Cyp26b1 restrains murine heart valve growth during development

Endothelial cells (ECs) are critical to proper heart valve development, directly contributing to the mesenchyme of the cardiac cushions, which progressively transform into mature valves. To date, investigators have lacked useful markers of valve ECs to fully evaluate their contributions during valve morphogenesis. As a result, it has been unclear whether the well-characterized regional differentiation of valves correlates with any endothelial domains in the heart. Furthermore, it has been difficult to ascertain whether endothelial heterogeneity in the heart influences underlying mesenchymal zones in an angiocrine manner. To identify regionally expressed EC genes in the heart valves, we screened publicly available databases and assembled a toolkit of endothelial-enriched genes. We identified Cyp26b1 as one of many endothelial enriched genes found to be expressed in the endocardium of the developing cushions and valves. Here, we show that Cyp26b1 is required for normal heart valve development. Genetic ablation of Cyp26b1 in mouse embryos leads to abnormally thickened aortic valve leaflets, which is due in part to increased endothelial and mesenchymal cell proliferation in the remodeling valves. In addition, Cyp26b1 mutant hearts display ventricular septal defects (VSDs) in a portion of null embryos. We show that loss of Cyp26b1 results in upregulation of retinoic acid (RA) target genes, supporting the observation that Cyp26b1 has RA-dependent roles. Together, this work identifies a novel role for Cyp26b1 in heart valve morphogenesis. Understanding the spatiotemporal expression dynamics of cardiac EC genes will likely prove useful to the investigation of both normal as well as dysfunctional heart valve development. HIGHLIGHTS{middle dot} A mouse heart valve gene expression atlas can be generated with publicly available online tools, such as Genepaint and other gene expression databases. {middle dot} Endothelium of developing mouse heart valves is regionally heterogeneous. {middle dot} Cyp26b1 is expressed in the endocardial/endothelial lining of developing heart valves. {middle dot} Loss of Cyp26b1 leads to significant enlargement of aortic valves and to ventricular septal defects. {middle dot} Cyp26b1 represses cell proliferation in valve mesenchyme. {middle dot} Retinoic acid targets are upregulated in Cyp26b1-/- heart valves, indicating dysregulation of RA metabolism.