Lipid compositional analysis of pulmonary surfactant monolayers and monolayer-associated reservoirs

Abstract

Pulmonary surfactant is a lipid:protein complex containing dipalmitoyl-phosphatidylcholine (DPPC) as the major component. Recent studies indicate adsorbed surfactant films consist of a surface monolayer and a monolayerassociated reservoir. It has been hypothesized that the monolayer and its functionally contiguous reservoir may be enriched in DPPC relative to bulk phase surfactant. We investigated the compositional relationship between the monolayer and its reservoir using paper-supported wet bridges to transfer films from adsorbing dishes to clean surfaces on spreading dishes. Spreading films appear to form monolayers in the spreading dishes. We employed bovine lipid ex- It is generally agreed that the alveolar surface is covered by a continuous thin layer of water that supports a surface active film of pulmonary surfactant (1, 2) [for review see (3-7)]. Through its ability to reduce the surface tension of this air-water interface, pulmonary surfactant stabilizes the terminal air spaces. Considerable evidence has accumulated indicating that surfactant films are composed of more than a single monolayer. Pattle (8) first proposed that the surfactant film overlying the alveolar lining layer consists of a monomolecular layer and underlying material that serves as a reservoir. Using electron microscopy, Weibel and Gil (9) observed the presence of lamellar layers of phospholipids with three to six repeating distances of 38-51 Å on the alveolar epithelial surface of rat lungs. Studies by Manabe (2) and, more recently, Bastacky et al. (1) using scanning electron microscopic studies indicate that the alveolar lining layer is continuous and its surface contains many lipidic structures. In vitro studies involving surface films adsorbed from surfactant dispersions have also provided evidence indicating the surface monolayer is accompanied by a functional continuous reservoir (10-13). Surfactant reservoirs that can provide phospholipids to the air-water interface during surface area expansion can also be created during film compression (14)(15)(16)(17) [as reviewed in refs. (18, 19)].Pulmonary surfactant consists of ‫ف‬ 90% lipids and ‫ف‬ 10% protein. The phospholipid composition of bovine pulmonary surfactant, which is representative of mammalian species, consists of ‫ف‬ 80% total phosphatidylcholines (PCs), 10-15% phosphatidylglycerol (PG), 2-3% each of phosphatidylethanolamine, phosphatidylinositol, and sphingomyelin, and 1-2% lyso-bis -phosphatidic acid (20). Dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) are major molecular species, while dipalmitoylphosphatidylglycerol (DPPG) and 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG) are present at significant levels (20)(21)(22). Bovine pulmonary surfactant also contains ‫ف‬ 4% neutral lipids, of which ‫ف‬ 90% is cholesterol. Pulmonary surfactant proteins (SPs) consist of two small hydrophobic proteins, SP-B and SP-C, and two hydrophilic complex glycoproteins, .

Cloning and Characterization of Mouse Lung-type Acyl-CoA:Lysophosphatidylcholine Acyltransferase 1 (LPCAT1)

“…It is interesting to note that mLPCAT1 also used LPG as an acceptor (Fig. 5A), because pulmonary surfactant contains PG, in addition to DPPC (41). Elucidation of the roles of mLPCAT1 in the production of components of surfactant, and in other biological processes awaits gene-targeting studies.…”

“…Whereas there has been extensive analyses of surfactant proteins A, B, C, and D (38,40), very little information is available on the mechanisms of production of surfactant lipids. Extensive analysis of surfactant composition revealed that there is 40% DPPC, and other PC, and that it contains 10% PG (41). Essentially, three distinct biosynthetic pathways are proposed to contribute to the composition of surfactant: 1) de novo biosynthesis, 2) remodeling by LPC or LPG acyltransferases, and 3) CoA-independent transacylation with 2 molecules of LPC (1, 2, 42-48).…”

Equation of state for a coarse-grained DPPC monolayer at the air/water interface

“…Recent experimental studies have questioned the classical model [12,13]. Yu and Possmayer conclude that there is no evidence of lipid selectivity during the formation of surface films by adsorption.…”

Synthetic Pulmonary Surfactant Preparations: New Developments and Future Trends

“…In spite that cholesterol is present in proportions in the order of 5-10% of natural surfactants, it is systematically removed from all the clinical surfactant preparations used today, either synthetic or from animal origin. The reason is that cholesterol showed to impair surface activity in simple lipid systems tested in surface balances [47]. However, cholesterol is well recognized as a component that provides mechanical stability to cell membranes [48] and an important structural element to modulate their lateral structure, including segregation of specialized domains such as lipid rafts [49,50] contents from liposomes as well as lipid vesicle aggregation and fusion [79,80] …”