Background Zebrafish are able to fully regenerate their hearts after injury by resolving the initial scar and replenishing the injury site with functional cardiomyocytes. Transforming growth factor β and platelet-derived growth factor BB are known to be important for zebrafish cardiac regeneration. They are also ligands that trigger intracellular responses via receptor complexes including neuropilins (NRPs). NRP1 and NRP2 are co-receptors for vascular endothelial growth factors and other cytokines, essential for developmental angiogenesis and implicated in diverse pathophysiological processes, but little is known of their role following myocardial injury.
Hypothesis Because several NRPs binding partners are involved in zebrafish heart repair, we hypothesised that neuropilins play a role in zebrafish heart regeneration. We used the zebrafish cryoinjury model of myocardial infarction to characterise neuropilin spatio-temporal expression in the regenerating heart.
Results Quantitative PCR shows nrp1a, nrp1b and nrp2a are significantly upregulated at 1 and 3 days post cryoinjury (dpci), whereas nrp2b, the mostly highly expressed isoform in the heart, does not change after cardiac damage. In situ hybridisation shows expression of nrp1a mRNA localised to the activated epicardium, proximal to the injury. Immunofluorescence imaging also indicated strong upregulation of NRP1 by epicardial cells overlaying the injured region, and expression by the endocardium. Furthermore, epicardial NRP1 expression was demonstrated in epicardial cells in vitro. NRP2 is expressed in leucocytes during the initial inflammatory phase following injury and by cardiomyocytes migrating into the injured zone during the regenerative phase.
Conclusions Neuropilin isoforms exhibit marked regional differences in expression following cryoinjury. Our results indicate a novel role for NRP1 in the activation of the epicardium, a region of the heart with an essential role in cardiac regeneration, whereas NRP2 may be more important for inflammation and cardiomyocyte replacement.