RASSF8, CYRI and Gukh - novel regulators in invasive collective cell migration

Collective cell migration is an essential process in developmental processes and is involved in tumorigenesis and wound closure. To migrate collectively, actin dynamics have to be controlled both temporally and spatially. Here, Drosophila border cells are used as an established model for in vivo cell migration to study the functions of Ras-association domain containing family member 8 (RASSF8), CYFIP-related Rac interactor (CYRI) and Gukholder (Gukh) in invasive collective cell migration. The Drosophila egg chamber consist of an oocyte which is nourished by 15 nurse cells and protected by a surrounding epithelium. During oogenesis a cluster of around eight cells delaminates from the epithelium and migrates through the nurse cells to the oocyte. This group of cells is called the border cell cluster and plays a vital part in the formation of a pore in the micropyle, the entry point for the sperm. An important driver of cell migration in single and collective cell migration is the small GTPase Rac1, which activates the WAVE regulatory complex (WRC) and its name-giving protein WASP family verprolin homologue (WAVE). The WRC is an activator of the Arp2/3 complex, a conserved actin nucleation machine essential for the formation of branched actin networks in eukaryotic cells, necessary for the formation of the lamellipodium and efficient cell migration. CYRI is a negative regulator of the Rac-WRC-Arp2/3 pathway and has been described in vitro as a competitive inhibitor of the WRC. Here, for the first time it is shown, that loss of CYRI leads to a mislocalisation of integrins within the border cell cluster, leading to cohesion defects, resulting in single border cells lagging behind the cluster. A similar defect could also be observed upon loss of the tumor suppressor RASSF8, which we previously identified as a novel interaction partner of WAVE. A genetic interaction between RASSF8 and WAVE is relevant in wing and bristle development, where loss of one copy of wave in a rassf8 mutant background leads to a drastic augmentation of wing morphology defects and the number of bent bristles. Additionally, rassf8 mutant females display a reduced fertility and frequent defects in border cell migration, marked by a reduction in cell cohesion and lagging border cells. Interestingly, RASSF8 is necessary for the proper localisation of the cell adhesion protein Echinoid and the septate junction protein Coracle. Additionally, here a new function in cell adhesion can be shown for Gukh, the only Drosophila homologue for the human Nance-Horan Syndrome (NHS) protein. Gukh contains a WAVE homology domain (WHD), is a negative regulator of the WRC and was first described as an interaction partner of Scribble and therefore important for the maintenance of cell polarity. Even though Scribble localisation is unchanged in gukh mutant border cell clusters, gukh mutant clusters display prominent cohesion defects during migration. Taken together my work impressively demonstrates a new complexity of the signalling pathways that collectively migrating cells receive and process. The three proteins RASSF8, CYRI and GUKH are of particular importance here, because they can influence actin dynamics via regulation of the WRC and additionally impact cell adhesion.