Virus-modified exosomes for targeted RNA delivery; a new approach in nanomedicine

Abstract

A major goal in biomedical research is to clinically reverse the cause of disease rather than treating the symptoms. Gene therapy has the potential to meet this goal and the discovery of RNA interference (RNAi) has lead to a new class of highly selective therapeutics. However, initial enthusiasm is reduced due to safety concerns associated with virus-based delivery vectors that are used for in vivo delivery. Viral vectors for siRNA delivery into target cells are used because of their high target specificity and delivery efficacy (endosomal escape). Recent discoveries suggest that a specialized form of nano-sized lipid vesicles called exosomes can incorporate and transport functional RNAs into target cells and may serve as an attractive alternative. Evidence is accumulating that most pluricellular organisms sustain exosome-based communications via inter-cellular exchange of mRNA and miRNAs between cells. We discovered that viruses have found ways to exploit this communication pathway and we argue here that adaptations of exosomes imposed by viruses maybe exploited for superior delivery of RNA in vivo. We discuss recent discoveries in exosome biogenesis their physical properties, targeting and delivery strategies and how the knowledge of exosome production in virus infected cells could propel their entry into clinical settings.

Graphical abstract

Fig. 1

Formation and release of virus-modified exosomes. Intraluminal vesicles the presumed precursors of exosomes, are formed by inward budding in the lumen of multivesicular bodies (MVBs). The process of internal budding leads to an encapsulation of cytoplasmic content from the cell of origin, including host and/or viral components such as nucleic acids (e.g. small non-coding RNA and mRNA) and proteins (e.g. proteins situated in the cytoplasm, membrane bound proteins, viral (glyco)proteins). The exact mechanism of how nucleic acids are packaged into exosomes is unclear and maybe due to simple (random) inclusion of molecules or by active and selective packaging of certain nucleic acid species such as miRNAs. Induced exosome release and selective cargo incorporation are evident in virus-infected cells. When released to the extracellular environment, exosomes have the same orientation as the cell membrane and have been shown to display many of the surface markers from their cell of origin. While incorporation of viral proteins into exosomes appears to be selective; the criterion determining their exosomal faith is yet to be defined. (Nucleus, N; Epstein - Barr virus, EBV).

Fig. 2

Extracellular exosomes, expressing a defined set of proteins and lipids, deliver small non-coding RNA to a target cell. When exosomes leave the cell of origin, some will enter the blood stream or other bodily fluids where they can be taken up by other cells as a means of cell-to-cell communication. Depending on a targeting strategy, bioengineered or virus-modified exosomes are destined to engage cell-specific receptors. Exosomes that are taken up by endocytosis will fuse with the endosomal membrane to release their genetic cargo into the cytoplasm where they might associate with the RNAi (RISC) machinery to block mRNA translation into protein. (Nucleus, N).