9600539 Bejsovec The Wnt gene family encode secreted growth factor-like molecules that regulate cell behavior through cell-cell communication. Inappropriate expression of Wnt-l in the mammary glands of adult female mice results in mammary tumors, or breast cancer. Normal expression of Wnt-1 in developing mouse embryos regulates cell proliferation and specifies cellular identities. This proposal exploits the fruitfly, Drosophila melanogaster, as a model system to study the cellular mechanisms of Wnt gene action. The Drosophila wingless (wg) gene is highly homologous to the mouse Wnt-1 gene and performs similar signaling functions that influence cell behavior during development. The powerful genetic system of Drosophila has revealed that wg specifies different aspects of embryonic pattern at different times during development. These temporally distinct signaling activities are separately mutable within the Wg protein, suggesting that structural features of the signaling protein influence the cellular response. Furthermore, secretion and transport of Wg protein within and between cells appears to be essential to Wg function. The nature of the distinct Wg signaling activities and the role of intercellular Wg transport will be examined through the directed expression of mutant transgenes in living embryos. Mutant Wg molecules that possess defined subsets of the normal Wg signaling activities will be expressed either ubiquitously, under the control of the heat shock promoter, or in defined domains, using the GAL4-UAS binary expression system. The phenotypic consequences on embryonic pattern and on molecular markers of wg signaling activity will be assessed to determine the cellular basis for the distinct Wg signaling activities observed. In addition, molecules that interact with Wg protein during signaling and transport will be identified by characterizing mutations that modulate wg signaling function. A small collection of mutations that modify the wg mutant phenotype have been isolated and several mutations show features that strongly suggest a direct involvement in the wg signaling pathway. These modifier mutations will be characterized genetically and molecularly to determine the nature of their interaction with Wg. These two approaches, in the genetically tractable Drosophila system, will provide insight into the cellular machinery through which Wg/Wnt molecules specify cell fate and control cell proliferation.
