Effects of Lipid Interactions on Model Vesicle Engulfment by Alveolar Macrophages

Abstract

The engulfment function of macrophages relies on complex molecular interactions involving both lipids and proteins. In particular, the clearance of apoptotic bodies (efferocytosis) is enabled by externalization on the cell target of phosphatidylserine lipids, which activate receptors on macrophages, suggesting that (local) specific lipid-protein interactions are required at least for the initiation of efferocytosis. However, in addition to apoptotic cells, macrophages can engulf foreign bodies that vary substantially in size from a few nanometers to microns, suggesting that nonspecific interactions over a wide range of length scales could be relevant. Here, we use model lipid membranes (made of phosphatidylcholine, phosphatidylserine, and ceramide) and rat alveolar macrophages to show how lipid bilayer properties probed by small-angle x-ray scattering and solid-state (2)H NMR correlate with engulfment rates measured by flow cytometry. We find that engulfment of protein-free model lipid vesicles is promoted by the presence of phosphatidylserine lipids but inhibited by ceramide, in accord with a previous study of apoptotic cells. We conclude that the roles of phosphatidylserine and ceramide in phagocytosis is based, at least in part, on lipid-mediated modification of membrane physical properties, including interactions at large length scales as well as local lipid ordering and possible domain formation.

Structural and functional characterization of endothelial microparticles released by cigarette smoke

“…The inhibition of efferocytosis by high concentrations of AC-derived EMPs together with the weaker effects of CS-derived EMPs on efferocytosis during conditions of aSMase loss of function suggest a competitive effect of EMPs with apoptotic cells for phagocyte engulfment. In addition, critical levels of ceramide in EMPs may hinder efferocytosis, which is consistent with our report that increasing ceramide content in a target lipid vesicle bilayer significantly inhibited macrophages efferocytosis, due to increased membrane stiffness and impaired intercellular membrane fusion 24 . We did detect sphingosine-1 phosphate (S1P) within cMPs (ranging from 0.83 to 1.65 pmoles/cMP in healthy to 1.02 to 2.38 pmoles/cMP in COPD individuals).…”

“…Since the role of CS-induced EMPs has not been reported yet, we focused our investigation on their impact on specialized phagocytes. Others and we have shown that CS and ceramides decrease apoptotic cell clearance by macrophages (efferocytosis), via alterations of the cytoskeleton and impaired intercellular membranes fusion 24 . We therefore hypothesized that MPs released during CS exposure will inhibit PBMDM efferocytosis.…”

Guidelines for the use and interpretation of assays for monitoring autophagy (3rd edition)

“…587 Sphingolipids participate in the formation of different membrane structures and subcellular organelles, such as mitochondria and ER, and are also involved in the fusion and biophysical properties of cell membranes. 588 Ceramides, positioned at the core of sphingolipid metabolism, play several roles that affect multiple steps of macroautophagy, by inhibition of nutrient transporters, 589 by modulation of BCL2-BECN1 association at the level of AKT signaling, 590 and by regulation of mitophagy. 591 The latter function is regulated by a particular ceramide species, steroyl (C18:0)-ceramide, a sphingolipid generated by CERS1 (ceramide synthase 1).…”

Alveolar Dynamics and Beyond – The Importance of Surfactant Protein C and Cholesterol in Lung Homeostasis and Fibrosis

“…Both, cholesterol-loaded vesicles, as well as their combination with SP-C, led to a higher lipid engulfment by AMs (Ruwisch et al, 2020). Increased cholesterol content, however, seemed to increase lipid uptake by AMs regardless of the presence of SP-C. Vesicle size and membrane fluidity can affect lipid uptake by AMs (Justice et al, 2014), and thus, the combined effect of cholesterol and SP-C on membrane structure and fluidity (Roldan et al, 2016(Roldan et al, , 2017 is a factor that must be taken into account. The study of the transcriptional response of MH-S cells to lipid administration revealed that genes associated to lipid metabolism and cholesterol transport were altered upon lipid uptake.…”