Abstract:The ability of cells to follow gradients of extracellular matrix stiffnessdurotaxis-has been implicated in development, fibrosis and cancer. Durotaxis is established as a single cell phenomenon but whether it can direct the motion of cell collectives is unknown. Here we found that multicellular clusters exhibited durotaxis even if isolated constituent cells did not. This emergent mode of directed collective cell migration applied to a variety of epithelial cell types, and required the action of myosin motors and the integrity of cell-cell junctions. By extending traction microscopy to extracellular matrices of arbitrary stiffness profiles we showed that collective durotaxis originated from supracellular transmission of contractile physical forces. To explain the observed phenomenology, we developed a generalized clutch model in which local stickslip dynamics of cell-matrix adhesions is integrated to the tissue level through cell-cell junctions. Collective durotaxis is far more efficient than single cell durotaxis; it thus emerges as a robust mechanism to direct cell migration during development, wound healing, and collective cancer cell invasion. One Sentence Summary: Mechanical cooperation between cells enables an emergent mode of collective movement -3- Main Text:The ability of living cells to migrate following environmental gradients underlies a broad range of phenomena in development, homeostasis, and disease (1, 2). The best understood mode of directed cell migration is chemotaxis, the well-established ability of cells to follow gradients of soluble chemical cues (1). Some cell types are also able to follow gradients in the stiffness of their extracellular matrix (ECM), a process known as durotaxis (3-10).Durotaxis has been implicated in development (11), fibrosis (12) and cancer (13), but its underlying mechanisms remain unclear.Most of our understanding of directed cell migration has been obtained in single isolated cells. However, fundamental processes during development, wound healing, tissue regeneration, and some forms of cancer cell invasion are driven by directed migration of cell groups (14-16). Cell-cell interactions within these groups provide cooperative mechanisms of cell guidance that are altogether inaccessible to single cells (14-20). Here we investigated whether cell groups undergo collective durotaxis, and the cooperative nature of underlying mechanisms.Using stencils of magnetic PDMS, we micropatterned rectangular clusters (500 µm width) of human mammary epithelial cells (MCF-10A) on fibronectin-coated polyacrylamide gel substrates exhibiting uniform stiffness or a stiffness gradient (51 ± 17 kPa/mm, Fig. S1) (21). Upon removal of the PDMS stencil, clusters migrating on uniform gels displayed symmetric expansion (Fig. 1A,C,E,G, Fig. S2, Movie S1), whereas clusters migrating on stiffness gradients displayed a robust asymmetry characterized by faster, more persistent expansion towards the stiff edge (Fig. 1B-D-F-H, Fig. S2, Movie S1). This result was also -4-observed in clusters of...

A fundamental feature of multicellular organisms is their ability to self-repair wounds through the movement of epithelial cells into the damaged area. This collective cellular movement is commonly attributed to a combination of cell crawling and “purse-string” contraction of a supracellular actomyosin ring. Here we show by direct experimental measurement that these two mechanisms are insufficient to explain force patterns observed during wound closure. At early stages of the process, leading actin protrusions generate traction forces that point away from the wound, showing that wound closure is initially driven by cell crawling. At later stages, we observed unanticipated patterns of traction forces pointing towards the wound. Such patterns have strong force components that are both radial and tangential to the wound. We show that these force components arise from tensions transmitted by a heterogeneous actomyosin ring to the underlying substrate through focal adhesions. The structural and mechanical organization reported here provides cells with a mechanism to close the wound by cooperatively compressing the underlying substrate.

Dynamics of epithelial tissues determines key processes in development, tissue healing, and cancer invasion. These processes are critically influenced by cell-cell adhesion forces. However, the identity of the proteins that resist and transmit forces at cell-cell junctions remains unclear, and how these proteins control tissue dynamics is largely unknown. Here we provide a systematic study of the interplay between cell-cell adhesion proteins, intercellular forces, and epithelial tissue dynamics. We show that collective cellular responses to selective perturbations of the intercellular adhesome conform to three mechanical phenotypes. These phenotypes are controlled by different molecular modules and characterized by distinct relationships between cellular kinematics and intercellular forces. We show that these forces and their rates can be predicted by the concentrations of cadherins and catenins. Unexpectedly, we identified different mechanical roles for P-cadherin and E-cadherin; while P-cadherin predicts levels of intercellular force, E-cadherin predicts the rate at which intercellular force builds up.

Epithelial monolayers are one-cell thick tissue sheets that separate internal and external environments. As part of their function, they have to withstand extrinsic mechanical stresses applied at high strain rates. However, little is known about how monolayers respond to mechanical deformations. Here, by subjecting suspended epithelial monolayers to stretch, we find that they dissipate stresses on a minute timescale in a process that involves an increase in monolayer length, pointing to active remodelling of cell architecture during relaxation. Strikingly, monolayers consisting of tens of thousands of cells relax stress with similar dynamics to single rounded cells and both respond similarly to perturbations of actomyosin. By contrast, cell-cell junctional complexes and intermediate filaments do not relax tissue stress, but form stable connections between cells, allowing monolayers to behave rheologically as single cells. Taken together our data show that actomyosin dynamics governs the rheological properties of epithelial monolayers, dissipating applied stresses, and enabling changes in monolayer length. the Rosetrees Trust, the UCL Graduate School, the EPSRC funded doctoral training program CoMPLEX, and the European Research Council (ERC-CoG MolCellTissMech, agreement 647186 to GC). N.K. was in receipt of a UCL Overseas Research Scholarship. N.K. was supported by the Prof Rob Seymour Travel Bursary Fund for research visits to Barcelona. J.F. and A.B. were funded by BBSRC grant (BB/M003280 and BB/M002578) to G.

Ventral furrow formation is the first large-scale movement in the Drosophila cellular blastoderm which involves the co-ordinated shape change of cells, and as such is an ideal system to use as a proof of principle for simulation methods to study the mechanics of morphogenesis. We have developed a 3D finite element method model of ventral furrow formation by decomposing the total deformation into two parts: an imposed active deformation, and an elastic passive deformation superimposed onto the latter. The model imposes as boundary conditions (i) Vito, C., Muñoz, J.J., Miodownik M., A 3D finite element model of ventral furrow invagination in the Drosophila melanogaster embryo, Journal of the Mechanical Behavior of Biomedical Materials, Vol. 1, Issue 2, pp. 188-198, 2008 compared the model with a 2D model and shown that it exhibits a more robust invagination phenomenon. The 3D model has also revealed that invagination causes a yolk flow from the central region to the anterior and posterior ends of the embryo, causing an accordion-like global compression and expansion wave to move through the embryo. Such phenomenon cannot be described by 2D models.