Effect of hydrophobic proteins in modulating the mechanical properties of lung surfactant membranes

Pulmonary surfactant is a membranous complex that enables breathing dynamics at the respiratory surface. Extremely low values of surface tension are achieved at end-expiration thanks to a unique mixture of lipids and proteins. In particular, the hydrophobic surfactant proteins, specially the protein SP-B, are crucial for surfactant biophysical function, in order to provide the surfactant lipid matrix with the ability to form membranous multi-layered interfacial films that sustain optimal mechanical properties. To analyse the contribution of the proteins to modulate the resistance to mechanical forces of surfactant membrane-based structures, atomic force microscopy of supported lipid bilayers has been used here to determine quantitative mechanical parameters defining the effect of the presence of proteins SP-B and/or SP-C on phospholipid membranes intended to model at least part of the structures integrated into pulmonary surfactant complexes. The results show clear differences introduced by proteins in membrane thickness, lateral packing and elasticity, providing evidence supporting protein-promoted modulating of the mechanical properties of surfactant membranes. These effects are found consistent with the behaviour of two relevant native materials: whole pulmonary surfactant isolated from porcine bronchoalveolar lavages and freshly produced human pulmonary surfactant isolated from amniotic fluid, where it is transferred from the foetal lung before the respiratory air-liquid interface has been established.