Abstract 1392: Single Cardiac Stem Cells in Adult Mammals Exhibit Mesenchymal Features and Proliferate through Akt/GSK3 Pathway
Recent studies and our work have shown the existence of intrinsic stem cells from the adult heart based on the expression of stem cell antigen-1 (Sca-1); however, whether these cardiac stem cells are derived from a single entity remains unknown as is the question of which signaling pathways are involved in stem cell self-renewal. In the present study, we investigated the presence of cardiac stem cells in the adult mammalian heart on the basis of sphere-forming capacity without progenitor selection using specific cell surface markers and identified a novel population of cells on the hierarchy characteristic of mesenchymal stem cells (MSCs) which heterogeneously expressed Sca-1. To investigate the role of Sca-1 in cardiac MSCs (ca-MSCs), we generated Sca-1 knockdown (KD) mice in which double stranded Sca-1 RNA was expressed under the control of an RNA polymerase II promoter. Although the morphological configuration at baseline was comparable between the hearts of KD and wild-type (WT) mice, KD mice displayed significantly increased mortality as compared with WT mice, with 70% of the KD mice dying by 7 days after myocardial infarction, whereas 75% of the WT mice remained alive at this time. Histological examination demonstrated LV remodeling and increase in fibrotic area in KD hearts after infarction. To determine the mechanisms of early cardiac death in ischemic KD mice, ca-MSCs were isolated from KD mice and showed significantly impaired growth kinetics concomitant with the reduction of BrdU incorporation and p53 upregulation as compared with WT controls. In addition, phosphorylation of Akt by EGF/bFGF stimulation was inhibited in ca-MSCs failing to express Sca-1 but the activation of ERK1/2 was unaffected. In cell transfer experiments, ca-MSCs lacking Sca-1 transplanted into ischemic hearts resulted in larger LV volume and infarct size as compared with WT ca-MSC implantation. In human cell culture, Akt inhibitor decreased the self-renewal capacity of ca-MSCs, whereas GSK3 inhibitor, BIO, increased their replicative growth. In conclusion, resident ca-MSC population constitutes a novel source of cardiac stem cells in the adult mammalian heart and Akt/GSK3 may be crucial regulators for maintaining the self-renewal of ca-MSCs for myocardial regeneration.