Polycomb condensates can promote epigenetic marks but are not required for sustained chromatin compaction

Nucleosome remodeler exclusion by histone deacetylation enforces heterochromatic silencing and epigenetic inheritance

Heterochromatin enforces transcriptional gene silencing and can be epigenetically inherited, but the underlying mechanisms remain unclear. Here, we show that histone deacetylation, a conserved feature of heterochromatin domains, blocks SWI/SNF subfamily remodelers involved in chromatin unraveling, thereby stabilizing modified nucleosomes that preserve gene silencing. Histone hyperacetylation, resulting from either the loss of histone deacetylase (HDAC) activity or the direct targeting of a histone acetyltransferase to heterochromatin, permits remodeler access, leading to silencing defects. The requirement for HDAC in heterochromatin silencing can be bypassed by impeding SWI/SNF activity. Highlighting the crucial role of remodelers, merely targeting SWI/SNF to heterochromatin, even in cells with functional HDAC, increases nucleosome turnover, causing defective gene silencing and compromised epigenetic inheritance. This study elucidates a fundamental mechanism whereby histone hypoacetylation, maintained by high HDAC levels in heterochromatic regions, ensures stable gene silencing and epigenetic inheritance, providing insights into genome regulatory mechanisms relevant to human diseases.

Epigenetic memory is governed by an effector recruitment specificity toggle in Heterochromatin Protein 1

HP1 proteins are essential for establishing and maintaining transcriptionally silent heterochromatin. They dimerize, forming a binding interface to recruit diverse chromatin-associated factors. Although HP1 proteins are known to rapidly evolve, the extent of variation required to achieve functional specialization is unknown. To investigate how changes in amino acid sequence impacts heterochromatin formation, we performed a targeted mutagenesis screen of the S. pombe HP1 homolog, Swi6. Substitutions within an auxiliary surface adjacent to the HP1 dimerization interface produce Swi6 variants with divergent maintenance properties. Remarkably, substitutions at a single amino acid position lead to the persistent gain or loss of epigenetic inheritance. These substitutions increase Swi6 chromatin occupancy in vivo and altered Swi6-protein interactions that reprogram H3K9me maintenance. We show how relatively minor changes in Swi6 amino acid composition in an auxiliary surface can lead to profound changes in epigenetic inheritance providing a redundant mechanism to evolve HP1-effector specificity.

Phase separation and inheritance of repressive chromatin domains

Polycomb-associated chromatin and pericentromeric heterochromatin form genomic domains important for the epigenetic regulation of gene expression. Both Polycomb complexes and heterochromatin factors rely on 'read and write' mechanisms, which, on their own, are not sufficient to explain the formation and the maintenance of these epigenetic domains. Microscopy has revealed that they form specific nuclear compartments separated from the rest of the genome. Recently, some subunits of these molecular machineries have been shown to undergo phase separation, both in vitro and in vivo, suggesting that phase separation might play important roles in the formation and the function of these two kinds of repressive chromatin. In this review, we will present the recent advances in the field of facultative and constitutive heterochromatin formation and maintenance through phase separation.

GSK-126 Attenuates Cell Apoptosis in Ischemic Brain Injury by Modulating the EZH2-H3K27me3-Bcl2l1 Axis

Whether epigenetic modifications participate in the cell apoptosis after ischemic stroke remains unclear. Histone 3 tri-methylation at lysine 27 (H3K27me3) is a histone modification that leads to gene silencing and is involved in the pathogenesis of ischemic stroke. Since the expression of many antiapoptotic genes is inhibited in the ischemic brains, here we aimed to offer an epigenetic solution to cell apoptosis after stroke by reversing H3K27me3 levels after ischemia. GSK-126, a specific inhibitor of enhancer of zeste homolog 2 (EZH2), significantly decreased H3K27me3 levels and inhibited middle cerebral artery occlusion (MCAO) induced and oxygen glucose deprivation (OGD) induced cell apoptosis. Moreover, GSK-126 attenuated the apoptosis caused by oxidative stress, excitotoxicity, and excessive inflammatory responses in vitro. The role of H3K27me3 in regulating of the expression of the antiapoptotic molecule B cell lymphoma-2 like 1 (Bcl2l1) explained the antiapoptotic effect of GSK-126. In conclusion, we found that GSK-126 could effectively protect brain cells from apoptosis after cerebral ischemia, and this role of GSK-126 is closely related to an axis that regulates Bcl2l1 expression, beginning with the regulation of EZH2-dependent H3K27me3 modification.

Attraction and disruption: how loop extrusion and compartmentalisation shape the nuclear genome

Chromatin loops, which bring two distal loci of the same chromosome into close physical proximity, are the ubiquitous units of the three-dimensional genome. Recent advances in understanding the spatial organisation of chromatin suggest that several distinct mechanisms control chromatin interactions, such as loop extrusion by cohesin complexes, compartmentalisation by phase separation, direct protein-protein interactions and others. Here, we review different types of chromatin loops and highlight the factors and processes involved in their regulation. We discuss how loop extrusion and compartmentalisation shape chromatin interactions and how these two processes can either positively or negatively influence each other.

Chromatin compaction by Polycomb group proteins revisited

The chromatin compaction activity of Polycomb group proteins has traditionally been considered essential for transcriptional repression. However, there is very little information on how Polycomb group proteins compact chromatin at the molecular level and no causal link between the compactness of chromatin and transcriptional repression. Recently, a more complete picture of Polycomb-dependent chromatin architecture has started to emerge, owing to advanced methods for imaging and chromosome conformation capture. Discoveries into Polycomb-driven phase separation add another layer of complexity. Recent observations generally imply that Polycomb group proteins modulate chromatin structure at multiple scales to reduce its dynamics and segregate it from active domains. Hence, it is reasonable to hypothesise that Polycomb group proteins maintain the energetically favourable state of compacted chromatin, rather than actively compact it.

The SUN-family protein Sad1 mediates heterochromatin spatial organization through interaction with histone H2A-H2B

Heterochromatin is generally associated with the nuclear periphery, but how the spatial organization of heterochromatin is regulated to ensure epigenetic silencing remains unclear. Here we found that Sad1, an inner nuclear membrane SUN-family protein in fission yeast, interacts with histone H2A-H2B but not H3-H4. We solved the crystal structure of the histone binding motif (HBM) of Sad1 in complex with H2A-H2B, revealing the intimate contacts between Sad1HBM and H2A-H2B. Structure-based mutagenesis studies revealed that the H2A-H2B-binding activity of Sad1 is required for the dynamic distribution of Sad1 throughout the nuclear envelope (NE). The Sad1-H2A-H2B complex mediates tethering telomeres and the mating-type locus to the NE. This complex is also important for heterochromatin silencing. Mechanistically, H2A-H2B enhances the interaction between Sad1 and HDACs, including Clr3 and Sir2, to maintain epigenetic identity of heterochromatin. Interestingly, our results suggest that Sad1 exhibits the histone-enhanced liquid-liquid phase separation property, which helps recruit heterochromatin factors to the NE. Our results uncover an unexpected role of SUN-family proteins in heterochromatin regulation and suggest a nucleosome-independent role of H2A-H2B in regulating Sad1's functionality.

Epigenetic memory is governed by an effector recruitment specificity toggle in Heterochromatin Protein 1

HP1 proteins are essential for establishing and maintaining transcriptionally silent heterochromatin. They dimerize, forming a binding interface to recruit diverse chromatin-associated factors. HP1 proteins are specialized and rapidly evolve, but the extent of variation required to achieve functional specialization is unknown. To investigate how changes in amino acid sequence impacts epigenetic inheritance, we performed a targeted mutagenesis screen of the S. pombe HP1 homolog, Swi6. Substitutions within an auxiliary surface adjacent to the HP1 dimerization interface produced Swi6 variants with divergent maintenance properties. Remarkably, substitutions at a single amino acid position led to the persistent gain or loss of epigenetic inheritance. These substitutions increased Swi6 chromatin occupancy in vivo and altered Swi6-protein interactions that reprogram H3K9me maintenance. We show that relatively minor changes in Swi6 amino acid composition can lead to profound changes in epigenetic inheritance which provides a redundant mechanism to evolve novel effector specificity. .

Dynamical phase transition in models that couple chromatin folding with histone modifications

Genomic regions can acquire heritable epigenetic states through unique histone modifications, which lead to stable gene expression patterns without altering the underlying DNA sequence. However, the relationship between chromatin conformational dynamics and epigenetic stability is poorly understood. In this paper, we propose kinetic models to investigate the dynamic fluctuations of histone modifications and the spatial interactions between nucleosomes. Our model explicitly incorporates the influence of chemical modifications on the structural stability of chromatin and the contribution of chromatin contacts to the cooperative nature of chemical reactions. Through stochastic simulations and analytical theory, we have discovered distinct steady-state outcomes in different kinetic regimes, resembling a dynamical phase transition. Importantly, we have validated that the emergence of this transition, which occurs on biologically relevant timescales, is robust against variations in model design and parameters. Our findings suggest that the viscoelastic properties of chromatin and the timescale at which it transitions from a gel-like to a liquidlike state significantly impact dynamic processes that occur along the one-dimensional DNA sequence.

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.

Engineering Material Properties of Transcription Factor Condensates to Control Gene Expression in Mammalian Cells and Mice

Phase separation of biomolecules into condensates is a key mechanism in the spatiotemporal organization of biochemical processes in cells. However, the impact of the material properties of biomolecular condensates on important processes, such as the control of gene expression, remains largely elusive. Here, the material properties of optogenetically induced transcription factor condensates are systematically tuned, and probed for their impact on the activation of target promoters. It is demonstrated that transcription factors in rather liquid condensates correlate with increased gene expression levels, whereas stiffer transcription factor condensates correlate with the opposite effect, reduced activation of gene expression. The broad nature of these findings is demonstrated in mammalian cells and mice, as well as by using different synthetic and natural transcription factors. These effects are observed for both transgenic and cell-endogenous promoters. The findings provide a novel materials-based layer in the control of gene expression, which opens novel opportunities in optogenetic engineering and synthetic biology.

Mechanism of phase condensation for chromosome architecture and function

Chromosomal phase separation is involved in a broad spectrum of chromosome organization and functional processes. Nonetheless, the intricacy of this process has left its molecular mechanism unclear. Here, we introduce the principles governing phase separation and its connections to physiological roles in this context. Our primary focus is contrasting two phase separation mechanisms: self-association-induced phase separation (SIPS) and bridging-induced phase separation (BIPS). We provide a comprehensive discussion of the distinct features characterizing these mechanisms and offer illustrative examples that suggest their broad applicability. With a detailed understanding of these mechanisms, we explore their associations with nucleosomes and chromosomal biological functions. This comprehensive review contributes to the exploration of uncharted territory in the intricate interplay between chromosome architecture and function.

Decoding Chromatin Ubiquitylation: A Chemical Biology Perspective

Since Strahl and Allis proposed the "language of covalent histone modifications", a host of experimental studies have shed light on the different facets of chromatin regulation by epigenetic mechanisms. Initially proposed as a concept for controlling gene transcription, the regulation of deposition and removal of histone post-translational modifications (PTMs), such as acetylation, methylation, and phosphorylation, have been implicated in many chromatin regulation pathways. However, large PTMs such as ubiquitylation challenge research on many levels due to their chemical complexity. In recent years, chemical tools have been developed to generate chromatin in defined ubiquitylation states in vitro. Chemical biology approaches are now used to link specific histone ubiquitylation marks with downstream chromatin regulation events on the molecular level. Here, we want to highlight how chemical biology approaches have empowered the mechanistic study of chromatin ubiquitylation in the context of gene regulation and DNA repair with attention to future challenges.

Inheritance of epigenetic transcriptional memory

Epigenetic memory allows organisms to stably alter their transcriptional program in response to developmental or environmental stimuli. Such transcriptional programs are mediated by heritable regulation of the function of enhancers and promoters. Memory involves read-write systems that enable self-propagation and mitotic inheritance of cis-acting epigenetic marks to induce stable changes in transcription. Also, in response to environmental cues, cells can induce epigenetic transcriptional memory to poise inducible genes for faster induction in the future. Here, we discuss modes of epigenetic inheritance and the molecular basis of epigenetic transcriptional memory.

Polycomb repression of Hox genes involves spatial feedback but not domain compaction or phase transition

Polycomb group proteins have a critical role in silencing transcription during development. It is commonly proposed that Polycomb-dependent changes in genome folding, which compact chromatin, contribute directly to repression by blocking the binding of activating complexes. Recently, it has also been argued that liquid-liquid demixing of Polycomb proteins facilitates this compaction and repression by phase-separating target genes into a membraneless compartment. To test these models, we used Optical Reconstruction of Chromatin Architecture to trace the Hoxa gene cluster, a canonical Polycomb target, in thousands of single cells. Across multiple cell types, we find that Polycomb-bound chromatin frequently explores decompact states and partial mixing with neighboring chromatin, while remaining uniformly repressed, challenging the repression-by-compaction or phase-separation models. Using polymer simulations, we show that these observed flexible ensembles can be explained by 'spatial feedback'-transient contacts that contribute to the propagation of the epigenetic state (epigenetic memory), without inducing a globular organization.

Epigenetic regulatory layers in the 3D nucleus

Nearly 7 decades have elapsed since Francis Crick introduced the central dogma of molecular biology, as part of his ideas on protein synthesis, setting the fundamental rules of sequence information transfer from DNA to RNAs and proteins. We have since learned that gene expression is finely tuned in time and space, due to the activities of RNAs and proteins on regulatory DNA elements, and through cell-type-specific three-dimensional conformations of the genome. Here, we review major advances in genome biology and discuss a set of ideas on gene regulation and highlight how various biomolecular assemblies lead to the formation of structural and regulatory features within the nucleus, with roles in transcriptional control. We conclude by suggesting further developments that will help capture the complex, dynamic, and often spatially restricted events that govern gene expression in mammalian cells.

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.

Interplay of dynamic genome organization and biomolecular condensates

After 60 years of chromatin investigation, our understanding of chromatin organization has evolved from static chromatin fibers to dynamic nuclear compartmentalization. Chromatin is embedded in a heterogeneous nucleoplasm in which molecules are grouped into distinct compartments, partitioning nuclear space through phase separation. Human genome organization affects transcription which controls euchromatin formation by excluding inactive chromatin. Chromatin condensates have been described as either liquid-like or solid-like. In this short review, we discuss the dynamic nature of chromatin from the perspective of biomolecular condensates and highlight new live-cell synthetic tools to probe and manipulate chromatin organization and associated condensates.

Augmenting L3MBTL2-induced condensates suppresses tumor growth in osteosarcoma

Osteosarcoma is a highly aggressive cancer and lacks effective therapeutic targets. We found that L3MBTL2 acts as a tumor suppressor by transcriptionally repressing IFIT2 in osteosarcoma. L3MBTL2 recruits the components of Polycomb repressive complex 1.6 to form condensates via both Pho-binding pockets and polybasic regions within carboxyl-terminal intrinsically disordered regions; the L3MBTL2-induced condensates are required for its tumor suppression. Multi-monoubiquitination of L3MBTL2 by UBE2O results in its proteasomal degradation, and the UBE2O/L3MBTL2 axis was crucial for osteosarcoma growth. There is a reverse correlation between L3MBTL2 and UBE2O in osteosarcoma tissues, and higher UBE2O and lower L3MBTL2 are associated with poorer prognosis in osteosarcoma. Pharmacological blockage of UBE2O by arsenic trioxide can enhance L3MBTL2-induced condensates and consequently suppress osteosarcoma growth. Our findings unveil a crucial biological function of L3MBTL2-induced condensates in mediating tumor suppression, proposing the UBE2O-L3MBTL2 axis as a potential cancer therapeutic target in osteosarcoma.

Regulation of the heterochromatin spreading reaction by trans-acting factors

Heterochromatin is a gene-repressive protein-nucleic acid ultrastructure that is initially nucleated by DNA sequences. However, following nucleation, heterochromatin can then propagate along the chromatin template in a sequence-independent manner in a reaction termed spreading. At the heart of this process are enzymes that deposit chemical information on chromatin, which attracts the factors that execute chromatin compaction and transcriptional or co/post-transcriptional gene silencing. Given that these enzymes deposit guiding chemical information on chromatin they are commonly termed 'writers'. While the processes of nucleation and central actions of writers have been extensively studied and reviewed, less is understood about how the spreading process is regulated. We discuss how the chromatin substrate is prepared for heterochromatic spreading, and how trans-acting factors beyond writer enzymes regulate it. We examine mechanisms by which trans-acting factors in Suv39, PRC2, SETDB1 and SIR writer systems regulate spreading of the respective heterochromatic marks across chromatin. While these systems are in some cases evolutionarily and mechanistically quite distant, common mechanisms emerge which these trans-acting factors exploit to tune the spreading reaction.

How a disordered linker in the Polycomb protein Polyhomeotic tunes phase separation and oligomerization

The Polycomb Group (PcG) complex PRC1 represses transcription, forms condensates in cells, and modifies chromatin architecture. These processes are connected through the essential, polymerizing Sterile Alpha Motif (SAM) present in the PRC1 subunit Polyhomeotic (Ph). In vitro, Ph SAM drives formation of short oligomers and phase separation with DNA or chromatin in the context of a Ph truncation ("mini-Ph"). Oligomer length is controlled by the long disordered linker (L) that connects the SAM to the rest of Ph--replacing Drosophila PhL with the evolutionarily diverged human PHC3L strongly increases oligomerization. How the linker controls SAM polymerization, and how polymerization and the linker affect condensate formation are not know. We analyzed PhL and PHC3L using biochemical assays and molecular dynamics (MD) simulations. PHC3L promotes mini-Ph phase separation and makes it relatively independent of DNA. In MD simulations, basic amino acids in PHC3L form contacts with acidic amino acids in the SAM. Engineering the SAM to make analogous charge-based contacts with PhL increased polymerization and phase separation, partially recapitulating the effects of the PHC3L. Ph to PHC3 linker swaps and SAM surface mutations alter Ph condensate formation in cells, and Ph function in Drosophila imaginal discs. Thus, SAM-driven phase separation and polymerization are conserved between flies and mammals, but the underlying mechanisms have diverged through changes to the disordered linker.

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.

Dispersal of PRC1 condensates disrupts polycomb chromatin domains and loops

Polycomb repressive complex 1 (PRC1) strongly influences 3D genome organization, mediating local chromatin compaction and clustering of target loci. Several PRC1 subunits have the capacity to form biomolecular condensates through liquid-liquid phase separation in vitro and when tagged and over-expressed in cells. Here, we use 1,6-hexanediol, which can disrupt liquid-like condensates, to examine the role of endogenous PRC1 biomolecular condensates on local and chromosome-wide clustering of PRC1-bound loci. Using imaging and chromatin immunoprecipitation, we show that PRC1-mediated chromatin compaction and clustering of targeted genomic loci-at different length scales-can be reversibly disrupted by the addition and subsequent removal of 1,6-hexanediol to mouse embryonic stem cells. Decompaction and dispersal of polycomb domains and clusters cannot be solely attributable to reduced PRC1 occupancy detected by chromatin immunoprecipitation following 1,6-hexanediol treatment as the addition of 2,5-hexanediol has similar effects on binding despite this alcohol not perturbing PRC1-mediated 3D clustering, at least at the sub-megabase and megabase scales. These results suggest that weak hydrophobic interactions between PRC1 molecules may have a role in polycomb-mediated genome organization.

Biomolecular condensates tunes immune signaling at the Host-Pathogen interface

Membraneless organelles participate in diverse spatiotemporal regulation of cellular signal transduction by recruiting necessary signaling factors. During host-pathogen interactions, the plasma membrane (PM) at the interface between the plant and microbes serves as a central platform for forming multicomponent immune signaling hubs. The macromolecular condensation of the immune complex and regulators is important in regulating immune signaling outputs regarding strength, timing, and crosstalk between signaling pathways. This review discusses mechanisms that regulate specific and crosstalk of plant immune signal transduction pathways through macromolecular assembly and condensation.

Sticky, Adaptable, and Many-sided: SAM protein versatility in normal and pathological hematopoietic states

With decades of research seeking to generalize sterile alpha motif (SAM) biology, many outstanding questions remain regarding this multi-tool protein module. Recent data from structural and molecular/cell biology has begun to reveal new SAM modes of action in cell signaling cascades and biomolecular condensation. SAM-dependent mechanisms underlie blood-related (hematologic) diseases, including myelodysplastic syndromes and leukemias, prompting our focus on hematopoiesis for this review. With the increasing coverage of SAM-dependent interactomes, a hypothesis emerges that SAM interaction partners and binding affinities work to fine tune cell signaling cascades in developmental and disease contexts, including hematopoiesis and hematologic disease. This review discusses what is known and remains unknown about the standard mechanisms and neoplastic properties of SAM domains and what the future might hold for developing SAM-targeted therapies.

Proximity-dependent recruitment of Polycomb repressive complexes by the lncRNA Airn

During mouse embryogenesis, expression of the long non-coding RNA (lncRNA) Airn leads to gene repression and recruitment of Polycomb repressive complexes (PRCs) to varying extents over a 15-Mb domain. The mechanisms remain unclear. Using high-resolution approaches, we show in mouse trophoblast stem cells that Airn expression induces long-range changes to chromatin architecture that coincide with PRC-directed modifications and center around CpG island promoters that contact the Airn locus even in the absence of Airn expression. Intensity of contact between the Airn lncRNA and chromatin correlated with underlying intensity of PRC recruitment and PRC-directed modifications. Deletion of CpG islands that contact the Airn locus altered long-distance repression and PRC activity in a manner that correlated with changes in chromatin architecture. Our data imply that the extent to which Airn expression recruits PRCs to chromatin is controlled by DNA regulatory elements that modulate proximity of the Airn lncRNA product to its target DNA.

Intrinsic mesoscale properties of a Polycomb protein underpin heterochromatin fidelity

Little is understood about how the two major types of heterochromatin domains (HP1 and Polycomb) are kept separate. In the yeast Cryptococcus neoformans, the Polycomb-like protein Ccc1 prevents deposition of H3K27me3 at HP1 domains. Here we show that phase separation propensity underpins Ccc1 function. Mutations of the two basic clusters in the intrinsically disordered region or deletion of the coiled-coil dimerization domain alter phase separation behavior of Ccc1 in vitro and have commensurate effects on formation of Ccc1 condensates in vivo, which are enriched for PRC2. Notably, mutations that alter phase separation trigger ectopic H3K27me3 at HP1 domains. Supporting a direct condensate-driven mechanism for fidelity, Ccc1 droplets efficiently concentrate recombinant C. neoformans PRC2 in vitro whereas HP1 droplets do so only weakly. These studies establish a biochemical basis for chromatin regulation in which mesoscale biophysical properties play a key functional role.

Critical Beginnings: Selective Tuning of Solubility and Structural Accuracy of Newly Synthesized Proteins by the Hsp70 Chaperone System

Proteins are particularly prone to aggregation immediately after release from the ribosome, and it is therefore important to elucidate the role of chaperones during these key steps of protein life. The Hsp70 and trigger factor (TF) chaperone systems interact with nascent proteins during biogenesis and immediately post-translationally. It is unclear, however, whether these chaperones can prevent formation of soluble and insoluble aggregates. Here, we address this question by monitoring the solubility and structural accuracy of globin proteins biosynthesized in an Escherichia coli cell-free system containing different concentrations of the bacterial Hsp70 and TF chaperones. We find that Hsp70 concentrations required to grant solubility to newly synthesized proteins are extremely sensitive to client-protein sequence. Importantly, Hsp70 concentrations yielding soluble client proteins are insufficient to prevent formation of soluble aggregates. In fact, for some aggregation-prone protein variants, avoidance of soluble-aggregate formation demands Hsp70 concentrations that exceed cellular levels in E. coli. In all, our data highlight the prominent role of soluble aggregates upon nascent-protein release from the ribosome and show the limitations of the Hsp70 chaperone system in the case of highly aggregation-prone proteins. These results demonstrate the need to devise better strategies to prevent soluble-aggregate formation upon release from the ribosome.

Loss of CBX2 causes genomic instability and Wnt activation in high grade serous ovarian carcinoma cells

High grade serous ovarian carcinoma (HGSOC) is lethal with insidious onset, rapid progression, poor prognosis, and limited treatment options. Polycomb repressor complexes (PRC) 1 and 2 are intimately involved in progression of many types of cancer including HGSOC. Unlike the consistent constitution of PRC2, PRC1 consists of diverse components whose clinical significance in HGSOC are not entirely clear. Here, prognosis-associated PRC1 components were identified through data-mining. CBX2 promoted proliferation and reduced apoptosis of HGSOC cell lines OVCAR4, OVCAR3, and CAOV3. Complete loss of CBX2 by CRISPR-cas9 editing (CBX2^KO ) destabilized genome stability with increased spontaneous chromosomal breaks and tendency to polyploidy accompanied by disrupted cell cycle especially stalled G2/M transition and caused severe cell death. Wnt/β-catenin/LEF1/TCF7L1 was activated in surviving OVCAR4-CBX2^KO clones to bypass the crisis caused by loss of CBX2. The relieve of TCF7L1 core-promoter region occupied by CBX2 might be one of the possible explanations to TCF7L1 increase in OVCAR4-CBX2^KO clones. Subcutaneous tumor model further validated that depletion of CBX2 repressed HGSOC cell line derived tumor growth. High immunohistochemistry score of CBX2 in primary ovarian cancer tissue associated with advanced clinical stage (p = 0.033), poor overall survival (HR = 3.056, 95% CI: 1.024-9.123), and progression free survival (HR = 4.455, 95% CI: 1.513-13.118) in HGSOC. Overall, our results suggested that CBX2 was a promising prognostic factor and therapeutic target in HGSOC.

4D epigenomics: deciphering the coupling between genome folding and epigenomic regulation with biophysical modeling

Recent experimental observations suggest a strong coupling between the 3D nuclear chromosome organization and epigenomics. However, the mechanistic and functional bases of such interplay remain elusive. In this review, we describe how biophysical modeling has been instrumental in characterizing how genome folding may impact the formation of epigenomic domains and, conversely, how epigenomic marks may affect chromosome conformation. Finally, we discuss how this mutual feedback loop between chromatin organization and epigenome regulation, via the formation of physicochemical nanoreactors, may represent a key functional role of 3D compartmentalization in the assembly and maintenance of stable - but yet plastic - epigenomic landscapes.

Phase separation in fungi

Phase separation, in which macromolecules partition into a concentrated phase that is immiscible with a dilute phase, is involved with fundamental cellular processes across the tree of life. We review the principles of phase separation and highlight how it impacts diverse processes in the fungal kingdom. These include the regulation of autophagy, cell signalling pathways, transcriptional circuits and the establishment of asymmetry in fungal cells. We describe examples of stable, phase-separated assemblies including membraneless organelles such as the nucleolus as well as transient condensates that also arise through phase separation and enable cells to rapidly and reversibly respond to important environmental cues. We showcase how research into phase separation in model yeasts, such as Saccharomyces cerevisiae and Schizosaccharomyces pombe, in conjunction with that in plant and human fungal pathogens, such as Ashbya gossypii and Candida albicans, is continuing to enrich our understanding of fundamental molecular processes.

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.

Chromatin accessibility: methods, mechanisms, and biological insights

Access to DNA is a prerequisite to the execution of essential cellular processes that include transcription, replication, chromosomal segregation, and DNA repair. How the proteins that regulate these processes function in the context of chromatin and its dynamic architectures is an intensive field of study. Over the past decade, genome-wide assays and new imaging approaches have enabled a greater understanding of how access to the genome is regulated by nucleosomes and associated proteins. Additional mechanisms that may control DNA accessibility in vivo include chromatin compaction and phase separation - processes that are beginning to be understood. Here, we review the ongoing development of accessibility measurements, we summarize the different molecular and structural mechanisms that shape the accessibility landscape, and we detail the many important biological functions that are linked to chromatin accessibility.

3D Genome Plasticity in Normal and Diseased Neurodevelopment

Non-random spatial organization of the chromosomal material inside the nuclei of brain cells emerges as an important regulatory layer of genome organization and function in health and disease. Here, we discuss how integrative approaches assessing chromatin in context of the 3D genome is providing new insights into normal and diseased neurodevelopment. Studies in primate (incl. human) and rodent brain have confirmed that chromosomal organization in neurons and glia undergoes highly dynamic changes during pre- and early postnatal development, with potential for plasticity across a much wider age window. For example, neuronal 3D genomes from juvenile and adult cerebral cortex and hippocampus undergo chromosomal conformation changes at hundreds of loci in the context of learning and environmental enrichment, viral infection, and neuroinflammation. Furthermore, locus-specific structural DNA variations, such as micro-deletions, duplications, repeat expansions, and retroelement insertions carry the potential to disrupt the broader epigenomic and transcriptional landscape far beyond the boundaries of the site-specific variation, highlighting the critical importance of long-range intra- and inter-chromosomal contacts for neuronal and glial function.

Modulation of the high-order chromatin structure by Polycomb complexes

The multi-subunit Polycomb Repressive Complex (PRC) 1 and 2 act, either independently or synergistically, to maintain and enforce a repressive state of the target chromatin, thereby regulating the processes of cell lineage specification and organismal development. In recent years, deep sequencing-based and imaging-based technologies, especially those tailored for mapping three-dimensional (3D) chromatin organization and structure, have allowed a better understanding of the PRC complex-mediated long-range chromatin contacts and DNA looping. In this review, we review current advances as for how Polycomb complexes function to modulate and help define the high-order chromatin structure and topology, highlighting the multi-faceted roles of Polycomb proteins in gene and genome regulation.

Histone post-translational modifications - cause and consequence of genome function

Much has been learned since the early 1960s about histone post-translational modifications (PTMs) and how they affect DNA-templated processes at the molecular level. This understanding has been bolstered in the past decade by the identification of new types of histone PTM, the advent of new genome-wide mapping approaches and methods to deposit or remove PTMs in a locally and temporally controlled manner. Now, with the availability of vast amounts of data across various biological systems, the functional role of PTMs in important processes (such as transcription, recombination, replication, DNA repair and the modulation of genomic architecture) is slowly emerging. This Review explores the contribution of histone PTMs to the regulation of genome function by discussing when these modifications play a causative (or instructive) role in DNA-templated processes and when they are deposited as a consequence of such processes, to reinforce and record the event. Important advances in the field showing that histone PTMs can exert both direct and indirect effects on genome function are also presented.

Structure of a nucleosome-bound MuvB transcription factor complex reveals DNA remodelling

Genes encoding the core cell cycle machinery are transcriptionally regulated by the MuvB family of protein complexes in a cell cycle-specific manner. Complexes of MuvB with the transcription factors B-MYB and FOXM1 activate mitotic genes during cell proliferation. The mechanisms of transcriptional regulation by these complexes are still poorly characterised. Here, we combine biochemical analysis and in vitro reconstitution, with structural analysis by cryo-electron microscopy and cross-linking mass spectrometry, to functionally examine these complexes. We find that the MuvB:B-MYB complex binds and remodels nucleosomes, thereby exposing nucleosomal DNA. This remodelling activity is supported by B-MYB which directly binds the remodelled DNA. Given the remodelling activity on the nucleosome, we propose that the MuvB:B-MYB complex functions as a pioneer transcription factor complex. In this work, we rationalise prior biochemical and cellular studies and provide a molecular framework of interactions on a protein complex that is key for cell cycle regulation.

De Novo Polycomb Recruitment and Repressive Domain Formation

Every cell of an organism shares the same genome; even so, each cellular lineage owns a different transcriptome and proteome. The Polycomb group proteins (PcG) are essential regulators of gene repression patterning during development and homeostasis. However, it is unknown how the repressive complexes, PRC1 and PRC2, identify their targets and elicit new Polycomb domains during cell differentiation. Classical recruitment models consider the pre-existence of repressive histone marks; still, de novo target binding overcomes the absence of both H3K27me3 and H2AK119ub. The CpG islands (CGIs), non-core proteins, and RNA molecules are involved in Polycomb recruitment. Nonetheless, it is unclear how de novo targets are identified depending on the physiological context and developmental stage and which are the leading players stabilizing Polycomb complexes at domain nucleation sites. Here, we examine the features of de novo sites and the accessory elements bridging its recruitment and discuss the first steps of Polycomb domain formation and transcriptional regulation, comprehended by the experimental reconstruction of the repressive domains through time-resolved genomic analyses in mammals.

Rheology and Viscoelasticity of Proteins and Nucleic Acids Condensates

Phase separation is as familiar as watching vinegar separating from oil in vinaigrette. The observation that phase separation of proteins and nucleic acids is widespread in living cells has opened an entire field of research into the biological significance and the biophysical mechanisms of phase separation and protein condensation in biology. Recent evidence indicates that certain proteins and nucleic acids condensates are not simple liquids and instead display both viscous and elastic behaviors, which in turn may have biological significance. The aim of this Perspective is to review the state-of-the-art of this quickly emerging field focusing on the material and rheological properties of protein condensates. Finally, we discuss the different techniques that can be employed to quantify the viscoelasticity of condensates and highlight potential future directions and opportunities for interdisciplinary cross-talk between chemists, physicists, and biologists.

KDM4C Contributes to Trophoblast-Like Stem Cell Conversion from Porcine-Induced Pluripotent Stem Cells (piPSCs) Via Regulating CDX2

Studies on ESRRB-regulating porcine-induced pluripotent stem cells (piPSCs) converted to trophoblast-like stem cells (TLSCs) contribute to the understanding of early embryo development. However, the epigenetic modification regulation network during the conversion is poorly understood. Here, the global change in histone H3 Lysine 4, 9, 27, 36 methylation and Lysine 27 acetylation was investigated in piPSCs and TLSCs. We found a high modification profile of H3K36me2 in TLSCs compared to that of piPSCs, whereas the profiles of other modifications remained constant. KDM4C, a H3K36me3/2 demethylase, whose gene body region was combined with ESRRB, was upregulated in TLSCs. Moreover, KDM4 inhibitor supplementation rescued the AP-negative phenotype observed in TLSCs, confirming that KDM4C could regulate the pluripotency of TLSCs. Subsequently, KDM4C replenishment results show the significantly repressed proliferation and AP-positive staining of TLSCs. The expressions of CDX2 and KRT8 were also upregulated after KDM4C overexpression. In summary, these results show that KDM4C replaced the function of ESRRB. These findings reveal the unique and crucial role of KDM4C-mediated epigenetic chromatin modifications in determination of piPSCs’ fate and expand the understanding of the connection between piPSCs and TSCs.

The mechanobiology of nuclear phase separation

The cell nucleus can be thought of as a complex, dynamic, living material, which functions to organize and protect the genome and coordinate gene expression. These functions are achieved via intricate mechanical and biochemical interactions among its myriad components, including the nuclear lamina, nuclear bodies, and the chromatin itself. While the biophysical organization of the nuclear lamina and chromatin have been thoroughly studied, the concept that liquid–liquid phase separation and related phase transitions play a role in establishing nuclear structure has emerged only recently. Phase transitions are likely to be intimately coupled to the mechanobiology of structural elements in the nucleus, but their interplay with one another is still not understood. Here, we review recent developments on the role of phase separation and mechanics in nuclear organization and discuss the functional implications in cell physiology and disease states.

Characterizing locus specific chromatin structure and dynamics with correlative conventional and super-resolution imaging in living cells

The dynamic rearrangement of chromatin is critical for gene regulation, but mapping both the spatial organization of chromatin and its dynamics remains a challenge. Many structural conformations are too small to be resolved via conventional fluorescence microscopy and the long acquisition time of super-resolution photoactivated localization microscopy (PALM) precludes the structural characterization of chromatin below the optical diffraction limit in living cells due to chromatin motion. Here we develop a correlative conventional fluorescence and PALM imaging approach to quantitatively map time-averaged chromatin structure and dynamics below the optical diffraction limit in living cells. By assigning localizations to a locus as it moves, we reliably discriminate between bound and unbound dCas9 molecules, whose mobilities overlap. Our approach accounts for changes in DNA mobility and relates local chromatin motion to larger scale domain movement. In our experimental system, we show that compacted telomeres move faster and have a higher density of bound dCas9 molecules, but the relative motion of those molecules is more restricted than in less compacted telomeres. Correlative conventional and PALM imaging therefore improves the ability to analyze the mobility and time-averaged nanoscopic structural features of locus specific chromatin with single molecule sensitivity and yields unprecedented insights across length and time scales.

SSX2 promotes the formation of a novel type of intranuclear lamin bodies

SSX proteins are normally restricted to spermatogenic cells, but ectopic expression can be observed in many types of human cancer. We recently demonstrated that SSX family members may contribute to tumorigenesis by modifying chromatin structure and, in specific settings, compromise chromatin stability. Here, we used normal and tumorigenic breast epithelial cell line models to further study the effect of ectopic expression of SSX2 on nuclear organization. We show that SSX2 induces the formation of a novel type of nucleoplasmic lamin bodies. Ectopic expression of SSX2 in various breast epithelial cell lines led to the formation of a previously undescribed type of intranuclear bodies containing both A and B type lamins but no other components of the nuclear lamina. SSX2-expressing cells contained a highly variable number of lamin bodies distributed throughout the nuclear space. SSX2-mediated establishment of intranuclear lamin bodies could not be linked to previous molecular interactions of SSX proteins, including polycomb proteins and the Mediator complex, but was, however, dependent on S-phase progression. These results reveal a novel interaction between SSX2 and lamins in the nucleoplasmic space. They further suggest that SSX2 promotes the formation of chromatin neighborhoods supporting the organization of lamins into nuclear bodies. We speculate that this may have implications for the organization and functional regulation of chromatin in cancer cells. Our study contributes to the further understanding of the biology of SSX proteins in tumorigenesis.