Developmental regulation of the toxin sensitivity of Ca(2+)-permeable AMPA receptors in cortical glia

Technical approaches and challenges to study AMPA receptors in oligodendrocyte lineage cells: Past, present, and future

Receptors for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPARs) are ligand-gated ionotropic receptors for glutamate that is a major excitatory neurotransmitter in the central nervous system. AMPARs are located at postsynaptic sites of neuronal synapses where they mediate fast synaptic signaling and synaptic plasticity. Remarkably, AMPARs are also expressed by glial cells. Their expression by the oligodendrocyte (OL) lineage cells is of special interest because AMPARs mediate fast synaptic communication between neurons and oligodendrocyte progenitor cells (OPCs), modulate proliferation and differentiation of OPCs, and may also be involved in regulation of myelination. On the other hand, during pathological conditions, AMPARs may mediate damage of the OL lineage cells. In the present review, we focus on the technical approaches that have been used to study AMPARs in the OL lineage cells, and discuss future perspectives of AMPAR research in these glial cells.

HIV-induced neuroinflammation inhibits oligodendrocyte maturation via glutamate-dependent activation of the PERK arm of the integrated stress response

Despite combined antiretroviral therapy (cART), HIV-associated neurocognitive disorder (HAND) affects 30-50% of HIV-positive patients. Importantly, persistent white matter pathologies, specifically corpus callosum thinning and disruption of white matter microstructures observed in patients with HAND despite viral control through cART, raise the possibility that HIV infection in the setting of suboptimal cART may perturb oligodendrocyte (OL) maturation, function and/or survival, influencing HAND persistence in the cART era. To examine the effect of HIV infection on OL maturation, we used supernatants of primary human monocyte-derived macrophages infected with HIV (HIV/MDMs) to treat primary cultures of rat oligodendrocyte precursor cells (OPCs) during their differentiation to mature OLs. Using immunostaining for lineage-specific markers, we found that HIV/MDMs significantly inhibited OPC maturation. Based on our previous studies, we examined the potential role of several signaling pathways, including ionotropic glutamate receptors and the integrated stress response (ISR), and found that AMPA receptors (AMPAR)/kainic acid (KA) receptors (KARs) mediated the HIV/MDMs-induced defect in OL maturation. We also found that the treatment of OPC cultures with glutamate or AMPAR/KAR agonists phenocopied this effect. Blocking ISR activation, specifically the PERK arm of the ISR, protected OPCs from HIV/MDMs-mediated inhibition of OL maturation. Further, while glutamate, AMPA, and KA activated the ISR, inhibition of AMPAR/KAR activation prevented ISR induction in OPCs and rescued OL maturation. Collectively, these data identify glutamate signaling via ISR activation as a potential therapeutic pathway to ameliorate white matter pathologies in HAND and highlight the need for further investigation of their contribution to cognitive impairment.

Polyamine-mediated channel block of ionotropic glutamate receptors and its regulation by auxiliary proteins

Most excitatory neurotransmission in the mammalian brain is mediated by a family of plasma membrane-bound signaling proteins called ionotropic glutamate receptors (iGluRs). iGluRs assemble at central synapses as tetramers, forming a central ion-channel pore whose primary function is to rapidly transport Na⁺ and Ca²⁺ in response to binding the neurotransmitter l-glutamic acid. The pore of iGluRs is also accessible to bulkier cytoplasmic cations, such as the polyamines spermine, spermidine, and putrescine, which are drawn into the permeation pathway, but get stuck and block the movement of other ions. The degree of this polyamine-mediated channel block is highly regulated by processes that control the free cytoplasmic polyamine concentration, the membrane potential, or the iGluR subunit composition. Recently, an additional regulation by auxiliary proteins, most notably transmembrane AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor regulatory proteins (TARPs), cornichons, and neuropilin and tolloid-like proteins (NETOs), has been identified. Here, I review what we have learned of polyamine block of iGluRs and its regulation by auxiliary subunits. TARPs, cornichons, and NETOs attenuate the channel block by enabling polyamines to exit the pore. As a result, polyamine permeation occurs at more negative and physiologically relevant membrane potentials. The structural basis for enhanced polyamine transport remains unresolved, although alterations in both channel architecture and charge-screening mechanisms have been proposed. That auxiliary subunits can attenuate the polyamine block reveals an unappreciated impact of polyamine permeation in shaping the signaling properties of neuronal AMPA- and kainate-type iGluRs. Moreover, enhanced polyamine transport through iGluRs may have a role in regulating cellular polyamine levels.

Elevated Pressure Increases Ca2+ Influx Through AMPA Receptors in Select Populations of Retinal Ganglion Cells

The predominate type of AMPA receptor expressed in the CNS is impermeable to Ca²⁺ (CI-AMPAR). However, some AMPA receptors are permeable to Ca²⁺ (CP-AMPAR) and play important roles in development, plasticity and disease. In the retina, ganglion cells (RGCs) are targets of disease including glaucoma and diabetic retinopathy, but there are many types of RGCs and not all types are targeted equally. In the present study, we sought to determine if there are differences in expression of AMPARs amongst RGC subtypes, and if these differences might contribute to differential vulnerability in a model of stress. Using cultured RGCs we first show that acute exposure to elevated pressure increased expression of Ca²⁺-permeable AMPA receptors (CP-AMPARs) in some, but not all classes of RGCs. When RGCs were sampled without regard to subtype, AMPA currents, measured using patch clamp recording, were blocked by the CP-AMPAR blocker PhTX-74 to a greater extent in pressure-treated RGCs vs. control. Furthermore, imaging experiments revealed an increase in Ca²⁺ influx during AMPA application in pressure-treated RGCs. However, examination of specific RGC subtypes using reporter lines revealed striking differences in both baseline AMPAR composition and modulation of this baseline composition by stress. Notably, ON alpha RGCs identified using the Opn4 mouse line and immunohistochemistry, had low expression of CP-AMPARs. Conversely, an ON-OFF direction selective RGC and putative OFF alpha RGC each expressed high levels of CP-AMPARs. These differences between RGC subtypes were also observed in RGCs from whole retina. Elevated pressure further lowered expression of CP-AMPARs in ON alpha RGCs, but raised expression in ON-OFF and OFF RGCs. Changes in CP-AMPAR expression following challenge with elevated pressure were correlated with RGC survival: ON alpha RGCs were unaffected by application of pressure, while the number of putative OFF alpha RGCs declined by approximately 50% following challenge with pressure. Differences in expression of CP-AMPARs between RGC subtypes may form the underpinnings for subtype-specific synaptic plasticity. Furthermore, the differential responses of these RGC subtypes to elevated pressure may contribute to the reported resistance of ON alpha, and susceptibility of OFF and ON-OFF RGCs to injury in models of glaucoma.

Maturation and electrophysiological properties of human pluripotent stem cell-derived oligodendrocytes

Rodent‐based studies have shown that the membrane properties of oligodendrocytes play prominent roles in their physiology and shift markedly during their maturation from the oligodendrocyte precursor cell (OPC) stage. However, the conservation of these properties and maturation processes in human oligodendrocytes remains unknown, despite their dysfunction being implicated in human neurodegenerative diseases such as multiple sclerosis (MS) and amyotrophic lateral sclerosis (ALS). Here, we have defined the membrane properties of human oligodendrocytes derived from pluripotent stem cells as they mature from the OPC stage, and have identified strong conservation of maturation‐specific physiological characteristics reported in rodent systems. We find that as human oligodendrocytes develop and express maturation markers, they exhibit a progressive decrease in voltage‐gated sodium and potassium channels and a loss of tetrodotoxin‐sensitive spiking activity. Concomitant with this is an increase in inwardly rectifying potassium channel activity, as well as a characteristic switch in AMPA receptor composition. All these steps mirror the developmental trajectory observed in rodent systems. Oligodendrocytes derived from mutant C9ORF72‐carryng ALS patient induced pluripotent stem cells did not exhibit impairment to maturation and maintain viability with respect to control lines despite the presence of RNA foci, suggesting that maturation defects may not be a primary feature of this mutation. Thus, we have established that the development of human oligodendroglia membrane properties closely resemble those found in rodent cells and have generated a platform to enable the impact of human neurodegenerative disease‐causing mutations on oligodendrocyte maturation to be studied. Stem Cells2016;34:1040–1053

Agonist-induced down-regulation of AMPA receptors in oligodendrocyte progenitors

Prolonged exposure of oligodendrocyte progenitor cultures to non-toxic concentrations of glutamate receptor agonists for 24 h decreased cellular proliferation mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Since prolonged agonist stimulation can regulate the expression of various families of receptors, we examined this possibility. Pretreatment of progenitor cultures with 100 μM kainic acid (KA) for 1-24 h caused a time-dependent decrease in AMPA receptor activity, determined by agonist-induced (45)Ca(2+) uptake. The maximum effect (70-80% decrease), observed in the 24 h-pretreated cells, was accompanied by a significant reduction in AMPA receptor subunits, as determined by Western blotting. GluR2/3 and GluR4 subunits were the most affected. Receptor down-regulation and (45)Ca(2+) uptake were only partially reversible upon KA removal. Furthermore, 24 h co-treatment of cultures with CNQX blocked the KA-induced decreases in calcium uptake. To address whether calpain, a calcium-activated protease, was implicated in the regulation of the AMPA receptor subunits, cultures were treated with the specific inhibitor PD150606 alone or in combination with KA for 24 h. Calpain inhibition significantly increased GluR1 in both conditions and partly reversed downregulation of GluR4 by KA. Collectively, these results indicate that calpain is not involved in the agonist-induced down-regulation of AMPA receptors subunits 2/3 in oligodendrocyte progenitors, while it downregulates GluR1 and GluR4.

Redefining the classification of AMPA-selective ionotropic glutamate receptors

AMPA-type ionotropic glutamate receptors (iGluRs) represent the major excitatory neurotransmitter receptor in the developing and adult vertebrate CNS. They are crucial for the normal hardwiring of glutamatergic circuits but also fine tune synaptic strength by cycling into and out of synapses during periods of sustained patterned activity or altered homeostasis. AMPARs are grouped into two functionally distinct tetrameric assemblies based on the inclusion or exclusion of the GluA2 receptor subunit. GluA2-containing receptors are thought to be the most abundant AMPAR in the CNS, typified by their small unitary events, Ca(2+) impermeability and insensitivity to polyamine block. In contrast, GluA2-lacking AMPARs exhibit large unitary conductance, marked divalent permeability and nano- to micromolar polyamine affinity. Here, I review evidence for the existence of a third class of AMPAR which, though similarly Ca(2+) permeable, is characterized by its near-insensitivity to internal and external channel block by polyamines. This novel class of AMPAR is most notably found at multivesicular release synapses found in the avian auditory brainstem and mammalian retina. Curiously, these synapses lack NMDA-type iGluRs, which are conventionally associated with controlling AMPAR insertion. The lack of NMDARs suggests that a different set of rules may govern AMPAR cycling at these synapses. AMPARs with similar functional profiles are also found on some glial cells suggesting they may have a more widespread distribution in the mammalian CNS. I conclude by noting that modest changes to the ion-permeation pathway might be sufficient to retain divalent permeability whilst eliminating polyamine sensitivity. Consequently, this emerging AMPAR subclass need not be assembled from novel subunits, yet to be cloned, but could simply occur by varying the stoichiometry of existing proteins.

GluR2-free ?-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors intensify demyelination in experimental autoimmune encephalomyelitis

We adopted a genetic approach to test the importance of edited GluR2-free, Ca(2+)-permeable, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors in the pathophysiology of experimental autoimmune encephalomyelitis, an inflammatory demyelinative disorder resembling multiple sclerosis. Initial studies showed that oligodendroglial lineage cells from mice lacking functional copies of the gene encoding the GluR3 AMPA receptor subunit (Gria3) had a diminished capacity to assemble edited GluR2-free AMPA receptors, and were resistant to excitotoxicity in vitro. Neurological deficits and spinal cord demyelination elicited by immunization with myelin oligodendrocyte glycoprotein peptide were substantially milder in these Gria3 mutant mice than in their wild-type littermates. These results support the hypothesis that oligodendroglial excitotoxicity mediated by AMPA receptors that do not contain edited GluR2 subunits contributes to demyelination in experimental autoimmune encephalomyelitis, and suggest that inhibiting these Ca(2+)-permeable AMPA receptors would be therapeutic in multiple sclerosis.

Glutamate-induced calcium transients in rat neurons of the dorsal motor nucleus of the vagus

The dorsal motor nucleus of the vagus (DMNV) integrates peripheral and central signals and sends efferent output to the gastrointestinal system. Glutamate, the major excitatory neurotransmitter of the central nervous system, causes increases in intracellular calcium in DMNV neurons. The mechanisms by which glutamate activates calcium signaling in the DMNV were examined. DMNV neurons were isolated from neonatal rat brainstem using microdissection and enzymatic digestion. Exposure to glutamate caused intracellular Ca(2+) increments in greater than 80% of cells. Removal of extracellular Ca(2+) abolished intracellular Ca(2+) transients. Kynurenic acid, a nonspecific glutamate receptor antagonist, abolished intracellular Ca(2+) transients. Exposure to glutamate while blocking AMPA receptors with GYKI 52466 abolished the Ca(2+) response. Exposure to (S)AMPA, an AMPA receptor agonist, caused intracellular Ca(2+) increments in 97% of cells. Activation and antagonism of NMDA and kainate receptors produced no changes compared to control experiments. NiCl, a nonspecific Ca(2+) channel blocker, abolished intracellular Ca(2+) transients. Blocking T-type Ca(2+) channels with mibefradil abolished the Ca(2+) response in 76% of cells. Blockade of L-type and N-type Ca(2+) channels did not affect the Ca(2+) response. Glutamate mediates intracellular Ca(2+) currents in DMNV neurons via the AMPA receptor and T-type Ca(2+) channels, allowing influx of extracellular Ca(2+).

HIV-1 gp120 enhances giant depolarizing potentials via chemokine receptor CXCR4 in neonatal rat hippocampus

In the immature hippocampus, the giant depolarizing potentials (GDPs) are recurrent network-driven synaptic events generated by gamma-aminobutyric acid (GABA), which in neonatal life is depolarizing and excitatory. The GDPs enable a high degree of synchrony in immature neurons and participate in activity-dependent growth and synapse formation. To understand how human immunodeficiency virus type one (HIV-1) infection in the immature brain impairs brain growth and development, we studied the effects of HIV-1 envelope glycoprotein, gp120, a viral toxin shed in abundance by infected cells, on spontaneous occurring GDPs recorded in the CA3 pyramidal cells in neonatal (P2-P6) Sprague-Dawley rat hippocampal slices using whole-cell patch technique. Bath application of gp120 produced a sustained enhancement of GDP frequency in a concentration-dependent manner without affecting passive membrane properties, suggesting that the site of action is most likely on neural network, other than on the recorded neurons. The gp120-induced enhancement of GDPs was blocked by T140, a highly specific antagonist for the chemokine receptor, CXCR4, indicating the involvement of CXCR4 in the gp120-induced increase of GDPs. Bath application of stromal cell-derived factor-1alpha (SDF-1alpha), the only CXCR4 ligand, mimicked the effects of gp120 on GDPs, supporting the engagement of CXCR4 receptors in the gp120-induced increase of GDP occurrence. Further studies revealed the involvement of protein kinase A/C in the gp120-induced enhancement of GDPs. These results demonstrate that gp120 enhances GDPs in the neonatal rat hippocampus. This enhancement may cause an excessive increase in intracellular calcium and resultant neuronal injury, leading to retardation of the brain and behavioural development as seen in paediatric AIDS patients.

Evidence for Ca(2+)-permeable AMPA receptors in the olfactory bulb

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs), a subtype of glutamate receptor, contribute to olfactory processing in the olfactory bulb (OB). These ion channels consist of various combinations of the subunits GluR1-GluR4, which bestow certain properties. For example, AMPARs that lack GluR2 are highly permeable to Ca(2+) and generate inwardly rectifying currents. Because increased intracellular Ca(2+) could trigger a host of Ca(2+)-dependent odor-encoding processes, we used whole cell recording as well as histological and immunocytochemical (ICC) techniques to investigate whether AMPARs on rat OB neurons flux Ca(2+). Application of 1-naphthylacetyl spermine (NAS), a selective antagonist of Ca(2+)-permeable AMPARs (CP-AMPARs), inhibited AMPAR-mediated currents in subsets of interneurons and principal cells in cultures and slices. The addition of spermine to the electrode yielded inwardly rectifying current-voltage plots in some cells. In OB slices, olfactory nerve stimulation elicited excitatory responses in juxtaglomerular and mitral cells. Bath application of NAS with d,l-2-amino-5-phosphonovaleric acid (AP5) to isolate AMPARs suppressed the amplitudes of these synaptic responses compared with responses obtained using AP5 alone. Co(2+) staining, which involves the kainate-stimulated influx of Co(2+) through CP-AMPARs, produced diverse patterns of labeling in cultures and slices as did ICC techniques used with a GluR2-selective antibody. These results suggest that subsets of OB neurons express CP-AMPARs, including functional CP-AMPARs at synapses. Ca(2+) entry into cells via these receptors could influence odor encoding by modulating K(+) channels, N-methyl-d-aspartate receptors, and Ca(2+)-binding proteins, or it could facilitate synaptic vesicle fusion.

Astrocytic glutamate release-induced transient depolarization and epileptiform discharges in hippocampal CA1 pyramidal neurons

A paroxysmal depolarization shift (PDS) has been suggested to be a hallmark for epileptic activity in partial-onset seizures. By monitoring membrane potentials and currents in pairs of pyramidal neurons and astrocytes with dual patch-clamp recording and exocytosis of vesicles from astrocytes with two-photon laser scanning microscopy in hippocampal slices, we found that infusion of inositol 1,4,5-trisphosphate (IP(3)) into astrocytes by patch pipettes induced astrocytic glutamate release that triggered a transient depolarization (TD) and epileptiform discharges in CA1 pyramidal neurons. The TD is due to a tetrodotoxin (TTX)-insensitive slowly decaying transient inward current (STC). Astrocytic glutamate release simultaneously triggers both the STC in pyramidal neurons and a transport current (TC) in astrocytes. The neuronal STC is mediated by ionotropic glutamate receptors leading to the TD and epileptiform discharges; while the astrocytic TC is a glutamate reuptake current resulting from transporting released glutamate into the patched astrocyte. Fusion of a large vesicle in astrocytes was immediately followed by an astrocytic TC, suggesting that the fused vesicle contains glutamate. Both fusion of large vesicles and astrocytic TCs were blocked by tetanus toxin (TeNT), suggesting that astrocytic glutamate release is via SNARE-dependent exocytosis of glutamate-containing vesicles. In the presence of TTX, the epileptogenic reagent, 4-AP, also induced similar neuronal STCs and astrocytic TCs, suggesting that astrocytic glutamate release may play an epileptogenic role in initiation of epileptic seizures under pathological conditions. Our study provides a novel mechanism, astrocytic release of glutamate, for seizure initiation.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Glutamate-mediated [Ca2+]c dynamics in spontaneously firing dopamine neurons of the rat substantia nigra pars compacta

The mechanism by which glutamate regulates the cytosolic free Ca2+ concentration ([Ca2+]c) in spontaneously firing dopamine neurons is not clear. Thus we have investigated the glutamate-mediated [Ca2+]c dynamics in the acutely isolated dopamine neurons from the rat substantia nigra pars compacta by measuring [Ca2+]c and spontaneously occurring action potentials (SAPs). The freshly isolated dopamine neurons showed tetrodotoxin (TTX)-sensitive spontaneous firing of 2-3 Hz and the resting [Ca2+]c decreased with abolition of the SAPs. The level of [Ca2+]c was affected by the spontaneous firing rate. In the presence of the Na+ channel antagonist, TTX (0.5 microM), glutamate increased [Ca2+]c by activating different glutamate receptors depending on the glutamate concentration used. Addition of glutamate at low concentrations (<3 microM) raised [Ca2+]c mainly by activating metabotropic glutamate receptors (mGluR), whereas at high concentrations (>10 microM) it raised [Ca2+]c mainly by activating AMPA/kainate receptors. The contribution of NMDA receptors to the glutamate-mediated [Ca2+]c rises was largest at intermediate concentrations of glutamate. Activation of mGluR elicited a Ca2+ release from intracellular Ca2+ stores and continuous Ca2+ influx out of the cell. The spontaneous firing activities were highly enhanced by submicromolar levels of glutamate and abolished at levels above 10 microM. From these results, we conclude that at low glutamate concentrations the [Ca2+]c in the dopamine neurons is mainly governed by mGluR and the firing activities, whose rate is regulated at submicromolar glutamate concentrations, but at higher glutamate concentrations [Ca2+]c is dominantly affected by AMPA/kainate receptors.

Low editing efficiency of GluR2 mRNA is associated with a low relative abundance of ADAR2 mRNA in white matter of normal human brain

The ionotropic glutamate receptor (GluR) subunits GluR2, GluR5 and GluR6 are subject to RNA editing at their Q/R sites, resulting in significant alterations in the channel properties of the receptors. RNA editing at the Q/R site of GluRs is both developmentally and regionally regulated. Here we provide the first quantitative measurements of both mRNAs of the GluR subunits and mRNAs of the RNA editing enzymes ADAR1-ADAR3 in a comparison of the efficiency of editing at the Q/R site with the expression levels of ADAR mRNA in human brain. We demonstrate that the Q/R site of GluRs in white matter is edited significantly less than in grey matter. In addition, by means of quantitative reverse transcription-polymerase chain reaction methods, we demonstrate that the relative abundance of ADAR2 mRNA to GluR2 mRNA is significantly lower in white matter than in grey matter and that the GluR2 Q/R site editing decreased only when the ratio of ADAR2 mRNA (not that of ADAR1 mRNA) to GluR2 mRNA dropped below a threshold (20 x 10(-3)). These results suggest that Q/R site of GluRs editing is regulated in a regional, and hence presumably cell-specific, manner and that the GluR2 Q/R site editing is critically regulated by ADAR2 in human brain.

Bcl-2-related protein family gene expression during oligodendroglial differentiation

Oligodendroglial lineage cells (OLC) vary in susceptibility to both necrosis and apoptosis depending on their developmental stages, which might be regulated by differential expression of Bcl-2-related genes. As an initial step to test this hypothesis, we examined the expression of 19 Bcl-2-related genes in purified cultures of rat oligodendroglial progenitors, immature and mature oligodendrocytes. All 'multidomain' anti-apoptotic members (Bcl-x, Bcl-2, Mcl-1, Bcl-w and Bcl2l10/Diva/Boo) except Bcl2a1/A1 are expressed in OLC. Semiquantitative and real-time RT-PCR revealed that Bcl-xL and Mcl-1 mRNAs are the dominant anti-apoptotic members and increase four- and twofold, respectively, with maturation. Bcl-2 mRNA is less abundant than Bcl-xL mRNA in progenitors and falls an additional 10-fold during differentiation. Bcl-w mRNA also increases, with significant changes in its splicing pattern, as OLC mature. Transfection studies demonstrated that Bcl-xL overexpression protects against kainate-induced excitotoxicity, whereas Bcl-2 overexpression does not. As for 'multidomain' pro-apoptotic members (Bax, Bad and Bok/Mtd), Bax and Bak are highly expressed throughout differentiation. Among 'BH3 domain-only' members examined (Bim, Biklk, DP5/Hrk, Bad, Bid, Noxa, Puma/Bbc3, Bmf, BNip3 and BNip3L), BNip3 and Bmf mRNAs increase markedly during differentiation. These results provide basic information to guide further studies on the roles for Bcl-2-related family proteins in OLC death.

Changes in splice variant expression and subunit assembly of AMPA receptors during maturation of hippocampal astrocytes

Astrocytes in the hippocampus express glutamate receptors of the AMPA subtype. An increasing body of evidence suggests a contribution of astroglial AMPA receptors to a direct signaling between neurons and glial cells in vivo. Here, we have combined functional analysis with singlecell RT-PCR to investigate whether hippocampal astrocytes express Ca(2+)-permeable AMPA receptors. We show that by postnatal day 5, a mosaic of Ca(2+)-permeable and less Ca(2+)-permeable AMPA receptors coexists in individual astrocytes, while receptors with a more uniform, low divalent permeability dominate in older cells. Moreover, we report an upregulation of the flip form of the GluR2 subunit during maturation, while the splicing status of GluR1 and GluR4 remains unchanged. Due to its specific properties, Ca(2+)-permeable AMPA receptors in astrocytes might strengthen neuron-to-glia signaling and enable proper formation of structural and functional connections between glial cells and glutamatergic synapses in the developing hippocampus.

Type-2 astrocyte-like cells are more resistant than oligodendrocyte-like cells against non-N-methyl-D-aspartate glutamate receptor-mediated excitotoxicity

Glutamate causes excitotoxicity via non-N-methyl-D-aspartate (NMDA) glutamate receptors (GluR) in oligodendrocytes. Because both oligodendrocytes and type 2 astrocytes are differentiated from oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells, we investigated whether astrocytes are also vulnerable to non-NMDA GluR-mediated excitotoxicity. For these studies, oligodendrocyte-like cells (OLC) and type 2 astrocyte-like cells (2ALC) were derived from CG-4 cells, an immortalized rat O-2A progenitor cell line. About 50% of 2ALC were positive for glial fibrillary acidic protein and 90% were positive for A2B5, verifying that these cells have an type 2 astrocytic phenotype. A 24-hr exposure of OLC to 2 mM kainate, an activator of non-NMDA GluR, caused cell damage as shown by the release of lactate dehydrogenase. The extent of kainate-induced OLC damage was increased by cyclothiazide. In contrast, exposure of 2ALC to 2 mM kainate alone did not induce injury, though mild 2ALC injury was elicited by exposure to 2 mM kainate plus 100 microM cyclothiazide. Furthermore, we found that the kainate induced Ca(2+) uptake by 2ALC was 27.5% of that induced by kainate in OLC. Finally, both OLC and 2ALC expressed non-NMDA GluR subunit mRNAs, including GluR2, GluR3, GluR4, GluR6, GluR7, KA1, and KA2, but quantitative Western blot analysis revealed higher immunodetectable GluR2 and lower immunodetectable GluR3 and GluR4 in 2ALC than in OLC. Together, these results suggest that astrocytes are relatively resistant to non-NMDA GluR-mediated excitotoxicity because they have a higher expression of GluR2 and lower expression of GluR3 and GluR4.

Blockage of Ca(2+)-permeable AMPA receptors suppresses migration and induces apoptosis in human glioblastoma cells

Glioblastoma multiforme is the most undifferentiated type of brain tumor, and its prognosis is extremely poor. Glioblastoma cells exhibit highly migratory and invasive behavior, which makes surgical intervention unsuccessful. Here, we showed that glioblastoma cells express Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors assembled from the GluR1 and/or GluR4 subunits, and that their conversion to Ca(2+)-impermeable receptors by adenovirus-mediated transfer of the GluR2 cDNA inhibited cell locomotion and induced apoptosis. In contrast, overexpression of Ca(2+)-permeable AMPA receptors facilitated migration and proliferation of the tumor cells. These findings indicate that Ca(2+)-permeable AMPA receptors have crucial roles in growth of glioblastoma. Blockage of these Ca(2+)-permeable receptors may be a useful therapeutic strategy for the prevention of glioblastoma invasion.

AMPA receptor-mediated toxicity in oligodendrocyte progenitors involves free radical generation and activation of JNK, calpain and caspase 3

The molecular mechanisms underlying AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) receptor-mediated excitotoxicity were characterized in rat oligodendrocyte progenitor cultures. Activation of AMPA receptors, in the presence of cyclothiazide to selectively block desensitization, produced a massive Ca(2+) influx and cytotoxicity which were blocked by the antagonists CNQX and GYKI 52466. A role for free radical generation in oligodendrocyte progenitor cell death was deduced from three observations: (i) treatment with AMPA agonists decreased intracellular glutathione; (ii) depletion of intracellular glutathione with buthionine sulfoximine potentiated cell death; and (iii) the antioxidant N -acetylcysteine replenished intracellular glutathione and protected cultures from AMPA receptor-mediated toxicity. Cell death displayed some characteristics of apoptosis, including DNA fragmentation, chromatin condensation and activation of caspase-3 and c-Jun N-terminal kinase (JNK). A substrate of calpain and caspase-3, alpha-spectrin, was cleaved into characteristic products following treatment with AMPA agonists. In contrast, inhibition of either caspase-3 by DEVD-CHO or calpain by PD 150606 protected cells from excitotoxicity. Our results indicate that overactivation of AMPA receptors causes apoptosis in oligodendrocyte progenitors through mechanisms involving Ca(2+) influx, depletion of glutathione, and activation of JNK, calpain, and caspase-3.

Discovery of diaminobutane derivatives as Ca(2+)-permeable AMPA receptor antagonists

We designed and synthesized a series of the polyamine derivatives as potent Ca(2+)-permeable AMPA receptor antagonists. In the course of this study, we found that the polyamine derivatives exhibited strong hypotensive activity which was undesirable activity for neuroprotective agents. Therefore, we tried to find non-hypotensive antagonists by structural modification of such compounds. Through this derivatization, we obtained the diamine compounds having desired profiles. Especially, compound 8f, which was non-hypotensive and potent Ca(2+)-permeable AMPA receptor antagonist, showed neuroprotective effects in transient global ischemia models in gerbils.

AMPA glutamate receptor-mediated calcium signaling is transiently enhanced during development of oligodendrocytes

Cells of the oligodendroglial lineage express Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-preferring glutamate receptors (AMPA-GluR) during development. Prolonged activation of their AMPA-GluR causes Ca2+ overload, resulting in excitotoxic death. Prior studies have shown that oligodendroglial progenitors and immature oligodendrocytes are susceptible to excitotoxicity, whereas mature oligodendrocytes are resistant. An unresolved issue has been why Ca2+-permeability of AMPA-GluR varies so markedly with oligodendroglial development, although the level of expression of edited GluR2, an AMPA-GluR subunit which blocks Ca2+ entry, is relatively constant. To address this question, we performed Ca2+ imaging, molecular and electrophysiological analyses using purified cultures of the rat oligodendroglial lineage. We demonstrate that transient up-regulation of expression of GluR3 and GluR4 subunits in oligodendroglial progenitors and immature oligodendrocytes results in the assembly by these cells, but not by oligodendroglial pre-progenitors or mature oligodendrocytes, of a population of AMPA-GluR which lack GluR2. This stage-specific up-regulation of edited GluR2-free, and hence Ca2+-permeable, AMPA-GluR explains the selective susceptibility to excitotoxicity of cells at these stages of oligodendroglial differentiation, and is likely to be important to these cells in the trans-synaptic Ca2+-signaling from glutamatergic neurons, which occurs in hippocampus

Role of GluR2 expression in AMPA-induced toxicity in cultured murine cerebral cortical neurons

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R)-mediated neurotoxicity was studied in relation to subunit expression and the presence of Ca(2+)-permeable receptor channels. AMPA-mediated toxicity had two components: 1) a direct AMPA-R-mediated component, which was not due to Ca(2+) influx through voltage-gated Ca(2+) channels, reversal of the Na(+)/Ca(2+) exchanger or release of calcium from dantrolene-sensitive intracellular Ca(2+) stores, and 2) a minor, indirect component involving activation of NMDA receptor channels, because of glutamate release and removal of the Mg(2+) block of the NMDA receptor on AMPA-R stimulation. The involvement of Ca(2+) influx through AMPA-R was also examined. The number of neurons possessing Ca(2+)-permeable AMPA-R increased during culture development, concurrently with an increasing susceptibility for AMPA-induced toxicity during development. GluR2(R) levels also increased during development, and channel blockers of Ca(2+)-permeable AMPA-R lacking the GluR2(R) subunit (spermine and philanthotoxin) failed to prevent neurotoxicity or increases in [Ca(2+)](i). Thus, the direct AMPA-R-mediated toxicity may be explained by initiation of cell death by Ca(2+) fluxing through AMPA-R containing GluR2(R). The components of direct AMPA-R-mediated toxicity are proposed to be 1) toxicity mediated by GluR2(R)-lacking AMPA-R and 2) toxicity mediated by low-Ca(2+)-permeability AMPA-R containing GluR2(R).

Ampa/kainate receptor activation mediates hypoxic oligodendrocyte death and axonal injury in cerebral white matter

We developed an in situ model to investigate the hypothesis that AMPA/kainate (AMPA/KA) receptor activation contributes to hypoxic-ischemic white matter injury in the adult brain. Acute coronal brain slices, including corpus callosum, were prepared from adult mice. After exposure to transient oxygen and glucose deprivation (OGD), white matter injury was assessed by electrophysiology and immunofluorescence for oligodendrocytes and axonal neurofilaments. White matter cellular components and the stimulus-evoked compound action potential (CAP) remained stable for 12 hr after preparation. OGD for 30 min resulted in an irreversible loss of the CAP as well as structural disruption of axons and subsequent loss of neurofilament immunofluorescence. OGD also caused widespread oligodendrocyte death, demonstrated by the loss of APC labeling and the gain of pyknotic nuclear morphology and propidium iodide labeling. Blockade of AMPA/KA receptors with 30 microm NBQX or the AMPA-selective antagonist 30 microm GYKI 52466 prevented OGD-induced oligodendrocyte death. Oligodendrocytes also were preserved by the removal of Ca(2+), but not by a blockade of voltage-gated Na(+) channels. The protective action of NBQX was still present in isolated corpus callosum slices. CAP areas and axonal structure were preserved by Ca(2+) removal and partially protected by a blockade of voltage-gated Na(+) channels. NBQX prevented OGD-induced CAP loss and preserved axonal structure. These observations highlight convergent pathways leading to hypoxic-ischemic damage of cerebral white matter. In accordance with previous suggestions, the activation of voltage-gated Na(+) channels contributes to axonal damage. Overactivation of glial AMPA/KA receptors leads to oligodendrocyte death and also plays an important role in structural and functional disruption of axons.

Mitogen‐activated protein kinase signalling in oligodendrocytes: a comparison of primary cultures and CG‐4

Oligodendrocytes play a significant role in the central nervous system, as these cells are responsible for myelinating axons and allowing for the efficient conduction of nerve impulses. Therefore, any understanding we can gain about the functional biology of oligodendrocytes will give us important insights into demyelinating diseases such as multiple sclerosis, where oligodendrocytes and myelin are damaged or destroyed. Currently, much attention has focussed on the role of a family of mitogen-activated protein kinases in OL. This kinase family includes the extracellular signal-regulated protein kinases (ERKs), the stress-activated c-Jun N-terminal kinase (JNK), and the 38 kDa high osmolarity glycerol response kinase (p38). The actions of mitogen-activated protein kinases in oligodendrocytes appear to range from proliferation and cell survival to differentiation and cell death. In the past, studies on oligodendrocytes have been hampered by the difficulties inherent in producing large enough quantities of these cells for experimentation. This problem arises in large part due to the post-mitotic nature of mature oligodendrocytes. Over the years, a cell line known as Central Glia-4 (CG-4) has become a popular oligodendrocyte model due to its potentially unlimited capacity for self-renewal. In this review, we will look at the suitability of the Central Glia-4 cell line as an oligodendrocyte model, specifically in respect to studies on mitogen-activated protein kinase signalling in oligodendrocytes.

Synaptic and non-synaptic AMPA receptors permeable to calcium

For a long time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors permeable to calcium have been considered to be either non-existent or as "atypical". There is now ample evidence that these receptors exist in numerous regions of the nervous system and in many neuronal as well as non-neuronal cell populations. This evidence has been accumulated by several methods, including electrophysiological recording, calcium imaging and cobalt-loading. Functional AMPA receptors permeable to calcium are already expressed at very early stages of embryonic development, well before the onset of synaptogenesis. They are probably involved in the paracrine signaling necessary for construction of the nervous system before becoming involved in synaptic transmission. In immature cells, cyclothiazide strongly increases the steady-state level of responses not only to AMPA, but also to kainate. Ingestion, during pregnancy, of food or drug substances that can cross the placental barrier and act upon the embryonic receptors may constitute a risk for normal development. In the adult nervous system, synaptic as well as non-synaptic (paracrine) AMPA receptors permeable to calcium are probably widely expressed in both glial and neuronal cells. They may also participate in controlling some aspects related to adult neurogenesis, in particular the migration of newly formed neurons.

Disruption of the GluR2-NSF interaction protects primary hippocampal neurons from ischemic stress

A specific interaction between the AMPA receptor subunits GluR2 and GluR3 and the fusion protein NSF has recently been identified. Disruption of this interaction by adenoviral-mediated expression of a peptide (pep2m) corresponding to the NSF-binding region of GluR2 results in a dramatic reduction in surface expression of AMPA receptors in primary hippocampal neurons. Here we report that expression of pep2m from a recently developed neuronal-specific adenoviral system gave significant neuroprotection to primary CA1-CA3 hippocampal neurons following stimulation with kainate (KA) and this was accompanied by a reduction in Ca(2+) influx. Protection was also observed following glucose deprivation and exposure to ischemic buffer in the absence of any NMDA receptor antagonists. These results provide strong evidence that AMPA receptors play a direct role in mediating postischemic neurotoxicity.

Coupling of AMPA receptors with the Na(+)/Ca(2+) exchanger in cultured rat astrocytes

Astrocytes exhibit three transmembrane Ca(2+) influx pathways: voltage-gated Ca(2+) channels (VGCCs), the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) class of glutamate receptors, and Na(+)/Ca(2+) exchangers. Each of these pathways is thought to be capable of mediating a significant increase in Ca(2+) concentration ([Ca(2+)](i)); however, the relative importance of each and their interdependence in the regulation astrocyte [Ca(2+)](i) is not known. We demonstrate here that 100 microM AMPA in the presence of 100 microM cyclothiazide (CTZ) causes an increase in [Ca(2+)](i) in cultured cerebral astrocytes that requires transmembrane Ca(2+) influx. This increase of [Ca(2+)](i) is blocked by 100 microM benzamil or 0.5 microM U-73122, which inhibit reverse-mode operation of the Na(+)/Ca(2+) exchanger by independent mechanisms. This response does not require Ca(2+) influx through VGCCs, nor does it depend upon a significant Ca(2+) influx through AMPA receptors (AMPARs). Additionally, AMPA in the presence of CTZ causes a depletion of thapsigargin-sensitive intracellular Ca(2+) stores, although depletion of these Ca(2+) stores does not decrease the peak [Ca(2+)](i) response to AMPA. We propose that activation of AMPARs in astrocytes can cause [Ca(2+)](i) to increase through the reverse mode operation of the Na(+)/Ca(2+) exchanger with an associated release of Ca(2+) from intracellular stores. This proposed mechanism requires neither Ca(2+)-permeant AMPARs nor the activation of VGCCs to be effective.

NBQX attenuates excitotoxic injury in developing white matter

The excitatory neurotransmitter glutamate is released from axons and glia under hypoxic/ischemic conditions. In vitro, oligodendrocytes (OLs) express non-NMDA glutamate receptors (GluRs) and are susceptible to GluR-mediated excitotoxicity. We evaluated the role of GluR-mediated OL excitotoxicity in hypoxic/ischemic white matter injury in the developing brain. Hypoxic/ischemic white matter injury is thought to mediate periventricular leukomalacia, an age-dependent white matter lesion seen in preterm infants and a common antecedent to cerebral palsy. Hypoxia/ischemia in rat pups at postnatal day 7 (P7) produced selective white matter lesions and OL death. Furthermore, OLs in pericallosal white matter express non-NMDA GluRs at P7. Unilateral carotid ligation in combination with hypoxia (6% O(2) for 1 hr) resulted in selective, subcortical white matter injury with a marked ipsilateral decrease in immature and myelin basic protein-expressing OLs that was also significantly attenuated by 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX). Intracerebral AMPA demonstrated greater susceptibility to OL injury at P7 than in younger or older pups, and this was attenuated by systemic pretreatment with the AMPA antagonist NBQX. These results indicate a parallel, maturation-dependent susceptibility of immature OLs to AMPA and hypoxia/ischemia. The protective efficacy of NBQX suggests a role for glutamate receptor-mediated excitotoxic OL injury in immature white matter in vivo.

Glutamate receptors in glia: new cells, new inputs and new functions

Functional glutamate receptors are expressed on the majority of glial cell types in the developing and mature brain. Although glutamate receptors on glia are activated by glutamate released from neurons, their physiological role remains largely unknown. Potential roles for these receptors in glia include regulation of proliferation and differentiation, and modulation of synaptic efficacy. Recent anatomical and functional evidence indicates that glutamate receptors on immature glia are activated through direct synaptic inputs. Therefore, glutamate and its receptors appear to be involved in a continuous crosstalk between neurons and glia during development and also in the mature brain.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

Non-N-methyl-D-aspartate glutamate receptors mediate oxygen--glucose deprivation-induced oligodendroglial injury

Cells of oligodendroglial lineage are susceptible to oxygen and glucose deprivation. When oligodendrocyte-like cells differentiated from CG-4-immortalized rat O-2A progenitor cells were exposed to hypoxia alone or glucose deprivation alone for 48 h, release of lactate dehydrogenase (LDH) into the culture medium did not increase. However, when cells were deprived of both oxygen and glucose for 6 or 12 h preceding reoxygenation for 2 h, LDH release increased. Adding glucose to the medium protected against cell death and increased lactate production in a concentration-dependent manner. Cell damage induced by deprivation of oxygen and glucose was prevented by calcium-free medium or by non-N-methyl-D-aspartate glutamate receptor (GluR) antagonists, such as 6-cyano-7-nitroquinoxaline-2,3-dione or LY293558, but not by the voltage-dependent calcium channel blocker, nimodipine, or by the N-methyl-D-aspartate GluR antagonist, MK-801. The glutamate concentration in the medium from cells exposed to oxygen-glucose deprivation for 12 h was 49.70+/-3.04 microM/l, which is sufficient to activate GluRs during deprivation of oxygen and glucose. Apoptotic cells detected by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) or Hoechst 33258 staining did not increase in cells exposed to oxygen-glucose deprivation for 12 h and subsequent reoxygenation for 2 h. No DNA laddering was detected by agarose gel electrophoresis from cells exposed to deprivation of oxygen and glucose. Neither acetyl-YVAD-CHO, an inhibitor of caspase-1-like proteases, nor acetyl-DEVD-CHO, an inhibitor of caspase-3-like proteases, prevented oxygen-glucose deprivation-induced injury. Thus, oxygen and glucose deprivation causes calcium-influx-induced necrotic cell damage in cells of oligodendroglial lineage via non-N-methyl-D-aspartate GluR channels.

Characterization of the AMPA-activated receptors present on motoneurons

Motoneurons have been shown to be particularly sensitive to Ca2+-dependent glutamate excitotoxicity, mediated via AMPA receptors (AMPARs). To determine the molecular basis for this susceptibility we have used immunocytochemistry, RT-PCR, and electrophysiology to profile AMPARs on embryonic day 14.5 rat motoneurons. Motoneurons show detectable AMPAR-mediated calcium permeability in vitro and in vivo as determined by cobalt uptake and electrophysiology. Motoneurons express all four AMPAR subunit mRNAs, with glutamate receptor (GluR) 2 being the most abundant (63.9+/-4.8%). GluR2 is present almost exclusively in the edited form, and electrophysiology confirms that most AMPARs present are calcium-impermeant. However, the kainate current in motoneurons was blocked an average of 32.0% by Joro spider toxin, indicating that a subset of the AM PARs is Ca2+-permeable. Therefore, heterogeneity of AMPARs, rather than the absence of GluR2 or the presence of unedited GluR2, explains AMPAR-mediated Ca2+ permeability. The relative levels of flip/flop isoforms of each subunit were also examined by semiquantitative PCR. Both isoforms were present, but the relative proportion varied for each subunit, and the flip isoform predominated. Thus, our data show that despite high levels of edited GluR2 mRNA, some AMPARs are Ca2+-permeable, and this subset of AMPARs can account for the AMPAR-mediated Ca2+ inflow inferred from cobalt uptake and electrophysiology studies.

The open channel blocking drug, IEM-1460, reveals functionally distinct alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors in rat brain neurons

The properties of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors were examined in various cell types isolated from young rat hippocampus, striatum and cerebellum using patch-clamp and fast application techniques. A dicationic adamantane derivative, IEM-1460, reversibly inhibited kainate-induced currents. In the presence of 100 microM IEM-1460, kainate currents in striatal giant cholinergic interneurons and hippocampal non-pyramidal neurons were inhibited by 95% and 81%, respectively, at Vh = - 70 mV. Striatal GABAergic principal cells, hippocampal pyramidal neurons and cerebellar Purkinje cells had low sensitivity to IEM-1460 (inhibition by 4-15%). Analysis of averaged data from the cell types studied revealed a highly significant positive correlation (r= 0.93, P < 0.01) between percentage inhibition by 100 microM IEM-1460 and relative calcium permeability of AMPA receptors, P(Ca)/P(Na). Also, within each brain structure, the sensitivity of IEM-1460 block was lower the stronger the outward rectification of kainate currents. Some hippocampal neurons exhibited intermediate sensitivity to IEM-1460. Kainate currents were suppressed by 40% in the presence of 100 microM IEM-1460. Meanwhile, AMPA receptors in this cell type had low calcium permeability (P(Ca)/P(Na) = 0.13) and demonstrated outwardly rectifying kainate currents. The interrelation of different properties of AMPA receptors considering their assembly is discussed. The data obtained suggest that IEM-1460 may be a convenient and promising marker of native AMPA receptor assembly: it selectively inhibits Ca(2+)-permeable, GluR2-lacking AMPA receptors.

Cerebral cortical neuron apoptosis after mild excitotoxic injury in vitro: different roles of mesencephalic and cortical astrocytes

OBJECTIVE

Increasing evidence supports the presence of neuronal apoptosis after ischemic or excitotoxic brain injury. Astrocytes, which exhibit significant regional differences in function, may exert a protective effect on neurons exposed to ischemic injury. We examined the effects of astrocytes derived from different regions of the central nervous system on neuronal apoptosis after mild excitotoxic injury in tissue culture.

METHODS

Purified astrocyte cultures derived from P4 rat cerebral cortex or mesencephalon showed transient cell swelling but no cell death when exposed to 50 micromol/L glutamate for 5 minutes. When mixed neuronal/glial cocultures were exposed to the same glutamate dose, neuron death was observed. Necrotic and apoptotic cell death during 24 hours was examined using morphological criteria, nuclear staining, triphosphate nick end labeling, and trypan blue exclusion.

RESULTS

We found that cortical neurons that elaborate a more extensive dendritic arbor when grown on homotypic astrocytes are more likely to undergo apoptosis than neurons with a limited dendritic arbor grown on heterotypic astrocytes. By contrast, a similar number of neurons undergo necrotic cell death.

CONCLUSION

This finding may be associated with 1) increased vulnerability of neurons with a more elaborate dendrite structure to mild excitotoxic injury, or 2) regional differences in the ability of astrocytes to attenuate apoptosis.

Ca(2+) permeation of AMPA receptors in cerebellar neurons expressing glu receptor 2

AMPA receptors in cultured cerebellar neurons were characterized by whole-cell electrophysiological studies and single cell PCR-based quantitation of subunit mRNA expression. Purkinje neurons consistently expressed high levels of Glu receptor 2 (GluR2) mRNA and AMPA receptors with low but nonzero Ca(2+) permeability. Other cerebellar neurons expressed AMPA receptors with a wide range of Ca(2+) permeability and of fractional GluR2. These properties correlated on a cell-by-cell basis. Their relationship was well fit by a model that assumed stochastic assembly of subunits and GluR2 dominance in controlling divalent cation permeation, suggesting that AMPA receptor properties in individual neurons may be determined primarily by relative levels of subunit transcription. A fraction of receptors, lacking GluR2, can contribute a highly Ca(2+)-permeable component to AMPA receptor responses, even in cells expressing GluR2.

Characterization of AMPA receptor populations in rat brain cells by the use of subunit-specific open channel blocking drug, IEM-1460

Dicationic adamantane derivative, IEM-1460, which selectively blocks GluR2-lacking, Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors, was used to characterize the distribution of AMPA receptors among populations of rat brain cells. IEM-1460 inhibited kainate-induced inward currents (at -80 mV) in a dose-dependent manner. IEM-1460 concentrations producing 50% inhibition of kainate-induced current amplitude (IC50) varied greatly depending on the cell type studied. Striatal giant cholinergic interneurons and putative Bergmann glial cells isolated from the cerebellum were found to be highly sensitive to IEM-1460 block (IC50=2.6 microM), indicating the expression of GluR2-lacking AMPA receptor subtype. Among hippocampal and cortical non-pyramidal neurons, there were cell-to-cell differences in the pattern of AMPA receptor subtype expression. Some cells which are known to express AMPA receptors lacking GluR2 subunit exhibited high sensitivity of IEM-1460 block (IC50 about 1 microM) but in the others, the part of AMPA receptor population seemed to be represented by GluR2-having receptor subtype. The latter subtype was mainly expressed by pyramidal neurons isolated from hippocampus (IC50=1102 microM) and sensorimotor cortex (IC50=357 microM) which showed low affinity for IEM-1460 block. In conclusion, IEM-1460 can be utilized as an indicator of the distribution of AMPA receptor subtypes among populations of rat brain cells, and pharmacological detection of the absence of GluR2 subunit in AMPA receptor assembly can provide useful information for the interpretation of physiological events.

Calcium dynamics in the extracellular space of mammalian neural tissue

In the brain, hundreds of intracellular processes are known to depend on calcium influx; hence any substantial fluctuation in external calcium ([Ca2+]o) is likely to engender important functional effects. Employing the known scales and parameters of mammalian neural tissue, we introduce and justify a computational approach to the hypothesis that large changes in local [Ca2+]o will be part of normal neural activity. Using this model, we show that the geometry of the extracellular space in combination with the rapid movement of calcium through ionic channels can cause large external calcium fluctuations, up to 100% depletion in many cases. The exact magnitude of a calcium fluctuation will depend on 1) the size of the consumption zone, 2) the local diffusion coefficient of calcium, and 3) the geometrical arrangement of the consuming elements. Once we have shown that using biologically relevant parameters leads to calcium changes, we focus on the signaling capacity of such concentration fluctuations. Given the sensitivity of neurotransmitter release to [Ca2+]o, the exact position and timing of neural activity will delimit the terminals that are able to release neurotransmitter. Our results indicate that mammalian neural tissue is engineered to generate significant changes in external calcium concentrations during normal activity. This design suggests that such changes play a role in neural information processing.

Molecular pathways mediating activation by kainate of mitogen-activated protein kinase in oligodendrocyte progenitors

Oligodendroglial cells express ionotropic glutamate receptors of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid hydrobromide (AMPA) and kainate (KA) subtypes. Recently, we reported that AMPA receptor agonists increased 45Ca2+ uptake and phospholipase C (PLC) activity. To further elucidate the intracellular signaling mechanisms, we examined the effects of AMPA and KA on mitogen-activated protein kinase (MAPK). KA caused a time- and concentration-dependent increase in MAPK activity (predominantly the p42mapk or ERK2) and the effect was blocked by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX), a competitive AMPA/KA receptor antagonist. Furthermore, the noncompetitive antagonists of AMPA receptor GYKI 52466 and LY 303070 prevented the actions of the agonists, indicating that the effect of KA on MAPK activation is mediated through AMPA receptors in oligodendrocyte progenitors. Chelation of extracellular Ca2+ by EDTA or inhibition of PLC with U73122 abolished MAPK activation by KA. In addition, KA-stimulated MAPK activation was reduced by the protein kinase C (PKC) inhibitors, H7 and bisindolylmaleimide, as well as downregulation of PKC by prolonged exposure to phorbol esters. The involvement of PKC in the signal transduction pathways was further supported by the ability of KA to induce translocation of PKC measured by [3H]PDBu binding. Interestingly, a wortmannin-sensitive phosphatidylinositol 3-kinase and a pertussis toxin (PTX)-sensitive G protein form part of the molecular pathways mediating MAPK activation by AMPA receptor. A specific inhibitor of MAPK kinase, PD 098059, blocked MAPK activation and reduced KA-induced c-fos gene expression. All together, these results indicate that MAPK is implicated in the transmission of AMPA signaling to the nucleus and requires extracellular Ca2+, and PLC/PKC activation.

Dissociation between the Joro spider toxin sensitivity of recombinant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and their ability to increase intracellular calcium

We compared the toxin sensitivity, Ca2+ flux response and rectification properties of recombinant alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptors obtained by transfecting human embryonic kidney (HEK) 293 cells with different ratios of GluR1 and GluR2 cDNAs (10:1 to 1:10). Simultaneous measurements of kainate-activated Ca2+ fluxes and inward currents, using fura-2 microfluorimetry under voltage clamp conditions, suggested the existence of GluR2 containing channels which are permeable to Ca2+ and insensitive to Joro spider toxin (JSTx). Imaging experiments showed that JSTx inhibition of the Ca2+ response induced by kainate was reduced by increasing the relative amount of GluR2. However, even at GluR1/GluR2(R) ratios of 1:1 and 1:4, cells were still able to flux Ca2+ when stimulated by kainate. GluR2 similarly inhibited the ability of JSTx to reduce kainate-evoked inward currents in whole cell patch-clamp experiments. Variations in the rectification properties of the AMPA currents, induced by changes in the cDNA ratio, were not always correlated with the changes in toxin sensitivity and [Ca2+]i response. Thus, cells with almost linear I-V relationships were partially blocked by JSTx and still Ca2+ permeable. Our results indicate a dissociation between the toxin sensitivity and Ca2+ flux through GluR2 containing AMPA receptors and suggest that receptors with diverse Ca2+ permeabilities are generated by the expression of variable amounts of GluR2.

Ca2+ permeability and joro spider toxin sensitivity of AMPA and kainate receptors on cerebellar granule cells

We have investigated the Ca2+ permeability of native kainate- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate- (AMPA) receptors in cultured rat cerebellar granule cells. Intracellular Ca2+ ([Ca2+]i) increases and Mn2+ quench of fura-2 (a measure of Ca2+ entry) mediated by kainate receptors were completely dependent on the presence of extracellular Na+. Kainate receptor-mediated [Ca2+]i rises were reduced 37% by the L-type voltage-gated Ca2+ channel blocker nifedipine (1 microM). AMPA receptor-mediated [Ca2+]i rises observed in Na+-free buffer were sensitive to Joro spider toxin (500 nM) blockade showing a 65% reduction, while kainate receptor-mediated [Ca2+]i responses were largely insensitive. These results suggest that a component of AMPA receptor-mediated [Ca2+]i increases occurs through Ca2+ permeable receptors which lack the GluR2 subunit and are Joro spider toxin sensitive. In contrast, kainate receptors do not appear to directly gate significant Ca2+ but raise [Ca2+]i through activation of voltage-gated Ca2+ channels and seem largely insensitive to Joro spider toxin.

Characterization of ryanodine receptors in oligodendrocytes, type 2 astrocytes, and O-2A progenitors

In this study we have investigated the expression of ryanodine receptors (RyRs), and the ability of caffeine to evoke RyR-mediated elevation of intracellular Ca2+ levels ([Ca2+]i) in glial cells of the oligodendrocyte/type 2 astrocyte lineage. Immunocytochemistry with specific antibodies identified ryanodine receptors in cultured oligodendrocytes, type 2 astrocytes, and O-2A progenitor cells, at high levels in the perinuclear region and in a variegated pattern along processes. Glia acutely isolated from rat brain and in aldehydefixed sections of cortex were similarly found to express RyRs. Caffeine (5-50 mM) caused an increase in [Ca2+]i in most cultured type 2 astrocytes and in 50% of oligodendrocytes. Responses elicited by caffeine were inhibited by pretreatment with ryanodine (10 microM) or thapsigargin (1 microM), and the peak response was unaffected by removal of [Ca2+]o. O-2A progenitor cells, in contrast, were largely unresponsive to caffeine treatment. Pretreatment with kainate (200 microM) to activate Ca2+ entry increased the magnitude of caffeine-evoked [Ca2+]i elevations in type 2 astrocytes and oligodendrocytes, and caused caffeine to activate responses in a significant proportion of previously non-responding O-2A progenitors. In both type 2 astrocytes and oligodendrocytes, caffeine evoked Ca2+ changes which propagated as wavefronts from several initiation sites. These wave amplification sites were characterized by significantly higher local Ca2+ release kinetics. Our results indicate that several glial cell types express RyRs, and that their functionality differs within different cell types of the oligodendrocyte lineage. In addition, ionotropic glutamate receptor activation fills the caffeine-sensitive Ca2+ stores in these cells.