Regulation of TGFbeta Signaling by microRNAs during Cardiogenesis

Congenital heart diseases (CHDs) remain the leading noninfectious cause of infant morbidity/mortality. The TGFbeta cytokines play essential roles during cardiogenesis; mutations that alter activities of TGFbeta signaling cause various congenital cardiovascular defects in both animal models and human patients. Our preliminary data have revealed that the normal level of TGFbeta activities in embryonic hearts relies on proper micro-RNA (miRNA) biosynthesis. We found that the expression level of TGFBR1 is significantly increased in embryonic hearts with Dicer1 deleted. DICER1 is an RNase III enzyme essential for generation of functional miRNAs. Our further analysis suggested that the miRNA-mediated TGFbeta repression is critical for cardiomyocyte proliferation/survival and ventricular wall compaction. Defects in ventricular compaction cause spongy myocardium, which may lead to congestive heart failure and sudden cardiac death. We thus provide a potential novel mechanism accounting for this underexplored congenital cardiomyopathy. MiRNAs have emerged as attractive therapeutic targets for cardiovascular diseases; however, our knowledge of the cardiogenic roles played by miRNAs remains largely limited. Our central hypothesis is that miRNA-mediated repression of Tgfbr1 expression prevents excessive TGFbeta signaling in cardiomyocytes thereby supporting normal cardiogenesis. Three aims are proposed. In the 1st aim, we will test whether Tgfbr1 mRNA is a direct target of multiple miRNAs. We will identify miRNAs that directly target Tgfbr1 mRNA in embryonic hearts and test whether they are involved in modulating TGFbeta responsiveness in embryonic cardiomyocytes. In the 2nd aim, we will test whether reducing the dosage of Tgfbr1 can alleviate the cardiac defects caused by Dicer1 inactivation in hearts. We will perform Tgfbr1 and Dicer1 double-gene-inactivation experiments. In the 3rd aim, we will test whether myocardial-overexpression of Tgfbr1 can partially recapitulate the cardiac defects caused by Dicer1 inactivation. We will generate Tgfbr1 inducible overexpression mouse lines and determine how overexpression of Tgfbr1 in hearts will affect cardiogenesis in comparison with Dicer1 heart-inactivation embryos. Accomplishment of this project will significantly advance our knowledge of the molecular mechanisms governing normal cardiogenesis and provide key insights into the therapeutic potential of using miRNAs to target TGFbeta signaling for CHD treatment. (AHA Program: Grant-in-Aid)