Cell adhesion and migration depend on engagement of extracellular matrix ligands by integrins. Integrin activation is dynamically regulated by interactions of various cytoplasmic proteins, such as filamin and integrin activators, talin and kindlin, with the cytoplasmic tail of the integrin β subunit. Although filamin has been suggested to be an inhibitor of integrin activation, direct functional evidence for the inhibitory role of filamin is limited. Migfilin, a filamin-binding protein enriched at cell-cell and cell-extracellular matrix contact sites, can displace filamin from β1 and β3 integrins and promote integrin activation. However, its role in activation and functions of different β integrins in human vascular cells is unknown. In this study, using flow cytometry, we demonstrate that filamin inhibits β1 and αIIbβ3 integrin activation, and migfilin can overcome its inhibitory effect. Migfilin protein is widely expressed in different adherent and circulating blood cells and can regulate integrin activation in naturally-occurring vascular cells, endothelial cells and neutrophils. Migfilin can activate β1, β2 and β3 integrins and promote integrin mediated responses while migfilin depletion impairs the spreading and migration of endothelial cells. Thus, filamin can act broadly as an inhibitor and migfilin is a promoter of integrin activation.
In vitro analysis confirms talin binding is sufficient for activation and extension of membrane-embedded integrin.
Increased affinity of integrins for the extracellular matrix (activation) regulates cell adhesion and migration, extracellular matrix assembly, and mechanotransduction. Major uncertainties concern the sufficiency of talin for activation, whether conformational change without clustering leads to activation, and whether mechanical force is required for molecular extension. Here, we reconstructed physiological integrin activation in vitro and used cellular, biochemical, biophysical, and ultrastructural analyses to show that talin binding is sufficient to activate integrin αIIbβ3. Furthermore, we synthesized nanodiscs, each bearing a single lipid-embedded integrin, and used them to show that talin activates unclustered integrins leading to molecular extension in the absence of force or other membrane proteins. Thus, we provide the first proof that talin binding is sufficient to activate and extend membrane-embedded integrin αIIbβ3, thereby resolving numerous controversies and enabling molecular analysis of reconstructed integrin signaling.
Cell migration requires the coordination of adhesion site assembly and turnover. Canonical models for nascent adhesion formation postulate that integrin binding to extracellular matrix (ECM) proteins results in the rapid recruitment of cytoskeletal proteins such as talin and paxillin to integrin cytoplasmic domains. It is thought that integrin-talin clusters recruit and activate tyrosine kinases such as focal adhesion kinase (FAK). However, the molecular connections of this linkage remain unresolved. Our recent findings support an alternative model whereby FAK recruits talin to new sites of β1 integrin-mediated adhesion in mouse embryonic fibroblasts and human ovarian carcinoma cells. This is dependent on a direct binding interaction between FAK and talin and occurs independently of direct talin binding to β1 integrin. Herein, we discuss differences between nascent and mature adhesions, interactions between FAK, talin and paxillin, possible mechanisms of FAK activation and how this FAK-talin complex may function to promote cell motility through increased adhesion turnover.
adhesion assembly; focal adhesion kinase; focal adhesion turnover; motility; nascent adhesion; recruitment; talin; β1 integrin
Agonist stimulation of integrin receptors, composed of transmembrane α and β subunits, leads cells to regulate integrin affinity (‘activation’), a process that controls cell adhesion and migration, and extracellular matrix assembly. A final step in integrin activation is the binding of talin to integrin β cytoplasmic domains. We used forward, reverse and synthetic genetics to engineer and order integrin activation pathways of a prototypic integrin, platelet αIIbβ3. PMA activated αIIbβ3 only after expression of both PKCα (protein kinase Cα) and talin at levels approximating those in platelets. Inhibition of Rap1 GTPase reduced αIIbβ3 activation, whereas expression of constitutively active Rap1A(G12V) bypassed the requirement for PKCα. Overexpression of a Rap effector, RIAM (Rap1-GTP-interacting adaptor molecule), activated αIIbβ3 and bypassed the requirement for PKCα and Rap1. In addition, shRNA (short hairpin RNA)-mediated knockdown of RIAM blocked talin interaction with and activation of integrin αIIbβ3. Rap1 activation caused the formation of an ‘activation complex’ containing talin and RIAM that redistributed to the plasma membrane and activated αIIbβ3. The central finding was that this Rap1-induced formation of an ’integrin activation complex’ leads to the unmasking of the integrin-binding site on talin, resulting in integrin activation.
cell adhesion; integrin; Rap1; signal transduction; talin; thrombosis
Integrin-mediated cell adhesion is involved in many essential normal cellular and pathological functions including cell survival, growth, differentiation, migration, inflammatory responses, platelet aggregation, tissue repair and tumor invasion. 24 different heterodimerized transmembrane integrin receptors are combined from 18 different α and 8 different β subunits. Each integrin subunit contains a large extracellular domain, a single transmembrane domain and a usually short cytoplasmic domain. Integrins bind extracellular matrix (ECM) proteins through their large extracellular domain, and engage the cytoskeleton via their short cytoplasmic tails. These integrin-mediated linkages on either side of the plasma membrane are dynamically linked. Thus, integrins communicate over the plasma membrane in both directions, i.e., outside-in and inside-out signaling. In outside-in signaling through integrins, conformational changes of integrin induced by ligand binding on the extracellular domain altered the cytoplasmic domain structures to elicit various intracellular signaling pathways. Inside-out signaling originates from non-integrin cell surface receptors or cytoplasmic molecules and it activates signaling pathways inside the cells, ultimately resulting in the activation/deactivation of integrins. Integrins are one of key family proteins for cell adhesion regulation through binding to a large number of ECM molecules and cell membrane proteins. Lack of expression of integrins may result in a wide variety of effects ranging from blockage in pre-implantation to embryonic or perinatal lethality and developmental defects. Based on both the key role they played in angiogenesis, leukocytes function and tumor development and easy accessibility as cell surface receptors interacting with extracellular ligands, the integrin superfamily represents the best opportunity of targeting both antibodies and small-molecule antagonists for both therapeutic and diagnostic utility in various key diseases so far.
Integrin; inside-out signaling; outside-in signaling; cell adhesion molecule; angiogenesis.
Integrins have emerged as key sensory molecules that translate chemical and physical cues from the extracellular matrix (ECM) into biochemical signals that regulate cell behavior. Integrins function by clustering into adhesion plaques, but the molecular mechanisms that drive integrin clustering in response to interaction with the ECM remain unclear. To explore how deformations in the cell-ECM interface influence integrin clustering, we developed a spatial-temporal simulation that integrates the micro-mechanics of the cell, glycocalyx, and ECM with a simple chemical model of integrin activation and ligand interaction. Due to mechanical coupling, we find that integrin-ligand interactions are highly cooperative, and this cooperativity is sufficient to drive integrin clustering even in the absence of cytoskeletal crosslinking or homotypic integrin-integrin interactions. The glycocalyx largely mediates this cooperativity and hence may be a key regulator of integrin function. Remarkably, integrin clustering in the model is naturally responsive to the chemical and physical properties of the ECM, including ligand density, matrix rigidity, and the chemical affinity of ligand for receptor. Consistent with experimental observations, we find that integrin clustering is robust on rigid substrates with high ligand density, but is impaired on substrates that are highly compliant or have low ligand density. We thus demonstrate how integrins themselves could function as sensory molecules that begin sensing matrix properties even before large multi-molecular adhesion complexes are assembled.
Critical cell decisions, including whether to live, proliferate, or assemble into tissue structures, are directed by cues from the extracellular matrix, the external protein scaffold that surrounds cells. Integrin receptors on the cell surface bind to the extracellular matrix and cluster into complexes that translate matrix cues into the set of instructions a cell follows. Using a newly developed model of the cell-matrix interface, in this work we detail a simple yet efficient mechanism by which integrins could “sense” important matrix properties, including chemical composition and mechanical stiffness, and cluster appropriately. This mechanism relies on mechanical resistance to integrin-matrix interaction provided by the glycocalyx, the slimy sugar and protein coating on the cell, as well as the stiffness of the matrix and the cell itself. In general, the resistance alters integrin-ligand reaction rates, such that integrin clustering is favored for many physiologically relevant conditions. Interestingly, the mechanical properties of the cell and ECM are altered in many prevalent diseases, such as cancer, and our work suggests how these mechanical perturbations might adversely influence integrin function.
Integrins play a key role in cell–cell and cell–matrix interactions. Artificial surfaces grafted with integrin ligands, mimicking natural interfaces, have been used to study integrin-mediated cell adhesion. Here we report the use of a new chemical engineering technology in combination with single-molecule nanomechanical measurements to quantify peptide binding to integrins. We prepared latex beads with covalently-attached dextran. The beads were then functionalized with the bioactive peptides, cyclic RGDFK (cRGD) and the fibrinogen γC-dodecapeptide (H12), corresponding to the active sites for fibrinogen binding to the platelet integrin αIIbβ3. Using optical tweezers-based force spectroscopy to measure non-specific protein–protein interactions, we found the dextran-coated beads nonreactive towards fibrinogen, thus providing an inert platform for biospecific modifications. Using periodate oxidation followed by reductive amination, we functionalized the bead-attached dextran with either cRGD or H12 and used the peptide-grafted beads to measure single-molecule interactions with the purified αIIbβ3. Bimolecular force spectroscopy revealed that the peptide-functionalized beads were highly and specifically reactive with the immobilized αIIbβ3. Further, the cRGD- and H12-functionalized beads displayed a remarkable interaction profile with a bimodal force distribution up to 90 pN. The cRGD–αIIbβ3 interactions had greater binding strength than that of H12–αIIbβ3, indicating that they are more stable and resistant mechanically, consistent with the platelet reactivity of RGD-containing ligands. Thus, the results reported here describe the mechanistic characteristics of αIIbβ3–ligand interactions, confirming the utility of peptide-functionalized latex beads for the quantitative analysis of molecular recognition.
Integrins mediate cell adhesion to the extracellular matrix and transmit signals within the cell that stimulate cell spreading, retraction, migration, and proliferation. The mechanism of integrin outside-in signaling has been unclear. We found that the heterotrimeric guanine nucleotide-binding protein (G protein), Gα13, directly bound to the integrin β3 cytoplasmic domain, and that Gα13-integrin interaction was promoted by ligand binding to the integrin αIIbβ3 and by guanosine triphosphate (GTP)-loading of Gα13. Interference of Gα13 expression or a myristoylated fragment of Gα13 that inhibited interaction of αIIbβ3 with Gα13 diminished activation of protein kinase c-Src and stimulated the small GTPase RhoA, consequently inhibiting cell spreading and accelerating cell retraction. We conclude that integrins are non-canonical Gα13-coupled receptors that provide a mechanism for dynamic regulation of RhoA.
Developing neurons use a combination of guidance cues to assemble a functional neural network. A variety of proteins immobilized within the extracellular matrix (ECM) provide specific binding sites for integrin receptors on neurons. Integrin receptors on growth cones associate with a number of cytosolic adaptor and signaling proteins that regulate cytoskeletal dynamics and cell adhesion. Recent evidence suggests that soluble growth factors and classic axon guidance cues may direct axon pathfinding by controlling integrin-based adhesion. Moreover, since classic axon guidance cues themselves are immobilized within the ECM and integrins modulate cellular responses to many axon guidance cues, interactions between activated receptors modulate cell signals and adhesion. Ultimately, growth cones control axon outgrowth and pathfinding behaviors by integrating distinct biochemical signals to promote the proper assembly of the nervous system. In this review, we discuss our current understanding how ECM proteins and their associated integrin receptors control neural network formation.
axon pathfinding; actin; microtubules; laminin
Regulation of membrane-type 1 matrix metalloproteinase (MT1-MMP) by different extracellular matrices (ECMs) on human endothelial cells (ECs) has been investigated. First, MT1-MMP is found at the intercellular contacts of confluent ECs grown on β1 integrin–dependent matrix such as type 1 collagen (COL I), fibronectin (FN), or fibrinogen (FG), but not on gelatin (GEL) or vitronectin (VN). The novel localization of MT1-MMP at cell–cell contacts is assessed by confocal videomicroscopy of MT1-MMP-GFP–transfected ECs. Moreover, MT1-MMP colocalizes with β1 integrins at the intercellular contacts, whereas it is preferentially found with αvβ3 integrin at motility-associated structures on migrating ECs. In addition, clustered integrins recruit MT1-MMP and neutralizing anti-β1 or anti-αv integrin mAb displace MT1-MMP from its specific sites, pointing to a biochemical association that is finally demonstrated by coimmunoprecipitation assays. On the other hand, COL I, FN, or FG up-regulate cell surface MT1-MMP on confluent ECs by an impairment of its internalization, whereas expression and internalization are not modified on GEL or VN. In addition, MT1-MMP activity is diminished in confluent ECs on COL I, FN, or FG. Finally, MT1-MMP participates and cooperates with β1 and αvβ3 integrins in the migration of ECs on different ECM. These data show a novel mechanism by which ECM regulates MT1-MMP association with β1 or αvβ3 integrins at distinct cellular compartments, thus modulating its internalization, activity, and function on human ECs.
MMP; adhesion; endocytosis; angiogenesis; extracellular matrix
In this review, we discuss the role of focal adhesion kinase (FAK), an intracellular tyrosine kinase, in endothelial cells in relation to neovascularization. Genetic and in vitro studies have identified critical factors, receptor systems, and their intracellular signaling components that regulate the neovasculogenic phenotypes of endothelial cells. Among these factors, FAK appears to regulate several aspects of endothelial cellular behavior, including migration, survival, cytoskeletal organization, as well as cell proliferation. Upon adhesion of endothelial cells to extracellular matrix (ECM) ligands, integrins cluster on the plane of plasma-membrane, while cytoplasmic domains of integrins interact with cytoskeletal proteins and signaling molecules including FAK. However, FAK not only serves as a critical component of integrin signaling, but is also a downstream element of the VEGF/VEGF-receptor and other ligand-receptor systems that regulate neovascularization. A complete understanding of FAK-mediated neovascularization, therefore, should address the molecular and cellular mechanisms that regulate the biology of FAK. Continued research on FAK may, therefore, yield novel therapies to improve treatment modalities for the pathological neovascularization associated with diseases.
Angiogenesis; bFGF; FAK; neovascularization; Src; VEGF
Integrins are widely expressed plasma membrane adhesion molecules that tether cells to matrix proteins and to one another in cell–cell interactions. Integrins also transmit outside-in signals that regulate functional responses of cells, and are known to influence gene expression by regulating transcription. In previous studies we found that platelets, which are naturally occurring anucleate cytoplasts, translate preformed mRNA transcripts when they are activated by outside-in signals. Using strategies that interrupt engagement of integrin αIIbβ3 by fibrinogen and platelets deficient in this integrin, we found that αIIbβ3 regulates the synthesis of B cell lymphoma 3 (Bcl-3) when platelet aggregation is induced by thrombin. We also found that synthesis of Bcl-3, which occurs via a specialized translation control pathway regulated by mammalian target of rapamycin (mTOR), is induced when platelets adhere to immobilized fibrinogen in the absence of thrombin and when integrin αIIbβ3 is engaged by a conformation-altering antibody against integrin αIIbβ3. Thus, outside-in signals delivered by integrin αIIbβ3 are required for translation of Bcl-3 in thrombin-stimulated aggregated platelets and are sufficient to induce translation of this marker protein in the absence of thrombin. Engagement of integrin α2β1 by collagen also triggered synthesis of Bcl-3. Thus, control of translation may be a general mechanism by which surface adhesion molecules regulate gene expression.
adhesion; integrins; platelets; translation; gene regulation
Integrins interact with extracellular matrix (ECM) and deliver intracellular signaling for cell proliferation, survival and motility. During tumor metastasis, integrin-mediated cell adhesion to and migration on the ECM proteins are required for cancer cell survival and adaptation to the new microenvironment. Using SILAC-MS, we profiled the phosphoproteomic changes induced by the interactions of cell integrins with type I collagen, the most common ECM substratum. Integrin-ECM interactions modulate phosphorylation of 517 serine, threonine, or tyrosine residues in 513 peptides, corresponding to 357 proteins. Among these proteins, 33 key signaling mediators with kinase or phosphatase activity were subjected to siRNA-based functional screening. Three integrin-regulated kinases, DBF4, PAK2 and GRK6, were identified for their critical role in cell adhesion and migration possibly through their regulation of actin cytoskeleton arrangement. Altogether, we not only depict an integrin-modulated phosphorylation network during cell-ECM protein interactions but also reveal novel regulators for cell adhesion and migration.
phosphoproteomics; integrin; adhesion; metastasis; signaling
Integrins are the major adhesion receptors of leukocytes and platelets. β1 and β2 integrin function on leukocytes is crucial for a successful immune response and the platelet integrin αIIbβ3 initiates the process of blood clotting through binding fibrinogen1-3. Integrins on circulating cells bind poorly to their ligands but become active after ‘inside-out’ signaling through other membrane receptors4,5. Subjects with leukocyte adhesion deficiency-1 (LAD-I) do not express β2 integrins because of mutations in the gene specifying the β2 subunit, and they suffer recurrent bacterial infections6,7. Mutations affecting αIIbβ3 integrin cause the bleeding disorder termed Glanzmann’s thrombasthenia3. Subjects with LAD-III show symptoms of both LAD-I and Glanzmann’s thrombasthenia. Their hematopoietically-derived cells express β1, β2 and β3 integrins, but defective inside-out signaling causes immune deficiency and bleeding problems8. The LAD-III lesion has been attributed to a C→A mutation in the gene encoding calcium and diacylglycerol guanine nucleotide exchange factor (CALDAGGEF1; official symbol RASGRP2) specifying the CALDAG-GEF1 protein9, but we show that this change is not responsible for the LAD-III disorder. Instead, we identify mutations in the KINDLIN3 (official symbol FERMT3) gene specifying the KINDLIN-3 protein as the cause of LAD-III in Maltese and Turkish subjects. Two independent mutations result in decreased KINDLIN3 messenger RNA levels and loss of protein expression. Notably, transfection of the subjects’ lymphocytes with KINDLIN3 complementary DNA but not CALDAGGEF1 cDNA reverses the LAD-III defect, restoring integrin-mediated adhesion and migration.
R-Ras regulates integrin function, but its effects on integrin signaling pathways have not been well described. We demonstrate that activation of R-Ras promoted focal adhesion formation and altered localization of the α2β1 integrin from cell-cell to cell-matrix adhesions in breast epithelial cells. Constitutively activated R-Ras(38V) dramatically enhanced focal adhesion kinase (FAK) and p130Cas phosphorylation upon collagen stimulation or clustering of the α2β1 integrin, even in the absence of increased ligand binding. Signaling events downstream of R-Ras differed from integrins and K-Ras, since pharmacological inhibition of Src or disruption of actin inhibited integrin-mediated FAK and p130Cas phosphorylation, focal adhesion formation, and migration in control and K-Ras(12V)-expressing cells but had minimal effect in cells expressing R-Ras(38V). Therefore, signaling from R-Ras to FAK and p130Cas has a component that is Src independent and not through classic integrin signaling pathways and a component that is Src dependent. R-Ras effector domain mutants and pharmacological inhibition suggest a partial role for phosphatidylinositol 3-kinase (PI3K), but not Raf, in R-Ras signaling to FAK and p130Cas. However, PI3K cannot account for the Src-independent pathway, since simultaneous inhibition of both PI3K and Src did not completely block effects of R-Ras on FAK phosphorylation. Our results suggest that R-Ras promotes focal adhesion formation by signaling to FAK and p130Cas through a novel mechanism that differs from but synergizes with the α2β1 integrin.
Bidirectional signaling of integrin αIIbβ3 requires the β3 cytoplasmic domain. To determine the sequence in the β3 cytoplasmic domain that is critical to integrin signaling, cell lines were established that coexpress the platelet receptor for von Willebrand factor (vWF), glycoprotein Ib-IX, integrin αIIb, and mutants of β3 with truncations at sites COOH terminal to T741, Y747, F754, and Y759. Truncation at Y759 did not affect integrin activation, as indicated by vWF-induced fibrinogen binding, but affected cell spreading and stable adhesion. Thus, the COOH-terminal RGT sequence of β3 is important for outside-in signaling but not inside-out signaling. In contrast, truncation at F754, Y747, or T741 completely abolished integrin activation. A point mutation replacing Y759 with alanine also abolished integrin activation. Thus, the T755NITY759 sequence of β3, containing an NXXY motif, is critical to inside-out signaling, whereas the intact COOH terminus is important for outside-in signaling. In addition, we found that the calcium-dependent protease calpain preferentially cleaves at Y759 in a population of β3 during platelet aggregation and adhesion, suggesting that calpain may selectively regulate integrin outside-in signaling.
integrin; platelet; calpain; signaling; platelet activation
How intracellular cytoskeletal and signaling proteins connect and communicate with the extracellular matrix (ECM) is a fundamental question in cell biology. Recent biochemical, cell biological, and genetic studies have revealed important roles of cytoplasmic integrin-linked kinase (ILK) and its interactive proteins in these processes. Cell adhesion to ECM is an important process that controls cell shape change, migration, proliferation, survival, and differentiation. Upon adhesion to ECM, integrins and a selective group of cytoskeletal and signaling proteins are recruited to cell matrix contact sites where they link the actin cytoskeleton to the ECM and mediate signal transduction between the intracellular and extracellular compartments. In this review, we discuss the molecular activities and cellular functions of ILK, a protein that is emerging as a key component of the cell–ECM adhesion structures.
integrins; integrin-linked kinase; cell adhesion; cytoskeleton; signal transduction
The ability to control the placement of individual molecules promises to enable a wide range of applications and is a key challenge in nanoscience and nanotechnology. Many biological interactions, in particular, are sensitive to the precise geometric arrangement of proteins. We have developed a technique which combines molecular-scale nanolithography with site-selective biochemistry to create biomimetic arrays of individual protein binding sites. The binding sites can be arranged in heterogeneous patterns of virtually any possible geometry with a nearly unlimited number of degrees of freedom. We have used these arrays to explore how the geometric organization of the extracellular matrix (ECM) binding ligand RGD (Arg-Gly-Asp) affects cell adhesion and spreading. Systematic variation of spacing, density and cluster size of individual integrin binding sites was used to elicit different cell behavior. Cell spreading assays on arrays of different geometric arrangements revealed a dramatic increase in spreading efficiency when at least 4 liganded sites were spaced within 60 nm or less, with no dependence on global density. This points to the existence of a minimal matrix adhesion unit for fibronectin defined in space and stoichiometry. Developing an understanding of the ECM geometries that activate specific cellular functional complexes is a critical step toward controlling cell behavior. Potential practical applications range from new therapeutic treatments to the rational design of tissue scaffolds that can optimize healing without scarring. More broadly, spatial control at the single-molecule level can elucidate factors controlling individual molecular interactions and can enable synthesis of new systems based on molecular-scale architectures.
nanofabrication; nanobiology; mechanobiology; integrin clustering; cell adhesion
A method for measuring the microclustering of a class of cell surface receptors called integrins is reported. Integrins are proteins involved in bi-directional signaling across the cell membrane, and are important in cell adhesion, growth and survival. Their activity is regulated by changes in protein conformation and protein clustering. The developed in vivo clustering assay uses fluorescence resonance energy transfer (FRET), and has the benefit of requiring a single cloning step to generate FRET donors and acceptors that can be used to measure the clustering of a series of integrin mutants. The FRET reporters contain extracellular donor or acceptor fluorescent protein attached to native integrin cytoplasmic and transmembrane domains, and these are expressed along with wild-type or mutant integrins. Expression of the FRET reporters has no affect on the ligand binding properties of co-expressed integrins. FRET values are calculated for cell lines spreading on ligand coated surfaces, and these values are independent of fluorescent protein expression. No FRET is observed in cell lines expressing the reporters in the absence of integrins. Integrin dependent FRET values increase approximately 2- to 3-fold when the integrins contain mutations that result in increased ligand binding affinities.
The extracellular matrix (ECM) is an extracellular scaffold composed of complex mixtures of proteins that plays a pivotal role in tumor progression. ECM remodeling is crucial for tumor migration and invasion during the process of metastasis. ECM can be remodeled by several processes including synthesis, contraction and proteolytic degradation. In order to cross through the ECM barriers, malignant cells produce a spectrum of extracellular proteinases including matrix metalloproteinases (MMPs), serine proteases (mainly the urokinase plasminogen activator (uPA) system) and cysteine proteases to degrade ECM components. As major adhesion molecules to support cell attachment to ECM, integrins play critical roles in tumor progression by enhancing tumor cell survival, migration and invasion. Previous studies have shown that integrins can regulate the expression and activity of these proteases through different pathways. This review summarizes the roles of MMPs and uPA system in ECM remodeling and discusses the regulatory functions of integrins on these proteases in invasive tumors.
ECM remodeling; Metastasis; Integrins; MMPs; uPA system
Focal adhesion kinase (FAK) is an essential nonreceptor tyrosine kinase regulating cell migration, adhesive signaling, and mechanosensing. Using FAK-null cells expressing FAK under an inducible promoter, we demonstrate that FAK regulates the time-dependent generation of adhesive forces. During the early stages of adhesion, FAK expression in FAK-null cells enhances integrin activation to promote integrin binding and, hence, the adhesion strengthening rate. Importantly, FAK expression regulated integrin activation, and talin was required for the FAK-dependent effects. A role for FAK in integrin activation was confirmed in human fibroblasts with knocked-down FAK expression. The FAK autophosphorylation Y397 site was required for the enhancements in adhesion strengthening and integrin-binding responses. This work demonstrates a novel role for FAK in integrin activation and the time-dependent generation of cell–ECM forces.
The speed of cell migration on 2-dimensional (2D) surfaces is determined by the rate of assembly and disassembly of clustered integrin receptors known as focal adhesions. Different modes of cell migration that have been described in 3D environments are distinguished by their dependence on integrin-mediated interactions with the extra-cellular matrix. In particular, the mesenchymal invasion mode is the most dependent on focal adhesion dynamics. The focal adhesion protein NEDD9 is a key signalling intermediary in mesenchymal cell migration, however whether NEDD9 plays a role in regulating focal adhesion dynamics has not previously been reported. As NEDD9 effects on 2D migration speed appear to depend on the cell type examined, in the present study we have used mouse embryo fibroblasts (MEFs) from mice in which the NEDD9 gene has been depleted (NEDD9 −/− MEFs). This allows comparison with effects of other focal adhesion proteins that have previously been demonstrated using MEFs. We show that focal adhesion disassembly rates are increased in the absence of NEDD9 expression and this is correlated with increased paxillin phosphorylation at focal adhesions. NEDD9−/− MEFs have increased rates of migration on 2D surfaces, but conversely, migration of these cells is significantly reduced in 3D collagen gels. Importantly we show that myosin light chain kinase is activated in 3D in the absence of NEDD9 and is conversely inhibited in 2D cultures. Measurement of adhesion strength reveals that NEDD9−/− MEFs have decreased adhesion to fibronectin, despite upregulated α5β1 fibronectin receptor expression. We find that β1 integrin activation is significantly suppressed in the NEDD9−/−, suggesting that in the absence of NEDD9 there is decreased integrin receptor activation. Collectively our data suggest that NEDD9 may promote 3D cell migration by slowing focal adhesion disassembly, promoting integrin receptor activation and increasing adhesion force to the ECM.
Cell spreading, adhesion and remodelling of the extracellular matrix (eCM) involve bi-directional signalling and physical linkages between the eCM, integrins and the cell cytoskeleton1–3. the actin-binding proteins talin1 and 2 link ligand-bound integrins to the actin cytoskeleton and increase the affinity of integrin for the eCM4–6. Here we report that depletion of talin2 in talin1-null (talin1−/−) cells did not affect the initiation of matrix-activated spreading or src family kinase (sFK) activation, but abolished the eCM–integrin–cytoskeleton linkage and sustained cell spreading and adhesion. specifically, focal adhesion assembly, focal adhesion kinase (FAK) signalling and traction force generation on substrates were severely affected. the talin1 head domain restored β1 integrin activation but only full-length talin1 restored the eCM–cytoskeleton linkage and normal cytoskeleton organization. Our results demonstrate three biochemically distinct steps in fibronectin-activated cell spreading and adhesion: 1) fibronectin–integrin binding and initiation of spreading, 2) fast cell spreading and 3) focal adhesion formation and substrate traction. We suggest that talin is not required for initial cell spreading. However, talin provides the important mechanical linkage between ligand-bound integrins and the actin cytoskeleton required to catalyse focal adhesion-dependent pathways.
Integrins link the cell's cytoskeleton to the
extracellular matrix, as well as to receptors on other cells. These
links occur not only at focal contacts but also at smaller
integrin-containing protein complexes outside of focal
contacts. We previously demonstrated the importance of focal
contact-independent integrin–cytoskeleton interactions of
β2 integrins: activation of adhesion resulted
from a release of integrins from cytoskeletal constraints. To
determine whether changes in integrin–cytoskeleton
interactions were related to activation of the integrin, we
used single particle tracking to examine focal contact-independent
cytoskeletal associations of
αIIbβ3-integrin, in which
activation results in a large conformational change. Direct activation
of αIIbβ3 by mutation did not mimic
activation of lymphocytes with phorbol ester, because it enhanced
integrin–cytoskeleton interactions, whereas activation of
lymphocytes decreased them. Using additional integrin mutants,
we found that both α- and β-cytoplasmic domains were required for
these links. This suggests that 1) both β2- and
β3-integrins interact with the cytoskeleton
outside of focal contacts; 2) activation of a cell and activation of an
integrin are distinct processes, and both can affect
integrin–cytoskeleton interactions; and 3) the role of the
α-subunit in integrin–cytoskeleton interactions in at least
some circumstances is more direct than generally supposed.
Integrins are used as prognostic indicators in breast cancer. Following engagement with extracellular matrix proteins, their signaling influences numerous cellular processes including migration, proliferation, and death. Integrin signaling varies between cell types through differential expression of integrin subunits, and changes within a given cell upon exposure to a cell agonist or through changes in its surroundings. These variations in signaling can profoundly affect the phenotypic, tumorogenecity and metastatic properties of cancer cells. In the present study, we investigated if there were differences in the expression of integrins, integrin structures, and integrin co-receptors within three breast cancer cells and if these differences effected integrin signaling.
Expression of integrins, urokinase receptor and vascular endothelial cell growth factor receptor (VEGFR) in metastatic MDA-MB-435 and MDA-MB-231, non-metastatic MCF7 and non-breast cancer Hek-293 cells was measured by flow cytometry. Cell adhesion was assessed using collagen, fibrinogen, fibronectin and vitronectin coated plates. Changes in kinase levels following PMA stimulation, and cell adhesion-induced activation of kinases were determined by western blot analysis. Distribution of actin stress fibers and focal adhesions was assessed by immunocytochemistry.
All cells expressed αv integrins, while high β5 and αvβ5 expression was restricted to the cancer cells and high β3 and αvβ3 expression was restricted to MDA-MB-435 cells. The two metastatic cells were the least adhesive, but all cells adhered well to most proteins in the absence of PMA. All proliferating cells expressed activated pSrc, but only proliferating metastatic cells expressed high pMEK levels. PMA treatment resulted in time-dependent changes in activated kinase levels, and only MDA-MB-231 cells constitutively expressed high levels of activated pMEK. MDA-MB-435 cells formed more stress fibers and focal adhesions and only exhibited adhesion-induced activation of pMEK and pFAK. All cells expressed the urokinase receptor, but MCF7 cells had markedly higher VEGFR expression. Adhesion induced differential expression of pFAK, pMEK and pERK.
This study demonstrates that breast cancers vary in their expression of integrins, their capacity to form focal adhesion and to signal through integrins. These differences likely contribute to phenotypic variations between cancer lines and account for some of the heterogeneity of breast cancer.