Abstract
RATIONALE: The resiliency of embryonic development to genetic and environmental perturbations has been long appreciated; however, little is known about the mechanisms underlying the robustness of developmental processes. Aberrations resulting in neonatal lethality are exemplified by congenital heart disease (CHD) arising from defective morphogenesis of pharyngeal arch arteries (PAA) and their derivatives. OBJECTIVE: To uncover mechanisms underlying the robustness of PAA morphogenesis. METHODS AND RESULTS: The second heart field (SHF) gives rise to the PAA endothelium. Here, we show that the number of SHF-derived ECs is regulated by VEGFR2 and Tbx1 . Remarkably, when SHF-derived EC number is decreased, PAA development can be rescued by the compensatory endothelium. Blocking such compensatory response leads to embryonic demise. To determine the source of compensating ECs and mechanisms regulating their recruitment, we investigated three-dimensional EC connectivity, EC fate, and gene expression. Our studies demonstrate that the expression of VEGFR2 by the SHF is required for the differentiation of SHF-derived cells into PAA ECs. The deletion of one VEGFR2 allele (VEGFR2 (SHF-HET) ) reduces SHF contribution to the PAA endothelium, while the deletion of both alleles (VEGFR2 (SHF-KO) ) abolishes it. The decrease in SHF-derived ECs in VEGFR2 (SHF-HET) and VEGFR2 (SHF-KO) embryos is complemented by the recruitment of ECs from the nearby veins. Compensatory ECs contribute to PAA derivatives, giving rise to the endothelium of the aortic arch and the ductus in VEGFR2 (SHF-KO) mutants. Blocking the compensatory response in VEGFR2 (SHF-KO) mutants results in embryonic lethality shortly after mid-gestation. The compensatory ECs are absent in Tbx1 (+/-) embryos, a model for 22q11 deletion syndrome, leading to unpredictable arch artery morphogenesis and CHD. Tbx1 regulates the recruitment of the compensatory endothelium in an SHF-non-cell-autonomous manner. CONCLUSIONS: Our studies uncover a novel buffering mechanism underlying the resiliency of PAA development and remodeling. NONSTANDARD ABBREVIATIONS AND ACRONYMS IN ALPHABETICAL ORDER: CHD - congenital heart disease; ECs - endothelial cells; IAA-B - interrupted aortic arch type B; PAA - pharyngeal arch arteries; RERSA - retro-esophageal right subclavian artery; SHF - second heart field; VEGFR2 - Vascular endothelial growth factor receptor 2.