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We show that expression of p57Kip2, a potent tight-binding inhibitor of several G1 cyclin–cyclin-dependent kinase (Cdk) complexes, increases markedly during C2C12 myoblast differentiation. We examined the effect of p57Kip2 on the activity of the transcription factor MyoD. In transient transfection assays, transcriptional transactivation of the mouse muscle creatine kinase promoter by MyoD was enhanced by the Cdk inhibitors. In addition, p57Kip2, p21Cip1, and p27Kip1 but not p16Ink4a induced an increased level of MyoD protein, and we show that MyoD, an unstable nuclear protein, was stabilized by p57Kip2. Forced expression of p57Kip2 correlated with hypophosphorylation of MyoD in C2C12 myoblasts. A dominant-negative Cdk2 mutant arrested cells at the G1 phase transition and induced hypophosphorylation of MyoD. Furthermore, phosphorylation of MyoD by purified cyclin E-Cdk2 complexes was inhibited by p57Kip2. In addition, the NH2 domain of p57Kip2 necessary for inhibition of cyclin E-Cdk2 activity was sufficient to inhibit MyoD phosphorylation and to stabilize it, leading to its accumulation in proliferative myoblasts. Taken together, our data suggest that repression of cyclin E-Cdk2-mediated phosphorylation of MyoD by p57Kip2 could play an important role in the accumulation of MyoD at the onset of myoblast differentiation.

The DNA sequence between position +36 and -1907 of the murine myelin basic protein gene contains the enhancer and promoter elements necessary for abundant and cell specific expression in transgenic mice. Surprisingly, the pattern of expression promoted by this DNA fragment is a subset of that exhibited by the endogenous myelin basic protein (MBP) gene. Fusion genes prepared with this promoter/enhancer and a Lac Z reporter gene are expressed only in oligodendrocytes and not in Schwann cells, whereas the endogenous MBP gene is expressed in both cell types. The level of transgene expression measured by nuclear run- on experiments is very substantial and rivals that of the endogenous MBP gene. Furthermore, this 1.9-kb DNA fragment directs transcription on the same (or very similar) developmental schedule as the endogenous gene. These results indicate that the MBP promoter/enhancer sequences are at least tripartite: a core promoter, the oligodendrocyte enhancer elements, and a third component that either expands the specificity of the oligodendrocyte enhancer to include Schwann cells or acts independently to specifically stimulate transcription in Schwann cells.

A transcriptional regulator, Hes1, plays crucial roles in the control of differentiation and proliferation of neuronal, endocrine, and T-lymphocyte progenitors during development. Mechanisms for the regulation of cell proliferation by Hes1, however, remain to be verified. In embryonic carcinoma cells, endogenous Hes1 expression was repressed by retinoic acid in concord with enhanced p27Kip1 expression and cell cycle arrest. Conversely, conditional expression of a moderate but not maximal level of Hes1 in HeLa cells by a tetracycline-inducible system resulted in reduced p27Kip1 expression, which was attributed to decreased basal transcript rather than enhanced proteasomal degradation, with concomitant increases in the growth rate and saturation density. Hes1 induction repressed the promoter activity of a 5′ flanking basal enhancer region of p27Kip1 gene in a manner dependent on Hes1 expression levels, and this was mediated by its binding to class C sites in the promoter region. Finally, hypoplastic fetal thymi, as well as livers and brains of Hes1-deficient mice, showed significantly increased p27Kip1 transcripts compared with those of control littermates. These results have suggested that Hes1 directly contributes to the promotion of progenitor cell proliferation through transcriptional repression of a cyclin-dependent kinase inhibitor, p27Kip1.

The gene encoding myelin basic protein (MBP) contains multiple activator sequences spanning upstream of its transcriptional initiation site which differentially promote transcription in glial cells. The proximal activator sequence, designated MB1, activates transcription in a glial cell type specific manner. This sequence resides between -14 to -50 with respect to the RNA initiation site of the MBP gene. We have identified within the MB1 sequence a 10-nucleotide domain, 5'-ACCTTCAAAG-3', that increases transcription of a test promoter in glial and Schwann cells. This proximal motif functions in both orientations and specifically interacts with a nuclear protein derived from glial cells. Results of in vivo competition experiments indicate that this 10-nucleotide motif positively contributes to the overall transcriptional activity obtained from the entire MBP promoter in glial cells.

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Background

The cyclin-dependent kinase inhibitor p27Kip1 is essential for proper control of cell cycle progression. The levels of p27Kip1 are regulated by several mechanisms including transcriptional and translational controls. In order to delineate the molecular details of these regulatory mechanisms it is important to identify the transcription initiation site within the p27Kip1 gene, thereby defining the promoter region of the gene and the 5'-untranslated region of the p27Kip1 mRNA. Although several previous studies have attempted to map p27Kip1 transcription start sites, the results vary widely for both the mouse and human genes. In addition, even though the mouse and human p27Kip1 gene sequences are very highly conserved, the reported start sites are notably different.

Results

In this report, using a method that identifies capped ends of mRNA molecules together with RNase protection assays, we demonstrate that p27Kip1 transcription is initiated predominantly from a single site which is conserved in the human and mouse genes. Initiation at this site produces a 5'-untranslated region of 472 nucleotides in the human p27Kip1 mRNA and 502 nucleotides in the mouse p27Kip1 mRNA. In addition, several minor transcription start sites were identified for both the mouse and human genes.

Conclusions

These results demonstrate that the major transcription initiation sites in the mouse and human p27Kip1 genes are conserved and that the 5'-UTR of the p27Kip1 mRNA is much longer than generally believed. It will be important to consider these findings when designing experiments to identify elements that are involved in regulating the cellular levels of p27Kip1.

The cortactin oncoprotein is frequently overexpressed in head and neck squamous cell carcinoma (HNSCC), often due to amplification of the encoding gene (CTTN). While cortactin overexpression enhances invasive potential, recent research indicates that it also promotes cell proliferation, but how cortactin regulates the cell cycle machinery is unclear. In this article we report that stable short hairpin RNA-mediated cortactin knockdown in the 11q13-amplified cell line FaDu led to increased expression of the Cip/Kip cyclin-dependent kinase inhibitors (CDKIs) p21WAF1/Cip1, p27Kip1, and p57Kip2 and inhibition of S-phase entry. These effects were associated with increased binding of p21WAF1/Cip1 and p27Kip1 to cyclin D1- and E1-containing complexes and decreased retinoblastoma protein phosphorylation. Cortactin regulated expression of p21WAF1/Cip1 and p27Kip1 at the transcriptional and posttranscriptional levels, respectively. The direct roles of p21WAF1/Cip1, p27Kip1, and p57Kip2 downstream of cortactin were confirmed by the transient knockdown of each CDKI by specific small interfering RNAs, which led to partial rescue of cell cycle progression. Interestingly, FaDu cells with reduced cortactin levels also exhibited a significant diminution in RhoA expression and activity, together with decreased expression of Skp2, a critical component of the SCF ubiquitin ligase that targets p27Kip1 and p57Kip2 for degradation. Transient knockdown of RhoA in FaDu cells decreased expression of Skp2, enhanced the level of Cip/Kip CDKIs, and attenuated S-phase entry. These findings identify a novel mechanism for regulation of proliferation in 11q13-amplified HNSCC cells, in which overexpressed cortactin acts via RhoA to decrease expression of Cip/Kip CDKIs, and highlight Skp2 as a downstream effector for RhoA in this process.

The expression levels of the p21Cip1 family CDK inhibitors (CKIs), p21Cip1, p27Kip1 and p57Kip2, play a pivotal role in the precise regulation of cyclin-dependent kinase (CDK) activity, which is instrumental to proper cell cycle progression. The stabilities of p21Cip1, p27Kip1 and p57Kip2 are all tightly and differentially regulated by ubiquitylation and proteasome-mediated degradation during various stages of the cell cycle, either in steady state or in response to extracellular stimuli, which often elicit site-specific phosphorylation of CKIs triggering their degradation.

Mounting evidence indicates cyclin-dependent kinase (CDK) inhibitors (CKIs) of the Cip/Kip family, including p57Kip2 and p27Kip1, control not only cell cycle exit but also corticogenesis. Nevertheless, distinct activities of p57Kip2 remain poorly defined. Using in vivo and culture approaches, we show p57Kip2 overexpression at E14.5–15.5 elicits precursor cell cycle exit, promotes transition from proliferation to neuronal differentiation, and enhances process outgrowth, while opposite effects occur in p57Kip2-deficient precursors. Studies at later ages indicate p57Kip2 overexpression also induces precocious glial differentiation, suggesting stage-dependent effects. In embryonic cortex, p57Kip2 overexpression advances cell radial migration and alters postnatal laminar positioning. While both CKIs induce differentiation, p57Kip2 was twice as effective as p27Kip1 in inducing neuronal differentiation and was not permissive to astrogliogenic effects of ciliary neurotrophic factor, suggesting that the CKIs differentially modulate cell fate decisions. At molecular levels, although highly conserved N-terminal regions of both CKIs elicit cycle withdrawal and differentiation, the C-terminal region of p57Kip2 alone inhibits in vivo migration. Furthermore, p57Kip2 effects on neurogenesis and gliogenesis require the N-terminal cyclin/CDK binding/inhibitory domains, while previous p27Kip1 studies report cell cycle-independent functions. These observations suggest p57Kip2 coordinates multiple stages of corticogenesis and exhibits distinct and common activities compared with related family member p27Kip1.

Expression of myelin basic protein (MBP) in mice is regulated in a cell- and stage-specific manner during brain development. The MBP control region contains multiple cis-acting elements, shown by in vivo and in vitro assays, which are responsible for its unique pattern of transcription. Using synthetic DNA fragments spanning the MBP control region, we have analyzed nuclear proteins obtained from newborn (2-3 d), young adult (18-30 d), and adult (60 d) animals; these nuclear proteins form DNA-protein complexes with the MBP regulatory region. Brain extracts from young adult and adult mice showed enhanced binding activities with the sequences supporting transcriptional activation in glial cells. Deletion analysis of the proximal activating sequence located at position -14 to -50 with respect to the RNA initiation site resulted in identification of a small region, located between nucleotides -14 to -37, which is required for formation of the complexes. Southwestern assay revealed a major 39-kD protein from young adult brain extract that recognizes the sequences between nucleotides -14 to -37. An additional minor 37-kD protein, derived from young adult brain extract, was also found to be associated with this proximal activating region. Of particular interest is the observation that the minor 37-kD protein became more abundant in the extract derived from adult brain, whereas the major 39-kD protein became less abundant. The possible role of these proteins in cell/stage-specific transcription of MBP is discussed.

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HTLV-1 Tax can induce senescence by up-regulating the levels of cyclin-dependent kinase inhibitors p21CIP1/WAF1 and p27KIP1. Tax increases p27KIP1 protein stability by activating the anaphase promoting complex/cyclosome (APC/C) precociously, causing degradation of Skp2 and inactivation of SCFSkp2, the E3 ligase that targets p27KIP1. The rate of p21CIP1/WAF1 protein turnover, however, is unaffected by Tax. Rather, the mRNA of p21CIP1/WAF1 is greatly up-regulated. Here we show that Tax increases p21 mRNA expression by transcriptional activation and mRNA stabilization. Transcriptional activation of p21CIP1/WAF1 by Tax occurs in a p53-independent manner and requires two tumor growth factor-β-inducible Sp1 binding sites in the -84 to -60 region of the p21CIP1/WAF1 promoter. Tax binds Sp1 directly, and the CBP/p300-binding activity of Tax is required for p21CIP1/WAF1 trans-activation. Tax also increases the stability of p21CIP1/WAF1 transcript. Several Tax mutants trans-activated the p21 promoter, but were attenuated in stabilizing p21CIP1/WAF1 mRNA, and were less proficient in increasing p21CIP1/WAF1 expression. The possible involvement of Tax-mediated APC/C activation in p21CIP1/WAF1 mRNA stabilization is discussed.

The clinical picture of experimental autoimmune encephalomyelitis (EAE) is critically dependent on the nature of the target autoantigen and the genetic background of the experimental animals. Potentially lethal EAE is mediated by myelin basic protein (MBP)–specific T cells in Lewis rats, whereas transfer of S100β- or myelin oligodendrocyte glycoprotein (MOG)–specific T cells causes intense inflammatory response in the central nervous system (CNS) with minimal disease. However, in Dark Agouti rats, the pathogenicity of MOG-specific T cells resembles the one of MBP-specific T cells in the Lewis rat. Using retrovirally transduced green fluorescent T cells, we now report that differential disease activity reflects different levels of autoreactive effector T cell activation in their target tissue. Irrespective of their pathogenicity, the migratory activity, gene expression patterns, and immigration of green fluorescent protein+ T cells into the CNS were similar. However, exclusively highly pathogenic T cells were significantly reactivated within the CNS. Without local effector T cell activation, production of monocyte chemoattractants was insufficient to initiate and propagate a full inflammatory response. Low-level reactivation of weakly pathogenic T cells was not due to anergy because these cells could be activated by specific antigen in situ as well as after isolation ex vivo.

p57 (Kip2, cyclin-dependent kinase inhibitor 1C), often found downregulated in cancer, is reported to hold tumor suppressor properties. Originally described as a cyclin-dependent kinase (cdk) inhibitor, p57KIP2 has since been shown to influence other cellular processes, beyond cell cycle regulation, including cell death and cell migration. Inhibition of cell migration by p57KIP2 is attributed to the stabilization of the actin cytoskeleton through the activation of LIM domain kinase-1 (LIMK-1). Furthermore, p57KIP2 is able to enhance mitochondrial-mediated apoptosis. Here, we report that the cell death promoting effect of p57KIP2 is linked to its effect on the actin cytoskeleton. Indeed, whereas Jasplakinolide, an actin cytoskeleton-stabilizing agent, mimicked p57KIP2's pro-apoptotic effect, destabilizing the actin cytoskeleton with cytochalsin D reversed p57KIP2's pro-apoptotic function. Conversely, LIMK-1, the enzyme mediating p57KIP2's effect on the actin cytoskeleton, was required for p57KIP2's death promoting effect. Finally, p57KIP2-mediated stabilization of the actin cytoskeleton was associated with the displacement of hexokinase-1, an inhibitor of the mitochondrial voltage-dependent anion channel, from the mitochondria, providing a possible mechanism for the promotion of the mitochondrial apoptotic cell death pathway. Altogether, our findings link together two tumor suppressor properties of p57KIP2, by showing that the promotion of cell death by p57KIP2 requires its actin cytoskeleton stabilization function.

The universal cyclin-Cdk inhibitor p27Kip1 functions as a tumor suppressor and reduced levels of p27Kip1 connote poor prognosis in several human malignancies. p27Kip1 levels are predominately regulated by ubiquitin-mediated turnover of the protein, which is marked for destruction by the E3 ubiquitin ligase SCFSkp2 complex following its phosphorylation by the cyclin E-Cdk2 complex. Binding of phospho-p27Kip1 is directed by the Skp2 F-box protein, and this is greatly augmented by its allosteric regulator Cks1. We have established that programmed expression of c-Myc in the B cells of Eμ-Myc transgenic mice triggers p27Kip1 destruction by inducing Cks1, that this response controls Myc-driven proliferation, and that loss of Cks1 markedly delays Myc-induced lymphomagenesis and cancels the dissemination of these tumors. Here, we report that elevated levels of Skp2 are a characteristic of Eμ-Myc lymphomas and of human Burkitt lymphoma that bear MYC/immunoglobulin chromosomal translocations. As expected, Myc-mediated suppression of p27Kip1 was abolished in Skp2-null Eμ-Myc B cells. However, the impact of Skp2 loss on Myc-driven proliferation and lymphomagenesis was surprisingly modest compared to the effects of Cks1 loss. Collectively these findings suggest that Cks1 targets in addition to p27Kip1 are critical for Myc-driven proliferation and tumorigenesis.

Tregs are important mediators of immune tolerance to self antigens, and it has been suggested that Treg inactivation may cause autoimmune disease. Therefore, immunotherapy approaches that aim to restore or expand autoantigen-specific Treg activity might be beneficial for the treatment of autoimmune disease. Here we report that Treg-mediated suppression of autoimmune disease can be achieved in vivo by taking advantage of the ability of the liver to promote immune tolerance. Expression of the neural autoantigen myelin basic protein (MBP) in the liver was accomplished stably in liver-specific MBP transgenic mice and transiently using gene transfer to liver cells in vivo. Such ectopic MBP expression induced protection from autoimmune neuroinflammation in a mouse model of multiple sclerosis. Protection from autoimmunity was mediated by MBP-specific CD4+CD25+Foxp3+ Tregs, as demonstrated by the ability of these cells to prevent disease when adoptively transferred into nontransgenic mice and to suppress conventional CD4+CD25– T cell proliferation after antigen-specific stimulation with MBP in vitro. The generation of MBP-specific CD4+CD25+Foxp3+ Tregs in vivo depended on expression of MBP in the liver, but not in skin, and occurred by TGF-β–dependent peripheral conversion from conventional non-Tregs. Our findings indicate that autoantigen expression in the liver may generate autoantigen-specific Tregs. Thus, targeting of autoantigens to hepatocytes may be a novel approach to prevention or treatment of autoimmune diseases.

BACKGROUND: Low levels of the cyclin-dependent kinase inhibitor p27Kip1 are associated with poor prognosis in cancer. It is unclear whether this is related strictly to p27Kip1-mediated cell cycle inhibition or to other, possibly extranuclear, roles of this protein. In this study, we examined p27Kip1 expression in quiescent and activated lymphocytes. T-cell membranes have been shown to possess sphingolipid and cholesterol-rich microdomains that are insoluble in non-ionic detergents. These "rafts" provide a scaffold for signaling proteins. Signal transduction coincides with coalescence of these microdomains into larger complexes. METHODS: Localization of p27Kip1 was studied by electron and confocal microscopy. Association of p27Kip1 with membrane microdomains in unstimulated and stimulated lymphocytes was determined using Western blots analysis of isolated membranes variably treated with detergents. RESULTS: We demonstrated that p27Kip1 was present in clusters associated with the plasma membrane in normal lymphocytes. The solubility profile of p27Kip1 in isolated membranes indicated that it was localized to raft structures. When lymphocytes were stimulated, however, p27Kip1 was excluded from aggregated raft complexes. CONCLUSIONS: This study identifies, for the first time, the localization of p27 within a membrane microdomain associated with signaling. Because some cell surface signaling complexes lose p27Kip1 upon cellular activation, p27Kip1 may play a functional role in modulating membrane signaling.

Oligodendroglial cells undergo rapid transcriptional and dynamic morphological transformation in order to effectively myelinate neuronal axons. Olig1, a basic helix-loop-helix transcription factor, functions to promote the transcription of myelin-specific genes and promotes differentiation and (re)myelination. While the role for nuclear Olig1 is well established, the function for cytoplasmic Olig1 remains uncertain. We observe that translocation of Olig1 into the cytosol highly correlates with differentiation of oligodendrocytes both in vivo and in vitro. By enforcing expression of a nuclear-specific form of Olig1 into OPCs in a null-Olig1 background, we demonstrate that nuclear Olig1 is sufficient to facilitate MBP expression, but with greatly diminished membrane volume and area. We demonstrate that serine 138 in the helix-loop-helix domain of Olig1 is phosphorylated and that this form resides in the cytosol. Mutating serine 138 to alanine restricts Olig1 to the nucleus, facilitating MBP expression but limiting membrane expansion. However, a serine to aspartic acid mutation results in the cytoplasmic localization of Olig1 enhancing membrane expansion. Our results suggest a novel role for a phosphorylated cytosolic Olig1 in membrane expansion and maturation of oligodendrocytes.

Promoter elements of the mouse myelin basic protein (MBP) gene were analyzed by in vitro transcription using HeLa cell extracts. We demonstrated the MBTE (MBP transcription element), GC-box core and TATA-box elements, at -130, -93 and -34, respectively. The TATA-box was indispensable for the promoter function. The GC-box was suggested to function co-operatively with far upstream sequences including the MBTE. The MBTE was crucial to direct maximal transcription, and also functioned with a heterologous promoter irrespective of its orientation. We identified a ubiquitous binding factor which interacted specifically with the MBTE and activated transcription. Intensive foot-printing studies demonstrated that the MBTE had a NFI-binding sequence. The MBTE was considered to be one of the strongest NFI-binding motif among known cellular genes. Interestingly, similar strong NFI-binding motifs were suggested to be present in the enhancer of JC virus whose gene is expressed like the MBP gene, in the nervous system.

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During development, differentiating oligodendrocytes progress in distinct maturation steps from premyelinating to myelinating cells. Such maturing oligodendrocytes express both receptors mediating signaling via extracellular lysophosphatidic acid (LPA) and the major enzyme generating extracellular LPA, namely phosphodiesterase-Iα/autotaxin (PD-Iα/ATX). However, the biological role of extracellular LPA during the maturation of differentiating oligodendrocytes is currently unclear. Here, we demonstrate that application of exogenous LPA induced an increase in the area occupied by the oligodendrocytes’ process network, but only when PD-Iα/ATX expression was down-regulated. This increase in network area was caused primarily by the formation of membranous structures. In addition, LPA increased the number of cells positive for myelin basic protein (MBP). This effect was associated by an increase in the mRNA levels coding for MBP but not myelin oligodendrocyte glycoprotein (MOG). Taken together, these data suggest that LPA may play a crucial role in regulating the later stages of oligodendrocyte maturation.

Mechanisms coordinating neural progenitor cell cycle exit and differentiation are incompletely understood. The cyclin-dependent kinase inhibitor p27Kip1 is transcriptionally induced, switching specific neural progenitors from proliferation to differentiation. However, neuronal differentiation-specific transcription factors mediating p27Kip1 transcription have not been identified. We demonstrate the homeodomain transcription factor Phox2a, required for central nervous system (CNS)- and neural crest (NC)-derived noradrenergic neuron differentiation, coordinates cell cycle exit and differentiation by inducing p27Kip1 transcription. Phox2a transcription and activation in the CNS-derived CAD cell line and primary NC cells is mediated by combined cyclic AMP (cAMP) and bone morphogenetic protein 2 (BMP2) signaling. In the CAD cellular model, cAMP and BMP2 signaling initially induces proliferation of the undifferentiated precursors, followed by p27Kip1 transcription, G1 arrest, and neuronal differentiation. Small interfering RNA silencing of either Phox2a or p27Kip1 suppresses p27Kip1 transcription and neuronal differentiation, suggesting a causal link between p27Kip1 expression and differentiation. Conversely, ectopic Phox2a expression via the Tet-off expression system promotes accelerated CAD cell neuronal differentiation and p27Kip1 transcription only in the presence of cAMP signaling. Importantly, endogenous or ectopically expressed Phox2a activated by cAMP signaling binds homeodomain cis-acting elements of the p27Kip1 promoter in vivo and mediates p27Kip1-luciferase expression in CAD and NC cells. We conclude that developmental cues of cAMP signaling causally link Phox2a activation with p27Kip1 transcription, thereby coordinating neural progenitor cell cycle exit and differentiation.

p27kip1 regulates cell proliferation by binding to and inhibiting the activity of cyclin-dependent kinases and its expression oscillates with cell cycle. Recently, it has been suggested from studies using the traditional dicistronic DNA assay that the expression of p27kip1 is regulated by internal ribosome entry site (IRES)-mediated translation initiation, and several RNA-binding protein factors were thought to play some role in this regulation. Considering the inevitable drawbacks of the dicistronic DNA assay, which could mislead a promoter activity or alternative splicing to IRES as previously demonstrated, we decided to reanalyze the 5′-untranslated region (5′-UTR) sequence of p27kip1 and test whether it contains an IRES element or a promoter using more stringent methods, such as dicistronic RNA and promoterless dicistronic and monocistronic DNA assays. We found that the 5′-UTR sequence of human p27kip1 does not have any significant IRES activity. The previously observed IRES activities are likely generated from the promoter activities present in the 5′-UTR sequences of p27kip1. The findings in this study indicate that transcriptional regulation likely plays an important role in p27kip1 expression, and the mechanism of regulation of p27 expression by RNA-binding factors needs to be re-examined. The findings in this study also further enforce the importance that more stringent studies, such as promoterless dicistronic and monocistronic DNA and dicistronic RNA tests, are required to safeguard any future claims of cellular IRES.

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

Transcription of the myelin basic protein (MBP) gene is regulated in a cell-type-specific and developmental stage-specific manner during myelin formation in the murine central nervous system. The 5'-flanking region of the MBP gene contains several regulatory elements that differentially contribute to the cell-type-specific transcription of MBP in cells derived from the central nervous system. The proximal element, termed MB1, which is located between nucleotides -14 and -50 with respect to the RNA start site, has previously been shown to have characteristics of a cell-type-specific enhancer element. In this study, we used band shift and UV cross-linking assays to identify DNA-binding proteins in mouse brain nuclear extract which interact with the MB1 element. Fractionation of these extracts has allowed the identification of a 38- to 41-kDa nuclear protein, derived from mouse brain tissue at the peak of myelination, which specifically binds the MB1 DNA sequence. Fractions enriched in the MB1-binding protein have been shown to stimulate transcription of the MBP promoter in extract derived from HeLa cells. MB1 binding protein activity is expressed in a tissue-specific and development stage-specific pattern which coincides with the pattern of MBP transcription, suggesting that this protein may be a biologically relevant transcription factor for the MBP gene in vivo.

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The MB1 regulatory sequence of the myelin basic protein (MBP) gene spanning between nucleotides -14 to -50 with respect to the transcription start site is critical for cell type-specific transcription of the MBP gene, which encodes the major protein component of myelin sheath in cells derived from the central nervous system (CNS). This regulatory sequence has the ability to interact with a developmentally controlled DNA-binding protein from mouse brain that stimulates transcription of MBP promoter in an in vitro system (Haas, S., J. Gordon, and K. Khalili. 1993. Mol. Cell. Biol. 13:3103-3112). Here, we report the purification of a 39-kD protein from mouse brain tissue at the peak of myelination and MBP production that binds to the MB1 regulatory motif. Following partial amino acid sequence analysis, we have identified a complementary DNA encoding a 39-kD DNA-binding protein called pur alpha. Expression of pur alpha cDNA in the prokaryotic and eukaryotic cells resulted in the synthesis of a protein with characteristics similar to the purified brain-derived 39-kD protein in band shift competition assays. Cotransfection of the recombinant pur alpha expressor plasmid with MBP promoter construct indicated that Pur alpha stimulates transcription of the MBP promoter in oligodendrocytic cells, and that the nucleotide sequence required for binding of the 39-kD Pur alpha to DNA within the MB1 region is crucial for this activity. Moreover, transient expression of Pur alpha caused elevation in the level of endogenous MBP RNA in oligodendrocytic cells. Thus, Pur alpha, a sequence-specific DNA-binding protein upon binding to MB1 regulatory region may play a significant role in determining the cell type-specific expression of MBP in brain.

The myelin basic proteins (MBPs) mediate the cytoplasmic apposition of the oligodendrocyte plasma membrane to form the major dense line of central nervous system myelin. Four major isoforms of murine MBP, obtained by alternative splicing of seven exons from a single primary transcript, display distinct developmental profiles. We expressed these major MBPs individually in HeLa cells and mapped their distributions by immunofluorescence and confocal microscopy. The 14- and 18.5-kD MBPs that are the predominant forms in compact myelin distributed primarily in the perinuclear regions of the cell in configurations highly suggestive of close association with membranes. We infer that these MBP isoforms possess strong, nonspecific membrane-binding properties that have been adapted by the oligodendrocyte to mediate compaction of the sheaths of plasma membrane that form myelin. In contrast, the 17- and 21.5-kD isoforms distributed diffusely in both the cytoplasm and the nucleoplasm and often accumulated within the nucleus. This distribution can be correlated with the presence of the peptide segment encoded by exon II, which is unique to these isoforms. The physiological significance of the nuclear targeting displayed by the 17- and 21.5-kD MBP isoforms in HeLa cells remains to be determined.

Both in vivo and in vitro studies indicate a correlation between reduced acetylation of histone core proteins and oligodendrocyte development. The nature of these histone modifications and the mechanisms mediating them remain undefined. To address these issues we utilized OL-1 cells, a rat non-transformed oligodendrocyte cell line, and primary oligodendrocyte cultures. We found that the acetylated histone H3 at lysine 9 and lysine 14 (H3K9/K14ac) is reduced in both the myelin basic protein (MBP) and proteolipid protein (PLP) genes of maturing oligodendroglial OL-1 cells, and furthermore, this temporally correlates with increases in MBP, PLP, and histone deacetylase (HDAC) 11 expression. Disruption of developmentally-regulated histone H3 deacetylation within the MBP and PLP genes by the HDAC inhibitor trichostatin A blunts MBP and PLP expression. With its increased expression, interaction of HDAC 11 with acetylated histone H3 and recruitment of HDAC 11 to the MBP and PLP genes markedly increases in maturing OL-1 cells. Moreover, suppressing HDAC 11 expression with small interfering RNA significantly: 1) increases H3K9/K14ac globally and within the MBP and PLP genes, 2) decreases MBP and PLP mRNA expression, and 3) blunts the morphological changes associated with oligodendrocyte development. Our data strongly support a specific role for HDAC 11 in histone deacetylation and in turn the regulation of oligodendrocyte-specific protein gene expression and oligodendrocyte development.