Endocardial primary cilia and blood flow regulate EndoMT during endocardial cushion development [Single Nuc-RNAseq_IFT]
Blood flow is critical for heart valve formation, and cellular mechanosensors are essential to translate flow into transcriptional regulation of development.  Here, we identify a role for primary cilia in vivo in the spatial regulation of cushion formation, the first stage of valve development, by regionally controlling endothelial to mesenchymal transition (EndoMT) via modulation of Kruppel-like Factor 4 (Klf4). We find that high shear stress intracardiac regions decrease endocardial ciliation over cushion development, correlating with KLF4 downregulation and EndoMT progression. Mouse embryos constitutively lacking cilia exhibit a blood-flow dependent accumulation of KLF4 in these regions, independent of upstream left-right abnormalities, resulting in impaired cushion cellularization. snRNA-seq revealed that cilia KO endocardium fails to progress to late-EndoMT, retains endothelial markers and has reduced EndoMT/mesenchymal genes that KLF4 antagonizes. Together, these data identify a mechanosensory role for endocardial primary cilia in cushion development through regional regulation of KLF4. Overall design: Whole hearts of e9.5 mouse embryos (Ift20+/+ (wildtype), Ift20+/-, Ift20-/-) were microdissected and flashfrozen. Five flashfrozen hearts of each genotype were pooled and prepared for single nuclei RNA-sequencing. Ift20+/+ had 3 replicates of this process; Ift20+/- and Ift20-/- had 2.