Mild Hypothermia Enhances Lung Surfactant Activity: Delving into the Molecular Mechanisms

Abstract

Pulmonary surfactant (PS) is a lipid-protein complex that reduces the surface tension at the respiratory air-liquid interface of alveoli, minimizing the work of breathing. The composition and structure of PS are directly responsible for its mechanical properties in a healthy lung or under pathological conditions. We previously demonstrated that PS activity improves after 72h of Whole Body Hypothermia (WBH, 33 °C) in neonates with and without lung injury. PS performance is significantly better when samples are tested under cooling condition. Moreover, decreasing neonatal lung temperature, SP-C and poli-unsaturated phospholipids are significantly reduced in surfactant large aggregates, promoting the simultaneous increase in DPPC percentage. To understand the molecular mechanisms under this temperature-dependent improvement, we designed a surfactant in vitro model. We combined surfactant lipids at different percentages (DPPC, POPG, POPC and DOPC) with purified porcine surfactant proteins SP-B and SP-C. We tested the biophysical activity of the resulting synthetic models upon breathing-like dynamics in a Captive Bubble Surfactometer, at both 37 °C and 33 °C. We also analyzed the thermotropic profiles of these protein-lipid mixtures by Differential Scanning Calorimetry. Finally, to further confirm our results, we studied both the lateral structure and the lipid composition of porcine PS once subjected to compression at the air-liquid interface. To do so, we coupled Langmuir-Blodgett Balance experiments with Epifluorescence Microscopy and Lipidomic Analysis. Our in vitro data suggests that the improvement in PS activity at 33 °C depends on a better and preferential exclusion from the air-liquid interface of less active phospholipids. This process occurs during compression (at expiration) and SP-C may play a crucial role.