Changes in viscoelastic properties of rat lung parenchymal strips with maturation

Proteomic analysis reveals the aging-related pathways contribute to pulmonary fibrogenesis

Aging usually causes lung-function decline and susceptibility to chronic lung diseases, such as pulmonary fibrosis. However, how aging affects the lung-fibrosis pathways and leads to the occurrence of pulmonary fibrosis is not completely understood. Here, mass spectrometry-based proteomics was used to chart the lung proteome of young and old mice. Micro computed tomography imaging, RNA immunoprecipitation, dual-fluorescence mRFP-GFP-LC3 adenovirus monitoring, transmission electron microscopy, and other experiments were performed to explore the screened differentially expressed proteins related to abnormal ferroptosis, autophagy, mitochondria, and mechanical force in vivo, in vitro, and in healthy people. Combined with our previous studies on pulmonary fibrosis, we further demonstrated that these biological processes and underlying molecular players were also involved in the aging process. Our work depicted a comprehensive cellular and molecular atlas of the aging lung and attempted to explain why aging is a risk factor for pulmonary fibrosis and the role that aging plays in the progression of pulmonary fibrosis. The abnormalities of aging triggered an increase in mechanical force and ferroptosis, autophagy blockade, and mitochondrial dysfunction, which often appear during pulmonary fibrogenesis. We hope that the elucidation of these anomalies will help to enhance our understanding of senescence-inducing pulmonary fibrosis, thereby guiding the use of anti-senescence as an entry point for early intervention in pulmonary fibrosis and age-related diseases.

Mineralocorticoid effects in the late gestation ovine fetal lung

This study was designed to determine the effects of corticosteroids at MR in the late‐gestation fetal lung. Since both the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) are expressed at relatively high levels in the fetal lung, endogenous corticosteroids may act at MR as well as GR in the preterm fetal lung. The GR agonist, betamethasone, the MR agonist, aldosterone, or both were infused intravenously for 48 h in ovine fetuses of approximately 130 days gestation. Effects on airway pressures during stepwise inflation of the in situ lung, expression of ENaC alpha (SCNN1A), ENaC beta (SCNN1B), and Na,K ATPase (ATP1A1), and elastin and collagen content were determined after the infusions. We found that aldosterone significantly reduced the airway pressure measured during the initial step in inflation of the lung, although aldosterone had no overall effect on lung compliance, nor did aldosterone induce expression of ENaCα, ENaCβ or Na,K ATPaseα1. Betamethasone significantly increased expression of the epithelial sodium channel (ENaC) subunit mRNAs, and collagen and elastin content in the lungs, although this dose of betamethasone also had no effect on lung compliance. There was no synergy between effects of the MR and GR agonists. Transcriptomic analysis suggested that although aldosterone did not alter genes in pathways related to epithelial sodium transport, aldosterone did alter genes in pathways involved in cell proliferation in the lungs. The results are consistent with corticosteroid‐induced fluid reabsorption at birth through GR rather than MR, but suggest that MR facilitates lung maturation, and may contribute to inflation with the first breaths via mechanisms distinct from known aldosterone effects in other epithelia.

Infusion of the mineralocorticoid receptor agonist, aldosterone, to the ovine fetus resulted in reduced airway pressures with initial lung inflation. However, aldosterone did not alter lung surfactant or epithelial sodium transport genes which are classical MR gene targets. Transcriptomic analysis revealed an aldosterone effect on genes related to cell cycle, suggesting that MR have a role distinct form that of GR in the maturing lung.

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.

Lung parenchymal mechanics in health and disease

The mechanical properties of lung tissue are important determinants of lung physiological functions. The connective tissue is composed mainly of cells and extracellular matrix, where collagen and elastic fibers are the main determinants of lung tissue mechanical properties. These fibers have essentially different elastic properties, form a continuous network along the lungs, and are responsible for passive expiration. In the last decade, many studies analyzed the relationship between tissue composition, microstructure, and macrophysiology, showing that the lung physiological behavior reflects both the mechanical properties of tissue individual components and its complex structural organization. Different lung pathologies such as acute respiratory distress syndrome, fibrosis, inflammation, and emphysema can affect the extracellular matrix. This review focuses on the mechanical properties of lung tissue and how the stress-bearing elements of lung parenchyma can influence its behavior.

Effects of leptin deficiency on postnatal lung development in mice

Leptin modulates energy metabolism and lung development. We hypothesize that the effects of leptin on postnatal lung development are volume dependent from 2 to 10 wk of age and are independent of hypometabolism associated with leptin deficiency. To test the hypotheses, effects of leptin deficiency on lung maturation were characterized in age groups of C57BL/6J mice with varying Lep(ob) genotypes. Quasi-static pressure-volume curves and respiratory impedance measurements were performed to profile differences in respiratory system mechanics. Morphometric analysis was conducted to estimate alveolar size and number. Oxygen consumption was measured to assess metabolic rate. Lung volume at 40-cmH(2)O airway pressure (V(40)) increased with age in each genotypic group, and V(40) was significantly (P < 0.05) lower in leptin-deficient (ob/ob) mice beginning at 2 wk. Differences were amplified through 7 wk of age relative to wild-type (+/+) mice. Morphometric analysis showed that alveolar surface area was lower in ob/ob compared with +/+ and heterozygote (ob/+) mice beginning at 2 wk. Unlike the other genotypic groups, alveolar size did not increase with age in ob/ob mice. In another experiment, ob/ob at 4 wk received leptin replacement (5 microg.g(-1) x day(-1)) for 8 days, and expression levels of the Col1a1, Col3a1, Col6a3, Mmp2, Tieg1, and Stat1 genes were significantly increased concomitantly with elevated V(40). Leptin-induced increases in V(40) corresponded with enlarged alveolar size and surface area. Gene expression suggested a remodeling event of lung parenchyma after exogenous leptin replacement. These data support the hypothesis that leptin is critical to postnatal lung remodeling, particularly related to increased V(40) and enlarged alveolar surface area.

Extracellular matrix mechanics in lung parenchymal diseases

In this review, we examine how the extracellular matrix (ECM) of the lung contributes to the overall mechanical properties of the parenchyma, and how these properties change in disease. The connective tissues of the lung are composed of cells and ECM, which includes a variety of biological macromolecules and water. The macromolecules that are most important in determining the mechanical properties of the ECM are collagen, elastin, and proteoglycans. We first discuss the various components of the ECM and how their architectural organization gives rise to the mechanical properties of the parenchyma. Next, we examine how mechanical forces can affect the physiological functioning of the lung parenchyma. Collagen plays an especially important role in determining the homeostasis and cellular responses to injury because it is the most important load-bearing component of the parenchyma. We then demonstrate how the concept of percolation can be used to link microscopic pathologic alterations in the parenchyma to clinically measurable lung function during the progression of emphysema and fibrosis. Finally, we speculate about the possibility of using targeted tissue engineering to optimize treatment of these two major lung diseases.

Comparison of postnatal lung growth and development between C3H/HeJ and C57BL/6J mice

Previous work by our group has demonstrated substantial differences in lung volume and morphometric parameters between inbred mice. Specifically, adult C3H/HeJ (C3) have a 50% larger lung volume and 30% greater mean linear intercept than C57BL/6J (B6) mice. Although much of lung development occurs postnatally in rodents, it is uncertain at what age the differences between these strains become manifest. In this study, we performed quasi-static pressure-volume curves and morphometric analysis on neonatal mice. Lungs from anesthetized mice were degassed in vivo using absorption of 100% O2. Pressure-volume curves were then recorded in situ. The lungs were then fixed by instillation of Zenker's solution at a constant transpulmonary pressure. The left lung from each animal was used for morphometric determination of mean air space chord length (Lma). We found that the lung volume of C3 mice was substantially greater than that of B6 mice at all ages. In contrast, there was no difference in Lma (62.7 microm in C3 and 58.5 microm in B6) of 3-day-old mice. With increasing age (8 days), there was a progressive decrease in the Lma of both strains, with the magnitude of the decrease in B6 Lma mice exceeding that of C3. C3 lung volume remained 50% larger. The combination of parenchymal architectural similarity with lung air volume differences and different rates of alveolar septation support the hypothesis that lung volume and alveolar dimensions are independently regulated.

Biomechanics of the lung parenchyma: critical roles of collagen and mechanical forces

The biomechanical properties of connective tissues play fundamental roles in how mechanical interactions of the body with its environment produce physical forces at the cellular level. It is now recognized that mechanical interactions between cells and the extracellular matrix (ECM) have major regulatory effects on cellular physiology and cell-cycle kinetics that can lead to the reorganization and remodeling of the ECM. The connective tissues are composed of cells and the ECM, which includes water and a variety of biological macromolecules. The macromolecules that are most important in determining the mechanical properties of these tissues are collagen, elastin, and proteoglycans. Among these macromolecules, the most abundant and perhaps most critical for structural integrity is collagen. In this review, we examine how mechanical forces affect the physiological functioning of the lung parenchyma, with special emphasis on the role of collagen. First, we overview the composition of the connective tissue of the lung and their complex structural organization. We then describe how mechanical properties of the parenchyma arise from its composition as well as from the architectural organization of the connective tissue. We argue that, because collagen is the most important load-bearing component of the parenchymal connective tissue, it is also critical in determining the homeostasis and cellular responses to injury. Finally, we overview the interactions between the parenchymal collagen network and cellular remodeling and speculate how mechanotransduction might contribute to disease propagation and the development of small- and large-scale heterogeneities with implications to impaired lung function in emphysema.

Effects of Cold Preservation on the Lung Mechanical Properties in Rats

In lung transplantation, cold preservation is an important process. However, the mechanical changes in the airway and tissue during cold preservation, especially before reperfusion, are unknown. To test the hypothesis that the mechanical changes in the airway and lung parenchyma start during cold preservation, we investigated the mechanical properties of the rat lung as a whole organ and in excised lung strips. In the 0 h group, the lungs were not preserved. In the 9 and 24 h group, the lungs were preserved for 9 and 24 h at 4 degrees C. After preservation, we evaluated the static compliance (Csta) of the whole lung as obtained from the pressure volume curves (n=5 in each group). Also, we measured the input impedance taken by a computer-controlled small-animal ventilator (n=9 in each group). All data were analyzed using a homogeneous linear model, which includes airway resistance (Raw), tissue elastance (H), and tissue resistance (G). Hysteresivity (eta) was calculated as G/H. Moreover, the tissue elasticity (Eqs) obtained from the quasi-static stress-strain curves was compared. There was no significant difference in Csta among the three groups. Raw was significantly lower in the 24 h group than in the 0 h group (p<0.01). Eqs was significantly higher in the preserved groups than in the 0 h group (p<0.01). These results demonstrated that the changes in the three mechanical properties of the airway and the tissue started within 9 h of preservation.

Developmental changes in airway and tissue mechanics in mice

Most studies using mice to model human lung diseases are carried out in adults, although there is emerging interest in the effects of allergen, bacterial, and viral exposure early in life. This study aims to characterize lung function in BALB/c mice from infancy (2 wk) through to adulthood (8 wk). The low-frequency forced oscillation technique was used to obtain impedance data, partitioned into components representing airway resistance, tissue damping, tissue elastance, and hysteresivity (tissue damping/tissue elastance). Measurements were made at end-expiratory pause (transrespiratory system pressure = 2 cmH2O) and during relaxed slow expiration from 20 to 0 cmH2O. Airway resistance decreased with age from 0.63 cmH2O x ml(-1) x s at 2 wk to 0.24 cmH2O x ml(-1) x s at 8 wk (P < 0.001). Both tissue damping and tissue elastance decreased with age (P < 0.001) from 2 to 5 wk, then plateaued through to 8 wk (P < 0.001). This pattern was seen both in measurements taken at end-expiratory pause and during expiration. There were no age-related changes seen in hysteresivity when measured at end-expiratory pause, but the pattern of volume dependence did differ with the age of the mice. These changes in respiratory mechanics parallel the reported structural changes of the murine lung from the postnatal period into adulthood.

Lung Development and Susceptibility to Ventilator-induced Lung Injury

RATIONALE

Ventilator-induced lung injury has been predominantly studied in adults.

OBJECTIVES

To explore the effects of age and lung development on susceptibility to such injury.

METHODS

Ex vivo isolated nonperfused rat lungs (infant, juvenile, and adult) were mechanically ventilated where VT was based on milliliters per kilogram of body weight or as a percentage of the measured total lung capacity (TLC). In vivo anesthetized rats (infant, adult) were mechanically ventilated with pressure-limited VTs. Allocation to ventilation strategy was randomized.

MEASUREMENTS

Ex vivo injury was assessed by pressure-volume analysis, reduction in TLC, and histology, and in vivo injury by lung compliance, cytokine production, and wet- to dry-weight ratio.

MAIN RESULTS

Ex vivo ventilation (VT 30 ml.kg(-1)) resulted in a significant reduction (36.0 +/- 10.1%, p < 0.05) in TLC in adult but not in infant lungs. Ex vivo ventilation (VT 50% TLC) resulted in a significant reduction in TLC in both adult (27.8 +/- 2.8%) and infant (10.6 +/- 7.0%) lungs, but more so in the adult lungs (p < 0.05); these changes were paralleled by histology and pressure-volume characteristics. After high stretch in vivo ventilation, adult but not infant rats developed lung injury (total lung compliance, wet/dry ratio, tumor necrosis factor alpha). Surface video microscopy demonstrated greater heterogeneity of alveolar distension in ex vivo adult versus infant lungs.

CONCLUSION

These data provide ex vivo and in vivo evidence that comparable ventilator settings are significantly more injurious in the adult than infant rat lung, probably reflecting differences in intrinsic susceptibility or inflation pattern.

Mechanics, nonlinearity, and failure strength of lung tissue in a mouse model of emphysema: possible role of collagen remodeling

Enlargement of the respiratory air spaces is associated with the breakdown and reorganization of the connective tissue fiber network during the development of pulmonary emphysema. In this study, a mouse (C57BL/6) model of emphysema was developed by direct instillation of 1.2 IU of porcine pancreatic elastase (PPE) and compared with control mice treated with saline. The PPE treatment caused 95% alveolar enlargement ( P = 0.001) associated with a 29% lower elastance along the quasi-static pressure-volume curves ( P < 0.001). Respiratory mechanics were measured at several positive end-expiratory pressures in the closed-chest condition. The dynamic tissue elastance was 19% lower ( P < 0.001), hysteresivity was 9% higher ( P < 0.05), and harmonic distortion, a measure of collagen-related dynamic nonlinearity, was 33% higher in the PPE-treated group ( P < 0.001). Whole lung hydroxyproline content, which represents the total collagen content, was 48% higher ( P < 0.01), and α-elastin content was 13% lower ( P = 0.16) in the PPE-treated group. There was no significant difference in airway resistance ( P = 0.7). The failure stress at which isolated parenchymal tissues break during stretching was 40% lower in the PPE-treated mice ( P = 0.002). These findings suggest that, after elastolytic injury, abnormal collagen remodeling may play a significant role in all aspects of lung functional changes and mechanical forces, leading to progressive emphysema.

Maturational changes in extracellular matrix and lung tissue mechanics

The viscoelastic properties of the pulmonary parenchyma change rapidly postparturition. We compared changes in mechanical properties with changes in tissue composition of rat lung parenchymal strips in three groups of Sprague-Dawley rats: baby (B; 10-14 days), young (Y; approximately 3 wk), and adult (A; approximately 8 wk). Strips were suspended in an organ bath, and resistance (R), elastance (E), and hysteresivity (eta) were calculated during sinusoidal oscillations before and after the addition of acetylcholine (ACh) (10(-3) M). Strips were then fixed in formalin, and sections were stained with hematoxylin and eosin, Verhoff's elastic stain, or Van Gieson's picric acid-fuchsin stain for collagen. The volume proportion of collagen (%Col), the length density of elastic fibers (L(V)/Pr(alv)), and the arithmetic mean thickness of alveolar septae (T(a)) were calculated by morphometry. Tissue was also stained for alpha-smooth muscle actin (ASMA), and the volume proportion of ASMA (%ASMA) was calculated. Hyaluronic acid (HA) was quantitated by radioimmunoassay in separate strips. R and E in B strips were significantly higher, whereas eta was significantly smaller than in Y or A strips. Changes in these parameters with ACh were greater in B strips. T(a), %ASMA, and HA were greatest in B strips, whereas %Col and L(V)/Pr(alv) were least. There were significant positive correlations between R and E vs. T(a) and between percent change in R and eta post-ACh vs. T(a) and vs. %ASMA, and significant negative correlations between R and E vs. %Col and vs. L(V)/Pr(alv) and percent increase in all three mechanical parameters post-ACh vs. %Col. These data suggest that the relatively high stiffness, R, and contractile responsiveness of parenchymal tissues observed in newborns are not directly attributable to the amount of collagen and elastic fibers in the tissue, but rather they are related to the thickened alveolar wall and the relatively greater percent of contractile cells.

Respiratory mechanics and lung development in the rat from early age to adulthood

The purpose of the present study was to establish how the dependence of respiratory mechanics on lung inflation changes during development. We studied seven groups of rats from 10 days to 3 mo of age at five levels of positive end-expiratory pressure (PEEP) from 0 to 7 hPa (1 hPa = 0.1 kPa approximately 1 cmH(2)O). At each PEEP level, we measured respiratory system resistance and elastance at both 0.9 and 4.8 Hz to partition the mechanical properties into its airway and tissue components. Elastance increased more rapidly with PEEP in the younger animals, which we interpret as reflecting a more pronounced strain stiffening of the younger parenchyma. However, the decrease in airway resistance with PEEP was more pronounced in the older animals. Morphometric analysis showed that mean tissue density decreased and total alveolar surface area increased with age. Our data suggest that the mechanical interdependence between airways and parenchyma is weaker in very young animals compared with mature animals. This may play a role in the hyperresponsiveness of immaturity.

Effects of gestation and antenatal steroid on airway and tissue mechanics in newborn lambs

The aim of this study was to partition airway and parenchymal mechanics in newborn lambs at different gestations and following variable exposure to antenatal maternal betamethasone using the forced oscillation technique (FOT). Pulmonary impedance data were collected in 37 sedated and intubated apneic lambs with the FOT between 0.5 and 20 Hz and fitted by a model to estimate airway resistance (Raw) and inertance (Iaw) and the coefficients of tissue resistance (GL) and elastance (HL). Total respiratory resistance (Rrs) was also determined during tidal ventilation by using the multiple linear regression technique. Advancing gestation or increasing antenatal steroid exposure had no clinically significant effect on the values of Raw and Iaw, whereas Rrs and both GL and HL decreased markedly. There was a decrease in tissue hysteresivity (GL/HL) with repeated antenatal steroid exposure. Partitioning of lung mechanics highlights the dominant contribution of the tissues to the total respiratory resistance in the immature ovine lung. Clinically relevant changes in lung mechanics associated with structural and functional maturation of the immature ovine lung are primarily confined to the tissue compartment.

Changes in extracellular matrix and tissue viscoelasticity in bleomycin-induced lung fibrosis. Temporal aspects

Bleomycin-induced lung fibrosis results in changes in tissue mechanical properties due to alterations in the extracellular matrix (ECM). How oscillatory mechanics and changes in the matrix evolve over time has not been addressed. Sprague-Dawley rats were instilled with bleomycin sulfate (BM) (1.5 U) intratracheally; control animals (C) received saline. At 7, 14, and 28 d after BM, parenchymal strips (7 x 2 x 2 mm) were obtained and strips suspended in a Krebs-filled organ bath. One end of the strip was attached to a force (F) transducer and the other to a lever arm that effected sinusoidal length (L) oscillations. Strips were oscillated at varying amplitudes (1, 3, and 10% of resting L) and frequencies (f = 0.3, 1, 3, and 10 Hz) at an operating stress of 2 kPa. Resistance (R) and elastance (E) were estimated by fitting changes in F and L to the equation of motion. Hysteresivity (eta) was calculated as eta = (R/E) 2pif. Strips were then fixed for morphological study of collagen, elastic fibers, and the small proteoglycans (PGs), biglycan and fibromodulin (FM). R and E were significantly greater and eta significantly less in BM versus C strips (p < 0.001). The increase in R and E peaked at 14 d after BM; the decrement in eta was maximal at Day 7. Biglycan was increased in BM lung strips at all time points, FM and elastic fibers were increased at 14 and 28 d, and collagen was increased at 28 d only. Hence, changes in mechanics were maximal before collagen content had increased. In addition, we demonstrated a significant correlation between biglycan and all mechanical parameters. These data suggest that changes in PGs may be critical in determining changes in lung tissue viscoelastic behavior in this fibrosis model