Surfactant protein B: disulfide bridges, structural properties, and kringle similarities

Emulation of the structure of the Saposin protein fold by a lung surfactant peptide construct of surfactant Protein B

The three-dimensional structure of the synthetic lung Surfactant Protein B Peptide Super Mini-B was determined using an integrative experimental approach, including mass spectrometry and isotope enhanced Fourier-transform infrared (FTIR) spectroscopy. Mass spectral analysis of the peptide, oxidized by solvent assisted region-specific disulfide formation, confirmed that the correct folding and disulfide pairing could be facilitated using two different oxidative structure-promoting solvent systems. Residue specific analysis by isotope enhanced FTIR indicated that the N-terminal and C-terminal domains have well defined α-helical amino acid sequences. Using these experimentally derived measures of distance constraints and disulfide connectivity, the ensemble was further refined with molecular dynamics to provide a medium resolution, residue-specific structure for the peptide construct in a simulated synthetic lung surfactant lipid multilayer environment. The disulfide connectivity combined with the α-helical elements stabilize the peptide conformationally to form a helical hairpin structure that resembles critical elements of the Saposin protein fold of the predicted full-length Surfactant Protein B structure.

Disulfide bonds in the SAPA domain of the pulmonary surfactant protein B precursor

The disulfide bonds formed in the SAPA domain of a recombinant version of the NH₂-terminal propeptide (SP-B_N) from the precursor of human pulmonary surfactant protein B (SP-B) were identified through sequential digestion of SP-B_N with GluC/trypsin or thermolysin/GluC, followed by mass spectrometry (MS) analysis. MS spectra allowed identification of disulfide bonds between Cys³²-Cys⁴⁹ and Cys⁴⁰-Cys⁵⁵, and we propose a disulfide connectivity pattern of 1-3 and 2-4 within the SAPA domain, with the Cys residues numbered according to their position from the N-terminus of the propeptide sequence. The peaks with m/z ∼ 2136 and ∼ 1780 in the MS spectrum of the GluC/trypsin digest were assigned to peptides ²⁴AWTTSSLACAQGPE³⁷ and ⁴⁵QALQCR⁵⁰ linked by Cys³²-Cys⁴⁹ and ³⁸FWCQSLE⁴⁴ and ⁵¹ALGHCLQE⁵⁸ linked by Cys⁴⁰-Cys⁵⁵ respectively. Tandem mass spectrometry (MS/MS) analysis verified the position of the bonds. The results of the series ions, immonium ions and internal fragment ions were all compatible with the proposed 1-3/2-4 position of the disulfide bonds in the SAPA domain. This X-pattern differs from the kringle-type found in the SAPB domain of the SAPLIP proteins, where the first Cys in the sequence links to the last, the second to the penultimate and the third to the fourth one. Regarding the SAPB domain of the SP-B_N propeptide, the MS analysis of both digests identified the bond Cys¹⁰⁰-Cys¹¹², numbered 7-8, which is coincident with the bond position in the kringle motif. SIGNIFICANCE: The SAPLIP (saposin-like proteins) family encompasses several proteins with homology to saposins (sphingolipids activator proteins). These are proteins with mainly alpha-helical folds, compact packing including well conserved disulfide bonds and ability to interact with phospholipids and membranes. There are two types of saposin-like domains termed as Saposin A (SAPA) and Saposin B (SAPB) domains. While disulfide connectivity has been well established in several SAPB domains, the position of disulfide bonds in SAPA domains is still unknown. The present study approaches a detailed proteomic study to determine disulfide connectivity in the SAPA domain of the precursor of human pulmonary surfactant-associated protein SP-B. This task has been a challenge requiring the combination of different sequential proteolytic treatments followed by MS analysis including MALDI-TOF and tandem mass MS/MS spectrometry. The determination for first time of the position of disulfide bonds in SAPA domains is an important step to understand the structural determinants defining its biological functions.

The lung surfactant activity probed with molecular dynamics simulations

The surface of pulmonary alveolar subphase is covered with a mixture of lipids and proteins. This lung surfactant plays a crucial role in lung functioning. It shows a complex phase behavior which can be altered by the interaction with third molecules such as drugs or pollutants. For studying multicomponent biological systems, it is of interest to couple experimental approach with computational modelling yielding atomic-scale information. Simple two, three, or four-component model systems showed to be useful for getting more insight in the interaction between lipids, lipids and proteins or lipids and proteins with drugs and impurities. These systems were studied theoretically using molecular dynamic simulations and experimentally by means of the Langmuir technique. A better understanding of the structure and behavior of lung surfactants obtained from this research is relevant for developing new synthetic surfactants for efficient therapies, and may contribute to public health protection.

Treatment of Respiratory Distress Syndrome with Single Recombinant Polypeptides that Combine Features of SP-B and SP-C

Treatment of respiratory distress syndrome (RDS) with surfactant replacement therapy in prematurely born infants was introduced more than 30 years ago; however, the surfactant preparations currently in clinical use are extracts from animal lungs. A synthetic surfactant that matches the currently used nature-derived surfactant preparations and can be produced in a cost-efficient manner would enable worldwide treatment of neonatal RDS and could also be tested against lung diseases in adults. The major challenge in developing fully functional synthetic surfactant preparations is to recapitulate the properties of the hydrophobic lung surfactant proteins B (SP-B) and SP-C. Here, we have designed single polypeptides that combine properties of SP-B and SP-C and produced them recombinantly using a novel solubility tag based on spider silk production. These Combo peptides mixed with phospholipids are as efficient as nature-derived surfactant preparations against neonatal RDS in premature rabbit fetuses.

The role of SP-B1–25 peptides in lung surfactant monolayers exposed to gold nanoparticles

Lung surfactant monolayer’s (acts as the first line barrier for inhaled nanoparticles) components (lipids and peptides) rearrange themselves by the influence of exposed gold nanoparticles at various stages of the breathing cycle.

Efficient delipidation of a recombinant lung surfactant lipopeptide analogue by liquid-gel chromatography

Pulmonary surfactant preparations extracted from natural sources have been used to treat millions of newborn babies with respiratory distress syndrome (RDS) and can possibly also be used to treat other lung diseases. Due to costly production and limited supply of animal-derived surfactants, synthetic alternatives are attractive. The water insolubility and aggregation-prone nature of the proteins present in animal-derived surfactant preparations have complicated development of artificial surfactant. A non-aggregating analog of lung surfactant protein C, SP-C33Leu is used in synthetic surfactant and we recently described an efficient method to produce rSP-C33Leu in bacteria. Here rSP-C33Leu obtained by salt precipitation of bacterial extracts was purified by two-step liquid gel chromatography and analyzed using mass spectrometry and RP-HPLC, showing that it is void of modifications and adducts. Premature New Zealand White rabbit fetuses instilled with 200mg/kg of 2% of rSP-C33Leu in phospholipids and ventilated with a positive end expiratory pressure showed increased tidal volumes and lung gas volumes compared to animals treated with phospholipids only. This shows that rSP-C33Leu can be purified from bacterial lipids and that rSP-C33Leu surfactant is active against experimental RDS.

Synthetic surfactants with SP-B and SP-C analogues to enable worldwide treatment of neonatal respiratory distress syndrome and other lung diseases

Treatment of neonatal respiratory distress syndrome (RDS) using animal-derived lung surfactant preparations has reduced the mortality of handling premature infants with RDS to a 50th of that in the 1960s. The supply of animal-derived lung surfactants is limited and only a part of the preterm babies is treated. Thus, there is a need to develop well-defined synthetic replicas based on key components of natural surfactant. A synthetic product that equals natural-derived surfactants would enable cost-efficient production and could also facilitate the development of the treatments of other lung diseases than neonatal RDS. Recently the first synthetic surfactant that contains analogues of the two hydrophobic surfactant proteins B (SP-B) and SP-C entered clinical trials for the treatment of neonatal RDS. The development of functional synthetic analogues of SP-B and SP-C, however, is considerably more challenging than anticipated 30 years ago when the first structural information of the native proteins became available. For SP-B, a complex three-dimensional dimeric structure stabilized by several disulphides has necessitated the design of miniaturized analogues. The main challenge for SP-C has been the pronounced amyloid aggregation propensity of its transmembrane region. The development of a functional non-aggregating SP-C analogue that can be produced synthetically was achieved by designing the amyloidogenic native sequence so that it spontaneously forms a stable transmembrane α-helix.

Disulphide Bridges in Surfactant Protein B Analogues Affect Their Activity in Synthetic Surfactant Preparations

BACKGROUND

Limited supply and complicated manufacturing procedure of animal-derived surfactants make the development of synthetic surfactants warranted. The synthesis of surfactant protein (SP)-B and SP-C is complicated and several analogues have been developed. Mini-BLeu is an analogue that corresponds to the first and last helix of SP-B joined by a loop and linked by 2 disulphide bridges. SP-C33Leu is an SP-C analogue that can be cost-efficiently produced, but no such analogue has yet been described for SP-B.

OBJECTIVE

To design short SP-B analogues which lack disulphide bridges, are easy to produce and are efficacious in a preterm rabbit fetus model of neonatal RDS.

METHODS

Synthetic surfactants were prepared by adding 2 or 8% (w/w) of synthetic variants of Mini-B27, similar to Mini-BLeu but with a short loop, or different peptides covering helix 1 of SP-B to 2% (w/w) of SP-C33Leu in 80 mg/mL of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/egg yolk phosphatidylcholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol, 50: 40: 10 (by weight). Premature newborn rabbit fetuses were treated with 200 mg/kg of the surfactant preparations and ventilated with defined pressures for 30 min without positive end-expiratory pressure. Tidal volumes were registered during the experiments and lung gas volumes were measured at the end of the ventilation period.

RESULTS

Synthetic surfactant containing the Mini-B27 analogue with 2 disulphides gives similar lung gas volumes as treatment with an animal-derived surfactant preparation, but all other SP-B analogues gave lower lung gas volumes. All synthetic surfactants studied gave no significant differences in compliances except the surfactant containing the Mini-B27 analogue without cysteines that performed somewhat better at 30 min.

CONCLUSION

The helix-loop-helix SP-B analogues tested in this study require the presence of 2 disulphide bridges for optimal activity in a rabbit RDS model.

Quenching of tryptophan fluorescence in a highly scattering solution: Insights on protein localization in a lung surfactant formulation

CHF5633 (Chiesi Farmaceutici, Italy) is a synthetic surfactant developed for respiratory distress syndrome replacement therapy in pre-term newborn infants. CHF5633 contains two phospholipids (dipalmitoylphosphatidylcholine and 1-palmitoyl-2oleoyl-sn-glycero-3-phosphoglycerol sodium salt), and peptide analogues of surfactant protein C (SP-C analogue) and surfactant protein B (SP-B analogue). Both proteins are fundamental for an optimal surfactant activity in vivo and SP-B genetic deficiency causes lethal respiratory failure after birth. Fluorescence emission of the only tryptophan residue present in SP-B analogue (SP-C analogue has none) could in principle be exploited to probe SP-B analogue conformation, localization and interaction with other components of the pharmaceutical formulation. However, the high light scattering activity of the multi-lamellar vesicles suspension characterizing the pharmaceutical surfactant formulation represents a challenge for such studies. We show here that quenching of tryptophan fluorescence and Singular Value Decomposition analysis can be used to accurately calculate and subtract background scattering. The results indicate, with respect to Trp microenvironment, a conformationally homogeneous population of SP-B. Trp is highly accessible to the water phase, suggesting a surficial localization on the membrane of phospholipid vesicles, similarly to what observed for full length SP-B in natural lung surfactant, and supporting an analogous role in protein anchoring to the lipid phase.

Surfactant protein B: From biochemistry to its potential role as diagnostic and prognostic marker in heart failure

Growing interest raised on circulating biomarkers of structural alveolar-capillary unit damage and very recent data support surfactant protein type B (SP-B) as the most promising candidate in this setting. With respect to other proteins proposed as possible markers of lung damage, SP-B has some unique qualities: it is critical for the assembly of pulmonary surfactant, making its lack incompatible with life; it has no other known site of synthesis except alveolar epithelial cells different from other surfactant proteins; and, it undergoes a proteolytic processing in a pulmonary-cell-specific manner. In the recent years circulating SP-B isoforms, mature or immature, have been demonstrated to be detectable in the circulation depending on the magnitude of the damage of alveolar capillary membrane. In the present review, we summarize the recent knowledge on SP-B regulation, function and we discuss its potential role as reliable biological marker of alveolar capillary membrane (dys)function in the context of heart failure.

Hydrophobic surfactant proteins strongly induce negative curvature

The hydrophobic surfactant proteins SP-B and SP-C greatly accelerate the adsorption of vesicles containing the surfactant lipids to form a film that lowers the surface tension of the air/water interface in the lungs. Pulmonary surfactant enters the interface by a process analogous to the fusion of two vesicles. As with fusion, several factors affect adsorption according to how they alter the curvature of lipid leaflets, suggesting that adsorption proceeds via a rate-limiting structure with negative curvature, in which the hydrophilic face of the phospholipid leaflets is concave. In the studies reported here, we tested whether the surfactant proteins might promote adsorption by inducing lipids to adopt a more negative curvature, closer to the configuration of the hypothetical intermediate. Our experiments used x-ray diffraction to determine how the proteins in their physiological ratio affect the radius of cylindrical monolayers in the negatively curved, inverse hexagonal phase. With binary mixtures of dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC), the proteins produced a dose-related effect on curvature that depended on the phospholipid composition. With DOPE alone, the proteins produced no change. With an increasing mol fraction of DOPC, the response to the proteins increased, reaching a maximum 50% reduction in cylindrical radius at 5% (w/w) protein. This change represented a doubling of curvature at the outer cylindrical surface. The change in spontaneous curvature, defined at approximately the level of the glycerol group, would be greater. Analysis of the results in terms of a Langmuir model for binding to a surface suggests that the effect of the lipids is consistent with a change in the maximum binding capacity. Our findings show that surfactant proteins can promote negative curvature, and support the possibility that they facilitate adsorption by that mechanism.

A unique story in neonatal research: the development of a porcine surfactant

Surfactant deficiency was identified as the cause of respiratory distress syndrome (RDS) as long ago as 1959. Trials of surfactant replacement in the 1960s were unsuccessful because the preparations used contained only phospholipids and they were administered inefficiently by nebulization. In the 1970s Bengt Robertson and Göran Enhörning showed that natural surfactant, containing both phospholipids and proteins, could ameliorate the signs of RDS in immature rabbits. In the 1980s Bengt Robertson and Tore Curstedt developed a porcine surfactant, Curosurf (named after their surnames), which was effective in immature animals and was used in a pilot clinical trial beginning in 1983. Subsequent randomized clinical trials were planned a year later by Bengt Robertson, Tore Curstedt and Henry Halliday, and the first trial was begun in 1985. This showed that Curosurf reduced pulmonary air leaks and neonatal mortality in preterm infants with severe RDS. A second trial, coordinated by Christian Speer, demonstrated that multiple doses of Curosurf were more effective than a single dose. Subsequent trials conducted by the Collaborative European Multicenter Study Group, which included among others Guilio Bevilacqua, Janna Koppe, Ola Saugstad, Nils Svenningsen and Jean-Pierre Relier, showed that early treatment was more effective than later administration and that infants treated at birth had similar neurodevelopmental status to untreated controls at a corrected age of 2 years. Members of the Collaborative European Multicenter Study Group in Denmark and Sweden performed studies to demonstrate the benefits of a combination of surfactant treatment and early continuous positive airway pressure. Curosurf has also been compared with several synthetic and natural surfactants, and at a dose of 200 mg/kg Curosurf has been shown to be superior to either Survanta or Curosurf used at a dose of 100 mg/kg. Recently, new-generation synthetic surfactants containing both phospholipids and proteins have been developed. After preclinical testing, CHF5633 (developed by Tore Curstedt and Jan Johansson in collaboration with Chiesi Farmaceutici) has undergone a preliminary first study in humans under the guidance of Christian Speer. If effective, this new surfactant preparation could revolutionize the treatment of preterm infants worldwide as it could be made consistently and safely in almost unlimited quantities. This story of a porcine surfactant preparation has been truly remarkable, and many thousands of preterm babies worldwide are now alive and well because of it.

Synthetic surfactant containing SP-B and SP-C mimics is superior to single-peptide formulations in rabbits with chemical acute lung injury

Background. Chemical spills are on the rise and inhalation of toxic chemicals may induce chemical acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). Although the pathophysiology of ALI/ARDS is well understood, the absence of specific antidotes has limited the effectiveness of therapeutic interventions.

Objectives. Surfactant inactivation and formation of free radicals are important pathways in (chemical) ALI. We tested the potential of lipid mixtures with advanced surfactant protein B and C (SP-B and C) mimics to improve oxygenation and lung compliance in rabbits with lavage- and chemical-induced ALI/ARDS.

Methods. Ventilated young adult rabbits underwent repeated saline lung lavages or underwent intratracheal instillation of hydrochloric acid to induce ALI/ARDS. After establishment of respiratory failure rabbits were treated with a single intratracheal dose of 100 mg/kg of synthetic surfactant composed of 3% Super Mini-B (S-MB), a SP-B mimic, and/or SP-C33 UCLA, a SP-C mimic, in a lipid mixture (DPPC:POPC:POPG 5:3:2 by weight), the clinical surfactant Infasurf^®, a bovine lung lavage extract with SP-B and C, or synthetic lipids alone. End-points consisted of arterial oxygenation, dynamic lung compliance, and protein and lipid content in bronchoalveolar lavage fluid. Potential mechanism of surfactant action for S-MB and SP-C33 UCLA were investigated with captive bubble surfactometry (CBS) assays.

Results. All three surfactant peptide/lipid mixtures and Infasurf equally lowered the minimum surface tension on CBS, and also improved oxygenation and lung compliance. In both animal models, the two-peptide synthetic surfactant with S-MB and SP-C33 UCLA led to better arterial oxygenation and lung compliance than single peptide synthetic surfactants and Infasurf. Synthetic surfactants and Infasurf improved lung function further in lavage- than in chemical-induced respiratory failure, with the difference probably due to greater capillary-alveolar protein leakage and surfactant dysfunction after HCl instillation than following lung lavage. At the end of the duration of the experiments, synthetic surfactants provided more clinical stability in ALI/ARDS than Infasurf, and the protein content of bronchoalveolar lavage fluid was lowest for the two-peptide synthetic surfactant with S-MB and SP-C33 UCLA.

Conclusion. Advanced synthetic surfactant with robust SP-B and SP-C mimics is better equipped to tackle surfactant inactivation in chemical ALI than synthetic surfactant with only a single surfactant peptide or animal-derived surfactant.

Interaction of the C-terminal peptide of pulmonary surfactant protein B (SP-B) with a bicellar lipid mixture containing anionic lipid

The hydrophobic lung surfactant SP-B is essential for respiration. SP-B promotes spreading and adsorption of surfactant at the alveolar air-water interface and may facilitate connections between the surface layer and underlying lamellar reservoirs of surfactant material. SP-B_63–78 is a cationic and amphipathic helical peptide containing the C-terminal helix of SP-B. ²H NMR has been used to examine the effect of SP-B_63–78 on the phase behavior and dynamics of bicellar lipid dispersions containing the longer chain phospholipids DMPC-d₅₄ and DMPG and the shorter chain lipid DHPC mixed with a 3∶1∶1 molar ratio. Below the gel-to-liquid crystal phase transition temperature of the longer chain components, bicellar mixtures form small, rapidly reorienting disk-like particles with shorter chain lipid components predominantly found around the highly curved particle edges. With increasing temperature, the particles coalesce into larger magnetically-oriented structures and then into more extended lamellar phases. The susceptibility of bicellar particles to coalescence and large scale reorganization makes them an interesting platform in which to study peptide-induced interactions between lipid assemblies. SP-B_63–78 is found to lower the temperature at which the orientable phase transforms to the more extended lamellar phase. The peptide also changes the spectrum of motions contributing to quadrupole echo decay in the lamellar phase. The way in which the peptide alters interactions between bilayered micelle structures may provide some insight into some aspects of the role of full-length SP-B in maintaining a functional surfactant layer in lungs.

An anionic phospholipid enables the hydrophobic surfactant proteins to alter spontaneous curvature

The hydrophobic surfactant proteins, SP-B and SP-C, greatly accelerate the adsorption of the surfactant lipids to an air/water interface. Previous studies of factors that affect curvature suggest that vesicles may adsorb via a rate-limiting structure with prominent negative curvature, in which the hydrophilic face of the lipid leaflets is concave. To determine if SP-B and SP-C might promote adsorption by inducing negative curvature, we used small-angle x-ray scattering to test whether the physiological mixture of the two proteins affects the radius of cylindrical monolayers in the inverse hexagonal phase. With dioleoyl phosphatidylethanolamine alone, the proteins had no effect on the hexagonal lattice constant, suggesting that the proteins fail to insert into the cylindrical monolayers. The surfactant lipids also contain ∼10% anionic phospholipids, which might allow incorporation of the cationic proteins. With 10% of the anionic dioleoyl phosphatidylglycerol added to dioleoyl phosphatidylethanolamine, the proteins induced a dose-related decrease in the hexagonal lattice constant. At 30°C, the reduction reached a maximum of 8% relative to the lipids alone at ∼1% (w/w) protein. Variation of NaCl concentration tested whether the effect of the protein represented a strictly electrostatic effect that screening by electrolyte would eliminate. With concentrations up to 3 M NaCl, the dose-related change in the hexagonal lattice constant decreased but persisted. Measurements at different hydrations determined the location of the pivotal plane and proved that the change in the lattice constant produced by the proteins resulted from a shift in spontaneous curvature. These results provide the most direct evidence yet that the surfactant proteins can induce negative curvature in lipid leaflets. This finding supports the model in which the proteins promote adsorption by facilitating the formation of a negatively curved, rate-limiting structure.

Role of lipid ordered/disordered phase coexistence in pulmonary surfactant function

The respiratory epithelium has evolved to produce a complicated network of extracellular membranes that are essential for breathing and, ultimately, survival. Surfactant membranes form a stable monolayer at the air-liquid interface with bilayer structures attached to it. By reducing the surface tension at the air-liquid interface, surfactant stabilizes the lung against collapse and facilitates inflation. The special composition of surfactant membranes results in the coexistence of two distinct micrometer-sized ordered/disordered phases maintained up to physiological temperatures. Phase coexistence might facilitate monolayer folding to form three-dimensional structures during exhalation and hence allow the film to attain minimal surface tension. These folded structures may act as a membrane reserve and attenuate the increase in membrane tension during inspiration. The present review summarizes what is known of ordered/disordered lipid phase coexistence in lung surfactant, paying attention to the possible role played by domain boundaries in the monolayer-to-multilayer transition, and the correlations of biophysical inactivation of pulmonary surfactant with alterations in phase coexistence.

Lung surfactant protein SP-B promotes formation of bilayer reservoirs from monolayer and lipid transfer between the interface and subphase

We investigated the possible role of SP-B proteins in the function of lung surfactant. To this end, lipid monolayers at the air/water interface, bilayers in water, and transformations between them in the presence of SP-B were simulated. The proteins attached bilayers to monolayers, providing close proximity of the reservoirs with the interface. In the attached aggregates, SP-B mediated establishment of the lipid-lined connection similar to the hemifusion stalk. Via this connection, a lipid flow was initiated between the monolayer at the interface and the bilayer in water in a surface-tension-dependent manner. On interface expansion, the flow of lipids to the monolayer restored the surface tension to the equilibrium spreading value. SP-B induced formation of bilayer folds from the monolayer at positive surface tensions below the equilibrium. In the absence of proteins, lipid monolayers were stable at these conditions. Fold nucleation was initiated by SP-B from the liquid-expanded monolayer phase by local bending, and the proteins lined the curved perimeter of the growing fold. No effect on the liquid-condensed phase was observed. Covalently linked dimers resulted in faster kinetics for monolayer folding. The simulation results are in line with existing hypotheses on SP-B activity in lung surfactant and explain its molecular mechanism.

Synthetic surfactant based on analogues of SP-B and SP-C is superior to single-peptide surfactants in ventilated premature rabbits

BACKGROUND

Respiratory distress syndrome (RDS) is currently treated with surfactant preparations obtained from natural sources and attempts to develop equally active synthetic surfactants have been unsuccessful. One difference in composition is that naturally derived surfactants contain the two hydrophobic proteins SP-B and SP-C while synthetic preparations contain analogues of either SP-B or SP-C. It was recently shown that both SP-B and SP-C (or SP-C33, an SP-C analogue) are necessary to establish alveolar stability at end-expiration in a rabbit RDS model, as reflected by high lung gas volumes without application of positive end-expiratory pressure.

OBJECTIVES

To study the efficacy of fully synthetic surfactants containing analogues of both SP-B and SP-C compared to surfactants with only one protein analogue.

METHODS

Premature newborn rabbits, treated with synthetic surfactants, were ventilated for 30 min without positive end-expiratory pressure. Tidal volumes as well as lung gas volumes at end-expiration were determined.

RESULTS

Treatment with 2% Mini-B (a short-cut version of SP-B) and 2% SP-C33, or its C-terminally truncated form SP-C30, in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol, 68:31 (w/w) resulted in median lung gas volumes of 8-9 ml/kg body weight, while animals treated with 2% Mini-B surfactant or 2% SP-C33/SP-C30 surfactant had lung gas volumes of 3-4 ml/kg, and those treated with Curosurf, a porcine surfactant, 15-17 ml/kg. In contrast, mixing SP-C33 with peptides with different distributions of positively charged and hydrophobic residues did not improve lung gas volumes.

CONCLUSIONS

The data indicate that synthetic surfactants containing analogues of both SP-B and SP-C might be superior to single-peptide surfactants in the treatment of RDS.

Novel peptoid building blocks: synthesis of functionalized aromatic helix-inducing submonomers

Peptoids, oligo-N-substituted glycines, can fold into well-defined helical secondary structures. The design and synthesis of new peptoid building blocks that are capable of both (a) inducing a helical secondary structure and (b) decorating the helices with chemical functionalities are reported. Peptoid heptamers containing carboxamide, carboxylic acid or thiol functionalities were synthesized, and the resulting peptoids were shown to form stable helices. A thiol-containing peptoid readily formed the homodisulfide, providing a convenient route to prepare peptoid helix homodimers.

Critical structural and functional roles for the N-terminal insertion sequence in surfactant protein B analogs

Background

Surfactant protein B (SP-B; 79 residues) belongs to the saposin protein superfamily, and plays functional roles in lung surfactant. The disulfide cross-linked, N- and C-terminal domains of SP-B have been theoretically predicted to fold as charged, amphipathic helices, suggesting their participation in surfactant activities. Earlier structural studies with Mini-B, a disulfide-linked construct based on the N- and C-terminal regions of SP-B (i.e., ∼residues 8–25 and 63–78), confirmed that these neighboring domains are helical; moreover, Mini-B retains critical in vitro and in vivo surfactant functions of the native protein. Here, we perform similar analyses on a Super Mini-B construct that has native SP-B residues (1–7) attached to the N-terminus of Mini-B, to test whether the N-terminal sequence is also involved in surfactant activity.

Methodology/Results

FTIR spectra of Mini-B and Super Mini-B in either lipids or lipid-mimics indicated that these peptides share similar conformations, with primary α-helix and secondary β-sheet and loop-turns. Gel electrophoresis demonstrated that Super Mini-B was dimeric in SDS detergent-polyacrylamide, while Mini-B was monomeric. Surface plasmon resonance (SPR), predictive aggregation algorithms, and molecular dynamics (MD) and docking simulations further suggested a preliminary model for dimeric Super Mini-B, in which monomers self-associate to form a dimer peptide with a “saposin-like” fold. Similar to native SP-B, both Mini-B and Super Mini-B exhibit in vitro activity with spread films showing near-zero minimum surface tension during cycling using captive bubble surfactometry. In vivo, Super Mini-B demonstrates oxygenation and dynamic compliance that are greater than Mini-B and compare favorably to full-length SP-B.

Conclusion

Super Mini-B shows enhanced surfactant activity, probably due to the self-assembly of monomer peptide into dimer Super Mini-B that mimics the functions and putative structure of native SP-B.

Surfactant protein B propeptide contains a saposin-like protein domain with antimicrobial activity at low pH

Surfactant protein B (SP-B) proprotein contains three saposin-like protein (SAPLIP) domains: a SAPLIP domain corresponding to the mature SP-B peptide is essential for lung function and postnatal survival; the function of SAPLIP domains in the N-terminal (SP-BN) and C-terminal regions of the proprotein is not known. In the current study, SP-BN was detected in the supernatant of mouse bronchoalveolar lavage fluid (BALF) and in nonciliated bronchiolar cells, alveolar type II epithelial cells, and alveolar macrophages. rSP-BN indirectly promoted the uptake of bacteria by macrophage cell lines and directly killed bacteria at acidic pH, consistent with a lysosomal, antimicrobial function. Native SP-BN isolated from BALF also killed bacteria but only at acidic pH; the bactericidal activity of BALF at acidic pH was completely blocked by SP-BN Ab. Transgenic mice overexpressing SP-BN and mature SP-B peptide had significantly decreased bacterial burden and increased survival following intranasal inoculation with bacteria. These findings support the hypothesis that SP-BN contributes to innate host defense of the lung by supplementing the nonoxidant antimicrobial defenses of alveolar macrophages.

The effect of a C-terminal peptide of surfactant protein B (SP-B) on oriented lipid bilayers, characterized by solid-state 2H- and 31P-NMR

SP-B(CTERM), a cationic, helical peptide based on the essential lung surfactant protein B (SP-B), retains a significant fraction of the function of the full-length protein. Solid-state (2)H- and (31)P-NMR were used to examine the effects of SP-B(CTERM) on mechanically oriented lipid bilayer samples. SP-B(CTERM) modified the multilayer structure of bilayers composed of POPC, POPG, POPC/POPG, or bovine lipid extract surfactant (BLES), even at relatively low peptide concentrations. The (31)P spectra of BLES, which contains approximately 1% SP-B, and POPC/POPG with 1% SP-B(CTERM), look very similar, supporting a similarity in lipid interactions of SP-B(CTERM) and its parent protein, full-length SP-B. In the model systems, although the peptide interacted with both the oriented and unoriented fractions of the lipids, it interacted differently with the two fractions, as demonstrated by differences in lipid headgroup structure induced by the peptide. On the other hand, although SP-B(CTERM) induced similar disruptions in overall bilayer orientation in BLES, there was no evidence of lipid headgroup conformational changes in either the oriented or the unoriented fractions of the BLES samples. Notably, in the model lipid systems the peptide did not induce the formation of small, rapidly tumbling lipid structures, such as micelles, or of hexagonal phases, the observation of which would have provided support for functional mechanisms involving peptide-induced lipid flip-flop or stabilization of curved lipid structures, respectively.

Perturbation of DPPC/POPG bilayers by the N-terminal helix of lung surfactant protein SP-B: a (2)H NMR study

SP-B(8-25) is a synthetic peptide comprising the N-terminal helix of the essential lung surfactant protein SP-B. Rat lung oxygenation studies have shown that SP-B(8-25) retains some of the function of full-length SP-B. We have used deuterium nuclear magnetic resonance ((2)H-NMR) to examine the influence of SP-B(8-25) on the mixing properties of saturated PC and unsaturated PG lipids in model mixed lipid bilayers containing dipalmitoylphosphatidylcholine (DPPC) and palmitoyl-oleoyl-phosphatidylglycerol (POPG), in a molar ratio of 7:3. In the absence of the peptide, (2)H-NMR spectra of DPPC/POPG mixtures, with one or the other lipid component deuterated, indicate coexistence of large liquid crystal and gel domains over a range of about 10 degrees C through the liquid crystal to gel transition of the bilayer. Addition of SP-B(8-25) has little effect on the width of the transition but the spectra through the transition range cannot be resolved into distinct liquid crystal and gel spectral components suggesting that the peptide interferes with the tendency of the DPPC and POPG lipid components in this mixture to phase separate near the bilayer transition temperature. Quadrupole echo decay observations suggest that the peptide may also reduce differences in the correlation times for local reorientation of the two lipids. These observations suggest that SP-B(8-25) promotes a more thorough mixing of saturated PC and unsaturated PG components and may be relevant to understanding the behaviour of lung surfactant material under conditions of lateral compression which might be expected to enhance the propensity for saturated and unsaturated surfactant lipid components to segregate.

Self-aggregation of a recombinant form of the propeptide NH2-terminal of the precursor of pulmonary surfactant protein SP-B: a conformational study

A recombinant form of the peptide N-terminally positioned from proSP-B (SP-BN) has been produced in Escherichia coli as fusion with the Maltose Binding Protein, separated from it by Factor Xa cleavage and purified thereafter. This protein module is thought to control assembly of mature SP-B, a protein essential for respiration, in pulmonary surfactant as it progress through the progressively acidified secretory pathway of pneumocytes. Self-aggregation studies of the recombinant propeptide have been carried out as the pH of the medium evolved from neutral to moderately acid, again to neutral and finally basic. The profile of aggregation versus subsequent changes in pH showed differences depending on the ionic strength of the medium, low or moderate, and the presence of additives such as L-arginine (a known aggregation suppressor) and Ficoll 70 (a macromolecular crowder). Circular dichroism studies of SP-BN samples along the aggregation process showed a decrease in alpha-helical content and a concomitant increase in beta-sheet. Intrinsic fluorescence emission of SP-BN was dominated by the emission of Trp residues in neutral medium, being its emission maximum shifted to red at low pH, suggesting that the protein undergoes a pH-dependent conformational change that increases the exposure of their Trp to the environment. A marked increase in the fluorescence emission of the extrinsic probe bis-ANS indicated the exposure of hydrophobic regions of SP-BN at pH 5. The fluorescence of bis-ANS decreased slightly at low ionic strength, but to a great extent at moderate ionic strength when the pH was reversed to neutrality, suggesting that self-aggregation properties of the SP-BN module could be tightly modulated by the conditions of pH and the ionic environment encountered by pulmonary surfactant during assembly and secretion.

Structure of SP-B/DPPC mixed films studied by neutron reflectometry

The structures of films of pulmonary surfactant protein B (SP-B) and mixtures of SP-B and dipalmitoylphosphatidylcholine (DPPC) at the air/water interface have been studied by neutron reflectometry and Langmuir film balance methods. From the film balance studies, we observe that the isotherms of pure DPPC and SP-B/DPPC mixtures very nearly overlay one another at very high pressures, suggesting that the SP-B is being excluded from the film. The use of multiple contrasts with neutron reflectometry at a range of surface pressures has enabled the mixing and squeeze out of the DPPC and SP-B mixtures to be studied. We can identify the SP-B component of the interfacial structure and its position as a function of surface pressure. The mixtures are initially a homogeneous layer at low surface pressures. At higher surface pressures, the SP-B is squeezed out of the lipid layer into the subphase, with the first signs detected at 30 mN m(-1). At 50 mN m(-1), the subphase is almost completely excluded from the DPPC layer, with the SP-B content significantly reduced. Only a small amount of DPPC appears to be associated with the squeezed out SP-B.

Surfactant proteins B and C are both necessary for alveolar stability at end expiration in premature rabbits with respiratory distress syndrome

Modified natural surfactant preparations, used for treatment of respiratory distress syndrome in premature infants, contain phospholipids and the hydrophobic surfactant protein (SP)-B and SP-C. Herein, the individual and combined effects of SP-B and SP-C were evaluated in premature rabbit fetuses treated with airway instillation of surfactant and ventilated without positive end-expiratory pressure. Artificial surfactant preparations composed of synthetic phospholipids mixed with either 2% (wt/wt) of porcine SP-B, SP-C, or a synthetic poly-Leu analog of SP-C (SP-C33) did not stabilize the alveoli at the end of expiration, as measured by low lung gas volumes of ∼5 ml/kg after 30 min of ventilation. However, treatment with phospholipids containing both SP-B and SP-C/SP-C33 approximately doubled lung gas volumes. Doubling the SP-C33 content did not affect lung gas volumes. The tidal volumes were similar in all groups receiving surfactant. This shows that SP-B and SP-C exert different physiological effects, since both proteins are needed to establish alveolar stability at end expiration in this animal model of respiratory distress syndrome, and that an optimal synthetic surfactant probably requires the presence of mimics of both SP-B and SP-C.

Characterization of bovine surfactant proteins B and C by electrospray ionization mass spectrometry

Bovine surfactant proteins B (SP-B) and C (SP-C) were analyzed by nano-electrospray ionization mass spectrometry (nano-ESI-MS). The observed molecular masses showed discrepancies compared to the calculated molecular masses using the published amino acid sequences. The number of cysteine residues in the published bovine SP-B amino acid sequences also failed to match the observed mass shift upon reduction of the SP-B dimer. To determine the amino acid sequences of two proteins, SP-B was first digested with trypsin and analyzed by liquid chromatography/tandem mass spectrometry (LC/MS/MS), while SP-C was analyzed by MS/MS in its intact form. The amino acid sequence of bovine SP-B determined here matches the observed molecular mass. The sequence is almost identical to the sheep SP-B except for two amino acid residues, consistent with the proximity of the two species. The correct sequence contains seven cysteine residues. Bovine SP-B exists as dimers and all cysteines are oxidized to form disulfide bonds in physiological conditions, which is in agreement with the observed mass shift upon reduction of the SP-B dimer. These cysteine residues are completely conserved across all species indicating their importance for the biological functions of this surfactant protein. The sequence of SP-C determined here also reveals an L to V substitution at its position 22 compared with the published bovine SP-B sequence.

Structure and conformation of the disulfide bond in dimeric lung surfactant peptides SP-B1–25 and SP-B8–25

Raman spectroscopy was used to determine the conformation of the disulfide linkage between cysteine residues in the homodimeric construct of the N-terminal alpha helical domain of surfactant protein B (dSP-B(1-25)). The conformation of the disulfide bond between cysteine residues in position 8 of the homodimer of dSP-B(1-25) was compared with that of a truncated homodimer (dSP-B(8-25)) of the peptide having a disulfide linkage at the same position in the alpha helix. Temperature-dependent Raman spectra of the S-S stretching region centered at approximately 500 cm(-1) indicated a stable, although highly strained disulfide conformation with a chi(CS-SC) dihedral angle of +/-10 degrees for the dSP-B(1-25) dimer. In contrast, the truncated dimer dSP-B(8-25) exhibited a series of disulfide conformations with the chi(CS-SC) dihedral angle taking on values of either +/-30 degrees or 85+/-20 degrees . For conformations with chi(CS-SC) close to the +/-90 degrees value, the Raman spectra of the 8-25 truncated dimers exhibited chi(SS-CC) dihedral angles of 90/180 degrees and 20-30 degrees . In the presence of a lipid mixture, both constructs showed a nu(S-S) band at approximately 488 cm(-1), corresponding to a chi(CS-SC) dihedral angle of +/-10 degrees . Polarized infrared spectroscopy was also used to determine the orientation of the helix and beta-sheet portion of both synthetic peptides. These calculations indicated that the helix was oriented primarily in the plane of the surface, at an angle of approximately 60-70 degrees to the surface normal, while the beta structure had approximately 40 degrees tilt. This orientation direction did not change in the presence of a lipid mixture or with temperature. These observations suggest that: (i) the conformational flexibility of the disulfide linkage is dependent on the amino acid residues that flank the cysteine disulfide bond, and (ii) in both constructs, the presence of a lipid matrix locks the disulfide bond into a preferred conformation.

Lipid composition greatly affects the in vitro surface activity of lung surfactant protein mimics

A crucial aspect of developing a functional, biomimetic lung surfactant (LS) replacement is the selection of the synthetic lipid mixture and surfactant proteins (SPs) or suitable mimics thereof. Studies elucidating the roles of different lipids and surfactant proteins in natural LS have provided critical information necessary for the development of synthetic LS replacements that offer performance comparable to the natural material. In this study, the in vitro surface-active behaviors of peptide- and peptoid-based mimics of the lung surfactant proteins, SP-B and SP-C, were investigated using three different lipid formulations. The lipid mixtures were chosen from among those commonly used for the testing and characterization of SP mimics--(1) dipalmitoyl phosphatidylcholine:palmitoyloleoyl phosphatidylglycerol 7:3 (w/w) (PCPG), (2) dipalmitoyl phosphatidylcholine:palmitoyloleoyl phosphatidylglycerol:palmitic acid 68:22:9 (w/w) (TL), and (3) dipalmitoyl phosphatidylcholine:palmitoyloleoyl phosphatidylcholine:palmitoyloleoyl phosphatidylglycerol:palmitoyloleoyl phosphatidylethanolamine:palmitoyloleoyl phosphatidylserine:cholesterol 16:10:3:1:3:2 (w/w) (IL). The lipid mixtures and lipid/peptide or lipid/peptoid formulations were characterized in vitro using a Langmuir-Wilhelmy surface balance, fluorescent microscopic imaging of surface film morphology, and a pulsating bubble surfactometer. Results show that the three lipid formulations exhibit significantly different surface-active behaviors, both in the presence and absence of SP mimics, with desirable in vitro biomimetic behaviors being greatest for the TL formulation. Specifically, the TL formulation is able to reach low-surface tensions at physiological temperature as determined by dynamic PBS and LWSB studies, and dynamic PBS studies show this to occur with a minimal amount of compression, similar to natural LS.

Hydrophobic surfactant proteins and their analogues

Lung surfactant is a complex mixture of phospholipids and four surfactant-associated proteins (SP-A, SP-B, SP-C and SP-D). Its major function in the lung alveolus is to reduce surface tension at the air-water interface in the terminal airways by the formation of a surface-active film enriched in surfactant lipids, hence preventing cellular collapse during respiration. Surfactant therapy using bovine or porcine lung surfactant extracts, which contain only polar lipids and native SP-B and SP-C, has dramatically improved the therapeutic outcomes of preterm infants with respiratory distress syndrome (RDS). One important goal of surfactant researchers is to replace animal-derived therapies with fully synthetic preparations based on SP-B and SP-C, produced by recombinant technology or peptide synthesis, and reconstituted with selected synthetic lipids. Here, we review recent research developments with peptide analogues of SP-B and SP-C, designed using either the known primary sequence and three-dimensional (3D) structure of the native proteins or, alternatively, the known 3D structures of closely homologous proteins. Such SP-B and SP-C mimics offer the possibility of studying the mechanisms of action of the respective native proteins, and may allow the design of optimized surfactant formulations for specific pulmonary diseases (e.g., acute lung injury (ALI) or acute respiratory distress syndrome (ARDS)). These synthetic surfactant preparations may also be a cost-saving therapeutic approach, with better quality control than may be obtained with animal-based treatments.

The role of charged amphipathic helices in the structure and function of surfactant protein B

Surfactant protein B (SP-B) is essential for normal lung surfactant function. Theoretical models predict that the disulfide cross-linked, N- and C-terminal domains of SP-B fold as charged amphipathic helices, and suggest that these adjacent helices participate in critical surfactant activities. This hypothesis is tested using a disulfide-linked construct (Mini-B) based on the primary sequences of the N- and C-terminal domains. Consistent with theoretical predictions of the full-length protein, both isotope-enhanced Fourier transform infrared (FTIR) spectroscopy and molecular modeling confirm the presence of charged amphipathic alpha-helices in Mini-B. Similar to that observed with native SP-B, Mini-B in model surfactant lipid mixtures exhibits marked in vitro activity, with spread films showing near-zero minimum surface tensions during cycling using captive bubble surfactometry. In vivo, Mini-B shows oxygenation and dynamic compliance that compare favorably with that of full-length SP-B. Mini-B variants (i.e. reduced disulfides or cationic residues replaced by uncharged residues) or Mini-B fragments (i.e. unlinked N- and C-terminal domains) produced greatly attenuated in vivo and in vitro surfactant properties. Hence, the combination of structure and charge for the amphipathic alpha-helical N- and C-terminal domains are key to SP-B function.

Simple, Helical Peptoid Analogs of Lung Surfactant Protein B

The helical, amphipathic surfactant protein, SP-B, is a critical element of pulmonary surfactant and hence is an important therapeutic molecule. However, it is difficult to isolate from natural sources in high purity. We have created and studied three different, nonnatural analogs of a bioactive SP-B fragment (SP-B(1-25)), using oligo-N-substituted glycines (peptoids) with simple, repetitive sequences designed to favor the formation of amphiphilic helices. For comparison, a peptide with a similar repetitive sequence previously shown to be a good SP mimic was also studied, along with SP-B(1-25) itself. Surface pressure-area isotherms, surfactant film phase morphology, and dynamic adsorption behavior all indicate that the peptoids are promising mimics of SP-B(1-25). The extent of biomimicry appears to correlate with peptoid helicity and lipophilicity. These biostable oligomers could serve in a synthetic surfactant replacement to treat respiratory distress syndrome.

Surfactant therapy for respiratory distress syndrome in premature neonates: a comparative review

Exogenous surfactant therapy has been part of the routine care of preterm neonates with respiratory distress syndrome (RDS) since the beginning of the 1990s. Discoveries that led to its development as a therapeutic agent span the whole of the 20th century but it was not until 1980 that the first successful use of exogenous surfactant therapy in a human population was reported. Since then, randomized controlled studies demonstrated that surfactant therapy was not only well tolerated but that it significantly reduced both neonatal mortality and pulmonary air leaks; importantly, those surviving neonates were not at greater risk of subsequent neurological impairment. Surfactants may be of animal or synthetic origin. Both types of surfactants have been extensively studied in animal models and in clinical trials to determine the optimum timing, dose size and frequency, route and method of administration. The advantages of one type of surfactant over another are discussed in relation to biophysical properties, animal studies and results of randomized trials in neonatal populations. Animal-derived exogenous surfactants are the treatment of choice at the present time with relatively few adverse effects related largely to changes in oxygenation and heart rate during surfactant administration. The optimum dose of surfactant is usually 100 mg/kg. The use of surfactant with high frequency oscillation and continuous positive pressure modes of respiratory support presents different problems compared with its use with conventional ventilation. The different components of surfactant have important functions that influence its effectiveness both in the primary function of the reduction of surface tension and also in secondary, but nonetheless just as important, role of lung defense. With greater understanding of the individual surfactant components, particularly the surfactant-associated proteins, development of newer synthetic surfactants has been made possible. Despite being an effective therapy for RDS, surfactant has failed to have a significant impact on the incidence of chronic lung disease in survivors. Paradoxically the cost of care has increased as surviving neonates are more immature and consume a greater proportion of neonatal intensive care resources. Despite this, surfactant is considered a cost-effective therapy for RDS compared with other therapeutic interventions in premature infants.

Conformational changes in SP-B as a function of surface pressure

X-ray reflectivity of bovine and sheep surfactant-associated protein B (SP-B) monolayers is used in conjunction with pressure-area isotherms and protein models to suggest that the protein undergoes changes in its tertiary structure at the air/water interface under the influence of surface pressure, indicating the likely importance of such changes to the phenomena of protein squeeze out as well as lipid exchange between the air-water interface and subphase structures. We describe an algorithm based on the well-established box- or layer-models that greatly assists the fitting of such unknown scattering-length density profiles, and which takes the available instrumental resolution into account. Scattering-length density profiles from neutron reflectivity of bovine SP-B monolayers on aqueous subphases are shown to be consistent with the exchange of a large number of labile protons as well as the inclusion of a significant amount of water, which is partly squeezed out of the protein monolayer at elevated surface pressures.

Topographical organization of the N-terminal segment of lung pulmonary surfactant protein B (SP-B(1-25)) in phospholipid bilayers

The location and depth of each residue of lung pulmonary surfactant protein B (SP-B(1-25)) in a phospholipid bilayer (PB) was determined by fluorescence quenching using synthesized single-residue-substituted peptides that were reconstituted into 1,2-dipalmitoyl phosphatidylcholine (DPPC)-enriched liposomes. The single-residue substitutions in peptides were either aspartate or tryptophan. The aspartate was subsequently labeled with the N-cyclohexyl-N'-(4-(dimethylamino)naphthyl)carbodiimide (NCD-4) fluorophore, whereas tryptophan is autofluorescent. Spin-labeled compounds, 5-doxylstearic acid (5-DSA), 7-doxylstearic acid (7-DSA), 12-doxylstearic acid (12-DSA), 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (CAT-16), and 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxy iodide (CAT-1), were used in the quenching experiments. The effective quenching order is determined by the accessibility of the quencher to a fluorescent group on the peptide. The order of quenching efficiency provides information about the relative locations of individual residues in the PB. Our data indicate that residues Phe1-Pro6 are located at the surface of PB, residues Tyr7-Trp9 are embedded in PB, and residues Leu10-Ile22 are involved in an amphipathic alpha-helix with its axis parallel to the surface of PB; residues Pro23-Gly25 reside at the surface. The effects of intermolecular disulfide bond formation in the SP-B(1-25) dimer were also investigated. The experiments suggest that the SP-B helix A has to rotate at an angle to form a disulfide bond with the neighboring cysteine, which makes the hydrophobic sides of the amphipathic helices face each other, thus forming a hydrophobic domain. The detailed topographical mapping of SP-B(1-25) and its dimer in PB provides new insights into the conformational organization of the lung pulmonary surfactant proteins in the environment that mimics the native state. The environment-specific conformational flexibility of the hydrophobic domain created by SP-B folding may explain the key functional properties of SP-B including their impact on phospholipid transport between the lipid phases and in modulating the cell inflammatory response during respiratory distress syndrome.

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.

Function of surfactant proteins B and C

SP-B is the only surfactant-associated protein absolutely required for postnatal lung function and survival. Complete deficiency of SP-B in mice and humans results in lethal, neonatal respiratory distress syndrome and is characterized by a virtual absence of lung compliance, highly disorganized lamellar bodies, and greatly diminished levels of SP-C mature peptide; in contrast, lung structure and function in SP-C null mice is normal. This review attempts to integrate recent findings in humans and transgenic mice with the results of in vitro studies to provide a better understanding of the functions of SP-B and SP-C and the structural basis for their actions.

Membrane activity of (Cys48Ser) lung surfactant protein B increases with dimerisation

One of the possible functions of lung surfactant protein B (SP-B), an hydrophobic membrane-associated saposin-like protein, is to reduce the alveolar surface tension by promoting insertion of phospholipids into the air/liquid interface of the lung. SP-B is a covalent homodimer; Cys48 of two polypeptides form an intermolecular disulphide bond. In order to test whether dimerisation of SP-B is important for surfactant function, transgenic mice which express (Cys48Ser) human SP-B in a mouse SP-B null background were generated. In previous studies (Cys48Ser)SP-B showed a concentration-dependent in vitro activity, suggesting that it may form non-covalent dimers. Here (Cys48Ser)SP-B isolated from bronchoalveolar lavage of transgenic mice was studied at different concentrations by circular dichroism (CD) spectroscopy, pulsating bubble surfactometry, mass spectrometry and reversed-phase HPLC. The results indicate that (Cys48Ser)SP-B, both in a phospholipid environment and in organic solvents, is largely monomeric and exhibits low activity at concentrations lower than 1 -2 microM, while at higher concentrations it forms non-covalent dimers, which are nearly functionally equivalent to native SP-B in vitro. Furthermore, electrospray mass spectrometry showed that more dimers were found relative to the monomer when the polarity of the solvent was decreased, and when the concentration of SP-B increased. (Cys48Ser)SP-B also eluted earlier than native SP-B in reversed-phase HPLC. Taken together, these results indicate that a polar surface is buried upon dimerisation, thereby promoting formation of interchain ion pairs between Glu51-Arg52' and Glu51'-Arg52.

The amoebapore superfamily

Amoebapores, synthesized by human protozoan parasites, form ion channels in target cells and artificial lipid membranes. The major pathogenic effect of these proteins is due to their cytolytic capability which results in target cell death. They comprise a coherent family and are homologous to other proteins and protein domains found in eight families. These families include in addition to the amoebapores (1) the saposins, (2) the NK-lysins and granulysins, (3) the pulmonary surfactant proteins B, (4) the acid sphingomyelinases, (5) acyloxyacyl hydrolases and (6) the aspartic proteases. These amoebapore homologues have many properties in common including membrane binding and stability. We note for the first time that a new protein, countin, from the cellular slime mold, Dictyostelium discoideum, comprises the eighth family within this superfamily. All currently sequenced members of these eight families are identified, and the structural, functional and phylogenetic properties of these proteins are discussed.

Surfactant protein B and C analogues

Mammalian lung surfactant is a mixture of phospholipids and four surfactant-associated proteins (SP-A, SP-B, SP-C, and SP-D). Its major function is to reduce surface tension at the air-water interface in the terminal airways by the formation of a surface-active film highly enriched in dipalmitoyl phosphatidylcholine (DPPC), thereby preventing alveolar collapse during expiration. SP-A and SP-D are large hydrophilic proteins, which play an important role in host defense, whereas the small hydrophobic peptides SP-B and SP-C interact with DPPC to generate and maintain a surface-active film. Surfactant replacement therapy with bovine and porcine lung surfactant extracts, which contain only polar lipids and SP-B and SP-C, has revolutionized the clinical management of premature infants with respiratory distress syndrome. Newer surfactant preparations will probably be based on SP-B and SP-C, produced by recombinant technology or peptide synthesis, and reconstituted with selected synthetic lipids. The development of peptide analogues of SP-B and SP-C offers the possibility to study their molecular mechanism of action and will allow the design of surfactant formulations for specific pulmonary diseases and better quality control. This review describes the hydrophobic peptide analogues developed thus far and their potential for use in a new generation of synthetic surfactant preparations.

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.

Identification of a novel sequence involved in lysosomal sorting of the sphingolipid activator protein prosaposin

Prosaposin is synthesized as a 53-kDa protein, post-translationally modified to a 65-kDa form and further glycosylated to a 70-kDa secretory product. The 65-kDa protein is associated to Golgi membranes and is targeted to lysosomes, where four smaller nonenzymatic saposins implicated in the hydrolysis of sphingolipids are generated by its partial proteolysis. The targeting of the 65-kDa protein to lysosomes is not mediated by the mannose 6-phosphate receptor. The Golgi apparatus appears to accomplish the molecular sorting of the 65-kDa prosaposin by decoding a signal from its amino acid backbone. This investigation deals with the characterization of the sequence involved in this process by deleting the saposin functional domains A, B, C, and D and the highly conserved N and C termini of prosaposin. The truncated cDNAs were subcloned into expression vectors and transfected to COS-7 cells. The destination of the mutated proteins was assessed by immunocytochemistry. Deletion of the C terminus did not interfere with the secretion of prosaposin but abolished its transport to lysosomes. Deletion of saposins and the N-terminal domain did not affect the lysosomal or secretory routing of prosaposin. A chimeric construct of albumin and the C terminus of prosaposin was not directed to lysosomes. However, albumin connected to the C terminus and one or more functional domains of prosaposin reached lysosomes, indicating that the C terminus and at least one saposin domain are required for this process. In summary, we are reporting a novel sequence involved in the targeting of prosaposin to lysosomes.

Conformational mapping of the N-terminal segment of surfactant protein B in lipid using 13C-enhanced Fourier transform infrared spectroscopy

Synthetic peptides based on the N-terminal domain of human surfactant protein B (SP-B1-25; 25 amino acid residues; NH2-FPIPLPYCWLCRALIKRIQAMIPKG) retain important lung activities of the full-length, 79-residue protein. Here, we used physical techniques to examine the secondary conformation of SP-B1-25 in aqueous, lipid and structure-promoting environments. Circular dichroism and conventional, 12C-Fourier transform infrared (FTIR) spectroscopy each indicated a predominate alpha-helical conformation for SP-B1-25 in phosphate-buffered saline, liposomes of 1-palmitoyl-2-oleoyl phosphatidylglycerol and the structure-promoting solvent hexafluoroisopropanol; FTIR spectra also showed significant beta- and random conformations for peptide in these three environments. In further experiments designed to map secondary structure to specific residues, isotope-enhanced FTIR spectroscopy was performed with 1-palmitoyl-2-oleoyl phosphatidylglycerol liposomes and a suite of SP-B1-25 peptides labeled with 13C-carbonyl groups at either single or multiple sites. Combining these 13C-enhanced FTIR results with energy minimizations and molecular simulations indicated the following model for SP-B1-25 in 1-palmitoyl-2-oleoyl phosphatidylglycerol: beta-sheet (residues 1-6), alpha-helix (residues 8-22) and random (residues 23-25) conformations. Analogous structural motifs are observed in the corresponding homologous N-terminal regions of several proteins that also share the 'saposin-like' (i.e. 5-helix bundle) folding pattern of full-length, human SP-B. In future studies, 13C-enhanced FTIR spectroscopy and energy minimizations may be of general use in defining backbone conformations at amino acid resolution, particularly for peptides or proteins in membrane environments.