Characterization of Nfatc1 regulation identifies an enhancer required for gene expression that is specific to pro-valve endocardial cells in the developing heart

Nfatc1 is an endocardial transcription factor required for development of cardiac valves. Herein, we describe identification and characterization of a tissue-specific enhancer in the first intron of murine Nfatc1 that activates a heterogenic promoter and directs gene expression in a subpopulation of endocardial cells of the developing heart: the pro-valve endocardial cells. This enhancer activity begins on embryonic day (E) 8.5 in endocardial cells at the ventricular end of the atrioventricular canal, intensifies and extends from E9.5 to E11.5 in endocardium along the atrioventricular canal and outflow tract. By E12.5, the enhancer activity is accentuated in endocardial cells of forming valves. Sequential deletion analysis identified that a 250 bp DNA fragment at the 3' end of the intron 1 is required for endocardial-specific activity. This region contains two short conserved sequences hosting a cluster of binding sites for transcription factors, including Nfat and Hox proteins. Electrophoresis mobility shift and chromatin immunoprecipitation assays demonstrated binding of Nfatc1 to the Nfat sites, and inactivation of Nfatc1 downregulated the enhancer activity in pro-valve endocardial cells. By contrast, mutation of the Hox site abolished its specificity, allowing gene expression in non pro-valve endocardium and extracardiac vasculature. Thus, autoregulation of Nfatc1 is required for maintaining high Nfatc1 expression in pro-valve endocardial cells, while suppression through the Hox site prevents its expression outside pro-valve endocardial cells during valve development. Our data demonstrate the first autonomous cell-specific enhancer for pro-valve endocardial cells and delineate a unique transcriptional mechanism that regulates endocardial Nfatc1 expression within developing cardiac valves.