Rescue of SP-B knockout mice with a truncated SP-B proprotein. Function of the C-terminal propeptide

A dominant negative variant of RAB5B disrupts maturation of surfactant protein B and surfactant protein C

The Rab5 GTPase functions in early endosome (EE) fusion in the endocytic pathway. Here, we propose that RAB5B also has a noncanonical vesicular fusion function in the regulated secretion pathway that produces mature surfactant proteins SP-B and SP-C in the lung. This function was revealed from investigation of a proband with interstitial lung disease suggestive of a surfactant dysfunction disorder who carried a de novo Asp136His variant in the RAB5B gene. Our modeling in C. elegans provided information on the genetic and cell biological mechanism, and analyses of proband and normal lung biopsies suggested a function for RAB5B and EEs in surfactant protein processing/trafficking. This work indicates that RAB5B p.Asp136His causes a surfactant dysfunction disorder.

Pathogenic variants in surfactant proteins SP-B and SP-C cause surfactant deficiency and interstitial lung disease. Surfactant proteins are synthesized as precursors (proSP-B, proSP-C), trafficked, and processed via a vesicular-regulated secretion pathway; however, control of vesicular trafficking events is not fully understood. Through the Undiagnosed Diseases Network, we evaluated a child with interstitial lung disease suggestive of surfactant deficiency. Variants in known surfactant dysfunction disorder genes were not found in trio exome sequencing. Instead, a de novo heterozygous variant in RAB5B was identified in the Ras/Rab GTPases family nucleotide binding domain, p.Asp136His. Functional studies were performed in Caenorhabditis elegans by knocking the proband variant into the conserved position (Asp135) of the ortholog, rab-5. Genetic analysis demonstrated that rab-5[Asp135His] is damaging, producing a strong dominant negative gene product. rab-5[Asp135His] heterozygotes were also defective in endocytosis and early endosome (EE) fusion. Immunostaining studies of the proband’s lung biopsy revealed that RAB5B and EE marker EEA1 were significantly reduced in alveolar type II cells and that mature SP-B and SP-C were significantly reduced, while proSP-B and proSP-C were normal. Furthermore, staining normal lung showed colocalization of RAB5B and EEA1 with proSP-B and proSP-C. These findings indicate that dominant negative–acting RAB5B Asp136His and EE dysfunction cause a defect in processing/trafficking to produce mature SP-B and SP-C, resulting in interstitial lung disease, and that RAB5B and EEs normally function in the surfactant secretion pathway. Together, the data suggest a noncanonical function for RAB5B and identify RAB5B p.Asp136His as a genetic mechanism for a surfactant dysfunction disorder.

Gene Therapy Potential for Genetic Disorders of Surfactant Dysfunction

Pulmonary surfactant is critically important to prevent atelectasis by lowering the surface tension of the alveolar lining liquid. While respiratory distress syndrome (RDS) is common in premature infants, severe RDS in term and late preterm infants suggests an underlying genetic etiology. Pathogenic variants in the genes encoding key components of pulmonary surfactant including surfactant protein B (SP-B, SFTPB gene), surfactant protein C (SP-C, SFTPC gene), and the ATP-Binding Cassette transporter A3 (ABCA3, ABCA3 gene) result in severe neonatal RDS or childhood interstitial lung disease (chILD). These proteins play essential roles in pulmonary surfactant biogenesis and are expressed in alveolar epithelial type II cells (AEC2), the progenitor cell of the alveolar epithelium. SP-B deficiency most commonly presents in the neonatal period with severe RDS and requires lung transplantation for survival. SFTPC mutations act in an autosomal dominant fashion and more commonly presents with chILD or idiopathic pulmonary fibrosis than neonatal RDS. ABCA3 deficiency often presents as neonatal RDS or chILD. Gene therapy is a promising option to treat monogenic lung diseases. Successes and challenges in developing gene therapies for genetic disorders of surfactant dysfunction include viral vector design and tropism for target cell types. In this review, we explore adeno-associated virus (AAV), lentiviral, and adenoviral (Ad)-based vectors as delivery vehicles. Both gene addition and gene editing strategies are compared to best design treatments for lung diseases resulting from pathogenic variants in the SFTPB, SFTPC, and ABCA3 genes.

Revisiting the role of pulmonary surfactant in chronic inflammatory lung diseases and environmental exposure

Pulmonary surfactant is a crucial and dynamic lung structure whose primary functions are to reduce alveolar surface tension and facilitate breathing. Though disruptions in surfactant homeostasis are typically thought of in the context of respiratory distress and premature infants, many lung diseases have been noted to have significant surfactant abnormalities. Nevertheless, preclinical and clinical studies of pulmonary disease too often overlook the potential contribution of surfactant alterations - whether in quantity, quality or composition - to disease pathogenesis and symptoms. In inflammatory lung diseases, whether these changes are cause or consequence remains a subject of debate. This review will outline 1) the importance of pulmonary surfactant in the maintenance of respiratory health, 2) the diseases associated with primary surfactant dysregulation, 3) the surfactant abnormalities observed in inflammatory pulmonary diseases and, finally, 4) the available research on the interplay between surfactant homeostasis and smoking-associated lung disease. From these published studies, we posit that changes in surfactant integrity and composition contribute more considerably to chronic inflammatory pulmonary diseases and that more work is required to determine the mechanisms underlying these alterations and their potential treatability.

AP-3-dependent targeting of flippase ATP8A1 to lamellar bodies suppresses activation of YAP in alveolar epithelial type 2 cells

Lamellar bodies (LBs) are lysosome-related organelles (LROs) of surfactant-producing alveolar type 2 (AT2) cells of the distal lung epithelium. Trafficking pathways to LBs have been understudied but are likely critical to AT2 cell homeostasis given associations between genetic defects of endosome to LRO trafficking and pulmonary fibrosis in Hermansky Pudlak syndrome (HPS). Our prior studies uncovered a role for AP-3, defective in HPS type 2, in trafficking Peroxiredoxin-6 to LBs. We now show that the P4-type ATPase ATP8A1 is sorted by AP-3 from early endosomes to LBs through recognition of a C-terminal dileucine-based signal. Disruption of the AP-3/ATP8A1 interaction causes ATP8A1 accumulation in early sorting and/or recycling endosomes, enhancing phosphatidylserine exposure on the cytosolic leaflet. This in turn promotes activation of Yes-activating protein, a transcriptional coactivator, augmenting cell migration and AT2 cell numbers. Together, these studies illuminate a mechanism whereby loss of AP-3-mediated trafficking contributes to a toxic gain-of-function that results in enhanced and sustained activation of a repair pathway associated with pulmonary fibrosis.

Fusion Expression and Fibrinolytic Activity of rPA/SP-B

BACKGROUND

Pulmonary surfactant dysfunction is an important pathological factor in acute respiratory distress syndrome (ARDS) and pulmonary fibrosis (PF).

OBJECTIVE

In this study, the characteristics of recombinant mature surfactant protein B (SP-B) and reteplase (rPA) fusion protein maintaining good pulmonary surface activity and rPA fibrinolytic activity in acute lung injury cell model were studied.

METHODS

We studied the characteristics of SP-B fusion expression, cloned rPA gene and N-terminal rPA/C-terminal SP-B co-expression gene, and constructed them into eukaryotic expression vector pEZ-M03 to obtain recombinant plasmids pEZ-rPA and pEZ-rPA/SP-B. The recombinant plasmids was transfected into Chinese hamster ovary (CHO) K1 cells and the expression products were analyzed by Western Blot. Lipopolysaccharide (LPS) was used to induce CCL149 (an alveolar epithelial cell line) cell injury model. Fluorescence staining of rPA and rPA/SP-B was carried out with the enhanced green fluorescent protein (eGFP) that comes with pEZ-M03; the cell Raman spectroscopy technique was used to analyze the interaction between rPA/SP-B fusion protein and the phospholipid structure of cell membrane in CCL149 cells. The enzyme activity of rPA in the fusion protein was determined by fibrin-agarose plate method.

RESULTS

The rPA/SP-B fusion protein was successfully expressed. In the CCL149 cell model of acute lung injury (ALI), the green fluorescence of rPA/SP-B is mainly distributed on the CCL149 cell membrane. The rPA/SP-B fusion protein can reduce the disorder of phospholipid molecules and reduce cell membrane damage. The enzyme activity of rPA/SP-B fusion protein was 3.42, and the fusion protein still had good enzyme activity.

CONCLUSION

The recombinant eukaryotic plasmid pEZ-rPA/SP-B is constructed and can be expressed in the eukaryotic system. Studies have shown that rPA/SP-B fusion protein maintains good SP-B lung surface activity and rPA enzyme activity in acute lung injury cell model.

Mechanism of Lamellar Body Formation by Lung Surfactant Protein B

Breathing depends on pulmonary surfactant, a mixture of phospholipids and proteins, secreted by alveolar type II cells. Surfactant requires lamellar bodies (LBs), organelles containing densely packed concentric membrane layers, for storage and secretion. LB biogenesis remains mysterious but requires surfactant protein B (SP-B), which is synthesized as a precursor (pre-proSP-B) that is cleaved during trafficking into three related proteins. Here, we elucidate the functions and cooperation of these proteins in LB formation. We show that the N-terminal domain of proSP-B is a phospholipid-binding and -transfer protein whose activities are required for proSP-B export from the endoplasmic reticulum (ER) and sorting to LBs, the conversion of proSP-B into lipoprotein particles, and neonatal viability in mice. The C-terminal domain facilitates ER export of proSP-B. The mature middle domain, generated after proteolytic cleavage of proSP-B, generates the striking membrane layers characteristic of LBs. Together, our results lead to a mechanistic model of LB biogenesis.

Genetic Association of Pulmonary Surfactant Protein Genes, SFTPA1, SFTPA2, SFTPB, SFTPC, and SFTPD With Cystic Fibrosis

Surfactant proteins (SP) are involved in surfactant function and innate immunity in the human lung. Both lung function and innate immunity are altered in CF, and altered SP levels and genetic association are observed in Cystic Fibrosis (CF). We hypothesized that single nucleotide polymorphisms (SNPs) within the SP genes associate with CF or severity subgroups, either through single SNP or via SNP-SNP interactions between two SNPs of a given gene (intragenic) and/or between two genes (intergenic). We genotyped a total of 17 SP SNPs from 72 case-trio pedigree (SFTPA1 (5), SFTPA2 (4), SFTPB (4), SFTPC (2), and SFTPD (2)), and identified SP SNP associations by applying quantitative genetic principles. The results showed (a) Two SNPs, SFTPB rs7316 (p = 0.0083) and SFTPC rs1124 (p = 0.0154), each associated with CF. (b) Three intragenic SNP-SNP interactions, SFTPB (rs2077079, rs3024798), and SFTPA1 (rs1136451, rs1059057 and rs4253527), associated with CF. (c) A total of 34 intergenic SNP-SNP interactions among the 4 SP genes to be associated with CF. (d) No SNP-SNP interaction was observed between SFTPA1 or SFTPA2 and SFTPD. (e) Equal number of SNP-SNP interactions were observed between SFTPB and SFTPA1/SFTPA2 (n = 7) and SP-B and SFTPD (n = 7). (f) SFTPC exhibited significant SNP-SNP interactions with SFTPA1/SFTPA2 (n = 11), SFTPB (n = 4) and SFTPD (n = 3). (g) A single SFTPB SNP was associated with mild CF after Bonferroni correction, and several intergenic interactions that are associated (p < 0.01) with either mild or moderate/severe CF were observed. These collectively indicate that complex SNP-SNP interactions of the SP genes may contribute to the pulmonary disease in CF patients. We speculate that SPs may serve as modifiers for the varied progression of pulmonary disease in CF and/or its severity.

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 proteins in pediatric interstitial lung disease

BACKGROUND

Children's interstitial lung diseases (chILD) comprise a broad spectrum of diseases. Besides the genetically defined surfactant dysfunction disorders, most entities pathologically involve the alveolar surfactant region, possibly affecting the surfactant proteins SP-B and SP-C. Therefore, our objective was to determine the value of quantitation of SP-B and SP-C levels in bronchoalveolar lavage fluid (BALF) for the diagnosis of chILD.

METHODS

Levels of SP-B and SP-C in BALF from 302 children with chILD and in controls were quantified using western blotting. In a subset, single-nucleotide polymorphisms (SNPs) in the SFTPC promoter were genotyped by direct sequencing.

RESULTS

While a lack of dimeric SP-B was found only in the sole subject with hereditary SP-B deficiency, low or absent SP-C was observed not only in surfactant dysfunction disorders but also in patients with other diffuse parenchymal lung diseases pathogenetically related to the alveolar surfactant region. Genetic analysis of the SFTPC promoter showed association of a single SNP with SP-C level.

CONCLUSION

SP-B levels may be used for screening for SP-B deficiency, while low SP-C levels may point out diseases caused by mutations in TTF1, SFTPC, ABCA3, and likely in other genes involved in surfactant metabolism that remain to be identified. We conclude that measurement of levels of SP-B and SP-C was useful for the differential diagnosis of chILD, and for the precise molecular diagnosis, sequencing of the genes is necessary.

Acceleration of Lung Regeneration by Platelet-Rich Plasma Extract through the Low-Density Lipoprotein Receptor-Related Protein 5-Tie2 Pathway

Angiogenesis, the growth of new blood vessels, plays a key role in organ development, homeostasis, and regeneration. The cooperation of multiple angiogenic factors, rather than a single factor, is required for physiological angiogenesis. Recently, we have reported that soluble platelet-rich plasma (PRP) extract, which contains abundant angiopoietin-1 and multiple other angiogenic factors, stimulates angiogenesis and maintains vascular integrity in vitro and in vivo. In this report, we have demonstrated that mouse PRP extract increases phosphorylation levels of the Wnt coreceptor low-density lipoprotein receptor-related protein 5 (LRP5) and thereby activates angiogenic factor receptor Tie2 in endothelial cells (ECs) and accelerates EC sprouting and lung epithelial cell budding in vitro. PRP extract also increases phosphorylation levels of Tie2 in the mouse lungs and accelerates compensatory lung growth and recovery of exercise capacity after unilateral pneumonectomy in mice, whereas soluble Tie2 receptor or Lrp5 knockdown attenuates the effects of PRP extract. Because human PRP extract is generated from autologous peripheral blood and can be stored at -80°C, our findings may lead to the development of novel therapeutic interventions for various angiogenesis-related lung diseases and to the improvement of strategies for lung regeneration.

Platelet-rich plasma extract prevents pulmonary edema through angiopoietin-Tie2 signaling

Increased vascular permeability contributes to life-threatening pathological conditions, such as acute respiratory distress syndrome. Current treatments for sepsis-induced pulmonary edema rely on low-tidal volume mechanical ventilation, fluid management, and pharmacological use of a single angiogenic or chemical factor with antipermeability activity. However, it is becoming clear that a combination of multiple angiogenic/chemical factors rather than a single factor is required for maintaining stable and functional blood vessels. We have demonstrated that mouse platelet-rich plasma (PRP) extract contains abundant angiopoietin (Ang) 1 and multiple other factors (e.g., platelet-derived growth factor), which potentially stabilize vascular integrity. Here, we show that PRP extract increases tyrosine phosphorylation levels of Tunica internal endothelial cell kinase (Tie2) and attenuates disruption of cell-cell junctional integrity induced by inflammatory cytokine in cultured human microvascular endothelial cells. Systemic injection of PRP extract also increases Tie2 phosphorylation in mouse lung and prevents endotoxin-induced pulmonary edema and the consequent decreases in lung compliance and exercise intolerance resulting from endotoxin challenge. Soluble Tie2 receptor, which inhibits Ang-Tie2 signaling, suppresses the ability of PRP extract to inhibit pulmonary edema in mouse lung. These results suggest that PRP extract prevents endotoxin-induced pulmonary edema mainly through Ang-Tie2 signaling, and PRP extract could be a potential therapeutic strategy for sepsis-induced pulmonary edema and various lung diseases caused by abnormal vascular permeability.

Surfactant protein B propeptide contains a saposin-like protein domain with antimicrobial activity at low pH

Surfactant protein B (SP-B) proprotein contains three saposin-like protein (SAPLIP) domains: a SAPLIP domain corresponding to the mature SP-B peptide is essential for lung function and postnatal survival; the function of SAPLIP domains in the N-terminal (SP-BN) and C-terminal regions of the proprotein is not known. In the current study, SP-BN was detected in the supernatant of mouse bronchoalveolar lavage fluid (BALF) and in nonciliated bronchiolar cells, alveolar type II epithelial cells, and alveolar macrophages. rSP-BN indirectly promoted the uptake of bacteria by macrophage cell lines and directly killed bacteria at acidic pH, consistent with a lysosomal, antimicrobial function. Native SP-BN isolated from BALF also killed bacteria but only at acidic pH; the bactericidal activity of BALF at acidic pH was completely blocked by SP-BN Ab. Transgenic mice overexpressing SP-BN and mature SP-B peptide had significantly decreased bacterial burden and increased survival following intranasal inoculation with bacteria. These findings support the hypothesis that SP-BN contributes to innate host defense of the lung by supplementing the nonoxidant antimicrobial defenses of alveolar macrophages.

Anterograde transport of surfactant protein C proprotein to distal processing compartments requires PPDY-mediated association with Nedd4 ubiquitin ligases

Biosynthesis of surfactant protein C (SP-C) by alveolar type 2 cells requires proteolytic processing of a 21-kDa propeptide (proSP-C21) in post-Golgi compartments to yield a 3.7-kDa mature form. Scanning alanine mutagenesis, binding assays, and co-immunoprecipitation were used to characterize the proSP-C targeting domain. Delivery of proSP-C21 to distal processing organelles is dependent upon the NH2-terminal cytoplasmic SP-C propeptide, which contains a conserved PPDY motif. In A549 cells, transfection of EGFP/proSP-C21 constructs containing polyalanine substitution for Glu11-Thr18, 13PPDY16, or 14P,16Y produced endoplasmic reticulum retention of the fusion proteins. Protein-protein interactions of proSP-C with known WW domains were screened using a solid-phase array that revealed binding of the proSP-C NH2 terminus to several WW domains found in the Nedd4 family of E3 ligases. Specificity of the interaction was confirmed by co-immunoprecipitation of proSP-C and Nedd4 or Nedd4-2 in epithelial cell lines. By Western blotting and reverse transcription-PCR, both forms were detected in primary human type 2 cells. Knockdown of Nedd4-2 by small interference RNA transfection of cultured human type 2 cells blocked processing of 35S-labeled proSP-C21. Mutagenesis of potential acceptor sites for ubiquitination in the cytosolic domain of proSP-C (Lys6, Lys34, or both) failed to inhibit trafficking of EGFP/proSP-C21. These results indicate that PPDY-mediated interaction with Nedd4 E3-ligases is required for trafficking of proSP-C. We speculate that the Nedd4/proSP-C tandem is part of a larger protein complex containing a ubiquitinated component that further directs its transport.

Developmental and genetic regulation of human surfactant protein B in vivo

BACKGROUND

Genetic and developmental disruption of surfactant protein B (SP-B) expression causes neonatal respiratory distress syndrome (RDS).

OBJECTIVES

To assess developmental and genetic regulation of SP-B expression in vivo.

METHODS

To evaluate in vivo developmental regulation of SP-B, we used immunoblotting to compare frequency of detection of mature and pro-SP-B peptides in developmentally distinct cohorts: 24 amniotic fluid samples, unfractionated tracheal aspirates from 101 infants >or=34 weeks' gestation with (75) and without (26) neonatal RDS, and 6 nonsmoking adults. To examine genetic regulation, we used univariate and logistic regression analyses to detect associations between common SP-B (SFTPB) genotypes and SP-B peptides in the neonatal RDS cohort.

RESULTS

We found pro-SP-B peptides in 24/24 amniotic fluid samples and in 100/101 tracheal aspirates from newborn infants but none in bronchoalveolar lavage from normal adults (0/6) (p < 0.001). We detected an association (p = 0.0011) between pro-SP-B peptides (M(r) 40 and 42 kDa) and genotype of a nonsynonymous single nucleotide polymorphism at genomic position 1580 that regulates amino-terminus glycosylation.

CONCLUSIONS

Pro-SP-B peptides are more common in developmentally less mature humans. Association of genotype at genomic position 1580 with pro-SP-B peptides (M(r) 40 and 42 kDa) suggests genetic regulation of amino terminus glycosylation in vivo.

Self-aggregation of a recombinant form of the propeptide NH2-terminal of the precursor of pulmonary surfactant protein SP-B: a conformational study

A recombinant form of the peptide N-terminally positioned from proSP-B (SP-BN) has been produced in Escherichia coli as fusion with the Maltose Binding Protein, separated from it by Factor Xa cleavage and purified thereafter. This protein module is thought to control assembly of mature SP-B, a protein essential for respiration, in pulmonary surfactant as it progress through the progressively acidified secretory pathway of pneumocytes. Self-aggregation studies of the recombinant propeptide have been carried out as the pH of the medium evolved from neutral to moderately acid, again to neutral and finally basic. The profile of aggregation versus subsequent changes in pH showed differences depending on the ionic strength of the medium, low or moderate, and the presence of additives such as L-arginine (a known aggregation suppressor) and Ficoll 70 (a macromolecular crowder). Circular dichroism studies of SP-BN samples along the aggregation process showed a decrease in alpha-helical content and a concomitant increase in beta-sheet. Intrinsic fluorescence emission of SP-BN was dominated by the emission of Trp residues in neutral medium, being its emission maximum shifted to red at low pH, suggesting that the protein undergoes a pH-dependent conformational change that increases the exposure of their Trp to the environment. A marked increase in the fluorescence emission of the extrinsic probe bis-ANS indicated the exposure of hydrophobic regions of SP-BN at pH 5. The fluorescence of bis-ANS decreased slightly at low ionic strength, but to a great extent at moderate ionic strength when the pH was reversed to neutrality, suggesting that self-aggregation properties of the SP-BN module could be tightly modulated by the conditions of pH and the ionic environment encountered by pulmonary surfactant during assembly and secretion.

Identification of a segment in the precursor of pulmonary surfactant protein SP-B, potentially involved in pH-dependent membrane assembly of the protein

In the present work, the hydrophobic properties of proSP-B, the precursor of pulmonary surfactant protein SP-B, have been analyzed under different pH conditions, and the sequence segment at position 111-135 of the N-terminal domain of the precursor has been detected as potentially possessing pH-dependent hydrophobic properties. We have studied the structure and lipid-protein interactions of the synthetic peptides BpH, with sequence corresponding to the segment 111-135 of proSP-B, and BpH-W, bearing the conservative substitution F127W to use the tryptophan as an intrinsic fluorescent probe. Peptide BpH-W interacts with both zwitterionic and anionic phospholipid vesicles at neutral pH, as monitored by the blue-shifted maximum emission of its tryptophan reporter. Insertion of tryptophan into the membranes is further improved at pH 5.0, especially in negatively-charged membranes. Peptides BpH and BpH-W also showed pH-dependent properties to insert into phospholipid monolayers. We have also found that the single sequence variation F120K decreases substantially the interaction of this segment with phospholipid surfaces as well as its pH-dependent insertion into deeper regions of the membranes. We hypothesize that this region could be involved in pH-triggered conformational changes occurring in proSP-B along the exocytic pathway of surfactant in type II cells, leading to the exposure of the appropriate segments for processing and assembly of SP-B within surfactant lipids.

A major deletion in the surfactant protein-B gene causing lethal respiratory distress

BACKGROUND

Loss of function mutations in the surfactant protein-B gene (SFTPB) cause lethal neonatal respiratory distress due to reduced or absent expression of mature surfactant protein B (SP-B, encoded in exons 6 and 7). No large deletions in SFTPB have been previously identified.

AIM

Genomic, proteomic and immunohistochemical characterization of a 3 kb deletion in SFTPB.

METHODS

A full-term newborn presented with refractory respiratory failure. We amplified and sequenced SFTPB from the infant and both parents, determined SP-B protein expression in tracheal aspirate samples using Western-blot analysis, and performed immunohistochemical staining and electron microscopy of lung biopsy tissue.

RESULTS

The infant was homozygous for a 2958 bp deletion in SFTPB that included exons 7 and 8. Both asymptomatic parents were heterozygous for the deletion. A truncated mature SP-B peptide was detected on Western blotting of tracheal aspirate. Amino acid sequence specific to that encoded in exon 5 was present, but that encoded by exon 7 was absent. ProSP-B expression was robust within alveolar type II cells and lamellar body structure was disrupted.

CONCLUSIONS

This deletion in SFTPB resulted in SP-B deficiency due to absence of elements in mature SP-B that are critical for appropriate peptide folding, trafficking and processing.

Genetically engineered mice in understanding the basis of neonatal lung disease

Advances in genetic engineering have allowed the creation of animals with additional or deleted genes. New genes may be inserted in mice, specific genes inactivated or "knocked out," and more complex animals created in which genes can be turned on or off at different times in development or in different tissues. These animal models allow for more detailed studies of the proteins encoded by the manipulated gene, an improved understanding of the pathophysiology of diseases resulting from the genetic alterations, and model organisms in which to study potential new therapies. Multiple mouse models involving genes important in surfactant production and regulation relevant to lung disease observed in human newborns have been created. This review will discuss the creation of such animals and illustrate their utility in understanding human disease.

Production in Escherichia coli of a recombinant C-terminal truncated precursor of surfactant protein B (rproSP-BΔc). Structure and interaction with lipid interfaces

SP-B, a protein absolutely required to maintain the lungs open after birth, is synthesized in the pneumocytes as a precursor containing C-terminal and N-terminal domains flanking the mature sequence. These flanking-domains are cleaved to produce mature SP-B, coupled with its assembly into pulmonary surfactant lipid-protein complexes. In the present work we have optimized over-expression in Escherichia coli and purification of rproSP-B(DeltaC), a recombinant form of human proSP-B lacking the C-terminal flanking peptide, which is still competent to restore SP-B function in vivo. rProSP-B(DeltaC) has been solubilized, purified and refolded from bacterial inclusion bodies in amounts of about 4 mg per L of culture. Electrophoretic mobility, immunoreactivity, N-terminal sequencing and peptide fingerprinting all confirmed that the purified protein had the expected mass and sequence. Once refolded, the protein was soluble in aqueous buffers. Circular dichroism and fluorescence emission spectra of bacterial rproSP-B(DeltaC) indicated that the protein is properly folded, showing around 32% alpha-helix and a mainly hydrophobic environment of its tryptophan residues. Presence of zwitterionic or anionic phospholipids vesicles caused changes in the fluorescence emission properties of rproSP-B(DeltaC) that were indicative of lipid-protein interaction. The association of this SP-B precursor with membranes suggests an intrinsic amphipathic character of the protein, which spontaneously adsorbs at air-liquid interfaces either in the absence or in the presence of phospholipids. The analysis of the structure and properties of recombinant proSP-B(DeltaC) in surfactant-relevant environments will open new perspectives on the investigation of the mechanisms of lipid and protein assembly in surfactant complexes.

Increased and prolonged pulmonary fibrosis in surfactant protein C-deficient mice following intratracheal bleomycin

Recent reports have linked mutations in the surfactant protein C gene (SFTPC) to familial forms of pulmonary fibrosis, but it is uncertain whether deficiency of mature SP-C contributes to disease pathogenesis. In this study, we evaluated bleomycin-induced lung fibrosis in mice with genetic deletion of SFTPC. Compared with wild-type (SFTPC+/+) controls, mice lacking surfactant protein C (SFTPC-/-) had greater lung neutrophil influx at 1 week after intratracheal bleomycin, greater weight loss during the first 2 weeks, and increased mortality. At 3 and 6 weeks after bleomycin, lungs from SFTPC-/- mice had increased fibroblast numbers, augmented collagen accumulation, and greater parenchymal distortion. Furthermore, resolution of fibrosis was delayed. Although remodeling was near complete in SFTPC+/+ mice by 6 weeks, SFTPC-/- mice did not return to baseline until 9 weeks after bleomycin. By terminal dUTP nick-end labeling staining, widespread cell injury was observed in SFTPC-/- and SFTPC+/+ mice 1 week after bleomycin; however, ongoing apoptosis of epithelial and interstitial cells occurred in lungs of SFTPC-/- mice, but not SFTPC+/+ mice, 6 weeks after bleomycin. Thus, SP-C functions to limit lung inflammation, inhibit collagen accumulation, and restore normal lung structure after bleomycin.

Partial SP-B deficiency perturbs lung function and causes air space abnormalities

Surfactant protein B (SP-B) is required for function of newborn and adult lung, and partial deficiency has been associated with susceptibility to lung injury. In the present study, transgenic mice were produced in which expression of SP-B in type II epithelial cells was conditionally regulated. Concentrations of SP-B were maintained at 60–70% of that normally present in control. Immunostaining for SP-B demonstrated cellular heterogeneity in expression of the protein. In subsets of type II cells in which SP-B staining was decreased, immunostaining for pro-SP-C was increased and lamellar body ultrastructure was disrupted, consistent with focal SP-B deficiency. Fluorescence-activated cell sorting analyses of freshly isolated type II cells identified a population of cells with low SP-B content and a smaller population with increased SP-B content, confirming nonuniform expression of the SP-B transgene. Focal air space enlargement, without cellular infiltration or inflammation, was observed. Pressure-volume curves indicated that maximal tidal volume was unchanged; however, hysteresis was modestly altered and residual volumes were significantly decreased in the SP-B-deficient mice. Chronic, nonuniform SP-B deficiency perturbed pulmonary function and caused air space enlargement.

Multilayer structures in lipid monolayer films containing surfactant protein C: effects of cholesterol and POPE

The influence of cholesterol and POPE on lung surfactant model systems consisting of DPPC/DPPG (80:20) and DPPC/DPPG/surfactant protein C (80:20:0.4) has been investigated. Cholesterol leads to a condensation of the monolayers, whereas the isotherms of model lung surfactant films containing POPE exhibit a slight expansion combined with an increased compressibility at medium surface pressure (10-30 mN/m). An increasing amount of liquid-expanded domains can be visualized by means of fluorescence light microscopy in lung surfactant monolayers after addition of either cholesterol or POPE. At surface pressures of 50 mN/m, protrusions are formed which differ in size and shape as a function of the content of cholesterol or POPE, but only if SP-C is present. Low amounts of cholesterol (10 mol %) lead to an increasing number of protrusions, which also grow in size. This is interpreted as a stabilizing effect of cholesterol on bilayers formed underneath the monolayer. Extreme amounts of cholesterol (30 mol %), however, cause an increased monolayer rigidity, thus preventing reversible multilayer formation. In contrast, POPE, as a nonbilayer lipid thought to stabilize the edges of protrusions, leads to more narrow protrusions. The lateral extension of the protrusions is thereby more influenced than their height.

Surfactant protein C gene variation in the Finnish population - association with perinatal respiratory disease

Surfactant protein C (SP-C) is a small hydrophobic protein component of alveolar surfactant, a lipid-protein complex lining the alveolar surface of the lung. Surfactant deficiency is the main cause of respiratory distress syndrome (RDS) in premature infants. RDS is a major risk factor of a chronic lung disease called bronchopulmonary dysplasia (BPD). The dominant mutations of the SP-C gene have recently been associated with interstitial lung diseases. However, the common genetic variation in the surfactant protein C gene has not been studied in detail. In the present study, the exonic variation of the SP-C gene in the Finnish population (n=472) was defined, and the association of the allelic variants with the susceptibility to RDS and BPD was examined. Conformation-sensitive gel electrophoresis (CSGE) was used to determine the extent of exonic variation in the SP-C gene. Methods of genotyping were generated for three biallelic polymorphisms of the SP-C gene's exons 1, 4 and 5, which encode proSP-C. The frequencies of these polymorphisms were evaluated in a study population consisting of 158 DNA samples from full-term infants. In addition, the linkage disequilibrium between the SP-C alleles was evaluated by haplotype analysis of parent-infant triplets. The role of SP-C gene variation in RDS and in BPD was evaluated in a high-risk population of 245 premature infants. According to the present results, the SP-C polymorphisms were associated with RDS and with very premature birth. The strength of allelic associations differed according to the gender of the premature infants.

N-terminally extended surfactant protein (SP) C isolated from SP-B-deficient children has reduced surface activity and inhibited lipopolysaccharide binding

In both humans and mice, a deficiency of surfactant protein B (SP-B) is associated with a decreased concentration of mature SP-C and accumulation of a larger SP-C peptide, denoted SP-C(i), which is not observed under normal conditions. Isolation of hydrophobic polypeptides from the lungs of children who died with two different SP-B mutations yielded pure SP-C(i) and showed only trace amounts of mature SP-C. Determination of the SP-C(i) covalent structure revealed a 12-residue N-terminal peptide segment, followed by a 35-residue segment that is identical to mature SP-C. The SP-C(i) structure determined herein is similar to that of a proposed late intermediate in the processing of proSP-C, suggesting that SP-C(i) is the immediate precursor of SP-C. In bronchoalveolar lavage fluid from transgenic mice with a focal deficiency of SP-B, SP-C(i) was detected in the biophysically active, large aggregate fraction and was associated with membrane structures that are typical for a large aggregate surfactant. However, unlike SP-C, SP-C(i) exhibited a very poor ability to promote phospholipid adsorption, gave high surface tension during cyclic film compression, and did not bind lipopolysaccharide in vitro. SP-C(i) is thus capable of associating with surfactant lipids, but its N-terminal dodecapeptide segment must be proteolytically removed to generate a biologically functional peptide. The results of this study indicate that the early postnatal fatal respiratory distress seen in SP-B-deficient children is combined with the near absence of active variants of SP-C.

Alterations in SP-B and SP-C expression in neonatal lung disease

The hydrophobic surfactant proteins, SP-B and SP-C, have important roles in surfactant function. The importance of these proteins in normal lung function is highlighted by the lung diseases associated with abnormalities in their expression. Mutations in the gene encoding SP-B result in severe, fatal neonatal lung disease, and mutations in the gene encoding SP-C are associated with chronic interstitial lung diseases in newborns, older children, and adults. This work reviews the current state of knowledge concerning the lung diseases associated with mutations in the SP-B and SP-C genes, and the potential roles of abnormal SP-B and SP-C expression and genetic variation in these genes in other lung diseases.

Processing of Pulmonary Surfactant Protein B by Napsin and Cathepsin H

Surfactant protein B (SP-B) is an essential constituent of pulmonary surfactant. SP-B is synthesized in alveolar type II cells as a preproprotein and processed to the mature peptide by the cleavage of NH2- and COOH-terminal peptides. An aspartyl protease has been suggested to cleave the NH2-terminal propeptide resulting in a 25-kDa intermediate. Napsin, an aspartyl protease expressed in alveolar type II cells, was detected in fetal lung homogenates as early as day 16 of gestation, 1 day before the onset of SP-B expression and processing. Napsin was localized to multivesicular bodies, the site of SP-B proprotein processing in type II cells. Incubation of SP-B proprotein from type II cells with a crude membrane extract from napsin-transfected cells resulted in enhanced levels of a 25-kDa intermediate. Purified napsin cleaved a recombinant SP-B/EGFP fusion protein within the NH2-terminal propeptide between Leu178 and Pro179, 22 amino acids upstream of the NH2 terminus of mature SP-B. Cathepsin H, a cysteine protease also implicated in pro-SP-B processing, cleaved SP-B/EGFP fusion protein 13 amino acids upstream of the NH2 terminus of mature SP-B. Napsin did not cleave the COOH-terminal peptide, whereas cathepsin H cleaved the boundary between mature SP-B and the COOH-terminal peptide and at several other sites within the COOH-terminal peptide. Knockdown of napsin by small interfering RNA resulted in decreased levels of mature SP-B and mature SP-C in type II cells. These results suggest that napsin, cathepsin H, and at least one other enzyme are involved in maturation of the biologically active SP-B peptide.

Expression of a human surfactant protein C mutation associated with interstitial lung disease disrupts lung development in transgenic mice

Surfactant Protein C (SP-C) is a secreted transmembrane protein that is exclusively expressed by alveolar type II epithelial cells of the lung. SP-C associates with surfactant lipids to reduce surface tension within the alveolus, maintaining lung volume at end expiration. Mutations in the gene encoding SP-C (SFTPC) have recently been linked to chronic lung disease in children and adults. The goal of this study was to determine whether a disease-linked mutation in SFTPC causes lung disease in transgenic mice. The SFTPC mutation, designated g.1728 G --> A, results in the deletion of exon4, generating a truncated form of SP-C (SP-C(Deltaexon4)). cDNA encoding SP-C(Deltaexon4) was constitutively expressed in type II epithelial cells of transgenic mice. Viable F0 transgene-positive mice were not generated after two separate rounds of pronuclear injections. Histological analysis of lung tissue harvested from embryonic day 17.5 F0 transgene-positive fetuses revealed that SP-C(Deltaexon4) caused a dose-dependent disruption in branching morphogenesis of the lung associated with epithelial cell cytotoxicity. Transient expression of SP-C(Deltaexon4) in isolated type II epithelial cells or HEK293 cells resulted in incomplete processing of the mutant proprotein, a dose-dependent increase in BiP transcription, trapping of the proprotein in the endoplasmic reticulum, and rapid degradation via a proteasome-dependent pathway. Taken together, these data suggest that the g.1728 G --> A mutation causes misfolding of the SP-C proprotein with subsequent induction of the unfolded protein response and endoplasmic reticulum-associated degradation pathways ultimately resulting in disrupted lung morphogenesis.

Effect of exogenous surfactant on the development of surfactant synthesis in premature rabbit lung

Surfactant replacement is an effective therapy for neonatal respiratory distress syndrome. Full recovery from respiratory distress syndrome requires development of endogenous surfactant synthesis and metabolism. The influence of exogenous surfactant on the development of surfactant synthesis in premature lungs is not known. We hypothesized that different exogenous surfactants have different effects on the development of endogenous surfactant production in the premature lung. We treated organ cultures of d 25 fetal rabbit lung for 3 d with 100 mg/kg body weight of natural rabbit surfactant, Survanta, and Exosurf and measured their effects on the development of surfactant synthesis. Additional experiments tested how these surfactants and Curosurf affected surfactant protein (SP) SP-A, SP-B, and SP-C mRNA expression. Surfactant synthesis was measured as the incorporation of 3H-choline and 14C-glycerol into disaturated phosphatidylcholine recovered from lamellar bodies. Randomized-block ANOVA showed significant differences among treatments for incorporation of both labels (p < 0.01), with natural rabbit surfactant less than control, Survanta greater than control, and Exosurf unchanged. Additional experiments with natural rabbit surfactant alone showed no significant effects in doses up to 1000 mg/kg. Survanta stimulated disaturated phosphatidylcholine synthesis (173 +/- 41% of control; p = 0.01), increased total lamellar body disaturated phosphatidylcholine by 22% (p < 0.05), and increased 14C-disat-PC specific activity by 35% (p < 0.05). The response to Survanta was dose-dependent up to 1000 mg/kg. Survanta did not affect surfactant release. No surfactant altered the expression of mRNA for SP-A, SP-B, or SP-C. We conclude that surfactant replacement therapy can enhance the maturation of surfactant synthesis, but this potential benefit differs with different surfactant preparations.

Surfactant protein B inhibits endotoxin-induced lung inflammation

Transgenic mice, in which the level of surfactant protein (SP)-B mature peptide varied 5.6-fold between SP-B(+/-) and SP-B-overexpressing lines (SP-B+/+/+), were used to test the hypothesis that SP-B protects against endotoxin-induced lung inflammation. Intratracheal administration of endotoxin resulted in significantly lower concentration of SP-B mature peptide and elevated levels of total protein in bronchoalveolar lavage fluid of SP-B(+/-) mice compared with SP-B-overexpressing mice, indicating that endotoxin treatment leads to impairment of SP-B expression coincident with increased lung injury in SP-B(+/-) mice. Recruitment of inflammatory cells and elaboration of proinflammatory cytokines in bronchoalveolar lavage fluid were reduced in SP-B-overexpressing mice compared with SP-B(+/-) mice, suggesting that SP-B inhibited endotoxin-induced lung inflammation. Lung compliance and tissue damping were significantly decreased in SP-B(+/+) and SP-B(+/-) mice, but were not changed in SP-B(+/+/+) mice, consistent with a protective effect of SP-B. The minimum surface tension of large aggregate surfactant was significantly lower for surfactant isolated from SP-B-overexpressing mice, both in the absence and the presence of added plasma proteins. These data suggest that SP-B protected against endotoxin-induced lung inflammation by enhancing surfactant function, resulting in reduced lung injury, decreased influx of inflammatory cells, and lower cytokine levels; in contrast, levels of SP-B in SP-B(+/-) mice were further decreased by endotoxin treatment, likely exacerbating lung injury in this group.

Maintenance of the mouse type II cell phenotype in vitro

The purpose of this study was to identify culture conditions for maintenance of isolated mouse type II cells with intact surfactant protein (SP) and phospholipid production. Type II cells were isolated from 6-wk-old mice and cultured on Matrigel matrix-rat tail collagen (70:30 vol/vol) in bronchial epithelial cell growth medium minus hydrocortisone plus 5% charcoal-stripped FBS and 10 ng/ml keratinocyte growth factor. Under these conditions, type II cells actively produced surfactant phospholipids and proteins for at least 7 days. Synthesis and secretion of surfactant phospholipids and SP-A, -B, -C, and -D declined on day 1 of culture but recovered by day 3, reaching levels comparable to or exceeding freshly isolated cells by day 5. Abundant lamellar bodies were readily apparent in cells examined on days 5 and 7, and a surfactant pellet was recovered by centrifugation of media harvested on each day of culture. Secretion of SP-B, SP-C, and phosphatidylcholine was stimulated by phorbol 12-myristate 13-acetate and was inhibited by compound 48/80. When tested with a bubble surfactometer, surfactant secreted by type II cells on day 5 of culture lowered surface tension to 5.2 +/- 2.3 mN/m. This is the first description of the synthesis and secretion of a functional surfactant complex by mouse type II cells after 7 days in primary culture.

A near-field microscopy study of submicron domain structure in a model lung surfactant monolayer

The submicron domain structure of coexisting liquid condensed (LC) and liquid expanded (LE) phases in monolayers composed of palmitic acid and 20 wt% of a lung surfactant protein B fragment has been investigated. Near-field microscopy was used to simultaneously measure topography and fluorescence images of monolayers that were prepared at a surface pressure of 15 mN/m and a temperature of 22 degrees C. The use of a fluorescently tagged peptide allowed for unambiguous determination of the peptide location in the two-component system. The LC and LE phases in the monolayers are measured on the submicron length scale. A 6-11 A height difference between the LC and LE phases was evident in the height images. Gradual transitions between the LC and LE domains were observed across a 1.3 microm length scale in the near-field fluorescence images, but were significantly sharper in the simultaneously collected topography images and in the separately measured AFM images. These results may reflect the occurrence of peptide encroachment into the LC domains.

Genetic disorders of neonatal respiratory function

Genetic risk for respiratory distress in infancy has been recognized with increasing frequency in neonatal intensive care units. Reports of family clusters of affected infants and of ethnic- and gender-based respiratory phenotypes point to the contribution of inheritance. Similarly, different outcomes among gestationally matched infants with comparable exposures to oxygen, mechanical ventilation, or nutritional deficiency also suggest a genetic risk for respiratory distress. Examples of inherited deficiency of surfactant protein B in both humans and genetically engineered murine lineages illustrate the importance of identifying markers of genetic risk. In contrast to developmental, inflammatory, or nutritional causes of respiratory distress that may resolve as infants mature, genetic causes result in both acute and chronic (and potentially irreversible) respiratory failure. The availability of clinically useful genetic markers of risk for respiratory distress in infancy will permit development of rational strategies for treatment of genetic lung disorders of infancy and more accurate counseling of families whose infants are at genetic risk for development of respiratory distress at birth or during early childhood. We review examples of genetic variations known to be associated with or cause respiratory distress in infancy.

Function of surfactant proteins B and C

SP-B is the only surfactant-associated protein absolutely required for postnatal lung function and survival. Complete deficiency of SP-B in mice and humans results in lethal, neonatal respiratory distress syndrome and is characterized by a virtual absence of lung compliance, highly disorganized lamellar bodies, and greatly diminished levels of SP-C mature peptide; in contrast, lung structure and function in SP-C null mice is normal. This review attempts to integrate recent findings in humans and transgenic mice with the results of in vitro studies to provide a better understanding of the functions of SP-B and SP-C and the structural basis for their actions.

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.

Protein-protein interaction of retinoic acid receptor alpha and thyroid transcription factor-1 in respiratory epithelial cells

Surfactant protein B (SP-B) is a 79-amino acid peptide critical to postnatal respiratory adaptation and is developmentally regulated. Previous studies demonstrated that retinoic acid receptors (RARs) and thyroid transcription factor 1 (TTF-1) stimulated SP-B gene expression in respiratory epithelial cells. Clustered retinoic acid-responsive element and TTF-1 binding sites were identified in the enhancer region of the SP-B gene and were required for retinoic acid stimulation of the human SP-B (hSP-B) promoter. In addition, RAR and TTF-1 were colocalized in mouse bronchiolar and alveolar type II epithelial cells, the cellular site of SP-B synthesis. In the present studies, RAR and TTF-1 were colocalized in the nucleus of H441 cells. RAR and TTF-1 synergistically stimulated the hSP-B promoter in H441 cells. Direct protein-protein interactions between RAR and TTF-1 were demonstrated by the glutathione S-transferase pull-down assay and the mammalian cell two hybrid assay. Truncation/deletion studies showed that the RAR-TTF-1 interaction was mediated through the RAR DNA binding domain (DBD) and the TTF-1 homeodomain. RAR DBD greatly enhanced TTF-1 homeodomain DNA binding activity to a hSP-B enhancer oligonucleotide, in which retinoic acid-responsive element and TTF-1 DNA binding sites overlap. Chromatin immunoprecipitation assay demonstrated that retinoic acid treatment of H441 cells greatly stimulated both RAR and TTF-1 DNA binding to the hSP-B enhancer region in H441 cells. These findings support a model in which RAR/retinoid X receptor, TTF-1, and coactivators (p160 members and CBP) form an enhanceosome in the enhancer region of the hSP-B gene.

Imaging of monolayers composed of palmitic acid and lung surfactant protein B

Near-field scanning optical microscopy and atomic force microscopy are used to probe the sub-micrometre phase structure in palmitic acid monolayers containing the 25 peptide amino terminus of lung surfactant protein B (SP-B(1-25)). Monolayers deposited onto mica substrates at a surface pressure of 15 mN m-1 exhibit a two-phase coexistence across a broad range of SP-B(1-25) concentrations. Monolayers containing 5 wt.% SP-B(1-25) or less exhibit an expanse of liquid condensed phase in which elliptical liquid expanded (LE) domains with areas of approximately 25 microm2 coexist. By contrast, monolayers containing 20 wt.% SP-B(1-25) exhibit an expanse of liquid expanded phase in which circular liquid condensed domains coexist. The phase distribution dependence on SP-B(1-25) concentration suggests that the peptide induces disorder in the monolayer.

Surfactant proteins in the digestive tract, mesentery, and other organs: evolutionary significance

For years, the so-called surfactant proteins (SPs) that were discovered in the phospholipid-rich material designated pulmonary surfactant, were considered to be lung-specific. The fact that surfactant-like materials composed of phospholipids are secreted by a number of other organs recently prompted several groups to search for SP expression in these organs also. The hydrophilic proteins SP-A and SP-D and their transcripts have been found in a number of tissues, including gastric and intestinal mucosae, mesothelial tissues (mesentery, peritoneum, and pleura), synovial cells, Eustachian tube and sinus, and possibly in salivary glands, pancreas, and urinary tract. By contrast, the hydrophobic proteins SP-B and SP-C actually appear to be expressed in lung epithelium only. SP-A and SP-D belong to the innate defence system against pathogens and play a role as opsonins for facilitating phagocytosis. Their expression appears as a general feature of organs exposed to pathogens because they present an interface with the external milieu. Although this function has thus far been investigated in the lung only through the gene-targeting approach, increased expression of SP-A in the infected middle ear and of SP-D in the Helicobacter-infected antrum argues for such a function also in other organs. In organs that are not exposed to external pathogens, their role is likely to exert anti-inflammatory and immunomodulatory functions, as suggested by increased SP-A immunoreactivity in rheumatoid disease. SP-A and SP-B have been found in association with phospholipids in the lung of all air-breathing vertebrates, including the most primitive forms represented by lungfish, which implies that the surfactant system had a single evolutionary origin. Immunochemical proximity of the proteins among vertebrates indicates considerable conservation during evolution. Moreover, the finding of an SP-A-like protein in intestine and swim bladder of actinopterygian fish implies that the ancestral form of the protein was already present before the emergence of lung structures.

Post-translational processing of surfactant protein-C proprotein: targeting motifs in the NH(2)-terminal flanking domain are cleaved in late compartments

Rat surfactant protein (SP)-C is a 3.7-kD hydrophobic lung-specific protein generated from proteolytic processing of a 21-kD propeptide (SP-C(21)). We have demonstrated that initial post-translational processing of SP-C(21) involves two cleavages of the COOH-terminus (Beers and colleagues, J. Biol. Chem. 1994;269:20,318--20,328). The goal of the current study was to define processing and function of the NH(2)-terminal flanking domain. Epitope-specific antisera directed against spatially distinct regions of the NH(2) terminus, NPROSP-C(2-9) (epitope = D(2)-L(9)) and NPROSP-C(11-23) (= E(11)-Q(23)) were produced. By Western blotting, both antisera identified SP-C(21) in microsomes. A 6-kD form (SP-C(6)), enriched in lamellar bodies (LBs), was detected only by NPROSP-C(11-23) and not extractable with NaCO(3) treatment. Immunogold staining of ultrathin lung sections with NPROSP-C(11-23) identified proSP-C in both multivesicular bodies (mvb) and LBs whereas NPROSP-C(2-9) labeled only mvb. (35)S-pulse chase analysis demonstrated synthesis of SP-C(21) and three intermediate forms (SP-C(16), SP-C(7), and SP-C(6)). Complete processing involved four separate cleavages with a precursor- product relationship between the low molecular weight forms SP-C(7) and SP-C(6). Fluorescence microscopy of A549 cells expressing fusion proteins of enhanced green fluorescent protein (EGFP) and proSP-C NH(2)-terminal deletion mutants showed targeting of EGFP/SP-C(1-194) and EGFP/SP-C(10-194) to early endosomal antigen-1-negative, CD-63-positive cytoplasmic vesicles whereas EGFP/SP-C(19-194), EGFP/SP-C(Delta 10-18), and EGFP/SP-C(24-194) were restricted to the endoplasmic reticulum (ER). We conclude that synthetic processing includes a previously unrecognized cleavage of the proximal NH(2) terminus (M(1)-L(9)), which occurs after removal of COOH-flanking domains (H(59)-I(194)) but before packaging in LBs, and that the region M(10)-T(18) is required for targeting of proSP-C to post-ER vesicular compartments in the biosynthetic pathway.

Bacterial killing is enhanced by expression of lysozyme in the lungs of transgenic mice

To assess the role of lysozyme in pulmonary host defense in vivo, transgenic mice expressing rat lysozyme cDNA in distal respiratory epithelial cells were generated. Two transgenic mouse lines were established in which the level of lysozyme protein in bronchoalveolar (BAL) lavage fluid was increased 2- or 4-fold relative to that in WT mice. Lung structure and cellular composition of BAL were not altered by the expression of lysozyme. Lysozyme activity in BAL was significantly increased (6.6- and 17-fold) in 5-wk-old animals from each transgenic line. To determine whether killing of bacteria was enhanced by expression of rat lysozyme, 5-wk-old transgenic mice and WT littermates were infected with 10(6) CFU of group B streptococci or 10(7) CFU of a mucoid strain of Pseudomonas aeruginosa by intratracheal injection. Killing of group B streptococci was significantly enhanced (2- and 3-fold) in the mouse transgenic lines at 6 h postinfection and was accompanied by a decrease in systemic dissemination of pathogen. Killing of Pseudomonas aeruginosa was also enhanced in the transgenic lines (5- and 30-fold). Twenty-four hours after administration of Pseudomonas aeruginosa, all transgenic mice survived, whereas 20% of the WT mice died. Increased production of lysozyme in respiratory epithelial cells of transgenic mice enhanced bacterial killing in the lung in vivo, and was associated with decreased systemic dissemination of pathogen and increased survival following infection.

Prolonged survival in hereditary surfactant protein B (SP-B) deficiency associated with a novel splicing mutation

Hereditary surfactant protein B (SP-B) deficiency has been lethal in the first year of life without lung transplantation. We tested the hypothesis that SP-B gene mutations may result in milder phenotypes by investigating the mechanisms for lung disease in two children with less severe symptoms than have been previously observed in SP-B deficiency. Immunostaining patterns for pulmonary surfactant proteins were consistent with SP-B deficiency in both children. DNA sequence analysis indicated that both children were homozygous for a mutation in exon 5 that created an alternative splice site. Reverse transcriptase PCR and sequence analysis confirmed use of this splice site, which resulted in a frameshift and a premature termination codon in exon 7. The predominant reverse transcriptase PCR product, however, lacked exon 7, which restored the reading frame but would not allow translation of the exons that encode mature SP-B. Western blot analysis detected reduced amounts of mature SP-B as well as an aberrant SP-B proprotein that corresponded to the size expected from translation of the abnormal transcript. We conclude that a novel splicing mutation was the cause of lung disease in these children and that hereditary SP-B deficiency can be the cause of lung disease in older children.

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.

Absence of lamellar bodies with accumulation of dense bodies characterizes a novel form of congenital surfactant defect

Two female sibling full-term newborns developed respiratory distress shortly after birth, which progressed to respiratory failure. Tracheal lavage demonstrated presence of surfactant protein A (SP-A), but little surfactant protein B (SP-B), without aberrant surfactant protein C (SP-C). On a lung biopsy performed in both infants, prominent type II pneumocyte hyperplasia was evident. Through ultrastructural examination an absence of normally formed lamellar bodies was determined, with numerous irregular electron dense bodies within the type II pneumocytes. These electron dense bodies could also be identified in the alveolar spaces and alveolar macrophages. No alveolar tubular myelin was present. Abnormally high immunoreactivity for surfactant proteins SP-A, proSP-B, SP-B, and proSP-C was demonstrated by light microscopy. Presence of incompletely processed immunopositive proSP-B, but not proSP-C was observed in the alveolar lumina. No mutations in either the SP-B or SP-C gene were identified by sequence analysis of amplified cDNA. We conclude that these siblings exhibit an inherited surfactant deficiency characterized by abnormal accumulations of surfactant proteins within the pneumocytes. This abnormal accumulation may be due to a primary secretory defect, a defect in surfactant phospholipids, or an abnormal interaction between the phospholipids and surfactant proteins.

Aberrant SP-B mRNA in lung tissue of patients with congenital alveolar proteinosis (CAP)

Mutations in the surfactant protein (SP)-B gene are responsible for SP-B deficiency in congenital alveolar proteinosis (CAP) (Nogee et al. J Clin Invest 1994: 93: 1860-1883; Lin et al. Mol Genet Metab 1998: 64: 25-35; Klein et al. Pediatrics 1998: 132: 244-248; Ballard et al. Pediatrics 1995: 96: 1046-1052). The multigenerational consanguineous pedigree under study does not carry any of the known mutations, although this pedigree had 14 infant deaths following respiratory distress at birth. Immunostaining of the lungs from three such infants revealed decreased or absent SP-B. By sequencing of SP-B exons, exon-intron junctions, and the 5' and 3' flanking regions, nine polymorphisms were found in this pedigree, but none of them could explain the observed SP-B deficiency. Further analysis of SP-B mRNA by reverse transcription-polymerase chain reaction from paraffin-embedded lung tissue of CAP patients showed that SP-B mRNA is not intact. Although the sequence of mRNA from exon 1-exon 7 and from exon 8-exon 10 could be amplified, the region between exons 7 and 8 could not. From fluorescence in situ hybridization of the short arm of chromosome 2p, only 2 signals were identified, eliminating the possibility of translocation as the cause of the SP-B mRNA aberrance. Although the nature of the genetic basis of SP-B deficiency in this family is currently unknown, the existence of aberrant SP-B mRNA may, at least in part, be responsible for the SP-B deficiency in this pedigree.

Intracellular localization of processing events in human surfactant protein B biosynthesis

Surfactant protein B (SP-B) is essential to the function of pulmonary surfactant and to alveolar type 2 cell phenotype. Human SP-B is the 79-amino acid product of extensive post-translational processing of a 381-amino acid preproprotein. Processing involves modification of the primary translation product from 39 to 42 kDa and at least 3 subsequent proteolytic cleavages to produce the mature 8-kDa SP-B. To examine the intracellular sites of SP-B processing, we carried out immunofluorescence cytochemistry and inhibitor studies on human fetal lung in explant culture and isolated type 2 cells in monolayer culture using polyclonal antibodies to human SP-B(8) (Phe(201)-Met(279)) and specific epitopes within the N- (NFProx, Ser(145)-Leu(160); NFlank Gln(186)-Gln(200)) and C-terminal (CFlank, Gly(284)-Ser(304)) propeptides of pro-SP-B. Fluorescence immunocytochemistry using epitope-specific antisera showed colocalization of pro-SP-B with the endoplasmic reticulum resident protein BiP. The 25-kDa intermediate was partially endo H-sensitive, colocalized with the medial Golgi resident protein MG160, and shifted into the endoplasmic reticulum in the presence of brefeldin A, which interferes with anterograde transport from endoplasmic reticulum to Golgi. The 9-kDa intermediate colocalized in part with MG160 but not with Lamp-1, a transmembrane protein resident in late endosomes and lamellar bodies. Brefeldin A induced a loss of colocalization between MG160 and NFlank, shifting NFlank immunostaining to a juxtanuclear tubular array. In pulse-chase studies, brefeldin A blocked all processing of 42-kDa pro-SP-B whereas similar studies using monensin blocked the final N-terminal processing event of 9 to 8 kDa SP-B. We conclude that: 1) the first enzymatic cleavage of pro-SP-B to the 25-kDa intermediate is in the brefeldin A-sensitive, medial Golgi; 2) cleavage of the 25-kDa intermediate to a 9-kDa form is a trans-Golgi event that is slowed but not blocked by monensin; 3) the final cleavage of 9 to 8 kDa SP-B is a monensin-sensitive, post-Golgi event occurring prior to transfer of SP-B to lamellar bodies.

Allelic heterogeneity in hereditary surfactant protein B (SP-B) deficiency

Inability to produce surfactant protein B (SP-B) causes fatal neonatal respiratory disease. A frame-shift mutation (121ins2) is the predominant but not exclusive cause of disease. To determine the range of mechanisms responsible for SP-B deficiency, both alleles from 32 affected infants were characterized. Sixteen infants were homozygous for the 121ins2 mutation, 10 infants were heterozygous for the 121ins2 and another mutation, and six infants were homozygous for other mutations. Thirteen novel SP-B gene mutations were identified, which were not found in a control population. One novel mutation was found in two unrelated families. Surfactant protein expression was evaluated by immunohistochemistry and/or protein blotting. Absence of proSP-B and mature SP-B was associated with nonsense and frame-shift mutations. In contrast, proSP-B expression was associated with missense mutations, or mutations causing in-frame deletions or insertions, and low levels of mature SP-B expression were associated with four mutations. Extracellular staining for proSP-C and/or aberrantly processed SP-C was observed in lungs of all infants with SP-B gene mutations. Hereditary SP-B deficiency is caused by a variety of distinct mutations in the SP-B gene and may be associated with reduced, as well as absent, levels of mature SP-B, likely caused by impaired processing of proSP-B.

Population-based estimates of surfactant protein B deficiency

OBJECTIVE

Surfactant protein B deficiency is a lethal cause of respiratory distress in infancy that results most commonly from a homozygous frameshift mutation (121ins2). Using independent clinical ascertainment and molecular methods in different populations, we sought to determine allele frequency.

STUDY DESIGN

Using clinical characteristics of the phenotype of affected infants, we screened the Missouri linked birth-death database (n = 1 052 544) to ascertain potentially affected infants. We used molecular amplification and restriction enzyme digestion of DNA samples from a metropolitan New York birth cohort (n = 6599) to estimate allele frequency.

RESULTS

The point estimate and 95% confidence interval of the 121ins2 allele frequency in the Missouri cohort are 1/1000 individuals (.03-5.6/1000) and in the New York cohort are.15/1000 (. 08-.25/1000). These estimates are not statistically different.

CONCLUSIONS

The close approximation of these independent estimates suggests accurate gene frequency (approximately one 121ins2 mutation per 1000-3000 individuals) despite its rare occurrence and that this mutation does not account for the majority of full-term infants with lethal respiratory distress.

The role of homodimers in surfactant protein B function in vivo

Surfactant protein B (SP-B) is detected in the airways as a sulfhydryl-dependent dimer (M(r) approximately 16,000). To test the hypothesis that formation of homodimers is critical for SP-B function, the cysteine residue reported to be involved in SP-B dimerization was mutated to serine (Cys(248) --> Ser) and the mutated protein was targeted to the distal respiratory epithelium of transgenic mice. Transgenic lines which demonstrated appropriate processing, sorting, and secretion of human SP-B monomer were crossed with SP-B +/- mice to achieve expression of human monomer in the absence of endogenous SP-B dimer (hSP-B(mon), mSP-B-/-). In two of three transgenic lines, hSP-B(mon), mSP-B-/- mice had normal lung structure, complete processing of SP-C proprotein, well formed lamellar bodies, and normal longevity. Pulmonary function studies revealed an altered hysteresis curve for hSP-B(mon), mSP-B-/- mice relative to wild type mice. Large aggregate surfactant fractions from hSP-B(mon), mSP-B-/- mice resulted in higher minimum surface tension in vitro compared with surfactant from wild type mice. Surfactant lipids supplemented with 2% hSP-B monomer resulted in slower adsorption and higher surface tension than surfactant with 2% hSP-B dimer. Taken together, these data indicate a role for SP-B dimer in surface tension reduction in the alveolus.

Ablation of a critical surfactant protein B intramolecular disulfide bond in transgenic mice

The 79-amino acid, mature SP-B peptide contains three intramolecular disulfide bonds shared by all saposin-like proteins. This study tested the hypothesis that the disulfide bond formed between cysteine residues 35 and 46 (residues 235 and 246 of the SP-B proprotein) is essential for proper function of SP-B. To test the role of this bridge in SP-B function in vivo, a construct was generated in which cysteine residues 235 and 246 of the human SP-B proprotein were mutated to serine and cloned under the control of the 3.7-kilobase hSP-C promoter (hSP-B(C235S/C246S)). In two transgenic mouse lines, expression of the mutant peptide in the wild-type murine SP-B background was invariably lethal in the neonatal period. In four additional lines, survival was inversely related to the level of transgene expression. To test the ability of the mutant peptide to functionally replace the wild-type protein, transgenic mice were crossed into the SP-B null background. No animals that expressed hSP-B(C235S/C246S) in the murine SP-B-/- background survived the neonatal period. hSP-B(C235S/C246S) proprotein accumulated in the endoplasmic reticulum and was not processed to the mature, biologically active peptide. The results of these studies demonstrate that the intramolecular bridge between residues 235 and 246 is critical for intracellular trafficking of SP-B and suggest that overexpression of mutant SP-B in the wild-type background may be lethal.

Structural requirements for intracellular targeting of SP-C proprotein

Rat surfactant protein (SP) C is synthesized as a 194-amino acid proprotein that is proteolytically processed to a 35-amino acid mature form in subcellular compartments distal to the medial Golgi compartment. To identify domains of SP-C proprotein (proSP-C) necessary for endoplasmic reticulum translocation and for targeting to cytosolic processing compartments, we characterized expression patterns of heterologous SP-C fusion proteins in A549 lung epithelial cells and in the rat pheochromocytoma cell line PC-12. cDNA constructs were produced; these constructs encoded fusion proteins consisting of enhanced green fluorescent protein (EGFP) and wild-type proSP-C (EGFP/SP-C(1-194)), mature SP-C (EGFP/SP-C(24-59)), or progressive deletions of the NH(2)- or COOH-terminal flanking domains. By fluorescence microscopy, EGFP/SP-C(1-194) transfected into A549 cells was translocated and expressed in acidic cytoplasmic vesicles. By deletional analysis, a functional signal peptide was mapped to the domain Phe(24) to His(59), whereas a motif for targeting to cytosolic vesicular compartments was localized to the NH(2) flanking domain Met(10) to Gln(23). Truncations of the distal COOH terminus were retained in the endoplasmic reticulum/Golgi compartment; however, the COOH flanking region alone was insufficient for targeting. In PC-12 cells, EGFP/SP-C(1-194) was expressed in peripheral cytosolic vesicles, whereas EGFP/SP-C(24-194) and EGFP/SP-C(24-59) were each translocated but not targeted. We conclude that two domains in the proSP-C sequence are required for targeting: mature SP-C (Phe(24) to Leu(58)) contains a functional signal sequence active in epithelial and nonepithelial cells, whereas Met(10) to Gln(23), but not the COOH flanking peptide, is required for targeting to late vesicular compartments.