Interfacial sensing regulates alveolar barrier function in lung epithelial monolayer
Alveolar Type II cells (AT II cells) are located in the alveoli of the lung and are in close proximity to an air-liquid-interface. An important function of AT II cells is the production and secretion of pulmonary surfactant. Pulmonary surfactant is an essential substance, which reduces the high surface tension acting at the alveolar interface. Different stimuli and forces act on this nanometer scaled gas-blood-barrier. As AT II cells are responsible for the regulation and maintenance of alveolar structures and processes, we suggest that changes of the extracellular environment lead to a fast feedback response via functional and morphological changes in the cells. In order to test this hypothesis, primary AT II cells isolated from rat lung were grown on cell culture inserts with porous membranes for longer time periods, applying different culture conditions. We found out that in contrast to many cell lines, applying culture media supplemented with different concentrations of growth factor, nutrients or corticoids had no effect on the barrier function of primary lung cells. However, we discovered that ATII cells cultured in immersed-liquid-interface (ILI) conditions show a significantly higher transepithelial electrical resistance (TER) in comparison to cells that are cultured under air-liquid-interface (ALI) conditions. These results were confirmed by testing transport capacity of different-sized (< 1 – 70kDa) dextran-conjugates from the basolateral to the apical compartment. Interestingly, the loss of intercellular tightness was not principally based on changes in tight junction protein expression. Instead, we observed dramatic and quick alterations in actin cytoskeleton formation. The TER effect between the two cultures was not observed when ALI treated cells were covered with a hyperthin layer of surfactant. Therefore, we suggest that the surface tension, which acts on the air exposed cells, induce a fast cell response via calcium influx, which leads to actin reformation followed by functional and morphological cell differentiation. We propose that this newly discovered mechanism of interfacial sensing is essential for the AT II cells for maintaining the sensitive alveolar lining fluid homeostasis during physiological and pathophysiological setting.