UC Santa Barbara

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 was 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 found that knocking down septins significantly impacted border cell migration and cluster morphology through detachment failure and failure to form stable forward-directed protrusions. Overexpressing septins also dramatically impacted migration, with an inverse effect on morphology. We used high-resolution Airyscan imaging paired with Tissue Cartography to generate 3D models of the surface of the cluster and measure spectral decomposition with changes in septin expression. We also created 2D binary masks of border cells and quantified morphological changes due to septin expression.

To uncover the mechanistic role of septins in border cell migration, we tested candidates that may interact with septins. Myosin colocalized dynamically with septins, but surprisingly they do not directly influence each other. Instead, septins and myosin are independently regulated by Rho. Septins and myosin regulate border cell membrane texture and contractility to allow for proper cortical tension, protrusion formation, and migration.