TGF- β Signaling Is Necessary and Sufficient for Pharyngeal Arch Artery Angioblast Formation

The pharyngeal arch arteries (PAAs) are transient embryonic blood vessels that mature into critical segments of the aortic arch and its branches. Although defects in PAA development cause life-threating congenital cardiovascular defects, the molecular mechanisms that orchestrate PAA morphogenesis remain unclear. Through small-molecule screening in zebrafish, we identified TGF-beta signaling as indispensable for PAA development. Specifically, chemical inhibition of the TGF-beta type I receptor ALK5 impairs PAA development because nkx2.5(+) PAA progenitor cells fail to differentiate into tie1(+) angioblasts. Consistent with this observation, we documented a burst of ALK5-mediated Smad3 phosphorylation within PAA progenitors that foreshadows angioblast emergence. Remarkably, premature induction of TGF-beta receptor activity stimulates precocious angioblast differentiation, thereby demonstrating the sufficiency of this pathway for initiating the PAA progenitor to angioblast transition. More broadly, these data uncover TGF-b as a rare signaling pathway that is necessary and sufficient for angioblast lineage commitment.