Solubility versus electrostatics: what determines lipid/protein interaction in lung surfactant

Headgroup-Specific Interaction of Biological Lipid Monolayer/Water Interface with Perfluorinated Persistent Organic Pollutant (f-POP): As Observed with Interface-Selective Vibrational Spectroscopy

Perfluoro compounds are widely used in various manufacturing processes, which leads to their bioaccumulation and subsequent adverse effects on human health. Using interface-selective vibrational spectroscopy (heterodyne-detected vibrational sum frequency generation (HD-VSFG)), we have elucidated the molecular mechanism of the perturbation of lipid monolayers on the water surface using a prototype perfluorinated persistent organic pollutant, perfluoroheptanoic acid (PFHA). PFHA disrupts the well-ordered all-trans conformation of a cationic lipid (1,2-dipalmitoyl-3-trimethylammonium propane (DPTAP)) monolayer and reduces the interfacial electric field at the lipid/water interface. In contrast, the hydrophobic packing of an anionic lipid (1,2-dipalmitoyl-sn-glycero-3-phospoglycerol (DPPG)) monolayer remains largely unaffected in the presence of PFHA, though the interfacial electric field is reduced. For a zwitterionic lipid (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC))/water interface, both alkyl chain ordering and interfacial electric field are fairly perturbed by PFHA. Lipid headgroup-specific interaction of PFHA and the repulsive interaction of oleophobic fluoroalkyl chain with the lipid alkyl chains govern these distinct perturbations of the lipid monolayers on the water surface.

Structural hallmarks of lung surfactant: Lipid-protein interactions, membrane structure and future challenges

Lung surfactant (LS) is an outstanding example of how a highly regulated and dynamic membrane-based system has evolved to sustain a wealth of structural reorganizations in order to accomplish its biophysical function, as it coats and stabilizes the respiratory air-liquid interface in the mammalian lung. The present review dissects the complexity of the structure-function relationships in LS through an updated description of the lipid-protein interactions and the membrane structures that sustain its synthesis, secretion, interfacial performance and recycling. We also revise the current models and the biophysical techniques employed to study the membranous architecture of LS. It is important to consider that the structure and functional properties of LS are often studied in bulk or under static conditions, in spite that surfactant function is strongly connected with a highly dynamic behaviour, sustained by very polymorphic structures and lipid-lipid, lipid-protein and protein-protein interactions that reorganize in precise spatio-temporal coordinates. We have tried to underline the evidences available of the existence of such structural dynamism in LS. A last important aspect is that the synthesis and assembly of LS is a strongly regulated intracellular process to ensure the establishment of the proper interactions driving LS surface activity, while protecting the integrity of other cell membranes. The use of simplified lipid models or partial natural materials purified from animal tissues could be too simplistic to understand the true molecular mechanisms defining surfactant function in vivo. In this line, we will bring into the attention of the reader the methodological challenges and the questions still open to understand the structure-function relationships of LS at its full biological relevance.

Lipophilic Allergens, Different Modes of Allergen-Lipid Interaction and Their Impact on Asthma and Allergy

Molecular allergology research has provided valuable information on the structure and function of single allergenic molecules. There are several allergens in food and inhalant allergen sources that are able to interact with lipid ligands via different structural features: hydrophobic pockets, hydrophobic cavities, or specialized domains. For only a few of these allergens information on their associated ligands is already available. Several of the allergens are clinically relevant, so that it is highly probable that the individual structural features with which they interact with lipids have a direct effect on their allergenic potential, and thus on allergy development. There is some evidence for a protective effect of lipids delaying the enzymatic digestion of the peanut (Arachis hypogaea) allergen Ara h 8 (hydrophobic pocket), probably allowing this molecule to get to the intestinal immune system intact (sensitization). Oleosins from different food allergen sources are part of lipid storage organelles and potential marker allergens for the severity of the allergic reaction. House dust mite (HDM), is more often associated with allergic asthma than other sources of inhalant allergens. In particular, lipid-associated allergens from Dermatophagoides pteronyssinus which are Der p 2, Der p 5, Der p 7, Der p 13, Der p 14, and Der p 21 have been reported to be associated with severe allergic reactions and respiratory symptoms such as asthma. The exact mechanism of interaction of these allergens with lipids still has to be elucidated. Apart from single allergens glycolipids have been shown to directly induce allergic inflammation. Several-in parts conflicting-data exist on the lipid (and allergen) and toll-like receptor interactions. For only few single allergens mechanistic studies were performed on their interaction with the air-liquid interface of the lungs, in particular with the surfactant components SP-A and SP-D. The increasing knowledge on protein-lipid-interaction for lipophilic and hydrophobic food and inhalant allergens on the basis of their particular structure, of their capacity to be integral part of membranes (like the oleosins), and their ability to interact with membranes, surfactant components, and transport lipids (like the lipid transfer proteins) are essential to eventually clarify allergy and asthma development.

Solubilization kinetics determines the pulsatory dynamics of lipid vesicles exposed to surfactant

We establish a biophysical model for the dynamics of lipid vesicles exposed to surfactants. The solubilization of the lipid membrane due to the insertion of surfactant molecules induces a reduction of membrane surface area at almost constant vesicle volume. This results in a rate-dependent increase of membrane tension and leads to the opening of a micron-sized pore. We show that solubilization kinetics due to surfactants can determine the regime of pore dynamics: either the pores open and reseal within a second (short-lived pore), or the pore stays open up to a few minutes (long-lived pore). First, we validate our model with previously published experimental measurements of pore dynamics. Then, we investigate how the solubilization kinetics and membrane properties affect the dynamics of the pore and construct a phase diagram for short and long-lived pores. Finally, we examine the dynamics of sequential pore openings and show that cyclic short-lived pores occur with a period inversely proportional to the solubilization rate. By deriving a theoretical expression for the cycle period, we provide an analytical tool to estimate the solubilization rate of lipid vesicles by surfactants. Our findings shed light on some fundamental biophysical mechanisms that allow simple cell-like structures to sustain their integrity against environmental stresses, and have the potential to aid the design of vesicle-based drug delivery systems. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Model Lung Surfactant Films: Why Composition Matters

Lung surfactant replacement therapies, Survanta and Infasurf, and two lipid-only systems both containing saturated and unsaturated phospholipids and one containing additional palmitic acid were used to study the impact of buffered saline on the surface activity, morphology, rheology, and structure of Langmuir monolayer model membranes. Isotherms and Brewster angle microscopy show that buffered saline subphases induce a film expansion, except when the cationic protein, SP-B, is present in sufficient quantities to already screen electrostatic repulsion, thus limiting the effect of changing pH and adding counterions. Grazing incidence X-ray diffraction results indicate an expansion not only of the liquid expanded phase but also an expansion of the lattice of the condensed phase. The film expansion corresponded in all cases with a significant reduction in the viscosity and elasticity of the films. The viscoelastic parameters are dominated by liquid expanded phase properties and do not appear to be dependent on the structure of the condensed phase domains in a phase separated film. The results highlight that the choice of subphase and film composition is important for meaningful interpretations of measurements using model systems.

Electrostatically driven lipid-protein interaction: Answers from FRET

Electrostatics govern the association of a large number of proteins with cellular membranes. In some cases, these proteins present specialized lipid-binding modules or membrane targeting domains while in other cases association is achieved through nonspecific interaction of unstructured clusters of basic residues with negatively charged lipids. Given its spatial resolution in the nanometer range, Förster resonance energy transfer (FRET) is a powerful tool to give insight into protein-lipid interactions and provide molecular level information which is difficult to retrieve with other spectroscopic techniques. In this review we present and discuss the basic formalisms of both hetero- and homo-FRET pertinent to the most commonly encountered problems in lipid-protein interaction studies and highlight some examples of implementations of different FRET methodologies to characterize lipid/protein systems in which electrostatic interactions play a crucial role. This article is part of a Special Issue entitled: Lipid-protein interactions.

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.

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

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

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

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

A ToF-SIMS study of the lateral organization of lipids and proteins in pulmonary surfactant systems

Pulmonary surfactant is a complex lipid-protein mixture whose main function is to reduce the surface tension at the air-liquid interface of alveoli to minimize the work of breathing. The exact mechanism by which surfactant monolayers and multilayers are formed and how they lower surface tension to very low values during lateral compression remains uncertain. We used time-of-flight secondary ion mass spectrometry to study the lateral organization of lipids and peptide in surfactant preparations ranging in complexity. We show that we can successfully determine the location of phospholipids, cholesterol and a peptide in surfactant Langmuir-Blodgett films and we can determine the effect of cholesterol and peptide addition. A thorough understanding of the lateral organization of PS interfacial films will aid in our understanding of the role of each component as well as different lipid-lipid and lipid-protein interactions. This may further our understanding of pulmonary surfactant function.

Surface view of the lateral organization of lipids and proteins in lung surfactant model systems-a ToF-SIMS approach

The lateral organization of domain structures is an extremely significant aspect of biomembrane research. Chemical imaging by mass spectrometry with its recent advancement in sensitivity and lateral resolution has become a highly promising tool in biological research. In this review, we focus briefly on the instrumentation, working principle and important concepts related to time-of-flight secondary ion mass spectrometry followed by an overview of lipid/protein fragmentation patterns and chemical mapping. The key issues addressed are the applications of time-of-flight secondary ion mass spectrometry in biological membrane research. Additionally, we briefly review our recent investigations based on time-of-flight secondary ion mass spectrometry to unravel the lateral distribution of lipids and surfactant proteins in lung surfactant model systems as an example that highlights the importance of fluidity and ionic conditions on lipid phase behavior and lipid-protein interactions.

Nanoparticle interaction with model lung surfactant monolayers

One of the most important functions of the lung surfactant monolayer is to form the first line of defence against inhaled aerosols such as nanoparticles (NPs), which remains largely unexplored. We report here, for the first time, the interaction of polyorganosiloxane NPs (AmorSil20: 22 nm in diameter) with lipid monolayers characteristic of alveolar surfactant. To enable a better understanding, the current knowledge about an established model surface film that mimics the surface properties of the lung is reviewed and major results originating from our group are summarized. The pure lipid components dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol have been used to study the biophysical behaviour of their monolayer films spread at the air-water interface in the presence of NPs. Film balance measurements combined with video-enhanced fluorescence microscopy have been used to investigate the formation of domain structures and the changes in the surface pattern induced by NPs. We are able to show that NPs are incorporated into lipid monolayers with a clear preference for defect structures at the fluid-crystalline interface leading to a considerable monolayer expansion and fluidization. NPs remain at the air-water interface probably by coating themselves with lipids in a self-assembly process, thereby exhibiting hydrophobic surface properties. We also show that the domain structure in lipid layers containing surfactant protein C, which is potentially responsible for the proper functioning of surfactant material, is considerably affected by NPs.

Calcium ions as "miscibility switch": colocalization of surfactant protein B with anionic lipids under absolute calcium free conditions

One of the main determinants of lung surfactant function is the complex interplay between its protein and lipid components. The lipid specificity of surfactant protein B (SP-B), however, and the protein's ability to selectively squeeze out lipids, has remained contradictory. In this work we present, for the first time to our knowledge, by means of time-of-flight secondary ion mass spectrometry chemical imaging, a direct evidence for colocalization of SP-B as well as its model peptide KL(4) with negatively charged dipalmitoylphosphatidylglycerol under absolute calcium free conditions. Our results prove that protein/lipid localization depends on the miscibility of all surfactant components, which itself is influenced by subphase ionic conditions. In contrast to our earlier studies reporting SP-B/KL(4) colocalization with zwitterionic dipalmitoylphosphatidylcholine, in the presence of even the smallest traces of calcium, we finally evidence an apparent reversal of protein/lipid mixing behavior upon calcium removal with ethylene diamine tetraacetic acid. In addition, scanning force microscopy measurements reveal that by depleting the subphase from calcium ions the protrusion formation ability of SP-B or KL(4) is not hampered. However, in the case of KL(4), distinct differences in protrusion morphology and height are visible. Our results support the idea that calcium ions act as a "miscibility switch" in surfactant model systems and probably are one of the major factors steering lipid/protein mixing behavior as well as influencing the protein's protrusion formation ability.

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

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

Pulmonary surfactant model systems catch the specific interaction of an amphiphilic peptide with anionic phospholipid

Interfacial behavior was studied in pulmonary surfactant model systems containing an amphiphilic alpha-helical peptide (Hel 13-5), which consists of 13 hydrophobic and five hydrophilic amino acid residues. Fully saturated phospholipids of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) were utilized to understand specific interactions between anionic DPPG and cationic Hel 13-5 for pulmonary functions. Surface pressure (pi)-molecular area (A) and surface potential (DeltaV)-A isotherms of DPPG/Hel 13-5 and DPPC/DPPG (4:1, mol/mol)/Hel 13-5 preparations were measured to obtain basic information on the phase behavior under compression and expansion processes. The interaction leads to a variation in squeeze-out surface pressures against a mole fraction of Hel 13-5, where Hel 13-5 is eliminated from the surface on compression. The phase behavior was visualized by means of Brewster angle microscopy, fluorescence microscopy, and atomic force microscopy. At low surface pressures, the formation of differently ordered domains in size and shape is induced by electrostatic interactions. The domains independently grow upon compression to high surface pressures, especially in the DPPG/Hel 13-5 system. Under the further compression process, protrusion masses are formed in AFM images in the vicinity of squeeze-out pressures. The protrusion masses, which are attributed to the squeezed-out Hel 13-5, grow larger in lateral size with increasing DPPG content in phospholipid compositions. During subsequent expansion up to 35 mN m(-1), the protrusions retain their height and lateral diameter for the DPPG/Hel 13-5 system, whereas the protrusions become smaller for the DPPC/Hel 13-5 and DPPC/DPPG/Hel 13-5 systems due to a reentrance of the ejected Hel 13-5 into the surface. In this work we detected for the first time, to our knowledge, a remarkably large hysteresis loop for cyclic DeltaV-A isotherms of the binary DPPG/Hel 13-5 preparation. This exciting phenomenon suggests that the specific interaction triggers two completely independent processes for Hel 13-5 during repeated compression and expansion: 1), squeezing-out into the subsolution; and 2), and close packing as a monolayer with DPPG at the interface. These characteristic processes are also strongly supported by atomic force microscopy observations. The data presented here provide complementary information on the mechanism and importance of the specific interaction between the phosphatidylglycerol headgroup and the polarized moiety of native surfactant protein B for biophysical functions of pulmonary surfactants.

Interactions of surfactants with lipid membranes

Surfactants are surface-active, amphiphilic compounds that are water-soluble in the micro- to millimolar range, and self-assemble to form micelles or other aggregates above a critical concentration. This definition comprises synthetic detergents as well as amphiphilic peptides and lipopeptides, bile salts and many other compounds. This paper reviews the biophysics of the interactions of surfactants with membranes of insoluble, naturally occurring lipids. It discusses structural, thermodynamic and kinetic aspects of membrane-water partitioning, changes in membrane properties induced by surfactants, membrane solubilisation to micelles and other phases formed by lipid-surfactant systems. Each section defines and derives key parameters, mentions experimental methods for their measurement and compiles and discusses published data. Additionally, a brief overview is given of surfactant-like effects in biological systems, technical applications of surfactants that involve membrane interactions, and surfactant-based protocols to study biological membranes.

Structure of pulmonary surfactant membranes and films: the role of proteins and lipid-protein interactions

The pulmonary surfactant system constitutes an excellent example of how dynamic membrane polymorphism governs some biological functions through specific lipid-lipid, lipid-protein and protein-protein interactions assembled in highly differentiated cells. Lipid-protein surfactant complexes are assembled in alveolar pneumocytes in the form of tightly packed membranes, which are stored in specialized organelles called lamellar bodies (LB). Upon secretion of LBs, surfactant develops a membrane-based network that covers rapidly and efficiently the whole respiratory surface. This membrane-based surface layer is organized in a way that permits efficient gas exchange while optimizing the encounter of many different molecules and cells at the epithelial surface, in a cross-talk essential to keep the whole organism safe from potential pathogenic invaders. The present review summarizes what is known about the structure of the different forms of surfactant, with special emphasis on current models of the molecular organization of surfactant membrane components. The architecture and the behaviour shown by surfactant structures in vivo are interpreted, to some extent, from the interactions and the properties exhibited by different surfactant models as they have been studied in vitro, particularly addressing the possible role played by surfactant proteins. However, the limitations in structural complexity and biophysical performance of surfactant preparations reconstituted in vitro will be highlighted in particular, to allow for a proper evaluation of the significance of the experimental model systems used so far to study structure-function relationships in surfactant, and to define future challenges in the design and production of more efficient clinical surfactants.

Atomic force microscopy studies of functional and dysfunctional pulmonary surfactant films. I. Micro- and nanostructures of functional pulmonary surfactant films and the effect of SP-A

Monolayers of a functional pulmonary surfactant (PS) can reach very low surface tensions well below their equilibrium value. The mechanism by which PS monolayers reach such low surface tensions and maintain film stability remains unknown. As shown previously by fluorescence microscopy, phospholipid phase transition and separation seem to be important for the normal biophysical properties of PS. This work studied phospholipid phase transitions and separations in monolayers of bovine lipid extract surfactant using atomic force microscopy. Atomic force microscopy showed phospholipid phase separation on film compression and a monolayer-to-multilayer transition at surface pressure 40-50 mN/m. The tilted-condensed phase consisted of domains not only on the micrometer scale, as detected previously by fluorescence microscopy, but also on the nanometer scale, which is below the resolution limits of conventional optical methods. The nanodomains were embedded uniformly within the liquid-expanded phase. On compression, the microdomains broke up into nanodomains, thereby appearing to contribute to tilted-condensed and liquid-expanded phase remixing. Addition of surfactant protein A altered primarily the nanodomains and promoted the formation of multilayers. We conclude that the nanodomains play a predominant role in affecting the biophysical properties of PS monolayers and the monolayer-to-multilayer transition.

The molecular mechanism of monolayer-bilayer transformations of lung surfactant from molecular dynamics simulations

The aqueous lining of the lung surface exposed to the air is covered by lung surfactant, a film consisting of lipid and protein components. The main function of lung surfactant is to reduce the surface tension of the air-water interface to the low values necessary for breathing. This function requires the exchange of material between the lipid monolayer at the interface and lipid reservoirs under dynamic compression and expansion of the interface during the breathing cycle. We simulated the reversible exchange of material between the monolayer and lipid reservoirs under compression and expansion of the interface. We used a mixture of dipalmitoyl-phosphatidylcholine, palmitoyl-oleoyl-phosphatidylglycerol, cholesterol, and surfactant-associated protein C as a functional analog of mammalian lung surfactant. In our simulations, the monolayer collapses into the water subphase on compression and forms bilayer folds. On monolayer reexpansion, the material is transferred from the folds back to the interface. The simulations indicate that the connectivity of the bilayer aggregates to the monolayer is necessary for the reversibility of the monolayer-bilayer transformation. The simulations also show that bilayer aggregates are unstable in the air subphase and stable in the water subphase.