The evolutionarily conserved Wnt (Wingless and INT-1) family proteins are secreted cysteine-rich glycoproteins essential for embryonic patterning and adult homeostasis. Wnt signaling is highly regulated in all aspects, including the biogenesis and processing of active Wnt ligands, Wnt secretion and gradient formation on the cell surface and in the extracellular matrix, and perception and signal transduction within the signal-receiving cells. Dysregulation of Wnt signaling underlies many human diseases, in particular, cancers.
To gain deeper insights into Wnt signaling, I used Drosophila as a model system and focused on the best studied Drosophila Wnt homolog, Wingless (Wg). My studies have shed light on three aspects of the regulation of Wg/Wnt signaling. First, I systematically analyzed the roles of two major post-translational modifications of Wg. By using multiple in vitro and in vivo systems, my work provides solid and thorough data toward the question ‘how lipid modification and N-glycosylation contribute to Wingless secretion and signaling’. Furthermore, I investigated the process of Wntless (Wls) trafficking and its role in Wingless secretion. I found that Wls undergoes ubiquitination on the cell surface. Importantly, from a genetic RNAi screen, I identified two essential components in the control of Wls ubiquitiation, the E3 ligase Su(dx) and the deubiquitinating enzyme USP8. My work suggests that Wls ubiquitination is involved in the trafficking of Wls and the sorting of Wg following Wls endocytosis. Finally, I identified a novel component essential in the signal transduction of canonical Wg/Wnt signalling, the hyperplastic discs (hyd). Further epistasis analysis indicates that hyd acts in parallel with or downstream of Armadillo (Arm) to regulate nuclear Arm activity. Taken together, this dissertation work will help us to better understand the orchestrated regulation of Wg/Wnt pathway at multiple levels.