Role of pulmonary surfactant components in surface film formation and dynamics

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.

Temperature and pressure dependent growth and morphology of DMPC/DSPC domains studied by Brewster angle microscopy

In this work, the temperature and pressure dependent growth of domains in DMPC/DSPC monolayers at various molar ratios was studied by Brewster angle microscopy. Upon compression, roughly discoidal domains with some branching are formed. Further compression leads to an increase in both the number and the average size of the domains, which range between ca. 5 and 20 microm. The isobaric heating of the monolayers results in a gradual decrease of the domain size until their disappearance. The size and morphology of the domains depend not only on equilibrium parameters such as temperature, pressure and composition, but appear to be also strongly dependent on non-equilibrium parameters such as the rate of perturbation. The comparison between our results and those previously published for bilayers allows us to infer that the growth behaviour in monolayers can be qualitatively but not quantitatively extrapolated to bilayers.

Structural model for an AxxxG-mediated dimer of surfactant-associated protein C

The pulmonary surfactant prevents alveolar collapse and is required for normal pulmonary function. One of the important components of the surfactant besides phospholipids is surfactant-associated protein C (SP-C). SP-C shows complex oligomerization behavior and a transition to beta-amyloid-like fibril structures, which are not yet fully understood. Besides this nonspecific oligomerization, MS and chemical cross-linking data combined with CD spectra provide evidence of a specific, mainly alpha-helical, dimer at low to neutral pH. Furthermore, resistance to CNBr cleavage and dual NMR resonances of porcine and human recombinant SP-C with Met32 replaced by isoleucine point to a dimerization site located at the C-terminus of the hydrophobic alpha-helix of SP-C, where a strictly conserved heptapeptide sequence is found. Computational docking of two SP-C helices, described here, reveals a dimer with a helix-helix interface that strikingly resembles that of glycophorin A and is mediated by an AxxxG motif similar to the experimentally determined GxxxG pattern of glycophorin A. It is highly likely that mature SP-C adopts such a dimeric structure in the lamellar bilayer systems found in the surfactant. Dimerization has been shown in previous studies to have a role in sorting and trafficking of SP-C and may also be important to the surfactant function of this protein.

Apocrine Glands in the Eyelid of Primates Contribute to the Ocular Host Defense

Apocrine glands of Moll are regular components of primate eyelids. We studied the distribution and localization of these glands in three different primate species, the common marmoset, the rhesus monkey, and the hamadryas baboon. In addition, we tested the primate glands of Moll with antibodies against antimicrobial proteins, cytoskeletal proteins and the androgen receptor. The glands of Moll differ in abundance and distribution in different monkeys. In the common marmoset, a representative of the New World monkeys, Platyrrhini, the apocrine glands are frequently found at the lid margin and in the overlying epidermis of the lid. In the rhesus monkey and the hamadryas baboon, representatives of Old World monkeys, Catarrhini, apocrine glands are rarer and located predominantly at the margin of the lid. The immunohistochemical analysis indicates the presence of a variety of antimicrobial proteins, e.g. lysozyme, beta-defensin-2, adrenomedullin, lactoferrin, and IgA, in these glands. Interestingly, there are basically no androgen receptors in the nuclei of apocrine glands at the lid margin in all three monkey species. In the common marmoset, however, androgen receptors are found in apocrine glands of the overlying epidermis of the lid. We speculate that the glands of Moll are derived from apocrine glands as found in the skin of the entire body in New World monkeys which developed at the lid margins of higher primates and humans into specialized glands secreting agents of host defense in the eye.

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.

Effects of fixatives on function of pulmonary surfactant

The structure of pulmonary surfactant films remains ill defined. Although plausible film fragments have been imaged by electron microscopy, questions about the significance of the findings and even about the true fixability of surfactant films by the usual fixatives glutaraldehyde (GA), osmium tetroxide (OsO(4)), and uranyl acetate (UA) have not been settled. We exposed functioning natural surfactant films to fixatives within a captive bubble surfactometer and analyzed the effect of fixatives on surfactant function. The capacity of surfactant to reach near-zero minimum surface tension on film compression was barely impaired after exposure to GA or OsO(4). Although neither GA nor OsO(4) prevented the surfactant from forming a surface active film, GA increased the equilibrium surface tension to above 30 mN/m, and both GA and OsO(4) decreased film stability as seen in the slowly rising minimum surface tension from 1 to ~5 mN/m in 10 min. In contrast, the effect of UA seriously impaired surface activity in that both adsorption and minimum surface tension were substantially increased. In conclusion, the fixatives tested in this study are not suitable to fix, i.e., to solidify, surfactant films. Evidently, however, OsO(4) and UA may serve as staining agents.

Effect of surfactant on pulmonary expression of type IIA PLA(2) in an animal model of acute lung injury

We previously showed that the seminatural surfactant Curosurf inhibits the in vitro synthesis of secretory type IIA phospholipase A(2) (sPLA(2)-IIA) in alveolar macrophages (AM). These cells are the main source of sPLA(2)-IIA in a guinea pig model of lipopolysaccharide (LPS)-induced acute lung injury (ALI). Here, we investigate the effect of Curosurf on the pulmonary synthesis of sPLA(2)-IIA in this ALI model. Our results showed that intratracheal administration of LPS (330 microg/kg) induced an increase in pulmonary expression of sPLA(2)-IIA, which was inhibited when animals received Curosurf (16 mg/guinea pig) 30 min or 8 h after LPS instillation. When AM were isolated from LPS-treated animals and cultured in conditioned medium, they expressed higher levels of sPLA(2)-IIA than AM from saline-treated animals. This ex vivo sPLA(2)-IIA expression was significantly reduced when guinea pigs received Curosurf 30 min after LPS instillation. Finally, we examined the effect of Curosurf on pulmonary inflammation measured 8 or 24 h after LPS administration. Curosurf instillation 30 min or 8 h after LPS reversed the increase in tumor necrosis factor-alpha expression, polymorphonuclear cell extravasation, and protein concentration in bronchoalveolar lavage fluids. Curosurf also decreased the bronchial reactivity induced by LPS. We conclude that Curosurf inhibits the pulmonary expression of sPLA(2)-IIA and exhibits palliative anti-inflammatory effects in an animal model of ALI.

Mechanical ventilation of isolated rat lungs changes the structure and biophysical properties of surfactant

Mechanical ventilation is an essential but potentially harmful therapeutic intervention for patients with acute lung injury. The objective of this study was to investigate the effects of mechanical ventilation on large-aggregate surfactant (LA) structure and function. Isolated rat lungs were randomized to either a nonventilated control group, a relatively noninjuriously ventilated group [1 h, 10 ml/kg tidal volume, 3 cmH(2)O positive end-expiratory pressure (PEEP)], or an injuriously ventilated group (1 h, 20 ml/kg tidal volume, 0 cmH(2)O PEEP). Injurious ventilation resulted in significantly decreased lung compliance compared with the other two groups. LA structure, as determined by electron microscopy, revealed that LA from the injurious group had significantly lower amounts of organized lipid-protein structures compared with LA obtained from the other groups. Analysis of the biophysical properties by using a captive bubble surfactometer demonstrated that adsorption and surface tension reduction were significantly impaired with LA from the injuriously ventilated lungs. We conclude that the injurious mechanical ventilation impairs LA function and that this impairment is associated with significant morphological alterations.

Superficial disposition of the N-terminal region of the surfactant protein SP-C and the absence of specific SP-B-SP-C interactions in phospholipid bilayers

A dansylated form of porcine surfactant-associated protein C (Dns-SP-C), bearing a single dansyl group at its N-terminal end, has been used to characterize the lipid-protein and protein-protein interactions of SP-C reconstituted in phospholipid bilayers, using fluorescence spectroscopy. The fluorescence emission spectrum of Dns-SP-C in phospholipid bilayers is similar to the spectrum of dansyl-phosphatidylethanolamine, and indicates that the N-terminal end of the protein is located at the surface of the membranes and is exposed to the aqueous environment. In membranes containing phosphatidylglycerol (PG), the fluorescence of Dns-SP-C shows a 3-fold increase with respect to the fluorescence of phosphatidylcholine (PC), suggesting that electrostatic lipid-protein interactions induce important effects on the structure and disposition of the N-terminal segment of the protein in these membranes. This effect saturates above 20% PG molar content in the bilayers. The parameters for the interaction of Dns-SP-C with PC or PG have been estimated from the changes induced in the fluorescence emission spectrum of the protein. The protein had similar K(d) values for its interaction with the different phospholipids tested, of the order of a few micromolar. Cooling of Dns-SP-C-containing dipalmitoyl PC bilayers to temperatures below the phase transition of the phospholipid produced a progressive blue-shift of the fluorescence emission of the protein. This effect is interpreted as a consequence of the transfer of the N-terminal segment of the protein into less polar environments that originate during protein lateral segregation. This suggests that conformation and interactions of the N-terminal segment of SP-C could be important in regulating the lateral distribution of the protein in surfactant bilayers and monolayers. Potential SP-B-SP-C interactions have been explored by analysing fluorescence resonance energy transfer (RET) from the single tryptophan in porcine SP-B to dansyl in Dns-SP-C. RET has been detected in samples where native SP-B and Dns-SP-C were concurrently reconstituted in PC or PG bilayers. However, the analysis of the dependence of RET on the protein density excluded specific SP-B-Dns-SP-C associations.

Discrepancy between phase behavior of lung surfactant phospholipids and the classical model of surfactant function

The studies reported here used fluorescence microscopy and Brewster angle microscopy to test the classical model of how pulmonary surfactant forms films that are metastable at high surface pressures in the lungs. The model predicts that the functional film is liquid-condensed (LC) and greatly enriched in dipalmitoyl phosphatidylcholine (DPPC). Both microscopic methods show that, in monolayers containing the complete set of phospholipids from calf surfactant, an expanded phase persists in coexistence with condensed domains at surface pressures approaching 70 mN/m. Constituents collapsed from the interface above 45 mN/m, but the relative area of the two phases changed little, and the LC phase never occupied more than 30% of the interface. Calculations based on these findings and on isotherms obtained on the continuous interface of a captive bubble estimated that collapse of other constituents increased the mol fraction of DPPC to no higher than 0.37. We conclude that monolayers containing the complete set of phospholipids achieve high surface pressures without forming a homogeneous LC film and with a mixed composition that falls far short of the nearly pure DPPC predicted previously. These findings contradict the classical model.

Thermodynamic effects of the hydrophobic surfactant proteins on the early adsorption of pulmonary surfactant

We determined the influence of the two hydrophobic proteins, SP-B and SP-C, on the thermodynamic barriers that limit adsorption of pulmonary surfactant to the air-water interface. We compared the temperature and concentration dependence of adsorption, measured by monitoring surface tension, between calf lung surfactant extract (CLSE) and the complete set of neutral and phospholipids (N&PL) without the proteins. Three stages generally characterized the various adsorption isotherms: an initial delay during which surface tension remained constant, a fall in surface tension at decreasing rates, and, for experiments that reached approximately 40 mN/m, a late acceleration of the fall in surface tension to approximately 25 mN/m. For the initial change in surface tension, the surfactant proteins accelerated adsorption for CLSE relative to N&PL by more than ten-fold, reducing the Gibbs free energy of transition (DeltaG(O)) from 119 to 112 kJ/mole. For the lipids alone in N&PL, the enthalpy of transition (DeltaH(O), 54 kJ/mole) and entropy (-T. DeltaS, 65 kJ/mole at 37 degrees C) made roughly equal contributions to DeltaG(O). The proteins in CLSE had little effect on -T. DeltaS(O) (68 kJ/mole), but lowered DeltaG(O) for CLSE by reducing DeltaH(O) (44 kJ/mole). Models of the detailed mechanisms by which the proteins facilitate adsorption must meet these thermodynamic constraints.

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.

Pulmonary surfactant protein SP-B is significantly more immunoreactive in anionic than in zwitterionic bilayers

Binding of polyclonal and monoclonal antibodies, quantitated by enzyme-linked immunosorbent assay, to porcine SP-B reconstituted in different phospholipid bilayers has been used to assess differences in protein structure due to lipid-protein interactions. SP-B bound significantly more antibodies when it was reconstituted in bilayers made of anionic phospholipids (phosphatidic acid, cardiolipin, phosphatidylglycerol, phosphatidylinositol or phosphatidylserine) than in zwitterionic bilayers (phosphatidylcholine, phosphatidylcholine/cholesterol, or phosphatidylethanolamine) or in fatty acid micelles (made of salts of palmitic or stearic acids). These differences in immunoreactivity can be important in the development of quantitation methods for SP-B in clinical samples based on immunological techniques.