Characterization of the secretion of mesenchymal stem cells and its relevance to cardioprotection

Abstract

Acute myocardial infarction (AMI), which is caused by occlusion of coronary artery, results in myocardial infarction and this may eventually contribute to the development of heart failure. Ironically, reperfusion therapy, which restores blood flow and significantly limits ischemic injury, causes reperfusion injury and contributes to the final infarct size. Amelioration of reperfusion injury will therefore improve the efficacy of reperfusion therapy. However, there is still no effective treatment to limit reperfusion injury, and this is contributing to a growing epidemic of heart failure. Recent developments have indicated that secretion of mesenchymal stem cells (MSCs) can reduce reperfusion injury. However, the cardioprotective factor in the secretion and underlying mechanism of its cardioprotection remains to be elucidated. To identify the active component in MSC secretion, 0.2 µM filtered culture medium conditioned by human embryonic stem cell-derived MSCs was filtered sequentially through filters with decreasing pore sizes. Only the >1000 kDa fraction reduced infarct size in a mouse MI/R injury model. This physically limited the size of cardioprotective factor to 100-220 ¿m and the candidate factor to exosome. Electron microscopy showed the presence of 100 ¿m particles in the conditioned medium. Further analysis revealed the presence of co-immunoprecipitating exosome-associated proteins and the co-sedimentation of these proteins with membrane lipids after ultracentrifugation. These proteins were determined to have an exosome-like flotation density of 1.10-1.16 µg/ml by sucrose gradient centrifugation. These exosomes could be purified by size exclusion on HPLC and this purified exosome significantly reduced infarct size in the same mouse model. To assess if the secretion of cardioprotective exosome was restricted to hESC-derived MSCs, we derived 5 MSCs cultures from various tissues of 3 first-trimester aborted fetuses. These MSCs were highly expandable, displayed typical MSC surface antigen and gene expression profile, and possessed the MSC tri-lineage differentiation potential. Like hESC-MSCs, they produced exosomes that were cardioprotective in mouse MI/R injury model. Therefore, production of cardioprotective exosomes was not restricted to hESC-MSCs but was common to all MSCs. To understand the cardioprotective mechanism of MSC exosome, the biochemical potential of exosome in vitro and in vivo was assessed. Proteomic profiling of exosome identified 866 proteins that together had the potential to drive diverse biological processes. Several of these processes had the potential to reduce injury during reperfusion including enhancing glycolysis, inhibiting the formation of membrane attack complex, reducing oxidative stress and activating pro-survival kinases. Consistent with the in vitro data, exosome treatment in mouse model promoted pro-survival signaling, enhanced ATP production and redox balance. These probably contributed to the reduced infarct size and preserved cardiac function and geometry that observed in the exosomes treated group. In summary, we identified exosome as the cardioprotective component in MSCs secretion. We further demonstrated that secretion of cardioprotective exosomes was not restricted to hESC-MSCs and suggested potential mechanisms underlying this cardioprotection. These findings not only redefined the paracrine mechanism of MSCs, more importantly they might lead to the development of adjunctive reperfusion therapy.