Organogenesis requires the carefully orchestrated arrangement of multiple cell types into a specific three-dimensional form that is essential for the organ's effective function.  The embryonic vertebrate heart provides a clear example of the relationship between organ form and function: it is composed of two major chambers, a ventricle and an atrium, each with discrete morphological and physiological characteristics that define its functional capacity.  Proper execution of cardiac chamber formation is critical for efficient circulation, and congenital heart defects are often associated with errors in chamber formation.  Therefore, the identification of genetic pathways controlling cardiac chamber formation is likely to illuminate the causes of congenital heart disease.  The goal of our laboratory’s research is to understand the molecular mechanisms responsible for the specific size, shape, and attributes of each cardiac chamber. The zebrafish is an excellent subject for our studies, by virtue of the accessibility of the embryonic zebrafish heart and the ease of classical genetic analyses. Through forward genetic screens in zebrafish, we have isolated a panel of over 50 mutations that disrupt cardiac chamber formation.  Using these mutations as tools, we have identified pivotal regulators of the two major phases of chamber formation: specification and morphogenesis.  Current projects focus on the signals that specify cardiac chamber lineages and the morphogenetic processes that create chamber form.

Chamber specification
Cardiac chamber formation begins with the process of chamber specification, the assignment of ventricular and atrial identities to discrete progenitor populations.  Through high-resolution fate mapping, we have located cardiac progenitors in the early embryo, demonstrating that ventricular and atrial progenitor populations are spatially organized prior to gastrulation.  By applying our fate map technique to the analysis of mutant phenotypes, we have identified signaling pathways that play essential roles during chamber specification.  Specifically, we have demonstrated that Nodal signaling has a potent influence on chamber specification: fate map analyses suggest that a marginal-to-animal gradient of Nodal signaling establishes the marginal-animal organization of ventricular and atrial progenitors.  Additionally, we have shown that retinoic acid signaling plays a critical role in restricting cardiac progenitor specification: fate map analyses indicate that retinoic acid limits the density of cardiac progenitors emerging from a competent region, enforcing a balance between cardiac and non-cardiac identities.  Together, our studies demonstrate that cardiac progenitor specification is achieved through interplay of inductive and repressive pathways.  Our current studies investigate the molecular mechanisms by which multiple specification signals converge to influence components of the cardiac differentiation pathway (such as hand2, spt5, and spt6).

Chamber morphogenesis
Following progenitor specification, a complex series of cell movements organize the ventricular and atrial precursors into a heart tube.  Subsequently, additional morphogenetic processes generate the characteristic shape of each cardiac chamber.  Using time-lapse confocal microscopy, we have analyzed the trajectories of individual cardiomyocytes during tube assembly.  Our comparisons of wild-type and mutant embryos indicate spatiotemporally regulated patterns of directed cell movement, influenced differentially by interactions with the endoderm and the endocardium.  Within the heart tube, our analyses of cardiomyocyte morphology and organization indicate that regional changes in cell shape and size contribute to the emergence of distinct chamber morphologies.  These cellular traits are regulated cell-autonomously by chamber-specific gene expression programs; additionally, cardiomyocyte and chamber morphology can be influenced by extrinsic factors, such as blood flow.  Our current studies further explore the pathways that integrate genetic and epigenetic regulation of cardiac chamber morphology.