The fusion of the gametes upon fertilization results in the formation of a totipotent cell. Embryonic chromatin is expected to be able to support a large degree of plasticity. However, whether this plasticity relies on a particular conformation of the embryonic chromatin is unknown. Moreover, whether chromatin plasticity is functionally linked to cellular potency has not been addressed. Here, we adapted fluorescence recovery after photobleaching (FRAP) in the developing mouse embryo and show that mobility of the core histones H2A, H3.1, and H3.2 is unusually high in two-cell stage embryos and decreases as development proceeds. The transition toward pluripotency is accompanied by a decrease in histone mobility, and, upon lineage allocation, pluripotent cells retain higher mobility than the differentiated trophectoderm. Importantly, totipotent two-cell-like embryonic stem cells also display high core histone mobility, implying that reprogramming toward totipotency entails changes in chromatin mobility. Our data suggest that changes in chromatin dynamics underlie the transitions in cellular plasticity and that higher chromatin mobility is at the nuclear foundations of totipotency.

The zonula adherens (ZA), a type of adherens junction (AJ), plays a major role in epithelial cell–cell adhesions. It remains unknown how the ZA is remodeled during epithelial reorganization. Here we found that the ZA was converted to another type of AJ with punctate morphology (pAJ) at the margins of epithelial colonies. The F-actin–stabilizing protein EPLIN (epithelial protein lost in neoplasm), which functions to maintain the ZA via its association with αE-catenin, was lost in the pAJs. Consistently, a fusion of αE-catenin and EPLIN contributed to the formation of ZA but not pAJs. We show that junctional tension was important for retaining EPLIN at AJs, and another force derived from actin fibers laterally attached to the pAJs inhibited EPLIN–AJ association. Vinculin was required for general AJ formation, and it cooperated with EPLIN to maintain the ZA. These findings suggest that epithelial cells remodel their junctional architecture by responding to mechanical forces, and the αE-catenin–bound EPLIN acts as a mechanosensitive regulator for this process.

Polarized epithelial cells exhibit a characteristic array of microtubules that are oriented along the apicobasal axis of the cells. The minus-ends of these microtubules face apically, and the plus-ends face toward the basal side. The mechanisms underlying this epithelial-specific microtubule assembly remain unresolved, however. Here, using mouse intestinal cells and human Caco-2 cells, we show that the microtubule minus-end binding protein CAMSAP3 (calmodulin-regulated–spectrin-associated protein 3) plays a pivotal role in orienting the apical-to-basal polarity of microtubules in epithelial cells. In these cells, CAMSAP3 accumulated at the apical cortices, and tethered the longitudinal microtubules to these sites. Camsap3 mutation or depletion resulted in a random orientation of these microtubules; concomitantly, the stereotypic positioning of the nucleus and Golgi apparatus was perturbed. In contrast, the integrity of the plasma membrane was hardly affected, although its structural stability was decreased. Further analysis revealed that the CC1 domain of CAMSAP3 is crucial for its apical localization, and that forced mislocalization of CAMSAP3 disturbs the epithelial architecture. These findings demonstrate that apically localized CAMSAP3 determines the proper orientation of microtubules, and in turn that of organelles, in mature mammalian epithelial cells.