Using Conditional Knockout Technology Smad4 Gene Function In Heart Development And Disease

TGF-β superfamily molecules play important roles in regulating cell proliferation, lineage determination, motility, adhesion and apoptosis. Smad4 is the central mediator of the TGF-β superfamily signalings. In order to comprehensively study the function of TGF-β superfamily molecules in the heart development and related diseases, we generated the cardiomyocyte-specific Smad4 gene knockout mice using the Cre-loxP system.We generated a mouse that specifically expresses Cre recombinase in cardiomyocyte (α-MHC-Cre) and determined the tissue distribution and the activity of Cre recombinase in the transgenic mouse. The results showed that the Cre recombinase was expressed exclusively in the cardiac tissues of the α-MHC-Cre transgenic mice, indicating that the α-MHC-Cre transgenic mice was a useful tool for making cardiomyocyte specific gene-knockout mice.The cardiomyocyte-specific Smad4 knockout mice were obtained by crossing the α-MHC-Cre transgenic mice and Smad4^Co/Co mice. The deletion of Smad4 in cardiomyocytes did not result in embryonic lethality, approximately 70% of Smad4^Co/Co;α-MHC-Cxe mice died suddenly between 5 and 12 months of age. The mutant mice exhibited cardiac hypertrophy at 1-month-old, with the increased ratio of Heart Weight/Body Weight(HW/BW), Heart Weight/Tibia Length(HW/TL) and upregulation of the hypertrophic marker genes, such as MHCp, ANF and BNP. The analysis of histological sections and isolated cardiomyocytes showed the size of mutant cardiomyocytes was increased. The mutant mice developed obvious fibrosis with advanced age . Electron microscopic analysis showed the abnormal mitochondria and disorganization of Z bands in the mutant cardiac myocytes. The M-mode echocardiography and invasive hemodynamic study revealed that the Smad4 mutant mice also exhibited remarkably decreased cardiac contractility. To analyze the molecular mechanism of the cardiac hypertrophy in Smad4 mutants, we examined some important signaling pathways related to cardiac hypertrophy, such as MAPK and PI3K, and found...