Studies of Tight Junctions and Airway Surface Liquid in Airway Epithelium with Relevance to Cystic Fibrosis

Abstract: Cystic fibrosis (CF) is a multi-organ autosomal recessive disease of fluid-transporting epithelia, due to a mutation in the gene coding for the cystic fibrosis transmembrane conductance regulator (CFTR) protein. CFTR is a cAMP-regulated Cl-channel involved in various regulatory processes. Salt and water transport depend on CFTR and the epithelial sodium channel (ENaC), operating in concert with the paracellular pathway through the tight junctions (TJ). The ionic composition of the ASL has been assumed to be altered in CF, resulting in a fatal accumulation of viscous mucus in the airways.ASL samples were collected from tracheal and nasal fluid in normal and transgenic CF mice and from the fluid covering the apical surface of normal bronchial cells (16HBE14o-) and a CF human bronchial cell line (CFBE41o-). Analysis of the elemental content of the ASL was performed by X-ray microanalysis. The ASL contained more Na and Cl in CFTR-deficient or DF508-CFTR-containing cells than in control cells with wild- type CFTR.The relation between osmolarity and TJ permeability was examined by the addition of salt or sugar (295-700 mOsm) to 16HBE14o- cells, where the integrity of TJ was evaluated by transepithelial electrical resistance (TEER) measurements. Studies of interaction between the activity of CFTR, TJ and cytoskeleton were performed in CFBE41o-, plasmid corrected CFBE41o- (CFBE41o pCep4), and 16HBE14o- cells exposed to an inhibitor of CFTR (CFTRinh-172). The TJ were investigated by determining the paracellular permeability to lanthanum ions or with [14C] mannitol. Cytoskeletal changes were evaluated by immunofluorescence.Hyperosmotic stress resulted in opening of TJ. Inhalation of hypertonic salt or sugar solutions may open TJ, leading to enhanced paracellular water transport and increased ASL volume, diluted mucus and enhanced mucociliary clearance. This may explain the beneficial effect of this treatment for CF-patients.In healthy airway epithelial cells, inhibition of CFTR by CFTRinh-172 resulted in an increased TEER, whereas stimulation of CFTR by IBMX/forskolin caused a decrease. The paracellular permeability was inversely proportional to TEER. Immunofluorescence revealed a disorganization of cytoskeletal proteins in CF-cells. These results point toward a possible interaction between the activity of CFTR and TJ protein complex, presumably via the cytoskeleton.