Essential role of Smad4 in maintaining cardiomyocyte proliferation during murine embryonic heart development

Abstract Transforming growth factor-β/bone morphogenetic protein (TGF-β/BMP) signaling pathway is essential for embryonic and postnatal heart development and remodeling. The intracellular factor Smad4 plays a pivotal role in mediating TGF-β/BMP signal transduction in the nucleus. To examine the function of Smad4 in embryonic cardiac development during mid-gestation, we specifically deleted the Smad4 gene in embryonic cardiomyocytes using the Cre –Lox P system. Deletion of Smad4 as early as E9.5, led to embryonic lethality between E12.5 and E15.5, and embryos exhibited severe morphological defects in the heart, including a thin compact layer, disorganized trabeculae, and ventricular septum defects (VSD). Smad4 deletion also led to a dramatic decrease in cardiomyocyte proliferation accompanied by downregulation of contractile protein-encoding genes such as α-myosin heavy chain , β-myosin heavy chain , ventricular myosin light chain 2 , and α-cardiac actin . In addition, deletion of Smad4 resulted in perturbation of TGF-β/BMP ligand expression and signaling, and defects in expression of several cardiac transcription factor genes such as Nkx2.5 , GATA4 , and MEF2c . These results provide direct genetic evidences that Smad4 is essential for regulating cardiomyocyte proliferation and differentiation during murine cardiogenesis, and provides new insights into potential causes of congenital heart disease.