Search

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. [ABSTRACT FROM AUTHOR]

Am J Physiol Lung Cell Mol Physiol 298: L117-L125, 2010. First published November 6, 2009; doi: 10.1152/ajplung.00218.2009.--Mechanical ventilation may lead to an impairment of the endogenous surfactant system, which is one of the mechanisms by which this intervention contributes to the progression of acute lung injury. The most extensively studied mechanism of surfactant dysfunction is serum protein inhibition. However, recent studies indicate that hydrophobic components of surfactant may also contribute. It was hypothesized that elevated levels of cholesterol significantly contribute to surfactant dysfunction in ventilation-induced lung injury. Sprague-Dawley rats (n = 30) were randomized to either high-tidal volume or low-tidal volume ventilation and monitored for 2 h. Subsequently, the lungs were lavaged, surfactant was isolated, and the biophysical properties of this isolated surfactant were analyzed on a captive bubble surfactometer with and without the removal of cholesterol using methyl-[beta]-cyclodextrin. The results showed lower oxygenation values in the high-tidal volume group during the last 30 rain of ventilation compared with the low-tidal volume group. Surfactant obtained from the high-tidal volume animals had a significant impairment in function compared with material from the low-tidal volume group. Removal of cholesterol from the high-tidal volume group improved the ability of the surfactant to reduce the surface tension to low values. Subsequent reconstitution of high-cholesterol values led to an impairment in surface activity. It is concluded that increased levels of cholesterol associated with endogenous surfactant represent a major contributor to the inhibition of surfactant function in ventilation-induced lung injury. mechanical ventilation; surface tension

Lung injury due to mechanical ventilation is associated with an impairment of endogenous surfactant. It is unknown whether this impairment is a consequence of or an active contributor to the development and progression of lung injury. To investigate this issue, the present study addressed three questions: Do alterations to surfactant precede physiological lung dysfunction during mechanical ventilation? Which components are responsible for surfactant's biophysical dysfunction? Does exogenous surfactant supplementation offer a physiological benefit in ventilation-induced lung injury? Adult rats were exposed to either a low-stretch [tidal volume ([V.sub.T]) = 8 ml/kg, positive end-expiratory pressure (PEEP) = 5 cm[H.sub.2]0, respiratory rate (RR) = 54-56 breaths/rain (bpm), fractional inspired oxygen ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]) = 1.0] or high-stretch ([V.sub.T] = 30 ml/kg, PEEP = 0 cm[H.sub.2]O, RR = 14-16 bpm, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] = 1.0) ventilation strategy and monitored for either 1 or 2 h. Subsequently, animals were lavaged and the composition and function of surfactant was analyzed. Separate groups of animals received exogenous surfactant after 1 h of high-stretch ventilation and were monitored for an additional 2 h. High stretch induced a significant decrease in blood oxygenation after 2 h of ventilation. Alterations in surfactant pool sizes and activity were observed at 1 h of high-stretch ventilation and progressed over time. The functional impairment of surfactant appeared to be caused by alterations to the hydrophobic components of surfactant. Exogenous surfactant treatment after a period of high-stretch ventilation mitigated subsequent physiological lung dysfunction. Together, these results suggest that alterations of surfactant are a consequence of the ventilation strategy that impair the biophysical activity of this material and thereby contribute directly to lung dysfunction over time. mechanical ventilation; biophysical activity; ventilation-induced lung injury

(Orgeig and Daniels) This surfactant symposium reflects the integrative and multidisciplinary aims of the 1st ICRB, by encompassing in vitro and in vivo research, studies of vertebrates and invertebrates, and research across multiple disciplines. We explore the physical and structural challenges that face gas exchange surfaces in vertebrates and insects, by focusing on the role of the surfactant system. Pulmonary surfactant is a complex mixture of lipids and proteins that lines the air-liquid interface of the lungs of all air-breathing vertebrates, where it functions to vary surface tension with changing lung volume. We begin with a discussion of the extraordinary conservation of the blood-gas barrier among vertebrate respiratory organs, which has evolved to be extremely thin, thereby maximizing gas exchange, but simultaneously strong enough to withstand significant distension forces. The principal components of pulmonary surfactant are highly conserved, with a mixed phospholipid and neutral lipid interfacial film that is established, maintained and dynamically regulated by surfactant proteins (SP). A wide variation in the concentrations of individual components exists, however, and highlights lipidomic as well as proteomic adaptations to different physiological needs. As SP-B deficiency in mammals is lethal, oxidative stress to SP-B is detrimental to the biophysical function of pulmonary surfactant and SP-B is evolutionarily conserved across the vertebrates. It is likely that SP-B was essential for the evolutionary origin of pulmonary surfactant. We discuss three specific issues of the surfactant system to illustrate the diversity of function in animal respiratory structures. (1) Temperature: In vitro analyses of the behavior of different model surfactant films under dynamic conditions of surface tension and temperature suggest that, contrary to previous beliefs, the alveolar film may not have to be substantially enriched in the disaturated phospholipid, dipalmitoylphosphatidylcholine (DPPC), but that similar properties of rate of film formation can be achieved with more fluid films. Using an in vivo model of temperature change, a mammal that enters torpor, we show that film structure and function varies between surfactants isolated from torpid and active animals. (2) Spheres versus tubes: Surfactant is essential for lung stabilization in vertebrates, but its function is not restricted to the spherical alveolus. Instead, surfactant is also important in narrow tubular respiratory structures such as the terminal airways of mammals and the air capillaries of birds. (3). Insect tracheoles: We investigate the structure and function of the insect tracheal system and ask whether pulmonary surfactant also has a role in stabilizing these minute tubules. Our theoretical analysis suggests that a surfactant system may be required, in order to cope with surface tension during processes, such as molting, when the tracheae collapse and fill with water. Hence, despite observations by Wigglesworth in the 1930s of fluid-filled tracheoles, the challenge persists into the 21st century to determine whether this fluid is associated with a pulmonary-type surfactant system. Finally, we summarize the current status of the field and provide ideas for future research.

Exposing bovine lipid extract surfactant (BLES), a clinical surfactant to reactive oxygen species (ROS) alters surfactant protein B (SP-B). ROS treatment of BLES clinical surfactant results in delayed absorption and an impairment of the ability to attain low surface tensions during film compression.

Pulmonary surfactant, a complex mixture of lipids and proteins, lowers the surface tension in terminal air spaces and is crucial for lung function. Within an animal species, surfactant composition can be influenced by development, disease, respiratory rate, and/or body temperature. Here, we analyzed the composition of surfactant in three heterothermic mammals (dunnart, bat, squirrel), displaying different torpor patterns, to determine: 1) whether increases in surfactant cholesterol (Chol) and phospholipid (PL) saturation occur during long-term torpor in squirrels, as in bats and dunnarts; 2) whether surfactant proteins change during torpor; and 3) whether PL molecular species (molsp) composition is altered. In addition, we analyzed the molsp composition of a further nine mammals (including placental/marsupial and hetero-/homeothermic contrasts) to determine whether phylogeny or thermal behavior determines molsp composition in mammals. We discovered that like bats and dunnarts, surfactant Chol increases during torpor in squirrels. However, changes in PL saturation during torpor may not be universal. Torpor was accompanied by a decrease in surfactant protein A in dunnarts and squirrels, but not in bats, whereas surfactant protein B did not change in any species. Phosphatidylcholine (PC)16:0/16:0 is highly variable between mammals and is not the major PL in the wombat, dunnart, shrew, or Tasmanian devil. An inverse relationship exists between PC16:0/16:0 and two of the major fluidizing components, PC16:0/16:1 and PC16:0/14:0. The PL molsp profile of an animal species is not determined by phylogeny or thermal behavior. We conclude that there is no single PL molsp composition that functions optimally in all mammals; rather, surfactant from each animal is unique and tailored to the biology of that animal. lung; temperature; surfactant proteins; electrospray ionization mass spectrometry; cholesterol

C-reactive protein (CRP) and surfactant protein A (SP-A) are phosphatidylcholine (PC) binding proteins that function in the innate host defense system. We examined the effects of CRP and SP-A on the surface activity of bovine lipid extract surfactant (BLES), a clinically applied modified natural surfactant. CRP inhibited BLES adsorption to form a surface-active film and the film's ability to lower surface tension ([gamma]) to low values near 0 mN/m during surface area reduction. The inhibitory effects of CRP were reversed by phosphorylcholine, a water-soluble CRP ligand. SP-A enhanced BLES adsorption and its ability to lower [gamma] to low values. Small amounts of SP-A blocked the inhibitory effects of CRP. Electron microscopy showed CRP has little effect on the lipid structure of BLES. SP-A altered BLES multilamellar vesicular structure by generating large, loose bilayer structures that were separated by a fuzzy amorphous material, likely SP-A. These studies indicate that although SP-A and CRP both bind PC, there is a difference in the manner in which they interact with surface films. acute respiratory distress syndrome; captive bubble tensiometer; electron microscopy; surface tension; surfactant biophysical impairment

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is [Mg.sup.2+] dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is [Mg.sup.2+] independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair. epidermal growth factor receptors; caveolae; lysophospholipids; phospholipase D; surfactant secretion

Acute lung injury and lung transplantation influence in vitro subtype conversion of pulmonary surfactant. Am J Physiol Lung Cell Mol Physiol 282: L67-L74, 2002.--The effects of surfactant treatment on surfactant subtype conversion after lung injury were examined. Dogs were subjected to hyperventilation for 8 h with or without surfactant treatment. Lungs were stored for 17 h, and the right lung was transplanted and reperfused for 6 h. Conversion of large aggregate (LA) surfactant to small aggregates was investigated using in vitro surface area cycling. LA from transplanted lungs (Transplant-LA) from the nontreated group converted more rapidly than Transplant-LA from the treated group. Transplant-LA from both groups converted more rapidly than LA from normal lungs. Calculations based on [[sup.3]H]dipalmitoylphosphatidylcholine in the administered surfactant [bovine lipid extract surfactant (BLES)] showed that the endogenous component of Transplant-LA converted more rapidly than the exogenous component. This indicates exogenous BLES did not equilibrate completely with endogenous surfactant. LA from hyperventilated, stored donor right lungs and from the recipients' native lungs from the nontreated group converted more rapidly than corresponding LA in the BLES-treated group. Similar relative conversions were observed with exogenous components from all lungs. Relative conversion of endogenous component from Transplant-LA was more rapid than that from LA from donor's stored right lung or from the recipient's native right lung. Low levels of phenylmethylsulfonyl fluoride inhibited conversion of Transplant-LA to a greater extent than normal LA. LA from all experimental groups had similar protein levels. These studies show acute lung injury, transplant, ischemia-reperfusion, and surfactant treatment have major effects on surfactant subtype integrity. carboxyesterase; convertase; dipalmitoylphosphatidylcholine; ischemia-reperfusion; surfactant subtype conversion

Nanjundan, Meera, and Fred Possmayer. Molecular cloning and expression of pulmonary lipid phosphate phosphohydrolases. Am J Physiol Lung Cell Mol Physiol 281: L1484-L1493, 2001.--Pulmonary lipid phosphate phosphohydrolase (LPP) was shown previously to hydrolyze phosphatidic acid and lysophosphatidic acid in purified rat lung plasma membranes. To better investigate the nature of pulmonary LPP isoforms and their role in the lung, LPPs were cloned by RT-PCR from both adult rat lung and type II cell RNA. The RT-PCR generated LPP1 (849 bp), up to three LPP1 variants, and LPP3 (936 bp) cDNAs. The three LPP1 variants include LPP1a (852 bp) and two novel isoforms, LPP1b (697 bp) and LPP1c (1004 bp). The pulmonary LPP1 and LPP3 isoforms are essentially identical to the previously cloned rat liver and intestinal LPPs, respectively, and the LPP1a isoform has 80% sequence identity to the human homolog. The LPP2 isoform was not detected in lung by RT-PCR. Northern analyses revealed that the mRNAs for LPP1 and LPP3 increase in fetal rat lung in late gestation to day 1 after birth. These mRNAs decrease somewhat during the neonatal period but increase slightly during postnatal development. Expression of LPP1, LPP1a, and LPP3 cDNAs in HEK 293 cells established that they encode functional LPP. In contrast, the novel isoforms LPP1b and LPP1c contain frameshifts that would result in premature termination, producing putative catalytically inactive polypeptides of 30 and 76 amino acids, respectively. Further investigation of the LPP1b isoform revealed that it was present across a variety of tissues, although at lower levels than LPP1/1a. Transient mammalian expression of LPP1b failed to increase phosphatidate phosphohydrolase activity in HEK 293 cells. lung growth; novel isoforms; rat lung; type II cells; tissue expression; overexpression; surfactant secretion

Rat surfactant protein A (SP-A) trimers in association with phospholipid monolayers have been analyzed to determine their three-dimensional structure. The glycosylation site of SP-A was at the side of each subunit, which would suggest that the covalently linked carbohydrate moiety likely is in the spaces between the adjacent globular domains, which would not sterically interfere with ligand binding.

A series of transmission-electron-microscopic studies were conducted using purified surfactant protein-A and lipid vesicles made in vitro as well as native surfactant from bovine lung to elucidate the interaction of SP-A and various lipids commonly found in tubular myelin (TM). Dipalmitoylphosphatidylcholine-egg phosphatidylcholine bilayers formed corrugations, folds, and predominantly laticelike structures. These results offer evidence that SPs and lipid composition play crucial roles in the formation of the lipid membranes of TM and TM-like structures.

Research was conducted to examine the hypothesis that chronic placental insufficiency resulting in fetal growth restriction (FGR) leads to an increase in fetal lung surfactant-related protein (SP) gene expression. A further objective was to determine if the potential increase in surfactant apoprotein gene expression with FGR was associated with changes in fetal cortisol levels in late gestation. Results indicate that FGR is related to changes in fetal lung SP.

A study was conducted to describe the mechanisms of pulmonary surfactant protein A (SP-A) as an immune suppressive agent. SP-A was found to inhibit human T lymphocyte proliferation in a dose dependent manner and that the effect is not mediated by the carbohydrate recognition domain of SP-A. Results implied a potential role for SP-A in inhibiting lymphocyte responses to exogenous stimuli, as well as support the hypothesis that SP-A contributes to the inhibition of in vivo T cell proliferation and that the effect helps maintain the hyporesponsive state of pulmonary leukocytes.

Palmitoyled or depalmitoyled bovine surfactant-associated protein C (SP-C) were analyzed by utilizing captive bubble surfactometer to determine the effects of palmitoylation on SP-C function and structure. Captive bubble surfactometer analysis indicated the importance of palmitoylation on the full activity of SP-C as a pulmonary surfactant. Furthermore, palmitoylation enhanced SP-C lipid absorption compared to deplamitoyled SP-C.

Respiratory distress syndrome in premature infants (RDS) results in severely impaired breathing and is the leading cause of death in premature infants born in the US. Pulmonary surfactant is a phosphate and lipid compound that lowers the surface tension of the air and liquid emulsion in the lungs. Surfactant has been administered to prevent RDS in high-risk infants or to treat infants with confirmed RDS. The effect of timing of administering surfactant on the outcome of RDS was assessed in 182 infants who were born at fewer than 30 weeks gestation. Subgroups of 60 infants in each were designated and were given surfactant at birth; surfactant at less than six hours of age; or received no surfactant. Surfactant was not given to infants with clear chest X-rays and whose who did not require supplemental oxygen. Twenty-eight of 60 infants assigned to the late surfactant group, or the group receiving treatment at less than six hours of age, did not require surfactant. The remaining 31 infants of the late surfactant group required surfactant at an average of 2.9 hours of age. Infants treated with surfactant had better gas exchange, and lower incidence of pulmonary air leak and severe chronic lung disease compared with untreated infants. The infants receiving early and late surfactant treatment had similar measures of lung function, although the early treatment group had a higher frequency of mild chronic lung disease. In addition, infants receiving late surfactant treatment required shorter duration of intensive medical treatment and oxygen support. These findings show that surfactant treatment of infants born at less than 30 weeks' gestation decreases the requirements for oxygen and supportive breathing interventions during the first week of life, and also decreases the incidence of pulmonary air leak and severe chronic lung disease. Prophylactic treatment with surfactant was associated with a higher incidence of mild chronic lung disease compared with those receiving late treatment. These results indicate that prophylactic surfactant treatment may not be routinely needed among premature infants. (Consumer Summary produced by Reliance Medical Information, Inc.)

Abstract Gastrointestinal symptoms and altered blood phospholipid profiles have been reported in patients with autism spectrum disorders (ASD). Most of the phospholipid analyses have been conducted on the fatty acid composition of isolated phospholipid classes following hydrolysis. A paucity of information exists on how the intact phospholipid molecular species are altered in ASD. We applied ESI/MS to determine how brain and blood intact phospholipid species were altered during the induction of ASD-like behaviors in rats following intraventricular infusions with the enteric bacterial metabolite propionic acid. Animals were infused daily for 8 days, locomotor activity assessed, and animals killed during the induced behaviors. Propionic acid infusions increased locomotor activity. Lipid analysis revealed treatment altered 21 brain and 30 blood phospholipid molecular species. Notable alterations were observed in the composition of brain SM, diacyl mono and polyunsaturated PC, PI, PS, PE, and plasmalogen PC and PE molecular species. These alterations suggest that the propionic acid rat model is a useful tool to study aberrations in lipid metabolism known to affect membrane fluidity, peroxisomal function, gap junction coupling capacity, signaling, and neuroinflammation, all of which may be associated with the pathogenesis of ASD.

Respiratory distress syndrome (RDS) is the severe impairment of respiration in the premature newborn. This disorder is the leading cause of death among prematurely born infants in the United States. Pulmonary surfactant is a phosphate- and lipid-containing substance that controls the surface tension of air-liquid emulsion present in the lungs. Studies have shown that surfactant helps to prevent or treat respiratory distress syndrome by reducing the oxygen and ventilatory (breathing) requirements of newborns. However, most studies have focused on infants with very low birth weights and have explored the effects of a single or double dose of surfactant. It has been demonstrated that, following a single dose of surfactant, lung function deteriorates after initial improvement. Hence, the effects of multiple doses of surfactant were compared with those of a single dose in treating respiratory distress syndrome in 75 newborns delivered at 30 to 36 weeks' gestation. Oxygenation (oxygen saturation of the blood in the lungs) of the newborns improved within 10 minutes of administering either a single or multiple dose of surfactant. However, oxygenation and ventilation deteriorated within 6 to 12 hours after the first dose of surfactant. This deterioration could be reduced by administering multiple doses of surfactant. Although additional doses of surfactant did not improve ventilatory requirements or the duration of assisted ventilation, additional doses of surfactant were beneficial in sustaining improved oxygenation. These results indicate that surfactant is effective in reducing oxygen and ventilatory requirements in premature infants less than six hours of age with respiratory distress syndrome. The most effective dose and plan for additional treatment requires further investigation. (Consumer Summary produced by Reliance Medical Information, Inc.)