Cell cycle arrest induced by ectopic expression of p27 is not sufficient to promote oligodendrocyte differentiation

Regulators of Oligodendrocyte Differentiation

Myelination has evolved as a mechanism to ensure fast and efficient propagation of nerve impulses along axons. Within the central nervous system (CNS), myelination is carried out by highly specialized glial cells, oligodendrocytes. The formation of myelin is a prolonged aspect of CNS development that occurs well into adulthood in humans, continuing throughout life in response to injury or as a component of neuroplasticity. The timing of myelination is tightly tied to the generation of oligodendrocytes through the differentiation of their committed progenitors, oligodendrocyte precursor cells (OPCs), which reside throughout the developing and adult CNS. In this article, we summarize our current understanding of some of the signals and pathways that regulate the differentiation of OPCs, and thus the myelination of CNS axons.

Ion Channel Drugs Suppress Cancer Phenotype in NG108-15 and U87 Cells: Toward Novel Electroceuticals for Glioblastoma

Glioblastoma is a lethal brain cancer that commonly recurs after tumor resection and chemotherapy treatment. Depolarized resting membrane potentials and an acidic intertumoral extracellular pH have been associated with a proliferative state and drug resistance, suggesting that forced hyperpolarization and disruption of proton pumps in the plasma membrane could be a successful strategy for targeting glioblastoma overgrowth. We screened 47 compounds and compound combinations, most of which were ion-modulating, at different concentrations in the NG108-15 rodent neuroblastoma/glioma cell line. A subset of these were tested in the U87 human glioblastoma cell line. A FUCCI cell cycle reporter was stably integrated into both cell lines to monitor proliferation and cell cycle response. Immunocytochemistry, electrophysiology, and a panel of physiological dyes reporting voltage, calcium, and pH were used to characterize responses. The most effective treatments on proliferation in U87 cells were combinations of NS1643 and pantoprazole; retigabine and pantoprazole; and pantoprazole or NS1643 with temozolomide. Marker analysis and physiological dye signatures suggest that exposure to bioelectric drugs significantly reduces proliferation, makes the cells senescent, and promotes differentiation. These results, along with the observed low toxicity in human neurons, show the high efficacy of electroceuticals utilizing combinations of repurposed FDA approved drugs.

The Quaking RNA-binding proteins as regulators of cell differentiation

The RNA-binding protein Quaking (QKI) has emerged as a potent regulator of cellular differentiation in developmental and pathological processes. The QKI gene is itself alternatively spliced to produce three major isoforms, QKI-5, QKI-6, and QKI-7, that possess very distinct functions. Here, we highlight roles of the different QKI isoforms in neuronal, vascular, muscle, and monocyte cell differentiation, and during epithelial-mesenchymal transition in cancer progression. QKI isoforms control cell differentiation through regulating alternative splicing, mRNA stability and translation, with activities in gene transcription now also becoming evident. These diverse functions of the QKI isoforms contribute to their broad influences on RNA metabolism and cellular differentiation. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Development.

Motor Learning and Physical Exercise in Adaptive Myelination and Remyelination

The idea that myelination is driven by both intrinsic and extrinsic cues has gained much traction in recent years. Studies have demonstrated that myelination occurs in an intrinsic manner during early development and continues through adulthood in an activity-dependent manner called adaptive myelination. Motor learning, the gradual acquisition of a specific novel motor skill, promotes adaptive myelination in both the healthy and demyelinated central nervous system (CNS). On the other hand, exercise, a physical activity that involves planned, structured and repetitive bodily movements that expend energy and benefits one's fitness, promotes remyelination in pathology, but it is less clear whether it promotes adaptive myelination in healthy subjects. Studies on these topics have also investigated whether the timing of motor learning or physical exercise is important for successful addition of myelin. Here we review our current understanding of the relationship of motor skill learning and physical exercise on myelination.

Remyelinating Oligodendrocyte Precursor Cell miRNAs from the Sfmbt2 Cluster Promote Cell Cycle Arrest and Differentiation

Oligodendrocyte (OL) loss contributes to the functional deficits underlying diseases with a demyelinating component. Remyelination by oligodendrocyte progenitor cells (OPCs) can restore these deficits. To understand the role that microRNAs (miRNAs) play in remyelination, 2',3'-cyclic-nucleotide 3'-phosphodiesterase-EGFP(+) mice were treated with cuprizone, and OPCs were sorted from the corpus callosum. Microarray analysis revealed that Sfmbt2 family miRNAs decreased during cuprizone treatment. One particular Sfmbt2 miRNA, miR-297c-5p, increased during mouse OPC differentiation in vitro and during callosal development in vivo. When overexpressed in both mouse embryonic fibroblasts and rat OPCs (rOPCs), cell cycle analysis revealed that miR-297c-5p promoted G1/G0 arrest. Additionally, miR-297c-5p transduction increased the number of O1(+) rOPCs during differentiation. Luciferase reporter assays confirmed that miR-297c-5p targets cyclin T2 (CCNT2), the regulatory subunit of positive transcription elongation factor b, a complex that inhibits OL maturation. Furthermore, CCNT2-specific knockdown promoted rOPC differentiation while not affecting cell cycle status. Together, these data support a dual role for miR-297c-5p as both a negative regulator of OPC proliferation and a positive regulator of OL maturation via its interaction with CCNT2.

SIGNIFICANCE STATEMENT

This work describes the role of oligodendrocyte progenitor cell (OPC) microRNAs (miRNAs) during remyelination and development in vivo and differentiation in vitro. This work highlights the importance of miRNAs to OPC biology and describes miR-297c-5p, a novel regulator of OPC function. In addition, we identified CCNT2 as a functional target, thus providing a mechanism by which miR-297c-5p imparts its effects on differentiation. These data are important, given our lack of understanding of OPC miRNA regulatory networks and their potential clinical value. Therefore, efforts to understand the role of miR-297c-5p in pathological conditions and its potential for facilitating repair may provide future therapeutic strategies to treat demyelination.

Use of a small molecule cell cycle inhibitor to control cell growth and improve specific productivity and product quality of recombinant proteins in CHO cell cultures

The continued need to improve therapeutic recombinant protein productivity has led to ongoing assessment of appropriate strategies in the biopharmaceutical industry to establish robust processes with optimized critical variables, that is, viable cell density (VCD) and specific productivity (product per cell, qP). Even though high VCD is a positive factor for titer, uncontrolled proliferation beyond a certain cell mass is also undesirable. To enable efficient process development to achieve consistent and predictable growth arrest while maintaining VCD, as well as improving qP, without negative impacts on product quality from clone to clone, we identified an approach that directly targets the cell cycle G1-checkpoint by selectively inhibiting the function of cyclin dependent kinases (CDK) 4/6 with a small molecule compound. Results from studies on multiple recombinant Chinese hamster ovary (CHO) cell lines demonstrate that the selective inhibitor can mediate a complete and sustained G0/G1 arrest without impacting G2/M phase. Cell proliferation is consistently and rapidly controlled in all recombinant cell lines at one concentration of this inhibitor throughout the production processes with specific productivities increased up to 110 pg/cell/day. Additionally, the product quality attributes of the mAb, with regard to high molecular weight (HMW) and glycan profile, are not negatively impacted. In fact, high mannose is decreased after treatment, which is in contrast to other established growth control methods such as reducing culture temperature. Microarray analysis showed major differences in expression of regulatory genes of the glycosylation and cell cycle signaling pathways between these different growth control methods. Overall, our observations showed that cell cycle arrest by directly targeting CDK4/6 using selective inhibitor compound can be utilized consistently and rapidly to optimize process parameters, such as cell growth, qP, and glycosylation profile in recombinant antibody production cultures.

E2F1 coregulates cell cycle genes and chromatin components during the transition of oligodendrocyte progenitors from proliferation to differentiation

Cell cycle exit is an obligatory step for the differentiation of oligodendrocyte progenitor cells (OPCs) into myelinating cells. A key regulator of the transition from proliferation to quiescence is the E2F/Rb pathway, whose activity is highly regulated in physiological conditions and deregulated in tumors. In this paper we report a lineage-specific decline of nuclear E2F1 during differentiation of rodent OPC into oligodendrocytes (OLs) in developing white matter tracts and in cultured cells. Using chromatin immunoprecipitation (ChIP) and deep-sequencing in mouse and rat OPCs, we identified cell cycle genes (i.e., Cdc2) and chromatin components (i.e., Hmgn1, Hmgn2), including those modulating DNA methylation (i.e., Uhrf1), as E2F1 targets. Binding of E2F1 to chromatin on the gene targets was validated and their expression assessed in developing white matter tracts and cultured OPCs. Increased expression of E2F1 gene targets was also detected in mouse gliomas (that were induced by retroviral transformation of OPCs) compared with normal brain. Together, these data identify E2F1 as a key transcription factor modulating the expression of chromatin components in OPC during the transition from proliferation to differentiation.

Pax6 is required for normal cell-cycle exit and the differentiation kinetics of retinal progenitor cells

The coupling between cell-cycle exit and onset of differentiation is a common feature throughout the developing nervous system, but the mechanisms that link these processes are mostly unknown. Although the transcription factor Pax6 has been implicated in both proliferation and differentiation of multiple regions within the central nervous system (CNS), its contribution to the transition between these successive states remains elusive. To gain insight into the role of Pax6 during the transition from proliferating progenitors to differentiating precursors, we investigated cell-cycle and transcriptomic changes occurring in Pax6 (-) retinal progenitor cells (RPCs). Our analyses revealed a unique cell-cycle phenotype of the Pax6-deficient RPCs, which included a reduced number of cells in the S phase, an increased number of cells exiting the cell cycle, and delayed differentiation kinetics of Pax6 (-) precursors. These alterations were accompanied by coexpression of factors that promote (Ccnd1, Ccnd2, Ccnd3) and inhibit (P27 (kip1) and P27 (kip2) ) the cell cycle. Further characterization of the changes in transcription profile of the Pax6-deficient RPCs revealed abrogated expression of multiple factors which are known to be involved in regulating proliferation of RPCs, including the transcription factors Vsx2, Nr2e1, Plagl1 and Hedgehog signaling. These findings provide novel insight into the molecular mechanism mediating the pleiotropic activity of Pax6 in RPCs. The results further suggest that rather than conveying a linear effect on RPCs, such as promoting their proliferation and inhibiting their differentiation, Pax6 regulates multiple transcriptional networks that function simultaneously, thereby conferring the capacity to proliferate, assume multiple cell fates and execute the differentiation program into retinal lineages.

Events at the transition between cell cycle exit and oligodendrocyte progenitor differentiation: the role of HDAC and YY1

The complexity of the adult brain is the result of an integrated series of developmental events that depends on appropriate timing of differentiation. The importance of transcriptional regulatory networks and epigenetic mechanisms of regulation of gene expression is becoming increasingly evident. Among these mechanisms, previous work has revealed the importance of histone deacetylation in oligodendrocyte differentiation. In this manuscript we define the region of interaction between transcription factor Yin-Yang 1 (YY1) and histone deacetylase 1, and characterize the functional consequences of YY1 overexpression on the differentiation of oligodendrocyte progenitors.

Pelizaeus-Merzbacher disease-associated proteolipid protein 1 inhibits oligodendrocyte precursor cell differentiation via extracellular-signal regulated kinase signaling

Oligodendrocytes (OLs) are myelin-forming glial cells in the central nervous system (CNS) and their dysfunction causes neuropathies such as demyelinating diseases. Proteolipid protein 1 (PLP1) is an oligodendrocyte myelin-rich tetraspan membrane protein and aberration of the plp1 gene is known to be responsible for dysmyelinating Pelizaeus-Merzbacher disease (PMD). Among previously identified gene alternations, multiplication of the plp1 gene causes increased expression of PLP1, resulting in a phenotype with severe dysmyelination in human and also rodent models. Yet little is known about the relationship between increased PLP1 expression and oligodendrocyte precursor cell (OPC) differentiation and the intracellular molecular mechanism. Here we show that expression of PLP1 in OPCs markedly inhibits their differentiation, and that this inhibitory effect is effectively improved by inhibition of extracellular signal-regulated kinase (ERK) activity. Furthermore, in cocultures with dorsal root ganglion (DRG) neurons, ERK inhibition also improves PLP1-induced dysmyelination. Thus, ERK inhibition helps to improve defective OPC differentiation induced by PLP1 expression, suggesting that molecules belonging to ERK signaling cascade may be new PMD therapeutic targets.

Differential requirement of cyclin-dependent kinase 2 for oligodendrocyte progenitor cell proliferation and differentiation

Cyclin-dependent kinases (Cdks) and their cyclin regulatory subunits control cell growth and division. Cdk2-cyclin E complexes, phosphorylating the retinoblastoma protein, drive cells through the G1/S transition into the S phase of the cell cycle. Despite its fundamental role, Cdk2 was found to be indispensable only in specific cell types due to molecular redundancies in its function. Converging studies highlight involvement of Cdk2 and associated cell cycle regulatory proteins in oligodendrocyte progenitor cell proliferation and differentiation. Giving the contribution of this immature cell type to brain plasticity and repair in the adult, this review will explore the requirement of Cdk2 for oligodendrogenesis, oligodendrocyte progenitor cells proliferation and differentiation during physiological and pathological conditions.

Loss of protein-tyrosine phosphatase α (PTPα) increases proliferation and delays maturation of oligodendrocyte progenitor cells

Tightly controlled termination of proliferation determines when oligodendrocyte progenitor cells (OPCs) can initiate differentiation and mature into myelin-forming cells. Protein-tyrosine phosphatase α (PTPα) promotes OPC differentiation, but its role in proliferation is unknown. Here we report that loss of PTPα enhanced in vitro proliferation and survival and decreased cell cycle exit and growth factor dependence of OPCs but not neural stem/progenitor cells. PTPα(-/-) mice have more oligodendrocyte lineage cells in embryonic forebrain and delayed OPC maturation. On the molecular level, PTPα-deficient mouse OPCs and rat CG4 cells have decreased Fyn and increased Ras, Cdc42, Rac1, and Rho activities, and reduced expression of the Cdk inhibitor p27Kip1. Moreover, Fyn was required to suppress Ras and Rho and for p27Kip1 accumulation, and Rho inhibition in PTPα-deficient cells restored expression of p27Kip1. We propose that PTPα-Fyn signaling negatively regulates OPC proliferation by down-regulating Ras and Rho, leading to p27Kip1 accumulation and cell cycle exit. Thus, PTPα acts in OPCs to limit self-renewal and facilitate differentiation.

Oligodendrocyte progenitors reversibly exit the cell cycle and give rise to astrocytes in response to interferon-γ

Oligodendrocyte-type 2 astrocyte progenitor cells (O-2A/OPCs) populate the CNS and generate oligodendrocytes and astrocytes in vitro and in vivo. Understanding how O-2A/OPCs respond to their environment is crucial to understanding how these cells function in the CNS and how to best promote their therapeutic proliferation and differentiation. We show that interferon-γ (IFN-γ) was not toxic to highly purified perinatal or adult rat O-2A/OPCs. IFN-γ treatment led to downregulation of PDGFR-α (platelet-derived growth factor receptor-α) and Ki-67 and decreased self-renewal in clonal populations. IFN-γ also significantly increased the proportion of cells in the G(0)/G(1) phase of the cell cycle, decreased BrdU (5-bromo-2'-deoxyuridine) incorporation, and led to increased expression of the cell cycle inhibitors Rb and p27(kip1). Although p27(kip1) expression was not necessary for IFN-γ-mediated quiescence, its upstream regulator IRF-1 was required. The quiescent state of O-2A/OPCs caused by IFN-γ was reversible as the withdrawal of IFN-γ allowed O-2A/OPCs to appropriately respond to both proliferation and differentiation signals. Differentiation into oligodendrocytes induced by either thyroid hormone or CNTF was also abrogated by IFN-γ. This inhibition was specific to the oligodendrocyte pathway, as O-2A/OPC differentiation into astrocytes was not inhibited. IFN-γ alone also led to the generation of GFAP-positive astrocytes in a subset of O-2A/OPCs. Together, these results demonstrate a reversible inhibitory effect of IFN-γ on O-2A/OPC proliferation with a concomitant generation of astrocytes. We propose that neuroinflammation involving increased IFN-γ can reduce progenitor numbers and inhibit differentiation, which has significant clinical relevance for injury repair, but may also contribute to the generation of astrocytes.

Hes1 functions downstream of growth factors to maintain oligodendrocyte lineage cells in the early progenitor stage

Expansion of the progenitor pool of oligodendrocytes (OLs) is a critical process for obtaining appropriate amounts of mature myelin-forming OLs in the developing and regenerating central nervous system. In vitro, fibroblast growth factor-2 (FGF2), together with platelet-derived growth factor (PDGF), is required to expand oligodendrocyte progenitor cells (OLPs) in an unlimited manner, maintaining them in the early progenitor stage. However, the intracellular mechanisms that prevent OLP maturation remain elusive. In order to investigate these mechanisms, we established a mouse OLP primary culture, which enabled us to undertake biochemical analyses. We found that the suppressive effects on maturation of early OLP to the late O4(+) progenitor by PDGF+FGF2 treatment was abrogated by Mek inhibitor, while transfecting cells with a constitutively active Mek1 construct prevented OLP maturation, suggesting that the Mek-Erk pathway is implicated in the effects of the growth factor treatment. The activation of Mek-Erk pathway promoted proliferation of OLP suggesting that cell cycle progression has suppressive effects to the maturation of OLP. Furthermore, molecular screening using DNA microarrays revealed that Hes1, a negative regulator of bHLH transcription factors, is one of the downstream molecules induced by PDGF+FGF2 treatment. We confirmed that forced activation of Mek-Erk pathway is sufficient to induce Hes1 expression and that Hes1, in turn, exerts suppressive effects on the maturation of OL lineage by itself. Our observations thus indicate that Mek-Erk pathway plays pivotal role in preventing early OLP maturation to late OLPs and the effect is mediated by cell cycle progression as well as Hes1 induction.

Laminin alters fyn regulatory mechanisms and promotes oligodendrocyte development

Mutations in LAMA2, the gene for the extracellular matrix protein laminin-alpha2, cause a severe muscular dystrophy termed congenital muscular dystrophy type-1A (MDC1A). MDC1A patients have accompanying CNS neural dysplasias and white matter abnormalities for which the underlying mechanisms remain unknown. Here, we report that in laminin-deficient mice, oligodendrocyte development was delayed such that oligodendrocyte progenitors accumulated inappropriately in adult brains. Conversely, laminin substrates were found to promote the transition of oligodendrocyte progenitors to newly formed oligodendrocytes. Laminin-enhanced differentiation was Src family kinase-dependent and resulted in the activation of the Src family kinase Fyn. In laminin-deficient brains, however, increased Fyn repression was accompanied by elevated levels of the Src family kinase negative regulatory proteins, Csk (C-terminal Src kinase), and its transmembrane adaptor, Cbp (Csk-binding protein). These findings indicate that laminin deficiencies delay oligodendrocyte maturation by causing dysregulation of signaling pathways critical for oligodendrocyte development, and suggest that a normal role for CNS laminin is to promote the development of oligodendrocyte progenitors into myelin-forming oligodendrocytes via modulation of Fyn regulatory molecules.

Essential function, sophisticated regulation and pathological impact of the selective RNA-binding protein QKI in CNS myelin development

The selective RNA-binding protein QKI play a key role in advancing oligodendrocyte-dependent myelination, which is essential for the function and development of the CNS. The emerging evidence that QKI abnormalities are associated with schizophrenia and may underlie myelin impairment in this devastating disease has greatly increased interest in understanding the function of QKI. Despite the discovery of the biochemical basis for QKI-RNA interaction, a comprehensive model is currently missing regarding how QKI regulates its mRNA ligands to promote normal myelinogenesis and how deficiency of the QKI pathway is involved in the pathogenesis of human diseases that affect CNS myelin. In this review, we will focus on the role of QKI in regulating distinct mRNA targets at critical developmental steps to promote oligodendrocyte differentiation and myelin formation. In addition, we will discuss molecular mechanisms that control QKI expression and activity during normal myelinogenesis as well as the pathological impact of QKI deficiency in dysmyelination mutant animals and in human myelin disorders.

Post-translational modifications of nucleosomal histones in oligodendrocyte lineage cells in development and disease

The role of epigenetics in modulating gene expression in the development of organs and tissues and in disease states is becoming increasingly evident. Epigenetics refers to the several mechanisms modulating inheritable changes in gene expression that are independent of modifications of the primary DNA sequence and include post-translational modifications of nucleosomal histones, changes in DNA methylation, and the role of microRNA. This review focuses on the epigenetic regulation of gene expression in oligodendroglial lineage cells. The biological effects that post-translational modifications of critical residues in the N-terminal tails of nucleosomal histones have on oligodendroglial cells are reviewed, and the implications for disease and repair are critically discussed.

The transcription factor Yin Yang 1 is essential for oligodendrocyte progenitor differentiation

The progression of progenitors to oligodendrocytes requires proliferative arrest and the activation of a transcriptional program of differentiation. While regulation of cell cycle exit has been extensively characterized, the molecular mechanisms responsible for the initiation of differentiation remain ill-defined. Here, we identify the transcription factor Yin Yang 1 (YY1) as a critical regulator of oligodendrocyte progenitor differentiation. Conditional ablation of yy1 in the oligodendrocyte lineage in vivo induces a phenotype characterized by defective myelination, ataxia, and tremor. At the cellular level, lack of yy1 arrests differentiation of oligodendrocyte progenitors after they exit from the cell cycle. At the molecular level, YY1 acts as a lineage-specific repressor of transcriptional inhibitors of myelin gene expression (Tcf4 and Id4), by recruiting histone deacetylase-1 to their promoters during oligodendrocyte differentiation. Thus, we identify YY1 as an essential component of the transcriptional network regulating the transition of oligodendrocyte progenitors from cell cycle exit to differentiation.

A crucial role for p57(Kip2) in the intracellular timer that controls oligodendrocyte differentiation

The intracellular molecular mechanism that controls the timing of oligodendrocyte differentiation remains unknown. Temple and Raff (1986) previously showed that an oligodendrocyte precursor cell (OPC) can divide a maximum of approximately eight times before its daughter cells simultaneously cease proliferating and differentiate into oligodendrocytes. They postulated that over time the level of an intracellular molecule might synchronously change in each daughter cell, ultimately reaching a level that prohibited additional proliferation. Here, we report the discovery of such a molecule, the cyclin-dependent kinase inhibitor p57(Kip2) (Cdkn1c). We show in vitro that all daughters of a clone of OPCs express similar levels of p57(Kip2), that p57(Kip2) levels increase over time in proliferating OPCs, and that p57(Kip2) levels regulate how many times an OPC can divide before differentiating. These findings reveal a novel part of the mechanism by which OPCs measure time and are likely to extend to similar timers in many other precursor cell types.

The selective RNA-binding protein quaking I (QKI) is necessary and sufficient for promoting oligodendroglia differentiation

Quaking I (QKI) is a selective RNA-binding protein essential for myelination of the central nervous system. Three QKI isoforms with distinct C termini and subcellular localization, namely QKI-5, QKI-6, and QKI-7, are expressed in oligodendroglia progenitor cells (OPCs) prior to the initiation of myelin formation and implicated in promoting oligodendrocyte lineage development. However, the functional requirement for each QKI isoform and the mechanisms by which QKI isoforms govern OPC development still remain elusive. We report here that exogenous expression of each QKI isoform is sufficient to enhance differentiation of OPCs with different efficiency, which is abolished by a point mutation that abrogates the RNA binding activity of QKI. Reciprocally, small interfering RNA-mediated QKI knockdown blocks OPC differentiation, which can be partly rescued by QKI-5 and QKI-6 but not by QKI-7, indicating the differential requirement of QKI isoform function in advancing OPC differentiation. Furthermore, we found that abrogation of OPC differentiation, as a result of QKI deficiency, is not due to altered proliferation capacity or cell cycle progression. These results indicate that QKI isoforms are necessary and sufficient for promoting OPC development, which must involve direct influence of QKI on differentiation/maturation of OPCs independent of cell cycle exit, likely via regulating the expression of the target mRNAs of QKI that support OPC differentiation.

The Yin and Yang of cell cycle progression and differentiation in the oligodendroglial lineage

In white matter disorders such as leukodystrophies (LD), periventricular leucomalacia (PVL), or multiple sclerosis (MS), the hypomyelination or the remyelination failure by oligodendrocyte progenitor cells involves errors in the sequence of events that normally occur during development when progenitors proliferate, migrate through the white matter, contact the axon, and differentiate into myelin-forming oligodendrocytes. Multiple mechanisms underlie the eventual progressive deterioration that typifies the natural history of developmental demyelination in LD and PVL and of adult-onset demyelination in MS. Over the past few years, pathophysiological studies have mostly focused on seeking abnormalities that impede oligodendroglial maturation at the level of migration, myelination, and survival. In contrast, there has been a strikingly lower interest for early proliferative and differentiation events that are likely to be equally critical for white matter development and myelin repair. This review highlights the Yin and Yang principles of interactions between intrinsic factors that coordinately regulate progenitor cell division and the onset of differentiation, i.e. the initial steps of oligodendrocyte lineage progression that are obviously crucial in health and diseases.

MEK-ERK signaling is involved in interferon-gamma-induced death of oligodendroglial progenitor cells

Oligodendrocytes are exposed to various cytokines in inflammatory lesions in the central nervous system. In this study, we focused on the direct effects of interferon-gamma (IFNG) on highly purified rat oligodendroglial cultures at different developmental stages. Among the three stages tested, IFNG had direct cytotoxic effects on actively proliferating oligodendrocyte progenitors but much less on immature oligodendrocytes and none on mature oligodendrocytes. This stage-specific susceptibility of progenitors to IFNG-induced cytotoxicity consisted of two components, delay in the G(1)/S transition of the cell cycle and increased cell death at least partly mediated by apoptosis, suggesting that progression of the cell cycle was tightly linked to this toxic mechanism. There was no functional difference in the signal transducers and activators of transcription (STAT) pathways between progenitors and mature oligodendrocytes as determined by induction of IRF1 mRNA in response to IFNG. We found that partial inhibition of the MEK-ERK pathway, one of the mitogen-activated protein kinase phosphorelay modules, by U0126 partially reversed the IFNG-induced cytotoxicity in progenitors. In addition, ERK activity was quickly down-regulated after in vitro differentiation of progenitors to immature oligodendrocytes. Therefore, we concluded that simultaneous activation of the STAT pathway by IFNG and of the ERK pathway by exogenous trophic factors played a role in the stage-specific IFNG-induced cytotoxicity in oligodendroglial progenitors. Our study has implications with respect to the mechanisms of periventricular leukomalacia in infants and of persistent demyelination in multiple sclerosis lesions in adults.

Number of oligodendrocyte progenitors recruited to the lesioned spinal cord is modulated by the levels of the cell cycle regulatory protein p27Kip-1

Remyelination is a critical step for recovery of function after demyelination and defines the ability to generate new myelin. This repair process is dependent on the presence of resident oligodendrocyte progenitors (OLPs) that have been shown to remyelinate axons after demyelination. We have previously shown that the levels of the cell cycle inhibitor p27Kip-1 modulate the number of neonatal cortical OLPs. We now asked whether this cell cycle molecule plays also a role in regulating the number of adult OLP in the spinal cord after demyelination induced by lysolecithin injection. The proliferative response of OLP in the spinal cord of injected wild-type (wt) and p27Kip-1 null mice was evaluated 3 days after lesion. In vivo labeling with bromodeoxyuridine (BrdU) was used to identify cells in S phase. Double immunofluorescence for the OLP marker NG2, and for BrdU was used to count the number of proliferating progenitors. Consistent with a role of p27Kip-1 in regulating the number of adult OLP in the injured spinal cord, a larger number of proliferating OLPs was observed in p27Kip-1null mice compared with wild-type controls. These cells were able to differentiate as assessed by the presence of MBP+ cells in the spinal cord 14 days after injury. We conclude that the cellular levels of the cell cycle inhibitor p27Kip-1 modulate the repair response of OLPs to injury in the adult spinal cord.

Oligodendrocyte maturation is inhibited by bone morphogenetic protein

Mature oligodendrocytes myelinate axons in the CNS. The development of the myelin sheath is dependent on the proper maturation of oligodendrocytes from precursors cells, a spatially restricted process that is regulated by inductive and repressive cues. Several members of the bone morphogenetic protein family (BMP2 and 4) have been implicated as repressors of oligodendrocyte development in vitro by shifting oligodendrocyte precursors into the astrocyte lineage. We now report on a second role of BMPs in oligodendrocyte development, regulation of myelin protein expression in immature oligodendrocytes. Purified immature rodent oligodendrocytes treated with BMP4 maintained galactocerebroside (GalC) expression, whereas the expression of three key myelin proteins, proteolipid protein (PLP), myelin basic protein (MBP), and 2'-3'-cyclic nucleotide 3'-phosphodiesterase (CNP), was severely decreased. Paradoxically, BMP-treated oligodendrocytes show increased process extension and complexity, normally a feature of oligodendrocyte maturation. We also investigated whether BMP4 could inhibit myelin protein expression in an E 12.5 mouse explant culture of cervical spinal cord and hindbrain that maintains the in vivo cellular relationships and architecture. Beads soaked in BMP protein implanted into these explants inhibited the expression of myelin proteins, proteolipid protein, and myelin-associated glycoprotein (MAG), in the local area surrounding the bead. Since these explants also contained precursors cells, expression of galactocerebroside and O4, an oligodendrocyte marker, were also decreased by BMP treatment but to a much lesser degree than the myelin markers. Together, these data indicate that BMPs have multiple roles in oligodendrocyte development. At earlier stages, they affect cell lineage decisions and at later stages, they inhibit cell specialization.

IGF-I and FGF-2 coordinately enhance cyclin D1 and cyclin E-cdk2 association and activity to promote G1 progression in oligodendrocyte progenitor cells

A critical question in developmental neurobiology is how stem and progenitor cells interpret multiple signals to decide whether to proliferate or exit the cell cycle. Insulin-like growth factor (IGF)-I and fibroblast growth factor (FGF)-2 have known functions individually in development of neural stem cells as well as more restricted neuronal and glial progenitor cells. The goal of this study was to elucidate how IGF-I and FGF-2 coordinately regulate the cell cycle machinery in primary oligodendrocyte progenitors (OPs). IGF-I/FGF-2 synergistically increased the numbers of OP cells recruited into S phase. IGF-I enhanced FGF-2 induction of cyclin D1, activation of G(1) cyclin-cyclin-dependent kinase (cdk) complexes, and hyperphosphorylation of retinoblastoma protein (pRb). Moreover, IGF-I was required for G(2)/M progression. In contrast, FGF-2 decreased levels of the cdk inhibitor p27(Kip1) associated with cyclin E-cdk2. These studies provide a mechanistic basis for coordinate regulation of cell cycle progression in progenitor cells by multiple growth factors.

Chemical genetics: adding to the developmental biology toolbox

Recent advances in cell and molecular biology have generated important tools to probe developmental questions. In addition, new genetic model systems such as Danio rerio now make large-scale vertebrate early developmental mutant screens feasible. Nonetheless, some developmental questions remain difficult to study because of the need for finer temporal, spatial, or tuneable control of gene function within a developmental system. New uses for old teratogens as well as novel chemical modulators of development have begun to fill this void.

The Sp1 family of transcription factors is involved in p27(Kip1)-mediated activation of myelin basic protein gene expression

p27(Kip1) levels increase in many cells as they leave the cell cycle and begin to differentiate. The increase in p27(Kip1) levels generally precedes the expression of differentiation-specific genes. Previous studies from our laboratory showed that the overexpression of p27(Kip1) enhances myelin basic protein (MBP) promoter activity. This activation is specific to p27(Kip1). Additionally, inhibition of cyclin-dependent kinase activity alone is not sufficient to increase MBP expression. In this study, we focused on understanding how p27(Kip1) can activate gene transcription by using the MBP gene in oligodendrocytes as a model. We show that the enhancement of MBP promoter activity by p27(Kip1) is mediated by a proximal region of the MBP promoter that contains a conserved GC box binding sequence. This sequence binds transcription factors Sp1 and Sp3. Increased expression of p27(Kip1) increases the level of Sp1 promoter binding to the GC box but does not change the level of Sp3 binding. The binding of Sp1 to this element activates the MBP promoter. p27(Kip1) leads to increased Sp1 binding through a decrease in Sp1 protein turnover. Enhancement of MBP promoter activity by an increase in the level of p27(Kip1) involves a novel mechanism that is mediated through the stabilization and binding of transcription factor Sp1.

Histone deacetylase activity is necessary for oligodendrocyte lineage progression

Gene expression can be modulated by chromatin changes induced by histone acetylation and deacetylation. Acetylation of histone lysine residues by acetyltransferases is associated with transcriptionally active chromatin, whereas the removal of acetyl groups by histone deacetylases (HDACs) correlates with repressed chromatin. Recent evidence has shown that histone deacetylation is responsible for restricting neuronal gene expression, whereas histone acetylation is necessary for astrocytic differentiation We now asked whether histone acetylation or deacetylation was necessary for oligodendrocyte differentiation. Neonatal rat cortical progenitors were kept proliferating and undifferentiated in the presence of mitogens and induced to stop proliferating and differentiate into oligodendrocytes by mitogen removal. Histone deacetylation was observed during the temporal window between exit from the cell cycle and onset of differentiation, which was characterized by acquisition of branched morphology and myelin gene expression. Blocking HDAC activity during this critical window using the inhibitor trichostatin A (TSA) prevented the progression of progenitors into mature oligodendrocytes. TSA-treated progenitors were able to exit from the cell cycle but did not progress to oligodendrocytes. Their development was arrested at the progenitor stage, characterized by simple morphology and lack of myelin gene expression. The effect of TSA on progenitor differentiation was lineage specific, because TSA did not affect the ability of these cells to differentiate into type II astrocytes when cultured in the presence of serum. From these data, we conclude that histone deacetylation is a necessary component of the oligodendrocyte differentiation program.

A PLP splicing abnormality is associated with an unusual presentation of PMD

We report that a deletion of 19 base pairs (bp) in intron 3 of the proteolipid protein (PLP/DM20) gene causes a neurological disease characterized by mild developmental delay, followed by progressive decline of acquired motor and cognitive milestones. The clinical features are associated with mild delay in myelination demonstrated by magnetic resonance imaging studies and with ongoing demyelination and axonal loss demonstrated by magnetic resonance spectroscopy. We demonstrate that the purine-rich 19bp element regulates PLP-specific splice site selection in transient transfections of chimeric constructs into cultured oligodendrocytes. Runs of 4 and 5 Gs centered in the 19bp element are critical for efficient PLP-specific splicing. The intronic element is sequence specific in oligodendrocytes and is not a repressor of PLP-specific splicing in nonglial cells. These data support the conclusion that deletion of the 19bp purine-rich region in PLP intron 3 causes a reduction in PLP message and protein, which affects myelin stability and axonal integrity.

Posttranscriptional regulation of p18 and p27 Cdk inhibitor proteins and the timing of oligodendrocyte differentiation

A cell-intrinsic timer helps control when rodent oligodendrocyte precursor cells (OPCs) exit the cell cycle and terminally differentiate when cultured in platelet-derived growth factor (PDGF) and thyroid hormone (TH). There is evidence that the cyclin-dependent kinase inhibitor (CKI) p27/Kip1 (p27) is a component of this TH-regulated timer, as it increases as OPCs proliferate and is required for the timer to operate accurately. Here, we provide evidence that another CKI, p18/INK (p18), may also be a component of the timer: it increases as OPCs proliferate, and its overexpression in OPCs accelerates the timer, causing the cells to differentiate prematurely. We also show that the overexpression of p27 accelerates the timer and that the increases in both p27 and p18 that occur in proliferating OPCs are controlled posttranscriptionally. By contrast, we show that the overexpression of either p18 or p27 in OPCs proliferating in PDGF and the absence of TH greatly slows the cell cycle but fails to accelerate the spontaneous differentiation that normally occurs independently of TH.

Down-regulation of the retinoblastoma protein (rb) is associated with rat oligodendrocyte differentiation

Terminal differentiation of oligodendrocytes is associated with permanent withdrawal from the cell cycle. We studied the expression of the retinoblastoma protein, expression and activity of G1 cyclins and kinases in oligodendrocyte progenitor cells cultured in vitro. We found that Rb stopped to be expressed concomitantly with the activation of CNPase in oligodendrocytes differentiated with thyroid hormone. In contrast, Rb continued to be expressed at reduced levels in oligodendrocytes that were arrested in G1 by removal of mitogens. Cyclin D1, cdk2, and cdk4 kinase activities were decreased in G1-arrested and differentiated oligodendrocytes. Cyclin E, however, continued to be expressed in G1-arrested oligodendrocytes. Inhibition of differentiation induced by mitogens in oligodendrocytes arrested in G1 by Ad-p27 was accompanied by continued expression of Rb, D1, and E cyclins. After removal of mitogens and addition of thyroid hormone, Rb stopped being expressed and CNPase expression was activated with a temporal course similar to that of oligodendrocytes infected with a control adenovirus. Our results indicate that Rb may play an important function in differentiation of oligodendrocytes in response to external mitogens and differentiation factors.

p27(Kip1) enhances myelin basic protein gene promoter activity

The process of oligodendrocyte differentiation is a complex event that requires cell cycle withdrawal, followed by the activation of a specific transcriptional program responsible for the synthesis of myelin genes. Because growth arrest precedes differentiation, we sought to investigate the role of cell cycle molecules in the activation of myelin gene promoters. We hypothesized that the cell cycle inhibitor p27(Kip1), which is primarily responsible for arresting proliferating oligodendrocyte progenitors, may be involved in the transcriptional regulation of myelin genes. In agreement with this hypothesis, overexpression of p27(Kip1) in the CG4 cell line, but not in 3T3 fibroblasts, enhances the expression of luciferase driven by the myelin basic protein (MBP) promoter. Interestingly, this effect is specific for p27(Kip1); overexpression of other cell cycle inhibitors had no effect. Additionally, this effect is independent of halting the cell cycle; treatment with the cell cycle blocker roscovitine did not affect MBP promoter usage. We conclude that p27(Kip1) contributes to oligodendrocyte differentiation by regulating transcription of the MBP gene.

Inhibition of cyclin E-cyclin-dependent kinase 2 complex formation and activity is associated with cell cycle arrest and withdrawal in oligodendrocyte progenitor cells

Stimulatory and inhibitory signals regulate cell proliferation through the activity of specific enzymes that operate in distinct phases of the cell cycle. We have studied cell cycle progression, arrest, and withdrawal in the oligodendrocyte progenitor (OP) cell model system, focusing on the G(1) phase and G(1)-S transition. Not only were proliferating OPs found to display higher protein levels of cyclin E and D and cyclin-dependent kinases (cdk) 2, 4, and 6 than cells that had permanently withdrawn from the cycle, but the kinase activities of both cyclin D-cdk4/6 and cyclin E-cdk2 were also higher in dividing OPs. This was associated with a decrease in the formation of the cyclin E-cdk2 and cyclin D-cdk4/cyclin D-cdk6 complexes in differentiated oligodendrocytes that had permanently withdrawn from the cell cycle. Reversible cell cycle arrest in G(1) induced by glutamatergic and beta-adrenergic receptor activation or cell depolarization, however, did not modify cyclin E and cdk2 protein expression compared with proliferating OPs. Instead, these agents caused a selective decrease in cdk2 activity and an impairment of cyclin E-cdk2 complex formation. Although cyclin D protein levels were higher than in proliferating cells, cyclin D-associated kinase activity was not modified in G(1)-arrested OPs. Analysis in corpus callosum in vivo showed that cyclin E-cdk2 activity increased between postnatal days 3 and 15 and decreased between postnatal days 15 and 30. Our results indicate that the cyclin E-cdk2 complex is a major regulator of OP cell cycle progression and that the cdks involved in reversible cell cycle arrest are distinct from those implicated in permanent cell cycle withdrawal.

Coordinating cell proliferation and differentiation

Cell proliferation and differentiation are highly coordinated processes during development. Recent studies have revealed that this coordination may result from dual functions residing in the central regulators of proliferation, allowing them to also regulate differentiation. Studies have also shown that some terminally differentiated cells can be made to divide beyond their normal capacity.

Cip/Kip cell-cycle inhibitors: a neuro-oncological perspective

The cell cycle is a precisely controlled cellular program that ensures normal cellular proliferation and development. The cyclin-dependant kinases (CDK) are molecules central to the continued progression through the cell-cycle checkpoints and as such are regulated by various mechanisms including cyclin levels, phosphorylation/dephosphorylation and cyclin-dependant kinase inhibitors (CKI). The CKIs are grouped into two families based on their structure and function, four lnk4 CKIs and three Cip/Kip CKIs. Abnormalities in these proteins can give rise to developmental defects and cancer. In this review, we will discuss the biochemistry and cell biology of the each of the Cip/Kip CKIs, their role in development as evidenced by targeted mutations in mice, and their role as possible tumor suppressor genes.

Expression and role of p27(kip1) in neuronal differentiation of embryonal carcinoma cells

We examined the expression and the regulation of p21(waf1) and p27(kip1) cdk inhibitors in P19 mouse embryonal carcinoma (EC) cells following treatment with all-trans retinoic acid (ATRA) to induce neuronal differentiation. The levels of p27 mRNA and protein increased within 24 h of treatment with ATRA, reaching a plateau 4-5 days later prior to neurite formation. In contrast, levels of p21 expression remained low until after neurites were extensively formed. Induction of muscle differentiation from P19 cells by treatment with dimethyl sulfoxide caused only transient increases in p27 levels. In a mutant P19 cell line, RAC65, treatment with ATRA induced neither p27 accumulation nor neuronal differentiation, but p21 mRNA expression increased markedly. In contrast, treatment of RAC65 cells with 9-cis retinoic acid induced both p27 expression and neuronal differentiation. Correlation between p27 expression and neuronal differentiation was also observed in NT2/D1 human EC cells. Luciferase reporter assays showed that p27 promoter activity increased in ATRA-treated cells, consistent with the elevation of p27 mRNA levels. Arrest of neuronal differentiation of P19 cells by okadaic acid resulted in inhibition of p27 expression, whereas p21 mRNA expression was greatly enhanced. Conversely, inhibition of p27 expression by antisense p27 oligonucleotides resulted in blockade of neuronal differentiation. Taken together, these results strongly suggest that the expression of p27 is indispensable for neuronal differentiation of EC cells.

To differentiate or not to differentiate?

Developmental regulation of the cell cycle is an important determinant of tissue size and shape. Equally important is regulated withdrawal from the cell cycle to allow cells to differentiate. Recent evidence supports a direct link between transcriptional regulation of the cell cycle machinery and cell differentiation.