Impaired mucociliary clearance in COPD: A role for mitochondrial dysfunction
Salil Srivastava, Selome Mitiku, Olivier Bonneau, Kaushik Subramanian, Mark Dowling and David J Rowlands
Novartis Institutes for Biomedical Research Inc., Respiratory Disease Area, 700 Main St, Cambridge, MA, 02139, USA
Chronic obstructive pulmonary disease (COPD) is the third leading cause of death in the United States, and with an aging population, declining air quality and increased tobacco use, the incidence of COPD is expected to increase. Mucus hypersecretion is a hallmark of COPD and is coupled with severely impaired mucociliary clearance (MCC), predisposing patients to recurrent airway infections, thereby exacerbating COPD. Ciliated epithelial cells account for ~50% of epithelial cells in the normal conducting airway and have approximately 200-300 cilia per cell on the luminal surface. Vast quantities of mitochondria are present in close proximity to the apical surface, providing energy required for coordinated cilia beating. Mitochondrial dysfunction has been implicated to play a pivotal role in the pathogenesis of COPD however, its contribution to impaired MCC remains to be fully elucidated. Here, we sought to determine whether treatment of ex vivo tissue and human epithelial cell cultures with cigarette smoke extract (CSE), could act as a model of COPD, induced mitochondrial dysfunction and impaired MCC. Using ex vivo murine tracheal strips cultured for 24h with CSE (5%), a significant reduction in fluorescent bead displacement (~80%, P<0.01, n=3-6) was observed, indicating impaired MCC. Notably, treatment of the immortalized human bronchial epithelial cell line, BEAS-2B, with CSE (5%, 24h) led to a 2-fold decline in ΔψM (P<0.01, n=6), increased cell death (P<0.01, n=3), and increased expression of the mitophagy associated protein PINK1 (P<0.05, n=3), thus strongly indicating CSE mediated mitochondrial dysfunction. To determine whether mitochondrial dysfunction plays a role in impaired MCC, murine tracheal strips incubated with the complex III inhibitor antimycin A (1μM, 1h) demonstrated a significant reduction in bead displacement (P<0.01, n=3-4). Moreover, treatment of primary human bronchial epithelial cells (cultured at air-liquid interface), with antimycin A (1μM, 1h) also led to a decline in cilia beat frequency (P<0.01, n=4), providing evidence that functioning mitochondria are pivotal for cilia function and active MCC. Together, our data demonstrate that cigarette smoke mediated decline in MCC associated with COPD may occur as a result of mitochondrial dysfunction. Strategies aimed at restoring the pool of healthy mitochondria may led to novel therapies for COPD.