Genome-wide transcriptional profiling of human embryonic stem cells differentiating to cardiomyocytes

Mammals are unable to regenerate their heart after major cardiomyocyte loss caused by myocardial infarction. Human embryonic stem cells (hESCs) can give rise to functional cardiomyocytes and therefore have exciting potential as a source of cells for replacement therapy. Understanding the molecular regulation of cardiomyocyte differentiation from stem cells is crucial for the stepwise enhancement and scaling of cardiomyocyte production that will be necessary for transplantation therapy. Our novel hESC differentiation protocol is now efficient enough for meaningful genome-wide transcriptional profiling by microarray technology of hESCs, differentiating toward cardiomyocytes. Here, we have identified and validated time-dependent gene expression patterns and shown a reflection of early embryonic events; induction of genes of the primary mesoderm and endodermal lineages is followed by those of cardiac progenitor cells and fetal cardiomyocytes in consecutive waves of known and novel genes. Collectively, these results permit enhancement of stepwise differentiation and facilitate isolation and expansion of cardiac progenitor cells. Furthermore, these genes may provide new clinically relevant clues for identifying causes of congenital heart defects.

Increased cardiomyocyte differentiation from human embryonic stem cells in serum-free cultures

Human embryonic stem cells (hESCs) can differentiate into cardiomyocytes, but the efficiency of this process is low. We routinely induce cardiomyocyte differentiation of the HES-2 cell line by coculture with a visceral endoderm-like cell line, END-2, in the presence of 20% fetal calf serum (FCS). In this study, we demonstrate a striking inverse relationship between cardiomyocyte differentiation and the concentration of FCS during HES-2-END-2 coculture. The number of beating areas in the cocultures was increased 24-fold in the absence of FCS compared with the presence of 20% FCS. An additional 40% increase in the number of beating areas was observed when ascorbic acid was added to serum-free cocultures. The increase in serum-free cocultures was accompanied by increased mRNA and protein expression of cardiac markers and of Isl1, a marker of cardiac progenitor cells. The number of beating areas increased up to 12 days after initiation of coculture of HES-2 with END-2 cells. However, the number of alpha-actinin-positive cardiomyocytes per beating area did not differ significantly between serum-free cocultures (503 +/- 179; mean +/- standard error of the mean) and 20% FCS cocultures (312 +/- 227). The stimulating effect of serum-free coculture on cardiomyocyte differentiation was observed not only in HES-2 but also in the HES-3 and HES-4 cell lines. To produce sufficient cardiomyocytes for cell replacement therapy in the future, upscaling cardiomyocyte formation from hESCs is essential. The present data provide a step in this direction and represent an improved in vitro model, without interfering factors in serum, for testing other factors that might promote cardiomyocyte differentiation.