| The discovery of genes that direct mammalian development has been greatly advanced by forward genetic screens. Because of its unbiased nature, forward genetic screen often uncovers unexpected gene functions and novel mechanisms of mouse development. Here I present the characterization of two mouse mutants from forward genetic screens: tenuous (tenu) and canopus (canp). The analysis of these two mutants reveals novel roles of Giantin and Axin2 in mouse development.;The tenu mutation causes specific heart defects at midgestion such as reduced formation of the outflow tract and right ventricle, thin cardiac jelly and and a thick myocardium. These phenotypes are very similar to those seen in mouse mutants devoid of hyaluronic acid due to a null mutation in haluronic acid synthase 2. Phenotypic analysis revealed that there was, indeed, a specific loss of hyaluornic acid in the tenu mutant embryos.;The tenu mutation is a null allele of Giantin , which encodes a protein previously implicated in the maintenance of the Golgi structure and the support of intracellular trafficking. In contrast to the data from cultured cells, the analysis of tenu shows that Giantin is dispensable for the structure of the Golgi apparatus and most trafficking events in mouse embryos, and defines a specific role of Giantin in regulating the early heart development through the production of hyaluronic acid.;Axin and Axin2 are essential negative regulators of the canonical Wnt pathway. They serve as scaffolds for the beta-catenin destruction complex, and bind specifically to several Wnt pathway components such as DvI, Gsk3beta, beta-catenin and APC. In the absence of a Wnt signal, Axins promote the phosphorylation and the consequent degradation of beta-catenin. Upon Wnt stimulation, the destruction complex falls apart and beta-catenin accumulates and enters the nucleus to turn on target gene expression. In addition to its negative role in transducing Wnt signaling, the expression of Axin2 is induced by the Wnt signaling such that Axin2 acts in a negative feedback loop to regulate the duration of Wnt signalling. Emerging evidence shows that Axin and Axin2 are degraded in response to the Wnt signaling, but it is unclear whether the degradation of Axin and Axin2 is required for the transduction of the canonical Wnt pathway.;Here I show that a hypermorphic recessive mutation of Axin2, canp, stabilizes Axin2 in mouse embryos. The stabilized Axin2, canp, not only inhibits the Wnt signal transduction in mouse embryos, but also disrupts the negative feedback loop, which leads to decreased but temporally extended Wnt signaling in the presomitic mesoderm and primitive streak. As a result of the loss of Wnt signaling, the canp mutants display specific morphological defects with variable penetrance, including cardia bifida, discontinuous notochord, fused somites and an absence of neural plate elevation. A fraction of canp mutants show mesoderm migration defects at e7.5, but display a striking recovery from e7.5 to e9.5, concomitant with the prolonged Wnt signaling in the streak. Thus our analysis of canp mutants demonstrates that the degradation of Axin2 is required for the proper regulation of Wnt signaling in mouse embryos and uncovers novel roles of canonical Wnt signaling in cardiac migration, notochord formation and the elevation of neural plate. |
