A Comparative Analysis of Mouse Imprinted and Random X-Chromosome Inactivation.

The mammalian sexes are distinguished by the X and Y chromosomes. Whereas males harbor one X and one Y chromosome, females harbor two X chromosomes. To equalize X-linked gene expression between the sexes, therian mammals have evolved X-chromosome inactivation as a dosage compensation mechanism. During X-inactivation, most genes on one of the two X chromosomes in females are transcriptionally silenced, thus equalizing X-linked gene expression between the sexes. Two forms of X-inactivation characterize eutherian mammals, imprinted and random. Imprinted X-inactivation is defined by the exclusive inactivation of the paternal X chromosome in all cells, whereas random X-inactivation results in the silencing of genes on either the paternal or maternal X chromosome in individual cells. Both forms of X-inactivation have been studied intensively in the mouse model system, which undergoes both imprinted and random X-inactivation early in embryonic development. Stable imprinted and random X-inactivation requires the induction of the Xist long non-coding RNA. Following its induction, Xist RNA recruits proteins and complexes that silence genes on the inactive-X. In this review, we present a current understanding of the mechanisms of Xist RNA induction, and, separately, the establishment and maintenance of gene silencing on the inactive-X by Xist RNA during imprinted and random X-inactivation.

Quiescence enables unrestricted cell fate in naive embryonic stem cells.

Quiescence in stem cells is traditionally considered as a state of inactive dormancy or with poised potential. Naive mouse embryonic stem cells (ESCs) can enter quiescence spontaneously or upon inhibition of MYC or fatty acid oxidation, mimicking embryonic diapause in vivo. The molecular underpinning and developmental potential of quiescent ESCs (qESCs) are relatively unexplored. Here we show that qESCs possess an expanded or unrestricted cell fate, capable of generating both embryonic and extraembryonic cell types (e.g., trophoblast stem cells). These cells have a divergent metabolic landscape comparing to the cycling ESCs, with a notable decrease of the one-carbon metabolite S-adenosylmethionine. The metabolic changes are accompanied by a global reduction of H3K27me3, an increase of chromatin accessibility, as well as the de-repression of endogenous retrovirus MERVL and trophoblast master regulators. Depletion of methionine adenosyltransferase Mat2a or deletion of Eed in the polycomb repressive complex 2 results in removal of the developmental constraints towards the extraembryonic lineages. Our findings suggest that quiescent ESCs are not dormant but rather undergo an active transition towards an unrestricted cell fate.

Activation of Xist by an evolutionarily conserved function of KDM5C demethylase.

XX female and XY male therian mammals equalize X-linked gene expression through the mitotically-stable transcriptional inactivation of one of the two X chromosomes in female somatic cells. Here, we describe an essential function of the X-linked homolog of an ancestral X-Y gene pair, Kdm5c-Kdm5d, in the expression of Xist lncRNA, which is required for stable X-inactivation. Ablation of Kdm5c function in females results in a significant reduction in Xist RNA expression. Kdm5c encodes a demethylase that enhances Xist expression by converting histone H3K4me2/3 modifications into H3K4me1. Ectopic expression of mouse and human KDM5C, but not the Y-linked homolog KDM5D, induces Xist in male mouse embryonic stem cells (mESCs). Similarly, marsupial (opossum) Kdm5c but not Kdm5d also upregulates Xist in male mESCs, despite marsupials lacking Xist, suggesting that the KDM5C function that activates Xist in eutherians is strongly conserved and predates the divergence of eutherian and metatherian mammals. In support, prototherian (platypus) Kdm5c also induces Xist in male mESCs. Together, our data suggest that eutherian mammals co-opted the ancestral demethylase KDM5C during sex chromosome evolution to upregulate Xist for the female-specific induction of X-inactivation.

Preventing erosion of X-chromosome inactivation in human embryonic stem cells.

X-chromosome inactivation is a paradigm of epigenetic transcriptional regulation. Female human embryonic stem cells (hESCs) often undergo erosion of X-inactivation upon prolonged culture. Here, we investigate the sources of X-inactivation instability by deriving new primed pluripotent hESC lines. We find that culture media composition dramatically influenced the expression of XIST lncRNA, a key regulator of X-inactivation. hESCs cultured in a defined xenofree medium stably maintained XIST RNA expression and coating, whereas hESCs cultured in the widely used mTeSR1 medium lost XIST RNA expression. We pinpointed lithium chloride in mTeSR1 as a cause of XIST RNA loss. The addition of lithium chloride or inhibitors of GSK-3 proteins that are targeted by lithium to the defined hESC culture medium impeded XIST RNA expression. GSK-3 inhibition in differentiating female mouse embryonic stem cells and epiblast stem cells also resulted in a loss of XIST RNA expression. Together, these data may reconcile observed variations in X-inactivation in hESCs and inform the faithful culture of pluripotent stem cells.

Highly Resolved Detection of Long Non-coding RNAs In Situ.

Long non-coding RNAs (lncRNAs) have been postulated to function in a number of DNA-based processes, most notably transcription. The detection of lncRNAs in situ can offer insights into their function. Fluorescence in situ hybridization (FISH) enables the detection of specific nucleic acid sequences, including lncRNAs, within individual cells. Current RNA FISH techniques can inform both the localization and expression level of RNA transcripts. Together with advances in microscopy, these in situ techniques now allow for visualization and quantification of even lowly expressed or unstable lncRNAs. When combined with detection of associated proteins and chromatin modifications by immunofluorescence, RNA FISH can lend essential insights into lncRNA function. Here, we describe an integrated set of protocols to detect, individually or in combination, specific RNAs, DNAs, proteins, and histone modifications in single cells at high sensitivity using conventional fluorescence microscopy.

A PRC2-independent function for EZH2 in regulating rRNA 2'-O methylation and IRES-dependent translation.

Dysregulated translation is a common feature of cancer. Uncovering its governing factors and underlying mechanism are important for cancer therapy. Here, we report that enhancer of zeste homologue 2 (EZH2), previously known as a transcription repressor and lysine methyltransferase, can directly interact with fibrillarin (FBL) to exert its role in translational regulation. We demonstrate that EZH2 enhances rRNA 2'-O methylation via its direct interaction with FBL. Mechanistically, EZH2 strengthens the FBL-NOP56 interaction and facilitates the assembly of box C/D small nucleolar ribonucleoprotein. Strikingly, EZH2 deficiency impairs the translation process globally and reduces internal ribosome entry site (IRES)-dependent translation initiation in cancer cells. Our findings reveal a previously unrecognized role of EZH2 in cancer-related translational regulation.

Generating primed pluripotent epiblast stem cells: A methodology chapter.

At least two distinct pluripotent states, referred to as naïve and primed, define the early mammalian embryo. In the mouse, the pluripotent epiblast cells in the pre/peri-implantation embryo are the source of naïve embryonic stem cells (ESCs). After the embryo implants, the epiblast lineage generates a restricted or primed population of stem cells, referred to as epiblast stem cells (EpiSCs). ESCs can be cultured in EpiSC media to generate epiblast-like cells (EpiLCs). The differentiation of naive ESCs into primed EpiLCs permits insights into the development and differentiation of the pluripotent epiblast lineage. This chapter describes the generation and characterization of EpiSCs as well as EpiLCs.

Epigenomic analysis of gastrulation identifies a unique chromatin state for primed pluripotency.

Around implantation, the epiblast (Epi) transits from naïve to primed pluripotency, before giving rise to the three germ layers. How chromatin is reconfigured during this developmental window remains poorly understood. We performed a genome-wide investigation of chromatin landscapes during this period. We find that enhancers in ectoderm are already pre-accessible in embryonic day 6.5 (E6.5) Epi when cells enter a primed pluripotent state. Unexpectedly, strong trimethylation of histone H3 at lysine 4 (H3K4me3) emerges at developmental gene promoters in E6.5 Epi and positively correlates with H3K27me3, thus establishing bivalency. These genes also show enhanced spatial interactions. Both the strong bivalency and spatial clustering are virtually absent in preimplantation embryos and are markedly reduced in fate-committed lineages. Finally, we show that KMT2B is essential for establishing bivalent H3K4me3 at E6.5 but becomes partially dispensable later. Its deficiency leads to impaired activation of developmental genes and subsequent embryonic lethality. Thus, our data characterize lineage-specific chromatin reconfiguration and a unique chromatin state for primed pluripotency.

Conversion of random X-inactivation to imprinted X-inactivation by maternal PRC2.

Imprinted X-inactivation silences genes exclusively on the paternally-inherited X-chromosome and is a paradigm of transgenerational epigenetic inheritance in mammals. Here, we test the role of maternal vs. zygotic Polycomb repressive complex 2 (PRC2) protein EED in orchestrating imprinted X-inactivation in mouse embryos. In maternal-null (Eedm-/-) but not zygotic-null (Eed-/-) early embryos, the maternal X-chromosome ectopically induced Xist and underwent inactivation. Eedm-/- females subsequently stochastically silenced Xist from one of the two X-chromosomes and displayed random X-inactivation. This effect was exacerbated in embryos lacking both maternal and zygotic EED (Eedmz-/-), suggesting that zygotic EED can also contribute to the onset of imprinted X-inactivation. Xist expression dynamics in Eedm-/- embryos resemble that of early human embryos, which lack oocyte-derived maternal PRC2 and only undergo random X-inactivation. Thus, expression of PRC2 in the oocyte and transmission of the gene products to the embryo may dictate the occurrence of imprinted X-inactivation in mammals.

Experimental Analysis of Imprinted Mouse X-Chromosome Inactivation.

X-chromosome inactivation is a dosage compensation mechanism that equalizes X-linked gene expression between male and female mammals through the transcriptional silencing of most genes on one of the two X-chromosomes in females. With a few key exceptions, once the X-chromosome is inactivated replicated copies of that X-chromosome are maintained as inactive in all descendant cells. X-inactivation is therefore a paradigm of epigenetic inheritance. Imprinted X-inactivation is a specialized form of X-inactivation that results in the silencing of the paternally derived X-chromosome. Due to its parent-of-origin-specific pattern of inactivation, imprinted X-inactivation is a model of mitotic as well as meiotic, i.e., transgenerational, epigenetic inheritance. All cells of the early mouse embryo undergo imprinted X-inactivation, a pattern that is subsequently maintained in extraembryonic cell types in vivo and in vitro. Here, we describe both high- and low-throughput approaches to interrogate imprinted X-inactivation in the mouse embryo as well in cultured extraembryonic stem cells.

An apicosome initiates self-organizing morphogenesis of human pluripotent stem cells.

Human pluripotent stem cells (hPSCs) self-organize into apicobasally polarized cysts, reminiscent of the lumenal epiblast stage, providing a model to explore key morphogenic processes in early human embryos. Here, we show that apical polarization begins on the interior of single hPSCs through the dynamic formation of a highly organized perinuclear apicosome structure. The membrane surrounding the apicosome is enriched in apical markers and displays microvilli and a primary cilium; its lumenal space is rich in Ca2+ Time-lapse imaging of isolated hPSCs reveals that the apicosome forms de novo in interphase, retains its structure during mitosis, is asymmetrically inherited after mitosis, and relocates to the recently formed cytokinetic plane, where it establishes a fully polarized lumen. In a multicellular aggregate of hPSCs, intracellular apicosomes from multiple cells are trafficked to generate a common lumenal cavity. Thus, the apicosome is a unique preassembled apical structure that can be rapidly used in single or clustered hPSCs to initiate self-organized apical polarization and lumenogenesis.

PRC2 represses transcribed genes on the imprinted inactive X chromosome in mice.

BACKGROUND: Polycomb repressive complex 2 (PRC2) catalyzes histone H3K27me3, which marks many transcriptionally silent genes throughout the mammalian genome. Although H3K27me3 is associated with silenced gene expression broadly, it remains unclear why some but not other PRC2 target genes require PRC2 and H3K27me3 for silencing. RESULTS: Here we define the transcriptional and chromatin features that predict which PRC2 target genes require PRC2/H3K27me3 for silencing by interrogating imprinted mouse X-chromosome inactivation. H3K27me3 is enriched at promoters of silenced genes across the inactive X chromosome. To abrogate PRC2 function, we delete the core PRC2 protein EED in F1 hybrid trophoblast stem cells (TSCs), which undergo imprinted inactivation of the paternally inherited X chromosome. Eed -/- TSCs lack H3K27me3 and Xist lncRNA enrichment on the inactive X chromosome. Despite the absence of H3K27me3 and Xist RNA, only a subset of the inactivated X-linked genes is derepressed in Eed -/- TSCs. Unexpectedly, in wild-type (WT) TSCs these genes are transcribed and are enriched for active chromatin hallmarks on the inactive-X, including RNA PolII, H3K27ac, and H3K36me3, but not the bivalent mark H3K4me2. By contrast, PRC2 targets that remain repressed in Eed -/- TSCs are depleted for active chromatin characteristics in WT TSCs. CONCLUSIONS: A comparative analysis of transcriptional and chromatin features of inactive X-linked genes in WT and Eed -/- TSCs suggests that PRC2 acts as a brake to prevent induction of transcribed genes on the inactive X chromosome, a mode of PRC2 function that may apply broadly.

MLL1 Inhibition Reprograms Epiblast Stem Cells to Naive Pluripotency.

The interconversion between naive and primed pluripotent states is accompanied by drastic epigenetic rearrangements. However, it is unclear whether intrinsic epigenetic events can drive reprogramming to naive pluripotency or if distinct chromatin states are instead simply a reflection of discrete pluripotent states. Here, we show that blocking histone H3K4 methyltransferase MLL1 activity with the small-molecule inhibitor MM-401 reprograms mouse epiblast stem cells (EpiSCs) to naive pluripotency. This reversion is highly efficient and synchronized, with more than 50% of treated EpiSCs exhibiting features of naive embryonic stem cells (ESCs) within 3 days. Reverted ESCs reactivate the silenced X chromosome and contribute to embryos following blastocyst injection, generating germline-competent chimeras. Importantly, blocking MLL1 leads to global redistribution of H3K4me1 at enhancers and represses lineage determinant factors and EpiSC markers, which indirectly regulate ESC transcription circuitry. These findings show that discrete perturbation of H3K4 methylation is sufficient to drive reprogramming to naive pluripotency.

Sex-specific silencing of X-linked genes by Xist RNA.

X-inactive specific transcript (Xist) long noncoding RNA (lncRNA) is thought to catalyze silencing of X-linked genes in cis during X-chromosome inactivation, which equalizes X-linked gene dosage between male and female mammals. To test the impact of Xist RNA on X-linked gene silencing, we ectopically induced endogenous Xist by ablating the antisense repressor Tsix in mice. We find that ectopic Xist RNA induction and subsequent X-linked gene silencing is sex specific in embryos and in differentiating embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs). A higher frequency of X(ΔTsix)Y male cells displayed ectopic Xist RNA coating compared with X(ΔTsix)X female cells. This increase reflected the inability of X(ΔTsix)Y cells to efficiently silence X-linked genes compared with X(ΔTsix)X cells, despite equivalent Xist RNA induction and coating. Silencing of genes on both Xs resulted in significantly reduced proliferation and increased cell death in X(ΔTsix)X female cells relative to X(ΔTsix)Y male cells. Thus, whereas Xist RNA can inactivate the X chromosome in females it may not do so in males. We further found comparable silencing in differentiating X(ΔTsix)Y and 39,X(ΔTsix) (X(ΔTsix)O) ESCs, excluding the Y chromosome and instead implicating the X-chromosome dose as the source of the sex-specific differences. Because X(ΔTsix)X female embryonic epiblast cells and EpiSCs harbor an inactivated X chromosome prior to ectopic inactivation of the active X(ΔTsix) X chromosome, we propose that the increased expression of one or more X-inactivation escapees activates Xist and, separately, helps trigger X-linked gene silencing.

Visualizing Long Noncoding RNAs on Chromatin.

Fluorescence in situ hybridization (FISH) enables the detection of specific nucleic acid sequences within single cells. For example, RNA FISH provides information on both the expression level and localization of RNA transcripts and, when combined with detection of associated proteins and chromatin modifications, can lend essential insights into long noncoding RNA (lncRNA) function. Epigenetic effects have been postulated for many lncRNAs, but shown for only a few. Advances in in situ techniques and microscopy, however, now allow for visualization of lncRNAs that are expressed at very low levels or are not very stable. FISH-based detections of RNA and DNA coupled with immunological staining of proteins/histone modifications offer the possibility to connect lncRNAs to epigenetic effects. Here, we describe an integrated set of protocols to detect, individually or in combination, specific RNAs, DNAs, proteins, and histone modifications in single cells at a high level of sensitivity using conventional fluorescence microscopy.

Lumen Formation Is an Intrinsic Property of Isolated Human Pluripotent Stem Cells.

We demonstrate that dissociated human pluripotent stem cells (PSCs) are intrinsically programmed to form lumens. PSCs form two-cell cysts with a shared apical domain within 20 hr of plating; these cysts collapse to form monolayers after 5 days. Expression of pluripotency markers is maintained throughout this time. In two-cell cysts, an apical domain, marked by EZRIN and atypical PKCζ, is surrounded by apically targeted organelles (early endosomes and Golgi). Molecularly, actin polymerization, regulated by ARP2/3 and mammalian diaphanous-related formin 1 (MDIA), promotes lumen formation, whereas actin contraction, mediated by MYOSIN-II, inhibits this process. Finally, we show that lumenal shape can be manipulated in bioengineered micro-wells. Since lumen formation is an indispensable step in early mammalian development, this system can provide a powerful model for investigation of this process in a controlled environment. Overall, our data establish that lumenogenesis is a fundamental cell biological property of human PSCs.