Exosome: A New Player in Translational Nanomedicine

Abstract

Exosomes are extracellular vesicles released by the vast majority of cell types both in vivo and ex vivo, upon the fusion of multivesicular bodies (MVBs) with the cellular plasma membrane. Two main functions have been attributed to exosomes: their capacity to transport proteins, lipids and nucleic acids between cells and organs, as well as their potential to act as natural intercellular communicators in normal biological processes and in pathologies. From a clinical perspective, the majority of applications use exosomes as biomarkers of disease. A new approach uses exosomes as biologically active carriers to provide a platform for the enhanced delivery of cargo in vivo. One of the major limitations in developing exosome-based therapies is the difficulty of producing sufficient amounts of safe and efficient exosomes. The identification of potential proteins involved in exosome biogenesis is expected to directly cause a deliberate increase in exosome production. In this review, we summarize the current state of knowledge regarding exosomes, with particular emphasis on their structural features, biosynthesis pathways, production techniques and potential clinical applications.

Keywords: CARs; cancer; exosomes; gene editing; immunotherapy; liquid biopsies.

Figure 1

Different exosome biogenesis pathways. Exosome formation begins with syntenin-syndecan interactions which require direct interaction between ALIX and CHMP4 proteins. The intervention of two additional components, Tsg 101 (ESCRT-1) and Vps22 (ESCRT-II), has also been reported, although their mode of action remains little understood. Exosome formation is further regulated by heparanase, an enzyme that cleaves syndecan heparan sulfate, while the small GTase Arf6 also plays a crucial role. The small GTPase ADP ribosylation factor 6 (Arf6) and its effector phospholipase D2 (PLD2) regulate the syntenin pathway. The interaction of Arf6 and PLD2 affects exosome formation by controlling the budding of intraluminal vesicles (ILVs) in multivesicular bodies (MVBs). The silencing of hepatocyte growth-factor-regulated tyrosine kinase substrate (Hrs)proteins, which interact with the tumour susceptibility gene 101 (tsg101) in exosome biogenesis, decreases the number of vesicles [31]. As interferon-stimulated gene 15 (Isg15) expression inhibits Tsg101 ubiquitination, the disruption of tsg15 may increase exosome release. The upregulation of the tumor-suppressor-activated pathway 6 (TSAP6), a p53-inducible transmembrane protein, has been shown to increase exosome production [32]. Two other possibilities are involved in ESCRT-independent pathway: the ceramide-based sphingomyelinase (SMase) pathway, in which sphingomyelin is hydrolysed into phosphorylcoline, and ceramide, which contributes to alternative exosome production. The third pathway is a tetraspanin-dependent pathway that involves CD63, belonging to the superfamily of tetraspanins, which, along with their partner molecules, form tetraspanin-enriched microdomains that contribute to exosome formation. Furthermore, exosome trafficking is regulated by the small GTPase, a member of the Rab and Ral protein superfamilies. For instance, Rab11, together with Rab27a/b, facilitate exovesicular secretion in a calcium-dependent manner [33]. Finally, SNARE and syntaxin 5 proteins enable vesicles to dock and fuse with the plasma membrane and to release exosomes into the external medium.

Figure 2

Secretion of exosomes associated with immune cells types and their modes of action.