Strain of pulmonary alveolar cells

Alveoli are the site of gas exchange between air and blood. During each inspiration, fresh air flows into the terminal airways, driven by the distension of the lung. Lung distension is accompanied by mechanical unfolding and deformation of alveoli, including deformation of the alveolar epithelium. This causes strain (defined as the change of length in relation to initial length) of alveolar epithelial cells. Strain of type II cells is an important stimulus for surfactant secretion, thereby keeping the lung easily "distensible": It was observed back in 1959 by Mead and Collier that a "stiff lung" during mechanical ventilation with small tidal volumes could be normalized by a single large inflation. Strain on cells and membranes in the lung has also an important pathophysiological role for ventilator-induced lung injury. Finally, physical forces in the lung play an integral part in maintaining the alveolar structure by modulating maturation, differentiation and apoptosis. The pulmonary alveolar epithelium consists of two cell types: type I cells and type II cells. Type II cells control alveolar distensibility and fluid homeostasis in various ways. Their main functions are the secretion of surfactant and active electrolyte and fluid transport. Secretion of surfactant occurs by exocytosis of surfactant, which is stored in vesicles called "lamellar bodies". Surfactant is a lipid-rich, lipoprotein-like substance which forms a barrier at the air-liquid interface, thereby reducing the surface tension of the lung and facilitating inspiration. Using new techniques which we have developed in our lab, we aim at elucidating the cellular/molecular mechanisms by which a single stretch of alveolar cells mediates the secretion of surfactant into the extracellular space. Furthermore, these studies are designed to yield substantial new insights into the pathophysiological mechanisms of membrane stress during hyperinflation. We seek to delimit - on a cellular level - the amount of strain which is physiologically important for the control of secretion from that which results in pathophysiological responses such as membrane stress failure.

It was the aim of this project to elucidate mechanical forces that regulate and are involved in the secretion of surfactant. Type II cells store surfactant, a substance of lipids and proteins, in intracellular vesicles termed "lamellar bodies" (LBs). Surfactant is secreted into the alveolar lumen via exocytosis of LBs, i.e. via fusion of the limiting LB membrane with the plasma membrane. This exocytotic process is fundamental for lung function, respiration and survival, because surfactant enables inspiration by lowering the surface tension at the air-liquid- interface. Dysfunction of this process may cause respiratory failure and death. Mechanical forces, such as a single cell stretch during a deep breath (yawning, sighing), exert profound effects on surfactant secretion. In this project we isolated type II cells from rat, mouse and human lung and investigated the exocytotic pathway of surfactant with particular focus on mechanical forces which might influence vesicle fusion, fusion pore expansion and surfactant release. We made use of special fluorescence techniques developed earlier by us in a previous FWF project. With these techniques, we were able to visualize surfactant and its secretion from type II cells with high spatial and temporal resolution. In the course of this project the following major achievements were obtained: 1. Elucidation of basic cellular effects of cell stretch on Ca2+ signalling and LB fusion with the plasma membrane. 2. Discovery of a new post-fusion signalling pathway confined to single fused vesicles. The importance of these findings resulted in a multitude of scientific activities, awards and promotions. Among them are several publications in peer-reviewed journals, invited talks and plenary lectures in international scientific meetings, invited reviews in top journals of physiology, a patent and - last but not least - promotions such as doctoral degrees, a "habilitation", and the move of the project leader from an assistant professor position in Austria to a full professor position in Germany. Furthermore, findings of this project were published in an Austrian newspaper ("Profil") as a transfer of knowledge between scientific community and the public. The results and achievements of this project were also essential for a subsequent EU project (Pulmo-Net), which is co-ordinated by the project leader, and a subsequent grant by the Deutsche Forschungsgemeinschaft. They gave rise to numerous new international scientific collaborations and collaboration with industry (Boehringer Ingelheim).