The heart is the first functional organ to form during vertebrate development and it is evolutionarily conserved across species. It is crucial for the proper delivering of essential nutrients and oxygen throughout the embryo's body. Its development is complex and requires fine-tuning processes at levels involving growth, differentiation, and morphogenesis. First, a linear heart tube is formed, followed by cardiac looping, chamber formation, and maturation. As for many organs, the heart arises from a simple epithelium with planar polarity properties. The genetic and molecular programs involved in heart formation have been studied for a long time. However, besides the genetic and cellular contributions to heart formation, little is known about the molecular and cellular components involved in generating tissue and tension forces required in heart morphogenesis. Embryonic heart tube remodeling requires coordination of actomyosin-dependent tissue forces fundamental to the emergence of cardiac chambers and looped heart. It has been established that cardiac chamber remodeling is coordinated through tissue-scale polarization of actomyosin. Here, using zebrafish as a model, I describe the role of actomyosin in generating and distributing the tension forces necessary across the ventricular myocardium during cardiac looping and chamber formation. I describe the spatiotemporal distribution of phosphorylated myosin during embryonic heart formation. A mathematical model was generated to demonstrate that the tissue-scale supracellular polarization of actomyosin within the myocardial epithelium is essential for heart formation. The mathematical model serves as a predictive tool of cardiac looping and chamber formation and supports its dependence on the proper actomyosin distribution. Examining the molecular mechanisms governing the actomyosin activity along the heart tube, I demonstrate that both Rho-associated Protein Kinase 2a (Rock2a) and cardiac-specific Myosin Light Chain Kinase 3 (Mylk3) regulate the actomyosin-based tissue forces through the phosphorylation state of the Myosin Regulatory Light Chain (MRLC). I show that the preferential basal activity of Mylk3 and the apical activity of Rock2a mediate the proper levels of phosphorylated myosin (pMyo) and its polarized distribution along the apicobasal axis within the myocardium. I propose that the antagonistic force-generating activities of Mylk3 and Rock2a facilitate mechano-molecular control of heart tube morphogenesis. Moreover, I show Mylk3 and Rock2a are under the genetic control of Planar Cell Polarity signaling, identifying Mylk3 as a novel tissue-specific effector, downstream of the Vangl2 branch of this signaling pathway. Altogether, these findings describe for the first time a mechano-molecular mechanism necessary for proper looping and chamber formation during heart development.
