Genome-edited human stem cells expressing fluorescently labeled endocytic markers allow quantitative analysis of clathrin-mediated endocytosis during differentiation

We developed a general approach for investigation of how cellular processes become adapted for specific cell types during differentiation. Previous studies reported substantial differences in the morphology and dynamics of clathrin-mediated endocytosis (CME) sites. However, associating specific CME properties with distinct differentiated cell types and determining how these properties are developmentally specified during differentiation have been elusive. Using genome-edited human embryonic stem cells, and isogenic fibroblasts and neuronal progenitor cells derived from them, we established by live-cell imaging and platinum replica transmission electron microscopy that CME site dynamics and ultrastructure on the plasma membrane are precisely reprogrammed during differentiation. Expression levels for the endocytic adaptor protein AP2 mu 2 were found to underlie dramatic changes in CME dynamics and structure. Additionally, CME dependency on actin assembly and phosphoinositide-3 kinase activity are distinct for each cell type. Collectively, our results demonstrate that key CME properties are reprogrammed during differentiation at least in part through AP2 mu 2 expression regulation.