Patients with a congenital bicuspid aortic valve (BAV), a valve with two instead of three aortic leaflets, have an increased risk of developing thoracic aneurysms and aortic dissection. The mechanisms underlying BAV-associated aortopathy are poorly understood. This study examined BAV-associated aortopathy in Nos3(-/-) mice, a model with congenital BAV formation. A combination of histological examination and in vivo ultrasound imaging was used to investigate aortic dilation and dissections in Nos3(-/-) mice. Moreover, cell lineage analysis and single-cell RNA sequencing were used to observe the molecular anomalies within vascular smooth muscle cells (VSMCs) of Nos3(-/-) mice. Spontaneous aortic dissections were found in ascending aortas located at the sinotubular junction in similar to 13% of Nos3(-/-) mice. Moreover, Nos3(-/-) mice were prone to developing aortic dilations in the proximal and distal ascending aorta during early adulthood. Lower volumes of elastic fibres were found within vessel walls of the ascending aortas of Nos3(-/-) mice, as well as incomplete coverage of the aortic inner media by neural crest cell (NCC)-derived VSMCs. VSMCs of Nos3(-/-) mice showed downregulation of 15 genes, of which seven were associated with aortic aneurysms and dissections in the human population. Elastin mRNA was most markedly downregulated, followed by fibulin-5 expression, both primary components of elastic fibres. This study demonstrates that, in addition to congenital BAV formation, disrupted endothelial-mediated nitric oxide (NO) signalling in Nos3(-/-) mice also causes aortic dilation and dissection, as a consequence of inhibited elastic fibre formation in VSMCs within the ascending aorta.

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.

Objective-Maternal hypercholesterolemia is associated with a higher incidence and faster progression of atherosclerotic lesions in neonatal offspring. We aimed to determine whether an in utero environment exposing a fetus to maternal hypercholesterolemia and associated risk factors can prime the murine vessel wall to accelerated development of cardiovascular disease in adult life.Methods and Results-To investigate the epigenetic effect in utero, we generated genetically identical heterozygous apolipoprotein E-deficient progeny from mothers with a wild-type or apolipoprotein E-deficient background. A significant increase in loss of endothelial cell volume was observed in the carotid arteries of fetuses of apolipoprotein E-deficient mothers, but fatty streak formation was absent. Spontaneous atherosclerosis development was absent in the aorta and carotid arteries in adult life. We unilaterally placed a constrictive collar around the carotid artery to induce lesion formation. In offspring from apolipoprotein E-deficient mothers, collar placement resulted in severe neointima formation in 9 of 10 mice analyzed compared with only minor lesion volume (2 of 10) in the progeny of wild-type mothers.Conclusions-We conclude that the susceptibility to neointima formation of morphologically normal adult arteries is already imprinted during prenatal development and manifests itself in the presence of additional atherogenic risk factors in adult life. Future research will concentrate on the mechanisms involved in this priming process, as well as on prevention strategies.