Adult bone marrow-derived cells do not acquire functional attributes of cardiomyocytes when transplanted into peri-infarct myocardium

The cardiomyogenic potential of adult bone marrow ( BM) cells after being directly transplanted into the ischemically injured heart remains a controversial issue. In this study, we investigated the ability of transplanted BM cells to develop intracellular calcium ([Ca2+](i)) transients in response to membrane depolarization in situ. Low-density mononuclear ( LDM) BM cells, c-kit-enriched ( c-kit(enr)) BM cells, and highly enriched lin(-)c-kit(+) BM cells were obtained from adult transgenic mice ubiquitously expressing enhanced green fluorescent protein ( EGFP), and injected into peri-infarct myocardiums of nontransgenic mice. After 9-10 days the mice were killed, and the hearts were removed, perfused in Langendorff mode, loaded with the calcium-sensitive fluorophore rhod-2, and subjected to two-photon laser scanning fluorescence microscopy ( TPLSM) to monitor action potential-induced [Ca2+](i) transients in EGFP-expressing donor-derived cells and non-expressing host cardiomyocytes. Whereas spontaneous and electrically evoked [Ca2+](i) transients were found to occur synchronously in host cardiomyocytes along the graft-host border and in areas remote from the infarct, they were absent in all of the > 3,000 imaged BM-derived cells that were located in clusters throughout the infarct scar or peri-infarct zone. We conclude that engrafted BM-derived cells lack attributes of functioning cardiomyocytes, calling into question the concept that adult BM cells can give rise to substantive cardiomyocyte regeneration within the infarcted heart.