Rotation and asymmetric development of the zebrafish heart requires directed migration of cardiac progenitor cells

Nkx genes regulate heart tube extension and exert differential effects on ventricular and atrial cell number

Heart formation is a complex morphogenetic process, and perturbations in cardiac morphogenesis lead to congenital heart disease. NKX2-5 is a key causative gene associated with cardiac birth defects, presumably because of its essential roles during the early steps of cardiogenesis. Previous studies in model organisms implicate NKX2-5 homologs in numerous processes, including cardiac progenitor specification, progenitor proliferation, and chamber morphogenesis. By inhibiting function of the zebrafish NKX2-5 homologs, nkx2.5 and nkx2.7, we show that nkx genes are essential to establish the original dimensions of the linear heart tube. The nkx-deficient heart tube fails to elongate normally: its ventricular portion is atypically short and wide, and its atrial portion is disorganized and sprawling. This atrial phenotype is associated with a surplus of atrial cardiomyocytes, whereas ventricular cell number is normal at this stage. However, ventricular cell number is decreased in nkx-deficient embryos later in development, when cardiac chambers are emerging. Thus, we conclude that nkx genes regulate heart tube extension and exert differential effects on ventricular and atrial cell number. Our data suggest that morphogenetic errors could originate during early stages of heart tube assembly in patients with NKX2-5 mutations.

Direct and indirect roles for Nodal signaling in two axis conversions during asymmetric morphogenesis of the zebrafish heart

The Nodal signaling pathway plays a conserved role in determining left-sided identity in vertebrates with this early left-right (L/R) patterning influencing the asymmetric development and placement of visceral organs. We have studied the role of Nodal signaling in asymmetric cardiac morphogenesis in zebrafish and describe two distinct rotations occurring within the heart. The first is driven by an asymmetric migration of myocardial cells during cardiac jogging, resulting in the conversion of the L/R axis to the dorsal-ventral (D/V) axis of the linear heart. This first rotation is directly influenced by the laterality of asymmetric gene expression. The second rotation occurs before cardiac looping and positions the original left cells exposed to Nodal signaling back to the left of the wild-type (WT) heart by 48 hours postfertilization (hpf). The direction of this second rotation is determined by the laterality of cardiac jogging and is not directly influenced by asymmetric gene expression. Finally, we have identified a role for Nodal signaling in biasing the location of the inner ventricular and outer atrial curvature formations. These results suggest that Nodal signaling directs asymmetric cardiac morphogenesis through establishing and subsequently reinforcing laterality information over the course of cardiac development.