Activation of metabotropic glutamate receptor 3 enhances interleukin (IL)-1beta-stimulated release of IL-6 in cultured human astrocytes

Astroglial calcium signaling and homeostasis in tuberous sclerosis complex

Tuberous Sclerosis Complex (TSC) is a multisystem genetic disorder characterized by the development of benign tumors in various organs, including the brain, and is often accompanied by epilepsy, neurodevelopmental comorbidities including intellectual disability and autism. A key hallmark of TSC is the hyperactivation of the mechanistic target of rapamycin (mTOR) signaling pathway, which induces alterations in cortical development and metabolic processes in astrocytes, among other cellular functions. These changes could modulate seizure susceptibility, contributing to the progression of epilepsy and its associated comorbidities. Epilepsy is characterized by dysregulation of calcium (Ca2+) channels and intracellular Ca2+ dynamics. These factors contribute to hyperexcitability, disrupted synaptogenesis, and altered synchronization of neuronal networks, all of which contribute to seizure activity. This study investigates the intricate interplay between altered Ca2+ dynamics, mTOR pathway dysregulation, and cellular metabolism in astrocytes. The transcriptional profile of TSC patients revealed significant alterations in pathways associated with cellular respiration, ER and mitochondria, and Ca2+ regulation. TSC astrocytes exhibited lack of responsiveness to various stimuli, compromised oxygen consumption rate and reserve respiratory capacity underscoring their reduced capacity to react to environmental changes or cellular stress. Furthermore, our study revealed significant reduction of store operated calcium entry (SOCE) along with strong decrease of basal mitochondrial Ca2+ concentration and Ca2+ influx in TSC astrocytes. In addition, we observed alteration in mitochondrial membrane potential, characterized by increased depolarization in TSC astrocytes. Lastly, we provide initial evidence of structural abnormalities in mitochondria within TSC patient-derived astrocytes, suggesting a potential link between disrupted Ca2+ signaling and mitochondrial dysfunction. Our findings underscore the complexity of the relationship between Ca2+ signaling, mitochondria dynamics, apoptosis, and mTOR hyperactivation. Further exploration is required to shed light on the pathophysiology of TSC and on TSC associated neuropsychiatric disorders offering further potential avenues for therapeutic development.

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.

Blockade of mGluR5 in astrocytes derived from human iPSCs modulates astrocytic function and increases phagocytosis

TNF-α is essential for induction and maintenance of inflammatory responses and its dysregulation is associated with susceptibility to various pathogens that infect the central nervous system. Activation of both microglia and astrocytes leads to TNF-α production, which in turn triggers further activation of these cells. Astrocytes have been implicated in the pathophysiology of a wide range of neurodegenerative diseases with either harmful or protective roles, as these cells are capable of secreting several inflammatory factors and also promote synapse elimination and remodeling. These responses are possible because they sense their surroundings via several receptors, including the metabotropic glutamate receptor 5 (mGluR5). Under neuroinflammatory conditions, mGluR5 activation in astrocytes can be neuroprotective or have the opposite effect. In the current study, we investigated the role of mGluR5 in hiPSC-derived astrocytes subjected to pro-inflammatory stimulation by recombinant TNF-α (rTNF-α). Our results show that mGluR5 blockade by CTEP decreases the secreted levels of pro-inflammatory cytokines (IL-6 and IL-8) following short rTNF-α stimulation, although this effect subsides with time. Additionally, CTEP enhances synaptoneurosome phagocytosis by astrocytes in both non-stimulated and rTNF-α-stimulated conditions, indicating that mGluR5 blockade alone is enough to drive synaptic material engulfment. Finally, mGluR5 antagonism as well as rTNF-α stimulation augment the expression of the reactivity marker SERPINA3 and reduces the expression of synaptogenic molecules. Altogether, these data suggest a complex role for mGluR5 in human astrocytes, since its blockade may have beneficial and detrimental effects under inflammatory conditions.

Alpha and beta adrenoceptors activate interleukin-6 transcription through different pathways in cultured astrocytes from rat spinal cord

In brain astrocytes, noradrenaline (NA) has been shown to up-regulate IL-6 production via β-adrenoceptors (ARs). However, the underlying intracellular mechanisms for this regulation are not clear, and it remains unknown whether α-ARs are involved. In this study, we investigated the AR-mediated regulation of IL-6 mRNA levels in the cultured astrocytes from rat spinal cord. NA, the α1-agonist phenylephrine, and the β-agonist isoproterenol increased IL-6 mRNA levels. The phenylephrine-induced IL-6 increase was accompanied by an increase in ERK phosphorylation, and these effects were blocked by inhibitors of PKC and ERK. The isoproterenol-induced IL-6 increase was accompanied by an increase in CREB phosphorylation, and these effects were blocked by a PKA inhibitor. Our results indicate that IL-6 increases by α1- and β-ARs are mediated via the PKC/ERK and cAMP/PKA/CREB pathways, respectively. Moreover, conditioned medium collected from astrocytes treated with the α2-AR agonist dexmedetomidine, increased IL-6 mRNA in other astrocytes. In this study, we elucidate that α1- and α2-ARs, in addition to β-ARs, promote IL-6 transcription through different pathways in spinal cord astrocytes.

IL-1β Induced Cytokine Expression by Spinal Astrocytes Can Play a Role in the Maintenance of Chronic Inflammatory Pain

It is now widely accepted that the glial cells of the central nervous system (CNS) are key players in many processes, especially when they are activated via neuron-glia or glia-glia interactions. In turn, many of the glia-derived pro-inflammatory cytokines contribute to central sensitization during inflammation or nerve injury-evoked pathological pain conditions. The prototype of pro-inflammatory cytokines is interleukin-1beta (IL-1β) which has widespread functions in inflammatory processes. Our earlier findings showed that in the spinal cord (besides neurons) astrocytes express the ligand binding interleukin-1 receptor type 1 (IL-1R1) subunit of the IL-1 receptor in the spinal dorsal horn in the chronic phase of inflammatory pain. Interestingly, spinal astrocytes are also the main source of the IL-1β itself which in turn acts on its neuronal and astrocytic IL-1R1 leading to cell-type specific responses. In the initial experiments we measured the IL-1β concentration in the spinal cord of C57BL/6 mice during the course of complete Freund adjuvant (CFA)-induced inflammatory pain and observed a peak of IL-1β level at the time of highest mechanical sensitivity. In order to further study astrocytic activation, primary astrocyte cultures from spinal cords of C57BL/6 wild type and IL-1R1 deficient mice were exposed to IL-1β in concentrations corresponding to the spinal levels in the CFA-induced pain model. By using cytokine array method we observed significant increase in the expressional level of three cytokines: interleukin-6 (IL-6), granulocyte-macrophage colony stimulating factor (GM-CSF) and chemokine (C-C motif) ligand 5 (CCL5 or RANTES). We also observed that the secretion of the three cytokines is mediated by the NFkB signaling pathway. Our data completes the picture of the IL-1β-triggered cytokine cascade in spinal astrocytes, which may lead to enhanced activation of the local cells (neurons and glia as well) and can lead to the prolonged maintenance of chronic pain. All these cytokines and the NFkB pathway can be possible targets of pain therapy.

mGlu2/3 receptor antagonists

Abnormalities of glutamatergic transmission are implicated in neuropsychiatric disorders. Among the glutamate receptors, metabotropic (mGlu) 2/3 receptors have recently gained much attention as molecular targets for the treatment of several neuropsychiatric disorders including depression and anxiety. Both orthosteric and allosteric antagonists of mGlu2/3 receptors have been synthesized, and their therapeutic potential has been examined. These research activities have demonstrated the promise of mGlu2/3 receptor antagonists as potential treatment agents for the above-mentioned neuropsychiatric disorders. In particular, it has been considered that the antidepressant effects of mGlu2/3 receptor antagonists are worthy of pursuing, since the antidepressant profiles as well as synaptic/neural mechanisms involved in the actions of mGlu2/3 receptor antagonists are similar to those of ketamine, which has been demonstrated to show potent, rapid and sustained efficacy in patients with depression, even those resistant to the conventionally prescribed antidepressants. In this chapter, the general pharmacology of mGlu2/3 receptor antagonists and their therapeutic potential are reviewed. In particular, I focus on the usefulness of mGlu2/3 receptor antagonists as novel antidepressants, in comparison with ketamine.

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.

Chloride ions stabilize the glutamate-induced active state of the metabotropic glutamate receptor 3

Due to the essential roles of glutamate, detection and response to a large range of extracellular concentrations of this excitatory amino acid are necessary for the fine-tuning of brain functions. Metabotropic glutamate receptors (mGluRs) are implicated in shaping the activity of many synapses in the central nervous system. Among the eight mGluR subtypes, there is increasing interest in studying the mGlu₃ receptor which has recently been linked to various diseases, including psychiatric disorders. This receptor displays striking functional properties, with a high and, often, full basal activity, making its study elusive in heterologous systems. Here, we demonstrate that Cl^- ions exert strong positive allosteric modulation of glutamate on the mGlu₃ receptor. We have also identified the molecular and structural determinants lying behind this allostery: a unique interactive "chloride-lock" network. Indeed, Cl^- ions dramatically stabilize the glutamate-induced active state of the extracellular domain of the mGlu₃ receptor. Thus, the mGlu₃ receptors' large basal activity does not correspond to a constitutive activity in absence of agonist. Instead, it results mostly from a Cl^-mediated amplified response to low ambient glutamate concentrations, such as those measured in cell media. This strong interaction between glutamate and Cl^- ions allows the mGlu₃ receptor to sense and efficiently react to sub-micromolar concentrations of glutamate, making it the most sensitive member of mGluR family.

Ameliorative potential of pioglitazone and ceftriaxone alone and in combination in rat model of neuropathic pain: Targeting PPARγ and GLT-1 pathways

BACKGROUND

The relation between glutamate homeostasis and PPAR gamma has got tremendous importance in nerve trauma and pain. Present study has been designed to elucidate the interaction between the GLT-1 activator (ceftriaxone) and PPAR gamma agonist (pioglitazone) in the spinal nerve ligation induced neuropathic pain.

METHODS

Male SD rats were subjected to spinal nerve ligation to induce neuropathic pain. Pioglitazone, ceftriaxone and their combination treatments were given for 28 days. Various behavioral, biochemical, neuroinflammatory and apoptotic mediators were assessed subsequently.

RESULTS

In the present study, ligation of L5 and L6 spinal nerves resulted in marked hyperalgesia and allodynia to different mechanical and thermal stimuli. In addition there is marked increase in oxidative-nitrosative stress parameters, inflammatory and apoptotic markers in spinal cord of spinal nerve ligated rats. Treatment with pioglitazone and ceftriaxone significantly prevented these behavioral, biochemical, mitochondrial and cellular alterations in rats. Further, combination of pioglitazone (10mg/kg, ip) with ceftriaxone (100mg/kg, ip) significantly potentiated the protective effects as compared to their effects per se.

CONCLUSION

Based on these results we propose that possible interplay between the neuroprotective effects of pioglitazone and ceftriaxone exists in suppressing the behavioral, biochemical, mitochondrial, neuroinflammatory and apoptotic cascades in spinal nerve ligation induced neuropathic pain in rats.

Astrocyte-derived BDNF supports myelin protein synthesis after cuprizone-induced demyelination

It is well established that BDNF may enhance oligodendrocyte differentiation following a demyelinating lesion, however, the endogenous sources of BDNF that may be harnessed to reverse deficits associated with such lesions are poorly defined. Here, we investigate roles of astrocytes in synthesizing and releasing BDNF. These cells are known to express BDNF following injury in vivo. In culture, they increase BDNF synthesis and release in response to glutamate metabotropic stimulation. Following cuprizone-elicited demyelination in mice, astrocytes contain BDNF and increase levels of metabotropic receptors. The metabotropic agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD), was therefore injected into the demyelinating lesion. Increases in BDNF, as well as myelin proteins, were observed. Effects of ACPD were eliminated by coinjection of trkB-Fc to locally deplete BDNF and by deletion of astrocyte-derived BDNF. The data indicate that astrocyte-derived BDNF may be a source of trophic support that can be used to reverse deficits elicited following demyelination.

Group I metabotropic glutamate receptor mediated dynamic immune dysfunction in children with fragile X syndrome

Background

Fragile X syndrome (FXS) is the leading cause of inheritable intellectual disability in male children, and is predominantly caused by a single gene mutation resulting in expanded trinucleotide CGG-repeats within the 5’ untranslated region of the fragile X mental retardation (FMR1) gene. Reports have suggested the presence of immune dysregulation in FXS with evidence of altered plasma cytokine levels; however, no studies have directly assessed functional cellular immune responses in children with FXS. In order to ascertain if immune dysregulation is present in children with FXS, dynamic cellular responses to immune stimulation were examined.

Methods

Peripheral blood mononuclear cells (PBMC) were from male children with FXS (n = 27) and from male aged-matched typically developing (TD) controls (n = 8). PBMC were cultured for 48 hours in media alone or with lipopolysaccharides (LPS; 1 μg/mL) to stimulate the innate immune response or with phytohemagglutinin (PHA; 8 μg/mL) to stimulate the adaptive T-cell response. Additionally, the group I mGluR agonist, DHPG, was added to cultures to ascertain the role of mGluR signaling in the immune response in subject with FXS. Supernatants were harvested and cytokine levels were assessed using Luminex multiplexing technology.

Results

Children with FXS displayed similar innate immune response following challenge with LPS alone when compared with TD controls; however, when LPS was added in the presence of a group I mGluR agonist, DHPG, increased immune response were observed in children with FXS for a number of pro-inflammatory cytokines including IL-6 (P = 0.02), and IL-12p40 (P < 0.01). Following PHA stimulation, with or without DHPG, no significant differences between subjects with FXS and TD were seen.

Conclusions

In unstimulated cultures, subjects with FXS did not display altered dynamic immune response to LPS or PHA alone; however, subjects with FXS showed an altered response to co-current stimulation of LPS and DHPG, such that subjects with FXS failed to inhibit production of pro-inflammatory cytokines, suggesting a role of group I mGluR signaling in innate immune responses in FXS.

P2X7 receptor mediates activation of microglial cells in prostate of chemically irritated rats

PURPOSE

Evidence shows that adenosine triphosphate (ATP) is involved in the transmission of multiple chronic pain via P2X7 receptor. This study was to investigate the P2X7 and microglial cells in the chronic prostatitis pain.

MATERIALS AND METHODS

Rats were divided into control group and chronic prostatitis group (n = 24 per group). A chronic prostatitis animal model was established by injecting complete Freund's adjuvant (CFA) to the prostate of rats, and the thermal withdrawal latency (TWL) was detected on days 0, 4, 12 and 24 (n = 6 at each time point in each group). Animals were sacrificed and the pathological examination of the prostate, detection of mRNA expression of P2X7 and ionized calcium binding adaptor molecule 1 (IBA-1) and measurement of content of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) in the dorsal horn of L5-S2 spinal cord were performed on days 0, 4, 12 and 24. In addition, the content of TNF-α and IL-1β in the dorsal horn of L5-S2 spinal cord was measured after intrathecal injection of inhibitors of microglial cells and/or P2X7 for 5 days.

RESULTS

The chronic prostatitis was confirmed by pathological examination. The expression of P2X7 and IBA-1 and the content of TNF-α and IL-1β in rats with chronic prostatitis were significantly higher than those in the control group. On day 4, the expressions of pro-inflammatory cytokines became to increase, reaching a maximal level on day 12 and started to reduce on day 24, but remained higher than that in the control group. Following suppression of microglial cells and P2X7 receptor, the secretion of TNF-α and IL-1β was markedly reduced.

CONCLUSION

In chronic prostatitis pain, the microglial cells and P2X7 receptor are activated resulting in the increased expression of TNF-α and IL-1β in the L5-S2 spinal cord, which might attribute to the maintenance and intensification of pain in chronic prostatitis.

MicroRNA-146a: a key regulator of astrocyte-mediated inflammatory response

Increasing evidence supports the involvement of microRNAs (miRNA) in the regulation of inflammation in human neurological disorders. In the present study we investigated the role of miR-146a, a key regulator of the innate immune response, in the modulation of astrocyte-mediated inflammation. Using Taqman PCR and in situ hybridization, we studied the expression of miR-146a in epilepsy-associated glioneuronal lesions which are characterized by prominent activation of the innate immune response. In addition, cultured human astrocytes were used to study the regulation of miR-146a expression in response to proinflammatory cytokines. qPCR and western blot were used to evaluate the effects of overexpression or knockdown of miR-146a on IL-1β signaling. Downstream signaling in the IL-1β pathway, as well as the expression of IL-6 and COX-2 were evaluated by western blot and ELISA. Release several cytokines was evaluated using a human magnetic multiplex cytokine assay on a Luminex® 100™/200™ platform. Increased expression of miR-146a was observed in glioneuronal lesions by Taqman PCR. MiR-146a expression in human glial cell cultures was strongly induced by IL-1β and blocked by IL-1β receptor antagonist. Modulation of miR-146a expression by transfection of astrocytes with anti-miR146a or mimic, regulated the mRNA expression levels of downstream targets of miR-146a (IRAK-1, IRAK-2 and TRAF-6) and the expression of IRAK-1 protein. In addition, the expression of IL-6 and COX-2 upon IL-1β stimulation was suppressed by increased levels of miR-146a and increased by the reduction of miR-146a. Modulation of miR-146a expression affected also the release of several cytokines such as IL-6 and TNF-α. Our observations indicate that in response to inflammatory cues, miR-146a was induced as a negative-feedback regulator of the astrocyte-mediated inflammatory response. This supports an important role of miR-146a in human neurological disorders associated with chronic inflammation and suggests that this miR may represent a novel target for therapeutic strategies.

Therapeutic potential of metabotropic glutamate receptor modulators

Glutamate is the main excitatory neurotransmitter in the central nervous system (CNS) and is a major player in complex brain functions. Glutamatergic transmission is primarily mediated by ionotropic glutamate receptors, which include NMDA, AMPA and kainate receptors. However, glutamate exerts modulatory actions through a family of metabotropic G-protein-coupled glutamate receptors (mGluRs). Dysfunctions of glutamatergic neurotransmission have been implicated in the etiology of several diseases. Therefore, pharmacological modulation of ionotropic glutamate receptors has been widely investigated as a potential therapeutic strategy for the treatment of several disorders associated with glutamatergic dysfunction. However, blockade of ionotropic glutamate receptors might be accompanied by severe side effects due to their vital role in many important physiological functions. A different strategy aimed at pharmacologically interfering with mGluR function has recently gained interest. Many subtype selective agonists and antagonists have been identified and widely used in preclinical studies as an attempt to elucidate the role of specific mGluRs subtypes in glutamatergic transmission. These studies have allowed linkage between specific subtypes and various physiological functions and more importantly to pathological states. This article reviews the currently available knowledge regarding the therapeutic potential of targeting mGluRs in the treatment of several CNS disorders, including schizophrenia, addiction, major depressive disorder and anxiety, Fragile X Syndrome, Parkinson’s disease, Alzheimer’s disease and pain.

mGlu3 receptor and astrocytes: partners in neuroprotection

Astrocytes are currently studied intensively because of their now highlighted relevance as key players with neurons that modulate a wide range of central functions, from synaptic plasticity and synaptogenesis to regulation of metabolic and neuroinflammatory processes. Since the discovery of mGlu3 receptors on astrocytes, accumulating evidence supports a role of these receptors not only in maintaining synaptic homeostasis and treating psychiatric disorders but also in promoting astrocyte survival in several pathologic conditions. This review focuses on providing up-to-date knowledge regarding effects of activating astroglial mGlu3 receptors on psychiatric disorders, astrocyte and neuronal survival, and neurodegenerative diseases. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Functional role of astrocyte glutamate receptors and carbon monoxide in cerebral vasodilation response to glutamate

In newborn pigs, vasodilation of pial arterioles in response to glutamate is mediated via carbon monoxide (CO), a gaseous messenger endogenously produced from heme degradation by a heme oxygenase (HO)-catalyzed reaction. We addressed the hypothesis that ionotropic glutamate receptors (iGluRs), including N-methyl-D-aspartic acid (NMDA)- and 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl) propanoic acid (AMPA)/kainate-type receptors, expressed in cortical astrocytes mediate glutamate-induced astrocyte HO activation that leads to cerebral vasodilation. Acute vasoactive effects of topical iGluR agonists were determined by intravital microscopy using closed cranial windows in anesthetized newborn pigs. iGluR agonists, including NMDA, (±)1-aminocyclopentane-cis-1,3-dicarboxylic acid (cis-ACPD), AMPA, and kainate, produced pial arteriolar dilation. Topical L-2-aminoadipic acid, a gliotoxin that selectively disrupts glia limitans, reduced vasodilation caused by iGluR agonists, but not by hypercapnia, bradykinin, or sodium nitroprusside. In freshly isolated and cultured cortical astrocytes constitutively expressing HO-2, iGluR agonists NMDA, cis-ACPD, AMPA, and kainate rapidly increased CO production two- to threefold. Astrocytes overexpressing inducible HO-1 had high baseline CO but were less sensitive to glutamate stimulation of CO production when compared with HO-2-expressing astrocytes. Glutamate-induced astrocyte HO-2-mediated CO production was inhibited by either the NMDA receptor antagonist (R)-3C4HPG or the AMPA/kainate receptor antagonist DNQX. Accordingly, either antagonist abolished pial arteriolar dilation in response to glutamate, NMDA, and AMPA, indicating functional interaction among various subtypes of astrocytic iGluRs in response to glutamate stimulation. Overall, these data indicate that the astrocyte component of the neurovascular unit is responsible for the vasodilation response of pial arterioles to topically applied glutamate via iGluRs that are functionally linked to activation of constitutive HO in newborn piglets.

Astrocyte immune responses in epilepsy

Astrocytes, the major glial cell type of the central nervous system (CNS), are known to play a major role in the regulation of the immune/inflammatory response in several human CNS diseases. In epilepsy-associated pathologies, the presence of astrogliosis has stimulated extensive research focused on the role of reactive astrocytes in the pathophysiological processes that underlie the development of epilepsy. In brain tissue from patients with epilepsy, astrocytes undergo significant changes in their physiological properties, including the activation of inflammatory pathways. Accumulating experimental evidence suggests that proinflammatory molecules can alter glio-neuronal communications contributing to the generation of seizures and seizure-related neuronal damage. In particular, both in vitro and in vivo data point to the role of astrocytes as both major source and target of epileptogenic inflammatory signaling. In this context, understanding the astroglial inflammatory response occurring in epileptic brain tissue may provide new strategies for targeting astrocyte-mediated epileptogenesis. This article reviews current evidence regarding the role of astrocytes in the regulation of the innate immune responses in epilepsy. Both clinical observations in drug-resistant human epilepsies and experimental findings in clinically relevant models will be discussed and elaborated, highlighting specific inflammatory pathways (such as interleukin-1β/toll-like receptor 4) that could be potential targets for antiepileptic, disease-modifying therapeutic strategies.

Spinal upregulation of glutamate transporter GLT-1 by ceftriaxone: therapeutic efficacy in a range of experimental nervous system disorders

Glutamate neurotransmission is highly regulated, largely by glutamate transporters. In the spinal cord, the glutamate transporter GLT-1 is primarily responsible for glutamate clearance. Downregulation of GLT-1 can occur in activated astrocytes, and is associated with increased extracellular glutamate and neuroexcitation. Among other conditions, astrocyte activation occurs following repeated opioids and in models of chronic pain. If GLT-1 downregulation occurs in these states, GLT-1 could be a pharmacological target for improving opioid efficacy and controlling chronic pain. The present studies explored whether daily intrathecal treatment of rats with ceftriaxone, a beta-lactam antibiotic that upregulates GLT-1 expression, could prevent development of hyperalgesia and allodynia following repeated morphine, reverse pain arising from central or peripheral neuropathy, and reduce glial activation in these models. Ceftriaxone pre-treatment attenuated the development of hyperalgesia and allodynia in response to repeated morphine, and prevented associated astrocyte activation. In a model of multiple sclerosis (experimental autoimmune encephalomyelitis; EAE), ceftriaxone reversed tactile allodynia and halted the progression of motor weakness and paralysis. Similarly, ceftriaxone reversed tactile allodynia induced by chronic constriction nerve injury (CCI). EAE and CCI each significantly reduced the expression of membrane-bound, dimerized GLT-1 protein in lumbar spinal cord, an effect normalized by ceftriaxone. Lastly, ceftriaxone normalized CCI- and EAE-induced astrocyte activation in lumbar spinal cord. Together, these data indicate that increasing spinal GLT-1 expression attenuates opioid-induced paradoxical pain, alleviates neuropathic pain, and suppresses associated glial activation. GLT-1 therefore may be a therapeutic target that could improve available treatment options for patients with chronic pain.

Dynamic regulation of spinal pro-inflammatory cytokine release in the rat in vivo following peripheral nerve injury

Spinal release of cytokines may play a critical role in the maladapted nociceptive signaling underlying chronic pain states. In order to investigate this biology, we have developed a novel 'high flux' intrathecal microdialysis approach in combination with multiplex bead-based immunoassay technology to concurrently monitor the spinal release of interleukin (IL)-1beta, IL-6 and tumour necrosis factor (TNF)alpha in rats with unilateral sciatic nerve chronic constriction injury (CCI). Intrathecal microdialysis was performed under isoflurane/N(2)O anaesthesia in rats with confirmed mechanical hypersensitivity. In a first study, C-fiber strength electrical stimulation of the operated nerve in neuropathic rats was found to evoke a dramatic increase in IL-1beta efflux ( approximately 15-fold) that was significantly greater than that observed in the sham-operated group. Spinal IL-6 efflux was also responsive to primary afferent stimulation, whereas TNFalpha was not. In a second study, treatment with the glial inhibitor propentofylline for 7days normalized CCI-induced mechanical hypersensitivity. In the same animals, this treatment also significantly reduced intrathecal IL-1beta, IL-6 and TNFalpha and prevented afferent stimulation-evoked cytokine release of both IL-1beta and IL-6. These results provide support for glia as the source of the majority of intrathecal IL-1beta, IL-6 and TNFalpha that accompanies mechanical hypersensitivity in the CCI rat. Moreover, our studies demonstrate the ability of a neurone-glia signaling mechanism to dynamically modulate this release and support a role of spinal IL-1beta in the phasic transmission of abnormal pain signals.

Pathological and protective roles of glia in chronic pain

Glia have emerged as key contributors to pathological and chronic pain mechanisms. On activation, both astrocytes and microglia respond to and release a number of signalling molecules, which have protective and/or pathological functions. Here we review the current understanding of the contribution of glia to pathological pain and neuroprotection, and how the protective, anti-inflammatory actions of glia are being harnessed to develop new drug targets for neuropathic pain control. Given the prevalence of chronic pain and the partial efficacy of current drugs, which exclusively target neuronal mechanisms, new strategies to manipulate neuron-glia interactions in pain processing hold considerable promise.

Up-regulation of astrocyte metabotropic glutamate receptor 5 by amyloid-β peptide

The effects of amyloid-beta peptide (Aβ) on astrocyte responses to activation of mGlu5 receptors have been investigated using calcium imaging. Pre-incubation with Aβ1-40 peptide for up to 72 h produced a time- and concentration-dependent 2-4 fold enhancement in the magnitude of the intracellular calcium mobilization response to the group I metabotropic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine (DHPG). In contrast, pre-treatment with Aβ1-40 did not alter the calcium responses induced by other G protein coupled- or ion channel-receptors. Aβ 1-40-enhanced DHPG responses were blocked by the mGlu5 antagonist MPEP but not by inhibitors of voltage dependent calcium channels or by the AMPA/KA receptor antagonist CNQX. Up-regulation of mGlu5 coupled responses was associated with significant increases in astrocyte mGlu5 receptor-mRNA and-protein expression after preincubation with Aβ . The changes observed in vitro were consistent with results obtained from human Alzheimer's disease (AD) patients.Immunostaining for mGlu5 receptors was increased on astrocytes which were colocalized with Aβ plaques in hippocampal tissue from AD patients compared to age-matched controls. These results suggest that modulation of mGlu5 receptors in astrocytes could be an important mechanism in determining the progression of pathology in AD.

Cellular localization of metabotropic glutamate receptors in cortical tubers and subependymal giant cell tumors of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder associated with cortical malformations (cortical tubers) and the development of glial tumors (subependymal giant-cell tumors, SGCTs). Expression of metabotropic glutamate receptor (mGluR) subtypes is developmentally regulated and several studies suggest an involvement of mGluR-mediated glutamate signaling in the regulation of proliferation and survival of neural stem-progenitor cells, as well as in the control of tumor growth. In the present study, we have investigated the expression and cell-specific distribution of group I (mGluR1, mGluR5), group II (mGluR2/3) and group III (mGluR4 and mGluR8) mGluR subtypes in human TSC specimens of both cortical tubers and SGCTs, using immunocytochemistry. Strong group I mGluR immunoreactivity (IR) was observed in the large majority of TSC specimens in dysplastic neurons and in giant cells within cortical tubers, as well as in tumor cells within SGCTs. In particular mGluR5 appeared to be most frequently expressed, whereas mGluR1alpha was detected in a subpopulation of neurons and giant cells. Cells expressing mGluR1alpha and mGluR5, demonstrate IR for phospho-S6 ribosomal protein (PS6), which is a marker of the mammalian target of rapamycin (mTOR) pathway activation. Group II and particularly group III mGluR IR was less frequently observed than group I mGluRs in dysplastic neurons and giant cells of tubers and tumor cells of SGCTs. Reactive astrocytes were mainly stained with mGluR5 and mGluR2/3. These findings expand our knowledge concerning the cellular phenotype in cortical tubers and in SGCTs and highlight the role of group I mGluRs as important mediators of glutamate signaling in TSC brain lesions. Individual mGluR subtypes may represent potential pharmacological targets for the treatment of the neurological manifestations associated with TSC brain lesions.

Group I metabotropic glutamate receptors: a role in neurodevelopmental disorders?

Group I metabotropic glutamate receptors (mGlu1 and mGlu5) are coupled to polyphosphoinositide hydrolysis and are involved in activity-dependent forms of synaptic plasticity, both during development and in the adult life. Group I mGlu receptors can also regulate proliferation, differentiation, and survival of neural stem/progenitor cells, which further support their role in brain development. An exaggerated response to activation of mGlu5 receptors may underlie synaptic dysfunction in Fragile X syndrome, the most common inherited form of mental retardation. In addition, group I mGlu receptors are overexpressed in dysplastic neurons of focal cortical dysplasia and hemimegaloencephaly, which are disorders of cortical development associated with chronic epilepsy. Drugs that block the activity of group I mGlu receptors (in particular, mGlu5 receptors) are potentially helpful for the treatment of Fragile X syndrome and perhaps other neurodevelopmental disorders.

A neuropathological study of two autopsy cases of syndromic hemimegalencephaly

Hemimegalencephaly (HMEG) is a malformation of cortical development characterized by unilateral enlargement of the cerebral hemisphere, severe architectural and cellular abnormalities and association with intractable epilepsy. HMEG may represent an isolated lesion of the central nervous system, but may also be associated with several neurocutaneous syndromes. In the present study we discuss the neuropathological findings of two autopsy cases of HMEG associated with linear naevus sebaceous syndrome. Both cases showed the presence of linear naevus sebaceous on extensive areas of the face. The neurochemical profile of the glial and neuronal components in the affected hemisphere was determined using immunocytochemical markers and was compared with the unaffected contralateral hemisphere and normal control tissue. The observed cytomegalic neurones expressed receptors for distinct neurotransmitters, neuropeptides and growth factors. Analysis of components of the phosphoinositide 3-kinase pathway revealed expression of phospho-S6 ribosomal protein in cytomegalic neurones. Autopsy findings confirm the complexity of the histologic phenotypic manifestations in HMEG and proved useful in determining the spectrum of cytoarchitectural and neurochemical abnormalities, underlying the molecular pathogenesis and epileptogenesis of this brain malformation.

A critical role of toll-like receptor 2 in nerve injury-induced spinal cord glial cell activation and pain hypersensitivity

The activation of spinal cord glial cells has been implicated in the development of neuropathic pain upon peripheral nerve injury. The molecular mechanisms underlying glial cell activation, however, have not been clearly elucidated. In this study, we found that damaged sensory neurons induce the expression of tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, and inducible nitric-oxide synthase genes in spinal cord glial cells, which is implicated in the development of neuropathic pain. Studies using primary glial cells isolated from toll-like receptor 2 knock-out mice indicate that damaged sensory neurons activate glial cells via toll-like receptor 2. In addition, behavioral studies using toll-like receptor 2 knock-out mice demonstrate that the expression of toll-like receptor 2 is required for the induction of mechanical allodynia and thermal hyperalgesia due to spinal nerve axotomy. The nerve injury-induced spinal cord microglia and astrocyte activation is reduced in the toll-like receptor 2 knock-out mice. Similarly, the nerve injury-induced pro-inflammatory gene expression in the spinal cord is also reduced in the toll-like receptor 2 knock-out mice. These data demonstrate that toll-like receptor 2 contributes to the nerve injury-induced spinal cord glial cell activation and subsequent pain hypersensitivity.

Metabotropic glutamate receptors mediate lipopolysaccharide-induced fever and sickness behavior

Several mechanisms have been proposed for neuroimmune communication supporting the sickness syndrome (fever, anorexia, inactivity, and cachexia) following infection. We examined the role of glutamate as a neurochemical intermediary of sickness behavior induced by intraperitoneal lipopolysaccharide (LPS). Mice implanted with biotelemetry devices capable of detecting body temperature (Tb) were administered LPS (50 or 500 microg/kg i.p., serotype 0111:B4) with or without i.p. pretreatment with vehicle or broad-spectrum antagonists selective for N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic (AMPA)/kainite, or metabotropic glutamate (mGlu) receptors. While NMDA and AMPA/kainate receptor antagonism failed to attenuate LPS-induced sickness behavior, antagonism of metabotropic receptors with l(+)-AP3 reduced the febrile (0-11h: control: 37.32+/-0.16 degrees C, l(+)-AP3: 36.66+/-0.27), anorexic (control: -87+/-5%, l(+)-AP3: 48+/-12% scotophase food intake), and cachexic (control: -8.9+/-0.4%, l(+)-AP3: -6.1+/-1.3% body weight) effects of 500 microg/kg LPS, and produced a biphasic Tb effect in response to 50 microg/kg LPS (1h: -0.90+/-0.26; 6h: 1.78+/-0.35 degrees C relative to baseline). At this dose the Tb of l(+)-AP3-treated mice was 1.18 degrees C lower than controls 2h post-injection, and 0.68 degrees C greater that controls 8h post-injection. These results suggest a role for mGlu receptors in mediating fever, anorexia, and cachexia possibly via activation of extra-vagal pathways, since the attenuating effect of l(+)-AP3 increased with increasing dosages of LPS. Given the critical role ascribed to mGlu receptors in neurotransmitter release and astrocytic processes, it is possible that these observations reflect an l(+)-AP3-induced attenuation of these systems.