Formation of functional blood and its conduit vascular system is requisite for successful embryogenesis. The ETS transcription factor ETV2 has emerged as a critical regulator of hematopoietic and vascular development. In particular, ETV2 is required at the earliest stage in the formation of hematopoietic and endothelial cell lineages with its deficiency leading to a complete block in blood and vessel formation. At the molecular level, ETV2 positively activates genes essential for hematopoietic and endothelial cell lineage specification. At the cellular level, ETV2 is crucial for restricting FLK1+ mesoderm to the FLK1+ hemangiogenic mesoderm, which generates hematopoietic and endothelial cell lineages. Importantly, ETV2 function is evolutionary conserved, as mouse or human ETV2 can induce hematopoietic and vascular program in zebrafish. Moreover, hematopoietic and endothelial cells can be more efficiently generated from mouse and human pluripotent stem cells by enforced Etv2 expression. Human somatic cells can also be reprogrammed to endothelial cells by the inclusion of Etv2. These findings suggest a unique and essential function played by this factor in vertebrates and relevance in humans with the strong potential for translational applications. However, we currently have very limited knowledge on the upstream regulators of Etv2 expression. Whether ETV2 requires cofactor(s) in controlling its target gene expression is also not known. Our preliminary data suggests that ETV2 specifies hemangiogenic fate in a threshold-dependent manner and that coordinated function of forkhead transcription factor Foxh1 and Flk1 is required for achieving Etv2 threshold expression. Our preliminary data also suggests that chromatin remodeling of the target genes may be required for ETV2 function, as we identified the core regulatory subunit of SWI/SNF chromatin remodeling complex, BAF155 (aka SRG3 or SMARCC1) to form a complex with ETV2. Importantly, conditional deletion of Baf155 using Flk1-Cre resulted in embryonic lethality with hematopoietic defects, further supporting the notion that chromatin remodeling is integral to ETV2 transcriptional function. From these fundamental findings, we will investigate whether the hemangiogenic fate specification is achieved through Etv2 threshold expression and the requirements of Foxh1 and Flk1 in achieving the Etv2 threshold expression. We will also investigate whether ETV2 requires SWI/SNF chromatin remodelers in activating hematopoietic and endothelial cell lineage program. Successful completion of the work will establish the role for Foxh1, Flk1, Etv2 and the chromatin remodelers in the hemangiogenic fate specification and will lead to deeper mechanistic understanding of how hematopoietic and vascular development is regulated. The outcome will also be instrumental for generating hematopoietic and endothelial cells from pluripotent stem cells or somatic cell reprogramming and function of such cells in a wide range of applications in regenerative medicine.
This grant proposal is to understand how blood and blood vessels are established in the developing mouse embryo. By focusing on the ETS transcription factor ETV2 (aka ER71), which is required at the earliest stage of the blood and blood vessel formation, we will characterize upstream transcription factors, signaling pathways and chromatin remodeling mechanisms that regulate ETV2 expression and function. The outcome of this study is highly relevant to basic research and regenerative medicine.
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