A basic question in vertebrate development is the identity of the signals and signal transduction mechanisms that establish the position and polarity of the embryonic axes. In amphibians, the proteins in the Wnt pathway, including GSK-3, beta-catenin, and APC play important roles in this process. Similar roles in mammalian development have yet to be established. The Principal Investigator and his colleaques have characterized mouse "Fused" gene, which appears to be a novel regulator of the Wnt pathway. Preliminary data suggests that Fused acts as a negative regulator of axis development and exerts its effects in the Wnt pathway. The Principal Investigator now proposes to further investigate the role of Fused in embryonic development, and the molecular mechanisms by which it regulates signaling in the Wnt Pathway. To accomplish this goal, five specific aims are listed. The first is to use Xenopus laevis embryos to study the role of Fused in the specification of the dorsal axis through injection of wild type or mutant Fused with factors in the Wnt pathway or in other signaling pathways. The second aim will characterize the sizes, subcellular location, covalent modification, and expression during embryogenesis of the Fused proteins, a smaller protein potentially encoded by a minor transcript, and two spontaneous mutants. The third aim is to investigate how Fused functions at the molecular level by studying its physical and functional interaction with other proteins. The fourth aim will characterize further the recessive embryonic defects caused by two mutants alleles in an effort to understand the role of Fused in mouse embryogenesis. Finally, new types of mutants and transgenic mice will be generated to study the role of Fused in mouse embryogenesis.