Stromal Cell Derived Factor-1 Increases Capillary Density But Does Not Contribute To Cardiomyocyte Replacement After Experimental Myocardial Infarction

Background. The mechanisms by which regenerative therapies improve cardiac function are incompletely defined. Numerous laboratories have demonstrated that the stem cell chemoattractant Stromal cell derived factor-1 (SDF-1) improves cardiac function after myocardial infarction (MI). This study used a genetic fate-mapping approach to ask the question: Is the beneficial effect of SDF-1 delivery due to cardiomyocyte regeneration, increased capillary density, or both? Methods. We used a genetic fate-mapping system that allows “pulse-chase” studies of cardiomyocyte turnover in mice. We crossed an inducible cardiomyocyte-specific Mer-Cre-Mer transgenic mouse with Z/EG reporter mice. Using a high-efficiency 4-OH-tamoxifen protocol, we then induced Cre recombination and expression of GFP in cardiomyocytes only. SDF-1(S4V), a protease-resistant form of SDF-1, was delivered with nanofibers after coronary ligation. Results. The percentage of pulse-labeled GFP+ cardiomyocytes was 81±4% (n=6) in sham-operated mice, as anticipated with this system. GFP+ cardiomyocytes were 60±5% and 69±4% (n=11) respectively at MI border and MI remote areas of the control nanofiber group, consistent with our previous study and suggesting a significant stem/precursor cell contribution to cardiomyocyte replacement after injury (P<0.0001). GFP+ cardiomyocytes were 65±9% and 76±6% (n=10) respectively at MI border and MI remote areas of the SDF-1(S4V)-RAD group (p=ns vs nanofiber control group), indicating no significantly increase in cardiomyocyte refreshment attributable to SDF-1. However, capillary density increased from 204.7 ± 10.1/mm2 in the nanofiber control group to 308.9 ± 21.9/mm2 in SDF-1(S4V)-RAD + nanofiber group (p =0.0003). Conclusion. Using genetic “pulse-chase” fate mapping, these data indicate that the stem cell chemoattractant SDF-1 does not induce measurable adult mammalian cardiomyocyte replacement by stem/precursor cells following injury. However, SDF-1 significantly increases myocardial capillary density, suggesting that increased angiogenesis but not cardiogenesis is responsible for the beneficial effects of SDF-1 on cardiac function.