Exosomes from Human Placenta Purified by Affinity Chromatography on Sepharose Bearing Immobilized Antibodies Against CD81 Tetraspanin Contain Many Peptides and Small Proteins

Exosomes are nanovesicles (40-100 nm) containing various RNAs and different proteins. Exosomes are involved in intracellular communication and immune system function. Exosomes from different sources are usually isolated using standard methods-centrifugation and ultracentrifugations. Exosomes isolated by these procedures were reported to contain from a few dozen to thousands of different proteins. Here crude vesicle preparations from five placentas (normal pregnancy) were first obtained using standard centrifugation procedures. According to electron-microscopic studies, these preparations contained vesicles of different size (30-225 nm), particles of round shape of average electron density ("nonvesicles" 20-40 nm) (A), structured clusters of associated proteins and shapeless aggregations (B), as well as ring-shaped 10-14 nm structures formed by ferritin (C). After additional purification of the vesicle preparations by gel filtration on Sepharose 4B, the main part of protein structures was removed; however, the preparations still contained small admixtures of components A-C. Further purification of the preparations by affinity chromatography on Sepharose bearing immobilized antibodies against exosome surface protein CD81 led to isolation of highly purified exosomes (40-100 nm). These exosomes according to electron microscopy data contained tetraspanin embedded in the membrane, which was stained with antibodies against CD81 conjugated with 10-12 nm gold nanoparticles. SDS-PAGE and MALDI MS and MS/MS mass spectrometry of tryptic hydrolysates of proteins contained in these exosomes revealed eleven major proteins (>10 kDa): hemoglobin subunits, CD81, interleukin-1 receptor, annexin A5, cytoplasmic actin, alpha-actin-4, alkaline phosphatase, human serum albumin, serotransferrin, and lactotrasferrin. Using MALDI mass analysis of the highly purified exosomes, we for the first time found that in addition to the large proteins (>10 kDa), exosomes having affinity to CD81 contain more than 27 different peptides and small proteins of 2-10 kDa. This finding can be useful for revealing biological functions of pure exosomes. © 2018 IUBMB Life, 70(11):1144-1155, 2018.