Connections Between Metabolism and Epigenetics in Programming Cellular Differentiation.

Researchers are intensifying efforts to understand the mechanisms by which changes in metabolic states influence differentiation programs. An emerging objective is to define how fluctuations in metabolites influence the epigenetic states that contribute to differentiation programs. This is because metabolites such as S-adenosylmethionine, acetyl-CoA, α-ketoglutarate, 2-hydroxyglutarate, and butyrate are donors, substrates, cofactors, and antagonists for the activities of epigenetic-modifying complexes and for epigenetic modifications. We discuss this topic from the perspective of specialized CD4+ T cells as well as effector and memory T cell differentiation programs. We also highlight findings from embryonic stem cells that give mechanistic insight into how nutrients processed through pathways such as glycolysis, glutaminolysis, and one-carbon metabolism regulate metabolite levels to influence epigenetic events and discuss similar mechanistic principles in T cells. Finally, we highlight how dysregulated environments, such as the tumor microenvironment, might alter programming events.

Allele-Specific Chromosome Removal after Cas9 Cleavage in Human Embryos.

Correction of disease-causing mutations in human embryos holds the potential to reduce the burden of inherited genetic disorders and improve fertility treatments for couples with disease-causing mutations in lieu of embryo selection. Here, we evaluate repair outcomes of a Cas9-induced double-strand break (DSB) introduced on the paternal chromosome at the EYS locus, which carries a frameshift mutation causing blindness. We show that the most common repair outcome is microhomology-mediated end joining, which occurs during the first cell cycle in the zygote, leading to embryos with non-mosaic restoration of the reading frame. Notably, about half of the breaks remain unrepaired, resulting in an undetectable paternal allele and, after mitosis, loss of one or both chromosomal arms. Correspondingly, Cas9 off-target cleavage results in chromosomal losses and hemizygous indels because of cleavage of both alleles. These results demonstrate the ability to manipulate chromosome content and reveal significant challenges for mutation correction in human embryos.

A Peninsular Structure Coordinates Asynchronous Differentiation with Morphogenesis to Generate Pancreatic Islets.

The pancreatic islets of Langerhans regulate glucose homeostasis. The loss of insulin-producing ß cells within islets results in diabetes, and islet transplantation from cadaveric donors can cure the disease. In vitro production of whole islets, not just ß cells, will benefit from a better understanding of endocrine differentiation and islet morphogenesis. We used single-cell mRNA sequencing to obtain a detailed description of pancreatic islet development. Contrary to the prevailing dogma, we find islet morphology and endocrine differentiation to be directly related. As endocrine progenitors differentiate, they migrate in cohesion and form bud-like islet precursors, or "peninsulas" (literally "almost islands"). α cells, the first to develop, constitute the peninsular outer layer, and ß cells form later, beneath them. This spatiotemporal collinearity leads to the typical core-mantle architecture of the mature, spherical islet. Finally, we induce peninsula-like structures in differentiating human embryonic stem cells, laying the ground for the generation of entire islets in vitro.

The Implication of Early Chromatin Changes in X Chromosome Inactivation.

During development, the precise relationships between transcription and chromatin modifications often remain unclear. We use the X chromosome inactivation (XCI) paradigm to explore the implication of chromatin changes in gene silencing. Using female mouse embryonic stem cells, we initiate XCI by inducing Xist and then monitor the temporal changes in transcription and chromatin by allele-specific profiling. This reveals histone deacetylation and H2AK119 ubiquitination as the earliest chromatin alterations during XCI. We show that HDAC3 is pre-bound on the X chromosome and that, upon Xist coating, its activity is required for efficient gene silencing. We also reveal that first PRC1-associated H2AK119Ub and then PRC2-associated H3K27me3 accumulate initially at large intergenic domains that can then spread into genes only in the context of histone deacetylation and gene silencing. Our results reveal the hierarchy of chromatin events during the initiation of XCI and identify key roles for chromatin in the early steps of transcriptional silencing.

Nudt21 Controls Cell Fate by Connecting Alternative Polyadenylation to Chromatin Signaling.

Cell fate transitions involve rapid gene expression changes and global chromatin remodeling, yet the underlying regulatory pathways remain incompletely understood. Here, we identified the RNA-processing factor Nudt21 as a novel regulator of cell fate change using transcription-factor-induced reprogramming as a screening assay. Suppression of Nudt21 enhanced the generation of induced pluripotent stem cells, facilitated transdifferentiation into trophoblast stem cells, and impaired differentiation of myeloid precursors and embryonic stem cells, suggesting a broader role for Nudt21 in cell fate change. We show that Nudt21 directs differential polyadenylation of over 1,500 transcripts in cells acquiring pluripotency, although only a fraction changed protein levels. Remarkably, these proteins were strongly enriched for chromatin regulators, and their suppression neutralized the effect of Nudt21 during reprogramming. Collectively, our data uncover Nudt21 as a novel post-transcriptional regulator of cell fate and establish a direct, previously unappreciated link between alternative polyadenylation and chromatin signaling.

Insights into Nucleosome Organization in Mouse Embryonic Stem Cells through Chemical Mapping.

Nucleosome organization influences gene activity by controlling DNA accessibility to transcription machinery. Here, we develop a chemical biology approach to determine mammalian nucleosome positions genome-wide. We uncovered surprising features of nucleosome organization in mouse embryonic stem cells. In contrast to the prevailing model, we observe that for nearly all mouse genes, a class of fragile nucleosomes occupies previously designated nucleosome-depleted regions around transcription start sites and transcription termination sites. We show that nucleosomes occupy DNA targets for a subset of DNA-binding proteins, including CCCTC-binding factor (CTCF) and pluripotency factors. Furthermore, we provide evidence that promoter-proximal nucleosomes, with the +1 nucleosome in particular, contribute to the pausing of RNA polymerase II. Lastly, we find a characteristic preference for nucleosomes at exon-intron junctions. Taken together, we establish an accurate method for defining the nucleosome landscape and provide a valuable resource for studying nucleosome-mediated gene regulation in mammalian cells.

Selective chemical tracking of Dnmt1 catalytic activity in live cells.

Enzymatic methylation of cytosine to 5-methylcytosine in DNA is a fundamental epigenetic mechanism involved in mammalian development and disease. DNA methylation is brought about by collective action of three AdoMet-dependent DNA methyltransferases, whose catalytic interactions and temporal interplay are poorly understood. We used structure-guided engineering of the Dnmt1 methyltransferase to enable catalytic transfer of azide tags onto DNA from a synthetic cofactor analog, Ado-6-azide, in vitro. We then CRISPR-edited the Dnmt1 locus in mouse embryonic stem cells to install the engineered codon, which, following pulse internalization of the Ado-6-azide cofactor by electroporation, permitted selective azide tagging of Dnmt1-specific genomic targets in cellulo. The deposited covalent tags were exploited as "click" handles for reading adjoining sequences and precise genomic mapping of the methylation sites. The proposed approach, Dnmt-TOP-seq, enables high-resolution temporal tracking of the Dnmt1 catalysis in mammalian cells, paving the way to selective studies of other methylation pathways in eukaryotic systems.

Oct4:Sox2 binding is essential for establishing but not maintaining active and silent states of dynamically regulated genes in pluripotent cells.

Much has been learned about the mechanisms of action of pluripotency factors Oct4 and Sox2. However, as with other regulators of cell identity, little is known about the impact of disrupting their binding motifs in a native environment or the characteristics of genes they regulate. By quantitatively examining dynamic ranges of gene expression instead of focusing on conventional measures of differential expression, we found that Oct4 and Sox2 enhancer binding is strongly enriched near genes subject to large dynamic ranges of expression among cell types, with binding sites near these genes usually within superenhancers. Mutagenesis of representative Oct4:Sox2 motifs near such active, dynamically regulated genes revealed critical roles in transcriptional activation during reprogramming, with more limited roles in transcriptional maintenance in the pluripotent state. Furthermore, representative motifs near silent genes were critical for establishing but not maintaining the fully silent state, while genes whose transcript levels varied by smaller magnitudes among cell types were unaffected by nearby Oct4:Sox2 motifs. These results suggest that Oct4 and Sox2 directly establish both active and silent transcriptional states in pluripotent cells at a large number of genes subject to dynamic regulation during mammalian development, but are less important than expected for maintaining transcriptional states.

Transcription factors specifically control change.

Transcription factors are defined by their sequence-specific binding to DNA and by their selective impacts on gene expression, depending on specific binding sites. The factor binding motifs in the DNA should thus represent a blueprint of regulatory logic, suggesting that transcription factor binding patterns on the genome (e.g., measured by ChIP-seq) should indicate which target genes the factors are directly controlling. However, although genetic data confirm high impacts of transcription factor perturbation in embryology, transcription factors bind to far more sites than the number of genes they dynamically regulate, when measured by direct perturbation in a given cell type. Also, deletion of carefully chosen transcription factor binding sites often gives disappointingly weak results. In a new study in the previous issue of Genes & Development, Lo and colleagues (pp. 1079-1095) reconcile these contradictions by using an elegant experimental system to directly compare the roles of transcription factor-binding site interaction in gene regulation maintenance with roles of the same factor-site interactions in gene regulation through developmental change. They examine Oct4:Sox2 shared target genes under maintained versus reinduced pluripotency conditions within the same cell clone. The results show that the same factor-site interaction impacts can appear modest in assays in developmental steady-state but are far more important as regulatory catalysts of developmental change.

RNA Pol II pausing facilitates phased pluripotency transitions by buffering transcription.

Promoter-proximal RNA Pol II pausing is a critical step in transcriptional control. Pol II pausing has been predominantly studied in tissue culture systems. While Pol II pausing has been shown to be required for mammalian development, the phenotypic and mechanistic details of this requirement are unknown. Here, we found that loss of Pol II pausing stalls pluripotent state transitions within the epiblast of the early mouse embryo. Using Nelfb -/- mice and a NELFB degron mouse pluripotent stem cell model, we show that embryonic stem cells (ESCs) representing the naïve state of pluripotency successfully initiate a transition program but fail to balance levels of induced and repressed genes and enhancers in the absence of NELF. We found an increase in chromatin-associated NELF during transition from the naïve to later pluripotent states. Overall, our work defines the acute and long-term molecular consequences of NELF loss and reveals a role for Pol II pausing in the pluripotency continuum as a modulator of cell state transitions.

NMD is required for timely cell fate transitions by fine-tuning gene expression and regulating translation.

Cell fate transitions depend on balanced rewiring of transcription and translation programs to mediate ordered developmental progression. Components of the nonsense-mediated mRNA decay (NMD) pathway have been implicated in regulating embryonic stem cell (ESC) differentiation, but the exact mechanism is unclear. Here we show that NMD controls expression levels of the translation initiation factor Eif4a2 and its premature termination codon-encoding isoform (Eif4a2PTC ). NMD deficiency leads to translation of the truncated eIF4A2PTC protein. eIF4A2PTC elicits increased mTORC1 activity and translation rates and causes differentiation delays. This establishes a previously unknown feedback loop between NMD and translation initiation. Furthermore, our results show a clear hierarchy in the severity of target deregulation and differentiation phenotypes between NMD effector KOs (Smg5 KO > Smg6 KO > Smg7 KO), which highlights heterodimer-independent functions for SMG5 and SMG7. Together, our findings expose an intricate link between mRNA homeostasis and mTORC1 activity that must be maintained for normal dynamics of cell state transitions.

Multiomics analysis of the NAD+-PARP1 axis reveals a role for site-specific ADP-ribosylation in splicing in embryonic stem cells.

The differentiation of embryonic stem cells (ESCs) into a lineage-committed state is a dynamic process involving changes in cellular metabolism, epigenetic modifications, post-translational modifications, gene expression, and RNA processing. Here we integrated data from metabolomic, proteomic, and transcriptomic assays to characterize how alterations in NAD+ metabolism during the differentiation of mouse ESCs lead to alteration of the PARP1-mediated ADP-ribosylated (ADPRylated) proteome and mRNA isoform specialization. Our metabolomic analyses indicate that mESCs use distinct NAD+ biosynthetic pathways in different cell states: the de novo pathway in the pluripotent state, and the salvage and Preiss-Handler pathways as differentiation progresses. We observed a dramatic induction of PARP1 catalytic activity driven by enhanced nuclear NAD+ biosynthesis during the early stages of mESC differentiation (e.g., within 12 h of LIF removal). PARP1-modified proteins in mESCs are enriched for biological processes related to stem cell maintenance, transcriptional regulation, and RNA processing. The PARP1 substrates include core spliceosome components, such as U2AF35 and U2AF65, whose splicing functions are modulated by PARP1-mediated site-specific ADP-ribosylation. Finally, we observed that splicing is dysregulated genome-wide in Parp1 knockout mESCs. Together, these results demonstrate a role for the NAD+-PARP1 axis in the maintenance of mESC state, specifically in the splicing program during differentiation.

Studying lineage decision-making in vitro: emerging concepts and novel tools.

Correct and timely lineage decisions are critical for normal embryonic development and homeostasis of adult tissues. Therefore, the search for fundamental principles that underlie lineage decision-making lies at the heart of developmental biology. Here, we review attempts to understand lineage decision-making as the interplay of single-cell heterogeneity and gene regulation. Fluctuations at the single-cell level are an important driving force behind cell-state transitions and the creation of cell-type diversity. Gene regulatory networks amplify such fluctuations and define stable cell types. They also mediate the influence of signaling inputs on the lineage decision. In this review, we focus on insights gleaned from in vitro differentiation of embryonic stem cells. We discuss emerging concepts, with an emphasis on transcriptional regulation, dynamical aspects of differentiation, and functional single-cell heterogeneity. We also highlight some novel tools to study lineage decision-making in vitro.

SALL4 controls cell fate in response to DNA base composition.

Mammalian genomes contain long domains with distinct average compositions of A/T versus G/C base pairs. In a screen for proteins that might interpret base composition by binding to AT-rich motifs, we identified the stem cell factor SALL4, which contains multiple zinc fingers. Mutation of the domain responsible for AT binding drastically reduced SALL4 genome occupancy and prematurely upregulated genes in proportion to their AT content. Inactivation of this single AT-binding zinc-finger cluster mimicked defects seen in Sall4 null cells, including precocious differentiation of embryonic stem cells (ESCs) and embryonic lethality in mice. In contrast, deletion of two other zinc-finger clusters was phenotypically neutral. Our data indicate that loss of pluripotency is triggered by downregulation of SALL4, leading to de-repression of a set of AT-rich genes that promotes neuronal differentiation. We conclude that base composition is not merely a passive byproduct of genome evolution and constitutes a signal that aids control of cell fate.

Distinct PRC2 subunits regulate maintenance and establishment of Polycomb repression during differentiation.

The Polycomb repressive complex 2 (PRC2) is an essential epigenetic regulator that deposits repressive H3K27me3. PRC2 subunits form two holocomplexes-PRC2.1 and PRC2.2-but the roles of these two PRC2 assemblies during differentiation are unclear. We employed auxin-inducible degradation to deplete PRC2.1 subunit MTF2 or PRC2.2 subunit JARID2 during differentiation of embryonic stem cells (ESCs) to neural progenitors (NPCs). Depletion of either MTF2 or JARID2 resulted in incomplete differentiation due to defects in gene regulation. Distinct sets of Polycomb target genes were derepressed in the absence of MTF2 or JARID2. MTF2-sensitive genes were marked by H3K27me3 in ESCs and remained silent during differentiation, whereas JARID2-sensitive genes were preferentially active in ESCs and became newly repressed in NPCs. Thus, MTF2 and JARID2 contribute non-redundantly to Polycomb silencing, suggesting that PRC2.1 and PRC2.2 have distinct functions in maintaining and establishing, respectively, Polycomb repression during differentiation.

Integrated loss- and gain-of-function screens define a core network governing human embryonic stem cell behavior.

Understanding the genetic control of human embryonic stem cell function is foundational for developmental biology and regenerative medicine. Here we describe an integrated genome-scale loss- and gain-of-function screening approach to identify genetic networks governing embryonic stem cell proliferation and differentiation into the three germ layers. We identified a deep link between pluripotency maintenance and survival by showing that genetic alterations that cause pluripotency dissolution simultaneously increase apoptosis resistance. We discovered that the chromatin-modifying complex SAGA and in particular its subunit TADA2B are central regulators of pluripotency, survival, growth, and lineage specification. Joint analysis of all screens revealed that genetic alterations that broadly inhibit differentiation across multiple germ layers drive proliferation and survival under pluripotency-maintaining conditions and coincide with known cancer drivers. Our results show the power of integrated multilayer genetic screening for the robust mapping of complex genetic networks.

TEAD2 initiates ground-state pluripotency by mediating chromatin looping.

The transition of mouse embryonic stem cells (ESCs) between serum/LIF and 2i(MEK and GSK3 kinase inhibitor)/LIF culture conditions serves as a valuable model for exploring the mechanisms underlying ground and confused pluripotent states. Regulatory networks comprising core and ancillary pluripotency factors drive the gene expression programs defining stable naïve pluripotency. In our study, we systematically screened factors essential for ESC pluripotency, identifying TEAD2 as an ancillary factor maintaining ground-state pluripotency in 2i/LIF ESCs and facilitating the transition from serum/LIF to 2i/LIF ESCs. TEAD2 exhibits increased binding to chromatin in 2i/LIF ESCs, targeting active chromatin regions to regulate the expression of 2i-specific genes. In addition, TEAD2 facilitates the expression of 2i-specific genes by mediating enhancer-promoter interactions during the serum/LIF to 2i/LIF transition. Notably, deletion of Tead2 results in reduction of a specific set of enhancer-promoter interactions without significantly affecting binding of chromatin architecture proteins, CCCTC-binding factor (CTCF), and Yin Yang 1 (YY1). In summary, our findings highlight a novel prominent role of TEAD2 in orchestrating higher-order chromatin structures of 2i-specific genes to sustain ground-state pluripotency.

Haploid mouse embryonic stem cells: rapid genetic screening and germline transmission.

Most animal genomes are diploid, and mammalian development depends on specific adaptations that have evolved secondary to diploidy. Genomic imprinting and dosage compensation restrict haploid development to early embryos. Recently, haploid mammalian development has been reinvestigated since the establishment of haploid embryonic stem cells (ESCs) from mouse embryos. Haploid cells possess one copy of each gene, facilitating the generation of loss-of-function mutations in a single step. Recessive mutations can then be assessed in forward genetic screens. Applications of haploid mammalian cell systems in screens have been illustrated in several recent publications. Haploid ESCs are characterized by a wide developmental potential and can contribute to chimeric embryos and mice. Different strategies for introducing genetic modifications from haploid ESCs into the mouse germline have been further developed. Haploid ESCs therefore introduce new possibilities in mammalian genetics and could offer an unprecedented tool for genome exploration in the future.

Cdk1 Controls Global Epigenetic Landscape in Embryonic Stem Cells.

The cyclin-dependent kinase 1 (Cdk1) drives cell division. To uncover additional functions of Cdk1, we generated knockin mice expressing an analog-sensitive version of Cdk1 in place of wild-type Cdk1. In our study, we focused on embryonic stem cells (ESCs), because this cell type displays particularly high Cdk1 activity. We found that in ESCs, a large fraction of Cdk1 substrates is localized on chromatin. Cdk1 phosphorylates many proteins involved in epigenetic regulation, including writers and erasers of all major histone marks. Consistent with these findings, inhibition of Cdk1 altered histone-modification status of ESCs. High levels of Cdk1 in ESCs phosphorylate and partially inactivate Dot1l, the H3K79 methyltransferase responsible for placing activating marks on gene bodies. Decrease of Cdk1 activity during ESC differentiation de-represses Dot1l, thereby allowing coordinated expression of differentiation genes. These analyses indicate that Cdk1 functions to maintain the epigenetic identity of ESCs.

A central role for canonical PRC1 in shaping the 3D nuclear landscape.

Polycomb group (PcG) proteins silence gene expression by chemically and physically modifying chromatin. A subset of PcG target loci are compacted and cluster in the nucleus; a conformation that is thought to contribute to gene silencing. However, how these interactions influence gross nuclear organization and their relationship with transcription remains poorly understood. Here we examine the role of Polycomb-repressive complex 1 (PRC1) in shaping 3D genome organization in mouse embryonic stem cells (mESCs). Using a combination of imaging and Hi-C analyses, we show that PRC1-mediated long-range interactions are independent of CTCF and can bridge sites at a megabase scale. Impairment of PRC1 enzymatic activity does not directly disrupt these interactions. We demonstrate that PcG targets coalesce in vivo, and that developmentally induced expression of one of the target loci disrupts this spatial arrangement. Finally, we show that transcriptional activation and the loss of PRC1-mediated interactions are separable events. These findings provide important insights into the function of PRC1, while highlighting the complexity of this regulatory system.