Structure and functions of a dimeric form of surfactant protein SP-C: a Fourier transform infrared and surfactometry study

Exploring protein-protein interactions and oligomerization state of pulmonary surfactant protein C (SP-C) through FRET and fluorescence self-quenching

Pulmonary surfactant (PS) is a lipid-protein complex that forms films reducing surface tension at the alveolar air-liquid interface. Surfactant protein C (SP-C) plays a key role in rearranging the lipids at the PS surface layers during breathing. The N-terminal segment of SP-C, a lipopeptide of 35 amino acids, contains two palmitoylated cysteines, which affect the stability and structure of the molecule. The C-terminal region comprises a transmembrane α-helix that contains a ALLMG motif, supposedly analogous to a well-studied dimerization motif in glycophorin A. Previous studies have demonstrated the potential interaction between SP-C molecules using approaches such as Bimolecular Complementation assays or computational simulations. In this work, the oligomerization state of SP-C in membrane systems has been studied using fluorescence spectroscopy techniques. We have performed self-quenching and FRET assays to analyze dimerization of native palmitoylated SP-C and a non-palmitoylated recombinant version of SP-C (rSP-C) using fluorescently labeled versions of either protein reconstituted in different lipid systems mimicking pulmonary surfactant environments. Our results reveal that doubly palmitoylated native SP-C remains primarily monomeric. In contrast, non-palmitoylated recombinant SP-C exhibits dimerization, potentiated at high concentrations, especially in membranes with lipid phase separation. Therefore, palmitoylation could play a crucial role in stabilizing the monomeric α-helical conformation of SP-C. Depalmitoylation, high protein densities as a consequence of membrane compartmentalization, and other factors may all lead to the formation of protein dimers and higher-order oligomers, which could have functional implications under certain pathological conditions and contribute to membrane transformations associated with surfactant metabolism and alveolar homeostasis.

Pulmonary Surfactant: A Mighty Thin Film

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

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.

Surfactant protein C peptides with salt-bridges ("ion-locks") promote high surfactant activities by mimicking the α-helix and membrane topography of the native protein

Background. Surfactant protein C (SP-C; 35 residues) in lungs has a cationic N-terminal domain with two cysteines covalently linked to palmitoyls and a C-terminal region enriched in Val, Leu and Ile. Native SP-C shows high surface activity, due to SP-C inserting in the bilayer with its cationic N-terminus binding to the polar headgroup and its hydrophobic C-terminus embedded as a tilted, transmembrane α-helix. The palmitoylcysteines in SP-C act as ‘helical adjuvants’ to maintain activity by overriding the β-sheet propensities of the native sequences.

Objective. We studied SP-C peptides lacking palmitoyls, but containing glutamate and lysine at 4-residue intervals, to assess whether SP-C peptides with salt-bridges (“ion-locks”) promote surface activity by mimicking the α-helix and membrane topography of native SP-C.

Methods. SP-C mimics were synthesized that reproduce native sequences, but without palmitoyls (i.e., SP-Css or SP-Cff, with serines or phenylalanines replacing the two cysteines). Ion-lock SP-C molecules were prepared by incorporating single or double Glu⁻–Lys⁺ into the parent SP-C’s. The secondary structures of SP-C mimics were studied with Fourier transform infrared (FTIR) spectroscopy and PASTA, an algorithm that predicts β-sheet propensities based on the energies of the various β-sheet pairings. The membrane topography of SP-C mimics was investigated with orientated and hydrogen/deuterium (H/D) exchange FTIR, and also Membrane Protein Explorer (MPEx) hydropathy analysis. In vitro surface activity was determined using adsorption surface pressure isotherms and captive bubble surfactometry, and in vivo surface activity from lung function measures in a rabbit model of surfactant deficiency.

Results. PASTA calculations predicted that the SP-Css and SP-Cff peptides should each form parallel β-sheet aggregates, with FTIR spectroscopy confirming high parallel β-sheet with ‘amyloid-like’ properties. The enhanced β-sheet properties for SP-Css and SP-Cff are likely responsible for their low surfactant activities in the in vitro and in vivo assays. Although standard ¹²C-FTIR study showed that the α-helicity of these SP-C sequences in lipids was uniformly increased with Glu⁻–Lys⁺ insertions, elevated surfactant activity was only selectively observed. Additional results from oriented and H/D exchange FTIR experiments indicated that the high surfactant activities depend on the SP-C ion-locks recapitulating both the α-helicity and the membrane topography of native SP-C. SP-Css ion-lock 1, an SP-Css with a salt-bridge for a Glu⁻–Lys⁺ ion-pair predicted from MPEx hydropathy calculations, demonstrated enhanced surfactant activity and a transmembrane helix simulating those of native SP-C.

Conclusion. Highly active SP-C mimics were developed that replace the palmitoyls of SP-C with intrapeptide salt-bridges and represent a new class of synthetic surfactants with therapeutic interest.

Purifying selection drives the evolution of surfactant protein C (SP-C) independently of body temperature regulation in mammals

The pulmonary surfactant system of heterothermic mammals must be capable of dealing with the effect of low body temperatures on the physical state of the lipid components. We have shown previously that there is a modest increase in surfactant cholesterol during periods of torpor, however these changes do not fully explain the capacity of surfactant to function under the wide range of physical conditions imposed by torpor. Here we examine indirectly the role of surfactant protein C (SP-C) in adapting to variable body temperatures by testing for the presence of positive (adaptive) selection during evolutionary transitions between heterothermy and homeothermy. We sequenced SP-C from genomic DNA of 32 mammalian species from groups of closely related heterothermic and homeothermic species (contrasts). We used phylogenetic analysis by maximum likelihood estimates of rates of non-synonymous to synonymous substitutions and fully Bayesian inference of these sequences to determine whether the mode of body temperature regulation exerts a selection pressure driving the molecular adaptation of SP-C. The protein sequence of SP-C is highly conserved with synonymous or highly conservative amino acid substitutions being predominant. The evolution of SP-C among mammals is characterised by high codon usage bias and high rates of transition/transversion. The only contrast to show evidence of positive selection was that of the bears (Ursus americanus and U. maritimus). The significance of this result is unclear. We show that SP-C is under strong evolutionary constraints, driven by purifying selection, presumably to maintain protein function despite variation in the mode of body temperature regulation.

Molecular dynamics of surfactant protein C: from single molecule to heptameric aggregates

Surfactant protein C (SP-C) is a membrane-associated protein essential for normal respiration. It has been found that the alpha-helix form of SP-C can undergo, under certain conditions, a transformation from an alpha-helix to a beta-strand conformation that closely resembles amyloid fibrils, which are possible contributors to the pathogenesis of pulmonary alveolar proteinosis. Molecular dynamics simulations using the NAMD2 package were performed for systems containing from one to seven SP-C molecules to study their behavior in water. The results of our simulations show that unfolding of the protein occurs at the amino terminal, and despite this unfolding, no transition from alpha-helix to beta-strand was observed.

Surfactant protein C biosynthesis and its emerging role in conformational lung disease

Surfactant protein C (SP-C) is a hydrophobic 35-amino acid peptide that co-isolates with the phospholipid fraction of lung surfactant. SP-C represents a structurally and functionally challenging protein for the alveolar type 2 cell, which must synthesize, traffic, and process a 191-197-amino acid precursor protein through the regulated secretory pathway. The current understanding of SP-C biosynthesis considers the SP-C proprotein (proSP-C) as a hybrid molecule that incorporates structural and functional features of both bitopic integral membrane proteins and more classically recognized luminal propeptide hormones, which are subject to post-translational processing and regulated exocytosis. Adding to the importance of a detailed understanding of SP-C biosynthesis has been the recent association of mutations in the proSP-C sequence with chronic interstitial pneumonias in children and adults. Many of these mutations involve either missense or deletion mutations located in a region of the proSP-C molecule that has structural homology to the BRI family of proteins linked to inherited degenerative dementias. This review examines the current state of SP-C biosynthesis with a focus on recent developments related to molecular and cellular mechanisms implicated in the emerging role of SP-C mutations in the pathophysiology of diffuse parenchymal lung disease.

Effects of various forms of surfactant protein C on tidal volume in ventilated immature newborn rabbits

Surfactant protein (SP)-C is characterized by alpha-helix structure and palmitoyl groups attached to two cysteine residues. We examined the function of palmitoylation and dimerization in promotion of tidal volume in immature newborn rabbits. Reconstituted surfactants were made from a mixture of synthetic phospholipids and porcine SP-B (basic mixture) by adding various forms of SP-Cs: normal SP-C isolated from porcine lungs and monomeric or dimeric forms of SP-C. These latter two were isolated from patients with pulmonary alveolar proteinosis and were less palmitoylated. Animals were ventilated at an inspiratory pressure of 25 cmH2O. Median tidal volumes were <2 ml/kg in nontreated controls, 7.7 ml/kg in animals receiving the basic mixture without SP-C, and >18 ml/kg in animals treated with reconstituted surfactants containing 3% normal or 2% dimeric SP-C (P < 0.05 vs. basic mixture). The physiological effect of basic mixture was not improved by monomeric SP-C. We conclude that palmitoyl groups are important for the physiological effects of SP-C and that the dimeric form also improves physiological effects.

Effects of oligomerization and secondary structure on the surface behavior of pulmonary surfactant proteins SP-B and SP-C

The relationship among protein oligomerization, secondary structure at the interface, and the interfacial behavior was investigated for spread layers of native pulmonary surfactant associated proteins B and C. SP-B and SP-C were isolated either from butanol or chloroform/methanol lipid extracts that were obtained from sheep lung washings. The proteins were separated from other components by gel exclusion chromatography or by high performance liquid chromatography. SDS gel electrophoresis data indicate that the SP-B samples obtained using different solvents showed different oligomerization states of the protein. The CD and FTIR spectra of SP-B isolated from all extracts were consistent with a secondary structure dominated by alpha-helix. The CD and FTIR spectra of the first SP-C corresponded to an alpha-helical secondary structure and the spectra of the second SP-C corresponded to a mixture of alpha-helical and beta-sheet conformation. In contrast, the spectra of the third SP-C corresponded to antiparallel beta-sheets. The interfacial behavior was characterized by surface pressure/area (pi-A) isotherms. Differences in the oligomerization state of SP-B as well as in the secondary structure of SP-C all produce significant differences in the surface pressure/area isotherms. The molecular cross sections determined from the pi-A isotherms and from dynamic cycling experiments were 6 nm(2)/dimer molecule for SP-B and 1.15 nm(2)/molecule for SP-C in alpha-helical conformation and 1.05 nm(2)/molecule for SP-C in beta-sheet conformation. Both the oligomer ratio of SP-B and the secondary structure of SP-C strongly influence organization and behavior of these proteins in monolayer assemblies. In addition, alpha-helix --> beta-sheet conversion of SP-C occurs simply by an increase of the summary protein/lipid concentration in solution.

Heterozygosity for a surfactant protein C gene mutation associated with usual interstitial pneumonitis and cellular nonspecific interstitial pneumonitis in one kindred

Familial pulmonary fibrosis is a heterogeneous group of interstitial lung diseases of unknown cause that is associated with multiple pathologic subsets. Mutations in the surfactant protein C (SP-C) gene (SFTPC) are associated with familial desquamative and nonspecific interstitial pneumonitis. Genetic studies in familial usual interstitial pneumonitis have been inconclusive. Using a candidate gene approach, we found a heterozygous exon 5 + 128 T-->A transversion of SFTPC in a large familial pulmonary fibrosis kindred, including adults with usual interstitial pneumonitis and children with cellular nonspecific interstitial pneumonitis. The mutation is predicted to substitute a glutamine for a conserved leucine residue and may hinder processing of SP-C precursor protein. SP-C precursor protein displayed aberrant subcellular localization by immunostaining. Electron microscopy of affected lung revealed alveolar type II cell atypia, with numerous abnormal lamellar bodies. Mouse lung epithelial cells transfected with the SFTPC mutation were notable for similar electron microscopy findings and for exaggerated cellular toxicity. We show that an SFTPC mutation segregates with the pulmonary fibrosis phenotype in this kindred and may cause type II cellular injury. The presence of two different pathologic diagnoses in affected relatives sharing this mutation indicates that in this kindred, these diseases may represent pleiotropic manifestations of the same central pathogenesis.

Overexpression of surfactant protein-C mature peptide causes neonatal lethality in transgenic mice

Surfactant replacement preparations containing either surfactant protein (SP)-B or SP-C significantly improve lung function in surfactant-deficient infants, suggesting that these peptides may be functionally redundant. SP-B is absent and SP-C is greatly diminished in the airspaces of SP-B (-/-) mice, which die of respiratory distress syndrome (RDS) shortly after birth. The goal of this study was to determine if elevated expression of SP-C mature peptide could reverse the neonatal lethality in SP-B (-/-) mice. SP-C peptide (residues 24-57 of mouse SP-C proprotein) with a hemagglutinin epitope (SP-C(24-57)HA) was expressed in type II cells of transgenic mice, with the goal of crossing these animals into the SP-B (-/-) background. Unexpectedly, expression of the SP-C(24-57)HA transgene resulted in delayed/arrested lung development and lethal, neonatal RDS of all transgenic progeny in two independent transgenic lines. In transgenic mice, SP-C(24-57)HA was localized predominantly to the endoplasmic reticulum and Golgi; in contrast, SP-B and SP-C were very difficult to detect in the endoplasmic reticulum of wild-type mice. These results suggest that elevated expression of SP-C(24-57)HA in type II cells resulted in aggregation of SP-C in the early secretory pathway, leading to cytotoxicity and, ultimately, altered lung development.

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.

Free energy barrier estimation of unfolding the alpha-helical surfactant-associated polypeptide C

Molecular dynamics simulations were conducted to estimate the free energy barrier of unfolding surfactant-associated polypeptide C (SP-C) from an alpha-helical conformation. Experimental studies indicate that while the helical fold of SP-C is thermodynamically stable in phospholipid micelles, it is metastable in a mixed organic solvent of CHCl3/CH3OH/0.1 M HCl at 32:64:5 (v/v/v), in which it undergoes an irreversible transformation to an insoluble aggregate that contains beta-sheet. On the basis of experimental observations, the free energy barrier was estimated to be approximately 100 kJ/mole by applying Eyring's transition state theory to the experimental rate of unfolding [Protein Sci 1998;7:2533-2540]. These studies prompted us to carry out simulations to investigate the unwinding process of two helical turns encompassing residues 25-32 in water and in methanol. The results give an upper bound estimation for the free energy barrier of unfolding of SP-C of approximately 20 kJ/mole. The results suggest a need to reconsider the applicability of a single-mode activated process theory to protein unfolding.

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.

Pulmonary surfactant-associated polypeptide C in a mixed organic solvent transforms from a monomeric alpha-helical state into insoluble beta-sheet aggregates

In the 35-residue pulmonary surfactant-associated lipopolypeptide C (SP-C), the stability of the valyl-rich alpha-helix comprising residues 9-34 has been monitored by circular dichroism, nuclear magnetic resonance, and Fourier transform infrared spectroscopy in both a mixed organic solvent and in phospholipid micelles. The alpha-helical form of SP-C observed in freshly prepared solutions in a mixed solvent of CHCl3/CH3OH/0.1 M HCl 32:64:5 (v/v/v) at 10 degrees C undergoes within a few days an irreversible transformation to an insoluble aggregate that contains beta-sheet secondary structure. Hydrogen exchange experiments revealed that this conformational transition proceeds through a transition state with an Eyring free activation enthalpy of about 100 kJ mol(-1), in which the polypeptide segment 9-27 largely retains a helical conformation. In dodecylphosphocholine micelles, the helical form of SP-C was maintained after seven weeks at 50 degrees C. The alpha-helical form of SP-C thus seems to be the thermodynamically most stable state in this micellar environment, whereas its presence in freshly prepared samples in the aforementioned mixed solvent is due to a high kinetic barrier for unfolding. These observations support a previously proposed pathway for in vivo synthesis of SP-C through proteolytic processing from a 21-kDa precursor protein.

Structure and properties of surfactant protein C

Pulmonary surfactant contains less than 1 wt% of the very non-polar surfactant protein C (SP-C). In most animal species the major form of SP-C is a 35-residue peptide chain which contains two thioester-linked palmitoyl groups, giving a total molecular mass of 4.2 kDa. Several minor variants of SP-C exist, formed from N-terminal truncation, lysine palmitoylation, methionine oxidation and C-terminal esterification. The primary structure is evolutionarily conserved and SP-C appears to be the only constituent which is unique to pulmonary surfactant, indicating important and specific functions. The three-dimensional structure in an aqueous mixed organic solvent determined by NMR spectroscopy revealed one continuous 37 A long alpha-helix encompassing residues 9-34 as the only regular structural element. The central 23 A of the helix contains exclusively aliphatic residues with branched side-chains, mainly valines, and exposes an all-hydrophobic regular surface. The size of the entire helix perfectly matches the thickness of a fluid dipalmitoylphosphatidylcholine membrane, and the all-hydrophobic part of the helix matches the acyl-chain part of such a bilayer. This supports a transmembrane orientation of SP-C in pulmonary surfactant bilayers. In a phospholipid monolayer, the SP-C helix is tilted, thereby maximizing the interactions with the lipid acyl-chains also in this environment. The palmitoylcysteines of SP-C, which are located in the flexibly disordered N-terminal octapeptide segment, appear to be important both for integrity of the alpha-helical structure and for functional properties. Since the conformation of the N-terminal part in a phospholipid environment is not known, the mechanisms whereby the SP-C thioester-linked palmitoyl chains affect structure and function remain to be determined.

Removal of a dimeric form of surfactant protein C from mouse lungs: its acceleration by reduction

Clearance of hydrophobic surfactant-associated protein C (SP-C) and its dimeric form ([SP-C]2) was investigated. SP-C and [SP-C]2 obtained from proteinosis patients were fluorescently labeled and were instilled into mouse lungs as lipid-protein complexes. [SP-C]2 was removed more slowly than SP-C, with apparent half-lives of 30 and 18 h, respectively. A significant amount of [SP-C]2 was removed as SP-C, and the conversion rate was 0.22 micrograms.h-1.mouse-1. By correcting the removal as SP-C, we obtained 38 h for a possible half-life of [SP-C]2. Conversion from SP-C to [SP-C]2 seemed very slow. Decrease in glutathione (GSH) in the lung inhibited the conversion of [SP-C]2 to SP-C and GSH-treatment of liposomes accelerated clearance of [SP-C]2. These results suggest that the removal of [SP-C]2 from lung is accelerated by reduction and that GSH acts as a reducing agent in the lung.

Conformational equilibria of terminally blocked single amino acids at the water-hexane interface. A molecular dynamics study

The conformational equilibria of the acetyl and methyl amide terminally blocked L-alanine, L-leucine and L-glutamine amino acids are examined in vacuum, in bulk water, and at the water-hexane interface, using multi-nanosecond molecular dynamics simulations. The two-dimensional probability distribution functions of finding the peptides at different dihedral angles of the backbone, phi and psi, are calculated, and free energy differences between different conformational states are determined. All three peptides are interfacially active, i.e. tend to accumulate at the interface even though they are not amphiphilic. Conformational states stable in both gas phase and water are also stable in the interfacial environment. Their populations, however, cannot be simply predicted from the knowledge of conformational equilibria in the bulk phases, indicating that the interface exerts a unique effect on the peptides. Conformational preferences in the interfacial environment arise from the interplay between electrostatic and hydrophobic effects. As in an aqueous solution, electrostatic solute-solvent interactions lead to the stabilization of more polar peptide conformations. The hydrophobic effect is manifested at the interface by a tendency to segregate polar and nonpolar moieties of the solute into the aqueous and the hexane phases, respectively. For the terminally blocked glutamine, this favors conformations for which such a segregation is compatible with the formation of strong, backbone-side chain intramolecular hydrogen bonds on the hexane side of the interface. The influence of the hydrophobic effect can be also noted in the orientational preferences of the peptides at the interface. The terminally blocked leucine is oriented such that its nonpolar side chain is buried in hexane, whereas the polar side chain of glutamine is immersed in water. The free energies of rotating the peptides along the axis parallel to the interface by more than 90 degrees are substantial. This indicates that peptide folding at interfaces is strong by driven by the tendency to adopt amphiphilic structures.

Reverse-phase HPLC of the hydrophobic pulmonary surfactant proteins: detection of a surfactant protein C isoform containing Nepsilon-palmitoyl-lysine

A reverse-phase HPLC protocol for analysis of strictly hydrophobic peptides and proteins was developed. Peptide aggregation is minimized by using only 25-40% water in methanol or ethanol as initial solvents and subsequent elution with a gradient of propan-2-ol. Analysis of the pulmonary surfactant-associated proteins B (SP-B) and C (SP-C) with this method reveals several features. (1) SP-B and SP-C retain their secondary structures and separate by about 15 min over a 40 min gradient. SP-B is more hydrophilic than SP-C, which in turn behaves chromatographically like palmitoyl-ethyl ester. (2) SP-C exhibits isoforms additional to the major form characterized previously, which contains two thioester-linked palmitoyl groups. The isoforms now observed contain one or three palmitoyl moieties and constitute together 15-20% of the major form. The tripalmitoylated species contains a palmitoyl group linked to the epsilon-amino group of Lys-11, as concluded from the elution position,MS and amino acid sequence analysis. The tripalmitoylated form increases relative to the dipalmitoylated form on incubation of SP-C ina phospholipid environment. An Nepsilon-bound palmitoyl moiety constitutes a third mode of fatty acyl modification of proteins, in addition to the established Nalpha-bound myristoyl groups and S-bound palmitoyl chains. (3) The dimeric structure of SP-B, lacking covalent modifications, is confirmed by MS detection of the dimer. No SP-B isoforms were detected. (4) Denatured, non-helical SP-C can be distinguished chromatographically from the native alpha-helical peptide. (5) HPLC of SP-C at 60-75 degrees C reveals an isoform containing an extra 14 Da moiety compared with the main form. This is concluded to arise from inadvertent methyl esterification of the C-terminal carboxy group. In conclusion, this HPLC method affords a sensitive means of assessing modifications and conformations of SP-B or SP-C in different disease states and before functional studies. It might also prove useful for analysis of other strictly hydrophobic polypeptides.

Spiking Survanta with synthetic surfactant peptides improves oxygenation in surfactant-deficient rats

The hypothesis that the in vivo function of Survanta (Beractant) can be improved by supplementation with synthetic surfactant peptides B and C was tested in a surfactant-deficient rat model. Full length surfactant protein-B (SP-B1-78) (B) and palmitoylated surfactant protein-C (SP-C1-35) (C), and synthetic KL4 peptide were added to Survanta after extraction, creating extracted Survanta (ES) with 1% B, 2% B, and 2% B plus 1% C, or mixed with Survanta without extraction, creating modified Survanta (S) with 2% B, 2% B plus 1% C, and 2% KL4. Adult rats were ventilated with 100% oxygen, tidal volumes (VT) of 7.5 ml/kg and a rate of 60/min, and were lavaged until the PaO2 dropped below 80 mm Hg, when 100 mg/kg of surfactant was instilled. After 15 to 60 min of ventilation, pressure-volume (P-V) curves were generated in situ. Instillation of ES or S with 2% B plus 1% C led to the greatest increase in oxygenation, closely followed by ES and S with 2% B, and more distantly by S plus 2% KL4. TLC was comparable among the ES and S groups, but greater than that of air-placebo controls. These data suggest that spiking of Survanta with synthetic SP-B and SP-C increased oxygenation more effectively than B or KL4 alone in this surfactant-deficient rat model.

Molecular structures and interactions of pulmonary surfactant components

The dominating functional property of pulmonary surfactant is to reduce the surface tension at the alveolar air/liquid interface, and thereby prevent the lungs from collapsing at the end of expiration. In addition, the system exhibits host-defense properties. Insufficient amounts of pulmonary surfactant in premature infants causes respiratory distress syndrome, a serious threat which nowadays can be effectively treated by airway instillation of surfactant preparations. Surfactant is a mixture of many molecular species, mainly phospholipids and specific proteins, surfactant protein A (SP-A), SP-B, SP-C and SP-D. SP-A and SP-D are water-soluble and belong to the collectins, a family of large multimeric proteins which structurally exhibit collagenous/lectin hybrid properties and functionally are Ca2+-dependent carbohydrate binding proteins involved in innate host-defence functions. SP-A and SP-D also bind lipids and SP-A is involved in organization of alveolar surfactant phospholipids. SP-B belongs to another family of proteins, which includes also lipid-interacting polypeptides with antibacterial and lytic properties. SP-B is a 17.4-kDa homodimer and each subunit contains three intrachain disulphides and has been proposed to contain four amphipathic helices oriented pairwise in an antiparallel fashion. SP-A, SP-B and SP-D all have been detected also in the gastrointestinal tract. SP-C, in contrast, appears to be a unique protein with extreme structural and stability properties and to exist exclusively in the lungs. SP-C is a lipopeptide containing covalently linked palmitoyl chains and is folded into a 3.7-nm alpha-helix with a central 2.3-nm all-aliphatic part, making it perfectly suited to interact in a transmembranous way with a fluid bilayer composed of dipalmitoylglycerophosphocholine, the main component of surfactant. Homozygous genetic deficiency of proSP-B causes lethal respiratory distress soon after birth and is associated with aberrant processing of the precursor of SP-C. This review focuses on the chemical composition, structures and interactions of the pulmonary surfactant, in particular the associated proteins.

Acylation of pulmonary surfactant protein-C is required for its optimal surface active interactions with phospholipids

This study investigates the importance of thioester-linked acyl groups in lung surfactant protein C (SP-C) in facilitating interactions with phospholipids that yield functionally important surface active behaviors. Native SP-C, palmitoylated at cysteine residues at positions 5 and 6, was isolated from bovine lung surfactant by liquid chromatography. Deacylated SP-C (dSP-C), unchanged in composition and sequence from SP-C but having a decreased alpha-helical content in films with dipalmitoyl phosphatidylcholine (DPPC) of 52 versus 70%, was obtained by treatment with 0.1 M sodium carbonate buffer at pH 10. Surface activity was studied for SP-C and dSP-C combined with column-purified phospholipids (PPL) from calf lung surfactant or with synthetic phospholipids (DPPC or a synthetic phospholipid mixture (SPL) containing 50:35:15, DPPC:egg phosphatidylcholine:egg phosphatidylglycerol). Interfacial measurements included surface pressure time adsorption isotherms for dispersed surfactants with diffusion minimized, dynamic surface pressure area isotherms and respreading for films in the Wilhelmy balance, and overall surface tension lowering at physiologic cycling rate in oscillating bubble experiments. Dispersions of PPL:SP-C and SPL:SP-C rapidly adsorbed to high equilibrium surface pressures of 47-48 mN/m, significantly better than corresponding dispersions containing dSP-C. The adsorption of PPL:dSP-C was essentially unchanged from that of PPL alone, and the adsorption of SPL:dSP-C was improved only slightly over SPL alone. In Wilhelmy balance studies, dynamic respreading was significantly improved over phospholipids alone in films of SP-C plus PPL, SPL, or DPPC. Respreading was improved less markedly by dSP-C in corresponding films with SPL or DPPC and not at all in films with PPL. Maximum surface pressures were also higher in cycled films of SP-C versus dSP-C combined with PPL or SPL. In bubble experiments (37 degrees C, 20 cycles/min), dispersions of PPL:SP-C and SPL:SP-C reached low minimum surface tensions of <1 and 5 mN/m, respectively, whereas PPL:dSP-C and SPL:dSP-C only reached minima of approximately 20 mN/m as did PPL and SPL alone. Acylation in SP-C is crucial for its interactions with phospholipids over the full spectrum of adsorption and dynamic surface behaviors important for lung surfactant.

External reflection absorption infrared spectroscopy study of lung surfactant proteins SP-B and SP-C in phospholipid monolayers at the air/water interface

The interactions of the hydrophobic pulmonary surfactant proteins SP-B and SP-C with 1,2-dipalmitoylphosphatidylcholine in mixed, spread monolayer films have been studied in situ at the air/water interface with the technique of external reflection absorption infrared spectroscopy (IRRAS). SP-C has a mostly alpha-helical secondary structure both in the pure state and in the presence of lipids, whereas SP-B secondary structure is a mixture of alpha-helical and disordered forms. When films of SP-B/1,2-dipalmitoylphosphatidylcholine are compressed to surface pressures (pi) greater than approximately 40-43 mN/m, the protein is partially (15-35%) excluded from the surface, as measured by intensity ratios of the peptide bond amide l/lipid C==O stretching vibrations. The extent of exclusion increases as the protein/lipid ratio in the film increases. In contrast, SP-C either remains at the surface at high pressures or leaves accompanied by lipids. The amide l peak of SP-C becomes asymmetric as a result of the formation of intermolecular sheet structures (1615-1630 cm-1) suggestive of peptide aggregation. The power of the IRRAS experiment for determination of film composition and molecular structure, i.e., as a direct test of the squeeze-out hypothesis of pulmonary surfactant function, is evident from this work.

Fourier-transform infrared spectroscopic analysis of rabbit lung surfactant: subfraction-associated phospholipid and protein profiles

Surfactant obtained from bronchoalveolar lavage (BAL) can be separated into subfractions based on sedimentation characteristics. It has been suggested that the 10,000 x g, 60,000 x g and 100,000 x g subfractions isolated by this approach represent stages of surfactant extracellular processing. These three subfractions have been reported to differ in their morphology, composition and ability to lower surface tension. We wished to determine if infrared spectroscopy, which may be applied as a non-invasive technique could potentially prove useful for characterization and quantification of bronchoalveolar lavage (BAL) protein and phospholipid, and if this approach could detect differences in intermediate surfactant processing stages. Subfractions were collected from adult rabbit lungs by BAL and differential centrifugation and analyzed by Fourier transform infrared (FT-IR) spectroscopy. Biochemical assay of phospholipid and protein showed differences between subfractions that correlated well with the phospholipid/protein ratios obtained from FT-IR spectra (r = 0.939; r2 = 0.882). The subfraction sedimenting at 100,000 x g (P100) exhibited spectral shifts in the Amide I band, suggesting that the protein secondary structure was different compared to other fractions. Spectra obtained after separation of lipids and protein components showed an apparent disordering of protein secondary structure but little or no effect on the structure or mobility of phospholipids. These results support the idea that subfractions represent various processing stages of surfactant. In addition, they show that results from FT-IR analyses correlate significantly with traditional biochemical assay methods which may prove of clinical use.(ABSTRACT TRUNCATED AT 250 WORDS)

Neutralization of the positive charges of surfactant protein C. Effects on structure and function

Pulmonary surfactant protein C (SP-C) is a small, extremely hydrophobic peptide with a highly conservative primary structure. The protein is characterized by two adjacent palmitoylated cysteine residues, two positively charged residues (one arginine residue and one lysine residue) in the N-terminal region, and a long hydrophobic stretch. SP-C enhances the adsorption of phospholipids into an air-water interface. To determine the importance of the positively charged residues, we carried out experiments with natural porcine SP-C and modified porcine SP-C (SP-Cm) in which the positive charges had been blocked by phenylglyoxal. Circular dichroism experiments showed that SP-Cm had an increased content of alpha-helix. Natural SP-C, but not SP-Cm, catalyzed insertion of phospholipids into a monolayer at the airwater interface. This reduced insertion was due to a strong reduction of binding of phospholipid vesicles to the monolayer. The insertion catalyzed by the natural porcine SP-C was decreased by an increased pH of the subphase. In contrast to natural SP-C, SP-Cm induced lipid mixing between phospholipid vesicles. The extent of lipid mixing was a function of the SP-C content. We conclude that the positively charged residues of SP-C are important for the binding of phospholipid vesicles to the monolayer, a process that precedes the insertion of phospholipids into the monolayer.

Exclusion of SP-C, but not SP-B, by gel phase palmitoyl lipids

The interactions of the hydrophobic pulmonary surfactant proteins, SP-C and SP-B, with lipid bilayers were assessed by fluorescence energy transfer. SP-C and SP-B were labeled with the fluorescent probe, succinimidyl nitrobenzoxadiazolyl amino hexanoate (NBD). Fluorescence energy transfer from NBD-SP-C and NBD-SP-B to four distinct indocarbocyanine probes (CnDiI) was utilized to determine the association of the surfactant proteins with various lipid acyl chains. In lipid mixtures including DPPC and DPPG, SP-C was associated with shorter chain and unsaturated lipids below the bulk lipid phase transition. Longer chain saturated CnDiI were excluded from SP-C aggregates. In contrast, SP-B demonstrated little acyl chain preference. The association of SP-C with shorter chain and unsaturated lipids below the bulk phase transition is interpreted to arise from a mismatch in the length of the hydrophobic region of the SP-C alpha-helix relative to the length of the hydrophobic region of dipalmitoyl lipids in the gel phase.

Secondary structure and biophysical activity of synthetic analogues of the pulmonary surfactant polypeptide SP-C

Native pulmonary-surfactant-associated lipopolypeptide SP-C, its chemically depalmitoylated form and several synthetic analogues lacking the palmitoylcysteine residues were analysed for secondary structure in phospholipid micelles and for biophysical activity in 1,2-dipalmitoyl-sn-glycero-3- phosphocholine/phosphatidylglycerol/palmitic acid (68:22:9, by wt.). Compared with the native molecule, with the entire poly-valyl part in a known alpha-helical conformation, depalmitoylated SP-C was found to be still mainly alpha-helical, but with an approx. 20% decrease in the helical content. A synthetic hybrid polypeptide where the entire poly-valyl alpha-helical part of native SP-C had been replaced with the amino acid sequence of a transmembrane helix of bacteriorhodopsin is also predominantly alpha-helical. In contrast, synthetic SP-C analogues lacking only the palmitoyl groups, by replacement of the palmitoylcysteine residues with cysteine, phenylalanine or serine, or lacking the positively charged amino acids by replacement with alanine, are considerably less alpha-helical than both native and depalmitoylated SP-C. The data indicate that the SP-C palmitoyl groups are important for maintenance of the alpha-helical conformation in parts of the polypeptide, and that the poly-valyl alpha-helical conformation is not fully formed in synthetic SP-C polypeptides. Furthermore, the helical structure of both native and depalmitoylated SP-C in dodecylphosphocholine micelles is very resistant to thermal denaturation, exhibiting ordered structure at 90 degrees C. The alpha-helical content grossly parallels the peptide-induced acceleration of the spreading of phospholipids at an air/water interface and the increase of surface pressure. The data suggest that the alpha-helical conformation itself, rather than just the covalent structure, is of prime importance for the biological function of synthetic pulmonary-surfactant peptides.

The effect of environment on the stability of an integral membrane helix: molecular dynamics simulations of surfactant protein C in chloroform, methanol and water

A series of three molecular dynamics simulations at 300 K in explicit solvent environments of chloroform, methanol and water has been performed on the pulmonary surfactant lipoprotein, SP-C, comprising several consecutive valine residues in order to investigate the stability of the alpha-helical conformation. Two additional simulations were performed on truncated SP-C with a five-residue N-terminal deletion at 300 K and 500 K in water, the high temperature run in order to increase the rate of peptide denaturation. Indications of destabilization appear in chloroform during 1 ns while the SP-C alpha-helix is remarkably stable during 1 ns in methanol and water. In particular the polyvalyl part comprising residues Val15 to Val21 remains intact even at elevated temperature, and the valines do not disrupt the alpha-helical conformation. The valyl-rotamer sampling is partly restricted. Unfolding takes place successively along the primary sequence starting from the C-terminal end. Factors affecting polypeptide stability in molecular dynamics simulations are addressed. The intrinsic helix-forming tendency of valine residues and its dependence on the sequence context, and the role of the solvent environment in stabilizing or destabilizing an alpha-helical fold, are discussed.

Conformational flexibility of pulmonary surfactant proteins SP-B and SP-C, studied in aqueous organic solvents

The structure of hydrophobic pulmonary surfactant-associated proteins SP-B and SP-C have been studied in different acetonitrile (ACN)/water and trifluorethanol (TFE)/water mixtures by circular dichroism and fluorescence spectroscopy to analyze the conformational flexibility of these proteins in response to changes in solvent composition. SP-B presented a very stable conformation in all the assayed ACN/water mixtures and in TFE/water mixtures containing until 70% TFE, showing around 40% alpha-helix. When SP-B was transferred to mixtures containing more than 70% TFE, the percent of alpha-helix in SP-B increased up to 60%. The fluorescence emission spectra of SP-B in the different solvents showed that tryptophan residues are more sensitive to solvent changes than those of tyrosine, reflecting differential effects on different protein microenvironments. The effect of solvent changes on the two tryptophan populations detected by fluorescence spectra was also different. A model for the folding of SP-B dimers, dominated by intra- and intermolecular disulphide bonds, is proposed. Surfactant protein SP-C revealed a secondary structure much more sensitive to solvent composition than SP-B. It had a main alpha-helical conformation in ACN/water solvents which was up to 63% in mixtures containing more than 60% ACN. When the protein was transferred to solvents containing less than 60% ACN, its secondary structure possessed less percent of alpha-helix and an increased percent of beta-structure. On the other hand, SP-C had a main beta-sheet secondary structure in all the assayed TFE/water mixtures, with 30-40% alpha-helix and around 50% beta-structure. The strong dependence of SP-C conformation on the nature of the solvent is interpreted to arise from its high hydrophobicity and the possible occurrence of protein-protein interactions.

Characterization of a dimeric canine form of surfactant protein C (SP-C)

Canine pulmonary surfactant protein C (SP-C) is a small hydrophobic peptide which has one palmitoylated cysteine residue. SP-C enhances the insertion of phospholipids into a monolayer. Two forms of canine SP-C were isolated using Sephadex LH-60 chromatography. It was found that canine SP-C exists in a palmitoylated monomeric form of 3.5 kDa, and a non-acylated dimeric form of 7 kDa. Circular dichroism showed that both forms of SP-C exhibit similar secondary structures at the air/water interface. Both forms of SP-C were able to induce the insertion of phospholipids into a monolayer as measured with the Wilhelmy plate technique. In contrast to the palmitoylated monomeric form of SP-C, the non-acylated dimeric form of SP-C does not require calcium ions to insert phospholipids into a monolayer without the negatively charged phosphatidylglycerol. It is concluded that two forms of canine SP-C exist, but the physiological significance of these different forms remains to be established.

The NMR structure of the pulmonary surfactant-associated polypeptide SP-C in an apolar solvent contains a valyl-rich alpha-helix

The nuclear magnetic resonance (NMR) structure of the pulmonary surfactant-associated lipoplypeptide C (SP-C) was determined in a mixed solvent of C2H3Cl/C2H3OH/ 1 M HCl 32:64:5 (v/v). Sequence-specific 1H NMR assignments and the collection of conformational constraints were achieved with two-dimensional 1H NMR, and the structure was calculated with the distance geometry program DIANA. The root mean square deviations for the well-defined polypeptide segment of residues 9-34 calculated for the 20 best energy-minimized DIANA conformers relative to their mean are 0.5 and 1.3 A for the polypeptide backbone atoms N, C alpha, and C', and for all heavy atoms, respectively. The 35-residue polypeptide chain of SP-C forms an alpha-helix between positions 9 and 34, which includes two segments of seven and four consecutive valyls that are separated by a single leucyl residue. The N-terminal hexapeptide segment, which includes two palmitoylcysteinyls, is flexibly disordered. The length of the alpha-helix is about 37 A, and the helical segment of residues 13-28, which contains exclusively aliphatic residues with branched side chains, is 23-A long and about 10 A in diameter. The alpha-helix is outstandingly regular, with virtually identical chi 1 angles for all valyl residues. The observation of a helical structure of SP-C was rather unexpected, considering that Val is generally underrepresented in alpha-helices, and it provides intriguing novel insights into the structural basis of SP-C functions as well as into general structural aspects of protein-lipid interactions in biological membranes.

Utilization of modified surfactant-associated protein B for delivery of DNA to airway cells in culture

Pulmonary surfactant lines the airway epithelium and creates a potential barrier to successful transfection of the epithelium in vivo. Based on the functional properties of pulmonary surfactant protein B (SP-B) and the fact that this protein is neither toxic nor immunogenic in the airway, we hypothesized that SP-B could be modified to deliver DNA to airway cells. We have modified native bovine SP-B by the covalent linkage of poly(lysine) (average molecular mass of 3.3 or 10 kDa) to the N terminus of SP-B and formed complexes between a test plasmid and the modified SP-B. Transfection efficiency was determined by transfection of pulmonary adenocarcinoma cells (H441) in culture with the test plasmid pCPA-RSV followed by measurement of activity of the reporter gene encoding chloramphenicol acetyltransferase (CAT). Transfections were performed with DNA.protein complexes using poly(lysine)10kDa-SP-B ([Lys]10kDa-SP-B) or poly(lysine)3.3kDa-SP-B ([Lys]3.3kDa-SP-B), and results were compared with transfections using unmodified poly(lysine).DNA, unmodified SP-B.DNA, or DNA only. For [Lys]10kDa-SP-B.pCPA-RSV preparations, CAT activity was readily detectable above the background of [Lys]3.3kDa-SP-B or unmodified SP-B. The SP-B-poly(lysine) conjugates were effective over a broad range of protein-to-DNA molar ratios, although they were optimal at approximately 500:1-1000:1. Transfection efficiency varied with the tested cell line but was not specific to airway cells. Addition of replication-defective adenovirus to the [Lys]10kDa-SP-B.pCPA-RSV complex enhanced CAT activity about 30-fold with respect to that produced by the [Lys]10kDa-SP-B.pCPA-RSV complex alone. This increase suggests routing of the adenoviral.[Lys]10kDa-SP-B.pCPA-RSV complex through an endosomal pathway. Effects of covalent modification on the secondary structure of SP-B were examined by Fourier transform infrared spectrometry (FTIR). Results of FTIR indicated that the conformation of [Lys]10kDa-SP-B was comprised primarily of alpha-helical structure compared with a predominantly aggregated structure of unmodified poly(lysine). We conclude that poly(lysine) conjugates of SP-B effectively deliver DNA in vitro and may have utility as DNA delivery vehicles to the airway in vivo.

Production of immortalized distal respiratory epithelial cell lines from surfactant protein C/simian virus 40 large tumor antigen transgenic mice

Murine lung epithelial (MLE) cell lines representing the distal bronchiolar and alveolar epithelium were produced from lung tumors generated in transgenic mice harboring the viral oncogene simian virus 40 (SV40) large tumor antigen under transcriptional control of a promoter region from the human surfactant protein C (SP-C) gene. The cell lines exhibited rapid growth, lack of contact inhibition, and an epithelial cell morphology for 30-40 passages in culture. Microvilli, cytoplasmic multivesicular bodies, and multilamellar inclusion bodies (morphologic characteristics of alveolar type II cells) were detected in some of the MLE cell lines by electron microscopic analysis. The MLE cells also maintained functional characteristics of distal respiratory epithelial cells including the expression of surfactant proteins and mRNAs and the ability to secrete phospholipids. Expression of the exogenous SV40 large tumor antigen gene was detected in all of the generated cell lines. The SP-C/SV40 large tumor antigen transgenic mice and the MLE cell lines will be useful for the study of pulmonary surfactant production and regulation as well as lung development and tumorigenesis.

Lipid effects on aggregation of pulmonary surfactant protein SP-C studied by fluorescence energy transfer

The self-association of pulmonary surfactant protein SP-C in lipid vesicles was studied using fluorescence energy transfer. Bovine SP-C was labeled with two fluorescent probes, succinimidyl 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]hexanoate and eosin isothiocyanate, on the amino terminus of the protein, producing NBD-SP-C and EITC-SP-C, respectively. The N-terminus of SP-C was relatively immobile between 20 and 37 degrees C, as demonstrated by high fluorescence anisotropy of NBD-SP-C and EITC-SP-C. The mobility increased at the transition of the lipid to the fluid phase. Using fluorescence energy transfer, with NBD-SP-C as the donor and EITC-SP-C as the acceptor, a high degree of SP-C/SP-C association was found below 25 degrees C, decreasing to very little self-association above 42 degrees C in 7:1 1,2-dipalmitoylphosphatidylcholine-1,2-dipalmitoylphosphatidylglycerol (DPPC-DPPG) vesicles. The fraction of SP-C aggregated below 37 degrees C in 7:1 DPPC-DPPG was estimated from the observed energy transfer to be more than 70% of total SP-C. In various lipid mixtures, self-association of SP-C was dependent on the presence of at least some gel-phase lipids. In a lipid mixture resembling pulmonary surfactant, gradually increasing self-association was observed below 38 degrees C. The relation of the present data to the state of aggregation of SP-C in pulmonary surfactant is discussed.