ROS as targets for therapy development in the area of chronic neurodegeneration of the retina.

Recent advances in genetically modified animal models of glaucoma and their roles in drug repositioning

Glaucoma is one of the leading causes of vision loss in the world. Currently, pharmacological intervention for glaucoma therapy is limited to eye drops that reduce intraocular pressure (IOP). Recent studies have shown that various factors as well as IOP are involved in the pathogenesis of glaucoma, especially in the subtype of normal tension glaucoma. To date, various animal models of glaucoma have been established, including glutamate/aspartate transporter knockout (KO) mice, excitatory amino acid carrier 1 KO mice, optineurin E50K knock-in mice, DBA/2J mice and experimentally induced models. These animal models are very useful for elucidating the pathogenesis of glaucoma and for identifying potential therapeutic targets. However, each model represents only some aspects of glaucoma, never the whole disease. This review will summarise the benefits and limitations of using disease models of glaucoma and recent basic research in retinal protection using existing drugs.

DJ-1 in Ocular Diseases: A Review

Protein deglycase DJ-1 (Parkinson disease protein 7) is a 20 kDa protein encoded by PARK7 gene. It is also known as a redox-sensitive chaperone and sensor that protect cells against oxidative stress-induced cell death in many human diseases. Though increasing evidence implicates that DJ-1 may also participate in ocular diseases, the overview of DJ-1 in ocular diseases remains elusive. In this review, we discuss the role as well as the underlying molecular mechanisms of DJ-1 in ocular diseases, including Fuchs endothelial corneal dystrophy (FECD), age-related macular degeneration (AMD), cataracts, and ocular neurodegenerative diseases, highlighting that DJ-1 may serve as a very striking therapeutic target for ocular diseases.

Heat Shock Proteins and Autophagy Pathways in Neuroprotection: from Molecular Bases to Pharmacological Interventions

Neurodegenerative diseases (NDDs) such as Alzheimer's disease, Parkinson's disease and Huntington's disease (HD), amyotrophic lateral sclerosis, and prion diseases are all characterized by the accumulation of protein aggregates (amyloids) into inclusions and/or plaques. The ubiquitous presence of amyloids in NDDs suggests the involvement of disturbed protein homeostasis (proteostasis) in the underlying pathomechanisms. This review summarizes specific mechanisms that maintain proteostasis, including molecular chaperons, the ubiquitin-proteasome system (UPS), endoplasmic reticulum associated degradation (ERAD), and different autophagic pathways (chaperon mediated-, micro-, and macro-autophagy). The role of heat shock proteins (Hsps) in cellular quality control and degradation of pathogenic proteins is reviewed. Finally, putative therapeutic strategies for efficient removal of cytotoxic proteins from neurons and design of new therapeutic targets against the progression of NDDs are discussed.

Increased intraocular pressure alters the cellular distribution of HuR protein in retinal ganglion cells - A possible sign of endogenous neuroprotection failure

The RNA-binding protein, HuR, modulates mRNA processing and gene expression of several stress response proteins i.e. Hsp70 and p53 that have been postulated to be involved in the pathogenesis of glaucoma, a chronic optic neuropathy leading to irreversible blindness. We evaluated HuR protein expression in retinas and optic nerves of glaucomatous rats and human primary open angle glaucoma patients and its possible impact on stress response mechanisms. We found that the cytoplasmic content of HuR was reduced more extensively in glaucomatous retinas than in optic nerves and this was linked with a declined cytoplasmic Hsp70 level and p53 nuclear translocation. In the optic nerve, the p53 content was decreased as a feature of reactive gliosis. Based on our findings, we conclude that the alteration in the HuR content, observed both in rat glaucoma model and human glaucoma samples, affects post-transcriptionally the expression of genes crucial for maintaining cell homeostasis; therefore, we postulate that HuR may be involved in the pathogenesis of glaucoma.

Targeting Oxidative Stress for Treatment of Glaucoma and Optic Neuritis

Glaucoma is a neurodegenerative disease of the eye and it is one of the leading causes of blindness. Glaucoma is characterized by progressive degeneration of retinal ganglion cells (RGCs) and their axons, namely, the optic nerve, usually associated with elevated intraocular pressure (IOP). Current glaucoma therapies target reduction of IOP, but since RGC death is the cause of irreversible vision loss, neuroprotection may be an effective strategy for glaucoma treatment. One of the risk factors for glaucoma is increased oxidative stress, and drugs with antioxidative properties including valproic acid and spermidine, as well as inhibition of apoptosis signal-regulating kinase 1, an enzyme that is involved in oxidative stress, have been reported to prevent glaucomatous retinal degeneration in mouse models of glaucoma. Optic neuritis is a demyelinating inflammation of the optic nerve that presents with visual impairment and it is commonly associated with multiple sclerosis, a chronic demyelinating disease of the central nervous system. Although steroids are commonly used for treatment of optic neuritis, reduction of oxidative stress by approaches such as gene therapy is effective in ameliorating optic nerve demyelination in preclinical studies. In this review, we discuss oxidative stress as a therapeutic target for glaucoma and optic neuritis.