Expression of a-cardiac myosin heavy chain gene (alpha MHC) is developmentally regulated in normal embryonic hearts and down-regulated in cardiac myopathy and failing hearts. Jumonji (JMJ) has been shown to be critical for normal cardiovascular development and functions as a transcriptional repressor. Here, we demonstrate that JMJ represses of alpha MHC expression through inhibition of myocyte enhancer factor 2 (MEF2) activity. In primary cardiomyocytes, overexpression of JMJ leads to marked reduction of endogenous alpha MHC expression. JMJ represses the synergistic activation of alpha MHC by MEF2 and thyroid hormone receptor (TR). Interestingly, JMJ inhibits transcriptional activities of all MEF2 isoforms, but not the TR-dependent activation. The transcriptional repression domain of JMJ interacts with the N-terminal part of MEF2A. resulting in the repression of MEF2A activities. These results suggest that JMJ represses alpha MHC expression via protein-protein interaction with MEF2A. (c) 2005 Elsevier Inc. All rights reserved.