Abstract 3875: Dedifferentiation of Mammalian Cardiomyocytes Into Cardiac Stem Cells
BACKGROUND: Resident cardiac stem cells (CSCs) underlie the heart’s ability to regenerate. However, the origin of CSCs remains unclear. Prior studies have focused on seeding from circulating blood pools, versus endogenous origin in the embryonic heart.
HYPOTHESIS: We considered the alternative possibility that CSCs arise from dedifferentiation of mature heart muscle cells.
METHODS AND RESULTS: Within days in culture, highly-purified rat cardiomyocytes lost their characteristic striations, flattened and began to divide. Proliferation markers Ki67 and Histone 3, and BrdU incorporation confirmed cell cycle re-entry. Within three weeks in culture, dedifferentiated cardiomyocytes went on to express the stem cell antigen c-kit (61±20%), and the early cardiac transcription factors GATA4 and NKx2.5. Such myocyte-derived cells (MDCs) spontaneously aggregated into free-floating spheres which often beat. RT-PCR revealed that α-MHC, a signature of mature cardiomyocytes, went down in MDCs but rebounded in MDC-formed spheres; conversely, c-kit was undetectable in fresh cardiomyocytes, rich in MDCs, but weakly-expressing in spheres. In prolonged culture at sphere stage (3–5d), MDCs could not only re-differentiate into myocytes but also could become endothelial cells, as revealed by immunocytochemistry and RT-PCR. The finding that cardiomyocytes could dedifferentiate to express stem cell antigens was verified by tracing cell fate of GFP+ cardiomyocytes from bi-transgenic mice in which cardiomyocytes and progeny were specifically and permanently labeled with GFP after cardiac-specific gene recombination induced by tamoxifen. When freshly-isolated, GFP+ cardiomyocytes were rod-shaped with striations but did not express c-kit; over time in culture, however, GFP+ cells lost cardiac filament proteins, became small and round, and richly expressed c-kit (e.g. GFP+/c-kit+ cells were 0.4±0.2% of the cardiac outgrowth at 10d and 20.3±2.8% at 3 weeks).
CONCLUSION: Contradicting the prevailing view that they are terminally-differentiated, mammalian cardiomyocytes are instead capable of substantial plasticity.