Noncoding transcription at enhancers: general principles and functional models

Long non-coding RNAs in corticogenesis: deciphering the non-coding code of the brain

Evidence on the role of long non-coding (lnc) RNAs has been accumulating over decades, but it has been only recently that advances in sequencing technologies have allowed the field to fully appreciate their abundance and diversity. Despite this, only a handful of lncRNAs have been phenotypically or mechanistically studied. Moreover, novel lncRNAs and new classes of RNAs are being discovered at growing pace, suggesting that this class of molecules may have functions as diverse as protein-coding genes. Interestingly, the brain is the organ where lncRNAs have the most peculiar features including the highest number of lncRNAs that are expressed, proportion of tissue-specific lncRNAs and highest signals of evolutionary conservation. In this work, we critically review the current knowledge about the steps that have led to the identification of the non-coding transcriptome including the general features of lncRNAs in different contexts in terms of both their genomic organisation, evolutionary origin, patterns of expression, and function in the developing and adult mammalian brain.

Activation of muscle enhancers by MyoD and epigenetic modifiers

The early 1980s revelation of cis-acting genomic elements, known as transcriptional enhancers, is still regarded as one of the fundamental discoveries in the genomic field. However, only with the emergence of genome-wide techniques has the genuine biological scope of enhancers begun to be fully uncovered. Massive scientific efforts of multiple laboratories rapidly advanced the overall perception that enhancers are typified by common epigenetic characteristics that distinguish their activating potential. Broadly, chromatin modifiers and transcriptional regulators lay down the essential foundations necessary for constituting enhancers in their activated form. Basing on genome-wide ChIP-sequencing of enhancer-related marks we identified myogenic enhancers before and after muscle differentiation and discovered that MyoD was bound to nearly a third of condition-specific enhancers. Experimental studies that tested the deposition patterns of enhancer-related epigenetic marks in MyoD-null myoblasts revealed the high dependency that a specific set of muscle enhancers have towards this transcriptional regulator. Re-expression of MyoD restored the deposition of enhancer-related marks at myotube-specific enhancers and partially at myoblasts-specific enhancers. Our proposed mechanistic model suggests that MyoD is involved in recruitment of methyltransferase Set7, acetyltransferase p300 and deposition of H3K4me1 and H3K27ac at myogenic enhancers. In addition, MyoD binding at enhancers is associated with PolII occupancy and with local noncoding transcription. Modulation of muscle enhancers is suggested to be coordinated via transcription factors docking, including c-Jun and Jdp2 that bind to muscle enhancers in a MyoD-dependent manner. We hypothesize that distinct transcription factors may act as placeholders and mediate the assembly of newly formed myogenic enhancers.

Genome-wide analysis of enhancer RNA in gene regulation across 12 mouse tissues

Enhancers play a crucial role in gene regulation but the participation of enhancer transcripts (i.e. enhancer RNA, eRNAs) in regulatory systems remains unclear. We provide a computational analysis on eRNAs using genome-wide data across 12 mouse tissues. The expression of genes targeted by transcribing enhancer is positively correlated with eRNA expression and significantly higher than expression of genes targeted by non-transcribing enhancers. This result implies eRNA transcription indicates a state of enhancer that further increases gene expression. This state of enhancer is tissue-specific, as the same enhancer differentially transcribes eRNAs across tissues. Therefore, the presence of eRNAs describes a tissue-specific state of enhancer that is generally associated with higher expressed target genes, surmising as to whether eRNAs have gene activation potential. We further found a large number of eRNAs contain regions in which sequences and secondary structures are similar to microRNAs. Interestingly, an increasing number of recent studies hypothesize that microRNAs may switch from their general repressive role to an activating role when targeting promoter sequences. Collectively, our results provide speculation that eRNAs may be associated with the selective activation of enhancer target genes.

Expanding the p53 regulatory network: LncRNAs take up the challenge

Long noncoding RNAs (lncRNAs) are rapidly emerging as important regulators of gene expression in a wide variety of physiological and pathological cellular processes. In particular, a number of studies revealed that some lncRNAs participate in the p53 pathway, the unquestioned protagonist of tumor suppressor response. Indeed, several lncRNAs are not only part of the large pool of genes coordinated by p53 transcription factor, but are also required by p53 to fine-tune its response and to fully accomplish its tumor suppressor program. In this review we will discuss the current and fast growing knowledge about the contribution of lncRNAs to the complexity of the p53 network, the different mechanisms by which they affect gene regulation in this context, and their involvement in cancer. The incipient impact of lncRNAs in the p53 biological response may encourage the development of therapies and diagnostic methods focused on these noncoding molecules. This article is part of a Special Issue entitled: Clues to long noncoding RNA taxonomy1, edited by Dr. Tetsuro Hirose and Dr. Shinichi Nakagawa.

Genome-Wide Definition of Promoter and Enhancer Usage during Neural Induction of Human Embryonic Stem Cells

Genome-wide mapping of transcriptional regulatory elements is an essential tool for understanding the molecular events orchestrating self-renewal, commitment and differentiation of stem cells. We combined high-throughput identification of transcription start sites with genome-wide profiling of histones modifications to map active promoters and enhancers in embryonic stem cells (ESCs) induced to neuroepithelial-like stem cells (NESCs). Our analysis showed that most promoters are active in both cell types while approximately half of the enhancers are cell-specific and account for most of the epigenetic changes occurring during neural induction, and most likely for the modulation of the promoters to generate cell-specific gene expression programs. Interestingly, the majority of the promoters activated or up-regulated during neural induction have a “bivalent” histone modification signature in ESCs, suggesting that developmentally-regulated promoters are already poised for transcription in ESCs, which are apparently pre-committed to neuroectodermal differentiation. Overall, our study provides a collection of differentially used enhancers, promoters, transcription starts sites, protein-coding and non-coding RNAs in human ESCs and ESC-derived NESCs, and a broad, genome-wide description of promoter and enhancer usage and of gene expression programs characterizing the transition from a pluripotent to a neural-restricted cell fate.

De Novo Reconstruction of Adipose Tissue Transcriptomes Reveals Long Non-coding RNA Regulators of Brown Adipocyte Development

Brown adipose tissue (BAT) protects against obesity by promoting energy expenditure via uncoupled respiration. To uncover BAT-specific long non-coding RNAs (lncRNAs), we used RNA-seq to reconstruct de novo transcriptomes of mouse brown, inguinal white, and epididymal white fat and identified ∼1,500 lncRNAs, including 127 BAT-restricted loci induced during differentiation and often targeted by key regulators PPARγ, C/EBPα, and C/EBPβ. One of them, lnc-BATE1, is required for establishment and maintenance of BAT identity and thermogenic capacity. lnc-BATE1 inhibition impairs concurrent activation of brown fat and repression of white fat genes and is partially rescued by exogenous lnc-BATE1 with mutated siRNA-targeting sites, demonstrating a function in trans. We show that lnc-BATE1 binds heterogeneous nuclear ribonucleoprotein U and that both are required for brown adipogenesis. Our work provides an annotated catalog for the study of fat depot-selective lncRNAs and establishes lnc-BATE1 as a regulator of BAT development and physiology.

The distal upstream promoter in Ly49 genes, Pro1, is active in mature NK cells and T cells, does not require TATA boxes, and displays enhancer activity

Missing self recognition of MHC class I molecules is mediated in murine species primarily through the stochastic expression of CD94/NKG2 and Ly49 receptors on NK cells. Previous studies have suggested that the stochastic expression of Ly49 receptors is achieved through the use of an alternate upstream promoter, designated Pro1, that is active only in immature NK cells and operates via the mutually exclusive binding of transcription initiation complexes to closely opposed forward and reverse TATA boxes, with forward transcription being transiently required to activate the downstream promoters, Pro2/Pro3, that are subsequently responsible for transcription in mature NK cells. In this study, we report that Pro1 transcripts are not restricted to immature NK cells but are also found in mature NK cells and T cells, and that Pro1 fragments display strong promoter activity in mature NK cell and T cell lines as well as in immature NK cells. However, the strength of promoter activity in vitro does not correlate well with Ly49 expression in vivo and forward promoter activity is generally weak or undetectable, suggesting that components outside of Pro1 are required for efficient forward transcription. Indeed, conserved sequences immediately upstream and downstream of the core Pro1 region were found to inhibit or enhance promoter activity. Most surprisingly, promoter activity does not require either the forward or reverse TATA boxes, but is instead dependent on residues in the largely invariant central region of Pro1. Importantly, Pro1 displays strong enhancer activity, suggesting that this may be its principal function in vivo.

Chromatin mechanisms in the developmental control of imprinted gene expression

Hundreds of protein-coding genes and regulatory non-coding RNAs (ncRNAs) are subject to genomic imprinting. The mono-allelic DNA methylation marks that control imprinted gene expression are somatically maintained throughout development, and this process is linked to specific chromatin features. Yet, at many imprinted genes, the mono-allelic expression is lineage or tissue-specific. Recent studies provide mechanistic insights into the developmentally-restricted action of the 'imprinting control regions' (ICRs). At several imprinted domains, the ICR expresses a long ncRNA that mediates chromatin repression in cis (and probably in trans as well). ICRs at other imprinted domains mediate higher-order chromatin structuration that enhances, or prevents, transcription of close-by genes. Here, we present how chromatin and ncRNAs contribute to developmental control of imprinted gene expression and discuss implications for disease. This article is part of a Directed Issue entitled: Epigenetics dynamics in development and disease.

Long Intergenic Non-Coding RNAs: Novel Drivers of Human Lymphocyte Differentiation

Upon recognition of a foreign antigen, CD4(+) naïve T lymphocytes proliferate and differentiate into subsets with distinct functions. This process is fundamental for the effective immune system function, as CD4(+) T cells orchestrate both the innate and adaptive immune response. Traditionally, this differentiation event has been regarded as the acquisition of an irreversible cell fate so that memory and effector CD4(+) T subsets were considered terminally differentiated cells or lineages. Consequently, these lineages are conventionally defined thanks to their prototypical set of cytokines and transcription factors. However, recent findings suggest that CD4(+) T lymphocytes possess a remarkable phenotypic plasticity, as they can often re-direct their functional program depending on the milieu they encounter. Therefore, new questions are now compelling such as which are the molecular determinants underlying plasticity and stability and how the balance between these two opposite forces drives the cell fate. As already mentioned, in some cases, the mere expression of cytokines and master regulators could not fully explain lymphocytes plasticity. We should consider other layers of regulation, including epigenetic factors such as the modulation of chromatin state or the transcription of non-coding RNAs, whose high cell-specificity give a hint on their involvement in cell fate determination. In this review, we will focus on the recent advances in understanding CD4(+) T lymphocytes subsets specification from an epigenetic point of view. In particular, we will emphasize the emerging importance of non-coding RNAs as key players in these differentiation events. We will also present here new data from our laboratory highlighting the contribution of long non-coding RNAs in driving human CD4(+) T lymphocytes differentiation.

Deciphering the regulatory logic of an ancient, ultraconserved nuclear receptor enhancer module

Cooperative, synergistic gene regulation by nuclear hormone receptors can increase sensitivity and amplify cellular responses to hormones. We investigated thyroid hormone (TH) and glucocorticoid (GC) synergy on the Krüppel-like factor 9 (Klf9) gene, which codes for a zinc finger transcription factor involved in development and homeostasis of diverse tissues. We identified regions of the Xenopus and mouse Klf9 genes 5-6 kb upstream of the transcription start sites that supported synergistic transactivation by TH plus GC. Within these regions, we found an orthologous sequence of approximately 180 bp that is highly conserved among tetrapods, but absent in other chordates, and possesses chromatin marks characteristic of an enhancer element. The Xenopus and mouse approximately 180-bp DNA element conferred synergistic transactivation by hormones in transient transfection assays, so we designate this the Klf9 synergy module (KSM). We identified binding sites within the mouse KSM for TH receptor, GC receptor, and nuclear factor κB. TH strongly increased recruitment of liganded GC receptor and serine 5 phosphorylated (initiating) RNA polymerase II to chromatin at the KSM, suggesting a mechanism for transcriptional synergy. The KSM is transcribed to generate long noncoding RNAs, which are also synergistically induced by combined hormone treatment, and the KSM interacts with the Klf9 promoter and a far upstream region through chromosomal looping. Our findings support that the KSM plays a central role in hormone regulation of vertebrate Klf9 genes, it evolved in the tetrapod lineage, and has been maintained by strong stabilizing selection.

Suppression of pervasive noncoding transcription in embryonic stem cells by esBAF

Hainer et al. show that esBAF, a SWI/SNF family nucleosome remodeling factor, suppresses transcription of noncoding RNAs (ncRNAs) from ∼57,000 nucleosome-depleted regions (NDRs) throughout the genome of mouse embryonic stem cells. esBAF’s function to enforce nucleosome occupancy adjacent to NDRs, but not its function to maintain NDRs in a nucleosome-free state, is necessary for silencing transcription over ncDNA. Finally, the ability of a strongly positioned nucleosome to repress ncRNA depends on its translational positioning.

Approximately 75% of the human genome is transcribed, the majority of which does not encode protein. However, many noncoding RNAs (ncRNAs) are rapidly degraded after transcription, and relatively few have established functions, questioning the significance of this observation. Here we show that esBAF, a SWI/SNF family nucleosome remodeling factor, suppresses transcription of ncRNAs from ∼57,000 nucleosome-depleted regions (NDRs) throughout the genome of mouse embryonic stem cells (ESCs). We show that esBAF functions to both keep NDRs nucleosome-free and promote elevated nucleosome occupancy adjacent to NDRs. Reduction of adjacent nucleosome occupancy upon esBAF depletion is strongly correlated with ncRNA expression, suggesting that flanking nucleosomes form a barrier to pervasive transcription. Upon forcing nucleosome occupancy near two NDRs using a nucleosome-positioning sequence, we found that esBAF is no longer required to silence transcription. Therefore, esBAF’s function to enforce nucleosome occupancy adjacent to NDRs, and not its function to maintain NDRs in a nucleosome-free state, is necessary for silencing transcription over ncDNA. Finally, we show that the ability of a strongly positioned nucleosome to repress ncRNA depends on its translational positioning. These data reveal a novel role for esBAF in suppressing pervasive transcription from open chromatin regions in ESCs.

Long noncoding RNAs in cardiovascular diseases

In recent year, increasing evidence suggests that noncoding RNAs play important roles in the regulation of tissue homeostasis and pathophysiological conditions. Besides small noncoding RNAs (eg, microRNAs), >200-nucleotide long transcripts, namely long noncoding RNAs (lncRNAs), can interfere with gene expressions and signaling pathways at various stages. In the cardiovascular system, studies have detected and characterized the expression of lncRNAs under normal physiological condition and in disease states. Several lncRNAs are regulated during acute myocardial infarction (eg, Novlnc6) and heart failure (eg, Mhrt), whereas others control hypertrophy, mitochondrial function and apoptosis of cardiomyocytes. In the vascular system, the endothelial-expressed lncRNAs (eg, MALAT1 and Tie-1-AS) can regulate vessel growth and function, whereas the smooth-muscle-expressed lncRNA smooth muscle and endothelial cell-enriched migration/differentiation-associated long noncoding RNA was recently shown to control the contractile phenotype of smooth muscle cells. This review article summarizes the data on lncRNA expressions in mouse and human and highlights identified cardiovascular lncRNAs that might play a role in cardiovascular diseases. Although our understanding of lncRNAs is still in its infancy, these examples may provide helpful insights how lncRNAs interfere with cardiovascular diseases.

An anti-silencer- and SATB1-dependent chromatin hub regulates Rag1 and Rag2 gene expression during thymocyte development

Rag1 and Rag2 gene expression in CD4(+)CD8(+) double-positive (DP) thymocytes depends on the activity of a distant anti-silencer element (ASE) that counteracts the activity of an intergenic silencer. However, the mechanistic basis for ASE activity is unknown. Here, we show that the ASE physically interacts with the distant Rag1 and Rag2 gene promoters in DP thymocytes, bringing the two promoters together to form an active chromatin hub. Moreover, we show that the ASE functions as a classical enhancer that can potently activate these promoters in the absence of the silencer or other locus elements. In thymocytes lacking the chromatin organizer SATB1, we identified a partial defect in Tcra gene rearrangement that was associated with reduced expression of Rag1 and Rag2 at the DP stage. SATB1 binds to the ASE and Rag promoters, facilitating inclusion of Rag2 in the chromatin hub and the loading of RNA polymerase II to both the Rag1 and Rag2 promoters. Our results provide a novel framework for understanding ASE function and demonstrate a novel role for SATB1 as a regulator of Rag locus organization and gene expression in DP thymocytes.

Long noncoding RNA transcriptome of plants

Since their discovery more than two decades ago, animal long noncoding RNAs (lncRNAs) have emerged as important regulators of many biological processes. Recently, a large number of lncRNAs have also been identified in higher plants, and here, we review their identification, classification and known regulatory functions in various developmental events and stress responses. Knowledge gained from a deeper understanding of this special group of noncoding RNAs may lead to biotechnological improvement of crops. Some possible examples in this direction are discussed.

RNA transcribed from a distal enhancer is required for activating the chromatin at the promoter of the gonadotropin α-subunit gene

Since the discovery that many transcriptional enhancers are transcribed into long noncoding RNAs termed "enhancer RNAs" (eRNAs), their putative role in enhancer function has been debated. Very recent evidence has indicted that some eRNAs play a role in initiating or activating transcription, possibly by helping recruit and/or stabilize binding of the general transcription machinery to the proximal promoter of their target genes. The distal enhancer of the gonadotropin hormone α-subunit gene, chorionic gonadotropin alpha (Cga), is responsible for Cga cell-specific expression in gonadotropes and thyrotropes, and we show here that it encodes two bidirectional nonpolyadenylated RNAs whose levels are increased somewhat by exposure to gonadotropin-releasing hormone but are not necessarily linked to Cga transcriptional activity. Knockdown of the more distal eRNA led to a drop in Cga mRNA levels, initially without effect on the forward eRNA levels. With time, however, the repression on the Cga increased, and the forward eRNA levels were suppressed also. We demonstrate that the interaction of the enhancer with the promoter is lost after eRNA knockdown. Dramatic changes also were seen in the chromatin, with an increase in total histone H3 occupancy throughout this region and a virtual loss of histone H3 Lys 4 trimethylation at the promoter following the eRNA knockdown. Moreover, histone H3 Lys 27 (H3K27) acetylation, which was found at both enhancer and promoter in wild-type cells, appeared to have been replaced by H3K27 trimethylation at the enhancer. Thus, the Cga eRNA mediates the physical interaction between these genomic regions and determines the chromatin structure of the proximal promoter to allow gene expression.

Role of Tet1 and 5-hydroxymethylcytosine in cocaine action

Ten-eleven translocation (TET) enzymes mediate the conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), which is enriched in brain, and its ultimate DNA demethylation. However, the influence of TET and 5hmC on gene transcription in brain remains elusive. We found that ten-eleven translocation protein 1 (TET1) was downregulated in mouse nucleus accumbens (NAc), a key brain reward structure, by repeated cocaine administration, which enhanced behavioral responses to cocaine. We then identified 5hmC induction in putative enhancers and coding regions of genes that have pivotal roles in drug addiction. Such induction of 5hmC, which occurred similarly following TET1 knockdown alone, correlated with increased expression of these genes as well as with their alternative splicing in response to cocaine administration. In addition, 5hmC alterations at certain loci persisted for at least 1 month after cocaine exposure. Together, these reveal a previously unknown epigenetic mechanism of cocaine action and provide new insight into how 5hmC regulates transcription in brain in vivo.

Long non-coding RNAs control hematopoietic stem cell function

Hematopoietic stem cells (HSCs) possess unique gene expression programs that enforce their identity and regulate lineage commitment. Long non-coding RNAs (lncRNAs) have emerged as important regulators of gene expression and cell fate decisions, although their functions in HSCs are unclear. Here we profiled the transcriptome of purified HSCs by deep sequencing and identified 323 unannotated lncRNAs. Comparing their expression in differentiated lineages revealed 159 lncRNAs enriched in HSCs, some of which are likely HSC specific (LncHSCs). These lncRNA genes share epigenetic features with protein-coding genes, including regulated expression via DNA methylation, and knocking down two LncHSCs revealed distinct effects on HSC self-renewal and lineage commitment. We mapped the genomic binding sites of one of these candidates and found enrichment for key hematopoietic transcription factor binding sites, especially E2A. Together, these results demonstrate that lncRNAs play important roles in regulating HSCs, providing an additional layer to the genetic circuitry controlling HSC function.

Comparative validation of the D. melanogaster modENCODE transcriptome annotation

Accurate gene model annotation of reference genomes is critical for making them useful. The modENCODE project has improved the D. melanogaster genome annotation by using deep and diverse high-throughput data. Since transcriptional activity that has been evolutionarily conserved is likely to have an advantageous function, we have performed large-scale interspecific comparisons to increase confidence in predicted annotations. To support comparative genomics, we filled in divergence gaps in the Drosophila phylogeny by generating draft genomes for eight new species. For comparative transcriptome analysis, we generated mRNA expression profiles on 81 samples from multiple tissues and developmental stages of 15 Drosophila species, and we performed cap analysis of gene expression in D. melanogaster and D. pseudoobscura. We also describe conservation of four distinct core promoter structures composed of combinations of elements at three positions. Overall, each type of genomic feature shows a characteristic divergence rate relative to neutral models, highlighting the value of multispecies alignment in annotating a target genome that should prove useful in the annotation of other high priority genomes, especially human and other mammalian genomes that are rich in noncoding sequences. We report that the vast majority of elements in the annotation are evolutionarily conserved, indicating that the annotation will be an important springboard for functional genetic testing by the Drosophila community.

An integrative analysis of TFBS-clustered regions reveals new transcriptional regulation models on the accessible chromatin landscape

DNase I hypersensitive sites (DHSs) define the accessible chromatin landscape and have revolutionised the discovery of distinct cis-regulatory elements in diverse organisms. Here, we report the first comprehensive map of human transcription factor binding site (TFBS)-clustered regions using Gaussian kernel density estimation based on genome-wide mapping of the TFBSs in 133 human cell and tissue types. Approximately 1.6 million distinct TFBS-clustered regions, collectively spanning 27.7% of the human genome, were discovered. The TFBS complexity assigned to each TFBS-clustered region was highly correlated with genomic location, cell selectivity, evolutionary conservation, sequence features, and functional roles. An integrative analysis of these regions using ENCODE data revealed transcription factor occupancy, transcriptional activity, histone modification, DNA methylation, and chromatin structures that varied based on TFBS complexity. Furthermore, we found that we could recreate lineage-branching relationships by simple clustering of the TFBS-clustered regions from terminally differentiated cells. Based on these findings, a model of transcriptional regulation determined by TFBS complexity is proposed.

Expression and regulation of long noncoding RNAs in TLR4 signaling in mouse macrophages

Background

Though long non-coding RNAs (lncRNAs) are emerging as critical regulators of immune responses, whether they are involved in LPS-activated TLR4 signaling pathway and how is their expression regulated in mouse macrophages are still unexplored.

Results

By repurposing expression microarray probes, we identified 994 lncRNAs in bone marrow-derived macrophages (BMDMs) and classified them to enhancer-like lncRNAs (elncRNAs) and promoter-associated lncRNAs (plncRNAs) according to chromatin signatures defined by relative levels of H3K4me1 and H3K4me3. Fifteen elncRNAs and 12 plncRNAs are differentially expressed upon LPS stimulation. The expression change of lncRNAs and their neighboring protein-coding genes are significantly correlated. Also, the regulation of both elncRNAs and plncRNAs expression is associated with H3K4me3 and H3K27Ac. Crucially, many identified LPS-regulated lncRNAs, such as lncRNA-Nfkb2 and lncRNA-Rel, locate near to immune response protein-coding genes. The majority of LPS-regulated lncRNAs had at least one binding site among the transcription factors p65, IRF3, JunB and cJun.

Conclusions

We established an integrative microarray analysis pipeline for profiling lncRNAs. Also, our results suggest that lncRNAs can be important regulators of LPS-induced innate immune response in BMDMs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1270-5) contains supplementary material, which is available to authorized users.

Repression of RNA Polymerase II Transcription by B2 RNA Depends on a Specific Pattern of Structural Regions in the RNA

B2 RNA is a mouse non-coding RNA that binds directly to RNA polymerase II (Pol II) and represses transcription by disrupting critical interactions between the polymerase and promoter DNA. How the structural regions within B2 RNA work together to mediate transcriptional repression is not well understood. To address this question, we systematically deleted structural regions from B2 RNA and determined the effects on transcriptional repression using a highly purified Pol II in vitro transcription system. Deletions that compromised the ability of B2 RNA to function as a transcriptional repressor were also tested for their ability to bind directly to Pol II, which enabled us to distinguish regions uniquely important for repression from those important for binding. We found that transcriptional repression requires a pattern of RNA structural motifs consisting of an extended single-stranded region bordered by two stem-loops. Hence, there is modularity in the function of the stem-loops in B2 RNA-when one stem-loop is deleted, another can take its place to enable transcriptional repression.

A shared architecture for promoters and enhancers

A new study detects unstable nascent RNAs and uncovers thousands of transcription initiation sites in promoters and enhancers. Detailed analysis shows that these initiation sites have a similar architecture and that they are differentiated by post-transcriptional regulation rather than transcription initiation.

Chromatin remodelling and autocrine TNFα are required for optimal interleukin-6 expression in activated human neutrophils

Controversy currently exists about the ability of human neutrophils to produce IL-6. Here, we show that the chromatin organization of the IL-6 genomic locus in human neutrophils is constitutively kept in an inactive configuration. However, we also show that upon exposure to stimuli that trigger chromatin remodelling at the IL-6 locus, such as ligands for TLR8 or, less efficiently, TLR4, highly purified neutrophils express and secrete IL-6. In TLR8-activated neutrophils, but not monocytes, IL-6 expression is preceded by the induction of a latent enhancer located 14 kb upstream of the IL-6 transcriptional start site. In addition, IL-6 induction is potentiated by endogenous TNFα, which prolongs the synthesis of the IκBζ co-activator and sustains C/EBPβ recruitment and histone acetylation at IL-6 regulatory regions. Altogether, these data clarify controversial literature on the ability of human neutrophils to generate IL-6 and uncover chromatin-dependent layers of regulation of IL-6 in these cells.

Ligand-dependent enhancer activation regulated by topoisomerase-I activity

The discovery that enhancers are regulated transcription units, encoding eRNAs, has raised new questions about the mechanisms of their activation. Here, we report an unexpected molecular mechanism that underlies ligand-dependent enhancer activation, based on DNA nicking to relieve torsional stress from eRNA synthesis. Using dihydrotestosterone (DHT)-induced binding of androgen receptor (AR) to prostate cancer cell enhancers as a model, we show rapid recruitment, within minutes, of DNA topoisomerase I (TOP1) to a large cohort of AR-regulated enhancers. Furthermore, we show that the DNA nicking activity of TOP1 is a prerequisite for robust eRNA synthesis and enhancer activation and is kinetically accompanied by the recruitment of ATR and the MRN complex, followed by additional components of DNA damage repair machinery to the AR-regulated enhancers. Together, our studies reveal a linkage between eRNA synthesis and ligand-dependent TOP1-mediated nicking-a strategy exerting quantitative effects on eRNA expression in regulating AR-bound enhancer-dependent transcriptional programs.

RNA switch at enhancers

Polycomb/Trithorax response elements (PRE/TREs) are genetic elements that can stably silence or activate genes. A new study describes how long noncoding RNAs (lncRNAs) transcribed from opposite strands of the Drosophila melanogaster vestigial PRE/TRE throw the switch between these two opposing epigenetic states.

Inflammation-sensitive super enhancers form domains of coordinately regulated enhancer RNAs

Enhancers are critical genomic elements that define cellular and functional identity through the spatial and temporal regulation of gene expression. Recent studies suggest that key genes regulating cell type-specific functions reside in enhancer-dense genomic regions (i.e., super enhancers, stretch enhancers). Here we report that enhancer RNAs (eRNAs) identified by global nuclear run-on sequencing are extensively transcribed within super enhancers and are dynamically regulated in response to cellular signaling. Using Toll-like receptor 4 (TLR4) signaling in macrophages as a model system, we find that transcription of super enhancer-associated eRNAs is dynamically induced at most of the key genes driving innate immunity and inflammation. Unexpectedly, genes repressed by TLR4 signaling are also associated with super enhancer domains and accompanied by massive repression of eRNA transcription. Furthermore, we find each super enhancer acts as a single regulatory unit within which eRNA and genic transcripts are coordinately regulated. The key regulatory activity of these domains is further supported by the finding that super enhancer-associated transcription factor binding is twice as likely to be conserved between human and mouse than typical enhancer sites. Our study suggests that transcriptional activities at super enhancers are critical components to understand the dynamic gene regulatory network.

Understanding the Complex Circuitry of lncRNAs at the X-inactivation Center and Its Implications in Disease Conditions

Balanced gene expression is a high priority in order to maintain optimal functioning since alterations and variations could result in acute consequences. X chromosome inactivation (X-inactivation) is one such strategy utilized by mammalian species to silence the extra X chromosome in females to uphold a similar level of expression between the two sexes. A functionally versatile class of molecules called long noncoding RNA (lncRNA) has emerged as key regulators of gene expression and plays important roles during development. An lncRNA that is indispensable for X-inactivation is X-inactive specific transcript (Xist), which induces a repressive epigenetic landscape and creates the inactive X chromosome (Xi). With recent advents in the field of X-inactivation, novel positive and negative lncRNA regulators of Xist such as Jpx and Tsix, respectively, have broadened the regulatory network of X-inactivation. Xist expression failure or dysregulation has been implicated in producing developmental anomalies and disease states. Subsequently, reactivation of the Xi at a later stage of development has also been associated with certain tumors. With the recent influx of information about lncRNA biology and advancements in methods to probe lncRNA, we can now attempt to understand this complex network of Xist regulation in development and disease. It has become clear that the presence of an extra set of genes could be fatal for the organism. Only by understanding the precise ways in which lncRNAs function can treatments be developed to bring aberrations under control. This chapter summarizes our current understanding and knowledge with regard to how lncRNAs are orchestrated at the X-inactivation center (Xic), with a special focus on how genetic diseases come about as a consequence of lncRNA dysregulation.

Epidaurus: aggregation and integration analysis of prostate cancer epigenome

Integrative analyses of epigenetic data promise a deeper understanding of the epigenome. Epidaurus is a bioinformatics tool used to effectively reveal inter-dataset relevance and differences through data aggregation, integration and visualization. In this study, we demonstrated the utility of Epidaurus in validating hypotheses and generating novel biological insights. In particular, we described the use of Epidaurus to (i) integrate epigenetic data from prostate cancer cell lines to validate the activation function of EZH2 in castration-resistant prostate cancer and to (ii) study the mechanism of androgen receptor (AR) binding deregulation induced by the knockdown of FOXA1. We found that EZH2's noncanonical activation function was reaffirmed by its association with active histone markers and the lack of association with repressive markers. More importantly, we revealed that the binding of AR was selectively reprogramed to promoter regions, leading to the up-regulation of hundreds of cancer-associated genes including EGFR. The prebuilt epigenetic dataset from commonly used cell lines (LNCaP, VCaP, LNCaP-Abl, MCF7, GM12878, K562, HeLa-S3, A549, HePG2) makes Epidaurus a useful online resource for epigenetic research. As standalone software, Epidaurus is specifically designed to process user customized datasets with both efficiency and convenience.

Divergent signalling pathways regulate lipopolysaccharide-induced eRNA expression in human monocytic THP1 cells

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eRNAs are expressed from enhancers and have been shown to regulate gene expression.

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Expression of eRNAs is widespread upon activation of the innate immune response.

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We show that the NF-κB signalling pathway regulates LPS-induced eRNAs.

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Expression of individual eRNAs is also dependent on ERK-1/2 and p38.

Recent studies have indicated that non-coding RNAs transcribed from enhancer regions are important regulators of enhancer function and gene expression. In this report, we have characterised the expression of six enhancer RNAs (eRNAs) induced in human monocytic THP1 cells following activation of the innate immune response by lipopolysaccharide (LPS). Specifically, we have demonstrated that LPS-induced expression of individual eRNAs is mediated through divergent intracellular signalling pathways that includes NF-κB and the mitogen activated protein kinases, extracellular regulated kinase-1/2 and p38.

Convergent transcription at intragenic super-enhancers targets AID-initiated genomic instability

Activation-induced cytidine deaminase (AID) initiates both somatic hypermutation (SHM) for antibody affinity maturation and DNA breakage for antibody class switch recombination (CSR) via transcription-dependent cytidine deamination of single-stranded DNA targets. Though largely specific for immunoglobulin genes, AID also acts on a limited set of off-targets, generating oncogenic translocations and mutations that contribute to B cell lymphoma. How AID is recruited to off-targets has been a long-standing mystery. Based on deep GRO-seq studies of mouse and human B lineage cells activated for CSR or SHM, we report that most robust AID off-target translocations occur within highly focal regions of target genes in which sense and antisense transcription converge. Moreover, we found that such AID-targeting "convergent" transcription arises from antisense transcription that emanates from super-enhancers within sense transcribed gene bodies. Our findings provide an explanation for AID off-targeting to a small subset of mostly lineage-specific genes in activated B cells.

TNFα signalling primes chromatin for NF-κB binding and induces rapid and widespread nucleosome repositioning

BACKGROUND

The rearrangement of nucleosomes along the DNA fiber profoundly affects gene expression, but little is known about how signalling reshapes the chromatin landscape, in three-dimensional space and over time, to allow establishment of new transcriptional programs.

RESULTS

Using micrococcal nuclease treatment and high-throughput sequencing, we map genome-wide changes in nucleosome positioning in primary human endothelial cells stimulated with tumour necrosis factor alpha (TNFα) - a proinflammatory cytokine that signals through nuclear factor kappa-B (NF-κB). Within 10 min, nucleosomes reposition at regions both proximal and distal to NF-κB binding sites, before the transcription factor quantitatively binds thereon. Similarly, in long TNFα-responsive genes, repositioning precedes transcription by pioneering elongating polymerases and appears to nucleate from intragenic enhancer clusters resembling super-enhancers. By 30 min, widespread repositioning throughout megabase pair-long chromosomal segments, with consequential effects on three-dimensional structure (detected using chromosome conformation capture), is seen.

CONCLUSIONS

Whilst nucleosome repositioning is viewed as a local phenomenon, our results point to effects occurring over multiple scales. Here, we present data in support of a TNFα-induced priming mechanism, mostly independent of NF-κB binding and/or elongating RNA polymerases, leading to a plastic network of interactions that affects DNA accessibility over large domains.

Epigenetic targets for novel therapies of lung diseases

In spite of substantial advances in defining the immunobiology and function of structural cells in lung diseases there is still insufficient knowledge to develop fundamentally new classes of drugs to treat many lung diseases. For example, there is a compelling need for new therapeutic approaches to address severe persistent asthma that is insensitive to inhaled corticosteroids. Although the prevalence of steroid-resistant asthma is 5-10%, severe asthmatics require a disproportionate level of health care spending and constitute a majority of fatal asthma episodes. None of the established drug therapies including long-acting beta agonists or inhaled corticosteroids reverse established airway remodeling. Obstructive airways remodeling in patients with chronic obstructive pulmonary disease (COPD), restrictive remodeling in idiopathic pulmonary fibrosis (IPF) and occlusive vascular remodeling in pulmonary hypertension are similarly unresponsive to current drug therapy. Therefore, drugs are needed to achieve long-acting suppression and reversal of pathological airway and vascular remodeling. Novel drug classes are emerging from advances in epigenetics. Novel mechanisms are emerging by which cells adapt to environmental cues, which include changes in DNA methylation, histone modifications and regulation of transcription and translation by noncoding RNAs. In this review we will summarize current epigenetic approaches being applied to preclinical drug development addressing important therapeutic challenges in lung diseases. These challenges are being addressed by advances in lung delivery of oligonucleotides and small molecules that modify the histone code, DNA methylation patterns and miRNA function.

Unbiased analysis of potential targets of breast cancer susceptibility loci by Capture Hi-C

Genome-wide association studies have identified more than 70 common variants that are associated with breast cancer risk. Most of these variants map to non-protein-coding regions and several map to gene deserts, regions of several hundred kilobases lacking protein-coding genes. We hypothesized that gene deserts harbor long-range regulatory elements that can physically interact with target genes to influence their expression. To test this, we developed Capture Hi-C (CHi-C), which, by incorporating a sequence capture step into a Hi-C protocol, allows high-resolution analysis of targeted regions of the genome. We used CHi-C to investigate long-range interactions at three breast cancer gene deserts mapping to 2q35, 8q24.21, and 9q31.2. We identified interaction peaks between putative regulatory elements ("bait fragments") within the captured regions and "targets" that included both protein-coding genes and long noncoding (lnc) RNAs over distances of 6.6 kb to 2.6 Mb. Target protein-coding genes were IGFBP5, KLF4, NSMCE2, and MYC; and target lncRNAs included DIRC3, PVT1, and CCDC26. For one gene desert, we were able to define two SNPs (rs12613955 and rs4442975) that were highly correlated with the published risk variant and that mapped within the bait end of an interaction peak. In vivo ChIP-qPCR data show that one of these, rs4442975, affects the binding of FOXA1 and implicate this SNP as a putative functional variant.

Noncoding RNAs and the control of hormonal signaling via nuclear receptor regulation

Despite its identification over 100 years ago, new discoveries continue to add to the complexity of the regulation of the endocrine system. Today the nuclear receptors (NRs) that play such a pivotal role in the extensive communication networks of hormones and gene expression remain an area of intense research. By orchestrating core processes, from metabolism to organismal development, the gene expression programs they control are dependent on their cellular context, their own levels, and those of numerous co-regulatory proteins. A previously unknown component of these networks, noncoding RNAs (ncRNAs) are now recognized as potent regulators of NR signaling, influencing receptor and co-factor levels and functions while being reciprocally regulated by the NRs themselves. This review explores the regulation enacted by microRNAs and long ncRNAs on NR function, using representative examples to show the varied roles of ncRNAs, in turn producing significant effects on the NR functional network in health and disease.

ICR noncoding RNA expression controls imprinting and DNA replication at the Dlk1-Dio3 domain

Imprinted genes play essential roles in development, and their allelic expression is mediated by imprinting control regions (ICRs). The Dlk1-Dio3 locus is among the few imprinted domains controlled by a paternally methylated ICR. The unmethylated maternal copy activates imprinted expression early in development through an unknown mechanism. We find that in mouse embryonic stem cells (ESCs) and in blastocysts, this function is linked to maternal, bidirectional expression of noncoding RNAs (ncRNAs) from the ICR. Disruption of ICR ncRNA expression in ESCs affected gene expression in cis, led to acquisition of aberrant histone and DNA methylation, delayed replication timing along the domain on the maternal chromosome, and changed its subnuclear localization. The epigenetic alterations persisted during differentiation and affected the neurogenic potential of the stem cells. Our data indicate that monoallelic expression at an ICR of enhancer RNA-like ncRNAs controls imprinted gene expression, epigenetic maintenance processes, and DNA replication in embryonic cells.

Regulation of transcription by long noncoding RNAs

Over the past decade there has been a greater understanding of genomic complexity in eukaryotes ushered in by the immense technological advances in high-throughput sequencing of DNA and its corresponding RNA transcripts. This has resulted in the realization that beyond protein-coding genes, there are a large number of transcripts that do not encode for proteins and, therefore, may perform their function through RNA sequences and/or through secondary and tertiary structural determinants. This review is focused on the latest findings on a class of noncoding RNAs that are relatively large (>200 nucleotides), display nuclear localization, and use different strategies to regulate transcription. These are exciting times for discovering the biological scope and the mechanism of action for these RNA molecules, which have roles in dosage compensation, imprinting, enhancer function, and transcriptional regulation, with a great impact on development and disease.

Transcriptional control of inflammatory responses

The inflammatory response requires the activation of a complex transcriptional program that is both cell-type- and stimulus-specific and involves the dynamic regulation of hundreds of genes. In the context of an inflamed tissue, extensive changes in gene expression occur in both parenchymal cells and infiltrating cells of the immune system. Recently, basic transcriptional mechanisms that control inflammation have been clarified at a genome scale, particularly in macrophages and conventional dendritic cells. The regulatory logic of distinct groups of inflammatory genes can be explained to some extent by identifiable sequence-encoded features of their chromatin organization, which impact on transcription factor (TF) accessibility and impose different requirements for gene activation. Moreover, it has become apparent that the interplay between TFs activated by inflammatory stimuli and master regulators exerts a crucial role in controlling cell-type-specific transcriptional outputs.

Tissue-specific RNA expression marks distant-acting developmental enhancers

Short non-coding transcripts can be transcribed from distant-acting transcriptional enhancer loci, but the prevalence of such enhancer RNAs (eRNAs) within the transcriptome, and the association of eRNA expression with tissue-specific enhancer activity in vivo remain poorly understood. Here, we investigated the expression dynamics of tissue-specific non-coding RNAs in embryonic mouse tissues via deep RNA sequencing. Overall, approximately 80% of validated in vivo enhancers show tissue-specific RNA expression that correlates with tissue-specific enhancer activity. Globally, we identified thousands of tissue-specifically transcribed non-coding regions (TSTRs) displaying various genomic hallmarks of bona fide enhancers. In transgenic mouse reporter assays, over half of tested TSTRs functioned as enhancers with reproducible activity in the predicted tissue. Together, our results demonstrate that tissue-specific eRNA expression is a common feature of in vivo enhancers, as well as a major source of extragenic transcription, and that eRNA expression signatures can be used to predict tissue-specific enhancers independent of known epigenomic enhancer marks.

Up to 80% of mammalian genomes are actively transcribed, producing large numbers of non-coding RNAs without known functions. One particularly exciting category of such non-coding transcripts are the recently discovered enhancer RNAs (eRNAs) transcribed from distant-acting enhancer elements. Studies in cell-based paradigms suggest a functional requirement for such eRNA in enhancer-mediated gene regulation. In this study, we explored the in vivo expression dynamics of tissue-specific non-coding RNAs in embryonic mouse tissues via in-depth transcriptome profiling. Our results suggest that enhancers may be a predominant function associated with differentially expressed non-coding loci across developing tissues, and that differential eRNA expression signatures from total RNA-Seq can be used to identify uncharacterized tissue-specific in vivo enhancers independent of known epigenomic marks. Our results highlight the widespread and potentially important role of eRNAs in orchestrating gene expression and the necessity for functional studies in interpreting genome-wide enhancer predictions.

Probabilistic bidirectional promoter switches: noncoding RNA takes control

The discovery of probabilistic promoter switches in genes that code for class I major histocompatibility complex receptors in mouse and human provides a useful paradigm to explain programmed cell fate decisions. These switches have preset probabilities of transcribing in either the sense or antisense direction, and the characteristics of individual switches are programmed by the relative affinity of competing transcription factor-binding sites. The noncoding RNAs produced from these switches can either activate or suppress gene transcription, based on their location relative to the promoter responsible for gene expression in mature cells. The switches are active in a developmental phase that precedes gene expression by mature cells, thus temporally separating the stochastic events that determine gene activation from the protein expression phase. This allows the probabilistic generation of variegated gene expression in the absence of selection and ensures that mature cells have stable expression of the genes. Programmed probabilistic switches may control cell fate decisions in many developmental systems, and therefore, it is important to investigate noncoding RNAs expressed by progenitor cells to determine if they are expressed in a stochastic manner at the single cell level. This review provides a summary of current knowledge regarding murine and human switches, followed by speculation on the possible involvement of probabilistic switches in other systems of programmed differentiation.

Effects of supercoiling on enhancer-promoter contacts

Using Brownian dynamics simulations, we investigate here one of possible roles of supercoiling within topological domains constituting interphase chromosomes of higher eukaryotes. We analysed how supercoiling affects the interaction between enhancers and promoters that are located in the same or in neighbouring topological domains. We show here that enhancer–promoter affinity and supercoiling act synergistically in increasing the fraction of time during which enhancer and promoter stay in contact. This stabilizing effect of supercoiling only acts on enhancers and promoters located in the same topological domain. We propose that the primary role of recently observed supercoiling of topological domains in interphase chromosomes of higher eukaryotes is to assure that enhancers contact almost exclusively their cognate promoters located in the same topological domain and avoid contacts with very similar promoters but located in neighbouring topological domains.

Long non-coding RNAs in the regulation of the immune response

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Widespread changes in lncRNA expresssion are associated with the immune response.

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lncRNAs regulate the inflammatory response following activation of innate immunity.

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lncRNAs regulate T cell differentiation and migration.

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The action of long non-coding RNAs is mediated via diverse mechanisms.

It is increasingly clear that long non-coding RNAs (lncRNAs) regulate a variety biological responses, and that they do so by a diverse range of mechanisms. In the field of immunology, recent publications have shown widespread changes in the expression of lncRNAs during the activation of the innate immune response and T cell development, differentiation, and activation. These lncRNAs control important aspects of immunity such as production of inflammatory mediators, differentiation, and cell migration through regulating protein–protein interactions or via their ability to basepair with RNA and DNA. We review the current understanding of the mechanism of action of these immune-related lncRNAs, discuss their impact on physiological and pathological processes, and highlight important areas of inquiry at the intersection between immunology and lncRNA biology.

A conserved noncoding sequence can function as a spermatocyte-specific enhancer and a bidirectional promoter for a ubiquitously expressed gene and a testis-specific long noncoding RNA

Tissue-specific gene expression is tightly regulated by various elements such as promoters, enhancers, and long noncoding RNAs (lncRNAs). In the present study, we identified a conserved noncoding sequence (CNS1) as a novel enhancer for the spermatocyte-specific mouse testicular cell adhesion molecule 1 (Tcam1) gene. CNS1 was located 3.4kb upstream of the Tcam1 gene and associated with histone H3K4 mono-methylation in testicular germ cells. By the in vitro reporter gene assay, CNS1 could enhance Tcam1 promoter activity only in GC-2spd(ts) cells, which were derived from mouse spermatocytes. When we integrated the 6.9-kb 5'-flanking sequence of Tcam1 with or without a deletion of CNS1 linked to the enhanced green fluorescent protein gene into the chromatin of GC-2spd(ts) cells, CNS1 significantly enhanced Tcam1 promoter activity. These results indicate that CNS1 could function as a spermatocyte-specific enhancer. Interestingly, CNS1 also showed high bidirectional promoter activity in the reporter assay, and consistent with this, the Smarcd2 gene and lncRNA, designated lncRNA-Tcam1, were transcribed from adjacent regions of CNS1. While Smarcd2 was ubiquitously expressed, lncRNA-Tcam1 expression was restricted to testicular germ cells, although this lncRNA did not participate in Tcam1 activation. Ubiquitous Smarcd2 expression was correlated to CpG hypo-methylation of CNS1 and partially controlled by Sp1. However, for lncRNA-Tcam1 transcription, the strong association with histone acetylation and histone H3K4 tri-methylation also appeared to be required. The present data suggest that CNS1 is a spermatocyte-specific enhancer for the Tcam1 gene and a bidirectional promoter of Smarcd2 and lncRNA-Tcam1.