Cardiac regeneration: effect of nerve growth factor

Heart failure (HF) is a leading cause of death, disability and hospitalization particularly in individuals over 65 years of age. Current treatments provide symptomatic benefit and can also improve survival as well as reversing maladaptive ventricular remodelling. However, HF frequently progressively worsens in association with the ongoing loss of cardiomyocytes. Cardiomyocytes rarely proliferate and cardiac progenitor cells (CPCs) are scarce in the mammalian adult heart. As such, it is likely that insufficient cardiac regeneration following injury to the heart contributes to HF. In contrast to mammals, zebrafish are able to robustly regenerate their hearts sufficiently to prevent the onset of HF. Our group has shown that a key growth factor, Nerve growth factor (NGF), is lowered in failing hearts. Therefore, in this thesis, I hypothesised and investigated whether NGF may have a regenerative effect on the heart. To address this, a cardiotoxic model of heart failure/regeneration was established in larval zebrafish. I first optimised the exposure duration and concentration of a cardiotoxin (aristolochic acid, AA) to cause approximately half of the exposed larval zebrafish to develop HF. The addition of NGF following AA exposure significantly decreased the proportion of zebrafish from developing HF and dying prematurely. Conversely, the inhibition of the high affinity NGF (TrkA) receptor following AA exposure increased the incidence of HF and death. A common feature of HF is apoptosis in the heart, and NGF was previously shown to have an anti-apoptotic role in cardiomyocytes. However, in the current paradigm AA exposure increased apoptosis in zebrafish hearts, but surprisingly NGF did not attenuate apoptosis. My studies showed that NGF decreased the incidence of AA induced HF and death in zebrafish by promoting cardiac regeneration via cardiomyocyte proliferation. AA exposure decreased the total number of cardiomyocytes and inhibited cardiomyocyte proliferation. In contrast, the addition of NGF following AA exposure increased cardiomyocyte proliferation and total cardiomyocytes. In support of this finding, zebrafish in egg water supplemented with NGF also increased cardiomyocyte proliferation and total cardiomyocytes. Interestingly, isl1 mRNA was also increased in hearts from zebrafish exposed to AA with subsequent treatment to NGF compared to fish only exposed to AA. The regenerative effects of NGF were also validated in a mammalian system, using a mouse embryonic (E13.5) heart organ culture model. NGF promoted mouse embryonic cardiomyocytes to proliferate in vitro. In addition, NGF transiently upregulated Nrg1 and ErbB4 mRNA (a known signalling pathway involved in cardiomyocyte proliferation). Also, NGF increased Isl1 mRNA (at a developmental stage when Isl1+ CPCs are abundant), which suggests that NGF might also increase Isl1+ cardiac progenitor cells. This thesis has identified a novel cardiovascular role for NGF. NGF promotes cardiomyocyte proliferation in zebrafish and mice, possibly through the Nrg1-ErbB4 pathway. As such, the deficiency of NGF in the failing heart may contribute to the progressive nature of HF due to blunted regeneration. Further studies should address the therapeutic value of NGF in established HF