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

Area of Science:

• Biochemistry
• Membrane Biology
• Extracellular Vesicles

Background:

• Exosomes are increasingly studied, with growing knowledge of their proteins, nucleic acids, and lipids.
• The dynamic properties of exosomal membranes, such as fluidity and polarity, remain under-explored.
• Milk exosomes serve as a model to investigate exosomal membrane characteristics.

Purpose of the Study:

• To characterize the membrane fluidity and polarity of milk exosomes.
• To determine the lipid composition and phase state of milk exosomal membranes.
• To propose a model for exosomal vesicle cargo based on mimicking liposome compositions.

Main Methods:

• Milk exosomes were analyzed for lipid composition and phase state.
• Model liposomes mimicking milk exosome composition (cholesterol, milk sphingomyelin, POPC) were created.
• Fluorescent probes (DPH and Laurdan) were used to investigate membrane microenvironments.

Main Results:

• Milk exosomes showed enriched cholesterol (43.6 mol%), promoting a liquid-ordered (l o) phase.
• Milk exosome membrane fluidity was slightly higher than comparable model liposomes, indicating unsaturated lipids.
• Optimal membrane properties were achieved with a liposome composition of 40% cholesterol, 15% milk SM, and 45% POPC.

Conclusions:

• Cholesterol is a fundamental component of milk exosomal membranes, essential for the liquid-ordered phase.
• The liquid-ordered phase enhances membrane rigidity, contributing to exosome function.
• This research provides insights into exosome membrane dynamics and lipid-based cargo delivery models.