GSE104846
SCL establishes a global repressive environment and cooperates with Polycomb RYBP-PRC1 to suppress alternative lineages in blood-fated cells [ChIP-seq]
Understanding the molecular basis of cell fate decision is critical to shed light onto biological processes supporting tissue formation. In particular, whether lineage-fated cells derive from multi-primed progenitors and need to establish active fate restriction mechanisms to engage in a differentiation pathway is still matter of debate. To characterise the mechanisms underlying specification of blood-fated cells from mesodermal precursors, we studied the transcriptional activity of the key hematopoietic regulator SCL/TAL1. Scl-null mouse embryos die at embryonic day E9.5 from absence of blood formation, and exhibit mis-specification of mesodermal cells into cardiomyocytes, thus offering an excellent entry point for mechanistic studies of fate determination. Using high-throughput assays from mouse ES cell-derived mesodermal cells, we showed that, in parallel with activating the blood program and repressing the cardiac and paraxial programs, SCL strongly activates expression of repressors, such as ETO2 (known partner of SCL) and RYBP (member of Polycomb repressive complex PRC1). ETO2's association with chromatin reflects SCL's overall genomic binding, pointing to ETO2's modulatory, rather than purely repressive, transcriptional activities. In contrast, SCL/RYBP chromatin co-occupancy is preferentially enriched at genes normally repressed in blood-fated cells and involved in early developmental processes and alternative lineages, highlighting a relationship between SCL and RYBP/PRC1 in repressive processes. Remarkably, SCL is critical for maintaining high levels of PRC1-associated repressive histone mark H2AK119ub genome-wide. It is also required for maximal levels of repressive mark H3K27me3 on genes normally repressed. Finally, Rybp knock-down in differentiating ES cells phenocopies the cardiac phenotype of Scl-null cells, supporting a functional interaction between SCL and a critical repressive chromatin remodeling complex. Altogether, these data unveil the establishment of a repressive environment in mesodermal progenitors by a hematopoietic-specific transcriptional regulator. This leads to restriction of expression of regulators of alternative fates and uncompromised commitment towards a hematopoietic identity. Overall design: mcherry+ cells from D4 EBs were sorted from Scl-mcherry reporter cell lines expressing either wild type SCL (Scl:mcherry) or basic helix-loop-helix deleted, non-functional SCL protein (Scl d/d:mcherry). Mouse mcherry positive sorted cells were mixed in 2:1 ratio to drosophila S2 cell line. Once the mouse and drosophila cells were combined, the sample was treated as a single Chipseq sample throughout the experiment to completion of DNA sequencing. Standard Chip was then carried out and histone/DNA complexes were isolated with the antibodies listed below.