Adaptation to low body temperature influences pulmonary surfactant composition thereby increasing fluidity while maintaining appropriately ordered membrane structure and surface activity

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®.

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Predicting CPAP failure after less invasive surfactant administration (LISA) in preterm infants by machine learning model on vital parameter data: a pilot study

Objective. Less invasive surfactant administration (LISA) has been introduced to preterm infants with respiratory distress syndrome on continuous positive airway pressure (CPAP) support in order to avoid intubation and mechanical ventilation. However, after this LISA procedure, a significant part of infants fails CPAP treatment (CPAP-F) and requires intubation in the first 72 h of life, which is associated with worse complication free survival chances. The aim of this study was to predict CPAP-F after LISA, based on machine learning (ML) analysis of high resolution vital parameter monitoring data surrounding the LISA procedure.Approach. Patients with a gestational age (GA) <32 weeks receiving LISA were included. Vital parameter data was obtained from a data warehouse. Physiological features (HR, RR, peripheral oxygen saturation (SpO2) and body temperature) were calculated in eight 0.5 h windows throughout a period 1.5 h before to 2.5 h after LISA. First, physiological data was analyzed to investigate differences between the CPAP-F and CPAP-Success (CPAP-S) groups. Next, the performance of two types of ML models (logistic regression: LR, support vector machine: SVM) for the prediction of CPAP-F were evaluated.Main results. Of 51 included patients, 18 (35%) had CPAP-F. Univariate analysis showed lower SpO2, temperature and heart rate variability (HRV) before and after the LISA procedure. The best performing ML model showed an area under the curve of 0.90 and 0.93 for LR and SVM respectively in the 0.5 h window directly after LISA, with GA, HRV, respiration rate and SpO2as most important features. Excluding GA decreased performance in both models.Significance. In this pilot study we were able to predict CPAP-F with a ML model of patient monitor signals, with best performance in the first 0.5 h after LISA. Using ML to predict CPAP-F based on vital signals gains insight in (possibly modifiable) factors that are associated with LISA failure and can help to guide personalized clinical decisions in early respiratory management.

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.

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.

Compositional, structural and functional properties of discrete coexisting complexes within bronchoalveolar pulmonary surfactant

Lung surfactant (LS) stabilizes the respiratory surface by forming a film at the alveolar air-liquid interface that reduces surface tension and minimizes the work of breathing. Typically, this surface-active agent has been isolated from animal lungs both for research and biomedical applications. However, these materials are constituted by complex membranous architectures including surface-active and inactive lipid/protein assemblies. In this work, we describe the composition, structure and surface activity of discrete membranous entities that are part of a LS preparation isolated from bronchoalveolar lavages of porcine lungs. Seven different fractions could be resolved from whole surfactant subjected to sucrose density gradient centrifugation. Detailed compositional characterization revealed differences in protein and cholesterol content but no distinct saturated:unsaturated phosphatidylcholine ratios. Moreover, no significant differences were detected regarding apparent hydration at the headgroup region of membranes, as reported by the probe Laurdan, and lipid chain mobility analysed by electron spin resonance (ESR) in spite of the variety of membranous assemblies observed by transmission electron microscopy. In addition, six of the seven separated LS subfractions formed similar, essentially disordered-like, interfacial films and performed efficient surface activity, under physiologically relevant conditions. Altogether, our work show that a LS isolated from porcine lungs is comprised by a heterogenous population of membranous assemblies lacking freshly secreted unused LS complexes sustaining highly dehydrated and ordered membranous assemblies as previously reported. We propose that surfactant subfractions may illustrate intermediates in sequential structural steps within the structural transformations occurring along the respiratory compression-expansion cycles.

Molecular and biophysical mechanisms behind the enhancement of lung surfactant function during controlled therapeutic hypothermia

Therapeutic hypothermia (TH) enhances pulmonary surfactant performance in vivo by molecular mechanisms still unknown. Here, the interfacial structure and the composition of lung surfactant films have been analysed in vitro under TH as well as the molecular basis of its improved performance both under physiological and inhibitory conditions. The biophysical activity of a purified porcine surfactant was tested under slow and breathing-like dynamics by constrained drop surfactometry (CDS) and in the captive bubble surfactometer (CBS) at both 33 and 37 °C. Additionally, the temperature-dependent surfactant activity was also analysed upon inhibition by plasma and subsequent restoration by further surfactant supplementation. Interfacial performance was correlated with lateral structure and lipid composition of films made of native surfactant. Lipid/protein mixtures designed as models to mimic different surfactant contexts were also studied. The capability of surfactant to drastically reduce surface tension was enhanced at 33 °C. Larger DPPC-enriched domains and lower percentages of less active lipids were detected in surfactant films exposed to TH-like conditions. Surfactant resistance to plasma inhibition was boosted and restoration therapies were more effective at 33 °C. This may explain the improved respiratory outcomes observed in cooled patients with acute respiratory distress syndrome and opens new opportunities in the treatment of acute lung injury.

Understanding the Functional Properties of Lipid Heterogeneity in Pulmonary Surfactant Monolayers at the Atomistic Level

Pulmonary surfactant is a complex mixture of lipids and proteins lining the interior of the alveoli, and constitutes the first barrier to both oxygen and pathogens as they progress toward blood circulation. Despite decades of study, the behavior of the pulmonary surfactant at the molecular scale is poorly understood, which hinders the development of effective surfactant replacement therapies, useful in the treatment of several lung-related diseases. In this work, we combined all-atom molecular dynamics simulations, Langmuir trough measurements, and AFM imaging to study synthetic four-component lipid monolayers designed to model protein-free pulmonary surfactant. We characterized the structural and dynamic properties of the monolayers with a special focus on lateral heterogeneity. Remarkably, simulations reproduce almost quantitatively the experimental data on pressure-area isotherms and the presence of lateral heterogeneities highlighted by AFM. Quite surprisingly, the pressure-area isotherms do not show a plateau region, despite the presence of liquid-condensed nanometer-sized domains at surface pressures larger than 20 mN/m. In the simulations, the liquid-condensed domains were small and transient, but they did not coalesce to yield a separate phase. They were only slightly enriched in DPPC and cholesterol, and their chemical composition remained very similar to the overall composition of the monolayer membrane. Instead, they differed from liquid-expanded regions in terms of membrane thickness (in agreement with AFM data), diffusion rates, as well as acyl chain packing and orientation. We hypothesize that such lateral heterogeneities are crucial for lung surfactant function, as they allow both efficient packing, to achieve low surface tension, and sufficient fluidity, critical for rapid adsorption to the air–liquid interface during the breathing cycle.

Structure and Thermotropic Behavior of Bovine- and Porcine-Derived Exogenous Lung Surfactants

Two commercial exogenous pulmonary surfactants, Curosurf and Survanta, are investigated. Their thermotropic behavior and associated structural changes for the samples in bulk are characterized and described. For Survanta, the obtained results of differential scanning calorimetry showed a thermogram with three peaks on heating and only a single peak on cooling. Curosurf on the other hand, presents calorimetric thermograms with only one peak in both the heating and cooling scans. This distinct thermotropic behavior between the two pulmonary surfactants, a consequence of their particular compositions, is associated with structural changes that were evaluated by simultaneous small- and wide-angle X-ray scattering experiments with in situ temperature variation. Interestingly, for temperatures below ∼35 °C for Curosurf and ∼53 °C for Survanta, the scattering data indicated the coexistence of two lamellar phases with different carbon chain organizations. For temperatures above these limits, the coexistence of phases disappears, giving rise to a fluid phase in both pulmonary surfactants, with multilamelar vesicles for Curosurf and unilamellar vesicles for Survanta. This process is quasi-reversible under cooling, and advanced data analysis for the scattering data indicated differences in the structural and elastic properties of the pulmonary surfactants. The detailed and systematic investigation shown in this work expands on the knowledge of the structure and thermodynamic behavior of Curosurf and Survanta, being relevant from both physiological and biophysical perspectives and also providing a basis for further studies on other types of pulmonary surfactants.

Cellular, Molecular, and Physiological Adaptations of Hibernation: The Solution to Environmental Challenges

Thriving in times of resource scarcity requires an incredible flexibility of behavioral, physiological, cellular, and molecular functions that must change within a relatively short time. Hibernation is a collection of physiological strategies that allows animals to inhabit inhospitable environments, where they experience extreme thermal challenges and scarcity of food and water. Many different kinds of animals employ hibernation, and there is a spectrum of hibernation phenotypes. Here, we focus on obligatory mammalian hibernators to identify the unique challenges they face and the adaptations that allow hibernators to overcome them. This includes the cellular and molecular strategies used to combat low environmental and body temperatures and lack of food and water. We discuss metabolic, neuronal, and hormonal cues that regulate hibernation and how they are thought to be coordinated by internal clocks. Last, we touch on questions that are left to be addressed in the field of hibernation research. Studies from the last century and more recent work reveal that hibernation is not simply a passive reduction in body temperature and vital parameters but rather an active process seasonally regulated at the molecular, cellular, and organismal levels.

Surfactant replacement therapy: from biological basis to current clinical practice

This review summarizes the current knowledge on the physiological action of endogenous and exogenous pulmonary surfactant, the role of different types of animal-derived and synthetic surfactants for RDS therapy, different modes of administration, potential risks and strategies of ventilation, and highlights the most promising aims for future development. Scientists have clarified the physicochemical properties and functions of the different components of surfactant, and part of this successful research is derived from the characterization of genetic diseases affecting surfactant composition or function. Knowledge from functional tests of surfactant action, its immunochemistry, kinetics and homeostasis are important also for improving therapy with animal-derived surfactant preparations and for the development of modified surfactants. In the past decade newly designed artificial surfactants and additives have gained much attention and have proven different advantages, but their particular role still has to be defined. For clinical practice, alternative administration techniques as well as postsurfactant ventilation modes, taking into account alterations in lung mechanics after surfactant placement, may be important in optimizing the potential of this most important drug in neonatology.

Angiogenic signaling in the lungs of a metabolically suppressed hibernating mammal (Ictidomys tridecemlineatus)

To conserve energy in times of limited resource availability, particularly during cold winters, hibernators suppress even the most basic of physiologic processes. Breathing rates decrease from 40 breaths/minute to less than 1 breath/min as they decrease body temperature from 37 °C to ambient. Nevertheless, after months of hibernation, these incredible mammals emerge from torpor unscathed. This study was conducted to better understand the protective and possibly anti-inflammatory adaptations that hibernator lungs may use to prevent damage associated with entering and emerging from natural torpor. We postulated that the differential protein expression of soluble protein receptors (decoy receptors that sequester soluble ligands to inhibit signal transduction) would help identify inhibited inflammatory signaling pathways in metabolically suppressed lungs. Instead, the only two soluble receptors that responded to torpor were sVEGFR1 and sVEGFR2, two receptors whose full-length forms are bound by VEGF-A to regulate endothelial cell function and angiogenesis. Decreased sVEGFR1/2 correlated with increased total VEGFR2 protein levels. Maintained or increased levels of key γ-secretase subunits suggested that decreased sVEGFR1/2 protein levels were not due to decreased levels of intramembrane cleavage complex subunits. VEGF-A protein levels did not change, suggesting that hibernators may regulate VEGFR1/2 signaling at the level of the receptor instead of increasing relative ligand abundance. A panel of angiogenic factors used to identify biomarkers of angiogenesis showed a decrease in FGF-1 and an increase in BMP-9. Torpid lungs may use VEGF and BMP-9 signaling to balance angiogenesis and vascular stability, possibly through the activation of SMAD signaling for adaptive tissue remodeling.

Determining the fluid ordered and disordered phases in a pulmonary surfactant by electron spin resonance technique

Pulmonary surfactant main function is to reduce surface tension at alveolar interface. Two lipids phases coexist in surfactant membranes: a liquid-ordered (Lo) and a liquid-disordered (Ld) phases. This coexistence of phases would be crucial for the surfactant activity. Until now, the proportion of phases was determined qualitatively. We design an electronic spin resonance technique to quantify the lipid fraction in Ld phase. An exogenous pulmonary surfactant (EPS) with or without extra Cho was labeled with 5-doxil stearic acid to estimate the membrane fluidity and with TEMPO to determine the PL in Ld phase. A unique equation was established for the calculation of PL in Ld phase with an error of less than 3%. TEMPO partition coefficient was (0.78 ± 0.03). Cholesterol added to EPS did not modify this coefficient. The equation is valid for different batches of surfactant regardless of the cholesterol content. The proposed method is simple, precise and allows evaluating changes in lateral structure that could affect surfactant biophysical properties.

Melting transitions in biomembranes

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

Effect of cooling on lung secretory phospholipase A2 activity in vitro, ex vivo, and in vivo

Hypothermia can modify surfactant composition and function. Secretory phospholipase A2 (sPLA2) hydrolyses surfactant phospholipids and is important in the pathobiology of several critical respiratory disorders. We hypothesize that sPLA2 activity might be influenced by the temperature partially explaining surfactant changes. This study aims to evaluate comprehensively the effect of hypothermia on sPLA2 activity. We measured sPLA2 activity at different temperatures, alone or combined with bile acids, in vitro (incubating human recombinant sPLA2-IIA and porcine sPLA2-IB), ex vivo (by cooling bronchoalveolar lavage samples from neonates with respiratory distress syndrome or no lung disease), and in vivo (using lavage samples obtained before and after 72 h of whole body cooling in neonates with hypoxic-ischemic encephalopathy). We also measured concentrations of various sPLA2 subtypes and natural sPLA2 inhibitors in in vivo cooled samples. Results were corrected for protein content and dilution. In vitro cooling did not show any effect of hypothermia on sPLA2. Ex vivo cooling did not alter total sPLA2 activity, and the addition of bile acids increased sPLA2 activity irrespective of the temperature and the type of sampled patient. In vivo hypothermia reduced median sPLA2 activity from 16.6 [15.2–106.7] IU/mg to 3.3 [2.7–8.5] IU/mg ( P = 0.026) and mean sPLA2-IIA from 1.1 (0.8) pg/μg to 0.6 (0.4) pg/μg ( P = 0.047), whereas dioleylphosphatidylglycerol increased from 8.3 (3.9)% to 12.8 (5.1)% ( P = 0.02). Whole body hypothermia decreases in vivo global sPLA2 activity in bronchoalveolar lavage fluids through the reduction of sPLA2-IIA and increment of dioleylphosphatidylglycerol. This effect is absent during in vitro or ex vivo hypothermia.

Tracheal acid or surfactant instillation raises alveolar surface tension

Whether alveolar liquid surface tension, T, is elevated in the acute respiratory distress syndrome (ARDS) has not been demonstrated in situ in the lungs. Neither is it known how exogenous surfactant, which has failed to treat ARDS, affects in situ T. We aim to determine T in an acid-aspiration ARDS model before and after exogenous surfactant administration. In isolated rat lungs, we combine servo-nulling pressure measurement and confocal microscopy to determine alveolar liquid T according to the Laplace relation. Administering 0.01 N (pH 1.9) HCl solution by alveolar injection or tracheal instillation, to model gastric liquid aspiration, raises T. Subsequent surfactant administration fails to normalize T. Furthermore, in normal lungs, tracheal instillation of control saline or exogenous surfactant raises T. Lavaging the trachea with saline and injecting the lavage solution into the alveolus raises T, suggesting that tracheal instillation may wash T-raising airway contents to the alveolus. Adding 0.01 N HCl or 5 mM CaCl₂-either of which aggregates mucins-to tracheal lavage solution reduces or eliminates the effect of lavage solution on alveolar T. Following tracheal saline instillation, liquid suctioned directly out of alveoli through a micropipette contains mucins. Additionally, alveolar injection of gastric mucin solution raises T. We conclude that 1) tracheal liquid instillation likely washes T-raising mucins to the alveolus and 2) even exogenous surfactant that could be delivered mucin-free to the alveolus might not normalize T in acid-aspiration ARDS. NEW & NOTEWORTHY We demonstrate in situ in isolated lungs that surface tension is elevated in an acid-aspiration acute respiratory distress syndrome (ARDS) model. Following tracheal liquid instillation, also in isolated lungs, we directly sample alveolar liquid. We find that liquid instillation into normal lungs washes mucins to the alveolus, thereby raising alveolar surface tension. Furthermore, even if exogenous surfactant could be delivered mucin-free to the alveolus, exogenous surfactant might fail to normalize alveolar surface tension in acid-aspiration ARDS.

Melting of the Dipalmitoylphosphatidylcholine Monolayer

Langmuir monolayer self-assembled at the air-water interface represents an excellent model for studying phase transition and lipid polymorphism in two dimensions. Compared with numerous studies of phospholipid phase transitions induced by isothermal compression, there are very scarce reports on two-dimensional phase transitions induced by isobaric heating. This is mainly due to technical difficulties of continuously regulating temperature variations while maintaining a constant surface pressure in a classical Langmuir-type film balance. Here, with technological advances in constrained drop surfactometry and closed-loop axisymmetric drop shape analysis, we studied the isobaric heating process of the dipalmitoylphosphatidylcholine (DPPC) monolayer. It is found that temperature and surface pressure are two equally important intensive properties that jointly determine the phase behavior of the phospholipid monolayer. We have determined a critical point of the DPPC monolayer at a temperature of 44 °C and a surface pressure of 57 mN/m. Beyond this critical point, no phase transition can exist in the DPPC monolayer, either by isothermal compression or by isobaric heating. The melting process of the DPPC monolayer studied here provides novel insights into the understanding of a wide range of physicochemical and biophysical phenomena, such as surface thermodynamics, critical phenomena, and biophysical study of pulmonary surfactants.

Unitary Physiology

The common relationships among a great variety of biological phenomena seem enigmatic when considered solely at the level of the phenotype. The deep connections in physiology, for example, between the effects of maternal food restriction in utero and the subsequent incidence of metabolic syndrome in offspring, the effects of microgravity on cell polarity and reproduction in yeast, stress effects on jellyfish, and their endless longevity, or the relationship between nutrient abundance and the colonial form in slime molds, are not apparent by phenotypic observation. Yet all of these phenomena are ultimately determined by the Target of Rapamycin (TOR) gene and its associated signaling complexes. In the same manner, the unfolding of evolutionary physiology can be explained by a comparable application of the common principle of cell-cell signaling extending across complex developmental and phylogenetic traits. It is asserted that a critical set of physiologic and phenotypic adaptations emanated from a few crucial, ancestral receptor gene duplications that enabled the successful terrestrial transition of vertebrates from water to land. In combination, mTor and its cognate receptors and a few crucial genetic duplications provide a mechanistic common denominator across a diverse spectrum of biological responses. The proper understanding of their purpose yields a unified concept of physiology and its evolutionary development. © 2018 American Physiological Society. Compr Physiol 8:761-771, 2018.

Controlled hypothermia may improve surfactant function in asphyxiated neonates with or without meconium aspiration syndrome

Background

Whole-body hypothermia (WBH) is used to improve neurological outcomes in perinatal asphyxia. Recent studies suggested a beneficial effect of hypothermia for some types of acute respiratory failure. However, no data are available about the biophysical function of human surfactant during WBH. We investigated whether WBH improves surfactant biophysical properties in asphyxiated neonates with or without meconium aspiration syndrome (MAS).

Methods

Non-bronchoscopic bronchoalveolar lavage (BAL) has been collected from 10 asphyxiated neonates (2 with MAS, 8 with no lung disease (NLD)) at different time-points (pre-WBH, 24h, 48h, 72h of WBH and post-WBH). Surfactant was extracted and tested by captive bubble surfactometry (CBS) in triplicate, at 37°C and 33.5°C, through initial adsorption and dynamic compression-expansion cycling. Phosphatidylcholine and cholesterol were assayed using enzymatic methods. Clinical data were recorded in real-time.

Results

Minimum surface tension under dynamic testing was significantly improved as assessed at 33.5°C compared with its behavior at 37°C in NLD neonates: the difference was evident after at least 72h of WBH and remained significant at 6h after rewarming (72h: p = 0.009; rewarming: p = 0.040). Similar results were obtained in MAS patients whose surfactant activity improved already at 48h of hypothermia. Total cholesterol showed a trend to increase at the first 24-48h of hypothermia in NLD patients. Conversely, hypothermia seemed to reduce the excess of exogenous cholesterol in MAS surfactant.

Conclusions

Surfactant biophysical properties may improve after 48-72h of WBH in asphyxiated neonates and the improvement is maintained shortly after rewarming. Due to study limitations, further studies are warranted to better clarify the effects of hypothermia on surfactant activity.

The role of mucus as an invisible cloak to transepithelial drug delivery by nanoparticles

Mucosal administration of drugs and drug delivery systems has gained increasing interest. However, nanoparticles intended to protect and deliver drugs to epithelial surfaces require transport through the surface-lining mucus. Translation from bench to bedside is particularly challenging for mucosal administration since a variety of parameters will influence the specific barrier properties of the mucus including the luminal fluids, the microbiota, the mucus composition and clearance rate, and the condition of the underlying epithelia. Besides, after administration, nanoparticles interact with the mucosal components, forming a biomolecular corona that modulates their behavior and fate after mucosal administration. These interactions are greatly influenced by the nanoparticle properties, and therefore different designs and surface-engineering strategies have been proposed. Overall, it is essential to evaluate these biomolecule-nanoparticle interactions by complementary techniques using complex and relevant mucus barrier matrices.

A compendium and functional characterization of mammalian genes involved in adaptation to Arctic or Antarctic environments

Background

Many mammals are well adapted to surviving in extremely cold environments. These species have likely accumulated genetic changes that help them efficiently cope with low temperatures. It is not known whether the same genes related to cold adaptation in one species would be under selection in another species. The aims of this study therefore were: to create a compendium of mammalian genes related to adaptations to a low temperature environment; to identify genes related to cold tolerance that have been subjected to independent positive selection in several species; to determine promising candidate genes/pathways/organs for further empirical research on cold adaptation in mammals.

Results

After a search for publications containing keywords: “whole genome”, “transcriptome or exome sequencing data”, and “genome-wide genotyping array data” authors looked for information related to genetic signatures ascribable to positive selection in Arctic or Antarctic mammalian species. Publications related to Human, Arctic fox, Yakut horse, Mammoth, Polar bear, and Minke whale were chosen. The compendium of genes that potentially underwent positive selection in >1 of these six species consisted of 416 genes. Twelve of them showed traces of positive selection in three species. Gene ontology term enrichment analysis of 416 genes from the compendium has revealed 13 terms relevant to the scope of this study. We found that enriched terms were relevant to three major groups: terms associated with collagen proteins and the extracellular matrix; terms associated with the anatomy and physiology of cilium; terms associated with docking. We further revealed that genes from compendium were over-represented in the lists of genes expressed in the lung and liver.

Conclusions

A compendium combining mammalian genes involved in adaptation to cold environment was designed, based on the intersection of positively selected genes from six Arctic and Antarctic species. The compendium contained 416 genes that have been positively selected in at least two species. However, we did not reveal any positively selected genes that would be related to cold adaptation in all species from our list. But, our work points to several strong candidate genes involved in mechanisms and biochemical pathways related to cold adaptation response in different species.

Electronic supplementary material

The online version of this article (10.1186/s12863-017-0580-9) contains supplementary material, which is available to authorized users.

Palmitic Acid: Physiological Role, Metabolism and Nutritional Implications

Palmitic acid (PA) has been for long time negatively depicted for its putative detrimental health effects, shadowing its multiple crucial physiological activities. PA is the most common saturated fatty acid accounting for 20–30% of total fatty acids in the human body and can be provided in the diet or synthesized endogenously via de novo lipogenesis (DNL). PA tissue content seems to be controlled around a well-defined concentration, and changes in its intake do not influence significantly its tissue concentration because the exogenous source is counterbalanced by PA endogenous biosynthesis. Particular physiopathological conditions and nutritional factors may strongly induce DNL, resulting in increased tissue content of PA and disrupted homeostatic control of its tissue concentration. The tight homeostatic control of PA tissue concentration is likely related to its fundamental physiological role to guarantee membrane physical properties but also to consent protein palmitoylation, palmitoylethanolamide (PEA) biosynthesis, and in the lung an efficient surfactant activity. In order to maintain membrane phospholipids (PL) balance may be crucial an optimal intake of PA in a certain ratio with unsaturated fatty acids, especially PUFAs of both n-6 and n-3 families. However, in presence of other factors such as positive energy balance, excessive intake of carbohydrates (in particular mono and disaccharides), and a sedentary lifestyle, the mechanisms to maintain a steady state of PA concentration may be disrupted leading to an over accumulation of tissue PA resulting in dyslipidemia, hyperglycemia, increased ectopic fat accumulation and increased inflammatory tone via toll-like receptor 4. It is therefore likely that the controversial data on the association of dietary PA with detrimental health effects, may be related to an excessive imbalance of dietary PA/PUFA ratio which, in certain physiopathological conditions, and in presence of an enhanced DNL, may further accelerate these deleterious effects.

Non-linear van't Hoff behavior in pulmonary surfactant model membranes

Pulmonary surfactant exhibits phase coexistence over a wide range of surface pressure and temperature. Less is known about the effect of temperature on pulmonary surfactant models. Given the lack of studies on this issue, we used electron paramagnetic resonance (EPR) and nonlinear least-squares (NLLS) simulations to investigate the thermotropic phase behavior of the matrix that mimics the pulmonary surfactant lipid complex, i.e., the lipid mixture composed of dipalmitoyl phosphatidylcholine (DPPC), palmitoyl-oleoyl phosphatidylcholine (POPC) and palmitoyl-oleoyl phosphatidylglycerol (POPG). Irrespective of pH, the EPR spectra recorded from 5°C to 25°C in the DPPC/POPC/POPG (4:3:1) model membrane contain two spectral components corresponding to lipids in gel-like and fluid-like phases, indicating a coexistence of two domains in that range. The temperature dependence of the distribution of spin labels between the domains yielded nonlinear van't Hoff plots. The thermodynamic parameters evaluated were markedly different for DPPC and for the ternary DPPC/POPC/POPG (4:3:1) membranes and exhibited a dependence on chemical environment. While enthalpy and entropy changes for DPPC were always positive and presented a quadratic behavior with temperature, those of the ternary mixture were linearly dependent on temperature and changed from negative to positive values. Despite that, enthalpy-entropy compensation takes place in the two systems. The thermotropic process associated with the coexistence of the two domains is entropically-driven in DPPC and either entropically- or enthalpically-driven in the pulmonary surfactant membrane depending on the pH, ionic strength and temperature. The significance of these results to the structure and function of the pulmonary surfactant lipid matrix is discussed.

The Molecular Apgar Score: A Key to Unlocking Evolutionary Principles

One of the first "tools" used for systematically evaluating successful newborn transitional physiology at birth was the Apgar Score, devised by Virginia Apgar in 1953. This objective assessment tool allowed clinicians to immediately gauge the relative success of a newborn infant making the transition from the in utero liquid immersive environment to the ex utero gas environment in the delivery room during the first minutes after birth. The scoring system, although eponymous, is generally summarized as an acronym based on Appearance, Pulse, Grimace, Activity, and Respiration, criteria evaluated and scored at 1 and 5 min after birth. This common clinical appraisal is a guide for determining the elements of integrated physiology involved as the infant makes the transition from a "sea water" environment of 3% oxygen to a "land" environment in 21% oxygen. Appearance determines the perfusion of the skin with oxygenated blood-turning it pink; Pulse is the rate of heart beat, reflecting successful oxygen delivery to organs; Grimace, or irritability, is a functional marker for nervous system integration; Activity represents locomotor capacity; and, of course, Respiration represents pulmonary function as well as the successful neuro-feedback-mediated drive to breathe, supplying oxygen by inspiring atmospheric gas. Respiration, locomotion, and metabolism are fundamental processes adapted for vertebrate evolution from a water-based to an atmosphere-based life and are reflected by the Apgar Score. These physiologic processes last underwent major phylogenetic changes during the water-land transition some 300-400 million years ago, during which specific gene duplications occurred that facilitated terrestrial adaptation, in particular the parathyroid hormone-related protein receptor, the β-adrenergic receptor, and the glucocorticoid receptor. All these genetic traits and the gene regulatory networks they comprise represent the foundational substructure of the Apgar Score. As such, these molecular elements can be examined using a Molecular Apgar evaluation of keystone evolutionary events that predict successful evolutionary adaptation of physiologic functions necessary for neonatal transition and survival.

Pulmonary surfactant metabolism in the alveolar airspace: Biogenesis, extracellular conversions, recycling

Pulmonary surfactant is a lipid-protein complex that lines and stabilizes the respiratory interface in the alveoli, allowing for gas exchange during the breathing cycle. At the same time, surfactant constitutes the first line of lung defense against pathogens. This review presents an updated view on the processes involved in biogenesis and intracellular processing of newly synthesized and recycled surfactant components, as well as on the extracellular surfactant transformations before and after the formation of the surface active film at the air-water interface. Special attention is paid to the crucial regulation of surfactant homeostasis, because its disruption is associated with several lung pathologies.

On the Evolution of the Mammalian Brain

Hobson and Friston have hypothesized that the brain must actively dissipate heat in order to process information (Hobson et al., 2014). This physiologic trait is functionally homologous with the first instantation of life formed by lipids suspended in water forming micelles- allowing the reduction in entropy (heat dissipation). This circumvents the Second Law of Thermodynamics permitting the transfer of information between living entities, enabling them to perpetually glean information from the environment, that is felt by many to correspond to evolution per se. The next evolutionary milestone was the advent of cholesterol, embedded in the cell membranes of primordial eukaryotes, facilitating metabolism, oxygenation and locomotion, the triadic basis for vertebrate evolution. Lipids were key to homeostatic regulation of calcium, forming calcium channels. Cell membrane cholesterol also fostered metazoan evolution by forming lipid rafts for receptor-mediated cell-cell signaling, the origin of the endocrine system. The eukaryotic cell membrane exapted to all complex physiologic traits, including the lung and brain, which are molecularly homologous through the function of neuregulin, mediating both lung development and myelinization of neurons. That cooption later exapted as endothermy during the water-land transition (Torday, 2015a), perhaps being the functional homolog for brain heat dissipation and conscious/mindful information processing. The skin and brain similarly share molecular homologies through the “skin-brain” hypothesis, giving insight to the cellular-molecular “arc” of consciousness from its unicellular origins to integrated physiology. This perspective on the evolution of the central nervous system clarifies self-organization, reconciling thermodynamic and informational definitions of the underlying biophysical mechanisms, thereby elucidating relations between the predictive capabilities of the brain and self-organizational processes.

Does Whole-Body Hypothermia in Neonates with Hypoxic-Ischemic Encephalopathy Affect Surfactant Disaturated-Phosphatidylcholine Kinetics?

Background

It is unknown whether Whole-Body Hypothermia (WBH) affects pulmonary function. In vitro studies, at relatively low temperatures, suggest that hypothermia may induce significant changes to the surfactant composition. The effect of WBH on surfactant kinetics in newborn infants is unknown. We studied in vivo kinetics of disaturated-phosphatidylcholine (DSPC) in asphyxiated newborns during WBH and in normothermic controls (NTC) with no or mild asphyxia. Both groups presented no clinically apparent lung disease.

Methods

Twenty-seven term or near term newborns requiring mechanical ventilation were studied (GA 38.6±2.2 wks). Fifteen during WBH and twelve NTC. All infants received an intra-tracheal dose of ¹³C labelled DSPC and tracheal aspirate were performed. DSPC amount, DSPC half-life (HL) and pool size (PS) were calculated.

Results

DSPC amount in tracheal aspirates was 0.42 [0.22–0.54] and 0.36 [0.10–0.58] mg/ml in WBH and NTC respectively (p = 0.578). DSPC HL was 24.9 [15.7–52.5] and 25.3 [15.8–59.3] h (p = 0.733) and DSPC PS was 53.2 [29.4–91.6] and 40.2 [29.8–64.6] mg/kg (p = 0.598) in WBH and NTC respectively.

Conclusions

WBH does not alter DSPC HL and PS in newborn infants with no clinical apparent lung disease.

Computer simulations of lung surfactant

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

Heterochrony as Diachronically Modified Cell-Cell Interactions

Heterochrony is an enabling concept in evolution theory that metaphorically captures the mechanism of biologic change due to mechanisms of growth and development. The spatio-temporal patterns of morphogenesis are determined by cell-to-cell signaling mediated by specific soluble growth factors and their cognate receptors on nearby cells of different germline origins. Subsequently, down-stream production of second messengers generates patterns of form and function. Environmental upheavals such as Romer’s hypothesized drying up of bodies of water globally caused the vertebrate water-land transition. That transition caused physiologic stress, modifying cell-cell signaling to generate terrestrial adaptations of the skeleton, lung, skin, kidney and brain. These tissue-specific remodeling events occurred as a result of the duplication of the Parathyroid Hormone-related Protein Receptor (PTHrPR) gene, expressed in mesodermal fibroblasts in close proximity to ubiquitously expressed endodermal PTHrP, amplifying this signaling pathway. Examples of how and why PTHrPR amplification affected the ontogeny, phylogeny, physiology and pathophysiology of the lung are used to substantiate and further our understanding through insights to the heterochronic mechanisms of evolution, such as the fish swim bladder evolving into the vertebrate lung, interrelated by such functional homologies as surfactant and mechanotransduction. Instead of the conventional description of this phenomenon, lung evolution can now be understood as adaptive changes in the cellular-molecular signaling mechanisms underlying its ontogeny and phylogeny.

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

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

A REVIEW OF THE EFFECTS OF BODY TEMPERATURE VARIATIONS ON RESPIRATORY MECHANICS: MEASUREMENTS BY THE END-INFLATION OCCLUSION METHOD IN THE RAT

The temperature of body fluids is expected to affect tissues mechanical properties, including respiratory system tissues. This is because of the changes in airway smooth muscle tone and contractile properties, influencing airway frictional resistance to airflow, and because of the temperature effects on the stress–strain relationships of elastin and collagen, which determinates the elastic behavior of the lungs as reflected by their pressure–volume relationship. Alveolar surfactant biological and physical properties have also been shown to be affected by temperature changes, suggesting influences on the respiratory system hysteretic properties.

Experimental works describing the effects of body temperature variations on respiratory mechanics are reviewed, including recent findings dealing with investigations on respiratory mechanics carried out by the end-inflation occlusion method in the rat. This method allows to determine, together with the elastance of the respiratory system, its resistive properties too. In particular, both the ohmic airway resistance due to frictional forces in the airway and the additional visco-elastic resistance exerted because of tissues stress-relaxation may be quantified.

The effects of body temperature variations were assessed, and experimentally induced temperature increments and/or decrements allowed to conclude that respiratory system tissues stiffness, both the ohmic and the stress-relaxation linked resistances, and the hysteretic behavior of the respiratory system, decrease with temperature increments. The mechanisms responsible for these effects are analyzed.

Upper critical solution temperature behavior of cinnamic acid and polyethyleneimine mixture and its effect on temperature-dependent release of liposome

The mixture of polyethyleneimine (PEI) and cinnamic acid (CA) in HEPES buffer (pH 7.0) exhibited an upper critical solution temperature in the temperature range of 20-50 °C. CA would be electrostatically conjugated with PEI and the PEI-CA conjugate is thought to act as a thermo-sensitive polymer. On the optical microscope image of PEI/CA mixture, microparticles were found at 25 °C, disappeared when heated to 50 °C, and formed again upon cooling to 25 °C. PEI-CA conjugate was immobilized on the surface of egg phosphatidylcholine (EPC) liposome by adding PEI to the suspension of liposome incorporating CA. The size and the zeta potential of the liposome markedly increased by cooling the liposomal suspension from 50 °C to 20 °C. This could be ascribed to the cooling-induced self-assembling property of PEI-CA conjugate. The release profile of Rhodamine B base from liposome incorporating CA with PEI was investigated while the liposome suspension of 50 °C was exposed to the release medium of 20 °C, 30 °C, 40 °C and 50 °C. The release degree was higher at a lower temperature. When exposed to a lower temperature (20 °C, 30 °C, 40 °C), PEI-CA could be self-assembled and change its configuration on the surface of liposome, promoting the release from the liposome.

Pleiotropy as the Mechanism for Evolving Novelty: Same Signal, Different Result

In contrast to the probabilistic way of thinking about pleiotropy as the random expression of a single gene that generates two or more distinct phenotypic traits, it is actually a deterministic consequence of the evolution of complex physiology from the unicellular state. Pleiotropic novelties emerge through recombinations and permutations of cell-cell signaling exercised during reproduction based on both past and present physical and physiologic conditions, in service to the future needs of the organism for its continued survival. Functional homologies ranging from the lung to the kidney, skin, brain, thyroid and pituitary exemplify the evolutionary mechanistic strategy of pleiotropy. The power of this perspective is exemplified by the resolution of evolutionary gradualism and punctuated equilibrium in much the same way that Niels Bohr resolved the paradoxical duality of light as Complementarity.

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.

Physiological variables affecting surface film formation by native lamellar body-like pulmonary surfactant particles

Pulmonary surfactant (PS) is a surface active complex of lipids and proteins that prevents the alveolar structures from collapsing and reduces the work of breathing by lowering the surface tension at the alveolar air-liquid interface (ALI). Surfactant is synthesized by the alveolar type II (AT II) cells, and it is stored in specialized organelles, the lamellar bodies (LBs), as tightly packed lipid bilayers. Upon secretion into the alveolar lining fluid, a large fraction of these particles retain most of their packed lamellar structure, giving rise to the term lamellar body like-particles (LBPs). Due to their stability in aqueous media, freshly secreted LBPs can be harvested from AT II cell preparations. However, when LBPs get in contact with an ALI, they quickly and spontaneously adsorb into a highly organized surface film. In the present study we investigated the adsorptive capacity of LBPs at an ALI under relevant physiological parameters that characterize the alveolar environment in homeostatic or in pathological conditions. Adsorption of LBPs at an ALI is highly sensitive to pH, temperature and albumin concentration and to a relatively lesser extent to changes in osmolarity or Ca(2+) concentrations in the physiological range. Furthermore, proteolysis of LBPs significantly decreases their adsorptive capacity confirming the important role of surfactant proteins in the formation of surface active films.

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

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

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

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

Compositional and structural characterization of monolayers and bilayers composed of native pulmonary surfactant from wild type mice

This work comprises a structural and dynamical study of monolayers and bilayers composed of native pulmonary surfactant from mice. Spatially resolved information was obtained using fluorescence (confocal, wide field and two photon excitation) and atomic force microscopy methods. Lipid mass spectrometry experiments were also performed in order to obtain relevant information on the lipid composition of this material. Bilayers composed of mice pulmonary surfactant showed coexistence of distinct domains at room temperature, with morphologies and lateral packing resembling the coexistence of liquid ordered (lo)/liquid disordered (ld)-like phases reported previously in porcine lung surfactant. Interestingly, the molar ratio of saturated (mostly DPPC)/non-saturated phospholipid species and cholesterol measured in the innate material corresponds with that of a DOPC/DPPC/cholesterol mixture showing lo/ld phase coexistence at a similar temperature. This suggests that at quasi-equilibrium conditions, key lipid classes in this complex biological material are still able to produce the same scaffold observed in relevant but simpler model lipid mixtures. Also, robust structural and dynamical similarities between mono- and bi-layers composed of mice pulmonary surfactant were observed when the monolayers reach a surface pressure of 30mN/m. This value is in line with theoretically predicted and recently measured surface pressures, where the monolayer-bilayer equivalence occurs in samples composed of single phospholipids. Finally, squeezed out material attached to pulmonary surfactant monolayers was observed at surface pressures near the beginning of the monolayer reversible exclusion plateau (~40mN/m). Under these conditions this material adopts elongated tubular shapes and displays ordered lateral packing as indicated by spatially resolved LAURDAN GP measurements.

The effect of body cooling on respiratory system mechanics and hysteresis in rats

Literature reports and theoretical considerations suggest that body cooling may affect respiratory mechanics in vivo. To examine this hypothesis, healthy rats were studied using the end-inflation occlusion method under control conditions and after total body cooling. Respiratory mechanics parameters, hysteresis areas, the inspiratory work of breathing, and its elastic and resistive components, were calculated. After body cooling (mean rectal temperature from 36.6 ± 0.25 to 32.1 ± 0.26 °C), the ohmic and the additional visco-elastic respiratory system resistances, the hysteresis, the total inspiratory work of breathing, and its resistive components, were all increased. No significant changes were detected for the static and dynamic respiratory system elastance mean values, and the related elastic component of the work of breathing. These data indicate that body cooling increases the mechanical inspiratory work of breathing by increasing the resistive pressures dissipation. This effect is evident even for limited temperature variations, and it is suggested that it may occur in the event of accidental or therapeutic hypothermia.

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

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