Maintaining the presynaptic glutamate supply for excitatory neurotransmission

Astrocyte coverage of excitatory synapses correlates to measures of synapse structure and function in ferret primary visual cortex

Most excitatory synapses in the mammalian brain are contacted or ensheathed by astrocyte processes, forming tripartite synapses. Astrocytes are thought to be critical regulators of the structural and functional dynamics of synapses. While the degree of synaptic coverage by astrocytes is known to vary across brain regions and animal species, the reason for and implications of this variability remains unknown. Further, how astrocyte coverage of synapses relates to in vivo functional properties of individual synapses has not been investigated. Here, we characterized astrocyte coverage of synapses of pyramidal neurons in the ferret visual cortex and, using correlative light and electron microscopy, examined their relationship to synaptic strength and sensory-evoked Ca2+ activity. Nearly, all synapses were contacted by astrocytes, and most were contacted along the axon-spine interface. Structurally, we found that the degree of synaptic astrocyte coverage directly scaled with synapse size and postsynaptic density complexity. Functionally, we found that the amount of astrocyte coverage scaled with how selectively a synapse responds to a particular visual stimulus and, at least for the largest synapses, scaled with the reliability of visual stimuli to evoke postsynaptic Ca2+ events. Our study shows astrocyte coverage is highly correlated with structural metrics of synaptic strength of excitatory synapses in the visual cortex and demonstrates a previously unknown relationship between astrocyte coverage and reliable sensory activation.

Subcellular functions of tau mediates repair response and synaptic homeostasis in injury

Injury responses in terminally differentiated cells such as neurons is tightly regulated by pathways aiding homeostatic maintenance. Cancer patients subjected to neuronal injury in brain radiation experience cognitive declines similar to those seen in primary neurodegenerative diseases. Numerous studies have investigated the effect of radiation in proliferating cells of the brain, yet the impact in differentiated, post-mitotic neurons, especially the structural and functional alterations remain largely elusive. We identified that microtubule-associated tau is a critical player in neuronal injury response via compartmentalized functions in both repair-centric and synaptic regulatory pathways. Ionizing radiation-induced injury acutely induces increase in phosphorylated tau in the nucleus and directly interacts with histone 2AX (H2AX), a DNA damage repair (DDR) marker. Loss of tau significantly reduced H2AX after irradiation, indicating that tau may play an important role in neuronal DDR response. We also observed that loss of tau increases eukaryotic elongation factor levels after irradiation, the latter being a positive regulator of protein translation. This cascades into a significant increase in synaptic proteins, resulting in disrupted homeostasis. Consequently, novel object recognition test showed decrease in learning and memory in tau-knockout mice after irradiation, and electroencephalographic activity showed increase in delta and theta band oscillations, often seen in dementia patients. Our findings demonstrate tau's previously undefined, multifunctional role in acute responses to injury, ranging from DDR response in the nucleus to synaptic function within a neuron. Such knowledge is vital to develop therapeutic strategies targeting neuronal injury in cognitive decline for at risk and vulnerable populations.

Astrocyte coverage of excitatory synapses correlates to measures of synapse structure and function in primary visual cortex

Most excitatory synapses in the mammalian brain are contacted by astrocytes, forming the tripartite synapse. This interface is thought to be critical for glutamate turnover and structural or functional dynamics of synapses. While the degree of synaptic contact of astrocytes is known to vary across brain regions and animal species, the implications of this variability remain unknown. Furthermore, precisely how astrocyte coverage of synapses relates to in vivo functional properties of individual dendritic spines has yet to be investigated. Here, we characterized perisynaptic astrocyte processes (PAPs) contacting synapses of pyramidal neurons of the ferret visual cortex and, using correlative light and electron microscopy, examined their relationship to synaptic strength and to sensory-evoked Ca2+ activity. Nearly all synapses were contacted by PAPs, and most were contacted along the axon-spine interface (ASI). Structurally, we found that the degree of PAP coverage scaled with synapse size and complexity. Functionally, we found that PAP coverage scaled with the selectivity of Ca2+ responses of individual synapses to visual stimuli and, at least for the largest synapses, scaled with the reliability of visual stimuli to evoke postsynaptic Ca2+ events. Our study shows astrocyte coverage is highly correlated with structural properties of excitatory synapses in the visual cortex and implicates astrocytes as a contributor to reliable sensory activation.

Ca2+-regulated expression of high affinity methylaminoisobutryic acid transport in hippocampal neurons inhibited by riluzole and novel neuroprotective aminothiazoles

High affinity methylaminoisobutyric acid(MeAIB)/glutamine(Gln) transport activity regulated by neuronal firing occurs at the plasma membrane in mature rat hippocampal neuron-enriched cultures. Spontaneous Ca2+-regulated transport activity was similarly inhibited by riluzole, a benzothiazole anticonvulsant agent, and by novel naphthalenyl substituted aminothiazole derivatives such as SKA-378. Here, we report that spontaneous transport activity is stimulated by 4-aminopyridine (4-AP) and that phorbol-myristate acetate (PMA) increases high K+ stimulated transport activity that is inhibited by staurosporine. 4-AP-stimulated spontaneous and PMA-stimulated high K+-induced transport is not present at 7 days in vitro (DIV) and is maximal by DIV∼21. The relative affinity for MeAIB is similar for spontaneous and high K+-stimulated transport (Km ∼ 50 μM) suggesting that a single transporter is involved. While riluzole and SKA-378 inhibit spontaneous transport with equal potency (IC50 ∼ 1 μM), they exhibit decreased (∼3-5 X) potency for 4-AP-stimulated spontaneous transport. Interestingly, high K+-stimulated MeAIB transport displays lower and differential sensitivity to the two compounds. SKA-378-related halogenated derivatives of SKA-75 (SKA-219, SKA-377 and SKA-375) preferentially inhibit high K+-induced expression of MeAIB transport activity at the plasma membrane (IC50 < 25 μM), compared to SKA-75 and riluzole (IC50 > 100 μM). Ca2+-dependent spontaneous and high K+-stimulated MeAIB transport activity is blocked by ω-conotoxin MVIIC, ω-agatoxin IVA, ω-agatoxin TK (IC50 ∼ 500 nM) or cadmium ion (IC50 ∼ 20 μM) demonstrating that P/Q-type CaV channels that are required for activity-regulated presynaptic vesicular glutamate (Glu) release are also required for high-affinity MeAIB transport expression at the plasma membrane. We suggest that neural activity driven and Ca2+ dependent trafficking of the high affinity MeAIB transporter to the plasma membrane is a unique target to understand mechanisms of Glu/Gln recycling in synapses and acute neuroprotection against excitotoxic presynaptic Glu induced neural injury.

EAAT5 glutamate transporter rapidly binds glutamate with micromolar affinity in mouse rods

Light responses of rod photoreceptor cells in the retina are encoded by changes in synaptic glutamate release that is in turn shaped by reuptake involving EAAT5 plasma membrane glutamate transporters. Heterologously expressed EAAT5 activates too slowly upon glutamate binding to support significant uptake. We tested EAAT5 activation in mouse rods in vivo by stimulating glutamate transporter anion currents (IA(glu)) with UV flash photolysis of MNI-glutamate, varying flash intensity to vary glutamate levels. Responses to uncaging rose rapidly with time constants of 2-3 ms, similar to IA(glu) events arising from spontaneous release. Spontaneous release events and IA(glu) evoked by weak flashes also declined with similar time constants of 40-50 ms. Stronger flashes evoked responses that decayed more slowly. Time constants were twofold faster at 35°C, suggesting that they reflect transporter kinetics, not diffusion. Selective EAAT1 and EAAT2 inhibitors had no significant effect, suggesting IA(glu) in rods arises solely from EAAT5. We calibrated glutamate levels attained during flash photolysis by expressing a fluorescent glutamate sensor iGluSnFr in cultured epithelial cells. We compared fluorescence at different glutamate concentrations to fluorescence evoked by photolytic uncaging of MNI-glutamate. The relationship between flash intensity and glutamate yielded EC50 values for EAAT5 amplitude, decay time, and rise time of ∼10 μM. Micromolar affinity and rapid activation of EAAT5 in rods show it can rapidly bind synaptic glutamate. However, we also found that EAAT5 currents are saturated by the synchronous release of only a few vesicles, suggesting limited capacity and a role for glial uptake at higher release rates.

Glial Glutamine Homeostasis in Health and Disease

Glutamine is an essential cerebral metabolite. Several critical brain processes are directly linked to glutamine, including ammonia homeostasis, energy metabolism and neurotransmitter recycling. Astrocytes synthesize and release large quantities of glutamine, which is taken up by neurons to replenish the glutamate and GABA neurotransmitter pools. Astrocyte glutamine hereby sustains the glutamate/GABA-glutamine cycle, synaptic transmission and general brain function. Cerebral glutamine homeostasis is linked to the metabolic coupling of neurons and astrocytes, and relies on multiple cellular processes, including TCA cycle function, synaptic transmission and neurotransmitter uptake. Dysregulations of processes related to glutamine homeostasis are associated with several neurological diseases and may mediate excitotoxicity and neurodegeneration. In particular, diminished astrocyte glutamine synthesis is a common neuropathological component, depriving neurons of an essential metabolic substrate and precursor for neurotransmitter synthesis, hereby leading to synaptic dysfunction. While astrocyte glutamine synthesis is quantitatively dominant in the brain, oligodendrocyte-derived glutamine may serve important functions in white matter structures. In this review, the crucial roles of glial glutamine homeostasis in the healthy and diseased brain are discussed. First, we provide an overview of cellular recycling, transport, synthesis and metabolism of glutamine in the brain. These cellular aspects are subsequently discussed in relation to pathological glutamine homeostasis of hepatic encephalopathy, epilepsy, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the multifaceted roles of cerebral glutamine will not only increase our understanding of the metabolic collaboration between brain cells, but may also aid to reveal much needed therapeutic targets of several neurological pathologies.

Mitochondrial dysfunction in cognitive neurodevelopmental disorders: Cause or effect?

Mitochondria have a crucial role in brain development and neurogenesis, both in embryonic and adult brains. Since the brain is the highest energy consuming organ, it is highly vulnerable to mitochondrial dysfunction. This has been implicated in a range of brain disorders including, neurodevelopmental conditions, psychiatric illnesses, and neurodegenerative diseases. Genetic variations in mitochondrial DNA (mtDNA), and nuclear DNA encoding mitochondrial proteins, have been associated with several cognitive disorders. However, it is not yet clear whether mitochondrial dysfunction is a primary cause of these conditions or a secondary effect. Our review article deals with this topic, and brings out recent advances in mitochondria-oriented therapies. Mitochondrial dysfunction could be involved in the pathogenesis of a subset of disorders involving cognitive impairment. In these patients, mitochondrial dysfunction could be the cause of the condition, rather than the consequence. There are vast areas in this topic that remains to be explored and elucidated.

Riluzole and novel naphthalenyl substituted aminothiazole derivatives prevent acute neural excitotoxic injury in a rat model of temporal lobe epilepsy

Epileptogenic seizures, or status epilepticus (SE), leads to excitotoxic injury in hippocampal and limbic neurons in the kainic acid (KA) animal model of temporal lobe epilepsy (TLE). Here, we have further characterized neural activity regulated methylaminoisobutryic acid (MeAIB)/glutamine transport activity in mature rat hippocampal neurons in vitro that is inhibited by riluzole (IC₅₀ = 1 μM), an anti-convulsant benzothiazole agent. We screened a library of riluzole derivatives and identified SKA-41 followed by a second screen and synthesized several novel chlorinated aminothiazoles (SKA-377, SKA-378, SKA-379) that are also potent MeAIB transport inhibitors in vitro, and brain penetrant following systemic administration. When administered before KA, SKA-378 did not prevent seizures but still protected the hippocampus and several other limbic areas against SE-induced neurodegeneration at 3d. When SKA-377 - 379, (30 mg/kg) were administered after KA-induced SE, acute neural injury in the CA3, CA1 and CA4/hilus was also largely attenuated. Riluzole (10 mg/kg) blocks acute neural injury. Kinetic analysis of SKA-378 and riluzoles' blockade of Ca²⁺-regulated MeAIB transport in neurons in vitro indicates that inhibition occurs via a non-competitive, indirect mechanism. Sodium channel Na_V1.6 antagonism blocks neural activity regulated MeAIB/Gln transport in vitro (IC₅₀ = 60 nM) and SKA-378 is the most potent inhibitor of Na_V1.6 (IC₅₀ = 28 μM) compared to Na_V1.2 (IC₅₀ = 118 μM) in heterologous cells. However, pharmacokinetic analysis suggests that sodium channel blockade may not be the predominant mechanism of neuroprotection here. Riluzole and our novel aminothiazoles are agents that attenuate acute neural hippocampal injury following KA-induced SE and may help to understand mechanisms involved in the progression of epileptic disease.

Aryl Hydrocarbon Receptor in Glia Cells: A Plausible Glutamatergic Neurotransmission Orchestrator

Glutamate is the major excitatory amino acid in the vertebrate brain. Glutamatergic signaling is involved in most of the central nervous system functions. Its main components, namely receptors, ion channels, and transporters, are tightly regulated at the transcriptional, translational, and post-translational levels through a diverse array of extracellular signals, such as food, light, and neuroactive molecules. An exquisite and well-coordinated glial/neuronal bidirectional communication is required for proper excitatory amino acid signal transactions. Biochemical shuttles such as the glutamate/glutamine and the astrocyte-neuronal lactate represent the fundamental involvement of glial cells in glutamatergic transmission. In fact, the disruption of any of these coordinated biochemical intercellular cascades leads to an excitotoxic insult that underlies some aspects of most of the neurodegenerative diseases characterized thus far. In this contribution, we provide a comprehensive summary of the involvement of the Aryl hydrocarbon receptor, a ligand-dependent transcription factor in the gene expression regulation of glial glutamate transporters. These receptors might serve as potential targets for the development of novel strategies for the treatment of neurodegenerative diseases.

Contribution of cyclooxygenase-2 overexpression to enhancement in tonically active glutamatergic inputs to the rostral ventrolateral medulla in hypertension

OBJECTIVE

Cyclooxygenase (COX) is critical in regulating cardiovascular function, but its role involved in the central control of blood pressure (BP) is uncovered. The tonic glutamatergic inputs to the rostral ventrolateral medulla (RVLM) are enhanced in hypertension. Here, the present study was designed to investigate the effect and mechanism of central COX on tonic glutamatergic inputs to the RVLM and BP regulation.

METHODS

Wistar-Kyoto (WKY) rats and spontaneous hypertensive rats (SHRs) received RVLM microinjection of adeno-associated viral vectors to promote or inhibit the COX2 expression were subjected to subsequent experiments. Glutamate level and glutaminase expression were detected by ELISA and western blot, respectively. The function of tonic glutamatergic inputs was assessed by BP response to microinjection of the glutamate receptor antagonist into the RVLM. PC12 cells were used to detect the underlying signal pathway.

RESULTS

The RVLM COX2 expression and prostaglandin E2 level were significant higher in SHRs than in WKY rats. Overexpression of COX2 in the RVLM produced an increase in basal BP, RVLM glutamate level, and glutaminase expression in WKY rats, while they were significantly reduced by interfering with COX2 expression in SHRs. Microinjections of the glutamate receptor antagonist into the RVLM produced a significant BP decrease in WKY rats with COX2 overexpression pretreatment. Furthermore, the increased levels of BP, glutamate content, and glutaminase activity in the RVLM evoked by central infusion of angiotensin II were attenuated in COX2 knockout mice. It was also found that prostaglandin E2 increased supernatant glutamate level and phosphorylation of signal transducer and activator of transcription 3 in PC12 cells.

CONCLUSION

Our findings suggest that upregulated COX2 expression enhances the tonically active glutamatergic inputs to the RVLM, which is associated with cardiovascular regulation in hypertension.

Hypoxia-triggered O-GlcNAcylation in the brain drives the glutamate-glutamine cycle and reduces sensitivity to sevoflurane in mice

BACKGROUND

Hypersensitivity to general anaesthetics predicts adverse postoperative outcomes in patients. Hypoxia exerts extensive pathophysiological effects on the brain; however, whether hypoxia influences sevoflurane sensitivity and its underlying mechanisms remain poorly understood.

METHODS

Mice were acclimated to hypoxia (oxygen 10% for 8 h day^-1) for 28 days and anaesthetised with sevoflurane; the effective concentrations for 50% of the animals (EC₅₀) showing loss of righting reflex (LORR) and loss of tail-pinch withdrawal response (LTWR) were determined. Positron emission tomography-computed tomography, O-glycoproteomics, seahorse analysis, carbon-13 tracing, site-specific mutagenesis, and electrophysiological techniques were performed to explore the underlying mechanisms.

RESULTS

Compared with the control group, the hypoxia-acclimated mice required higher concentrations of sevoflurane to present LORR and LTWR (EC50_LORR: 1.61 [0.03]% vs 1.46 [0.04]%, P<0.01; EC50_LTWR: 2.46 [0.14]% vs 2.22 [0.06]%, P<0.01). Hypoxia-induced reduction in sevoflurane sensitivity was correlated with elevation of protein O-linked N-acetylglucosamine (O-GlcNAc) modification in brain, especially in the thalamus, and could be abolished by 6-diazo-5-oxo-l-norleucine, a glutamine fructose-6-phosphate amidotransferase inhibitor, and mimicked by thiamet-G, a selective O-GlcNAcase inhibitor. Mechanistically, O-GlcNAcylation drives de novo synthesis of glutamine from glucose in astrocytes and promotes the glutamate-glutamine cycle, partially via glycolytic flux and activation of glutamine synthetase.

CONCLUSIONS

Intermittent hypoxia exposure decreased mouse sensitivity to sevoflurane anaesthesia through enhanced O-GlcNAc-dependent modulation of the glutamate-glutamine cycle in the brain.

Contributions of Astrocyte and Neuronal Volume to CA1 Neuron Excitability Changes in Elevated Extracellular Potassium

Rapid increases in cell volume reduce the size of the extracellular space (ECS) and are associated with elevated brain tissue excitability. We recently demonstrated that astrocytes, but not neurons, rapidly swell in elevated extracellular potassium (^∧[K⁺]_o) up to 26 mM. However, effects of acute astrocyte volume fluctuations on neuronal excitability in ^∧[K⁺]_o have been difficult to evaluate due to direct effects on neuronal membrane potential and generation of action potentials. Here we set out to isolate volume-specific effects occurring in ^∧[K⁺]_o on CA1 pyramidal neurons in acute hippocampal slices by manipulating cell volume while recording neuronal glutamate currents in 10.5 mM [K⁺]_o + tetrodotoxin (TTX) to prevent neuronal firing. Elevating [K⁺]_o to 10.5 mM induced astrocyte swelling and produced significant increases in neuronal excitability in the form of mixed α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/N-methyl-D-aspartate (NMDA) receptor mEPSCs and NMDA receptor-dependent slow inward currents (SICs). Application of hyperosmolar artificial cerebrospinal fluid (ACSF) by addition of mannitol in the continued presence of 10.5 mM K⁺ forced shrinking of astrocytes and to a lesser extent neurons, which resisted swelling in ^∧[K⁺]_o. Cell shrinking and dilation of the ECS significantly dampened neuronal excitability in 10.5 mM K⁺. Subsequent removal of mannitol amplified effects on neuronal excitability and nearly doubled the volume increase in astrocytes, presumably due to continued glial uptake of K⁺ while mannitol was present. Slower, larger amplitude events mainly driven by NMDA receptors were abolished by mannitol-induced expansion of the ECS. Collectively, our findings suggest that cell volume regulation of the ECS in elevated [K⁺]_o is driven predominantly by astrocytes, and that cell volume effects on neuronal excitability can be effectively isolated in elevated [K⁺]_o conditions.

Physiological synaptic activity and recognition memory require astroglial glutamine

Presynaptic glutamate replenishment is fundamental to brain function. In high activity regimes, such as epileptic episodes, this process is thought to rely on the glutamate-glutamine cycle between neurons and astrocytes. However the presence of an astroglial glutamine supply, as well as its functional relevance in vivo in the healthy brain remain controversial, partly due to a lack of tools that can directly examine glutamine transfer. Here, we generated a fluorescent probe that tracks glutamine in live cells, which provides direct visual evidence of an activity-dependent glutamine supply from astroglial networks to presynaptic structures under physiological conditions. This mobilization is mediated by connexin43, an astroglial protein with both gap-junction and hemichannel functions, and is essential for synaptic transmission and object recognition memory. Our findings uncover an indispensable recruitment of astroglial glutamine in physiological synaptic activity and memory via an unconventional pathway, thus providing an astrocyte basis for cognitive processes.

The authors present a fluorescent probe that tracks glutamine in live cells. They demonstrate the capabilities of the probe by providing direct visual evidence of an activity-dependent glutamine supply from astroglial networks to presynaptic structures under physiological conditions.

Ion dynamics at the energy-deprived tripartite synapse

The anatomical and functional organization of neurons and astrocytes at 'tripartite synapses' is essential for reliable neurotransmission, which critically depends on ATP. In low energy conditions, synaptic transmission fails, accompanied by a breakdown of ion gradients, changes in membrane potentials and cell swelling. The resulting cellular damage and cell death are causal to the often devastating consequences of an ischemic stroke. The severity of ischemic damage depends on the age and the brain region in which a stroke occurs, but the reasons for this differential vulnerability are far from understood. In the present study, we address this question by developing a comprehensive biophysical model of a glutamatergic synapse to identify key determinants of synaptic failure during energy deprivation. Our model is based on fundamental biophysical principles, includes dynamics of the most relevant ions, i.e., Na+, K+, Ca2+, Cl- and glutamate, and is calibrated with experimental data. It confirms the critical role of the Na+/K+-ATPase in maintaining ion gradients, membrane potentials and cell volumes. Our simulations demonstrate that the system exhibits two stable states, one physiological and one pathological. During energy deprivation, the physiological state may disappear, forcing a transit to the pathological state, which can be reverted when blocking voltage-gated Na+ and K+ channels. Our model predicts that the transition to the pathological state is favoured if the extracellular space fraction is small. A reduction in the extracellular space volume fraction, as, e.g. observed with ageing, will thus promote the brain's susceptibility to ischemic damage. Our work provides new insights into the brain's ability to recover from energy deprivation, with translational relevance for diagnosis and treatment of ischemic strokes.

Huntingtin and the Synapse

Huntington disease (HD) is a monogenic disease that results in a combination of motor, psychiatric and cognitive symptoms. HD is caused by a CAG trinucleotide repeat expansion in the huntingtin (HTT) gene, which results in the production of a pathogenic mutant HTT protein (mHTT). Although there is no cure at present for HD, a number of RNA-targeting therapies have recently entered clinical trials which aim to lower mHTT production through the use of antisense oligonucleotides (ASOs) and RNAi. However, many of these treatment strategies are non-selective in that they cannot differentiate between non-pathogenic wild type HTT (wtHTT) and the mHTT variant. As HD patients are already born with decreased levels of wtHTT, these genetic therapies may result in critically low levels of wtHTT. The consequence of wtHTT reduction in the adult brain is currently under debate, and here we argue that wtHTT loss is not well-tolerated at the synaptic level. Synaptic dysfunction is an extremely sensitive measure of subsequent cell death, and is known to precede neurodegeneration in numerous brain diseases including HD. The present review focuses on the prominent role of wtHTT at the synapse and considers the consequences of wtHTT loss on both pre- and postsynaptic function. We discuss how wtHTT is implicated in virtually all major facets of synaptic neurotransmission including anterograde and retrograde transport of proteins to/from terminal buttons and dendrites, neurotransmitter release, endocytic vesicle recycling, and postsynaptic receptor localization and recycling. We conclude that wtHTT presence is essential for proper synaptic function.

Glutamate homeostasis and dopamine signaling: Implications for psychostimulant addiction behavior

Cocaine, amphetamine, and methamphetamine abuse disorders are serious worldwide health problems. To date, there are no FDA-approved medications for the treatment of these disorders. Elucidation of the biochemical underpinnings contributing to psychostimulant addiction is critical for the development of effective therapies. Excitatory signaling and glutamate homeostasis are well known pathophysiological substrates underlying addiction-related behaviors spanning multiple types of psychostimulants. To alleviate relapse behavior to psychostimulants, considerable interest has focused on GLT-1, the major glutamate transporter in the brain. While many brain regions are implicated in addiction behavior, this review focuses on two regions well known for their role in mediating the effects of cocaine and amphetamines, namely the nucleus accumbens (NAc) and the ventral tegmental area (VTA). In addition, because many investigators have utilized Cre-driver lines to selectively control gene expression in defined cell populations relevant for psychostimulant addiction, we discuss potential off-target effects of Cre-recombinase that should be considered in the design and interpretation of such experiments.

MECP2 and the Biology of MECP2 Duplication Syndrome

MECP2 duplication syndrome (MDS), a rare X-linked genomic disorder affecting predominantly males, is caused by duplication of the chromosomal region containing the methyl CpG binding protein-2 (MECP2) gene, which encodes methyl-CpG-binding protein 2 (MECP2), a multi-functional protein required for proper brain development and maintenance of brain function during adulthood. Disease symptoms include severe motor and cognitive impairment, delayed or absent speech development, autistic features, seizures, ataxia, recurrent respiratory infections and shortened lifespan. The cellular and molecular mechanisms by which a relatively modest increase in MECP2 protein causes such severe disease symptoms are poorly understood and consequently there are no treatments available for this fatal disorder. This review summarizes what is known to date about the structure and complex regulation of MECP2 and its many functions in the developing and adult brain. Additionally, recent experimental findings on the cellular and molecular underpinnings of MDS based on cell culture and mouse models of the disorder are reviewed. The emerging picture from these studies is that MDS is a neurodegenerative disorder in which neurons die in specific parts of the central nervous system, including the cortex, hippocampus, cerebellum and spinal cord. Neuronal death likely results from astrocytic dysfunction, including a breakdown of glutamate homeostatic mechanisms. The role of elevations in the expression of glial acidic fibrillary protein (GFAP) in astrocytes and the microtubule-associated protein, Tau, in neurons to the pathogenesis of MDS is discussed. Lastly, potential therapeutic strategies to potentially treat MDS are discussed.

Herpes Simplex Virus Type 1 Neuronal Infection Triggers the Disassembly of Key Structural Components of Dendritic Spines

Herpes simplex virus type 1 (HSV-1) is a widespread neurotropic virus. Primary infection of HSV-1 in facial epithelium leads to retrograde axonal transport to the central nervous system (CNS) where it establishes latency. Under stressful conditions, the virus reactivates, and new progeny are transported anterogradely to the primary site of infection. During the late stages of neuronal infection, axonal damage can occur, however, the impact of HSV-1 infection on the morphology and functional integrity of neuronal dendrites during the early stages of infection is unknown. We previously demonstrated that acute HSV-1 infection in neuronal cell lines selectively enhances Arc protein expression - a major regulator of long-term synaptic plasticity and memory consolidation, known for being a protein-interaction hub in the postsynaptic dendritic compartment. Thus, HSV-1 induced Arc expression may alter the functionality of infected neurons and negatively impact dendritic spine dynamics. In this study we demonstrated that HSV-1 infection induces structural disassembly and functional deregulation in cultured cortical neurons, an altered glutamate response, Arc accumulation within the somata, and decreased expression of spine scaffolding-like proteins such as PSD-95, Drebrin and CaMKIIβ. However, whether these alterations are specific to the HSV-1 infection mechanism or reflect a secondary neurodegenerative process remains to be determined.

Molecular, Structural, Functional, and Pharmacological Sites for Vesicular Glutamate Transporter Regulation

Vesicular glutamate transporters (VGLUTs) control quantal size of glutamatergic transmission and have been the center of numerous studies over the past two decades. VGLUTs contain two independent transport modes that facilitate glutamate packaging into synaptic vesicles and phosphate (Pi) ion transport into the synaptic terminal. While a transmembrane proton electrical gradient established by a vacuolar-type ATPase powers vesicular glutamate transport, recent studies indicate that binding sites and flux properties for chloride, potassium, and protons within VGLUTs themselves regulate VGLUT activity as well. These intrinsic ionic binding and flux properties of VGLUTs can therefore be modulated by neurophysiological conditions to affect levels of glutamate available for release from synapses. Despite their extraordinary importance, specific and high-affinity pharmacological compounds that interact with these sites and regulate VGLUT function, distinguish between the various modes of transport, and the different isoforms themselves, are lacking. In this review, we provide an overview of the physiologic sites for VGLUT regulation that could modulate glutamate release in an over-active synapse or in a disease state.

Modulation on Glutamic Pathway of Frontal-Striatum-Thalamus by rs11146020 and rs3813296 Gene Polymorphism in First-Episode Negative Schizophrenia

Objectives

The frontal-striatum-thalamus pathway is important in the glutamic neural circuit. The hypofunction of GRIN1 and GRIA2 subunits from glutamic receptors has been hypothesized as the primary process in the etiology of schizophrenia. Identified gene polymorphism involved in the pathogenesis of schizophrenia may uncover relevant mechanism pathways.

Methods

We selected two loci of rs11146020 and rs3813296 distributed in GRIN1 and GRIA2 genes and tested their main and interaction effects on causality connections and structural characteristics in the frontal-striatum-thalamus pathway in 55 Han Chinese first-episode negative schizophrenia patients.

Results

We found that: (1) rs11146020 has a significant main effect on the causality connections between the bilateral dorsolateral prefrontal cortex, and rs3813296 mainly influences those of the descending pathway from the prefrontal cortex to the striatum; (2) interaction effect of rs11146020 and rs3813296 on causality connections are located in the ascending pathway from the pallidum to the dorsolateral prefrontal cortex; and (3) the two loci have effects on the volumes of several regions of this pathway.

Conclusion

Our results suggested there is modulation on glutamic frontal-striatum-thalamus pathway by rs11146020 and rs3813296 gene polymorphism. Patients with different genotypes have different neuroimaging characteristics, which indirectly reminded clinicians those patients should receive different clinical interventions.

GLAST Activity is Modified by Acute Manganese Exposure in Bergmann Glial Cells

Glutamate is the major excitatory amino acid neurotransmitter in the vertebrate brain. It exerts its actions through the activation of specific plasma membrane receptors expressed in neurons and glial cells. Overactivation of glutamate receptors results in neuronal death, known as excitotoxicity. A family of sodium-dependent glutamate transporters enriched in glial cells are responsible of the vast majority of the removal of this amino acid form the synaptic cleft. Therefore, a precise and exquisite regulation of these proteins is required not only for a proper glutamatergic transmission but also for the prevention of an excitotoxic insult. Manganese is a trace element essential as a cofactor for several enzymatic systems, although in high concentrations is involved in the disruption of brain glutamate homeostasis. The molecular mechanisms associated to manganese neurotoxicity have been focused on mitochondrial function, although energy depletion severely compromises the glutamate uptake process. In this context, in this contribution we analyze the effect of manganese exposure in glial glutamate transporters function. To this end, we used the well-established model of chick cerebellar Bergmann glia cultures. A time and dose dependent modulation of [³H]-D-aspartate uptake was found. An increase in the transporter catalytic efficiency, most probably linked to a discrete increase in the affinity of the transporter was detected upon manganese exposure. Interestingly, glucose uptake was reduced by this metal. These results favor the notion of a direct effect of manganese on glial cells, this in turn alters their coupling with neurons and might lead to changes in glutamatergic transmission.

Glutaminase activity in GLS1 Het mouse brain compared to putative pharmacological inhibition by ebselen using ex vivo MRS

Glutaminase mediates the recycling of neurotransmitter glutamate, supporting most excitatory neurotransmission in the mammalian central nervous system. A constitutive heterozygous reduction in GLS1 engenders in mice a model of schizophrenia resilience and associated increases in Gln, reductions in Glu and activity-dependent attenuation of excitatory synaptic transmission. Hippocampal brain slices from GLS1 heterozygous mice metabolize less Gln to Glu. Whether glutaminase activity is diminished in the intact brain in GLS1 heterozygous mice has not been assessed, nor the regional impact. Moreover, it is not known whether pharmacological inhibition would mimic the genetic reduction. We addressed this using magnetic resonance spectroscopy to assess amino acid content and ¹³C-acetate loading to assess glutaminase activity, in multiple brain regions. Glutaminase activity was reduced significantly in the hippocampus of GLS1 heterozygous mice, while acute treatment with the putative glutaminase inhibitor ebselen did not impact glutaminase activity, but did significantly increase GABA. This approach identifies a molecular imaging strategy for testing target engagement by comparing genetic and pharmacological inhibition, across brain regions.

Brain Glucose Metabolism: Integration of Energetics with Function

Glucose is the long-established, obligatory fuel for brain that fulfills many critical functions, including ATP production, oxidative stress management, and synthesis of neurotransmitters, neuromodulators, and structural components. Neuronal glucose oxidation exceeds that in astrocytes, but both rates increase in direct proportion to excitatory neurotransmission; signaling and metabolism are closely coupled at the local level. Exact details of neuron-astrocyte glutamate-glutamine cycling remain to be established, and the specific roles of glucose and lactate in the cellular energetics of these processes are debated. Glycolysis is preferentially upregulated during brain activation even though oxygen availability is sufficient (aerobic glycolysis). Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in excess of oxygen, and adrenergic regulation of aerobic glycolysis draws attention to astrocytic metabolism, particularly glycogen turnover, which has a high impact on the oxygen-carbohydrate mismatch. Aerobic glycolysis is proposed to be predominant in young children and specific brain regions, but re-evaluation of data is necessary. Shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation, neurotransmission, and memory consolidation are controversial topics for which alternative mechanisms are proposed. Nutritional therapy and vagus nerve stimulation are translational bridges from metabolism to clinical treatment of diverse brain disorders.

Neurointegrity and neurophysiology: astrocyte, glutamate, and carbon monoxide interactions

Astrocyte contributions to brain function and prevention of neuropathologies are as extensive as that of neurons. Astroglial regulation of glutamate, a primary neurotransmitter, is through uptake, release through vesicular and non-vesicular pathways, and catabolism to intermediates. Homeostasis by astrocytes is considered to be of primary importance in determining normal central nervous system health and central nervous system physiology - glutamate is central to dynamic physiologic changes and central nervous system stability. Gasotransmitters may affect diverse glutamate interactions positively or negatively. The effect of carbon monoxide, an intrinsic central nervous system gasotransmitter, in the complex astrocyte homeostasis of glutamate may offer insights to normal brain development, protection, and its use as a neuromodulator and neurotherapeutic. In this article, we will review the effects of carbon monoxide on astrocyte homeostasis of glutamate.

Regenerative glutamate release in the hippocampus of Rett syndrome model mice

Excess glutamate during intense neuronal activity is not instantly cleared and may accumulate in the extracellular space. This has various long-term consequences such as ectopic signaling, modulation of synaptic efficacy and excitotoxicity; the latter implicated in various neurodevelopmental and neurodegenerative diseases. In this study, the quantitative imaging of glutamate homeostasis of hippocampal slices from methyl-CpG binding protein 2 knock-out (Mecp2-/y) mice, a model of Rett syndrome (RTT), revealed unusual repetitive glutamate transients. They appeared in phase with bursts of action potentials in the CA1 neurons. Both glutamate transients and bursting activity were suppressed by the blockade of sodium, AMPA and voltage-gated calcium channels (T- and R-type), and enhanced after the inhibition of HCN channels. HCN and calcium channels in RTT and wild-type (WT) CA1 neurons displayed different voltage-dependencies and kinetics. Both channels modulated postsynaptic integration and modified the pattern of glutamate spikes in the RTT hippocampus. Spontaneous glutamate transients were much less abundant in the WT preparations, and, when observed, had smaller amplitude and frequency. The basal ambient glutamate levels in RTT were higher and transient glutamate increases (spontaneous and evoked by stimulation of Schaffer collaterals) decayed slower. Both features indicate less efficient glutamate uptake in RTT. To explain the generation of repetitive glutamate spikes, we designed a novel model of glutamate-induced glutamate release. The simulations correctly predicted the patterns of spontaneous glutamate spikes observed under different experimental conditions. We propose that pervasive spontaneous glutamate release is a hallmark of Mecp2-/y hippocampus, stemming from and modulating the hyperexcitability of neurons.

PKC-Mediated Modulation of Astrocyte SNAT3 Glutamine Transporter Function at Synapses in Situ

Astrocytes are glial cells that have an intimate physical and functional association with synapses in the brain. One of their main roles is to recycle the neurotransmitters glutamate and gamma-aminobutyric acid (GABA), as a component of the glutamate/GABA-glutamine cycle. They perform this function by sequestering neurotransmitters and releasing glutamine via the neutral amino acid transporter SNAT3. In this way, astrocytes regulate the availability of neurotransmitters and subsequently influence synaptic function. Since many plasma membrane transporters are regulated by protein kinase C (PKC), the aim of this study was to understand how PKC influences SNAT3 glutamine transport in astrocytes located immediately adjacent to synapses. We studied SNAT3 transport by whole-cell patch-clamping and fluorescence pH imaging of single astrocytes in acutely isolated brainstem slices, adjacent to the calyx of the Held synapse. Activation of SNAT3-mediated glutamine transport in these astrocytes was reduced to 77 ± 6% when PKC was activated with phorbol 12-myristate 13-acetate (PMA). This effect was very rapid (within ~20 min) and eliminated by application of bisindolylmaleimide I (Bis I) or 7-hydroxystaurosporine (UCN-01), suggesting that activation of conventional isoforms of PKC reduces SNAT3 function. In addition, cell surface biotinylation experiments in these brain slices show that the amount of SNAT3 in the plasma membrane is reduced by a comparable amount (to 68 ± 5%) upon activation of PKC. This indicates a role for PKC in dynamically controlling the trafficking of SNAT3 transporters in astrocytes in situ. These data demonstrate that PKC rapidly regulates the astrocytic glutamine release mechanism, which would influence the glutamine availability for adjacent synapses and control levels of neurotransmission.

The role of glutamate transporters in the pathophysiology of neuropsychiatric disorders

Altered glutamate transporter expression is a common feature of many neuropsychiatric conditions, including schizophrenia. Excitatory amino acid transporters (EAATs) are responsible for the reuptake of glutamate, preventing non-physiological spillover from the synapse. Postmortem studies have revealed significant dysregulation of EAAT expression in various brain regions at the cellular and subcellular level. Recent animal studies have also demonstrated a role for glutamate spillover as a mechanism of disease. In this review, we describe current evidence for the role of glutamate transporters in regulating synaptic plasticity and transmission. In neuropsychiatric conditions, EAAT splice variant expression is altered. There are changes in the localization of the transporters and disruption of the metabolic and structural protein network that supports EAAT activity. This results in aberrant neuroplasticity and excitatory signaling, contributing to the symptoms associated with neuropsychiatric disease. Understanding the complex functions of glutamate transporters will clarify the relevance of their role in the pathophysiology of neuropsychiatric disorders.

Dopamine neuron dependent behaviors mediated by glutamate cotransmission

Dopamine neurons in the ventral tegmental area use glutamate as a cotransmitter. To elucidate the behavioral role of the cotransmission, we targeted the glutamate-recycling enzyme glutaminase (gene Gls1). In mice with a dopamine transporter (Slc6a3)-driven conditional heterozygous (cHET) reduction of Gls1 in their dopamine neurons, dopamine neuron survival and transmission were unaffected, while glutamate cotransmission at phasic firing frequencies was reduced, enabling a selective focus on the cotransmission. The mice showed normal emotional and motor behaviors, and an unaffected response to acute amphetamine. Strikingly, amphetamine sensitization was reduced and latent inhibition potentiated. These behavioral effects, also seen in global GLS1 HETs with a schizophrenia resilience phenotype, were not seen in mice with an Emx1-driven forebrain reduction affecting most brain glutamatergic neurons. Thus, a reduction in dopamine neuron glutamate cotransmission appears to mediate significant components of the GLS1 HET schizophrenia resilience phenotype, and glutamate cotransmission appears to be important in attribution of motivational salience.

Functional identification of activity-regulated, high-affinity glutamine transport in hippocampal neurons inhibited by riluzole

Glutamine (Gln) is considered the preferred precursor for the neurotransmitter pool of glutamate (Glu), the major excitatory transmitter in the mammalian CNS. Here, an activity-regulated, high-affinity Gln transport system is described in developing and mature neuron-enriched hippocampal cultures that is potently inhibited by riluzole (IC₅₀ 1.3 ± 0.5 μM), an anti-glutamatergic drug, and is blocked by low concentrations of 2-(methylamino)isobutyrate (MeAIB), a system A transport inhibitor. K⁺ -stimulated MeAIB transport displays an affinity (K_m ) for MeAIB of 37 ± 1.2 μM, saturates at ~ 200 μM, is dependent on extracellular Ca²⁺ , and is blocked by inhibition of voltage-gated Ca²⁺ channels. Spontaneous MeAIB transport is also dependent on extracellullar Ca²⁺ and voltage-gated calcium channels, but is also blocked by the Na⁺ channel blocker tetrodotoxin, by Glu receptor antagonists, and by GABA indicating its dependence on intact neural circuits driven by endogenous glutamatergic activity. The transport of MeAIB itself does not rely on Ca²⁺ , but on Na⁺ ions, and is pH sensitive. Activity-regulated, riluzole-sensitive spontaneous and K⁺ -stimulated transport is minimal at 7-8 days in vitro, coordinately induced during the next 2 weeks and is maximally expressed by days in vitro > 20; the known period for maturation of the Glu/Gln cycle and regulated pre-synaptic Glu release. Competition analyses with various amino acids indicate that Gln is the most likely physiological substrate. Activity-regulated Gln/MeAIB transport is not observed in astrocytes. The functional identification of activity-regulated, high-affinity, riluzole-sensitive Gln/MeAIB transport in hippocampal neurons may have important ramifications in the neurobiology of activity-stimulated pre-synaptic Glu release, the Glu/Gln cycle between astrocytes and neurons, and neuronal Glu-induced excitotoxicity. Cover Image for this issue: doi: 10.1111/jnc.13805.

The conversion of glutamate by glutamine synthase in neocortical astrocytes from juvenile rat is important to limit glutamate spillover and peri/extrasynaptic activation of NMDA receptors

Glutamate transporters (EAATs) are important to maintain spatial and temporal specificity of synaptic transmission. Their efficiency to uptake and transport glutamate into the intracellular space depends on several parameters including the intracellular concentrations of Na⁺ and glutamate, the elevations of which may slow down the cycling rate of EAATs. In astrocytes, glutamate is maintained at low concentration due to the presence of specific enzymes such as glutamine synthase (GS). GS inhibition results in cytosolic accumulation of glutamate suggesting that the conversion of glutamate by GS is important for EAATs operation. Here we recorded astrocytes from juvenile rat neocortical slices and analyzed the consequences of elevated intracellular glutamate concentrations and of GS inhibition on the time course of synaptically evoked transporter current (STC). In slices from rats treated with methionine sulfoximine (MSO), a GS inhibitor, STC evoked by short burst of high frequency stimulation (HFS; 100 Hz for 100 ms) but not by low frequency stimulation (LFS; 0.1 Hz) was twice slower than STC evoked from saline injected rats. Same results were obtained for astrocytes recorded with pipette containing 3-10 mM glutamate and compared with cells recorded with 0 or1 mM glutamate in the patch pipette. We also showed that HFS elicited significantly larger NMDAR-excitatory postsynaptic currents (EPSCs) with a stronger peri/extrasynaptic component in pyramidal cells from MSO-treated compared with saline treated rats. Taken together our data demonstrate that the conversion of glutamate by GS is fundamental to ensure an efficient clearance of glutamate by EAATs and to prevent glutamate spillover. GLIA 2017;65:401-415.

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.

Functional magnetic resonance spectroscopy of glutamate in schizophrenia and major depressive disorder: anterior cingulate activity during a color-word Stroop task

Background:

Glutamate abnormalities have been suggested to be associated with symptoms of schizophrenia. Using functional magnetic resonance spectroscopy (¹H-fMRS), it is possible to monitor glutamate dynamically in the activated brain areas, which has yet to be reported in schizophrenia. It was hypothesized that subjects with schizophrenia would have weaker glutamatergic responses in the anterior cingulate to a color-word Stroop Task.

AIMS:

The aim of this study was to gain insight into the health of GLU neurotransmission and the GLU-GLN cycle in SZ using a ¹H-fMRS protocol.

Methods:

Spectra were acquired from the anterior cingulate of 16 participants with schizophrenia, 16 healthy controls and 16 participants with major depressive disorder (MDD) while performing the Stroop task in a 7T magnetic resonance imaging scanner. ¹H-fMRS spectra were acquired for 20 min in which there were three 4-min blocks of cross fixation interleaved with two 4-min blocks of the Stroop paradigm.

Results:

A repeated-measures analysis of variance revealed a main effect of time for glutamate concentrations of all groups (P<0.001). The healthy control group increased glutamate concentrations in the first run of the Stroop task (P=0.006) followed by a decrease in the recovery period (P=0.007). Neither the schizophrenia (P=0.107) nor MDD (P=0.081) groups had significant glutamate changes in the first run of the task, while the schizophrenia group had a significant increase in glutamine (P=0.005). The MDD group decreased glutamate concentrations in the second run of the task (P=0.003), as did all the groups combined (P=0.003).

Conclusions:

¹H-fMRS data were successfully acquired from psychiatric subjects with schizophrenia and mood disorder using a cognitive paradigm for the first time. Future study designs should further elucidate the glutamatergic response to functional activation in schizophrenia.

Alterations of cerebral glutamate in the euthymic state of patients with bipolar disorder

The pathophysiology of bipolar disorder (BD) mostly remains unclear. However, some findings argue for a dysfunction in glutamatergic neurotransmission in BD. Proton magnetic resonance spectroscopy at 3T was used to determine glutamate concentrations in the anterior cingulate cortex (ACC) and the hippocampus (HC) of euthymic outpatients with BP-I disorder and age- and sex-matched healthy controls. In patients with BD, glutamate concentrations were significantly increased in the ACC and decreased in the HC compared with concentrations in controls. Significant group differences were also measured for N-acetyl aspartate and choline; no differences were found for other metabolites examined. An inverse correlation was observed for glutamate concentrations in the ACC and number of episodes. The findings of the study add to the concept of abnormalities in glutamatergic regulation in the ACC and HC in patients with BD.

GLAST But Not Least--Distribution, Function, Genetics and Epigenetics of L-Glutamate Transport in Brain--Focus on GLAST/EAAT1

Synaptically released L-glutamate, the most important excitatory neurotransmitter in the CNS, is removed from extracellular space by fast and efficient transport mediated by several transporters; the most abundant ones are EAAT1/GLAST and EAAT2/GLT1. The review first summarizes their location, functions and basic characteristics. We then look at genetics and epigenetics of EAAT1/GLAST and EAAT2/GLT1 and perform in silico analyses of their promoter regions. There is one CpG island in SLC1A2 (EAAT2/GLT1) gene and none in SLC1A3 (EAAT1/GLAST) suggesting that DNA methylation is not the most important epigenetic mechanism regulating EAAT1/GLAST levels in brain. There are targets for specific miRNA in SLC1A2 (EAAT2/GLT1) gene. We also note that while defects in EAAT2/GLT1 have been associated with various pathological states including chronic neurodegenerative diseases, very little is known on possible contributions of defective or dysfunctional EAAT1/GLAST to any specific brain disease. Finally, we review evidence of EAAT1/GLAST involvement in mechanisms of brain response to alcoholism and present some preliminary data showing that ethanol, at concentrations which may be reached following heavy drinking, can have an effect on the distribution of EAAT1/GLAST in cultured astrocytes; the effect is blocked by baclofen, a GABA-B receptor agonist and a drug potentially useful in the treatment of alcoholism. We argue that more research effort should be focused on EAAT1/GLAST, particularly in relation to alcoholism and drug addiction.