COUNTERPOINT: AIRWAY SMOOTH MUSCLE IS NOT USEFUL

Sensitivity of the airway smooth muscle in terms of force, shortening and stiffness

Eight pig tracheal strips were stimulated to contract with log increments of methacholine from 10-8 to 10-5 M. For each strip, the concentration-response was repeated four times in a randomized order to measure isometric force, isotonic shortening against a load corresponding to either 5 or 10 % of a reference force, and average force, stiffness, elastance and resistance over one cycle while the strip length was oscillating sinusoidally by 5 % at 0.2 Hz. For each readout, the logEC50 was calculated and compared. Isotonic shortening with a 5 % load had the lowest logEC50 (-7.13), yielding a greater sensitivity than any other contractile readout (p<0.05). It was followed by isotonic shortening with a 10 % load (-6.66), elastance (-6.46), stiffness (-6.46), resistance (-6.38), isometric force (-6.32), and average force (-6.30). Some of these differences were significant. For example, the EC50 with the average force was 44 % greater than with the elastance (p=0.001). The methacholine sensitivity is thus affected by the contractile readout being measured.

A life off the beaten track in biomechanics: Imperfect elasticity, cytoskeletal glassiness, and epithelial unjamming

Textbook descriptions of elasticity, viscosity, and viscoelasticity fail to account for certain mechanical behaviors that typify soft living matter. Here, we consider three examples. First, strong empirical evidence suggests that within lung parenchymal tissues, the frictional stresses expressed at the microscale are fundamentally not of viscous origin. Second, the cytoskeleton (CSK) of the airway smooth muscle cell, as well as that of all eukaryotic cells, is more solid-like than fluid-like, yet its elastic modulus is softer than the softest of soft rubbers by a factor of 104-105. Moreover, the eukaryotic CSK expresses power law rheology, innate malleability, and fluidization when sheared. For these reasons, taken together, the CSK of the living eukaryotic cell is reminiscent of the class of materials called soft glasses, thus likening it to inert materials such as clays, pastes slurries, emulsions, and foams. Third, the cellular collective comprising a confluent epithelial layer can become solid-like and jammed, fluid-like and unjammed, or something in between. Esoteric though each may seem, these discoveries are consequential insofar as they impact our understanding of bronchospasm and wound healing as well as cancer cell invasion and embryonic development. Moreover, there are reasons to suspect that certain of these phenomena first arose in the early protist as a result of evolutionary pressures exerted by the primordial microenvironment. We have hypothesized, further, that each then became passed down virtually unchanged to the present day as a conserved core process. These topics are addressed here not only because they are interesting but also because they track the journey of one laboratory along a path less traveled by.

Structure and function of small airways in asthma patients revisited

Small airways (<2 mm in diameter) are probably involved across almost all asthma severities and they show proportionally more structural and functional abnormalities with increasing asthma severity. The structural and functional alterations of the epithelium, extracellular matrix and airway smooth muscle in small airways of people with asthma have been described over many years using in vitro studies, animal models or imaging and modelling methods. The purpose of this review was to provide an overview of these observations and to outline several potential pathophysiological mechanisms regarding the role of small airways in asthma.

Microfluidic chest cavities reveal that transmural pressure controls the rate of lung development

Mechanical forces are increasingly recognized to regulate morphogenesis, but how this is accomplished in the context of the multiple tissue types present within a developing organ remains unclear. Here, we use bioengineered 'microfluidic chest cavities' to precisely control the mechanical environment of the fetal lung. We show that transmural pressure controls airway branching morphogenesis, the frequency of airway smooth muscle contraction, and the rate of developmental maturation of the lungs, as assessed by transcriptional analyses. Time-lapse imaging reveals that branching events are synchronized across distant locations within the lung, and are preceded by long-duration waves of airway smooth muscle contraction. Higher transmural pressure decreases the interval between systemic smooth muscle contractions and increases the rate of morphogenesis of the airway epithelium. These data reveal that the mechanical properties of the microenvironment instruct crosstalk between different tissues to control the development of the embryonic lung.

Pharmacological investigation on the anti-oxidant and anti-inflammatory activity of N-acetylcysteine in an ex vivo model of COPD exacerbation

Background

Oxidative stress is recognized to be one of predisposing factor in the pathogenesis of COPD. The oxidant/antioxidant imbalance is significantly pronounced in patients with COPD exacerbation. N-acetylcysteine (NAC) seems to be able to reduce COPD exacerbations by modulating the oxidative stress in addition to its well-known mucolytic activity, but there are discordant findings on the actual anti-oxidant activity of NAC.

Methods

The anti-oxidant effect of NAC and its impact on the inflammatory response have been pharmacologically characterized on a human ex vivo model of COPD exacerbation induced by lipopolysaccharide (LPS).

Results

NAC prevented the desensitization induced by LPS incubation on the contractile tone in linear concentration-response manner. Concentrations of NAC ≥1 μM reduced the pro-oxidant response (peroxidase activity, hydrogen peroxide, malondialdehyde, nitric oxide), and improved the anti-oxidant response (total anti-oxidant capacity, glutathione, superoxide dismutase) induced by LPS. Lower concentrations of NAC (<1 μM) did not modulate the bronchial oxidative imbalance. Concentrations of NAC ≥300 μM inhibited the inflammatory response (release of IL-1β, IL-8, and TNF-α) of human airways induced by the overnight stimulation with LPS, whereas lower concentrations of NAC (≥1 μM) were sufficient to reduce the release of IL-6 elicited by LPS. Both the anti-oxidant effect and the anti-inflammatory effect of NAC were inversely correlated with the release of NKA.

Conclusions

The findings of this study suggest that NAC may have a role in modulating the detrimental effect induced by LPS in course of COPD exacerbation. It may elicit both anti-oxidant and anti-inflammatory effects when administered at high concentrations.

Smooth muscle in human bronchi is disposed to resist airway distension

Studying airway smooth muscle (ASM) in conditions that emulate the in vivo environment within which the bronchi normally operate may provide important clues regarding its elusive physiological function. The present study examines the effect of lengthening and shortening of ASM on tension development in human bronchial segments. ASM from each bronchial segment was set at a length approximating in situ length (Linsitu). Bronchial tension was then measured during a slow cyclical strain (0.004Hz, from 0.7Linsitu to 1.3Linsitu) in the relaxed state and at graded levels of activation by methacholine. In all cases, tension was greater at longer ASM lengths, and greater during lengthening than shortening. The threshold of methacholine concentration that was required for ASM to account for bronchial tension across the entire range of ASM lengths tested was on average smaller by 2.8 logs during lengthening than during shortening. The length-dependency of ASM tension, together with this lower threshold of methacholine concentration during lengthening versus shortening, suggest that ASM has a greater ability to resist airway dilation during lung inflation than to narrow the airways during lung deflation. More than serving to narrow the airway, as has long been thought, these data suggest that the main function of ASM contraction is to limit airway wall distension during lung inflation.

Quantitative assessment of bronchiolar smooth muscle in healthy and diseased porcine lungs

Smooth muscle cells are major components of bronchiolar wall. Bronchiolar smooth muscle is reported to increase in some veterinary pulmonary disorders, but such assumption is not supported by detailed morphometric analyses. The present investigation aimed at quantitatively evaluating bronchiolar smooth muscle in healthy and diseased pig lungs. Our results suggest that bronchiolar smooth muscle cells significantly modify in size and number under different disease conditions, namely parasitic bronchopneumonia and Mycoplasma hyopneumoniae-induced enzootic pneumonia. Further studies are needed in order to understand the pathogenesis and the functional impact of such changes.

Bronchial thermoplasty for severe asthma

PURPOSE OF REVIEW

the present article will address the potential for bronchial thermoplasty to be used in addition to conventional medications to help us treat our patients with severe asthma.

RECENT FINDINGS

two recently published studies report on the use of bronchial thermoplasty in patients with severe asthma. Now that patients with a range of asthma severity have been treated with bronchial thermoplasty, we are better able to comment on the appropriate selection of patients for this therapy that should optimize benefits and limit complications. In addition, studies reporting longer term follow-up are now available indicating the persistence of benefit and the absence of late developing adverse events.

SUMMARY

bronchial thermoplasty represents a novel approach to asthma treatment that is complementary to anti-inflammatory and bronchodilating therapies. Criteria for selecting appropriate patients are established and experience with bronchial thermoplasty is expanding since US Food and Drug Administration approval was obtained in April 2010.

Multi-scale lung modeling

Multi-scale modeling of biological systems has recently become fashionable due to the growing power of digital computers as well as to the growing realization that integrative systems behavior is as important to life as is the genome. While it is true that the behavior of a living organism must ultimately be traceable to all its components and their myriad interactions, attempting to codify this in its entirety in a model misses the insights gained from understanding how collections of system components at one level of scale conspire to produce qualitatively different behavior at higher levels. The essence of multi-scale modeling thus lies not in the inclusion of every conceivable biological detail, but rather in the judicious selection of emergent phenomena appropriate to the level of scale being modeled. These principles are exemplified in recent computational models of the lung. Airways responsiveness, for example, is an organ-level manifestation of events that begin at the molecular level within airway smooth muscle cells, yet it is not necessary to invoke all these molecular events to accurately describe the contraction dynamics of a cell, nor is it necessary to invoke all phenomena observable at the level of the cell to account for the changes in overall lung function that occur following methacholine challenge. Similarly, the regulation of pulmonary vascular tone has complex origins within the individual smooth muscle cells that line the blood vessels but, again, many of the fine details of cell behavior average out at the level of the organ to produce an effect on pulmonary vascular pressure that can be described in much simpler terms. The art of multi-scale lung modeling thus reduces not to being limitlessly inclusive, but rather to knowing what biological details to leave out.

Stress and strain in the contractile and cytoskeletal filaments of airway smooth muscle

Stress and strain are omnipresent in the lung due to constant lung volume fluctuation associated with respiration, and they modulate the phenotype and function of all cells residing in the airways including the airway smooth muscle (ASM) cell. There is ample evidence that the ASM cell is very sensitive to its physical environment, and can alter its structure and/or function accordingly, resulting in either desired or undesired consequences. The forces that are either conferred to the ASM cell due to external stretching or generated inside the cell must be borne and transmitted inside the cytoskeleton (CSK). Thus, maintaining appropriate levels of stress and strain within the CSK is essential for maintaining normal function. Despite the importance, the mechanisms regulating/dysregulating ASM cytoskeletal filaments in response to stress and strain remained poorly understood until only recently. For example, it is now understood that ASM length and force are dynamically regulated, and both can adapt over a wide range of length, rendering ASM one of the most malleable living tissues. The malleability reflects the CSK's dynamic mechanical properties and plasticity, both of which strongly interact with the loading on the CSK, and all together ultimately determines airway narrowing in pathology. Here we review the latest advances in our understanding of stress and strain in ASM cells, including the organization of contractile and cytoskeletal filaments, range and adaptation of functional length, structural and functional changes of the cell in response to mechanical perturbation, ASM tone as a mediator of strain-induced responses, and the novel glassy dynamic behaviors of the CSK in relation to asthma pathophysiology.

Exploiting mechanical stimuli to rescue growth of the hypoplastic lung

Impaired lung development afflicts a range of newborns cared for by paediatric surgeons. As a result the speciality has led in the development of surgical models that illustrate the biomechanical regulation of lung growth. Using transgenic mutants, biologists have similarly discovered much about the biochemical regulation of prenatal lung growth. Airway smooth muscle (ASM) and its prenatal contractility airway peristalsis (AP) represent a novel link between these areas: ASM progenitors produce an essential biochemical factor for lung morphogenesis, whilst calcium-driven biomechanical ASM activity appears to regulate the same. In this invited paper, I take the opportunity both to review our recent findings on lung growth and prenatal ASM, and also to discuss mechanisms by which ASM contractility can regulate growth. Finally, I will introduce some novel ideas for exploration: ASM contractility could help to schedule parturition (pulmonary parturition clock) and could even be a generic model for smooth muscle regulation of morphogenesis in similar organs.