Phase transitions in films of lung surfactant at the air-water interface

The biophysical function of pulmonary surfactant

The type II pneumocytes of the lungs secrete a mixture of lipids and proteins that together acts as a surfactant. The material forms a thin film on the surface of the liquid layer that lines the alveolar air sacks. When compressed by the decreasing alveolar surface area during exhalation, the films reduce surface tension to exceptionally low levels. Pulmonary surfactant is essential for preserving the integrity of the barrier between alveolar air and capillary blood during normal breathing. This review focuses on the major biophysical processes by which endogenous pulmonary surfactant achieves its function and the mechanisms involved in those processes. Vesicles of pulmonary surfactant adsorb rapidly from the alveolar liquid to form the interfacial film. Interfacial insertion, which requires the hydrophobic surfactant protein SP-B, proceeds by a process analogous to the fusion of two vesicles. When compressed, the adsorbed film desorbs slowly. Constituents remain at the surface at high interfacial concentrations that reduce surface tensions well below equilibrium levels. We review the models proposed to explain how pulmonary surfactant achieves both the rapid adsorption and slow desorption characteristic of a functional film.

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.

Unraveling Complex Hysteresis Phenomenon in 1,2-Dipalmitoyl-sn-Glycero-3-Phosphocholine Monolayer: Insight into Factors Influencing Surface Dynamics

This study explores the hysteresis phenomenon in DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) monolayers, considering several variables, including temperature, compression and expansion rates, residence time, and subphase content. The investigation focuses on analyzing the influence of these variables on key indicators such as the π-A isotherm curve, loop area, and compression modulus. By employing the Langmuir-Blodgett technique, the findings reveal that all the examined factors significantly affect the aforementioned parameters. Notably, the hysteresis loop, representing dissipated energy, provides valuable insights into the monolayer's viscoelasticity, molecular packing, phase transition changes, and resistance during the isocycle process. These findings contribute to a comprehensive understanding of the structural and dynamic properties of DPPC monolayers, offering insights into their behavior under varying conditions. Moreover, the knowledge gained from this study can aid in the development of precise models and strategies for controlling and manipulating monolayer properties, with potential applications in drug delivery systems, surface coatings, as well as further investigation into air penetration into alveoli and the blinking mechanism.

Phase transitions of the pulmonary surfactant film at the perfluorocarbon-water interface

Pulmonary surfactant is a lipid-protein complex that forms a thin film at the air-water surface of the lungs. This surfactant film defines the elastic recoil and respiratory mechanics of the lungs. One generally accepted rationale of using oxygenated perfluorocarbon (PFC) as a respiratory medium in liquid ventilation is to take advantage of its low surface tensions (14-18 mN/m), which was believed to make PFC an ideal replacement of the exogenous surfactant. Compared with the extensive studies of the phospholipid phase behavior of the pulmonary surfactant film at the air-water surface, its phase behavior at the PFC-water interface is essentially unknown. Here, we reported the first detailed biophysical study of phospholipid phase transitions in two animal-derived natural pulmonary surfactant films, Infasurf and Survanta, at the PFC-water interface using constrained drop surfactometry. Constrained drop surfactometry allows in situ Langmuir-Blodgett transfer from the PFC-water interface, thus permitting direct visualization of lipid polymorphism in pulmonary surfactant films using atomic force microscopy. Our data suggested that regardless of its low surface tension, the PFC cannot be used as a replacement of pulmonary surfactant in liquid ventilation where the air-water surface of the lungs is replaced with the PFC-water interface that features an intrinsically high interfacial tension. The pulmonary surfactant film at the PFC-water interface undergoes continuous phase transitions at surface pressures less than the equilibrium spreading pressure of 50 mN/m and a monolayer-to-multilayer transition above this critical pressure. These results provided not only novel biophysical insight into the phase behavior of natural pulmonary surfactant at the oil-water interface but also translational implications into the further development of liquid ventilation and liquid breathing techniques.

Curvature and shape relaxation in surface-viscous domains

The mechanics of curved, heterogeneous, surfactant-laden surfaces and interfaces are important to a variety of engineering and biological applications. To date, most models of rheologically complex interfaces have focused on homogeneous systems of planar or fixed curvature. In this study, we investigate a simple, dynamical model of a two-phase surface fluid on a curved interface: a condensed, surface-viscous domain embedded within a surface-inviscid, spherical interface of time-varying radius of curvature. Our aim is to understand how changes in surface curvature generate two-dimensional Stokes flows inside the domain, thereby resisting curvature deformation and distorting the domain shape. We model the surface stress within the domain using the classical Boussinesq-Scriven constitutive equation, simplified for a near-spherical cap undergoing a small-amplitude curvature deformation. We then analyze the frequency-dependent dynamics of the surface stress and curvature within the domain when the pressure difference across the surface is sinusoidally oscillated. We find that the curvature relaxes diffusively, and thus define a Peclet number (Pe) relating the rate of diffusion to the oscillation frequency. At small enough Pe, the surface deforms quasi-statically, whereas at high Pe, the curvature varies sharply within a thin boundary layer adjacent to the domain border. Consequently, the curvature of the domain appears discontinuous from the rest of the surface under rapid oscillation. We then examine the linear stability of the domain shape to small, non-axisymmetric perturbations when the surface is steadily compressed (i.e., the pressure difference across it is increased). While the line tension at the domain border tends to maintain circular symmetry, surface-viscous stresses generated by surface compression tend to destabilize the perimeter. A shape instability arises above a critical surface capillary number (Ca) relating surface-viscous stresses to line tension. Moreover, we show that the mechanism of instability is distinct from that of the famous Saffman-Taylor fingering instability. Various extensions of our model are discussed, including materials with finite dilatational surface viscosity, linear and nonlinear (visco)elasticity, and large-amplitude deformations.

Acinar micromechanics in health and lung injury: what we have learned from quantitative morphology

Within the pulmonary acini ventilation and blood perfusion are brought together on a huge surface area separated by a very thin blood-gas barrier of tissue components to allow efficient gas exchange. During ventilation pulmonary acini are cyclically subjected to deformations which become manifest in changes of the dimensions of both alveolar and ductal airspaces as well as the interalveolar septa, composed of a dense capillary network and the delicate tissue layer forming the blood-gas barrier. These ventilation-related changes are referred to as micromechanics. In lung diseases, abnormalities in acinar micromechanics can be linked with injurious stresses and strains acting on the blood-gas barrier. The mechanisms by which interalveolar septa and the blood-gas barrier adapt to an increase in alveolar volume have been suggested to include unfolding, stretching, or changes in shape other than stretching and unfolding. Folding results in the formation of pleats in which alveolar epithelium is not exposed to air and parts of the blood-gas barrier are folded on each other. The opening of a collapsed alveolus (recruitment) can be considered as an extreme variant of septal wall unfolding. Alveolar recruitment can be detected with imaging techniques which achieve light microscopic resolution. Unfolding of pleats and stretching of the blood-gas barrier, however, require electron microscopic resolution to identify the basement membrane. While stretching results in an increase of the area of the basement membrane, unfolding of pleats and shape changes do not. Real time visualization of these processes, however, is currently not possible. In this review we provide an overview of septal wall micromechanics with focus on unfolding/folding as well as stretching. At the same time we provide a state-of-the-art design-based stereology methodology to quantify microarchitecture of alveoli and interalveolar septa based on different imaging techniques and design-based stereology.

Carbon Nanoparticle-Induced Changes to Lipid Monolayer Structure at Water-Air Interfaces

Surface specific vibrational spectroscopy experiments together with surface tension measurements and spectroscopic ellipsometry data were used to characterize the effects of soluble carbon particulates on compressed and partially compressed lipid monolayers adsorbed to the water-air interface. The lipid monolayers consisted of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DPPC), and measurements were made for both tightly packed monolayers (40 Ų/molecule) and monolayers in their liquid condensed state (55 Ų/molecule). Langmuir trough data show that very small amounts of PHF (0.0075 mg/mL or 6.4 × 10^-6 M) decrease lipid film compressibility. This finding supports a cooperative adsorption mechanism whereby the soluble PHFs are drawn to the surface and associate with the insoluble DPPC monolayer. Excess free energies (ΔG_mix^E) were positive, consistent with the cooperative adsorption mechanism, and although the excess free energies are small (≤1 kJ/mol), adsorbed PHF has measurable effects on monolayer structure. Further support for the cooperative adsorption mechanism at the water-air interface comes from vibrational sum frequency generation (VSFG) experiments. Low PHF concentrations (≤0.06 mg/mL) increase DPPC acyl chain ordering in liquid condensed lipid films and decrease DPPC acyl chain ordering and film thickness in tightly packed lipid films.

A recipe for a good clinical pulmonary surfactant

The lives of thousands premature babies have been saved along the last thirty years thanks to the establishment and consolidation of pulmonary surfactant replacement therapies (SRT). It took some time to close the gap between the identification of the biophysical and molecular causes of the high mortality associated with respiratory distress syndrome in very premature babies and the development of a proper therapy. Closing the gap required the elucidation of some key questions defining the structure–function relationships in surfactant as well as the particular role of the different molecular components assembled into the surfactant system. On the other hand, the application of SRT as part of treatments targeting other devastating respiratory pathologies, in babies and adults, is depending on further extensive research still required before enough amounts of good humanized clinical surfactants will be available. This review summarizes our current concepts on the compositional and structural determinants defining pulmonary surfactant activity, the principles behind the development of efficient natural animal-derived or recombinant or synthetic therapeutic surfactants, as well as a the most promising lines of research that are already opening new perspectives in the application of tailored surfactant therapies to treat important yet unresolved respiratory pathologies.

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.

Structural Changes in Films of Pulmonary Surfactant Induced by Surfactant Vesicles

When compressed by the shrinking alveolar surface area during exhalation, films of pulmonary surfactant in situ reduce surface tension to levels at which surfactant monolayers collapse from the surface in vitro. Vesicles of pulmonary surfactant added below these monolayers slow collapse. X-ray scattering here determined the structural changes induced by the added vesicles. Grazing incidence X-ray diffraction on monolayers of extracted calf surfactant detected an ordered phase. Mixtures of dipalmitoyl phosphatidylcholine and cholesterol, but not the phospholipid alone, mimic that structure. At concentrations that stabilize the monolayers, vesicles in the subphase had no effect on the unit cell, and X-ray reflection showed that the film remained monomolecular. The added vesicles, however, produced a concentration-dependent increase in the diffracted intensity. These results suggest that the enhanced resistance to collapse results from enlargement by the additional material of the ordered phase.

Shape morphology of dipolar domains in planar and spherical monolayers

We present a continuum theory for predicting the equilibrium shape and size of dipolar domains formed during liquid-liquid phase coexistence in planar and spherical monolayers. Our main objective is to assess the impact of the monolayer surface curvature on domain morphology. Following previous investigators, we base our analysis around minimizing the free energy, with contributions from line tension and electrostatic dipolar repulsions. Assuming a monodisperse system of circularly symmetric domains, we calculate self-energies and interaction energies for planar and spherical monolayers and determine the equilibrium domain size from the energy minima. We subsequently evaluate the stability of the circularly symmetric domain shapes to an arbitrary, circumferential distortion of the perimeter via a linear stability analysis. We find that the surface curvature generally promotes the formation of smaller, circularly symmetric domains instead of larger, elongated domains. We rationalize these results by examining the effect of the curvature on the intra- and inter-domain dipolar repulsions. We then present a phase diagram of domain shape morphologies, parameterized in terms of the domain area fraction and the monolayer curvature. For typical domain dimensions of 1-30 µm, our theoretical results are relevant to monolayers (and possibly also bilayers) in liquid-liquid phase coexistence with radii of curvature of 1-100 µm.

Establishment of a Synthetic In Vitro Lung Surfactant Model for Particle Interaction Studies on a Langmuir Film Balance

With the intention to provide a robust and economical model that can be used for predicting particle interactions with the pulmonary surfactant, this study was aimed to find an artificial surfactant model that perfectly mimics the properties of the natural pulmonary surfactant. A surfactant model should be reproducible, robust, and able to predict interactions between the pulmonary surfactant and exogenous influences from air and the aqueous site. We compared three synthetic models with the natural bovine surfactant Alveofact. The lung conditions were simulated by spreading the surfactants at the air/aqueous interface on a Langmuir trough with movable barriers. All three artificial surfactant models showed properties very similar to that of Alveofact. Visualization of the monolayers by atomic force microscopy revealed very similar structures with domain formation. The Tanaka lipid mixture has already shown good results in vitro and in vivo in previous studies. The 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)-1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) model has large conformations in the surface pressure isotherms and showed a biomimetic exclusion plateau, indicative of an effective lung surfactant formulation. Also, the equilibrium spreading pressure was similar. DPPC-1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-1'-rac-glycerol (POPG) had the greatest similarities with Alveofact in the hysteresis areas. The kinetic constants of the relaxation experiments during desorption showed that the PCPG model (at 30 mN/m) had almost identical diffusion and dissolution values as Alveofact. As a proof of concept, the interaction of the models with PLGA nanoparticles showed promising results in all experiments for all the three surfactant models. The results show that the choice of components in a model play a crucial role in obtaining reproducible results. The selected models can be used for further studies as synthetic in vitro lung models.

SP-D attenuates LPS-induced formation of human neutrophil extracellular traps (NETs), protecting pulmonary surfactant inactivation by NETs

An exacerbated amount of neutrophil extracellular traps (NETs) can cause dysfunction of systems during inflammation. However, host proteins and factors that suppress NET formation (NETosis) are not clearly identified. Here we show that an innate immune collectin, pulmonary surfactant protein-D (SP-D), attenuates lipopolysaccharide (LPS)-mediated NETosis in human neutrophils by binding to LPS. SP-D deficiency in mice (Sftpd-/-) leads to excess NET formation in the lungs during LPS-mediated inflammation. In the absence of SP-D, NETs inhibit the surface-active properties of lung surfactant, essential to prevent the collapse of alveoli, the air breathing structures of the lungs. SP-D reverses NET-mediated inhibition of surfactant and restores the biophysical properties of surfactant. To the best of our knowledge, this study establishes for the first time that (i) SP-D suppresses LPS-mediated NETosis, (ii) NETs inhibit pulmonary surfactant function in the absence of SP-D, and (iii) SP-D can restore NET-mediated inhibition of the surfactant system.

Melting transitions in biomembranes

We investigated melting transitions in native biological membranes containing their membrane proteins. The membranes originated from E. coli, B. subtilis, lung surfactant and nerve tissue from the spinal cord of several mammals. For some preparations, we studied the pressure, pH and ionic strength dependence of the transition. For porcine spine, we compared the transition of the native membrane to that of the extracted lipids. All preparations displayed melting transitions of 10-20° below physiological or growth temperature, independent of the organism of origin and the respective cell type. We found that the position of the transitions in E. coli membranes depends on the growth temperature. We discuss these findings in the context of the thermodynamic theory of membrane fluctuations close to transition that predicts largely altered elastic constants, an increase in fluctuation lifetime and in membrane permeability. We also discuss how to distinguish lipid melting from protein unfolding transitions. Since the feature of a transition slightly below physiological temperature is conserved even when growth conditions change, we conclude that the transitions are likely to be of major biological importance for the survival and the function of the cell.

The Lγ Phase of Pulmonary Surfactant

To determine how different components affect the structure of pulmonary surfactant, we measured X-ray scattering by samples derived from calf surfactant. The surfactant phospholipids demonstrated the essential characteristics of the L_γ phase: a unit cell with a lattice constant appropriate for two bilayers, and crystalline chains detected by wide-angle X-ray scattering (WAXS). The electron density profile, obtained from scattering by oriented films at different relative humidities (70-97%), showed that the two bilayers, arranged as mirror images, each contain two distinct leaflets with different thicknesses and profiles. The detailed structures suggest one ordered leaflet that would contain crystalline chains and one disordered monolayer likely to contain the anionic compounds, which constitute ∼10% of the surfactant phospholipids. The spacing and temperature dependence detected by WAXS fit with an ordered leaflet composed of dipalmitoyl phosphatidylcholine. Physiological levels of cholesterol had no effect on this structure. Removing the anionic phospholipids prevented formation of the L_γ phase. The cationic surfactant proteins inhibited L_γ structures, but at levels unlikely related to charge. Because the L_γ phase, if arranged properly, could produce a self-assembled ordered interfacial monolayer, the structure could have important functional consequences. Physiological levels of the proteins, however, inhibit formation of the L_γ structures at high relative humidities, making their physiological significance uncertain.

Homo- and hetero-oligomerization of hydrophobic pulmonary surfactant proteins SP-B and SP-C in surfactant phospholipid membranes

Pulmonary surfactant is a lipid/protein mixture that reduces surface tension at the respiratory air-water interface in lungs. Among its nonlipidic components are pulmonary surfactant-associated proteins B and C (SP-B and SP-C, respectively). These highly hydrophobic proteins are required for normal pulmonary surfactant function, and whereas past literature works have suggested possible SP-B/SP-C interactions and a reciprocal modulation effect, no direct evidence has been yet identified. In this work, we report an extensive fluorescence spectroscopy study of both intramolecular and intermolecular SP-B and SP-C interactions, using a combination of quenching and FRET steady-state and time-resolved methodologies. These proteins are compartmentalized in full surfactant membranes but not in pure 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) vesicles, in accordance with their previously described preference for liquid disordered phases. From the observed static self-quenching and homo-FRET of BODIPY-FL labeled SP-B, we conclude that this protein forms homoaggregates at low concentration (lipid:protein ratio, 1:1000). Increases in polarization of BODIPY-FL SP-B and steady-state intensity of WT SP-B were observed upon incorporation of under-stoichiometric amounts of WT SP-C. Conversely, Marina Blue-labeled SP-C is quenched by over-stoichiometric amounts of WT SP-B, whereas under-stoichiometric concentrations of the latter actually increase SP-C emission. Time-resolved hetero-FRET from Marina Blue SP-C to BODIPY-FL SP-B confirm distinct protein aggregation behaviors with varying SP-B concentration. Based on these multiple observations, we propose a model for SP-B/SP-C interactions, where SP-C might induce conformational changes on SP-B complexes, affecting its aggregation state. The conclusions inferred from the present work shed light on the synergic functionality of both proteins in the pulmonary surfactant system.

Applications of Brewster angle microscopy from biological materials to biological systems

Brewster angle microscopy (BAM) is a powerful technique that allows for real-time visualization of Langmuir monolayers. The lateral organization of these films can be investigated, including phase separation and the formation of domains, which may be of different sizes and shapes depending on the properties of the monolayer. Different molecules or small changes within a molecule such as the molecule's length or presence of a double bond can alter the monolayer's lateral organization that is usually undetected using surface pressure-area isotherms. The effect of such changes can be clearly observed using BAM in real-time, under full hydration, which is an experimental advantage in many cases. While previous BAM reviews focused more on selected compounds or compared the impact of structural variations on the lateral domain formation, this review provided a broader overview of BAM application using biological materials and systems including the visualization of amphiphilic molecules, proteins, drugs, extracts, DNA, and nanoparticles at the air-water interface.

Life Is Simple-Biologic Complexity Is an Epiphenomenon

Life originated from unicellular organisms by circumventing the Second Law of Thermodynamics using the First Principles of Physiology, namely negentropy, chemiosmosis and homeostatic regulation of calcium and lipids. It is hypothesized that multicellular organisms are merely contrivances or tools, used by unicellular organisms as agents for the acquisition of epigenetic inheritance. The First Principles of Physiology, which initially evolved in unicellular organisms are the exapted constraints that maintain, sustain and perpetuate that process. To ensure fidelity to this mechanism, we must return to the first principles of the unicellular state as the determinants of the primary level of selection pressure during the life cycle. The power of this approach is reflected by examples of its predictive value. This perspective on life is a "game changer", mechanistically rendering transparent many dogmas, teleologies and tautologies that constrain the current descriptive view of Biology.

Computer simulations of lung surfactant

Lung surfactant lines the gas-exchange interface in the lungs and reduces the surface tension, which is necessary for breathing. Lung surfactant consists mainly of lipids with a small amount of proteins and forms a monolayer at the air-water interface connected to bilayer reservoirs. Lung surfactant function involves transfer of material between the monolayer and bilayers during the breathing cycle. Lipids and proteins are organized laterally in the monolayer; selected species are possibly preferentially transferred to bilayers. The complex 3D structure of lung surfactant and the exact roles of lipid organization and proteins remain important goals for research. We review recent simulation studies on the properties of lipid monolayers, monolayers with phase coexistence, monolayer-bilayer transformations, lipid-protein interactions, and effects of nanoparticles on lung surfactant. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Pre-transition effects mediate forces of assembly between transmembrane proteins

We present a mechanism for a generic, powerful force of assembly and mobility for transmembrane proteins in lipid bilayers. This force is a pre-transition (or pre-melting) effect for the first-order transition between ordered and disordered phases in the membrane. Using large-scale molecular simulation, we show that a protein with hydrophobic thickness equal to that of the disordered phase embedded in an ordered bilayer stabilizes a microscopic order–disorder interface. The stiffness of that interface is finite. When two such proteins approach each other, they assemble because assembly reduces the net interfacial energy. Analogous to the hydrophobic effect, we refer to this phenomenon as the 'orderphobic effect'. The effect is mediated by proximity to the order–disorder phase transition and the size and hydrophobic mismatch of the protein. The strength and range of forces arising from this effect are significantly larger than those that could arise from membrane elasticity for the membranes considered.

DOI:http://dx.doi.org/10.7554/eLife.13150.001

The membrane that surrounds cells provides a selective barrier that allows some molecules through, but blocks the path of others. A cell’s membrane is made up of two layers of molecules with oily tails, and is therefore known as a bilayer. Many proteins are dotted within and on the inner and outer surfaces of the bilayer: some act as channels that control what goes in and out of the cell, while others protrude outside the cell so that they can sense changes in the environment.

Membrane proteins can move and interact within the bilayer, and various models have emerged to try to explain this dynamic system. These models are based on the membrane having some fluidity but also having regions where there is more structure, and typically describe the proteins as ordered clusters floating in an otherwise disordered fluid membrane. However, many researchers now think some proteins that pass through both layers of the bilayer (i.e., transmembrane proteins) make membranes more ordered, with a possibly gel-like state. However, it is not clear how transmembrane proteins can move and assemble together within such a relatively rigid membrane.

To investigate this, Katira, Mandadapu, Vaikuntanathan et al. carried out computer simulations using a model of a simple bilayer membrane. This membrane can exist in an ordered state, where the oily tails are neatly aligned, or a disordered state, where they are irregularly packed. Virtual ‘heating’ of the membrane caused it to shift from an ordered to a disordered state. When a simple transmembrane protein favoring the disordered state was inserted into the ordered state of the modeled membrane, disordered regions formed locally around the protein and the protein was able to diffuse within the membrane.

Modeling what would happen if two transmembrane proteins approached each other revealed that a consequence of the order–disorder transition is a strong attractive force that assembles the proteins together. Katira, Mandadapu, Vaikuntanathan et al. named this new phenomenon the 'orderphobic effect'. The forces arising from this effect were much greater than those currently believed to contribute to the assembly of membrane protein complexes, such as those generated by the elasticity of the membrane. This means that the orderphobic effect may be responsible for generating the protein clusters commonly seen in cell membranes.

Future work should next explore the opposite effect, where proteins favoring the ordered state are inserted into the disordered state of a membrane. This is expected to cause clustering of such proteins and thus large ordered regions in an otherwise disordered membrane.

DOI:http://dx.doi.org/10.7554/eLife.13150.002

Evolution, Development, and Function of the Pulmonary Surfactant System in Normal and Perturbed Environments

Surfactant lipids and proteins form a surface active film at the air-liquid interface of internal gas exchange organs, including swim bladders and lungs. The system is uniquely positioned to meet both the physical challenges associated with a dynamically changing internal air-liquid interface, and the environmental challenges associated with the foreign pathogens and particles to which the internal surface is exposed. Lungs range from simple, transparent, bag-like units to complex, multilobed, compartmentalized structures. Despite this anatomical variability, the surfactant system is remarkably conserved. Here, we discuss the evolutionary origin of the surfactant system, which likely predates lungs. We describe the evolution of surfactant structure and function in invertebrates and vertebrates. We focus on changes in lipid and protein composition and surfactant function from its antiadhesive and innate immune to its alveolar stability and structural integrity functions. We discuss the biochemical, hormonal, autonomic, and mechanical factors that regulate normal surfactant secretion in mature animals. We present an analysis of the ontogeny of surfactant development among the vertebrates and the contribution of different regulatory mechanisms that control this development. We also discuss environmental (oxygen), hormonal and biochemical (glucocorticoids and glucose) and pollutant (maternal smoking, alcohol, and common "recreational" drugs) effects that impact surfactant development. On the adult surfactant system, we focus on environmental variables including temperature, pressure, and hypoxia that have shaped its evolution and we discuss the resultant biochemical, biophysical, and cellular adaptations. Finally, we discuss the effect of major modern gaseous and particulate pollutants on the lung and surfactant system.

The Equilibrium Spreading Tension of Pulmonary Surfactant

Monomolecular films at an air/water interface coexist at the equilibrium spreading tension (γ(e)) with the bulk phase from which they form. For individual phospholipids, γ(e) is single-valued, and separates conditions at which hydrated vesicles adsorb from tensions at which overcompressed monolayers collapse. With pulmonary surfactant, isotherms show that monolayers compressed on the surface of bubbles coexist with the three-dimensional collapsed phase over a range of surface tensions. γ(e) therefore represents a range rather than a single value of surface tension. Between the upper and lower ends of this range, rates of collapse for spread and adsorbed films decrease substantially. Changes during adsorption across this narrow region of coexistence between the two- and three-dimensional structures at least partially explain how alveolar films of pulmonary surfactant become resistant to collapse.

The mechanism of collapse of heterogeneous lipid monolayers

Collapse of homogeneous lipid monolayers is known to proceed via wrinkling/buckling, followed by folding into bilayers in water. For heterogeneous monolayers with phase coexistence, the mechanism of collapse remains unclear. Here, we investigated collapse of lipid monolayers with coexisting liquid-liquid and liquid-solid domains using molecular dynamics simulations. The MARTINI coarse-grained model was employed to simulate monolayers of ∼80 nm in lateral dimension for 10-25 μs. The monolayer minimum surface tension decreased in the presence of solid domains, especially if they percolated. Liquid-ordered domains facilitated monolayer collapse due to the spontaneous curvature induced at a high cholesterol concentration. Upon collapse, bilayer folds formed in the liquid (disordered) phase; curved domains shifted the nucleation sites toward the phase boundary. The liquid (disordered) phase was preferentially transferred into bilayers, in agreement with the squeeze-out hypothesis. As a result, the composition and phase distribution were altered in the monolayer in equilibrium with bilayers compared to a flat monolayer at the same surface tension. The composition and phase behavior of the bilayers depended on the degree of monolayer compression. The monolayer-bilayer connection region was enriched in unsaturated lipids. Percolation of solid domains slowed down monolayer collapse by several orders of magnitude. These results are important for understanding the mechanism of two-to-three-dimensional transformations in heterogeneous thin films and the role of lateral organization in biological membranes. The study is directly relevant for the function of lung surfactant, and can explain the role of nanodomains in its surface activity and inhibition by an increased cholesterol concentration.

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.

An investigation of the carbon nanotube--Lipid interface and its impact upon pulmonary surfactant lipid function

Multiwalled carbon nanotubes (MWCNTs) are now synthesized on a large scale, increasing the risk of occupational inhalation. However, little is known of the MWCNT-pulmonary surfactant (PS) interface and its effect on PS functionality. The Langmuir-Blodgett trough was used to evaluate the impact of MWCNTs on fundamental properties of PS lipids which influence PS function, i.e. compression resistance and maximum obtainable pressure. Changes were found to be MWCNT length-dependent. 'Short' MWCNTs (1.1 μm, SD = 0.61) penetrated the lipid film, reducing the maximum interfacial film pressure by 10 mN/m (14%) in dipalmitoylphosphatidylcholine (DPPC) and PS, at an interfacial MWCNT-PS lipid mass ratio range of 50:1 to 1:1. 'Long' commercial MWCNTs (2.1 μm, SD = 1.2) caused compression resistance at the same mass loadings. 'Very long' MWCNTs (35 μm, SD = 19) sequestered DPPC and were squeezed out of the DPPC film. High resolution transmission electron microscopy revealed that all MWCNT morphologies formed DPPC coronas with ordered arrangements. These results provide insight into how nanoparticle aspect ratio affects the interaction mechanisms with PS, in its near-native state at the air-water interface.

Direct observation of nanometer-scale pores of melittin in supported lipid monolayers

Melittin is the most studied membrane-active peptide and archetype within a large and diverse group of pore formers. However, the molecular characteristics of melittin pores remain largely unknown. Herein, we show by atomic force microscopy (AFM) that lipid monolayers in the presence of melittin are decorated with numerous regularly shaped circular pores that can be distinguished from nonspecific monolayer defects. The specificity of these pores is reinforced through a statistical evaluation of depressions found in Langmuir-Blodgett monolayers in the presence and absence of melittin, which eventually allows characterization of the melittin-induced pores at a quantitative low-resolution level. We observed that the large majority of pores exhibit near-circular symmetry and a Gaussian distribution in size, with a mean diameter of ∼8.7 nm. A distinctive feature is a ring of material found around the pores, made by, on average, three positive peaks, with a height over the level of the lipidic background of ∼0.23 nm. This protruding rim is most likely due to the presence of melittin near the pore border. Although the current resolution of the AFM images in the {x, y} plane does not allow distinction of the specific organization of the peptide molecules, these results provide an unprecedented view of melittin pores formed in lipidic interfaces and open new perspectives for future structural investigations of these and other pore-forming peptides and proteins using supported monolayers.

Computer simulations of phase separation in lipid bilayers and monolayers

Studying phase coexistence in lipid bilayers and monolayers is important for understanding lipid-lipid interactions underlying lateral organization in biological membranes. Computer simulations follow experimental approaches and use model lipid mixtures of simplified composition. Atomistic simulations give detailed information on the specificity of intermolecular interactions, while coarse-grained simulations achieve large time and length scales and provide a bridge towards state-of-the-art experimental techniques. Computer simulations allow characterizing the structure and composition of domains during phase transformations at Angstrom and picosecond resolution, and bring new insights into phase behavior of lipid membranes.

Composition, structure and mechanical properties define performance of pulmonary surfactant membranes and films

The respiratory surface in the mammalian lung is stabilized by pulmonary surfactant, a membrane-based system composed of multiple lipids and specific proteins, the primary function of which is to minimize the surface tension at the alveolar air-liquid interface, optimizing the mechanics of breathing and avoiding alveolar collapse, especially at the end of expiration. The goal of the present review is to summarize current knowledge regarding the structure, lipid-protein interactions and mechanical features of surfactant membranes and films and how these properties correlate with surfactant biological function inside the lungs. Surfactant mechanical properties can be severely compromised by different agents, which lead to surfactant inhibition and ultimately contributes to the development of pulmonary disorders and pathologies in newborns, children and adults. A detailed comprehension of the unique mechanical and rheological properties of surfactant layers is crucial for the diagnostics and treatment of lung diseases, either by analyzing the contribution of surfactant impairment to the pathophysiology or by improving the formulations in surfactant replacement therapies. Finally, a short review is also included on the most relevant experimental techniques currently employed to evaluate lung surfactant mechanics, rheology, and inhibition and reactivation processes.

Mechanism of action of lung damage caused by a nanofilm spray product

Inhalation of waterproofing spray products has on several occasions caused lung damage, which in some cases was fatal. The present study aims to elucidate the mechanism of action of a nanofilm spray product, which has been shown to possess unusual toxic effects, including an extremely steep concentration-effect curve. The nanofilm product is intended for application on non-absorbing flooring materials and contains perfluorosiloxane as the active film-forming component. The toxicological effects and their underlying mechanisms of this product were studied using a mouse inhalation model, by in vitro techniques and by identification of the binding interaction. Inhalation of the aerosolized product gave rise to increased airway resistance in the mice, as evident from the decreased expiratory flow rate. The toxic effect of the waterproofing spray product included interaction with the pulmonary surfactants. More specifically, the active film-forming components in the spray product, perfluorinated siloxanes, inhibited the function of the lung surfactant due to non-covalent interaction with surfactant protein B, a component which is crucial for the stability and persistence of the lung surfactant film during respiration. The active film-forming component used in the present spray product is also found in several other products on the market. Hence, it may be expected that these products may have a toxicity similar to the waterproofing product studied here. Elucidation of the toxicological mechanism and identification of toxicological targets are important to perform rational and cost-effective toxicological studies. Thus, because the pulmonary surfactant system appears to be an important toxicological target for waterproofing spray products, study of surfactant inhibition could be included in toxicological assessment of this group of consumer products.

Structure-function relationships in pulmonary surfactant membranes: from biophysics to therapy

Pulmonary surfactant is an essential lipid-protein complex to maintain an operative respiratory surface at the mammalian lungs. It reduces surface tension at the alveolar air-liquid interface to stabilise the lungs against physical forces operating along the compression-expansion breathing cycles. At the same time, surfactant integrates elements establishing a primary barrier against the entry of pathogens. Lack or deficiencies of the surfactant system are associated with respiratory pathologies, which treatment often includes supplementation with exogenous materials. The present review summarises current models on the molecular mechanisms of surfactant function, with particular emphasis in its biophysical properties to stabilise the lungs and the molecular alterations connecting impaired surfactant with diseased organs. It also provides a perspective on the current surfactant-based strategies to treat respiratory pathologies. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Segregated ordered lipid phases and protein-promoted membrane cohesivity are required for pulmonary surfactant films to stabilize and protect the respiratory surface

Pulmonary surfactant is a lipid-protein complex essential to stabilize alveoli, by forming surface active films able to reach and sustain very low surface tensions (< 2 mN m(-1)) during the film compression that occurs at end-expiration. The particular lipid composition of surfactant, including a high proportion of dipalmitoylphosphatidylcholine (DPPC), induces segregation of fluid ordered and disordered phases in surfactant membranes and films at physiological temperatures. The segregation of DPPC-enriched ordered phase has been related with the ability of surfactant films to produce very low tensions, while the presence in surfactant of two specific hydrophobic polypeptides, SP-B and SP-C, is absolutely required to facilitate surfactant dynamics, including film formation and re-spreading during expansion at inspiration. In the present study, we have used X-ray scattering to analyze the structure of (1) whole native surfactant membranes purified from porcine lungs, (2) membranes reconstituted from the organic extract of surfactant containing the full lipid complement and the physiological proportion of SP-B and SP-C, and (3) membranes reconstituted from the lipid fraction of surfactant depleted of proteins. Small angle X-ray scattering data from whole surfactant or from membranes reconstituted from surfactant organic extract indicated the co-existence of two lamellar phases with different thicknesses. Such phase coexistence disappeared upon heating of the samples at temperatures above physiological values. When assessed in a captive bubble surfactometer, which mimics interfacial compression-expansion dynamics, the ability of surfactant films to produce very low tensions is only maintained at temperatures permitting the coexistence of the two lamellar phases. On the other hand, membranes reconstituted in the absence of proteins produced diffractograms indicative of the existence of a single dominant lamellar phase at all temperatures. These data suggest that SP-B and SP-C establish membrane-membrane interactions coupling the stacks of different segregated phases. The low compressibility of surfactant films that leads to the maximal pressures (minimal tensions) is supported on one hand by the highly packed solid-like character of segregated DPPC-enriched domains and, on the other hand, by a high cohesivity of multilayered structures promoted by hydrophoblic surfactant proteins, in particular SP-B, at the more dynamic disordered membrane regions, in which SP-B selectively partitions. Cryo-electron microscopy has shown that SP-B induces formation of tight membrane-membrane contacts, a finding that supports our inference concerning the role of these surfactant proteins.

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.

Adaptations to hibernation in lung surfactant composition of 13-lined ground squirrels influence surfactant lipid phase segregation properties

Pulmonary surfactant lines the entire alveolar surface, serving primarily to reduce the surface tension at the air-liquid interface. Surfactant films adsorb as a monolayer interspersed with multilayers with surfactant lipids segregating into different phases or domains. Temperature variation, which influences lipid physical properties, affects both the lipid phase segregation and the surface activity of surfactants. In hibernating animals, such as 13-lined ground squirrels, which vary their body temperature, surfactant must be functional over a wide range of temperatures. We hypothesised that surfactant from the 13-lined ground squirrel, Ictidomys tridecemlineatus, would undergo appropriate lipid structural re-arrangements at air-water interfaces to generate phase separation, sufficient to attain the low surface tensions required to remain stable at both low and high body temperatures. Here, we examined pressure-area isotherms at 10, 25 and 37°C and found that surfactant films from both hibernating and summer-active squirrels reached their highest surface pressure on the Wilhelmy-Langmuir balance at 10°C. Epifluorescence microscopy demonstrated that films of hibernating squirrel surfactant display different lipid micro-domain organisation characteristics than surfactant from summer-active squirrels. These differences were also reflected at the nanoscale as determined by atomic force microscopy. Such re-arrangement of lipid domains in the relatively more fluid surfactant films of hibernating squirrels may contribute to overcoming collapse pressures and support low surface tension during the normal breathing cycle at low body temperatures.

Phase-field model for the morphology of monolayer lipid domains

Phase-separated domains exist in multicomponent lipid monolayers and bilayers. We present here a phase-field model that takes into account the competition between lipid dipole-dipole interactions and line tension to define the domain morphology. A dynamic equation for the phase-field is solved numerically showing stationary non-circular shapes like starfish shapes. This phase-field model could be applied to study the dynamic properties of complex problems like phase segregation in pulmonary surfactant membranes and films.

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.

Influence of hydrophobic alkylated gold nanoparticles on the phase behavior of monolayers of DPPC and clinical lung surfactant

The effect of hydrophobic alkylated gold nanoparticles (Au NPs) on the phase behavior and structure of Langmuir monolayers of dipalmitoylphosphatidylcholine (DPPC) and Survanta, a naturally derived commercial pulmonary surfactant that contains DPPC as the main lipid component and hydrophobic surfactant proteins SP-B and SP-C, has been investigated in connection with the potential implication of inorganic NPs in pulmonary surfactant dysfunction. Hexadecanethiolate-capped Au NPs (C(16)SAu NPs) with an average core diameter of 2 nm have been incorporated into DPPC monolayers in concentrations ranging from 0.1 to 0.5 mol %. Concentrations of up to 0.2 mol % in DPPC and 16 wt % in Survanta do not affect the monolayer phase behavior at 20 °C, as evidenced by surface pressure-area (π-A) and ellipsometric isotherms. The monolayer structure at the air/water interface was imaged as a function of the surface pressure by Brewster angle microscopy (BAM). In the liquid-expanded/liquid-condensed phase coexistence region of DPPC, the presence of 0.2 mol % C(16)SAu NPs causes the formation of many small, circular, condensed lipid domains, in contrast to the characteristic larger multilobes formed by pure lipid. Condensed domains of similar size and shape to those of DPPC with 0.2 mol % C(16)SAu NPs are formed by compressing Survanta, and these are not affected by the C(16)SAu NPs. Atomic force microscopy images of Langmuir-Schaefer-deposited films support the BAM observations and reveal, moreover, that at high surface pressures (i.e., 35 and 45 mN m(-1)) the C(16)SAu NPs form honeycomb-like aggregates around the polygonal condensed DPPC domains. In the Survanta monolayers, the C(16)SAu NPs were found to accumulate together with the proteins in the liquid-expanded phase around the circular condensed lipid domains. In conclusion, the presence of hydrophobic C(16)SAu NPs in amounts that do not influence the π-A isotherm alters the nucleation, growth, and morphology of the condensed domains in monolayers of DPPC but not of those of Survanta. Systematic investigations of the effect of the interaction of chemically defined NPs with the lipid and protein components of lung surfactant on the physicochemical properties of surfactant films are pertinent to understanding how inhaled NPs impact pulmonary function.

A modified squeeze-out mechanism for generating high surface pressures with pulmonary surfactant

The exact mechanism by which pulmonary surfactant films reach the very low surface tensions required to stabilize the alveoli at end expiration remains uncertain. We utilized the nanoscale sensitivity of atomic force microscopy (AFM) to examine phospholipid (PL) phase transition and multilayer formation for two Langmuir-Blodgett (LB) systems: a simple 3 PL surfactant-like mixture and the more complex bovine lipid extract surfactant (BLES). AFM height images demonstrated that both systems develop two types of liquid condensed (LC) domains (micro- and nano-sized) within a liquid expanded phase (LE). The 3 PL mixture failed to form significant multilayers at high surface pressure (π while BLES forms an extensive network of multilayer structures containing up to three bilayers. A close examination of the progression of multilayer formation reveals that multilayers start to form at the edge of the solid-like LC domains and also in the fluid-like LE phase. We used the elemental analysis capability of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to show that multilayer structures are enriched in unsaturated PLs while the saturated PLs are concentrated in the remaining interfacial monolayer. This supports a modified squeeze-out model where film compression results in the hydrophobic surfactant protein-dependent formation of unsaturated PL-rich multilayers which remain functionally associated with a monolayer enriched in disaturated PL species. This allows the surface film to attain low surface tensions during compression and maintain values near equilibrium during expansion.

Meconium impairs pulmonary surfactant by a combined action of cholesterol and bile acids

Mechanisms for meconium-induced inactivation of pulmonary surfactant as part of the meconium aspiration syndrome in newborn infants, to our knowledge, are not clearly understood. Here we have studied the biophysical mechanisms of how meconium affects surface activity of pulmonary surfactant and whether the membrane-perturbing effects of meconium can be mimicked by exposure of surfactant to a mixture of bile acids and cholesterol. Surface activity of pulmonary surfactant complexes purified from animal lungs was analyzed in the absence and in the presence of meconium in standard surface balances and in a captive bubble surfactometer. We have also evaluated accumulation of surfactant at the air-liquid interface by what we believe to be a novel microtiter plate fluorescent assay, and the effect of meconium components on surfactant membrane fluidity using Laurdan fluorescence thermotropic profiles and differential scanning calorimetry thermograms. Rapid interfacial adsorption, low surface tension upon film compression, efficient film replenishment upon expansion, and thermotropic properties of surfactant complexes are all adversely affected by meconium, and, in a similar manner, they are affected by cholesterol/taurocholate mixtures but not by taurocholate alone. We conclude that inhibition of surfactant by meconium can be mimicked by a bile salt-promoted incorporation of excess cholesterol into surfactant complexes. These results highlight the potential pathogenic role of cholesterol-mobilizing agents as a crucial factor resulting in cholesterol induced alterations of structure and dynamics of surfactant membranes and films.