The role of myocyte enhancer factor 2 during cardiomyogenesis

Heart development requires the complex integration of appropriate signals in a defined spatial and temporal pattern. The temporal expression of cardiac specific factors during embryonic heart development is recapitulated in P19 embryonic carcinoma cells that are aggregated in the presence of dimethylsulfoxide. Double knock out of histone deacetylase-5 and -9, HDAC5 and HDAC9, in mice resulted in severe heart defects. We hypothesized that HDAC activity at early stages of cardiomyogenesis would be inhibitory. In the first chapter, inactivation of HDAC activity in P19 cells promoted the commitment of mesoderm cells into the cardiac lineage and their differentiation into cardiac muscle, while the overexpression of HDAC4 enhanced mesoderm formation and prevented the entrance of cells into the cardiac lineage. Therefore, this thesis defines a role for HDACs as early inhibitory mediators of cardiomyogenesis. Although Myocyte Enhancer Factor 2C, MEF2C, has been generally viewed as an activator of gene expression in both skeletal and cardiac muscle development, a possible repressor role for MEF2C during the commitment stage of cardiomyogenesis has also been shown. Mef2c-/- mice in previous studies died of heart defects, however cardiomyocytes still formed which suggested a relatively late role for MEF2C activity during heart development. We hypothesized that an earlier role may have been masked by compensation of other MEF2 members. In the second chapter, a dominant negative mutant of MEF2C was used to examine the role of MEF2C during cardiomyogenesis. Here, we showed that the expression of a dominant negative mutant of MEF2C in cardiac destined cells resulted in the loss of cardiac muscle development. Therefore, wild type MEF2 activity was essential for the differentiation of cardiomyoblasts into cardiomyocytes. Lastly, while BMP signaling has been shown to be sufficient to upregulate the expression of cardiomyoblast marker genes such as Nkx2-5, gata-4 , and mef2c, whether BMP can modulate MEF2C function has not been examined. In the third chapter, BMP signaling was found to positively regulate MEF2C activity and to promote cardiomyogenesis. Thus, P19 cells have allowed for the closer examination of factors involved in the commitment and differentiation stages of cardiomyogenesis.; Keywords. Cardiomyogenesis, P19 cells, MEF2C, Nkx2-5, GATA-4, HDAC, CaMKIV, BMP, development...