Members of the Wnt family of secreted signaling molecules play critical roles in the specification of the mammalian body plan. Nineteen Wnt genes have been identified in the mouse and human genomes and many are co-expressed during embryogenesis begging the question of how specificity arises. One way specificity could be achieved is through the selective activation of different signal transduction pathways. One well-characterized pathway, the canonical Wnt/beta-catenin pathway, signals through beta-catenin, while other alternative signaling pathways, known as the Wnt/planar cell polarity (PCP) pathway and the Wnt/Ca2+ pathway are less well-defined and utilize different signal transducers. The main goals of this project are to understand how Wnts participate in the formation and elaboration of the body plan during early mouse development, and the molecular pathways through which these Wnts signal. ? ? Wnt3a Links Left-Right Determination with Segmentation and Anterior-Posterior Axis Elongation ? ? The alignment of the left-right (LR) body axis relative to the anteroposterior (AP) and dorsoventral (DV) body 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. Wnt3a is known to regulate somitogenesis by controlling the segmentation clock and the molecular oscillations of the Notch pathway. We have discovered that Wnt3a is required for LR asymmetry. Investigations into the molecular mechanisms underlying LR determination by Wnt3a demonstrate that it directly activates the target gene Dll1, which encodes the Notch ligand Delta. Activation of the Notch pathway in turn activates expression of the left determinant, Nodal, in the node while simultaneously controlling the segmentation clock and the molecular oscillations of the Wnt/beta-catenin and Notch pathways. We showed that Wnt3a, expressed in the primitive streak and dorsal posterior node, acts as a longe-range signaling molecule, directly regulating target gene expression throughout the node and presomitic mesoderm. Wnt3a may also modulate the symmetry-breaking 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. A revised manuscript describing this work is currently under consideration by the journal editor. ? ? Beta-catenin specifies the presomitic mesoderm and controls the segmentation clock ? ? Our analysis of Wnt3a function implicates the Wnt/beta-catenin pathway as the primary transducer of Wnt3a signals during LR determination and segmentation. To better understand the mechanisms underlying the coordinate regulation of these complex developmental processes, we have taken two approaches: ? ? 1) We have examined the function of beta-catenin in these processes using conditional loss and gain of function alleles. Conditional approaches were necessitated by the early pre-gastrulation lethality of beta-catenin null alleles. We used a fragment of the T(Brachyury) promoter, which is activated after gastrulation has begun, to drive expression of Cre recombinase in the posterior embryo, allowing us to bypass the pre-gastrulation requirement for beta-catenin. Conditional inactivation of beta-catenin revealed that it is indeed required for asymmetric heart-looping (LR asymmetry) and somitogenesis. Histological analysis indicated that overt somite boundaries did not form and presomitic mesoderm was absent, while molecular analyses could find no evidence for oscillating gene expression in the Wnt or Notch pathways. ?
| Nakaya, Masa-aki; Biris, Kristin; Tsukiyama, Tadasuke et al. (2005) Wnt3a links left-right determination with segmentation and anteroposterior axis elongation. Development 132:5425-36 |
