Exosomes secreted by human cardiac progenitor cells inhibit cardiomyocyte apoptosis and improve cardiac function after myocardial infarction

Introduction: Cardiac progenitor cell (CPC) transplantation improves cardiac function after myocardial infarction (MI). This effect is mediated, at least in part, by secreted factors. Exosomes (Exo) are secreted nano-sized membrane vesicles acting as intercellular carriers of proteins and nucleic acids, including microRNA (miRNA). Here, we investigated the role of Exo in the paracrine activity of human CPCs. Methods: Atrial appendage specimens were obtained from patients who underwent heart valve surgery. CPCs were derived from the cellular outgrowth of these specimens using the primary ex vivo tissue culture technique. Exo were purified from CPC-conditioned medium (CM-CPC) using ExoQuick and analysed by transmission electron microscopy (TEM). Functional effects of CM-CPC and purified Exo were assessed in vitro using angiogenesis and apoptosis models in human endothelial cells (HUVECs) and mouse HL-1 cardiomyocytes (CMC), respectively. Apoptosis was assessed by activated caspase 3/7 immunostaining. The effects of cryopreserved Exo derived from human CPCs were compared with those from normal human dermal fibroblasts (NHDF). In vivo studies were performed in a rat model of myocardial infarction (MI) induced by permanent left anterior coronary artery ligation. Exo were injected into the infarct border zone 1 h after coronary ligation. Cardiac function was assessed by echocardiography. Apoptosis was detected by TUNEL. Results: Culture medium conditioned by CPCs (CM-CPC) stimulated tube formation by endothelial cells (HUVECs) and inhibited apoptosis of mouse HL-1 cardiomyocytes after serum deprivation in vitro. Depletion of Exo abolished the proangiogenic and antiapoptotic activities of CM-CPC. Supplementation of Exo-depleted CM with Exo-CPC restored this activity. Exo-CPC inhibited cardiomyocyte apoptosis in a dose-dependent manner, whereas Exo secreted by normal human dermal fibroblasts (Exo-F) did not. miRNA analysis identified miR-146a-3p, miR-181a, miR-132 and miR-210-3p among the most highly upregulated miRNAs in Exo-CPC versus Exo-F (15-, 11-, 5- and 4-fold, respectively). In a rat model of MI, Exo-CPC significantly reduced cardiomyocyte apoptosis and scar, enhanced angiogenesis and preserved LVEF 7 days post-MI (0.8±6.8%) compared with controls (–21.3±4.5%; p<0.05) and Exo-F-injected hearts (–12.0±6.3%). Summary/conclusion: Exo accounts for proangiogenic and antiapoptotic activities of the paracrine secretion of human CPCs, which preserve cardiac function post-MI. As a cell-free product, Exo-CPC may circumvent many of the limitations of cell transplantation for cardiac repair.