The role of Müller cells in the formation of the blood-retinal barrier

Visual stimulation and brain-derived neurotrophic factor (BDNF) have protective effects in experimental autoimmune uveoretinitis

AIMS

To investigate the therapeutic potential of visual stimulation (VS) and BDNF in murine experimental autoimmune uveoretinitis (EAU).

MAIN METHODS

Mice were immunized by subcutaneous injection of interphotoreceptor retinoid-binding protein in Freund's complete adjuvant and intravenous injection of pertussis toxin, and were then exposed to high-contrast VS 12 h/day (days 1-14 post-immunization). EAU severity was assessed by examining clinical score, visual acuity, inflammatory markers, and immune cells in the retina. The transcriptome of activated retinal cells was determined by RNA-seq using RNA immunoprecipitated in complex with phosphorylated ribosomal protein S6. The retinal levels of protein products of relevant upregulated genes were quantified. The effect of BDNF on EAU was tested in unstimulated mice by its daily topical ocular administration (days 8-14 post-immunization).

KEY FINDINGS

VS attenuated EAU development and decreased the expression of pro-inflammatory cytokines/chemokines and numbers of immune cells in the retina (n = 10-20 eyes/group for each analysis). In activated retinal cells of control mice (n = 30 eyes/group), VS upregulated genes encoding immunomodulatory neuropeptides, of which BDNF and vasoactive intestinal peptide (VIP) also showed increased mRNA and protein levels in the retina of VS-treated EAU mice (n = 6-10 eyes/group for each analysis). In unstimulated EAU mice, BDNF treatment mimicked the protective effects of VS by modulating the inflammatory and stem cell properties of Müller cells (n = 5 eyes/group for each analysis).

SIGNIFICANCE

VS effectively suppresses EAU, at least through enhancing retinal levels of anti-inflammatory and neuroprotective factors, VIP and BDNF. Our findings also suggest BDNF as a promising therapeutic agent for uveitis treatment.

Roles of noncoding RNAs in diabetic retinopathy: Mechanisms and therapeutic implications

Diabetic retinopathy (DR) is a microvascular complication of diabetes that leads to vision loss. The striking features of DR are hard exudate, cotton-wool spots, hemorrhage, and neovascularization. The dysregulated retinal cells, encompassing microvascular endothelial cells, pericytes, Müller cells, and adjacent retinal pigment epithelial cells, are involved in the pathological processes of DR. According to recent research, oxidative stress, inflammation, ferroptosis, pyroptosis, apoptosis, and angiogenesis contribute to DR. Recent advancements have highlighted that noncoding RNAs could regulate diverse targets in pathological processes that contribute to DR. Noncoding RNAs, including long noncoding RNAs, microRNAs (miRNA), and circular RNAs, are dysregulated in DR, and interact with miRNA, mRNA, or proteins to control the pathological processes of DR. Hence, modulation of noncoding RNAs may have therapeutic effects on DR. Small extracellular vesicles may be valuable tools for transferring noncoding RNAs and regulating the genes involved in progression of DR. However, the roles of noncoding RNA in developing DR are not fully understood; it is critical to summarize the mechanisms for noncoding RNA regulation of pathological processes and pathways related to DR. This review provides a fundamental understanding of the relationship between noncoding RNAs and DR, exploring the mechanism of how noncoding RNA modulates different signaling pathways, and pave the way for finding potential therapeutic strategies for DR.

Relapse-Independent disease activity in neuromyelitis optica spectrum disorder: A systematic review

INTRODUCTION

Neuromyelitis Optica Spectrum Disorders (NMOSD) is a neuroinflammatory condition characterized by optic neuritis and transverse myelitis. While the current approach to NMOSD focuses on relapse-associated worsening (RAW), recent evidence indicates Relapse-Independent Disease Activity (RIDA) in patients.

METHOD

Databases including Embase, PubMed, Scopus, and Web of Sciences were systematically searched up to December 2023. No restrictions were applied. Inclusion criteria focused on studies reporting evidence of RIDA in NMOSD patients. Data extraction involved details such as study title, author, participant characteristics, treatment, evaluation methods, positive findings according to RIDA, and prevalence of findings in NMOSD patients. This study is conducted following the PRISMA guidelines with a registered protocol on PROSPERO (ID = CRD42023492352).

RESULT

Of 802 studies, 38 were included in the systematic review, covering 1881 NMOSD patients. AQP4-IGg status was positive in 90.6 % of the patients. Ocular findings indicative of RIDA were reported in 23 studies, including thinning of GCIPL, RNFL, GCC, and GCL layers, foveal and macular shape and volume abnormalities, vessel loss, and visual evoked potentials (VEPs) abnormalities. MRI findings supporting the RIDA were reported in 13 studies, including new lesion incidence and brain and spinal cord atrophy. Serum and CSF RIDA-supporting findings were reported in five studies, including elevation in sGFAP and sNFL. Biopsies and autopsies suggested inflammatory processes in relapse-free patients in 2 studies. The predominant manifestation of RIDA in NMOSD was identified in the visual system, suggesting the impaired retinal glial cells like Müller cells during the relapse-free period in NMOSD.

INTERPRETATION

Our systematic review provides valuable insights into RIDA in NMOSD. Establishing guidelines for the diagnosis and treatment of RIDA is crucial. Further studies are needed to provide robust evidence on RIDA in NMOSD patients.

Diabetic Müller-Glial-Cell-Specific Il6ra Knockout Mice Exhibit Accelerated Retinal Functional Decline and Thinning of the Inner Nuclear Layer

Purpose

Interleukin-6 (IL-6) is implicated in the pathology of diabetic retinopathy (DR). IL-6 trans-signaling via soluble IL-6 receptor (IL-6R) is primarily responsible for its pro-inflammatory functions, whereas cis-signaling via membrane-bound IL-6R is anti-inflammatory. Using a Müller-glial-cell-specific Il6ra-/- mouse, we examined how loss of IL-6 cis-signaling in Müller glial cells (MGCs) affected retinal thinning and electroretinography (ERG) response over 9 months of diabetes.

Methods

Diabetes was induced in wildtype and knockout mice with streptozotocin (40 mg/kg, daily for 5 days). Spectral domain optical coherence tomography (SD-OCT), ERG, and fundoscopy/fluorescein angiography (FA) were assessed at 2, 6, and 9 months of diabetes. MGCs and bipolar neurons were examined in retinal tissue sections by immunofluorescence.

Results

Diabetic MGC Il6ra-/- mice had significantly thinner retinas than diabetic wildtype mice at 2 (-7.6 µm), 6 (-12.0 µm), and 9 months (-5.0 µm) of diabetes, as well as significant thinning of the inner nuclear layer (INL). Diabetic MGC Il6ra-/- mice also showed a reduction in scotopic B-wave amplitude and B-wave/A-wave ratio earlier than wildtype diabetic mice. In retinal sections, we found a decrease in bipolar neuronal marker PKCα only in diabetic MGC Il6ra-/- mice, which was significantly lower than both controls and diabetic wildtype mice. Glutamine synthetase, a Müller cell marker, was reduced in both wildtype and MGC Il6ra-/- diabetic mice compared to their respective controls.

Conclusions

IL-6 cis-signaling in MGCs contributes to maintenance of the INL in diabetes, and loss of the IL-6 receptor reduces MGC-mediated neuroprotection of bipolar neurons in the diabetic retina.

Rare intercellular material transfer as a confound to interpreting inner retinal neuronal transplantation following internal limiting membrane disruption

Intercellular cytoplasmic material transfer (MT) occurs between transplanted and developing photoreceptors and ambiguates cell origin identification in developmental, transdifferentiation, and transplantation experiments. Whether MT is a photoreceptor-specific phenomenon is unclear. Retinal ganglion cell (RGC) replacement, through transdifferentiation or transplantation, holds potential for restoring vision in optic neuropathies. During careful assessment for MT following human stem cell-derived RGC transplantation into mice, we identified RGC xenografts occasionally giving rise to labeling of donor-derived cytoplasmic, nuclear, and mitochondrial proteins within recipient Müller glia. Critically, nuclear organization is distinct between human and murine retinal neurons, which enables unequivocal discrimination of donor from host cells. MT was greatly facilitated by internal limiting membrane disruption, which also augments retinal engraftment following transplantation. Our findings demonstrate that retinal MT is not unique to photoreceptors and challenge the isolated use of species-specific immunofluorescent markers for xenotransplant identification. Assessment for MT is critical when analyzing neuronal replacement interventions.

Twelve protections evolved for the brain, and their roles in extending its functional life

As human longevity has increased, we have come to understand the ability of the brain to function into advanced age, but also its vulnerability with age, apparent in the age-related dementias. Against that background of success and vulnerability, this essay reviews how the brain is protected by (by our count) 12 mechanisms, including: the cranium, a bony helmet; the hydraulic support given by the cerebrospinal fluid; the strategically located carotid body and sinus, which provide input to reflexes that protect the brain from blood-gas imbalance and extremes of blood pressure; the blood brain barrier, an essential sealing of cerebral vessels; the secretion of molecules such as haemopexin and (we argue) the peptide Aβ to detoxify haemoglobin, at sites of a bleed; autoregulation of the capillary bed, which stabilises metabolites in extracellular fluid; fuel storage in the brain, as glycogen; oxygen storage, in the haemoprotein neuroglobin; the generation of new neurones, in the adult, to replace cells lost; acquired resilience, the stress-induced strengthening of cell membranes and energy production found in all body tissues; and cognitive reserve, the ability of the brain to maintain function despite damage. Of these 12 protections, we identify 5 as unique to the brain, 3 as protections shared with all body tissues, and another 4 as protections shared with other tissues but specialised for the brain. These protections are a measure of the brain's vulnerability, of its need for protection. They have evolved, we argue, to maintain cognitive function, the ability of the brain to function despite damage that accumulates during life. Several can be tools in the hands of the individual, and of the medical health professional, for the lifelong care of our brains.

Glial, Neuronal, Vascular, Retinal Pigment Epithelium, and Inflammatory Cell Damage in a New Western Diet-Induced Primate Model of Diabetic Retinopathy

This study investigated retinal changes in a Western diet (WD)-induced nonhuman primate model of type 2 diabetes. Rhesus nonhuman primates, aged 15 to 17 years, were fed a high-fat diet (n = 7) for >5 years reflective of the traditional WD. Age-matched controls (n = 6) were fed a standard laboratory primate diet. Retinal fundus photography, optical coherence tomography, autofluorescence imaging, and fluorescein angiography were performed before euthanasia. To assess diabetic retinopathy (DR), eyes were examined using trypsin digests, lipofuscin autofluorescence, and multimarker immunofluorescence on cross-sections and whole mounts. Retinal imaging showed venous engorgement and tortuosity, aneurysms, macular exudates, dot and blot hemorrhages, and a marked increase in fundus autofluorescence. Post-mortem changes included the following: decreased CD31 blood vessel density (P < 0.05); increased acellular capillaries (P < 0.05); increased density of ionized calcium-binding adaptor molecule expressing amoeboid microglia/macrophage; loss of regular distribution in stratum and spacing typical of ramified microglia; and increased immunoreactivity of aquaporin 4 and glial fibrillary acidic protein (P < 0.05). However, rhodopsin immunoreactivity (P < 0.05) in rods and neuronal nuclei antibody-positive neuronal density of 50% (P < 0.05) were decreased. This is the first report of a primate model of DR solely induced by a WD that replicates key features of human DR.

Baseline demographic, clinical and multimodal imaging features of young patients with type 2 macular telangiectasia

PURPOSE

Macular telangiectasia (MacTel) type 2 is observed in patients in their 5th-8th decades of life. The clinical and imaging findings in younger patients is unknown in larger cohorts. The study purpose is to report prevalence, baseline clinical and spectral domain optical coherence tomography (SDOCT) findings in young MacTel patients below 40 years.

METHODS

This hospital-based, multicentre, retrospective, cross-sectional study included patients between 2011 and 2023. Retinal photographs from multiple imaging techniques were evaluated to diagnose and stage type 2 MacTel and describe their SDOCT findings. Imaging characteristics were correlated with clinical stages and visual acuity.

RESULTS

Among all MacTel patients seen in hospital, prevalence of young MacTel cases less than age 40 was 1.77% (32/1806; 62 eyes). Youngest participant was 34 years, while mean age was 38.44 ± 1.795 years. Sixteen patients (50%) were diabetics. Perifoveal greying (n = 56, 90%) and perifoveal hyperreflective middle retinal layers (n = 47, 76%) were the most prevalent clinical and SDOCT imaging finding respectively. Less than 10% (n = 6) eyes had proliferative disease. Presence of retinal pigment clumps (RPC) (7% vs. 67%; p = 0.002) coincided with proliferative MacTel. Poor vision was associated with presence of outer retinal layer SDOCT findings like outward bending of inner retinal layers (p = 0.047), RPC (p = 0.007), subfoveal neurosensory detachment (p = 0.048) and subretinal neovascular membrane (p = 0.001).

CONCLUSION

Type 2 MacTel before age 40 is rare, common in women and diabetics, and affects vision in advanced stage. Disease symmetry, comparison with older cases, and longitudinal SDOCT changes in such patients require further study.

Knockout of PERK protects rat Müller glial cells against OGD-induced endoplasmic reticulum stress-related apoptosis

BACKGROUND

The pathological basis for many retinal diseases, retinal ischemia is also one of the most common causes of visual impairment. Numerous ocular diseases have been linked to Endoplasmic reticulum(ER)stress. However, there is still no clear understanding of the relationship between ER stress and Müller glial cells during retinal ischemia and hypoxia. This study examined the effects of ER stress on autophagy and apoptosis-related proteins, as well as the microtubule-related protein tau in rMC-1 cells.

METHODS

rMC-1 cells were cultured in vitro. RT-PCR、immunofluorescence and Western blotting revealed the expression levels of associated mRNAs and proteins, and the CCK-8 and flow cytometry assays detected cell apoptosis.

RESULTS

The results showed that under OGD(Oxygen-glucose deprivation) conditions, the number of rMC-1 cells was decreased, the PERK/eIF2a pathway was activated, and the expressions of p-tau, LC3、Beclin1 and Caspase-12 proteins were increased. After the PERK knockout, the expression of the above proteins was decreased, and the apoptosis was also decreased.

CONCLUSION

According to the findings of this study, specific downregulation of PERK expression had an anti-apoptotic effect on OGD-conditioned rMC-1 cells. There is a possibility that this is one of the mechanisms of MG cell apoptosis during retinal ischemic injury.

Human glial müller and umbilical vein endothelial cell coculture as an in vitro model to investigate retinal oxidative damage. A morphological and molecular assessment

The aim of this study was to optimize a coculture in vitro model established between the human Müller glial cells and human umbilical vein endothelial cells, mimicking the inner blood-retinal barrier, and to explore its resistance to damage induced by oxidative stress. A spontaneously immortalized human Müller cell line MIO-M1 and human umbilical vein endothelial cells (HUVEC) were plated together at a density ratio 1:1 and maintained up to the 8th passage (p8). The MIO-M1/HUVECs p1 through p8 were treated with increasing concentrations (range 200-800 μM) of H₂ O₂ to evaluate oxidative stress induced damage and comparing data with single cell cultures. The following features were assayed p1 through p8: doubling time maintenance, cell viability using MTS assay, ultrastructure of cell-cell contacts, immunofluorescence for Vimentin and GFAP, molecular biology (q-PCR) for GFAP and CD31 mRNA. MIO-M1/HUVECs cocultures maintained distinct cell cytotype up to p8 as shown by flow cytometry analysis, without evidence of cross activation, displaying cell-cell tight junctions mimicking those found in human retina, only acquiring a slight resistance to oxidative stress induction over the passages. This MIO-M1/HUVECs coculture represents a simple, reproducible and affordable model for in vitro studies on oxidative stress-induced retinal damages.

Mechanisms of macular edema

Macular edema is the pathological accumulation of fluid in the central retina. It is a complication of many retinal diseases, including diabetic retinopathy, retinal vascular occlusions and uveitis, among others. Macular edema causes decreased visual acuity and, when chronic or refractory, can cause severe and permanent visual impairment and blindness. In most instances, it develops due to dysregulation of the blood-retinal barrier which permits infiltration of the retinal tissue by proteins and other solutes that are normally retained in the blood. The increase in osmotic pressure in the tissue drives fluid accumulation. Current treatments include vascular endothelial growth factor blockers, corticosteroids, and non-steroidal anti-inflammatory drugs. These treatments target vasoactive and inflammatory mediators that cause disruption to the blood-retinal barrier. In this review, a clinical overview of macular edema is provided, mechanisms of disease are discussed, highlighting processes targeted by current treatments, and areas of opportunity for future research are identified.

MEK/ERK/RUNX2 Pathway-Mediated IL-11 Autocrine Promotes the Activation of Müller Glial Cells during Diabetic Retinopathy

PURPOSE

To uncover the role of the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK)/runt-related transcription factor 2 (RUNX2)/interleukin-11 (IL-11) pathway in the activation of Müller glial cells (MGCs) and the breakdown of blood-retina barrier (BRB) during diabetic retinopathy (DR).

METHODS

Western blot (WB) detected the activation of MEK/ERK/RUNX2/IL-11 pathway, and quantitative reverse transcription polymerase chain reaction (qRT-PCR) detected IL-11 mRNA levels in high glucose (HG)-exposed MIO-M1 cells. Co-immunoprecipitation (Co-IP) identified the interaction between ERK and RUNX2. Immunofluorescence (IF) measured the co-localization of ERK and RUNX2. Luciferase reporter gene assay identified the transcription activity of IL-11 promoter under HG conditions. Enzyme-linked immunosorbent assay (ELISA) detected IL-11 levels in MIO-M1 cell culture supernatant. WB detected IL-RA protein levels, and Immunofluorescence measured the co-localization of IL-11 and IL-11RA. WB detected MGCs activation marker glial fibrillary acidic protein (GFAP) protein levels. 5-ethynyl-2'-deoxyuridine (EdU) incorporation assay detected the proliferation of MGCs. WB detected the activation of MEK/ERK/RUNX2/IL-11 pathway in streptozotocin (STZ)-induced diabetic mice. ELISA detected IL-11 and IL-11RA levels in mouse retina tissues. QRT-PCR and WB detected tight junction-associated molecules claudin-5, occluding and tight junction protein 1 (ZO-1) mRNA and protein levels in mouse retina tissues, respectively.

RESULTS

MEK/ERK/RUNX2/IL-11 pathway was activated in HG-exposed MIO-M1 cells. Additionally, IL-11 bound to IL-11RA on MIO-M1 cells to promote MIO-M1 cell activation and proliferation. In the mouse STZ-induced diabetic model, MEK/ERK/RUNX2/IL-11/IL-11RA pathway was also activated. Finally, the blockade of the pathway mitigated the activation of MGCs and the breakdown of BRB.

CONCLUSION

The data suggested that activated MEK/ERK/RUNX2/IL-11/IL-11RA autocrine pathway can promote the activation of MGCs and the breakdown of BRB during DR, implying novel anti-molecular strategies for the treatment of DR.

Studies of the retinal microcirculation using human donor eyes and high-resolution clinical imaging: Insights gained to guide future research in diabetic retinopathy

The microcirculation plays a key role in delivering oxygen to and removing metabolic wastes from energy-intensive retinal neurons. Microvascular changes are a hallmark feature of diabetic retinopathy (DR), a major cause of irreversible vision loss globally. Early investigators have performed landmark studies characterising the pathologic manifestations of DR. Previous works have collectively informed us of the clinical stages of DR and the retinal manifestations associated with devastating vision loss. Since these reports, major advancements in histologic techniques coupled with three-dimensional image processing has facilitated a deeper understanding of the structural characteristics in the healthy and diseased retinal circulation. Furthermore, breakthroughs in high-resolution retinal imaging have facilitated clinical translation of histologic knowledge to detect and monitor progression of microcirculatory disturbances with greater precision. Isolated perfusion techniques have been applied to human donor eyes to further our understanding of the cytoarchitectural characteristics of the normal human retinal circulation as well as provide novel insights into the pathophysiology of DR. Histology has been used to validate emerging in vivo retinal imaging techniques such as optical coherence tomography angiography. This report provides an overview of our research on the human retinal microcirculation in the context of the current ophthalmic literature. We commence by proposing a standardised histologic lexicon for characterising the human retinal microcirculation and subsequently discuss the pathophysiologic mechanisms underlying key manifestations of DR, with a focus on microaneurysms and retinal ischaemia. The advantages and limitations of current retinal imaging modalities as determined using histologic validation are also presented. We conclude with an overview of the implications of our research and provide a perspective on future directions in DR research.

Lipid metabolism and retinal diseases

PURPOSE

The retina has enormous lipids demands and must meet those needs. Retinal lipid homeostasis appears to be based on the symbiosis between neurons, Müller glial cells (MGC), and retinal pigment epithelium (RPE) cells, which can be impacted in several retinal diseases. The current research challenge is to better understand lipid-related mechanisms involved in retinal diseases, such as age-related macular degeneration (AMD) and glaucoma.

RESULTS

In a first axis, in vitro and focus on Müller glial cell, we aimed to characterize whether the 24S-hydroxycholesterol (24S-OHC), an overexpressed end-product of cholesterol elimination pathway in neural tissue and likely produced by suffering retinal ganglion cells in glaucoma, may modulate MGC membrane organization, such as lipid rafts, to trigger cellular signalling pathways related to retinal gliosis. We have found that lipid composition appears to be a key factor of membrane architecture, especially for lipid raft microdomain formation, in MGC. However, 24S-OHC did not appear to trigger retinal gliosis via the modulation of lipid or protein composition within lipid rafts microdomains. This study provided a better understanding of the complex mechanisms involved in the pathophysiology of glaucoma. On a second clinical ax, we focused on the lipid-related mechanisms involved in the dysfunction of aging RPE and the appearance of drusenoid deposits in AMD. Using the Montrachet population-based study, we intend to report the frequency of reticular pseudodrusen (RPD) and its ocular and systemic risk factors, particularly related to lipid metabolisms, such as plasma lipoprotein levels, carotenoids levels, and lipid-lowering drug intake. Our study showed that RPD was less common in subjects taking lipid-lowering drugs. Lipid-lowering drugs, such as statins, may reduce the risk of RPD through their effect on the production and function of lipoproteins. This observation highlights the potential role of retinal lipid trafficking via lipoproteins between photoreceptors and retinal pigment epithelium cells in RPD formation. Those findings have been complemented with preliminary results on the analysis of plasma fatty acid (FA) profile, a surrogate marker of short-term dietary lipid intake, according to the type of predominant drusenoid deposit, soft drusen or RPD, in age-related maculopathy.

CONCLUSION

Further research on lipid metabolism in retinal diseases is warranted to better understand the pathophysiology of retinal diseases and develop new promising diagnostic, prognostic, and therapeutic tools for our patients.

AUTOSOMAL DOMINANT MÜLLER CELL SHEEN DYSTROPHY: Clinical, Histopathologic, and Genetic Assessment in an Extended Family With Long Follow-Up

BACKGROUND

Autosomal dominant Müller cell dystrophy is a rare condition we described in 1991. It is characterized by a striking sheen appearance on the retinal surface with progressive retinal changes leading to disorganization and atrophy with a decreased b-wave electroretinograms.

MATERIALS AND METHODS

We examined 45 members of a 4-generation family. Fifteen subjects from three generations were found with the disease, without gender predilection. Seven patients underwent ophthalmic examination including fundus examination, intravenous fluorescein angiogram, spectral-domain optical coherence tomography, and electroretinogram. Six patients have a 30-year follow-up. Histopathology examination was performed on eyes of the eldest patient. Whole exome sequencing was done in four affected subjects.

RESULTS

Findings include a decreased visual acuity, abnormal cellophane-like sheen of the vitreoretinal interface, a "plush" nerve fiber layer, and characteristic macular changes. Electroretinogram showed a selective b-wave diminution. Intravenous fluorescein angiogram presented perifoveal hyperfluorescence and capillary leakage. Spectral-domain optical coherence tomography revealed cavitations involving inner and later outer retinal layers with later disorganization. Histopathologic findings included Müller cell abnormalities with cystic disruption of inner retinal layers, pseudoexfoliation in anterior segment, and amyloidosis of extraocular vessels. Pedigree analysis suggests an autosomal dominant inheritance with late onset. DNA analysis demonstrated a previously undescribed heterozygous missense p.Glu109Val mutation in transthyretin.

CONCLUSION

To the best of our knowledge, this is the first family reported with this disorder. Our data support the hypothesis that autosomal dominant Müller cell dystrophy is a distinct retinal dystrophy affecting Müller cells. Mutations in transthyretin gene may manifest as a predominantly retinal disorder.

The Mineralocorticoid receptor signal could be a new molecular target for the treatment of diabetic retinal complication

BACKGROUND

Activation of the mineralocorticoid receptor (MR) is involved in the pathophysiology of diabetic vascular complications. In recent years, it has been indicated that the MR expressed in retinal Müller cells plays an important role in regulating the potassium and water balance in the retina. Therefore, it has also been speculated that abnormal MR signaling contributes to edematous diseases of the retina.

RESEARCH DESIGN AND METHODS

We examined the effect of high-glucose conditions on MR protein and mRNA levels in human retinal Müller cells and changes in cell size in vitro. We also investigated MR transcriptional activity and signaling in high-glucose conditions.

RESULTS

The MR protein increased by 2.2-fold with high-glucose treatment. Additionally, high-glucose treatment induced Müller cell swelling. Aldosterone-induced MR transcriptional activity was enhanced in high-glucose conditions, resulting in the upregulation of αENaC, AQP4 and Kir4.1 mRNA. Treatment with an MR antagonist led to the suppression of aldosterone-induced cell swelling, MR transcriptional activity and upregulation of the target genes in high-glucose conditions.

CONCLUSIONS

High glucose induces Müller cell swelling through activation of MR signaling, which could be associated with aggravation of macular edema. Thus, Müller cell swelling and diabetic macular edema may represent a target for treatment with MR antagonists.

Biochemical mechanism underlying the pathogenesis of diabetic retinopathy and other diabetic complications in humans: the methanol-formaldehyde-formic acid hypothesis

Hyperglycemia in diabetic patients is associated with abnormally-elevated cellular glucose levels. It is hypothesized that increased cellular glucose will lead to increased formation of endogenous methanol and/or formaldehyde, both of which are then metabolically converted to formic acid. These one-carbon metabolites are known to be present naturally in humans, and their levels are increased under diabetic conditions. Mechanistically, while formaldehyde is a cross-linking agent capable of causing extensive cytotoxicity, formic acid is an inhibitor of mitochondrial cytochrome oxidase, capable of inducing histotoxic hypoxia, ATP deficiency and cytotoxicity. Chronic increase in the production and accumulation of these toxic one-carbon metabolites in diabetic patients can drive the pathogenesis of ocular as well as other diabetic complications. This hypothesis is supported by a large body of experimental and clinical observations scattered in the literature. For instance, methanol is known to have organ- and species-selective toxicities, including the characteristic ocular lesions commonly seen in humans and non-human primates, but not in rodents. Similarly, some of the diabetic complications (such as ocular lesions) also have a characteristic species-selective pattern, closely resembling methanol intoxication. Moreover, while alcohol consumption or combined use of folic acid plus vitamin B is beneficial for mitigating acute methanol toxicity in humans, their use also improves the outcomes of diabetic complications. In addition, there is also a large body of evidence from biochemical and cellular studies. Together, there is considerable experimental support for the proposed hypothesis that increased metabolic formation of toxic one-carbon metabolites in diabetic patients contributes importantly to the development of various clinical complications.

Retinal Changes in Astrocytes and Müller Glia in a Mouse Model of Laser-Induced Glaucoma: A Time-Course Study

Macroglia (astrocytes and Müller glia) may play an important role in the pathogenesis of glaucoma. In a glaucoma mouse model, we studied the effects of unilateral laser-induced ocular hypertension (OHT) on macroglia in OHT and contralateral eyes at different time points after laser treatment (1, 3, 5, 8 and 15 days) using anti-GFAP and anti-MHC-II, analyzing the morphological changes, GFAP-labelled retinal area (GFAP-PA), and GFAP and MHC-II immunoreactivity intensities ((GFAP-IRI and MHC-II-IRI)). In OHT and contralateral eyes, with respect to naïve eyes, at all the time points, we found the following: (i) astrocytes with thicker somas and more secondary processes, mainly in the intermediate (IR) and peripheral retina (PR); (ii) astrocytes with low GFAP-IRI and only primary processes near the optic disc (OD); (iii) an increase in total GFAP-RA, which was higher at 3 and 5 days, except for at 15 days; (iv) an increase in GFAP-IRI in the IR and especially in the PR; (v) a decrease in GFAP-IRI near the OD, especially at 1 and 5 days; (vi) a significant increase in MHC-II-IRI, which was higher in the IR and PR; and (vii) the Müller glia were GFAP+ and MHC-II+. In conclusion, in this model of glaucoma, there is a bilateral macroglial activation maintained over time involved in the inflammatory glaucoma process.

Neurovascular abnormalities in retinopathy of prematurity and emerging therapies

Blood vessels in the developing retina are formed in concert with neural growth, resulting in functional neurovascular network. Disruption of the neurovascular coordination contributes to the pathogenesis of retinopathy of prematurity (ROP), a potentially blinding retinal neovascular disease in preterm infants that currently lacks an approved drug therapy in the USA. Despite vasculopathy as predominant clinical manifestations, an increasing number of studies revealed complex neurovascular interplays among neurons, glial cells and blood vessels during ROP. Coordinated expression of glia-derived vascular endothelial growth factor (VEGF) in spatio-temporal gradients is pivotal to the formation of well-organized vascular plexuses in the healthy retina, whereas uncoordinated VEGF expression triggers pathological angiogenesis with disorganized vascular tufts in ROP. In contrast with VEGF driving both pathological and physiological angiogenesis, neuron-derived angiogenic factor secretogranin III (Scg3) stringently regulates ROP but not healthy retinal vessels in animal models. Anti-VEGF and anti-Scg3 therapies confer similar high efficacies to alleviate ROP in preclinical studies but are distinct in their disease selectivity and safety. This review discusses neurovascular communication among retinal blood vessels, neurons and glial cells during retinal development and ROP pathogenesis and summarizes the current and emerging therapies to address unmet clinical needs for the disease.

Spectral domain OCT features in type 2 macular telangiectasia (type 2 MacTel): its relevance with clinical staging and visual acuity

BACKGROUND

To report spectral domain optical coherence tomography (SDOCT) imaging findings in type 2 macular telangiectasia (MacTel) and correlate them with clinical stages and visual acuity.

METHODS

This retrospective, cross-sectional study included type 2 MacTel cases who underwent SDOCT imaging with Spectralis machine. Macular SDOCT images were analysed. Imaging features were tested for correlation with different clinicals stages and visual acuity.

RESULTS

212 eyes of 108 type 2 MacTel patients were included. Hyperreflective middle retinal layer (87%) was the most frequently detected abnormality. This was followed by inner retinal cavities (49%), outward bending of inner retinal layers (35%), retinal pigment clumps (35%) and foveal contour irregularity (31%). Hyperreflective middle retinal layers (p < 0.001), inner (p = 0.032) and outer retinal (p = 0.002) cavities and internal limiting membrane drape (p = 0.031) were associated with poor vision in non-proliferative group and presence of retinal pigment clumps (p = 0.002), subretinal fluid (p = 0.037) and foveal contour irregularity (p < 0.001) were associated with poor vision in proliferative group.

CONCLUSION

The described SDOCT features are practical for the diagnosis and staging in type 2 MacTel. Presence of hyperreflective middle retinal layers, hyporeflective inner and outer retinal cavities and internal limiting membrane drape were associated with poor vision in the non-proliferative group while retinal pigment clumps and subretinal neovascular membrane were associated with proliferative group and poor vision. Further long-term studies are required to describe the progressive and sequential changes on SDOCT.

Contribution of Müller Cells in the Diabetic Retinopathy Development: Focus on Oxidative Stress and Inflammation

Diabetic retinopathy is a neurovascular complication of diabetes and the main cause of vision loss in adults. Glial cells have a key role in maintenance of central nervous system homeostasis. In the retina, the predominant element is the Müller cell, a specialized cell with radial morphology that spans all retinal layers and influences the function of the entire retinal circuitry. Müller cells provide metabolic support, regulation of extracellular composition, synaptic activity control, structural organization of the blood-retina barrier, antioxidant activity, and trophic support, among other roles. Therefore, impairments of Müller actions lead to retinal malfunctions. Accordingly, increasing evidence indicates that Müller cells are affected in diabetic retinopathy and may contribute to the severity of the disease. Here, we will survey recently described alterations in Müller cell functions and cellular events that contribute to diabetic retinopathy, especially related to oxidative stress and inflammation. This review sheds light on Müller cells as potential therapeutic targets of this disease.

Variability in Retinal Neuron Populations and Associated Variations in Mass Transport Systems of the Retina in Health and Aging

Aging is associated with a broad range of visual impairments that can have dramatic consequences on the quality of life of those impacted. These changes are driven by a complex series of alterations affecting interactions between multiple cellular and extracellular elements. The resilience of many of these interactions may be key to minimal loss of visual function in aging; yet many of them remain poorly understood. In this review, we focus on the relation between retinal neurons and their respective mass transport systems. These metabolite delivery systems include the retinal vasculature, which lies within the inner portion of the retina, and the choroidal vasculature located externally to the retinal tissue. A framework for investigation is proposed and applied to identify the structures and processes determining retinal mass transport at the cellular and tissue levels. Spatial variability in the structure of the retina and changes observed in aging are then harnessed to explore the relation between variations in neuron populations and those seen among retinal metabolite delivery systems. Existing data demonstrate that the relation between inner retinal neurons and their mass transport systems is different in nature from that observed between the outer retina and choroid. The most prominent structural changes observed across the eye and in aging are seen in Bruch's membrane, which forms a selective barrier to mass transfers at the interface between the choroidal vasculature and the outer retina.

Clinical Features of Untreated Type 2 Macular Telangiectasia and Efficacy of Anti-Vascular Endothelial Growth Factor Therapy in Macular Neovascularization

Objectives:

To compare best corrected visual acuity (BCVA), central macular thickness (CMT), and central choroidal thickness (CCT) in patients with type 2 macular telangiectasia (MacTel 2) and a control group and to evaluate the efficacy of intravitreal anti-vascular endothelial growth factor (anti-VEGF) treatment in MacTel 2 patients with macular neovascularization (MNV).

Materials and Methods:

We conducted a retrospective chart review of consecutive MacTel 2 patients who underwent a full ophthalmologic examination including BCVA and dilated fundus examination with slit-lamp biomicroscopy, fluorescein angiography, and optical coherence tomography imaging at baseline and follow-up visits. BCVA, CMT, and CCT were compared between all identified patients (n=26) and a control group (n=30). A subgroup analysis was performed among eyes with MNV (n=7) before and after treatment.

Results:

CMT and CCT were significantly lower in the MacTel 2 group compared to the control group. Forty-one treatment-naive eyes without MNV proliferation showed no significant change in BCVA, CMT, or CCT during follow-up. Eight eyes of 7 MacTel 2 patients developed MNV during follow-up. All of the patients were treated with intravitreal anti-VEGF.

Conclusion:

It is important to closely follow MacTel 2 patients for MNV development. To avoid adverse effects, we prefer to monitor patients who have not yet developed MNV. Patients with proliferative MacTel 2 with decreasing visual function may benefit from intravitreal anti-VEGF treatment.

Contributory factors for developing foveal neovascularization in proliferative diabetic retinopathy

Purpose

Foveal neovascularisation (NV) in proliferative diabetic retinopathy (PDR) is uncommon. The study aim is to analyse a series of cases of foveal NV in PDR and ascertain the factors leading to its development.

Methods

In this retrospective case-control study, optical coherence tomography (OCT) and OCT-angiography (OCTA) images of PDR cases with/without foveal NV diagnosed on fluorescein angiography were analysed.

Results

From 124 consecutive PDR eyes, foveal NV was identified in 12 (10%) eyes. Eyes with foveal NV showed thin choroid (p = 0.001), increased FAZ area and reduced vessel density at the macula compared to control group on OCT and OCTA. After regression analysis, an increased FAZ in the superficial capillary plexus slab (p = 0.002) was associated with foveal NV development.

Conclusion

Our case series suggest that foveal NV is an uncommon finding, occurring due to reduced choroidal and inner retinal perfusion at the macula. Further studies are required to assess the treatment outcomes in such eyes.

Faim knockout leads to gliosis and late-onset neurodegeneration of photoreceptors in the mouse retina

Fas Apoptotic Inhibitory Molecule protein (FAIM) is a death receptor antagonist and an apoptosis regulator. It encodes two isoforms, namely FAIM-S (short) and FAIM-L (long), both with significant neuronal functions. FAIM-S, which is ubiquitously expressed, is involved in neurite outgrowth. In contrast, FAIM-L is expressed only in neurons and it protects them from cell death. Interestingly, FAIM-L is downregulated in patients and mouse models of Alzheimer's disease before the onset of neurodegeneration, and Faim transcript levels are decreased in mouse models of retinal degeneration. However, few studies have addressed the role of FAIM in the central nervous system, yet alone the retina. The retina is a highly specialized tissue, and its degeneration has proved to precede pathological mechanisms of neurodegenerative diseases. Here we describe that Faim depletion in mice damages the retina persistently and leads to late-onset photoreceptor death in older mice. Immunohistochemical analyses showed that Faim knockout (Faim^-/- ) mice present ubiquitinated aggregates throughout the retina from early ages. Moreover, retinal cells released stress signals that can signal to Müller cells, as shown by immunofluorescence and qRT-PCR. Müller cells monitor retinal homeostasis and trigger a gliotic response in Faim^-/- mice that becomes pathogenic when sustained. In this regard, we observed pronounced vascular leakage at later ages, which may be caused by persistent inflammation. These results suggest that FAIM is an important player in the maintenance of retinal homeostasis, and they support the premise that FAIM is a plausible early marker for late photoreceptor and neuronal degeneration.

The "Neuro-Glial-Vascular" Unit: The Role of Glia in Neurovascular Unit Formation and Dysfunction

The neurovascular unit (NVU) is a complex multi-cellular structure consisting of endothelial cells (ECs), neurons, glia, smooth muscle cells (SMCs), and pericytes. Each component is closely linked to each other, establishing a structural and functional unit, regulating central nervous system (CNS) blood flow and energy metabolism as well as forming the blood-brain barrier (BBB) and inner blood-retina barrier (BRB). As the name suggests, the “neuro” and “vascular” components of the NVU are well recognized and neurovascular coupling is the key function of the NVU. However, the NVU consists of multiple cell types and its functionality goes beyond the resulting neurovascular coupling, with cross-component links of signaling, metabolism, and homeostasis. Within the NVU, glia cells have gained increased attention and it is increasingly clear that they fulfill various multi-level functions in the NVU. Glial dysfunctions were shown to precede neuronal and vascular pathologies suggesting central roles for glia in NVU functionality and pathogenesis of disease. In this review, we take a “glio-centric” view on NVU development and function in the retina and brain, how these change in disease, and how advancing experimental techniques will help us address unanswered questions.

Insights on the Regeneration Potential of Müller Glia in the Mammalian Retina

Müller glia, the major glial cell types in the retina, maintain retinal homeostasis and provide structural support to retinal photoreceptors. They also possess regenerative potential that might be used for retinal repair in response to injury or disease. In teleost fish (such as zebrafish), the Müller glia response to injury involves reprogramming events that result in a population of proliferative neural progenitors that can regenerate the injured retina. Recent studies have revealed several important mechanisms for the regenerative capacity of Müller glia in fish, which may shed more light on the mechanisms of Müller glia reprogramming and regeneration in mammals. Mammalian Müller glia can adopt stem cell characteristics, and in response to special conditions, be persuaded to proliferate and regenerate, although their native regeneration potential is limited. In this review, we consider the work to date revealing the regenerative potential of the mammalian Müller glia and discuss whether they are a potential source for cell regeneration therapy in humans.

Low-Dose Recombinant Adeno-Associated Virus-Mediated Inhibition of Vascular Endothelial Growth Factor Can Treat Neovascular Pathologies Without Inducing Retinal Vasculitis

The wet form of age-related macular degeneration is characterized by neovascular pathologies that, if untreated, can result in edemas followed by rapid vision loss. Inhibition of vascular endothelial growth factor (VEGF) has been used to successfully treat neovascular pathologies of the eye. Nonetheless, some patients require frequent intravitreal injections of anti-VEGF drugs, increasing the burden and risk of complications from the procedure to affected individuals. Recombinant adeno-associated virus (rAAV)-mediated expression of anti-VEGF proteins is an attractive alternative to reduce risk and burden to patients. However, controversy remains as to the safety of prolonged VEGF inhibition in the eye. Here, we show that two out of four rAAV serotypes tested by intravitreal delivery to express the anti-VEGF drug conbercept lead to a dose-dependent vascular sheathing pathology that is characterized by immune cell infiltrates, reminiscent of vasculitis in humans. We show that this pathology is accompanied by increased expression in vascular cell adhesion molecule 1 (VCAM1) and intercellular adhesion molecule 1 (ICAM1), both of which promote extravasation of immune cells from the vasculature. While formation of the vascular sheathing pathology is prevented in immunodeficient Rag-1 mice that lack B and T cells, increased expression of VACM1 and ICAM1 still occurs, indicating that inhibition of VEGF function leads to expression changes in cell adhesion molecules that promote extravasation of immune cells. Importantly, a 10-fold lower dose of one of the vectors that cause a vascular sheathing pathology is still able to reduce edemas resulting from choroidal neovascularization without causing any vascular sheathing pathology and only a minimal increase in VCAM1 expression. The data suggest that treatments of neovascular eye pathologies with rAAV-mediated expression of anti VEGF drugs can be developed safely. However, viral load needs to be adjusted to the tropisms of the serotype and the expression pattern of the promoter.

Choroidal vascularity index: an enhanced depth optical coherence tomography-based parameter to determine vascular status in patients with proliferative and non-proliferative macular telangiectasia

KEY MESSAGES

The pathogenesis of subretinal neovascularization (SRNV) due to macular telengiectasia (MacTel 2) has not fully elucidated. This optical coherence tomography (OCT)-based method can provide better understanding of the pathogenesis of SRNV due to MacTel 2.

PURPOSE

To evaluate the choroidal vascular index (CVI) through optical coherence tomography (OCT) on eyes with proliferative macular telangiectasia type 2 (MacTel 2) or non-proliferative MacTel 2, and in healthy individuals.

METHODS

Macular enhanced depth imaging OCT scans on 42 eyes of 21 patients with non-proliferative MacTel 2, on 32 eyes of 20 patients with proliferative MacTel 2, and on 38 eyes of 32 control patients were analyzed by adjusting for age-gender-axial length. Proliferative MacTel 2 was diagnosed when subretinal neovascularization (SRNV) was simultaneously observed in the non-proliferative phase. Binarization methods of ImageJ software were used to analyze images, and total choroid area (TCA), luminal area (LA) and stromal area (SA) were obtained. CVI was characterized as the ratio of LA to TCA.

RESULTS

The mean TCA and SA were significantly higher in group 1 and group 2 when compared with group 3 (3.36 ± 0.29 mm² vs. 3.27 ± 0.76 mm² vs. 2.49 ± 0.24 mm², p < 0.001; 1.15 ± 0.31 mm² vs. 1.10 ± 0.69 mm² vs. 0.35 ± 0.23 mm², respectively; p < 0.001). Although LA was relatively higher in group 1 and group 2 than group 3, no statistically significant difference was observed (2.22 ± 0.14 mm² vs. 2.17 ± 0.15 mm² vs. 2.13 ± 0.21 mm²) (p = 0.088). CVI was significantly lower in group 1 than other groups (0.65 ± 0.01 vs 0.67 ± 0.02 vs 0.68 ± 0.02) (p < 0.001).

CONCLUSION

As an OCT screening method, CVI may be used to assess the vascular status of the choroid on the eyes which are naive for or were exposed to SRNV secondary to MacTel 2, and to elucidate the pathogenesis of this disease.

HK2 Mediated Glycolytic Metabolism in Mouse Photoreceptors Is Not Required to Cause Late Stage Age-Related Macular Degeneration-Like Pathologies

Age-related macular degeneration (AMD) is a multifactorial disease of unclear etiology. We previously proposed that metabolic adaptations in photoreceptors (PRs) play a role in disease progression. We mimicked these metabolic adaptations in mouse PRs through deletion of the tuberous sclerosis complex (TSC) protein TSC1. Here, we confirm our previous findings by deletion of the other complex protein, namely TSC2, in rod photoreceptors. Similar to deletion of Tsc1, mice with deletion of Tsc2 in rods develop AMD-like pathologies, including accumulation of apolipoproteins, migration of microglia, geographic atrophy, and neovascular pathologies. Subtle differences between the two mouse models, such as a significant increase in microglia activation with loss of Tsc2, were seen as well. To investigate the role of altered glucose metabolism in disease pathogenesis, we generated mice with simulation deletions of Tsc2 and hexokinase-2 (Hk2) in rods. Although retinal lactate levels returned to normal in mice with Tsc2-Hk2 deletion, AMD-like pathologies still developed. The data suggest that the metabolic adaptations in PRs that cause AMD-like pathologies are independent of HK2-mediated aerobic glycolysis.

Inflammatory Regulation of CNS Barriers After Traumatic Brain Injury: A Tale Directed by Interleukin-1

Several barriers separate the central nervous system (CNS) from the rest of the body. These barriers are essential for regulating the movement of fluid, ions, molecules, and immune cells into and out of the brain parenchyma. Each CNS barrier is unique and highly dynamic. Endothelial cells, epithelial cells, pericytes, astrocytes, and other cellular constituents each have intricate functions that are essential to sustain the brain's health. Along with damaging neurons, a traumatic brain injury (TBI) also directly insults the CNS barrier-forming cells. Disruption to the barriers first occurs by physical damage to the cells, called the primary injury. Subsequently, during the secondary injury cascade, a further array of molecular and biochemical changes occurs at the barriers. These changes are focused on rebuilding and remodeling, as well as movement of immune cells and waste into and out of the brain. Secondary injury cascades further damage the CNS barriers. Inflammation is central to healthy remodeling of CNS barriers. However, inflammation, as a secondary pathology, also plays a role in the chronic disruption of the barriers' functions after TBI. The goal of this paper is to review the different barriers of the brain, including (1) the blood-brain barrier, (2) the blood-cerebrospinal fluid barrier, (3) the meningeal barrier, (4) the blood-retina barrier, and (5) the brain-lesion border. We then detail the changes at these barriers due to both primary and secondary injury following TBI and indicate areas open for future research and discoveries. Finally, we describe the unique function of the pro-inflammatory cytokine interleukin-1 as a central actor in the inflammatory regulation of CNS barrier function and dysfunction after a TBI.

Dysfunction of retinal neurons and glia during diabetes

Diabetic retinopathy is the leading cause of blindness in those of working age. It is well known that the retinal vasculature is altered during diabetes. More recently, it has emerged that neuronal and glial dysfunction occurs in those with diabetes. Current research is directed at understanding these neuronal and glial changes because they may be an early manifestation of disease processes that ultimately lead to vascular abnormality. This review will highlight the recent advances in our understanding of the neuronal and glial changes that occur during diabetes.

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.

Regulation of Endothelium-Reticulum-Stress-Mediated Apoptotic Cell Death by a Polymethoxylated Flavone, Nobiletin, Through the Inhibition of Nuclear Translocation of Glyceraldehyde 3-Phosphate Dehydrogenase in Retinal Müller Cells

In the early stages of diabetic retinopathy (DR), subtle biochemical and functional alterations occur in Müller cells, which are one of the components of the blood-retinal barrier (BRB). Müller cells are the principal glia of the retina and have shown a strong involvement in the maintenance of homeostasis and the development of retinal tissue. Their functional abnormalities and eventual loss have been correlated with a decrease in the tight junctions between endothelial cells and a consequent breakdown of the BRB, leading to the development of DR. We demonstrated that the endothelium reticulum (ER) triggers Müller cell death and that nuclear accumulation of glyceraldehyde 3-phosphate dehydrogenase is closely associated with ER-induced Müller cell death. In addition, induction of ER stress in Müller cells increased vascular endothelial growth factor expression but decreased pigment-epithelium-derived factor (PEDF) expression in Müller cells. We found that nobiletin, a polymethoxylated flavone from citrus explants, exerts protective action against ER-stress-induced Müller cell death. In addition, nobiletin was found to augment PEDF expression in Müller cells, which may lead to the protection of BRB integrity. These results suggest that nobiletin can be an attractive candidate for the protection of the BRB from breakdown in DR.

Reproducing diabetic retinopathy features using newly developed human induced-pluripotent stem cell-derived retinal Müller glial cells

Muller glial cells (MGCs) are responsible for the homeostatic and metabolic support of the retina. Despite the importance of MGCs in retinal disorders, reliable and accessible human cell sources to be used to model MGC-associated diseases are lacking. Although primary human MGCs (pMGCs) can be purified from post-mortem retinal tissues, the donor scarcity limits their use. To overcome this problem, we developed a protocol to generate and bank human induced pluripotent stem cell-derived MGCs (hiMGCs). Using a transcriptome analysis, we showed that the three genetically independent hiMGCs generated were homogeneous and showed phenotypic characteristics and transcriptomic profile of pMGCs. These cells expressed key MGC markers, including Vimentin, CLU, DKK3, SOX9, SOX2, S100A16, ITGB1, and CD44 and could be cultured up to passage 8. Under our culture conditions, hiMGCs and pMGCs expressed low transcript levels of RLPB1, AQP4, KCNJ1, KCJN10, and SLC1A3. Using a disease modeling approach, we showed that hiMGCs could be used to model the features of diabetic retinopathy (DR)-associated dyslipidemia. Indeed, palmitate, a major free fatty acid with elevated plasma levels in diabetic patients, induced the expression of inflammatory cytokines found in the ocular fluid of DR patients such as CXCL8 (IL-8) and ANGPTL4. Moreover, the analysis of palmitate-treated hiMGC secretome showed an upregulation of proangiogenic factors strongly related to DR, including ANG2, Endoglin, IL-1β, CXCL8, MMP-9, PDGF-AA, and VEGF. Thus, hiMGCs could be an alternative to pMGCs and an extremely valuable tool to help to understand and model glial cell involvement in retinal disorders, including DR.

Microglia increase tight-junction permeability in coordination with Müller cells under hypoxic condition in an in vitro model of inner blood-retinal barrier

Microglia and Müller cells (MCs) are believed to be critically involved in hypoxia-induced blood-retinal barrier (BRB) disruption, which is a major pathogenic factor of various retinopathies. However, the underlying mechanism remains poorly defined. The inner BRB (iBRB) is primarily formed of microvascular endothelial cells (ECs) with tight junction (TJ), which are surrounded and supported by retinal glial cells. We developed a novel in vitro iBRB model sheet by sandwiching Transwell membrane with layered mouse brain microvascular ECs (bEnd.3) and mouse retinal MCs (QMMuC-1) on each side of the membrane. Using this model, we tested the hypothesis that under hypoxic condition, activated microglia produce inflammatory cytokines such as interleukin (IL)-1β, which may promote vascular endothelial growth factor (VEGF) production from MCs, leading to TJ disruption. The iBRB model cell sheets were exposed to 1% oxygen for 6 h with or without mouse brain microglia (BV2) or IL-1β. TJ structure and function were examined by zonula occludens (ZO)-1 immunostaining and fluorescein isothiocyanate permeability assay, respectively. Relative gene expression of IL-1β in BV2 under normoxic and hypoxic conditions was examined by real-time reverse transcription-polymerase chain reaction. VEGF protein concentration in QMMuC-1 supernatants was measured by enzyme-linked immunosorbent assay. The bEnd.3 cell sheet incubated with BV2 in hypoxic condition or with IL-1β in normoxic condition showed abnormal localization of ZO-1 and aberrated barrier function. Under normoxic condition, EC-MC iBRB model cell sheet showed lower permeability than bEnd.3 cell sheet. Under hypoxic conditions, the barrier function of EC-MC iBRB model cell sheet was more deteriorated compared to bEnd.3 cell sheet. Under hypoxic condition, incubation of EC-MC iBRB model cell sheet with BV2 cells or IL-1β significantly increased barrier permeability, and hypoxia-treated BV2 cells expressed significantly higher levels of IL-1β mRNA. Incubation of QMMuC-1 with IL-1β increased VEGF production. These results suggest that under hypoxic condition, microglia are activated to release proinflammatory cytokines such as IL-1β that promote VEGF production from MCs, leading to disruption of iBRB function. Modulating microglia and MCs function may be a novel approach to treat hypoxia-induced retinal BRB dysfunction.

PDGF Receptor Alpha Signaling Is Key for Müller Cell Homeostasis Functions

Müller cells, the major retinal macroglia, are key to maintaining vascular integrity as well as retinal fluid and ion homeostasis. Although platelet derived growth factor (PDGF) receptor expression in Müller glia has been reported earlier, their actual role for Müller cell function and intimate interaction with cells of the retinal neurovascular unit remains unclear. To close this gap of knowledge, Müller cell-specific PDGF receptor alpha (PDGFRα) knockout (KO) mice were generated, characterized, and subjected to a model of choroidal neovascularization (CNV). PDGFRα-deficient Müller cells could not counterbalance hypoosmotic stress as efficiently as their wildtype counterparts. In wildtypes, the PDGFRα ligand PDGF-BB prevented Müller cell swelling induced by the administration of barium ions. This effect could be blocked by the PDGFR family inhibitor AC710. PDGF-BB could not restore the capability of an efficient volume regulation in PDGFRα KO Müller cells. Additionally, PDGFRα KO mice displayed reduced rod and cone-driven light responses. Altogether, these findings suggest that Müller glial PDGFRα is central for retinal functions under physiological conditions. In contrast, Müller cell-specific PDGFRα KO resulted in less vascular leakage and smaller lesion areas in the CNV model. Of note, the effect size was comparable to pharmacological blockade of PDGF signaling alone or in combination with anti-vascular endothelial growth factor (VEGF) therapy—a treatment regimen currently being tested in clinical trials. These data imply that targeting PDGF to treat retinal neovascular diseases may have short-term beneficial effects, but may elicit unwarranted side effects given the putative negative effects on Müller cell homeostatic functions potentially interfering with a long-term positive outcome.

The Impact of Oxidative Stress on Blood-Retinal Barrier Physiology in Age-Related Macular Degeneration

The blood retinal barrier (BRB) is a fundamental eye component, whose function is to select the flow of molecules from the blood to the retina and vice-versa, and its integrity allows the maintenance of a finely regulated microenvironment. The outer BRB, composed by the choriocapillaris, the Bruch's membrane, and the retinal pigment epithelium, undergoes structural and functional changes in age-related macular degeneration (AMD), the leading cause of blindness worldwide. BRB alterations lead to retinal dysfunction and neurodegeneration. Several risk factors have been associated with AMD onset in the past decades and oxidative stress is widely recognized as a key factor, even if the exact AMD pathophysiology has not been exactly elucidated yet. The present review describes the BRB physiology, the BRB changes occurring in AMD, the role of oxidative stress in AMD with a focus on the outer BRB structures. Moreover, we propose the use of cerium oxide nanoparticles as a new powerful anti-oxidant agent to combat AMD, based on the relevant existing data which demonstrated their beneficial effects in protecting the outer BRB in animal models of AMD.

Influences of advanced glycosylation end products on the inner blood-retinal barrier in a co-culture cell model in vitro

Advanced glycosylation end products (AGEs) are harmful factors that can damage the inner blood–retinal barrier (iBRB). Rat retinal microvascular endothelial cells (RMECs) were isolated and cultured, and identified by anti-CD31 and von Willebrand factor polyclonal antibodies. Similarly, rat retinal Müller glial cells (RMGCs) were identified by H&E staining and with antibodies of glial fibrillary acidic protein and glutamine synthetase. The transepithelial electrical resistance (TEER) value was measured with a Millicell electrical resistance system to observe the leakage of the barrier. Transwell cell plates for co-culturing RMECs with RMGCs were used to construct an iBRB model, which was then tested with the addition of AGEs at final concentrations of 50 and 100 mg/L for 24, 48, and 72 h. AGEs in the in vitro iBRB model constructed by RMEC and RMGC co-culture led to the imbalance of the vascular endothelial growth factor (VEGF) and pigment epithelial derivative factor (PEDF), and the permeability of the RMEC layer increased because the TEER decreased in a dose- and time-dependent manner. AGEs increased VEGF but lowered PEDF in a dose- and time-dependent manner. The intervention with AGEs led to the change of the transendothelial resistance of the RMEC layer likely caused by the increased ratio of VEGF/PEDF.

Retinal energy metabolism in health and glaucoma

Energy metabolism refers to the processes by which life transfers energy to do cellular work. The retina's relatively large energy demands make it vulnerable to energy insufficiency. In addition, evolutionary pressures to optimize human vision have been traded against retinal ganglion cell bioenergetic fragility. Details of the metabolic profiles of the different retinal cells remain poorly understood and are challenging to resolve. Detailed immunohistochemical mapping of the energy pathway enzymes and substrate transporters has provided some insights and highlighted interspecies differences. The different spatial metabolic patterns between the vascular and avascular retinas can account for some inconsistent data in the literature. There is a consilience of evidence that at least some individuals with glaucoma have impaired RGC energy metabolism, either due to impaired nutrient supply or intrinsic metabolic perturbations. Bioenergetic-based therapy for glaucoma has a compelling pathophysiological foundation and is supported by recent successes in animal models. Recent demonstrations of visual and electrophysiological neurorecovery in humans with glaucoma is highly encouraging and motivates longer duration trials investigating bioenergetic neuroprotection.

Elucidation of Pathophysiology and Novel Treatment for Diabetic Macular Edema Derived from the Concept of Neurovascular Unit

The retina transmits light signals to the brain via a complex structure composed of photoreceptor cells, neurons including ganglion cells, glial cells such as astrocytes and Mueller cells, as well as retinal blood vessels that feed the retina. The retina performs such high-level physiological function and maintains homeostasis effectively through interactions among the cells that form the neurovascular units (NVUs). Furthermore, as a component of the blood‒retinal barrier (BRB), the vascular structure of the retina is functionally based on the NVUs, in which the nervous system and the vascular tissues collaborate in a mutually supportive relationship. Retinal neurons such as ganglion cells and amacrine cells are traditionally considered to be involved only in visual function, but multiple functionality of neurons attracted attention lately, and retinal neurons play an important role in the formation and function of retinal blood vessels. In other words, damage to neurons indirectly affects retinal blood vessels. Diabetic macular edema is the leading cause of vision loss in diabetic retinopathy, and this type of edema results in neurological and vascular disorders. In this article, the regulatory mechanism of retinal capillaries in diabetic macular edema is reviewed from the viewpoint of NVU.

Destruction of the blood-retina barrier in diabetic retinopathy depends on angiotensin-converting enzyme-mediated TGF-β1/Smad signaling pathway activation

Diabetes mellitus (DM) is a systemic, chronic metabolic disease that is related to heredity and autoimmunity and is often accompanied by complications of retinopathy. However, the causative mechanism involved in the pathological process remains unclear. In this research, treatment with fosinopril or LY2109761 was found to be responsible for the improvement of the pathological processes, serum biochemical indexes and retinopathy in rats with streptozotocin-induced diabetes. In addition, the upregulation of angiotensin-converting enzyme (ACE) in the serum and the increased expression of TGF-β1 in the pathological outer nuclear layer (ONL) and inner nuclear layer (INL) of the retina were also reduced. In vitro experiments demonstrated that ACE enhanced cell damage and TGF-β1/Smad signaling pathway activation in retinal capillary endothelial cells (RCECs) under high glucose conditions. In addition, the activity of ACE was also considered to be related to the increasing levels of activated TGF-β1 in both rat retinal Müller cells (RMCs) and RCECs, but ACE activity had no effect on the high glucose-mediated upregulation of total TGF-β1 in RMCs. Coculture experiments with RCECs and RMCs showed that the barrier that was established under normal conditions was significantly impaired when exposed to high glucose combined with ACE, and damage of barrier can be prevented by adding fosinopril or LY2109761. Finally, a similar auxiliary effect of ACE was also observed in the activated TGF-β1-mediated barrier damage in blood-retinal barrier model in vitro. In summary, ACE-mediated TGF-β1/Smad signaling pathway activation was found to be involved in the destruction of the blood-retina barrier during diabetic retinopathy in a model of streptozotocin-induced diabetes, and these data may provide evidence to guide the treatment of the complications of diabetes mellitus.

Retinal Dystrophies and the Road to Treatment: Clinical Requirements and Considerations

: Retinal dystrophies (RDs) comprise relatively rare but devastating causes of progressive vision loss. They represent a spectrum of diseases with marked genetic and clinical heterogeneity. Mutations in the same gene may lead to different diagnoses, for example, retinitis pigmentosa or cone dystrophy. Conversely, mutations in different genes may lead to the same phenotype. The age at symptom onset, and the rate and characteristics of peripheral and central vision decline, may vary widely per disease group and even within families. For most RD cases, no effective treatment is currently available. However, preclinical studies and phase I/II/III gene therapy trials are ongoing for several RD subtypes, and recently the first retinal gene therapy has been approved by the US Food and Drug Administration for RPE65-associated RDs: voretigene neparvovec-rzyl (Luxturna). With the rapid advances in gene therapy studies, insight into the phenotypic spectrum and long-term disease course is crucial information for several RD types. The vast clinical heterogeneity presents another important challenge in the evaluation of potential efficacy in future treatment trials, and in establishing treatment candidacy criteria. This perspective describes these challenges, providing detailed clinical descriptions of several forms of RD that are caused by genes of interest for ongoing and future gene or cell-based therapy trials. Several ongoing and future treatment options will be described.

Nox (NADPH Oxidase) 1, Nox4, and Nox5 Promote Vascular Permeability and Neovascularization in Retinopathy

Hypertension is a risk factor for the vascular permeability and neovascularization that threatens vision in diabetic retinopathy. Excess reactive oxygen species derived from the Nox (NADPH oxidase) isoforms, Nox1 and Nox4, contributes to vasculopathy in diabetic retinopathy; however, if Nox1/4 inhibition is beneficial in hypertensive diabetic retinopathy is unknown. Here, we determined that diabetic spontaneously hypertensive rats had exacerbated retinal vascular permeability and expression of angiogenic and inflammatory factors, compared with normotensive diabetic Wistar Kyoto rats. GKT136901, a specific dual inhibitor of Nox1 and Nox4, prevented these events in diabetic Wistar Kyoto rats and spontaneously hypertensive rats. Retinal neovascularization does not develop in diabetic rodents, and therefore, the oxygen-induced retinopathy model is used to evaluate this pathology. We previously demonstrated that Nox1/4 inhibition reduced retinal neovascularization in oxygen-induced retinopathy. However, although Nox5 is expressed in human retina, its contribution to retinopathy has not been studied in vivo, largely due to its absence from the rodent genome. We generated transgenic mice with inducible human Nox5 expressed in endothelial cells (vascular endothelial-cadherin⁺Nox5⁺ mice). In vascular endothelial-cadherin⁺Nox5⁺ mice with oxygen-induced retinopathy, retinal vascular permeability and neovascularization, as well as the expression of angiogenic and inflammatory factors, were increased compared with wild-type littermates. In bovine retinal endothelial cells, which express Nox1, Nox4, and Nox5, Nox1/4 inhibition, as well as Nox5 silencing RNA, reduced the high glucose-induced upregulation of oxidative stress, angiogenic, and inflammatory factors. Collectively, these data indicate the potential of Nox1, Nox4, and Nox5 inhibition to reduce vision-threatening damage to the retinal vasculature.

Iron Accumulates in Retinal Vascular Endothelial Cells But Has Minimal Retinal Penetration After IP Iron Dextran Injection in Mice

Purpose

Iron supplementation therapy is used for iron-deficiency anemia but has been associated with macular degeneration in a 43-year-old patient. Iron entry into the neurosensory retina (NSR) can be toxic. It is important to determine conditions under which serum iron might cross the blood retinal barrier (BRB) into the NSR. Herein, an established mouse model of systemic iron overload using high-dose intraperitoneal iron dextran (IP FeDex) was studied. In addition, because the NSR expresses the iron regulatory hormone hepcidin, which could limit iron influx into the NSR, we gave retina-specific hepcidin knockout (RS-HepcKO) mice IP FeDex to test this possibility.

Methods

Wild-type (WT) and RS-HepcKO mice were given IP FeDex. In vivo retina imaging was performed. Blood and tissues were analyzed for iron levels. Quantitative PCR was used to measure levels of mRNAs encoding iron regulatory and photoreceptor-specific genes. Ferritin and albumin were localized in the retina by immunofluorescence.

Results

IP FeDex in both WT and RS-HepcKO mice induced high levels of iron in the liver, serum, retinal vascular endothelial cells (rVECs), and RPE, but not the NSR. The BRB remained intact. Retinal degeneration did not occur.

Conclusions

Following injection of high-dose IP FeDex, iron accumulated in the BRB, but not the NSR. Thus, the BRB can shield the NSR from iron delivered in this manner. This ability is not dependent on NSR hepcidin production.

Comparison of anatomical and visual outcomes following different anti-vascular endothelial growth factor treatments in subretinal neovascular membrane secondary to type 2 proliferative macular telangiectasia

PURPOSE

To evaluate central macular thickness (CMT), subfoveal choroidal thickness (SFCT), and visual outcomes following different intravitreal anti-vascular endothelial growth factor (VEGF) treatments in eyes with subretinal neovascular membrane (SRNVM) due to type 2 proliferative macular telangiectasia (Mac Tel 2).

MATERIALS AND METHODS

A total of 38 eyes of 34 patients who underwent intravitreal aflibercept (IVA), intravitreal ranibizumab (IVR), or intravitreal bevacizumab (IVB) injections secondary to SRNVM due to type 2 proliferative MacTel were retrospectively reviewed. The CMT, central macular volume (CMV), best corrected visual acuity (BCVA), and SFCT were evaluated at baseline and at 2 weeks, at 1 month, and at final visits following treatment. Spectral-domain optical coherence tomography and enhanced depth optical coherence tomography were used for the analysis.

RESULTS

The mean age of the patients was 58.34 ± 12.48 years (range, 27-79 years). The mean follow-up time was 15.97 ± 6.79 months (range 5-32 months). The mean BCVA showed a statistically significant increase in each group (< 0.001). There was no statistically significant difference in BCVA changes between groups in follow-up periods. There was a significant decrease in CMT following IVA (326.4 ± 168.03 μm to 236 ± 58.33 μm) and IVB (383.71 ± 156.79 μm to 343.85 ± 146.25 μm) (p < 0.001, p = 0.004, respectively) whereas no significant decrease in CMT was observed following IVR (374.57 ± 124.28 μm to 339.71 ± 126.10 μm) (p = 0.65) between baseline and final visit. The SFCT significantly decreased following both IVB and IVR treatments (p = 0.009, p = 0.03, respectively).

CONCLUSIONS

The IVA, IVR, and IVB were found to be effective with regards to anatomical and visual outcomes in proliferative Mac Tel type 2 patients related with SRNVM. Patients receiving both IVA and IVB needed less injections compared to patients who received IVR. Moreover, IVB and IVR lead to significant decrease in SFCT whereas IVA did not show significant effect on SFCT.

Radial Hemorrhage in Henle Layer in Macular Telangiectasia Type 2

Importance

Radial hemorrhage in the outer plexiform layer of Henle may be a complication of macular telangiectasia type 2 (MacTel 2) and may occur because of microvascular abnormalities of the deep retinal capillary plexus in the absence of subretinal neovascularization.

Objective

To describe the multimodal imaging findings, including cross-sectional and en face optical coherence tomography (OCT), of radial hemorrhage in the outer plexiform layer of Henle, which may be a complication of MacTel 2.

Design, Setting, and Participants

This retrospective case series from 2 tertiary referral centers (Stein Eye Institute, Los Angeles, California; New England Eye Center, Boston, Massachusetts) between January 1, 2012, and December 31, 2017, describes 3 patients with MacTel 2 complicated by characteristic radial hemorrhage in the outer plexiform layer of Henle.

Main Outcomes and Measures

Color fundus photography, cross-sectional and en face OCT, OCT angiography (OCTA), fundus autofluorescence, and fluorescein angiography.

Results

Three male patients presented with sudden vision loss in the right eye. A characteristic radial pattern of hemorrhage was noted with color fundus photography. Cross-sectional and en face OCT and OCTA localized the hemorrhage to the outer plexiform layer of Henle in the absence of subretinal neovascularization. Optical coherence tomography findings consistent with MacTel 2 were identified in the fellow eye in each patient. At the follow-up visit 1 to 2 months after presentation, spontaneous resolution of the hemorrhage was noted in all 3 patients, and OCTA illustrated underlying microvascular abnormalities in the deep retinal capillary plexus in 2 patients.

Conclusions and Relevance

This report describes 3 patients with MacTel 2 complicated by characteristic radial hemorrhage in the outer plexiform layer of Henle, which may represent a characteristic finding in MacTel 2 that may develop as a result of microvascular abnormalities of the deep retinal capillary plexus in the absence of subretinal neovascularization.

Neural activity regulates autoimmune diseases through the gateway reflex

The brain, spinal cord and retina are protected from blood-borne compounds by the blood-brain barrier (BBB), blood-spinal cord barrier (BSCB) and blood-retina barrier (BRB) respectively, which create a physical interface that tightly controls molecular and cellular transport. The mechanical and functional integrity of these unique structures between blood vessels and nervous tissues is critical for maintaining organ homeostasis. To preserve the stability of these barriers, interplay between constituent barrier cells, such as vascular endothelial cells, pericytes, glial cells and neurons, is required. When any of these cells are defective, the barrier can fail, allowing blood-borne compounds to encroach neural tissues and cause neuropathologies. Autoimmune diseases of the central nervous system (CNS) and retina are characterized by barrier disruption and the infiltration of activated immune cells. Here we review our recent findings on the role of neural activity in the regulation of these barriers at the vascular endothelial cell level in the promotion of or protection against the development of autoimmune diseases. We suggest nervous system reflexes, which we named gateway reflexes, are fundamentally involved in these diseases. Although their reflex arcs are not completely understood, we identified the activation of specific sensory neurons or receptor cells to which barrier endothelial cells respond as effectors that regulate gateways for immune cells to enter the nervous tissue. We explain this novel mechanism and describe its role in neuroinflammatory conditions, including models of multiple sclerosis and posterior autoimmune uveitis.

Neuroprotective effect of levetiracetam in mouse diabetic retinopathy: Effect on glucose transporter-1 and GAP43 expression

AIMS

Retinopathy is a neurodegenerative complication associating diabetes mellitus. Diabetic retinopathy (DR) is the primary reason of visual loss during early adulthood. DR has a complicated multifactorial pathophysiology initiated by hyperglycaemia-induced ischaemic neurodegenerative retinal changes, followed by vision-threatening consequences. The main therapeutic modalities for DR involve invasive delivery of intravitreal antiangiogenic agents as well as surgical interventions. The current work aimed to explore the potential anti-inflammatory and retinal neuroprotective effects of levetiracetam.

MAIN METHODS

This study was performed on alloxan-induced diabetes in mice (n: 21). After 10 weeks, a group of diabetic animals (n: 7) was treated with levetiracetam (25 mg/kg) for six weeks. Retinal tissues were dissected and paraffin-fixed for examination using (1) morphometric analysis with haematoxylin and eosin (HE), (2) immunohistochemistry (GLUT1, GFAP and GAP43), and (3) RT-PCR-detected expression of retinal inflammatory and apoptotic mediators (TNF-α, IL6, iNOS, NF-κB and Tp53).

KEY FINDINGS

Diabetic mice developed disorganized and debilitated retinal layers with upregulation of the gliosis marker GFAP and downregulation of the neuronal plasticity marker GAP43. Additionally, diabetic retinae showed increased transcription of NF-κB, TNF-α, IL6, iNOS and Tp53. Levetiracetam-treated mice showed downregulation of retinal GLUT1 with relief and regression of retinal inflammation and improved retinal structural organization.

SIGNIFICANCE

Levetiracetam may represent a potential neuroprotective agent in DR. The data presented herein supported an anti-inflammatory role of levetiracetam. However, further clinical studies may be warranted to confirm the effectiveness and safety of levetiracetam in DR patients.