The role of Müller cell glucocorticoid signaling in diabetic retinopathy

Regulating the formation of Müller glia-derived progenitor cells in the retina

We summarize recent findings in different animal models regarding the different cell-signaling pathways and gene networks that influence the reprogramming of Müller glia into proliferating, neurogenic progenitor cells in the retina. Not surprisingly, most of the cell-signaling pathways that guide the proliferation and differentiation of embryonic retinal progenitors also influence the ability of Müller glia to become proliferating Müller glia-derived progenitor cells (MGPCs). Further, the neuronal differentiation of MGPC progeny is potently inhibited by networks of neurogenesis-suppressing genes in chick and mouse models but occurs freely in zebrafish. There are important differences between the model systems, particularly pro-inflammatory signals that are active in mature Müller glia in damaged rodent and chick retinas, but less so in fish retinas. These pro-inflammatory signals are required to initiate the process of reprogramming, but if sustained suppress the potential of Müller glia to become neurogenic MGPCs. Further, there are important differences in how activated Müller glia up- or downregulate pro-glial transcription factors in the different model systems. We review recent findings regarding regulatory cell signaling and gene networks that influence the activation of Müller glia and the transition of these glia into proliferating progenitor cells with neurogenic potential in fish, chick, and mouse model systems.

Fluocinolone Acetonide Implant for Uveitis: Dissecting Responder and Non-Responder Outcomes at a Tertiary Center

Macular edema (ME) remains a primary cause of visual deterioration in uveitis. Visual acuity (VA) can often be maintained using corticosteroid depot systems. This study evaluated the efficacy of a fluocinolone acetonide (FAc) intravitreal implant (ILUVIEN®) in treating non-infectious uveitis using real-world data. This retrospective analysis included 135 eyes subdivided into responders and non-responders. Central retinal thickness (CRT), VA, and intraocular pressure (IOP) were followed over time. A significant decrease in CRT and an increase in VA were observed in all eyes throughout the follow-up period (p < 0.01). An IOP increase (p = 0.028) necessitated treatment in 43% of eyes by Month 6. Non-responders were older (p = 0.004) and had been treated with more dexamethasone (DEX) implants (p = 0.04); 89.3% had a defect in the external limiting membrane (ELM) and inner/outer segment (IS/OS) zone (p < 0.001). Immunomodulatory therapy had no impact on treatment response. Pars plana vitrectomy (PPV) patients had a mean CRT reduction of 47.55 µm and a reduced effect by Month 24 (p = 0.046) versus non-PPV patients. We conclude that the FAc implant achieves long-term control of CRT and improves VA. Increases in IOP were manageable. Eyes with a previous PPV showed milder results. Data showed a correlation between older age, a damaged ELM and IS/OS zone, frequent DEX inserts, and poorer outcome measures.

GMFB/AKT/TGF-β3 in Müller cells mediated early retinal degeneration in a streptozotocin-induced rat diabetes model

Retinal degeneration, characterized by Müller cell gliosis and photoreceptor apoptosis, is considered an early event in diabetic retinopathy (DR). Our previous study proposed that GMFB may mediate diabetic retinal degeneration. This study identified GMFB as a sensitive and functional gliosis marker for DR. Compared to the wild type (WT) group, Gmfb knockout (KO) significantly improved visual function, attenuated gliosis, reduced the apoptosis of neurons, and decreased the mRNA levels of tumor necrosis factor α (Tnf-α) and interleukin-1β (Il-1β) in diabetic retinas. Tgf-β3 was enriched by hub genes using RNA sequencing in primary WT and KO Müller cells. Gmfb KO significantly upregulated the transforming growth factor (TGF)-β3 protein level via the AKT pathway. The protective effect of TGF-β3 in the vitreous resulted in significantly improved visual function and decreased the number of apoptotic cells in the diabetic retina. The protection of Gmfb KO in primary Müller cells against high glucose (HG)-induced photoreceptor apoptosis was partially counteracted by TGF-β3 antibody and administration of TGFBR1/2 inhibitors. Nuclear receptor subfamily 3 group C member 1 (NR3C1) binds to the promoter region of Gmfb and regulates Gmfb mRNA at the transcriptional level. NR3C1 was increased in the retinas of early diabetic rats but decreased in the retinas of late diabetic rats. N'-[(1E)-(3-Methoxyphenyl)Methylene]-3-Methyl-1H-Pyrazole-5-Carbohydrazide (DS-5) was identified as an inhibitor of GMFB, having a protective role in DR. We demonstrated that GMFB/AKT/TGF-β3 mediated early diabetic retinal degeneration in diabetic rats. This study provides a novel therapeutic strategy for treating retinal degeneration in patients with DR.

The glucocorticoid receptor as a master regulator of the Müller cell response to diabetic conditions in mice

Diabetic retinopathy (DR) is considered a primarily microvascular complication of diabetes. Müller glia cells are at the centre of the retinal neurovascular unit and play a critical role in DR. We therefore investigated Müller cell-specific signalling pathways that are altered in DR to identify novel targets for gene therapy. Using a multi-omics approach on purified Müller cells from diabetic db/db mice, we found the mRNA and protein expression of the glucocorticoid receptor (GR) to be significantly decreased, while its target gene cluster was down-regulated. Further, oPOSSUM TF analysis and ATAC- sequencing identified the GR as a master regulator of Müller cell response to diabetic conditions. Cortisol not only increased GR phosphorylation. It also induced changes in the expression of known GR target genes in retinal explants. Finally, retinal functionality was improved by AAV-mediated overexpression of GR in Müller cells. Our study demonstrates an important role of the glial GR in DR and implies that therapeutic approaches targeting this signalling pathway should be aimed at increasing GR expression rather than the addition of more ligand.

Probucol attenuates high glucose-induced Müller cell damage through enhancing the Nrf2/p62 signaling pathway

PURPOSE

This study investigated the protective effect of probucol on Müller cells exposed to high glucose conditions and examined potential mechanisms of action.

METHODS

Primary human retinal Müller cells were incubated with high glucose (HG, 35 mM) in the present or absence of different concentrations of probucol for 24 h. Cell viability was determined using the CCK-8 method. Mitochondrial membrane potential (MMP) was measured using JC-1 staining and cell cycle by flow cytometry. The expression of nuclear factor E2-related factor 2 (Nrf2), glutamate-cysteine ligase catalytic subunit, and p62 was quantified using quantitative polymerase chain reaction and western blot.

RESULTS

We found that HG inhibited cell proliferation, arrested cell cycle, and increased MMP in human Müller cells. Probucol activated the Nrf2/p62 pathway and upregulated the anti-apoptotic protein, Bcl2, and attenuated HG-mediated damage in Müller cells.

CONCLUSIONS

Our results suggest that probucol may protect Müller cells from HG-induced damage through enhancing the Nrf2/p62 signaling pathway.

SRY-box transcription factor 9 modulates Müller cell gliosis in diabetic retinopathy by upregulating TXNIP transcription

Diabetic retinopathy (DR), a common complication of diabetes, involves excessive proliferation and inflammation of Muller cells and ultimately leads to vision loss and blindness. SRY-box transcription factor 9 (SOX9) has been reported to be highly expressed in Müller cells in light-induced retinal damage rats, but the functional role of SOX9 in DR remains unclear. To explore this issue, the DR rat model was successfully constructed via injection with streptozotocin (65 mg/kg) and the retinal thicknesses and blood glucose levels were evaluated. Müller cells were treated with 25 mmol/l glucose to create a cell model in vitro. The results indicated that SOX9 expression was significantly increased in DR rat retinas and in Müller cells stimulated with a high glucose (HG) concentration. HG treatment promoted the proliferation and migration capabilities of Müller cells, whereas SOX9 knockdown reversed those behaviors. Moreover, SOX9 knockdown provided protection against an HG-induced inflammatory response, as evidenced by reduced tumor necrosis factor-α, IL-1β, and IL-6 levels in serum and decreased NLRP3 inflammasome activation. Notably, SOX9 acted as a transcription factor that positively regulated thioredoxin-interacting protein (TXNIP), a positive regulator of Müller cells gliosis under HG conditions. A dual-luciferase assay demonstrated that SOX9 could enhance TXNIP expression at the transcriptional level through binding to the promoter of TXNIP. Moreover, TXNIP overexpression restored the effects caused by SOX9 silencing. In conclusion, these findings demonstrate that SOX9 may accelerate the progression of DR by promoting glial cell proliferation, metastasis, and inflammation, which involves the transcriptional regulation of TXNIP, providing new theoretical fundamentals for DR therapy.

Retinal regions shape human and murine Müller cell proteome profile and functionality

The human macula is a highly specialized retinal region with pit-like morphology and rich in cones. How Müller cells, the principal glial cell type in the retina, are adapted to this environment is still poorly understood. We compared proteomic data from cone- and rod-rich retinae from human and mice and identified different expression profiles of cone- and rod-associated Müller cells that converged on pathways representing extracellular matrix and cell adhesion. In particular, epiplakin (EPPK1), which is thought to play a role in intermediate filament organization, was highly expressed in macular Müller cells. Furthermore, EPPK1 knockout in a human Müller cell-derived cell line led to a decrease in traction forces as well as to changes in cell size, shape, and filopodia characteristics. We here identified EPPK1 as a central molecular player in the region-specific architecture of the human retina, which likely enables specific functions under the immense mechanical loads in vivo.

Embryonic hyperglycemia perturbs the development of specific retinal cell types, including photoreceptors

Diabetes is linked to various long-term complications in adults, such as neuropathy, nephropathy and diabetic retinopathy. Diabetes poses additional risks for pregnant women, because glucose passes across the placenta, and excess maternal glucose can result in diabetic embryopathy. While many studies have examined the teratogenic effects of maternal diabetes on fetal heart development, little is known about the consequences of maternal hyperglycemia on the development of the embryonic retina. To address this question, we investigated retinal development in two models of embryonic hyperglycemia in zebrafish. Strikingly, we found that hyperglycemic larvae displayed a significant reduction in photoreceptors and horizontal cells, whereas other retinal neurons were not affected. We also observed reactive gliosis and abnormal optokinetic responses in hyperglycemic larvae. Further analysis revealed delayed retinal cell differentiation in hyperglycemic embryos that coincided with increased reactive oxygen species (ROS). Our results suggest that embryonic hyperglycemia causes abnormal retinal development via altered timing of cell differentiation and ROS production, which is accompanied by visual defects. Further studies using zebrafish models of hyperglycemia will allow us to understand the molecular mechanisms underlying these effects.

Understanding Neurodegeneration from a Clinical and Therapeutic Perspective in Early Diabetic Retinopathy

Recent evidence indicates that neurodegeneration is a critical element of diabetic retinopathy (DR) pathogenesis. The neuronal cells’ apoptosis contributes to microvascular impairment and blood–retinal barrier breakdown. Therefore, neurodegeneration represents an early intervention target to slow and prevent the development of microvascular alterations visible on clinical examination. Multimodal imaging features and functional assessment can permit the identification of neuronal damage in a subclinical stage before the recognition of DR signs. Clinical features of neurodegeneration are crucial in identifying patients at high risk of developing a vascular impairment and, thus, serve as outcome measures to understand the efficacy of supplementation. The optimal approach for targeting neurodegeneration contemplates the use of topical compounds that possibly act on different elements of the pathogenic cascade. To date, clinical trials available on humans tested three different topical agents, including brimonidine, somatostatin, and citicoline, with promising results.

Proteomic Phenotyping of Stimulated Müller Cells Uncovers Profound Pro-Inflammatory Signaling and Antigen-Presenting Capacity

Müller cells are the main macroglial cells of the retina exerting a wealth of functions to maintain retinal homoeostasis. Upon pathological changes in the retina, they become gliotic with both protective and detrimental consequences. Accumulating data also provide evidence for a pivotal role of Müller cells in the pathogenesis of diabetic retinopathy (DR). While microglial cells, the resident immune cells of the retina are considered as main players in inflammatory processes associated with DR, the implication of activated Müller cells in chronic retinal inflammation remains to be elucidated. In order to assess the signaling capacity of Müller cells and their role in retinal inflammation, we performed in-depth proteomic analysis of Müller cell proteomes and secretomes after stimulation with INFγ, TNFα, IL-4, IL-6, IL-10, VEGF, TGFβ1, TGFβ2 and TGFβ3. We used both, primary porcine Müller cells and the human Müller cell line MIO-M1 for our hypothesis generating approach. Our results point towards an intense signaling capacity of Müller cells, which reacted in a highly discriminating manner upon treatment with different cytokines. Stimulation of Müller cells resulted in a primarily pro-inflammatory phenotype with secretion of cytokines and components of the complement system. Furthermore, we observed evidence for mitochondrial dysfunction, implying oxidative stress after treatment with the various cytokines. Finally, both MIO-M1 cells and primary porcine Müller cells showed several characteristics of atypical antigen-presenting cells, as they are capable of inducing MHC class I and MHC class II with co-stimulatory molecules. In line with this, they express proteins associated with formation and maturation of phagosomes. Thus, our findings underline the importance of Müller cell signaling in the inflamed retina, indicating an active role in chronic retinal inflammation.

Role of microRNA-25 in high glucose cultured Müller glia

AIM

To investigate the role of microRNA-25 (miR-25) in proliferation and apoptosis of retinal Müller glia (MG) under high glucose condition.

METHODS

The purity of the cultured cells was verified by immunocytochemistry and flow cytometry using antibodies that specifically recognized MG. The expression level of miR-25 under normal and high glucose conditions were validated by quantitative reverse transcription polymerase chain reaction (RT-qPCR). miR-25 mimics and negative control were transfected into MG and multiple functional experiments including cell counting kit-8 assay, EDU assay, and flow cytometry were conducted to explore the effects of miR-25 on the proliferation and apoptosis of high glucose cultured MG (HGMG).

RESULTS

Immunocytochemistry and flow cytometry confirmed the high purity of primary cultured MG. RT-PCR results showed that the expression level of miR-25 was significantly repressed in HGMG, while over-expression of miR-25 by miR-25 mimic markedly inhibited the high glucose induced cell apoptosis and promoted the proliferation of MG.

CONCLUSION

The expression level of miR-25 is significantly downregulated in HGMG and its overexpression could attenuate the high glucose damages on MG by promoting proliferation and reducing apoptosis.

Cell Surface Profiling of Retinal Müller Glial Cells Reveals Association to Immune Pathways after LPS Stimulation

Retinal Müller glial cells (RMG) are involved in virtually every retinal disease; however, the role of these glial cells in neuroinflammation is still poorly understood. Since cell surface proteins play a decisive role in immune system signaling pathways, this study aimed at characterizing the changes of the cell surface proteome of RMG after incubation with prototype immune system stimulant lipopolysaccharide (LPS). While mass spectrometric analysis of the human Müller glia cell line MIO-M1 revealed 507 cell surface proteins in total, with 18 proteins significantly more abundant after stimulation (ratio ≥ 2), the surfaceome of primary RMG comprised 1425 proteins, among them 79 proteins with significantly higher abundance in the stimulated state. Pathway analysis revealed notable association with immune system pathways such as “antigen presentation”, “immunoregulatory interactions between a lymphoid and a non-lymphoid cell” and “cell migration”. We could demonstrate a higher abundance of proteins that are usually ascribed to antigen-presenting cells (APCs) and function to interact with T-cells, suggesting that activated RMG might act as atypical APCs in the course of ocular neuroinflammation. Our data provide a detailed description of the unstimulated and stimulated RMG surfaceome and offer fundamental insights regarding the capacity of RMG to actively participate in neuroinflammation in the retina.

VEGF-Independent Activation of Müller Cells by the Vitreous from Proliferative Diabetic Retinopathy Patients

Proliferative diabetic retinopathy (PDR), a major complication of diabetes mellitus, results from an inflammation-sustained interplay among endothelial cells, neurons, and glia. Even though anti-vascular endothelial growth factor (VEGF) interventions represent the therapeutic option for PDR, they are only partially efficacious. In PDR, Müller cells undergo reactive gliosis, produce inflammatory cytokines/chemokines, and contribute to scar formation and retinal neovascularization. However, the impact of anti-VEGF interventions on Müller cell activation has not been fully elucidated. Here, we show that treatment of MIO-M1 Müller cells with vitreous obtained from PDR patients stimulates cell proliferation and motility, and activates various intracellular signaling pathways. This leads to cytokine/chemokine upregulation, a response that was not mimicked by treatment with recombinant VEGF nor inhibited by the anti-VEGF drug ranibizumab. In contrast, fibroblast growth factor-2 (FGF2) induced a significant overexpression of various cytokines/chemokines in MIO-M1 cells. In addition, the FGF receptor tyrosine kinase inhibitor BGJ398, the pan-FGF trap NSC12, the heparin-binding protein antagonist N-tert-butyloxycarbonyl-Phe-Leu-Phe-Leu-Phe Boc2, and the anti-inflammatory hydrocortisone all inhibited Müller cell activation mediated by PDR vitreous. These findings point to a role for various modulators beside VEGF in Müller cell activation and pave the way to the search for novel therapeutic strategies in PDR.

Prevalence and Severity of Diabetic Retinopathy in Patients with Macular Telangiectasia Type 2

PURPOSE

To study the prevalence and severity of diabetic retinopathy (DR) in patients with macular telangiectasia type 2 (MacTel 2).

DESIGN

Retrospective case series.

PARTICIPANTS

Patients with a diagnosis of MacTel 2 treated at the Rotterdam Eye Hospital or Erasmus Medical Center between 2014 and 2018 were included.

METHODS

The following information was retrieved from patient files: demographics, history of diabetes mellitus and hypertension, presence of DR, and severity of DR, that is, mild, moderate, severe, or proliferative. Presence of diabetic macular edema (DME) was assessed using OCT.

MAIN OUTCOME MEASURES

Presence and severity of DR.

RESULTS

Two hundred six eyes of 103 patients were included. At the onset of MacTel 2, the mean age was 61 years (standard deviation [SD], 9.8 years) and 64 (62%) were women. Mean follow-up was 71 months (SD, 60 months). Diabetes mellitus type 2 was present in 50 patients (49%) and hypertension was present in 47 patients (46%). Mild DR was present in 22 eyes (11%), of which 14 eyes (7%) showed signs at baseline and 8 eyes (4%) showed signs at a later time during follow-up. Ten eyes (5%) demonstrated remission of mild DR during follow-up. Both eyes (1%) in 1 patient progressed to moderate DR. Severe DR, proliferative DR, and DME did not occur.

CONCLUSIONS

Although diabetes mellitus was highly prevalent among MacTel 2 patients, no patients showed severe or proliferative DR or DME. These findings suggest that MacTel 2 could have a protective effect on the progression of DR. We hypothesize that our results may be explained by the role of Müller cells in the development of MacTel 2 and DR, and therefore a link between both diseases warrants additional studies.