Research in the Burns Laboratory is devoted to understanding how cardiovascular progenitor cells build the vertebrate heart during embryonic development and how, in certain vertebrate species such as the zebrafish, the heart can fully regenerate following injury. In the embryo, we are particularly interested in a recently identified late-differentiating pool of cardiovascular progenitors, called the anterior second heart field (SHF), that is responsible for building the right ventricle and outflow tract in higher vertebrates. Defects in human SHF development cause common complex congenital cardiac malformations including Tetralogy of Fallot and Double Outlet Right Ventricle. Even though the zebrafish heart does not have the anatomic equivalent of a right ventricle, we discovered that the SHF is conserved in zebrafish. Using lineage tracing, we determined that the zebrafish SHF gives rise to a significant portion of cardiomyocytes in the ventricle and three cardiovascular lineages in the outflow tract. Using genetic and small molecule loss-of-function analysis, we implicated TGFb-signaling in promoting proliferation of SHF progenitors to balance SHF differentiation. More recently, we have demonstrated that the timing and location of SHF specification are conserved in zebrafish and that the essential functions of two cardiac transcription factors during SHF development are also conserved in zebrafish. Current laboratory efforts are devoted to using unbiased approaches (genetic and small molecule screens) to identify novel regulators of SHF development. We are also generating the necessary tools to perform in vivo clonal analysis to determine when SHF progenitors are segregated from other cardiovascular progenitors.

The second embryonic process of interest to our laboratory is pharyngeal arch artery (PAA) development. PAAs are paired bilateral embryonic vessels that undergo remodeling and give rise to essential segments of the aorta, pulmonary artery, and carotid arteries in higher vertebrates. In humans, defects in PAA development or remodeling are common causes of congenital malformations of the aorta. The zebrafish PAAs are not remodeled but the initial configuration of the PAAs is conserved. In recent work, we have identified the classically defined heart field as the embryonic source of PAA endothelium. Using loss of function analysis, we have implicated the cardiac transcription factor Nkx2.5 in controlling the endothelial differentiation of angioblasts. Current efforts are devoted to discovering novel small molecules that disrupt PAA formation as an unbiased approach to identify novel regulators of this process.

Another major focus of the laboratory is cardiac regeneration. In humans, cardiac injury induces scar formation rather than regeneration. The holy grail of cardiovascular medicine is to understand the barriers to cardiomyocyte regeneration in humans and devise new therapeutic approaches to overcome them. The zebrafish heart regenerates myocardium robustly following several different forms of injury. Myocardial regeneration relies on proliferation of uninjured cardiomyocytes but the extrinsic and intrinsic factors that stimulate cardiomyocyte proliferation are not well defined. We have recently implicated Notch signaling in the control of zebrafish cardiomyocyte proliferation and cardiac regeneration. Notch receptors are induced in the endocardium and epicardium, but not the myocardium, following injury. Inhibition of Notch signaling using a dominant negative approach impairs cardiac regeneration by inhibiting cardiomyocyte proliferation. Because Notch signaling is high in the endocardium and epicardium but the proliferation defect is seen in the myocardium, we hypothesize that a Notch dependent myocardial proliferation signal is released from the endocardium and epicardium following injury. Current efforts are underway to identify this Notch dependent proliferation signal. We are also devising novel transgenic tools to probe the genetic pathways controlling cardiomyocyte proliferation in the embryo on the assumption that many of the same pathways are controlling cardiomyocyte proliferation in the embryo and regenerating heart.