Histone variants on the move: substrates for chromatin dynamics

CENP-B protects centromere chromatin integrity by facilitating histone deposition via the H3.3-specific chaperone Daxx

Background

The main chromatin unit, the nucleosome, can be modulated by the incorporation of histone variants that, in combination with posttranslational histones modifications, determine epigenetics properties of chromatin. Understanding the mechanism that creates a histone variants landscape at different genomic elements is expected to elevate our comprehension of chromatin assembly and function. The Daxx chaperone deposits transcription-associated histone H3.3 at centromeres, but mechanism of centromere-specific Daxx targeting remains unclear.

Results

In this study, we identified an unexpected function of the constitutive centromeric protein CENP-B that serves as a “beacon” for H3.3 incorporation. CENP-B depletion reduces Daxx association and H3.3 incorporation at centromeres. Daxx/CENP-B interaction and Daxx centromeric association are SUMO dependent and requires SIMs of Daxx. Depletion of SUMO-2, but not SUMO-1, decreases Daxx/CENP-B interaction and reduces centromeric accumulation of Daxx and H3.3, demonstrating distinct functions of SUMO paralogs in H3.3 chaperoning. Finally, disruption of CENP-B/Daxx-dependent H3.3 pathway deregulates heterochromatin marks H3K9me3, ATRX and HP1α at centromeres and elevates chromosome instability.

Conclusion

The demonstrated roles of CENP-B and SUMO-2 in H3.3 loading reveal a novel mechanism controlling chromatin maintenance and genome stability. Given that CENP-B is the only centromere protein that binds centromere-specific DNA elements, our study provides a new link between centromere DNA and unique epigenetic landscape of centromere chromatin.

Electronic supplementary material

The online version of this article (10.1186/s13072-017-0164-y) contains supplementary material, which is available to authorized users.

Shaping Chromatin in the Nucleus: The Bricks and the Architects

Chromatin organization in the nucleus provides a vast repertoire of information in addition to that encoded genetically. Understanding how this organization impacts genome stability and influences cell fate and tumorigenesis is an area of rapid progress. Considering the nucleosome, the fundamental unit of chromatin structure, the study of histone variants (the bricks) and their selective loading by histone chaperones (the architects) is particularly informative. Here, we report recent advances in understanding how relationships between histone variants and their chaperones contribute to tumorigenesis using cell lines and Xenopus development as model systems. In addition to their role in histone deposition, we also document interactions between histone chaperones and other chromatin factors that govern higher-order structure and control DNA metabolism. We highlight how a fine-tuned assembly line of bricks (H3.3 and CENP-A) and architects (HIRA, HJURP, and DAXX) is key in adaptation to developmental and pathological changes. An example of this conceptual advance is the exquisite sensitivity displayed by p53-null tumor cells to modulation of HJURP, the histone chaperone for CENP-A (CenH3 variant). We discuss how these findings open avenues for novel therapeutic paradigms in cancer care.

Resetting the Yeast Epigenome with Human Nucleosomes

Humans and yeast are separated by a billion years of evolution, yet their conserved histones retain central roles in gene regulation. Here, we "reset" yeast to use core human nucleosomes in lieu of their own (a rare event taking 20 days), which initially only worked with variant H3.1. The cells adapt by acquiring suppressor mutations in cell-division genes or by acquiring certain aneuploid states. Converting five histone residues to their yeast counterparts restored robust growth. We reveal that humanized nucleosomes are positioned according to endogenous yeast DNA sequence and chromatin-remodeling network, as judged by a yeast-like nucleosome repeat length. However, human nucleosomes have higher DNA occupancy, globally reduce RNA content, and slow adaptation to new conditions by delaying chromatin remodeling. These humanized yeasts (including H3.3) pose fundamental new questions about how chromatin is linked to many cell processes and provide a platform to study histone variants via yeast epigenome reprogramming.

Regulation of DNA replication-coupled histone gene expression

The expression of core histone genes is cell cycle regulated. Large amounts of histones are required to restore duplicated chromatin during S phase when DNA replication occurs. Over-expression and excess accumulation of histones outside S phase are toxic to cells and therefore cells need to restrict histone expression to S phase. Misregulation of histone gene expression leads to defects in cell cycle progression, genome stability, DNA damage response and transcriptional regulation. Here, we discussed the factors involved in histone gene regulation as well as the underlying mechanism. Understanding the histone regulation mechanism will shed lights on elucidating the side effects of certain cancer chemotherapeutic drugs and developing potential biomarkers for tumor cells.

Catching global interactions in vivo

Histone proteins and transcription factors (TFs) play critical roles in gene transcription and development of multicellular organisms. Although antibody mediated protein isolation couple with mass spectrometry approach has been a standard method to identify TF interacting partners and characterize their functional molecular complexes, it becomes urge to develop a robust method to functional characterize how these transcription factors act during biological process in the post-human genome project era. Here, Dr. Zhao and his colleagues in the National Heart, Lung, and Blood Institute of NIH develop a sensitive and robust strategy to globally identify and characterize in vivo protein-protein interactions termed bait protein-protein interaction-sequencing (bPPI-seq) (Zhang et al. in Cell Res doi:10.1038/cr.2017.112, 2017). As a proof-of-principle, they demonstrated that genome-wide interacting partners of histone variant H2A.Z are mainly involved in transcriptional regulation which is distinct from the interacting proteins of canonical histone H2A. Thus, their results suggest that bPPI-seq can be widely used to globally characterize protein complexes especially transcription factor interacting partners and molecular networks formed.

Molecular structures guide the engineering of chromatin

Chromatin is a system of proteins, RNA, and DNA that interact with each other to organize and regulate genetic information within eukaryotic nuclei. Chromatin proteins carry out essential functions: packing DNA during cell division, partitioning DNA into sub-regions within the nucleus, and controlling levels of gene expression. There is a growing interest in manipulating chromatin dynamics for applications in medicine and agriculture. Progress in this area requires the identification of design rules for the chromatin system. Here, we focus on the relationship between the physical structure and function of chromatin proteins. We discuss key research that has elucidated the intrinsic properties of chromatin proteins and how this information informs design rules for synthetic systems. Recent work demonstrates that chromatin-derived peptide motifs are portable and in some cases can be customized to alter their function. Finally, we present a workflow for fusion protein design and discuss best practices for engineering chromatin to assist scientists in advancing the field of synthetic epigenetics.

Nucleosome stability measured in situ by automated quantitative imaging

Current approaches have limitations in providing insight into the functional properties of particular nucleosomes in their native molecular environment. Here we describe a simple and powerful method involving elution of histones using intercalators or salt, to assess stability features dependent on DNA superhelicity and relying mainly on electrostatic interactions, respectively, and measurement of the fraction of histones remaining chromatin-bound in the individual nuclei using histone type- or posttranslational modification- (PTM-) specific antibodies and automated, quantitative imaging. The method has been validated in H3K4me3 ChIP-seq experiments, by the quantitative assessment of chromatin loop relaxation required for nucleosomal destabilization, and by comparative analyses of the intercalator and salt induced release from the nucleosomes of different histones. The accuracy of the assay allowed us to observe examples of strict association between nucleosome stability and PTMs across cell types, differentiation state and throughout the cell-cycle in close to native chromatin context, and resolve ambiguities regarding the destabilizing effect of H2A.X phosphorylation. The advantages of the in situ measuring scenario are demonstrated via the marked effect of DNA nicking on histone eviction that underscores the powerful potential of topological relaxation in the epigenetic regulation of DNA accessibility.

Understanding nucleosome dynamics and their links to gene expression and DNA replication

Advances in genomics technology have provided the means to probe myriad chromatin interactions at unprecedented spatial and temporal resolution. This has led to a profound understanding of nucleosome organization within the genome, revealing that nucleosomes are highly dynamic. Nucleosome dynamics are governed by a complex interplay of histone composition, histone post-translational modifications, nucleosome occupancy and positioning within chromatin, which are influenced by numerous regulatory factors, including general regulatory factors, chromatin remodellers, chaperones and polymerases. It is now known that these dynamics regulate diverse cellular processes ranging from gene transcription to DNA replication and repair.

The histone H3 variant H3.3 regulates gene body DNA methylation in Arabidopsis thaliana

BACKGROUND

Gene bodies of vertebrates and flowering plants are occupied by the histone variant H3.3 and DNA methylation. The origin and significance of these profiles remain largely unknown. DNA methylation and H3.3 enrichment profiles over gene bodies are correlated and both have a similar dependence on gene transcription levels. This suggests a mechanistic link between H3.3 and gene body methylation.

RESULTS

We engineered an H3.3 knockdown in Arabidopsis thaliana and observed transcription reduction that predominantly affects genes responsive to environmental cues. When H3.3 levels are reduced, gene bodies show a loss of DNA methylation correlated with transcription levels. To study the origin of changes in DNA methylation profiles when H3.3 levels are reduced, we examined genome-wide distributions of several histone H3 marks, H2A.Z, and linker histone H1. We report that in the absence of H3.3, H1 distribution increases in gene bodies in a transcription-dependent manner.

CONCLUSIONS

We propose that H3.3 prevents recruitment of H1, inhibiting H1's promotion of chromatin folding that restricts access to DNA methyltransferases responsible for gene body methylation. Thus, gene body methylation is likely shaped by H3.3 dynamics in conjunction with transcriptional activity.

Total chemical synthesis of methylated analogues of histone 3 revealed KDM4D as a potential regulator of H3K79me3

Histone H3 methylation plays an important role in regulating gene expression. In histones in general, this mark is dynamically regulated via various demethylases, which found to control cell fate decisions as well as linked to several diseases, including neurological and cancer. Despite major progress in studying methylation mark at various positions in H3 histone proteins, less is known about the regulation of methylated H3 at Lys79. Methylation at this site is known to have direct cross-talk with monoubiquitination of histone H2B at positions Lys120 and 34, as well as with acetylated H3 at Lys9. Herein we applied convergent total chemical protein synthesis to prepare trimethylated H3 at Lys79 to perform initial studies related to the regulation of this mark. Our study enabled us to identify KDM4D lysine demethylase as a potential regulator for trimethylated H3 at Lys79.

Molecular signature of anastasis for reversal of apoptosis

Anastasis (Greek for "rising to life") is a cell recovery phenomenon that rescues dying cells from the brink of cell death. We recently discovered anastasis to occur after the execution-stage of apoptosis in vitro and in vivo. Promoting anastasis could in principle preserve injured cells that are difficult to replace, such as cardiomyocytes and neurons. Conversely, arresting anastasis in dying cancer cells after cancer therapies could improve treatment efficacy. To develop new therapies that promote or inhibit anastasis, it is essential to identify the key regulators and mediators of anastasis - the therapeutic targets. Therefore, we performed time-course microarray analysis to explore the molecular mechanisms of anastasis during reversal of ethanol-induced apoptosis in mouse primary liver cells. We found striking changes in transcription of genes involved in multiple pathways, including early activation of pro-cell survival, anti-oxidation, cell cycle arrest, histone modification, DNA-damage and stress-inducible responses, and at delayed times, angiogenesis and cell migration. Validation with RT-PCR confirmed similar changes in the human liver cancer cell line, HepG2, during anastasis. Here, we present the time-course whole-genome gene expression dataset revealing gene expression profiles during the reversal of apoptosis. This dataset provides important insights into the physiological, pathological, and therapeutic implications of anastasis.

Molecular signature of anastasis for reversal of apoptosis

Anastasis (Greek for "rising to life") is a cell recovery phenomenon that rescues dying cells from the brink of cell death. We recently discovered anastasis to occur after the execution-stage of apoptosis in vitro and in vivo. Promoting anastasis could in principle preserve injured cells that are difficult to replace, such as cardiomyocytes and neurons. Conversely, arresting anastasis in dying cancer cells after cancer therapies could improve treatment efficacy. To develop new therapies that promote or inhibit anastasis, it is essential to identify the key regulators and mediators of anastasis – the therapeutic targets. Therefore, we performed time-course microarray analysis to explore the molecular mechanisms of anastasis during reversal of ethanol-induced apoptosis in mouse primary liver cells. We found striking changes in transcription of genes involved in multiple pathways, including early activation of pro-cell survival, anti-oxidation, cell cycle arrest, histone modification, DNA-damage and stress-inducible responses, and at delayed times, angiogenesis and cell migration. Validation with RT-PCR confirmed similar changes in the human liver cancer cell line, HepG2, during anastasis. Here, we present the time-course whole-genome gene expression dataset revealing gene expression profiles during the reversal of apoptosis. This dataset provides important insights into the physiological, pathological, and therapeutic implications of anastasis.

Plant Deubiquitinases and Their Role in the Control of Gene Expression Through Modification of Histones

Selective degradation of proteins in the cell occurs through ubiquitination, which consists of post-translational deposition of ubiquitin on proteins to target them for degradation by proteases. However, ubiquitination does not only impact on protein stability, but promotes changes in their functions. Whereas the deposition of ubiquitin has been amply studied and discussed, the antagonistic activity, deubiquitination, is just emerging and the full model and players involved in this mechanism are far from being completely understood. Nevertheless, it is the dynamic balance between ubiquitination and deubiquitination that is essential for the development and homeostasis of organisms. In this review, we present a detailed analysis of the members of the deubiquitinase (DUB) superfamily in plants and its division in different clades. We describe current knowledge in the molecular and functional characterisation of DUB proteins, focusing primarily on Arabidopsis thaliana. In addition, the striking function of the duality between ubiquitination and deubiquitination in the control of gene expression through the modification of chromatin is discussed and, using the available information of the activities of the DUB superfamily in yeast and animals as scaffold, we propose possible scenarios for the role of these proteins in plants.

Chromatin Replication and Histone Dynamics

Inheritance of the DNA sequence and its proper organization into chromatin is fundamental for genome stability and function. Therefore, how specific chromatin structures are restored on newly synthesized DNA and transmitted through cell division remains a central question to understand cell fate choices and self-renewal. Propagation of genetic information and chromatin-based information in cycling cells entails genome-wide disruption and restoration of chromatin, coupled with faithful replication of DNA. In this chapter, we describe how cells duplicate the genome while maintaining its proper organization into chromatin. We reveal how specialized replication-coupled mechanisms rapidly assemble newly synthesized DNA into nucleosomes, while the complete restoration of chromatin organization including histone marks is a continuous process taking place throughout the cell cycle. Because failure to reassemble nucleosomes at replication forks blocks DNA replication progression in higher eukaryotes and leads to genomic instability, we further underline the importance of the mechanistic link between DNA replication and chromatin duplication.