Bone morphogenetic protein signaling pathways during mouse heart development: Roles for CHD7 and Mycn

Bone Morphogenetic Protein (BMP) signaling pathways are imperative for proper heart development. BMP ligands bind serine threonine kinase receptors, which activate intracellular receptor-regulated SMAD proteins. SMAD1, SMAD5, and SMAD8 transduce BMP signals from the cytoplasm to the nucleus, where they regulate transcription. We have investigated two aspects of BMP signaling during mouse cardiogenesis: identifying SMAD1-interacting proteins and exploring the roles of a known BMP target, Mycn, in the developing myocardium.

Chromodomain helicase DNA binding protein 7 (CHD7) is a highly conserved transcription factor that promotes protein synthesis, proliferation, and differentiation. Haploinsufficiency for CHD7 causes CHARGE syndrome, a developmental disorder characterized by diverse heart defects. CHD7 was identified as a SMAD1-interacting protein in a yeast two-hybrid screen. The interaction was confirmed with glutathione Stransferase (GST) pull-down assays in mammalian cells and in vitro. Future studies are needed to verify the functional significance of the SMAD1-CHD7 interaction and to delineate Chd7’s roles during mouse heart development with conditional gene inactivation.

MYCN is a conserved transcription factor with roles in development and disease. Mutations in MYCN are associated with Feingold syndrome, a disorder associated with congenital heart defects. To uncover the roles of Mycn in the developing mouse myocardium, we used a novel transgenic mouse model with Mycn deleted from the myocardium. Conditional deletion of Mycn from the myocardium resulted in embryonic lethality at E12.5. Histological examination of mutant embryos revealed a thin ventricular myocardial wall defect, which likely reduced contractility and resulted in cardioinsufficiency. Mutants had hypocellular myocardial walls with significantly decreased cardiomyocyte proliferation within the ventricles, but no detectable changes in apoptosis. Expression of cell cycle regulators and MYCN targets, CCND1, CCND2, and ID2 was reduced within the mutant ventricles. Depletion of MYCN from the myocardium also caused a significant reduction in ventricular cardiomyocyte size along with reduced expression of p70(S6K), a key regulator of ribosome biogenesis and protein synthesis. MYCN was also necessary for the proper expression of a subset of myofilament proteins that are important for cardiomyocyte structure and function. These results reveal that Mycn is a critical mediator of cardiomyocyte proliferation, size, and gene expression.