Sook Hyun Chung

Conditional Müller Cell Ablation Causes Independent Neuronal and Vascular Pathologies in a Novel Transgenic Model

Müller cells are the major glia of the retina that serve numerous functions essential to retinal homeostasis, yet the contribution of Müller glial dysfunction to retinal diseases remains largely unknown. We have developed a transgenic model using a portion of the regulatory region of the retinaldehyde binding protein 1 gene for conditional Müller cell ablation and the consequences of primary Müller cell dysfunction have been studied in adult mice. We found that selective ablation of Müller cells led to photoreceptor apoptosis, vascular telangiectasis, blood–retinal barrier breakdown and, later, intraretinal neovascularization. These changes were accompanied by impaired retinal function and an imbalance between vascular endothelial growth factor-A (VEGF-A) and pigment epithelium-derived factor. Intravitreal injection of ciliary neurotrophic factor inhibited photoreceptor injury but had no effect on the vasculopathy. Conversely, inhibition of VEGF-A activity attenuated vascular leak but did not protect photoreceptors. Our findings show that Müller glial deficiency may be an important upstream cause of retinal neuronal and vascular pathologies in retinal diseases. Combined neuroprotective and anti-angiogenic therapies may be required to treat Müller cell deficiency in retinal diseases and in other parts of the CNS associated with glial dysfunction.

The role of glia in retinal vascular disease

Retinal vascular diseases collectively represent a leading cause of blindness. Unsurprisingly, pathological characterisation and treatment of retinal ‘vascular’ diseases have primarily focused on the aetiology and consequences of vascular dysfunction. Far less research has addressed the contribution of neuronal and glial dysfunction to the disease process of retinal vascular disorders. Ample evidence now suggests that retinal vasculopathy only uncommonly occurs in isolation, usually existing in concert with neuropathy and gliopathy. Retinal glia (Müller cells, astrocytes and microglia) have been reported to exhibit morphological and functional changes in both early and advanced phases of almost every retinal vascular disease. It is anticipated that identifying the causes of glial activation and dysfunction, and their contribution to loss of vision in retinal vascular disease, will lead to a better understanding of retinal vascular diseases, which might ultimately be translated into novel clinical therapies.

Tyrosine phosphorylation of VE-cadherin and claudin-5 is associated with TGF-β1-induced permeability of centrally derived vascular endothelium.

Breakdown of the inner blood-retinal barrier and the blood-brain barrier is associated with changes in tight and adherens junction-associated proteins that link vascular endothelial cells. This study aimed to test the hypothesis that transforming growth factor (TGF)-β1 increases the paracellular permeability of vascular endothelial monolayers through tyrosine phosphorylation of VE-cadherin and claudin-5. Bovine retinal and human brain capillary endothelial cells were grown as monolayers on coated polycarbonate membranes. Paracellular permeability was studied by measuring the equilibration of (14)C-inulin or fluorescence-labelled dextran. Changes in VE-cadherin and claudin-5 expression were studied by immunocytochemistry (ICC) and quantified by cell-based enzyme linked immunosorbent assays (ELISA). Tyrosine phosphorylation of VE-cadherin and claudin-5 was studied by ICC, immunoprecipitation and Western blotting. We found that exposure of endothelial cells to TGF-β1 caused a dose-dependent increase in paracellular permeability as reflected by increases in the equilibration of (14)C-inulin. This effect was enhanced by the tyrosine phosphatase inhibitor orthovanadate and attenuated by the tyrosine kinase inhibitor lavendustin A. ICC and cell-based ELISA revealed that TGF-β1 induced both dose- and time-dependent decreases in VE-cadherin and claudin-5 expression. Assessment of cell viability indicated that changes in these junction-associated proteins were not due to endothelial death or injury. ICC revealed that tyrosine phosphorylation of endothelial monolayers was greatly enhanced by TGF-β1 treatment, and immunoprecipitation of cell lysates showed increased tyrosine phosphorylation of VE-cadherin and claudin-5. Our results suggest that tyrosine phosphorylation of VE-cadherin and claudin-5 is involved in the increased paracellular permeability of central nervous system-derived vascular endothelium induced by TGF-β1.

Retinal vascular changes after glial disruption in rats

Glial dysfunction is found in a number of retinal vascular diseases but its link with blood‐retinal barrier (BRB) breakdown remains poorly understood. The present study tested the hypothesis that glial dysfunction is a major contributor to the BRB breakdown that is a hallmark of retinal vascular diseases. We investigated thespecificity of the purportedly selective glial toxin, DL‐α‐aminoadipic acid (DL‐α‐AAA) on different types of ocular cells in vitro and then tested the effect of glial disruption on retinal vasculature after intraocular injection of DL‐α‐AAA or siRNA targeting glutamine synthetase (GS) in rats. DL‐α‐AAA was toxic to astrocytes and Müller cells but not to other types of BRB‐related cells in vitro. Subretinal injection of DL‐α‐AAA disrupted retinal glial cells, induced vascular telangiectasis and increased vascular permeability from 4 days to over 2 months post‐injection. Vascular changes induced by DL‐α‐AAA were observed predominantly in regions of glial disruption, as reflected by reduced expression of GS and increased expression of glial fibrillary acidic protein and vimentin. Confocal microscopy showed changes in all three layers of the retinal vasculature, which co‐localised with areas of Müller cell disruption. Double labeling immunohistochemistry revealed that retinal glial disruption after DL‐α‐AAA injection was accompanied by increased expression of vascular endothelial growth factor and reduced expression of the tight junction protein claudin‐5. Intravitreal injection of GS siRNA induced similar changes in Müller cells and BRB breakdown. Our data are consistent with the hypothesis that glial dysfunction is a primary contributor to the BRB breakdown in retinal vascular diseases.

Profiling of microRNAs involved in retinal degeneration caused by selective Muller cell ablation.

Dysfunction of Müller cells has been implicated in the pathogenesis of several retinal diseases. In order to understand the potential contribution of Müller cells to retinal disease better, we have developed a transgenic model in which foci of Müller cell ablation can be selectively induced. MicroRNAs (miRNAs), small non-coding RNAs that are involved in post-transcriptional modulation, have critical functions in various biological processes. The aim of this study was to profile differential expression of miRNAs and to examine changes in their target genes 2 weeks after Müller cell ablation. We identified 20 miRNAs using the miScript HC PCR array. Data analysis using two target gene prediction databases (TargetScan and mirTarBase) revealed 78 overlapping target genes. DAVID and KEGG pathway analysis suggested that the target genes were generally involved in cell apoptosis, p53, neurotrophin, calcium, chemokine and Jak-STAT signalling pathways. Changes in seven target genes including Cyclin D2, Caspase 9, insulin-like growth factor 1, IL-1 receptor-associated kinase (IRAK), calmodulin (CALM) and Janus kinase 2 (Jak2), were validated with qRT-PCR and western blots. The cellular localisation of cleaved-caspase 9, Cyclin D2, Jak2 and CALM was examined by immunofluorescence studies. We found that the transcription of some miRNAs was positively, rather than negatively, correlated with their target genes. After confirming that overexpressed miR-133a-3p was localised to the outer nuclear layer in the damaged retina, we validated the correlation between miR-133a-3p and one of its predicted target genes, cyclin D2, with a luciferase assay in 661 photoreceptor cells. Results revealed by miRNA profiling, target gene analysis and validation were generally consistent with our previous findings that selective Müller cell ablation causes photoreceptor degeneration and neuroinflammation. Our data on alterations of miRNAs and their target gene expression after Müller cell ablation provide further insights into the potential role of Müller cell dysfunction in retinal disease.

Effect of glucocorticoids on neuronal and vascular pathology in a transgenic model of selective Müller cell ablation.

Retinal diseases such as macular telangiectasis type 2 (MacTel), age‐related macular degeneration (AMD) and diabetic retinopathy (DR) affect both neurons and blood vessels. Treatments addressing both at the same time might have advantages over more specific approaches, such as vascular endothelial growth factor (VEGF) inhibitors, which are used to treat vascular leak but are suspected to have a neurotoxic effect. Here, we studied the effects of an intravitreal injection of triamcinolone acetonide (TA) in a transgenic model in which patchy Müller cell ablation leads to photoreceptor degeneration, vascular leak, and intraretinal neovascularization. TA was injected 4 days before Müller cell ablation. Changes in photoreceptors, microglia and Müller cells, retinal vasculature, differential expression of p75 neurotrophin receptor (p75NTR), tumor necrosis factor‐α (TNFα), the precursor and mature forms of neurotrophin 3 (pro‐NT3 and mature NT3) and activation of the p53 and p38 stress‐activated protein kinase (p38/SAPK) signaling pathways were examined. We found that TA prevented photoreceptor degeneration and inhibited activation of microglial and Müller cells. TA attenuated Müller cell loss and inhibited overexpression of p75NTR, TNFα, pro‐NT, and the activation of p53 and p38/SAPK signaling pathways. TA not only prevented the development of retinal vascular lesions but also inhibited fluorescein leakage from established vascular lesions. TA inhibited overexpression of VEGF in transgenic mice but without affecting its basal level expression in the normal retina. Our data suggest that glucocorticoid treatment may be beneficial for treatment of retinal diseases such as MacTel, AMD, and DR that affect both neurons and the vasculature. GLIA 2014;62:1110–1124

Submacular DL-α-Aminoadipic Acid Eradicates Primate Photoreceptors but Does Not Affect Luteal Pigment or the Retinal Vasculature

PURPOSE Macular telangiectasia type 2 (MT2) is a condition of uncertain etiology characterized by retinal vascular abnormalities, depletion of luteal pigment, and photoreceptor loss. To model this condition, the authors recently used a purportedly glial-selective toxin, DL-α-aminoadipic acid (DL-α-AAA), to test the effect of Müller cell disruption on the blood-retinal barrier in rats. In this study, they investigated macular changes after subretinal injection of DL-α-AAA in monkeys. METHODS Various doses of DL-α-AAA were injected beneath the macula in eight monkey eyes. Eyes were examined by multifocal electroretinography (mfERG), optical coherence tomography (OCT), fundus autofluorescence, color photography, and fluorescein angiography. Five months after injection, eyes were examined by histology and immunohistochemistry for changes in photoreceptors and the retinal glia. In vitro studies evaluated the effect of DL-α-AAA on 661W cone photoreceptor viability. RESULTS Subretinal injection of DL-α-AAA resulted in virtually complete ablation of photoreceptors in the injected area, as shown by OCT and histology, and severely impaired mfERG responses. Müller cells, albeit activated, survived the injury. Macular pigment remained unchanged in the central fovea. Subretinal injection of DL-α-AAA did not induce vascular leakage, though it increased the fundus autofluorescence. DL-α-AAA had a dose-dependent toxic effect on 661W photoreceptors. CONCLUSIONS Submacular injection of DL-α-AAA induced severe damage to photoreceptors but failed to eliminate Müller cells in monkeys. Central macular pigment persisted despite loss of photoreceptors, and the retinal vasculature was unaffected. These observations may have significance in studying the roles of different cellular components in the pathogenesis of MT2.

Systemic Administration of Erythropoietin Inhibits Retinopathy in RCS Rats

Objective Royal College of Surgeons (RCS) rats develop vasculopathy as photoreceptors degenerate. The aim of this study was to examine the effect of erythropoietin (EPO) on retinopathy in RCS rats. Methods Fluorescein angiography was used to monitor retinal vascular changes over time. Changes in retinal glia and vasculature were studied by immunostaining. To study the effects of EPO on retinal pathology, EPO (5000 IU/kg) was injected intraperitoneally in 14 week old normal and RCS rats twice a week for 4 weeks. Changes in the retinal vasculature, glia and microglia, photoreceptor apoptosis, differential expression of p75 neurotrophin receptor (p75NTR), pro-neurotrophin 3 (pro-NT3), tumour necrosis factor-α (TNFα), pigment epithelium derived factor (PEDF) and vascular endothelial growth factor-A (VEGF-A), the production of CD34+ cells and mobilization of CD34+/VEGF-R2+ cells as well as recruitment of CD34+ cells into the retina were examined after EPO treatment. Results RCS rats developed progressive capillary dropout and subretinal neovascularization which were accompanied by retinal gliosis. Systemic administration of EPO stabilized the retinal vasculature and inhibited the development of focal vascular lesions. Further studies showed that EPO modulated retinal gliosis, attenuated photoreceptor apoptosis and p75NTR and pro-NT3 upregulation, promoted the infiltration of ramified microglia and stimulated VEGF-A expression but had little effect on TNFα and PEDF expression. EPO stimulated the production of red and white blood cells and CD34+ cells along with effective mobilization of CD34+/VEGF-R2+ cells. Immunofluorescence study demonstrated that EPO enhanced the recruitment of CD34+ cells into the retina. Conclusions Our results suggest that EPO has therapeutic potentials in treatment of neuronal and vascular pathology in retinal disease. The protective effects of EPO on photoreceptors and the retinal vasculature may involve multiple mechanisms including regulation of retinal glia and microglia, inhibition of p75NTR-pro-NT3 signaling together with stimulation of production and mobilization of bone marrow derived cells.

Laser capture microdissection-directed profiling of glycolytic and mTOR pathways in areas of selectively ablated Müller cells in the murine retina.

PURPOSE We have reported previously down-regulation of key metabolic pathways, the glycolytic and mTOR pathways, from a global retinal microarray analysis after selective Müller cell ablation in a novel transgenic model. The purpose of the present study was to examine changes in expression of key molecules of glycolytic and mTOR pathways specifically in patches of Müller cell loss. METHODS Eyes were enucleated 1 and 3 months after induced Müller cell ablation, directly embedded in optimal cutting temperature medium, and snap frozen in liquid nitrogen. Laser capture microdissection (LCM) was conducted to dissect patches of Müller cell loss for quantitative RT-PCR (qRT-PCR) analysis of key genes of the glycolytic (glyceraldehyde-3-phosphate dehydrogenase, enolase 1 and 2, lactate dehydrogenase A and B) and mTOR pathways (insulin-like growth factor receptor 1, phosphatidylinositide-3-kinase, Akt1, and regulatory-associated protein of mTOR). Protein validations were performed by immunohistochemistry. RESULTS The LCM-directed qRT-PCR analysis of Müller cell ablated specimens demonstrated reduced transcription of genes involved in the glycolytic and mTOR metabolic pathways. Of the proteins we chose to study, only enolase 1 was expressed by Müller cells. Other glycolytic and mTOR pathway proteins were expressed by photoreceptor inner and outer segments, which were lost in patches of Müller cell ablation. CONCLUSIONS We found suppression of genes encoding various glycolytic and mTOR pathway-associated enzymes in areas of Müller cell loss. This appeared mainly to be due to loss of photoreceptor inner and outer segments. The consequences of metabolic derangement caused by Müller cell ablation warrant further investigation.

Laser capture microdissection： from its principle to applications in research on neurodegeneration

History and the principle of laser capture microdissection (LCM): LCM, also known as laser microdissection and pressure catapulting, is one of the most powerful and useful techniques in various research areas where isolation of heterogeneous cell population is required. The first use of laser as a cell operation method, was originated in early 1920s (Gilbrich-Wille, 2013), and it became widely used as a microsurgical tool in early 1960s. In the need of extremely small tissue isolation, laser beam of LCM has been modified and developed over several decades from Ultraviolet (UV) laser beam to high-energy nitrogen, infrared and carbon dioxide lasers (Gilbrich-Wille, 2013). Due to the development of its hardware, which combines a laser unit and a microscope, tissue preparation technique also has improved from manual preparation to histological sections in 1980s and this has brought numerous advantages in biochemistry and molecular biology research (Gilbrich-Wille, 2013).