Lipid and Protein Effects on Monolayer Stability

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Acute respiratory distress syndrome (ARDS) is a diffuse, inflammatory lung disease characterized by increased vascular permeability, decreased lung compliance, and loss of aerated tissue affecting 200,000 in the US annually with 40% mortality. Covid-19 infections have significantly increased these numbers over the past year. The mechanisms through which ARDS arises and how host factors confer an increased risk of developing severe disease remain unclear. It is known that inflammation due to underlying disease is linked to ARDS progression and severity. We hypothesize that lipase-catalyzed degradation of pathogen phospholipids to soluble lysolipids and free fatty acids can lead to a mechanical lung instability known as the “Laplace Instability by inactivating healthy lung surfactant. The Laplace Instability occurs because the intra-alveolar pressure is increased by the Laplace pressure, ∆𝑃 = 2𝛾⁄𝑅 ; ∆𝑃 is higher in alveoli with smaller radii, 𝑅, than larger alveoli if 𝛾, the surface tension, is constant. This would cause smaller alveoli to deflate and fill with fluid, while the larger alveolar become distended, both symptoms of ARDS. To prevent this, 𝛾 must decrease with decreasing alveolar radius such that 2𝐸∗(𝜔) − 𝛾 > 0 in which 𝐸∗(𝜔) = 𝐴(𝜔)(𝜕𝛾⁄𝜕𝐴) is the dilatational modulus. For healthy lung surfactant 2𝐸∗(𝜔) -g >> 0. However, during inflammation, lysolipid concentrations increase by orders of magnitude in the alveolar fluids due to lipases acting on pathogen membranes. We find that lysopalmitoylphosphatidylcholine concentrations above its critical micelle concentration (CMC) cause 𝐸∗(𝜔) of lung surfactant monolayers to decrease dramatically leading to 2𝐸∗(𝜔) − 𝛾 ≤ 0 at breathing frequencies, which can lead to the Laplace Instability and compromise uniform lung inflation. Theoretical models show 𝐸∗ decreases due to diffusive exchange of lysolipids between the monolayer and the micelles in the adjacent fluid. This project will use a combined experimental and theoretical approach to understand how pathogen derived lysolipids with widely varying CMC’s influence 𝐸∗(𝜔) so that chemical or physical interventions might be developed to reverse the Laplace Instability. In Aim 1, we will use the unique experimental techniques developed in our labs to establish how the dilatational properties of lung surfactant change due to contact with lysolipids representative of various pathogens at concentrations above and below the CMC. We will determine the impact of 𝐸∗(𝜔) on the dynamics of model alveolar surfaces, and how this leads to physiological dysfunction. In Aim 2, we will examine the liquid-solid domain morphology of clinical and model lung surfactants to understand the role of cholesterol and interfacial curvature in modifying domain shapes and sizes to maximize LS respreading during inhalation and minimize lysolipid adsorption. These first of their kind experiments and fundamental theoretical approaches should determine the optimal lung surfactant composition that is resistant to lysolipid adsorption and provide a new way of stabilizing the ARDS lung.