Force loading explains spatial sensing of ligands by cells

The formation of cellular adhesion complexes is important in normal and pathological cell activity, and is determined by the force imposed by the combined effect of the distribution of extracellular matrix molecules and substrate rigidity. Forcing cellular sticking points Integrin-mediated cell adhesion is a critical parameter in many physiological and pathological processes, and can be harnessed to modulate cellular responses to synthetic biomaterials for various applications. This paper reports that the formation of cellular focal adhesions is regulated by both the distribution of extracellular matrix (ECM) ligands and substrate rigidity. The authors suggest that these parameters dictate how much force is loaded onto each ECM–integrin bond, which they call a 'molecular clutch', from myosin motors within the cell pulling on the bonds via the actin filaments. They propose a model to explain how cells attempt to regulate the force loaded onto each molecular clutch by recruiting more integrins to the adhesion to generate new molecular clutches. If the maximum number of available clutches is generated but the force per clutch is too high, the focal adhesion collapses. Cells can sense the density and distribution of extracellular matrix (ECM) molecules by means of individual integrin proteins and larger, integrin-containing adhesion complexes within the cell membrane. This spatial sensing drives cellular activity in a variety of normal and pathological contexts 1 , 2 . Previous studies of cells on rigid glass surfaces have shown that spatial sensing of ECM ligands takes place at the nanometre scale, with integrin clustering and subsequent formation of focal adhesions impaired when single integrin–ligand bonds are separated by more than a few tens of nanometres 3 , 4 , 5 , 6 . It has thus been suggested that a crosslinking ‘adaptor’ protein of this size might connect integrins to the actin cytoskeleton, acting as a molecular ruler that senses ligand spacing directly 3 , 7 , 8 , 9 . Here, we develop gels whose rigidity and nanometre-scale distribution of ECM ligands can be controlled and altered. We find that increasing the spacing between ligands promotes the growth of focal adhesions on low-rigidity substrates, but leads to adhesion collapse on more-rigid substrates. Furthermore, disordering the ligand distribution drastically increases adhesion growth, but reduces the rigidity threshold for adhesion collapse. The growth and collapse of focal adhesions are mir