Ynuk Bossé

Mechanical properties of asthmatic airway smooth muscle

Airway smooth muscle (ASM) is the major effector of excessive airway narrowing in asthma. Changes in some of the mechanical properties of ASM could contribute to excessive narrowing and have not been systematically studied in human ASM from nonasthmatic and asthmatic subjects. Human ASM strips (eight asthmatic and six nonasthmatic) were studied at in situ length and force was normalised to maximal force induced by electric field stimulation (EFS). Measurements included: passive and active force versus length before and after length adaptation, the force–velocity relationship, maximal shortening and force recovery after length oscillation. Force was converted to stress by dividing by cross-sectional area of muscle. The only functional differences were that the asthmatic tissue was stiffer at longer lengths (p<0.05) and oscillatory strain reduced isometric force in response to EFS by 19% as opposed to 36% in nonasthmatics (p<0.01). The mechanical properties of human ASM from asthmatic and nonasthmatic subjects are comparable except for increased passive stiffness and attenuated decline in force generation after an oscillatory perturbation. These data may relate to reduced bronchodilation induced by a deep inspiration in asthmatic subjects.

Adaptation of Airway Smooth Muscle to Basal Tone

Lung inflammation and airway hyperresponsiveness (AHR) are hallmarks of asthma, but their interrelationship is unclear. Excessive shortening of airway smooth muscle (ASM) in response to bronchoconstrictors is likely an important determinant of AHR. Hypercontractility of ASM could stem from a change in the intrinsic properties of the muscle, or it could be due to extrinsic factors such as chronic exposure of the muscle to inflammatory mediators in the airways. The latter could be the link between lung inflammation and AHR. The present study was designed to examine the influence of chronic exposure to a contractile agonist on the force-generating capacity of ASM. Force generation in response to electric field stimulation (EFS) was measured in ovine trachealis with or without a basal tone induced by acetylcholine (ACh). While the tone was maintained, the EFS-induced force decreased transiently but increased over time to reach a plateau in approximately 50 minutes. The total force (ACh tone + EFS force) increased monotonically and in proportion to ACh concentration. The results indicate that the muscle adapted to the basal tone and regained its contractile ability in response to a second stimulus (EFS) over time. Analysis suggests that this is due to a cytoskeletal transformation that allows the cytoskeleton to bear force, thus freeing up actomyosin crossbridges to generate more force. Force adaptation in ASM as a consequence of prolonged exposure to the many spasmogens found in asthmatic airways could be a mechanism contributing to AHR seen in asthma.

It's Not All Smooth Muscle: Non-Smooth-Muscle Elements in Control of Resistance to Airflow

To achieve gas exchange, inspired air must pass through an intricate and dynamic tracheobronchial tree. The tree offers resistance to airflow, and increased resistance is the most important functional change in lung disease. Numerous mechanisms contribute to increased resistance by causing airway narrowing, closure, occlusion, and/or obliteration. Although airway smooth muscle (ASM) contraction and shortening are an important cause of increased resistance, non-ASM elements can also contribute. Nonmuscle elements can modify the amount of airway narrowing for any given level of ASM shortening and the amount of shortening for a given level of ASM activation. In this review, we outline the physiological basis for airflow resistance and describe how changes in the lung parenchyma, the airways, and their luminal contents can contribute to increased airflow resistance. A detailed understanding of the mechanisms and consequences of increased airway resistance is vital to our attempts to alleviate the enormous burden of suffering caused by obstructive lung diseases.

MicroRNA‐19a enhances proliferation of bronchial epithelial cells by targeting TGFβR2 gene in severe asthma

Allergic asthma is characterized by inflammation and airway remodeling. Bronchial epithelium is considered a key player in coordinating airway wall remodeling. In mild asthma, the epithelium is damaged and fails to proliferate and to repair, whereas in severe asthma, the epithelium is highly proliferative and thicker. This may be due to different regulatory mechanisms. The purpose of our study was to determine the role of miRNAs in regulating proliferation of bronchial epithelial cells obtained from severe asthmatic subjects in comparison with cells obtained from mild asthmatics and healthy controls.

Signaling by the cysteinyl-leukotriene receptor 2. Involvement in chemokine gene transcription.

Cysteinyl-leukotrienes are involved in inflammation and act on at least two G-protein-coupled receptors, CysLT1 and CysLT2. However, the role of the CysLT2 receptor as well as its signaling remain poorly understood. Here we show that leukotriene (LT)C4 induced the production of the chemokine interleukin (IL)-8 in endothelial cells. To further study the signaling cascade involved, HEK293 cells were stably transfected with CysLT2 and used to study the transcriptional regulation of the IL-8 promoter. Stimulation of the cells with increasing concentrations of LTC4 resulted in a time- and concentration-dependent induction of IL-8 transcription and protein synthesis. Use of IL-8 promoter mutants with substitutions in their NF-κB, AP-1, or NF-IL-6 binding elements revealed an almost total requirement for NF-κB and AP-1 elements, and a lesser requirement for the NF-IL-6 element. Overexpression of dominant-negative IκBα prevented the IL-8 transactivation induced by LTC4. LTC4 stimulation induced NF-κB and AP-1 DNA binding, which involved the formation of a p50/p65 and a c-JUN·c-FOS complex, respectively. Transfection of the cells with a dominant negative (dn) form of PKCϵ prevented p65 phosphorylation, whereas dnPKCδ prevented AP-1 binding. Moreover, dnPKCδ, dnPKCϵ, and dnPKCζ prevented LTC4-induced IL-8 transcription in response to LTC4. Our data show for the first time that LTC4 can act via the CysLT2 receptor to transcriptionally activate chemokine production through induction of NF-κB and AP-1 transcription factors. These findings suggest the potential implication of CysLT2 in the inflammatory response through the modulation of chemokine gene transcription.

Airway contractility and remodeling : Links to asthma symptoms

Respiratory symptoms are largely caused by obstruction of the airways. In asthma, airway narrowing mediated by airway smooth muscle (ASM) contraction contributes significantly to obstruction. The spasmogens produced following exposure to environmental triggers, such as viruses or allergens, are initially responsible for ASM activation. However, the extent of narrowing of the airway lumen due to ASM shortening can be influenced by many factors and it remains a real challenge to decipher the exact role of ASM in causing asthmatic symptoms. Innovative tools, such as the forced oscillation technique, continue to develop and have been proven useful to assess some features of ASM function in vivo. Despite these technologic advances, it is still not clear whether excessive narrowing in asthma is driven by ASM abnormalities, by other alterations in non-muscle factors or simply because of the overexpression of spasmogens. This is because a multitude of forces are acting on the airway wall, and because not only are these forces constantly changing but they are also intricately interconnected. To counteract these limitations, investigators have utilized in vitro and ex vivo systems to assess and compare asthmatic and non-asthmatic ASM contractility. This review describes: 1- some muscle and non-muscle factors that are altered in asthma that may lead to airway narrowing and asthma symptoms; 2- some technologies such as the forced oscillation technique that have the potential to unveil the role of ASM in airway narrowing in vivo; and 3- some data from ex vivo and in vitro methods that probe the possibility that airway hyperresponsiveness is due to the altered environment surrounding the ASM or, alternatively, to a hypercontractile ASM phenotype that can be either innate or acquired.

Human airway smooth muscle is structurally and mechanically similar to that of other species

Airway smooth muscle (ASM) plays a vital role in the exaggerated airway narrowing seen in asthma. However, whether asthmatic ASM is mechanically different from nonasthmatic ASM is unclear. Much of our current understanding about ASM mechanics comes from measurements made in other species. Limited data on human ASM mechanics prevents proper comparisons between healthy and asthmatic tissues, as well as human and animal tissues. In the current study, we sought to define the mechanical properties of healthy human ASM using tissue from intact lungs and compare these properties to measurements in other species. The mechanical properties measured included: maximal stress generation, force–length properties, the ability of the muscle to undergo length adaptation, the ability of the muscle to recover from an oscillatory strain, shortening velocity and maximal shortening. The ultrastructure of the cells was also examined. Healthy human ASM was found to be mechanically and ultrastructurally similar to that of other species. It is capable of undergoing length adaptation and responds to mechanical perturbation like ASM from other species. Force generation, shortening capacity and velocity were all similar to other mammalian ASM. These results suggest that human ASM shares similar contractile mechanisms with other animal species and provides an important dataset for comparisons with animal models of disease and asthmatic ASM.

Transforming growth factor-β1 in asthmatic airway smooth muscle enlargement: is fibroblast growth factor-2 required?

Enlargement of airway smooth muscle (ASM) tissue around the bronchi/bronchioles is a histopathological signature of asthmatic airway remodelling and has been suggested to play a critical role in the increased lung resistance and airway hyperresponsiveness seen in asthmatic patients. The pleiotropic cytokine, TGF‐β1, is believed to contribute to several aspects of asthmatic airway remodelling and is known to influence the growth of many cell types. Increased TGF‐β1 expression/signalling and ASM growth have been shown to occur concurrently in animal models of asthma. Abundant studies further substantiate this association by showing that therapeutic strategies that reduce or prevent TGF‐β1 overexpression/signalling lead to a parallel decrease or prevention of ASM enlargement. Finally, recent findings have supported a direct link of causality between TGF‐β1 overexpression/signalling and the overgrowth of ASM tissue. To follow‐up on these in vivo studies, many investigators have pursued detailed investigation of ASM in cell culture conditions, assessing the direct role of TGF‐β1 on cellular proliferation and/or hypertrophy. Inconsistencies among the in vitro studies suggest that the effect of TGF‐β1 on ASM cell proliferation/hypertrophy is contextual. A hypothesis focusing on fibroblast growth factor‐2 is presented at the end of this review, which could potentially reconcile the apparent discrepancy between the conflicting in vitro findings with the consistent in vivo finding that TGF‐β1 is required for ASM enlargement in asthma.

Increase in passive stiffness at reduced airway smooth muscle length: potential impact on airway responsiveness

The amplitude of strain in airway smooth muscle (ASM) produced by oscillatory perturbations such as tidal breathing or deep inspiration (DI) influences the force loss in the muscle and is therefore a key determinant of the bronchoprotective and bronchodilatory effects of these breathing maneuvers. The stiffness of unstimulated ASM (passive stiffness) directly influences the amplitude of strain. The nature of the passive stiffness is, however, not clear. In this study, we measured the passive stiffness of ovine ASM at different muscle lengths (relative to in situ length, which was used as a reference length, L(ref)) and states of adaptation to gain insights into the origin of this muscle property. The results showed that the passive stiffness was relatively independent of muscle length, possessing a constant plateau value over a length range from 0.62 to 1.25 L(ref). Following a halving of ASM length, passive stiffness decreased substantially (by 71%) but redeveloped over time ( approximately 30 min) at the shorter length to reach 65% of the stiffness value at L(ref), provided that the muscle was stimulated to contract at least once over a approximately 30-min period. The redevelopment and maintenance of passive stiffness were dependent on the presence of Ca(2+) but unaffected by latrunculin B, an inhibitor of actin filament polymerization. The maintenance of passive stiffness was also not affected by blocking myosin cross-bridge cycling using a myosin light chain kinase inhibitor or by blocking the Rho-Rho kinase (RhoK) pathway using a RhoK inhibitor. Our results suggest that the passive stiffness of ASM is labile and capable of redevelopment following length reduction. Redevelopment and maintenance of passive stiffness following muscle shortening could contribute to airway hyperresponsiveness by attenuating the airway wall strain induced by tidal breathing and DI.

Decrease of airway smooth muscle contractility induced by simulated breathing maneuvers is not simply proportional to strain.

The lung is a dynamic organ and the oscillating stress applied to the airway wall during breathing maneuvers can decrease airway smooth muscle (ASM) contractility. However, it is unclear whether it is the stress or the attendant strain that is responsible for the decline of ASM force associated with breathing maneuvers, and whether tone can prevent the decline of force by attenuating the strain. To investigate these questions, ovine tracheal strips were subjected to oscillating stress that simulates breathing maneuvers, and the resulting strain and decline of force were measured in the absence or presence of different levels of tone elicited by acetylcholine. In relaxed ASM, high stress simulating 20 cm H(2)O-transpulmonary pressure excursions strained ASM strips by 20.7% and decreased force by 17.1%. When stress oscillations were initiated during measurement of ACh concentration-response curves, tone almost abrogated strain at an ACh concentration of 10(-6) M (1.1%) but the decline of force was not affected (18.9%). When stress oscillations were initiated after ACh-induced contraction had reached its maximal force, strain was almost abrogated at an ACh concentration of 10(-6) M (0.9%) and the decline of force was attenuated (10.1%). However, even at the highest ACh concentration (10(-4) M), substantial decline of force (6.1%) was still observed despite very small strain (0.7%). As expected, the results indicate that tone attenuated the strain experienced by ASM during breathing maneuver simulations. More surprisingly, the reduction of strain induced by tone was not proportional to its effect on the decline of force induced by simulated breathing maneuvers.