Hyaluronan reduces surfactant inhibition and improves rat lung function after meconium injury

The unremarkable alveolar epithelial glycocalyx: a thorium dioxide-based electron microscopic comparison after heparinase or pneumolysin treatment

Recent investigations analyzed in depth the biochemical and biophysical properties of the endothelial glycocalyx. In comparison, this complex cell-covering structure is largely understudied in alveolar epithelial cells. To better characterize the alveolar glycocalyx ultrastructure, unaffected versus injured human lung tissue explants and mouse lungs were analyzed by transmission electron microscopy. Lung tissue was treated with either heparinase (HEP), known to shed glycocalyx components, or pneumolysin (PLY), the exotoxin of Streptococcus pneumoniae not investigated for structural glycocalyx effects so far. Cationic colloidal thorium dioxide (cThO₂) particles were used for glycocalyx glycosaminoglycan visualization. The level of cThO₂ particles orthogonal to apical cell membranes (≙ stained glycosaminoglycan height) of alveolar epithelial type I (AEI) and type II (AEII) cells was stereologically measured. In addition, cThO₂ particle density was studied by dual-axis electron tomography (≙ stained glycosaminoglycan density in three dimensions). For untreated samples, the average cThO₂ particle level was ≈ 18 nm for human AEI, ≈ 17 nm for mouse AEI, ≈ 44 nm for human AEII and ≈ 35 nm for mouse AEII. Both treatments, HEP and PLY, resulted in a significant reduction of cThO₂ particle levels on human and mouse AEI and AEII. Moreover, a HEP- and PLY-associated reduction in cThO₂ particle density was observed. The present study provides quantitative data on the differential glycocalyx distribution on AEI and AEII based on cThO₂ and demonstrates alveolar glycocalyx shedding in response to HEP or PLY resulting in a structural reduction in both glycosaminoglycan height and density. Future studies should elucidate the underlying alveolar epithelial cell type-specific distribution of glycocalyx subcomponents for better functional understanding.

Targeting of Glycosaminoglycans in Genetic and Inflammatory Airway Disease

In the lung, glycosaminoglycans (GAGs) are dispersed in the extracellular matrix (ECM) occupying the interstitial space between the capillary endothelium and the alveolar epithelium, in the sub-epithelial tissue and in airway secretions. In addition to playing key structural roles, GAGs contribute to a number of physiologic processes ranging from cell differentiation, cell adhesion and wound healing. Cytokine and chemokine–GAG interactions are also involved in presentation of inflammatory molecules to respective receptors leading to immune cell migration and airway infiltration. More recently, pathophysiological roles of GAGs have been described. This review aims to discuss the biological roles and molecular interactions of GAGs, and their impact in the pathology of chronic airway diseases, such as cystic fibrosis and chronic obstructive pulmonary disease. Moreover, the role of GAGs in respiratory disease has been heightened by the current COVID-19 pandemic. This review underlines the essential need for continued research aimed at exploring the contribution of GAGs in the development of inflammation, to provide a better understanding of their biological impact, as well as leads in the development of new therapeutic agents.

Bilayer aggregate microstructure determines viscoelasticity of lung surfactant suspensions

Neonatal respiratory distress syndrome (NRDS) is treated by intratracheal delivery of suspensions of animal-derived lung surfactant in saline. Lung surfactants are extracted via organic solvents from animal lung lavage, followed by solvent removal and surfactant re-hydration to form multi-bilayer particles suspended in saline. Following intra-tracheal administration, the surfactant suspension spreads throughout the lungs by surface tension gradient induced flow; the spreading rate is limited by suspension viscoelasticity. Here we examine the rheology of three clinical lung surfactant suspensions: Survanta (bovine lung), Curosurf (porcine lung), and Infasurf (calf lung). These surfactants have widely different rheological properties that depend on the lipid composition and bilayer organization. The steady shear viscosity is related to the bilayer particle volume fraction as for a suspension of hard spheres, but the lipid volume fraction is not simply related to the mass loading. Optical and electron microscopy and small angle X-ray scattering show that the viscosity variation is due to the temperature and composition dependent bilayer aggregate shapes and internal particle organization. Survanta forms crystalline bilayers at 37 °C, resulting in high aspect ratio asymmetric particles. Infasurf forms aggregates of unilamellar vesicles containing water pockets, while Curosurf forms onion-like multi-layered liposomes. While the mass loading of the three clinical surfactants is different, the different bilayer organization causes the particle volume fractions to be similar. Adding polyethylene glycol dehydrates and partially flocculates the bilayer aggregates in all suspensions, leading to smaller particle volume fractions and a reduced suspension viscosity even though the solvent viscosity increases almost six-fold.

Immunomodulatory biomaterials and their application in therapies for chronic inflammation-related diseases

The degree of tissue injuries such as the level of scarring or organ dysfunction, and the immune response against them primarily determine the outcome and speed of healing process. The successful regeneration of functional tissues requires proper modulation of inflammation-producing immune cells and bioactive factors existing in the damaged microenvironment. In the tissue repair and regeneration processes, different types of biomaterials are implanted either alone or by combined with other bioactive factors, which will interact with the immune systems including immune cells, cytokines and chemokines etc. to achieve different results highly depending on this interplay. In this review article, the influences of different types of biomaterials such as nanoparticles, hydrogels and scaffolds on the immune cells and the modification of immune-responsive factors such as reactive oxygen species (ROS), cytokines, chemokines, enzymes, and metalloproteinases in tissue microenvironment are summarized. In addition, the recent advances of immune-responsive biomaterials in therapy of inflammation-associated diseases such as myocardial infarction, spinal cord injury, osteoarthritis, inflammatory bowel disease and diabetic ulcer are discussed.

High Molecular Weight Hyaluronan Suppresses Macrophage M1 Polarization and Enhances IL-10 Production in PM2.5-Induced Lung Inflammation

PM_2.5 is particulate matter with a diameter of 2.5 μm or less. Airway macrophages are the key players regulating PM_2.5-induced inflammation. High molecular weight hyaluronan (HMW-HA) has previously been shown to exert protective effects on PM_2.5-induced acute lung injury and inflammation. However, little is known about the detailed mechanism. In this study, we aimed to determine whether HMW-HA alleviates PM_2.5-induced pulmonary inflammation by modulating macrophage polarization. The levels of M1 biomarkers TNF-α, IL-1β, IL-6, CXCL1, CXCL2, NOS2 and CD86, as well as M2 biomarkers IL-10, MRC1, and Arg-1 produced by macrophages were measured by ELISA, qPCR, and flow cytometry. In addition, the amount of M1 macrophages in lung tissues was examined by immunofluorescence of CD68 and NOS2. We observed a decline in PM_2.5-induced M1 polarization both in macrophages and lung tissues when HMW-HA was administered simultaneously. Meanwhile, western blot analysis revealed that PM_2.5-induced JNK and p38 phosphorylation was suppressed by HMW-HA. Furthermore, in vitro and in vivo studies showed that co-stimulation with HMW-HA and PM_2.5 promoted the expression and release of IL-10, but exhibited limited effects on the transcription of MRC1 and ARG1. In conclusion, our results demonstrated that HMW-HA ameliorates PM_2.5-induced lung inflammation by repressing M1 polarization through JNK and p38 pathways and promoting the production of pro-resolving cytokine IL-10.

Inhibition and counterinhibition of Surfacen, a clinical lung surfactant of natural origin

Inactivation of pulmonary surfactant by different components such as serum, cholesterol or meconium contributes to severe respiratory pathologies through destabilization and collapse of airspaces. Recent studies have analyzed in detail how the interfacial properties of natural surfactant purified from animal lungs are altered as a consequence of its exposure to serum proteins or meconium-mobilized cholesterol. It has been also demonstrated that pre-exposure of surfactant to polymers such as hyaluronic acid provides resistance to inactivation by multiple inhibitory agents. In the current work, we have extended these studies to the analysis of Surfacen, a clinical surfactant currently in use to rescue premature babies suffering or at risk of respiratory distress due to congenital lack of surfactant. This surfactant is also strongly inhibited by both meconium and serum when tested in the captive bubble surfactometer (CBS) under conditions mimicking respiratory dynamics. As it occurs with native surfactant, Surfacen is markedly protected from inhibition by pre-exposure to hyaluronic acid, confirming that clinical surfactants can be improved to treat pathologies associated with strongly deactivating contexts, such as those associated with lung injury and inflammation. Remarkably, we found that, under physiologically-mimicking conditions, a cholesterol-free clinical surfactant such as Surfacen is less susceptible to inhibition by cholesterol-mobilizing environments than cholesterol-containing natural surfactant, as a consequence of a markedly reduced susceptibility to incorporation of exogenous cholesterol.

High molecular weight hyaluronan attenuates fine particulate matter-induced acute lung injury through inhibition of ROS-ASK1-p38/JNK-mediated epithelial apoptosis

Inhalation of fine particulate matter (PM_2.5) is asscoiated with lung injury. High molecular weight hyaluronan (HMW-HA) is an essential constituent of extracellular matrix (ECM), exhibiting anti-oxidative and anti-inflammatory properties when administered by injection, inhalation, nebulization or gene delivery of HA synthases. The aim of the present study is to determine whether HMW-HA alleviates PM_2.5-induced acute lung injury (ALI) and investigate the underlying mechanisms. We observed that HMW-HA suppressed pathological injury, inflammation, oxidative stress, edema and epithelial damage caused by PM_2.5 in the lungs of the rats. The protective mechanism of HMW-HA was further explored in vitro. The results elucidated that reactive oxygen species (ROS) was involved in PM_2.5-induced cell apoptosis, and HMW-HA mitigated the oxidative potential of PM_2.5, subsequently inhibiting phosphorylation of ASK1 at Thr845, downstream phosphorylation of p38 and JNK, and eventual apoptosis. Our study indicates that HMW-HA is a promising strategy in the prevention of PM_2.5-induced pulmonary damage.

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.

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.

Hyaluronan with dextran added to therapeutic lung surfactants improves effectiveness in vitro and in vivo

Surfactants in current clinical use are largely ineffective in treating acute lung injury (ALI)/ acute respiratory distress syndrome. In part, this ineffectiveness is due to inactivation of surfactant by serum leakage into the alveoli. Previously, we reported that adding hyaluronan and some nonionic polymers to synthetic lipids combined with native SP-B and SP-C enhanced surface activity. In this study, we first tested two therapeutic lung surfactants and then retested after adding hyaluronan, polyethylene glycol or dextran alone or in two-polymer combinations including hyaluronan in the absence or presence of serum. Surface activities were measured in a modified bubble surfactometer. Results indicate that the inhibition threshold (defined as the amount of serum required to produce a minimum surface tension above 10 mN/m after 5 minutes of cycling) was 35 times higher with hyaluronan plus dextran added to Infasurf than with Infasurf alone, and better than all other mixtures tested. The threshold for Survanta with hyaluronan plus polyethylene glycol was 7 times higher than Survanta alone. We next tested selected surfactant mixtures in an animal model that mimicked ALI. All measurements of lung function showed significant improvement (P ≤ .05) with hyaluronan, or with hyaluronan and dextran added to Infasurf compared to Infasurf alone. Also, for these two groups, lung function was still improving at the end of the experiment. We conclude that certain polymers added to clinical surfactants can greatly increase resistance to inactivation in vitro, while in vivo, both Infasurf mixtures containing hyaluronan tended to normalize measures of lung function unlike other mixtures tested.

Exposure to polymers reverses inhibition of pulmonary surfactant by serum, meconium, or cholesterol in the captive bubble surfactometer

Dysfunction of pulmonary surfactant in the lungs is associated with respiratory pathologies such as acute respiratory distress syndrome or meconium aspiration syndrome. Serum, cholesterol, and meconium have been described as inhibitory agents of surfactant's interfacial activity once these substances appear in alveolar spaces during lung injury and inflammation. The deleterious action of these agents has been only partly evaluated under physiologically relevant conditions. We have optimized a protocol to assess surfactant inhibition by serum, cholesterol, or meconium in the captive bubble surfactometer. Specific measures of surface activity before and after native surfactant was exposed to inhibitors included i), film formation, ii), readsorption of material from surface-associated reservoirs, and iii), interfacial film dynamics during compression-expansion cycling. Results show that serum creates a steric barrier that impedes surfactant reaching the interface. A mechanical perturbation of this barrier allows native surfactant to compete efficiently with serum to form a highly surface-active film. Exposure of native surfactant to cholesterol or meconium, on the other hand, modifies the compressibility of surfactant films though optimal compressibility properties recover on repetitive compression-expansion cycling. Addition of polymers like dextran or hyaluronic acid to surfactant fully reverses inhibition by serum. These polymers also prevent surfactant inhibition by cholesterol or meconium, suggesting that the protective action of polymers goes beyond the mere enhancement of interfacial adsorption as described by depletion force theories.

The impact of hyaluronan on monocyte Toll-like receptor expression in term infant cord blood

AIM

To explore the possible effects of hyaluronan, an endogenous mediator of inflammation, on monocyte surface expression of Toll-like receptors 2 and 4 in human umbilical cord blood ex vivo, and in a model mimicking Gram-negative neonatal sepsis.

METHODS

Term infant cord blood was obtained after elective caesarean sections, n = 15. Both unstimulated and lipopolysaccharide-stimulated (10 ng/mL) blood was incubated with 500 μg/mL high- or low-molecular-weight hyaluronan for 6 h. Expression of Toll-like receptors 2 and 4 on monocytes was measured using flow cytometry, and plasma concentrations of proinflammatory cytokines and matrix metalloproteinase 9 were analysed. RESULTS (MEAN ± SEM): We found a significant decrease in Toll-like receptor 4 expression in the presence of high-molecular-weight hyaluronan (HMW HA) in unstimulated blood (median fluorescence intensity 141 ± 7.3 vs. 163 ± 9.8, p = 0.019). There were no significant changes in Toll-like receptor 2 expression. Levels of cytokines and matrix metalloproteinase 9 increased in the presence of both forms of hyaluronan.

CONCLUSIONS

Our results confirm that hyaluronan affects the neonatal immune response. The biological significance of these findings requires further clarification. More studies are needed to validate the possible down-modulation of Toll-like receptor 4 exerted by HMW HA.

Pulmonary surfactant proteins and polymer combinations reduce surfactant inhibition by serum

Acute respiratory distress syndrome (ARDS) is an inflammatory condition that can be associated with capillary leak of serum into alveoli causing inactivation of surfactant. Resistance to inactivation is affected by types and concentrations of surfactant proteins, lipids, and polymers. Our aim was to investigate the effects of different combinations of these three components. A simple lipid mixture (DPPC/POPG) or a more complex lipid mixture (DPPC/POPC/POPG/cholesterol) was used. Native surfactant proteins SP-B and SP-C obtained from pig lung lavage were added either singly or combined at two concentrations. Also, non-ionic polymers polyethylene glycol and dextran and the anionic polymer hyaluronan were added either singly or in pairs with hyaluronan included. Non-ionic polymers work by different mechanisms than anionic polymers, thus the purpose of placing them together in the same surfactant mixture was to evaluate if the combination would show enhanced beneficial effects. The resulting surfactant mixtures were studied in the presence or absence of serum. A modified bubble surfactometer was used to evaluate surface activities. Mixtures that included both SP-B and SP-C plus hyaluronan and either dextran or polyethylene glycol were found to be the most resistant to inhibition by serum. These mixtures, as well as some with either SP-B or SP-C with combined polymers were as or more resistant to inactivation than native surfactant. These results suggest that improved formulations of lung surfactants are possible and may be useful in reducing some types of surfactant inactivation in treating lung injuries.

An albumin-associated PLA2-like activity inactivates surfactant phosphatidylcholine secreted from fetal type II pneumocytes

Type II pneumocytes are responsible for the synthesis and secretion of pulmonary surfactant, which reduces surface tension in lung alveoli, thus decreasing their tendency to collapse during expiration. For this effect to be sustained, the integrity of the surface-active components of surfactant must be maintained. This study has shown that, when cultured type II pneumocytes are exposed to lipoprotein-free serum (LFS), the level of lyso-phosphatidylcholine (lyso-PC) in the secreted surfactant phospholipids is markedly elevated with a concomitant decline in the level of phosphatidylcholine (PC). This effect is the result of hydrolysis of surfactant PC by a phospholipase A(2) (PLA(2))-like activity present within serum. Anion-exchange chromatography, gel filtration chromatography and preparative electrophoresis of human LFS have shown that this PLA(2)-like activity coelutes with albumin and is biochemically distinct from the secretory form of PLA(2). Furthermore, specific inhibitors of PLA(2) such as p-bromophenacyl bromide, aristolochic acid, and palmitoyl trifluoromethyl ketone do not inhibit this activity of serum. Commercially purified human serum albumin fraction V and recombinant human serum albumin (rHSA) are almost as effective as LFS in enhancing the level of lyso-PC in the media. The latter finding implies that rHSA directly generates lyso-PC from secreted PC and suggests that this PLA(2)-like activity may be an intrinsic attribute of albumin.

The effect of hyaluronan on airway mucus transport and airway epithelial barrier integrity: potential application to the cytoprotection of airway tissue

The lubricating abilities and the protective functions of hyaluronan, a structural component of interstitial and connective tissues, were assessed in in vitro models of airway mucus transport and epithelial barrier. We found that hyaluronan enhanced the transport of airway mucus by cilia and by cough: the lower the hyaluronan molecular weight, the higher the increase. By immunofluorescence and western blot, we observed a significant dose-dependent (0.1, 1, 5 and 10 mg/ml) increase by low molecular weight hyaluronan (40 kDa) in the expression of tight junction proteins such as ZO-1, as well as an increase in the trans-epithelial resistance. Incubation of airway epithelial cells with hyaluronan 40 kDa also significantly increased the gap junction functionality. Finally, we demonstrated that hyaluronan 40 kDa protects the airway epithelium against injury induced by bacterial products during infection. These results demonstrate that the expression and functionality of intercellular adhesion molecules are increased by hyaluronan which can also act as a lubricant at the airway epithelium surface and suggest that hyaluronan may play a therapeutic role in a variety of respiratory diseases.

Overcoming rapid inactivation of lung surfactant: analogies between competitive adsorption and colloid stability

Lung surfactant (LS) is a mixture of lipids and proteins that line the alveolar air-liquid interface, lowering the interfacial tension to levels that make breathing possible. In acute respiratory distress syndrome (ARDS), inactivation of LS is believed to play an important role in the development and severity of the disease. This review examines the competitive adsorption of LS and surface-active contaminants, such as serum proteins, present in the alveolar fluids of ARDS patients, and how this competitive adsorption can cause normal amounts of otherwise normal LS to be ineffective in lowering the interfacial tension. LS and serum proteins compete for the air-water interface when both are present in solution either in the alveolar fluids or in a Langmuir trough. Equilibrium favors LS as it has the lower equilibrium surface pressure, but the smaller proteins are kinetically favored over multi-micron LS bilayer aggregates by faster diffusion. If albumin reaches the interface, it creates an energy barrier to subsequent LS adsorption that slows or prevents the adsorption of the necessary amounts of LS required to lower surface tension. This process can be understood in terms of classic colloid stability theory in which an energy barrier to diffusion stabilizes colloidal suspensions against aggregation. This analogy provides qualitative and quantitative predictions regarding the origin of surfactant inactivation. An important corollary is that any additive that promotes colloid coagulation, such as increased electrolyte concentration, multivalent ions, hydrophilic non-adsorbing polymers such as PEG, dextran, etc. added to LS, or polyelectrolytes such as chitosan, also promotes LS adsorption in the presence of serum proteins and helps reverse surfactant inactivation. The theory provides quantitative tools to determine the optimal concentration of these additives and suggests that multiple additives may have a synergistic effect. A variety of physical and chemical techniques including isotherms, fluorescence microscopy, electron microscopy and X-ray diffraction show that LS adsorption is enhanced by this mechanism without substantially altering the structure or properties of the LS monolayer.

Rediscovering the Schulze-Hardy rule in competitive adsorption to an air-water interface

The ratio of divalent to monovalent ion concentration necessary to displace the surface-active protein, albumin, by lung surfactant monolayers and multilayers at an air-water interface scales as 2(-6), the same concentration dependence as the critical flocculation concentration (CFC) for colloids with a high surface potential. Confirming this analogy between competitive adsorption and colloid stability, polymer-induced depletion attraction and electrostatic potentials are additive in their effects; the range of the depletion attraction, twice the polymer radius of gyration, must be greater than the Debye length to have an effect on adsorption.

Kinematic viscosity of therapeutic pulmonary surfactants with added polymers

The addition of various polymers to pulmonary surfactants improves surface activity in experiments both in vitro and in vivo. Although the viscosity of surfactants has been investigated, the viscosity of surfactant polymer mixtures has not. In this study, we have measured the viscosities of Survanta and Infasurf with and without the addition of polyethylene glycol, dextran or hyaluronan. The measurements were carried out over a range of surfactant concentrations using two concentrations of polymers at two temperatures. Our results indicate that at lower surfactant concentrations, the addition of any polymers increased the viscosity. However, the addition of polyethylene glycol and dextran to surfactants at clinically used concentrations can substantially lower viscosity. Addition of hyaluronan at clinical surfactant concentrations slightly increased Infasurf viscosity and produced little change in Survanta viscosity. Effects of polymers on viscosity correlate with changes in size and distribution of surfactant aggregates and the apparent free volume of liquid as estimated by light microscopy. Aggregation of surfactant vesicles caused by polymers may therefore not only improve surface activity as previously shown, but may also affect viscosity in ways that could improve surfactant distribution in vivo.

Selective NF-kappaB inhibition, but not dexamethasone, decreases acute lung injury in a newborn piglet airway inflammation model

Acute respiratory failure in neonates (e.g. ARDS, meconium aspiration pneumonitis, pneumonia) is characterized by an excessive inflammatory response, governing the migration of polymorpho-nuclear leukocytes (PMNLs) into lung tissue and causing consecutive impairment of gas exchange and lung function. Critical to this inflammatory response is the activation of nuclear factor-kappaB (NF-kappaB) that is required for transcription of the genes for many pro-inflammatory mediators. We asked whether the inhibition of NF-kappaB activity using either a selective inhibitor (IKK-NBD peptide) or dexamethasone would be more effective in decreasing NF-kappaB activity and chemokine expression in pulmonary cells. Changes in lung function were repeatedly assessed for 24h following induction of acute respiratory failure and therapeutic intervention. We conducted a randomized, controlled, prospective animal study with mechanically ventilated newborn piglets which underwent repeated airway lavage (20+/-2 [SEM]) to remove surfactant and to induce lung inflammation. Admixed to 100 mg kg(-1) surfactant, piglets then received either IKK-NBD peptide (S+IKK), a selective inhibitor of NF-kappaB activation, its control peptide without intrinsic activity, dexamethasone (S+Dexa), its solvent aqua, or an air bolus only (all groups n=8). After 24h of mechanical ventilation, the following differences were measured: PaO(2)/FiO(2) (S+IKK 230+/-9 mm Hg vs. S+Dexa 188+/-14, p<0.05); ventilation efficiency index (0.18+/-0.01 [3800/(PIP-PEEP)(*)f(*)PaCO(2)] vs. 0.14+/-0.01, p<0.05); extravascular lung water (24+/-1 ml kg(-1) vs. 29+/-2, p<0.05); PMNL in BAL fluid (112+/-21 cells microl(-1) vs. 208+/-34, p<0.05), IL-8 (351+/-117 pg ml(-1) vs. 491+/-144, p=ns) and leukotriene B(4) (23+/-7 pg ml(-1) vs. 71+/-11, p<0.01) in BAL fluid. NF-kappaB activity in the nucleus of pulmonary cells differed by 32+/-5% vs. 55+/-3, p<0.001. Differences between these two intervention groups were more pronounced in the second half of the observation period (hours 12-24). At 24h of mechanical ventilation, inhibition of NF-kappaB activity by IKK-NBD peptide admixed to surfactant as a carrier caused improved gas exchange, lung function and reduced pulmonary inflammation, as evidenced by reduction in PMNL migration into lung tissue due to reduced nuclear NF-kappaB activity. We conclude that IKK-NBD admixture to surfactant in acute neonatal respiratory failure is superior to dexamethasone administration within the first 24h.

Mechanisms of polyelectrolyte enhanced surfactant adsorption at the air-water interface

Chitosan, a naturally occurring cationic polyelectrolyte, restores the adsorption of the clinical lung surfactant Survanta to the air-water interface in the presence of albumin at much lower concentrations than uncharged polymers such as polyethylene glycol. This is consistent with the positively charged chitosan forming ion pairs with negative charges on the albumin and lung surfactant particles, reducing the net charge in the double-layer, and decreasing the electrostatic energy barrier to adsorption to the air-water interface. However, chitosan, like other polyelectrolytes, cannot perfectly match the charge distribution on the surfactant, which leads to patches of positive and negative charge at net neutrality. Increasing the chitosan concentration further leads to a reduction in the rate of surfactant adsorption consistent with an over-compensation of the negative charge on the surfactant and albumin surfaces, which creates a new repulsive electrostatic potential between the now cationic surfaces. This charge neutralization followed by charge inversion explains the window of polyelectrolyte concentration that enhances surfactant adsorption; the same physical mechanism is observed in flocculation and re-stabilization of anionic colloids by chitosan and in alternate layer deposition of anionic and cationic polyelectrolytes on charged colloids.

Toxic effects of different meconium fractions on lung function: new therapeutic strategies for meconium aspiration syndrome?

To review and summarize experimental data examining the effects of different fractions of meconium, and to test the effect of albumin on meconium aspiration both as prophylactic and rescue treatment. Newborn piglets 2 to 5 days of age were made hypoxic and then instilled meconium or fractions of meconium intratracheally. Meconium-added albumin and albumin instilled after meconium were also tested. Lung function and inflammatory cytokines were measured. Both the lipid- and water-soluble fractions induce inflammation in the lungs with elevation of inflammatory cytokines. When meconium was mixed with albumin, the inflammatory effects of meconium were significantly ameliorated. Rescue therapy with intratracheal albumin 5 min after the meconium aspiration syndrome was induced also improved lung function. These results indicate that at least part of the symptoms seen in the meconium aspiration syndrome could be prevented by blocking the active substances of meconium such as bile acids and free fatty acids.

Molecular weight dependence of the depletion attraction and its effects on the competitive adsorption of lung surfactant

Albumin competes with lung surfactant for the air-water interface, resulting in decreased surfactant adsorption and increased surface tension. Polyethylene glycol (PEG) and other hydrophilic polymers restore the normal rate of surfactant adsorption to the interface, which re-establishes low surface tensions on compression. PEG does so by generating an entropic depletion attraction between the surfactant aggregates and interface, reducing the energy barrier to adsorption imposed by the albumin. For a fixed composition of 10 g/L (1% wt.), surfactant adsorption increases with the 0.1 power of PEG molecular weight from 6 kDa-35 kDa as predicted by simple excluded volume models of the depletion attraction. The range of the depletion attraction for PEG with a molecular weight below 6 kDa is less than the dimensions of albumin and there is no effect on surfactant adsorption. PEG greater than 35 kDa reaches the overlap concentration at 1% wt. resulting in both decreased depletion attraction and decreased surfactant adsorption. Fluorescence images reveal that the depletion attraction causes the surfactant to break through the albumin film at the air-water interface to spread as a monolayer. During this transition, there is a coexistence of immiscible albumin and surfactant domains. Surface pressures well above the normal equilibrium surface pressure of albumin are necessary to force the albumin from the interface during film compression.

Bronchoalveolar lavage with pulmonary surfactant/dextran mixture improves meconium clearance and lung functions in experimental meconium aspiration syndrome

Surfactant lung lavage is a promising approach in the treatment of meconium aspiration syndrome (MAS). We hypothesise that the enrichment of modified natural surfactant with dextran will enhance meconium clearance from the airspaces during lung lavage and improve lung function in experimental MAS. Human meconium (30 mg/ml; 4 ml/kg) was instilled into the tracheal cannula of anaesthetised and paralysed adult rabbits to induce respiratory failure. The animals were then lavaged with saline (Sal), surfactant without (Surf) and with dextran (Surf+dex). Lung lavage (10 ml/kg in three portions) was performed with diluted surfactant (Curosurf, 10 mg/ml, 100 mg/kg) without or with dextran (3 mg/mg of surfactant phospholipids) or saline and the animals were conventionally ventilated with 100% O(2) for an additional hour. Lung functions were measured prior to and after meconium instillation, and 10, 30 and 60 min after lavage. The recovery of meconium in bronchoalveolar lavage (BAL) fluid was quantified. More meconium solids was recovered in the surfactant-lavaged than in the saline-lavaged groups (Surf: 12.4 +/- 3.9% and Surf+dex: 17.5 +/- 3.5% vs. Sal: 4.8 +/- 1.0%; both P < 0.01). Moreover, more meconium solids was obtained by Curosurf/dextran than by Curosurf-only lavage (P < 0.05). In the Surf group, the values for PaO(2)/FiO(2) were significantly higher than in the controls (at 60 min: 24.5 +/- 4.2 kPa vs.9.1 +/- 2.2 kPa, P < 0.01). An additional increase in oxygenation was seen in the Surf+dex group (at 60 min: 34.2 +/- 8.1 kPa, P vs. Surf group <0.01). The lung-thorax compliance was higher in the Surf+dex group in comparison with the Sal and Surf groups (at 60 min: 9.6 +/- 0.9 vs.7.6 +/- 1.2, P < 0.01 and 8.0 +/- 0.7 ml/kPa/kg, P < 0.05). The enrichment of Curosurf with dextran improves meconium clearance and lung functions in surfactant-lavaged rabbits with meconium aspiration.

Current perspectives in pulmonary surfactant--inhibition, enhancement and evaluation

Pulmonary surfactant (PS) is a complicated mixture of approximately 90% lipids and 10% proteins. It plays an important role in maintaining normal respiratory mechanics by reducing alveolar surface tension to near-zero values. Supplementing exogenous surfactant to newborns suffering from respiratory distress syndrome (RDS), a leading cause of perinatal mortality, has completely altered neonatal care in industrialized countries. Surfactant therapy has also been applied to the acute respiratory distress syndrome (ARDS) but with only limited success. Biophysical studies suggest that surfactant inhibition is partially responsible for this unsatisfactory performance. This paper reviews the biophysical properties of functional and dysfunctional PS. The biophysical properties of PS are further limited to surface activity, i.e., properties related to highly dynamic and very low surface tensions. Three main perspectives are reviewed. (1) How does PS permit both rapid adsorption and the ability to reach very low surface tensions? (2) How is PS inactivated by different inhibitory substances and how can this inhibition be counteracted? A recent research focus of using water-soluble polymers as additives to enhance the surface activity of clinical PS and to overcome inhibition is extensively discussed. (3) Which in vivo, in situ, and in vitro methods are available for evaluating the surface activity of PS and what are their relative merits? A better understanding of the biophysical properties of functional and dysfunctional PS is important for the further development of surfactant therapy, especially for its potential application in ARDS.

What's new in surfactant? A clinical view on recent developments in neonatology and paediatrics

Surfactant therapy has significantly changed clinical practice in neonatology over the last 25 years. Recent trials in infants with respiratory distress syndrome (RDS) have not shown superiority of any natural surfactant over another. Advancements in the development of synthetic surfactants are promising, yet to date none has been shown to be superior to natural preparations. Ideally, surfactant would be administered without requiring mechanical ventilation. An increasing number of studies investigate the roles of alternative modes of administration and the use of nasal continuous positive airway pressure to minimise the need for mechanical ventilation. Whether children with other lung diseases benefit from surfactant therapy is less clear. Evidence suggests that infants with meconium aspiration syndrome and children with acute lung injury/acute respiratory distress syndrome may benefit, while no positive effect of surfactant is seen in infants with congenital diaphragmatic hernia. However, more research is needed to establish potential beneficial effects of surfactant administration in children with lung diseases other than RDS. Furthermore, genetic disorders of surfactant metabolism have recently been linked to respiratory diseases of formerly unknown origin. It is important to consider these disorders in the differential diagnosis of unexplained respiratory distress although no established treatment is yet available besides lung transplantation for the most severe cases.

CONCLUSION

Research around surfactant is evolving and recent developments include further evolution of synthetic surfactants, evaluation of surfactant as a therapeutic option in lung diseases other than RDS and the discovery of genetic disorders of surfactant metabolism. Ongoing research is essential to continue to improve therapeutic prospects for children with serious respiratory disease involving disturbances in surfactant.

A freeze-fracture transmission electron microscopy and small angle x-ray diffraction study of the effects of albumin, serum, and polymers on clinical lung surfactant microstructure

Freeze-fracture transmission electron microscopy shows significant differences in the bilayer organization and fraction of water within the bilayer aggregates of clinical lung surfactants, which increases from Survanta to Curosurf to Infasurf. Albumin and serum inactivate all three clinical surfactants in vitro; addition of the nonionic polymers polyethylene glycol, dextran, or hyaluronic acid also reduces inactivation in all three. Freeze-fracture transmission electron microscopy shows that polyethylene glycol, hyaluronic acid, and albumin do not adsorb to the surfactant aggregates, nor do these macromolecules penetrate the interior water compartments of the surfactant aggregates. This results in an osmotic pressure difference that dehydrates the bilayer aggregates, causing a decrease in the bilayer spacing as shown by small angle x-ray scattering and an increase in the ordering of the bilayers as shown by freeze-fracture electron microscopy. Small angle x-ray diffraction shows that the relationship between the bilayer spacing and the imposed osmotic pressure for Curosurf is a screened electrostatic interaction with a Debye length consistent with the ionic strength of the solution. The variation in surface tension due to surfactant adsorption measured by the pulsating bubble method shows that the extent of surfactant aggregate reorganization does not correlate with the maximum or minimum surface tension achieved with or without serum in the subphase. Albumin, polymers, and their mixtures alter the surfactant aggregate microstructure in the same manner; hence, neither inhibition reversal due to added polymer nor inactivation due to albumin is caused by alterations in surfactant microstructure.

Bench-to-bedside review: the role of glycosaminoglycans in respiratory disease

The extracellular matrix (ECM) plays a significant role in the mechanical behaviour of the lung parenchyma. The ECM is composed of a three-dimensional fibre mesh that is filled with various macromolecules, among which are the glycosaminoglycans (GAGs). GAGs are long, linear and highly charged heterogeneous polysaccharides that are composed of a variable number of repeating disaccharide units. There are two main types of GAGs: nonsulphated GAG (hyaluronic acid) and sulphated GAGs (heparan sulphate and heparin, chondroitin sulphate, dermatan sulphate, and keratan sulphate). With the exception of hyaluronic acid, GAGs are usually covalently attached to a protein core, forming an overall structure that is referred to as proteoglycan. In the lungs, GAGs are distributed in the interstitium, in the sub-epithelial tissue and bronchial walls, and in airway secretions. GAGs have important functions in lung ECM: they regulate hydration and water homeostasis; they maintain structure and function; they modulate the inflammatory response; and they influence tissue repair and remodelling. Given the great diversity of GAG structures and the evidence that GAGs may have a protective effect against injury in various respiratory diseases, an understanding of changes in GAG expression that occur in disease may lead to opportunities to develop innovative and selective therapies in the future.

Expanded use of surfactant therapy in newborns

Although there is no doubt that administration of exogenous surfactant to very preterm babies who have respiratory distress syndrome is safe and efficacious, surfactant inactivation or deficiency plays a role in the pathophysiology of other pulmonary disorders affecting newborn infants. Preliminary data suggest that there may be a role for surfactant administration to babies who have meconium aspiration syndrome, pneumonia, and possibly bronchopulmonary dysplasia. Further investigation is necessary but seems warranted.

Enhanced surfactant adsorption via polymer depletion forces: a simple model for reversing surfactant inhibition in acute respiratory distress syndrome

Lung surfactant adsorption to an air-water interface is strongly inhibited by an energy barrier imposed by the competitive adsorption of albumin and other surface-active serum proteins that are present in the lung during acute respiratory distress syndrome. This reduction in surfactant adsorption results in an increased surface tension in the lung and an increase in the work of breathing. The reduction in surfactant adsorption is quantitatively described using a variation of the classical Smolukowski analysis of colloid stability. Albumin adsorbed to the interface induces an energy barrier to surfactant diffusion of order 5 k(B)T, leading to a reduction in adsorption equivalent to reducing the surfactant concentration by a factor of 100. Adding hydrophilic, nonadsorbing polymers such as polyethylene glycol to the subphase provides a depletion attraction between the surfactant aggregates and the interface that eliminates the energy barrier. Surfactant adsorption increases exponentially with polymer concentration as predicted by the simple Asakura and Oosawa model of depletion attraction. Depletion forces can likely be used to overcome barriers to adsorption at a variety of liquid-vapor and solid-liquid interfaces.

Chitosan enhances the in vitro surface activity of dilute lung surfactant preparations and resists albumin-induced inactivation

Chitosan is a natural, cationic polysaccharide derived from fully or partially deacetylated chitin. Chitosan is capable of inducing large phospholipid aggregates, closely resembling the function of nonionic polymers tested previously as additives to therapeutic lung surfactants. The effects of chitosan on improving the surface activity of a dilute lung surfactant preparation, bovine lipid extract surfactant (BLES), and on resisting albumin-induced inactivation were studied using a constrained sessile drop (CSD) method. Also studied in parallel were the effects of polyethylene glycol (PEG, 10 kD) and hyaluronan (HA, 1240 kD). Both adsorption and dynamic cycling studies showed that chitosan is able to significantly enhance the surface activity of 0.5 mg/mL BLES and to resist albumin-induced inactivation at an extremely low concentration of 0.05 mg/mL, 1000 times smaller than the usual concentration of PEG and 20 times smaller than HA. Optical microscopy found that chitosan induced large surfactant aggregates even in the presence of albumin. Cytotoxicity tests confirmed that chitosan has no deleterious effect on the viability of lung epithelial cells. The experimental results suggest that chitosan may be a more effective polymeric additive to lung surfactant than the other polymers tested so far.

Morphological alterations of exogenous surfactant inhibited by meconium can be prevented by dextran

Background

Surfactant dysfunction due to inhibition is involved in the pathophysiology of meconium aspiration syndrome. Dextran addition has been shown to reverse exogenous surfactant inactivation by meconium, but the precise mechanisms and the morphological correlate of this effect are yet unknown. Morphological surfactant analysis by transmission electron microscopy (TEM) and stereology allows the differentiation of active (large aggregates = LA) and inactive (small aggregates = SA) subtypes.

Methods

To determine the in vitro effects of meconium and dextran addition on the morphology of a modified porcine natural surfactant (Curosurf), Curosurf samples were either incubated alone or together with meconium or with meconium and dextran, fixed and processed for TEM. Volume fractions of surfactant subtypes [lamellar body-like forms (LBL), multilamellar vesicles (MV), unilamellar vesicles (UV)] were determined stereologically.

Results

All preparations contained LBL and MV (corresponding to LA) as well as UV (corresponding to SA). The volume fraction of UV increased with addition of meconium and decreased with further addition of dextran. Correspondingly, the UV/(LBL+MV) ratio (resembling the SA/LA ratio) increased when meconium was added and decreased when dextran was added to the surfactant-meconium mixture.

Conclusion

Meconium causes alterations in the ultrastructural composition of Curosurf that can be visualized and analyzed by TEM and stereology. These alterations resemble an increase in the SA/LA ratio and are paralleled by an increase in minimum surface tension. Dextran prevents these effects and may therefore be a useful additive to exogenous surfactant preparations to preserve their structural and functional integrity, thereby improving their resistance to inactivation.

Effects of Hyaluronan-Fortified Surfactant in Ventilated Premature Piglets with Respiratory Distress

We hypothesized that enriching surfactant with hyaluronan would restore lung function when tested in a premature animal model. Newborn piglets (85% gestation, term 112-114 days) were delivered by cesarean section, subjected to mechanical ventilation (tidal volume 6- 8 ml/kg) and randomly assigned to treatment with 50 or 100 mg/kg Curosurf (C50 and C100), 50 or 100 mg/kg Curosurf mixed with 2.5% HA (w/w, CH50 and CH100). A ventilated and not treated group (Cont) and a not treated and not ventilated group (Non) were included as controls. Six hours after treatment the lungs were removed and biochemical, biophysical, cytological and histological analyses were carried out. The CH100, CH50, C100 and C50 groups had variable but significantly improved alveolar phospholipid content, minimal surface tension, alveolar aeration and wet/dry lung weight ratios, but little histological evidence of lung injury. CH100, CH50 and C100 groups had the best effects in terms of oxygenation, lung compliance and histology and evidence of decreased inflammation (IL-8 and TNF-alpha mRNA expression). We conclude that HA added to 50 mg/kg Curosurf or use of 100 mg/kg Curosurf with or without HA provides the best effects in terms of lung function and reduction of inflammation.

Polyethylene glycol-surfactant for lavage lung injury in rats

Addition of ionic and nonionic water-soluble polymers to pulmonary surfactants in the presence of inactivating substances prevents surfactant inactivation in vitro and improves lung function in several models of lung injury. However, a recent report found opposite effects when surfactant plus polyethylene glycol (PEG) was used to treat lung injury caused by saline lung lavage. Therefore, we examined the reasons why the polymer effect is less evident in the saline lung lavage lung injury model. We treated rats with lavage lung injury with a commercial lung surfactant extract derived from bovine lung (Survanta) with or without addition of PEG. Groups treated with Survanta + PEG had significantly higher static post mortem lung volumes than groups treated with Survanta. However, groups treated with Survanta + PEG had more tracheal fluid and no significant benefit in arterial oxygenation compared with the group treated with Survanta, despite our use of measures to reduce pulmonary edema. Measurements after intravascular injections of (125)I-labeled albumin confirmed that addition of PEG increased extravascular lung water and that this effect is mitigated by furosemide. We conclude that surfactant + PEG mixtures are less effective in lavage injury than in other forms of lung injury because of increased extravascular lung water.

Inactivation of pulmonary surfactant due to serum-inhibited adsorption and reversal by hydrophilic polymers: experimental

The rate of change of surface pressure, pi, in a Langmuir trough following the deposition of surfactant suspensions on subphases containing serum, with or without polymers, is used to model a likely cause of surfactant inactivation in vivo: inhibition of surfactant adsorption due to competitive adsorption of surface active serum proteins. Aqueous suspensions of native porcine surfactant, organic extracts of native surfactant, and the clinical surfactants Curosurf, Infasurf, and Survanta spread on buffered subphases increase the surface pressure, pi, to approximately 40 mN/m within 2 min. The variation with concentration, temperature, and mode of spreading confirmed Brewster angle microscopy observations that subphase to surface adsorption of surfactant is the dominant form of surfactant transport to the interface. However (with the exception of native porcine surfactant), similar rapid increases in pi did not occur when surfactants were applied to subphases containing serum. Components of serum are surface active and adsorb reversibly to the interface increasing pi up to a concentration-dependent saturation value, pi(max). When surfactants were applied to subphases containing serum, the increase in pi was significantly slowed or eliminated. Therefore, serum at the interface presents a barrier to surfactant adsorption. Addition of either hyaluronan (normally found in alveolar fluid) or polyethylene glycol to subphases containing serum reversed inhibition by restoring the rate of surfactant adsorption to that of the clean interface, thereby allowing surfactant to overcome the serum-induced barrier to adsorption.