Pulmonary surfactant and its components inhibit secretion of phosphatidylcholine from cultured rat alveolar type II cells

Sigmar1 ablation leads to lung pathological changes associated with pulmonary fibrosis, inflammation, and altered surfactant proteins levels

Sigma1 receptor protein (Sigmar1) is a small, multifunctional molecular chaperone protein ubiquitously expressed in almost all body tissues. This protein has previously shown its cardioprotective roles in rodent models of cardiac hypertrophy, heart failure, and ischemia-reperfusion injury. Extensive literature also suggested its protective functions in several central nervous system disorders. Sigmar1's molecular functions in the pulmonary system remained unknown. Therefore, we aimed to determine the expression of Sigmar1 in the lungs. We also examined whether Sigmar1 ablation results in histological, ultrastructural, and biochemical changes associated with lung pathology over aging in mice. In the current study, we first confirmed the presence of Sigmar1 protein in human and mouse lungs using immunohistochemistry and immunostaining. We used the Sigmar1 global knockout mouse (Sigmar1-/-) to determine the pathophysiological role of Sigmar1 in lungs over aging. The histological staining of lung sections showed altered alveolar structures, higher immune cells infiltration, and upregulation of inflammatory markers (such as pNFκB) in Sigmar1-/- mice compared to wildtype (Wt) littermate control mice (Wt). This indicates higher pulmonary inflammation resulting from Sigmar1 deficiency in mice, which was associated with increased pulmonary fibrosis. The protein levels of some fibrotic markers, fibronectin, and pSMAD2 Ser 245/250/255 and Ser 465/467, were also elevated in mice lungs in the absence of Sigmar1 compared to Wt. The ultrastructural analysis of lungs in Wt mice showed numerous multilamellar bodies of different sizes with densely packed lipid lamellae and mitochondria with a dark matrix and dense cristae. In contrast, the Sigmar1-/- mice lung tissues showed altered multilamellar body structures in alveolar epithelial type-II pneumocytes with partial loss of lipid lamellae structures in the lamellar bodies. This was further associated with higher protein levels of all four surfactant proteins, SFTP-A, SFTP-B, SFTP-C, and SFTP-D, in the Sigmar1-/- mice lungs. This is the first study showing Sigmar1's expression pattern in human and mouse lungs and its association with lung pathophysiology. Our findings suggest that Sigmar1 deficiency leads to increased pulmonary inflammation, higher pulmonary fibrosis, alterations of the multilamellar body stuructures, and elevated levels of lung surfactant proteins.

Platform Effects on Regeneration by Pulmonary Basal Cells as Evaluated by Single-Cell RNA Sequencing

Cell-based therapies have shown promise for treating myriad chronic pulmonary diseases through direct application of epithelial progenitors or by way of engineered tissue grafts or whole organs. To elucidate environmental effects on epithelial regenerative outcomes in vitro, here, we isolate and culture a population of pharmacologically expanded basal cells (peBCs) from rat tracheas. At peak basal marker expression, we simultaneously split peBCs into four in vitro platforms: organoid, air-liquid interface (ALI), engineered trachea, and engineered lung. Following differentiation, these samples are evaluated using single-cell RNA sequencing (scRNA-seq) and computational pipelines are developed to compare samples both globally and at the population level. A sample of native rat tracheal epithelium is also evaluated by scRNA-seq as a control for engineered epithelium. Overall, this work identifies platform-specific effects that support the use of engineered models to achieve the most physiologic differential outcomes in pulmonary epithelial regenerative applications.

Surfactant protein A and D polymorphisms and methylprednisolone pharmacogenetics in donor lungs

OBJECTIVE

Surfactant proteins A and D are important molecules involved in lung allograft innate immunity. Genetic polymorphisms of surfactant proteins A and D are associated with various lung diseases. In this study, surfactant protein A and D expression responses were investigated during pharmacogenetics upon methylprednisolone treatment as observed during lung transplantation.

METHODS

A human cell line (NCI-H441) and precision-cut lung slices from 16 human donors were incubated with methylprednisolone, and surfactant protein A1, surfactant protein A2, and surfactant protein D messenger RNA and surfactant protein A protein expression were assayed. Surfactant protein A1, A2, and D polymorphisms and surfactant protein A gene and protein expressions were determined.

RESULTS

In NCI-H441 cells, methylprednisolone treatment at 10^-5 M and 10^-6 M reduced surfactant protein A1 and surfactant protein A2 messenger RNA and surfactant protein A protein expression (P < .05). A pharmacogenetic relationship was observed in human donor precision-cut lung slices between the surfactant protein A2 (1A^x) variants: Surfactant protein A1, A2, and D messenger RNA expression were greater for 1A⁰ versus 1A¹ (P < .05); surfactant protein A1/surfactant protein A2 genotype 6A²6A²/1A⁰1A⁰ (n = 5) showed greater surfactant protein A1, A2, and D messenger RNA expression and surfactant protein A protein expression compared with the other surfactant protein A1/surfactant protein A2 genotypes (n = 11) (P < .05).

CONCLUSIONS

The surfactant protein A genotype and methylprednisolone stimuli influence donor lung surfactant protein A and D expression. Lungs carrying the surfactant protein A2 variant 1A⁰ have a greater expression of surfactant protein A when treated with methylprednisolone. Surfactant protein A polymorphisms could be used to personalize immunosuppressive regimens.

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.

Surfactant protein-A (SP-A) selectively inhibits prostaglandin F2alpha (PGF2alpha) production in term decidua: implications for the onset of labor

CONTEXT

Labor is characterized by "decidual activation" with production of inflammatory mediators. Recent data suggest that surfactant protein-A (SP-A) may be critical to the onset of labor in mice. Whether this is also true in humans is unclear.

OBJECTIVES

The aim was to investigate: 1) the expression of SP-A at the maternal-fetal interface; 2) the effect of SP-A on the production of inflammatory mediators by human decidua; and 3) the association between single nucleotide polymorphisms in maternal SP-A genes and spontaneous preterm birth.

RESEARCH DESIGN AND METHODS

In situ expression of SP-A was investigated by immunohistochemistry and quantitative RT-PCR. Term decidual stromal cells were isolated, purified, and treated with/without SP-A (1-100 μg/ml), IL-1β, and/or thrombin. Levels of inflammatory mediators [IL-6, IL-8, TNFα, matrix metalloproteinase-3, monocyte chemotactic protein-1, IL-1β, PGE(2), prostaglandin F(2α) (PGF(2α))] and angiogenic factors (soluble fms-like tyrosine kinase-1, vascular endothelial growth factor) were measured in conditioned supernatant by ELISA and corrected for protein content. The effect of SP-A on eicosanoid gene expression was measured by quantitative RT-PCR.

RESULTS

SP-A localized to endometrium/decidua. High-dose SP-A (100 μg/ml) inhibited PGF(2α) by term decidual stromal cells without affecting the production of other inflammatory mediators, and this effect occurred at a posttranscriptional level. Decidual SP-A expression decreased significantly with labor. Single nucleotide polymorphisms in the SP-A genes do not appear to be associated with preterm birth.

CONCLUSIONS

SP-A is produced by human endometrium/decidua, where it significantly and selectively inhibits PGF(2α) production. Its expression decreases with labor. These novel observations suggest that decidual SP-A likely plays a critical role in regulating prostaglandin production within the uterus, culminating at term in decidual activation and the onset of labor.

Hydrolysis of phosphatidylcholine by cerium(IV) releases significant amounts of choline and inorganic phosphate at lysosomal pH

Niemann-Pick disease and drug-induced phospholipidosis are examples of lysosomal storage disorders in which serious respiratory infections are brought on by high levels of the phospholipid phosphatidylcholine in the acidic lamellar bodies and lysosomes of pulmonary cells. One approach to developing an effective therapeutic agent could involve the use of a metal to preferentially hydrolyze phospholipid phosphate ester bonds at mildly acidic, lysosomal pH values (~pH 4.8). Towards this end, here we have investigated phosphatidylcholine hydrolysis by twelve metal ion salts at 60°C. Using a malachite green/molybdate-based colorimetric assay to detect inorganic phosphate released upon metal-assisted phosphate ester bond hydrolysis, Ce(IV) was shown to possess outstanding reactivity in comparison to the eleven other metals. We then utilized cerium(IV) to hydrolyze phosphatidylcholine at normal, core body temperature (37°C). The malachite green/molybdate assay was used to quantitate free phosphate and an Amplex® Red-based colorimetric assay and matrix-assisted laser desorption ionization time-of-flight mass spectrometry were employed to detect choline. Ce(IV) hydrolyzed phosphatidylcholine more efficiently at lysosomal pH: i.e., at a Triton X-100:phosphatidylcholine molar mixing ratio of 1.57, yields of choline and phosphate were 51±4% and 40±4% at ~pH 4.8, compared to 28±4% and 27±5% at ~pH 7.2.

Transient in utero disruption of cystic fibrosis transmembrane conductance regulator causes phenotypic changes in alveolar type II cells in adult rats

Background

Mechanicosensory mechanisms regulate cell differentiation during lung organogenesis. We have previously demonstrated that cystic fibrosis transmembrane conductance regulator (CFTR) was integral to stretch-induced growth and development and that transient expression of antisense-CFTR (ASCFTR) had negative effects on lung structure and function. In this study, we examined adult alveolar type II (ATII) cell phenotype after transient knock down of CFTR by adenovirus-directed in utero expression of ASCFTR in the fetal lung.

Results

In comparison to (reporter gene-treated) Controls, ASCFTR-treated adult rat lungs showed elevated phosphatidylcholine (PC) levels in the large but not in the small aggregates of alveolar surfactant. The lung mRNA levels for SP-A and SP-B were lower in the ASCFTR rats. The basal PC secretion in ATII cells was similar in the two groups. However, compared to Control ATII cells, the cells in ASCFTR group showed higher PC secretion with ATP or phorbol myristate acetate. The cell PC pool was also larger in the ASCFTR group. Thus, the increased surfactant secretion in ATII cells could cause higher PC levels in large aggregates of surfactant. In freshly isolated ATII cells, the expression of surfactant proteins was unchanged, suggesting that the lungs of ASCFTR rats contained fewer ATII cells. Gene array analysis of RNA of freshly isolated ATII cells from these lungs showed altered expression of several genes including elevated expression of two calcium-related genes, Ca²⁺-ATPase and calcium-calmodulin kinase kinase1 (CaMkk1), which was confirmed by real-time PCR. Western blot analysis showed increased expression of calmodulin kinase I, which is activated following phosphorylation by CaMkk1. Although increased expression of calcium regulating genes would argue in favor of Ca²⁺-dependent mechanisms increasing surfactant secretion, we cannot exclude contribution of alternate mechanisms because of other phenotypic changes in ATII cells of the ASCFTR group.

Conclusion

Developmental changes due to transient disruption of CFTR in fetal lung reflect in altered ATII cell phenotype in the adult life.

The concentration of surfactant protein-A in amniotic fluid decreases in spontaneous human parturition at term

OBJECTIVE

The fetus is thought to play a central role in the onset of labor. Pulmonary surfactant protein (SP)-A, secreted by the maturing fetal lung, has been implicated in the mechanisms initiating parturition in mice. The present study was conducted to determine whether amniotic fluid concentrations of SP-A and SP-B change during human parturition.

STUDY DESIGN

Amniotic fluid SP-A and SP-B concentrations were measured with a sensitive and specific ELISA in the following groups of pregnant women: (1) mid-trimester of pregnancy, between 15 and 18 weeks of gestation (n = 29), (2) term pregnancy not in labor (n = 28), and (3) term pregnancy in spontaneous labor (n = 26). Non-parametric statistics were used for analysis.

RESULTS

SP-A was detected in all amniotic fluid samples. SP-B was detected in 24.1% (7/29) of mid-trimester samples and in all samples at term. The median amniotic fluid concentrations of SP-A and SP-B were significantly higher in women at term than in women in the mid-trimester (SP-A term no labor: median 5.6 microg/mL, range 2.2-15.2 microg/mL vs. mid-trimester: median 1.64 microg/mL, range 0.1-4.7 microg/mL, and SP-B term no labor: median 0.54 microg/mL, range 0.17-1.99 microg/mL vs. mid-trimester: median 0 microg/mL, range 0-0.35 microg/mL; both p < 0.001). The median amniotic fluid SP-A concentration in women at term in labor was significantly lower than that in women at term not in labor (term in labor: median 2.7 microg/mL, range 1.2-10.1 microg/mL vs. term no labor: median 5.6 microg/mL, range 2.2-15.2 microg/mL; p < 0.001). There was no significant difference in the median amniotic fluid SP-B concentrations between women in labor and those not in labor (term in labor: median 0.47 microg/mL, range 0.04-1.32 microg/mL vs. term no labor: median 0.54 microg/mL, range 0.17-1.99 microg/mL; p = 0.2).

CONCLUSION

The amniotic fluid concentration of SP-A decreases in spontaneous human parturition at term.

Isolated rat alveolar type II cells protrude intracellular lamellar bodies by forming bubble-like structures during surfactant secretion

Pulmonary surfactant is synthesized and secreted by pulmonary alveolar type II epithelial cells (type II cells). It passes through the alveolar lining fluid and adsorbs to the air-liquid interface. The process from secretion to adsorption is not yet entirely understood. To acquire a detailed understanding of this process, we used multiple observations of type II cells isolated from rat lungs under electron microscopy (EM) and confocal laser scanning microscopy (CLSM). Transmission EM observation demonstrated a loosening process of the intracellular lamellar bodies from the inside to the outside of the cell. Scanning EM observation revealed bubble-like protrusions from the cell surface, and differential interference contrast microscopy illustrated the protrusions expanding with time. CLSM observation with FM 1-43, a fluorescent membrane probe, revealed that the bubble-like protrusions were composed of phospholipids. Thus, we have demonstrated that isolated rat type II cells protrude intracellular lamellar bodies by forming bubble-like structures, possibly enabling them to adsorb to the air-liquid interface directly. These observations suggest a new mechanism for surfactant secretion from type II cells.

Surfactant protein-A plays an important role in lung surfactant clearance: evidence using the surfactant protein-A gene-targeted mouse

Previous studies with the isolated perfused rat lung showed that both clathrin- and actin-mediated pathways are responsible for endocytosis of dipalmitoylphosphatidylcholine (DPPC)-labeled liposomes by granular pneumocytes in the intact lung. Using surfactant protein-A (SP-A) gene-targeted mice, we examined the uptake of [(3)H]DPPC liposomes by isolated mouse lungs under basal and secretagogue-stimulated conditions. Unilamellar liposomes composed of [(3)H]DPPC: phosphatidylcholine:cholesterol:egg phosphatidylglycerol (10:5:3:2 mol fraction) were instilled into the trachea of anesthetized mice, and the lungs were perfused (2 h). Uptake was calculated as percentage of instilled disintegrations per minute in the postlavaged lung. Amantadine, an inhibitor of clathrin and, thus, receptor-mediated endocytosis via clathrin-coated pits, decreased basal [(3)H]DPPC uptake by 70% in SP-A +/+ but only by 20% in SP-A -/- lung, data compatible with an SP-A/receptor-regulated lipid clearance pathway in the SP-A +/+ mice. The nonclathrin, actin-dependent process was low in the SP-A +/+ lung but accounted for 55% of liposome endocytosis in the SP-A -/- mouse. With secretagogue (8-bromoadenosine 3',5'-cyclic monophosphate) treatment, both clathrin- and actin-dependent lipid clearance were elevated in the SP-A +/+ lungs while neither pathway responded in the SP-A -/- lungs. Binding of iodinated SP-A to type II cells isolated from both genotypes of mice was similar indicating a normal SP-A receptor status in the SP-A -/- lung. Inclusion of SP-A with instilled liposomes served to "rescue" the SP-A -/- lungs by reestablishing secretagogue-dependent enhancement of liposome uptake. These data are compatible with a major role for receptor-mediated endocytosis of DPPC by granular pneumocytes, a process critically dependent on SP-A.

Regulation of surfactant secretion in alveolar type II cells

Molecular mechanisms of surfactant delivery to the air/liquid interface in the lung, which is crucial to lower the surface tension, have been studied for more than two decades. Lung surfactant is synthesized in the alveolar type II cells. Its delivery to the cell surface is preceded by surfactant component synthesis, packaging into specialized organelles termed lamellar bodies, delivery to the apical plasma membrane and fusion. Secreted surfactant undergoes reuptake, intracellular processing, and finally resecretion of recycled material. This review focuses on the mechanisms of delivery of surfactant components to and their secretion from lamellar bodies. Lamellar bodies-independent secretion is also considered. Signal transduction pathways involved in regulation of these processes are discussed as well as disorders associated with their malfunction.

Effect of SP-B peptides on the uptake of liposomes by alveolar cells

BACKGROUND

Exogenous surfactant has been accepted worldwide as a therapy of RDS in premature and term infants. Exogenous surfactant is usually derived from lung extracts containing phospholipids and the surfactant proteins SP-B and SP-C. Synthetic peptides of SP-B and SP-C are being tested with the aim to develop a completely synthetic surfactant preparation. Nevertheless, the effects of these peptides on the endogenous surfactant metabolism remain unknown.

OBJECTIVES

The effect of synthetic SP-B peptides on uptake of surfactant-like liposomes was investigated in alveolar cells. Native SP-B and seven SP-B peptides were included: monomeric and dimeric SP-B(1-25) (Cys-11 --> Ala-11), SP-B(63-78)and Ala-SP-B(63-78) (Cys-71 --> Ala-71;Cys-77 --> Ala-77)and their serine mutants.

METHODS

In vitro, alveolar macrophages (AM) and alveolar type II cells (ATII) were incubated with liposomes containing SP-B or one of its peptides. In vivo, rats received intratracheally various SP-B peptides (SP-B/lipid ratio 1:33 w/w) incorporated in fluorescent surfactant-like liposomes. One hour after instillation, AM and ATII were isolated and cell-associated fluorescence was determined using flow cytometry. Confocal laser microscopy was performed to ensure internalization of the liposomes.

RESULTS

In vitro uptake by AM or ATII was not influenced by the SP-B peptides. In vivo, SP-B(1-25) and Ser-SP-B(1-25) increased the uptake by AM whereas dSP-B(1-25) decreased the uptake. Neither SP-B(1-25) nor dSP-B(1-25 )affected total uptake by ATII. The overall uptake by SP-B(63-78) variants was not changed.

CONCLUSIONS

Surface-active synthetic SP-B peptides do not interfere with the normal uptake of surfactant by ATII.

Surfactant Protein A Directly Interacts with TLR4 and MD-2 and Regulates Inflammatory Cellular Response

The purpose of the current study was to examine the binding of pulmonary surfactant protein A (SP-A) to TLR4 and MD-2, which are critical signaling receptors for lipopolysaccharides (LPSs). The direct binding of SP-A to the recombinant soluble form of extracellular TLR4 domain (sTLR4) and MD-2 was detected using solid-phase binding, immunoprecipitation, and BIAcore. SP-A bound to sTLR4 and MD-2 in a Ca2+-dependent manner, and an anti-SP-A monoclonal antibody whose epitope lies in the region Thr184-Gly194 blocked the SP-A binding to sTLR4 and MD-2, indicating the involvement of the carbohydrate recognition domain (CRD) in the binding. SP-A avidly bound to the deglycosylated forms of sTLR4 and MD-2, suggesting a protein/protein interaction. In addition, SP-A attenuated cell surface binding of smooth LPS and smooth LPS-induced NF-kappaB activation in TLR4/MD-2-expressing cells. To know the role of oligomerization in the interaction of SP-A with TLR4 and MD-2, the collagenase-resistant fragment (CRF), which consisted of CRD plus neck domain of SP-A, was isolated. CRF assembled as a trimer, whereas SP-A assembled as a higher order oligomer. Although CRD was suggested to be involved in the binding, CRF exhibited approximately 600- and 155-fold higher KD for the binding to TLR4 and MD-2, respectively, when compared with SP-A. Consistently significantly higher molar concentrations of CRF were required to inhibit smooth LPS-induced NF-kappaB activation and tumor necrosis factor-alpha secretion. These results demonstrate for the first time the direct interaction between SP-A and TLR4/MD-2 and suggest the importance of supratrimeric oligomerization in the immunomodulatory function of SP-A.

A fluorescent microplate assay for exocytosis in alveolar type II cells

The authors describe a simple, reliable, and quantitative assay to monitor exocytotic fusion of lamellar bodies (LBs) in adherent rat alveolar type II (AT II) cells. The assay is based on fluorescence measurements of LB-plasma membrane (PM) fusions modified for the use in multiwell culture plates to obtain a high-sample throughput. In particular, it is based on the presence of a highly light-absorbing dye in the cell supernatants to increase the specificity of fluorescence signals and to yield pseudo-confocal information from the cells. When the assay was tested with agonist-(ATP) and phorbolester-induced stimulation of LB-PM fusions, the authors found a good correlation with direct microscopic investigations based on single cell recordings. To further validate the assay, they used Curosurf at 10 mg/ml. However, it influenced neither the basal nor the ATP-stimulated rate of LB-PM fusions. This was corroborated by the fact that Curosurf had no effect on resting Ca (2+) levels nor the ATP induced Ca (2+) signals. The results cast new light on previous findings that surfactant phospholipids decrease the rate of secretion in AT II cells in a dose-dependent way. The authors conclude that the inhibitory effect exerted by phospholipids might be due to action on a later step in exocytosis, probably associated with exocytotic fusion pore expansion and content release out of fused vesicles.

Current concepts on the pulmonary surfactant in infants

Surfactant has been a main topic of neonatology in the last 20 years. Many studies have been conducted since the discovery of its role in the pathogenesis of respiratory distress syndrome and the knowledge on its composition and metabolism has become complex. In this article we review the current concepts of its metabolism, ways of acting, properties of its proteins and activities other than the ability of reducing surface tension within the lung as a basis to understand the development of disease in case of its deficiency.

The genetics of neonatal respiratory disease

This chapter reviews some of the genetic predispositions that may govern the presence or severity of neonatal respiratory disorders. Respiratory disease is common in the neonatal period, and genetic factors have been implicated in some rare and common respiratory diseases. Among the most common respiratory diseases are respiratory distress syndrome of the newborn and transient tachypnoea of the newborn, whereas less common ones are cystic fibrosis, congenital alveolar proteinosis and primary ciliary dyskinesias. A common complication of neonatal respiratory distress syndrome is bronchopulmonary dysplasia or neonatal chronic lung disease. This review examines the evidence linking known genetic contributions to these diseases. The value and success of neonatal screening for cystic fibrosis is reviewed, and the recently characterised contribution of polymorphisms and mutations in the surfactant protein genes to neonatal respiratory disease is evaluated. The evidence that known variability in the expression of surfactant protein genes may contribute to the risk of development of neonatal chronic lung disease or bronchopulmonary dysplasia is examined.

Exocytosis of lung surfactant: from the secretory vesicle to the air-liquid interface

Exocytosis is fundamental in biology and requires an orchestra of proteins and other constituents to fuse a vesicle with the plasma membrane. Although the molecular fusion machinery appears to be well conserved in evolution, the process itself varies considerably with regard to the diversity of physico-chemical and structural factors that govern the delay between stimulus and fusion, the expansion of the fusion pore, the release of vesicle content, and, finally, its extracellular dispersion. Exocytosis of surfactant is unique in many of these aspects. This review deals with the secretory pathway of pulmonary surfactant from the type II cell to the air-liquid interface, with focus on the distinct mechanisms and regulation of lamellar body (LB) fusion and release. We also discuss the fate of secreted material until it is rearranged into units that finally function to reduce the surface tension in the lung.

Surfactant protein A and B genetic variants in respiratory distress syndrome in singletons and twins

Interactive genetic and environmental factors may influence the differentiation of surfactant and the risk of respiratory distress syndrome (RDS). DNA samples from 441 premature singleton infants and 480 twin or multiple infants were genotyped for surfactant-specific protein (SP)-A1, SP-A2, and SP-B exon 4 polymorphisms and intron 4 size variants in a homogeneous white population. Distributions of the SP-A and SP-B gene variants between RDS and no-RDS infants were determined alone and in combination. SP-A1 allele 6A2 (p = 0.009) and the homozygous genotype 6A2/6A2 (p = 0.003) were overrepresented in RDS of singletons when the SP-B exon 4 genotype was Thr/Thr, and underrepresented in RDS of multiples when the SP-B genotype was Ile/Thr (p = 0.012 for 6A2 and p = 0.03 for 6A2/6A2) or Thr/Thr (p = 0.12 for 6A2 and p = 0.018 for 6A2/6A2, respectively). The SP-A 6A2 allele in the SP-B Thr131 background predisposed the smallest singleton infants to RDS, whereas near-term multiples were protected from RDS. There was a continuous association between fetal mass and risk of RDS, defined by the SP-A and SP-B variants. Labeled lung explants with the Thr/Thr genotype showed proSP-B amino-terminal glycosylation, which was absent in Ile/Ile samples. Genetic and environmental variation may influence intracellular processing of surfactant complex and the susceptibility to RDS.

Effect of surfactant protein A on granular pneumocyte surfactant secretion in vitro

Surfactant secretion by lung type II cells occurs when lamellar bodies (LBs) fuse with the plasma membrane and surfactant is released into the alveolar lumen. Surfactant protein A (SP-A) blocks secretagogue-stimulated phospholipid (PL) release, even in the presence of surfactant-like lipid. The mechanism of action is not clear. We have shown previously that an antibody to LB membranes (MAb 3C9) can be used to measure LB membrane trafficking. Although the ATP-stimulated secretion of PL was blocked by SP-A, the cell association of iodinated MAb 3C9 was not altered, indicating no effect on LB movement. FM1-43 is a hydrophobic dye used to monitor the formation of fusion pores. After secretagogue exposure, the threefold enhancement of the number of FM1-43 fluorescent LBs (per 100 cells) was not altered by the presence of SP-A. Finally, there was no evidence of a large PL pool retained on the cell surface through interaction with SP-A. Thus SP-A exposure does not affect these stages in the surfactant secretory pathway of type II cells.

Pulmonary surfactant proteins A and D directly suppress CD3+/CD4+ cell function: evidence for two shared mechanisms

Pulmonary surfactant is a lipoprotein complex that lowers surface tension at the air-liquid interface of the lung and participates in pulmonary host defense. Surfactant proteins (SP), SP-A and SP-D, modulate a variety of immune cell functions, including the production of cytokines and free radicals. Previous studies showed that SP-A and SP-D inhibit lymphocyte proliferation in the presence of accessory cells. The goal of this study was to determine whether SP-A and SP-D directly suppress Th cell function. Both proteins inhibited CD3(+)/CD4(+) lymphocyte proliferation induced by PMA and ionomycin in an IL-2-independent manner. Both proteins decreased the number of cells entering the S and mitotic phases of the cell cycle. Neither SP-A nor SP-D altered cell viability, apoptosis, or secretion of IL-2, IL-4, or IFN-gamma when Th cells were treated with PMA and ionomycin. However, both proteins attenuated ionomycin-induced cytosolic free calcium ([Ca(2+) ](i)), but not thapsigargin-induced changes in [Ca(2+)](i). In summary, inhibition of T cell proliferation by SP-A and SP-D occurs via two mechanisms, an IL-2-dependent mechanism observed with accessory cell-dependent T cell mitogens and specific Ag, as well as an IL-2-independent mechanism of suppression that potentially involves attenuation of [Ca(2+)](i).

Surfactant protein A regulates surfactant phospholipid clearance after LPS-induced injury in vivo

Previous in vitro studies have suggested that surfactant protein A (SP-A) may play a role in pulmonary surfactant homeostasis by mediating surfactant secretion and clearance. However, mice made deficient in SP-A [SP-A (-/-) animals] have relatively normal levels of surfactant compared with wild-type SP-A (+/+) animals. We hypothesize that SP-A may play a role in surfactant homeostasis after acute lung injury. Bacterial lipopolysaccharide was instilled into the lungs of SP-A (-/-) mice and SP-A (+/+) mice to induce injury. Surfactant phospholipid levels were increased 1.6-fold in injured SP-A (-/-) animals, although injury did not alter [3H]choline or [14C]palmitate incorporation into dipalmitoylphosphatidylcholine (DPPC), suggesting no change in surfactant synthesis/secretion 12 h after injury. Clearance of [3H]DPPC from the lungs of injured SP-A (-/-) animals was decreased by approximately 40%. Instillation of 50 microg of exogenous SP-A rescued both the clearance defect and the increased phospholipid defect in injured SP-A (-/-) animals, suggesting that SP-A may play a role in regulating clearance of surfactant phospholipids after acute lung injury.

The collagen-like region of surfactant protein A (SP-A) is required for correction of surfactant structural and functional defects in the SP-A null mouse

Pulmonary surfactant isolated from gene-targeted surfactant protein A null mice (SP-A(-/-)) is deficient in the surfactant aggregate tubular myelin and has surface tension-lowering activity that is easily inhibited by serum proteins in vitro. To further elucidate the role of SP-A and its collagen-like region in surfactant function, we used the human SP-C promoter to drive expression of rat SP-A (rSPA) or SP-A containing a deletion of the collagen-like domain (DeltaG8-P80) in the Clara cells and alveolar type II cells of SP-A(-/-) mice. The level of the SP-A in the alveolar wash of the SP-A(-/-,rSP-A) and SP-A(-/-,DeltaG8-P80) mice was 6.1-and 1.3-fold higher, respectively, than in the wild type controls. Tissue levels of saturated phosphatidylcholine were slightly reduced in the SP-A(-/-,rSP-A) mice compared with SP-A(-/-) littermates. Tubular myelin was present in the large surfactant aggregates isolated from the SP-A(-/-,rSP-A) lines but not in the SP-A(-/-,DeltaG8-P80) mice or SP-A(-/-) controls. The equilibrium and minimum surface tensions of surfactant from the SP-A(-/-,rSP-A) mice were similar to SP-A(-/-) controls, but both were markedly elevated in the SP-A(-/-,DeltaG8-P80) mice. There was no defect in the surface tension-lowering activity of surfactant from SP-A(+/+,DeltaG8-P80) mice, indicating that the inhibitory effect of DeltaG8-P80 on surface activity can be overcome by wild type levels of mouse SP-A. The surface activity of surfactant isolated from the SP-A(-/-,rSP-A) but not the SP-A(-/-,DeltaG8-P80) mice was more resistant than SP-A(-/-) littermate control animals to inhibition by serum proteins in vitro. Pressure volume relationships of lungs from the SP-A(-/-), SP-A(-/-,rSP-A), and SP-A(-/-,DeltaG8-P80) lines were very similar. These data indicate that expression of SP-A in the pulmonary epithelium of SP-A(-/-) animals restores tubular myelin formation and resistance of isolated surfactant to protein inhibition by a mechanism that is dependent on the collagen-like region.

Surfactant proteins A and D in Eustachian tube epithelium

Surfactant protein (SP) A and SP-D are collectins that have roles in host defense. The Eustachian tube (ET) maintains the patency between the upper airways and the middle ear. Dysfunction of local mucosal immunity in ET may predispose infants to recurrent otitis media. We recently described preliminary evidence of the expression of SP-A and SP-D in the ET. Our present aim was to establish the sites of SP-A and SP-D expression within the epithelium of the ET in vivo. With in situ hybridization, electron microscopy, and immunoelectron microscopy, the cells responsible for SP-A and SP-D expression and storage were identified. SP-A expression was localized within the ET epithelium, and the protein was found in the electron-dense granules of microvillar epithelial cells. Being concentrated in the epithelial lining, only a few cells revealed intracellular SP-D, and it was not associated with granules. The SP-A and SP-D immunoreactivities in ET lavage fluid, as shown by Western blot analyses, were similar to those in bronchoalveolar lavage fluid. We propose that there are specialized cells in the ET epithelium expressing and secreting SP-A and SP-D. SP-A and SP-D may be important for antibody-independent protection of the middle ear against infections.

Surfactant-associated proteins: functions and structural variation

Pulmonary surfactant is a barrier material of the lungs and has a dual role: firstly, as a true surfactant, lowering the surface tension; and secondly, participating in innate immune defence of the lung and possibly other mucosal surfaces. Surfactant is composed of approximately 90% lipids and 10% proteins. There are four surfactant-specific proteins, designated surfactant protein A (SP-A), SP-B, SP-C and SP-D. Although the sequences and post-translational modifications of SP-B and SP-C are quite conserved between mammalian species, variations exist. The hydrophilic surfactant proteins SP-A and SP-D are members of a family of collagenous carbohydrate binding proteins, known as collectins, consisting of oligomers of trimeric subunits. In view of the different roles of surfactant proteins, studies determining the structure-function relationships of surfactant proteins across the animal kingdom will be very interesting. Such studies may reveal structural elements of the proteins required for surface film dynamics as well as those required for innate immune defence. Since SP-A and SP-D are also present in extrapulmonary tissues, the hydrophobic surfactant proteins SP-B and SP-C may be the most appropriate indicators for the evolutionary origin of surfactant. SP-B is essential for air-breathing in mammals and is therefore largely conserved. Yet, because of its unique structure and its localization in the lung but not in extrapulmonary tissues, SP-C may be the most important indicator for the evolutionary origin of surfactant.

The pulmonary collectins and surfactant metabolism

Lung surfactant covers and stabilizes a large, delicate surface at the interface between the host and the environment. The surfactant system is placed at risk by a number of environmental challenges such as inflammation, infection, or oxidant stress, and perhaps not surprisingly, it demonstrates adaptive changes in metabolism in response to alterations in the alveolar microenvironment. Recent experiments have shown that certain components of the surfactant system are active participants in the regulation of the alveolar response to a wide variety of environmental challenges. These components are capable not only of maintaining a low interfacial surface tension but also of amplifying or dampening inflammatory responses. These observations suggest that regulatory molecules are capable of both sensing the environment of the alveolus and providing feedback to the cells regulating surfactant synthesis, secretion, alveolar conversion, and clearance. In this review we examine the evidence from in vitro systems and gene-targeted mice that two surfactant-associated collectins (SP-A and SP-D) may serve in these roles and help modify surfactant homeostasis as part of a coordinated host response to environmental challenges.

Alveolar epithelial type II cell: defender of the alveolus revisited

In 1977, Mason and Williams developed the concept of the alveolar epithelial type II (AE2) cell as a defender of the alveolus. It is well known that AE2 cells synthesise, secrete, and recycle all components of the surfactant that regulates alveolar surface tension in mammalian lungs. AE2 cells influence extracellular surfactant transformation by regulating, for example, pH and [Ca2+] of the hypophase. AE2 cells play various roles in alveolar fluid balance, coagulation/fibrinolysis, and host defence. AE2 cells proliferate, differentiate into AE1 cells, and remove apoptotic AE2 cells by phagocytosis, thus contributing to epithelial repair. AE2 cells may act as immunoregulatory cells. AE2 cells interact with resident and mobile cells, either directly by membrane contact or indirectly via cytokines/growth factors and their receptors, thus representing an integrative unit within the alveolus. Although most data support the concept, the controversy about the character of hyperplastic AE2 cells, reported to synthesise profibrotic factors, proscribes drawing a definite conclusion today.

Treatment with exogenous surfactant stimulates endogenous surfactant synthesis in premature infants with respiratory distress syndrome

OBJECTIVE

Treatment of preterm infants with respiratory distress syndrome (RDS) with exogenous surfactant has greatly improved clinical outcome. Some infants require multiple doses, and it has not been studied whether these large amounts of exogenous surfactant disturb endogenous surfactant metabolism in humans. We studied endogenous surfactant metabolism in relation to different amounts of exogenous surfactant, administered as rescue therapy for RDS.

DESIGN

Prospective clinical study.

SETTING

Neonatal intensive care unit in a university hospital.

PATIENTS

A total of 27 preterm infants intubated and mechanically ventilated for respiratory insufficiency.

INTERVENTIONS

Infants received a 24-hr infusion with the stable isotope [U-13C]glucose starting 5.3 +/- 0.5 hrs after birth. The 13C-incorporation into palmitic acid in surfactant phosphatidylcholine (PC) isolated from serial tracheal aspirates was measured. Infants received either zero (n = 5), one (n = 4), two (n = 15), or three (n = 3) doses of Survanta (100 mg/kg) when clinically indicated.

MEASUREMENTS AND MAIN RESULTS

Using multiple regression analysis, the absolute synthesis rate (ASR) of surfactant PC from plasma glucose increased with 1.3 +/- 0.4 mg/kg/day per dose of Survanta (p = .007) (mean +/- SEM). The ASR of surfactant PC from glucose was increased by prenatal corticosteroid treatment with 1.3 +/- 0.4 mg/kg/day per dose corticosteroid (p = .004), and by the presence of a patent ductus arteriosus with 2.1 +/- 0.7 mg/ kg/day (p = .01).

CONCLUSION

These data are reassuring and show for the first time in preterm infants that multiple doses of exogenous surfactant for RDS are tolerated well by the developing lung and stimulate endogenous surfactant synthesis.

Role of pulmonary surfactant components in surface film formation and dynamics

Pulmonary surfactant is a mixture of lipids and proteins which is secreted by the epithelial type II cells into the alveolar space. Its main function is to reduce the surface tension at the air/liquid interface in the lung. This is achieved by forming a surface film that consists of a monolayer which is highly enriched in dipalmitoylphosphatidylcholine and bilayer lipid/protein structures closely attached to it. The molecular mechanisms of film formation and of film adaptation to surface changes during breathing in order to remain a low surface tension at the interface, are unknown. The results of several model systems give indications for the role of the surfactant proteins and lipids in these processes. In this review, we describe and compare the model systems that are used for this purpose and the progress that has been made. Despite some conflicting results using different techniques, we conclude that surfactant protein B (SP-B) plays the major role in adsorption of new material into the interface during inspiration. SP-C's main functions are to exclude non-DPPC lipids from the interface during expiration and to attach the bilayer structures to the lipid monolayer. Surfactant protein A (SP-A) appears to promote most of SP-B's functions. We describe a model proposing that SP-A and SP-B create DPPC enriched domains which can readily be adsorbed to create a DPPC-rich monolayer at the interface. Further enrichment in DPPC is achieved by selective desorption of non-DPPC lipids during repetitive breathing cycles.

Formation of three-dimensional protein-lipid aggregates in monolayer films induced by surfactant protein B

This study focuses on the structural organization of surfactant protein B (SP-B) containing lipid monolayers. The artificial system is composed of the saturated phospholipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) in a molar ratio of 4:1 with 0.2 mol% SP-B. The different "squeeze-out" structures of SP-B were visualized by scanning probe microscopy and compared with structures formed by SP-C. Particularly, the morphology and material properties of mixed monolayers containing 0.2 mol% SP-B in a wide pressure range of 10 to 54 mN/m were investigated revealing that filamentous domain boundaries occur at intermediate surface pressure (15-30 mN/m), while disc-like protrusions prevail at elevated pressure (50-54 mN/m). In contrast, SP-C containing lipid monolayers exhibit large flat protrusions composed of stacked bilayers in the plateau region (app. 52 mN/m) of the pressure-area isotherm. By using different scanning probe techniques (lateral force microscopy, force modulation, phase imaging) it was shown that SP-B is dissolved in the liquid expanded rather than in the liquid condensed phase of the monolayer. Although artificial, the investigation of this system contributes to further understanding of the function of lung surfactant in the alveolus.

Tolerance of SP-A-deficient mice to hyperoxia or exercise

Mice carrying a null mutation of the surfactant-associated protein A (SP-A) gene have normal respiratory function, but their surfactant lacks tubular myelin, is sensitive to protein inactivation in vitro, and contains decreased pool sizes of the biophysically active large-aggregate surfactant. We hypothesized that SP-A-deficient mice would be more susceptible to exercise-induced stress and O(2)-induced lung injury. SP-A-(-/-) and SP-A-(+/+) mice tolerated 1 h of swimming or 45 min of running on a treadmill at 15 m/min equivalently, without alterations of the amount of alveolar saturated phosphatidylcholine. After 3 days of hyperoxia, SP-A-(-/-) mice had increased alveolar protein, but pressure-volume curves were not different between groups. Alveolar protein concentration was similarly increased in SP-A-(-/-) and SP-A-(+/+) mice after 4 days of exposure to hyperoxia. Survival rates were similar after 4 days of hyperoxia. SP-A-(-/-) mice were equally tolerant to exercise and 4 days of hyperoxia, indicating that the SP-A-dependent alterations in surfactant did not result in functional deficits.

Surfactant proteins A and D bind CD14 by different mechanisms

Surfactant proteins A (SP-A) and D (SP-D) are lung collectins that are constituents of the innate immune system of the lung. Recent evidence (Sano, H., Sohma, H., Muta, T., Nomura, S., Voelker, D. R., and Kuroki, Y. (1999) J. Immunol. 163, 387-395) demonstrates that SP-A modulates lipopolysaccharide (LPS)-induced cellular responses by direct interaction with CD14. In this report we examined the structural elements of the lung collectins involved in CD14 recognition and the consequences for CD14/LPS interaction. Rat SP-A and SP-D bound CD14 in a concentration-dependent manner. Mannose and EDTA inhibited SP-D binding to CD14 but did not decrease SP-A binding. The SP-A binding to CD14 was completely blocked by a monoclonal antibody that binds to the SP-A neck domain but only partially blocked by an antibody that binds to the SP-A lectin domain. SP-A but not SP-D bound to deglycosylated CD14. SP-D decreased CD14 binding to both smooth and rough LPS, whereas SP-A enhanced CD14 binding to rough LPS and inhibited binding to smooth LPS. SP-A also altered the migration profile of LPS on a sucrose density gradient in the presence of CD14. From these results, we conclude that 1) lung collectins bind CD14, 2) the SP-A neck domain and SP-D lectin domain participate in CD14 binding, 3) SP-A recognizes a peptide component and SP-D recognizes a carbohydrate moiety of CD14, and 4) lung collectins alter LPS/CD14 interactions.

Incorporation of biotinylated SP-A into rat lung surfactant layer, type II cells, and clara cells

The goal of this study was to compare the functions of Clara and type II cells during alveolar clearance and recycling of surfactant protein (SP) A, a secretory product of both cell types. We examined the incorporation of instilled biotinylated SP-A (bSP-A) into rat lung type II and Clara cells as a measure of clearance and recycling of the protein. Ultrastructural localization of bSP-A was accomplished by an electron-microscopic immunogold technique at 7, 30, and 120 min after intratracheal instillation. Localization of bSP-A was quantitatively evaluated within extracellular surfactant components (lipid-rich forms: myelin figures, vesicles, and tubular myelin; and lipid-poor hypophase) and in compartments of type II and Clara cells. bSP-A was incorporated into myelinic and vesicular forms of extracellular surfactant, but tubular myelin and hypophase had little bSP-A. Lamellar bodies of type II cells demonstrated a significant time-dependent increase in their incorporation of bSP-A. There was a concentration of bSP-A in the secretory granules and mitochondria of Clara cells, but no Clara cell compartment showed a pattern of time-dependent change in immunolabeling. Our immunolabeling data demonstrated a time-dependent movement of exogenous SP-A from extracellular components into type II cells and their secretory granules. Clara cells did not demonstrate a time-dependent incorporation of bSP-A into their secretory granules during the period of this study. If Clara cells recycle SP-A, they must reach a steady state very quickly or very slowly.

Binding and uptake of surfactant protein D by freshly isolated rat alveolar type II cells

Alveolar type II cells secrete, internalize, and recycle pulmonary surfactant, a lipid and protein complex that increases alveolar compliance and participates in pulmonary host defense. Surfactant protein (SP) D, a collagenous C-type lectin, has recently been described as a modulator of surfactant homeostasis. Mice lacking SP-D accumulate surfactant in their alveoli and type II cell lamellar bodies, organelles adapted for recycling and secretion of surfactant. The goal of current study was to characterize the interaction of SP-D with rat type II cells. Type II cells bound SP-D in a concentration-, time-, temperature-, and calcium-dependent manner. However, SP-D binding did not alter type II cell surfactant lipid uptake. Type II cells internalized SP-D into lamellar bodies and degraded a fraction of the SP-D pool. Our results also indicated that SP-D binding sites on type II cells may differ from those on alveolar macrophages. We conclude that, in vitro, type II cells bind and recycle SP-D to lamellar bodies, but SP-D may not directly modulate surfactant uptake by type II cells.

Internalized SP-A and lipid are differentially resecreted by type II pneumocytes

Biochemical and morphological assays were developed to study surfactant protein A (SP-A) and lipid resecretion kinetics by isolated type II cells in vitro. After a 10-min uptake period with SP-A (3 microg/10(6) cells) in combination with liposomes (60 microg/10(6) cells), the cells were allowed to resecrete. After 5 min of resecretion, only 21.7 +/- 4.6% of the internalized SP-A remained intracellularly compared with 54 +/- 2.9% of the lipids. Extracellular SP-A present during the resecretion period partially inhibited resecretion (SP-A, 36% at 5 min; lipid, approximately 16% at 5 min). Lipid resecretion was also dependent on the SP-A concentration present during the uptake period. Although, as shown by confocal laser scanning microscopy, after a 10-min uptake period at 37 degrees C, most of the fluorescein isothiocyanate-labeled SP-A and rhodamine-phosphatidylethanolamine-labeled lipids colocalized within the cells, after an additional 10 min of resecretion, both the strength of the fluorescence signals and the extent of colocalization had markedly decreased. These data indicate that internalized lipid and SP-A can be resecreted rapidly by type II cells, likely via different pathways.

Surfactant protein A regulates pulmonary surfactant secretion via activation of phosphatidylinositol 3-kinase in type II alveolar cells

Pulmonary surfactant is secreted by the type II alveolar cells of the lung, and this secretion is induced by secretagogues of several types (e.g., ionomycin, phorbol esters, and terbutaline). Secretagogue-induced secretion is inhibited by surfactant-associated protein A (SP-A), which binds to a specific receptor (SPAR) on the surface of type II cells. The mechanism of SP-A-activated SPAR signaling is completely unknown. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 rescued surfactant secretion from inhibition by SP-A. In order to directly demonstrate a role for PI3K in SPAR signaling, PI3K activity was immunoprecipitated from type II cell extracts. PI3K activity increased rapidly after SP-A addition to type II cells. Since many receptors that activate PI3K do so through tyrosine-specific protein phosphorylation, antisera to phosphotyrosine, insulin-receptor substrate-1 (IRS-1), or SPAR were also examined. These antisera coimmunoprecipitated PI3K activity that was stimulated by SP-A. In addition, the tyrosine-specific protein kinase inhibitors genistein and herbimycin A blocked the action of SP-A on surfactant secretion. We conclude that SP-A signals to regulate surfactant secretion through SPAR, via pathways that involve tyrosine phosphorylation, include IRS-1, and entail activation of PI3K. This activation leads to inhibition of secretagogue-induced secretion of pulmonary surfactant.

Cellular activation by Ca2+ release from stores in the endoplasmic reticulum but not by increased free Ca2+ in the cytosol

Ca(2+) release from intracellular stores and/or transmembrane influx can increase the cytosolic free Ca(2+) concentration ([Ca(2+)](i)). Such changes in [Ca(2+)](i) might transduce signals regulating transcription, motility, secretion, and so on. Surfactant secretagogues such as ATP and ionomycin stimulate the release and transmembrane influx of Ca(2+), both of which increase [Ca(2+)](i). The addition of surfactant protein A (SP-A) or depleting cellular Ca(2+) inhibited both surfactant secretion and Ca(2+) transients. Current results suggest that Ca(2+) signalling stimulates surfactant secretion by type II pneumocytes, but not via increased [Ca(2+)](i). Treatment of cells with a Ca(2+) chelator, bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM), stimulated secretion but decreased [Ca(2+)](i). Adding SP-A or depleting Ca(2+) inhibited BAPTA-AM-induced secretion. When studied directly, Ca(2+) in the endoplasmic reticulum store ([Ca(2+)](l)) decreased in response to BAPTA, ionomycin and thapsigargin, and increased in response to SP-A. Phorbol ester (PMA) induced surfactant secretion without altering [Ca(2+)](i) or [Ca(2+)](l) and was unaffected by Ca(2+) depletion. The addition of PMA to Ca(2+)-releasing secretagogues increased secretion, but combining two Ca(2+)-releasing secretagogues did not. These results suggest that (1) Ca(2+) signalling of type II cell surfactant secretion reflects changes in [Ca(2+)](l), not [Ca(2+)](i), (2) PMA elicits secretion differently from Ca(2+)-releasing secretagogues, and (3) SP-A inhibits secretion by enhancing Ca(2+) sequestration within endoplasmic reticulum stores. Whether other cell types signal via changes in [Ca(2+)](l) is unknown.