Expression of glutamate transporters in human and rat retina and rat optic nerve

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.

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.

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.

Age-related differences in corneal nerve regeneration after SMILE and the mechanism revealed by metabolomics

PURPOSE

To investigate the effect of age on wound healing after small incision lenticule extraction (SMILE) and the underlying metabolomic mechanisms.

METHODS

This prospective study was conducted on 216 patients in four groups: the 18-20 (n = 38, Group I), 21-30 (n = 84, Group Ⅱ), 31-40 (n = 58, Group Ⅲ), and 41-50 (n = 36, Group IV) age groups. The density of corneal epithelial wing cells, basal cells, corneal stromal cells, endothelial cells and corneal nerves were examined with a laser confocal microscope (HRT III-RCM) before and 1 month, 3 month, 6 month and 1 year after SMILE. The central nerve fiber length (CNFL), the central corneal nerve fibre density (CNFD), and the central corneal nerve branch density (CNBD) were analyzed by Nero J. The corneal stroma lenticules were obtained from SMILE to analyze metabolites by high-performance liquid chromatography coupled with quadrupole time-of-flight tandem mass spectrometry (HPLC-QTOF-MS).

RESULTS

The density of corneal wing epithelial cells and basal epithelial cells have no significant difference among the four groups. The CNFL was 21.90 ± 1.68 mm/mm² in Group Ⅰ and 21.63 ± 2.09 mm/mm² in Group Ⅱ after 1 year of SMILE, which represented a return to the preoperative level, whereas the CNFL of Group Ⅲ (19.40 ± 0.98 mm/mm²) and Group Ⅳ (18.94 ± 0.72 mm/mm²) were lower than that preoperation (P ˂0.01). CNFL repair had a negative correlation with age after surgery (Pearson's R = -0.572, P ˂0.01). The CNFD and the CNBD showed the same trend with the CNFL (Pearson's R = -0.602 and -0.531, P ˂0.05). Through screening the significantly different metabolites between the 18-30 age group (including Group I and Group Ⅱ) and other two groups, 6 common remarkably different metabolites were identified. Meanwhile, 5 unique different metabolites were identified only between the 18-30 age group and the 31-40 age group. Six unique different metabolites were identified only between the 18-30 age group and the 41-50 age group.

CONCLUSION

Corneal nerve repair after SMILE was significantly affected by age. The identified age-associated differences in metabolites were mainly related to inflammation, oxidation, nerve protection and regeneration.

Glutamate transporters: Critical components of glutamatergic transmission

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. Once released, it binds to specific membrane receptors and transporters activating a wide variety of signal transduction cascades, as well as its removal from the synaptic cleft in order to avoid its extracellular accumulation and the overstimulation of extra-synaptic receptors that might result in neuronal death through a process known as excitotoxicity. Although neurodegenerative diseases are heterogenous in clinical phenotypes and genetic etiologies, a fundamental mechanism involved in neuronal degeneration is excitotoxicity. Glutamate homeostasis is critical for brain physiology and Glutamate transporters are key players in maintaining low extracellular Glutamate levels. Therefore, the characterization of Glutamate transporters has been an active area of glutamatergic research for the last 40 years. Transporter activity its regulated at different levels: transcriptional and translational control, transporter protein trafficking and membrane mobility, and through extensive post-translational modifications. The elucidation of these mechanisms has emerged as an important piece to shape our current understanding of glutamate actions in the nervous system.

Glutamate Transporters: Expression and Function in Oligodendrocytes

Glutamate, the main excitatory neurotransmitter of the vertebrate central nervous system (CNS), is well known as a regulator of neuronal plasticity and neurodevelopment. Such glutamate function is thought to be mediated primarily by signaling through glutamate receptors. Thus, it requires a tight regulation of extracellular glutamate levels and a fine-tuned homeostasis that, when dysregulated, has been associated with a wide range of central pathologies including neuropsychiatric, neurodevelopmental, and neurodegenerative disorders. In the mammalian CNS, extracellular glutamate levels are controlled by a family of sodium-dependent glutamate transporters belonging to the solute carrier family 1 (SLC1) that are also referred to as excitatory amino acid transporters (EAATs). The presumed main function of EAATs has been best described in the context of synaptic transmission where EAATs expressed by astrocytes and neurons effectively regulate extracellular glutamate levels so that synapses can function independently. There is, however, increasing evidence that EAATs are expressed by cells other than astrocytes and neurons, and that they exhibit functions beyond glutamate clearance. In this review, we will focus on the expression and functions of EAATs in the myelinating cells of the CNS, oligodendrocytes. More specifically, we will discuss potential roles of oligodendrocyte-expressed EAATs in contributing to extracellular glutamate homeostasis, and in regulating oligodendrocyte maturation and CNS myelination by exerting signaling functions that have traditionally been associated with glutamate receptors. In addition, we will provide some examples for how dysregulation of oligodendrocyte-expressed EAATs may be involved in the pathophysiology of neurologic diseases.

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.

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.

Neuronal vs glial glutamate uptake: Resolving the conundrum

Neither normal brain function nor the pathological processes involved in neurological diseases can be adequately understood without knowledge of the release, uptake and metabolism of glutamate. The reason for this is that glutamate (a) is the most abundant amino acid in the brain, (b) is at the cross-roads between several metabolic pathways, and (c) serves as the major excitatory neurotransmitter. In fact most brain cells express glutamate receptors and are thereby influenced by extracellular glutamate. In agreement, brain cells have powerful uptake systems that constantly remove glutamate from the extracellular fluid and thereby limit receptor activation. It has been clear since the 1970s that both astrocytes and neurons express glutamate transporters. However the relative contribution of neuronal and glial transporters to the total glutamate uptake activity, however, as well as their functional importance, has been hotly debated ever since. The present short review provides (a) an overview of what we know about neuronal glutamate uptake as well as an historical description of how we got there, and (b) a hypothesis reconciling apparently contradicting observations thereby possibly resolving the paradox.

Glial cell interactions and glaucoma

Purpose of review

The present review describes new advances in our understanding of the role of glial cells in the pathogenesis of glaucoma. It is becoming clear that retinal glia should not be studied in isolation in glaucoma because glia have dynamic and diverse interactions with a range of different cell types that could influence the disease process.

Recent findings

Microglial activity is modulated by signals from retinal ganglion cells and macroglia that influence RGC survival in various models of injury. New studies suggest that circulating monocytic populations may play a role in mediating the immune response to glaucoma. Astrocytes have been found to develop discrete localized processes that interact with a specific subset of retinal ganglion cells, possibly responding to the expression of phagocytic signals by stressed retinal ganglion cells.

Summary

Retinal glia constitute a highly versatile population that interacts with various cells to maintain homeostasis and limit disease. Defining the mechanisms that underlie glial communication could enable the development of more selective therapeutic targets, with great potential clinical applications.

Functional properties of the retinal glutamate transporters GLT-1c and EAAT5

In the mammalian retina, glutamate uptake is mediated by members of a family of glutamate transporters known as "excitatory amino acid transporters (EAATs)." Here we cloned and functionally characterized two retinal EAATs from mouse, the GLT-1/EAAT2 splice variant GLT-1c, and EAAT5. EAATs are glutamate transporters and anion-selective ion channels, and we used heterologous expression in mammalian cells, patch-clamp recordings and noise analysis to study and compare glutamate transport and anion channel properties of both EAAT isoforms. We found GLT-1c to be an effective glutamate transporter with high affinity for Na(+) and glutamate that resembles original GLT-1/EAAT2 in all tested functional aspects. EAAT5 exhibits glutamate transport rates too low to be accurately measured in our experimental system, with significantly lower affinities for Na(+) and glutamate than GLT-1c. Non-stationary noise analysis demonstrated that GLT-1c and EAAT5 also differ in single-channel current amplitudes of associated anion channels. Unitary current amplitudes of EAAT5 anion channels turned out to be approximately twice as high as single-channel amplitudes of GLT-1c. Moreover, at negative potentials open probabilities of EAAT5 anion channels were much larger than for GLT-1c. Our data illustrate unique functional properties of EAAT5, being a low-affinity and low-capacity glutamate transport system, with an anion channel optimized for anion conduction in the negative voltage range.

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.

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.

Role of retinal glial cell glutamate transporters in retinal ganglion cell survival following stimulation of NMDA receptor

PURPOSE

To investigate the role of glutamate transporters (GLTs)in retinal glial cells that were treated with N-methyl-D-aspartate (NMDA), in retinal ganglion cell (RGC) survival.

METHODS

Primary cultures of retinal glial cells or RGCs from 3-day-old Sprague-Dawley rats were employed in the present study. Retinal glial cells were treated with NMDA and changes in GLT mRNA and protein expression were analyzed. The effects of pretreating retinal glial cells with the GLAST-specific inhibitor, rottlerin (ROT), and the GLT-1-specific inhibitor, dihydrokainic acid (DHK), on RGC survival were investigated under exposure to NMDA. The amount of glutamate in the culture medium of retinal glial cells was measured by high-performance liquid chromatography.

RESULTS

NMDA treatment increased GLAST and GLT-1 expression. GLAST and GLT-1 mRNA expression increased by 2.94-fold and 3.36-fold at 12 h after treatment with the highest concentration of NMDA (33 mM), and by 1.41-fold and 1.39-fold at 24 h, respectively. GLT-1 and GLAST protein expression also increased. MK801, an NMDA-receptor antagonist, inhibited the NMDA-induced upregulation of GLT mRNA expression. Co-culture with retinal glial cells increased the survival rate of RGCs. ROT decreased the survival rate of RGCs, whereas DHK significantly increased the survival rate of RGCs treated with 33 mM NMDA. NMDA treatment reduced the total amount of glutamate in the culture medium, particularly when 33 mM NMDA was added to the medium. ROT pretreatment increased the amount of glutamate in the culture medium, whereas DHK pretreatment decreased it.

CONCLUSION

GLAST and GLT-1 may have different roles in the survival of RGCs mediated by retinal glial cells. These results suggest that the NMDA-associated induction of GLTs plays an important role in RGC survival.

A new splice variant of the glutamate-aspartate transporter: cloning and immunolocalization of GLAST1c in rat, pig and human brains

GLAST (EAAT1) is an abundant glial glutamate transporter in the mammalian brain. It plays important roles in terminating excitatory transmission in grey matter, as well as pathophysiological roles, including protecting white matter from excitotoxic injury. In normal brain, alternative splicing of GLAST has been described: GLAST1a and GLAST1b arise from the splicing out of exons 3 and 9, respectively. This study describes the isolation of a novel cDNA clone from neonatal hypoxic pig brain, referred to as GLAST1c, where exons 5 and 6 are skipped. GLAST1c encodes a protein of 430 amino acids. RT-PCR analysis showed that GLAST1c mRNA was readily detectable in control and hypoxic pig cortex, as well as in various brain regions of rat (cortex, mid, hind and cerebellum), and human cortex, retina and optic nerve. We have raised antibodies that selectively recognize GLAST1c and demonstrate expression of this novel splice variant in astrocytes and oligodendrocytes in rat brain, pig brain and human brain, including grey and white matter. Similarly expression of GLAST1c was observed in primary astrocyte cultures and in cultured oligodendrocytes. In unstimulated astrocytes GLAST1c exhibited an intracellular peri-nuclear distribution similar to that observed when GFP-tagged GLAST1c was transfected into COS 7 cells. In astrocytes this protein rapidly redistributed to the surface upon stimulation of protein kinase with phorbol esters. We conclude that GLAST1c may represent an astrocyte and oligodendrocyte glutamate transporter, though this could not be formally validated by D-aspartate uptake studies, due to the low transfection efficiency of constructs into COS 7 cells.

The discovery of slowness: low-capacity transport and slow anion channel gating by the glutamate transporter EAAT5

Excitatory amino acid transporters (EAATs) control the glutamate concentration in the synaptic cleft by glial and neuronal glutamate uptake. Uphill glutamate transport is achieved by the co-/countertransport of Na(+) and other ions down their concentration gradients. Glutamate transporters also display an anion conductance that is activated by the binding of Na(+) and glutamate but is not thermodynamically coupled to the transport process. Of the five known glutamate transporter subtypes, the retina-specific subtype EAAT5 has the largest conductance relative to glutamate uptake activity. Our results suggest that EAAT5 behaves as a slow-gated anion channel with little glutamate transport activity. At steady state, EAAT5 was activated by glutamate, with a K(m)= 61 ± 11 μM. Binding of Na(+) to the empty transporter is associated with a K(m) = 229 ± 37 mM, and binding to the glutamate-bound form is associated with a K(m) = 76 ± 40 mM. Using laser-pulse photolysis of caged glutamate, we determined the pre-steady-state kinetics of the glutamate-induced anion current of EAAT5. This was characterized by two exponential components with time constants of 30 ± 1 ms and 200 ± 15 ms, which is an order of magnitude slower than those observed in other glutamate transporters. A voltage-jump analysis of the anion currents indicates that the slow activation behavior is caused by two slow, rate-limiting steps in the transport cycle, Na(+) binding to the empty transporter, and translocation of the fully loaded transporter. We propose a kinetic transport scheme that includes these two slow steps and can account for the experimentally observed data. Overall, our results suggest that EAAT5 may not act as a classical high-capacity glutamate transporter in the retina; rather, it may function as a slow-gated glutamate receptor and/or glutamate buffering system.

Quantitative iTRAQ analysis of retinal ganglion cell degeneration after optic nerve crush

Retinal ganglion cells (RGCs) are central nervous system (CNS) neurons that transmit visual information from the retina to the brain. Apoptotic RGC degeneration causes visual impairment that can be modeled by optic nerve crush. Neuronal apoptosis is also a salient feature of CNS trauma, ischemia (stroke), and diseases of the CNS such as Alzheimer's, Parkinson's, multiple sclerosis, and amyotrophic lateral sclerosis. Optic nerve crush induces the apoptotic cell death of ∼ 70% of RGCs within the first 14 days after injury. This model is particularly attractive for studying adult neuron apoptosis because the time-course of RGC death is well established and axon regeneration within the myelinated optic nerve can be concurrently evaluated. Here, we performed a large scale iTRAQ proteomic study to identify and quantify proteins of the rat retina at 1, 3, 4, 7, 14, and 21 days after optic nerve crush. In total, 337 proteins were identified, and 110 were differentially regulated after injury. Of these, 58 proteins were upregulated (>1.3 ×), 46 were downregulated (<0.7 ×), and 6 showed both positive and negative regulation over 21 days, relative to normal retinas. Among the differentially expressed proteins, Thymosin-β4 showed an early upregulation at 3 days, the time-point that immediately precedes the induction of RGC apoptosis after injury. We examined the effect of exogenous Thymosin-β4 administration on RGC death after optic nerve injury. Intraocular injections of Thymosin-β4 significantly increased RGC survival by ∼ 3-fold compared to controls and enhanced axon regeneration after crush, demonstrating therapeutic potential for CNS insults. Overall, our study identified numerous proteins that are differentially regulated at key time-points after optic nerve crush, and how the temporal profiles of their expression parallel RGC death. This data will aid in the future development of novel therapeutics to promote neuronal survival and regeneration in the adult CNS.

Characterizing light-regulated retinal microRNAs reveals rapid turnover as a common property of neuronal microRNAs

Adaptation to different levels of illumination is central to the function of the retina. Here, we demonstrate that levels of the miR-183/96/182 cluster, miR-204, and miR-211 are regulated by different light levels in the mouse retina. Concentrations of these microRNAs were downregulated during dark adaptation and upregulated in light-adapted retinas, with rapid decay and increased transcription being responsible for the respective changes. We identified the voltage-dependent glutamate transporter Slc1a1 as one of the miR-183/96/182 targets in photoreceptor cells. We found that microRNAs in retinal neurons decay much faster than microRNAs in nonneuronal cells. The high turnover is also characteristic of microRNAs in hippocampal and cortical neurons, and neurons differentiated from ES cells in vitro. Blocking activity reduced turnover of microRNAs in neuronal cells while stimulation with glutamate accelerated it. Our results demonstrate that microRNA metabolism in neurons is higher than in most other cells types and linked to neuronal activity.

Glutamate, excitatory amino acid transporters, Xc- antiporter, glutamine synthetase, and gamma-glutamyltranspeptidase in human corneal epithelium

PURPOSE

The viability and functions of the corneal epithelium are dependent in large measure on the active uptake of nutrients, growth factors, and amino acids from stroma and tear. The present study presents the cellular distribution(s) of glutamate, the Na(+)-dependent glutamate/aspartate transporters (excitatory amino acid transporters; EAAT1-5), Na(+)-independent glutamate/cystine exchanger (Xc(-) antiporter) subunits (xCT light chain and 4F2hc heavy chain), glutamine synthetase (GS), and gamma-glutamyltranspeptidase (GGT) in human corneal epithelium.

METHODS

Glutamate, EAAT1-5, xCT/4F2hc, GS, and GGT immunoreactive proteins were detected by immunofluorescence microscopy. Human corneal GGT activity was quantified using a standard colorimetric assay.

RESULTS

Glutamate, EAAT3>2>1, xCT/4F2hc, and GGT proteins were detected in the columnar and wing cells. Glutamate was reduced or absent in the EAAT negative, Xc(-) antiporter, and GS positive outer wing cell and flat superficial epithelial cell layers. All EAATs (EAAT3>4/5>1/2), xCT/4F2hc, GS, and GGT were detected in flat superficial epithelial cell layer.

CONCLUSIONS

The localization of glutamate, multiple EAATs, Xc(-) antiporter proteins, and GGT to columnar and superficial epithelial cell layers suggests uptake of glutamate and cystine from the stroma and tear and supports their importance in regulation of glutamate/cystine and glutathione (GSH; a tripeptide of glutamate, cystine, and glycine) in the human cornea epithelium. In addition, the low glutamate levels in outer wing and flat superficial epithelial cells positive for Xc(-) antiporter and GS are consistent with exchange of glutamate by Xc(-) antiporter for extracellular cystine utilized in GSH synthesis and support coupling of ammonia detoxification with glutamate degradation by GS.

Glutamate metabolic pathways and retinal function

Glutamate is a major neurotransmitter in the CNS but is also a key metabolite intimately coupled to amino acid production/degradation. We consider the effect of inhibition of two key glutamate metabolic enzymes: glutamine synthetase (GS) and aspartate aminotransferase on retinal function assessed using the electroretinogram to consider photoreceptoral (a-wave) and post-receptoral (b-wave) amplitudes. Quantitative immunocytochemistry was used to assess amino acid levels within photoreceptors, ganglion and Müller cells secondary to GS inhibition. Intravitreal injections of methionine sulfoximine reduced GS immunoreactivity in the rat retina. Additionally, glutamate and its precursor aspartate was reduced in photoreceptors and ganglion cells, but elevated in Müller cells. This reduction in neuronal glutamate was consistent with a deficit in neurotransmission (-75% b-wave reduction). Exogenous glutamine supply completely restored the b-wave, whereas other amino acid substrates (lactate, pyruvate, alpha-ketoglutarate, and succinate) only partially restored the b-wave (16-20%). Inhibition of the aminotranferases using aminooxyacetic acid had no effect on retinal function. However, aminooxyacetic acid application after methionine sulfoximine further reduced the b-wave (from -75% to -92%). The above data suggest that de novo glutamate synthesis involving aspartate aminotransferase can partially sustain neurotransmission when glutamate recycling is impaired. We also show that altered glutamate homeostasis results in a greater change in amino acid distribution in ganglion cells compared with photoreceptors.

Functional glutamate transport in rodent optic nerve axons and glia

Recent findings suggest that synaptic-type glutamate signaling operates between axons and their supporting glial cells. Glutamate reuptake will be a necessary component of such a system. Evidence for glutamate-mediated damage of oligodendroglia somata and processes in white matter suggests that glutamate regulation in white matter structures is also of clinical importance. The expression of glutamate transporters was examined in postnatal Day 14-17 (P14-17) mouse and in mature mouse and rat optic nerve using immuno-histochemistry and immuno-electron microscopy. EAAC1 was the major glutamate transporter detected in oligodendroglia cell membranes in both developing and mature optic nerve, while GLT-1 was the most heavily expressed transporter in the membranes of astrocytes. Both EAAC1 and GLAST were also seen in adult astrocytes, but there was little membrane expression of either at P14-17. GLAST, EAAC1, and GLT-1 were expressed in P14-17 axons with marked GLT-1 expression in the axolemma, while in mature axons EAAC1 was abundant at the node of Ranvier. Functional glutamate transport was probed in P14-17 mouse optic nerve revealing Na+-dependent, TBOA-blockable uptake of D-aspartate in astrocytes, axons, and oligodendrocytes. The data show that in addition to oligodendroglia and astrocytes, axons represent a potential source for extracellular glutamate in white matter during ischaemic conditions, and have the capacity for Na(+)-dependent glutamate uptake. The findings support the possibility of functional synaptic-type glutamate release from central axons, an event that will require axonal glutamate reuptake.

Expression and distribution of 'high affinity' glutamate transporters GLT1, GLAST, EAAC1 and of GCPII in the rat peripheral nervous system

l-Glutamate is one of the major excitatory neurotransmitters in the mammalian central nervous system, but recently it has been shown to have a role also in the transduction of sensory input at the periphery, and in particular in the nociceptive pathway. An excess of glutamate is implicated in cases of peripheral neuropathies as well. Conventional therapeutic approaches for treating these diseases have focused on blocking glutamate receptors with small molecules or on reducing its synthesis of the receptors through the inhibition of glutamate carboxypeptidase II (GCPII), the enzyme that generates glutamate. In vivo studies have demonstrated that the pharmacological inhibition of GCPII can either prevent or treat the peripheral nerve changes in both BB/Wor and chemically induced diabetes in rats. In this study, we characterized the expression and distribution of glutamate transporters GLT1, GLAST, EAAC1 and of the enzyme GCPII in the peripheral nervous system of female Wistar rats. Immunoblotting results demonstrated that all glutamate transporters and GCPII are present in dorsal root ganglia (DRG) and the sciatic nerve. Immunofluorescence localization studies revealed that both DRG and sciatic nerves were immunopositive for all glutamate transporters and for GCPII. In DRG, satellite cells were positive for GLT1 and GCPII, whereas sensory neurons were positive for EAAC1. GLAST was localized in both neurons and satellite cells. In the sciatic nerve, GLT1 and GCPII were expressed in the cytoplasm of Schwann cells, whereas GLAST and EAAC1 stained the myelin layer. Our results give for the first time a complete characterization of the glutamate transporter system in the peripheral nervous system. Therefore, they are important both for understanding glutamatergic signalling in the PNS and for establishing new strategies to treat peripheral neuropathies.

Synaptic regulation of the light-dependent oscillatory currents in starburst amacrine cells of the mouse retina

Responses of on-center starburst amacrine cells to steady light stimuli were recorded in the dark-adapted mouse retina. The response to spots of dim white light appear to show two components, an initial peak that correspond to the onset of the light stimulus and a series of oscillations that ride on top of the initial peak relaxation. The frequency of oscillations during light stimulation was three time higher than the frequency of spontaneous oscillations recorded in the dark. The light-evoked responses in starburst cells were exclusively dependent on the release of glutamate likely from presynaptic bipolar axon terminals and the binding of glutamate to AMPA/kainate receptors because they were blocked by 6-cyano-7-nitroquinoxalene-2,3-dione. The synaptic pathway responsible for the light responses was blocked by AP4, an agonist of metabotropic glutamate receptors that hyperpolarize on-center bipolar cells on activation. Light responses were inhibited by the calcium channel blockers cadmium ions and nifedipine, suggesting that the release of glutamate was calcium dependent. The oscillatory component of the response was specifically inhibited by blocking the glutamate transporter with d-threo-beta-benzyloxyaspartic acid, suggesting that glutamate reuptake is necessary for the oscillatory release. GABAergic antagonists bicuculline, SR 95531, and picrotoxin increased the amplitude of the initial peak while they inhibit the frequency of oscillations. TTX had a similar effect. Strychnine, the blocker of glycine receptors did not affect the initial peak but strongly decreased the oscillations frequency. These inhibitory inputs onto the bipolar axon terminals shape and synchronize the oscillatory component.

EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

The excitatory amino acid transporter 2 (EAAT2) is responsible for the majority of glutamate uptake in the brain and its dysregulation has been associated with multiple psychiatric and neurological disorders. However, investigation of this molecule has been complicated by its complex pattern of alternative splicing, including three coding isoforms and multiple 5'- and 3'-UTRs that may have a regulatory function. It is likely that these sequences permit modulation of EAAT2 expression with spatial, temporal and or activity-dependent specificity; however, few studies have attempted to delineate the function of these sequences. Additionally, there are problems with the use of antibodies to study protein localization, possibly due to posttranslational modification of critical amino acid residues. This review describes what is currently known about the regulation of EAAT2 mRNA and protein isoforms and concludes with a summary of studies showing dysregulation of EAAT2 in psychiatric and neurological disorders. EAAT2 has been either primarily or secondarily implicated in a multitude of neuropsychiatric diseases in addition to the normal physiology of learning and memory. Thus, this molecule represents an intriguing therapeutic target once we improve our understanding of how it is regulated under normal conditions.

Proteomic analyses of retina of excitatory amino acid carrier 1 deficient mice

BACKGROUND

Excitatory amino acid carrier 1 (EAAC1) is a glutamate transporter found in neuronal tissues and is extensively expressed in the retina. EAAC1 plays a role in a variety of neural functions, but its biological functions in the retina has not been fully determined. The purpose of this study was to identify proteins regulated by EAAC1 in the retina of mice. To accomplish this, we used a proteomics-based approach to identify proteins that are up- or down-regulated in EAAC1-deficient (EAAC1-/-) mice.

RESULTS

Proteomic analyses and two-dimensional gel electorphoresis were performed on the retina of EAAC1-/- mice, and the results were compared to that of wild type mice. The protein spots showing significant differences were selected for identification by mass spectrometric analyses. Thirteen proteins were differentially expressed; nine proteins were up-regulated and five proteins were down-regulated in EAAC1-/- retina. Functional clustering showed that identified proteins are involved in various cellular process, e.g. cell cycle, cell death, transport and metabolism.

CONCLUSION

We identified thirteen proteins whose expression is changed in EAAC-/- mice retinas. These proteins are known to regulate cell proliferation, death, transport, metabolism, cell organization and extracellular matrix.

GLAST1b, the exon-9 skipping form of the glutamate-aspartate transporter EAAT1 is a sensitive marker of neuronal dysfunction in the hypoxic brain

In normal brain, we previously demonstrated that the exon-9 skipping form of glutamate-aspartate transporter (GLAST; which we refer to as GLAST1b) is expressed by small populations of neurons that appear to be sick or dying and suggested that these cells were subject to inappropriate local glutamate-mediated excitation. To test this hypothesis we examined the expression of GLAST1b in the hypoxic pig brain. In this model glial glutamate transporters such as GLAST and glutamate transporter 1 (GLT-1) are down-regulated in susceptible regions, leading to regional loss of glutamate homeostasis and thus to brain damage. We demonstrate by immunohistochemistry that in those brain regions where astroglial glutamate transporters are lost, GLAST1b expression is induced in populations of neurons and to a lesser extent in some astrocytes. These neurons were also immunolabeled by antibodies against the carboxyl-terminal region of GLAST but did not label with antibodies directed against the amino-terminal region. Our Western blotting data indicate that GLAST1b expressed by neurons lacks the normal GLAST amino-terminal region and may be further cleaved to a smaller approximately 30-kDa fragment. We propose that GLAST1b represents a novel and sensitive marker for the detection of neurons at risk of dying in response to hypoxic and other excitotoxic insults and may have wider applicability in experimental and clinical contexts.

Muller cell expression of glutamate cycle related proteins and anti-apoptotic proteins in early human retinal development

AIMS

The distribution of glutamate cycle related proteins (glutamine synthetase (GS) and GLAST) and anti-apoptotic proteins (Bcl-2 and Bcl-X) was investigated in Müller cells during early human retinal development, relative to the onset of expression of synaptophysin, a presynaptic vesicle protein.

METHODS

Using frozen sections of human fetal eyes (13-22 weeks gestation) (n = 10), Bcl-2, Bcl-X, GS, GLAST, and synaptophysin immunoreactivities (IR) were imaged using fluorescence microscopy and plotted as a function of eccentricity from the incipient fovea. Frozen sections of adult human retina (n = 4) were immunolabelled with antibodies to Bcl-2 and Bcl-X.

RESULTS

Müller cell immunoreactivity for GS, GLAST, and Bcl-2 was initially detected in the incipient fovea, and then at more peripheral locations with increasing age. Synaptophysin-IR appeared earlier than all other target proteins. Within the synaptophysin-IR region, mature (differentiated) Müller cells expressed both Bcl-2 and Bcl-X-IR from 13 weeks gestation, ahead of GS-IR and GLAST-IR that were first seen at 14 weeks gestation. Additionally, from as early as 13 weeks gestation, ganglion cells and immature neuronal progenitor cells across the entire retina expressed Bcl-2-IR and Bcl-X-IR, respectively. In adult retina, ganglion cells and some bipolar cells expressed Bcl-X but not Bcl-2.

CONCLUSION

Müller cells express Bcl-2 and Bcl-X after synaptogenesis has commenced, but before the onset of GS and GLAST expression, suggesting a protective role for these proteins in Müller cells during the onset of glutamatergic transmission in early human retinal development.

Different expression role among glutamate transporters in rat retinal glial cells under various culture conditions

Using cultured rat retinal glial cells, the changes in the expression of glutamate transporters (GLTs) under such conditions as the degree of confluence of the cells, hypoxia, glutamate loading, and potassium loading, were assessed. After the retinal glial cells were isolated from 3-day-old Sprague-Dawley rats, GLAST, GLT-1, and EAAC1 mRNA expression was confirmed by reverse transcription-polymerase chain reaction (RT-PCR) and their protein expression was confirmed by Western blot analysis and immunostaining. Changes in the expression of the GLTs at 6 h after passage and at confluence, during culture in 5% oxygen, during glutamate loading and during potassium loading were assessed using real-time PCR. Although the GLAST mRNA expression was increased during glutamate and potassium loading, no changes in the expression were observed during hypoxia and at confluence. By contrast, the GLT-1 mRNA expression was increased during hypoxia and at confluence, but not during glutamate and potassium loading, and the EAAC1 mRNA expression was not changed only during glutamate loading. The expression of EAAC1 in the cultured retinal glial cells was confirmed. The expression of the GLTs varied according to the environment and the type of load suggests that the involvement of the GLTs in retinal physiology and pathology varies depending on the subtype.

News and views in Histochemistry and Cell Biology

Advances in histochemical methodology and ingenious applications of novel and improved methods continue to confirm the standing of morphological means and approaches in research efforts, and contribute significantly to increasing our knowledge about structures and functions in all areas of the life sciences from cell biology to pathology. Reports published during recent months documenting this progress are summarized in the present review.

Expression of glutamate transporters in rat cardiomyocytes and their localization in the T-tubular system

Glutamate and aspartate play important roles in the intermediary metabolism of the myocardium and have been shown to improve cardiac recovery after hypoxia or ischemia. Limited data are available about the expression of glutamate transporters that are involved in the uptake of glutamate and aspartate in cardiomyocytes. In this study, non-radioactive in situ hybridization (ISH) using complementary RNA probes was applied to detect the glutamate transporters GLT1 variant (GLT1v) and EAAC1 mRNA in rat cardiomyocytes. The transporter proteins were demonstrated by Western blotting and immunocytochemistry using affinity-purified antibodies against transporter peptides. ISH and immunocytochemistry showed that both glutamate transporters are coexpressed in cardiomyocytes. The ISH labeling indicates the distribution of transporter mRNA throughout the cytoplasm of cardiomyocytes. GLT1v and EAAC1 proteins, which showed in Western blots a molecular mass of approximately 60 kD, are strongly enriched and colocalized in the transverse (T)-tubular system of cardiomyocytes. These results may indicate that glutamate/aspartate uptake into cardiomyocytes could be mediated by the high-affinity transporters GLT1v and EAAC1. A high efficiency of glutamate/aspartate transport into cardiomyocytes could be achieved by their localization in the T-tubular system, which consists of tubular invaginations of the sarcolemma extending deep into the cell.

Differential cellular and subcellular distribution of glutamate transporters in the cat retina

Retrieval of glutamate from extracellular sites in the retina involves at least five excitatory amino acid transporters. Immunocytochemical analysis of the cat retina indicates that each of these transporters exhibits a selective distribution which may reflect its specific function. The uptake of glutamate into Müller cells or astrocytes appears to depend upon GLAST and EAAT4, respectively. Staining for EAAT4 was also seen in the pigment epithelium. The remaining transporters are neuronal with GLT-1α localized to a number of cone bipolar, amacrine, and ganglion cells and GLT-1v in cone photoreceptors and several populations of bipolar cells. The EAAC1 transporter was found in horizontal, amacrine, and ganglion cells. Staining for EAAT5 was seen in the axon terminals of both rod and cone photoreceptors as well as in numerous amacrine and ganglion cells. Although some of the glutamate transporter molecules are positioned for presynaptic or postsynaptic uptake at glutamatergic synapses, others with localizations more distant from such contacts may serve in modulatory roles or provide protection against excitoxic or oxidative damage.

Developmental expression of glutamate transporters and glutamate dehydrogenase in astrocytes of the postnatal rat hippocampus

Glutamate is the major excitatory transmitter in the CNS and plays distinct roles in a number of developmental events. Its extracellular concentration, which mediates these activities, is regulated by glutamate transporters in glial cells and neurons. In the present study, we have used nonradioactive in situ hybridization, immunocytochemistry, and immunoblotting to show the cellular and regional expression of the high-affinity glutamate transporters GLAST (EAAT1) and generic GLT1 (EAAT2; glial form of GLT1) in the rat hippocampus during postnatal development (P1-60). The results of transporter expression were compared with the localization and activity pattern of glutamate dehydrogenase (GDH), an important glutamate-metabolizing enzyme. The study showed that both transporters and GDH were demonstrable at P1 (day of birth). The expression of GLAST (detected by in situ hybridization and immunocytochemistry) in the early postnatal development was higher than GLT1. Thereafter, the expression of both transporters increased, showing adult levels at between P20 and P30 (detected by in situ hybridization and immunoblotting). At these time points, the expression of GLT1 appeared to be significantly higher than the GLAST expression. GLT1 and GLAST proteins were demonstrable only in astrocytes. The increase of GDH activities (steepest increase from P5-P8), which were localized preferentially in astrocytes, was in agreement with the increase of transporter expression, preferentially with that of GLT1. These observations suggest that the extent of glutamate transporter expression and of glutamate-metabolizing GDH activity in astrocytes is intimately correlated with the formation of glutamatergic synapses in the developing hippocampus.

Innovative techniques and applications in histochemistry and cell biology

Recent studies documenting novel histochemical methods and applications in cell biology and in other areas of the life sciences have again rendered insights into structure and functions of tissues, cells, and cellular components to the level of proteins and genes. Particularly, sophisticated microscopic techniques have proved to be able to significantly advance our knowledge. Findings of recent investigations representing this progress are summarized in the present review.