Non-Hematopoietic Bone Marrow Cells Can Be Mobilized and Differentiate into Cardiomyocytes after Myocardial Infarction: Possible Contribution of Mesenchymal Stem Cells.

Recent studies have suggested that bone marrow (BM) cells can contribute to regeneration processes in various tissues. Cardiomyocytes derived from BM cells have been observed after myocardial infarction (MI), and BM-derived cells mobilized by cytokines were capable of regenerating the myocardial tissue, leading to an improvement in survival or cardiac function after MI. However, the responsible BM cells have not been fully identified. The most likely candidates for the BM-derived stem cells with the ability to regenerate myocardial tissue are hematopoietic stem cells (HSC) and mesenchymal stem cells (MSC). The aim of this study was to determine the precise origin of the BM cells mobilized by cytokines to repair infarcted myocardium. Here, we used two independent clonal studies; we transplanted genetically marked single HSC or clonal MSC into lethally-irradiated recipient mice, induced MI, treated the mice with G-CSF, and analyzed the cardiac tissues. First, we transplanted single CD34⁻c-kit⁺Sca-1⁺lineage⁻ side population (CD34⁻KSL-SP) cells or whole BM cells from mice ubiquitously expressing enhanced green fluorescent protein (EGFP) into lethally-irradiated mice, induced myocardial infarction (MI), and treated the animals with G-CSF to mobilize stem cells to the damaged myocardium. At 8 weeks post-MI, from 100 specimens we counted only 3 EGFP⁺actinin⁺ cells in myocardium of CD34⁻KSL-SP cells-transplanted mice, but more than 5,000 EGFP⁺actinin⁺ cells in whole BM cell-transplanted mice, suggesting that most of EGFP⁺actinin⁺ cells derived from non-hematopoietic BM cells. These results suggested that the major population of cells mobilized from the BM and active in the regeneration of cardiomyocytes was non-hematopoietic in origin. Next, clonally purified mesenchymal stem cells, CMG cells, transfected with a pMLC2v-EGFP plasmid encoding EGFP driven by the myosin light chain promoter were transplanted directly into BM of lethally-irradiated mice, MI was induced, and they were treated with G-CSF. As a result, a number of EGFP⁺actinin⁺ cells were observed in the ischemic myocardium, indicating that CMG cells had been mobilized and differentiated into cardiomyocytes. Together, these results suggested that the vast majority of BM-derived cardiomyocytes are of mesenchymal origin.