Integrin-linked kinase regulates smooth muscle differentiation marker gene expression in airway tissue

Membrane adhesion junctions regulate airway smooth muscle phenotype and function

The local environment surrounding airway smooth muscle (ASM) cells has profound effects on the physiological and phenotypic properties of ASM tissues. ASM is continually subjected to the mechanical forces generated during breathing and to the constituents of its surrounding extracellular milieu. The smooth muscle cells within the airways continually modulate their properties to adapt to these changing environmental influences. Smooth muscle cells connect to the extracellular cell matrix (ECM) at membrane adhesion junctions that provide mechanical coupling between smooth muscle cells within the tissue. Membrane adhesion junctions also sense local environmental signals and transduce them to cytoplasmic and nuclear signaling pathways in the ASM cell. Adhesion junctions are composed of clusters of transmembrane integrin proteins that bind to ECM proteins outside the cell and to large multiprotein complexes in the submembranous cytoplasm. Physiological conditions and stimuli from the surrounding ECM are sensed by integrin proteins and transduced by submembranous adhesion complexes to signaling pathways to the cytoskeleton and nucleus. The transmission of information between the local environment of the cells and intracellular processes enables ASM cells to rapidly adapt their physiological properties to modulating influences in their extracellular environment: mechanical and physical forces that impinge on the cell, ECM constituents, local mediators, and metabolites. The structure and molecular organization of adhesion junction complexes and the actin cytoskeleton are dynamic and constantly changing in response to environmental influences. The ability of ASM to rapidly accommodate to the ever-changing conditions and fluctuating physical forces within its local environment is essential for its normal physiological function.

The proprotein convertase furin inhibits IL-13-induced inflammation in airway smooth muscle by regulating integrin-associated signaling complexes

Furin is a proprotein convertase that regulates the activation and the inactivation of multiple proteins including matrix metalloproteinases, integrins, and cytokines. It is a serine endoprotease that localizes to the plasma membrane and can be secreted into the extracellular space. The role of furin in regulating inflammation in isolated canine airway smooth muscle tissues was investigated. The treatment of airway tissues with recombinant furin (rFurin) inhibited the activation of Akt and eotaxin secretion induced by IL-13, and it prevented the IL-13-induced suppression of smooth muscle myosin heavy chain expression. rFurin promoted a differentiated phenotype by activating β₁-integrin proteins and stimulating the activation of the adhesome proteins vinculin and paxillin by talin. Activated paxillin induced the binding of Akt to β-parvin IPP [integrin-linked kinase (ILK), PINCH, parvin] complexes, which inhibits Akt activation. Treatment of tissues with a furin inhibitor or the depletion of endogenous furin using shRNA resulted in Akt activation and inflammatory responses similar to those induced by IL-13. Furin inactivation or IL-13 caused talin cleavage and integrin inactivation, resulting in the inactivation of vinculin and paxillin. Paxillin inactivation resulted in the coupling of Akt to α-parvin IPP complexes, which catalyze Akt activation and an inflammatory response. The results demonstrate that furin inhibits inflammation in airway smooth muscle induced by IL-13 and that the anti-inflammatory effects of furin are mediated by activating integrin proteins and integrin-associated signaling complexes that regulate Akt-mediated pathways to the nucleus. Furin may have therapeutic potential for the treatment of inflammatory conditions of the lungs and airways.

S100A4 is activated by RhoA and catalyses the polymerization of non-muscle myosin, adhesion complex assembly and contraction in airway smooth muscle

KEY POINTS

S100A4 is expressed in many tissues, including smooth muscle (SM), but its physiologic function is unknown. S100A4 regulates the motility of metastatic cancer cells by binding to non-muscle (NM) myosin II. Contractile stimulation causes the polymerization of NM myosin in airway SM, which is necessary for tension development. NM myosin regulates the assembly of adhesion junction signalling complexes (adhesomes) that catalyse actin polymerization. In airway SM, ACh (acetylcholine) stimulated the binding of S100A4 to the NM myosin heavy chain, which was catalysed by RhoA GTPase via the RhoA-binding protein, rhotekin. The binding of S100A4 to NM myosin was required for NM myosin polymerization, adhesome assembly and actin polymerization. S100A4 plays a critical function in the regulation of airway SM contraction by catalysing NM myosin filament assembly. The interaction of S100A4 with NM myosin may also play an important role in the physiologic function of other tissues.

ABSTRACT

S100A4 binds to the heavy chain of non-muscle (NM) myosin II and can regulate the motility of crawling cells. S100A4 is widely expressed in many tissues including smooth muscle (SM), although its role in the regulation of their physiologic function is not known. We hypothesized that S100A4 contributes to the regulation of contraction in airway SM by regulating a pool of NM myosin II at the cell cortex. NM myosin II undergoes polymerization in airway SM and regulates contraction by catalysing the assembly of integrin-associated adhesome complexes that activate pathways that catalyse actin polymerization. ACh stimulated the interaction of S100A4 with NM myosin II in airway SM at the cell cortex and catalysed NM myosin filament assembly. RhoA GTPase regulated the activation of S100A4 via rhotekin, which facilitated the formation of a complex between RhoA, S100A4 and NM myosin II. The depletion of S100A4, RhoA or rhotekin from airway SM tissues using short hairpin RNA or small interfering RNA prevented NM myosin II polymerization as well as the recruitment of vinculin and paxillin to adhesome signalling complexes in response to ACh, and inhibited actin polymerization and tension development. S100A4 depletion did not affect ACh-stimulated SM myosin regulatory light chain phosphorylation. The results show that S100A4 plays a critical role in tension development in airway SM tissue by catalysing NM myosin filament assembly, and that the interaction of S100A4 with NM myosin in response to contractile stimulation is activated by RhoA GTPase. These results may be broadly relevant to the physiologic function of S100A4 in other cell and tissue types.

S100A4 is secreted by airway smooth muscle tissues and activates inflammatory signaling pathways via receptors for advanced glycation end products

S100A4 is a low-molecular-mass (12 kDa) EF-hand Ca²⁺-binding S100 protein that is expressed in a broad range of normal tissue and cell types. S100A4 can be secreted from some cells to act in an autocrine or paracrine fashion on target cells and tissues. S100A4 has been reported in the extracellular fluids of subjects with several inflammatory diseases, including asthma. Airway smooth muscle plays a critical role in airway inflammation by synthesizing and secreting inflammatory cytokines. We hypothesized that S100A4 may play an immunomodulatory role in airway smooth muscle. Trachealis smooth muscle tissues were stimulated with recombinant His-S100A4, and the effects on inflammatory responses were evaluated. S100A4 induced the activation of Akt and NF-κB and stimulated eotaxin secretion. It also increased the expression of RAGE and endogenous S100A4 in airway tissues. Stimulation of airway smooth muscle tissues with IL-13 or TNF-α induced the secretion of S100A4 from the tissues and promoted the expression of endogenous receptors for advanced glycation end products (RAGE) and S100A4. The role of RAGE in mediating the responses to S100A4A was evaluated by expressing a mutant nonfunctional RAGE (RAGEΔcyto) in tracheal muscle tissues and by treating tissues with a RAGE inhibitor. S100A4 did not activate NF-κB or Akt in tissues that were expressing RAGEΔcyto or treated with a RAGE inhibitor, indicating that S100A4 mediates its effects by acting on RAGE. Our results demonstrate that inflammatory mediators stimulate the synthesis and secretion of S100A4 in airway smooth muscle tissues and that extracellular S100A4 acts via RAGE to mediate airway smooth muscle inflammation.

Phenotype transitions induced by mechanical stimuli in airway smooth muscle are regulated by differential interactions of parvin isoforms with paxillin and Akt

Mechanical tension and humoral stimuli can induce transitions in airway smooth muscle phenotype between a synthetic inflammatory state that promotes cytokine secretion and a differentiated state that promotes the expression of smooth muscle phenotype-specific proteins. When tissues are maintained under high tension, Akt activation and eotaxin secretion are suppressed, but expression of the differentiation marker protein, smooth muscle myosin heavy chain (SmMHC), is promoted. When tissues are maintained under low tension, Akt activation and eotaxin secretion are stimulated, and the differentiated phenotype is suppressed. We hypothesized that mechanical stimuli are differentially transduced to Akt-mediated signaling pathways that regulate phenotype expression by α-parvin and β-parvin integrin-linked kinase/PINCH/parvin (IPP) signaling complexes within integrin adhesomes. High tension or ACh triggered paxillin phosphorylation and the binding of phospho-paxillin to β-parvin IPP complexes. This inhibited Akt activation and promoted SmMHC expression. Low tension or IL-4 did not elicit paxillin phosphorylation and triggered the binding of unphosphorylated paxillin to α-parvin IPP complexes, which promoted Akt activation and eotaxin secretion and suppressed SmMHC expression. Expression of a nonphosphorylatable paxillin mutant or β-parvin depletion by siRNA promoted the inflammatory phenotype, whereas the depletion of α-parvin promoted the differentiated phenotype. Results demonstrate that phenotype expression is regulated by the differential interaction of phosphorylated and unphosphorylated paxillin with α-parvin and β-parvin IPP complexes and that these complexes have opposite effects on the activation of Akt. Our results describe a novel molecular mechanism for transduction of mechanical and humoral stimuli within integrin signaling complexes to regulate phenotype expression in airway smooth muscle.

Laminin α4 contributes to airway remodeling and inflammation in asthma

Airway inflammation and remodeling are characteristic features of asthma, with both contributing to airway hyperresponsiveness (AHR) and lung function limitation. Airway smooth muscle (ASM) accumulation and extracellular matrix deposition are characteristic features of airway remodeling, which may contribute to persistent AHR. Laminins containing the α2-chain contribute to characteristics of ASM remodeling in vitro and AHR in animal models of asthma. The role of other laminin chains, including the laminin α4 and α5 chains, which contribute to leukocyte migration in other diseases, is currently unknown. The aim of the current study was to investigate the role of these laminin chains in ASM function and in AHR, remodeling, and inflammation in asthma. Expression of both laminin α4 and α5 was observed in the human and mouse ASM bundle. In vitro, laminin α4 was found to promote a pro-proliferative, pro-contractile, and pro-fibrotic ASM cell phenotype. In line with this, treatment with laminin α4 and α5 function-blocking antibodies reduced allergen-induced increases in ASM mass in a mouse model of allergen-induced asthma. Moreover, eosinophilic inflammation was reduced by the laminin α4 function-blocking antibody as well. Using airway biopsies from healthy subjects and asthmatic patients, we found inverse correlations between ASM α4-chain expression and lung function and AHR, whereas eosinophil numbers correlated positively with expression of laminin α4 in the ASM bundle. This study, for the first time, indicates a prominent role for laminin α4 in ASM function and in inflammation, AHR, and remodeling in asthma, whereas the role of laminin α5 is more subtle.

Quantitative in situ proximity ligation assays examining protein interactions and phosphorylation during smooth muscle contractions

Antibody-based in situ proximity ligation assays (isPLA) have the potential to study protein phosphorylation and protein interactions with spatial resolution in intact tissues. However, the application of isPLA at the tissue level is limited by a lack of appropriate positive and negative controls and the difficulty in accounting for changes in tissue shape. Here we demonstrate a set of experimental and computational approaches using gastric fundus smooth muscles to improve the validity of quantitative isPLA. Appropriate positive and negative biological controls and PLA technical controls were selected to ensure experimental rigor. To account for changes in morphology between relaxed and contracted smooth muscles, target PLA spots were normalized to smooth muscle myosin light chain 20 PLA spots or the cellular cross-sectional areas. We describe the computational steps necessary to filter out false-positive improperly sized spots and set the thresholds for counting true positive PLA spots to quantify the PLA signals. We tested our approach by examining protein phosphorylation and protein interactions in smooth muscle myofilament Ca²⁺ sensitization pathways from resting and contracted gastric fundus smooth muscles. In conclusion, our tissue-level isPLA method enables unbiased quantitation of protein phosphorylation and protein-protein interactions in intact smooth muscle tissues, suggesting the potential for quantitative isPLA applications in other types of intact tissues.

Molecular Mechanisms for the Mechanical Modulation of Airway Responsiveness

The smooth muscle of the airways is exposed to continuously changing mechanical forces during normal breathing. The mechanical oscillations that occur during breathing have profound effects on airway tone and airway responsiveness both in experimental animals and humans in vivo and in isolated airway tissues in vitro. Experimental evidence suggests that alterations in the contractile and mechanical properties of airway smooth muscle tissues caused by mechanical perturbations result from adaptive changes in the organization of the cytoskeletal architecture of the smooth muscle cell. The cytoskeleton is a dynamic structure that undergoes rapid reorganization in response to external mechanical and pharmacologic stimuli. Contractile stimulation initiates the assembly of cytoskeletal/extracellular matrix adhesion complex proteins into large macromolecular signaling complexes (adhesomes) that undergo activation to mediate the polymerization and reorganization of a submembranous network of actin filaments at the cortex of the cell. Cortical actin polymerization is catalyzed by Neuronal-Wiskott–Aldrich syndrome protein (N-WASP) and the Arp2/3 complex, which are activated by pathways regulated by paxillin and the small GTPase, cdc42. These processes create a strong and rigid cytoskeletal framework that may serve to strengthen the membrane for the transmission of force generated by the contractile apparatus to the extracellular matrix, and to enable the adaptation of smooth muscle cells to mechanical stresses. This model for the regulation of airway smooth muscle function can provide novel perspectives to explain the normal physiologic behavior of the airways and pathophysiologic properties of the airways in asthma.

Rho kinase collaborates with p21-activated kinase to regulate actin polymerization and contraction in airway smooth muscle

KEY POINTS

The mechanisms by which Rho kinase (ROCK) regulates airway smooth muscle contraction were determined in tracheal smooth muscle tissues. ROCK may mediate smooth muscle contraction by inhibiting myosin regulatory light chain (RLC) phosphatase. ROCK can also regulate F-actin dynamics during cell migration, and actin polymerization is critical for airway smooth muscle contraction. Our results show that ROCK does not regulate airway smooth muscle contraction by inhibiting myosin RLC phosphatase or by stimulating myosin RLC phosphorylation. We find that ROCK regulates airway smooth muscle contraction by activating the serine-threonine kinase Pak, which mediates the activation of Cdc42 and neuronal Wiskott-Aldrich syndrome protein (N-WASp). N-WASP transmits signals from Cdc42 to the Arp2/3 complex for the nucleation of actin filaments. These results demonstrate a novel molecular function for ROCK in the regulation of Pak and Cdc42 activation that is critical for the processes of actin polymerization and contractility in airway smooth muscle.

ABSTRACT

Rho kinase (ROCK), a RhoA GTPase effector, can regulate the contraction of airway and other smooth muscle tissues. In some tissues, ROCK can inhibit myosin regulatory light chain (RLC) phosphatase, which increases the phosphorylation of myosin RLC and promotes smooth muscle contraction. ROCK can also regulate cell motility and migration by affecting F-actin dynamics. Actin polymerization is stimulated by contractile agonists in airway smooth muscle tissues and is required for contractile tension development in addition to myosin RLC phosphorylation. We investigated the mechanisms by which ROCK regulates the contractility of tracheal smooth muscle tissues by expressing a kinase-inactive mutant of ROCK, ROCK-K121G, in the tissues or by treating them with the ROCK inhibitor H-1152P. Our results show no role for ROCK in the regulation of non-muscle or smooth muscle myosin RLC phosphorylation during contractile stimulation in this tissue. We found that ROCK regulates airway smooth muscle contraction by mediating activation of p21-activated kinase (Pak), a serine-threonine kinase, to promote actin polymerization. Pak catalyses paxillin phosphorylation on Ser273 and coupling of the GIT1-βPIX-Pak signalling module to paxillin, which activates the guanine nucleotide exchange factor (GEF) activity of βPIX towards Cdc42. Cdc42 is required for the activation of neuronal Wiskott-Aldrich syndrome protein (N-WASp), which transmits signals from Cdc42 to the Arp2/3 complex for the nucleation of actin filaments. Our results demonstrate a novel molecular function for ROCK in the regulation of Pak and Cdc42 activation that is critical for the processes of actin polymerization and contractility in airway smooth muscle.

Large-scale profiling of signalling pathways reveals an asthma specific signature in bronchial smooth muscle cells

Background

Bronchial smooth muscle (BSM) cells from asthmatic patients maintain in vitro a distinct hyper-reactive (“primed”) phenotype, characterized by increased release of pro-inflammatory factors and mediators, as well as hyperplasia and/or hypertrophy. This “primed” phenotype helps to understand pathogenesis of asthma, as changes in BSM function are essential for manifestation of allergic and inflammatory responses and airway wall remodelling.

Objective

To identify signalling pathways in cultured primary BSMs of asthma patients and non-asthmatic subjects by genome wide profiling of differentially expressed mRNAs and activated intracellular signalling pathways (ISPs).

Methods

Transcriptome profiling by cap-analysis-of-gene-expression (CAGE), which permits selection of preferentially capped mRNAs most likely to be translated into proteins, was performed in human BSM cells from asthmatic (n=8) and non-asthmatic (n=6) subjects and OncoFinder tool were then exploited for identification of ISP deregulations.

Results

CAGE revealed >600 RNAs differentially expressed in asthma vs control cells (p≤0.005), with asthma samples showing a high degree of similarity among them. Comprehensive ISP activation analysis revealed that among 269 pathways analysed, 145 (p<0.05) or 103 (p<0.01) are differentially active in asthma, with profiles that clearly characterize BSM cells of asthmatic individuals. Notably, we identified 7 clusters of coherently acting pathways functionally related to the disease, with ISPs down-regulated in asthma mostly targeting cell death-promoting pathways and up-regulated ones affecting cell growth and proliferation, inflammatory response, control of smooth muscle contraction and hypoxia-related signalization.

Conclusions

These first-time results can now be exploited toward development of novel therapeutic strategies targeting ISP signatures linked to asthma pathophysiology.

Elastase alters contractility and promotes an inflammatory synthetic phenotype in airway smooth muscle tissues

Neutrophil elastase is secreted by inflammatory cells during airway inflammation and can elicit airway hyperreactivity in vivo. Elastase can degrade multiple components of the extracellular matrix. We hypothesized that elastase might disrupt the connections between airway smooth muscle (ASM) cells and the extracellular matrix and that this might have direct effects on ASM tissue responsiveness and inflammation. The effect of elastase treatment on ASM contractility was assessed in vitro in isolated strips of canine tracheal smooth muscle by stimulation of tissues with cumulatively increasing concentrations of acetylcholine (ACh) and measurement of contractile force. Elastase treatment potentiated contractile responses to ACh at low concentrations but suppressed the maximal contractile force generated by the tissues without affecting the phosphorylation of myosin regulatory light chain (RLC). Elastase also promoted the secretion of eotaxin and the activation of Akt in ASM tissues and decreased expression of smooth muscle myosin heavy chain, consistent with promotion of a synthetic inflammatory phenotype. As the degradation of matrix proteins can alter integrin engagement, we evaluated the effect of elastase on the assembly and activation of integrin-associated adhesion junction complexes in ASM tissues. Elastase led to talin cleavage, reduced talin binding to vinculin, and suppressed activation of the adhesome proteins paxillin, focal adhesion kinase, and vinculin, indicating that elastase causes the disassembly of adhesion junction complexes and the inactivation of adhesome signaling proteins. We conclude that elastase promotes an inflammatory phenotype and increased sensitivity to ACh in ASM tissues by disrupting signaling pathways mediated by integrin-associated adhesion complexes.

Focal adhesion kinase (FAK) and mechanical stimulation negatively regulate the transition of airway smooth muscle tissues to a synthetic phenotype

The effects of mechanical forces and focal adhesion kinase (FAK) in regulating the inflammatory responses of airway smooth muscle (ASM) tissues to stimulation with interleukin (IL)-13 were investigated. Canine tracheal tissues were subjected to different mechanical loads in vitro, and the effects of mechanical load on eotaxin secretion and inflammatory signaling pathways in response to IL-13 were determined. Eotaxin secretion by tissues in response to IL-13 was significantly inhibited in muscles maintained at a higher (+) load compared with those at a lower (-) load as assessed by ELISA, and Akt activation was also reduced in the higher (+) loaded tissues. Conversely the (+) mechanical load increased activation of the focal adhesion proteins FAK and paxillin in the tissues. The role of FAK in regulating the mechanosensitive responses was assessed by overexpressing FAK-related nonkinase in the tissues, by expressing the FAK kinase-dead mutant FAK Y397F, or by treating tissues with the FAK inhibitor PF-573228. FAK inactivation potentiated Akt activity and increased eotaxin secretion in response to IL-13. FAK inhibition also suppressed the mechanosensitivity of Akt activation and eotaxin secretion. In addition, FAK inactivation suppressed smooth muscle myosin heavy chain expression induced by the higher (+) mechanical load. The results demonstrate that the imposition of a higher mechanical load on airway smooth muscle stimulates FAK activation, which promotes the expression of the differentiated contractile phenotype and suppresses the synthetic phenotype and the inflammatory responses of the muscle tissue.

Airway Hyperresponsiveness in Asthma Model Occurs Independently of Secretion of β1 Integrins in Airway Wall and Focal Adhesions Proteins Down Regulation

The extracellular domains of some membrane proteins can be shed from the cell. A similar phenomenon occurs with β1 integrins (α1β1 and α2β1) in guinea pig. The putative role of β1 integrin subunit alterations due to shedding in airway smooth muscle (ASM) in an allergic asthma model was evaluated. Guinea pigs were sensitized and challenged with antigen. Antigenic challenges induced bronchoobstruction and hyperresponsiveness at the third antigenic challenge. Immunohistochemistry and immunoelectronmicroscopy studies showed that the cytosolic and extracellular domains of the β1 integrin subunit shared the same distribution in airway structures in both groups. Various polypeptides with similar molecular weights were detected with both the cytosolic and extracellular β1 integrin subunit antibodies in isolated airway myocytes and the connective tissue that surrounds the ASM bundle. Flow cytometry and Western blot studies showed that the expression of cytosolic and extracellular β1 integrin subunit domains in ASM was similar between groups. An increment of ITGB1 mRNA in ASM was observed in the asthma model group. RACE-PCR of ITGB1 in ASM did not show splicing variants. The expression levels of integrin-linked kinase (ILK) and paxillin diminished in the asthma model, but not talin. The levels of phosphorylation of myosin phosphatase target subunit 1 (MYPT1) at Thr(696) increased in asthma model. Our work suggests that β1 integrin is secreted in guinea pig airway wall. This secretion is not altered in asthma model; nevertheless, β1 integrin cytodomain assembly proteins in focal cell adhesions in which ILK and paxillin are involved are altered in asthma model. J. Cell. Biochem. 117: 2385-2396, 2016. © 2016 Wiley Periodicals, Inc.

Critical role of actin-associated proteins in smooth muscle contraction, cell proliferation, airway hyperresponsiveness and airway remodeling

Asthma is characterized by airway hyperresponsiveness and airway remodeling, which are largely attributed to increased airway smooth muscle contractility and cell proliferation. It is known that both chemical and mechanical stimulation regulates smooth muscle contraction. Recent studies suggest that contractile activation and mechanical stretch induce actin cytoskeletal remodeling in smooth muscle. However, the mechanisms that control actin cytoskeletal reorganization are not completely elucidated. This review summarizes our current understanding regarding how actin-associated proteins may regulate remodeling of the actin cytoskeleton in airway smooth muscle. In particular, there is accumulating evidence to suggest that Abelson tyrosine kinase (Abl) plays a critical role in regulating airway smooth muscle contraction and cell proliferation in vitro, and airway hyperresponsiveness and remodeling in vivo. These studies indicate that Abl may be a novel target for the development of new therapy to treat asthma.

Contribution of SRF, Elk-1, and myocardin to airway smooth muscle remodeling in heaves, an asthma-like disease of horses

Myocyte hyperplasia and hypertrophy contribute to the increased mass of airway smooth muscle (ASM) in asthma. Serum-response factor (SRF) is a transcription factor that regulates myocyte differentiation in vitro in vascular and intestinal smooth muscles. When SRF is associated with phosphorylated (p)Elk-1, it promotes ASM proliferation while binding to myocardin (MYOCD) leading to the expression of contractile elements in these tissues. The objective of this study was therefore to characterize the expression of SRF, pElk-1, and MYOCD in ASM cells from central and peripheral airways in heaves, a spontaneously occurring asthma-like disease of horses, and in controls. Six horses with heaves and five aged-matched controls kept in the same environment were studied. Nuclear protein expression of SRF, pElk-1, and MYOCD was evaluated in peripheral airways and endobronchial biopsies obtained during disease remission and after 1 and 30 days of naturally occurring antigenic exposure using immunohistochemistry and immunofluorescence techniques. Nuclear expression of SRF (P = 0.03, remission vs. 30 days) and MYOCD (P = 0.05, controls vs. heaves at 30 days) increased in the peripheral airways of horses with heaves during disease exacerbation, while MYOCD (P = 0.04, remission vs. 30 days) decreased in the central airways of control horses. No changes were observed in the expression of pElk-1 protein in either tissue. In conclusion, SRF and its cofactor MYOCD likely contribute to the hypertrophy of peripheral ASM observed in equine asthmatic airways, while the remodeling of the central airways is more static or involves different transcription factors.

Integrins: therapeutic targets in airway hyperresponsiveness and remodelling?

Integrins are a group of transmembrane heterodimeric proteins that mediate cell-cell and cell-extracellular matrix (ECM) interactions. Integrins have been under intense investigation for their role in inflammation in asthma. Clinical trials investigating integrin antagonists, however, have shown that these compounds are relatively ineffective. Airway remodelling is another pathological feature of asthma that is thought to make an important contribution to airway hyperresponsiveness (AHR) and lung function decline. Recent studies have identified integrins as important players in this process, with a particular role for β1 and αv integrins. Here we review the role of these integrins in airway remodelling and hyperresponsiveness in obstructive airway disease and their potential as pharmacological targets for future treatment.

Beyond TGFβ--novel ways to target airway and parenchymal fibrosis

Within the lungs, fibrosis can affect both the parenchyma and the airways. Fibrosis is a hallmark pathological change in the parenchyma in patients with idiopathic pulmonary fibrosis (IPF), whilst in asthma or chronic obstructive pulmonary disease (COPD) fibrosis is a component of the remodelling of the airways. In the past decade, significant advances have been made in understanding the disease behaviour and pathogenesis of parenchymal and airway fibrosis and as a result a variety of novel therapeutic targets for slowing or preventing progression of these fibrotic changes have been identified. This review highlights a number of these targets and discusses the potential for treating parenchymal or airway fibrosis through these mediators/pathways in the future.

Slack length reduces the contractile phenotype of the Swine carotid artery

Contraction is the primary function of adult arterial smooth muscle. However, in response to vessel injury or inflammation, arterial smooth muscle is able to phenotypically modulate from the contractile state to several 'synthetic' states characterized by proliferation, migration and/or increased cytokine secretion. We examined the effect of tissue length (L) on the phenotype of intact, isometrically held, initially contractile swine carotid artery tissues. Tissues were studied (1) without prolonged incubation at the optimal length for force generation (1.0 Lo, control), (2) with prolonged incubation for 17 h at 1.0 Lo, or (3) with prolonged incubation at slack length (0.6 Lo) for 16 h and then restoration to 1.0 Lo for 1 h. Prolonged incubation at 1.0 Lo minimally reduced the contractile force without substantially altering the mediators of contraction (crossbridge phosphorylation, shortening velocity or stimulated actin polymerization). Prolonged incubation of tissues at slack length (0.6 Lo), despite return of length to 1.0 Lo, substantially reduced contractile force, reduced crossbridge phosphorylation, nearly abolished crossbridge cycling (shortening velocity) and abolished stimulated actin polymerization. These data suggest that (1) slack length treatment significantly alters the contractile phenotype of arterial tissue, and (2) slack length treatment is a model to study acute phenotypic modulation of intact arterial smooth muscle.

Focal adhesion kinase regulates collagen I-induced airway smooth muscle phenotype switching

Increased extracellular matrix (ECM) deposition and airway smooth muscle (ASM) mass are major contributors to airway remodeling in asthma. Recently, we demonstrated that the ECM protein collagen I, which is increased surrounding asthmatic ASM, induces a proliferative, hypocontractile ASM phenotype. Little is known, however, about the signaling pathways involved. Using bovine tracheal smooth muscle, we investigated the role of focal adhesion kinase (FAK) and downstream signaling pathways in collagen I-induced ASM phenotype modulation. Phosphorylation of FAK was increased during adhesion to both uncoated and collagen I-coated culture dishes, without differences between these matrices. Nor were any differences found in cellular adhesion. Inhibition of FAK activity by overexpression of the FAK deletion mutants FAT (focal adhesion targeting domain) and FRNK (FAK-related nonkinase) attenuated adhesion. After attachment, FAK phosphorylation increased in a time-dependent manner in cells cultured on collagen I, whereas no activation was found on an uncoated plastic matrix. In addition, collagen I increased in a time- and concentration-dependent manner the cell proliferation, which was fully inhibited by FAT and FRNK. Similarly, the specific pharmacologic FAK inhibitor PF-573228 [6-((4-((3-(methanesulfonyl)benzyl)amino)-5-trifluoromethylpyrimidin-2-yl) amino)-3,4-dihydro-1H-quinolin-2-one] as well as specific inhibitors of p38 mitogen-activated protein kinase (MAPK) and Src also fully inhibited collagen I-induced proliferation, whereas partial inhibition was observed by inhibition of phosphatidylinositol-3-kinase (PI3-kinase) and mitogen-activated protein kinase kinase (MEK). The inhibition of cell proliferation by these inhibitors was associated with attenuation of the collagen I-induced hypocontractility. Collectively, the results indicate that induction of a proliferative, hypocontractile ASM phenotype by collagen I is mediated by FAK and downstream signaling pathways.

Increased mechanical strain imposed on murine lungs during ventilation in vivo depresses airway responsiveness and activation of protein kinase Akt

Continuous positive airway pressure (CPAP) administered to tracheostomized rabbits and ferrets for 4 days or 2 wk suppresses bronchial reactivity in vivo and suppresses airway reactivity in lobes and tracheal segments isolated from these animals. In vitro studies of canine tracheal smooth muscle tissues indicate that mechanical loading suppresses the activation of the growth regulatory kinase, Akt, and that Akt is a negative regulator of smooth muscle differentiation. The transduction of mechanical signals in the tracheal tissues in vitro is mediated by integrin-associated adhesion complexes. To determine whether airway responsiveness and Akt activation are modulated by mechanical loads applied for short time periods to the airways of living animals in vivo, mice were mechanically ventilated for 2 h with high (5 cmH2O) or low (0-1 cmH2O) positive end-expiratory pressure (PEEP) and then ventilated at low PEEP for 30 min. Ventilation of mice with PEEP in vivo for 2 h depressed airway responsiveness to methacholine measured in vivo subsequent to the PEEP treatment. Airway narrowing in vitro in intraparenchymal airways in isolated lung slices and contractile responses of isolated tracheal segments in vitro were suppressed for at least 6 h subsequent to the in vivo exposure to PEEP. Tracheal segments isolated from high PEEP-treated mice exhibited significantly lower levels of Akt activation than tracheae from low PEEP-treated mice. The results indicate that mechanical loads imposed in vivo result in physiological and biochemical changes in the airway tissues after a relatively short 2-h period of in vivo loading.

Effects of integrin-linked kinase on human corpus cavernosum smooth muscle cell cytoskeletal organisation

We investigated the effects of integrin-linked kinase (ILK) on the in vitro attachment, spreading, migration and microfilament dynamics of human corpus cavernosum smooth muscle cells. ILK small interfering RNA (siRNA) was used to transfect human corpus cavernosum smooth muscle cells; and cell attachment, spreading and migration were assessed. Additionally, microfilament dynamics were evaluated using Alexa Fluor 488 and phalloidin staining. We found that ILK gene knock-down significantly inhibited human corpus cavernosum smooth muscle cell attachment, spreading and migration. Moreover, blocking the expression of ILK disturbed actin cytoskeleton reorganisation and morphology in human corpus cavernosum smooth muscle cells. These results show that the targeting of ILK with siRNA significantly inhibited cell attachment, spreading, migration and microfilament dynamics in human corpus cavernosum smooth muscle cells. These findings indicate that ILK might be a potential therapeutic molecular target for the treatment of erectile dysfunction.

Signaling mechanisms that regulate smooth muscle cell differentiation

Extensive studies over the last 30 years have demonstrated that vascular smooth muscle cell (SMC) differentiation and phenotypic modulation is controlled by a dynamic array of environmental cues. The identification of the signaling mechanisms by which these environmental cues regulate SMC phenotype has been more difficult because of our incomplete knowledge of the transcription mechanisms that regulate SMC-specific gene expression. However, recent advances in this area have provided significant insight, and the goal of this review is to summarize the signaling mechanisms by which extrinsic cues control SMC differentiation.

Mechanical stimuli and IL-13 interact at integrin adhesion complexes to regulate expression of smooth muscle myosin heavy chain in airway smooth muscle tissue

Airway smooth muscle phenotype may be modulated in response to external stimuli under physiological and pathophysiological conditions. The effect of mechanical forces on airway smooth muscle phenotype were evaluated in vitro by suspending weights of 0.5 or 1 g from the ends of canine tracheal smooth muscle tissues, incubating the weighted tissues for 6 h, and then measuring the expression of the phenotypic marker protein, smooth muscle myosin heavy chain (SmMHC). Incubation of the tissues at a high load significantly increased expression of SmMHC compared with incubation at low load. Incubation of the tissues at a high load also decreased activation of PKB/Akt, as indicated by its phosphorylation at Ser 473. Inhibition of Akt or phosphatidylinositol-3,4,5 triphosphate-kinase increased SmMHC expression in tissues at low load but did not affect SmMHC expression at high load. IL-13 induced a significant increase in Akt activation and suppressed the expression of SmMHC protein at both low and high loads. The role of integrin signaling in mechanotransduction was evaluated by expressing a PINCH (LIM1-2) fragment in the muscle tissues that prevents the membrane localization of the integrin-binding IPP complex (ILK/PINCH/α-parvin), and also by expressing an inactive integrin-linked kinase mutant (ILK S343A) that inhibits endogenous ILK activity. Both mutants inhibited Akt activation and increased expression of SmMHC protein at low load but had no effect at high load. These results suggest that mechanical stress and IL-13 both act through an integrin-mediated signaling pathway to oppositely regulate the expression of phenotypic marker proteins in intact airway smooth muscle tissues. The stimulatory effects of mechanical stress on contractile protein expression oppose the suppression of contractile protein expression mediated by IL-13; thus the imposition of mechanical strain may inhibit changes in airway smooth muscle phenotype induced by inflammatory mediators.

Chronic continuous positive airway pressure (CPAP) reduces airway reactivity in vivo in an allergen-induced rabbit model of asthma

Previous studies have demonstrated that chronic mechanical strain produced by continuous positive airway pressure (CPAP) reduces in vivo airway reactivity in rabbits and ferrets. For CPAP to potentially have a therapeutic benefit for asthmatic subjects, the reduction in airway responsiveness would need to persist for 12-24 h after its discontinuation, require application for only part of the day, and be effective in the presence of atopic airway inflammation. In the present study, airway responsiveness to acetylcholine or methacholine was measured during mechanical ventilation following three different protocols in which active, nonanesthetized, tracheotomized rabbits were treated with High vs. Low CPAP (6 vs. 0 cmH(2)O). 1) High CPAP was applied continuously for 4 days followed by 1 day of Low CPAP; 2) High CPAP was applied at night and Low CPAP during the daytime for 4 days, and 3) High CPAP was applied for 4 days in animals following ovalbumin (Ova) sensitization and challenge. For all three protocols, treatment with High CPAP resulted in significantly reduced airway responsiveness compared with treatment with Low CPAP. Cumulatively, our in vivo results in rabbits suggest that high CPAP, even when applied only at night, produces a persistent reduction of airway responsiveness. In addition, CPAP reduces airway responsiveness even in the presence of atopic airway inflammation.

From molecule to man: integrating molecular biology with whole organ physiology in studying respiratory disease

Chronic lung diseases such as asthma, chronic obstructive pulmonary disease (COPD), and idiopathic pulmonary fibrosis (IPF) are all characterized by structural changes of the airways and/or lungs that limit airflow and/or gas exchange. Currently, there is no therapy available that adequately targets the structural remodeling of the airways and lungs in these diseases. This underscores the great need for insight into the mechanisms that underpin the development of airway remodeling, fibrosis and emphysema in these diseases, in order to identify suitable drug targets. It is increasingly evident that structural cell-cell communication within the lung is central to the development of remodeling, indicating that a more integrative approach should be considered when studying molecular and cellular mechanisms of remodeling. Therefore, there is a great need to study molecular and cellular physiological and pathophysiological mechanisms in as much detail as possible, but with as little as possible loss of the physiological context. Here, we will review the use of models such as cellular co-culture, tissue culture, and lung slice culture, in which cell-cell communication and tissue architecture are better preserved or mimicked than in cell culture, and zoom in on the usefulness of molecular and cellular biological tools in these complex model systems to read out or control signaling and gene/protein regulation.

Myosin light chain kinase is necessary for tonic airway smooth muscle contraction

Different interacting signaling modules involving Ca(2+)/calmodulin-dependent myosin light chain kinase, Ca(2+)-independent regulatory light chain phosphorylation, myosin phosphatase inhibition, and actin filament-based proteins are proposed as specific cellular mechanisms involved in the regulation of smooth muscle contraction. However, the relative importance of specific modules is not well defined. By using tamoxifen-activated and smooth muscle-specific knock-out of myosin light chain kinase in mice, we analyzed its role in tonic airway smooth muscle contraction. Knock-out of the kinase in both tracheal and bronchial smooth muscle significantly reduced contraction and myosin phosphorylation responses to K(+)-depolarization and acetylcholine. Kinase-deficient mice lacked bronchial constrictions in normal and asthmatic airways, whereas the asthmatic inflammation response was not affected. These results indicate that myosin light chain kinase acts as a central participant in the contractile signaling module of tonic smooth muscle. Importantly, contractile airway smooth muscles are necessary for physiological and asthmatic airway resistance.