Interaction of pulmonary surfactant protein A with dipalmitoylphosphatidylcholine and cholesterol at the air/water interface

Vaping additives negatively impact the stability and lateral film organization of lung surfactant model systems

Aims: Inhalation of vaping additives has recently been shown to impair respiratory function, leading to e-cigarette or vaping product use associated with lung injuries. This work was designed to understand the impact of additives (vitamin E, vitamin E acetate, tetrahydrocannabinol and cannabidiol) on model lung surfactants. Materials & methods: Lipid monofilms at the air-water interface and Brewster angle microscopy were used to assess the impact of vaping additives on model lung surfactant films. Results & conclusion: The addition of 5 mol % of vaping additives, and even more so mixtures of vitamins and cannabinoids, negatively impacts lipid packing and film stability, induces material loss upon cycling and significantly reduces functionally relevant lipid domains. This range of detrimental effects could affect proper lung function.

Ionic polymeric amphiphiles with cholesterol mesogen: adsorption and organization characteristics at the air/water interface from Langmuir film balance studies

Ionic polymeric amphiphiles consisting of cholesterol mesogen were investigated for the interfacial adsorption characteristics at the air/water interface using a Langmuir film balance with an aim to understand the influence of ionic segment from 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) on the packing behavior of cholesterol at the interface. From surface pressure (pi)-area (A) isotherm characteristics, it is demonstrated that the homopolymer and the copolymer C consisting of 0.15 mol fraction CAB segments exhibit the most expanded structures contributing to surface area of about 84 A(2)/molecule. It is shown that the copolymer B with 0.1 mol fraction CAB provides optimum hydrophilic liphophilic balance to form the most compact structures contributing to a surface area of 35.75 A(2)/molecule. The high surface pressure, >40 mN/m, in contrast to that of PAMPS demonstrates significant adsorption of the copolymers at the interface. An interesting correlation among interfacial packing characteristics, thermal behavior, and solution structures is demonstrated. From molecular models developed for CAB, it is shown that the horizontal orientation of the linker group with respect to cholesterol chain in CAB underlies the expanded structures observed in PCAB and copolymer C.

Interactions of serum with lung surfactant extract in the bronchiolar and alveolar airway models

Lung surfactant (LS) a lipid-protein mixture is secreted by type-II pneumocytes and prevents alveolar collapse as well as maintains upper airway patency. In certain lung pathophysiology dysfunction of the LS occurs due to leakage of serum derived materials interacting with surfactant at the respiratory air-water interface. Bovine lipid extract surfactant (BLES) with and without foetal calf serum (FCS) were studied as models of bronchiolar airway patency using a capillary surfactometer, and in alveolar (terminal) airway using adsorbed Langmuir films in a surface balance. About 5 wt.% of serum was found to maximally decrease airway patency of BLES by 90%, as well as the surface films ability to reach low surface tension below 25 mN/m. In fact, FCS was found to be about 200-fold more potent inhibitor of the surfactant extract compared to a major serum component, albumin. Also serum but not albumin significantly reduced the gel-phase structures found in BLES films under compression at low amounts (5-10 wt.%), and eventually abolished these organized structures as imaged by fluorescence and atomic force microscopy. This fact suggests that serum caused complete molecular re-organization of the surfactant lipids in films at an air-water interface, and the ability of such films to reduce surface tension or maintain airway patency. The study may provide a novel structure-function disruption model for lung surfactant inactivation in the airways in pathophysiology.

Effect of cholesterol on the biophysical and physiological properties of a clinical pulmonary surfactant

Pulmonary surfactant is a complex mixture of lipids and proteins that forms a surface-active film at the air-water interface of alveoli capable of reducing surface tension to near 0 mN/m. The role of cholesterol, the major neutral lipid component of pulmonary surfactant, remains uncertain. We studied the physiological effect of cholesterol by monitoring blood oxygenation levels of surfactant-deficient rats treated or not treated with bovine lipid extract surfactant (BLES) containing zero or physiological amounts of cholesterol. Our results indicate no significant difference between BLES and BLES containing cholesterol immediately after treatment; however, during ventilation, BLES-treated animals maintained higher PaO2 values compared to BLES+cholesterol-treated animals. We used a captive bubble tensiometer to show that physiological amounts of cholesterol do not have a detrimental effect on the surface activity of BLES at 37 degrees C. The effect of cholesterol on topography and lateral organization of BLES Langmuir-Blodgett films was also investigated using atomic force microscopy. Our data indicate that cholesterol induces the formation of domains within liquid-ordered domains (Lo). We used time-of-flight-secondary ion mass spectrometry and principal component analysis to show that cholesterol is concentrated in the Lo phase, where it induces structural changes.

Effects of albumin and erythrocyte membranes on spread monolayers of lung surfactant lipids

Dipalmitoyl phosphatidylcholine (DPPC), one of the main constituents of lung surfactant is mainly responsible for reduction of surface tension to near 0 mN/m during expiration, resisting alveolar collapse. Other unsaturated phospholipids like palmitoyloleoyl phosphatidylglycerol (PG), palmitoyloleoyl phosphatidylcholine (POPC) and neutral lipids help in adsorption of lung surfactant to the air-aqueous interface. Lung surfactant lipids may interact with plasma proteins and hematological agents flooding the alveoli in diseased states. In this study, we evaluated the effects of albumin and erythrocyte membranes on spread films of DPPC alone and mixtures of DPPC with each of PG, POPC, palmitoyloleoyl phosphatidylethanolamine (PE), cholesterol (CHOL) and palmitic acid (PA) in 9:1 molar ratios. Surface tension-area isotherms were recorded using a Langmuir-Blodgett (LB) trough at 37 degrees C with 0.9% saline as the sub-phase. In the presence of erythrocyte membranes, DPPC and DPPC+PA monolayers reached minimum surface tensions of 7.3+/-0.9 and 9.6+/-1.4 mN/m, respectively. Other lipid combinations reached significantly higher minimum surface tensions >18 mN/m in presence of membranes (Newman Keul's test, p<0.05). The relative susceptibility to membrane inhibition was [(DPPC+PG, 7:3)=(DPPC+PG, 9:1)=(DPPC+POPC)=(DPPC+PE)=(DPPC+CHOL)]>[(DPPC+PA)=(DPPC)]. The differential response was more pronounced in case of albumin with DPPC and DPPC+PA monolayers reaching minimum surface tensions less than 2.4 mN/m in presence of albumin, whereas DPPC+PG and DPPC+POPC reached minimum surface tensions of around 20 mN/m in presence of albumin. Descending order of susceptibility of the spread monolayers of lipid mixtures to albumin destabilization was as follows: [(DPPC+PG, 7:3)=(DPPC+PG, 9:1)=(DPPC+POPC)]>[(DPPC+PE)=(DPPC+CHOL)]>[(DPPC+PA)=(DPPC)] The increase in minimum surface tension in presence of albumin and erythrocyte membranes was accompanied by sudden increases in compressibility at surface tensions of 15-30 mN/m. This suggests a monolayer destabilization and could be indicative of phase transitions in the mixed lipid films due to the presence of the hydrophobic constituents of erythrocyte membranes.

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.

Pulmonary surfactant function is abolished by an elevated proportion of cholesterol

A molecular film of pulmonary surfactant strongly reduces the surface tension of the lung epithelium-air interface. Human pulmonary surfactant contains 5-10% cholesterol by mass, among other lipids and surfactant specific proteins. An elevated proportion of cholesterol is found in surfactant, recovered from acutely injured lungs (ALI). The functional role of cholesterol in pulmonary surfactant has remained controversial. Cholesterol is excluded from most pulmonary surfactant replacement formulations, used clinically to treat conditions of surfactant deficiency. This is because cholesterol has been shown in vitro to impair the surface activity of surfactant even at a physiological level. In the current study, the functional role of cholesterol has been re-evaluated using an improved method of evaluating surface activity in vitro, the captive bubble surfactometer (CBS). Cholesterol was added to one of the clinically used therapeutic surfactants, BLES, a bovine lipid extract surfactant, and the surface activity evaluated, including the adsorption rate of the substance to the air-water interface, its ability to produce a surface tension close to zero and the area compression needed to obtain that low surface tension. No differences in the surface activity were found for BLES samples containing either none, 5 or 10% cholesterol by mass with respect to the minimal surface tension. Our findings therefore suggest that the earlier-described deleterious effects of physiological amounts of cholesterol are related to the experimental methodology. However, at 20%, cholesterol effectively abolished surfactant function and a surface tension below 15 mN/m was not obtained. Inhibition of surface activity by cholesterol may therefore partially or fully explain the impaired lung function in the case of ALI. We discuss a molecular mechanism that could explain why cholesterol does not prevent low surface tension of surfactant films at physiological levels but abolishes surfactant function at higher levels.

The surface activity of pulmonary surfactant from diving mammals

Pinnipeds (seals and sea lions) have developed a specialised respiratory system to cope with living in a marine environment. They have a highly reinforced lung that can completely collapse and reinflate during diving without any apparent side effects. These animals may also have a specialised surfactant system to augment the morphological adaptations. The surface activity of surfactant from four species of pinniped (California sea lion, Northern elephant seal, Northern fur seal and Ringed seal) was measured using a captive bubble surfactometer (CBS), and compared to two terrestrial species (sheep and cow). The surfactant of Northern elephant seal, Northern fur seal and Ringed seal was unable to reduce surface tension (gamma) to normal levels after 5 min adsorption (61.2, 36.7, and 46.2 +/- 1.7 mN/m, respectively), but California sea lion was able to reach the levels of the cow and sheep (23.4 mN/m for California sea lion, 21.6 +/- 0.3 and 23.0 +/- 1.5 mN/m for cow and sheep, respectively). All pinnipeds were also unable to obtain the very low gamma(min) achieved by cow (1.4 +/- 0.1 mN/m) and sheep (1.5 +/- 0.4 mN/m). These results suggest that reducing surface tension to very low values is not the primary function of surfactant in pinnipeds as it is in terrestrial mammals, but that an anti-adhesive surfactant is more important to enable the lungs to reopen following collapse during deep diving.

The pattern of surfactant cholesterol during vertebrate evolution and development: does ontogeny recapitulate phylogeny?

Pulmonary surfactant is a complex mixture of phospholipids (PLs), neutral lipids and proteins that lines the inner surface of the lung. Here it modulates surface tension, thereby increasing lung compliance and preventing the transudation of fluid. In humans, pulmonary surfactant is comprised of approximately 80% PLs, 12% neutral lipids and 8% protein. In most eutherian (i.e. placental) mammals, cholesterol (Chol) comprises approximately 8-10% by weight or 14-20 mol% of both alveolar and lamellar body surfactant. It is regarded as an integral component of pulmonary surfactant, yet few studies have concentrated on its function or control. The lipid composition is highly conserved within the vertebrates, except that surfactant of teleost fish is dominated by cholesterol, whereas tetrapod pulmonary surfactant contains a high proportion of disaturated phospholipids (DSPs). The primitive Australian dipnoan lungfish Neoceratodus forsterii demonstrates a 'fish-type' surfactant profile, whereas the other derived dipnoans demonstrate a surfactant profile similar to that of tetrapods. Homology of the surfactant proteins within the vertebrates points to a single evolutionary origin for the system and indicates that fish surfactant is a 'protosurfactant'. Among the terrestrial tetrapods, the relative proportions of DSPs and cholesterol vary in response to lung structure, habitat and body temperature (Tb), but not in relation to phylogeny. The cholesterol content of surfactant is elevated in species with simple saccular lungs or in aquatic species or in species with low Tb. The DSP content is highest in complex lungs, particularly of aquatic species or species with high Tb. Cholesterol is controlled separately from the PL component in surfactant. For example, in heterothermic mammals (i.e. mammals that vary their body temperature), the relative amount of cholesterol increases in cold animals. The rapid changes in the Chol to PL ratio in response to various physiological stimuli suggest that these two components have different turnover rates and may be packaged and processed differently. In mammals, the pulmonary surfactant system develops towards the end of gestation and is characterized by an increase in the saturation of PLs in lung washings and the appearance of surfactant proteins in amniotic fluid. In general, the pattern of surfactant development is highly conserved among the amniotes. This conservation of process is demonstrated by an increase in the amount and saturation of the surfactant PLs in the final stages (>75%) of development. Although the ratios of surfactant components (Chol, PL and DSP) are remarkably similar at the time of hatching/birth, the relative timing of the maturation of the lipid profiles differs dramatically between species. The uniformity of composition between species, despite differences in lung morphology, birthing strategy and relationship to each other, implies that the ratios are critical for the onset of pulmonary ventilation. The differences in the timing, on the other hand, appear to relate primarily to birthing strategy and the onset of air breathing. As the amount of cholesterol relative to the phospholipids is highly elevated in immature lungs, the pattern of cholesterol during development and evolution represents an example of ontogeny recapitulating phylogeny. The fact that cholesterol is an important component of respiratory structures that are primitive, when they are not in use or developing in an embryo, demonstrates that this substance has important and exciting roles in surfactant. These roles still remain to be explored.

Multilayer structures in lipid monolayer films containing surfactant protein C: effects of cholesterol and POPE

The influence of cholesterol and POPE on lung surfactant model systems consisting of DPPC/DPPG (80:20) and DPPC/DPPG/surfactant protein C (80:20:0.4) has been investigated. Cholesterol leads to a condensation of the monolayers, whereas the isotherms of model lung surfactant films containing POPE exhibit a slight expansion combined with an increased compressibility at medium surface pressure (10-30 mN/m). An increasing amount of liquid-expanded domains can be visualized by means of fluorescence light microscopy in lung surfactant monolayers after addition of either cholesterol or POPE. At surface pressures of 50 mN/m, protrusions are formed which differ in size and shape as a function of the content of cholesterol or POPE, but only if SP-C is present. Low amounts of cholesterol (10 mol %) lead to an increasing number of protrusions, which also grow in size. This is interpreted as a stabilizing effect of cholesterol on bilayers formed underneath the monolayer. Extreme amounts of cholesterol (30 mol %), however, cause an increased monolayer rigidity, thus preventing reversible multilayer formation. In contrast, POPE, as a nonbilayer lipid thought to stabilize the edges of protrusions, leads to more narrow protrusions. The lateral extension of the protrusions is thereby more influenced than their height.

Foetal rat lung epithelial (FRLE) cells: partial characterisation and response to pneumotoxins

Cultured cell lines are routinely used for in vitro toxicity screens, reducing the requirement for animal studies during the development of new pharmaceutical, agrochemical and cosmetic products. The foetal rat lung epithelial (FRLE) cell line was originally derived from alveolar type II cells (ATII) of the lung. The aims of this study were to further characterise FRLE cells and investigate their potential for screening for pneumotoxins. The cells were found to have retained some of the features of their progenitor cells, namely the expression of cytokeratin proteins, specifically cytokeratin 18, and the ability to actively accumulate the non-selective contact herbicide paraquat. However, the cells have lost the ability to synthesise surfactant protein mRNA and no longer contain multiple lamellar bodies. Toxins that damage ATII cells in vivo (cadmium chloride, cobalt chloride and paraquat) were found to induce cytotoxicity in FRLE cells, as did the non-specific pneumotoxin nitrofurantoin, and hydrogen peroxide. However, the cells were less sensitive to the effects of compounds that require metabolic activation (1-nitronaphthalene, coumarin and butylated hydroxytoluene) and the hepatotoxin bromobenzene. Thus, FRLE cells appear to be a good in vitro model for monitoring the potential toxicity to ATII cells and could be used as an initial screen for pneumotoxicity.

Surfactant protein A and B genetic variants in respiratory distress syndrome in singletons and twins

Interactive genetic and environmental factors may influence the differentiation of surfactant and the risk of respiratory distress syndrome (RDS). DNA samples from 441 premature singleton infants and 480 twin or multiple infants were genotyped for surfactant-specific protein (SP)-A1, SP-A2, and SP-B exon 4 polymorphisms and intron 4 size variants in a homogeneous white population. Distributions of the SP-A and SP-B gene variants between RDS and no-RDS infants were determined alone and in combination. SP-A1 allele 6A2 (p = 0.009) and the homozygous genotype 6A2/6A2 (p = 0.003) were overrepresented in RDS of singletons when the SP-B exon 4 genotype was Thr/Thr, and underrepresented in RDS of multiples when the SP-B genotype was Ile/Thr (p = 0.012 for 6A2 and p = 0.03 for 6A2/6A2) or Thr/Thr (p = 0.12 for 6A2 and p = 0.018 for 6A2/6A2, respectively). The SP-A 6A2 allele in the SP-B Thr131 background predisposed the smallest singleton infants to RDS, whereas near-term multiples were protected from RDS. There was a continuous association between fetal mass and risk of RDS, defined by the SP-A and SP-B variants. Labeled lung explants with the Thr/Thr genotype showed proSP-B amino-terminal glycosylation, which was absent in Ile/Ile samples. Genetic and environmental variation may influence intracellular processing of surfactant complex and the susceptibility to RDS.

Pulmonary surfactant protein-A (SP-A) restores the surface properties of surfactant after oxidation by a mechanism that requires the Cys6 interchain disulfide bond and the phospholipid binding domain

Reactive oxygen species produced by activated leuko-cytes in the alveolar epithelial lining fluid have been implicated in the inactivation of pulmonary surfactant and the impairment of lung function. Oxidation of bovine lipid extract surfactant (BLES), a therapeutic surfactant, with hypochlorous acid (H-BLES) or the Fenton reaction (F-BLES) led to temporary increases in conjugated dienes and formation of malondialdehyde and 4-hydroxy-2-nonenal. Electrospray ionization mass spectrometry revealed the appearance of lipid hydroperoxides, peroxides, lysophospholipids, and free fatty acids. Captive bubble tensiometer studies of H-BLES demonstrated prolonged adsorption times, film instability at low surface tensions during film compression, and reduced respreadability during film expansion. F-BLES exhibited prolonged adsorption times, a marked effect on increasing compressibility during compression, and a lesser effect on reducing respreadability on expansion. Addition of native bovine or rat surfactant-associated protein A (SP-A) reversed the effects of oxidation on surfactant biophysical properties. Studies using mutant recombinant rat SP-As indicated that an intact carbohydrate recognition domain and disulfide-dependent oligomeric assembly are critical for these effects, but the collagen-like region is not required. We conclude that SP-A can reverse the detrimental effects of surfactant oxidation on the biophysical properties of surfactant, by a mechanism that is dependent on interchain disulfide bond formation and the C-terminal domains of the protein.

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

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 monolayer-associated 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 extract surfactant [BLES(chol)] containing [3H]DPPC and either [14C]palmitoyl, oleoyl-phosphatidylcholine (POPC), [14C]dipalmitoyl-phosphatidylglycerol (DPPG), [14C]palmitoyl, oleoyl-phosphatidylglycerol (POPG), or [14C]cholesterol. Radiolabeled phosphatidylglycerols were prepared using phospholipase D. The studies demonstrated that the [3H]DPPC-[14C] POPC ratios were the same in the prepared BLES dispersions as in Langmuir-Blodgett films, indicating a lack of DPPC selectivity during film formation. Furthermore, identical 3H-14C isotopic ratios were observed with DPPC and either 14C-labeled POPC, DPPG, POPG, or cholesterol in the original dispersions, the bulk phases in adsorption dish D1, and monolayers recovered from spreading dish D2. These relationships remained unperturbed with 2-fold increases in bulk concentrations in D1 and 10-fold variations in D1-D2 surface area. These results indicate adsorbed surfactant monolayers and their associated reservoirs possess similar lipid compositions and argue against selective adsorption of DPPC.

Torpor-associated fluctuations in surfactant activity in Gould's wattled bat

The primary function of pulmonary surfactant is to reduce the surface tension (ST) created at the air-liquid interface in the lung. Surfactant is a complex mixture of lipids and proteins and its function is influenced by physiological parameters such as metabolic rate, body temperature and breathing. In the microchiropteran bat Chalinolobus gouldii these parameters fluctuate throughout a 24 h period. Here we examine the surface activity of surfactant from warm-active and torpid bats at both 24 degrees C and 37 degrees C to establish whether alterations in surfactant composition correlate with changes in surface activity. Bats were housed in a specially constructed bat room at Adelaide University, at 24 degrees C and on a 8:16 h light:dark cycle. Surfactant was collected from bats sampled during torpor (25<T(b)<28 degrees C), and while active (T(b)>35 degrees C). Alterations in the lipid composition of surfactant occur with changes in the activity cycle. Most notable is an increase in surfactant cholesterol (Chol) with decreases in body temperature [Codd et al., Physiol. Biochem. Zool. 73 (2000) 605-612]. Surfactant from active bats was more surface active at higher temperatures, indicated by lower ST(min) and less film area compression required to reach ST(min) at 37 degrees C than at 24 degrees C. Conversely, surfactant from torpid bats was more active at lower temperatures, indicated by lower ST(min) and less area compression required to reach ST(min) at 24 degrees C than at 37 degrees C. Alterations in the Chol content of bat surfactant appear to be crucial to allow it to achieve low STs during torpor.

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.

The roles of cholesterol in pulmonary surfactant: insights from comparative and evolutionary studies

In most eutherian mammals, cholesterol (Chol) comprises approximately 8-10 wt.% or 14-20 mol.% of both alveolar and lamellar body surfactant. It is regarded as an integral component of pulmonary surfactant, yet few studies have concentrated on its function or control. Throughout the evolution of the vertebrates, the contribution of cholesterol relative to surfactant phospholipids decreases, while that of the disaturated phospholipids (DSP) increases. Chol generally appears to dominate in animals with primitive bag-like lungs that lack septation, in the saccular lung of snakes or swimbladders which are not used predominantly for respiration, and also in immature lungs. It is possible that in these systems, cholesterol represents a protosurfactant. Cholesterol is controlled separately from the phospholipid (PL) component in surfactant. For example, in heterothermic mammals such as the fat-tailed dunnart, Sminthopsis crassicaudata, and the microchiropteran bat, Chalinolobus gouldii, and also in the lizard, Ctenophorus nuchalis, the relative amount of Chol increases in cold animals. During the late stages of embryonic development in chickens and lizards, the Chol to PL and Chol to DSP ratios decrease dramatically. While in isolated lizard lungs, adrenaline and acetylcholine stimulate the secretion of surfactant PL, Chol secretion remains unaffected. This is also supported in isolated cell studies of lizards and dunnarts. The rapid changes in the Chol to PL ratio in response to various physiological stimuli suggest that these two components have different turnover rates and may be packaged and processed differently. Infusion of [3H]cholesterol into the rat tail vein resulted in a large increase in Chol specific activity within 30 min in the lamellar body (LB) fraction, but over a 48-h period, failed to appear in the alveolar surfactant fraction. Analysis of the limiting membrane of the lamellar bodies revealed a high (76%) concentration of LB cholesterol. The majority of lamellar body Chol is, therefore, not released into the alveolar compartment, as the limiting membrane fuses with the cell membrane upon exocytosis. It appears unlikely, therefore, that lamellar bodies are the major source of alveolar Chol. It is possible that the majority of alveolar Chol is synthesised endogenously within the lung and stored independently from surfactant phospholipids. The role of cholesterol in the limiting membrane of the lamellar body may be to enable fast and easy processing by maintaining the membrane in a relatively fluid state.

Domains of surfactant protein A that affect protein oligomerization, lipid structure and surface tension

Surfactant protein A (SP-A) is an abundant protein found in pulmonary surfactant which has been reported to have multiple functions. In this review, we focus on the structural importance of each domain of SP-A in the functions of protein oligomerization, the structural organization of lipids and the surface-active properties of surfactant, with an emphasis on ultrastructural analyses. The N-terminal domain of SP-A is required for disulfide-dependent protein oligomerization, and for binding and aggregation of phospholipids, but there is no evidence that this domain directly interacts with lipid membranes. The collagen-like domain is important for the stability and oligomerization of SP-A. It also contributes shape and dimension to the molecule, and appears to determine membrane spacing in lipid aggregates such as common myelin and tubular myelin. The neck domain of SP-A is primarily involved in protein trimerization, which is critical for many protein functions, but it does not appear to be directly involved in lipid interactions. The globular C-terminal domain of SP-A clearly plays a central role in lipid binding, and in more complex functions such as the formation and/or stabilization of curved membranes. In recent work, we have determined that the maintenance of low surface tension of surfactant in the presence of serum protein inhibitors requires cooperative interactions between the C-terminal and N-terminal domains of the molecule. This effect of SP-A requires a high degree of oligomeric assembly of the protein, and may be mediated by the activity of the protein to alter the form or physical state of surfactant lipid aggregates.

The role of surfactant proteins in DPPC enrichment of surface films

A pressure-driven captive bubble surfactometer was used to determine the role of surfactant proteins in refinement of the surface film. The advantage of this apparatus is that surface films can be spread at the interface of an air bubble with a different lipid/protein composition than the subphase vesicles. Using different combinations of subphase vesicles and spread surface films a clear correlation between dipalmitoylphosphatidylcholine (DPPC) content and minimum surface tension was observed. Spread phospholipid films containing 50% DPPC over a subphase containing 50% DPPC vesicles did not form stable surface films with a low minimum surface tension. Addition of surfactant protein B (SP-B) to the surface film led to a progressive decrease in minimum surface tension toward 1 mN/m upon cycling, indicating an enrichment in DPPC. Surfactant protein C (SP-C) had no such detectable refining effect on the film. Surfactant protein A (SP-A) had a positive effect on refinement when it was present in the subphase. However, this effect was only observed when SP-A was combined with SP-B and incubated with subphase vesicles before addition to the air bubble containing sample chamber. Comparison of spread films with adsorbed films indicated that refinement induced by SP-B occurs by selective removal of non-DPPC lipids upon cycling. SP-A, combined with SP-B, induces a selective adsorption of DPPC from subphase vesicles into the surface film. This is achieved by formation of large lipid structures which might resemble tubular myelin.

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.

Lipid monolayers: why use half a membrane to characterize protein-membrane interactions?

Variants of membrane-active proteins and peptides are increasingly available through synthesis and molecular engineering. When determining the effects of structural changes upon the interaction of these proteins with lipid membranes, monomolecular films of lipids at the air-water interface have significant advantages over bilayers and other lipid dispersions. In the past year, a variety of protein-lipid interactions has been characterized successfully using relatively simple surface measurements.

Alterations in pulmonary surfactant after rapid arousal from torpor in the marsupial Sminthopsis crassicaudata

Torpor in the dunnart, Sminthopsis crassicaudata, alters surfactant lipid composition and surface activity. Here we investigated changes in surfactant composition and surface activity over 1 h after rapid arousal from torpor (15-30 degrees C at 1 degrees C/min). We measured total phospholipid (PL), disaturated PL (DSP), and cholesterol (Chol) content of surfactant lavage and surface activity (measured at both 15 and 37 degrees C in the captive bubble surfactometer). Immediately after arousal, Chol decreased (from 4.1 +/- 0.05 to 2.8 +/- 0.3 mg/g dry lung) and reached warm-active levels by 60 min after arousal. The Chol/DSP and Chol/PL ratios both decreased to warm-active levels 5 min after arousal because PL, DSP, and the DSP/PL ratio remained elevated over the 60 min after arousal. Minimal surface tension and film compressibility at 17 mN/m at 37 degrees C both decreased 5 min after arousal, correlating with rapid changes in surfactant Chol. Therefore, changes in lipids matched changes in surface activity during the postarousal period.

Filaments of surfactant protein A specifically interact with corrugated surfaces of phospholipid membranes

Pulmonary surfactant, a mixture of lipids and surfactant proteins (SPs), plays an important role in respiration and gas exchange. SP-A, the major SP, exists as an octadecamer that can self-associate to form elongated protein filaments in vitro. We have studied here the association of purified bovine SP-A with lipid vesicle bilayers in vitro with negative staining with uranyl acetate and transmission electron microscopy. Native bovine surfactant was also examined by transmission electron microscopy of thinly sectioned embedded material. Lipid vesicles made from dipalmitoylphosphatidylcholine and egg phosphatidylcholine (1:1 wt/wt) generally showed a smooth surface morphology, but some large vesicles showed a corrugated one. On the smooth-surfaced vesicles, SP-As primarily interacted in the form of separate octadecamers or as multidirectional protein networks. On the surfaces of the striated vesicles, SP-As primarily formed regularly spaced unidirectional filaments. The mean spacing between adjacent striations and between adjacent filaments was 49 nm. The striated surfaces were not essential for the formation of filaments but appeared to stabilize them. In native surfactant preparations, SP-A was detected in the dense layers. This latter arrangement of the lipid bilayer-associated SP-As supported the potential relevance of the in vitro structures to the in vivo situation.

Formation of membrane lattice structures and their specific interactions with surfactant protein A

Biological membranes exist in many forms, one of which is known as tubular myelin (TM). This pulmonary surfactant membranous structure contains elongated tubes that form square lattices. To understand the interaction of surfactant protein (SP) A and various lipids commonly found in TM, we undertook a series of transmission-electron-microscopic studies using purified SP-A and lipid vesicles made in vitro and also native surfactant from bovine lung. Specimens from in vitro experiments were negatively stained with 2% uranyl acetate, whereas fixed native surfactant was delipidated, embedded, and sectioned. We found that dipalmitoylphosphatidylcholine-egg phosphatidylcholine (1:1 wt/wt) bilayers formed corrugations, folds, and predominantly 47-nm-square latticelike structures. SP-A specifically interacted with these lipid bilayers and folds. We visualized other proteolipid structures that could act as intermediates for reorganizing lipids and SP-As. Such a reorganization could lead to the localization of SP-A in the lattice corners and could explain, in part, the formation of TM-like structures in vivo.

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.

Structural changes of surfactant protein A induced by cations reorient the protein on lipid bilayers

Surfactant protein A (SP-A) is an octadecameric hydrophilic glycoprotein and is the major protein component of pulmonary surfactant. This protein complex plays several roles in the body, such as regulation of surfactant secretion, recycling and adsorption of surfactant lipids, and non-serum-induced immune response. Many of SP-A's activities are dependent upon the presence of cations, especially calcium. Here, we have studied in vitro the effect of cations on the interaction of purified bovine SP-A with phospholipid vesicles made of dipalmitoylphosphatidylcholine and unsaturated phosphatidylcholine. We have found that SP-A octadecamers exist in an "opened-bouquet" conformation in the absence of cations and interact with lipid membranes via one or two globular headgroups. Calcium-induced structural changes in SP-A lead to the formation of a clearly identifiable stem in a "closed-bouquet" conformation. This change, in turn, seemingly results in all of SP-A's globular headgroups interacting with the lipid membrane surface and with the stem pointing away from the membrane surface. These results represent direct evidence that the headgroups of SP-A (comprising carbohydrate recognition domains), and not the stem (comprising the amino-terminus and collagen-like region), interact with lipid bilayers. Our data support models of tubular myelin in which the headgroups, not the tails, interact with the lipid walls of the lattice.