Long-term activation of c-Fos and c-Jun in optic nerve head astrocytes in experimental ocular hypertension in monkeys and after exposure to elevated pressure in vitro

MicroRNA-146a-5p protects retinal ganglion cells through reducing neuroinflammation in experimental glaucoma

Neuroinflammation plays important roles in retinal ganglion cell (RGC) degeneration in glaucoma. MicroRNA-146 (miR-146) has been shown to regulate inflammatory response in neurodegenerative diseases. In this study, whether and how miR-146 could affect RGC injury in chronic ocular hypertension (COH) experimental glaucoma were investigated. We showed that in the members of miR-146 family only miR-146a-5p expression was upregulated in COH retinas. The upregulation of miR-146a-5p was observed in the activated microglia and Müller cells both in primary cultured conditions and in COH retinas, but mainly occurred in microglia. Overexpression of miR-146a-5p in COH retinas reduced the levels pro-inflammatory cytokines and upregulated the levels of anti-inflammatory cytokines, which were further confirmed in the activated primary cultured microglia. Transfection of miR-146a-5p mimic increased the percentage of anti-inflammatory phenotype in the activated BV2 microglia, while transfection of miR-146a-5p inhibitor resulted in the opposite effects. Transfection of miR-146a-5p mimic/agomir inhibited the levels of interleukin-1 receptor associated kinase (IRAK1) and TNF receptor associated factor 6 (TRAF6) and phosphorylated NF-κB subunit p65. Dual luciferase reporter gene assay confirmed that miR-146a-5p could directly target IRAK1 and TRAF6. Moreover, downregulation of IRAK1 and TRAF6 by siRNA techniques or blocking NF-κB by SN50 in cultured microglia reversed the miR-146a-5p inhibitor-induced changes of inflammatory cytokines. In COH retinas, overexpression of miR-146a-5p reduced RGC apoptosis, increased RGC survival, and partially rescued the amplitudes of photopic negative response. Our results demonstrate that overexpression of miR-146a-5p attenuates RGC injury in glaucoma by reducing neuroinflammation through downregulating IRAK1/TRAF6/NF-κB signaling pathway in microglia.

IOP and glaucoma damage: The essential role of optic nerve head and retinal mechanosensors

There are many unanswered questions on the relation of intraocular pressure to glaucoma development and progression. IOP itself cannot be distilled to a single, unifying value, because IOP level varies over time, differs depending on ocular location, and can be affected by method of measurement. Ultimately, IOP level creates mechanical strain that affects axonal function at the optic nerve head which causes local extracellular matrix remodeling and retinal ganglion cell death - hallmarks of glaucoma and the cause of glaucomatous vision loss. Extracellular tissue strain at the ONH and lamina cribrosa is regionally variable and differs in magnitude and location between healthy and glaucomatous eyes. The ultimate targets of IOP-induced tissue strain in glaucoma are retinal ganglion cell axons at the optic nerve head and the cells that support axonal function (astrocytes, the neurovascular unit, microglia, and fibroblasts). These cells sense tissue strain through a series of signals that originate at the cell membrane and alter cytoskeletal organization, migration, differentiation, gene transcription, and proliferation. The proteins that translate mechanical stimuli into molecular signals act as band-pass filters - sensing some stimuli while ignoring others - and cellular responses to stimuli can differ based on cell type and differentiation state. Therefore, to fully understand the IOP signals that are relevant to glaucoma, it is necessary to understand the ultimate cellular targets of IOP-induced mechanical stimuli and their ability to sense, ignore, and translate these signals into cellular actions.

EphA4/ephrinA3 reverse signaling induced Müller cell gliosis and production of pro-inflammatory cytokines in experimental glaucoma

Previous work showed that ephrinA3/EphA4 forward signaling contributed to retinal ganglion cell (RGC) damage in experimental glaucoma. Since up-regulated patterns of ephrinA3 and EphA4 were observed in Müller cells and RGCs, an EphA4/ephrinA3 reverse signaling may exist in Müller cells of chronic ocular hypertension (COH) retina. We investigated effects of EphA4/ephrinA3 reverse signaling activation on Müller cells in COH retina. Intravitreal injection of the ephrinA3 agonist EphA4-Fc increased glial fibrillary acidic protein (GFAP) levels in normal retinas, suggestive of Müller cell gliosis, which was confirmed in purified cultured Müller cells treated with EphA4-Fc. These effects were mediated by intracellular STAT3 signaling pathway as phosphorylated STAT3 (p-STAT3) levels and ratios of p-STAT3/STAT3 were significantly increased in both COH retinas and EphA4-Fc intravitreally injected retinas, as well as in EphA4-Fc treated purified cultured Müller cells. The increase of GFAP protein levels in EphA4-Fc-injected retinas and EphA4-Fc treated purified cultured Müller cells could be partially eliminated by stattic, a selective STAT3 blocker. Co-immunoprecipitation results testified to the presence of interaction between ephrinA3 and STAT3/p-STAT3. In addition, intravitreal injection of EphA4-Fc or EphA4-Fc treatment of cultured Müller cells significantly up-regulated mRNA and protein contents of pro-inflammatory cytokines. Moreover, intravitreal injection of EphA4-Fc increased the number of apoptotic RGCs, which could be reversed by the tyrosine kinase blocker PP2. Overall, EphA4/ephrinA3 reverse signaling may induce Müller cell gliosis and increases release of pro-inflammatory factors, which could contribute to RGC death in glaucoma. Inhibition of EphA4/ephrinA3 signaling may provide an effective neuroprotection in glaucoma.

Integrated analysis of long non-coding RNAs and mRNAs associated with glaucoma in vitro

INTRODUCTION

In recent years, the biological functions and important roles of long non-coding RNAs (lncRNAs) have been widely reported in many diseases. Although glaucoma is the leading cause of blindness worldwide, the specific mechanisms of lncRNAs in the pathogenesis and progression of glaucoma remain unclear. Our research aims to elucidate the differentially expressed lncRNAs and mRNAs in glaucoma and to provide a basis for further exploration of the specific mechanism of action of lncRNAs in the progression of glaucoma.

METHODS

We performed RNA sequencing on samples from a pressurized model of R28 cells and performed bioinformatics analyses on the sequencing results. The expression consistency of lncRNAs in clinical samples from patients with glaucoma or cataracts was detected using real-time quantitative polymerase chain reaction (RT-qPCR).

RESULTS

RNA sequencing results showed that lncRNAs in cluster 5 were upregulated with increasing stress after typing all significantly altered lncRNAs using k-means in a cellular stress model. KEGG analysis indicated that they were associated with neurodegenerative diseases. Differentially expressed lncRNAs were verified by RT-qPCR, and the lncRNA expression levels of AC120246.2 and XLOC_006247 were significantly higher in the aqueous humor (AH) of patients with glaucoma than in those with cataracts. For LOC102551819, there was almost no expression in the AH and trabecular meshwork in patients with glaucoma but high expression was observed in the iris.

CONCLUSION

Our research proposes potential diagnostic or intervention targets for clinical applications as well as a theoretical basis for more in-depth research on the function of lncRNAs in glaucoma.

Optic Nerve Head Myelin-Related Protein, GFAP, and Iba1 Alterations in Non-Human Primates With Early to Moderate Experimental Glaucoma

Purpose

The purpose of this study was to test if optic nerve head (ONH) myelin basic protein (MBP), 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNPase), glial fibrillary acidic protein (GFAP), and ionized calcium binding adaptor molecule 1 (Iba1) proteins are altered in non-human primate (NHP) early/moderate experimental glaucoma (EG).

Methods

Following paraformaldehyde perfusion, control and EG eye ONH tissues from four NHPs were paraffin embedded and serially (5 µm) vertically sectioned. Anti-MBP, CNPase, GFAP, Iba1, and nuclear dye-stained sections were imaged using sub-saturating light intensities. Whole-section images were segmented creating anatomically consistent laminar (L) and retrolaminar (RL) regions/sub-regions. EG versus control eye intensity/pixel-cluster density data within L and two RL regions (RL1 [1-250 µm]/RL2 [251-500 µm] from L) were compared using random effects models within the statistical program “R.”

Results

EG eye retinal nerve fiber loss ranged from 0% to 20%. EG eyes’ MBP and CNPase intensity were decreased within the RL1 (MBP = 31.4%, P < 0.001; CNPase =62.3%, P < 0.001) and RL2 (MBP = 19.6%, P < 0.001; CNPase = 56.1%, P = 0.0004) regions. EG eye GFAP intensity was decreased in the L (41.6%, P < 0.001) and RL regions (26.7% for RL1, and 28.4% for RL2, both P < 0.001). Iba1+ and NucBlue pixel-cluster density were increased in the laminar (28.2%, P = 0.03 and 16.6%, P = 0.008) and both RL regions (RL1 = 37.3%, P = 0.01 and 23.7%, P = 0.0002; RL2 = 53.7%, P = 0.002 and 33.2%, P < 0.001).

Conclusions

Retrolaminar myelin disruption occurs early in NHP EG and may be accompanied by laminar and retrolaminar decreases in astrocyte process labeling and increases in microglial/ macrophage density. The mechanistic and therapeutic implications of these findings warrant further study.

Divergent transcriptional regulation of astrocyte reactivity across disorders

Astrocytes respond to injury and disease in the central nervous system with reactive changes that influence the outcome of the disorder1-4. These changes include differentially expressed genes (DEGs) whose contextual diversity and regulation are poorly understood. Here we combined biological and informatic analyses, including RNA sequencing, protein detection, assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and conditional gene deletion, to predict transcriptional regulators that differentially control more than 12,000 DEGs that are potentially associated with astrocyte reactivity across diverse central nervous system disorders in mice and humans. DEGs associated with astrocyte reactivity exhibited pronounced heterogeneity across disorders. Transcriptional regulators also exhibited disorder-specific differences, but a core group of 61 transcriptional regulators was identified as common across multiple disorders in both species. We show experimentally that DEG diversity is determined by combinatorial, context-specific interactions between transcriptional regulators. Notably, the same reactivity transcriptional regulators can regulate markedly different DEG cohorts in different disorders; changes in the access of transcriptional regulators to DNA-binding motifs differ markedly across disorders; and DEG changes can crucially require multiple reactivity transcriptional regulators. We show that, by modulating reactivity, transcriptional regulators can substantially alter disorder outcome, implicating them as therapeutic targets. We provide searchable resources of disorder-related reactive astrocyte DEGs and their predicted transcriptional regulators. Our findings show that transcriptional changes associated with astrocyte reactivity are highly heterogeneous and are customized from vast numbers of potential DEGs through context-specific combinatorial transcriptional-regulator interactions.

Immediate Early Gene c-fos in the Brain: Focus on Glial Cells

The c-fos gene was first described as a proto-oncogene responsible for the induction of bone tumors. A few decades ago, activation of the protein product c-fos was reported in the brain after seizures and other noxious stimuli. Since then, multiple studies have used c-fos as a brain activity marker. Although it has been attributed to neurons, growing evidence demonstrates that c-fos expression in the brain may also include glial cells. In this review, we collect data showing that glial cells also express this proto-oncogene. We present evidence demonstrating that at least astrocytes, oligodendrocytes, and microglia express this immediate early gene (IEG). Unlike neurons, whose expression changes used to be associated with depolarization, glial cells seem to express the c-fos proto-oncogene under the influence of proliferation, differentiation, growth, inflammation, repair, damage, plasticity, and other conditions. The collected evidence provides a complementary view of c-fos as an activity marker and urges the introduction of the glial cell perspective into brain activity studies. This glial cell view may provide additional information related to the brain microenvironment that is difficult to obtain from the isolated neuron paradigm. Thus, it is highly recommended that detection techniques are improved in order to better differentiate the phenotypes expressing c-fos in the brain and to elucidate the specific roles of c-fos expression in glial cells.

Transcriptional control of retinal ganglion cell death after axonal injury

Injury to the axons of retinal ganglion cells (RGCs) is a key pathological event in glaucomatous neurodegeneration. The transcription factors JUN (the target of the c-Jun N-terminal kinases, JNKs) and DDIT3/CHOP (a mediator of the endoplasmic reticulum stress response) have been shown to control the majority of proapoptotic signaling after mechanical axonal injury in RGCs and in other models of neurodegeneration. The downstream transcriptional networks controlled by JUN and DDIT3, which are critical for RGC death, however, are not well defined. To determine these networks, RNA was isolated from the retinas of wild-type mice and mice deficient in Jun, Ddit3, and both Jun and Ddit3 three days after mechanical optic nerve crush injury (CONC). RNA-sequencing data analysis was performed and immunohistochemistry was used to validate potential transcriptional signaling changes after axonal injury. This study identified downstream transcriptional changes after injury including both neuronal survival and proinflammatory signaling that were attenuated to differing degrees by loss of Ddit3, Jun, and Ddit3/Jun. These data suggest proinflammatory signaling in the retina might be secondary to activation of pro-death pathways in RGCs after acute axonal injury. These results determine the downstream transcriptional networks important for apoptotic signaling which may be important for ordering and staging the pro-degenerative signals after mechanical axonal injury.

Modeling the Mechanical Parameters of Glaucoma

Glaucoma is a major eye disease characterized by a progressive loss of retinal ganglion cells (RGCs). Biomechanical forces as a result of hydrostatic pressure and strain play a role in this disease. Decreasing intraocular pressure is the only available therapy so far, but is not always effective and does not prevent blindness in many cases. There is a need for drugs that protect RGCs from dying in glaucoma; to develop these, we need valid glaucoma and drug screening models. Since in vivo models are unsuitable for screening purposes, we focus on in vitro and ex vivo models in this review. Many groups have studied pressure and strain model systems to mimic glaucoma, to investigate the molecular and cellular events leading to mechanically induced RGC death. Therefore, the focus of this review is on the different mechanical model systems used to mimic the biomechanical forces in glaucoma. Most models use either cell or tissue strain, or fluid- or gas-controlled hydrostatic pressure application and apply it to the relevant cell types such as trabecular meshwork cells, optic nerve head astrocytes, and RGCs, but also to entire eyes. New model systems are warranted to study concepts and test experimental compounds for the development of new drugs to protect vision in glaucoma patients. Impact Statement The outcome of currently developed models to investigate mechanically induced retinal ganglion cell death by applying different mechanical strains varies widely. This suggests that a robust glaucoma model has not been developed yet. However, a comprehensive overview of current developments is not available. In this review, we have therefore assessed what has been done before and summarized the available knowledge in the field, which can be used to develop improved models for glaucoma research.

Lycium barbarum (Wolfberry) Increases Retinal Ganglion Cell Survival and Affects both Microglia/Macrophage Polarization and Autophagy after Rat Partial Optic Nerve Transection

The rat partial optic nerve transection (PONT) model has been used for studying secondary degeneration of retinal ganglion cells (RGCs) in recent years. In this study, we carried out PONT of the temporal side of rat optic nerves, whereas PONT was carried out of the superior side in the previous publication. We found that this surgery is better and easier than the previous method and can produce a repeatable and reliable model. We detected significant changes in the polarization of microglia/macrophages and the level of autophagy in optic nerves after PONT. We also used this model to detect the effects of the polysaccharides extracted from Lycium barbarum (LBP) on the survival of RGCs and the changes in the polarization of microglia/macrophages and the level of autophagy after PONT. We find that LBP can delay secondary degeneration of RGCs after temporal injury of optic nerves, promote the M2 polarization of microglia/macrophages, and down-regulate the level of autophagy after PONT. In conclusion, we find that the polarization of microglia/macrophages and the autophagy level change after PONT; LBP treatment delays secondary degeneration of RGCs; and the polarization of microglia/macrophages and the level of autophagy are also altered after LBP treatment.

Mkk4 and Mkk7 are important for retinal development and axonal injury-induced retinal ganglion cell death

The mitogen-activated protein kinase (MAPK) pathway has been shown to be involved in both neurodevelopment and neurodegeneration. c-Jun N-terminal kinase (JNK), a MAPK important in retinal development and after optic nerve crush injury, is regulated by two upstream kinases: MKK4 and MKK7. The specific requirements of MKK4 and MKK7 in retinal development and retinal ganglion cell (RGC) death after axonal injury, however, are currently undefined. Optic nerve injury is an important insult in many neurologic conditions including traumatic, ischemic, inflammatory, and glaucomatous optic neuropathies. Mice deficient in Mkk4, Mkk7, and both Mkk4 and Mkk7 were generated. Immunohistochemistry was used to study the distribution and structure of retinal cell types and to assess RGC survival after optic nerve injury (mechanical controlled optic nerve crush (CONC)). Adult Mkk4- and Mkk7-deficient retinas had all retinal cell types, and with the exception of small areas of disrupted photoreceptor lamination in Mkk4-deficient mice, the retinas of both mutants were grossly normal. Deficiency of Mkk4 or Mkk7 reduced JNK signaling in RGCs after axonal injury and resulted in a significantly greater percentage of surviving RGCs 35 days after CONC as compared to wild-type controls (Mkk4: 51.5%, Mkk7: 29.1%, WT: 15.2%; p < 0.001). Combined deficiency of Mkk4 and Mkk7 caused failure of optic nerve formation, irregular retinal axonal trajectories, disruption of retinal lamination, clumping of RGC bodies, and dendritic fasciculation of dopaminergic amacrine cells. These results suggest that MKK4 and MKK7 may serve redundant and unique roles in molecular signaling important for retinal development and injury response following axonal insult.

AP-1 and the injury response of the GFAP gene

Increased GFAP gene expression is a common feature of CNS injury, resulting in its use as a reporter to investigate mechanisms producing gliosis. AP-1 transcription factors are among those proposed to participate in mediating the reactive response. Prior studies found a consensus AP-1 binding site in the GFAP promoter to be essential for activity of reporter constructs transfected into cultured cells, but to have little to no effect on basal transgene expression in mice. Since cultured astrocytes display some properties of reactive astrocytes, these findings suggested that AP-1 transcription factors are critical for the upregulation of GFAP in injury, but not for its resting level of expression. We have examined this possibility by comparing the injury response in mice of lacZ transgenes driven by human GFAP promoters that contain the wild-type AP-1 binding site to those in which the site is mutated. An intact AP-1 site was found critical for a GFAP promoter response to the three different injury models used: physical trauma produced by cryoinjury, seizures produced by kainic acid, and chronic gliosis produced in an Alexander disease model. An unexpected additional finding was that the responses of the lacZ transgenes driven by the wild-type promoters were substantially less than that of the endogenous mouse GFAP gene. This suggests that the GFAP gene has previously unrecognized injury-responsive elements that reside further upstream of the transcription start site than the 2.2 kb present in the GFAP promoter segments used here.

Expression and activation of mitogen-activated protein kinases in the optic nerve head in a rat model of ocular hypertension

BACKGROUND

Glaucoma is a leading cause of irreversible blindness manifesting as an age-related, progressive optic neuropathy with associated retinal ganglion cell (RGC) loss. Mitogen-activated protein kinases (MAPKs: p42/44 MAPK, SAPK/JNK, p38 MAPK) are activated in various retinal disease models and likely contribute to the mechanisms of RGC death. Although MAPKs play roles in the development of retinal pathology, their action in the optic nerve head (ONH), where the initial insult to RGC axons likely resides in glaucoma, remains unexplored.

METHODS

An experimental paradigm representing glaucoma was established by induction of chronic ocular hypertension (OHT) via laser-induced coagulation of the trabecular meshwork in Sprague-Dawley rats. MAPKs were subsequently investigated over the following days for expression and activity alterations, using RT-PCR, immunohistochemistry and Western immunoblot.

RESULTS

p42/44 MAPK expression was unaltered after intraocular pressure (IOP) elevation, but there was a significant activation of this enzyme in ONH astrocytes after 6-24 h. Activated SAPK/JNK isoforms were present throughout healthy RGC axons but after IOP elevation or optic nerve crush, they both accumulated at the ONH, likely due to RGC axon transport disruption, and were subject to additional activation. p38 MAPK was expressed by a population of microglia which were significantly more populous following IOP elevation. However it was only significantly activated in microglia after 3 days, and then only in the ONH and optic nerve; in the retina it was solely activated in RGC perikarya.

CONCLUSIONS

In conclusion, each of the MAPKs showed a specific spatio-temporal expression and activation pattern in the retina, ONH and optic nerve as a result of IOP elevation. These findings likely reflect the roles of the individual enzymes, and the cells in which they reside, in the developing pathology following IOP elevation. These data have implications for understanding the mechanisms of ocular pathology in diseases such as glaucoma.

Investigations into Hypoxia and Oxidative Stress at the Optic Nerve Head in a Rat Model of Glaucoma

The vascular hypothesis of glaucoma proposes that retinal ganglion cell axons traversing the optic nerve head (ONH) undergo oxygen and nutrient insufficiency as a result of compromised local blood flow, ultimately leading to their degeneration. To date, evidence for the hypothesis is largely circumstantial. Herein, we made use of an induced rat model of glaucoma that features reproducible and widespread axonal transport disruption at the ONH following chronic elevation of intraocular pressure. If vascular insufficiency plays a role in the observed axonal transport failure, there should exist a physical signature at this time point. Using a range of immunohistochemical and molecular tools, we looked for cellular events indicative of vascular insufficiency, including the presence of hypoxia, upregulation of hypoxia-inducible, or antioxidant-response genes, alterations to antioxidant enzymes, increased formation of superoxide, and the presence of oxidative stress. Our data show that ocular hypertension caused selective hypoxia within the laminar ONH in 11/13 eyes graded as either medium or high for axonal transport disruption. Hypoxia was always present in areas featuring injured axons, and, the greater the abundance of axonal transport disruption, the greater the likelihood of a larger hypoxic region. Nevertheless, hypoxic regions were typically focal and were not necessarily evident in sections taken deeper within the same ONH, while disrupted axonal transport was frequently encountered without any discernible hypoxia. Ocular hypertension caused upregulation of heme oxygenase-1—an hypoxia-inducible and redox-sensitive enzyme—in ONH astrocytes. The distribution and abundance of heme oxygenase-1 closely matched that of axonal transport disruption, and encompassed hypoxic regions and their immediate penumbra. Ocular hypertension also caused upregulations in the iron-regulating protein ceruloplasmin, the anaerobic glycolytic enzyme lactate dehydrogenase, and the transcription factors cFos and p-cJun. Moreover, ocular hypertension increased the generation of superoxide radicals in the retina and ONH, as well as upregulating the active subunit of the superoxide-generating enzyme NADPH oxidase, and invoking modest alterations to antioxidant-response enzymes. The results of this study provide further indirect support for the hypothesis that reduced blood flow to the ONH contributes to axonal injury in glaucoma.

Involvement of AP-1 and C/EBPβ in upregulation of endothelin B (ETB) receptor expression in a rodent model of glaucoma

Previous studies showed that the endothelin B receptor (ET_B) expression was upregulated and played a key role in neurodegeneration in rodent models of glaucoma. However, the mechanisms underlying upregulation of ET_B receptor expression remain largely unknown. Using promoter-reporter assays, the 1258 bp upstream the human ET_B promoter region was found to be essential for constitutive expression of ET_B receptor gene in human non-pigmented ciliary epithelial cells (HNPE). The −300 to −1 bp and −1258 to −600 bp upstream promoter regions of the ET_B receptor appeared to be the key binding regions for transcription factors. In addition, the crucial AP-1 binding site located at −615 to −624 bp upstream promoter was confirmed by luciferase assays and CHIP assays which were performed following overexpression of c-Jun in HNPE cells. Overexpression of either c-Jun or C/EBPβ enhanced the ET_B receptor promoter activity, which was reflected in increased mRNA and protein levels of ET_B receptor. Furthermore, knock-down of either c-Jun or C/EBPβ in HNPE cells was significantly correlated to decreased mRNA levels of both ET_B and ET_A receptor. These observations suggest that c-Jun and C/EBPβ are important for regulated expression of the ET_B receptor in HNPE cells. In separate experiments, intraocular pressure (IOP) was elevated in one eye of Brown Norway rats while the corresponding contralateral eye served as control. Two weeks of IOP elevation produced increased expression of c-Jun and C/EBPβ in the retinal ganglion cell (RGC) layer from IOP-elevated eyes. The mRNA levels of c-Jun, ET_A and ET_B receptor were upregulated by 2.2-, 3.1- and 4.4-fold in RGC layers obtained by laser capture microdissection from retinas of eyes with elevated IOP, compared to those from contralateral eyes. Taken together, these data suggest that transcription factor AP-1 plays a key role in elevation of ET_B receptor in a rodent model of ocular hypertension.

Gene therapy for retinal ganglion cell neuroprotection in glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide. The primary cause of glaucoma is not known, but several risk factors have been identified, including elevated intraocular pressure and age. Loss of vision in glaucoma is caused by the death of retinal ganglion cells (RGCs), the neurons that convey visual information from the retina to the brain. Therapeutic strategies aimed at delaying or halting RGC loss, known as neuroprotection, would be valuable to save vision in glaucoma. In this review, we discuss the significant progress that has been made in the use of gene therapy to understand mechanisms underlying RGC degeneration and to promote the survival of these neurons in experimental models of optic nerve injury.

Meningothelial cells react to elevated pressure and oxidative stress

BACKGROUND

Meningothelial cells (MECs) are the cellular components of the meninges enveloping the brain. Although MECs are not fully understood, several functions of these cells have been described. The presence of desmosomes and tight junctions between MECs hints towards a barrier function protecting the brain. In addition, MECs perform endocytosis and, by the secretion of cytokines, are involved in immunological processes in the brain. However, little is known about the influence of pathological conditions on MEC function; e.g., during diseases associated with elevated intracranial pressure, hypoxia or increased oxidative stress.

METHODS

We studied the effect of elevated pressure, hypoxia, and oxidative stress on immortalized human as well as primary porcine MECs. We used MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) bioreduction assays to assess the proliferation of MECs in response to treatment and compared to untreated control cells. To assess endocytotic activity, the uptake of fluorescently labeled latex beads was analyzed by fluorescence microscopy.

RESULTS

We found that exposure of MECs to elevated pressure caused significant cellular proliferation and a dramatic decrease in endocytotic activity. In addition, mild oxidative stress severely inhibited endocytosis.

CONCLUSION

Elevated pressure and oxidative stress impact MEC physiology and might therefore influence the microenvironment of the subarachnoid space and thus the cerebrospinal fluid within this compartment with potential negative impact on neuronal function.

Targeted preventive measures and advanced approaches in personalised treatment of glaucoma neuropathy

Glaucoma is a major cause of vision loss worldwide with nearly 8 million people bilaterally blind from the disease. This number is estimated to increase over the next 10 years. The key to preventing blindness from glaucoma is effective diagnosis and treatment. The classical glaucoma treatment focuses on intraocular pressure (IOP) reduction. Better knowledge of the pathogenesis has opened up additional therapeutical approaches often called non-IOP lowering treatment. Whilst most of these new avenues of treatment are still in the experimental phase, others are already used by some physicians. These new therapeutic approaches allow a more personalised patient treatment. Non-IOP lowering treatment includes improvements of ocular blood flow, particularly blood flow regulation. This can be achieved by improving the regulation of ocular blood flow (improving autoregulation) by drugs such as carbonic anhydrase inhibitors, magnesium or calcium channel blockers. It can also be improved by decreasing blood pressure over-dips. Blood pressure can be increased by an increase in salt intake or in rare cases by treatment with fludrocortisone. Experimentally, glaucomatous optic neuropathy can be prevented by inhibition of astrocyte activation, either by blockage of epidermal growth factor receptor or by counteracting Endothelin. Glaucomatous optic neuropathy can also be prevented by nitric oxide-2 synthase inhibition. Suppression of matrix metalloproteinase-9 inhibits apoptosis of retinal ganglion cells and tissue remodelling. Upregulation of heat shock proteins protects the retinal ganglion cells and the optic nerve head. Reduction of oxidative stress especially at the level of mitochondria also seems to be protective. This can be achieved by gingko, dark chocolate, polyphenolic flavonoids occurring in tea, coffee or red wine and anthocyanosides found in bilberries as well as by ubiquinone and melatonin. This review describes the individual mechanisms which may be targeted by non-IOP lowering treatment based on our pathogenic scheme.

Hydrostatic pressure-induced release of stored calcium in cultured rat optic nerve head astrocytes

PURPOSE

Elevated intraocular pressure is associated with glaucomatous optic nerve damage. Other investigators have shown functional changes in optic nerve head astrocytes subjected to elevated hydrostatic pressure (HP) for 1 to 5 days. Recently, the authors reported ERK1/2, p90(RSK) and NHE1 phosphorylation after 2 hours. Here they examine calcium responses at the onset of HP to determine what precedes ERK1/2 phosphorylation.

METHODS

Cytoplasmic calcium concentration ([Ca(2+)](i)) was measured in cultured rat optic nerve astrocytes loaded with fura-2. The cells were placed in a closed imaging chamber and subjected to an HP increase of 15 mm Hg. Protein phosphorylation was detected by Western blot analysis.

RESULTS

The increase of HP caused an immediate slow increase in [Ca(2+)](i). The response persisted in calcium-free solution and when nickel chloride (4 mM) was added to suppress channel-mediated calcium entry. Previous depletion of the ER calcium stores by cyclopiazonic acid abolished the HP-induced calcium level increase. The HP-induced increase persisted in cells exposed to xestospongin C, an inhibitor of IP3R-mediated calcium release. In contrast, ryanodine receptor (RyR) antagonist ruthenium red (10 microM) or dantrolene (25 microM) inhibited the HP-induced calcium increase. The HP-induced calcium increase was abolished when ryanodine-sensitive calcium stores were pre-depleted with caffeine (3 mM). HP caused ERK1/2 phosphorylation. The magnitude of the ERK1/2 phosphorylation response was reduced by ruthenium red and dantrolene.

CONCLUSIONS

Increasing HP causes calcium release from a ryanodine-sensitive cytoplasmic store and subsequent ERK1/2 activation. Calcium store release appears to be a required early step in the initial astrocyte response to an HP increase.

Network analysis of human glaucomatous optic nerve head astrocytes

Background

Astrocyte activation is a characteristic response to injury in the central nervous system, and can be either neurotoxic or neuroprotective, while the regulation of both roles remains elusive.

Methods

To decipher the regulatory elements controlling astrocyte-mediated neurotoxicity in glaucoma, we conducted a systems-level functional analysis of gene expression, proteomic and genetic data associated with reactive optic nerve head astrocytes (ONHAs).

Results

Our reconstruction of the molecular interactions affected by glaucoma revealed multi-domain biological networks controlling activation of ONHAs at the level of intercellular stimuli, intracellular signaling and core effectors. The analysis revealed that synergistic action of the transcription factors AP-1, vitamin D receptor and Nuclear Factor-kappaB in cross-activation of multiple pathways, including inflammatory cytokines, complement, clusterin, ephrins, and multiple metabolic pathways. We found that the products of over two thirds of genes linked to glaucoma by genetic analysis can be functionally interconnected into one epistatic network via experimentally-validated interactions. Finally, we built and analyzed an integrative disease pathology network from a combined set of genes revealed in genetic studies, genes differentially expressed in glaucoma and closely connected genes/proteins in the interactome.

Conclusion

Our results suggest several key biological network modules that are involved in regulating neurotoxicity of reactive astrocytes in glaucoma, and comprise potential targets for cell-based therapy.

Elevated hydrostatic pressure activates sodium/hydrogen exchanger-1 in rat optic nerve head astrocytes

Optic nerve head astrocytes become abnormal in eyes that have elevated intraocular pressure, and cultured astrocytes display altered protein expression after being subjected for > or = 1 days to elevated hydrostatic pressure. Here we show that 2-h elevated hydrostatic pressure (15 or 30 mmHg) causes phosphorylation of ERK1/2, ribosomal S6 protein kinase (p90(RSK)), and Na/H exchanger (NHE)1 in cultured rat optic nerve head astrocytes as judged by Western blot analysis. The MEK/ERK inhibitor U0126 abolished phosphorylation of NHE1 and p90(RSK) as well as ERK1/2. To examine NHE1 activity, cytoplasmic pH (pH(i)) was measured with BCECF and, in some experiments, cells were acidified by 5-min exposure to 20 mM ammonium chloride. Although baseline pH(i) was unaltered, the rate of pH(i) recovery from acidification was fourfold higher in pressure-treated astrocytes. In the presence of either U0126 or dimethylamiloride (DMA), an NHE inhibitor, hydrostatic pressure did not change the rate of pH(i) recovery. The findings are consistent with NHE1 activation due to phosphorylation of ERK1/2, p90(RSK), and NHE1 that occurs in response to hydrostatic pressure. These responses may precede long-term changes of protein expression known to occur in pressure-stressed astrocytes.

Bioinformatic and statistical analysis of the optic nerve head in a primate model of ocular hypertension

BACKGROUND

The nonhuman primate model of glaucomatous optic neuropathy most faithfully reproduces the human disease. We used high-density oligonucleotide arrays to investigate whole genome transcriptional changes occurring at the optic nerve head during primate experimental glaucoma.

RESULTS

Laser scarification of the trabecular meshwork of cynomolgus macaques produced elevated intraocular pressure that was monitored over time and led to varying degrees of damage in different samples. The macaques were examined clinically before enucleation and the myelinated optic nerves were processed post-mortem to determine the degree of neuronal loss. Global gene expression was examined in dissected optic nerve heads with Affymetrix GeneChip microarrays. We validated a subset of differentially expressed genes using qRT-PCR, immunohistochemistry, and immuno-enriched astrocytes from healthy and glaucomatous human donors. These genes have previously defined roles in axonal outgrowth, immune response, cell motility, neuroprotection, and extracellular matrix remodeling.

CONCLUSION

Our findings show that glaucoma is associated with increased expression of genes that mediate axonal outgrowth, immune response, cell motility, neuroprotection, and ECM remodeling. These studies also reveal that, as glaucoma progresses, retinal ganglion cell axons may make a regenerative attempt to restore lost nerve cell contact.

Susceptibility to glaucoma: differential comparison of the astrocyte transcriptome from glaucomatous African American and Caucasian American donors

Comparison of gene expression in normal and glaucomatous eyes from Caucasian American and African American donors reveals differences that might reflect different susceptibility to glaucoma.

Background

Epidemiological and genetic studies indicate that ethnic/genetic background plays an important role in susceptibility to primary open angle glaucoma (POAG). POAG is more prevalent among the African-descent population compared to the Caucasian population. Damage in POAG occurs at the level of the optic nerve head (ONH) and is mediated by astrocytes. Here we investigated differences in gene expression in primary cultures of ONH astrocytes obtained from age-matched normal and glaucomatous donors of Caucasian American (CA) and African American (AA) populations using oligonucleotide microarrays.

Results

Gene expression data were obtained from cultured astrocytes representing 12 normal CA and 12 normal AA eyes, 6 AA eyes with POAG and 8 CA eyes with POAG. Data were normalized and significant differential gene expression levels detected by using empirical Bayesian shrinkage moderated t-statistics. Gene Ontology analysis and networks of interacting proteins were constructed using the BioGRID database. Network maps included regulation of myosin, actin, and protein trafficking. Real-time RT-PCR, western blots, ELISA, and functional assays validated genes in the networks.

Conclusion

Cultured AA and CA glaucomatous astrocytes retain differential expression of genes that promote cell motility and migration, regulate cell adhesion, and are associated with structural tissue changes that collectively contribute to neural degeneration. Key upregulated genes include those encoding myosin light chain kinase (MYLK), transforming growth factor-β receptor 2 (TGFBR2), rho-family GTPase-2 (RAC2), and versican (VCAN). These genes along with other differentially expressed components of integrated networks may reflect functional susceptibility to chronic elevated intraocular pressure that is enhanced in the optic nerve head of African Americans.

Endothelin-1 regulates astrocyte proliferation and reactive gliosis via a JNK/c-Jun signaling pathway

Reactive gliosis is characterized by enhanced glial fibrillary acidic protein (GFAP) expression, cellular hypertrophy, and astrocyte proliferation. The cellular and molecular mechanisms underlying this process are still largely undefined. We investigated the role of endothelin-1 (ET-1) in reactive gliosis in corpus callosum after lysolecithin (LPC)-induced focal demyelination and in cultured astrocytes. We show that ET-1 levels are upregulated in demyelinated lesions within 5 d after LPC injection, together with enhanced astrocyte proliferation, GFAP expression, and JNK phosphorylation. Infusion of the pan-ET-receptor (ET-R) antagonist Bosentan or the selective ET(B)-R antagonist BQ788 into the corpus callosum prevented postlesion astrocyte proliferation and JNK phosphorylation. In cultured astrocytes, ET-1-induced activation of ET(B)-Rs promotes a reactive phenotype by enhancing both GFAP expression and astrocyte proliferation. In the same cells, ET-1 activates both JNK and p38MAPK pathways, and induces c-Jun expression at the mRNA and protein levels. By using selective pharmacological inhibitors, we also provide evidence that ET-1 induces astrocyte proliferation and GFAP expression through activation of ERK- and JNK-dependent pathways, consistent with the previous observation of ET-1-induced activation of ERK (Schinelli et al., 2001). Finally, we show by gain and loss of function that increased c-Jun expression enhances the proliferative response of astrocytes to ET-1, whereas c-jun siRNA prevents ET-1-induced cell proliferation. Our results indicate that the effects of ET-1 on astrocyte proliferation depend on c-Jun induction and activation through ERK- and JNK-dependent pathways, and suggest that ET-R-associated pathways might represent important targets to control reactive gliosis.

Is there more to glaucoma treatment than lowering IOP?

Classic glaucoma treatment focuses on intraocular pressure (IOP) reduction. Better knowledge of the pathogenesis of the disease has opened up new therapeutical approaches. Whereas most of these new avenues of treatment are still in the experimental phase, others, such as magnesium, gingko, salt and fludrocortisone, are already used by some physicians. Blood pressure dips can be avoided by intake of salt or fludrocortisone. Vascular regulation can be improved locally by carbonic anhydrase inhibitors, and systemically with magnesium or with low doses of calcium channel blockers. Experimentally, glaucomatous optic neuropathy can be prevented by inhibition of astrocyte activation, either by blockage of epidermal growth factor receptor or by counteracting endothelin. Glaucomatous optic neuropathy can also be prevented by nitric oxide-2 synthase inhibition. Inhibition of matrix metalloproteinase-9 inhibits apoptosis of retinal ganglion cells and tissue remodeling. Upregulation of heat shock proteins protects the retinal ganglion cells and the optic nerve head. Reduction of oxidative stress especially at the level of mitochondria also seems to be protective. This can be achieved by gingko; dark chocolate; polyphenolic flavonoids occurring in tea, coffee, or red wine; anthocyanosides found in bilberries; as well as by ubiquinone and melatonin.

Regulation of c-fos, c-jun and c-myc Gene Expression by Angiotensin II in Primary Cultured Rat Astrocytes: Role of ERK1/2 MAP Kinases

We have previously shown that angiotensin II (Ang II) stimulates astrocyte growth through activation of ERK1/2 mitogen activated protein (MAP) kinases. In the current study, we determined whether Ang II stimulates the expression of c-fos, c-jun and c-myc in brainstem astrocyte cultures. Reverse transcriptase-PCR analysis showed c-fos, c-jun, and c-myc mRNAs were induced by Ang II. The EC50 values for Ang II stimulation of c-fos, c-jun and c-myc were 1.3, 1.68 and 1.4 nM, respectively. Ang II (100 nM) induced peak stimulation for all genes by 45 min followed by a gradual decline. Inhibition of ERK1/2 by PD98059 attenuated Ang II-induced c-fos and c-myc mRNA expression (by 75% and 100%, respectively) but was ineffective in preventing Ang II induction of c-jun. These studies show for the first time in brainstem astrocytes that Ang II induces the expression of c-fos, c-myc and c-jun, and showed that ERK1/2 mediate Ang II stimulation of c-fos and c-myc. These data implicate the ERK1/2 MAP kinase pathway as a divergent point in controlling Ang II stimulation of immediate early response genes in the central nervous system.

Optic Nerve Restoration

Neural regeneration and repair in the central nervous system are currently hot topics in neuroscience. For many years there has been a hope that neurodegenerative diseases which are resistant to current therapies may be treated by the selective replacement of cells. Yet it is only recently that we have started to acquire the knowledge, tools, and techniques that may translate such optimism into new therapies. In this article, we will consider the potential to restore function to the damaged optic nerve. We will consider the technical issues involved and suggest a strategy for research progress.

Signal transduction pathways for epidermal growth factor stimulated cyclooxygenase-2 induction in astrocytes

Cyclooxygenase-2 (COX-2) derived prostaglandins (PGs) are pathophysiological mediators in various disease states. Recently, we have demonstrated the rapid, epidermal growth factor receptor (EGFR)-dependent induction of COX-2 and PGE(2) synthesis in astrocytes following optic nerve injury and in culture. We have now investigated the signal transduction pathways activated by EGFR to accomplish the expression of COX-2 in primary optic nerve astrocytes. When astrocytes were exposed to EGF, marked, rapid gene expression of COX-2 was observed. Activation of EGFR caused an increase in the phosphorylation of extracellular signal-regulated kinase (ERK), p38 MAPK (p38) and c-Jun NH (2)-terminal kinase (JNK). Furthermore, U0126, an ERK pathway inhibitor, and SB203580, a p38 MAPK inhibitor, diminished EGF-induced COX-2 expression; whereas, a JNK inhibitor did not suppress COX-2 expression by EGF. Using inhibitors of several other signaling cascades, we found that, unlike epithelial and cancer cells, NF-kappaB, PI 3-kinase/Akt and PKC were not signaling pathways for EGFR-dependent induction of COX-2 in optic nerve astrocytes. Taken together, these data suggest that ERK and p38 are key components of the intracellular signaling switch that transduces EGFR activation into COX-2 induction and PGE(2) biosynthesis in optic nerve astrocytes.

4-Hydroxynonenal, a product of oxidative stress, leads to an antioxidant response in optic nerve head astrocytes

Oxidative stress has been implicated in the pathogenesis of several neurodegenerative disorders including primary open-angle glaucoma (POAG) an optic neuropathy characterized by loss of retinal ganglion cell (RGC) axons and remodeling of the optic nerve head (ONH). Previous findings in glaucomatous astrocytes suggested increased oxidative stress and lipid peroxidation in human optic nerves. We studied the dose and time dependent effects of 4-hydroxynonenal (HNE), a by-product of lipid peroxidation, on the viability of primary cultures of human ONH astrocyte. A significant depletion of glutathione (GSH) level was observed in normal astrocytes after exposure to HNE for 1 h and 3 h. Untreated glaucomatous astrocytes exhibited depleted levels of GSH which increased slightly after exposure to HNE. Both normal and glaucomatous astrocytes recovered GSH levels after 24 h of removal of HNE. HNE caused significant increases in expression of antioxidant enzymes, glutamate cysteine ligase catalytic subunit (GCLC), aldo-keto reductase 1C family member 1 (AKR1C1) and glutathione S-transferase-alpha4 (GSTA4). HNE induced expression of the transcription factor Nrf2, which coordinates the upregulation of detoxification enzymes. In addition, ONH astrocytes responded to HNE by activation and transcription of cFOS and NFkB, which regulate physiological protective responses against oxidative stress. Our results indicate that ONH astrocytes exhibit a strong antioxidant response to HNE treatment by inducing the transcription factors cFOS, NFkB, and Nrf2, which upregulate the expression of GCLC, to produce more GSH in the cell. AKR1C1 was also upregulated after HNE treatment to inactivate HNE, independent of GSH availability in the cells. Collectively these data indicate that ONH astrocytes can efficiently counteract the neurotoxic effects of HNE offering protection in the optic nerve by releasing GSH and antioxidant enzymes to eliminate the products of chronic oxidative stress.