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Characterization of induced cohesin loop extrusion trajectories in living cells [4C-seq]

ABSTRACT: The cohesin complex shapes chromosomes by DNA loop extrusion, but individual extrusion trajectories were so far unappreciable in vivo. Here, we developed and validated TArgeted Cohesin Loader (TACL), a system enabling the strong activation of anchored loop extrusion from dozens of defined genomic sites in living cells. Studying their individual loop extrusion trajectories revealed that extruding cohesin-STAG2 stops not only at domain boundaries but at all flanking CTCF sites, engaging them in a complex transient looping network that supports intradomain contacts. Cohesin-STAG1 cannot associate with weak CTCF binding sites and fails to similarly support intradomain interactions. NIPBL-MAU2 remains associated with cohesin when stalled at looping CTCF sites, suggesting these factors may also be required for loop stabilization. TACL induces cohesin traffic jams and illegal loops with divergent CTCF sites, demonstrating that stalled cohesin can block extruding cohesin in vivo. Genes exposed to TACL-induced loop extrusion were collectively hindered in transcription and the underlying chromatin altered its accessibility and reduced its H3K27ac marks. Thus, by enabling the study of individual loop extrusion trajectories in vivo, we could assign new functions to players, identify new looping networks and uncover an interplay between loop extrusion, gene transcription and chromatin composition.

ORGANISM(S): Homo sapiens

PROVIDER: GSE278982 | GEO | 2025/06/26

REPOSITORIES: GEO

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Project description:Cohesin organizes mammalian interphase chromosomes by reeling chromatin fibers into dynamic loops. "Loop extrusion" is obstructed when cohesin encounters a properly oriented CTCF protein. It has been proposed that transcription relocalizes or interferes with cohesin, and that active transcription start sites function as cohesin loading sites, but how these effects, and transcription in general, shape chromatin is unknown. To determine whether transcription can modulate loop extrusion, we studied cells in which the primary extrusion barriers could be removed by CTCF depletion and cohesin’s residence time and abundance on chromatin could be increased by Wapl knockout. We found evidence that transcription directly interacts with loop extrusion through a novel "moving barrier" mechanism, but not by loading cohesin at active promoters. Hi-C experiments showed intricate, cohesin-dependent genomic contact patterns near actively transcribed genes, and in CTCF-Wapl double knockout (DKO) cells, genomic contacts were enriched between sites of transcription-driven cohesin localization ("cohesin islands"). Similar patterns also emerged in polymer simulations in which transcribing RNA polymerases (RNAPs) acted as "moving barriers" by impeding, slowing, or pushing loop-extruding cohesins. The model predicts that cohesin does not load preferentially at promoters and instead accumulates at TSSs due to the barrier function of RNAPs. We tested this prediction by new ChIP-seq experiments, which revealed that the "cohesin loader" Nipbl co-localizes with cohesin, but, unlike in previous reports, Nipbl did not accumulate at active promoters. We propose that RNAP acts as a new type of barrier to loop extrusion that, unlike CTCF, is not stationary in its precise genomic position, but is itself dynamically translocating and relocalizes cohesin along DNA. In this way, loop extrusion could enable translocating RNAPs to maintain contacts with distal regulatory elements, allowing transcriptional activity to shape genomic functional organization.

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Characterization of induced cohesin loop extrusion trajectories in living cells [4C-seq]

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