Histone deacetylase inhibition-mediated differentiation of RGC-5 cells and interaction with survival

Retinal ganglion cell repopulation for vision restoration in optic neuropathy: a roadmap from the RReSTORe Consortium

Retinal ganglion cell (RGC) death in glaucoma and other optic neuropathies results in irreversible vision loss due to the mammalian central nervous system's limited regenerative capacity. RGC repopulation is a promising therapeutic approach to reverse vision loss from optic neuropathies if the newly introduced neurons can reestablish functional retinal and thalamic circuits. In theory, RGCs might be repopulated through the transplantation of stem cell-derived neurons or via the induction of endogenous transdifferentiation. The RGC Repopulation, Stem Cell Transplantation, and Optic Nerve Regeneration (RReSTORe) Consortium was established to address the challenges associated with the therapeutic repair of the visual pathway in optic neuropathy. In 2022, the RReSTORe Consortium initiated ongoing international collaborative discussions to advance the RGC repopulation field and has identified five critical areas of focus: (1) RGC development and differentiation, (2) Transplantation methods and models, (3) RGC survival, maturation, and host interactions, (4) Inner retinal wiring, and (5) Eye-to-brain connectivity. Here, we discuss the most pertinent questions and challenges that exist on the path to clinical translation and suggest experimental directions to propel this work going forward. Using these five subtopic discussion groups (SDGs) as a framework, we suggest multidisciplinary approaches to restore the diseased visual pathway by leveraging groundbreaking insights from developmental neuroscience, stem cell biology, molecular biology, optical imaging, animal models of optic neuropathy, immunology & immunotolerance, neuropathology & neuroprotection, materials science & biomedical engineering, and regenerative neuroscience. While significant hurdles remain, the RReSTORe Consortium's efforts provide a comprehensive roadmap for advancing the RGC repopulation field and hold potential for transformative progress in restoring vision in patients suffering from optic neuropathies.

A Case Study from the Past: "The RGC-5 vs. the 661W Cell Line: Similarities, Differences and Contradictions-Are They Really the Same?"

In the pursuit of identifying the underlying pathways of ocular diseases, the use of cell lines such as (retinal ganglion cell-5) RGC-5 and 661W became a valuable tool, including pathologies like retinal degeneration and glaucoma. In 2001, the establishment of the RGC-5 cell line marked a significant breakthrough in glaucoma research. Over time, however, concerns arose about the true nature of RGC-5 cells, with conflicting findings in the literature regarding their identity as retinal ganglion cells or photoreceptor-like cells. This study aimed to address the controversy surrounding the RGC-5 cell line's origin and properties by comparing it with the 661W cell line, a known cone photoreceptor model. Both cell lines were differentiated according to two prior published redifferentiation protocols under the same conditions using 500 nM of trichostatin A (TSA) and investigated for their morphological and neuronal marker properties. The results demonstrated that both cell lines are murine, and they exhibited distinct morphological and neuronal marker properties. Notably, the RGC-5 cells showed higher expression of the neuronal marker β-III tubulin and increased Thy-1-mRNA compared with the 661W cells, providing evidence of their different properties. The findings emphasize the importance of verifying the authenticity of cell lines used in ocular research and highlight the risks of contamination and altered cell properties.

Roles of Histone Acetyltransferases and Deacetylases in the Retinal Development and Diseases

The critical role of epigenetic modification of histones in maintaining the normal function of the nervous system has attracted increasing attention. Among these modifications, the level of histone acetylation, modulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs), is essential in regulating gene expression. In recent years, the research progress on the function of HDACs in retinal development and disease has advanced remarkably, while that regarding HATs remains to be investigated. Here, we overview the roles of HATs and HDACs in regulating the development of diverse retinal cells, including retinal progenitor cells, photoreceptor cells, bipolar cells, ganglion cells, and Müller glial cells. The effects of HATs and HDACs on the progression of various retinal diseases are also discussed with the highlight of the proof-of-concept research regarding the application of available HDAC inhibitors in treating retinal diseases.

Histone Deacetylases Inhibitors in Neurodegenerative Diseases, Neuroprotection and Neuronal Differentiation

Histone deacetylases (HADC) are the enzymes that remove acetyl group from lysine residue of histones and non-histone proteins and regulate the process of transcription by binding to transcription factors and regulating fundamental cellular process such as cellular proliferation, differentiation and development. In neurodegenerative diseases, the histone acetylation homeostasis is greatly impaired, shifting towards a state of hypoacetylation. The histone hyperacetylation produced by direct inhibition of HDACs leads to neuroprotective actions. This review attempts to elaborate on role of small molecule inhibitors of HDACs on neuronal differentiation and throws light on the potential of HDAC inhibitors as therapeutic agents for treatment of neurodegenerative diseases. The role of HDACs in neuronal cellular and disease models and their modulation with HDAC inhibitors are also discussed. Significance of these HDAC inhibitors has been reviewed on the process of neuronal differentiation, neurite outgrowth and neuroprotection regarding their potential therapeutic application for treatment of neurodegenerative diseases.

The Histone Deacetylase Inhibitor AN7, Attenuates Choroidal Neovascularization in a Mouse Model

: Choroidal neovascularization (CNV) is a complication of age-related macular degeneration and a major contributing factor to vision loss. In this paper, we show that in a mouse model of laser-induced CNV, systemic administration of Butyroyloxymethyl-diethyl phosphate (AN7), a histone deacetylase inhibitor (HDACi), significantly reduced CNV area and vascular leakage, as measured by choroidal flatmounts and fluorescein angiography. CNV area reduction by systemic AN7 treatment was similar to that achieved by intravitreal bevacizumab treatment. The expression of vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF-2), and the endothelial cells marker CD31, was lower in the AN7 treated group in comparison to the control group at the laser lesion site. In vitro, AN7 facilitated retinal pigmented epithelium (RPE) cells tight junctions' integrity during hypoxia, by protecting the hexagonal pattern of ZO-1 protein in the cell borders, hence reducing RPE permeability. In conclusion, systemic AN7 should be further investigated as a possible effective treatment for CNV.

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.

Epigenetics and Signaling Pathways in Glaucoma

Glaucoma is the most common cause of irreversible blindness worldwide. This neurodegenerative disease becomes more prevalent with aging, but predisposing genetic and environmental factors also contribute to increased risk. Emerging evidence now suggests that epigenetics may also be involved, which provides potential new therapeutic targets. These three factors work through several pathways, including TGF-β, MAP kinase, Rho kinase, BDNF, JNK, PI-3/Akt, PTEN, Bcl-2, Caspase, and Calcium-Calpain signaling. Together, these pathways result in the upregulation of proapoptotic gene expression, the downregulation of neuroprotective and prosurvival factors, and the generation of fibrosis at the trabecular meshwork, which may block aqueous humor drainage. Novel therapeutic agents targeting these pathway members have shown preliminary success in animal models and even human trials, demonstrating that they may eventually be used to preserve retinal neurons and vision.

Early changes in staurosporine-induced differentiated RGC-5 cells indicate cellular injury response to nonlethal blue light exposure

PURPOSE

Blue light has been previously demonstrated to induce injury of retinal cells. The cellular responses to nonlethal blue light exposure for each type of retinal cell are of particular interest but remain undetermined. Based on the doses of blue light reported in previous research to be nonlethal to retinal pigment epithelial cells, here we investigated whether and to what extent such doses of blue light are cytotoxic to staurosporine-differentiated RGC-5 cells.

METHODS

RGC-5 cells were differentiated for 24 hours using 200 nM staurosporine. The resulting cells were cultured and exposed to blue light at three different energy levels (1, 10, and 50 J cm(-2)). Cellular morphologies were investigated with an inverted microscope and cell viability was assessed with a Cell Counting Kit-8 (CCK-8) assay. The generation of intracellular reactive oxygen species (ROS) was evaluated by H2DCFDA. After loading of MitoTracker Green FM dye, the mitochondrial contents were analyzed using flow cytometry. The lactate dehydrogenase (LDH) activities in the media were also measured. The level of lipid peroxidation was determined by measuring the amount of malondialdehyde (MDA).

RESULTS

Treatment of the cells for 24 hours with 200 nM staurosporine successfully induced the differentiation of RGC-5 cells. No morphological changes were observed in the ssdRGC-5 cells exposed to blue light at 50 J cm(-2), which was the highest energy level tested. Exposure of the ssdRGC-5 cells to this energy level of blue light did, however, decrease their numbers by approximately 72.1% compared to the numbers of such cells found after being left in the dark. Remarkably, the levels of ROS generation and mitochondrial contents were, respectively, increased to 142% and 118% of those of the control by a 10 J cm(-2) exposure of blue light. The LDH activities and MDA levels exhibited no obvious changes in the blue light-exposed ssdRGC-5 cells compared to the control cells.

CONCLUSIONS

In vitro nonlethal blue light exposure led to cellular damage of staurosporine-differentiated RGC-5 cells. These increases in oxidative stress and mitochondrial content were the early steps of the cellular response to the exposure of relatively low doses (10 J cm(-2)) of blue light.

Histone Deacetylases Inhibitors in the Treatment of Retinal Degenerative Diseases: Overview and Perspectives

Retinal degenerative diseases are one of the important refractory ophthalmic diseases, featured with apoptosis of photoreceptor cells. Histone acetylation and deacetylation can regulate chromosome assembly, gene transcription, and posttranslational modification, which are regulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively. The histone deacetylase inhibitors (HDACis) have the ability to cause hyperacetylation of histone and nonhistone proteins, resulting in a variety of effects on cell proliferation, differentiation, anti-inflammation, and anti-apoptosis. Several HDACis have been approved for clinical trials to treat cancer. Studies have shown that HDACis have neuroprotective effects in nervous system damage. In this paper, we will summarize the neuroprotective effects of common HDACis in retinal degenerative diseases and make a prospect to the applications of HDACis in the treatment of retinal degenerative diseases in the future.

Interaction and colocalization of HERMES/RBPMS with NonO, PSF, and G3BP1 in neuronal cytoplasmic RNP granules in mouse retinal line cells

HERMES, also called RBPMS, is a conserved RNA binding protein with a single RNA recognition motif (RRM) that is abundantly expressed in retinal ganglion cells (RGCs) and in the heart in vertebrates. Here, we identified NonO and PSF as the interacting proteins of HERMES only when the neuronal differentiation of the retinal cell line RGC-5 was induced. Although NonO and PSF are nuclear paraspeckle components, these proteins formed cytoplasmic granules with HERMES in the neurites. G3BP1, a component of stress granules, was also colocalized to the granules, interacting with NonO and HERMES even in the absence of cellular stress. Consistent with a previous report that KIF5 interacts with neuronal granules, the localization of KIF5A overlapped with the cytoplasmic granules in differentiated RGC-5 cells. Thus, our study strongly suggests that the cytoplasmic granule containing HERMES, NonO, PSF, and G3BP1 is a neuronal RNA-protein granule that is transported in neurites during retinal differentiation.

Acetylation preserves retinal ganglion cell structure and function in a chronic model of ocular hypertension

PURPOSE

The current studies investigate if the histone deacetylase (HDAC) inhibitor, valproic acid (VPA), can limit retinal ganglion cell (RGC) degeneration in an ocular-hypertensive rat model.

METHODS

Intraocular pressure (IOP) was elevated unilaterally in Brown Norway rats by hypertonic saline injection. Rats received either vehicle or VPA (100 mg/kg) treatment for 28 days. Retinal ganglion cell function and number were assessed by pattern electroretinogram (pERG) and retrograde FluoroGold labeling. Western blotting and a fluorescence assay were used for determination of histone H3 acetylation and HDAC activity, respectively, at 3-day, 1-week, and 2-week time points.

RESULTS

Hypertonic saline injections increased IOPs by 7 to 14 mm Hg. In vehicle-treated animals, ocular hypertension resulted in a 29.1% and 39.4% decrease in pERG amplitudes at 2 and 4 weeks, respectively, and a 42.9% decrease in mean RGC density at 4 weeks. In comparison, VPA treatment yielded significant amplitude preservation at 2 and 4 weeks and showed significant RGC density preservation at 4 weeks. No significant difference in RGC densities or IOPs was measured between control eyes of vehicle- and VPA-treated rats. In ocular-hypertensive eyes, class I HDAC activity was significantly elevated within 1 week (13.3 ± 2.2%) and histone H3 acetylation was significantly reduced within 2 weeks following the induction of ocular hypertension.

CONCLUSIONS

Increase in HDAC activity is a relatively early retinal event induced by elevated IOP, and suppressing HDAC activity can protect RGCs from ocular-hypertensive stress. Together these data provide a basis for developing HDAC inhibitors for the treatment of optic neuropathies.

The histone deacetylase inhibitor trichostatin A induces neurite outgrowth in PC12 cells via the epigenetically regulated expression of the nur77 gene

Histone deacetylase (HDAC) inhibitors induce histone acetylation and gene expression by changing local chromatin structures. They can thereby influence various cells to proliferate or differentiate. It has been reported that trichostatin A (TSA) or valproic acid (VPA) can induce the neuronal differentiation of mouse embryonic neural stem cells and rat cerebellar granule cells. It is unclear however which gene is responsible for the neuronal differentiation induced by HDAC inhibitors. In this study, we investigated the contribution of immediate early gene (IEG) nur77 to the neuronal differentiation induced by TSA. We report that TSA induces neurite outgrowth in PC12 cells, and C646, an inhibitor of HAT (histone acetyl transferase) (p300), prevents TSA-induced neurite formation. The acetylation of the Lys14 residue of histone H3, and mRNA and protein expression of nur77 gene were found to be stimulated after treatment with TSA, but not in the presence of C646. A knock-down of nur77 inhibits the neurite outgrowth induced by TSA. Furthermore, the ectopic expression of nur77 significantly elicits neurite formation in PC12 cells. These results suggest that the expression of nur77, which is up-regulated via the TSA-induced acetylation of Lys14 on histone H3, is essential for the neuronal differentiation in TSA-induced PC12 cells.

Acetylation: a lysine modification with neuroprotective effects in ischemic retinal degeneration

Neuroretinal ischemic injury contributes to several degenerative diseases in the eye and the resulting pathogenic processes involving a series of necrotic and apoptotic events. This study investigates the time and extent of changes in acetylation, and whether this influences function and survival of neuroretinal cells following injury. Studies evaluated the time course of changes in histone deacetylase (HDAC) activity, histone-H3 acetylation and caspase-3 activation levels as well as retinal morphology and function (electroretinography) following ischemia. In addition, the effect of two HDAC inhibitors, trichostatin-A and valproic acid were also investigated. In normal eyes, retinal ischemia produced a significant increase in HDAC activity within 2 h that was followed by a corresponding significant decrease in protein acetylation by 4 h. Activated caspase-3 levels were significantly elevated by 24 h. Treatment with HDAC inhibitors blocked the early decrease in protein acetylation and activation of caspase-3. Retinal immunohistochemistry demonstrated that systemic administration of trichostatin-A or valproic acid, resulted in hyperacetylation of all retinal layers after systemic treatment. In addition, HDAC inhibitors provided a significant functional and structural neuroprotection at seven days following injury relative to vehicle-treated eyes. These results provide evidence that increases in HDAC activity is an early event following retinal ischemia, and are accompanied by corresponding decreases in acetylation in advance of caspase-3 activation. In addition to preserving acetylation status, the administration of HDAC inhibitors suppressed caspase activation and provided structural and functional neuroprotection in model of ischemic retinal injury. Taken together these data provide evidence that decrease in retinal acetylation status is a central event in ischemic retinal injury, and the hyperacetylation induced by HDAC inhibition can provide acute neuroprotection.

Histone deacetylase expression patterns in developing murine optic nerve

Background

Histone deacetylases (HDACs) play important roles in glial cell development and in disease states within multiple regions of the central nervous system. However, little is known about HDAC expression or function within the optic nerve. As a first step in understanding the role of HDACs in optic nerve, this study examines the spatio-temporal expression patterns of methylated histone 3 (K9), acetylated histone 3 (K18), and HDACs 1–6 and 8–11 in the developing murine optic nerve head.

Results

Using RT-qPCR, western blot and immunofluorescence, three stages were analyzed: embryonic day 16 (E16), when astrocyte precursors are found in the optic stalk, postnatal day 5 (P5), when immature astrocytes and oligodendrocytes are found throughout the optic nerve, and P30, when optic nerve astrocytes and oligodendrocytes are mature. Acetylated and methylated histone H3 immunoreactivity was co-localized in the nuclei of most SOX2 positive glia within the optic nerve head and adjacent optic nerve at all developmental stages. HDACs 1–11 were expressed in the optic nerve glial cells at all three stages of optic nerve development in the mouse, but showed temporal differences in overall levels and subcellular localization. HDACs 1 and 2 were predominantly nuclear throughout optic nerve development and glial cell maturation. HDACs 3, 5, 6, 8, and 11 were predominantly cytoplasmic, but showed nuclear localization in at least one stage of optic nerve development. HDACs 4, 9 and10 were predominantly cytoplasmic, with little to no nuclear expression at any time during the developmental stages examined.

Conclusions

Our results showing that HDACs 1, 2, 3, 5, 6, 8, and 11 were each localized to the nuclei of SOX2 positive glia at some stages of optic nerve development and maturation and extend previous reports of HDAC expression in the aging optic nerve. These HDACs are candidates for further research to understand how chromatin remodeling through acetylation, deacetylation and methylation contributes to glial development as well as their injury response.

Dysfunction of the stress-responsive FOXC1 transcription factor contributes to the earlier-onset glaucoma observed in Axenfeld-Rieger syndrome patients

Mutations in the Forkhead Box C1 (FOXC1) transcription factor gene are associated with Axenfeld-Rieger syndrome (ARS), a developmental disorder affecting structures in the anterior segment of the eye. Approximately 75% of ARS patients with FOXC1 mutations develop earlier-onset glaucoma. Constant exposure of the trabecular meshwork (TM), located in the anterior segment of the eye, to oxidative stress is predicted to be a risk factor for developing glaucoma. Stress-induced death of TM cells results in dysfunction of the TM, leading to elevated intraocular pressure, which is a major risk factor for developing glaucoma. FOXC1 is predicted to maintain homeostasis in TM cells by regulating genes that are important for stress response. In this study, we show that a member of the heat-shock 70 family of proteins, HSPA6, is a target gene of FOXC1. HSPA6 protein, which is only induced under severe oxidative stress conditions, has a protective function in human trabecular meshwork (HTM) cells. We also show that FOXC1 is anti-apoptotic as knocking down FOXC1 significantly decreases HTM cell viability. In addition, we show that FOXC1 itself responds to stress as exposure of cells to H₂O₂-induced oxidative stress reduces FOXC1 levels and activity. Conditions that decrease FOXC1 function, such as exposure of cells to oxidative stress and FOXC1 ARS mutations, compromise the ability of TM cells to effectively respond to environmental stresses. Dysfunction of FOXC1 contributes to the death of TM cells, an important step in the development of glaucoma.

What is the nature of the RGC-5 cell line?

The immortalized RGC-5 cell line has been widely used as a cell culture model to study the neurobiology of retinal ganglion cells (RGCs). The cells were originally introduced as derived from rat RGC showing expression of various neuronal markers, in particular the RGC-characteristic proteins Brn3 and Thy1. Recent data gave rise to concerns regarding the origin and nature of the cells. RGC-5 cells were identified to be of mouse origin and their expression of RGC characteristics was questioned by some laboratories. This article summarizes the available data on the properties of RGC-5, discusses common protocols for their differentiation and is aimed at providing researchers some guidelines on the benefits and limitations of RGC-5 for research.

Neuritogenic activity of trichostatin A in adult rat retinal ganglion cells through acetylation of histone H3 lysine 9 and RARβ induction

Like other CNS neurons, mature retinal ganglion cells (RGCs) cannot regenerate their axons after nerve injury due to loss of regenerative capacity. One of the reasons why they lose their capacity seems to be a dramatic shift in gene expression of RGCs under epigenetic modulation. In here, we found that levels of histone H3 lysine 9 acetylation decreased after birth in RGCs. This decrease showed good correlation with restriction of retinoic acid receptor β (RARβ) expression in RGCs after birth. Furthermore, we demonstrated that a histone deacetylase inhibitor, trichostatin A, induced axonal regeneration of adult rat RGCs through RARβ induction.

Requirement of retinoic acid receptor β for genipin derivative-induced optic nerve regeneration in adult rat retina

Like other CNS neurons, mature retinal ganglion cells (RGCs) are unable to regenerate their axons after nerve injury due to a diminished intrinsic regenerative capacity. One of the reasons why they lose the capacity for axon regeneration seems to be associated with a dramatic shift in RGCs’ program of gene expression by epigenetic modulation. We recently reported that (1R)-isoPropyloxygenipin (IPRG001), a genipin derivative, has both neuroprotective and neurite outgrowth activities in murine RGC-5 retinal precursor cells. These effects were both mediated by nitric oxide (NO)/S-nitrosylation signaling. Neuritogenic activity was mediated by S-nitrosylation of histone deacetylase-2 (HDAC2), which subsequently induced retinoic acid receptor β (RARβ) expression via chromatin remodeling in vitro. RARβ plays important roles of neural growth and differentiation in development. However, the role of RARβ expression during adult rat optic nerve regeneration is not clear. In the present study, we extended this hypothesis to examine optic nerve regeneration by IPRG001 in adult rat RGCs in vivo. We found a correlation between RARβ expression and neurite outgrowth with age in the developing rat retina. Moreover, we found that IPRG001 significantly induced RARβ expression in adult rat RGCs through the S-nitrosylation of HDAC2 processing mechanism. Concomitant with RARβ expression, adult rat RGCs displayed a regenerative capacity for optic axons in vivo by IPRG001 treatment. These neuritogenic effects of IPRG001 were specifically suppressed by siRNA for RARβ. Thus, the dual neuroprotective and neuritogenic actions of genipin via S-nitrosylation might offer a powerful therapeutic tool for the treatment of RGC degenerative disorders.

Inhibition of HDAC2 protects the retina from ischemic injury

PURPOSE

Protein acetylation is an essential mechanism in regulating transcriptional and inflammatory events. Studies have shown that nonselective histone deacetylase (HDAC) inhibitors can protect the retina from ischemic injury in rats. However, the role of specific HDAC isoforms in retinal degenerative processes remains obscure. The purpose of this study was to investigate the role of HDAC2 isoform in a mouse model of ischemic retinal injury.

METHODS

Localization of HDAC2 in mice retinas was evaluated by immunohistochemical analyses. To investigate whether selective reduction in HDAC2 activity can protect the retina from ischemic injury, Hdac2⁺/⁻ mice were utilized. Electroretinographic (ERG) and morphometric analyses were used to assess retinal function and morphology.

RESULTS

Our results demonstrated that HDAC2 is primarily localized in nuclei in inner nuclear and retinal ganglion cell layers, and HDAC2 activity accounted for approximately 35% of the total activities of HDAC1, 2, 3, and 6 in the retina. In wild-type mice, ERG a- and b-waves from ischemic eyes were significantly reduced when compared to pre-ischemia baseline values. Morphometric examination of these eyes revealed significant degeneration of inner retinal layers. In Hdac2⁺/⁻ mice, ERG a- and b-waves from ischemic eyes were significantly greater than those measured in ischemic eyes from wild-type mice. Morphologic measurements demonstrated that Hdac2⁺/⁻ mice exhibit significantly less retinal degeneration than wild-type mice.

CONCLUSIONS

This study demonstrated that suppressing HDAC2 expression can effectively reduce ischemic retinal injury. Our results support the idea that the development of selective HDAC2 inhibitors may provide an efficacious treatment for ischemic retinal injury.

Nuclear morphology and c-Jun N-terminal kinase 1 expression differentiate serum-starved oxidative stress signalling from hydrogen peroxide-induced apoptosis in retinal neuronal cell line

Oxidative stress induced by serum starvation and H2O2 exposure, both triggers apoptosis in retinal neuronal cell line RGC-5 (retinal ganglion cell-5). We have examined whether, despite excess generation of ROS (reactive oxygen species) and apoptosis induction, there is any dissimilarity in nuclear morphology and apoptotic signalling pathway in RGC-5 under these conditions. Sub-confluent cells were treated either with H2O2 or maintained in SFM (serum-free medium). ROS level was detected along with nuclear morphology and ultrastructural analysis. Generation of excess intracellular ROS, nuclear localization of Bax and caspase 3 activation along with decrease of cellular viability, confirmed apoptosis induction in RGC-5 by 72 h serum starvation and 500 M H2O2 exposure for 1 h. Nuclear swelling as supported by nuclear cytoplasmic ratio and conspicuous black spots with nuclear remodelling were observed only upon SFM, but not with H2O2 treatment. Serum starvation did not alter JNK1 (c-Jun N-terminal kinase 1) expression, although nuclear translocation and higher level of pJNK (phospho-JNK) was evident. Conversely, H2O2 exposure blocked the expression and activation of JNK1 to phospho-JNK as a negligible level of pJNK was present in the cytoplasm. Despite similar ROS generation in both the conditions, difference in nuclear morphology and JNK1 expression leads to the hypothesis that RGC-5 cells may follow different signalling pathways when challenged with serum starvation and H2O2.

Establishment of inducible wild type and mutant myocilin-GFP-expressing RGC5 cell lines

Background

Myocilin is a gene linked directly to juvenile- and adult-onset open angle glaucoma. Mutations including Gln368stop (Q368X) and Pro370Leu (P370L) have been identified in patients. The exact role of myocilin and its functional association with glaucoma are still unclear. In the present study, we established tetracycline-inducible (Tet-on) wild type and mutant myocilin-green fluorescence protein (GFP) expressing RGC5 stable cell lines and studied the changes in cell migration and barrier function upon induction.

Methodology/Principal Findings

After several rounds of selection, clones that displayed low, moderate, or high expression of wild type, Q368X or P370L myocilin-GFP upon doxycycline (Dox) induction were obtained. The levels of wild type and mutant myocilin-GFP in various clones were confirmed by Western blotting. Compared to non-induced controls, the cell migration was retarded, the actin stress fibers were fewer and shorter, and the trypsinization time needed for cells to round up was reduced when wild type or mutant myocilin was expressed. The barrier function was in addition aberrant following induced expression of wild type, Q368X or P370L myocilin. Immunoblotting further showed that tight junction protein occludin was downregulated in induced cells.

Conclusions/Significance

Tet-on inducible, stable RGC5 cell lines were established. These cell lines, expressing wild type or mutant (Q368X or P370L) myocilin-GFP upon Dox induction, are valuable in facilitating studies such as proteomics, as well as functional and pathogenesis investigations of disease-associated myocilin mutants. The barrier function was found impaired and the migration of cells was hindered with induced expression of wild type and mutant myocilin in RGC5 cell lines. The reduction in barrier function might be related to the declined level of occludin. The retarded cell migration was consistent with demonstrated myocilin phenotypes including the loss of actin stress fibers, lowered RhoA activities and compromised cell-matrix adhesiveness.

Histone deacetylase inhibitors sodium butyrate and valproic acid delay spontaneous cell death in purified rat retinal ganglion cells

PURPOSE

Histone deacetylase inhibitors (HDACi) have neuroprotective effects under various neurodegenerative conditions, e.g., after optic nerve crush (ONC). HDACi-mediated protection of central neurons by increased histone acetylation has not previously been demonstrated in rat retinal ganglion cells (RGCs), although epigenetic changes were shown to be associated with cell death after ONC. We investigated whether HDACi can delay spontaneous cell death in purified rat RGCs and analyzed concomitant histone acetylation levels.

METHODS

RGCs were purified from newborn (postnatal day [P] 0-P2) rat retinas by immunopanning with antibodies against Thy-1.1 and culturing in serum-free medium for 2 days. RGCs were treated with HDACi, each at several different concentrations: 0.1-10 mM sodium butyrate (SB), 0.1-2 mM valproic acid (VPA), or 0.5-10 nM trichostatin A (TSA). Negative controls were incubated in media alone, while positive controls were incubated in 0.05-0.4 IU/µl erythropoietin. Survival was quantified by counting viable cells using phase-contrast microscopy. The expression of acetylated histone proteins (AcH) 3 and 4 was analyzed in RGCs by immunohistochemistry.

RESULTS

SB and VPA enhanced RGC survival in culture, with both showing a maximum effect at 0.1 mM (increase in survival to 188% and 163%, respectively). Their neuroprotective effect was comparable to that of erythropoietin at 0.05 IU/µl. TSA 0.5-1.0 nM showed no effect on RGC survival, and concentrations ≥ 5 nM increased RGC death. AcH3 and AcH4 levels were only significantly increased in RGCs treated with 0.1 mM SB. VPA 0.1 mM produced only a slight effect on histone acetylation.

CONCLUSIONS

Millimolar concentrations of SB and VPA delayed spontaneous cell death in purified RGCs; however, significantly increased histone acetylation levels were only detectable in RGCs after SB treatment. As the potent HDACi TSA was not neuroprotective, mechanisms other than histone acetylation may be the basis on which SB and VPA are acting in this model. Additional studies are necessary to identify HDACi-targeted genes and pathways involved in RGC protection.

Inhibition of histone deacetylase protects the retina from ischemic injury

PURPOSE. The pathogenesis of retinal ischemia results from a series of events involving changes in gene expression and inflammatory cytokines. Protein acetylation is an essential mechanism in regulating transcriptional and inflammatory events. The purpose of this study was to investigate the neuroprotective action of the histone deacetylase (HDAC) inhibitor trichostatin A (TSA) in a retinal ischemic model. METHODS. To investigate whether HDAC inhibition can reduce ischemic injury, rats were treated with TSA (2.5 mg/kg intraperitoneally) twice daily on days 0, 1, 2, and 3. Seven days after ischemic injury, morphometric and electroretinographic (ERG) analyses were used to assess retinal structure and function. Western blot and immunohistochemical analyses were used to evaluate TSA-induced changes in histone-H3 acetylation and MMP secretion. RESULTS. In vehicle-treated animals, ERG a- and b-waves from ischemic eyes were significantly reduced compared with contralateral responses. In addition, histologic examination of these eyes revealed significant degeneration of inner retinal layers. In rats treated with TSA, amplitudes of ERG a- and b-waves from ischemic eyes were significantly increased, and normal inner retina morphology was preserved. Ischemia also increased the levels of retinal TNF-alpha, which was blocked by TSA treatment. In astrocyte cultures, the addition of TNF-alpha (10 ng/mL) stimulated the secretion of MMP-1 and MMP-3, which were blocked by TSA (100 nM). CONCLUSIONS. These studies provide the first evidence that suppressing HDAC activity can protect the retina from ischemic injury. This neuroprotective response is associated with the suppression of retinal TNF-alpha expression and signaling. The use of HDAC inhibitors may provide a novel treatment for ischemic retinal injury.

Posttranslational modifications, localization, and protein interactions of optineurin, the product of a glaucoma gene

Background

Glaucoma is a major blinding disease. The most common form of this disease, primary open angle glaucoma (POAG), is genetically heterogeneous. One of the candidate genes, optineurin, is linked principally to normal tension glaucoma, a subtype of POAG. The present study was undertaken to illustrate the basic characteristics of optineurin.

Methodology/Principal Findings

Lysates from rat retinal ganglion RGC5 cells were subjected to N- or O-deglycosylation or membrane protein extraction. The phosphorylation status was evaluated after immunoprecipitation. It was found that while phosphorylated, optineurin was neither N- nor O-glycosylated, and was by itself not a membrane protein. RGC5 and human retinal pigment epithelial cells were double stained with anti-optineurin and anti-GM130. The endogenous optineurin exhibited a diffuse, cytoplasmic distribution, but a population of the protein was associated with the Golgi apparatus. Turnover experiments showed that the endogenous optineurin was relatively short-lived, with a half-life of approximately 8 hours. Native blue gel electrophoresis revealed that the endogenous optineurin formed homohexamers. Optineurin also interacted with molecules including Rab8, myosin VI, and transferrin receptor to assemble into supermolecular complexes. When overexpressed, optineurin–green fluorescence protein (GFP) fusion protein formed punctate structures termed “foci” in the perinuclear region. Treatment of nocadazole resulted in dispersion of the optineurin foci. In addition, tetracycline-regulated optineurin-GFPs expressing RGC5 stable cell lines were established for the first time.

Conclusions/Significance

The present study provides new information regarding basic characteristics of optineurin that are important for future efforts in defining precisely how optineurin functions normally and how mutations may result in pathology. The inducible optineurin-GFP–expressing cell lines are also anticipated to facilitate in-depth studies of optineurin. Furthermore, the demonstrations that optineurin is an aggregation-prone protein and that the foci formation is microtubule-dependent bear similarities to features documented in neurodegenerative diseases, supporting a neurodegenerative paradigm for glaucoma.

Neuronal differentiation by analogs of staurosporine

RGC-5 cells are transformed cells that express several surface markers characteristic of neuronal precursor cells, but resemble glial cells morphologically and divide in culture. When treated with the apoptosis-inducing agent staurosporine, RGC-5 cells assume a neuronal morphology, extend neurites, stop dividing, and express ion channels without acute signs of apoptosis. This differentiation with staurosporine is similar to what has been described for certain other neuronal cell lines, and occurs by a mechanism not yet understood. Inhibition of several kinases known to be inhibited by staurosporine fails to differentiate RGC-5 cells, and examination of the kinome associated with staurosporine-dependent differentiation has been unhelpful so far. To better understand the mechanism of staurosporine-mediated differentiation of neuronal precursor cells, we studied the effects of the following structurally similar molecules on differentiation of neuronal and non-neuronal cell lines, comparing them to staurosporine: 9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid, 2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-, methyl ester, (9S,10R,12R)-(K252a), (5R,6S,8S)-6-hydroxy-5-methyl-13-oxo-6,7,8,13,14,15-hexahydro-5H-16-oxa-4b,8a,14-triaza-5,8-methanodibenzo[b,h]cycloocta[jkl]cyclopenta[e]-as-indacene-6-carboxylic acid (K252b), staurosporine aglycone (K252c), 7-hydroxystaurosporine (UCN-01), and 4'-N-benzoylstaurosporine (PKC-412). Morphological differentiation, indicated by neurite extension and somal rounding, was quantitatively assessed with NeuronJ. We found that the critical structural component for differentiation in RGC-5 cells is a basic amine adjacent to an accessible methoxy group at the 3' carbon. Given that UCN-01 and similar compounds are potent anti-cancer drugs, examination of molecules that share similar structural features may yield insights into the design of other drugs for differentiation.

SIRT2-mediated protein deacetylation: An emerging key regulator in brain physiology and pathology

Protein function is considerably altered by posttranslational modification. In recent years, cycles of acetylation/deacetylation emerged as fundamental regulators adjusting biological activity of many proteins. Particularly, protein deacetylation by Sirtuins, a family of atypical histone deacetylases (HDACs), was demonstrated to regulate fundamental cell biological processes including gene expression, genome stability, mitosis, nutrient metabolism, aging, mitochondrial function and cell motility. Given this wealth of biological functions, perhaps not unexpectedly then, pharmacological compounds targeting Sirtuin activity are now prime therapeutic agents for alleviating severity of major diseases encompassing diabetes, cancer, cardiovascular and neurodegenerative disorders in many organs. In this review, we will focus on the brain and its physiological and pathological processes governed by Sirtuin-mediated deacetylation. Besides discussing Sirtuin function in neurodegenerative diseases, emphasis will be given on the mounting evidence deciphering key developmental brain functions for Sirtuins in neuronal motility, neuroprotection and oligodendrocyte differentiation. In this respect, we will particularly highlight functions of the unconventional family member SIRT2 in post-mitotic neurons and glial cells.

Differential effects of myocilin and optineurin, two glaucoma genes, on neurite outgrowth

Myocilin and optineurin are two genes linked to glaucoma, a major blinding disease characterized by progressive loss of retinal ganglion cells (RGCs) and their axons. To investigate the effects of force-expressed wild-type and mutant myocilin and optineurin on neurite outgrowth in neuronal cells, we transiently transfected cells with pEGFP-N1 (mock control) as well as myocilin and optineurin plasmids including pMYOC(WT)-EGFP, pMYOC(P370L)-EGFP, pMYOC(1-367)-EGFP, pOPTN(WT)-EGFP, and pOPTN(E50K)-EGFP. PC12 cells transfected with pEGFP-N1 produced, as anticipated, long and extensive neuritis on nerve growth factor induction. The neurite length in those cells transfected with myocilin constructs was shortened and the number of neurites was also reduced. A similar inhibitory effect on neurite outgrowth was also elicited by myocilin transfection in RGC5 cells. In contrast, neither transfection of the optineurin constructs pOPTN(WT)-EGFP and pOPTN(E50K)-EGFP nor the myocilin and optineurin small-interfering RNA treatments induced significant alterations in neurite outgrowth. Transfection with the wild-type optineurin construct, but not with that of the wild-type myocilin, increased the apoptotic activity in cells. These results demonstrated that the two glaucoma genes, myocilin and optineurin, exhibited differential effects on neurite outgrowth. They may contribute to the development of neurodegenerative glaucoma via distinct mechanisms.