Acetylation mimics within individual core histone tail domains indicate distinct roles in regulating the stability of higher-order chromatin structure

HDAC4 influences the DNA damage response and counteracts senescence by assembling with HDAC1/HDAC2 to control H2BK120 acetylation and homology-directed repair

Access to DNA is the first level of control in regulating gene transcription, a control that is also critical for maintaining DNA integrity. Cellular senescence is characterized by profound transcriptional rearrangements and accumulation of DNA lesions. Here, we discovered an epigenetic complex between HDAC4 and HDAC1/HDAC2 that is involved in the erase of H2BK120 acetylation. The HDAC4/HDAC1/HDAC2 complex modulates the efficiency of DNA repair by homologous recombination, through dynamic deacetylation of H2BK120. Deficiency of HDAC4 leads to accumulation of H2BK120ac, impaired recruitment of BRCA1 and CtIP to the site of lesions, accumulation of damaged DNA and senescence. In senescent cells this complex is disassembled because of increased proteasomal degradation of HDAC4. Forced expression of HDAC4 during RAS-induced senescence reduces the genomic spread of γH2AX. It also affects H2BK120ac levels, which are increased in DNA-damaged regions that accumulate during RAS-induced senescence. In summary, degradation of HDAC4 during senescence causes the accumulation of damaged DNA and contributes to the activation of the transcriptional program controlled by super-enhancers that maintains senescence.

Role of the histone variant H2A.Z.1 in memory, transcription, and alternative splicing is mediated by lysine modification

Creating long-lasting memories requires learning-induced changes in gene expression, which are impacted by epigenetic modifications of DNA and associated histone proteins. Post-translational modifications (PTMs) of histones are key regulators of transcription, with different PTMs producing unique effects on gene activity and behavior. Although recent studies implicate histone variants as novel regulators of memory, effects of PTMs on the function of histone variants are rarely considered. We previously showed that the histone variant H2A.Z suppresses memory, but it is unclear if this role is impacted by H2A.Z acetylation, a PTM that is typically associated with positive effects on transcription and memory. To answer this question, we used a mutation approach to manipulate acetylation on H2A.Z without impacting acetylation of other histone types. Specifically, we used adeno-associated virus (AAV) constructs to overexpress mutated H2A.Z.1 isoforms that either mimic acetylation (acetyl-mimic) by replacing lysines 4, 7 and 11 with glutamine (KQ), or H2A.Z.1 with impaired acetylation (acetyl-defective) by replacing the same lysines with alanine (KA). Expressing the H2A.Z.1 acetyl-mimic (H2A.Z.1KQ) improved memory under weak learning conditions, whereas expressing the acetyl-defective H2A.Z.1KA generally impaired memory, indicating that the effect of H2A.Z.1 on memory depends on its acetylation status. RNA sequencing showed that H2A.Z.1KQ and H2A.Z.1KA uniquely impact the expression of different classes of genes in both females and males. Specifically, H2A.Z.1KA preferentially impacts genes involved in synaptic function, suggesting that acetyl-defective H2A.Z.1 impairs memory by altering synaptic regulation. Finally, we describe, for the first time, that H2A.Z is also involved in alternative splicing of neuronal genes, whereby H2A.Z depletion, as well as expression of H2A.Z.1 lysine mutants influence transcription and splicing of different gene targets, suggesting that H2A.Z.1 can impact behavior through effects on both splicing and gene expression. This is the first study to demonstrate that direct manipulation of H2A.Z post-translational modifications regulates memory, whereby acetylation adds another regulatory layer by which histone variants can fine tune higher brain functions through effects on gene expression and splicing.

Structural basis of nucleosome deacetylation and DNA linker tightening by Rpd3S histone deacetylase complex

In Saccharomyces cerevisiae, cryptic transcription at the coding region is prevented by the activity of Sin3 histone deacetylase (HDAC) complex Rpd3S, which is carried by the transcribing RNA polymerase II (RNAPII) to deacetylate and stabilize chromatin. Despite its fundamental importance, the mechanisms by which Rpd3S deacetylates nucleosomes and regulates chromatin dynamics remain elusive. Here, we determined several cryo-EM structures of Rpd3S in complex with nucleosome core particles (NCPs), including the H3/H4 deacetylation states, the alternative deacetylation state, the linker tightening state, and a state in which Rpd3S co-exists with the Hho1 linker histone on NCP. These structures suggest that Rpd3S utilizes a conserved Sin3 basic surface to navigate through the nucleosomal DNA, guided by its interactions with H3K36 methylation and the extra-nucleosomal DNA linkers, to target acetylated H3K9 and sample other histone tails. Furthermore, our structures illustrate that Rpd3S reconfigures the DNA linkers and acts in concert with Hho1 to engage the NCP, potentially unraveling how Rpd3S and Hho1 work in tandem for gene silencing.

Cytochrome c lysine acetylation regulates cellular respiration and cell death in ischemic skeletal muscle

Skeletal muscle is more resilient to ischemia-reperfusion injury than other organs. Tissue specific post-translational modifications of cytochrome c (Cytc) are involved in ischemia-reperfusion injury by regulating mitochondrial respiration and apoptosis. Here, we describe an acetylation site of Cytc, lysine 39 (K39), which was mapped in ischemic porcine skeletal muscle and removed by sirtuin5 in vitro. Using purified protein and cellular double knockout models, we show that K39 acetylation and acetylmimetic K39Q replacement increases cytochrome c oxidase (COX) activity and ROS scavenging while inhibiting apoptosis via decreased binding to Apaf-1, caspase cleavage and activity, and cardiolipin peroxidase activity. These results are discussed with X-ray crystallography structures of K39 acetylated (1.50 Å) and acetylmimetic K39Q Cytc (1.36 Å) and NMR dynamics. We propose that K39 acetylation is an adaptive response that controls electron transport chain flux, allowing skeletal muscle to meet heightened energy demand while simultaneously providing the tissue with robust resilience to ischemia-reperfusion injury.

Ketohexokinase-C regulates global protein acetylation to decrease carnitine palmitoyltransferase 1a-mediated fatty acid oxidation

BACKGROUND & AIMS

The consumption of sugar and a high-fat diet (HFD) promotes the development of obesity and metabolic dysfunction. Despite their well-known synergy, the mechanisms by which sugar worsens the outcomes associated with a HFD are largely elusive.

METHODS

Six-week-old, male, C57Bl/6 J mice were fed either chow or a HFD and were provided with regular, fructose- or glucose-sweetened water. Moreover, cultured AML12 hepatocytes were engineered to overexpress ketohexokinase-C (KHK-C) using a lentivirus vector, while CRISPR-Cas9 was used to knockdown CPT1α. The cell culture experiments were complemented with in vivo studies using mice with hepatic overexpression of KHK-C and in mice with liver-specific CPT1α knockout. We used comprehensive metabolomics, electron microscopy, mitochondrial substrate phenotyping, proteomics and acetylome analysis to investigate underlying mechanisms.

RESULTS

Fructose supplementation in mice fed normal chow and fructose or glucose supplementation in mice fed a HFD increase KHK-C, an enzyme that catalyzes the first step of fructolysis. Elevated KHK-C is associated with an increase in lipogenic proteins, such as ACLY, without affecting their mRNA expression. An increase in KHK-C also correlates with acetylation of CPT1α at K508, and lower CPT1α protein in vivo. In vitro, KHK-C overexpression lowers CPT1α and increases triglyceride accumulation. The effects of KHK-C are, in part, replicated by a knockdown of CPT1α. An increase in KHK-C correlates negatively with CPT1α protein levels in mice fed sugar and a HFD, but also in genetically obese db/db and lipodystrophic FIRKO mice. Mechanistically, overexpression of KHK-C in vitro increases global protein acetylation and decreases levels of the major cytoplasmic deacetylase, SIRT2.

CONCLUSIONS

KHK-C-induced acetylation is a novel mechanism by which dietary fructose augments lipogenesis and decreases fatty acid oxidation to promote the development of metabolic complications.

IMPACT AND IMPLICATIONS

Fructose is a highly lipogenic nutrient whose negative consequences have been largely attributed to increased de novo lipogenesis. Herein, we show that fructose upregulates ketohexokinase, which in turn modifies global protein acetylation, including acetylation of CPT1a, to decrease fatty acid oxidation. Our findings broaden the impact of dietary sugar beyond its lipogenic role and have implications on drug development aimed at reducing the harmful effects attributed to sugar metabolism.

Acetylation-Mimic Mutation of TRIM28-Lys304 to Gln Attenuates the Interaction with KRAB-Zinc-Finger Proteins and Affects Gene Expression in Leukemic K562 Cells

TRIM28/KAP1/TIF1β is a crucial epigenetic modifier. Genetic ablation of trim28 is embryonic lethal, although RNAi-mediated knockdown in somatic cells yields viable cells. Reduction in TRIM28 abundance at the cellular or organismal level results in polyphenism. Posttranslational modifications such as phosphorylation and sumoylation have been shown to regulate TRIM28 activity. Moreover, several lysine residues of TRIM28 are subject to acetylation, but how acetylation of TRIM28 affects its functions remains poorly understood. Here, we report that, compared with wild-type TRIM28, the acetylation-mimic mutant TRIM28-K304Q has an altered interaction with Krüppel-associated box zinc-finger proteins (KRAB-ZNFs). The TRIM28-K304Q knock-in cells were created in K562 erythroleukemia cells by CRISPR-Cas9 (Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein nuclease 9) gene editing method. Transcriptome analysis revealed that TRIM28-K304Q and TRIM28 knockout K562 cells had similar global gene expression profiles, yet the profiles differed considerably from wild-type K562 cells. The expression levels of embryonic-related globin gene and a platelet cell marker integrin-beta 3 were increased in TRIM28-K304Q mutant cells, indicating the induction of differentiation. In addition to the differentiation-related genes, many zinc-finger-proteins genes and imprinting genes were activated in TRIM28-K304Q cells; they were inhibited by wild-type TRIM28 via binding with KRAB-ZNFs. These results suggest that acetylation/deacetylation of K304 in TRIM28 constitutes a switch for regulating its interaction with KRAB-ZNFs and alters the gene regulation as demonstrated by the acetylation mimic TRIM28-K304Q.

Roles of histone acetylation sites in cardiac hypertrophy and heart failure

Heart failure results from various physiological and pathological stimuli that lead to cardiac hypertrophy. This pathological process is common in several cardiovascular diseases and ultimately leads to heart failure. The development of cardiac hypertrophy and heart failure involves reprogramming of gene expression, a process that is highly dependent on epigenetic regulation. Histone acetylation is dynamically regulated by cardiac stress. Histone acetyltransferases play an important role in epigenetic remodeling in cardiac hypertrophy and heart failure. The regulation of histone acetyltransferases serves as a bridge between signal transduction and downstream gene reprogramming. Investigating the changes in histone acetyltransferases and histone modification sites in cardiac hypertrophy and heart failure will provide new therapeutic strategies to treat these diseases. This review summarizes the association of histone acetylation sites and histone acetylases with cardiac hypertrophy and heart failure, with emphasis on histone acetylation sites.

Design and Characterization of Mutated Variants of the Oncotoxic Parvoviral Protein NS1

Oncotoxic proteins such as the non-structural protein 1 (NS1), a constituent of the rodent parvovirus H1 (H1-PV), offer a novel approach for treatment of tumors that are refractory to other treatments. In the present study, mutated NS1 variants were designed and tested with respect to their oncotoxic potential in human hepatocellular carcinoma cell lines. We introduced single point mutations of previously described important residues of the wild-type NS1 protein and a deletion of 114 base pairs localized within the N-terminal domain of NS1. Cell-viability screening with HepG2 and Hep3B hepatocarcinoma cells transfected with the constructed NS1-mutants led to identification of the single-amino acid NS1-mutant NS1-T585E, which led to a 30% decrease in cell viability as compared to NS1 wildtype. Using proteomics analysis, we could identify new interaction partners and signaling pathways of NS1. We could thus identify new oncotoxic NS1 variants and gain insight into the modes of action of NS1, which is exclusively toxic to human cancer cells. Our in-vitro studies provide mechanistic explanations for the observed oncolytic effects. Expression of NS1 variants had no effect on cell viability in NS1 unresponsive control HepG2 cells or primary mouse hepatocytes. The availability of new NS1 variants in combination with a better understanding of their modes of action offers new possibilities for the design of innovative cancer treatment strategies.

Effects of Hst3p inhibition in Candida albicans: a genome-wide H3K56 acetylation analysis

Candida spp. represent the third most frequent worldwide cause of infection in Intensive Care Units with a mortality rate of almost 40%. The classes of antifungals currently available include azoles, polyenes, echinocandins, pyrimidine derivatives, and allylamines. However, the therapeutical options for the treatment of candidiasis are drastically reduced by the increasing antifungal resistance. The growing need for a more targeted antifungal therapy is limited by the concern of finding molecules that specifically recognize the microbial cell without damaging the host. Epigenetic writers and erasers have emerged as promising targets in different contexts, including the treatment of fungal infections. In C. albicans, Hst3p, a sirtuin that deacetylates H3K56ac, represents an attractive antifungal target as it is essential for the fungus viability and virulence. Although the relevance of such epigenetic regulator is documented for the development of new antifungal therapies, the molecular mechanism behind Hst3p-mediated epigenetic regulation remains unrevealed. Here, we provide the first genome-wide profiling of H3K56ac in C. albicans resulting in H3K56ac enriched regions associated with Candida sp. pathogenicity. Upon Hst3p inhibition, 447 regions gain H3K56ac. Importantly, these genomic areas contain genes encoding for adhesin proteins, degradative enzymes, and white-opaque switching. Moreover, our RNA-seq analysis revealed 1330 upregulated and 1081 downregulated transcripts upon Hst3p inhibition, and among them, we identified 87 genes whose transcriptional increase well correlates with the enrichment of H3K56 acetylation on their promoters, including some well-known regulators of phenotypic switching and virulence. Based on our evidence, Hst3p is an appealing target for the development of new potential antifungal drugs.

Mechanism of histone deacetylases in cardiac hypertrophy and its therapeutic inhibitors

Cardiac hypertrophy is a key process in cardiac remodeling development, leading to ventricle enlargement and heart failure. Recently, studies show the complicated relation between cardiac hypertrophy and epigenetic modification. Post-translational modification of histone is an essential part of epigenetic modification, which is relevant to multiple cardiac diseases, especially in cardiac hypertrophy. There is a group of enzymes related in the balance of histone acetylation/deacetylation, which is defined as histone acetyltransferase (HAT) and histone deacetylase (HDAC). In this review, we introduce an important enzyme family HDAC, a key regulator in histone deacetylation. In cardiac hypertrophy HDAC I downregulates the anti-hypertrophy gene expression, including Kruppel-like factor 4 (Klf4) and inositol-5 phosphatase f (Inpp5f), and promote the development of cardiac hypertrophy. On the contrary, HDAC II binds to myocyte-specific enhancer factor 2 (MEF2), inhibit the assemble ability to HAT and protect against cardiac hypertrophy. Under adverse stimuli such as pressure overload and calcineurin stimulation, the HDAC II transfer to cytoplasm, and MEF2 can bind to nuclear factor of activated T cells (NFAT) or GATA binding protein 4 (GATA4), mediating inappropriate gene expression. HDAC III, also known as SIRTs, can interact not only to transcription factors, but also exist interaction mechanisms to other HDACs, such as HDAC IIa. We also present the latest progress of HDAC inhibitors (HDACi), as a potential treatment target in cardiac hypertrophy.

Cysteine-Based Mimic of Arginylation Reproduces Neuroprotective Effects of the Authentic Post-Translational Modification on α-Synuclein

Arginylation is an understudied post-translational modification (PTM) involving the transfer of arginine to aspartate or glutamate sidechains in a protein. Among the targets of this PTM is α-synuclein (αS), a neuronal protein involved in regulating synaptic vesicles. The aggregation of αS is implicated in neurodegenerative diseases, particularly in Parkinson's disease, and arginylation has been found to protect against this pathological process. Arginylated αS has been studied through semisynthesis involving multipart native chemical ligation (NCL), but this can be very labor-intensive with low yields. Here, we present a facile way to introduce a mimic of the arginylation modification into a protein of interest, compatible with orthogonal installation of labels such as fluorophores. We synthesize bromoacetyl arginine and react it with recombinant, site-specific cysteine mutants of αS. We validate the mimic by testing the vesicle binding affinity of mimic-arginylated αS, as well as its aggregation kinetics and monomer incorporation into fibrils, and comparing these results to those of authentically arginylated αS produced through NCL. In cultured neurons, we compare the fibril seeding capabilities of preformed fibrils carrying a small percentage of arginylated αS. We find that, consistent with authentically arginylated αS, mimic-arginylated αS does not perturb the protein's native function but alters aggregation kinetics and monomer incorporation. Both mimic and authentically modified αS suppress aggregation in neuronal cells. Our results provide further insight into the neuroprotective effects of αS arginylation, and our alternative strategy to generate arginylated αS enables the study of this PTM in proteins not accessible through NCL.

Histone H3 and H4 tails play an important role in nucleosome phase separation

Chromatin organization and its dynamic regulation are crucial in governing the temporal and spatial accessibility of DNA for proper gene expression. Disordered chains of nucleosomes comprise the basis of eukaryotic chromatin, forming higher-level organization across a range of length scales. Models of chromatin organization involving phase separation driven by chromatin-associating proteins have been proposed. More recently, evidence has emerged that nucleosome arrays can phase separate in the absence of other protein factors, yet questions remain regarding the molecular basis of chromatin phase separation that governs this dynamic nuclear organization. Here, we break chromatin down into its most basic subunit, the nucleosome core particle, and investigate phase separation using turbidity assays in conjunction with differential interference contrast microscopy. We show that, at physiologically-relevant concentrations, this fundamental subunit of chromatin undergoes phase separation. Individually removing the H3 and H4 tails abrogates phase separation under the same conditions. Taking a reductionist approach to investigate H3 and H4 tail peptide interactions in-trans with DNA and nucleosome core particles supports the direct involvement of these tails in chromatin phase separation. These results provide insight into fundamental mechanisms underlying phase separation of chromatin, which starts at the level of the nucleosome core particle, and support that long-range inter-nucleosomal interactions are sufficient to drive phase separation at nuclear concentrations. Additionally, our data have implications for understanding crosstalk between histone tails and provide a lens through which to interpret the effect of histone post-translational modifications and sequence variants. STATEMENT OF SIGNIFICANCE: Emerging models propose that chromatin organization is based in phase separation, however, mechanisms that drive this dynamic nuclear organization are only beginning to be understood. Previous focus has been on phase separation driven by chromatin-associating proteins, but this has recently shifted to recognize a direct role of chromatin in phase separation. Here, we take a fundamental approach in understanding chromatin phase separation and present new findings that the basic subunit of chromatin, the nucleosome core particle, undergoes phase separation under physiological concentrations of nucleosome and monovalent salt. Furthermore, the histone H3 and H4 tails are involved in phase separation in a manner independent of histone-associating proteins. These data suggest that H3 and H4 tail epigenetic factors may modulate chromatin phase separation.

H3.1K27me1 maintains transcriptional silencing and genome stability by preventing GCN5-mediated histone acetylation

Epigenetic mechanisms play diverse roles in the regulation of genome stability in eukaryotes. In Arabidopsis thaliana, genome stability is maintained during DNA replication by the H3.1K27 methyltransferases ARABIDOPSIS TRITHORAX-RELATED PROTEIN 5 (ATXR5) and ATXR6, which catalyze the deposition of K27me1 on replication-dependent H3.1 variants. The loss of H3.1K27me1 in atxr5 atxr6 double mutants leads to heterochromatin defects, including transcriptional de-repression and genomic instability, but the molecular mechanisms involved remain largely unknown. In this study, we identified the transcriptional co-activator and conserved histone acetyltransferase GCN5 as a mediator of transcriptional de-repression and genomic instability in the absence of H3.1K27me1. GCN5 is part of a SAGA-like complex in plants that requires the GCN5-interacting protein ADA2b and the chromatin remodeler CHR6 to mediate the heterochromatic defects in atxr5 atxr6 mutants. Our results also indicate that Arabidopsis GCN5 acetylates multiple lysine residues on H3.1 variants, but H3.1K27 and H3.1K36 play essential functions in inducing genomic instability in the absence of H3.1K27me1. Finally, we show that H3.1K36 acetylation by GCN5 is negatively regulated by H3.1K27me1 in vitro. Overall, this work reveals a key molecular role for H3.1K27me1 in maintaining transcriptional silencing and genome stability in heterochromatin by restricting GCN5-mediated histone acetylation in plants.

Role of histone deacetylase on nonalcoholic fatty liver disease

Introduction: Nonalcoholic fatty liver disease (NAFLD) is a group of diseases related to metabolic abnormalities, which severely impairs the life and health of patients, and brings great pressure to the society and medical resources. Currently, there is no specific treatment. Histone deacetylases (HDACs) have recently been reported to be involved in the pathogenesis of NAFLD and are considered as new targets for the treatment of NAFLD.Area covered: In this review, we summarized the role of HDACs in the pathogenesis of NAFLD and proposed possible therapeutic targets in order to provide new strategies for the treatment of NAFLD.Expert commentary: HDACs and related signal pathways are widely involved in the pathogenesis of NAFLD and have the potential to become therapeutic targets. However, based on current research alone, HDACs cannot be practical applied to the treatment of NAFLD. Therefore, more research on the pathogenesis of NAFLD and the mechanism of HDACs is what we need most now.

Histone Tail Conformations: A Fuzzy Affair with DNA

The core histone tails are critical in chromatin structure and signaling. Studies over the past several decades have provided a wealth of information on the histone tails and their interaction with chromatin factors. However, the conformation of the histone tails in a chromatin relevant context has remained elusive. Only recently has enough evidence emerged to start to build a structural model of the tails in the context of nucleosomes and nucleosome arrays. Here, we review these studies and propose that the histone tails adopt a high-affinity fuzzy complex with DNA, characterized by robust but dynamic association. Furthermore, we discuss how these DNA-bound conformational ensembles promote distinct chromatin structure and signaling, and that their fuzzy nature is important in transitioning between functional states.

Interactomic affinity profiling by holdup assay: Acetylation and distal residues impact the PDZome-binding specificity of PTEN phosphatase

Protein domains often recognize short linear protein motifs composed of a core conserved consensus sequence surrounded by less critical, modulatory positions. PTEN, a lipid phosphatase involved in phosphatidylinositol 3-kinase (PI3K) pathway, contains such a short motif located at the extreme C-terminus capable to recognize PDZ domains. It has been shown that the acetylation of this motif could modulate the interaction with several PDZ domains. Here we used an accurate experimental approach combining high-throughput holdup chromatographic assay and competitive fluorescence polarization technique to measure quantitative binding affinity profiles of the PDZ domain-binding motif (PBM) of PTEN. We substantially extended the previous knowledge towards the 266 known human PDZ domains, generating the full PDZome-binding profile of the PTEN PBM. We confirmed that inclusion of N-terminal flanking residues, acetylation or mutation of a lysine at a modulatory position significantly altered the PDZome-binding profile. A numerical specificity index is also introduced as an attempt to quantify the specificity of a given PBM over the complete PDZome. Our results highlight the impact of modulatory residues and post-translational modifications on PBM interactomes and their specificity.

Histone Variant H3.3 Mutations in Defining the Chromatin Function in Mammals

The systematic mutation of histone 3 (H3) genes in model organisms has proven to be a valuable tool to distinguish the functional role of histone residues. No system exists in mammalian cells to directly manipulate canonical histone H3 due to a large number of clustered and multi-loci histone genes. Over the years, oncogenic histone mutations in a subset of H3 have been identified in humans, and have advanced our understanding of the function of histone residues in health and disease. The oncogenic mutations are often found in one allele of the histone variant H3.3 genes, but they prompt severe changes in the epigenetic landscape of cells, and contribute to cancer development. Therefore, mutation approaches using H3.3 genes could be relevant to the determination of the functional role of histone residues in mammalian development without the replacement of canonical H3 genes. In this review, we describe the key findings from the H3 mutation studies in model organisms wherein the genetic replacement of canonical H3 is possible. We then turn our attention to H3.3 mutations in human cancers, and discuss H3.3 substitutions in the N-terminus, which were generated in order to explore the specific residue or associated post-translational modification.

Distinct contributions of DNA methylation and histone acetylation to the genomic occupancy of transcription factors

Epigenetic modifications on chromatin play important roles in regulating gene expression. Although chromatin states are often governed by multilayered structure, how individual pathways contribute to gene expression remains poorly understood. For example, DNA methylation is known to regulate transcription factor binding but also to recruit methyl-CpG binding proteins that affect chromatin structure through the activity of histone deacetylase complexes (HDACs). Both of these mechanisms can potentially affect gene expression, but the importance of each, and whether these activities are integrated to achieve appropriate gene regulation, remains largely unknown. To address this important question, we measured gene expression, chromatin accessibility, and transcription factor occupancy in wild-type or DNA methylation-deficient mouse embryonic stem cells following HDAC inhibition. We observe widespread increases in chromatin accessibility at retrotransposons when HDACs are inhibited, and this is magnified when cells also lack DNA methylation. A subset of these elements has elevated binding of the YY1 and GABPA transcription factors and increased expression. The pronounced additive effect of HDAC inhibition in DNA methylation-deficient cells demonstrates that DNA methylation and histone deacetylation act largely independently to suppress transcription factor binding and gene expression.

Cisplatin inhibits SIRT3-deacetylation MTHFD2 to disturb cellular redox balance in colorectal cancer cell

The folate-coupled metabolic enzyme MTHFD2 (the mitochondrial methylenetetrahydrofolate dehydrogenase/cyclohydrolase) confers redox homeostasis and drives cancer cell proliferation and migration. Here, we show that MTHFD2 is hyperacetylated and lysine 88 is the critical acetylated site. SIRT3, the major deacetylase in mitochondria, is responsible for MTHFD2 deacetylation. Interestingly, chemotherapeutic agent cisplatin inhibits expression of SIRT3 to induce acetylation of MTHFD2 in colorectal cancer cells. Cisplatin-induced acetylated K88 MTHFD2 is sufficient to inhibit its enzymatic activity and downregulate NADPH levels in colorectal cancer cells. Ac-K88-MTHFD2 is significantly decreased in human colorectal cancer samples and is inversely correlated with the upregulated expression of SIRT3. Our findings reveal an unknown regulation axis of cisplatin-SIRT3-MTHFD2 in redox homeostasis and suggest a potential therapeutic strategy for cancer treatments by targeting MTHFD2.

Ubiquitin ligase SMURF2 enhances epidermal growth factor receptor stability and tyrosine-kinase inhibitor resistance

The discovery of activating epidermal growth factor receptor (EGFR) mutations spurred the use of EGFR tyrosine kinase inhibitors (TKIs), such as erlotinib, as the first-line treatment of lung cancers. We previously reported that differential degradation of TKI-sensitive (e.g. L858R) and resistant (T790M) EGFR mutants upon erlotinib treatment correlates with drug sensitivity. We also reported that SMAD ubiquitination regulatory factor 2 (SMURF2) ligase activity is important in stabilizing EGFR. However, the molecular mechanisms involved remain unclear. Here, using in vitro and in vivo ubiquitination assays, MS, and superresolution microscopy, we show SMURF2-EGFR functional interaction is important for EGFR stability and response to TKI. We demonstrate that L858R/T790M EGFR is preferentially stabilized by SMURF2-UBCH5 (an E3-E2)-mediated polyubiquitination. We identified four lysine residues as the sites of ubiquitination and showed that replacement of one of them with acetylation-mimicking glutamine increases the sensitivity of mutant EGFR to erlotinib-induced degradation. We show that SMURF2 extends membrane retention of EGF-bound EGFR, whereas SMURF2 knockdown increases receptor sorting to lysosomes. In lung cancer cell lines, SMURF2 overexpression increased EGFR levels, improving TKI tolerance, whereas SMURF2 knockdown decreased EGFR steady-state levels and sensitized lung cancer cells. Overall, we propose that SMURF2-mediated polyubiquitination of L858R/T790M EGFR competes with acetylation-mediated receptor internalization that correlates with enhanced receptor stability; therefore, disruption of the E3-E2 complex may be an attractive target to overcome TKI resistance.

Tuning Transcription Factor Availability through Acetylation-Mediated Genomic Redistribution

It is widely assumed that decreasing transcription factor DNA-binding affinity reduces transcription initiation by diminishing occupancy of sequence-specific regulatory elements. However, in vivo transcription factors find their binding sites while confronted with a large excess of low-affinity degenerate motifs. Here, using the melanoma lineage survival oncogene MITF as a model, we show that low-affinity binding sites act as a competitive reservoir in vivo from which transcription factors are released by mitogen-activated protein kinase (MAPK)-stimulated acetylation to promote increased occupancy of their regulatory elements. Consequently, a low-DNA-binding-affinity acetylation-mimetic MITF mutation supports melanocyte development and drives tumorigenesis, whereas a high-affinity non-acetylatable mutant does not. The results reveal a paradoxical acetylation-mediated molecular clutch that tunes transcription factor availability via genome-wide redistribution and couples BRAF to tumorigenesis. Our results further suggest that p300/CREB-binding protein-mediated transcription factor acetylation may represent a common mechanism to control transcription factor availability.

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Reducing transcription factor DNA-binding affinity increases activity in vivo

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Acetylation is triggered by MAPK signaling

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Acetylation leads to genome-wide transcription factor redistribution

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Acetylation of MITF drives tumorigenesis and melanocyte development

Lysine acetylation of the housekeeping sigma factor enhances the activity of the RNA polymerase holoenzyme

Protein lysine acetylation, one of the most abundant post-translational modifications in eukaryotes, occurs in prokaryotes as well. Despite the evidence of lysine acetylation in bacterial RNA polymerases (RNAPs), its function remains unknown. We found that the housekeeping sigma factor (HrdB) was acetylated throughout the growth of an actinobacterium, Streptomyces venezuelae, and the acetylated HrdB was enriched in the RNAP holoenzyme complex. The lysine (K259) located between 1.2 and 2 regions of the sigma factor, was determined to be the acetylated residue of HrdB in vivo by LC–MS/MS analyses. Specifically, the label-free quantitative analysis revealed that the K259 residues of all the HrdB subunits were acetylated in the RNAP holoenzyme. Using mutations that mimic or block acetylation (K259Q and K259R), we found that K259 acetylation enhances the interaction of HrdB with the RNAP core enzyme as well as the binding activity of the RNAP holoenzyme to target promoters in vivo. Taken together, these findings provide a novel insight into an additional layer of modulation of bacterial RNAP activity.

Nucleosomes as allosteric scaffolds for genetic regulation

Nucleosomes are stable yet highly dynamic complexes exhibiting diverse types of motions, such as sliding, DNA unwrapping, and disassembly, encoding a landscape with a large number of metastable states. In this review, describing recent studies on these nucleosome structure changes, we propose that the nucleosome can be viewed as an ideal allosteric scaffold: regulated by effector molecules such as transcription factors and chromatin remodelers, the nucleosome controls the downstream gene activity. Binding of transcription factors to the nucleosome can enhance DNA unwrapping or slide the DNA, altering either the binding or the unbinding of other transcription factors to nearby sites. ATP-dependent chromatin remodelers induce a series of DNA deformations, which allosterically propagate throughout the nucleosome to induce DNA sliding or histone exchange.

Acetylation of the histone H3 tail domain regulates base excision repair on higher-order chromatin structures

Despite recent evidence suggesting that histone lysine acetylation contributes to base excision repair (BER) in cells, their exact mechanistic role remains unclear. In order to examine the influence of histone acetylation on the initial steps of BER, we assembled nucleosome arrays consisting of homogeneously acetylated histone H3 (H3K18 and H3K27) and measured the repair of a site-specifically positioned 2′-deoxyuridine (dU) residue by uracil DNA glycosylase (UDG) and apurinic/apyrimidinic endonuclease 1 (APE1). We find that H3K18ac and H3K27ac differentially influence the combined activities of UDG/APE1 on compact chromatin, suggesting that acetylated lysine residues on the H3 tail domain play distinct roles in regulating the initial steps of BER. In addition, we show that the effects of H3 tail domain acetylation on UDG/APE1 activity are at the nucleosome level and do not influence higher-order chromatin folding. Overall, these results establish a novel regulatory role for histone H3 acetylation during the initiation of BER on chromatin.

Characterization of a metazoan ADA acetyltransferase complex

The Gcn5 acetyltransferase functions in multiple acetyltransferase complexes in yeast and metazoans. Yeast Gcn5 is part of the large SAGA (Spt-Ada-Gcn5 acetyltransferase) complex and a smaller ADA acetyltransferase complex. In flies and mammals, Gcn5 (and its homolog pCAF) is part of various versions of the SAGA complex and another large acetyltransferase complex, ATAC (Ada2A containing acetyltransferase complex). However, a complex analogous to the small ADA complex in yeast has never been described in metazoans. Previous studies in Drosophila hinted at the existence of a small complex which contains Ada2b, a partner of Gcn5 in the SAGA complex. Here we have purified and characterized the composition of this complex and show that it is composed of Gcn5, Ada2b, Ada3 and Sgf29. Hence, we have named it the metazoan 'ADA complex'. We demonstrate that the fly ADA complex has histone acetylation activity on histones and nucleosome substrates. Moreover, ChIP-Sequencing experiments identified Ada2b peaks that overlap with another SAGA subunit, Spt3, as well as Ada2b peaks that do not overlap with Spt3 suggesting that the ADA complex binds chromosomal sites independent of the larger SAGA complex.

Symmetric dimethylation on histone H4R3 associates with histone deacetylation to maintain properly polarized cell growth

Yeast Hsl7 is recognized as a homolog of human arginine methyltransferase 5 (PRMT5) and shows type II PRMT activity by forming symmetric dimethylarginine residues on histones. Previously, we reported that Hsl7 is responsible for in vivo symmetric dimethylation on histone H4 arginine 3 (H4R3me2s) in a transcriptionally repressed state, possibly in association with histone deacetylation by Rpd3. Here, we investigated the function of Hsl7 during cell cycle progression. We found that the accumulation of Hsl7-mediated H4R3me2s is maintained by the histone deacetylase Rpd3 during transcriptional repression and that the low level of H4R3me2s is required for proper asymmetric cell growth during cell division. Our results suggest that the hypoacetylated state of histones is connected to the function of Hsl7 in regulating properly polarized cell growth during cell division and provide new insight into the epigenetic modifications that are important for cell cycle morphogenesis checkpoint control based on the repressive histone crosstalk between symmetric arginine methylation of H4 and histone deacetylation.

MEK inhibition remodels the active chromatin landscape and induces SOX10 genomic recruitment in BRAF(V600E) mutant melanoma cells

Background

The MAPK/ERK signaling pathway is an essential regulator of numerous cell processes that are crucial for normal development as well as cancer progression. While much is known regarding MAPK/ERK signal conveyance from the cell membrane to the nucleus, the transcriptional and epigenetic mechanisms that govern gene expression downstream of MAPK signaling are not fully elucidated.

Results

This study employed an integrated epigenome analysis approach to interrogate the effects of MAPK/ERK pathway inhibition on the global transcriptome, the active chromatin landscape, and protein–DNA interactions in 501mel melanoma cells. Treatment of these cells with the small-molecule MEK inhibitor AZD6244 induces hyperpigmentation, widespread gene expression changes including alteration of genes linked to pigmentation, and extensive epigenomic reprogramming of transcriptionally distinct regulatory regions associated with the active chromatin mark H3K27ac. Regulatory regions with differentially acetylated H3K27ac regions following AZD6244 treatment are enriched in transcription factor binding motifs of ETV/ETS and ATF family members as well as the lineage-determining factors MITF and SOX10. H3K27ac-dense enhancer clusters known as super-enhancers show similar transcription factor motif enrichment, and furthermore, these super-enhancers are associated with genes encoding MITF, SOX10, and ETV/ETS proteins. Along with genome-wide resetting of the active enhancer landscape, MEK inhibition also results in widespread SOX10 recruitment throughout the genome, including increased SOX10 binding density at H3K27ac-marked enhancers. Importantly, these MEK inhibitor-responsive enhancers marked by H3K27ac and occupied by SOX10 are located near melanocyte lineage-specific and pigmentation genes and overlap numerous human SNPs associated with pigmentation and melanoma phenotypes, highlighting the variants located within these regions for prioritization in future studies.

Conclusions

These results reveal the epigenetic reprogramming underlying the re-activation of melanocyte pigmentation and developmental transcriptional programs in 501mel cells in response to MEK inhibition and suggest extensive involvement of a MEK-SOX10 axis in the regulation of these processes. The dynamic chromatin changes identified here provide a rich genomic resource for further analyses of the molecular mechanisms governing the MAPK pathway in pigmentation- and melanocyte-associated diseases.

Electronic supplementary material

The online version of this article (10.1186/s13072-019-0297-2) contains supplementary material, which is available to authorized users.

An acetylation mimicking mutation, K274Q, in tau imparts neurotoxicity by enhancing tau aggregation and inhibiting tubulin polymerization

In Alzheimer's disease, tau is predominantly acetylated at K174, K274, K280, and K281 residues. The acetylation of K274-tau is linked with memory loss and dementia. In this study, we have examined the molecular mechanism of the toxicity of acetylated K274-tau. We incorporated an acetylation mimicking mutation at K274 (K→Q) residue of tau. The mutation (K274Q) strongly reduced the ability of tau to bind to tubulin and also to polymerize tubulin while K274R mutation did not reduce the ability of tau either to bind or polymerize tubulin. In addition, K274Q-tau displayed a higher aggregation propensity than wild-type tau as evident from thioflavin S fluorescence, tryptophan fluorescence, and electron microscopic images. Furthermore, dynamic light scattering, atomic force microscopy, and dot blot analysis using an oligomer-specific antibody suggested that K274Q mutation enhanced the oligomerization of tau. The K274Q mutation also strongly decreased the critical concentration for the liquid-liquid phase separation of tau. The oligomeric forms of K274Q-tau were found to be more toxic than wild tau to neuroblastoma cells. Using circular dichroism and fluorescence spectroscopy, we provide evidence indicating that the acetylation mimicking mutation (K274Q) induced conformational changes in tau. The results suggested that the acetylation of tau at 274 residues can increase tau aggregation and enhance the cytotoxicity of tau oligomers.

Gene-Specific H1 Eviction through a Transcriptional Activator→p300→NAP1→H1 Pathway

Linker histone H1 has been correlated with transcriptional inhibition, but the mechanistic basis of the inhibition and its reversal during gene activation has remained enigmatic. We report that H1-compacted chromatin, reconstituted in vitro, blocks transcription by abrogating core histone modifications by p300 but not activator and p300 binding. Transcription from H1-bound chromatin is elicited by the H1 chaperone NAP1, which is recruited in a gene-specific manner through direct interactions with activator-bound p300 that facilitate core histone acetylation (by p300) and concomitant eviction of H1 and H2A-H2B. An analysis in B cells confirms the strong dependency on NAP1-mediated H1 eviction for induction of the silent CD40 gene and further demonstrates that H1 eviction, seeded by activator-p300-NAP1-H1 interactions, is propagated over a CCCTC-binding factor (CTCF)-demarcated region through a distinct mechanism that also involves NAP1. Our results confirm direct transcriptional inhibition by H1 and establish a gene-specific H1 eviction mechanism through an activator→p300→NAP1→H1 pathway.

YfmK is an Nε-lysine acetyltransferase that directly acetylates the histone-like protein HBsu in Bacillus subtilis

N^ε-lysine acetylation is an abundant and dynamic regulatory posttranslational modification that remains poorly characterized in bacteria. In bacteria, hundreds of proteins are known to be acetylated, but the biological significance of the majority of these events remains unclear. Previously, we characterized the Bacillus subtilis acetylome and found that the essential histone-like protein HBsu contains seven previously unknown acetylation sites in vivo. Here, we investigate whether acetylation is a regulatory component of the function of HBsu in nucleoid compaction. Using mutations that mimic the acetylated and unacetylated forms of the protein, we show that the inability to acetylate key HBsu lysine residues results in a more compacted nucleoid. We further investigated the mechanism of HBsu acetylation. We screened deletions of the ∼50 putative GNAT domain-encoding genes in B. subtilis for their effects on DNA compaction, and identified five candidates that may encode acetyltransferases acting on HBsu. Genetic bypass experiments demonstrated that two of these, YfmK and YdgE, can acetylate Hbsu, and their potential sites of action on HBsu were identified. Additionally, purified YfmK was able to directly acetylate HBsu in vitro, suggesting that it is the second identified protein acetyltransferase in B. subtilis We propose that at least one physiological function of the acetylation of HBsu at key lysine residues is to regulate nucleoid compaction, analogous to the role of histone acetylation in eukaryotes.

A nucleosome-free region locally abrogates histone H1-dependent restriction of linker DNA accessibility in chromatin

Eukaryotic genomes are packaged into linker-oligonucleosome assemblies, providing compaction of genomic DNA and contributing to gene regulation and genome integrity. To define minimal requirements for initial steps in the transition of compact, closed chromatin to a transcriptionally active, open state, we developed a model in vitro system containing a single, unique, "target" nucleosome in the center of a 25-nucleosome array and evaluated the accessibility of the linker DNA adjacent to this target nucleosome. We found that condensation of H1-lacking chromatin results in ∼60-fold reduction in linker DNA accessibility and that mimics of acetylation within all four core histone tail domains of the target nucleosome synergize to increase accessibility ∼3-fold. Notably, stoichiometric binding of histone H1 caused >2 orders of magnitude reduction in accessibility that was marginally diminished by histone acetylation mimics. Remarkably, a nucleosome-free region (NFR) in place of the target nucleosome completely abrogated H1-dependent restriction of linker accessibility in the immediate vicinity of the NFR. Our results suggest that linker DNA is as inaccessible as DNA within the nucleosome core in fully condensed, H1-containing chromatin. They further imply that an unrecognized function of NFRs in gene promoter regions is to locally abrogate the severe restriction of linker DNA accessibility imposed by H1s.

Deacetylation of serine hydroxymethyl-transferase 2 by SIRT3 promotes colorectal carcinogenesis

The conversion of serine and glycine that is accomplished by serine hydroxymethyltransferase 2 (SHMT2) in mitochondria is significantly upregulated in various cancers to support cancer cell proliferation. In this study, we observed that SHMT2 is acetylated at K95 in colorectal cancer (CRC) cells. SIRT3, the major deacetylase in mitochondria, is responsible for SHMT2 deacetylation. SHMT2-K95-Ac disrupts its functional tetramer structure and inhibits its enzymatic activity. SHMT2-K95-Ac also promotes its degradation via the K63-ubiquitin–lysosome pathway in a glucose-dependent manner. TRIM21 acts as an E3 ubiquitin ligase for SHMT2. SHMT2-K95-Ac decreases CRC cell proliferation and tumor growth in vivo through attenuation of serine consumption and reduction in NADPH levels. Finally, SHMT2-K95-Ac is significantly decreased in human CRC samples and is inversely associated with increased SIRT3 expression, which is correlated with poorer postoperative overall survival. Our study reveals the unknown mechanism of SHMT2 regulation by acetylation which is involved in colorectal carcinogenesis.

Serine hydroxymethyltransferase 2 (SHMT2) converts serine to glycine in mitochondria and is upregulated in a variety of cancers. Here the authors show that acetylation of the lysine-95 (K95) residue negatively regulates SHMT2 expression and activity and is deacetylated by SIRT3 in colorectal cancer.

The conformation of the histone H3 tail inhibits association of the BPTF PHD finger with the nucleosome

Histone tails harbor a plethora of post-translational modifications that direct the function of chromatin regulators, which recognize them through effector domains. Effector domain/histone interactions have been broadly studied, but largely using peptide fragments of histone tails. Here, we extend these studies into the nucleosome context and find that the conformation adopted by the histone H3 tails is inhibitory to BPTF PHD finger binding. Using NMR spectroscopy and MD simulations, we show that the H3 tails interact robustly but dynamically with nucleosomal DNA, substantially reducing PHD finger association. Altering the electrostatics of the H3 tail via modification or mutation increases accessibility to the PHD finger, indicating that PTM crosstalk can regulate effector domain binding by altering nucleosome conformation. Together, our results demonstrate that the nucleosome context has a dramatic impact on signaling events at the histone tails, and highlights the importance of studying histone binding in the context of the nucleosome.

Novel function of HATs and HDACs in homologous recombination through acetylation of human RAD52 at double-strand break sites

The p300 and CBP histone acetyltransferases are recruited to DNA double-strand break (DSB) sites where they induce histone acetylation, thereby influencing the chromatin structure and DNA repair process. Whether p300/CBP at DSB sites also acetylate non-histone proteins, and how their acetylation affects DSB repair, remain unknown. Here we show that p300/CBP acetylate RAD52, a human homologous recombination (HR) DNA repair protein, at DSB sites. Using in vitro acetylated RAD52, we identified 13 potential acetylation sites in RAD52 by a mass spectrometry analysis. An immunofluorescence microscopy analysis revealed that RAD52 acetylation at DSBs sites is counteracted by SIRT2- and SIRT3-mediated deacetylation, and that non-acetylated RAD52 initially accumulates at DSB sites, but dissociates prematurely from them. In the absence of RAD52 acetylation, RAD51, which plays a central role in HR, also dissociates prematurely from DSB sites, and hence HR is impaired. Furthermore, inhibition of ataxia telangiectasia mutated (ATM) protein by siRNA or inhibitor treatment demonstrated that the acetylation of RAD52 at DSB sites is dependent on the ATM protein kinase activity, through the formation of RAD52, p300/CBP, SIRT2, and SIRT3 foci at DSB sites. Our findings clarify the importance of RAD52 acetylation in HR and its underlying mechanism.

A systematic analysis of nucleosome core particle and nucleosome-nucleosome stacking structure

Chromatin condensation is driven by the energetically favourable interaction between nucleosome core particles (NCPs). The close NCP-NCP contact, stacking, is a primary structural element of all condensed states of chromatin in vitro and in vivo. However, the molecular structure of stacked nucleosomes as well as the nature of the interactions involved in its formation have not yet been systematically studied. Here we undertake an investigation of both the structural and physico-chemical features of NCP structure and the NCP-NCP stacking. We introduce an “NCP-centred” set of parameters (NCP-NCP distance, shift, rise, tilt, and others) that allows numerical characterisation of the mutual positions of the NCPs in the stacking and in any other structures formed by the NCP. NCP stacking in more than 140 published NCP crystal structures were analysed. In addition, coarse grained (CG) MD simulations modelling NCP condensation was carried out. The CG model takes into account details of the nucleosome structure and adequately describes the long range electrostatic forces as well as excluded volume effects acting in chromatin. The CG simulations showed good agreement with experimental data and revealed the importance of the H2A and H4 N-terminal tail bridging and screening as well as tail-tail correlations in the stacked nucleosomes.

Acetylation of TBX5 by KAT2B and KAT2A regulates heart and limb development

TBX5 plays a critical role in heart and forelimb development. Mutations in TBX5 cause Holt-Oram syndrome, an autosomal dominant condition that affects the formation of the heart and upper-limb. Several studies have provided significant insight into the role of TBX5 in cardiogenesis; however, how TBX5 activity is regulated by other factors is still unknown. Here we report that histone acetyltransferases KAT2A and KAT2B associate with TBX5 and acetylate it at Lys339. Acetylation potentiates its transcriptional activity and is required for nuclear retention. Morpholino-mediated knockdown of kat2a and kat2b transcripts in zebrafish severely perturb heart and limb development, mirroring the tbx5a knockdown phenotype. The phenotypes found in MO-injected embryos were also observed when we introduced mutations in the kat2a or kat2b genes using the CRISPR-Cas system. These studies highlight the importance of KAT2A and KAT2B modulation of TBX5 and their impact on heart and limb development.

HMGN1 and 2 remodel core and linker histone tail domains within chromatin

The structure of the nucleosome, the basic building block of the chromatin fiber, plays a key role in epigenetic regulatory processes that affect DNA-dependent processes in the context of chromatin. Members of the HMGN family of proteins bind specifically to nucleosomes and affect chromatin structure and function, including transcription and DNA repair. To better understand the mechanisms by which HMGN 1 and 2 alter chromatin, we analyzed their effect on the organization of histone tails and linker histone H1 in nucleosomes. We find that HMGNs counteract linker histone (H1)-dependent stabilization of higher order ‘tertiary’ chromatin structures but do not alter the intrinsic ability of nucleosome arrays to undergo salt-induced compaction and self-association. Surprisingly, HMGNs do not displace H1s from nucleosomes; rather these proteins bind nucleosomes simultaneously with H1s without disturbing specific contacts between the H1 globular domain and nucleosomal DNA. However, HMGNs do alter the nucleosome-dependent condensation of the linker histone C-terminal domain, which is critical for stabilizing higher-order chromatin structures. Moreover, HMGNs affect the interactions of the core histone tail domains with nucleosomal DNA, redirecting the tails to more interior positions within the nucleosome. Our studies provide new insights into the molecular mechanisms whereby HMGNs affect chromatin structure.

Structure and Dynamics of a 197 bp Nucleosome in Complex with Linker Histone H1

Linker histones associate with nucleosomes to promote the formation of higher-order chromatin structure, but the underlying molecular details are unclear. We investigated the structure of a 197 bp nucleosome bearing symmetric 25 bp linker DNA arms in complex with vertebrate linker histone H1. We determined electron cryo-microscopy (cryo-EM) and crystal structures of unbound and H1-bound nucleosomes and validated these structures by site-directed protein cross-linking and hydroxyl radical footprinting experiments. Histone H1 shifts the conformational landscape of the nucleosome by drawing the two linkers together and reducing their flexibility. The H1 C-terminal domain (CTD) localizes primarily to a single linker, while the H1 globular domain contacts the nucleosome dyad and both linkers, associating more closely with the CTD-distal linker. These findings reveal that H1 imparts a strong degree of asymmetry to the nucleosome, which is likely to influence the assembly and architecture of higher-order structures.

Myc Regulates Chromatin Decompaction and Nuclear Architecture during B Cell Activation

50 years ago, Vincent Allfrey and colleagues discovered that lymphocyte activation triggers massive acetylation of chromatin. However, the molecular mechanisms driving epigenetic accessibility are still unknown. We here show that stimulated lymphocytes decondense chromatin by three differentially regulated steps. First, chromatin is repositioned away from the nuclear periphery in response to global acetylation. Second, histone nanodomain clusters decompact into mononucleosome fibers through a mechanism that requires Myc and continual energy input. Single-molecule imaging shows that this step lowers transcription factor residence time and non-specific collisions during sampling for DNA targets. Third, chromatin interactions shift from long range to predominantly short range, and CTCF-mediated loops and contact domains double in numbers. This architectural change facilitates cognate promoter-enhancer contacts and also requires Myc and continual ATP production. Our results thus define the nature and transcriptional impact of chromatin decondensation and reveal an unexpected role for Myc in the establishment of nuclear topology in mammalian cells.

Regulation, Function, and Detection of Protein Acetylation in Bacteria

N^ε-Lysine acetylation is now recognized as an abundant posttranslational modification (PTM) that influences many essential biological pathways. Advancements in mass spectrometry-based proteomics have led to the discovery that bacteria contain hundreds of acetylated proteins, contrary to the prior notion of acetylation events being rare in bacteria. Although the mechanisms that regulate protein acetylation are still not fully defined, it is understood that this modification is finely tuned via both enzymatic and nonenzymatic mechanisms. The opposing actions of Gcn5-related N-acetyltransferases (GNATs) and deacetylases, including sirtuins, provide the enzymatic control of lysine acetylation. A nonenzymatic mechanism of acetylation has also been demonstrated and proven to be prominent in bacteria, as well as in mitochondria. The functional consequences of the vast majority of the identified acetylation sites remain unknown. From studies in mammalian systems, acetylation of critical lysine residues was shown to impact protein function by altering its structure, subcellular localization, and interactions. It is becoming apparent that the same diversity of functions can be found in bacteria. Here, we review current knowledge of the mechanisms and the functional consequences of acetylation in bacteria. Additionally, we discuss the methods available for detecting acetylation sites, including quantitative mass spectrometry-based methods, which promise to promote this field of research. We conclude with possible future directions and broader implications of the study of protein acetylation in bacteria.

The Influence of Ionic Environment and Histone Tails on Columnar Order of Nucleosome Core Particles

The nucleosome core particle (NCP) is the basic building block of chromatin. Nucleosome-nucleosome interactions are instrumental in chromatin compaction, and understanding NCP self-assembly is important for understanding chromatin structure and dynamics. Recombinant NCPs aggregated by multivalent cations form various ordered phases that can be studied by x-ray diffraction (small-angle x-ray scattering). In this work, the effects on the supramolecular structure of aggregated NCPs due to lysine histone H4 tail acetylations, histone H2A mutations (neutralizing the acidic patch of the histone octamer), and the removal of histone tails were investigated. The formation of ordered mainly hexagonal columnar NCP phases is in agreement with earlier studies; however, the highly homogeneous recombinant NCP systems used in this work display a more compact packing. The long-range order of the NCP columnar phase was found to be abolished or reduced by acetylation of the H4 tails, acidic patch neutralization, and removal of the H3 and H2B tails. Loss of nucleosome stacking upon removal of the H3 tails in combination with other tails was observed. In the absence of the H2A tails, the formation of an unknown highly ordered phase was observed.

Regulation of Nucleosome Stacking and Chromatin Compaction by the Histone H4 N-Terminal Tail-H2A Acidic Patch Interaction

Chromatin folding and dynamics are critically dependent on nucleosome-nucleosome interactions with important contributions from internucleosome binding of the histone H4 N-terminal tail K16-R23 domain to the surface of the H2A/H2B dimer. The H4 Lys16 plays a pivotal role in this regard. Using in vitro reconstituted 12-mer nucleosome arrays, we have investigated the mechanism of the H4 N-terminal tail in maintaining nucleosome-nucleosome stacking and mediating intra- and inter-array chromatin compaction, with emphasis on the role of K16 and the positive charge region, R17-R23. Analytical ultracentrifugation sedimentation velocity experiments and precipitation assays were employed to analyze effects on chromatin folding and self-association, respectively. Effects on chromatin folding caused by various mutations and modifications at position K16 in the H4 histone were studied. Additionally, using charge-quenching mutations, we characterized the importance of the interaction of the residues within the H4 positive charge region R17-R23 with the H2A acidic patch of the adjacent nucleosome. Furthermore, crosslinking experiments were conducted to establish the proximity of the basic tail region to the acidic patch. Our data indicate that the positive charge and length of the side chain of H4 K16 are important for its access to the adjacent nucleosome in the process of nucleosome-nucleosome stacking and array folding. The location and orientation of the H4 R17-R23 domain on the H2A/H2B dimer surface of the neighboring nucleosome core particle (NCP) in the compacted chromatin fiber were established. The dominance of electrostatic interactions in maintaining intra-array interaction was demonstrated.

Physarum polycephalum for Studying the Function of Histone Modifications In Vivo

Histone modifications have been widely correlated with genetic activities. However, how these posttranslational modifications affect the dynamics and the structure of chromatin is poorly understood. Here, we describe the incorporation of the exogenous histone proteins into the slime mold Physarum polycephalum, which has been revealed to be a valuable tool for examining different facets of the function histones in chromatin dynamics like replication-coupled chromatin assembly, histone exchange, and nucleosome turnover.

Reactivation of Fetal Hemoglobin for Treating β-Thalassemia and Sickle Cell Disease

Reactivation of fetal hemoglobin (HbF) in adult hematopoietic cells has the potential for great clinical benefit in patients bearing deleterious mutations in the β-globin gene, such as β-thalassemia and sickle cell disease (SCD), since increasing the production of HbF can compensate for underproduction of β-globin chains (in β-thalassemia) and it can also disrupt sickle hemoglobin polymerization (in SCD). Thus for the past few decades, concerted efforts have been made to identify an effective way to induce the synthesis of HbF in adult erythroid cells for potential therapeutic relief from the effects of these β-globinopathies. Chemical inducers of HbF as well as a number of transcription factors that are able to reactivate HbF synthesis in vitro and in vivo in adult erythroid cells have been identified. However, there has been only limited success in attempts to manipulate either the drugs or regulatory proteins, and in only a fraction of patients, and there is wide variation in individual response to these drugs or transcription factors. These studies highlight the importance for understanding the molecular mechanisms underlying hemoglobin switching so that future studies can be designed to treat these disorders.

Acetylation Mimics Within a Single Nucleosome Alter Local DNA Accessibility In Compacted Nucleosome Arrays

The activation of a silent gene locus is thought to involve pioneering transcription factors that initiate changes in the local chromatin structure to increase promoter accessibility and binding of downstream effectors. To better understand the molecular requirements for the first steps of locus activation, we investigated whether acetylation of a single nucleosome is sufficient to alter DNA accessibility within a condensed 25-nucleosome array. We found that acetylation mimics within the histone H4 tail domain increased accessibility of the surrounding linker DNA, with the increased accessibility localized to the immediate vicinity of the modified nucleosome. In contrast, acetylation mimics within the H3 tail had little effect, but were able to synergize with H4 tail acetylation mimics to further increase accessibility. Moreover, replacement of the central nucleosome with a nucleosome free region also resulted in increased local, but not global DNA accessibility. Our results indicate that modification or disruption of only a single target nucleosome results in significant changes in local chromatin architecture and suggest that very localized chromatin modifications imparted by pioneer transcription factors are sufficient to initiate a cascade of events leading to promoter activation.

Histone acetylation dependent energy landscapes in tri-nucleosome revealed by residue-resolved molecular simulations

Histone tail acetylation is a key epigenetic marker that tends to open chromatin folding and activate transcription. Despite intensive studies, precise roles of individual lysine acetylation in chromatin folding have only been poorly understood. Here, we revealed structural dynamics of tri-nucleosomes with several histone tail acetylation states and analyzed histone tail interactions with DNA by performing molecular simulations at an unprecedentedly high resolution. We found versatile acetylation-dependent landscapes of tri-nucleosome. The H4 and H2A tail acetylation reduced the contact between the first and third nucleosomes mediated by the histone tails. The H3 tail acetylation reduced its interaction with neighboring linker DNAs resulting in increase of the distance between consecutive nucleosomes. Notably, two copies of the same histone in a single nucleosome have markedly asymmetric interactions with DNAs, suggesting specific pattern of nucleosome docking albeit high inherent flexibility. Estimated transcription factor accessibility was significantly high for the H4 tail acetylated structures.

Abnormal histone acetylation of CD8+ T cells in patients with severe aplastic anemia

Severe aplastic anemia (SAA) is a rare autoimmune disease characterized by severe pancytopenia and bone marrow failure, which is caused by activated T lymphocytes. In the present study, we evaluated histone H3 acetylation levels of bone marrow CD8⁺ T cells in SAA patients, and analyzed its correlation with clinical condition parameters. We found that the percentages of CD8⁺ T cell histone H3 acetylation in patients with untreated SAA, recovering SAA (R-SAA) and normal control, were 1.21 ± 0.08, 1.05 ± 0.36, and 1.00 ± 0.41, respectively, with no significant statistical differences. However, the amount of CD8⁺ T cell histone H3 acetylation from untreated SAA was 176.21 ± 32.22 μg/mg protein, which was significantly higher than that of complete response (CR)-SAA (104.29 ± 62.06 μg/mg protein) and normal control (133.94 ± 56.27 μg/mg protein) (P < 0.05) groups. Moreover, histone H3 acetylation amount of CD8⁺ T cell was significantly and negatively correlated with absolute neutrophil count, proportion of reticulocytes, ratio of CD4⁺ to CD8⁺ T cell in peripheral blood, and percentage of bone marrow erythroid (P < 0.05). To some extent, it also negatively correlated with hemoglobin level, platelet count, percentage of bone marrow granulocyte, and megakaryocyte count. Abnormal histone H3 acetylation of CD8⁺ T cells may thus play a role in the immune pathogenesis of SAA.

Genome maintenance in the context of 4D chromatin condensation

The eukaryotic genome is packaged in the three-dimensional nuclear space by forming loops, domains, and compartments in a hierarchical manner. However, when duplicated genomes prepare for segregation, mitotic cells eliminate topologically associating domains and abandon the compartmentalized structure. Alongside chromatin architecture reorganization during the transition from interphase to mitosis, cells halt most DNA-templated processes such as transcription and repair. The intrinsically condensed chromatin serves as a sophisticated signaling module subjected to selective relaxation for programmed genomic activities. To understand the elaborate genome-epigenome interplay during cell cycle progression, the steady three-dimensional genome requires a time scale to form a dynamic four-dimensional and a more comprehensive portrait. In this review, we will dissect the functions of critical chromatin architectural components in constructing and maintaining an orderly packaged chromatin environment. We will also highlight the importance of the spatially and temporally conscious orchestration of chromatin remodeling to ensure high-fidelity genetic transmission.