Surfactant-associated proteins perturb lipid membrane architecture: insights from nanoplasmonic sensing and differential scanning calorimetry.

Abstract

This study investigates the biophysical interactions between pulmonary surfactant proteins (SP) and model lipid membranes. Using a combination of state-of-the-art nanoplasmonic sensing (NPS) and micro differential scanning calorimetry (MicroCal DSC), the effects of surfactant protein B (SP-B), surfactant protein C (SP-C), and their mixture on the structural and thermodynamic properties of liposomes composed of neutral phosphatidylcholines and anionic phosphatidylglycerol, (POPC : POPG and DPPC : DMPG, 80 : 20 molar ratio) were assessed. In a first-of-its-kind application for this system, NPS measurements provided real-time, label-free monitoring of the mass-sensing shifts associated with protein-lipid binding and membrane reorganization. Our results suggest that while SP-B primarily influences inter-membrane connectivity and surface-level interactions, SP-C penetrates deeper into the hydrophobic core. The combined SP-B and SP-C system exhibited non-additive effects, affecting the fluidity and stability of the surfactant model. Complementary MicroCal DSC analysis revealed that protein reconstitution into lung-mimicking lipid membranes is a kinetic process requiring extended timeframes to establish stable protein-lipid interactions. The inclusion of 2% (protein-to-lipid ratio) of each SP (and their equimolar mixtures) significantly modulated the thermotropic phase behavior of the anionic bilayers. These changes were characterized by a significant elevation in the gel-to-liquid crystalline transition temperature (Tm) and a concomitant broadening of the main transition endotherm. These thermotropic shifts suggest enhanced protein-lipid interactions, resulting in reduced transition enthalpy (ΔH) and attenuated molecular cooperativity within the lipid bilayer. Together, these findings provide critical insights into the molecular mechanisms by which these proteins maintain lung function, potentially offering a framework for the development of advanced synthetic surfactant therapies.