Figure 4.

Key signalling pathways in zebrafish endocardium and cardiac vessels during regeneration. Vessels at the injury border (orange circle) undergo a glycolytic switch and use glucose as a primary energy source. Border zone vessels express Apelin and Neuropilin 1 (Nrp1) and respond to Cxcl12b signals that originate at least in part from the activated epicardium. The activated endocardium (pink circle) expresses Notch, which regulates expression of endocardial maturation genes, endothelial differentiation, integrity, and pro-angiogenic genes (e.g., vegfc, klf2a/b). Additionally, endocardial Notch induces expression of Wnt antagonists wif1 and notum1b that in turn promote cardiomyocyte (CM) proliferation (Prolif). Activated endocardial cells express nrp1, aldh1a2, and runx1. A subpopulation of Runx1-positive cells expresses smooth muscle cell genes myh11a and taglnI. Runx1 also regulates anxa2 and serpine1 and limits fibrinolysis of scar tissue. Within the injury (blue circle), it is hypothesised that recruited macrophages and neutrophils combined with increased Hif1 activity upregulate Vegfa expression. An additional Vegfa source is thought to arise from proteolytic enzyme activity within the injury that releases extracellular matrix (ECM)–bound Vegfa. The activated epicardium (purple circle) secretes Cxcl12b chemokines that bind to the Cxcr4a receptor of endothelial cells in infiltrating vessels. Endothelial cell proliferation is further promoted by epicardial expression of Duox and Nox enzymes that catalyse the generation of hydrogen peroxide (H₂O₂). This subsequently inhibits Dusp6 activity in endothelial cells, thus relieving Dusp6 suppression of Erk signalling and enhancing endothelial proliferation. Fgf signalling promotes revascularisation and epicardial activation. Pdgf and Nrp1 promote epicardial activation, a subpopulation of epicardial cells undergo epicardial to mesenchymal transition (EpiMT). These epicardial-derived cells (EPDCs) become fibroblasts secreting ECM and perivascular cells. Nrg1 expression by EPDCs further promotes angiogenesis. Prox1a-expressing lymphatic vessels are detected in the injury and respond to Vegfc/d via Vegfr3. Inflammatory cells, necrotic and apoptotic cells, and extracellular matrix debris are removed via the lymphatic vessels (green circle) to aid the regenerative process. Figure generated with BioRender.com.