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Key Event Title
Inhibition of lung surfactant function
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP||Point of Contact||Author Status||OECD Status|
|Lung surfactant function inhibition leading to decreased lung function||MolecularInitiatingEvent||Brendan Ferreri-Hanberry (send email)||Open for comment. Do not cite||Under Development|
|All life stages||High|
Key Event Description
Airborne substances that penetrate deep into the lungs and reach the alveoli will come into contact with the thin layer of lung surfactant prior to encountering the alveolar epithelial cells. In addition, blood components (such as albumin) that cross the alveolar-capillary membrane and reach the alveolar airspace can interact with the lung surfactant. The nature of this interaction between substances and lung surfactant depends on the origin (intrinsic versus extrinsic) of the substance, its molecular structure, size, and other physicochemical properties such as hydrophobicity, charge, etc. The interaction can be direct, with certain components of the lung surfactant film at the air-liquid interface i.e. by oxidation or cleaving of the phospholipids (Seeds, Grier et al. 2012, Stachowicz-Kusnierz, Cwiklik et al. 2018), or indirect, via competition with the adsorption of lung surfactant. In many cases, the interaction of substances with lung surfactant at the molecular level is responsible for lung surfactant function inhibition.
How It Is Measured or Detected
Measurements of lung surfactant function inhibition
The inhibition of lung surfactant function can be measured in vitro by evaluating the surface activity in dynamic assays that mimic the continuous compression and expansion of the surfactant films at the air-liquid interface in the alveoli during breathing. Values of minimum surface tension, i.e. the lowest value of surface tension reached upon compression of the surfactant film, is a good indicator of the proper functioning of the lung surfactant. Maximum surface tension, i.e. the highest value of surface tension reached upon expansion of the lung surfactant film, reflects the effective re-adsorption of the lung surfactant at the interface. This parameter was shown to be less sensitive than the minimum surface tension to identify inhibitors of lung surfactant function (Valle, Wu et al. 2015, Da Silva, Autilio et al. 2021). These tests can be performed in different setups.
Constrained drop surfactometer
In the constrained drop surfactometer (CDS) a droplet of lung surfactant is deposited on a sharp-edged pedestal, so that a surfactant film is formed at its air-water surface. The adsorbed lung surfactant film is cycled continuously, to mimic breathing (Zuo, Veldhuizen et al. 2008, Valle, Wu et al. 2015, Sørli, Da Silva et al. 2016, Yang, Wu et al. 2018). A camera continuously takes pictures of the droplet before and during exposure to aerosols of the test substance at the air-liquid interface. Alternatively, the lung surfactant and the test substance can be mixed prior to deposition on the pedestal (Sørli, Låg et al. 2020). Surface tension values are obtained by analysis of the drop shape in real-time (Yu, Yang et al. 2016). The main advantages of this method include the accessibility of the air-liquid interface for exposure to airborne substances, flexibility in controlling cycling rates, and ease of determination of the surface tension in real-time while cycling the surfactant film.
Captive bubble surfactometer
In the captive bubble surfactometer (CBS), the lung surfactant film is formed at the air-liquid interface of an air bubble suspended in liquid. The function can be studied by injecting the test substance in the proximity of the surfactant layer at the interface between the air bubble and the surrounding liquid, or by mixing the substance and the surfactant prior to injecting the lung surfactant into the chamber. The captive bubble surfactometer allows study of the rapid initial adsorption of the lung surfactant at the air-liquid interface, post-expansion adsorption, surface activity during dynamic and quasi-static cycles, and stability of the surfactant film to mechanical perturbations (Autilio and Perez-Gil 2019).
Pulsating bubble surfactometer
In the pulsating bubble surfactometer (PBS), an air bubble suspended on a capillary tube is formed in a chamber containing lung surfactant and is periodically compressed and expanded by a piston pulsator (Enhorning 2001, Autilio and Perez-Gil 2019). The method has been used to study the effects of nanoparticles (Schleh, Muhlfeld et al. 2009), bacterial lipopolysaccharides (Kolomaznik, Liskayova et al. 2018), glucocorticoids (Cimato, Facorro et al. 2018), or meconium (Stichtenoth, Jung et al. 2006) on lung surfactant. The pulsating bubble surfactometer was also used to investigate the surface activity of lung surfactant from patients with acute respiratory distress syndrome (Gregory, Longmore et al. 1991, Markart, Ruppert et al. 2007).
In the capillary surfactometer (CS), surfactant is deposited in a capillary tube of uneven diameter that simulate the cylindrical surfaces of the terminal conducting airways a constant airflow is led through the capillary. The percent of time with an open passage is used to assess the functionality of lung surfactant (Enhorning 2001, Larsen, Dallot et al. 2014, Sørli, Da Silva et al. 2016).
Surfactant adsorption test
The surfactant adsorption test is a fluorescence-based method that measures the extent and rate of adsorption of lung surfactant at the air-liquid interface. Lung surfactant is labelled with a fluorescent probe, and injected into the wells of a multi-well plate containing a light-absorbing agent (typically brilliant black). The plates are shaken and the fluorescence (of the lung surfactant sample reaching the surface of the wells) is measured. The fluorescence of the lung surfactant sample in the bulk (not adsorbed at the interface) is quenched by the light-adsorbing agent. This method is high-throughput compared to the biophysical assays described above and it allows to measure the effects of physiologically relevant factors, such as temperature, surfactant concentration, or presence of inhibitors in a high number of samples (Ravasio, Cruz et al. 2008). However, this assay does not measure other biophysical properties like pressure-area isotherms, compressibility etc.
Investigation of the interaction of a substance with lung surfactant
The interaction between a substance (exogenous airborne substances or biological components) and lung surfactant components can be investigated at the molecular level in vitro and estimated in silico. The methods rely on lung surfactant models, ranging from simple monolayers of dipalmitoylphosphatidylcholine (DPPC, the main surface-active component of lung surfactant), to the most complex native surfactant, obtained from broncho-alveolar lavage fluid or minced lung tissue. In most methods, a film of lung surfactant is formed at air-liquid interfaces and exposed to the substance of interest via aerosolisation or deposition. In some cases, the lung surfactant model is mixed directly with the test substance before spreading of the film.
Atomic force microscopy
The topography of surfactant structures formed at respiratory-like air-liquid interfaces upon exposure to test substances can be studied by atomic force microscopy on fixed samples.
Langmuir-Blodgett films are interfacial films of surfactant transferred from the air-liquid interface onto solid supports. They are used to gain information about the distribution of lipids and proteins within the surfactant film and the effect of the interaction with test substances (Cruz and Perez-Gil 2007). Surfactant films deposited at the air-liquid interface of a trough filled with liquid can be compressed by reducing the surface area of the trough. A sensor plate measures the variation in surface pressure over compression to yield surface pressure – area isotherms. It should be noted that in addition to the traditional Langmuir trough, the Langmuir-Blodgett technique has been adapted in the constrained drop surfactometer to study adsorbed surfactant films (Xu, Yang et al. 2020). The comparison of such isotherms in the presence or absence of the test substance gives insights in the interaction of a substance with lung surfactant at the molecular level. Shifts in the surface pressure-area isotherms are identified most easily using simple models such as DPPC monolayers, but can also be seen using the more complex lung surfactant. Structural changes can be identified during compression of the film when combined with epifluorescence or atomic force microscopy.
Cryogenic transmission electron microscopy
In aqueous dispersions, lung surfactant forms vesicles. Cryogenic transmission electron microscopy allows visualizing morphological and structural changes at the single membrane vesicle level. After incubation with the test substance, the surfactant model is applied onto a carbon grid and vitrified in liquid ethane cooled by liquid nitrogen. Changes in the size, circularity or lamerallity of the vesicles indicate disruption of the three-dimensional surfactant structures.
Differential scanning calorimetry
Differential scanning calorimetry allows the study of phase transitions occurring in lipid membranes (such as lung surfactant) over changes in temperature (Demetzos 2008). It can be used to characterize the thermotropic phase behaviours of phospholipids in the surfactant models in the absence or presence of interacting substances. Associated enthalpy, transition temperature, and cooperativity can be estimated from the thermograms. It is a very sensitive method when working with simple models such as pure DPPC bilayers. The method is much less sensitive when using complex lung surfactant models. This is because several transitions overlap in membranes made of complex mixtures, each occurring at different temperature so it is difficult to identify one specific variation.
Domain of Applicability
The applicability domain is restricted to the groups of organisms where the structure and the functioning of the pulmonary system, including the lung surfactant, are conserved and relevant. Lung surfactant is a vital component of the lungs found in all major vertebrate groups, but particularly, to sustain the delicate structure of the mammalian lung. The lung surfactant system has a single point of origin and was a prerequisite for the evolution of air breathing (Sullivan, Daniels et al. 1998). While the composition and function of lung surfactant are conserved in vertebrates, changes in composition among non-vertebrates are noted and likely reflect differences in the structure of the respiratory units (Veldhuizen, Nag et al. 1998). Decreased lung function has been observed after exposure to airborne toxicants in humans of all sexes and ages, and in common experimental animal species, such as mice, rats, and rabbits.
Al-Saiedy, M., L. Gunasekara, F. Green, R. Pratt, A. Chiu, A. Yang, J. Dennis, C. Pieron, C. Bjornson, B. Winston and M. Amrein (2018). "Surfactant Dysfunction in ARDS and Bronchiolitis is Repaired with Cyclodextrins." Mil Med 183(suppl_1): 207-215.
Autilio, C., M. Echaide, A. Cruz, C. Mouton, A. Hidalgo, E. Da Silva, D. De Luca, B. S. Jorid and J. Perez-Gil (2021). "Molecular and biophysical mechanisms behind the enhancement of lung surfactant function during controlled therapeutic hypothermia." Sci Rep 11(1): 728.
Autilio, C. and J. Perez-Gil (2019). "Understanding the principle biophysics concepts of pulmonary surfactant in health and disease." Arch Dis Child Fetal Neonatal Ed 104(4): F443-F451.
Bakshi, M. S., L. Zhao, R. Smith, F. Possmayer and N. O. Petersen (2008). "Metal nanoparticle pollutants interfere with pulmonary surfactant function in vitro." Biophys J 94(3): 855-868.
Cimato, A., G. Facorro and M. Martinez Sarrasague (2018). "Developing an exogenous pulmonary surfactant-glucocorticoids association: Effect of corticoid concentration on the biophysical properties of the surfactant." Respir Physiol Neurobiol 247: 80-86.
Cruz, A. and J. Perez-Gil (2007). "Langmuir films to determine lateral surface pressure on lipid segregation." Methods Mol Biol 400: 439-457.
Da Silva, E., C. Autilio, K. S. Hougaard, A. Baun, A. Cruz, J. Perez-Gil and J. B. Sørli (2021). "Molecular and biophysical basis for the disruption of lung surfactant function by chemicals." Biochim Biophys Acta Biomembr 1863(1): 183499.
Da Silva, E., C. Hickey, G. Ellis, K. S. Hougaard and J. B. Sørli (2021). "In vitro prediction of clinical signs of respiratory toxicity in rats following inhalation exposure." Under review.
Demetzos, C. (2008). "Differential Scanning Calorimetry (DSC): a tool to study the thermal behavior of lipid bilayers and liposomal stability." J Liposome Res 18(3): 159-173.
Enhorning, G. (2001). "Pulmonary surfactant function studied with the pulsating bubble surfactometer (PBS) and the capillary surfactometer (CS)." Comp Biochem Physiol A Mol Integr Physiol 129(1): 221-226.
Fan, Q., Y. E. Wang, X. Zhao, J. S. Loo and Y. Y. Zuo (2011). "Adverse biophysical effects of hydroxyapatite nanoparticles on natural pulmonary surfactant." ACS Nano 5(8): 6410-6416.
Fang, Q., Q. Zhao, X. Chai, Y. Li and S. Tian (2020). "Interaction of industrial smelting soot particles with pulmonary surfactant: Pulmonary toxicity of heavy metal-rich particles." Chemosphere 246: 125702.
Gasser, M., B. Rothen-Rutishauser, H. F. Krug, P. Gehr, M. Nelle, B. Yan and P. Wick (2010). "The adsorption of biomolecules to multi-walled carbon nanotubes is influenced by both pulmonary surfactant lipids and surface chemistry." Journal of Nanobiotechnology 8: 31.
Gomez-Gil, L., D. Schurch, E. Goormaghtigh and J. Perez-Gil (2009). "Pulmonary surfactant protein SP-C counteracts the deleterious effects of cholesterol on the activity of surfactant films under physiologically relevant compression-expansion dynamics." Biophys J 97(10): 2736-2745.
Gregory, T. J., W. J. Longmore, M. A. Moxley, J. A. Whitsett, C. R. Reed, A. A. Fowler, 3rd, L. D. Hudson, R. J. Maunder, C. Crim and T. M. Hyers (1991). "Surfactant chemical composition and biophysical activity in acute respiratory distress syndrome." J Clin Invest 88(6): 1976-1981.
Gross, T., E. Zmora, Y. Levi-Kalisman, O. Regev and A. Berman (2006). "Lung-surfactant-meconium interaction: in vitro study in bulk and at the air-solution interface." Langmuir 22(7): 3243-3250.
Gunasekara, L., S. Schurch, W. M. Schoel, K. Nag, Z. Leonenko, M. Haufs and M. Amrein (2005). "Pulmonary surfactant function is abolished by an elevated proportion of cholesterol." Biochim Biophys Acta 1737(1): 27-35.
Hidalgo, A., F. Salomone, N. Fresno, G. Orellana, A. Cruz and J. Perez-Gil (2017). "Efficient Interfacially Driven Vehiculization of Corticosteroids by Pulmonary Surfactant." Langmuir 33(32): 7929-7939.
Hobi, N., G. Siber, V. Bouzas, A. Ravasio, J. Perez-Gil and T. Haller (2014). "Physiological variables affecting surface film formation by native lamellar body-like pulmonary surfactant particles." Biochim Biophys Acta 1838(7): 1842-1850.
Hu, G., B. Jiao, X. Shi, R. P. Valle, Q. Fan and Y. Y. Zuo (2013). "Physicochemical properties of nanoparticles regulate translocation across pulmonary surfactant monolayer and formation of lipoprotein corona." ACS Nano 7(12): 10525-10533.
Hu, Q., X. Bai, G. Hu and Y. Y. Zuo (2017). "Unveiling the Molecular Structure of Pulmonary Surfactant Corona on Nanoparticles." ACS Nano 11(7): 6832-6842.
Jagalski, V., R. Barker, D. Topgaard, T. Gunther-Pomorski, B. Hamberger and M. Cardenas (2016). "Biophysical study of resin acid effects on phospholipid membrane structure and properties." Biochim Biophys Acta 1858(11): 2827-2838.
Kapralov, A. A., W. H. Feng, A. A. Amoscato, N. Yanamala, K. Balasubramanian, D. E. Winnica, E. R. Kisin, G. P. Kotchey, P. P. Gou, L. J. Sparvero, P. Ray, R. K. Mallampalli, J. Klein-Seetharaman, B. Fadeel, A. Star, A. A. Shvedova and V. E. Kagan (2012). "Adsorption of Surfactant Lipids by Single-Walled Carbon Nanotubes in Mouse Lung upon Pharyngeal Aspiration." Acs Nano 6(5): 4147-4156.
Kolomaznik, M., G. Liskayova, N. Kanjakova, L. Hubcik, D. Uhrikova and A. Calkovska (2018). "The Perturbation of Pulmonary Surfactant by Bacterial Lipopolysaccharide and Its Reversal by Polymyxin B: Function and Structure." Int J Mol Sci 19(7).
Larsen, S. T., E. Da Silva, J. S. Hansen, A. C. O. Jensen, I. K. Koponen and J. B. Sørli (2020). "Acute Inhalation Toxicity After Inhalation of ZnO Nanoparticles: Lung Surfactant Function Inhibition In Vitro Correlates With Reduced Tidal Volume in Mice." Int J Toxicol 39(4): 321-327.
Larsen, S. T., C. Dallot, S. W. Larsen, F. Rose, S. S. Poulsen, A. W. Nørgaard, J. S. Hansen, J. B. Sørli, G. D. Nielsen and C. Foged (2014). "Mechanism of action of lung damage caused by a nanofilm spray product." Toxicol Sci 140(2): 436-444.
Lopez-Rodriguez, E., A. Cruz, R. P. Richter, H. W. Taeusch and J. Perez-Gil (2013). "Transient exposure of pulmonary surfactant to hyaluronan promotes structural and compositional transformations into a highly active state." J Biol Chem 288(41): 29872-29881.
Lopez-Rodriguez, E., M. Echaide, A. Cruz, H. W. Taeusch and J. Perez-Gil (2011). "Meconium impairs pulmonary surfactant by a combined action of cholesterol and bile acids." Biophys J 100(3): 646-655.
Lopez-Rodriguez, E., O. L. Ospina, M. Echaide, H. W. Taeusch and J. Perez-Gil (2012). "Exposure to polymers reverses inhibition of pulmonary surfactant by serum, meconium, or cholesterol in the captive bubble surfactometer." Biophys J 103(7): 1451-1459.
Lugones, Y., O. Blanco, E. Lopez-Rodriguez, M. Echaide, A. Cruz and J. Perez-Gil (2018). "Inhibition and counterinhibition of Surfacen, a clinical lung surfactant of natural origin." PLoS One 13(9): e0204050.
Markart, P., C. Ruppert, M. Wygrecka, T. Colaris, B. Dahal, D. Walmrath, H. Harbach, J. Wilhelm, W. Seeger, R. Schmidt and A. Guenther (2007). "Patients with ARDS show improvement but not normalisation of alveolar surface activity with surfactant treatment: putative role of neutral lipids." Thorax 62(7): 588-594.
Przybyla, R. J., J. Wright, R. Parthiban, S. Nazemidashtarjandi, S. Kaya and A. M. Farnoud (2017). "Electronic cigarette vapor alters the lateral structure but not tensiometric properties of calf lung surfactant." Respir Res 18(1): 193.
Ravasio, A., A. Cruz, J. Perez-Gil and T. Haller (2008). "High-throughput evaluation of pulmonary surfactant adsorption and surface film formation." J Lipid Res 49(11): 2479-2488.
Roldan, N., J. Perez-Gil, M. R. Morrow and B. Garcia-Alvarez (2017). "Divide & Conquer: Surfactant Protein SP-C and Cholesterol Modulate Phase Segregation in Lung Surfactant." Biophys J 113(4): 847-859.
Sachan, A. K., R. K. Harishchandra, C. Bantz, M. Maskos, R. Reichelt and H. J. Galla (2012). "High-resolution investigation of nanoparticle interaction with a model pulmonary surfactant monolayer." ACS Nano 6(2): 1677-1687.
Schleh, C., C. Muhlfeld, K. Pulskamp, A. Schmiedl, M. Nassimi, H. D. Lauenstein, A. Braun, N. Krug, V. J. Erpenbeck and J. M. Hohlfeld (2009). "The effect of titanium dioxide nanoparticles on pulmonary surfactant function and ultrastructure." Respir Res 10: 90.
Stenger, P. C., C. Alonso, J. A. Zasadzinski, A. J. Waring, C. L. Jung and K. E. Pinkerton (2009). "Environmental tobacco smoke effects on lung surfactant film organization." Biochim Biophys Acta 1788(2): 358-370.
Stichtenoth, G., P. Jung, G. Walter, J. Johansson, B. Robertson, T. Curstedt and E. Herting (2006). "Polymyxin B/pulmonary surfactant mixtures have increased resistance to inactivation by meconium and reduce growth of gram-negative bacteria in vitro." Pediatr Res 59(3): 407-411.
Sullivan, L. C., C. B. Daniels, I. D. Phillips, S. Orgeig and J. A. Whitsett (1998). "Conservation of surfactant protein A: evidence for a single origin for vertebrate pulmonary surfactant." J Mol Evol 46(2): 131-138.
Sørli, J. B., K. Balogh Sivars, E. Da Silva, K. S. Hougaard, I. K. Koponen, Y. Y. Zuo, I. E. K. Weydahl, P. M. Åberg and R. Fransson (2018). "Bile salt enhancers for inhalation: Correlation between in vitro and in vivo lung effects." Int J Pharm 550(1-2): 114-122.
Sørli, J. B., E. Da Silva, P. Backman, M. Levin, B. L. Thomsen, I. K. Koponen and S. T. Larsen (2016). "A Proposed In Vitro Method to Assess Effects of Inhaled Particles on Lung Surfactant Function." Am J Respir Cell Mol Biol 54(3): 306-311.
Sørli, J. B., Y. Huang, E. Da Silva, J. S. Hansen, Y. Y. Zuo, M. Frederiksen, A. W. Nørgaard, N. E. Ebbehøj, S. T. Larsen and K. S. Hougaard (2018). "Prediction of acute inhalation toxicity using in vitro lung surfactant inhibition." ALTEX 35(1): 26-36.
Sørli, J. B., M. Låg, L. Ekeren, J. Perez-Gil, L. S. Haug, E. Da Silva, M. N. Matrod, K. B. Gutzkow and B. Lindeman (2020). "Per- and polyfluoroalkyl substances (PFASs) modify lung surfactant function and pro-inflammatory responses in human bronchial epithelial cells." Toxicol In Vitro 62: 104656.
Taeusch, H. W., J. Bernardino de la Serna, J. Perez-Gil, C. Alonso and J. A. Zasadzinski (2005). "Inactivation of pulmonary surfactant due to serum-inhibited adsorption and reversal by hydrophilic polymers: experimental." Biophys J 89(3): 1769-1779.
Tatur, S. and A. Badia (2012). "Influence of hydrophobic alkylated gold nanoparticles on the phase behavior of monolayers of DPPC and clinical lung surfactant." Langmuir 28(1): 628-639.
Valle, R. P., T. Wu and Y. Y. Zuo (2015). "Biophysical influence of airborne carbon nanomaterials on natural pulmonary surfactant." ACS Nano 9(5): 5413-5421.
Veldhuizen, R., K. Nag, S. Orgeig and F. Possmayer (1998). "The role of lipids in pulmonary surfactant." Biochim Biophys Acta 1408(2-3): 90-108.
Wang, F., J. Liu and H. Zeng (2020). "Interactions of particulate matter and pulmonary surfactant: Implications for human health." Adv Colloid Interface Sci 284: 102244.
Wang, Y. E., H. Zhang, Q. Fan, C. R. Neal and Y. Y. Zuo (2012). "Biophysical interaction between corticosteroids and natural surfactant preparation: implications for pulmonary drug delivery using surfactant a a carrier." Soft Matter 8(2): 504-511.
Xu, L., Y. Yang and Y. Y. Zuo (2020). "Atomic Force Microscopy Imaging of Adsorbed Pulmonary Surfactant Films." Biophys J 119(4): 756-766.
Xu, Y., Z. Luo, S. Li, W. Li, X. Zhang, Y. Y. Zuo, F. Huang and T. Yue (2017). "Perturbation of the pulmonary surfactant monolayer by single-walled carbon nanotubes: a molecular dynamics study." Nanoscale 9(29): 10193-10204.
Yang, Y., Y. K. Wu, Q. Z. Ren, L. G. Zhang, S. J. Liu and Y. Y. Zuo (2018). "Biophysical Assessment of Pulmonary Surfactant Predicts the Lung Toxicity of Nanomaterials." Small Methods 2(4).
Yang, Y., L. Xu, S. Dekkers, L. G. Zhang, F. R. Cassee and Y. Y. Zuo (2018). "Aggregation State of Metal-Based Nanomaterials at the Pulmonary Surfactant Film Determines Biophysical Inhibition." Environ Sci Technol 52(15): 8920-8929.
Yu, K., J. Yang and Y. Y. Zuo (2016). "Automated Droplet Manipulation Using Closed-Loop Axisymmetric Drop Shape Analysis." Langmuir 32(19): 4820-4826.
Yuan, Y., X. Liu, T. Liu, W. Liu, Y. Zhu, H. Zhang and C. Zhao (2020). "Molecular dynamics exploring of atmosphere components interacting with lung surfactant phospholipid bilayers." Sci Total Environ 743: 140547.
Zhang, H., Y. E. Wang, C. R. Neal and Y. Y. Zuo (2012). "Differential effects of cholesterol and budesonide on biophysical properties of clinical surfactant." Pediatr Res 71(4 Pt 1): 316-323.
Zhao, Q., Y. Li, X. Chai, L. Xu, L. Zhang, P. Ning, J. Huang and S. Tian (2019). "Interaction of inhalable volatile organic compounds and pulmonary surfactant: Potential hazards of VOCs exposure to lung." J Hazard Mater 369: 512-520.
Zuo, Y. Y., R. A. Veldhuizen, A. W. Neumann, N. O. Petersen and F. Possmayer (2008). "Current perspectives in pulmonary surfactant--inhibition, enhancement and evaluation." Biochim Biophys Acta 1778(10): 1947-1977.