Pharyngeal pouches provide a niche microenvironment for arch artery progenitor specification

Morphogenetic processes in the development and evolution of the arteries of the pharyngeal arches: their relations to congenital cardiovascular malformations

The heart and aortic arch arteries in amniotes form a double circulation, taking oxygenated blood from the heart to the body and deoxygenated blood to the lungs. These major vessels are formed in embryonic development from a series of paired and symmetrical arteries that undergo a complex remodelling process to form the asymmetric arch arteries in the adult. These embryonic arteries form in the pharyngeal arches, which are symmetrical bulges on the lateral surface of the head. The pharyngeal arches, and their associated arteries, are found in all classes of vertebrates, but the number varies, typically with the number of arches reducing through evolution. For example, jawed vertebrates have six pairs of pharyngeal arch arteries but amniotes, a clade of tetrapod vertebrates, have five pairs. This had led to the unusual numbering system attributed to each of the pharyngeal arch arteries in amniotes (1, 2, 3, 4, and 6). We, therefore, propose that these instead be given names to reflect the vessel: mandibular (1st), hyoid (2nd), carotid (3rd), aortic (4th) and pulmonary (most caudal). Aberrant arch artery formation or remodelling leads to life-threatening congenital cardiovascular malformations, such as interruption of the aortic arch, cervical origin of arteries, and vascular rings. We discuss why an alleged fifth arch artery has erroneously been used to interpret congenital cardiac lesions, which are better explained as abnormal collateral channels, or remodelling of the aortic sac.

Chemokine signaling synchronizes angioblast proliferation and differentiation during pharyngeal arch artery vasculogenesis

Developmentally, the great vessels of the heart originate from the pharyngeal arch arteries (PAAs). During PAA vasculogenesis, PAA precursors undergo sequential cell fate decisions that are accompanied by proliferative expansion. However, how these two processes are synchronized remains poorly understood. Here, we find that the zebrafish chemokine receptor Cxcr4a is expressed in PAA precursors, and genetic ablation of either cxcr4a or the ligand gene cxcl12b causes PAA stenosis. Cxcr4a is required for the activation of the downstream PI3K/AKT cascade, which promotes not only PAA angioblast proliferation, but also differentiation. AKT has a well-known role in accelerating cell-cycle progression through the activation of cyclin-dependent kinases. Despite this, we demonstrate that AKT phosphorylates Etv2 and Scl, the key regulators of angioblast commitment, on conserved serine residues, thereby protecting them from ubiquitin-mediated proteasomal degradation. Altogether, our study reveals a central role for chemokine signaling in PAA vasculogenesis through orchestrating angioblast proliferation and differentiation.

A phenotypic rescue approach identifies lineage regionalization defects in a mouse model of DiGeorge syndrome

TBX1 is a key regulator of pharyngeal apparatus (PhAp) development. Vitamin B12 (vB12) treatment partially rescues aortic arch patterning defects of Tbx1^+/− embryos. Here, we show that it also improves cardiac outflow tract septation and branchiomeric muscle anomalies of Tbx1 hypomorphic mutants. At the molecular level, in vivo vB12 treatment enabled us to identify genes that were dysregulated by Tbx1 haploinsufficiency and rescued by treatment. We found that SNAI2, also known as SLUG, encoded by the rescued gene Snai2, identified a population of mesodermal cells that was partially overlapping with, but distinct from, ISL1⁺ and TBX1⁺ populations. In addition, SNAI2⁺ cells were mislocalized and had a greater tendency to aggregate in Tbx1^+/− and Tbx1^−/− embryos, and vB12 treatment restored cellular distribution. Adjacent neural crest-derived mesenchymal cells, which do not express TBX1, were also affected, showing enhanced segregation from cardiopharyngeal mesodermal cells. We propose that TBX1 regulates cell distribution in the core mesoderm and the arrangement of multiple lineages within the PhAp.