Characterization of determinants of airway smooth muscle cell heterogeneity

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Halayko, Andrew John
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Airway smooth muscle contraction plays an indisputable role in exacerbations of bronchial asthma, a disease in which morbidity and mortality are increasing. Changes in airway smooth muscle function are associated with yperresponsiveness and remodelling of the airways, including airway smooth muscle cell (SMC) proliferation. Airway myocytes are thought to be similar, but recent data indicate that arterial SMC constitute phenotypically disparate subpopulations of myocytes. Primary cultures of canine airway SMC were established to identify phenotype-specific protein markers. Mature airway SMC were characterized by abundant contractile protein content, whereas proliferative, cultured cells expressed non-muscle isoforms of cytostructural proteins. Adult tr cheal SMC were distinct from less contractile, pulmonary arterial SMC, on the basis of marker protein content. Analysis of cell cycle progression by flow cytometry revealed airway SMC cultures to be composed of several distinct subpopulations differing in responsiveness to mitogenic and antiproliferative agents. Distinct subgroups of acutely dissociated canine airway myocytes were discriminated on the basis of ploidy and contractile protein content using immunocytochemistry and flow cytometry. The fraction of SMC possessing high levels of contractile proteins was greatest in muscle from large airways indicating that inter-airways heterogeneity in contractile properties may be due to differences in the distribution of functionally divergent airway myocytes. Flow cytometry was used to sort two populations of canine tracheal SMC, called Type A and Type B, that differed morphologically and in marker protein content. Primary cultures of each population were established. Type A myocytes were small, grew well in culture and expressed an immature phenotype. Type B cells were large, fusiform cells that were resistant to cell culture and presented a terminally differentiated, contractile phenotype. Collectively, these findings demonstrate that airway smooth muscle is composed of distinct, heterogeneous subpopulations of SMC that differ in contractile and proliferative potential. The distribution of myocyte subpopulations may be important in determining the normal physiology and pathophysiology in different areas of the lung. Future studies aimed at elucidation of the developmental and molecular mechanisms that determine SMC phenotype should provide valuable insights that may be used therapeutically for human asthma.