Critical evaluation of saponin as a permeabilizer for anandamide encapsulation in extracellular vesicles

Extracellular Vesicles (EV) have received considerable attention as drug delivery systems. Research into the loading of various exogenous cargoes in EV has been expanding in recent years. While bioactive lipids are a class of highly interesting molecules, their loading in EV remains much less explored. Here, EV isolated from a microglial cell line were loaded with the endocannabinoid anandamide (AEA), using different approaches: passive incubation, sonication and permeabilization by saponin. The addition of saponin increased the amount of AEA detected in the EV suspension, as compared to passive incubation or sonication. However, our subsequent observations demonstrated that the higher quantity of AEA measured when using saponin was due to AEA solubilization into saponin micelles. The elimination of residual saponin differs according to the purification method used. Here, ultrafiltration (UF) did not eliminate saponin micelles, leading to a co-isolation of AEA-loaded EV and AEA-loaded micelles. In addition to AEA quantification by UPLC-MS/MS, we applied the generalized polarization (GP) value of Laurdan to study the impact of AEA incubation on the EV membrane. Our data show that AEA increased EV membrane fluidity, supporting AEA insertion into the membrane of the EV. More generally, this work raises awareness about the use of saponin as a permeabilizer for bioactive lipids encapsulation in EV.