Regulation of chamber-specific gene expression in the developing heart by Irx4

The vertebrate heart consists of two types of chambers, the atria and the v entricles, which differ in their contractile and electrophysiological prope rties. Little is known of the molecular mechanisms by which these chambers are specified during embryogenesis. Here a chicken iroquois-related homeobo x gene, lrx4, was identified that has a ventricle-restricted expression pat tern at all stages of heart development. lrx4 protein was shown to regulate the chamber-specific expression of myosin isoforms by activating the expre ssion of the ventricle myosin heavy chain-1 (VMHC1) and suppressing the exp ression of the atrial myosin heavy chain-1 (AMHC1) in the ventricles. Thus, lrx4 may play a critical role in establishing chamber-specific gene expres sion in the developing heart.