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

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

Dynamic 1D search and processive nucleosome translocations by RSC and ISW2 chromatin remodelers

Eukaryotic gene expression is linked to chromatin structure and nucleosome positioning by ATP-dependent chromatin remodelers that establish and maintain nucleosome-depleted regions (NDRs) near transcription start sites. Conserved yeast RSC and ISW2 remodelers exert antagonistic effects on nucleosomes flanking NDRs, but the temporal dynamics of remodeler search, engagement, and directional nucleosome mobilization for promoter accessibility are unknown. Using optical tweezers and two-color single-particle imaging, we investigated the Brownian diffusion of RSC and ISW2 on free DNA and sparse nucleosome arrays. RSC and ISW2 rapidly scan DNA by one-dimensional hopping and sliding, respectively, with dynamic collisions between remodelers followed by recoil or apparent co-diffusion. Static nucleosomes block remodeler diffusion resulting in remodeler recoil or sequestration. Remarkably, both RSC and ISW2 use ATP hydrolysis to translocate mono-nucleosomes processively at ~30 bp/s on extended linear DNA under tension. Processivity and opposing push-pull directionalities of nucleosome translocation shown by RSC and ISW2 shape the distinctive landscape of promoter chromatin.

Unnatural Amino Acid Crosslinking for Increased Spatiotemporal Resolution of Chromatin Dynamics

The utilization of an expanded genetic code and in vivo unnatural amino acid crosslinking has grown significantly in the past decade, proving to be a reliable system for the examination of protein-protein interactions. Perhaps the most utilized amino acid crosslinker, p-benzoyl-(l)-phenylalanine (pBPA), has delivered a vast compendium of structural and mechanistic data, placing it firmly in the upper echelons of protein analytical techniques. pBPA contains a benzophenone group that is activated with low energy radiation (~365 nm), initiating a diradical state that can lead to hydrogen abstraction and radical recombination in the form of a covalent bond to a neighboring protein. Importantly, the expanded genetic code system provides for site-specific encoding of the crosslinker, yielding spatial control for protein surface mapping capabilities. Paired with UV-activation, this process offers a practical means for spatiotemporal understanding of protein-protein dynamics in the living cell. The chromatin field has benefitted particularly well from this technique, providing detailed mapping and mechanistic insight for numerous chromatin-related pathways. We provide here a brief history of unnatural amino acid crosslinking in chromatin studies and outlooks into future applications of the system for increased spatiotemporal resolution in chromatin related research.

Nucleosome Remodeling at the Yeast PHO8 and PHO84 Promoters without the Putatively Essential SWI/SNF Remodeler

Chromatin remodeling by ATP-dependent remodeling enzymes is crucial for all genomic processes, like transcription or replication. Eukaryotes harbor many remodeler types, and it is unclear why a given chromatin transition requires more or less stringently one or several remodelers. As a classical example, removal of budding yeast PHO8 and PHO84 promoter nucleosomes upon physiological gene induction by phosphate starvation essentially requires the SWI/SNF remodeling complex. This dependency on SWI/SNF may indicate specificity in remodeler recruitment, in recognition of nucleosomes as remodeling substrate or in remodeling outcome. By in vivo chromatin analyses of wild type and mutant yeast under various PHO regulon induction conditions, we found that overexpression of the remodeler-recruiting transactivator Pho4 allowed removal of PHO8 promoter nucleosomes without SWI/SNF. For PHO84 promoter nucleosome removal in the absence of SWI/SNF, an intranucleosomal Pho4 site, which likely altered the remodeling outcome via factor binding competition, was required in addition to such overexpression. Therefore, an essential remodeler requirement under physiological conditions need not reflect substrate specificity, but may reflect specific recruitment and/or remodeling outcomes.

The role of SWI/SNF chromatin remodelers in the repair of DNA double strand breaks and cancer therapy

Switch/Sucrose non-fermenting (SWI/SNF) chromatin remodelers hydrolyze ATP to push and slide nucleosomes along the DNA thus modulating access to various genomic loci. These complexes are the most frequently mutated epigenetic regulators in human cancers. SWI/SNF complexes are well known for their function in transcription regulation, but more recent work has uncovered a role for these complexes in the repair of DNA double strand breaks (DSBs). As radiotherapy and most chemotherapeutic agents kill cancer cells by inducing double strand breaks, by identifying a role for these complexes in double strand break repair we are also identifying a DNA repair vulnerability that can be exploited therapeutically in the treatment of SWI/SNF-mutated cancers. In this review we summarize work describing the function of various SWI/SNF subunits in the repair of double strand breaks with a focus on homologous recombination repair and discuss the implication for the treatment of cancers with SWI/SNF mutations.

Impact of Saccharomyces cerevisiae on the Field of Single-Molecule Biophysics

Cellular functions depend on the dynamic assembly of protein regulator complexes at specific cellular locations. Single Molecule Tracking (SMT) is a method of choice for the biochemical characterization of protein dynamics in vitro and in vivo. SMT follows individual molecules in live cells and provides direct information about their behavior. SMT was successfully applied to mammalian models. However, mammalian cells provide a complex environment where protein mobility depends on numerous factors that are difficult to control experimentally. Therefore, yeast cells, which are unicellular and well-studied with a small and completely sequenced genome, provide an attractive alternative for SMT. The simplicity of organization, ease of genetic manipulation, and tolerance to gene fusions all make yeast a great model for quantifying the kinetics of major enzymes, membrane proteins, and nuclear and cellular bodies. However, very few researchers apply SMT techniques to yeast. Our goal is to promote SMT in yeast to a wider research community. Our review serves a dual purpose. We explain how SMT is conducted in yeast cells, and we discuss the latest insights from yeast SMT while putting them in perspective with SMT of higher eukaryotes.

SMNDC1 links chromatin remodeling and splicing to regulate pancreatic hormone expression

Insulin expression is primarily restricted to the pancreatic β cells, which are physically or functionally depleted in diabetes. Identifying targetable pathways repressing insulin in non-β cells, particularly in the developmentally related glucagon-secreting α cells, is an important aim of regenerative medicine. Here, we perform an RNA interference screen in a murine α cell line to identify silencers of insulin expression. We discover that knockdown of the splicing factor Smndc1 triggers a global repression of α cell gene-expression programs in favor of increased β cell markers. Mechanistically, Smndc1 knockdown upregulates the β cell transcription factor Pdx1 by modulating the activities of the BAF and Atrx chromatin remodeling complexes. SMNDC1's repressive role is conserved in human pancreatic islets, its loss triggering enhanced insulin secretion and PDX1 expression. Our study identifies Smndc1 as a key factor connecting splicing and chromatin remodeling to the control of insulin expression in human and mouse islet cells.

Coordinated Epigenetic Regulation in Plants: A Potent Managerial Tool to Conquer Biotic Stress

Plants have evolved variable phenotypic plasticity to counteract different pathogens and pests during immobile life. Microbial infection invokes multiple layers of host immune responses, and plant gene expression is swiftly and precisely reprogramed at both the transcriptional level and post-transcriptional level. Recently, the importance of epigenetic regulation in response to biotic stresses has been recognized. Changes in DNA methylation, histone modification, and chromatin structures have been observed after microbial infection. In addition, epigenetic modifications may be preserved as transgenerational memories to allow the progeny to better adapt to similar environments. Epigenetic regulation involves various regulatory components, including non-coding small RNAs, DNA methylation, histone modification, and chromatin remodelers. The crosstalk between these components allows precise fine-tuning of gene expression, giving plants the capability to fight infections and tolerant drastic environmental changes in nature. Fully unraveling epigenetic regulatory mechanisms could aid in the development of more efficient and eco-friendly strategies for crop protection in agricultural systems. In this review, we discuss the recent advances on the roles of epigenetic regulation in plant biotic stress responses.

Cornelia de Lange Syndrome as Paradigm of Chromatinopathies

Chromatinopathies can be defined as a class of neurodevelopmental disorders caused by mutations affecting proteins responsible for chromatin remodeling and transcriptional regulation. The resulting dysregulation of gene expression favors the onset of a series of clinical features such as developmental delay, intellectual disability, facial dysmorphism, and behavioral disturbances. Cornelia de Lange syndrome (CdLS) is a prime example of a chromatinopathy. It is caused by mutations affecting subunits or regulators of the cohesin complex, a multisubunit protein complex involved in various molecular mechanisms such as sister chromatid cohesion, transcriptional regulation and formation of topologically associated domains. However, disease-causing variants in non-cohesin genes with overlapping functions have also been described in association with CdLS. Notably, the majority of these genes had been previously found responsible for distinct neurodevelopmental disorders that also fall within the category of chromatinopathies and are frequently considered as differential diagnosis for CdLS. In this review, we provide a systematic overview of the current literature to summarize all mutations in non-cohesin genes identified in association with CdLS phenotypes and discuss about the interconnection of proteins belonging to the chromatinopathies network.

Chromatin remodelers and lineage-specific factors interact to target enhancers to establish proneurosensory fate within otic ectoderm

Specification of Sox2⁺ proneurosensory progenitors within otic ectoderm is a prerequisite for the production of sensory cells and neurons for hearing. However, the underlying molecular mechanisms driving this lineage specification remain unknown. Here, we show that the Brg1-based SWI/SNF chromatin-remodeling complex interacts with the neurosensory-specific transcriptional regulators Eya1/Six1 to induce Sox2 expression and promote proneurosensory-lineage specification. Ablation of the ATPase-subunit Brg1 or both Eya1/Six1 results in loss of Sox2 expression and lack of neurosensory identity, leading to abnormal apoptosis within the otic ectoderm. Brg1 binds to two of three distal 3' Sox2 enhancers occupied by Six1, and Brg1-binding to these regions depends on Eya1-Six1 activity. We demonstrate that the activity of these Sox2 enhancers in otic neurosensory cells specifically depends on binding to Six1. Furthermore, genome-wide and transcriptome profiling indicate that Brg1 may suppress apoptotic factor Map3k5 to inhibit apoptosis. Together, our findings reveal an essential role for Brg1, its downstream pathways, and their interactions with Six1/Eya1 in promoting proneurosensory fate induction in the otic ectoderm and subsequent neuronal lineage commitment and survival of otic cells.

R-Loops and Its Chro-Mates: The Strange Case of Dr. Jekyll and Mr. Hyde

Since their discovery, R-loops have been associated with both physiological and pathological functions that are conserved across species. R-loops are a source of replication stress and genome instability, as seen in neurodegenerative disorders and cancer. In response, cells have evolved pathways to prevent R-loop accumulation as well as to resolve them. A growing body of evidence correlates R-loop accumulation with changes in the epigenetic landscape. However, the role of chromatin modification and remodeling in R-loops homeostasis remains unclear. This review covers various mechanisms precluding R-loop accumulation and highlights the role of chromatin modifiers and remodelers in facilitating timely R-loop resolution. We also discuss the enigmatic role of RNA:DNA hybrids in facilitating DNA repair, epigenetic landscape and the potential role of replication fork preservation pathways, active fork stability and stalled fork protection pathways, in avoiding replication-transcription conflicts. Finally, we discuss the potential role of several Chro-Mates (chromatin modifiers and remodelers) in the likely differentiation between persistent/detrimental R-loops and transient/benign R-loops that assist in various physiological processes relevant for therapeutic interventions.

ARID1 proteins: from transcriptional and post-translational regulation to carcinogenesis and potential therapeutics

The ARID1 proteins are mutually exclusive subunits of the BRG1/BRM-associated factor (BAF) complexes that play an important role in chromatin remodeling and regulate many fundamental cell functions. The role of ARID1s is well defined as a tumor-suppressive. The cancer cells evolve different mechanisms to downregulate ARID1s and inactivate their functions. ARID1s are frequently mutated in human cancer. The recent findings of ARID1A/B downregulation at transcriptional and translational levels along with their low levels in human cancers indicate the significance of regulatory mechanisms of ARID1s in cancers. In this review, we present the current knowledge on the regulation and alterations of ARID1 protein expression in human cancers and indicate the importance of regulators of ARID1s as a prognostic marker and in potential therapeutic strategies.

Large 1p36 Deletions Affecting Arid1a Locus Facilitate Mycn-Driven Oncogenesis in Neuroblastoma

Loss of heterozygosity (LOH) at 1p36 occurs in multiple cancers, including neuroblastoma (NBL). MYCN amplification and 1p36 deletions tightly correlate with markers of tumor aggressiveness in NBL. Although distal 1p36 losses associate with single-copy MYCN tumors, larger deletions correlate with MYCN amplification, indicating two tumor suppressor regions in 1p36, only one of which facilitates MYCN oncogenesis. To better define this region, we genome-edited the syntenic 1p36 locus in primary mouse neural crest cells (NCCs), a putative NBL cell of origin. In in vitro cell transformation assays, we show that Chd5 loss confers most of the MYCN-independent tumor suppressor effects of 1p36 LOH. In contrast, MYCN-driven tumorigenesis selects for NCCs with Arid1a deletions from a pool of NCCs with randomly sized 1p36 deletions, establishing Arid1a as the MYCN-associated tumor suppressor. Our findings reveal that Arid1a loss collaborates with oncogenic MYCN and better define the tumor suppressor functions of 1p36 LOH in NBL.

Remodeler Catalyzed Nucleosome Repositioning: Influence of Structure and Stability

The packaging of the eukaryotic genome into chromatin regulates the storage of genetic information, including the access of the cell’s DNA metabolism machinery. Indeed, since the processes of DNA replication, translation, and repair require access to the underlying DNA, several mechanisms, both active and passive, have evolved by which chromatin structure can be regulated and modified. One mechanism relies upon the function of chromatin remodeling enzymes which couple the free energy obtained from the binding and hydrolysis of ATP to the mechanical work of repositioning and rearranging nucleosomes. Here, we review recent work on the nucleosome mobilization activity of this essential family of molecular machines.

Calcium signaling instructs NIPBL recruitment at active enhancers and promoters via distinct mechanisms to reconstruct genome compartmentalization

In this study, Zhu et al. sought to understand the signaling pathways and epigenetic control of nuclear architecture during developmental progression of immune cells. They found that in activated neutrophils calcium influx rapidly recruited the cohesin-loading factor NIPBL to thousands of active enhancers and promoters to dictate widespread changes in compartment segregation.

During developmental progression the genomes of immune cells undergo large-scale changes in chromatin folding. However, insights into signaling pathways and epigenetic control of nuclear architecture remain rudimentary. Here, we found that in activated neutrophils calcium influx rapidly recruited the cohesin-loading factor NIPBL to thousands of active enhancers and promoters to dictate widespread changes in compartment segregation. NIPBL recruitment to enhancers and promoters occurred with distinct kinetics. The induction of NIPBL-binding was coordinate with increased P300, BRG1 and RNA polymerase II occupancy. NIPBL-bound enhancers were associated with NFAT, PU.1, and CEBP cis elements, whereas NIPBL-bound promoters were enriched for GC-rich DNA sequences. Using an acute degradation system, we found that the histone acetyltransferases P300 and CBP maintained H3K27ac abundance and facilitated NIPBL occupancy at enhancers and that active transcriptional elongation is essential to maintain H3K27ac abundance. Chromatin remodelers, containing either of the mutually exclusive BRG1 and BRM ATPases, promoted NIPBL recruitment at active enhancers. Conversely, at active promoters, depletion of BRG1 and BRM showed minimal effect on NIPBL occupancy. Finally, we found that calcium signaling in both primary innate and adaptive immune cells swiftly induced NIPBL occupancy. Collectively, these data reveal how transcriptional regulators, histone acetyltransferases, chromatin remodelers, and transcription elongation promote NIPBL occupancy at active enhancers while the induction of NIPLB occupancy at promoters is primarily associated with GC-rich DNA sequences.

Pioneer-like factor GAF cooperates with PBAP (SWI/SNF) and NURF (ISWI) to regulate transcription

Transcriptionally silent genes must be activated throughout development. This requires nucleosomes be removed from promoters and enhancers to allow transcription factor (TF) binding and recruitment of coactivators and RNA polymerase II (Pol II). Specialized pioneer TFs bind nucleosome-wrapped DNA to perform this chromatin opening by mechanisms that remain incompletely understood. Here, we show that GAGA factor (GAF), a Drosophila pioneer-like factor, functions with both SWI/SNF and ISWI family chromatin remodelers to allow recruitment of Pol II and entry to a promoter-proximal paused state, and also to promote Pol II's transition to productive elongation. We found that GAF interacts with PBAP (SWI/SNF) to open chromatin and allow Pol II to be recruited. Importantly, this activity is not dependent on NURF as previously proposed; however, GAF also synergizes with NURF downstream from this process to ensure efficient Pol II pause release and transition to productive elongation, apparently through its role in precisely positioning the +1 nucleosome. These results demonstrate how a single sequence-specific pioneer TF can synergize with remodelers to activate sets of genes. Furthermore, this behavior of remodelers is consistent with findings in yeast and mice, and likely represents general, conserved mechanisms found throughout eukarya.

Meta-analysis of Chromatin Programming by Steroid Receptors

Transcription factors (TFs) must access chromatin to bind to their response elements and regulate gene expression. A widely accepted model proposes that only a special subset of TFs, pioneer factors, can associate with condensed chromatin and initiate chromatin opening. We previously reported that steroid receptors (SRs), not considered pioneer factors, can assist the binding of an archetypal pioneer, the forkhead box protein 1 (FOXA1), at a subset of receptor-activated enhancers. These findings have been challenged recently, with the suggestion that newly acquired data fully support the prevailing pioneer model. Here, we reexamine our results and confirm the original conclusions. We also analyze and discuss a number of available datasets relevant to chromatin penetration by SRs and find a general consensus supporting our original observations. Hence, we propose that chromatin opening at some sites can be initiated by SRs, with a parallel recruitment of factors often treated as having a unique pioneer function. This Matters Arising paper is in response to Glont et al. (2019), published in Cell Reports.

The SWI/SNF ATPase BRG1 stimulates DNA end resection and homologous recombination by reducing nucleosome density at DNA double strand breaks and by promoting the recruitment of the CtIP nuclease

DNA double strand breaks (DSBs) are among the most toxic DNA lesions and can be repaired accurately through homologous recombination (HR). HR requires processing of the DNA ends by nucleases (DNA end resection) in order to generate the required single-stranded DNA (ssDNA) regions. The SWI/SNF chromatin remodelers are 10–15 subunit complexes that contain one ATPase (BRG1 or BRM). Multiple subunits of these complexes have recently been identified as a novel family of tumor suppressors. These complexes are capable of remodeling chromatin by pushing nucleosomes along the DNA. More recent studies have identified these chromatin remodelers as important factors in DNA repair. Using the DR-U2OS reporter system, we show that the down regulation of BRG1 significantly reduces HR efficiency, while BRM has a minor effect. Inactivation of BRG1 impairs DSB repair and results in a defect in DNA end resection, as measured by the amount of BrdU-containing ssDNA generated after DNA damage. Inactivation of BRG1 also impairs the activation of the ATR kinase, reduces the levels of chromatin-bound RPA, and reduces the number of RPA and RAD51 foci after DNA damage. This defect in DNA end resection is explained by the defective recruitment of GFP-CtIP to laser-induced DSBs in the absence of BRG1. Importantly, we show that BRG1 reduces nucleosome density at DSBs. Finally, inactivation of BRG1 renders cells sensitive to anti-cancer drugs that induce DSBs. This study identifies BRG1 as an important factor for HR, which suggests that BRG1-mutated cancers have a DNA repair vulnerability that can be exploited therapeutically.

Chromatin remodelers in oligodendroglia

Oligodendrocytes, the myelinating cells in the vertebrate central nervous system, produce myelin sheaths to enable saltatory propagation of action potentials. The process of oligodendrocyte myelination entails a stepwise progression from precursor specification to differentiation, which is coordinated by a series of transcriptional and chromatin remodeling events. ATP-dependent chromatin remodeling enzymes, which utilize ATP as an energy source to control chromatin dynamics and regulate the accessibility of chromatin to transcriptional regulators, are critical for oligodendrocyte lineage development and regeneration. In this review, we focus on the latest insights into the spatial and temporal specificity of chromatin remodelers during oligodendrocyte development, myelinogenesis, and regeneration. We will also bring together various plausible mechanisms by which lineage specific transcriptional regulators coordinate with chromatin remodeling factors for programming genomic landscapes to specifically modulate these different processes during developmental myelination and remyelination upon injury.

Epigenetic regulation of oligodendrocyte myelination in developmental disorders and neurodegenerative diseases

Oligodendrocytes are the critical cell types giving rise to the myelin nerve sheath enabling efficient nerve transmission in the central nervous system (CNS). Oligodendrocyte precursor cells differentiate into mature oligodendrocytes and are maintained throughout life. Deficits in the generation, proliferation, or differentiation of these cells or their maintenance have been linked to neurological disorders ranging from developmental disorders to neurodegenerative diseases and limit repair after CNS injury. Understanding the regulation of these processes is critical for achieving proper myelination during development, preventing disease, or recovering from injury. Many of the key factors underlying these processes are epigenetic regulators that enable the fine tuning or reprogramming of gene expression during development and regeneration in response to changes in the local microenvironment. These include chromatin remodelers, histone-modifying enzymes, covalent modifiers of DNA methylation, and RNA modification-mediated mechanisms. In this review, we will discuss the key components in each of these classes which are responsible for generating and maintaining oligodendrocyte myelination as well as potential targeted approaches to stimulate the regenerative program in developmental disorders and neurodegenerative diseases.

Chromatin Remodeling and Epigenetic Regulation in Plant DNA Damage Repair

DNA damage response (DDR) in eukaryotic cells is initiated in the chromatin context. DNA damage and repair depend on or have influence on the chromatin dynamics associated with genome stability. Epigenetic modifiers, such as chromatin remodelers, histone modifiers, DNA (de-)methylation enzymes, and noncoding RNAs regulate DDR signaling and DNA repair by affecting chromatin dynamics. In recent years, significant progress has been made in the understanding of plant DDR and DNA repair. SUPPRESSOR OF GAMMA RESPONSE1, RETINOBLASTOMA RELATED1 (RBR1)/E2FA, and NAC103 have been proven to be key players in the mediation of DDR signaling in plants, while plant-specific chromatin remodelers, such as DECREASED DNA METHYLATION1, contribute to chromatin dynamics for DNA repair. There is accumulating evidence that plant epigenetic modifiers are involved in DDR and DNA repair. In this review, I examine how DDR and DNA repair machineries are concertedly regulated in Arabidopsis thaliana by a variety of epigenetic modifiers directing chromatin remodeling and epigenetic modification. This review will aid in updating our knowledge on DDR and DNA repair in plants.

ATP-dependent chromatin remodeling complexes in embryonic vascular development and hypertension

Hypertension, a chronic elevation in blood pressure, is the largest single contributing factor to mortality worldwide and the most common preventable risk factor for cardiovascular disease. High blood pressure increases the risk for someone to experience a number of adverse cardiovascular events including heart failure, stroke, or aneurysm. Despite advancements in understanding factors that contribute to hypertension, the etiology remains elusive and there remains a critical need to develop innovative study approaches to develop more effective therapeutics. ATP-dependent chromatin remodelers are dynamic regulators of DNA-histone bonds and thus gene expression. The goal of this review is to highlight and summarize reports of ATP-dependent chromatin remodelers contribution to the development or maintenance of hypertension. Emerging evidence from hypertensive animal models suggests that induction of chromatin remodeler activity increases proinflammatory genes and increases blood pressure, whereas human studies demonstrate how chromatin remodelers may act as stress-response sensors to harmful physiological stimuli. Importantly, genomic studies have linked patients with hypertension to mutations in chromatin remodeler genes. Collectively, evidence linking chromatin remodelers and hypertension warrants additional research and ultimately could reveal novel therapeutic approaches for treating this complex and devastating disease.

Uncovering a New Step in Sliding Nucleosomes

Chromatin remodelers are ATP-driven motors that pump double-stranded DNA around the histone core of the nucleosome. Recent work by Chen and coworkers (Li et al., Nature, 2019 and Yan et al., Nat. Struct. Mol. Biol., 2019) has revealed an unexpected intermediate where initial translocation involves only one of the two DNA strands.

General Regulatory Factors Control the Fidelity of Transcription by Restricting Non-coding and Ectopic Initiation

The fidelity of transcription initiation is essential for accurate gene expression, but the determinants of start site selection are not fully understood. Rap1 and other general regulatory factors (GRFs) control the expression of many genes in yeast. We show that depletion of these factors induces widespread ectopic transcription initiation within promoters. This generates many novel non-coding RNAs and transcript isoforms with diverse stability, drastically altering the coding potential of the transcriptome. Ectopic transcription initiation strongly correlates with altered nucleosome positioning. We provide evidence that Rap1 can suppress ectopic initiation by a "place-holder" mechanism whereby it physically occludes inappropriate sites for pre-initiation complex formation. These results reveal an essential role for GRFs in the fidelity of transcription initiation and in the suppression of pervasive transcription, profoundly redefining current models for their function. They have important implications for the mechanism of transcription initiation and the control of gene expression.

Epigenetic Modifications in Alzheimer's Neuropathology and Therapeutics

Transcriptional activation is a highly synchronized process in eukaryotes that requires a series of cis- and trans-acting elements at promoter regions. Epigenetic modifications, such as chromatin remodeling, histone acetylation/deacetylation, and methylation, have frequently been studied with regard to transcriptional regulation/dysregulation. Recently however, it has been determined that implications in epigenetic modification seem to expand into various neurodegenerative disease mechanisms. Impaired learning and memory deterioration are cognitive dysfunctions often associated with a plethora of neurodegenerative diseases, including Alzheimer's disease. Through better understanding of the epigenetic mechanisms underlying these dysfunctions, new epigenomic therapeutic targets, such as histone deacetylases, are being explored. Here we review the intricate packaging of DNA in eukaryotic cells, and the various modifications in epigenetic mechanisms that are now linked to the neuropathology and the progression of Alzheimer's disease (AD), as well as potential therapeutic interventions.

Nutrient-Dependent Changes of Protein Palmitoylation: Impact on Nuclear Enzymes and Regulation of Gene Expression

Diet is the main environmental stimulus chronically impinging on the organism throughout the entire life. Nutrients impact cells via a plethora of mechanisms including the regulation of both protein post-translational modifications and gene expression. Palmitoylation is the most-studied protein lipidation, which consists of the attachment of a molecule of palmitic acid to residues of proteins. S-palmitoylation is a reversible cysteine modification finely regulated by palmitoyl-transferases and acyl-thioesterases that is involved in the regulation of protein trafficking and activity. Recently, several studies have demonstrated that diet-dependent molecules such as insulin and fatty acids may affect protein palmitoylation. Here, we examine the role of protein palmitoylation on the regulation of gene expression focusing on the impact of this modification on the activity of chromatin remodeler enzymes, transcription factors, and nuclear proteins. We also discuss how this physiological phenomenon may represent a pivotal mechanism underlying the impact of diet and nutrient-dependent signals on human diseases.

Epigenetic modifications-insight into oligodendrocyte lineage progression, regeneration, and disease

Myelination by oligodendrocytes in the central nervous system permits high-fidelity saltatory conduction from neuronal cell bodies to axon terminals. Dysmyelinating and demyelinating disorders impair normal nervous system functions. Consequently, an understanding of oligodendrocyte differentiation that moves beyond the genetic code into the field of epigenetics is essential. Chromatin reprogramming is critical for steering stage-specific differentiation processes during oligodendrocyte development. Fine temporal control of chromatin remodeling through ATP-dependent chromatin remodelers and sequential histone modifiers shapes a chromatin regulatory landscape conducive to oligodendrocyte fate specification, lineage differentiation, and maintenance of cell identity. In this Review, we will focus on the biological functions of ATP-dependent chromatin remodelers and histone deacetylases in myelinating oligodendrocyte development and implications for myelin regeneration in neurodegenerative diseases.

MAP3K4 Controls the Chromatin Modifier HDAC6 during Trophoblast Stem Cell Epithelial-to-Mesenchymal Transition

The first epithelial-to-mesenchymal transition (EMT) occurs in trophoblast stem (TS) cells during implantation. Inactivation of the serine/threonine kinase MAP3K4 in TS cells (TS^KI4 cells) induces an intermediate state of EMT, where cells retain stemness, lose epithelial markers, and gain mesenchymal characteristics. Investigation of relationships among MAP3K4 activity, stemness, and EMT in TS cells may reveal key regulators of EMT. Here, we show that MAP3K4 activity controls EMT through the ubiquitination and degradation of HDAC6. Loss of MAP3K4 activity in TS^KI4 cells results in elevated HDAC6 expression and the deacetylation of cytoplasmic and nuclear targets. In the nucleus, HDAC6 deacetylates the promoters of tight junction genes, promoting the dissolution of tight junctions. Importantly, HDAC6 knockdown in TS^KI4 cells restores epithelial features, including cell-cell adhesion and barrier formation. These data define a role for HDAC6 in regulating gene expression during transitions between epithelial and mesenchymal phenotypes.

Altered nucleosome positions in maize haplotypes and mutants of a subset of SWI/SNF-like proteins

Chromatin remodelers alter DNA-histone interactions in eukaryotic organisms and have been well characterized in yeast and Arabidopsis. While there are maize proteins with similar domains as known remodelers, the ability of the maize proteins to alter nucleosome position has not been reported. Mutant alleles of several maize proteins (RMR1, CHR101, CHR106, CHR127, and CHR156) with similar functional domains to known chromatin remodelers were identified. Altered gene expression of Chr101, Chr106, Chr127, and Chr156 was demonstrated in plants homozygous for the mutant alleles. These mutant genotypes were subjected to nucleosome position analysis to determine whether misregulation of putative maize chromatin proteins would lead to altered DNA-histone interactions. Nucleosome position changes were observed in plants homozygous for chr101, chr106, chr127, and chr156 mutant alleles, suggesting that CHR101, CHR106, CHR127, and CHR156 may affect chromatin structure. The role of RNA polymerases in altering DNA-histone interactions was also tested. Changes in nucleosome position were demonstrated in homozygous mop2-1 individuals. These changes were demonstrated at the b1 tandem repeats and at newly identified loci. Additionally, differential DNA-histone interactions and altered gene expression of putative chromatin remodelers were demonstrated between different maize haplotypes.

Expansion of the ISWI chromatin remodeler family with new active complexes

ISWI chromatin remodelers mobilize nucleosomes to control DNA accessibility. Complexes isolated to date pair one of six regulatory subunits with one of two highly similar ATPases. However, we find that each endogenously expressed ATPase co-purifies with every regulatory subunit, substantially increasing the diversity of ISWI complexes, and we additionally identify BAZ2B as a novel, seventh regulatory subunit. Through reconstitution of catalytically active human ISWI complexes, we demonstrate that the new interactions described here are stable and direct. Finally, we profile the nucleosome remodeling functions of the now expanded family of ISWI chromatin remodelers. By revealing the combinatorial nature of ISWI complexes, we provide a basis for better understanding ISWI function in normal settings and disease.

Constitutive turnover of histone H2A.Z at yeast promoters requires the preinitiation complex

The assembly of the preinitiation complex (PIC) occurs upstream of the +1 nucleosome which, in yeast, obstructs the transcription start site and is frequently assembled with the histone variant H2A.Z. To understand the contribution of the transcription machinery in the disassembly of the +1 H2A.Z nucleosome, conditional mutants were used to block PIC assembly. A quantitative ChIP-seq approach, which allows detection of global occupancy change, was employed to measure H2A.Z occupancy. Blocking PIC assembly resulted in promoter-specific H2A.Z accumulation, indicating that the PIC is required to evict H2A.Z. By contrast, H2A.Z eviction was unaffected upon depletion of INO80, a remodeler previously reported to displace nucleosomal H2A.Z. Robust PIC-dependent H2A.Z eviction was observed at active and infrequently transcribed genes, indicating that constitutive H2A.Z turnover is a general phenomenon. Finally, sites with strong H2A.Z turnover precisely mark transcript starts, providing a new metric for identifying cryptic and alternative sites of initiation.

DOI:http://dx.doi.org/10.7554/eLife.14243.001

To fit the genetic information of an animal, yeast or other eukaryote into cells, DNA is tightly wound around proteins called histones to form repeating units known as nucleosomes. However, this tight winding prevents proteins from accessing the DNA, and so prevents gene transcription – the first stage of producing the molecules encoded by a gene. For transcription to take place, nucleosomes at DNA sequences called promoters must be reorganized and disassembled, thereby allowing proteins to bind to and engage these sequences and to turn nearby genes on.

H2A is a histone protein that is found in the majority of nucleosomes in yeast cells. A different form of this histone – called H2A.Z – is found in nucleosomes near the promoter of almost every gene. It is thought that nucleosomes that contain H2A.Z are recognized and disassembled as the gene turns on, but it is unclear how this happens.

To investigate how H2A.Z nucleosomes are disassembled, Tramantano et al. depleted yeast cells of various proteins thought to play a role in the disassembly process. This indicated that the proteins that transcribe genes play crucial roles in the process of disassembling the H2A.Z nucleosomes, because H2A.Z accumulated at promoters in cells that are depleted of these proteins.

Further investigation revealed that disassembled H2A.Z nucleosomes are reassembled with H2A histones, before being converted back to the H2A.Z form by an enzyme called SWR1. This turnover of H2A.Z was seen at active genes and those that are infrequently transcribed, suggesting that it is a general phenomenon.

Tramantano et al. also found that the turnover rate of H2A.Z can be used to accurately predict the sites in the DNA where transcription starts. This observation could therefore help to identify previously unknown transcription start sites. Future work could address further questions about how H2A.Z nucleosomes are disassembled. For example, what is the mechanical force that drives this process? And at what step of the transcription process does it occur?

DOI:http://dx.doi.org/10.7554/eLife.14243.002

Dependence of the sperm/oocyte decision on the nucleosome remodeling factor complex was acquired during recent Caenorhabditis briggsae evolution

The major families of chromatin remodelers have been conserved throughout eukaryotic evolution. Because they play broad, pleiotropic roles in gene regulation, it was not known if their functions could change rapidly. Here, we show that major alterations in the use of chromatin remodelers are possible, because the nucleosome remodeling factor (NURF) complex has acquired a unique role in the sperm/oocyte decision of the nematode Caenorhabditis briggsae. First, lowering the activity of C. briggsae NURF-1 or ISW-1, the core components of the NURF complex, causes germ cells to become oocytes rather than sperm. This observation is based on the analysis of weak alleles and null mutations that were induced with TALENs and on RNA interference. Second, qRT-polymerase chain reaction data show that the C. briggsae NURF complex promotes the expression of Cbr-fog-1 and Cbr-fog-3, two genes that control the sperm/oocyte decision. This regulation occurs in the third larval stage and affects the expression of later spermatogenesis genes. Third, double mutants reveal that the NURF complex and the transcription factor TRA-1 act independently on Cbr-fog-1 and Cbr-fog-3. TRA-1 binds both promoters, and computer analyses predict that these binding sites are buried in nucleosomes, so we suggest that the NURF complex alters chromatin structure to allow TRA-1 access to Cbr-fog-1 and Cbr-fog-3. Finally, lowering NURF activity by mutation or RNA interference does not affect this trait in other nematodes, including the sister species C. nigoni, so it must have evolved recently. We conclude that altered chromatin remodeling could play an important role in evolutionary change.

The XNP remodeler targets dynamic chromatin in Drosophila

Heterochromatic gene silencing results from the establishment of a repressive chromatin structure over reporter genes. Gene silencing is often variegated, implying that chromatin may stochastically switch from repressive to permissive structures as cells divide. To identify remodeling enzymes involved in reorganizing heterochromatin, we tested 11 SNF2-type chromatin remodelers in Drosophila for effects on gene silencing. Overexpression of five remodelers affects gene silencing, and the most potent de-repressor is the alpha-thalassaemia mental retardation syndrome X-linked (ATRX) homolog X-linked nuclear protein (XNP). Although the mammalian ATRX protein localizes to heterochromatin, Drosophila XNP is not a general component of heterochromatin. Instead, XNP localizes to active genes and to a major focus near the heterochromatin of the X chromosome. The XNP focus corresponds to an unusual decondensed satellite DNA block, and both active genes and the XNP focus are sites of ongoing nucleosome replacement. We suggest that the XNP remodeler modulates nucleosome dynamics at its target sites to limit chromatin accessibility. Although XNP at active genes may contribute to gene silencing, we find that a single focus is present across Drosophila species and that perturbation of this site cripples heterochromatic gene silencing. Thus, the XNP focus appears to be a functional genetic element that can contribute to gene silencing throughout the nucleus.