HIRA-dependent boundaries between H3 variants shape early replication in mammals

Behaviors of nucleosomes with mutant histone H4s in euchromatic domains of living human cells

Since Robert Feulgen first stained DNA in the cell, visualizing genome chromatin has been a central issue in cell biology to uncover how chromatin is organized and behaves in the cell. To approach this issue, we have developed single-molecule imaging of nucleosomes, a basic unit of chromatin, to unveil local nucleosome behavior in living cells. In this study, we investigated behaviors of nucleosomes with various histone H4 mutants in living HeLa cells to address the role of H4 tail acetylation, including H4K16Ac and others, which are generally associated with more transcriptionally active chromatin regions. We ectopically expressed wild-type (wt) or mutated H4s (H4K16 point; H4K5,8,12,16 quadruple; and H4 tail deletion) fused with HaloTag in HeLa cells. Cells that expressed wtH4-Halo, H4K16-Halo mutants, and multiple H4-Halo mutants had euchromatin-concentrated distribution. Consistently, the genomic regions of the wtH4-Halo nucleosomes corresponded to Hi-C contact domains (or topologically associating domains, TADs) with active chromatin marks (A-compartment). Utilizing single-nucleosome imaging, we found that none of the H4 deacetylation or acetylation mimicked H4 mutants altered the overall local nucleosome motion. This finding suggests that H4 mutant nucleosomes embedded in the condensed euchromatic domains with excess endogenous H4 nucleosomes cannot cause an observable change in the local motion. Interestingly, H4 with four lysine-to-arginine mutations displayed a substantial freely diffusing fraction in the nucleoplasm, whereas H4 with a truncated N-terminal tail was incorporated in heterochromatic regions as well as euchromatin. Our study indicates the power of single-nucleosome imaging to understand individual histone/nucleosome behavior reflecting chromatin environments in living cells.

The many faces of H3.3 in regulating chromatin in embryonic stem cells and beyond

H3.3 is a highly conserved nonreplicative histone variant. H3.3 is enriched in promoters and enhancers of active genes, but it is also found within suppressed heterochromatin, mostly around telomeres. Accordingly, H3.3 is associated with seemingly contradicting functions: It is involved in development, differentiation, reprogramming, and cell fate, as well as in heterochromatin formation and maintenance, and the silencing of developmental genes. The emerging view is that different cellular contexts and histone modifications can promote opposing functions for H3.3. Here, we aim to provide an update with a focus on H3.3 functions in early mammalian development, considering the context of embryonic stem cell maintenance and differentiation, to finally conclude with emerging roles in cancer development and cell fate transition and maintenance.

MLL3/MLL4 enzymatic activity shapes DNA replication timing

Mammalian genomes are replicated in a precise order during S phase, which is cell-type-specific1-3 and correlates with local transcriptional activity2,4-8, chromatin modifications9 and chromatin architecture1,10,11,12. However, the causal relationships between these features and the key regulators of DNA replication timing (RT) are largely unknown. Here, machine learning was applied to quantify chromatin features, including epigenetic marks, histone variants and chromatin architectural factors, best predicting local RT under steady-state and RT changes during early embryonic stem (ES) cell differentiation. About one-third of genome exhibited RT changes during the differentiation. Combined, chromatin features predicted steady-state RT and RT changes with high accuracy. Of these features, histone H3 lysine 4 monomethylation (H3K4me1) catalyzed by MLL3/4 (also known as KMT2C/D) emerged as a top predictor. Loss of Mll3/4 (but not Mll3 alone) or their enzymatic activity resulted in erasure of genome-wide RT dynamics during ES cell differentiation. Sites that normally gain H3K4me1 in a MLL3/4-dependent fashion during the transition failed to transition towards earlier RT, often with transcriptional activation unaffected. Further analysis revealed a requirement for MLL3/4 in promoting DNA replication initiation zones through MCM2 recruitment, providing a direct link for its role in regulating RT. Our results uncover MLL3/4-dependent H3K4me1 as a functional regulator of RT and highlight a causal relationship between the epigenome and RT that is largely uncoupled from transcription. These findings uncover a previously unknown role for MLL3/4-dependent chromatin functions which is likely relevant to the numerous diseases associated with MLL3/4 mutations.

The cell-cycle choreography of H3 variants shapes the genome

Histone variants provide versatility in the basic unit of chromatin, helping to define dynamic landscapes and cell fates. Maintaining genome integrity is paramount for the cell, and it is intimately linked with chromatin dynamics, assembly, and disassembly during DNA transactions such as replication, repair, recombination, and transcription. In this review, we focus on the family of H3 variants and their dynamics in space and time during the cell cycle. We review the distinct H3 variants' specific features along with their escort partners, the histone chaperones, compiled across different species to discuss their distinct importance considering evolution. We place H3 dynamics at different times during the cell cycle with the possible consequences for genome stability. Finally, we examine how their mutation and alteration impact disease. The emerging picture stresses key parameters in H3 dynamics to reflect on how when they are perturbed, they become a source of stress for genome integrity.

Mitotic inheritance of genetic and epigenetic information via the histone H3.1 variant

The replication-dependent histone H3.1 variant, ubiquitous in multicellular eukaryotes, has been proposed to play key roles during chromatin replication due to its unique expression pattern restricted to the S phase of the cell cycle. Here, we describe recent discoveries in plants regarding molecular mechanisms and cellular pathways involving H3.1 that contribute to the maintenance of genomic and epigenomic information. First, we highlight new advances concerning the contribution of the histone chaperone CAF-1 and the TSK-H3.1 DNA repair pathway in preventing genomic instability during replication. We then summarize the evidence connecting H3.1 to specific roles required for the mitotic inheritance of epigenetic states. Finally, we discuss the recent identification of a specific interaction between H3.1 and DNA polymerase epsilon and its functional implications.

Histone exchange sensors reveal variant specific dynamics in mouse embryonic stem cells

Eviction of histones from nucleosomes and their exchange with newly synthesized or alternative variants is a central epigenetic determinant. Here, we define the genome-wide occupancy and exchange pattern of canonical and non-canonical histone variants in mouse embryonic stem cells by genetically encoded exchange sensors. While exchange of all measured variants scales with transcription, we describe variant-specific associations with transcription elongation and Polycomb binding. We found considerable exchange of H3.1 and H2B variants in heterochromatin and repeat elements, contrasting the occupancy and little exchange of H3.3 in these regions. This unexpected association between H3.3 occupancy and exchange of canonical variants is also evident in active promoters and enhancers, and further validated by reduced H3.1 dynamics following depletion of H3.3-specific chaperone, HIRA. Finally, analyzing transgenic mice harboring H3.1 or H3.3 sensors demonstrates the vast potential of this system for studying histone exchange and its impact on gene expression regulation in vivo.

Dynamics of nuclear architecture during early embryonic development and lessons from liveimaging

Nuclear organization has emerged as a potential key regulator of genome function. During development, the deployment of transcriptional programs must be tightly coordinated with cell division and is often accompanied by major changes in the repertoire of expressed genes. These transcriptional and developmental events are paralleled by changes in the chromatin landscape. Numerous studies have revealed the dynamics of nuclear organization underlying them. In addition, advances in live-imaging-based methodologies enable the study of nuclear organization with high spatial and temporal resolution. In this Review, we summarize the current knowledge of the changes in nuclear architecture in the early embryogenesis of various model systems. Furthermore, to highlight the importance of integrating fixed-cell and live approaches, we discuss how different live-imaging techniques can be applied to examine nuclear processes and their contribution to our understanding of transcription and chromatin dynamics in early development. Finally, we provide future avenues for outstanding questions in this field.

Mapping histone variant genomic distribution: Exploiting SNAP-tag labeling to follow the dynamics of incorporation of H3 variants

In the eukaryotic cell nucleus, in addition to the genomic information, chromatin organization provides an additional set of information which is more versatile and associates with distinct cell identities. In particular, the marking of the nucleosomes by a choice of specific histone variants can potentially confer distinct functional properties critical for genome function and stability. To understand how this unique marking operates we need to access to the genomic distribution of each variant. A general approach based on ChIP-Seq, relies on the specific isolation of DNA bound to the variant of interest, usually using cross-linked material and specific antibodies. The availability of reliable specific antibodies recognizing with high affinity crosslinked antigen represents a limitation. Here, we describe an experimental approach exploiting a tag fused to the protein of interest. The chose protein is a histone variant and we use native conditions for the selective capture of the histone variant in a nucleosome. Most importantly, we describe how to use a particular labeling system, with a SNAP tag enabling to specifically capture nucleosomes comprising newly synthesized histones. This method allows to follow the newly deposited histone variant at various times thereby offering a unique opportunity to evaluate the dynamics of histone deposition genome wide. We describe the method here for H3 variant, but it can be adapted to any histone variant with the appropriate fused tag to address genome wide a turn-over associated to the biological context of interest.

Histones and their chaperones: Adaptive remodelers of an ever-changing chromatinic landscape

Chromatin maintenance and remodeling are processes that take place alongside DNA repair, replication, or transcription to ensure the survival and adaptability of a cell. The environment and the needs of the cell dictate how chromatin is remodeled; particularly where and which histones are deposited, thus changing the canonical histone array to regulate chromatin structure and gene expression. Chromatin is highly dynamic, and histone variants and their chaperones play a crucial role in maintaining the epigenetic regulation at different genomic regions. Despite the large number of histone variants reported to date, studies on their roles in physiological processes and pathologies are emerging but continue to be scarce. Here, we present recent advances in the research on histone variants and their chaperones, with a focus on their importance in molecular mechanisms such as replication, transcription, and DNA damage repair. Additionally, we discuss the emerging role they have in transposable element regulation, aging, and chromatin remodeling syndromes. Finally, we describe currently used methods and their limitations in the study of these proteins and highlight the importance of improving the experimental approaches to further understand this epigenetic machinery.

Transcription-coupled H3.3 recycling: A link with chromatin states

Histone variant H3.3 is incorporated into chromatin throughout the cell cycle and even in non-cycling cells. This histone variant marks actively transcribed chromatin regions with high nucleosome turnover, as well as silent pericentric and telomeric repetitive regions. In the past few years, significant progress has been made in our understanding of mechanisms involved in the transcription-coupled deposition of H3.3. Here we review how, during transcription, new H3.3 deposition intermingles with the fate of the old H3.3 variant and its recycling. First, we describe pathways enabling the incorporation of newly synthesized vs old H3.3 histones in the context of transcription. We then review the current knowledge concerning differences between these two H3.3 populations, focusing on their PTMs composition. Finally, we discuss the implications of H3.3 recycling for the maintenance of the transcriptional state and underline the emerging importance of H3.3 as a potent epigenetic regulator for both maintaining and switching a transcriptional state.