The discovery of small non-coding microRNAs has revealed novel mechanisms of post-translational regulation of gene expression,
the implications of which are still incompletely understood. We focused on microRNA 21 (miR-21), which is expressed in cardiac
valve endothelium during development, in order to better understand its mechanistic role in cardiac valve development. Using
a combination of in vivo gene knockdown in zebrafish and in vitro assays in human cells, we show that miR-21 is necessary for proper development of the atrioventricular valve (AV). We identify
pdcd4b as a relevant in vivo target of miR-21 and show that protection of pdcd4b from miR-21 binding results in failure of AV development. In vitro experiments using human pulmonic valve endothelial cells demonstrate that miR-21 overexpression augments endothelial cell
migration. PDCD4 knockdown alone was sufficient to enhance endothelial cell migration. These results demonstrate that miR-21
plays a necessary role in cardiac valvulogenesis, in large part due to an obligatory downregulation of PDCD4.