Surfactant--a review for pediatric surgeons

Biomarker tests for fetal lung maturity

The production of surfactant is a key step in fetal lung development. Surfactant decreases alveolar surface tension, thereby preventing alveolar collapse and allowing efficient gas exchange. The lack of adequate amounts of lung surfactant results in respiratory distress syndrome. Tests that assess surfactant concentrations in amniotic fluid are good predictors of infants that will not develop respiratory distress syndrome. The most frequently used test to assess fetal lung maturity (TDx FLM II) will not be available after December 2011. Therefore, we review the currently available tests for fetal lung maturity including lecithin:sphingomyelin ratio, phosphatidyl glycerol, surfactant:albumin ratio and lamellar body counts. Herein, we discuss their clinical utility and consider a suitable replacement for the future.

Metal nanoparticle pollutants interfere with pulmonary surfactant function in vitro

Reported associations between air pollution and pulmonary and cardiovascular diseases prompted studies on the effects of gold nanoparticles (Au NP) on pulmonary surfactant function. Low levels (3.7 mol % Au/lipid, 0.98% wt/wt) markedly inhibited adsorption of a semisynthetic pulmonary surfactant (dipalmitoyl-phosphatidylcholine (DPPC)/palmitoyl-oleoyl-phosphatidylglycerol/surfactant protein B (SP-B); 70:30:1 wt %). Au NP also impeded the surfactant's ability to reduce surface tension (gamma) to low levels during film compression and to respread during film expansion. Transmission electron microscopy showed that Au NP generated by a seed-growth method were spherical with diameters of approximately 15 nm. Including palmitoyl-oleoyl-phosphatidylglycerol appeared to coat the NP with at least one lipid bilayer but did not affect NP shape or size. Similar overall observations occurred with dimyristoyl phosphatidylglycerol. Dipalmitoyl-phosphatidylglycerol was less effective in NP capping, although similar sized NP were formed. Including SP-B (1% wt/wt) appears to induce the formation of elongated strands of interacting threads with the fluid phosphatidylglycerols (PG). Including DPPC resulted in formation of aggregated, less spherical NP with a larger size distribution. With DPPC, strand formation due to SP-B was not observed. Agarose gel electrophoresis studies demonstrated that the aggregation induced by SP-B blocked migration of PG-coated NP. Migration was also influenced by the fluidity of the PGs. It is concluded that Au NP can interact with and sequester pulmonary surfactant phospholipids and, if inhaled from the atmosphere, could impede pulmonary surfactant function in the lung.

Fetal Tracheal Occlusion for the Treatment of Congenital Diaphragmatic Hernia

BACKGROUND

Congenital diaphragmatic hernia (CDH) continues to be associated with significant mortality and morbidity rates despite advances in neonatal care. Fetal intervention for CDH has been studied for 25 years. After initial difficulties encountered with open fetal repair, attention has turned to tracheal occlusion (TO) as a method to correct pulmonary hypoplasia before birth. This article reviews our contribution to this field of research and outlines the current status of this treatment modality.

MATERIALS AND METHODS

Using the fetal lamb model, we have studied the effects of fetal TO on tracheal fluid pressure, lung growth and type II pneumocyte maturation, and surfactant production. We developed a minimally invasive and reversible technique of TO, using a detachable balloon placed using single-port tracheoscopy. We examined differential lung growth, structural maturation, pulmonary artery remodeling, and lung function during an 8-h resuscitation period in lambs, comparing normal controls, lambs with a surgically created CDH, those with CDH+TO, and those with CDH+TO and release of TO 1 week before delivery. We also studied the potential benefits of maternal betamethasone administration and the administration of surfactant at birth. Using a neonatal piglet model, we examined the effect of postnatal pulmonary distension with perfluorocarbon on lung growth. More recently, we turned to the rat nitrofen-induced CDH model to study the effects of TO on bronchial branching and some molecular markers of lung growth (Shh and LGL1).

CONCLUSIONS

Fetal TO is being used to treat human CDH, but its application remains limited by the absence of reliable and widely reproducible prenatal prognostic criteria. A better understanding of the molecular events guiding the lung growth seen with TO may help to refine its use in humans.

Modification of Tryptophan and Methionine Residues Is Implicated in the Oxidative Inactivation of Surfactant Protein B

Exposing BLES (bovine lipid extract surfactant), a clinical surfactant, to reactive oxygen species (ROS) alters surfactant protein B (SP-B), as indicated by Coomassie Blue staining, silver staining, and Western analysis. Hypochlorous acid (HOCl) treatment leads to elevated maximum surface tension (gammamax) and a deterioration in minimum gamma (gammamin) during surface area cycling. Fenton reaction resulted in immediate increases in gammamin and gammamax. Intrinsic fluorescence measurements indicated Fenton, but not HOCl, induced conversion of Trp9 of SP-B to hydroxyTrp (OHTrp), N-formylkynurenine (NFKyn), and kynurenine (Kyn). Electrospray ionization mass spectrometry (ESI-MS) revealed molecular weight alterations consistent with oxidation of Met (HOCl, Fenton) and Trp (Fenton) residues. Oxidative alterations to Met29 and Met65 (HOCl, Fenton) and to Trp9 (OHTrp with HOCL and NFKyn plus Kyn with Fenton) were confirmed by matrix-assisted laser desorption mass spectrometry (MALDI-MS) studies on SP-B tryptic fragments. Some Met oxidation was observed with control SP-B. When taken together with captive bubble tensiometer measurements, these studies suggest that Met oxidation of SP-B by HOCl or Fenton interferes with phospholipid respreading during compression-expansion of surfactant films, while Fenton oxidation, which produces more extensive Met oxidation and disruption of the indole ring of Trp9, further abrogated the ability of such films to attain low surface tensions during compression. These studies provide insight into the manner by which ROS generated during acute lung injury and the acute respiratory distress syndrome act to inhibit not only endogenous surfactant but also therapeutic surfactants administered to counteract these conditions.

Temporal expression of Notch in preterm rat lungs exposed to hyperoxia

To explore the mechanism of Notch in hyperoxia-induced preterm rat lung injury, 2-days-old preterm SD rats were randomized into control and hyperoxia group (FiO2 > or = 0.85). On day 1, 7, 14 and 21, 8 rat pups of each time point were used to assess histopathological changes of lung with HE staining and to evaluate the expression of Notch1 and Notch3 with immunohistochemistry. Notch1, Notch3, Aquaprin5 (AQP5) and surfactant protein C (SP-C) mRNA were measured by reverse transcription polymerase chain reaction (RT-PCR). The results showed that the lung injury in the hyperoxia group was characterized by retarded lung alveolization and differentiation of alveolar epithelial type II cells (AEC II). Positive staining of Notch1 in hyperoxia group was weaker than controls at every time point (except for day 7), while positive staining of Notch3 was much stronger (P < 0.05, P < 0.01). Notch1, Notch3 mRNA level showed similar change as protein level. AQP5, SP-C mRNA decreased significantly as compared with that of the controls (P < 0.01). We are led to conclude that hyperoxia results in abnormal expression of Notch, which is likely to contribute to the pathogenesis of lung injury through regulating proliferation and transdifferentiation of alveolar epithelial cells.

Lung-surfactant-meconium interaction: in vitro study in bulk and at the air-solution interface

Lung surfactants (LSs) form a monolayer at the lung's alveoli air-solution interface and play a crucial role in making normal breathing possible by reducing the surface tension. LS are affected by various agents that hamper their normal functioning. Tobacco smoke [Bringezu, F.; Pinkerton, K. E.; Zasadzinski, J. A. Langmuir 2003, 19, 2900-2907] and meconium, the first excrement of the newborn, are examples for such LS poison. In neonates, intrauterine aspiration of meconium is a known cause for morbidity and mortality. We studied in vitro the interactions between modified porcine LSs (Curosurf), used as LS replacement, and meconium, as well as between their artificial analogues, phospholipids mixture, and taurocholic acid (TA), respectively. The interactions were examined both in the bulk solution and at the air-water interface, representing the pre- and postnatal situations. It was found that the artificial analogues represent the natural system reliably and exhibit similar effects. TA, a principle component of bile, is an amphiphilic sterol compound in which the hydrophilic and hydrophobic moieties are presented at different faces of the sterol plane. Here we found that TA affects the structure of both monolayers at the interface and surfactant aggregates in solution. A likely poisoning mechanism is by stereoselective penetration of TA into the lamellar or monolayer structures, thus disrupting the contiguous structure of the intact monolayer or the bilayer vesicle structure.

Fetal lung maturity

Respiratory distress syndrome of the newborn infant caused by immaturity of the fetal lung continues to be a clinical problem. Measurement of pulmonary surfactant production is the most effective way to evaluate pulmonary maturity. Since the first fetal lung maturity test was described more than two decades ago, advances in methodology have produced diagnostically sensitive tests that are both rapid and precise. Unfortunately, currently available tests continue to demonstrate low diagnostic specificity and remain poor predictors of fetal lung immaturity. We review the background, methodology, pre-analytical and analytical concerns, and clinical performance of various fetal lung maturity assays, and discuss the appropriate use and interpretation of these tests.

Fetal tracheal occlusion in lambs with congenital diaphragmatic hernia: role of exogenous surfactant at birth

Fetal tracheal occlusion (TO) has been used to reverse the lung hypoplasia associated with congenital diaphragmatic hernia (CDH). However, TO has a detrimental effect on type II pneumocyte function and surfactant production. Previously, we have shown that in surgically created CDH lambs, TO improved markedly the response to resuscitation even though the lungs remain surfactant deficient. The goal of this investigation was to assess the effects of exogenous surfactant administered at birth to CDH lambs with or without fetal TO during 8 h of resuscitation. Lambs were divided into five groups: CDH, CDH+surfactant (SURF), CDH+TO, CDH+TO+SURF, and nonoperated controls. A left-sided CDH was created in fetal lambs at 80 d gestation. TO was performed at 108 d, and the lambs were delivered by hysterotomy at 136 d. Bovine lipid extract surfactant was administered before the first breath and again at 4 h of life. All CDH+SURF lambs, but only three of five CDH lambs, survived up to 8 h. When compared with the corresponding nonsurfactant-treated group, surfactant-treated CDH and CDH+TO lambs did not demonstrate improved alveolar-arterial oxygen gradients, pH, or Pco(2). In fact, in the CDH+TO group, surfactant treatment significantly worsened ventilation efficiency as measured by the ventilation efficiency index. The observed improvement in pulmonary compliance secondary to surfactant treatment was not significant. This investigation demonstrates that prophylactic surfactant treatment at birth does not improve gas exchange or ventilation efficiency in CDH lambs with or without TO.

Postnatal pulmonary distension for the treatment of pulmonary hypoplasia: pilot study in the neonatal piglet model

BACKGROUND

Accelerated lung growth has previously been demonstrated after fetal tracheal occlusion. The purpose of this study was to determine if short-term perfluorocarbon (PFC) distension could increase lung growth postnatally in neonatal piglets.

METHODS

Eleven piglets aged 5 to 8 days were divided into 3 groups: (a) controls (n = 4), (b) PFC x 6 hours (n = 3), and (c) PFC x 12 hours (n = 4). A right posterolateral thoracotomy was performed and a pressure-monitoring catheter was placed in the posterior segment of the right upper lobe. Perfluorocarbon was infused and a mean intrabronchial pressure of 12 mm Hg (range, 5-21 mm Hg) was maintained. The control piglets also had a thoracotomy with right upper lobe bronchus dissection without ligation or PFC distension. All piglets were injected with [3H]-thymidine 3 hours before killing. Both right and left posterior segments of each upper lobe were analyzed for their respective amount of total DNA by fluorometry. Rates of DNA synthesis for each segment were determined by precipitating incorporated [3H]-thymidine with 5% trichloroacetic acid and reporting this value by the total amount of DNA. The differential lung DNA synthesis rate was calculated as (right posterior segment/left posterior segment) x 100. Statistical analysis consisted of 1-way analysis of variance and Student's t tests with significance at P < or = .05.

RESULTS

Heart rate, mean arterial pressure, temperature, oxygen saturation, pH, PCO2 , and PO2 were similar in all 3 groups. Lung DNA synthesis was nearly doubled in the PFC x 6 hours group compared with controls (302% vs 165%, P = .05). Animals in the PFC x 12 hours group experienced a 261% increase (P = NS).

CONCLUSION

Short-term PFC distension in neonatal piglets resulted in increased DNA synthesis within 6 hours presumably because of stretch-induced mechanisms.

Simple, Helical Peptoid Analogs of Lung Surfactant Protein B

The helical, amphipathic surfactant protein, SP-B, is a critical element of pulmonary surfactant and hence is an important therapeutic molecule. However, it is difficult to isolate from natural sources in high purity. We have created and studied three different, nonnatural analogs of a bioactive SP-B fragment (SP-B(1-25)), using oligo-N-substituted glycines (peptoids) with simple, repetitive sequences designed to favor the formation of amphiphilic helices. For comparison, a peptide with a similar repetitive sequence previously shown to be a good SP mimic was also studied, along with SP-B(1-25) itself. Surface pressure-area isotherms, surfactant film phase morphology, and dynamic adsorption behavior all indicate that the peptoids are promising mimics of SP-B(1-25). The extent of biomimicry appears to correlate with peptoid helicity and lipophilicity. These biostable oligomers could serve in a synthetic surfactant replacement to treat respiratory distress syndrome.

Meconium is a potent activator of complement in human serum and in piglets

Meconium aspiration syndrome (MAS) is a clinical condition in the newborn infant with a significant morbidity and mortality. The complex pathophysiology of MAS, leading to both pulmonary and systemic complications, is characterized by an incompletely understood inflammatory reaction. Treatment is symptomatic, mainly limited to airway cleaning and ventilatory support. In this study, we show for the first time that meconium is a potent activator of complement, a key mediator of inflammation. In vitro, meconium activated the alternative complement pathway in human umbilical cord serum as judged by a substantial increase in the alternative pathway convertase C3bBbP. The activation proceeded through C3 (C3bc) and the terminal C5-9 pathway (terminal SC5b-9 complement complex), whereas the classical and lectin pathways were not activated (C1rs-C1-inhibitor complexes and C4bc). The lipid fraction, containing, e.g. free fatty acids, and the water fraction, containing, e.g. bile acids, contributed equally to the complement activation. A blocking antibody to factor D (alternative pathway) completely inhibited the meconium-induced complement activation, whereas blocking antibodies to mannose-binding lectin (lectin pathway) and C2 (classical and lectin pathway) had no effect. In vivo, meconium induced systemic complement activation in a piglet model of MAS, paralleling the increase in lung dysfunction. In conclusion, meconium is a potent activator of the complement system both in vitro and in vivo. Complement may be important in the pathogenesis of MAS, and specific complement inhibition might be a possible treatment approach in MAS.

Lipid compositional analysis of pulmonary surfactant monolayers and monolayer-associated reservoirs

Pulmonary surfactant is a lipid:protein complex containing dipalmitoyl-phosphatidylcholine (DPPC) as the major component. Recent studies indicate adsorbed surfactant films consist of a surface monolayer and a monolayer-associated reservoir. It has been hypothesized that the monolayer and its functionally contiguous reservoir may be enriched in DPPC relative to bulk phase surfactant. We investigated the compositional relationship between the monolayer and its reservoir using paper-supported wet bridges to transfer films from adsorbing dishes to clean surfaces on spreading dishes. Spreading films appear to form monolayers in the spreading dishes. We employed bovine lipid extract surfactant [BLES(chol)] containing [3H]DPPC and either [14C]palmitoyl, oleoyl-phosphatidylcholine (POPC), [14C]dipalmitoyl-phosphatidylglycerol (DPPG), [14C]palmitoyl, oleoyl-phosphatidylglycerol (POPG), or [14C]cholesterol. Radiolabeled phosphatidylglycerols were prepared using phospholipase D. The studies demonstrated that the [3H]DPPC-[14C] POPC ratios were the same in the prepared BLES dispersions as in Langmuir-Blodgett films, indicating a lack of DPPC selectivity during film formation. Furthermore, identical 3H-14C isotopic ratios were observed with DPPC and either 14C-labeled POPC, DPPG, POPG, or cholesterol in the original dispersions, the bulk phases in adsorption dish D1, and monolayers recovered from spreading dish D2. These relationships remained unperturbed with 2-fold increases in bulk concentrations in D1 and 10-fold variations in D1-D2 surface area. These results indicate adsorbed surfactant monolayers and their associated reservoirs possess similar lipid compositions and argue against selective adsorption of DPPC.