Abstract
Exosomes are small extracellular vesicles (EVs) found in culture supernatants, blood, and breast milk. The size of these nanocomplexes limits the methods of EV analyses. In this study, nitrobenzoxadiazole (NBD), a fluorophore, conjugated endosome-lysosome imager, GIF-2250 and its derivative, GIF-2276, were evaluated for exosome analyses. A correlation was established between GIF-2250 intensity and protein maker levels in bovine milk exosomes. We found that high-temperature sterilization milk may not contain intact exosomes. For precise analysis, we synthesized GIF-2276, which allows for the covalent attachment of NBD to the Lys residue of exosome proteins, and labeled milk exosomes were separated using a gel filtration system. GIF-2276 showed chromatographic peaks of milk exosomes containing >3 ng protein. The area (quantity) and retention time (size) of the exosome peaks were correlated to biological activity (NO synthesis suppression in RAW264.7 murine macrophages). Heat denaturation of purified milk-derived exosomes disrupted these indicators. Proteome analyses revealed GIF-2276-labeled immunomodulators, such as butyrophilin subfamily 1 member A1 and polymeric immunoglobulin receptor. The immunogenicity and quantity of these factors decreased by heat denaturation. When milk exosomes were purified from market-sourced milk we found that raw and low-temperature sterilization milk samples, contained exosomes (none in high-temperature sterilization milk). These results were also supported by transmission electron microscopy analyses. We also found that GIF-2276 could monitor exosome transportation into HEK293 cells. These results suggested that GIF-2250/2276 may be helpful to evaluate milk exosomes.