Mechanisms of glutamate release from astrocytes

Histamine elicits glutamate release from cultured astrocytes

Astrocytes play key roles in regulating brain homeostasis and neuronal activity. This is, in part, accomplished by the ability of neurotransmitters in the synaptic cleft to bind astrocyte membrane receptors, activating signalling cascades that regulate concentration of intracellular Ca²⁺ ([Ca²⁺]_i) and gliotransmitter release, including ATP and glutamate. Gliotransmitters contribute to dendrite formation and synaptic plasticity, and in some cases, exacerbate neurodegeneration. The neurotransmitter histamine participates in several physiological processes, such as the sleep-wake cycle and learning and memory. Previous studies have demonstrated the expression of histamine receptors on astrocytes, but until now, only a few studies have examined the effects of histamine on astrocyte intracellular signalling and gliotransmitter release. Here, we used the human astrocytoma cell line 1321N1 to study the role of histamine in astrocyte intracellular signalling and gliotransmitter release. We found that histamine activated astrocyte signalling through histamine H₁ and H₂ receptors, leading to distinct cellular responses. Activation of histamine H₁ receptors caused concentration-dependent release of [Ca²⁺]_i from internal stores and concentration-dependent increase in glutamate release. Histamine H₂ receptor activation increased cyclic adenosine monophosphate (cAMP) levels and phosphorylation of transcription factor cAMP response-element binding protein. Taken together, these data emphasize a role for histamine in neuron-glia communication.

Reactive Astrocytes as Drug Target in Alzheimer's Disease

Alzheimer's disease is a neurodegenerative disease characterized by deposition of extracellular amyloid-β, intracellular neurofibrillary tangles, and loss of cortical neurons. However, the mechanism underlying neurodegeneration in Alzheimer's disease (AD) remains to be explored. Many of the researches on AD have been primarily focused on neuronal changes. Current research, however, broadens to give emphasis on the importance of nonneuronal cells, such as astrocytes. Astrocytes play fundamental roles in several cerebral functions and their dysfunctions promote neurodegeneration and, eventually, retraction of neuronal synapses, which leads to cognitive deficits found in AD. Astrocytes become reactive as a result of deposition of Aβ, which in turn have detrimental consequences, including decreased glutamate uptake due to reduced expression of uptake transporters, altered energy metabolism, altered ion homeostasis (K⁺ and Ca⁺), increased tonic inhibition, and increased release of cytokines and inflammatory mediators. In this review, recent insights on the involvement of, tonic inhibition, astrocytic glutamate transporters and aquaporin in the pathogenesis of Alzheimer's disease are provided. Compounds which increase expression of GLT1 have showed efficacy for AD in preclinical studies. Tonic inhibition mediated by GABA could also be a promising target and drugs that block the GABA synthesizing enzyme, MAO-B, have shown efficacy. However, there are contradictory evidences on the role of AQP4 in AD.

A computational study of astrocytic glutamate influence on post-synaptic neuronal excitability

The ability of astrocytes to rapidly clear synaptic glutamate and purposefully release the excitatory transmitter is critical in the functioning of synapses and neuronal circuits. Dysfunctions of these homeostatic functions have been implicated in the pathology of brain disorders such as mesial temporal lobe epilepsy. However, the reasons for these dysfunctions are not clear from experimental data and computational models have been developed to provide further understanding of the implications of glutamate clearance from the extracellular space, as a result of EAAT2 downregulation: although they only partially account for the glutamate clearance process. In this work, we develop an explicit model of the astrocytic glutamate transporters, providing a more complete description of the glutamate chemical potential across the astrocytic membrane and its contribution to glutamate transporter driving force based on thermodynamic principles and experimental data. Analysis of our model demonstrates that increased astrocytic glutamate content due to glutamine synthetase downregulation also results in increased postsynaptic quantal size due to gliotransmission. Moreover, the proposed model demonstrates that increased astrocytic glutamate could prolong the time course of glutamate in the synaptic cleft and enhances astrocyte-induced slow inward currents, causing a disruption to the clarity of synaptic signalling and the occurrence of intervals of higher frequency postsynaptic firing. Overall, our work distilled the necessity of a low astrocytic glutamate concentration for reliable synaptic transmission of information and the possible implications of enhanced glutamate levels as in epilepsy.

The role of astrocytes in the excitability and hyperexcitability of neurons is a subject which has gained a lot of attention, particularly in the pathology of neurological disorders including epilepsy. Although not completely understood, the control of glutamate homeostasis is believed to play a role in paroxysmal neuronal hyperexcitability known to precede seizure activity. We have developed a computational model which explores two of the astrocytic homeostatic mechanisms, namely glutamate clearance and gliotransmission, and connect them with a common controlling factor, astrocytic cytoplasmic glutamate concentration. In our model simulations we demonstrate both a slower clearance rate of synaptic glutamate and enhanced astrocytic glutamate release where cytoplasmic glutamate is elevated, both of which contribute to high frequency neuronal firing and conditions for seizure generation. We also describe a viable role for astrocytes as a “high pass” filter, where astrocytic activation in the form of intracellular calcium oscillations is possible for only a certain range of presynaptic neuronal firing rates, the lower bound of the range being reduced where astrocytic glutamate is elevated. In physiological terms this perhaps indicates not only neuronal but also astrocytic glutamate-mediated excitation in the neural-astrocytic network.

Glial loss of the metallo β-lactamase domain containing protein, SWIP-10, induces age- and glutamate-signaling dependent, dopamine neuron degeneration

Across phylogeny, glutamate (Glu) signaling plays a critical role in regulating neural excitability, thus supporting many complex behaviors. Perturbed synaptic and extrasynaptic Glu homeostasis in the human brain has been implicated in multiple neuropsychiatric and neurodegenerative disorders including Parkinson's disease, where theories suggest that excitotoxic insults may accelerate a naturally occurring process of dopamine (DA) neuron degeneration. In C. elegans, mutation of the glial expressed gene, swip-10, results in Glu-dependent DA neuron hyperexcitation that leads to elevated DA release, triggering DA signaling-dependent motor paralysis. Here, we demonstrate that swip-10 mutations induce premature and progressive DA neuron degeneration, with light and electron microscopy studies demonstrating the presence of dystrophic dendritic processes, as well as shrunken and/or missing cell soma. As with paralysis, DA neuron degeneration in swip-10 mutants is rescued by glial-specific, but not DA neuron-specific expression of wildtype swip-10, consistent with a cell non-autonomous mechanism. Genetic studies implicate the vesicular Glu transporter VGLU-3 and the cystine/Glu exchanger homolog AAT-1 as potential sources of Glu signaling supporting DA neuron degeneration. Degeneration can be significantly suppressed by mutations in the Ca2+ permeable Glu receptors, nmr-2 and glr-1, in genes that support intracellular Ca2+ signaling and Ca2+-dependent proteolysis, as well as genes involved in apoptotic cell death. Our studies suggest that Glu stimulation of nematode DA neurons in early larval stages, without the protective actions of SWIP-10, contributes to insults that ultimately drive DA neuron degeneration. The swip-10 model may provide an efficient platform for the identification of molecular mechanisms that enhance risk for Parkinson's disease and/or the identification of agents that can limit neurodegenerative disease progression.

TNFα and IL-1β modify the miRNA cargo of astrocyte shed extracellular vesicles to regulate neurotrophic signaling in neurons

Astrocytes are known to be critical regulators of neuronal function. However, relatively few mediators of astrocyte to neuron communication have been identified. Recent advancements in the biology of extracellular vesicles have begun to implicate astrocyte derived extracellular vesicles (ADEV) as mediators of astrocyte to neuron communication, suggesting that alterations in the release and/or composition of ADEVs could influence gliotransmission. TNFα and IL-1β are key mediators of glial activation and neuronal damage, but the effects of these cytokines on the release or molecular composition of ADEVs is unknown. We found that ADEVs released in response to IL-1β (ADEV-IL-1β) and TNFα (ADEV-TNFα) were enriched with miRNAs that target proteins involved in neurotrophin signaling. We confirmed that miR-125a-5p and miR-16-5p (both enriched in ADEV-IL-1β and ADEV-TNFα) targeted NTKR3 and its downstream effector Bcl2. Downregulation of these targets in neurons was associated with reductions in dendritic growth, dendritic complexity, reduced spike rates, and burst activity. Molecular interference of miR-125a-5p and miR-16-5p prevented ADEV-IL-1β from reducing dendritic complexity, spike, and burst rates. These findings suggest that astrocytes respond to inflammatory challenge by modifying the miRNA cargo of ADEVs to diminish the activity of target neurons by regulating the translational expression of proteins controlling programs essential for synaptic stability and neuronal excitability.

Expression of pannexin 1 and 2 in cortical lesions from intractable epilepsy patients with focal cortical dysplasia

Focal cortical dysplasia (FCD) is a major cause of intractable epilepsy in children however the mechanisms underlying the pathogenesis of FCD and FCD induced epilepsy remain unclear. Increasing evidence suggests that the large-pore ion channels, pannexin 1 (Panx1) and 2 (Panx2), are involved in epilepsy and brain development. In this study, we investigated the expression of Panx1 and Panx2 in surgical samples from patients with FCD type Ia (FCDIa), type IIa (FCDIIa), and type IIb (FCDIIb) and in age-matched autopsy control samples. We found Panx1 mRNA and protein levels were both increased in all these FCD samples. Immunohistochemical analyses revealed that Panx1 was mainly distributed in microcolumn neurons, dysmorphic neurons (DNs), balloon cells (BCs) and reactive astrocytes. Double-labeled staining showed that the Panx1-positive neurons were mostly glutamatergic DNs and occasionally GABAergic normal-appearing neurons. Importantly, the protein levels of Panx1 positively correlated with the frequency of seizures. Intriguingly, the Panx2 mRNA and protein levels were only upregulated in FCDIIb lesions and characteristically expressed on SOX2-positive multipotential BCs. Immunofluorescent experiments identified that Panx2-positive BCs mainly expressed the neuronal differentiation transcription factor MASH1 but not the immature glial marker vimentin. Taken together, our results established a potential role of the specific expression and cellular distribution patterns of Panx1 and Panx2 in FCD-associated epileptogenesis and pathogenesis.

GL261 glioma tumor cells respond to ATP with an intracellular calcium rise and glutamate release

Glioblastoma (GBM) is an aggressive brain cancer with an average survival rate of 15 months. The composition of the GBM tumor microenvironment-its pH, the presence of growth and immune factors, neurotransmitters, and gliotransmitters-plays an important role in GBM pathophysiology and facilitates tumor survival and growth. In particular, GBM tumor cells produce glutamate, which is toxic to healthy tissue and is associated with increased tumor invasion into adjacent brain regions. The conditions that lead to this excitotoxic release of glutamate are not completely understood. Previous studies have demonstrated that extracellular ATP is present at high levels in the tumor microenvironment, and that ATP stimulates the release of glutamate from astrocytes in culture. Here we examine the functional effects of extracellular ATP on the GL261 cell line, a model system for high-grade astrocytomas such as GBM. We show that treatment with ATP leads to an immediate, dose-dependent influx of calcium into the cell that is partially inhibited by an antagonist (o-ATP) of the ionotropic ATP receptor P2X7. In addition, GL261 cells respond to extracellular ATP with a dose-dependent release of glutamate. Consistent with other reports, we find that ATP is toxic to GL261 cells at high concentrations. Together, these results provide insight into the mechanisms responsible for glutamate production by tumor cells and inform future studies that will identify how the GBM tumor microenvironment facilitates tumor invasion into healthy areas of the brain.

The signaling role for chloride in the bidirectional communication between neurons and astrocytes

It is well known that the electrical signaling in neuronal networks is modulated by chloride (Cl^-) fluxes via the inhibitory GABA_A and glycine receptors. Here, we discuss the putative contribution of Cl^- fluxes and intracellular Cl^- to other forms of information transfer in the CNS, namely the bidirectional communication between neurons and astrocytes. The manuscript (i) summarizes the generic functions of Cl^- in cellular physiology, (ii) recaps molecular identities and properties of Cl^- transporters and channels in neurons and astrocytes, and (iii) analyzes emerging studies implicating Cl^- in the modulation of neuroglial communication. The existing literature suggests that neurons can alter astrocytic Cl^- levels in a number of ways; via (a) the release of neurotransmitters and activation of glial transporters that have intrinsic Cl^- conductance, (b) the metabotropic receptor-driven changes in activity of the electroneutral cation-Cl^- cotransporter NKCC1, and (c) the transient, activity-dependent changes in glial cell volume which open the volume-regulated Cl^-/anion channel VRAC. Reciprocally, astrocytes are thought to alter neuronal [Cl^-]_i through either (a) VRAC-mediated release of the inhibitory gliotransmitters, GABA and taurine, which open neuronal GABA_A and glycine receptor/Cl^- channels, or (b) the gliotransmitter-driven stimulation of NKCC1. The most important recent developments in this area are the identification of the molecular composition and functional heterogeneity of brain VRAC channels, and the discovery of a new cytosolic [Cl^-] sensor - the Wnk family protein kinases. With new work in the field, our understanding of the role of Cl^- in information processing within the CNS is expected to be significantly updated.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

Contrasting effect of the latency-reversing agents bryostatin-1 and JQ1 on astrocyte-mediated neuroinflammation and brain neutrophil invasion

Background

Despite effectiveness of the combined antiretroviral therapy, HIV-1 persists in long-lived latently infected cells. Consequently, new therapeutic approaches aimed at eliminating this latent reservoir are currently being developed. A “shock and kill” strategy using latency-reversing agents (LRA) to reactivate HIV-1 has been proposed. However, the impact of LRA on the central nervous system (CNS) remains elusive.

Methods

We used human fetal astrocytes and investigated the effects of several LRA on their functional and secretory activities. Astrocytes were infected with VSV-G-pseudotyped HIV-1 before treatment with various blood-brain barrier (BBB)-permeable LRA at subcytotoxic doses, which allow HIV-1 reactivation based on previous in vitro and clinical studies. Cells and supernatants were then used to evaluate effects of infection and LRA on (i) viability and metabolic activity of astrocytes using a colorimetric MTS assay; (ii) chemokines and proinflammatory cytokines secretion and gene expression by astrocytes using ELISA and RT-qPCR, respectively; (iii) expression of complement component 3 (C3), a proxy for astrogliosis, by RT-qPCR; (iv) glutamate uptake capacity by a fluorometric assay; and (v) modulation of neutrophil transmigration across an in vitro BBB model.

Results

We demonstrate that bryostatin-1 induces secretion of chemokines CCL2 and IL-8 and proinflammatory cytokines IL-6 and GM-CSF, whereas their production is repressed by JQ1. Bryostatin-1 also increases expression of complement component 3 and perturbs astrocyte glutamate homeostasis. Lastly, bryostatin-1 enhances transmigration of neutrophils across an in vitro blood-brain barrier model and induces formation of neutrophil extracellular traps.

Conclusions

These observations highlight the need to carefully assess the potential harmful effect to the CNS when selecting LRA for HIV-1 reactivation strategies.

Electronic supplementary material

The online version of this article (10.1186/s12974-017-1019-y) contains supplementary material, which is available to authorized users.

HIV-1-associated inflammation and antiretroviral therapy regulate astrocyte endoplasmic reticulum stress responses

Antiretroviral (ARV) therapy (ART) has effectively suppressed the incidence of human immunodeficiency virus (HIV)-associated dementia in HIV-1 positive individuals. However, the prevalence of more subtle forms of neurocognitive dysfunction continues to escalate. Recently, endoplasmic reticulum (ER) stress has been linked to many neurological diseases; yet, its role in HIV/neuroAIDS remains largely unexplored. Furthermore, upregulation of astrocyte elevated gene-1 (AEG-1), a novel HIV-1 inducible gene, along with ER stress markers in a Huntington’s disease model, suggests a possible role in HIV-associated ER stress. The current study is focused on unfolded protein responses (UPRs) and AEG-1 regulation in primary human astrocytes exposed to HIV-associated neurocognitive disorders (HAND)-relevant stimuli (HIV-1 virions, inflammation and ARV drugs). Interleukin (IL)-1β and the nucleoside reverse transcriptase inhibitor abacavir upregulated expression of ER stress markers in human astrocytes, including binding immunoglobulin protein (BiP), C/EBP homologous protein (CHOP), and calnexin. In addition, IL-1β activated all three well-known UPR pathways: protein kinase RNA-like ER kinase (PERK); activating transcription factor 6 (ATF-6); and inositol-requiring enzyme 1α (IRE1α). AEG-1 upregulation correlated to ER stress and demonstrated astrocyte AEG-1 interaction with the calcium-binding chaperone, calnexin. IL-1β and abacavir enhanced intracellular calcium signaling in astrocytes in the absence of extracellular calcium, illustrating ER-associated calcium release. Alternatively, calcium evoked in response to HAND-relevant stimuli led to mitochondrial permeability transition pore (mPTP) opening in human astrocytes. Importantly, IL-1β- and abacavir-induced UPR and mPTP opening were inhibited by the intracellular calcium chelation, indicating the critical role of calcium signaling in HAND-relevant ER stress in astrocytes. In summary, our study highlights that ARV drugs and IL-1β induced UPR, AEG-1 expression, intracellular calcium, and mitochondrial depolarization in astrocytes. This study uncovers astrocyte ER stress as a novel therapeutic target in the management of HIV-1-associated neurotoxicity and possibly in the treatment of neuroAIDS.

The role of astrocytes in amyloid production and Alzheimer's disease

Alzheimer's disease (AD) is marked by the presence of extracellular amyloid beta (Aβ) plaques, intracellular neurofibrillary tangles (NFTs) and gliosis, activated glial cells, in the brain. It is thought that Aβ plaques trigger NFT formation, neuronal cell death, neuroinflammation and gliosis and, ultimately, cognitive impairment. There are increased numbers of reactive astrocytes in AD, which surround amyloid plaques and secrete proinflammatory factors and can phagocytize and break down Aβ. It was thought that neuronal cells were the major source of Aβ. However, mounting evidence suggests that astrocytes may play an additional role in AD by secreting significant quantities of Aβ and contributing to overall amyloid burden in the brain. Astrocytes are the most numerous cell type in the brain, and therefore even minor quantities of amyloid secretion from individual astrocytes could prove to be substantial when taken across the whole brain. Reactive astrocytes have increased levels of the three necessary components for Aβ production: amyloid precursor protein, β-secretase (BACE1) and γ-secretase. The identification of environmental factors, such as neuroinflammation, that promote astrocytic Aβ production, could redefine how we think about developing therapeutics for AD.

Blunted mGluR Activation Disinhibits Striatopallidal Transmission in Parkinsonian Mice

The prevailing circuit model predicts that hyperactivity of the striatopallidal pathway and subsequently increased inhibition of external globus pallidus (GPe) neurons lead to the hypokinetic symptoms of Parkinson's disease (PD). It is believed that hyperactivity of the striatopallidal pathway is due to inactivity of dopamine receptors on the somatodendritic membrane of striatopallidal neurons, but the exact cellular underpinnings remain unclear. In this study, we show that mouse GPe astrocytes critically control ambient glutamate level, which in turn gates striatopallidal transmission via the activation of presynaptic metabotropic glutamate receptors. This presynaptic inhibition of striatopallidal transmission is diminished after the chronic loss of dopamine. Elevation of intracellular glutamate content in astrocytes restores the proper regulation of the striatopallidal input in PD models. These findings argue that astrocytes are key regulators of the striatopallidal synapse. Targeting this cell class may serve as an alternative therapeutic strategy for PD.

Glial and neuroinflammatory targets for treating substance use disorders

BACKGROUND

The plenary session at the 2016 Behavior, Biology and Chemistry: Translational Research in Addiction Conference focused on glia as potential players in the development, persistence and treatment of substance use disorders. Glia partake in various functions that are important for healthy brain activity. Drugs of abuse alter glial cell activity producing several perturbations in brain function that are thought to contribute to behavioral changes associated with substance use disorders. Consequently, drug-induced changes in glia-driven processes in the brain represent potential targets for pharmacotherapeutics treating substance use disorders.

METHODS

Four speakers presented preclinical and clinical research illustrating the effects that glial modulators have on abuse-related behavioral effects of psychostimulants and opioids. This review highlights some of these findings and expands its focus to include other research focused on drug-induced glia abnormalities and glia-focused treatment approaches in substance use disorders.

RESULTS

Preclinical findings show that drugs of abuse induce neuroinflammatory signals and disrupt glutamate homeostasis through their interaction with microglia and astrocytes. Preclinical and clinical studies testing the effects of glial modulators show general effectiveness in reducing behaviors associated with substance use disorders.

CONCLUSIONS

The contribution of drug-induced glial activity continues to emerge as an intriguing target for substance use disorder treatments. Clinical investigations of glial modulators have yielded promising results on substance use measures and indicate that they are generally safe and well-tolerated. However, results have not been entirely positive and more questions remain for continued exploration in the development and testing of glial-directed treatments for substance use disorders.

Astrocytes mediated the nootropic and neurotrophic effects of Sarsasapogenin-AA13 via upregulating brain-derived neurotrophic factor

Rhizoma Anemarrhena, a widely used traditional Chinese medicine, has previously been shown to have neuroprotective effect. Sarsasapogenin-AA13 (AA13) is a novel synthetic derivative of Sarsasapogenin, which is extracted from Rhizoma Anemarrhena. The aim of this study is to investigate the nootropic and neurotrophic effects of AA13 and underlying mechanisms. In vitro, cell viability of rat primary astrocytes treated with AA13 and neurons cultured with conditioned medium of AA13-treated rat primary astrocytes was tested by MTT assays. In vivo, a pharmacological model of cognitive impairment induced by scopolamine was employed and spatial memory of the mice was assessed by Morris water maze. This study found that AA13 increased cell viability of primary astrocytes and AA13-treated astrocyte-conditioned medium enhanced the survival rate of primary neurons. Interestingly, AA13 markedly enhanced the level of BDNF in astrocytes. Furthermore, AA13 (6 mg/kg) improved the cognitive deficits in animal models (p<0.05) and BDNF and PSD95 levels were increased in brain. Therefore, we hypothesize that AA13 exerts nootropic and neurotrophic activities through astrocytes mediated upregulation of BDNF secretion. The results suggest that AA13 could be a potential compound for cognitive impairment after further research.

Molecular composition and heterogeneity of the LRRC8-containing swelling-activated osmolyte channels in primary rat astrocytes

KEY POINTS

The volume-regulated anion channel (VRAC) is a swelling-activated chloride channel that is permeable to inorganic anions and a variety of small organic molecules. VRAC is formed via heteromerization of LRRC8 proteins, among which LRRC8A is essential, while LRRC8B/C/D/E serve as exchangeable complementary partners. We used an RNAi approach and radiotracer assays to explore which LRRC8 isoforms contribute to swelling-activated release of diverse organic osmolytes in rat astrocytes. Efflux of uncharged osmolytes (myo-inositol and taurine) was suppressed by deletion of LRRC8A or LRRC8D, but not by deletion of LRRC8C+LRRC8E. Conversely, release of charged osmolytes (d-aspartate) was strongly reduced by deletion of LRRC8A or LRRC8C+LRRC8E, but largely unaffected by downregulation of LRRC8D. Our findings point to the existence of multiple heteromeric VRACs in the same cell type: LRRC8A/D-containing heteromers appear to dominate release of uncharged osmolytes, while LRRC8A/C/E, with the additional contribution of LRRC8D, creates a conduit for movement of charged molecules.

ABSTRACT

The volume-regulated anion channel (VRAC) is the ubiquitously expressed vertebrate Cl^- /anion channel that is composed of proteins belonging to the LRRC8 family and activated by cell swelling. In the brain, VRAC contributes to physiological and pathological release of a variety of small organic molecules, including the amino acid neurotransmitters glutamate, aspartate and taurine. In the present work, we explored the role of all five LRRC8 family members in the release of organic osmolytes from primary rat astrocytes. Expression of LRRC8 proteins was modified using an RNAi approach, and amino acid fluxes via VRAC were quantified by radiotracer assays in cells challenged with hypoosmotic medium (30% reduction in osmolarity). Consistent with our prior work, knockdown of LRRC8A potently and equally suppressed the release of radiolabelled d-[¹⁴ C]aspartate and [³ H]taurine. Among other LRRC8 subunits, downregulation of LRRC8D strongly inhibited release of the uncharged osmolytes [³ H]taurine and myo-[³ H]inositol, without major impact on the simultaneously measured efflux of the charged d-[¹⁴ C]aspartate. In contrast, the release of d-[¹⁴ C]aspartate was preferentially sensitive to deletion of LRRC8C+LRRC8E, but unaffected by downregulation of LRRC8D. Finally, siRNA knockdown of LRRC8C+LRRC8D strongly inhibited the release of all osmolytes. Overall, our findings suggest the existence of at least two distinct heteromeric VRACs in astroglial cells. The LRRC8A/D-containing permeability pathway appears to dominate the release of uncharged osmolytes, while an alternative channel (or channels) is composed of LRRC8A/C/D/E and responsible for the loss of charged molecules.

TRPA1 channels promote astrocytic Ca2+ hyperactivity and synaptic dysfunction mediated by oligomeric forms of amyloid-β peptide

BACKGROUND

Excessive synaptic loss is thought to be one of the earliest events in Alzheimer's disease (AD). However, the key mechanisms that maintain plasticity of synapses during adulthood or initiate synapse dysfunction in AD remain unknown. Recent studies suggest that astrocytes contribute to functional changes observed during synaptic plasticity and play a major role in synaptic dysfunction but astrocytes behavior and involvement in early phases of AD remained largely undefined.

METHODS

We measure astrocytic calcium activity in mouse CA1 hippocampus stratum radiatum in both the global astrocytic population and at a single cell level, focusing in the highly compartmentalized astrocytic arbor. Concurrently, we measure excitatory post-synaptic currents in nearby pyramidal neurons.

RESULTS

We find that application of soluble Aβ oligomers (Aβo) induced fast and widespread calcium hyperactivity in the astrocytic population and in the microdomains of the astrocyte arbor. We show that astrocyte hyperactivity is independent of neuronal activity and is repaired by transient receptor potential A1 (TRPA1) channels blockade. In return, this TRPA1 channels-dependent hyperactivity influences neighboring CA1 neurons triggering an increase in glutamatergic spontaneous activity. Interestingly, in an AD mouse model (APP/PS1-21 mouse), astrocyte calcium hyperactivity equally takes place at the beginning of Aβ production, depends on TRPA1 channels and is linked to CA1 neurons hyperactivity.

CONCLUSIONS

Our experiments demonstrate that astrocytes contribute to early Aβo toxicity exhibiting a global and local Ca2+ hyperactivity that involves TRPA1 channels and is related to neuronal hyperactivity. Together, our data suggest that astrocyte is a frontline target of Aβo and highlight a novel mechanism for the understanding of early synaptic dysregulation induced by soluble Aβo species.

Astrocytes Control Circadian Timekeeping in the Suprachiasmatic Nucleus via Glutamatergic Signaling

The suprachiasmatic nucleus (SCN) of the hypothalamus orchestrates daily rhythms of physiology and behavior in mammals. Its circadian (∼24 hr) oscillations of gene expression and electrical activity are generated intrinsically and can persist indefinitely in temporal isolation. This robust and resilient timekeeping is generally regarded as a product of the intrinsic connectivity of its neurons. Here we show that neurons constitute only one “half” of the SCN clock, the one metabolically active during circadian daytime. In contrast, SCN astrocytes are active during circadian nighttime, when they suppress the activity of SCN neurons by regulating extracellular glutamate levels. This glutamatergic gliotransmission is sensed by neurons of the dorsal SCN via specific pre-synaptic NMDA receptor assemblies containing NR2C subunits. Remarkably, somatic genetic re-programming of intracellular clocks in SCN astrocytes was capable of remodeling circadian behavioral rhythms in adult mice. Thus, SCN circuit-level timekeeping arises from interdependent and mutually supportive astrocytic-neuronal signaling.

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SCN neurons are active during circadian day, but SCN astrocytes are active at night

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Astrocytes direct circadian cycles of extracellular glutamate to inhibit SCN neurons

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Astrocyte-derived inhibition is mediated by NMDAR2C complexes on dorsal SCN neurons

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Genetic re-programming of the clock in SCN astrocytes reshapes circadian behavior

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.

Genome-Scale Reconstruction of the Human Astrocyte Metabolic Network

Astrocytes are the most abundant cells of the central nervous system; they have a predominant role in maintaining brain metabolism. In this sense, abnormal metabolic states have been found in different neuropathological diseases. Determination of metabolic states of astrocytes is difficult to model using current experimental approaches given the high number of reactions and metabolites present. Thus, genome-scale metabolic networks derived from transcriptomic data can be used as a framework to elucidate how astrocytes modulate human brain metabolic states during normal conditions and in neurodegenerative diseases. We performed a Genome-Scale Reconstruction of the Human Astrocyte Metabolic Network with the purpose of elucidating a significant portion of the metabolic map of the astrocyte. This is the first global high-quality, manually curated metabolic reconstruction network of a human astrocyte. It includes 5,007 metabolites and 5,659 reactions distributed among 8 cell compartments, (extracellular, cytoplasm, mitochondria, endoplasmic reticle, Golgi apparatus, lysosome, peroxisome and nucleus). Using the reconstructed network, the metabolic capabilities of human astrocytes were calculated and compared both in normal and ischemic conditions. We identified reactions activated in these two states, which can be useful for understanding the astrocytic pathways that are affected during brain disease. Additionally, we also showed that the obtained flux distributions in the model, are in accordance with literature-based findings. Up to date, this is the most complete representation of the human astrocyte in terms of inclusion of genes, proteins, reactions and metabolic pathways, being a useful guide for in-silico analysis of several metabolic behaviors of the astrocyte during normal and pathologic states.

Inflammation-Associated Co-morbidity Between Depression and Cardiovascular Disease

Morbidity and mortality of cardiovascular disease (CVD) is exceedingly high worldwide. Depressive illness is a serious psychiatric illness that afflicts a significant portion of the world population. Epidemiological studies have confirmed the high co-morbidity between these two disease entities. The co-morbidity is bidirectional and the mechanisms responsible for it are complex and multifaceted. In addition to genetic, biological systems, psychosocial, and behavioral factors that are involved include the central and autonomic nervous systems, the neuroendocrine, immune, and the vascular and hematologic systems. Specific pathophysiologic factors across these systems include homeostatic imbalance between the sympathetic and the parasympathetic systems with loss of heart rate variability (HRV) in depression, sympathoadrenal activation, hypothalamic-pituitary-adrenal (HPA) axis activation, immune system dysregulation resulting in a pro-inflammatory status, platelet activation, and endothelial dysfunction. These abnormalities have been demonstrated in most individuals diagnosed with major depressive disorder (MDD), bipolar disorder (BPD), and probably in other psychiatric disorders. A likely common instigator underlying the co-morbidity between cardiovascular pathology and depression is mental stress. Chronic stress shifts the homeostatic balance in the autonomic nervous system with sustained sympathetic overdrive and diminished vagal tone. Diminished vagal tone contributes to a pro-inflammatory status with associated sequelae. Stress hormones and certain pro-inflammatory substances released by macrophages and microglia upregulate the rate-limiting enzymes in the metabolic pathway of tryptophan (TRP). This enzymatic upregulation stimulates the kynurenine (KYN) pathway resulting in the formation of neurotoxic metabolites. Inflammation occurs in cardiac, cardiovascular, and cerebrovascular pathology independent of the presence or absence of depression. Inflammation is closely associated with endothelial dysfunction, a preamble to atherosclerosis and atherothrombosis. Endothelial dysfunction has been detected in depression and may prove to be a trait marker for this illness. Thus understanding vascular biology in conjunction with psychiatric co-morbidity will be of critical importance. Antidepressant drug therapy is of definite benefit to patients with medical and psychiatric co-morbidity and may reverse the pro-inflammatory status associated with depression. There is, however, an urgent need to develop novel pharmacotherapeutic approaches to benefit a much larger proportion of patients suffering from these disease entities.

Inflammation, Glutamate, and Glia: A Trio of Trouble in Mood Disorders

Increasing data indicate that inflammation and alterations in glutamate neurotransmission are two novel pathways to pathophysiology in mood disorders. The primary goal of this review is to illustrate how these two pathways may converge at the level of the glia to contribute to neuropsychiatric disease. We propose that a combination of failed clearance and exaggerated release of glutamate by glial cells during immune activation leads to glutamate increases and promotes aberrant extrasynaptic signaling through ionotropic and metabotropic glutamate receptors, ultimately resulting in synaptic dysfunction and loss. Furthermore, glutamate diffusion outside the synapse can lead to the loss of synaptic fidelity and specificity of neurotransmission, contributing to circuit dysfunction and behavioral pathology. This review examines the fundamental role of glia in the regulation of glutamate, followed by a description of the impact of inflammation on glial glutamate regulation at the cellular, molecular, and metabolic level. In addition, the role of these effects of inflammation on glia and glutamate in mood disorders will be discussed along with their translational implications.

Inflammation Effects on Brain Glutamate in Depression: Mechanistic Considerations and Treatment Implications

There has been increasing interest in the role of glutamate in mood disorders, especially given the profound effect of the glutamate receptor antagonist ketamine in improving depressive symptoms in patients with treatment-resistant depression. One pathway by which glutamate alterations may occur in mood disorders involves inflammation. Increased inflammation has been observed in a significant subgroup of patients with mood disorders, and inflammatory cytokines have been shown to influence glutamate metabolism through effects on astrocytes and microglia. In addition, the administration of the inflammatory cytokine interferon-alpha has been shown to increase brain glutamate in the basal ganglia and dorsal anterior cingulate cortex as measured by magnetic resonance spectroscopy (MRS). Moreover, MRS studies in patients with major depressive disorder have revealed that increased markers of inflammation including C-reactive protein correlate with increased basal ganglia glutamate, which in turn was associated with anhedonia and psychomotor retardation. Finally, human and laboratory animal studies have shown that the response to glutamate antagonists such as ketamine is predicted by increased inflammatory cytokines. Taken together, these data make a strong case that inflammation may influence glutamate metabolism to alter behavior, leading to depressive symptoms including anhedonia and psychomotor slowing.

IL-17 signalling in astrocytes promotes glutamate excitotoxicity: Indications for the link between inflammatory and neurodegenerative events in multiple sclerosis

OBJECTIVE

Th-17 cells have been exclusively referred to inflammatory events in multiple sclerosis (MS), while their importance in the development of glutamate excitotoxicity and the consequent neurodegeneration has been a completely unexplored concept. Accordingly, the objective of our study was to assess IL-17A effect on astrocyte ability to metabolize and release glutamate, considering that astrocytes had the central role in glutamate homeostasis.

METHODS

By using primary rat astrocyte cultures, astrocyte ability to uptake glutamate was estimated by the alterations of glutamate transporters (GLAST and GLT-1) expression, whereas changes in glutamine synthetase expression were used to estimate the ability to metabolize glutamate. Gene expression was determined by real time polymerase chain reaction (rtPCR). IL-17A effect on astrocyte ability to produce glutamate was investigated directly, by measuring the level of released glutamate using high performance liquid chromatography (HPLC).

RESULTS

Lower concentrations of IL-17A reduced the expressions of both glutamate transporters and glutamine synthetase; however, this effect was lost when IL-17A was applied in a higher dose. IL-17A did not significantly modify glutamate release from astrocyte in basal conditions, but following Ca²⁺ stimulation, as well as Ca²⁺ removal from the culture medium, IL-17A stimulated glutamate release in dose-dependent manner.

CONCLUSION

Together, these results support that IL-17A could promote glutamate excitotoxicity by decreasing astrocyte ability to uptake and convert glutamate to non-toxic glutamine, but also by stimulating Ca²⁺ dependent glutamate release. Such interactions between IL-17A and glutamate excitotoxicity implicate the potential link between inflammation and neurodegeneration during MS pathogenesis, and identify astrocytes as a potential target in achieving neuroprotective effects in MS.

Role of Cl- -HCO3- exchanger AE3 in intracellular pH homeostasis in cultured murine hippocampal neurons, and in crosstalk to adjacent astrocytes

KEY POINTS

A polymorphism of human AE3 is associated with idiopathic generalized epilepsy. Knockout of AE3 in mice lowers the threshold for triggering epileptic seizures. The explanations for these effects are elusive. Comparisons of cells from wild-type vs. AE3^-/- mice show that AE3 (present in hippocampal neurons, not astrocytes; mediates HCO₃^- efflux) enhances intracellular pH (pH_i ) recovery (decrease) from alkali loads in neurons and, surprisingly, adjacent astrocytes. During metabolic acidosis (MAc), AE3 speeds initial acidification, but limits the extent of pH_i decrease in neurons and astrocytes. AE3 speeds re-alkalization after removal of MAc in neurons and astrocytes, and speeds neuronal pH_i recovery from an ammonium prepulse-induced acid load. We propose that neuronal AE3 indirectly increases acid extrusion in (a) neurons via Cl^- loading, and (b) astrocytes by somehow enhancing NBCe1 (major acid extruder). The latter would enhance depolarization-induced alkalinization of astrocytes, and extracellular acidification, and thereby reduce susceptibility to epileptic seizures.

ABSTRACT

The anion exchanger AE3, expressed in hippocampal (HC) neurons but not astrocytes, contributes to intracellular pH (pH_i ) regulation by facilitating the exchange of extracellular Cl^- for intracellular HCO₃^- . The human AE3 polymorphism A867D is associated with idiopathic generalized epilepsy. Moreover, AE3 knockout (AE3^-/- ) mice are more susceptible to epileptic seizure. The mechanism of these effects has been unclear because the starting pH_i in AE3^-/- and wild-type neurons is indistinguishable. The purpose of the present study was to use AE3^-/- mice to investigate the role of AE3 in pH_i homeostasis in HC neurons, co-cultured with astrocytes. We find that the presence of AE3 increases the acidification rate constant during pH_i recovery from intracellular alkaline loads imposed by reducing [CO₂ ]. The presence of AE3 also speeds intracellular acidification during the early phase of metabolic acidosis (MAc), not just in neurons but, surprisingly, in adjacent astrocytes. Additionally, AE3 contributes to braking the decrease in pH_i later during MAc in both neurons and astrocytes. Paradoxically, AE3 enhances intracellular re-alkalization after MAc removal in neurons and astrocytes, and pH_i recovery from an ammonium prepulse-induced acid load in neurons. The effects of AE3 knockout on astrocytic pH_i homeostasis in MAc-related assays require the presence of neurons, and are consistent with the hypothesis that the AE3 knockout reduces functional expression of astrocytic NBCe1. These findings suggest a new type of neuron-astrocyte communication, based on the expression of AE3 in neurons, which could explain how AE3 reduces seizure susceptibility.

Optogenetic and Chemogenetic Approaches for Studying Astrocytes and Gliotransmitters

The brain consists of heterogeneous populations of neuronal and non-neuronal cells. The revelation of their connections and interactions is fundamental to understanding normal brain functions as well as abnormal changes in pathological conditions. Optogenetics and chemogenetics have been developed to allow functional manipulations both in vitro and in vivo to examine causal relationships between cellular changes and functional outcomes. These techniques are based on genetically encoded effector molecules that respond exclusively to exogenous stimuli, such as a certain wavelength of light or a synthetic ligand. Activation of effector molecules provokes diverse intracellular changes, such as an influx or efflux of ions, depolarization or hyperpolarization of membranes, and activation of intracellular signaling cascades. Optogenetics and chemogenetics have been applied mainly to the study of neuronal circuits, but their use in studying non-neuronal cells has been gradually increasing. Here we introduce recent studies that have employed optogenetics and chemogenetics to reveal the function of astrocytes and gliotransmitters.

Conceptual convergence: increased inflammation is associated with increased basal ganglia glutamate in patients with major depression

Inflammation and altered glutamate metabolism are two pathways implicated in the pathophysiology of depression. Interestingly, these pathways may be linked given that administration of inflammatory cytokines such as interferon-α to otherwise non-depressed controls increased glutamate in the basal ganglia and dorsal anterior cingulate cortex (dACC) as measured by magnetic resonance spectroscopy (MRS). Whether increased inflammation is associated with increased glutamate among patients with major depression is unknown. Accordingly, we conducted a cross-sectional study of 50 medication-free, depressed outpatients using single-voxel MRS, to measure absolute glutamate concentrations in basal ganglia and dACC. Multivoxel chemical shift imaging (CSI) was used to explore creatine-normalized measures of other metabolites in basal ganglia. Plasma and cerebrospinal fluid (CSF) inflammatory markers were assessed along with anhedonia and psychomotor speed. Increased log plasma C-reactive protein (CRP) was significantly associated with increased log left basal ganglia glutamate controlling for age, sex, race, body mass index, smoking status and depression severity. In turn, log left basal ganglia glutamate was associated with anhedonia and psychomotor slowing measured by the finger-tapping test, simple reaction time task and the Digit Symbol Substitution Task. Plasma CRP was not associated with dACC glutamate. Plasma and CSF CRP were also associated with CSI measures of basal ganglia glutamate and the glial marker myoinositol. These data indicate that increased inflammation in major depression may lead to increased glutamate in the basal ganglia in association with glial dysfunction and suggest that therapeutic strategies targeting glutamate may be preferentially effective in depressed patients with increased inflammation as measured by CRP.

Translational potential of astrocytes in brain disorders

Fundamentally, all brain disorders can be broadly defined as the homeostatic failure of this organ. As the brain is composed of many different cells types, including but not limited to neurons and glia, it is only logical that all the cell types/constituents could play a role in health and disease. Yet, for a long time the sole conceptualization of brain pathology was focused on the well-being of neurons. Here, we challenge this neuron-centric view and present neuroglia as a key element in neuropathology, a process that has a toll on astrocytes, which undergo complex morpho-functional changes that can in turn affect the course of the disorder. Such changes can be grossly identified as reactivity, atrophy with loss of function and pathological remodeling. We outline the pathogenic potential of astrocytes in variety of disorders, ranging from neurotrauma, infection, toxic damage, stroke, epilepsy, neurodevelopmental, neurodegenerative and psychiatric disorders, Alexander disease to neoplastic changes seen in gliomas. We hope that in near future we would witness glial-based translational medicine with generation of deliverables for the containment and cure of disorders. We point out that such as a task will require a holistic and multi-disciplinary approach that will take in consideration the concerted operation of all the cell types in the brain.

Disentangling the Role of Astrocytes in Alcohol Use Disorder

Several laboratories recently identified that astrocytes are critical regulators of addiction machinery. It is now known that astrocyte pathology is a common feature of ethanol (EtOH) exposure in both humans and animal models, as even brief EtOH exposure is sufficient to elicit long-lasting perturbations in astrocyte gene expression, activity, and proliferation. Astrocytes were also recently shown to modulate the motivational properties of EtOH and other strongly reinforcing stimuli. Given the role of astrocytes in regulating glutamate homeostasis, a crucial component of alcohol use disorder (AUD), astrocytes might be an important target for the development of next-generation alcoholism treatments. This review will outline some of the more prominent features displayed by astrocytes, how these properties are influenced by acute and long-term EtOH exposure, and future directions that may help to disentangle astrocytic from neuronal functions in the etiology of AUD.

Glial Na(+) -dependent ion transporters in pathophysiological conditions

Sodium dynamics are essential for regulating functional processes in glial cells. Indeed, glial Na(+) signaling influences and regulates important glial activities, and plays a role in neuron-glia interaction under physiological conditions or in response to injury of the central nervous system (CNS). Emerging studies indicate that Na(+) pumps and Na(+) -dependent ion transporters in astrocytes, microglia, and oligodendrocytes regulate Na(+) homeostasis and play a fundamental role in modulating glial activities in neurological diseases. In this review, we first briefly introduced the emerging roles of each glial cell type in the pathophysiology of cerebral ischemia, Alzheimer's disease, epilepsy, Parkinson's disease, Amyotrophic Lateral Sclerosis, and myelin diseases. Then, we discussed the current knowledge on the main roles played by the different glial Na(+) -dependent ion transporters, including Na(+) /K(+) ATPase, Na(+) /Ca(2+) exchangers, Na(+) /H(+) exchangers, Na(+) -K(+) -Cl(-) cotransporters, and Na(+) - HCO3- cotransporter in the pathophysiology of the diverse CNS diseases. We highlighted their contributions in cell survival, synaptic pathology, gliotransmission, pH homeostasis, and their role in glial activation, migration, gliosis, inflammation, and tissue repair processes. Therefore, this review summarizes the foundation work for targeting Na(+) -dependent ion transporters in glia as a novel strategy to control important glial activities associated with Na(+) dynamics in different neurological disorders. GLIA 2016;64:1677-1697.

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.

Intercellular signal communication among odontoblasts and trigeminal ganglion neurons via glutamate

Various stimuli to the exposed surface of dentin induce changes in the hydrodynamic force inside the dentinal tubules resulting in dentinal pain. Recent evidences indicate that mechano-sensor channels, such as the transient receptor potential channels, in odontoblasts receive these hydrodynamic forces and trigger the release of ATP to the pulpal neurons, to generate dentinal pain. A recent study, however, has shown that odontoblasts also express glutamate receptors (GluRs). This implies that cells in the dental pulp tissue have the ability to release glutamate, which acts as a functional intercellular mediator to establish inter-odontoblast and odontoblast-trigeminal ganglion (TG) neuron signal communication. To investigate the intercellular signal communication, we applied mechanical stimulation to odontoblasts and measured the intracellular free Ca²⁺ concentration ([Ca²⁺]_i). During mechanical stimulation in the presence of extracellular Ca²⁺, we observed a transient [Ca²⁺]_i increase not only in single stimulated odontoblasts, but also in adjacent odontoblasts. We could not observe these responses in the absence of extracellular Ca²⁺. [Ca²⁺]_i increases in the neighboring odontoblasts during mechanical stimulation of single odontoblasts were inhibited by antagonists of metabotropic glutamate receptors (mGluRs) as well as glutamate-permeable anion channels. In the odontoblast-TG neuron coculture, we observed an increase in [Ca²⁺]_i in the stimulated odontoblasts and TG neurons, in response to direct mechanical stimulation of single odontoblasts. These [Ca²⁺]_i increases in the neighboring TG neurons were inhibited by antagonists for mGluRs. The [Ca²⁺]_i increases in the stimulated odontoblasts were also inhibited by mGluRs antagonists. We further confirmed that the odontoblasts express group I, II, and III mGluRs. However, we could not record any currents evoked from odontoblasts near the mechanically stimulated odontoblast, with or without extracellular Mg²⁺, indicating that N-methyl-d-aspartic acid receptor does not contribute to inter-odontoblast signal communication. The results suggest that a mechanically stimulated odontoblast is capable of releasing glutamate into the extracellular space via glutamate-permeable anion channels. The released glutamate activates mGluRs on the odontoblasts in an autocrine/paracrine manner, forming an inter-odontoblasts communication, which drives dentin formation via odontoblast-odontoblast signal communication. Glutamate and mGluRs also mediate neurotransmission between the odontoblasts and neurons in the dental pulp to modulate sensory signal transmission for dentinal sensitivity.

Glutamate Neurotransmission in Rodent Models of Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading cause of death and disability in people younger than 45 and is a significant public health concern. In addition to primary mechanical damage to cells and tissue, TBI involves additional molecular mechanisms of injury, termed secondary injury, that continue to evolve over hours, days, weeks, and beyond. The trajectory of recovery after TBI is highly unpredictable and in many cases results in chronic cognitive and behavioral changes. Acutely after TBI, there is an unregulated release of glutamate that cannot be buffered or cleared effectively, resulting in damaging levels of glutamate in the extracellular space. This initial loss of glutamate homeostasis may initiate additional changes in glutamate regulation. The excitatory amino acid transporters (EAATs) are expressed on both neurons and glia and are the principal mechanism for maintaining extracellular glutamate levels. Diffusion of glutamate outside the synapse due to impaired uptake may lead to increased extrasynaptic glutamate signaling, secondary injury through activation of cell death pathways, and loss of fidelity and specificity of synaptic transmission. Coordination of glutamate release and uptake is critical to regulating synaptic strength, long-term potentiation and depression, and cognitive processes. In this review, we will discuss dysregulation of extracellular glutamate and glutamate uptake in the acute stage of TBI and how failure to resolve acute disruptions in glutamate homeostatic mechanisms may play a causal role in chronic cognitive symptoms after TBI.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Peripherally restricted viral challenge elevates extracellular glutamate and enhances synaptic transmission in the hippocampus

Peripheral infections increase the propensity and severity of seizures in susceptible populations. We have previously shown that intraperitoneal injection of a viral mimic, polyinosinic-polycytidylic acid (PIC), elicits hypersusceptibility of mice to kainic acid (KA)-induced seizures. This study was undertaken to determine whether this seizure hypersusceptibility entails alterations in glutamate signaling. Female C57BL/6 mice were intraperitoneally injected with PIC, and after 24 h, glutamate homeostasis in the hippocampus was monitored using the enzyme-based microelectrode arrays. PIC challenge robustly increased the level of resting extracellular glutamate. While pre-synaptic potassium-evoked glutamate release was not affected, glutamate uptake was profoundly impaired and non-vesicular glutamate release was augmented, indicating functional alterations of astrocytes. Electrophysiological examination of hippocampal slices from PIC-challenged mice revealed a several fold increase in the basal synaptic transmission as compared to control slices. PIC challenge also increased the probability of pre-synaptic glutamate release as seen from a reduction of paired-pulse facilitation and synaptic plasticity as seen from an enhancement of long-term potentiation. Altogether, our results implicate a dysregulation of astrocytic glutamate metabolism and an alteration of excitatory synaptic transmission as the underlying mechanism for the development of hippocampal hyperexcitability, and consequently seizure hypersusceptibility following peripheral PIC challenge. Peripheral infections/inflammations enhance seizure susceptibility. Here, we explored the effect of peritoneal inflammation induced by a viral mimic on glutamate homeostasis and glutamatergic neurotransmission in the mouse hippocampus. We found that peritoneal inflammation elevated extracellular glutamate concentration and enhanced the probability of pre-synaptic glutamate release resulting in hyperexcitability of neuronal networks. These mechanisms are likely to underlie the enhanced seizure propensity.

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.

Miro1 Regulates Activity-Driven Positioning of Mitochondria within Astrocytic Processes Apposed to Synapses to Regulate Intracellular Calcium Signaling

It is fast emerging that maintaining mitochondrial function is important for regulating astrocyte function, although the specific mechanisms that govern astrocyte mitochondrial trafficking and positioning remain poorly understood. The mitochondrial Rho-GTPase 1 protein (Miro1) regulates mitochondrial trafficking and detachment from the microtubule transport network to control activity-dependent mitochondrial positioning in neurons. However, whether Miro proteins are important for regulating signaling-dependent mitochondrial dynamics in astrocytic processes remains unclear. Using live-cell confocal microscopy of rat organotypic hippocampal slices, we find that enhancing neuronal activity induces transient mitochondrial remodeling in astrocytes, with a concomitant, transient reduction in mitochondrial trafficking, mediated by elevations in intracellular Ca(2+). Stimulating neuronal activity also induced mitochondrial confinement within astrocytic processes in close proximity to synapses. Furthermore, we show that the Ca(2+)-sensing EF-hand domains of Miro1 are important for regulating mitochondrial trafficking in astrocytes and required for activity-driven mitochondrial confinement near synapses. Additionally, activity-dependent mitochondrial positioning by Miro1 reciprocally regulates the levels of intracellular Ca(2+) in astrocytic processes. Thus, the regulation of intracellular Ca(2+) signaling, dependent on Miro1-mediated mitochondrial positioning, could have important consequences for astrocyte Ca(2+) wave propagation, gliotransmission, and ultimately neuronal function.

Adrenergic activation attenuates astrocyte swelling induced by hypotonicity and neurotrauma

Edema in the central nervous system can rapidly result in life-threatening complications. Vasogenic edema is clinically manageable, but there is no established medical treatment for cytotoxic edema, which affects astrocytes and is a primary trigger of acute post-traumatic neuronal death. To test the hypothesis that adrenergic receptor agonists, including the stress stimulus epinephrine protects neural parenchyma from damage, we characterized its effects on hypotonicity-induced cellular edema in cortical astrocytes by in vivo and in vitro imaging. After epinephrine administration, hypotonicity-induced swelling of astrocytes was markedly reduced and cytosolic 3'-5'-cyclic adenosine monophosphate (cAMP) was increased, as shown by a fluorescence resonance energy transfer nanosensor. Although, the kinetics of epinephrine-induced cAMP signaling was slowed in primary cortical astrocytes exposed to hypotonicity, the swelling reduction by epinephrine was associated with an attenuated hypotonicity-induced cytosolic Ca(2+) excitability, which may be the key to prevent astrocyte swelling. Furthermore, in a rat model of spinal cord injury, epinephrine applied locally markedly reduced neural edema around the contusion epicenter. These findings reveal new targets for the treatment of cellular edema in the central nervous system.

Astrocytes: a central element in neurological diseases

The neurone-centred view of the past disregarded or downplayed the role of astroglia as a primary component in the pathogenesis of neurological diseases. As this concept is changing, so is also the perceived role of astrocytes in the healthy and diseased brain and spinal cord. We have started to unravel the different signalling mechanisms that trigger specific molecular, morphological and functional changes in reactive astrocytes that are critical for repairing tissue and maintaining function in CNS pathologies, such as neurotrauma, stroke, or neurodegenerative diseases. An increasing body of evidence shows that the effects of astrogliosis on the neural tissue and its functions are not uniform or stereotypic, but vary in a context-specific manner from astrogliosis being an adaptive beneficial response under some circumstances to a maladaptive and deleterious process in another context. There is a growing support for the concept of astrocytopathies in which the disruption of normal astrocyte functions, astrodegeneration or dysfunctional/maladaptive astrogliosis are the primary cause or the main factor in neurological dysfunction and disease. This review describes the multiple roles of astrocytes in the healthy CNS, discusses the diversity of astroglial responses in neurological disorders and argues that targeting astrocytes may represent an effective therapeutic strategy for Alexander disease, neurotrauma, stroke, epilepsy and Alzheimer's disease as well as other neurodegenerative diseases.

Astrocytes as secretory cells of the central nervous system: idiosyncrasies of vesicular secretion

Astrocytes are housekeepers of the central nervous system (CNS) and are important for CNS development, homeostasis and defence. They communicate with neurones and other glial cells through the release of signalling molecules. Astrocytes secrete a wide array of classic neurotransmitters, neuromodulators and hormones, as well as metabolic, trophic and plastic factors, all of which contribute to the gliocrine system. The release of neuroactive substances from astrocytes occurs through several distinct pathways that include diffusion through plasmalemmal channels, translocation by multiple transporters and regulated exocytosis. As in other eukaryotic cells, exocytotic secretion from astrocytes involves divergent secretory organelles (synaptic-like microvesicles, dense-core vesicles, lysosomes, exosomes and ectosomes), which differ in size, origin, cargo, membrane composition, dynamics and functions. In this review, we summarize the features and functions of secretory organelles in astrocytes. We focus on the biogenesis and trafficking of secretory organelles and on the regulation of the exocytotic secretory system in the context of healthy and diseased astrocytes.

Glial Cells and Integrity of the Nervous System

Today, there is enormous progress in understanding the function of glial cells, including astroglia, oligodendroglia, Schwann cells, and microglia. Around 150 years ago, glia were viewed as a glue among neurons. During the course of the twentieth century, microglia were discovered and neuroscientists' views evolved toward considering glia only as auxiliary cells of neurons. However, over the last two to three decades, glial cells' importance has been reconsidered because of the evidence on their involvement in defining central nervous system architecture, brain metabolism, the survival of neurons, development and modulation of synaptic transmission, propagation of nerve impulses, and many other physiological functions. Furthermore, increasing evidence shows that glia are involved in the mechanisms of a broad spectrum of pathologies of the nervous system, including some psychiatric diseases, epilepsy, and neurodegenerative diseases to mention a few. It appears safe to say that no neurological disease can be understood without considering neuron-glia crosstalk. Thus, this book aims to show different roles played by glia in the healthy and diseased nervous system, highlighting some of their properties while considering that the various glial cell types are essential components not only for cell function and integration among neurons, but also for the emergence of important brain homeostasis.

Interleukin 1β Regulation of the System xc- Substrate-specific Subunit, xCT, in Primary Mouse Astrocytes Involves the RNA-binding Protein HuR

System xc(-) is a heteromeric amino acid cystine/glutamate antiporter that is constitutively expressed by cells of the CNS, where it functions in the maintenance of intracellular glutathione and extracellular glutamate levels. We recently determined that the cytokine, IL-1β, increases the activity of system xc(-) in CNS astrocytes secondary to an up-regulation of its substrate-specific light chain, xCT, and that this occurs, in part, at the level of transcription. However, an in silico analysis of the murine xCT 3'-UTR identified numerous copies of adenine- and uridine-rich elements, raising the possibility that undefined trans-acting factors governing mRNA stability and translation may also contribute to xCT expression. Here we show that IL-1β increases the level of mRNA encoding xCT in primary cultures of astrocytes isolated from mouse cortex in association with an increase in xCT mRNA half-life. Additionally, IL-1β induces HuR translocation from the nucleus to the cytoplasm. RNA immunoprecipitation analysis reveals that HuR binds directly to the 3'-UTR of xCT in an IL-1β-dependent manner. Knockdown of endogenous HuR protein abrogates the IL-1β-mediated increase in xCT mRNA half-life, whereas overexpression of HuR in unstimulated primary mouse astrocytes doubles the half-life of constitutive xCT mRNA. This latter effect is accompanied by an increase in xCT protein levels, as well as a functional increase in system xc(-) activity. Altogether, these data support a critical role for HuR in mediating the IL-1β-induced stabilization of astrocyte xCT mRNA.

Purinergic signalling in brain ischemia

Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia a primary damage due to the early massive increase of extracellular glutamate is followed by activation of resident immune cells, i.e microglia, and production or activation of inflammation mediators. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. Extracellular concentrations of ATP and adenosine in the brain increase dramatically during ischemia in concentrations able to stimulate their respective specific P2 and P1 receptors. Both ATP P2 and adenosine P1 receptor subtypes exert important roles in ischemia. Although adenosine exerts a clear neuroprotective effect through A1 receptors during ischemia, the use of selective A1 agonists is hampered by undesirable peripheral effects. Evidence up to now in literature indicate that A2A receptor antagonists provide protection centrally by reducing excitotoxicity, while agonists at A2A (and possibly also A2B) and A3 receptors provide protection by controlling massive infiltration and neuroinflammation in the hours and days after brain ischemia. Among P2X receptors most evidence indicate that P2X7 receptor contribute to the damage induced by the ischemic insult due to intracellular Ca(2+) loading in central cells and facilitation of glutamate release. Antagonism of P2X7 receptors might represent a new treatment to attenuate brain damage and to promote proliferation and maturation of brain immature resident cells that can promote tissue repair following cerebral ischemia. Among P2Y receptors, antagonists of P2Y12 receptors are of value because of their antiplatelet activity and possibly because of additional anti-inflammatory effects. Moreover strategies that modify adenosine or ATP concentrations at injury sites might be of value to limit damage after ischemia. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Peripheral nerve injury induces aquaporin-4 expression and astrocytic enlargement in spinal cord

Aquaporin-4 (AQP4), a water channel protein, is expressed mainly in the perivascular end-feet of astrocytes in the brain and spinal cord. Dysregulation of AQP4 is critically associated with abnormal water transport in the astrocytes. We aimed to examine whether peripheral nerve injury (PNI) could induce the changes of AQP4 expression and astrocytic morphology in the spinal cord. Two different PNI models [partial sciatic nerve transection (PST) and chronic constriction injury (CCI)] were established on the left sciatic nerve in Sprague-Dawley rats, which decreased the pain withdrawal threshold in the ipsilateral hind paws. Both PNI models were associated with a persistent up-regulation of AQP4 in the ipsilateral dorsal horn at the lower lumbar region over 3 weeks, despite an absence of direct injury to the spinal cord. Three-dimensional reconstruction of astrocytes was made and morphometric analysis was done. Up-regulation of AQP4 was accompanied by a significant increase in the length and volume of astrocytic processes and the number of branch points. The most prominent changes were present in the distal processes of the astrocytes and the changes were maintained throughout the whole experimental period. Extravasation of systemically administered tracers Evans Blue and sodium fluorescein was not seen in both models. Taken together, PNI was associated with a long-lasting AQP4 up-regulation and enlargement of astrocytic processes in the spinal cord in rats, both of which were not related to the disruption of blood-spinal cord barrier. The findings could provide novel insights on the understanding of pathophysiology of spinal cords after PNI.