Several proteins are expressed very early during heart development (see Figure 1). Nkx2-5 and Mesp1 are also critical in initiating a self-sustaining gene regulatory network. One of the genes active in this network encodes the Gata4 transcription factor, which is first seen in the precardiac cells of chicks and mice when these cells emerge from the primitive streak. Gata4 is necessary for activating numerous heart-specific genes as well as for activating expression of the gene for N-cadherin, a protein that is critical for both the formation of the cardiac epithelium and the fusion of the two heart rudiments into one tube (Linask 1992; Zhang et al. 2003).
In addition to activating a group of core heart-forming genes, Mesp1 also helps activate different patterns of protein synthesis in the heart fields on each side of the embryo. Mesp1 and Nkx2-5 instruct the cells of the second heart field to express the Foxh1 gene, which commits these heart precursor cells to become the right ventricle and outflow tract (von Both et al. 2004). In the first heart field, Mesp1 activates the Tbx5 gene, whose product is critical for heart tube and left ventricle development (see Figure 18.16; Koshiba-Takeuchi et al. 2009). In these early cells, Tbx5 acts with Gata4 and Nkx2-5 to activate numerous genes involved in heart specification. Later, Tbx5 becomes restricted to the atria and left ventricle. The ventricular septum (the wall separating the left and right ventricles) is formed at the boundary between those cells that express Tbx5 and those that do not. Tbx5 protein works antagonistically to Tbx20, which becomes expressed in the right ventricle. When the Tbx5 expression domain is ectopically expanded, the location of the ventricular septum shifts to this new location. Moreover, a conditional knockout of the mouse Tbx5 gene—specifically inactivating it during ventricular development—leads to the formation of a lizardlike ventricle that lacks any septum (Takeuchi et al. 2003; Koshiba-Takeuchi et al. 2009). Thus, Tbx5 is extremely important in separating the left and right ventricles. Mutations in the human TBX5 gene cause Holt-Oram syndrome (Bruneau et al. 1999), characterized by abnormalities of the heart and upper limbs.