Opposing actomyosin pools generate cortical flows that establish epithelial polarity

Epithelial cell polarity, defined by distinct apical and basolateral domains, is fundamental for animal embryonic development and organ function. During organogenesis, epithelia often develop from unpolarized precursor cells. How mammalian epithelial cells establish polarity de novo from an initially unpolarized state has remained unclear, in part due to an inability to observe this process in real time in non-transformed cellular systems. Here, we leverage recent advances in 3D spheroid culture of mouse embryonic stem cells, fluorescent protein knock-in and live imaging techniques to study the process of epithelial polarity establishment. We show that apical myosin activity, regulated by Myosin Light Chain Kinase (MLCK), is crucial for the establishment and maintenance of epithelial polarity. Actomyosin cortical flows transport ZO-1, a tight junction component that interacts with apical polarity proteins, to establish the apical membrane. A second pool of myosin, regulated by Rho kinase, localizes basally and balances apically directed flow. Our result imply that epithelial polarity emerges as a consequence of actomyosin-driven cytoskeletal rearrangements.