Ahead of the Curve: Mechanisms of Left-Right Asymmetric Stomach Morphogenesis

Project Summary
Birth defects are a leading cause of infant mortality, yet in most cases, their etiology is unknown. Some
of the most common and complex malformations are found in families with abnormal left-right (LR) asymmetry,
suggesting that many congenital defects may result from perturbed laterality. The initial embryonic events that
determine the LR body axis, including the early breaking of bilateral symmetry, and subsequent left-sided
expression of determinants such as nodal and Pitx2, are now well understood. However, the genetic and
morphogenetic events involved in the final phases of LR development, at the organ level, remain largely
unknown. The long term goal is to ascertain the mechanisms of LR asymmetric organogenesis. The objective
in this application is to identify the molecular and cellular processes that generate LR asymmetry (curvature)
within an individual organ, the stomach. Preliminary analyses identified LR asymmetries in radial cell
rearrangements in the developing stomach as the driving force for its curvature. To identify the proximate
effectors of this novel asymmetric morphogenetic program, a new model organism (Lepidobatrachus laevis)
was employed. The extra-large embryos of this species facilitated a gene-discovery approach that would be
intractable in most models: transcriptome profiling (RNASeq) of tissues dissected from left vs. right halves of
the embryonic stomach. Pilot datasets include genes with LR asymmetric expression patterns and functions
during stomach curvature. The central hypothesis is that stomach curvature is determined by distinct left and
right regulatory networks which differentially modulate the cellular events controlling radial cell rearrangement.
The unique experimental amenability of frog embryos will be used to test this hypothesis via three specific
aims: 1) Generate molecular signatures of normal and abnormal stomach curvature. Comprehensive
spatiotemporal profiles of LR stomach genes will be generated and compared in the context of both normal
LR asymmetry and experimentally-induced LR axis defects. 2) Determine the cellular function of stomach-
specific LR genes. Select genes will be tested in loss- and gain-of-function assays to determine their
influence on radial cell rearrangement in the developing stomach. 3) Determine the regulatory hierarchy
that controls stomach curvature. Experimental perturbations combined with spatiotemporal profiling will
reveal core gene regulatory interactions governing asymmetric morphogenesis. The overall approach is
innovative because it takes advantage of distinctive attributes of a unique species to address one of the key
unanswered questions in LR development: what are the proximate mechanisms by which developing organs
become LR asymmetric entities? The proposed research is significant because it will immediately advance
our understanding of normal laterality and laterality-related birth defects by defining the genes, morphogenetic
processes and regulatory logic that govern the emergence of LR asymmetry at the organ level.