Remodelers organize cellular chromatin by counteracting intrinsic histone-DNA sequence preferences in a class-specific manner

Neuronal and Muscle Differentiation of Mammalian Cells Is Accompanied by a Change in PHF10 Isoform Expression

The PBAF chromatin remodeling complex of the SWI/SNF family plays a critical role in the regulation of gene expression during tissue differentiation and organism development. The subunits of the PBAF complex have domains responsible for binding to N-terminal histone sequences. It determines the specificity of binding of the complex to chromatin. PHF10, a specific subunit of the PBAF complex, contains a DPF domain, which is a unique chromatin interaction domain. A PHF10 isoform that lacks the DPF domain is also present in vertebrate cells. This work shows that during neuronal and muscle differentiation of human and mouse cells, the expression of PHF10 isoforms changes: the form that does not have DPF replaces the form in which it is present. Replacement of PHF10 isoforms in the PBAF complex may affect its selectivity in the regulation of genes in differentiating cells.

RNA inhibits dMi-2/CHD4 chromatin binding and nucleosome remodeling

The ATP-dependent nucleosome remodeler Mi-2/CHD4 broadly modulates chromatin landscapes to repress transcription and to maintain genome integrity. Here we use individual nucleotide resolution crosslinking and immunoprecipitation (iCLIP) to show that Drosophila Mi-2 associates with thousands of mRNA molecules in vivo. Biochemical data reveal that recombinant dMi-2 preferentially binds to G-rich RNA molecules using two intrinsically disordered regions of unclear function. Pharmacological inhibition of transcription and RNase digestion approaches establish that RNA inhibits the association of dMi-2 with chromatin. We also show that RNA inhibits dMi-2-mediated nucleosome mobilization by competing with the nucleosome substrate. Importantly, this activity is shared by CHD4, the human homolog of dMi-2, strongly suggesting that RNA-mediated regulation of remodeler activity is an evolutionary conserved mechanism. Our data support a model in which RNA serves to protect actively transcribed regions of the genome from dMi-2/CHD4-mediated establishment of repressive chromatin structures.

In vivo analysis reveals that ATP-hydrolysis couples remodeling to SWI/SNF release from chromatin

ATP-dependent chromatin remodelers control the accessibility of genomic DNA through nucleosome mobilization. However, the dynamics of genome exploration by remodelers, and the role of ATP hydrolysis in this process remain unclear. We used live-cell imaging of Drosophila polytene nuclei to monitor Brahma (BRM) remodeler interactions with its chromosomal targets. In parallel, we measured local chromatin condensation and its effect on BRM association. Surprisingly, only a small portion of BRM is bound to chromatin at any given time. BRM binds decondensed chromatin but is excluded from condensed chromatin, limiting its genomic search space. BRM-chromatin interactions are highly dynamic, whereas histone-exchange is limited and much slower. Intriguingly, loss of ATP hydrolysis enhanced chromatin retention and clustering of BRM, which was associated with reduced histone turnover. Thus, ATP hydrolysis couples nucleosome remodeling to remodeler release, driving a continuous transient probing of the genome.

Subunits of the PBAP Chromatin Remodeler Are Capable of Mediating Enhancer-Driven Transcription in Drosophila

The chromatin remodeler SWI/SNF is an important participant in gene activation, functioning predominantly by opening the chromatin structure on promoters and enhancers. Here, we describe its novel mode of action in which SWI/SNF factors mediate the targeted action of an enhancer. We studied the functions of two signature subunits of PBAP subfamily, BAP170 and SAYP, in Drosophila. These subunits were stably tethered to a transgene reporter carrying the hsp70 core promoter. The tethered subunits mediate transcription of the reporter in a pattern that is generated by enhancers close to the insertion site in multiple loci throughout the genome. Both tethered SAYP and BAP170 recruit the whole PBAP complex to the reporter promoter. However, we found that BAP170-dependent transcription is more resistant to the depletion of other PBAP subunits, suggesting that BAP170 may play a more critical role in establishing enhancer-dependent transcription.

Chromatin Modifiers in Transcriptional Regulation: New Findings and Prospects

Histone-modifying and remodeling complexes are considered the main coregulators that affect transcription by changing the chromatin structure. Coordinated action by these complexes is essential for the transcriptional activation of any eukaryotic gene. In this review, we discuss current trends in the study of histone modifiers and chromatin remodelers, including the functional impact of transcriptional proteins/ complexes i.e., "pioneers"; remodeling and modification of non-histone proteins by transcriptional complexes; the supplementary functions of the non-catalytic subunits of remodelers, and the participation of histone modifiers in the "pause" of RNA polymerase II. The review also includes a scheme illustrating the mechanisms of recruitment of the main classes of remodelers and chromatin modifiers to various sites in the genome and their functional activities.

Remodeler Catalyzed Nucleosome Repositioning: Influence of Structure and Stability

The packaging of the eukaryotic genome into chromatin regulates the storage of genetic information, including the access of the cell’s DNA metabolism machinery. Indeed, since the processes of DNA replication, translation, and repair require access to the underlying DNA, several mechanisms, both active and passive, have evolved by which chromatin structure can be regulated and modified. One mechanism relies upon the function of chromatin remodeling enzymes which couple the free energy obtained from the binding and hydrolysis of ATP to the mechanical work of repositioning and rearranging nucleosomes. Here, we review recent work on the nucleosome mobilization activity of this essential family of molecular machines.

Nucleosome Positioning around Transcription Start Site Correlates with Gene Expression Only for Active Chromatin State in Drosophila Interphase Chromosomes

We analyzed the whole-genome experimental maps of nucleosomes in Drosophila melanogaster and classified genes by the expression level in S2 cells (RPKM value, reads per kilobase million) as well as the number of tissues in which a gene was expressed (breadth of expression, BoE). Chromatin in 5′-regions of genes we classified on four states according to the hidden Markov model (4HMM). Only the Aquamarine chromatin state we considered as Active, while the rest three states we defined as Non-Active. Surprisingly, about 20/40% of genes with 5′-regions mapped to Active/Non-Active chromatin possessed the minimal/at least modest RPKM and BoE. We found that regardless of RPKM/BoE the genes of Active chromatin possessed the regular nucleosome arrangement in 5′-regions, while genes of Non-Active chromatin did not show respective specificity. Only for genes of Active chromatin the RPKM/BoE positively correlates with the number of nucleosome sites upstream/around TSS and negatively with that downstream TSS. We propose that for genes of Active chromatin, regardless of RPKM value and BoE the nucleosome arrangement in 5′-regions potentiates transcription, while for genes of Non-Active chromatin, the transcription machinery does not require the substantial support from nucleosome arrangement to influence gene expression.

The regulatory function of dIno80 correlates with its DNA binding activity

The INO80 complex, including the Ino80 protein, forms a highly conserved canonical complex that remodels chromatin in the context of multiple cellular functions. The Drosophila homologue, dIno80, is involved in homeotic gene regulation during development as a canonical Pho-dIno80 complex. Previously, we found that dIno80 regulates homeotic genes by interacting with epigenetic regulators, such as polycomb and trithorax, suggesting the occurrence of non-canonical Ino80 complexes. Here using spectroscopic methods and gel retardation assays, we identified a set of consensus DNA sequences that DNA binding domain of dIno80 (DBINO) interacts with having differential affinity and high specificity. Testing these sequences in reporter assays, showed that this interaction can positively regulate transcription. These results suggest that, dIno80 has a sequence preference for interaction with DNA leading to transcriptional changes.

Dangerous liaisons: interplay between SWI/SNF, NuRD, and Polycomb in chromatin regulation and cancer

In this review, Bracken et al. discuss the functional organization and biochemical activities of remodelers and Polycomb and explore how they work together to control cell differentiation and the maintenance of cell identity. They also discuss how mutations in the genes encoding these various chromatin regulators contribute to oncogenesis by disrupting the chromatin equilibrium.

Changes in chromatin structure mediated by ATP-dependent nucleosome remodelers and histone modifying enzymes are integral to the process of gene regulation. Here, we review the roles of the SWI/SNF (switch/sucrose nonfermenting) and NuRD (nucleosome remodeling and deacetylase) and the Polycomb system in chromatin regulation and cancer. First, we discuss the basic molecular mechanism of nucleosome remodeling, and how this controls gene transcription. Next, we provide an overview of the functional organization and biochemical activities of SWI/SNF, NuRD, and Polycomb complexes. We describe how, in metazoans, the balance of these activities is central to the proper regulation of gene expression and cellular identity during development. Whereas SWI/SNF counteracts Polycomb, NuRD facilitates Polycomb repression on chromatin. Finally, we discuss how disruptions of this regulatory equilibrium contribute to oncogenesis, and how new insights into the biological functions of remodelers and Polycombs are opening avenues for therapeutic interventions on a broad range of cancer types.

ADF1 and BEAF-32 chromatin proteins affect nucleosome positioning and DNA decompaction in 61C7/C8 interband region of Drosophila melanogaster polytene chromosomes

The formation of interphase chromosomes is a multi-level process in which DNA is compacted several thousandfold by association with histones and non-histone proteins. The first step of compaction includes the formation of nucleosomes – the basic repeating units of chromatin. Further packaging occurs due to DNA binding to histone H1 and non-histone proteins involved in enhancer-promoter and insulator interactions. Under these conditions, the genome retains its functionality due to the dynamic and uneven DNA compaction along the chromatin fiber. Since the DNA compaction level affects the transcription activity of a certain genomic region, it is important to understand the interplay between the factors acting at different levels of the packaging process. Drosophila polytene chromosomes are an excellent model system for studying the molecular mechanisms that determine DNA compaction degree. The unevenness of DNA packaging along the chromatin fiber is easily observed along these chromosomes due to their large size and specific banding pattern. The purpose of this study was to figure out the role of two non-histone regulatory proteins, ADF1 and BEAF-32, in the DNA packaging process from nucleosome positioning to the establishment of the final chromosome structure. We studied the impact of mutations that affect ADF1 and BEAF-32 binding sites on the formation of 61C7/C8 interband – one of the decompacted regions of Drosophila polytene chromosomes. We show that such mutations led to the collapse of an interband, which was accompanied with increased nucleosome stability. We also find that ADF1 and BEAF-32 binding sites are essential for the rescue of lethality caused by the null allele of bantam microRNA gene located in the region 61C7/C8.

Characterizing the nuclease accessibility of DNA in human cells to map higher order structures of chromatin

Packaging of DNA into chromatin regulates DNA accessibility and consequently all DNA-dependent processes. The nucleosome is the basic packaging unit of DNA forming arrays that are suggested, by biochemical studies, to fold hierarchically into ordered higher-order structures of chromatin. This organization has been recently questioned using microscopy techniques, proposing an irregular structure. To address the principles of chromatin organization, we applied an in situ differential MNase-seq strategy and analyzed in silico the results of complete and partial digestions of human chromatin. We investigated whether different levels of chromatin packaging exist in the cell. We assessed the accessibility of chromatin within distinct domains of kb to Mb genomic regions, performed statistical analyses and computer modelling. We found no difference in MNase accessibility, suggesting no difference in fiber folding between domains of euchromatin and heterochromatin or between other sequence and epigenomic features of chromatin. Thus, our data suggests the absence of differentially organized domains of higher-order structures of chromatin. Moreover, we identified only local structural changes, with individual hyper-accessible nucleosomes surrounding regulatory elements, such as enhancers and transcription start sites. The regulatory sites per se are occupied with structurally altered nucleosomes, exhibiting increased MNase sensitivity. Our findings provide biochemical evidence that supports an irregular model of large-scale chromatin organization.

The CHD6 chromatin remodeler is an oxidative DNA damage response factor

Cell survival after oxidative DNA damage requires signaling, repair and transcriptional events often enabled by nucleosome displacement, exchange or removal by chromatin remodeling enzymes. Here, we show that Chromodomain Helicase DNA-binding protein 6 (CHD6), distinct to other CHD enzymes, is stabilized during oxidative stress via reduced degradation. CHD6 relocates rapidly to DNA damage in a manner dependent upon oxidative lesions and a conserved N-terminal poly(ADP-ribose)-dependent recruitment motif, with later retention requiring the double chromodomain and central core. CHD6 ablation increases reactive oxygen species persistence and impairs anti-oxidant transcriptional responses, leading to elevated DNA breakage and poly(ADP-ribose) induction that cannot be rescued by catalytic or double chromodomain mutants. Despite no overt epigenetic or DNA repair abnormalities, CHD6 loss leads to impaired cell survival after chronic oxidative stress, abnormal chromatin relaxation, amplified DNA damage signaling and checkpoint hypersensitivity. We suggest that CHD6 is a key regulator of the oxidative DNA damage response.

Oxidative DNA damage is associated with nucleosome respacing and transcriptional changes requiring chromatin remodeling enzymes. Here, the authors reveal that the CHD6 remodeler is a DNA damage response factor that relocates to damaged sites and promotes cell survival following oxidative damage.

The Nucleosome Remodeling and Deacetylation Complex Modulates Chromatin Structure at Sites of Active Transcription to Fine-Tune Gene Expression

Chromatin remodeling complexes play essential roles in metazoan development through widespread control of gene expression, but the precise molecular mechanisms by which they do this in vivo remain ill defined. Using an inducible system with fine temporal resolution, we show that the nucleosome remodeling and deacetylation (NuRD) complex controls chromatin architecture and the protein binding repertoire at regulatory regions during cell state transitions. This is primarily exerted through its nucleosome remodeling activity while deacetylation at H3K27 follows changes in gene expression. Additionally, NuRD activity influences association of RNA polymerase II at transcription start sites and subsequent nascent transcript production, thereby guiding the establishment of lineage-appropriate transcriptional programs. These findings provide a detailed molecular picture of genome-wide modulation of lineage-specific transcription by an essential chromatin remodeling complex as well as insight into the orchestration of molecular events involved in transcriptional transitions in vivo.

Video Abstract

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NuRD increases nucleosome density, expelling TFs or inhibiting recruitment

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NuRD displaces RNA Pol II from TSSs, reducing nascent transcription

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Local gains in TF and Mediator occupancy can be indirect effects of NuRD activity

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Resetting protein binding at regulatory elements can promote or suppress transcription

Covalent Modifications of Histone H3K9 Promote Binding of CHD3

Chromatin remodeling is required for genome function and is facilitated by ATP-dependent complexes, such as nucleosome remodeling and deacetylase (NuRD). Among its core components is the chromodomain helicase DNA binding protein 3 (CHD3) whose functional significance is not well established. Here, we show that CHD3 co-localizes with the other NuRD subunits, including HDAC1, near the H3K9ac-enriched promoters of the NuRD target genes. The tandem PHD fingers of CHD3 bind histone H3 tails and posttranslational modifications that increase hydrophobicity of H3K9-methylation or acetylation (H3K9me3 or H3K9ac)-enhance this interaction. Binding of CHD3 PHDs promotes H3K9_Cme3-nucleosome unwrapping in vitro and perturbs the pericentric heterochromatin structure in vivo. Methylation or acetylation of H3K9 uniquely alleviates the intra-nucleosomal interaction of histone H3 tails, increasing H3K9 accessibility. Collectively, our data suggest that the targeting of covalently modified H3K9 by CHD3 might be essential in diverse functions of NuRD.

CHD3 and CHD4 form distinct NuRD complexes with different yet overlapping functionality

CHD3 and CHD4 (Chromodomain Helicase DNA binding protein), two highly similar representatives of the Mi-2 subfamily of SF2 helicases, are coexpressed in many cell lines and tissues and have been reported to act as the motor subunit of the NuRD complex (nucleosome remodeling and deacetylase activities). Besides CHD proteins, NuRD contains several repressors like HDAC1/2, MTA2/3 and MBD2/3, arguing for a role as a transcriptional repressor. However, the subunit composition varies among cell- and tissue types and physiological conditions. In particular, it is unclear if CHD3 and CHD4 coexist in the same NuRD complex or whether they form distinct NuRD complexes with specific functions. We mapped the CHD composition of NuRD complexes in mammalian cells and discovered that they are isoform-specific, containing either the monomeric CHD3 or CHD4 ATPase. Both types of complexes exhibit similar intranuclear mobility, interact with HP1 and rapidly accumulate at UV-induced DNA repair sites. But, CHD3 and CHD4 exhibit distinct nuclear localization patterns in unperturbed cells, revealing a subset of specific target genes. Furthermore, CHD3 and CHD4 differ in their nucleosome remodeling and positioning behaviour in vitro. The proteins form distinct CHD3- and CHD4-NuRD complexes that do not only repress, but can just as well activate gene transcription of overlapping and specific target genes.

EcR recruits dMi-2 and increases efficiency of dMi-2-mediated remodelling to constrain transcription of hormone-regulated genes

Gene regulation by steroid hormones plays important roles in health and disease. In Drosophila, the hormone ecdysone governs transitions between key developmental stages. Ecdysone-regulated genes are bound by a heterodimer of ecdysone receptor (EcR) and Ultraspiracle. According to the bimodal switch model, steroid hormone receptors recruit corepressors in the absence of hormone and coactivators in its presence. Here we show that the nucleosome remodeller dMi-2 is recruited to ecdysone-regulated genes to limit transcription. Contrary to the prevalent model, recruitment of the dMi-2 corepressor increases upon hormone addition to constrain gene activation through chromatin remodelling. Furthermore, EcR and dMi-2 form a complex that is devoid of Ultraspiracle. Unexpectedly, EcR contacts the dMi-2 ATPase domain and increases the efficiency of dMi-2-mediated nucleosome remodelling. This study identifies a non-canonical EcR-corepressor complex with the potential for a direct regulation of ATP-dependent nucleosome remodelling by a nuclear hormone receptor.

Chromatin remodeling during the in vivo glial differentiation in early Drosophila embryos

Chromatin remodeling plays a critical role in gene regulation and impacts many biological processes. However, little is known about the relationship between chromatin remodeling dynamics and in vivo cell lineage commitment. Here, we reveal the patterns of histone modification change and nucleosome positioning dynamics and their epigenetic regulatory roles during the in vivo glial differentiation in early Drosophila embryos. The genome-wide average H3K9ac signals in promoter regions are decreased in the glial cells compared to the neural progenitor cells. However, H3K9ac signals are increased in a group of genes that are up-regulated in glial cells and involved in gliogenesis. There occurs extensive nucleosome remodeling including shift, loss, and gain. Nucleosome depletion regions (NDRs) form in both promoters and enhancers. As a result, the associated genes are up-regulated. Intriguingly, NDRs form in two fashions: nucleosome shift and eviction. Moreover, the mode of NDR formation is independent of the original chromatin state of enhancers in the neural progenitor cells.

Human INO80/YY1 chromatin remodeling complex transcriptionally regulates the BRCA2- and CDKN1A-interacting protein (BCCIP) in cells

The BCCIP (BRCA2- and CDKN1A-interacting protein) is an important cofactor for BRCA2 in tumor suppression. Although the low expression of BCCIP is observed in multiple clinically diagnosed primary tumor tissues such as ovarian cancer, renal cell carcinoma and colorectal carcinoma, the mechanism of how BCCIP is regulated in cells is still unclear. The human INO80/YY1 chromatin remodeling complex composed of 15 subunits catalyzes ATP-dependent sliding of nucleosomes along DNA. Here, we first report that BCCIP is a novel target gene of the INO80/YY1 complex by presenting a series of experimental evidence. Gene expression studies combined with siRNA knockdown data locked candidate genes including BCCIP of the INO80/YY1 complex. Silencing or over-expressing the subunits of the INO80/YY1 complex regulates the expression level of BCCIP both in mRNA and proteins in cells. Also, the functions of INO80/YY1 complex in regulating the transactivation of BCCIP were confirmed by luciferase reporter assays. Chromatin immunoprecipitation (ChIP) experiments clarify the enrichment of INO80 and YY1 at +0.17 kb downstream of the BCCIP transcriptional start site. However, this enrichment is significantly inhibited by either knocking down INO80 or YY1, suggesting the existence of both INO80 and YY1 is required for recruiting the INO80/YY1 complex to BCCIP promoter region. Our findings strongly indicate that BCCIP is a potential target gene of the INO80/YY1 complex.

Interaction of the Chromatin Remodeling Protein hINO80 with DNA

The presence of a highly conserved DNA binding domain in INO80 subfamily predicted that INO80 directly interacts with DNA and we demonstrated its DNA binding activity in vitro. Here we report the consensus motif recognized by the DBINO domain identified by SELEX method and demonstrate the specific interaction of INO80 with the consensus motif. We show that INO80 significantly down regulates the reporter gene expression through its binding motif, and the repression is dependent on the presence of INO80 but not YY1 in the cell. The interaction is lost if specific residues within the consensus motif are altered. We identify a large number of potential target sites of INO80 in the human genome through in silico analysis that can grouped into three classes; sites that contain the recognition sequence for INO80 and YY1, only YY1 and only INO80. We demonstrate the binding of INO80 to a representative set of sites in HEK cells and the correlated repressive histone modifications around the binding motif. In the light of the role of INO80 in homeotic gene regulation in Drosophila as an Enhancer of trithorax and polycomb protein (ETP) that can modify the effect of both repressive complexes like polycomb as well as the activating complex like trithorax, it remains to be seen if INO80 can act as a recruiter of chromatin modifying complexes.

Transcriptional Regulators Compete with Nucleosomes Post-replication

Every nucleosome across the genome must be disrupted and reformed when the replication fork passes, but how chromatin organization is re-established following replication is unknown. To address this problem, we have developed Mapping In vivo Nascent Chromatin with EdU and sequencing (MINCE-seq) to characterize the genome-wide location of nucleosomes and other chromatin proteins behind replication forks at high temporal and spatial resolution. We find that the characteristic chromatin landscape at Drosophila promoters and enhancers is lost upon replication. The most conspicuous changes are at promoters that have high levels of RNA polymerase II (RNAPII) stalling and DNA accessibility and show specific enrichment for the BRM remodeler. Enhancer chromatin is also disrupted during replication, suggesting a role for transcription factor (TF) competition in nucleosome re-establishment. Thus, the characteristic nucleosome landscape emerges from a uniformly packaged genome by the action of TFs, RNAPII, and remodelers minutes after replication fork passage.

Ino80 is essential for proximal-distal axis asymmetry in part by regulating Bmp4 expression

Background

Understanding how embryos specify asymmetric axes is a major focus of biology. While much has been done to discover signaling pathways and transcription factors important for axis specification, comparatively little is known about how epigenetic regulators are involved. Epigenetic regulators operate downstream of signaling pathways and transcription factors to promote nuclear processes, most prominently transcription. To discover novel functions for these complexes in axis establishment during early embryonic development, we characterized phenotypes of a mouse knockout (KO) allele of the chromatin remodeling Ino80 ATPase.

Results

Ino80 KO embryos implant, but fail to develop beyond the egg cylinder stage. Ino80 KO embryonic stem cells (ESCs) are viable and maintain alkaline phosphatase activity, which is suggestive of pluripotency, but they fail to fully differentiate as either embryoid bodies or teratomas. Gene expression analysis of Ino80 KO early embryos by in situ hybridization and embryoid bodies by RT-PCR shows elevated Bmp4 expression and reduced expression of distal visceral endoderm (DVE) markers Cer1, Hex, and Lefty1. In culture, Bmp4 maintains stem cell pluripotency and when overexpressed is a known negative regulator of DVE differentiation in the early embryo. Consistent with the early embryo, we observed upregulated Bmp4 expression and down-regulated Cer1, Hex, and Lefty1 expression when Ino80 KO ESCs are differentiated in a monolayer. Molecular studies in these same cells demonstrate that Ino80 bound to the Bmp4 promoter regulates its chromatin structure, which correlates with enhanced SP1 binding. These results in combination suggest that Ino80 directly regulates the chromatin structure of the Bmp4 promoter with consequences to gene expression.

Conclusions

In contrast to Ino80 KO differentiated cells, our experiments show that undifferentiated Ino80 KO ESCs are viable, but fail to differentiate in culture and in the early embryo. Ino80 KO ESCs and the early embryo up-regulate Bmp4 expression and down-regulate the expression of DVE markers Cer1, Hex and Lefty1. Based on this data, we propose a model where the Ino80 chromatin remodeling complex represses Bmp4 expression in the early embryo, thus promoting DVE differentiation and successful proximal-distal axis establishment. These results are significant because they show that epigenetic regulators have specific roles in establishing embryonic axes. By further characterizing these complexes, we will deepen our understanding of how the mammalian embryo is patterned by epigenetic regulators.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0238-5) contains supplementary material, which is available to authorized users.

Genome-wide profiling of nucleosome sensitivity and chromatin accessibility in Drosophila melanogaster

Nucleosomal DNA is thought to be generally inaccessible to DNA-binding factors, such as micrococcal nuclease (MNase). Here, we digest Drosophila chromatin with high and low concentrations of MNase to reveal two distinct nucleosome types: MNase-sensitive and MNase-resistant. MNase-resistant nucleosomes assemble on sequences depleted of A/T and enriched in G/C-containing dinucleotides, whereas MNase-sensitive nucleosomes form on A/T-rich sequences found at transcription start and termination sites, enhancers and DNase I hypersensitive sites. Estimates of nucleosome formation energies indicate that MNase-sensitive nucleosomes tend to be less stable than MNase-resistant ones. Strikingly, a decrease in cell growth temperature of about 10°C makes MNase-sensitive nucleosomes less accessible, suggesting that observed variations in MNase sensitivity are related to either thermal fluctuations of chromatin fibers or the activity of enzymatic machinery. In the vicinity of active genes and DNase I hypersensitive sites nucleosomes are organized into periodic arrays, likely due to 'phasing' off potential barriers formed by DNA-bound factors or by nucleosomes anchored to their positions through external interactions. The latter idea is substantiated by our biophysical model of nucleosome positioning and energetics, which predicts that nucleosomes immediately downstream of transcription start sites are anchored and recapitulates nucleosome phasing at active genes significantly better than sequence-dependent models.

Constraint of gene expression by the chromatin remodelling protein CHD4 facilitates lineage specification

Chromatin remodelling proteins are essential for different aspects of metazoan biology, yet functional details of why these proteins are important are lacking. Although it is possible to describe the biochemistry of how they remodel chromatin, their chromatin-binding profiles in cell lines, and gene expression changes upon loss of a given protein, in very few cases can this easily translate into an understanding of how the function of that protein actually influences a developmental process. Here, we investigate how the chromatin remodelling protein CHD4 facilitates the first lineage decision in mammalian embryogenesis. Embryos lacking CHD4 can form a morphologically normal early blastocyst, but are unable to successfully complete the first lineage decision and form functional trophectoderm (TE). In the absence of a functional TE, Chd4 mutant blastocysts do not implant and are hence not viable. By measuring transcript levels in single cells from early embryos, we show that CHD4 influences the frequency at which unspecified cells in preimplantation stage embryos express lineage markers prior to the execution of this first lineage decision. In the absence of CHD4, this frequency is increased in 16-cell embryos, and by the blastocyst stage cells fail to properly adopt a TE gene expression programme. We propose that CHD4 allows cells to undertake lineage commitment in vivo by modulating the frequency with which lineage-specification genes are expressed. This provides novel insight into both how lineage decisions are made in mammalian cells, and how a chromatin remodelling protein functions to facilitate lineage commitment.

Active promoters give rise to false positive 'Phantom Peaks' in ChIP-seq experiments

Chromatin immunoprecipitation (ChIP) is widely used to identify chromosomal binding sites. Chromatin proteins are cross-linked to their target sequences in living cells. The purified chromatin is sheared and the relevant protein is enriched by immunoprecipitation with specific antibodies. The co-purifying genomic DNA is then determined by massive parallel sequencing (ChIP-seq).

We applied ChIP-seq to map the chromosomal binding sites for two ISWI-containing nucleosome remodeling factors, ACF and RSF, in Drosophila embryos. Employing several polyclonal and monoclonal antibodies directed against their signature subunits, ACF1 and RSF-1, robust profiles were obtained indicating that both remodelers co-occupied a large set of active promoters.

Further validation included controls using chromatin of mutant embryos that do not express ACF1 or RSF-1. Surprisingly, the ChIP-seq profiles were unchanged, suggesting that they were not due to specific immunoprecipitation. Conservative analysis lists about 3000 chromosomal loci, mostly active promoters that are prone to non-specific enrichment in ChIP and appear as ‘Phantom Peaks’. These peaks are not obtained with pre-immune serum and are not prominent in input chromatin.

Mining the modENCODE ChIP-seq profiles identifies potential Phantom Peaks in many profiles of epigenetic regulators. These profiles and other ChIP-seq data featuring prominent Phantom Peaks must be validated with chromatin from cells in which the protein of interest has been depleted.

Chromatin remodeler mutations in human cancers: epigenetic implications

Chromatin remodeler complexes exhibit the ability to alter nucleosome composition and positions, with seemingly divergent roles in the regulation of chromatin architecture and gene expression. The outcome is directed by subunit variation and interactions with accessory factors. Recent studies have revealed that subunits of chromatin remodelers display an unexpectedly high mutation rate and/or are inactivated in a number of cancers. Consequently, a repertoire of epigenetic processes are likely to be affected, including interactions with histone modifying factors, as well as the ability to precisely modulate nucleosome positions, DNA methylation patterns and potentially, higher-order genome structure. However, the true significance of chromatin remodeler genetic aberrations in promoting a cascade of epigenetic changes, particularly during initiation and progression of cancer, remains largely unknown.

Chromatin Remodelers: From Function to Dysfunction

Chromatin remodelers are key players in the regulation of chromatin accessibility and nucleosome positioning on the eukaryotic DNA, thereby essential for all DNA dependent biological processes. Thus, it is not surprising that upon of deregulation of those molecular machines healthy cells can turn into cancerous cells. Even though the remodeling enzymes are very abundant and a multitude of different enzymes and chromatin remodeling complexes exist in the cell, the particular remodeling complex with its specific nucleosome positioning features must be at the right place at the right time in order to ensure the proper regulation of the DNA dependent processes. To achieve this, chromatin remodeling complexes harbor protein domains that specifically read chromatin targeting signals, such as histone modifications, DNA sequence/structure, non-coding RNAs, histone variants or DNA bound interacting proteins. Recent studies reveal the interaction between non-coding RNAs and chromatin remodeling complexes showing importance of RNA in remodeling enzyme targeting, scaffolding and regulation. In this review, we summarize current understanding of chromatin remodeling enzyme targeting to chromatin and their role in cancer development.

Mi-2/NuRD chromatin remodeling complexes regulate B and T-lymphocyte development and function

Mi-2/nucleosomal remodeling and deacetylase (NuRD) complexes are important epigenetic regulators of chromatin structure and gene expression. Mi-2/NuRD complexes are an assemblage of proteins that combine key epigenetic regulators necessary for (i) histone deacetylation and demethylation, (ii) binding to methylated DNA, (iii) mobilization of nucleosomes, and (iv) recruitment of additional regulatory proteins. Depending on their context in chromatin, Mi-2/NuRD complexes either activate or repress gene transcription. In this regard, they are important regulators of hematopoiesis and lymphopoiesis. Mi-2/NuRD complexes maintain pools of hematopoietic stem cells. Specifically, components of these complexes control multiple stages of B-cell development by regulating B-cell specific transcription. With one set of components, they inhibit terminal differentiation of germinal center B cells into plasma B cells. They also mediate gene repression together with Blimp-1 during plasma cell differentiation. In cooperation with Ikaros, Mi-2/NuRD complexes also play important roles in T-cell development, including CD4 versus CD8 fate decisions and peripheral T-cell responses. Dysregulation of NuRD during lymphopoiesis promotes leukemogenesis. Here, we review general properties of Mi-2/NuRD complexes and focus on their functions in gene regulation and development of lymphocytes.

The Drosophila Helicase Maleless (MLE) is Implicated in Functions Distinct From its Role in Dosage Compensation

Helicases are ubiquitous enzymes that unwind or remodel single or double-stranded nucleic acids, and that participate in a vast array of metabolic pathways. The ATP-dependent DEXH-box RNA/DNA helicase MLE was first identified as a core member of the chromatin remodeling MSL complex, responsible for dosage compensation in Drosophila males. Although this complex does not assemble in females, MLE is present. Given the multiplicity of functions attributed to its mammalian ortholog RNA helicase A, we have carried out an analysis for the purpose of determining whether MLE displays the same diversity. We have identified a number of different proteins that associate with MLE, implicating its role in specific pathways. We have documented this association in selected examples that include the spliceosome complex, heterogeneous Nuclear Ribonucleoproteins involved in RNA Processing and in Heterochromatin Protein 1 deposition, and the NuRD complex.

A map of directional genetic interactions in a metazoan cell

Gene–gene interactions shape complex phenotypes and modify the effects of mutations during development and disease. The effects of statistical gene–gene interactions on phenotypes have been used to assign genes to functional modules. However, directional, epistatic interactions, which reflect regulatory relationships between genes, have been challenging to map at large-scale. Here, we used combinatorial RNA interference and automated single-cell phenotyping to generate a large genetic interaction map for 21 phenotypic features of Drosophila cells. We devised a method that combines genetic interactions on multiple phenotypes to reveal directional relationships. This network reconstructed the sequence of protein activities in mitosis. Moreover, it revealed that the Ras pathway interacts with the SWI/SNF chromatin-remodelling complex, an interaction that we show is conserved in human cancer cells. Our study presents a powerful approach for reconstructing directional regulatory networks and provides a resource for the interpretation of functional consequences of genetic alterations.

DOI:http://dx.doi.org/10.7554/eLife.05464.001

Genes encode instructions that control our physical characteristics, known as traits. Although some traits are controlled by the activity of a single gene, most traits are influenced by the activities of multiple genes.

The genes that influence a particular trait may work independently of each other. However, it is also possible for the genes to interact so that one gene may mask or amplify the effect of another gene. Although gene interactions were first described almost 100 years ago, it has been difficult to identify them and work out the direction of these interactions (i.e., does gene A affect gene B, or vice versa?).

Fischer, Sandmann et al. have now studied the interactions between the genes involved in 21 different traits of fruit fly cells. A technique called RNA interference was used to lower the expression of the genes in different combinations, which made it possible to analyze any changes in the traits that occurred when particular genes were not working properly. Fischer, Sandmann et al. took hundreds of thousands images of the cells and analyzed the changes in cell shape, cell size, cell division and other traits. Next, they developed a method to infer the directions of the interactions between individual pairs of genes from the data and then made a map of the genetic interactions for the traits.

This map was able to reconstruct the known order of activity of genes during cell division and other cell processes. Furthermore, it revealed previously unknown interactions between genes. For example, genes involved in the Ras signaling pathway—which promotes cell growth and is frequently mutated in human tumors—interacted with genes that encode a group of proteins called the SWI/SNF complex. This complex alters how DNA is packaged in cells to control the expression of genes, and these gene interactions may play an important role in the control of cell growth by Ras signaling.

The approach developed by Fischer, Sandmann et al. can shed light on the interactions between genes that produce complex traits of cells. In future, this approach might be helpful to find out which genetic differences between individuals alter the effectiveness of drug treatments, and the impact of using combinations of drugs to treat diseases.

DOI:http://dx.doi.org/10.7554/eLife.05464.002

Opposing ISWI- and CHD-class chromatin remodeling activities orchestrate heterochromatic DNA repair

Chromatin compaction mediated by CHD3.1 must be counteracted by ACF1–SNF2H and RNF20 in order to allow DNA double-strand break repair in heterochromatin of postreplicative cells.

Heterochromatin is a barrier to DNA repair that correlates strongly with elevated somatic mutation in cancer. CHD class II nucleosome remodeling activity (specifically CHD3.1) retained by KAP-1 increases heterochromatin compaction and impedes DNA double-strand break (DSB) repair requiring Artemis. This obstruction is alleviated by chromatin relaxation via ATM-dependent KAP-1S824 phosphorylation (pKAP-1) and CHD3.1 dispersal from heterochromatic DSBs; however, how heterochromatin compaction is actually adjusted after CHD3.1 dispersal is unknown. In this paper, we demonstrate that Artemis-dependent DSB repair in heterochromatin requires ISWI (imitation switch)-class ACF1–SNF2H nucleosome remodeling. Compacted chromatin generated by CHD3.1 after DNA replication necessitates ACF1–SNF2H–mediated relaxation for DSB repair. ACF1–SNF2H requires RNF20 to bind heterochromatic DSBs, underlies RNF20-mediated chromatin relaxation, and functions downstream of pKAP-1–mediated CHD3.1 dispersal to enable DSB repair. CHD3.1 and ACF1–SNF2H display counteractive activities but similar histone affinities (via the plant homeodomains of CHD3.1 and ACF1), which we suggest necessitates a two-step dispersal and recruitment system regulating these opposing chromatin remodeling activities during DSB repair.

Baculoviruses and nucleosome management

Negatively-supercoiled-ds DNA molecules, including the genomes of baculoviruses, spontaneously wrap around cores of histones to form nucleosomes when present within eukaryotic nuclei. Hence, nucleosome management should be essential for baculovirus genome replication and temporal regulation of transcription, but this has not been documented. Nucleosome mobilization is the dominion of ATP-dependent chromatin-remodeling complexes. SWI/SNF and INO80, two of the best-studied complexes, as well as chromatin modifier TIP60, all contain actin as a subunit. Retrospective analysis of results of AcMNPV time course experiments wherein actin polymerization was blocked by cytochalasin D drug treatment implicate actin-containing chromatin modifying complexes in decatenating baculovirus genomes, shutting down host transcription, and regulating late and very late phases of viral transcription. Moreover, virus-mediated nuclear localization of actin early during infection may contribute to nucleosome management.

Functional interactions between NURF and Ctcf regulate gene expression

Gene expression frequently requires chromatin-remodeling complexes, and it is assumed that these complexes have common gene targets across cell types. Contrary to this belief, we show by genome-wide expression profiling that Bptf, an essential and unique subunit of the nucleosome-remodeling factor (NURF), predominantly regulates the expression of a unique set of genes between diverse cell types. Coincident with its functions in gene expression, we observed that Bptf is also important for regulating nucleosome occupancy at nucleosome-free regions (NFRs), many of which are located at sites occupied by the multivalent factors Ctcf and cohesin. NURF function at Ctcf binding sites could be direct, because Bptf occupies Ctcf binding sites in vivo and has physical interactions with CTCF and the cohesin subunit SA2. Assays of several Ctcf binding sites using reporter assays showed that their regulatory activity requires Bptf in two different cell types. Focused studies at H2-K1 showed that Bptf regulates the ability of Klf4 to bind near an upstream Ctcf site, possibly influencing gene expression. In combination, these studies demonstrate that gene expression as regulated by NURF occurs partly through physical and functional interactions with the ubiquitous and multivalent factors Ctcf and cohesin.

A functional insulator screen identifies NURF and dREAM components to be required for enhancer-blocking

Chromatin insulators of higher eukaryotes functionally divide the genome into active and inactive domains. Furthermore, insulators regulate enhancer/promoter communication, which is evident from the Drosophila bithorax locus in which a multitude of regulatory elements control segment specific gene activity. Centrosomal protein 190 (CP190) is targeted to insulators by CTCF or other insulator DNA-binding factors. Chromatin analyses revealed that insulators are characterized by open and nucleosome depleted regions. Here, we wanted to identify chromatin modification and remodelling factors required for an enhancer blocking function. We used the well-studied Fab-8 insulator of the bithorax locus to apply a genome-wide RNAi screen for factors that contribute to the enhancer blocking function of CTCF and CP190. Among 78 genes required for optimal Fab-8 mediated enhancer blocking, all four components of the NURF complex as well as several subunits of the dREAM complex were most evident. Mass spectrometric analyses of CTCF or CP190 bound proteins as well as immune precipitation confirmed NURF and dREAM binding. Both co-localise with most CP190 binding sites in the genome and chromatin immune precipitation showed that CP190 recruits NURF and dREAM. Nucleosome occupancy and histone H3 binding analyses revealed that CP190 mediated NURF binding results in nucleosomal depletion at CP190 binding sites. Thus, we conclude that CP190 binding to CTCF or to other DNA binding insulator factors mediates recruitment of NURF and dREAM. Furthermore, the enhancer blocking function of insulators is associated with nucleosomal depletion and requires NURF and dREAM.

Drosophila Brahma complex remodels nucleosome organizations in multiple aspects

ATP-dependent chromatin remodeling complexes regulate nucleosome organizations. In Drosophila, gene Brm encodes the core Brahma complex, the ATPase subunit of SWI/SNF class of chromatin remodelers. Its role in modulating the nucleosome landscape in vivo is unclear. In this study, we knocked down Brm in Drosophila third instar larvae to explore the changes in nucleosome profiles and global gene transcription. The results show that Brm knockdown leads to nucleosome occupancy changes throughout the entire genome with a bias in occupancy decrease. In contrast, the knockdown has limited impacts on nucleosome position shift. The knockdown also alters another important physical property of nucleosome positioning, fuzziness. Nucleosome position shift, gain or loss and fuzziness changes are all enriched in promoter regions. Nucleosome arrays around the 5' ends of genes are reorganized in five patterns as a result of Brm knockdown. Intriguingly, the concomitant changes in the genes adjacent to the Brahma-dependent remodeling regions have important roles in development and morphogenesis. Further analyses reveal abundance of AT-rich motifs for transcription factors in the remodeling regions.

Nucleosomes shape DNA polymorphism and divergence

An estimated 80% of genomic DNA in eukaryotes is packaged as nucleosomes, which, together with the remaining interstitial linker regions, generate higher order chromatin structures [1]. Nucleosome sequences isolated from diverse organisms exhibit ∼10 bp periodic variations in AA, TT and GC dinucleotide frequencies. These sequence elements generate intrinsically curved DNA and help establish the histone-DNA interface. We investigated an important unanswered question concerning the interplay between chromatin organization and genome evolution: do the DNA sequence preferences inherent to the highly conserved histone core exert detectable natural selection on genomic divergence and polymorphism? To address this hypothesis, we isolated nucleosomal DNA sequences from Drosophila melanogaster embryos and examined the underlying genomic variation within and between species. We found that divergence along the D. melanogaster lineage is periodic across nucleosome regions with base changes following preferred nucleotides, providing new evidence for systematic evolutionary forces in the generation and maintenance of nucleosome-associated dinucleotide periodicities. Further, Single Nucleotide Polymorphism (SNP) frequency spectra show striking periodicities across nucleosomal regions, paralleling divergence patterns. Preferred alleles occur at higher frequencies in natural populations, consistent with a central role for natural selection. These patterns are stronger for nucleosomes in introns than in intergenic regions, suggesting selection is stronger in transcribed regions where nucleosomes undergo more displacement, remodeling and functional modification. In addition, we observe a large-scale (∼180 bp) periodic enrichment of AA/TT dinucleotides associated with nucleosome occupancy, while GC dinucleotide frequency peaks in linker regions. Divergence and polymorphism data also support a role for natural selection in the generation and maintenance of these super-nucleosomal patterns. Our results demonstrate that nucleosome-associated sequence periodicities are under selective pressure, implying that structural interactions between nucleosomes and DNA sequence shape sequence evolution, particularly in introns.

In eukaryotic cells, the majority of DNA is packaged in nucleosomes comprised of ∼147 bp of DNA wound tightly around the highly conserved histone octamer. Nucleosomal DNA from diverse organisms shows an anti-correlated ∼10 bp periodicity of AT-rich and GC-rich dinucleotides. These sequence features influence DNA bending and shape, facilitating structural interactions. We asked whether natural selection mediated through the periodic sequence preferences of nucleosomes shapes the evolution of non-protein-coding regions of D. melanogaster by examining the inter- and intra-species genomic variation relative to these fundamental chromatin building blocks. The sequence changes across nucleosome-bound regions on the melanogaster lineage mirror the observed nucleosome dinucleotide periodicities. Importantly, we show that the frequencies of polymorphisms in natural populations vary across these regions, paralleling divergence, with higher frequencies of preferred alleles. These patterns are most evident for intronic regions and indicate that non-protein coding regions are evolving toward sequences that facilitate the canonical association with the histone core. This result is consistent with the hypothesis that interactions between DNA and the core have systematic impacts on function that are subject to natural selection and are not solely due to mutational bias. These ubiquitous interactions with the histone core partially account for the evolutionary constraint observed in unannotated genomic regions, and may drive broad changes in base composition.

Mammalian cells contain two functionally distinct PBAF complexes incorporating different isoforms of PHF10 signature subunit

The PBAF subtype of the mammalian chromatin remodeling SWI/SNF complex has wide and diverse functions in transcription regulation and development, being both transcription activator and repressor. However, a mechanism accounting for such functional diversity remains unclear. Human PHF10/BAF45a subunit of the PBAF complex plays an important role in brain development but has not been studied sufficiently. We have shown that the PHF10 gene encodes 2 types of evolutionarily conserved, ubiquitously expressed isoforms that are incorporated into the PBAF complex in a mutually exclusive manner. One isoform contains C-terminal tandem PHD fingers, which in the other isoform are replaced by the consensus sequence for phosphorylation-dependent SUMO 1 conjugation (PDSM). PBAF complexes containing different PHF10 isoforms can bind to the promoters of the same genes but produce different effects on the recruitment of Pol II to the promoter and on the level of gene transcription. In addition, it is only the PBAF with PHD-containing isoform that activates proliferation. Our study demonstrates the existence of functionally different PBAF complexes in mammalian cell. It also provides an insight into the molecular structure and role of human PHF10/BAF45a and characterizes it as an essential PBAF subunit.

INO80-dependent regression of ecdysone-induced transcriptional responses regulates developmental timing in Drosophila

Sequential pulses of the steroid hormone ecdysone regulate the major developmental transitions in Drosophila, and the duration of each developmental stage is determined by the length of time between ecdysone pulses. Ecdysone regulates biological responses by directly initiating target gene transcription. In turn, these transcriptional responses are known to be self-limiting, with mechanisms in place to ensure regression of hormone-dependent transcription. However, the biological significance of these transcriptional repression mechanisms remains unclear. Here we show that the chromatin remodeling protein INO80 facilitates transcriptional repression of ecdysone-regulated genes during prepupal development. In ino80 mutant animals, inefficient repression of transcriptional responses to the late larval ecdysone pulse delays the onset of the subsequent prepupal ecdysone pulse, resulting in a significantly longer prepupal stage. Conversely, increased expression of ino80 is sufficient to shorten the prepupal stage by increasing the rate of transcriptional repression. Furthermore, we demonstrate that enhancing the rate of regression of the mid-prepupal competence factor βFTZ-F1 is sufficient to determine the timing of head eversion and thus the duration of prepupal development. Although ino80 is conserved from yeast to humans, this study represents the first characterization of a bona fide ino80 mutation in any metazoan, raising the possibility that the functions of ino80 in transcriptional repression and developmental timing are evolutionarily conserved.

Regulation of ISWI chromatin remodelling activity

The packaging of the eukaryotic genome into chromatin facilitates the storage of the genetic information within the nucleus, but prevents the access to the underlying DNA sequences. Structural changes in chromatin are mediated by several mechanisms. Among them, ATP-dependent remodelling complexes belonging to ISWI family provides one of the best examples that eukaryotic cells evolved to finely regulate these changes. ISWI-containing complexes use the energy derived from ATP hydrolysis to rearrange nucleosomes on chromatin in order to favour specific nuclear reactions. The combination of regulatory nuclear factors associated with the ATPase subunit as well as its modulation by specific histone modifications, specializes the nuclear function of each ISWI-containing complex. Here we review the different ways by which ISWI enzymatic activity can be modulated and regulated in the nucleus of eukaryotic cells.

Overlapping chromatin-remodeling systems collaborate genome wide at dynamic chromatin transitions

ATP-dependent chromatin remodeling is an essential process required for the dynamic organization of chromatin structure. Here we describe the genome-wide location and activity of three remodeler proteins with diverse physiological functions in the mouse genome: Brg1, Chd4, and Snf2h. The localization patterns of all three proteins significantly overlap with one another and with regions of accessible chromatin. Furthermore, using inducible mutant variants, we demonstrate that the catalytic activity of these proteins contributes to the remodeling of chromatin genome-wide, and that each of these remodelers can independently regulate chromatin reorganization at distinct sites. Many regions require the activity of more than one remodeler to regulate accessibility. These findings provide a dynamic view of chromatin organization, and highlight the differential contributions of remodelers to chromatin maintenance in higher eukaryotes.

The spring-loaded genome: nucleosome redistributions are widespread, transient, and DNA-directed

Nucleosome occupancy plays a key role in regulating access to eukaryotic genomes. Although various chromatin regulatory complexes are known to regulate nucleosome occupancy, the role of DNA sequence in this regulation remains unclear, particularly in mammals. To address this problem, we measured nucleosome distribution at high temporal resolution in human cells at hundreds of genes during the reactivation of Kaposi's sarcoma–associated herpesvirus (KSHV). We show that nucleosome redistribution peaks at 24 h post-KSHV reactivation and that the nucleosomal redistributions are widespread and transient. To clarify the role of DNA sequence in these nucleosomal redistributions, we compared the genes with altered nucleosome distribution to a sequence-based computer model and in vitro–assembled nucleosomes. We demonstrate that both the predicted model and the assembled nucleosome distributions are concordant with the majority of nucleosome redistributions at 24 h post-KSHV reactivation. We suggest a model in which loci are held in an unfavorable chromatin architecture and “spring” to a transient intermediate state directed by DNA sequence information. We propose that DNA sequence plays a more considerable role in the regulation of nucleosome positions than was previously appreciated. The surprising findings that nucleosome redistributions are widespread, transient, and DNA-directed shift the current perspective regarding regulation of nucleosome distribution in humans.

Histone chaperone NAP1 mediates sister chromatid resolution by counteracting protein phosphatase 2A

Chromosome duplication and transmission into daughter cells requires the precisely orchestrated binding and release of cohesin. We found that the Drosophila histone chaperone NAP1 is required for cohesin release and sister chromatid resolution during mitosis. Genome-wide surveys revealed that NAP1 and cohesin co-localize at multiple genomic loci. Proteomic and biochemical analysis established that NAP1 associates with the full cohesin complex, but it also forms a separate complex with the cohesin subunit stromalin (SA). NAP1 binding to cohesin is cell-cycle regulated and increases during G2/M phase. This causes the dissociation of protein phosphatase 2A (PP2A) from cohesin, increased phosphorylation of SA and cohesin removal in early mitosis. PP2A depletion led to a loss of centromeric cohesion. The distinct mitotic phenotypes caused by the loss of either PP2A or NAP1, were both rescued by their concomitant depletion. We conclude that the balanced antagonism between NAP1 and PP2A controls cohesin dissociation during mitosis.

DNA thermodynamics shape chromosome organization and topology

How much information is encoded in the DNA sequence of an organism? We argue that the informational, mechanical and topological properties of DNA are interdependent and act together to specify the primary characteristics of genetic organization and chromatin structures. Superhelicity generated in vivo, in part by the action of DNA translocases, can be transmitted to topologically sensitive regions encoded by less stable DNA sequences.

Nucleosome remodeling and epigenetics

Eukaryotic chromatin is kept flexible and dynamic to respond to environmental, metabolic, and developmental cues through the action of a family of so-called "nucleosome remodeling" ATPases. Consistent with their helicase ancestry, these enzymes experience conformation changes as they bind and hydrolyze ATP. At the same time they interact with DNA and histones, which alters histone-DNA interactions in target nucleosomes. Their action may lead to complete or partial disassembly of nucleosomes, the exchange of histones for variants, the assembly of nucleosomes, or the movement of histone octamers on DNA. "Remodeling" may render DNA sequences accessible to interacting proteins or, conversely, promote packing into tightly folded structures. Remodeling processes participate in every aspect of genome function. Remodeling activities are commonly integrated with other mechanisms such as histone modifications or RNA metabolism to assemble stable, epigenetic states.