Introduction: The term “extracellular vesicle” (EV) is broadly used to describe naturally released cellular vesicles with a lipid bilayer and without a nucleus. EV represent a heterogeneous population that is generally categorized into subpopulations based on characteristics such as size, biogenesis, function, and composition. The field of extracellular vesicle (EV) research is growing rapidly, but current metrics to delineate differences between vesicle subpopulations are limited. Recently, a type of EV embedded within the extracellular matrix of soft tissues termed matrix-bound nanovesicles (MBV) has been described but defining characteristics of MBV relative to other types of EV have not been established. The present study compared three distinct subpopulations of EV, namely mineralization-competent matrix vesicles (cMV), exosomes (Exo), and MBV.
Methods: MBV were isolated from mouse skeletal muscle that was decellularized, minced, and digested enzymatically to release the MBV from the extracellular matrix. cMV were isolated from the matrix of mineralizing mouse 17IIA11 pre-odontoblast cells. Exo were isolated from mouse plasma. The three types of vesicles were quantified using NanoSight Nanoparticle Tracking Analysis. The vesicle size, physical characteristics, protein cargo, miRNA cargo, and lipid membrane composition were compared. The immunomodulatory activity on naïve and pro-inflammatory bone-marrow derived macrophages was also evaluated.
Results: NanoSight and transmission electron microscopy (TEM) showed similar size and morphology of each vesicle type. Characterization of protein cargo showed that MBV contain low levels of proteins commonly associated with both Exo and cMV, including CD63, CD81, Annexin V, and alkaline phosphatase. Each vesicle type was associated with a unique profile of cytokine cargo. Lipidomic analysis showed clear differences in membrane composition of MBV relative to both Exo and cMV. The miRNA cargo of MBV and cMV were more similar while Exo-associated miRNA were highly distinct. Characterization of the immunomodulatory activity of each vesicle type showed that cMV induce pro-inflammatory activation of macrophages, while MBV induced a more anti-inflammatory phenotype.
Conclusion: The present study shows that despite similar physical characteristics, these three types represent distinctly unique subpopulations of EV. These results inform metrics for proper categorization of EV and guides choice of EV based on desired functional parameters.