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 (PKC?) 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 PKC? 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 PKC? expression in astrocytes from wild‐type animals with specific PKC?‐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 PKC? is required to generate Ca²⁺ oscillations following mGluR5 activation, which support the regulation of astrocytic glutamate uptake. Reduced expression of astrocytic PKC? could impair this neuroprotective process and participate in the progression of ALS.     

Enhanced astroglial Ca2+ signaling increases excitatory synaptic strength in the epileptic brain

The fine‐tuning of synaptic transmission by astrocyte signaling is crucial to CNS physiology. However, how exactly astroglial excitability and gliotransmission are affected in several neuropathologies, including epilepsy, remains unclear. Here, using a chronic model of temporal lobe epilepsy (TLE) in rats, we found that astrocytes from astrogliotic hippocampal slices displayed an augmented incidence of TTX‐insensitive spontaneous slow Ca²⁺ transients (STs), suggesting a hyperexcitable pattern of astroglial activity. As a consequence, elevated glutamate‐mediated gliotransmission, observed as increased slow inward current (SICs) frequency, up‐regulates the probability of neurotransmitter release in CA3‐CA1 synapses. Selective blockade of spontaneous astroglial Ca²⁺ elevations as well as the inhibition of purinergic P2Y1 or mGluR5 receptors relieves the abnormal enhancement of synaptic strength. Moreover, mGluR5 blockade eliminates any synaptic effects induced by P2Y1R inhibition alone, suggesting that the Pr modulation via mGluR occurs downstream of P2Y1R‐mediated Ca²⁺‐dependent glutamate release from astrocyte. Our findings show that elevated Ca²⁺‐dependent glutamate gliotransmission from hyperexcitable astrocytes up‐regulates excitatory neurotransmission in epileptic hippocampus, suggesting that gliotransmission should be considered as a novel functional key in a broad spectrum of neuropathological conditions. GLIA 2015;63:1507–1521     

β‐amyloid and ATP‐induced diffusional trapping of astrocyte and neuronal metabotropic glutamate type‐5 receptors

β‐Amyloid (Aβ) oligomers initiate synaptotoxicity following their interaction with the plasma membrane. Several proteins including metabotropic glutamate type 5 receptors (mGluR5s) contribute to this process. We observed an overexpression of mGluR5s in reactive astrocytes surrounding Aβ plaques in brain sections from an Alzheimer's disease mouse model. In a simplified cell culture system, using immunocytochemistry and single molecule imaging, we demonstrated a rapid binding of Aβ oligomers on the plasma membrane of astrocytes. The resulting aggregates of Aβ oligomers led to the diffusional trapping and clustering of mGluR5s. Further, Aβ oligomers induced an increase in ATP release following activation of astroglial mGluR5s by its agonist. ATP slowed mGluR5s diffusion in astrocytes as well as in neurons co‐cultured with astrocytes. This effect, which is purinergic receptor‐dependent, was not observed in pure neuronal cultures. Thus, Aβ oligomer‐ and mGluR5‐dependent ATP release by astrocytes may contribute to the overall deleterious effect of mGluR5s in Alzheimer's disease. GLIA 2013;61:1673–1686     

Ultralow concentrations of bupivacaine exert anti‐inflammatory effects on inflammation‐reactive astrocytes

Bupivacaine is a widely used, local anesthetic agent that blocks voltage‐gated Na⁺ channels when used for neuro‐axial blockades. Much lower concentrations of bupivacaine than in normal clinical use, < 10^?8 m , evoked Ca²⁺ transients in astrocytes from rat cerebral cortex, that were inositol trisphosphate receptor‐dependent. We investigated whether bupivacaine exerts an influence on the Ca²⁺ signaling and interleukin‐1β (IL‐1β) secretion in inflammation‐reactive astrocytes when used at ultralow concentrations, < 10^?8 m . Furthermore, we wanted to determine if bupivacaine interacts with the opioid‐, 5‐hydroxytryptamine‐ (5‐HT) and glutamate‐receptor systems. With respect to the μ‐opioid‐ and 5‐HT‐receptor systems, bupivacaine restored the inflammation‐reactive astrocytes to their normal non‐inflammatory levels. With respect to the glutamate‐receptor system, bupivacaine, in combination with an ultralow concentration of the μ‐opioid receptor antagonist naloxone and μ‐opioid receptor agonists, restored the inflammation‐reactive astrocytes to their normal non‐inflammatory levels. Ultralow concentrations of bupivacaine attenuated the inflammation‐induced upregulation of IL‐1β secretion. The results indicate that bupivacaine interacts with the opioid‐, 5‐HT‐ and glutamate‐receptor systems by affecting Ca²⁺ signaling and IL‐1β release in inflammation‐reactive astrocytes. These results suggest that bupivacaine may be used at ultralow concentrations as an anti‐inflammatory drug, either alone or in combination with opioid agonists and ultralow concentrations of an opioid antagonist.     

Expression and functional role of mGluR3 and mGluR5 in human astrocytes and glioma cells: opposite regulation of glutamate transporter proteins

We examined the regulation of glutamate transporter protein expression after stimulation with selective metabotropic glutamate receptor (mGluR) agonists in cultured human glial cells. mGluR3 and mGluR5 are expressed in human astrocytes and in human glioma cells in vivo as well as in vitro, as shown by either RT-PCR or western blot analysis. The selective group I agonist (S)-3,5-dihydroxyphenylglycine produced a significant down-regulation of both GLAST and GLT-1 protein expression in astrocytes cultured in the presence of growth factors. This condition mimics the morphology of reactive glial cells in vivo including an increased expression of mGluR5 protein (observed in pathological conditions). In contrast, (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine, a selective agonist of group II metabotropic glutamate receptors, positively modulates the expression of GLAST and GLT-1 proteins. A similar opposite effect of (S)-3,5-dihydroxyphenylglycine and (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine was observed for the expression of EAAT3 protein in U373 glioblastoma cell line. Selective group I and II antagonists prevented these effects. Pharmacological inhibition of mitogen-activated protein kinase and phosphatidylinositol-3-K pathways reduces the induction of GLT-1 observed in response to the group II metabotropic glutamate receptor agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine. Thus, mGluR3 and mGluR5 can critically and differentially modulate the expression of glutamate transporters and may represent interesting pharmacological targets to regulate the extracellular levels of glutamate in pathological conditions.     

Protein kinase Cɛ activity regulates mGluR5 surface expression in the rat nucleus accumbens

Type 5 metabotropic glutamate receptors (mGluR5) activate protein kinase C (PKC) via coupling to G_αq/11 protein signaling. We have previously demonstrated that the epsilon isoform of PKC (PKCɛ) is a critical downstream target of mGluR5 in regulating behavioral and biochemical responses to alcohol. Recent evidence suggests that PKC‐mediated phosphorylation of mGluR5 can lead to receptor desensitization and internalization. We therefore sought to examine the specific involvement of PKCɛ in the regulation of mGluR5 surface expression in the nucleus accumbens (NAc), a key regulator of alcohol‐associated behaviors. Coronal brain sections from male Wistar rats were analyzed for either colocalization of mGluR5 and PKCɛ via immunohistochemistry or changes in mGluR5 surface expression and PKCɛ phosphorylation following local application of PKCɛ translocation activator or inhibitor peptides and/or an orthosteric mGluR5 agonist. We observed colocalization of mGluR5 and PKCɛ in the NAc. We also showed that intra‐NAc infusion of the PKCɛ translocation inhibitor ɛV1–2 increased mGluR5 surface expression under baseline conditions. Stimulation of mGluR5 with an orthosteric agonist DHPG, dose dependently increased ERK1/2 and PKCɛ phosphorylation as well as mGluR5 internalization in acute NAc slices. Finally, we observed that activation of PKCɛ translocation with Tat‐ΨɛRACK peptide mediates agonist‐independent mGluR5 internalization, whereas PKCɛ translocation inhibitor ɛV1–2 prevents agonist‐dependent internalization of mGluR5 in NAc slice preparations. These findings suggest that the subcellular localization of mGluR5 in the NAc is regulated by PKCɛ under basal and stimulation conditions, which may influence the role of mGluR5–PKCɛ signaling in alcohol‐related behaviors. © 2016 Wiley Periodicals, Inc.     

Calmodulin activity regulates group I metabotropic glutamate receptor‐mediated signal transduction and synaptic depression

Group I metabotropic glutamate receptors (mGluR), including mGluR1 and mGluR 5 (mGluR1/5), are coupled to Gq and modulate activity‐dependent synaptic plasticity. Direct activation of mGluR1/5 causes protein translation‐dependent long‐term depression (LTD). Although it has been established that intracellular Ca²⁺ and the Gq‐regulated signaling molecules are required for mGluR1/5 LTD, whether and how Ca²⁺ regulates Gq signaling and upregulation of protein expression remain unknown. Through pharmacological inhibition, we tested the function of the Ca²⁺ sensor calmodulin (CaM) in intracellular signaling triggered by the activation of mGluR1/5. CaM inhibitor N‐[4‐aminobutyl]‐5‐chloro‐2‐naphthalenesulfonamide hydrochloride (W13) suppressed the mGluR1/5‐stimulated activation of extracellular signal‐regulated kinase 1/2 (ERK1/2) and p70‐S6 kinase 1 (S6K1) in hippocampal neurons. W13 also blocked the mGluR1/5 agonist‐induced synaptic depression in hippocampal slices and in anesthetized mice. Consistent with the function of CaM, inhibiting the downstream targets Ca²⁺/CaM‐dependent protein kinases (CaMK) blocked ERK1/2 and S6K1 activation. Furthermore, disruption of the CaM–CaMK–ERK1/2 signaling cascade suppressed the mGluR1/5‐stimulated upregulation of Arc expression. Altogether, our data suggest CaM as a new Gq signaling component for coupling Ca²⁺ and protein upregulation and regulating mGluR1/5‐mediated synaptic modification. © 2016 Wiley Periodicals, Inc.     

Expression of a functional metabotropic glutamate receptor 5 on enteric glia is altered in states of inflammation

The metabotropic glutamate receptor 5 (mGluR5) is expressed by astrocytes and its expression is modulated by inflammation. Enteric glia have many similarities to astrocytes and are the most numerous cell in the enteric nervous system (ENS). We investigated whether enteric glia express a functional mGluR5 and whether expression of this receptor was altered in colitis. In both enteric plexuses of the ileum and colon of guinea pigs and mice, we observed widespread glial mGluR5 expression. Incubation of isolated segments of the guinea pig ileum with the mGluR5 specific agonist RS-2-chloro-5-hydroxyphenylglycine (CHPG) caused a dose-dependent increase in the glial expression of c-Fos and the phosphorylated form of the extracellular signal-regulated kinase 1/2. Preincubation of tissues with the group I metabotropic glutamate receptor antagonist, S-4-carboxyphenylglycine, abolished the effects of CHPG. We examined mGluR5 expression in the guinea pig trinitrobenzene sulfonic acid and the IL-10 gene-deficient (IL-10(-/-)) mouse models of colitis. In guinea pigs, mGluR5 immunoreactivity became diffusely localized over the colonic myenteric ganglia, suggesting a change in receptor distribution. In contrast, glial mGluR5 expression was significantly reduced in the colonic myenteric plexus of IL-10(-/-) mice, as assessed with both real-time quantitative RT-PCR as well as immunohistochemistry and image analysis. These changes occurred without concomitant changes to enteric ganglia or glial fibrillary acidic protein expression in the IL-10(-/-) mouse. Our data suggest that enteric glia are a functional target of the glutamatergic neurotransmitter system in the ENS and that changes in mGluR5 expression may be of physiological significance during colitis.     

Autism‐associated Shank3 mutations alter mGluR expression and mGluR‐dependent but not NMDA receptor‐dependent long‐term depression

SHANK3 is a postsynaptic structural protein localized at excitatory glutamatergic synapses in which deletions and mutations have been implicated in patients with autism spectrum disorders (ASD). The expression of Shank3 ASD mutations causes impairments in ionotropic glutamate receptor‐mediated synaptic responses in neurons, which is thought to underlie ASD‐related behaviors, thereby indicating glutamatergic synaptopathy as one of the major pathogenic mechanisms. However, little is known about the functional consequences of ASD‐associated mutations in Shank3 on another important set of glutamate receptors, group I metabotropic glutamate receptors (mGluRs). Here, we further assessed how Shank3 mutations identified in patients with ASD (one de novo InsG mutation and two inherited point mutations, R87C and R375C) disrupt group I mGluR (mGluR1 and mGluR5) expression and function. To identify potential isoform‐specific deficits induced by ASD‐associated Shank3 mutations on group I mGluRs, we surface immunolabeled mGluR1 and mGluR5 independently. We also induced mGluR‐dependent synaptic plasticity (R,S‐3,5‐dihydroxyphenylglycine [DHPG]‐induced long‐term depression [LTD]) as well as N‐methyl‐D‐aspartate receptor (NMDAR)‐dependent LTD. ASD‐associated mutations in Shank3 differentially interfered with the ability of cultured hippocampal neurons to express mGluR5 and mGluR1 at synapses. Intriguingly, all ASD Shank3 mutations impaired mGluR‐dependent LTD without altering NMDAR‐dependent LTD. Our data show that the specific perturbation in mGluR‐dependent synaptic plasticity occurs in neurons expressing ASD‐associated Shank3 mutations, which may underpin synaptic dysfunction and subsequent behavioral deficits in ASD.     

Upregulation of metabotropic glutamate receptor subtype mGluR3 and mGluR5 in reactive astrocytes in a rat model of mesial temporal lobe epilepsy

Reactive gliosis is a prominent morphological feature of mesial temporal lobe epilepsy. Because astrocytes express glutamate receptors, we examined changes in metabotropic glutamate receptor (mGluR) 2/3, mGluR5 and transforming growth factor (TGF)-beta in glial cells of the hippocampal regions in an experimental rat model of spontaneous seizures. Rats that exhibited behavioural status epilepticus (SE) directly after 1 h of electrical angular bundle stimulation, displayed chronic spontaneous seizures after a latent period of 1-2 weeks as observed using continuous electrographic monitoring. SE resulted in hypertrophy of astrocytes and microglia activation throughout the hippocampus as revealed by immunolabelling studies. A dramatic, seizure intensity-dependent increase in vimentin immunoreactivity (a marker for reactive astrocytes) was revealed in CA3 and hilar regions where prominent neuronal loss occurs. Increased vimentin labelling was first apparent 24 h after onset of SE and persisted up to 3 months. mGluR2/3 and mGluR5 protein expression increased markedly in glial cells of CA3 and hilus by 1 week after SE, and persisted up to 3 months after SE. Double immunolabelling of brain sections with vimentin confirmed co-localization with glial fibrillary acidic protein (GFAP), mGluR2/3 and mGluR5 in reactive astrocytes. TGF-beta, a cytokine implicated in mGluR3-mediated neuroprotection, was also upregulated during the first 3 weeks after SE throughout the hippocampus. This study demonstrates seizure-induced upregulation of two mGluR subtypes in reactive astrocytes, which - together with the increased production of TGF-beta - may represent a novel mechanism for modulation of glial function and for changes in glial-neuronal communication in the course of epileptogenesis.     

Astrocyte‐mediated synapse remodeling in the pathological brain

Astrocytes, a major type of glia, reciprocally influence synaptic transmission and connectivity, forming the “tripartite synapses”. Astrocytic metabotropic glutamate receptor (mGluR)‐mediated Ca²⁺ waves and release of gliotransmitters or synaptogenic molecules mediate this neuron‐glia interaction in the developing brain, but this signaling has been challenged for adult brain. However, cumulative evidence has suggested that mature astrocytes exhibit re‐awakening of such immature phenotype in the pathological adult brain. This phenotypic change in astrocytes in response to injury may induce neural circuit and synapse plasticity. In this review article, we summarize astrocyte‐mediated synapse remodeling during physiological development, discuss re‐emergence of immature astrocytic signaling in adult pathological brain, and finally highlight its contribution to significant modification of synaptic connections correlating with functional progress of brain pathology.     

Signaling mechanisms downstream of quinolinic acid targeting the cytoskeleton of rat striatal neurons and astrocytes

The studies of signaling mechanisms involved in the disruption of the cytoskeleton homeostasis were performed in a model of quinolinic acid (QUIN) neurotoxicity in vitro. This investigation focused on the phosphorylation level of intermediate filament (IF) subunits of astrocytes (glial fibrillary acidic protein - GFAP) and neurons (low, medium and high molecular weight neurofilament subunits - NFL, NFM and NFH, respectively). The activity of the phosphorylating system associated with the IFs was investigated in striatal slices of rat exposed to QUIN or treated simultaneously with QUIN plus glutamate receptor antagonists, calcium channel blockers or kinase inhibitors. Results showed that in astrocytes, the action of 100 μM QUIN was mainly due to increased Ca(2+) influx through NMDA and L-type voltage-dependent Ca(2+) channels (L-VDCC). In neuronal cells QUIN acted through metabotropic glutamate receptor (mGluR) activation and influx of Ca(2+) through NMDA receptors and L-VDCC, as well as Ca(2+) release from intracellular stores. These mechanisms then set off a cascade of events including activation of PKA, PKCaMII and PKC, which phosphorylate head domain sites on GFAP and NFL. Also, Cdk5 was activated downstream of mGluR5, phosphorylating the KSP repeats on NFM and NFH. mGluR1 was upstream of phospholipase C (PLC) which, in turn, produced diacylglycerol (DAG) and inositol 3,4,5 triphosphate (IP3). DAG is important to activate PKC and phosphorylate NFL, while IP(3) contributed to Ca(2+) release from internal stores promoting hyperphosphorylation of KSP repeats on the tail domain of NFM and NFH. The present study supports the concept of glutamate and Ca(2+) contribution in excitotoxic neuronal damage provoked by QUIN associated to dysfunction of the cytoskeleton homeostasis and highlights the differential signaling mechanisms elicited in striatal astrocytes and neurons.     

Metabotropic glutamate receptor agonists reduce glutamate release from cultured astrocytes

Astrocytes are thought to control extracellular glutamate concentrations ([Glu]_o) in the brain, thereby protecting neurons from excitotoxic injury. We investigated the effects of metabotropic glutamate receptor (mGluR) agonists on glutamate transport and [Glu]_o in primary hippocampal astrocytic cultures. Acute or chronic exposure of astrocytes to the mGluR agonist trans‐1‐aminocyclopentane‐1,3‐dicarboxylic acid (trans‐ACPD) or its active isomer 1S,3R‐ACPD reduced [Glu]_o in a time‐ and dose‐dependent manner (44.5 ± 3.6% reductions of [Glu]_o in astrocytes from P0–P10 rats and 65.9 ± 4.1 % from rats P20 by 100 μM 1S,3R‐ACPD, EC₅₀ ∼ 5 μM). 1S,3R‐ACPD effects developed slowly (median effective at ∼60 min) and persisted for several hours after agonist removal. ACPD‐pretreated astrocytes established lower steady‐state [Glu]_o levels. ACPD effects persisted in the presence of the glutamate uptake inhibitors D ,L ‐threo‐β‐hydroxyaspartate (THA) and L ‐trans‐pyrrolidine‐2,4‐dicarboxylate (PDC) but were impaired by disruption of the transmembrane Na⁺, K⁺, or H⁺ gradients. In addition, 1S,3R‐ACPD had no effects on intracellular glutamate content and did not directly block glutamate transport. Furthermore, ACPD effects could be mimicked by glutamate per se and several other compounds presumed to be mGluR agonists, although (S)‐3,5‐dihydroxyphenylglycine (DHPG), (2S,2R,3R)‐2‐(2,3‐dicarboxycyclopropyl)glycine (DCG‐IV), and L ‐(+)‐2‐amino‐4‐phosphonobutyric acid (L ‐AP4) were without effect. These data suggest that glutamate and certain mGluR agonists may regulate [Glu]_o by modulating the transmembrane equilibrium of glutamate transport, especially by attenuating glutamate release. GLIA 25:270–281, 1999. © 1999 Wiley‐Liss, Inc.     

Rapid manifestation of reactive astrogliosis in acute hippocampal brain slices

A flurry of studies over the past decade has shown that astrocytes play a more active role in neural function than previously recognized. Hippocampal slices prepared from young rodent pups have served as a popular model for studying the pathways by which astrocytes participate in synaptic transmission. It is, however, not known how well astrocytes tolerate traumatic injury and hypoxia, which are unavoidable when preparing acute slices. We here showed that astrocytes exhibit striking changes in expression of several receptors and structural proteins, including re‐expression of the developmental marker nestin within 90 min following preparation of live vibratome slices. Moreover, immunoelectron microscopy showed a 2.7‐fold loss of astrocytic processes in acute hippocampal slices prepared from glial fibrillary acidic protein‐green fluorescent protein reporter mice. A sharp decrease in the number of mitochondria was also noted in acute slices, concurrently with an increase in mitochondrial size. Glycogen content decreased 3‐fold upon slice preparation and did not recover despite stable recordings of field excitatory postsynaptic current. Analysis of Ca²⁺ signaling showed that astrocytic responses to purine receptor and mGluR5 agonists differed in slice versus in vivo. These observations suggest that the functional properties and the fine structure of astrocytes in slices may be reflective of early stages of reactive gliosis and should be confirmed in vivo when possible. GLIA 2013;62:78–95     

Characterization of mGluR5R,a novel,metabotropic glutamate receptor 5-related gene

We report here the isolation of a novel gene termed mGluR5R (mGluR5-related). The N-terminus of mGluR5R is highly similar to the extracellular domain of metabotropic glutamate receptor 5 (mGluR5) whereas the C-terminus bears similarity to the testis-specific gene, RNF18. mGluR5R is expressed in the human CNS in a coordinate fashion with mGluR5. Although the sequence suggests that mGluR5R may be a secreted glutamate binding protein, we found that when expressed in HEK293 cells it was membrane associated and not secreted. Furthermore, mGluR5R was incapable of binding the metabotropic glutamate receptor class I selective agonist, quisqualate. Although mGluR5R could not form disulfide-mediated covalent homodimers, it was able to form a homomeric complex, presumably through noncovalent interactions. mGluR5R also formed noncovalent heteromeric associations with an engineered construct of the extracellular domain of mGluR5 as well as with full-length mGluR5 and mGluR1alpha. The ability of mGluR5R to associate with mGluR1alpha and mGluR5 suggests that it may be a modulator of class I metabotropic glutamate receptor function.     

Interleukin-1 beta down-regulates the expression of metabotropic glutamate receptor 5 in cultured human astrocytes

Expression of metabotropic glutamate receptor 5 (mGluR5) protein is known to be plastic and to depend critically on the astrocytes' microenvironment. In the present study we investigated whether interleukins, which are involved in the immune response following brain injury, could contribute to the regulation of mGluR5 protein in human astrocytes in culture. Using Western blotting and immunocytochemistry, no detectable changes in the expression of the mGluR5 protein were observed with both interleukin 1β and interleukin 6 in undifferentiated cultures (growing in serum free media). In contrast, in cultures that had been morphologically differentiated by exposure to epidermal growth factor (EGF), addition of interleukin 1β (but not interleukin 6) reduced mGluR5 protein expression. In addition, stimulation of phosphoinositide hydrolysis by the selective group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) was reduced after exposure to interleukin 1β. The suppressive effect on mGluR5 was prevented by the interleukin 1 receptor antagonist. Thus, interleukin 1β may represent an additional pathway through which mGluR5 expression and function can be modulated in astrocytes under different pathological conditions associated with an inflammatory response.     

Regulation of radial glial process growth by glutamate via mGluR5/TRPC3 and neuregulin/ErbB4

Radial glial cells play an essential role through their function as guides for neuronal migration during development. Disruption of metabotropic glutamate receptor 5 (mGluR5) function retards the growth of radial glial processes in vitro. Neuregulins (NRG) are activated by proteolytic cleavage and regulate (radial) glial maintenance via ErbB3/ErbB4 receptors. We show here that blocking ErbB4 disrupts radial process extension. Soluble NRG acting on ErbB4 receptors is able to promote radial process extension in particular where process elongation has been impeded by blockade of mGluR5, the nonselective cation channel canonical transient receptor potential 3 (TRPC3), or matrix metalloproteases (MMP). NRG does not restore retarded process growth caused by ErbB4 blockade. Stimulation of muscarinic receptors restores process elongation due to mGluR5 blockade but not that caused by TRPC3, MMP or ErbB4 blockade suggesting that muscarinic receptors can replace mGluR5 with respect to radial process extension. Additionally, NRG/ErbB4 causes Ca²⁺ mobilization in a population of cells through cooperation with ErbB1 receptors. Our results indicate that mGluR5 promotes radial process growth via NRG activation by a mechanism involving TRPC3 channels and MMPs. Thus neurotransmitters acting on G‐protein coupled receptors could play a central role in the maintenance of the radial glial scaffold through activation of NRG/ErbB4 signaling.