The effects of repeated allergen challenge on airway smooth muscle structural and molecular remodeling in a rat model of allergic asthma

Abstract

The effects of remodeling of airway smooth muscle (SM) by hyperplasia on airway SM contractility in vivo are poorly explored. The aim of this study was to investigate the relationship between allergen-induced airway SM hyperplasia and its contractile phenotype. Brown Norway rats were sensitized with ovalbumin (OVA) or saline on day 0 and then either OVA-challenged once on day 14 and killed 24 h later or OVA-challenged 3 times (on days 14, 19, and 24) and killed 2 or 7 days later. Changes in SM mass, expression of total myosin, SM myosin heavy chain fast isoform (SM-B) and myosin light chain kinase (MLCK), tracheal contractions ex vivo, and airway responsiveness to methacholine (MCh) in vivo were assessed. One day after a single OVA challenge, the number of SM cells positive for PCNA was greater than for control animals, whereas the SM mass, contractile phenotype, and tracheal contractility were unchanged. Two days after three challenges, SM mass and PCNA immunoreactive cells were increased (3-and 10-fold, respectively; P Ͻ 0.05), but airway responsiveness to MCh was unaffected. Lower expression in total myosin, SM-B, and MLCK was observed at the mRNA level (P Ͻ 0.05), and total myosin and MLCK expression were lower at the protein level (P Ͻ 0.05) after normalization for SM mass. Normalized tracheal SM force generation was also significantly lower 2 days after repeated challenges (P Ͻ 0.05). Seven days after repeated challenges, features of remodeling were restored toward control levels. Allergen-induced hyperplasia of SM cells was associated with a loss of contractile phenotype, which was offset by the increase in mass. phenotype; animal model; myosin heavy chain isoform STRUCTURAL AIRWAY REMODELING has been widely reported in both human asthma (4, 13) and animal models of experimental asthma (11,23,37). Most tissues of the airway wall are affected by remodeling, and the observed changes include shedding of the epithelium, thickening of the reticular basement membrane, increased mucus gland size and number, goblet cell differentiation, angiogenesis, and augmentation of the airway smooth muscle (SM) mass (4). The changes in airway wall tissues are likely to be of clinical importance since they may control airway caliber, airway elasticity, and responsiveness and may cause symptoms related to mucus overproduction (4). Airway SM remodeling is a reported feature of several animal models of allergic asthma (11,12,22), including the Brown Norway (BN) rat (19,26,31). The increase in SM mass, observed in asthma of varying severity (5, 29, 36), has been proposed to be sufficient to account for altered airway responsiveness (16). The augmentation of airway SM mass has been attributed to hyperplasia through evidence of cell proliferation quantified by bromodeoxyuridine incorporation or PCNA expression in vivo (11,19,28,37). Susceptibility to SM remodeling may be a risk factor for asthma; cultured airway SM cells from asthmatic subjects were shown to have a higher proliferation rate than cells from normal subjects (14), sim...

Treatment with a sphingosine-1-phosphate analog inhibits airway remodeling following repeated allergen exposure

“…We first examined the potential of S1PR stimulation with FTY720 to prevent the increase in airway smooth muscle observed following repeated exposure to OVA. In this model, as previously described (18,31,40,44,48), repeated OVA but not PBS challenges led to an increase in airway smooth muscle mass observed morphometrically on ␣-SMA-stained sections. This effect was inhibited following treatment with FTY720 (1, 10, 30, or 100 g/kg) 1 h prior to each OVA challenge (Fig.…”

“…All experiments were performed with the approval of the Animal Care Committee of McGill University. Sensitization and exposure to ovalbumin (OVA) or vehicle (saline) as well as bronchoalveolar lavage (BAL) and BAL protein concentration were performed as described previously (18,31,40,44,48). Briefly, rats were sensitized to OVA or PBS on days 0 and 7 and challenged with nebulized OVA (5%) or PBS on days 14,19, and 21.…”

“…Increased airway smooth muscle has been associated with asthma severity, airway hyperresponsiveness (AHR), fixed airflow limitation, and risk of fatal asthma (3,7,17,28). It has been postulated that an increase in airway smooth muscle mass is sufficient to account for AHR in asthma (19), although loss of the contractile phenotype of remodeled airway smooth muscle could offset the changes in airway responsiveness that would be expected otherwise (18,24). The mechanisms of airway remodeling are potentially complex and likely also multifactorial.…”

“…Our hypothesis is that limiting the capacity of T cells to migrate to the airway smooth muscle bundles would lead to a reduction in airway remodeling and consequently a reduction in AHR to methacho-line. Using a synthetic analog of S1P, FTY720, this study aimed to investigate the effects of repeated stimulation of S1PR1 on airway remodeling in a well-characterized model of repeated allergen exposure that presents with hallmarks of chronic asthma (18,31,40,44,48).…”

Human Trachealis and Main Bronchi Smooth Muscle Are Normoresponsive in Asthma

“…Furthermore, the increase in ASM mass found in asthma is caused by hypertrophy and hyperplasia, and both have been shown to reduce the contractility of ASM. Our laboratory has shown that rat tracheal ASM responds to repeated allergen challenge with a reduced contractility of SM cells and a commensurate increase in SM cell number (17). Hypertrophy of ASM has also been shown to result in reduced contractility in some studies (18), although not consistently (19).…”