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Constitutive heterochromatin undergoes a dynamic clustering and spatial reorganization during myogenic differentiation. However the detailed mechanisms and its role in cell differentiation remain largely elusive. Here, we report the identification of a muscle-specific long non-coding RNA, ChRO1, involved in constitutive heterochromatin reorganization. ChRO1 is induced during terminal differentiation of myoblasts, and is specifically localized to the chromocenters in myotubes. ChRO1 is required for efficient cell differentiation, with global impacts on gene expression. It influences DNA methylation and chromatin compaction at peri/centromeric regions. Inhibition of ChRO1 leads to defects in the spatial fusion of chromocenters, and mislocalization of H4K20 trimethylation, Suv420H2, HP1, MeCP2 and cohesin. In particular, ChRO1 specifically associates with ATRX/DAXX/H3.3 complex at chromocenters to promote H3.3 incorporation and transcriptional induction of satellite repeats, which is essential for chromocenter clustering. Thus, our results unveil a mechanism involving a lncRNA that plays a role in large-scale heterochromatin reorganization and cell differentiation.

Cyclin-dependent kinase 1 (Cdk1) is indispensable for embryonic stem cell (ESC) maintenance and embryo development. Even though some reports have described a connection between Cdk1 and Oct4, there is no evidence that Cdk1 activity is directly linked to the ESC pluripotency transcription program. We recently reported that Aurkb/PP1-mediated Oct4 resetting is important to cell cycle maintenance and pluripotency in mouse ESCs (mESCs). In this study, we show that Cdk1 is an upstream regulator of the Oct4 phosphorylation state during cell cycle progression, and it coordinates the chromatin associated state of Oct4 for pluripotency-related gene expression within the cell cycle. Upon entry into mitosis, Aurkb in the chromosome passenger complex becomes fully activated and PP1 activity is inhibited downstream of Cdk1 activation, leading to sustaining Oct4(S229) phosphorylation and dissociation of Oct4 from chromatin during the mitotic phase. Cdk1 inhibition at the mitotic phase abnormally results in Oct4 dephosphorylation, chromosome decondensation and chromatin association of Oct4, even in replicated chromosome. Our study results suggest a molecular mechanism by which Cdk1 directly links the cell cycle to the pluripotency transcription program in mESCs.

Post-translational modifications of core histones affect various cellular processes, primarily through transcription. However, their relationship with the termination of transcription has remained largely unknown. In this study, we show that DNA damage-activated AKT phosphorylates threonine 45 of core histone H3 (H3-T45). By genome-wide chromatin immunoprecipitation sequencing (ChIP-seq) analysis, H3-T45 phosphorylation was distributed throughout DNA damage-responsive gene loci, particularly immediately after the transcription termination site. H3-T45 phosphorylation pattern showed close-resemblance to that of RNA polymerase II C-terminal domain (CTD) serine 2 phosphorylation, which establishes the transcription termination signal. AKT1 was more effective than AKT2 in phosphorylating H3-T45. Blocking H3-T45 phosphorylation by inhibiting AKT or through amino acid substitution limited RNA decay downstream of mRNA cleavage sites and decreased RNA polymerase II release from chromatin. Our findings suggest that AKT-mediated phosphorylation of H3-T45 regulates the processing of the 3′ end of DNA damage-activated genes to facilitate transcriptional termination.

Transcription factors and chromatin remodeling proteins control the transcriptional variability for ESC lineage commitment. During ESC differentiation, chromatin modifiers are recruited to the regulatory regions by transcription factors, thereby activating the lineage-specific genes or silencing the transcription of active ESC genes. However, the underlying mechanisms that link transcription factors to exit from pluripotency are yet to be identified. In this study, we show that the Ctbp2-interacting zinc finger proteins, Zfp217 and Zfp516, function as linkers for the chromatin regulators during ESC differentiation. CRISPR-Cas9-mediated knock-outs of both Zfp217 and Zfp516 in ESCs prevent the exit from pluripotency. Both zinc finger proteins regulate the Ctbp2-mediated recruitment of the NuRD complex and polycomb repressive complex 2 (PRC2) to active ESC genes, subsequently switching the H3K27ac to H3K27me3 during ESC differentiation for active gene silencing. We therefore suggest that some zinc finger proteins orchestrate to control the concise epigenetic states on active ESC genes during differentiation, resulting in natural lineage commitment.

Myocyte enhancer factor 2 (MEF2) is a family of transcription factors that regulates many processes, including muscle differentiation. Due to its many target genes, MEF2D requires tight regulation of transcription activity over time and by location. Epigenetic modifiers have been suggested to regulate MEF2-dependent transcription via modifications to histones and MEF2. However, the modulation of MEF2 activity by lysine methylation, an important posttranslational modification that alters the activities of transcription factors, has not been studied. We report the reversible lysine methylation of MEF2D by G9a and LSD1 as a regulatory mechanism of MEF2D activity and skeletal muscle differentiation. G9a methylates lysine-267 of MEF2D and represses its transcriptional activity, but LSD1 counteracts it. This residue is highly conserved between MEF2 members in mammals. During myogenic differentiation of C2C12 mouse skeletal muscle cells, the methylation of MEF2D by G9a decreased, on which MEF2D-dependent myogenic genes were upregulated. We have also identified lysine-267 as a methylation/demethylation site and demonstrate that the lysine methylation state of MEF2D regulates its transcriptional activity and skeletal muscle cell differentiation.

The mammalian genome encodes multiple variants of histone H3 including H3.1/H3.2 and H3.3. In contrast to H3.1/H3.2, H3.3 is enriched in the actively transcribed euchromatin and the telomeric heterochromatins. However, the mechanism for H3.3 to incorporate into the different domains of chromatin is not known. Here, taking the advantage of well-defined transcription analysis system of yeast, we attempted to understand the molecular mechanism of selective deposition of human H3.3 into actively transcribed genes. We show that there are systemic H3 substrate-selection mechanisms operating even in yeasts, which encode a single type of H3. Yeast HIR complex mediated H3-specific recognition specificity for deposition of H3.3 in the transcribed genes. A critical component of this process was the H3 A-IG code composed of amino acids 87, 89 and 90. The preference toward H3.3 was completely lost when HIR subunits were absent and partially suppressed by human HIRA. Asf1 allows the influx of H3, regardless of H3 type. We propose that H3.3 is introduced into the active euchromatin by targeting the recycling pathway that is mediated by HIRA (or HIR), and this H3-selection mechanism is highly conserved through the evolution. These results also uncover an unexpected role of RI chaperones in evolution of variant H3s.

CABIN1 acts as a negative regulator of p53 by keeping p53 in an inactive state on chromatin. Genotoxic stress causes rapid dissociation of CABIN1 and activation of p53. However, its molecular mechanism is still unknown. Here, we reveal the phosphorylation- and ubiquitination-dependent degradation of CABIN1 upon DNA damage, releasing p53 for transcriptional activation. The DNA-damage-signaling kinases, ATM and CHK2, phosphorylate CABIN1 and increase the degradation of CABIN1 protein. Knockdown or overexpression of these kinases influences the stability of CABIN1 protein showing that their activity is critical for degradation of CABIN1. Additionally, CABIN1 was found to undergo ubiquitin-dependent proteasomal degradation mediated by the CRL4DDB2 ubiquitin ligase complex. Both phosphorylation and ubiquitination of CABIN1 appear to be relevant for controlling the level of CABIN1 protein upon genotoxic stress.

Parafibromin, a component of the RNA polymerase II-associated PAF1 complex, is a tumor suppressor linked to hyperparathyroidism-jaw tumor syndrome and sporadic parathyroid carcinoma. Parafibromin induces cell cycle arrest by repressing cyclin D1 via an unknown mechanism. Here, we show that parafibromin interacts with the histone methyltransferase, SUV39H1, and functions as a transcriptional repressor. The central region (128-227 amino acids) of parafibromin is important for both the interaction with SUV39H1 and transcriptional repression. Parafibromin associated with the promoter and coding regions of cyclin D1 and was required for the recruitment of SUV39H1 and the induction of H3 K9 methylation but not H3 K4 methylation. RNA interference analysis showed that SUV39H1 was critical for cyclin D1 repression. These data suggest that parafibromin plays an unexpected role as a repressor in addition to its widely known activity associated with transcriptional activation. Parafibromin as a part of the PAF1 complex might downregulate cyclin D1 expression by integrating repressive H3 K9 methylation during transcription.