The interaction of human serum components with model membranes containing phospholipids and lipopolysaccharides

Lipoproteins, key mediators of lipid transport, facilitate the bidirectional transfer of lipids such as fatty acids, triglycerides, and cholesterol between soluble particles and cell membranes. High-density lipoproteins (HDL) primarily engage in reverse cholesterol transport, while low-density lipoproteins (LDL) predominantly deposit lipids, affecting cardiovascular health with a well-known role in the formation of the atherosclerotic plaque. In addition, lipoproteins play an important role in neutralizing bacterial lipopolysaccharides (LPS), the major component of Gram-negative bacterial outer membranes, which act as potent TLR4 agonists and can trigger severe immune responses. Lipoproteins bind LPS in plasma, with HDL showing strong binding affinity and LDL contributing to LPS clearance under specific conditions. Here, we explore the interaction of LDL and human serum albumin (HSA), another serum lipid-binding protein, with model lipid bilayers containing either phospholipids or LPS. Using neutron reflectometry and attenuated total reflection infrared spectroscopy, we characterize lipid transfer processes influenced by calcium levels and lipid composition. Calcium plays a key role in receptor-mediated LDL binding, but less is known on its effect on LDL-mediated lipid transfer in the absence of LDL receptors. Our results show that elevated calcium levels enhance stable LDL adsorption onto mammalian phospholipid-cholesterol membranes, promoting lipid cargo deposition despite the absence of specific LDL-receptors. Conversely, LDL showed no stable binding to LPS reconstituted in asymmetric outer membrane models but was able to deposit phospholipids in the membrane. In contrast, HSA removed lipids from mammalian membranes and exhibited minimal interaction with LPS-containing models. The findings elucidate the distinct lipid exchange mechanisms of LDL and HSA and their roles in modulating lipid transfer at membrane interfaces. Receptor-free enhanced LDL lipid deposition in calcium-enriched environments may have implications for cardiovascular disease progression. Conversely, the minimal interaction of LDL with bacterial LPS suggests a limited ability to extract LPS from membrane environments. This study provides structural insights into the interplay between lipoproteins, calcium, and membrane composition, with relevance to atherosclerosis and systemic endotoxemia.