The biophysics of asthmatic airway smooth muscle

It is clear that significant advances have been made in the understanding of the physiology, biochemistry and molecular biology of airway smooth muscle (ASM) contraction and how the knowledge obtained from these approaches may be used to elucidate the pathogenesis of asthma. Not to belittle other theories of smooth muscle contraction extant in the field, perhaps the most outstanding development has been the formulation of plasticity theory. This may radically alter our understanding of smooth muscle contraction. Its message is that while shortening velocity and capacity are linear functions of length, active force is length independent. These changes are explained by the ability of thick filament protein to depolymerize at short lengths and to increase numbers of contractile units in series at lengths greater than optimal length or L(ref). Other advances are represented by the report that the major part of ASM shortening is complete within the initial first 20% of contraction time, that the nature and history of loading determine the extent of shortening and that these findings can be explained by the finding that the crossbridges are cycling four times faster than in the remaining time. Another unexpected finding is that late in the course of isotonic relaxation the muscle undergoes spontaneous activation which delays relaxation and smoothes it out; speculatively this could minimize turbulence of airflow. On the applied front evidence now shows the shortening ability of bronchial smooth muscle of human subjects of asthma is significantly increased. Measurements also indicate that increased smooth muscle myosin light chain kinase content, via increased actomyosin ATPase activity could be responsible for the changes in contractility.     

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

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

哮喘气道平滑肌细胞表观遗传学调控的研究进展

哮喘气道重塑中一个主要方面即是平滑肌细胞的增生肥大,近些年,人们逐渐发现表观遗传学对气道平滑肌细胞的增殖和分泌炎性因子方面有着重要的调节作用,其中包括DNA甲基转移酶抑制剂可以抑制其表型转换;组蛋白乙酰化与其增生肥大相关;另外,microRNA可以调控哮喘模型中气道平滑肌细胞的各种生理功能,包括抑制其增殖及炎性因子的释放。希望表观遗传学能够成为治疗哮喘的新型靶点。     

Mitogenic signaling pathways in airway smooth muscle

Increased airway smooth muscle mass has been demonstrated in patients with asthma, bronchopulmonary dysplasia and most recently, cystic fibrosis. These observations emphasize the need for further knowledge of the events involved in airway smooth muscle mitogenesis and hypertrophy. Workers in the field have developed cell culture systems involving tracheal and bronchial myocytes from different species. An emergent body of literature indicates that mutual signal transduction pathways control airway smooth muscle cell cycle entry across species lines. This article reviews what is known about mitogen-activated signal transduction in airway myocytes. The extracellular signal regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI 3-kinase) pathways appear to be key positive regulators of airway smooth muscle mitogenesis; recent studies have also demonstrated specific roles for reactive oxygen and the JAK/STAT pathway. It is also possible that growth factor stimulation of airway smooth muscle concurrently elicits signaling through negative regulatory intermediates such as p38 mitogen-activated protein (MAP) kinase and protein kinase C (PKC) delta, conceivably as a defense against extreme growth.     

Regulation of β-adrenergic responses in airway smooth muscle

Decreased responsiveness to β-adrenergic receptor agonists is a characteristic feature of human asthma. This review summarizes data regarding the impact of chronic beta agonist stimulation, cytokines, prostanoids and other factors on β-adrenergic responses in human airway smooth muscle, as well as the impact of polymorphisms of the β₂-adrenergic receptor on these responses. Effects of β-agonists on both airway smooth muscle relaxation and gene expression are considered. Understanding the regulation of β-adrenergic responses in airway smooth muscle cells may prove to be an important step in improving the efficacy of β-agonists for the treatment of asthma.     

Maturational changes in airway smooth muscle shortening and relaxation. Implications for asthma

Greater airway responsiveness in healthy juveniles is considered a factor in the higher asthma prevalence at a young age compared with adults. Several studies on the contractile response of airway smooth muscle (ASM) from birth to adulthood have addressed the hypothesis that a maturation of ASM plays a role in juvenile airway hyperresponsiveness. Maturation of distinct ASM properties, i.e. force generation, shortening, and relaxation, has been reported, although the majority of the studies have focused on maturation of maximum force and/or sensitivity to contractile agonists. However, in most animal species maturation of the ability to generate force does not correlate with maturation of airway responsiveness. Ontogenesis of ASM shortening has been less extensively studied and the existing reports emphasize an increase during maturation of tissue passive forces opposing shortening. ASM spontaneous relaxation has been very minimally investigated. We have recently demonstrated that the ability of ASM to spontaneously relax during stimulation is sharply reduced in juvenile airway tissue. It remains to be determined the role of these ASM properties in the onset of childhood asthma and whether specific alterations are induced by the occurrence of obstructive airway diseases in young individuals.     

转染Kv1.5基因抑制人气道平滑肌细胞的增殖

 目的：转染Kv1.5基因对人气道平滑肌细胞(HASMCs)增殖及凋亡的影响。方法:通过脂质体介导瞬时转染Kv1.5基因于培养的HASMCs中，以转染空载体pRc/CMV的细胞及未转染质粒的细胞为对照；用Western blotting 检测平滑肌细胞Kv1.5蛋白表达；用荧光光度法检测HASMCs胞内钙浓度；用流式细胞术观察细胞周期；用MTT法检测HASMCs 增殖及DNA 末端转移酶介导的原位缺口末端标记技术(TUNEL)检测细胞的凋亡。 结果: (1) 转染质粒组Kv1.5蛋白质的表达明显高于未转染组及空载体转染组(P<0.01); (2) 转染质粒组细胞胞内钙浓度明显低于未转染组及空载体转染组(P<0.05)，且其细胞周期中的G₀/G₁期细胞比例明显高于、细胞增殖率显著低于未转染组及空载体转染组(P<0.01)；同时，转染质粒组细胞的凋亡率明显高于未转染组及空载体转染组 (P<0.01)。 结论: 转染Kv1.5基因能抑制HASMCs的增殖、促进其凋亡，为进一步探讨哮喘气道重塑的机制及其治疗提供实验依据。     

Deep inspiration and airway smooth muscle adaptation to length change

In normal subjects a deep inspiration (DI) taken during bronchoconstriction substantially reduces airway narrowing (bronchodilation) and a DI taken prior to bronchoconstriction attenuates subsequent airway narrowing (bronchoprotection). Although the exact mechanism(s) for these phenomena are unclear the time course of these effects supports the hypothesis that they are mediated through actions of airway smooth muscle (ASM). There is convincing evidence that both the bronchodilation and bronchoprotection actions of DI are deficient or absent in asthmatic subjects. Various theories have been proposed such as a failure of transmission of stress and strain to the ASM in asthma, stretch-induced contraction of smooth muscle in asthmatics, a failure to release bronchodilating substances and differential effects on cross-bridge dynamics or contractile element rearrangement. In this brief review we focus on the mechanical consequences of DI on the ASM. We suggest that a failure of plastic rearrangement of the contractile apparatus following DI is at the basis of the abnormal response to DI in asthma.     

Airway smooth muscle growth from the perspective of animal models

Airway smooth muscle maintains airway tone and may assist in adjusting ventilation distribution within the normal lung. Alterations in the properties or the quantity of ASM are likely responsible for some instances of airways hyperresponsiveness to bronchoconstrictive stimuli that is a characteristic of diseases such as asthma. Morphometric studies have shown an increase in the mass of ASM in human asthmatic airways. Animal models have been developed that confirm that ASM can be induced to grow by allergic sensitization and challenge. Growth is in large part by hyperplasia as measured by incorporation of bromodeoxyuridine as a marker of the S-phase of the cell cycle. T cells, in particular CD4+ cells, may participate in the stimulation of growth of ASM by allergen challenge. The growth factors responsible for the increase in ASM are as yet unidentified but two mediators associated with allergic airway responses, cysteinyl leukotrienes and endothelin, have been implicated using specific receptor antagonists. The links between T cells and the biochemical mediators of growth have not been established.     

白细胞介素-4对大鼠气道平滑肌细胞的促增生作用

气道平滑肌增殖是支气管哮喘的特征性病理改变。本研究用体外培养的大鼠气道平滑肌细胞 (ASMC) ,观察了白细胞介素 4(IL 4 )对其增殖的影响。实验用四唑盐比色法 (MTT法 )和3 H TdR掺入法。MTT检测结果以OD值表示 ,加入IL 4的 2 4h组和 48h组分别为 0 32 3± 0 0 2 6 (x—±s,下同 )和 0 4 5 3± 0 0 48,比相应时间对照组的 0 191±0 0 18和 0 335± 0 0 6 3明显增加 ,(P均 <0 0 0 1) ;3 H TdR掺入也得到类似结果 ,IL 4 2 4h组 3 H掺入量为 76 5 8± 6 34 ,高于对照组的 6 0 6 0± 6 71counts min(P <0 0 1) ;用MTT检测法还观察到作用 2 4h ,IL 4 +胸腺肽组为 0 30 8± 0 0 0 7、IL 4 +地塞米松组为 0 2 45± 0 0 0 8比IL 4组 0 32 4± 0 0 14降低 (分别P <0 0 5及P <0 0 0 1) ,说明两种药物均可抑制IL 4的促增殖作用。实验表明IL 4可能还通过促进ASMC的增殖参与哮喘发病 ;而抑制IL 4对ASMC的促增殖作用可能是胸腺肽和地塞米松治疗哮喘的机制之一。     

Ontogeny of airway smooth muscle: structure, innervation, myogenesis and function in the fetal lung

Airway smooth muscle (ASM) is an integral component of the primordial lung. It differentiates from the mesenchyme as a ring of cells around the base of the epithelial bud that express smooth muscle-specific proteins. These rapidly form into interlocking bundles that progressively become wider and more compact along the bronchial tree to the trachea. Their orientation is perpendicular to the long axis of the airway. The ASM exhibits rhythmic contractility (i.e. it is a phasic-type smooth muscle) soon after formation, and the spontaneous airway narrowing shifts the lung liquid distally causing expansion of the tubule walls. This stretching is the mechanical stimulus to smooth muscle (SM) myogenesis and lung growth. Neural tissue, i.e. precursor ganglia interconnected by nerve trunks and smaller bundles, forms a sheath over the ASM layer with varicose fibres descending to the muscle. These are guided by glial-derived neurotrophic factor (GDNF) that appears to be produced by ASM. Maturation of neural tissue is slower than the ASM; functional cholinergic innervation is manifest by the early canalicular stage when most neurotransmitters appear.     

Airway smooth muscle chemokine receptor expression and function in asthma

Background Chemokine receptors play an important role in cell migration and wound repair. In asthma, CCR3 and 7 are expressed by airway smooth muscle (ASM) and CCR7 has been implicated in the development of ASM hyperplasia. The expression profile of other chemokine receptors by ASM and their function needs to be further explored.
Objective We sought to investigate ASM chemokine receptor expression and function in asthma.
Methods ASM cells were derived from 17 subjects with asthma and 36 non-asthmatic controls. ASM chemokine receptor expression was assessed by flow cytometry and immunofluorescence. The function of chemokine receptors expressed by more than 10% of ASM cells was investigated by intracellular calcium measurements, chemotaxis, wound healing, proliferation and survival assays.
Results In addition to CCR3 and 7, CXCR1, 3 and 4 were highly expressed by ASM. These CXC chemokine receptors were functional with an increase in intracellular calcium following ligand activation and promotion of wound healing [CXCL10 (100 ng/mL) 34 ± 2 cells/high-powered field (hpf) vs. control 29 ± 1; P =0.03; n =8]. Spontaneous wound healing was inhibited by CXCR3 neutralizing antibody (mean difference 7 ± 3 cells/hpf; P =0.03; n =3). CXC chemokine receptor activation did not modulate ASM chemotaxis, proliferation or survival. No differences in chemokine receptor expression or function were observed between ASM cells derived from asthmatic or non-asthmatic donors.
Conclusions Our findings suggest that the chemokine receptors CXCR1, 3 and 4 modulate some aspects of ASM function but their importance in asthma is uncertain.