Embryonic stem (ES) cells are promising for cardiac repair, but directing their differentiation toward cardiomyocytes remains challenging. We investigated whether the heart guides ES cells toward cardiomyocytes in vivo and whether allogeneic ES cells were immunologically tolerated. Undifferentiated mouse ES cells consistently formed cardiac teratomas in nude or immunocompetent syngeneic mice. Cardiac teratomas contained no more cardiomyocytes than hind-limb teratomas, suggesting lack of guided differentiation. ES cells also formed teratomas in infarcted hearts, indicating injury-related signals did not direct cardiac differentiation. Allogeneic ES cells also caused cardiac teratomas, but these were immunologically rejected after several weeks, in association with increased inflammation and up-regulation of class I and II histocompatibility antigens. Fusion between ES cells and cardiomyocytes occurred in vivo, but was rare. Infarct autofluorescence was identified as an artifact that might be mistaken for enhanced GFP expression and true regeneration. Hence, undifferentiated ES cells were not guided toward a cardiomyocyte fate in either normal or infarcted hearts, and there was no evidence for allogeneic immune tolerance of ES cell derivatives. Successful cardiac repair strategies involving ES cells will need to control cardiac differentiation, avoid introducing undifferentiated cells, and will likely require immune modulation to avoid rejection.

Abstract-Transcriptome-wide analysis of dynamically regulated progenitor cell populations has the potential to elucidate key aspects of cardiac development. The heart, as the first organ to develop in the mammal, is a technically challenging but clinically relevant target for study. To define the transcriptional program of the cardiac progenitor, we used a novel transgenic strategy and fluorescence-activated cell sorting to reliably label and isolate cardiac progenitors directly from mouse embryos. Pure populations of cardiac progenitor cells were isolated from the cardiac crescent and 2 subsequent stages of heart development: the linear heart tube and the looping heart. RNA was isolated from stage-specific cardiac progenitors and subjected to transcriptome analysis by oligonucleotide array hybridization. The cardiac transcriptional regulatory programs were compared with the molecular programs of age-matched noncardiac embryonic cells, embryonic stem cells, adult cardiomyocytes, and each other to identify sets of genes exhibiting differential expression in the cardiac progenitor cell population. These results define the transcriptional profile of mammalian cardiac progenitor cells and provide insight into the molecular regulation of the earliest periods of heart development.

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Recent studies suggest that embryonic and somatic stem cell populations hold promise as sources for tissue engineering. The use of cell biological and molecular technologies will enhance our understanding of embryonic and somatic stem cell populations and their molecular regulatory events that promote multipotentiation.