Small extracellular vesicles in milk (sMEVs) have attracted attention in drug delivery and as bioactive food compounds. Previous studies implicate galactose residues on the sMEV surface in sMEV transport across intestinal and endothelial barriers in humans, but details of glycoprotein-dependent transport are unknown. Researchers at the University of Nebraska-Lincoln used a combination of cell biology and genetics protocols to identify glycoproteins on the sMEV surface that facilitate sMEV absorption. The researchers identified 256 proteins on the bovine sMEVs surface by using LC-MS/MS, and bioinformatics analysis suggested that 42, 13, and 13 surface proteins were N-, O-, and 13 C-glycosylated, respectively. Lectin blots confirmed the presence of mannose, galactose, N-acetyl galactose, fucose, and neuraminate. When surface proteins were removed by various treatment with various proteases, sMEV uptake decreased by up to 58% and 67% in FHs-74 Int and Caco-2 cells, respectively, compared with controls (P < 0.05). When glycans were removed by treatment with various glycosidases, sMEV uptake decreased by up to 54% and 74% in FHs-74 Int and Caco-2 cells, respectively (P < 0.05). When galactose and N-acetyl galactosamine residues were blocked with agglutinins, sMEV uptake decreased by more than 50% in FHs-74 Int cells (P < 0.05). When bovine sMEVs were administered to Galectin-3 knockout mice by oral gavage, hepatic sMEV accumulation decreased by 56% compared with wild-type mice (P < 0.05), consistent with a role of β-galactoside glycan structures in the absorption of sMEVs. The researchers conclude that sMEVs are decorated with glycoproteins, and Galectin-3 and its galactose ligands are particularly important for sMEV absorption.
This is the first paper to assess the role of unique glycans and their Galectin-3 receptor in the transport and distribution of small extracellular vesicles (“exosomes”) from milk in mammals. The research assessed milk exosome transport and distribution by using multiple approaches and platforms including cell cultures, various exosome labels, knockout and mutant mice, enzymatic removal of surface proteins and glycans, and lectin blocking of glycans.