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

Understanding Abnormal c-JNK/p38MAPK Signaling Overactivation Involved in the Progression of Multiple Sclerosis: Possible Therapeutic Targets and Impact on Neurodegenerative Diseases

Demyelination, immune dysregulation, and neuroinflammation are the most common triggers of motor neuron disorders such as multiple sclerosis (MS). MS is a chronic demyelinating neurodegenerative disease of the central nervous system caused by abnormal immune activation, which causes myelin sheath damage. Cell signal transduction pathways are required for a variety of physiological and pathological processes in the brain. When these signaling systems become overactive, they can lead to disease progression. In various physiological conditions, abnormal mitogen-activated protein kinase (MAPK) activation is associated with several physiological dysfunctions that cause neurodegeneration. Previous research indicates that c-JNK and p38MAPK signaling play critical roles in neuronal growth and differentiation. c-JNK/p38MAPK is a member of the MAPK family, which regulates metabolic pathways, cell proliferation, differentiation, and apoptosis that control certain neurological activities. During brain injuries, c-JNK/p38MAPK also affects neuronal elastic properties, nerve growth, and cognitive processing. This review systematically linked abnormal c-JNK/p38MAPK signaling activation to multiple neuropathological pathways in MS and related neurological dysfunctions. MS progression is linked to genetic defects, oligodendrocyte destruction, glial overactivation, and immune dysregulation. We concluded that inhibiting both the c-JNK/p38MAPK signaling pathways can promote neuroprotection and neurotrophic effects against the clinical-pathological presentation of MS and influence other neurological disorders. As a result, the potential benefits of c-JNK/p38MAPK downregulation for the development of disease-modifying treatment interventions in the future could include MS prevention and related neurocomplications.

Vitamins D3 and D2 have marked but different global effects on gene expression in a rat oligodendrocyte precursor cell line

Background

Vitamin D deficiency increases the risk of developing multiple sclerosis (MS) but it is unclear whether vitamin D supplementation improves the clinical course of MS, and there is uncertainty about the dose and form of vitamin D (D2 or D3) to be used. The mechanisms underlying the effects of vitamin D in MS are not clear. Vitamin D3 increases the rate of differentiation of primary oligodendrocyte precursor cells (OPCs), suggesting that it might help remyelination in addition to modulating the immune response. Here we analyzed the transcriptome of differentiating rat CG4 OPCs treated with vitamin D2 or with vitamin D3 at 24 h and 72 h following onset of differentiation.

Methods

Gene expression in differentiating CG4 cells in response to vitamin D2 or D3 was quantified using Agilent DNA microarrays (n = 4 replicates), and the transcriptome data were processed and analysed using the R software environment. Differential expression between the experimental conditions was determined using LIMMA, applying the Benjamini and Hochberg multiple testing correction to p-values, and significant genes were grouped into co-expression clusters by hierarchical clustering. The functional significance of gene groups was explored by pathway enrichment analysis using the clusterProfiler package.

Results

Differentiation alone changed the expression of about 10% of the genes at 72 h compared to 24 h. Vitamin D2 and D3 exerted different effects on gene expression, with D3 influencing 1272 genes and D2 574 at 24 h. The expression of the vast majority of these genes was either not changed in differentiating cells not exposed to vitamin D or followed the same trajectory as the latter. D3-repressed genes were enriched for Gene Ontology (GO) categories including transcription factors and the Notch pathway, while D3-induced genes were enriched for the Ras pathway.

Conclusions

This study shows that vitamin D3, compared with D2, changes the expression of a larger number of genes in OLs. Identification of genes affected by D3 in OLs should help to identify mechanisms mediating its action in MS.

Erythropoietin Increases Myelination in Oligodendrocytes: Gene Expression Profiling Reveals Early Induction of Genes Involved in Lipid Transport and Metabolism

Several studies have shown that erythropoietin (EPO) has neuroprotective or neuroreparative actions on diseases of the nervous system and that improves oligodendrocyte (OL) differentiation and myelination in vivo and in vitro. This study aims at investigating the early molecular mechanisms for the pro-myelinating action of EPO at the gene expression level. For this purpose, we used a differentiating OL precursor cell line, rat central glia-4 cells. Cells were differentiated or not, and then treated with EPO for 1 or 20 h. RNA was extracted and changes in the gene expression profile were assessed using microarray analysis. Experiments were performed in biological replicates of n = 4. Differentiation alone changed the expression of 11% of transcripts (2,663 out of 24,272), representing 2,436 genes, half of which were upregulated and half downregulated. At 20 h of treatment, EPO significantly affected the expression of 99 genes that were already regulated by differentiation and of 150 genes that were not influenced by differentiation alone. Analysis of the transcripts most upregulated by EPO identified several genes involved in lipid transport (e.g., Cd36) and lipid metabolism (Ppargc1a/Pgc1alpha, Lpin1, Pnlip, Lpin2, Ppard, Plin2) along with Igf1 and Igf2, growth factors known for their pro-myelinating action. All these genes were only induced by EPO and not by differentiation alone, except for Pnlip which was highly induced by differentiation and augmented by EPO. Results were validated by quantitative PCR. These findings suggest that EPO might increase remyelination by inducing insulin-like growth factors and increasing lipid metabolism.

Myelin management by the 18.5-kDa and 21.5-kDa classic myelin basic protein isoforms

The classic myelin basic protein (MBP) splice isoforms range in nominal molecular mass from 14 to 21.5 kDa, and arise from the gene in the oligodendrocyte lineage (Golli) in maturing oligodendrocytes. The 18.5-kDa isoform that predominates in adult myelin adheres the cytosolic surfaces of oligodendrocyte membranes together, and forms a two-dimensional molecular sieve restricting protein diffusion into compact myelin. However, this protein has additional roles including cytoskeletal assembly and membrane extension, binding to SH3-domains, participation in Fyn-mediated signaling pathways, sequestration of phosphoinositides, and maintenance of calcium homeostasis. Of the diverse post-translational modifications of this isoform, phosphorylation is the most dynamic, and modulates 18.5-kDa MBP's protein-membrane and protein-protein interactions, indicative of a rich repertoire of functions. In developing and mature myelin, phosphorylation can result in microdomain or even nuclear targeting of the protein, supporting the conclusion that 18.5-kDa MBP has significant roles beyond membrane adhesion. The full-length, early-developmental 21.5-kDa splice isoform is predominantly karyophilic due to a non-traditional P-Y nuclear localization signal, with effects such as promotion of oligodendrocyte proliferation. We discuss in vitro and recent in vivo evidence for multifunctionality of these classic basic proteins of myelin, and argue for a systematic evaluation of the temporal and spatial distributions of these protein isoforms, and their modified variants, during oligodendrocyte differentiation.

Nucleus-localized 21.5-kDa myelin basic protein promotes oligodendrocyte proliferation and enhances neurite outgrowth in coculture, unlike the plasma membrane-associated 18.5-kDa isoform

The classic myelin basic protein (MBP) family of central nervous system (CNS) myelin arises from transcription start site 3 of the Golli (gene of oligodendrocyte lineage) complex and comprises splice isoforms ranging in nominal molecular mass from 14 kDa to (full-length) 21.5 kDa. We have determined here a number of distinct functional differences between the major 18.5-kDa and minor 21.5-kDa isoforms of classic MBP with respect to oligodendrocyte (OLG) proliferation. We have found that, in contrast to 18.5-kDa MBP, 21.5-kDa MBP increases proliferation of early developmental immortalized N19-OLGs by elevating the levels of phosphorylated ERK1/2 and Akt1 kinases and of ribosomal protein S6. Coculture of N2a neuronal cells with N19-OLGs transfected with the 21.5-kDa isoform (or conditioned medium from), but not the 18.5-kDa isoform, caused the N2a cells to have increased neurite outgrowth and process branching complexity. These roles were dependent on subcellular localization of 21.5-kDa MBP to the nucleus and on the exon II-encoded segment, suggesting that the nuclear localization of early minor isoforms of MBP may play a crucial role in regulating and/or initiating myelin and neuronal development in the mammalian CNS.

Olanzapine stimulates proliferation but inhibits differentiation in rat oligodendrocyte precursor cell cultures

In the developing brain, oligodendrocyte progenitor cells (OPCs) proliferate, migrate, and differentiate into mature oligodendrocytes (OLs) capable of myelinating axons. Recently, OPCs have been identified as an abundant and widespread population in the adult as well as in the developing animal. Current research indicates that these OPCs in the adult brain can proliferate and differentiate into myelinating OLs, albeit with different potentialities from those in developing animals. Multiple lines of evidence, from neuroimaging, postmortem, and genetic association studies, have implicated OL and myelin dysfunction in the pathogenesis of schizophrenia. If altered OL function is involved in pathogenesis, OPCs may thus respond to antipsychotic drugs during the recovery process. In the present study, we used primary OPC cultures from optic nerve of newborn Wistar rat pups to investigate the direct effects of haloperidol (HPD; a typical antipsychotic) and olanzapine (OLZ; an atypical antipsychotic) on the proliferation and differentiation of OPCs. Our results showed that 1) OLZ treatment significantly increased the number of viable OPCs when compared to HPD treatment at relatively high concentrations, 2) OLZ treatment suppressed the expression of myelin basic protein (MBP), and to a greater extent than HPD treatment, and 3) these pharmacological effects may be mediated via the ERK signaling pathway. Our findings suggest a glial mechanism for the antipsychotic action of OLZ, and a role for oligodendrocyte-lineage cells in the pathogenesis and treatment of schizophrenia.

Mechanisms of regulation of oligodendrocyte development by p38 mitogen-activated protein kinase

Many extracellular and intrinsic factors regulate oligodendrocyte development, but their signaling pathways remain poorly understood. Although the p38 mitogen-activated protein kinase (MAPK)-dependent pathway is implicated in oligodendrocyte progenitor cell (OPC) lineage progression, its molecular targets involved in myelinogenesis are mostly unidentified. We have analyzed mechanisms by which p38MAPK regulates oligodendrocyte development and demonstrate that p38MAPK inhibition prevents OPC lineage progression and inhibits MBP (myelin basic protein) promoter activity and Sox10 function. In white-matter tissue, differential levels of MAPK phosphorylation are observed in oligodendrocyte lineage cells. Phosphorylated p38MAPK was found in CC1- and CNP-expressing differentiated oligodendrocytes of the adult brain and was temporally associated with a decline in the levels of phosphorylated extracellular signal-regulated kinase (ERK) in cells of this lineage. PDGF stimulates the phosphorylation of ERK, p38MAPK, and c-Jun N-terminal kinase (JNK), and p38MAPK inhibition was associated with increased ERK, JNK, and c-Jun phosphorylation. In the presence of PDGF, simultaneous inhibition of p38MAPK and either MAPK kinase (MEK) or JNK significantly alleviates the repression of myelin gene expression and lineage progression induced by p38MAPK inhibition alone. Dominant-negative c-Jun reverses the inhibition of myelin promoter activity by active MEK1 or dominant-negative p38MAPKalpha mutants, and phosphorylated c-Jun was detected at the MBP promoter after p38MAPK inhibition, indicating c-Jun as a negative mediator of p38MAPK action. Our findings indicate that p38MAPK activity in the brain supports myelin gene expression through distinct mechanisms via positive and negative regulatory targets. We show that oligodendrocyte differentiation involves p38-mediated Sox10 regulation and cross talk with parallel ERK and JNK pathways to repress c-Jun activity.

Inhibiting poly(ADP-ribose) polymerase: a potential therapy against oligodendrocyte death

Oligodendrocyte loss and demyelination are major pathological hallmarks of multiple sclerosis. In pattern III lesions, inflammation is minor in the early stages, and oligodendrocyte apoptosis prevails, which appears to be mediated at least in part through mitochondrial injury. Here, we demonstrate poly(ADP-ribose) polymerase activation and apoptosis inducing factor nuclear translocation within apoptotic oligodendrocytes in such multiple sclerosis lesions. The same morphological and molecular pathology was observed in an experimental model of primary demyelination, induced by the mitochondrial toxin cuprizone. Inhibition of poly(ADP-ribose) polymerase in this model attenuated oligodendrocyte depletion and decreased demyelination. Poly(ADP-ribose) polymerase inhibition suppressed c-Jun N-terminal kinase and p38 mitogen-activated protein kinase phosphorylation, increased the activation of the cytoprotective phosphatidylinositol-3 kinase-Akt pathway and prevented caspase-independent apoptosis inducing factor-mediated apoptosis. Our data indicate that poly(ADP-ribose) polymerase activation plays a crucial role in the pathogenesis of pattern III multiple sclerosis lesions. Since poly(ADP-ribose) polymerase inhibition was also effective in the inflammatory model of multiple sclerosis, it may target all subtypes of multiple sclerosis, either by preventing oligodendrocyte death or attenuating inflammation.

In vitro characterization of a murine oligodendrocyte precursor cell line (BO-1) following spontaneous immortalization

The understanding of oligodendrocyte differentiation is crucial for designing therapies of demyelinating diseases. Oligodendrocyte precursor cells are of particular interest in this context, because of their remyelinating potential. Permanent cell lines, which are a versatile tool for studying oligodendrocyte physiology, have been so far mainly established from the rat CNS. In the present study, we describe a novel murine oligodendrocyte precursor cell line (BO-1) established by spontaneous immortalization using light microscopy, immunocytochemical phenotyping and genetic analysis. BO-1 cells displayed a bi- to multipolar morphology and expressed early oligodendrocytic lineage markers, such as A2B5 and NG-2. Expression of pre-oligodendrocyte (O4, CNPase) and mature oligodendrocyte markers (e.g. myelin basic protein) was found in about 30% and 1.5% of the cells, respectively. Addition of serum, known to promote type-2 astrocyte differentiation, significantly increased the number of GFAP-positive cells, while thyroid hormones, (T3/T4) known to foster oligodendrocyte differentiation, did not substantially alter the antigenic and gene expression of myelin markers. This deficiency might be related to the high intrinsic proliferation rate of BO-1 cells that was unaltered upon removal of mitogenic factors. Expression of O4 and CNPase in BO-1 cells could be significantly increased by co-culture with primary astrocytes suggesting that the differentiating potential of BO-1 cells was influenced by environmental factors and may have to be fully explored in future studies. In summary, the novel murine BO-1 cell line shares several characteristics with oligodendrocyte precursor cells but displays a restricted differentiation into mature oligodendrocytes.

The effect of CXCL1 on human fetal oligodendrocyte progenitor cells

Chemokine CXCL1 is abundantly present in proliferative zones during brain development and in regions of remyelination, suggesting that it influences development of oligodendrocyte progenitors (OPC) in these regions. We studied in vitro the effects and possible mechanisms by which CXCL1 acts on human fetal OPC. In organotypic slice cultures of human fetal cortical ventricular/subventricular (VZ/SVZ) zones, blocking of CXCL1 signaling reduced significantly the proliferation of OPC. Moreover, exogenously added CXCL1 induced increase of OPC proliferation. Treatments of purified OPC cultures and cell depletion experiments demonstrated that this effect of CXCL1 was mainly indirect, mediated through astrocytes. We identified that CXCL1 acted through the extracellular signal regulated kinase (ERK1/2) pathway, activated primarily in astrocytes. In vitro, astrocytes stimulated with CXCL1 released several cytokines, but only the release of interleukin-6 (IL-6) was completely blocked by inhibition of ERK1/2 pathway. When released IL-6 was neutralized in slices, a decrease in OPC proliferation was demonstrated, while addition of IL-6 was able to return OPC proliferation in astrocyte-depleted slices to the control level. These results suggest that in the human fetal brain CXCL1 promotes proliferation of early OPC, acting in part through an ERK1/2-dependent pathway and release of IL-6 from astrocytes.

JC virus agnoprotein inhibits in vitro differentiation of oligodendrocytes and promotes apoptosis

Productive infection of oligodendrocytes, which are responsible for the formation of myelin sheath in the central nervous system, with the human neurotropic virus JC virus (JCV) causes the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). In addition to encoding T antigen and the capsid proteins, which are produced at the early and late phases of the infection cycle, respectively, JCV encodes a small regulatory protein named agnoprotein that is important for successful completion of the virus life cycle. Here we used bipotential CG-4 cells to examine the impact of agnoprotein on oligodendrocyte differentiation and survival in the absence of JCV lytic infection. We demonstrate that the expression of agnoprotein delayed the formation of complex outgrowth networks of the cells during oligodendrocyte differentiation. These alterations were accompanied by high levels of DNA damage, induction of proapoptotic proteins, and suppression of prosurvival signaling. Accordingly, apoptosis was significantly increased upon the induction of CG-4 cells toward differentiation in cells expressing agnoprotein. These observations provide the first evidence for the possible involvement of agnoprotein, independent from its role in viral replication, in a series of biological events that may contribute to the pathological features seen in PML lesions.

Inhibition of p38 mitogen-activated protein kinase-induced apoptosis in cultured mature oligodendrocytes using SB202190 and SB203580

p38 Mitogen-activated protein kinase (p38 MAPK) is expressed in the oligodendrocyte lineage, and its activity has been implicated in the proliferation and transition of early progenitors into late progenitors. Although p38 MAPK expression has been found in the myelin sheath, however, its role in mature oligodendrocytes remains unknown. In the present study, in order to address the role of p38 MAPK in mature oligodendrocytes, selective inhibitors of p38 MAPK, SB202190, and SB203580 were added to primary cultures of mature oligodendrocytes. After 24h of exposure to the inhibitors, the appearance, and number of A2B5-positive progenitors were unchanged. However, the 2',3'-cyclic nucleotide-3'-phosphohydrolase-positive mature oligodendrocytes disappeared, and the numbers of living cells decreased in comparison to the control cells treated with SB202474, a negative analog of SB203580. Increases in the number of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive nuclei and in the activity of caspase-3/7 were detected 16 h after exposure to the inhibitors, thus causing the mature oligodendrocytes to die due to apoptosis. Similar results were obtained using a differentiated rat oligodendrocyte precursor cell (OPC) line, central glia-4 (CG-4). These findings indicate that p38 MAPK is vital for mature oligodendrocyte survival.

Role of ERK 1/2 signaling in neuronal differentiation of cultured embryonic stem cells

Embryonic stem (ES) cells represent an ideal source for cell engraftment in the damaged central nervous system (CNS). Understanding key signals that control ES cell differentiation may improve cell type-specific differentiation that is suitable for transplantation therapy. We tested the hypothesis that extracellular signal-regulated kinase (ERK) 1/2 phosphorylation is an early signaling event required for the neuronal differentiation of ES cells. Cultured mouse ES cells were treated with an all-trans-retinoic-acid (RA) protocol to generate neurally induced progenitor cells. Western blot analysis showed a dramatic increase in ERK 1/2 phosphorylation (p-ERK 1/2) 1-5 days after RA induction, which was attenuated in the presence of the p-ERK 1/2-specific inhibitor UO126. Phospho-ERK 1/2 inhibition significantly reduced the number of NeuN-positive cells and the expression of associated cytoskeletal proteins. In differentiating ES cells, there was increased nuclear translocation of STAT3 and decreased protein expression levels of GDNF, BDNF and NGF. STAT3 translocation was attenuated by UO126. Finally, caspase-3 activation was observed in the presence of UO126, suggesting that the ERK pathway also contributes to the survival of differentiating ES cells. These data indicate that ERK 1/2 phosphorylation is a key event required for early neuronal differentiation and survival of ES cells.

Partitioning of myelin basic protein into membrane microdomains in a spontaneously demyelinating mouse model for multiple sclerosisThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

We have characterized the lipid rafts in myelin from a spontaneously demyelinating mouse line (ND4), and from control mice (CD1 background), as a function of age and severity of disease. Myelin was isolated from the brains of CD1 and ND4 mice at various ages, and cold lysed with 1.5% CHAPS (3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulphonate). The lysate was separated by low-speed centrifugation into supernatant and pellet fractions, which were characterized by Western blotting for myelin basic protein (MBP) isoforms and their post-translationally modified variants. We found that, with maturation and with disease progression, there was a specific redistribution of the 14–21.5 kDa MBP isoforms (classic exon-II-containing vs exon-II-lacking) and phosphorylated forms into the supernatant and pellet. Further fractionation of the supernatant to yield detergent-resistant membranes (DRMs), representing coalesced lipid rafts, showed these to be highly enriched in exon-II-lacking MBP isoforms, and deficient in methylated MBP variants, in mice of both genotypes. The DRMs from the ND4 mice appeared to be enriched in MBP phosphorylated by MAP kinase at Thr95 (murine 18.5 kDa numbering). These studies indicate that different splice isoforms and post-translationally modified charge variants of MBP are targeted to different microdomains in the myelin membrane, implying multifunctionality of this protein family in myelin maintenance.

Developmental regulation of activated ERK expression in the spinal cord and dorsal root ganglion of the chick embryo

Mitogen-activated protein kinases (MAPKs) are involved in the intracellular pathways that respond to various extracellular signals. Extracellular signal-regulated kinase (ERK) is a member of MAPKs and has various functions in neural development. However, the in vivo distribution of the activated form of ERK (p-ERK) in the developing nervous system is not well understood. Here, we investigated the expression of p-ERK in the spinal cord and dorsal root ganglion (DRG) of chick embryos. In the spinal cord, p-ERK-positive cells appeared in the ventral ventricular zone on embryonic day 4 (E4). From E6 onward, they appeared in the gray matter and in the white matter, suggesting migration from the ventricular zone. A double labeling method revealed that these p-ERK-positive cells included oligodendrocyte precursors. In the dorsal horn, p-ERK-positive small cells appeared on E6. Subsequently, the positive cells in the dorsal horn increased transiently in number and then decreased markedly by E10. Motoneurons also expressed p-ERK transiently on E7. In the DRG, weak p-ERK immunoreaction appeared in the ventrolateral region on E5. From E6, the immunoreactivity became stronger and by E9 intense p-ERK-positive cells were observed throughout the DRG. These data provide a neuroanatomical framework to begin to examine the in vivo role of ERK in neural development.

NF-kappaB prevents TNF-alpha-induced apoptosis in an oligodendrocyte cell line

Nuclear factor kappa beta (NF-kappaB) inhibits apoptosis in sensory, hippocampal, and striatal neurons of the central nervous system. Although several apoptotic stimuli have been shown to activate NF-kappaB in oligodendrocytes, the function of NF-kappaB in this cell type remains unknown. In this study, we introduced plasmids expressing either the p50- or p65-subunit of human NF-kappaB into Central Glia-4 (CG-4)--a rat oligodendrocyte precursor cell line-and determined the influence of NF-kappaB function on tumor necrosis factor-alpha (TNF-alpha)-induced apoptosis. Expression of NF-kappaB markedly prevented CG-4 apoptosis, with p50 being more effective than p65. This anti-apoptotic activity was repressed by IkappaB-alpha, an inhibitor of NF-kappaB. These results imply that NF-kappaB acts as a potent inhibitor of TNF-induced apoptosis in oligodendrocytes.

Lysophosphatidic acid induces process retraction in CG-4 line oligodendrocytes and oligodendrocyte precursor cells but not in differentiated oligodendrocytes

Lysophosphatidic acid is a growth factor-like signalling phospholipid. We demonstrate here that lysophosphatidic acid induces process retraction in central glia-4 cells and oligodendrocyte precursors. This lysophosphatidic acid effect is rapid and concentration-dependent and results in cell rounding. It is inhibited by pre-treatment of cells with C3 exoenzyme, a specific inhibitor of Rho, or with Y-27632, a specific inhibitor of ROCK, a downstream kinase of Rho. Processes of differentiated central glia-4 oligodendrocytes were insensitive to lysophosphatidic acid treatment but cell bodies became phase dark, indicating cell spreading on the poly-l-lysine substratum. RT-PCR and Western blot analyses indicate that oligodendrocyte precursors and mature oligodendrocytes express mRNA and protein for LPA1, one of several LPA receptors. Thus lysophosphatidic acid may be signalling to Rho and stimulating actomyosin contraction in precursor oligodendrocytes by this family of receptors. The results show that lysophosphatidic acid signalling pathways influence retraction of processes in oligodendrocyte precursors but that this effect changes as oligodendrocytes differentiate.

Microfilament and microtubule organization and dynamics in process extension by central glia-4 oligodendrocytes: evidence for a microtubule organizing center

Microfilaments in freshly adhering CG-4 cells and differentiated CG-4 oligodendrocytes are concentrated at the tips and edges of rapidly forming processes while microtubules are concentrated in new processes and extend from a concentrated spot of alpha-tubulin staining in the cell body to the cell periphery. In motile bipolar CG-4 cells, microfilaments are heavily concentrated at the flattened end of one process and along the rim of processes and the cell body: microtubules are concentrated along main processes and splay out into process tips and the cell body. In differentiated CG-4 oligodendrocytes, microfilaments are concentrated at the many process tips, in filopodia and in fine processes, but are not obvious in main processes where separate bundles of microtubules, which diverge at process branch points, are concentrated. gamma-tubulin, involved in microtubule nucleation, is concentrated at a small discrete area in the cell body, indicative of a microtubule organizing center. Polymerization of both actin and tubulin is required for initial process elaboration. Depolymerization of microtubules, but not of microfilaments, causes complete retraction of bipolar CG-4 cell processes. This process retraction does not occur if microfilaments are depolymerized first, indicating that process extension/retraction in motile bipolar CG-4 cells may occur by a balance of motor protein-driven forces as suggested for growth cone motility. Cytoskeleton organization in CG-4 cells is very similar to that reported for oligodendrocytes. CG-4 cells are thus a useful model for investigating the signals and mechanisms regulating oligodendrocyte process dynamics.

Protein kinase C inhibitors counteract the ethanol effects on myelin basic protein expression in differentiating CG-4 oligodendrocytes

Abnormal formation of myelin appears to be one defect contributing to the development of the neuropathology associated with the fetal alcohol syndrome. Using the CG-4 cell line we previously showed that 25-75 mM EtOH downregulates the expression of myelin basic protein (MBP) in differentiating oligodendrocytes (OLGs) without affecting morphological development (Dev. Brain Res. 128 (2001) 9). Here we showed that a relatively low concentration of 12-phorbol-13-myristate acetate (PMA) mimicked the EtOH-caused inhibition of MBP expression without affecting morphology. The inhibition of MBP expression by 100 mM EtOH or 1 nM PMA was completely counteracted by three inhibitors of protein kinase C (PKC), bisindolylmaleimide I, chelerythrine chloride, and calphostin C, indicating that EtOH downregulated MBP expression by activating PKC. We investigated whether the EtOH activation resulted, in part, from upregulation of the expression of PKC isozymes. Of 11 PKC isozymes examined, CG-4 OLGs expressed nine; PKCs alpha, beta1, beta2; delta, epsilon, eta; lambda, zeta; mu; while PKC isozymes gamma and theta were not detected. Only five PKC isozymes, alpha, beta1, beta2, eta, and mu, displayed developmental changes in expression. However, EtOH did not upregulate the early expression of any PKC isozyme during the first 2 days of differentiation, the developmental stage when it downregulates MBP expression in CG-4 cells. The similar effects of PMA and EtOH indicate that EtOH delays MBP expression by activating at least one phorbol ester-sensitive PKC isozyme in oligodendrocytes without upregulating its expression.

Myelin proteolipid protein, basic protein, the small isoform of myelin-associated glycoprotein, and p42MAPK are associated in the Triton X-100 extract of central nervous system myelin

To further our understanding of the functions of the major myelin proteins, myelin basic protein (MBP) and proteolipid protein (PLP), and other myelin proteins, such as 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and myelin-associated glycoprotein (MAG), bovine brain myelin was extracted with Triton X-100, and protein complexes in the detergent-soluble fraction were isolated by coimmunoprecipitation and sucrose density gradient sedimentation. MBP, PLP, and the small isoform of MAG (S-MAG) were coimmunoprecipitated from the detergent-soluble fraction by anti-PLP, anti-MBP or anti-MAG monoclonal antibodies. Additionally, a 30 kDa phosphoserine-containing protein and two phosphotyrosine-containing proteins (M(r) 30 and 42 kDa) were found in the coimmunoprecipitates. The 42 kDa protein is probably p42MAPK, in that MAPK was shown also to be present in the immunoprecipitated complex. CNP, the small PLP isoform DM20, the large MAG isoform L-MAG, MOG, CD44, MEK, p44MAPK, and actin were not present in the immunoprecipitates, although they were present in the detergent-soluble fraction. Lipid analysis revealed that the PLP-MBP-S-MAG coimmunoprecipitated with some phospholipids and sulfatide but not cholesterol or galactosylceramide. However, the complex had a high density, indicating that the lipid/protein ratio is low, and it was retained on a Sepharose CL6B column, indicating that it is not a large membrane fragment. Given that MAG is localized mainly in the periaxonal region of myelin, where it interacts with axonal ligands, the PLP-MBP-S-MAG complex may come from these regions, where it could participate in dynamic functions in the myelin sheath and myelin-axonal interactions.