Straining to understand mechanotransduction in the lung

Dynamic airway constriction in rats: heterogeneity and response to deep inspiration

Airway narrowing due to hyperresponsiveness severely limits gas exchange in patients with asthma. Imaging studies in humans and animals have shown that bronchoconstriction causes patchy patterns of ventilation defects throughout the lungs, and several computational models have predicted that these regions are due to constriction of smaller airways. However, these imaging approaches are often limited in their ability to capture dynamic changes in small airways, and the patterns of constriction are heterogeneous. To directly investigate regional variations in airway narrowing and the response to deep inspirations (DIs), we utilized tantalum dust and microfocal X-ray imaging of rat lungs to obtain dynamic images of airways in an intact animal model. Airway resistance was simultaneously measured using the flexiVent system. Custom-developed software was used to track changes in airway diameters up to generation 19 (~0.3-3 mm). Changes in diameter during bronchoconstriction were then measured in response to methacholine (MCh) challenge. In contrast with the model predictions, we observed significantly greater percent constriction in larger airways in response to MCh challenge. Although there was a dose-dependent increase in total respiratory resistance with MCh, the percent change in airway diameters was similar for increasing doses. A single DI following MCh caused a significant reduction in resistance but did not cause a significant increase in airway diameters. Multiple DIs did, however, cause significant increases in airway diameters. These measurements allowed us to directly quantify dynamic changes in airways during bronchoconstriction and demonstrated greater constriction in larger airways.

In Vitro Bone Cell Models: Impact of Fluid Shear Stress on Bone Formation

This review describes the role of bone cells and their surrounding matrix in maintaining bone strength through the process of bone remodeling. Subsequently, this work focusses on how bone formation is guided by mechanical forces and fluid shear stress in particular. It has been demonstrated that mechanical stimulation is an important regulator of bone metabolism. Shear stress generated by interstitial fluid flow in the lacunar-canalicular network influences maintenance and healing of bone tissue. Fluid flow is primarily caused by compressive loading of bone as a result of physical activity. Changes in loading, e.g., due to extended periods of bed rest or microgravity in space are associated with altered bone remodeling and formation in vivo. In vitro, it has been reported that bone cells respond to fluid shear stress by releasing osteogenic signaling factors, such as nitric oxide, and prostaglandins. This work focusses on the application of in vitro models to study the effects of fluid flow on bone cell signaling, collagen deposition, and matrix mineralization. Particular attention is given to in vitro set-ups, which allow long-term cell culture and the application of low fluid shear stress. In addition, this review explores what mechanisms influence the orientation of collagen fibers, which determine the anisotropic properties of bone. A better understanding of these mechanisms could facilitate the design of improved tissue-engineered bone implants or more effective bone disease models.

Novel therapeutic strategies for lung disorders associated with airway remodelling and fibrosis

Inflammatory cell infiltration, cytokine release, epithelial damage, airway/lung remodelling and fibrosis are central features of inflammatory lung disorders, which include asthma, chronic obstructive pulmonary disease, acute respiratory distress syndrome and idiopathic pulmonary fibrosis. Although the lung has some ability to repair itself from acute injury, in the presence of ongoing pathological stimuli and/or insults that lead to chronic disease, it no longer retains the capacity to heal, resulting in fibrosis, the final common pathway that causes an irreversible loss of lung function. Despite inflammation, genetic predisposition/factors, epithelial-mesenchymal transition and mechanotransduction being able to independently contribute to airway remodelling and fibrosis, current therapies for inflammatory lung diseases are limited by their ability to only target the inflammatory component of the disease without having any marked effects on remodelling (epithelial damage and fibrosis) that can cause lung dysfunction independently of inflammation. Furthermore, as subsets of patients suffering from these diseases are resistant to currently available therapies (such as corticosteroids), novel therapeutic approaches are required to combat all aspects of disease pathology. This review discusses emerging therapeutic approaches, such as trefoil factors, relaxin, histone deacetylase inhibitors and stem cells, amongst others that have been able to target airway inflammation and airway remodelling while improving related lung dysfunction. A better understanding of the mode of action of these therapies and their possible combined effects may lead to the identification of their clinical potential in the setting of lung disease, either as adjunct or alternative therapies to currently available treatments.

Mechanosensitive ATP release maintains proper mucus hydration of airways

The clearance of mucus from the airways protects the lungs from inhaled noxious and infectious materials. Proper hydration of the mucus layer enables efficient mucus clearance through beating of cilia on airway epithelial cells, and reduced clearance of excessively concentrated mucus occurs in patients with chronic obstructive pulmonary disease and cystic fibrosis. Key steps in the mucus transport process are airway epithelia sensing and responding to changes in mucus hydration. We reported that extracellular adenosine triphosphate (ATP) and adenosine were important luminal autocrine and paracrine signals that regulated the hydration of the surface of human airway epithelial cultures through their action on apical membrane purinoceptors. Mucus hydration in human airway epithelial cultures was sensed by an interaction between cilia and the overlying mucus layer: Changes in mechanical strain, proportional to mucus hydration, regulated ATP release rates, adjusting fluid secretion to optimize mucus layer hydration. This system provided a feedback mechanism by which airways maintained mucus hydration in an optimum range for cilia propulsion. Understanding how airway epithelia can sense and respond to changes in mucus properties helps us to understand how the mucus clearance system protects the airways in health and how it fails in lung diseases such as cystic fibrosis.

Lung heparan sulfates modulate K(fc) during increased vascular pressure: evidence for glycocalyx-mediated mechanotransduction

Lung endothelial cells respond to changes in vascular pressure through mechanotransduction pathways that alter barrier function via non-Starling mechanism(s). Components of the endothelial glycocalyx have been shown to participate in mechanotransduction in vitro and in systemic vessels, but the glycocalyx's role in mechanosensing and pulmonary barrier function has not been characterized. Mechanotransduction pathways may represent novel targets for therapeutic intervention during states of elevated pulmonary pressure such as acute heart failure, fluid overload, and mechanical ventilation. Our objective was to assess the effects of increasing vascular pressure on whole lung filtration coefficient (K(fc)) and characterize the role of endothelial heparan sulfates in mediating mechanotransduction and associated increases in K(fc). Isolated perfused rat lung preparation was used to measure K(fc) in response to changes in vascular pressure in combination with superimposed changes in airway pressure. The roles of heparan sulfates, nitric oxide, and reactive oxygen species were investigated. Increases in capillary pressure altered K(fc) in a nonlinear relationship, suggesting non-Starling mechanism(s). nitro-l-arginine methyl ester and heparanase III attenuated the effects of increased capillary pressure on K(fc), demonstrating active mechanotransduction leading to barrier dysfunction. The nitric oxide (NO) donor S-nitrosoglutathione exacerbated pressure-mediated increase in K(fc). Ventilation strategies altered lung NO concentration and the K(fc) response to increases in vascular pressure. This is the first study to demonstrate a role for the glycocalyx in whole lung mechanotransduction and has important implications in understanding the regulation of vascular permeability in the context of vascular pressure, fluid status, and ventilation strategies.

TNF-α-converting enzyme/a disintegrin and metalloprotease-17 mediates mechanotransduction in murine tracheal epithelial cells

Bronchoconstriction applies compressive stress to airway epithelial cells. We show that the application of compressive stress to cultured murine tracheal epithelial cells elicits the increased phosphorylation of extracellular signal-regulated kinase (ERK) and Akt through an epidermal growth factor receptor (EGFR)-dependent process, consistent with previous observations of the bronchoconstriction-induced activation of EGFR in both human and murine airways. Mechanotransduction requires metalloprotease activity, indicating a pivotal role for proteolytic EGF-family ligand shedding. However, cells derived from mice with targeted deletions of the EGFR ligands Tgfα and Hb-egf showed only modest decreases in responses, even when combined with neutralizing antibodies to the EGFR ligands epiregulin and amphiregulin, suggesting redundant or compensatory roles for individual EGF family members in mechanotransduction. In contrast, cells harvested from mice with a conditional deletion of the gene encoding the TNF-α-converting enzyme (TACE/ADAM17), a sheddase for multiple EGF-family proligands, displayed a near-complete attenuation of ERK and Akt phosphorylation responses and compressive stress-induced gene regulation. Our data provide strong evidence that TACE plays a critical central role in the transduction of compressive stress.

Effects of distension on airway inflammation and venular P-selectin expression

We previously have shown in mice and rats, enhanced leukocyte recruitment to airway postcapillary venules after excessive distention imposed by the application of positive end-expiratory pressure. Because P-selectin was shown to be essential for this outcome, we sought to establish an in vitro endothelial cell model and determine the mechanisms whereby mechanical distension alters adhesion molecule expression. P-selectin surface expression on mouse jugular vein endothelial cells exposed to cyclic stretch (5 or 20% elongation for 5 min; Flexercell) were compared with static cells. The larger, pathophysiological regimen of cyclic stretch showed a 54% increase in P-selectin expression after stretch compared with static cells. This response was attenuated but confirmed in tracheal venular endothelium (29% increase). We questioned whether these changes were dependent on increases in intracellular Ca(2+) through voltage-gated Ca(2+) channels. The stretch-induced increase in P-selectin expression was substantially decreased by pretreatment with the T-type Ca(2+) channel inhibitor mibefradil (76% inhibition). Furthermore, when the Ca(v)3.1 T-type Ca(2+) channel expression was decreased in both endothelial cell subtypes with specific small-interfering RNA, the distension-induced expression of P-selectin decreased to levels less than that observed in static cells. We conclude that P-selectin expression on systemic venular endothelial cells contributes to a proinflammatory phenotype after mechanical stretch and can be selectively modulated by voltage-gated calcium channel inhibition.

Tubulin acetylation and histone deacetylase 6 activity in the lung under cyclic load

Previous studies from our lab have demonstrated that upon exposure to physiologic levels of cyclic stretch, alveolar epithelial cells demonstrate a significant decrease in the amount of polymerized tubulin (Geiger et al., Gene Therapy 2006;13:725-731). However, not all microtubules are disassembled, although the mechanisms or implications of this were unknown. Using immunofluorescence microscopy, Western blotting, and immunohistochemistry approaches, we have compared the levels of acetylated tubulin in stretched and unstretched A549 cells and in murine lungs. In cultured cells exposed to cyclic stretch (10% change in basement membrane surface area at 0.25 Hz), nearly all of the remaining microtubules were acetylated, as demonstrated using immunofluorescence microscopy. In murine lungs ventilated for 20 minutes at 12 to 20 ml/kg followed by 48 hours of spontaneous breathing or for 3 hours at 16 to 40 ml/kg, levels of acetylated tubulin were increased in the peripheral lung. In both our in vitro and in vivo studies, we have found that mild to moderate levels of cyclic stretch significantly increases tubulin acetylation in a magnitude- and duration-dependent manner. This appears to be due to a decrease in histone deacetylase 6 activity (HDAC6), the major tubulin deacetylase. Since it has been previously shown that acetylated microtubules are positively correlated to a more stable population of microtubules, this result suggests that microtubule stability may be increased by cyclic stretch, and that tubulin acetylation is one way in which cells respond to changes in exogenous mechanical forces.

Role of mechanical stress in regulating airway surface hydration and mucus clearance rates

Effective clearance of mucus is a critical innate airway defense mechanism, and under appropriate conditions, can be stimulated to enhance clearance of inhaled pathogens. It has become increasingly clear that extracellular nucleotides (ATP and UTP) and nucleosides (adenosine) are important regulators of mucus clearance in the airways as a result of their ability to stimulate fluid secretion, mucus hydration, and cilia beat frequency (CBF). One ubiquitous mechanism to stimulate ATP release is through external mechanical stress. This article addresses the role of physiologically relevant mechanical forces in the lung and their effects on regulating mucociliary clearance (MCC). The effects of mechanical forces on the stimulating ATP release, fluid secretion, CBF, and MCC are discussed. Also discussed is evidence suggesting that airway hydration and stimulation of MCC by stress-mediated ATP release may play a role in several therapeutic strategies directed at improving mucus clearance in patients with obstructive lung diseases, including cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD).

Differential effects of cyclic and constant stress on ATP release and mucociliary transport by human airway epithelia

In the lungs, the first line of defence against bacterial infection is the thin layer of airway surface liquid (ASL) lining the airway surface. The superficial airway epithelium exhibits complex regulatory pathways that blend ion transport to adjust ASL volume to maintain proper mucociliary clearance (MCC). We hypothesized that stresses generated by airflow and transmural pressures during breathing govern ASL volume by regulating the rate of epithelial ATP release. Luminal ATP, via interactions with apical membrane P2-purinoceptors, regulates the balance of active ion secretion versus absorption to maintain ASL volume at optimal levels for MCC. In this study we tested the hypothesis that cyclic compressive stress (CCS), mimicking normal tidal breathing, regulates ASL volume in airway epithelia. Polarized tracheobronchial epithelial cultures from normal and cystic fibrosis (CF) subjects responded to a range of CCS by increasing the rate of ATP release. In normal airway epithelia, the CCS-induced increase in ASL ATP concentration was sufficient to induce purinoceptor-mediated increases in ASL height and MCC, via inhibition of epithelial Na(+)-channel-mediated Na(+) absorption and stimulation of Cl(-) secretion through CFTR and the Ca(2+)-activated chloride channels. In contrast, static, non-oscillatory stress did not stimulate ATP release, ion transport or MCC, emphasizing the importance of rhythmic mechanical stress for airway defence. In CF airway cultures, which exhibit basal ASL depletion, CCS was partially effective, producing less ASL volume secretion than in normal cultures, but a level sufficient to restore MCC. The present data suggest that CCS may (1) regulate ASL volume in the normal lung and (2) improve clearance in the lungs of CF patients, potentially explaining the beneficial role of exercise in lung defence.

Effects of airway distension on leukocyte recruitment in the mouse tracheal microvasculature

We have shown previously that excessive distention of the rat trachea during mechanical ventilation results in enhanced leukocyte recruitment to the airway (Lim LH and Wagner EM. Am J Respir Crit Care Med 168:1068-1074, 2003). The objectives of this study were to develop a mouse model of positive end-expiratory pressure (PEEP)-induced leukocyte recruitment to the airway and begin to pursue molecular mechanisms that may contribute to the in vivo observation of increased leukocyte adhesion after PEEP exposure. We studied C57BL/6 wild-type mice and mice deficient in P-selectin or intercellular adhesion molecule-1 (ICAM-1) exposed to intermittent PEEP (8 cmH(2)O) applied five times for a 1-min duration, at 10-min intervals. After the imposed ventilatory stress, during normal ventilation (0.2 ml/breath, no PEEP), leukocyte adhesion in tracheal postcapillary venules was determined using intravital microscopy. PEEP induced a time-dependent increase in leukocyte adhesion that was significantly increased between 0 and 60 min (P < 0.01). Furthermore, PEEP-induced leukocyte adhesion at 60 min was ablated in P-selectin- and ICAM-1-deficient mice. These findings demonstrate the essential nature of both P-selectin and ICAM-1 within airway postcapillary venular endothelium for leukocyte recruitment after airway distension.

Cyclic stretch attenuates effects of hyperoxia on cell proliferation and viability in human alveolar epithelial cells

The treatment of severe lung disease often requires the use of high concentrations of oxygen coupled with the need for assisted ventilation, potentially exposing the pulmonary epithelium to both reactive oxygen species and nonphysiological cyclic stretch. Whereas prolonged hyperoxia is known to cause increased cell injury, cyclic stretch may result in either cell proliferation or injury depending on the pattern and degree of exposure to mechanical deformation. How hyperoxia and cyclic stretch interact to affect the pulmonary epithelium in vitro has not been previously investigated. This study was performed using human alveolar epithelial A549 cells to explore the combined effects of cyclic stretch and hyperoxia on cell proliferation and viability. Under room air conditions, cyclic stretch did not alter cell viability at any time point and increased cell number after 48 h compared with unstretched controls. After exposure to prolonged hyperoxia, cell number and [(3)H]thymidine incorporation markedly decreased, whereas evidence of oxidative stress and nonapoptotic cell death increased. The combination of cyclic stretch with hyperoxia significantly mitigated the negative effects of prolonged hyperoxia alone on measures of cell proliferation and viability. In addition, cyclic stretch resulted in decreased levels of oxidative stress over time in hyperoxia-exposed cells. Our results suggest that cyclic stretch, as applied in this study, can minimize the detrimental effects of hyperoxia on alveolar epithelial A549 cells.

What increases type III procollagen mRNA levels in lung tissue: stress induced by changes in force or amplitude?

We hypothesized that stress determined by force could induce higher type III procollagen (PCIII) mRNA expression than the stress determined by amplitude. To that end, rat lung tissue strips were oscillated for 1h under different amplitudes [1, 5 and 10% of resting length (L(B)), at 0.5 x 10(-2) N] and forces (0.25 x 10(-2), 0.5 x 10(-2) and 10(-2)N, at 5% L(B)). Resistance (R), elastance (E) and hysteresivity (eta) were analysed during sinusoidal oscillations at 1Hz. After 1h of oscillation, PCIII mRNA expression was determined by Northern-blot and semiquantitative RT-PCR. Control value of PCIII mRNA was obtained from unstressed strips. E and R increased with augmenting force and decreased with increasing amplitude, while eta remained unaltered. PCIII mRNA expression increased significantly after 1h of oscillation at 10(-2)N and 5% L(B) and remained unchanged for 6h. In conclusion, the stress induced by force but not by amplitude led to the increment in PCIII mRNA expression.

Airway distension promotes leukocyte recruitment in rat tracheal circulation

Mechanical distortion of blood vessels is known to activate endothelial cells. Whether airway distension likewise activates the vascular endothelium within the airway wall is unknown. Using intravital microscopy in the rat trachea, we investigated if airway distention with the application of positive end-expiratory pressure (PEEP) caused leukocyte recruitment to the airway. Tracheal postcapillary venules were visualized and leukocyte kinetics monitored in anesthetized, mechanically ventilated rats (80 breaths/minute, 6 ml/kg VT, 1 cm H(2)O PEEP). Leukocyte rolling velocity (Vwbc) and the number of adherent cells were not altered with normal ventilation over the course of 2 hours. Ventilation with sustained PEEP (8 cm H(2)O for 1 hour reduced Vwbc and increased adhesion, reaching a maximum at 1 hour of PEEP. Intermittent (2x and 5x) 8 cm H(2)O PEEP also induced a similar reduction in Vwbc, accompanied by an increase in adhesion. However, leukocyte recruitment after airway distension is localized to the airways because increased PEEP did not induce leukocyte recruitment in the mesenteric microcirculation or when PEEP was applied to the lung distal to the site of measurement. Pretreatment with endothelin receptor and selectin inhibitors blocked the effects of distension on leukocyte recruitment, suggesting their involvement in the proinflammatory response.

"The force be with you": ATP in gut mechanosensory transduction

Nucleotides such as ATP and UTP are everywhere. They play a key role in transducing mechanosensory signals via P2Y receptors in the large intestine or colon, leading to secretion of the sensory mediator 5-HT, and act as autocrine, paracrine, or neurocrine mediators in neural reflexes regulating chloride secretion.