Reversible calcium-dependent interaction of liposomes with pulmonary surfactant protein A. Analysis by resonant mirror technique and near-infrared light scattering

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.

Alveolar epithelial type II cell: defender of the alveolus revisited

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

Pulmonary surfactant protein A interacts with gel-like regions in monolayers of pulmonary surfactant lipid extract

Epifluorescence microscopy was used to investigate the interaction of pulmonary surfactant protein A (SP-A) with spread monolayers of porcine surfactant lipid extract (PSLE) containing 1 mol % fluorescent probe (NBD-PC) spread on a saline subphase (145 mM NaCl, 5 mM Tris-HCl, pH 6.9) containing 0, 0.13, or 0.16 microg/ml SP-A and 0, 1.64, or 5 mM CaCl(2). In the absence of SP-A, no differences were noted in PSLE monolayers in the absence or presence of Ca(2+). Circular probe-excluded (dark) domains were observed against a fluorescent background at low surface pressures (pi approximately 5 mN/m) and the domains grew in size with increasing pi. Above 25 mN/m, the domain size decreased with increasing pi. The amount of observable dark phase was maximal at 18% of the total film area at pi approximately 25 mN/m, then decreased to approximately 3% at pi approximately 40 mN/m. The addition of 0.16 microg/ml SP-A with 0 or 1.64 mM Ca(2+) in the subphase caused an aggregation of dark domains into a loose network, and the total amount of dark phase was increased to approximately 25% between pi of 10-28 mN/m. Monolayer features in the presence of 5 mM Ca(2+) and SP-A were not substantially different from those spread in the absence of SP-A, likely due to a self-association and aggregation of SP-A in the presence of higher concentrations of Ca(2+). PSLE films were spread on a subphase containing 0.16 microg/ml SP-A with covalently bound Texas Red (TR-SP-A). In the absence of Ca(2+), TR-SP-A associated with the reorganized dark phase (as seen with the lipid probe). The presence of 5 mM Ca(2+) resulted in an appearance of TR-SP-A in the fluid phase and of aggregates at the fluid/gel phase boundaries of the monolayers. This study suggests that SP-A associates with PSLE monolayers, particularly with condensed or solid phase lipid, and results in some reorganization of rigid phase lipid in surfactant monolayers.

Self-aggregation of surfactant protein A

Environmental factors of physiological relevance such as pH, calcium, ionic strength, and temperature can affect the state of self-aggregation of surfactant protein A (SP-A). We have studied the secondary structure of different SP-A aggregates and analyzed their fluorescence characteristics. (a) We found that self-aggregation of SP-A can be Ca(2+)-dependent. The concentration of Ca(2+) needed for half-maximal self-association (K(a)(Ca)()2+) depended on the presence of salts. Thus, at low ionic strength, K(a)(Ca)()2+ was 2.3 mM, whereas at physiological ionic strength, K(a)(Ca)()2+ was 2.35 microM. Circular dichroism and fluorescence measurements of Ca(2+)-dependent SP-A aggregates indicated that those protein aggregates formed in the absence of NaCl are structurally different from those formed in its presence. (b) We found that self-aggregation of SP-A can be pH-dependent. Self-aggregation of SP-A induced by H(+) was highly influenced by the presence of salts, which reduced the extent of self-association of the protein. The presence of both salts and Ca(2+) attenuated even more the effects of acidic media on SP-A self-aggregation. (c) We found that self-aggregation of SP-A can be temperature-dependent. At 20 degrees C, SP-A underwent self-aggregation at physiological but not at low ionic strength, in the presence of EDTA. All of these aggregates were dissociated by either adding EDTA (a), increasing the pH to neutral pH (b), or increasing the temperature to 37 degrees C (c). Dissociation of Ca(2+)-induced protein aggregates at low ionic strength was accompanied by an irreversible loss of both SP-A secondary structure and SP-A-dependent lipid aggregation properties. On the other hand, temperature-dependent experiments indicated that a structurally intact collagen-like domain was required for either Ca(2+)- or Ca(2+)/Na(+)-induced SP-A self-aggregation but not for H(+)-induced protein aggregation.

Interaction of pulmonary surfactant protein A with phospholipid liposomes: a kinetic study on head group and fatty acid specificity

Recent work on surfactant protein A (SP-A) has shown that Ca(2+) induces an active conformation, SP-A, which binds rapidly to liposomes and mediates their aggregation. Employing sensitive real time assays, we have now studied the lipid binding characteristics of the SP-A liposome interaction. From the final equilibrium level of the resonant mirror binding signal, an apparent dissociation constant of ca. K(d)=5 microM is obtained for the complex between SP-A and dipalmitoylphosphatidylcholine (DPPC) liposomes. At nanomolar SP-A concentrations, this complex is formed with a subsecond (0.3 s) reaction time, as measured by light-scattering signals evoked by photolysis of caged Ca(2+). With palmitoyloleoylphosphatidylcholine (POPC), the complex formation proceeds at half the rate, compared to DPPC, leading to a lower final equilibrium level of SP-A lipid interaction. Distearoylphosphatidylcholine (DSPC) shows a stronger interaction than DPPC. Regarding the phospholipid headgroups, phosphatidylinositol (PI) and sphingomyelin (SM) interact comparable to DPPC, while less interaction is seen with phosphatidylethanolamine (PE) or with phosphatidylglycerol (PG). Thus both headgroup and fatty acid composition determine SP-A phospholipid interaction. However, the protein does not exhibit high specificity for either the polar or the apolar moiety of phospholipids.

Cation-mediated conformational variants of surfactant protein A

Surfactant protein A (SP-A) is the major protein of pulmonary surfactant. This protein is implicated in regulating surfactant secretion, alveolar processing, recycling, and in non-serum-induced immune response. An increasing body of work indicates the importance of cations, particularly calcium, on SP-A function. However, little information exists on the effects of cations on SP-A quaternary structure. Here, the quaternary organisation of bovine surfactant protein A in the presence of cations has been quantitatively and systematically studied by transmission electron microscopy. The conformation of SP-A is altered by the presence of cations, especially calcium, then sodium, and to a small extent, magnesium. There is a transition concentration, unique for each cation, at which a conformational switch occurs. These transition concentrations are: 5 mM for CaCl2, 100 mM for NaCl and 1 mM for MgCl2. Below these concentrations, SP-A exists primarily in an opened form with a large head diameter of 20 nm; above it, SP-A is mostly in a closed form due to a compaction of the headgroups resulting in a head diameter of 11 nm. There is a corresponding increase in particle length from 17 nm for opened SP-A to 20 nm for closed SP-A. The fact that the transition concentrations are within physiological range suggests that cation-mediated conformational changes of SP-A could be operative in vivo.

Structure, processing and properties of surfactant protein A

Surfactant protein A (SP-A) is a highly ordered, oligomeric glycoprotein that is secreted into the airspaces of the lung by the pulmonary epithelium. The in vitro activities of protein suggest diverse roles in pulmonary host defense and surfactant homeostasis, structure and surface activity. Functional mapping of SP-A using directed mutagenesis has identified domains which interact with surfactant phospholipids, alveolar type II cells and microbes. Recently developed genetically manipulated animal models are beginning to clarify the critical physiological roles for SP-A in the normal lung, and in the pathophysiology of pulmonary disease.

Calcium-triggered selective intermembrane exchange of phospholipids by the lung surfactant protein SP-A

It is shown that human lung surfactant protein (SP-A) mediates selective exchange of phospholipid probes with unlabeled phospholipid in excess vesicles in the presence of calcium and NaCl. The exchange occurs without leakage of vesicle contents, or transbilayer movement (flip-flop) of the phospholipid probes, or fusion of vesicles. Individual steps preceding the exchange are dissected by a combination of protocols, and the results are operationally interpreted in terms of a model where a calcium-dependent change in SP-A triggers aggregation of vesicles followed by probe exchange between the vesicles in contact through SP-A. The contacts remain stable in the presence of calcium; i.e., the vesicles in contact do not change their partners on the time scale of several minutes. The binding of SP-A to vesicles and the aggregation of vesicles are rapid, and the aggregation is rapidly reversed by EGTA; i.e., both the forward and reverse aggregation reactions are complete in about 1 min. The exchange rate of the various probes between aggregated vesicles below 1 mM calcium in the presence of NaCl shows selectivity, i.e., a modest dependence on the net anionic charge on vesicles and for the headgroup of the probe. Exchange with lower selectivity is seen at >2 mM Ca in the absence of NaCl. SP-A binding to vesicles does not show an absolute specificity for the phospholipid structure, but the time course of the subsequent changes does. The results suggest that SP-A contacts between phospholipid interfaces could mediate the exchange of phospholipid species (trafficking and sorting) between lung surfactant pools in the hypophase and all accessible phospholipid interfaces of the alveolar space.