Characterization of induced cohesin loop extrusion trajectories in living cells

Cohesin (SMC1-SMC3-RAD21) constantly extrudes DNA loops to organize chromosomes into structural domains, pausing and anchoring at specific DNA-bound CTCF molecules. To study the detailed consequences of cohesin loop extrusion, we developed TArgeted Cohesin Loader (TACL) for controlled pan-cellular activation of chromatin loop formation at defined genomic locations in living cells. With TACL, we show that highly complex looping networks can exist, with extruding cohesin complexes that block each other, drive cohesin queuing and induce loop anchoring at nearly all CTCF-bound sites. TACL loops extend upon acute depletion of STAG2, PDS5A or WAPL. Activated cohesin loop extrusion hinders local gene transcription and can alter chromatin accessibility and H3K27ac distribution. TACL shows that the loading/extrusion complex NIPBL-MAU2 can be transported by cohesin to CTCF sites but, together with SMC1, to enhancers in a RAD21-independent manner. TACL thus enables studying the consequences of activated loop extrusion at defined genomic locations.