Bicuspid aortic valve formation: Nos3 mutation leads to abnormal lineage patterning of neural crest cells and the second heart field

Abstract

The bicuspid aortic valve (BAV), a valve with two instead of three aortic leaflets, belongs to the most prevalent congenital heart diseases in the world, occurring in 0.5-2% of the general population. We aimed to understand how changes in early cellular contributions result in BAV formation and impact cardiovascular outflow tract development. Detailed 3D reconstructions, immunohistochemistry and morphometrics determined that, during valvulogenesis, the non-coronary leaflet separates from the parietal outflow tract cushion instead of originating from an intercalated cushion. Nos3−/− mice develop a BAV without a raphe as a result of incomplete separation of the parietal outflow tract cushion into the right and non-coronary leaflet. Genetic lineage tracing of endothelial, second heart field and neural crest cells revealed altered deposition of neural crest cells and second heart field cells within the parietal outflow tract cushion of Nos3−/− embryos. The abnormal cell lineage distributions also affected the positioning of the aortic and pulmonary valves at the orifice level. The results demonstrate that the development of the right and non-coronary leaflets are closely related. A small deviation in the distribution of neural crest and second heart field populations affects normal valve formation and results in the predominant right-non-type BAV in Nos3−/− mice.

Development and evolution of the metazoan heart

“…In both mammals and birds, but not in crocodilians, a solution for the addition of the third cusp is found by adding an intercalated swelling . All these authors agree on the importance of the SHF but vary in their explanation for the development of bicuspidy by describing either a non‐anlage of intercalated cusps or a non‐disjunction from the main endocardial OFT cushion .…”

“…These include epicardial cells migrating from the AV junction in the case of AV cushions, and epicardial cells and NCCs in the case of OFT cushions . Each individual cushion probably receives its own balanced set of mesenchymal cells . In chicken embryos, it is shown that the formation of the semilunar valves highly depends on the interaction of endocardial and NC‐derived mesenchymal cells guided by hemodynamical forces sculpting the OFT .…”

“…103,132 Each individual cushion probably receives its own balanced set of mesenchymal cells. 130,131,133 In chicken embryos, it is shown that the formation of the semilunar valves highly depends on the interaction of endocardial and NC-derived mesenchymal cells guided by hemodynamical forces sculpting the OFT. 67 It is noteworthy that also in these events of cardiogenesis, asymmetric development is apparent, although the main leaflets of the aortic and pulmonary semilunar valves originate from the same two OFT endocardial cushions.…”

“…67 It is noteworthy that also in these events of cardiogenesis, asymmetric development is apparent, although the main leaflets of the aortic and pulmonary semilunar valves originate from the same two OFT endocardial cushions. Only the so-called nonfacing leaflets 117,133 may have a different contribution of NCCs. Indeed, aortic bicuspidy in humans is far more common than pulmonary bicuspidy, as is also supported by studying a hamster strain.…”

“…In both mammals and birds, but not in crocodilians, a solution for the addition of the third cusp is found by adding an intercalated swelling. 133,226,227 All these authors agree on the importance of the SHF but vary in their explanation for the development of bicuspidy by describing either a non-anlage of intercalated cusps or a non-disjunction from the main endocardial OFT cushion. 133 The difference in incidence of bicuspidy in the aortic vs the pulmonary orifice might have a hemodynamic background related to the difference in mass addition of SHF cells between the aortic and pulmonary sides (as might be inferred from Scherptong et al 89 ).…”