Cohesin and CTCF control chromosomal contact dynamics in living cells [capture C]
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ABSTRACT: Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.
Project description:Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.
Project description:Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.
Project description:Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.
Project description:Cohesin-mediated DNA loop extrusion enables gene regulation by distal enhancers through the establishment of chromosome structure and long-range enhancer-promoter interactions. The best characterized cohesin-related structures, such as topologically associating domains (TADs) anchored at convergent CTCF binding sites, represent static conformations. Consequently, loop extrusion dynamics remain poorly understood. To better characterize static and dynamically extruding chromatin loop structures, we use MNase-based 3D genome assays to simultaneously determine CTCF and cohesin localization as well as the 3D contacts they mediate. Here we present CTCF Analyzer (with) Multinomial Estimation (CAMEL), a tool that identifies CTCF footprints at near base-pair resolution in CTCF MNase HiChiP. We also use Region Capture Micro-C to identify a CTCF-adjacent footprint that is attributed to cohesin occupancy. We leverage this substantial advance in resolution to determine that the fully extruded (CTCF-CTCF loop) state is rare genome-wide with locus-specific variation from ~1-10%. We further investigate the impact of chromatin state on loop extrusion dynamics and find that active regulatory elements impede cohesin extrusion. These findings support a model of topological regulation whereby the transient, partially extruded state facilitates enhancer-promoter contacts that can regulate transcription.
Project description:Cohesin-mediated DNA loop extrusion enables gene regulation by distal enhancers through the establishment of chromosome structure and long-range enhancer-promoter interactions. The best characterized cohesin-related structures, such as topologically associating domains (TADs) anchored at convergent CTCF binding sites, represent static conformations. Consequently, loop extrusion dynamics remain poorly understood. To better characterize static and dynamically extruding chromatin loop structures, we use MNase-based 3D genome assays to simultaneously determine CTCF and cohesin localization as well as the 3D contacts they mediate. Here we present CTCF Analyzer (with) Multinomial Estimation (CAMEL), a tool that identifies CTCF footprints at near base-pair resolution in CTCF MNase HiChiP. We also use Region Capture Micro-C to identify a CTCF-adjacent footprint that is attributed to cohesin occupancy. We leverage this substantial advance in resolution to determine that the fully extruded (CTCF-CTCF loop) state is rare genome-wide with locus-specific variation from ~1-10%. We further investigate the impact of chromatin state on loop extrusion dynamics and find that active regulatory elements impede cohesin extrusion. These findings support a model of topological regulation whereby the transient, partially extruded state facilitates enhancer-promoter contacts that can regulate transcription.
Project description:Enhancers and promoters predominantly interact within large-scale topologically associating domains (TADs), which are formed by loop extrusion mediated by cohesin and CTCF. However, it is unclear whether complex chromatin structures exist at sub-kilobase-scale and to what extent fine-scale regulatory interactions depend on loop extrusion. To address these questions, we present an MNase-based chromosome conformation capture (3C) approach, which has enabled us to generate the most detailed local interaction data to date and precisely investigate the effects of cohesin and CTCF depletion on chromatin architecture. Our data reveal that cis-regulatory elements have distinct internal nano-scale structures, within which local insulation is dependent on CTCF, but which are independent of cohesin. In contrast, we find that depletion of cohesin causes a subtle reduction in longer-range enhancer-promoter interactions and that CTCF depletion can cause rewiring of regulatory contacts. Together, our data show that loop extrusion is not essential for enhancer-promoter interactions, but contributes to their robustness and specificity and to precise regulation of gene expression.
Project description:Enhancers are tissue-specific regulatory DNA elements that can activate transcription of genes over distance. Their target genes most often locate in the same topologically associating domain (TAD). TADs are structural entities within chromosomes in which cohesin DNA loop extrusion supports intra-domain DNA contacts and CTCF-bound boundaries serve to halt and concentrate paralogous genes. Enhancers shared by multiple unrelated genes may be more common than those acting on paralogous gene clusters, but are underexplored. Here, we analyzed the interplay between an enhancer and two distal, functionally unrelated, genes residing at opposite domain boundaries. The enhancer stimulated boundary and gene-gene contacts and required cohesin as well as two intact domain boundaries to support gene activation. The two genes preferentially transcribed when spatially together, but did not rely on each other’s transcription, nor showed gene competition. Placing them and the enhancer in smaller domains caused their upregulation. Domain boundaries, therefore, can support long-range enhancer-promoter communication inside domains. Boundary proximity thereby matters as the distal enhancer functions more effectively inside smaller domains. We propose that smaller domains have more concentrated cohesin loop extrusion activity to facilitate enhancer action over distance.
Project description:Enhancers are tissue-specific regulatory DNA elements that can activate transcription of genes over distance. Their target genes most often locate in the same topologically associating domain (TAD). TADs are structural entities within chromosomes in which cohesin DNA loop extrusion supports intra-domain DNA contacts and CTCF-bound boundaries serve to halt and concentrate paralogous genes. Enhancers shared by multiple unrelated genes may be more common than those acting on paralogous gene clusters, but are underexplored. Here, we analyzed the interplay between an enhancer and two distal, functionally unrelated, genes residing at opposite domain boundaries. The enhancer stimulated boundary and gene-gene contacts and required cohesin as well as two intact domain boundaries to support gene activation. The two genes preferentially transcribed when spatially together, but did not rely on each other’s transcription, nor showed gene competition. Placing them and the enhancer in smaller domains caused their upregulation. Domain boundaries, therefore, can support long-range enhancer-promoter communication inside domains. Boundary proximity thereby matters as the distal enhancer functions more effectively inside smaller domains. We propose that smaller domains have more concentrated cohesin loop extrusion activity to facilitate enhancer action over distance.
Project description:Enhancers are tissue-specific regulatory DNA elements that can activate transcription of genes over distance. Their target genes most often locate in the same topologically associating domain (TAD). TADs are structural entities within chromosomes in which cohesin DNA loop extrusion supports intra-domain DNA contacts and CTCF-bound boundaries serve to halt and concentrate paralogous genes. Enhancers shared by multiple unrelated genes may be more common than those acting on paralogous gene clusters, but are underexplored. Here, we analyzed the interplay between an enhancer and two distal, functionally unrelated, genes residing at opposite domain boundaries. The enhancer stimulated boundary and gene-gene contacts and required cohesin as well as two intact domain boundaries to support gene activation. The two genes preferentially transcribed when spatially together, but did not rely on each other’s transcription, nor showed gene competition. Placing them and the enhancer in smaller domains caused their upregulation. Domain boundaries, therefore, can support long-range enhancer-promoter communication inside domains. Boundary proximity thereby matters as the distal enhancer functions more effectively inside smaller domains. We propose that smaller domains have more concentrated cohesin loop extrusion activity to facilitate enhancer action over distance.
Project description:Mammalian genomes are folded by the distinct actions of SMC complexes which include the chromatin loop-extruding cohesin, the sister-chromatid cohesive cohesin, and the mitotic chromosome-associated condensins 1-3. While these complexes function at different stages of the cell cycle, they co-exist on chromatin during the G2/M-phase transition, when genome structure undergoes a dramatic reorganization 1,2. Yet, how distinct SMC complexes affect each other and how their mutual interplay orchestrates the dynamic folding of 3D genome remains elusive. Here, we engineered all possible cohesin/condensin configurations on mitotic chromosomes to delineate the concerted, mutually influential action of SMC complexes. We find that: (i) Condensin disrupts extrusive-cohesin binding at CTCF sites, thereby promoting the disassembly of interphase TADs and loops during mitotic progression. Conversely, extrusive-cohesin impedes condensin mediated mitotic chromosome spiralization. (ii) Condensin diminishes cohesive-cohesin peaks and, conversely, cohesive-cohesin antagonizes condensin-mediated mitotic chromosome longitudinal shortening. Co-presence of extrusive- and cohesive-cohesin synergizes these effects and dramatically inhibits mitotic chromosome condensation. (iii) Extrusive-cohesin positions cohesive-cohesin at CTCF binding sites. However, cohesive-cohesin by itself cannot be arrested by CTCF molecules, is insufficient to establish TADs or loops and lacks loop extrusion capacity, implying non-overlapping function with extrusive-cohesin. (iv) Cohesive-cohesin restricts extrusive-cohesin mediated chromatin loop expansion. Collectively, our data describe a comprehensive three-way interplay among major SMC complexes that dynamically sculpts chromatin architecture during cell cycle progression.