Heterogeneity of airway smooth muscle cells and its implications in pathogenesis of asthma

Abstract

The present project commenced with mechanical studies of airway smooth muscles from different generations of the canine airways. Two biophysically distinct groups of airway smooth muscle were detected along the canine airway tree: an extrapulmonary group, including airway smooth muscles from trachea and bronchial generations 1 and 2, displayed relatively high contractility, and an intrapulmonary group, consisting of muscles from bronchial generation 3 to 6, showed a relatively low contractility. Enhanced expression of smooth muscle myosin light chain kinase and N-terminal isoform of smooth muscle myosin heavy chain in extrapulmonary airway smooth muscle could probably contribute to the enhanced contractility of extrapulmonary muscle. At the single cell level, three morphologically, biophysically, and biochemically different types of contractile smooth muscle cells were identified in airway smooth muscle. Significant differences in proportions of these cells were detected between extra- and intrapulmonary airway smooth muscles. Increased numbers of cells with decreased contractility in intrapulmonary muscles might contribute to their reduced contractility compared with extrapulmonary muscles. Heterogeneous alterations in shortening properties were also observed in sensitized airway smooth muscle at tissue and single cell levels, these in turn may contribute to the non-homogeneous bronchospasm observed in asthmatic airways. Heterogeneity was also displayed in cultured tracheal smooth muscle cells subjected to prolonged serum deprivation. Typical patterns of hill-and-valley disappeared in serum-deprived smooth muscle cell in culture, instead, two phenotypically distinct populations of cells developed: (1) Elongated spindle-shaped cells, which expressed large amounts of smooth muscle contractile and regulatory proteins, usually formed bundles, and showed a surpri ing shortening capacity and velocity which was even greater than that of the freshly isolated cells; (2) Flat and stellate cells, which expressed very few smooth muscle contractile proteins and showed no contraction. A considerable increase of smooth muscle light chain kinase was detected in the newly developed super-contractile cells, but the myosin heavy chain N-terminal isoform which is believed to be responsible for the higher shortening velocity was not detected. Finally the shortening properties of human asthmatic airway smooth muscle were evaluated in single cells isolated from endobronchoscopic biopsies. Significantly increased shortening capacity and velocity were found in asthmatic cells, which unraveled the long-lasting puzzle of whether the asthmatic airway smooth muscle possesses altered contractility. In general these systematic studies demonstrated a clear heterogeneity of airway smooth muscle (cells) with respect to structural, biophysical and biochemical properties; this was of pathophysiological importance. (Abstract shortened by UMI.)