Abstract 356: Scl Represses Cardiogenesis via Distant Enhancers during Hemogenic Endothelium Specification

Abstract

Understanding the mechanisms directing mesoderm specification holds a great potential to advance the development of cell-based therapies for cardiovascular and blood disorders. The bHLH transcription factor Scl is known as the master regulator of the hematopoietic fate. We recently discovered that, in addition to its critical function in promoting the establishment of hemogenic endothelium during hematopoietic stem/progenitor cell (HS/PC) development, Scl is also required to repress cardiomyogenesis in endothelium in hematopoietic tissues and endocardium in the heart. However, the mechanisms for the cardiac repression have remained unknown. Using ChIP-sequencing and microarray analysis of Flk+ mesoderm differentiated from mouse ES cells, we show that Scl both directly activates a broad gene regulatory network required for hemogenic endothelium and HS/PC development (e.g. Runx1, cMyb, Lyl1, Mef2C, Sox7 etc.), and directly represses transcriptional regulators required for cardiogenesis (e.g. Gata4, Gata6, Myocd, etc.) and mesoderm development (Eomes, Mixl1, Etv2, etc.). Repression of cardiac and mesodermal programs occurs during a short developmental window through Scl binding to distant enhancers, while binding to hematopoietic regulators extends throughout HS/PC and red blood cell development and encompasses both distant and proximal binding sites. We also discovered that, surprisingly, Scl complex partners Gata 1 and 2 are dispensable for hematopoietic vs. cardiac specification and Scl binding to majority of its target genes. Nevertheless, Gata factors co-operate with Scl to activate selected transcription factors that facilitate HS/PC emergence from hemogenic endothelium. These results denote Scl as a true master regulator of hematopoietic vs. cardiac fate choice and suggest a mechanism by which lineage-specific bHLH factors direct the divergence of competing fates.