A small key unlocks a heavy door: The essential function of the small hydrophobic proteins SP-B and SP-C to trigger adsorption of pulmonary surfactant lamellar bodies

• Published in: Biochimica et biophysica acta Vol. 1863; no. 8; pp. 2124 - 2134
• Main Authors: Hobi, Nina, Giolai, Michael, Olmeda, Bárbara, Miklavc, Pika, Felder, Edward, Walther, Paul, Dietl, Paul, Frick, Manfred, Pérez-Gil, Jesus, Haller, Thomas
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.08.2016
• Subjects: Adsorption; Alveolar Epithelial Cells - drug effects; Alveolar Epithelial Cells - metabolism; Alveolar Epithelial Cells - secretion; Alveolar Epithelial Cells - ultrastructure; Animals; ARDS; Boron Compounds; Cells, Cultured; Compliance; Exocytosis; Fluorescent Dyes; Heterocyclic Compounds, 3-Ring; Hydrophobic and Hydrophilic Interactions; Lamellar body; Lung injury; Microscopy, Confocal; Organelles - drug effects; Organelles - metabolism; Pulmonary Surfactant-Associated Protein B - antagonists & inhibitors; Pulmonary Surfactant-Associated Protein B - pharmacology; Pulmonary Surfactant-Associated Protein B - physiology; Pulmonary Surfactant-Associated Protein C - antagonists & inhibitors; Pulmonary Surfactant-Associated Protein C - pharmacology; Pulmonary Surfactant-Associated Protein C - physiology; Pulmonary Surfactants - chemistry; Rats; Rats, Sprague-Dawley; Surface Properties; Surface Tension; Surface tension; Compliance; ARDS; Lamellar body; Lung injury
• ISSN: 0167-4889; 0006-3002; 1879-2596
• DOI: 10.1016/j.bbamcr.2016.04.028

The molecular basis involving adsorption of pulmonary surfactant at the respiratory air–liquid interface and the specific roles of the surfactant proteins SP-B and SP-C in this process have not been completely resolved. The reasons might be found in the largely unknown structural assembly in which surfactant lipids and proteins are released from alveolar type II cells, and the difficulties to sample, manipulate and visualize the adsorption of these micron-sized particles at an air–liquid interface under appropriate physiological conditions. Here, we introduce several approaches to overcome these problems. First, by immunofluorescence we could demonstrate the presence of SP-B and SP-C on the surface of exocytosed surfactant particles. Second, by sampling the released particles and probing their adsorptive capacity we could demonstrate a remarkably high rate of interfacial adsorption, whose rate and extent was dramatically affected by treatment with antibodies against SP-B and SP-C. The effect of both antibodies was additive and specific. Third, direct microscopy of an inverted air–liquid interface revealed that the blocking effect is due to a stabilization of the released particles when contacting the air–liquid interface, precluding their transformation and the formation of surface films. We conclude that SP-B and SP-C are acting as essential, preformed molecular keys in the initial stages of surfactant unpacking and surface film formation. We further propose that surfactant activation might be transduced by a conformational change of the surfactant proteins upon contact with surface forces acting on the air–liquid interface. •SP-B and SP-C proteins are exposed at the surface of exocytosed surfactant particles.•Antibodies against SP-B and SP-C prevent interfacial adsorption of surfactant particles.•Molecular interactions between SP-B and SP-C could constitute the lipid transfer machinery.•High surface tension is the likely factor to activate the interfacial lipid transfer.