The functional consequences of airway remodeling in asthma

Structural changes in the airway walls involving extracellular matrix remodelling are prominent features of asthma. These changes are probably driven by mediators released as a consequence of chronic allergic inflammation. It is clear that changes in the extracellular matrix have the capacity to influence airway function in asthma. However, it is not clear how each of the many changes that occur in the airway wall contribute to altered airway function in asthma. Collagen deposition in the subepithelial matrix, and hyaluronan and versican deposition around and internal to the smooth muscle would be expected to oppose the effect of smooth muscle contraction. Conversely, geometric considerations would result in exaggerated airway narrowing for a given degree of smooth muscle shortening, as the airway wall is thickened by the deposition of these molecules internal to the smooth muscle. Elastin and cartilage reorganization and degradation in the airway walls would be expected to result in decreased airway wall stiffness and increased airway narrowing for a given amount of force generated by the smooth muscle. Degradation of matrix associated with the smooth muscle may both decrease the stiffness of the parallel elastic component and uncouple smooth muscle from the load provided by lung recoil, allowing exaggerated smooth muscle shortening. Increase in muscle mass may be associated with an increase, a decrease or no change in smooth muscle contractility. If an increase in muscle mass was associated with preservation of its contractile capacity modelling studies suggest that it could be the most important contributor to exaggerated airway narrowing. Modelling studies also suggest that the pattern of mucosal folding during smooth muscle contraction may be an important determinant of airway narrowing. The greater the number of folds, and the stiffer the subepithelial collagenous layer the more resistant the airway will be to narrowing.