Pulmonary Surfactant Protein A Interacts with Gel-Like Regions in Monolayers of Pulmonary Surfactant Lipid Extract

Abstract

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 μg/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 ( π ∼5 mN/m) and the domains grew in size with increasing π. Above 25 mN/m, the domain size decreased with increasing π. The amount of observable dark phase was maximal at 18% of the total film area at π ∼25 mN/m, then decreased to ∼3% at π ∼40 mN/m. The addition of 0.16 μg/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 ∼25% between π 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 μg/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.