Epigenetic Modification at Notch Responsive Promoters Blunts Efficacy of Inducing Notch Pathway Reactivation After Myocardial Infarction

Rationale: The Notch pathway plays a key role in stimulating mammalian cardiomyocyte proliferation during development and in the early postnatal life; in adult zebrafish, reactivation of this pathway is also essential to drive cardiac regeneration after injury. Objective: We wanted to assess efficacy of Notch pathway stimulation in neonatal and adult hearts as a means to induce cardiac regeneration after myocardial infarction in mice. Methods and Results: In early postnatal life, cardiomyocyte exit from the cell cycle was paralleled by decreased Notch signaling and the establishment of a repressive chromatin environment at Notch-responsive genes, characterized by recruitment of the polycomb group enhancer of zeste homolog 2 methyltransferase and the acquisition of the histone 3 Lysine 27 trimethylation histone mark, as detected by chromatin immunoprecipitation. Forced Notch pathway activation by adenoassociated virus gene transfer of activated Notch1 or its ligand Jagged1 expanded the proliferative capacity of neonatal cardiomyocytes; this correlated with increased transcription of Notch target genes and maintenance of an open chromatin conformation at their promoters. The same adenoassociated virus vectors, however, were largely ineffective in stimulating cardiac repair after myocardial infarction in adult mice, despite optimal and long-lasting transgene expression. Analysis of Notch-responsive promoters in adult cardiomyocytes showed marks of repressed chromatin and irreversible CpG DNA methylation. Induction of adult cardiomyocyte re-entry into the cell cycle with microRNAs was independent from Notch pathway reactivation. Conclusions: Notch pathway activation is crucial in regulating cardiomyocyte proliferation during the early postnatal life, but it is largely ineffective in driving cardiac regeneration in adults, because of permanent epigenetic modification at Notch-responsive promoters.