Injured endocardium obtains characteristics of haemogenic endothelium during adult zebrafish heart regeneration

Reactivation of embryonic developmental pathways during regeneration aims to restore tissue architecture and functionality. We previously reported that following cryoinjury, a heterogeneous population of Runx1-expressing endocardial cells differentially upregulates genes associate with scarring and myofibroblast identity. Further analysis of our published RNAseq data alongside 5 publicly available datasets now identifies additional heterogeneity in the Runx1-positive injured endocardium. Here, we show that the endocardium also reactivates a dormant endocardial-to-haematopoietic transition (EHT) mechanism. Runx1-expressing endocardial cells upregulate genes associated with haemogenesis and morphologically display features of EHT. Live imaging shows cells budding off the endocardium and lineage analysis identifies overlap with leukocyte markers. Ablation of runx1 function further shifts differentiation of the endocardium towards the EHT fate. The identification of transient runx1 -expressing cells transitioning towards myofibroblast or haemogenic endocardium identities demonstrates the complexity of the zebrafish endocardial injury response and highlights the role of Runx1 in regulating cell fate decisions in the endocardium.