Injury-activated endocardium plays structural and signalling roles in zebrafish heart regeneration

The zebrafish heart remarkably regenerates after a severe ventricular damage followed by inflammation, fibrotic tissue deposition and removal concomitant with cardiac muscle replacement. We have investigated the role of the endocardium in this regeneration process. 3D-whole mount imaging in injured hearts revealed that GFP-labelled endocardial cells in ET33mi-60A transgenic fish become rapidly activated and highly proliferative at 3 days post cryoinjury (dpci). Endocardial cells extensively expand within the injury site and organize to form a coherent structure at 9 dpci that persists throughout the regeneration process. Upon injury, endocardial cells strongly up-regulate the Notch pathway ligand delta like4 (dll4) and the Notch receptors notch1b, notch2 and notch3. Expression profiling showed that Notch signalling inhibition affects endocardial gene expression and genes related to extracellular matrix remodelling and inflammation. Gain- and loss-of-function experiments revealed that Notch is required for the organization of the endocardium, attenuation of the inflammatory response and cardiomyocyte proliferation. These results demonstrate a novel structural and signalling role for the endocardium during heart regeneration.