Stretch magnitude and frequency-dependent actin cytoskeleton remodeling in alveolar epithelia

Abstract

Alveolar epithelial cells (AEC) maintain integrity of the blood-gas barrier with gasket-like intercellular tight junctions (TJ) that are anchored internally to the actin cytoskeleton. We hypothesize that stretch rapidly reorganizes actin (<10 min) into a perijunctional actin ring (PJAR) in a manner that is dependent on magnitude and frequency of the stretch, accompanied by spontaneous movement of actin-anchored receptors at the plasma membrane. Primary AEC monolayers were stretched biaxially to create a change in surface area (DeltaSA) of 12%, 25%, or 37% in a cyclic manner at 0.25 Hz for up to 60 min, or held tonic at 25% DeltaSA for up to 60 min, or left unstretched. By 10 min of stretch PJARs were evident in 25% and 37% DeltaSA at 0.25 Hz, but not for 12% DeltaSA at 0.25 Hz, or at tonic 25% DeltaSA, or with no stretch. Treatment with 1 muM jasplakinolide abolished stretch-induced PJAR formation, however. As a rough index of remodeling rate, we measured spontaneous motions of 5-mum microbeads bound to actin focal adhesion complexes on the apical membrane surfaces; within 1 min of exposure to DeltaSA of 25% and 37%, these motions increased substantially, increased with increasing stretch frequency, and were consistent with our mechanistic hypothesis. With a tonic stretch, however, the spontaneous motion of microbeads attenuated back to unstretched levels, whereas PJAR remained unchanged. Stretch did not increase spontaneous microbead motion in human alveolar epithelial adenocarcinoma A549 monolayers, confirming that this actin remodeling response to stretch was a cell-type specific response. In summary, stretch of primary rat AEC monolayers forms PJARs and rapidly reorganized actin binding sites at the plasma membrane in a manner dependent on stretch magnitude and frequency.