Malformation of the outflow tract (OFT) is one of the most common congenital heart defects (CHD) in humans and a leading cause of childhood lethality. The long-term objectives of this project are to identify signaling pathways important for OFT development and to understand how different pathways interact to regulate OFT morphogenesis. A thorough understanding of the developmental mechanisms underlying OFT formation will be critical for designing diagnostic and therapeutic approaches in the future. OFT development relies on the second heart field (SHF) lineage located in the pharyngeal/ splanchnic mesoderm, outside of the initial heart tube. Studies have delineated elegantly how transcriptional and signaling networks integrate to coordinate progenitor expansion and cardiac differentiation in the SHF. But how SHF cells are deployed to the OFT has remained largely unclear. The objective of the current proposal is to understand the role of the planar cell polarity (PCP) pathway, a branch of the non- canonical Wnt pathway, in the deployment of the SHF splanchnic mesoderm in mice. The objective will be achieved by specifically testing (1) whether PCP signaling is activated by non- canonical Wnt ligand Wnt5a in the SHF splanchnic mesoderm, (2) whether PCP effector Daam1, a novel Formin homology protein, functions downstream of Wnt5a and Wnt/PCP gene Disheveled (Dvl) in the SHF and (3) whether the deployment of SHF splanchnic mesodermal cells to the OFT is perturbed by Wnt5a or Dvl mutations. First, imaging experiments will be performed to test whether Wnt5a mutation disrupts plasma membrane recruitment of Dvl2 and its interaction with cytoskeletal regulators Daam1, Profilin1 and WGEF, a Rho GTPase activator. Secondly, we will test whether DIX transgenes, encoding a Dvl2 mutant form that specifically activates PCP signaling, can suppress the OFT defects in Wnt5a mutants. Thirdly, we will use a dominant negative form of Daam1 to test whether it is functionally important for SHF and OFT development, and use a constitutively active form of Daam1 to test whether its expression in the SHF can suppress the OFT defects in Wnt5a and Dvl mutants. Lastly, we will use a genetic approach to label Wnt5a expressing cells in the splanchnic mesoderm and track their behavior by time-lapse imaging. We will determine whether mutation in Wnt5a or Dvl alters their behavior and reduces their deployment to the OFT. Collectively, these studies will define the cellular and molecular mechanisms through which PCP signaling regulates SHF deployment to the OFT.
Understanding the role of the PCP pathway in mouse SHF development will help us elucidate the genetic control underlying OFT morphogenesis and identify the etiology of OFT defects in humans. Defining the pathogenic mechanisms of OFT malformation is the critical first step in developing early detection, prevention and therapies for these common, devastating CHD in humans.
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