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Extracellular molecules, such as nucleotides, lipids, short peptides or proteins, are released by cells and bind to receptors on the other cells, which are important mediators in cell-to-cell communications in multicellular organisms. In addition to single molecules, eukaryotic cells can also release membrane vesicles into extracellular environment, such as microvesicles, apoptotic blebs and exosomes. Exosomes represent a specific subtype of secreted membrane vesicles, showing a characteristic cup-shaped or well delimited round morphology when observed by electron microscopy. They are small membrane vesicles with a diameter of 30~100 nm. Derived from the endosomal compartments called multivesicular bodies (MVBs), exosomes contain diverse nucleic acids (mRNAs and miRNAs), lipids, peptides and proteins derived from the parent cells. Therefore, exosomes are thought to play an important role in intercellular communications, mediating numerous physiological processes (beyond immunology to neurobiology, stem cell and tumor biology) of the recipient cells. These features have prompted extensive research to exploit them as a source of biomarkers for several pathologies, such as inflammatory diseases, autoimmune disorders, neurodegenerative disorders and cancer.
Figure 1. Schematic of protein and RNA transfer by exosomes. (Graça R., et al. 2013)
In addition to the morphological properties, exosomes are usually identified based on their unique protein and lipid composition. Origin from endosomal system, exosomes contain proteins involved in membrane transport and fusion (e.g. RAB, annexins and flotillins), MVBs biogenesis (soluble N-ethylmaleimide-sensitive factor attachment protein receptors, SNAREs), integrins and tetraspanins (e.g. CD63, CD9, CD81 and CD82), cytoskeleton (e.g. β-actin, myosin, cofilin and tubulins), major histocompatibility complex (MHC) class I and II molecules, and mediators of intercellular cell signaling (e.g. IL-1β, TNF-α or TGF-β).
Exosomes are generally considered as an intermediate compartment between the plasma membrane (where endocytosis takes place) and lysosomes (where degradation occurs). They are formed inside the secreting cells in endosomal compartments MVBs, which is coordinated by the endosomal sorting complex required for transport (ESCRT). ESCRT complex is composed of approximately thirty proteins that assemble into four complexes, namely ESCRT-0, ESCRT-I, ESCRT-II and ESCRT-III. ESCRT complex is usually recruited to the cytosolic side of the endosomal membrane for the sorting of selected protein to intraluminal vesicles (ILVs), which requires ubiquitination of the cytosolic tail of endocytosed receptors. The ESCRT-0 complex recognizes and sequesters ubiquitinated transmembrane proteins in the endosomal membrane, whereas the ESCRT-I binds to the ubiquitinated cargo proteins and activates the ESCRT-II complex. ESCRT-I and -II complexes, therefore, appear to be responsible for membrane deformation into buds with sorted cargo. The activation of ESCRT-II further initiates the oligomerization and the formation of the ESCRT-III components, which is subsequently disassembled by an ATPase, driving vesicle scission.
Figure 2. Model for the ubiquitin-dependent sorting of proteins by the ESCRT machinery. (Vingtdeux V. et al., 2013)
Exosome secretion is regulated by MVB fusion with the plasma membrane, where Rab proteins play an essential role in intracellular vesicle transport between different compartments. Rab proteins belong to a family of small GTPases which is composed of more than 60 members. Each of the Rab proteins is preferentially associated with one intracellular compartment, involving in either vesicle budding, mobility through interaction with the cytoskeleton, or tethering to the membrane of an acceptor compartment. The final step of exosome secretion is known to possibly involves a specific combination of Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs). Vesicular SNAREs (v-SNAREs), which are localized on MVBs, interact with target SNAREs (t-SNAREs) that are localized on the intracellular side of the plasma membrane, to form a membrane-bridging SNARE complex, responsible for membrane fusion It has also been reported that calcium influx can induce exosome release in mast cells, human erythroleukemia cells, cultured cortical neurons and oligodendrocytes.
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