Background:Definite identification of the cell types and the mechanism relevant to cardiomyogenesis is essential for effective cardiac regenerative medicine. We aimed to identify the cell populations that can generate cardiomyocytes and to clarify whether generation of donor-marker+ cardiomyocytes requires cell fusion between BM-derived cells and recipient cardiomyocytes.Methodology/Principal Findings:Purified BM stem/progenitor cells from green fluorescence protein (GFP) mice were transplanted into C57BL/6 mice or cyan fluorescence protein (CFP)-transgenic mice. Purified human hematopoietic stem cells (HSCs) from cord blood were transplanted into immune-compromised NOD/SCID/IL2rγnull mice. GFP+ cells in the cardiac tissue were analyzed for the antigenecity of a cardiomyocyte by confocal microscopy following immunofluorescence staining. GFP+ donor-derived cells, GFP+CFP+ fused cells, and CFP+ recipient-derived cells were distinguished by linear unmixing analysis. Hearts of xenogeneic recipients were evaluated for the expression of human cardiomyocyte genes by real-time quantitative polymerase chain reaction. In C57BL/6 recipients, Lin-/lowCD45+ hematopoietic cells generated greater number of GFP+ cardiomyocytes than Lin-/lowCD45- mesenchymal cells (37.0+/-23.9 vs 0.00+/-0.00 GFP+ cardiomyocytes per a recipient, P = 0.0095). The number of transplanted purified HSCs (Lin-/lowSca-1+ or Lin-Sca-1+c-Kit+ or CD34-Lin-Sca-1+c-Kit+) showed correlation to the number of GFP+ cardiomyocytes (P<0.05 in each cell fraction), and the incidence of GFP+ cardiomyocytes per injected cell dose was greatest in CD34-Lin-Sca-1+c-Kit+ recipients. Of the hematopoietic progenitors, total myeloid progenitors generated greater number of GFP+ cardiomyocytes than common lymphoid progenitors (12.8+/-10.7 vs 0.67+/-1.00 GFP+ cardiomyocytes per a recipient, P = 0.0021). In CFP recipients, all GFP+ cardiomyocytes examined coexpressed CFP. Human troponin C and myosin heavy chain 6 transcripts were detected in the cardiac tissue of some of the xenogeneic recipients.Conclusions/Significance:Our results indicate that HSCs resulted in the generation of cardiomyocytes via myeloid intermediates by fusion-dependent mechanism. The use of myeloid derivatives as donor cells could potentially allow more effective cell-based therapy for cardiac repair.
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