Mitotic bookmarking redundancy by nuclear receptors in pluripotent cells.

Mitotic bookmarking transcription factors (TFs) are thought to mediate rapid and accurate reactivation after mitotic gene silencing. However, the loss of individual bookmarking TFs often leads to the deregulation of only a small proportion of their mitotic targets, raising doubts on the biological significance and importance of their bookmarking function. Here we used targeted proteomics of the mitotic bookmarking TF ESRRB, an orphan nuclear receptor, to discover a large redundancy in mitotic binding among members of the protein super-family of nuclear receptors. Focusing on the nuclear receptor NR5A2, which together with ESRRB is essential in maintaining pluripotency in mouse embryonic stem cells, we demonstrate conjoint bookmarking activity of both factors on promoters and enhancers of a large fraction of active genes, particularly those most efficiently reactivated in G1. Upon fast and simultaneous degradation of both factors during mitotic exit, hundreds of mitotic targets of ESRRB/NR5A2, including key players of the pluripotency network, display attenuated transcriptional reactivation. We propose that redundancy in mitotic bookmarking TFs, especially nuclear receptors, confers robustness to the reestablishment of gene regulatory networks after mitosis.

A gene subset requires CTCF bookmarking during the fast post-mitotic reactivation of mouse ES cells.

Mitosis leads to global downregulation of transcription that then needs to be efficiently resumed. In somatic cells, this is mediated by a transient hyper-active state that first reactivates housekeeping and then cell identity genes. Here, we show that mouse embryonic stem cells, which display rapid cell cycles and spend little time in G1, also display accelerated reactivation dynamics. This uniquely fast global reactivation lacks specificity towards functional gene families, enabling the restoration of all regulatory functions before DNA replication. Genes displaying the fastest reactivation are bound by CTCF, a mitotic bookmarking transcription factor. In spite of this, the post-mitotic global burst is robust and largely insensitive to CTCF depletion. There are, however, around 350 genes that respond to CTCF depletion rapidly after mitotic exit. Remarkably, these are characterised by promoter-proximal mitotic bookmarking by CTCF. We propose that the structure of the cell cycle imposes distinct constrains to post-mitotic gene reactivation dynamics in different cell types, via mechanisms that are yet to be identified but that can be modulated by mitotic bookmarking factors.

H3K9 tri-methylation at Nanog times differentiation commitment and enables the acquisition of primitive endoderm fate.

Mouse embryonic stem cells have an inherent propensity to explore gene regulatory states associated with either self-renewal or differentiation. This property depends on ERK, which downregulates pluripotency genes such as Nanog. Here, we aimed at identifying repressive histone modifications that would mark Nanog for inactivation in response to ERK activity. We found that the transcription factor ZFP57, which binds methylated DNA to nucleate heterochromatin, is recruited upstream of Nanog, within a region enriched for histone H3 lysine 9 tri-methylation (H3K9me3). Whereas before differentiation H3K9me3 at Nanog depends on ERK, in somatic cells it becomes independent of ERK. Moreover, the loss of H3K9me3 at Nanog, induced by deleting the region or by knocking out DNA methyltransferases or Zfp57, is associated with reduced heterogeneity of NANOG, delayed commitment into differentiation and impaired ability to acquire a primitive endoderm fate. Hence, a network axis centred on DNA methylation, ZFP57 and H3K9me3 links Nanog regulation to ERK activity for the timely establishment of new cell identities. We suggest that establishment of irreversible H3K9me3 at specific master regulators allows the acquisition of particular cell fates during differentiation.

OCT4 activates a Suv39h1-repressive antisense lncRNA to couple histone H3 Lysine 9 methylation to pluripotency.

Histone H3 Lysine 9 (H3K9) methylation, a characteristic mark of heterochromatin, is progressively implemented during development to contribute to cell fate restriction as differentiation proceeds. Accordingly, in undifferentiated and pluripotent mouse Embryonic Stem (ES) cells the global levels of H3K9 methylation are rather low and increase only upon differentiation. How global H3K9 methylation levels are coupled with the loss of pluripotency remains largely unknown. Here, we identify SUV39H1, a major H3K9 di- and tri-methylase, as an indirect target of the pluripotency network of Transcription Factors (TFs). We find that pluripotency TFs, principally OCT4, activate the expression of Suv39h1as, an antisense long non-coding RNA to Suv39h1. In turn, Suv39h1as downregulates Suv39h1 transcription in cis via a mechanism involving the modulation of the chromatin status of the locus. The targeted deletion of the Suv39h1as promoter region triggers increased SUV39H1 expression and H3K9me2 and H3K9me3 levels, affecting all heterochromatic regions, particularly peri-centromeric major satellites and retrotransposons. This increase in heterochromatinization efficiency leads to accelerated and more efficient commitment into differentiation. We report, therefore, a simple genetic circuitry coupling the genetic control of pluripotency with the global efficiency of H3K9 methylation associated with a major cell fate restriction, the irreversible loss of pluripotency.

The combined action of Esrrb and Nr5a2 is essential for murine naïve pluripotency.

The maintenance of pluripotency in mouse embryonic stem cells (ESCs) is governed by the action of an interconnected network of transcription factors. Among them, only Oct4 and Sox2 have been shown to be strictly required for the self-renewal of ESCs and pluripotency, particularly in culture conditions in which differentiation cues are chemically inhibited. Here, we report that the conjunct activity of two orphan nuclear receptors, Esrrb and Nr5a2, parallels the importance of that of Oct4 and Sox2 in naïve mouse ESCs. By occupying a large common set of regulatory elements, these two factors control the binding of Oct4, Sox2 and Nanog to DNA. Consequently, in their absence the pluripotency network collapses and the transcriptome is substantially deregulated, leading to the differentiation of ESCs. Altogether, this work identifies orphan nuclear receptors, previously thought to be performing supportive functions, as a set of core regulators of naïve pluripotency.

CTCF confers local nucleosome resiliency after DNA replication and during mitosis.

The access of Transcription Factors (TFs) to their cognate DNA binding motifs requires a precise control over nucleosome positioning. This is especially important following DNA replication and during mitosis, both resulting in profound changes in nucleosome organization over TF binding regions. Using mouse Embryonic Stem (ES) cells, we show that the TF CTCF displaces nucleosomes from its binding site and locally organizes large and phased nucleosomal arrays, not only in interphase steady-state but also immediately after replication and during mitosis. Correlative analyses suggest this is associated with fast gene reactivation following replication and mitosis. While regions bound by other TFs (Oct4/Sox2), display major rearrangement, the post-replication and mitotic nucleosome positioning activity of CTCF is not unique: Esrrb binding regions are also characterized by persistent nucleosome positioning. Therefore, selected TFs such as CTCF and Esrrb act as resilient TFs governing the inheritance of nucleosome positioning at regulatory regions throughout the cell-cycle.

The molecular logic of Nanog-induced self-renewal in mouse embryonic stem cells.

Transcription factor networks, together with histone modifications and signalling pathways, underlie the establishment and maintenance of gene regulatory architectures associated with the molecular identity of each cell type. However, how master transcription factors individually impact the epigenomic landscape and orchestrate the behaviour of regulatory networks under different environmental constraints is only partially understood. Here, we show that the transcription factor Nanog deploys multiple distinct mechanisms to enhance embryonic stem cell self-renewal. In the presence of LIF, which fosters self-renewal, Nanog rewires the pluripotency network by promoting chromatin accessibility and binding of other pluripotency factors to thousands of enhancers. In the absence of LIF, Nanog blocks differentiation by sustaining H3K27me3, a repressive histone mark, at developmental regulators. Among those, we show that the repression of Otx2 plays a preponderant role. Our results underscore the versatility of master transcription factors, such as Nanog, to globally influence gene regulation during developmental processes.

Transcription factor activity and nucleosome organization in mitosis.

Mitotic bookmarking transcription factors (BFs) maintain the capacity to bind to their targets during mitosis, despite major rearrangements of the chromatin. While they were thought to propagate gene regulatory information through mitosis by statically occupying their DNA targets, it has recently become clear that BFs are highly dynamic in mitotic cells. This represents both a technical and a conceptual challenge to study and understand the function of BFs: First, formaldehyde has been suggested to be unable to efficiently capture these transient interactions, leading to profound contradictions in the literature; and second, if BFs are not permanently bound to their targets during mitosis, it becomes unclear how they convey regulatory information to daughter cells. Here, comparing formaldehyde to alternative fixatives we clarify the nature of the chromosomal association of previously proposed BFs in embryonic stem cells: While ESRRB can be considered as a canonical BF that binds at selected regulatory regions in mitosis, SOX2 and POU5F1 (also known as OCT4) establish DNA sequence-independent interactions with the mitotic chromosomes, either throughout the chromosomal arms (SOX2) or at pericentromeric regions (POU5F1). Moreover, we show that ordered nucleosomal arrays are retained during mitosis at ESRRB bookmarked sites, whereas regions losing transcription factor binding display a profound loss of order. By maintaining nucleosome positioning during mitosis, ESRRB might ensure the rapid post-mitotic re-establishment of functional regulatory complexes at selected enhancers and promoters. Our results provide a mechanistic framework that reconciles dynamic mitotic binding with the transmission of gene regulatory information across cell division.

Mitotic binding of Esrrb marks key regulatory regions of the pluripotency network.

Pluripotent mouse embryonic stem cells maintain their identity throughout virtually infinite cell divisions. This phenomenon, referred to as self-renewal, depends on a network of sequence-specific transcription factors (TFs) and requires daughter cells to accurately reproduce the gene expression pattern of the mother. However, dramatic chromosomal changes take place in mitosis, generally leading to the eviction of TFs from chromatin. Here, we report that Esrrb, a major pluripotency TF, remains bound to key regulatory regions during mitosis. We show that mitotic Esrrb binding is highly dynamic, driven by specific recognition of its DNA-binding motif and is associated with early transcriptional activation of target genes after completion of mitosis. These results indicate that Esrrb may act as a mitotic bookmarking factor, opening another perspective to molecularly understand the role of sequence-specific TFs in the epigenetic control of self-renewal, pluripotency and genome reprogramming.

A rapid passage through a two-active-X-chromosome state accompanies the switch of imprinted X-inactivation patterns in mouse trophoblast stem cells.

BACKGROUND: In female mice, while the presence of two-active X-chromosomes characterises pluripotency, it is not tolerated in most other cellular contexts. In particular, in the trophoblastic lineage, impairment of paternal X (X(P)) inactivation results in placental defects. RESULTS: Here, we show that Trophoblast Stem (TS) cells can undergo a complete reversal of imprinted X-inactivation without detectable change in cell-type identity. This reversal occurs through a reactivation of the X(P) leading to TS clones showing two active Xs. Intriguingly, within such clones, all the cells rapidly and homogeneously either re-inactivate the X(P) or inactivate, de novo, the X(M). CONCLUSION: This secondary non-random inactivation suggests that the two-active-X states in TS and in pluripotent contexts are epigenetically distinct. These observations also reveal a pronounced plasticity of the TS epigenome allowing TS cells to dramatically and accurately reprogram gene expression profiles. This plasticity may serve as a back-up system when X-linked mono-allelic gene expression is perturbed.

Lineage-specific regulation of imprinted X inactivation in extraembryonic endoderm stem cells.

BACKGROUND: Silencing of the paternal X chromosome (Xp), a phenomenon known as imprinted X-chromosome inactivation (I-XCI), characterises, amongst mouse extraembryonic lineages, the primitive endoderm and the extraembryonic endoderm (XEN) stem cells derived from it. RESULTS: Using a combination of chromatin immunoprecipitation characterisation of histone modifications and single-cell expression studies, we show that whilst the Xp in XEN cells, like the inactive X chromosome in other cell types, globally accumulates the repressive histone mark H3K27me3, a large number of Xp genes locally lack H3K27me3 and escape from I-XCI. In most cases this escape is specific to the XEN cell lineage. Importantly, the degree of escape and the genes concerned remain unchanged upon XEN conversion into visceral endoderm, suggesting stringent control of I-XCI in XEN derivatives. Surprisingly, chemical inhibition of EZH2, a member of the Polycomb repressive complex 2 (PRC2), and subsequent loss of H3K27me3 on the Xp, do not drastically perturb the pattern of silencing of Xp genes in XEN cells. CONCLUSIONS: The observations that we report here suggest that the maintenance of gene expression profiles of the inactive Xp in XEN cells involves a tissue-specific mechanism that acts partly independently of PRC2 catalytic activity.

Spontaneous reactivation of clusters of X-linked genes is associated with the plasticity of X-inactivation in mouse trophoblast stem cells.

Random epigenetic silencing of the X-chromosome in somatic tissues of female mammals equalizes the dosage of X-linked genes between the sexes. Unlike this form of X-inactivation that is essentially irreversible, the imprinted inactivation of the paternal X, which characterizes mouse extra-embryonic tissues, appears highly unstable in the trophoblast giant cells of the placenta. Here, we wished to determine whether such instability is already present in placental progenitor cells prior to differentiation toward lineage-specific cell types. To this end, we analyzed the behavior of a GFP transgene on the paternal X both in vivo and in trophoblast stem (TS) cells derived from the trophectoderm of XX(GFP) blastocysts. Using single-cell studies, we show that not only the GFP transgene but also a large number of endogenous genes on the paternal X are subject to orchestrated cycles of reactivation/de novo inactivation in placental progenitor cells. This reversal of silencing is associated with local losses of histone H3 lysine 27 trimethylation extending over several adjacent genes and with the topological relocation of the hypomethylated loci outside of the nuclear compartment of the inactive X. The "reactivated" state is maintained through several cell divisions. Our study suggests that this type of "metastable epigenetic" states may underlie the plasticity of TS cells and predispose specific genes to relaxed regulation in specific subtypes of placental cells.

Molecular coupling of Tsix regulation and pluripotency.

The reprogramming of X-chromosome inactivation during the acquisition of pluripotency in vivo and in vitro is accompanied by the repression of Xist, the trigger of X-inactivation, and the upregulation of its antisense counterpart Tsix. We have shown that key factors supporting pluripotency-Nanog, Oct4 and Sox2-bind within Xist intron 1 in undifferentiated embryonic stem cells (ESC) to repress Xist transcription. However, the relationship between transcription factors of the pluripotency network and Tsix regulation has remained unclear. Here we show that Tsix upregulation in embryonic stem cells depends on the recruitment of the pluripotent marker Rex1, and of the reprogramming-associated factors Klf4 and c-Myc, by the DXPas34 minisatellite associated with the Tsix promoter. Upon deletion of DXPas34, binding of the three factors is abrogated and the transcriptional machinery is no longer efficiently recruited to the Tsix promoter. Additional analyses including knockdown experiments further demonstrate that Rex1 is critically important for efficient transcription elongation of Tsix. Hence, distinct embryonic-stem-cell-specific complexes couple X-inactivation reprogramming and pluripotency, with Nanog, Oct4 and Sox2 repressing Xist to facilitate the reactivation of the inactive X, and Klf4, c-Myc and Rex1 activating Tsix to remodel Xist chromatin and ensure random X-inactivation upon differentiation. The holistic pattern of Xist/Tsix regulation by pluripotent factors that we have identified suggests a general direct governance of complex epigenetic processes by the machinery dedicated to pluripotency.

2-D structure of the A region of Xist RNA and its implication for PRC2 association.

In placental mammals, inactivation of one of the X chromosomes in female cells ensures sex chromosome dosage compensation. The 17 kb non-coding Xist RNA is crucial to this process and accumulates on the future inactive X chromosome. The most conserved Xist RNA region, the A region, contains eight or nine repeats separated by U-rich spacers. It is implicated in the recruitment of late inactivated X genes to the silencing compartment and likely in the recruitment of complex PRC2. Little is known about the structure of the A region and more generally about Xist RNA structure. Knowledge of its structure is restricted to an NMR study of a single A repeat element. Our study is the first experimental analysis of the structure of the entire A region in solution. By the use of chemical and enzymatic probes and FRET experiments, using oligonucleotides carrying fluorescent dyes, we resolved problems linked to sequence redundancies and established a 2-D structure for the A region that contains two long stem-loop structures each including four repeats. Interactions formed between repeats and between repeats and spacers stabilize these structures. Conservation of the spacer terminal sequences allows formation of such structures in all sequenced Xist RNAs. By combination of RNP affinity chromatography, immunoprecipitation assays, mass spectrometry, and Western blot analysis, we demonstrate that the A region can associate with components of the PRC2 complex in mouse ES cell nuclear extracts. Whilst a single four-repeat motif is able to associate with components of this complex, recruitment of Suz12 is clearly more efficient when the entire A region is present. Our data with their emphasis on the importance of inter-repeat pairing change fundamentally our conception of the 2-D structure of the A region of Xist RNA and support its possible implication in recruitment of the PRC2 complex.