Atomic Force Microscopy of Interfacial Monomolecular Films of Pulmonary Surfactant

Pulmonary surfactant (PS) is a lipid protein complex secreted at the terminal airways of the lung. The material is secreted as lipid rich multilamellate bodies, which transforms into lipid—protein tubules, planar bilayers, and monomolecular films at the alveolar air—aqueous interface (1 ,2 ). The films reduce the surface tension of the interface and prevents lung collapse during end expiration (3 ). PS layers also act as a protective barrier against inhaled particles and bacteria and keeps the upper airways or bronchioles open during respiration (3 ). Dysfunction of PS has been implicated in various lung diseases, such as asthma, acute respiratory distress syndrome, cystic fibrosis, and pneumonia (4 ). The composition of PS is conserved in most air-breathing species; however, its high content of saturated phosphatidylcholine (PC) and phosphatidylglycerol (PG) is unique compared with other secretory materials and cell membranes, which lack these phospholipids (1 ,5 ). Specifically, PS contains significant amounts of dipalmitoylphosphatidylcholine (DPPC), palmitoyl-oleyl-PC (POPC) and PG (POPG), cholesterol, and small amounts (10%) of surfactant proteins SP-A, SP-B, SP-C, and SP-D (1 ). It is not clear to date how this lipid—protein complex functions by forming alveolar films or barrier in situ because such fragile and dynamic films are difficult to preserve for traditional electron microscopy (2 ,3 ). In vitro studies have focused on model lipid—protein films of PS and also by extracting the material out of lungs and studying interfacial properties of surface tension of such material using Langmuir and other surface balances (6 –8 ).