Ctbp2-mediated β-catenin regulation is required for exit from pluripotency

The canonical Wnt pathway is critical for embryonic stem cell (ESC) pluripotency and aberrant control of β-catenin leads to failure of exit from pluripotency and lineage commitments. Hence, maintaining the appropriate level of β-catenin is important for the decision to commit to the appropriate lineage. However, how β-catenin links to core transcription factors in ESCs remains elusive. C-terminal-binding protein (CtBP) in Drosophila is essential for Wnt-mediated target gene expression. In addition, Ctbp acts as an antagonist of β-catenin/TCF activation in mammals. Recently, Ctbp2, a core Oct4-binding protein in ESCs, has been reported to play a key role in ESC pluripotency. However, the significance of the connection between Ctbp2 and β-catenin with regard to ESC pluripotency remains elusive. Here, we demonstrate that C-terminal-binding protein 2 (Ctbp2) associates with major components of the β-catenin destruction complex and limits the accessibility of β-catenin to core transcription factors in undifferentiated ESCs. Ctbp2 knockdown leads to stabilization of β-catenin, which then interacts with core pluripotency-maintaining factors that are occupied by Ctbp2, leading to incomplete exit from pluripotency. These findings suggest a suppressive function for Ctbp2 in reducing the protein level of β-catenin, along with priming its position on core pluripotency genes to hinder β-catenin deposition, which is central to commitment to the appropriate lineage.

Methylation and demethylation of DNA and histones in chromatin: the most complicated epigenetic marker

No abstract available.

Differential regulation of the histone chaperone HIRA during muscle cell differentiation by a phosphorylation switch

Replication-independent incorporation of variant histone H3.3 has a profound impact on chromatin function and numerous cellular processes, including the differentiation of muscle cells. The histone chaperone HIRA and H3.3 have essential roles in MyoD regulation during myoblast differentiation. However, the precise mechanism that determines the onset of H3.3 deposition in response to differentiation signals is unclear. Here we show that HIRA is phosphorylated by Akt kinase, an important signaling modulator in muscle cells. By generating a phosphospecific antibody, we found that a significant amount of HIRA was phosphorylated in myoblasts. The phosphorylation level of HIRA and the occupancy of phosphorylated protein on muscle genes gradually decreased during cellular differentiation. Remarkably, the forced expression of the phosphomimic form of HIRA resulted in reduced H3.3 deposition and suppressed the activation of muscle genes in myotubes. Our data show that HIRA phosphorylation limits the expression of myogenic genes, while the dephosphorylation of HIRA is required for proficient H3.3 deposition and gene activation, demonstrating that the phosphorylation switch is exploited to modulate HIRA/H3.3-mediated muscle gene regulation during myogenesis.

A phosphorylation pattern-recognizing antibody specifically reacts to RNA polymerase II bound to exons

The C-terminal domain of RNA polymerase II is an unusual series of repeated residues appended to the C-terminus of the largest subunit and serves as a flexible binding scaffold for numerous nuclear factors. The binding of these factors is determined by the phosphorylation patterns on the repeats in the domain. In this study, we generated a synthetic antibody library by replacing the third heavy chain complementarity-determining region of an anti-HER2 (human epidermal growth factor receptor 2) antibody (trastuzumab) with artificial sequences of 7–18 amino-acid residues. From this library, antibodies were selected that were specific to serine phosphopeptides that represent typical phosphorylation patterns on the functional unit (YSPTSPS)₂ of the RNA polymerase II C-terminal domain (CTD). Antibody clones pCTD-1stS2 and pCTD-2ndS2 showed specificity for peptides with phosphoserine at the second residues of the first or second heptamer repeat, respectively. Additional clones specifically reacted to peptides with phosphoserine at the fifth serine of the first repeat (pCTD-1stS5), the seventh residue of the first repeat and fifth residue of the second repeat (pCTD-S7S5) or the seventh residue of either the first or second repeat (pCTD-S7). All of these antibody clones successfully reacted to RNA polymerase II in immunoblot analysis. Interestingly, pCTD-2ndS2 precipitated predominately RNA polymerase II from the exonic regions of genes in genome-wide chromatin immunoprecipitation sequencing analysis, which suggests that the phosphoserine at the second residue of the second repeat of the functional unit (YSPTSPS)2 is a mediator of exon definition.

Down syndrome critical region 1 enhances the proteolytic cleavage of calcineurin

Down syndrome critical region 1 (DSCR1), an oxidative stress-response gene, interacts with calcineurin and represses its phosphatase activity. Recently it was shown that hydrogen peroxide inactivates calcineurin by proteolytic cleavage. Based on these facts, we investigated whether oxidative stress affects DSCR1-mediated inactivation of calcineurin. We determined that overexpression of DSCR1 leads to increased proteolytic cleavage of calcineurin. Convertsely, knockdown of DSCR1 abolished calcineurin cleavage upon treatment with hydrogen peroxide. The PXIIXT motif in the COOH-terminus of DSCR1 is responsible for both binding and cleavage of calcineurin. The knockdown of overexpressed DSCR1 in DS fibroblast cells also abrogated calcineurin proteolysis by hydrogen peroxide. These results suggest that DSCR1 has the ability to inactivate calcineurin by inducing proteolytic cleavage of calcineurin upon oxidative stress.

Nur77 upregulates HIF-alpha by inhibiting pVHL-mediated degradation

In this study, we investigated the role of Nur77, an orphan nuclear receptor, in HIF-alpha transcriptional activity. We found that Nur77 associates and stabilizes HIF-1alpha via indirect interaction. Nur77 was found to interact with pVHL in vivo via the alpha-domain of pVHL. By binding to pVHL, Nur77 competed with elongin C for pVHL binding. Moreover, Nur77-binding to pVHL inhibited the pVHL-mediated ubiquitination of HIF-1alpha and ultimately increased the stability and transcriptional activity of HIF-1alpha. The ligand-binding domain of Nur77 was found to interact with pVHL and the expression of this ligand-binding domain was sufficient to stabilize and transactivate HIF-1alpha. Under the conditions that cobalt chloride was treated or pVHL was knocked down, Nur77 could not stabilize HIF-alpha. Moreover, Nur77 could not further stabilize HIF-2alpha in A498/VHL stable cells, which is consistent with our finding that Nur77 indirectly stabilizes HIF-alpha by binding to pVHL. Thus, our results suggest that an orphan nuclear receptor Nur77 binds to pVHL, thereby stabilizes and increases HIF-alpha transcriptional activity under the non- hypoxic conditions.

Menin represses JunD transcriptional activity in protein kinase Ctheta-mediated Nur77 expression

TCR signaling leading to thymocyte apoptosis is mediated through the expression of the Nur77 family of orphan nuclear receptors. It has been shown that the Nur77 promoter is activated by at least two signaling pathways, one mediated by calcium and the other by protein kinase C (PKC). MEF2D has been known to regulate Nur77 expression in a calcium- dependent manner. The mechanism by which calcium regulates MEF2D is through dissociation of calcium-sensitive MEF2 corepressors (Cabin1/ HDACs, HDAC4/5) and the association with calcineurin-activated transcription factor NF-AT and the coactivator p300. However, little is known about how PKC activates the Nur77 promoter. Herein, we report that PKC theta targets AP-1 like response element in the Nur77 promoter where JunD constitutively binds. PKC theta triggers mitogen-activated protein kinase- inediated phosphorylation of JunD, and increases transcriptional activity of JunD, cooperatively with p300. Menin is identified as the transcriptional corepressor for JunD via recruitment of mSin3-istone deacetylases. In fact, Menin represses PKC theta/ p300-mediated transcriptional activity of JunD in T cell. Its dynamic regulation of histone modifiers with JunD is responsible for PKCq-synergistic effect on Nur77 expression in T cell.