The palmitoyl groups of lung surfactant protein C reduce unfolding into a fibrillogenic intermediate

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.

Role of pulmonary surfactant protein Sp-C dimerization on membrane fragmentation: An emergent mechanism involved in lung defense and homeostasis

Surfactant protein C (SP-C) is a protein present in the pulmonary surfactant system that is involved in the biophysical properties of this lipoprotein complex, but it also has a role in lung defense and homeostasis. In this article, we propose that the link between both functions could rely on the ability of SP-C to induce fragmentation of phospholipid membranes and generate small vesicles that serve as support to present different ligands to cells in the lungs. Our results using bimolecular fluorescence complementation and tunable resistive pulse sensing setups suggest that SP-C oligomerization could be the triggering event that causes membrane budding and nanovesiculation. As shown by fluorescence microscopy and flow cytometry, these vesicles are differentially assimilated by alveolar macrophages and alveolar type II cells, indicating distinct roles of these alveoli-resident cells in the processing of the SP-C- induced vesicles and their cargo. These results depict a more accurate picture of the mechanisms of this protein, which could be relevant for the comprehension of pulmonary pathologies and the development of new therapeutic approaches.

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.

Divide & Conquer: Surfactant Protein SP-C and Cholesterol Modulate Phase Segregation in Lung Surfactant

Lung surfactant (LS) is an essential system supporting the respiratory function. Cholesterol can be deleterious for LS function, a condition that is reversed by the presence of the lipopeptide SP-C. In this work, the structure of LS-mimicking membranes has been analyzed under the combined effect of SP-C and cholesterol by deuterium NMR and phosphorus NMR and by electron spin resonance. Our results show that SP-C induces phase segregation at 37°C, resulting in an ordered phase with spectral features resembling an interdigitated state enriched in dipalmitoylphosphatidylcholine, a liquid-crystalline bilayer phase, and an extremely mobile phase consistent with small vesicles or micelles. In the presence of cholesterol, POPC and POPG motion seem to be more hindered by SP-C than dipalmitoylphosphatidylcholine. The use of deuterated cholesterol did not show signs of specific interactions that could be attributed to SP-C or to the other hydrophobic surfactant protein SP-B. Palmitoylation of SP-C had an indirect effect on the extent of protein-lipid perturbations by stabilizing SP-C structure, and seemed to be important to maximize differences among the lipids participating in each phase. These results shed some light on how SP-C-induced lipid perturbations can alter membrane structure to sustain LS functionality at the air-liquid interface.

Computer simulation study of amyloid fibril formation by palindromic sequences in prion peptides

We simulate the aggregation of large systems containing palindromic peptides from the Syrian hamster prion protein SHaPrP 113-120 (AGAAAAGA) and the mouse prion protein MoPrP 111-120 (VAGAAAAGAV) and eight sequence variations: GAAAAAAG, (AG)(4) , A8, GAAAGAAA, A10, V10, GAVAAAAVAG, and VAVAAAAVAV The first two peptides are thought to act as the Velcro that holds the parent prion proteins together in amyloid structures and can form fibrils themselves. Kinetic events along the fibrillization pathway influence the types of structures that occur and variations in the sequence affect aggregation kinetics and fibrillar structure. Discontinuous molecular dynamics simulations using the PRIME20 force field are performed on systems containing 48 peptides starting from a random coil configuration. Depending on the sequence, fibrillar structures form spontaneously over a range of temperatures, below which amorphous aggregates form and above which no aggregation occurs. AGAAAAGA forms well organized fibrillar structures whereas VAGAAAAGAV forms less well organized structures that are partially fibrillar and partially amorphous. The degree of order in the fibrillar structure stems in part from the types of kinetic events leading up to its formation, with AGAAAAGA forming less amorphous structures early in the simulation than VAGAAAAGAV. The ability to form fibrils increases as the chain length and the length of the stretch of hydrophobic residues increase. However as the hydrophobicity of the sequence increases, the ability to form well-ordered structures decreases. Thus, longer hydrophobic sequences form slightly disordered aggregates that are partially fibrillar and partially amorphous. Subtle changes in sequence result in slightly different fibril structures.

Palmitoylation of pulmonary surfactant protein SP-C is critical for its functional cooperation with SP-B to sustain compression/expansion dynamics in cholesterol-containing surfactant films

Recent data suggest that a functional cooperation between surfactant proteins SP-B and SP-C may be required to sustain a proper compression-expansion dynamics in the presence of physiological proportions of cholesterol. SP-C is a dually palmitoylated polypeptide of 4.2 kDa, but the role of acylation in SP-C activity is not completely understood. In this work we have compared the behavior of native palmitoylated SP-C and recombinant nonpalmitoylated versions of SP-C produced in bacteria to get a detailed insight into the importance of the palmitic chains to optimize interfacial performance of cholesterol-containing surfactant films. We found that palmitoylation of SP-C is not essential for the protein to promote rapid interfacial adsorption of phospholipids to equilibrium surface tensions (∼22 mN/m), in the presence or absence of cholesterol. However, palmitoylation of SP-C is critical for cholesterol-containing films to reach surface tensions ≤1 mN/m at the highest compression rates assessed in a captive bubble surfactometer, in the presence of SP-B. Interestingly, the ability of SP-C to facilitate reinsertion of phospholipids during expansion was not impaired to the same extent in the absence of palmitoylation, suggesting the existence of palmitoylation-dependent and -independent functions of the protein. We conclude that palmitoylation is key for the functional cooperation of SP-C with SP-B that enables cholesterol-containing surfactant films to reach very low tensions under compression, which could be particularly important in the design of clinical surfactants destined to replacement therapies in pathologies such as acute respiratory distress syndrome.

Alterations of the C-terminal end do not affect in vitro or in vivo activity of surfactant protein C analogs

The secondary structure, orientation and hydrogen/deuterium exchange of SP-C33, a surfactant protein C analog, in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/egg phosphatidylglycerol (8:2, wt./wt.) bilayers, was studied by attenuated total reflection Fourier transform infrared spectroscopy. This showed a transmembrane α-helix, in which about 55% of the amide hydrogens do not exchange for up to 20 h. Moreover, C-terminally modified SP-C33, either truncated after position 30, or having the methionine at position 31 exchanged for either lysine or isoleucine, showed the same secondary structure and orientation. The different peptides, suspended in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol 68:31 (wt./wt.), were tested for surfactant activity in vitro in a captive bubble surfactometer and in vivo in an animal model of respiratory distress syndrome using premature rabbit fetuses. All preparations showed similar surface activity in the captive bubble surfactometer. Also, in the rabbit model, all preparations performed equally well and significantly better than non-treated controls, both regarding tidal volumes and lung gas volumes. Thus, truncation or residue replacements in the C-terminal part of SP-C33 do not seem to affect membrane association or surfactant activity.

Pulmonary surfactant pathophysiology: current models and open questions

Pulmonary surfactant is an essential lipid-protein complex that stabilizes the respiratory units (alveoli) involved in gas exchange. Quantitative or qualitative derangements in surfactant are associated with severe respiratory pathologies. The integrated regulation of surfactant synthesis, secretion, and metabolism is critical for air breathing and, ultimately, survival. The goal of this review is to summarize our current understanding and highlight important knowledge gaps in surfactant homeostatic mechanisms.

Stabilization of conformationally dynamic helices by covalently attached acyl chains

Acylation of proteins is known to mediate membrane attachment and to influence subcellular sorting. Here, we report that acylation can stabilize secondary structure. Circular dichroism spectroscopy showed that N-terminal attachment of acyl chains decreases the ability of an intrinsically flexible hydrophobic model peptide to refold from an alpha-helical state to beta-sheet in response to changing solvent conditions. Acylation also stabilized the membrane-embedded alpha-helix. This increase of global helix stability did not result from decreased local conformational dynamics of the helix backbone as assessed by deuterium/hydrogen-exchange experiments. We concluded that acylation can stabilize the structure of intrinsically dynamic helices and may thus prevent misfolding.

The Brichos domain of prosurfactant protein C can hold and fold a transmembrane segment

Prosurfactant protein C (proSP-C) is a 197-residue integral membrane protein, in which the C-terminal domain (CTC, positions 59-197) is localized in the endoplasmic reticulum (ER) lumen and contains a Brichos domain (positions 94-197). Mature SP-C corresponds largely to the transmembrane (TM) region of proSP-C. CTC binds to SP-C, provided that it is in nonhelical conformation, and can prevent formation of intracellular amyloid-like inclusions of proSP-C that harbor mutations linked to interstitial lung disease (ILD). Herein it is shown that expression of proSP-C (1-58), that is, the N-terminal propeptide and the TM region, in HEK293 cells results in virtually no detectable protein, while coexpression of CTC in trans yields SDS-soluble monomeric proSP-C (1-58). Recombinant human (rh) CTC binds to cellulose-bound peptides derived from the nonpolar TM region, but not the polar cytosolic part, of proSP-C, and requires >/=5-residues for maximal binding. Binding of rhCTC to a nonhelical peptide derived from SP-C results in alpha-helix formation provided that it contains a long TM segment. Finally, rhCTC and rhCTC Brichos domain shows very similar substrate specificities, but rhCTC(L188Q), a mutation linked to ILD is unable to bind all peptides analyzed. These data indicate that the Brichos domain of proSP-C is a chaperone that induces alpha-helix formation of an aggregation-prone TM region.

Nedd4-2-mediated ubiquitination facilitates processing of surfactant protein-C

We previously proposed a model of surfactant protein (SP)-C biosynthesis in which internalization of the proprotein from the limiting membrane of the multivesicular body to internal vesicles represents a key step in the processing and secretion of SP-C. To test this hypothesis, alanine mutagenesis of the N-terminal propeptide of SP-C was performed. Adenoviruses encoding mutant proproteins were infected into type II cells isolated from Sftpc(-/-) mice, and media analyzed for secreted SP-C 24 hours after infection. Mutation of S(12)PPDYS(17) completely blocked secretion of SP-C. PPDY (PY motif) has previously been shown to bind WW domains of neural precursor cell-expressed developmentally down-regulated (Nedd) 4-like E3 ubiquitin ligases. Purified recombinant glutathione S-transferase-SP-C propeptide (residues 1-35) bound recombinant Nedd4-2 strongly, and Nedd4 weakly; the S(12)PPDYS(17)mutation abrogated binding of SP-C to Nedd4-2. Immobilized recombinant Nedd4-2 WW domain captured SP-C proprotein from mouse type II cell lysates; in the reverse pulldown, endogenous SP-C in type II cells was captured by recombinant Nedd4-2. To determine if the interaction of Nedd4-2 and SP-C resulted in ubiquitination, the SP-C proprotein was immunoprecipitated from transiently transfected human embryonic kidney 293 cells, and analyzed by SDS-PAGE/Western blotting with ubiquitin antibody. Two ubiquitinated forms of SP-C were detected; ubiquitination was blocked by mutation of K6, but not K34, in the SP-C propeptide. Mutation of K6 also inhibited processing of SP-C proprotein to the mature peptide in human embryonic kidney 293 cells. Nedd4-2-mediated ubiquitination regulates lumenal relocation of SP-C, leading to processing and, ultimately, secretion of SP-C.

Biomimicry of surfactant protein C

Since the widespread use of exogenous lung surfactant to treat neonatal respiratory distress syndrome, premature infant survival and respiratory morbidity have dramatically improved. Despite the effectiveness of the animal-derived surfactant preparations, there still remain some concerns and difficulties associated with their use. This has prompted investigation into the creation of synthetic surfactant preparations. However, to date, no clinically used synthetic formulation is as effective as the natural material. This is largely because the previous synthetic formulations lacked analogues of the hydrophobic proteins of the lung surfactant system, SP-B and SP-C, which are critical functional constituents. As a result, recent investigation has turned toward the development of a new generation of synthetic, biomimetic surfactants that contain synthetic phospholipids along with a mimic of the hydrophobic protein portion of lung surfactant. In this Account, we detail our efforts in creating accurate mimics of SP-C for use in a synthetic surfactant replacement therapy. Despite SP-C's seemingly simple structure, the predominantly helical protein is extraordinarily challenging to work with given its extreme hydrophobicity and structural instability, which greatly complicates the creation of an effective SP-C analogue. Drawing inspiration from Nature, two promising biomimetic approaches have led to the creation of rationally designed biopolymers that recapitulate many of SP-C's molecular features. The first approach utilizes detailed SP-C structure-activity relationships and amino acid folding propensities to create a peptide-based analogue, SP-C33. In SP-C33, the problematic and metastable polyvaline helix is replaced with a structurally stable polyleucine helix and includes a well-placed positive charge to prevent aggregation. SP-C33 is structurally stable and eliminates the association propensity of the native protein. The second approach follows the same design considerations but makes use of a non-natural, poly-N-substituted glycine or "peptoid" scaffold to circumvent the difficulties associated with SP-C. By incorporating unique biomimetic side chains in a non-natural backbone, the peptoid mimic captures both SP-C's hydrophobic patterning and its helical secondary structure. Despite the differences in structure, both SP-C33 and the SP-C peptoid mimic capture many requisite features of SP-C. In a surfactant environment, these analogues also replicate many of the key surface activities necessary for a functional biomimetic surfactant therapy while overcoming the difficulties associated with the natural protein. With improved stability, greater production potential, and elimination of possible pathogenic contamination, these biomimetic surfactant formulations offer not only the potential to improve the treatment of respiratory distress syndrome but also the opportunity to treat other respiratory-related disorders.

Effects of palmitoylation on dynamics and phospholipid-bilayer-perturbing properties of the N-terminal segment of pulmonary surfactant protein SP-C as shown by 2H-NMR

It has been proposed that palmitoylation of the N-terminal segment of surfactant protein SP-C is important for maintaining association of pulmonary surfactant complexes with interfacial films compressed to high pressures at the end of expiration. In this study, we examined surfactant membrane models containing palmitoylated and nonpalmitoylated synthetic peptides, based on the N-terminal SP-C sequence, in dipalmitoylphosphatidylcholine (DPPC)/egg phosphatidylglycerol (7:3, w/w) by (2)H-NMR. Perturbations of lipid properties by the peptide versions were compared in samples containing chain- and headgroup-deuterated lipid (DPPC-d(62) and DPPC-d(4) respectively). Also, deuterated peptide palmitate chains were compared with those of DPPC in otherwise identical lipid-protein mixtures. Palmitoylated peptide increased average DPPC-d(62) chain orientational order slightly, particularly for temperatures spanning gel and liquid crystalline coexistence, implying penetration of palmitoylated peptide into ordered membrane. In contrast, the nonpalmitoylated peptide had a small disordering effect in this temperature range. Both peptide versions perturbed DPPC-d(4) headgroup orientation similarly, suggesting little effect of palmitoylation on the largely electrostatic peptide-headgroup interaction. Deuterated acyl chains attached to the SP-C N-terminal segment displayed a qualitatively different distribution of chain order, and lower average order, than DPPC-d(62) in the same membranes. This likely reflects local perturbation of lipid headgroup spacing by the peptide portion interacting with the bilayer near the peptide palmitate chains. This study suggests that SP-C-attached acyl chains could be important for coupling of lipid and protein motions in surfactant bilayers and monolayers, especially in the context of ordered phospholipid structures such as those potentially formed during exhalation, when stabilization of the respiratory surface by surfactant is the most crucial.

Production and characterisation of recombinant forms of human pulmonary surfactant protein C (SP-C): Structure and surface activity

Surfactant protein C (SP-C) is an essential component for the surface tension-lowering activity of the pulmonary surfactant system. It contains a valine-rich alpha helix that spans the lipid bilayer, and is one of the most hydrophobic proteins known so far. SP-C is also an essential component of various surfactant preparations of animal origin currently used to treat neonatal respiratory distress syndrome (NRDS) in preterm infants. The limited supply of this material and the risk of transmission of infectious agents and immunological reactions have prompted the development of synthetic SP-C-derived peptides or recombinant humanized SP-C for inclusion in new preparations for therapeutic use. We describe herein the recombinant production in bacterial cultures of SP-C variants containing phenylalanines instead of the palmitoylated cysteines of the native protein, as fusions to the hydrophilic nuclease A (SN) from Staphylococcus aureus. The resulting chimerae were partially purified by affinity chromatography and subsequently subjected to protease digestion. The SP-C forms were recovered from the digestion mixtures by organic extraction and further purified by size exclusion chromatography. The two recombinant SP-C variants so obtained retained more than 50% alpha-helical content and showed surface activity comparable to the native protein, as measured by surface spreading of lipid/protein suspensions and from compression pi-A isotherms of lipid/protein films. Compared to the protein purified from porcine lungs, the recombinant SP-C forms improved movement of phospholipid molecules into the interface (during adsorption), or out from the interfacial film (during compression), suggesting new possibilities to develop improved therapeutic preparations.

Structure and influence on stability and activity of the N-terminal propeptide part of lung surfactant protein C

Mature lung surfactant protein C (SP-C) corresponds to residues 24-58 of the 21 kDa proSP-C. A late processing intermediate, SP-Ci, corresponding to residues 12-58 of proSP-C, lacks the surface activity of SP-C, and the SP-Ci alpha-helical structure does not unfold in contrast to the metastable nature of the SP-C helix. The NMR structure of an analogue of SP-Ci, SP-Ci(1-31), with two palmitoylCys replaced by Phe and four Val replaced by Leu, in dodecylphosphocholine micelles and in ethanol shows that its alpha-helix vs. that of SP-C is extended N-terminally. The Arg-Phe part in SP-Ci that is cleaved to generate SP-C is localized in a turn structure, which is followed by a short segment in extended conformation. Circular dichroism spectroscopy of SP-Ci(1-31) in microsomal or surfactant lipids shows a mixture of helical and extended conformation at pH 6, and a shift to more unordered structure at pH 5. Replacement of the N-terminal hexapeptide segment SPPDYS (known to constitute a signal in intracellular targeting) of SP-Ci with AAAAAA results in a peptide that is mainly unstructured, independent of pH, in microsomal and surfactant lipids. Addition of a synthetic dodecapeptide, corresponding to the propeptide part of SP-Ci, to mature SP-C results in slower aggregation kinetics and altered amyloid fibril formation, and reduces the surface activity of phospholipid-bound SP-C. These data suggest that the propeptide part of SP-Ci prevents unfolding by locking the N-terminal part of the helix, and that acidic pH results in structural disordering of the region that is proteolytically cleaved to generate SP-C.

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.

Deacylated pulmonary surfactant protein SP-C transforms from alpha-helical to amyloid fibril structure via a pH-dependent mechanism: an infrared structural investigation

Bovine pulmonary surfactant protein C (SP-C) is a hydrophobic, alpha-helical membrane-associated lipoprotein in which cysteines C4 and C5 are acylated with palmitoyl chains. Recently, it has been found that the alpha-helix form of SP-C is metastable, and under certain circumstances may transform from an alpha-helix to a beta-strand conformation that resembles amyloid fibrils. This transformation is accelerated when the protein is in its deacylated form (dSP-C). We have used infrared spectroscopy to study the structure of dSP-C in solution and at membrane interfaces. Our results show that dSP-C transforms from an alpha-helical to a beta-type amyloid fibril structure via a pH-dependent mechanism. In solution at low pH, dSP-C is alpha-helical in nature, but converts to an amyloid fibril structure composed of short beta-strands or beta-hairpins at neutral pH. The alpha-helix structure of dSP-C is fully recoverable from the amyloid beta-structure when the pH is once again lowered. Attenuated total reflectance infrared spectroscopy of lipid-protein monomolecular films showed that the fibril beta-form of dSP-C is not surface-associated at the air-water interface. In addition, the lipid-associated alpha-helix form of dSP-C is only retained at the surface at low surface pressures and dissociates from the membrane at higher surface pressures. In situ polarization modulation infrared spectroscopy of protein and lipid-protein monolayers at the air-water interface confirmed that the residual dSP-C helix conformation observed in the attenuated total reflectance infrared spectra of transferred films is randomly or isotropically oriented before exclusion from the membrane interface. This work identifies pH as one of the mechanistic causes of amyloid fibril formation for dSP-C, and a possible contributor to the pathogenesis of pulmonary alveolar proteinosis.

Alterations in SP-B and SP-C expression in neonatal lung disease

The hydrophobic surfactant proteins, SP-B and SP-C, have important roles in surfactant function. The importance of these proteins in normal lung function is highlighted by the lung diseases associated with abnormalities in their expression. Mutations in the gene encoding SP-B result in severe, fatal neonatal lung disease, and mutations in the gene encoding SP-C are associated with chronic interstitial lung diseases in newborns, older children, and adults. This work reviews the current state of knowledge concerning the lung diseases associated with mutations in the SP-B and SP-C genes, and the potential roles of abnormal SP-B and SP-C expression and genetic variation in these genes in other lung diseases.

The N-terminal Propeptide of Lung Surfactant Protein C is Necessary for Biosynthesis and Prevents Unfolding of a Metastable α-Helix

The lung surfactant-associated protein C (SP-C) consists mainly of a polyvaline alpha-helix, which is stable in a lipid membrane. However, in agreement with the predicted beta-strand conformation of a polyvaline segment, helical SP-C unfolds and transforms into beta-sheet aggregates and amyloid fibrils within a few days in aqueous organic solvents. SP-C fibril formation and aggregation have been associated with lung disease. Here, we show that in a recently isolated biosynthetic precursor of SP-C (SP-Ci), a 12 residue N-terminal propeptide locks the metastable polyvaline part in a helical conformation. The SP-Ci helix does not aggregate or unfold during several weeks of incubation, as judged by hydrogen/deuterium exchange and mass spectrometry. Hydrogen/deuterium exchange experiments further indicate that the propeptide reduces exchange in parts corresponding to mature SP-C. Finally, in an acidic environment, SP-Ci unfolds and aggregates into amyloid fibrils like SP-C. These data suggest a direct role of the N-terminal propeptide in SP-C biosynthesis.

A synthetic surfactant based on a poly-Leu SP-C analog and phospholipids: effects on tidal volumes and lung gas volumes in ventilated immature newborn rabbits

Available surfactants for treatment of respiratory distress syndrome in newborn infants are derived from animal lungs, which limits supply and poses a danger of propagating infectious material. Poly-Val-->poly-Leu analogs of surfactant protein (SP)-C can be synthesized in large quantities and exhibit surface activity similar to SP-C. Here, activity of synthetic surfactants containing a poly-Leu SP-C analog (SP-C33) was evaluated in ventilated premature newborn rabbits. Treatment with 2.5 ml/kg body wt of 2% (wt/wt) SP-C33 in 1,2-dipalmitoyl-sn-3-glycero phosphoryl choline (DPPC)-1-palmitoyl-2-oleoyl-sn-3-glycero phosphoryl choline (POPC)-1-palmitoyl-2-oleoyl-sn-3-glycero phosphoryl glycerol (POPG), 68:0:31, 68:11:20, or 68:16:15 (wt/wt/wt) suspended at 80 mg/ml gave tidal volumes (Vt) of 20-25 ml/kg body wt, with an insufflation pressure of 25 cmH2O and no positive end-expiratory pressure (PEEP), comparable to the Vt for animals treated with the porcine surfactant Curosurf. Nontreated littermates had a Vt of approximately 2 ml/kg body wt. The Vt for SP-C33 in DPPC-egg phosphatidylglycerol-palmitic acid [68:22:9 (wt/wt/wt)], DPPC-POPG-palmitic acid [68:22:9 (wt/wt/wt)], and DPPC-POPC-POPG [6:2:2 (wt/wt/wt)] was 15-20 ml/kg body wt. Histological examination of lungs from animals treated with SP-C33-based surfactants showed incomplete, usually patchy air expansion of alveolar spaces associated with only mild airway epithelial damage. Lung gas volume after 30 min of mechanical ventilation were more than threefold larger in animals treated with Curosurf than in those receiving SP-C33 in DPPC-POPC-POPG, 68:11:20. This difference could be largely counterbalanced by ventilation with PEEP (3-4 cmH2O). An artificial surfactant based on SP-C33 improves Vt in immature newborn animals ventilated with standardized peak pressure but requires PEEP to build up adequate lung gas volumes.

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.

Improving hydrogen/deuterium exchange mass spectrometry by reduction of the back-exchange effect

The measurement of deuterium incorporation kinetics using hydrogen/deuterium (H/D) exchange experiments is a valuable tool for the investigation of the conformational dynamics of biomolecules in solution. Experiments consist of two parts when using H/D exchange mass spectrometry to analyse the deuterium incorporation. After deuterium incorporation at high D(2)O concentration, it is necessary to decrease the D(2)O concentration before the mass analysis to avoid deuterium incorporation under artificial conditions of mass spectrometric preparation and measurement. A low D(2)O concentration, however, leads to back-exchange of incorporated deuterons during mass analysis. This back-exchange is one of the major problems in H/D exchange mass spectrometry and must be reduced as much as possible. In the past, techniques using electrospray ionization (ESI) had the lowest back-exchange values possible in H/D exchange mass spectrometry. Methods for the measurement of H/D exchange by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) that have been developed since 1998 have some significant advantages, but they could not achieve the back-exchange minima of ESI methods. Here, we present a protocol for H/D exchange MALDI-MS which allows for greater minimization of back-exchange compared with H/D exchange ESI-MS under similar conditions.

Protein surface mapping by chemical oxidation: structural analysis by mass spectrometry

The solvent-accessible surface area of proteins is important in biological function for many reasons, including protein-protein interactions, protein folding, and catalytic sites. Here we present a chemical technique to oxidize amino acid side chains in a model protein, apomyoglobin, and subsequent elucidation of the effect of solvent accessibility on the sites of oxidation. Under conditions of low protein oxidation (zero to three oxygen atoms added per apomyoglobin molecule), we have positively identified five oxidation sites by liquid chromatography-tandem mass spectrometry and high-resolution Fourier transform mass spectrometry. Our results indicate that all oxidized amino acids, with the exception of methionine, have highly solvent-accessible side chains, but the rate of oxidation may not be dictated solely by solvent accessibility and amino acid identity.

Palmitoylation and processing of the lipopeptide surfactant protein C

Pulmonary surfactant, a mixture of lipids and proteins, reduces the surface tension at the air-water interface of the lung alveoli by forming a surface active film. This way, it prevents alveoli from collapsing and facilitates the work of breathing. Surfactant protein C (SP-C) plays an important role in this surfactant function. SP-C is expressed as a proprotein (proSP-C), which becomes posttranslationally modified with palmitate and undergoes several rounds of proteolytical cleavage. This results in the formation of mature SP-C, which is stored in the lamellar bodies (LB) and finally secreted into the alveolar space. Recently, new insights into the sorting, processing and palmitoylation of proSP-C have been obtained by mutagenesis studies. Moreover, reports on the association of development of lung disease with SP-C deficiency have led to new insights into the importance of SP-C for proper surfactant homeostasis. In addition, new information has become available on the role of the palmitoyl chains of SP-C in surface activity. This review summarizes these recent developments in the processing and function of SP-C, with particular emphasis on the signals for and role of palmitoylation of SP-C.

Hydrogen/deuterium exchange and aggregation of a polyvaline and a polyleucine alpha-helix investigated by matrix-assisted laser desorption ionization mass spectrometry

The membrane-associated pulmonary surfactant protein C (SP-C), containing a polyvaline alpha-helix, and a synthetic SP-C analogue with a polyleucine helix (SP-C(Leu)) were studied by hydrogen/deuterium exchange matrix-assisted laser desorption ionization (MALDI) mass spectrometry. SP-C, but not SP-C(Leu), formed abundant amyloid fibrils under experimental conditions. In CD(3)OD/D(2)O, 91:9 (v/v), containing 2 mM ammonium acetate, SP-C(Leu) and SP-C exchanged 40% of their exchangeable hydrogens within 1 min. This corresponds to exchange of labile side-chain hydrogen atoms, hydrogens on the N- and C-terminal heteroatoms, and amide hydrogen atoms in the unstructured N-terminal regions. After approximately 300 h, four exchangeable hydrogen atoms in SP-C(Leu) and 10 in SP-C remained unexchanged. During this time period the ion current corresponding to singly charged SP-C decreased to <10% of the initial value due to the formation of insoluble aggregates that are not detected by MALDI mass spectrometry. In contrast, the ion current for SP-C(Leu) was maintained over this time period, although the peptides were incubated together. In combination, hydrogen/deuterium exchange and aggregation data indicate that the polyleucine peptide refolds into a helix after opening, while the unfolded polyvaline peptide forms insoluble beta-sheet aggregates rather than refolding into a helix. The SP-C helix, but not the SP-C(Leu) helix, is thus in a metastable state, which may contribute to the recently observed tendency of SP-C and its precursor to misfold and aggregate in vivo.

Structural basis for interactions between lung surfactant protein C and bacterial lipopolysaccharide

In the respiratory tract, recognition of bacterial endotoxin (lipopolysacharide, LPS) is a critical step of the innate host defense system directed against invading pathogens. Secretions of the airways contain proteins that have direct antimicrobial activity (lysozyme, lactoferrin, defensins, and cathelicidins) as well as complement factors and surfactant proteins that contribute to host defense. The hydrophobic surfactant protein C (SP-C) recognizes LPS (Augusto, L., Le Blay, K., Auger, G., Blanot, D., and Chaby, R. (2001) Am. J. Physiol. 281, L776-L785). In the present study, using synthetic analogs of SP-C, we demonstrate that the palmitoyl residues of SP-C are not required for the interaction with LPS and that both the hydrophilic and hydrophobic regions of SP-C are required for specific binding of a radiolabeled rough-type LPS. In addition, using LPS submitted to different chemical treatments as well as synthetic analogs of the lipid A moiety of LPS, we established that the terminal phosphate group at the reducing end of the lipid A disaccharide in alpha configuration is of crucial importance for recognition by SP-C. The N-linked fatty acyl chain on the reducing glucosamine of lipid A also takes part in the interaction. Dipalmitoyl phosphatidylcholine is not specifically required for the LPS-binding activity of SP-C, although a lipid environment significantly increases the binding. These results provide a basis for experiments on the role of SP-C in presentation of LPS to alveolar cells and for the design of drugs for the management of endotoxin-induced lung injury.

Studies of biomolecular conformations and conformational dynamics by mass spectrometry

In the post-genomic era, a wealth of structural information has been amassed for proteins from NMR and crystallography. However, static protein structures alone are not a sufficient description: knowledge of the dynamic nature of proteins is essential to understand their wide range of functions and behavior during the life cycle from synthesis to degradation. Furthermore, few proteins have the ability to act alone in the crowded cellular environment. Assemblies of multiple proteins governed by complex signaling pathways are often required for the tasks of target recognition, binding, transport, and function. Mass spectrometry has emerged over the past several years as a powerful tool to address many of these questions. Recent improvements in "soft" ionization techniques have enabled researchers to study proteins and biomolecular complexes, both directly and indirectly. Likewise, continuous improvements in instrumental design in recent years have resulted in a dramatic expansion of the m/z range and resolution, enabling observation of large multi-protein assemblies whose structures are retained in the gas phase. In this article, we discuss some of the mass spectrometric techniques applied to investigate the nature of the conformations and dynamical properties that govern protein function.