PROJECT SUMMARY Congenital heart disease (CHD) is a significant cause of neonatal mortality and morbidity worldwide. Defects in the aortic arch artery (AAA) and its major branches are among the most severe malformations that cause CHD. The AAA and its branches arise through the asymmetrical remodeling of the three bilaterally symmetrical pairs of the pharyngeal arch arteries (PAAs), 3, 4, and 6. Defects in the left 4th PAA are particularly devastating, leading to the interruption of the aortic arch type B (IAA-B). IAA-B disrupts the systemic circulation and is lethal after birth. The genetic causes of IAA-B are mostly unknown, underscoring a critical need to understand genes and mechanisms regulating the formation of the left 4th PAA. PAA formation is a multi-step process: PAA endothelial cell (EC) progenitors arise in the second heart field (SHF) by E7.5 and populate pharyngeal arches within the next two days, culminating in the formation of a plexus of small SHF- derived blood vessels between by E10.0 in the 4th arch. Following its formation, the EC plexus gradually remodels by merging in the middle of the 4th arch into one large vessel, the 4th PAA. The deletion of Fn1 or integrin a5b1 in the Isl1 lineages results in the defective formation of the right and left 4th PAA and leads to IAA-B. My lab has demonstrated that cell-ECM interactions regulated by Fn1 and integrin a5b1 function reiteratively, at multiple stages to regulate the 4th PAAs’ formation and remodeling in a cell-type-specific manner. In this grant, we present data implicating Fn1 and integrin a5b1 in maintaining the ipsilateral patterning of the SHF-derived vasculature in the pharynx at E9.5. Our data show that the deletion of Fn1 or integrin a5b1 in the Isl1 lineage disrupts symmetrical allocation of SHF-derived cells to the left and right pharyngeal arches, causing an imbalance in the number of SHF-derived cells on the left vs. the right. At a later stage, between E10 and E10.75, Fn1 and integrin a5b1 regulate vascular patterning by mediating the remodeling of the pharyngeal EC plexus into the 4th PAA. Our published data show that Fn1 and integrin a5b1 mediate the plexus-to-PAA remodeling in an EC-non-cell- autonomous manner by regulating the expression of negative vascular guidance cues and the activation of VEGFR2 and Erk1/2 in the 4th arch. To determine the mechanisms by which Fn1 and integrin a5b1 regulate the patterning of SHF-derived vasculature, we will use live imaging, quantitative immunofluorescence microscopy, genetic epistasis analysis, and single-cell RNAseq to test two hypotheses 1) that Fn1 and integrin a5b1 expressed in the Isl1 lineage maintain the balanced allocation of SHF-derived cells in the pharynx by regulating ECM assembly and ipsilateral SHF cell migration and 2) that Fn1 and integrin a5b1 regulate plexus-to-PAA remodeling by modulating the expression of EC guidance cues and the activation of Erk1/2. Completing the proposed studies will provide essential insights into the functions of cell-ECM interactions in the development of SHF-derived vasculature and CHD pathogenesis.

PROJECT NARRATIVE Congenital heart disease (CHD) is a leading cause of newborn mortality worldwide. Defects in the development of the aortic arch arteries are common in CHD patients. Completing the studies outlined in this proposal will provide valuable insights into the physiological regulation of aortic arch artery formation and the alterations causing CHD.