Exosomes and Microvesicles

Exosomes and microvesicles (EMVs) are secreted organelles that have the same topology as the cell and a diameter of ~50–250 nm. EMVs contain a wide variety of proteins, lipids, carbohydrates, and RNAs. Although exosomes are assumed to arise via multivesicular body (MVB)-like structures, many cells target exosomal proteins and lipids to discrete domains of the plasma membrane that serve as sites of exosome budding. Furthermore, it is now known that (1) the budding of EMV proteins requires their targeting to the plasma membrane, (2) targeting EMV proteins to endosome membranes does induce their budding, and (3) inhibiting MVB biogenesis does not inhibit exosome release. Based on these and other observations, it appears that the plasma membrane is a major site of exosome biogenesis, perhaps even more than endosomes. Cells manufacture EMVs to facilitate protein quality control at the plasma membrane, and to assist in the development of cell polarity. In addition, free EMVs can transmit signals to neighboring cells, and in some instances they complete a pathway of intercellular vesicle traffic, delivering proteins and RNAs from cell to cell. EMVs have been implicated in several physiological processes, particularly during immune signaling, and in multiple diseases, particularly HIV/AIDS and cancer.