Emphysema and mechanical stress-induced lung remodeling

Mechanical and morphological characterization of the emphysematous lung tissue

Irreversible alveolar airspace enlargement is the main characteristic of pulmonary emphysema, which has been extensively studied using animal models. While the alterations in lung mechanics associated with these morphological changes have been documented in the literature, the study of the mechanical behavior of parenchymal tissue from emphysematous lungs has been poorly investigated. In this work, we characterize the mechanical and morphological properties of lung tissue in elastase-induced emphysema rat models under varying severity conditions. We analyze the non-linear tissue behavior using suitable hyperelastic constitutive models that enable to compare different non-linear responses in terms of hyperelastic material parameters. We further analyze the effect of the elastase dose on alveolar morphology and tissue material parameters and study their connection with respiratory-system mechanical parameters. Our results show that while the lung mechanical function is not significantly influenced by the elastase treatment, the tissue mechanical behavior and alveolar morphology are markedly affected by it. We further show a strong association between alveolar enlargement and tissue softening, not evidenced by respiratory-system compliance. Our findings highlight the importance of understanding tissue mechanics in emphysematous lungs, as changes in tissue properties could detect the early stages of emphysema remodeling. STATEMENT OF SIGNIFICANCE: Gas exchange is vital for life and strongly relies on the mechanical function of the lungs. Pulmonary emphysema is a prevalent respiratory disease where alveolar walls are damaged, causing alveolar enlargement that induces harmful changes in the mechanical response of the lungs. In this work, we study how the mechanical properties of lung tissue change during emphysema. Our results from animal models show that tissue properties are more sensitive to alveolar enlargement due to emphysema than other mechanical properties that describe the function of the whole respiratory system.

Antenatal Endotoxin Induces Dysanapsis in Experimental Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD), the chronic lung disease of prematurity, is characterized by impaired lung development with sustained functional abnormalities due to alterations of airways and the distal lung. Although clinical studies have shown striking associations between antenatal stress and BPD, little is known about the underlying pathogenetic mechanisms. Whether dysanapsis, the concept of discordant growth of the airways and parenchyma, contributes to late respiratory disease as a result of antenatal stress is unknown. We hypothesized that antenatal endotoxin (ETX) impairs juvenile lung function as a result of altered central airway and distal lung structure, suggesting the presence of dysanapsis in this preclinical BPD model. Fetal rats were exposed to intraamniotic ETX (10 μg) or saline solution (control) 2 days before term. We performed extensive structural and functional evaluation of the proximal airways and distal lung in 2-week-old rats. Distal lung structure was quantified by stereology. Conducting airway diameters were measured using micro-computed tomography. Lung function was assessed during invasive ventilation to quantify baseline mechanics, response to methacholine challenge, and spirometry. ETX-exposed pups exhibited distal lung simplification, decreased alveolar surface area, and decreased parenchyma-airway attachments. ETX-exposed pups exhibited decreased tracheal and second- and third-generation airway diameters. ETX increased respiratory system resistance and decreased lung compliance at baseline. Only Newtonian resistance, specific to large airways, exhibited increased methacholine reactivity in ETX-exposed pups compared with controls. ETX-exposed pups had a decreased ratio of FEV in 0.1 second to FVC and a normal FEV in 0.1 second, paralleling the clinical definition of dysanapsis. Antenatal ETX causes abnormalities of the central airways and distal lung growth, suggesting that dysanapsis contributes to abnormal lung function in juvenile rats.

Collagen Tubular Airway-on-Chip for Extended Epithelial Culture and Investigation of Ventilation Dynamics

The lower respiratory tract is a hierarchical network of compliant tubular structures that are made from extracellular matrix proteins with a wall lined by an epithelium. While microfluidic airway-on-a-chip models incorporate the effects of shear and stretch on the epithelium, week-long air-liquid-interface culture at physiological shear stresses, the circular cross-section, and compliance of native airway walls have yet to be recapitulated. To overcome these limitations, a collagen tube-based airway model is presented. The lumen is lined with a confluent epithelium during two-week continuous perfusion with warm, humid air while presenting culture medium from the outside and compensating for evaporation. The model recapitulates human small airways in extracellular matrix composition and mechanical microenvironment, allowing for the first time dynamic studies of elastocapillary phenomena associated with regular breathing and mechanical ventilation, as well as their impacts on the epithelium. A case study reveales increasing damage to the epithelium during repetitive collapse and reopening cycles as opposed to overdistension, suggesting expiratory flow resistance to reduce atelectasis. The model is expected to promote systematic comparisons between different clinically used ventilation strategies and, more broadly, to enhance human organ-on-a-chip platforms for a variety of tubular tissues.

Downregulated BMP-Smad1/5/8 signaling causes emphysema via dysfunction of alveolar type II epithelial cells

Bone morphogenetic protein (BMP)-Smad1/5/8 signaling plays a crucial regulatory role in lung development and adult lung homeostasis. However, it remains elusive whether BMP-Smad1/5/8 signaling is involved in the pathogenesis of emphysema. In this study, we downregulated BMP-Smad1/5/8 signaling by overexpressing its antagonist Noggin in adult mouse alveolar type II epithelial cells (AT2s), resulting in an emphysematous phenotype mimicking the typical pathological features of human emphysema, including distal airspace enlargement, pulmonary inflammation, extracellular matrix remodeling, and impaired lung function. Dysregulation of BMP-Smad1/5/8 signaling in AT2s leads to inflammatory destruction dominated by macrophage infiltration, associated with reduced secretion of surfactant proteins and inhibition of AT2 proliferation and differentiation. Reactivation of BMP-Smad1/5/8 signaling by genetics or chemotherapy significantly attenuated the morphology and pathophysiology of emphysema and improved the lung function in Noggin-overexpressing lungs. We also found that BMP-Smad1/5/8 signaling was downregulated in cigarette smoke-induced emphysema, and that enhancing its activity in AT2s prevented or even reversed emphysema in the mouse model. Our data suggest that BMP-Smad1/5/8 signaling, located at the top of the signaling cascade that regulates lung homeostasis, represents a key molecular regulator of alveolar stem cell secretory and regenerative function, and could serve as a potential target for future prevention and treatment of pulmonary emphysema. © 2023 The Pathological Society of Great Britain and Ireland.

Anti-CELA1 antibody KF4 prevents emphysema by inhibiting stretch-mediated remodeling

There are no therapies to prevent emphysema progression. Chymotrypsin-like elastase 1 (CELA1) is a serine protease that binds and cleaves lung elastin in a stretch-dependent manner and is required for emphysema in a murine antisense oligonucleotide model of α-1 antitrypsin (AAT) deficiency. This study tested whether CELA1 is important in strain-mediated lung matrix destruction in non-AAT-deficient emphysema and the efficacy of CELA1 neutralization. Airspace simplification was quantified after administration of tracheal porcine pancreatic elastase (PPE), after 8 months of cigarette smoke (CS) exposure, and in aging. In all 3 models, Cela1-/- mice had less emphysema and preserved lung elastin despite increased lung immune cells. A CELA1-neutralizing antibody was developed (KF4), and it inhibited stretch-inducible lung elastase in ex vivo mouse and human lung and immunoprecipitated CELA1 from human lung. In mice, systemically administered KF4 penetrated lung tissue in a dose-dependent manner and 5 mg/kg weekly prevented emphysema in the PPE model with both pre- and postinjury initiation and in the CS model. KF4 did not increase lung immune cells. CELA1-mediated lung matrix remodeling in response to strain is an important contributor to postnatal airspace simplification, and we believe that KF4 could be developed as a lung matrix-stabilizing therapy in emphysema.

Integrating mechanical cues with engineered platforms to explore cardiopulmonary development and disease

Mechanical forces provide critical biological signals to cells during healthy and aberrant organ development as well as during disease processes in adults. Within the cardiopulmonary system, mechanical forces, such as shear, compressive, and tensile forces, act across various length scales, and dysregulated forces are often a leading cause of disease initiation and progression such as in bronchopulmonary dysplasia and cardiomyopathies. Engineered in vitro models have supported studies of mechanical forces in a number of tissue and disease-specific contexts, thus enabling new mechanistic insights into cardiopulmonary development and disease. This review first provides fundamental examples where mechanical forces operate at multiple length scales to ensure precise lung and heart function. Next, we survey recent engineering platforms and tools that have provided new means to probe and modulate mechanical forces across in vitro and in vivo settings. Finally, the potential for interdisciplinary collaborations to inform novel therapeutic approaches for a number of cardiopulmonary diseases are discussed.

Crystal ribcage: a platform for probing real-time lung function at cellular resolution

Understanding the dynamic pathogenesis and treatment response in pulmonary diseases requires probing the lung at cellular resolution in real time. Despite advances in intravital imaging, optical imaging of the lung during active respiration and circulation has remained challenging. Here, we introduce the crystal ribcage: a transparent ribcage that allows multiscale optical imaging of the functioning lung from whole-organ to single-cell level. It enables the modulation of lung biophysics and immunity through intravascular, intrapulmonary, intraparenchymal and optogenetic interventions, and it preserves the three-dimensional architecture, air-liquid interface, cellular diversity and respiratory-circulatory functions of the lung. Utilizing these capabilities on murine models of pulmonary pathologies we probed remodeling of respiratory-circulatory functions at the single-alveolus and capillary levels during disease progression. The crystal ribcage and its broad applications presented here will facilitate further studies of nearly any pulmonary disease as well as lead to the identification of new targets for treatment strategies.

Synergistic Effects of Resistive Breathing on Endotoxin-Induced Lung Injury in Mice

Introduction

Resistive breathing (RB) is characterized by forceful contractions of the inspiratory muscles, mainly the diaphragm, resulting in large negative intrathoracic pressure and mechanical stress imposed on the lung. We have shown that RB induces lung injury in healthy animals. Whether RB exerts additional injurious effects when added to pulmonary or extrapulmonary lung injury is unknown. Our aim was to study the synergistic effect of RB on lipopolysaccharide (LPS)-induced lung injury.

Methods

C57BL/6 mice inhaled an LPS aerosol (10mg/3mL) or received an intraperitoneal injection of LPS (10 mg/kg). Mice were then anaesthetized, the trachea was surgically exposed, and a nylon band of a specified length was sutured around the trachea, to provoke a reduction of the surface area at 50%. RB through tracheal banding was applied for 24 hours. Respiratory system mechanics were measured, BAL was performed, and lung sections were evaluated for histological features of lung injury.

Results

LPS inhalation increased BAL cellularity, mainly neutrophils (p < 0.001 to ctr), total protein and IL-6 in BAL (p < 0.001 and p < 0.001, respectively) and increased the lung injury score (p = 0.001). Lung mechanics were not altered. Adding RB to inhaled LPS further increased BAL cellularity (p < 0.001 to LPS inh.), total protein (p = 0.016), lung injury score (p = 0.001) and increased TNFa levels in BAL (p = 0.011). Intraperitoneal LPS increased BAL cellularity, mainly macrophages (p < 0.001 to ctr.), total protein levels (p = 0.017), decreased static compliance (p = 0.004) and increased lung injury score (p < 0.001). Adding RB further increased histological features of lung injury (p = 0.022 to LPS ip).

Conclusion

Resistive breathing exerts synergistic injurious effects when combined with inhalational LPS-induced lung injury, while the additive effect on extrapulmonary lung injury is less prominent.

Lung stiffness of C57BL/6 versus BALB/c mice

This study was undertaken to determine whether a smaller lung volume or a stiffer lung tissue accounts for the greater lung elastance of C57BL/6 than BALB/c mice. The mechanical properties of the respiratory system and lung volumes were measured with the flexiVent and compared between male C57BL/6 and BALB/c mice (n = 9). The size of the excised lung was also measured by volume liquid displacement. One lobe was then subjected to sinusoidal strains in vitro to directly assess the mechanical properties of the lung tissue, and another one was used to quantify the content of hydroxyproline. In vivo elastance was markedly greater in C57BL/6 than BALB/c mice based on 5 different readouts. For example, respiratory system elastance was 24.5 ± 1.7 vs. 21.5 ± 2.4 cmH2O/mL in C57BL/6 and BALB/c mice, respectively (p = 0.007). This was not due to a different lung volume measured by displaced liquid volume. On the isolated lobes, both elastance and the hydroxyproline content were significantly greater in C57BL/6 than BALB/c mice. These results suggest that the lung elastance of C57BL/6 mice is greater than BALB/c mice not because of a smaller lung volume but because of a stiffer lung tissue due to a greater content of collagen.

Glucocorticoid Alleviates Mechanical Stress-Induced Airway Inflammation and Remodeling in COPD via Transient Receptor Potential Canonical 1 Channel

Background

Increased airway resistance and hyperinflation in chronic obstructive pulmonary disease (COPD) are associated with increased mechanical stress that modulate many essential pathophysiological functions including airway remodeling and inflammation. Our present study aimed to investigate the role of transient receptor potential canonical 1 (TRPC1), a mechanosensitive cation channel in airway remodeling and inflammation in COPD and the effect of glucocorticoid on this process.

Methods

In patients, we investigated the effect of pathological high mechanical stress on the expression of airway remodeling-related cytokines transforming growth factor β1 (TGF-β1), matrix metalloproteinase-9 (MMP9) and the count of inflammatory cells in endotracheal aspirates (ETAs) by means of different levels of peak inspiratory pressure (PIP) under mechanical ventilation, and analyzed their correlation with TRPC1. Based on whether patients regularly used inhaled corticosteroid (ICS), COPD patients were further divided into ICS group (n = 12) and non-ICS group (n=15). The ICS effect on the expression of TRPC1 was detected by Western blot. In vitro, we imitated the mechanical stress using cyclic stretch and examined the levels of TGF-β1 and MMP-9. The role of TRPC1 was further explored by siRNA transfection and dexamethasone administration.

Results

Our results revealed that the TRPC1 level and the inflammatory cells counts were significantly higher in COPD group. After mechanical ventilation, the expression of TGF-β1 and MMP-9 in all COPD subgroups was significantly increased, while in the control group, only high PIP subgroup increased. Meanwhile, TRPC1 expression was positively correlated with the counts of inflammatory cells and the levels of TGF-β1 and MMP-9. In vitro, mechanical stretch significantly increased TGF-β1 and MMP-9 levels and such increase was greatly attenuated by TRPC1 siRNA transfection and dexamethasone administration.

Conclusion

Our results suggest that the increased TRPC1 may play a role in the airway inflammation and airway remodeling in COPD under high airway pressure. Glucocorticoid could in some degree alleviate airway remodeling via inhibition of TRPC1.

Animal models and mechanisms of tobacco smoke-induced chronic obstructive pulmonary disease (COPD)

Chronic obstructive pulmonary disease (COPD) is the third leading cause of death worldwide, and its global health burden is increasing. COPD is characterized by emphysema, mucus hypersecretion, and persistent lung inflammation, and clinically by chronic airflow obstruction and symptoms of dyspnea, cough, and fatigue in patients. A cluster of pathologies including chronic bronchitis, emphysema, asthma, and cardiovascular disease in the form of hypertension and atherosclerosis variably coexist in COPD patients. Underlying causes for COPD include primarily tobacco use but may also be driven by exposure to air pollutants, biomass burning, and workplace related fumes and chemicals. While no single animal model might mimic all features of human COPD, a wide variety of published models have collectively helped to improve our understanding of disease processes involved in the genesis and persistence of COPD. In this review, the pathogenesis and associated risk factors of COPD are examined in different mammalian models of the disease. Each animal model included in this review is exclusively created by tobacco smoke (TS) exposure. As animal models continue to aid in defining the pathobiological mechanisms of and possible novel therapeutic interventions for COPD, the advantages and disadvantages of each animal model are discussed.

Green tea extract ameliorates macrophage-driven emphysematous lesions in chronic obstructive pulmonary disease induced by cigarette smoke condensate

Chronic obstructive pulmonary disease (COPD) is an important lung disease characterized by complicated symptoms including emphysema. We aimed to explore the mechanisms underlying the protective effect of green tea extract (GTE) on cigarette smoke condensate (CSC)-induced emphysema by demonstrating the reduction of macrophage-induced protease expression through GTE treatment in vivo and in vitro. Mice were intranasally administered 50 mg/kg CSC once a week for 4 weeks, and doses of 100 or 300 mg/kg GTE were administered orally once daily for 4 weeks. GTE significantly reduced macrophage counts in bronchoalveolar lavage fluid and emphysematous lesions in lung tissues in CSC-exposed mice. In addition, GTE suppressed CSC-induced extracellular signal-regulated kinase (ERK)/activator protein (AP)-1 phosphorylation followed by matrix metalloproteinases (MMP)-9 expression as revealed by western blotting, immunohistochemistry, and zymography in CSC-instilled mice. These underlying mechanisms related to reduced protease expression were confirmed in NCI-H292 cells stimulated by CSC. Taken together, GTE effectively inhibits macrophage-driven emphysematous lesions induced by CSC treatment, and these protective effects of GTE are closely related to the ERK/AP-1 signaling pathway, followed by a reduced protease/antiprotease imbalance. These results suggest that GTE can be used as a supplementary agent for the prevention of emphysema progression in COPD patients.

A Model to Predict Residual Volume from Forced Spirometry Measurements in Chronic Obstructive Pulmonary Disease

Background

Lung hyperinflation with elevated residual volume (RV) is associated with poor prognosis in adults with chronic obstructive pulmonary disease (COPD) and is a critical criterion for lung volume reduction selection. Here, we proposed that patterns within spirometric measures could represent the degree of hyperinflation.

Methods

Fractional polynomial multivariate regression was used to develop a prediction model based on age, biological sex, forced expiratory volume in 1 second (FEV₁), and forced vital capacity (FVC) to estimate plethysmography measured RV in patients in the Pittsburgh Specialized Center for Clinically Oriented Research (SCCOR) cohort (n=450). Receiver operating characteristic area under the curve (ROC-AUC) and optimal cut-points from the model were identified. The model was validated in a separate cohort (n=793).

Results

The best fit model: RV %est=[FVC %predicted] x 3.46-[FEV₁/FVC] x 179.80- [FVC % (sqrt)] x 79.53-[age] x 0.98- [sex] x 10.88 + 737.06, where [sex], m=1. R² of observed versus %predicted RV was 0.71. The optimal cut-point to predict an RV % >175% was 161. At this cut-point, ROC-AUC was 0.95, with a sensitivity 0.95, specificity 0.86, positive predictive value (PPV) of 97%, negative predictive value (NPV) of 76%, positive likelihood ratio (LR) of 6.6, and negative LR of 0.06. In a validation cohort of COPD patients (n=793), the model performed similarly, with a sensitivity of 0.82, specificity of 0.83, PPV of 85%, NPV of 79%, positive LR of 4.7, and negative LR of 0.21.

Conclusion

In patients with COPD, a model using only spirometry, age, and biological sex can estimate elevated RV. This tool could facilitate the identification of candidates for lung volume reduction procedures and can be integrated into existing epidemiologic databases to investigate the clinical impact of hyperinflation.

Pharmacological Activation of YAP/TAZ by Targeting LATS1/2 Enhances Periodontal Tissue Regeneration in a Murine Model

Due to their multi-differentiation potential, periodontal ligament fibroblasts (PDLF) play pivotal roles in periodontal tissue regeneration in vivo. Several in vitro studies have suggested that PDLFs can transmit mechanical stress into favorable basic cellular functions. However, the application of mechanical force for periodontal regeneration therapy is not expected to exhibit an effective prognosis since mechanical forces, such as traumatic occlusion, also exacerbate periodontal tissue degeneration and loss. Herein, we established a standardized murine periodontal regeneration model and evaluated the regeneration process associated with cementum remodeling. By administering a kinase inhibitor of YAP/TAZ suppressor molecules, such as large tumor suppressor homolog 1/2 (LATS1/2), we found that the activation of YAP/TAZ, a key downstream effector of mechanical signals, accelerated periodontal tissue regeneration due to the activation of PDLF cell proliferation. Mechanistically, among six kinds of MAP4Ks previously reported as upstream kinases that suppressed YAP/TAZ transcriptional activity through LATS1/2 in various types of cells, MAP4K4 was identified as the predominant MAP4K in PDLF and contributed to cell proliferation and differentiation depending on its kinase activity. Ultimately, pharmacological activation of YAP/TAZ by inhibiting upstream inhibitory kinase in PDLFs is a valuable strategy for improving the clinical outcomes of periodontal regeneration therapies.

Remodeling of the Aged and Emphysematous Lungs: Roles of Microenvironmental Cues

Aging is a slow process that affects all organs, and the lung is no exception. At the alveolar level, aging increases the airspace size with thicker and stiffer septal walls and straighter and thickened collagen and elastic fibers. This creates a microenvironment that interferes with the ability of cells in the parenchyma to maintain normal homeostasis and respond to injury. These changes also make the lung more susceptible to disease such as emphysema. Emphysema is characterized by slow but progressive remodeling of the deep alveolar regions that leads to airspace enlargement and increased but disorganized elastin and collagen deposition. This remodeling has been attributed to ongoing inflammation that involves inflammatory cells and the cytokines they produce. Cellular senescence, another consequence of aging, weakens the ability of cells to properly respond to injury, something that also occurs in emphysema. These factors conspire to make alveolar walls more prone to mechanical failure, which can set emphysema in motion by driving inflammation through immune stimulation by protein fragments. Both aging and emphysema are influenced by microenvironmental conditions such as local inflammation, chemical makeup, tissue stiffness, and mechanical stresses. Although aging and emphysema are not equivalent, they have the potential to influence each other in synergistic ways; aging sets up the conditions for emphysema to develop, while emphysema may accelerate cellular senescence and thus aging itself. This article focuses on the similarities and differences between the remodeled microenvironment of the aging and emphysematous lung, with special emphasis on the alveolar septal wall. © 2022 American Physiological Society. Compr Physiol 12:3559-3574, 2022.

Chronic lung diseases: entangled in extracellular matrix

The extracellular matrix (ECM) is the scaffold that provides structure and support to all organs, including the lung; however, it is also much more than this. The ECM provides biochemical and biomechanical cues to cells that reside or transit through this micro-environment, instructing their responses. The ECM structure and composition changes in chronic lung diseases; how such changes impact disease pathogenesis is not as well understood. Cells bind to the ECM through surface receptors, of which the integrin family is one of the most widely recognised. The signals that cells receive from the ECM regulate their attachment, proliferation, differentiation, inflammatory secretory profile and survival. There is extensive evidence documenting changes in the composition and amount of ECM in diseased lung tissues. However, changes in the topographical arrangement, organisation of the structural fibres and stiffness (or viscoelasticity) of the matrix in which cells are embedded have an undervalued but strong impact on cell phenotype. The ECM in diseased lungs also changes in physical and biomechanical ways that drive cellular responses. The characteristics of these environments alter cell behaviour and potentially orchestrate perpetuation of lung diseases. Future therapies should target ECM remodelling as much as the underlying culprit cells.

Effects of time-controlled adaptive ventilation on cardiorespiratory parameters and inflammatory response in experimental emphysema

The time-controlled adaptive ventilation (TCAV) method attenuates lung damage in acute respiratory distress syndrome. However, so far, no study has evaluated the impact of the TCAV method on ventilator-induced lung injury (VILI) and cardiac function in emphysema. We hypothesized that the use of the TCAV method to achieve an expiratory flow termination/expiratory peak flow (E_FT/E_PF) of 25% could reduce VILI and improve right ventricular function in elastase-induced lung emphysema in rats. Five weeks after the last intratracheal instillation of elastase, animals were anesthetized and mechanically ventilated for 1 h using TCAV adjusted to either E_FT/E_PF 25% or E_FT/E_PF 75%, the latter often applied in ARDS. Pressure-controlled ventilation (PCV) groups with positive end-expiratory pressure levels similar to positive end-release pressure in TCAV with E_FT/E_PF 25% and E_FT/E_PF 75% were also analyzed. Echocardiography and lung ultrasonography were monitored. Lung morphometry, alveolar heterogeneity, and biological markers related to inflammation (interleukin [IL]-6, CINC-1), alveolar pulmonary stretch (amphiregulin), lung matrix damage (metalloproteinase [MMP]-9) were assessed. E_FT/E_PF 25% reduced respiratory system peak pressure, mean linear intercept, B lines at lung ultrasonography, and increased pulmonary acceleration time/pulmonary ejection time ratio compared with E_FT/E_PF 75%. The volume fraction of mononuclear cells, neutrophils, and expression of IL-6, CINC-1, amphiregulin, and MMP-9 were lower with E_FT/E_PF 25% than with E_FT/E_PF 75%. In conclusion, TCAV with E_FT/E_PF 25%, compared with E_FT/E_PF 75%, led to less lung inflammation, hyperinflation, and pulmonary arterial hypertension, which may be a promising strategy for patients with emphysema.

A Personalized Spring Network Representation of Emphysematous Lungs From CT Images

Emphysema is a progressive disease characterized by irreversible tissue destruction and airspace enlargement, which manifest as low attenuation area (LAA) on CT images. Previous studies have shown that inflammation, protease imbalance, extracellular matrix remodeling and mechanical forces collectively influence the progression of emphysema. Elastic spring network models incorporating force-based mechanical failure have been applied to investigate the pathogenesis and progression of emphysema. However, these models were general without considering the patient-specific information on lung structure available in CT images. The aim of this work was to develop a novel approach that provides an optimal spring network representation of emphysematous lungs based on the apparent density in CT images, allowing the construction of personalized networks. The proposed method takes into account the size and curvature of LAA clusters on the CT images that correspond to a pre-stressed condition of the lung as opposed to a naïve method that excludes the effects of pre-stress. The main findings of this study are that networks constructed by the new method 1) better preserve LAA cluster sizes and their distribution than the naïve method; and 2) predict different course of emphysema progression compared to the naïve method. We conclude that our new method has the potential to predict patient-specific emphysema progression which needs verification using clinical data.

Cellular modulation by the mechanical cues from biomaterials for tissue engineering

Mechanical cues from the extracellular matrix (ECM) microenvironment are known to be significant in modulating the fate of stem cells to guide developmental processes and maintain bodily homeostasis. Tissue engineering has provided a promising approach to the repair or regeneration of damaged tissues. Scaffolds are fundamental in cell-based regenerative therapies. Developing artificial ECM that mimics the mechanical properties of native ECM would greatly help to guide cell functions and thus promote tissue regeneration. In this review, we introduce various mechanical cues provided by the ECM including elasticity, viscoelasticity, topography, and external stimuli, and their effects on cell behaviours. Meanwhile, we discuss the underlying principles and strategies to develop natural or synthetic biomaterials with different mechanical properties for cellular modulation, and explore the mechanism by which the mechanical cues from biomaterials regulate cell function toward tissue regeneration. We also discuss the challenges in multimodal mechanical modulation of cell behaviours and the interplay between mechanical cues and other microenvironmental factors.

How Do Mechanics Guide Fibroblast Activity? Complex Disruptions during Emphysema Shape Cellular Responses and Limit Research

The emphysema death toll has steadily risen over recent decades, causing the disease to become the third most common cause of death worldwide in 2019. Emphysema is currently incurable and could be due to a genetic condition (Alpha-1 antitrypsin deficiency) or exposure to pollutants/irritants, such as cigarette smoke or poorly ventilated cooking fires. Despite the growing burden of emphysema, the mechanisms behind emphysematous pathogenesis and progression are not fully understood by the scientific literature. A key aspect of emphysematous progression is the destruction of the lung parenchyma extracellular matrix (ECM), causing a drastic shift in the mechanical properties of the lung (known as mechanobiology). The mechanical properties of the lung such as the stiffness of the parenchyma (measured as the elastic modulus) and the stretch forces required for inhalation and exhalation are both reduced in emphysema. Fibroblasts function to maintain the structural and mechanical integrity of the lung parenchyma, yet, in the context of emphysema, these fibroblasts appear incapable of repairing the ECM, allowing emphysema to progress. This relationship between the disturbances in the mechanical cues experienced by an emphysematous lung and fibroblast behaviour is constantly overlooked and consequently understudied, thus warranting further research. Interestingly, the failure of current research models to integrate the altered mechanical environment of an emphysematous lung may be limiting our understanding of emphysematous pathogenesis and progression, potentially disrupting the development of novel treatments. This review will focus on the significance of emphysematous lung mechanobiology to fibroblast activity and current research limitations by examining: (1) the impact of mechanical cues on fibroblast activity and the cell cycle, (2) the potential role of mechanical cues in the diminished activity of emphysematous fibroblasts and, finally, (3) the limitations of current emphysematous lung research models and treatments as a result of the overlooked emphysematous mechanical environment.

Interplay between Extracellular Matrix and Neutrophils in Diseases

The extracellular matrix (ECM) is a highly dynamic and complex network structure, which exists in almost all tissues and is the microenvironment that cells rely on for survival. ECM interacts with cells to regulate diverse functions, including differentiation, proliferation, and migration. Neutrophils are the most abundant immune cells in circulation and play key roles in orchestrating a complex series of events during inflammation. Neutrophils can also mediate ECM remodeling by providing specific matrix-remodeling enzymes (such as neutrophil elastase and metalloproteinases), generating neutrophil extracellular traps, and releasing exosomes. In turn, ECM can remodel the inflammatory microenvironment by regulating the function of neutrophils, which drives disease progression. Both the presence of ECM and the interplay between neutrophils and their extracellular matrices are considered an important and outstanding mechanistic aspect of inflammation. In this review, the importance of ECM will be considered, together with the discussion of recent advances in understanding the underlying mechanisms of the intricate interplay between ECM and neutrophils. A better comprehension of immune cell-matrix reciprocal dependence has exciting implications for the development of new therapeutic options for neutrophil-associated infectious and inflammatory diseases.

New Insights into the Clinical Implications of Yes-Associated Protein in Lung Cancer: Roles in Drug Resistance, Tumor Immunity, Autophagy, and Organoid Development

Simple Summary

Innovative advancements in lung cancer treatment have developed over the past decade with the advent of targeted and immune therapies. Yes-associated protein (YAP), an effector of the Hippo pathway, promotes the resistance of these targeted drugs and modulates tumor immunity in lung cancer. YAP is involved in autophagy in lung cancer and plays a prominent role in forming the tubular structure in lung organoids and alveolar differentiation. In this review, we discuss the central roles of YAP in lung cancer and present YAP as a novel target for treating resistance to targeted therapies and immunotherapies in lung cancer.

Abstract

Despite significant innovations in lung cancer treatment, such as targeted therapy and immunotherapy, lung cancer is still the principal cause of cancer-associated death. Novel strategies to overcome drug resistance and inhibit metastasis in cancer are urgently needed. The Hippo pathway and its effector, Yes-associated protein (YAP), play crucial roles in lung development and alveolar differentiation. YAP is known to mediate mechanotransduction, an important process in lung homeostasis and fibrosis. In lung cancer, YAP promotes metastasis and confers resistance against chemotherapeutic drugs and targeted agents. Recent studies revealed that YAP directly controls the expression of programmed death-ligand 1 (PD-L1) and modulates the tumor microenvironment (TME). YAP not only has a profound relationship with autophagy in lung cancer but also controls alveolar differentiation, and is responsible for tubular structure formation in lung organoids. In this review, we discuss the various roles and clinical implications of YAP in lung cancer and propose that targeting YAP can be a promising strategy for treating lung cancer.

Septal destruction enhances chaotic mixing and increases cellular doses of nanoparticles in emphysematous acinus

One hallmark of emphysema is the breakdown of inter-alveolar septal walls in pulmonary acini. How the acinar dosimetry of environmental aerosols varies at different stages of emphysema remains unclear; this is specifically pertinent to users of tobacco products, which is the leading cause of emphysema. The objective of this study is to systematically assess the impacts of septal destruction on the behavior and fate of nanoparticles (1–800 nm) in a pyramid-shaped sub-acinar model consisting of 496 alveoli. Four diseased geometry variants were created by gradually removing the septal walls from the base model. Particle motions within the acinar region were tracked for particles raging 1–800 nm at four emphysema stages using a well-tested Lagrangian tracking model. Both spatial profile and temporal variation of particle deposition were predicted in healthy and diseased sub-acinar geometries on both a total and regional basis. Results show large differences in airflow and particle dynamics among different emphysema stages. Large differences in particle dynamics are also observed among different particle sizes, with one order of magnitude’s variation in the speeds of particles of 1, 10, and 200 nm. The destruction of septal walls also changed the deposition mechanisms, shifting from connective diffusion to chaotic mixing with emphysema progression. The sub-acinar dosimetry became less sensitive to particle size variation with more septal destructions. The lowest retention rate was found at 200–500 nm in the healthy sub-acinar geometry, but at 800 nm in all emphysematous models considered. The acinus-averaged dose for nanoparticles (1–800 nm) increases with aggravating septal destructions, indicating an even higher risk to the acinus at later emphysema stages.

Biological Evaluation of Azetidine-2-One Derivatives of Ferulic Acid as Promising Anti-Inflammatory Agents

The purpose of this study was to evaluate the in vivo biological potential of new azetidine-2-one derivatives of ferulic acid (6a–f). First, the in vivo acute toxicity of azetidine-2-one derivatives of ferulic acid on Swiss white mice was investigated and, based on the obtained results, it can be stated that the studied derivatives belong to compounds with moderate toxicity. The in vivo anti-inflammatory potential of these derivatives was determined in a model of acute inflammation induced by carrageenan in rats and in a chronic inflammation model induced in rats using the granuloma test. In the acute inflammation model, all the studied compounds had a maximum anti-inflammatory effect 24 h after administration, which suggests that these compounds may be classified, from a pharmacokinetic point of view, in the category of long-acting compounds. The most active compound in the series was found to be compound 6b. In the case of the chronic inflammation model, it was observed that the studied compounds (6a–f) reduced the formation of granulation tissue compared to the control group, having an intense effect of inhibiting the proliferative component. The most important inhibitory effect of inhibiting the proliferative component was recorded for compound 6b. Additionally, the investigation of liver function was performed by determining the serum levels of liver enzymes aspartate transaminase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH) and bilirubin (total and direct). The results showed that, in the series of azetidin-2-one derivatives, the liver enzymes concentration values were close to those recorded for the reference anti-inflammatories (diclofenac sodium and indomethacin) and slightly higher compared to the values for the healthy control group. At the end of the experiment, the animals were euthanized and fragments of liver, lung, and kidney tissue were taken from all groups in the study. These were processed for histopathological examination, and we noticed no major changes in the groups treated with the azetidine 2-one derivatives of ferulic acid compared to the healthy groups.

Spontaneous Breathing Through Increased Airway Resistance Augments Elastase-Induced Pulmonary Emphysema

Introduction

Resistive breathing (RB), the pathophysiologic hallmark of chronic obstructive pulmonary disease (COPD), especially during exacerbations, is associated with significant inflammation and mechanical stress on the lung. Mechanical forces are implicated in the progression of emphysema that is a major pathologic feature of COPD. We hypothesized that resistive breathing exacerbates emphysema.

Methods

C57BL/6 mice were exposed to 0.75 units of pancreatic porcine elastase intratracheally to develop emphysema. Resistive breathing was applied by suturing a nylon band around the trachea to reduce surface area to half for the last 24 or 72 hours of a 21-day time period after elastase treatment in total. Following RB (24 or 72 hours), lung mechanics were measured and bronchoalveolar lavage (BAL) was performed. Emphysema was quantified by the mean linear intercept (Lm) and the destructive index (DI) in lung tissue sections.

Results

Following 21 days of intratracheal elastase exposure, Lm and DI increased in lung tissue sections [Lm (μm), control 39.09±0.76, elastase 62.05±2.19, p=0.003 and DI, ctr 30.95±2.75, elastase 73.12±1.75, p<0.001]. RB for 72 hours further increased Lm by 64% and DI by 19%, compared to elastase alone (p<0.001 and p=0.02, respectively). RB induced BAL neutrophilia in elastase-treated mice. Static compliance (C_st) increased in elastase-treated mice [C_st (mL/cmH2O), control 0.067±0.001, elastase 0.109±0.006, p<0.001], but superimposed RB decreased C_st, compared to elastase alone [C_st (mL/cmH2O), elastase+RB24h 0.090±0.004, p=0.006 to elastase, elastase+RB72h 0.090±0.005, p=0.006 to elastase].

Conclusion

Resistive breathing augments pulmonary inflammation and emphysema in an elastase-induced emphysema mouse model.

Differential lung inflammation and injury with tobacco smoke exposure in Wistar Kyoto and spontaneously hypertensive rats

OBJECTIVE

Chronic obstructive pulmonary disease (COPD) is the third leading cause of death worldwide and has been associated with periods of intense lung inflammation. The objective of this study was to characterize whether similar rat strains, possessing different genetic predispositions, might play a role in exacerbating the pathophysiology of COPD-like cellular and structural changes with progressive 12-week exposure to tobacco smoke (TS). Normotensive Wistar Kyoto (WKY) and spontaneously hypertensive (SH) rats were compared.

MATERIALS AND METHODS

WKY and SH rats were exposed to filtered air or to tobacco smoke at a particulate concentration of 80 mg/m3 for 4, 8, or 12 weeks. Necropsy was performed 24 h after the last exposure to obtain cells by bronchoalveolar lavage for total cell and differential counts. Scoring of lung tissues and immunohistochemical staining for M1 (pro-inflammatory) and M2 (anti-inflammatory) macrophages were performed on paraffin-embedded lung sections.

RESULTS AND DISCUSSION

With progressive exposure, TS-exposed SH rats demonstrated significant airspace enlargement, mucin production, and lung inflammation compared to their FA control and TS-matched WKY rats. Moreover, SH rats also demonstrated increased expression of the M1 marker in alveolar macrophages compared to FA control, as well as the M2 marker compared to controls and TS-exposed WKY rats.

CONCLUSION

The progressive tobacco smoke exposure contributes to persistent lung injury and inflammation that can be significantly enhanced by rat strain susceptibility in the genesis of COPD.

A High-Throughput System for Cyclic Stretching of Precision-Cut Lung Slices During Acute Cigarette Smoke Extract Exposure

Rationale

Precision-cut lung slices (PCLSs) are a valuable tool in studying tissue responses to an acute exposure; however, cyclic stretching may be necessary to recapitulate physiologic, tidal breathing conditions.

Objectives

To develop a multi-well stretcher and characterize the PCLS response following acute exposure to cigarette smoke extract (CSE).

Methods

A 12-well stretching device was designed, built, and calibrated. PCLS were obtained from male Sprague-Dawley rats (N = 10) and assigned to one of three groups: 0% (unstretched), 5% peak-to-peak amplitude (low-stretch), and 5% peak-to-peak amplitude superimposed on 10% static stretch (high-stretch). Lung slices were cyclically stretched for 12 h with or without CSE in the media. Levels of Interleukin-1β (IL-1β), matrix metalloproteinase (MMP)-1 and its tissue inhibitor (TIMP1), and membrane type-MMP (MT1-MMP) were assessed via western blot from tissue homogenate.

Results

The stretcher system produced nearly identical normal Lagrangian strains (E_xx and E_yy, p > 0.999) with negligible shear strain (E_xy < 0.0005) and low intra-well variability 0.127 ± 0.073%. CSE dose response curve was well characterized by a four-parameter logistic model (R² = 0.893), yielding an IC₅₀ value of 0.018 cig/mL. Cyclic stretching for 12 h did not decrease PCLS viability. Two-way ANOVA detected a significant interaction between CSE and stretch pattern for IL-1β (p = 0.017), MMP-1, TIMP1, and MT1-MMP (p < 0.001).

Conclusion

This platform is capable of high-throughput testing of an acute exposure under tightly-regulated, cyclic stretching conditions. We conclude that the acute mechano-inflammatory response to CSE exhibits complex, stretch-dependence in the PCLS.

Autotitrating external positive end-expiratory airway pressure to abolish expiratory flow limitation during tidal breathing in patients with severe COPD: a physiological study

Background

The optimal noninvasive application of external positive end-expiratory pressure (EPAP) to abolish tidal-breathing expiratory flow limitation (EFLT) and minimise intrinsic positive end-expiratory pressure (PEEPi) is challenging in COPD patients. We investigated whether auto-titrating EPAP, using the forced oscillation technique (FOT) to detect and abolish EFLT, would minimise PEEPi, work of breathing and neural respiratory drive (NRD) in patients with severe COPD.

Methods

Patients with COPD with chronic respiratory failure underwent auto-titration of EPAP using a FOT-based algorithm that detected EFLT. Once optimal EPAP was identified, manual titration was performed to assess NRD (using diaphragm and parasternal intercostal muscle electromyography, EMGdi and EMGpara, respectively), transdiaphragmatic inspiratory pressure swings (ΔPdi), transdiaphragmatic pressure–time product (PTPdi) and PEEPi, between EPAP levels 2 cmH2O below to 3 cmH2O above optimal EPAP.

Results

Of 10 patients enrolled (age 65±6 years; male 60%; body mass index 27.6±7.2 kg.m−2; forced expiratory volume in 1 s 28.4±8.3% predicted), eight had EFLT, and optimal EPAP was 9 (range 4–13) cmH2O. NRD was reduced from baseline EPAP at 1 cmH2O below optimal EPAP on EMGdi and at optimal EPAP on EMGpara. In addition, at optimal EPAP, PEEPi (0.80±1.27 cmH2O versus 1.95± 1.70 cmH2O; p<0.05) was reduced compared with baseline. PTPdi (10.3±7.8 cmH2O·s−1versus 16.8±8.8 cmH2O·s−1; p<0.05) and ΔPdi (12.4±7.8 cmH2O versus 18.2±5.1 cmH2O; p<0.05) were reduced at optimal EPAP+1 cmH2O compared with baseline.

Conclusion

Autotitration of EPAP, using a FOT-based algorithm to abolish EFLT, minimises transdiaphragmatic pressure swings and NRD in patients with COPD and chronic respiratory failure.

Transient receptor potential canonical 1 channel mediates the mechanical stress‑induced epithelial‑mesenchymal transition of human bronchial epithelial (16HBE) cells

Airway remodeling is a central event in the pathology of chronic obstructive pulmonary disease (COPD) that leads to airway narrowing and subsequently, to increased mechanical pressure. High mechanical pressure can exacerbate airway remodeling. Thus, a treatment regimen aimed at disrupting this high-pressure airway remodeling vicious cycle may improve the prognosis of patients with COPD. Recent studies have demonstrated that mechanical stress induces lung epithelial-mesenchymal transition (EMT), which is commonly present in airway epithelial cells of patients with COPD. As TRPC1 functions as a mechanosensitive channel that mediates non-selective cation entry in response to increased membrane stretch, the present study investigated the role of TRPC1 in the occurrence of EMT induced by mechanical stress. In the present study, the expression of TRPC1 in the bronchial epithelium was examined in vivo by immunohistochemistry. In vitro, human bronchial epithelial (16HBE) cells were subjected to mechanical stretching for up to 48 h, and TRPC1 expression was then examined by RT-qPCR and western blot analysis. In addition, TRPC1 receptor function was assessed by Ca²⁺ imaging and siRNA transfection. EMT was identified using immunofluorescence, western blot analysis and RT-qPCR. It was found that TRPC1 expression was upregulated in patients with COPD and in 16HBE cells subjected to mechanical stretch. The mechanical stress-induced activation of TRPC1 in 16HBE cells increased the intracellular calcium concentration and subsequently decreased the expression of cytokeratin 8 and E-cadherin, and increased the expression of α-smooth muscle actin, indicating the occurrence of EMT. On the whole, the findings of the present study demonstrate that TRPC1 plays a key role in the occurrence of EMT in human lung epithelial cells in response to mechanical stretch; thus, this protein may serve as a novel therapeutic target for progressive airway remodeling in COPD.

The role of three-dimensionality and alveolar pressure in the distribution and amplification of alveolar stresses

Alveolar stresses are fundamental to enable the respiration process in mammalians and have recently gained increasing attention due to their mechanobiological role in the pathogenesis and development of respiratory diseases. Despite the fundamental physiological role of stresses in the alveolar wall, the determination of alveolar stresses remains challenging, and our current knowledge is largely drawn from 2D studies that idealize the alveolar septal wall as a spring or a planar continuum. Here we study the 3D stress distribution in alveolar walls of normal lungs by combining ex-vivo micro-computed tomography and 3D finite-element analysis. Our results show that alveolar walls are subject to a fully 3D state of stresses rather than to a pure axial stress state. To understand the contributions of the different components and deformation modes, we decompose the stress tensor field into hydrostatic and deviatoric components, which are associated with isotropic and distortional stresses, respectively. Stress concentrations arise in localized regions of the alveolar microstructure, with magnitudes that can be up to 27 times the applied alveolar pressure. Interestingly, we show that the stress amplification factor strongly depends on the level of alveolar pressure, i.e, stresses do not scale proportional to the applied alveolar pressure. In addition, we show that 2D techniques to assess alveolar stresses consistently overestimate the stress magnitude in alveolar walls, particularly for lungs under high transpulmonary pressure. These findings take particular relevance in the study of stress-induced remodeling of the emphysematous lung and in ventilator-induced lung injury, where the relation between transpulmonary pressure and alveolar wall stress is key to understand mechanotransduction processes in pneumocytes.

Role of TRPC1 channels in pressure-mediated activation of airway remodeling

Background

Bronchoconstriction and cough, a characteristic of the asthmatic response, leads to development of compressive stresses in the airway wall. We hypothesized that progressively pathological high mechanical stress could act on mechanosensitive cation channels, such as transient receptor potential channel 1 (TRPC1) and then contributes to airway remodeling.

Methods

We imitate the pathological airway pressure in vitro using cyclic stretch at 10 and 15% elongation. Ca²⁺ imaging was applied to measure the activity of TRPC1 after bronchial epithelial cells exposed to cyclic stretch for 0, 0.5, 1, 1.5, 2, 2.5 h. To further clarify the function of channnel TRPC1 in the process of mechano-transduction in airway remodeling, the experiment in vivo was implemented. The TRPC1 siRNA and budesonide were applied separately to asthmatic models. The morphological changes were measured by HE and Massion method. The expression levels of TRPC1 were evaluated by real-time PCR, western blot and immunohistochemistry. The protein expression level of IL-13, TGF-β₁ and MMP-9 in BALF were measured by ELISA.

Results

The result showed that cyclic stretch for 15% elongation at 1.5 h could maximize the activity of TRPC1 channel. This influx in Ca²⁺ was blocked by TRPC1 siRNA. Higher TRPC1 expression was observed in the bronchial epithelial layer of ovalbumin induced asthmatic models. The knockdown of TRPC1 with TRPC1 siRNA was associated with a hampered airway remodeling process, such as decreased bronchial wall thickness and smooth muscle hypertrophy/hyperplasia, a decreased ECM deposition area and inflammation infiltration around airway wall. Meantime, expression of IL-13, TGF-β₁ and MMP-9 in OVA+TRPC1 siRNA also showed reduced level. TRPC1 intervention treatment showed similar anti-remodeling therapeutic effect with budesonide.

Conclusions

These results demonstrate that most TRPC1 channels expressed in bronchial epithelial cells mediate the mechanotransduction mechanism. TRPC1 inducing abnormal Ca²⁺ signal mediates receptor-stimulated and mechanical stimulus-induced airway remodeling. The inhibition of TRPC1 channel could produce similar therapeutic effect as glucocortisteroid to curb the development of asthmatic airway remodeling.

Constituent-specific material behavior of soft biological tissue: experimental quantification and numerical identification for lung parenchyma

In this study, we present a method to experimentally quantify and numerically identify the constituent-specific material behavior of soft biological tissues. This allows the clear identification of the individual contributions of major load-bearing constituents and their interactions in the constitutive law. While the overall approach is applicable for many tissues, here it will be presented for the identification of a sophisticated constituent-specific material model of viable lung parenchyma. This material model will help to better model the effects of various lung diseases that feature altered fiber content in the lungs, such as emphysema or fibrosis. To experimentally quantify the mechanical properties of collagen, elastin, collagen-elastin-fiber interactions, and ground substance, we examined 18 collagenase and elastase treated rat lung parenchymal slices. The mechanical contributions of the collagen and elastin fibers in the living tissue were inferred from uniaxial tension tests comparing the behavior before and after the selective digestion of the respective fibers. In order to also obtain the mechanical influence of the ground substance, we consecutively treated the samples with both proteases. Collagen and elastin fibers are morphologically interconnected. Thus, a mechanical interaction between these fibers appears likely, but has not yet been experimentally verified. In this paper, we propose an experimental method to quantitatively assess the mechanical behavior of these collagen-elastin-fiber interactions. Based on our experiments, we have identified individual material models within a nonlinear continuum mechanics framework for each load-bearing component via an inverse analysis. The proposed constituent-specific material law can be incorporated into computational models of the respiratory system to simulate and even predict the behavior and alteration of the individual constituents and their effect on the whole respiratory system during normal and artificial breathing, in particular in the case of diseases that alter the fibers in the tissue.

Mesenchymal Stromal Cell Exosomes Ameliorate Experimental Bronchopulmonary Dysplasia and Restore Lung Function through Macrophage Immunomodulation

RATIONALE

Mesenchymal stem/stromal cell (MSC) therapies have shown promise in preclinical models of pathologies relevant to newborn medicine, such as bronchopulmonary dysplasia (BPD). We have reported that the therapeutic capacity of MSCs is comprised in their secretome, and demonstrated that the therapeutic vectors are exosomes produced by MSCs (MSC-exos).

OBJECTIVES

To assess efficacy of MSC-exo treatment in a preclinical model of BPD and to investigate mechanisms underlying MSC-exo therapeutic action.

METHODS

Exosomes were isolated from media conditioned by human MSC cultures. Newborn mice were exposed to hyperoxia (HYRX; 75% O₂), treated with exosomes on Postnatal Day (PN) 4 and returned to room air on PN7. Treated animals and appropriate controls were harvested on PN7, -14, or -42 for assessment of pulmonary parameters.

MEASUREMENTS AND MAIN RESULTS

HYRX-exposed mice presented with pronounced alveolar simplification, fibrosis, and pulmonary vascular remodeling, which was effectively ameliorated by MSC-exo treatment. Pulmonary function tests and assessment of pulmonary hypertension showed functional improvements after MSC-exo treatment. Lung mRNA sequencing demonstrated that MSC-exo treatment induced pleiotropic effects on gene expression associated with HYRX-induced inflammation and immune responses. MSC-exos modulate the macrophage phenotype fulcrum, suppressing the proinflammatory "M1" state and augmenting an antiinflammatory "M2-like" state, both in vitro and in vivo.

CONCLUSIONS

MSC-exo treatment blunts HYRX-associated inflammation and alters the hyperoxic lung transcriptome. This results in alleviation of HYRX-induced BPD, improvement of lung function, decrease in fibrosis and pulmonary vascular remodeling, and amelioration of pulmonary hypertension. The MSC-exo mechanism of action is associated with modulation of lung macrophage phenotype.

CT Imaging-Based Low-Attenuation Super Clusters in Three Dimensions and the Progression of Emphysema

BACKGROUND

Distributions of low-attenuation areas in two-dimensional (2-D) CT lung slices are used to quantify parenchymal destruction in patients with COPD. However, these segmental approaches are limited and may not reflect the true three-dimensional (3-D) tissue processes that drive emphysematous changes in the lung. The goal of this study was to instead evaluate distributions of 3-D low-attenuation volumes, which we hypothesized would follow a power law distribution and provide a more complete assessment of the mechanisms underlying disease progression.

METHODS

CT scans and pulmonary function test results were acquired from an observational database for N = 12 patients with COPD and N = 12 control patients. The data set included baseline and two annual follow-up evaluations in patients with COPD. Three-dimensional representations of the lungs were reconstructed from 2-D axial CT slices, with low-attenuation volumes identified as contiguous voxels < -960 Hounsfield units.

RESULTS

Low-attenuation sizes generally followed a power law distribution, with the exception of large, individual outliers termed "super clusters," which deviated from the expected distribution. Super cluster volume was correlated with disease severity (% total low attenuation, ρ = 0.950) and clinical measures of lung function including FEV₁ (ρ = -0.849) and diffusing capacity of the lung for carbon monoxide Dlco (ρ = -0.874). To interpret these results, we developed a personalized computational model of super cluster emergence. Simulations indicated disease progression was more likely to occur near existing emphysematous regions, giving rise to a biomechanical, force-induced mechanism of super cluster growth.

CONCLUSIONS

Low-attenuation super clusters are defining, quantitative features of parenchymal destruction that dominate disease progression, particularly in advanced COPD.

Variability in Tidal Volume Affects Lung and Cardiovascular Function Differentially in a Rat Model of Experimental Emphysema

In experimental elastase-induced emphysema, mechanical ventilation with variable tidal volumes (V_T) set to 30% coefficient of variation (CV) may result in more homogenous ventilation distribution, but might also impair right heart function. We hypothesized that a different CV setting could improve both lung and cardiovascular function. Therefore, we investigated the effects of different levels of V_T variability on cardiorespiratory function, lung histology, and gene expression of biomarkers associated with inflammation, fibrogenesis, epithelial cell damage, and mechanical cell stress in this emphysema model. Wistar rats (n = 35) received repeated intratracheal instillation of porcine pancreatic elastase to induce emphysema. Seven animals were not ventilated and served as controls (NV). Twenty-eight animals were anesthetized and assigned to mechanical ventilation with a V_T CV of 0% (BASELINE). After data collection, animals (n = 7/group) were randomly allocated to V_T CVs of 0% (VV₀); 15% (VV₁₅); 22.5% (VV_22.5); or 30% (VV₃₀). In all groups, mean V_T was 6 mL/kg and positive end-expiratory pressure was 3 cmH₂O. Respiratory system mechanics and cardiac function (by echocardiography) were assessed continuously for 2 h (END). Lung histology and molecular biology were measured post-mortem. VV_22.5 and VV₃₀ decreased respiratory system elastance, while VV₁₅ had no effect. VV₀, VV₁₅, and VV_22.5, but not VV₃₀, increased pulmonary acceleration time to pulmonary ejection time ratio. VV_22.5 decreased the central moment of the mean linear intercept (D2 of Lm) while increasing the homogeneity index (1/β) compared to NV (77 ± 8 μm vs. 152 ± 45 μm; 0.85 ± 0.06 vs. 0.66 ± 0.13, p < 0.05 for both). Compared to NV, VV₃₀ was associated with higher interleukin-6 expression. Cytokine-induced neutrophil chemoattractant-1 expression was higher in all groups, except VV_22.5, compared to NV. IL-1β expression was lower in VV_22.5 and VV₃₀ compared to VV₀. IL-10 expression was higher in VV_22.5 than NV. Club cell protein 16 expression was higher in VV_22.5 than VV₀. SP-D expression was higher in VV₃₀ than NV, while SP-C was higher in VV₃₀ and VV_22.5 than VV_0. In conclusion, VV_22.5 improved respiratory system elastance and homogeneity of airspace enlargement, mitigated inflammation and epithelial cell damage, while avoiding impairment of right cardiac function in experimental elastase-induced emphysema.

Platelet-derived growth factor receptor-α and Ras-related C3 botulinum toxin substrate-1 regulate mechano-responsiveness of lung fibroblasts

Platelet-derived growth factor (PDGF)-A, which only signals through PDGF-receptor-α (PDGFR-α), is required for secondary alveolar septal formation. Although PDGFR-α distinguishes mesenchymal progenitor cells during the saccular stage, PDGFR-α-expressing alveolar cells persist through adulthood. PDGF-A sustains proliferation, limits apoptosis, and maintains α-smooth muscle actin (α-SMA)-containing alveolar cells, which congregate at the alveolar entry ring at postnatal day (P)12. PDGFR-α-expressing, α-SMA-containing alveolar cells redistribute in the elongating septum, suggesting that they migrate to the alveolar entry rings, where mechanical tension is higher. We hypothesized that PDGFR-α and Ras-related C3 botulinum toxin substrate 1(Rac1) are required for mechanosensitive myofibroblast migration. Spreading of PDGFR-α-deficient lung fibroblasts was insensitive to increased rigidity, and their migration was not reduced by Rac1-guanine exchange factor (GEF)-inhibition. PDGFR-α-expressing fibroblasts migrated toward stiffer regions within two-dimensional substrates by increasing migrational persistence (durotaxis). Using a Förster resonance energy transfer (FRET) biosensor for Rac1-GTP, we observed that PDGFR-α was required for fibroblast Rac1 responsiveness to stiffness within a three-dimensional collagen substrate, which by itself increased Rac1-FRET. Rho-GTPase stabilized, whereas Rac1-GTPase increased the turnover of focal adhesions. Under conditions that increased Rac1-GTP, PDGFR-α signaled through both phosphoinositide-3-kinase (PIK) or Src to engage the Rac1 GEF dedicator of cytokinesis-1 (Dock180) and p21-activated-kinase interacting exchange factor-β (βPIX). In cooperation with collagen fibers, these signaling pathways may guide fibroblasts toward the more rigid alveolar entry ring during secondary septation. Because emphysema and interstitial fibrosis disrupt the parenchymal mechanical continuum, understanding how mechanical factors regulate fibroblast migration could elicit strategies for alveolar repair and regeneration.

Genome-Wide Association Study of the Genetic Determinants of Emphysema Distribution

RATIONALE

Emphysema has considerable variability in the severity and distribution of parenchymal destruction throughout the lungs. Upper lobe-predominant emphysema has emerged as an important predictor of response to lung volume reduction surgery. Yet, aside from alpha-1 antitrypsin deficiency, the genetic determinants of emphysema distribution remain largely unknown.

OBJECTIVES

To identify the genetic influences of emphysema distribution in non-alpha-1 antitrypsin-deficient smokers.

METHODS

A total of 11,532 subjects with complete genotype and computed tomography densitometry data in the COPDGene (Genetic Epidemiology of Chronic Obstructive Pulmonary Disease [COPD]; non-Hispanic white and African American), ECLIPSE (Evaluation of COPD Longitudinally to Identify Predictive Surrogate Endpoints), and GenKOLS (Genetics of Chronic Obstructive Lung Disease) studies were analyzed. Two computed tomography scan emphysema distribution measures (difference between upper-third and lower-third emphysema; ratio of upper-third to lower-third emphysema) were tested for genetic associations in all study subjects. Separate analyses in each study population were followed by a fixed effect metaanalysis. Single-nucleotide polymorphism-, gene-, and pathway-based approaches were used. In silico functional evaluation was also performed.

MEASUREMENTS AND MAIN RESULTS

We identified five loci associated with emphysema distribution at genome-wide significance. These loci included two previously reported associations with COPD susceptibility (4q31 near HHIP and 15q25 near CHRNA5) and three new associations near SOWAHB, TRAPPC9, and KIAA1462. Gene set analysis and in silico functional evaluation revealed pathways and cell types that may potentially contribute to the pathogenesis of emphysema distribution.

CONCLUSIONS

This multicohort genome-wide association study identified new genomic loci associated with differential emphysematous destruction throughout the lungs. These findings may point to new biologic pathways on which to expand diagnostic and therapeutic approaches in chronic obstructive pulmonary disease. Clinical trial registered with www.clinicaltrials.gov (NCT 00608764).

The instructive extracellular matrix of the lung: basic composition and alterations in chronic lung disease

The pulmonary extracellular matrix (ECM) determines the tissue architecture of the lung, and provides mechanical stability and elastic recoil, which are essential for physiological lung function. Biochemical and biomechanical signals initiated by the ECM direct cellular function and differentiation, and thus play a decisive role in lung development, tissue remodelling processes and maintenance of adult homeostasis. Recent proteomic studies have demonstrated that at least 150 different ECM proteins, glycosaminoglycans and modifying enzymes are expressed in the lung, and these assemble into intricate composite biomaterials. These highly insoluble assemblies of interacting ECM proteins and their glycan modifications can act as a solid phase-binding interface for hundreds of secreted proteins, which creates an information-rich signalling template for cell function and differentiation. Dynamic changes within the ECM that occur upon injury or with ageing are associated with several chronic lung diseases. In this review, we summarise the available data about the structure and function of the pulmonary ECM, and highlight changes that occur in idiopathic pulmonary fibrosis (IPF), pulmonary arterial hypertension (PAH), chronic obstructive pulmonary disease (COPD), asthma and lung cancer. We discuss potential mechanisms of ECM remodelling and modification, which we believe are relevant for future diagnosis and treatment of chronic lung disease.

Astragalin Inhibits Allergic Inflammation and Airway Thickening in Ovalbumin-Challenged Mice

Lung inflammation and oxidative stress are the major contributors to the development of obstructive pulmonary diseases. Macrophages are involved in pulmonary inflammation and alveolar damage in emphysema. Astragalin is an anti-inflammatory flavonoid present in persimmon leaves and green tea seeds. This study elucidated that astragalin inhibited inflammatory cell infiltration induced by 20 μM H₂O₂ and blocked airway thickening and alveolar emphysema induced by 20 μg of ovalbumin (OVA) in mice. OVA induced mouse pulmonary MCP-1, and H₂O₂ enhanced the expression of MCP-1/ICAM-1/αv integrin in bronchial airway epithelial BEAS-2B cells. Such induction was inhibited by supplying 10-20 mg/kg of astragalin to OVA-challenged mice and 1-20 μM astragalin to oxidant-stimulated cells. Oral administration of 20 mg/kg of astragalin reduced the induction of F4/80/CD68/CD11b in airways of mice challenged with OVA. Additionally, emphysema tissue damage was observed in OVA-exposed alveoli. Mast cell recruitment in the airway subepithelium was blocked by supplementing astragalin to OVA-challenged mice. Orally treating 20 mg/kg of astragalin reduced α-SMA induction in inflammation-occurring airways and appeared to reverse airway thickening and constriction induced by an OVA episode. These results revealed that astragalin may improve airway thickening and alveolar destruction with blockade of allergic inflammation in airways. Therefore, astragalin may be a therapeutic agent antagonizing asthma and obstructive pulmonary diseases.

Biophysical Regulation of Cell Behavior-Cross Talk between Substrate Stiffness and Nanotopography

The stiffness and nanotopographical characteristics of the extracellular matrix (ECM) influence numerous developmental, physiological, and pathological processes in vivo. These biophysical cues have therefore been applied to modulate almost all aspects of cell behavior, from cell adhesion and spreading to proliferation and differentiation. Delineation of the biophysical modulation of cell behavior is critical to the rational design of new biomaterials, implants, and medical devices. The effects of stiffness and topographical cues on cell behavior have previously been reviewed, respectively; however, the interwoven effects of stiffness and nanotopographical cues on cell behavior have not been well described, despite similarities in phenotypic manifestations. Herein, we first review the effects of substrate stiffness and nanotopography on cell behavior, and then focus on intracellular transmission of the biophysical signals from integrins to nucleus. Attempts are made to connect extracellular regulation of cell behavior with the biophysical cues. We then discuss the challenges in dissecting the biophysical regulation of cell behavior and in translating the mechanistic understanding of these cues to tissue engineering and regenerative medicine.

Effects of transient receptor potential canonical 1 (TRPC1) on the mechanical stretch-induced expression of airway remodeling-associated factors in human bronchial epithelioid cells

Research has shown that mechanical stress stimulation can cause airway remodeling. We investigate the effects of mechanical stretch on the expression of the airway remodeling-associated factors interleukin-13 (IL-13) and matrix metalloprotein-9 (MMP-9) and signaling pathways in human bronchial epithelioid (16HBE) cells under mechanical stretch. A Flexcell FX-4000 Tension System with a flexible substrate was applied to stretch 16HBE cells at a 15% elongation amplitude and 1Hz frequency, with stretching for 0.5h, 1h, 1.5h and 2h. The experimental group with higher IL-13, MMP-9, and TRPC1 expression and higher Ca²⁺ levels was selected for performing intervention experiment. These cells were pretreated with the transient receptor potential canonical 1 (TRPC1) channel antagonist SKF96365 and TRPC1-specific siRNA, and then mechanical stretch was applied. Our results provided evidences that mechanical pressure significantly increased IL-13, MMP-9, and TRPC1 protein and mRNA expression levels and intracellular Ca²⁺ fluorescence intensity at 4 time points compared with the control group. The peak IL-13, MMP-9, and TRPC1 expression levels were observed at 0.5h after exposure to mechanical pressure. IL-13 and MMP-9 expression levels and Ca²⁺ fluorescence intensity in the stretch+SKF96365 group and in the stretch+TRPC1 siRNA group were significantly lower than those were in the mechanical stretch group. By incubating the cells with the intracellular calcium chelator BAPTA-AM, the expression of IL-13 and MMP9 was significantly decreased, and the expression level of TRPC1 remained unchanged. These observations suggest that mechanical stretch may induce an influx of Ca²⁺ and up-regulation of IL-13 and MMP-9 expression in 16HBE cells via activation of TRPC1.

Probing Cellular and Molecular Mechanisms of Cigarette Smoke-Induced Immune Response in the Progression of Chronic Obstructive Pulmonary Disease Using Multiscale Network Modeling

Chronic obstructive pulmonary disease (COPD) is a chronic inflammatory disorder characterized by progressive destruction of lung tissues and airway obstruction. COPD is currently the third leading cause of death worldwide and there is no curative treatment available so far. Cigarette smoke (CS) is the major risk factor for COPD. Yet, only a relatively small percentage of smokers develop the disease, showing that disease susceptibility varies significantly among smokers. As smoking cessation can prevent the disease in some smokers, quitting smoking cannot halt the progression of COPD in others. Despite extensive research efforts, cellular and molecular mechanisms of COPD remain elusive. In particular, the disease susceptibility and smoking cessation effects are poorly understood. To address these issues in this work, we develop a multiscale network model that consists of nodes, which represent molecular mediators, immune cells and lung tissues, and edges describing the interactions between the nodes. Our model study identifies several positive feedback loops and network elements playing a determinant role in the CS-induced immune response and COPD progression. The results are in agreement with clinic and laboratory measurements, offering novel insight into the cellular and molecular mechanisms of COPD. The study in this work also provides a rationale for targeted therapy and personalized medicine for the disease in future.

Comparison between Variable and Conventional Volume-Controlled Ventilation on Cardiorespiratory Parameters in Experimental Emphysema

Emphysema is characterized by loss of lung tissue elasticity and destruction of structures supporting alveoli and capillaries. The impact of mechanical ventilation strategies on ventilator-induced lung injury (VILI) in emphysema is poorly defined. New ventilator strategies should be developed to minimize VILI in emphysema. The present study was divided into two protocols: (1) characterization of an elastase-induced emphysema model in rats and identification of the time point of greatest cardiorespiratory impairment, defined as a high specific lung elastance associated with large right ventricular end-diastolic area; and (2) comparison between variable (VV) and conventional volume-controlled ventilation (VCV) on lung mechanics and morphometry, biological markers, and cardiac function at that time point. In the first protocol, Wistar rats (n = 62) received saline (SAL) or porcine pancreatic elastase (ELA) intratracheally once weekly for 4 weeks, respectively. Evaluations were performed 1, 3, 5, or 8 weeks after the last intratracheal instillation of saline or elastase. After identifying the time point of greatest cardiorespiratory impairment, an additional 32 Wistar rats were randomized into the SAL and ELA groups and then ventilated with VV or VCV (n = 8/group) [tidal volume (V_T) = 6 mL/kg, positive end-expiratory pressure (PEEP) = 3 cmH₂O, fraction of inspired oxygen (FiO₂) = 0.4] for 2 h. VV was applied on a breath-to-breath basis as a sequence of randomly generated V_T values (mean V_T = 6 mL/kg), with a 30% coefficient of variation. Non-ventilated (NV) SAL and ELA animals were used for molecular biology analysis. The time point of greatest cardiorespiratory impairment, was observed 5 weeks after the last elastase instillation. At this time point, interleukin (IL)-6, cytokine-induced neutrophil chemoattractant (CINC)-1, amphiregulin, angiopoietin (Ang)-2, and vascular endothelial growth factor (VEGF) mRNA levels were higher in ELA compared to SAL. In ELA animals, VV reduced respiratory system elastance, alveolar collapse, and hyperinflation compared to VCV, without significant differences in gas exchange, but increased right ventricular diastolic area. Interleukin-6 mRNA expression was higher in VCV and VV than NV, while surfactant protein-D was increased in VV compared to NV. In conclusion, VV improved lung function and morphology and reduced VILI, but impaired right cardiac function in this model of elastase induced-emphysema.

Computed Tomography Image Matching in Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD), characterized by progressive airflow obstruction due to the combined effects of emphysema and small airways disease, is associated with high morbidity and mortality. The complex link between emphysema and airways disease is associated with significant heterogeneity in clinical presentation. Spirometry is the current gold standard for diagnosis and stratification of the severity of airflow obstruction in COPD. Although spirometry is simple to use, it does not enable the separation of emphysema from airways disease. Computed tomography (CT), on the other hand, provides the anatomic localization of disease and has been increasingly used to phenotype COPD. The majority of current CT measures are extracted from a single-volume CT scan and although useful to characterize emphysema and airways disease, they do not link structural and functional abnormalities. Alternatively, CT image matching combines information from both inspiratory and expiratory CT scans, thus enabling determination of functional changes such as regional ventilation and mechanical properties of the lung. In this review, we discuss recent applications of CT image matching that provide clinically meaningful information beyond spirometry and single-volume CT scan measures.

Matrix, mesenchyme, and mechanotransduction

The extracellular matrix (ECM) of the lung serves as both a scaffold for resident cells and a mechanical support for respiratory function. The ECM is deposited during development and undergoes continuous turnover and maintenance during organ growth and homeostasis. Cells of the mesenchyme, including the tissue resident fibroblast, take a leading role in depositing and organizing the matrix and do so in an anatomically distinct fashion, with differing composition, organization, and mechanical properties within the airways, vessels, and alveoli of the lung. Recent technological advancements have allowed the lung's ECM biochemical composition and mechanical properties to be studied with improved resolution, thereby identifying novel disease-related changes in ECM characteristics. In parallel, efforts to study cells seeded on normal and disease-derived matrices have illustrated the powerful role the ECM can play in altering key functions of lung resident cells. The mechanical properties of the matrix have been identified as an important modifier of cell-matrix adhesions, with matrices of pathologic stiffness promoting profibrotic signaling and cell function. Ongoing work is identifying both mechanically activated pathways in mesenchymal cells and disease-related ECM molecules that biochemically regulate cell function. Uncovering the control systems by which cells respond to and regulate the matrix, and the failures in these systems that underlie aberrant repair, remains a major challenge. Progress in this area will be an essential element in efforts to engineer functional lung tissue for regenerative approaches and will be key to identifying new therapeutic strategies for lung diseases characterized by disturbed matrix architecture.

Airway and Extracellular Matrix Mechanics in COPD

Chronic obstructive pulmonary disease (COPD) is one of the most common lung diseases worldwide, and is characterized by airflow obstruction that is not fully reversible with treatment. Even though airflow obstruction is caused by airway smooth muscle contraction, the extent of airway narrowing depends on a range of other structural and functional determinants that impact on active and passive tissue mechanics. Cells and extracellular matrix in the airway and parenchymal compartments respond both passively and actively to the mechanical stimulation induced by smooth muscle contraction. In this review, we summarize the factors that regulate airway narrowing and provide insight into the relative contributions of different constituents of the extracellular matrix and their biomechanical impact on airway obstruction. We then review the changes in extracellular matrix composition in the airway and parenchymal compartments at different stages of COPD, and finally discuss how these changes impact airway narrowing and the development of airway hyperresponsiveness. Finally, we position these data in the context of therapeutic research focused on defective tissue repair. As a conclusion, we propose that future works should primarily target mild or early COPD, prior to the widespread structural changes in the alveolar compartment that are more characteristic of severe COPD.

The key role of exudative lesions and their encapsulation: lessons learned from the pathology of human pulmonary tuberculosis

A review of the pathology of human pulmonary TB cases at different stages of evolution in the pre-antibiotic era suggests that neutrophils play an instrumental role in the progression toward active TB. This progression is determined by the type of lesion generated. Thus, exudative lesions, in which neutrophils are the major cell type, are both triggered by and induce local high bacillary load, and tend to enlarge and progress toward liquefaction and cavitation. In contrast, proliferative lesions are triggered by low bacillary loads, mainly comprise epithelioid cells and fibroblasts and tend to fibrose, encapsulate and calcify, thus controlling the infection. Infection of the upper lobes is key to the progression toward active TB for two main reasons, namely poor breathing amplitude, which allows local bacillary accumulation, and the high mechanical stress to which the interlobular septae (which enclose secondary lobes) are submitted, which hampers their ability to encapsulate lesions. Overall, progressing factors can be defined as internal (exudative lesion, local bronchogenous dissemination, coalescence of lesions), with lympho-hematological dissemination playing a very limited role, or external (exogenous reinfection). Abrogating factors include control of the bacillary load and the local encapsulation process, as directed by interlobular septae. The age and extent of disease depend on the quality and speed with which lesions liquefy and disseminate bronchially, the volume of the slough, and the amount and distribution of the sloughing debris dispersed.

Localization and stretch-dependence of lung elastase activity in development and compensatory growth

Synthesis and remodeling of the lung matrix is necessary for primary and compensatory lung growth. Because cyclic negative force is applied to developing lung tissue during the respiratory cycle, we hypothesized that stretch is a critical regulator of lung matrix remodeling. By using quantitative image analysis of whole-lung and whole-lobe elastin in situ zymography images, we demonstrated that elastase activity increased twofold during the alveolar stage of postnatal lung morphogenesis in the mouse. Remodeling was restricted to alveolar walls and ducts and was nearly absent in dense elastin band structures. In the mouse pneumonectomy model of compensatory lung growth, elastase activity increased threefold, peaking at 14 days postpneumonectomy and was higher in the accessory lobe compared with other lobes. Remodeling during normal development and during compensatory lung growth was different with increased major airway and pulmonary arterial remodeling during development but not regeneration, and with homogenous remodeling throughout the parenchyma during development, but increased remodeling only in subpleural regions during compensatory lung growth. Left lung wax plombage prevented increased lung elastin during compensatory lung growth. To test whether the adult lung retains an innate capacity to remodel elastin, we developed a confocal microscope-compatible stretching device. In ex vivo adult mouse lung sections, lung elastase activity increased exponentially with strain and in peripheral regions of lung more than in central regions. Our study demonstrates that lung elastase activity is stretch-dependent and supports a model in which externally applied forces influence the composition, structure, and function of the matrix during periods of alveolar septation.