ABSTRACT: Myocardial infarction is a major cause of morbidity and mortality worldwide. The limited ability of the surviving cardiac cells to proliferate following an ischemic attack renders the damaged heart susceptible to unfavorable remodeling processes and heart failure. Currently, pharmaceutical and implantable device management of heart failure seek only to preserve existing viable myocardium after an ischemic attack, and thus merely slows the progression of cardiac dysfunction. Ultimately, heart transplantation is the only viable treatment option for end-stage heart failure patients. To “regenerate” the heart and not only preserve cardiac function but also recover lost or diseased muscle, stem cell therapy has emerged as a promising therapy for heart disease because it can provide a virtually unlimited source of cardiomyocytes, endothelial cells, and other differentiated cell types. The hope is to use these cells to replace diseased myocardium that would otherwise progress to outright failure and regenerate the heart to its former, healthy self. Recently, human embryonic stem cells (hESCs) have generated much interest because of their capacity for self-renewal and pluripotency. In practical terms, hESCs can be cultured indefinitely ex vivo, and can differentiate into virtually any cell type in the adult body. hESCs are thus an attractive source for the derivation of large numbers of cells to be used in various tissue repair and cell replacement therapies. However, upon transplantation into living organisms, undifferentiated hESCs can spontaneously differentiate into rapidly proliferating teratomas, which are disordered amalgams of all three germs layers. Nevertheless, under the appropriate conditions, ex vivo hESCs can be directed to differentiate into beating cardiomyocytes via an embryoid body (EB) intermediate. Subsequently, the cardiomyocyte sub-population is enriched several-fold using discontinuous density gradient separation. Therefore, coaxing hESCs into cardiomyocytes for therapeutic applications is an innovative and feasible strategy that can minimize the risk of cellular misbehavior and teratoma formation. In order to define at a molecular level the changes occurring at each stage of hESC differentiation into cardiomyocytes, we performed transcriptional profiling of the cells using whole human genome microarrays. This allowed us to examine the activation of specific genes as well as broader developmental processes during the progression from hESC to fetal cardiomyocyte, and to identify novel genes that are potentially important in mediating differentiation and development as well as potential novel markers of each stage. In the future, such genes may prove vital in efforts to more closely direct and assess differentiation of potential therapeutic pre-cardiomyocytes or cardiomyocytes in the repair of injured cardiac tissues. In our microarray analysis, we observed high expression of pluripotency-related genes involved in the core hESC regulatory circuitry, including OCT4, SOX2, and NANOG, as well as CRYPTO 1 and 3, LCK, and HESX1. Differentiation into beating EBs was accompanied by mesodermal differentiation and dramatic activation of TWIST1, TBX5, and MEOX transcription, as well as the very clear induction of nearly all of the early cardiogenic genes, including FOXC1, ISL2, HAND1, GATA4, 5, and 6, FOXH1, and MEF2C. While it is clear that other developmental lineages are still present in the EB population, it is also clear from the high levels of cardiac gene expression that this population is significantly enriched for the cardiac lineage even at an early stage. The transcriptional analysis of the final differentiation and selection of the hESC-derived CMs indicates that this enrichment continues, with the CM population expressing differentiated cardiomyocyte genes at levels similar to our more advanced FH cells. Importantly, because of the cell type heterogeneity in the fetal heart, we specifically isolated cardiomyocytes from the fetal left ventricles for microarray analysis. In summary, hESC-CMs hold potential promise for treatment of cardiovascular disease. The molecular processes that control stem cell pluripotency, differentiation, and proliferation are complex, justifying the need for a broad investigation that integrates systems biological tools for transcriptome analysis. We found that the enriched hESC-CMs expresses cardiomyocyte genes at levels similar to 20-week fetal heart cells, making this population a good source of potential replacement cells in the in vivo setting.
