Transcriptional dynamics of the murine heart during perinatal development at single-cell resolution.

Heart maturation and remodelling during the foetal and early postnatal period are critical for the proper survival and growth of the foetus, yet our knowledge of the molecular processes involved is lacking for many cardiac cell types. To gain a deeper understanding of the transcriptional dynamics of the heart during the perinatal period, we performed single-cell RNA sequencing on foetal and early postnatal mouse hearts to establish a catalogue of 49,769 single-cell transcriptomes and used this for bioinformatics analyses. Pseudotime analyses and RNAscope fluorescence in situ hybridisation showed that while lncRNA H19 expression decreased over time in multiple cardiac cell types, it remained stably expressed in endocardial and valve endothelial cells. To further investigate this in a human setting, we generated human induced pluripotent stem cell (hiPSC)-derived endothelial cells and used single-cell data to develop a sorting strategy to separate endocardial from vascular endothelial cells, ensuring endothelial subtype purity. Knockdown of H19 in hiPSC-derived endocardial cells resulted in morphological changes, decreased expression of endothelial markers, and increased expression of mesenchymal markers, consistent with the onset of endothelial-to-mesenchymal transition. Together, our data suggest a role for H19 in maintaining endocardial and valve endothelial cell states during perinatal heart maturation.