Endocardial-myocardial coordination during cardiogenesis.

The embryonic heart undergoes a complex series of morphogenetic steps in order to mold a properly functioning organ. The precise coordination of the development of two major cardiac cell types, the myocardial cells and the endocardial cells, is necessary to drive key steps in cardiac morphogenesis. Before the heart starts to function, cardiac cells migrate toward the midline and form a tube through a process called cardiac fusion. The primitive heart tube drives early embryonic circulation while proceeding with the next step of chamber emergence. One aspect of chamber emergence involves specialized differentation of the atrioventricular (AV) canal, where the endocardial cushions (ECs) form to prevent retrograde blood flow. Finally, the embryonic ventricular chamber must mature and become more structurally elaborate through the process of trabeculation. Although multiple pathways have been implicated in coordinating cardiac morphogenesis, our knowledge of the regulation of these crucial steps in cardiogenesis is incomplete. My thesis work focused on enriching our comprehension of the regulation of cardiac morphogenesis in the zebrafish. By studying the frozen ventricle (frv) mutant, I discovered a role for the previously uncharacterized Tmem2 protein during cardiogenesis. Tmem2 functions first to promote cardiac fusion and later to restrict AV differentiation to a defined region. Tmem2 is a transmembrane protein that is highly conserved in vertebrates yet has unknown molecular or cellular functions. Tissue-specific rescue experiments demonstrate that Tmem2 function in the myocardium is sufficient to promote cardiac fusion, and that Tmem2 function in the endocardium is sufficient to repress AV differentiation in the ventricle. Together, these data reveal that Tmem2 is an essential mediator of myocardial-endocardial coordination during cardiac morphogenesis. Turning my attention to ventricular chamber maturation, I contributed to the first characterization of the process of trabeculation in the zebrafish embryo. Through live image analysis, we demonstrate a stereotyped initiation of trabeculation that is followed by three progressive phases of morphogenesis that result in the final complex trabecular meshwork. Analysis in cloche (clo) mutants and pharmacological inhibition of Neuregulin (Nrg) shows that endocardial-myocardial Neuregulin signaling is required for initiation of trabeculation. Finally, studies in the weak atrium (wea) mutant reveal that optimal blood flow is necessary for the progression of trabeculation. Altogether, these data highlight the importance of endocardial-myocardial coordination during key steps of cardiac morphogenesis.