Bone marrow-derived side population cells are capable of functional cardiomyogenic differentiation

It has been reported that bone marrow (BM)-side population (SP) cells, with hematopoietic stem cell activity, can transdifferentiate into cardiomyocytes and contribute to myocardial repair. However, this has been questioned by recent studies showing that hematopoietic stem cells (HSCs) adopt a hematopoietic cell lineage in the ischemic myocardium. The present study was designed to investigate whether BM-SP cells can in fact transdifferentiate into functional cardiomyocytes. Phenotypically, BM-SP cells were 19.59% (+/-)9.00 CD14+, 8.22% (+/-)2.72 CD34+, 92.93% (+/-)2.68 CD44+, 91.86% (+/-)4.07 CD45+, 28.48% (+/-)2.24 c-kit+, 71.09% (+/-)3.67 Sca-1+. Expression of endothelial cell markers (CD31, Flk-1, Tie-2 and VEGF-A) was higher in BM-SP cells than whole BM cells. After five days of co-culture with neonatal cardiomyocytes, 7.2% (+/-)1.2 of the BM-SP cells expressed sarcomeric alpha-actinin as measured by flow cytometry. Moreover, BM-SP cells co-cultured on neonatal cardiomyocytes fixed to inhibit cell fusion also expressed sarcomeric alpha-actinin. The co-cultured BM-SP cells showed neonatal cardiomyocyte-like action potentials of relatively long duration and shallow resting membrane potential. They also generated calcium transients with amplitude and duration similar to those of neonatal cardiomyocytes. These results show that BM-SP cells are capable of functional cardiomyogenic differentiation when co-cultured with neonatal cardiomyocytes.