Mechanisms of surface-tension-induced epithelial cell damage in a model of pulmonary airway reopening

Influence of airway wall compliance on epithelial cell injury and adhesion during interfacial flows

Interfacial flows during cyclic airway reopening are an important source of ventilator-induced lung injury. However, it is not known how changes in airway wall compliance influence cell injury during airway reopening. We used an in vitro model of airway reopening in a compliant microchannel to investigate how airway wall stiffness influences epithelial cell injury. Epithelial cells were grown on gel substrates with different rigidities, and cellular responses to substrate stiffness were evaluated in terms of metabolic activity, mechanics, morphology, and adhesion. Repeated microbubble propagations were used to simulate cyclic airway reopening, and cell injury and detachment were quantified via live/dead staining. Although cells cultured on softer gels exhibited a reduced elastic modulus, these cells experienced less plasma membrane rupture/necrosis. Cells on rigid gels exhibited a minor, but statistically significant, increase in the power law exponent and also exhibited a significantly larger height-to-length aspect ratio. Previous studies indicate that this change in morphology amplifies interfacial stresses and, therefore, correlates with the increased necrosis observed during airway reopening. Although cells cultured on stiff substrates exhibited more plasma membrane rupture, these cells experienced significantly less detachment and monolayer disruption during airway reopening. Western blotting and immunofluorescence indicate that this protection from detachment and monolayer disruption correlates with increased focal adhesion kinase and phosphorylated paxillin expression. Therefore, changes in cell morphology and focal adhesion structure may govern injury responses during compliant airway reopening. In addition, these results indicate that changes in airway compliance, as occurs during fibrosis or emphysema, may significantly influence cell injury during mechanical ventilation.

A micromechanical model for estimating alveolar wall strain in mechanically ventilated edematous lungs

To elucidate the micromechanics of pulmonary edema has been a significant medical concern, which is beneficial to better guide ventilator settings in clinical practice. In this paper, we present an adjoining two-alveoli model to quantitatively estimate strain and stress of alveolar walls in mechanically ventilated edematous lungs. The model takes into account the geometry of the alveolus, the effect of surface tension, the length-tension properties of parenchyma tissue, and the change in thickness of the alveolar wall. On the one hand, our model supports experimental findings (Perlman CE, Lederer DJ, Bhattacharya J. Am J Respir Cell Mol Biol 44: 34-39, 2011) that the presence of a liquid-filled alveolus protrudes into the neighboring air-filled alveolus with the shared septal strain amounting to a maximum value of 1.374 (corresponding to the maximum stress of 5.12 kPa) even at functional residual capacity; on the other hand, it further shows that the pattern of alveolar expansion appears heterogeneous or homogeneous, strongly depending on differences in air-liquid interface tension on alveolar segments. The proposed model is a preliminary step toward picturing a global topographical distribution of stress and strain on the scale of the lung as a whole to prevent ventilator-induced lung injury.

Spatial pattern formation in the lung

Clustered ventilation defects are a hallmark of asthma, typically seen via imaging studies during asthma attacks. The mechanisms underlying the formation of these clusters is of great interest in understanding asthma. Because the clusters vary from event to event, many researchers believe they occur due to dynamic, rather than structural, causes. To study the formation of these clusters, we formulate and analyze a lattice-based model of the lung, considering both the role of airway bistability and a mechanism for organizing the spatial structure. Within this model we show how and why the homogeneous ventilation solution becomes unstable, and under what circumstances the resulting heterogeneous solution is a clustered solution. The size of the resulting clusters is shown to arise from structure of the eigenvalues and eigenvectors of the system, admitting not only clustered solutions but also (aphysical) checkerboard solutions. We also consider the breathing efficiency of clustered solutions in severely constricted lungs, showing that stabilizing the homogeneous solution may be advantageous in some circumstances. Extensions to hexagonal and cubic lattices are also studied.

Computational Models of Ventilator Induced Lung Injury and Surfactant Dysfunction

Managing acute respiratory distress syndrome (ARDS) invariably involves the administration of mechanical ventilation, the challenge being to avoid the iatrogenic sequellum known as ventilator-induced lung injury (VILI). Devising individualized ventilation strategies in ARDS requires that patient-specific lung physiology be taken into account, and this is greatly aided by the use of computational models of lung mechanical function that can be matched to physiological measurements made in a given patient. In this review, we discuss recent models that have the potential to serve as the basis for devising minimally injurious modes of mechanical ventilation in ARDS patients.

Microfluidic shear stress-regulated surfactant secretion in alveolar epithelial type II cells in vitro

We investigated the role of flow-induced shear stress on the mechanisms regulating surfactant secretion in type II alveolar epithelial cells (ATII) using microfluidic models. Following flow stimulation spanning a range of wall shear stress (WSS) magnitudes, monolayers of ATII (MLE-12 and A549) cells were examined for surfactant secretion by evaluating essential steps of the process, including relative changes in the number of fusion events of lamellar bodies (LBs) with the plasma membrane (PM) and intracellular redistribution of LBs. F-actin cytoskeleton and calcium levels were analyzed in A549 cells subjected to WSS spanning 4-20 dyn/cm(2). Results reveal an enhancement in LB fusion events with the PM in MLE-12 cells upon flow stimulation, whereas A549 cells exhibit no foreseeable changes in the monitored number of fusion events for WSS levels ranging up to a threshold of ∼8 dyn/cm(2); above this threshold, we witness instead a decrease in LB fusion events in A549 cells. However, patterns of LB redistribution suggest that WSS can potentially serve as a stimulus for A549 cells to trigger the intracellular transport of LBs toward the cell periphery. This observation is accompanied by a fragmentation of F-actin, indicating that disorganization of the F-actin cytoskeleton might act as a limiting factor for LB fusion events. Moreover, we note a rise in cytosolic calcium ([Ca(2+)]c) levels following stimulation of A549 cells with WSS magnitudes ranging near or above the experimental threshold. Overall, WSS stimulation can influence key components of molecular machinery for regulated surfactant secretion in ATII cells in vitro.

The unusual symmetric reopening effect induced by pulmonary surfactant

This study investigates the stability of a finger of air as it propagates into a liquid-filled model of a liquid-filled model of a pulmonary bifurcation. We seek to elucidate the stability characteristics of the reopening of daughter airways, an event that may be important to the treatment of acute lung disease. To do so, we investigated the symmetry of reopening under conditions of nearly constant surface tension with 1) purified H2O or 2) an anionic surfactant (sodium dodecyl sulfate). Dynamic surface tension was investigated using pulmonary surfactant (Infasurf) with and without the presence of albumin. Flow visualization was accomplished using a microparticle image velocimetry (μ-PIV)/shadowgraph system through which we measured 1) the propagation velocity of the finger of air that reopens each daughter branch, and 2) the instantaneous and averaged velocity field of liquid phase surrounding the tip of the propagating bubble. Only pulmonary surfactant demonstrated the ability of maintaining a nearly symmetric propagation in the daughter channels, which is likely to lead to homogeneous airway reopening. In contrast, when pulmonary surfactant was inactivated by albumin or when the system was held at a nearly constant surface tension, reopening occurred asymmetrically. Our analysis suggests that Infasurf's dynamic surface tension qualities are important to stabilize the removal of liquid obstructions. This demonstrates a new important function of pulmonary surfactant for airway reopening of a multibranched network.

Surfactant before the first inflation at birth improves spatial distribution of ventilation and reduces lung injury in preterm lambs

The interrelationship between the role of surfactant and a sustained inflation (SI) to aid ex utero transition of the preterm lung is unknown. We compared the effect of surfactant administered before and after an initial SI on gas exchange, lung mechanics, spatial distribution of ventilation, and lung injury in preterm lambs. Gestational-age lambs (127 days; 9 per group) received 100 mg/kg of a surfactant (Curosurf) either prior (Surf+SI) or 10 min after birth (SI+Surf). At birth, a 20-s, 35 cmH2O SI was applied, followed by 70 min of positive pressure ventilation. Oxygenation, carbon dioxide removal, respiratory system compliance, end-expiratory thoracic volume (via respiratory inductive plethysmography), and distribution of end-expiratory volume and ventilation (via electrical impedance tomography) were measured throughout. Early markers of lung injury were analyzed using quantitative RT-PCR. During the first 15 min, oxygenation, carbon dioxide removal, and compliance were better in the Surf+SI group (all P < 0.05). End-expiratory volume on completion of the sustained inflation was higher in the Surf+SI group than the SI+Surf group; 11 ± 1 ml/kg vs. 7 ± 1 ml/kg (mean ± SE) (P = 0.043; t-test), but was not different at later time points. Although neither achieved homogenous aeration, spatial ventilation was more uniform in the Surf+SI group throughout; 50.1 ± 10.9% of total ventilation in the left hemithorax at 70 min vs. 42.6 ± 11.1% in the SI+Surf group. Surf+SI resulted in lower mRNA levels of CYR61 and EGR1 compared with SI+Surf (P < 0.001, one-way ANOVA). Surfactant status of the fetal preterm lung at birth influences the mechanical and injury response to a sustained inflation and ventilation by changing surface tension of the air/fluid interface.

Gas exchange in the respiratory distress syndromes

This article describes the gas exchange abnormalities occurring in the acute respiratory distress syndrome seen in adults and children and in the respiratory distress syndrome that occurs in neonates. Evidence is presented indicating that the major gas exchange abnormality accounting for the hypoxemia in both conditions is shunt, and that approximately 50% of patients also have lungs regions in which low ventilation-to-perfusion ratios contribute to the venous admixture. The various mechanisms by which hypercarbia may develop and by which positive end-expiratory pressure improves gas exchange are reviewed, as are the effects of vascular tone and airway narrowing. The mechanisms by which surfactant abnormalities occur in the two conditions are described, as are the histological findings that have been associated with shunt and low ventilation-to-perfusion.

Physiologically relevant organs on chips

Recent advances in integrating microengineering and tissue engineering have generated promising microengineered physiological models for experimental medicine and pharmaceutical research. Here we review the recent development of microengineered physiological systems, or also known as "ogans-on-chips", that reconstitute the physiologically critical features of specific human tissues and organs and their interactions. This technology uses microengineering approaches to construct organ-specific microenvironments, reconstituting tissue structures, tissue-tissue interactions and interfaces, and dynamic mechanical and biochemical stimuli found in specific organs, to direct cells to assemble into functional tissues. We first discuss microengineering approaches to reproduce the key elements of physiologically important, dynamic mechanical microenvironments, biochemical microenvironments, and microarchitectures of specific tissues and organs in microfluidic cell culture systems. This is followed by examples of microengineered individual organ models that incorporate the key elements of physiological microenvironments into single microfluidic cell culture systems to reproduce organ-level functions. Finally, microengineered multiple organ systems that simulate multiple organ interactions to better represent human physiology, including human responses to drugs, is covered in this review. This emerging organs-on-chips technology has the potential to become an alternative to 2D and 3D cell culture and animal models for experimental medicine, human disease modeling, drug development, and toxicology.

Reabsorption atelectasis in a porcine model of ARDS: regional and temporal effects of airway closure, oxygen, and distending pressure

Little is known about the small airways dysfunction in acute respiratory distress syndrome (ARDS). By computed tomography (CT) imaging in a porcine experimental model of early ARDS, we aimed at studying the location and magnitude of peripheral airway closure and alveolar collapse under high and low distending pressures and high and low inspiratory oxygen fraction (FIO2). Six piglets were mechanically ventilated under anesthesia and muscle relaxation. Four animals underwent saline-washout lung injury, and two served as healthy controls. Beyond the site of assumed airway closure, gas was expected to be trapped in the injured lungs, promoting alveolar collapse. This was tested by ventilation with an FIO2 of 0.25 and 1 in sequence during low and high distending pressures. In the most dependent regions, the gas/tissue ratio of end-expiratory CT, after previous ventilation with FIO2 0.25 low-driving pressure, was significantly higher than after ventilation with FIO2 1; with high-driving pressure, this difference disappeared. Also, significant reduction in poorly aerated tissue and a correlated increase in nonaerated tissue in end-expiratory CT with FIO2 1 low-driving pressure were seen. When high-driving pressure was applied or after previous ventilation with FIO2 0.25 and low-driving pressure, this pattern disappeared. The findings suggest that low distending pressures produce widespread dependent airway closure and with high FIO2, subsequent absorption atelectasis. Low FIO2 prevented alveolar collapse during the study period because of slow absorption of gas behind closed airways.

Ventilation-induced lung injury

Mechanical ventilation (MV) is, by definition, the application of external forces to the lungs. Depending on their magnitude, these forces can cause a continuum of pathophysiological alterations ranging from the stimulation of inflammation to the disruption of cell-cell contacts and cell membranes. These side effects of MV are particularly relevant for patients with inhomogeneously injured lungs such as in acute lung injury (ALI). These patients require supraphysiological ventilation pressures to guarantee even the most modest gas exchange. In this situation, ventilation causes additional strain by overdistension of the yet non-injured region, and additional stress that forms because of the interdependence between intact and atelectatic areas. Cells are equipped with elaborate mechanotransduction machineries that respond to strain and stress by the activation of inflammation and repair mechanisms. Inflammation is the fundamental response of the host to external assaults, be they of mechanical or of microbial origin and can, if excessive, injure the parenchymal tissue leading to ALI. Here, we will discuss the forces generated by MV and how they may injure the lungs mechanically and through inflammation. We will give an overview of the mechanotransduction and how it leads to inflammation and review studies demonstrating that ventilator-induced lung injury can be prevented by blocking pathways of mechanotransduction or inflammation.

Biophysical determinants of alveolar epithelial plasma membrane wounding associated with mechanical ventilation

Mechanical ventilation may cause harm by straining lungs at a time they are particularly prone to injury from deforming stress. The objective of this study was to define the relative contributions of alveolar overdistension and cyclic recruitment and "collapse" of unstable lung units to membrane wounding of alveolar epithelial cells. We measured the interactive effects of tidal volume (VT), transpulmonary pressure (PTP), and of airspace liquid on the number of alveolar epithelial cells with plasma membrane wounds in ex vivo mechanically ventilated rat lungs. Plasma membrane integrity was assessed by propidium iodide (PI) exclusion in confocal images of subpleural alveoli. Cyclic inflations of normal lungs from zero end-expiratory pressure to 40 cmH2O produced VT values of 56.9 ± 3.1 ml/kg and were associated with 0.12 ± 0.12 PI-positive cells/alveolus. A preceding tracheal instillation of normal saline (3 ml) reduced VT to 49.1 ± 6 ml/kg but was associated with a significantly greater number of wounded alveolar epithelial cells (0.52 ± 0.16 cells/alveolus; P < 0.01). Mechanical ventilation of completely saline-filled lungs with saline (VT = 52 ml/kg) to pressures between 10 and 15 cmH2O was associated with the least number of wounded epithelial cells (0.02 ± 0.02 cells/alveolus; P < 0.01). In mechanically ventilated, partially saline-filled lungs, the number of wounded cells increased substantially with VT, but, once VT was accounted for, wounding was independent of maximal PTP. We found that interfacial stress associated with the generation and destruction of liquid bridges in airspaces is the primary biophysical cell injury mechanism in mechanically ventilated lungs.

Effects of ventilation strategy on distribution of lung inflammatory cell activity

Introduction

Leukocyte infiltration is central to the development of acute lung injury, but it is not known how mechanical ventilation strategy alters the distribution or activation of inflammatory cells. We explored how protective (vs. injurious) ventilation alters the magnitude and distribution of lung leukocyte activation following systemic endotoxin administration.

Methods

Anesthetized sheep received intravenous endotoxin (10 ng/kg/min) followed by 2 h of either injurious or protective mechanical ventilation (n = 6 per group). We used positron emission tomography to obtain images of regional perfusion and shunting with infused ¹³N[nitrogen]-saline and images of neutrophilic inflammation with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). The Sokoloff model was used to quantify ¹⁸F-FDG uptake (K_i), as well as its components: the phosphorylation rate (k₃, a surrogate of hexokinase activity) and the distribution volume of ¹⁸F-FDG (F_e) as a fraction of lung volume (K_i = F_e × k₃). Regional gas fractions (f_gas) were assessed by examining transmission scans.

Results

Before endotoxin administration, protective (vs. injurious) ventilation was associated with a higher ratio of partial pressure of oxygen in arterial blood to fraction of inspired oxygen (PaO₂/FiO₂) (351 ± 117 vs. 255 ± 74 mmHg; P < 0.01) and higher whole-lung f_gas (0.71 ± 0.12 vs. 0.48 ± 0.08; P = 0.004), as well as, in dependent regions, lower shunt fractions. Following 2 h of endotoxemia, PaO₂/FiO₂ ratios decreased in both groups, but more so with injurious ventilation, which also increased the shunt fraction in dependent lung. Protective ventilation resulted in less nonaerated lung (20-fold; P < 0.01) and more normally aerated lung (14-fold; P < 0.01). K_i was lower during protective (vs. injurious) ventilation, especially in dependent lung regions (0.0075 ± 0.0043/min vs. 0.0157 ± 0.0072/min; P < 0.01). ¹⁸F-FDG phosphorylation rate (k₃) was twofold higher with injurious ventilation and accounted for most of the between-group difference in K_i. Dependent regions of the protective ventilation group exhibited lower k₃ values per neutrophil than those in the injurious ventilation group (P = 0.01). In contrast, F_e was not affected by ventilation strategy (P = 0.52). Lung neutrophil counts were not different between groups, even when regional inflation was accounted for.

Conclusions

During systemic endotoxemia, protective ventilation may reduce the magnitude and heterogeneity of pulmonary inflammatory cell metabolic activity in early lung injury and may improve gas exchange through its effects predominantly in dependent lung regions. Such effects are likely related to a reduction in the metabolic activity, but not in the number, of lung-infiltrating neutrophils.

Combined effects of ventilation mode and positive end-expiratory pressure on mechanics, gas exchange and the epithelium in mice with acute lung injury

The accepted protocol to ventilate patients with acute lung injury is to use low tidal volume (V(T)) in combination with recruitment maneuvers or positive end-expiratory pressure (PEEP). However, an important aspect of mechanical ventilation has not been considered: the combined effects of PEEP and ventilation modes on the integrity of the epithelium. Additionally, it is implicitly assumed that the best PEEP-V(T) combination also protects the epithelium. We aimed to investigate the effects of ventilation mode and PEEP on respiratory mechanics, peak airway pressures and gas exchange as well as on lung surfactant and epithelial cell integrity in mice with acute lung injury. HCl-injured mice were ventilated at PEEPs of 3 and 6 cmH(2)O with conventional ventilation (CV), CV with intermittent large breaths (CV(LB)) to promote recruitment, and a new mode, variable ventilation, optimized for mice (VV(N)). Mechanics and gas exchange were measured during ventilation and surfactant protein (SP)-B, proSP-B and E-cadherin levels were determined from lavage and lung homogenate. PEEP had a significant effect on mechanics, gas exchange and the epithelium. The higher PEEP reduced lung collapse and improved mechanics and gas exchange but it also down regulated surfactant release and production and increased epithelial cell injury. While CV(LB) was better than CV, VV(N) outperformed CV(LB) in recruitment, reduced epithelial injury and, via a dynamic mechanotransduction, it also triggered increased release and production of surfactant. For long-term outcome, selection of optimal PEEP and ventilation mode may be based on balancing lung physiology with epithelial injury.

A model of surfactant-induced surface tension effects on the parenchymal tethering of pulmonary airways

We developed a computational model of lung parenchyma, which is comprised of individual alveolar chamber models. Each alveolus is modeled by a truncated octahedron. Considering the force balance between the elastin and collagen fibers laying on the alveolar membrane and the pressures acting on the membrane, we computed the deformations of the parenchyma with a finite element method. We focused on the effect of surfactant on the force of parenchymal tethering an airway. As the lung inflates, the parenchyma becomes stiffer and the tethering force becomes stronger. As the alveolar surfactant concentration is reduced, the lung volume at a fixed alveolar pressure decreases, and thus, the tethering force becomes weaker. The distortion of parenchyma caused by the deformation of an airway extends widely around the airway. The displacement of parenchyma decays with distance from the airway wall, but deviates from the prediction based on a theory for a continuum material. Using results obtained from the present lung parenchyma model, we also developed a simple 1-dimensional model for parenchyma tethering force on an airway, which could be utilized for the analysis of liquid/gas transports in an axis-symmetric elastic airway. The effective shear modulus was calculated from the pressure-volume relation of parenchyma. By manipulating the pressure-volume curve, this simple model may be used to predict the parenchyma tethering force in diseased lungs.

Atelectrauma disrupts pulmonary epithelial barrier integrity and alters the distribution of tight junction proteins ZO-1 and claudin 4

Mechanical ventilation inevitably exposes the delicate tissues of the airways and alveoli to abnormal mechanical stresses that can induce pulmonary edema and exacerbate conditions such as acute respiratory distress syndrome. The goal of our research is to characterize the cellular trauma caused by the transient abnormal fluid mechanical stresses that arise when air is forced into a liquid-occluded airway (i.e., atelectrauma). Using a fluid-filled, parallel-plate flow chamber to model the "airway reopening" process, our in vitro study examined consequent increases in pulmonary epithelial plasma membrane rupture, paracellular permeability, and disruption of the tight junction (TJ) proteins zonula occludens-1 and claudin-4. Computational analysis predicts the normal and tangential surface stresses that develop between the basolateral epithelial membrane and underlying substrate due to the interfacial stresses acting on the apical cell membrane. These simulations demonstrate that decreasing the velocity of reopening causes a significant increase in basolateral surface stresses, particularly in the region between neighboring cells where TJs concentrate. Likewise, pulmonary epithelial wounding, paracellular permeability, and TJ protein disruption were significantly greater following slower reopening. This study thus demonstrates that maintaining a higher velocity of reopening, which reduces the damaging fluid stresses acting on the airway wall, decreases the mechanical stresses on the basolateral cell surface while protecting cells from plasma membrane rupture and promoting barrier integrity.

Respiratory physiology on a chip

Our current understanding of respiratory physiology and pathophysiological mechanisms of lung diseases is often limited by challenges in developing in vitro models faithful to the respiratory environment, both in cellular structure and physiological function. The recent establishment and adaptation of microfluidic-based in vitro devices (μFIVDs) of lung airways have enabled a wide range of developments in modern respiratory physiology. In this paper, we address recent efforts over the past decade aimed at advancing in vitro models of lung structure and airways using microfluidic technology and discuss their applications. We specifically focus on μFIVDs covering four major areas of respiratory physiology, namely, artificial lungs (AL), the air-liquid interface (ALI), liquid plugs and cellular injury, and the alveolar-capillary barrier (ACB).

Interfacial stress affects rat alveolar type II cell signaling and gene expression

Previous work from our group (Ravasio A, Hobi N, Bertocchi C, Jesacher A, Dietl P, Haller T. Am J Physiol Cell Physiol 300: C1456-C1465, 2011.) showed that contact of alveolar epithelial type II cells with an air-liquid interface (I(AL)) leads to a paradoxical situation. It is a potential threat that can cause cell injury, but also a Ca(2+)-dependent stimulus for surfactant secretion. Both events can be explained by the impact of interfacial tensile forces on cellular structures. Here, the strength of this mechanical stimulus became also apparent in microarray studies by a rapid and significant change on the transcriptional level. Cells challenged with an I(AL) in two different ways showed activation/inactivation of cellular pathways involved in stress response and defense, and a detailed Pubmatrix search identified genes associated with several lung diseases and injuries. Altogether, they suggest a close relationship of interfacial stress sensation with current models in alveolar micromechanics. Further similarities between I(AL) and cell stretch were found with respect to the underlying signaling events. The source of Ca(2+) was extracellular, and the transmembrane Ca(2+) entry pathway suggests the involvement of a mechanosensitive channel. We conclude that alveolar type II cells, due to their location and morphology, are specific sensors of the I(AL), but largely protected from interfacial stress by surfactant release.

The influence of surfactant on the propagation of a semi-infinite bubble through a liquid-filled compliant channel

We investigate the influence of a soluble surfactant on the steady-state motion of a finger of air through a compliant channel. This study provides a basic model from which to understand the fluid-structure interactions and physicochemical hydrodynamics of pulmonary airway reopening. Airway closure occurs in lung diseases such as respiratory distress syndrome and acute respiratory distress syndrome as a result of fluid accumulation and surfactant insufficiency. This results in 'compliant collapse' with the airway walls buckled and held in apposition by a liquid occlusion that blocks the passage of air. Airway reopening is essential to the recovery of adequate ventilation, but has been associated with ventilator-induced lung injury because of the exposure of airway epithelial cells to large interfacial flow-induced pressure gradients. Surfactant replacement is helpful in modulating this deleterious mechanical stimulus, but is limited in its effectiveness owing to slow surfactant adsorption. We investigate the effect of surfactant on micro-scale models of reopening by computationally modelling the steady two-dimensional motion of a semi-infinite bubble propagating through a liquid-filled compliant channel doped with soluble surfactant. Many dimensionless parameters affect reopening, but we primarily investigate how the reopening pressure p(b) depends upon the capillary number Ca (the ratio of viscous to surface tension forces), the adsorption depth parameter λ (a bulk concentration parameter) and the bulk Péclet number Pe(b) (the ratio of bulk convection to diffusion). These studies demonstrate a dependence of p(b) on λ, and suggest that a critical bulk concentration must be exceeded to operate as a low-surface-tension system. Normal and tangential stress gradients remain largely unaffected by physicochemical interactions - for this reason, further biological studies are suggested that will clarify the role of wall flexibility and surfactant on the protection of the lung from atelectrauma.

Plasma membrane disruptions with different modes of injurious mechanical ventilation in normal rat lungs*

OBJECTIVES

Plasma membrane disruptions are caused by excessive mechanical stress and thought to be involved in inflammatory mediator upregulation. Presently, plasma membrane disruption formation has been studied only during mechanical ventilation with large tidal volumes and limitedly to subpleural alveoli. No information is available concerning the distribution of plasma membrane disruptions within the lung or the development of plasma membrane disruptions during another modality of injurious mechanical ventilation, i.e., mechanical ventilation with eupneic tidal volume (7 mL · kg) at low end-expiratory lung volume. The aim of this study is to assess whether 1) mechanical ventilation with eupneic tidal volume at low end-expiratory lung volume causes plasma membrane disruptions; and 2) the distribution of plasma membrane disruptions differs from that of mechanical ventilation with large tidal volume at normal end-expiratory lung volume.

DESIGN

Experimental animal model.

SUBJECTS

Sprague-Dawley rats.

INTERVENTIONS

Plasma membrane disruptions have been detected as red spots in gelatin-included slices of rat lungs stained with ethidium homodimer-1 shortly after anesthesia (control) after prolonged mechanical ventilation with eupneic tidal volume at low end-expiratory lung volume followed or not by the restoration of physiological end-expiratory lung volume and after prolonged mechanical ventilation with large tidal volumes and normal end-expiratory lung volume.

MEASUREMENTS AND MAIN RESULTS

Plasma membrane disruptions increased during mechanical ventilation at low end-expiratory lung volume, mainly at the bronchiolar level. Resealing of most plasma membrane disruptions occurred on restoration of normal end-expiratory lung volume. Mechanical ventilation with large tidal volume caused the appearance of plasma membrane disruptions, both bronchiolar and parenchymal, the latter to a much greater extent than with mechanical ventilation at low end-expiratory lung volume. The increase of plasma membrane disruptions correlated with the concomitant increase of airway resistance with both modes of mechanical ventilation.

CONCLUSIONS

: Amount and distribution of plasma membrane disruptions between small airways and lung parenchyma depends on the type of injurious mechanical ventilation. This could be relevant to the release of inflammatory mediators.

Tidal recruitment assessed by electrical impedance tomography and computed tomography in a porcine model of lung injury*

OBJECTIVES

To determine the validity of electrical impedance tomography to detect and quantify the amount of tidal recruitment caused by different positive end-expiratory pressure levels in a porcine acute lung injury model.

DESIGN

Randomized, controlled, prospective experimental study.

SETTING

Academic research laboratory.

SUBJECTS

Twelve anesthetized and mechanically ventilated pigs.

INTERVENTIONS

Acute lung injury was induced by central venous oleic acid injection and abdominal hypertension in seven animals. Five healthy pigs served as control group. Animals were ventilated with positive end-expiratory pressure of 0, 5, 10, 15, 20, and 25 cm H2O, respectively, in a randomized order.

MEASUREMENTS AND MAIN RESULTS

At any positive end-expiratory pressure level, electrical impedance tomography was obtained during a slow inflation of 12 mL/kg of body weight. Regional-ventilation-delay indices quantifying the time until a lung region reaches a certain amount of impedance change were calculated for lung quadrants and for every single electrical impedance tomography pixel, respectively. Pixel-wise calculated regional-ventilation-delay indices were plotted in a color-coded regional-ventilation-delay map. Regional-ventilation-delay inhomogeneity that quantifies heterogeneity of ventilation time courses was evaluated by calculating the scatter of all pixel-wise calculated regional-ventilation-delay indices. End-expiratory and end-inspiratory computed tomography scans were performed at each positive end-expiratory pressure level to quantify tidal recruitment of the lung. Tidal recruitment showed a moderate inter-individual (r = .54; p < .05) and intra-individual linear correlation (r = .46 up to r = .73 and p < .05, respectively) with regional-ventilation-delay obtained from lung quadrants. Regional-ventilation-delay inhomogeneity was excellently correlated with tidal recruitment intra- (r = .90 up to r = .99 and p < .05, respectively) and inter-individually (r = .90; p < .001).

CONCLUSIONS

Regional-ventilation-delay can be noninvasively measured by electrical impedance tomography during a slow inflation of 12 mL/kg of body weight and visualized using ventilation delay maps. Our experimental data suggest that the impedance tomography-based analysis of regional-ventilation-delay inhomogeneity provides a good estimate of the amount of tidal recruitment and may be useful to individualize ventilatory settings.

Influence of embryo transfer on embryo preimplantation development

OBJECTIVE

To investigate the impact of injection speeds of the transferred load on embryo development.

DESIGN

A laboratory model for in vitro simulation of ET was developed to investigate the impact of varying injection speeds of the transferred load on embryo development.

SETTING

Academic research institutes of reproduction biotechnology and private centers of reproductive medicine.

ANIMAL(S)

Mouse hybrid F(1) females (C57bl/10 J × CBA-H; N = 15) aged 2-3 months.

INTERVENTION(S)

In vitro exposure of mouse embryos with either the fast ET (ejection speed, >1 m/s) or slow ET (ejection speed, <0.1 m/s) and consecutive culture for 36 hours.

MAIN OUTCOME MEASURE(S)

Development rate, morphology and apoptotic index of embryos.

RESULT(S)

The development rate was the slowest in embryos exposed to the fast ET. Morphological changes in response to ET were observed only among embryos exposed to the fast ET. The mean apoptotic index was 17.6% in the group exposed to the fast ET, 5.6% in the group exposed to the slow ET, and 2.58% in the control group.

CONCLUSION(S)

A reduction of the ejection speed of the transferred load allows avoidance of a developmental delay and diminishes injury of the embryos. Therefore, it is reasonable to suggest transferring the embryos at the lowest possible ejection speed.

Simulation of cell seeding within a three-dimensional porous scaffold: a fluid-particle analysis

Cell seeding is a critical step in tissue engineering. A high number of cells evenly distributed in scaffolds after seeding are associated with a more functional tissue culture. Furthermore, high cell densities have shown the possibility to reduce culture time or increase the formation of tissue. Experimentally, it is difficult to predict the cell-seeding process. In this study, a new methodology to simulate the cell-seeding process under perfusion conditions is proposed. The cells are treated as spherical particles dragged by the fluid media, where the physical parameters are computed through a Lagrangian formulation. The methodology proposed enables to define the kinetics of cell seeding continuously over time. An exponential relationship was found to optimize the seeding time and the number of cells seeded in the scaffold. The cell distribution and cell efficiency predicted using this methodology were similar to the experimental results of Melchels et al. One of the main advantages of this method is to be able to determine the three-dimensional position of all the seeded cells and to, therefore, better know the initial conditions for further cell proliferation and differentiation studies. This study opens up the field of numerical predictions related to the interactions between biomaterials, cells, and dynamics media.

Precisely targeted delivery of cells and biomolecules within microchannels using aqueous two-phase systems

Laminar and pulsatile flow of aqueous solutions in microfluidic channels can be useful for controlled delivery of cells and molecules. Dispersion effects resulting from diffusion and convective disturbances, however, result in reagent delivery profiles becoming blurred over the length of the channels. This issue is addressed partially by using oil-in-water phase systems. However, there are limitations in terms of the biocompatibility of these systems for adherent cell culture. Here we present a fully biocompatible aqueous two-phase flow system that can be used to pattern cells within simple microfluidic channel designs, as well as to deliver biochemical treatments to cells according to discrete boundaries. We demonstrate that aqueous two-phase systems are capable of precisely delivering cells as laminar patterns, or as islands by way of forced droplet formation. We also demonstrate that these systems can be used to precisely control chemical delivery to preformed monolayers of cells growing within channels. Treatments containing trypsin were localized more reliably using aqueous two-phase delivery than using conventional delivery in aqueous medium.

Computational simulation of hematocrit effects on arterial gas embolism dynamics

BACKGROUND

Recent computational investigations have shed light into the various hydrodynamic mechanisms at play during arterial gas embolism that may result in endothelial cell (EC) injury. Other recent studies have suggested that variations in hematocrit level may play an important role in determining the severity of neurological complications due to decompression sickness associated with gas embolism.

METHODS

To develop a comprehensive picture, we computationally modeled the effect of hematocrit variations on the motion of a nearly occluding gas bubble in arterial blood vessels of various sizes. The computational methodology is based on an axisymmetric finite difference immersed boundary numerical method to precisely track the blood-bubble dynamics of the interface. Hematocrit variations are taken to be in the range of 0.2-0.6. The chosen blood vessel sizes correspond to small arteries and small and large arterioles in normal humans.

RESULTS

Relevant hydrodynamic interactions between the gas bubble and EC-lined vessel lumen have been characterized and quantified as a function of hematocrit levels. In particular, the variations in shear stress, spatial and temporal shear stress gradients, and the gap between bubble and vascular endothelium surfaces that contribute to EC injury have been computed.

DISCUSSION

The results suggest that in small arteries, the deleterious hydrodynamic effects of the gas embolism on an EC-lined cell wall are significantly amplified as the hematocrit levels increase. However, such pronounced variations with hematocrit levels are not observed in the arterioles.

Mechanobiology in lung epithelial cells: measurements, perturbations, and responses

Epithelial cells of the lung are located at the interface between the environment and the organism and serve many important functions including barrier protection, fluid balance, clearance of particulate, initiation of immune responses, mucus and surfactant production, and repair following injury. Because of the complex structure of the lung and its cyclic deformation during the respiratory cycle, epithelial cells are exposed to continuously varying levels of mechanical stresses. While normal lung function is maintained under these conditions, changes in mechanical stresses can have profound effects on the function of epithelial cells and therefore the function of the organ. In this review, we will describe the types of stresses and strains in the lungs, how these are transmitted, and how these may vary in human disease or animal models. Many approaches have been developed to better understand how cells sense and respond to mechanical stresses, and we will discuss these approaches and how they have been used to study lung epithelial cells in culture. Understanding how cells sense and respond to changes in mechanical stresses will contribute to our understanding of the role of lung epithelial cells during normal function and development and how their function may change in diseases such as acute lung injury, asthma, emphysema, and fibrosis.

Lagrangian transport properties of pulmonary interfacial flows

Disease states characterized by airway fluid occlusion and pulmonary surfactant insufficiency, such as respiratory distress syndrome, have a high mortality rate. Understanding the mechanics of airway reopening, particularly involving surfactant transport, may provide an avenue to increase patient survival via optimized mechanical ventilation waveforms. We model the occluded airway as a liquid-filled rigid tube with the fluid phase displaced by a finger of air that propagates with both mean and sinusoidal velocity components. Finite-time Lyapunov exponent (FTLE) fields are employed to analyse the convective transport characteristics, taking note of Lagrangian coherent structures (LCSs) and their effects on transport. The Lagrangian perspective of these techniques reveals flow characteristics that are not readily apparent by observing Eulerian measures. These analysis techniques are applied to surfactant-free velocity fields determined computationally, with the boundary element method, and measured experimentally with micro particle image velocimetry (μ-PIV). We find that the LCS divides the fluid into two regimes, one advected upstream (into the thin residual film) and the other downstream ahead of the advancing bubble. At higher oscillatory frequencies particles originating immediately inside the LCS experience long residence times at the air-liquid interface, which may be conducive to surfactant transport. At high frequencies a well-mixed attractor region is identified; this volume of fluid cyclically travels along the interface and into the bulk fluid. The Lagrangian analysis is applied to velocity data measured with 0.01 mg ml(-1) of the clinical pulmonary surfactant Infasurf in the bulk fluid, demonstrating flow field modifications with respect to the surfactant-free system that were not visible in the Eulerian frame.

Organs-on-a-chip: a focus on compartmentalized microdevices

Advances in microengineering technologies have enabled a variety of insights into biomedical sciences that would not have been possible with conventional techniques. Engineering microenvironments that simulate in vivo organ systems may provide critical insight into the cellular basis for pathophysiologies, development, and homeostasis in various organs, while curtailing the high experimental costs and complexities associated with in vivo studies. In this article, we aim to survey recent attempts to extend tissue-engineered platforms toward simulating organ structure and function, and discuss the various approaches and technologies utilized in these systems. We specifically focus on microtechnologies that exploit phenomena associated with compartmentalization to create model culture systems that better represent the in vivo organ microenvironment.

PTEN inhibition improves wound healing in lung epithelia through changes in cellular mechanics that enhance migration

The phosphoinositide-3 kinase/Akt pathway is a vital survival axis in lung epithelia. We previously reported that inhibition of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a major suppressor of this pathway, results in enhanced wound repair following injury. However, the precise cellular and biomechanical mechanisms responsible for increased wound repair during PTEN inhibition are not yet well established. Using primary human lung epithelia and a related lung epithelial cell line, we first determined whether changes in migration or proliferation account for wound closure. Strikingly, we observed that cell migration accounts for the majority of wound recovery following PTEN inhibition in conjunction with activation of the Akt and ERK signaling pathways. We then used fluorescence and atomic force microscopy to investigate how PTEN inhibition alters the cytoskeletal and mechanical properties of the epithelial cell. PTEN inhibition did not significantly alter cytoskeletal structure but did result in large spatial variations in cell stiffness and in particular a decrease in cell stiffness near the wound edge. Biomechanical changes, as well as migration rates, were mediated by both the Akt and ERK pathways. Our results indicate that PTEN inhibition rapidly alters biochemical signaling events that in turn provoke alterations in biomechanical properties that enhance cell migration. Specifically, the reduced stiffness of PTEN-inhibited cells promotes larger deformations, resulting in a more migratory phenotype. We therefore conclude that increased wound closure consequent to PTEN inhibition occurs through enhancement of cell migration that is due to specific changes in the biomechanical properties of the cell.

In situ enhancement of pulmonary surfactant function using temporary flow reversal

Acute respiratory distress syndrome is a pulmonary disease with a mortality rate of ∼40% and 75,000 deaths annually in the United States. Mechanical ventilation restores airway patency and gas transport but leads to ventilator-induced lung injury. Furthermore, surfactant replacement therapy is ineffective due to surfactant delivery difficulties and deactivation by vascular proteins leaking into the airspace. Here, we demonstrated that surfactant function can be substantially improved (up to 50%) in situ in an in vitro pulmonary airway model using unconventional flows that incorporate a short-term retraction of the air-liquid interface, leading to a net decrease in cellular damage. Computational fluid dynamic simulations provided insights into this method and demonstrated the physicochemical hydrodynamic foundation for the improved surfactant microscale transport and mobility. This study may provide a starting point for developing novel ventilation waveforms to improve surfactant function in edematous airways.

Effects of surfactant depletion on regional pulmonary metabolic activity during mechanical ventilation

Inflammation during mechanical ventilation is thought to depend on regional mechanical stress. This can be produced by concentration of stresses and cyclic recruitment in low-aeration dependent lung. Positron emission tomography (PET) with (18)F-fluorodeoxyglucose ((18)F-FDG) allows for noninvasive assessment of regional metabolic activity, an index of neutrophilic inflammation. We tested the hypothesis that, during mechanical ventilation, surfactant-depleted low-aeration lung regions present increased regional (18)F-FDG uptake suggestive of in vivo increased regional metabolic activity and inflammation. Sheep underwent unilateral saline lung lavage and were ventilated supine for 4 h (positive end-expiratory pressure = 10 cmH(2)O, tidal volume adjusted to plateau pressure = 30 cmH(2)O). We used PET scans of injected (13)N-nitrogen to compute regional perfusion and ventilation and injected (18)F-FDG to calculate (18)F-FDG uptake rate. Regional aeration was quantified with transmission scans. Whole lung (18)F-FDG uptake was approximately two times higher in lavaged than in nonlavaged lungs (2.9 ± 0.6 vs. 1.5 ± 0.3 10(-3)/min; P < 0.05). The increased (18)F-FDG uptake was topographically heterogeneous and highest in dependent low-aeration regions (gas fraction 10-50%, P < 0.001), even after correction for lung density and wet-to-dry lung ratios. (18)F-FDG uptake in low-aeration regions of lavaged lungs was higher than that in low-aeration regions of nonlavaged lungs (P < 0.05). This occurred despite lower perfusion and ventilation to dependent regions in lavaged than nonlavaged lungs (P < 0.001). In contrast, (18)F-FDG uptake in normally aerated regions was low and similar between lungs. Surfactant depletion produces increased and heterogeneously distributed pulmonary (18)F-FDG uptake after 4 h of supine mechanical ventilation. Metabolic activity is highest in poorly aerated dependent regions, suggesting local increased inflammation.

Ventilator-induced lung injury: historical perspectives and clinical implications

Mechanical ventilation can produce lung physiological and morphological alterations termed ventilator-induced lung injury (VILI). Early experimental studies demonstrated that the main determinant of VILI is lung end-inspiratory volume. The clinical relevance of these experimental findings received resounding confirmation with the results of the acute respiratory distress syndrome (ARDS) Network study, which showed a 22% reduction in mortality in patients with the acute respiratory distress syndrome through a simple reduction in tidal volume. In contrast, the clinical relevance of low lung volume injury remains debated and the application of high positive end-expiratory pressure levels can contribute to lung overdistension and thus be deleterious. The significance of inflammatory alterations observed during VILI is debated and has not translated into clinical application. This review examines seminal experimental studies that led to our current understanding of VILI and contributed to the current recommendations in the respiratory support of ARDS patients.

Lung parenchymal mechanics

The lung parenchyma comprises a large number of thin-walled alveoli, forming an enormous surface area, which serves to maintain proper gas exchange. The alveoli are held open by the transpulmonary pressure, or prestress, which is balanced by tissues forces and alveolar surface film forces. Gas exchange efficiency is thus inextricably linked to three fundamental features of the lung: parenchymal architecture, prestress, and the mechanical properties of the parenchyma. The prestress is a key determinant of lung deformability that influences many phenomena including local ventilation, regional blood flow, tissue stiffness, smooth muscle contractility, and alveolar stability. The main pathway for stress transmission is through the extracellular matrix. Thus, the mechanical properties of the matrix play a key role both in lung function and biology. These mechanical properties in turn are determined by the constituents of the tissue, including elastin, collagen, and proteoglycans. In addition, the macroscopic mechanical properties are also influenced by the surface tension and, to some extent, the contractile state of the adherent cells. This chapter focuses on the biomechanical properties of the main constituents of the parenchyma in the presence of prestress and how these properties define normal function or change in disease. An integrated view of lung mechanics is presented and the utility of parenchymal mechanics at the bedside as well as its possible future role in lung physiology and medicine are discussed.

Should mechanical ventilation be guided by esophageal pressure measurements?

PURPOSE OF REVIEW

Despite the well recognized role of mechanical ventilation in lung injury, appropriate surrogate markers to guide titration of ventilator settings remain elusive. One would like to strike a balance between protecting aerated units from overdistension while recruiting unstable units, thereby reducing tissue damage associated with their cyclic recruitment and derecruitment. To do so requires some estimate of the topographical distribution of parenchymal stress and strain.

RECENT FINDINGS

Recent studies have highlighted the importance of chest wall recoil and its effect on pleural pressure (Ppl) in determining lung stress. Although esophageal pressure (Pes) has traditionally been used to measure the average Ppl in normal upright patients, in recumbent acute lung injury/acute respiratory distress syndrome patients Pes-based estimates of Ppl are subject to untestable assumptions. Nevertheless, Pes measurements in recumbent patients with injured lungs strongly suggest that Ppl over dependent parts of the lung can exceed airway pressure by substantial amounts. Moreover, results of a pilot study in which Pes was used to titrate positive end-expiratory pressure (PEEP) suggest clinical benefit.

SUMMARY

Notwithstanding its theoretical limitations, esophageal manometry has shown promise in PEEP titration and deserves further evaluation in a larger trial on patients with injured lungs.

Beyond volutrauma in ARDS: the critical role of lung tissue deformation

Ventilator-induced lung injury (VILI) consists of tissue damage and a biological response resulting from the application of inappropriate mechanical forces to the lung parenchyma. The current paradigm attributes VILI to overstretching due to very high-volume ventilation (volutrauma) and cyclic changes in aeration due to very low-volume ventilation (atelectrauma); however, this model cannot explain some research findings. In the present review, we discuss the relevance of cyclic deformation of lung tissue as the main determinant of VILI. Parenchymal stability resulting from the interplay of respiratory parameters such as tidal volume, positive end-expiratory pressure or respiratory rate can explain the results of different clinical trials and experimental studies that do not fit with the classic volutrauma/atelectrauma model. Focusing on tissue deformation could lead to new bedside monitoring and ventilatory strategies.

Procedure for lung engineering

Lung tissue, including lung cancer and chronic lung diseases such as chronic obstructive pulmonary disease, cumulatively account for some 280,000 deaths annually; chronic obstructive pulmonary disease is currently the fourth leading cause of death in the United States. Contributing to this mortality is the fact that lungs do not generally repair or regenerate beyond the microscopic, cellular level. Therefore, lung tissue that is damaged by degeneration or infection, or lung tissue that is surgically resected is not functionally replaced in vivo. To explore whether lung tissue can be generated in vitro, we treated lungs from adult rats using a procedure that removes cellular components to produce an acellular lung extracellular matrix scaffold. This scaffold retains the hierarchical branching structures of airways and vasculature, as well as a largely intact basement membrane, which comprises collagen IV, laminin, and fibronectin. The scaffold is mounted in a bioreactor designed to mimic critical aspects of lung physiology, such as negative pressure ventilation and pulsatile vascular perfusion. By culturing pulmonary epithelium and vascular endothelium within the bioreactor-mounted scaffold, we are able to generate lung tissue that is phenotypically comparable to native lung tissue and that is able to participate in gas exchange for short time intervals (45-120 minutes). These results are encouraging, and suggest that repopulation of lung matrix is a viable strategy for lung regeneration. This possibility presents an opportunity not only to work toward increasing the supply of lung tissue for transplantation, but also to study respiratory cell and molecular biology in vitro for longer time periods and in a more accurate microenvironment than has previously been possible.

Quantitative imaging of alveolar recruitment with hyperpolarized gas MRI during mechanical ventilation

The aim of this study was to assess the utility of (3)He MRI to noninvasively probe the effects of positive end-expiratory pressure (PEEP) maneuvers on alveolar recruitment and atelectasis buildup in mechanically ventilated animals. Sprague-Dawley rats (n = 13) were anesthetized, intubated, and ventilated in the supine position ((4)He-to-O(2) ratio: 4:1; tidal volume: 10 ml/kg, 60 breaths/min, and inspiration-to-expiration ratio: 1:2). Recruitment maneuvers consisted of either a stepwise increase of PEEP to 9 cmH(2)O and back to zero end-expiratory pressure or alternating between these two PEEP levels. Diffusion MRI was performed to image (3)He apparent diffusion coefficient (ADC) maps in the middle coronal slices of lungs (n = 10). ADC was measured immediately before and after two recruitment maneuvers, which were separated from each other with a wait period (8-44 min). We detected a statistically significant decrease in mean ADC after each recruitment maneuver. The relative ADC change was -21.2 ± 4.1 % after the first maneuver and -9.7 ± 5.8 % after the second maneuver. A significant relative increase in mean ADC was observed over the wait period between the two recruitment maneuvers. The extent of this ADC buildup was time dependent, as it was significantly related to the duration of the wait period. The two postrecruitment ADC measurements were similar, suggesting that the lungs returned to the same state after the recruitment maneuvers were applied. No significant intrasubject differences in ADC were observed between the corresponding PEEP levels in two rats that underwent three repeat maneuvers. Airway pressure tracings were recorded in separate rats undergoing one PEEP maneuver (n = 3) and showed a significant relative difference in peak inspiratory pressure between pre- and poststates. These observations support the hypothesis of redistribution of alveolar gas due to recruitment of collapsed alveoli in presence of atelectasis, which was also supported by the decrease in peak inspiratory pressure after recruitment maneuvers.

Spatiotemporal dynamics of actin remodeling and endomembrane trafficking in alveolar epithelial type I cell wound healing

Alveolar epithelial type I cell (ATI) wounding is prevalent in ventilator-injured lungs and likely contributes to pathogenesis of "barotrauma" and "biotrauma." In experimental models most wounded alveolar cells repair plasma membrane (PM) defects and survive insults. Considering the force balance between edge energy at the PM wound margins and adhesive interactions of the lipid bilayer with the underlying cytoskeleton (CSK), we tested the hypothesis that subcortical actin depolymerization is a key facilitator of PM repair. Using real-time fluorescence imaging of primary rat ATI transfected with a live cell actin-green fluorescent protein construct (Lifeact-GFP) and loaded with N-rhodamine phosphatidylethanolamine (PE), we examined the spatial and temporal coordination between cytoskeletal remodeling and PM repair following micropuncture. Membrane integrity was inferred from the fluorescence intensity profiles of the cytosolic label calcein AM. Wounding led to rapid depolymerization of the actin CSK near the wound site, concurrent with accumulation of endomembrane-derived N-rhodamine PE. Both responses were sustained until PM integrity was reestablished, which typically occurs between ∼10 and 40 s after micropuncture. Only thereafter did the actin CSK near the wound begin to repolymerize, while the rate of endomembrane lipid accumulation decreased. Between 60 and 90 s after successful PM repair, after translocation of the actin nucleation factor cortactin, a dense actin fiber network formed. In cells that did not survive micropuncture injury, actin remodeling did not occur. These novel results highlight the importance of actin remodeling in ATI cell repair and suggest molecular targets for modulating the repair process.

Bioreactor for the long-term culture of lung tissue

In this article we describe the design and validation of a bioreactor for the in vitro culture of whole rodent lung tissue. Many current systems only enable large segments of lung tissue to be studied ex vivo for up to a few hours in the laboratory. This limitation restricts the study of pulmonary biology in controlled laboratory settings, and also impacts the ability to reliably culture engineered lung tissues in the laboratory. Therefore, we designed, built, and validated a bioreactor intended to provide sufficient nutrient supply and mechanical stimulation to support cell survival and differentiation in cultured lung tissue. We also studied the effects of perfusion and ventilation on pulmonary cell survival and maintenance of cell differentiation state. The final bioreactor design described herein is capable of supporting the culture of whole native lung tissue for up to 1 week in the laboratory, and offers promise in the study of pulmonary biology and the development of engineered lung tissues in the laboratory.

Determinants of plasma membrane wounding by deforming stress

Once excess liquid gains access to air spaces of an injured lung, the act of breathing creates and destroys foam and thereby contributes to the wounding of epithelial cells by interfacial stress. Since cells are not elastic continua, but rather complex network structures composed of solid as well as liquid elements, we hypothesize that plasma membrane (PM) wounding is preceded by a phase separation, which results in blebbing. We postulate that interventions such as a hypertonic treatment increase adhesive PM-cytoskeletal (CSK) interactions, thereby preventing blebbing as well as PM wounds. We formed PM tethers in alveolar epithelial cells and fibroblasts and measured their retractive force as readout of PM-CSK adhesive interactions using optical tweezers. A 50-mOsm increase in media osmolarity consistently increased the tether retractive force in epithelial cells but lowered it in fibroblasts. The osmo-response was abolished by pretreatment with latrunculin, cytochalasin D, and calcium chelation. Epithelial cells and fibroblasts were exposed to interfacial stress in a microchannel, and the fraction of wounded cells were measured. Interventions that increased PM-CSK adhesive interactions prevented blebbing and were cytoprotective regardless of cell type. Finally, we exposed ex vivo perfused rat lungs to injurious mechanical ventilation and showed that hypertonic conditioning reduced the number of wounded subpleural alveolus resident cells to baseline levels. Our observations support the hypothesis that PM-CSK adhesive interactions are important determinants of the cellular response to deforming stress and pave the way for preclinical efficacy trials of hypertonic treatment in experimental models of acute lung injury.

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.