RNAPII and DNA supercoiling regulate cohesin engagement in neurons [RNA-Seq]

Chromatin looping by CTCF and cohesin is thought to be crucial for chromosome organization and gene transcription. Yet the precise relationships between CTCF, cohesin, and transcription in neurons are still poorly understood. To address this issue, we compared the occupancy of CTCF and the cohesin subunit, SMC1, relative to transcriptionally engaged RNAPII and as a function of stimulus-dependent transcription in cultured mouse cortical neurons. We show that CTCF and cohesin are enriched at transcription start sites (TSS) and that their levels increase with the level of transcriptionally engaged RNAPII, suggesting that RNAPII facilitates the engagement of CTCF and cohesin. Unexpectedly, while neuronal stimulation causes widespread transcriptional activation, it resulted in the rapid genome-wide loss of SMC1 signals, including at the TSS of genes, loop anchors, and TAD boundaries. Activity-dependent reductions in cohesin were independent of CTCF but were mimicked by inhibiting topoisomerase IIb (TOP2B), which resolves torsional stress from DNA supercoiling. We show that neuronal stimulation elevates DNA supercoiling and that increasing torsional stress triggers the dissociation of SMC1 from chromatin. Overall, these results suggest that modulation of torsional stress could be a physiologically relevant mechanism of regulating cohesin engagement and chromatin architecture in neurons.