Abstract 710: Notch1 Receptor Enhances Myocyte Differentiation of Cardiac Progenitor Cells and Myocardial Regeneration After Infarction
Cardiac progenitor cells (CPCs) have been identified in the adult heart. However, the molecular mechanisms involved in the commitment of CPCs to the myocyte lineage remain to be determined. Notch-1 is a transmembrane receptor activated by the DSL family of ligands which include Jagged1 and Delta-4. Upon ligand binding, the activated receptor undergoes cleavage by γ-secretase, and its intracellular portion (Notch intracellular domain, NICD) is released, translocates to the nucleus and exerts its function as a transcriptional regulator. The objective of this study was to determine whether the components of the Notch pathway are present in the CPCs of the adult mammalian heart and whether activation of the Notch-1 receptor promotes the differentiation of CPCs into myocytes. For this purpose, c-kit-positive CPCs were isolated from the mouse heart and analyzed by FACS and immunocytochemistry. Notch-1 receptor was detected in ~50% of c-kit-positive CPCs. CPCs were then plated on culture dishes coated with immobilized Jagged1 or Delta-4 and maintained in low-serum medium. Additional groups of cells were similarly exposed to the ligands but were also treated with γ-secretase inhibitor. After 5 days in culture, the number of CPCs was markedly lower in the presence of the Notch ligands and significantly higher in the presence of the γ-secretase inhibitor. After 8 days in culture, cells became confluent and did not express any longer c-kit. With respect to cells treated with the γ-secretase inhibitor, exposure to Jagged1 and Delta-4 resulted respectively in a 10-fold and 20-fold increase in the fraction of CPCs positive for Nkx2.5. These findings were consistent with a positive effect of Notch on CPC differentiation and Nkx2.5 upregulation. To establish whether Notch influenced cardiomyogenesis in vivo, infarcted mice were treated for 11 days with the γ-secretase inhibitor. The regenerative response of the infarcted heart, defined by the percentage of BrdU-positive myocytes distributed in the border zone, was 50% lower in animals that received the γ-secretase inhibitor. Thus, Notch1 modulates CPC differentiation in vitro and myocardial regeneration in vivo after infarction.