Epithelial tricellular junctions act as interphase cell shape sensors to orient mitosis

As fruitfly epithelial cells round up during mitosis, tricellular junctions serve as spatial landmarks, encoding information about interphase cell shape directionality to orient mitosis, and promoting geometric and mechanical sensing in epithelial tissues. The orientation of cell division is important for controlling tissue development and homeostasis. In non-dividing interphase cells, the long axis determines the division orientation. But as the cells enter the mitosis stage to divide, they lose this axis as they round up to ensure faithful chromosome segregation. So how can the dividing cells remember their division orientation? At a cellular level, it is thought that the default orientation is regulated by the mitotic-cell shape. Yohanns Bellaïche and colleagues have determined what happens at the tissue level. They find that as fruitfly epithelial cells round up during mitosis, tricellular junctions serve as spatial landmarks, encoding information about interphase cell-shape directionality. The authors provide molecular insights into this process, showing that tricellular junctions promote geometry and mechanical sensing in epithelial tissues. The orientation of cell division along the long axis of the interphase cell—the century-old Hertwig’s rule—has profound roles in tissue proliferation, morphogenesis, architecture and mechanics1,2. In epithelial tissues, the shape of the interphase cell is influenced by cell adhesion, mechanical stress, neighbour topology, and planar polarity pathways3,4,5,6,7,8,9,10,11,12. At mitosis, epithelial cells usually adopt a rounded shape to ensure faithful chromosome segregation and to promote morphogenesis1. The mechanisms underlying interphase cell shape sensing in tissues are therefore unknown. Here we show that in Drosophila epithelia, tricellular junctions (TCJs) localize force generators, pulling on astral microtubules and orienting cell division via the Dynein-associated protein Mud independently of the classical Pins/Gαi pathway. Moreover, as cells round up during mitosis, TCJs serve as spatial landmarks, encoding information about interphase cell shape anisotropy to orient division in the rounded mitotic cell. Finally, experimental and simulation data show that shape and mechanical strain sensing by the TCJs emerge from a general geometric property of TCJ distributions in epithelial tissues. Thus, in addition to their function as epithelial barrier structures, TCJs serve as polarity cues promoting geometry and mechanical sensing in epithelial tissues.