APOE-NOTCH Axis Governs Elastogenesis During Human Cardiac Valve Remodeling
Valve remodeling is a complex process involving extracellular matrix organization, development of trilaminar structures, and physical elongation of valve leaflets. However, the cellular and molecular mechanisms regulating valve remodeling and their roles in congenital valve disorders remain poorly understood. Semilunar valves and atrioventricular valves from healthy and age-matched human fetal hearts with pulmonary stenosis (PS) were collected. Single-Cell RNA-sequencing (scRNA-seq) was performed to determine the transcriptomic landscape of multiple valvular cell subtypes in valve remodeling and disease. Spatial localization of newly-identified cell subtypes was determined via immunofluorescence and RNA in situ hybridization. The molecular mechanisms mediating valve development was investigated utilizing primary human fetal valve interstitial cells (VICs) and endothelial cells (VECs). scRNA-seq analysis of healthy human fetal valves identified a novel APOE+ elastin-producing VIC subtype (Elastin-VIC) spatially located underneath VECs sensing the unidirectional flow. Knockdown of APOE in fetal VICs resulted in significant elastogenesis defects. In pulmonary valve with PS, we observed decreased expression of APOE and other genes regulating elastogenesis such as EMILIN1 and LOXL1, as well as elastin fragmentation. These findings suggested the crucial role of APOE in regulating elastogenesis during valve remodeling. Furthermore, cell-cell interaction analysis revealed that JAG1 from unidirectional VECs activates NOTCH signaling in Elastin-VICs through NOTCH3. In vitro Jag1 treatment in VICs increased the expression of elastogenesis-related genes and enhanced contractile functions with related gene expression. This was accompanied by activation of NOTCH signaling and elastogenesis observed in VICs co-cultured with VECs in the presence of unidirectional flow. Notably, we found that the JAG1-NOTCH3 signaling pair was drastically reduced in the PS valves. Lastly, we demonstrated that APOE is indispensable for JAG1-induced NOTCH activation in VICs, reinforcing the presence of a synergistic intrinsic and external regulatory network involving APOE and NOTCH signaling that is responsible for regulating elastogenesis during human valve remodeling. scRNA-seq analysis of human fetal valves identified a novel Elastin-VIC subpopulation, and revealed mechanism of intrinsic APOE and external NOTCH signaling from VECs sensing unidirectional flow in regulating elastogenesis during valve remodeling. These mechanisms may contribute to the pathogenesis of elastic malformation in congenital valve disease. Overall design: scRNA-seq analysis of four human fetal valves, and comparison between healthy pulmonary valve and age-matched malformed pulmonary valve were also performed. ***RAW data not provided for GSM7134268 due to patient privacy concerns***