Introduction: Earlier, in a first study of its kind, we have demonstrated a novel mechanism that therapeutically significant human CD34+ stem cells secrete membrane bound nano-vesicles called exosomes (CD34Exo). CD34Exo are angiogenic and constitute a critical component of the pro-angiogenic paracrine activity of the cells. Further, when transplanted locally, cell-free CD34Exo induce ischemic tissue repair in a murine hindlimb ischemia model.
Here, we hypothesize that exosomes released via paracrine secretion from human CD34+ cells mediate myocardial repair by direct transfer of microRNAs to target cells in the heart.
Methods and Results: When injected into mouse ischemic myocardium, cell-free CD34Exo replicated the therapeutic activity of human CD34+ cells by significantly improving ischemia (ejection fraction, 42±4 v 22±6%; capillary density, 113±7 v 66±6/HPF; fibrosis, 27±2 v 48±7%; p<0.05, n=7-12) compared with PBS control. Interestingly, confocal imaging and flow cytometry analyses of the exosomes-injected ischemic myocardial tissue revealed that CD34Exo was selectively internalized into endothelial cells and cardiomyocytes. CD34Exo, which is enriched with miR126, induced the expression of miR126 and several pro-angiogenic mRNAs in the exosomes-treated ischemic myocardium, but did not affect the endogenous synthesis of miR126. CD34Exo lacking miR126 had decreased angiogenic activity in vitro and decreased proangiogenic gene expression in vivo indicating that miR126 is important for CD34Exo function. Imaging using fluorescent miR126 confirms that CD34Exo directly transferred miR126 and possibly other yet to be identified moieties from its cargo, selectively to endothelial cells and cardiomyocytes in the ischemic heart.
Conclusion: Our results reveal a novel molecular and trafficking mechanism of CD34Exo that may be responsible for intercellular transfer of genetic information such as miRNAs from human CD34+ stem cells, selectively to endothelial cells and cardiomyocytes inducing changes in gene expression, angiogenesis and myocardial recovery. Exosomes-shuttled miRNAs may signify amplification of stem cell function and may explain the therapeutic benefits associated with human CD34+ cell therapy.