Long-term expansion of basal cells and the novel differentiation methods identify mechanisms for switching Claudin expression in normal epithelia.

Epithelia are tightly connected cellular sheets, that shield our body from the external environment. They are continuously maintained by a pooled population of undifferentiated cells through differentiation. However, the maintenance mechanisms remain incompletely understood due to the difficulty of experimentally observing the differentiation process. To address this issue, we developed a culture method for long-term expansion of primary mammary basal cells with a set of compounds, that includes undifferentiated cells. An effective differentiation method regarding Claudin expression was also developed by simply removing compounds. To verify this differentiation-switching technique, we obtained microarray data comparing each differentiation state. Subsequent cellular analysis confirmed key transcription factors in each state: (1) EGR1 in undifferentiated basal cells is important for suppressing Claudin expression through maintaining the epithelial-mesenchymal transition (EMT) transcription factor TWIST1, (2) ELF3 in differentiated cells is important for actin organization and subsequent Claudin localization at the cell-cell border, that corresponds to the amount of GRHL3, an actin organizer. Their relevance was also observed in tissues and organoids. In summary, we present an effective tool for verifying molecular mechanisms that determine Claudin status in normal basal cell differentiation, that would be beneficial in epithelial cell biology, cancer biology, physiology, and regeneration research.