Results from several recent studies in lower vertebrates

Results from several recent studies in lower vertebrates are consistent with an important role for ECM-derived signals for heart regeneration (Fig. 2). Fibronectin, a major component of the ECM, is secreted by epicardial ag1478 and deposited at the injury site after cardiac injury in zebrafish (Wang et al., 2013). Loss-of-function experiments established that fibronectin is required for cardiac regeneration in zebrafish, although it does not affect cardiomyocyte proliferation in this species (Wang et al., 2013). The mechanism by which ECM-derived fibronectin influences heart regeneration in zebrafish is currently unknown but could be due to a potential role of fibronectin signaling on cardiomyocyte migration or epicardial cell function. On the other hand, transforming growth factor beta (TGFβ) ligands and receptors (tgfbr1b) are induced in fibroblasts and cardiomyocytes following cryoinjury in zebrafish and are required for cardiac regeneration (Chablais and Jazwinska, 2012). Transient scar formation was found to be an essential step in the zebrafish regenerative response following cryoinjury and pharmacological inhibition of TGFβ signaling abolished the regenerative response (Chablais and Jazwinska, 2012). Thus, early ECM deposition and TGFβ-mediated cardiomyocyte proliferative signals are required for zebrafish heart regeneration. Furthermore, a recent study in adult newts has established that early up-regulation of ECM components in the epicardium precedes cardiomyocyte proliferation and migration in this species. Interestingly, the ECM-derived component tenascin-C was shown to be sufficient to induce cardiomyocyte proliferation in vitro, suggesting that ECM-derived signals in the adult newt are also required for induction of cardiomyocyte proliferation during cardiac regeneration (Mercer et al., 2013).
Do neonatal immune responses dictate cardiac regenerative outcomes? Cardiac injury in lower vertebrates, neonatal mice and adult mammals is associated with inflammation, yet the physiological consequences of injury in neonatal and adult mammals are vastly different. The immune response is important for clearing cellular debris following myocardial infarction in adults but it also facilitates scar formation, which ultimately compromises cardiac function (Frangogiannis, 2012). Indeed, multiple lines of evidence support the notion that the adult mammalian immune system constrains regenerative capacity and recent studies suggest that the immune response to injury supports regeneration in lower vertebrates (King et al., 2012; Godwin et al., 2013; Kyritsis et al., 2012). In mammals, the adaptive and innate immune systems undergo drastic alterations and maturation during the neonatal period, coinciding with the developmental loss of cardiac regenerative capacity (Adkins et al., 2004; Kincade et al., 2002; Lu et al., 2002; Garcia et al., 2000). A recent study by Aurora et al. suggests that these postnatal changes in the immune system may also play an important role in dictating regeneration versus repair responses of the neonatal heart following injury (Aurora et al., 2014). Through systematic profiling of the cellular immune response to cardiac injury, Aurora et al. identified a number of qualitative and quantitative differences in the cellular immune response of 1-day-old (regenerative) and 14-day-old (non-regenerative) mice following myocardial infarction. Differences in the monocyte and macrophage responses to cardiac injury in neonatal mice were particularly prevalent. Subsequent depletion of macrophages in neonatal mice using a clodronate-liposome depletion method revealed that macrophages are required for neonatal heart regeneration following myocardial infarction. In contrast to the pro-fibrotic role of macrophages in the adult heart, neonatal macrophages were found to be required for heart regeneration. Interestingly, macrophages did not influence cardiomyocyte proliferation following infarction but were essential for angiogenesis (Fig. 3). Molecular profiling of macrophages in the regenerating (P1) and non-regenerating (P14) heart following infarction revealed a unique transcriptional signature of neonatal macrophages, including the expression of several potentially important pro-angiogenic cytokines. Future studies will be required to uncover the underlying mechanisms that govern these developmentally distinct macrophage responses to injury. In addition, whether the absence of regeneration in adult mammals is due to differences in the types of inflammatory cells that are recruited, the specific signals secreted from inflammatory cells, or the responsiveness of different cardiac cell types to these inflammatory stimuli remains to be determined.