Dynamic Expression Profiles of Sox9 in Embryonic, Post Natal, and Adult Heart Valve Cell Populations.

Heart valves are dynamic structures and abnormalities during embryonic development can lead to premature lethality or congenital malformations present at birth. The transcription factor Sox9 has been shown to be critical for early and late stages of valve formation, but its defined expression pattern throughout embryonic, post natal, and adult growth and maturation is incomplete. Here we use an antibody to detect 1-100 amino acids of Sox9 and show that in the developing embryo, Sox9 is not detected in valve endothelial cells (VECs) lining the primitive valve structures, but is highly expressed in the endothelial-derived valve interstitial cell population following endothelial-to-mesenchymal transformation. Expression is maintained in this cell population after birth, but is additionally detected in VECs from post natal day 1. Using a specific antibody to detect a phosphorylated form of Sox9 at Serine 181 (pSox9), we note enrichment of pSox9 in VECs at post natal days 1 and 10 and this pattern correlates with the known upstream kinase RockI, and downstream target, Aggrecan. The contribution of Sox9 to post natal growth and maturation of the valve is not known, but this study provides insights for future work examining the differential functions of Sox9 protein in valve cell populations. Anat Rec, 302:108-116, 2019.