MED12 is a Transcriptional Mediator of Cardiac Contractile Function

Abstract

The Mediator complex plays an essential role in transcription for nearly all genes, and functions by transmitting physiological and developmental signals to RNA polymerase II. The Mediator complex is regulated by a kinase submodule made up of four proteins: MED12, MED13, CDK8, and Cyclin C. MED12 is a pivotal member of this submodule as it is required for CDK8 activity, which regulates Polymerase II‐driven transcription. The objective of this study is to identify the role of MED12 in the heart and determine how it regulates transcription through interactions with transcription factors. MED12 protein levels are increased in human heart failure biopsies, and in the mouse heart after transverse aortic constriction (TAC)‐induced heart failure. Therefore, we hypothesize that increased levels of MED12 contributes to development of heart failure. To test this hypothesis we generated cardiac‐specific MED12 transgenic (cTg) mice by driving increased MED12 expression in cardiomyocytes using the aMHC promoter. We performed serial echocardiography and observed that increased MED12 expression leads to decreased cardiac function, measured as fractional shortening. Decreased cardiac function was accompanied by increased cardiac chamber size and dilation, without evidence of cardiac fibrosis. We performed RNA sequencing on cTg and control cardiomyocytes, to investigate mechanisms of MED12‐induced cardiac dysfunction, and found that increased MED12 levels lead to dysregulated calcium handing gene expression. MED12 is a transcriptional regulator, but does not directly bind DNA. To determine how MED12 regulates expression of calcium handling genes we used cardiomyocytes from our mouse models to screen for transcription factors that interact with MED12. We immunoprecipitated MED12 and identified interacting proteins by mass spectrometry, many of which are regulators of transcription. In follow‐up experiments we demonstrated that MED12 interacts with the transcription factor (TF) MEF2 to regulate calcium handling genes. Preliminary experiments suggest that MED12 also interacts with TFs CREB and SRF. Additional mechanistic studies are ongoing to determine how MED12 coordinates gene expression through multiple transcription factors in cardiomyocytes. Collectively, our data demonstrate that tight regulation of MED12 levels is necessary to maintain normal cardiac function and regulate gene expression in the heart, and increased MED12 expression leads to the development of heart failure.