. Combining neuropeptide Y and mesenchymal stem cells reverses remodeling after myocardial infarction. Am J Physiol Heart Circ Physiol 298: H275-H286, 2010. First published November 6, 2009 doi:10.1152/ajpheart.00765.2009.-Neuropeptide Y (NPY) induced reentry of differentiated rat neonatal and adult cardiomyocytes into the cell cycle. NPY also induced differentiation of bone marrow-derived mesenchymal stem cells (MSC) into cardiomyocytes following transplantation into infarcted myocardium. Rat neonatal and adult cardiomyocytes were treated in vitro with vehicle, NPY, fibroblast growth factor (FGF; 100 ng/ml), or FGF plus NPY. DNA synthesis, mitosis, and cytokinesis were determined by immunocytochemistry. NPY-induced MSC gene expression, cell migration, tube formation, and endothelial cell differentiation were analyzed. Male rat green fluorescent protein-MSC (2 ϫ 10 6 ), pretreated with either vehicle or NPY (10 Ϫ8 M) for 72 h, were injected into the border zone of the female myocardium following left anterior descending artery ligation. On day 30, heart function was assessed, and hearts were harvested for histological and immunohistochemical analyses. NPY increased 5-bromo-2Ј-deoxy-uridine incorporation and promoted both cytokinesis and mitosis in rat neonatal and adult myocytes. NPY also upregulated several genes required for mitosis in MSC, including aurora B kinase, FGF-2, cycline A2, eukaryotic initiation factor 4 E, and stromal cell-derived factor-1␣. NPY directly induced neonatal and adult cardiomyocyte cell-cycle reentry and enhanced the number of differentiated cardiomyocytes from MSC in the infarcted myocardium, which corresponded to improved cardiac function, reduced fibrosis, ventricular remodeling, and increased angiomyogenesis. It is concluded that a combined treatment of NPY with MSC is a novel approach for cardiac repair. deoxyribonucleic acid synthesis FETAL AND NEONATAL CARDIAC myocytes are capable of undergoing DNA synthesis and cell division. However, their ability to divide diminishes progressively during postnatal development (2). Adult mammalian cardiomyocytes are considered terminally differentiated, incapable of proliferation and irreversibly withdrawn from the cell cycle soon after birth (28, 29). The permanent loss of cardiomyocytes following myocardial infarction (MI) often results in heart failure. Potential therapies could include stimulating the myocytes to divide and adding new cells via pharmacological and genetic manipulations or delivering stem cells to multiply and subsequently differentiate into cardiomyocytes (7). Recent reports have shown that adult cardiac myocytes can be induced to reenter into cell cycle with periostin (17), p38 MAP kinase inhibitor (8), cyclin D1/CDK4 (27), cyclin A2 (3), and transforming growth factor- (4).Although the adult heart has a limited capacity for myocyte proliferation, increasing the number of remaining cardiomyocytes by activating their proliferative potential via extrinsic factors could result in the repair of damaged myocardium. Therefor...