A novel role for primary cilia in airway remodeling

Airway ciliated cells in adult lung homeostasis and COPD

Cilia are organelles emanating from the cell surface, consisting of an axoneme of microtubules that extends from a basal body derived from the centrioles. They are either isolated and nonmotile (primary cilia), or grouped and motile (motile cilia). Cilia are at the centre of fundamental sensory processes and are involved in a wide range of human disorders. Pulmonary cilia include motile cilia lining the epithelial cells of the conductive airways to orchestrate mucociliary clearance, and primary cilia found on nondifferentiated epithelial and mesenchymal cells acting as sensors and cell cycle keepers. Whereas cilia are essential along the airways, their regulatory molecular mechanisms remain poorly understood, resulting in a lack of therapeutic strategies targeting their structure or functions. This review summarises the current knowledge on cilia in the context of lung homeostasis and COPD to provide a comprehensive overview of the (patho)biology of cilia in respiratory medicine with a particular emphasis on COPD.

Primary ciliogenesis is a crucial step for multiciliated cell determinism in the respiratory epithelium

The alteration of the mucociliary clearance is a major hallmark of respiratory diseases related to structural and functional cilia abnormalities such as chronic obstructive pulmonary diseases (COPD), asthma and cystic fibrosis. Primary cilia and motile cilia are the two principal organelles involved in the control of cell fate in the airways. We tested the effect of primary cilia removal in the establishment of a fully differentiated respiratory epithelium. Epithelial barrier integrity was not altered while multiciliated cells were decreased and mucous-secreting cells were increased. Primary cilia homeostasis is therefore paramount for airway epithelial cell differentiation. Primary cilia-associated pathophysiologic implications require further investigations in the context of respiratory diseases.

Increased Primary Cilia in Idiopathic Pulmonary Fibrosis

Primary cilia are solitary, non-motile, axonemal microtubule-based antenna-like organelles that project from the plasma membrane of most mammalian cells and are implicated in transducing hedgehog signals during development. It was recently proposed that aberrant SHH signaling may be implicated in the progression of idiopathic pulmonary fibrosis (IPF). However, the distribution and role of primary cilia in IPF remains unclear. Here, we clearly observed the primary cilia in alveolar epithelial cells, fibroblasts, and endothelial cells of human normal lung tissue. Then, we investigated the distribution of primary cilia in human IPF tissue samples using immunofluorescence. Tissues from six IPF cases showed an increase in the number of primary cilia in alveolar cells and fibroblasts. In addition, we observed an increase in ciliogenesis related genes such as IFT20 and IFT88 in IPF. Since major components of the SHH signaling pathway are known to be localized in primary cilia, we quantified the mRNA expression of the SHH signaling components using qRT-PCR in both IPF and control lung. mRNA levels of SHH, the coreceptor SMO, and the transcription factors GLI1 and GLI2 were upregulated in IPF compared with control. Furthermore, the nuclear localization of GLI1 was observed mainly in alveolar epithelia and fibroblasts. In addition, we showed that defective KIF3A-mediated ciliary loss in human type II alveolar epithelial cell lines leads to disruption of SHH signaling. These results indicate that a significant increase in the number of primary cilia in IPF contributes to the upregulation of SHH signals.