Nucleosome Movement Research Articles

Does TFIIH move nucleosomes?

Transcription factor (TF) IIH is a factor involved in transcription, DNA repair, mitosis, and telomere stability. These functions stem from its helicase/ATPase and kinase activities. Recent reports on the structure and function of the transcription machinery, as well as chromosome compaction during mitosis, suggest that TFIIH also influences nucleosome movement, are explored here.

The AT-hook is an evolutionarily conserved auto-regulatory domain of SWI/SNF required for cell lineage priming

The SWI/SNF ATP-dependent chromatin remodeler is a master regulator of the epigenome, controlling pluripotency and differentiation. Towards the C-terminus of the catalytic subunit of SWI/SNF is a motif called the AT-hook that is evolutionary conserved. The AT-hook is present in many chromatin modifiers and generally thought to help anchor them to DNA. We observe however that the AT-hook regulates the intrinsic DNA-stimulated ATPase activity aside from promoting SWI/SNF recruitment to DNA or nucleosomes by increasing the reaction velocity a factor of 13 with no accompanying change in substrate affinity (KM). The changes in ATP hydrolysis causes an equivalent change in nucleosome movement, confirming they are tightly coupled. The catalytic subunit’s AT-hook is required in vivo for SWI/SNF remodeling activity in yeast and mouse embryonic stem cells. The AT-hook in SWI/SNF is required for transcription regulation and activation of stage-specific enhancers critical in cell lineage priming. Similarly, growth assays suggest the AT-hook is required in yeast SWI/SNF for activation of genes involved in amino acid biosynthesis and metabolizing ethanol. Our findings highlight the importance of studying SWI/SNF attenuation versus eliminating the catalytic subunit or completely shutting down its enzymatic activity.

Abstract 1137: Identification of a high selective SMARCA2 degrader which effectively suppresses the SMARCA4-deficient tumors in vitro and in vivo

Abstract The SWI/SNF chromatin-remodeling complex plays essential roles in a variety of cellular processes including differentiation, proliferation and DNA repair. Mammalian SWI/SNF complexes always contain one of two mutually exclusive and structurally highly related ATPases: SMARCA2/BRM or SMARCA4/BRG1, as its catalytic subunit, which hydrolyze ATP to drive nucleosome movement. It is reported that absence of SMARCA4 are present in several cancers and these SMARCA4 deficient cells highly depend on its paralog SMARCA2 for survival. Furthermore, concurrent inhibition of SMARCA2 and SMARCA4 is fatal to normal cells. Therefore, it holds great therapeutic promise to treat SMARCA4 deficient cancers with selective SMARCA2 degraders while leaving SMARCA4 wild type cells unaffected. Here, we report that SCR-9140 is a high selective SMARCA2 degrader which demonstrates superior anti-tumor activity and low cytotoxicity against non-malignant cells. SCR-9140 strongly degrades SMARCA2 with DC50 of <1 nM and achieves the complete protein degradation (Dmax of >95%) and also shows 100-fold degradation selectivity over SMARCA4 with DC50 of around 100 nM and Dmax of <60% in HiBiT knock-in SW1573 cells. Across a panel of SMARCA4 deficient tumor cell lines, SCR-9140 potently inhibits proliferation with IC50 values ranging from 1 to 50 nM. Due to the high selectivity over SMARCA4, SCR-9140 neither inhibits proliferation of SW1573 (SMARCA2-WT SMARCA4-WT) cells nor NCI-H1581 (SMARCA2-DEL SMARCA4-DEL) cells with IC50s of over 2000 nM. The degraders with poor selectivity of SMARCA2/4 (DC50 ratio: < 10 folds, Dmax of SMARCA4 > 80%) strongly inhibit the SW1573 cells proliferation with IC50s < 100nM, while there are no significant effects on the NCI-H1581 cells ascribing to the absence of targets. Furthermore, SCR-9140 demonstrates weak cytotoxicity on non-malignant cells, such as WI-38 (human lung fibroblast cells), THLE-2 (human liver cells), and human PBMC, rendering it the greater therapeutic window. In a CDX model of NSCLC, SCR-9140 effectively inhibits tumor growth and TGI correlates well with SMARCA2 degradation in tumors. In conclusion, our potent and selective SMARCA2 degrader, SCR-9140 induces strong synthetic lethality in SMARCA4 deficient cells in vitro and in vivo and shows great safety margin in normal cells. Citation Format: Feng Zhou, Zhiyong Yu, Ruonan Chen, Feng Tang, Peng Gu, Wenqing Yang, Liting Xue, Ping Chen, Renhong Tang. Identification of a high selective SMARCA2 degrader which effectively suppresses the SMARCA4-deficient tumors in vitro and in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1137.

Dimensional Reduction for Single-Molecule Imaging of DNA and Nucleosome Condensation by Polyamines, HP1α and Ki-67.

Macromolecules organize themselves into discrete membrane-less compartments. Mounting evidence has suggested that nucleosomes as well as DNA itself can undergo clustering or condensation to regulate genomic activity. Current in vitro condensation studies provide insight into the physical properties of condensates, such as surface tension and diffusion. However, methods that provide the resolution needed for complex kinetic studies of multicomponent condensation are desired. Here, we use a supported lipid bilayer platform in tandem with total internal reflection microscopy to observe the two-dimensional movement of DNA and nucleosomes at the single-molecule resolution. This dimensional reduction from three-dimensional studies allows us to observe the initial condensation events and dissolution of these early condensates in the presence of physiological condensing agents. Using polyamines, we observed that the initial condensation happens on a time scale of minutes while dissolution occurs within seconds upon charge inversion. Polyamine valency, DNA length, and GC content affect the threshold polyamine concentration for condensation. Protein-based nucleosome condensing agents, HP1α and Ki-67, have much lower threshold concentrations for condensation than charge-based condensing agents, with Ki-67 being the most effective, requiring as low as 100 pM for nucleosome condensation. In addition, we did not observe condensate dissolution even at the highest concentrations of HP1α and Ki-67 tested. We also introduce a two-color imaging scheme where nucleosomes of high density labeled in one color are used to demarcate condensate boundaries and identical nucleosomes of another color at low density can be tracked relative to the boundaries after Ki-67-mediated condensation. Our platform should enable the ultimate resolution of single molecules in condensation dynamics studies of chromatin components under defined physicochemical conditions.

Perspectives on ATP-dependent chromatin remodeling.

Nucleosomes are intrinsically immobile, and thus, ATP-dependent chromatin remodeling factors are needed to alter nucleosomes to facilitate DNA-directed processes such as transcription. More generally, chromatin remodeling factors mediate chromatin dynamics, which encompasses nucleosome assembly, movement, and disruption as well as histone exchange. Here, I present selected thoughts and perspectives on the past, present, and future of these fascinating ATP-driven motor proteins.

Histone protein surface accessibility dictates direction of RSC-dependent nucleosome mobilization.

Chromatin remodeling enzymes use energy derived from ATP hydrolysis to mobilize nucleosomes and alter their structure to facilitate DNA access. The Remodels the Structure of Chromatin (RSC) complex has been extensively studied, yet aspects of how this complex functionally interacts with nucleosomes remain unclear. We introduce a steric mapping approach to determine how RSC activity depends on interaction with specific surfaces within the nucleosome. We find that blocking SHL + 4.5/–4.5 via streptavidin binding to the H2A N-terminal tail domains results in inhibition of RSC nucleosome mobilization. However, restriction enzyme assays indicate that remodeling-dependent exposure of an internal DNA site near the nucleosome dyad is not affected. In contrast, occlusion of both protein faces of the nucleosome by streptavidin attachment near the acidic patch completely blocks both remodeling-dependent nucleosome mobilization and internal DNA site exposure. However, we observed partial inhibition when only one protein surface is occluded, consistent with abrogation of one of two productive RSC binding orientations. Our results indicate that nucleosome mobilization requires RSC access to the trailing but not the leading protein surface, and reveals a mechanism by which RSC and related complexes may drive unidirectional movement of nucleosomes to regulate cis-acting DNA sequences in vivo.

Direct Observation of Dynamic Interactions between Orientation-Controlled Nucleosomes in a DNA Origami Frame.

The nucleosome is one of the most fundamental units involved in gene expression and consequent cell development, differentiation, and expression of cell functions. We report here a method to place reconstituted nucleosomes into a DNA origami frame for direct observation using high-speed atomic-force microscopy (HS-AFM). By using this method, multiple nucleosomes can be incorporated into a DNA origami frame and real-time movement of nucleosomes can be visualized. The arrangement and conformation of nucleosomes and the distance between two nucleosomes can be designed and controlled. In addition, four nucleosomes can be placed in a DNA frame. Multiple nucleosomes were well accessible in each conformation. Dynamic movement of the individual nucleosomes were precisely monitored in the DNA frame, and their assembly and interaction were directly observed. Neither mica surface modification nor chemical fixation of nucleosomes is used in this method, meaning that the DNA frame not only holds nucleosomes, but also retains their natural state. This method offers a promising platform for investigating nucleosome interactions and for studying chromatin structure.

Nucleosome movement analysis based on second-order information entropy and density functional theory

Dynamics of +1 and -1 nucleosomes near TSS of yeast chromosome 2 were analyzed by using second-order information entropy and density functional theory method. Second-order information entropy can measure the interaction intensity between nucleosome sequences and nucleosome histones based on the intensity of base association. In addition, density functional theory method can be used to obtain the global interaction intensity between nucleosome sequences and nucleosome histones based on energy state size and active or non-active state of binucleoside pairs. Our results showed asymmetry of interaction intensity on both sides of the nucleosome central site, and that +1 nucleosomes tend to move toward the 5'-end and -1 nucleosomes tend to move toward the 3'-end. Under the dynamic balance of nucleosome movement, in roder to shut down gene transcription, +1 and -1 nucleosomes will cover TSS. If the dynamic balance is destroyed, +1 and -1 nucleosomes stay away from each other to expose TSS to restart gene transcription. The movement trend of +1 and -1 nucleosomes coincides with the biological mechanism of gene transcription and non-transcription, and the nucleosome sequences contain the dynamic information of nucleosome movement, which provides effective technical support for the study of gene transcription regulation mechanism.

Opposing chromatin remodelers control transcription initiation frequency and start site selection.

Precise nucleosome organization at eukaryotic promoters is thought to be generated by multiple chromatin remodeler (CR) enzymes and to affect transcription initiation. Using an integrated analysis of chromatin remodeler binding and nucleosome occupancy following rapid remodeler depletion, we investigated the interplay between these enzymes and their impact on transcription in yeast. We show that many promoters are affected by multiple CRs that operate in concert or in opposition to position the key transcription start site (TSS)-associated +1 nucleosome. We also show that nucleosome movement after CR inactivation usually results from the activity of another CR and that in the absence of any remodeling activity, +1 nucleosomes largely maintain their positions. Finally, we present functional assays suggesting that +1 nucleosome positioning often reflects a trade-off between maximizing RNA polymerase recruitment and minimizing transcription initiation at incorrect sites. Our results provide a detailed picture of fundamental mechanisms linking promoter nucleosome architecture to transcription initiation.

Histone Octamer Structure Is Altered Early in ISW2 ATP-Dependent Nucleosome Remodeling

Nucleosomes are the fundamental building blocks ofchromatin that regulate DNA access and are composed of histone octamers. ATP-dependent chromatin remodelers like ISW2 regulate chromatin access by translationally moving nucleosomes to different DNA regions. We find that histone octamers are more pliable than previously assumed and distorted by ISW2 early in remodeling before DNA enters nucleosomes and the ATPase motor moves processively on nucleosomal DNA. Uncoupling the ATPase activity of ISW2 from nucleosome movement with deletion of the SANT domain from the C terminus of the Isw2 catalytic subunit traps remodeling intermediates in which the histone octamer structure is changed. We find restricting histone movement by chemical crosslinking also traps remodeling intermediates resembling those seen early in ISW2 remodeling with loss of the SANT domain. Other evidence shows histone octamers are intrinsically prone to changing their conformation and can be distorted merely by H3-H4 tetramer disulfide crosslinking.

The base pair-scale diffusion of nucleosomes modulates binding of transcription factors

The structure of promoter chromatin determines the ability of transcription factors (TFs) to bind to DNA and therefore has a profound effect on the expression levels of genes. However, the role of spontaneous nucleosome movements in this process is not fully understood. Here, we developed a single-molecule optical tweezers assay capable of simultaneously characterizing the base pair-scale diffusion of a nucleosome on DNA and the binding of a TF, using the luteinizing hormone β subunit gene (Lhb) promoter and Egr-1 as a model system. Our results demonstrate that nucleosomes undergo confined diffusion, and that the incorporation of the histone variant H2A.Z serves to partially relieve this confinement, inducing a different type of nucleosome repositioning. The increase in diffusion leads to exposure of a TF's binding site and facilitates its association with the DNA, which, in turn, biases the subsequent movement of the nucleosome. Our findings suggest the use of mobile nucleosomes as a general transcriptional regulatory mechanism.

Dynamic Nucleosome Movement Provides Structural Information of Topological Chromatin Domains in Living Human Cells.

The mammalian genome is organized into submegabase-sized chromatin domains (CDs) including topologically associating domains, which have been identified using chromosome conformation capture-based methods. Single-nucleosome imaging in living mammalian cells has revealed subdiffusively dynamic nucleosome movement. It is unclear how single nucleosomes within CDs fluctuate and how the CD structure reflects the nucleosome movement. Here, we present a polymer model wherein CDs are characterized by fractal dimensions and the nucleosome fibers fluctuate in a viscoelastic medium with memory. We analytically show that the mean-squared displacement (MSD) of nucleosome fluctuations within CDs is subdiffusive. The diffusion coefficient and the subdiffusive exponent depend on the structural information of CDs. This analytical result enabled us to extract information from the single-nucleosome imaging data for HeLa cells. Our observation that the MSD is lower at the nuclear periphery region than the interior region indicates that CDs in the heterochromatin-rich nuclear periphery region are more compact than those in the euchromatin-rich interior region with respect to the fractal dimensions as well as the size. Finally, we evaluated that the average size of CDs is in the range of 100–500 nm and that the relaxation time of nucleosome movement within CDs is a few seconds. Our results provide physical and dynamic insights into the genome architecture in living cells.

Assembly of the Arp5 (Actin-related Protein) Subunit Involved in Distinct INO80 Chromatin Remodeling Activities

ATP-dependent chromatin remodeling, which repositions and restructures nucleosomes, is essential to all DNA-templated processes. The INO80 chromatin remodeling complex is an evolutionarily conserved complex involved in diverse cellular processes, including transcription, DNA repair, and replication. The functional diversity of the INO80 complex can, in part, be attributed to specialized activities of distinct subunits that compose the complex. Furthermore, structural analyses have identified biochemically discrete subunit modules that assemble along the Ino80 ATPase scaffold. Of particular interest is the Saccharomyces cerevisiae Arp5-Ies6 module located proximal to the Ino80 ATPase and the Rvb1-Rvb2 helicase module needed for INO80-mediated in vitro activity. In this study we demonstrate that the previously uncharacterized Ies2 subunit is required for Arp5-Ies6 association with the catalytic components of the INO80 complex. In addition, Arp5-Ies6 module assembly with the INO80 complex is dependent on distinct conserved domains within Arp5, Ies6, and Ino80, including the spacer region within the Ino80 ATPase domain. Arp5-Ies6 interacts with chromatin via assembly with the INO80 complex, as IES2 and INO80 deletion results in loss of Arp5-Ies6 chromatin association. Interestingly, ectopic addition of the wild-type Arp5-Ies6 module stimulates INO80-mediated ATP hydrolysis and nucleosome sliding in vitro. However, the addition of mutant Arp5 lacking unique insertion domains facilitates ATP hydrolysis in the absence of nucleosome sliding. Collectively, these results define the requirements of Arp5-Ies6 assembly, which are needed to couple ATP hydrolysis to productive nucleosome movement.

The Chd1 Chromatin Remodeler can Sense a Protein Bound at the Edge of the Nucleosome and is Sensitive to DNA Unwrapping

Chromatin remodelers are essential for establishing and maintaining the placement of nucleosomes along genomic DNA. However, how remodelers respond to transcription factors and other bound factors that might influence chromatin organization is poorly understood. Here we use the Lac repressor to investigate how the Chd1 remodeler responds to a protein bound to the edge of a nucleosome. We found that Lac repressor effectively provided a barrier for nucleosome sliding by Chd1. This barrier did not require an absolute block in sliding, but instead was achieved through the bidirectional movement of nucleosomes, with a higher preference for sliding nucleosomes away from occupied Lac repressor sites. The presence of Lac repressor did not markedly diminish the affinity of Chd1 for nucleosomes, suggesting that Lac repressor is sensed after nucleosome sliding has been initiated. Nucleosome sliding rates were also reduced by DNA unwrapping, suggesting a mechanism by which Chd1 may indirectly sense factors bound at the edge of the nucleosome.

Using FRET to Monitor Nucleosome Movement by the Chd1 Chromatin Remodeler

The chromatin remodeler, Chd1, creates regularly spaced arrays of nucleosomes. This spacing behavior correlates with a preference for shifting mononucleosomes away from DNA ends. However, the mechanism by which Chd1 senses extranucleosomal DNA is not understood. To investigate how sliding activity may be regulated, we are adopting a FRET-based nucleosome sliding assay to measure the kinetics of Chd1 directed nucleosome sliding. By labeling the end of the DNA with Cy3 and the nucleosome with Cy5, we should be able to monitor nucleosome sliding as the loss of quenching of Cy3 by Cy5 as the dye pair separates. Using a similar system, others have observed single or double exponential increases in Cy3 fluorescence corresponding to nucleosome sliding. Curiously, with Chd1 we observe an initial increasing phase followed by a pause and then a second increasing phase. In addition, after an initial drop in Cy5 fluorescence expected from FRET, a later phase shows an increase in Cy5 fluorescence above its initial level. These results suggest that not all of the fluorescence signals are reporting on DNA movement alone, and are likely being influenced by Chd1 binding to the labeled nucleosome. We are investigating other dye pairs and nucleosome constructs to isolate nucleosome sliding activity from binding or other molecular interactions that may occur during the reaction.

Poly-dA:dT tracts form an in vivo nucleosomal turnstile.

Nucleosomes regulate many DNA-dependent processes by controlling the accessibility of DNA, and DNA sequences such as the poly-dA:dT element are known to affect nucleosome binding. We demonstrate that poly-dA:dT tracts form an asymmetric barrier to nucleosome movement in vivo, mediated by ATP-dependent chromatin remodelers. We theorize that nucleosome transit over poly-A elements is more energetically favourable in one direction, leading to an asymmetric arrangement of nucleosomes around these sequences. We demonstrate that different arrangements of poly-A and poly-T tracts result in very different outcomes for nucleosome occupancy in yeast, mouse, and human, and show that yeast takes advantage of this phenomenon in its promoter architecture.

ISWI chromatin remodeling: one primary actor or a coordinated effort?

The ISWI family of ATP-dependent chromatin remodelers regulates transcription of coding and noncoding RNA by mobilizing nucleosomes and controlling the length of linker DNA separating nucleosomes (spacing). Nucleosome movement is tightly coupled to the DNA translocation activity of the helicase domain in the catalytic subunit. There may be other domains besides the helicase domain needed to move DNA in and out of nucleosomes. The C terminus of the ISWI catalytic subunit with the conserved HAND, SANT, and SLIDE domains may be involved in nucleosome spacing. There are several models of how the C terminus may facilitate in ISWI remodeling such as regulating the activity of the helicase domain and causing the helicase domain to translocate more efficiently on DNA or to enhance its selectivity for nucleosomes. Another possibility is that domains like SLIDE promote linker DNA entering into nucleosomes in a coordinated manner with the helicase domain.

Mechanism of Nucleosome Remodeling by INO80 from S. Cerevisiae

Chromatin remodeling enzymes use the energy of ATP to alter chromatin structure by disrupting DNA-histone contacts. These enzymes play a major role in the regulation of all nuclear processes, including transcription, replication, and DNA repair. A major goal in chromatin biology is to understand how the basic biochemical mechanisms of these chromatin remodelers relate to, or even dictate their regulatory roles in vivo. INO80 is a 15-subunit chromatin remodeling complex from S. cerevisiae. INO80 is involved in the transcription of over 1000 yeast genes, and has additional roles in processes that promote genomic stability such as DNA repair. Unlike most remodeling enzymes INO80 contains several active ATPases. In addition to the canonical remodeling ATPase, Ino80p, the complex also contains two hexameric AAA+ ATPases, Rvb1 and Rvb2. Our work seeks to understand the mechanism of nucleosome remodeling by INO80 as well as the roles that Rvb1 and Rvb2 have in this process. We employ several types of remodeling and ATPase assays to gather information about the kinetics of the different steps in remodeling. We have discovered that nucleosome movement by INO80 involves a fast step that is detectable by FRET, followed by a slow step that is detectable by a change in migration of the nucleosome on a native gel. Both steps are greatly affected by the length of extra-nucleosomal DNA. Interestingly, this length sensing behavior by INO80 is not strongly coupled to ATPase activity of the complex.

Nucleosome sliding mechanisms: new twists in a looped history

Nucleosomes, the basic organizational units of chromatin, package and regulate eukaryotic genomes. ATP-dependent nucleosome-remodeling factors endow chromatin with structural flexibility by promoting assembly or disruption of nucleosomes and the exchange of histone variants. Furthermore, most remodeling factors induce nucleosome movements through sliding of histone octamers on DNA. We summarize recent progress toward unraveling the basic nucleosome sliding mechanism and the interplay of the remodelers' DNA translocases with accessory domains. Such domains optimize and regulate the basic sliding reaction and exploit sliding to achieve diverse structural effects, such as nucleosome positioning or eviction, or the regular spacing of nucleosomes in chromatin.

Nucleosome mapping across the CFTR locus identifies novel regulatory factors.

Nucleosome positioning on the chromatin strand plays a critical role in regulating accessibility of DNA to transcription factors and chromatin modifying enzymes. Hence, detailed information on nucleosome depletion or movement at cis-acting regulatory elements has the potential to identify predicted binding sites for trans-acting factors. Using a novel method based on enrichment of mononucleosomal DNA by bacterial artificial chromosome hybridization, we mapped nucleosome positions by deep sequencing across 250 kb, encompassing the cystic fibrosis transmembrane conductance regulator (CFTR) gene. CFTR shows tight tissue-specific regulation of expression, which is largely determined by cis-regulatory elements that lie outside the gene promoter. Although multiple elements are known, the repertoire of transcription factors that interact with these sites to activate or repress CFTR expression remains incomplete. Here, we show that specific nucleosome depletion corresponds to well-characterized binding sites for known trans-acting factors, including hepatocyte nuclear factor 1, Forkhead box A1 and CCCTC-binding factor. Moreover, the cell-type selective nucleosome positioning is effective in predicting binding sites for novel interacting factors, such as BAF155. Finally, we identify transcription factor binding sites that are overrepresented in regions where nucleosomes are depleted in a cell-specific manner. This approach recognizes the glucocorticoid receptor as a novel trans-acting factor that regulates CFTR expression in vivo.