Non-vesicular release of glutamate from glial cells by reversed electrogenic glutamate uptake

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Homeostasis of the Intraparenchymal-Blood Glutamate Concentration Gradient: Maintenance, Imbalance, and Regulation

It is widely accepted that glutamate is the most important excitatory neurotransmitter in the central nervous system (CNS). However, there is also a large amount of glutamate in the blood. Generally, the concentration gradient of glutamate between intraparenchymal and blood environments is stable. However, this gradient is dramatically disrupted under a variety of pathological conditions, resulting in an amplifying cascade that causes a series of pathological reactions in the CNS and peripheral organs. This eventually seriously worsens a patient’s prognosis. These two “isolated” systems are rarely considered as a whole even though they mutually influence each other. In this review, we summarize what is currently known regarding the maintenance, imbalance and regulatory mechanisms that control the intraparenchymal-blood glutamate concentration gradient, discuss the interrelationships between these systems and further explore their significance in clinical practice.

A mathematical model of recurrent spreading depolarizations

A detailed biophysical model for a neuron/astrocyte network is developed in order to explore mechanisms responsible for the initiation and propagation of recurrent cortical spreading depolarizations. The model incorporates biophysical processes not considered in the earlier models. This includes a model for the Na⁺-glutamate transporter, which allows for a detailed description of reverse glutamate uptake. In particular, we consider the specific roles of elevated extracellular glutamate and K⁺ in the initiation, propagation and recurrence of spreading depolarizations.

Obstructive sleep apnea is associated with altered midbrain chemical concentrations

Obstructive sleep apnea (OSA) is accompanied by altered structure and function in cortical, limbic, brainstem, and cerebellar regions. The midbrain is relatively unexamined, but contains many integrative nuclei which mediate physiological functions that are disrupted in OSA. We therefore assessed the chemistry of the midbrain in OSA in this exploratory study. We used a recently developed accelerated 2D magnetic resonance spectroscopy (2D-MRS) technique, compressed sensing-based 4D echo-planar J-resolved spectroscopic imaging (4D-EP-JRESI), to measure metabolites in the midbrain of 14 OSA (mean age±SD:54.6±10.6years; AHI:35.0±19.4; SAO2 min:83±7%) and 26 healthy control (50.7±8.5years) subjects. High-resolution T1-weighted scans allowed voxel localization. MRS data were processed with custom MATLAB-based software, and metabolite ratios calculated with respect to the creatine peak using a prior knowledge fitting (ProFit) algorithm. The midbrain in OSA showed decreased N-acetylaspartate (NAA; OSA:1.24±0.43, Control:1.47±0.41; p=0.03; independent samples t-test), a marker of neuronal viability. Increased levels in OSA over control subjects appeared in glutamate (Glu; OSA:1.23±0.57, Control:0.98±0.33; p=0.03), ascorbate (Asc; OSA:0.56±0.28, Control:0.42±0.20; (50.7±8.5years; p=0.03), and myo-inositol (mI; OSA:0.96±0.48, Control:0.72±0.35; p=0.03). No differences between groups appeared in γ-aminobutyric acid (GABA) or taurine. The midbrain in OSA patients shows decreased NAA, indicating neuronal injury or dysfunction. Higher Glu levels may reflect excitotoxic processes and astrocyte activation, and higher mI is also consistent with glial activation. Higher Asc levels may result from oxidative stress induced by intermittent hypoxia in OSA. Additionally, Asc and Glu are involved with glutamatergic processes, which are likely upregulated in the midbrain nuclei of OSA patients. The altered metabolite levels help explain dysfunction and structural deficits in the midbrain of OSA patients.

The role of glutamate in neuronal ion homeostasis: A case study of spreading depolarization

Simultaneous changes in ion concentrations, glutamate, and cell volume together with exchange of matter between cell network and vasculature are ubiquitous in numerous brain pathologies. A complete understanding of pathological conditions as well as normal brain function, therefore, hinges on elucidating the molecular and cellular pathways involved in these mostly interdependent variations. In this paper, we develop the first computational framework that combines the Hodgkin-Huxley type spiking dynamics, dynamic ion concentrations and glutamate homeostasis, neuronal and astroglial volume changes, and ion exchange with vasculature into a comprehensive model to elucidate the role of glutamate uptake in the dynamics of spreading depolarization (SD)-the electrophysiological event underlying numerous pathologies including migraine, ischemic stroke, aneurysmal subarachnoid hemorrhage, intracerebral hematoma, and trauma. We are particularly interested in investigating the role of glutamate in the duration and termination of SD caused by K+ perfusion and oxygen-glucose deprivation. Our results demonstrate that glutamate signaling plays a key role in the dynamics of SD, and that impaired glutamate uptake leads to recovery failure of neurons from SD. We confirm predictions from our model experimentally by showing that inhibiting astrocytic glutamate uptake using TFB-TBOA nearly quadruples the duration of SD in layers 2-3 of visual cortical slices from juvenile rats. The model equations are either derived purely from first physical principles of electroneutrality, osmosis, and conservation of particles or a combination of these principles and known physiological facts. Accordingly, we claim that our approach can be used as a future guide to investigate the role of glutamate, ion concentrations, and dynamics cell volume in other brain pathologies and normal brain function.

Suppression of SNARE-dependent exocytosis in retinal glial cells and its effect on ischemia-induced neurodegeneration

Nervous tissue is characterized by a tight structural association between glial cells and neurons. It is well known that glial cells support neuronal functions, but their role under pathologic conditions is less well understood. Here, we addressed this question in vivo using an experimental model of retinal ischemia and transgenic mice for glia‐specific inhibition of soluble N‐ethylmaleimide‐sensitive factor attachment protein receptor (SNARE)‐dependent exocytosis. Transgene expression reduced glutamate, but not ATP release from single Müller cells, impaired glial volume regulation under normal conditions and reduced neuronal dysfunction and death in the inner retina during the early stages of ischemia. Our study reveals that the SNARE‐dependent exocytosis in glial cells contributes to neurotoxicity during ischemia in vivo and suggests glial exocytosis as a target for therapeutic approaches.

Astrocytic modulation of neuronal excitability through K+ spatial buffering

The human brain contains two major cell populations, neurons and glia. While neurons are electrically excitable and capable of discharging short voltage pulses known as action potentials, glial cells are not. However, astrocytes, the prevailing subtype of glia in the cortex, are highly connected and can modulate the excitability of neurons by changing the concentration of potassium ions in the extracellular environment, a process called K⁺ clearance. During the past decade, astrocytes have been the focus of much research, mainly due to their close association with synapses and their modulatory impact on neuronal activity. It has been shown that astrocytes play an essential role in normal brain function including: nitrosative regulation of synaptic release in the neocortex, synaptogenesis, synaptic transmission and plasticity. Here, we discuss the role of astrocytes in network modulation through their K⁺ clearance capabilities, a theory that was first raised 50 years ago by Orkand and Kuffler. We will discuss the functional alterations in astrocytic activity that leads to aberrant modulation of network oscillations and synchronous activity.

Molecular and cellular physiology of sodium-dependent glutamate transporters

Glutamate is the major excitatory transmitter in the vertebrate brain. After its release from presynaptic nerve terminals, it is rapidly taken up by high-affinity sodium-dependent plasma membrane transporters. While both neurons and glial cells express these excitatory amino acid transporters (EAATs), the majority of glutamate uptake is accomplished by astrocytes, which convert synaptically-released glutamate to glutamine or feed it into their own metabolism. Glutamate uptake by astrocytes not only shapes synaptic transmission by regulating the availability of glutamate to postsynaptic neuronal receptors, but also protects neurons from hyper-excitability and subsequent excitotoxic damage. In the present review, we provide an overview of the molecular and cellular characteristics of sodium-dependent glutamate transporters and their associated anion permeation pathways, with a focus on astrocytic glutamate transport. We summarize their functional properties and roles within tripartite synapses under physiological and pathophysiological conditions, exemplifying the intricate interactions and interrelationships between neurons and glial cells in the brain.

The role of inversely operating glutamate transporter in the paradoxical analgesia produced by glutamate transporter inhibitors

Controlling extracellular glutamate level in a physiological range is important to maintain normal sensory transmission. Here, we investigated the paradoxical action of glutamate transporters in the rat formalin test to elucidate a possible role of inversely operating transporters in its analgesic mechanism. The effects of glutamate transporter inhibitor on formalin-induced pain behavior were examined. Then we performed a microdialysis study to clarify the differential change in extracellular glutamate concentration by intrathecal administration of transportable and non-transportable blockers. And we further investigated the mechanism pharmacologically via pretreatment with antagonists of various receptors and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL) staining. Intrathecally-injected glutamate transporter inhibitors, non-transportable DL-threo-β-benzyloxyaspartat (TBOA) and transportable trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC), produced paradoxical antinociception in the formalin test. In normal rats, inhibition of the glutamate transporter increased extracellular glutamate. In the formalin model rats, TBOA suppressed while t-PDC enhanced glutamate release. When tPDC was pretreated 30min prior to formalin injection, glutamate release was blocked. Blocking α-2 adrenergic receptors reversed the tPDC analgesia. Increased apoptosis was not apparent in the spinal dorsal horn of tPDC-treated rats compared to the control group. These data suggest that glutamate transporters in a formalin-induced pain state work in a reverse mode and can be blocked from releasing glutamate by TBOA and preloaded tPDC. The analgesic mechanism of TBOA may be related to the blockade of inversely operating transporter, and that of tPDC may be associated with the activation of noradrenergic neurotransmission but not with dorsal horn neurotoxicity.

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.

Role of Astroglial Hemichannels and Pannexons in Memory and Neurodegenerative Diseases

Under physiological conditions, astroglial hemichannels and pannexons allow the release of gliotransmitters from astrocytes. These gliotransmitters are critical in modulating synaptic transmission, plasticity and memory. However, recent evidence suggests that under pathological conditions, they may be central in the development of various neurodegenerative diseases. Here we review current literature on the role of astroglial hemichannels and pannexons in memory, stress and the development of neurodegenerative diseases, and propose that they are not only crucial for normal brain function, including memory, but also a potential target for the treatment of neurodegenerative diseases.

Regulation of Glutamate Transporter Function

Normal neuronal function and neuronal survival require that brain extracellular glutamate concentrations be maintained at low micromolar levels. This is accomplished by a family of Na+-dependent glutamate transporters. These transporters are expressed on both glia and neurons, but uptake by glia seems to predominate. Several transporter subtypes have been identified that differ in anatomical distribution, cell type of expression, and electrophysiological properties. Activity of the transporters can be influenced by changes in the uptake driving forces (thermodynamic forces) and by phosphorylation and other modulations that alter their kinetic properties. An understanding of the modulatory mech anisms and signal transduction systems that govern glutamate transport is now beginning to take shape. NEUROSCIENTIST 5:280-282, 1999

Glial Influences on Neuronal Signaling

Since their discovery in the late nineteenth century, glial cells have taken a backstage role to neurons in our investigations of the nervous system. Yet, in the mammalian brain, glial cells outnumber neurons and comprise 50% of the brain volume. A number of proteins, including important synthetic enzymes, are located almost exclusively in glia, suggesting that glial cells may serve functions beyond providing structural support. Speculations of more active roles for glial cells in brain function began at the turn of the century, at which time it had been suggested that they serve nutritive functions (Golgi, 1885), insulate nerve fibers (Ramon y Cajal, 1909), phagocytose dying neurons (Marinesco, 1896), secrete fac tors (Achucarro, 1915), and remove neurotransmitters (the substances by which one nerve cell excites another) from the synaptic cleft (Lugaro, 1907). Many of these early hypotheses have subsequently gained experimental support; however, our understanding of glial roles in brain function is still limited. The Neuroscientist 1:123-126, 1995

Review : Glial Neuronal Interactions: A Physiological Perspective

Throughout the nervous system, neurons are closely surrounded by glial cells, leaving only a 20-nm wide extracellular space filled with interstitial fluid. Ions, transmitters, hormones, nutrients, and waste products all share this narrow diffusion pathway. Because the interstitial space occupies only a small volume, neuronal activity can lead to appreciable changes in the extracellular concentration of ions, protons, and neurotrans mitters. These changes can affect neuronal activity and are believed to be influenced by glial cells. The proximity of glial processes to synapses and axons make glial cells ideal partners to sequester ions and transmitters released by neurons. The failure of glial cells to perform such essential homeostatic functions can have profound effects, and these homeostatic activities may constitute one way in which glial cells can influence neuronal signaling. In addition, glial cells, which, unlike most neurons, are coupled to each other through gap-junctions, communicate with each other and possibly also with adjacent neurons through prop agated intracellular Ca2+waves. The importance of such interglial signaling is not understood. Additionally, glial cells and neurons mutually modulate their expression of ion channels, most likely through factors re leased into the extracellular space. The range of responses observed in glial cells and their intimate anatomical relationship with neurons suggest a broader role for glia than is currently appreciated. It also emphasizes the importance of a better understanding of glial-neuronal interactions to an understanding of brain function. The Neuroscientist 1:328-337, 1995

Aquaporins: Multiple Roles in the Central Nervous System

Aquaporins (AQPs) represent a diverse family of membrane proteins found in prokaryotes and eukaryotes. The primary aquaporins expressed in the mammalian brain are AQP1, which is densely packed in choroid plexus cells lining the ventricles, and AQP4, which is abundant in astrocytes and concentrated especially in the end-feet structures that surround capillaries throughout the brain and are present in glia limitans structures, notably in osmosensory areas such the supraoptic nucleus. Water movement in brain tissues is carefully regulated from the micro- to macroscopic levels, with aquaporins serving key roles as multifunctional elements of complex signaling assemblies. Intriguing possibilities suggest links for AQP1 in Alzheimer's disease, AQP4 as a target for therapy in brain edema, and a possible contribution of AQP9 in Parkinson's disease. For all the aquaporins, new contributions to physiological functions are likely to continue to be discovered with ongoing work in this rapidly expanding field of research. NEUROSCIENTIST 13(5):470—485, 2007.

Neuroprotective Effects of the Glutamate Transporter Activator (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153) following Traumatic Brain Injury in the Adult Rat

Traumatic brain injury (TBI) in humans and in animals leads to an acute and sustained increase in tissue glutamate concentrations within the brain, triggering glutamate-mediated excitotoxicity. Excitatory amino acid transporters (EAATs) are responsible for maintaining extracellular central nervous system glutamate concentrations below neurotoxic levels. Our results demonstrate that as early as 5 min and up to 2 h following brain trauma in brain-injured rats, the activity (Vmax) of EAAT2 in the cortex and the hippocampus was significantly decreased, compared with sham-injured animals. The affinity for glutamate (KM) and the expression of glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST) were not altered by the injury. Administration of (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline (MS-153), a GLT-1 activator, beginning immediately after injury and continuing for 24 h, significantly decreased neurodegeneration, loss of microtubule-associated protein 2 and NeuN (+) immunoreactivities, and attenuated calpain activation in both the cortex and the hippocampus at 24 h after the injury; the reduction in neurodegeneration remained evident up to 14 days post-injury. In synaptosomal uptake assays, MS-153 up-regulated GLT-1 activity in the naïve rat brain but did not reverse the reduced activity of GLT-1 in traumatically-injured brains. This study demonstrates that administration of MS-153 in the acute post-traumatic period provides acute and long-term neuroprotection for TBI and suggests that the neuroprotective effects of MS-153 are related to mechanisms other than GLT-1 activation, such as the inhibition of voltage-gated calcium channels.

1,25-Dihydroxyvitamin D induces the glutamate transporter SLC1A1 and alters glutamate handling in non-transformed mammary cells

Genomic profiling of immortalized human mammary epithelial (hTERT-HME1) cells identified several metabolic genes, including the membrane glutamate transporter, SLC1A1, as 1,25-dihydroxyvitamin D3 (1,25D) regulated. In these studies we have surveyed the effects of 1,25D on known glutamate transporters and evaluated its impact on cellular glutamate handling. We confirm that expression of SLC1A1 and all of its known transcript variants are significantly upregulated in hTERT-HME1 cells following 1,25D treatment. Expression of the full-length cognate protein, EAAT3, is correspondingly increased in 1,25D treated hTERT-HME1 cells. Under the same conditions, the expression of two other glutamate transporters--SLC1A6 (EAAT4) and SLC1A2 (EAAT2 or GLT-1)--is enhanced by 1,25D while that of SLC1A3 (EAAT1 or GLAST) and SLC7A11 (xCT) is decreased. Glutamate is not essential for growth of hTERT-HME1 cells, and supplemental glutamate (up to 0.5 mM) does not abrogate the growth inhibitory effects of 1,25D. These data suggest that extracellular glutamate is not a major contributor to cellular energy metabolism in hTERT-HME1 cells under basal conditions and that the growth inhibitory effects of 1,25D are not secondary to its effects on glutamate handling. Instead, the effects of 1,25D on glutamate transporters translated to a decrease in cellular glutamate concentration and an increase in media glutamate concentration, suggesting that one or more of these transporters functions to export glutamate in response to 1,25D exposure. The reduced cellular glutamate concentration may also reflect its incorporation into the cellular glutathione (GSH) pool, which is increased upon 1,25D treatment. In support of this concept, the expression of GCLC (which codes for the rate-limiting enzyme in GSH synthesis) and genes which generate reducing equivalents in the form of NADPH (ie, G6PD, PGD, IDH2) are elevated in 1,25D-treated cells. Taken together, these data identify 1,25D as a physiological regulator of multiple membrane glutamate transporters that impacts on overall cellular glutamate handling.

Prevalence and audiological profiles of GJB2 mutations in a large collective of hearing impaired patients

Mutations in the GJB2 gene are known to represent the commonest cause of hereditary and congenital hearing loss. In this study, a complete sequencing of the GJB2 gene in a cohort of 506 patients from a single, large cochlear implant program in Europe was performed. Audiological testing for those patients who could actively participate was performed using pure tone audiometry (PTA). Those unable to undergo PTA were measured using click-auditory brainstem response (ABR). Data analysis was performed to determine genotype-phenotype correlations of the mutational status vs. audiological profiles and vs. age at the time of presentation. An overall prevalence of biallelic mutations of 13.4% was found for the total collective. When subsets of younger patients were examined, the prevalence increased to 27% of those up to age 18 and 35% of those up to age 5 at the time of testing, respectively. This increase was found to be highly significant (p < 0.001). Analysis of the mean PTA thresholds revealed a strong correlation between allele combination status and mean PTA (p = 0.021). The prevalence of simple heterozygotes was found to be approximately 10.1%, which is around 3.3 times the value expected in the general population. As GJB2 follows a recessive pattern of inheritance, the question arises as to why such a large fraction of simple heterozygotes was observed among the hearing impaired patients included in this study.

Astrocytic Actions on Extrasynaptic Neuronal Currents

In the last few decades, knowledge about astrocytic functions has significantly increased. It was demonstrated that astrocytes are not passive elements of the central nervous system (CNS), but active partners of neurons. There is a growing body of knowledge about the calcium excitability of astrocytes, the actions of different gliotransmitters and their release mechanisms, as well as the participation of astrocytes in the regulation of synaptic functions and their contribution to synaptic plasticity. However, astrocytic functions are even more complex than being a partner of the “tripartite synapse,” as they can influence extrasynaptic neuronal currents either by releasing substances or regulating ambient neurotransmitter levels. Several types of currents or changes of membrane potential with different kinetics and via different mechanisms can be elicited by astrocytic activity. Astrocyte-dependent phasic or tonic, inward or outward currents were described in several brain areas. Such currents, together with the synaptic actions of astrocytes, can contribute to neuromodulatory mechanisms, neurosensory and -secretory processes, cortical oscillatory activity, memory, and learning or overall neuronal excitability. This mini-review is an attempt to give a brief summary of astrocyte-dependent extrasynaptic neuronal currents and their possible functional significance.

Volume-regulated anion channel--a frenemy within the brain

The volume-regulated anion channel (VRAC) is a ubiquitously expressed yet highly enigmatic member of the superfamily of chloride/anion channels. It is activated by cellular swelling and mediates regulatory cell volume decrease in a majority of vertebrate cells, including those in the central nervous system (CNS). In the brain, besides its crucial role in cellular volume regulation, VRAC is thought to play a part in cell proliferation, apoptosis, migration, and release of physiologically active molecules. Although these roles are not exclusive to the CNS, the relative significance of VRAC in the brain is amplified by several unique aspects of its physiology. One important example is the contribution of VRAC to the release of the excitatory amino acid neurotransmitters glutamate and aspartate. This latter process is thought to have impact on both normal brain functioning (such as astrocyte-neuron signaling) and neuropathology (via promoting the excitotoxic death of neuronal cells in stroke and traumatic brain injury). In spite of much work in the field, the molecular nature of VRAC remained unknown until less than 2 years ago. Two pioneer publications identified VRAC as the heterohexamer formed by the leucine-rich repeat-containing 8 (LRRC8) proteins. These findings galvanized the field and are likely to result in dramatic revisions to our understanding of the place and role of VRAC in the brain, as well as other organs and tissues. The present review briefly recapitulates critical findings in the CNS and focuses on anticipated impact on the LRRC8 discovery on further progress in neuroscience research.

Age-specific localization of NMDA receptors on oligodendrocytes dictates axon function recovery after ischemia

Oligodendrocytes and axons are the main targets of an ischemic white matter injury and the resultant loss of axon function underlies the clinical disability in patients who survive a stroke. The cellular mechanisms of ischemic injury change as a function of age in concordance with age-mediated structural changes in white matter. Shorter periods of injury cause rapid and robust loss of axon function together with widespread oligodendrocyte death. While blockade of NMDA receptors fails to benefit axon function, removal of extracellular Ca²⁺ during ischemia remarkably promotes axon function recovery in young white matter. However, these same approaches hinder axon function recovery and fail to protect oligodendrocytes in aging white matter. The obligatory GluN1 subunit of the NMDA receptor exhibits an age-specific expression pattern such that in young adult white matter, it is mostly localized on oligodendrocyte cell bodies, while in aging white matter, it is also observed on myelin processes. This age-dependent re-localization and redistribution pattern mimics GluN1 expression observed during development, but in reverse order. During development, GluN1 immunoreactivity traffics from astrocytes at postnatal day 4-11 (P4-11) to myelin processes at P12-18 and to oligodendrocytes cell bodies at P19-21. Although immature axons are more resistant to ischemia, blockade of NMDA receptors during ischemia at P4-11 and P12-18 worsens axon function recovery and fails to benefit axons at P19-21. Thus, age-specific expression patterns of NMDA receptor localization may seem to modulate the plasticity of oligodendrocytes and myelin in response to ischemia as a function of age in white matter. This article is part of the Special Issue entitled 'Oligodendrocytes in Health and Disease'.

Glutamate transporter EAAT2: regulation, function, and potential as a therapeutic target for neurological and psychiatric disease

Glutamate is the predominant excitatory neurotransmitter in the central nervous system. Excitatory amino acid transporter 2 (EAAT2) is primarily responsible for clearance of extracellular glutamate to prevent neuronal excitotoxicity and hyperexcitability. EAAT2 plays a critical role in regulation of synaptic activity and plasticity. In addition, EAAT2 has been implicated in the pathogenesis of many central nervous system disorders. In this review, we summarize current understanding of EAAT2, including structure, pharmacology, physiology, and functions, as well as disease relevancy, such as in stroke, Parkinson's disease, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, major depressive disorder, and addiction. A large number of studies have demonstrated that up-regulation of EAAT2 protein provides significant beneficial effects in many disease models suggesting EAAT2 activation is a promising therapeutic approach. Several EAAT2 activators have been identified. Further understanding of EAAT2 regulatory mechanisms could improve development of drug-like compounds that spatiotemporally regulate EAAT2.

Proteomic analysis of the hippocampus in naïve and ischemic-preconditioned rat

The hippocampus exhibits regional differences in vulnerability to ischemia, wherein pyramidal cells in the CA1 region are vulnerable to ischemia while pyramidal cells in the CA3 region and granule cells in the dentate gyrus (DG) region are relatively ischemia resistant. However, pyramidal cells in the CA1 region reportedly exhibit ischemic tolerance following exposure to a brief non-lethal period of ischemia known as ischemic preconditioning. In this study, we used proteomic analysis to examine the difference in protein expression between naïve rat CA1 and CA3/DG regions, as well as the altered protein expression in the CA1 region after 3min of ischemic preconditioning. Proteomic analysis identified ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCH-L1), glutathione S-transferase μ5 (GSTμ5), glutamine synthetase (GS), and dynamin-1 as proteins with differential expression levels in naïve CA1 and CA3/DG regions. The difference in expression levels of GSTμ5 and GS between these two regions was further confirmed by western blot. Our analysis also identified aconitase2, α-tubulin, protein-l-isoaspartate O-methiltransferase (PIMT), and voltage-dependent anion channel 1 (VDCA1) as proteins with down-regulated expression levels in the CA1 region following 3min ischemic preconditioning. The decrease in the expression of aconitase2 was also confirmed by western blot and immunohistochemical staining. The present results suggest that GSTμ5 and GS may be associated with ischemia-resistance in the CA3/DG region and that aconitase2 may play a part in the ischemic tolerance in the CA1 region.

Neuroinflammatory responses in diabetic retinopathy

Diabetic retinopathy (DR) is a common complication of diabetes and has been recognized as a vascular dysfunction leading to blindness in working-age adults. It becomes increasingly clear that neural cells in retina play an important role in the pathogenesis of DR. Neural retina located at the back of the eye is part of the brain and a representative of the central nervous system. The neurosensory deficits seen in DR are related to inflammation and occur prior to the clinically identifiable vascular complications. The neural deficits are associated with abnormal reactions of retina glial cells and neurons in response to hyperglycemia. Improper activation of the innate immune system may also be an important contributor to the pathophysiology of DR. Therefore, DR manifests characteristics of both vasculopathy and chronic neuroinflammatory diseases. In this article, we attempt to provide an overview of the current understanding of inflammation in neural retina abnormalities in diabetes. Inhibition of neuroinflammation may represent a novel therapeutic strategy to the prevention of the progression of DR.

Spreading depolarizations mediate excitotoxicity in the development of acute cortical lesions

Spreading depolarizations (SD) are mass depolarizations of neurons and astrocytes that occur spontaneously in acute brain injury and mediate time-dependent lesion growth. Glutamate excitotoxicity has also been extensively studied as a mechanism of neuronal injury, although its relevance to in vivo pathology remains unclear. Here we hypothesized that excitotoxicity in acute lesion development occurs only as a consequence of SD. Using glutamate-sensitive microelectrodes, we found that SD induced by KCl in normal rat cortex elicits increases in extracellular glutamate (11.6±1.3μM) that are synchronous with the onset, sustainment, and resolution of the extracellular direct-current shift of SD. Inhibition of glutamate uptake with d,l-threo-β-benzyloxyaspartate (TBOA, 0.5 and 1mM) significantly prolonged the duration of the direct-current shift (148% and 426%, respectively) and the glutamate increase (167% and 374%, respectively) in a dose-dependent manner (P<0.05). These prolonged events produced significant cortical lesions as indicated by Fluoro-Jade staining (P<0.05), while no lesions were observed after SD in control conditions or after cortical injection of 1mM glutamate (extracellular increase: 243±50.8μM) or 0.5mM TBOA (glutamate increase: 8.5±1.6μM) without SD. We then used an embolic focal ischemia model to determine whether glutamate elevations occur independent of SD in the natural evolution of a cortical lesion. In both the ischemic core and penumbra, glutamate increased only in synchrony with anoxic terminal SD (6.1±1.1μM) and transient SDs (11.8±2.4μM), and not otherwise. Delayed terminal SDs were also observed in two animals at 98 and 150min after ischemic onset and induced similar glutamate elevations. Durations of SDs and glutamate increases were significantly correlated in both normal and ischemic animals (P<0.05). These data suggest that pathologically prolonged SDs are a required mechanism of acute cortical lesion development and that glutamate elevations and the mass electrochemical changes of SD and are merely different facets of the same pathophysiologic process.

Astrocytic vesicles and gliotransmitters: Slowness of vesicular release and synaptobrevin2-laden vesicle nanoarchitecture

Neurotransmitters released at synapses activate neighboring astrocytes, which in turn, modulate neuronal activity by the release of diverse neuroactive substances that include classical neurotransmitters such as glutamate, GABA or ATP. Neuroactive substances are released from astrocytes through several distinct molecular mechanisms, for example, by diffusion through membrane channels, by translocation via plasmalemmal transporters or by vesicular exocytosis. Vesicular release regulated by a stimulus-mediated increase in cytosolic calcium involves soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptor (SNARE)-dependent merger of the vesicle membrane with the plasmalemma. Up to 25 molecules of synaptobrevin 2 (Sb2), a SNARE complex protein, reside at a single astroglial vesicle; an individual neuronal, i.e. synaptic, vesicle contains ∼70 Sb2 molecules. It is proposed that this paucity of Sb2 molecules in astrocytic vesicles may determine the slow secretion. In the present essay we shall overview multiple aspects of vesicular architecture and types of vesicles based on their cargo and dynamics in astroglial cells.

Transport rates of a glutamate transporter homologue are influenced by the lipid bilayer

The aspartate transporter from Pyrococcus horikoshii (GltPh) is a model for the structure of the SLC1 family of amino acid transporters. Crystal structures of GltPh provide insight into mechanisms of ion coupling and substrate transport; however, structures have been solved in the absence of a lipid bilayer so they provide limited information regarding interactions that occur between the protein and lipids of the membrane. Here, we investigated the effect of the lipid environment on aspartate transport by reconstituting GltPh into liposomes of defined lipid composition where the primary lipid is phosphatidylethanolamine (PE) or its methyl derivatives. We showed that the rate of aspartate transport and the transmembrane orientation of GltPh were influenced by the primary lipid in the liposomes. In PE liposomes, we observed the highest transport rate and showed that 85% of the transporters were orientated right-side out, whereas in trimethyl PE liposomes, 50% of transporters were right-side out, and we observed a 4-fold reduction in transport rate. Differences in orientation can only partially explain the lipid composition effect on transport rate. Crystal structures of GltPh revealed a tyrosine residue (Tyr-33) that we propose interacts with lipid headgroups during the transport cycle. Based on site-directed mutagenesis, we propose that a cation-π interaction between Tyr-33 and the lipid headgroups can influence conformational flexibility of the trimerization domain and thus the rate of transport. These results provide a specific example of how interactions between membrane lipids and membrane-bound proteins can influence function and highlight the importance of the role of the membrane in transporter function.

Distinct functional states of astrocytes during sleep and wakefulness: Is norepinephrine the master regulator?

Astrocytes are the chief supportive cells in the central nervous system, but work over the past 20 years have documented that astrocytes also contribute to complex neural processes, such as working memory. Recent discoveries of norepinephrine-mediated astrocytic Ca²⁺ responses have raised the possibility that astrocytic activity in the adult brain is driven by global responses to changes in behavioral state. Moreover, analysis of the interstitial space volume suggests that astrocytes may undergo changes in cell volume in response to activation of norepinephrine receptors. This review will focus on what is known about astrocytic functions within the nervous system, and how these functions interrelate with rapid changes in behavioral state mediated by norepinephrine signaling.

Glutamate Transporters/Na(+), K(+)-ATPase Involving in the Neuroprotective Effect as a Potential Regulatory Target of Glutamate Uptake

The glutamate (Glu) transporters GLAST and GLT-1, as the two most important transporters in brain tissue, transport Glu from the extracellular space into the cell protecting against Glu toxicity. Furthermore, GLAST and GLT-1 are sodium-dependent Glu transporters (GluTs) that rely on sodium and potassium gradients generated principally by Na(+), K(+)-ATPase to generate ion gradients that drive Glu uptake. There is an interaction between Na(+), K(+)-ATPase and GluTs to modulate Glu uptake, and Na(+), K(+)-ATPase α, β or γ subunit can be directly coupled to GluTs, co-localizing with GLAST or GLT-1 in vivo to form a macromolecular complex and operate as a functional unit to regulate glutamatergic neurotransmission. Therefore, GluTs/Na(+), K(+)-ATPase may be involved in the neuroprotective effect as a potential regulatory target of Glu uptake in neurodegenerative diseases induced by Glu-mediated neurotoxicity as the final common pathway.

Functional changes in glutamate transporters and astrocyte biophysical properties in a rodent model of focal cortical dysplasia

Cortical dysplasia is associated with intractable epilepsy and developmental delay in young children. Recent work with the rat freeze-induced focal cortical dysplasia (FCD) model has demonstrated that hyperexcitability in the dysplastic cortex is due in part to higher levels of extracellular glutamate. Astrocyte glutamate transporters play a pivotal role in cortical maintaining extracellular glutamate concentrations. Here we examined the function of astrocytic glutamate transporters in a FCD model in rats. Neocortical freeze lesions were made in postnatal day (PN) 1 rat pups and whole cell electrophysiological recordings and biochemical studies were performed at PN 21–28. Synaptically evoked glutamate transporter currents in astrocytes showed a near 10-fold reduction in amplitude compared to sham operated controls. Astrocyte glutamate transporter currents from lesioned animals were also significantly reduced when challenged exogenously applied glutamate. Reduced astrocytic glutamate transport clearance contributed to increased NMDA receptor-mediated current decay kinetics in lesioned animals. The electrophysiological profile of astrocytes in the lesion group was also markedly changed compared to sham operated animals. Control astrocytes demonstrate large-amplitude linear leak currents in response to voltage-steps whereas astrocytes in lesioned animals demonstrated significantly smaller voltage-activated inward and outward currents. Significant decreases in astrocyte resting membrane potential and increases in input resistance were observed in lesioned animals. However, Western blotting, immunohistochemistry and quantitative PCR demonstrated no differences in the expression of the astrocytic glutamate transporter GLT-1 in lesioned animals relative to controls. These data suggest that, in the absence of changes in protein or mRNA expression levels, functional changes in astrocytic glutamate transporters contribute to neuronal hyperexcitability in the FCD model.

Quinine enhances the behavioral stimulant effect of cocaine in mice

The Na(+)-dependent dopamine transporter (DAT) is primarily responsible for regulating free dopamine (DA) concentrations in the brain by participating in the majority of DA uptake; however, other DA transporters may also participate, especially if cocaine or other drugs of abuse compromise DAT. Recently, such cocaine-insensitive low-affinity mono- and poly-amine OCT transporters were described in astrocytes which use DA as a substrate. These transporters are from a different transporter family and while insensitive to cocaine, they are specifically blocked by quinine and some steroids. Quinine is inexpensive and is often found in injected street drugs as an "adulterant". The present study was designed to determine the participation of OCTs in cocaine dependent behavioral and physiological changes in mice. Using FVB mice we showed, that daily single injections of quinine (10 mg/kg, i.p.) co-administered with cocaine (15 mg/kg, i.p.) for 10 days significantly enhanced cocaine-induced locomotor behavioral sensitization. Quinine had no significant effect on the time course of behavioral activation. In astrocytes from the ventral tegmental area of mice, transporter currents of quinine-sensitive monoamine transporters were also augmented after two weeks of cocaine administration. The importance of low-affinity high-capacity transporters for DA clearance is discussed, explaining the known ability of systemically administered DAT inhibitors to anomalously increase DA clearance.

Rat nucleus accumbens core astrocytes modulate reward and the motivation to self-administer ethanol after abstinence

Our understanding of the active role that astrocytes play in modulating neuronal function and behavior is rapidly expanding, but little is known about the role that astrocytes may play in drug-seeking behavior for commonly abused substances. Given that the nucleus accumbens is critically involved in substance abuse and motivation, we sought to determine whether nucleus accumbens astrocytes influence the motivation to self-administer ethanol following abstinence. We found that the packing density of astrocytes that were expressing glial fibrillary acidic protein increased in the nucleus accumbens core (NAcore) during abstinence from EtOH self-administration. No change was observed in the nucleus accumbens shell. This increased NAcore astrocyte density positively correlated with the motivation for ethanol. Astrocytes can communicate with one another and influence neuronal activity through gap-junction hemichannels. Because of this, the effect of blocking gap-junction hemichannels on the motivation for ethanol was examined. The motivation to self-administer ethanol after 3 weeks abstinence was increased following microinjection of gap-junction hemichannel blockers into the NAcore at doses that block both neuronal and astrocytic channels. In contrast, no effect was observed following microinjection of doses that are not thought to block astrocytic channels or following microinjection of either dose into the nucleus accumbens shell. Additionally, the motivation for sucrose after 3 weeks abstinence was unaffected by NAcore gap-junction hemichannel blockers. Next, Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) were selectively expressed in NAcore astrocytes to test the effect of astrocyte stimulation. DREADD activation increased cytosolic calcium in primary astrocytes, facilitated responding for rewarding brain stimulation, and reduced the motivation for ethanol after 3 weeks abstinence. This is the first work to modulate drug-seeking behavior with astrocyte-specific DREADDs. Taken together, our findings demonstrate that NAcore astrocytes can shape the motivation to self-administer ethanol; suggesting that the development of ligands which selectively stimulate astrocytes may be a successful strategy to abate ethanol-seeking behavior.

Ceftriaxone modulates uptake activity of glial glutamate transporter-1 against global brain ischemia in rats

Ceftriaxone(Cef) selectively increases the expression of glial glutamate transporter-1 (GLT-1), which was thought to be neuroprotective in some circumstances. However, the effect of Cef on glutamate uptake of GLT-1 was mostly assayed using in vitro studies such as primary neuron/astrocyte cultures or brain slices. In addition, the effect of Cef on neurons in different ischemic models was still discrepant. Therefore, this study was undertaken to observe the effect of Cef on neurons in global brain ischemia in rats, and especially to provide direct evidence of the up-regulation of GLT-1 uptake for glutamate contributing to the neuronal protection of Cef against brain ischemia. Neuropathological evaluation indicated that administration of Cef, especially pre-treatment protocols, significantly prevented delayed neuronal death in hippocampal CA1 subregion normally induced by global brain ischemia. Simultaneously, pre-administration of Cef significantly up-regulated the expression of GLT-1. Particularly, GLT-1 uptake assay with (3) H-glutamate in living cells from adult rats showed that up-regulation in glutamate uptake accompanied up-regulated GLT-1 expression. Inhibition of GLT-1 by antisense oligodeoxynucleotides or dihydrokainate significantly inhibited the Cef-induced up-regulation in GLT-1 uptake and the neuroprotective effect against global ischemia. Thus, we may conclude that Cef protects neurons against global brain ischemia via up-regulation of the expression and glutamate uptake of GLT-1. Glutamate uptake by glial glutamate transporter-1 (GLT-1) is the principal way to regulate extracellular glutamate homeostasis in central nervous system. Over-accumulation of glutamate results in excitotoxicity and injures neurons after cerebral ischemia. Ceftriaxone up-regulates GLT-1 expression and uptake of glutamate, diminishes the excitotoxicity of glutamate and then protects neurons against global brain ischemia.

Astrocyte glutamine synthetase: pivotal in health and disease

The multifunctional properties of astrocytes signify their importance in brain physiology and neurological function. In addition to defining the brain architecture, astrocytes are primary elements of brain ion, pH and neurotransmitter homoeostasis. GS (glutamine synthetase), which catalyses the ATP-dependent condensation of ammonia and glutamate to form glutamine, is an enzyme particularly found in astrocytes. GS plays a pivotal role in glutamate and glutamine homoeostasis, orchestrating astrocyte glutamate uptake/release and the glutamate-glutamine cycle. Furthermore, astrocytes bear the brunt of clearing ammonia in the brain, preventing neurotoxicity. The present review depicts the central function of astrocytes, concentrating on the importance of GS in glutamate/glutamine metabolism and ammonia detoxification in health and disease.

Changes in the expression of the glutamate transporter EAAT3/EAAC1 in health and disease

Excitatory amino acid transporters (EAATs) are high-affinity Na(+)-dependent carriers of major importance in maintaining glutamate homeostasis in the central nervous system. EAAT3, the human counterpart of the rodent excitatory amino acid carrier 1 (EAAC1), is encoded by the SLC1A1 gene. EAAT3/EAAC1 is ubiquitously expressed in the brain, mostly in neurons but also in other cell types, such as oligodendrocyte precursors. While most of the glutamate released in the synapses is taken up by the "glial-type" EAATs, EAAT2 (GLT-1 in rodents) and EAAT1 (GLAST), the functional role of EAAT3/EAAC1 is related to the subtle regulation of glutamatergic transmission. Moreover, because it can also transport cysteine, EAAT3/EAAC1 is believed to be important for the synthesis of intracellular glutathione and subsequent protection from oxidative stress. In contrast to other EAATs, EAAT3/EAAC1 is mostly intracellular, and several mechanisms have been described for the rapid regulation of the membrane trafficking of the transporter. Moreover, the carrier interacts with several proteins, and this interaction modulates transport activity. Much less is known about the slow regulatory mechanisms acting on the expression of the transporter, although several recent reports have identified changes in EAAT3/EAAC1 protein level and activity related to modulation of its expression at the gene level. Moreover, EAAT3/EAAC1 expression is altered in pathological conditions, such as hypoxia/ischemia, multiple sclerosis, schizophrenia, and epilepsy. This review summarizes these results and provides an overall picture of changes in EAAT3/EAAC1 expression in health and disease.

Glutamate involvement in calcium-dependent migration of astrocytoma cells

BACKGROUND

Astrocytoma are known to have altered glutamate machinery that results in the release of large amounts of glutamate into the extracellular space but the precise role of glutamate in favoring cancer processes has not yet been fully established. Several studies suggested that glutamate might provoke active killing of neurons thereby producing space for cancer cells to proliferate and migrate. Previously, we observed that calcium promotes disassembly of integrin-containing focal adhesions in astrocytoma, thus providing a link between calcium signaling and cell migration. The aim of this study was to determine how calcium signaling and glutamate transmission cooperate to promote enhanced astrocytoma migration.

METHODS

The wound-healing model was used to assay migration of human U87MG astrocytoma cells and allowed to monitor calcium signaling during the migration process. The effect of glutamate on calcium signaling was evaluated together with the amount of glutamate released by astrocytoma during cell migration.

RESULTS

We observed that glutamate stimulates motility in serum-starved cells, whereas in the presence of serum, inhibitors of glutamate receptors reduce migration. Migration speed was also reduced in presence of an intracellular calcium chelator. During migration, cells displayed spontaneous Ca(2+) transients. L-THA, an inhibitor of glutamate re-uptake increased the frequency of Ca(2+) oscillations in oscillating cells and induced Ca(2+) oscillations in quiescent cells. The frequency of migration-associated Ca(2+) oscillations was reduced by prior incubation with glutamate receptor antagonists or with an anti-β1 integrin antibody. Application of glutamate induced increases in internal free Ca(2+) concentration ([Ca(2+)]i). Finally we found that compounds known to increase [Ca(2+)]i in astrocytomas such as thapsigagin, ionomycin or the metabotropic glutamate receptor agonist t-ACPD, are able to induce glutamate release.

CONCLUSION

Our data demonstrate that glutamate increases migration speed in astrocytoma cells via enhancement of migration-associated Ca(2+) oscillations that in turn induce glutamate secretion via an autocrine mechanism. Thus, glutamate receptors are further validated as potential targets for astrocytoma cancer therapy.

Effects of pharmacological inhibition of AMP-activated protein kinase on GLUT3 expression and the development of ischemic tolerance in astrocytes

Ischemic tolerance resulting from preconditioning ischemia is a neuroprotective mechanism. In cultured astrocytes, its development depends on regulation of the expression of glucose transporter 3 (GLUT3) by the stress sensor/effector AMP-activated protein kinase (AMPK). Here we demonstrate that GLUT3 is upregulated during preconditioning and then downregulated during recovery. We also found that, although AMPK inhibition during preconditioning initially suppressed the upregulation of GLUT3 as shown previously, this was followed by a period of GLUT3 upregulation, enhanced glycogen accumulation, and enhanced tolerance to a subsequent ischemic challenge. These results reveal that AMPK has a complex influence on ischemic tolerance.