Septins Coordinate with Microtubules and Actin to Initiate Cell Morphogenesis

Abstract Many organs are formed by a process of branching morphogenesis, which begins with the formation of cytoplasmic extensions from the basal surface of polarized cells in an epithelial sheet. To study this process, we used a system of polarized epithelial spheroids, which emit cytoplasmic extensions in response to treatment with hepatocyte growth factor. We found that these extensions contain both actin and microtubules, but also septins, which are localized to microtubule bundles and appear to be important in maintaining microtubule organization. We found that these extensions are highly dynamic and form at a non-linear rate. We also demonstrated that the coordinated activity of microtubules, actin, and septins is necessary for the formation and dynamic behavior of extensions. Each cytoskeletal system plays a district role in this process, with microtubules enabling persistent growth of the extensions, actin enabling extension dynamics, and septins organizing microtubules in the extensions and supporting the extension formation. Together, our data offer insights into the dynamics of early morphogenic extensions and the distinct, but coordinated, roles of cytoskeleton in early morphogenesis.