Probing the mechanism of Wnt signal transduction at the Drosophila neuromuscular junction

Intercellular communication is a basic phenomenon of life, essential for cells to choose their correct fates and manifest the appropriate morphologies and functional relationships with their neighbors. This proposal focuses on how Wnt (Wingless/Wnt1) signaling, a fundamental mechanism of intercellular communication, controls the development of neuromuscular junctions (NMJ’s) - the specialized synapses by which the central nervous system controls muscle activity.In canonical Wnt signaling, Wnts are received by the Frizzled (Fz) family of 7-pass transmembrane receptors. Fz receptors then transduce Wnt signals by controlling the stability and nuclear accumulation of the transcription factor Armadillo/ß-catenin (Arm). A decade ago an alternative, non-canonical mechanism of Wnt signal transduction, termed the Frizzled Nuclear Import (FNI) pathway, was proposed to explain how Drosophila Wingless (Wg), a founding member of the Wnt superfamily, is transduced in NMJ’s. It posits that Wg from motoneurons acts on the Fz family receptor dFz2 in muscle to induce the cleavage and nuclear import of its cytosolic C-terminal fragment, which acts as a transcription factor. As proposed, the FNI pathway constitutes an independent, and more direct, route of Wg transduction compared to the canonical Arm/ß-catenin pathway. This view is now widely cited and generally accepted, with major implications for our understanding of synapse development and function, including our understanding of neuromuscular disease.The central aim of this proposal is to test the FNI Wg signaling model and more generally, to determine both the role and molecular mechanism of Wnt transduction in the NMJ. To do so, I am using the powerful genetics of Drosophila together with diverse cellular and molecular techniques to determine: (i) the requirement for the cytosolic C-terminal fragment of the receptor in transducing Wg in the NMJ - a key stipulation of the FNI model, (ii) the role of the canonical Arm/ß-catenin pathway - the most likely alternative to the FNI pathway, and (iii) the source(s) and role(s) of Wg in NMJ development. The outcome of this project will have an impact on both our general understanding of how synapses form and function, as well as the etiology of neuromuscular disease.