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

ABSTRACT: 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

ORGANISM(S): Homo sapiens

PROVIDER: GSE207110 | GEO | 2023/01/30

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.

Project description:The cohesin complex organizes the genome forming dynamic chromatin loops that impact on all DNA-mediates processes. There are two different cohesin complexes in vertebrate somatic cells, carrying the STAG1 or STAG2 subunit, and two versions of the regulatory subunit PDS5, PDS5A and PDS5B. Mice deficient for any of the variant subunits are embryonic lethal, which indicates that they are not functionally redundant. However, their specific behavior at the molecular level is not fully understood. The genome-wide distribution of cohesin provides important information with functional consequences. Here, we have characterized the distribution of cohesin subunits and regulators in mouse embryo fibroblasts (MEFs) either wild type or deficient for cohesin subunits and regulators by chromatin immunoprecipitation and deep sequencing. We identify non-CTCF cohesin binding sites in addition to the commonly detected CTCF cohesin sites and show that cohesin-STAG2 is the preferred variant at these positions. Moreover, this complex has a more dynamic association with chromatin as judged by fluorescent recovery after photobleaching (FRAP), associates preferentially with WAPL and is more easily extracted from chromatin with salt than cohesin-STAG1. We observe that both PDS5A and PDS5B are exclusively located at cohesin-CTCF positions, that ablation of a single paralog has no noticeable consequences for cohesin distribution, while double knocked out cells show decreased accumulation of cohesin at all its binding sites. With the exception of a fraction of cohesin positions in which we find binding of all regulators-including CTCF and WAPL-, the presence of NIPBL and PDS5 is mutually exclusive, consistent with results of immunoprecipitation reactions in mammalian cells, as suggested previously from results in vitro. Our findings support the idea that non-CTCF cohesin binding sites represent sites of cohesin loading or pausing and are preferentially occupied by the more dynamic cohesin-STAG2. PDS5 proteins redundantly contribute to arrest cohesin at CTCF sites, possibly by preventing binding of NIPBL, but are not essential for this arrest. These results add important insights towards understanding how cohesin regulates genome folding and the specific contributions of the different variants that coexist in the cell.

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

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