[show abstract][hide abstract] ABSTRACT: While stem cells can sense and respond to physical properties of their environment, the molecular aspects how physical information is translated into biochemical signals remain unknown. Here we show that human mesenchymal stem cells (hMSCs) harvest and assemble plasma fibronectin into their extracellular matrix (ECM) fibrils within 24 hours. hMSCs pro-actively pull on newly assembled fibronectin ECM fibrils, and the fibers are more stretched on rigid than on soft fibronectin-coated polyacrylamide gels. Culturing hMSCs on single stretched fibronectin fibers upregulates hMSC osteogenesis. Osteogenesis was increased when αvβ3 integrins were blocked on relaxed fibronectin fibers, and decreased when α5β1 integrins were blocked or when epidermal growth factor (EGF) receptor signaling was inhibited on stretched fibronectin fibers. This suggests that hMSCs utilize their own contractile forces to translate environmental cues into differential biochemical signals by stretching fibronectin fibrils. Mechanoregulation of fibronectin fibrils may thus serve as check point to regulate hMSC osteogenesis.
[show abstract][hide abstract] ABSTRACT: Substrate-exploring functions of filopodia were previously suggested based on cell studies on flat surfaces, but their role in topography sensing especially within nanofibrillar environments remained elusive. Here we have grown highly flexible hairy silicon nanowires on micropatterned islands on otherwise flat glass surfaces and coated them both with the extracellular matrix (ECM) protein fibronectin. This allowed us to visualize how filopodia steer fundamental cell functions such as cell adhesion, spreading, migration and division in the absence of lamellipodia. Shortly after seeding, transient filopodia protrude from the still spherical cells. Once filopodia contact nanowires, they bend and align them, while most filopodia peel off from flat surfaces. A zipping mechanism regulated by traction forces is proposed to explain how force-induced changes in filopodia-substrate contact angles enable topography sensing, including the still elusive phenomenon of contact guidance. Filopodia thus play a central role in steering transient topographic preferences.
[show abstract][hide abstract] ABSTRACT: 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.
Nature Material 05/2012; 11(7):642-9. · 35.75 Impact Factor