Glutamate Receptor Stimulation Up-Regulates Glutamate Uptake in Human Müller Glia Cells

NADPH and NAC synergistically inhibits chronic ocular hypertension-induced neurodegeneration and neuroinflammation through regulating p38/MAPK pathway and peroxidation

Glaucoma, the leading cause of irreversible blindness worldwide, is characterized by neurodegeneration and neuroinflammation with retinal NAD/NADP and GSH decline. Nicotinamide adenine dinucleotide (NAD)/NAD phosphate (NADP) and glutathione (GSH) are two redox reducers in neuronal and glial metabolism. However, therapeutic strategies targeting NAD/NADP or GSH do not exert ideal effects, and the underlying mechanisms are still poorly understood. We assessed morphological changes in retinal ganglion cells (RGCs), the affected neurons in glaucoma, and Müller cells, the major glial cells in the retina, as well as the levels of phosphorylated p38 (p-p38) and Caspase-3 in glaucoma patients. We constructed a modified chronic ocular hypertensive rat model and an oxygen-glucose deprivation (OGD) cell model. After applying NADPH and N-acetylcysteine (NAC), a precursor to cysteine, the rate-limiting substrate in GSH biosynthesis, to cells, apoptosis, axonal damage and peroxidation were reduced in the RGCs of the NAC group and p-p38 levels were decreased in the RGCs of the NADPH group, while in stimulated Müller cells cultured individually or cocultured with RGCs, gliosis and p38/MAPK, rather than JNK/MAPK, activation were inhibited. The results were more synergistic in the rat model, where either NADPH or NAC showed crossover effects on inhibiting peroxidation and p38/MAPK pathway activation. Moreover, the combination of NADPH and NAC ameliorated RGC electrophysiological function and prevented Müller cell gliosis to the greatest extent. These data illustrated conjoined mechanisms in glaucomatous RGC injury and Müller cell gliosis and suggested that NADPH and NAC collaborate as a neuroprotective and anti-inflammatory combination treatment for glaucoma and other underlying human neurodegenerative diseases.

The Rax homeoprotein in Müller glial cells is required for homeostasis maintenance of the postnatal mouse retina

Müller glial cells, which are the most predominant glial subtype in the retina, play multiple important roles, including the maintenance of structural integrity, homeostasis, and physiological functions of the retina. We have previously found that the Rax homeoprotein is expressed in postnatal and mature Müller glial cells in the mouse retina. However, the function of Rax in postnatal and mature Müller glial cells remains to be elucidated. In the current study, we first investigated Rax function in retinal development using retroviral lineage analysis and found that Rax controls the specification of late-born retinal cell types, including Müller glial cells in the postnatal retina. We next generated Rax tamoxifen-induced conditional KO (Rax iCKO) mice, where Rax can be depleted in mTFP-labeled Müller glial cells upon tamoxifen treatment, by crossing Raxflox/flox mice with Rlbp1-CreERT2 mice, which we have produced. Immunohistochemical analysis showed a characteristic of reactive gliosis and enhanced gliosis of Müller glial cells in Rax iCKO retinas under normal and stress conditions, respectively. We performed RNA-seq analysis on mTFP-positive cells purified from the Rax iCKO retina and found significantly reduced expression of suppressor of cytokinesignaling-3 (Socs3). Reporter gene assays showed that Rax directly transactivates the Socs3 promoter. We observed decreased expression of Socs3 in Müller glial cells of Rax iCKO retinas by immunostaining. Taken together, the present results suggest that Rax suppresses inflammation in Müller glial cells by transactivating Socs3. This study sheds light on the transcriptional regulatory mechanisms underlying retinal Müller glial cell homeostasis.

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.

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.

RNA Foci Formation in a Retinal Glial Model for Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disorder characterized by cerebellar ataxia and retinopathy. SCA7 is caused by a CAG expansion in the ATXN7 gene, which results in an extended polyglutamine (polyQ) tract in the encoded protein, the ataxin-7. PolyQ expanded ataxin-7 elicits neurodegeneration in cerebellar Purkinje cells, however, its impact on the SCA7-associated retinopathy remains to be addressed. Since Müller glial cells play an essential role in retinal homeostasis, we generate an inducible model for SCA7, based on the glial Müller MIO-M1 cell line. The SCA7 pathogenesis has been explained by a protein gain-of-function mechanism, however, the contribution of the mutant RNA to the disease cannot be excluded. In this direction, we found nuclear and cytoplasmic foci containing mutant RNA accompanied by subtle alternative splicing defects in MIO-M1 cells. RNA foci were also observed in cells from different lineages, including peripheral mononuclear leukocytes derived from SCA7 patient, suggesting that this molecular mark could be used as a blood biomarker for SCA7. Collectively, our data showed that our glial cell model exhibits the molecular features of SCA7, which makes it a suitable model to study the RNA toxicity mechanisms, as well as to explore therapeutic strategies aiming to alleviate glial dysfunction.

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.

Müller Glia in Retinal Development: From Specification to Circuit Integration

Müller glia of the retina share many features with astroglia located throughout the brain including maintenance of homeostasis, modulation of neurotransmitter spillover, and robust response to injury. Here we present the molecular factors and signaling events that govern Müller glial specification, patterning, and differentiation. Next, we discuss the various roles of Müller glia in retinal development, which include maintaining retinal organization and integrity as well as promoting neuronal survival, synaptogenesis, and phagocytosis of debris. Finally, we review the mechanisms by which Müller glia integrate into retinal circuits and actively participate in neuronal signaling during development.

The ERM Complex: A New Player Involved in Diabetes-induced Vascular Leakage

BACKGROUND

Microvascular complications remain an important cause of morbidity in diabetic patients, and they are associated with a significant economic burden for healthcare systems. Vascular leakage is one of the earlier hallmarks in diabetic microvascular complications. Ezrin, Radixin and Moesin (ERM) proteins have recently been involved in vascular dysfunction under the effect of molecular mediators of diabetes complications. In this review, we will present the available evidence regarding the role of these proteins in vascular leakage and their putative implication in diabetic microvascular complications.

METHODS AND RESULTS

A comprehensive literature search of the electronic MEDLINE database was performed between November 2017 and January 2018. As a result, 36 articles have been reviewed and discussed.

DISCUSSION

ERM proteins are cytoskeleton-membrane linkers, and when activated in endothelial cells are able to induce cytoskeleton reorganization in stress fibers leading to the disassembly of focal adhesions and the formation of paracellular gaps which result in an increase of vascular permeability. The activation of these proteins is induced by mediators involved in diabetic complications such as PKC activation, TNF-α, AGEs and oxidative stress. In conclusion, ERMs play an essential role in endothelium homeostasis and can be envisaged as a new therapeutic molecular target for preventing or arresting diabetes-induced vascular leakage.

A miRNA screen procedure identifies garz as an essential factor in adult glia functions and validates Drosophila as a beneficial 3Rs model to study glial functions and GBF1 biology

Invertebrate glia performs most of the key functions controlled by mammalian glia in the nervous system and provides an ideal model for genetic studies of glial functions. To study the influence of adult glial cells in ageing we have performed a genetic screen in Drosophila using a collection of transgenic lines providing conditional expression of micro-RNAs (miRNAs). Here, we describe a methodological algorithm to identify and rank genes that are candidate to be targeted by miRNAs that shorten lifespan when expressed in adult glia. We have used four different databases for miRNA target prediction in Drosophila but find little agreement between them, overall. However, top candidate gene analysis shows potential to identify essential genes involved in adult glial functions. One example from our top candidates' analysis is gartenzwerg ( garz). We establish that garz is necessary in many glial cell types, that it affects motor behaviour and, at the sub-cellular level, is responsible for defects in cellular membranes, autophagy and mitochondria quality control. We also verify the remarkable conservation of functions between garz and its mammalian orthologue, GBF1, validating the use of Drosophila as an alternative 3Rs-beneficial model to knock-out mice for studying the biology of GBF1, potentially involved in human neurodegenerative diseases.

A miRNA screen procedure identifies garz as an essential factor in adult glia functions and validates Drosophila as a beneficial 3Rs model to study glial functions and GBF1 biology

Invertebrate glia performs most of the key functions controlled by mammalian glia in the nervous system and provides an ideal model for genetic studies of glial functions. To study the influence of adult glial cells in ageing we have performed a genetic screen in Drosophila using a collection of transgenic lines providing conditional expression of micro-RNAs (miRNAs). Here, we describe a methodological algorithm to identify and rank genes that are candidate to be targeted by miRNAs that shorten lifespan when expressed in adult glia. We have used four different databases for miRNA target prediction in Drosophila but find little agreement between them, overall. However, top candidate gene analysis shows potential to identify essential genes involved in adult glial functions. One example from our top candidates' analysis is gartenzwerg ( garz). We establish that garz is necessary in many glial cell types, that it affects motor behaviour and, at the sub-cellular level, is responsible for defects in cellular membranes, autophagy and mitochondria quality control. We also verify the remarkable conservation of functions between garz and its mammalian orthologue, GBF1, validating the use of Drosophila as an alternative 3Rs-beneficial model to knock-out mice for studying the biology of GBF1, potentially involved in human neurodegenerative diseases.

IPL Sublamination in Chicken Retinal Spheroids Is Initiated via Müller Cells and Cholinergic Differentiation, and Is Disrupted by NMDA Signaling

Purpose

Reaggregates from E6 embryonic chicken retina exhibit areas corresponding to an inner plexiform layer (IPL), which presents an ideal in vitro model to test conditions and constraints of cholinergic and glutamatergic network formation, providing a basis for retinal tissue engineering. Here, we show that ipl formation is regulated by cholinergic starburst amacrine cells (SACs), a glial scaffold and by L-glutamate.

Methods

Rosetted spheroids were cultured in absence or presence of 0.2 to 0.4 mM L-glutamate and analyzed by immuno- and enzyme histochemistry, proliferation, and apoptosis assays.

Results

After 2 days in vitro (div), ipl formation was announced by acetylcholinesterase+ (AChE) and choline acetyltransferase+ (ChAT) cells. Individual vimentin+ or transitin+ Müller glial cell precursors (MCPs) in ipl centers coexpressed ChAT. Comparable to in vivo, pairwise arranged ChAT+ SACs formed two laminar subbands. Projections of calretinin+ amacrine cells (ACs) into ipl associated with MCP processes. In L-glutamate-, or NMDA-treated spheroids ipls were disrupted, including loss of SACs and MCs; coincubation with NMDA receptor inhibitor MK-801 prevented these effects. Also, many Pax6+ cells, comprising most ACs, were lost, while rho4D2+ rod photoreceptors were increased. Cell proliferation was slightly increased, while apoptosis remained unaffected.

Conclusions

This demonstrated: (1) a far-advanced differentiation of an IPL in retinal spheroids, as never described before; (2) ipl sublamination was initiated by cholinergic precursor cells, which-functioning as "ipl founder cells"-(3) gave rise to neurons and glial cells; (4) these SACs and MCPs together organized ipl formation; and (5) this process was counteracted by NMDA-dependent glutamate actions.

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.

Perspectives for Ezrin and Radixin in Astrocytes: Kinases, Functions and Pathology

Astrocytes are increasingly perceived as active partners in physiological brain function and behaviour. The structural correlations of the glia–synaptic interaction are the peripheral astrocyte processes (PAPs), where ezrin and radixin, the two astrocytic members of the ezrin-radixin-moesin (ERM) family of proteins are preferentially localised. While the molecular mechanisms of ERM (in)activation appear universal, at least in mammalian cells, and have been studied in great detail, the actual ezrin and radixin kinases, phosphatases and binding partners appear cell type specific and may be multiplexed within a cell. In astrocytes, ezrin is involved in process motility, which can be stimulated by the neurotransmitter glutamate, through activation of the glial metabotropic glutamate receptors (mGluRs) 3 or 5. However, it has remained open how this mGluR stimulus is transduced to ezrin activation. Knowing upstream signals of ezrin activation, ezrin kinase(s), and membrane-bound binding partners of ezrin in astrocytes might open new approaches to the glial role in brain function. Ezrin has also been implicated in invasive behaviour of astrocytomas, and glial activation. Here, we review data pertaining to potential molecular interaction partners of ezrin in astrocytes, with a focus on PKC and GRK2, and in gliomas and other diseases, to stimulate further research on their potential roles in glia-synaptic physiology and pathology.

Pin1 Is Regulated by CaMKII Activation in Glutamate-Induced Retinal Neuronal Regulated Necrosis

In our previous study, we reported that peptidyl-prolyl isomerase 1 (Pin1)-modulated regulated necrosis (RN) occurred in cultured retinal neurons after glutamate injury. In the current study, we investigated the role of calcium/calmodulin-dependent protein kinase II (CaMKII) in Pin1-modulated RN in cultured rat retinal neurons, and in an animal in vivo model. We first demonstrated that glutamate might lead to calcium overloading mainly through ionotropic glutamate receptors activation. Furthermore, CaMKII activation induced by overloaded calcium leads to Pin1 activation and subsequent RN. Inactivation of CaMKII by KN-93 (KN, i.e., a specific CaMKII inhibitor) application can decrease the glutamate-induced retinal neuronal RN. Finally, by using an animal in vivo model, we also demonstrated the important role of CaMKII in glutamate-induced RN in rat retina. In addition, flash electroretinogram results provided evidence that the impaired visual function induced by glutamate can recover after CaMKII inhibition. In conclusion, CaMKII is an up-regulator of Pin1 and responsible for the RN induced by glutamate. This study provides further understanding of the regulatory pathway of RN and is a complementary mechanism for Pin1 activation mediated necrosis. This finding will provide a potential target to protect neurons from necrosis in neurodegenerative diseases, such as glaucoma, diabetic retinopathy, and even central nervous system diseases.

Glutamate transport system as a key constituent of glutamosome: Molecular pathology and pharmacological modulation in chronic pain

Neural uptake of glutamate is executed by the structurally related members of the SLC1A family of solute transporters: GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3, EAAT4, ASCT2. These plasma membrane proteins ensure supply of glutamate, aspartate and some neutral amino acids, including glutamine and cysteine, for synthetic, energetic and signaling purposes, whereas effective removal of glutamate from the synaptic cleft shapes excitatory neurotransmission and prevents glutamate toxicity. Glutamate transporters (GluTs) possess also receptor-like properties and can directly initiate signal transduction. GluTs are physically linked to other glutamate signaling-, transporting- and metabolizing molecules (e.g., glutamine transporters SNAT3 and ASCT2, glutamine synthetase, NMDA receptor, synaptic vesicles), as well as cellular machineries fueling the transmembrane transport of glutamate (e.g., ion gradient-generating Na/K-ATPase, glycolytic enzymes, mitochondrial membrane- and matrix proteins, glucose transporters). We designate this supramolecular functional assembly as 'glutamosome'. GluTs play important roles in the molecular pathology of chronic pain, due to the predominantly glutamatergic nature of nociceptive signaling in the spinal cord. Down-regulation of GluTs often precedes or occurs simultaneously with development of pain hypersensitivity. Pharmacological inhibition or gene knock-down of spinal GluTs can induce/aggravate pain, whereas enhancing expression of GluTs by viral gene transfer can mitigate chronic pain. Thus, functional up-regulation of GluTs is turning into a prospective pharmacotherapeutic approach for the management of chronic pain. A number of novel positive pharmacological regulators of GluTs, incl. pyridazine derivatives and β-lactams, have recently been introduced. However, design and development of new analgesics based on this principle will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of the glutamate transport system in nociceptive circuits. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Effect of Adenosine and Adenosine Receptor Antagonists on Retinal Müller Cell Inwardly Rectifying Potassium Channels under Exogenous Glutamate Stimulation

The vitreousness of glaucoma subjects contains elevated glutamate, and excessive extracellular glutamate is toxic to retinal neurons. Therefore, glutamate clearance is potentially impaired in the retina of glaucoma subjects. Müller cells play an important role in maintaining low extracellular levels of neurotransmitters, such as glutamate. A better understanding of the cross-talk between adenosine and glutamate may provide a better characterization of the regulatory network in Müller cells. Here, Müller cells were purified from the rat retina on postnatal day 5 using the papain digestion method. Application of increasing concentrations of glutamate (0-20 mmol/L) caused a dose-dependent decrease in the expression levels of Kir4.1, Kir2.1, GLAST, and GS. Exogenous adenosine regulated Kir channels and subsequently promoted GLAST and GS expression levels in Müller cells under exogenous glutamate stimulation. These effects were partly dependent on adenosine receptors.

Glutamate-Dependent Translational Control of Glutamine Synthetase in Bergmann Glia Cells

Glutamate is the major excitatory transmitter of the vertebrate brain. It exerts its actions through the activation of specific plasma membrane receptors expressed both in neurons and in glial cells. Recent evidence has shown that glutamate uptake systems, particularly enriched in glia cells, trigger biochemical cascades in a similar fashion as receptors. A tight regulation of glutamate extracellular levels prevents neuronal overstimulation and cell death, and it is critically involved in glutamate turnover. Glial glutamate transporters are responsible of the majority of the brain glutamate uptake activity. Once internalized, this excitatory amino acid is rapidly metabolized to glutamine via the astrocyte-enriched enzyme glutamine synthetase. A coupling between glutamate uptake and glutamine synthesis and release has been commonly known as the glutamate/glutamine shuttle. Taking advantage of the established model of cultured Bergmann glia cells, in this contribution, we explored the gene expression regulation of glutamine synthetase. A time- and dose-dependent regulation of glutamine synthetase protein and activity levels was found. Moreover, glutamate exposure resulted in the transient shift of glutamine synthetase mRNA from the monosomal to the polysomal fraction. These results demonstrate a novel mode of glutamate-dependent glutamine synthetase regulation and strengthen the notion of an exquisite glia neuronal interaction in glutamatergic synapses.