Glutamate transport by retinal Müller cells in glutamate/aspartate transporter-knockout mice

Müller cells are activated in response to retinal outer nuclear layer degeneration in rats subjected to simulated weightlessness conditions

JOURNAL/nrgr/04.03/01300535-202507000-00032/figure1/v/2024-09-09T124005Z/r/image-tiff A microgravity environment has been shown to cause ocular damage and affect visual acuity, but the underlying mechanisms remain unclear. Therefore, we established an animal model of weightlessness via tail suspension to examine the pathological changes and molecular mechanisms of retinal damage under microgravity. After 4 weeks of tail suspension, there were no notable alterations in retinal function and morphology, while after 8 weeks of tail suspension, significant reductions in retinal function were observed, and the outer nuclear layer was thinner, with abundant apoptotic cells. To investigate the mechanism underlying the degenerative changes that occurred in the outer nuclear layer of the retina, proteomics was used to analyze differentially expressed proteins in rat retinas after 8 weeks of tail suspension. The results showed that the expression levels of fibroblast growth factor 2 (also known as basic fibroblast growth factor) and glial fibrillary acidic protein, which are closely related to Müller cell activation, were significantly upregulated. In addition, Müller cell regeneration and Müller cell gliosis were observed after 4 and 8 weeks, respectively, of simulated weightlessness. These findings indicate that Müller cells play an important regulatory role in retinal outer nuclear layer degeneration during weightlessness.

Glaucoma Animal Models beyond Chronic IOP Increase

Glaucoma is a complex and multifactorial disease defined as the loss of retinal ganglion cells (RGCs) and their axons. Besides an elevated intraocular pressure (IOP), other mechanisms play a pivotal role in glaucoma onset and progression. For example, it is known that excitotoxicity, immunological alterations, ischemia, and oxidative stress contribute to the neurodegeneration in glaucoma disease. To study these effects and to discover novel therapeutic approaches, appropriate animal models are needed. In this review, we focus on various glaucoma animal models beyond an elevated IOP. We introduce genetically modified mice, e.g., the optineurin E50K knock-in or the glutamate aspartate transporter (GLAST)-deficient mouse. Excitotoxicity can be mimicked by injecting the glutamate analogue N-methyl-D-aspartate intravitreally, which leads to rapid RGC degeneration. To explore the contribution of the immune system, the experimental autoimmune glaucoma model can serve as a useful tool. Here, immunization with antigens led to glaucoma-like damage. The ischemic mechanism can be mimicked by inducing a high IOP for a certain amount of time in rodents, followed by reperfusion. Thereby, damage to the retina and the optic nerve occurs rapidly after ischemia/reperfusion. Lastly, we discuss the importance of optic nerve crush models as model systems for normal-tension glaucoma. In summary, various glaucoma models beyond IOP increase can be utilized.

Postnatal exposure to trimethyltin chloride induces retinal developmental neurotoxicity in mice via glutamate and its transporter related changes

Exposure to toxic substances during postnatal period is one of the major factors causing retinal developmental defects. The developmental toxicity of trimethyltin chloride (TMT), a byproduct of an organotin compound widely used in agriculture and industrial fields, has been reported; however, the effect on the mammalian retina during postnatal development and the mechanism have not been elucidated to date. We exposed 0.75 and 1.5 mg/kg of TMT to neonatal ICR mice (1:1 ratio of male and female) up to postnatal day 14 and performed analysis of the retina: histopathology, apoptosis, electrophysiological function, glutamate concentration, gene expression, and fluorescence immunostaining. Exposure to TMT caused delayed eye opening, eye growth defect and thinning of retinal layer. In addition, apoptosis occurred in the retina along with b-wave and spiking activity changes in the micro-electroretinogram. These changes were accompanied by an increase in the concentration of glutamate, upregulation of astrocyte-related genes, and increased expression of glial excitatory amino acid transporter (EAAT) 1 and 2. Conversely, EAAT 3, 4, and 5, mainly located in the neurons, were decreased. Our results are the first to prove postnatal retinal developmental neurotoxicity of TMT at the mammalian model and analyze the molecular, functional as well as morphological aspects to elucidate possible mechanisms: glutamate toxicity with EAAT expression changes. These mechanisms may suggest not only a strategy to treat but also a clue to prevent postnatal retina developmental toxicity of toxic substances.

Activation of retinal glial cells contributes to the degeneration of ganglion cells in experimental glaucoma

Elevation of intraocular pressure (IOP) is a major risk factor for neurodegeneration in glaucoma. Glial cells, which play an important role in normal functioning of retinal neurons, are well involved into retinal ganglion cell (RGC) degeneration in experimental glaucoma animal models generated by elevated IOP. In response to elevated IOP, mGluR I is first activated and Kir4.1 channels are subsequently inhibited, which leads to the activation of Müller cells. Müller cell activation is followed by a complex process, including proliferation, release of inflammatory and growth factors (gliosis). Gliosis is further regulated by several factors. Activated Müller cells contribute to RGC degeneration through generating glutamate receptor-mediated excitotoxicity, releasing cytotoxic factors and inducing microglia activation. Elevated IOP activates microglia, and following morphological and functional changes, these cells, as resident immune cells in the retina, show adaptive immune responses, including an enhanced release of pro-inflammatory factors (tumor neurosis factor-α, interleukins, etc.). These ATP and Toll-like receptor-mediated responses are further regulated by heat shock proteins, CD200R, chemokine receptors, and metabotropic purinergic receptors, may aggravate RGC loss. In the optic nerve head, astrogliosis is initiated and regulated by a complex reaction process, including purines, transmitters, chemokines, growth factors and cytokines, which contributes to RGC axon injury through releasing pro-inflammatory factors and changing extracellular matrix in glaucoma. The effects of activated glial cells on RGCs are further modified by the interplay among different types of glial cells. This review is concluded by presenting an in-depth discussion of possible research directions in this field in the future.

EphA4/ephrinA3 reverse signaling mediated downregulation of glutamate transporter GLAST in Müller cells in an experimental glaucoma model

Deficiency of glutamate transporter GLAST in Müller cells may be culpable for excessive extracellular glutamate, which involves in retinal ganglion cell (RGC) damage in glaucoma. We elucidated how GLAST was regulated in rat chronic ocular hypertension (COH) model. Western blot and whole-cell patch-clamp recordings showed that GLAST proteins and GLAST-mediated current densities in Müller cells were downregulated at the early stages of COH. In normal rats, intravitreal injection of the ephrinA3 activator EphA4-Fc mimicked the changes of GLAST in COH retinas. In purified cultured Müller cells, EphA4-Fc treatment reduced GLAST expression at mRNA and protein levels, which was reversed by the tyrosine kinase inhibitor PP2 or transfection with ephrinA3-siRNA (Si-EFNA3), suggesting that EphA4/ephrinA3 reverse signaling mediated GLAST downregulation. EphA4/ephrinA3 reverse signaling-induced GLAST downregulation was mediated by inhibiting PI3K/Akt/NF-κB pathways since EphA4-Fc treatment of cultured Müller cells reduced the levels of p-Akt/Akt and NF-κB p65, which were reversed by transfecting Si-EFNA3. In Müller cells with ephrinA3 knockdown, the PI3K inhibitor LY294002 still decreased the protein levels of NF-κB p65 in the presence of EphA4-Fc, and the mRNA levels of GLAST were reduced by LY294002 and the NF-κB inhibitor SN50, respectively. Pre-injection of the PI3K/Akt pathway activator 740 Y-P reversed the GLAST downregulation in COH retinas. Western blot and TUNEL staining showed that transfecting of Si-EFNA3 reduced Müller cell gliosis and RGC apoptosis in COH retinas. Our results suggest that activated EphA4/ephrinA3 reverse signaling induces GLAST downregulation in Müller cells via inhibiting PI3K/Akt/NF-κB pathways, thus contributing to RGC damage in glaucoma.

Synapse Dysfunctions in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic neuroinflammatory disease of the central nervous system (CNS) affecting nearly three million humans worldwide. In MS, cells of an auto-reactive immune system invade the brain and cause neuroinflammation. Neuroinflammation triggers a complex, multi-faceted harmful process not only in the white matter but also in the grey matter of the brain. In the grey matter, neuroinflammation causes synapse dysfunctions. Synapse dysfunctions in MS occur early and independent from white matter demyelination and are likely correlates of cognitive and mental symptoms in MS. Disturbed synapse/glia interactions and elevated neuroinflammatory signals play a central role. Glutamatergic excitotoxic synapse damage emerges as a major mechanism. We review synapse/glia communication under normal conditions and summarize how this communication becomes malfunctional during neuroinflammation in MS. We discuss mechanisms of how disturbed glia/synapse communication can lead to synapse dysfunctions, signaling dysbalance, and neurodegeneration in MS.

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.

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).

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

Cellular Physiology and Pathophysiology of EAAT Anion Channels

Excitatory amino acid transporters (EAATs) optimize the temporal resolution and energy demand of mammalian excitatory synapses by quickly removing glutamate from the synaptic cleft into surrounding neuronal and glial cells and ensuring low resting glutamate concentrations. In addition to secondary active glutamate transport, EAATs also function as anion channels. The channel function of these transporters is conserved in all homologs ranging from archaebacteria to mammals; however, its physiological roles are insufficiently understood. There are five human EAATs, which differ in their glutamate transport rates. Until recently the high-capacity transporters EAAT1, EAAT2, and EAAT3 were believed to conduct only negligible anion currents, with no obvious function in cell physiology. In contrast, the low-capacity glutamate transporters EAAT4 and EAAT5 are thought to regulate neuronal signaling as glutamate-gated channels. In recent years, new experimental approaches and novel animal models, together with the discovery of a human genetic disease caused by gain-of-function mutations in EAAT anion channels have enabled identification of the first physiological and pathophysiological roles of EAAT anion channels.

Excitatory Amino Acid Transporter EAAT5 Improves Temporal Resolution in the Retina

Excitatory amino acid transporters (EAATs) remove glutamate from the synaptic cleft. In the retina, EAAT1 and EAAT2 are considered the major glutamate transporters. However, it has not yet been possible to determine how EAAT5 shapes the retinal light responses because of the lack of a selective EAAT5 blocker or EAAT5 knock-out (KO) animal model. In this study, EAAT5 was found to be expressed in a punctate manner close to release sites of glutamatergic synapses in the mouse retina. Light responses from retinae of wild-type (WT) and of a newly generated model with a targeted deletion of EAAT5 (EAAT5^-/-) were recorded in vitro using multielectrode arrays (MEAs). Flicker resolution was considerably lower in EAAT5^-/- retinae than in WT retinae. The close proximity to the glutamate release site makes EAAT5 an ideal tool to improve temporal information processing in the retina by controlling information transfer at glutamatergic synapses.

EAAT5 Glutamate Transporter-Mediated Inhibition in the Vertebrate Retina

Glutamate transporters typically remove glutamate from the synaptic cleft. In addition, all glutamate transporters have a chloride channel, which is opened upon glutamate binding to the transporter. There are five types of glutamate transporter (EAATs 1–5, excitatory amino acid transporters), which have distinct chloride conductances. Some EAATs that have low chloride conductances, remove glutamate from the synaptic cleft most effectively (e.g., EAAT1). By contrast, EAATs that have high chloride conductances, remove glutamate less effectively (e.g., EAAT5). We have studied EAAT5 in the retina. In the retina, light activates a chloride current, mediated by the glutamate activation of EAAT5. EAAT5 is not a significant contributor to lateral inhibition in the retina. Instead, it is the main source of autoinhibition to rod bipolar cells (RBCs). EAAT5-mediated inhibition has a substantial effect on synaptic transmission from RBCs to downstream retinal neurons.

Targeted pharmacotherapy against neurodegeneration and neuroinflammation in early diabetic retinopathy

Diabetic retinopathy (DR), the most frequent complication of diabetes, is one of the leading causes of irreversible blindness in working-age adults and has traditionally been regarded as a microvascular disease. However, increasing evidence has revealed that synaptic neurodegeneration of retinal ganglion cells (RGCs) and activation of glial cells may represent some of the earliest events in the pathogenesis of DR. Upon diabetes-induced metabolic stress, abnormal glycogen synthase kinase-3β (GSK-3β) activation drives tau hyperphosphorylation and β-catenin downregulation, leading to mitochondrial impairment and synaptic neurodegeneration prior to RGC apoptosis. Moreover, glial cell activation triggers enhanced inflammation and oxidative stress, which may accelerate the deterioration of diabetic RGCs neurodegeneration. These findings have opened up opportunities for therapies, such as inhibition of GSK-3β, glial cell activation, glutamate excitotoxicity and the use of neuroprotective drugs targeting early neurodegenerative processes in the retina and halting the progression of DR before the manifestation of microvascular abnormalities. Such interventions could potentially remedy early neurodegeneration and help prevent vision loss in people suffering from DR.

Potential of Müller Glia for Retina Neuroprotection

Müller glia constitute the main glial cells of the retina. They are spatially distributed along this tissue, facilitating their close membrane interactions with all retinal neurons. Müller glia are characterized by their active metabolic functions, which are neuroprotective in nature. Although they can become reactive under pathological conditions, leading to their production of inflammatory and neurotoxic factors, their main metabolic functions confer neuroprotection to the retina, resulting in the promotion of neural cell repair and survival. In addition to their protective metabolic features, Müller glia release several neurotrophic factors and antioxidants into the retinal microenvironment, which are taken up by retinal neurons for their survival. This review summarizes the Müller glial neuroprotective mechanisms and describes advances made on the clinical application of these factors for the treatment of retinal degenerative diseases. It also discusses prospects for the use of these cells as a vehicle to deliver neuroprotective factors into the retina.

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.

Wide field retinal imaging and the detection of drug associated retinal toxicity

Background

To describe the peripheral retinal findings associated with systemic medication toxicity and to outline the importance of ultra-widefield imaging in the detection, analysis and monitoring of these abnormalities.

Main text

This review highlights the retinal manifestations associated with the more common drug toxicities, with emphasis on the peripheral features and the indications for wide field imaging. The presenting findings, underlying pathophysiology, and retinal alterations in hydroxychloroquine, thioridazine, didanosine, tamoxifen, MEK-inhibitor, and immune checkpoint inhibitor associated drug toxicity will be described and the importance of wide field imaging in the evaluation of these abnormalities will be emphasized.

Conclusions

Wide field retinal imaging can improve the detection of peripheral retinal abnormalities associated with drug toxicity and may be an important tool in the diagnosis and management of these disorders.

SOCS1-Derived Peptide Administered by Eye Drops Prevents Retinal Neuroinflammation and Vascular Leakage in Experimental Diabetes

Current treatments for diabetic retinopathy (DR) target late stages when vision has already been significantly affected. Accumulating evidence suggests that neuroinflammation plays a major role in the pathogenesis of DR, resulting in the disruption of the blood-retinal barrier. Suppressors of cytokine signaling (SOCS) are cytokine-inducible proteins that function as a negative feedback loop regulating cytokine responses. On this basis, the aim of the present study was to evaluate the effect of a SOCS1-derived peptide administered by eye drops (2 weeks) on retinal neuroinflammation and early microvascular abnormalities in a db/db mouse model. In brief, we found that SOCS1-derived peptide significantly reduced glial activation and neural apoptosis induced by diabetes, as well as retinal levels of proinflammatory cytokines. Moreover, a significant improvement of electroretinogram parameters was observed, thus revealing a clear impact of the histological findings on global retinal function. Finally, SOCS1-derived peptide prevented the disruption of the blood-retinal barrier. Overall, our results suggest that topical administration of SOCS1-derived peptide is effective in preventing retinal neuroinflammation and early microvascular impairment. These findings could open up a new strategy for the treatment of early stages of DR.

Localizing Proton-Mediated Inhibitory Feedback at the Retinal Horizontal Cell-Cone Synapse with Genetically-Encoded pH Probes

Lateral inhibition in the vertebrate retina depends on a negative feedback synapse between horizontal cells (HCs) and rod and cone photoreceptors. A change in pH is thought to be the signal for negative feedback, but its spatial profile in the synaptic cleft is unknown. Here we use three different membrane proteins, each fused to the same genetically-encoded pH-sensitive Green Fluorescent Protein (GFP) (pHluorin), to probe synaptic pH in retina from transgenic zebrafish (Danio rerio) of either sex. We used the cone transducin promoter to express SynaptopHluorin (pHluorin on vesicle-associated membrane protein (VAMP2)) or CalipHluorin (pHluorin on an L-type Ca²⁺ channel) and the HC-specific connexin-55.5 promoter to express AMPApHluorin (pHluorin on an AMPA receptor). Stimulus light led to increased fluorescence of all three probes, consistent with alkalinization of the synaptic cleft. The receptive field size, sensitivity to surround illumination, and response to activation of an alien receptor expressed exclusively in HCs, are consistent with lateral inhibition as the trigger for alkalinization. However, SynaptopHluorin and AMPApHluorin, which are displaced farther from cone synaptic ribbons than CalipHluorin, reported a smaller pH change. Hence, unlike feedforward glutamatergic transmission, which spills over to allow cross talk between terminals in the cone network, the pH change underlying HC feedback is compartmentalized to individual synaptic invaginations within a cone terminal, consistent with private line communication.SIGNIFICANCE STATEMENT Lateral inhibition (LI) is a fundamental feature of information processing in sensory systems, enhancing contrast sensitivity and enabling edge discrimination. Horizontal cells (HCs) are the first cellular substrate of LI in the vertebrate retina, but the synaptic mechanisms underlying LI are not completely understood, despite decades of study. This paper makes a significant contribution to our understanding of LI, by showing that each HC-cone synapse is a "private-line" that operates independently from other HC-cone connections. Using transgenic zebrafish expressing pHluorin, a pH-sensitive GFP variant spliced onto three different protein platforms expressed either in cones or HCs we show that the feedback pH signal is constrained to individual cone terminals, and more stringently, to individual synaptic contact sites within each terminal.

PTEN knockdown with the Y444F mutant AAV2 vector promotes axonal regeneration in the adult optic nerve

The lack of axonal regeneration is the major cause of vision loss after optic nerve injury in adult mammals. Activating the PI3K/AKT/mTOR signaling pathway has been shown to enhance the intrinsic growth capacity of neurons and to facilitate axonal regeneration in the central nervous system after injury. The deletion of the mTOR negative regulator phosphatase and tensin homolog (PTEN) enhances regeneration of adult corticospinal neurons and ganglion cells. In the present study, we used a tyrosine-mutated (Y444F) AAV2 vector to efficiently express a short hairpin RNA (shRNA) for silencing PTEN expression in retinal ganglion cells. We evaluated cell survival and axonal regeneration in a rat model of optic nerve axotomy. The rats received an intravitreal injection of wildtype AAV2 or Y444F mutant AAV2 (both carrying shRNA to PTEN) 4 weeks before optic nerve axotomy. Compared with the wildtype AAV2 vector, the Y444F mutant AAV2 vector enhanced retinal ganglia cell survival and stimulated axonal regeneration to a greater extent 6 weeks after axotomy. Moreover, post-axotomy injection of the Y444F AAV2 vector expressing the shRNA to PTEN rescued ~19% of retinal ganglion cells and induced axons to regenerate near to the optic chiasm. Taken together, our results demonstrate that PTEN knockdown with the Y444F AAV2 vector promotes retinal ganglion cell survival and stimulates long-distance axonal regeneration after optic nerve axotomy. Therefore, the Y444F AAV2 vector might be a promising gene therapy tool for treating optic nerve injury.

Immunohistochemical Localization of GFAP and Glutamate Regulatory Proteins in Chick Retina and Their Levels of Expressions in Altered Photoperiods

Moderate to intense light is reported to damage the chick retina, which is cone dominated. Light damage alters neurotransmitter pools, such as those of glutamate. Glutamate level in the retina is regulated by glutamate-aspartate transporter (GLAST) and glutamine synthetase (GS). We examined immunolocalization patterns and the expression levels of both markers and of glial fibrillary acidic protein (GFAP, a marker of neuronal stress) in chick retina exposed to 2000 lux under 12-h light:12-h dark (12L:12D; normal photoperiod), 18L:6D (prolonged photoperiod), and 24L:0D (constant light) at post-hatch day 30. Retinal damage (increased death of photoreceptors and inner retinal neurons and Müller cell hypertrophy) and GFAP expression in Müller cells were maximal in 24L:0D condition compared to that seen in 12L:12D and 18L:6D conditions. GS was present in Müller cells and GLAST expressed in Müller cell processes and photoreceptor inner segments. GLAST expression was decreased in 24L:0D condition, and the expression levels between 12L:12D and 18L:6D, though increased marginally, were statistically insignificant. Similar was the case with GS expression that significantly decreased in 24L:0D condition. Our previous study with chicks exposed to 2000 lux reported increased retinal glutamate level in 24L:0D condition. The present results indicate that constant light induces decreased expressions of GLAST and GS, a condition that might aggravate glutamate-mediated neurotoxicity and delay neuroprotection in a cone-dominated retina.

Calcium Dobesilate Prevents Neurodegeneration and Vascular Leakage in Experimental Diabetes

PURPOSE

The mechanisms involved in the reported beneficial effects of Calcium dobesilate monohydrate (CaD) for the treatment of diabetic retinopathy (DR) remain to be elucidated. The main aim of the present study is to examine whether CaD prevents early events in the pathogenesis of DR such as neurodegeneration and vascular leakage. In addition, putative mediators of both neurodegeneration (glutamate/GLAST, ET-1/ETB receptor) and early microvascular impairment (ET-1/ETA receptor, oxidative stress, VEGF, and the PKC-delta-p38 MAPK pathway) have been examined.

METHODS

Diabetic (db/db) mice were randomly assigned to daily oral treatment with CaD (200 mg/Kg/day) (n = 12) or vehicle (n = 12) for 14 days. In addition, 12 non-diabetic (db/+) mice matched by age were used as the control group. Functional abnormalities were assessed by electroretinography. Neurodegeneration and microvascular abnormalities were evaluated by immunohistochemistry and Western blot. Glutamate was determined by HPLC.

RESULTS

CaD significantly decreased glial activation and apoptosis and produced a significant improvement in the electroretinogram parameters. Mechanistically, CaD prevented the diabetes-induced up-regulation of ET-1 and its cognate receptors (ETA-R and ETB-R), which are involved in microvascular impairment and neurodegeneration, respectively. In addition, treatment with CaD downregulated GLAST, the main glutamate transporter, and accordingly prevented the increase in glutamate. Finally, CaD prevented oxidative stress, and the upregulation of VEGF and PKC delta-p38 MAPK pathway induced by diabetes, thus resulting in a significant reduction in vascular leakage.

CONCLUSIONS

Our findings demonstrate for the first time that CaD exerts neuroprotection in an experimental model of DR. In addition, we provide first evidence that CaD prevents the overexpression of ET-1 and its receptors in the diabetic retina. These beneficial effects on the neurovascular unit could pave the way for clinical trials addressed to confirm the effectiveness of CaD in very early stages of DR.

Tyrosine-mutated AAV2-mediated shRNA silencing of PTEN promotes axon regeneration of adult optic nerve

Activating PI3K/AKT/mTOR signaling pathway via deleting phosphatase and tensin homolog (PTEN) has been confirmed to enhance intrinsic growth capacity of neurons to facilitate the axons regeneration of central nervous system after injury. Considering conditional gene deletion is currently not available in clinical practice, we exploited capsid residue tyrosine 444 to phenylalanine mutated single-stranded adeno-associated virus serotype 2 (AAV2) as a vector delivering short hairpin RNA to silence PTEN to promote retinal ganglion cells (RGCs) survival and axons regeneration in adult rat optic nerve axotomy paradigm. We found that mutant AAV2 displayed higher infection efficiency to RGCs and Müller cells by intravitreal injection, mediated PTEN suppression, resulted in much more RGCs survival and more robust axons regeneration compared with wild type AAV2, due to the different extent of the mTOR complex-1 activation and glutamate aspartate transporter (GLAST) regulation. These results suggest that high efficiency AAV2-mediated PTEN knockdown represents a practicable therapeutic strategy for optic neuropathy.

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Retinal Remodeling and Metabolic Alterations in Human AMD

Age-related macular degeneration (AMD) is a progressive retinal degeneration resulting in central visual field loss, ultimately causing debilitating blindness. AMD affects 18% of Americans from 65 to 74, 30% older than 74 years of age and is the leading cause of severe vision loss and blindness in Western populations. While many genetic and environmental risk factors are known for AMD, we currently know less about the mechanisms mediating disease progression. The pathways and mechanisms through which genetic and non-genetic risk factors modulate development of AMD pathogenesis remain largely unexplored. Moreover, current treatment for AMD is palliative and limited to wet/exudative forms. Retina is a complex, heterocellular tissue and most retinal cell classes are impacted or altered in AMD. Defining disease and stage-specific cytoarchitectural and metabolic responses in AMD is critical for highlighting targets for intervention. The goal of this article is to illustrate cell types impacted in AMD and demonstrate the implications of those changes, likely beginning in the retinal pigment epithelium (RPE), for remodeling of the the neural retina. Tracking heterocellular responses in disease progression is best achieved with computational molecular phenotyping (CMP), a tool that enables acquisition of a small molecule fingerprint for every cell in the retina. CMP uncovered critical cellular and molecular pathologies (remodeling and reprogramming) in progressive retinal degenerations such as retinitis pigmentosa (RP). We now applied these approaches to normal human and AMD tissues mapping progression of cellular and molecular changes in AMD retinas, including late-stage forms of the disease.

Topical Administration of GLP-1 Receptor Agonists Prevents Retinal Neurodegeneration in Experimental Diabetes

Retinal neurodegeneration is an early event in the pathogenesis of diabetic retinopathy (DR). Since glucagon-like peptide 1 (GLP-1) exerts neuroprotective effects in the central nervous system and the retina is ontogenically a brain-derived tissue, the aims of the current study were as follows: 1) to examine the expression and content of GLP-1 receptor (GLP-1R) in human and db/db mice retinas; 2) to determine the retinal neuroprotective effects of systemic and topical administration (eye drops) of GLP-1R agonists in db/db mice; and 3) to examine the underlying neuroprotective mechanisms. We have found abundant expression of GLP-1R in the human retina and retinas from db/db mice. Moreover, we have demonstrated that systemic administration of a GLP-1R agonist (liraglutide) prevents retinal neurodegeneration (glial activation, neural apoptosis, and electroretinographical abnormalities). This effect can be attributed to a significant reduction of extracellular glutamate and an increase of prosurvival signaling pathways. We have found a similar neuroprotective effect using topical administration of native GLP-1 and several GLP-1R agonists (liraglutide, lixisenatide, and exenatide). Notably, this neuroprotective action was observed without any reduction in blood glucose levels. These results suggest that GLP-1R activation itself prevents retinal neurodegeneration. Our results should open up a new approach in the treatment of the early stages of DR.

Sigma 1 receptor regulates the oxidative stress response in primary retinal Müller glial cells via NRF2 signaling and system xc(-), the Na(+)-independent glutamate-cystine exchanger

Oxidative stress figures prominently in retinal diseases, including diabetic retinopathy, and glaucoma. Ligands for σ1R, a unique transmembrane protein localized to the endoplasmic reticulum, mitochondria, and nuclear and plasma membranes, have profound retinal neuroprotective properties in vitro and in vivo. Studies to determine the mechanism of σ1R-mediated retinal neuroprotection have focused mainly on neurons. Little is known about the effects of σ1R on Müller cell function, yet these radial glial cells are essential for homeostatic support of the retina. Here we investigated whether σ1R mediates the oxidative stress response of Müller cells using wild-type (WT) and σ1R-knockout (σ1RKO) mice. We observed increased endogenous reactive oxygen species (ROS) levels in σ1RKO Müller cells compared to WT, which was accompanied by decreased expression of Sod1, catalase, Nqo1, Hmox1, Gstm6, and Gpx1. The protein levels of SOD1, CAT, NQO1, and GPX1 were also significantly decreased. The genes encoding these antioxidants contain an antioxidant response element (ARE), which under stress is activated by NRF2, a transcription factor that typically resides in the cytoplasm bound by KEAP1. In the σ1RKO Müller cells Nrf2 expression was decreased significantly at the gene (and protein) level, whereas Keap1 gene (and protein) levels were markedly increased. NRF2-ARE binding affinity was decreased markedly in σ1RKO Müller cells. We investigated system xc(-), the cystine-glutamate exchanger important for synthesis of glutathione (GSH), and observed decreased function in σ1RKO Müller cells compared to WT as well as decreased GSH and GSH/GSSG ratios. This was accompanied by decreased gene and protein levels of xCT, the unique component of system xc(-). We conclude that Müller glial cells lacking σ1R manifest elevated ROS, perturbation of antioxidant balance, suppression of NRF2 signaling, and impaired function of system xc(-). The data suggest that the oxidative stress-mediating function of retinal Müller glial cells may be compromised in the absence of σ1R. The neuroprotective role of σ1R may be linked directly to the oxidative stress-mediating properties of supportive glial cells.

Glutamatergic Transmission: A Matter of Three

Glutamatergic transmission in the vertebrate brain requires the involvement of glia cells, in a continuous molecular dialogue. Glial glutamate receptors and transporters are key molecules that sense synaptic activity and by these means modify their physiology in the short and long term. Posttranslational modifications that regulate protein-protein interactions and modulate transmitter removal are triggered in glial cells by neuronal released glutamate. Moreover, glutamate signaling cascades in these cells are linked to transcriptional and translational control and are critically involved in the control of the so-called glutamate/glutamine shuttle and by these means in glutamatergic neurotransmission. In this contribution, we summarize our current understanding of the biochemical consequences of glutamate synaptic activity in their surrounding partners and dissect the molecular mechanisms that allow neurons to take control of glia physiology to ensure proper glutamate-mediated neuronal communication.

Effect of adenosine on GLAST expression in the retina of a chronic ocular hypertension rat model

This study was performed to evaluate the effect of adenosine and an adenosine receptor antagonist on the expression of the L-glutamate/L-aspartate transporter (GLAST) in the retina of a chronic ocular hypertension (COH) rat model. COH models were established via the cauterization of three episcleral veins. Measurements of the intraocular pressure of the right eye (COH eye) were taken weekly by a handheld digital tonometer. A total of 10 µM adenosine or 10 µM adenosine + 100 nM SCH442416 solution (2 µl) was injected into the rat vitreous space. The reverse transcription-quantitative polymerase chain reaction, western blotting and immunohistochemistry were used to detect GLAST expression. Compared with the COH group, GLAST mRNA expression was decreased by 33.6% in the group treated with adenosine (n=6, P=0.020) and was increased by 159.6% in the group treated with SCH442416 (n=6, P=0.001). Administration of adenosine decreased GLAST protein expression by 34.7% (n=6, P<0.001), while treatment with the adenosine A_2A receptor antagonist SCH442416 increased GLAST protein expression by 48.3% compared with the control COH group (n=6, P<0.001). Immunohistochemical experiments showed that administration of adenosine decreased GLAST protein expression, as compared with expression in the control COH rat retina. Administration of SCH442416 markedly increased GLAST protein expression. The results of the present study may provide a novel method for retinal neuron protection.

Effects of 17 α-methyltestosterone on transcriptome, gonadal histology and sex steroid hormones in rare minnow Gobiocypris rarus

The 17α-methyltestosterone (MT), a synthetic androgen, is known for its interference effects on the endocrine system. Aiming to investigate the transcriptome profiling of gonads induced by MT and to understand the molecular mechanism by which MT causes adverse effects in fish, transcriptome profiling of gonads, gonadal histology and the sex steroid hormones in response to MT were analyzed in Gobiocypris rarus. Eight libraries, 4 from the ovary and 4 from the testis, were constructed and sequenced and then a total number of clean reads per sample ranging from 7.03 to 9.99 million were obtained. In females, a total of 191 transcripts were differentially regulated by MT, consisting of 102 up-regulated transcripts and 89 down-regulated transcripts. In males, 268 differentially expressed genes with 108 up-regulated and 160 down-regulated were detected upon MT exposure. Testosterone serves as the major sex steroid hormone content in G. rarus of both sexes. The concentrations of 17β-estradiol, testosterone and 11-ketotestosterone were significantly increased in females and decreased in males after MT exposure. Interestingly, MT caused a decreased number of vitellogenic oocytes in the ovary and spermatozoa in the testis. After MT exposure, four differentially expressed genes (ndufa4, slc1a3a, caskin-2 and rpt3) were found in G. rarus of both sexes. Overall, we suggest that MT seemed to affect genes involved in pathways related to physiological processes in the gonads of G. rarus. These processes include the electron transfer of Complex IV, endothelial cell activation, axon growth and guidance, and proteasome assembly and glutamate transport metabolic.

Effect of fenofibrate on retinal neurodegeneration in an experimental model of type 2 diabetes

There is now consistent evidence from two major clinical trials (the Fenofibrate Intervention and Event Lowering in Diabetes and the Action to Control Cardiovascular Risk in Diabetes Eye) that fenofibrate arrests the progression of diabetic retinopathy in type 2 diabetic patients. However, the underlying mechanisms of this beneficial effect remain to be elucidated. The aim of the study was to evaluate the potential effect of fenofibric acid (FA), the active metabolite of fenofibrate, in preventing retinal neurodegeneration in an experimental mouse model of type 2 diabetes. For this purpose, we evaluated a total of 24 diabetic mice (db/db) aged 8 weeks that were randomly assigned to daily oral treatment (by gavage) with FA (100 mg/kg/day) (n = 12) or vehicle (n = 12) for 1 week. Ten non-diabetic mice (db/+) were used as control group. Retinal neurodegeneration was evaluated by measuring glial activation (immunofluorescence and Western blot) and apoptosis. Glutamate/aspartate transporter (GLAST) was assessed by immunofluorescence. Functional abnormalities were assessed by electroretinography (ERG). We observed that diabetic mice presented significantly higher glial activation and apoptosis in ganglion cell layer (GCL) than in age-matched non-diabetic mice. Treatment with FA resulted in a significant decrease in both glial activation and the rate of apoptosis in GCL in comparison with diabetic mice treated with vehicle. In addition, FA prevented GLAST downregulation induced by diabetes. Furthermore, a significant improvement of ERG parameters (oscillatory potential amplitudes and b-wave implicit time) was observed. We conclude that FA prevents retinal neurodegeneration induced by diabetes. Our results suggest that neuroprotection is one of the underlying mechanisms by which fenofibrate exerts its beneficial actions in diabetic retinopathy.

The db/db mouse: a useful model for the study of diabetic retinal neurodegeneration

Background

To characterize the sequential events that are taking place in retinal neurodegeneration in a murine model of spontaneous type 2 diabetes (db/db mouse).

Methods

C57BLKsJ-db/db mice were used as spontaneous type 2 diabetic animal model, and C57BLKsJ-db/+ mice served as the control group. To assess the chronological sequence of the abnormalities the analysis was performed at different ages (8, 16 and 24 weeks). The retinas were evaluated in terms of morphological and functional abnormalities [electroretinography (ERG)]. Histological markers of neurodegeneration (glial activation and apoptosis) were evaluated by immunohistochemistry. In addition glutamate levels and glutamate/aspartate transporter (GLAST) expression were assessed. Furthermore, to define gene expression changes associated with early diabetic retinopathy a transcriptome analyses was performed at 8 week. Furthermore, an additional interventional study to lower blood glucose levels was performed.

Results

Glial activation was higher in diabetic than in non diabetic mice in all the stages (p<0.01). In addition, a progressive loss of ganglion cells and a significant reduction of neuroretinal thickness were also observed in diabetic mice. All these histological hallmarks of neurodegeneration were less pronounced at week 8 than at week 16 and 24. Significant ERG abnormalities were present in diabetic mice at weeks 16 and 24 but not at week 8. Moreover, we observed a progressive accumulation of glutamate in diabetic mice associated with an early downregulation of GLAST. Morphological and ERG abnormalities were abrogated by lowering blood glucose levels. Finally, a dysregulation of several genes related to neurotransmission and oxidative stress such as UCP2 were found at week 8.

Conclusions

Our results suggest that db/db mouse reproduce the features of the neurodegenerative process that occurs in the human diabetic eye. Therefore, it seems an appropriate model for investigating the underlying mechanisms of diabetes-induced retinal neurodegeneration and for testing neuroprotective drugs.

SLC1 glutamate transporters

The plasma membrane transporters for the neurotransmitter glutamate belong to the solute carrier 1 family. They are secondary active transporters, taking up glutamate into the cell against a substantial concentration gradient. The driving force for concentrative uptake is provided by the cotransport of Na(+) ions and the countertransport of one K(+) in a step independent of the glutamate translocation step. Due to eletrogenicity of transport, the transmembrane potential can also act as a driving force. Glutamate transporters are expressed in many tissues, but are of particular importance in the brain, where they contribute to the termination of excitatory neurotransmission. Glutamate transporters can also run in reverse, resulting in glutamate release from cells. Due to these important physiological functions, glutamate transporter expression and, therefore, the transport rate, are tightly regulated. This review summarizes recent literature on the functional and biophysical properties, structure-function relationships, regulation, physiological significance, and pharmacology of glutamate transporters. Particular emphasis is on the insight from rapid kinetic and electrophysiological studies, transcriptional regulation of transporter expression, and reverse transport and its importance for pathophysiological glutamate release under ischemic conditions.

New functions of Müller cells

Müller cells, the major type of glial cells in the retina, are responsible for the homeostatic and metabolic support of retinal neurons. By mediating transcellular ion, water, and bicarbonate transport, Müller cells control the composition of the extracellular space fluid. Müller cells provide trophic and anti-oxidative support of photoreceptors and neurons and regulate the tightness of the blood-retinal barrier. By the uptake of glutamate, Müller cells are more directly involved in the regulation of the synaptic activity in the inner retina. This review gives a survey of recently discoved new functions of Müller cells. Müller cells are living optical fibers that guide light through the inner retinal tissue. Thereby they enhance the signal/noise ratio by minimizing intraretinal light scattering and conserve the spatial distribution of light patterns in the propagating image. Müller cells act as soft, compliant embedding for neurons, protecting them in case of mechanical trauma, and also as soft substrate required for neurite growth and neuronal plasticity. Müller cells release neuroactive signaling molecules which modulate neuronal activity, are implicated in the mediation of neurovascular coupling, and mediate the homeostasis of the extracellular space volume under hypoosmotic conditions which are a characteristic of intense neuronal activity. Under pathological conditions, a subset of Müller cells may differentiate to neural progenitor/stem cells which regenerate lost photoreceptors and neurons. Increasing knowledge of Müller cell function and responses in the normal and diseased retina will have great impact for the development of new therapeutic approaches for retinal diseases.

Molecular mechanisms of retinal ganglion cell degeneration in glaucoma and future prospects for cell body and axonal protection

Glaucoma, which affects more than 70 million people worldwide, is a heterogeneous group of disorders with a resultant common denominator; optic neuropathy, eventually leading to irreversible blindness. The clinical manifestations of primary open-angle glaucoma (POAG), the most common subtype of glaucoma, include excavation of the optic disc and progressive loss of visual field. Axonal degeneration of retinal ganglion cells (RGCs) and apoptotic death of their cell bodies are observed in glaucoma, in which the reduction of intraocular pressure (IOP) is known to slow progression of the disease. A pattern of localized retinal nerve fiber layer (RNFL) defects in glaucoma patients indicates that axonal degeneration may precede RGC body death in this condition. The mechanisms of degeneration of neuronal cell bodies and their axons may differ. In this review, we addressed the molecular mechanisms of cell body death and axonal degeneration in glaucoma and proposed axonal protection in addition to cell body protection. The concept of axonal protection may become a new therapeutic strategy to prevent further axonal degeneration or revive dying axons in patients with preperimetric glaucoma. Further study will be needed to clarify whether the combination therapy of axonal protection and cell body protection will have greater protective effects in early or progressive glaucomatous optic neuropathy (GON).

GABA and Glutamate Uptake and Metabolism in Retinal Glial (Müller) Cells

Müller cells, the principal glial cells of the retina, support the synaptic activity by the uptake and metabolization of extracellular neurotransmitters. Müller cells express uptake and exchange systems for various neurotransmitters including glutamate and γ-aminobutyric acid (GABA). Müller cells remove the bulk of extracellular glutamate in the inner retina and contribute to the glutamate clearance around photoreceptor terminals. By the uptake of glutamate, Müller cells are involved in the shaping and termination of the synaptic activity, particularly in the inner retina. Reactive Müller cells are neuroprotective, e.g., by the clearance of excess extracellular glutamate, but may also contribute to neuronal degeneration by a malfunctioning or even reversal of glial glutamate transporters, or by a downregulation of the key enzyme, glutamine synthetase. This review summarizes the present knowledge about the role of Müller cells in the clearance and metabolization of extracellular glutamate and GABA. Some major pathways of GABA and glutamate metabolism in Müller cells are described; these pathways are involved in the glutamate-glutamine cycle of the retina, in the defense against oxidative stress via the production of glutathione, and in the production of substrates for the neuronal energy metabolism.

Abnormalities in glutamate metabolism and excitotoxicity in the retinal diseases

In the physiological condition, glutamate acts as an excitatory neurotransmitter in the retina. However, excessive glutamate can be toxic to retinal neurons by overstimulation of the glutamate receptors. Glutamate excess is primarily attributed to perturbation in the homeostasis of the glutamate metabolism. Major pathway of glutamate metabolism consists of glutamate uptake by glutamate transporters followed by enzymatic conversion of glutamate to nontoxic glutamine by glutamine synthetase. Glutamate metabolism requires energy supply, and the energy loss inhibits the functions of both glutamate transporters and glutamine synthetase. In this review, we describe the present knowledge concerning the retinal glutamate metabolism under the physiological and pathological conditions.

The expression of Toll-like receptors in murine Müller cells, the glial cells in retina

Müller cells, the principal glial cells of the retina, play an important role in immune responses. Toll-like receptors (TLRs) are members of the pattern recognition receptor family and mediate innate and adaptive immune responses. In this study, we isolated, characterized Müller cells from mouse retina, and analyzed the expression of TLRs in these cells. We found that the mRNA of TLR2, TLR3, TLR4, and TLR5 was highly expressed by Müller cells. PAM3 and LPS, the agonists for TLR2 and TLR4, promoted Müller cells to produce the inflammatory cytokine Interleukine-6 and the chemokine MIP-2/CXCL2. These results suggest that Müller cells may be involved in innate and adaptive responses via TLR signaling in the eye. Our study should facilitate further study of the role of Müller cell in eye diseases and identification of the potential therapeutic targets.

Altered phenotype of the vestibular organ in GLAST-1 null mice

Various studies point to a crucial role of the high-affinity sodium-coupled glutamate aspartate transporter GLAST-1 for modulation of excitatory transmission as shown in the retina and the CNS. While 2-4-month-old GLAST-1 null mice did not show any functional vestibular abnormality, we observed profound circling behavior in older (7 months) animals lacking GLAST-1. An unchanged total number of otoferlin-positive vestibular hair cells (VHCs), similar ribbon numbers in VHCs, and an unchanged VGLUT3 expression in type II VHCs were detected in GLAST-1 null compared to wild-type mice. A partial loss of supporting cells and an apparent decline of a voltage-gated channel potassium subunit (KCNQ4) was observed in postsynaptic calyceal afferents contacting type I VHCs, together with a reduction of neurofilament- (NF200-) and vesicular glutamate transporter 1- (VGLUT1-) positive calyces in GLAST-1 null mice. Taken together, GLAST-1 deletion appeared to preferentially affect the maintenance of a normal postsynaptic/neuronal phenotype, evident only with increasing age.

Glutamate spillover between mammalian cone photoreceptors

Cone photoreceptors transmit signals at high temporal frequencies and mediate fine spatial vision. High-frequency transmission requires a high rate of glutamate release, which could promote spillover to neighboring cells, whereas spatial vision requires that cones within a tightly packed array signal light to postsynaptic bipolar cells with minimal crosstalk. Glutamate spread from the cone terminal is thought to be limited by presynaptic transporters and nearby glial processes. In addition, there is no ultrastructural evidence for chemical synapses between mammalian cones, although such synapses have been described in lower vertebrate retinas. We tested for cone-cone glutamate diffusion by recording from adjacent cone pairs in the ground squirrel retina, and instead found that the glutamate released by one cone during electrical stimulation activates glutamate transporter Cl(-) conductances on neighboring cones. Unlike in other systems, where crosstalk is diminished by increasing the temperature and by moving to a more intact preparation, glutamate spread persisted at physiological temperatures (37°C) and in retinal flat mounts. The glutamate-gated anion conductance in cones has a reversal potential of ∼-30 mV compared with a cone resting potential of ∼-50 mV; thus, crosstalk should have a depolarizing effect on the cone network. Cone-cone glutamate spread is regulated by the physiological stimulus, light, and under physiological conditions can produce a response of ∼2 mV, equivalent to 13-20% of a cone's light response. We conclude that in the absence of discrete chemical synapses, glutamate flows between cones during a light response and may mediate a spatially distributed positive feedback.

LHON: Mitochondrial Mutations and More

Leber’s hereditary optic neuropathy (LHON) is a mitochondrial disorder leading to severe visual impairment or even blindness by death of retinal ganglion cells (RGCs). The primary cause of the disease is usually a mutation of the mitochondrial genome (mtDNA) causing a single amino acid exchange in one of the mtDNA-encoded subunits of NADH:ubiquinone oxidoreductase, the first complex of the electron transport chain. It was thus obvious to accuse neuronal energy depletion as the most probable mediator of neuronal death. The group of Valerio Carelli and other authors have nicely shown that energy depletion shapes the cell fate in a LHON cybrid cell model. However, the cybrids used were osteosarcoma cells, which do not fully model neuronal energy metabolism. Although complex I mutations may cause oxidative stress, a potential pathogenetic role of the latter was less taken into focus. The hypothesis of bioenergetic failure does not provide a simple explanation for the relatively late disease onset and for the incomplete penetrance, which differs remarkably between genders. It is assumed that other genetic and environmental factors are needed in addition to the ‘primary LHON mutations’ to elicit RGC death. Relevant nuclear modifier genes have not been identified so far. The review discusses the unresolved problems of a pathogenetic hypothesis based on ATP decline and/or ROS-induced apoptosis in RGCs.

Müller glial cells in retinal disease

Virtually all pathogenic stimuli activate Müller cells. Reactive Müller cells exert protective and toxic effects on photoreceptors and neurons. They contribute to oxidative stress and glutamate toxicity due to malfunctions of glutamate uptake and glutathione synthesis. Downregulation of potassium conductance disrupts transcellular potassium and water transport, resulting in neuronal hyperexcitability and edema. Protective effects of reactive Müller cells include upregulation of adenosine 5'-triphosphate (ATP)-degrading ectoenzymes, which enhances the extracellular availability of the neuroprotectant adenosine, abrogation of the osmotic release of ATP, which might protect retinal ganglion cells from apoptosis, and the release of antioxidants and neurotrophic factors. The dedifferentiation of reactive Müller cells to progenitor-like cells might have an impact on future therapeutic approaches. A better understanding of the gliotic mechanisms will be helpful in developing efficient therapeutic strategies aiming at increased protective and regenerative properties and decreased toxicity of reactive Müller cells.

Downregulation of glutamine synthetase via GLAST suppression induces retinal axonal swelling in a rat ex vivo hydrostatic pressure model

PURPOSE. High levels of glutamate can be toxic to retinal GCs. Thus, effective buffering of extracellular glutamate is important in preserving retinal structure and function. GLAST, a major glutamate transporter in the retina, and glutamine synthetase (GS) regulate extracellular glutamate accumulation and prevent excitotoxicity. This study was an examination of changes in function and expression of GLAST and GS in ex vivo rat retinas exposed to acute increases in ambient pressure. METHODS. Ex vivo rat retinas were exposed to elevated hydrostatic pressure for 24 hours. The expression of GLAST and GS were examined using immunochemistry and real-time PCR analysis. Also examined were the effects of (2S,3S)-3-[3-[4-(trifluoromethyl) benzoylamino] benzyloxy] aspartate (TFB-TBOA), an inhibitor of glutamate transporters, and l-methionine-S-sulfoximine (MSO), an inhibitor of GS. RESULTS. In this acute model, Western blot and real-time RT-PCR analyses revealed that substantially (75 mm Hg), but not moderately (35 mm Hg), elevated pressure depressed GLAST expression, diminished GS activity, and induced axonal swelling between the GC layer and the inner limiting membrane. However, at the moderately elevated pressure (35 mm Hg), administration of either TFB-TBOA or MSO also induced axonal swelling and excitotoxic neuronal damage. MSO did not depress GLAST expression but TFB-TBOA significantly suppressed GS, suggesting that downregulation of GS during pressure loading may result from impaired GLAST expression. CONCLUSIONS. The retina is at risk during acute intraocular pressure elevation due to downregulation of GS activity resulting from depressed GLAST expression.

Altered electroretinograms in patients with KCNJ10 mutations and EAST syndrome

The K+ channel expressed by the KCNJ10 gene (Kir4.1) has previously demonstrated importance in retinal function in animal experiments. Recently, mutations in KCNJ10 were recognised as pathogenic in man, causing a constellation of symptoms, including epilepsy, ataxia, sensorineural deafness and a renal tubulopathy designated as EAST syndrome. We have studied the impact of KCNJ10 mutations on the human electroretinogram (ERG) in four unrelated patients with EAST syndrome. Corneal ganzfeld ERGs were elicited in response to flash stimuli of strengths of 0.001–10 phot cd s/m2 presented scotopically, and 0.3–10 phot cd s/m2 presented photopically. ERG waveforms from light-adapted retinae of all patients showed reduced amplitudes of the photopic negative response (PhNR) (P < 0.001). The photopic ERGs showed a delay in b-wave time to peak, but the photopic hill, i.e. the relative variation of time to peak and amplitude with luminance flash strength, was preserved. Scotopic ERGs to flash strengths 0.01 to 0.1 phot cd s/m2 showed a delay of up to 20 ms before the onset of the b-wave in two patients compared to controls. Stimulus–response functions were fitted by Michaelis–Menten equations and showed significantly lower retinal sensitivity in two patients than in controls (P < 0.001). Our study for the first time in the human ERG shows changes in association with KCNJ10 mutations affecting a Muller cell K+ channel. These data illustrate the role of KCNJ10 function in the physiology of proximal and possibly also the distal human retina.

Interleukin-1 attenuates normal tension glaucoma-like retinal degeneration in EAAC1-deficient mice

Glaucoma, one of the leading causes of irreversible blindness, is characterized by progressive degeneration of retinal ganglion cells (RGCs) and optic nerves. Although glaucoma is often associated with elevated intraocular pressure, recent studies have shown a relatively high prevalence of normal tension glaucoma (NTG) in glaucoma patient populations. In the mammalian retina, glutamate/aspartate transporter (GLAST) is localized to Müller glial cells, whereas excitatory amino acid carrier 1 (EAAC1) is expressed in neural cells, including RGCs. Since the loss of GLAST or EAAC1 leads to retinal degeneration similar to that seen in NTG, we examined the effects of interleukin-1 (IL-1) on RGC death in GLAST- and EAAC1-deficient mice. IL-1 promoted increased glutamate uptake in Müller cells by suppressing intracellular Na(+) accumulation, which is necessary to counteract Na(+)-glutamate cotransport. The observed trends for the glutamate uptake increase in the wild-type (WT), GLAST- and EAAC1-deficient mice were similar; however, the baseline glutamate uptake and intracellular Na(+) concentration in the GLAST-deficient mice were significantly lower than those in the wild-type mice. Consistently, pretreatment with IL-1 exhibited no beneficial effects on glutamate-induced RGC degeneration in the GLAST-deficient mice. In contrast, IL-1 significantly increased glutamate uptake by Müller cells and the number of surviving RGCs in the wild-type and EAAC1-deficient mice. Our findings suggest that the use of IL-1 for enhancing the function of glutamate transporters may be useful for neuroprotection in retinal degenerative disorders including NTG.

Protein Kinase C-Mediated Modulation of Glutamate Transporter Activity in Rat Retina

It has previously been shown that inhibitors of protein kinase C (PKC) attenuate retinal glutamate uptake in situ. The aim of the current study was to determine whether PKCdelta-mediated inhibition differentially reduces the transport of glutamate into retinal Müller cells when compared with retinal neurons. The influence of two different types of PKC inhibitors on the uptake of [3H]D-aspartate was therefore compared in the intact retina, mixed retinal cultures, and Müller cell-enriched retinal cultures. It was found that 25 microM of the pan-isoform PKC inhibitor, chelerythrine, reduced [3H]D-aspartate uptake by 78%, 71%, and 68% in isolated retinas, mixed neuronal/glial cultures, and Müller cell-enriched cultures, respectively. Importantly, 20 microM of the PKCdelta-selective inhibitor rottlerin also reduced the uptake of D-aspartate to similar extents in all three systems, and the reductions were statistically similar to those found for the pan-specific PKC inhibitor. Neither pan-isoform nor PKCdelta-selective activators stimulated glutamate uptake in either culture system or the intact retina. The current results suggest that specific PKC inhibitors are quantitatively similar in reducing the uptake of glutamate into retinal neurons and Müller cells.

Effects of hyperglycemia and oxidative stress on the glutamate transporters GLAST and system xc- in mouse retinal Müller glial cells

Elevated glutamate levels have been reported in humans with diabetic retinopathy. Retinal Müller glial cells regulate glutamate levels via the GLAST transporter and system x(c)(-) (cystine-glutamate exchanger). We have investigated whether transporter function and gene and/or protein expression are altered in mouse Müller cells cultured under conditions of hyperglycemia or oxidative stress (two factors implicated in diabetic retinopathy). Cells were subjected to hyperglycemic conditions (35 mM glucose) over an 8-day period or to oxidative stress conditions (induced by exposure to various concentrations of xanthine:xanthine oxidase) for 6 h. The Na(+)-dependent and -independent uptake of [(3)H] glutamate was assessed as a measure of GLAST and system x(c)(-) function, respectively. Hyperglycemia did not alter the uptake of [(3)H] glutamate by GLAST or system x(c)(-); neither gene nor protein expression decreased. Oxidative stress (70:14 or 100:20 microM xanthine:mU/ml xanthine oxidase) decreased GLAST activity by approximately 10% but increased system x(c)(-) activity by 43% and 89%, respectively. Kinetic analysis showed an oxidative-stress-induced change in V(max), but not K(m). Oxidative stress caused a 2.4-fold increase in mRNA encoding xCT, the unique component of system x(c)(-). Of the two isoforms of xCT (40 and 50 kDa), oxidative stress induced a 3.6-fold increase in the 40-kDa form localized to the plasma membrane. This is the first report of the differential expression and localization of xCT isoforms as caused by cellular stress. Increased system x(c)(-) activity in Müller cells subjected to conditions associated with diabetic retinopathy may be beneficial, as this exchanger is important for the synthesis of the antioxidant glutathione.