Pulmonary surfactant protein C containing lipid films at the air-water interface as a model for the surface of lung alveoli

Abstract

The pulmonary surfactant lines as a complex monolayer of lipids and proteins the alveolar epithelial surface. The monolayer dynamically adapts the surface tension of this interface to the varying surface areas during inhalation and exhalation. Its presence in the alveoli is thus a prerequisite for a proper lung function. The lipid moiety represents about 90% of the surfactant and contains mainly dipalmitoylphosphatidylcholine (DPPC) and phosphatidylglycerol (PG). The surfactant proteins involved in the surface tension adaption are called SP-A, SP-B and SP-C. The aim of the present investigation is to analyse the properties of monolayer films made from pure SP-C and from mixtures of DPPC, DPPG and SP-C in order to mimic the surfactant monolayer with minimal compositional requirement. Pressure-area diagrams were taken. Ellipsometric measurements at the air-water interface of a Langmuir film balance allowed measurement of the changes in monolayer thickness upon compression. Isotherms of pure SP-C monolayers exhibit a plateau between 22 and 25 mN/m. A further plateau is reached at higher compression. Structures of the monolayer formed during compression are reversible during expansion. Together with ellipsometric data which show a stepwise increase in film thickness (coverage) during compression, we conclude that pure SP-C films rearrange reversibly into multilayers of homogenous thickness. Lipid monolayers collapse locally and irreversibly if films are compressed to approximately 0.4 nm2/molecule. In contrast, mixed DPPG/SP-C monolayers with less than 5 mol% protein collapse in a controlled and reversible way. The pressure-area diagrams exhibit a plateau at 20 mN/m, indicating partial demixing of SP-C and DPPG.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescently labeled pulmonary surfactant protein C in spread phospholipid monolayers

“…This implies that the protein had a greater partial area at the higher 7r in the mixed films than it would occupy in a monolayer of the protein alone. It was shown recently by ellipsometry and infrared spectroscopy (Post et al, 1995, Pastrana-Rios et al, 1995 that native acylated SP-C in lipid films possibly undergoes conformational changes at the air-water interface in the surface pressure range of 10-25 mN/m, accompanied by a possible change of orientation of the a-helix. Whereas such changes might be responsible for an increase in partial area of the protein, the size of the change estimated from the surface data is large, and it is difficult to envisage how changes of such a magnitude would accompany the conformational and orientational effects noted by Post et al (1995) and Pastrana-Rios et al (1995).…”

“…Hydrophobic SP-C may have a different organization from water-soluble proteins in phospholipid monolayers. Recently Post et al (1995) showed by ellipsometry of DPPC/SP-C monolayers that a change of thickness of the lipid-protein monolayers occurred between 10 and 25 mN/m in the presence of SP-C, possibly indicating that some change of molecular alignment of this protein in monolayers occurred between those ir. SP-C is insoluble in aqueous systems, so it can be spread in monolayers in pure form (Fig.…”

Effects of surfactant proteins SP-B and SP-C on dynamic and static mechanics of immature lungs

“…These findings are clearly disparate from those of tidal volume and may support the assertion that findings obtained in static measurements do not always show the overall activity of a surfactant (11). Several investigators have addressed the possible roles of SP-C in monolayer function (4,18). In addition, some researchers are speculating that SP-C accelerates the adsorption of the phospholipid bilayer to an interfacial monolayer (1,17,27,29).…”

An Elevated Level of Cholesterol Impairs Self-Assembly of Pulmonary Surfactant into a Functional Film

“…In fluorescence light microscopy, surfactant containing no cholesterol segregates into domains. Condensed domains, typically several micrometers across, are surrounded by the more fluid components in the film (e.g., (35)(36)(37)(38)(39)). Surfactants containing cholesterol, on the other hand, appear homogenous by light microscopy (38,39).…”