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(Circulation Research. 2008;103:1430.)
© 2008 American Heart Association, Inc.

Molecular Medicine

MEKK3 Initiates Transforming Growth Factor β₂–Dependent Epithelial-to-Mesenchymal Transition During Endocardial Cushion Morphogenesis

Mark V. Stevens, Derrick M. Broka, Patti Parker, Elisa Rogowitz, Richard R. Vaillancourt, Todd D. Camenisch

From the Department of Pharmacology and Toxicology (M.V.S., D.M.B., P.P., E.R., R.R.V., T.D.C.), Department of Molecular and Cellular Biology (T.D.C.), Steele Children’s Research Center (T.D.C.), and BIO5 Institute (R.R.V., T.D.C.), University of Arizona, Tucson.

Correspondence to Todd D. Camenisch, PhD, University of Arizona, Department of Pharmacology and Toxicology, College of Pharmacy, 1703 E Mabel St, Tucson, AZ 85721. E-mail camenisch{at}pharmacy.arizona.edu

Congenital heart defects occur at a rate of 5% and are the most prevalent birth defects. A better understanding of the complex signaling networks regulating heart development is necessary to improve repair strategies for congenital heart defects. The mitogen-activated protein 3 kinase (MEKK3) is important to early embryogenesis, but developmental processes affected by MEKK3 during heart morphogenesis have not been fully examined. We identify MEKK3 as a critical signaling molecule during endocardial cushion development. We report the detection of MEKK3 transcripts to embryonic hearts before, during, and after cardiac cushion cells have executed epithelial-to-mesenchymal transition (EMT). MEKK3 is observed to endocardial cells of the cardiac cushions with a diminishing gradient of expression into the cushions. These observations suggest that MEKK3 may function during production of cushion mesenchyme as required for valvular development and septation of the heart. We used a kinase inactive form of MEKK3 (MEKK3^KI) in an in vitro assay that recapitulates in vivo EMT and show that MEKK3^KI attenuates mesenchyme formation. Conversely, constitutively active MEKK3 (ca-MEKK3) triggers mesenchyme production in ventricular endocardium, a tissue that does not normally undergo EMT. MEKK3-driven mesenchyme production is further substantiated by increased expression of EMT-relevant genes, including TGFβ₂, Has2, and periostin. Furthermore, we show that MEKK3 stimulates EMT via a TGFβ₂-dependent mechanism. Thus, the activity of MEKK3 is sufficient for developmental EMT in the heart. This knowledge provides a basis to understand how MEKK3 integrates signaling cascades activating endocardial cushion EMT.

Key Words: epithelial-to-mesenchymal transition • MEKK3 • TGFβ₂ • endocardial cushions • heart

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