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.

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深入了解外泌体膜：从膜流动性和极性的角度来看

已经对外泌体进行了大量研究，特别是针对膜蛋白和内含核酸的研究，关于外泌体膜中脂质的知识量一直在增加。然而，几乎没有研究外泌体膜的动态特性。以乳外泌体为例，本文中外泌体膜的表征重点在于膜的流动性和极性。估计了牛奶外泌体（来自牛乳的外泌体）的脂质组成和相状态。乳外泌体含有丰富的 Chol（占总脂质提取物的 43.6 mol%），这使得膜在液体中有序（l _o) 相通过与磷脂相互作用。为了提出外泌体囊泡货物模型，研究了模拟乳外泌体的脂质体成分：脂质体由胆固醇（Chol）、乳鞘磷脂（乳 SM）和 1-palmitoyl-2-oleoyl- sn制成-甘油-3-磷酸胆碱 (POPC)。通过使用荧光探针1,6-二苯基-1,3,5-己三烯和6-十二烷酰基-2-二甲氨基萘，研究了外泌体和模型脂质体的亚微米级膜的微环境。牛奶外泌体的膜流动性略高于具有相似 Chol 含量的 Chol/牛奶 SM/POPC 脂质体，表明存在富集的不饱和脂质。最有目的的膜特性是通过 Chol/milk SM/POPC = 40/15/45 的脂质体组合物获得的。综上所述，Chol 是乳外泌体膜的基本成分，可以构建富集的l _o相，可以增加膜的刚性并有助于外泌体的功能。