Effect of pulmonary surfactant protein B (SP-B) and calcium on phospholipid adsorption and squeeze-out of phosphatidylglycerol from binary phospholipid monolayers containing dipalmitoylphosphatidylcholine

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Effect of Molecular Structure on Electrochemical Phase Behavior of Phospholipid Bilayers on Au(111)

Lipid bilayers form the basis of biological cell membranes, selective and responsive barriers vital to the function of the cell. The structure and function of the bilayer are controlled by interactions between the constituent molecules and so vary with the composition of the membrane. These interactions also influence how a membrane behaves in the presence of electric fields they frequently experience in nature. In this study, we characterize the electrochemical phase behavior of dipalmitoylphosphatidylcholine (DPPC), a glycerophospholipid prevalent in nature and often used in model systems and healthcare applications. DPPC bilayers were formed on Au(111) electrodes using Langmuir-Blodgett and Langmuir-Schaefer deposition and studied with electrochemical methods, atomic force microscopy (AFM) and in situ polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). The coverage of the substrate determined with AFM is in accord with that estimated from differential capacitance measurements, and the bilayer thickness is slightly higher than for bilayers of the similar but shorter-chained lipid, dimyristoylphosphatidylcholine (DMPC). DPPC bilayers exhibit similar electrochemical response to DMPC bilayers, but the organization of molecules differs, particularly at negative charge densities. Infrared spectra show that DPPC chains tilt as the charge density on the metal is increased in the negative direction, but, unlike in DMPC, the chains then return to their original tilt angle at the most negative potentials. The onset of the increase in the chain tilt angle coincides with a decrease in solvation around the ester carbonyl groups, and the conformation around the acyl chain linkage differs from that in DMPC. We interpret the differences in behavior between bilayers formed from these structurally similar lipids in terms of stronger dispersion forces between DPPC chains and conclude that relatively subtle changes in molecular structure may have a significant impact on a membrane's response to its environment.

Mitigation of Hydrochloric Acid (HCl)-Induced Lung Injury in Mice by Aerosol Therapy of Surface-Active Nanovesicles Containing Antioxidant and Anti-inflammatory Drugs

Acute lung injury leading to alveolar inflammation and surfactant dysfunction remains a medical challenge. Surface-active lipid nanovesicles of 200-250 nm size with antioxidant D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) and anti-inflammatory drug dexamethasone disodium phosphate (DXP) dual combination (Dual-NV) were developed for delivery as aerosols by nebulization in acid lung injury models. Drug deposition studies showed Dual-NV deposited ∼2.5 times more DXP compared to equivalent DXP solution. Nanovesicles are actively internalized by A549 cells through ATP- and clathrin-dependent pathways. The nanovesicles could be phagocytosed by RAW 264.7 macrophages and were nonimmunogenic and did not elicit overproduction of TNF-α, IL-1β, and IL-6. Dual-NV aerosol therapy at 200 mg/kg body weight, in HCl acid-induced lung injury in mice, markedly reduced pulmonary hemorrhage and protein leakage and improved capillary (airway) patency to ∼96%. Dual-NV aerosol therapy also significantly lowered production of inflammatory cytokine IL-1β, IL-6, and TNF-α and reduced oxidative stress by ∼95% in the injured group. Surface-active Dual-NV aerosol therapy is promising for replenishing the dysfunctional surfactant pool and mitigating inflammation and oxidative stress in lung injuries.

Quantitative lipidomic analysis of mouse lung during postnatal development by electrospray ionization tandem mass spectrometry

Lipids play very important roles in lung biology, mainly reducing the alveolar surface tension at the air-liquid interface thereby preventing end-expiratory collapse of the alveoli. In the present study we performed an extensive quantitative lipidomic analysis of mouse lung to provide the i) total lipid quantity, ii) distribution pattern of the major lipid classes, iii) composition of individual lipid species and iv) glycerophospholipid distribution pattern according to carbon chain length (total number of carbon atoms) and degree of unsaturation (total number of double bonds). We analysed and quantified 160 glycerophospholipid species, 24 sphingolipid species, 18 cholesteryl esters and cholesterol from lungs of a) newborn (P1), b) 15-day-old (P15) and c) 12-week-old adult mice (P84) to understand the changes occurring during postnatal pulmonary development. Our results revealed an increase in total lipid quantity, correlation of lipid class distribution in lung tissue and significant changes in the individual lipid species composition during postnatal lung development. Interestingly, we observed significant stage-specific alterations during this process. Especially, P1 lungs showed high content of monounsaturated lipid species; P15 lungs exhibited myristic and palmitic acid containing lipid species, whereas adult lungs were enriched with polyunsaturated lipid species. Taken together, our study provides an extensive quantitative lipidome of the postnatal mouse lung development, which may serve as a reference for a better understanding of lipid alterations and their functions in lung development and respiratory diseases associated with lipids.

Biomechanics of liquid-epithelium interactions in pulmonary airways

The delicate structure of the lung epithelium makes it susceptible to surface tension induced injury. For example, the cyclic reopening of collapsed and/or fluid-filled airways during the ventilation of injured lungs generates hydrodynamic forces that further damage the epithelium and exacerbate lung injury. The interactions responsible for epithelial injury during airway reopening are fundamentally multiscale, since air-liquid interfacial dynamics affect global lung mechanics, while surface tension forces operate at the molecular and cellular scales. This article will review the current state-of-knowledge regarding the effect of surface tension forces on (a) the mechanics of airway reopening and (b) epithelial cell injury. Due to the complex nature of the liquid-epithelium system, a combination of computational and experimental techniques are being used to elucidate the mechanisms of surface-tension induced lung injury. Continued research is leading to an integrated understanding of the biomechanical and biological interactions responsible for cellular injury during airway reopening. This information may lead to novel therapies that minimize ventilation induced lung injury.

Metal nanoparticle pollutants interfere with pulmonary surfactant function in vitro

Reported associations between air pollution and pulmonary and cardiovascular diseases prompted studies on the effects of gold nanoparticles (Au NP) on pulmonary surfactant function. Low levels (3.7 mol % Au/lipid, 0.98% wt/wt) markedly inhibited adsorption of a semisynthetic pulmonary surfactant (dipalmitoyl-phosphatidylcholine (DPPC)/palmitoyl-oleoyl-phosphatidylglycerol/surfactant protein B (SP-B); 70:30:1 wt %). Au NP also impeded the surfactant's ability to reduce surface tension (gamma) to low levels during film compression and to respread during film expansion. Transmission electron microscopy showed that Au NP generated by a seed-growth method were spherical with diameters of approximately 15 nm. Including palmitoyl-oleoyl-phosphatidylglycerol appeared to coat the NP with at least one lipid bilayer but did not affect NP shape or size. Similar overall observations occurred with dimyristoyl phosphatidylglycerol. Dipalmitoyl-phosphatidylglycerol was less effective in NP capping, although similar sized NP were formed. Including SP-B (1% wt/wt) appears to induce the formation of elongated strands of interacting threads with the fluid phosphatidylglycerols (PG). Including DPPC resulted in formation of aggregated, less spherical NP with a larger size distribution. With DPPC, strand formation due to SP-B was not observed. Agarose gel electrophoresis studies demonstrated that the aggregation induced by SP-B blocked migration of PG-coated NP. Migration was also influenced by the fluidity of the PGs. It is concluded that Au NP can interact with and sequester pulmonary surfactant phospholipids and, if inhaled from the atmosphere, could impede pulmonary surfactant function in the lung.

Synthesis and surface activity of diether-linked phosphoglycerols: Potential applications for exogenous lung surfactants

The synthesis of three phosphoglycerols is described, one of which contains the previously unknown phosphonoglycerol headgroup. The surface tension-lowering capabilities of synthetic lung surfactant mixtures containing the PG analogs were measured on the pulsating bubble surfactometer and compared to known controls. The PG-containing mixtures exhibited superior surface tension-lowering properties indicating the significant potential of these analogs as components in synthetic exogenous lung surfactants.

Reactive oxygen species inactivation of surfactant involves structural and functional alterations to surfactant proteins SP-B and SP-C

Exposing bovine lipid extract surfactant (BLES), a clinical surfactant, to reactive oxygen species arising from hypochlorous acid or the Fenton reaction resulted in an increase in lipid (conjugated dienes, lipid aldehydes) and protein (carbonyls) oxidation products and a reduction in surface activity. Experiments where oxidized phospholipids (PL) were mixed with BLES demonstrated that this addition hampered BLES biophysical activity. However the effects were only moderately greater than with control PL. These results imply a critical role for protein oxidation. BLES oxidation by either method resulted in alterations in surfactant proteins SP-B and SP-C, as evidenced by altered Coomassie blue and silver staining. Western blot analyses showed depressed reactivity with specific antibodies. Oxidized SP-C showed decreased palmitoylation. Reconstitution experiments employing PL, SP-B, and SP-C isolated from control or oxidized BLES demonstrated that protein oxidation was more deleterious than lipid oxidation. Furthermore, addition of control SP-B can improve samples containing oxidized SP-C, but not vice versa. We conclude that surfactant oxidation arising from reactive oxygen species generated by air pollution or leukocytes interferes with surfactant function through oxidation of surfactant PL and proteins, but that protein oxidation, in particular SP-B modification, produces the major deleterious effects.

Interfacial properties of pulmonary surfactant layers

The composition of the pulmonary surfactant and the border conditions of normal human breathing are relevant to characterize the interfacial behavior of pulmonary layers. Based on experimental data methods are reviewed to investigate interfacial properties of artificial pulmonary layers and to explain the behavior and interfacial structures of the main components during compression and expansion of the layers observed by epifluorescence and scanning force microscopy. Terms like over-compression, collapse, and formation of the surfactant reservoir are discussed. Consequences for the viscoelastic surface rheological behavior of such layers are elucidated by surface pressure relaxation and harmonic oscillation experiments. Based on a generalized Volmer isotherm the interfacial phase transition is discussed for the hydrophobic surfactant proteins, SP-B and SP-C, as well as for the mixtures of dipalmitoylphosphatidylcholine (DPPC) with these proteins. The behavior of the layers depends on both the oligomerisation state and the secondary structure of the hydrophobic surfactant proteins, which are controlled by the preparation of the proteins. An example for the surface properties of bronchoalveolar porcine lung washings of uninjured, injured, and Curosurf treated lavage is discussed in the light of surface behavior. An outlook summarizes the present knowledge and the main future development in this field of surface science.

Molecular dynamics simulations of the anchoring and tilting of the lung-surfactant peptide SP-B1-25 in palmitic acid monolayers

We have performed molecular dynamics simulations of multiple copies of the lung-surfactant peptide SP-B1-25 in a palmitic acid (PA) monolayer. SP-B1-25 is a shorter version of lung-surfactant protein B, an important component of lung surfactant. Up to 30 ns simulations of 20 wt % SP-B1-25 in the PA monolayers were performed with different surface areas of PA, extents of PA ionization, and various initial configurations of the peptides. Starting with initial peptide orientation perpendicular to the monolayer, the predicted final tilt angles average 54 degrees approximately 62 degrees with respect to the monolayer normal, similar to those measured experimentally by Lee et al. (Biophysical Journal. 2001. Synchrotron x-ray study of lung surfactant-specific protein SP-B in lipid monolayers. 81:572-585). In their final conformations, hydrogen-bond analysis and amino acid mutation studies show that the peptides are anchored by hydrogen bond interactions between the cationic residues Arg-12 and Arg-17 and the hydrogen bond acceptors of the ionized PA headgroup, and the tilt angle is affected by the interactions of Tyr-7 and Gln-19 with the PA headgroup. Our work indicates that the factors controlling orientation of small peptides in lipid layers can now be uncovered through molecular dynamics simulations.

Molecular dynamics study of the lung surfactant peptide SP-B1-25 with DPPC monolayers: insights into interactions and peptide position and orientation

We have performed molecular dynamics simulations of the interactions of the peptide SP-B(1-25), which is a truncated version of the full pulmonary surfactant protein SP-B, with dipalmitoylphosphatidylcholine monolayers, which are the major lipid components of lung surfactant. Simulations of durations of 10-20 ns show that persistent hydrogen bonds form between the donor atoms of the protein and the acceptors of the lipid headgroup and that these bonds determine the position, orientation, and secondary structure of the peptide in the membrane environment. From an ensemble of initial conditions, the most probable equilibrium orientation of the alpha-helix of the peptide is predicted to be parallel to the interface, matching recent experimental results on model lipid mixtures. Simulations of a few mutated analogs of SP-B(1-25) also suggest that the charged amino acids are important in determining the position of the peptide in the interface. The first eight amino acids of the peptide, also known as the insertion sequence, are found to be essential in reducing the fluctuations and anchoring the peptide in the lipid/water interface.

Poly(ethylene glycol) (PEG) enhances dynamic surface activity of a bovine lipid extract surfactant (BLES)

Shortage or malfunction of pulmonary surfactant in alveolar space leads to a critical condition termed respiratory distress syndrome (RDS). Surfactant replacement therapy, the major method to treat RDS, is an expensive treatment. In this paper, the effect of poly(ethylene glycol) (PEG) to improve dynamic surface activity of a bovine lipid extract surfactant (BLES) was studied by axisymmetric drop shape analysis (ADSA) and a captive bubble method. The activity of BLES+PEG mixtures was compared to that of a natural surfactant containing surfactant proteins A and D. When PEG was added into BLES mixtures, the surface tension hysteresis of BLES films was minimized when the films were compressed by more than 50%. PEG also helps to quickly restore surfactant films after film collapse. Thus, as far as surface tension effects go, the findings suggest that PEG might be used as a substitute for surfactant-associated protein SP-A in therapeutic surfactant products, and might also be used to reduce the amount of BLES required in clinical applications.

The role of exogenous surfactant in the treatment of acute lung injury

A number of conditions, such as pneumonia, trauma, or systemic sepsis arising from the gut, may result in the acute respiratory distress syndrome (ARDS). Because of its significant morbidity and mortality, ARDS has been the focus of extensive research. One specific area of interest has been the investigation of the role of the surfactant system in the pathophysiology of this disease. Several studies have demonstrated that alterations of surfactant contribute to the lung dysfunction associated with ARDS, which has led to investigations into the use of exogenous surfactant as a therapy for this syndrome. Clinical experience with surfactant therapy has been variable owing to a number of factors including the nature of the injury at the time of treatment, the specific surfactant preparation utilized, the dose and delivery method chosen, the timing of surfactant administration over the course of the disease, and the mode of ventilation used during and after surfactant administration.

Effect of hydrophobic surfactant protein SP-C on binary phospholipid monolayers. Molecular machinery at the air/water interface

Fluorescent and modified dark-field microscopies were used to investigate the phase behavior of physiologically relevant lipid/protein monomolecular films containing surfactant protein C(SP-C). Synthetic human SP-C(1-34) was labeled at its N-terminus using the fluorescent probe 6-(((4(4,4-difluoro-5-(2-thienyl)-4-bora-3a,4a-diaza-s-indacene-3-yl)phenoxy)acetyl)amino)hexanoic acid (BODIPY/TR-X). Using dual fluorescent labeling (lipid and protein) in the monolayers, we have correlated (at physiologically small concentrations of the protein) the lipid phase separation and protein distribution in situ. A comparison of the lipid and protein dye fluorescent micrographs indicates that SP-C(1-34) is preferentially associated with the disordered lipid phase. Three concepts arise from our results. (1) The presence of SP-C alone does not result in the complete dissolution of condensed phase domains in a fashion that we have previously reported for the entire hydrophobic surfactant protein (SP-B/C) fraction (Biophys. J. 77 (1999) 903). Rather, the use of relatively high amounts ( approximately 10 wt.%) of the labeled SP-C protein is needed to reproduce the fluorescence monolayer morphology previously observed for small concentrations ( approximately 1 wt.%) of the natural SP-B/C mixture. (2) Scattered light, dark-field microscopy performed using grazing angle laser illumination reveals the presence of surface-associated, three-dimensional (3D) structures of micrometer-sized dimensions when the labeled BODIPY/TR-X:SP-C(1-34) protein is included in the monolayer, as previously observed with the naturally isolated SP-B/C mixture. The 3D structures are associated exclusively with the presence of the SP-C protein in disordered monolayer phases. (3) To explain these results, we have derived a molecular model accounting for the structure and physico-chemical properties of the SP-C protein in terms of its energetics. The molecular events involved in the SP-C-mediated production of the 3D surface particles are explained using the analogy of a simple molecular machine, namely a loaded spring. This interpretation is supported by an energetic analysis that suggests the major factor contributing to the formation of the 3D particles is the energy liberated by re-expansion of the surrounding phospholipid film into the area vacated by the SP-C protein as it re-orients away from the surface.

Specific mode of interaction between components of model pulmonary surfactants using computer simulations

Atomistic molecular dynamics simulations and structural bioinformatics tools enable the identification of the exact mode of interaction between model pulmonary surfactant components. Two nanosecond long simulations of the N-terminal region of human surfactant protein-B (SP-B(1-25)) in dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) monolayers of different lipid surface densities reveal the preferential affinity of SP-B(1-25) for anionic phospholipids. In particular, arginine 12 and lysine 24 interact strongly and with high specificity with the phosphate group of the DPPG lipids, stabilizing the position, the orientation, and the secondary structure of the peptide in the monolayer. The peptide lies at an oblique angle to the interfacial plane, ranging between 47 degrees and 62 degrees, increasing with decreasing lipid surface density. In DPPC monolayers the interaction is largely determined by hydrophobic interactions. The non-specific nature of DPPC-SP-B(1-25) interactions allows for significant flexibility in the topology of the peptide in the lipid matrix. Bioinformatics tools are employed to generalize the simulation results to the sequences of SP-B(1-25) in other organisms. The importance of specific residues, and the role of the largely helical and amphiphilic nature of the peptide in the functionality of SP-B(1-25) are established. The synergy of classical mechanics tools with bioinformatics methods greatly enhances the molecular-level interpretation of pulmonary surfactant action and facilitates the development of design rules for synthetic surfactant analogues.

Multilayer formation upon compression of surfactant monolayers depends on protein concentration as well as lipid composition. An atomic force microscopy study

The determinants for the formation of multilayers upon compression of surfactant monolayers were investigated by compressing films, beyond the squeeze-out plateau, to a surface tension of 22 millinewtons/m. Atomic force microscopy was used to visualize the topography of lipid films containing varying amounts of native surfactant protein B (SP-B). These films were compared with films containing synthetic peptides based on the N terminus of human SP-B: monomeric mSP-B-(1-25) or dimeric dSP-B-(1-25). The formation of typical hexagonal network structures as well as the height of protrusions were shown to depend on the concentration of SP-B. Protrusions of bilayer height were formed from physiologically relevant concentrations of 0.2-0.4 mol % (4.5-8.5 wt %) SP-B upwards. Much higher concentrations of SP-B-(1-25) peptides were needed to obtain network structures, and protrusion heights were not equal to those found for films with native SP-B. A striking observation was that while protrusions formed in films of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/1,2-dipalmitoyl-sn-glycero-3-(phospho-rac-(1-glycerol)) (DPPG) (80/20) had single bilayer thickness, those formed in DPPC/1-palmitoyl-2-oleoyl-sn-glycero-3-(phospho-rac-(1-glycerol)) (80/20) had various heights of multilayers, whereas those seen in DPPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/DPPG (60/20/20) were mainly of bilayer height. For the first time direct observations by atomic force microscopy show (i) that a certain minimal concentration of SP-B is required for the formation of layered protrusions upon film compression, (ii) that protrusion height depends on whether the phospholipids contain an unsaturated fatty acyl chain, and (iii) that protrusion height also depends on whether the unsaturated acyl chain is present in phosphatidylcholine or in phosphatidylglycerol.

The effect of dextran to restore the activity of pulmonary surfactant inhibited by albumin

Pulmonary surfactant is crucial to maintain the proper functioning of the respiration system. Certain types of blood proteins (e.g. albumin) were found to inhibit the activity of pulmonary surfactant. Axisymmetric Drop Shape Analysis (ADSA) was used to study the effect of dextran to restore the activity of an albumin-inhibited pulmonary surfactant. It was found that dextran could effectively restore surface tension properties of the inhibited surfactant in vitro. Furthermore, dextran improved the performance of pulmonary surfactants when albumin was absent. It was found that when a surfactant film was under high compression (e.g. above 70% surface area reduction), the presence of dextran increased film stability, so that the film could sustain high surface pressures without being collapsing.

Phosphatidylcholine molecular species in lung surfactant: composition in relation to respiratory rate and lung development

Surfactant reduces surface tension at the air-liquid interface of lung alveoli. While dipalmitoylphosphatidylcholine (PC16:0/ 16:0) is its main component, proteins and other phospholipids contribute to the dynamic properties and homeostasis of alveolar surfactant. Among these components are significant amounts of palmitoylmyristoylphosphatidylcholine (PC16:0/ 14:0) and palmitoylpalmitoleoylphosphatidylcholine (PC16:0/ 16:1), whereas in surfactant from the rigid tubular bird lung, PC16:0/14:0 is absent and PC16:0/16:1 strongly diminished. We therefore hypothesized that the concentrations of PC16:0/14:0 and PC16:0/16:1 in surfactants correlate with differences in the respiratory physiology of mammalian species. In surfactants from newborn and adult mice, rats, and pigs, molar fractions of PC16:0/14:0 and PC16:0/16:1 correlated with respiratory rate. Labeling experiments with [methyl-(3)H]choline in mice and perfused rat lungs demonstrated identical alveolar proportions of total and newly synthesized PC16:0/14:0, PC16:0/16:1, and PC16:0/16:0, which were much higher than those of other phosphatidylcholine species. In surfactant from human term and preterm neonates, fractional concentrations not only of PC16:0/16:0 but also of PC16:0/14:0 and PC16:0/ 16:1 increased with maturation. Our data emphasize that PC16:0/14:0 and PC16:0/16:1 may be important surfactant components in alveolar lungs, and that their concentrations are adapted to respiratory physiology.

Surface activity in vitro: role of surfactant proteins

Pattle, who provided some of the initial direct evidence for the presence of pulmonary surfactant in the lung, was also the first to show surfactant was susceptible to proteases such as trypsin. Pattle concluded surfactant was a lipoprotein. Our group has investigated the roles of the surfactant proteins (SP-) SP-A, SP-B, and SP-C using a captive bubble tensiometer. These studies show that SP-C>SP-B>SP-A in enhancing surfactant lipid adsorption (film formation) to the equilibrium surface tension of approximately 22-25 mN/m from the 70 mN/m of saline at 37 degrees C. In addition to enhancing adsorption, surfactant proteins can stabilize surfactant films so that lateral compression induced through surface area reduction results in the lowering of surface tension (gamma) from approximately 25 mN/m (equilibrium) to values near 0 mN/m. These low tensions, which are required to stabilize alveoli during expiration, are thought to arise through exclusion of fluid phospholipids from the surface monolayer, resulting in an enrichment in the gel phase component dipalmitoylphosphatidylcholine (DPPC). The results are consistent with DPPC enrichment occurring through two mechanisms, selective DPPC adsorption and preferential squeeze-out of fluid components such as unsaturated phosphatidylcholine (PC) and phosphatidylglycerol (PG) from the monolayer. Evidence for selective DPPC adsorption arises from experiments showing that the surface area reductions required to achieve gamma near 0 mN/m with DPPC/PG samples containing SP-B or SP-A plus SP-B films were less than those predicted for a pure squeeze-out mechanism. Surface activity improves during quasi-static or dynamic compression-expansion cycles, indicating the squeeze-out mechanism also occurs. Although SP-C was not as effective as SP-B in promoting selective DPPC adsorption, this protein is more effective in promoting the reinsertion of lipids forced out of the surface monolayer following overcompression at low gamma values. Addition of SP-A to samples containing SP-B but not SP-C limits the increase in gamma(max) during expansion. It is concluded that the surfactant apoproteins possess distinct overlapping functions. SP-B is effective in selective DPPC insertion during monolayer formation and in PG squeeze-out during monolayer compression. SP-A can promote adsorption during film formation, particularly in the presence of SP-B. SP-C appears to have a superior role to SP-B in formation of the surfactant reservoir and in reinsertion of collapse phase lipids.

Surfactant-associated proteins: functions and structural variation

Pulmonary surfactant is a barrier material of the lungs and has a dual role: firstly, as a true surfactant, lowering the surface tension; and secondly, participating in innate immune defence of the lung and possibly other mucosal surfaces. Surfactant is composed of approximately 90% lipids and 10% proteins. There are four surfactant-specific proteins, designated surfactant protein A (SP-A), SP-B, SP-C and SP-D. Although the sequences and post-translational modifications of SP-B and SP-C are quite conserved between mammalian species, variations exist. The hydrophilic surfactant proteins SP-A and SP-D are members of a family of collagenous carbohydrate binding proteins, known as collectins, consisting of oligomers of trimeric subunits. In view of the different roles of surfactant proteins, studies determining the structure-function relationships of surfactant proteins across the animal kingdom will be very interesting. Such studies may reveal structural elements of the proteins required for surface film dynamics as well as those required for innate immune defence. Since SP-A and SP-D are also present in extrapulmonary tissues, the hydrophobic surfactant proteins SP-B and SP-C may be the most appropriate indicators for the evolutionary origin of surfactant. SP-B is essential for air-breathing in mammals and is therefore largely conserved. Yet, because of its unique structure and its localization in the lung but not in extrapulmonary tissues, SP-C may be the most important indicator for the evolutionary origin of surfactant.

Surfactant protein interactions with neutral and acidic phospholipid films

The captive bubble tensiometer was employed to study interactions of phospholipid (PL) mixtures of dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) at 50 microg/ml with physiological levels of the surfactant protein (SP) A SP-B, and SP-C alone and in combination at 37 degrees C. All surfactant proteins enhanced lipid adsorption to equilibrium surface tension (gamma), with SP-C being most effective. Kinetics were consistent with the presence of two adsorption phases. Under the conditions employed, SP-A did not affect the rate of film formation in the presence of SP-B or SP-C. Little difference in gamma(min) was observed between the acidic POPG and the neutral POPC systems with SP-B or SP-C with and without SP-A. However, gamma(max) was lower with the acidic POPG system during dynamic, but not during quasi-static, cycling. Considerably lower compression ratios were required to generate low gamma(min) values with SP-B than SP-C. DPPC-POPG-SP-B was superior to the neutral POPC-SP-B system. Although SP-A had little effect on film formation with SP-B, surface activity during compression was enhanced with both PL systems. In the presence of SP-C, lower compression ratios were required with the acidic system, and with this mixture, SP-A addition adversely affected surface activity. The results suggest specific interactions between SP-B and phosphatidylglycerol, and between SP-B and SP-A. These observations are consistent with the presence of a surface-associated surfactant reservoir which is involved in generating low gamma during film compression and lipid respreading during film expansion.

Effect of the hydrophobic surfactant proteins on the surface activity of spread films in the captive bubble surfactometer

The main function of pulmonary surfactant, a mixture of lipids and proteins, is to reduce the surface tension at the air/liquid interface of the lung. The hydrophobic surfactant proteins SP-B and SP-C are required for this process. When testing their activity in spread films in a captive bubble surfactometer, both SP-B and SP-C showed concentration dependence for lipid insertion as well as for lipid film refinement. Higher activity in DPPC refinement of the monolayer was observed for SP-B compared with SP-C. Further differences between both proteins were found, when subphase phospholipid vesicles, able to create a monolayer-attached lipid reservoir, were omitted. SP-C containing monolayers showed gradually increasing minimum surface tensions upon cycling, indicating that a lipid reservoir is required to prevent loss of material from the monolayer. Despite reversible cycling dynamics, SP-B containing monolayers failed to reach near-zero minimum surface tensions, indicating that the reservoir is required for stable films.

Functional Tests for the Characterization of Surfactant Protein B (SP-B) and a Fluorescent SP-B Analog

Surfactant protein B (SP-B) enhances lipid insertion into the alveolar air/liquid interface upon inhalation. The aim of this study was (i) to apply a palette of tests for a detailed biochemical and biophysical characterization of SP-B and (ii) to use these tests to compare native SP-B with a fluorescent (Bodipy) SP-B analog. The method of labeling was fast and resulted in a covalent fluorophore-protein bond. The ability of both proteins to spread a surfactant film on top of a buffer surface was determined in a spreading tray using the Wilhelmy plate technique to allow detection of alterations in surface tension and calculation of spreading velocities. In a captive bubble surfactometer surface tensions of spread films were measured. Similar biophysical properties were found for both native and Bodipy-labeled SP-B. It is concluded that the combination of tests used allows detection of small differences in structure and activity between the two proteins.

Commercial versus native surfactants. Surface activity, molecular components, and the effect of calcium

Despite their broad clinical use, there is no standardized comparative study on the functional, biochemical, and morphologic differences of the various commercial surfactants in relation to native surfactant. We investigated these parameters in Alveofact, Curosurf, Exosurf, and Survanta, and compared them with native bovine (NBS) and porcine (NPS) surfactant. For Curosurf and Alveofact the concentrations necessary for minimal surface tensions < 5 mN/m were six to 12 times higher (1.5 and 3 mg/ml, respectively) than with NPS and NBS. Exosurf and Survanta only reached 22 and 8 mN/m, respectively. Increasing calcium to nonphysiologic concentrations artificially improved the function of Alveofact and Curosurf, but it had little effect on Exosurf and Survanta. Impaired surface activity of commercial versus native surfactants corresponded with their lack in surfactant protein SP-A and decreased SP-B/C. The higher surface activity of Curosurf compared with Alveofact corresponded with its higher concentration of dipalmitoylphosphatidylcholine (DPPC). Despite their enrichment in DPPC Survanta and Exosurf exhibited poor surface activity because of low or absent SP-B/C. Ultrastructurally, Curosurf and Alveofact consisted mainly of lamellar and vesicular structures, which were also present in NPS and NBS. Exosurf contained crystalline structures only, whereas the DPPC-enriched Survanta contained separate lamellar/vesicular and crystalline structures. We conclude that in vitro surface activity of commercial surfactants is impaired compared with native surfactants at physiologic calcium concentrations. In the presence of SP-B/C, surface activity corresponds to the concentration of DPPC. Our data underscore the importance of a standardized protocol at physiologic calcium concentrations for the in vitro assessment of commercial surfactants.

Role of pulmonary surfactant components in surface film formation and dynamics

Pulmonary surfactant is a mixture of lipids and proteins which is secreted by the epithelial type II cells into the alveolar space. Its main function is to reduce the surface tension at the air/liquid interface in the lung. This is achieved by forming a surface film that consists of a monolayer which is highly enriched in dipalmitoylphosphatidylcholine and bilayer lipid/protein structures closely attached to it. The molecular mechanisms of film formation and of film adaptation to surface changes during breathing in order to remain a low surface tension at the interface, are unknown. The results of several model systems give indications for the role of the surfactant proteins and lipids in these processes. In this review, we describe and compare the model systems that are used for this purpose and the progress that has been made. Despite some conflicting results using different techniques, we conclude that surfactant protein B (SP-B) plays the major role in adsorption of new material into the interface during inspiration. SP-C's main functions are to exclude non-DPPC lipids from the interface during expiration and to attach the bilayer structures to the lipid monolayer. Surfactant protein A (SP-A) appears to promote most of SP-B's functions. We describe a model proposing that SP-A and SP-B create DPPC enriched domains which can readily be adsorbed to create a DPPC-rich monolayer at the interface. Further enrichment in DPPC is achieved by selective desorption of non-DPPC lipids during repetitive breathing cycles.

Formation of three-dimensional protein-lipid aggregates in monolayer films induced by surfactant protein B

This study focuses on the structural organization of surfactant protein B (SP-B) containing lipid monolayers. The artificial system is composed of the saturated phospholipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) in a molar ratio of 4:1 with 0.2 mol% SP-B. The different "squeeze-out" structures of SP-B were visualized by scanning probe microscopy and compared with structures formed by SP-C. Particularly, the morphology and material properties of mixed monolayers containing 0.2 mol% SP-B in a wide pressure range of 10 to 54 mN/m were investigated revealing that filamentous domain boundaries occur at intermediate surface pressure (15-30 mN/m), while disc-like protrusions prevail at elevated pressure (50-54 mN/m). In contrast, SP-C containing lipid monolayers exhibit large flat protrusions composed of stacked bilayers in the plateau region (app. 52 mN/m) of the pressure-area isotherm. By using different scanning probe techniques (lateral force microscopy, force modulation, phase imaging) it was shown that SP-B is dissolved in the liquid expanded rather than in the liquid condensed phase of the monolayer. Although artificial, the investigation of this system contributes to further understanding of the function of lung surfactant in the alveolus.

SP-B refining of pulmonary surfactant phospholipid films

Pulmonary surfactant stabilizes the alveoli by lining the air-fluid interface with films that reduce surface tension to near 0 mN/m (gamma(min)). Surfactant protein B (SP-B) enhances the surface activity of surfactant phospholipids. A captive bubble tensiometer (CBT) was used to study the properties of adsorbed films of dipalmitoylphosphatidylcholine (DPPC) with acidic 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) or neutral 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine with (7:3) and without 1% dimeric SP-B. SP-B enhanced the adsorption rate of DPPC-containing neutral or acidic lipid suspensions (1 mg/ml) to a similar extent. Quasi-static cycling of these films revealed that SP-B significantly decreased the film area reduction required to reach gamma(min) for the acidic but not for the neutral system. The results obtained with DPPC-phosphatidylglycerol (PG)-SP-B were consistent with selective DPPC adsorption into the surface monolayer during film formation. Film area reduction required to reach gamma(min) with this system (with and without calcium) approached that of pure DPPC, suggesting selective DPPC insertion and PG squeeze-out. Dynamic cycling of such films showed that larger film area reductions were required to reach gamma(min) for the neutral than for acidic system, even after 20 cycles. Fluorescence microscopy of solvent-spread DPPC-POPG-SP-B planar films revealed highly condensed structures at approximately 25 mN/m, although no specific PG phase-segregated structures could be identified. The study suggests that specific interactions of SP-B with acidic phospholipids of surfactant may be involved in the generation and maintenance of DPPC-rich films in the alveoli.

Dissociation of surfactant protein B from canine surfactant large aggregates during formation of small surfactant aggregates by in vitro surface area cycling

Pulmonary surfactant isolated by lavage can be separated into large aggregates (LA) and small aggregates (SA). Pulse labeling experiments have shown that the LA subtype is the precursor of the SA subtype. Conversion of LA to SA can be demonstrated in vitro using the technique of surface area cycling. The precise mechanisms of surfactant subtype conversion remain unknown. We have previously reported a decline in surfactant-associated protein B (SP-B) during in vitro subtype conversion of canine surfactant. This led to the hypothesis that SP-B may be degraded by a serine protease 'convertase' during cycling. The current studies used a quantitative slot-blot assay to investigate the fates of SP-A and SP-B during in vitro cycling. These studies confirmed some SP-A is present in SA, but SP-B is confirmed to LA. Conversion leads to an apparent loss of SP-B during cycling. However, SP-B can be recovered from the walls of polypropylene and Teflon tubes by washing with chloroform:methanol. Recovered SP-B migrated on non-reducing tricine gels as a single band with an apparent molecular weight of 17 kDa, corresponding to intact SP-B dimer. Reconstitution studies demonstrated that the recovered SP-B retained its surface active properties as determined on a pulsating bubble surfactometer. We conclude in vitro surface area cycling of canine LA results in the dissociation of SP-B from surfactant lipids resulting in an apparent decline in SP-B levels.

Effect of the compression ratio on properties of lung surfactant (bovine lipid extract surfactant) films

Using axisymmetric drop shape analysis (ADSA) and a captive bubble technique, the properties of lung surfactant (BLES) films under different compression ratios (i.e. the ratio of maximum to minimum surface area of the bubble) were investigated. In particular, the effect of the compression ratio of the films on the shape of the volume area surface tension-time (VAST) graphs and on the shape of the surface tension relative area (gamma-A ) graphs was examined at the air-liquid interface. Under relatively low compression ratios of the films, the VAST and gamma-A graphs indicate that the cycling is reversible (i.e. the gamma-A graphs are identical for expansion and compression). When the compression ratio of the films increases, failure tends to occur when the surface film can not support the increasing surface pressure, causing a hysteresis loop in the gamma-A graph. It was also found that low surface tension (i.e. below 5 mJ/m2) at the interface could be attained with a 30% compression ratio and collapse of the films always occurs near or below 1 mJ/m2.

The role of lipids in pulmonary surfactant

Pulmonary surfactant is composed of approx. 90% lipids and 10% protein. This review article focusses on the lipid components of surfactant. The first sections will describe the lipid composition of mammalian surfactant and the techniques that have been utilized to study the involvement of these lipids in reducing the surface tension at an air-liquid interface, the main function of pulmonary surfactant. Subsequently, the roles of specific lipids in surfactant will be discussed. For the two main surfactant phospholipids, phosphatidylcholine and phosphatidylglycerol, specific contributions to the overall surface tension reducing properties of surfactant have been indicated. In contrast, the role of the minor phospholipid components and the neutral lipid fraction of surfactant is less clear and requires further study. Recent technical advances, such as fluorescent microscopic techniques, hold great potential for expanding our knowledge of how surfactant lipids, including some of the minor components, function. Interesting information regarding surfactant lipids has also been obtained in studies evaluating the surfactant system in non-mammalian species. In certain non-mammalian species (and at least one marsupial), surfactant lipid composition, most notably disaturated phosphatidylcholine and cholesterol, changes drastically under different conditions such as an alteration in body temperature. The impact of these changes on surfactant function provide insight into the function of these lipids, not only in non-mammalian lungs but also in the surfactant from mammalian species.

Effects of a cationic and hydrophobic peptide, KL4, on model lung surfactant lipid monolayers

We report on the surface behavior of a hydrophobic, cationic peptide, [lysine-(leucine)4]4-lysine (KL4), spread at the air/water interface at 25 degrees C and pH 7.2, and its effect at very low molar ratios on the surface properties of the zwitterionic phospholipid 1,2-dipalmitoylphosphatidylcholine (DPPC), and the anionic forms of 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG) and palmitic acid (PA), in various combinations. Surface properties were evaluated by measuring equilibrium spreading pressures (pi(e)) and surface pressure-area isotherms (pi-A) with the Wilhelmy plate technique. Surface phase separation was observed with fluorescence microscopy. KL4 itself forms a single-phase monolayer, stable up to a surface pressure pi of 30 mN/m, and forms an immiscible monolayer mixture with DPPC. No strong interaction was detected between POPG and KL4 in the low pi region, whereas a stable monolayer of the PA/KL4 binary mixture forms, which is attributed to ionic interactions between oppositely charged PA and KL4. KL4 has significant effects on the DPPC/POPG mixture, in that it promotes surface phase separation while also increasing pi(e) and pi(max), and these effects are greatly enhanced in the presence of PA. In the model we have proposed, KL4 facilitates the separation of DPPC-rich and POPG/PA-rich phases to achieve surface refinement. It is these two phases that can fulfill the important lung surfactant functions of high surface pressure stability and efficient spreading.

Surfactant protein B: effects on lipid domain formation and intermembrane lipid flow

Pulmonary surfactant is a mixture of (phospho)lipids and surfactant specific proteins, lining the alveolar space. During each respiration cycle phospholipids are transferred between the phospholipid monolayer at the air/water interface and a variety of underlying membranes. Surfactant proteins may play a role in facilitating the insertion and removal of phospholipids by affecting the lipid organization of the bilayer and monolayer. The experiments described in this article were carried out in order to investigate the influence of surfactant protein B (SP-B) on the distribution of phospholipids in membranes and on the mixing of lipids between membranes. To determine the distribution of the non-labeled phospholipids in small unilamellar vesicles (SUV), the relative clustering of pyrene-labeled phospholipids was used, by measuring the ratio of excimer-to-monomer (E/M) pyrene fluorescence. In the absence of SP-B it was found that the clustering of the pyrenePC molecules was dependent on the proportion of saturated acyl chains and not on the proportion of negative charges. Addition of the positively charged SP-B to a mixture of DPPC and PG, led to an increase of approximately 20% in E/M ratio, indicating a clustering of the negatively charged PG molecules. This effect was intensified by addition of calcium ions. If pyrenePC-containing SUV were mixed with excess non-labeled SUV in the presence of SP-B and calcium ions, the E/M ratio decreased, corresponding with a flow of the pyrenePC molecules into the acceptor membranes. It is concluded that presence of domains of phospholipids can be detected with the use of pyrene-labeled phospholipids. Furthermore, SP-B showed a concentrating effect on the distribution of the negatively charged phospholipids, a process that could be important in regulating the phospholipid composition of the monolayer.

Phase and morphology changes in lipid monolayers induced by SP-B protein and its amino-terminal peptide

Both human lung surfactant protein, SP-B, and its amino-terminal peptide, SP-B1-25, inhibit the formation of condensed phases in monolayers of palmitic acid, resulting in a new fluid phase. This fluid phase forms a network, separating condensed-phase domains at coexistence. The network persists to high surface pressures, altering the nucleation, growth, and morphology of monolayer collapse structures, leading to lower surface tensions on compression and more reversible respreading on expansion. The network is stabilized by the low line tension between the fluid phase and the condensed phase as confirmed by the formation of "stripe" phases.

Calcium ions and interactions of pulmonary surfactant proteins SP-B and SP-C with phospholipids in spread monolayers at the air/water interface

Spread monolayers containing hydrophobic pulmonary surfactant protein, SP-B or SP-C, or SP-B/SP-C (2:1, w/w), alone or mixed with dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylglycerol (DPPG), were formed on saline subphases containing calcium ions. Surface pressure-area characteristics of the films of the proteins were not affected by the presence of Ca2+ in the subphase. Calcium ions did not alter the surface properties of the binary and ternary films of DPPC plus either SP-B, or SP-C, or SP-B/SP-C (2:1, w/w). Surface pressure-area isotherms for the spread films of DPPG plus hydrophobic surfactant protein were Ca(2+)-dependent. The exclusion pressures of SP-B, SP-C and SP-B/SP-C (2:1, w/w) from protein-DPPG films in the presence of calcium were lower than the exclusion pressures in the absence of Ca2+. The divalent cation appeared to suppress the ability of SP-C and SP-B/SP-C (2:1, w/w) to remove phospholipid during squeeze-out from their mixed films with DPPG. The effects of Ca2+ on the monolayers of DPPG plus hydrophobic surfactant proteins were consistent with calcium producing diminished lipid-protein interactions, possibly resulting from Ca(2+)-induced changes in the ionization state and molecular packing of DPPG.

Regulation of mRNA levels for pulmonary surfactant-associated proteins in developing rabbit lung

Gene transcriptional activities and steady-state mRNA levels have been examined for the surfactant-associated proteins SP-A, SP-B and SP-C in developing rabbit lung. It was observed SP-C mRNA levels increase early in gestation, while SP-A and SP-B mRNA levels increase rapidly between 26 and 30 days gestation. Transcriptional activities for all three surfactant apoproteins increase between 26 and 30 days. Studies conducted with fetal lung explants of 26 days gestation demonstrated exposure to low doses of dexamethasone increases SP-A and SP-C mRNA levels, while high doses stimulate transcription, although this only significant for SP-C. Time course studies revealed different temporal patterns and glucocorticoid responses for SP-A and SP-C mRNAs. SP-A and SP-C mRNA production and steady-state levels were reduced after treatment with cycloheximide. In contrast, SP-B gene transcription was selectively stimulated, suggesting involvement of a labile negative regulatory factory. It is concluded that expression of the three surfactant apoproteins is independently regulated. Early in gestation, SP-C mRNA levels may be regulated in vivo through message stabilization. Glucocorticoids can affect SP-A and SP-C mRNA levels in culture at both transcriptional and post-transcriptional levels. The ability of glucocorticoids to influence these processes declines during fetal development.

Surface-area cycling of different surfactant preparations: SP-A and SP-B are essential for large-aggregate integrity

Surface-area cycling is an in vitro procedure for the conversion of large into small surfactant aggregates. In this procedure a tube containing a surfactant suspension is rotated end-over-end at 37 degrees C so that the surface area of the suspension changes twice each cycle. We have utilized this method to study the mechanisms involved in aggregate conversion. Several different surfactant preparations were analysed: (1) bovine natural surfactant, a sucrose-gradient-purified material containing surfactant phospholipid and surfactant-associated proteins (SP-) SP-A, SP-B and SP-C; (2) bovine lipid-extract surfactant, which contains the surfactant phospholipids and SP-B and SP-C; (3) mixtures of dipalmitoyl phosphatidylcholine and phosphatidylglycerol (7:3, w/w) reconstituted with one or more surfactant proteins. Aggregate conversion was measured by phosphorus analysis of a 40,000 g supernatant (small aggregate) and pellet (large aggregates) before and after surface-area cycling. Surface-area cycling of lipid extract surfactant or lipids plus SP-B or SP-C resulted in rapid aggregate conversion. Lipids alone were not converted. Only a small percentage of purified natural surfactant was converted into small aggregates. Addition of SP-A to lipid extract surfactant could inhibit aggregate conversion of this material, but this was only observed when an additional 1% (w/w) of SP-B was added to the lipid extract. It is concluded that SP-A is important for large-aggregate integrity. It appears that SP-A acts in conjunction with SP-B. The presence of SP-B and/or SP-C is required for aggregate conversion; it is proposed that this reflects the necessity for lipid adsorption in aggregate conversion.

Pulmonary surfactant proteins SP-B and SP-C in spread monolayers at the air-water interface: I. Monolayers of pulmonary surfactant protein SP-B and phospholipids

The effects of pulmonary surfactant protein SP-B on the properties of monolayers of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG), and a mixture of DPPC:DPPG (7:3, mol:mol) were studied using spread films at the air-water interface. The addition of SP-B to the phospholipid monolayers gave positive deviations from additivity of the mean areas in the films. At low protein concentrations (less than 45% amino acid residues which corresponds to 0.5 mol% or 10 weight% SP-B) monolayers of SP-B/DPPC, SP-B/DPPG and SP-B/(DPPC:DPPG) collapsed at surface pressures of about 70 mN.m-1, comparable to those of the lipids alone. At higher concentrations of SP-B in the protein-lipid monolayers, kink points appeared in the isotherms at about 40-45 mN.m-1, implying possible exclusion of material from the films, hence, changes in the original monolayer compositions. Calculated analyses of the monolayer compositions as a function of surface pressure indicated that nearly pure SP-B, associated with small amounts of phospholipid (2-3 lipid molecules per SP-B dimer), was lost from SP-B/DPPC, SP-B/DPPG, and SP-B/(DPPC:DPPG) films at surface pressures higher than 40-45 mN.m-1. The results are consistent with a low effectiveness of SP-B in removing saturated phospholipids, DPPC or DPPG, from the spread SP-B/phospholipid films.

Effect of pulmonary surfactant protein A (SP-A) and calcium on the adsorption of cholesterol and film stability

The effect of exogenous cholesterol on the stability of surface films at 37 degrees C from various surfactants was studied with the pulsating bubble surfactometer. Addition of cholesterol (5%, w/w) to bovine lipid extract surfactant (bLES) or mixtures of dipalmitoylphosphatidylcholine/1-palmitoyl-2-oleoyl-phosphatidylglycerol /SP-B (7:3:1%) dispersed in 1.5 mM CaCl2/0.9% NaCl resulted in unstable surface films. Although 10% cholesterol only partially impaired the surface activity of bLES, it virtually abolished that of the reconstituted surfactant. The inhibitory effects of cholesterol were significantly repressed by SP-A (10%, w/w of lipid) and 3 mM CaCl2 or 5 mM CaCl2 without SP-A. Adsorption of cholesterol from various surfactants into the air/water interface was examined by measuring the surface radioactivity of [14C]cholesterol. Cholesterol alone dispersed in 1.5 mM CaCl2/0.9% NaCl could not adsorb to the interface, but it adsorbed readily when mixed with bLES. Cholesterol adsorption was markedly suppressed by SP-A in 3 mM CaCl2/0.9% NaCl or 5 mM CaCl2/0.9% NaCl without SP-A. Electron microscopy revealed striking ultrastructural differences between bLES/5% cholesterol/10% SP-A in 3 mM CaCl2/0.9% NaCl and bLES/5% cholesterol in 3 or 5 mM CaCl2/0.9% NaCl. The former exhibited large multilayer and small unilamellar vesicles, while the latter displayed condensed patches of aggregates. Adsorption studies showed aggregated patches adsorbed more rapidly than vesicles but attained lower equilibrium surface pressures. These results indicate SP-A and calcium limit the adsorption of surfactant cholesterol to the air-water interface.

Interaction of phospholipids with proteins and peptides. New advances IV

1. The review deals with the newest achievements in the field of the various interactions between phospholipids and proteins and peptides. 2. Interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecular assemblies is also discussed.

Degradation of surfactant-associated protein B (SP-B) during in vitro conversion of large to small surfactant aggregates

Pulmonary surfactant obtained from lung lavages can be separated by differential centrifugation into two distinct subfractions known as large surfactant aggregates and small surfactant aggregates. The large-aggregate fraction is the precursor of the small-aggregate fraction. The ratio of the small non-surface-active to large surface-active surfactant aggregates increases after birth and in several types of lung injury. We have utilized an in vitro system, surface area cycling, to study the conversion of large into small aggregates. Small aggregates generated by surface area cycling were separated from large aggregates by centrifugation at 40,000 g for 15 min rather than by the normal sucrose gradient centrifugation. This new separation method was validated by morphological studies. Surface-tension-reducing activity of total surfactant extracts, as measured with a pulsating-bubble surfactometer, was impaired after surface area cycling. This impairment was related to the generation of small aggregates. Immunoblot analysis of large and small aggregates separated by sucrose gradient centrifugation revealed the presence of detectable amounts of surfactant-associated protein B (SP-B) in large aggregates but not in small aggregates. SP-A was detectable in both large and small aggregates. PAGE of cycled and non-cycled surfactant showed a reduction in SP-B after surface area cycling. We conclude that SP-B is degraded during the formation of small aggregates in vitro and that a change in surface area appears to be necessary for exposing SP-B to protease activity.

Exposure of rabbit fetal lung to glucocorticoids in vitro does not enhance transcription of the gene encoding pulmonary surfactant-associated protein-B (SP-B)

We have investigated the ontogeny and hormonal regulation of both synthesis rates and cellular accumulation of the mRNA for surfactant-associated protein B (SP-B) in rabbit fetal lung. The developmental pattern for SP-B mRNA synthesis increased as a function of gestational age and paralleled that for SP-B mRNA levels except on days 22-26 of gestation where relatively higher levels of gene transcription were observed. Time-course studies with explants from 26- and 30-day fetal lung maintained in culture revealed a gradual increase in mRNA levels and a much smaller increase in gene transcription relative to adult values. Within 48 h of exposure of 26-day explants to dexamethasone at 10(-8) M there was a rapid increase in SP-B mRNA levels to 7-fold adult levels. A similar overall although somewhat slower and attenuated pattern was observed with 30-day explants. Dexamethasone at 10(-8) M had no effect on SP-B gene transcription with explants of either gestational age. We conclude that the major effect of dexamethasone treatment in vitro on SP-B mRNA levels appears to be post-transcriptional and there are small but distinct differences in the effects of glucocorticoids on SP-B mRNA levels with explant cultures from early and late stages of fetal lung maturation.