Linking Hedgehog and Nodal/TGF-beta signaling in the establishment of left-right asymmetry

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Project summary/abstract The establishment of left-right (LR) asymmetry is a critical event required for the correct positioning of internal organs. Defects in human LR axis formation cause birth defects of the heart, vasculature, lungs, and intestinal tract. The gene network contributing to the generation of LR asymmetry is highly conserved across vertebrates. In the mouse, the initial asymmetric signals establishing LR axis are determined in the node by cilia-driven leftward fluid flow (nodal flow). These signals are then transferred to the left lateral plate mesoderm (LPM), which will undergo asymmetric organ morphogenesis. Nodal, a secretory protein that belongs to the transforming growth factor-b (TGF-b) superfamily, is expressed in the node and travels a long distance to the left LPM, where it initiates a transient auto-regulatory circuit (involving Nodal and Lefty) that propagates Nodal signaling and activates expression of the left-sided determinant Paired-Like Homeodomain transcription factor 2 (Pitx2). The Hedgehog (Hh) signaling pathway also plays a crucial role in LR patterning. In the mouse, Hh signaling is required to establish the midline that separates the left and right sides of the embryo, as well as to activate the Nodal-dependent auto-regulatory circuit in the LPM. However, the mechanism by which Hh signaling regulates the competence of the LPM for Nodal response is not well understood. Thus, identifying the specific convergence point of the Hh and TGF-b pathways is critical for understanding the in-depth mechanism underlying LR asymmetry determination, and therefore to provide better diagnosis, preventive and therapeutic intervention against LR asymmetry-related birth defects. We recently found that a novel target of the Hh pathway, Neuron-Derived Neurotrophic Factor (Ndnf), regulates axial rotation and intestinal looping in the mouse. In the proposed work, we will incorporate biochemical, molecular, and genetic approaches to (Aim 1) assess the source, (Aim 2) determine the transcriptional regulation, and (Aim 3) investigate the underlying molecular mechanisms of Ndnf in LR determination.