As Long As Your Heart Is In the Left Place: A Study of Left-Right Patterning In Cardiac Morphogenesis in Zebrafish

Abstract

Improper left-right axis establishment during embryonic development is known to result in congenital heart defects in humans. But how exactly does the asymmetric patterning of an embryo facilitate asymmetric cardiac morphogenesis? The full mechanism is as of yet undetailed. The current understanding follows that in the zebrafish model, Nodal signaling acts on the left to differentially increase migration velocities of cardiac precursor cells. A faster rate of migration of these cells compared to their right counterparts results in the displacement of the heart tube to the left of the embryonic midline. Based on the understanding that actin polymerization can modulate cell motility, an investigation ensued to determine if actin polymerization regulated by Nodal plays a role in the stereotypic leftward cardiac jog. Inhibition of actin polymerization with the drug Cytochalasin B was used to determine the impact of the loss of actin polymerization in wild type and mutant zebrafish. Reduction of F-actin resulted in decreased heart tube elongation and less severe, if any, leftward migration of cardiac cells. This finding was backed by cell tracking of the same embryos under similar drug conditions. In order for actin polymerization to facilitate migration, the extracellular matrix (ECM) must be degraded. Matrix Metalloproteinase-2 (MMP-2) was identified as a target of Nodal signaling in a previous microarray analysis, verified by qt-PCR. Matrix Metalloproteinase-2 (MMP-2) was inhibited in this study by Batimastat and ARP-100. The result was delayed cardiac jogging and shortened heart tubes. The totality of these results points towards the implication that Nodal acts to upregulate a number of factors, including MMP-2 and actin polymerization in order to asymmetrically induce cell migration during cardiac morphogenesis. This understanding may impact the current comprehension of how congenital heart defects arise in humans.