Cell Adhesion: Integrating the Integrin Response

RESEARCH HIGHLIGHTS

Nature Reviews Molecular Cell Biology | AOP, published online 12 June 2013; doi:10.1038/nrm3605 NPG

CELL ADHESION Integrating the integrin response The 24 αβ integrin heterodimers encoded by colocalized with fibronectin associated with mammals mediate cellular adhesion to the F-actin bundles at the cell periphery. The extracellular matrix and to neighbouring cells. β3 integrin clusters, but not β1 integrin staining, actomyosin- They are often co-expressed and bind similar could be disrupted with a myosin II inhibitor. mediated factors — for example, α5β1 integrin and all This indicates that actomyosin-mediated tension αv class integrins bind fibronectin — but mediate stabilizes αvβ3–fibronectin bonds, which could tension both specific and redundant functions. contribute to rigidity sensing. Indeed, the stabilizes Schiller et al. have provided an explanation for contractile energy of pKO-αv/β1 cells, but not αvβ3– this specificity by correlating the integrin type pKO‑β1 cells, increased with the rigidity of with the protein composition of and the the substrate. fibronectin downstream signalling from particular Consistent with the dependence of αvβ3 bonds, integrin-based adhesions. The results show how integrin-mediated adhesion on myosin II-mediated which could α5β1 and αv integrins cooperate to sense the tension, myosin II inhibition completely prevented contribute to rigidity of fibronectin-based microenvironments protein recruitment to focal adhesions in pKO-αv through the interplay between signalling outputs cells, whereas pKO-αv/β1 and pKO‑β1 cells were rigidity sensing and feedback amplification of myosin II activity. still able to recruit integrin‑proximal proteins. The authors generated pan-knockout (pKO) Using quantitative mass spectrometry to fibroblast clones lacking β1, β2, β7 and αv integrins, determine the protein composition of which fail to express any integrin heterodimers. integrin-mediated adhesions and quantitative These cells were transduced with αv- and/or phosphoproteomics to visualize adhesion- β1‑encoding sequences to produce cells mediated signalling, the authors could account for expressing αv (pKO-αv), β1 (pKO‑β1) or αv and β1 the specific and synergistic effects of αv and (pKO-αv/β1) integrins. The three cell lines were β1 integrins. They found that binding shown to bind fibronectin through αvβ3 and of α5β1 integrin to fibronectin activates αvβ5 integrins (pKO-αv cells), α5β1 integrin RAC1–‌WAVE–ARP2/3‑driven actin polymerization (pKO‑β1 cells) or αvβ3, αvβ5 and α5β1 integrins to induce membrane protrusions as well as (pKO-αv/β1 cells). generating RHOA–RHO kinase-mediated myosin II On a fibronectin substrate, pKO‑β1 cells activity. They propose that the resulting formed small nascent adhesions and showed actomyosin-mediated tension stabilizes binding of increased protrusive activity and migration speed αv integrins to fibronectin. RHOA activity compared with pKO-αv cells, which formed larger downstream of αv integrins does not activate focal adhesions; pKO-αv/β1 cells formed both small myosin II directly but instead activates DIAP1 and large adhesions and showed an intermediate (Drosophila melanogaster inhibitor of apoptosis 1). migration speed. Immunofluorescence analysis This drives stress fibre formation and thus provides revealed that myosin II activity was highest in a structural basis for force generation by myosin II, pKO-αv/β1 cells and lowest in pKO-αv cells. which leads to larger and more stable focal Treatment of pKO-αv/β‌ 1 cells with an αv-specific adhesions. inhibitor decreased myosin II activity to the So, co-expression of both α5β1 and αv integrins intermediate level found in pKO‑β1 cells. So, the is required for an optimized response of cells to larger focal adhesions formed by αvβ3 and αvβ5 the stiffness of a fibronectin-based extracellular integrins seemed to synergize with the small matrix, which might have implications for adhesions formed by α5β1 integrin to induce pathologies such as fibrosis and tumour optimal myosin II-mediated cell contractility. metastasis. The authors went on to show that this optimal Kirsty Minton cell contractility can be regulated according to ORIGINAL RESEARCH PAPER Schiller, H. B. et al. β1- and αv-class the rigidity of the substrate. Whereas β1 integrin integrins cooperate to regulate myosin II during rigidity sensing of staining colocalized with fibronectin uniformly fibronectin-based microenvironments. Nature Cell Biol. 15, 625–636 (2013) throughout pKO-αv/β1 cells, β3 integrin staining

NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 14 | JULY 2013