MicroRNA-34a Plays a Key Role in Cardiac Repair and Regeneration Following Myocardial Infarction.

RATIONALE In response to injury, the rodent heart is capable of virtually full regeneration via cardiomyocyte proliferation early in life. This regenerative capacity, however, is diminished as early as 1 week postnatal and remains lost in adulthood. The mechanisms that dictate postinjury cardiomyocyte proliferation early in life remain unclear. OBJECTIVE To delineate the role of miR-34a, a regulator of age-associated physiology, in regulating cardiac regeneration secondary to myocardial infarction (MI) in neonatal and adult mouse hearts. METHODS AND RESULTS Cardiac injury was induced in neonatal and adult hearts through experimental MI via coronary ligation. Adult hearts demonstrated overt cardiac structural and functional remodeling, whereas neonatal hearts maintained full regenerative capacity and cardiomyocyte proliferation and recovered to normal levels within 1-week time. As early as 1 week postnatal, miR-34a expression was found to have increased and was maintained at high levels throughout the lifespan. Intriguingly, 7 days after MI, miR-34a levels further increased in the adult but not neonatal hearts. Delivery of a miR-34a mimic to neonatal hearts prohibited both cardiomyocyte proliferation and subsequent cardiac recovery post MI. Conversely, locked nucleic acid-based anti-miR-34a treatment diminished post-MI miR-34a upregulation in adult hearts and significantly improved post-MI remodeling. In isolated cardiomyocytes, we found that miR-34a directly regulated cell cycle activity and death via modulation of its targets, including Bcl2, Cyclin D1, and Sirt1. CONCLUSIONS miR-34a is a critical regulator of cardiac repair and regeneration post MI in neonatal hearts. Modulation of miR-34a may be harnessed for cardiac repair in adult myocardium.