Distinct and Overlapping Function for Scl, Lmo2 and Ldb1 in Regulating the Divergence of Hematopoietic and Cardiac Fates

Lmo2 and Ldb1 are non-DNA binding components of a multi-factor complex containing Scl/Tal1, the master transcription factor that regulates hematopoietic fate commitment during embryogenesis. Scl has been shown to be critical for the activation of the hematopoietic transcription program in hemogenic endothelium, and repression of cardiac transcription program in endothelium and endocardium, and hence responsible for the divergence of these mutually exclusive mesodermal fates. However, the specific functions of Lmo2 and Ldb1 in this complex, especially in the repressive function, are poorly understood. Here, we employ a combination of germline and conditionally-deleted Scl, Lmo2 and Ldb1 mouse model to study the dynamic roles of each complex member. Our data suggests that similar to Scl, both Lmo2 and Ldb1 are critical for primitive and definitive erythropoiesis. However, whereas the deletion of Scl completely ablates HSPC specification, the presence of residual CD41+cKit+ HSPCs in Lmo2 and Ldb1 knockout embryos suggests that they may have partially redundant roles during HSPC specification. Lmo2, similar to Scl, is critical for the repression of cardiac fate in hemogenic endothelium and endocardium, but becomes dispensable before Scl for continued cardiac repression in the endocardium. Ldb1-/- embryos lack heart and instead show accumulation of CD31+Pdgfra+ cells in the rostral half. Moreover, hemogenic tissues of Ldb1 mutants do not show accumulation of cardiogenic progenitors but exhibit an expansion of CD31+cKit+ endothelial cells, suggesting that Ldb1 critically regulates the activation of both hematopoietic and cardiac transcriptional program in the endothelium. ChIP-seq revealed that Lmo2 predominantly acts through Scl-bound enhancers. Ldb1 also has an Scl independent function in activating cardiac genes, as well as a distinct, but Scl-dependent, function in regulating genes associated with polycomb-repression such as Ezh2 and Suz12. Our data demonstrates the dynamic and temporal interactions as well as individual functions between the members of Scl complex during the divergence of hematopoietic and cardiac lineages.