UCLA

Cardiovascular and blood are the first functioning organ systems that need to be established during embryogenesis. Although individual regulators governing blood, vascular and heart development have been identified, we lack understanding of how these factors cooperate to enable precisely timed fate choices. Here, genetically modified mouse embryonic stem cells and embryoid body differentiation system were used to investigate how lineage specifying transcription factors (SCL, GATA1, GATA2, ETV2) exploit epigenetic landscape to regulate gene expression during mesoderm diversification to hemato-vascular and cardiac lineages. Our lab previously showed that transcription factor SCL specifies hematopoiesis and represses cardiogenesis during embryogenesis. In this thesis work, we determined that SCL directly binds to enhancers of both hematopoietic and cardiac genes that were epigenetically primed for activation in mesoderm prior to SCL binding. The repression of cardiac fate was not caused by SCL dependent recruitment of co-repressors but possibly by SCL blocking the activation of these genes by cardiac transcription factors. Moreover, ChIP-seq, RNA-seq and functional analysis showed that SCL complex partners GATA1 and GATA2 are critical for SCL mediated endothelial to hematopoietic cell transition, but are dispensable for cardiac suppression by SCL. Finally, we demonstrated that SCL activates hematopoiesis and represses ectopic cardiogenesis in hemogenic endothelium independent of its upstream transcription factor ETV2. However, ETV2 cooperates with SCL in the activation of key hemato-vascular genes by establishing chromatin accessibility at its binding sites.

These data have increased our understanding of the transcriptional events regulating specification of mesodermal cells to hemato-vascular and cardiac fates, and provides new insights to the basic mechanisms of cardiovascular lineage choice. This knowledge may help develop better protocols for hematopoietic, endothelial and cardiac cell generation in vitro and thereby advance cell-based therapies for blood and cardiovascular disease.