flt1 inactivation promotes zebrafish cardiac regeneration by enhancing endothelial activity and limiting the fibrotic response

VEGFA administration has been explored as a pro-angiogenic therapy for cardiovascular diseases including heart failure for several years; however, many challenges remain. Here we investigate a different approach to augmenting VEGFA bioavailability, one that achieves more physiological VEGFA concentrations by deleting VEGFR1/FLT1, a VEGFA decoy receptor. We find that, following cryoinjury, zebrafish flt1 mutant hearts display enhanced coronary revascularization and endocardial expansion, increased cardiomyocyte dedifferentiation and proliferation, and decreased scarring. Suppressing Vegfa signaling in flt1 mutants abrogates the beneficial effects of flt1 deletion. Transcriptomic analyses of cryoinjured flt1 mutant hearts revealed enhanced endothelial MAPK/ERK signaling and downregulation of the transcription factor gene egr3. Using genetic tools, we observe egr3 upregulation in the regenerating endocardium and find that Egr3 promotes myofibroblast differentiation. These data suggest that with enhanced VEGFA bioavailability, the cardiac endothelium limits myofibroblast differentiation via egr3 downregulation, thereby providing a more permissive microenvironment for cardiomyocyte replenishment after injury. Overall design: Comparative gene expression analysis between cryoinjured wild-type zebrafish ventricles and flt1 mutant ventricles at 96 hours post-cryoinjury.