A spatiotemporal observation of EndMT and mesenchymal cell colonization at the onset of human cardiac valve development

Abstract

Elucidation of mechanisms in semilunar valve development may enable the development of new therapies. Here, we found differences in proliferation-associated genes and genes repressed by vascular endothelial growth factor between human semilunar valves from first and second trimester valve leaflets. The proliferation of valve interstitial cells and ventricular valve endothelial cells (VECs) and cellular density declined from the first to the second trimester. Cytoplasmic expression of nuclear factor of activated T-cells cytoplasmic 1 (NFATc-1) in VECs (4 weeks), and later cells in the leaflet/annulus junction mesenchyme expressing inactive NFATc-1 (5.5-9 weeks) were detected, indicative of EndMT in valvulogenesis. At this leaflet/annulus junction CD44+ cells clustered during elongation (11 weeks), extending toward the tip along the fibrosal layer in second trimester leaflets. Differing patterns of maturation in the fibrosa and ventricularis were detected via increased fibrosal periostin content, which tracked the presence of the CD44+ cells in the second trimester. We revealed that spatiotemporal NFATc-1 expression actively regulates EndMT during human valvulogenesis, as early as 4 weeks. Additionally, CD44+ cells play a role in leaflet maturation toward the trilaminar structure, possibly via migration of VECs undergoing EndMT, which subsequently ascend from the leaflet/annulus junction.