p27(Kip1) enhances myelin basic protein gene promoter activity

Inhibition of miR-221-3p promotes axonal regeneration and repair of primary sensory neurons via regulating p27 expression

This study aimed to explore the key microRNA (miRNA) playing a vital role in axonal regeneration with a hostile microenvironment after spinal cord injury. Based on the theory that sciatic nerve conditioning injury (SNCI) could promote the repair of the injured dorsal column. Differentially expressed miRNAs were screened according to the microarray, revealing that 47 known miRNAs were differentially expressed after injury and perhaps involved in nerve regeneration. Among the 47 miRNAs, the expression of miR-221-3p decreased sharply in the SNCI group compared with the simple dorsal column lesion (SDCL) group. Subsequently, it was confirmed that p27 was the target gene of miR-221-3p from luciferase reporter assay. Further, we found that inhibition of miR-221-3p expression could specifically target p27 to upregulate the expression of growth-associated protein 43 (GAP-43), α-tubulin acetyltransferase (α-TAT1) together with α-tubulin, and advance the regeneration of dorsal root ganglion (DRG) neuronal axons. Chondroitin sulfate proteoglycans (CSPGs) are the main components of glial scar, which can hinder the extension and growth of damaged neuronal axons. After CSPGs were used in this study, the results demonstrated that restrained miR-221-3p expression also via p27 promoted the upregulation of GAP-43, α-TAT1, and α-tubulin and enhanced the axonal growth of DRG neurons. Hence, miR-221-3p could contribute significantly to the regeneration of DRG neurons by specifically regulating p27 in the p27/CDK2/GAP-43 and p27/α-TAT1/α-tubulin pathways even in the inhibitory environment with CSPGs.

The rates of adult neurogenesis and oligodendrogenesis are linked to cell cycle regulation through p27-dependent gene repression of SOX2

Cell differentiation involves profound changes in global gene expression that often has to occur in coordination with cell cycle exit. Because cyclin-dependent kinase inhibitor p27 reportedly regulates proliferation of neural progenitor cells in the subependymal neurogenic niche of the adult mouse brain, but can also have effects on gene expression, we decided to molecularly analyze its role in adult neurogenesis and oligodendrogenesis. At the cell level, we show that p27 restricts residual cyclin-dependent kinase activity after mitogen withdrawal to antagonize cycling, but it is not essential for cell cycle exit. By integrating genome-wide gene expression and chromatin accessibility data, we find that p27 is coincidentally necessary to repress many genes involved in the transit from multipotentiality to differentiation, including those coding for neural progenitor transcription factors SOX2, OLIG2 and ASCL1. Our data reveal both a direct association of p27 with regulatory sequences in the three genes and an additional hierarchical relationship where p27 repression of Sox2 leads to reduced levels of its downstream targets Olig2 and Ascl1. In vivo, p27 is also required for the regulation of the proper level of SOX2 necessary for neuroblasts and oligodendroglial progenitor cells to timely exit cell cycle in a lineage-dependent manner.

Releasing 'brakes' to nerve regeneration: intrinsic molecular targets

Restoring critical neuronal architecture after peripheral nerve injury is challenging. Although immediate regenerative responses to peripheral axon injury involve the synthesis of regeneration-associated proteins in neurons and Schwann cells, an unfavorable balance between growth facilitatory and growth inhibitory signaling impairs the growth continuum of injured peripheral nerves. Molecules involved with the signaling network of tumor suppressors play crucial roles in shifting the balance between growth and restraint during axon regeneration. An understanding of the molecular framework of tumor suppressor molecules in injured neurons and its impact on stage-specific regeneration events may expose therapeutic intervention points. In this review we discuss how signaling networks of the specific tumor suppressors PTEN, Rb1, p53, p27 and p21 are altered in injured peripheral nerves and how this impacts peripheral nerve regeneration. Insights into the roles and importance of these pathways may open new avenues for improving the neurological deficits associated with nerve injury.

Intracellular Protein Shuttling: A Mechanism Relevant for Myelin Repair in Multiple Sclerosis?

A prominent feature of demyelinating diseases such as multiple sclerosis (MS) is the degeneration and loss of previously established functional myelin sheaths, which results in impaired signal propagation and axonal damage. However, at least in early disease stages, partial replacement of lost oligodendrocytes and thus remyelination occur as a result of resident oligodendroglial precursor cell (OPC) activation. These cells represent a widespread cell population within the adult central nervous system (CNS) that can differentiate into functional myelinating glial cells to restore axonal functions. Nevertheless, the spontaneous remyelination capacity in the adult CNS is inefficient because OPCs often fail to generate new oligodendrocytes due to the lack of stimulatory cues and the presence of inhibitory factors. Recent studies have provided evidence that regulated intracellular protein shuttling is functionally involved in oligodendroglial differentiation and remyelination activities. In this review we shed light on the role of the subcellular localization of differentiation-associated factors within oligodendroglial cells and show that regulation of intracellular localization of regulatory factors represents a crucial process to modulate oligodendroglial maturation and myelin repair in the CNS.

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.

Vsx2 controls eye organogenesis and retinal progenitor identity via homeodomain and non-homeodomain residues required for high affinity DNA binding

The homeodomain and adjacent CVC domain in the visual system homeobox (VSX) proteins are conserved from nematodes to humans. Humans with missense mutations in these regions of VSX2 have microphthalmia, suggesting both regions are critical for function. To assess this, we generated the corresponding mutations in mouse Vsx2. The homeodomain mutant protein lacked DNA binding activity and the knock-in mutant phenocopied the null mutant, ocular retardation J. The CVC mutant protein exhibited weakened DNA binding; and, although the corresponding knock-in allele was recessive, it unexpectedly caused the strongest phenotype, as indicated by severe microphthalmia and hyperpigmentation of the neural retina. This occurred through a cryptic transcriptional feedback loop involving the transcription factors Mitf and Otx1 and the Cdk inhibitor p27^Kip1. Our data suggest that the phenotypic severity of the CVC mutant depends on the weakened DNA binding activity elicited by the CVC mutation and a previously unknown protein interaction between Vsx2 and its regulatory target Mitf. Our data also suggest that an essential function of the CVC domain is to assist the homeodomain in high-affinity DNA binding, which is required for eye organogenesis and unhindered execution of the retinal progenitor program in mammals. Finally, the genetic and phenotypic behaviors of the CVC mutation suggest it has the characteristics of a recessive neomorph, a rare type of genetic allele.

Problems with the early development of the mammalian retina can cause congenital eye defects such as microphthalmia, in which the eye is dramatically smaller and functionally compromised. Severe microphthalmia is associated with mutations in the retinal-expressed visual system homeobox 2 (Vsx2) gene, but how Vsx2 controls retinal development, and ultimately eye formation, has remained unclear. We assessed the impact of two missense mutations, discovered in humans, on Vsx2 function and eye development in mice. One mutation altered a highly conserved residue of the homeodomain, and the other altered a highly conserved residue in the CVC domain, a region of unresolved function. Both mutations impacted the DNA binding properties of the protein, although to differing extents. Likewise, both mutations caused microphthalmia and disruptions in retinal development, also to differing extents and by distinct mechanisms. Our data suggest that Vsx2 acts as a gatekeeper of the retinal gene expression program by preventing the activation of interfering or competing gene expression programs. We propose that the evolutionary stable association between the VSX-class homeodomain and CVC domain set the stage for Vsx2 or its archetype to assume a gatekeeper function for retinal development and ultimately eye organogenesis.

Peroxisome-proliferator-activated receptor gamma coactivator 1 α contributes to dysmyelination in experimental models of Huntington's disease

The peroxisome-proliferator-activated receptor gamma coactivator 1 α (PGC1α) has been implicated in the pathogenesis of several neurodegenerative disorders, including Huntington's disease (HD). Recent data demonstrating white matter abnormalities in PGC1α knock-out (KO) mice prompted us to examine the role of PGC1α in CNS myelination and its relevance to HD pathogenesis. We found deficient postnatal myelination in the striatum of PGC1α KO mice, accompanied by a decrease in myelin basic protein (MBP). In addition, brain cholesterol, its precursors, and the rate-limiting enzymes for cholesterol synthesis, HMG CoA synthase (HMGCS1) and HMG CoA reductase (HMGCR), were also reduced in PGC1α KO mice. Moreover, knockdown of PGC1α in oligodendrocytes by lentiviral shRNA led to a decrease in MBP, HMGCS1, and Hmgcr mRNAs. Chromatin immunoprecipitations revealed the recruitment of PGC1α to MBP promoter in mouse brain, and PGC1α over-expression increased MBP and SREBP-2 promoter activity, suggesting that PGC1α regulates MBP and cholesterol synthesis at the transcriptional level. Importantly, expression of mutant huntingtin (Htt) in primary oligodendrocytes resulted in decreased expression of PGC1α and its targets HmgcS1, Hmgcr, and MBP. Decreased expression of MBP and deficient myelination were found postnatally and in adult R6/2 mouse model of HD. Diffusion tensor imaging detected white matter abnormalities in the corpus callosum of R6/2 mice, and electron microscopy revealed thinner myelin sheaths and increased myelin periodicity in BACHD [bacterial artificial chromosome (BAC)-mediated transgenic model for Huntington's disease] mice expressing full-length mutant Htt. Together, these data suggest that PGC1α plays a role in postnatal myelination and that deficient PGC1α activity in oligodendrocytes may contribute to abnormal myelination in HD.

Conditional Tet-regulated over-expression of Hoxa2 in CG4 cells increases their proliferation and delays their differentiation into oligodendrocyte-like cells expressing myelin basic protein

Hoxa2 gene was reported to be expressed by oligodendrocytes (OLs) and down-regulated at the terminal differentiation stage during oligodendrogenesis in mice (Nicolay et al. 2004b). To further investigate the role of Hoxa2 in oligodendroglial development, a tetracycline regulated controllable expression system was utilized to establish a stable cell line (CG4-SHoxa2 [sense Hoxa2]), where the expression level of Hoxa2 gene could be up-regulated. The impact of Hoxa2 over-expression on the proliferation and differentiation of CG4-SHoxa2 cells was investigated. Up-regulation of Hoxa2 increased the proliferation of CG4-SHoxa2 cells. The mRNA levels of PDGFαR (platelet-derived growth factor [PDGF] alpha receptor), which is expressed by OL progenitor cells, were not different in CG4-SHoxa2 cells compared to wild-type CG4 cells. Semi-quantitative RT-PCR revealed that the mRNA levels of myelin basic protein (MBP) was lower in CG4-SHoxa2 cells than in wild-type CG4 cells indicating the differentiation of CG4-SHoxa2 cells was delayed when the Hoxa2 gene was up-regulated.

Expression of mutant human DISC1 in mice supports abnormalities in differentiation of oligodendrocytes

Abnormalities in oligodendrocyte (OLG) differentiation and OLG gene expression deficit have been described in schizophrenia (SZ). Recent studies revealed a critical requirement for Disrupted-in-Schizophrenia 1 (DISC1) in neural development. Transgenic mice with forebrain restricted expression of mutant human DISC1 (ΔhDISC1) are characterized by neuroanatomical and behavioral abnormalities reminiscent of some features of SZ. We sought to determine whether the expression of ΔhDISC1 may influence the development of OLGs in this mouse model. OLG- and cell cycle-associated gene and protein expression were characterized in the forebrain of ΔhDISC1 mice during different stages of neurodevelopment (E15 and P1 days) and in adulthood. The results suggest that the expression of ΔhDISC1 exerts a significant influence on oligodendrocyte differentiation and function, evidenced by premature OLG differentiation and increased proliferation of their progenitors. Additional findings showed that neuregulin 1 and its receptors may be contributing factors to the observed upregulation of OLG genes. Thus, OLG function may be perturbed by mutant hDISC1 in a model system that provides new avenues for studying aspects of the pathogenesis of SZ.

The novel BTB/POZ and zinc finger factor Zbtb45 is essential for proper glial differentiation of neural and oligodendrocyte progenitor cells

Understanding the regulatory mechanisms controlling the fate decisions of neural stem cells (NSCs) is a crucial issue to shed new light on mammalian central nervous system (CNS) development in health and disease. We have investigated a possible role for the previously uncharacterized BTB/POZ-domain containing zinc finger factor Zbtb45 in the differentiation of NSCs and postnatal oligodendrocyte precursors. In situ hybridization histochemistry and RT-qPCR analysis revealed that Zbtb45 mRNA was ubiquitously expressed in the developing CNS in mouse embryos at embryonic day (E) 12.5 and 14.5. Zbtb45 mRNA knockdown in embryonic forebrain NSCs by siRNA resulted in a rapid decrease in the expression of oligodendrocyte-characteristic genes after mitogen (FGF2) withdrawal, whereas the expression of astrocyte-associated genes such as CD44 and GFAP increased compared to control. Accordingly, the number of astrocytes was significantly increased seven days after Zbtb45 siRNA delivery to NSCs, in contrast to the numbers of neuronal and oligodendrocyte-like cells. Surprisingly, mRNA knockdown of the Zbtb45-associated factor Med31, a subunit of the Mediator complex, did not result in any detectable effect on NSC differentiation. Similar to NSCs, Zbtb45 mRNA knockdown in oligodendrocyte precursors (CG-4) reduced oligodendrocyte maturation upon mitogen withdrawal associated with down-regulation of the mRNA expression and protein levels of markers for oligodendrocytic differentiation. Zbtb45 mRNA knockdown did not significantly affect proliferation or cell death in any of the cell types. Based on these observations, we propose that Zbtb45 is a novel regulator of glial differentiation.

Neurotrophin-3 targets the translational initiation machinery in oligodendrocytes

Neurotrophin-3 (NT-3) regulates oligodendrocyte (OLG) differentiation by mechanisms that remain poorly understood. Exposure of OLGs to NT-3 induces a significant increase in the levels of myelin basic protein (MBP). However, we found that this stimulation occurs in the absence of measurable effects on MBP gene promoter activation or mRNA expression, suggesting that NT-3 upregulates MBP protein expression by a posttranscriptional mechanism. Furthermore, NT-3 also causes an increase in the levels of myelin-associated glycoprotein (MAG) and myelin OLG glycoprotein (MOG), raising the possibility of a more general effect on myelin protein synthesis. Surprisingly, (35)S-methionine incorporation into total OLG proteins demonstrated a 50% increase in labeling following only a brief, 15-min treatment with NT-3. Such a remarkably fast response is unlikely due to transcriptional activation, reinforcing the possibility that NT-3 may play a crucial role in regulating protein expression by a posttranscriptional mechanism. In support of this idea, we found that NT-3 stimulates the phosphorylation of essential regulators of the initiation machinery, eukaryotic initiation factor 4E (eIF4E), and its inhibitory binding partner 4E binding protein 1 (4EBP1), two crucial players in controlling cap-dependent protein synthesis. This stimulation involves the activation of pathways mediated by ERK1/2 and PI3K/mTOR, implicating these two kinase systems as modulators of protein synthesis in developing OLGs. Altogether, these observations show for the first time that NT-3 has the capacity of targeting the translational machinery and suggest a potential stimulatory effect of this neurotrophin on myelination by direct action on protein translation in the OLGs.

The tumor suppressor functions of p27(kip1) include control of the mesenchymal/amoeboid transition

In many human cancers, p27 downregulation correlates with a worse prognosis, suggesting that p27 levels could represent an important determinant in cell transformation and cancer development. Using a mouse model system based on v-src-induced transformation, we show here that p27 absence is always linked to a more aggressive phenotype. When cultured in three-dimensional contexts, v-src-transformed p27-null fibroblasts undergo a morphological switch from an elongated to a rounded cell shape, accompanied by amoeboid-like morphology and motility. Importantly, the acquisition of the amoeboid motility is associated with a greater ability to move and colonize distant sites in vivo. The reintroduction of different p27 mutants in v-src-transformed p27-null cells demonstrates that the control of cell proliferation and motility represents two distinct functions of p27, both necessary for it to fully act as a tumor suppressor. Thus, we highlight here a new p27 function in driving cell plasticity that is associated with its C-terminal portion and does not depend on the control of cyclin-dependent kinase activity.

HDAC1 and HDAC2 regulate oligodendrocyte differentiation by disrupting the beta-catenin-TCF interaction

Oligodendrocyte development is regulated by the interaction of repressors and activators in a complex transcriptional network. We found that two histone-modifying enzymes, HDAC1 and HDAC2, were required for oligodendrocyte formation. Genetic deletion of both Hdac1 and Hdac2 in oligodendrocyte lineage cells resulted in stabilization and nuclear translocation of beta-catenin, which negatively regulates oligodendrocyte development by repressing Olig2 expression. We further identified the oligodendrocyte-restricted transcription factor TCF7L2/TCF4 as a bipartite co-effector of beta-catenin for regulating oligodendrocyte differentiation. Targeted disruption of Tcf7l2 in mice led to severe defects in oligodendrocyte maturation, whereas expression of its dominant-repressive form promoted precocious oligodendrocyte specification in developing chick neural tube. Transcriptional co-repressors HDAC1 and HDAC2 compete with beta-catenin for TCF7L2 interaction to regulate downstream genes involved in oligodendrocyte differentiation. Thus, crosstalk between HDAC1/2 and the canonical Wnt signaling pathway mediated by TCF7L2 serves as a regulatory mechanism for oligodendrocyte differentiation.

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.

p38 mitogen-activated protein kinase is required for central nervous system myelination

The p38 MAPKs are a family of kinases that regulate a number of cellular functions including cell migration, proliferation, and differentiation. Here, we report that p38 regulates oligodendrocyte differentiation. Inhibition of p38 with PD169316 and SB203580 prevented accumulation of protein and mRNA of cell-stage specific markers characteristic of differentiated oligodendrocytes, including myelin basic protein, myelin-associated glycoprotein, and the glycosphingolipids, galactosylceramide and sulfatide. In addition, the cell cycle regulator p27(kip1) and the transcription factor Sox10 were also significantly reduced. Most significantly, p38 inhibitors completely and irreversibly blocked myelination of dorsal root ganglion neurons by oligodendrocytes and prevented the axolemmal organization of the axo-glial adhesion molecule Caspr. Our results suggest a role(s) for this kinase in key regulatory steps in the maturation of OLGs and initiation of myelination.

p38 Mitogen-activated protein kinase regulates myelination

The p38 mitogen-activated protein kinase family is emerging as a crucial signaling molecule for a vast number of cellular functions including cell migration, proliferation, and differentiation. The function of p38 in myelination has only been recently addressed. Using pyridinyl imidazole-based p38 alpha/beta selective inhibitors, we have reported a critical role for this kinase in the regulation of myelination, specifically, in controlling the differentiation of Schwann cells, and oligodendrocytes, the myelinating glia of the peripheral and central nervous systems, respectively. These compounds inhibited the accumulation of myelin-cell-specific markers, including myelin-specific glycosphingolipids, myelin-associated glycoprotein, and myelin basic protein. More significantly, myelination of dorsal root ganglia neurons by oligodendrocytes was irreversibly blocked by p38 inhibitors. Our current studies are focusing on the molecular mechanisms by which p38 regulates oligodendrocyte and Schwann cell differentiation and its role in models of myelination and remyelination.

Optic nerve alterations in P27(Kip1) knockout mice

PURPOSE

To study the morphologic characteristics of the optic nerve (ON) by using an experimental model of knockout mice for the expression of the P27(Kip1) gene, mainly involved in cell cycle arrest, apoptosis control, and retinoblastoma protein phosphorylation.

METHODS

Eyeballs with the retrobulbar ON attached were obtained from 26-week-old mice. By using morphologic and morphometric techniques, light and electron transmission microscopy, the ON characteristics were determined in two groups of mice: 1) wild type mice as the control group (n=15), 2) homozygous knockout mice (-/-) for the P27(Kip1) gene as the knockout group (n=15). Glial fibrillary acidic protein (GFAP) and myelin basic protein (MBP) were studied using Western blot and immunoblotting approaches.

RESULTS

The ON cross-sectional area was significantly larger in the P27(Kip1) knockout mice group than in the control group (p<0.001). The axon sizes in knockout animals were much larger than in wild-type mice (p<0.001). Higher number of axons forming the ON, intra-axonal degeneration, myelin sheath, and axoplasm density alterations were found in P27(Kip1) knockout mice when compared with control group (p<0.001). Analysis of lysates of optic nerves by Western blot showed less expression of myelin basic protein and GFAP in P27(Kip1) knockout mice as compared to wild type mice (p<0.005, p<0.01, respectively).

CONCLUSIONS

The morphologic and morphometric results suggest that homozygous P27(Kip1) knock-out mice had hypertrophic, hyperplastic, and dystrophic ON.

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.

Galectin-4 is involved in p27-mediated activation of the myelin basic protein promoter

Our previous studies have found that expression of p27 in oligodendrocytes enhances myelin basic protein (MBP) gene expression through a mechanism that involves the transcription factor Sp1. In this study we show that this activation only requires the N-terminal 45 amino acids of p27 containing a functional cyclin-binding motif. In an effort to identify other cofactors that are involved in the p27-mediated activation of MBP gene expression, a yeast two-hybrid assay was performed using an N-terminal truncated p27 and a mouse embryo cDNA library. Galectin-4 was found to interact with p27 in the yeast two-hybrid assay. This novel interaction was also confirmed using a glutathione-S-transferase interaction assay and immunoprecipitation assays. Expression of galectin-4 in primary oligodendrocytes was confirmed by western blot. Additionally, the MBP promoter could be activated by expression of galectin-4 in CG4 oligodendrocytes, similar to the effects of increased p27 levels. We also show that Sp1 and galectin-4 interact in cells, while a complex of all three proteins could not be found. We conclude that galectin-4 is involved in the p27-mediated activation of the MBP gene, possibly through modulation of the glycosylation status of the transcription factor Sp1.

Multiple roles of Id4 in developmental myelination: predicted outcomes and unexpected findings

Myelination in the central nervous system is a complex process requiring the integration of oligodendrocyte progenitor differentiation and the coordinate expression of myelin genes. This study addresses the role of the helix-loop-helix protein Id4 in these two events. Overexpression of Id4 in oligodendrocyte progenitors prevents differentiation and consequently decreases the endogenous expression of all myelin genes. Conversely, progenitors lacking Id4 display precocious differentiation both in vitro and in vivo, and this phenotype is partially compensated by increased apoptosis. Besides this role, Id4 also has the ability to decrease the activity of specific myelin promoters, since Id4 overexpression decreases the activity of luciferase reporter genes driven by the ceramide galactosyltransferase (CGT) or myelin basic protein (MBP) promoter, but not by a myelin proteolipid protein (PLP) promoter. Consistent with these results, the expression levels of MBP and CGT are greater in neonatal Id4 null mice when compared with wild-type siblings and correlate with the early detection of MBP immunoreactive myelinated fibers. In contrast, the levels of other myelin proteins, such as PLP and myelin associated glycoprotein (MAG) are decreased in the Id4 null mice. MAG expression is localized to the soma rather than the fibers of immunoreactive cells in the neonatal brain and compensated at later developmental stages. These data support the role of Id4 as oligodendrocyte differentiation inhibitor with the ability to differentially regulate the expression and subcellular distribution of myelin gene products.

A molecular insight of Hes5-dependent inhibition of myelin gene expression: old partners and new players

This study identifies novel mechanisms of Hes5 function in developmental myelination. We report here upregulation of myelin gene expression in Hes5-/- mice compared to wild-type siblings and downregulation in overexpressing progenitors. This effect was only partially explained by the ability to regulate the levels of Mash1 and bind to N boxes in myelin promoters, as deletion of the DNA-binding domain of Hes5 did not suppress its inhibitory role on myelin gene expression. Novel mechanisms of Hes5 function in the oligodendrocyte lineage include the regulation of feedback loops with the cell-specific transcriptional activator Sox10. In progenitors with low levels of Sox10, Hes5 further decreases the bioavailability of this protein by transcriptional inhibition and direct sequestration of this activator. Increasing levels of Sox10 in progenitors, in turn, bind to Hes5 and titrate out its inhibitory effect by sequestration and displacement of the repressive complexes from myelin promoters. Thus, Hes5-dependent modulation of myelin gene expression involves old players (i.e. Mash1) and novel mechanisms of transcriptional regulation that include cell-specific regulatory loops with transcriptional activators (i.e. Sox10).

Expression of myelin basic protein in two oligodendroglial cell lines is modulated by apotransferrin through different transcription factors

We have shown that apotransferrin (aTf) promotes the differentiation of two oligodendroglial cell (OLGc) lines, N19 and N20.1, representing different stages of OLGc maturation. Although in both cell lines aTf promoted myelin basic protein (MBP) expression, an increase in cAMP levels and CREB phosphorylation was observed only in the less mature cells (N19), suggesting that the maturation induced by aTf is achieved probably through different signaling pathways. We transfected both cell lines with the proximal region of the human MBP promoter fused to the lacZ reporter gene. In both transfected cell lines, addition of aTf produced an activation of the promoter. To elucidate the mechanisms involved in this action, Western blot analysis, EMSAs, and RT-PCR were performed for different transcription factors involved in mbp regulation. In the N20.1 line, treatment with aTf increased the expression and the DNA-binding capacity of thyroid hormone (TH) receptors, Sp1, and nuclear factor-kappaB (NFkappaB). For these cells we found that an inductor of NFkappaB (tumor necrosis factor-alpha) promoted MBP messenger synthesis, whereas mithramycin, a specific inibitor of Sp1, and a cAMP analog (db-cAMP) inhibited its transcription. In the N19 cell line, aTf stimulated NF-I and NFkappaB activation, but, aside from aTf, only db-cAMP induced mbp transcription. These data suggest that, depending on the OLGc maturational stage, aTf modulates MBP expression and OLGc differentiation through different signaling pathways and different transcription factors.

Human QKI, a potential regulator of mRNA expression of human oligodendrocyte-related genes involved in schizophrenia

The quaking viable mouse mutation (qk(v)) is a deletion including the 5' regulatory region of the quaking gene (Qki), which causes body tremor and severe dysmyelination in mouse. The function of the human quaking gene, called quaking homolog KH domain RNA-binding (mouse) (QKI), is not well known. We have previously shown that QKI is a new candidate gene for schizophrenia. Here we show that human QKI mRNA levels can account for a high proportion (47%) of normal interindividual mRNA expression variation (and covariation) of six oligodendrocyte-related genes (PLP1, MAG, MBP, TF, SOX10, and CDKN1B) in 55 human brain autopsy samples from individuals without psychiatric diagnoses. In addition, the tightly coexpressed myelin-related genes (PLP1, MAG, and TF) have decreased mRNA levels in 55 schizophrenic patients, as compared with 55 control individuals, and most of this difference (68-96%) can be explained by variation in the relative mRNA levels of QKI-7kb, the same QKI splice variant previously shown to be down-regulated in patients with schizophrenia. Taken together, our results suggest that QKI levels may regulate oligodendrocyte differentiation and maturation in human brain, in a similar way as in mouse. Moreover, we hypothesize that previously observed decreased activity of myelin-related genes in schizophrenia might be caused by disturbed QKI splicing.

Combinatorial profiles of oligodendrocyte-selective classes of transcriptional regulators differentially modulate myelin basic protein gene expression

Recent studies suggest that specific neural basic helix-loop-helix (HLH; i.e., Olig1 and Olig2, Mash1), associated inhibitory HLH (i.e., Id2 and Id4), high-mobility group domain (i.e., Sox10), and homeodomain (i.e., Nkx2.2) transcription factors are involved in oligodendrocyte (OL) lineage specification and progressive stages of maturation including myelination. However, the developmental interplay among these lineage-selective determinants, in a cell- and maturational stage-specific context, has not yet been defined. We show here in vivo and in vitro developmental expression profiles for these distinct classes of transcriptional regulators of OLs. We show that progressive stages of OL lineage maturation are characterized by dynamic changes in the subcellular distribution of these transcription factors and by different permutations of combinatorial transcriptional codes. Transient transfections of these precise combinatorial codes with a luciferase reporter gene driven by the myelin basic protein promoter define how changes in the molecular composition of these transcriptional complexes modulate myelin gene expression. Our overall findings suggest that the dynamic interplay between developmental stage-specific classes of transcriptional activators and associated inhibitory factors orchestrate myelin gene expression during terminal maturation of the mammalian CNS.

Interferon-gamma inhibits cell cycle exit in differentiating oligodendrocyte progenitor cells

The developmental processes of the oligodendrocyte progenitor cell (OPC) lineage that are targeted by interferon-gamma (IFN-gamma) were studied in primary rat OPC cultures. Under conditions of thyroid hormone-mediated oligodendrocyte differentiation, IFN-gamma produced a dose-dependent apoptotic response in OPCs. The lowest dose tested (15 ng/ml or 75 U/ml) was nonapoptotic, but activated detectable STAT1 DNA-binding. At this dose, IFN-gamma reduced the percentage of mature O1+ cells and increased the percentage of immature A2B5+ OPCs. This was observed without significant change in total cell number and cytotoxicity, and was accompanied by an increase in BrdU-labeled A2B5+ and O4+ cells. FACS analysis confirmed a lack of apoptotic sub-G1 cells and revealed a greater percentage of S- and G2/M-phase OPCs with IFN-gamma treatment. Dual immunostaining with Ki-67 and Olig2 showed a smaller percentage of Olig2+ cells in G0 phase in IFN-gamma-treated OPCs, indicating loss of G1 control. Instead, increased levels and phosphorylation of the checkpoint protein p34cdc2 by IFN- suggested increased partial arrest in G2. IFN-gamma not only sustained expression of PCNA and the G1-S regulators retinoblastoma protein, cyclin D1, cyclin E, and cdk2, but also decreased p27 levels. In addition to changes in cell proliferation and differentiation, IFN-gamma attenuated myelin basic protein (MBP) expression significantly, which was associated with decreased expression of both MBP and Sox10 RNAs. These findings indicate that IFN-gamma not only maintains cell cycle activity that could predispose OPCs to apoptosis, but also overrides G1-G0 signals leading to thyroid hormone-mediated terminal differentiation and myelin gene expression.

Sox10 acts as a tissue-specific transcription factor enhancing activation of the myelin basic protein gene promoter by p27Kip1 and Sp1

Myelin basic protein (MBP) is one of the major components of the myelin sheath that insulates axons. In the central nervous system, MBP is synthesized by differentiating oligodendrocytes. The expression of MBP in oligodendrocytes is regulated mainly at the transcriptional level. The Sp1 family of transcription factors has been shown to be important in the regulation of many genes. Binding of Sp1 to the GC box in the proximal MBP promoter has been shown to be indispensable for the activation of MBP gene expression. Previous results from our laboratory have shown that the increase in p27Kip1 that accompanies oligodendrocyte differentiation is paralleled by an increase in Sp1. We also have shown that the increase in MBP expression resulting from elevated p27Kip1 levels is mediated through Sp1 and that this effect occurs specifically in oligodendrocytes. In this study, we found that increased expression of p27Kip1 together with the nervous-system-specific transcription factor Sox10 can activate the MBP promoter even in nonoligodendrocyte cells. This indicates that Sox10 confers cell type specificity on the expression of MBP. Both Sp1 and Sox10 can enhance MBP promoter activity when expressed alone. Cotransfection of plasmids encoding Sp1 and Sox10 induces increased activation of the MBP promoter over expression of either transcription factor alone. This effect is not limited to oligodendrocyte cell lines, in that Sp1 and Sox10 can also synergistically activate the MBP promoter when expressed in NIH3T3 fibroblasts. Mutation of the Sp1 binding sites in the MBP promoter eliminates Sox10 stimulated activation, suggesting that the MBP promoter is activated, at least in part, through protein-protein interactions between Sp1 and Sox10.

Stage-specific expression of myelin basic protein in oligodendrocytes involves Nkx2.2-mediated repression that is relieved by the Sp1 transcription factor

The homeodomain-containing protein Nkx2.2 is critical for the development of oligodendrocyte lineage cells, but the target genes of Nkx2.2 regulation have not been identified. In the present study, we found that the myelin basic protein gene is one of the genes that is regulated by Nkx2.2. Expression of Nkx2.2 represses the expression of myelin basic protein in oligodendrocyte progenitors. Two regulatory elements in the myelin basic protein promoter were identified and found to interact with Nkx2.2 in vitro. Despite their sequence divergence, both sites were involved in the Nkx2.2-mediated repression of the myelin basic protein promoter. Binding of Nkx2.2 also blocked and disrupted the binding of the transcriptional activator Puralpha to the myelin basic protein promoter. Additionally Nkx2.2 recruited a histone deacetylase 1-mSin3A complex to the myelin basic protein promoter. We also found that the transcription factor Sp1 was able to compete off the binding of Nkx2.2 to its consensus binding site in vitro and reversed the repressive effect of Nkx2.2 in vivo. Our data revealed a novel role for Nkx2.2 in preventing the precocious expression of myelin basic protein in immature oligodendrocytes. Based on this study and our previous reports, a model for myelin basic protein gene control is proposed.

Cloning and characterization of the rat myelin basic protein gene promoter

Expression of myelin basic protein in differentiating oligodendrocytes is mainly regulated at the transcriptional level. To better understand the regulation of myelin basic protein gene expression in mammalian cells, we cloned and characterized the rat myelin basic protein promoter by a genome walking technique. Extensive sequence homology has been found among mouse, rat and human MBP promoters. Alignment of the proximal core promoter of mouse and rat reveals highly conserved cis-elements that are important for regulating myelin basic protein gene transcription. One major transcription start site along with two minor sites have been identified in both mouse and rat myelin basic protein gene promoters using RNA ligase-mediated rapid amplification of 5' cDNA ends. The amplified rat myelin basic protein promoter was cloned into a luciferase reporter construct. Transient transfection experiments show that both mouse and rat myelin basic protein promoters yield increased expression when oligodendrocytes differentiate. The sequence and characterization of the rat MBP promoter provide a useful tool to investigate MBP gene regulation in mammalian cells.

Role of Gadd45alpha in the density-dependent G1 arrest induced by p27(Kip1)

p27(Kip1), an inhibitor of cyclin-dependent kinases, is an important regulator of cell cycle progression. We have previously shown that p27(Kip1) inhibits the G0 to S transition when ectopically expressed in p27-47 mouse fibroblasts arrested at high but not low densities. In the study described here, we identify Gadd45alpha, a member of the growth arrest- and DNA damage-inducible family of proteins, as a potential mediator of the density-dependent effects of p27(Kip1) on cell proliferation. Gadd45alpha mRNA and protein were more abundant in p27-47 cells arrested at high densities than at low densities. Amounts of both decreased and remained low when cells arrested at high densities were exposed to mitogens in the absence, but not in the presence, of ectopically expressed p27(Kip1). Importantly, enforced expression of Gadd45alpha prevented density-arrested mouse fibroblasts from initiating DNA synthesis in response to mitogens. We suggest that amounts of Gadd45alpha above a certain threshold are growth inhibitory and that such amounts are achieved in cells arrested at high but not low densities. For cultures arrested at high densities, the resumption of cell cycle traverse requires a sustained reduction in Gadd45alpha abundance, a process that is induced by mitogens and inhibited by p27(Kip1).

Inhibition of the proteasome by lactacystin enhances oligodendroglial cell differentiation

We have used lactacystin, a specific inhibitor of the 26S proteasome, in oligodendroglial cell (OLGc) primary cultures to explore the possible participation of the proteasome-ubiquitin-dependent pathway in the decision of the OLGcs to arrest their proliferation and start differentiation. Addition of lactacystin at various concentrations to cultures containing a majority of OLGc was found to produce their withdrawal from the cell cycle and to induce their biochemical and morphological differentiation, with the appearance of extensive myelin-like sheets. The three classic proteolytic activities of the proteasome were significantly decreased in the lactacystin-treated cultures, and the immunocytochemical analysis showed an increase in the number of O4-, O1-, myelin basic protein-, and myelin proteolipid protein-positive cells and a decrease in A2B5-reacting cells. Quantitative immunochemical evaluation of the expression of certain proteins controlling the cell cycle showed an increase in p27kip1-, cyclin D-, and cdk4-positive cells, with a decrease in cyclin E- and cdk2-positive cells. In the lactacystin-treated OLGcs, there was a dose-dependent decrease in the number of cells incorporating bromodeoxyuridine and in the activity of the complexes cyclin D-cdk4 and cyclin E-cdk2. Furthermore, increased levels of expression of several STAT factors were found, suggesting that proteasome inhibition in OLGcs could stabilize signals of survival and differentiation that might be processed through the JAK/STAT signaling cascade.

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.

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.