Cell-ECM interactions play distinct and essential roles at multiple stages during the development of the aortic arch arteries

RationaleDefects in the morphogenesis of the 4th pharyngeal arch arteries (PAAs) give rise to lethal birth defects. Understanding genes and mechanisms regulating PAA formation will provide important insights into the etiology and treatments for congenital heart disease. ObjectiveCell-ECM interactions play essential roles in the morphogenesis of PAAs and their derivatives, the aortic arch artery (AAA) and its major branches; however, their specific functions are not well-understood. Previously, we demonstrated that integrin 5{beta}1 and fibronectin (Fn1) expressed in the Isl1 lineages regulate PAA formation. The objective of these studies was to investigate cellular mechanisms by which integrin 5{beta}1 and Fn1 regulate AAA morphogenesis. Methods and ResultsUsing temporal lineage tracing, whole-mount confocal imaging, and quantitative analysis of the second heart field (SHF) and endothelial cell (EC) dynamics, we show that the majority of PAA EC progenitors arise by E7.5 in the SHF and populate pharyngeal arch mesenchyme between E7.5 and E9.5. Consequently, SHF-derived ECs in the pharyngeal arches become organized into a uniform plexus of small blood vessels, which becomes remodeled into the PAAs between 31 - 35 somites. The remodeling of the vascular plexus is orchestrated by signals dependent on pharyngeal ECM microenvironment extrinsic to the endothelium. Conditional ablation of integrin 5{beta}1 or Fn1 in the Isl1 lineages showed that signaling by the ECM regulates AAA morphogenesis at multiple steps: 1) the recruitment of the SHF-derived ECs into the pharyngeal arches, 2) the remodeling of the uniform EC plexus in the 4th arches into the PAAs; and 3) differentiation of neural crest-derived cells abutting the PAA endothelium into vascular smooth muscle cells. ConclusionsPAA formation is a multi-step process entailing dynamic contribution of SHF-derived ECs to pharyngeal arches, the remodeling of endothelial plexus into the PAAs, and the remodeling of the PAAs into the AAA and its major branches. Cell-ECM interactions regulated by integrin 5{beta}1 and Fn1 play essential roles at each of these developmental stages.