Mutations in the Novel Drosophila Tricellular Junction Protein M6 Induce Metastatic Behaviors via Apical Cell Delaminatio

Despite the obvious health impacts of metastasis, remarkably little is known about one of the first active steps of metastasis: departure of a tumor cell from its originating tissue, which is referred to as cell delamination. Our lack of knowledge about this process is due to the inherent difficulties in studying the dynamic and transient process of cell delamination in vivo. To study cell delamination in more depth we utilized the power of Drosophila genetics and performed a genetic screen for metastatic invasion in Drosophila and discovered that mutations in the protein M6 synergize with oncogenic RasV12 to drive invasion and metastasis. We further found that loss of M6 induces prominent apical cell delamination. In contrast to the common belief that metastatic behaviors such as invasion are only initiated through basal delamination, we found that these apically delaminated clones are migratory, can cross tissue layers, and spread into other tissues. Mechanistically, we observed that M6 deficient RasVI2 clones delaminate apically as a result of alterations in a Canoe-RhoA-myosin II axis which is necessary for both the delamination and invasion phenotypes. To uncover the cellular roles of M6 we show that it localizes to tricellular junctions in epithelial tissues. Further, we show that M6 is necessary for the structural integrity of multicellular contacts as evidenced by holes present at tricellular junctions in M6 mutant tissue when viewed by electron microscopy. We then expand these findings to show that alterations in other tricellular junction proteins can also drive apical delamination and metastasis of RasV12 clones. This work provides evidence that apical delamination can initiate metastatic behaviors in certain contexts, and highlights the important role that tricellular junction integrity can play in this process.