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

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Postnatal development of the bronchiolar club cells of distal airways in the mouse lung: stereological and molecular biological studies

Club (Clara) cells are nonciliated secretory epithelial cells present in bronchioles of distal pulmonary airways. So far, no information is available on the postnatal differentiation of club cells by a combination of molecular biological, biochemical, and stereological approaches in the murine lung. Therefore, the present study was designed to investigate the changes in the club cell secretory proteins (CC10, surfactant proteins A, B and D) and club cell abundance within the epithelium of bronchioles of distal airways during the postnatal development of the mouse lung. Perfusion-fixed murine lungs of three developmental stages (newborn, 15-day-old and adult) were used. Frozen, unfixed lungs were used for cryosectioning and subsequent laser-assisted microdissection of bronchiolar epithelial cells and RT-PCR analyses. High resolution analyses of the three-dimensional structures and composition of lung airways were obtained by scanning electron microscopy. Finally, using design-based stereology, the total and average club cell volume and the volume of secretory granules were quantified by light and transmission electron microscopy. Our results reveal that murine club cells are immature at birth and differentiate postnatally. Further, increase of the club cell volume and number of intracellular granules are closely correlated to the total lung volume enlargement. However, secretory granule density was only increased within the first 15 days of postnatal development. The differentiation is accompanied by a decrease in glycogen content, and a close positive relationship between CC10 expression and secretory granule abundance. Taken together, our data are consistent with the concept that the morphological and functional differentiation of club cells is a postnatal phenomenon.

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

OBJECTIVE

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

STUDY DESIGN

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

RESULTS

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

CONCLUSION

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

Surfactant protein A regulates IgG-mediated phagocytosis in inflammatory neutrophils

Surfactant proteins (SP)-A and SP-D have been shown to affect the functions of a variety of innate immune cells and to interact with various immune proteins such as complement and immunoglobulins. The goal of the current study is to test the hypothesis that SP-A regulates IgG-mediated phagocytosis by neutrophils, which are major effector cells of the innate immune response that remove invading pathogens by phagocytosis and by extracellular killing mediated by reactive oxygen and nitrogen. We have previously shown that SP-A stimulates chemotaxis by inflammatory, but not peripheral, neutrophils. To evaluate the ability of SP-A to modulate IgG-mediated phagocytosis, polystyrene beads were coated with BSA and treated with anti-BSA IgG. SP-A significantly and specifically enhanced IgG-mediated phagocytosis by inflammatory neutrophils, but it had no effect on beads not treated with IgG. SP-A bound to IgG-coated beads and enhanced their uptake via direct interactions with the beads as well as direct interactions with the neutrophils. SP-A did not affect reactive oxygen production or binding of IgG to neutrophils and had modest effects on polymerization of actin. These data suggest that SP-A plays an important role in mediating the phagocytic response of neutrophils to IgG-opsonized particles.

Effects of keratinocyte growth factor on intra-alveolar surfactant fixed in situ: Quantitative ultrastructural and immunoelectron microscopic analysis

Quantitative (immuno) transmission electron microscopy using design-based stereology was performed on specimens collected by means of systematic uniform random sampling of rat lungs, which were fixed by vascular perfusion to stabilize intra-alveolar surfactant in situ. This procedure ensures that the data recorded are representative of the whole organ. Ultrathin sections of specimens embedded at low temperature in Lowicryl HM20 were labeled by indirect immuno-gold staining for surfactant protein A. We observed that, 3 days after treatment of lungs in vivo with truncated keratinocyte growth factor (DeltaN23-KGF), a potent mitogen of alveolar epithelial type II cells, surfactant protein A associated with the tubular myelin fraction of intra-alveolar surfactant was increased by 47% in comparison with buffer-treated control lungs. Despite the marked type II cell hyperplasia, the relative amount of ultrastructural surfactant subtypes was not significantly affected. Because surfactant protein A reduces the sensitivity to inhibition of the biophysical activity of surfactant by exudating plasma proteins, we propose that pretreatment of lungs with DeltaN23-KGF ameliorates adverse effects observed in acute lung injury following, for example, ischemia and reperfusion.

SP-A binds alpha1-antitrypsin in vitro and reduces the association rate constant for neutrophil elastase

Background

α1-antitrypsin and surfactant protein-A (SP-A) are major lung defense proteins. With the hypothesis that SP-A could bind α1-antitrypsin, we designed a series of in vitro experiments aimed at investigating the nature and consequences of such an interaction.

Methods and results

At an α1-antitrypsin:SP-A molar ratio of 1:1, the interaction resulted in a calcium-dependent decrease of 84.6% in the association rate constant of α1-antitrypsin for neutrophil elastase. The findings were similar when SP-A was coupled with the Z variant of α1-antitrypsin. The carbohydrate recognition domain of SP-A appeared to be a major determinant of the interaction, by recognizing α1-antitrypsin carbohydrate chains. However, binding of SP-A carbohydrate chains to the α1-antitrypsin amino acid backbone and interaction between carbohydrates of both proteins are also possible. Gel filtration chromatography and turnover per inactivation experiments indicated that one part of SP-A binds several molar parts of α1-antitrypsin.

Conclusion

We conclude that the binding of SP-A to α1-antitrypsin results in a decrease of the inhibition of neutrophil elastase. This interaction could have potential implications in the physiologic regulation of α1-antitrypsin activity, in the pathogenesis of pulmonary emphysema, and in the defense against infectious agents.

Improved lung preservation relates to an increase in tubular myelin-associated surfactant protein A

Background

Declining levels of surfactant protein A (SP-A) after lung transplantation are suggested to indicate progression of ischemia/reperfusion (IR) injury. We hypothesized that the previously described preservation-dependent improvement of alveolar surfactant integrity after IR was associated with alterations in intraalveolar SP-A levels.

Methods

Using immuno electron microscopy and design-based stereology, amount and distribution of SP-A, and of intracellular surfactant phospholipids (lamellar bodies) as well as infiltration by polymorphonuclear leukocytes (PMNs) and alveolar macrophages were evaluated in rat lungs after IR and preservation with EuroCollins or Celsior.

Results

After IR, labelling of tubular myelin for intraalveolar SP-A was significantly increased. In lungs preserved with EuroCollins, the total amount of intracellular surfactant phospholipid was reduced, and infiltration by PMNs and alveolar macrophages was significantly increased. With Celsior no changes in infiltration or intracellular surfactant phospholipid amount occurred. Here, an increase in the number of lamellar bodies per cell was associated with a shift towards smaller lamellar bodies. This accounts for preservation-dependent changes in the balance between surfactant phospholipid secretion and synthesis as well as in inflammatory cell infiltration.

Conclusion

We suggest that enhanced release of surfactant phospholipids and SP-A represents an early protective response that compensates in part for the inactivation of intraalveolar surfactant in the early phase of IR injury. This beneficial effect can be supported by adequate lung preservation, as e.g. with Celsior, maintaining surfactant integrity and reducing inflammation, either directly (via antioxidants) or indirectly (via improved surfactant integrity).

Intracellular and intraalveolar localization of surfactant protein A (SP-A) in the parenchymal region of the human lung

Although it is clearly established that surfactant protein A (SP-A) is secreted by type II pneumocytes as a component of pulmonary surfactant, its secretion pathway as well as its subcellular localization in the human lung are uncertain. We therefore studied the intracellular and intra-alveolar localization of SP-A in eight adult human lungs by immunohistochemistry and immunoelectron microscopy. Only type II pneumocytes could be identified as SP-A positive cells within the parenchymal region. SP-A was localized mainly in small vesicles and multivesicular bodies close to the apical plasma membrane. Only few lamellar bodies were weakly labeled at their outer membranes. Stereologic analysis showed this weak signal to be due to specific labeling. In the alveolar space, lamellar body-like surfactant forms in close proximity to tubular myelin were labeled for SP-A at their periphery. The strongest SP-A labeling was found over tubular myelin figures. Labeling for SP-A was also found in close association with the surface film and unilamellar vesicles. Our results support the hypothesis that, in the human lung, SP-A is mainly secreted into the alveolar space via an alternative pathway that largely bypasses the lamellar bodies. After secretion, the outer membranes of unwinding lamellar bodies become enriched with SP-A when tubular myelin formation is initiated. SP-A may also be involved in the transition of tubular myelin into the surface film.

Macrophage and type II cell catabolism of SP-A and saturated phosphatidylcholine in mouse lungs

Type II cells and macrophages are the major cells involved in the alveolar clearance and catabolism of surfactant. We measured type II cell and macrophage contributions to the catabolism of saturated phosphatidylcholine and surfactant protein A (SP-A) in mice. We used intratracheally administered SP-A labeled with residualizing125I-dilactitol-tyramine, radiolabeled dipalmitoylphosphatidylcholine ([3H]DPPC), and its degradation-resistant analog [14C]DPPC-ether. At 15 min and 7, 19, 29, and 48 h after intratracheal injection, the mice were killed; alveolar lavage was then performed to recover macrophages and surfactant. Type II cells and macrophages not recovered by the lavage were subsequently isolated by enzymatic digestion of the lung. Radioactivity was measured in total lung, lavage fluid macrophages, alveolar washes, type II cells, and lung digest macrophages. Approximately equal amounts of125I-dilactitol-tyramine-SP-A and [14C]DPPC-ether associated with the macrophages (lavage fluid plus lung digest) and type II cells when corrected for the efficiency of type II cell isolation. Eighty percent of the macrophage-associated radiolabel was recovered from lung digest macrophages. We conclude that macrophages and type II cells contribute equally to saturated phosphatidylcholine and SP-A catabolism in mice.

Domains of surfactant protein A that affect protein oligomerization, lipid structure and surface tension

Surfactant protein A (SP-A) is an abundant protein found in pulmonary surfactant which has been reported to have multiple functions. In this review, we focus on the structural importance of each domain of SP-A in the functions of protein oligomerization, the structural organization of lipids and the surface-active properties of surfactant, with an emphasis on ultrastructural analyses. The N-terminal domain of SP-A is required for disulfide-dependent protein oligomerization, and for binding and aggregation of phospholipids, but there is no evidence that this domain directly interacts with lipid membranes. The collagen-like domain is important for the stability and oligomerization of SP-A. It also contributes shape and dimension to the molecule, and appears to determine membrane spacing in lipid aggregates such as common myelin and tubular myelin. The neck domain of SP-A is primarily involved in protein trimerization, which is critical for many protein functions, but it does not appear to be directly involved in lipid interactions. The globular C-terminal domain of SP-A clearly plays a central role in lipid binding, and in more complex functions such as the formation and/or stabilization of curved membranes. In recent work, we have determined that the maintenance of low surface tension of surfactant in the presence of serum protein inhibitors requires cooperative interactions between the C-terminal and N-terminal domains of the molecule. This effect of SP-A requires a high degree of oligomeric assembly of the protein, and may be mediated by the activity of the protein to alter the form or physical state of surfactant lipid aggregates.