Point:Counterpoint: Alterations in airway smooth muscle phenotype do/do not cause airway hyperresponsiveness in asthma POINT: ALTERATIONS IN AIRWAY SMOOTH MUSCLE PHENOTYPE DO CAUSE AIRWAY HYPERRESPONSIVENESS IN ASTHMA

Airway hyperresponsiveness and obstruction are cardinal features of asthma. Compelling evidence now suggests that the key features of asthma in part are due to intrinsic differences in airway smooth muscle (ASM) function. Inflammatory mediators alter ASM phenotype, which directly modulates ASM hyperresponsiveness and promotes airway remodeling that further contributes to amplified airway obstruction. For decades, the most effective and common therapies in the management of asthma have focused on bronchodilation and reversal of ASM shortening. Accordingly, alterations in the phenotype of ASM, the pivotal cell regulating bronchomotor tone, fundamentally contribute to airway hyperresponsiveness in asthma. Asthma, a disease characterized by airway inflammation, hyperresponsiveness, and reversible airflow obstruction, remains a highly prevalent respiratory disease accounting for profound morbidity. Airway hyperresponsiveness and reversible airflow obstruction are primarily due to shortening of ASM, the pivotal cell responsible for regulating bronchomotor tone. Over the past 20 years, evidence has accumulated that airway hyperresponsiveness results from intrinsic abnormalities in the ASM of people with asthma that modulate excitation-contraction coupling mechanisms, enhance ASM cell proliferation, alter the production of extracellular matrix proteins, and enhance the production of inflammatory chemokines and cytokines. Collectively these phenotypic alterations in the ASM cell may be responsible for many of the pathologic properties of the asthmatic airway and are likely to be the primary mechanism for airway hyperresponsiveness. A substantial body of evidence indicates that asthmatic ASM manifests enhanced shortening and contractility in vitro compared with ASM from normal subjects. Several studies have directly compared the in vitro contractility of ASM cells and tissues isolated from asthmatic and normal subjects, and found that the shortening or contraction of ASM isolated from asthmatic subjects is greater than that from normal subjects. Bronchi prepared from surgical specimens from asthmatic lungs have also been found to exhibit increased sensitivity to adenosine compared with those from normal subjects (6, 33). Furthermore, the contractility of human bronchial smooth muscle cells from non-atopic subjects can be enhanced by incubation in vitro with human serum from atopic individuals containing immunoglobulin E (IgE) or by passive sensitization using human serum containing IgE (33). Newer methodologies in deriving ASM cell lines from subjects with asthma compared with ageand sex-matched controls have resulted in similar findings: bronchial smooth muscle cell lines derived from subjects with asthma exhibit enhanced shortening compared with cell lines from subjects without asthma (30, 41). ASM cells from patients with asthma embedded in collagen gels exhibited greater contraction in response to histamine than cells from normal subjects (31). All of these studies point to phenotypic differences in the ASM cells from asthmatics compared with normals that result in enhanced contractility. Investigators have shown that asthmatic-derived ASM has an amplified expression of CD38 (24). CD38 is a cell surface hydrolase and cyclase whose activation generates cADPR, a putative activator of the ryanodine receptor. Accordingly, increases in cADPR enhance agonist-induced ASM calcium mobilization (14). Importantly, CD38 expression is also induced by IL-13 and TNF, suggesting that CD38 may also play an important role in mediating airway hyperresponsiveness (24, 40). These findings in human asthmatic ASM cells and tissues are supported by parallel findings in studies of ASM cells derived from animal models of airway hyperreactivity and asthma. The enhanced contractility and shortening in vitro of ASM isolated from animals with intrinsic airway hyperreactivity is well documented: ASM from the hyperresponsive Fisher rats is more reactive in vitro than that of Lewis rats (42); ASM from the hyperreactive Basenji greyhound dogs exhibits enhanced sensitivity to contractile agents compared with that of less reactive dogs (15); ASM from hyperreactive mouse strains exhibits enhanced contractility in vitro (5). Differences in ASM responsiveness also exist in airways isolated from animal models of allergen-induced airway hyperreactivity, indicating that the airway hyperresponsiveness observed in these animals can be attributed directly to the effects of sensitization on the excitation-contraction coupling in the ASM (11, 12, 32). Inflammatory mediators present in the bronchoalveolar lavage of subjects with asthma (IL-4, IL-13, and TNF) also increase in vitro responsiveness of ASM to bronchoconstricting agents and decrease the relaxation of ASM to relaxing agents (11, 16, 17, 19, 26, 37). Although the precise mechanisms by which cytokines enhance agonist-induced excitationcontraction coupling remain unclear, a variety of calcium signaling pathways have been impugned (40). Furthermore, increases in shortening velocity also alter the responses of ASM to mechanical stretch that has been described as dysfunctional in asthma (1). Collectively, overwhelming evidence suggests that ASM derived from subjects with asthma manifest an enhanced excitation-contraction coupling response to bronchoconstricting agents. It is likely that this enhanced bronchoconstricting response coupled with an insensitivity to bronchodilators directly regulate airway hyperresponsiveness in asthma. In addition to alterations in calcium mobilization pathways and contractile protein responses to agonists, ASM derived from asthma subjects also manifests a hyperproliferative phenotype and chemokine/cytokine secretion profiles that play important roles in mediating airway hyperresponsiveness. ASM cells derived from subjects with asthma compared with non-asthmatic subjects manifest greater cell proliferation at baseline and in response to mitogens. Importantly, the hyperproliferative ASM is less responsive to steroids and cAMPmobilizing agents that are effective in inhibiting mitogeninduced ASM proliferation in cells derived from non-asthma subjects (13, 23, 43, 45). Because increases in ASM mass are one of the most commonly reported findings in the histopaJ Appl Physiol 113: 837–843, 2012; doi:10.1152/japplphysiol.00483.2012. Point:Counterpoint