Monitoring the mass, eigenfrequency, and quality factor of mammalian cells.

The regulation of mass is essential for the development and homeostasis of cells and multicellular organisms. However, cell mass is also tightly linked to cell mechanical properties, which depend on the time scales at which they are measured and change drastically at the cellular eigenfrequency. So far, it has not been possible to determine cell mass and eigenfrequency together. Here, we introduce microcantilevers oscillating in the Ångström range to monitor both fundamental physical properties of the cell. If the oscillation frequency is far below the cellular eigenfrequency, all cell compartments follow the cantilever motion, and the cell mass measurements are accurate. Yet, if the oscillating frequency approaches or lies above the cellular eigenfrequency, the mechanical response of the cell changes, and not all cellular components can follow the cantilever motions in phase. This energy loss caused by mechanical damping within the cell is described by the quality factor. We use these observations to examine living cells across externally applied mechanical frequency ranges and to measure their total mass, eigenfrequency, and quality factor. The three parameters open the door to better understand the mechanobiology of the cell and stimulate biotechnological and medical innovations.

Talin and kindlin use integrin tail allostery and direct binding to activate integrins.

Integrin affinity regulation, also termed integrin activation, is essential for metazoan life. Although talin and kindlin binding to the ß-integrin cytoplasmic tail is indispensable for integrin activation, it is unknown how they achieve this function. By combining NMR, biochemistry and cell biology techniques, we found that talin and kindlin binding to the ß-tail can induce a conformational change that increases talin affinity and decreases kindlin affinity toward it. We also discovered that this asymmetric affinity regulation is accompanied by a direct interaction between talin and kindlin, which promotes simultaneous binding of talin and kindlin to ß-tails. Disrupting allosteric communication between the ß-tail-binding sites of talin and kindlin or their direct interaction in cells severely compromised integrin functions. These data show how talin and kindlin cooperate to generate a small but critical population of ternary talin-ß-integrin-kindlin complexes with high talin-integrin affinity and high dynamics.

Engineered Biomimetic Fibrillar Fibronectin Matrices Regulate Cell Adhesion Initiation, Migration, and Proliferation via α5ß1 Integrin and Syndecan-4 Crosstalk.

Cells regulate adhesion to the fibrillar extracellular matrix (ECM) of which fibronectin is an essential component. However, most studies characterize cell adhesion to globular fibronectin substrates at time scales long after cells polarize and migrate. To overcome this limitation, a simple and scalable method to engineer biomimetic 3D fibrillar fibronectin matrices is introduced and how they are sensed by fibroblasts from the onset of attachment is characterized. Compared to globular fibronectin substrates, fibroblasts accelerate adhesion initiation and strengthening within seconds to fibrillar fibronectin matrices via α5ß1 integrin and syndecan-4. This regulation, which additionally accelerates on stiffened fibrillar matrices, involves actin polymerization, actomyosin contraction, and the cytoplasmic proteins paxillin, focal adhesion kinase, and phosphoinositide 3-kinase. Furthermore, this immediate sensing and adhesion of fibroblast to fibrillar fibronectin guides migration speed, persistency, and proliferation range from hours to weeks. The findings highlight that fibrillar fibronectin matrices, compared to widely-used globular fibronectin, trigger short- and long-term cell decisions very differently and urge the use of such matrices to better understand in vivo interactions of cells and ECMs. The engineered fibronectin matrices, which can be printed onto non-biological surfaces without loss of function, open avenues for various cell biological, tissue engineering and medical applications.

In mitosis integrins reduce adhesion to extracellular matrix and strengthen adhesion to adjacent cells.

To enter mitosis, most adherent animal cells reduce adhesion, which is followed by cell rounding. How mitotic cells regulate adhesion to neighboring cells and extracellular matrix (ECM) proteins is poorly understood. Here we report that, similar to interphase, mitotic cells can employ integrins to initiate adhesion to the ECM in a kindlin- and talin-dependent manner. However, unlike interphase cells, we find that mitotic cells cannot engage newly bound integrins to actomyosin via talin or vinculin to reinforce adhesion. We show that the missing actin connection of newly bound integrins leads to transient ECM-binding and prevents cell spreading during mitosis. Furthermore, ß1 integrins strengthen the adhesion of mitotic cells to adjacent cells, which is supported by vinculin, kindlin, and talin1. We conclude that this dual role of integrins in mitosis weakens the cell-ECM adhesion and strengthens the cell-cell adhesion to prevent delamination of the rounding and dividing cell.

Tailoring the Sensitivity of Microcantilevers To Monitor the Mass of Single Adherent Living Cells.

Microcantilevers are widely employed as mass sensors for biological samples, from single molecules to single cells. However, the accurate mass quantification of living adherent cells is impaired by the microcantilever's mass sensitivity and cell migration, both of which can lead to detect masses mismatching by ≫50%. Here, we design photothermally actuated microcantilevers to optimize the accuracy of cell mass measurements. By reducing the inertial mass of the microcantilever using a focused ion beam, we considerably increase its mass sensitivity, which is validated by finite element analysis and experimentally by gelatin microbeads. The improved microcantilevers allow us to instantly monitor at much improved accuracy the mass of both living HeLa cells and mouse fibroblasts adhering to different substrates. Finally, we show that the improved cantilever design favorably restricts cell migration and thus reduces the large measurement errors associated with this effect.

New insights into the phosphorylation of the threonine motif of the ß1 integrin cytoplasmic domain.

Integrins require an activation step before ligand binding and signaling that is mediated by talin and kindlin binding to the ß integrin cytosolic domain (ß-tail). Conflicting reports exist about the contribution of phosphorylation of a conserved threonine motif in the ß1-tail (ß1-pT788/pT789) to integrin activation. We show that widely used and commercially available antibodies against ß1-pT788/pT789 integrin do not detect specific ß1-pT788/pT789 integrin signals in immunoblots of several human and mouse cell lysates but bind bi-phosphorylated threonine residues in numerous proteins, which were identified by mass spectrometry experiments. Furthermore, we found that fibroblasts and epithelial cells expressing the phospho-mimicking ß1-TT788/789DD integrin failed to activate ß1 integrins and displayed reduced integrin ligand binding, adhesion initiation and cell spreading. These cellular defects are specifically caused by the inability of kindlin to bind ß1-tail polypeptides carrying a phosphorylated threonine motif or phospho-mimicking TT788/789DD substitutions. Our findings indicate that the double-threonine motif in ß1-class integrins is not a major phosphorylation site but if phosphorylated would curb integrin function.

Rheology of rounded mammalian cells over continuous high-frequencies.

Understanding the viscoelastic properties of living cells and their relation to cell state and morphology remains challenging. Low-frequency mechanical perturbations have contributed considerably to the understanding, yet higher frequencies promise to elucidate the link between cellular and molecular properties, such as polymer relaxation and monomer reaction kinetics. Here, we introduce an assay, that uses an actuated microcantilever to confine a single, rounded cell on a second microcantilever, which measures the cell mechanical response across a continuous frequency range ≈ 1-40 kHz. Cell mass measurements and optical microscopy are co-implemented. The fast, high-frequency measurements are applied to rheologically monitor cellular stiffening. We find that the rheology of rounded HeLa cells obeys a cytoskeleton-dependent power-law, similar to spread cells. Cell size and viscoelasticity are uncorrelated, which contrasts an assumption based on the Laplace law. Together with the presented theory of mechanical de-embedding, our assay is generally applicable to other rheological experiments.

αv-Class integrin binding to fibronectin is solely mediated by RGD and unaffected by an RGE mutation.

Fibronectin (FN) is an essential glycoprotein of the extracellular matrix; binds integrins, syndecans, collagens, and growth factors; and is assembled by cells into complex fibrillar networks. The RGD motif in FN facilitates cell binding- and fibrillogenesis through binding to α5ß1 and αv-class integrins. However, whether RGD is the sole binding site for αv-class integrins is unclear. Most notably, substituting aspartate with glutamate (RGE) was shown to eliminate integrin binding in vitro, while mouse genetics revealed that FNRGE preserves αv-class integrin binding and fibrillogenesis. To address this conflict, we employed single-cell force spectroscopy, engineered cells, and RGD motif-deficient mice (Fn1ΔRGD/ΔRGD) to search for additional αv-class integrin-binding sites. Our results demonstrate that α5ß1 and αv-class integrins solely recognize the FN-RGD motif and that αv-class, but not α5ß1, integrins retain FN-RGE binding. Furthermore, Fn1ΔRGD/ΔRGD tissues and cells assemble abnormal and dysfunctional FNΔRGD fibrils in a syndecan-dependent manner. Our data highlight the central role of FN-RGD and the functionality of FN-RGE for αv-class integrins.

Protease-activated receptor signalling initiates α5ß1-integrin-mediated adhesion in non-haematopoietic cells.

Haematopoietic cells and platelets employ G-protein-coupled receptors (GPCRs) to sense extracellular information and respond by initiating integrin-mediated adhesion. So far, such processes have not been demonstrated in non-haematopoietic cells. Here, we report that the activation of protease-activated receptors PAR1 and PAR2 induce multiple signalling pathways to establish α5ß1-integrin-mediated adhesion. First, PARs signal via Gßγ and PI3K to α5ß1-integrins to adopt a talin- and kindlin-dependent high-affinity conformation, which triggers fibronectin binding and initiates cell adhesion. Then, within 60 s, PARs signal via Gα13, Gαi, ROCK and Src to strengthen the α5ß1-integrin-mediated adhesion. Furthermore, PAR signalling changes the abundance of numerous proteins in the adhesome assembled by α5ß1-integrins, including Gα13, vacuolar protein-sorting-associated protein 36, and band 4.1-like protein 4B or 5, and accelerates cell adhesion maturation, spreading and migration. The mechanistic insights describe how agonist binding to PAR employs GPCR and integrin-signalling pathways to initiate and regulate adhesion and to guide physiological responses of non-haematopoietic cells.

Corrigendum: Fibronectin-bound α5ß1 integrins sense load and signal to reinforce adhesion in less than a second.

This corrects the article DOI: 10.1038/nmat5023.

Fibronectin-bound α5ß1 integrins sense load and signal to reinforce adhesion in less than a second.

Integrin-mediated mechanosensing of the extracellular environment allows cells to control adhesion and signalling. Whether cells sense and respond to force immediately upon ligand-binding is unknown. Here, we report that during adhesion initiation, fibroblasts respond to mechanical load by strengthening integrin-mediated adhesion to fibronectin (FN) in a biphasic manner. In the first phase, which depends on talin and kindlin as well as on the actin nucleators Arp2/3 and mDia, FN-engaged α5ß1 integrins activate focal adhesion kinase (FAK) and c-Src in less than 0.5 s to steeply strengthen α5ß1- and αV-class integrin-mediated adhesion. When the mechanical load exceeds a certain threshold, fibroblasts decrease adhesion and initiate the second phase, which is characterized by less steep adhesion strengthening. This unique, biphasic cellular adhesion response is mediated by α5ß1 integrins, which form catch bonds with FN and signal to FN-binding integrins to reinforce cell adhesion much before visible adhesion clusters are formed.

αV-class integrins exert dual roles on α5ß1 integrins to strengthen adhesion to fibronectin.

Upon binding to the extracellular matrix protein, fibronectin, αV-class and α5ß1 integrins trigger the recruitment of large protein assemblies and strengthen cell adhesion. Both integrin classes have been functionally specified, however their specific roles in immediate phases of cell attachment remain uncharacterized. Here, we quantify the adhesion of αV-class and/or α5ß1 integrins expressing fibroblasts initiating attachment to fibronectin (≤120 s) by single-cell force spectroscopy. Our data reveals that αV-class integrins outcompete α5ß1 integrins. Once engaged, αV-class integrins signal to α5ß1 integrins to establish additional adhesion sites to fibronectin, away from those formed by αV-class integrins. This crosstalk, which strengthens cell adhesion, induces α5ß1 integrin clustering by RhoA/ROCK/myosin-II and Arp2/3-mediated signalling, whereas overall cell adhesion depends on formins. The dual role of both fibronectin-binding integrin classes commencing with an initial competition followed by a cooperative crosstalk appears to be a basic cellular mechanism in assembling focal adhesions to the extracellular matrix.

Spontaneous and electric field-controlled front-rear polarization of human keratinocytes.

It has long been known that electrical fields (EFs) are able to influence the direction of migrating cells, a process commonly referred to as electrotaxis or galvanotaxis. Most studies have focused on migrating cells equipped with an existing polarity before EF application, making it difficult to delineate EF-specific pathways. Here we study the initial events in front-rear organization of spreading keratinocytes to dissect the molecular requirements for random and EF-controlled polarization. We find that Arp2/3-dependent protrusive forces and Rac1/Cdc42 activity were generally required for both forms of polarization but were dispensable for controlling the direction of EF-controlled polarization. By contrast, we found a crucial role for extracellular pH as well as G protein coupled-receptor (GPCR) or purinergic signaling in the control of directionality. The normal direction of polarization toward the cathode was reverted by lowering extracellular pH. Polarization toward the anode was also seen at neutral pH when GPCR or purinergic signaling was inhibited. However, the stepwise increase of extracellular pH in this scenario led to restoration of cathodal polarization. Overall our work puts forward a model in which the EF uses distinct polarization pathways. The cathodal pathway involves GPCR/purinergic signaling and is dominant over the anodal pathway at neutral pH.

Increasing throughput of AFM-based single cell adhesion measurements through multisubstrate surfaces.

Mammalian cells regulate adhesion by expressing and regulating a diverse array of cell adhesion molecules on their cell surfaces. Since different cell types express distinct sets of cell adhesion molecules, substrate-specific adhesion is cell type- and condition-dependent. Single-cell force spectroscopy is used to quantify the contribution of cell adhesion molecules to adhesion of cells to specific substrates at both the cell and single molecule level. However, the low throughput of single-cell adhesion experiments greatly limits the number of substrates that can be examined. In order to overcome this limitation, segmented polydimethylsiloxane (PDMS) masks were developed, allowing the measurement of cell adhesion to multiple substrates. To verify the utility of the masks, the adhesion of four different cell lines, HeLa (Kyoto), prostate cancer (PC), mouse kidney fibroblast and MDCK, to three extracellular matrix proteins, fibronectin, collagen I and laminin 332, was examined. The adhesion of each cell line to different matrix proteins was found to be distinct; no two cell lines adhered equally to each of the proteins. The PDMS masks improved the throughput limitation of single-cell force spectroscopy and allowed for experiments that previously were not feasible. Since the masks are economical and versatile, they can aid in the improvement of various assays.

Assay for characterizing the recovery of vertebrate cells for adhesion measurements by single-cell force spectroscopy.

Single-cell force spectroscopy (SCFS) is becoming a widely used method to quantify the adhesion of a living cell to a substrate, another cell or tissue. The high sensitivity of SCFS permits determining the contributions of individual cell adhesion molecules (CAMs) to the adhesion force of an entire cell. However, to prepare adherent cells for SCFS, they must first be detached from tissue-culture flasks or plates. EDTA and trypsin are often applied for this purpose. Because cellular properties can be affected by this treatment, cells need to recover before being further characterized by SCFS. Here we introduce atomic force microscopy (AFM)-based SCFS to measure the mechanical and adhesive properties of HeLa cells and mouse embryonic kidney fibroblasts while they are recovering after detachment from tissue-culture. We find that mechanical and adhesive properties of both cell lines recover quickly (<10 min) after detachment using EDTA, while trypsin-detached fibroblasts require >60 min to fully recover. Our assay introduced to characterize the recovery of mammalian cells after detachment can in future be used to estimate the recovery behavior of other adherent cell types.

Dynamic coupling of ALCAM to the actin cortex strengthens cell adhesion to CD6.

At the immunological synapse, the activated leukocyte cell adhesion molecule (ALCAM) on a dendritic cell (DC) and CD6 molecules on a T cell contribute to sustained DC-T-cell contacts. However, little is known about how ALCAM-CD6 bonds resist and adapt to mechanical stress. Here, we combine single-cell force spectroscopy (SCFS) with total-internal reflection fluorescence microscopy to examine ALCAM-CD6-mediated cell adhesion. The combination of cells expressing ALCAM constructs with certain cytoplasmic tail mutations and improved SCFS analysis processes reveal that the affinity of ALCAM-CD6 bonds is not influenced by the linking of the intracellular domains of ALCAM to the actin cortex. By contrast, the recruitment of ALCAM to adhesion sites and the propensity of ALCAM to anchor plasma membrane tethers depend on actin cytoskeletal interactions. Furthermore, linking ALCAM to the actin cortex through adaptor proteins stiffens the cortex and strengthens cell adhesion. We propose a framework for how ALCAMs contribute to DC-T-cell adhesion, stabilize DC-T-cell contacts and form a mechanical link between CD6 and the actin cortex to strengthen cell adhesion at the immunological synapse.

αV-integrins are required for mechanotransduction in MDCK epithelial cells.

The properties of epithelial cells within tissues are regulated by their immediate microenvironment, which consists of neighboring cells and the extracellular matrix (ECM). Integrin heterodimers orchestrate dynamic assembly and disassembly of cell-ECM connections and thereby convey biochemical and mechanical information from the ECM into cells. However, the specific contributions and functional hierarchy between different integrin heterodimers in the regulation of focal adhesion dynamics in epithelial cells are incompletely understood. Here, we have studied the functions of RGD-binding αV-integrins in a Madin Darby Canine Kidney (MDCK) cell model and found that αV-integrins regulate the maturation of focal adhesions (FAs) and cell spreading. αV-integrin-deficient MDCK cells bound collagen I (Col I) substrate via α2ß1-integrins but failed to efficiently recruit FA components such as talin, focal adhesion kinase (FAK), vinculin and integrin-linked kinase (ILK). The apparent inability to mature α2ß1-integrin-mediated FAs and link them to cellular actin cytoskeleton led to disrupted mechanotransduction in αV-integrin deficient cells seeded onto Col I substrate.