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

Surfactant protein A modulates neuroinflammation in adult mice upon pulmonary infection

BACKGROUND

One of the most common entry gates for systemic infection is the lung. In humans, pulmonary infections can lead to significant neurological impairment, ranging from acute sickness behavior to long-term disorders. Surfactant proteins (SP), essential parts of the pulmonary innate immune defense, have been detected in the brain of rats and humans. Recent evidence suggests that SP-A, the major protein component of surfactant, also plays a functional role in modulating neuroinflammation. This study aimed to determine whether SP-A deficiency affects the inflammatory response in the brain of adult mice during pulmonary infection.

EXPERIMENTAL PROCEDURE

Adult male wild-type (WT, n = 72) and SP-A-deficient (SP-A-/-, n = 72) mice were oropharyngeally challenged with lipopolysaccharide (LPS), Pseudomonas aeruginosa (P. aeruginosa), or PBS (control). Both, behavioral assessment and subsequent brain tissue analysis, were performed 24, 48, and 72 h after challenge. The brain concentrations of the pro-inflammatory cytokines TNF-α, IL-6, and IL-1β were determined by ELISA. Quantitative rtPCR was used to detect SP-A mRNA expression in brain homogenates and immunohistochemistry was applied for the detection of SP-A protein expression in brain coronal slices.

RESULTS

SP-A mRNA and histological evidence of protein expression were detected in both the lungs and brains of WT mice, with significantly higher amounts in lung samples. SP-A-/- mice exhibited significantly higher baseline concentrations of brain TNF-α, IL-6, and IL-1β compared to WT mice. Oropharyngeal application of either LPS or P. aeruginosa elicited significantly higher brain levels of TNF-α and IL-1β in SP-A-/- mice compared to WT mice at all time points. In comparison, behavioral impairment as a measure of sickness behavior, was significantly stronger in WT than in SP-A-/- mice, particularly after LPS application.

CONCLUSION

SP-A is known for its anti-inflammatory role in the pulmonary immune response to bacterial infection. Recent evidence suggests that in an abdominal sepsis model SP-A deficiency can lead to increased cytokine levels in the brain. Our results extend this perception and provide evidence for an anti-inflammatory role of SP-A in the brain of adult WT mice after pulmonary infection.

Activation of alveolar epithelial ER stress by β-coronavirus infection disrupts surfactant homeostasis in mice: implications for COVID-19 respiratory failure

COVID-19 syndrome is characterized by acute lung injury, hypoxemic respiratory failure, and high mortality. Alveolar type 2 (AT2) cells are essential for gas exchange, repair, and regeneration of distal lung epithelium. We have shown that the causative agent, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and other members of the β-coronavirus genus induce an endoplasmic reticulum (ER) stress response in vitro; however, the consequences for host AT2 cell function in vivo are less understood. To study this, two murine models of coronavirus infection were used-mouse hepatitis virus-1 (MHV-1) in A/J mice and a mouse-adapted SARS-CoV-2 strain. MHV-1-infected mice exhibited dose-dependent weight loss with histological evidence of distal lung injury accompanied by elevated bronchoalveolar lavage fluid (BALF) cell counts and total protein. AT2 cells showed evidence of both viral infection and increased BIP/GRP78 expression, consistent with activation of the unfolded protein response (UPR). The AT2 UPR included increased inositol-requiring enzyme 1α (IRE1α) signaling and a biphasic response in PKR-like ER kinase (PERK) signaling accompanied by marked reductions in AT2 and BALF surfactant protein (SP-B and SP-C) content, increases in surfactant surface tension, and emergence of a reprogrammed epithelial cell population (Krt8+ and Cldn4+). The loss of a homeostatic AT2 cell state was attenuated by treatment with the IRE1α inhibitor OPK-711. As a proof-of-concept, C57BL6 mice infected with mouse-adapted SARS-CoV-2 demonstrated similar lung injury and evidence of disrupted surfactant homeostasis. We conclude that lung injury from β-coronavirus infection results from an aberrant host response, activating multiple AT2 UPR stress pathways, altering surfactant metabolism/function, and changing AT2 cell state, offering a mechanistic link between SARS-CoV-2 infection, AT2 cell biology, and acute respiratory failure.NEW & NOTEWORTHY COVID-19 syndrome is characterized by hypoxemic respiratory failure and high mortality. In this report, we use two murine models to show that β-coronavirus infection produces acute lung injury, which results from an aberrant host response, activating multiple epithelial endoplasmic reticular stress pathways, disrupting pulmonary surfactant metabolism and function, and forcing emergence of an aberrant epithelial transition state. Our results offer a mechanistic link between SARS-CoV-2 infection, AT2 cell biology, and respiratory failure.

Overstretching alveolar epithelial type II cells decreases surfactant secretion via actin polymerization and intracellular trafficking alteration

Pulmonary surfactant is essential for maintaining proper lung function. Alveolar epithelial type II (AE2) cells secrete surfactants via lamellar bodies (LBs). In tidal loading during each breath, the physiological cyclic stretching of AE2 cells promotes surfactant secretion. Excessive stretching inhibits surfactant secretion, which is considered to contribute to the development of lung damage. However, its precise mechanism remains unknown. This study tested whether actin polymerization and intracellular transport are required for pulmonary surfactant secretion and the association of actin polymerization and transport in identical human AE2-derived A549 cells using live-cell imaging, not in the bulk cells population. We found that overstretching approximately doubled actin polymerization into filaments (F-actin) and suppressed LB secretion by half in the fluorescent area ratio, compared with physiological stretching (F-actin: 1.495 vs 0.643 (P < 0.01); LB: 0.739 vs 0.332 (P < 0.01)). An inhibitor of actin polymerization increased LB secretion. Intracellular tracking using fluorescent particles revealed that cyclic stretching shifted the particle motion perpendicularly to the direction of stretching according to the orientation of the F-actin (proportion of perpendicular axis motion prior particle: 0h 40.12 % vs 2h 63.13 % (P < 0.01)), and particle motion was restricted over time in the cells subjected to overstretching, indicating that overstretching regulates intracellular transport dynamics (proportion of stop motion particle: 0h 1.01 % vs 2h 11.04 % (P < 0.01)). These findings suggest that overstretching changes secretion through the cytoskeleton: overstretching AE2 cells inhibits pulmonary surfactant secretion, at least through accelerating actin polymerization and decreasing intracellular trafficking, and the change in actin orientation would modulate intracellular trafficking.

Nanomaterials-assisted gene editing and synthetic biology for optimizing the treatment of pulmonary diseases

The use of nanomaterials in gene editing and synthetic biology has emerged as a pivotal strategy in the pursuit of refined treatment methodologies for pulmonary disorders. This review discusses the utilization of nanomaterial-assisted gene editing tools and synthetic biology techniques to promote the development of more precise and efficient treatments for pulmonary diseases. First, we briefly outline the characterization of the respiratory system and succinctly describe the principal applications of diverse nanomaterials in lung ailment treatment. Second, we elaborate on gene-editing tools, their configurations, and assorted delivery methods, while delving into the present state of nanomaterial-facilitated gene-editing interventions for a spectrum of pulmonary diseases. Subsequently, we briefly expound on synthetic biology and its deployment in biomedicine, focusing on research advances in the diagnosis and treatment of pulmonary conditions against the backdrop of the coronavirus disease 2019 pandemic. Finally, we summarize the extant lacunae in current research and delineate prospects for advancement in this domain. This holistic approach augments the development of pioneering solutions in lung disease treatment, thereby endowing patients with more efficacious and personalized therapeutic alternatives.

A Review on Investigating the Interactions between Nanoparticles and the Pulmonary Surfactant Monolayer with Coarse-Grained Molecular Dynamics Method

Pulmonary drug delivery has garnered significant attention due to its targeted local lung action, minimal toxic side effects, and high drug utilization. However, the physicochemical properties of inhaled nanoparticles (NPs) used as drug carriers can influence their interactions with the pulmonary surfactant (PS) monolayer, potentially altering the fate of the NPs and impairing the biophysical function of the PS monolayer. Thus, the objective of this review is to summarize how the physicochemical properties of NPs affect their interactions with the PS monolayer. Initially, the definition and properties of NPs, as well as the composition and characteristics of the PS monolayer, are introduced. Subsequently, the coarse-grained molecular dynamics (CGMD) simulation method for studying the interactions between NPs and the PS monolayer is presented. Finally, the implications of the hydrophobicity, size, shape, surface charge, surface modification, and aggregation of NPs on their interactions with the PS monolayer and on the composition of biomolecular corona are discussed. In conclusion, gaining a deeper understanding of the effects of the physicochemical properties of NPs on their interactions with the PS monolayer will contribute to the development of safer and more effective nanomedicines for pulmonary drug delivery.

Structural characterization of lateral phase separation in polymer-lipid hybrid membranes

Hierarchic self-assembly is the main mechanism used to create diverse structures using soft materials. This is a case for both synthetic materials and biomolecular systems, as exemplified by the non-covalent organization of lipids into membranes. In nature, lipids often assemble into single bilayers, but other nanostructures are encountered, such as bilayer stacks and tubular and vesicular aggregates. Synthetic block copolymers can be engineered to recapitulate many of the structures, forms, and functions of lipid systems. When block copolymers are amphiphilic, they can be inserted or co-assembled into hybrid membranes that exhibit synergistic structural, permeability, and mechanical properties. One example is the emergence of lateral phase separation akin to the raft formation in biomembranes. When higher-order structures, such as hybrid membranes, are formed, this lateral phase separation can be correlated across membranes in the stack. This chapter outlines a set of important methods, such as X-ray Scattering, Atomic Force Microscopy, and Cryo-Electron Microscopy, that are relevant to characterizing and evaluating lateral and correlated phase separation in hybrid membranes at the nano and mesoscales. Understanding the phase behavior of polymer-lipid hybrid materials could lead to innovative advancements in biomimetic membrane separation systems.

Exogenous surfactant for lung contusion causing ARDS: A systematic review of clinical and experimental reports

This systematic review aimed to summarize the available data on the treatment of pulmonary contusions with exogenous surfactants, determine whether this treatment benefits patients with severe pulmonary contusions, and evaluate the optimal type of surfactant, method of administration, and drug concentration. Three databases (MEDline, Scopus, and Web of Science) were searched using the following keywords: pulmonary surfactant, surface-active agents, exogenous surfactant, pulmonary contusion, and lung contusion for articles published between 1945 and February 2023, with no language restrictions. Four reviewers independently rated the studies for inclusion, and the other four reviewers resolved conflicts. Of the 100 articles screened, six articles were included in the review. Owing to the limited number of papers on this topic, various types of studies were included (two clinical studies, two experiments, and two case reports). In all the studies, surfactant administration improved the selected ventilation parameters. The most frequently used type of surfactant was Curosurf® in the concentration of 25 mg/kg of ideal body weight. In most studies, the administration of a surfactant by bronchoscopy into the segmental bronchi was the preferable way of administration. In both clinical studies, patients who received surfactants required shorter ventilation times. The administration of exogenous surfactants improved ventilatory parameters and, thus, reduced the need for less aggressive artificial lung ventilation and ventilation days. The animal-derived surfactant Curosurf® seems to be the most suitable substance; however, the ideal concentration remains unclear. The ideal route of administration involves a bronchoscope in the segmental bronchi.

Multivalent, calcium-independent binding of surfactant protein A and D to sulfated glycosaminoglycans of the alveolar epithelial glycocalyx

Lung surfactant collectins, surfactant protein A (SP-A) and D (SP-D), are oligomeric C-type lectins involved in lung immunity. Through their carbohydrate recognition domain, they recognize carbohydrates at pathogen surfaces and initiate lung innate immune response. Here, we propose that they may also be able to bind to other carbohydrates present in typical cell surfaces, such as the alveolar epithelial glycocalyx. To test this hypothesis, we analyzed and quantified the binding affinity of SP-A and SP-D to different sugars and glycosaminoglycans (GAGs) by microscale thermophoresis (MST). In addition, by changing the calcium concentration, we aimed to characterize any consequences on the binding behavior. Our results show that both oligomeric proteins bind with high affinity (in nanomolar range) to GAGs, such as hyaluronan (HA), heparan sulfate (HS) and chondroitin sulfate (CS). Binding to HS and CS was calcium-independent, as it was not affected by changing calcium concentration in the buffer. Quantification of GAGs in bronchoalveolar lavage (BAL) fluid from animals deficient in either SP-A or SP-D showed changes in GAG composition, and electron micrographs showed differences in alveolar glycocalyx ultrastructure in vivo. Taken together, SP-A and SP-D bind to model sulfated glycosaminoglycans of the alveolar epithelial glycocalyx in a multivalent and calcium-independent way. These findings provide a potential mechanism for SP-A and SP-D as an integral part of the alveolar epithelial glycocalyx binding and interconnecting free GAGs, proteoglycans, and other glycans in glycoproteins, which may influence glycocalyx composition and structure.NEW & NOTEWORTHY SP-A and SP-D function has been related to innate immunity of the lung based on their binding to sugar residues at pathogen surfaces. However, their function in the healthy alveolus was considered as limited to interaction with surfactant lipids. Here, we demonstrated that these proteins bind to glycosaminoglycans present at typical cell surfaces like the alveolar epithelial glycocalyx. We propose a model where these proteins play an important role in interconnecting alveolar epithelial glycocalyx components.

LIRAGLUTIDE ALLEVIATES ACUTE LUNG INJURY AND MORTALITY IN PNEUMONIA-INDUCED SEPSIS THROUGH REGULATING SURFACTANT PROTEIN EXPRESSION AND SECRETION

ABSTRACT

Glucagon-like peptide 1 (GLP-1) analogs are used to treat type 2 diabetes, and they can regulate insulin secretion, energy homeostasis, inflammation, and immune cell function. This study sought to determine whether the GLP-1 analog liraglutide exerts a beneficial action in an acute lung injury model of pneumonia-induced sepsis. Methods: Wild-type FVB/NJ mice (n = 114) were infected by intratracheal injection with Pseudomonas aeruginosa Xen5 (4 × 10 4 CFU/mouse) or an equal volume (50 μL) of saline (control) with or without a subcutaneous injection of liraglutide (2 mg/kg, 30 min after infection). Mice were killed 24 h after infection. Lung tissues and BALF were analyzed. In separate experiments, the dynamic growth of bacteria and animal mortality was monitored using in vivo imaging system within 48 h after infection. In addition, primary lung alveolar type II cells isolated from mice were used to study the mechanism of liraglutide action. Result: Liraglutide improved survival ( P < 0.05), decreased bacterial loads in vivo , and reduced lung injury scores ( P < 0.01) in septic mice. Liraglutide-treated mice showed decreased levels of inflammatory cells ( P < 0.01) and proinflammatory cytokines (TNF-α and IL-6) ( P < 0.01) in the lung compared with septic controls. Liraglutide significantly increased pulmonary surfactant proteins (SP-A and SP-B) expression/secretion ( P < 0.01) and phospholipid secretion ( P < 0.01) in vivo . Primary alveolar type II cells pretreated with liraglutide improved SP-A and SP-B expression after LPS exposure ( P < 0.01). Conclusion: Liraglutide attenuates mortality and lung inflammation/injury in pneumonia-induced sepsis. The increased surfactant expression/secretion and anti-inflammatory effects of liraglutide represent potential mechanisms by GLP-1 agonists potentiate host defense and maintain alveolar respiratory function in acute lung injury.

Thermodynamic and Structural Study of Budesonide-Exogenous Lung Surfactant System

The clinical benefits of using exogenous pulmonary surfactant (EPS) as a carrier of budesonide (BUD), a non-halogenated corticosteroid with a broad anti-inflammatory effect, have been established. Using various experimental techniques (differential scanning calorimetry DSC, small- and wide- angle X-ray scattering SAXS/WAXS, small- angle neutron scattering SANS, fluorescence spectroscopy, dynamic light scattering DLS, and zeta potential), we investigated the effect of BUD on the thermodynamics and structure of the clinically used EPS, Curosurf®. We show that BUD facilitates the Curosurf® phase transition from the gel to the fluid state, resulting in a decrease in the temperature of the main phase transition (Tm) and enthalpy (ΔH). The morphology of the Curosurf® dispersion is maintained for BUD < 10 wt% of the Curosurf® mass; BUD slightly increases the repeat distance d of the fluid lamellar phase in multilamellar vesicles (MLVs) resulting from the thickening of the lipid bilayer. The bilayer thickening (~0.23 nm) was derived from SANS data. The presence of ~2 mmol/L of Ca2+ maintains the effect and structure of the MLVs. The changes in the lateral pressure of the Curosurf® bilayer revealed that the intercalated BUD between the acyl chains of the surfactant's lipid molecules resides deeper in the hydrophobic region when its content exceeds ~6 wt%. Our studies support the concept of a combined therapy utilising budesonide-enriched Curosurf®.

Life cell imaging of amiodarone sequestration into lamellar bodies of alveolar type II cells

Amiodarone is widely used to treat cardiac arrhythmias and is very effective in preventing these disorders. However, its use is limited by a wide range of adverse effects, mainly affecting the lungs, and ranging from mild shortness of breath to pulmonary fibrosis. Amiodarone has been shown to accumulate strongly in lung tissue, exceeding its plasma concentration by a hundredfold. However, the site of accumulation and the mechanisms of transport are not fully understood. In this study, we used live cell imaging of primary rat alveolar type II cells to show that amiodarone specifically accumulates in large amounts in lamellar bodies, the surfactant storage organelles. Fluorescence imaging and correlation, and colocalization studies combined with confocal Raman microscopy identified these organelles as a major target for sequestration. Accumulation was rapid, on the order of a few hours, while storage was much more persistent. Partial uptake was observed in chemically fixed, dead cells, or cells treated with bafilomycin A1. Not only was uptake pH dependent, but intraluminal pH, measured with lysosomotropic pH sensitive dyes, was also affected. From these observations and from the physicochemical properties of amiodarone, we propose that passive diffusion, ion-trapping and lipophilic interactions are the main mechanisms for intracellular bioaccumulation. Furthermore, we demonstrate that measurement of amiodarone autofluorescence is highly useful for tracking cellular uptake and sequestration.

Structure of graphene oxide-phospholipid monolayers: A grazing incidence X-ray diffraction and neutron and X-ray reflectivity study

HYPOTHESIS

Graphene oxide-based nanotechnology has aroused a great interest due to its applications in the biomedical and optoelectronic fields. The wide use of these materials makes it necessary to study its potential toxicity associated with the inhalation of Graphene Oxide (GO) nanoparticles and its interaction with the lung surfactant. Langmuir monolayers have proven to be an excellent tool for studying the properties of the lung surfactant and the effect of intercalation of nanoparticles on its structure and properties. Therefore, to know the origin of the phospholipids/GO interaction and the structure of the lipid layer with GO, in this work we study the effect of the insertion of GO sheets on a Langmuir film of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC).

EXPERIMENTS

Surface pressure-area isotherms, Neutron (NR) and X-ray Reflectivity (XRR) and Grazing Incidence X-ray Diffraction (GIXD) measurements of hydrogenated and deuterated DPPC monolayers with and without GO have been carried out.

FINDINGS

The results outline a strong interaction between the GO and the zwitterionic form of DPPC and prove that GO is in three regions of the DPPC monolayer, the aliphatic chains of DPPC, the head groups and water in the subphase. Comparison between results obtained with hydrogenated and deuterated DPPC allows concluding that both, electrostatic attractions, and dispersion forces are responsible of the interaction GO/DPPC. Results also demonstrated that the insertion of GO into the DPPC aliphatic chains does not induce significant changes on unit cell of DPPC.

Structural changes in layers of lipid mixtures at low surface tensions

Layers of pulmonary lipids on an aqueous substrate at non-equilibrium conditions can decrease the surface tension of water to quite low values. This is connected with different relaxation processes occurring at the interface and the associated changes in the surface layer structure. Results of measurements by the combination of methods like surface rheology, ellipsometry, Brewster angle microscopy, and IRRAS for spread layers of lipid mixtures open a possibility to specify the dynamics of structural changes at conditions close to the physiological state. At sufficiently low surface tension values (below 5 mN/m) significant changes in the ellipsometric signal were observed for pure DPPC layers, which can be related to a transition from 2D to 3D structures caused by the layer folding. The addition of other lipids can accelerate the relaxation processes connected with squeezing-out of molecules or multilayer stacks formation hampering thereby a decrease of surface tension down to low values corresponding to the folding of the monolayer.

Inhaled RNA drugs to treat lung diseases: Disease-related cells and nano-bio interactions

In recent years, RNA-based therapies have gained much attention as biomedicines due to their remarkable therapeutic effects with high specificity and potency. Lung diseases offer a variety of currently undruggable but attractive targets that could potentially be treated with RNA drugs. Inhaled RNA drugs for the treatment of lung diseases, including asthma, chronic obstructive pulmonary disease, cystic fibrosis, and acute respiratory distress syndrome, have attracted more and more attention. A variety of novel nanoformulations have been designed and attempted for the delivery of RNA drugs to the lung via inhalation. However, the delivery of RNA drugs via inhalation poses several challenges. It includes protection of the stability of RNA molecules, overcoming biological barriers such as mucus and cell membrane to the delivery of RNA molecules to the targeted cytoplasm, escaping endosomal entrapment, and circumventing unwanted immune response etc. To address these challenges, ongoing researches focus on developing innovative nanoparticles to enhance the stability of RNA molecules, improve cellular targeting, enhance cellular uptake and endosomal escape to achieve precise delivery of RNA drugs to the intended lung cells while avoiding unwanted nano-bio interactions and off-target effects. The present review first addresses the pathologic hallmarks of different lung diseases, disease-related cell types in the lung, and promising therapeutic targets in these lung cells. Subsequently we highlight the importance of the nano-bio interactions in the lung that need to be addressed to realize disease-related cell-specific delivery of inhaled RNA drugs. This is followed by a review on the physical and chemical characteristics of inhaled nanoformulations that influence the nano-bio interactions with a focus on surface functionalization. Finally, the challenges in the development of inhaled nanomedicines and some key aspects that need to be considered in the development of future inhaled RNA drugs are discussed.

New technique to quantify phospolipids in disordered phase in lung surfactant by electron spin resonance

In pulmonary surfactant (PS) the coexistence of the ordered (Lo) and disordered (Ld) lipid phases would be essential for an optimal activity. Electron spin resonance (ESR) of PS labeled with 5-doxil stearic acid shows two spectral components called S and W. S/W ratio could be understood as the ratio between the probe population in Lo and in Ld. Although the specificity of S/W as an indicator of Lo/Ld has not yet been demonstrated, S/W has been used qualitatively to study changes in Lo/Ld. The goal of this paper is to stablish the correlation between S/W parameter and the amount of lipids in Ld (%Ld) measured by the ESR TEMPO technique described in our previous work. S/W and %Ld were measured in different PS samples under different experimental conditions. The results showed an inverse correlation between S/W and %Ld (r = -0.983; p < 0.001). We demonstrated that the S/W is sensible to the changes of Lo/Ld and can be used to quantify the %Ld.

Effect of airflow rate and drainage on the properties of 2D smectic liquid crystal foams

As a two-phase system, foams are widely applied in the industry and exist ubiquitously in our daily lives. For this reason, studying them and investigating the parameters that affect their properties is crucial for the development of new and improved foam-based products. In this paper, we create 2D foam using an ordered fluid, the smectic liquid crystal (LC), and discuss the experimental parameters that affect their fabrication, including temperature and confining conditions. Then, we examine the influence of the injected airflow rate and drainage on their structure, size, liquid fraction, and stability. Finally, we compare their behavior to that of low-viscosity liquid foams and discuss the difference between them. Our findings indicate that surface tension is the dominant parameter in LC foam systems. Despite the strong elasticity of LCs, surface tension plays a crucial role in determining the properties of elastic foams. These results provide valuable insights that can be applied to different industrial applications. For instance, they may find relevance in the fields of cosmetics, thermal insulation, oil recovery, and sensing, where the fabrication of foams with high-viscosity fluids is required.

The anti-inflammatory and antiviral properties of anionic pulmonary surfactant phospholipids

The pulmonary surfactant system of the lung is a lipid and protein complex, which regulates the biophysical properties of the alveoli to prevent lung collapse and the innate immune system in the lung. Pulmonary surfactant is a lipoprotein complex consisting of 90% phospholipids and 10% protein, by weight. Two minor components of pulmonary surfactant phospholipids, phosphatidylglycerol (PG) and phosphatidylinositol (PI), exist at very high concentrations in the extracellular alveolar compartments. We have reported that one of the most dominant molecular species of PG, palmitoyl-oleoyl-phosphatidylglycerol (POPG) and PI inhibit inflammatory responses induced by multiple toll-like receptors (TLR2/1, TLR3, TLR4, and TLR2/6) by interacting with subsets of multiprotein receptor components. These lipids also exert potent antiviral effects against RSV and influenza A, in vitro, by inhibiting virus binding to host cells. POPG and PI inhibit these viral infections in vivo, in multiple animal models. Especially noteworthy, these lipids markedly attenuate SARS-CoV-2 infection including its variants. These lipids are natural compounds that already exist in the lung and, thus, are less likely to cause adverse immune responses by hosts. Collectively, these data demonstrate that POPG and PI have strong potential as novel therapeutics for applications as anti-inflammatory compounds and preventatives, as treatments for broad ranges of RNA respiratory viruses.

Barriers for orally inhaled therapeutic antibodies

INTRODUCTION

Respiratory diseases represent a worldwide health issue. The recent Sars-CoV-2 pandemic, the burden of lung cancer, and inflammatory respiratory diseases urged the development of innovative therapeutic solutions. In this context, therapeutic antibodies (Abs) offer a tremendous opportunity to benefit patients with respiratory diseases. Delivering Ab through the airways has been demonstrated to be relevant to improve their therapeutic index. However, few inhaled Abs are on the market.

AREAS COVERED

This review describes the different barriers that may alter the fate of inhaled therapeutic Abs in the lungs at steady state. It addresses both physical and biological barriers and discusses the importance of taking into consideration the pathological changes occurring during respiratory disease, which may reinforce these barriers.

EXPERT OPINION

The pulmonary route remains rare for delivering therapeutic Abs, with few approved inhaled molecules, despite promising evidence. Efforts must focus on the intertwined barriers associated with lung diseases to develop appropriate Ab-formulation-device combo, ensuring optimal Ab deposition in the respiratory tract. Finally, randomized controlled clinical trials should be carried out to establish inhaled Ab therapy as prominent against respiratory diseases.

Stratification of asthma by lipidomic profiling of induced sputum supernatant

BACKGROUND

Asthma is a chronic respiratory disease with significant heterogeneity in its clinical presentation and pathobiology. There is need for improved understanding of respiratory lipid metabolism in asthma patients and its relation to observable clinical features.

OBJECTIVE

We performed a comprehensive, prospective, cross-sectional analysis of the lipid composition of induced sputum supernatant obtained from asthma patients with a range of disease severities, as well as from healthy controls.

METHODS

Induced sputum supernatant was collected from 211 adults with asthma and 41 healthy individuals enrolled onto the U-BIOPRED (Unbiased Biomarkers for the Prediction of Respiratory Disease Outcomes) study. Sputum lipidomes were characterized by semiquantitative shotgun mass spectrometry and clustered using topologic data analysis to identify lipid phenotypes.

RESULTS

Shotgun lipidomics of induced sputum supernatant revealed a spectrum of 9 molecular phenotypes, highlighting not just significant differences between the sputum lipidomes of asthma patients and healthy controls, but also within the asthma patient population. Matching clinical, pathobiologic, proteomic, and transcriptomic data helped inform the underlying disease processes. Sputum lipid phenotypes with higher levels of nonendogenous, cell-derived lipids were associated with significantly worse asthma severity, worse lung function, and elevated granulocyte counts.

CONCLUSION

We propose a novel mechanism of increased lipid loading in the epithelial lining fluid of asthma patients resulting from the secretion of extracellular vesicles by granulocytic inflammatory cells, which could reduce the ability of pulmonary surfactant to lower surface tension in asthmatic small airways, as well as compromise its role as an immune regulator.

Role of PPARγ in dyslipidemia and altered pulmonary functioning in mice following ozone exposure

Exposure to ozone causes decrements in pulmonary function, a response associated with alterations in lung lipids. Pulmonary lipid homeostasis is dependent on the activity of peroxisome proliferator activated receptor gamma (PPARγ), a nuclear receptor that regulates lipid uptake and catabolism by alveolar macrophages (AMs). Herein, we assessed the role of PPARγ in ozone-induced dyslipidemia and aberrant lung function in mice. Exposure of mice to ozone (0.8 ppm, 3 h) resulted in a significant reduction in lung hysteresivity at 72 h post exposure; this correlated with increases in levels of total phospholipids, specifically cholesteryl esters, ceramides, phosphatidylcholines, phosphorylethanolamines, sphingomyelins, and di- and triacylglycerols in lung lining fluid. This was accompanied by a reduction in relative surfactant protein-B (SP-B) content, consistent with surfactant dysfunction. Administration of the PPARγ agonist, rosiglitazone (5 mg/kg/day, i.p.) reduced total lung lipids, increased relative amounts of SP-B, and normalized pulmonary function in ozone-exposed mice. This was associated with increases in lung macrophage expression of CD36, a scavenger receptor important in lipid uptake and a transcriptional target of PPARγ. These findings highlight the role of alveolar lipids as regulators of surfactant activity and pulmonary function following ozone exposure and suggest that targeting lipid uptake by lung macrophages may be an efficacious approach for treating altered respiratory mechanics.

LPCAT1 levels in the placenta, the maternal plasma and the fetal plasma do not predict fetal lung responses to glucocorticoids in a sheep model of pregnancy

INTRODUCTION

Lysophosphatidylcholine acyltransferase 1 (LPCAT1) is important for saturated phosphatidylcholine (Sat-PC) production in the lung. Sat-PC is a critical component of pulmonary surfactant, which maintains low alveolar surface tension, facilitating respiration. Previous studies have reported an association between maternal and fetal LPCAT1 levels and neonatal lung function. Using a sheep model of pregnancy, we investigated a potential correlation between glucocorticoid-induced lung maturation and LPCAT1 mRNA and/or protein levels in the fetal lung, the placenta, the fetal plasma, and the maternal plasma.

METHODS

Eighty seven single pregnant ewes received maternal intramuscular injections of betamethasone. A sub-group of five animals had both maternal and fetal catheters installed to allow for sequential sampling from both plasma compartments. Lambs were surgically delivered under terminal anaesthesia between 2 and 8 days after initial ANS treatment, at a gestational age of 121-123 days. Lambs were ventilated for 30 min to determine functional lung maturation before being euthanized for necropsy and sample collection. Fetal lung, placenta, and fetal and maternal plasma samples were used to analyse LPCAT1 gene expression and protein levels.

RESULTS

The expression of LPCAT1 mRNA in the fetal lung was significantly corelated to Sat-PC levels at 8 days (R² = 0.23, p < 0.001) and lung maturation status overall (gas exchange efficiency as determined by measurements of lamb PaCO₂ during ventilation, R² = 0.20, p < 0.001). Similarly, fetal lung LPCAT1 mRNA was also significantly correlated with the individual durability of ANS effects on fetal lung maturation (R² = 0.20, p < 0.001). Although ANS therapy altered LPCAT1 mRNA expression in the placenta, observed changes were independent of fetal lung maturation outcomes. Neither maternal nor fetal plasma LPCAT1 levels were changed by ANS therapy over the period, including in analysis of serial maternal and fetal samples from chronically catheterised animals.

DISCUSSION

LPCAT1 expression in the fetal lung was associated with the durability of glucocorticoid effects on fetal lung maturation. However, LPCAT1 expression in the placenta, the fetal plasma, and the maternal plasma was neither associated with, nor predictive of fetal lung maturation after glucocorticoid treatment in a sheep model of pregnancy.

N,N,N-Trimethyl chitosan as a permeation enhancer for inhalation drug delivery: Interaction with a model pulmonary surfactant

N,N,N-Trimethyl chitosan (TMC), a biocompatible and biodegradable derivative of chitosan, is currently used as a permeation enhancer to increase the translocation of drugs to the bloodstream in the lungs. This article discusses the effect of TMC on a mimetic pulmonary surfactant, Curosurf®, a low-viscosity lipid formulation administered to preterm infants with acute respiratory distress syndrome. Curosurf® exhibits a strong interaction with TMC, resulting in the formation of aggregates at electrostatic charge stoichiometry. At nanoscale, Curosurf® undergoes a profound reorganization of its lipid vesicles in terms of size and lamellarity. The initial micron-sized vesicles (average size 4.8 μm) give way to a froth-like network of unilamellar vesicles about 300 nm in size. Under such conditions, neutralization of the cationic charges by pulmonary surfactant may inhibit TMC permeation enhancer capacity, especially as electrostatic charge complexation is found at low TMC content. The permeation properties of pulmonary surfactant-neutralized TMC should then be evaluated for its applicability as a permeation enhancer for inhalation in the alveolar region.

Cholesterol enhances the negative impact of vaping additives on lung surfactant model systems

Aims: Vaping has given rise to e-cigarette or vaping product use-associated lung injury. Model lung surfactant films were used to assess the impact of vape additives (vitamin E, vitamin E acetate, tetrahydrocannabinol, cannabidiol). This work builds upon our previous findings, by incorporating cholesterol, to understand the interplay between the additives and the sterol in surfactant function. Materials & methods: Compression-expansion cycles of lipid monofilm at the air-water interface and Brewster angle microscopy allowed elucidating the effects of vape additives. Results & conclusion: Vape additives at 5 mol% inhibited proper lipid packing and reduced film stability. Cholesterol enhanced the additive effects, resulting in significantly destabilized films and altered domains. The observed impact could signify dysfunctional lung surfactant and impaired lung function.

The Relationship of Cholesterol Responses to Mitochondrial Dysfunction and Lung Inflammation in Chronic Obstructive Pulmonary Disease

Hyperlipidemia is frequently reported in chronic obstructive pulmonary disease (COPD) patients and is linked to the progression of the disease and its comorbidities. Hypercholesterolemia leads to cholesterol accumulation in many cell types, especially immune cells, and some recent studies suggest that cholesterol impacts lung epithelial cells' inflammatory responses and mitochondrial responses. Several studies also indicate that targeting cholesterol responses with either statins or liver X receptor (LXR) agonists may be plausible means of improving pulmonary outcomes. Equally, cholesterol metabolism and signaling are linked to mitochondrial dysfunction and inflammation attributed to COPD progression. Here, we review the current literature focusing on the impact of cigarette smoke on cholesterol levels, cholesterol efflux, and the influence of cholesterol on immune and mitochondrial responses within the lungs.

Beyond the Interface: Improved Pulmonary Surfactant-Assisted Drug Delivery through Surface-Associated Structures

Pulmonary surfactant (PS) has been proposed as an efficient drug delivery vehicle for inhaled therapies. Its ability to adsorb and spread interfacially and transport different drugs associated with it has been studied mainly by different surface balance designs, typically interconnecting various compartments by interfacial paper bridges, mimicking in vitro the respiratory air-liquid interface. It has been demonstrated that only a monomolecular surface layer of PS/drug is able to cross this bridge. However, surfactant films are typically organized as multi-layered structures associated with the interface. The aim of this work was to explore the contribution of surface-associated structures to the spreading of PS and the transport of drugs. We have designed a novel vehiculization balance in which donor and recipient compartments are connected by a whole three-dimensional layer of liquid and not only by an interfacial bridge. By combining different surfactant formulations and liposomes with a fluorescent lipid dye and a model hydrophobic drug, budesonide (BUD), we observed that the use of the bridge significantly reduced the transfer of lipids and drug through the air-liquid interface in comparison to what can be spread through a fully open interfacial liquid layer. We conclude that three-dimensional structures connected to the surfactant interfacial film can provide an important additional contribution to interfacial delivery, as they are able to transport significant amounts of lipids and drugs during surfactant spreading.

Comment on "Bilayer aggregate microstructure determines viscoelasticity of lung surfactant suspensions" by C. O. Ciutara and J. A. Zasadzinski, Soft Matter, 2021, , 5170-5182

For applications of pulmonary surfactant delivery to the lungs, the question of rheology of the existing clinical formulations is of upmost importance. Recently, Ciutara and Zasadsinky (C. O. Ciutara and J. A. Zasadzinski, Soft Matter, 2021, 17, 5170-5182.) measured the rheological properties of Infasurf®, Survanta® and Curosurf®, three of the most used pulmonary surfactant substitutes. This study revealed that these fluids are shear-thinning and characterized by a yield stress. The results obtained by Ciutara et al. on Curosurf® differ from our results published in L.-P.-A. Thai, F. Mousseau, E. Oikonomou, M. Radiom and J.-F. Berret, Colloids Surf., B, 2019, 178, 337-345. and in L.-P.-A. Thai, F. Mousseau, E. Oikonomou, M. Radiom and J.-F. Berret, ACS Nano, 2020, 14, 466-475. In contrast, we found that Curosurf® suspensions are viscous Newtonian or slightly shear-thinning fluids, with no evidence of yield stress. The purpose of this Comment is to discuss possible causes for the discrepancy between the two studies, and to suggest that for biological fluids such as surfactant substitutes, the microrheology technique of rotational magnetic spectroscopy (MRS) can provide valuable results.

The concentration-dependent effect of hydrocortisone on the structure of model lung surfactant monolayer by using an in silico approach

Understanding the adsorption mechanism of corticosteroids in the lung surfactant requires the knowledge of corticosteroid molecular interactions with lung surfactant monolayer (LSM). We employed coarse-grained molecular dynamics simulation to explore the action of hydrocortisone on an LSM comprised of a phospholipid, cholesterol and surfactant protein. The structural and dynamical morphology of the lung surfactant monolayer at different surface tensions were investigated to assess the monolayer compressibility. The simulations were also conducted at the two extreme ends of breathing cycles: exhalation (0 mN m^-1 surface tension) and inhalation (20 mN m^-1 surface tension). The impact of surface tension and hydrocortisone concentration on the monolayer compressibility and stability are significant, resulting the monolayer expansion at higher surface tension. However, at low surface tension, the highly compressed monolayer induces monolayer instability in the presence of the drug due to the accumulation of surfactant protein and drug. The constant area per lipid simulation results demonstrate that the surface pressure-area isotherms show a decrease in area-per-lipid with increased drug concentration. The drug-induced expansion causes considerable instability in the monolayer after a specific drug concentration is attained at inhalation breathing condition, whereas, for exhalation breathing, the monolayer gets more compressed, causing the LSM to collapse. The monolayer collapse occurs for inhalation due to the higher drug concentration, whereas for exhalation due to the accumulation of surfactant proteins and drugs. The findings from this study will aid in enhancing the knowledge of molecular interactions of corticosteroid drugs with lung surfactants to treat respiratory diseases.

Down-regulated surfactant protein B in obese asthmatics

BACKGROUND

Obesity is a common comorbid condition in adult asthmatics and known as a feature of asthma severity. However, the molecular mechanism under obesity-induced inflammation has not yet been fully understood.

OBJECTIVE

Considering the essential role of hydrophobic surfactant protein B (SP-B) in lung function, SP-B was targeted to examine its involvement in the development of obesity-induced airway inflammation in asthmatics.

METHODS

The aim was to examine an alteration in circulating SP-B according to obesity in adult asthmatics, 129 asthmatics were enrolled and classified into 3 groups (obese, overweight and normal-weight groups) according to body mass index (BMI). Circulating SP-B levels were determined by enzyme-linked immunosorbent assay. Four single nucleotide polymorphisms of SFTPB gene were genotyped. Serum ceramide levels were measured by liquid chromatography-tandem mass spectrometry.

RESULTS

Significantly lower serum SP-B levels were noted in the obese group than in the overweight or normal-weight group (p = .002). The serum SP-B level was significantly correlated with serum levels of C18:0 ceramide and transforming growth factor beta 1 as well as BMI (r = -0.200; r = -0.215; r = -0.332, p < .050 for all). An inverse correlation was noted between serum SP-B and fractional exhaled nitric oxide levels in female asthmatics (r = -0.287, p = .009). Genetic predisposition of the SFTPB gene at 9306 A>G to the obese and overweight groups was noted.

CONCLUSION

Obesity altered ceramide metabolism leading to pulmonary surfactant dysfunction and impaired resolution of airway inflammation, finally contributing to the phenotypes of obese asthmatics.

Single nucleotide polymorphisms in surfactant protein A1 are not associated with a lack of responsiveness to antenatal steroid therapy in a pregnant sheep model

Treatment with antenatal steroids (ANS) is standard practice for reducing the risk of respiratory distress in the preterm infant. Despite clear overall benefits when appropriately administered, many fetuses fail to derive benefit from ANS therapies. In standardized experiments using a pregnant sheep model, we have demonstrated that around 40% of ANS-exposed lambs did not have functional lung maturation significantly different from that of saline-treated controls. Surfactant protein A is known to play an important role in lung function. In this genotyping study, we investigated the potential correlation between polymorphisms in SFTPA1, messenger RNA and protein levels, and ventilation outcomes in animals treated with ANS. 45 preterm lambs were delivered 48 h after initial ANS therapy and 44 lambs were delivered 8 days after initial ANS therapy. The lambs were ventilated for 30 min after delivery. SFTPA1 mRNA expression in lung tissue was not correlated with arterial blood PaCO2 values at 30 min of ventilation in lambs delivered 48 h after treatment. SFTPA1 protein in lung tissue was significantly correlated with PaCO2 at 30 min of ventilation in lambs ventilated both 48 h and 8 days after ANS treatment. Six different single nucleotide polymorphisms (SNPs) in the Ovis aries SFTPA1 sequence were detected by Sanger Sequencing. No individual SNPs or SNP haplotypes correlated with alterations in PaCO2 at 30 min of ventilation or SFTPA1 protein levels in the lung. For the subset of animals analyzed in the present study, variable lung maturation responses to ANS therapy were not associated with mutations in SFTPA1.

Tailored lipopeptide surfactants as potentially effective drugs to treat SARS-CoV-2 infection

Finding effective drugs to treat SARS-CoV-2 infection as a complementary step to the extensive vaccination is of the great importance to overcome the current pandemic situation. It has been shown that some bio-active unsaturated fatty acids such as Arachidonic Acid (AA) can reduce the infection severity and even destroy the virus by disintegration of the virus lipid envelope. On the other hand, it has been reported that several designed peptides with an activity similar to the angiotensin converting enzyme 2 (ACE-2), which has a high affinity towards the novel corona virus spike protein, can inhibit the viral infection through concealing the spike proteins from the cell surfaces ACE-2. Binding the mentioned peptides to the bio-active lipids like AA will result in a lipopeptide surfactant molecule with the synergistic effect of both the active moieties in its structure to treat the novel corona infection. In addition, the peptide segment increases the aqueous solubility of the lipid segment and enables the targeted delivery of the surfactant molecule to the virus. The resultant lipopeptide would be a potentially effective drug for SARS-CoV-2 infection treatment with the minimum side effects.

Biomembrane lipids: When physics and chemistry join to shape biological activity

Biomembranes constitute the first lines of defense of cells. While small molecules can often permeate cell walls in bacteria and plants, they are generally unable to penetrate the barrier constituted by the double layer of phospholipids, unless specific receptors or channels are present. Antimicrobial or cell-penetrating peptides are in fact highly specialized molecules able to bypass this barrier and even discriminate among different cell types. This capacity is made possible by the intrinsic properties of its phospholipids, their distribution between the internal and external leaflet, and their ability to mutually interact, modulating the membrane fluidity and the exposition of key headgroups. Although common phospholipids can be found in the membranes of most organisms, some are characteristic of specific cell types. Here, we review the properties of the most common lipids and describe how they interact with each other in biomembrane. We then discuss how their assembly in bilayers determines some key physical-chemical properties such as permeability, potential and phase status. Finally, we describe how the exposition of specific phospholipids determines the recognition of cell types by membrane-targeting molecules.

Evaluating the Impact of Hydrophobic Silicon Dioxide in the Interfacial Properties of Lung Surfactant Films

The interaction of hydrophobic silicon dioxide particles (fumed silicon dioxide), as model air pollutants, and Langmuir monolayers of a porcine lung surfactant extract has been studied in order to try to shed light on the physicochemical bases underlying the potential adverse effects associated with pollutant inhalation. The surface pressure-area isotherms of lung surfactant (LS) films including increasing amounts of particles revealed that particle incorporation into LS monolayers modifies the organization of the molecules at the water/vapor interface, which alters the mechanical resistance of the interfacial films, hindering the ability of LS layers for reducing the surface tension, and reestablishing the interface upon compression. This influences the normal physiological function of LS as is inferred from the analysis of the response of the Langmuir films upon the incorporation of particles against harmonic changes of the interfacial area (successive compression-expansion cycles). These experiments evidenced that particles alter the relaxation mechanisms of LS films, which may be correlated to a modification of the transport of material within the interface and between the interface and the adjacent fluid during the respiratory cycle.

MiR-199a-3p in mouse bone marrow mesenchymal stem cell exosomes increases epithelial sodium channel expression in lung injury

Acute lung injury (ALI) causes significant morbidity and mortality in critically ill patients, which often presents with extensive accumulation of activated inflammatory cells and diffused alveolar damage accompanied by oxidative stress. Exosomes are nanovesicles, which have notable anti-inflammatory and repair properties, thus alleviating the symptoms of ALI. Epithelial sodium channel (ENaC) is essential for the transepithelial absorption of Na⁺ and fluid from alveolar spaces. We studied the effects of bone marrow mesenchymal stem cell exosomes (BMSC-exo) on the apoptosis and protein expression of ENaC in primary mouse alveolar epithelial type 2 cells (AT 2 cells). Moreover, the change of miR-199a-3p in AT 2 cells was detected by qRT-PCR, and we studied the regulation of miR-199a-3p on ENaC protein expression. Our results demonstrated that BMSC-exo could not only improve viability and reduce apoptosis in AT 2 cells, but also enhance the expression of ENaC protein and miR-199a-3p. Meanwhile, the upregulation of miR-199a-3p resulted in increased expression of ENaC protein. In summary, the BMSC-exo could participate in the regulation of ENaC through miR-199a-3p originated from BMSC-exo, thereby providing a new pharmacological tool for the treatment of ALI.

Reduced DMPC and PMPC in lung surfactant promote SARS-CoV-2 infection in obesity

OBJECTIVE

Obesity is an established risk factor for higher SARS-CoV-2 viral loads, severe COVID-19 pneumonia requiring hospitalization, and worse outcomes. However, the underlying mechanisms for the increased risk are not well understood. SARS-CoV-2 is a respiratory virus with the primary route of entry through the lungs, where the Spike protein of SARS-CoV-2 binds to the ACE2 receptor on pneumocytes. Lung surfactant produced by type II pneumocytes plays a major role in respiratory defense against infections. Surfactant predominantly contains lipids, especially phosphatidylcholines (PC), and obesity is characterized by aberrant lipid metabolism. We hypothesized that altered lipid composition in lung surfactant in obesity may promote SARS-CoV-2 infection, leading to severe COVID-19 disease.

METHODS

Lipidomic analysis of lung tissue and bronchoalveolar lavage fluid (BALF) was performed using LC-MS/MS. The effects of PCs on SARS-CoV-2 pseudovirus infection were studied in HEK293T cells with ACE2 overexpression and in Vero-E6 cells with endogenous ACE2 expression. For the cell-cell fusion assay, HEK293T-ACE2 and HEK293T expressing SARS-CoV-2 Spike/eGFP were used as the target and effector cells, respectively.

RESULTS

Lipidomic analysis revealed that myristic acid-containing dimyristoyl-PC (DMPC) and palmitoylmyristoyl-PC (PMPC) were reduced in lung tissue and BALF from high fat diet-induced obese mice. DMPC and PMPC markedly inhibited wild type and D614G mutant SARS-CoV-2 infection in HEK293T-ACE2 and Vero-E6 cells. Feeding obese mice with trimyristin, the triglycerides of myristic acid, increased DMPC and PMPC levels in lung surfactant. Lipid extract from BALF of trimyristin-treated obese mice mitigated the elevated wild type and D614G mutant SARS-CoV-2 infection. The inhibitory effects of DMPC and PMPC on SARS-CoV-2 infection were reversed by cholesterol.

CONCLUSIONS

The reduced DMPC and PMPC in lung surfactant may promote SARS-CoV-2 infection. Increasing DMPC and PMPC in lung surfactant could be an innovative strategy for preventing and treating severe COVID-19 disease in obesity.

Dimerization of the pulmonary surfactant protein C in a membrane environment

Surfactant protein C (SP-C) has several functions in pulmonary surfactant. These include the transfer of lipids between different membrane structures, a role in surfactant recycling and homeostasis, and involvement in modulation of the innate defense system. Despite these important functions, the structures of functional SP-C complexes have remained unclear. SP-C is known to exist as a primarily α-helical structure with an apparently unstructured N-terminal region, yet there is recent evidence that the functions of SP-C could be associated with the formation of SP-C dimers and higher oligomers. In this work, we used molecular dynamics simulations, two-dimensional umbrella sampling, and well-tempered metadynamics to study the details of SP-C dimerization. The results suggest that SP-C dimerizes in pulmonary surfactant membranes, forming dimers of different topologies. The simulations identified a dimerization motif region V²¹xxxVxxxGxxxM³³ that is much larger than the putative A³⁰xxxG³⁴ motif that is commonly assumed to control the dimerization of some α-helical transmembrane domains. The results provide a stronger basis for elucidating how SP-C functions in concert with other surfactant proteins.

Concentration-Dependent Effect of the Steroid Drug Prednisolone on a Lung Surfactant Monolayer

The lung surfactant monolayer (LSM) is the main barrier for particles entering the lung, including steroid drugs used to treat lung diseases. The present study combines Langmuir experiments and coarse-grained (CG) molecular dynamics simulations to investigate the concentration-dependent effect of steroid drug prednisolone on the structure and morphology of a model LSM. The surface pressure-area isotherms for the Langmuir monolayers reveal a concentration-dependent decrease in area per lipid (APL). Results from simulations at a fixed surface tension, representing inhalation and exhalation conditions, suggest that at high drug concentrations, prednisolone induces a collapse of the LSM, which is likely caused by the inability of the drug to diffuse into the bilayer. Overall, the monolayer is most susceptible to drug-induced collapse at surface tensions representing exhalation conditions. The presence of cholesterol also exacerbates the instability. The findings of this investigation might be helpful for better understanding the interaction between steroid drug prednisolone and lung surfactants in relation to off-target effects.

Pathological cardiolipin-promoted membrane hemifusion stiffens pulmonary surfactant membranes

Lower tract respiratory diseases such as pneumonia are pervasive, affecting millions of people every year. The stability of the air/water interface in alveoli and the mechanical performance during the breathing cycle are regulated by the structural and elastic properties of pulmonary surfactant membranes (PSMs). Respiratory dysfunctions and pathologies often result in, or are caused by, impairment of the PSMs. However, a gap remains between our knowledge of the etiology of lung diseases and the fundamental properties of PSMs. For example, bacterial pneumonia in humans and mice has been associated with aberrant levels of cardiolipin, a mitochondrial-specific, highly unsaturated 4-tailed anionic phospholipid, in lung fluid, which likely disrupts the structural and mechanical integrity of PSMs. Specifically, cardiolipin is expected to significantly alter PSM elasticity due to its intrinsic molecular properties favoring membrane folding away from a flat configuration. In this paper, we investigate the structural and mechanical properties of the lipidic components of PSMs using lipid-based models as well as bovine extracts affected by the addition of pathological cardiolipin levels. Specifically, using a combination of optical and atomic force microscopy with a surface force apparatus, we demonstrate that cardiolipin strongly promotes hemifusion of PSMs and that these local membrane contacts propagate at larger scales, resulting in global stiffening of lung membranes.

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.

Molecular hydrogen is a potential protective agent in the management of acute lung injury

Acute lung injury (ALI) and acute respiratory distress syndrome, which is a more severe form of ALI, are life-threatening clinical syndromes observed in critically ill patients. Treatment methods to alleviate the pathogenesis of ALI have improved to a great extent at present. Although the efficacy of these therapies is limited, their relevance has increased remarkably with the ongoing pandemic caused by the novel coronavirus disease 2019 (COVID-19), which causes severe respiratory distress syndrome. Several studies have demonstrated the preventive and therapeutic effects of molecular hydrogen in the various diseases. The biological effects of molecular hydrogen mainly involve anti-inflammation, antioxidation, and autophagy and cell death modulation. This review focuses on the potential therapeutic effects of molecular hydrogen on ALI and its underlying mechanisms and aims to provide a theoretical basis for the clinical treatment of ALI and COVID-19.

The Ability of Resveratrol to Attenuate Ovalbumin-Mediated Allergic Asthma Is Associated With Changes in Microbiota Involving the Gut-Lung Axis, Enhanced Barrier Function and Decreased Inflammation in the Lungs

Asthma is a chronic respiratory disease highly prevalent worldwide. Recent studies have suggested a role for microbiome-associated gut-lung axis in asthma development. In the current study, we investigated if Resveratrol (RES), a plant-based polyphenol, can attenuate ovalbumin (OVA)-induced murine allergic asthma, and if so, the role of microbiome in the gut-lung axis in this process. We found that RES attenuated allergic asthma with significant improvements in pulmonary functions in OVA-exposed mice when tested using plethysmography for frequency (F), mean volume (MV), specific airway resistance (sRaw), and delay time(dT). RES treatment also suppressed inflammatory cytokines in the lungs. RES modulated lung microbiota and caused an abundance of Akkermansia muciniphila accompanied by a reduction of LPS biosynthesis in OVA-treated mice. Furthermore, RES also altered gut microbiota and induced enrichment of Bacteroides acidifaciens significantly in the colon accompanied by an increase in butyric acid concentration in the colonic contents from OVA-treated mice. Additionally, RES caused significant increases in tight junction proteins and decreased mucin (Muc5ac) in the pulmonary epithelium of OVA-treated mice. Our results demonstrated that RES may attenuate asthma by inducing beneficial microbiota in the gut-lung axis and through the promotion of normal barrier functions of the lung.

Fluid Films as Models for Understanding the Impact of Inhaled Particles in Lung Surfactant Layers

Pollution is currently a public health problem associated with different cardiovascular and respiratory diseases. These are commonly originated as a result of the pollutant transport to the alveolar cavity after their inhalation. Once pollutants enter the alveolar cavity, they are deposited on the lung surfactant (LS) film, altering their mechanical performance which increases the respiratory work and can induce a premature alveolar collapse. Furthermore, the interactions of pollutants with LS can induce the formation of an LS corona decorating the pollutant surface, favoring their penetration into the bloodstream and distribution along different organs. Therefore, it is necessary to understand the most fundamental aspects of the interaction of particulate pollutants with LS to mitigate their effects, and design therapeutic strategies. However, the use of animal models is often invasive, and requires a careful examination of different bioethics aspects. This makes it necessary to design in vitro models mimicking some physico-chemical aspects with relevance for LS performance, which can be done by exploiting the tools provided by the science and technology of interfaces to shed light on the most fundamental physico-chemical bases governing the interaction between LS and particulate matter. This review provides an updated perspective of the use of fluid films of LS models for shedding light on the potential impact of particulate matter in the performance of LS film. It should be noted that even though the used model systems cannot account for some physiological aspects, it is expected that the information contained in this review can contribute on the understanding of the potential toxicological effects of air pollution.

Biophysical analysis of gelatin and PLGA nanoparticle interactions with complex biomimetic lung surfactant models

Biocompatible materials are increasingly used for pulmonary drug delivery, and it is essential to understand their potential impact on the respiratory system, notably their effect on lung surfactant, a monolayer of lipids and proteins, responsible for preventing alveolar collapse during breathing cycles. We have developed a complex mimic of lung surfactant composed of eight lipids mixed in ratios reported for native lung surfactant. A synthetic peptide based on surfactant protein B was added to better mimic the biological system. This model was used to evaluate the impact of biocompatible gelatin and poly(lactic-co-glycolic acid) nanoparticles. Surface pressure–area isotherms were used to assess lipid packing, film compressibility and stability, whereas the lateral organization was visualized by Brewster angle microscopy. Nanoparticles increased film fluidity and altered the monolayer collapse pressure. Bright protruding clusters formed in their presence indicate a significant impact on the lateral organization of the surfactant film. Altogether, this work indicates that biocompatible materials considered to be safe for drug delivery still need to be assessed for their potential detrimental impact before use in therapeutic applications

Biodegradable nanoparticles drastically alters lateral organization of lung surfactant lipid- peptide model system.

Surfaxin attenuates PM2.5-induced airway inflammation via restoring surfactant proteins in rats exposed to cigarette smoke

Epidemiologic studies have shown that the fine particulate matter 2.5 (PM2.5) exaggerates chronic airway inflammation involving in acute exacerbation of chronic obstructive pulmonary disease (AECOPD). Surfactant proteins (SPs) decreases significantly related to airflow limitation and airway inflammation. However, how to restore the reduction of SPs levels in airway inflammation exposed to PM2.5 has not been well understood. In the present study, the SPs including SPA, SPB, SPC and SPD levels in bronchoalveolar lavage fluid (BALF) were detected from patients with stable COPD. Rats were exposed to cigarette smoke and PM2.5. After given with Surfaxin, the expression of SPs, protein kinase C (PKC) and tight junction protein (ZO-1) in lung tissue and the levels of C-reactive protein (CRP) and fibrinogen (FIB) in plasma was observed. The results showed that SPA, SPB and SPD were significantly lower than those of the control group (p < 0.01). PM2.5 aggravated smoking-induced airway inflammation and oxidative stress demonstrated by pathological changes of lung tissue and increased levels of CRP and PKC in vivo. PM2.5 decreased the expression of all the SPs and ZO-1, which could be significantly restored by Surfaxin. These findings indicate that Surfaxin protects the alveolar epithelium from PM2.5 in airway inflammation through increasing SPs.

Channels and Transporters of the Pulmonary Lamellar Body in Health and Disease

The lamellar body (LB) of the alveolar type II (ATII) cell is a lysosome-related organelle (LRO) that contains surfactant, a complex mix of mainly lipids and specific surfactant proteins. The major function of surfactant in the lung is the reduction of surface tension and stabilization of alveoli during respiration. Its lack or deficiency may cause various forms of respiratory distress syndrome (RDS). Surfactant is also part of the innate immune system in the lung, defending the organism against air-borne pathogens. The limiting (organelle) membrane that encloses the LB contains various transporters that are in part responsible for translocating lipids and other organic material into the LB. On the other hand, this membrane contains ion transporters and channels that maintain a specific internal ion composition including the acidic pH of about 5. Furthermore, P2X4 receptors, ligand gated ion channels of the danger signal ATP, are expressed in the limiting LB membrane. They play a role in boosting surfactant secretion and fluid clearance. In this review, we discuss the functions of these transporting pathways of the LB, including possible roles in disease and as therapeutic targets, including viral infections such as SARS-CoV-2.

Alveolar type II cells and pulmonary surfactant in COVID-19 era

In this review, we discuss the role of pulmonary surfactant in the host defense against respiratory pathogens, including novel coronavirus SARS-CoV-2. In the lower respiratory system, the virus uses angiotensin-converting enzyme 2 (ACE2) receptor in conjunction with serine protease TMPRSS2, expressed by alveolar type II (ATII) cells as one of the SARS-CoV-2 target cells, to enter. ATII cells are the main source of surfactant. After their infection and the resulting damage, the consequences may be severe and may include injury to the alveolar-capillary barrier, lung edema, inflammation, ineffective gas exchange, impaired lung mechanics and reduced oxygenation, which resembles acute respiratory distress syndrome (ARDS) of other etiology. The aim of this review is to highlight the key role of ATII cells and reduced surfactant in the pathogenesis of the respiratory form of COVID-19 and to emphasize the rational basis for exogenous surfactant therapy in COVID-19 ARDS patients.

The highly packed and dehydrated structure of pre-formed unexposed human pulmonary surfactant isolated from amniotic fluid

By coating the alveolar air-liquid interface, lung surfactant overwhelms surface tension forces that, otherwise, would hinder the lifetime effort of breathing. Years of research have provided a picture of how highly hydrophobic and specialized proteins in surfactant promote rapid and efficient formation of phospholipid-based complex three-dimensional films at the respiratory surface, highly stable under the demanding breathing mechanics. However, recent evidence suggest that the structure and performance of surfactant typically isolated from bronchoalveolar lung lavages may be far from that of nascent, still unused, surfactant as freshly secreted by type II pneumocytes into the alveolar airspaces. In the present work, we report the isolation of lung surfactant from human amniotic fluid (amniotic fluid surfactant, AFS) and a detailed description of its composition, structure and surface activity in comparison to a natural surfactant (NS) purified from porcine bronchoalveolar lavages. We observe that the lipid/protein complexes in AFS exhibit a substantially higher lipid packing and dehydration than in NS. AFS shows melting transitions at higher temperatures than NS and a conspicuous presence of non-lamellar phases. The surface activity of AFS is not only comparable to that of NS under physiologically-meaningful conditions, but displays significantly higher resistance to inhibition by serum or meconium, agents that inactivate surfactant in the context of severe respiratory pathologies. We propose that AFS may be the optimal model to study the molecular mechanisms sustaining pulmonary surfactant performance in health and disease, and the reference material to develop improved therapeutic surfactant preparations to treat yet unresolved respiratory pathologies.