Summary Wnt 3 a links left-right determination with segmentation and anteroposterior axis elongation

The anteroposterior (AP) body axis is the first axis to be established during the formation of the mammalian body plan. The left-right (LR) axis is specified last, and is oriented orthogonally to the pre-existing AP and DV axes. The specification and coordination of all three vertebrate body axes is controlled by a small group of cells known as the SpemannMangold organizer (Niehrs, 2004). A transient structure, termed the node, is generally considered to be the murine equivalent of the Spemann-Mangold organizer; however, the node first forms at the anterior end of the primitive streak of the gastrulating embryo on embryonic day (E) 7.5, well after AP polarity has been established. The timing of node formation correlates well with LR axis specification, and with the beginning of somitogenesis and the development of the trunk. Somitogenesis generates the segmental structures of the trunk and is a major morphogenetic force driving the elongation of the AP axis. The node plays an important role in trunk development as node ablation results in the loss of LR and dorsoventral (DV) polarity, retarded somite formation and shortened trunks (Davidson et al., 1999). Thus, the node functions as a trunk organizer, coordinating axis determination with trunk elongation. Members of the transforming growth factor (Tgf ) family, specifically Nodal, Lefty1 and Lefty2, are the first genes to be asymmetrically expressed along the LR axis (Hamada et al., 2002). Nodal is expressed in the periphery of the node, where it functions as the left-determinant (Brennan et al., 2002; Saijoh et al., 2003). Nodal transcription is controlled by the Notch signaling pathway. Activation of Notch receptors by the ligand Delta-like 1 (Dll1), leads to the cleavage and nuclear translocation of the Notch intracellular domain, where it acts as a transcription factor when bound to the DNA-binding protein RBP-J (Rbpsuh – Mouse Genome Informatics) (Schweisguth, 2004). Loss of function mutations in components of the Notch pathway lead to loss of LR asymmetry, and RBP-J-binding sites found within the Nodal node-specific enhancer are required for Nodal expression in the node (Krebs et al., 2003; Raya et al., 2003). These data demonstrate that Nodal is a direct target gene of the Notch signaling pathway; however, the relationship between Notch activity and symmetry-breaking events in the node is not clear. Cilia emanating from the ventral surface of the node play a crucial role in the breaking of bilateral symmetry (McGrath and Brueckner, 2003). Embryos carrying mutations in genes required for cilia formation or motility display laterality defects (Marszalek et al., 1999; Nonaka et al., 1998, Supp et al., 1999). Motile cilia generate a leftward flow of extra-embryonic fluid at the node, termed nodal flow, that is necessary for the generation of LR asymmetry (Nonaka et al., 1998; Okada et al., 1999). Artificial reversal of nodal flow is sufficient to reorient the LR axis (Nonaka et al., 2002) demonstrating that nodal flow is both necessary and sufficient for LR axis specification. These experiments led to the development of the The alignment of the left-right (LR) body axis relative to the anteroposterior (AP) and dorsoventral (DV) axes is central to the organization of the vertebrate body plan and is controlled by the node/organizer. Somitogenesis plays a key role in embryo morphogenesis as a principal component of AP elongation. How morphogenesis is coupled to axis specification is not well understood. We demonstrate that Wnt3a is required for LR asymmetry. Wnt3a activates the Delta/Notch pathway to regulate perinodal expression of the left determinant Nodal, while simultaneously controlling the segmentation clock and the molecular oscillations of the Wnt/ -catenin and Notch pathways. We provide evidence that Wnt3a, expressed in the primitive streak and dorsal posterior node, acts as a long-range signaling molecule, directly regulating target gene expression throughout the node and presomitic mesoderm. Wnt3a may also modulate the symmetrybreaking activity of mechanosensory cilia in the node. Thus, Wnt3a links the segmentation clock and AP axis elongation with key left-determining events, suggesting that Wnt3a is an integral component of the trunk organizer.