Factors Promoting Cardiogenesis in Vertebrates

Transcription factors that initiate the cardiac fate include NKX2.5, GATA family members, T-box family members (Tbx5, Tbx20), MCMI, agamous, serum response factor (MADS), and MEF2 family members. Extracellular signals that act upstream of these factors include growth factor families BMP, FGF, and WNT and WNT antagonists such as Dkk1 and Crescent.FGF2 is required for the expression of cardiac transcription factors. BMP receptors are essential for cardiac organogenesis. In presence of BMPs, the type-II receptor (BMPRII) phosphorylates the type-I receptors (ALK3, ALK6), which activate signaling by intracellular effector SMADs and TAK1. SMAD1, SMAD5 and SMAD8 transduce signals from BMPs specifically, while SMAD4 is a general partner of ligand-specific SMADs. Co-overexpression of SMAD1 and SMAD4 induce differentiation of cardiac precursor cells into cardiomyocytes, while overexpression of SMAD6 inhibits differentiation of cardiac precursor cells into cardiomyocytes. The BMP pathway is negatively regulated by Noggin.TAK1 and the SMAD pathways promote cardiogenesis by activating several transcription factors like ATF2, NKX2.5 and GATA4. ATF2 stimulates β-MHC promoter activity and transactivates cardiac-specific genes such as NPPA and NPPB. NKX2.5 and GATA4 induce differentiation into cardiomyocytes cooperatively with unknown factors induced by dimethyl sulfoxide (DMSO). Some cardiac-specific genes, such as MEF2C and MLC2v are upregulated by NKX2.5 and GATA4.Besides BMPs, Cripto (TDGF1) is also involved in the induction of cardiogenesis. Cripto recruits Nodal to ALK4 or ALK7 and to ActRII receptors. Upon receptor activation, intracellular effectors SMAD2 or SMAD3 are phosphorylated and form a complex with SMAD4 in the nucleus, where they induce transcription of cardiac-specific genes. Cerberus Short (Cer1) blocks Nodal by direct binding and results in a strong inhibition of Cripto activity in promoting cardiogenesis.WNT family members also regulate cardiomyogenesis. WNTs bind to Frizzled receptors, whose activation leads to stabilization of β-catenin through inactivation of GSK3β, followed by β-catenin-dependent activation of TCF/LEF transcription factors and induction of WNT-responsive genes. In contrast, WNT11 signals through a β-catenin-independent non-canonical pathway involving PKC and JNK. Some common WNT inhibitors include Dickkopf (DKK3) and Crescent.In Mouse, activation of Ca2+/calmodulin-dependent kinase and a NOX4-dependent activation of p38 MAPK induce cardiac differentiation. These two signaling pathways converge at the level of MEF2C. NOX4-generated reactive oxygen species (ROS) lead to p38 MAPK phosphorylation and subsequent MEF2C nuclear translocation, which is responsible for the activation of MLC2v.Transcription factors such as Tbx5 and Tbx20 also play an important role in cardiac differentiation. Tbx5 induces the expression of EGF or FGF, which are required for cell cycle progression. Tbx5 also indirectly regulates Rb phosphorylation. In its hypophosphorylated state, Rb interacts with E2F, inhibiting its transcriptional activation activity. Upon phosphorylation, the Rb-E2F interaction is disrupted and E2F is released and able to activate its downstream genes required for S-Phase entry. Tbx5 depletion leads to a G1/S-phase arrest, which causes an increase in the expression of proteins associated with the cardiac cell cycle S-phase, including CDC6, Cyclin-E2, SLBP and PCNA. (Upgraded 01/2020)