Neutrophils facilitate the epicardial regenerative response after zebrafish heart injury

Despite extensive studies on endogenous heart regeneration within the past 20 years, the players involved in initiating early regeneration events are far from clear. Here, we assessed the function of neutrophils, the first-responder cells to tissue damage, during heart regeneration. We detected rapid neutrophil mobilization to the injury site after ventricular amputation, peaking at 1-day post-amputation (dpa) and resolving by 3 dpa. Further analyses indicated neutrophil mobilization coincides with peak epicardial cell proliferation and recruited neutrophils associated with activated, expanding epicardial cells at 1 dpa. Neutrophil depletion inhibited myocardial regeneration and significantly reduced epicardial cell expansion, proliferation, and activation. To explore the molecular mechanism of neutrophils on the epicardial regenerative response, we performed scRNA-seq analysis of 1 dpa neutrophils and identified enrichment of the FGF and ERK signaling pathways. Pharmacological inhibition of FGF signaling indicated its’ requirement for epicardial cell expansion, while neutrophil depletion blocked ERK signaling activation in epicardial cells. Altogether, our studies revealed that neutrophils quickly induce epicardial cells, which later accumulate at the injury site and contribute to heart regeneration. Neutrophils from the zebrafish heart were isolated by fluorescence-activated cell sorting (FACS) according to the presence or absence of eGFP signal and analyzed using scRNAseq.