Abstract 18132: Extracellular Vesicles Secreted From Mesenchymal Stem Cells Promote Mature Cardiomyocyte Proliferation

It is well known that the heart cannot regenerate adequately after injury due to limited regenerative capacity of myocardium. Previous studies indicated that the paracrine factors released from stem cells promote ischemic myocardium repair. Therefore, we hypothesized that extracellular vesicles (EVs) released from mesenchymal stem cells (MSCs) deliver bioactive molecules to induce proliferation of mature cardiomyocytes (CMs). MSCs were isolated from rat bone marrow and EVs were harvested from conditioned medium. Mature CMs were obtained from adult Sprague-Dawley rat ventricles and plated on laminin-coated cover-glass or Petri dishes. The dedifferentiation of mature CMs was recorded by a real-time imaging system which was installed inside a conventional cell-culture incubator. The shape of CMs changed from long-rod to short rod, and finally becoming round before they began to grow up (Fig. A). The cell number was significantly increased in CM cultured in the serum-free medium containing EVs after 7 days than that treated with BSA at same protein concentration (Fig. B). To confirm the proliferation of mature CM, cultured cells were stained with Ki67 and EdU, respectively. CMs were identified by staining with α-actinin. The percentage of Ki67 + CM and EdU + CM were significantly higher in the group treated with EVs compared to that treated with BSA control (Fig. C). Furthermore, microRNA-seq data showed that 358 miRs were found in EVs and the let-7 family members have high reads. To investigate whether EVs deliver these miRs into CMs, EVs pre-labeled with PKH26 were added into CM culture. It was found that EVs could be quickly internalized by CMs and the expression of let-7 miRs in CMs was significantly upregulated. Our studies support the hypothesis that EVs released from MSC promote mature CM re-entry cell-cycle and proliferation through let-7 miRs and possibly other bioactive molecules.