Altered stability of pulmonary surfactant in SP-C-deficient mice

Defective surfactant secretion in a mouse model of Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) in humans represents a family of disorders of lysosome-related organelle biogenesis associated with severe, progressive pulmonary disease. Human case reports and a mouse model of HPS, the pale ear/pearl mouse (ep/pe), exhibit giant lamellar bodies (GLB) in type II alveolar epithelial cells. We examined surfactant proteins and phospholipid from ep/pe mice to elucidate the process of GLB formation. The 2.8-fold enrichment of tissue phospholipids in ep/pe mice resulted from accumulation from birth through adulthood. Tissue surfactant protein (SP)-B and -C were increased in adult ep/pe mice compared with wild-type mice (WT), whereas SP-A and -D were not different. Large aggregate surfactant (LA) from adult ep/pe mice had decreased phospholipid, SP-B, and SP-C, with no differences in SP-A and -D compared with WT. Although LA from ep/pe animals exhibited an increased total protein-to-total phospholipid ratio compared with WT, surface tension was not compromised. Phospholipid secretion from isolated type II cells showed that basal and stimulated secretion from ep/pe cells were approximately 50% of WT cells. Together, our data indicate that GLB formation is not associated with abnormal trafficking or recycling of surfactant material. Instead, impaired secretion is an important component of GLB formation in ep/pe mice.

Insuficiencia respiratoria neonatal asociada a mutación en el gen de la proteína C del surfactante

Respiratory insufficiency in a term infant during the first weeks of life is unusual. Possible causes include interstitial or diffuse lung disease, which are a heterogeneous group of mostly idiopathic disorders, characterized by diffuse infiltrates, restrictive functional defect, and disordered gas exchange. A form of interstitial lung disease that can affect infants, children or young adults is that associated with congenital surfactant protein B or C deficiency, in which the inflammatory process leading to interstitial fibrosis is preceded by the accumulation of proteinaceous material in the alveolar space. We assessed the role of potential abnormalities in the surfactant proteins B and C in a Spanish family in which two infants showed progressive neonatal respiratory failure associated with radiological and pathological alterations compatible with interstitial lung disease. The father had a history of respiratory disease since childhood. The two affected children in this family had abnormal expression of surfactant C precursor protein, with markedly decreased levels of the mature protein. Moreover, a previously unreported mutation in the gene encoding surfactant protein C, which was found in this family, is described.

Surfactant protein C biosynthesis and its emerging role in conformational lung disease

Surfactant protein C (SP-C) is a hydrophobic 35-amino acid peptide that co-isolates with the phospholipid fraction of lung surfactant. SP-C represents a structurally and functionally challenging protein for the alveolar type 2 cell, which must synthesize, traffic, and process a 191-197-amino acid precursor protein through the regulated secretory pathway. The current understanding of SP-C biosynthesis considers the SP-C proprotein (proSP-C) as a hybrid molecule that incorporates structural and functional features of both bitopic integral membrane proteins and more classically recognized luminal propeptide hormones, which are subject to post-translational processing and regulated exocytosis. Adding to the importance of a detailed understanding of SP-C biosynthesis has been the recent association of mutations in the proSP-C sequence with chronic interstitial pneumonias in children and adults. Many of these mutations involve either missense or deletion mutations located in a region of the proSP-C molecule that has structural homology to the BRI family of proteins linked to inherited degenerative dementias. This review examines the current state of SP-C biosynthesis with a focus on recent developments related to molecular and cellular mechanisms implicated in the emerging role of SP-C mutations in the pathophysiology of diffuse parenchymal lung disease.

Monolayer–multilayer transitions in a lung surfactant model: IR reflection–absorption spectroscopy and atomic force microscopy

A hydrophobic pulmonary surfactant protein, SP-C, has been implicated in surface-associated activities thought to facilitate the work of breathing. Model surfactant films composed of lipids and SP-C display a reversible transition from a monolayer to surface-associated multilayers upon compression and expansion at the air/water (A/W) interface. The molecular-level mechanics of this process are not yet fully understood. The current work uses atomic force microscopy on Langmuir-Blodgett films to verify the formation of multilayers in a dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, cholesterol, and SP-C model system. Isotherms of SP-C-containing films are consistent with exclusion and essentially complete respreading during compression and expansion, respectively. Multilayer formation was not detected in the absence of SP-C. Most notable are the results from IR reflection-absorption spectroscopy (IRRAS) conducted at the A/W interface, where the position and intensity of the Amide I band of SP-C reveal that the predominantly helical structure changes its orientation in monolayers versus multilayers. IRRAS measurements indicate that the helix tilt angle changed from approximately 80 degrees in monolayers to a transmembrane orientation in multilayers. The results constitute the first quantitative measure of helix orientation in mixed monolayer/multilamellar domains at the A/W interface and provide insight into the molecular mechanism for SP-C-facilitated respreading of surfactant.

Nonspecific interstitial pneumonia, alveolar proteinosis, and abnormal proprotein trafficking resulting from a spontaneous mutation in the surfactant protein C gene

Human surfactant protein C (hSP-C(1-197)) is synthesized as a 197 amino acid proprotein and cleaved to a mature 3.7 kD form. Although interstitial lung disease in patients with mutations of the hSP-C gene is becoming increasingly recognized, the mechanisms linking molecular events with clinical pathogenesis are not fully defined. We describe a full-term infant with respiratory insufficiency associated with a spontaneous heterozygous mutation resulting in a substitution of lysine for glutamic acid at position 66 (= E66K) of the proximal hSP-C COOH flanking propeptide. Lung histology and biochemical studies of the index patient (hSP-C(E66K)) revealed nonspecific interstitial pneumonia, increased alveolar total phospholipid lacking phosphatidylglycerol, and increased surfactant protein A. Localization of proSP-C from lung sections prepared from this patient using immunofluorescence and immunogold electron microscopy revealed abnormal proSP-C staining in endosomal-like vesicles of type II cells distinct from SP-B. To evaluate the effect of the E66K substitution on intracellular trafficking of proSP-C, fusion proteins consisting of enhanced green fluorescent protein (EGFP) and hSP-C(1-197) (wild type) or mutant hSP-C(E66K) were generated and transfected into A549 cells. EGFP/hSP-C(1-197) was expressed within CD-63-positive, EEA-1-negative vesicles, whereas EGFP/hSP-C(E66K) localized to EEA-1 positive vesicles. The E66K substitution is representative of a new class of SP-C mutation associated with interstitial lung disease that is diverted from the normal biosynthetic pathway. We propose that, similar to other storage disorders, lung injury results from induction of a toxic gain of function induced by the mutant product that is subject to genetic modifiers and environmental influences.

Simple, Helical Peptoid Analogs of Lung Surfactant Protein B

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

Increased Phosphatidylcholine Production but Disrupted Glycogen Metabolism in Fetal Type II Cells of Mice That Overexpress CTP:Phosphocholine Cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCT) is a rate-determining enzyme in the de novo synthesis of phosphatidylcholine (PtdCho). Alveolar type II cells synthesize large quantities of disaturated PtdCho, the surface-active agent of pulmonary surfactant, particularly at late gestation when the lung prepares itself for postnatal air breathing. To clarify the role of CCTalpha in lung surfactant maturation, we overexpressed CCTalpha(1-367) using the surfactant protein-C promoter. Lungs of transgenic mice were analyzed at day 18 of gestation (term = 19 days). Overexpression of CCTalpha(1-367) increased the synthesis and content of PtdCho in fetal type II cells isolated from the transgenic mice. Also, PtdCho content of fetal lung fluid was increased. No changes in surfactant protein content were detected. Interestingly, fetal type II cells of transgenic mice contained more glycogen than control cells. Incorporation studies with [U-(14)C]glucose demonstrated that overexpression of CCTalpha(1-367) in fetal type II cells increased glycogen synthesis without affecting glycogen breakdown. To determine which domain contributes to this glycogen phenotype, two additional transgenes were created overexpressing either CCTalpha(1-239) or CCTalpha(239-367). Glycogen synthesis and content were increased in fetal type II cells expressing CCTalpha(239-367) but not CCTalpha(1-239)(.) We conclude that overexpression of CCTalpha increases surfactant PtdCho synthesis without affecting surfactant protein levels but that it disrupts glycogen metabolism in differentiating type II cells via its regulatory domain.

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.

Pulmonary pathology

Common causes of neonatal respiratory distress include meconium aspiration, pneumonia, persistent pulmonary hypertension of the newborn, pneumothorax and cystic adenomatoid malformation. Genomics and proteomics have enabled the recent recognition of several additional disorders that lead to neonatal death from respiratory disease. These are broadly classified as disorders of lung homeostasis and have pathological features of proteinosis, interstitial pneumonitis or lipidosis. These pathological changes result from inherited disorders of surfactant proteins or granulocyte-macrophage colony stimulating factor. Abnormal lung vascular development is the basis for another cause of fatal neonatal respiratory distress, alveolar capillary dysplasia with or without associated misalignment of veins. Diagnosis of these genetically transmitted disorders is important because of the serious implications for future siblings. There is also a critical need for establishing an archival tissue bank to permit future molecular biological studies.

Alveolus formation: what have we learned from genetic studies?

The respiratory system has two basic functions: air exchange and pathogen clearance. The conducting airway and alveolar parenchyma are the basic structures to fulfill these functions during respiratory cycles. In humans, there are ∼40 cell types in the lung that coordinately work together through various structural and signaling molecules. These molecules are vital for maintaining normal lung functions in response to environmental changes. Aberrant expression of these molecules can jeopardize human health and cause various pulmonary diseases. In this article, we will review some recent progress made in the pulmonary field, using genetic animal model systems to elucidate molecular mechanisms that are important for alveolar formation and lung diseases.

Inherited interstitial lung disease

This article focuses on recent advances in the identification of genes and genetic polymorphisms that have been implicated in the development of human interstitial lung diseases. It focuses on the inherited mendelian diseases in which pulmonary fibrosis is part of the clinical phenotype and the genetics of familial idiopathic pulmonary fibrosis and other rare inherited interstitial lung diseases. The article also reviews the association studies that have been published to date regarding the genetics of sporadic idiopathic pulmonary fibrosis. The reader is directed to recent reviews on human genetic predisposition of sarcoidosis, environmental-related, drug-related, connective tissue related pulmonary fibrosis, and genetic predisposition of fibrosis in animal models.

Distinct effects of SP-B and SP-C on the uptake of surfactant-like liposomes by alveolar cells in vivo and in vitro

The effects of surfactant protein B (SP-B) and SP-C on the uptake of surfactant-like liposomes by alveolar type II cells and alveolar macrophages were studied both in vivo and in vitro. In vivo, mechanically ventilated rats were intratracheally instilled with fluorescently labeled liposomes that had SP-B and/or SP-C incorporated in different concentrations. Consequently, the alveolar cells were isolated, and cell-associated fluorescence was determined using flow cytometry. The results show that the incorporation of SP-B does not influence the uptake, and it also does not in the presence of essential cofactors. The inclusion of SP-C in the liposomes enhanced the alveolar type II cells at a SP-C to lipid ratio of 2:100. If divalent cations (calcium and magnesium) were present at physiological concentrations in the liposome suspension, uptake of liposomes by alveolar macrophages was also enhanced. In vitro, the incorporation of SP-B affected uptake only at a protein-to-lipid ratio of 8:100, whereas the inclusion of SP-C in the liposomes leads to an increased uptake at a protein-to-lipid ratio of 1:100. From these results, it can be concluded that SP-B is unlikely to affect uptake of surfactant, whereas SP-C in combination with divalent cations and other solutes are capable of increasing the uptake.

Genetic mechanisms of surfactant deficiency

The production of pulmonary surfactant is necessary to maintain alveolar stability and normal lung function. Mutations in three different genes important for surfactant production and function have now been recognized to result in surfactant deficiency and acute and/or chronic lung disease. The clinical and laboratory features associated with these genetic disorders, along with their implications for the understanding of normal surfactant metabolism, are reviewed.

Progressive Lung Disease and Surfactant Dysfunction with a Deletion in Surfactant Protein C Gene

Mutations in the surfactant protein (SP)-C gene are responsible for familial and sporadic interstitial lung disease (ILD). The consequences of such mutations on pulmonary surfactant composition and function are poorly understood. To determine the effects of a mutation in the SP-C gene on surfactant, we obtained lung tissue at the time of transplantation from a 14-mo-old infant with progressive ILD. An in-frame 9-bp deletion spanning codons 91-93 in Exon 3 of the SP-C gene was present on one allele; neither parent carried this deletion. SP-C mRNA was present in normal size and amount. By immunofluorescence, proSP-C was aggregated within alveolar Type II cells in a compartment separate from SP-B. In airway surfactant, there was little or no mature SP-B or SP-C; SP-A content was increased. Minimum surface tension was increased (20 mN/m, normal < 5 mN/m). Type II cells contained normal and disorganized appearing lamellar bodies by electron microscopy. This spontaneous deletion on one allele of the SP-C gene was associated with sporadic ILD and abnormalities in surfactant composition and function. We propose that a dominant negative effect on surfactant protein metabolism and function results from aggregation of misfolded proSP-C and subsequent cell injury and inflammation.

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.

Mutation of SFTPC in infantile pulmonary alveolar proteinosis with or without fibrosing lung disease

Pulmonary surfactant protein C (SP-C) is a highly hydrophobic peptide produced by type-II alveolar cells through the processing of a high-molecular weight precursor (pro-SP-C), that enhances surface tension and facilitates the recycling of pulmonary surfactant in vitro. Recently, two seemingly dominant-negative mutations of the pro-SP-C-encoding gene (SFTPC, MIM 178620), were reported in families with vertically-inherited interstitial lung disease (Nogee et al. [2001: N Engl J Med 344:573-579]; Thomas et al. [2002: Am J Respir Crit Care Med 165:1322-1328]). We have examined the SP-C protein and its precursor as well as the encoding gene, in a cohort of 34 sporadic or familial cases with unexplained respiratory distress (URD) in which surfactant protein B (SP-B) deficiency related to SFTPB mutation had been ruled out. One patient with complete SP-C deficiency had no detectable mutation of SFTPC. Of the 10 patients with abnormal pro-SP-C processing, as suggested from analysis of broncho-alveolar lavage (BAL) fluid, two distinct heterozygous SFTPC missense mutations were identified. The first, g.1286T > C (p.I73T), was de novo and resulted in progressive respiratory failure with intra-alveolar storage of a granular, protein- and lipid-rich, periodic acid Schiff (PAS)-positive material (pulmonary alveolar proteinosis (PAP)), and interstitial lung disease. The second, g.2125G > A (p.R167Q), was found in two PAP patients from the endogamous white settler population of Réunion Island in which URD has an unexpectedly high prevalence. Since this mutation was diagnosed in subjects from this subpopulation who did not have evidence for lung disease, we propose environmental exposures or modifier genes to play a role in the phenotype, as suggested from murine models lacking the SP-C protein, although we cannot rule out a rare polymorphism, hitherto restricted to that subpopulation. Most remarkably, these observations extend the phenotypic spectrum related to SFTPC mutation from interstitial lung disease to PAP. Notably, the reported mutations do not appear to be dominant negatives. This article contains supplementary material, which may be viewed at the American Journal of Medical Genetics website at http://www.interscience.wiley.com/jpages/0148-7299/suppmat/index.html.

Stat-3 is required for pulmonary homeostasis during hyperoxia

Acute lung injury syndromes remain common causes of morbidity and mortality in adults and children. Cellular and physiologic mechanisms maintaining pulmonary homeostasis during lung injury remain poorly understood. In the present study, the Stat-3 gene was selectively deleted in respiratory epithelial cells by conditional expression of Cre-recombinase under control of the surfactant protein C gene promoter. Cell-selective deletion of Stat-3 in respiratory epithelial cells did not alter prenatal lung morphogenesis or postnatal lung function. However, exposure of adult Stat-3-deleted mice to 95% oxygen caused a more rapidly progressive lung injury associated with alveolar capillary leak and acute respiratory distress. Epithelial cell injury and inflammatory responses were increased in the Stat-3-deleted mice. Surfactant proteins and lipids were decreased or absent in alveolar lavage material. Intratracheal treatment with exogenous surfactant protein B improved survival and lung histology in Stat-3-deleted mice during hyperoxia. Expression of Stat-3 in respiratory epithelial cells is not required for lung formation, but plays a critical role in maintenance of surfactant homeostasis and lung function during oxygen injury.

Stat-3 is required for pulmonary homeostasis during hyperoxia

Acute lung injury syndromes remain common causes of morbidity and mortality in adults and children. Cellular and physiologic mechanisms maintaining pulmonary homeostasis during lung injury remain poorly understood. In the present study, the Stat-3 gene was selectively deleted in respiratory epithelial cells by conditional expression of Cre-recombinase under control of the surfactant protein C gene promoter. Cell-selective deletion of Stat-3 in respiratory epithelial cells did not alter prenatal lung morphogenesis or postnatal lung function. However, exposure of adult Stat-3-deleted mice to 95% oxygen caused a more rapidly progressive lung injury associated with alveolar capillary leak and acute respiratory distress. Epithelial cell injury and inflammatory responses were increased in the Stat-3-deleted mice. Surfactant proteins and lipids were decreased or absent in alveolar lavage material. Intratracheal treatment with exogenous surfactant protein B improved survival and lung histology in Stat-3-deleted mice during hyperoxia. Expression of Stat-3 in respiratory epithelial cells is not required for lung formation, but plays a critical role in maintenance of surfactant homeostasis and lung function during oxygen injury.

From birds to humans: new concepts on airways relative to alveolar surfactant

Pulmonary surfactant is a surface-active mixture of phospholipids and specific proteins that lines the epithelial surfaces of mammalian lungs. In the alveoli, its main function is to reduce surface tension to ensure that these structures can remain open during respiratory cycles of contraction and expansion. However, surfactant is also present in the conducting airways, even though they are relatively rigid and do not need a system capable of rapidly lowering surface tension in response to compression. This has raised the question whether there is a difference in composition and function between airway and alveolar surfactant. Interest in this question has been stimulated further by the recognition that surfactant also has important functions in the immune defenses of the respiratory tract. In this review, we describe differences that have been reported between human airway and alveolar surfactant. In addition, we draw parallels between human airway surfactant and surfactant from the lungs of birds. The latter are tubular and rigid and do not undergo cycles of contraction and expansion, thus more resembling the human conducting airways than alveoli. Using this as a model, we propose a new hypothesis to explain structural and functional differences between human airway and alveolar surfactant. We suggest that the molecular composition of surfactant is adapted to differences in the architecture of pulmonary surfaces and to the dynamics of surface area changes during respiration.

Surfactant therapy for respiratory distress syndrome in premature neonates: a comparative review

Exogenous surfactant therapy has been part of the routine care of preterm neonates with respiratory distress syndrome (RDS) since the beginning of the 1990s. Discoveries that led to its development as a therapeutic agent span the whole of the 20th century but it was not until 1980 that the first successful use of exogenous surfactant therapy in a human population was reported. Since then, randomized controlled studies demonstrated that surfactant therapy was not only well tolerated but that it significantly reduced both neonatal mortality and pulmonary air leaks; importantly, those surviving neonates were not at greater risk of subsequent neurological impairment. Surfactants may be of animal or synthetic origin. Both types of surfactants have been extensively studied in animal models and in clinical trials to determine the optimum timing, dose size and frequency, route and method of administration. The advantages of one type of surfactant over another are discussed in relation to biophysical properties, animal studies and results of randomized trials in neonatal populations. Animal-derived exogenous surfactants are the treatment of choice at the present time with relatively few adverse effects related largely to changes in oxygenation and heart rate during surfactant administration. The optimum dose of surfactant is usually 100 mg/kg. The use of surfactant with high frequency oscillation and continuous positive pressure modes of respiratory support presents different problems compared with its use with conventional ventilation. The different components of surfactant have important functions that influence its effectiveness both in the primary function of the reduction of surface tension and also in secondary, but nonetheless just as important, role of lung defense. With greater understanding of the individual surfactant components, particularly the surfactant-associated proteins, development of newer synthetic surfactants has been made possible. Despite being an effective therapy for RDS, surfactant has failed to have a significant impact on the incidence of chronic lung disease in survivors. Paradoxically the cost of care has increased as surviving neonates are more immature and consume a greater proportion of neonatal intensive care resources. Despite this, surfactant is considered a cost-effective therapy for RDS compared with other therapeutic interventions in premature infants.

Site of inflammation influences site of hyperresponsiveness in experimental asthma

BACKGROUND

Our recently developed murine asthma model is capable of inducing airway-specific chronic inflammatory changes and remodeling, features of human asthma commonly missing in conventional animal models.

OBJECTIVES

To validate this model by site-specific physiological evaluation of hyperresponsiveness.

METHODS

Non-sensitized and sensitized mice received either short-term uncontrolled or long-term controlled low-level exposures to aerosolized ovalbumin (OVA). Respiratory impedance (Zrs) was measured in response to increasing doses of methacholine (Mch). The constant-phase model was fitted to Zrs spectra to determine the specific site of hyperresponsiveness.

RESULTS

Sensitized acutely exposed mice had significantly increased tissue damping (G), tissue elastance (H) and hysteresivity (eta) in response to Mch, but no significant increase in airway resistance (Raw), indicating tissue-specific hyperresponsiveness. In contrast, sensitized chronically exposed mice had significantly elevated Raw at all concentrations of Mch but no increases in G, H or eta indicating airway-specific hyperresponsiveness.

CONCLUSIONS

Chronic inhalational exposure of sensitized mice to low-mass concentrations of OVA induces airway-specific hyperresponsiveness.

The N-terminal segment of pulmonary surfactant lipopeptide SP-C has intrinsic propensity to interact with and perturb phospholipid bilayers

In the present study, 13-residue peptides with sequences corresponding to the native N-terminal segment of pulmonary SP-C (surfactant protein C) have been synthesized and their interaction with phospholipid bilayers characterized. The peptides are soluble in aqueous media but associate spontaneously with bilayers composed of either zwitterionic (phosphatidylcholine) or anionic (phosphatidylglycerol) phospholipids. The peptides show higher affinity for anionic than for zwitterionic membranes. Interaction of the peptides with both zwitterionic and anionic membranes promotes phospholipid vesicle aggregation, and leakage of the aqueous content of the vesicles. The lipid-peptide interaction includes a significant hydrophobic component for both zwitterionic and anionic membranes, although the interaction with phosphatidylglycerol bilayers is also electrostatic in nature. The effects of the SP-C N-terminal peptides on the membrane structure are mediated by significant perturbations of the packing order and mobility of phospholipid acyl chain segments deep in the bilayer, as detected by differential scanning calorimetry and spin-label ESR. These results suggest that the N-terminal region of SP-C, even in the absence of acylation, possesses an intrinsic propensity to interact with and perturb phospholipid bilayers, thereby potentially facilitating SP-C promoting bilayer-monolayer transitions at the alveolar spaces.

Surfactant protein profile of pulmonary surfactant in premature infants

Although premature infants are known to be deficient in pulmonary surfactant, there is limited information regarding surfactant protein (SP) composition. To assess the postnatal profile of SPs, tracheal aspirate samples were collected from 35 intubated infants of 23-31 weeks of gestation between 8 and 80 days of age. In 71 large aggregate surfactant samples that had normal in vitro function (minimum surface tension of less than 1 mN/m by pulsating bubble surfactometry), mean +/- SEM contents of SP-A, SP-B, and SP-C (3.7 kD) were 7.1 +/- 1.4%, 1.8 +/- 0.2%, and 4.6 +/- 0.6%, respectively, of phospholipid. To assess SPs in the 1st week of life, we analyzed samples from additional infants receiving only synthetic replacement surfactant. On the 2nd day of life, contents of SP-A, SP-B, and SP-C were 13.4%, 8.4%, and 0.1%, respectively, of the mean levels for Day 8-80 samples. The major postnatal increases for SP-A, SP-B, and SP-C occurred during the 1st, 2nd, and 3rd weeks, respectively. We conclude that surfactant of newborn premature infants is markedly deficient in SPs, in particular SP-C. Despite continuing lung disease, some infants who are more than 1 week of age have surfactant with normal in vitro function that contains SPs at levels comparable to adult surfactant.

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.

Aberrant lung structure, composition, and function in a murine model of Hermansky-Pudlak syndrome

Hermansky-Pudlak syndrome (HPS) is a genetically heterogeneous inherited disease causing hypopigmentation and prolonged bleeding times. An additional serious clinical problem of HPS is the development of lung pathology, which may lead to severe lung disease and premature death. No cure for the disease exists, and previously, no animal model for the HPS lung abnormalities has been reported. A mouse model of HPS, which is homozygously recessive for both the Hps1 (pale ear) and Hps2 (pearl) genes, exhibits striking abnormalities of lung type II cells. Type II cells and lamellar bodies of this mutant are greatly enlarged, and the lamellar bodies are engorged with surfactant. Mutant lungs accumulate excessive autofluorescent pigment. The air spaces of mutant lungs contain age-related elevations of inflammatory cells and foamy macrophages. In vivo measurement of lung hysteresivity demonstrated aberrant lung function in mutant mice. All these features are similar to the lung pathology described in HPS patients. Morphometry of mutant lungs indicates a significant emphysema. These mutant mice provide a model to further investigate the lung pathology and therapy of HPS. We hypothesize that abnormal type II cell lamellar body structure/function may predict future lung pathology in HPS.

SP-B deficiency causes respiratory failure in adult mice

Targeted deletion of the surfactant protein (SP)-B locus in mice causes lethal neonatal respiratory distress. To assess the importance of SP-B for postnatal lung function, compound transgenic mice were generated in which the mouse SP-B cDNA was conditionally expressed under control of exogenous doxycycline in SP-B-/- mice. Doxycycline-regulated expression of SP-B fully corrected lung function in compound SP-B-/- mice and protected mice from respiratory failure at birth. Withdrawal of doxycycline from adult compound SP-B-/- mice resulted in decreased alveolar content of SP-B, causing respiratory failure when SP-B concentration was reduced to <25% of normal levels. Decreased SP-B was associated with low alveolar content of phosphatidylglycerol, accumulation of misprocessed SP-C proprotein in the air spaces, increased protein content in bronchoalveolar lavage fluid, and altered surfactant activity in vitro. Consistent with surfactant dysfunction, hysteresis, maximal tidal volumes, and end expiratory volumes were decreased. Reduction of alveolar SP-B content causes surfactant dysfunction and respiratory failure, indicating that SP-B is required for postnatal lung function.

Surfactant biology and clinical application

There is strong evidence that alterations in the pulmonary surfactant system play an important role in the pathophysiology of lung disease, including ARDS . Although it is still unclear whether mortality and morbidity of ARDS will be reduced, surfactant replacement therapy has been shown to improve oxygenation, improve lung compliance, and decrease the need for ventilatory support. The critical need for more standardized studies with one type of intratracheal surfactant and uniform measurements of surfactant proteins and phospholipids by BAL is evident. Further studies will also be needed to elucidate the optimal timing and dosage regimen for different disease processes. Some evidence supports the measurements of surfactant protein levels as markers for predicting the onset and outcome of ARDS and perhaps providing a window for early treatment of patients at risk to develop ARDS. Continued investigation into the role of surfactant in the immune regulation of the lung may also provide additional information to support the efficacy of surfactant replacement in lung disease.

The role of exogenous surfactant in the treatment of acute lung injury

A number of conditions, such as pneumonia, trauma, or systemic sepsis arising from the gut, may result in the acute respiratory distress syndrome (ARDS). Because of its significant morbidity and mortality, ARDS has been the focus of extensive research. One specific area of interest has been the investigation of the role of the surfactant system in the pathophysiology of this disease. Several studies have demonstrated that alterations of surfactant contribute to the lung dysfunction associated with ARDS, which has led to investigations into the use of exogenous surfactant as a therapy for this syndrome. Clinical experience with surfactant therapy has been variable owing to a number of factors including the nature of the injury at the time of treatment, the specific surfactant preparation utilized, the dose and delivery method chosen, the timing of surfactant administration over the course of the disease, and the mode of ventilation used during and after surfactant administration.

The transmembrane domain of surfactant protein C precursor determines the morphology of the induced membrane compartment in CHO cells

Surfactant protein C (SP-C) is a small lipopeptide of which the main part consists of a typical valyl-rich transmembrane domain. The protein is expressed as a propeptide (proSP-C) which is processed and sorted via the regulated secretory pathway to the lamellar body, where mature SP-C is stored before secretion into the alveolar space. In this study we investigated the identity of the compartment to which proSP-C is sorted in cells that do not have a regulated secretory pathway, such as CHO cells. By electron microscopy we determined that proSP-C was localized in an uncommon membrane compartment with very regular morphology, which was not present in control cells. This membrane compartment is not influenced by the palmitoylation of proSP-C and is probably derived from the endoplasmic reticulum. However, proSP-C chimeras with artificial transmembrane domains induced a membrane compartment with a different morphology. Therefore we propose that the typical amino acid sequence of the transmembrane domain of proSP-C plays a role in membrane formation and morphology, which may be relevant under physiological conditions.

Pneumonitis and emphysema in sp-C gene targeted mice

SP-C-deficient (SP-C -/-) mice developed a severe pulmonary disorder associated with emphysema, monocytic infiltrates, epithelial cell dysplasia, and atypical accumulations of intracellular lipids in type II epithelial cells and alveolar macrophages. Whereas alveolar and tissue surfactant phospholipid pools were increased, levels of other surfactant proteins were not altered (SP-B) or were modestly increased (SP-A and SP-D). Analysis of pressure-volume curves and forced oscillatory dynamics demonstrated abnormal respiratory mechanics typical of emphysema. Lung disease was progressive, causing weight loss and cardiomegaly. Extensive alveolar remodeling was accompanied by type II cell hyperplasia, obliteration of pulmonary capillaries, and widespread expression of alpha-smooth muscle actin, indicating myofibroblast transformation in the lung parenchyma. Dysplastic epithelial cells lining conducting airways stained intensely for the mucin, MUC5A/C. Tissue concentrations of proinflammatory cytokines were not substantially altered in the SP-C (-/-) mice. Production of matrix metalloproteinases (MMP-2 and MMP-9) was increased in alveolar macrophages from SP-C (-/-) mice. Absence of SP-C caused a severe progressive pulmonary disorder with histologic features consistent with interstitial pneumonitis.

Volume dependence of airway and tissue impedances in mice

We measured respiratory input impedance (1-25 Hz) in mice and obtained parameters for airway and tissue mechanics by model fitting. Lung volume was varied by inflating to airway opening pressure (Pao) between 0 and 20 cm H2O. The expected pattern of changes in respiratory mechanics with increasing lung volume was seen: a progressive fall in airway resistance and increases in the coefficients of tissue damping and elastance. A surprising pattern was seen in hysteresivity (eta), with a plateau at low lung volumes (Pao < 10 cm H2O), a sharp fall occurring between 10 and 15 cm H2O, and eta approaching a second (lower) plateau at higher lung volumes. Studies designed to elucidate the mechanism(s) behind this behavior revealed that this was not due to chest wall properties, differences in volume history at low lung volume, time dependence of volume recruitment, or surface-acting forces. Our data are consistent with the notion that at low lung volumes the mechanics of the tissue matrix determine eta, whereas at high lung volumes the properties of individual fibers (collagen) become more important.

Surfactant with SP-B and SP-C analogues improves lung function in surfactant-deficient rats

The use of mammalian lung surfactant extracts has sharply reduced mortality and morbidity from respiratory distress syndrome in premature infants. Synthesis of surfactant protein B and C (SP-B and SP-C) analogues may lead the way to a synthetic surfactant preparation. Dimeric SP-B(1-25) (dSP-B(1-25)) is based on the N-terminal domain of human SP-B and SP-Cfc is a modified human SP-C in which a single phenylalanine is substituted for a palmitoylated cysteine residue in the N-terminal segment (Phe-4 > Cys-4 variant). We tested the effects of synthetic surfactants with 1 or 2% dSP-B(1-25) and 1% SP-Cfc on lung function in surfactant-deficient rats. Four experimental surfactant preparations were prepared by mixing 1% dSP-B(1-25), 2% dSP-B(1-25), 1% dSP-B(1-25) +1% SP-Cfc, and 2% dSP-B(1-25) +1% SP-Cfc with phospholipids (PL). PL and Survanta, a bovine lung extract, were controls. Groups of 8 rats were ventilated, lavaged until surfactant deficiency, and treated with 100 mg/kg surfactant. Arterial blood gas values and dynamic compliance were measured every 15 min and after 2 h of ventilation, the rats were killed and pressure-volume curves performed. Oxygenation improved quickly after instillation of surfactant with synthetic peptides and Survanta. Oxygenation and lung volumes were consistently higher in the 2% than in the 1% dSP-B(1-25) groups. Addition of 1% SP-Cfc to the synthetic surfactants further improved oxygenation and lung volume, but to a lesser extent than increasing the dSP-B(1-25) content from 1 to 2%. These data indicate that improvements in oxygenation and lung volume in lavaged rats are dependent on the concentration of dSP-B(1-25) in the surfactant preparation and that the presence of SP-Cfc has a relative minor effect on these parameters.

Biogenesis of lamellar bodies, lysosome-related organelles involved in storage and secretion of pulmonary surfactant

Lamellar bodies are members of a subclass of lysosome-related organelles referred to as secretory lysosomes. The principal constituents of the lamellar body, surfactant phospholipids, are organized into tightly packed, bilayer membranes in a process that is strongly influenced by the lung-specific, hydrophobic peptide SP-B. Newly synthesized SP-B is transported from the Golgi to the lamellar body via multivesicular bodies; in contrast, preliminary evidence suggests that newly synthesized surfactant phospholipids are transported from the ER and incorporated into the internal membranes of the lamellar body via a distinct pathway.

In vivo measurements of changes in respiratory mechanics with age in mice deficient in surfactant protein D

Mice deficient in surfactant protein D [SP-D (-/-)] develop progressive emphysema with age, associated with loss of parenchymal tissue, subpleural fibrosis, and accumulation of abnormal elastin fibers. We measured the changes in lung function, partitioned into components for the airways and lung parenchyma, occurring with age in SP-D (-/-) mice at three ages (n = 8 per group) (5, 8, and 13 wk). Impedance spectra between 0.25 and 19.625 Hz were calculated and a model, consisting of an airway compartment [airway resistance (Raw) and inertance (Iaw)] and a constant-phase tissue compartment [coefficients of tissue damping (G) and elastance (H)], was fitted to the data. Hysteresivity was calculated as G/H. Adult values of Raw, G, and H are reached by 8 wk of age in wild-type controls. Raw and H were lower at all ages in SP-D (-/-) compared with the wild-type controls (p = 0.006 and 0.029, respectively), and a similar trend was seen in G (p = 0.060). The patterns of change in respiratory mechanics were similar in both SP-D (+/+) and (-/-) groups. There were no changes in hysteresivity with age and no differences between wild-type and SP-D (-/-) mice. These data demonstrate that the changes in lung structure in SP-D (-/-) mice are reflected in the mechanical properties of both airway and lung parenchyma measured in vivo.

Deletion of exon 4 from human surfactant protein C results in aggresome formation and generation of a dominant negative

Human surfactant protein C (hSP-C) is synthesized by the alveolar type 2 cell as a 197 amino acid integral membrane proprotein and proteolytically processed to a secreted 3.7 kDa mature form. Although the SP-C null mouse possesses a non-lethal phenotype, a heterozygous substitution of A for G in the first base of intron 4 of the human SP-C gene (c.460+1A>G) has been reported in association with familial interstitial lung disease and absence of mature protein. This mutation produces a splice deletion of exon 4 (deltaExon4) resulting in removal of a positionally conserved cysteine in the C-terminal flanking propeptide. Based on a prior study showing that an identical deletion in the rat isoform diverted mutant protein to stable aggregates, we hypothesized that expression of the deltaExon4 mutation would result in disruption of intracellular trafficking of both mutant and wild-type proSP-C. We tested this in vitro using fusion proteins of EGFP conjugated either to wild-type SP-C (EGFP/hSP-C(1-197)) or to SP-C deleted of Exon4 (EGFP/hSP-C(deltaExon4)). Fluorescence microscopy showed that EGFP/hSP-C(1-197) transfected into A549 cells was expressed in a punctuate pattern in CD63 (+) cytoplasmic vesicles, whereas EGFP/hSP-C(deltaExon4) accumulated in ubiquitinated perinuclear inclusions linked to the microtubule organizing center. A similar juxtanuclear pattern was observed following transfection of SP-C cDNA lacking only cysteine residues in the C-terminal propeptide encoded by Exon 4 (EGFP/hSP-C(C120/121G)). To evaluate whether mutant proSP-C could function as a dominant negative, EGFP/hSP-C(deltaExon4) was cotransfected with HA-tagged hSP-C(1-197) and resulted in the restriction of both forms to perinuclear compartments. Addition of Na(+) 4-phenylbutyrate, a facilitator of trafficking of other misfolded proteins, attenuated the aggregation of EGFP/hSP-C(deltaExon4). We conclude that c.460+1A>G mutation of human SP-C results in disruption of disulfide-mediated folding encoded by Exon 4 leading to diversion of unprocessed proSP-C to aggresomes. The heterotypic oligomerization of hSP-C(1-197) and hSP-C(deltaExon4) provides a molecular mechanism for the dominant-negative effect observed in vivo.

Genetic influences and neonatal lung disease

Neonatal lung diseases may have a genetic background. The available studies mainly concentrate on surfactant proteins (SP-A, SP-B) and respiratory distress syndrome. Specific alleles of the SP-A and SP-B genes associate interactively with susceptibility to respiratory distress syndrome. This genetic impact on the condition is influenced by environmental, acquired and inherited factors. Other alleles and genotypes of SP-A and SP-D associate with severe respiratory infections in early infancy. Rare mutations causing an absence of the SP-B protein result in progressive respiratory failure. Dominant mutations of SP-C associate with chronic lung disease, with variable manifestations. The first steps towards unraveling the genetic network influencing the susceptibility to neonatal lung diseases are now being taken. Genes encoding multifunctional proteins in the distal lung are prime candidates for causing susceptibility to neonatal lung disease, including bronchopulmonary dysplasia.

Surfactant in the middle ear and eustachian tube: a review

There has been a recent surge in research on surfactant and surfactant proteins. Fields ranging from immunology to surface chemistry are making contributions to our understanding of this multifunctional compound. This paper reviews the literature on the structure and function of Eustachian tube surfactant. It covers the proposed functions of endogenous surfactant in normal physiology, as well as the experimental applications of exogenous surfactant in the treatment of otitis media. The analysis is based on four platforms of research: the role of surfactant in the innate immune system, the effect of surfactant on surface tension and Eustachian tube opening pressure, the capacity of surfactant to alter the rheological properties of mucus and the efficiency of the mucociliary system, and the ability of surfactant to mitigate oxygen free radical damage.

Molecular interactions in pulmonary surfactant films

Pulmonary surfactant is a lipid-protein complex which has the essential physiological role of stabilizing the respiratory surface of lungs against collapse. To achieve this function, surfactant forms films at the air-liquid interface that reduce dramatically the surface tension of the thin aqueous layer lining the alveoli. Natural surfactant has optimised two important properties. Once secreted to the alveolar spaces, surfactant adsorbs rapidly to the interface. There, surfactant films reduce surface tension close to 0 mN/m when compressed during expiration. The design and production of efficient artificial surfactants for respiratory therapeutics is critically dependent on the knowledge of the molecular mechanisms governing efficient interfacial adsorption and optimal surface activity, in the context of respiratory cycling. The present review summarizes the data available today on the behaviour of the different molecular components of pulmonary surfactant at air-liquid interfaces. A working model is proposed of how surfactant films could modulate the respiratory dynamics.

Palmitoylation and processing of the lipopeptide surfactant protein C

Pulmonary surfactant, a mixture of lipids and proteins, reduces the surface tension at the air-water interface of the lung alveoli by forming a surface active film. This way, it prevents alveoli from collapsing and facilitates the work of breathing. Surfactant protein C (SP-C) plays an important role in this surfactant function. SP-C is expressed as a proprotein (proSP-C), which becomes posttranslationally modified with palmitate and undergoes several rounds of proteolytical cleavage. This results in the formation of mature SP-C, which is stored in the lamellar bodies (LB) and finally secreted into the alveolar space. Recently, new insights into the sorting, processing and palmitoylation of proSP-C have been obtained by mutagenesis studies. Moreover, reports on the association of development of lung disease with SP-C deficiency have led to new insights into the importance of SP-C for proper surfactant homeostasis. In addition, new information has become available on the role of the palmitoyl chains of SP-C in surface activity. This review summarizes these recent developments in the processing and function of SP-C, with particular emphasis on the signals for and role of palmitoylation of SP-C.

BRICHOS: a conserved domain in proteins associated with dementia, respiratory distress and cancer

A novel domain (the BRICHOS domain) of approximately 100 amino acids has been identified in several previously unrelated proteins that are linked to major diseases. These include BRI(2), which is related to familial British and Danish dementia (FBD and FDD); Chondromodulin-I (ChM-I), related to chondrosarcoma; CA11, related to stomach cancer; and surfactant protein C (SP-C), related to respiratory distress syndrome (RDS). In several of these, the conserved BRICHOS domain is located in the propeptide region that is removed after proteolytic processing. Experimental data suggest that the role of this domain could be related to the complex post-translational processing of these proteins.

Structural basis for interactions between lung surfactant protein C and bacterial lipopolysaccharide

In the respiratory tract, recognition of bacterial endotoxin (lipopolysacharide, LPS) is a critical step of the innate host defense system directed against invading pathogens. Secretions of the airways contain proteins that have direct antimicrobial activity (lysozyme, lactoferrin, defensins, and cathelicidins) as well as complement factors and surfactant proteins that contribute to host defense. The hydrophobic surfactant protein C (SP-C) recognizes LPS (Augusto, L., Le Blay, K., Auger, G., Blanot, D., and Chaby, R. (2001) Am. J. Physiol. 281, L776-L785). In the present study, using synthetic analogs of SP-C, we demonstrate that the palmitoyl residues of SP-C are not required for the interaction with LPS and that both the hydrophilic and hydrophobic regions of SP-C are required for specific binding of a radiolabeled rough-type LPS. In addition, using LPS submitted to different chemical treatments as well as synthetic analogs of the lipid A moiety of LPS, we established that the terminal phosphate group at the reducing end of the lipid A disaccharide in alpha configuration is of crucial importance for recognition by SP-C. The N-linked fatty acyl chain on the reducing glucosamine of lipid A also takes part in the interaction. Dipalmitoyl phosphatidylcholine is not specifically required for the LPS-binding activity of SP-C, although a lipid environment significantly increases the binding. These results provide a basis for experiments on the role of SP-C in presentation of LPS to alveolar cells and for the design of drugs for the management of endotoxin-induced lung injury.

Involvement of cathepsin H in the processing of the hydrophobic surfactant-associated protein C in type II pneumocytes

Surfactant protein C (SP-C) is synthesized by type II pneumocytes as a 21-kD propeptide (proSP-C) which is proteolytically processed to a 4.2-kD dipalmitoylated protein. To characterize the processing of proSP-C and the role of the cysteine protease cathepsin H, we studied the localization of proSP-C and cathepsin H in human as well as proSP-C in rat lungs, the enzymatic cathepsin H activity in isolated rat lamellar bodies, and the cleavage of human proSP-C by purified cathepsin H. Using antisera directed against the N-terminal E(11)-R(23) (NPROSP-C(11-23)), the C-terminal G(162)-G(174) domain (CPROSP-C(162-174)) of proSP-C, and against cathepsin H, immunogold labeling identified all three in electron-dense multivesicular bodies, but only NPROSP-C(11-23) and cathepsin H in composite as well as lamellar bodies of type II pneumocytes. Immuno double-labeling further distinguished electron-dense vesicles containing cathepsin H or electron light vesicles/multivesicular bodies containing proSP-C. Isolated lamellar bodies contained enzymatically active cathepsin H, a 6-kD proSP-C processing intermediate detected only by NPROSP-C(11-23), and mature SP-C. Using enzyme activities comparable to those in isolated lamellar bodies, purified cathepsin H generated a partially N-terminal processed proSP-C intermediate in vitro. In conclusion, our results indicate that after the fusion of electron-dense vesicles containing cathepsin H and electron-light vesicles or multivesicular bodies containing proSP-C, cathepsin H is involved in the first N-terminal processing step of proSP-C in electron-dense multivesicular bodies of type II pneumocytes.

Biosynthesis of surfactant protein C (SP-C). Sorting of SP-C proprotein involves homomeric association via a signal anchor domain

Rat surfactant protein C (SP-C) is synthesized as a 194-amino acid propeptide (SP-C-(1-194)) that is directed to the distal secretory pathway and proteolytically processed as an integral membrane protein to yield its mature form. We had shown previously that trafficking of proSP-C is mediated both by a signal anchor domain contained within the mature SP-C sequence and by a targeting domain in the NH(2)-flanking propeptide. Based on evidence from other integral membrane proteins, we hypothesized that proSP-C targeting is effected by oligomerization of proSP-C monomers. To evaluate this in vitro, cDNA constructs encoding for either wild type proSP-C (pcDNA3/SP-C-(1-194)) or heterologous fusion proteins containing green fluorescent protein (EGFP) linked to SP-C-(1-194) (EGFP/SP-C-(1-194)), to mutant proSP-C lacking the NH(2) targeting domain (EGFP/SP-C-(24-194)), or to mature SP-C alone (EGFP/SP-C-(24-58)) were produced. In transfected A549 cells, fluorescence microscopy revealed that pcDNA3/SP-C-(1-194) and EGFP/SP-C-(1-194) were each expressed in CD63 (+), EEA1 (-) cytoplasmic vesicles. Expression of EGFP/SP-C-(24-194) or EGFP/SP-C-(24-58) resulted in translocation but retention in early compartments. When co-transfected with pcDNA3/SP-C-(1-194), both EGFP/SP-C-(24-194) and EGFP/SP-C-(24-58) were directed to CD63 (+) vesicles that also contained SP-C-(1-194). In contrast, trafficking of a folding mutant that forms juxtanuclear aggregates, EGFP/SP-C(C122/186G), was not corrected by cotransfection with pcDNA3/SP-C-(1-194). Chemical cross-linking studies of transfected cell lysates with bismaleimidohexane produced multimeric forms of both EGFP/SP-C-(1-194) and EGFP/SP-C-(24-58). These results indicate that sorting involves oligomeric association of proSP-C monomers mediated by the mature SP-C domain. Heteromeric assembly allows wild type proSP-C to facilitate trafficking of SP-C mutants with intact transmembrane domains but lacking targeting signals. We speculate that heterotypic oligomerization of wild type with SP-C folding mutants produces a dominant negative thus contributing to the pathology of chronic lung disease associated with patients heterozygous for mutant SP-C alleles.

Heterozygosity for a surfactant protein C gene mutation associated with usual interstitial pneumonitis and cellular nonspecific interstitial pneumonitis in one kindred

Familial pulmonary fibrosis is a heterogeneous group of interstitial lung diseases of unknown cause that is associated with multiple pathologic subsets. Mutations in the surfactant protein C (SP-C) gene (SFTPC) are associated with familial desquamative and nonspecific interstitial pneumonitis. Genetic studies in familial usual interstitial pneumonitis have been inconclusive. Using a candidate gene approach, we found a heterozygous exon 5 + 128 T-->A transversion of SFTPC in a large familial pulmonary fibrosis kindred, including adults with usual interstitial pneumonitis and children with cellular nonspecific interstitial pneumonitis. The mutation is predicted to substitute a glutamine for a conserved leucine residue and may hinder processing of SP-C precursor protein. SP-C precursor protein displayed aberrant subcellular localization by immunostaining. Electron microscopy of affected lung revealed alveolar type II cell atypia, with numerous abnormal lamellar bodies. Mouse lung epithelial cells transfected with the SFTPC mutation were notable for similar electron microscopy findings and for exaggerated cellular toxicity. We show that an SFTPC mutation segregates with the pulmonary fibrosis phenotype in this kindred and may cause type II cellular injury. The presence of two different pathologic diagnoses in affected relatives sharing this mutation indicates that in this kindred, these diseases may represent pleiotropic manifestations of the same central pathogenesis.