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Exosomes: Distinctly Different?

Initial doubts arose from markers said to be unique to exosomes but which are also found on MP in the larger size range. For example, the “canonical exosome proteins” are said to include CD63 and flotillin [5], but CD63 is also a marker of platelet MP (PMP), and flotillin is a marker of lipid rafts of the plasma membrane, including red blood cells (RBC) [6]. Similarly, lysosomal protein LAMP-1 (CD107a) is often listed as exosomes-specific but is also a marker of PMP [7]. In addition, if exosomes originate from multivesicular endosomes, one wonders how to explain their lipid raft proteins or the fact that 60 nm “nanovesicles” were described long ago from RBC, which lack endosomes [8], and more recently [9].

Only in the last few years have these uncertainties been acknowledged and addressed. Xu et al. compared proteomic profiles between exosomes and MP (aka ECV) from a cancer cell line and observe that a majority of about 180 proteins listed were absent from one but not the other particle type [10], showing qualitative difference. With respect to functional differences, it was shown that exosomes from RBC, but not the larger MP fraction, induced a T-cell response [9]. Kanada et al. demonstrate qualitative differences between exosomes vs. the larger vesicles in their respective ability to transport and deliver functional RNA or DNA to target cells, finding exosomes but not MP deliver RNA whereas MP but not exosomes deliver DNA [11].

These and other reports confirm a qualitative distinction. Some authors speak of ectosomes as opposed to exosomes, to indicate shedding from the plasma membrane, e.g., [12, 13]. However, this distinction is not easily made, partly because exosomes fuse with plasma membrane (lipid rafts) before release, explaining how they acquire markers of the plasma membrane, e.g., [14]. Some authors avoid the term, exosomes, using the more inclusive “extracellular vesicles” [11]. In practice, distinguishing exosomes is not simple, as shown by many papers on various isolation methods, [15-17], among others.

 
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