Endocardial Notch Signaling Promotes Cardiomyocyte Proliferation in the Regenerating Zebrafish Heart through Wnt Pathway Antagonism

Previous studies demonstrate that the regenerative zebrafish heart responds to injury by upregulating Notch receptors in the endocardium and epicardium. Moreover, global suppression of Notch activity following injury impairs cardiomyocyte proliferation and induces scarring. However, the lineage-specific requirements for Notch signaling and full array of downstream targets remain unidentified. Here, we demonstrate that inhibition of endocardial Notch signaling following ventricular amputation compromises cardiomyocyte proliferation and stimulates fibrosis. RNA sequencing uncovered reduced levels of two transcripts encoding secreted Wnt antagonists, Wif1 and Notum1b, in Notch-suppressed hearts. Like Notch receptors, wif1 and notum1b are induced following injury in the endocardium and epicardium. Small-molecule-mediated activation of Wnt signaling is sufficient to impair cardiomyocyte proliferation and induce scarring. Last, Wnt pathway suppression partially restored cardiomyocyte proliferation in hearts experiencing endocardial Notch inhibition. Taken together, our data demonstrate that Notch signaling supports cardiomyocyte proliferation by dampening myocardial Wnt activity during zebrafish heart regeneration.