Acute up-regulation of glutamate uptake mediated by mGluR5a in reactive astrocytes

INFLUENCES OF HEAT STRESS ON GLUTAMATE TRANSMISSION-DEPENDENT EXPRESSION LEVELS OF IL-1β and IL-18 IN BV-2 MICROGLIAL CELLS

ABSTRACT

Objective: This study aimed to explore the impact of heat stress (HS) on glutamate transmission-dependent expression levels of interleukin-1β (IL-1β) and IL-18 in BV-2 microglial cells. Methods: BV-2 microglial cells were cultured in vitro , with cells maintained at 37°C serving as the control. The HS group experienced incubation at 40°C for 1 h, followed by further culturing at 37°C for 6 or 12 h. The experimental group was preincubated with glutamate, the glutamate antagonist riluzole, or the mGluR5 agonist, 2-chloro-5-hydroxyphenylglycine (CHPG), before HS. Glutamate content in BV-2 culture supernatant was assessed using colorimetric assay. Moreover, mRNA expression levels of EAAT3 and/or mGluR5 in BV-2 cells were determined via quantitative polymerase chain reaction. Interleukins (IL-1β and IL-18) in cell culture supernatant were measured using enzyme-linked immunosorbent assay. Western blot analysis was employed to assess protein levels of IL-1β and IL-18 in BV-2 cells. Results: HS induced a significant release of glutamate and increased the expression levels of mGluR5 and EAAT3 in BV-2 cells. It also triggered the expression levels and release of proinflammatory factors, such as IL-1β and IL-18, synergizing with the effects of glutamate treatment. Preincubation with both riluzole and CHPG significantly reduced HS-induced glutamate release and mitigated the increased expression levels and release of IL-1β and IL-18 induced by HS. Conclusion: The findings confirmed that microglia could be involved in HS primarily through glutamate metabolisms, influencing the expression levels and release of IL-1β and IL-18.

Modulation of Type 5 Metabotropic Glutamate Receptor-Mediated Intracellular Calcium Mobilization by Regulator of G Protein Signaling 4 (RGS4) in Cultured Astrocytes

Acting as GTPase activating proteins promoting the silencing of activated G-proteins, regulators of G protein signaling (RGSs) are generally considered negative modulators of cell signaling. In the CNS, the expression of RGS4 is altered in diverse pathologies and its upregulation was reported in astrocytes exposed to an inflammatory environment. In a model of cultured cortical astrocytes, we herein investigate the influence of RGS4 on intracellular calcium signaling mediated by type 5 metabotropic glutamate receptor (mGluR5), which is known to support the bidirectional communication between neurons and glial cells. RGS4 activity was manipulated by exposure to the inhibitor CCG 63802 or by infecting the cells with lentiviruses designed to achieve the silencing or overexpression of RGS4. The pharmacological inhibition or silencing of RGS4 resulted in a decrease in the percentage of cells responding to the mGluR5 agonist DHPG and in the proportion of cells showing typical calcium oscillations. Conversely, RGS4-lentivirus infection increased the percentage of cells showing calcium oscillations. While the physiological implication of cytosolic calcium oscillations in astrocytes is still under investigation, the fine-tuning of calcium signaling likely determines the coding of diverse biological events. Indirect signaling modulators such as RGS4 inhibitors, used in combination with receptor ligands, could pave the way for new therapeutic approaches for diverse neurological disorders with improved efficacy and selectivity.

The role of astrocytes response triggered by hyperglycaemia during spinal cord injury

Objective: This manuscript aimed to provide a comprehensive overview of the physiological, molecular, and cellular mechanisms triggered by reactive astrocytes (RA) in the context of spinal cord injury (SCI), with a particular focus on cases involving hyperglycaemia.Methods: The compilation of articles related to astrocyte responses in neuropathological conditions, with a specific emphasis on those related to SCI and hyperglycaemia, was conducted by searching through databases including Science Direct, Web of Science, and PubMed.Results and Conclusions: This article explores the dual role of astrocytes in both neurophysiological and neurodegenerative conditions within the central nervous system (CNS). In the aftermath of SCI and hyperglycaemia, astrocytes undergo a transformation into RA, adopting a distinct phenotype. While there are currently no approved therapies for SCI, various therapeutic strategies have been proposed to alleviate the detrimental effects of RAs following SCI and hyperglycemia. These strategies show promising potential in the treatment of SCI and its likely comorbidities.

mGluR5 in Astrocytes in the Ventromedial Hypothalamus Regulates Pituitary Adenylate Cyclase-Activating Polypeptide Neurons and Glucose Homeostasis

The ventromedial hypothalamus (VMH) is a functionally heterogeneous nucleus critical for systemic energy, glucose, and lipid balance. We showed previously that the metabotropic glutamate receptor 5 (mGluR5) plays essential roles regulating excitatory and inhibitory transmission in SF1+ neurons of the VMH and facilitating glucose and lipid homeostasis in female mice. Although mGluR5 is also highly expressed in VMH astrocytes in the mature brain, its role there influencing central metabolic circuits is unknown. In contrast to the glucose intolerance observed only in female mice lacking mGluR5 in VMH SF1 neurons, selective depletion of mGluR5 in VMH astrocytes enhanced glucose tolerance without affecting food intake or body weight in both adult female and male mice. The improved glucose tolerance was associated with elevated glucose-stimulated insulin release. Astrocytic mGluR5 male and female mutants also exhibited reduced adipocyte size and increased sympathetic tone in gonadal white adipose tissue. Diminished excitatory drive and synaptic inputs onto VMH Pituitary adenylate cyclase-activating polypeptide (PACAP+) neurons and reduced activity of these cells during acute hyperglycemia underlie the observed changes in glycemic control. These studies reveal an essential role of astrocytic mGluR5 in the VMH regulating the excitatory drive onto PACAP+ neurons and activity of these cells facilitating glucose homeostasis in male and female mice.SIGNIFICANCE STATEMENT Neuronal circuits within the VMH play chief roles in the regulation of whole-body metabolic homeostasis. It remains unclear how astrocytes influence neurotransmission in this region to facilitate energy and glucose balance control. Here, we explored the role of the metabotropic glutamate receptor, mGluR5, using a mouse model with selective depletion of mGluR5 from VMH astrocytes. We show that astrocytic mGluR5 critically regulates the excitatory drive and activity of PACAP-expressing neurons in the VMH to control glucose homeostasis in both female and male mice. Furthermore, mGluR5 in VMH astrocytes influences adipocyte size and sympathetic tone in white adipose tissue. These studies provide novel insight toward the importance of hypothalamic astrocytes participating in central circuits regulating peripheral metabolism.

Genetic Downregulation of the Metabotropic Glutamate Receptor Type 5 Dampens the Reactive and Neurotoxic Phenotype of Adult ALS Astrocytes

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by progressive degeneration of motor neurons (MNs). Astrocytes display a toxic phenotype in ALS, which results in MN damage. Glutamate (Glu)-mediated excitotoxicity and group I metabotropic glutamate receptors (mGluRs) play a pathological role in the disease progression. We previously demonstrated that in vivo genetic ablation or pharmacological modulation of mGluR5 reduced astrocyte activation and MN death, prolonged survival and ameliorated the clinical progression in the SOD1G93A mouse model of ALS. This study aimed to investigate in vitro the effects of mGluR5 downregulation on the reactive spinal cord astrocytes cultured from adult late symptomatic SOD1G93A mice. We observed that mGluR5 downregulation in SOD1G93A astrocytes diminished the cytosolic Ca2+ overload under resting conditions and after mGluR5 simulation and reduced the expression of the reactive glial markers GFAP, S100β and vimentin. In vitro exposure to an anti-mGluR5 antisense oligonucleotide or to the negative allosteric modulator CTEP also ameliorated the altered reactive astrocyte phenotype. Downregulating mGluR5 in SOD1G93A mice reduced the synthesis and release of the pro-inflammatory cytokines IL-1β, IL-6 and TNF-α and ameliorated the cellular bioenergetic profile by improving the diminished oxygen consumption and ATP synthesis and by lowering the excessive lactate dehydrogenase activity. Most relevantly, mGluR5 downregulation hampered the neurotoxicity of SOD1G93A astrocytes co-cultured with spinal cord MNs. We conclude that selective reduction in mGluR5 expression in SOD1G93A astrocytes positively modulates the astrocyte reactive phenotype and neurotoxicity towards MNs, further supporting mGluR5 as a promising therapeutic target in ALS.

Revealing the contribution of astrocytes to glutamatergic neuronal transmission

Research on glutamatergic neurotransmission has focused mainly on the function of presynaptic and postsynaptic neurons, leaving astrocytes with a secondary role only to ensure successful neurotransmission. However, recent evidence indicates that astrocytes contribute actively and even regulate neuronal transmission at different levels. This review establishes a framework by comparing glutamatergic components between neurons and astrocytes to examine how astrocytes modulate or otherwise influence neuronal transmission. We have included the most recent findings about the role of astrocytes in neurotransmission, allowing us to understand the complex network of neuron-astrocyte interactions. However, despite the knowledge of synaptic modulation by astrocytes, their contribution to specific physiological and pathological conditions remains to be elucidated. A full understanding of the astrocyte's role in neuronal processing could open fruitful new frontiers in the development of therapeutic applications.

Targeting the Type 5 Metabotropic Glutamate Receptor: A Potential Therapeutic Strategy for Neurodegenerative Diseases?

The type 5 metabotropic glutamate receptor, mGlu₅, has been proposed as a potential therapeutic target for the treatment of several neurodegenerative diseases. In preclinical neurodegenerative disease models, novel allosteric modulators have been shown to improve cognitive performance and reduce disease-related pathology. A common pathological hallmark of neurodegenerative diseases is a chronic neuroinflammatory response, involving glial cells such as astrocytes and microglia. Since mGlu₅ is expressed in astrocytes, targeting this receptor could provide a potential mechanism by which neuroinflammatory processes in neurodegenerative disease may be modulated. This review will discuss current evidence that highlights the potential of mGlu₅ allosteric modulators to treat neurodegenerative diseases, including Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Furthermore, this review will explore the role of mGlu₅ in neuroinflammatory responses, and the potential for this G protein-coupled receptor to modulate neuroinflammation.

Prevention of L-Dopa-Induced Dyskinesias by MPEP Blockade of Metabotropic Glutamate Receptor 5 Is Associated with Reduced Inflammation in the Brain of Parkinsonian Monkeys

Proinflammatory markers were found in brains of Parkinson’s disease (PD) patients. After years of L-Dopa symptomatic treatment, most PD patients develop dyskinesias. The relationship between inflammation and L-Dopa-induced dyskinesias (LID) is still unclear. We previously reported that MPEP (a metabotropic glutamate receptor 5 antagonist) reduced the development of LID in de novo MPTP-lesioned monkeys. We thus investigated if MPEP reduced the brain inflammatory response in these MPTP-lesioned monkeys and the relationship to LID. The panmacrophage/microglia marker Iba1, the phagocytosis-related receptor CD68, and the astroglial protein GFAP were measured by Western blots. The L-Dopa-treated dyskinetic MPTP monkeys had increased Iba1 content in the putamen, substantia nigra, and globus pallidus, which was prevented by MPEP cotreatment; similar findings were observed for CD68 contents in the putamen and globus pallidus. There was a strong positive correlation between dyskinesia scores and microglial markers in these regions. GFAP contents were elevated in MPTP + L-Dopa-treated monkeys among these brain regions and prevented by MPEP in the putamen and subthalamic nucleus. In conclusion, these results showed increased inflammatory markers in the basal ganglia associated with LID and revealed that MPEP inhibition of glutamate activity reduced LID and levels of inflammatory markers.

Responsivity of serotonin transporter knockout rats to short and long access to cocaine: modulation of the glutamate signaling in the nucleus accumbens shell

Background and Purpose

It has been well established that glutamate in the nucleus accumbens (NAc) plays a critical role in the motivation to take drugs of abuse. We have previously demonstrated that rats with ablation of the serotonin transporter (SERT^−/− rats) show increased cocaine intake reminiscent of compulsivity.

Experimental Approach

By comparing SERT^−/− to SERT^+/+ rats, we investigated whether SERT deletion influences glutamate homeostasis under control conditions as well as after short access (ShA: 1 h per session) or long access (LgA: 6 h per session) to cocaine self‐administration. Rats were killed at 24 h after the last self‐administration session for ex vivo molecular analyses of the main determinants of the glutamate system, including transporters (vesicular and glial), receptors (main post‐synaptic subunits of NMDA and AMPA receptors together with the metabotropic subunit mGLUR5), and scaffolding proteins (SAP102, SAP97, and GRIP) in the NAc shell (sNAc)

Key Results

In cocaine‐naive animals, SERT deletion was associated with changes indicative for a reduction in glutamate signalling. ShA and LgA exposure led to a further dysregulation of the glutamatergic synapse.

Conclusion

SERT deletion may render the glutamatergic synapses of the NAc shell more responsive to both ShA and LgA intake of cocaine.

Rapid Regulation of Glutamate Transport: Where Do We Go from Here?

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na⁺-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.

Dynamic expression of homeostatic ion channels in differentiated cortical astrocytes in vitro

The capacity of astrocytes to adapt their biochemical and functional features upon physiological and pathological stimuli is a fundamental property at the basis of their ability to regulate the homeostasis of the central nervous system (CNS). It is well known that in primary cultured astrocytes, the expression of plasma membrane ion channels and transporters involved in homeostatic tasks does not closely reflect the pattern observed in vivo. The individuation of culture conditions that promote the expression of the ion channel array found in vivo is crucial when aiming at investigating the mechanisms underlying their dynamics upon various physiological and pathological stimuli. A chemically defined medium containing growth factors and hormones (G5) was previously shown to induce the growth, differentiation, and maturation of primary cultured astrocytes. Here we report that under these culture conditions, rat cortical astrocytes undergo robust morphological changes acquiring a multi-branched phenotype, which develops gradually during the 2-week period of culturing. The shape changes were paralleled by variations in passive membrane properties and background conductance owing to the differential temporal development of inwardly rectifying chloride (Cl⁻) and potassium (K⁺) currents. Confocal and immunoblot analyses showed that morphologically differentiated astrocytes displayed a large increase in the expression of the inward rectifier Cl⁻ and K⁺ channels ClC-2 and Kir4.1, respectively, which are relevant ion channels in vivo. Finally, they exhibited a large diminution of the intermediate filaments glial fibrillary acidic protein (GFAP) and vimentin which are upregulated in reactive astrocytes in vivo. Taken together the data indicate that long-term culturing of cortical astrocytes in this chemical-defined medium promotes a quiescent functional phenotype. This culture model could aid to address the regulation of ion channel expression involved in CNS homeostasis in response to physiological and pathological challenges.

Blocking glutamate mGlu5 receptors with the negative allosteric modulator CTEP improves disease course in SOD1G93A mouse model of amyotrophic lateral sclerosis

BACKGROUND AND PURPOSE

The pathogenesis of amyotrophic lateral sclerosis (ALS) is not fully clarified, although excessive glutamate (Glu) transmission and the downstream cytotoxic cascades are major mechanisms for motor neuron death. Two metabotropic glutamate receptors (mGlu₁ and mGlu₅ ) are overexpressed in ALS and regulate cellular disease processes. Expression and function of mGlu₅ receptors are altered at early symptomatic stages in the SOD1^G93A mouse model of ALS and knockdown of mGlu5 receptors in SOD1^G93A mice improved disease progression.

EXPERIMENTAL APPROACH

We treated male and female SOD1^G93A mice with 2-chloro-4-((2,5-dimethyl-1-(4-(trifluoromethoxy)phenyl)-1H-imidazol-4-yl)ethynyl)pyridine (CTEP), an orally available mGlu₅ receptor negative allosteric modulator (NAM), using doses of 2 mg·kg^-1 per 48 h or 4 mg·kg^-1 per 24 h from Day 90, an early symptomatic disease stage. Disease progression was studied by behavioural and histological approaches.

KEY RESULTS

CTEP dose-dependently ameliorated clinical features in SOD1^G93A mice. The lower dose increased survival and improved motor skills in female mice, with barely positive effects in male mice. Higher doses significantly ameliorated disease symptoms and survival in both males and females, females being more responsive. CTEP also reduced motor neuron death, astrocyte and microglia activation, and abnormal glutamate release in the spinal cord, with equal effects in male and female mice. No differences were also observed in CTEP access to the brain.

CONCLUSION AND IMPLICATIONS

Our results suggest that mGlu₅ receptors are promising targets for the treatment of ALS and highlight mGlu5 receptor NAMs as effective pharmacological tools with translational potential.

Glia and Orofacial Pain: Progress and Future Directions

Orofacial pain is a universal predicament, afflicting millions of individuals worldwide. Research on the molecular mechanisms of orofacial pain has predominately focused on the role of neurons underlying nociception. However, aside from neural mechanisms, non-neuronal cells, such as Schwann cells and satellite ganglion cells in the peripheral nervous system, and microglia and astrocytes in the central nervous system, are important players in both peripheral and central processing of pain in the orofacial region. This review highlights recent molecular and cellular findings of the glia involvement and glia–neuron interactions in four common orofacial pain conditions such as headache, dental pulp injury, temporomandibular joint dysfunction/inflammation, and head and neck cancer. We will discuss the remaining questions and future directions on glial involvement in these four orofacial pain conditions.

Astrocyte Glutamate Uptake and Signaling as Novel Targets for Antiepileptogenic Therapy

Astrocytes regulate and respond to extracellular glutamate levels in the central nervous system (CNS) via the Na⁺-dependent glutamate transporters glutamate transporter-1 (GLT-1) and glutamate aspartate transporter (GLAST) and the metabotropic glutamate receptors (mGluR) 3 and mGluR5. Both impaired astrocytic glutamate clearance and changes in metabotropic glutamate signaling could contribute to the development of epilepsy. Dysregulation of astrocytic glutamate transporters, GLT-1 and GLAST, is a common finding across patients and preclinical seizure models. Astrocytic metabotropic glutamate receptors, particularly mGluR5, have been shown to be dysregulated in both humans and animal models of temporal lobe epilepsy (TLE). In this review, we synthesize the available evidence regarding astrocytic glutamate homeostasis and astrocytic mGluRs in the development of epilepsy. Modulation of astrocyte glutamate uptake and/or mGluR activation could lead to novel glial therapeutics for epilepsy.

Reactive Astrogliosis: Implications in Spinal Cord Injury Progression and Therapy

Astrocytes are the most populous glial cells in the central nervous system (CNS). They are essential to CNS physiology and play important roles in the maintenance of homeostasis, development of synaptic plasticity, and neuroprotection. Nevertheless, under the influence of certain factors, astrocytes may also exert detrimental effects through a process of reactive astrogliosis. Previous studies have shown that astrocytes have more than one type of polarization. Two types have been extensively researched. One is a damaging change that occurs under inflammation and has been termed A1 astrocyte, while the other is a restorative change that occurs under ischemic induction and was termed A2 astrocyte. Researchers are now increasingly paying attention to the role of astrocytes in spinal cord injury (SCI), degenerative diseases, chronic pain, neurological tumors, and other CNS disorders. In this review, we discuss (a) the characteristics of polarized astrocytes, (b) the relationship between astrocyte polarization and SCI, and (c) new implications of reactive astrogliosis for future SCI therapies.

Excitatory amino acid transporter (EAAT)1 and EAAT2 mRNA levels are altered in the prefrontal cortex of subjects with schizophrenia

Excitatory amino acid transporter (EAAT)1 and EAAT2 mediate glutamatergic neurotransmission and prevent excitotoxicity through binding and transportation of glutamate into glia. These EAATs may be regulated by metabotropic glutamate receptor 5 (mGluR5), which is also expressed by glia. Whilst we have data from an Affymetrix™ Human Exon 1.0 ST Array showing higher levels of EAAT1 mRNA (+36%) in Brodmann's are (BA)9 of subjects with schizophrenia, there is evidence that EAAT1 and EAAT2, as well as mGluR5 levels, are altered in the cortex of subjects with the disorder. Hence, we measured mRNA levels of these genes in other cortical regions in subjects with that disorder. EAAT1, EAAT2 and mGluR5 mRNA were measured, in triplicate, using Quantitative PCR in BA10 and BA46 from subjects with schizophrenia (n = 20) and age and sex matched controls (n = 18). Levels of mRNA were normalised to the geometric mean of two reference genes, transcription factor B1, mitochondrial (TFB1M) and S-phase kinase-associated protein 1A (SKP1A), for which mRNA did not vary between diagnostic groups in either region. Normalised levels of EAAT1 and EAAT2 mRNA were significantly higher in BA10 (EAAT1: U = 58, p = 0.0002; EAAT2 U = 70, p = 0.0009), but not BA46 (EAAT1: U = 122, p = 0.09; EAAT2: U = 136, p = 0.21), from subjects with schizophrenia compared to controls. mGluR5 levels in BA10 (U = 173, p=0.85) and BA46 (U = 178, p = 0.96) did not vary by cohort. Our data suggests that region-specific increases in cortical EAAT1 and EAAT2 mRNA are involved in schizophrenia pathophysiology and that disrupted glutamate uptake in schizophrenia may be of particular significance in BA10.

Glial Scar-a Promising Target for Improving Outcomes After CNS Injury

After central nervous system (CNS) injury, a series of stress responses induce astrocytes activation. Reactive astrocytes, which are typically different from astrocytes in normal conditions in altered morphology and gene expression, combine with extracellular matrix (ECM) components to form a glial scar at the lesion site, which walls of the injured region from neighboring healthier tissue. However, as a physical and molecular barrier, glial scar can impede patients' functional recovery in the late period of CNS injury. Thus, inhibiting glial scar formation in the chronic stage after CNS injury may be a promising target to improve outcomes. Since the therapeutic strategies targeting on mediating glial scar formation are regarded as an important part on improving functional recovery after CNS injury, in this review, we focus on the regulating effects of related signaling pathways and other molecules on glial scar, and the process of glial scar formation and the roles that it plays during the acute and chronic stages are also expounded in this article. We hope to get a comprehensive understanding of glial scar during CNS injury based on current researches and to open new perspectives for the therapies to promote functional recovery after CNS injury.

Metabotropic Glutamate Receptors in Glial Cells: A New Potential Target for Neuroprotection?

Neurodegenerative disorders are characterized by excitotoxicity and neuroinflammation that finally lead to slow neuronal degeneration and death. Although neurons are the principal target, glial cells are important players as they contribute by either exacerbating or dampening the events that lead to neuroinflammation and neuronal damage. A dysfunction of the glutamatergic system is a common event in the pathophysiology of these diseases. Metabotropic glutamate (mGlu) receptors belong to a large family of G protein-coupled receptors largely expressed in neurons as well as in glial cells. They often appear overexpressed in areas involved in neurodegeneration, where they can modulate glutamatergic transmission. Of note, mGlu receptor upregulation may involve microglia or, even more frequently, astrocytes, where their activation causes release of factors potentially able to influence neuronal death. The expression of mGlu receptors has been also reported on oligodendrocytes, a glial cell type specifically involved in the development of multiple sclerosis. Here we will provide a general overview on the possible involvement of mGlu receptors expressed on glial cells in the pathogenesis of different neurodegenerative disorders and the potential use of subtype-selective mGlu receptor ligands as candidate drugs for the treatment of neurodegenerative disorders. Negative allosteric modulators (NAM) of mGlu5 receptors might represent a relevant pharmacological tool to develop new neuroprotective strategies in these diseases. Recent evidence suggests that targeting astrocytes and microglia with positive allosteric modulators (PAM) of mGlu3 receptor or oligodendrocytes with mGlu4 PAMS might represent novel pharmacological approaches for the treatment of neurodegenerative disorders.

Constitutive downregulation protein kinase C epsilon in hSOD1G93A astrocytes influences mGluR5 signaling and the regulation of glutamate uptake

Accumulating evidence indicates that motor neuron degeneration in amyotrophic lateral sclerosis (ALS) is a non-cell-autonomous process and that impaired glutamate clearance by astrocytes, leading to excitotoxicity, could participate in progression of the disease. In astrocytes derived from an animal model of ALS (hSOD1^G93A rats), activation of type 5 metabotropic glutamate receptor (mGluR5) fails to increase glutamate uptake, impeding a putative dynamic neuroprotective mechanism involving astrocytes. Using astrocyte cultures from hSOD1^G93A rats, we have demonstrated that the typical Ca²⁺ oscillations associated with mGluR5 activation were reduced, and that the majority of cells responded with a sustained elevation of intracellular Ca²⁺ concentration. Since the expression of protein kinase C epsilon isoform (PKCɛ) has been found to be considerably reduced in astrocytes from hSOD1^G93A rats, the consequences of manipulating its activity and expression on mGluR5 signaling and on the regulation of glutamate uptake have been examined. Increasing PKCɛ expression was found to restore Ca²⁺ oscillations induced by mGluR5 activation in hSOD1^G93A -expressing astrocytes. This was also associated with an increase in glutamate uptake capacity in response to mGluR5 activation. Conversely, reducing PKCɛ expression in astrocytes from wild-type animals with specific PKCɛ-shRNAs was found to alter the mGluR5 associated oscillatory signaling profile, and consistently reduced the regulation of the glutamate uptake-mediated by mGluR5 activation. These results suggest that PKCɛ is required to generate Ca²⁺ oscillations following mGluR5 activation, which support the regulation of astrocytic glutamate uptake. Reduced expression of astrocytic PKCɛ could impair this neuroprotective process and participate in the progression of ALS.

Inflammation-associated regulation of RGS in astrocytes and putative implication in neuropathic pain

Background

Regulators of G-protein signaling (RGS) are major physiological modulators of G-protein-coupled receptors (GPCR) signaling. Several GPCRs expressed in both neurons and astrocytes participate in the central control of pain processing, and the reduced efficacy of analgesics in neuropathic pain conditions may rely on alterations in RGS function. The expression and the regulation of RGS in astrocytes is poorly documented, and we herein hypothesized that neuroinflammation which is commonly observed in neuropathic pain could influence RGS expression in astrocytes.

Methods

In a validated model of neuropathic pain, the spared nerve injury (SNI), the regulation of RGS2, RGS3, RGS4, and RGS7 messenger RNA (mRNA) was examined up to 3 weeks after the lesion. Changes in the expression of the same RGS were also studied in cultured astrocytes exposed to defined activation protocols or to inflammatory cytokines.

Results

We evidenced a differential regulation of these RGS in the lumbar spinal cord of animals undergoing SNI. In particular, RGS3 appeared upregulated at early stages after the lesion whereas expression of RGS2 and RGS4 was decreased at later stages. Decrease in RGS7 expression was already observed after 3 days and outlasted until 21 days after the lesion. In cultured astrocytes, we observed that changes in the culture conditions distinctly influenced the constitutive expression of these RGS. Also, brief exposures (4 to 8 h) to either interleukin-1β, interleukin-6, or tumor necrosis factor α caused rapid changes in the mRNA levels of the RGS, which however did not strictly recapitulate the regulations observed in the spinal cord of lesioned animals. Longer exposure (48 h) to inflammatory cytokines barely influenced RGS expression, confirming the rapid but transient regulation of these cell signaling modulators.

Conclusion

Changes in the environment of astrocytes mimicking the inflammation observed in the model of neuropathic pain can affect RGS expression. Considering the role of astrocytes in the onset and progression of neuropathic pain, we propose that the inflammation-mediated modulation of RGS in astrocytes constitutes an adaptive mechanism in a context of neuroinflammation and may participate in the regulation of nociception.

The role of system Xc- in methamphetamine-induced dopaminergic neurotoxicity in mice

The cystine/glutamate antiporter (system Xc^-, Sxc) transports cystine into cell in exchange for glutamate. Since xCT is a specific subunit of Sxc, we employed xCT knockout mice and investigated whether this antiporter affected methamphetamine (MA)-induced dopaminergic neurotoxicity. MA treatment significantly increased striatal oxidative burdens in wild type mice. xCT inhibitor [i.e., S-4-carboxy-phenylglycine (CPG), sulfasalazine] or an xCT knockout significantly protected against these oxidative burdens. MA-induced increases in Iba-1 expression and Iba-1-labeled microglial immunoreactivity (Iba-1-IR) were significantly attenuated by CPG or sulfasalazine administration or xCT knockout. CPG or sulfasalazine significantly attenuated MA-induced TUNEL-positive cell populations in the striatum of Taconic ICR mice. The decrease in excitatory amino acid transporter-2 (or glutamate transporter-1) expression and increase in glutamate release were attenuated by CPG, sulfasalazine or xCT knockout. In addition, CPG, sulfasalazine or xCT knockout significantly protected against dopaminergic loss (i.e., decreases in tyrosine hydroxylase expression and immunoreactivity, and an increase in dopamine turnover rate) induced by MA. However, CPG, sulfasalazine or xCT knockout did not significantly affect the impaired glutathione system [i.e., decrease in reduced glutathione (GSH) and increase in oxidized glutathione (GSSG)] induced by MA. Our results suggest that Sxc mediates MA-induced neurotoxicity via facilitating oxidative stress, microgliosis, proapoptosis, and glutamate-related toxicity.

PKC epsilon-dependent calcium oscillations associated with metabotropic glutamate receptor 5 prevent agonist-mediated receptor desensitization in astrocytes

A critical role has been assigned to protein kinase C (PKC)ε in the control of intracellular calcium oscillations triggered upon activation of type 5 metabotropic glutamate receptor (mGluR5) in cultured astrocytes. Nevertheless, the physiological significance of this particular signalling profile in the response of astrocytes to glutamate remains largely unknown. Considering that kinases are frequently involved in the regulation of G protein-coupled receptors, we have examined a putative link between the nature of the calcium signals and the response regulation upon repeated exposures of astrocytes to the agonist (S)-3,5-dihydroxyphenylglycine. We show that upon repeated mGluR5 activations, a robust desensitization was observed in astrocytes grown in culture conditions favouring the peak-plateau-type response. At variance, in cell cultures where calcium oscillations were predominating, the response was fully preserved even during repeated challenges with the agonist. Pharmacological inhibition of PKCε or genetic suppression of this isoform using shRNA was found to convert an oscillatory calcium profile to a sustained calcium mobilization and this latter profile was subject to desensitization upon repetitive mGluR5 activation. Our results suggest a yet undocumented scheme in which the activity of PKCε contributes to preserve the receptor sensitivity upon repeated or sustained activations. Cover Image for this issue: doi: 10.1111/jnc.13797.

The Role of Adenosine Signaling in Headache: A Review

Migraine is the third most prevalent disease on the planet, yet our understanding of its mechanisms and pathophysiology is surprisingly incomplete. Recent studies have built upon decades of evidence that adenosine, a purine nucleoside that can act as a neuromodulator, is involved in pain transmission and sensitization. Clinical evidence and rodent studies have suggested that adenosine signaling also plays a critical role in migraine headache. This is further supported by the widespread use of caffeine, an adenosine receptor antagonist, in several headache treatments. In this review, we highlight evidence that supports the involvement of adenosine signaling in different forms of headache, headache triggers, and basic headache physiology. This evidence supports adenosine A_2A receptors as a critical adenosine receptor subtype involved in headache pain. Adenosine A_2A receptor signaling may contribute to headache via the modulation of intracellular Cyclic adenosine monophosphate (cAMP) production or 5' AMP-activated protein kinase (AMPK) activity in neurons and glia to affect glutamatergic synaptic transmission within the brainstem. This evidence supports the further study of adenosine signaling in headache and potentially illuminates it as a novel therapeutic target for migraine.

Metabotropic glutamate receptor 5 may be involved in macrophage plasticity

Background

Macrophages are a functionally heterogeneous cell population and depending on microenvironments they polarize in two main groups: M1 and M2. Glutamic acid and glutamate receptors may participate in the regulation of macrophage plasticity. To investigate the role of glutamatergic systems in macrophages physiology, we performed the transfection of mGluR5 cDNAs into RAW-264.7 cells.

Results

Comparative analysis of modified (RAW-mGluR5 macrophages) and non-modified macrophages (RAW-macrophages) has shown that the RAW-mGluR5 macrophages absorbed more glutamate than control cells and the amount of intracellular glutamate correlated with the expression of excitatory amino acid transporters -2 (EAAT-2). Besides, our results have shown that RAW-mGluR5 macrophages expressed a higher level of peroxisome proliferator-activated receptor γ (PPAR-γ) and secreted more IL-10, high mobility group box 1 proteins (HMGB1) and Galectin-3 than control RAW-macrophages.

Conclusions

We propose that elevation of intracellular glutamate and expression of mGluR5 may initiate the metabolic rearrangement in macrophages that could contribute to the formation of an immunosuppressive phenotype.

Disinhibition of the intergeniculate leaflet network in the WAG/Rij rat model of absence epilepsy

The intergeniculate leaflet (IGL) of the thalamus is a retinorecipient structure implicated in orchestrating circadian rhythmicity. The IGL network is highly GABAergic and consists mainly of neuropeptide Y-synthesising and enkephalinergic neurons. A high density of GFAP-immunoreactive astrocytes has been observed in the IGL, with a probable function in guarding neuronal inhibition. Interestingly, putatively enkephalinergic IGL neurons generate action potentials with an infra-slow oscillatory (ISO) pattern in vivo in urethane anesthetised Wistar rats, under light-on conditions only. Absence epilepsy (AE) is a disease characterised by spike-wave discharges present in the encephalogram, directly caused by hypersynchronous thalamo-cortical oscillations. Many pathologies connected with the arousal system, such as abnormalities in sleep architecture and an insufficient brain sleep-promoting system accompany the epileptic phenotype. We hypothesise that disturbances in the function of biological clock structures, controlling this rhythmic physiological process, may be responsible for the observed pathomechanism. To test this hypothesis, we performed an in vitro patch-clamp study on WAG/Rij rats, a well-validated genetic model of AE, in order to assess dampened GABAergic synaptic transmission in the IGL expressed as a lower IPSC amplitude and reduced sIPSC frequency. Moreover, our in vivo extracellular recordings showed higher firing rate of ISO IGL neurons with an abnormal reaction to a change in constant illumination (maintenance of rhythmic neuronal activity in darkness) in the AE model. Additional immunohistochemical experiments indicated astrogliosis in the area of the IGL, which may partially underlie the observed changes in inhibition. Altogether, the data presented here show for the first time the disinhibition of IGL neurons in a model of AE, thereby proposing the possible involvement of circadian-related brain structures in the epileptic phenotype.

Molecular and cellular physiology of sodium-dependent glutamate transporters

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

Quantitative Analysis of Glutamate Receptors in Glial Cells from the Cortex of GFAP/EGFP Mice Following Ischemic Injury: Focus on NMDA Receptors

Cortical glial cells contain both ionotropic and metabotropic glutamate receptors. Despite several efforts, a comprehensive analysis of the entire family of glutamate receptors and their subunits present in glial cells is still missing. Here, we provide an overall picture of the gene expression of ionotropic (AMPA, kainate, NMDA) and the main metabotropic glutamate receptors in cortical glial cells isolated from GFAP/EGFP mice before and after focal cerebral ischemia. Employing single-cell RT-qPCR, we detected the expression of genes encoding subunits of glutamate receptors in GFAP/EGFP-positive (GFAP/EGFP(+)) glial cells in the cortex of young adult mice. Most of the analyzed cells expressed mRNA for glutamate receptor subunits, the expression of which, in most cases, even increased after ischemic injury. Data analyses disclosed several classes of GFAP/EGFP(+) glial cells with respect to glutamate receptors and revealed in what manner their expression correlates with the expression of glial markers prior to and after ischemia. Furthermore, we also examined the protein expression and functional significance of NMDA receptors in glial cells. Immunohistochemical analyses of all seven NMDA receptor subunits provided direct evidence that the GluN3A subunit is present in GFAP/EGFP(+) glial cells and that its expression is increased after ischemia. In situ and in vitro Ca(2+) imaging revealed that Ca(2+) elevations evoked by the application of NMDA were diminished in GFAP/EGFP(+) glial cells following ischemia. Our results provide a comprehensive description of glutamate receptors in cortical GFAP/EGFP(+) glial cells and may serve as a basis for further research on glial cell physiology and pathophysiology.

Astrocytic dynamin-like protein 1 regulates neuronal protection against excitotoxicity in Parkinson disease

Mitochondrial dynamics has recently become an area of piqued interest in neurodegenerative disorders, including Parkinson disease (PD); however, the contribution of astrocytes to these disorders remains unclear. Here, we show that the level of dynamin-like protein 1 (Dlp1; official name DNM1L), which promotes mitochondrial fission, is lower in astrocytes from the brains of PD patients, and that decreased astrocytic Dlp1 likely represents a relatively early event in PD pathogenesis. In support of this conclusion, we show that Dlp1 knockdown dramatically affects mitochondrial morphological characteristics and localization in astrocytes, impairs the ability of astrocytes to adequately protect neurons from the excitotoxic effects of glutamate, and increases intracellular Ca(2+) in response to extracellular glutamate, resulting from compromised intracellular Ca(2+) buffering. Taken together, our results suggest that astrocytic mitochondrial Dlp1 is a key protein in mitochondrial dynamics and decreased Dlp1 may interfere with neuron survival in PD by disrupting Ca(2+)-coupled glutamate uptake.

Synaptic rearrangement following axonal injury: Old and new players

Following axotomy, the contact between motoneurons and muscle fibers is disrupted, triggering a retrograde reaction at the neuron cell body within the spinal cord. Together with chromatolysis, a hallmark of such response to injury is the elimination of presynaptic terminals apposing to the soma and proximal dendrites of the injured neuron. Excitatory inputs are preferentially eliminated, leaving the cells under an inhibitory influence during the repair process. This is particularly important to avoid glutamate excitotoxicity. Such shift from transmission to a regeneration state is also reflected by deep metabolic changes, seen by the regulation of several genes related to cell survival and axonal growth. It is unclear, however, how exactly synaptic stripping occurs, but there is substantial evidence that glial cells play an active role in this process. In one hand, immune molecules, such as the major histocompatibility complex (MHC) class I, members of the complement family and Toll-like receptors are actively involved in the elimination/reapposition of presynaptic boutons. On the other hand, plastic changes that involve sprouting might be negatively regulated by extracellular matrix proteins such as Nogo-A, MAG and scar-related chondroitin sulfate proteoglycans. Also, neurotrophins, stem cells, physical exercise and several drugs seem to improve synaptic stability, leading to functional recovery after lesion. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Endothelin-1 is over-expressed in amyotrophic lateral sclerosis and induces motor neuron cell death

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by progressive loss of motor neurons (MNs) and astrogliosis. Recent evidence suggests that factors secreted by activated astrocytes might contribute to degeneration of MNs. We focused on endothelin-1 (ET-1), a peptide which is strongly up-regulated in reactive astrocytes under different pathological conditions. We show that ET-1 is abundantly expressed by reactive astrocytes in the spinal cord of the SOD1-G93A mouse model and sporadic ALS patients. To test if ET-1 might play a role in degeneration of MNs, we investigated its effect on MN survival in an in vitro model of mixed rat spinal cord cultures (MSCs) enriched of astrocytes exhibiting a reactive phenotype. ET-1 exerted a toxic effect on MNs in a time- and concentration-dependent manner, with an exposure to 100-200nM ET-1 for 48h resulting in 40-50% MN cell death. Importantly, ET-1 did not induce MN degeneration when administered on cultures treated with AraC (5μM) or grown in a serum-free medium that did not favor astrocyte proliferation and reactivity. We found that both ETA and ETB receptors are enriched in astrocytes in MSCs. The ET-1 toxic effect was mimicked by ET-3 (100nM) and sarafotoxin S6c (10nM), two selective agonists of endothelin-B receptors, and was not additive with that of ET-3 suggesting the involvement of ETB receptors. Surprisingly, however, the ET-1 effect persisted in the presence of the ETB receptor antagonist BQ-788 (200nM-2μM) and was slightly reversed by the ETA receptor antagonist BQ-123 (2μM), suggesting an atypical pharmacological profile of the astrocytic receptors responsible for ET-1 toxicity. The ET-1 effect was not undone by the ionotropic glutamate receptor AMPA antagonist GYKI 52466 (20μM), indicating that it is not caused by an increased glutamate release. Conversely, a 48-hour ET-1 treatment increased MN cell death induced by acute exposure to AMPA (50μM), which is indicative of two distinct pathways leading to neuronal death. Altogether these results indicate that ET-1 exerts a toxic effect on cultured MNs through mechanisms mediated by reactive astrocytes and suggest that ET-1 may contribute to MN degeneration in ALS. Thus, a treatment aimed at lowering ET-1 levels or antagonizing its effect might be envisaged as a potential therapeutic strategy to slow down MN degeneration in this devastating disease.

Lesion-induced alterations in astrocyte glutamate transporter expression and function in the hippocampus

Astrocytes express the sodium-dependent glutamate transporters GLAST and GLT-1, which are critical to maintain low extracellular glutamate concentrations. Here, we analyzed changes in their expression and function following a mechanical lesion in the CA1 area of organotypic hippocampal slices. 6-7 days after lesion, a glial scar had formed along the injury site, containing strongly activated astrocytes with increased GFAP and S100 β immunoreactivity, enlarged somata, and reduced capability for uptake of SR101. Astrocytes in the scar's periphery were swollen as well, but showed only moderate upregulation of GFAP and S100 β and efficiently took up SR101. In the scar, clusters of GLT-1 and GLAST immunoreactivity colocalized with GFAP-positive fibers. Apart from these, GLT-1 immunoreactivity declined with increasing distance from the scar, whereas GLAST expression appeared largely uniform. Sodium imaging in reactive astrocytes indicated that glutamate uptake was strongly reduced in the scar but maintained in the periphery. Our results thus show that moderately reactive astrocytes in the lesion periphery maintain overall glutamate transporter expression and function. Strongly reactive astrocytes in the scar, however, display clusters of GLAST and GLT-1 immunoreactivity together with reduced glutamate transport activity. This reduction might contribute to increased extracellular glutamate concentrations and promote excitotoxic cell damage at the lesion site.

cAMP response element-binding protein (CREB) and nuclear factor κB mediate the tamoxifen-induced up-regulation of glutamate transporter 1 (GLT-1) in rat astrocytes

Tamoxifen (TX), a selective estrogen receptor modulator, exerts antagonistic effects on breast tissue and is used to treat breast cancer. Recent evidence also suggests that it may act as an agonist in brain tissue. We reported previously that TX enhanced the expression and function of glutamate transporter 1 (GLT-1) in rat astrocytes, an effect that was mediated by TGF-α. To gain further insight into the mechanisms that mediate TX-induced up-regulation of GLT-1 (EAAT2 in humans), we investigated its effect on GLT-1 at the transcriptional level. TX phosphorylated the cAMP response element-binding protein (CREB) and recruited CREB to the GLT-1 promoter consensus site. The effect of TX on astrocytic GLT-1 was attenuated by the inhibition of PKA, the upstream activator of the CREB pathway. In addition, the effect of TX on GLT-1 promoter activity was abolished by the inhibition of the NF-κB pathway. Furthermore, TX recruited the NF-κB subunits p65 and p50 to the NF-κB binding domain of the GLT-1 promoter. Mutation of NF-κB (triple, -583/-282/-251) or CRE (-308) sites on the GLT-1 promoter led to significant repression of the promoter activity, but neither mutant completely abolished the TX-induced GLT-1 promoter activity. Mutation of both the NF-κB (-583/-282/-251) and CRE (-308) sites led to a complete abrogation of the effect of TX on GLT-1 promoter activity. Taken together, our findings establish that TX regulates GLT-1 via the CREB and NF-κB pathways.

Contribution of neuronal and glial circuit in intact hemisphere for functional remodeling after focal ischemia

The number of people who suffer from disabilities such as aphasia and/or paralysis after a focal brain stroke has not markedly decreased even in countries with established medical care systems. Functions such as speech can be lost following a stroke; however, such functions can sometimes be recovered. In this review, we focus on functional compensation that was achieved by the intact region contralateral to the stroke region. Using a mice stroke model, we used in vivo imaging techniques in combination with conventional electrophysiology and behavior tests, which showed that functional recovery was achieved through the specific synaptic (neuronal circuit) remodeling at the region contralateral to the focal stroke region 1 week after the stroke. During this period, astrocytes play a critical role in reducing the accumulation of synaptically released glutamate in the extracellular space, which would otherwise cause excitotoxicity. These findings indicate that the hemisphere that was intact after a stroke can potentially achieve bilateral functions even in adults when proper remodeling of neuronal circuits occurs. Activating the intact hemisphere may become a new therapeutic strategy for stroke patients.

Distinct increased metabotropic glutamate receptor type 5 (mGluR5) in temporal lobe epilepsy with and without hippocampal sclerosis

Metabotropic glutamate receptor type 5 (mGluR5) upregulation in temporal lobe epilepsy (TLE) and the correlation of its expression with features of hippocampal sclerosis (HS) remains unclear. Here we characterized mGluR5 immunoreactivity in hippocampus, entorhinal cortex (EC), and subiculum of TLE specimens with confirmed HS, with neocortical TLE (non-HS) and necropsy controls. We correlated mGluR5 immunoreactivity with neuronal density, mossy fiber sprouting, astrogliosis (GFAP), and dendritic alterations (MAP2). TLE specimens showed increased mGluR5 expression, which was most pronounced in the EC, subiculum, CA2, and dentate gyrus outer molecular layer. Increased mGluR5 expression was seen in hippocampal head and body segments and was independent of neuronal density, astrogliosis, or dendritic alterations. Positive correlation between mGluR5 expression with mossy fiber sprouting and with MAP2 in CA3 and CA1 was found only in HS specimens. Negative correlation between mGluR5 expression with seizure frequency and epilepsy duration was found only in non-HS cases. Specimens from HS patients without previous history of febrile seizure (FS) showed higher mGluR5 and MAP2 expression in CA2. Our study suggests that mGluR5 upregulation is part of a repertoire of post-synaptic adaptations that might control overexcitation and excessive glutamate release rather than a dysfunction that leads to seizure facilitation. That would explain why non-HS cases, on which seizures are likely to originate outside the hippocampal formation, also exhibit upregulated mGluR5. On the other hand, lower mGluR5 expression was related to increased seizure frequency. In addition to its role in hyperexcitability, mGluR5 upregulation could play a role in counterbalance mechanisms along the hyperexcitable circuitry uniquely altered in sclerotic hippocampal formation. Inefficient post-synaptic compensatory morphological (dendritic branching) and glutamatergic (mGluR5 expression) mechanisms in CA2 subfield could potentially underlie the association of FS with HS and TLE. © 2013 Wiley Periodicals, Inc.

Critical role of the astrocyte for functional remodeling in contralateral hemisphere of somatosensory cortex after stroke

After ischemic stroke, the corresponding area contralateral to the lesion may partly compensate for the loss of function. We previously reported the remodeling of neuronal circuits in the contralateral somatosensory cortex (SSC) during the first week after infarction for processing bilateral information, resulting in functional compensation. However, the underlying processes in the contralateral hemisphere after stroke have not yet been fully elucidated. Recent studies have shown that astrocytes may play critical roles in synaptic reorganization and functional compensation after a stroke. Thus, we aim to clarify the contribution of astrocytes using a rodent stroke model. In vivo calcium imaging showed a significantly large number of astrocytes in the contralateral SSC responding to ipsilateral limb stimulation at the first week after infarction. Simultaneously, extracellular glutamine level increased, indicating the involvement of astrocytes in the conversion of glutamate to glutamine, which may be an important process for functional recovery. This hypothesis was supported further by the observation that application of (2S,3S)-3-{3-[4-(trifluoromethyl)benzoylamino]benzyloxy} aspartate, a glial glutamate transporter blocker, disturbed the functional recovery. These findings indicate the involvement of astrocytes in functional remodeling/recovery in the area contralateral to the lesion. Our study has provided new insights into the mechanisms underlying synaptic remodeling after cerebral infarction, which contributes to the development of effective therapeutic approaches for patients after a stroke.

Reciprocal modulation between microglia and astrocyte in reactive gliosis following the CNS injury

Reactive gliosis, also known as glial scar formation, is an inflammatory response characterized by the proliferation of microglia and astrocytes as well as astrocytic hypertrophy following injury in the central nervous system (CNS). The glial scar forms a physical and molecular barrier to isolate the injured area from adjacent normal nervous tissue for re-establishing the integrity of the CNS. It prevents the further spread of cellular damage but represents an obstacle to regrowing axons. In this review, we integrated the current findings to elucidate the tightly reciprocal modulation between activated microglia and astrocytes in reactive gliosis and proposed that modification of cellular response to the injury or cellular reprogramming in the glial scar could lead advances in axon regeneration and functional recovery after the CNS injury.

Astrocyte GRK2 as a novel regulator of glutamate transport and brain damage

G protein-coupled receptor (GPCR) kinase 2 (GRK2) regulates cellular signaling via desensitization of GPCRs and by direct interaction with intracellular signaling molecules. We recently described that ischemic brain injury decreases cerebral GRK2 levels. Here we studied the effect of astrocyte GRK2-deficiency on neonatal brain damage in vivo. As astrocytes protect neurons by taking up glutamate via plasma-membrane transporters, we also studied the effect of GRK2 on the localization of the GLutamate ASpartate Transporter (GLAST). Brain damage induced by hypoxia-ischemia was significantly reduced in GFAP-GRK2(+/-) mice, which have a 60% reduction in astrocyte GRK2 compared to GFAP-WT littermates. In addition, GRK2-deficient astrocytes have higher plasma-membrane levels of GLAST and an increased capacity to take up glutamate in vitro. In search for the mechanism by which GRK2 regulates GLAST expression, we observed increased GFAP levels in GRK2-deficient astrocytes. GFAP and the cytoskeletal protein ezrin are known regulators of GLAST localization. In line with this evidence, GRK2-deficiency reduced phosphorylation of the GRK2 substrate ezrin and enforced plasma-membrane GLAST association after stimulation with the group I mGluR-agonist DHPG. When ezrin was silenced, the enhanced plasma-membrane GLAST association in DHPG-exposed GRK2-deficient astrocytes was prevented. In conclusion, we identified a novel role of astrocyte GRK2 in regulating plasma-membrane GLAST localization via an ezrin-dependent route. We demonstrate that the 60% reduction in astrocyte GRK2 protein level that is observed in GFAP-GRK2(+/-) mice is sufficient to significantly reduce neonatal ischemic brain damage. These findings underline the critical role of GRK2 regulation in astrocytes for dampening the extent of brain damage after ischemia.

Glia and epilepsy: excitability and inflammation

Epilepsy is characterized by recurrent spontaneous seizures due to hyperexcitability and hypersynchrony of brain neurons. Current theories of pathophysiology stress neuronal dysfunction and damage, and aberrant connections as relevant factors. Most antiepileptic drugs target neuronal mechanisms. However, nearly one-third of patients have seizures that are refractory to available medications; a deeper understanding of mechanisms may be required to conceive more effective therapies. Recent studies point to a significant contribution by non-neuronal cells, the glia--especially astrocytes and microglia--in the pathophysiology of epilepsy. This review critically evaluates the role of glia-induced hyperexcitability and inflammation in epilepsy.

Bidirectional scaling of astrocytic metabotropic glutamate receptor signaling following long-term changes in neuronal firing rates

Very little is known about the ability of astrocytic receptors to exhibit plasticity as a result of changes in neuronal activity. Here we provide evidence for bidirectional scaling of astrocytic group I metabotropic glutamate receptor signaling in acute mouse hippocampal slices following long-term changes in neuronal firing rates. Plasticity of astrocytic mGluRs was measured by recording spontaneous and evoked Ca²⁺ elevations in both astrocytic somata and processes. An exogenous astrocytic Gq G protein-coupled receptor was resistant to scaling, suggesting that the alterations in astrocyte Ca²⁺ signaling result from changes in activity of the surface mGluRs rather than a change in intracellular G protein signaling molecules. These findings suggest that astrocytes actively detect shifts in neuronal firing rates and adjust their receptor signaling accordingly. This type of long-term plasticity in astrocytes resembles neuronal homeostatic plasticity and might be important to ensure an optimal or expected level of input from neurons.

Phenotype, differentiation, and function differ in rat and mouse neocortical astrocytes cultured under the same conditions

The study of slowly progressing brain diseases in which glial cells play a pathogenic role requires astrocytes that have been cultured for several weeks. We characterized neocortical astrocytes, grown for up to 42 days in vitro (DIV), from newborn rats and mice by indirect immunofluorescence technique, Western blot, and real-time RT-PCR analyses. We obtained highly enriched rat and mouse astrocyte cultures, where most cells were positively stained for the astrocyte markers GFAP, vimentin, and S100β, whereas neuronal and oligodendrocyte markers were undetectable. The protein and mRNA levels of GFAP, vimentin, and nestin were higher in rat than in mouse astrocytes. From 28 to 42 DIV, the levels of vimentin and nestin, but not of GFAP, decreased in both species, with an increase in the vimentin-GFAP ratio of 1.7 for rat, and of 0.9 for mouse astrocytes suggesting that the rat cultures were more differentiated than the mouse cultures, although both remained partially immature. The protoplasmic appearance of the cells, the negative A2B5 immunoreactivity, and the expression of the glutamate transporters GLAST and GLT-1 indicate that the rat and mouse cultures contained mainly type I astrocytes. The protein levels of GLAST and GLT-1 decreased from 28 to 42 DIV in the mouse, but not in the rat astrocytes, suggesting that the rat cultures are suitable for functional studies. Thus, under the same culture conditions, astrocyte cultures from rats and mice differ in phenotype, differentiation, and functionality. This finding should be taken into account when long-lasting glial reaction patterns are being studied.

Reactive astrogliosis after spinal cord injury-beneficial and detrimental effects

Reactive astrogliosis is a pathologic hallmark of spinal cord injury (SCI). It is characterised by profound morphological, molecular, and functional changes in astrocytes that occur within hours of SCI and evolves as time elapses after injury. Astrogliosis is a defense mechanism to minimize and repair the initial damage but eventually leads to some detrimental effects. Reactive astrocytes secrete a plethora of both growth promoting and inhibitory factors after SCI. However, the production of inhibitory components surpasses the growth stimulating factors, thus, causing inhibitory effects. In severe cases of injury, astrogliosis results in the formation of irreversible glial scarring that acts as regeneration barrier due to the expression of inhibitory components such as chondroitin sulfate proteoglycans. Scar formation was therefore recognized from a negative perspective for many years. Accumulating evidence from pharmacological and genetic studies now signifies the importance of astrogliosis and its timing for spinal cord repair. These studies have advanced our knowledge regarding signaling pathways and molecular mediators, which trigger and modulate reactive astrocytes and scar formation. In this review, we discuss the recent advances in this field. We also review therapeutic strategies that have been developed to target astrocytes reactivity and glial scaring in the environment of SCI. Astrocytes play pivotal roles in governing SCI mechanisms, and it is therefore crucial to understand how their activities can be targeted efficiently to harness their potential for repair and regeneration after SCI.

Metabotropic glutamate receptor-mediated signaling in neuroglia

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors, which include eight subtypes that have been classified into three groups (I-III) based upon sequence homology, signal transduction mechanism and pharmacological profile. Although most studied with regard to neuronal function and modulation, mGlu receptors are also expressed by neuroglia-including astrocytes, microglia and oligodendrocytes. Activation of mGlu receptors on neuroglia under both physiologic and pathophysiologic conditions mediates numerous actions that are essential for intrinsic glial cell function, as well as for glial-neuronal interactions. Astrocyte mGlu receptors play important physiological roles in regulating neurotransmission and maintaining neuronal homeostasis. However, mGlu receptors on astrocytes and microglia also serve to modulate cell death and neurological function in a variety of pathophysiological conditions such as acute and chronic neurodegenerative disorders. The latter effects are complex and bi-directional, depending on which mGlu receptor sub-types are activated.

EAAT expression by macrophages and microglia: still more questions than answers

Glutamate is the main excitatory amino acid, but its presence in the extracellular milieu has deleterious consequences. It may induce excitotoxicity and also compete with cystine for the use of the cystine-glutamate exchanger, blocking glutathione neosynthesis and inducing an oxidative stress-induced cell death. Both mechanisms are critical in the brain where up to 20% of total body oxygen consumption occurs. In normal conditions, the astrocytes ensure that extracellular concentration of glutamate is kept in the micromolar range, thanks to their coexpression of high-affinity glutamate transporters (EAATs) and glutamine synthetase (GS). Their protective function is nevertheless sensitive to situations such as oxidative stress or inflammatory processes. On the other hand, macrophages and microglia do not express EAATs and GS in physiological conditions and are the principal effector cells of brain inflammation. Since the late 1990s, a number of studies have now shown that both microglia and macrophages display inducible EAAT and GS expression, but the precise significance of this still remains poorly understood. Brain macrophages and microglia are sister cells but yet display differences. Both are highly sensitive to their microenvironment and can perform a variety of functions that may oppose each other. However, in the very particular environment of the healthy brain, they are maintained in a repressed state. The aim of this review is to present the current state of knowledge on brain macrophages and microglial cells activation, in order to help clarify their role in the regulation of glutamate under pathological conditions as well as its outcome.

Insulin increases glutamate transporter GLT1 in cultured astrocytes

The astroglial cell-specific glutamate transporter subtype 2 (excitatory amino acid transporter 2, GLT1) plays an important role in excitotoxicity that develops after damage to the central nervous system (CNS) is incurred. Both the protein kinase C signaling pathway and the epidermal growth factor (EGF) pathway have been suggested to participate in the modulation of GLT1, but the modulatory mechanisms of GLT1 expression are not fully understood. In the present study, we aimed to evaluate the effects of insulin on GLT1 expression. We found that short-term stimulation of insulin led to the upregulation of both total and surface expressions of GLT1. Akt phosphorylation increased after insulin treatment, and triciribine, the inhibitor of Akt phosphorylation, significantly inhibited the effects of insulin. We also found that the upregulation of GLT1 expression correlated with increased kappa B motif-binding phosphoprotein (KBBP) and GLT1 mRNA levels. Our results suggest that insulin may modulate the expression of astrocytic GLT1, which might play a role in reactive astrocytes after CNS injuries.