Segregated ordered lipid phases and protein-promoted membrane cohesivity are required for pulmonary surfactant films to stabilize and protect the respiratory surface

Abstract

Pulmonary surfactant is a lipid-protein complex essential to stabilize alveoli, by forming surface active films able to reach and sustain very low surface tensions (<2 mN m À1 ) during the film compression that occurs at end-expiration. The particular lipid composition of surfactant, including a high proportion of dipalmitoylphosphatidylcholine (DPPC), induces segregation of fluid ordered and disordered phases in surfactant membranes and films at physiological temperatures. The segregation of DPPC-enriched ordered phase has been related with the ability of surfactant films to produce very low tensions, while the presence in surfactant of two specific hydrophobic polypeptides, SP-B and SP-C, is absolutely required to facilitate surfactant dynamics, including film formation and re-spreading during expansion at inspiration. In the present study, we have used X-ray scattering to analyze the structure of (1) whole native surfactant membranes purified from porcine lungs, (2) membranes reconstituted from the organic extract of surfactant containing the full lipid complement and the physiological proportion of SP-B and SP-C, and (3) membranes reconstituted from the lipid fraction of surfactant depleted of proteins. Small angle X-ray scattering data from whole surfactant or from membranes reconstituted from surfactant organic extract indicated the co-existence of two lamellar phases with different thicknesses. Such phase coexistence disappeared upon heating of the samples at temperatures above physiological values. When assessed in a captive bubble surfactometer, which mimics interfacial compression-expansion dynamics, the ability of surfactant films to produce very low tensions is only maintained at temperatures permitting the coexistence of the two lamellar phases. On the other hand, membranes reconstituted in the absence of proteins produced diffractograms indicative of the existence of a single dominant lamellar phase at all temperatures. These data suggest that SP-B and SP-C establish membranemembrane interactions coupling the stacks of different segregated phases. The low compressibility of surfactant films that leads to the maximal pressures

Human Pulmonary Surfactant Protein SP-A1 Provides Maximal Efficiency of Lung Interfacial Films

“…On one hand, multivalent binding capacities provided by the higher oligomerization state of SP-A1 could be associated with the formation of complex films by a highly cooperative adsorption of complexes in which each SP-A1 molecule could bring together several membrane aggregates. Compression-driven reorganization of such complex films, rich in associated membranes, could rapidly lead to the highly cohesive multilayer film that is competent to reach very low surface tensions with very little area reduction (64). On the other hand, the less oligomerized structure of SP-A2 could lead to formation of surface films with limited complexity, i.e., a lower density of associated membranes.…”

A model for the structure and mechanism of action of pulmonary surfactant protein B

“…On the other hand, the superposition of the 2 rings, one on top of the other as suggested by the electron micrographs, could reflect the association of 2 SP‐B complexes to approximate 2 neighboring membranes. SP‐B‐promoted membrane‐membrane connections have been documented (20, 50) and could be the basis for SP‐B to catalyze true fusion between membrane compartments (22). Due to the hydrophobicity of the dimer cavities, a phospholipid could be held inside each of them in a similar manner to how single phospholipid molecules have been visualized inside saposin B dimers (15).…”

Introductory Lecture: Basic quantities in model biomembranes

“…It is therefore certainly appropriate that this general theme of lateral organization is included in several papers in this Discussion . 48–50,53,54,84,85 …”

“…3,48,52,84,85,90,91 There are two broad foci for the information one would like to obtain in such systems. One focus addresses how the additive resides in the bilayer.…”