MEKK4 regulates developmental EMT in the embryonic heart.

Congenital heart malformations occur at a rate of one per one hundred births and are considered the most frequent birth defects. This high incidence of cardiac defects underscores the complex developmental processes required to form the first functioning organ in mammals. The molecular cues which govern heart development are poorly defined and require an improved understanding in order to advance repair strategies for heart defects. The cytoplasmic MAP kinase kinase kinase, MEKK4, is a critical effector in cellular stress responses; however, the function of MEKK4 during embryonic development and cardiogenesis is not well understood. We have identified MEKK4 as a critical signaling molecule during cardiovascular development. We report the detection of MEKK4 transcripts to early myocardium, endocardium and to cardiac cushion cells that have executed epithelial to mesenchymal transformation (EMT). These observations suggest that MEKK4 may function during production of the cushion mesenchyme as required to create valves and the septated heart. We used a kinase inactive form of MEKK4(MEKK4(KI)) in an in vitro assay that recapitulates in vivo EMT, and show that MEKK4(KI) attenuates mesenchyme production. However, addition of a constitutively active MEKK4 into ventricular explants, a system that does not normally undergo EMT, is not able to cause mesenchymal cell outgrowth. Thus, the kinase activity of MEKK4 is essential, but not sufficient, to support developmental EMT. This knowledge provides a basis to understand how MEKK4 may integrate signaling cascades controlling heart development.