How Polycomb-Mediated Cell Memory Deals With a Changing Environment

Alternative silencing states of transposable elements in Arabidopsis associated with H3K27me3

BACKGROUND

The DNA/H3K9 methylation and Polycomb-group proteins (PcG)-H3K27me3 silencing pathways have long been considered mutually exclusive and specific to transposable elements (TEs) and genes, respectively in mammals, plants, and fungi. However, H3K27me3 can be recruited to many TEs in the absence of DNA/H3K9 methylation machinery and sometimes also co-occur with DNA methylation.

RESULTS

In this study, we show that TEs can also be solely targeted and silenced by H3K27me3 in wild-type Arabidopsis plants. These H3K27me3-marked TEs not only comprise degenerate relics but also seemingly intact copies that display the epigenetic features of responsive PcG target genes as well as an active H3K27me3 regulation. We also show that H3K27me3 can be deposited on newly inserted transgenic TE sequences in a TE-specific manner indicating that silencing is determined in cis. Finally, a comparison of Arabidopsis natural accessions reveals the existence of a category of TEs-which we refer to as "bifrons"-that are marked by DNA methylation or H3K27me3 depending on the accession. This variation can be linked to intrinsic TE features and to trans-acting factors and reveals a change in epigenetic status across the TE lifespan.

CONCLUSIONS

Our study sheds light on an alternative mode of TE silencing associated with H3K27me3 instead of DNA methylation in flowering plants. It also suggests dynamic switching between the two epigenetic marks at the species level, a new paradigm that might extend to other multicellular eukaryotes.

Toward a comprehensive catalog of regulatory elements

Regulatory elements are the genomic regions that interact with transcription factors to control cell-type-specific gene expression in different cellular environments. A precise and complete catalog of functional elements encoded by the human genome is key to understanding mammalian gene regulation. Here, we review the current state of regulatory element annotation. We first provide an overview of assays for characterizing functional elements, including genome, epigenome, transcriptome, three-dimensional chromatin interaction, and functional validation assays. We then discuss computational methods for defining regulatory elements, including peak-calling and other statistical modeling methods. Finally, we introduce several high-quality lists of regulatory element annotations and suggest potential future directions.

DNA methylation in cell plasticity and malignant transformation in liver diseases

The liver possesses extraordinary regenerative capacity mainly attributable to the ability of hepatocytes (HCs) and biliary epithelial cells (BECs) to self-replicate. This ability is left over from their bipotent parent cell, the hepatoblast, during development. When this innate regeneration is compromised due to the absence of proliferative parenchymal cells, such as during cirrhosis, HCs and BEC can transdifferentiate; thus, adding another layer of complexity to the process of liver repair. In addition, dysregulated lineage maintenance in these two cell populations has been shown to promote malignant growth in experimental conditions. Here, malignant transformation, driven in part by insufficient maintenance of lineage reprogramming, contributes to end-stage liver disease. Epigenetic changes are key drivers for cell fate decisions as well as transformation by finetuning overall transcription and gene expression. In this review, we address how altered DNA methylation contributes to the initiation and progression of hepatic cell fate conversion and cancer formation. We also discussed the diagnostic and therapeutic potential of targeting DNA methylation in liver cancer, its current limitations, and what future research is necessary to facilitate its contribution to clinical translation.

Dynamic Interactome of PRC2-EZH1 Complex Using Tandem-Affinity Purification and Quantitative Mass Spectrometry

The Polycomb repressive complex 2 (PRC2) is a well-characterized chromatin regulator of transcription programs acting through H3K27me3 deposition. In mammals, there are two main versions of PRC2 complexes: PRC2-EZH2, which is prevalent in cycling cells, and PRC2-EZH1 where EZH1 replaces EZH2 in post-mitotic tissues. Stoichiometry of PRC2 complex is dynamically modulated during cellular differentiation and various stress conditions. Therefore, unraveling unique architecture of PRC2 complexes under specific biological context through comprehensive and quantitative characterization could provide insight into the underlying mechanistic molecular mechanism in regulation of transcription process. In this chapter, we describe an efficient method which combines tandem-affinity purification (TAP) with label-free quantitative proteomics strategy for studying PRC2-EZH1 complex architecture alterations and identifying novel protein regulators in post-mitotic C2C12 skeletal muscle cells.

Modulation of sirtuins during monolayer chondrocyte culture influences cartilage regeneration upon transfer to a 3D culture environment

This study examined the role of sirtuins in the regenerative potential of articular chondrocytes. Sirtuins (SIRT1-7) play a key role in regulating cartilage homeostasis. By inhibiting pro-inflammatory pathways responsible for cartilage degradation and promoting the expression of key matrix components, sirtuins have the potential to drive a favourable balance between anabolic and catabolic processes critical to regenerative medicine. When subjected to osmolarity and glucose concentrations representative of the in vivo niche, freshly isolated bovine chondrocytes exhibited increases in SIRT1 but not SIRT3 gene expression. Replicating methods adopted for the in vitro monolayer expansion of chondrocytes for cartilage regenerative therapies, we found that SIRT1 gene expression declined during expansion. Manipulation of sirtuin activity during in vitro expansion by supplementation with the SIRT1-specific activator SRT1720, nicotinamide mononucleotide, or the pan-sirtuin inhibitor nicotinamide, significantly influenced cartilage regeneration in subsequent 3D culture. Tissue mass, cellularity and extracellular matrix content were reduced in response to sirtuin inhibition during expansion, whilst sirtuin activation enhanced these measures of cartilage tissue regeneration. Modulation of sirtuin activity during monolayer expansion influenced H3K27me3, a heterochromatin mark with an important role in development and differentiation. Unexpectedly, treatment of primary chondrocytes with sirtuin activators in 3D culture reduced their matrix synthesis. Thus, modulating sirtuin activity during the in vitro monolayer expansion phase may represent a distinct opportunity to enhance the outcome of cartilage regenerative medicine techniques.

EZH2 endorses cell plasticity to non-small cell lung cancer cells facilitating mesenchymal to epithelial transition and tumour colonization

Reversible transition between the epithelial and mesenchymal states are key aspects of carcinoma cell dissemination and the metastatic disease, and thus, characterizing the molecular basis of the epithelial to mesenchymal transition (EMT) is crucial to find druggable targets and more effective therapeutic approaches in cancer. Emerging studies suggest that epigenetic regulators might endorse cancer cells with the cell plasticity required to conduct dynamic changes in cell state during EMT. However, epigenetic mechanisms involved remain mostly unknown. Polycomb Repressive Complexes (PRCs) proteins are well-established epigenetic regulators of development and stem cell differentiation, but their role in different cancer systems is inconsistent and sometimes paradoxical. In this study, we have analysed the role of the PRC2 protein EZH2 in lung carcinoma cells. We found that besides its described role in CDKN2A-dependent cell proliferation, EZH2 upholds the epithelial state of cancer cells by repressing the transcription of hundreds of mesenchymal genes. Chemical inhibition or genetic removal of EZH2 promotes the residence of cancer cells in the mesenchymal state during reversible epithelial-mesenchymal transition. In fitting, analysis of human patient samples and tumour xenograft models indicate that EZH2 is required to efficiently repress mesenchymal genes and facilitate tumour colonization in vivo. Overall, this study discloses a novel role of PRC2 as a master regulator of EMT in carcinoma cells. This finding has important implications for the design of therapies based on EZH2 inhibitors in human cancer patients.

Truncated jarid2 and kdm6b transcripts are associated with temperature-induced sex reversal during development in a dragon lizard

Sex determination and differentiation in reptiles are complex. In the model species, Pogona vitticeps, high incubation temperature can cause male to female sex reversal. To elucidate the epigenetic mechanisms of thermolabile sex, we used an unbiased genome-wide assessment of intron retention during sex reversal. The previously implicated chromatin modifiers (jarid2 and kdm6b) were two of three genes to display sex reversal-specific intron retention. In these species, embryonic intron retention resulting in C-terminally truncated jarid2 and kdm6b isoforms consistently occurs at low temperatures. High-temperature sex reversal is uniquely characterized by a high prevalence of N-terminally truncated isoforms of jarid2 and kdm6b, which are not present at low temperatures, or in two other reptiles with temperature-dependent sex determination. This work verifies that chromatin-modifying genes are involved in highly conserved temperature responses and can also be transcribed into isoforms with new sex-determining roles.

Polycomb Repressive Complex 2 in Eukaryotes-An Evolutionary Perspective

Polycomb repressive complex 2 (PRC2) represents a group of evolutionarily conserved multi-subunit complexes that repress gene transcription by introducing trimethylation of lysine 27 on histone 3 (H3K27me3). PRC2 activity is of key importance for cell identity specification and developmental phase transitions in animals and plants. The composition, biochemistry, and developmental function of PRC2 in animal and flowering plant model species are relatively well described. Recent evidence demonstrates the presence of PRC2 complexes in various eukaryotic supergroups, suggesting conservation of the complex and its function. Here, we provide an overview of the current understanding of PRC2-mediated repression in different representatives of eukaryotic supergroups with a focus on the green lineage. By comparison of PRC2 in different eukaryotes, we highlight the possible common and diverged features suggesting evolutionary implications and outline emerging questions and directions for future research of polycomb repression and its evolution.

Tbx15/18/22 shares a binding site with Tbx6-r.b to maintain expression of a muscle structural gene in ascidian late embryos

The ascidian larval tail contains muscle cells for swimming. Most of these muscle cells differentiate autonomously. The genetic program behind this autonomy has been studied extensively and the genetic cascade from maternal factors to initiation of expression of a muscle structural gene, Myl.c, has been uncovered; Myl.c expression is directed initially by transcription factor Tbx6-r.b at the 64-cell stage and then by the combined actions of Tbx6-r.b and Mrf from the gastrula to early tailbud stages. In the present study, we showed that transcription of Myl.c continued in late tailbud embryos and larvae, although a fusion protein of Tbx6-r.b and GFP was hardly detectable in late tailbud embryos. A knockdown experiment, reporter assay, and in vitro binding assay indicated that an essential cis-regulatory element of Myl.c that bound Tbx6-r.b in early embryos bound Tbx15/18/22 in late embryos to maintain expression of Myl.c. We also found that Tbx15/18/22 was controlled by Mrf, which constitutes a regulatory loop with Tbx6-r.b. Therefore, our data indicated that Tbx15/18/22 was activated initially under control of this regulatory loop as in the case of Myl.c, and then Tbx15/18/22 maintained the expression of Myl.c after Tbx6-r.b had disappeared. RNA-sequencing of Tbx15/18/22 morphant embryos revealed that many muscle structural genes were regulated similarly by Tbx15/18/22. Thus, the present study revealed the mechanisms of maintenance of transcription of muscle structural genes in late embryos in which Tbx15/18/22 takes the place of Tbx6-r.b.

Malat-1-PRC2-EZH1 interaction supports adaptive oxidative stress dependent epigenome remodeling in skeletal myotubes

PRC2-mediated epigenetic function involves the interaction with long non-coding RNAs (lncRNAs). Although the identity of some of these RNAs has been elucidated in the context of developmental programs, their counterparts in postmitotic adult tissue homeostasis remain uncharacterized. To this aim, we used terminally differentiated postmitotic skeletal muscle cells in which oxidative stress induces the dynamic activation of PRC2-Ezh1 through Embryonic Ectoderm Develpment (EED) shuttling to the nucleus. We identify lncRNA Malat-1 as a necessary partner for PRC2-Ezh1-dependent response to oxidative stress. We show that in this pathway, PRC2-EZH1 dynamic assembly, and in turn stress induced skeletal muscle targeted genes repression, depends specifically on Malat-1. Our study reports about PRC2-RNA interactions in the physiological context of adaptive oxidative stress response and identifies the first lncRNA involved in PRC2-Ezh1 function.

Two transcriptionally distinct pathways drive female development in a reptile with both genetic and temperature dependent sex determination

How temperature determines sex remains unknown. A recent hypothesis proposes that conserved cellular mechanisms (calcium and redox; 'CaRe' status) sense temperature and identify genes and regulatory pathways likely to be involved in driving sexual development. We take advantage of the unique sex determining system of the model organism, Pogona vitticeps, to assess predictions of this hypothesis. P. vitticeps has ZZ male: ZW female sex chromosomes whose influence can be overridden in genetic males by high temperatures, causing male-to-female sex reversal. We compare a developmental transcriptome series of ZWf females and temperature sex reversed ZZf females. We demonstrate that early developmental cascades differ dramatically between genetically driven and thermally driven females, later converging to produce a common outcome (ovaries). We show that genes proposed as regulators of thermosensitive sex determination play a role in temperature sex reversal. Our study greatly advances the search for the mechanisms by which temperature determines sex.

Polycomb mutant partially suppresses DNA hypomethylation-associated phenotypes in Arabidopsis

A mutation in Arabidopsis polycomb repressive complex 2 partially suppresses the transposon activity observed in a DNA methylation mutant, challenging expectations.

In plants and mammals, DNA methylation and histone H3 lysine 27 trimethylation (H3K27me3), which is deposited by the polycomb repressive complex 2, are considered as two specialized systems for the epigenetic silencing of transposable element (TE) and genes, respectively. Nevertheless, many TE sequences acquire H3K27me3 when DNA methylation is lost. Here, we show in Arabidopsis thaliana that the gain of H3K27me3 observed at hundreds of TEs in the ddm1 mutant defective in the maintenance of DNA methylation, essentially depends on CURLY LEAF (CLF), one of two partially redundant H3K27 methyltransferases active in vegetative tissues. Surprisingly, the complete loss of H3K27me3 in ddm1 clf double mutant plants was not associated with further reactivation of TE expression nor with a burst of transposition. Instead, ddm1 clf plants exhibited less activated TEs, and a chromatin recompaction as well as hypermethylation of linker DNA compared with ddm1. Thus, a mutation in polycomb repressive complex 2 does not aggravate the molecular phenotypes linked to ddm1 but instead partially suppresses them, challenging our assumptions of the relationship between two conserved epigenetic silencing pathways.

Persistent epigenetic reprogramming of sweet taste by diet

Diets rich in sugar, salt, and fat alter taste perception and food preference, contributing to obesity and metabolic disorders, but the molecular mechanisms through which this occurs are unknown. Here, we show that in response to a high sugar diet, the epigenetic regulator Polycomb Repressive Complex 2.1 (PRC2.1) persistently reprograms the sensory neurons of Drosophila melanogaster flies to reduce sweet sensation and promote obesity. In animals fed high sugar, the binding of PRC2.1 to the chromatin of the sweet gustatory neurons is redistributed to repress a developmental transcriptional network that modulates the responsiveness of these cells to sweet stimuli, reducing sweet sensation. Half of these transcriptional changes persist despite returning the animals to a control diet, causing a permanent decrease in sweet taste. Our results uncover a new epigenetic mechanism that, in response to the dietary environment, regulates neural plasticity and feeding behavior to promote obesity.

Post-Transcriptional Mechanisms Respond Rapidly to Ecologically Relevant Thermal Fluctuations During Temperature-Dependent Sex Determination

An organism's ability to integrate transient environmental cues experienced during development into molecular and physiological responses forms the basis for adaptive shifts in phenotypic trajectories. During temperature-dependent sex determination (TSD), thermal cues during discrete periods in development coordinate molecular changes that ultimately dictate sexual fate and contribute to patterns of inter- and intra-sexual variation. How these mechanisms interface with dynamic thermal environments in nature remain largely unknown. By deploying thermal loggers in wild nests of the American alligator (Alligator mississippiensis) over two consecutive breeding seasons, we observed that 80% of nests exhibit both male- and female-promoting thermal cues during the thermosensitive period, and of these nests, all exhibited both male- and female-promoting temperatures within the span of a single day. These observations raise a critical question-how are opposing environmental cues integrated into sexually dimorphic transcriptional programs across short temporal scales? To address this question, alligator embryos were exposed to fluctuating temperatures based on nest thermal profiles and sampled over the course of a daily thermal fluctuation. We examined the expression dynamics of upstream genes in the temperature-sensing pathway and find that post-transcriptional alternative splicing and transcript abundance of epigenetic modifier genes JARID2 and KDM6B respond rapidly to thermal fluctuations while transcriptional changes of downstream effector genes, SOX9 and DMRT1, occur on a delayed timescale. Our findings reveal how the basic mechanisms of TSD operate in an ecologically relevant context. We present a hypothetical hierarchical model based on our findings as well as previous studies, in which temperature-sensitive alternative splicing incrementally influences the epigenetic landscape to affect the transcriptional activity of key sex-determining genes.

Dynamic Competition of Polycomb and Trithorax in Transcriptional Programming

Predicting regulatory potential from primary DNA sequences or transcription factor binding patterns is not possible. However, the annotation of the genome by chromatin proteins, histone modifications, and differential compaction is largely sufficient to reveal the locations of genes and their differential activity states. The Polycomb Group (PcG) and Trithorax Group (TrxG) proteins are the central players in this cell type-specific chromatin organization. PcG function was originally viewed as being solely repressive and irreversible, as observed at the homeotic loci in flies and mammals. However, it is now clear that modular and reversible PcG function is essential at most developmental genes. Focusing mainly on recent advances, we review evidence for how PcG and TrxG patterns change dynamically during cell type transitions. The ability to implement cell type-specific transcriptional programming with exquisite fidelity is essential for normal development.

Activity of PRC1 and Histone H2AK119 Monoubiquitination: Revising Popular Misconceptions

Polycomb group proteins are evolutionary conserved chromatin-modifying complexes, essential for the regulation of developmental and cell-identity genes. Polycomb-mediated transcriptional regulation is provided by two multi-protein complexes known as Polycomb repressive complex 1 (PRC1) and 2 (PRC2). Recent studies positioned PRC1 as a foremost executer of Polycomb-mediated transcriptional control. Mammalian PRC1 complexes can form multiple sub-complexes that vary in their core and accessory subunit composition, leading to fascinating and diverse transcriptional regulatory mechanisms employed by PRC1 complexes. These mechanisms include PRC1-catalytic activity toward monoubiquitination of histone H2AK119, a well-established hallmark of PRC1 complexes, whose importance has been long debated. In this review, the central roles that PRC1-catalytic activity plays in transcriptional repression are emphasized and the recent evidence supporting a role for PRC1 complexes in gene activation is discussed.

Priming exposures to lipopolysaccharides do not affect the induction of Polycomb target genes upon re-exposure

The Polycomb group (PcG) proteins are chromatin factors underlying the process of transcriptional memory to preserve developmental decisions and keep cellular identities. However, not only developmental signals need to be memorized and thus maintained during the life of an organism. For host protection against pathogens, also a memory of previous exposures to an immunogenic stimulus is crucial to mount a more protective immune response upon re-exposure. The antigen-specific adaptive immunity in vertebrates is an example of such a memory to previous immunogenic stimulation. Recently, adaptive characteristics were also attributed to innate immunity, which was classically seen to lack memory. However, the mechanistic details of an adaptive innate immune response are yet to be fully understood and chromatin-based epigenetic mechanisms seem to play an important role in this phenomenon. Possibly, PcG proteins can contribute to such an epigenetic innate immune memory. In this study, we analyzed whether the PcG system can mediate a transcriptional memory of exposure to lipopolysaccharides (LPS). To this end, various forms of LPS pre-treatment were applied to reporter cells and expression kinetics of PcG target genes were analyzed after a second LPS exposure. Neither single nor multiple LPS pre-treatment affected the induction of endogenous LPS-responsive transcripts upon re-exposure. Altogether, our extensive analyses did not provide any evidence for a PcG system-mediated memory of LPS stimulation.

Ubiquitin ligases HUWE1 and NEDD4 cooperatively control signal-dependent PRC2-Ezh1α/β-mediated adaptive stress response pathway in skeletal muscle cells

BACKGROUND

While the role of Polycomb group protein-mediated "cell memory" is well established in developmental contexts, little is known about their role in adult tissues and in particular in post-mitotic cells. Emerging evidence assigns a pivotal role in cell plasticity and adaptation. PRC2-Ezh1α/β signaling pathway from cytoplasm to chromatin protects skeletal muscle cells from oxidative stress. However, detailed mechanisms controlling degradation of cytoplasmic Ezh1β and assembly of canonical PRC2-Ezh1α repressive complex remain to be clarified.

RESULTS

Here, we report NEDD4 ubiquitin E3 ligase, as key regulator of Ezh1β. In addition, we report that ubiquitination and degradation of Ezh1β is controlled by another layer of regulation, that is, one specific phosphorylation of serine 560 located at Ezh1β-specific C terminal. Finally, we demonstrate that also Ezh1α needs to be stabilized under stress condition and this stabilization process requires decreased association pattern between another E3 ubiquitin ligase HUWE1.

CONCLUSIONS

Together, these results shed light on key components that regulate PRC2-Ezh1α/β pathway to direct modulation of epigenome plasticity and transcriptional output in skeletal muscle cells.

Mechanotransduction, nuclear architecture and epigenetics in Emery Dreifuss Muscular Dystrophy: tous pour un, un pour tous

The alteration of the several roles that Lamin A/C plays in the mammalian cell leads to a broad spectrum of pathologies that – all together – are named laminopathies. Among those, the Emery Dreifuss Muscular Dystrophy (EDMD) is of particular interest as, despite the several known mutations of Lamin A/C, the genotype–phenotype correlation still remains poorly understood; this suggests that the epigenetic background of patients might play an important role during the time course of the disease. Historically, both a mechanical role of Lamin A/C and a regulative one have been suggested as the driving force of laminopathies; however, those two hypotheses are not mutually exclusive. Recent scientific evidence shows that Lamin A/C sustains the correct gene expression at the epigenetic level thanks to the Lamina Associated Domains (LADs) reorganization and the crosstalk with the Polycomb Group of Proteins (PcG). Furthermore, the PcG-dependent histone mark H3K27me3 increases under mechanical stress, finally pointing out the link between the mechano-properties of the nuclear lamina and epigenetics. Here, we summarize the emerging mechanisms that could explain the high variability seen in Emery Dreifuss muscular dystrophy.

The diverse roles of DNA methylation in mammalian development and disease

DNA methylation is of paramount importance for mammalian embryonic development. DNA methylation has numerous functions: it is implicated in the repression of transposons and genes, but is also associated with actively transcribed gene bodies and, in some cases, with gene activation per se. In recent years, sensitive technologies have been developed that allow the interrogation of DNA methylation patterns from a small number of cells. The use of these technologies has greatly improved our knowledge of DNA methylation dynamics and heterogeneity in embryos and in specific tissues. Combined with genetic analyses, it is increasingly apparent that regulation of DNA methylation erasure and (re-)establishment varies considerably between different developmental stages. In this Review, we discuss the mechanisms and functions of DNA methylation and demethylation in both mice and humans at CpG-rich promoters, gene bodies and transposable elements. We highlight the dynamic erasure and re-establishment of DNA methylation in embryonic, germline and somatic cell development. Finally, we provide insights into DNA methylation gained from studying genetic diseases.

Senomic view of the cell: Senome versus Genome

In the legacy of Thomas Henry Huxley, and his 'epigenetic' philosophy of biology, cells are proposed to represent a trinity of three memory-storing media: Senome, Epigenome, and Genome that together comprise a cell-wide informational architecture. Our current preferential focus on the Genome needs to be complemented by a similar focus on the Epigenome and a here proposed Senome, representing the sum of all the sensory experiences of the cognitive cell and its sensing apparatus. Only then will biology be in a position to embrace the whole complexity of the eukaryotic cell, understanding its true nature which allows the communicative assembly of cells in the form of sentient multicellular organisms.