To investigate how substrate properties influence stem-cell fate, we cultured single human epidermal stem cells on polydimethylsiloxane (PDMS) and polyacrylamide (PAAm) hydrogel surfaces, 0.1 kPa-2.3 MPa in stiffness, with a covalently attached collagen coating. Cell spreading and differentiation were unaffected by polydimethylsiloxane stiffness. However, cells on polyacrylamide of low elastic modulus (0.5 kPa) could not form stable focal adhesions and differentiated as a result of decreased activation of the extracellular-signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) signalling pathway. The differentiation of human mesenchymal stem cells was also unaffected by PDMS stiffness but regulated by the elastic modulus of PAAm. Dextran penetration measurements indicated that polyacrylamide substrates of low elastic modulus were more porous than stiff substrates, suggesting that the collagen anchoring points would be further apart. We then changed collagen crosslink concentration and used hydrogel-nanoparticle substrates to vary anchoring distance at constant substrate stiffness. Lower collagen anchoring density resulted in increased differentiation. We conclude that stem cells exert a mechanical force on collagen fibres and gauge the feedback to make cell-fate decisions.

Collective movement of epithelial cells drives essential multicellular organization during various fundamental physiological processes encompassing embryonic morphogenesis, cancer and wound healing. Yet the molecular mechanism that ensures the coordinated movement of many cells remains elusive. Here we show that a tumour suppressor protein, merlin, coordinates collective migration of tens of cells, by acting as a mechanochemical transducer. In a stationary epithelial monolayer and also in three-dimensional human skin, merlin localizes to cortical cell-cell junctions. During migration initiation, a fraction of cortical merlin relocalizes to the cytoplasm. This relocalization is triggered by the intercellular pulling force of the leading cell and depends on the actomyosin-based cell contractility. Then in migrating cells, taking its cue from the intercellular pulling forces, which show long-distance ordering, merlin coordinates polarized Rac1 activation and lamellipodium formation on the multicellular length scale. Together, these results provide a distinct molecular mechanism linking intercellular forces to collective cell movements in migrating epithelia.

We report on the creation of films of end-grafted hyaluronan, based on solid-supported lipid membranes. We characterize the layer thickness, the grafting density, the mechanical properties and the permeability of these highly hydrated and up to several hundred nanometer-thick monomolecular layers by quartz crystal microbalance with dissipation monitoring, by colloidal probe triple-wavelength reflection interference contrast microscopy, and by reflectometry. The two-dimensional assemblies thus created are expected to serve as versatile platforms to study, in a well-controlled and quantitative manner, the effect of hyaluronan-binding proteins on the structure, properties, and biological function of this type of films.