Thursday, April 25, 2024
4/25/2024
Hemangiogenesis, the formation of functional blood and endothelial cells (EC), is requisite for successful embryogenesis. We have so far demonstrated that blood and ECs develop from the PDGFRa-FLK1+ hemangiogenic mesoderm, which originates from the PDGFRa+ nascent mesoderm through a PDGFRa+FLK-1+ intermediary (i.e., PDGFRa+ ® PDGFRa+FLK1+ ® PDGFRa-FLK1+). We recently performed single-cell RNA sequencing and mapped a developmental pathway of the hemangiogenesis. In this process, we discovered that hemangiogenic lineage shares a close molecular ontogeny with the smooth muscle cell (SMC) lineage. Moreover, in vivo, yolk sac SMCs still retained the Flk1+ mesoderm transcriptome signature. Therefore, we proposed that the SMC lineage is the default pathway of the FLK1+ mesoderm. In further delineating mechanisms involved in the hemangiogenesis, we have shown that the transition from PDGFRa+FLK1+ to the PDGFRa-FLK1+ stage is dependent on the ETS transcription factor ETV2, a critical factor for the formation of hematopoietic and endothelial cell lineages. At the molecular level, ETV2 positively activates genes essential for hematopoietic and endothelial cell lineage specification. Two critical events occur in the transition from PDGFRa+FLK1+ to the PDGFRa-FLK1+ stage. First, Etv2 expression becomes higher, suggesting an Etv2 threshold mechanism in hemangiogenesis critical for this transition. Second, ETV2 target gene loci become accessible, suggesting epigenetic and chromatin changes occurring during this transition. We identified BAF155, a subunit of the mammalian SWI/SNF chromatin-remodeling BAF (BRG1/BRM associated factor) complex, as a cofactor of ETV2. This finding suggests that ETV2 utilizes BAF in reconfiguring chromatin accessibility of its target gene loci. A deeper understanding of the lineage relationship among FLK1+ populations and molecular processes occurring during the transition from PDGFRa+FLK1+ to PDGFRa-FLK1+ will be critical for further understanding how the hematopoietic and vascular systems are established in embryogenesis. This proposal's overarching goal is to gain deeper molecular insights into the regulation of Flk1+ lineage allocation concerning Etv2 and Baf155 expression and function. To this end, we will determine if hemangiogenic vs. SMC lineage fate of the FLK1+ mesoderm is controlled by the Etv2 dosage (aim 1), determine if PDGFRa+FLK1+ to the PDGFRa-FLK1+ transition requires ETV2 mediated Flk1 enhancer switching mechanisms (aim 2), and determine the chromatin remodeling requirements in the ETV2 regulation of hemangiogenesis (aim 3). The successful completion of the proposed studies will advance our understanding of how hematopoietic and EC development is regulated. Ultimately, this knowledge will be instrumental for generating hematopoietic and ECs from pluripotent stem cells or somatic cell reprogramming and the function of such cells in a wide range of regenerative medicine applications.
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