Shot Down Inflamed: Airway Smooth Muscle Bronchodilator Insensitivity in Cystic Fibrosis

Abstract

Globally, cystic fibrosis (CF) affects nearly 80,000 children and adults (1). This autosomal-recessive disorder includes multiple gene alterations of the chloride transporter CF transmembrane conductance regulator (CFTR), preventing its processing, translocation to cell membranes, and activity. CF presents as a progressive multiorgan disease with debilitating effects on the lung, including significant airflow resistance and airway obstruction that is associated with compromised mucus hydration, chronic bacterial and viral infections, and lingering inflammation. For these reasons, the airway epithelium has been a focus for understanding disease pathophysiology and developing new therapeutics. Indeed, very promising recent clinical trials using triple CFTR-targeting therapy involving a potentiator (to increase channel opening) and two small-molecule correctors (to promote CFTR translocation to the membrane) were born from preclinical in vitro studies with primary cultured airway epithelial cells (2, 3). In this issue of the Journal, Matusovsky and colleagues (pp. 434–444) look beyond the airway epithelium, setting their sights on airway smooth muscle that encircles intrapulmonary airways, to determine whether intrinsic changes in muscle contractility further underpin airway obstruction in CF (4). There is a compelling rationale for addressing this issue. The term “CF-asthma” was coined because more than 50% of patients with CF exhibit hallmark symptoms of asthma, such as cough, wheeze, and airway hyperresponsiveness, and 80% exhibit bronchodilator-reversible airway obstruction (5). This accounts for the overlap in standard clinical therapies for managing asthma and CF, with inhaled corticosteroids being used to control lung inflammation, and longor short-acting b-agonists being used to dampen and reverse recurrent airway smooth muscle bronchoconstriction. Airway remodeling that includes enlargement of airway smooth muscle mass is a pathologic hallmark of asthma (6) and CF (7), an observation that was reconfirmed by Matusovsky and colleagues in airways from patients with severe CF. Interestingly, the carrier frequency of CFTR mutations is anomalously high in patients with asthma (8). Moreover, in a porcine model, Cook and colleagues (9) showed that the absence of CFTR leads to airway smooth muscle hypercontractility due to increased muscle tone, prolonged actomyosin activation, and compromised reuptake of Ca to the sarcoplasmic reticulum after bronchoconstrictor stimulation. This is associated with the absence of CFTR in the sarcoplasmic reticulum membrane, resulting in loss of Cl current that would otherwise contribute to the driving potential for inward Ca flow (9). It is important to note that this work was completed using neonatal porcine lung material devoid of inflammation, and therefore it may not wholly represent the degree of real-life airway smooth muscle dysfunction experienced by adult patients with CF who have an airway phenotype that has been molded by ongoing inflammation and clinical intervention. Matusovsky and colleagues meet these questions and knowledge gaps head on, using freshly isolated airway smooth muscle preparations microdissected from intrapulmonary airways of human lung transplant donors to evaluate intrinsic contractile properties and the relaxation response to b2-receptor activation, and how these are affected by preexposure to IL-13. They used airways from eight adult CF transplant lungs with severe disease for contractile function studies. The donors included five individuals homozygous for the DF508 mutation and one individual with the DF508/R334W genotype. CFTR protein deficiency was confirmed by IB. All of the CF donor lungs were positive for bacterial infection. For comparison, control airways were obtained from bacteria-free lungs of donors who died of acute causes unrelated to resident lung disease. Surprisingly, despite the use of elegant and sensitive methods to investigate baseline contractility using servo-electric muscle lever systems, there were no differences in methacholine-mediated maximum force, stress, shortening velocity, or isoproterenol-induced relaxation of precontracted airway muscle strips. At first glance, these findings do not appear to support a paradigm in which CFTR defects significantly alter human airway smooth muscle contractility. However, this is where the story really gets interesting! The majority of CF lungs used in this study were obtained from donors who were being managed clinically with oral prednisone and inhaled bronchodilator. Lung tissue was acquired 24 hours before airway dissection in physiologic buffers, which likely eliminated and/or compromised the viability of resident inflammatory cells, and diluted local mediators. Chronic and acute exacerbationassociated lung inflammation in CF prominently includes T-helper cell type 2 (Th2) and Th17 cytokines, including IL-13, IL-8, and IFN-g (10). IL-13 is particularly germane because it directly promotes airway smooth muscle hypercontraction in response to histamine and acetylcholine (11, 12). IL-13 reportedly decreases the abundance of sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA) in human airway smooth muscle, an effect that leads to prolonged and elevated myoplasmic Ca (13). An additional mechanism for this effect involves dimeric IL-13 receptors that mediate increased expression of CD38, a membrane-associated enzyme that synthesizes cyclic adenine diphosphonucleotide ribose (cADPR) (11). cADPR is an activating ligand for ryanodine receptors that serve as ports for additional Ca entry into the myoplasm from the sarcoplasmic reticulum, which increases agonist-induced contractile force. In this context, Matusovsky and colleagues specifically interrogated how IL-13 preexposure affects airway smooth muscle contraction and relaxation. In so doing, they uncovered two