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Sensitivity of cohesin-chromatin association to high salt treatment corroborates non-topological mode of loop extrusion

ABSTRACT: Cohesin is a key organizer of chromatin folding in eukaryotic cells. Two basic activities of this ring-shaped protein complex are maintenance of sister chromatid cohesion and establishment of long-range DNA-DNA interactions through the process of loop extrusion. Though basic principles of both cohesion and loop extrusion have been described we still do not understand several crucial mechanistic details. One of such unresolved issues is the question of whether a cohesin ring topologically embraces DNA string(s) during loop extrusion. Here we show that cohesin complexes residing on CTCF-occupied genomic sites in mammalian cells do not interact with DNA topologically. We assessed stability of cohesin-dependent loops and cohesin association with chromatin in high ionic strength conditions in G1-synchronised HeLa cells. We found that increased salt concentration completely displaces cohesin from those genomic regions which correspond to CTCF-defined loop anchors. Unsurprisingly, CTCF-anchored cohesin loops also dissipate in these conditions. As topologically-engaged cohesin is considered to be salt-resistant, our data corroborate a non-topological model of loop extrusion.

ORGANISM(S): Homo sapiens

PROVIDER: GSE166387 | GEO | 2021/07/20

REPOSITORIES: GEO

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Cohesin mediates DNA loop extrusion and sister chromatid cohesion by distinct mechanisms [Hi-C]

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MCM complexes are barriers that restrict cohesin-mediated loop extrusion [RNA-Seq]

Project description:Eukaryotic genomes are compacted into loops and topologically associating domains (TADs), which contribute to transcription, recombination and genomic stability. Cohesin extrudes DNA into loops that are thought to lengthen until CTCF boundaries are encountered. Little is known about whether loop extrusion is impeded by DNA-bound machines. Here we show that the minichromosome maintenance (MCM) complex is a barrier that restricts loop extrusion in G1 phase. Single-nucleus Hi-C of mouse zygotes revealed that MCM loading reduces CTCF-anchored loops and decreases TAD boundary insulation, suggesting loop extrusion is impeded before reaching CTCF. This effect extends to HCT116 cells, where MCMs affect the number of CTCF-anchored loops and gene expression. Simulations suggest that MCMs are abundant, randomly positioned, partially permeable barriers. Single-molecule imaging shows that MCMs are physical barriers that frequently constrain cohesin translocation in vitro. Remarkably, chimaeric yeast MCMs harbouring a cohesin-interaction motif from human MCM3 induce cohesin pausing, suggesting that MCMs are “active” barriers with binding sites. These findings raise the possibility that cohesin can arrive by loop extrusion at MCMs, which determine the genomic sites at which sister chromatid cohesion is established. Based on in vivo, in silico and in vitro data, we conclude that distinct loop extrusion barriers shape the 3D genome.

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Response of cohesin STAG1 and STAG2 to reduction of NIPBL levels [Hi-C]

Project description:Cohesin mediates sister chromatid cohesion and organizes the genome through the formation of chromatin loops. Two versions of the complex carrying either STAG1 or STAG2 show overlapping and specific functions and both are required to fulfill embryonic development. Cohesin-STAG1 displays longer residence time on chromatin that depends on CTCF and ESCO1 and establishes longer, long-lived chromatin loops together with CTCF. Cohesin-STAG2 shows a preferential interaction with WAPL and mediates shorter loops involved in tissue-specific transcription independently of CTCF. Here we show that the two variants respond in opposite ways to knock down of NIPBL, the putative cohesin loader that is also essential for loop extrusion. Cohesin-STAG1 levels increase on chromatin under this condition and the complex accumulates further at CTCF positions while cohesin-STAG2 is diminished genome-wide. Our data support a model in which NIPBL is not required for association of cohesin with chromatin but it is for loop extrusion, which in turn facilitates stabilization of cohesin-STAG2 at CTCF positions after being loaded elsewhere. In contrast, cohesin-STAG1 is preferentially loaded at CTCF sites independently of NIPBL. Nevertheless, loop formation by these chromatin-bound complexes is impaired and gene expression is severely affected, resembling alterations in Cornelia de Lange patients

2023-01-30 | GSE207115 | GEO

Response of cohesin STAG1 and STAG2 to reduction of NIPBL levels [ChIP-Seq]

2023-01-30 | GSE207113 | GEO

Response of cohesin STAG1 and STAG2 to reduction of NIPBL levels [RNA-Seq]

2023-01-30 | GSE207110 | GEO

Chromatin jets define the properties of cohesin-driven in vivo loop extrusion

Project description:Complex genomes show intricate organization in three-dimensional (3D) nuclear space. Current models posit that cohesin extrudes loops to form self-interacting domains delimited by the DNA binding protein CTCF. Here, we describe and quantitatively characterize cohesin-propelled, jet-like chromatin contacts as landmarks of loop extrusion in quiescent mammalian lymphocytes. Experimental observations and polymer simulations indicate that narrow origins of loop extrusion favor jet formation. Unless constrained by CTCF, jets propagate symmetrically for 1-2 Mb, providing an estimate for the range of in vivo loop extrusion. Asymmetric CTCF binding deflects the angle of jet propagation as experimental evidence that cohesin-mediated loop extrusion can switch from bi- to unidirectional and is controlled independently in both directions. These data offer new insights into the physiological behavior of in vivo cohesin-mediated loop extrusion and further our understanding of the principles that underlie genome organization.

2022-08-15 | GSE199059 | GEO

Topologically associating domains and chromatin loops depend on cohesin and are regulated by CTCF, WAPL and PDS5 proteins

Project description:Mammalian genomes are organized into compartments, topologically-associating domains (TADs) and loops to facilitate gene regulation and other chromosomal functions. Compartments are formed by nucleosomal interactions, but how TADs and loops are generated is unknown. It has been proposed that cohesin forms these structures by extruding loops until it encounters CTCF, but direct evidence for this hypothesis is missing. Here we show that cohesin suppresses compartments but is essential for TADs and loops, that CTCF defines their boundaries, and that WAPL and its PDS5 binding partners control the length of chromatin loops. In the absence of WAPL and PDS5 proteins, cohesin passes CTCF sites with increased frequency, forms extended chromatin loops, accumulates in axial chromosomal positions (vermicelli) and condenses chromosomes to an extent normally only seen in mitosis. These results show that cohesin has an essential genome-wide function in mediating long-range chromatin interactions and support the hypothesis that cohesin creates these by loop extrusion, until it is delayed by CTCF in a manner dependent on PDS5 proteins, or until it is released from DNA by WAPL.

2017-12-07 | GSE102884 | GEO

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