Genetic causes of surfactant protein abnormalities.

PURPOSE OF REVIEW: Mutations in genes encoding proteins critical for the production and function of pulmonary surfactant cause diffuse lung disease. Timely recognition and diagnosis of affected individuals is important for proper counseling concerning prognosis and recurrence risk. RECENT FINDINGS: Involved genes include those encoding for surfactant proteins A, B, and C, member A3 of the ATP-binding cassette family, and for thyroid transcription factor 1. Clinical presentations overlap and range from severe and rapidly fatal neonatal lung disease to development of pulmonary fibrosis well into adult life. The inheritance patterns, course, and prognosis differ depending upon the gene involved, and in some cases the specific mutation. Treatment options are currently limited, with lung transplantation an option for patients with end-stage pulmonary fibrosis. Additional genetic disorders with overlapping pulmonary phenotypes are being identified through newer methods, although these disorders often involve other organ systems. SUMMARY: Genetic disorders of surfactant production are rare but associated with significant morbidity and mortality. Diagnosis can be made invasively through clinically available genetic testing. Improved treatment options are needed and better understanding of the molecular pathophysiology may provide insights into treatments for other lung disorders causing fibrosis.

Role for phospholipid acyl chains and cholesterol in pulmonary infections and inflammation.

Bacterial and viral respiratory tract infections result in millions of deaths worldwide and are currently the leading cause of death from infection. Acute inflammation is an essential element of host defense against infection, but can be damaging to the host when left unchecked. Effective host defense requires multiple lipid mediators, which collectively have proinflammatory and/or proresolving effects on the lung. During pulmonary infections, phospholipid acyl chains and cholesterol can be chemically and enzymatically oxidized, as well as truncated and modified, producing complex mixtures of bioactive lipids. We review recent evidence that phospholipids and cholesterol and their derivatives regulate pulmonary innate and adaptive immunity during infection. We first highlight data that oxidized phospholipids generated in the lung during infection stimulate pattern recognition receptors, such as TLRs and scavenger receptors, thereby amplifying the pulmonary inflammatory response. Next, we discuss evidence that oxidation of endogenous pools of cholesterol during pulmonary infections produces oxysterols that also modify the function of both innate and adaptive immune cells. Last, we conclude with data that n-3 polyunsaturated fatty acids, both in the form of phospholipid acyl chains and through enzymatic processing into endogenous proresolving lipid mediators, aid in the resolution of lung inflammation through distinct mechanisms. Unraveling the complex mechanisms of induction and function of distinct classes of bioactive lipids, both native and modified, may hold promise for developing new therapeutic strategies for improving pulmonary outcomes in response to infection.

Lessons from the biophysics of interfaces: lung surfactant and tear fluid.

The purpose of this review is to provide insight into the biophysical properties and functions of tear fluid and lung surfactant--two similar fluids covering the epithelium of two distinctive organs. Both fluids form a layer-like structure that essentially comprise of an aqueous layer next to the epithelium and an anterior lipid layer at the air-water interface. The aqueous layers contain soluble proteins and metabolites, and they are responsible for the host defence system and nutrition of the organ. However, many proteins also interact with the lipid layer and are important for the surface-active function of the fluid film. The lipid layer of lung surfactant comprises mainly of phospholipids, especially phosphatidylcholines, and only small amounts of non-polar lipids, mainly cholesterol. In contrast, tear fluid lipid layer comprises of a mixture of polar and non-polar lipids. However, the relative proportion and the spectrum of different polar and non-polar lipids seem to be more extensive in tear fluid than in lung surfactant. The differing lipid compositions generate distinctive lipid layer structures. Despite the structural differences, these lipid layers decrease the surface tension of the air-water interface. The structure of the tear film lipid layer also minimises the evaporation of the tear fluid. In lung surfactant surface activity is crucial for the function of the organ, as the lipid layer prevents the collapse of the lung alveoli during the compression-expansion cycle of breathing. Similarly the tear film experiences a compression-expansion cycle during blinking. The dynamics of this cycle have been studied to a lesser extent and are not as clear as those of lung surfactant. The common structure and properties suggest a similar behaviour under rapid compression-expansion for both fluids.

The effect of tissue elastic properties and surfactant on alveolar stability.

This paper presents a novel mathematical model of alveoli, which simulates the effects of tissue elasticity and surfactant on the stability of human alveoli. The model incorporates a spherical approximation to the alveolar geometry, the hysteretic behavior of pulmonary surfactant and tissue elasticity. The model shows that the alveolus without surfactant and the elastic properties of the lung tissue are always at an unstable equilibrium, with the capability both to collapse irreversibly and to open with infinite volume when the alveolus has small opening radii. During normal tidal breathing, the alveolus can becomes stable, if surfactant is added. Including the passive effect of tissue elasticity stabilizes the alveolus, further allowing the alveoli to be stable, even for lung volumes below residual volume. The model is the first to describe the combined effects of tissue elasticity and surfactant on alveolar stability. The model may be used as an integrated part of a more comprehensive model of the respiratory system, since it can predict opening pressures of alveoli.

Simulated transport alters surfactant homeostasis and causes dose-dependent changes in respiratory function in neonatal Sprague-Dawley rats.

OBJECTIVE: Forces transmitted to the neonate as a consequence of accelerations during transport have been associated with adverse neonatal outcomes including broncho-pulmonary dysplasia. In this study, we sought to determine the relationship between the duration of transport and respiratory performance in the rat model. METHODS: Four groups of Sprague-Dawley rat pups (10-12 pups/groups) were exposed to simulated medical transport on postnatal day of life 11 or 12. Each group was exposed to an average impulse of 27.4 m/s(2)/min for 0, 30, 60 or 90 min. During the exposure periods, impulse was monitored by computerized sampling using a digital accelerometer. Post-exposure, animals were immediately prepared, placed on mechanical ventilation and analyzed for elastance, tissue damping, airway resistance, ratio of damping to elastance (eta), hysteresivity, and inertance at positive end expiratory pressures (PEEPs) of 0, 3 and 6 cm(3) of H(2)O. Total phospholipid content and surfactant proteins A, B, and C mRNA levels in broncho-alveolar lavage fluid and lung tissue were obtained. RESULTS: Increased transport time resulted in a significant step-wise increase in airway resistance at all levels of PEEP (P<0.01). Static compliance decreased significantly after 60 min at PEEPs of 3 and 6 cm H(2)O (P<0.01). Eta significantly decreased with greater transport time at a PEEP of 6 cm H(2)O (P<0.05). Tissue damping increased with duration of transport time across all PEEP levels, but only exhibited statistical significance at a PEEP of 0 cm H(2)O (P<0.05). No differences were seen in hysteresivity or inertance. Compared with controls, transport was associated with significant reductions in total phospholipid content and mRNA levels of surfactant proteins B and C. CONCLUSION: Rat pups experienced significant deterioration of respiratory function with increasing duration of simulated transport.

Alveolar surfactant homeostasis and the pathogenesis of pulmonary disease.

The alveolar region of the lung creates an extensive epithelial surface that mediates the transfer of oxygen and carbon dioxide required for respiration after birth. Maintenance of pulmonary function depends on the function of type II epithelial cells that synthesize and secrete pulmonary surfactant lipids and proteins, reducing the collapsing forces created at the air-liquid interface in the alveoli. Genetic and acquired disorders associated with the surfactant system cause both acute and chronic lung disease. Mutations in the ABCA3, SFTPA, SFTPB, SFTPC, SCL34A2, and TERT genes disrupt type II cell function and/or surfactant homeostasis, causing neonatal respiratory failure and chronic interstitial lung disease. Defects in GM-CSF receptor function disrupt surfactant clearance, causing pulmonary alveolar proteinosis. Abnormalities in the surfactant system and disruption of type II cell homeostasis underlie the pathogenesis of pulmonary disorders previously considered idiopathic, providing the basis for improved diagnosis and therapies of these rare lung diseases.

Reduced surface tension normalizes static lung mechanics in a rodent chronic heart failure model.

RATIONALE: Chronic elevation of pulmonary microvascular pressure in chronic heart failure results in compensatory changes in the lung that reduce alveolar fluid filtration and protect against pulmonary microvascular rupture. OBJECTIVES: To determine whether these compensatory responses may have maladaptive effects on lung function. METHODS: Six weeks after myocardial infarction (chronic heart failure model) rat lung composition, both gross and histologic; air and saline mechanics; surfactant production; and immunological mediators were examined. MEASUREMENTS AND MAIN RESULTS: An increase in dry lung weight, due to increased insoluble protein, lipid and cellular infiltrate, without pulmonary edema was found. Despite this, both forced impedance and air pressure-volume mechanics were normal. However, there was increased tissue stiffness in the absence of surface tension (saline pressure-volume curve) with a concurrent increase in both surfactant content and alveolar type II cell numbers, suggesting a novel homeostatic phenomenon. CONCLUSIONS: These studies suggest a compensatory reduction in pulmonary surface tension that attenuates the effect of lung parenchymal remodeling on lung mechanics, hence work of breathing.

Diesel exhaust enhanced susceptibility to influenza infection is associated with decreased surfactant protein expression.

We have previously shown that exposure of respiratory epithelial cells to diesel exhaust (DE) enhances susceptibility to influenza infection and increases the production of interleukin (IL)-6 and interferon (IFN)-beta. The purpose of this study was to confirm and expand upon these in vitro results by assessing the effects of DE exposure on the progression of influenza infection and on development of associated pulmonary immune and inflammatory responses in vivo. BALB/c mice were exposed to air or to DE containing particulate matter at concentrations of 0.5 or 2 mg/m(3) for 4 h/day for 5 days and subsequently instilled with influenza A/Bangkok/1/79 virus. Exposure to 0.5 mg/m(3) (but not the higher 2-mg/m(3) dose) of DE increased susceptibility to influenza infection as demonstrated by a significant increase in hemagglutinin (HA) mRNA levels, a marker of influenza copies, and greater immunohistochemical staining for influenza virus protein in the lung. The enhanced susceptibility to infection observed in mice exposed to 0.5 mg/m(3) of DE was associated with a significant increase in the expression of IL-6, while antiviral lung IFN levels were unaffected. Analysis of the expression and production of surfactant proteins A and D, which are components of the interferon-independent antiviral defenses, showed that these factors were decreased following exposure to 0.5 mg/m(3) of DE but not to the higher 2-mg/m(3) concentration. Taken together, the results demonstrate that exposure to DE enhances the susceptibility to respiratory viral infections by reducing the expression and production of antimicrobial surfactant proteins.

Surfactant function and composition in premature infants treated with inhaled nitric oxide.

OBJECTIVES: We hypothesized that inhaled nitric oxide treatment of premature infants at risk for bronchopulmonary dysplasia would not adversely affect endogenous surfactant function or composition. METHODS: As part of the Nitric Oxide Chronic Lung Disease Trial of inhaled nitric oxide, we examined surfactant in a subpopulation of enrolled infants. Tracheal aspirate fluid was collected at specified intervals from 99 infants with birth weights <1250 g who received inhaled nitric oxide (20 ppm, weaned to 2 ppm) or placebo gas for 24 days. Large-aggregate surfactant was analyzed for surface activity with a pulsating bubble surfactometer and for surfactant protein contents with an immunoassay. RESULTS: At baseline, before administration of study gas, surfactant function and composition were comparable in the 2 groups, and there was a positive correlation between minimum surface tension and severity of lung disease for all infants. Over the first 4 days of treatment, minimum surface tension increased in placebo-treated infants and decreased in inhaled nitric oxide-treated infants. There were no significant differences between groups in recovery of large-aggregate surfactant or contents of surfactant protein A, surfactant protein B, surfactant protein C, or total protein, normalized to phospholipid. CONCLUSIONS: We conclude that inhaled nitric oxide treatment for premature infants at risk of bronchopulmonary dysplasia does not alter surfactant recovery or protein composition and may improve surfactant function transiently.

Hydrophobic surfactant proteins and their analogues.

Lung surfactant is a complex mixture of phospholipids and four surfactant-associated proteins (SP-A, SP-B, SP-C and SP-D). Its major function in the lung alveolus is to reduce surface tension at the air-water interface in the terminal airways by the formation of a surface-active film enriched in surfactant lipids, hence preventing cellular collapse during respiration. Surfactant therapy using bovine or porcine lung surfactant extracts, which contain only polar lipids and native SP-B and SP-C, has dramatically improved the therapeutic outcomes of preterm infants with respiratory distress syndrome (RDS). One important goal of surfactant researchers is to replace animal-derived therapies with fully synthetic preparations based on SP-B and SP-C, produced by recombinant technology or peptide synthesis, and reconstituted with selected synthetic lipids. Here, we review recent research developments with peptide analogues of SP-B and SP-C, designed using either the known primary sequence and three-dimensional (3D) structure of the native proteins or, alternatively, the known 3D structures of closely homologous proteins. Such SP-B and SP-C mimics offer the possibility of studying the mechanisms of action of the respective native proteins, and may allow the design of optimized surfactant formulations for specific pulmonary diseases (e.g., acute lung injury (ALI) or acute respiratory distress syndrome (ARDS)). These synthetic surfactant preparations may also be a cost-saving therapeutic approach, with better quality control than may be obtained with animal-based treatments.

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.

Surfactant protein polymorphisms and neonatal lung disease.

Here, we describe the approach of defining the genetic contribution to disease and discuss the polymorphisms of some genes that are associated with respiratory disease. The common allelic variants of SP-A1, SP-A2, SP-B, SP-C, and SP-D genes are associated with respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD), or respiratory syncytial virus (RSV) bronchiolitis. The main SP-A haplotype, interactively with SP-B Ile131Thr polymorphism and with constitutional and environmental factors, influences the risk of RDS. The polymorphisms of SP-A2 and SP-D are associated with the risk of severe RSV. The polymorphism may turn out to be important in susceptibility to influenza virus. The SP-B intron 4 deletion variant is the risk factor of BPD. Understanding the molecular mechanisms behind the hereditary risk may lead to new focused treatment strategies.

Surfactant and its role in chronic sinusitis.

Although numerous studies have focused on the nature and defensive role of surfactant in the lower airways, relatively little is known about its role in the upper airways. Decreased levels of the main component of surfactant--phospholipids--have been implicated in atrophic rhinitis. The lamellar body arrangement of phospholipids has now been demonstrated in both normal and diseased sinus tissue, resulting in the implication that these structures may play a crucial role in mucociliary clearance against inhaled pathogens, as well as in the regulation of mucous viscosity. Furthermore, they may be secreted from sinonasal ciliated epithelium. Surfactant proteins (SPs) make up a relatively smaller proportion of surfactant, but appear to have an important role in innate immunity. Altered levels of SPs have been observed in a number of respiratory tract diseases. These SPs may prove to play a significant role in chronic sinusitis. Demonstrated expression of SP-A and SP-D in diseased and normal sinus tissue may mean that these SPs are excreted into the airway-lining fluid of the sinuses. Additionally, initial contact and interaction between pathogens and SP-A and SP-D may occur relatively early after inhalation and deposition into the mucus of the respiratory tract. These findings may lead to potential therapeutic options for difficult-to-treat sinus disease in the future.

Evolution of pulmonary surfactants for the treatment of neonatal respiratory distress syndrome and paediatric lung diseases.

UNLABELLED: This review documents the evolution of surfactant therapy, beginning with observations of surfactant deficiency in respiratory distress syndrome, the basis of exogenous surfactant treatment and the development of surfactant-containing novel peptides patterned after SP-B. We critically analyse the molecular interactions of surfactant proteins and phospholipids contributing to surfactant function. CONCLUSION: Peptide-containing surfactant provides clinical efficacy in the treatment of respiratory distress syndrome and offers promise for treating other lung diseases in infancy.

Surfactant composition and function in a primate model of infant chronic lung disease: effects of inhaled nitric oxide.

Bronchopulmonary dysplasia, or chronic lung disease (CLD), of premature infants involves injury from hyperoxia and mechanical ventilation to an immature lung. We examined surfactant and nitric oxide (NO), which are developmentally deficient in premature infants, in the baboon model of developing CLD. Fetuses were delivered at 125 d gestation and were managed for 14 d with ventilation and oxygen prn without (controls) or with inhaled NO at 5 ppm. Compared with term infants, premature control infants had reduced maximal lung volume, decreased tissue content of surfactant proteins SP-A, -B, and -C, abnormal lavage surfactant as assessed by pulsating bubble surfactometer, and a low concentration of SP-B/phospholipid. NO treatment significantly increased maximal lung volume and tissue SP-A and SP-C, reduced recovery of lavage surfactant by 33%, decreased the total protein:phospholipid ratio of surfactant by 50%, and had no effect on phospholipid composition or SP content except for SP-C (50%). In both treatment groups, levels of SP-B and SP-C in surfactant were negatively correlated with STmin, with a 5-fold greater SP efficiency for NO versus control animals. By contrast, lung volume and compliance were not correlated with surfactant function. We conclude that surfactant is often dysfunctional in developing CLD secondary to SP-B deficiency. NO treatment improves the apparent ability of hydrophobic SP to promote low surface tension, perhaps secondary to less protein inactivation of surfactant, and improves lung volume by a process unrelated to surfactant function.

Current concepts on the pulmonary surfactant in infants.

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

Secretory leukoprotease inhibitor and pulmonary surfactant serve as principal defenses against influenza A virus infection in the airway and chemical agents up-regulating their levels may have therapeutic potential.

Influenza A virus (IAV) is one of the most common infectious pathogens in humans. Entry of this virus into cells is primarily determined by host cellular trypsin-type processing proteases, which proteolytically activate viral membrane fusion glycoprotein precursors. Human IAV and murine parainfluenza virus type 1 Sendai virus are exclusively pneumotropic, and the infectious organ tropism of these viruses is determined by the susceptibility of the viral envelope glycoprotein to cleavage by proteases in the airway. Proteases in the upper respiratory tract are suppressed by secretory leukoprotease inhibitor, and those in the lower respiratory tract are suppressed by pulmonary surfactant, which by adsorption inhibits the interaction between the proteases and viral membrane proteins. Although the protease activities are predominant over the activities of inhibitory compounds under normal airway conditions, intranasal administration of inhibitors was able to significantly suppress multi-cycles of viral replication in the airway. In addition, we identified chemical agents that could act as defensive factors by up-regulating the levels of the natural inhibitors and immunoglobulin A (IgA) in airway fluids. One of these compounds, ambroxol, is a mucolytic and anti-oxidant agent that stimulates the release of secretory leukoprotease inhibitor and pulmonary surfactant in the early phase, and IgA in the late phase of infection at an optimal dose, i.e. a dose sufficient to inhibit virus proliferation and increase the survival rate of animals after treatment with a lethal dose of IAV. Another agent, clarithromycin, is a macrolide antibiotic that increases IgA levels through augmentation of interleukin-12 levels and mucosal immunization in the airway. In addition to the sialidase inhibitors, which prevent the release of IAV from infected cells, inhibitors of the processing proteases and chemical agents that augment mucosal immunity and/or levels of the relevant defensive compounds may also ultimately prove to be useful as new anti-influenza agents.

Host defense functions of pulmonary surfactant.

Surfactant is a complex of lipids and proteins that reduces surface tension at the air/liquid interface of the lung and regulates immune cell function. Surfactant immune function is primarily attributed to two proteins: SP-A and SP-D. SP-A and SP-D are members of a protein family known as 'collectins', which are distinguished by their N-terminal collagen-like region and their C-terminal lectin domain. The lectin domain binds preferentially to sugars on the surface of pathogens and thereby opsonizes them for uptake by phagocytes. The collectins also modulate the functions of cells of the adaptive immune network including dendritic cells and T lymphocytes. In addition, recent studies show that bacterial products degrade surfactant. In summary, surfactant plays an important role in lung host defense. Surfactant degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection.

Surfactant therapy for acute lung injury/acute respiratory distress syndrome.

PURPOSE OF REVIEW: Currently, three phase III surfactant replacement trials for acute lung injury (ALI)/acute respiratory distress syndromes (ARDS) patients are underway. Although the efficacy of surfactant replacement therapy will first have to be proved in these phase III trials, recent reports indicate some enticing possibilities for the future of surfactant therapy. RECENT FINDINGS: Patients requiring mechanical ventilation show alterations in their endogenous surfactant composition. Depending on the type of lung injury or the elapsed time, modifications to surfactant preparations could enhance the efficacy of these preparations. Surfactants that closely resemble natural surfactant, especially those containing surfactant proteins (SP-B/C) and nonphospholipids (cholesterol), are able to restore normal surfactant physiology. Furthermore, lipids that are able to withstand degradation by lipases could further enhance surfactant therapy. SUMMARY: If surfactant therapy fulfills the promises expected from the ongoing phase III trials, future surfactant preparations may even enhance therapy efficacy and restore the altered endogenous surfactant pool as soon as possible.