Epithelial polarity and growth signaling by Drosophila tumor suppressor genes

Abstract

The Drosophila model system has been at the forefront of many scientific discoveries. Studies of tumor suppressor genes in fruit flies have provided major insights towards polarity and growth signaling, while also aiding the understanding of complex signaling in cancer and other human diseases. The research in this thesis describes and discusses findings regarding a multitude of Drosophila tumor suppressor genes, putting the results of these studies in a broader context of epithelial growth, polarity and cell signaling. In Chapter 1 I provide an overview of epithelial polarity and its regulators, including the Drosophila genes that are involved in controlling epithelial integrity and cause tumorous phenotypes when disrupted. The mutations that cause tumorigenesis in flies can provide clues to what cellular processes are connected to aberrant proliferation and allows us to interrogate signaling pathways that underlie malignant transformation. While many Drosophila mutants are known that generate epithelial overgrowth, the links between the different categories of genes are largely unknown. Chapter 2 establishes a connection between two of these major gene groups: the Scrib polarity module and endocytic trafficking regulators. This work shows that components of the retromer trafficking pathway genetically interact with the Scrib module, and retromer pathway cargo traffic is defective in Scrib mutant tissue. The Scrib module is necessary for proper retromer function and we propose that one of the major ways in which Scrib regulates polarity is through controlling endocytic itineraries of retromer- dependent cargo. Chapter 3 identifies a new player in Drosophila epithelial polarity regulation. Here we show that null mutants for a substrate adaptor of an SCF-class E3 ubiquitin ligase, called supernumerary limbs (slmb), display a loss of polarity and dramatic overproliferation in the imaginal discs of Drosophila larvae. Normally Slmb is required to antagonize the polarity regulator aPKC by restricting its activity to the apical membrane, independent of endocytic trafficking and parallel to the Scrib module. This implicates targeted protein degradation as a new mechanism involved in regulating Drosophila apicobasal polarity. In Chapter 4 we investigate molecular signaling underlying a known Drosophila TSG that has been relatively understudied. While many fly TSGs provided insight into the regulation of epithelial growth, polarity and architecture, some have remained obscure. An example of this is tumorous imaginal discs (tid), which was discovered along with many other classical tumor suppressors, though the functional mechanisms behind tid have remained elusive. We report that tid was originally miscloned, and the phenotypes are driven by the loss of the N-linked glycosylation pathway component ALG3. Tumorous growth is driven by altered Hippo growth signaling, which is induced by excessive JNK pathway activity. Surprisingly this JNK activation is caused by the defective glycosylation of a single protein - the fly TNF receptor homolog- inducing increased affinity for TNF. Our findings indicate that N-linked glycosylation can modulate TNFR signaling by adjusting the dynamic interaction threshold with its ligand, providing tissues with a mechanism to respond to altered physiological inputs.