Inhibition of the p38 MAP kinase pathway destabilizes smooth muscle length during physiological loading

Tristetraprolin and its role in regulation of airway inflammation

Chronic inflammatory diseases, such as asthma and chronic obstructive pulmonary disease (COPD), are clinically and socioeconomically important diseases globally. Currently the mainstay of anti-inflammatory therapy in respiratory diseases is corticosteroids. Although corticosteroids have proven clinical efficacy in asthma, many asthmatic inflammatory conditions (e.g., infection, exacerbation, and severe asthma) are not responsive to corticosteroids. Moreover, despite an understanding that COPD progression is driven by inflammation, we currently do not have effective anti-inflammatory strategies to combat this disease. Hence, alternative anti-inflammatory strategies are required. p38 mitogen-activated protein kinase (MAPK) has emerged as an important signaling molecule driving airway inflammation, and pharmacological inhibitors against p38 MAPK may provide potential therapies for chronic respiratory disease. In this review, we discuss some of the recent in vitro and in vivo studies targeting p38 MAPK, but suggest that p38 MAPK inhibitors may prove less effective than originally considered because they may block anti-inflammatory molecules along with proinflammatory responses. We propose that an alternative strategy may be to target an anti-inflammatory molecule farther downstream of p38 MAPK, i.e., tristetraprolin (TTP). TTP is an mRNA-destabilizing, RNA-binding protein that enhances the decay of mRNAs, including those encoding proteins implicated in chronic respiratory diseases. We suggest that understanding the molecular mechanism of TTP expression and its temporal regulation will guide future development of novel anti-inflammatory pharmacotherapeutic approaches to combat respiratory disease.

Airway smooth muscle: a potential target for asthma therapy

PURPOSE OF REVIEW

Asthma is a major public health problem that afflicts nearly one in 20 people worldwide. Despite available treatments, asthma symptoms remain poorly controlled in a significant minority of asthma patients, especially those with severe disease. Accordingly, much ongoing effort has been directed at developing new therapeutic strategies; these efforts are described in detail below.

RECENT FINDINGS

Although mucus hypersecretion is an important component of asthma pathobiology, the primary mechanism of morbidity and mortality in asthma is excessive narrowing of the airway. The key end- effector of excessive airway narrowing is airway smooth muscle (ASM) contraction; overcoming ASM contraction is therefore a prominent therapeutic strategy. Here, we review exciting new advances aimed at ASM relaxation.

SUMMARY

Exciting advances in ASM biology have identified new therapeutic targets for the prevention or reversal of bronchoconstriction in asthma.

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.

Airway smooth muscle in the pathophysiology and treatment of asthma

Airway smooth muscle (ASM) plays an integral part in the pathophysiology of asthma. It is responsible for acute bronchoconstriction, which is potentiated by constrictor hyperresponsiveness, impaired relaxation and length adaptation. ASM also contributes to airway remodeling and inflammation in asthma. In light of this, ASM is an important target in the treatment of asthma.

Effect of length oscillations on airway smooth muscle reactivity and cross-bridge cycling

Excessive airway narrowing due to airway smooth muscle (ASM) hyperconstriction is a major symptom in many respiratory diseases. In vitro imposition of length oscillations similar to those produced by tidal breathing on contracted ASM have shown to reduce muscle active forces, which is usually attributed to unconfirmed disruption of actomyosin cross-bridges. This research focuses on an in vitro investigation of the effect of mechanical oscillations on ASM reactivity and actomyosin cross-bridges. A computerized organ bath system was used to test maximally precontracted bovine ASM subjected to length oscillations at frequencies in the range of 10–100 Hz superimposed on tidal breathing oscillation. Using an immunofluorescence technique, two specific antibodies against the phospho-serine19 myosin light chain and the α-smooth muscle actin were used to analyze the colocalization between these two filaments. Data were processed using the plug-in “colocalization threshold” of ImageJ 1.43m software. The results demonstrate that both tidal and superimposed length oscillations reduce the active force in contracted ASM for a relatively long term and that the latter enhances the force reduction of the former. This reduction was also found to be frequency and time dependent. Additionally colocalization analysis indicates that length oscillations cause the detachment of the actomyosin connections and that this condition is sustained even after the cessation of the length oscillations.

Dilatation of the constricted human airway by tidal expansion of lung parenchyma

RATIONALE

In the normal lung, breathing and deep inspirations potently antagonize bronchoconstriction, but in the asthmatic lung this salutary effect is substantially attenuated or even reversed. To explain these findings, the prevailing hypothesis focuses on contracting airway smooth muscle and posits a nonlinear dynamic interaction between actomyosin binding and the tethering forces imposed by tidally expanding lung parenchyma.

OBJECTIVE

This hypothesis has never been tested directly in bronchial smooth muscle embedded within intraparenchymal airways. Our objective here is to fill that gap.

METHODS

We designed a novel system to image contracting intraparenchymal human airways situated within near-normal lung architecture and subjected to dynamic parenchymal expansion that simulates breathing.

MEASUREMENTS AND MAIN RESULTS

Reversal of bronchoconstriction depended on the degree to which breathing actually stretched the airway, which in turn depended negatively on severity of constriction and positively on the depth of breathing. Such behavior implies positive feedbacks that engender airway instability.

OVERALL CONCLUSIONS

These findings help to explain heterogeneity of airflow obstruction as well as why, in people with asthma, deep inspirations are less effective in reversing bronchoconstriction.

p38 mitogen-activated protein kinase pathways in asthma and COPD

The mitogen-activated protein kinase (MAPK) family includes the p38 kinases, which consist of highly conserved proline-directed serine-threonine protein kinases that are activated in response to inflammatory signals. Of the four isoforms, p38α is the most abundant in inflammatory cells and has been the most studied through mainly the availability of small molecule inhibitors. The p38 substrates include transcription factors; other protein kinases, which in turn phosphorylate transcription factors; cytoskeletal proteins and translational components; and other enzymes. Both asthma and COPD are characterized by chronic airflow obstruction, airway and lung remodeling, and chronic inflammation. p38 is involved in the inflammatory responses induced by cigarette smoke exposure, endotoxin, and oxidative stress through activation and release of proinflammatory cytokines/chemokines, posttranslational regulation of these genes, and activation of inflammatory cell migration. Inhibition of p38 MAPK prevented allergen-induced pulmonary eosinophilia, mucus hypersecretion, and airway hyperresponsiveness, effects that may partly result from p38 activation on eosinophil apoptosis and on airway smooth muscle cell production of cytokines/chemokines. In addition, p38 regulates the augmented contractile response induced by oxidative stress. The activation of p38 observed in epithelial cells and macrophages also may underlie corticosteroid insensitivity of severe asthma and COPD. Therefore, p38 inhibitors present a potential attractive treatment of these conditions. Second-generation p38 inhibitors have been disappointing in the treatment of rheumatoid arthritis. In two 6-week studies in patients with COPD, the results were encouraging. Side effects such as liver toxicity remain a possibility, and whether the beneficial effects of p38 inhibitors are clinically significant and sustained need to be determined.

The synergistic effect of sodium chlorite and bromochloroacetic acid on BrO3(-)-induced renal cell death

Bromate (BrO(3)(-)) is a drinking water disinfection by-product (DBP) that induces renal cell death via DNA damage-dependent and -independent mechanisms. Drinking water contains other DBPs in addition to BrO(3)(-). We tested the effect of two of these, sodium chlorite (NaClO(2)) and bromochloroacetic acid (BCAA), on BrO(3)(-) cytotoxicity in normal rat kidney (NRK) cells. NaClO(2) and BCAA alone induced cytotoxicity at concentrations of over 20ppm, while BrO(3)(-) was only moderately cytotoxic at concentrations of 200ppm. Combining BrO(3)(-) with NaClO(2) or BCAA alone enhanced cytotoxicity 1.5-4 fold. Exposing cells to all three compounds induced synergistic-like increases in cytotoxicity. This effect did not correlate to increases in reactive oxygen species (ROS), even though all three compounds induced ROS formation alone. NaClO(2), but not BCAA, increased BrO(3)(-)-mediated DNA damage as measured by 8-hydroxydeoxyguanosine (8-OHdG) staining. In addition, NaClO(2), but not BCAA, decreased BrO(3)(-)-induced G2/M cell cycle arrest. Both compounds increased apoptosis in the presence of BrO(3)(-) as assessed by annexin V, PI, and DAPI staining. This is in contrast to BrO(3)(-) treatment alone, which induced necrosis. Immunoblot analysis showed that both NaClO(2) and BCAA increased p38 activation; however, consistent with 8-OHdG staining, only NaClO(2) increased BrO(3)(-)-induced histone H2AX phosphorylation, a marker of DNA damage. In contrast, BCAA, but not NaClO(2), increased BrO(3)(-)-induced phosphorylation of p53. These data support the novel finding that mixtures of DBPs increase BrO(3)(-)-induced renal cell death by DNA-dependent and -independent mechanisms, and could alter how the risk of these DBPs towards humans is assessed.

Inhibition of p38 MAPK-dependent bronchial contraction after ozone by corticosteroids

We determined the role of p38 mitogen-activated protein kinase (MAPK) in the increased airway smooth muscle (ASM) contractile responses following ozone and modulation by corticosteroids. Mice were exposed to air or ozone (3 ppm for 3 h) and isometric contractile responses of bronchial rings to acetylcholine (ACh) were measured using a myograph in the presence of p38 MAPK inhibitor, SB239063 (10⁻⁶ M) or dexamethasone (10⁻⁶ M). Because MAPK phosphatase (MKP)-1 is a negative regulator of p38 MAPK, we also studied these effects in MKP-1(-/-) mice. Bronchial rings from ozone-exposed wild-type and MKP-1(-/-) mice showed increased contractile responses, with a leftward shift of the dose-response curve in MKP-1(-/-) mice. SB239063 inhibited bronchial contraction equally in air- and ozone-exposed C57/BL6 and MKP-1(-/-) mice. Dexamethasone inhibited ACh-induced bronchial contraction in both air- and ozone-exposed C57/BL6 mice, but not in air- or ozone-exposed MKP-1(-/-) mice. ACh-stimulated p38 MAPK and heat shock protein (HSP)27 phosphorylation, as measured by Western blotting, and this effect was suppressed by SB239063 in C57/BL6 and MKP-1(-/-) mice, but not by dexamethasone in either air- or ozone-exposed MKP-1(-/-) mice. p38 MAPK plays a role in maximal ACh-induced isometric contractile responses and increased contractility induced by ozone. Dexamethasone inhibits ACh-induced ASM contraction through phosphorylation of p38 MAPK and HSP27.

Emergence of airway smooth muscle functions related to structural malleability

The function of a complex system such as a smooth muscle cell is the result of the active interaction among molecules and molecular aggregates. Emergent macroscopic manifestations of these molecular interactions, such as the length-force relationship and its associated length adaptation, are well documented, but the molecular constituents and organization that give rise to these emergent muscle behaviors remain largely unknown. In this minireview, we describe emergent properties of airway smooth muscle that seem to have originated from inherent fragility of the cellular structures, which has been increasingly recognized as a unique and important smooth muscle attribute. We also describe molecular interactions (based on direct and indirect evidence) that may confer malleability on fragile structural elements that in turn may allow the muscle to adapt to large and frequent changes in cell dimensions. Understanding how smooth muscle works may hinge on how well we can relate molecular events to its emergent macroscopic functions.

Length oscillation mimicking periodic individual deep inspirations during tidal breathing attenuates force recovery and adaptation in airway smooth muscle

Airway smooth muscle (ASM) is able to generate maximal force under static conditions, and this isometric force can be maintained over a large length range due to length adaptation. The increased force at short muscle length could lead to excessive narrowing of the airways. Prolonged exposure of ASM to submaximal stimuli also increases the muscle's ability to generate force in a process called force adaptation. To date, the effects of length and force adaptation have only been demonstrated under static conditions. In the mechanically dynamic environment of the lung, ASM is constantly subjected to periodic stretches by the parenchyma due to tidal breathing and deep inspiration. It is not known whether force recovery due to muscle adaptation to a static environment could occur in a dynamic environment. In this study the effect of length oscillation mimicking tidal breathing and deep inspiration was examined. Force recovery after a length change was attenuated in the presence of length oscillation, except at very short lengths. Force adaptation was abolished by length oscillation. We conclude that in a healthy lung (with intact airway-parenchymal tethering) where airways are not allowed to narrow excessively, large stretches (associated with deep inspiration) may prevent the ability of the muscle to generate maximal force that would occur under static conditions irrespective of changes in mean length; mechanical perturbation on ASM due to tidal breathing and deep inspiration, therefore, is the first line of defense against excessive bronchoconstriction that may result from static length and force adaptation.

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.

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.

Steroids augment relengthening of contracted airway smooth muscle: potential additional mechanism of benefit in asthma

Breathing (especially deep breathing) antagonises development and persistence of airflow obstruction during bronchoconstrictor stimulation. Force fluctuations imposed on contracted airway smooth muscle (ASM) in vitro result in its relengthening, a phenomenon called force fluctuation-induced relengthening (FFIR). Because breathing imposes similar force fluctuations on contracted ASM within intact lungs, FFIR represents a likely mechanism by which breathing antagonises bronchoconstriction. While this bronchoprotective effect appears to be impaired in asthma, corticosteroid treatment can restore the ability of deep breaths to reverse artificially induced bronchoconstriction in asthmatic subjects. It has previously been demonstrated that FFIR is physiologically regulated through the p38 mitogen-activated protein kinase (MAPK) signalling pathway. While the beneficial effects of corticosteroids have been attributed to suppression of airway inflammation, the current authors hypothesised that alternatively they might exert their action directly on ASM by augmenting FFIR as a result of inhibiting p38 MAPK signalling. This possibility was tested in the present study by measuring relengthening in contracted canine tracheal smooth muscle (TSM) strips. The results indicate that dexamethasone treatment significantly augmented FFIR of contracted canine TSM. Canine tracheal ASM cells treated with dexamethasone demonstrated increased MAPK phosphatase-1 expression and decreased p38 MAPK activity, as reflected in reduced phosphorylation of the p38 MAPK downstream target, heat shock protein 27. These results suggest that corticosteroids may exert part of their therapeutic effect through direct action on airway smooth muscle, by decreasing p38 mitogen-activated protein kinase activity and thus increasing force fluctuation-induced relengthening.

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.

Expression of smooth muscle and extracellular matrix proteins in relation to airway function in asthma

BACKGROUND

Smooth muscle content is increased within the airway wall in patients with asthma and is likely to play a role in airway hyperresponsiveness. However, smooth muscle cells express several contractile and structural proteins, and each of these proteins may influence airway function distinctly.

OBJECTIVE

We examined the expression of contractile and structural proteins of smooth muscle cells, as well as extracellular matrix proteins, in bronchial biopsies of patients with asthma, and related these to lung function, airway hyperresponsiveness, and responses to deep inspiration.

METHODS

Thirteen patients with asthma (mild persistent, atopic, nonsmoking) participated in this cross-sectional study. FEV(1)% predicted, PC(20) methacholine, and resistance of the respiratory system by the forced oscillation technique during tidal breathing and deep breath were measured. Within 1 week, a bronchoscopy was performed to obtain 6 bronchial biopsies that were immunohistochemically stained for alpha-SM-actin, desmin, myosin light chain kinase (MLCK), myosin, calponin, vimentin, elastin, type III collagen, and fibronectin. The level of expression was determined by automated densitometry.

RESULTS

PC(20) methacholine was inversely related to the expression of alpha-smooth muscle actin (r = -0.62), desmin (r = -0.56), and elastin (r = -0.78). In addition, FEV(1)% predicted was positively related and deep inspiration-induced bronchodilation inversely related to desmin (r = -0.60), MLCK (r = -0.60), and calponin (r = -0.54) expression.

CONCLUSION

Airway hyperresponsiveness, FEV(1)% predicted, and airway responses to deep inspiration are associated with selective expression of airway smooth muscle proteins and components of the extracellular matrix.

Strange dynamics of a dynamic cytoskeleton

A novel physical perspective of molecular interactions within the cytoskeleton of the airway smooth muscle cell may help to explain why the most efficacious of all known bronchodilatory agencies-a simple deep inspiration-becomes abrogated during the spontaneous asthma attack and leads thereby to excessive airway narrowing. This perspective invites us to think of airway smooth muscle not only biochemically as a nidus of traditional cell signaling and immune modulation or mechanically as a motor for generation of active forces but also physically as a phase of soft condensed matter that can restrict airway stretch and dilation. This is perhaps a risky path and is surely an unconventional one, but it is where the trail of evidence leads. This line of investigation is unlikely by itself to provide an asthma cure but will lead to a new conceptual framework without which novel pathways, unsuspected phase transitions, and unanticipated mechanisms of action of target molecules would almost surely remain hidden. Glassy dynamics of the cytoskeleton are likely to be important in a wide range of biological functions and disease processes, but had it not been for their preeminent role in bronchospasm, they might never have been discovered.

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.

Airway hyperresponsiveness, remodeling, and smooth muscle mass: right answer, wrong reason?

We quantified the effects of airway wall remodeling upon airway smooth muscle (ASM) shortening. Isolated ASM from sheep was attached to a servo-controller that applied a physiologic load. This load could be altered to reflect specified changes of airway wall geometry, elasticity, parenchymal tethering, transpulmonary pressure (P(L)), and fluctuations in P(L) associated with breathing. Starting at a reference length (L(ref)), ASM was stimulated with acetlycholine and held at constant P(L) of 4 cm H(2)O for 2 h. When all compartments were thickened to simulate the asthmatic airway but P(L) was held fixed, ASM shortened much more than that in the normal airway (to 0.52 L(ref) versus 0.66 L(ref)). When breathing with deep inspirations (DIs) was initiated, within the first three DIs the ASM in the normal airway lengthened to 0.84 L(ref), whereas that in the asthmatic airway remained stuck at 0.53 L(ref). Thickening of the smooth muscle layer alone produced the greatest muscle shortening (to 0.47 L(ref)) when compared with thickening of only submucosal (to 0.67 L(ref)) or only adventitial (to 0.62 L(ref)) compartments. With increased ASM mass, the ASM failed to lengthen in response to DIs, whereas in the airway with thickened submucosal and adventitial layers ASM lengthened dramatically (to 0.83 L(ref)). These findings confirm the long-held conclusion that increased muscle mass is the functionally dominant derangement, but mechanisms accounting for this conclusion differ dramatically from those previously presumed. Furthermore, increased ASM mass explained both hyperresponsiveness and the failure of a DI to relax the asthmatic airway.

Mechanisms of mechanical strain memory in airway smooth muscle

We evaluated the hypothesis that mechanical deformation of airway smooth muscle induces structural remodeling of airway smooth muscle cells, thereby modulating mechanical performance in subsequent contractions. This hypothesis implied that past experience of mechanical deformation was retained (or "memorized") as structural changes in airway smooth muscle cells, which modulated the cell's subsequent contractile responses. We termed this phenomenon mechanical strain memory. Preshortening has been found to induce attenuation of both force and isotonic shortening velocity in cholinergic receptor-activated airway smooth muscle. Rapid stretching of cholinergic receptor-activated airway smooth muscle from an initial length to a final length resulted in post-stretch force and myosin light chain phosphorylation that correlated significantly with initial length. Thus post-stretch muscle strips appeared to retain memory of the initial length prior to rapid stretch (mechanical strain memory). Cytoskeletal recruitment of actin- and integrin-binding proteins and Erk 1/2 MAPK appeared to be important mechanisms of mechanical strain memory. Sinusoidal length oscillation led to force attenuation during oscillation and in subsequent contractions in intact airway smooth muscle, and p38 MAPK appeared to be an important mechanism. In contrast, application of local mechanical strain to cultured airway smooth muscle cells induced local actin polymerization and cytoskeletal stiffening. It is conceivable that deep inspiration-induced bronchoprotection may be a manifestation of mechanical strain memory such that mechanical deformation from past breathing cycles modulated the mechanical performance of airway smooth muscle in subsequent cycles in a continuous and dynamic manner.

Tidal breathing pattern differentially antagonizes bronchoconstriction in C57BL/6J vs. A/J mice

There is abundant evidence that tidal breathing, and especially tidal breathing at elevated minute ventilation, antagonizes the development and persistence of airflow obstruction during bronchoconstrictor stimulation in normal animals and people. Here, we studied the antiobstructive effect of different tidal breathing patterns in C57Bl/6J and A/J mice during bronchoconstriction induced by continuous or bolus infusion of methacholine. Anesthetized, paralyzed mice were mechanically ventilated at 1,500 ml.kg(-1).min(-1), using each of three breathing patterns: 5 ml/kg, 300 breath/min; 10 ml/kg, 150 breath/min; or 20 ml/kg, 75 breath/min. Changing from 10 ml/kg, 150 breath/min to 20 ml/kg, 75 breath/min, breathing functionally antagonized bronchoconstriction, reducing the level of airflow obstruction induced by methacholine infusion or boluses equivalently in both strains. In marked contrast, changing from 10 ml/kg, 150 breath/min to 5 ml/kg, 300 breath/min, breathing substantially exacerbated methacholine-induced airflow obstruction in A/J mice, whereas it had no significant effect in C57Bl/6J mice. Our results therefore demonstrate that 1) even at moderate, fixed minute ventilation, the precise breathing pattern can influence the degree of airflow obstruction substantially, and 2) the influence of breathing pattern on bronchoconstriction differs considerably between genetically diverse inbred mouse strains. These findings imply that differences in antiobstructive effects of breathing can contribute to differences in apparent airway constrictor responsiveness. Much attention has been placed on dysregulation of contractile function of airway smooth muscle in human disease. We suggest that important pathophysiology might also be found in impairment of the functional antagonist effect of tidal breathing on airflow obstruction.

A maturational model for the study of airway smooth muscle adaptation to mechanical oscillation

It has been shown that mechanical stretches imposed on airway smooth muscle (ASM) by deep inspiration reduce the subsequent contractile response of the ASM. This passive maneuver of lengthening and retraction of the muscle is beneficial in normal subjects to counteract bronchospasm. However, it is detrimental to hyperresponsive airways because it triggers further bronchoconstriction. Although the exact mechanisms for this contrary response by normal and hyperresponsive airways are unclear, it has been suggested that the phenomenon is related to changes in ASM adaptability to mechanical oscillation. Healthy immature airways of both human and animal exhibit hyperresponsiveness, but whether the adaptative properties of hyperresponsive airway differ from normal is still unknown. In this article, we review the phenomenon of ASM adaptation to mechanical oscillation and its relevance and implication to airway hyperresponsiveness. We demonstrate that the age-specific expression of ASM adaptation is prominent using an established maturational animal model developed in our laboratory. Our data on immature ASM showed potentiated contractile force shortly after a length oscillation compared with the maximum force generated before oscillation. Several potential mechanisms such as myogenic response, changes in actin polymerization, or changes in the quantity of the cytoskeletal regulatory proteins plectin and vimentin, which may underlie this age-specific force potentiation, are discussed. We suggest a working model of the structure of smooth muscle associated with force transmission, which may help to elucidate the mechanisms responsible for the age-specific expression of smooth muscle adaptation. It is important to study the maturational profile of ASM adaptation as it could contribute to juvenile hyperresponsiveness.Key words: ASM adaptation, maturation, bronchoprotection, airway hyperresponsiveness.

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.

Inhibition of cross-bridge formation has no effect on contraction-associated phosphorylation of p38 MAPK in mouse skeletal muscle

Mitogen-activated protein kinases (MAPKs), in particular p38 MAPK, are phosphorylated in response to contractile activity, yet the mechanism for this is not understood. We tested the hypothesis that the force of contraction is responsible for p38 MAPK phosphorylation in skeletal muscle. Extensor digitorum longus (EDL) muscles isolated from adult male Swiss Webster mice were stimulated at fixed length at 10 Hz for 15 min and then subjected to Western blot analysis for the phosphorylation of p38 MAPK and ERK1/2. Contralateral muscles were fixed at resting length and were not stimulated. Stimulated muscles showed a 2.5-fold increase in phosphorylated p38 MAPK relative to nonstimulated contralateral controls, and there was no change in the phosphorylation of ERK1/2. When contractile activity was inhibited with N-benzyl-p-toluene sulfonamide (BTS), a specific inhibitor of actomyosin ATPase, force production decreased in both a time- and concentration-dependent manner. Preincubation with 25, 75, and 150 microM BTS caused 78+/-4%, 97+/-0.2%, and 99+/-0.2% inhibition in contractile force, respectively, and was stable after 30 min of treatment. Fluorescence measurements demonstrated that Ca2+ cycling was minimally affected by BTS treatment. Surprisingly, BTS did not suppress the level of p38 MAPK phosphorylation in stimulated muscles. These data do not support the view that force generation per se activates p38 MAPK and suggest that other events associated with contraction must be responsible.

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.

Sinusoidal length oscillation- and receptor-mediated mRNA expression of myosin isoforms and alpha-SM actin in airway smooth muscle

We tested the hypothesis that sinusoidal length oscillation and receptor activation interactively regulate the abundance of mRNA encoding alpha-smooth muscle (alpha-SM) actin and myosin isoforms in intact bovine tracheal smooth muscle. We found that sinusoidal length oscillation significantly downregulated abundance of mRNA encoding alpha-SM actin mRNA in unstimulated tissues but not in histamine- and carbachol-activated tissues. This observation suggests antagonistic interactions between mechanical stretch and receptor-mediated signal transduction in regulating the abundance of mRNA encoding alpha-SM actin in intact airway smooth muscle. This pattern of antagonistic interaction was also observed in cholinergic receptor activation experiments. Whereas carbachol significantly upregulated myosin heavy chain SMA isoform expression in muscle strips held at slack length, carbachol did not significantly alter SMA expression in muscle strips at sinusoidal length oscillation. Carbachol also significantly upregulated GAPDH expression in bovine tracheal smooth muscle. However, unlike SMA expression, upregulation of GAPDH expression mediated by cholinergic receptor activation appeared to be insensitive to the mechanical state of airway smooth muscle. Unlike carbachol, histamine did not significantly alter the expression of GAPDH, myosin heavy chain SMA and SMB, myosin light chain LC17a and LC17b, and alpha-SM actin in bovine tracheal smooth muscle. U0126 (10 muM) completely inhibited carbachol-induced ERK1/2 MAPK phosphorylation but did not significantly affect carbachol-induced upregulation of GAPDH and SMA expression, suggesting that the ERK1/2 MAPK pathway was not the underlying mechanism. A potential implication of these findings is that periodic stretching of airways during respiratory cycles may modulate mRNA expression by receptor agonists in airway smooth muscle cells in vivo.

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.

Mechanisms of inflammation-mediated airway smooth muscle plasticity and airways remodeling in asthma

Recent evidence points to progressive structural change in the airway wall, driven by chronic local inflammation, as a fundamental component for development of irreversible airway hyperresponsiveness. Acute and chronic inflammation is orchestrated by cytokines from recruited inflammatory cells, airway myofibroblasts and myocytes. Airway myocytes exhibit functional plasticity in their capacity for contraction, proliferation, and synthesis of matrix protein and cytokines. This confers a principal role in driving different components of the airway remodeling process, and mediating constrictor hyperresponsiveness. Functional plasticity of airway smooth muscle (ASM) is regulated by an array of environmental cues, including cytokines, which mediate their effects through receptors and a number of intracellular signaling pathways. Despite numerous studies of the cellular effects of cytokines on cultured airway myocytes, few have identified how intracellular signaling pathways modulate or induce these cellular responses. This review summarizes current understanding of these concepts and presents a model for the effects of inflammatory mediators on functional plasticity of ASM in asthma.

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.

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

Bronchial hyperresponsiveness (BHR), the occurrence of excessive bronchoconstriction in response to relatively small constrictor stimuli, is a cardinal feature of asthma. Here, we consider the role that airway smooth muscle might play in the generation of BHR. The weight of evidence suggests that smooth muscle isolated from asthmatic tissues exhibits normal sensitivity to constrictor agonists when studied during isometric contraction, but the increased muscle mass within asthmatic airways might generate more total force than the lesser amount of muscle found in normal bronchi. Another salient difference between asthmatic and normal individuals lies in the effect of deep inhalation (DI) on bronchoconstriction. DI often substantially reverses induced bronchoconstriction in normals, while it often has much less effect on spontaneous or induced bronchoconstriction in asthmatics. It has been proposed that abnormal dynamic aspects of airway smooth muscle contraction velocity of contraction or plasticity- elasticity balance might underlie the abnormal DI response in asthma. We suggest a speculative model in which abnormally long actin filaments might account for abnormally increased elasticity of contracted airway smooth muscle.