Left-Right Patterning in the Vertebrate Embryo

In my laboratory we are using the zebrafish to study how the left-right (LR) axis and pattern is established. Vertebrates appear bilaterally symmetric, but have internal asymmetries along the LR axis. This axis is revealed by the asymmetric placement of organs along the midline. For example, the human heart is located on the left of the body cavity, while the liver is located on the right. While genes implicated in LR patterning have been identified, we do not know how the LR axis is established, how the axis is aligned with the existing dorsal-ventral and anterior-posterior axes, or how LR information is received and interpreted by developing organs. Proper LR axis formation is critical for organogenesis as correct organ placement allows for proper connectivity with the developing vasculature. In humans, defects in LR patterning often manifest as congenital heart disease. Our current studies focus on a pathway known to be involved in left-right patterning, and on identifying new genes involved in this process.

The role of Nodal signaling in left-right patterning

In all vertebrates, components of the Nodal signaling pathway are expressed asymmetrically in the left lateral plate mesoderm, a tissue that will give rise to many asymmetric visceral organs. These components include the nodal ligand, the feedback inhibitor lefty, and the downstream transcription factor pitx2. In zebrafish we have an additional asymmetric expression domain for these genes in the developing brain. We are using zebrafish one-eyed pinhead (oep) mutants to explore the role of Nodal signaling in LR patterning of the viscera and brain. Oep is a member of the EGF-CFC family of proteins, which act as co-factors that are absolutely required for Nodal signaling. In oep mutants, asymmetric organs in the viscera and brain are still asymmetrically placed, but their positioning is randomized compared to wild-type controls. For example, the pancreas is correctly positioned on the right in approximately half of oep embryos, and on the left in the other half. This suggests Nodal is not required to generate asymmetries, but is required to properly direct their asymmetric position such that a consistent pattern is achieved. Future work in the lab will focus on when and where the Nodal signaling pathway is required for proper LR patterning and how this pathway provides directional information to developing organs.

Understanding how organs obtain asymmetric positions

How an organ obtains its final asymmetric position is not understood. Organs such as the heart and pancreas form in the midline, and obtain asymmetric positions later in development. To examine the cell movements that occur during this process, we are taking advantage of the transparency of zebrafish embryos. We are using GFP transgenics to observe cell behaviors during organ morphogenesis in the living embryo. To complement this descriptive approach, we are studying new zebrafish mutants to identify additional genes that affect LR organ patterning. In Class I mutants, organs that are normally asymmetric remain in midline positions. The identification of the genes affected in these mutants will further our understanding of how organs are asymmetrically positioned. Class II mutants have a complete reversal of asymmetric organs in half of the mutant embryos, and wild-type LR patterning in the other half. Class III mutants have randomized organ positioning similar to what is observed in oep mutants. Some of the Class III mutations have additional defects in the kidney. Cloning and characterizing the genes affected in these mutants will provide insights into how the embryos establishes and patterns the LR axis, and how these genes may be used in other contexts, such as kidney formation.