Apical clathrin-coated endocytic pits control the growth and size of epithelial microvilli

Epithelial cells from diverse contexts assemble apical specializations to serve tissue-specific functions. In virtually all cases, these features consist of arrays of microvilli: micron-scale, actin bundle-supported protrusions that mediate biochemical and physical interactions with the external environment. Despite their importance for epithelial physiology, how microvilli grow during cellular differentiation remains poorly understood. Using genetic and small molecule perturbations, we found that an epithelial cell’s potential for growing microvilli of normal size is limited by an adjacent actin-dependent process: apical clathrin-mediated endocytosis. Timelapse imaging of individual microvillar growth events revealed tight spatial and temporal coupling to sites of clathrin-mediated endocytosis. Finally, ultrastructural analysis of undifferentiated epithelial monolayers showed that most nascent microvilli are immediately adjacent to or in contact with an apical endocytic pit. Based on these discoveries, we conclude that clathrin-mediated endocytosis controls the timing and location of microvillar growth, as well as the dimensions of the resulting protrusions.