Highly Purified Hematopoietic Stem Cell Can Generate Cardiomyocytes Via Myeloid Progenitors through Cell Fusion.

[Introduction] Recent reports demonstrating the ability of bone marrow (BM) cells to regenerate cardiomyocytes have prompted clinical as well as basic studies for the treatment of ischemic and non-ischemic cardiomyopathies. However, the responsible BM cells and underlying mechanism of regeneration remain unclear. In this study, we checked the cardiomyogenic potential of BM cells in vivo, to identify the cell populations in BM that possess the capacity to give rise to cardiomyocytes and clarify whether cardiomyogenic potential of BM-derived cells require cell fusion with host cardiomyocytes.

[Method and Result] BM cells of mice constitutively expressing green fluorescent protein (GFP) was intravenously injected into irradiated syngeneic newborn C57BL/6 mouse within 48hours after the birth. At 4–5 weeks after transplantation, the cardiac tissue of recipients was analyzed for the number of GFP⁺ cardiomyocytes per 40 sections. We first separated Linaege antigen⁻ BM cells to CD45⁺ cells (Lin⁻CD45⁺) and CD45⁻ cells (Lin⁻CD45⁻) and transplanted each. In the recipients of Lin⁻CD45⁺, 37.0±23.9 GFP⁺ cardiomyocytes were detected (n=6). While no GFP⁺ cardiomyocyte was detected in the recipients of Lin⁻CD45⁻ (n=4). Since hematopoietic cells generated cardiomyocytes, we next transplanted limiting numbers of purified HSCs and hematopoietic progenitors by cell sorting. Both HSCs and hematopoietic progenitors generated cardiomyocytes dose-dependently, suggested that HSC can generate cardiomyocytes via hematopoietic progenitors. To explore the hematopoietic lineage, we further transplanted purified myeloid progenitors and common lymphoid progenitors by cell sorting. The number of GFP⁺ cardiomyocytes were 12.8±10.7 and 0.91±1.20 in the recipients of myeloid progenitors (n=7) and common lymphoid progenitors (n=10) respectively (p<0.005), suggested that myeloid lineage is more strongly involved in the generation of cardiomyocytes than lymphoid lineage. Furthermore, we confirmed the contribution of myeloid cells to cardiomyocytes by transplanting common myeloid progenitors (CMPs) and granulocyte/macrophage lineage-restricted progenitors (GMPs). Finally, we transplanted GFP⁺ HSCs and myeloid progenitors into CFP (cyan fluorescent protein) transgenic mouse. Linear unmixing analysis using confocal microscopy revealed that donor-derived GFP⁺ cardiomyocytes coexpressed CFP, indicated cell fusion had occurred. We are now analyzing recipients transplanted with single GFP⁺ HSC.

[Conclusion] Our results suggested that HSC can contribute to the post-natal generation of cardiomyocytes via myeloid progenitors through cell fusion.