Mechanical properties of alveolar epithelial cells in culture

Nanoparticle-Cell Interaction: A Cell Mechanics Perspective

Progress in the field of nanoparticles has enabled the rapid development of multiple products and technologies; however, some nanoparticles can pose both a threat to the environment and human health. To enable their safe implementation, a comprehensive knowledge of nanoparticles and their biological interactions is needed. In vitro and in vivo toxicity tests have been considered the gold standard to evaluate nanoparticle safety, but it is becoming necessary to understand the impact of nanosystems on cell mechanics. Here, the interaction between particles and cells, from the point of view of cell mechanics (i.e., bionanomechanics), is highlighted and put in perspective. Specifically, the ability of intracellular and extracellular nanoparticles to impair cell adhesion, cytoskeletal organization, stiffness, and migration are discussed. Furthermore, the development of cutting-edge, nanotechnology-driven tools based on the use of particles allowing the determination of cell mechanics is emphasized. These include traction force microscopy, colloidal probe atomic force microscopy, optical tweezers, magnetic manipulation, and particle tracking microrheology.

Method for quantitative measurements of the elastic modulus of biological cells in AFM indentation experiments

Here we overview and further develop a quantitative method to measure mechanics of biological cells in indentation experiments, which is based on the use of atomic force microscopy (AFM). We demonstrate how the elastic modulus of the cell body should be measured when the cellular brush is taken into account. The brush is an essential inelastic part of the cell, which surrounds all eukaryotic (the brush is mostly microvilli and glycocalyx) and gram-negative prokaryotic cells (the brush is polysaccharides). The other main feature of the described method is the use of a relatively dull AFM probe to stay in the linear stress-strain regime. In particular, we show that the elastic modulus (aka the Young's modulus) of cells is independent of the indentation depth up to 10-20% deformation for the eukaryotic cells studied here. Besides the elastic modulus, the method presented allows obtaining the parameters of cellular brush, such as the effective length and grafting density of the brush. Although the method is demonstrated on eukaryotic cells, it is directly applicable for all types of cells, and even non-biological soft materials surrounded by either a brush or any field of long-range forces.

A method to measure mechanical properties of pulmonary epithelial cell layers

The lung has a huge inner alveolar surface composed of epithelial cell layers. The knowledge about mechanical properties of lung epithelia is helpful to understand the complex lung mechanics and biomechanical interactions. Methods have been developed to determine mechanical indices (e.g., tissue elasticity) which are both very complex and in need of costly equipment. Therefore, in this study, a mechanostimulator is presented to dynamically stimulate lung epithelial cell monolayers in order to determine their mechanical properties based on a simple mathematical model. First, the method was evaluated by comparison to classical tensile testing using silicone membranes as substitute for biological tissue. Second, human pulmonary epithelial cells (A549 cell line) were grown on flexible silicone membranes and stretched at a defined magnitude. Equal secant moduli were determined in the mechanostimulator and in a conventional tension testing machine (0.49 ± 0.05 MPa and 0.51 ± 0.03 MPa, respectively). The elasticity of the cell monolayer could be calculated by the volume-pressure relationship resulting from inflation of the membrane-cell construct. The secant modulus of the A549 cell layer was calculated as 0.04 ± 0.008 MPa. These findings suggest that the mechanostimulator may represent an adequate device to determine mechanical properties of cell layers.

Cyclic stretch of alveolar epithelial cells alters cytoskeletal micromechanics

Cytoplasmic transport of large molecules such as plasmid DNA (pDNA) has been shown to increase when cells are subjected to mild levels of cyclic stretch for brief periods. In the case of pDNA, this is in part due to the increased active transport of pDNA along stabilized, acetylated microtubules in the cytoplasm, whose levels are increased in response to stretch. It also has been shown that disruption of the dense actin network leads to increased pDNA and macromolecule diffusion as well. We hypothesize that stretch not only increases active transport of pDNA but also, similar to actin disrupting drugs, decreases cytoplasmic stiffness leading to a less restive pathway for macromolecules to diffuse. To test this we used particle tracking microrheology to measure cytoplasmic mechanics. We conclude that while cyclic stretch transiently decreases cytoplasmic stiffness and increases diffusivity, stretch-independent modulation of the levels of acetylated, stable microtubules has no effect on cytoplasmic stiffness. Furthermore, stretching cells that have maximally acetylated microtubules increases cytoplasmic trafficking of pDNA, without increasing levels of acetylated microtubules. These findings suggest that stretch-enhanced gene transfer may occur by two independent mechanisms: increased levels of acetylated microtubules for directed active transport, and reduced cytoplasmic stiffness for increased diffusion.

Mechanical properties of cells and ageing

Mechanical properties are fundamental properties of the cells and tissues of living organisms. The mechanical properties of a single cell as a biocomposite are determined by the interdependent combination of cellular components mechanical properties. Quantitative estimate of the cell mechanical properties depends on a cell state, method of measurement, and used theoretical model. Predominant tendency for the majority of cells with ageing is an increase of cell stiffness and a decrease of cell ability to undergo reversible large deformations. The mechanical signal transduction in old cells becomes less effective than that in young cells, and with ageing, the cells lose the ability of the rapid functional rearrangements of cellular skeleton. The article reviews the theoretical and experimental facts touching the age-related changes of the mechanical properties of cellular components and cells in the certain systems of an organism (the blood, the vascular system, the musculoskeletal system, the lens, and the epithelium). In fact, the cell mechanical parameters (including elastic modulii) can be useful as specific markers of cell ageing.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Image-based finite element modeling of alveolar epithelial cell injury during airway reopening

The acute respiratory distress syndrome (ARDS) is characterized by fluid accumulation in small pulmonary airways. The reopening of these fluid-filled airways involves the propagation of an air-liquid interface that exerts injurious hydrodynamic stresses on the epithelial cells (EpC) lining the airway walls. Previous experimental studies have demonstrated that these hydrodynamic stresses may cause rupture of the plasma membrane (i.e., cell necrosis) and have postulated that cell morphology plays a role in cell death. However, direct experimental measurement of stress and strain within the cell is intractable, and limited data are available on the mechanical response (i.e., deformation) of the epithelium during airway reopening. The goal of this study is to use image-based finite element models of cell deformation during airway reopening to investigate how cell morphology and mechanics influence the risk of cell injury/necrosis. Confocal microscopy images of EpC in subconfluent and confluent monolayers were used to generate morphologically accurate three-dimensional finite element models. Hydrodynamic stresses on the cells were calculated from boundary element solutions of bubble propagation in a fluid-filled parallel-plate flow channel. Results indicate that for equivalent cell mechanical properties and hydrodynamic load conditions, subconfluent cells develop higher membrane strains than confluent cells. Strain magnitudes were also found to decrease with increasing stiffness of the cell and membrane/cortex region but were most sensitive to changes in the cell's interior stiffness. These models may be useful in identifying pharmacological treatments that mitigate cell injury during airway reopening by altering specific biomechanical properties of the EpC.

Atomic force microscope elastography reveals phenotypic differences in alveolar cell stiffness

To understand the connection between alveolar mechanics and key biochemical events such as surfactant secretion, one first needs to characterize the underlying mechanical properties of the lung parenchyma and its cellular constituents. In this study, the mechanics of three major cell types from the neonatal rat lung were studied; primary alveolar type I (AT1) and type II (AT2) epithelial cells and lung fibroblasts were isolated using enzymatic digestion. Atomic force microscopy indentation was used to map the three-dimensional distribution of apparent depth-dependent pointwise elastic modulus. Histograms of apparent modulus data from all three cell types indicated non-Gaussian distributions that were highly skewed and appeared multimodal for AT2 cells and fibroblasts. Nuclear stiffness in all three cell types was similar (2.5+/-1.0 kPa in AT1 vs. 3.1+/-1.5 kPa in AT2 vs. 3.3+/-0.8 kPa in fibroblasts; n=10 each), whereas cytoplasmic moduli were significantly higher in fibroblasts and AT2 cells (6.0+/-2.3 and 4.7+/-2.9 kPa vs. 2.5+/-1.2 kPa). In both epithelial cell types, actin was arranged in sparse clusters, whereas prominent actin stress fibers were observed in lung fibroblasts. No systematic difference in actin or microtubule organization was noted between AT1 and AT2 cells. Atomic force microscope elastography, combined with live-cell fluorescence imaging, revealed that the stiffer measurements in AT2 cells often colocalized with lamellar bodies. These findings partially explain reported heterogeneity of alveolar cell deformation during in situ lung inflation and provide needed data for better understanding of how mechanical stretch influences surfactant release.

Localized elasticity measured in epithelial cells migrating at a wound edge using atomic force microscopy

Restoration of lung homeostasis following injury requires efficient wound healing by the epithelium. The mechanisms of lung epithelial wound healing include cell spreading and migration into the wounded area and later cell proliferation. We hypothesized that mechanical properties of cells vary near the wound edge, and this may provide cues to direct cell migration. To investigate this hypothesis, we measured variations in the stiffness of migrating human bronchial epithelial cells (16HBE cells) approximately 2 h after applying a scratch wound. We used atomic force microscopy (AFM) in contact mode to measure the cell stiffness in 1.5-microm square regions at different locations relative to the wound edge. In regions far from the wound edge (>2.75 mm), there was substantial variation in the elastic modulus in specific cellular regions, but the median values measured from multiple fields were consistently lower than 5 kPa. At the wound edge, cell stiffness was significantly lower within the first 5 microm but increased significantly between 10 and 15 microm before decreasing again below the median values away from the wound edge. When cells were infected with an adenovirus expressing a dominant negative form of RhoA, cell stiffness was significantly decreased compared with cells infected with a control adenovirus. In addition, expression of dominant negative RhoA abrogated the peak increase in stiffness near the wound edge. These results suggest that cells near the wound edge undergo localized changes in cellular stiffness that may provide signals for cell spreading and migration.

Alveolar expansion imaged by optical sectioning microscopy

During lung expansion, the pattern of alveolar perimeter distension is likely to be an important determinant of lung functions as, for example, surfactant secretion. However, the segmental characteristics of alveolar perimeter distension remain unknown. Here, we applied real-time confocal microscopy in the isolated, perfused rat lung to determine the micromechanics of alveolar perimeter distension. To image the alveolar perimeter, we loaded alveolar epithelial cells with a fluorescent dye that we microinjected into the alveolus. Then we viewed single alveoli in a 2-microm-thick optical section at a focal plane 20 mum deep to the pleural surface at baseline. In each alveolus, we identified five to eight segments of the perimeter. For each segment, we determined length (L(seg)) by means of image analysis. At baseline alveolar pressure (P(alv)) of 5 cmH(2)O, L(seg) averaged 46 microm. We hyperinflated the lung to P(alv) of 20 cmH(2)O and identified the same optical section as referenced against morphological landmarks. Hyperinflation increased mean L(seg) by 14%. However, segment distension was heterogeneous, even within the single alveolus. Furthermore, distension was greater in alveolar type 1 than type 2 epithelial cells. These findings indicate that alveoli expand nonuniformly, suggesting that segments that distend the most might be preferred alveolar locations for injury in conditions associated with lung overdistension.

Mechanosensitivity of mouse tracheal ciliary beat frequency: roles for Ca2+, purinergic signaling, tonicity, and viscosity

Mechanosensitivity is hypothesized to participate in the regulation of ciliary beat frequency (CBF) in airway epithelia. To investigate this hypothesis, CBF in excised mouse trachea was monitored (microscopy image analysis) while varying mucosal shear (perfusate velocity and/or viscosity; planar flow). CBF increased within minutes of step increase to steady shear stress as small as 10(-3) Pa and decreased within minutes of shear reduction (<or=10(-4) Pa). CBF response was directional, being less with cephalad vs. caudal flow, and was reduced in trachea from mutant mice lacking P2Y2 receptors, as well as by administration of the Ca2+ chelator EGTA, the Ca2+ channel inhibitor La3+, the nucleotide phosphohydrolase apyrase, the metabolically stabilized adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine, the osmotic agent mannitol, and the viscosity modifier dextran. Brief exposure to exogenous ATP, a candidate mediator, augmented CBF response, although augmentation declined with higher ATP concentration (5.0 vs. 0.1 mM) or longer ATP exposure before shear (55 vs. 20 min). Prolonged extended exposure (45 min) to the metabolically stabilized ATP analog ATPgammaS [adenosine 5'-(3-thiotriphosphate), 0.1 mM] inhibited CBF response to shear. Furthermore, neither ATP nor ATPgammaS substantially increased CBF in the relative absence of shear. With viscosity increase or shear withdrawal apyrase evoked CBF stimulation, inhibitable by the adenosine receptor antagonist 8-(p-sulfophenyl)theophylline. Thus CBF response to shear is finely tuned, directional, La3+ sensitive, likely dependent on extracellular Ca2+ and ATP, involving P2Y2 and adenosine receptor activations, influenced by shear history, tonicity, viscosity, and metabolism/exposure of ATP, and thus reflective of a complex interplay of physical and biochemical actions.

Alveolar macrophages contribute to alveolar barrier dysfunction in ventilator-induced lung injury

In patients requiring mechanical ventilation for acute lung injury or acute respiratory distress syndrome (ARDS), tidal volume reduction decreases mortality, but the mechanisms of the protective effect have not been fully explored. To test the hypothesis that alveolar macrophage activation is an early and critical event in the initiation of ventilator-induced lung injury (VILI), rats were ventilated with high tidal volume (HV(T)) for 10 min to 4 h. Alveolar macrophage counts in bronchoalveolar lavage (BAL) fluid decreased 45% by 20 min of HV(T) (P < 0.05) consistent with activation-associated adhesion. Depletion of alveolar macrophages in vivo with liposomal clodronate significantly decreased permeability and pulmonary edema following 4 h of HV(T) (P < 0.05). BAL fluid from rats exposed to 20 min of HV(T) increased nitric oxide synthase activity nearly threefold in naïve primary alveolar macrophages (P < 0.05) indicating that soluble factors present in the air spaces contribute to macrophage activation in VILI. Media from cocultures of alveolar epithelial cell monolayers and alveolar macrophages exposed to 30 min of stretch in vitro also significantly increased nitrite production in naïve macrophages (P < 0.05), but media from stretched alveolar epithelial cells or primary alveolar macrophages alone did not, suggesting alveolar epithelial cell-macrophage interaction was required for the subsequent macrophage activation observed. These data demonstrate that injurious mechanical ventilation rapidly activates alveolar macrophages and that alveolar macrophages play an important role in the initial pathogenesis of VILI.

Magnetic micro- and nanoparticle mediated activation of mechanosensitive ion channels

Most cells are known to respond to mechanical cues, which initiate biochemical signalling pathways and play a role in cell membrane electrodynamics. These cues can be transduced either via direct activation of mechanosensitive (MS) ion channels or through deformation of the cell membrane and cytoskeleton. Investigation of the function and role of these ion channels is a fertile area of research and studies aimed at characterizing and understanding the mechanoactive regions of these channels and how they interact with the cytoskeleton are fundamental to discovering the specific role that mechanical cues play in cells. In this review, we will focus on novel techniques, which use magnetic micro- and nanoparticles coupled to external applied magnetic fields for activating and investigating MS ion channels and cytoskeletal mechanics.

Epithelial cell deformation during surfactant-mediated airway reopening: a theoretical model

A theoretical model is presented describing the reopening by an advancing air bubble of an initially liquid-filled collapsed airway lined with deformable epithelial cells. The model integrates descriptions of flow-structure interaction (accounting for nonlinear deformation of the airway wall and viscous resistance of the airway liquid flow), surfactant transport around the bubble tip (incorporating physicochemical parameters appropriate for Infasurf), and cell deformation (due to stretching of the airway wall and airway liquid flows). It is shown how the pressure required to drive a bubble into a flooded airway, peeling apart the wet airway walls, can be reduced substantially by surfactant, although the effectiveness of Infasurf is limited by slow adsorption at high concentrations. The model demonstrates how the addition of surfactant can lead to the spontaneous reopening of a collapsed airway, depending on the degree of initial airway collapse. The effective elastic modulus of the epithelial layer is shown to be a key determinant of the relative magnitude of strains generated by flow-induced shear stresses and by airway wall stretch. The model also shows how epithelial-layer compressibility can mediate strains arising from flow-induced normal stresses and stress gradients.

Cellular stress failure in ventilator-injured lungs

The clinical and experimental literature has unequivocally established that mechanical ventilation with large tidal volumes is injurious to the lung. However, uncertainty about the micromechanics of injured lungs and the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. In this review we focus on experimental evidence for lung cells as injury targets and the relevance of these studies for human ventilator-associated lung injury. In vitro, the stress-induced mechanical interactions between matrix and adherent cells are important for cellular remodeling as a means for preventing compromise of cell structure and ultimately cell injury or death. In vivo, these same principles apply. Large tidal volume mechanical ventilation results in physical breaks in alveolar epithelial and endothelial plasma membrane integrity and subsequent triggering of proinflammatory signaling cascades resulting in the cytokine milieu and pathologic and physiologic findings of ventilator-associated lung injury. Importantly, though, alveolar cells possess cellular repair and remodeling mechanisms that in addition to protecting the stressed cell provide potential molecular targets for the prevention and treatment of ventilator-associated lung injury in the future.

Paracrine stimulation of surfactant secretion by extracellular ATP in response to mechanical deformation

We developed a heterologous system to study the effect of mechanical deformation on alveolar epithelial cells. First, isolated primary rat alveolar type II (ATII) cells were plated onto silastic substrata coated with fibronectin and maintained in culture under conditions where they become alveolar type I-like (ATI) cells. This was followed by a second set of ATII cells labeled with the nontransferable, vital fluorescent stain 5-chloromethylfluorescein diacetate to distinguish them from ATI cells. By morphometric analysis, equibiaxial deformation (stretch) of the silastic substratum induced comparable changes in cell surface area for both ATII and ATI cells. Surfactant lipid secretion was measured using cells metabolically labeled with [(3)H]choline. In response to 21% tonic stretch for 15 min, ATII cells seeded with ATI cells secreted nearly threefold more surfactant lipid compared with ATII cells seeded alone. ATI cells did not secrete lipid in response to stretch. The enhanced lipid secretion by ATII plus ATI cocultures was inhibited by treatment with apyrase and adenosine deaminase, suggesting that ATP release by ATI cells enhanced surfactant lipid secretion at 21% stretch. This was confirmed using a luciferase assay where, in response to 21% stretch, ATI cells released fourfold more ATP than ATII cells. Because ATI cells release significantly more ATP at a lower level of stretch than ATII cells, this supports the hypothesis that ATI cells are mechanosensors in the lung and that paracrine stimulation of ATII cells by extracellular ATP released from ATI cells plays a role in regulating surfactant secretion.

Viscoelasticity of human alveolar epithelial cells subjected to stretch

Alveolar epithelial cells undergo stretching during breathing and mechanical ventilation. Stretch can modify cell viscoelastic properties, which may compromise the balance of forces in the alveolar epithelium. We studied the viscoelasticity of alveolar epithelial cells (A549) subjected to equibiaxial distention with a novel experimental approach. Cells were cultured on flexible substrates and subjected to stepwise deformations of up to 17% with a device built on an inverted microscope. Simultaneously, cell storage (G') and loss (G'') moduli were measured (0.1-100 Hz) with optical magnetic twisting cytometry. G' and G'' increased with strain up to 64 and 30%, respectively, resulting in a decrease in G''/G' (15%). This stretch-induced response was inhibited by disruption of the actin cytoskeleton with latrunculin A. G' increased with frequency following a power law with exponent alpha = 0.197. G'' increased proportionally to G' but exhibited a more marked frequency dependence at high frequencies. Stretching (14%) caused a fall in alpha (13%). At high stretching amplitudes, actual cell strain (14.4%) was lower than the applied substrate strain (17.3%), which could indicate a partial cell detachment. These data suggest that cytoskeletal prestress modulates the elastic and frictional properties of alveolar epithelial cells in a coupled manner, according to soft glassy rheology. Stretch-induced cell stiffening could compromise the balance of forces at the cell-cell and cell-matrix adhesions.

Modeling airflow-related shear stress during heterogeneous constriction and mechanical ventilation

Ventilator-induced lung injury has been proposed as being caused by overdistention and closure and reopening of small airways and alveoli. Here we investigate the possibility that heterogeneous constriction increases airflow-related shear stress to a dangerously high level that may be sufficient to cause injury to the epithelial cells during mechanical ventilation. We employed an anatomically consistent model of the respiratory system, based on Horsfield morphometric data, and solved for the time evolution of pressure and flow along the airway tree during mechanical ventilation. We simulated constant-flow ventilation with passive expiration in two different conditions: baseline and highly heterogeneous constriction. The constriction was applied with two strategies: establishing a simple diameter reduction or adding also a length shortening. The shear stress distribution on airway walls was analyzed for airways ranging from the trachea to the acini. Our results indicate that 1). heterogeneous constriction can amplify the maximal values of shear stress up to 50-fold, with peak values higher than 0.6 cmH2O; 2). the highest shear stress is found in pathways constricted by 60-80%; 3). simultaneous diameter reduction and shortening amplifies the shear stresses by three- to fourfold, with shear stresses reaching 2 cmH2O; and 4). there is a range of airways (diameters from 0.6 to 0.3 mm at baseline) that appear to be at risk of very high stresses. We conclude that elevated airflow-related shear stress on the epithelial cell layer can occur during heterogeneous constriction and conjecture that this may constitute a mechanism contributing to ventilator-induced lung injury.

Microrheology of human lung epithelial cells measured by atomic force microscopy

Lung epithelial cells are subjected to large cyclic forces from breathing. However, their response to dynamic stresses is poorly defined. We measured the complex shear modulus (G(*)(omega)) of human alveolar (A549) and bronchial (BEAS-2B) epithelial cells over three frequency decades (0.1-100 Hz) and at different loading forces (0.1-0.9 nN) with atomic force microscopy. G(*)(omega) was computed by correcting force-indentation oscillatory data for the tip-cell contact geometry and for the hydrodynamic viscous drag. Both cell types displayed similar viscoelastic properties. The storage modulus G'(omega) increased with frequency following a power law with exponent approximately 0.2. The loss modulus G"(omega) was approximately 2/3 lower and increased similarly to G'(omega) up to approximately 10 Hz, but exhibited a steeper rise at higher frequencies. The cells showed a weak force dependence of G'(omega) and G"(omega). G(*)(omega) conformed to the power-law model with a structural damping coefficient of approximately 0.3, indicating a coupling of elastic and dissipative processes within the cell. Power-law behavior implies a continuum distribution of stress relaxation time constants. This complex dynamics is consistent with the rheology of soft glassy materials close to a glass transition, thereby suggesting that structural disorder and metastability may be fundamental features of cell architecture.

Role of deformation-induced lipid trafficking in the prevention of plasma membrane stress failure

Cells experience plasma membrane stress failure when the matrix to which they adhere undergoes large deformations. In the lung, such a mechanism might explain mechanical ventilation-associated cell injury. We have previously shown that in alveolar epithelial cells, deformation induces lipid trafficking to the plasma membrane, thereby accommodating the required increase in the cell surface area. We now show that cell wounding is strain amplitude and rate dependent and that under conditions of impaired exocytosis strain-induced cell wounding is significantly increased. In addition, the susceptibility of cells to mechanical injury was not correlated with changes in cell stiffness. Using a dual-labeling technique, we differentiated between cell populations that were reversibly and irreversibly injured and showed that interventions that impair deformation-induced lipid trafficking also reduce the likelihood of plasma membrane resealing. Our findings suggest that cell plasticity and remodeling responses such as deformation-induced lipid trafficking are more important for cytoprotection from strain injury than are the innate mechanical properties of the cell. We also conclude that in deformation experiments, tests of cell membrane integrity cannot be interpreted as tests of cell viability because an intact plasma membrane after deformation does not mean that no injury had occurred.