CTCF organizes inter-A compartment interactions through RYBP-dependent phase separation

Deciphering the role of RNA in regulating CTCF's DNA binding affinity in leukemia cells

BACKGROUND

CTCF, a highly studied transcription factor, is essential for chromatin interaction maintenance. Several independent studies report that CTCF interacts with RNAs in vitro and in cells. Yet continuous debates about the authenticity of the RNA-binding affinity of CTCF and its biological role remain in large part due to limited research techniques available, such as CLIP-seq.

RESULT

Here, we investigate RNA's role in CTCF's transcription factor function through its chromatin occupancy. To systematically explore whether RNAs affect CTCF's ability to bind DNA, we perturb CTCF-RNA interactions by three independent approaches and examine CTCF genome occupancy by ChIP-seq. Although RNase A and triptolide treatment each affect a certain number of CTCF-binding peaks, few peaks overlap between treatment groups indicating the effect of RNA in regulating CTCF's DNA binding affinity is variable between loci. In addition, limited transcriptional or chromatin accessibility changes occur between cells expressing wild-type CTCF or CTCF lacking the RNA binding region.

CONCLUSION

Our data provide a complementary approach and in silico evidence to consider the significance of RNA affecting CTCF's DNA binding affinity globally.

Chrombus-XMBD: a graph convolution model predicting 3D-genome from chromatin features

The 3D conformation of the chromatin is crucial for transcriptional regulation. However, current experimental techniques for detecting the 3D structure of the genome are costly and limited to the biological conditions. Here, we described "ChrombusXMBD," a graph convolution model capable of predicting chromatin interactions ab initio based on available chromatin features. Using dynamic edge convolution with multihead attention mechanism, Chrombus encodes the 2D-chromatin features into a learnable embedding space, thereby generating a genome-wide 3D-contactmap. In validation, Chrombus effectively recapitulated the topological associated domains, expression quantitative trait loci, and promoter/enhancer interactions. Especially, Chrombus outperforms existing algorithms in predicting chromatin interactions over 1-2 Mb, increasing prediction correlation by 11.8%-48.7%, and predicts long-range interactions over 2 Mb (Pearson's coefficient 0.243-0.582). Chrombus also exhibits strong generalizability across human and mouse-derived cell lines. Additionally, the parameters of Chrombus inform the biological mechanisms underlying cistrome. Our model provides a new, generalizable analytical tool for understanding the complex dynamics of chromatin interactions and the landscape of cis-regulation of gene expression.

YTHDC1 phase separation drives the nuclear export of m6A-modified lncNONMMUT062668.2 through the transport complex SRSF3-ALYREF-XPO5 to aggravate pulmonary fibrosis

Fibroblast-to-myofibroblast differentiation is the main cytopathologic characteristic of pulmonary fibrosis. However, its underlying molecular mechanism remains poorly understood. This study elucidated that the nuclear export of lncNONMMUT062668.2 (lnc668) exacerbated pulmonary fibrosis by activating fibroblast-to-myofibroblast differentiation. Mechanistic research revealed that histone H3K9 lactylation in the promoter region of the N6-methyladenosine (m6A) writer METTL3 was enriched to enhance METTL3 transcription, leading to the lnc668 m6A modification. Meanwhile, the m6A reader YTHDC1 recognized m6A-modified lnc668 and elevated the METTL3-mediated lnc668 modification. Subsequently, phase-separating YTHDC1 promoted the nuclear export of m6A-modified lnc668. In this process, the phase-separating YTHDC1 formed a nuclear pore complex with serine/arginine-rich splicing factor 3, Aly/REF export factor, and exportin-5 to assist the translocation of m6A-modified lnc668 from nucleus to cytoplasm. After nuclear export, lnc668 facilitated the translation and stability of its host gene phosphatidylinositol-binding clathrin assembly protein to activate fibroblast-to-myofibroblast differentiation, leading to the aggravation of pulmonary fibrosis, which also depended on YTHDC1 phase separation. This study first clarified that YTHDC1 phase separation is crucial for the m6A modification, nuclear export, and profibrotic role of lnc668 in exacerbating pulmonary fibrosis. These findings provide new insights into the nuclear export of cytoplasmic lncRNAs and identified potential targets for pulmonary fibrosis therapy.

Transcription regulation by biomolecular condensates

Biomolecular condensates regulate transcription by dynamically compartmentalizing the transcription machinery. Classic models of transcription regulation focus on the recruitment and regulation of RNA polymerase II by the formation of complexes at the 1-10 nm length scale, which are driven by structured and stoichiometric interactions. These complexes are further organized into condensates at the 100-1,000 nm length scale, which are driven by dynamic multivalent interactions often involving domain-ligand pairs or intrinsically disordered regions. Regulation through condensate-mediated organization does not supersede the processes occurring at the 1-10 nm scale, but it provides regulatory mechanisms for promoting or preventing these processes in the crowded nuclear environment. Regulation of transcription by transcriptional condensates is involved in cell state transitions during animal and plant development, cell signalling and cellular responses to the environment. These condensate-mediated processes are dysregulated in developmental disorders, cancer and neurodegeneration. In this Review, we discuss the principles underlying the regulation of transcriptional condensates, their roles in physiology and their dysregulation in human diseases.

Nuclear remodeling during cell fate transitions

Totipotent stem cells, the earliest cells in embryonic development, can differentiate into complete embryos and extra-embryonic tissues, making them essential for understanding both development and regenerative medicine. This review examines recent advances in the dynamic remodeling of nuclear structures during the transition between totipotency and pluripotency, as well as other cell fate transition processes. Additionally, we highlight innovative experimental and computational methods that elucidate the relationship between nuclear architecture and cell fate decisions. By integrating these insights, we aim to enhance our understanding of how nuclear remodeling influences totipotency and other cell fate transitions, paving the way for future research in this critical field.

Rbfox3 Promotes Transformation of MDSC-Like Tumor Cells to Shape Immunosuppressive Microenvironment

Myeloid-derived suppressor cells (MDSCs) within the tumor microenvironment (TME) contribute to the malignant progression of tumors by exerting immunosuppressive effects. Bacterial lipopolysaccharides (LPS) have been widely demonstrated in various types of solid tumors. LPS can promote the malignant progression of tumors, which mechanism has not yet been fully elucidated. In this study, a type of MDSC-like tumor cells (MLTCs) is found in tumor tissues induced by low-dose and long-term LPS stimulation. MLTCs can simultaneously express tumor cell and MDSCs markers. Similar to MDSCs, MLTCs can produce arginine, nitric oxide, and reactive oxygen species and inhibit the activity of NK and T cells to promote the formation of an immunosuppressive microenvironment. MLTCs can also promote tumor cell proliferation and vasculogenic mimicry formation. CRISPR-Cas9 activity screening studies identified RNA-binding Fox-1 homolog 3 (Rbfox3) as a critical protein for MLTCs formation after LPS treatment. Rbfox3 can transcriptionally regulate the expression of Ass1 in the form of phase-separated particles. Crocin can inhibit the generation of MLTCs by disrupting phase-separated particles of Rbfox3 and enhance the anti-tumor effects of immune checkpoint inhibitors (ICIs).

SiCLAT: simultaneous imaging of chromatin loops and active transcription in living cells

We present SiCLAT, which introduces a dCas9-dCas13d cassette into the mouse genome. This model enables the stable expression of both dCas9 and dCas13 proteins in diverse cell populations, facilitating concurrent labeling of DNA and RNA across various cell types. Using SiCLAT, we accurately labeled chromatin loop anchor interactions and associated gene transcription during myogenic differentiation. This imaging system offers a novel means of directly observing cis-element interactions and the corresponding gene transcription in living primary cells, thus providing real-time imaging for comprehensive mechanistic investigations of dynamic enhancer-promoter or enhancer-enhancer interactions in regulating transcription activation within living cells.

Phase-separated chromatin compartments: Orchestrating gene expression through condensation

Eukaryotic genomes are organized into distinct chromatin compartments, some of which exhibit properties of biomolecular condensates. These condensates primarily form due to chromatin-associated proteins/complexes (CAPs). CAPs play a crucial role in gene expression, functioning as either transcriptional repressors or activators. Phase separation, a well-established biophysical phenomenon, is a key driver of chromatin condensate formation by CAPs. Notably, multivalent CAPs with the ability to engage in diverse interactions promote chromatin compaction, leading to the formation of transcriptionally repressed compartments. Conversely, interactions between intrinsically disordered region (IDR)-containing transcriptional regulators, mediated by their multivalent IDRs, lead to the formation of protein-rich, transcriptionally active droplets on decondensed genomic regions. Interestingly, both repressive heterochromatin and activating euchromatin condensates exhibit spontaneous phase separation and selectively enrich components with concordant transcriptional functions. This review delves into the mechanisms by which transcriptionally repressive CAPs orchestrate the formation of repressed chromatin domains. We further explore how a diverse array of transcription-related CAPs or core histone variants, via phase separation, influence gene expression by inducing erroneous transcription events, regulating expression levels, and facilitating the interconversion of transcriptionally repressed and active regions.

Long-range transcription factor binding sites clustered regions may mediate transcriptional regulation through phase-separation interactions in early human embryo

In mammals, during the post-fertilization pre-implantation phase, the expression of cell type-specific genes is crucial for normal embryonic development, which is regulated by cis-regulatory elements (CREs). TFs control gene expression by interacting with CREs. Research shows that transcription factor binding sites (TFBSs) reflect the general characteristics of the regulatory genome. Here, we identified TFBSs from chromatin accessibility data in five stages of early human embryonic development, and quantified transcription factor binding sites-clustered regions (TFCRs) and their complexity (TC). Assigning TC values to TFCRs has made it possible to assess the functionality of these regulatory elements in a more quantitative way. Our findings reveal a robust correlation between TFCR complexity and gene expression starting from the 8Cell stage, which is when the zygotic genome is activated in humans. Furthermore, we have defined long-range TFCRs (LR-TFCRs) and conjecture that LR-TFCRs may regulate gene expression through phase-separation mechanisms during the early stages of human embryonic development.

Polycomb protein RYBP facilitates super-enhancer activity

BACKGROUND

Polycomb proteins are conventionally known as global repressors in cell fate determination. However, recent observations have shown their involvement in transcriptional activation, the mechanisms of which need further investigation.

METHODS

Herein, multiple data from ChIP-seq, RNA-seq and HiChIP before or after RYBP depletion in embryonic stem cell (ESC), epidermal progenitor (EPC) and mesodermal cell (MEC) were analyzed.

RESULTS

We found that Polycomb protein RYBP occupies super-enhancer (SE) in ESCs, where core Polycomb group (PcG) components such as RING1B and EZH2 are minimally enriched. Depletion of RYBP results in impaired deposition of H3K27ac, decreased expression of SE-associated genes, and reducing the transcription of enhancer RNA at SE regions (seRNA). Regarding the mechanism of seRNA transcription, the Trithorax group (TrxG) component WDR5 co-localizes with RYBP at SEs, and is required for seRNA expression. RYBP depletion reduces WDR5 deposition at SE regions. In addition, TrxG-associated H3K4me3 tends to be enriched at SEs with high levels of seRNA transcription, and RYBP deficiency impairs the deposition of H3K4me3 at SEs. Structurally, RYBP is involved in both intra- and inter-SE interactions. Finally, RYBP generally localizes at SEs in both in vitro cell lines and in vivo tissue-derived cells, dysfunction of RYBP is associated with various cancers and developmental diseases.

CONCLUSION

RYBP cooperates with TrxG component to regulate SE activity. Dysfunction of RYBP relates to various diseases. The findings provide new insights into the transcriptionally active function of Polycomb protein in cell fate determination.

Liquid-liquid Phase Separation in Aging: Novel Insights in the Pathogenesis and Therapeutics

The intricate organization of distinct cellular compartments is paramount for the maintenance of normal biological functions and the orchestration of complex biochemical reactions. These compartments, whether membrane-bound organelles or membraneless structures like Cajal bodies and RNA transport granules, play crucial roles in cellular function. Liquid-liquid phase separation (LLPS) serves as a reversible process that elucidates the genesis of membranelles structures through the self-assembly of biomolecules. LLPS has been implicated in a myriad of physiological and pathological processes, encompassing immune response and tumor genesis. But the association between LLPS and aging has not been clearly clarified. A recent advancement in the realm of aging research involves the introduction of a new edition outlining the twelve hallmarks of aging, categorized into three distinct groups. By delving into the role and mechanism of LLPS in the formation of membraneless structures at a molecular level, this review encapsulates an exploration of the interaction between LLPS and these aging hallmarks, aiming to offer novel perspectives of the intricate mechanisms underlying the aging process and deeper insights into aging therapeutics.

Multifaceted role of CTCF in X-chromosome inactivation

Therian female mammals compensate for the dosage of X-linked gene expression by inactivating one of the X-chromosomes. X-inactivation is facilitated by the master regulator Xist long non-coding RNA, which coats the inactive-X and facilitates heterochromatinization through recruiting different chromatin modifiers and changing the X-chromosome 3D conformation. However, many mechanistic aspects behind the X-inactivation process remain poorly understood. Among the many contributing players, CTCF has emerged as one of the key players in orchestrating various aspects related to X-chromosome inactivation by interacting with several other protein and RNA partners. In general, CTCF is a well-known architectural protein, which plays an important role in chromatin organization and transcriptional regulation. Here, we provide significant insight into the role of CTCF in orchestrating X-chromosome inactivation and highlight future perspectives.

Bioinformatic approaches of liquid-liquid phase separation in human disease

ABSTRACT

Biomolecular aggregation within cellular environments via liquid-liquid phase separation (LLPS) spontaneously forms droplet-like structures, which play pivotal roles in diverse biological processes. These structures are closely associated with a range of diseases, including neurodegenerative disorders, cancer and infectious diseases, highlighting the significance of understanding LLPS mechanisms for elucidating disease pathogenesis, and exploring potential therapeutic interventions. In this review, we delineate recent advancements in LLPS research, emphasizing its pathological relevance, therapeutic considerations, and the pivotal role of bioinformatic tools and databases in facilitating LLPS investigations. Additionally, we undertook a comprehensive analysis of bioinformatic resources dedicated to LLPS research in order to elucidate their functionality and applicability. By providing comprehensive insights into current LLPS-related bioinformatics resources, this review highlights its implications for human health and disease.

Decoding the genomic landscape of chromatin-associated biomolecular condensates

Biomolecular condensates play a significant role in chromatin activities, primarily by concentrating and compartmentalizing proteins and/or nucleic acids. However, their genomic landscapes and compositions remain largely unexplored due to a lack of dedicated computational tools for systematic identification in vivo. To address this, we develop CondSigDetector, a computational framework designed to detect condensate-like chromatin-associated protein co-occupancy signatures (CondSigs), to predict genomic loci and component proteins of distinct chromatin-associated biomolecular condensates. Applying this framework to mouse embryonic stem cells (mESC) and human K562 cells enable us to depict the high-resolution genomic landscape of chromatin-associated biomolecular condensates, and uncover both known and potentially unknown biomolecular condensates. Multi-omics analysis and experimental validation further verify the condensation properties of CondSigs. Additionally, our investigation sheds light on the impact of chromatin-associated biomolecular condensates on chromatin activities. Collectively, CondSigDetector provides an approach to decode the genomic landscape of chromatin-associated condensates, facilitating a deeper understanding of their biological functions and underlying mechanisms in cells.

Dynamic Changes in Histone Modifications Are Associated with Differential Chromatin Interactions

Eukaryotic genomes are organized into chromatin domains through long-range chromatin interactions which are mediated by the binding of architectural proteins, such as CTCF and cohesin, and histone modifications. Based on the published Hi-C and ChIP-seq datasets in human monocyte-derived macrophages, we identified 206 and 127 differential chromatin interactions (DCIs) that were not located within transcription readthrough regions in influenza A virus- and interferon β-treated cells, respectively, and found that the binding positions of CTCF and RAD21 within more than half of the DCI sites did not change. However, five histone modifications, H3K4me3, H3K27ac, H3K36me3, H3K9me3, and H3K27me3, showed significantly more dramatic changes than CTCF and RAD21 within the DCI sites. For H3K4me3, H3K27ac, H3K36me3, and H3K27me3, significantly more dramatic changes were observed outside than within the DCI sites. We further applied a motif scanning approach to discover proteins that might correlate with changes in histone modifications and chromatin interactions and found that PRDM9, ZNF384, and STAT2 frequently bound to DNA sequences corresponding to 1 kb genomic intervals with gains or losses of a histone modification within the DCI sites. This study explores the dynamic regulation of chromatin interactions and extends the current knowledge of the relationship between histone modifications and chromatin interactions.

Connecting Chromatin Structures to Gene Regulation Using Dynamic Polymer Simulations

The transfer of regulatory information between distal loci on chromatin is thought to involve physical proximity, but key biophysical features of these contacts remain unclear. For instance, it is unknown how close and for how long two loci need to be in order to productively interact. The main challenge is that it is currently impossible to measure chromatin dynamics with high spatiotemporal resolution at scale. Polymer simulations provide an accessible and rigorous way to test biophysical models of chromatin regulation, yet there is a lack of simple and general methods for extracting the values of model parameters. Here we adapt the Nelder-Mead simplex optimization algorithm to select the best polymer model matching a given Hi-C dataset, using the MYC locus as an example. The model's biophysical parameters predict a compartmental rearrangement of the MYC locus in leukemia, which we validate with single-cell measurements. Leveraging trajectories predicted by the model, we find that loci with similar Hi-C contact frequencies can exhibit widely different contact dynamics. Interestingly, the frequency of productive interactions between loci exhibits a non-linear relationship with their Hi-C contact frequency when we enforce a specific capture radius and contact duration. These observations are consistent with recent experimental observations and suggest that the dynamic ensemble of chromatin configurations, rather than average contact matrices, is required to fully predict productive long-range chromatin interactions.

CBX7C⋅PHC2 interaction facilitates PRC1 assembly and modulates its phase separation properties

CBX7 is a key component of PRC1 complex. Cbx7C is an uncharacterized Cbx7 splicing isoform specifically expressed in mouse embryonic stem cells (mESCs). We demonstrate that CBX7C functions as an epigenetic repressor at the classic PRC1 targets in mESCs, and its preferential interaction to PHC2 facilitates PRC1 assembly. Both Cbx7C and Phc2 are significantly upregulated during cell differentiation, and knockdown of Cbx7C abolishes the differentiation of mESCs to embryoid bodies. Interestingly, CBX7C⋅PHC2 interaction at low levels efficiently undergoes the formation of functional Polycomb bodies with high mobility, whereas the coordination of the two factors at high doses results in the formation of large, low-mobility, chromatin-free aggregates. Overall, these findings uncover the unique roles and molecular basis of the CBX7C⋅PHC2 interaction in PRC1 assembly on chromatin and Pc body formation and open a new avenue of controlling PRC1 activities via modulation of its phase separation properties.

The PTM profiling of CTCF reveals the regulation of 3D chromatin structure by O-GlcNAcylation

CCCTC-binding factor (CTCF), a ubiquitously expressed and highly conserved protein, is known to play a critical role in chromatin structure. Post-translational modifications (PTMs) diversify the functions of protein to regulate numerous cellular processes. However, the effects of PTMs on the genome-wide binding of CTCF and the organization of three-dimensional (3D) chromatin structure have not been fully understood. In this study, we uncovered the PTM profiling of CTCF and demonstrated that CTCF can be O-GlcNAcylated and arginine methylated. Functionally, we demonstrated that O-GlcNAcylation inhibits CTCF binding to chromatin. Meanwhile, deficiency of CTCF O-GlcNAcylation results in the disruption of loop domains and the alteration of chromatin loops associated with cellular development. Furthermore, the deficiency of CTCF O-GlcNAcylation increases the expression of developmental genes and negatively regulates maintenance and establishment of stem cell pluripotency. In conclusion, these results provide key insights into the role of PTMs for the 3D chromatin structure.

Precise prediction of phase-separation key residues by machine learning

Understanding intracellular phase separation is crucial for deciphering transcriptional control, cell fate transitions, and disease mechanisms. However, the key residues, which impact phase separation the most for protein phase separation function have remained elusive. We develop PSPHunter, which can precisely predict these key residues based on machine learning scheme. In vivo and in vitro validations demonstrate that truncating just 6 key residues in GATA3 disrupts phase separation, enhancing tumor cell migration and inhibiting growth. Glycine and its motifs are enriched in spacer and key residues, as revealed by our comprehensive analysis. PSPHunter identifies nearly 80% of disease-associated phase-separating proteins, with frequent mutated pathological residues like glycine and proline often residing in these key residues. PSPHunter thus emerges as a crucial tool to uncover key residues, facilitating insights into phase separation mechanisms governing transcriptional control, cell fate transitions, and disease development.

ADNP modulates SINE B2-derived CTCF-binding sites during blastocyst formation in mice

CTCF is crucial for chromatin structure and transcription regulation in early embryonic development. However, the kinetics of CTCF chromatin occupation in preimplantation embryos have remained unclear. In this study, we used CUT&RUN technology to investigate CTCF occupancy in mouse preimplantation development. Our findings revealed that CTCF begins binding to the genome prior to zygotic genome activation (ZGA), with a preference for CTCF-anchored chromatin loops. Although the majority of CTCF occupancy is consistently maintained, we identified a specific set of binding sites enriched in the mouse-specific short interspersed element (SINE) family B2 that are restricted to the cleavage stages. Notably, we discovered that the neuroprotective protein ADNP counteracts the stable association of CTCF at SINE B2-derived CTCF-binding sites. Knockout of Adnp in the zygote led to impaired CTCF binding signal recovery, failed deposition of H3K9me3, and transcriptional derepression of SINE B2 during the morula-to-blastocyst transition, which further led to unfaithful cell differentiation in embryos around implantation. Our analysis highlights an ADNP-dependent restriction of CTCF binding during cell differentiation in preimplantation embryos. Furthermore, our findings shed light on the functional importance of transposable elements (TEs) in promoting genetic innovation and actively shaping the early embryo developmental process specific to mammals.

Epigenetic inhibition of CTCF by HN1 promotes dedifferentiation and stemness of anaplastic thyroid cancer

Anaplastic thyroid cancer (ATC) is one of the deadliest cancers, whose important malignant feature is dedifferentiation. Chromatin remodeling is critical for tumorigenesis and progression, while its roles and regulator in facilitating dedifferentiation of ATC had been poorly understood. In our study, an emerging function of hematological and neurological expressed 1 (HN1) in promoting dedifferentiation of ATC cells was uncovered. HN1 expression was negatively correlated with the thyroid differentiation markers both at mRNA and protein level. Knockdown of HN1 in ATC cells effectively upregulated the thyroid differentiation markers and impeded the sphere formation capacity, accompanying with the loss of cancer stemness. In contrast, overexpression of HN1 drove the gain of stemness and the loss of thyroid differentiation markers. Nude mouse and zebrafish xenograft models showed that inhibition of HN1 in ATC cells effectively hindered tumor growth due to the loss of cancer stemness. Further study showed that HN1 was negatively correlated with CTCF in an independent thyroid-cancer cohort, and inhibition of HN1 enhanced the expression of CTCF in ATC cells. Overexpression of CTCF significantly reversed the dedifferentiation phenotypes of ATC cells, whereas simultaneously inhibiting HN1 and CTCF was unable to recover the level of thyroid differentiation markers. The combination of ATAC-seq and ChIP-seq analysis confirmed that CTCF regulated genes relating with thyroid gland development through influencing their chromatin accessibility. HN1 inhibited the acetylation of H3K27 at the promoter of CTCF by recruiting HDAC2, thereby inhibiting the transcriptional activation of CTCF. These findings demonstrated an essential role of HN1 in regulating the chromatin accessibility of thyroid differentiation genes during ATC dedifferentiation.

Through the lens of phase separation: intrinsically unstructured protein and chromatin looping

The establishment, maintenance and dynamic regulation of three-dimensional (3D) chromatin structures provide an important means for partitioning of genome into functionally distinctive domains, which helps to define specialized gene expression programs associated with developmental stages and cell types. Increasing evidence supports critical roles for intrinsically disordered regions (IDRs) harbored within transcription factors (TFs) and chromatin-modulatory proteins in inducing phase separation, a phenomenon of forming membrane-less condensates through partitioning of biomolecules. Such a process is also critically involved in the establishment of high-order chromatin structures and looping. IDR- and phase separation-driven 3D genome (re)organization often goes wrong in disease such as cancer. This review discusses about recent advances in understanding how phase separation of intrinsically disordered proteins (IDPs) modulates chromatin looping and gene expression.

Large-scale data-driven and physics-based models offer insights into the relationships among the structures, dynamics, and functions of chromosomes

The organized three-dimensional chromosome architecture in the cell nucleus provides scaffolding for precise regulation of gene expression. When the cell changes its identity in the cell-fate decision-making process, extensive rearrangements of chromosome structures occur accompanied by large-scale adaptations of gene expression, underscoring the importance of chromosome dynamics in shaping genome function. Over the last two decades, rapid development of experimental methods has provided unprecedented data to characterize the hierarchical structures and dynamic properties of chromosomes. In parallel, these enormous data offer valuable opportunities for developing quantitative computational models. Here, we review a variety of large-scale polymer models developed to investigate the structures and dynamics of chromosomes. Different from the underlying modeling strategies, these approaches can be classified into data-driven ('top-down') and physics-based ('bottom-up') categories. We discuss their contributions to offering valuable insights into the relationships among the structures, dynamics, and functions of chromosomes and propose the perspective of developing data integration approaches from different experimental technologies and multidisciplinary theoretical/simulation methods combined with different modeling strategies.

Be-1DCNN: a neural network model for chromatin loop prediction based on bagging ensemble learning

The chromatin loops in the three-dimensional (3D) structure of chromosomes are essential for the regulation of gene expression. Despite the fact that high-throughput chromatin capture techniques can identify the 3D structure of chromosomes, chromatin loop detection utilizing biological experiments is arduous and time-consuming. Therefore, a computational method is required to detect chromatin loops. Deep neural networks can form complex representations of Hi-C data and provide the possibility of processing biological datasets. Therefore, we propose a bagging ensemble one-dimensional convolutional neural network (Be-1DCNN) to detect chromatin loops from genome-wide Hi-C maps. First, to obtain accurate and reliable chromatin loops in genome-wide contact maps, the bagging ensemble learning method is utilized to synthesize the prediction results of multiple 1DCNN models. Second, each 1DCNN model consists of three 1D convolutional layers for extracting high-dimensional features from input samples and one dense layer for producing the prediction results. Finally, the prediction results of Be-1DCNN are compared to those of the existing models. The experimental results indicate that Be-1DCNN predicts high-quality chromatin loops and outperforms the state-of-the-art methods using the same evaluation metrics. The source code of Be-1DCNN is available for free at https://github.com/HaoWuLab-Bioinformatics/Be1DCNN.

Liquid-Liquid Phase Separation Sheds New Light upon Cardiovascular Diseases

Liquid-liquid phase separation (LLPS) is a biophysical process that mediates the precise and complex spatiotemporal coordination of cellular processes. Proteins and nucleic acids are compartmentalized into micron-scale membrane-less droplets via LLPS. These droplets, termed biomolecular condensates, are highly dynamic, have concentrated components, and perform specific functions. Biomolecular condensates have been observed to organize diverse key biological processes, including gene transcription, signal transduction, DNA damage repair, chromatin organization, and autophagy. The dysregulation of these biological activities owing to aberrant LLPS is important in cardiovascular diseases. This review provides a detailed overview of the regulation and functions of biomolecular condensates, provides a comprehensive depiction of LLPS in several common cardiovascular diseases, and discusses the revolutionary therapeutic perspective of modulating LLPS in cardiovascular diseases and new treatment strategies relevant to LLPS.

May the force be with you: Nuclear condensates function beyond transcription control: Potential nongenetic functions of nuclear condensates in physiological and pathological conditions

Over the past decade, research has revealed biomolecular condensates' relevance in diverse cellular functions. Through a phase separation process, they concentrate macromolecules in subcompartments shaping the cellular organization and physiology. In the nucleus, biomolecular condensates assemble relevant biomolecules that orchestrate gene expression. We here hypothesize that chromatin condensates can also modulate the nongenetic functions of the genome, including the nuclear mechanical properties. The importance of chromatin condensates is supported by the genetic evidence indicating that mutations in their members are causative of a group of rare Mendelian diseases named chromatinopathies (CPs). Despite a broad spectrum of clinical features and the perturbations of the epigenetic machinery characterizing the CPs, recent findings highlighted negligible changes in gene expression. These data argue in favor of possible noncanonical functions of chromatin condensates in regulating the genome's spatial organization and, consequently, the nuclear mechanics. In this review, we discuss how condensates may impact nuclear mechanical properties, thus affecting the cellular response to mechanical cues and, eventually, cell fate and identity. Chromatin condensates organize macromolecules in the nucleus orchestrating the transcription regulation and mutations in their members are responsible for rare diseases named chromatinopathies. We argue that chromatin condensates, in concert with the nuclear lamina, may also govern the nuclear mechanical properties affecting the cellular response to external cues.

Simultaneous profiling of chromatin architecture and transcription in single cells

The three-dimensional structure of chromatin plays a crucial role in development and disease, both of which are associated with transcriptional changes. However, given the heterogeneity in single-cell chromatin architecture and transcription, the regulatory relationship between the three-dimensional chromatin structure and gene expression is difficult to explain based on bulk cell populations. Here we develop a single-cell, multimodal, omics method allowing the simultaneous detection of chromatin architecture and messenger RNA expression by sequencing (single-cell transcriptome sequencing (scCARE-seq)). Applying scCARE-seq to examine chromatin architecture and transcription from 2i to serum single mouse embryonic stem cells, we observe improved separation of cell clusters compared with single-cell chromatin conformation capture. In addition, after defining the cell-cycle phase of each cell through chromatin architecture extracted by scCARE-seq, we find that periodic changes in chromatin architecture occur in parallel with transcription during the cell cycle. These findings highlight the potential of scCARE-seq to facilitate comprehensive analyses that may boost our understanding of chromatin architecture and transcription in the same single cell.

CTCF and Its Multi-Partner Network for Chromatin Regulation

Architectural proteins are essential epigenetic regulators that play a critical role in organizing chromatin and controlling gene expression. CTCF (CCCTC-binding factor) is a key architectural protein responsible for maintaining the intricate 3D structure of chromatin. Because of its multivalent properties and plasticity to bind various sequences, CTCF is similar to a Swiss knife for genome organization. Despite the importance of this protein, its mechanisms of action are not fully elucidated. It has been hypothesized that its versatility is achieved through interaction with multiple partners, forming a complex network that regulates chromatin folding within the nucleus. In this review, we delve into CTCF's interactions with other molecules involved in epigenetic processes, particularly histone and DNA demethylases, as well as several long non-coding RNAs (lncRNAs) that are able to recruit CTCF. Our review highlights the importance of CTCF partners to shed light on chromatin regulation and pave the way for future exploration of the mechanisms that enable the finely-tuned role of CTCF as a master regulator of chromatin.

(Tn5-)FISH-based imaging in the era of 3D/spatial genomics

3D genomics mainly focuses on the 3D position of single genes at the cell level, while spatial genomics focuses more on the tissue level. In this exciting new era of 3D/spatial genomics, half-century old FISH and its derivative methods, including Tn5-FISH, play important roles. In this review, we introduce the Tn5-FISH we developed recently, and present six different applications published by our collaborators and us, based on (Tn5-)FISH, which can be either general BAC clone-based FISH or Tn5-FISH. In these interesting cases, (Tn5-)FISH demonstrated its vigorous ability of targeting sub-chromosomal structures across different diseases and cell lines (leukemia, mESCs (mouse embryonic stem cells), and differentiation cell lines). Serving as an effective tool to image genomic structures at the kilobase level, Tn5-FISH holds great potential to detect chromosomal structures in a high-throughput manner, thus bringing the dawn for new discoveries in the great era of 3D/spatial genomics.

Phase Separation: Direct and Indirect Driving Force for High-Order Chromatin Organization

The multi-level spatial chromatin organization in the nucleus is closely related to chromatin activity. The mechanism of chromatin organization and remodeling attract much attention. Phase separation describes the biomolecular condensation which is the basis for membraneless compartments in cells. Recent research shows that phase separation is a key aspect to drive high-order chromatin structure and remodeling. In addition, chromatin functional compartmentalization in the nucleus which is formed by phase separation also plays an important role in overall chromatin structure. In this review, we summarized the latest work about the role of phase separation in spatial chromatin organization, focusing on direct and indirect effects of phase separation on 3D chromatin organization and its impact on transcription regulation.

The dynamics of three-dimensional chromatin organization and phase separation in cell fate transitions and diseases

Cell fate transition is a fascinating process involving complex dynamics of three-dimensional (3D) chromatin organization and phase separation, which play an essential role in cell fate decision by regulating gene expression. Phase separation is increasingly being considered a driving force of chromatin folding. In this review, we have summarized the dynamic features of 3D chromatin and phase separation during physiological and pathological cell fate transitions and systematically analyzed recent evidence of phase separation facilitating the chromatin structure. In addition, we discuss current advances in understanding how phase separation contributes to physical and functional enhancer-promoter contacts. We highlight the functional roles of 3D chromatin organization and phase separation in cell fate transitions, and more explorations are required to study the regulatory relationship between 3D chromatin organization and phase separation. 3D chromatin organization (shown by Hi-C contact map) and phase separation are highly dynamic and play functional roles during early embryonic development, cell differentiation, somatic reprogramming, cell transdifferentiation and pathogenetic process. Phase separation can regulate 3D chromatin organization directly, but whether 3D chromatin organization regulates phase separation remains unclear.

D2 Plot, a Matrix of DNA Density and Distance to Periphery, Reveals Functional Genome Regions

The execution of biological activities inside space-limited cell nuclei requires sophisticated organization. Current studies on the 3D genome focus on chromatin interactions and local structures, e.g., topologically associating domains (TADs). In this study, two global physical properties: DNA density and distance to nuclear periphery (DisTP), are introduced and a 2D matrix, D2 plot, is constructed for mapping genetic and epigenetic markers. Distinct patterns of functional markers on the D2 plot, indicating its ability to compartmentalize functional genome regions, are observed. Furthermore, enrichments of transcription-related markers are concatenated into a cross-species transcriptional activation model, where the nucleus is divided into four areas: active, intermediate, repress and histone, and repress and repeat. Based on the trajectories of the genomic regions on D2 plot, the constantly active and newly activated genes are successfully identified during olfactory sensory neuron maturation. The analysis reveals that the D2 plot effectively categorizes functional regions and provides a universal and transcription-related measurement for the 3D genome.

CTCF and Its Partners: Shaper of 3D Genome during Development

The 3D genome organization and its dynamic modulate genome function, playing a pivotal role in cell differentiation and development. CTCF and cohesin, acting as the core architectural components involved in chromatin looping and genome folding, can also recruit other protein or RNA partners to fine-tune genome structure during development. Moreover, systematic screening for partners of CTCF has been performed through high-throughput approaches. In particular, several novel protein and RNA partners, such as BHLHE40, WIZ, MAZ, Aire, MyoD, YY1, ZNF143, and Jpx, have been identified, and these partners are mostly implicated in transcriptional regulation and chromatin remodeling, offering a unique opportunity for dissecting their roles in higher-order chromatin organization by collaborating with CTCF and cohesin. Here, we review the latest advancements with an emphasis on features of CTCF partners and also discuss the specific functions of CTCF-associated complexes in chromatin structure modulation, which may extend our understanding of the functions of higher-order chromatin architecture in developmental processes.