What are Exosomes?

In recent years, research of extracellular vesicles (EVs) has been advancing at an accelerating pace. While the number of scientific articles on EVs published in 2011 was approximately two hundred, the number increased to more than one thousand in 2016 and involvement of EVs in various physiological functions and pathogenic mechanisms has been suggested. Although EVs are roughly classified into at least two categories: exosomes derived from endosomes and microvesicles derived from the plasma membrane, it is difficult to strictly separate them from each other by differential centrifugation, the technique most frequently used for purification of EVs at present, and the EVs not sedimenting at 10,000×g are called "small EVs" (mainly composed of exosomes) for convenience.¹⁾

Exosomes are small membrane vesicles (approximately 30-100 nm in diameter) secreted by various cells and present in most body fluids (e.g., blood, urine, and spinal fluid) and cell culture liquids. Exosomes, membrane vesicles surrounded by a lipid bilayer, are generated within intracellular vesicles called "multi-vesicular endosomes" and released into the extracellular space by fusion of multi-vesicular endosomes with the cell membrane. Exosomes contain proteins from secretory cells, including those of endosome origin (e.g., ESCRTs), those involved in intracellular transport (e.g., Rab GTPase), and those of cell membrane origin (e.g., CD63 and CD81), as well as RNAs. Exosomes also contain the cell membrane of secretory cells and lipids from the endosome membrane (cholesterol and sphingomyelin, etc.).²⁾

Although exosomes had long been considered to be involved in release of unnecessary cell contents, exosomes are recently attracting attentions of researchers as new mediators of cell-cell communication transporting biomolecules such as lipids, proteins, and RNAs in vivo. In addition to clarifi cation of physiological or pathophysiological functions of exosomes, research aiming at clinical application of these functions is rapidly in progress, particularly focusing on diagnostic and therapeutic application as well as development of biomarkers.

Current exosome research covers almost all research areas in biomedical science (immunology, neuroscience, oncology, endocrinology, and cardiovascular research). For example, exosomes derived from immune cells have been shown to contain antigen peptide/MHC complexes and various antigens, which suggests a possibility that exosomes might regulate various immune responses such as activation/inactivation of immune cells in addition to the exchange of antigenic information between immune cells.³⁾ In the nervous system, exosomes have been found to be involved not only in regulation of neural circuits⁴⁾ but also in extracellular release of proteins causing various neurodegenerative diseases for subsequent transmission to other cells, a process that might be deeply involved in disease progression.⁵⁾ Exosomes released by cancer cells contain many biomolecules related to angiogenesis and immune evasion, suggesting that they might contribute to construction of microenvironment optimal for cancer cell growth and promotion of cancer progression.⁶⁾ In addition, the expression profile of adhesion molecules on the surface of exosomes from cancer cells has been shown to determine the destination of cancer metastasis.⁷⁾ Recently, exosomes released from adipocytes have been reported to regulate gene expression in the liver.⁸⁾ Furthermore, while many viruses leave cells by utilizing the pathway for exosome production, bacteria and parasites infecting cells are likely to regulate activities of bacteria/parasites infecting other cells via exosomes.^9,10) Most of the above-mentioned functions are mediated by secretory cell-derived biomolecules located within exosomes. In particular, since mRNAs and miRNAs of secretory cell origin were identified in exosomes, potential involvement of exosomes in horizontal transmission of gene expression information between cells has been attracting great research interest.¹¹⁾ Since these RNAs are encapsulated within the lipid bilayer membrane of exosomes, they are not susceptible to degradation by RNase and remain intact in blood or other body fluid. Exosomes incorporated into target cells fuse with the endosome membrane to release encapsulated RNAs into the cytosol of target cells. Once released into the cytosol, mRNAs are translated into proteins while miRNAs suppress translation of target genes. Thus, exosomes regulate gene expression within target cells. A single exosome is considered to contain more than several ten-thousands of proteins and more than several thousands of mRNAs and miRNAs. The composition of these biomolecules may vary depending on the type and conditions of a secretory cell which originally harbored the exosome. In addition, the composition of proteins, mRNAs, and miRNAs within an exosome is different from that within the original secretory cell, which suggests the existence of a mechanism selectively loading exosome- specific proteins and mRNAs/miRNAs into exosomes. Such specific composition of exosome RNAs makes them attractive candidates for biomarkers and targets for therapeutic development. While mRNAs within exosomes incorporated into target cells are capable of inducing expression of functional proteins, most miRNA within exosomes are present as precursors of functional miRNA and their physiological significance is under extensive investigation. Thus, since exosomes contain a wide variety of proteins, RNAs, and lipids, construction of an exosome database "ExoCarta" is currently ongoing through claassification by cell types. Furthermore, while large scale analysis of exosomes utilizing cutting-edge methodologies in proteomics, transcriptomics, and system biology are separately conducted in laboratories all over the world, EV plugin for FunRich (a stand-alone functional enrichment analysis tool) is distributed aiming at a common and integrated analysis tool. Sharing information among researchers in a wide variety of research fields is essential for promotion of future exosome research.

References

1. Kowal, J. et al. : Proc. Natl. Acad. Sci. USA, 113 (8), E968-977 (2016).
2. Colombo, M., Raposo, G. and Thery, C. : Annu. Rev. Cell Dev. Biol., 30, 255-289 (2014).
3. Bobrie, A., Colombo, M., Raposo, G. and Thery, C. : Traffic, 12 (12), 1659-1668 (2011).
4. Bahrini, I., Song, J. H., Diez, D. and Hanayama, R. : Sci. Rep., 5, 7989 (2015).
5. Kramer-Albers, E. M. and Hill, A. F. : Curr. Opin. Neurobiol., 39, 101-107 (2016).
6. Tkach, M. and Thery, C. : Cell , 164 (6), 1226-1232 (2016).
7. Hoshino, A. et al. : Nature, 527 (7578), 329-335 (2015).
8. Thomou, T. et al. : Nature, 542 (7642), 450-455 (2017).
9. Izquierdo-Useros, N., Puertas, M. C., Borras, F. E., Blanco, J. and Martinez-Picado, J. : Cell. Microbiol., 13(1), 10-17 (2011).
10. Regev-Rudzki, N. et al. : Cell, 153 (5), 1120-1133 (2013).
11. Valadi, H. et al. : Nat. Cell Biol., 9 (6), 654-659 (2007).