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...

Cancer-associated fibroblasts (CAFs) promote tumour invasion and metastasis. We show that CAFs exert a physical force on cancer cells that enables their collective invasion. Force transmission is mediated by a heterophilic adhesion involving N-cadherin at the CAF membrane and E-cadherin at the cancer cell membrane. This adhesion is mechanically active; when subjected to force it triggers β-catenin recruitment and adhesion reinforcement dependent on α-catenin/vinculin interaction. Impairment of E-cadherin/N-cadherin adhesion abrogates the ability of CAFs to guide collective cell migration and blocks cancer cell invasion. N-cadherin also mediates repolarization of the CAFs away from the cancer cells. In parallel, nectins and afadin are recruited to the cancer cell/CAF interface and CAF repolarization is afadin dependent. Heterotypic junctions between CAFs and cancer cells are observed in patient-derived material. Together, our findings show that a mechanically active heterophilic adhesion between CAFs and cancer cells enables cooperative tumour invasion.

Podosomes are adhesion structures formed in monocyte-derived cells. They are F-actin-rich columns perpendicular to the substrate surrounded by a ring of integrins. Here, to measure podosome protrusive forces, we designed an innovative experimental setup named protrusion force microscopy (PFM), which consists in measuring by atomic force microscopy the deformation induced by living cells onto a compliant Formvar sheet. By quantifying the heights of protrusions made by podosomes onto Formvar sheets, we estimate that a single podosome generates a protrusion force that increases with the stiffness of the substratum, which is a hallmark of mechanosensing activity. We show that the protrusive force generated at podosomes oscillates with a constant period and requires combined actomyosin contraction and actin polymerization. Finally, we elaborate a model to explain the mechanical and oscillatory activities of podosomes. Thus, PFM shows that podosomes are mechanosensing cell structures exerting a protrusive force.