Regulation of Notch signaling in the heart by epigenetic modifications

Synopsis: Understanding the molecular mechanisms regulating cardiac cell proliferation during the embryonic, fetal and adult life is of paramount importance in view of developing innovative strategies aimed at inducing myocardial regeneration after cardiac damage. The Notch pathway plays a key role in the regulation of cardiomyocyte proliferation during mammalian embryonic life. Moreover, it is essentially involved in the cardiac regeneration process after injury in Zebrafish. Therefore, we assessed the efficacy of Notch pathway activation to sustain cardiac regeneration in a model of myocardial infarction in mice. During early postnatal life, cardiomyocytes exit the cell cycle. We demonstrated that this event is paralleled by a decrease of Notch signaling and by the establishment of a repressive chromatin environment at Notch target genes, characterized by Polycomb Group protein 2-mediated silencing. The stimulation of the Notch pathway through Adeno-associated virus-mediated gene transfer of activated Notch1 or of the soluble form of the ligand Jagged1 prolonged the capacity of cardiomyocytes to replicate, which correlated with an increased rate of Notch target gene expression and the maintenance of an open chromatin conformation at Notch target gene promoters. However, the same vectors were ineffective in stimulating cardiac repair in a model of myocardial infarction in adult mice, despite efficient transgene expression. We identified the molecular cause of the lack of action of Notch signaling stimulation in adults in the increased DNA methylation at Notch target gene promoters, which correlated with permanent switch off of the Notch pathway. Our results confirm that the Notch pathway is an important regulator of neonata adults, due to the permanent epigenetic modifications at the DNA level at Notch responsive genes l.