The pattern of surfactant cholesterol during vertebrate evolution and development: does ontogeny recapitulate phylogeny?

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.

The Potential Role of Bioactive Plasmalogens in Lung Surfactant

Neonatal respiratory distress syndrome (NRDS) is a type of newborn disorder caused by the deficiency or late appearance of lung surfactant, a mixture of lipids and proteins. Studies have shown that lung surfactant replacement therapy could effectively reduce the morbidity and mortality of NRDS, and the therapeutic effect of animal-derived surfactant preparation, although with its limitations, performs much better than that of protein-free synthetic ones. Plasmalogens are a type of ether phospholipids present in multiple human tissues, including lung and lung surfactant. Plasmalogens are known to promote and stabilize non-lamellar hexagonal phase structure in addition to their significant antioxidant property. Nevertheless, they are nearly ignored and underappreciated in the lung surfactant-related research. This report will focus on plasmalogens, a minor yet potentially vital component of lung surfactant, and also discuss their biophysical properties and functions as anti-oxidation, structural modification, and surface tension reduction at the alveolar surface. At the end, we boldly propose a novel synthetic protein-free lung surfactant preparation with plasmalogen modification as an alternative strategy for surfactant replacement therapy.

Properties of Lipid Models of Lung Surfactant Containing Cholesterol and Oxidized Lipids: A Mixed Experimental and Computational Study

We introduce and study a multicomponent lipid film mimicking lipid composition of the human lung surfactant. It consists of phospholipids with various lipid headgroups and tail saturation. Furthermore, it includes cholesterol and oxidized lipids. Langmuir trough and fluorescence microscopy experiments are combined with fully atomistic molecular dynamics simulations. The considered lipid mixtures form complex interfacial films with properties modulated by lateral compression. Cholesterol laterally condenses, and oxidized lipids laterally expand the films; both types of molecules increase film miscibility. Oxidized lipids also alter the lipid-water interface enhancing film hydration; this effect can be partially reversed by cholesterol. Regarding presentation of different chemical moieties toward the aqueous subphase, the zwitterionic phosphatidylcholine groups dominate at the lipid-water interface, while both the negatively charged phosphatidylglycerol and hydroxyl group of cholesterol are less exposed. The investigated synthetic lipid-only mimic of the lung surfactant may serve as a basis for further studies involving nonlipid pulmonary surfactant components.

A diachronic evolutionary biologic perspective: Reconsidering the role of the eukaryotic unicell offers a 'Timeless' biology

Biology has remained descriptive since its formalization by Linnaeus in the 18th Century. Dobzhansky has challenged us to think mechanistically by stating that 'Nothing in Biology makes sense except in the light of evolution', but NeoDarwinian evolution remains untestable or refutable. The physicist Bohm has encouraged us to recognize that our perception of 'reality' is mediated by our evolved, subjective senses, though there is a coherent Implicate Order just out of reach. Only recently has a novel understanding of physiologic evolution based on cell-cell communication for embryonic development and phylogeny offered the opportunity to mechanistically merge Quantum Mechanics with Evolutionary Biology.

Human amniotic membrane as newly identified source of amniotic fluid pulmonary surfactant

Pulmonary surfactant (PS) reduces surface tension at the air-liquid interface in the alveolar epithelium of the lung, which is required for breathing and for the pulmonary maturity of the developing foetus. However, the origin of PS had never been thoroughly investigated, although it was assumed to be secreted from the foetal developing lung. Human amniotic membrane (hAM), particularly its epithelial cell layer, composes the amniotic sac enclosing the amniotic fluid. In this study, we therefore aimed to investigate a potential contribution of the cellular components of the hAM to pulmonary surfactant found in amniotic fluid. We identified that cells within the native membrane contain lamellar bodies and express all four surfactant proteins as well as ABCA3. Lipidomic profiling by nanoESI – MS/MS revealed the presence of the essential lipid species as found in PS. Also, the biophysical activity of conditioned cell culture supernatant obtained from hAM was tested with captive bubble surfactometry. hAM supernatant showed the ability to reduce surface tension, similar to human PS obtained from bronchoalveolar lavage. This means that hAM produces the essential PS-associated components and can therefore contribute as second potential source of PS in amniotic fluid aside from the foetal lung.

The resolution of ambiguity as the basis for life: A cellular bridge between Western reductionism and Eastern holism

Boundary conditions enable cellular life through negentropy, chemiosmosis, and homeostasis as identifiable First Principles of Physiology. Self-referential awareness of status arises from this organized state to sustain homeostatic imperatives. Preferred homeostatic status is dependent upon the appraisal of information and its communication. However, among living entities, sources of information and their dissemination are always imprecise. Consequently, living systems exist within an innate state of ambiguity. It is presented that cellular life and evolutionary development are a self-organizing cellular response to uncertainty in iterative conformity with its basal initiating parameters. Viewing the life circumstance in this manner permits a reasoned unification between Western rational reductionism and Eastern holism.

Critical appraisal of some factors pertinent to the functional designs of the gas exchangers

Respiration acquires O₂ from the external fluid milieu and eliminates CO₂ back into the same. Gas exchangers evolved under certain immutable physicochemical laws upon which their elemental functional design is hardwired. Adaptive changes have occurred within the constraints set by such laws to satisfy metabolic needs for O₂, environmental conditions, respiratory medium utilized, lifestyle pursued and phylogenetic level of development: correlation between structure and function exists. After the inaugural simple cell membrane, as body size and structural complexity increased, respiratory organs formed by evagination or invagination: the gills developed by the former process and the lungs by the latter. Conservation of water on land was the main driver for invagination of the lungs. In gills, respiratory surface area increases by stratified arrangement of the structural components while in lungs it occurs by internal subdivision. The minuscule terminal respiratory units of lungs are stabilized by surfactant. In gas exchangers, respiratory fluid media are transported by convection over long distances, a process that requires energy. However, movement of respiratory gases across tissue barriers occurs by simple passive diffusion. Short distances and large surface areas are needed for diffusion to occur efficiently. Certain properties, e.g., diffusion of gases through the tissue barrier, stabilization of the respiratory units by surfactant and a thin tripartite tissue barrier, have been conserved during the evolution of the gas exchangers. In biology, such rare features are called Bauplans, blueprints or frozen cores. That several of them (Bauplans) exist in gas exchangers almost certainly indicates the importance of respiration to life.

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.

The Unicellular State as a Point Source in a Quantum Biological System

A point source is the central and most important point or place for any group of cohering phenomena. Evolutionary development presumes that biological processes are sequentially linked, but neither directed from, nor centralized within, any specific biologic structure or stage. However, such an epigenomic entity exists and its transforming effects can be understood through the obligatory recapitulation of all eukaryotic lifeforms through a zygotic unicellular phase. This requisite biological conjunction can now be properly assessed as the focal point of reconciliation between biology and quantum phenomena, illustrated by deconvoluting complex physiologic traits back to their unicellular origins.

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 central theory of biology

The history of physiologic cellular-molecular interrelationships can be traced all the way back to the unicellular state by following the pathway formed by lipids ubiquitously accommodating calcium homeostasis, and its consequent adaptive effects on oxygen uptake by cells, tissues and organs. As a result, a cohesive, mechanistically integrated view of physiology can be formulated by recognizing the continuum comprising conception, development, physiologic homeostasis and death mediated by soluble growth factor signaling. Seeing such seemingly disparate processes as embryogenesis, chronic disease and dying as the gain and subsequent loss of cell-cell signaling provides a novel perspective for physiology and medicine. It is emblematic of the self-organizing, self-referential nature of life, starting from its origins. Such organizing principles obviate the pitfalls of teleologic evolution, conversely providing a way of resolving such seeming dichotomies as holism and reductionism, genotype and phenotype, emergence and contingence, proximate and ultimate causation in evolution, cells and organisms. The proposed approach is scale-free and predictive, offering a Central Theory of Biology.

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.

Paramecium caudatum enhances transmission and infectivity of Mycobacterium marinum and M. chelonae in zebrafish Danio rerio

Mycobacterial infections in laboratory zebrafish Danio rerio are common and widespread in research colonies. Mycobacteria within free-living amoebae have been shown to be transmission vectors for mycobacteriosis. Paramecium caudatum are commonly used as a first food for zebrafish, and we investigated this ciliate's potential to serve as a vector of Mycobacterium marinum and M. chelonae. The ability of live P. caudatum to transmit these mycobacteria to larval, juvenile and adult zebrafish was evaluated. Infections were defined by histologic observation of granulomas containing acid-fast bacteria in extraintestinal locations. In both experiments, fish fed paramecia containing mycobacteria became infected at a higher incidence than controls. Larvae (exposed at 4 d post hatch) fed paramecia with M. marinum exhibited an incidence of 30% (24/80) and juveniles (exposed at 21 d post hatch) showed 31% incidence (14/45). Adult fish fed a gelatin food matrix containing mycobacteria within paramecia or mycobacteria alone for 2 wk resulted in infections when examined 8 wk after exposure as follows: M. marinum OSU 214 47% (21/45), M. marinum CH 47% (9/19), and M. chelonae 38% (5/13). In contrast, fish feed mycobacteria alone in this diet did not become infected, except for 2 fish (5%) in the M. marinum OSU 214 low-dose group. These results demonstrate that P. caudatum can act as a vector for mycobacteria. This provides a useful animal model for evaluation of natural mycobacterial infections and demonstrates the possibility of mycobacterial transmission in zebrafish facilities via contaminated paramecia cultures.

Evolution and Cell Physiology. 1. Cell signaling is all of biology

I hypothesize that the First Principles of Physiology (FPPs) were co-opted during the vertebrate transition from water to land, beginning with the acquisition of cholesterol by eukaryotes, facilitating unicellular evolution over the course of the first 4.5 billion years of the Earth's history, in service to the reduction in intracellular entropy, far from equilibrium. That mechanism was perpetuated by the advent of cholesterol in the cell membrane of unicellular eukaryotes, ultimately giving rise to the metazoan homologs of the gut, lung, kidney, skin, bone, and brain. Parathyroid hormone-related protein (PTHrP), whose cognate receptor underwent a gene duplication during the transition from fish to amphibians, facilitated gas exchange for the water-to-land transition, since PTHrP is necessary for the formation of lung alveoli: deletion of the PTHrP gene in mice causes the offspring to die within a few minutes of birth due to the absence of alveoli. Moreover, PTHrP is central to the development and homeostasis of the kidney, skin, gut, bone, and brain. Therefore, duplication of the PTHrP receptor gene is predicted to have facilitated the molecular evolution of all the necessary traits for land habitation through a common cellular and molecular motif. Subsequent duplication of the β-adrenergic receptor gene permitted blood pressure control within the lung microvasculature, allowing further evolution of the lung by increasing its surface area. I propose that such gene duplications were the result of shear stress on the microvasculature, locally generating radical oxygen species that caused DNA mutations, giving rise to duplication of the PTHrP and β-adrenergic receptor genes. I propose that one can determine the FPPs by systematically tracing the molecular homologies between the lung, skin, kidney, gut, bone, and brain across development, phylogeny, and pathophysiology as a type of "reverse evolution." By tracing such relationships back to unicellular organisms, one can use the underlying principles to predict homeostatic failure as disease, thereby also potentially forming the basis for maneuvers that can treat or even prevent such failure.

Comparative transcriptome analyses indicate molecular homology of zebrafish swimbladder and mammalian lung

The fish swimbladder is a unique organ in vertebrate evolution and it functions for regulating buoyancy in most teleost species. It has long been postulated as a homolog of the tetrapod lung, but the molecular evidence is scarce. In order to understand the molecular function of swimbladder as well as its relationship with lungs in tetrapods, transcriptomic analyses of zebrafish swimbladder were carried out by RNA-seq. Gene ontology classification showed that genes in cytoskeleton and endoplasmic reticulum were enriched in the swimbladder. Further analyses depicted gene sets and pathways closely related to cytoskeleton constitution and regulation, cell adhesion, and extracellular matrix. Several prominent transcription factor genes in the swimbladder including hoxc4a, hoxc6a, hoxc8a and foxf1 were identified and their expressions in developing swimbladder during embryogenesis were confirmed. By comparison of enriched transcripts in the swimbladder with those in human and mouse lungs, we established the resemblance of transcriptome of the zebrafish swimbladder and mammalian lungs. Based on the transcriptomic data of zebrafish swimbladder, the predominant functions of swimbladder are in its epithelial and muscular tissues. Our comparative analyses also provide molecular evidence of the relatedness of the fish swimbladder and mammalian lung.

Cholesterol as an evolutionary response to living with oxygen

Although often considered in a negative light, cholesterol is an essential molecule with unusually diverse functions. Cholesterol and related sterols (ergosterol in yeast, phytosterols in plants) is considered a hallmark of eukaryotes, and may even have triggered the evolution of multicellular organisms. Synthesis of cholesterol is an extremely oxygen-intensive process and requires sufficient terrestrial oxygen to proceed. In turn, several lines of evidence support the argument that cholesterol evolved at least in part as an adaptation to the hazards of oxygen. This evolutionary perspective usefully informs medical research on cholesterol to address health-related issues, as illustrated by examples drawn from three prominent human diseases: cataracts, heart disease, and cancer.

Identifying the spatial distribution of vitamin E, pulmonary surfactant and membrane lipids in cells and tissue by confocal Raman microscopy

Every organ compromises of several different cell types. When studying the effects of a chosen compound within this organ or tissue uptake, localisation, metabolism, and the effect itself can be expected to differ between cells. Using the example of Vitamin E in pulmonary tissue we introduce confocal Raman Microscopy as a superior method to localise lipid-soluble compounds within tissues and cells. We describe the analyses of vitamin E, its oxidation products, and metabolites as well as pulmonary surfactant phospholipids in fixed lung tissue sections. Examples of main structural membrane lipids (PC, cholesterol) and an example of a lipid-signalling molecule (ceramide) are also included. Confocal Raman microscopy is a non-destructive optical method of analysing chemical and physical composition of solids, liquids, gases, gels, and solutions. The method is rich in information allowing discrimination of chemically similar molecules (including geometric isomers) and sensitive monitoring of subtle physical interactions. Additionally, Raman spectroscopy is relatively insensitive to water allowing the analysis of aqueous solutions and suspensions typical in biochemistry. In contrast, Raman spectroscopy is sensitive to non-polar molecules making it ideal for lipidomics research.

Special relationship between sterols and oxygen: were sterols an adaptation to aerobic life?

A fascinating link between sterols and molecular oxygen (O(2)) has been a common thread running through the fundamental work of Konrad Bloch, who elucidated the biosynthetic pathway for cholesterol, to recent work supporting a role of sterols in the sensing of O(2). Synthesis of sterols by eukaryotes is an O(2)-intensive process. In this review, we argue that increased levels of O(2) in the atmosphere not only made the evolution of sterols possible, but that these sterols may in turn have provided the eukaryote with an early defence mechanism against O(2). The idea that nature crafted sterols as a feedback loop to adapt to, or help protect against, the hazards of O(2) is novel and enticing. We marshal several lines of evidence to support this thesis: (1) coincidence of atmospheric O(2) and sterol evolution; (2) sterols regulate O(2) entry into eukaryotic cells and organelles; (3) sterols act as O(2) sensors across eukaryotic life; (4) sterols serve as a primitive cellular defence against O(2) (including reactive oxygen species). Therefore, sterols may have evolved in eukaryotes partially as an adaptive response to the rise of terrestrial O(2), rather than merely as a consequence of it.

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

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

Comparison of the respiratory transition at birth or hatching in viviparous and oviparous amniote vertebrates

Regardless of the mode of reproduction, three things must occur at birth or hatching in amniote vertebrates: initiation of breathing, pulmonary fluid elimination and reabsorption, and adequate perfusion of pulmonary circulation. Although data on these events are few, there appears to be no fundamental difference in them that can be associated with the oviparity to viviparity transition. There are, however, differences in the timing of these events in oviparous and viviparous amniotes. The transition to neonatal respiration tends to be very quick in viviparous species because the vascular support for oxygen uptake provided by the mother is rapidly disassociated from the mechanism for uptake by the embryo. By contrast, hatching often is a slow process, taking 24 h or more in some species, as chorioallantoic blood flow slowly gives way to clearing of the lungs and pulmonary gas exchange. Little is known of the mechanisms of pulmonary fluid elimination and reabsorption or lung inflation in reptiles, but the cellular structures and surfactant systems are similar in all amniote vertebrates. Nevertheless, there are differences, particularly of timing and maturation of various systems, but there has been no exploration of the functional (or phylogenetic) bases of these differences. Perfusion of the neonatal pulmonary system to support respiration in reptiles remains to be investigated. In mammals and birds, closure of the ductus arteriosus is important, but the role played by the ductus arterioisus in reptilian hatching or birth is not known.

Collectins and cationic antimicrobial peptides of the respiratory epithelia

The respiratory epithelium is a primary site for the deposition of microorganisms that are acquired during inspiration. The innate immune system of the respiratory tract eliminates many of these potentially harmful agents preventing their colonization. Collectins and cationic antimicrobial peptides are antimicrobial components of the pulmonary innate immune system produced by respiratory epithelia, which have integral roles in host defense and inflammation in the lung. Synthesis and secretion of these molecules are regulated by the developmental stage, hormones, as well as many growth and immunoregulatory factors. The purpose of this review is to discuss antimicrobial innate immune elements within the respiratory tract of healthy and pneumonic lung with emphasis on hydrophilic surfactant proteins and beta-defensins.

Regulation of surfactant protein and defensin mRNA expression in cultured ovine type II pneumocytes by all-trans retinoic acid and VEGF

Beta-defensins and surfactant proteins are components of the pulmonary innate immune system. Their gene expression is regulated by development, hormones, growth and immunoregulatory factors. It was our hypothesis that growth and differentiation factors such as all-trans retinoic acid (RA) and vascular endothelial growth factor (VEGF) may affect expression of selected innate immune genes by respiratory epithelial cells. Ovine JS7 cells (alveolar type II pneumocytes) were incubated in serum-free Dulbecco's modified Eagle's medium (DMEM) complete media that contained: no treatment (negative control), RA (500 nM), or VEGF (100 ng/ml) for 6, 12 or 24 h incubation. Total RNA was isolated, cDNA synthesized, and relative mRNA levels of surfactant protein A (SP-A) and SP-D, and sheep beta-defensin-1 (SBD-1) were determined by real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Cells had significantly increased expression of SP-D mRNA at 6 h and 24 h, decreased expression of SP-A mRNA at 12 h, and unchanged levels of SBD-1 mRNA after the treatment with RA compared with their respective negative controls. VEGF did not alter the expression of the three innate immune genes. These findings suggest that SP-A and SP-D have different transcription regulation pathways, and that expression of SBD-1 is not inducible by RA similar to its human homolog HBD-1. The lack of changes induced by VEGF treatment suggests that VEGF does not have a direct effect on epithelial cells, but may affect gene expression indirectly.

Surfactant as an airway smooth muscle relaxant

A variety of clinical and experimental evidence indicates that surfactant may be important in the pathogenesis and treatment of asthma. The purpose of this study was to determine the pharmacologic effect of pulmonary surfactant and its major lipid and protein constituents on bronchial smooth muscle. First-generation bronchi from male Sprague-Dawley rats were contracted with methacholine and exposed to two kinds of surfactant: whole rat surfactant and two bovine surfactant extracts in clinical use. The latter lack the hydrophilic surfactant-associated proteins (SP)-A and SP-D. All the surfactants relaxed the rat bronchi in a concentration-dependent manner; however, whole rat surfactant was more potent than the bovine extracts. Both surfactant lipids and SP-A contributed to the bronchial relaxation. The relaxation response produced by the highest concentration (0.5 mg/ml) of whole rat surfactant was equivalent to that caused by substance P (5 microM) and approximately half of that caused by 1 microM isoproterenol. The relaxation response was epithelium-dependent and blocked by indomethacin but not by N-omega-nitro-L-arginine methyl ester. We conclude that surfactant can relax airway smooth muscle directly via a prostanoid-mediated, epithelium-dependent process that does not involve nitric oxide synthase.

Interfacial properties of pulmonary surfactant layers

The composition of the pulmonary surfactant and the border conditions of normal human breathing are relevant to characterize the interfacial behavior of pulmonary layers. Based on experimental data methods are reviewed to investigate interfacial properties of artificial pulmonary layers and to explain the behavior and interfacial structures of the main components during compression and expansion of the layers observed by epifluorescence and scanning force microscopy. Terms like over-compression, collapse, and formation of the surfactant reservoir are discussed. Consequences for the viscoelastic surface rheological behavior of such layers are elucidated by surface pressure relaxation and harmonic oscillation experiments. Based on a generalized Volmer isotherm the interfacial phase transition is discussed for the hydrophobic surfactant proteins, SP-B and SP-C, as well as for the mixtures of dipalmitoylphosphatidylcholine (DPPC) with these proteins. The behavior of the layers depends on both the oligomerisation state and the secondary structure of the hydrophobic surfactant proteins, which are controlled by the preparation of the proteins. An example for the surface properties of bronchoalveolar porcine lung washings of uninjured, injured, and Curosurf treated lavage is discussed in the light of surface behavior. An outlook summarizes the present knowledge and the main future development in this field of surface science.

The composition of pulmonary surfactant from diving mammals

Maintaining a functional pulmonary surfactant system at depth is critical for diving mammals to ensure that inspiration is possible upon re-emergence. The lipid and protein composition of lavage extracts from three pinniped species (California sea lion, Northern elephant seal and Ringed seal) were compared to several terrestrial species. Lavage samples were purified using a NaBr discontinuous gradient. Concentrations of phospholipid classes and molecular species were measured using electrospray ionisation mass spectrometry, cholesterol was measured using high-performance liquid chromatography, surfactant protein A (SP-A) and SP-B were measured using enzyme-linked immunosorbent assays. There were small differences in phospholipid classes, with a lower level of anionic surfactant phospholipids, PG and PI, between diving and terrestrial mammals. There were no differences in PL saturation or SP-A levels between species. PC16:0/14:0, PC16:0/16:1, PC16:0/16:0, long chain PI species and the total concentrations of alkyl-acyl species of PC and PG as a ratio of diacyl species were increased in diving mammals, whereas concentrations of PC16:0/18:1, PG16:0/16:0 and PG16:0/18:1 were decreased. Cholesterol levels were very variable between species and SP-B was very low in diving mammals. These differences may explain the very poor surface activity of pinniped surfactant that we have previously described [Miller, N.J., Daniels, C.B., Schürch, S., Schoel, W.M., Orgeig, S., 2005. The surface activity of pulmonary surfactant from diving mammals. Respir. Physiol. Neurobiol. 150 (2006) 220-232], supporting the hypothesis that pinniped surfactant has primarily an anti-adhesive function to meet the challenges of regularly collapsing lungs.

The lipid composition of autophagic vacuoles regulates expression of multilamellar bodies

Multilamellar bodies (MLBs) are responsible for surfactant secretion in type II alveolar cells but also accumulate in other cell types under pathological conditions, including cancer and lysosomal storage diseases such as Niemann-Pick C (NPC), a congenital disease where defective cholesterol transport leads to its accumulation in lysosomes. Mv1Lu type II alveolar cells transfected with Golgi β1,6 N-acetylglucosaminyltransferase V (Mgat5), enhancing the polylactosamine content of complex-type N-glycans, exhibit stable expression of MLBs whose formation requires lysosomal proteolysis within dense autophagic vacuoles. MLBs of Mgat5-transfected Mv1Lu cells are rich in phospholipids and have low levels of cholesterol. In Mv1Lu cells treated with the NPC-mimicking drug U18666A, cholesterol-rich MLBs accumulate independently of both Mgat5 expression and lysosomal proteolysis. Inhibition of autophagy by blocking the PI 3-kinase pathway with 3-methyladenine prevents MLB formation and results in the accumulation of non-lamellar, acidic lysosomal vacuoles. Treatment with 3-methyladenine inhibited the accumulation of monodansylcadaverine, a phospholipid-specific marker for autophagic vacuoles, but did not block endocytic access to the lysosomal vacuoles. Induction of autophagy via serum starvation resulted in an increased size of cholesterol-rich MLBs. Although expression of MLBs in the Mv1Lu cell line can be induced by modulating lysosomal cholesterol or protein glycosylation, an autophagic contribution of phospholipids is critical for the formation of concentric membrane lamellae within late lysosomal organelles.

Cholesterol rules: direct observation of the coexistence of two fluid phases in native pulmonary surfactant membranes at physiological temperatures

Pulmonary surfactant, the lipid-protein material that stabilizes the respiratory surface of the lungs, contains approximately equimolar amounts of saturated and unsaturated phospholipid species and significant proportions of cholesterol. Such lipid composition suggests that the membranes taking part in the surfactant structures could be organized heterogeneously in the form of inplane domains, originating from particular distributions of specific proteins and lipids. Here we report novel results concerning the lateral organization of bilayer membranes made of native pulmonary surfactant where the coexistence of two distinct micrometer sized fluid phases (fluid ordered and fluid disordered-like phases) is observed at physiological temperatures by using fluorescence microscopy and atomic force microscopy. Additional experiments using fluorescent-labeled proteins SP-B and SP-C show that at physiological temperatures these hydrophobic proteins are located exclusively in the fluid disordered-like phase. Most interestingly, the microscopic coexistence of fluid phases is maintained up to 37.5 degrees C, where most fluid ordered phases melt. This observation suggests that the particular composition of this material is naturally designed to be at the "edge" of a lateral structure transition under physiological conditions, likely providing particular structural and dynamic properties for its mechanical function. The observed lateral structure in native pulmonary surfactant membranes is dramatically affected by the extraction of cholesterol, an effect not observed upon extraction of the surfactant proteins. Furthermore, the spreading properties of the native surfactant material at the air-liquid interface were also greatly affected by cholesterol extraction, suggesting a connection between the observed lateral structure and a physiologically relevant function of the material. We suggest that the particular lipid composition of surfactant could be finely tuned to provide, under physiological conditions, a structural scaffold for surfactant proteins to act at appropriate local densities and lipid composition.

Surfactant from diving aquatic mammals

Diving mammals that descend to depths of 50-70 m or greater fully collapse the gas exchanging portions of their lungs and then reexpand these areas with ascent. To investigate whether these animals may have evolved a uniquely developed surfactant system to facilitate repetitive alveolar collapse and expansion, we have analyzed surfactant in bronchoalveolar lavage fluid (BAL) obtained from nine pinnipeds and from pigs and humans. In contrast to BAL from terrestrial mammals, BAL from pinnipeds has a higher concentration of phospholipid and relatively more fluidic phosphatidylcholine molecular species, perhaps to facilitate rapid spreading during alveolar reexpansion. Normalized concentrations of hydrophobic surfactant proteins B and C were not significantly different among pinnipeds and terrestrial mammals by immunologic assay, but separation of proteins by gel electrophoresis indicated a greater content of surfactant protein B in elephant seal surfactant than in human surfactant. Remarkably, surfactant from the deepest diving pinnipeds produced moderately elevated in vitro minimum surface tension measurements, a finding not explained by the presence of protein or neutral lipid inhibitors. Further study of the composition and function of pinniped surfactants may contribute to the design of optimized therapeutic surfactants.

The pulmonary surfactant: impact of tobacco smoke and related compounds on surfactant and lung development

Cigarette smoking, one of the most pervasive habits in society, presents many well established health risks. While lung cancer is probably the most common and well documented disease associated with tobacco exposure, it is becoming clear from recent research that many other diseases are causally related to smoking. Whether from direct smoking or inhaling environmental tobacco smoke (ETS), termed secondhand smoke, the cells of the respiratory tissues and the lining pulmonary surfactant are the first body tissues to be directly exposed to the many thousands of toxic chemicals in tobacco. Considering the vast surface area of the lung and the extreme attenuation of the blood-air barrier, it is not surprising that this organ is the primary route for exposure, not just to smoke but to most environmental contaminants. Recent research has shown that the pulmonary surfactant, a complex mixture of phospholipids and proteins, is the first site of defense against particulates or gas components of smoke. However, it is not clear what effect smoke has on the surfactant. Most studies have demonstrated that smoking reduces bronchoalveolar lavage phospholipid levels. Some components of smoke also appear to have a direct detergent-like effect on the surfactant while others appear to alter cycling or secretion. Ultimately these effects are reflected in changes in the dynamics of the surfactant system and, clinically in changes in lung mechanics. Similarly, exposure of the developing fetal lung through maternal smoking results in postnatal alterations in lung mechanics and higher incidents of wheezing and coughing. Direct exposure of developing lung to nicotine induces changes suggestive of fetal stress. Furthermore, identification of nicotinic receptors in fetal lung airways and corresponding increases in airway connective tissue support a possible involvement of nicotine in postnatal asthma development. Finally, at the level of the alveoli of the lung, colocalization of nicotinic receptors and surfactant-specific protein in alveolar cells is suggestive of a role in surfactant metabolism. Further research is needed to determine the mechanistic effects of smoke and its components on surfactant function and, importantly, the effects of smoke components on the developing pulmonary system.