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.
Preclinical Evaluation of the Engineered Stem Cell Chemokine Stromal Cell–Derived Factor 1α Analog in a Translational Ovine Myocardial Infarction Model