Diminished calcium homeostasis and increased susceptibility to excitotoxicity of JS 3/16 progenitor cells after differentiation to oligodendroglia

Electrophysiological properties of NG2(+) cells: Matching physiological studies with gene expression profiles

NG2(+) glial cells are a dynamic population of non-neuronal cells that give rise to myelinating oligodendrocytes in the central nervous system. These cells express numerous ion channels and neurotransmitter receptors, which endow them with a complex electrophysiological profile that is unique among glial cells. Despite extensive analysis of the electrophysiological properties of these cells, relatively little was known about the molecular identity of the channels and receptors that they express. The generation of new RNA-Seq datasets for NG2(+) cells has provided the means to explore how distinct genes contribute to the physiological properties of these progenitors. In this review, we systematically compare the results obtained through RNA-Seq transcriptional analysis of purified NG2(+) cells to previous physiological and molecular studies of these cells to define the complement of ion channels and neurotransmitter receptors expressed by NG2(+) cells in the mammalian brain and discuss the potential significance of the unique physiological properties of these cells. This article is part of a Special Issue entitled SI:NG2-glia(Invited only).

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.

Amyloid β1-42 oligomer inhibits myelin sheet formation in vitro

Accumulating evidence indicates that white matter degeneration contributes to the neural disconnections that underlie Alzheimer's disease pathophysiology. Although this white matter degeneration is partly attributable to axonopathy associated with neuronal degeneration, amyloid β (Aβ) protein-mediated damage to oligodendrocytes could be another mechanism. To test this hypothesis, we studied effects of soluble Aβ in oligomeric form on survival and differentiation of cells of the oligodendroglial lineage using highly purified oligodendroglial cultures from rats at different developmental stages. Aβ oligomer at 10 μM or higher reduced survival of mature oligodendrocytes, whereas oligodendroglial progenitor cells (OPCs) were relatively resistant to the Aβ oligomer-mediated cytotoxicity. Further study revealed that Aβ oligomer even at 1 μM accelerated 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) formazan exocytosis in mature oligodendrocytes, and, more significantly, inhibited myelin sheet formation after induction of in vitro differentiation of OPCs. These results imply a novel pathogenetic mechanism underlying Aβ oligomer-mediated white matter degeneration, which could impair myelin maintenance and remyelination by adult OPCs, resulting in accumulating damage to myelinating axons thereby contributing to neural disconnections.

Injury and differentiation following inhibition of mitochondrial respiratory chain complex IV in rat oligodendrocytes

Oligodendrocyte lineage cells are susceptible to a variety of insults including hypoxia, excitotoxicity, and reactive oxygen species. Demyelination is a well-recognized feature of several CNS disorders including multiple sclerosis, white matter strokes, progressive multifocal leukoencephalopathy, and disorders due to mitochondrial DNA mutations. Although mitochondria have been implicated in the demise of oligodendrocyte lineage cells, the consequences of mitochondrial respiratory chain defects have not been examined. We determine the in vitro impact of established inhibitors of mitochondrial respiratory chain complex IV or cytochrome c oxidase on oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes as well as on differentiation capacity of OPCs from P0 rat. Injury to mature oligodendrocytes following complex IV inhibition was significantly greater than to OPCs, judged by cell detachment and mitochondrial membrane potential (MMP) changes, although viability of cells that remained attached was not compromised. Active mitochondria were abundant in processes of differentiated oligodendrocytes and MMP was significantly greater in differentiated oligodendrocytes than OPCs. MMP dissipated following complex IV inhibition in oligodendrocytes. Furthermore, complex IV inhibition impaired process formation within oligodendrocyte lineage cells. Injury to and impaired process formation of oligodendrocytes following complex IV inhibition has potentially important implications for the pathogenesis and repair of CNS myelin disorders. © 2010 Wiley-Liss, Inc.

Bi-directional control of motor neuron dendrite remodeling by the calcium permeability of AMPA receptors

Motor neurons express particularly high levels of the AMPA receptor subunit GluR1(Q)flip (GluR1(Q)i) during the period in early postnatal life when their dendritic tree grows and becomes more branched. To investigate how GluR1-containing AMPA receptors contribute to dendrite morphogenesis, we characterized a mutant form of GluR1 (containing a histidine in the Q/R editing site) with unique electrophysiological properties. Most notably, AMPA receptors assembled from GluR1(H)i display less calcium permeability than AMPA receptors assembled from GluR1(Q)i. Expression of GluR1(Q)i in vivo or in vitro led to an increase in dendrite branching with no net change in the overall tree size while GluR1(H)i led to a loss of branches and a net reduction in overall tree size. GluR1(H)i-dependent dendrite atrophy is mediated by protein phosphatase 2B. The results suggest that the electrophysiological properties of cell surface AMPA receptors, specifically their permeability to calcium, can be a central determinant of whether the dendrites undergo activity-dependent branching or atrophy.

Interferon-gamma is up-regulated in the hippocampus in response to intermittent fasting and protects hippocampal neurons against excitotoxicity

Dietary restriction (DR) increases the life span of many different organisms, and recent findings have demonstrated neuroprotective effects of DR in rodent and nonhuman primate models of neurodegenerative disorders. The neuroprotective mechanism of action of DR is unknown, but it may result from a mild cellular stress response involving increased production of neurotrophic factors. Because several different cytokines are known to be up-regulated in brain cells in response to stress, we determined whether DR affected cytokine expression in the rat brain. Levels of expression of interferon-gamma (IFN-gamma) and its receptor were significantly increased in the hippocampus of rats that had been maintained on an intermittent fasting DR regimen compared with rats on the ad libitum control diet. Pretreatment of embryonic rat hippocampal cell cultures with IFN-gamma protected neurons against glutamate-induced death. IFN-gamma-mediated neuroprotection was associated with an enhanced recovery of intracellular Ca(2+) concentrations following exposure to glutamate. Our data show that intermittent fasting DR stimulates IFN-gamma-mediated neuroprotective signaling in the hippocampus, suggesting a role for this cytokine in the previously reported ability of DR to protect neurons in animal models of severe epileptic seizures, stroke, and neurodegenerative disorders.

Oligodendrocyte differentiation is increased in transferrin transgenic mice

Transferrin (Tf), the iron transport glycoprotein found in biological fluids of vertebrates, is synthesized mainly by hepatocytes. Tf is also synthesized by oligodendrocytes (Ol), and several lines of evidence indicate that brain Tf could be involved in myelinogenesis. Because Tf is postnatally expressed in the brain, we sought to investigate whether Tf could intervene in Ol differentiation. For this purpose, we analyzed transgenic mice overexpressing the complete human Tf gene in Ol. We show that the hTf transgene was expressed only from 5 days postpartum onward. In the brain of 14-day-old transgenic mice, the DM-20 mRNA level was decreased, whereas the PLP, MBP, CNP, and MAG mRNA levels were increased. We counted a higher proportion of Ol expressing the O4 (Ol-specific antigens) and PLP in brain cells cultured from transgenic mice. These results support the idea that overexpressing Tf in the brain accelerates the oligodendrocyte lineage maturation. Accordingly, by NMR imaging acquisition of diffusion tensor in hTf transgenic mice, we observed early maturation of the cerebellum and spinal cord and more myelination in the corpus callosum. In addition, hTf overexpression led to an increase in Sox10 mRNA and protein. Increases in Sox10 and in Tf expression occur simultaneously during brain development. The Olig1 mRNA level also increased, but long after the rise of hTf and Sox10. The Olig2 mRNA level remained unchanged in the brain of transgenic mice. Our findings suggest that Tf could influence oligodendrocyte progenitor differentiation in the CNS.

Oligodendrocyte excitotoxicity determined by local glutamate accumulation and mitochondrial function

Developing oligodendrocytes (OL precursors, pre-OLs) express alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype glutamate receptors (AMPARs) and are highly vulnerable to hypoxic-ischemic or oxygen-glucose deprivation (OGD)-induced excitotoxic injury, yet the mechanisms of injury remain unclear. Here we investigated the role of glutamate accumulation and mitochondrial function in OGD-induced pre-OL toxicity in vitro. Bulk glutamate concentration in the culture medium did not increase during OGD and OGD-conditioned medium did not transfer toxicity to naïve cells. Facilitation of glutamate diffusion by constant agitation of the culture reduced, while inhibition of glutamate diffusion by increasing medium viscosity with dextran enhanced, OGD-induced pre-OL injury. Depletion of extracellular glutamate by the glutamate scavenging system, glutamate-pyruvate transaminase plus pyruvate, attenuated pre-OL injury during OGD. Together these data suggest that local glutamate accumulation is critical for OGD toxicity. Interestingly, under normoxic conditions, addition of glutamate to pre-OLs did not cause receptor-mediated toxicity, but the toxicity could be unmasked by mitochondrial impairment with mitochondrial toxins. Furthermore, OGD caused mitochondrial potential collapse that was independent of AMPAR activation, and OGD toxicity was enhanced by mitochondrial toxins. These data demonstrate that pre-OL excitotoxicity is exacerbated by mitochondrial dysfunction during OGD. Overall, our results indicate that OGD-induced pre-OL injury is a novel form of excitotoxicity caused by the combination of local glutamate accumulation that occurs without an increase in bulk glutamate concentration and mitochondrial dysfunction. Therapeutic strategies targeting local glutamate concentration and mitochondrial injury during hypoxia-ischemia may be relevant to human disorders associated with pre-OL excitotoxicity.

Role of glutamate receptors in periventricular leukomalacia

Periventricular leukomalacia is a form of white-matter injury that occurs in the setting of either primary or secondary hypoxia-ischemia in the premature infant. Hypoxia-ischemia induces increases in cerebral extracellular glutamate levels, thereby activating glutamate receptors on a variety of cell types within the white matter. This review examines the evidence of a role for glutamate receptors in white-matter injury and periventricular leukomalacia. Multiple glutamate receptor subtypes exist, and these appear to play differential roles depending on cell type and time after injury. Glutamate receptors are developmentally regulated on neurons and glia, and certain subtypes are transiently overexpressed in developing rodent brain and are expressed on immature oligodendrocytes in human white matter in the premature period. Pharmacologic agents acting on glutamate receptors might represent age-specific therapeutic strategies for the treatment of periventricular leukomalacia.

Myelination and motor coordination are increased in transferrin transgenic mice

Myelin deficiency in the central nervous system (CNS) can cause severe disabling conditions. Most of the transgenic mice models overexpressing myelin components have limitations for investigators of myelin deficiency and myelin therapy as they severely alter CNS architecture. It has been postulated that transferrin (Tf) is involved in oligodendrocyte (OL) maturation and myelinogenesis. Because Tf is not an intrinsic myelin constituent, we decided to investigate if its overexpression could have an impact on the myelination process without affecting myelin integrity. We generated transgenic mice containing the complete human Tf gene specifically overexpressed in OLs. This overexpression leads to more than a 30% increase in myelin components, such as galactolipids, phospholipids, and proteins. Electron microscopy showed that myelin is structurally normal in terms of thickness and compaction. Behavior analysis showed that mice do not display significant modifications in their locomotion and cognitive and emotional abilities. Furthermore, in one of the genetic background, animals presented a significant increase in motor coordination. We did not find any modification in OL number during early postnatal development, suggesting that Tf does not act on OL proliferation. In addition, the levels of iron and ferritin remained unchanged in the brain of transgenic mice compared to control mice. Our findings indicate that, besides its known iron transport function, Tf is able to influence myelination process and induce behavioral improvements in mice.

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.

Oligodendroglia in developmental neurotoxicity

The developing nervous system has been long recognized as a primary target for a variety of toxicants. To date, most efforts to understand the impact of neurotoxic agents on the brain have focused primarily on neurons and to a lesser degree astroglia as cellular targets. The role of oligodendroglia, the myelin-forming cells in the central nervous system (CNS), in developmental neurotoxicity has been emphasized only in recent years. Oligodendrocytes originate from migratory, mitotic progenitors that mature progressively into postmitotic myelinating cells. During differentiation, oligodendroglial lineage cells pass through a series of distinct phenotypic stages that are characterized by different proliferative capacities and migratory abilities, as well as dramatic changes in morphology with sequential expression of unique developmental markers. In recent years, it has become appreciated that oligodendrocyte lineage cells have important functions other than those related to myelin formation and maintenance, including participation in neuronal survival and development, as well as neurotransmission and synaptic function. Substantial knowledge has accumulated on the control of oligodendroglial survival, migration, proliferation, and differentiation, as well as the cellular and molecular events involved in oligodendroglial development and myelin formation. Recently, studies have been initiated to address the role of oligodendrocyte lineage cells in neurotoxic processes. This article examines recent progress in oligodendroglial biology, focuses attention on the characteristic features of the oligodendrocyte developmental lineage as a model system for neurotoxicological studies, and explores the role of oligodendrocyte lineage cells in developmental neurotoxicity. The potential role of oligodendroglia in environmental lead neurotoxicity is presented to exemplify this thesis.

Remyelination-promoting antibodies activate distinct Ca2+ influx pathways in astrocytes and oligodendrocytes: relationship to the mechanism of myelin repair

Our laboratory has identified mouse and human monoclonal antibodies that promote myelin repair in multiple models of demyelinating disease. We have proposed that these antibodies promote remyelination by directly activating central nervous system glia. Intracellular calcium concentration was monitored using a Fura2 ratiometric assay. Repair-promoting antibodies induced distinct Ca2+ signals in both astrocytes and oligodendrocytes. Astrocyte Ca2+ signaling is mediated by a phospholipase C-dependent pathway while oligodendrocyte Ca2+ signaling is mediated via AMPA-sensitive glutamate receptors. An antibody's ability to induce Ca2+ signals is statistically correlated with promotion of myelin repair. These findings support the hypothesis that remyelination-promoting antibodies are acting directly at the surface of glial cells to induce calcium-dependent physiologic reparative function.

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.

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

ST8Sia IV mRNA corresponds with the biosynthesis of alpha2,8sialyl polymers but not oligomers in rat oligodendrocytes

As oligodendrocytes mature they progress through a series of distinct differentiation steps characterized by the expression of specific markers. One such marker, polysialic acid found on the neural cell adhesion molecule (NCAM), is detected by antibodies and is present on progenitor oligodendrocytes, but is not detected to the same extent on mature oligodendrocytes. Two closely related polysialyltransferases, ST8Sia II (STX) and ST8Sia IV (PST) have been cloned previously and shown to synthesize polysialic acid on NCAM and other glycoproteins. To determine whether or not polyalpha2,8sialyltransferases are downregulated during the differentiation of oligodendrocytes, the enzyme activity and expression of ST8Sia II and ST8Sia IV mRNA at two stages of maturation in JS12/1 and JS3/16 oligodendrocytes were examined. Differentiation in both oligodendroglial cell lines was accompanied by more than a 50% reduction in the biosynthesis of polymers of alpha2,8sialic acid when fetuin was used as substrate. Most interestingly, extracts of JS12/1 mature cells synthesized 60% more short oligomers of alpha2,8sialic acid than the progenitor cells, whereas JS3/16 mature cells synthesized barely detectable amounts of the short oligomers. Transcripts for ST8Sia IV mRNA were present in both JS12/1 and JS3/16 and were reduced when the biosynthesis was markedly reduced. In contrast ST8Sia II mRNA was barely detectable in JS3/16 cells and although detectable in JS12/1 cells, there was no clear modulation with maturation. These results were supported by the examination of the brains of rats from embryonic to Day 21 ages. The enzyme activity and mRNA experiments show that polyalpha2,8sialyltransferase itself is down regulated to cause the reduction in sialyl polymers on mature oligodendrocytes. Moreover, ST8Sia IV is responsible for the polysialylation of NCAM in oligodendrocytes.

Classification of demyelinating diseases at the interface between etiology and pathogenesis

The classical demyelinating diseases include the 'autoimmune' inflammatory demyelinating diseases, the inflammatory demyelinating diseases of infectious aetiology, and the demyelinating or dysmyelinating diseases of genetic/hereditary background. In addition, primary demyelination is present in other conditions, such as brain ischaemia and intoxication. Irrespective of the primary aetiology, selective demyelination can be mediated through various pathogenetic pathways: the immune-mediated inflammatory pathway; the metabolic pathway; and the ischaemic/excitotoxic pathway. These pathways are only partly segregated with distinct aetiologies of demyelinating diseases, but they also reflect the way in which the patient copes with the disease-inciting event in relation to their particular genetic background. For future therapeutic strategies it will be important to interfere with the specific pathogenetic pathways of demyelination, which may be common to various demyelinating diseases, but may differ in subgroups of patients who suffer from a particular clinical demyelinating disease entity.