Protein kinase C prevents oligodendrocyte differentiation: modulation of actin cytoskeleton and cognate polarized membrane traffic

Tamoxifen accelerates the repair of demyelinated lesions in the central nervous system

Enhancing central nervous system (CNS) myelin regeneration is recognized as an important strategy to ameliorate the devastating consequences of demyelinating diseases such as multiple sclerosis. Previous findings have indicated that myelin proteins, which accumulate following demyelination, inhibit remyelination by blocking the differentiation of rat oligodendrocyte progenitor cells (OPCs) via modulation of PKCα. We therefore screened drugs for their potential to overcome this differentiation block. From our screening, tamoxifen emerges as a potent inducer of OPC differentiation in vitro. We show that the effects of tamoxifen rely on modulation of the estrogen receptors ERα, ERβ, and GPR30. Furthermore, we demonstrate that administration of tamoxifen to demyelinated rats in vivo accelerates remyelination. Tamoxifen is a well-established drug and is thus a promising candidate for a drug to regenerate myelin, as it will not require extensive safety testing. In addition, Tamoxifen plays an important role in biomedical research as an activator of inducible genetic models. Our results highlight the importance of appropriate controls when using such models.

Antibody-mediated neutralization of myelin-associated EphrinB3 accelerates CNS remyelination

Remyelination in multiple sclerosis (MS) lesions often remains incomplete despite the presence of oligodendrocyte progenitor cells (OPCs). Amongst other factors, successful remyelination depends on the phagocytic clearance of myelin debris. However, the proteins in myelin debris that act as potent and selective inhibitors on OPC differentiation and inhibit CNS remyelination remain unknown. Here, we identify the transmembrane signalling protein EphrinB3 as important mediator of this inhibition, using a protein analytical approach in combination with a primary rodent OPC assay. In the presence of EphrinB3, OPCs fail to differentiate. In a rat model of remyelination, infusion of EphrinB3 inhibits remyelination. In contrast, masking EphrinB3 epitopes using antibodies promotes remyelination. Finally, we identify EphrinB3 in MS lesions and demonstrate that MS lesion extracts inhibit OPC differentiation while antibody-mediated masking of EphrinB3 epitopes promotes it. Our findings suggest that EphrinB3 could be a target for therapies aiming at promoting remyelination in demyelinating disease.

X MARCKS the spot: myristoylated alanine-rich C kinase substrate in neuronal function and disease

Intracellular protein-protein interactions are dynamic events requiring tightly regulated spatial and temporal checkpoints. But how are these spatial and temporal cues integrated to produce highly specific molecular response patterns? A helpful analogy to this process is that of a cellular map, one based on the fleeting localization and activity of various coordinating proteins that direct a wide array of interactions between key molecules. One such protein, myristoylated alanine-rich C-kinase substrate (MARCKS) has recently emerged as an important component of this cellular map, governing a wide variety of protein interactions in every cell type within the brain. In addition to its well-documented interactions with the actin cytoskeleton, MARCKS has been found to interact with a number of other proteins involved in processes ranging from intracellular signaling to process outgrowth. Here, we will explore these diverse interactions and their role in an array of brain-specific functions that have important implications for many neurological conditions.

Transcriptional expression of myelin basic protein in oligodendrocytes depends on functional syntaxin 4: a potential correlation with autocrine signaling

Myelination of axons by oligodendrocytes is essential for saltatory nerve conduction. To form myelin membranes, a coordinated synthesis and subsequent polarized transport of myelin components are necessary. Here, we show that as part of the mechanism to establish membrane polarity, oligodendrocytes exploit a polarized distribution of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) machinery components syntaxins 3 and 4, localizing to the cell body and the myelin membrane, respectively. Our data further reveal that the expression of myelin basic protein (MBP), a myelin-specific protein that is synthesized "on site" after transport of its mRNA, depends on the correct functioning of the SNARE machinery, which is not required for mRNA granule assembly and transport per se. Thus, downregulation and overexpression of syntaxin 4 but not syntaxin 3 in oligodendrocyte progenitor cells but not immature oligodendrocytes impeded MBP mRNA transcription, thereby preventing MBP protein synthesis. The expression and localization of another myelin-specific protein, proteolipid protein (PLP), was unaltered. Strikingly, conditioned medium obtained from developing oligodendrocytes was able to rescue the block of MBP mRNA transcription in syntaxin 4-downregulated cells. These findings indicate that the initiation of the biosynthesis of MBP mRNA relies on a syntaxin 4-dependent mechanism, which likely involves activation of an autocrine signaling pathway.

Myelin formation and remodeling

Myelin is a multilayer wrapping of insulation formed by glial cells around axons that is essential for rapid impulse transmission, but how glial cells accomplish this cellular choreography has long intrigued researchers. In this issue of Cell, Snaidero et al., provide new insights into how myelin forms and is remodeled.

Ethanol alters the expressions of c-Fos and myelin basic protein in differentiating oligodendrocytes

Myelination occurs in the central nervous system of the human fetus, adolescents, and young adults. Ethanol interferes with myelination in part by altering the composition of the myelin sheath. Here we show that ethanol also affected the expression of the transcription factor c-Fos in differentiating oligodendrocytes (OLGs). Central glial-4 OLG progenitors were induced to differentiate in the absence and presence of 100 mM ethanol, and ethanol-caused changes in the levels of c-Fos and myelin basic protein (MBP) were determined by Western blot analysis at selected developmental stages. The relatively high c-Fos level in progenitors did not immediately decrease to a low level at the onset of differentiation but displayed a downregulation at a later developmental stage. Ethanol delayed the developmental c-Fos downregulation maintaining c-Fos at a 45% higher level at 2 days of differentiation (DoD). Ethanol also decreased the rate of the burst of MBP expression that occurred between 1 and 2 DoD, reducing the MBP level by 47% at 2 DoD. The ethanol-caused delays of c-Fos downregulation and MBP upregulation were both blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIM). Likewise, treatment of OLGs with a low 5-nM concentration of the PKC activator by 12-O-tetradecanoylphorbol-13-acetate mimicked the ethanol effects on the expression of both proteins, effects that were also counteracted by BIM. The results indicate that ethanol-caused delays of the stage-specific c-Fos downregulation and the inhibition of MBP expression both occur through a PKC-mediated mechanism. The ethanol-caused delay in c-Fos downregulation may disrupt normal timing for expression of genes involved in OLG differentiation, and the inhibited MBP expression may alter the myelin sheath composition.

On the biogenesis of myelin membranes: sorting, trafficking and cell polarity

In the central nervous system, a multilayered membrane layer known as the myelin sheath enwraps axons, and is required for optimal saltatory signal conductance. The sheath develops from membrane processes that extend from the plasma membrane of oligodendrocytes and displays a unique lipid and protein composition. Myelin biogenesis is carefully regulated, and multiple transport pathways involving a variety of endosomal compartments are involved. Here we briefly summarize how the major myelin proteins proteolipid protein and myelin basic protein reach the sheath, and highlight potential mechanisms involved, including the role of myelin specific lipids and cell polarity related transport pathways.

Functional involvement of protein kinase C-betaII and its substrate, myristoylated alanine-rich C-kinase substrate (MARCKS), in insulin-stimulated glucose transport in L6 rat skeletal muscle cells

AIMS/HYPOTHESIS

Insulin stimulates phosphorylation cascades, including phosphatidylinositol-3-kinase (PI3K), phosphatidylinositol-dependent kinase (PDK1), Akt, and protein kinase C (PKC). Myristoylated alanine-rich C-kinase substrate (MARCKS), a PKCbetaII substrate, could link the effects of insulin to insulin-stimulated glucose transport (ISGT) via phosphorylation of its effector domain since MARCKS has a role in cytoskeletal rearrangements.

METHODS

We examined phosphoPKCbetaII after insulin treatment of L6 myocytes, and cytosolic and membrane phosphoMARCKS, MARCKS and phospholipase D1 in cells pretreated with LY294002 (PI3K inhibitor), CG53353 (PKCbetaII inhibitor) or W13 (calmodulin inhibitor), PI3K, PKCbetaII and calmodulin inhibitors, respectively, before insulin treatment, using western blots. ISGT was examined after cells had been treated with inhibitors, small inhibitory RNA (siRNA) for MARCKS, or transfection with MARCKS mutated at a PKC site. MARCKS, PKCbetaII, GLUT4 and insulin receptor were immunoblotted in subcellular fractions with F-actin antibody immunoprecipitates to demonstrate changes following insulin treatment. GLUT4 membrane insertion was followed after insulin with or without CG53353.

RESULTS

Insulin increased phosphoPKCbetaII(Ser660 and Thr641); LY294002 blocked this, indicating its activation by PI3K. Insulin treatment increased cytosolic phosphoMARCKS, decreased membrane MARCKS and increased membrane phospholipase D1 (PLD1), a protein regulating glucose transporter vesicle fusion resulted. PhosphoMARCKS was attenuated by CG53353 or MARCKS siRNA. MARCKS siRNA blocked ISGT. Association of PKCbetaII and GLUT4 with membrane F-actin was enhanced by insulin, as was that of cytosolic and membrane MARCKS. ISGT was attenuated in myocytes transfected with mutated MARCKS (Ser152Ala), whereas overproduction of wild-type MARCKS enhanced ISGT. CG53353 blocked insertion of GLUT4 into membranes of insulin treated cells.

CONCLUSIONS/INTERPRETATION

The results suggest that PKCbetaII is involved in mediating downstream steps of ISGT through MARCKS phosphorylation and cytoskeletal remodelling.

Myelin-mediated inhibition of oligodendrocyte precursor differentiation can be overcome by pharmacological modulation of Fyn-RhoA and protein kinase C signalling

Failure of oligodendrocyte precursor cell (OPC) differentiation contributes significantly to failed myelin sheath regeneration (remyelination) in chronic demyelinating diseases. Although the reasons for this failure are not completely understood, several lines of evidence point to factors present following demyelination that specifically inhibit differentiation of cells capable of generating remyelinating oligodendrocytes. We have previously demonstrated that myelin debris generated by demyelination inhibits remyelination by inhibiting OPC differentiation and that the inhibitory effects are associated with myelin proteins. In the present study, we narrow down the spectrum of potential protein candidates by proteomic analysis of inhibitory protein fractions prepared by CM and HighQ column chromatography followed by BN/SDS/SDS–PAGE gel separation using Nano-HPLC-ESI-Q-TOF mass spectrometry. We show that the inhibitory effects on OPC differentiation mediated by myelin are regulated by Fyn-RhoA-ROCK signalling as well as by modulation of protein kinase C (PKC) signalling. We demonstrate that pharmacological or siRNA-mediated inhibition of RhoA-ROCK-II and/or PKC signalling can induce OPC differentiation in the presence of myelin. Our results, which provide a mechanistic link between myelin, a mediator of OPC differentiation inhibition associated with demyelinating pathologies and specific signalling pathways amenable to pharmacological manipulation, are therefore of significant potential value for future strategies aimed at enhancing CNS remyelination.

Fibronectin impedes "myelin" sheet-directed flow in oligodendrocytes: a role for a beta 1 integrin-mediated PKC signaling pathway in vesicular trafficking

Differentiation of oligodendrocytes results in the formation of the myelin sheath, a dramatic morphological alteration that accompanies cell specialization. Here, we demonstrate that changes in the extracellular microenvironment may regulate these morphological changes by altering intracellular vesicular trafficking of myelin sheet-directed proteins. The data reveal that fibronectin, in contrast to laminin-2, decreased membrane-directed transport of endogenous NCAM 140 and the model viral protein VSV G, both proteins normally residing in the myelin membrane. The underlying mechanism relies on an integrin-mediated activation of PKC, which causes stable phosphorylation of MARCKS. As a result, dynamic reorganization of the cortical actin cytoskeleton necessary for the targeting of vesicular trafficking to the myelin sheet is precluded, a prerequisite for morphological differentiation. These data are discussed in the context of the demyelinating disease multiple sclerosis, i.e., that leakage of fibronectin across the blood-brain barrier may impede myelination by interference with intracellular myelin sheet-directed membrane transport.

Effects of selenocystine on lead-exposed Chinese hamster ovary (CHO) and PC-12 cells

Lead is a pervasive environmental toxin that affects multiple organ systems, including the nervous, renal, reproductive, and hematological systems. Even though it is probably the most studied toxic metal, some of the symptoms of lead toxicity still cannot be explained by known molecular mechanisms. Therefore, lead-induced oxidative stress has recently started to gain attention. This in vitro study confirms the existence of oxidative stress due to lead exposure. Administration of lead acetate (PbA) to cultures of Chinese hamster ovary cells (CHO) had a concentration-dependent inhibitory effect on colony formation and cell proliferation. This inhibition was eliminated by 5 microM selenocystine (SeCys). In order to evaluate the nature of SeCys's effect, we measured glutathione (GSH), its oxidized form glutathione disulfide (GSSG), malondialdehyde (MDA), catalase, and GSH peroxidase (GPx) activities in lead-exposed CHO cells both in the presence and absence of SeCys. Increases in MDA, catalase, and GPx activities were observed in cultures that received only PbA, but supplementation with SeCys returned these measures to pretreatment levels. The ratio of GSH to GSSG increased in lead-exposed cells incubated in SeCys-enhanced media but declined in cultures treated with PbA only. In order to determine whether SeCys also reverses lead-induced neurotoxicity, a neuronal cell line, PC-12 cells, was used. Lead's inhibition on neurite formation was significantly eliminated by SeCys in PC-12 cells. Our results suggest that SeCys can confer protection against lead-induced toxicity in CHO cells and neurotoxicity in PC-12 cells.

Physical and functional interaction of Fyn tyrosine kinase with a brain-enriched Rho GTPase-activating protein TCGAP

Fyn, a member of the Src family of tyrosine kinases, is implicated in both brain development and adult brain function. In the present study, we identified a Rho GTPase-activating protein (GAP), TCGAP (Tc10/Cdc42 GTPase-activating protein), as a novel Fyn substrate. TCGAP interacted with Fyn and was phosphorylated by Fyn, with Tyr-406 in the GAP domain as a major Fyn-mediated phosphorylation site. Fyn suppressed the GAP activity of wild-type TCGAP but not the Y406F mutant of TCGAP in a phosphorylation-dependent manner, suggesting that Fyn-mediated Tyr-406 phosphorylation negatively regulated the TCGAP activity. In situ hybridization analyses showed that TCGAP mRNA was expressed prominently in both immature and adult mouse brain, with high levels in cortex, corpus striatum, hippocampus, and olfactory bulb. Overexpression of wild-type TCGAP in PC12 cells suppressed nerve growth factor-induced neurite outgrowth, whereas a GAP-defective mutant of TCGAP enhanced the neurite outgrowth. Nerve growth factor enhanced tyrosine phosphorylation of TCGAP through activation of Src family kinases. These results suggest that TCGAP is involved in Fyn-mediated regulation of axon and dendrite outgrowth.

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.

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.

Myosin light chain kinase inhibitors induce retraction of mature oligodendrocyte processes

Mature oligodendrocytes emit numerous myelinating processes. Force generating molecules are required for process outgrowth and spreading. We have analyzed the effect of the myosin II light chain kinase inhibitors ML-7 and ML-9 in cultured oligodendrocytes. Both drugs affect oligodendrocyte cell shape, provoking a retraction of high order processes. Our results suggest that the adhesion of the myelinating processes to the substrate depends on MLC phosphorylation, thus likely implicating myosin IIA.

Hepatocyte growth factor stimulates the proliferation and migration of oligodendrocyte precursor cells

Hepatocyte growth factor (HGF) was initially identified as a potent mitogen for mature hepatocytes and has since been found to affect a variety of cells. Evidence suggests that HGF may also influence the nervous system, in that HGF stimulates the proliferation of myelin-forming Schwann cells and olfactory ensheathing cells. However, it is not known whether HGF influences oligodendrocytes. To address this issue, oligodendrocyte precursors were obtained from neonatal rat cerebra and cultured. Immunostaining and Western blotting revealed expression of both HGF and the HGF receptor (c-Met) by cultured oligodendrocytes. When the ability of HGF to stimulate oligodendrocyte division and migration was examined, we observed that treatment with HGF (10-50 ng/ml) elicited twofold increases in oligodendrocyte precursor proliferation. HGF also enhanced oligodendrocyte precursor migration, with 2.5-fold increases in rates of migration seen after treatment for 8 hr. HGF also influenced inducing the oligodendrocyte cytoskeleton by altering patterns of F-actin and beta-tubulin distribution and enhanced the expression of actin and beta-tubulin. These observations show that a functional HGF/c-Met system is present in oligodendrocytes, which can influence the growth, development, and cytoskeletal organization of oligodendrocytes.

The oligodendrocyte precursor mitogen PDGF stimulates proliferation by activation of alpha(v)beta3 integrins

Central nervous system development requires precise and localized regulation of neural precursor behaviour. Here we show how the interaction between growth factor and integrin signalling pathways provides a mechanism for such precision in oligodendrocyte progenitor (OP) proliferation. While physiological concentrations of platelet-derived growth factor (PDGF) were not in themselves sufficient to promote OP proliferation, they did so on extracellular matrix (ECM) substrates that bind alpha(v)beta3 integrin. Upon PDGF-AA exposure and alpha(v)beta3 engagement, a physical co-association between both receptors was demonstrated, confirming the interaction between these signalling pathways. Furthermore, we found that PDGFalphaR stimulated a protein kinase C-dependent activation of integrin alpha(v)beta3, which in turn induced OP proliferation via a phosphatidylinositol 3-kinase-dependent signalling pathway. These studies establish a mechanism by which OP proliferation is dependent on the availability of both an ECM ligand and a mitogenic growth factor. Growth factor- mediated integrin activation is the critical integrative step in proliferation signalling, and ensures that the response of neural precursor cells to long-range cues can be regulated by their cellular neighbours, allowing precise control of cell behaviour during development.

Protein kinase C activation is required for the lead-induced inhibition of proliferation and differentiation of cultured oligodendroglial progenitor cells

Lead (Pb) is a common neurotoxicant of major public health concern. Previous studies revealed that cultured oligodendrocyte progenitor cells (OPCs) are highly vulnerable to Pb toxicity. The present study examines the effect of Pb on the survival, proliferation and differentiation of OPCs in vitro. Dose-response studies showed that> or = l5-10 microM Pb is cytotoxic to OPCs within 24 h. However, 1 microM of Pb was found to inhibit the proliferation and differentiation of OPCs without affecting cell viability. Pb markedly decreased the proliferative capability of OPCs and inhibited cell-intrinsic lineage progression of OPCs at a late progenitor stage. The Pb-induced decrease of proliferation and differentiation was abolished by inhibition of protein kinase C (PKC) with bisindolylmaleimide I, while the effect of the PKC-activating agent phorbol-12,13-didecanoate was potentiated by Pb. Furthermore, Pb exposure of OPCs caused the translocation of PKC from the cytoplasm to membrane without an increase in total cellular PKC enzymic activity. These results indicate that Pb inhibits the proliferation and differentiation of oligodendrocyte lineage cells in vitro through a mechanism requiring PKC activation.

Sustained phosphorylation of MARCKS in differentiating neurogenic regions during chick embryo development

MARCKS, a substrate for several kinases, has critical functions in morphogenetic processes involved in the development of the nervous system. We previously described the purification of MARCKS from chick embryo brain, using a monoclonal antibody (mAb 3C3), raised against embryonic neural retina. Here we show that mAb 3C3 is an antibody sensitive to phosphorylation state. We used it to explore the appearance and developmental progression of phospho-MARCKS (ph-MARCKS) during initial stages of neurogenesis in retina and spinal cord, and compared its distribution with total MARCKS. Before the onset of neural differentiation, MARCKS protein was already accumulated in neural and non-neural embryonic tissues, while ph-MARCKS immunoreactivity was weak, although ubiquitous too. A sudden increase of ph-MARCKS, paralleling a total MARCKS augmentation, was particularly noticeable in the earliest differentiating neurons in the neural retina. Ganglion cells displayed a high ph-MARCKS signal in the soma, as well as in the growing axon. A short time thereafter, a similar increase of ph-MARCKS was present across the entire width of the neural retina, where the differentiation of other neurons and photoreceptors occurs. The increase of ph-MARCKS in cells took place before the detection of the transcription factor Islet-1/2, an early neuronal differentiation molecular marker, in cells of the same region. Analogous phenomena were observed in cervical regions of the spinal cord, where motor neurons were differentiating. Neurogenic regions in the spinal cord contained higher amounts of ph-MARCKS than the floor plate. Taken together, these results strongly suggest that the appearance and relatively long-lasting presence of ph-MARCKS polypeptides are related to specific signaling pathways active during neurogenesis.

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.

Protein kinase C and mitogen-activated protein kinase signalling in oligodendrocytes

Oligodendrocytes (OL) play a significant physiological role in the central nervous system by creating the myelin sheath that allows for the efficient conduction of nerve impulses. Therefore, it is important to understand which signalling cascades define the proliferation, differentiation, survival, and myelin formation potential of these cells. Currently, much of the knowledge in this field has focused on two sets of protein kinase signalling molecules: Protein kinase C (PKC) and the mitogen-activated protein kinases (MAPKs). The roles of these kinases in OL are complex, and appear to be highly dependent on the developmental stage of the OL. Even so, some broad conclusions can be drawn from the multitude of experiments conducted on the roles of PKC and MAPKs in OL. For instance, PKC appears to have a proliferative effect on immature OL, while at the same time having an inhibitory effect on OL differentiation. In mature OL, the effects of PKC include increased process extension and myelin formation. The extracellular signal-regulated (ERK) members of the MAPK family also appear to increase process extensions in mature OL. On the other hand, the c-Jun N-terminal kinase (JNK) and p38 kinase members of the MAPK family appear to regulate apoptotic events in OL.

PDGF and FGF-2 signaling in oligodendrocyte progenitor cells: regulation of proliferation and differentiation by multiple intracellular signaling pathways

In this paper we address the linking of platelet-derived growth factor (PDGF) and basic fibroblast growth factor (FGF-2) to intracellular signaling molecules in oligodendrocyte progenitors. It is demonstrated that both growth factors activate downstream targets similar to those shown for protein kinase C (PKC) activation. Yet, neither the arrest of terminal oligodendrocyte differentiation nor the proliferation induced by PDGF or FGF-2 can be antagonized by inhibition of PKC. Rather, p42/p44 mitogen-activated protein kinase (MAPK), p38 MAPK, and pp70 S6 kinase were found to be necessary for the mitogenic activity of PDGF and FGF-2. Paradoxically, these kinases were also necessary for the onset of oligodendrocyte differentiation in control cells. In addition, cAMP-dependent kinase A (PKA) activation inhibited the mitogenic response of oligodendrocyte progenitors to FGF-2. Taken together, the molecular mechanism that controls oligodendrocyte lineage progression is operated by at least two signal pathways, which interfere either with proliferation and/or differentiation of oligodendrocyte progenitors.