Abstract Body: Introduction
The cells that comprise the circulatory system not only share their origins but also mutually promote each other’s differentiation during the formation of functional circulatory system. We previously reported a subset of endocardial cells is hemogenic during early embryogenesis. They are enriched in the cushion region, the primordia of the cardiac valves and septa, where active remodeling via endothelial-mesenchymal transition takes place. Hemogenic endocardial cells undergo endocardial-hematopoietic transition (EHT) via Nkx2-5-dependent manner, suggesting that Drosophila tinman-dependent cardio-hematopoietic program is conserved in mammals.
Methods
In this study, we analyzed the regulatory network of Nkx2-5-dependent endocardial hematopoiesis using a scRNA-seq data from wildtype and Nkx2-5-null embryonic hearts. In vivo genetic analysis were performed to confirm the candidate signal pathways driven under Nkx2-5 transcription.
Results and conclusion
Nkx2-5-null hearts were devoid of clusters for hemogenic endocardium and cushion endocardium. Interestingly, scRNA-seq analysis further revealed that genes related to Notch signaling pathway are significantly downregulated in Nkx2-5-null endocardium. A further gene network analysis identified that Dhrs3, an enzyme that attenuates retinoic acid (RA) signal by catalyzing the reduction of all-trans-retinaldehyde to all-trans-retinol, is a signature gene of the hemogenic endocardial cells downstream of Nkx2-5. Although RA signal is known to induce the formation of multipotent progenitor, our ex vivo hematopoietic colony forming assay revealed that macrophage formation is strongly inhibited by RA signal. Notch inhibition also suppressed the formation of macrophages. Consistently, in vivo forced activation of NICD drastically increased the number of CD41⁺ hemogenic endocardial cells as well as macrophages in the cardiac cushion. Strikingly, impaired hematopoiesis and cushion defects in the Nkx2-5-null heart were both rescued by overexpression of NICD, suggesting that Notch signaling promotes endocardial hematopoiesis downstream of Nkx2-5. Taken together, our study demonstrated that the Nkx2-5/RA/Notch signaling axis plays a pivotal role in EHT during early embryogenesis, thereby facilitating local tissue remodeling by inducing macrophage differentiation.
The talk will include the latest findings on the specific role of endocardially-derived tissue macrophages.