Roles of microtubules and microtubule regulators in collective invasive migration of drosophila border cells

Abstract

Unlike actin, which is required for almost all eukaryotic cell migrations, the roles of another cytoskeleton component, the microtubule, are less clear. Much of our understanding of how cells migrate has largely come from studies in tissue culture assays, raising the concern that it may not always be applied in vivo. Border cells are a group of somatic follicle cells that perform a stereotypic migration between the nurse cells of the Drosphila ovary during oogenesis. During this migration the nurse cells act as the substrate over which the border cells migrate. This reproducible migration serves as a convenient model to study collective migration in vivo. Through imaging of both stable and dynamic microtubules, we found differential microtubule organization and dynamics within the cluster: microtubules are highly organized in the polar cells and form a microtubule organization center (MTOC)-like structure that is polarized towards the leading edge prior to migration. The outer border cells, in contrast, have some cortical microtubules, but are less organized. Tracking of the plus end marker EB1-GFP showed microtubules grow preferentially towards the center of the cluster.

We started investigations of general effects of microtubules in the border cell migration system by drugs. Net cluster movement was affected by both nocodazole and taxol which disrupt microtubules and microtubule dynamics. The specific microtubule depolymerization factor Stathmin had a subtle role in migration, and was found to be largely required in the substrate nurse cells. To find additional regulators, we conducted a RNAi screen against genes encoding known or potential microtubule regulators in the fly genome. Among about 70 genes screened, the dynein interactors Lissencephaly-1 (Lis-1) and nudE, together with dynein were found to be required both in the polar cells (in agreement with previous published results) and outer border cells. These genes have important roles in regulating the forward extensions that may generate traction force for cluster movement. In addition, compromising their activities severely disrupted the organization of the border cell cluster, as visualized by the abnormal distribution of adhesion molecules. In summary, we found microtubules do play roles in both migratory border cells as well as their interacting cells. Specifically, the Lis-1-NudE-Dynein complex was required, possibly through regulating front extensions and the reorganization of the follicular epithelium to ensure a properly organized migratory cluster.