View Program - 63rd Annual Drosophila Research Conference

Collective cell migration is crucial for development and the preferred mode of migration by metastatic tumors, but much about it is unknown. The border cell cluster in the Drosophila ovary is an ideal model for collective cell migration, as border cells migrate on and between nurse cells. The cytoskeleton is a critical regulator of cell migration. Septins, now considered the fourth cytoskeletal element, remain unexplored in collective systems. The objective of this study is to understand the functions and necessity of septins in border cell migration. Using RNAi, mutants, and over-expression, we investigated the impact of septins on border cell migration and cluster morphology through fixed and live imaging. We used high-resolution Airyscan imaging paired with Tissue Cartography to generate 3D models of the surface of the cluster. We found that knocking down any of the five Drosophila septins significantly impacted border cell migration and cluster morphology through detachment failure, failure to form stable forward-directed protrusions, and a loose blebby morphology. Overexpressing septins also dramatically impacted migration, with an inverse effect on morphology. As septin subunits function by forming higher order structures with each other, we investigated if septin monomers interacted. Clonal knockdown of Septin 1 (Sep1) or Septin 2 (Sep2) led to a significant loss of Peanut (Pnut, or Sep3) in the follicle and border cells. Similarly, knockdown of Sep1 or Pnut caused a loss of Sep2. These findings suggest that Sep1, Sep2, and Pnut form structures with each other in the follicle and border cells independently of Sep4 and Sep5, which have no impact on Pnut or Sep2 expression. To uncover the mechanistic role of septins in border cell migration, we tested candidates that may interact with septins. We observed co-localization between septins and nonmuscle myosin II in fixed imaging, and then explored further through live imaging. Amazingly, dynamic myosin flashes completely co-localized with dynamic septin expression. This suggests an interaction between septins and myosin. For example, septins may act as a scaffold for myosin, recruiting it and stabilizing protrusions. In conclusion, we found that septins are necessary for the detachment of the border cell cluster and for protrusion formation, while too much septin induces an excess of curvature and prevents migration. Sep1, Sep2, and Pnut interact to form structures and may interact with myosin.

Septins are necessary for detachment and protrusion formation in border cell migration