Microtubule-templated actin assembly by septin9 drives apical expansion of epithelial cells

Cells dynamically regulate the size of their apical domain during epithelial morphogenesis. Multiciliated cells (MCCs) represent an extreme case, undergoing massive apical expansion to accommodate hundreds of basal bodies that generate an array of motile cilia. While actin-generated forces drive this expansion, the molecular mechanisms that govern actin assembly and organization remain poorly understood. We show that in Xenopus, depletion of Septin9 results in failure of filamentous actin assembly, apical domain expansion, and ciliogenesis. Notably, human Septin9 (SEPT9) localizes to cortical microtubules rather than actin filaments. Live-cell imaging and pharmacological studies reveal that SEPT9 and microtubules form a mutually stabilizing scaffold that directs actin assembly. Using isoform-specific depletion and rescue experiments that genetically separate the microtubule from its actin-bound functions of SEPT9, we demonstrate that SEPT9 association with microtubules is required for actin assembly. These results provide the first in vivo demonstration that microtubule-templated actin assembly via Septin9 drives apical surface expansion during epithelial morphogenesis.