Exosomes and Their Role in the Life Cycle and Pathogenesis of RNA Viruses

Abstract

Exosomes are membrane-enclosed vesicles actively released into the extracellular space, whose content reflect the physiological/pathological state of the cells they originate from. These vesicles participate in cell-to-cell communication and transfer of biologically active proteins, lipids, and RNAs. Their role in viral infections is just beginning to be appreciated. RNA viruses are an important class of pathogens and affect millions of people worldwide. Recent studies on Human Immunodeficiency Virus (HIV), Hepatitis C Virus (HCV), human T-cell lymphotropic virus (HTLV), and Dengue Virus (DENV) have demonstrated that exosomes released from infected cells harbor and deliver many regulatory factors including viral RNA and proteins, viral and cellular miRNA, and other host functional genetic elements to neighboring cells, helping to establish productive infections and modulating cellular responses. Exosomes can either spread or limit an infection depending on the type of pathogen and target cells, and can be exploited as candidates for development of antiviral or vaccine treatments. This review summarizes recent progress made in understanding the role of exosomes in RNA virus infections with an emphasis on their potential contribution to pathogenesis.

Keywords: RNA virus; exosomes; infection; miRNA; microvesicles; pathogenesis.

Figure 1

Structure and composition of exosomes. Exosomes contain a plasma membrane-derived phospholipid bilayer membrane. The composition depends on the cell type of origin, state of health of the host, and extracellular stimuli. Exosome contents include mRNA, miRNA, DNA, and proteins like annexins, tetraspanins, Alix, TSG101, MHC molecules, Rab proteins, cytoskeletal proteins, enzymes, and signal transduction proteins. GAPDH: Glyceraldehyde 3-phosphate dehydrogenase PGK: 3-phosphoglycerate kinase, PK: pyruvate kinase, EGFR: epidermal growth factor receptor, CDC42: cell division control protein 42, PI3k: phosphatidylinositide 3-kinases, ARF1: ADP-ribosylation factor 1, MUC1: Mucin 1, vRNA: viral RNA, vmiRNA: viral miRNA.

Figure 2

Schematic representation of exosome biogenesis and release. Exosome generation starts with early endosome formation during endocytosis. The membrane proteins are internalized through clathrin-coated vesicles and delivered to early endosomes. This leads to intraluminal vesicles (ILVs) formation by inward budding of the limiting membrane and multi vesicular bodies (MVBs) are formed. Upon maturation the exosome-filled MVBs are either sent to lysosomes for degradation or fused with the plasma membrane to release exosomes to the extracellular milieu. RER: Rough Endoplasmic Reticulum, GC: Golgi complex.

Figure 3

Schematic representation of exosome isolation strategies. Graphical representation of exosome isolation by both ultracentrifugation- and precipitation reagent-based isolation procedures, and analytical tools for exosome identification.

Figure 4

Schematic representation of exosome immune-isolation. To obtain exosomes free of contaminants, exosomes can be subjected to immunomagnetic selection using anti-CD63 antibody conjugated beads and then characterized by immunoblot, electron microscopy, and size determination.