Exosomal miRNA-let-7i-5p from bone marrow mesenchymal stem cells protects against myocardial infarction by inhibiting myocardial apoptosis

OBJECTIVES: To elucidate the regulatory effect of exosomes secreted by bone marrow mesenchymal stem cells (BMSCs-exosomes) on cardiomyocyte apoptosis through miRNA-let-7i-5p in myocardial infarction. METHODS: BMSCs-exosomes were extracted, and their morphology and size distribution were analyzed using transmission electron microscope. Expression of exosome surface markers was determined by western blot. H9C2 cells were randomly assigned into five groups, namely control, OGD, OGD+exos, OGD+exos+miR-let-7i-5p inhibitor and OGD+exos+miR-let-7i-5p inhibitor NC. Hypoxic cardiomyocytes were induced using glucose-free Dulbecco's Modified Eagle Medium (DMEM). Mice were randomly assigned into sham, myocardial infarction (MI), MI+exos, MI+exos+miR-let-7i-5p inhibitor and MI+exos+miR-let-7i-5p inhibitor NC groups. MI model was established by ligation of the left anterior descending (LAD) coronary artery. Subsequently, BMSCs-exosomes or BMSCs-exosomes transfected with miRNA-let-7i-5p inhibitor were incubated with hypoxia cardiomyocytes or injected into the MI mouse model. Cell survival was accessed by CCK-8 assay. Cardiomyocyte apoptosis was accessed with V-FITC/PI and TUNEL. Heart function of MI mice was evaluated by echocardiography. Myocardial infarct size was calculated through TTC staining. Relative miRNA-let-7i-5p expression level was determined by RT-qPCR. Expression of apoptosis-related proteins in myocardial tissue were detected by western blot. RESULTS: Exosomes secreted from BMSCs were successfully extracted. In H9C2 cells, miRNA-let-7i-5p expression was significantly upregulated, cell survival rate was increased, and the apoptosis rate was decreased after incubation with BMSCs-exosomes. In MI mice, injection of BMSCs-exosomes markedly upregulated miRNA-let-7i-5p level, reduced infarct size, improved cardiac function, and decreased apoptotic rate. BMSCs-exosomes treatment downregulated Bax and upregulated Bcl-2 protein expression. These effects were reversed by transfection with the miRNA-let-7i-5p inhibitor. CONCLUSIONS: through miRNA-let-7i-5p.