Mitochondrial function in Müller cells - Does it matter?

The Circadian Clock of Müller Glia Is Necessary for Retinal Homeostasis and Neuronal Survival

Biological processes throughout the body are orchestrated in time through the regulation of local circadian clocks. The retina is among the most metabolically active tissues, with demands depending greatly on the light/dark cycle. Most cell types within the rodent retina are known to express the circadian clock; however, retinal clock expression in humans has not previously been localized. Moreover, the effect of local circadian clock dysfunction on retinal homeostasis is incompletely understood. The current study indicated an age-dependent decline in circadian clock gene and protein expression in the human retina. An animal model of targeted Bmal1 deficiency was used to identify the circadian clock of the retinal Müller glia as essential for neuronal survival, vascular integrity, and retinal function. These results suggest a potential role for the local retinal circadian clock within the Müller glia in age-related retinal disease and retinal degeneration.

Mitochondrial transfer between BMSCs and Müller promotes mitochondrial fusion and suppresses gliosis in degenerative retina

Mitochondrial dysfunction and Müller cells gliosis are significant pathological characteristics of retinal degeneration (RD) and causing blinding. Stem cell therapy is a promising treatment for RD, the recently accepted therapeutic mechanism is cell fusion induced materials transfer. However, whether materials including mitochondrial transfer between grafted stem cells and recipient's cells contribute to suppressing gliosis and mechanism are unclear. In present study, we demonstrated that bone marrow mesenchymal stem cells (BMSCs) transferred mitochondria to Müller cells by cell fusion and tunneling nanotubes. BMSCs-derived mitochondria (BMSCs-mito) were integrated into mitochondrial network of Müller cells, improving mitochondrial function, reducing oxidative stress and gliosis, which protected visual function partially in the degenerative rat retina. RNA sequencing analysis revealed that BMSCs-mito increased mitochondrial DNA (mtDNA) content and facilitated mitochondrial fusion in damaged Müller cells. It suggests that mitochondrial transfer from BMSCs remodels Müller cells metabolism and suppresses gliosis; thus, delaying the degenerative progression of RD.

Ferroptosis in Glaucoma: A Promising Avenue for Therapy

Glaucoma, a blind-leading disease largely since chronic pathological intraocular high pressure (ph-IOP). Hitherto, it is reckoned incurable for irreversible neural damage and challenges in managing IOP. Thus, it is significant to develop neuroprotective strategies. Ferroptosis, initially identified as an iron-dependent regulated death that triggers Fenton reactions and culminates in lipid peroxidation (LPO), has emerged as a focal point in multiple tumors and neurodegenerative diseases. Researches show that iron homeostasis play critical roles in the optic nerve (ON) and retinal ganglion cells (RGCs), suggesting targeted treatments could be effective. In glaucoma, apart from neural lesions, disrupted metal balance and increased oxidative stress in trabecular meshwork (TM) are observed. These disturbances lead to extracellular matrix excretion disorders, known as sclerotic mechanisms, resulting in refractory blockages. Importantly, oxidative stress, a significant downstream effect of ferroptosis, is also a key factor in cell senescence. It plays a crucial role in both the etiology and risk of glaucoma. Moreover, ferroptosis also induces non-infectious inflammation, which exacerbate glaucomatous injury. Therefore, the relevance of ferroptosis in glaucoma is extensive and multifaceted. In this review, the study delves into the current understanding of ferroptosis mechanisms in glaucoma, aiming to provide clues to inform clinical therapeutic practices.

Temporospatial dynamics of the morphogenesis of the rabbit retina from prenatal to postnatal life: Light and electron microscopic study

The retina consists of various cell types arranged in eight cell layers and two membranes that originate from the neuroectodermal cells. In this study, the timing of differentiation and distribution of the cellular components and the layers of the rabbit retina are investigated using light and electron microscopy and immunohistochemical techniques. There were 32 rabbit embryos and 12 rabbits used. The rabbit retina begins its prenatal development on the 10th day of gestation in the form of optic cup. The process of neuro- and gliogenesis occurs in several stages: In the first stage, the ganglionic cells are differentiated at the 15th day. The second stage includes the differentiation of Muller, amacrine, and cone cells on the 23rd day. The differentiation of bipolar, horizontal, and rod cells and formation of the inner segments of the photoreceptors consider the late stage that occurs by the 27th and 30th day of gestation. On the first week of age postnatally, the outer segments of the photoreceptors are developed. S100 protein is expressed by the Muller cells and its processes that traverse the retina from the outer to the inner limiting membranes. Calretinin is intensely labeled within the amacrine and displaced amacrine cells. Ganglionic cells exhibited moderate immunoreactivity for calretinin confined to their cytoplasm and dendrites. In conclusion, all stages of neuro- and gliogenesis of the rabbit retina occur during the embryonic period. Then, the retina continues its development postnatally by formation of the photoreceptor outer segments and all layers of the retina become established. RESEARCH HIGHLIGHTS: The aim of this study is to investigate the morphogenesis of the rabbit retina during pre- and postnatal life. The primordia of the retina could be observed in the form of the optic cup. The ganglionic cells are the first cells to differentiate, while the photoreceptor cells are the last. S100 protein is expressed by the Muller cells and its processes. Calretinin is intensely labeled in the amacrine and displaced amacrine cells and moderately expressed in the cytoplasm and dendrites of ganglionic cells.

Glaucoma: from pathogenic mechanisms to retinal glial cell response to damage

Glaucoma is a neurodegenerative disease of the retina characterized by the irreversible loss of retinal ganglion cells (RGCs) leading to visual loss. Degeneration of RGCs and loss of their axons, as well as damage and remodeling of the lamina cribrosa are the main events in the pathogenesis of glaucoma. Different molecular pathways are involved in RGC death, which are triggered and exacerbated as a consequence of a number of risk factors such as elevated intraocular pressure (IOP), age, ocular biomechanics, or low ocular perfusion pressure. Increased IOP is one of the most important risk factors associated with this pathology and the only one for which treatment is currently available, nevertheless, on many cases the progression of the disease continues, despite IOP control. Thus, the IOP elevation is not the only trigger of glaucomatous damage, showing the evidence that other factors can induce RGCs death in this pathology, would be involved in the advance of glaucomatous neurodegeneration. The underlying mechanisms driving the neurodegenerative process in glaucoma include ischemia/hypoxia, mitochondrial dysfunction, oxidative stress and neuroinflammation. In glaucoma, like as other neurodegenerative disorders, the immune system is involved and immunoregulation is conducted mainly by glial cells, microglia, astrocytes, and Müller cells. The increase in IOP produces the activation of glial cells in the retinal tissue. Chronic activation of glial cells in glaucoma may provoke a proinflammatory state at the retinal level inducing blood retinal barrier disruption and RGCs death. The modulation of the immune response in glaucoma as well as the activation of glial cells constitute an interesting new approach in the treatment of glaucoma.

Potential Retinal Biomarkers in Alzheimer's Disease

Alzheimer's disease (AD) represents a major diagnostic challenge, as early detection is crucial for effective intervention. This review examines the diagnostic challenges facing current AD evaluations and explores the emerging field of retinal alterations as early indicators. Recognizing the potential of the retina as a noninvasive window to the brain, we emphasize the importance of identifying retinal biomarkers in the early stages of AD. However, the examination of AD is not without its challenges, as the similarities shared with other retinal diseases introduce complexity in the search for AD-specific markers. In this review, we address the relevance of using the retina for the early diagnosis of AD and the complex challenges associated with the search for AD-specific retinal biomarkers. We provide a comprehensive overview of the current landscape and highlight avenues for progress in AD diagnosis by retinal examination.

Müller cell vulnerability in aging human retina: Implications on photoreceptor cell survival

Müller glial cells (MC) support various metabolic functions of the retinal neurons, and maintain the homeostasis. Oxidative stress is intensified with aging, and in human retina, MC and photoreceptors undergo lipid peroxidation and protein nitration. Information on how MC respond to oxidative stress is vital to understand the fate of aging retinal neurons. This study examined age-related changes in MC of donor human retina (age: 35-98 years; N = 18 donors). Ultrastructural and immunohistochemical observations indicate that MC undergo gliosis and increased lipid peroxidation, and show osmotic changes with advanced aging (>80 years). Photoreceptor cells also undergo oxidative-nitrosative stress with aging, and their synapses also show clear osmotic swelling. MC respond to oxidative stress via proliferation of smooth endoplasmic reticulum in their processes, and increased expression of aquaporin-4 in endfeet and outer retina. In advanced aged retinas (81-98 years), they showed mitochondrial disorganisation, accumulation of lipids and autophagosomes, lipofuscin granules and axonal remnants in phagolysosomes in their inner processes, suggesting a reduced phagocytotic potential in them with aging. Glutamine synthetase expression does not alter until advanced aging, when the retinas show its increased expression in endfeet and Henle fiber layer. It is evident that MC are vulnerable with normal aging and this could be a reason for photoreceptor cell abnormalities reported with aging of the human retina.

TGR5 supresses cGAS/STING pathway by inhibiting GRP75-mediated endoplasmic reticulum-mitochondrial coupling in diabetic retinopathy

Diabetic retinopathy (DR) is a serious and relatively under-recognized complication of diabetes. Müller glial cells extend throughout the retina and play vital roles in maintaining retinal homeostasis. Previous studies have demonstrated that TGR5, a member of the bile acid-activated GPCR family, could ameliorate DR. However, the role of TGR5 in regulating Müller cell function and the underlying mechanism remains to be ascertained. To address this, high glucose (HG)-treated human Müller cells and streptozotocin-treated Sprague-Dawley rats were used in the study. The IP3R1-GRP75-VDAC1 axis and mitochondrial function were assessed after TGR5 ablation or agonism. Cytosolic mitochondrial DNA (mtDNA)-mediated cGAS-STING activation was performed. The key markers of retinal vascular leakage, apoptosis, and inflammation were examined. We found that mitochondrial Ca2+ overload and mitochondrial dysfunction were alleviated by TGR5 agonist. Mechanically, TGR5 blocked the IP3R1-GRP75-VDAC1 axis mediated Ca2+ efflux from the endoplasmic reticulum into mitochondria under diabetic condition. Mitochondrial Ca2+ overload led to the opening of the mitochondrial permeability transition pore and the release of mitochondrial DNA (mtDNA) into the cytosol. Cytoplasmic mtDNA bound to cGAS and upregulated 2'3' cyclic GMP-AMP. Consequently, STING-mediated inflammatory responses were activated. TGR5 agonist prevented retinal injury, whereas knockdown of TGR5 exacerbated retinal damage in DR rats, which was rescued by the STING inhibitor. Based on the above results, we propose that TGR5 might be a novel therapeutic target for the treatment of DR.

Specific Deoxyceramide Species Correlate with Expression of Macular Telangiectasia Type 2 (MacTel2) in a SPTLC2 Carrier HSAN1 Family

Hereditary sensory and autonomic neuropathy type 1 (HSAN1/HSN1) is a peripheral neuropathy most commonly associated with pathogenic variants in the serine palmitoyltransferase complex (SPTLC1, SPTLC2) genes, which are responsible for sphingolipid biosynthesis. Recent reports have shown that some HSAN1 patients also develop macular telangiectasia type 2 (MacTel2), a retinal neurodegeneration with an enigmatic pathogenesis and complex heritability. Here, we report a novel association of a SPTLC2 c.529A>G p.(Asn177Asp) variant with MacTel2 in a single member of a family that otherwise has multiple members afflicted with HSAN1. We provide correlative data to suggest that the variable penetrance of the HSAN1/MacTel2-overlap phenotype in the proband may be explained by levels of certain deoxyceramide species, which are aberrant intermediates of sphingolipid metabolism. We provide detailed retinal imaging of the proband and his HSAN1+/MacTel2- brothers and suggest mechanisms by which deoxyceramide levels may induce retinal degeneration. This is the first report of HSAN1 vs. HSAN1/MacTel2 overlap patients to comprehensively profile sphingolipid intermediates. The biochemical data here may help shed light on the pathoetiology and molecular mechanisms of MacTel2.

Is fat the future for saving sight? Bioactive lipids and their impact on glaucoma

Glaucoma is characterized by a continuous loss of retinal ganglion cells. The cause of glaucoma is associated with an increase in intraocular pressure (IOP), but the underlying pathophysiology is diverse and, in most cases, unknown. There is an indisputable unmet need to identify new pathways involved in glaucoma pathogenesis. Increasing evidence suggests that bioactive lipids may be critical in the development and progression of glaucoma. Preclinical and clinical bioactive lipid targets exist and are being developed. In this review, we aim to shed light on the potential of bioactive lipids for the prevention, diagnosis, prognosis, and treatment of glaucoma by asking the question "is fat the future for saving sight".

Changes in glial cells and neurotrophic factors due to rotenone-induced oxidative stress in Nrf2 knockout mice

Glaucoma is one of the most common causes of blindness worldwide. It is thought to be a multifactorial disease with underlying mechanisms that include mitochondrial dysfunction and oxidative stress. Here, we used NF-E2 related factor 2 (Nrf2) knockout (KO) mice, which are vulnerable to oxidative stress, to examine a neuroprotective effect against oxidative stress due to rotenone, a mitochondrial complex I inhibitor. Wild-type (WT) and Nrf2 KO mice received an oral solution of rotenone for 30 days. We then extracted the retinas and performed immunohistochemistry and quantitative RT-PCR. We also prepared a primary Müller cell culture of samples from each mouse, added 30 μM rotenone, and then measured cell viability, cytotoxicity and CellRox absorbance. We also examined gene expression. We found a significant increase in the number of 8-OHdG-positive retinal ganglion cells (RGCs) after rotenone administration in both the WT and Nrf2 KO mice. There was no difference in the number of RNA-binding protein with multiple splicing (RBPMS)-positive RGCs in the WT and Nrf2 KO mice, but Nrf2 KO mice that were given rotenone had significantly less retinal gene expression of RBPMS than Nrf2 KO mice given a control. Moreover, there was significantly higher mRNA gene expression of vimentin and glial fibrillary acidic protein (GFAP) in Nrf2 KO mice that received rotenone than WT mice that received rotenone. A statistical analysis of the in vitro experiment showed that cell viability was lower, cytotoxicity was higher, and oxidative stress was higher in the Müller cells of the Nrf2 KO mice than the WT mice. Finally, brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF) were significantly higher in the Müller cells of the Nrf2 KO mice than the WT mice. These findings suggest that in Nrf2 KO mice under oxidative stress caused by rotenone, temporary neurotrophic factors are secreted from the Müller cells, conferring neuroprotection in these cells.

Single-cell transcriptome reveals diversity of Müller cells with different metabolic-mitochondrial signatures in normal and degenerated macula

Müller cell is the most abundant glial cell in mammalian retina, supporting the functions of photoreceptors and other retinal neurons via maintaining environmental homeostasis. In response to injury and/or neuronal degeneration, Müller cells undergo morphological and functional alternations, known as reactive gliosis documented in multiple retinal diseases, including age-related macular degeneration (AMD), retinitis pigmentosa, diabetic retinopathy, and traumatic retinal detachment. But the functional consequences of Müller glia cell reactivation or even the regulatory networks of the retinal gliosis are still controversial. In this study, we reveal different subpopulations of Müller cells with distinct metabolic-mitochondrial signatures by integrating single cell transcriptomic data from Early AMD patients and healthy donors. Our results show that a portion of Müller cells exhibits low mitochondrial DNA (mtDNA) expressions, reduced protein synthesis, impaired homeostatic regulation, decreased proliferative ability but enhanced proangiogenic function. Interestingly, the major alternation of Müller cells in Early AMD retina is the change of subpopulation abundance, rather than generation of new subcluster. Transcription factor enrichment analysis further highlights the key regulators of metabolic-mitochondrial states of Müller glias in Early AMD patients especially. Our study demonstrates new characteristics of retinal gliosis associated with Early AMD and suggests the possibility to prevent degeneration by intervening mitochondrial functions of Müller cells.

Destabilizing COXIV in Müller Glia Increases Retinal Glycolysis and Alters Scotopic Electroretinogram

Müller glia (MG), the principal glial cell of the retina, have a metabolism that defies categorization into glycolytic versus oxidative. We showed that MG mount a strong hypoxia response to ocular hypertension, raising the question of their relative reliance on mitochondria for function. To explore the role of oxidative phosphorylation (OXPHOS) in MG energy production in vivo, we generated and characterized adult mice in which MG have impaired cytochrome c oxidase (COXIV) activity through knockout of the COXIV constituent COX10. Histochemistry and protein analysis showed that COXIV protein levels were significantly lower in knockout mouse retina compared to control. Loss of COXIV activity in MG did not induce structural abnormalities, though oxidative stress was increased. Electroretinography assessment showed that knocking out COX10 significantly impaired scotopic a- and b-wave responses. Inhibiting mitochondrial respiration in MG also altered the retinal glycolytic profile. However, blocking OXPHOS in MG did not significantly exacerbate retinal ganglion cell (RGC) loss or photopic negative response after ocular hypertension (OHT). These results suggest that MG were able to compensate for reduced COXIV stability by maintaining fundamental processes, but changes in retinal physiology and metabolism-associated proteins indicate subtle changes in MG function.

Tert-butylhydroquinone protects the retina from oxidative stress in STZ-induced diabetic rats via the PI3K/Akt/eNOS pathway

This study aims to investigate whether tert-butylhydroquinone protects the retina from oxidative stress in STZ-induced experimental diabetic rats through the activation of phosphinositide 3-kinase (PI3K)/Akt/endothelial nitric oxide synthase (eNOS) pathway.In vitro, NO, reactive oxygen species(ROS), eNOS, p-eNOS Ser1179, Akt, p-Akt Ser473 and L-NAME protein expression was analyzed within rMC-1 cells cultivated within normal control(NC), high glucose (HG) and HG-containing tert-butyl hydroquinone (tBHQ) (5 μM) medium. We confirmed tBHQ's protection through administering inhibitors of PI3K and Akt. In vivo, tBHQ was administered at a ratio of 1% (w/w) to diabetic rats was induced through an STZ injection (65 mg/kg) for a 3-month period, and the retinal expression of eNOS, p-eNOS Ser1179, Akt, and p-Akt Ser473 proteins was measured using Western blotting (WB) assay. We also utilized the TUNEL kit for detecting retinal cell apoptosis. The changes of retinal morphology and visual function were measured by performing hematoxylin-eosin staining (HE staining) and electroretinograms. In vitro, ROS levels were increased in the high glucose group, NO levels were decreased, and the relative expression of Akt/p-Akt Ser473 and eNOs/p-eNOS Ser1179 was reduced. tBHQ abolished these changes, and these effects were suppressed by specific inhibitors. In vivo, tBHQ upregulated retinal protein expression in STZ-induced diabetic rats, reduced retinal apoptotic cell numbers, and partially prevented abnormalities in retinal function and structure caused by diabetes. tBHQ alleviates oxidative stress during diabetic retinopathy by upregulating the PI3K/Akt/eNOS pathway and partially restoring the structure and function of the retina. It may play a role in delaying vision loss caused by diabetic retinopathy.

Beyond Genetics: The Role of Metabolism in Photoreceptor Survival, Development and Repair

Vision commences in the retina with rod and cone photoreceptors that detect and convert light to electrical signals. The irreversible loss of photoreceptors due to neurodegenerative disease leads to visual impairment and blindness. Interventions now in development include transplanting photoreceptors, committed photoreceptor precursors, or retinal pigment epithelial (RPE) cells, with the latter protecting photoreceptors from dying. However, introducing exogenous human cells in a clinical setting faces both regulatory and supply chain hurdles. Recent work has shown that abnormalities in central cell metabolism pathways are an underlying feature of most neurodegenerative disorders, including those in the retina. Reversal of key metabolic alterations to drive retinal repair thus represents a novel strategy to treat vision loss based on cell regeneration. Here, we review the connection between photoreceptor degeneration and alterations in cell metabolism, along with new insights into how metabolic reprogramming drives both retinal development and repair following damage. The potential impact of metabolic reprogramming on retinal regeneration is also discussed, specifically in the context of how metabolic switches drive both retinal development and the activation of retinal glial cells known as Müller glia. Müller glia display latent regenerative properties in teleost fish, however, their capacity to regenerate new photoreceptors has been lost in mammals. Thus, re-activating the regenerative properties of Müller glia in mammals represents an exciting new area that integrates research into developmental cues, central metabolism, disease mechanisms, and glial cell biology. In addition, we discuss this work in relation to the latest insights gleaned from other tissues (brain, muscle) and regenerative species (zebrafish).

Restoring the oxidative balance in age-related diseases - An approach in glaucoma

As human life expectancy increases, age-related health issues including neurodegenerative diseases continue to rise. Regardless of genetic or environmental factors, many neurodegenerative conditions share common pathological mechanisms, such as oxidative stress, a hallmark of many age-related health burdens. In this review, we describe oxidative damage and mitochondrial dysfunction in glaucoma, an age-related neurodegenerative eye disease affecting 80 million people worldwide. We consider therapeutic approaches used to counteract oxidative stress in glaucoma, including untapped treatment options such as novel plant-derived antioxidant compounds that can reduce oxidative stress and prevent neuronal loss. We summarize the current pre-clinical models and clinical work exploring the therapeutic potential of a range of candidate plant-derived antioxidant compounds. Finally, we explore advances in drug delivery systems, particular those employing nanotechnology-based carriers which hold significant promise as a carrier for antioxidants to treat age-related disease, thus reviewing the key current state of all of the aspects required towards translation.

670nm photobiomodulation modulates bioenergetics and oxidative stress, in rat Müller cells challenged with high glucose

Diabetic retinopathy (DR), the most common complication of diabetes mellitus, is associated with oxidative stress, nuclear factor-κB (NFκB) activation, and excess production of vascular endothelial growth factor (VEGF) and intracellular adhesion molecule-1 (ICAM-1). Muller glial cells, spanning the entirety of the retina, are involved in DR inflammation. Mitigation of DR pathology currently occurs via invasive, frequently ineffective therapies which can cause adverse effects. The application of far-red to near-infrared (NIR) light (630-1000nm) reduces oxidative stress and inflammation in vitro and in vivo. Thus, we hypothesize that 670nm light treatment will diminish oxidative stress preventing downstream inflammatory mechanisms associated with DR initiated by Muller cells. In this study, we used an in vitro model system of rat Müller glial cells grown under normal (5 mM) or high (25 mM) glucose conditions and treated with a 670 nm light emitting diode array (LED) (4.5 J/cm2) or no light (sham) daily. We report that a single 670 nm light treatment diminished reactive oxygen species (ROS) production and preserved mitochondrial integrity in this in vitro model of early DR. Furthermore, treatment for 3 days in culture reduced NFκB activity to levels observed in normal glucose and prevented the subsequent increase in ICAM-1. The ability of 670nm light treatment to prevent early molecular changes in this in vitro high glucose model system suggests light treatment could mitigate early deleterious effects modulating inflammatory signaling and diminishing oxidative stress.

Reintroduction of DJ-1 in Müller Cells Inhibits Retinal Degeneration in the DJ-1 Deficient Retina

The eye is continuously under oxidative stress due to high metabolic activity and reactive oxygen species generated by daily light exposure. The redox-sensitive protein DJ-1 has proven to be essential in order to protect retina and retinal pigment epithelium (RPE) from oxidative-stress-induced degeneration. Here, we analyzed the specific role of Müller cell DJ-1 in the adult zebrafish retina by re-establishing Müller-cell-specific DJ-1 expression in a DJ-1 knockout retina. Loss of DJ-1 resulted in an age-dependent retinal degeneration, including loss of cells in the ganglion cell layer, retinal thinning, photoreceptor disorganization and RPE cell dysfunction. The degenerative phenotype induced by the absence of DJ-1 was inhibited by solely expressing DJ-1 in Müller cells. The protective effect was dependent upon the cysteine-106 residue of DJ-1, which has been shown to be an oxidative sensor of DJ-1. In a label-free proteomics analysis of isolated retinas, we identified proteins differentially expressed after DJ-1 knockout, but with restored levels after Müller cell DJ-1 re-insertion. Our data show that Müller cell DJ-1 has a major role in protecting the retina from age-dependent oxidative stress.

Oxidative Stress in Optic Neuropathies

Increasing evidence indicates that changes in the redox system may contribute to the pathogenesis of multiple optic neuropathies. Optic neuropathies are characterized by the neurodegeneration of the inner-most retinal neurons, the retinal ganglion cells (RGCs), and their axons, which form the optic nerve. Often, optic neuropathies are asymptomatic until advanced stages, when visual impairment or blindness is unavoidable despite existing treatments. In this review, we describe systemic and, whenever possible, ocular redox dysregulations observed in patients with glaucoma, ischemic optic neuropathy, optic neuritis, hereditary optic neuropathies (i.e., Leber's hereditary optic neuropathy and autosomal dominant optic atrophy), nutritional and toxic optic neuropathies, and optic disc drusen. We discuss aspects related to anti/oxidative stress biomarkers that need further investigation and features related to study design that should be optimized to generate more valuable and comparable results. Understanding the role of oxidative stress in optic neuropathies can serve to develop therapeutic strategies directed at the redox system to arrest the neurodegenerative processes in the retina and RGCs and ultimately prevent vision loss.

Role of glycolysis in retinal vascular endothelium, glia, pigment epithelium, and photoreceptor cells and as therapeutic targets for related retinal diseases

Glycolysis produces large amounts of adenosine triphosphate (ATP) in a short time. The retinal vascular endothelium feeds itself primarily through aerobic glycolysis with less ATP. But when it generates new vessels, aerobic glycolysis provides rapid and abundant ATP support for angiogenesis, and thus inhibition of glycolysis in endothelial cells can be a target for the treatment of neovascularization. Aerobic glycolysis has a protective effect on Müller cells, and it can provide with a target for visual protection and maintenance of the blood-retinal barrier. Under physiological conditions, the mitochondria of RPE can use lactic acid produced by photoreceptor cells as an energy source to provide ATP for survival. In pathological conditions, because RPE cells avoid their oxidative damage by increasing glycolysis, a large number of glycolysis products accumulate, which in turn has a toxic effect on photoreceptor cells. This shows that stabilizing the function of RPE mitochondria may become a target for the treatment of diseases such as retinal degeneration. The decrease of aerobic glycolysis leads to the decline of photoreceptor cell function and impaired vision; therefore, aerobic glycolysis of stable photoreceptor cells provides a reliable target for delaying vision loss. It is of great significance to study the role of glycolysis in various retinal cells for the targeted treatment of ocular fundus diseases.

High glucose treatment promotes extracellular matrix proteome remodeling in Mller glial cells

Background

The underlying pathomechanisms in diabetic retinopathy (DR) remain incompletely understood. The aim of this study was to add to the current knowledge about the particular role of retinal Mller glial cells (RMG) in the initial processes of DR.

Methods

Applying a quantitative proteomic workflow, we investigated changes of primary porcine RMG under short term high glucose treatment as well as glycolysis inhibition treatment.

Results

We revealed significant changes in RMG proteome primarily in proteins building the extracellular matrix (ECM) indicating fundamental remodeling processes of ECM as novel rapid response to high glucose treatment. Among others, Osteopontin (SPP1) as well as its interacting integrins were significantly downregulated and organotypic retinal explant culture confirmed the selective loss of SPP1 in RMG upon treatment. Since SPP1 in the retina has been described neuroprotective for photoreceptors and functions against experimentally induced cell swelling, its rapid loss under diabetic conditions may point to a direct involvement of RMG to the early neurodegenerative processes driving DR. Data are available via ProteomeXchange with identifier PXD015879.

Incomplete response to Anti-VEGF therapy in neovascular AMD: Exploring disease mechanisms and therapeutic opportunities

Intravitreal anti-vascular endothelial growth factor (VEGF) drugs have revolutionized the treatment of neovascular age-related macular degeneration (NVAMD). However, many patients suffer from incomplete response to anti-VEGF therapy (IRT), which is defined as (1) persistent (plasma) fluid exudation; (2) unresolved or new hemorrhage; (3) progressive lesion fibrosis; and/or (4) suboptimal vision recovery. The first three of these collectively comprise the problem of persistent disease activity (PDA) in spite of anti-VEGF therapy. Meanwhile, the problem of suboptimal vision recovery (SVR) is defined as a failure to achieve excellent functional visual acuity of 20/40 or better in spite of sufficient anti-VEGF treatment. Thus, incomplete response to anti-VEGF therapy, and specifically PDA and SVR, represent significant clinical unmet needs. In this review, we will explore PDA and SVR in NVAMD, characterizing the clinical manifestations and exploring the pathobiology of each. We will demonstrate that PDA occurs most frequently in NVAMD patients who develop high-flow CNV lesions with arteriolarization, in contrast to patients with capillary CNV who are highly responsive to anti-VEGF therapy. We will review investigations of experimental CNV and demonstrate that both types of CNV can be modeled in mice. We will present and consider a provocative hypothesis: formation of arteriolar CNV occurs via a distinct pathobiology, termed neovascular remodeling (NVR), wherein blood-derived macrophages infiltrate the incipient CNV lesion, recruit bone marrow-derived mesenchymal precursor cells (MPCs) from the circulation, and activate MPCs to become vascular smooth muscle cells (VSMCs) and myofibroblasts, driving the development of high-flow CNV with arteriolarization and perivascular fibrosis. In considering SVR, we will discuss the concept that limited or poor vision in spite of anti-VEGF may not be caused simply by photoreceptor degeneration but instead may be associated with photoreceptor synaptic dysfunction in the neurosensory retina overlying CNV, triggered by infiltrating blood-derived macrophages and mediated by Müller cell activation Finally, for each of PDA and SVR, we will discuss current approaches to disease management and treatment and consider novel avenues for potential future therapies.

Neuroinflammatory Mechanisms of Mitochondrial Dysfunction and Neurodegeneration in Glaucoma

The exact mechanism of retinal ganglion cell loss in the pathogenesis of glaucoma is yet to be understood. Mitochondrial damage-associated molecular patterns (DAMPs) resulting from mitochondrial dysfunction have been linked to Leber's hereditary optic neuropathy and autosomal dominant optic atrophy, as well as to brain neurodegenerative diseases. Recent evidence shows that, in conditions where mitochondria are damaged, a sustained inflammatory response and downstream pathological inflammation may ensue. Mitochondrial damage has been linked to the accumulation of age-related mitochondrial DNA mutations and mitochondrial dysfunction, possibly through aberrant reactive oxygen species production and defective mitophagy. The present review focuses on how mitochondrial dysfunction may overwhelm the ability of neurons and glial cells to adequately maintain homeostasis and how mitochondria-derived DAMPs trigger the immune system and induce neurodegeneration.

Mingmu Xiaomeng Tablets Restore Autophagy and Alleviate Diabetic Retinopathy by Inhibiting PI3K/Akt/mTOR Signaling

Objective: To investigate the effect of Mingmu Xiaomeng tablets (MMXM) on the expression of phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR)-related proteins in a diabetic rat model.

Methods: Thirty-two male Sprague Dawley rats were randomly divided into four groups: normal control (NC), diabetic model (DM) control, MMXM, and calcium dobesilate (CD) Rats injected with streptozotocin (STZ) were used as an experimental diabetes model. After 14 weeks, autophagy and PI3K/Akt/mTOR signaling pathway proteins were detected by western blot. Glial fibrillary acidic protein (GFAP) expression in Müller cells was examined by immunohistochemistry. Retinal function was evaluated with electroretinography, and retinal ultrastructure was observed by transmission electron microscopy. Serum cytokine levels were detected with protein chip technology.

Results: MMXM restored autophagy by decreasing the protein expression of LC3-II and p62 and reducing the phosphorylation of PI3K, Akt, and mTOR, thus promoting autophagy. MMXM decreased GFAP expression in retinal Müller cells; restored electrophysiology indexes and retinal ultrastructures; and reduced serum levels of interleukin (IL)-1β, IL-4, IL-6, tumor necrosis factor-α, and vascular endothelial growth factor.

Conclusion: MMXM may protect the diabetic retina by inhibiting PI3K/Akt/mTOR signaling and enhancing autophagy.

Mitochondrial DNA A3243G variant-associated retinopathy: a meta-analysis of the clinical course of visual acuity and correlation with systemic manifestations

PURPOSE

The mitochondrial DNA A3243G (m.3243A>G) variant causes a wide spectrum of phenotypes, with pigmentary retinopathy as the most common ocular finding. We undertook this meta-analysis to investigate the clinical course of visual acuity (VA) in patients with m.3243A>G variant and provide key clinical correlations with systemic manifestations.

METHODS

A PubMed literature search was performed and studies were selected after satisfying pre-set inclusion criteria. Demographic and clinical data, including retinal findings and systemic manifestations were recorded. Cross-sectional and linear regression analyses were used to investigate the relationship between VA and age, as well as between the age at diagnosis of retinopathy and the mean ages at diagnosis of sensorineural hearing loss or diabetes. The age and prevalence of systemic manifestations among patients with and without retinopathy were studied using t-tests and Mann-Whitney U-tests (performed on binarized data). Likelihood ratios were computed.

RESULTS

The mean VA (average of both eyes) of 90 patients (72.2% female; 65/90) were collected from 18 studies published between 1990 and 2018. The baseline mean age was 45.2 years (range 17 to 92). The mean logMAR VA was 0.10 (- 0.12 to 1.39). There was a statistically significant linear correlation between the logMAR VA and age (p = .008). The VA of patients less than or equal to 50 years of age was significantly better than that of patients older than 50 years (0.06 vs.0.18 logMAR, p = .002). 67 patients (74.4%) showed a characteristic pigmentary retinopathy with a mean age at diagnosis of 47.9 years (17 to 92) and VA of 0.14 logMAR (- 0.12 to 1.24). Age at diagnosis of retinopathy was linearly correlated with age at diagnosis of hearing loss or diabetes (p < .001). Patients with retinopathy were more likely to have hearing loss (83.6% vs. 56.5%, p = .03) or diabetes (56.7% vs. 17.4%, p = .001) than those without retinopathy. Those with both hearing loss and diabetes had an earlier onset of retinopathy than those without (46.4 vs. 60.4 years, p = .01). Patients without both hearing loss and diabetes were 5.3-fold less likely to develop a retinopathy.

CONCLUSIONS

Patients with m.3243A>G variant pigmentary retinopathy maintain highly functional VA until around the fifth decade of life, after which significant visual decline ensues. Patients without hearing loss and diabetes have a lower likelihood of exhibiting a retinopathy, which tends to appear about one decade after hearing loss and diabetes are diagnosed.

Glial Cells in Glaucoma: Friends, Foes, and Potential Therapeutic Targets

Glaucoma is the second leading cause of blindness worldwide, affecting ~80 million people by 2020 (1, 2). The condition is characterized by a progressive loss of retinal ganglion cells (RGCs) and their axons accompanied by visual field loss. The underlying pathophysiology of glaucoma remains elusive. Glaucoma is recognized as a multifactorial disease, and lowering intraocular pressure (IOP) is the only treatment that has been shown to slow the progression of the condition. However, a significant number of glaucoma patients continue to go blind despite intraocular pressure-lowering treatment (2). Thus, the need for alternative treatment strategies is indisputable. Accumulating evidence suggests that glial cells play a significant role in supporting RGC function and that glial dysfunction may contribute to optic nerve disease. Here, we review recent advances in understanding the role of glial cells in the pathophysiology of glaucoma. A particular focus is on the dynamic and essential interactions between glial cells and RGCs and potential therapeutic approaches to glaucoma by targeting glial cells.

Mitochondrial DNA A3243G variant-associated retinopathy: Current perspectives and clinical implications

Cellular function and survival are critically dependent on the proper functionality of the mitochondrion. Neurodegenerative cellular processes including cellular adenosine triphosphate production, intermediary metabolism control, and apoptosis regulation are all mitochondrially mediated. The A to G transition at position 3243 in the mitochondrial MTTL1 gene that encodes for the leucine transfer RNA (m.3243A>G) causes a variety of diseases, including maternally inherited loss of hearing and diabetes syndrome (MIDD), mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS). Ophthalmological findings-including posterior sub-capsular cataract, ptosis, external ophthalmoplegia, and pigmentary retinopathy- have all been associated with the m.3243A>G variant. Pigmentary retinopathy is, however, the most common ocular finding, occurring in 38% to 86% of cases. To date, little is known about the pathogenesis, natural history, and heteroplasmic and phenotypic correlations of m.3243A>G-associated pigmentary retinopathy. We summarize the current understanding of mitochondrial genetics and pathogenesis of some associated diseases. We then review the pathophysiology, histology, clinical features, treatment, and important ocular and systemic phenotypic manifestations of m.3243A>G variant associated retinopathy. Mitochondrial diseases require a multidisciplinary team approach to ensure effective treatment, regular follow-up, and accurate genetic counseling.

Mitochondria: The Retina's Achilles' Heel in AMD

Strong experimental evidence from studies in human donor retinas and animal models supports the idea that the retinal pathology associated with age-related macular degeneration (AMD) involves mitochondrial dysfunction and consequent altered retinal metabolism. This chapter provides a brief overview of mitochondrial structure and function, summarizes evidence for mitochondrial defects in AMD, and highlights the potential ramifications of these defects on retinal health and function. Discussion of mitochondrial haplogroups and their association with AMD brings to light how mitochondrial genetics can influence disease outcome. As one of the most metabolically active tissues in the human body, there is strong evidence that disruption in key metabolic pathways contributes to AMD pathology. The section on retinal metabolism reviews cell-specific metabolic differences and how the metabolic interdependence of each retinal cell type creates a unique ecosystem that is disrupted in the diseased retina. The final discussion includes strategies for therapeutic interventions that target key mitochondrial pathways as a treatment for AMD.

Inflammation in Glaucoma: From the back to the front of the eye, and beyond

The pathophysiology of glaucoma is complex, multifactorial and not completely understood. Elevated intraocular pressure (IOP) and/or impaired retinal blood flow may cause initial optic nerve damage. In addition, age-related oxidative stress in the retina concurrently with chronic mechanical and vascular stress is crucial for the initiation of retinal neurodegeneration. Oxidative stress is closely related to cell senescence, mitochondrial dysfunction, excitotoxicity, and neuroinflammation, which are involved in glaucoma progression. Accumulating evidence from animal glaucoma models and from human ocular samples suggests a dysfunction of the para-inflammation in the retinal ganglion cell layer and the optic nerve head. Moreover, quite similar mechanisms in the anterior chamber could explain the trabecular meshwork dysfunction and the elevated IOP in primary open-angle glaucoma. On the other hand, ocular surface disease due to topical interventions is the most prominent and visible consequence of inflammation in glaucoma, with a negative impact on filtering surgery failure, topical treatment efficacy, and possibly on inflammation in the anterior segment. Consequently, glaucoma appears as an outstanding eye disease where inflammatory changes may be present to various extents and consequences along the eye structure, from the ocular surface to the posterior segment, and the visual pathway. Here we reviewed the inflammatory processes in all ocular structures in glaucoma from the back to the front of the eye and beyond. Our approach was to explain how para-inflammation is necessary to maintain homoeostasis, and to describe abnormal inflammatory findings observed in glaucomatous patients or in animal glaucoma models, supporting the hypothesis of a dysregulation of the inflammatory balance toward a pro-inflammatory phenotype. Possible anti-inflammatory therapeutic approaches in glaucoma are also discussed.

Single-cell transcriptome analysis of the Akimba mouse retina reveals cell-type-specific insights into the pathobiology of diabetic retinopathy

AIMS/HYPOTHESIS

Diabetic retinopathy is a common complication of diabetes and a leading cause of visual impairment and blindness. Despite recent advances, our understanding of its pathophysiology remains incomplete. The aim of this study was to provide deeper insight into the complex network of molecular and cellular changes that underlie diabetic retinopathy by systematically mapping the transcriptional changes that occur in the different cellular compartments of the degenerating diabetic mouse retina.

METHODS

Single-cell RNA sequencing was performed on retinal tissue from 12-week-old wild-type and Akimba (Ins2^Akita×Vegfa^+/-) mice, which are known to replicate features of clinical diabetic retinopathy. This resulted in transcriptome data for 9474 retinal cells, which could be annotated to eight distinct retinal cell types. Using STRING analysis, we studied differentially expressed gene networks in neuronal, glial and immune cell compartments to create a comprehensive view on the pathological changes that occur in the Akimba retina. Using subclustering analysis, we further characterised macroglial and inflammatory cell subpopulations. Prominent findings were confirmed at the protein level using immunohistochemistry, western blotting and ELISA.

RESULTS

At 12 weeks, the Akimba retina was found to display degeneration of rod photoreceptors and presence of inflammatory cells, identified by subclustering analysis as monocyte, macrophage and microglial populations. Analysis of differentially expressed genes in the rod, cone, bipolar cell and macroglial compartments indicated changes in cell metabolism and ribosomal gene expression, gliosis, activation of immune system pathways and redox and metal ion dyshomeostasis. Experiments at the protein level supported a metabolic shift from glycolysis to oxidative phosphorylation (glyceraldehyde 3-phosphate dehydrogenase), activation of microglia/macrophages (isolectin-B4), metal ion and oxidative stress response (metallothionein and haem oxygenase-1) and reactive macroglia (glial fibrillary acidic protein and S100) in the Akimba retina, compared with wild-type mice. Our single-cell approach also indicates macroglial subpopulations with distinct fibrotic, inflammatory and gliotic profiles.

CONCLUSIONS/INTERPRETATION

Our study identifies molecular pathways underlying inflammatory, metabolic and oxidative stress-mediated changes in the Akimba mouse model of diabetic retinopathy and distinguishes distinct functional subtypes of inflammatory and macroglial cells.

DATA AVAILABILITY

RNA-seq data have been deposited in the ArrayExpress database at EMBL-EBI ( www.ebi.ac.uk/arrayexpress ) under accession number E-MTAB-9061. Graphical abstract.

Diabetic Retinopathy: The Role of Mitochondria in the Neural Retina and Microvascular Disease

Diabetic retinopathy (DR), a common chronic complication of diabetes mellitus and the leading cause of vision loss in the working-age population, is clinically defined as a microvascular disease that involves damage of the retinal capillaries with secondary visual impairment. While its clinical diagnosis is based on vascular pathology, DR is associated with early abnormalities in the electroretinogram, indicating alterations of the neural retina and impaired visual signaling. The pathogenesis of DR is complex and likely involves the simultaneous dysregulation of multiple metabolic and signaling pathways through the retinal neurovascular unit. There is evidence that microvascular disease in DR is caused in part by altered energetic metabolism in the neural retina and specifically from signals originating in the photoreceptors. In this review, we discuss the main pathogenic mechanisms that link alterations in neural retina bioenergetics with vascular regression in DR. We focus specifically on the recent developments related to alterations in mitochondrial metabolism including energetic substrate selection, mitochondrial function, oxidation-reduction (redox) imbalance, and oxidative stress, and critically discuss the mechanisms of these changes and their consequences on retinal function. We also acknowledge implications for emerging therapeutic approaches and future research directions to find novel mitochondria-targeted therapeutic strategies to correct bioenergetics in diabetes. We conclude that retinal bioenergetics is affected in the early stages of diabetes with consequences beyond changes in ATP content, and that maintaining mitochondrial integrity may alleviate retinal disease.

Mitochondrial Markers in Aging and Primary Open-Angle Glaucoma

This review focuses on recent progress in understanding the role of mitochondrial markers in the context of mitochondrial dysfunction in glaucoma and discussing new therapeutic approaches to modulate mitochondrial function and potentially lead to improved outcomes in glaucoma.

Lactate: More Than Merely a Metabolic Waste Product in the Inner Retina

The retina is an extension of the central nervous system and has been considered to be a simplified, more tractable and accessible version of the brain for a variety of neuroscience investigations. The optic nerve displays changes in response to underlying neurodegenerative diseases, such as stroke, multiple sclerosis, and Alzheimer's disease, as well as inner retinal neurodegenerative disease, e.g., glaucoma. Neurodegeneration has increasingly been linked to dysfunctional energy metabolism or conditions in which the energy supply does not meet the demand. Likewise, increasing lactate levels have been correlated with conditions consisting of unbalanced energy supply and demand, such as ischemia-associated diseases or excessive exercise. Lactate has thus been acknowledged as a metabolic waste product in organs with high energy metabolism. However, in the past decade, numerous beneficial roles of lactate have been revealed in the central nervous system. In this context, lactate has been identified as a valuable energy substrate, protecting against glutamate excitotoxicity and ischemia, as well as having signaling properties which regulate cellular functions. The present review aims to summarize and discuss protective roles of lactate in various model systems (in vitro, ex vivo, and in vivo) reflecting the inner retina focusing on lactate metabolism and signaling in inner retinal homeostasis and disease.

Potential metabolic markers in glaucoma and their regulation in response to hypoxia

PURPOSE

To assess novel differences in serum levels of glucose, lactate and amino acids in patients with normal-tension glaucoma (NTG) compared to age-matched controls, at baseline and in response to universal hypoxia.

METHODS

Twelve patients diagnosed with NTG and eleven control subjects underwent normobaric hypoxia for 2 hr. Peripheral venous blood samples were taken at baseline, during hypoxia and in the recovery phase. Serum glucose and lactate levels were measured by a blood gas analyser. Amino acids were analysed by high-performance liquid chromatography.

RESULTS

Baseline levels of lactate and total amino acids were significantly lower in patients with NTG compared to healthy controls. No differences were seen in blood glucose levels between the two groups. Lactate levels remained unchanged during hypoxia in the control group, but increased in patients with NTG. In the recovery phase, total amino acid levels were reduced in the control group, whereas no changes were found in patients with NTG.

CONCLUSION

Reduced serum levels of lactate and total amino acids were identified as potential markers for NTG. Moreover, significant differential regulatory patterns of certain amino acids were found in patients with NTG compared to control subjects. Overall, our results suggest a link between systemic energy metabolites and NTG and support a novel understanding of glaucoma as an inner retinal manifestation of a systemic condition.

Retinal ischemia induces α-SMA-mediated capillary pericyte contraction coincident with perivascular glycogen depletion

Increasing evidence indicates that pericytes are vulnerable cells, playing pathophysiological roles in various neurodegenerative processes. Microvascular pericytes contract during cerebral and coronary ischemia and do not relax after re-opening of the occluded artery, causing incomplete reperfusion. However, the cellular mechanisms underlying ischemia-induced pericyte contraction, its delayed emergence, and whether it is pharmacologically reversible are unclear. Here, we investigate i) whether ischemia-induced pericyte contractions are mediated by alpha-smooth muscle actin (α-SMA), ii) the sources of calcium rise in ischemic pericytes, and iii) if peri-microvascular glycogen can support pericyte metabolism during ischemia. Thus, we examined pericyte contractility in response to retinal ischemia both in vivo, using adaptive optics scanning light ophthalmoscopy and, ex vivo, using an unbiased stereological approach. We found that microvascular constrictions were associated with increased calcium in pericytes as detected by a genetically encoded calcium indicator (NG2-GCaMP6) or a fluoroprobe (Fluo-4). Knocking down α-SMA expression with RNA interference or fixing F-actin with phalloidin or calcium antagonist amlodipine prevented constrictions, suggesting that constrictions resulted from calcium- and α-SMA-mediated pericyte contractions. Carbenoxolone or a Cx43-selective peptide blocker also reduced calcium rise, consistent with involvement of gap junction-mediated mechanisms in addition to voltage-gated calcium channels. Pericyte calcium increase and capillary constrictions became significant after 1 h of ischemia and were coincident with depletion of peri-microvascular glycogen, suggesting that glucose derived from glycogen granules could support pericyte metabolism and delay ischemia-induced microvascular dysfunction. Indeed, capillary constrictions emerged earlier when glycogen breakdown was pharmacologically inhibited. Constrictions persisted despite recanalization but were reversible with pericyte-relaxant adenosine administered during recanalization. Our study demonstrates that retinal ischemia, a common cause of blindness, induces α-SMA- and calcium-mediated persistent pericyte contraction, which can be delayed by glucose driven from peri-microvascular glycogen. These findings clarify the contractile nature of capillary pericytes and identify a novel metabolic collaboration between peri-microvascular end-feet and pericytes.

Dual Properties of Lactate in Müller Cells: The Effect of GPR81 Activation

Purpose

Besides being actively metabolized, lactate may also function as a signaling molecule by activation of the G-protein-coupled receptor 81 (GPR81). Thus, we aimed to characterize the metabolic effects of GPR81 activation in Müller cells.

Method

Primary Müller cells from mice were treated with and without 10 mM L-lactate in the presence or absence of 6 mM glucose. The effects of lactate receptor GPR81 activation were evaluated by the addition of 5 mM 3,5-DHBA (3,5-dihydroxybenzoic acid), a GPR81 agonist. Western blot analyses were used to determine protein expression of GPR81. Cell survival was assessed through 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) viability assays. Lactate release was quantified by commercially available lactate kits. 13C-labeling studies via mass spectroscopy and Seahorse analyses were performed to evaluate metabolism of lactate and glucose, and mitochondrial function. Finally, Müller cell function was evaluated by measuring glutamate uptake.

Results

The lactate receptor, GPR81, was upregulated during glucose deprivation. Treatment with a GPR81 agonist did not affect Müller cell survival. However, GPR81 activation diminished lactate release allowing lactate to be metabolized intracellularly. Furthermore, GPR81 activation increased metabolism of glucose and mitochondrial function. Finally, maximal glutamate uptake decreased in response to GPR81 activation during glucose deprivation.

Conclusions

The present study revealed dual properties of lactate via functioning as an active metabolic energy substrate and a regulatory molecule by activation of the GPR81 receptor in primary Müller cells. Thus, combinational therapy of lactate and GPR81 agonists may be of future interest in maintaining Müller cell survival, ultimately leading to increased resistance toward retinal neurodegeneration.

Probucol promotes high glucose-induced proliferation and inhibits apoptosis by reducing reactive oxygen species generation in Müller cells

PURPOSE

To explore the protective effect of probucol on human retinal Müller cells cultured in high glucose.

METHODS

Primary Müller cells from human retinas were cultured in complete DMEM. Third-generation Müller cells were identified using glutamine synthetase (GS) antibody and randomly divided into three groups: normoglycemia (NG, 5.5 mmol/L); hyperglycemia (HG, 30 mmol/L); and hyperglycemia (30 mmol/L) with probucol (10 μmol/L; HGPB). After a 24-h intervention, cell proliferation, apoptosis, and cellular reactive oxygen species (ROS) were measured with a CCK-8 kit, flow cytometry, and DCFH-DA probe, respectively. Kelch-like ECH-associated protein 1 (Keap1), NF-E2-related factor 2 (Nrf2), and glutamate cysteine ligase catalytic subunit (GCLC) protein expression were detected by immunofluorescence staining.

RESULTS

For NG, HG, and HGPB, optical density (OD) values for cell proliferation were 0.98 ± 0.23, 0.58 ± 0.11, and 0.73 ± 0.11; apoptotic rates were 2.79 ± 0.52%, 7.70 ± 0.44%, and 4.00 ± 0.95%; and intracellular ROS were 20.89 ± 5.14, 55.17 ± 14.07, and 26.28 ± 4.73, respectively. Compared to NG, OD was markedly decreased (P < 0.01), apoptosis was increased (P < 0.001), and intracellular ROS level was significantly higher than in HG (P < 0.01). Compared to HG, OD was markedly increased (P < 0.01), apoptosis was meaningfully decreased (P < 0.01), and intracellular ROS level was significantly lower than in HGPB (P < 0.01). GS, Keap1, Nrf2, and GCLC had positive expression.

CONCLUSIONS

Probucol could inhibit intracellular ROS generation, promote proliferation, and decrease apoptosis of human retinal Müller cells cultured in high glucose. This might also be associated with Keap1/Nrf2/ARE oxidative stress signaling pathway activation.

Hyperhomocysteinemia-induced death of retinal ganglion cells: The role of Müller glial cells and NRF2

Hyperhomocysteinemia (Hhcy), or increased levels of the excitatory amino acid homocysteine (Hcy), is implicated in glaucoma, a disease characterized by increased oxidative stress and loss of retinal ganglion cells (RGCs). Whether Hhcy is causative or merely a biomarker for RGC loss in glaucoma is unknown. Here we analyzed the role of NRF2, a master regulator of the antioxidant response, in Hhcy-induced RGC death in vivo and in vitro. By crossing Nrf2^−/− mice and two mouse models of chronic Hhcy (Cbs^+/- and Mthfr^+/- mice), we generated Cbs^+/-Nrf2^−/− and Mthfr^+/-Nrf2^−/− mice and performed systematic analysis of retinal architecture and visual acuity followed by assessment of retinal morphometry and gliosis. We observed significant reduction of inner retinal layer thickness and reduced visual acuity in Hhcy mice lacking NRF2. These functional deficits were accompanied by fewer RGCs and increased gliosis. Given the key role of Müller glial cells in maintaining RGCs, we established an ex-vivo indirect co-culture system using primary RGCs and Müller cells. Hhcy-exposure decreased RGC viability, which was abrogated when cells were indirectly cultured with wildtype (WT) Müller cells, but not with Nrf2^−/− Müller cells. Exposure of WT Müller cells to Hhcy yielded a robust mitochondrial and glycolytic response, which was not observed in Nrf2^−/− Müller cells. Taken together, the in vivo and in vitro data suggest that deleterious effects of Hhcy on RGCs are likely dependent upon the health of retinal glial cells and the availability of an intact retinal antioxidant response mechanism.

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Oxidative stress is linked to homocysteine (Hcy)-induced retinal ganglion cell death.

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NRF2's role in protecting ganglion cells from excess Hcy was studied in vitro/vivo.

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Hyper-Hcy mice were crossed with Nrf2^−/− mice to study retinal function/structure.

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Ganglion cells co-cultured with primary WT Müller glial cells survived Hcy treatment.

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Nrf2^−/− Müller cells did not afford neuroprotective advantage to Hcy-treated cells.

Lactate-Mediated Protection of Retinal Ganglion Cells

Loss of retinal ganglion cells (RGCs) is a leading cause of blinding conditions. The purpose of this study was to evaluate the effect of extracellular l-lactate on RGC survival facilitated through lactate metabolism and ATP production. We identified lactate as a preferred energy substrate over glucose in murine RGCs and showed that lactate metabolism and consequently increased ATP production are crucial components in promoting RGC survival during energetic crisis. Lactate was released to the extracellular environment in the presence of glucose and detained intracellularly during glucose deprivation. Lactate uptake and metabolism was unaltered in the presence and absence of glucose. However, the ATP production declined significantly for 24 h of glucose deprivation and increased significantly in the presence of lactate. Finally, lactate exposure for 2 and 24 h resulted in increased RGC survival during glucose deprivation. In conclusion, the metabolic pathway of lactate in RGCs may be of great future interest to unravel potential pharmaceutical targets, ultimately leading to novel therapies in the prevention of blinding neurodegenerative diseases, for example, glaucoma.

Aloin Inhibits Müller Cells Swelling in a Rat Model of Thioacetamide-Induced Hepatic Retinopathy

Swelling of retinal Müller cells is implicated in retinal edema and neuronal degeneration. Müller cell swelling is observed in patients with liver failure and is referred to as hepatic retinopathy. In the present study, we evaluated the effects of aloin, an anthraquinone-C-glycoside present in various Aloe species, on Müller cell dysfunction in a rat model of thioacetamide (TAA)-induced hepatic retinopathy. Experimental hepatic retinopathy was induced by three injections of TAA (200 mg/kg/day, intraperitoneal injection) for 3 days in rats. After the last injection of TAA, aloin (50 and 100 mg/kg) was orally gavaged for 5 days. The effects of aloin on the liver injury, serum ammonia levels, Müller cell swelling, glial fibrillary acidic protein (GFAP) expression, and gene expression of Kir4.1 and aquaporin-4 were examined. TAA-injected rats exhibited liver failure and hyperammonemia. In the TAA-injected rats, Müller cell bodies were highly enlarged, and GFAP, an indicator of retinal stress, was highly expressed in the retinas, indicating a predominant Müller cell gliosis. However, administration of aloin suppressed liver injury as well as Müller cell swelling through the normalization of Kir4.1 and aquaporin-4 channels, which play a key role in potassium and water transport in Müller cells. These results indicate that aloin may be helpful to protect retinal injury associated with liver failure.

Essential Roles of Lactate in Müller Cell Survival and Function

Müller cells are pivotal in sustaining retinal ganglion cells, and an intact energy metabolism is essential for upholding Müller cell functions. The present study aimed to investigate the impact of lactate on Müller cell survival and function. Primary mice Müller cells and human Müller cell lines (MIO-M1) were treated with or without lactate (10 or 20 mM) for 2 and 24 hours. Simultaneously, Müller cells were incubated with or without 6 mM of glucose. L-lactate exposure increased Müller cell survival independently of the presence of glucose. This effect was abolished by the addition of the monocarboxylate inhibitor 4-cinnamic acid to the treatment media, whereas survival continued to increase in response to addition of D-lactate during glucose restriction. ATP levels decreased over time in MIO-M1 cells and remained stable over time in primary Müller cells. Lactate was preferably metabolized in MIO-M1 cells compared to glucose, and 10 mM of L-Lactate exposure prevented complete glycogen depletion in MIO-M1 cells. Glutamate uptake increased after 2 hours and decreased after 24 hours in glucose-restricted Müller cells compared to cells with glucose supplement. The addition of 10 mM of lactate to the treatment media increased glutamate uptake in glucose supplemented and restricted cells. In conclusion, lactate is a key component in maintaining Müller cell survival and function. Hence, lactate administration may be of great future interest, ultimately leading to novel therapies to rescue retinal ganglion cells.

The unfolded protein response signaling and retinal Müller cell metabolism

The retina is one of the most energy demanding tissues in the body. Like most neurons in the central nervous system, retinal neurons consume high amounts of adenosine-5'-triphosphate (ATP) to generate visual signal and transmit the information to the brain. Disruptions in retinal metabolism can cause neuronal dysfunction and degeneration resulting in severe visual impairment and even blindness. The homeostasis of retinal metabolism is tightly controlled by multiple signaling pathways, such as the unfolded protein response (UPR), and the close interactions between retinal neurons and other retinal cell types including vascular cells and Müller glia. The UPR is a highly conserved adaptive cellular response and can be triggered by many physiological stressors and pathophysiological conditions. Activation of the UPR leads to changes in glycolytic rate, ATP production, de novo serine synthesis, and the hexosamine biosynthetic pathway, which are considered critical components of Müller glia metabolism and provide metabolic support to surrounding neurons. When these pathways are disrupted, neurodegeneration occurs rapidly. In this review, we summarize recent advance in studies of the UPR in Müller glia and highlight the potential role of the UPR in retinal degeneration through regulation of Müller glia metabolism.

A Perspective on the Müller Cell-Neuron Metabolic Partnership in the Inner Retina

The Müller cells represent the predominant macroglial cell in the retina. In recent decades, Müller cells have been acknowledged to be far more influential on neuronal homeostasis in the retina than previously assumed. With its unique localization, spanning the entire retina being interposed between the vessels and neurons, Müller cells are responsible for the functional and metabolic support of the surrounding neurons. As a consequence of major energy demands in the retina, high levels of glucose are consumed and processed by Müller cells. The present review provides a perspective on the symbiotic relationship between Müller cells and inner retinal neurons on a cellular level by emphasizing the essential role of energy metabolism within Müller cells in relation to retinal neuron survival.

Mitochondrial dysfunction underlying outer retinal diseases

Dysfunction of photoreceptors, retinal pigment epithelium (RPE) or both contribute to the initiation and progression of several outer retinal disorders. Disrupted Müller glia function might additionally subsidize to these diseases. Mitochondrial malfunctioning is importantly associated with outer retina pathologies, which can be classified as primary and secondary mitochondrial disorders. This review highlights the importance of oxidative stress and mitochondrial DNA damage, underlying outer retinal disorders. Indeed, the metabolically active photoreceptors/RPE are highly prone to these hallmarks of mitochondrial dysfunction, indicating that mitochondria represent a weak link in the antioxidant defenses of outer retinal cells.