Pulmonary surfactant structure as solved by neutron reflectometry and atomic force microscopy

In order to sustain breathing mechanics, the alveolar surface needs to be covered by pulmonary surfactant, a lipid/protein system that provides with the required surface tension reduction to avoid alveolar collapse at the end of expiration. Any disruption of this system could impair its function leading to severe respiratory pathologies. Pulmonary surfactant is composed by 90 % lipids and 10 % hydrophilic (SP-A and SP-D) and hydrophobic (SP-B and SP-C) proteins. After secretion, surfactant lipids form a complex structure at the alveolar air-liquid interface consisting of an adsorbed monolayer connected to a membranous reservoir in the subphase maintained thanks to the action of both hydrophobic proteins. The molecular mechanism of surfactant proteins is still not fully understood, making the finding of treatments for some respiratory pathologies a challenge. Model lipid systems mimicking pulmonary surfactant composition (DPPC/POPC/POPG 50:25:15 w:w:w) in the presence of SP-B and/or SP-C, as well as a native surfactant isolated from animal lungs, have been used as models for the study of the structure and function of pulmonary surfactant films. In this work, we have characterized these models with different biophysical technics such as neutron reflectometry and atomic force microscopy. The results point to the importance of hydrophobic proteins in the formation of three-dimensional structures associated to highly compressed surfactant films. Furthermore, a recently isolated human amniotic fluid surfactant has also been studied. This material is thought to maintain the properties of a freshly synthetized and secreted surfactant that could cover the air-liquid interface in a more efficient way, with physiological importance to develop new surfactant therapies. Here we show that amniotic fluid surfactant exhibits a similar behavior than animal-derived surfactant once it coats the interface.