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The ability of animal cells to sense, adhere to and remodel their local extracellular matrix (ECM) is central to control of cell shape, mechanical responsiveness, motility and signalling, and hence to development, tissue formation, wound healing and the immune response. Cell–ECM interactions occur at various specialized, multi-protein adhesion complexes that serve to physically link the ECM to the cytoskeleton and the intracellular signalling apparatus. This occurs predominantly via clustered transmembrane receptors of the integrin family. Here we review how the interplay of mechanical forces, biochemical signalling and molecular self-organization determines the composition, organization, mechanosensitivity and dynamics of these adhesions. Progress in the identification of core multi-protein modules within the adhesions and characterization of rearrangements of their components in response to force, together with advanced imaging approaches, has improved understanding of adhesion maturation and turnover and the relationships between adhesion structures and functions. Perturbations of adhesion contribute to a broad range of diseases and to age-related dysfunction, thus an improved understanding of their molecular nature may facilitate therapeutic intervention in these conditions.Cell–extracellular matrix (ECM) interactions occur at specialized, multi-protein adhesion complexes, with clustered integrins as the predominant ECM receptors. Progress in characterization of adhesion composition, organization and dynamics in response to force has improved understanding of adhesion maturation and turnover and the relationships between adhesion structures and functions.

Focal adhesion kinase (FAK) is both a non-receptor tyrosine kinase and an adaptor protein that primarily regulates adhesion signalling and cell migration, but FAK can also promote cell survival in response to stress. FAK is commonly overexpressed in cancer and is considered a high-value druggable target, with multiple FAK inhibitors currently in development. Evidence suggests that in the clinical setting, FAK targeting will be most effective in combination with other agents so as to reverse failure of chemotherapies or targeted therapies and enhance efficacy of immune-based treatments of solid tumours. Here, we discuss the recent preclinical evidence that implicates FAK in anticancer therapeutic resistance, leading to the view that FAK inhibitors will have their greatest utility as combination therapies in selected patient populations.Focal adhesion kinase (FAK) is overexpressed in many cancers and is involved in a multitude of oncogenic processes and resistance mechanisms. This Review discusses the rationale and preclinical evidence for FAK-based combination therapies and strategies for future development.

Cell adhesion to the extracellular matrix is fundamental to metazoan multicellularity and is accomplished primarily through the integrin family of cell-surface receptors. Integrins are internalized and enter the endocytic–exocytic pathway before being recycled back to the plasma membrane. The trafficking of this extensive protein family is regulated in multiple context-dependent ways to modulate integrin function in the cell. Here, we discuss recent advances in understanding the mechanisms and cellular roles of integrin endocytic trafficking. Moreno-Layseca et al. discuss how integrins, key receptors that mediate cell adhesion to the extracellular matrix, are endocytosed and recycled to the cell surface to modulate cell and tissue behaviour.

Mechanotransduction is a process by which cells sense the mechanical properties of their surrounding environment and adapt accordingly to perform cellular functions such as adhesion, migration and differentiation. Integrin-mediated focal adhesions are major sites of mechanotransduction and their connection with the actomyosin network is crucial for mechanosensing as well as for the generation and transmission of forces onto the substrate. Despite having emerged as major regulators of cell adhesion and migration, the contribution of microtubules to mechanotransduction still remains elusive. Here, we show that talin- and actomyosin-dependent mechanosensing of substrate rigidity controls microtubule acetylation (a tubulin post-translational modification) by promoting the recruitment of α-tubulin acetyltransferase 1 (αTAT1) to focal adhesions. Microtubule acetylation tunes the mechanosensitivity of focal adhesions and Yes-associated protein (YAP) translocation. Microtubule acetylation, in turn, promotes the release of the guanine nucleotide exchange factor GEF-H1 from microtubules to activate RhoA, actomyosin contractility and traction forces. Our results reveal a fundamental crosstalk between microtubules and actin in mechanotransduction that contributes to mechanosensitive cell adhesion and migration. Substrate-rigidity-dependent microtubule acetylation is now shown to be triggered by mechanosensing at focal adhesions, and in turn controls the mechanosensitivity of Yes-associated protein (YAP) translocation, focal adhesion distribution, actomyosin contractility and cell migration.

The mechanical properties of extracellular matrices can control the function of cells. Studies of cellular responses to biomimetic soft materials have been largely restricted to hydrogels and elastomers that have stiffness values independent of time and extent of deformation, so the substrate stiffness can be unambiguously related to its effect on cells. Real tissues, however, often have loss moduli that are 10 to 20% of their elastic moduli and behave as viscoelastic solids. The response of cells to a time-dependent viscous loss is largely uncharacterized because appropriate viscoelastic materials are lacking for quantitative studies. Here we report the synthesis of soft viscoelastic solids in which the elastic and viscous moduli can be independently tuned to produce gels with viscoelastic properties that closely resemble those of soft tissues. Systematic alteration of the hydrogel viscosity demonstrates the time dependence of cellular mechanosensing and the influence of viscous dissipation on cell phenotype. Purely elastic biomimetic soft materials are used to characterize the mechanical response of cells, but do not resemble real tissues. Here the authors develop a viscoelastic solid hydrogel, based on polyacrylamide, that can be tuned to closely resemble soft tissue, and show the influence of viscous dissipation on cellular mechanical sensing.

Key Points Cells alter their migratory phenotypes and velocity in response to the physical properties of their extracellular environment. Confinement, adhesion, stiffness and topology of the extracellular environment are key physical variables influencing cell migration. Univariate profiles and phase diagrams enable an understanding of how physical variables influence cell migration. Numerical simulations enable systematic exploration of the phase space to highlight regions for experimental exploration. The physical properties of the extracellular environment — in terms of confinement, rigidity, surface topology and adhesion-ligand density — can have profound effects on the migration strategy and migration velocity of cells in different in vivo contexts. The way in which a cell migrates is influenced by the physical properties of its surroundings, in particular the properties of the extracellular matrix. How the physical aspects of the cell's environment affect cell migration poses a considerable challenge when trying to understand migration in complex tissue environments and hinders the extrapolation of in vitro analyses to in vivo situations. A comprehensive understanding of these problems requires an integrated biochemical and biophysical approach. In this Review, we outline the findings that have emerged from approaches that span these disciplines, with a focus on actin-based cell migration in environments with different stiffness, dimensionality and geometry.

Case and Waterman discuss how integrating extracellular-matrix-bound integrins and the actin cytoskeleton into a mechanosensitive molecular clutch transmits actin-cytoskeleton-generated forces to the extracellular matrix through focal adhesions. During cell migration, the forces generated in the actin cytoskeleton are transmitted across transmembrane receptors to the extracellular matrix or other cells through a series of mechanosensitive, regulable protein–protein interactions termed the molecular clutch. In integrin-based focal adhesions, the proteins forming this linkage are organized into a conserved three-dimensional nano-architecture. Here we discuss how the physical interactions between the actin cytoskeleton and focal-adhesion-associated molecules mediate force transmission from the molecular clutch to the extracellular matrix.

Directional cell movement through tissues is critical for multiple biological processes and requires maintenance of polarity in the face of complex environmental cues. Here we use intravital imaging to demonstrate that secretion of exosomes from late endosomes is required for directionally persistent and efficient in vivo movement of cancer cells. Inhibiting exosome secretion or biogenesis leads to defective tumour cell migration associated with increased formation of unstable protrusions and excessive directional switching. In vitro r escue experiments with purified exosomes and matrix coating identify adhesion assembly as a critical exosome function that promotes efficient cell motility. Live-cell imaging reveals that exosome secretion directly precedes and promotes adhesion assembly. Fibronectin is found to be a critical motility-promoting cargo whose sorting into exosomes depends on binding to integrins. We propose that autocrine secretion of exosomes powerfully promotes directionally persistent and effective cell motility by reinforcing otherwise transient polarization states and promoting adhesion assembly. How cells maintain directional polarity when migrating through a complex environment is not well understood. Here Sung et al. show that autocrine exosome secretion is required for persistent and efficient in vivo cancer cell motility and promotes assembly of adhesion complexes by delivering fibronectin-bound exosomes.

Key Points Focal adhesion kinase (FAK) is a non-receptor protein tyrosine kinase that drives tumour growth and metastasis through kinase-dependent and kinase-independent pathways. FAK promotes metastasis by regulating processes involved in tumour cell motility and invasion, including control of focal adhesion and cytoskeletal dynamics, as well as the regulation of matrix metalloproteinase (MMP) surface expression. Tumour growth is enhanced through pro-proliferative and anti-apoptotic functions of FAK. FAK is connected to cancer stem cell and progenitor cell maintenance through kinase-dependent and kinase-independent functions. FAK signals contribute to the malignant outgrowth of these cells. FAK favours tumour progression via the regulation of signalling pathways within cells of the tumour microenvironment, such as endothelial cells, haematopoietic cells, platelets, macrophages and fibroblasts. FAK activity promotes endothelial cell migration, proliferation and survival, and it stimulates tumour angiogenesis. FAK-mediated regulation of endothelial cell permeability can influence tumour metastasis. FAK expression and activity in tumour and endothelial cells is frequently upregulated and correlated with a poor patient prognosis. Several molecules that target FAK kinase activity or its kinase-independent scaffolding function are under investigation in preclinical trials. Promising drug candidates in Phase I or II clinical trials are small molecule ATP-competitive inhibitors. Focal adhesion kinase (FAK) can promote tumour growth and metastasis through various kinase-dependent and kinase-independent pathways. This Review discusses the roles of FAK in tumour cells and cells of the microenvironment, as well as the progress that is being made in the clinical development of FAK inhibitors. Focal adhesion kinase (FAK) is a cytoplasmic protein tyrosine kinase that is overexpressed and activated in several advanced-stage solid cancers. FAK promotes tumour progression and metastasis through effects on cancer cells, as well as stromal cells of the tumour microenvironment. The kinase-dependent and kinase-independent functions of FAK control cell movement, invasion, survival, gene expression and cancer stem cell self-renewal. Small molecule FAK inhibitors decrease tumour growth and metastasis in several preclinical models and have initial clinical activity in patients with limited adverse events. In this Review, we discuss FAK signalling effects on both tumour and stromal cell biology that provide rationale and support for future therapeutic opportunities.

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 mirrored by, respectively, the nuclear or cytosolic localization of the transcriptional regulator protein YAP. We explain these findings not through direct sensing of ligand spacing, but by using an expanded computational molecular-clutch model 10 , 11 , in which individual integrin–ECM bonds—the molecular clutches—respond to force loading by recruiting extra integrins, up to a maximum value. This generates more clutches, redistributing the overall force among them, and reducing the force loading per clutch. At high rigidity and high ligand spacing, maximum recruitment is reached, preventing further force redistribution and leading to adhesion collapse. Measurements of cellular traction forces and actin flow speeds support our model. Our results provide a general framework for how cells sense spatial and physical information at the nanoscale, precisely tuning the range of conditions at which they form adhesions and activate transcriptional regulation.