The orientation of cell division along the interphase cell long-axis, 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 pathways3–12. At mitosis, epithelial cells usually round up to ensure faithful chromosome segregation and to promote morphogenesis1. The mechanisms underlying interphase cell shape sensing in tissues are therefore unknown. We found that in Drosophila epithelia, tricellular junctions (TCJ) localize microtubule force generators, 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 TCJ 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.

Summary Life for all animals starts with a precise 3D-choreography of reductive divisions of the fertilized egg, known as cleavage patterns. These patterns exhibit conserved geometrical features and striking inter-species invariance within certain animal classes. To identify the generic rules that may govern these morphogenetic events, we developed a 3D-modelling framework that iteratively infers blastomere division positions and orientations, and consequent multicellular arrangements. From a minimal set of parameters, our model predicts detailed features of cleavage patterns in the embryos of fishes, amphibians, echinoderms and ascidians, as well as the genetic and physical perturbations that alter these patterns. This framework demonstrates that a geometrical system based on length-dependent microtubule forces that probe blastomere shape and yolk gradients, biased by cortical polarity domains, may dictate division patterns and overall embryo morphogenesis. These studies thus unravel the default self-organization rules governing early embryogenesis, and how they are altered by deterministic regulatory layers.

The contractile and enzymatic activities of myosin VI are regulated by calcium binding to associated calmodulin light chains. We have used transient phosphorescence anisotropy (TPA) to monitor the microsecond rotational dynamics of erythrosin iodoacetamide-labeled actin with strongly-bound myosin VI (MVI) and to evaluate the effect of MVI-bound calmodulin (CaM) light chain on actin filament dynamics. MVI binding lowers the amplitude but accelerates actin filament microsecond dynamics in a Ca2+ - and CaM-dependent manner, as indicated from an increase in the final anisotropy and a decrease in the correlation time of TPA decays. MVI with bound apo-CaM or Ca2+ - CaM weakly affects actin filament microsecond dynamics, relative to other myosins (e.g. muscle myosin II and myosin Va). CaM dissociation from bound MVI damps filament rotational dynamics (i.e. increases the torsional rigidity), such that the perturbation is comparable to that induced by other characterized myosins. Analysis of individual actin filament shape fluctuations imaged by fluorescence microscopy reveals a correlated effect on filament bending mechanics. These data support a model in which Ca2+ - dependent CaM binding to the IQ domain of MVI is linked to an allosteric reorganization of the actin-binding site(s), which alters the structural dynamics and the mechanical rigidity of actin filaments. Such modulation of filament dynamics may contribute to the Ca2+ – and CaM–dependent regulation of myosin VI motility and ATP utilization.

Chlamydomonas axonemal tubulin despite greater than 85% similarity in primary structure. Finally, we found that the instantaneous translocation speed of microtubules in the gliding assay is unsteady. Our analysis suggests that the source of this unsteadiness may arise from the same mechanochemical properties of dynein that have been predicted to be sufficient for coordination. Together these results suggest that the interactions between dynein and tubulin are important factors in axonemal dynein coordination.