What evidence implicates airway smooth muscle in the cause of BHR?

Smooth-muscle-derived WNT5A augments allergen-induced airway remodelling and Th2 type inflammation

Asthma is a heterogeneous disease characterized by chronic inflammation and structural changes in the airways. The airway smooth muscle (ASM) is responsible for airway narrowing and an important source of inflammatory mediators. We and others have previously shown that WNT5A mRNA and protein expression is higher in the ASM of asthmatics compared to healthy controls. Here, we aimed to characterize the functional role of (smooth muscle-derived) WNT5A in asthma. We generated a tet-ON smooth-muscle-specific WNT5A transgenic mouse model, enabling in vivo characterization of smooth-muscle-derived WNT5A in response to ovalbumin. Smooth muscle specific WNT5A overexpression showed a clear trend towards enhanced actin (α-SMA) expression in the ASM in ovalbumin challenged animals, but had no effect on collagen content. WNT5A overexpression in ASM also significantly enhanced the production of the Th2-cytokines IL4 and IL5 in lung tissue after ovalbumin exposure. In line with this, WNT5A increased mucus production, and enhanced eosinophilic infiltration and serum IgE production in ovalbumin-treated animals. In addition, CD4⁺ T cells of asthma patients and healthy controls were stimulated with WNT5A and changes in gene transcription assessed by RNA-seq. WNT5A promoted expression of 234 genes in human CD4⁺ T cells, among which the Th2 cytokine IL31 was among the top 5 upregulated genes. IL31 was also upregulated in response to smooth muscle-specific WNT5A overexpression in the mouse. In conclusion, smooth-muscle derived WNT5A augments Th2 type inflammation and remodelling. Our findings imply a pro-inflammatory role for smooth muscle-derived WNT5A in asthma, resulting in increased airway wall inflammation and remodelling.

Bronchial Thermoplasty

Bronchial thermoplasty is an advanced therapy for severe asthma. It is a bronchoscopic procedure in which radiofrequency energy is applied to the airway wall, resulting in decreased airway smooth muscle burden. Human trials have shown that bronchial thermoplasty may reduce asthma exacerbations and improve quality of life in patients with severe uncontrolled asthma. It has been demonstrated to be a safe procedure, with most adverse events being early and mild. More studies are required to understand the precise effects of bronchial thermoplasty on the asthmatic airway and optimal parameters to appropriately select patients for this novel procedure.

Emerging understanding of the mechanism of action of Bronchial Thermoplasty in asthma

Bronchial Thermoplasty (BT) is an endoscopic treatment for moderate-to-severe asthma patients who are uncontrolled despite optimal medical therapy. Effectiveness of BT has been demonstrated in several randomized clinical trials. However, the asthma phenotype that benefits most of this treatment is unclear, partly because the mechanism of action is incompletely understood. BT was designed to reduce the amount of airway smooth muscle (ASM), but additional direct and indirect effects on airway pathophysiology are expected. This review will provide an overview of the different components of airway pathophysiology including remodeling, with the ASM as the key player. Current concepts in the understanding of BT clinical effectiveness with a focus on its impact on airway remodeling will be reviewed.

Bronchial hyperresponsiveness

BACKGROUND

Bronchial hyperresponsiveness (BHR) is often regarded as a 'hallmark' of asthma and bronchoprovocation testing is frequently performed to support a diagnosis of asthma. However, BHR is also found in a spectrum of other lung diseases and can be provoked by a variety of specific stimuli.

AIMS

To review the pathophysiology of BHR, discuss various methods of testing for BHR and describe the epidemiology of BHR in a variety of previously studied populations.

METHODS

We performed a systematic review of references identified using Medline and hand searches of identified articles. Because of space limitations, we have included those reports that seem most representative of the overall BHR literature.

RESULTS

BHR can be induced by a variety of stimuli that trigger a number of different but overlapping physiological mechanisms. Bronchoprovocation testing can be performed using a variety of stimuli, various protocols and differing test criteria, yielding results that may be discordant. Elevated rates of BHR have been reported in studies of smokers, chronic obstructive pulmonary disease patients, atopics, athletes, exposed workers and the general population.

CONCLUSIONS

Due to the prevalence of BHR in a spectrum of clinical patients and working populations, clinicians should be aware that BHR is not specific for asthma. When performed correctly, the greatest clinical value of BHR testing is to rule out suspected asthma in patients in whom testing is negative. Assessment of BHR also provides insights into the pathological mechanisms of airway disease.

Patterns of heart rate variability and cardiac autonomic modulations in controlled and uncontrolled asthmatic patients

BACKGROUND

Previous heart rate variability (HRV) studies in asthmatic subjects (AS) demonstrate predominance of parasympathetic drive concomitant with low HRV, which is against the general belief that enhanced parasympathetic modulation improves HRV. The aim of this study was to compare patterns of HRV and cardiac autonomic modulations of AS to healthy control subjects (HS).

METHODS

Eighty AS and forty HS were enrolled in the study. Asthma control test and spirometry were used to discriminate uncontrolled (UA) from controlled (CA) asthmatic patients. Natural logarithmic (Ln) scale of total power (TP), very low frequency (VLF), low frequency (LF) and high frequency (HF) were used to evaluate HRV. Normalized low frequency (LF Norm) and high frequency (HF Norm) were used to determine sympathetic and parasympathetic autonomic modulations respectively.

RESULTS

CA patients achieved significantly higher LnTP, LnLF, LnHF and HF Norm but lower LF Norm and LnLF/HF compared with UA patients (p < 0.05). Although CA patients showed increased HRV and augmented vagal modulation compared with HS, these findings were no longer significant following adjustment for mean heart rates and anti-asthma treatment. All measured HRV parameters were not significantly different in UA patients compared with the HS (p > 0.05).

CONCLUSIONS

CA is associated with enhanced parasympathetic modulations and higher HRV compared with UA. However, neither CA nor UA patients had different autonomic modulations and/or HRV compared with HS.

Inelastic mechanics: A unifying principle in biomechanics

Many soft materials are classified as viscoelastic. They behave mechanically neither quite fluid-like nor quite solid-like - rather a bit of both. Biomaterials are often said to fall into this class. Here, we argue that this misses a crucial aspect, and that biomechanics is essentially damage mechanics, at heart. When deforming an animal cell or tissue, one can hardly avoid inducing the unfolding of protein domains, the unbinding of cytoskeletal crosslinkers, the breaking of weak sacrificial bonds, and the disruption of transient adhesions. We classify these activated structural changes as inelastic. They are often to a large degree reversible and are therefore not plastic in the proper sense, but they dissipate substantial amounts of elastic energy by structural damping. We review recent experiments involving biological materials on all scales, from single biopolymers over cells to model tissues, to illustrate the unifying power of this paradigm. A deliberately minimalistic yet phenomenologically very rich mathematical modeling framework for inelastic biomechanics is proposed. It transcends the conventional viscoelastic paradigm and suggests itself as a promising candidate for a unified description and interpretation of a wide range of experimental data. This article is part of a Special Issue entitled: Mechanobiology.

The Three A's in Asthma - Airway Smooth Muscle, Airway Remodeling & Angiogenesis

Asthma affects more than 300 million people worldwide and its prevalence is still rising. Acute asthma attacks are characterized by severe symptoms such as breathlessness, wheezing, tightness of the chest, and coughing, which may lead to hospitalization or death. Besides the acute symptoms, asthma is characterized by persistent airway inflammation and airway wall remodeling. The term airway wall remodeling summarizes the structural changes in the airway wall: epithelial cell shedding, goblet cell hyperplasia, hyperplasia and hypertrophy of the airway smooth muscle (ASM) bundles, basement membrane thickening and increased vascular density. Airway wall remodeling starts early in the pathogenesis of asthma and today it is suggested that remodeling is a prerequisite for other asthma pathologies. The beneficial effect of bronchial thermoplasty in reducing asthma symptoms, together with the increased potential of ASM cells of asthmatics to produce inflammatory and angiogenic factors, indicate that the ASM cell is a major effector cell in the pathology of asthma. In the present review we discuss the ASM cell and its role in airway wall remodeling and angiogenesis.

Mechanical and structural plasticity

Excessive narrowing of the airways due to airway smooth muscle (ASM) contraction is a major cause of asthma exacerbation. ASM is therefore a direct target for many drugs used in asthma therapy. The contractile mechanism of smooth muscle is not entirely clear. A major advance in the field in the last decade was the recognition and appreciation of the unique properties of smooth muscle--mechanical and structural plasticity, characterized by the muscle's ability to rapidly alter the structure of its contractile apparatus and cytoskeleton and adapt to the mechanically dynamic environment of the lung. This article describes a possible mechanism for smooth muscle to adapt and function over a large length range by adding or subtracting contractile units in series spanning the cell length; it also describes a mechanism by which actin-myosin-actin connectivity might be influenced by thin and thick filament lengths, thus altering the muscle response to mechanical perturbation. The new knowledge is extremely useful for our understanding of ASM behavior in the lung and could provide new and more effective targets for drugs aimed at relaxing the muscle or keeping the muscle from excessive shortening in the asthmatic airways.

Carbachol-induced volume adaptation in mouse bladder and length adaptation via rhythmic contraction in rabbit detrusor

The length-tension (L-T) relationships in rabbit detrusor smooth muscle (DSM) are similar to those in vascular and airway smooth muscles and exhibit short-term length adaptation characterized by L-T curves that shift along the length axis as a function of activation and strain history. In contrast to skeletal muscle, the length-active tension (L-T(a)) curve for rabbit DSM strips does not have a unique peak tension value with a single ascending and descending limb. Instead, DSM can exhibit multiple ascending and descending limbs, and repeated KCl-induced contractions at a particular muscle length on an ascending or descending limb display increasingly greater tension. In the present study, mouse bladder strips with and without urothelium exhibited KCl-induced and carbachol-induced length adaptation, and the pressure-volume relationship in mouse whole bladder displayed short-term volume adaptation. Finally, prostaglandin-E(2)-induced low-level rhythmic contraction produced length adaptation in rabbit DSM strips. A likely role of length adaptation during bladder filling is to prepare DSM cells to contract efficiently over a broad range of volumes. Mammalian bladders exhibit spontaneous rhythmic contraction (SRC) during the filling phase and SRC is elevated in humans with overactive bladder (OAB). The present data identify a potential physiological role for SRC in bladder adaptation and motivate the investigation of a potential link between short-term volume adaptation and OAB with impaired contractility.

Epithelial EGF receptor signaling mediates airway hyperreactivity and remodeling in a mouse model of chronic asthma

Increases in the epidermal growth factor receptor (EGFR) have been associated with the severity of airway thickening in chronic asthmatic subjects, and EGFR signaling is induced by asthma-related cytokines and inflammation. The goal of this study was to determine the role of EGFR signaling in a chronic allergic model of asthma and specifically in epithelial cells, which are increasingly recognized as playing an important role in asthma. EGFR activation was assessed in mice treated with intranasal house dust mite (HDM) for 3 wk. EGFR signaling was inhibited in mice treated with HDM for 6 wk, by using either the drug erlotinib or a genetic approach that utilizes transgenic mice expressing a mutant dominant negative epidermal growth factor receptor in the lung epithelium (EGFR-M mice). Airway hyperreactivity (AHR) was assessed by use of a flexiVent system after increasing doses of nebulized methacholine. Airway smooth muscle (ASM) thickening was measured by morphometric analysis. Sensitization to HDM (IgG and IgE), inflammatory cells, and goblet cell changes were also assessed. Increased EGFR activation was detected in HDM-treated mice, including in bronchiolar epithelial cells. In mice exposed to HDM for 6 wk, AHR and ASM thickening were reduced after erlotinib treatment and in EGFR-M mice. Sensitization to HDM and inflammatory cell counts were similar in all groups, except neutrophil counts, which were lower in the EGFR-M mice. Goblet cell metaplasia with HDM treatment was reduced by erlotinib, but not in EGFR-M transgenic mice. This study demonstrates that EGFR signaling, especially in the airway epithelium, plays an important role in mediating AHR and remodeling in a chronic allergic asthma model.

Length adaptation of the passive-to-active tension ratio in rabbit detrusor

The passive and active length-tension (L-T (p) and L-T (a)) relationships in airway, vascular, and detrusor smooth muscles can adapt with length changes and/or multiple contractions. The present objectives were to (1) determine whether short-term adaptation at one muscle length shifts the entire L-T (a) curve in detrusor smooth muscle (DSM), (2) compare adaptation at shorter versus longer lengths, and (3) determine the effect of adaptation on the T (p)/T (a) ratio. Results showed that multiple KCl-induced contractions on the descending limb of the original L-T (a) curve adapted DSM strips to that length and shifted the L-T (a) curve rightward. Peak T (a) at the new length was not different from the original peak T (a), and the L-T (p) curve shifted rightward with the L-T (a) curve. Multiple contractions on the ascending limb increased both T (a) and T (p). In contrast, multiple contractions on the descending limb increased T (a) but decreased T (p). The T (p)/T (a) ratio on the original descending limb adapted from 0.540 +/- 0.084 to 0.223 +/- 0.033 (mean +/- SE, n = 7), such that it was not different from the ratio of 0.208 +/- 0.033 at the original peak T (a) length, suggesting a role of length adaptation may be to maintain a desirable T (p)/T (a) ratio as the bladder fills and voids over a broad DSM length range.

MEK modulates force-fluctuation-induced relengthening of canine tracheal smooth muscle

Tidal breathing, and especially deep breathing, is known to antagonise bronchoconstriction caused by airway smooth muscle (ASM) contraction; however, this bronchoprotective effect of breathing is impaired in asthma. Force fluctuations applied to contracted ASM in vitro cause it to relengthen, force-fluctuation-induced relengthening (FFIR). Given that breathing generates similar force fluctuations in ASM, FFIR represents a likely mechanism by which breathing antagonises bronchoconstriction. Thus it is of considerable interest to understand what modulates FFIR, and how ASM might be manipulated to exploit this phenomenon. It was demonstrated previously that p38 mitogen-activated protein kinase (MAPK) signalling regulates FFIR in ASM strips. Here, it was hypothesised that the MAPK kinase (MEK) signalling pathway also modulates FFIR. In order to test this hypothesis, changes in FFIR were measured in ASM treated with the MEK inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene). Increasing concentrations of U0126 caused greater FFIR. U0126 reduced extracellular signal-regulated kinase 1/2 phosphorylation without affecting isotonic shortening or 20-kDa myosin light chain and p38 MAPK phosphorylation. However, increasing concentrations of U0126 progressively blunted phosphorylation of high-molecular-weight caldesmon (h-caldesmon), a downstream target of MEK. Thus changes in FFIR exhibited significant negative correlation with h-caldesmon phosphorylation. The present data demonstrate that FFIR is regulated through MEK signalling, and suggest that the role of MEK is mediated, in part, through caldesmon.

Adaptation of the length-active tension relationship in rabbit detrusor

Studies have shown that the length-tension (L-T) relationships in airway and vascular smooth muscles are dynamic and can adapt to length changes over a period of time. Our prior studies have shown that the passive L-T relationship in rabbit detrusor smooth muscle (DSM) is also dynamic and that DSM exhibits adjustable passive stiffness (APS) characterized by a passive L-T curve that can shift along the length axis as a function of strain history and activation history. The present study demonstrates that the active L-T curve for DSM is also dynamic and that the peak active tension produced at a particular muscle length is a function of both strain and activation history. More specifically, this study reveals that the active L-T relationship, or curve, does not have a unique peak tension value with a single ascending and descending limb, but instead reveals that multiple ascending and descending limbs can be exhibited in the same DSM strip. This study also demonstrates that for DSM strips not stretched far enough to reveal a descending limb, the peak active tension produced by a maximal KCl-induced contraction at a short, passively slack muscle length of 3 mm was reduced by 58.6 +/- 4.1% (n = 15) following stretches to and contractions at threefold the original muscle length, 9 mm. Moreover, five subsequent contractions at the short muscle length displayed increasingly greater tension; active tension produced by the sixth contraction was 91.5 +/- 9.1% of that produced by the prestretch contraction at that length. Together, these findings indicate for the first time that DSM exhibits length adaptation, similar to vascular and airway smooth muscles. In addition, our findings demonstrate that preconditioning, APS and adaptation of the active L-T curve can each impact the maximum total tension observed at a particular DSM length.

Disrupting actin-myosin-actin connectivity in airway smooth muscle as a treatment for asthma?

Breathing is known to functionally antagonize bronchoconstriction caused by airway muscle contraction. During breathing, tidal lung inflation generates force fluctuations that are transmitted to the contracted airway muscle. In vitro, experimental application of force fluctuations to contracted airway smooth muscle strips causes them to relengthen. Such force fluctuation-induced relengthening (FFIR) likely represents the mechanism by which breathing antagonizes bronchoconstriction. Thus, understanding the mechanisms that regulate FFIR of contracted airway muscle could suggest novel therapeutic interventions to increase FFIR, and so to enhance the beneficial effects of breathing in suppressing bronchoconstriction. Here we propose that the connectivity between actin filaments in contracting airway myocytes is a key determinant of FFIR, and suggest that disrupting actin-myosin-actin connectivity by interfering with actin polymerization or with myosin polymerization merits further evaluation as a potential novel approach for preventing prolonged bronchoconstriction in asthma.

The contribution of Ca2+ signaling and Ca2+ sensitivity to the regulation of airway smooth muscle contraction is different in rats and mice

To determine the relative contributions of Ca(2+) signaling and Ca(2+) sensitivity to the contractility of airway smooth muscle cells (SMCs), we compared the contractile responses of mouse and rat airways with the lung slice technique. Airway contraction was measured by monitoring changes in airway lumen area with phase-contrast microscopy, whereas changes in intracellular calcium concentration ([Ca(2+)](i)) of the SMCs were recorded with laser scanning microscopy. In mice and rats, methacholine (MCh) or serotonin induced concentration-dependent airway contraction and Ca(2+) oscillations in the SMCs. However, rat airways demonstrated greater contraction compared with mice, in response to agonist-induced Ca(2+) oscillations of a similar frequency. Because this indicates that rat airway SMCs have a higher Ca(2+) sensitivity compared with mice, we examined Ca(2+) sensitivity with Ca(2+)-permeabilized airway SMCs in which the [Ca(2+)](i) was experimentally controlled. In the absence of agonists, high [Ca(2+)](i) induced a sustained contraction in rat airways but only a transient contraction in mouse airways. This sustained contraction of rat airways was relaxed by Y-23672, a Rho kinase inhibitor, but not affected by GF-109203X, a PKC inhibitor. The subsequent exposure of Ca(2+)-permeabilized airway SMCs, with high [Ca(2+)](i), to MCh elicited a further contraction of rat airways and initiated a sustained contraction of mouse airways, without changing the [Ca(2+)](i) of the SMCs. Collectively, these results indicate that airway SMCs of rats have a substantially higher innate Ca(2+) sensitivity than mice and that this strongly influences the transduction of the frequency of Ca(2+) oscillations into the contractility of airway SMCs.

Bronchial hyper-responsiveness, subepithelial fibrosis, and transforming growth factor-beta(1) expression in patients with long-standing and recently diagnosed asthma

INTRODUCTION

Chronic inflammation in asthmatic airways leads to bronchial hyper-responsiveness (BHR) and the development of structural changes. Important features of remodeling include the formation of subepithelial fibrosis due to increased collagen deposition in the reticular basement membrane. Transforming growth factor (TGF)-beta might be a central mediator of tissue fibrosis and remodeling.

MATERIALS AND METHODS

Immunohistochemistry was used to measure collagen III deposition and TGF-beta(1) expression in biopsies from patients with long-standing asthma treated with inhaled corticosteroids, patients with recently diagnosed asthma, and control subjects. Computer-assisted image analysis was used to evaluate total basement membrane (TBM) thickness.

RESULTS

Asthmatics, particularly those with long-standing asthma, had thicker TBMs than healthy subjects. Collagen III deposition was comparable in the studied groups. BHR was not correlated with features of mucosal inflammation and was lower in steroid-treated patients with long-standing asthma than in subjects with newly diagnosed asthma untreated with steroids. Epithelial TGF-beta(1) expression negatively correlated with collagen III deposition and TBM thickness.

CONCLUSIONS

The study showed that TBM thickness, but not collagen III deposition, could be a differentiating marker of asthmatics of different disease duration and treatment. The lack of correlation between BHR and features of mucosal inflammation suggests the complexity of BHR development. Corticosteroids can reduce BHR in asthmatics, but it seems to be less effective in reducing subepithelial fibrosis. The role of epithelial TGF-beta(1) needs to be further investigated since the possibility that it plays a protective and anti-inflammatory role in asthmatic airways cannot be excluded.

Myosin, transgelin, and myosin light chain kinase: expression and function in asthma

RATIONALE

Airway smooth muscle (SM) of patients with asthma exhibits a greater velocity of shortening (Vmax) than that of normal subjects, and this is thought to contribute to airway hyperresponsiveness. A greater Vmax can result from increased myosin activation. This has been reported in sensitized human airway SM and in models of asthma. A faster Vmax can also result from the expression of specific contractile proteins that promote faster cross-bridge cycling. This possibility has never been addressed in asthma.

OBJECTIVES

We tested the hypothesis that the expression of genes coding for SM contractile proteins is altered in asthmatic airways and contributes to their increased Vmax.

METHODS

We quantified the expression of several genes that code for SM contractile proteins in mild allergic asthmatic and control human airway endobronchial biopsies. The function of these contractile proteins was tested using the in vitro motility assay.

MEASUREMENTS AND MAIN RESULTS

We observed an increased expression of the fast myosin heavy chain isoform, transgelin, and myosin light chain kinase in patients with asthma. Immunohistochemistry demonstrated the expression of these genes at the protein level. To address the functional significance of this overexpression, we purified tracheal myosin from the hyperresponsive Fisher rats, which also overexpress the fast myosin heavy chain isoform as compared with the normoresponsive Lewis rats, and found a faster rate of actin filament propulsion. Conversely, transgelin did not alter the rate of actin filament propulsion.

CONCLUSIONS

Selective overexpression of airway smooth muscle genes in asthmatic airways leads to increased Vmax, thus contributing to the airway hyperresponsiveness observed in asthma.

Airway smooth muscle in asthma

Airway smooth muscle plays a multifaceted role in the pathogenesis of asthma. We review the current understanding of the contribution of airway myocytes to airway inflammation, airway wall remodeling, and airflow obstruction in this prevalent disease syndrome. Together, these roles make airway smooth muscle an attractive target for asthma therapy.

Force fluctuation-induced relengthening of acetylcholine-contracted airway smooth muscle

Superimposition of force fluctuations on contracted tracheal smooth muscle (TSM) has been used to simulate normal breathing. Breathing has been shown to reverse lung resistance of individuals without asthma and animals given methacholine to contract their airways; computed tomography scans also demonstrated bronchial dilation after a deep inhalation in normal volunteers. This reversal of airway resistance and bronchial constriction are absent (or much diminished) in individuals with asthma. Many studies have demonstrated that superimposition of force oscillations on contracted airway smooth muscle results in substantial smooth muscle lengthening. Subsequent studies have shown that this force fluctuation-induced relengthening (FFIR) is a physiologically regulated phenomenon. We hypothesized that actin filament length in the smooth muscle of the airways regulates FFIR of contracted tissues. We based this hypothesis on the observations that bovine TSM strips contracted using acetylcholine (ACh) demonstrated amplitude-dependent FFIR that was sensitive to mitogen-activated protein kinase (p38 MAPK) inhibition- an upstream regulator of actin filament assembly. We demonstrated latrunculin B (sequesters actin monomers thus preventing their assimilation into filaments resulting in shorter filaments) greatly increases FFIR and jasplakinolide (an actin filament stabilizer) prevents the effects of latrunculin B incubation on strips of contracted canine TSM. We suspect that p38 MAPK inhibition and latrunculin B predispose to shorter actin filaments. These studies suggest that actin filament length may be a key determinant of airway smooth muscle relengthening and perhaps breathing-induced reversal of agonist-induced airway constriction.

Biophysical basis for airway hyperresponsiveness

Airway hyperresponsiveness is the excessive narrowing of the airway lumen caused by stimuli that would cause little or no narrowing in the normal individual. It is one of the cardinal features of asthma, but its mechanisms remain unexplained. In asthma, the key end-effector of acute airway narrowing is contraction of the airway smooth muscle cell that is driven by myosin motors exerting their mechanical effects within an integrated cytoskeletal scaffolding. In just the past few years, however, our understanding of the rules that govern muscle biophysics has dramatically changed, as has their classical relationship to airway mechanics. It has become well established, for example, that muscle length is equilibrated dynamically rather than statically, and that in a dynamic setting nonclassical features of muscle biophysics come to the forefront, including unanticipated interactions between the muscle and its time-varying load, as well as the ability of the muscle cell to adapt (remodel) its internal microstructure rapidly in response to its ever-changing mechanical environment. Here, we consider some of these emerging concepts and, in particular, focus on structural remodeling of the airway smooth muscle cell as it relates to excessive airway narrowing in asthma.

Smooth muscle molecular mechanics in airway hyperresponsiveness and asthma

Asthma is a respiratory disorder characterized by airway inflammation and hyperresponsiveness associated with reversible airway obstruction. The relative contributions of airway hyperresponsiveness and inflammation are still debated, but ultimately, airway narrowing mediated by airway smooth muscle contraction is the final pathway to asthma. Considerable effort has been devoted towards identifying the factors that lead to the airway smooth muscle hypercontractility observed in asthma, and this will be the focus of this review. Airway remodeling has been observed in severe and fatal asthma. However, it is unclear whether remodeling plays a protective role or worsens airway responsiveness. Smooth muscle plasticity is a mechanism likely implicated in asthma, whereby contractile filament rearrangements lead to maximal force production, independent of muscle length. Increased smooth muscle rate of shortening via altered signaling pathways or altered contractile protein expression has been demonstrated in asthma and in numerous models of airway hyperresponsiveness. Increased rate of shortening is implicated in counteracting the relaxing effect of tidal breathing and deep inspirations, thereby creating a contracted airway smooth muscle steady-state. Further studies are therefore required to understand the numerous mechanisms leading to the airway hyperresponsiveness observed in asthma as well as their multiple interactions.

Airway remodeling and airway smooth muscle in asthma

Airway remodeling in asthma has been recognized as structural changes of airways such as smooth muscle hypertrophy (an increase in size of airway smooth muscle cells) and hyperplasia (an increase in the number of airway smooth muscle cells), thickening and fibrosis of sub-epithelial basement membrane, hypertrophy of bronchial glands, goblet cell hyperplasia, and thickening of airway epithelium. In these pathological changes, airway smooth muscle remodeling has been recognized as one of the most important factors related to in vitro and in vitro airway responsiveness and the severity of asthma. Both hypertrophy and hyperplasia have been shown in asthmatic airways by morphometrical analyses, although there is a wide variation in the contribution of each mechanism in each patient. Such changes could also be recognized as a phenotypic modulation of airway smooth muscle. On the background of airway smooth muscle remodeling, the existence of several contributing factors, such as inflammatory mediators, growth factors, cytokines, extra-cellular matrix proteins, and genetic factors have been suggested. On the other hand, recent studies revealed that airway smooth muscle could also be a source of inflammatory mediators promoting airway inflammation. In this article, the recent understanding in the mechanisms of airway smooth muscle remodeling in asthma, its relations to airway inflammation and airway physiology, and possible usefulness of early intervention with inhaled glucocorticoids have been discussed.

Smooth muscle length adaptation and actin filament length: a network model of the cytoskeletal dysregulation

Length adaptation of the airway smooth muscle cell is attributable to cytoskeletal remodeling. It has been proposed that dysregulated actin filaments may become longer in asthma, and that such elongation would prevent a parallel-to-series transition of contractile units, thus precluding the well-known beneficial effects of deep inspirations and tidal breathing. To test the potential effect that actin filament elongation could have in overall muscle mechanics, we present an extremely simple model. The cytoskeleton is represented as a 2-D network of links (contractile filaments) connecting nodes (adhesion plaques). Such a network evolves in discrete time steps by forming and dissolving links in a stochastic fashion. Links are formed by idealized contractile units whose properties are either those from normal or elongated actin filaments. Oscillations were then imposed on the network to evaluate both the effects of breathing and length adaptation. In response to length oscillation, a network with longer actin filaments showed smaller decreases of force, smaller increases in compliance, and higher shortening velocities. Taken together, these changes correspond to a network that is refractory to the effects of breathing and therefore approximates an asthmatic scenario. Thus, an extremely simple model seems to capture some relatively complex mechanics of airway smooth muscle, supporting the idea that dysregulation of actin filament length may contribute to excessive airway narrowing.

Understanding the pathophysiology of severe asthma to generate new therapeutic opportunities

Although asthma is defined in terms of reversibility of airflow obstruction, as the disease becomes more severe and chronic, it adopts different characteristics, including a degree of fixed airflow obstruction and corticosteroid refractoriness. Underlying these phenotypes is evidence of airway wall remodeling, which should be distinguished from the increase in smooth muscle linked to airways hyperresponsiveness. Aberrant epithelial-mesenchymal communication leads to a chronic wound scenario, which is characterized by activation of the epithelial-mesenchymal trophic unit, epithelial damage, the laying down of new matrix, and greater involvement of neutrophils in the inflammatory response. In allergic asthmatic patients who remain symptomatic despite high-dose corticosteroid therapy, blockade of IgE with omalizumab confers appreciable clinical benefit. Chronic severe asthma is also accompanied by a marked increase in TNF-alpha production that might contribute to corticosteroid refractoriness. Based on this, TNF blockade with the soluble fusion protein entanercept produces improvement in asthma symptoms, lung function, and quality of life paralleled by a marked reduction in airways hyperresponsiveness. Identification of novel susceptibility genes, such as a disintegrin and metalloprotease 33 (ADAM33), will provide further targets against which to direct novel therapies for asthma, especially at the more severe end of the disease spectrum.

The contribution of transforming growth factor-beta and epidermal growth factor signalling to airway remodelling in chronic asthma

Asthma is increasing in prevalence in the developing world, affecting approximately 10% of the world's population. It is characterised by chronic lung inflammation and airway remodelling associated with wheezing, shortness of breath, acute bronchial hyperresponsiveness to a variety of innocuous stimuli and a more rapid decline in lung function over time. Airway remodelling, involving proliferation and differentiation of mesenchymal cells, particularly myofibroblasts and smooth muscle cells, is generally refractory to corticosteroids and makes a major contribution to disease chronicity. Transforming growth factor-beta is a potent profibrogenic factor whose expression is increased in the asthmatic airways and is a prime candidate for the initiation and persistence of airway remodelling in asthma. This review highlights the role of transforming growth factor-beta in the asthmatic lung, incorporating biosynthesis, signalling pathways and functional outcome. In vivo, however, it is the balance between transforming growth factor-beta and other growth factors, such as epidermal growth factor, which will determine the extent of fibrosis in the airways. A fuller comprehension of the actions of transforming growth factor-beta, and its interaction with other signalling pathways, such as the epidermal growth factor receptor signalling cascade, may enable development of therapies that control airway remodelling where there is an unmet clinical need.

Weight of evidence: a review of concept and methods

"Weight of evidence" (WOE) is a common term in the published scientific and policy-making literature, most often seen in the context of risk assessment (RA). Its definition, however, is unclear. A systematic review of the scientific literature was undertaken to characterize the concept. For the years 1994 through 2004, PubMed was searched for publications in which "weight of evidence" appeared in the abstract and/or title. Of the 276 papers that met these criteria, 92 were selected for review: 71 papers published in 2003 and 2004 (WOE appeared in abstract/title) and 21 from 1994 through 2002 (WOE appeared in title). WOE has three characteristic uses in this literature: (1) metaphorical, where WOE refers to a collection of studies or to an unspecified methodological approach; (2) methodological, where WOE points to established interpretative methodologies (e.g., systematic narrative review, meta-analysis, causal criteria, and/or quality criteria for toxicological studies) or where WOE means that "all" rather than some subset of the evidence is examined, or rarely, where WOE points to methods using quantitative weights for evidence; and (3) theoretical, where WOE serves as a label for a conceptual framework. Several problems are identified: the frequent lack of definition of the term "weight of evidence," multiple uses of the term and a lack of consensus about its meaning, and the many different kinds of weights, both qualitative and quantitative, which can be used in RA. A practical recommendation emerges: the WOE concept and its associated methods should be fully described when used. A research agenda should examine the advantages of quantitative versus qualitative weighting schemes, how best to improve existing methods, and how best to combine those methods (e.g., epidemiology's causal criteria with toxicology's quality criteria).

Length adaptation of airway smooth muscle: a stochastic model of cytoskeletal dynamics

To account for cytoskeleton remodeling as well as smooth muscle length adaptation, here we represent the cytoskeleton as a two-dimensional network of links (contractile filaments or stress fibers) that connect nodes (dense plaques or focal adhesions). The network evolves in continuous turnover with probabilities of link formation and dissolution. The probability of link formation increases with the available fraction of contractile units, increases with the degree of network activation, and decreases with increasing distance between nodes, d, as 1/d(s), where s controls the distribution of link lengths. The probability of link dissolution decays with time to mimic progressive cytoskeleton stabilization. We computed network force (F) as the vector summation of link forces exerted at all nodes, unloaded shortening velocity (V) as being proportional to the average link length, and network compliance (C) as the change in network length per change in elastic force. Imposed deformation caused F to decrease transiently and then recover dynamically; recovery ability decreased with increasing time after activation, mimicking observed biological behavior. Isometric contractions showed small sensitivity of F to network length, thus maintaining high force over a wide range of lengths; V and C increased with increasing length. In these behaviors, link length regulation, as described by the parameter s, was found to be crucial. Concerning length adaptation, all phenomena reported thus far in the literature were captured by this extremely simple network model.

Latrunculin B increases force fluctuation-induced relengthening of ACh-contracted, isotonically shortened canine tracheal smooth muscle

We hypothesized that differences in actin filament length could influence force fluctuation-induced relengthening (FFIR) of contracted airway smooth muscle and tested this hypothesis as follows. One-hundred micromolar ACh-stimulated canine tracheal smooth muscle (TSM) strips set at optimal reference length ( Lref) were allowed to shorten against 32% maximal isometric force (Fmax) steady preload, after which force oscillations of ±16% Fmax were superimposed. Strips relengthened during force oscillations. We measured hysteresivity and calculated FFIR as the difference between muscle length before and after 20-min imposed force oscillations. Strips were relaxed by ACh removal and treated for 1 h with 30 nM latrunculin B (sequesters G-actin and promotes depolymerization) or 500 nM jasplakinolide (stabilizes actin filaments and opposes depolymerization). A second isotonic contraction protocol was then performed; FFIR and hysteresivity were again measured. Latrunculin B increased FFIR by 92.2 ± 27.6% Lref and hysteresivity by 31.8 ± 13.5% vs. pretreatment values. In contrast, jasplakinolide had little influence on relengthening by itself; neither FFIR nor hysteresivity was significantly affected. However, when jasplakinolide-treated tissues were then incubated with latrunculin B in the continued presence of jasplakinolide for 1 more h and a third contraction protocol performed, latrunculin B no longer substantially enhanced TSM relengthening. In TSM treated with latrunculin B + jasplakinolide, FFIR increased by only 3.03 ± 5.2% Lref and hysteresivity by 4.14 ± 4.9% compared with its first (pre-jasplakinolide or latrunculin B) value. These results suggest that actin filament length, in part, determines the relengthening of contracted airway smooth muscle.

Methodologic advancements in the study of airway smooth muscle

The study of isolated airway myocytes has provided important information relative to specific processes that regulate contraction, proliferation, and synthetic properties of airway smooth muscle (ASM). To place this information in physiological context, however, improved methods to examine airway biology in vivo are needed. Advances in genetic, biochemical, and optical methods provide unprecedented opportunities to improve our understanding of in vivo physiology and pathophysiology. This article describes 4 important methodologic advances in the study of ASM: (1) the development of transgenic mice that could be used to investigate ASM proliferation and phenotype switching during the development of hypersensitivity, and to investigate excitation-contraction coupling; (2) the use of CD38-deficient mice to confirm the role of CD38-dependent, cyclic adenosine diphosphate-ribose-mediated calcium release in airway responsiveness; (3) investigation of the role of actin filament length and p38 mitogen-activated protein kinase activity in regulating the mechanical plasticity-elasticity balance in contracted ASM; and (d) the use of bronchial biopsies to study ASM structure and phenotype in respiratory science.

Localized mechanical stress induces time-dependent actin cytoskeletal remodeling and stiffening in cultured airway smooth muscle cells

Mechanical stress (MS) causes cytoskeletal (CSK) and phenotypic changes in cells. Such changes in airway smooth muscle (ASM) cells might contribute to the pathophysiology of asthma. We have shown that periodic mechanical strain applied to cultured ASM cells alters the structure and expression of CSK proteins and increases cell stiffness and contractility (Smith PG, Moreno R, and Ikebe M. Am J Physiol Lung Cell Mol Physiol 272: L20-L27, 1997; and Smith PG, Deng L, Fredberg JJ, and Maksym GN. Am J Physiol Lung Cell Mol Physiol 285: L456-L463, 2003). However, the mechanically induced CSK changes, altered cell function, and their time courses are not well understood. Here we applied MS to the CSK by magnetically oscillating ferrimagnetic beads bound to the CSK. We quantified CSK remodeling by measuring actin accumulation at the sites of applied MS using fluorescence microscopy. We also measured CSK stiffness using optical magnetic twisting cytometry. We found that, during MS of up to 120 min, the percentage of beads associated with actin structures increased with time. At 60 min, 68.1 +/- 1.6% of the beads were associated with actin structures compared with only 6.7 +/- 2.8% before MS and 38.4 +/- 5.5% in time-matched controls (P < 0.05). Similarly, CSK stiffness increased more than twofold in response to the MS compared with time-matched controls. These changes were more pronounced than observed with contractile stimulation by 80 mM KCl or 10(-4) M acetylcholine. Together, these findings imply that MS is a potent stimulus to enhance stiffness and contractility of ASM cells through CSK remodeling, which may have important implications in airway narrowing and dilation in asthma.

Bronchospasm and its biophysical basis in airway smooth muscle

Airways hyperresponsiveness is a cardinal feature of asthma but remains unexplained. In asthma, the airway smooth muscle cell is the key end-effector of bronchospasm and acute airway narrowing, but in just the past five years our understanding of the relationship of responsiveness to muscle biophysics has dramatically changed. It has become well established, for example, that muscle length is equilibrated dynamically rather than statically, and that non-classical features of muscle biophysics come to the forefront, including unanticipated interactions between the muscle and its time-varying load, as well as the ability of the muscle cell to adapt rapidly to changes in its dynamic microenvironment. These newly discovered phenomena have been described empirically, but a mechanistic basis to explain them is only beginning to emerge.

Do inflammatory mediators influence the contribution of airway smooth muscle contraction to airway hyperresponsiveness in asthma?

It is now accepted that a host of cytokines, chemokines, growth factors, and other inflammatory mediators contributes to the development of nonspecific airway hyperresponsiveness in asthma. Yet, relatively little is known about how inflammatory mediators might promote airway structural remodeling or about the molecular mechanisms by which they might exaggerate smooth muscle shortening as observed in asthmatic airways. Taking a deep inspiration, which provides relief of bronchodilation in normal subjects, is less effective in asthmatic subjects, and some have speculated that this deficiency stems directly from an abnormality of airway smooth muscle and results in airway hyperresponsiveness to constrictor agonists. Here, we consider some of the mechanisms by which inflammatory mediators might acutely or chronically induce changes in the contractile apparatus that in turn might contribute to hyperresponsive airways in asthma.