Structural insights into histone binding and nucleosome assembly by chromatin assembly factor-1

Structural insights into the interaction of Hir2 and Hpc2 in the yeast Hir histone chaperone complex

The HIRA complex, composed of HIRA, UBN1, and CABIN1 in humans, plays a central role in histone chaperone activity and chromatin regulation by depositing the H3.3 histone variant into nucleosomes. Proper subunit interactions are critical for complex stability and function. In this study, we examine the interaction between Hir2 and Hpc2, the yeast homologs of HIRA and UBN1, using biochemical and structural approaches. We show that the N-terminal to the Hpc2-related domain (NHRD) of Hpc2 binds to the WD40 domain of Hir2, consistent with the human HIRA-UBN1 interaction. The crystal structure of the Hir2_WD40-Hpc2_NHRD complex reveals a seven-bladed β-propeller fold in Hir2_WD40, with Hpc2_NHRD forming an antiparallel β sheet interface. Notably, a unique five-stranded blade in Hir2_WD40, stabilized by proline residue P228, is essential for Hpc2 binding. Mutational analysis confirms key interface residues, providing structural insights into the evolutionary conservation of the HIRA complex.

Always on the Move: Overview on Chromatin Dynamics within Nuclear Processes

Our genome is organized into chromatin, a dynamic and modular structure made of nucleosomes. Chromatin organization controls access to the DNA sequence, playing a fundamental role in cell identity and function. How nucleosomes enable these processes is an active area of study. In this review, we provide an overview of chromatin dynamics, its properties, mechanisms, and functions. We highlight the diverse ways by which chromatin dynamics is controlled during transcription, DNA replication, and repair. Recent technological developments have promoted discoveries in this area, to which we provide an outlook on future research directions.

Insights into the synchronization between DNA replication and parental histone recycling

Accurate parental histone recycling is of pivotal importance in epigenetic inheritance. Its proper functioning hinges on the precise co-ordination among a diverse array of proteins. During DNA replication, any aberration in the distribution of parental histones can potentially result in the loss of epigenetic memory. To date, although several key proteins involved in parental histone recycling have been identified, the detailed molecular mechanisms underlying their functions remain elusive. This mini-review focuses on summarizing the synchrony between DNA replication and parental histone recycling, along with the key participants in parental histone recycling. In the end, we provide an overview of the inherent connection between parental histone recycling and epigenetic inheritance, shedding light on the fundamental role of histone recycling in maintaining epigenetic information across cell divisions.

Structure and function of histone chaperones in replication-coupled chromatin assembly

Eukaryotic cell divisions pass on genetic and epigenetic information from parental to daughter cells through replication of the chromatin, which needs to be reestablished following DNA replication, as its building block, the nucleosome, is disrupted by the passage of the DNA replication fork. This replication-coupled (RC) nucleosome assembly process takes place in distinct pathways depending on whether newly synthesized or parental histones are used. This review highlights recent progress in structural and biochemical studies of RC nucleosome assembly, focusing on the roles of histone chaperones in both de novo assembly of nucleosomes from newly synthesized histones and the recycling of parental histones. We also discuss the interactions between histone chaperones and replisome components that govern the coupling of nucleosome assembly to chromatin replication. Finally, we offer our perspective on future efforts in advancing this important research direction.

Mechanisms of Inheritance of Chromatin States: From Yeast to Human

In this article I review mechanisms that underpin epigenetic inheritance of CpG methylation and histone H3 lysine 9 methylation (H3K9me) in chromatin in fungi and mammals. CpG methylation can be faithfully inherited epigenetically at some sites for a lifetime in vertebrates and, remarkably, can be propagated for millions of years in some fungal lineages. Transmission of methylation patterns requires maintenance-type DNA methyltransferases (DNMTs) that recognize hemimethylated CpG DNA produced by replication. DNMT1 is the maintenance enzyme in vertebrates; we recently identified DNMT5 as an ATP-dependent CpG maintenance enzyme found in fungi and protists. In vivo, CpG methylation is coupled to H3K9me. H3K9me is itself reestablished after replication via local histone H3-H4 tetramer recycling involving mobile and nonmobile chaperones, de novo nucleosome assembly, and read-write mechanisms that modify naive nucleosomes. Additional proteins recognize hemimethylated CpG or fully methylated CpG-containing motifs and enhance restoration of methylation by recruiting and/or activating the maintenance methylase.

Histone tetrasome dynamics affects chromatin transcription

During various DNA-centered processes in the cell nucleus, the minimal structural units of chromatin organization, nucleosomes, are often transiently converted to hexasomes and tetrasomes missing one or both H2A/H2B histone dimers, respectively. However, the structural and functional properties of the subnucleosomes and their impact on biological processes in the nuclei are poorly understood. Here, using biochemical approaches, molecular dynamics simulations, single-particle Förster resonance energy transfer microscopy, and nuclear magnetic resonance spectroscopy, we have shown that, surprisingly, removal of both dimers from a nucleosome results in much higher mobility of both histones and DNA in the tetrasome. Accordingly, DNase I footprinting shows that DNA-histone interactions in tetrasomes are greatly compromised, resulting in formation of a much lower barrier to transcribing RNA polymerase II than nucleosomes. The data suggest that tetrasomes are remarkably dynamic structures and their formation can strongly affect various biological processes.

Beyond the mono-nucleosome

Nucleosomes, the building block of chromatin, are responsible for regulating access to the DNA sequence. This control is critical for essential cellular processes, including transcription and DNA replication and repair. Studying chromatin can be challenging both in vitro and in vivo, leading many to use a mono-nucleosome system to answer fundamental questions relating to chromatin regulators and binding partners. However, the mono-nucleosome fails to capture essential features of the chromatin structure, such as higher-order chromatin folding, local nucleosome-nucleosome interactions, and linker DNA trajectory and flexibility. We briefly review significant discoveries enabled by the mono-nucleosome and emphasize the need to go beyond this model system in vitro. Di-, tri-, and tetra-nucleosome arrays can answer important questions about chromatin folding, function, and dynamics. These multi-nucleosome arrays have highlighted the effects of varying linker DNA lengths, binding partners, and histone post-translational modifications in a more chromatin-like environment. We identify various chromatin regulatory mechanisms yet to be explored with multi-nucleosome arrays. Combined with in-solution biophysical techniques, studies of minimal multi-nucleosome chromatin models are feasible.

The ortholog of human DNAJC9 promotes histone H3-H4 degradation and is counteracted by Asf1 in fission yeast

Mammalian J-domain protein DNAJC9 interacts with histones H3-H4 and is important for cell proliferation. However, its exact function remains unclear. Here, we show that, in the fission yeast Schizosaccharomyces pombe, loss of Djc9, the ortholog of DNAJC9, renders the histone chaperone Asf1 no longer essential for growth. Utilizing AlphaFold-based structural prediction, we identified a histone-binding surface on Djc9 that binds to helix α3 of H3 in a manner that precludes simultaneous helix α3-binding by Asf1. Djc9 and Asf1 indeed compete for binding to the H3-H4 dimer in vitro, and an H3-α3 mutation impeding Djc9 binding also renders Asf1 non-essential, indicating that the role of Asf1 needed for growth in fission yeast is to prevent histone binding by Djc9. In the absence of Asf1, cell growth is hindered due to unrestrained Djc9-mediated downregulation of H3 and H4. In the presence of Asf1, Djc9 confers resistance to the DNA replication inhibitor hydroxyurea and dominant negative disease-related histone mutants by promoting the degradation of superfluous or dysfunctional histones. Our findings provide new insights into the function and mechanism of this conserved histone-binding protein.

Placental trophoblast aging in advanced maternal age is related to increased oxidative damage and decreased YAP

Introduction

The advanced maternal age (AMA) pregnancies escalate rapidly, which are frequently linked to higher risks of adverse outcomes. Advanced maternal age (AMA) placenta exhibited premature aging, presumably resulting in trophoblast dysfunction, inadequate placentation. However, the precise reasons and mechanisms of trophoblast aging in AMA placenta remain unclear, posing a significant limitation to provide effective guidance for prenatal healthcare in clinical settings. Notably, the organism shows heightened vulnerability to oxidative damage as it ages. YAP (Yes-associated protein) was reported to play a critical role in regulation of aging and resisting oxidative damage, yet these roles had not been elucidated in the placenta. Therefore, this study explored the relationship between trophoblast cell aging and oxidative injury and YAP in AMA pregnancy, which not only provided an insight into the mechanisms of trophoblast cell aging, but also provide valuable directions for healthcare during AMA pregnancy.

Methods

In this study, human term placentas were collected from AMA and normal pregnancies for the analysis of aging, oxidative damage and YAP level. HTR8/SVneo cells were manipulated with (hydrogen peroxide) H2O2 to explore the effects of oxidative damage on trophoblast cell senescence and YAP levels. YAP expression in HTR8/SVneo cells was manipulated to investigate its role in trophoblastic senescence and oxidative damage.

Results

Compared with the control group, the AMA placenta exhibits increased aging biomarkers, which is coupled with an elevation in oxidative damage within placental trophoblast cells and a notable decline in YAP levels. Cellular experiments demonstrated that oxidative damage from H2O2 triggered trophoblast cell senescence and resulted in a reduction of YAP levels. Furthermore, employing molecular modification to silence YAP expression in these cells led to an induction of aging. Conversely, overexpressing YAP ameliorated both trophoblast cell aging and the associated DNA oxidative damage that arised from H2O2.

Conclusion

The decline of YAP in AMA pregnancy should be responsible for the increased oxidative injury and premature placenta aging, indicating that YAP plays a significant role in combating oxidative damage and delaying aging, thereby providing a new guidance for prenatal care in AMA pregnancies. Maintaining YAP levels or implementing anti-oxidative stress interventions could potentially mitigate the incidence of complications involved AMA pregnancy.

A tale of two strands: Decoding chromatin replication through strand-specific sequencing

DNA replication, a fundamental process in all living organisms, proceeds with continuous synthesis of the leading strand by DNA polymerase ε (Pol ε) and discontinuous synthesis of the lagging strand by polymerase δ (Pol δ). This inherent asymmetry at each replication fork necessitates the development of methods to distinguish between these two nascent strands in vivo. Over the past decade, strand-specific sequencing strategies, such as enrichment and sequencing of protein-associated nascent DNA (eSPAN) and Okazaki fragment sequencing (OK-seq), have become essential tools for studying chromatin replication in eukaryotic cells. In this review, we outline the foundational principles underlying these methodologies and summarize key mechanistic insights into DNA replication, parental histone transfer, epigenetic inheritance, and beyond, gained through their applications. Finally, we discuss the limitations and challenges of current techniques, highlighting the need for further technological innovations to better understand the dynamics and regulation of chromatin replication in eukaryotic cells.

H3K56 acetylation regulates chromatin maturation following DNA replication

Following DNA replication, the newly reassembled chromatin is disorganized and must mature to its steady state to maintain both genome and epigenome integrity. However, the regulatory mechanisms governing this critical process remain poorly understood. Here, we show that histone H3K56 acetylation (H3K56ac), a mark on newly-synthesized H3, facilitates the remodeling of disorganized nucleosomes in nascent chromatin, and its removal at the subsequent G2/M phase of the cell cycle marks the completion of chromatin maturation. In vitro, H3K56ac enhances the activity of ISWI chromatin remodelers, including yeast ISW1 and its human equivalent SNF2h. In vivo, a deficiency of H3K56ac in nascent chromatin results in the formation of closely packed di-nucleosomes and/or tetra-nucleosomes. In contrast, abnormally high H3K56ac levels disrupt chromatin maturation, leading to genome instability. These findings establish a central role of H3K56ac in chromatin maturation and reveal a mechanism regulating this critical aspect of chromosome replication.

Chromatin assembly factor subunit CHAF1A as a monogenic cause for oculo-auriculo-vertebral spectrum

Oculo-auriculo-vertebral spectrum (OAVS) is characterized by abnormal development of the 1st and 2nd branchial arches. Despite arguments against a monogenic condition, a few genes have been involved in a minority of cases. We now report heterozygous, presumably loss-of function variants in the CHAF1A gene in 8 individuals, including 3 members of the same family. Four cases fulfill stringent diagnostic criteria for OAVS, including asymmetric ear dysplasia, preauricular tags, mandibular asymmetry +/- vertebral malformations. Two patients also presented with kidney malformations. CHAF1A encodes a subunit of CAF-1 (chromatin assembly factor-1), a heterotrimeric protein complex responsible for the deposition of newly synthesized histones H3-H4 onto the newly synthetized DNA strand during replication. The identification of loss-of-unction variants in CHAF1A is consistent with the hypothesis of early developmental genes dysregulation driving OAVS and other associations recently lumped under the acronym Recurrent Constellations of Embryonic Malformations (RCEM).

Stepwise order in protein complex assembly: approaches and emerging themes

Protein-based nanomachines drive every cellular process. An explosion of high-resolution structures of multiprotein complexes has improved our understanding of what these machines look like and how they work, but we still know relatively little about how they assemble in living cells. For example, it has only recently been appreciated that many complexes assemble co-translationally, with at least one subunit still undergoing active translation while already interacting with other subunits. One aspect that is particularly understudied is assembly order, the idea that there is a stepwise order to the subunit-subunit associations that underlies the efficient assembly of the quaternary structure. Here, we integrate a review of the methodological approaches commonly used to query assembly order within a discussion of studies of the 20S proteasome core particle, septin protein complexes, and the histone octamer. We highlight shared and distinct properties of these complexes that illustrate general themes applicable to most other multisubunit assemblies.

Acute multi-level response to defective de novo chromatin assembly in S-phase

Long-term perturbation of de novo chromatin assembly during DNA replication has profound effects on epigenome maintenance and cell fate. The early mechanistic origin of these defects is unknown. Here, we combine acute degradation of chromatin assembly factor 1 (CAF-1), a key player in de novo chromatin assembly, with single-cell genomics, quantitative proteomics, and live microscopy to uncover these initiating mechanisms in human cells. CAF-1 loss immediately slows down DNA replication speed and renders nascent DNA hyper-accessible. A rapid cellular response, distinct from canonical DNA damage signaling, is triggered and lowers histone mRNAs. In turn, histone variants' usage and their modifications are altered, limiting transcriptional fidelity and delaying chromatin maturation within a single S-phase. This multi-level response induces a p53-dependent cell-cycle arrest after mitosis. Our work reveals the immediate consequences of defective de novo chromatin assembly during DNA replication, indicating how at later times the epigenome and cell fate can be altered.

Replication-coupled inheritance of chromatin states

During the development of eukaryote, faithful inheritance of chromatin states is central to the maintenance of cell fate. DNA replication poses a significant challenge for chromatin state inheritance because every nucleosome in the genome is disrupted as the replication fork passes. It has been found that many factors including DNA polymerases, histone chaperones, as well as, RNA Pol II and histone modifying enzymes coordinate spatially and temporally to maintain the epigenome during this progress. In this review, we provide a summary of the detailed mechanisms of replication-coupled nucleosome assembly and post-replication chromatin maturation, highlight the inheritance of chromatin states and epigenome during these processes, and discuss the future directions and challenges in this field.

The crosstalk between alternative splicing and circular RNA in cancer: pathogenic insights and therapeutic implications

RNA splicing is a fundamental step of gene expression. While constitutive splicing removes introns and joins exons unbiasedly, alternative splicing (AS) selectively determines the assembly of exons and introns to generate RNA variants corresponding to the same transcript. The biogenesis of circular RNAs (circRNAs) is inextricably associated with AS. Back-splicing, the biogenic process of circRNA, is a special form of AS. In cancer, both AS and circRNA deviate from the original track. In the present review, we delve into the intricate interplay between AS and circRNAs in the context of cancer. The relationship between AS and circRNAs is intricate, where AS modulates the biogenesis of circRNAs and circRNAs in return regulate AS events. Beyond that, epigenetic and posttranscriptional modifications concurrently regulate AS and circRNAs. On the basis of this modality, we summarize current knowledge on how splicing factors and other RNA binding proteins regulate circRNA biogenesis, and how circRNAs interact with splicing factors to influence AS events. Specifically, the feedback loop regulation between circRNAs and AS events contributes greatly to oncogenesis and cancer progression. In summary, resolving the crosstalk between AS and circRNA will not only provide better insight into cancer biology but also provoke novel strategies to combat cancer.

Molecular mechanism of parental H3/H4 recycling at a replication fork

In chromatin replication, faithful recycling of histones from parental DNA to replicated strands is essential for maintaining epigenetic information across generations. A previous experiment has revealed that disrupting interactions between the N-terminal tail of Mcm2, a subunit in DNA replication machinery, and a histone H3/H4 tetramer perturb the recycling. However, the molecular pathways and the factors that regulate the ratio recycled to each strand and the destination location are yet to be revealed. Here, we performed molecular dynamics simulations of yeast DNA replication machinery, an H3/H4 tetramer, and replicated DNA strands. The simulations demonstrated that histones are recycled via Cdc45-mediated and unmediated pathways without histone chaperones, as our in vitro biochemical assays supported. Also, RPA binding regulated the ratio recycled to each strand, whereas DNA bending by Pol ε modulated the destination location. Together, the simulations provided testable hypotheses, which are vital for elucidating the molecular mechanisms of histone recycling.

PHF8/KDM7B: A Versatile Histone Demethylase and Epigenetic Modifier in Nervous System Disease and Cancers

Many human diseases, such as malignant tumors and neurological diseases, have a complex pathophysiological etiology, often accompanied by aberrant epigenetic changes including various histone modifications. Plant homologous domain finger protein 8 (PHF8), also known as lysine-specific demethylase 7B (KDM7B), is a critical histone lysine demethylase (KDM) playing an important role in epigenetic modification. Characterized by the zinc finger plant homology domain (PHD) and the Jumonji C (JmjC) domain, PHF8 preferentially binds to H3K4me3 and erases repressive methyl marks, including H3K9me1/2, H3K27me1, and H4K20me1. PHF8 is indispensable for developmental processes and the loss of PHF8 enzyme activity is linked to neurodevelopmental disorders. Moreover, increasing evidence shows that PHF8 is highly expressed in multiple tumors as an oncogenic factor. These findings indicate that studying the role of PHF8 will facilitate the development of novel therapeutic agents by the manipulation of PHF8 demethylation activity. Herein, we summarize the current knowledge of PHF8 about its structure and demethylation activity and its involvement in development and human diseases, with an emphasis on nervous system disorders and cancer. This review will update our understanding of PHF8 and promote the clinical transformation of its predictive and therapeutic value.

HIRA complex deposition of histone H3.3 is driven by histone tetramerization and histone-DNA binding

The HIRA histone chaperone complex is comprised of four protein subunits: HIRA, UBN1, CABIN1, and transiently associated ASF1a. All four subunits have been demonstrated to play a role in the deposition of the histone variant H3.3 onto areas of actively transcribed euchromatin in cells. The mechanism by which these subunits function together to drive histone deposition has remained poorly understood. Here we present biochemical and biophysical data supporting a model whereby ASF1a delivers histone H3.3/H4 dimers to the HIRA complex, H3.3/H4 tetramerization drives the association of two HIRA/UBN1 complexes, and the affinity of the histones for DNA drives release of ASF1a and subsequent histone deposition. These findings have implications for understanding how other histone chaperone complexes may mediate histone deposition.

Interplay between histone variants and chaperones in plants

Histone chaperones and histone variants play crucial roles in DNA replication, gene transcription, and DNA repair in eukaryotes. Histone chaperones reversibly promote nucleosome assembly and disassembly by incorporating or evicting histones and histone variants to modulate chromatin accessibility, thereby altering the chromatin states and modulating DNA-related biological processes. Cofactors assist histone chaperones to target specific chromatin regions to regulate the exchange of histones and histone variants. In this review, we summarize recent progress in the interplay between histone variants and chaperones in plants. We discuss the structural basis of chaperone-histone complexes and the mechanisms of their cooperation in regulating gene transcription and plant development.

Structure of the Hir histone chaperone complex

The evolutionarily conserved HIRA/Hir histone chaperone complex and ASF1a/Asf1 co-chaperone cooperate to deposit histone (H3/H4)2 tetramers on DNA for replication-independent chromatin assembly. The molecular architecture of the HIRA/Hir complex and its mode of histone deposition have remained unknown. Here, we report the cryo-EM structure of the S. cerevisiae Hir complex with Asf1/H3/H4 at 2.9-6.8 Å resolution. We find that the Hir complex forms an arc-shaped dimer with a Hir1/Hir2/Hir3/Hpc2 stoichiometry of 2/4/2/4. The core of the complex containing two Hir1/Hir2/Hir2 trimers and N-terminal segments of Hir3 forms a central cavity containing two copies of Hpc2, with one engaged by Asf1/H3/H4, in a suitable position to accommodate a histone (H3/H4)2 tetramer, while the C-terminal segments of Hir3 harbor nucleic acid binding activity to wrap DNA around the Hpc2-assisted histone tetramer. The structure suggests a model for how the Hir/Asf1 complex promotes the formation of histone tetramers for their subsequent deposition onto DNA.

Histone Tetrasome Dynamics Affects Chromatin Transcription

During various DNA-centered processes in the cell nucleus, the minimal structural units of chromatin organization, nucleosomes, are often transiently converted to hexasomes and tetrasomes missing one or both H2A/H2B histone dimers, respectively. However, the structural and functional properties of the subnucleosomes and their impact on biological processes in the nuclei are poorly understood. Here, using biochemical approaches, molecular dynamics simulations, single-particle Förster resonance energy transfer (spFRET) microscopy and NMR spectroscopy, we have shown that, surprisingly, removal of both dimers from a nucleosome results in much higher mobility of both histones and DNA in the tetrasome. Accordingly, DNase I footprinting shows that DNA-histone interactions in tetrasomes are greatly compromised, resulting in formation of a much lower barrier to transcribing RNA polymerase II than nucleosomes. The data suggest that tetrasomes are remarkably dynamic structures and their formation can strongly affect various biological processes.

The PCNA-Pol δ complex couples lagging strand DNA synthesis to parental histone transfer for epigenetic inheritance

Inheritance of epigenetic information is critical for maintaining cell identity. The transfer of parental histone H3-H4 tetramers, the primary carrier of epigenetic modifications on histone proteins, represents a crucial yet poorly understood step in the inheritance of epigenetic information. Here, we show the lagging strand DNA polymerase, Pol δ, interacts directly with H3-H4 and that the interaction between Pol δ and the sliding clamp PCNA regulates parental histone transfer to lagging strands, most likely independent of their roles in DNA synthesis. When combined, mutations at Pol δ and Mcm2 that compromise parental histone transfer result in a greater reduction in nucleosome occupancy at nascent chromatin than mutations in either alone. Last, PCNA contributes to nucleosome positioning on nascent chromatin. On the basis of these results, we suggest that the PCNA-Pol δ complex couples lagging strand DNA synthesis to parental H3-H4 transfer, facilitating epigenetic inheritance.

Nucleosome Dynamics Derived at the Single-Molecule Level Bridges Its Structures and Functions

Nucleosome, the building block of chromatin, plays pivotal roles in all DNA-related processes. While cryogenic-electron microscopy (cryo-EM) has significantly advanced our understanding of nucleosome structures, the emerging field of single-molecule force spectroscopy is illuminating their dynamic properties. This technique is crucial for revealing how nucleosome behavior is influenced by chaperones, remodelers, histone variants, and post-translational modifications, particularly in their folding and unfolding mechanisms under tension. Such insights are vital for deciphering the complex interplay in nucleosome assembly and structural regulation, highlighting the nucleosome's versatility in response to DNA activities. In this Perspective, we aim to consolidate the latest advancements in nucleosome dynamics, with a special focus on the revelations brought forth by single-molecule manipulation. Our objective is to highlight the insights gained from studying nucleosome dynamics through this innovative approach, emphasizing the transformative impact of single-molecule manipulation techniques in the field of chromatin research.