Structure and properties of surfactant protein C

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.

Surfactant Proteins SP-B and SP-C in Pulmonary Surfactant Monolayers: Physical Properties Controlled by Specific Protein-Lipid Interactions

The lining of the alveoli is covered by pulmonary surfactant, a complex mixture of surface-active lipids and proteins that enables efficient gas exchange between inhaled air and the circulation. Despite decades of advancements in the study of the pulmonary surfactant, the molecular scale behavior of the surfactant and the inherent role of the number of different lipids and proteins in surfactant behavior are not fully understood. The most important proteins in this complex system are the surfactant proteins SP-B and SP-C. Given this, in this work we performed nonequilibrium all-atom molecular dynamics simulations to study the interplay of SP-B and SP-C with multicomponent lipid monolayers mimicking the pulmonary surfactant in composition. The simulations were complemented by z-scan fluorescence correlation spectroscopy and atomic force microscopy measurements. Our state-of-the-art simulation model reproduces experimental pressure-area isotherms and lateral diffusion coefficients. In agreement with previous research, the inclusion of either SP-B and SP-C increases surface pressure, and our simulations provide a molecular scale explanation for this effect: The proteins display preferential lipid interactions with phosphatidylglycerol, they reside predominantly in the lipid acyl chain region, and they partition into the liquid expanded phase or even induce it in an otherwise packed monolayer. The latter effect is also visible in our atomic force microscopy images. The research done contributes to a better understanding of the roles of specific lipids and proteins in surfactant function, thus helping to develop better synthetic products for surfactant replacement therapy used in the treatment of many fatal lung-related injuries and diseases.

Lung surfactant as a biophysical assay for inhalation toxicology

Lung surfactant (LS) is a mixture of lipids and proteins that forms a thin film at the gas-exchange surfaces of the alveoli. The components and ultrastructure of LS contribute to its biophysical and biochemical functions in the respiratory system, most notably the lowering of surface tension to facilitate breathing mechanics. LS inhibition can be caused by metabolic deficiencies or the intrusion of endogenous or exogenous substances. While LS has been sourced from animals or synthesized for clinical therapeutics, the biofluid mixture has also gained recent interest as a biophysical model for inhalation toxicity. Various methods can be used to evaluate LS function quantitatively or qualitatively after exposure to potential toxicants. A narrative review of the recent literature was conducted. Studies focused whether LS was inhibited by various environmental contaminants, nanoparticles, or manufactured products. A review is also conducted on synthetic lung surfactants (SLS), which have emerged as a promising alternative to conventional animal-sourced LS. The intrinsic advantages and recent advances of SLS make a strong case for more widespread usage in LS-based toxicological assays.

Changes in CSF Surface Tension in Relation to Surfactant Proteins in Children with Intraventricular Hemorrhage

The regulation of surface tension (ST) by surfactants plays an important role in the human respiratory system but is largely unexplored in brain homeostasis. The aim of this study was to evaluate changes in ST in relation to surfactant proteins (SPs) in children with intraventricular hemorrhage (IVH). CSF samples from 93 patients were analyzed for ST with a force tensiometer and SP-A-D and -G with ELISA assays. Patients belonged to six groups: (i) IVH before primary intervention (PI), (ii) IVH 4−28 days after PI, (iii) IVH 44−357 days after PI, (iv) hydrocephalus, (v) sepsis and (vi) controls. We found indirect correlations and significant differences in ST and SPs (all p < 0.001; except for SP-C, p = 0.007). Post hoc analyses showed significantly decreased ST in IVH patients before PI compared with patients with hydrocephalus, sepsis or controls (p < 0.001), but it increased in IVH patients over time. All SPs were significantly elevated when comparing IVH patients before PI with controls (all p < 0.001; except for SP-C, p = 0.003). Children suffering from IVH displayed an increase in SPs and a decrease in ST as coping mechanisms to preserve CSF flow. The increase in ST over time could serve as prognostic marker for the healing process.

Physicochemical properties of nanoparticles affecting their fate and the physiological function of pulmonary surfactants

Pulmonary drug delivery has drawn great attention due to its targeted local lung action, reduced side effects, and ease of administration. However, inhaled nanoparticles (NPs) could adsorb different pulmonary surfactants depending on their physicochemical properties, which may impair the physiological function of the pulmonary surfactants or alter the fate of the NPs. Thus, the objective of this review is to summarize how the physicochemical properties of NPs affecting the physiological function of pulmonary surfactants and their fate. First of all, the composition and characteristics of pulmonary surfactants, methods for studying pulmonary surfactant interaction with NPs are introduced. Thereafter, the influence of physicochemical properties of NPs on hydrophobic protein adsorption and strategies to decrease the interaction of NPs with pulmonary surfactants are discussed. Finally, the influence of physicochemical properties of NPs on lipids and hydrophilic protein adsorption and consequently their fate is described. In conclusion, a better understanding of the interaction of NPs with pulmonary surfactants will promote the faster development of safe and effective nanomedicine for pulmonary drug delivery. STATEMENT OF SIGNIFICANCE: Drug delivery carriers often face complex body fluid components after entering the human body. Pulmonary surfactants diffuse at the lung gas-liquid interface, and particles inevitably interact with pulmonary surfactants after pulmonary nanomedicine delivery. This review presents an overview of how the physicochemical properties of nanoparticles affecting their fate and physiological function of pulmonary surfactants. We believe that the information included in this review can provide important guiding for the development of safe and effective pulmonary delivery nanocarriers.

Effect of Hyaluronic Acid on the Differentiation of Mesenchymal Stem Cells into Mature Type II Pneumocytes

Hyaluronic acid (HA) is an essential component of the extracellular matrix (ECM) of the healthy lung, playing an important role in the structure of the alveolar surface stabilizing the surfactant proteins. Alveolar type II (ATII) cells are the fundamental element of the alveolus, specializing in surfactant production. ATII cells represent the main target of lung external lesion and a cornerstone in the repair process of pulmonary damage. In this context, knowledge of the factors influencing mesenchymal stem cell (MSC) differentiation in ATII cells is pivotal in fulfilling therapeutic strategies based on MSCs in lung regenerative medicine. To achieve this goal, the role of HA in promoting the differentiation of MSCs in mature Type II pneumocytes capable of secreting pulmonary surfactant was evaluated. Results demonstrated that HA, at a specific molecular weight can greatly increase the expression of lung surfactant protein, indicating the ability of HA to influence MSC differentiation in ATII cells.

Genetic disorders of the surfactant system: focus on adult disease

Genes involved in the production of pulmonary surfactant are crucial for the development and maintenance of healthy lungs. Germline mutations in surfactant-related genes cause a spectrum of severe monogenic pulmonary diseases in patients of all ages. The majority of affected patients present at a very young age, however, a considerable portion of patients have adult-onset disease. Mutations in surfactant-related genes are present in up to 8% of adult patients with familial interstitial lung disease (ILD) and associate with the development of pulmonary fibrosis and lung cancer.High disease penetrance and variable expressivity underscore the potential value of genetic analysis for diagnostic purposes. However, scarce genotype-phenotype correlations and insufficient knowledge of mutation-specific pathogenic processes hamper the development of mutation-specific treatment options.This article describes the genetic origin of surfactant-related lung disease and presents spectra for gene, age, sex and pulmonary phenotype of adult carriers of germline mutations in surfactant-related genes.

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.

Far from Inert: Membrane Lipids Possess Intrinsic Reactivity That Has Consequences for Cell Biology

In this article, it is hypothesized that a fundamental chemical reactivity exists between some non-lipid constituents of cellular membranes and ester-based lipids, the significance of which is not generally recognized. Many peptides and smaller organic molecules have now been shown to undergo lipidation reactions in model membranes in circumstances where direct reaction with the lipid is the only viable route for acyl transfer. Crucially, drugs like propranolol are lipidated in vivo with product profiles that are comparable to those produced in vitro. Some compounds have also been found to promote lipid hydrolysis. Drugs with high lytic activity in vivo tend to have higher toxicity in vitro. Deacylases and lipases are proposed as key enzymes that protect cells against the effects of intrinsic lipidation. The toxic effects of intrinsic lipidation are hypothesized to include a route by which nucleation can occur during the formation of amyloid fibrils.

All-Atom Molecular Dynamics Simulations of Dimeric Lung Surfactant Protein B in Lipid Multilayers

Although lung surfactant protein B (SP-B) is an essential protein that plays a crucial role in breathing, the details of its structure and mechanism are not well understood. SP-B forms covalent homodimers, and in this work we use all-atom molecular dynamics simulations to study dimeric SP-B’s structure and its behavior in promoting lipid structural transitions. Four initial system configurations were constructed based on current knowledge of SP-B’s structure and mechanism, and the protein maintained a helicity consistent with experiment in all systems. Several SP-B-induced lipid reorganization behaviors were observed, and regions of the protein particularly important for these activities included SP-B’s “central loop” and “hinge” regions. SP-B dimers with one subunit initially positioned in each of two adjacent bilayers appeared to promote close contact between two bilayers. When both subunits were initially positioned in the same bilayer, SP-B induced the formation of a defect in the bilayer, with water penetrating into the centre of the bilayer. Similarly, dimeric SP-B showed a propensity to interact with preformed interpores in the bilayer. SP-B dimers also promoted bilayer thinning and creasing. This work fleshes out the atomistic details of the dimeric SP-B structures and SP-B/lipid interactions that underlie SP-B’s essential functions.

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.

Paradoxical Bactericidal Effects of Hydrophobic Lung Surfactant Proteins and Their Peptide Mimics Using Liposome Molecular Trojan

Lung surfactant, besides alveolar stability, also provides defence against pathogens by surfactant proteins (SP), SP-A and SP-D. The hydrophobic proteins SP-B and SP-C enhance surface activity. An unusual and paradoxical effect of bovine LS and synthetic model LS with SP-B/-C was bactericidal to Staphylococcus aureus and Escherichia coli. Bacterial proliferation were investigated with bovine lung surfactant extract (BLES), dipalmitoylphosphatdylcholine, palmitooleylglycerol, in combination with SP-B/-C using standard microbiological colony forming unit (CFU) counts and structural imaging. BLES and other surfactant-SP-B/-C mixtures inhibit bacterial growth in the concentration range of 0 -7.5 mg/mL, at > 10 mg/mL paradoxical growth of both the bacterial species suggest antibiotic resistance. The lipid only LS have no effect on bacterial proliferation. Smaller peptide mimics of SP-B or SP-B_1-25, were less efficient than SP-C_ff. Ultra structural studies of the bacterial CFU using electron and atomic force microscopy suggest some membrane damage of S. aereus at inhibitory concentration of BLES, and some structural alteration of E. coli at dividing zones, suggesting utilization and incorporation of surfactant lipid species by both bacteria. The results depicted from in vitro studies are also in agreement with protein-protein interactions obtained from PatchDock, FireDock and ClasPro algorithm. The MD-simulation decipher a small range fluctuation of gyration radius of the LS proteins and their peptide mimics. The studies have alarming implications in the use of high dosages (100 mg/mL/kg body weight) of exogenous surfactant for treatment of respiratory distress syndrome, genetic knock-out abnormalities associated with these proteins, and the novel roles played by SP-B/C as bactericidal agents.

Elevated Surfactant Protein Levels and Increased Flow of Cerebrospinal Fluid in Cranial Magnetic Resonance Imaging

Surfactant proteins (SPs) are a multifunctional group of proteins, responsible for the regulation of rheological properties of body fluids, host defense, and cellular waste clearance. Their concentrations are changed in cerebrospinal fluid (CSF) of patients suffering from communicating hydrocephalus. Hydrocephalic conditions are accompanied by altered CSF flow dynamics; however, the association of CSF-SP concentrations and CSF flow has not yet been investigated. Hence, the aim of this study was to evaluate the association between SP concentrations in the CSF and marked CSF flow phenomena at different anatomical landmarks of CSF spaces. Sixty-one individuals (15 healthy subjects and 46 hydrocephalus patients) were included in this study. CSF specimens were analyzed for SP-A, SP-B, SP-C, and SP-D concentrations by the use of enzyme-linked immunosorbent assays (ELISA). CSF flow was evaluated in axial T2_turbo inversion recovery magnitude (TIRM)-weighted and sagittal T2-weighted magnetic resonance imaging sections using a 4-grade scale (1-no flow, 2-subtle flow, 3-moderate flow, and 4-strong flow). CSF-SP concentrations (mean ± standard deviation) of the overall collective were as follows: SP-A = 0.73 ± 0.58 ng/ml, SP-B = 0.17 ± 0.93 ng/ml, SP-C = 0.95 ± 0.75 ng/ml, and SP-D = 7.43 ± 5.17 ng/ml. The difference between healthy controls and hydrocephalic patients regarding CSF concentrations of SP-A (0.34 ± 0.22 vs. 0.81 ± 0.59 ng/ml) and SP-C (0.48 ± 0.29 vs. 1.10 ± 0.79 ng/ml) revealed to be statistically significant as calculated by means of ANOVA (p values of 0.022 and 0.007, respectively). CSF flow voids were detectable at all investigated landmarks of the CSF spaces (foramina of Monro, third ventricle, mesencephalic aqueduct, prepontine cistern, fourth ventricle, cisterna magna, and craniocervical junction). CSF flow voids, reported as mean ± standard deviation, revealed to be significantly increased in hydrocephalic patients compared to controls as calculated by means of ANOVA (respective p values are given in brackets following values of descriptive statistics) at the following sites: foramina of Monro (1.60 ± 0.91 vs. 2.37 ± 0.99, p = 0.01), fourth ventricle (1.67 ± 0.98 vs. 2.52 ± 1.05, p = 0.007), and the cisterna magna (1.93 ± 1.10 vs. 2.72 ± 1.13, p = 0.022). Spearman's rank order calculation identified significant correlations for CSF flow voids at the foramina of Monro and the third ventricle with SP-A (r = 0.429, p = 0.001 and r = 0.464, p < 0.001) and CSF flow void at the mesencephalic duct with SP-D (r = - 0.371, p = 0.039). Furthermore, SP-C showed a moderate inverse correlation with age (r = - 0.302, p = 0.022). The present study confirmed statistically significant differences in SP-CSF concentrations between healthy controls and hydrocephalic patients. Additionally, significant correlations between SP concentrations in CSF with increased CSF flow were identified. These findings underline the role of SPs as regulators of CSF rheology.

Correlations of Ventricular Enlargement with Rheologically Active Surfactant Proteins in Cerebrospinal Fluid

Purpose: Surfactant proteins (SPs) are involved in the regulation of rheological properties of body fluids. Concentrations of SPs are altered in the cerebrospinal fluid (CSF) of hydrocephalus patients. The common hallmark of hydrocephalus is enlargement of the brain ventricles. The relationship of both phenomena has not yet been investigated. The aim of this study was to evaluate the association between SP concentrations in the CSF and enlargement of the brain ventricles.

Procedures: Ninty-six individuals (41 healthy subjects and 55 hydrocephalus patients) were included in this retrospective analysis. CSF specimens were analyzed for SP-A, SP-B, SP-C and SP-D concentrations by use of enzyme linked immunosorbent assays (ELISA). Ventricular enlargement was quantified in T2 weighted (T2w) magnetic resonance imaging (MRI) sections using an uni-dimensional (Evans’ Index) and a two-dimensional approach (lateral ventricles area index, LVAI).

Results: CSF-SP concentrations (mean ± standard deviation in ng/ml) were as follows: SP-A 0.71 ± 0.58, SP-B 0.18 ± 0.43, SP-C 0.89 ± 0.77 and SP-D 7.4 ± 5.4. Calculated values of Evans’ Index were 0.37 ± 0.11, a calculation of LVAI resulted in 0.18 ± 0.15 (each mean ± standard deviation). Significant correlations were identified for Evans’ Index with SP-A (r = 0.388, p < 0.001) and SP-C (r = 0.392, p < 0.001), LVAI with SP-A (r = 0.352, p = 0.001), SP-C (r = 0.471, p < 0.001) and SP-D (r = 0.233, p = 0.025). Furthermore, SP-C showed a clear inverse correlation with age (r = −0.357, p = 0.011).

Conclusion: The present study confirmed significant correlations between SPs A, C and D in the CSF with enlargement of the inner CSF spaces. In conclusion, SPs clearly play an important role for CSF rheology. CSF rheology is profoundly altered in hydrocephalic diseases, however, diagnosis and therapy of hydrocephalic conditions are still almost exclusively based on ventricular enlargement. Until now it was unclear, whether the stage of the disease, as represented by the extent of ventricular dilatation, is somehow related to the changes of SP levels in the CSF. Our study is the first to provide evidence that increasing ventriculomegaly is accompanied by enhanced changes of rheologically active compounds in the CSF and therefore introduces completely new aspects for hydrocephalus testing and conservative therapeutic approaches.

Design of Surfactant Protein B Peptide Mimics Based on the Saposin Fold for Synthetic Lung Surfactants

Surfactant protein (SP)-B is a 79-residue polypeptide crucial for the biophysical and physiological function of endogenous lung surfactant. SP-B is a member of the saposin or saposin-like proteins (SAPLIP) family of proteins that share an overall three-dimensional folding pattern based on secondary structures and disulfide connectivity and exhibit a wide diversity of biological functions. Here, we review the synthesis, molecular biophysics and activity of synthetic analogs of saposin proteins designed to mimic those interactions of the parent proteins with lipids that enhance interfacial activity. Saposin proteins generally interact with target lipids as either monomers or multimers via well-defined amphipathic helices, flexible hinge domains, and insertion sequences. Based on the known 3D-structural motif for the saposin family, we show how bioengineering techniques may be used to develop minimal peptide constructs that maintain desirable structural properties and activities in biomedical applications. One important application is the molecular design, synthesis and activity of Saposin mimics based on the SP-B structure. Synthetic lung surfactants containing active SP-B analogs may be potentially useful in treating diseases of surfactant deficiency or dysfunction including the neonatal respiratory distress syndrome and acute lung injury/acute respiratory distress syndrome.

The Cerebral Surfactant System and Its Alteration in Hydrocephalic Conditions

Introduction

Pulmonary Surfactant reduces surface tension in the terminal airways thus facilitating breathing and contributes to host’s innate immunity. Surfactant Proteins (SP) A, B, C and D were recently identified as inherent proteins of the CNS. Aim of the study was to investigate cerebrospinal fluid (CSF) SP levels in hydrocephalus patients compared to normal subjects.

Patients and Methods

CSF SP A-D levels were quantified using commercially available ELISA kits in 126 patients (0–84 years, mean 39 years). 60 patients without CNS pathologies served as a control group. Hydrocephalus patients were separated in aqueductal stenosis (AQS, n = 24), acute hydrocephalus without aqueductal stenosis (acute HC w/o AQS, n = 16) and idiopathic normal pressure hydrocephalus (NPH, n = 20). Furthermore, six patients with pseudotumor cerebri were investigated.

Results

SP A—D are present under physiological conditions in human CSF. SP-A is elevated in diseases accompanied by ventricular enlargement (AQS, acute HC w/o AQS) in a significant manner (0.67, 1.21 vs 0.38 ng/ml in control, p<0.001). SP-C is also elevated in hydrocephalic conditions (AQS, acute HC w/o AQS; 0.87, 1.71 vs. 0.48 ng/ml in controls, p<0.001) and in Pseudotumor cerebri (1.26 vs. 0.48 ng/ml in controls, p<0.01). SP-B and SP-D did not show significant alterations.

Conclusion

The present study confirms the presence of SPs in human CSF. There are significant changes of SP-A and SP-C levels in diseases affecting brain water circulation and elevation of intracranial pressure. Cause of the alterations, underlying regulatory mechanisms, as well as diagnostic and therapeutic consequences of cerebral SP’s requires further thorough investigations.

Amyloid-β (1-40) restores adhesion properties of pulmonary surfactant, counteracting the effect of cholesterol

A pulmonary surfactant (PS) is a thin lipid-protein film covering the surface of the lung alveoli at the air/liquid interface. The primary purpose of a PS is to control the surface tension of the air/liquid interface and to reduce the work of breathing. High levels of cholesterol in a PS are associated with life-threatening acute respiratory distress syndrome (ARDS) and acute lung injury (ALI). Finding therapeutics to counteract the effect of cholesterol in a PS is a matter of contemporary research. In our earlier work, we showed that the addition of amyloid-β (1-40) (Aβ40), the protein implicated in Alzheimer's disease, can reverse the detrimental effects of cholesterol in surfactants by improving multilayer formation and restoring PS surface active properties. We hypothesized that this phenomenon was due to Aβ40 improving adhesion properties of a surfactant. In this work we used atomic force spectroscopy to demonstrate that Aβ40 counteracts the adhesive properties of a PS compromised by high levels of cholesterol in a PS and helps to restore the functionality of a PS.

The Effect of Membrane Environment on Surfactant Protein C Stability Studied by Constant-pH Molecular Dynamics

Pulmonary surfactant protein C (SP-C) is a small peptide with two covalently linked fatty acyl chains that plays a crucial role in the formation and stabilization of the pulmonary surfactant reservoirs during the compression and expansion steps of the respiratory cycle. Although its function is known to be tightly related to its highly hydrophobic character and key interactions maintained with specific lipid components, much is left to understand about its molecular mechanism of action. Also, although it adopts a mainly helical structure while associated with the membrane, factors as pH variation and deacylation have been shown to affect its stability and function. In this work, the conformational behavior of both the acylated and deacylated SP-C isoforms was studied in a DPPC bilayer under different pH conditions using constant-pH molecular dynamics simulations. Our findings show that both protein isoforms are remarkably stable over the studied pH range, even though the acylated isoform exhibits a labile helix-turn-helix motif rarely observed in the other isoform. We estimate similar tilt angles for the two isoforms over the studied pH range, with a generally higher degree of internalization of the basic N-terminal residues in the deacylated case, and observe and discuss some protonation-conformation coupling effects. Both isoforms establish contacts with the surrounding lipid molecules (preferentially with the sn-2 ester bonds) and have a local effect on the conformational behavior of the surrounding lipid molecules, the latter being more pronounced for acylated SP-C.

A non-foaming proteosurfactant engineered from Ranaspumin-2

Advances in biological surfactant proteins have already yielded a diverse range of benefits from dramatically improved survival rates for premature births to artificial photosynthesis. Presented here is the design, development, and analysis of a novel biosurfactant protein we call Surfactant Resisting Foam formatioN (SRFN). Starting with the Tungara frog's foam forming protein Ranaspumin-2, we have engineered a new surfactant protein with a destabilized hinge region to alter the kinetics and equilibrium of the protein structural transition from aqueous globular form to an extended surfactant structure at the air/water interface. SRFN is capable of approximately the same total surface tension reduction, but with the unique property of forming quickly collapsible foams. The difference in foam formation is attributed to the destabilizing glycine substitutions engineered into the hinge region. Surfactants used specifically to increase wettability, such as those used in agricultural applications would benefit from this new proteosurfactant since foamed liquid has greater wind resistance and decreased dispersal. Indeed, given growing concern of organsilicone surfactant effects on declining bee populations, biological surfactant proteins have several unique advantages over more common amphiphiles in that they can be renewably sourced, are environmentally friendly, degrade readily into non-toxic byproducts, and reduce surface tension without deleterious effects on cell membranes.

Overcoming inactivation of the lung surfactant by serum proteins: a potential role for fluorocarbons?

In many pulmonary conditions serum proteins interfere with the normal adsorption of components of the lung surfactant to the surface of the alveoli, resulting in lung surfactant inactivation, with potentially serious untoward consequences. Here, we review the strategies that have recently been designed in order to counteract the biophysical mechanisms of inactivation of the surfactant. One approach includes protein analogues or peptides that mimic the native proteins responsible for innate resistance to inactivation. Another perspective uses water-soluble additives, such as electrolytes and hydrophilic polymers that are prone to enhance adsorption of phospholipids. An alternative, more recent approach consists of using fluorocarbons, that is, highly hydrophobic inert compounds that were investigated for partial liquid ventilation, that modify interfacial properties and can act as carriers of exogenous lung surfactant. The latter approach that allows fluidisation of phospholipid monolayers while maintaining capacity to reach near-zero surface tension definitely warrants further investigation.

Investigating the effect of particle size on pulmonary surfactant phase behavior

We study the impact of the addition of particles of a range of sizes on the phase transition behavior of lung surfactant under compression. Charged particles ranging from micro- to nanoscale are deposited on lung surfactant films in a Langmuir trough. Surface area versus surface pressure isotherms and fluorescent microscope observations are utilized to determine changes in the phase transition behavior. We find that the deposition of particles close to 20 nm in diameter significantly impacts the coexistence of the liquid-condensed phase and liquid-expanded phase. This includes morphological changes of the liquid-condensed domains and the elimination of the squeeze-out phase in isotherms. Finally, a drastic increase of the domain fraction of the liquid-condensed phase can be observed for the deposition of 20-nm particles. As the particle size is increased, we observe a return to normal phase behavior. The net result is the observation of a critical particle size that may impact the functionality of the lung surfactant during respiration.

Establishment of LC-MS methods for the analysis of palmitoylated surfactant proteins

The surfactant proteins (SPs), SP-B and SP-C, are important components of pulmonary surfactant involved in the reduction of alveolar surface tension. Quantification of SP-B and SP-C in surfactant drugs is informative for their quality control and the evaluation of their biological activity. Western blot analysis enabled the quantification of SP-B, but not SP-C, in surfactant drugs. Here, we report a new procedure involving chemical treatments and LC-MS to analyze SP-C peptides. The procedure enabled qualitative analysis of SP-C from different species with discrimination of the palmitoylation status and the artificial modifications that occur during handling and/or storage. In addition, the method can be used to estimate the total amount of SP-C in pulmonary surfactant drugs. The strategy described here might serve as a prototype to establish analytical methods for peptides that are extremely hydrophobic and behave like lipids. The new method provides an easy measurement of SP-C from various biological samples, which will help the characterization of various experimental animal models and the quality control of surfactant drugs, as well as diagnostics of human samples.

Changes in Lung Surfactant Proteins in Rats With Lipopolysaccharide--Induced Fever

The study was designed to prove the hypothesis that lipopolysaccharide (LPS)-induced fever elicits the changes in surfactant specific proteins, potentially related to thermal tachypnea. In adult rats fever was induced by intraperitoneal administration of LPS at a dose 100 µg/kg of body weight; control group received saline. Respiratory parameters, arterial blood gases and pH and colonic body temperature (BT) were recorded. Five hours later, surfactant proteins (SP) A, B, C and D were evaluated in bronchoalveolar lavage fluid (BALF) and lung tissue (LT). LPS evoked monophasic thermic response (at 300 min 38.7±0.2 vs. 36.4±0.3 °C, P0.05) and an increase in minute ventilation due to changes in breathing rate and tidal volume. LPS-instilled animals had higher levels of SP-A and SP-D in LT (P0.05 and 0.01), and higher SP-D in BALF (P0.01) than controls. SP-B increased in LT and SP-C in BALF of animals with LPS (both P0.05 vs. controls). The changes in all surfactant specific proteins are present in LPS-induced fever. Alterations of proteins related to local immune mechanisms (SP-A, SP-D) are probably a part of general inflammatory response to pyrogen. Changes in proteins related to surface activity (SP-B and SP-C) might reflect the effort of the body to stabilize the lungs in thermal challenge.

Biophysical inhibition of pulmonary surfactant function by polymeric nanoparticles: role of surfactant protein B and C

The current study investigated the mechanisms involved in the process of biophysical inhibition of pulmonary surfactant by polymeric nanoparticles (NP). The minimal surface tension of diverse synthetic surfactants was monitored in the presence of bare and surface-decorated (i.e. poloxamer 407) sub-100 nm poly(lactide) NP. Moreover, the influence of NP on surfactant composition (i.e. surfactant protein (SP) content) was studied. Dose-elevations of SP advanced the biophysical activity of the tested surfactant preparation. Surfactant-associated protein C supplemented phospholipid mixtures (PLM-C) were shown to be more susceptible to biophysical inactivation by bare NP than phospholipid mixture supplemented with surfactant protein B (PLM-B) and PLM-B/C. Surfactant function was hindered owing to a drastic depletion of the SP content upon contact with bare NP. By contrast, surface-modified NP were capable of circumventing unwanted surfactant inhibition. Surfactant constitution influences the extent of biophysical inhibition by polymeric NP. Steric shielding of the NP surface minimizes unwanted NP-surfactant interactions, which represents an option for the development of surfactant-compatible nanomedicines.

Molecular understanding of a potential functional link between antimicrobial and amyloid peptides

Antimicrobial and amyloid peptides do not share common sequences, typical secondary structures, or normal biological activity but both the classes of peptides exhibit membrane-disruption ability to induce cell toxicity. Different membrane-disruption mechanisms have been proposed for antimicrobial and amyloid peptides, individually, some of which are not exclusive to either peptide type, implying that certain common principles may govern the folding and functions of different cytolytic peptides and associated membrane disruption mechanisms. Particularly, some antimicrobial and amyloid peptides have been identified to have dual complementary amyloid and antimicrobial properties, suggesting a potential functional link between amyloid and antimicrobial peptides. Given that some similar structural and membrane-disruption characteristics exist between the two classes of peptides, this review summarizes major findings, recent advances, and future challenges related to antimicrobial and amyloid peptides and strives to illustrate the similarities, differences, and relationships in the sequences, structures, and membrane interaction modes between amyloid and antimicrobial peptides, with a special focus on direct interactions of the peptides with the membranes. We hope that this review will stimulate further research at the interface of antimicrobial and amyloid peptides - which has been studied less intensively than either type of peptides - to decipher a possible link between both amyloid pathology and antimicrobial activity, which can guide drug design and peptide engineering to influence peptide-membrane interactions important in human health and diseases.

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.

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

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

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

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

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

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

Improvement of pulmonary surfactant activity by introducing D-amino acids into highly hydrophobic amphiphilic α-peptide Hel 13-5

The high costs of artificial pulmonary surfactants, ranging in hundreds per kilogram of body weight, used for treating the respiratory distress syndrome (RDS) premature babies have limited their applications. We have extensively studied soy lecithins and higher alcohols as lipid alternatives to expensive phospholipids such as DPPC and PG. As a substitute for the proteins, we have synthesized the peptide Hel 13-5D3 by introducing D-amino acids into a highly lipid-soluble, basic amphiphilic peptide, Hel 13-5, composed of 18 amino acid residues. Analysis of the surfactant activities of lipid-amphiphilic artificial peptide mixtures using lung-irrigated rat models revealed that a mixture (Murosurf SLPD3) of dehydrogenated soy lecithin, fractionated soy lecithin, palmitic acid (PA), and peptide Hel 13-5D3 (40:40:17.5:2.5, by weight) superior pulmonary surfactant activity than a commercially available pulmonary surfactant (beractant, Surfacten®). Experiments using ovalbumin-sensitized model animals revealed that the lipid-amphiphilic artificial peptide mixtures provided significant control over an increase in the pulmonary resistance induced by premature allergy reaction and reduced the number of acidocytes and neutrophils in lung-irrigated solution. The newly developed low-cost pulmonary surfactant system may be used for treatment of a wide variety of respiratory diseases.

Genetic risk factors associated with respiratory distress syndrome

Respiratory distress syndrome (RDS) among preterm infants is typically due to a quantitative deficiency of pulmonary surfactant. Aside from the degree of prematurity, diverse environmental and genetic factors can affect the development of RDS. The variance of the risk of RDS in various races/ethnicities or monozygotic/dizygotic twins has suggested genetic influences on this disorder. So far, several specific mutations in genes encoding surfactant-associated molecules have confirmed this. Specific genetic variants contributing to the regulation of pulmonary development, its structure and function, or the inflammatory response could be candidate risk factors for the development of RDS. This review summarizes the background that suggests the genetic predisposition of RDS, the identified mutations, and candidate genetic polymorphisms of pulmonary surfactant proteins associated with RDS.

The fetal sheep lung does not respond to cortisol infusion during the late canalicular phase of development

The prepartum surge in plasma cortisol concentrations in humans and sheep promotes fetal lung and surfactant system maturation in the support of air breathing after birth. This physiological process has been used to enhance lung maturation in the preterm fetus using maternal administration of betamethasone in the clinical setting in fetuses as young as 24 weeks gestation (term = 40 weeks). Here, we have investigated the impact of fetal intravenous cortisol infusion during the canalicular phase of lung development (from 109- to 116-days gestation, term = 150 ± 3 days) on the expression of genes regulating glucocorticoid (GC) activity, lung liquid reabsorption, and surfactant maturation in the very preterm sheep fetus and compared this to their expression near term. Cortisol infusion had no impact on mRNA expression of the corticosteroid receptors (GC receptor and mineralocorticoid receptor) or HSD11B-2, however, there was increased expression of HSD11B-1 in the fetal lung. Despite this, cortisol infusion had no effect on the expression of genes involved in lung sodium (epithelial sodium channel -α, -β, or -γ subunits and sodium–potassium ATPase-β1 subunit) or water (aquaporin 1, 3, and 5) reabsorption when compared to the level of expression during exposure to the normal prepartum cortisol surge. Furthermore, in comparison to late gestation, cortisol infusion does not increase mRNA expression of surfactant proteins (SFTP-A, -B, and -C) or the number of SFTP-B-positive cells present in the alveolar epithelium, the cells that produce pulmonary surfactant. These data suggest that there may be an age before which the lung is unable to respond biochemically to an increase in fetal plasma cortisol concentrations.

Role of the N-terminal seven residues of surfactant protein B (SP-B)

Breathing is enabled by lung surfactant, a mixture of proteins and lipids that forms a surface-active layer and reduces surface tension at the air-water interface in lungs. Surfactant protein B (SP-B) is an essential component of lung surfactant. In this study we probe the mechanism underlying the important functional contributions made by the N-terminal 7 residues of SP-B, a region sometimes called the “insertion sequence”. These studies employed a construct of SP-B, SP-B (1–25,63–78), also called Super Mini-B, which is a 41-residue peptide with internal disulfide bonds comprising the N-terminal 7-residue insertion sequence and the N- and C-terminal helices of SP-B. Circular dichroism, solution NMR, and solid state ²H NMR were used to study the structure of SP-B (1–25,63–78) and its interactions with phospholipid bilayers. Comparison of results for SP-B (8–25,63–78) and SP-B (1–25,63–78) demonstrates that the presence of the 7-residue insertion sequence induces substantial disorder near the centre of the lipid bilayer, but without a major disruption of the overall mechanical orientation of the bilayers. This observation suggests the insertion sequence is unlikely to penetrate deeply into the bilayer. The 7-residue insertion sequence substantially increases the solution NMR linewidths, most likely due to an increase in global dynamics.

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

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

SFTPC mutations cause SP-C degradation and aggregate formation without increasing ER stress

BACKGROUND

Mutations in the gene encoding surfactant protein C (SP-C) cause familial and sporadic interstitial lung disease (ILD), which is associated with considerable morbidity and mortality. Unfortunately, effective therapeutic options are still lacking due to a very limited understanding of pathomechanisms. Knowledge of mutant SP-C proprotein (proSP-C) trafficking, processing, intracellular degradation and aggregation is a crucial prerequisite for the development of specific therapies to correct aberrant trafficking and processing of proSP-C and to hinder accumulation of cytotoxic aggregates.

MATERIALS AND METHODS

To identify possible starting points for therapeutic intervention, we stably transfected A549 alveolar epithelial cells with several proSP-C mutations previously found in patients suffering from ILD. Effects of mutant proSP-C were assessed by Western blotting, immunofluorescence and Congo red staining.

RESULTS

A group of mutations (p.I73T, p.L110R, p.A116D and p.L188Q) resulted in aberrant proSP-C products, which were at least partially trafficked to lamellar bodies. Another group of mutations (p.P30L and p.P115L) was arrested in the endoplasmic reticulum (ER). Except for p.I73T, all mutations led to accumulation of intracellular Congo red-positive aggregates. Enhanced ER stress was detectable in none of these stably transfected cells.

CONCLUSIONS

Different SP-C mutations have unique consequences for alveolar epithelial cell biology. As these cannot be predicted based upon the localization of the mutation, our data emphasize the importance of studying individual mutations in detail in order to develop mutation-specific therapies.

Pulmonary Surfactant and its in vitro Assessment Using Axisymmetric Drop Shape Analysis (ADSA): A Review

Recent progress in the study of pulmonary surfactant is reviewed. The first half of this paper provides general background in both physiological and clinical perspectives. The second half focuses on the in vitro assessment of pulmonary surfactant using methods based on a drop shape technique, Axisymmetric Drop Shape Analysis (ADSA). Theories, experiments, and techniques of image analysis used in these ADSA methods are briefly described. Typical applications of these methods are discussed in detail. It is concluded that the accuracy, versatility, and simplicity of these ADSA methods render them suitable to the study of pulmonary surfactant.

Hydrophobic pulmonary surfactant proteins SP-B and SP-C induce pore formation in planar lipid membranes: evidence for proteolipid pores

Pulmonary surfactant is a complex mixture of lipids and specific surfactant proteins, including the hydrophobic proteins SP-B and SP-C, in charge of stabilizing the respiratory surface of mammalian lungs. The combined action of both proteins is responsible for the proper structure and dynamics of membrane arrays in the pulmonary surfactant network that covers the respiratory surface. In this study, we explore the possibility that proteins SP-B and SP-C induce the permeabilization of phospholipid membranes via pore formation. To this end, electrophysiological measurements have been carried out in planar lipid membranes prepared with different lipid/protein mixtures. Our main result is that channel-like structures are detected in the presence of SP-B, SP-C, or the native mixture of both proteins. Current traces show a high variety of conductance states (from pS to nS) that are dependent both on the lipid composition and the applied potential. We also show that the type of host lipid crucially determines the ionic selectivity of the observed pores: the anionic selectivity observed in zwitterionic membranes is inverted to cationic selectivity in the presence of negatively charged lipids. All those results suggest that SP-B and SP-C proteins promote the formation of proteolipid channels in which lipid molecules are functionally involved. We propose that proteolipidic membrane-permeabilizing structures may have an important role to tune ionic and lipidic flows through the pulmonary surfactant membrane network at the alveolar surfaces.

A novel conserved targeting motif found in ABCA transporters mediates trafficking to early post-Golgi compartments

The ATP binding cassette, class A (ABCA) proteins are homologous polytopic transmembrane transporters that function as lipid pumps at distinct subcellular sites in a variety of cells. Located within the N terminus of these transporters, there exists a highly conserved xLxxKN motif of unknown function. To define its role, human ABCA3 was employed as a primary model representing ABCA transporters, while mouse ABCA1 was utilized to support major findings. Transfection studies showed colocalization of both transporters with surfactant protein C (SP-C), a marker peptide for successful protein targeting to lysosomal-like organelles. In contrast, alanine mutation of xLxxKN resulted in endoplasmic reticulum retention. As proof of principle, swapping xLxxKN for the known lysosomal targeting motif of SP-C resulted in post-Golgi targeting of the SP-C chimera. However, these products failed to reach their terminal processing compartments, suggesting that the xLxxKN motif only serves as a Golgi exit signal. We propose a model whereby an N-terminal signal sequence, xLxxKN, directs ABCA transporters to a post-Golgi vesicular sorting station where additional signals may be required for selective delivery of individual transporters to final subcellular destinations.

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.

Combined and independent action of proteins SP-B and SP-C in the surface behavior and mechanical stability of pulmonary surfactant films

The hydrophobic proteins SP-B and SP-C are essential for pulmonary surfactant function, even though they are a relatively minor component (<2% of surfactant dry mass). Despite countless studies, their specific differential action and their possible concerted role to optimize the surface properties of surfactant films have not been completely elucidated. Under conditions kept as physiologically relevant as possible, we tested the surface activity and mechanical stability of several surfactant films of varying protein composition in vitro using a captive bubble surfactometer and a novel (to our knowledge) stability test. We found that in the naturally derived surfactant lipid mixtures, surfactant protein SP-B promoted film formation and reextension to lower surface tensions than SP-C, and in particular played a vital role in sustaining film stability at the most compressed states, whereas SP-C produced no stabilization. Preparations containing both proteins together revealed a slight combined effect in enhancing film formation. These results provide a qualitative and quantitative framework for the development of future synthetic therapeutic surfactants, and illustrate the crucial need to include SP-B or an efficient SP-B analog for optimal function.

Novel fatty acid acylation of lens integral membrane protein aquaporin-0

Fatty acid acylation of proteins is a well-studied co- or posttranslational modification typically conferring membrane trafficking signals or membrane anchoring properties to proteins. Commonly observed examples of protein acylation include N-terminal myristoylation and palmitoylation of cysteine residues. In the present study, direct tissue profiling mass spectrometry of bovine and human lens sections revealed an abundant signal tentatively assigned as a lipid-modified form of aquaporin-0. LC/MS/MS proteomic analysis of hydrophobic tryptic peptides from lens membrane proteins revealed both N-terminal and C-terminal peptides modified by 238 and 264 Da which were subsequently assigned by accurate mass measurement as palmitoylation and oleoylation, respectively. Specific sites of modification were the N-terminal methionine residue and lysine 238 revealing, for the first time, an oleic acid modification via an amide linkage to a lysine residue. The specific fatty acids involved reflect their abundance in the lens fiber cell plasma membrane. Imaging mass spectrometry indicated abundant acylated AQP0 in the inner cortical region of both bovine and human lenses and acylated truncation products in the lens nucleus. Additional analyses revealed that the lipid-modified forms partitioned exclusively to a detergent-resistant membrane fraction, suggesting a role in membrane domain targeting.