Tissue and matrix influences on airway smooth muscle function

Functional role of glial-derived neurotrophic factor in a mixed allergen murine model of asthma

Our previous study showed that glial-derived neurotrophic factor (GDNF) expression is upregulated in asthmatic human lungs, and GDNF regulates calcium responses through its receptor GDNF family receptor α1 (GFRα1) and RET receptor in human airway smooth muscle (ASM) cells. In this study, we tested the hypothesis that airway GDNF contributes to airway hyperreactivity (AHR) and remodeling using a mixed allergen mouse model. Adult C57BL/6J mice were intranasally exposed to mixed allergens (ovalbumin, Aspergillus, Alternaria, house dust mite) over 4 wk with concurrent exposure to recombinant GDNF, or extracellular GDNF chelator GFRα1-Fc. Airway resistance and compliance to methacholine were assessed using FlexiVent. Lung expression of GDNF, GFRα1, RET, collagen, and fibronectin was examined by RT-PCR and histology staining. Allergen exposure increased GDNF expression in bronchial airways including ASM and epithelium. Laser capture microdissection of the ASM layer showed increased mRNA for GDNF, GFRα1, and RET in allergen-treated mice. Allergen exposure increased protein expression of GDNF and RET, but not GFRα1, in ASM. Intranasal administration of GDNF enhanced baseline responses to methacholine but did not consistently potentiate allergen effects. GDNF also induced airway thickening, and collagen deposition in bronchial airways. Chelation of GDNF by GFRα1-Fc attenuated allergen-induced AHR and particularly remodeling. These data suggest that locally produced GDNF, potentially derived from epithelium and/or ASM, contributes to AHR and remodeling relevant to asthma.NEW & NOTEWORTHY Local production of growth factors within the airway with autocrine/paracrine effects can promote features of asthma. Here, we show that glial-derived neurotrophic factor (GDNF) is a procontractile and proremodeling factor that contributes to allergen-induced airway hyperreactivity and tissue remodeling in a mouse model of asthma. Blocking GDNF signaling attenuates allergen-induced airway hyperreactivity and remodeling, suggesting a novel approach to alleviating structural and functional changes in the asthmatic airway.

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.

Smooth muscle brain-derived neurotrophic factor contributes to airway hyperreactivity in a mouse model of allergic asthma

Recent studies have demonstrated an effect of neurotrophins, particularly brain-derived neurotrophic factor (BDNF), on airway contractility [ via increased airway smooth muscle (ASM) intracellular calcium [Ca2+]i] and remodeling (ASM proliferation and extracellular matrix formation) in the context of airway disease. In the present study, we examined the role of BDNF in allergen-induced airway inflammation using 2 transgenic models: 1) tropomyosin-related kinase B (TrkB) conditional knockin (TrkBKI) mice allowing for inducible, reversible disruption of BDNF receptor kinase activity by administration of 1NMPP1, a PP1 derivative, and 2) smooth muscle-specific BDNF knockout (BDNFfl/fl/SMMHC11Cre/0) mice. Adult mice were intranasally challenged with PBS or mixed allergen ( Alternaria alternata, Aspergillus fumigatus, house dust mite, and ovalbumin) for 4 wk. Our data show that administration of 1NMPP1 in TrkBKI mice during the 4-wk allergen challenge blunted airway hyperresponsiveness (AHR) and reduced fibronectin mRNA expression in ASM layers but did not reduce inflammation per se. Smooth muscle-specific deletion of BDNF reduced AHR and blunted airway fibrosis but did not significantly alter airway inflammation. Together, our novel data indicate that TrkB signaling is a key modulator of AHR and that smooth muscle-derived BDNF mediates these effects during allergic airway inflammation.-Britt, R. D., Jr., Thompson, M. A., Wicher, S. A., Manlove, L. J., Roesler, A., Fang, Y.-H., Roos, C., Smith, L., Miller, J. D., Pabelick, C. M., Prakash, Y. S. Smooth muscle brain-derived neurotrophic factor contributes to airway hyperreactivity in a mouse model of allergic asthma.

Effects of ozone stimulation of bronchial epithelial cells on proliferation and collagen synthesis of co-cultured lung fibroblasts

Ozone (O₃) as a major air pollutant is widely recognized for causing pathological changes of the airway system. However, it is not clear whether O₃ exposure of bronchial epithelial cells (BECs) influences the proliferation and collagen synthesis of submucosal fibroblasts and contributes to the pathogenesis of airway remodeling in diseases, including asthma. In the present study, a co-culture method was applied to culture human lung fibroblasts (HLFs) with human bronchial epithelial cells (HBECs) that were pre-stimulated with O₃. Following co-culture for up to 24 h, the proliferation of HLFs was measured using MTT colorimetry. Furthermore, the collagen synthesis capacity of HLFs was determined by the level of hydroxyproline. In addition, the protein expression levels of cytokines, including transforming growth factor (TGF)-β1, tumor necrosis factor (TNF)-α and prostaglandin E2 (PGE2) were assessed. Results indicated that the proliferation of HLFs co-cultured with HBECs was significantly inhibited when compared with HLFs cultured alone (P<0.05). By contrast, co-culture with O₃-stimulated HBECs significantly promoted the proliferation of HLFs compared with the HLFs cultured alone or those cultured with HBECs but no O₃ stimulation, respectively (P<0.05 and P<0.01). Furthermore, similar effects were observed regarding the collagen synthesis capacity of HLFs co-cultured with HBECs for 24. In the supernatant, TGF-β1 concentration was continuously increased over 24 h, whereas the concentration of PGE2 increased and plateaued between 12 to 24 h and TNF-α concentration was not significantly altered during the assessed time period. To conclude, the present results suggest that O₃ pre-exposure of HBECs may promote the transformation of HLFs from the typical inhibitory state into a promoting state with respect to proliferation and collagen synthesis, which may likely occur through a mechanism that influences the balance between pro- and anti-inflammatory factors, including TGF-β1 and PGE2. The present findings may improve the understanding of the mechanism involved in O₃-induced airway remodeling from a novel perspective of maintenance/loss of steady-state function of the airway epithelium.

CD34 Differentially Regulates Contractile and Noncontractile Elements of Airway Reactivity

Airway hyperresponsiveness (AHR), a major hallmark of asthma, results from alterations of contractile and noncontractile elements of airway reactivity. CD34 is a sialomucin that is expressed on various cells involved in asthma, such as eosinophils and airway smooth muscle precursors, highlighting its potential influence in AHR. To study the role of CD34 in regulating the contractile and noncontractile elements of AHR, AHR was induced by chronic exposure to house dust mite (HDM) antigen. To assess the role of CD34 on the contractile elements of AHR, airway reactivity and airway smooth muscle contractility in response to methacholine were measured. To assess CD34's role in regulating the noncontractile elements of AHR, a chimeric mouse model was used to determine the impact of CD34 expression on inflammatory versus microenvironmental cells in AHR development. Extracellular matrix production, mucus production, and mast cell degranulation were also measured. Whereas wild-type mice developed AHR in response to HDM, a loss of airway reactivity was observed in Cd34^-/- mice 24 hours after the last exposure to HDM compared with naive controls. This was reversed when airway reactivity was measured 1 week after the last HDM exposure. Additionally, mast cell degranulation and mucus production were altered in the absence of CD34 expression. Importantly, simultaneous expression of CD34 on cells originating from the hematopoietic compartment and the microenvironment was needed for expression of this phenotype. These results provide evidence that CD34 is required for AHR and airway reactivity maintenance in the early days after an inflammatory episode in asthma.

Brain-derived neurotrophic factor and airway fibrosis in asthma

Airway remodeling in asthma driven by inflammation involves proliferation of epithelial cells and airway smooth muscle (ASM), as well as enhanced extracellular matrix (ECM) generation and deposition, i.e., fibrosis. Accordingly, understanding profibrotic mechanisms is important for developing novel therapeutic strategies in asthma. Recent studies, including our own, have suggested a role for locally produced growth factors such as brain-derived neurotrophic factor (BDNF) in mediating and modulating inflammation effects. In this study, we explored the profibrotic influence of BDNF in the context of asthma by examining expression, activity, and deposition of ECM proteins in primary ASM cells isolated from asthmatic vs. nonasthmatic patients. Basal BDNF expression and secretion, and levels of the high-affinity BDNF receptor TrkB, were higher in asthmatic ASM. Exogenous BDNF significantly increased ECM production and deposition, especially of collagen-1 and collagen-3 (less so fibronectin) and the activity of matrix metalloproteinases (MMP-2, MMP-9). Exposure to the proinflammatory cytokine TNFα significantly increased BDNF secretion, particularly in asthmatic ASM, whereas no significant changes were observed with IL-13. Chelation of BDNF using TrkB-Fc reversed TNFα-induced increase in ECM deposition. Conditioned media from asthmatic ASM enhanced ECM generation in nonasthmatic ASM, which was blunted by BDNF chelation. Inflammation-induced changes in MMP-2, MMP-9, and tissue inhibitor metalloproteinases (TIMP-1, TIMP-2) were reversed in the presence of TrkB-Fc. These novel data suggest ASM as an inflammation-sensitive source of BDNF within human airways, with autocrine effects on fibrosis relevant to asthma.

Emerging concepts in smooth muscle contributions to airway structure and function: implications for health and disease

Airway structure and function are key aspects of normal lung development, growth, and aging, as well as of lung responses to the environment and the pathophysiology of important diseases such as asthma, chronic obstructive pulmonary disease, and fibrosis. In this regard, the contributions of airway smooth muscle (ASM) are both functional, in the context of airway contractility and relaxation, as well as synthetic, involving production and modulation of extracellular components, modulation of the local immune environment, cellular contribution to airway structure, and, finally, interactions with other airway cell types such as epithelium, fibroblasts, and nerves. These ASM contributions are now found to be critical in airway hyperresponsiveness and remodeling that occur in lung diseases. This review emphasizes established and recent discoveries that underline the central role of ASM and sets the stage for future research toward understanding how ASM plays a central role by being both upstream and downstream in the many interactive processes that determine airway structure and function in health and disease.

The Effects of Tumstatin on Vascularity, Airway Inflammation and Lung Function in an Experimental Sheep Model of Chronic Asthma

Tumstatin, a protein fragment of the alpha-3 chain of Collagen IV, is known to be significantly reduced in the airways of asthmatics. Further, there is evidence that suggests a link between the relatively low level of tumstatin and the induction of angiogenesis and inflammation in allergic airway disease. Here, we show that the intra-segmental administration of tumstatin can impede the development of vascular remodelling and allergic inflammatory responses that are induced in a segmental challenge model of experimental asthma in sheep. In particular, the administration of tumstatin to lung segments chronically exposed to house dust mite (HDM) resulted in a significant reduction of airway small blood vessels in the diameter range 10⁺–20 μm compared to controls. In tumstatin treated lung segments after HDM challenge, the number of eosinophils was significantly reduced in parenchymal and airway wall tissues, as well as in the bronchoalveolar lavage fluid. The expression of VEGF in airway smooth muscle was also significantly reduced in tumstatin-treated segments compared to control saline-treated segments. Allergic lung function responses were not attenuated by tumstatin administration in this model. The data are consistent with the concept that tumstatin can act to suppress vascular remodelling and inflammation in allergic airway disease.

Effects of antenatal lipopolysaccharide and postnatal hyperoxia on airway reactivity and remodeling in a neonatal mouse model

BACKGROUND

Antenatal inflammation and preterm birth are associated with the development of airway diseases such as wheezing and asthma. Utilizing a newborn mouse model, we assessed the effects of maternal inflammation and postnatal hyperoxia on the neonatal airway.

METHODS

Pregnant C57/Bl6 dams were injected with lipopolysaccharide (LPS) or saline on embryonic day 16. Offspring were placed in room air or hyperoxia (50% O2) for 7 d and then returned to normoxia. Airway mechanics, histology, and laser capture micro-dissection (LCM) were performed.

RESULTS

At postnatal day 21, maternal LPS- and 50% O2-exposed pups exhibited increased resistance and decreased compliance compared to 21% O2 pups; however their effects were not synergistic. LPS and hyperoxia each increased the thickness of airway smooth muscle (ASM), but not the airway epithelial layer. Structural changes were largely limited to the conducting airways. Upregulation of inflammatory markers in the lung was observed at birth. LCM revealed increased collagen-3, transforming growth factor β, and connective tissue growth factor expression with LPS and hyperoxia within the ASM layer.

CONCLUSION

These novel studies provide functional, structural, and molecular evidence that antenatal inflammation is detrimental to the developing airway. Exposure to moderate hyperoxia does not exacerbate LPS effects on the airway.

Vitamin D attenuates cytokine-induced remodeling in human fetal airway smooth muscle cells

Asthma in the pediatric population remains a significant contributor to morbidity and increasing healthcare costs. Vitamin D3 insufficiency and deficiency have been associated with development of asthma. Recent studies in models of adult airway diseases suggest that the bioactive Vitamin D3 metabolite, calcitriol (1,25-dihydroxyvitamin D3 ; 1,25(OH)2 D3 ), modulates responses to inflammation; however, this concept has not been explored in developing airways in the context of pediatric asthma. We used human fetal airway smooth muscle (ASM) cells as a model of the early postnatal airway to explore how calcitriol modulates remodeling induced by pro-inflammatory cytokines. Cells were pre-treated with calcitriol and then exposed to TNFα or TGFβ for up to 72 h. Matrix metalloproteinase (MMP) activity, production of extracellular matrix (ECM), and cell proliferation were assessed. Calcitriol attenuated TNFα enhancement of MMP-9 expression and activity. Additionally, calcitriol attenuated TNFα and TGFβ-induced collagen III expression and deposition, and separately, inhibited proliferation of fetal ASM cells induced by either inflammatory mediator. Analysis of signaling pathways suggested that calcitriol effects in fetal ASM involve ERK signaling, but not other major inflammatory pathways. Overall, our data demonstrate that calcitriol can blunt multiple effects of TNFα and TGFβ in developing airway, and point to a potentially novel approach to alleviating structural changes in inflammatory airway diseases of childhood.

Asthma is not only an airway disease, but also a vascular disease

Multiple studies have identified an expansion and morphological dysregulation of the bronchial vascular network in the airways of asthmatics. Increased number, size and density of blood vessels, as well as vascular leakage and plasma engorgement, have been reported in the airways of patients with all grades of asthma from mild to fatal. This neovascularisation is an increasingly commonly reported feature of airway remodelling; however, the pathophysiological impact of the increased vasculature in the bronchial wall and its significance to pulmonary function in asthma are unrecognised at this time. Multiple factors capable of influencing the development and persistence of the vascular network exist within asthmatic airway tissue. These include structural components of the altered extracellular matrix (ECM), imbalance of proteases and their endogenous inhibitors, release of active matrikines and the dysregulated levels of both soluble and matrix sequestered growth factors. This review will explore the features of the asthmatic airway which influence the development and persistence of the increased vascular network, as well as the effect of enhanced tissue perfusion on chronic inflammation and airway dynamics. The response of cells of the airways to the altered vascular profile and the subsequent influence on the features of airway remodelling will also be highlighted. We will explore the failure of current asthma therapeutics in "normalising" this vascular remodelling. Finally, we will summarize the outcomes of recent clinical trials which provide hope that anti-angiogenic therapies may be a potent asthma-resolving class of drugs and provide a new approach to asthma management in the future.

Gene expression profiling of laser microdissected airway smooth muscle tissue in asthma and atopy

BACKGROUND

Asthma and atopy share common characteristics including type 2 helper-T-cell-mediated inflammation. However, only asthma is associated with variable airways obstruction. The complex cellular and molecular pathways distinguishing asthma and atopy can now be captured by transcriptomic analysis (RNA-Seq). We hypothesized that the transcriptomic profile of airway smooth muscle (ASM) distinguishes atopic asthma from atopic healthy controls. First, we compared the ASM transcriptomic profiles of endobronchial biopsies between glucocorticoid-free, atopic asthma patients, and atopic and nonatopic healthy controls. Second, we investigated the association between ASM transcriptomic profiles and airway function.

METHODS

Twelve asthma patients and 12 control subjects (six atopic, six nonatopic) underwent bronchoscopy. RNA of laser-dissected ASM from 96 bronchial biopsy specimens was sequenced with Roche GS FLX. Gene networks were identified using Ingenuity Pathway Analysis. RNA-Seq reads were assumed to follow a negative binomial distribution. With the current sample size, the estimated false discovery rate was approximately 1%.

RESULTS

One hundred and seventy four ASM genes were differentially expressed between asthma patients and atopic controls, 108 between asthma patients and nonatopic controls, and 135 between atopic and nonatopic controls. A set of eight genes discriminated asthma patients from nonasthmatic controls, irrespective of atopy. Four of these genes (RPTOR, VANGL1, FAM129A, LEPREL1) were associated with airway hyper-responsiveness (P < 0.05).

CONCLUSION

Airway smooth muscle from asthma patients can be distinguished from that of atopic and nonatopic control subjects by a specific gene expression profile, which is associated with airway hyper-responsiveness.

Akt induces down regulation of MUC5AC production in NCI-H292 human airway epithelial cells cultured on extracellular matrix

MUC5AC mucin overproduction is a key feature of asthma as contributes to airway obstruction. The production of MUC5AC is regulated in part by signals from extracellular matrix via integrin pathways, but it remains largely unclear. We investigated the role of Akt, a typical signal transducer in the integrin pathway, in the regulation of MUC5AC production. When NCI-H292 human airway epithelial cells were cultured on laminin or Matrigel, we found that the activity of Akt was suppressed, as compared to control cells with upregulated MUC5AC production. In contrast, Akt was activated in cells cultured on type IV collagen with downregulated MUC5AC production. The Akt inhibitor induced upregulation of MUC5AC. In contrast, overexpression of active Akt induced downregulation of MUC5AC production. These results suggest that a signal from laminin or Matrigel induces upregulation of MUC5AC by suppressing Akt activity, whereas a signal from type IV collagen induces downregulation of MUC5AC, mediated by Akt activation.

ADAM8 in asthma. Friend or foe to airway inflammation?

Airway inflammation has been suggested as the pathological basis in asthma pathogenesis. Recruitment of leukocytes from the vasculature into airway sites is essential for induction of airway inflammation, a process thought to be mediated by a disintegrin and metalloprotease 8 (ADAM8). However, there is an apparent controversy about whether ADAM8 helps or hampers transmigration of leukocytes through endothelium in airway inflammation of asthma. This review outlines the current contradictory concepts concerning the role of ADAM8 in airway inflammation, particularly focusing on the recruitment of leukocytes during asthma, and attempts to bridge the existing experimental data on the basis of the functional analysis of different domains of ADAM8 and their endogenous processing in vivo. We suggest a possible hypothesis for the specific mechanism by which ADAM8 regulates the transmigration of leukocytes to explain the disparity existing in current studies, and we also raise some questions that require future investigations.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

It is now established that airway smooth muscle (ASM) has roles in determining airway structure and function, well beyond that as the major contractile element. Indeed, changes in ASM function are central to the manifestation of allergic, inflammatory, and fibrotic airway diseases in both children and adults, as well as to airway responses to local and environmental exposures. Emerging evidence points to novel signaling mechanisms within ASM cells of different species that serve to control diverse features, including 1) [Ca(2+)]i contractility and relaxation, 2) cell proliferation and apoptosis, 3) production and modulation of extracellular components, and 4) release of pro- vs. anti-inflammatory mediators and factors that regulate immunity as well as the function of other airway cell types, such as epithelium, fibroblasts, and nerves. These diverse effects of ASM "activity" result in modulation of bronchoconstriction vs. bronchodilation relevant to airway hyperresponsiveness, airway thickening, and fibrosis that influence compliance. This perspective highlights recent discoveries that reveal the central role of ASM in this regard and helps set the stage for future research toward understanding the pathways regulating ASM and, in turn, the influence of ASM on airway structure and function. Such exploration is key to development of novel therapeutic strategies that influence the pathophysiology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis.

The expression and activity of cathepsins D, H and K in asthmatic airways

Tumstatin is an anti-angiogenic collagen IV α3 fragment, levels of which are reduced in the airways of asthmatics. Its reduction may be due to the degradation by extracellular matrix (ECM) proteases. Cathepsins play a role in ECM remodelling, with cathepsin D, H and K (CTSD, CTSH and CTSK) being associated with lung diseases. CTSD modulates the NC1 domains of collagen molecules including tumstatin, while CTSH and CTSK are involved in ECM degradation. The role of these cathepsins in the regulation of tumstatin in the lung has not previously been examined. We demonstrated that CTSB, D, F, H, K, L and S mRNA was expressed in the airways. Quantification of immunohistochemistry showed that there is no difference in the global expression of CTSD, CTSH and CTSK between asthmatics and non-asthmatics. CTSD and CTSK, but not CTSH had the capacity to degrade tumstatin. No difference was observed in the activity of CTSD and H in bronchoalveolar lavage fluid of asthmatic and non-asthmatics, while CTSK was undetectable. This indicates that while CTSD possesses the potential to directly regulate tumstatin, and thus angiogenesis through this mechanism however, it is not likely to be involved in the dysregulation of tumstatin found in asthmatic airways.

The role of IgE-receptors in IgE-dependent airway smooth muscle cell remodelling

Background

In allergic asthma, IgE increases airway remodelling but the mechanism is incompletely understood. Airway remodelling consists of two independent events increased cell numbers and enhanced extracellular matrix deposition, and the mechanism by which IgE up-regulates cell proliferation and extracellular matrix deposition by human airway smooth muscle cells in asthma is unclear.

Objective

Characterise the role of the two IgE receptors and associated signalling cascades in airway smooth muscle cell remodelling.

Methods

Primary human airway smooth muscle cells (8 asthmatics, 8 non-asthmatics) were stimulated with human purified antibody-activated IgE. Proliferation was determined by direct cell counts. Total collagen deposition was determined by Sircol; collagen species deposition by ELISA. IgE receptors were silenced by siRNA and mitogen activated protein kinase (MAPK) signalling was blocked by chemical inhibitors.

Results

IgE dose-dependently increased extracellular matrix and collagen deposition by airway smooth muscle cells as well as their proliferation. Specifically in cells of asthma patients IgE increased the deposition of collagen-type-I, -III, –VII and fibronectin, but did not affect the deposition of collagens type-IV. IgE stimulated collagen type-I and type-VII deposition through IgE receptor-I and Erk1/2 MAPK. Proliferation and deposition of collagens type-III and fibronectin involved both IgE receptors as well as Erk1/2 and p38 MAPK. Pre-incubation (30 minutes) with Omalizumab prevented all remodelling effects completely. We observed no changes in gelatinase activity or their inhibitors.

Conclusion & Clincal Relevance

Our study provides the molecular biological mechanism by which IgE increases airway remodelling in asthma through increased airway smooth muscle cell proliferation and deposition of pro-inflammatory collagens and fibronectin. Blocking IgE action prevents several aspects of airway smooth muscle cell remodelling. Our findings may explain the recently described reduction of airway wall thickness in severe asthma patients treated with humanised anti-IgE antibodies.

Brain-derived neurotrophic factor induces proliferation of human airway smooth muscle cells

Airway diseases such as asthma involve increased airway smooth muscle (ASM) contractility and remodelling via enhanced proliferation. Neurotrophins (NTs) such as brain-derived neurotrophic factor (BDNF), well-known in the nervous system, can regulate Ca²⁺ signalling, and interact with cytokines in contributing to airway hyperreactivity. In this study, we determined whether and how BDNF regulates human ASM cell proliferation in the presence of inflammation, thus testing its potential role in airway remodelling. Cells were treated with 10 nM BDNF, 25 ng/ml tumour necrosis factor (TNF-α) or interleukin-13 (IL-13), or 10 ng/ml platelet-derived growth factor (PDGF). Proliferation was measured using CyQuant dye, with immunoblotting of cell cycle proteins predicted to change with proliferation. Forty-eight hours of BDNF enhanced ASM proliferation to ∼50% of that by PDGF or cytokines. Transfection with small interfering RNAs (siRNAs) targeting high-affinity tropomyosin-related kinase B receptor abolished BDNF effects on proliferation, whereas low-affinity 75 kD neurotrophin receptor (p75NTR) siRNA had no effect. Systematic pharmacologic inhibition of different components of ERK1/2 and PI3K/Akt1 pathways blunted BDNF or TNF-α–induced proliferation. BDNF also induced IκB phosphorylation and nuclear translocation of p50 and p65 NF-κB subunits, with electron mobility shift assay confirmation of NF-κB binding to consensus DNA sequence. These results demonstrate that NTs such as BDNF can enhance human ASM cell proliferation by activating proliferation-specific signalling pathways and a versatile transcription factor such as NF-κB, which are common to cytokines and growth factors involved in asthma.

The potential roles of a laminin receptor in adhesion and apoptosis of cells of the marine bivalve Meretrix meretrix

Background

The laminin receptors (LRs) play important roles in cell adhesion to the extracellular matrix, certain cell-cell adhesions, and the activation of many intracellular signaling pathways. Studies of LRs have primarily focused on mammals, while few studies of LRs in marine invertebrates have been reported. The functions of LRs in marine bivalve species are still unclear.

Methodology/Principal Findings

In this study, we cloned and sequenced an LR gene, MmeLR, from the clam Meretrix meretrix. The MmeLR mRNA and protein detected by realtime PCR and western blots were primarily distributed in muscle tissues. Far-western analysis showed a specific interaction between recombinant MmeLR and the LR ligand laminin. The results of the binding assay suggested a role of LR in cell adhesion and apoptosis in cultured primary cells of mantle tissues from M. meretrix. The Bcl-2 mRNA expression level in primary cells cultured in matrigel (mainly laminin) coated plates was significantly higher than in cells cultured in non-coated plates at 48 h of culture, while the p53 mRNA expression pattern was inversely related to that of bcl-2, suggesting that MmeLR is involved in p53-dependent apoptosis, and the binding between MmeLR and laminin inhibits apoptosis during primary cell culture.

Conclusions

Our results suggest that MmeLR may be involved in cell adhesion and apoptosis. This study may increase the understanding of the role of laminin receptor in cell adhesion and apoptosis and help to improve the culture of primary cells of marine invertebrates.

Functional phenotype of airway myocytes from asthmatic airways

In asthma, the airway smooth muscle (ASM) cell plays a central role in disease pathogenesis through cellular changes which may impact on its microenvironment and alter ASM response and function. The answer to the long debated question of what makes a 'healthy' ASM cell become 'asthmatic' still remains speculative. What is known of an 'asthmatic' ASM cell, is its ability to contribute to the hallmarks of asthma such as bronchoconstriction (contractile phenotype), inflammation (synthetic phenotype) and ASM hyperplasia (proliferative phenotype). The phenotype of healthy or diseased ASM cells or tissue for the most part is determined by expression of key phenotypic markers. ASM is commonly accepted to have different phenotypes: the contractile (differentiated) state versus the synthetic (dedifferentiated) state (with the capacity to synthesize mediators, proliferate and migrate). There is now accumulating evidence that the synthetic functions of ASM in culture derived from asthmatic and non-asthmatic donors differ. Some of these differences include an altered profile and increased production of extracellular matrix proteins, pro-inflammatory mediators and adhesion receptors, collectively suggesting that ASM cells from asthmatic subjects have the capacity to alter their environment, actively participate in repair processes and functionally respond to changes in their microenvironment.

Emerging mediators of airway smooth muscle dysfunction in asthma

Phenotypic changes in airway smooth muscle are integral to the pathophysiological changes that constitute asthma - namely inflammation, airway wall remodelling and bronchial hyperresponsiveness. In vitro and in vivo studies have shown that the proliferative, secretory and contractile functions of airway smooth muscle are dysfunctional in asthma. These functions can be modulated by various mediators whose levels are altered in asthma, derived from inflammatory cells or produced by airway smooth muscle itself. In this review, we describe the emerging roles of the CXC chemokines (GROs, IP-10), Th17-derived cytokines (IL-17, IL-22) and semaphorins, as well as the influence of viral infection on airway smooth muscle function, with a view to identifying new opportunities for therapeutic intervention in asthma.

Smooth muscle in tissue remodeling and hyper-reactivity: airways and arteries

Smooth muscle comprises a key functional component of both the airways and their supporting vasculature. Dysfunction of smooth muscle contributes to and exacerbates a host of breathing-associated pathologies such as asthma, chronic obstructive pulmonary disease and pulmonary hypertension. These diseases may be marked by airway and/or vascular smooth muscle hypertrophy, proliferation and hyper-reactivity, and related conditions such as fibrosis and extracellular matrix remodeling. This review will focus on the contribution of airway or vascular smooth dysfunction to common airway diseases.

Extracellular matrix in airway smooth muscle is associated with dynamics of airway function in asthma

BACKGROUND

Altered deposition of extracellular matrix (ECM) in the airway smooth muscle (ASM) layer as observed in asthma may influence ASM mechanical properties. We hypothesized that ECM in ASM is associated with airway function in asthma. First, we investigated the difference in ECM expression in ASM between asthma and controls. Second, we examined whether ECM expression is associated with bronchoconstriction and bronchodilation in vivo.

METHODS

Our cross-sectional study comprised 19 atopic mild asthma patients, 15 atopic and 12 nonatopic healthy subjects. Spirometry, methacholine responsiveness, deep-breath-induced bronchodilation (ΔR(rs) ) and bronchoscopy with endobronchial biopsies were performed. Positive staining of elastin, collagen I, III and IV, decorin, versican, fibronectin, laminin and tenascin in ASM was quantified as fractional area and mean density. Data were analysed using Pearson's or Spearman's correlation coefficient.

RESULTS

Extracellular matrix expression in ASM was not different between asthma and controls. In asthmatics, fractional area and mean density of collagen I and III were correlated with methacholine dose-response slope and ΔR(rs) , respectively (r = 0.71, P < 0.01; r = 0.60, P = 0.02). Furthermore, ASM collagen III and laminin in asthma were correlated with FEV(1) reversibility (r = -0.65, P = 0.01; r = -0.54, P = 0.04).

CONCLUSION

In asthma, ECM in ASM is related to the dynamics of airway function in the absence of differences in ECM expression between asthma and controls. This indicates that the ASM layer in its full composition is a major structural component in determining variable airways obstruction in asthma.

Elastin in asthma

Extracellular matrix is generally increased in asthma, causing thickening of the airways which may either increase or decrease airway responsiveness, depending on the mechanical requirements of the deposited matrix. However, in vitro studies have shown that the altered extracellular matrix produced by asthmatic airway smooth muscle cells is able to induce increased proliferation of non-asthmatic smooth muscle cells, which is a process believed to contribute to airway hyper-responsiveness in asthma. Elastin is an extracellular matrix protein that is altered in asthmatic airways, but there has been no systematic investigation of the functional effect of these changes. This review reveals divergent reports of the state of elastin in the airway wall in asthma. In some layers of the airway it has been described as increased, decreased and/or fragmented, or unchanged. There is also considerable evidence for an imbalance of matrix metalloproteinases, which degrade elastin, and their respective inhibitors the tissue inhibitors of metalloproteinases, which collectively help to explain observations of both increased elastin and elastin fragments. A loss of lung elastic recoil in asthma suggests a mechanical role for disordered elastin in the aetiology of the disease, but extensive studies of elastin in other tissues show that elastin fragments elicit cellular effects such as increased proliferation and inflammation. This review summarises the current understanding of the role of elastin in the asthmatic airway.

The pivotal role of airway smooth muscle in asthma pathophysiology

Asthma is characterized by the association of airway hyperresponsiveness (AHR), inflammation, and remodelling. The aim of the present article is to review the pivotal role of airway smooth muscle (ASM) in the pathophysiology of asthma. ASM is the main effector of AHR. The mechanisms of AHR in asthma may involve a larger release of contractile mediators and/or a lower release of relaxant mediators, an improved ASM cell excitation/contraction coupling, and/or an alteration in the contraction/load coupling. Beyond its contractile function, ASM is also involved in bronchial inflammation and remodelling. Whereas ASM is a target of the inflammatory process, it can also display proinflammatory and immunomodulatory functions, through its synthetic properties and the expression of a wide range of cell surface molecules. ASM remodelling represents a key feature of asthmatic bronchial remodelling. ASM also plays a role in promoting complementary airway structural alterations, in particular by its synthetic function.

Respiratory mechanics and lung tissue remodeling in a hepatopulmonary syndrome rat model

Intrapulmonary vasodilation is a hallmark of the hepatopulmonary syndrome (HPS). However, its effects on respiratory mechanical properties and lung morphology are unknown. To determine these effects, 28 rats were randomly divided to control and experimental HPS groups (eHPS). The spontaneous breathing pattern, gas exchange, respiratory system mechanical properties, and lung and liver morphology of the rats were evaluated. Tidal volume, minute ventilation and mean inspiratory flow were significantly reduced in the eHPS group. Chest wall pressure dissipation against the resistive and viscoelastic components and elastic elastance were increased in the eHPS group. The lung resistive pressure dissipation was lower but the viscoelastic pressure was higher in the eHPS group. The airway volume proportion of collagen and elastic fibers was increased in the eHPS animals (16% and 51.7%; P<0.05 and P<0.001, respectively). The proportion of collagen volume in the vasculature increased 29% in the eHPS animals (P<0.01). HPS presents with respiratory system mechanical disarray as well as airway and vascular remodeling.

Models and approaches to understand the role of airway remodelling in disease

Airway remodelling is a collective term for changes in the amount or organisation of the cellular and molecular constituents of the airway wall. Remodelling occurs in and is associated with the pathophysiology of airways diseases including asthma and chronic obstructive pulmonary disease. The remodelling that occurs in these diseases exhibits both shared and distinct features. Remodelling is generally considered to be deleterious to airway function but recent studies also indicate potential protective effects. However, the true impact of different aspects of the remodelling process on lung function, both negative and positive, is poorly understood. In addition, the genetic susceptibility and processes by which environmental insults drive the cell and molecular events which result in airway remodelling and the potential for therapeutic reversibility are also incompletely understood. The last 10-15 years has seen the development of animal models of airway remodelling which have been refined and modified as new factors such as exacerbations and early life influences have been recognised as being of importance. In addition, invertebrate models have been put forward and complex in vitro culture systems and lung slice preparations developed. In parallel, imaging technology has developed to an extent where it is feasible using a combination of techniques to image structural components, cells and proteins in the airway wall as well as to analyse biological processes, cell and receptor activation non-invasively over time. The integration of data from in vivo and in vitro models together with use of imaging techniques in man and animals should allow validation of models, further our understanding of the pathophysiology of airway remodelling and potentially improve predictive accuracy for the translation of novel therapeutic agents into the clinic.

Does airway smooth muscle express an inflammatory phenotype in asthma?

In addition to hyperresponsiveness in asthma, airway smooth muscle (ASM) also manifests an inflammatory phenotype characterized by augmented expression of mediators that enhance inflammation, contribute to tissue remodelling and augment leucocyte trafficking and activity. Our present review summarizes contemporary understanding of ASM-derived mediators and their paracrine and autocrine actions in airway diseases.

Models to understand contractile function in the airways

Although the role of contractile function in the airways is controversial, there is general consensus on the importance of airway smooth muscle (ASM) as a therapeutic target for diseases characterized by airway obstruction, such as asthma or chronic obstructive pulmonary disease. Indeed, the use of bronchodilators to relax ASM is the most common and effective practice to treat airflow obstruction. Excessive pathologic bronchoconstriction may originate from primary alterations of ASM mechanical function and/or from the effects exerted on ASM function by disease processes, such as inflammation and remodeling. An in depth knowledge of the potentially multiple mechanisms that distinctively regulate primary and secondary alterations in ASM contractile function would be essential for the development of new therapeutic approaches aimed at preventing the occurrence or reducing the severity of bronchoconstriction. The present review discusses studies that have addressed the mechanisms of altered ASM contractile function in models of airway hyperresponsiveness. Although not comprehensively, in the present review, animal models of intrinsic airway hyperresponsiveness, normal ontogenesis, and allergic sensitization are analyzed in the attempt to summarize the current knowledge on regulatory mechanisms of ASM contractile function in health and disease. Studies in human ASM and the need for additional models to understand contractile function in the airways are also discussed.

Pathological airway remodelling in inflammation

INTRODUCTION

Airway remodelling refers to a wide pattern of pathophysiological mechanisms involving smooth muscle cell hyperplasia, increase of activated fibroblasts and myofibroblasts with deposition of extracellular matrix. In asthma, it includes alterations of the epithelial cell layer with goblet cell hyperplasia, thickening of basement membranes, peri-bronchial and peri-bronchoalveolar fibrosis. Moreover, airway remodelling occurs not only in asthma but also in several pulmonary disorders such as chronic obstructive pulmonary disease, idiopathic pulmonary fibrosis and systemic sclerosis. Asthma treatment with inhaled corticosteroids does not fully prevent airway remodelling and thus have restricted influence on the natural course of the disease.

OBJECTIVES

This review highlights the role of different fibroblast phenotypes and potential origins of these cells in airway remodelling.

RESULTS

During inflammatory conditions, such as asthma, fibroblasts can differentiate into an active, more contractile phenotype termed myofibroblast, with expression of stress fibres and alpha-smooth muscle actin. The origin of myofibroblasts has lately been debated, and three sources have been identified: recruitment and differentiation of resident tissue fibroblasts; fibrocytes - circulating progenitor cells; and epithelial-mesenchymal transition.

CONCLUSION

It is clear that airway mesenchymal cells, including fibroblasts/myofibroblasts, are more dynamic in terms of differentiation and origin than has previously been recognised. Considering that these cells are key players in the remodelling process, it is of utmost importance to characterise specific markers for the various fibroblast phenotypes and to explore factors that drive the differentiation to develop future diagnostic and therapeutic tools for asthma patients.

Plasminogen activation by airway smooth muscle is regulated by type I collagen

Plasmin, the activated protease product of plasminogen, is involved in collagen remodeling, and is strongly implicated in asthma pathophysiology by recent genome-wide association studies. This study examines plasminogen "activation" by airway smooth muscle cells, and its regulation in a fibrotic environment created by culture on type I collagen and incubation with transforming growth factor (TGF)-β. Urokinase plasminogen activator (uPA) activity was detected in the supernatants of human airway smooth muscle cell cultures maintained in serum-free conditions. Incubation with plasminogen (1.5-50.0 μg/ml, 24 h) increased plasmin activity in a concentration-dependent manner (P < 0.001). uPA activity was higher in cultures maintained on fibrillar type I collagen substrata than in those on plastic, as was plasmin activity after incubation with plasminogen (20 μg/ml). Pretreatment with TGF-β (100 pM) for 18 hours inhibited plasminogen activation by airway smooth muscle cells maintained on plastic, but not on collagen. TGF-β stimulated an increase in the level of uPA mRNA in airway smooth muscle cells grown on collagen, but not on plastic. Reducing the levels of β1-integrin collagen receptor, using interference RNA, attenuated plasmin formation by airway smooth muscle cells grown on collagen, and restored the inhibitory effect of TGF-β. This study shows that airway smooth muscle activation of plasminogen by uPA is accelerated in a collagen-rich environment in which the inhibitory effect of TGF-β is attenuated in association with greater uPA expression induced via β1-integrin signaling. These findings suggest that the plasminogen-activation system involving uPA has the potential to contribute to airway wall remodeling in asthma.

Airway smooth muscle remodels pericellular collagen fibrils: implications for proliferation

Airway wall remodeling comprises a broad range of structural changes including increases in the volume of airway smooth muscle (ASM) and fibrillar collagen. The impact of fibrillar collagen remodeling on ASM proliferation was investigated. Human ASM cultured on type I fibrillar collagen remodeled the collagen substrate by both degradation (collagenolysis) and formation of networks comprised of thicker reticular collagen fibrils (fibrillogenesis). In cultures maintained on fibrillar collagen, the levels of matrix metalloproteases (MMPs) -1 and -14 mRNA and active MMP-2 were higher than in cultures maintained on nonfibrillar type I collagen (gelatin) or plastic. Although there was no apparent increase in cytotoxicity or apoptosis, the number of ASM was lower on fibrillar collagen than on gelatin or plastic for control conditions. Furthermore, maintenance on fibrillar collagen attenuated basic fibroblast growth factor-stimulated increases in cell number and the percentage of cells entering S-phase. In cultures maintained on fibrillar collagen, the MMP inhibitor ilomastat (2.5 μM) 1) attenuated collagenolysis, 2) enhanced fibrillogenesis, and 3) inhibited proliferation. In contrast, knockdown of the β1-integrin gene in ASM maintained on fibrillar collagen led to an increase in proliferation and reduced MMP-1 and -14 expression. Thus, ASM remodel the pericellular environment by degrading collagen fibrils and spinning them into larger collagen assemblies. Moreover, the collagen fibrils limit proliferation and activate autocrine MMPs in a β-integrin-dependent manner, suggesting a potential negative feedback on modeling executed through fibrillar collagen activation of β1-integrins.

MUC5AC production is downregulated in NCI-H292 lung cancer cells cultured on type-IV collagen

Mucus overproduction is an important feature of bronchial asthma. MUC5AC mucin is a major component of mucus and is overproduced in patients with asthma. Although regulation of MUC5AC production has been well investigated, its regulation through the signals from extracellular matrix (ECM) is less clear. In this study, we investigated whether the signals from ECM regulate MUC5AC production in the human lung epithelial cell line NCI-H292. We found that MUC5AC production is downregulated in NCI-H292 cells cultured on type-IV collagen, a major component of ECM, but shows no obvious changes when cultured on type-I collagen or fibronectin. In contrast, MUC5AC production was upregulated on laminin and on reconstituted basement membrane (Matrigel), a complex of ECM components. Antibody-mediated inhibition of integrin beta1-subunit, a major receptor involved in the adherence of cells to type-IV collagen, upregulated the MUC5AC production in NCI-H292 cells, and also in the cells cultured on type-IV collagen. Although the major signaling pathway from integrins is via Src kinase activation, treatment of cells with PP2, a Src kinase inhibitor, did not recover the downregulation of MUC5AC on type-IV collagen. In contrast, on Matrigel, the inhibition of integrin beta1-subunit did not abolish the upregulation of MUC5AC production, but PP2 reduced the upregulation. These results suggest that ECM and an integrin/Src pathway play an important role in the regulation of MUC5AC production in the cell line NCI-H292. The production of MUC5AC is downregulated on type-IV collagen through a Src-independent pathway. In contrast, MUC5AC is upregulated on Matrigel through a Src-dependent pathway in NCI-H292 cells.