Persistent truncus arteriosus and abnormal large vessels in mice lacking Alk5 in cardiac NCCs
To inactivate Alk5 in cardiac NCCs, mice homozygous for the floxed Alk5 allele (Alk5Flox/Flox) [21] were crossed with transgenic Wnt1-Cre mice [22], which were also heterozygous for the Alk5 knockout allele (Alk5KO) allele. The resulting mice heterozygous for the Alk5Flox and Alk5KO alleles, which also carried the Wnt1-Cre transgene, had the Alk5 gene specifically inactivated in NCCs (herein termed Alk5/Wnt1-Cre), while the littermates with remaining allelic combinations were phenotypically normal and served as controls (Alk5Flox/+, Alk5KO/+; Wnt1-Cre). When embryos were harvested during the last day of gestation, an expected number (25%) of Alk5/Wnt1-Cre mutants were recovered. However, all mutant offspring died either during the birth or during the first post-natal hours.
To determine, if ALK5-mediated TGF-β-signaling had a role in development of the OFT and large vessels of the aortic arch, we performed casting dye experiments on E17 embryos (Fig. 1A–D). In wild-type embryos (Fig. 1A), the aorta was clearly separated from the pulmonary trunk, and the right brachiocephalic, left carotid and left subclavian arteries branched directly off the aortic arch. In contrast, Alk5/Wnt1Cre mutant embryos consistently displayed a single prominent arterial trunk (Fig. 1C–D), while corresponding Tgfbr2 mutant embryos (Fig. 1B) displayed interrupted aortic arch, as reported earlier [8]. Approximately 40% of the Alk5 mutants had a right-sided outflow tract, with the retroesophageal arch connecting to the descending aorta and to the left subclavian artery. The carotid arteries originated either from a common bud located in the ventral side of the ascending arch, or from separate adjacent sites, as verified by serial sectioning (Fig. 1M–P). The remaining mutants displayed a left-sided aortic arch, where the right carotid arteries originated from the right lateral aspect of the ascending trunk, while the left carotid arteries budded from the ventral or right ventral aspects of the trunk (Fig. 1I–L). Both right and left subclavian arteries consistently originated from the descending part of the aortic arch. Similarly, in all mutants both left and right pulmonary arteries always branched out from the common arterial trunk. To conclude, Alk5/Wnt1-Cre mutants consistently displayed PTA, which differed significantly from the characteristic interrupted aortic arch phenotype seen in Tgfbr2/Wnt1-Cre mutants [8,9].
Figure 1  Abrogation of Alk5 in neural crest cells leads to persistent truncus arteriosus type A2. A-D, Casting-dye analysis of OFT morphogenesis at E17.0. Control (A), Tgfbr2/Wnt1-Cre mutant [8] (B) demonstrating the PTA type A4 (= truncus arteriosus with interrupted aortic arch [30]) and Alk5/Wnt1-Cre mutants demonstrating the right-sided (C) and left-sided (D) arches of the truncus. E-P, Histological cross-sections on four different levels (rostral to caudal) at E17.0. In a control (E-H), the ascending aorta (Ao) and pulmonary trunk (PT) are separated by the conotruncal septum. In Alk5/Wnt1-Cre mutants (I-P) the conotruncal septum fails to form, and either left-sided (I-L) or right-sided (M-P) aortic arch can be seen. Aberrant branching of carotid arteries from the truncus has been illustrated by black arrows (J and M). Ao, aorta; PT, pulmonary trunk; RSA, right subclavian artery; RCA, right carotid artery; LCA; left carotid artery; LSA, left subclavian artery; IAA, interrupted aortic arch; PTA, persistent truncus arteriosus.