Thrombin-induced contraction in alveolar epithelial cells probed by traction microscopy

Substrate stiffness induces nuclear localization of myosin regulatory light chain to suppress apoptosis

Stiffness of the extracellular matrix regulates various biological responses, but the response mechanisms are poorly understood. Here, we found that the nuclear diphosphorylated myosin regulatory light chain (2P-MRLC) is a critical mechanomediator that suppresses apoptosis in response to substrate stiffness. Stiff substrates promoted the nuclear localization of 2P-MRLC. Zipper-interacting protein kinase [ZIPK; also known as death-associated protein kinase 3 (DAPK3)], a kinase for MRLC, was localized in the nucleus in response to stiff substrates and promoted the nuclear localization of 2P-MRLC. Moreover, actin fiber formation induced by substrate stiffness promoted the nuclear localization of 2P-MRLC via ZIPK. 2P-MRLC in response to substrate stiffness suppressed the expression of MAF bZIP transcription factor B (MafB) and repressed apoptosis. These findings reveal a newly identified role of MRLC in mechanotransduction.

Dysregulation of TSP2-Rac1-WAVE2 axis in diabetic cells leads to cytoskeletal disorganization, increased cell stiffness, and dysfunction

Fibroblasts are a major cell population that perform critical functions in the wound healing process. In response to injury, they proliferate and migrate into the wound space, engaging in extracellular matrix (ECM) production, remodeling, and contraction. However, there is limited knowledge of how fibroblast functions are altered in diabetes. To address this gap, several state-of-the-art microscopy techniques were employed to investigate morphology, migration, ECM production, 2D traction, 3D contraction, and cell stiffness. Analysis of cell-derived matrix (CDM) revealed that diabetic fibroblasts produce thickened and less porous ECM that hindered migration of normal fibroblasts. In addition, diabetic fibroblasts were found to lose spindle-like shape, migrate slower, generate less traction force, exert limited 3D contractility, and have increased cell stiffness. These changes were due, in part, to a decreased level of active Rac1 and a lack of co-localization between F-actin and Waskott-Aldrich syndrome protein family verprolin homologous protein 2 (WAVE2). Interestingly, deletion of thrombospondin-2 (TSP2) in diabetic fibroblasts rescued these phenotypes and restored normal levels of active Rac1 and WAVE2-F-actin co-localization. These results provide a comprehensive view of the extent of diabetic fibroblast dysfunction, highlighting the regulatory role of the TSP2-Rac1-WAVE2-actin axis, and describing a new function of TSP2 in regulating cytoskeleton organization.

Cell contractile force measured using a deformable hollow capsule

There are several possible ways to measure the contraction of cells in vitro. Here, we report measurements of the contractile properties of 3T3-L1 cells grown to confluence on 3D hollow capsules. The capsules were fabricated using the layer-by-layer polyelectrolyte deposition technique on a polymer core. After the polyelectrolyte film was completed, the core was dissolved to leave the hollow capsule. The contractile force of the cells was determined from the deformation in the capsule size induced by interruption of the actin cytoskeleton of the cells that adhered to the outer surface of the hollow capsules, using prior measurements of the elastic modulus of the capsule. From the measurements of the compressive modulus for the capsules (of 6.52 μN), those capsule deformations indicate that the forskolin relaxed the layer of cells by 19.6 μN and the cytochalasin-D relaxed the layer of cells by 45.6 μN. The density of cells in the layer indicated that the force associated with the forskolin-induced relaxation of a single cell is 3.2 nN and the force associated with the cytochalasin-D-induced relaxation of a single cell is 7.5 nN. The mechanism of action of forskolin through second messenger pathways to disrupt the assembly of actin stress fibres also explains its reduced effect on cell contraction compared to that for cytochalasin-D, which is a compound that directly inhibits the polymerization of F-actin filaments.

Piezo1 activation attenuates thrombin-induced blebbing in breast cancer cells

Cancer cells exploit a variety of migration modes to leave primary tumors and establish metastases, including amoeboid cell migration, which is typically reliant on bleb formation. Here we demonstrate that thrombin induces dynamic blebbing in the MDA-MB-231 breast cancer cell line and confirm that protease-activated receptor 1 (PAR1) activation is sufficient to induce this effect. Cell confinement has been implicated as a driving force in bleb-based migration. Unexpectedly, we found that gentle contact compression, exerted using a custom built ‘cell press’ to mechanically stimulate cells, reduced thrombin-induced blebbing. Thrombin-induced blebbing was similarly attenuated using the small molecule Yoda1, an agonist of the mechanosensitive Ca²⁺ channel Piezo1, and this attenuation was impaired in Piezo1-depleted cells. Additionally, Piezo1 activation suppressed thrombin-induced phosphorylation of ezrin, radixin and moesin (ERM) proteins, which are implicated in the blebbing process. Our results provide mechanistic insights into Piezo1 activation as a suppressor of dynamic blebbing, specifically that which is induced by thrombin.

Summary: Thrombin and protease-activated receptor agonists induce dynamic blebbing in breast cancer cells, which can be attenuated by contact-mediated compression, and activation of the mechanosensitive ion channel Piezo1.

Extracellular Vesicles and Alveolar Epithelial-Capillary Barrier Disruption in Acute Respiratory Distress Syndrome: Pathophysiological Role and Therapeutic Potential

Extracellular vesicles (EVs) mediate intercellular communication by transferring genetic material, proteins and organelles between different cells types in both health and disease. Recent evidence suggests that these vesicles, more than simply diagnostic markers, are key mediators of the pathophysiology of acute respiratory distress syndrome (ARDS) and other lung diseases. In this review, we will discuss the contribution of EVs released by pulmonary structural cells (alveolar epithelial and endothelial cells) and immune cells in these diseases, with particular attention to their ability to modulate inflammation and alveolar-capillary barrier disruption, a hallmark of ARDS. EVs also offer a unique opportunity to develop new therapeutics for the treatment of ARDS. Evidences supporting the ability of stem cell-derived EVs to attenuate the lung injury and ongoing strategies to improve their therapeutic potential are also discussed.

Mechanical ventilation promotes lung tumor spread by modulation of cholesterol cell content

Mechanical stretch of cancer cells can alter their invasiveness. During mechanical ventilation, lungs may be exposed to an increased amount of stretch, but the consequences on lung tumors have not been explored. To characterize the influence of mechanical ventilation on the behavior of lung tumors, invasiveness assays and transcriptomic analyses were performed in cancer cell lines cultured in static conditions or under cyclic stretch. Mice harbouring lung melanoma implants were submitted to mechanical ventilation and metastatic spread was assessed. Additional in vivo experiments were performed to determine the mechano-dependent specificity of the response. Incidence of metastases was studied in a cohort of lung cancer patients that received mechanical ventilation compared with a matched group of non-ventilated patients. Stretch increases invasiveness in melanoma B16F10luc2 and lung adenocarcinoma A549 cells. We identified a mechanosensitive upregulation of pathways involved in cholesterol processing in vitro, leading to an increase in PCSK9 and LDLR expression, a decrease in intracellular cholesterol and preservation of cell stiffness. A course of mechanical ventilation in mice harboring melanoma implants increased brain and kidney metastases two weeks later. Blockade of PCSK9 using a monoclonal antibody increased cell cholesterol and stiffness and decreased cell invasiveness in vitro and metastasis in vivo In patients, mechanical ventilation increased PCSK9 abundance in lung tumors and the incidence of metastasis, thus decreasing survival. Our results suggest that mechanical stretch promote invasiveness of cancer cells, which may have clinically relevant consequences. Pharmacological manipulation of cholesterol endocytosis could be a novel therapeutic target in this setting.

Interleukin-1β Modulation of the Mechanobiology of Primary Human Pulmonary Fibroblasts: Potential Implications in Lung Repair

Pro-inflammatory cytokines like interleukin-1β (IL-1β) are upregulated during early responses to tissue damage and are expected to transiently compromise the mechanical microenvironment. Fibroblasts are key regulators of tissue mechanics in the lungs and other organs. However, the effects of IL-1β on fibroblast mechanics and functions remain unclear. Here we treated human pulmonary fibroblasts from control donors with IL-1β and used Atomic Force Microscopy to unveil that IL-1β significantly reduces the stiffness of fibroblasts concomitantly with a downregulation of filamentous actin (F-actin) and alpha-smooth muscle (α-SMA). Likewise, COL1A1 mRNA was reduced, whereas that of collagenases MMP1 and MMP2 were upregulated, favoring a reduction of type-I collagen. These mechanobiology changes were functionally associated with reduced proliferation and enhanced migration upon IL-1β stimulation, which could facilitate lung repair by drawing fibroblasts to sites of tissue damage. Our observations reveal that IL-1β may reduce local tissue rigidity by acting both intracellularly and extracellularly through the downregulation of fibroblast contractility and type I collagen deposition, respectively. These IL-1β-dependent mechanical effects may enhance lung repair further by locally increasing pulmonary tissue compliance to preserve normal lung distension and function. Moreover, our results support that IL-1β provides innate anti-fibrotic protection that may be relevant during the early stages of lung repair.

Activation of synovial fibroblasts from patients at revision of their metal-on-metal total hip arthroplasty

Background

The toxicity of released metallic particles generated in metal-on-metal (MoM) total hip arthroplasty (THA) using cobalt chromium (CoCr) has raised concerns regarding their safety amongst both surgeons and the public. Soft tissue changes such as pseudotumours and metallosis have been widely observed following the use of these implants, which release metallic by-products due to both wear and corrosion. Although activated fibroblasts, the dominant cell type in soft tissues, have been linked to many diseases, the role of synovial fibroblasts in the adverse reactions caused by CoCr implants remains unknown. To investigate the influence of implants manufactured from CoCr, the periprosthetic synovial tissues and synovial fibroblasts from patients with failed MoM THA, undergoing a revision operation, were analysed and compared with samples from patients undergoing a primary hip replacement, in order to elucidate histological and cellular changes.

Results

Periprosthetic tissue from patients with MoM implants was characterized by marked fibrotic changes, notably an increase in collagen content from less than 20% to 45–55%, an increase in α-smooth muscle actin positive cells from 4 to 9% as well as immune cells infiltration. Primary cell culture results demonstrated that MoM synovial fibroblasts have a decreased apoptosis rate from 14 to 6% compared to control synovial fibroblasts. In addition, synovial fibroblasts from MoM patients retained higher contractility and increased responsiveness to chemotaxis in matrix contraction. Their mechanical properties at a single cell level increased as observed by a 60% increase in contraction force and higher cell stiffness (3.3 kPa in MoM vs 2.18 kPa in control), as measured by traction force microscopy and atomic force microscopy. Further, fibroblasts from MoM patients promoted immune cell invasion by secreting monocyte chemoattractant protein 1 (MCP-1, CCL2) and induced monocyte differentiation, which could also be associated with excess accumulation of synovial macrophages.

Conclusion

Synovial fibroblasts exposed in vivo to MoM THA implants that release CoCr wear debris displayed dramatic phenotypic alteration and functional changes. These findings unravelled an unexpected effect of the CoCr alloy and demonstrated an important role of synovial fibroblasts in the undesired tissue reactions caused by MoM THAs.

Biomechanical Response of Lung Epithelial Cells to Iron Oxide and Titanium Dioxide Nanoparticles

Increasing evidence shows that lungs can be damaged by inhalation of nanoparticles (NPs) at environmental and occupational settings. Recent findings have associated the exposure to iron oxide (Fe₂O₃) and titanium dioxide (TiO₂) – NPs widely used in biomedical and clinical research – with pulmonary oxidative stress and inflammation. Although changes on cellular mechanics could contribute to pulmonary inflammation, there is no information regarding the effects of Fe₂O₃ and TiO₂ on alveolar epithelial cell biomechanics. The aim was to investigate the NPs-induced biomechanical effects in terms of cell stiffness and traction forces exerted by human alveolar epithelial cells. Cell Young’s modulus (E) measured by atomic force microscopy in alveolar epithelial cells significantly decreased after exposure to Fe₂O₃ and TiO₂ (∼28 and ∼25%, respectively) compared to control conditions. Moreover, both NPs induced a similar reduction in the traction forces exerted by the alveolar epithelial cells in comparison to the control conditions. Accordingly, immunofluorescence images revealed a reduction of actomyosin stress fibers in response to the exposure to NPs. However, no inflammatory response was detected. In conclusion, an acute exposure of epithelial pulmonary cells to Fe₂O₃ and TiO₂ NPs, which was mild since it was non-cytotoxic and did not induce inflammation, modified cell biomechanical properties which could be translated into damage of the epithelial barrier integrity, suggesting that mild environmental inhalation of Fe₂O₃ and TiO₂ NPs could not be innocuous.

Nonlinear elasticity of the lung extracellular microenvironment is regulated by macroscale tissue strain

The extracellular matrix (ECM) of the lung provides physical support and key mechanical signals to pulmonary cells. Although lung ECM is continuously subjected to different stretch levels, detailed mechanics of the ECM at the scale of the cell is poorly understood. Here, we developed a new polydimethylsiloxane (PDMS) chip to probe nonlinear mechanics of tissue samples with atomic force microscopy (AFM). Using this chip, we performed AFM measurements in decellularized rat lung slices at controlled stretch levels. The AFM revealed highly nonlinear ECM elasticity with the microscale stiffness increasing with tissue strain. To correlate micro- and macroscale ECM mechanics, we also assessed macromechanics of decellularized rat lung strips under uniaxial tensile testing. The lung strips exhibited exponential macromechanical behavior but with stiffness values one order of magnitude lower than at the microscale. To interpret the relationship between micro- and macromechanical properties, we carried out a finite element (FE) analysis which revealed that the stiffness of the alveolar cell microenvironment is regulated by the global strain of the lung scaffold. The FE modeling also indicates that the scale dependence of stiffness is mainly due to the porous architecture of the lung parenchyma. We conclude that changes in tissue strain during breathing result in marked changes in the ECM stiffness sensed by alveolar cells providing tissue-specific mechanical signals to the cells. STATEMENT OF SIGNIFICANCE: The micromechanical properties of the extracellular matrix (ECM) are a major determinant of cell behavior. The ECM is exposed to mechanical stretching in the lung and other organs during physiological function. Therefore, a thorough knowledge of the nonlinear micromechanical properties of the ECM at the length scale that cells probe is required to advance our understanding of cell-matrix interplay. We designed a novel PDMS chip to perform atomic force microscopy measurements of ECM micromechanics on decellularized rat lung slices at different macroscopic strain levels. For the first time, our results reveal that the microscale stiffness of lung ECM markedly increases with macroscopic tissue strain. Therefore, changes in tissue strain during breathing result in variations in ECM stiffness providing tissue-specific mechanical signals to lung cells.

Combined atomic force microscopy (AFM) and traction force microscopy (TFM) reveals a correlation between viscoelastic material properties and contractile prestress of living cells

Living cells exhibit a complex mechanical behavior, whose underlying mechanisms are still largely unknown. Emerging from the molecular structure and dynamics of the cytoskeleton, the mechanical behavior comprises "passive" viscoelastic material properties and "active" contractile prestress. To directly investigate the connection between these quantities at the single-cell level, we here present the combination of atomic force microscopy (AFM) with traction force microscopy (TFM). With this combination, we simultaneously measure viscoelastic material parameters (stiffness, fluidity) and contractile prestress of adherent fibroblast and epithelial cells. Although stiffness, fluidity, and contractile prestress greatly vary within a cell population, they are highly correlated: stiffer cells have a lower fluidity and a larger prestress than softer cells. We show that viscoelastic material properties and contractile prestress are both governed by the activity of the actomyosin machinery. Our results underline the connection between a cell's viscoelastic material properties and its contractile prestress and their importance in cell mechanics.

Hyaluronan-CD44 interactions mediate contractility and migration in periodontal ligament cells

The role of hyaluronan (HA) in periodontal healing has been speculated via its interaction with the CD44 receptor. While HA-CD44 interactions have previously been implicated in numerous cell types; effect and mechanism of exogenous HA on periodontal ligament (PDL) cells is less clear. Herein, we examine the effect of exogenous HA on contractility and migration in human and murine PDL cells using arrays of microposts and time-lapse microscopy. Our findings observed HA-treated human PDL cells as more contractile and less migratory than untreated cells. Moreover, the effect of HA on contractility and focal adhesion area was abrogated when PDL cells were treated with Y27632, an inhibitor of rho-dependent kinase, but not when these cells were treated with ML-7, an inhibitor of myosin light chain kinase. Our results provide insight into the mechanobiology of PDL cells, which may contribute towards the development of therapeutic strategies for periodontal healing and tissue regeneration.

Contractility of Airway Smooth Muscle Cell in Response to Zinc Oxide Nanoparticles by Traction Force Microscopy

Zinc oxide nanoparticles (ZnO-NPs) have been widely used in engineering and biomedicine. However, their adverse pathological effects and mechanisms, especially the biomechanical effects on respiratory system where airway smooth muscle cell (ASMC) contractility regulates the airway response and lung function, are not fully understood. Herein, we used traction force microscopy (TFM) method to investigate whether ZnO-NPs of different concentrations (0.1-10 μg/mL) can alter ASMC contractility (basal and agonist-stimulated) after a short-term exposure and the potential mechanisms. We found that ZnO-NPs exposure led to a decrease of ASMC viability in a dose-dependent manner. Notably, basal contractility was enhanced when the concentration of ZnO-NPs was less than 0.1 μg/mL and decreased afterwards, while KCl-stimulated contractility was reduced in all cases of ZnO-NPs treated groups. Cytoskeleton structure was also found to be significantly altered in ASMC with the stimulation of ZnO-NPs. More importantly, it seems that ZnO-NPs with low concentration (< 0.1 μg/mL) would change ASMC contractility without any apparent cytotoxicity through disruption of the microtubule assembly. Moreover, our results also emerged that ASMC contractility responses were regulated by clathrin-mediated endocytosis and cytoskeleton remodeling. Together, these findings indicate the susceptibility of cell mechanics to NPs exposure, suggesting that cell mechanical testing will contribute to uncover the pathological mechanisms of NPs in respiratory diseases.

New insights into the mechanisms of pulmonary edema in acute lung injury

Appearance of alveolar protein-rich edema is an early event in the development of acute respiratory distress syndrome (ARDS). Alveolar edema in ARDS results from a significant increase in the permeability of the alveolar epithelial barrier, and represents one of the main factors that contribute to the hypoxemia in these patients. Damage of the alveolar epithelium is considered a major mechanism responsible for the increased pulmonary permeability, which results in edema fluid containing high concentrations of extravasated macromolecules in the alveoli. The breakdown of the alveolar-epithelial barrier is a consequence of multiple factors that include dysregulated inflammation, intense leukocyte infiltration, activation of pro-coagulant processes, cell death and mechanical stretch. The disruption of tight junction (TJ) complexes at the lateral contact of epithelial cells, the loss of contact between epithelial cells and extracellular matrix (ECM), and relevant changes in the communication between epithelial and immune cells, are deleterious alterations that mediate the disruption of the alveolar epithelial barrier and thereby the formation of lung edema in ARDS.

Three-dimensional forces exerted by leukocytes and vascular endothelial cells dynamically facilitate diapedesis

Leukocyte transmigration across vessel walls is a critical step in the innate immune response. Upon their activation and firm adhesion to vascular endothelial cells (VECs), leukocytes preferentially extravasate across junctional gaps in the endothelial monolayer (paracellular diapedesis). It has been hypothesized that VECs facilitate paracellular diapedesis by opening their cell-cell junctions in response to the presence of an adhering leukocyte. However, it is unclear how leukocytes interact mechanically with VECs to open the VEC junctions and migrate across the endothelium. In this study, we measured the spatial and temporal evolution of the 3D traction stresses generated by the leukocytes and VECs to elucidate the sequence of mechanical events involved in paracellular diapedesis. Our measurements suggest that the contractile stresses exerted by the leukocytes and the VECs can separately perturb the junctional tensions of VECs to result in the opening of gaps before the initiation of leukocyte transmigration. Decoupling the stresses exerted by the transmigrating leukocytes and the VECs reveals that the leukocytes actively contract the VECs to open a junctional gap and then push themselves across the gap by generating strong stresses that push into the matrix. In addition, we found that diapedesis is facilitated when the tension fluctuations in the VEC monolayer were increased by proinflammatory thrombin treatment. Our findings demonstrate that diapedesis can be mechanically regulated by the transmigrating leukocytes and by proinflammatory signals that increase VEC contractility.

Effects of the decellularization method on the local stiffness of acellular lungs

Lung bioengineering, a novel approach to obtain organs potentially available for transplantation, is based on decellularizing donor lungs and seeding natural scaffolds with stem cells. Various physicochemical protocols have been used to decellularize lungs, and their performance has been evaluated in terms of efficient decellularization and matrix preservation. No data are available, however, on the effect of different decellularization procedures on the local stiffness of the acellular lung. This information is important since stem cells directly sense the rigidity of the local site they are engrafting to during recellularization, and it has been shown that substrate stiffness modulates cell fate into different phenotypes. The aim of this study was to assess the effects of the decellularization procedure on the inhomogeneous local stiffness of the acellular lung on five different sites: alveolar septa, alveolar junctions, pleura, and vessels' tunica intima and tunica adventitia. Local matrix stiffness was measured by computing Young's modulus with atomic force microscopy after decellularizing the lungs of 36 healthy rats (Sprague-Dawley, male, 250-300 g) with four different protocols with/without perfusion through the lung circulatory system and using two different detergents (sodium dodecyl sulfate [SDS] and 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate [CHAPS]). The local stiffness of the acellular lung matrix significantly depended on the site within the matrix (p<0.001), ranging from ∼ 15 kPa at the alveolar septum to ∼ 60 kPa at the tunica intima. Acellular lung stiffness (p=0.003) depended significantly, albeit modestly, on the decellularization process. Whereas perfusion did not induce any significant differences in stiffness, the use of CHAPS resulted in a ∼ 35% reduction compared with SDS, the influence of the detergent being more important in the tunica intima. In conclusion, lung matrix stiffness is considerably inhomogeneous, and conventional decellularization procedures do not result in substantially different local stiffness in the acellular lung.

Local micromechanical properties of decellularized lung scaffolds measured with atomic force microscopy

Bioartificial lungs re-engineered from decellularized organ scaffolds are a promising alternative to lung transplantation. Critical features for improving scaffold repopulation depend on the mechanical properties of the cell microenvironment. However, the mechanics of the lung extracellular matrix (ECM) is poorly defined. The local mechanical properties of the ECM were measured in different regions of decellularized rat lung scaffolds with atomic force microscopy. Lungs excised from rats (n=11) were decellularized with sodium dodecyl sulfate (SDS) and cut into ~7μm thick slices. The complex elastic modulus (G(∗)) of lung ECM was measured over a frequency band ranging from 0.1 to 11.45Hz. Measurements were taken in alveolar wall segments, alveolar wall junctions and pleural regions. The storage modulus (G', real part of G(∗)) of alveolar ECM was ~6kPa, showing small changes between wall segments and junctions. Pleural regions were threefold stiffer than alveolar walls. G' of alveolar walls and pleura increased with frequency as a weak power law with exponent 0.05. The loss modulus (G″, imaginary part of G(∗)) was 10-fold lower and showed a frequency dependence similar to that of G' at low frequencies (0.1-1Hz), but increased more markedly at higher frequencies. Local differences in mechanical properties and topology of the parenchymal site could be relevant mechanical cues for regulating the spatial distribution, differentiation and function of lung cells.

Barrier-protective effects of activated protein C in human alveolar epithelial cells

Acute lung injury (ALI) is a clinical manifestation of respiratory failure, caused by lung inflammation and the disruption of the alveolar-capillary barrier. Preservation of the physical integrity of the alveolar epithelial monolayer is of critical importance to prevent alveolar edema. Barrier integrity depends largely on the balance between physical forces on cell-cell and cell-matrix contacts, and this balance might be affected by alterations in the coagulation cascade in patients with ALI. We aimed to study the effects of activated protein C (APC) on mechanical tension and barrier integrity in human alveolar epithelial cells (A549) exposed to thrombin. Cells were pretreated for 3 h with APC (50 µg/ml) or vehicle (control). Subsequently, thrombin (50 nM) or medium was added to the cell culture. APC significantly reduced thrombin-induced cell monolayer permeability, cell stiffening, and cell contraction, measured by electrical impedance, optical magnetic twisting cytometry, and traction microscopy, respectively, suggesting a barrier-protective response. The dynamics of the barrier integrity was also assessed by western blotting and immunofluorescence analysis of the tight junction ZO-1. Thrombin resulted in more elongated ZO-1 aggregates at cell-cell interface areas and induced an increase in ZO-1 membrane protein content. APC attenuated the length of these ZO-1 aggregates and reduced the ZO-1 membrane protein levels induced by thrombin. In conclusion, pretreatment with APC reduced the disruption of barrier integrity induced by thrombin, thus contributing to alveolar epithelial barrier protection.

Epithelial cells utilize cortical actin/myosin to activate latent TGF-β through integrin α(v)β(6)-dependent physical force

Transforming Growth Factor Beta (TGF-β) is involved in regulating many biological processes and disease states. Cells secrete cytokine as a latent complex that must be activated for it to exert its biological functions. We previously discovered that the epithelial-restricted integrin α(v)β(6) activates TGF-β and that this process is important in a number of in vivo models of disease. Here, we show that agonists of G-protein coupled receptors (Sphingosine-1-Phosphate and Lysophosphatidic Acid) which are ligated under conditions of epithelial injury directly stimulate primary airway epithelial cells to activate latent TGF-β through a pathway that involves Rho Kinase, non-muscle myosin, the α(v)β(6) integrin, and the generation of mechanical tension. Interestingly, lung epithelial cells appear to exert force on latent TGF-β using sub-cortical actin/myosin rather than the stress fibers utilized by fibroblasts and other traditionally "contractile" cells. These findings extend recent evidence suggesting TGF-β can be activated by integrin-mediated mechanical force and suggest that this mechanism is important for an integrin (α(v)β(6)) and a cell type (epithelial cells) that have important roles in biologically relevant TGF-β activation in vivo.

Cytoskeleton fluidization versus resolidification: prestress effect

The differential elastic modulus of an active actomyosin network is computed as a function of applied stress, taking into account both thermal and motor contributions to filament compliance in the low-frequency domain. It is shown that, due to a dual nature of motor activity, increasing motor concentration may either stiffen the network due to stronger prestress or soften it due to motor agitation, in accordance with experimental data. Prestress anisotropy, which may be induced by redistribution of motors triggered by external force, causes anisotropy of the elastic moduli. This helps to explain the contradictory phenomena of cell fluidization and resolidification in response to transient stretch observed in recent experiments.

Thrombin promotes actin stress fiber formation in RPE through Rho/ROCK-mediated MLC phosphorylation

The retinal pigment epithelium (RPE) forms the outer blood-retina barrier (BRB). Most retinal diseases involve BRB breakdown, whereupon thrombin contained in serum directly contacts the RPE. Thrombin is known to promote actin stress fiber formation, an important determinant in eye diseases involving the epithelial-mesenchymal transition (EMT) and migration of RPE cells, such as proliferative vitreoretinopathy. We analyzed thrombin effect on signaling pathways leading to myosin light chain (MLC) phosphorylation and actin stress fiber formation in primary cultures of rat RPE cells, in order to support a role for thrombin in RPE transdifferentiation. MLC phosphorylation was measured by Western blot; actin cytoskeleton was visualized using immunofluorescent phalloidin, and Rho GTPase activation was assessed by ELISA. We showed that thrombin/PAR-1 induces the time- and dose-dependent phosphorylation of MLC through the activation of Rho/ROCK and myosin light chain kinase (MLCK). ROCK increased phospho-MLC by phosphorylating MLC and by inhibiting MLC phosphatase. Thrombin effect was abolished by the ROCK inhibitor Y-27632, whereas MLCK inhibitor ML-7 and PLC-β inhibitor U73122 attenuated MLC phosphorylation by ≈50%, suggesting the activation of MLCK by PLC-β-mediated calcium increase. Additionally, thrombin-induced MLC phosphorylation was blocked by the inhibitory PKCζ pseudosubstrate, wortmannin, and LY294002, indicating IP(3)/PKCζ involvement in the control of MLC phosphorylation. Moreover, we demonstrated that thrombin effect on MLC induces actin stress fiber formation, since this effect was prevented by inhibiting the pathways leading to MLC phosphorylation. We conclude that thrombin stimulation of MLC phosphorylation and actin stress fiber formation may be involved in thrombin-induced RPE cell transformation subsequent to BRB dysfunction.

Cigarette smoke exposure up-regulates endothelin receptor B in human pulmonary artery endothelial cells: molecular and functional consequences

BACKGROUND AND PURPOSE

Pulmonary arteries from smokers and chronic obstructive pulmonary disease patients show abnormal endothelium-dependent vascular reactivity. We studied the effect of cigarette smoke extract (CSE) on endothelin receptor B (ET(B) ) expression in human pulmonary artery endothelial cells (HPAECs) and its role in endothelial dysfunction.

EXPERIMENTAL APPROACH

ET(B) receptor expression was measured by real time RT-PCR, Western blot and immunofluorescence. Cell contraction, intracellular Ca(2+) , F/G-actin, RhoA activity, myosin light chain phosphorylation, ET, NO, thromboxane (Tx)A(2) and reactive oxygen species (ROS) were measured by traction microscopy, fluorescence microscopy, phalloidin fluorescence, colorimetric assay, Western blot, elisa and DCFDA fluorescence respectively.

KEY RESULTS

Cigarette smoke extract dose-dependently increased ET(B) receptor expression in HPAECs after 24h incubation. CSE-induced ET(B) expression was attenuated by bosentan, the ET(B) receptor antagonist BQ788, the Rho kinase antagonist Y27632 and the antioxidant N-acetylcysteine. A monoclonal antibody to ET-1 prevented CSE-induced ET(B) receptor overexpression. Twenty-four hour exposure to ET-1 dose-dependently increased ET(B) receptor expression, mimicking the effect of CSE. CSE-induced ET(B) receptor overexpression caused greater cell contraction; increased intracellular Ca(2+) ; increased F/G-actin and RhoA activity; increased myosin light chain phosphorylation; augmented TxA(2) and ROS production; and decreased NO after acute ET-1 (10nM). These effects were attenuated by bosentan, BQ788, Y27632 and N-acetylcysteine.

CONCLUSIONS AND IMPLICATION

Cigarette smoke extract induced ET(B) receptor overexpression by a feed forward mechanism mediated partly by ET release, promoting HPAEC dysfunction and attenuated by ET(B) receptor blockade, Rho kinase and ROS inhibition. These results provide support for the use of bosentan in CS-related endothelial dysfunction.

Optimization of traction force microscopy for micron-sized focal adhesions

To understand how adherent cells regulate traction forces on their surrounding extracellular matrix (ECM), quantitative techniques are needed to measure forces at the cell-ECM interface. Microcontact printing is used to create a substrate of 1 μm diameter circles of ECM ligand to experimentally study the reconstruction of traction stresses at constrained, point-like focal adhesions. Traction reconstruction with point forces (TRPF) and Fourier transform traction cytometry (FTTC) are used to calculate the traction forces and stress field, respectively, at isolated adhesions. We find that the stress field calculated with FTTC peaks near the center of individual adhesions but propagates several microns beyond the adhesion location. We find the optimal set of FTTC parameters that yield the highest stress magnitude, minimizing information lost from over-smoothing and sampling of the displacement or stress field. A positive correlation between the TRPF and FTTC measurements exists, but integrating the FTTC stress field over the adhesion area yields only a small fraction of the force calculated by TRPF. An effective area similar to that defined by the width of the stress distribution measured with FTTC is required to reconcile these measurements. These measurements set bounds on the spatial resolution and precision of FTTC measurements on micron-sized adhesions.

Nickel induces intracellular calcium mobilization and pathophysiological responses in human cultured airway epithelial cells

Environmental exposure to nickel is associated to respiratory disorders and potential toxicity in the lung but molecular mechanisms remain incompletely explored. The extracellular Ca(2+)-sensing receptor (CaSR) is widely distributed and may be activated by divalent cations. In this study, we investigated the presence of CaSR in human cultured airway epithelial cells and its activation by nickel. Nickel transiently increased intracellular calcium (-logEC(50)=4.67+/-0.06) in A549 and human bronchial epithelial cells as measured by epifluorescence microscopy. Nickel (20muM)-induced calcium responses were reduced after thapsigargin or ryanodine exposure but not by Ca(2+)-free medium. Inhibition of phospholipase-C or inositol trisphosphate release reduced intracellular calcium responses to nickel indicating activation of G(q)-signaling. CaSR mRNA and protein expression in epithelial cells was demonstrated by RT-PCR, western blot and immunofluorescence. Transfection of specific siRNA inhibited CaSR expression and suppressed nickel-induced intracellular calcium responses in A549 cells thus confirming nickel-CaSR activation. NPS2390, a CaSR antagonist, abolished the calcium response to nickel. Nickel-induced contraction, proliferation, alpha(1)(I)collagen production and inflammatory cytokines mRNA expression by epithelial cells as measured by traction microscopy, BrdU assay and RT-PCR, respectively. These responses were blocked by NPS2390. In conclusion, micromolar nickel concentrations, relevant to nickel found in the lung tissue of humans exposed to high environmental nickel, trigger intracellular Ca(2+) mobilization in human airway epithelial cells through the activation of CaSR which translates into pathophysiological outputs potentially related to pulmonary disease.

The temperature dependence of cell mechanics measured by atomic force microscopy

The cytoskeleton is a complex polymer network that regulates the structural stability of living cells. Although the cytoskeleton plays a key role in many important cell functions, the mechanisms that regulate its mechanical behaviour are poorly understood. Potential mechanisms include the entropic elasticity of cytoskeletal filaments, glassy-like inelastic rearrangements of cross-linking proteins and the activity of contractile molecular motors that sets the tensional stress (prestress) borne by the cytoskeleton filaments. The contribution of these mechanisms can be assessed by studying how cell mechanics depends on temperature. The aim of this work was to elucidate the effect of temperature on cell mechanics using atomic force microscopy. We measured the complex shear modulus (G*) of human alveolar epithelial cells over a wide frequency range (0.1-25.6 Hz) at different temperatures (13-37 degrees C). In addition, we probed cell prestress by mapping the contractile forces that cells exert on the substrate by means of traction microscopy. To assess the role of actomyosin contraction in the temperature-induced changes in G* and cell prestress, we inhibited the Rho kinase pathway of the myosin light chain phosphorylation with Y-27632. Our results show that with increasing temperature, cells become stiffer and more solid-like. Cell prestress also increases with temperature. Inhibiting actomyosin contraction attenuated the temperature dependence of G* and prestress. We conclude that the dependence of cell mechanics with temperature is dominated by the contractile activity of molecular motors.

Thermal activation and ATP dependence of the cytoskeleton remodeling dynamics

The cytoskeleton (CSK) is a nonequilibrium polymer network that uses hydrolyzable sources of free energy such as adenosine triphosphate (ATP) to remodel its internal structure. As in inert nonequilibrium soft materials, CSK remodeling has been associated with structural rearrangements driven by energy-activated processes. We carry out particle tracking and traction microscopy measurements of alveolar epithelial cells at various temperatures and ATP concentrations. We provide the first experimental evidence that the remodeling dynamics of the CSK is driven by structural rearrangements over free-energy barriers induced by thermally activated forces mediated by ATP. The measured activation energy of these forces is approximately 40k_{B}T_{r} ( k_{B} being the Boltzmann constant and T_{r} being the room temperature). Our experiments provide clues to understand the analogy between the dynamics of the living CSK and that of inert nonequilibrium soft materials.

Cell traction forces direct fibronectin matrix assembly

Interactions between cells and the surrounding matrix are critical to the development and engineering of tissues. We have investigated the role of cell-derived traction forces in the assembly of extracellular matrix using what we believe is a novel assay that allows for simultaneous measurement of traction forces and fibronectin fibril growth at discrete cell-matrix attachment sites. NIH3T3 cells were plated onto arrays of deformable cantilever posts for 2-24 h. Data indicate that developing fibril orientation is guided by the direction of the traction force applied to that fibril. In addition, cells initially establish a spatial distribution of traction forces that is largest at the cell edge and decreases toward the cell center. This distribution progressively shifts from a predominantly peripheral pattern to a more uniform pattern as compressive strain at the cell perimeter decreases with time. The impact of these changes on fibrillogenesis was tested by treating cells with blebbistatin or calyculin A to tonically block or augment, respectively, myosin II activity. Both treatments blocked the inward translation of traction forces, the dissipation of compressive strain, and fibronectin fibrillogenesis over time. These data indicate that dynamic spatial and temporal changes in traction force and local strain may contribute to successful matrix assembly.

Curcumin inhibits lung cancer cell invasion and metastasis through the tumor suppressor HLJ1

Curcumin (diferuloylmethane) is an active component of the spice turmeric and has a diversity of antitumor activities. In this study, we found that curcumin can inhibit cancer cell invasion and metastasis through activation of the tumor suppressor DnaJ-like heat shock protein 40 (HLJ1). Human lung adenocarcinoma cells (CL1-5) treated with curcumin (1-20 mumol/L) showed a concentration-dependent reduction in cell migration, invasion, and metastatic ability, and this was associated with increased HLJ1 expression. Knockdown of HLJ1 expression by siRNA was able to reverse the curcumin-induced anti-invasive and antimetastasis effects in vitro and in vivo. The HLJ1 promoter and enhancer in a luciferase reporter assay revealed that curcumin transcriptionally up-regulates HLJ1 expression through an activator protein (AP-1) site within the HLJ1 enhancer. JunD, one of the AP-1 components, was significantly up-regulated by curcumin (1-20 mumol/L) in a concentration- and time-dependent manner. Knockdown of JunD expression could partially reduce the curcumin-induced HLJ1 activation and diminish the anti-invasive effect of curcumin, indicating that JunD would seem to be involved in curcumin-induced HLJ1 expression. Curcumin was able to induce c-Jun NH(2)-kinase (JNK) phosphorylation, whereas the JNK inhibitor (SP-600125) could attenuate curcumin-induced JunD and HLJ1 expression. Activation of HLJ1 by curcumin further leads to up-regulation of E-cadherin and a suppression of cancer cell invasion. Our results show that curcumin induces HLJ1, through activation of the JNK/JunD pathway, and inhibits lung cancer cell invasion and metastasis by modulating E-cadherin expression. This is a novel mechanism and supports the application of curcumin in anti-cancer metastasis therapy.

Melatonin prevents oxidized low-density lipoprotein-induced increase of myosin light chain kinase activation and expression in HUVEC through ERK/MAPK signal transduction

Melatonin, the main secretary product of the pineal gland, is potentially effective in the prevention of a number of diseases in which free radical processes are involved. The development of hypercholesterolemia is a multifactorial process in which elevated oxidized low-density lipoprotein (ox-LDL) levels play a central role. The purpose of this study was to test whether melatonin prevents ox-LDL-induced increase of myosin light chain kinase (MLCK) activation and expression in human umbilical vein endothelial cells (HUVECs) through extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) signal transduction. HUVEC were cultured in vitro and treated with ox-LDL, melatonin, and PD98059 (a selective inhibitor of ERK), respectively. The expression, transcription, and activity of MLCK were measured by western blot, immunohistochemistry, reverse transcription-polymerase chain reaction and gamma-(32)P-adenosine triphosphate (ATP) incorporation, respectively. The results showed that the expression and activity of MLCK were increased in ox-LDL-treated HUVECs and this was decreased by melatonin and PD98059. The expression and activity of MLCK induced by ox-LDL was associated with the phosphorylation of ERK. These results indicate for the first time that hypercholesterolemia may be associated with MLCK expression and the activity which can be reduced by melatonin through ERK/MAPK signal transduction.

Mapping cell-matrix stresses during stretch reveals inelastic reorganization of the cytoskeleton

The mechanical properties of the living cell are intimately related to cell signaling biology through cytoskeletal tension. The tension borne by the cytoskeleton (CSK) is in part generated internally by the actomyosin machinery and externally by stretch. Here we studied how cytoskeletal tension is modified during stretch and the tensional changes undergone by the sites of cell-matrix interaction. To this end we developed a novel technique to map cell-matrix stresses during application of stretch. We found that cell-matrix stresses increased with imposition of stretch but dropped below baseline levels on stretch release. Inhibition of the actomyosin machinery resulted in a larger relative increase in CSK tension with stretch and in a smaller drop in tension after stretch release. Cell-matrix stress maps showed that the loci of cell adhesion initially bearing greater stress also exhibited larger drops in traction forces after stretch removal. Our results suggest that stretch partially disrupts the actin-myosin apparatus and the cytoskeletal structures that support the largest CSK tension. These findings indicate that cells use the mechanical energy injected by stretch to rapidly reorganize their structure and redistribute tension.

Micropatterning of single endothelial cell shape reveals a tight coupling between nuclear volume in G1 and proliferation

Shape-dependent local differentials in cell proliferation are considered to be a major driving mechanism of structuring processes in vivo, such as embryogenesis, wound healing, and angiogenesis. However, the specific biophysical signaling by which changes in cell shape contribute to cell cycle regulation remains poorly understood. Here, we describe our study of the roles of nuclear volume and cytoskeletal mechanics in mediating shape control of proliferation in single endothelial cells. Micropatterned adhesive islands were used to independently control cell spreading and elongation. We show that, irrespective of elongation, nuclear volume and apparent chromatin decondensation of cells in G1 systematically increased with cell spreading and highly correlated with DNA synthesis (percent of cells in the S phase). In contrast, cell elongation dramatically affected the organization of the actin cytoskeleton, markedly reduced both cytoskeletal stiffness (measured dorsally with atomic force microscopy) and contractility (measured ventrally with traction microscopy), and increased mechanical anisotropy, without affecting either DNA synthesis or nuclear volume. Our results reveal that the nuclear volume in G1 is predictive of the proliferative status of single endothelial cells within a population, whereas cell stiffness and contractility are not. These findings show that the effects of cell mechanics in shape control of proliferation are far more complex than a linear or straightforward relationship. Our data are consistent with a mechanism by which spreading of cells in G1 partially enhances proliferation by inducing nuclear swelling and decreasing chromatin condensation, thereby rendering DNA more accessible to the replication machinery.

The role of F-actin and myosin in epithelial cell rheology

Although actin and myosin are important contributors to cell-force generation, shape change, and motility, their contributions to cell stiffness and frequency-dependent rheology have not been conclusively determined. We apply several pharmacological interventions to cultured epithelial cells to elucidate the roles of actin and myosin in the mechanical response of cells and intracellular fluctuations. A suite of different methods is used to separately examine the mechanics of the deep cell interior and cortex, in response to depletion of intracellular ATP, depolymerization of F-actin, and inhibition of myosin II. Comparison of these results shows that F-actin plays a significant role in the mechanics of the cortical region of epithelial cells, but its disruption has no discernable effect on the rheology of the deeper interior. Moreover, we find that myosins do not contribute significantly to the rheology or ATP-dependent, non-Brownian motion in the cell interior. Finally, we investigate the broad distribution of apparent stiffness values reported by some microrheology methods, which are not observed with two-point microrheology. Based on our findings and a simple model, we conclude that heterogeneity of the tracer-cytoskeleton contacts, rather than the network itself, can explain the broad distribution of apparent stiffnesses.