Insight into the Exosomal Membrane: From Viewpoints of Membrane Fluidity and Polarity

Abstract

Numerous research studies have been done for exosomes, particularly focusing on membrane proteins and included nucleic acids, and the volume of the knowledge about the lipids in the exosomal membrane has been increasing. However, the dynamic property of the exosomal membrane is hardly studied. By employing milk exosome as an example, herein the exosomal membrane was characterized focusing on the membrane fluidity and polarity. The lipid composition and phase state of milk exosome (exosome from bovine milk) were estimated. The milk exosome contained enriched Chol (43.6 mol % in total lipid extracts), which made the membrane in the liquid-ordered (l o) phase by interacting with phospholipids. To suggest a model of exosomal vesicle cargo, the liposome compositions that mimic milk exosome were studied: liposomes were made of cholesterol (Chol), milk sphingomyelin (milk SM), and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). By using fluorescent probes 1,6-diphenyl-1,3,5-hexatriene and 6-dodecanoyl-2-dimethylaminonaphthalene, the microenvironments of submicron-sized membranes of exosome and model liposomes were investigated. The membrane fluidity of milk exosome was slightly higher than those of Chol/milk SM/POPC liposomes with a similar content of Chol, suggesting the presence of enriched unsaturated lipids. The most purposeful membrane property was obtained by the liposome composition of Chol/milk SM/POPC = 40/15/45. From the above, it is concluded that Chol is a fundamental component of the milk exosomal membrane to construct the enriched l o phase, which could increase the membrane rigidity and contribute to the function of exosome.

Milk Exosomes Facilitate Oral Delivery of Drugs against Intestinal Bacterial Infections

“…48 mExo contains enriched 43.6 mol % Chol in total lipids, which made the phospholipid bilayer in the liquid-ordered phase increase the membrane rigidity, resulting in high stability in the gastrointestinal environment. 19 The high rigidity of nanoparticles also overcame the morphological changes to increase the cellular uptake, consistent with the previous studies that rigidity regulated the phagocytosis of nanoparticles. 49,50 mExo could stay in the gastrointestinal tract for a long time, which is a favorable characterization for intestinal treatment.…”

“…Importantly, compared to the exosomes derived from cell lines and bacteria, mExo possesses advances in physicochemical stability, , special composition, , and production cost . mExo contains enriched 43.6 mol % Chol in total lipids, which made the phospholipid bilayer in the liquid-ordered phase increase the membrane rigidity, resulting in high stability in the gastrointestinal environment . The high rigidity of nanoparticles also overcame the morphological changes to increase the cellular uptake, consistent with the previous studies that rigidity regulated the phagocytosis of nanoparticles. , mExo could stay in the gastrointestinal tract for a long time, which is a favorable characterization for intestinal treatment. , Furthermore, mExo increases the mucus penetrability of cargoes by more than 4-fold, which were capable of reaching the epithelial monolayers of the intestine to improve the antibacterial efficiency at a site of hydropathicity .…”

“…mExo exhibit low systemic immunogenicity, significant bioavailability, and high mucus permeability . Furthermore, they have a unique composition, including enriched cholesterol (Chol) (43.6 mol %) and abundant milk fat globule membrane proteins, which qualify them to increase the stability under gastrointestinal conditions. , Compared to the low production of exosomes derived from cell lines, bovine milk contains large amounts of biocompatible exosomes, and the integrity of the phospholipid vesicles was stable in vitro for a long time . Moreover, targeted drug delivery systems have the potential to serve as powerful tools for enhancing the efficiency of a wide range of biomolecules and chemotherapeutic agents .…”

Fluorescence Spectroscopic Analysis of Lateral and Transbilayer Fluidity of Exosome Membranes

“…Previous studies have demonstrated that ordered and functional membrane domains, such as lipid rafts are present on exosome membranes, and relative sphingolipids and cholesterol compositions affect the difference in the order of the exosome membranes . A higher molar ratio of cholesterol (>40 mol %) has been reported in milk, PC3, and Oli-neu exosomes, whereas exosomes derived from Mast cells contain approximately 15 mol % cholesterol. The milk and PC3 exosome membranes also had higher sphingomyelin and PC contents than other exosomes secreted from Oli-neu and Mast cells.…”

“…The exosome membranes are often enriched with cholesterol, sphingolipids, and phosphatidylserine in different lipid compositions from the cell membranes. − The abundances of cholesterol and sphingolipids are similar to the lipid rafts in cell membranes and liquid-ordered (L o ) phase in model membranes . In fact, the exosome membranes exhibit high rigidity and stability comparable to the L o phase of raft-mimicking model membranes. ,,− In addition, exosome membranes contain lipid raft-associated proteins such as GPI-anchored proteins and flotillins . Therefore, exosome membranes have been assumed to relate to raft membranes and assist in delivering their cargo through the raft domain-dependent mechanism. , …”