Extracellular Vesicles Exploit Viral Entry Routes for Cargo Delivery

Abstract

Extracellular vesicles (EVs) have emerged as crucial mediators of intercellular communication, being involved in a wide array of key biological processes. Eukaryotic cells, and also bacteria, actively release heterogeneous subtypes of EVs into the extracellular space, where their contents reflect their (sub)cellular origin and the physiologic state of the parent cell. Within the past 20 years, presumed subtypes of EVs have been given a rather confusing diversity of names, including exosomes, microvesicles, ectosomes, microparticles, virosomes, virus-like particles, and oncosomes, and these names are variously defined by biogenesis, physical characteristics, or function. The latter category, functions, in particular the transmission of biological signals between cells in vivo and how EVs control biological processes, has garnered much interest. EVs have pathophysiological properties in cancer, neurodegenerative disorders, infectious disease, and cardiovascular disease, highlighting possibilities not only for minimally invasive diagnostic applications but also for therapeutic interventions, like macromolecular drug delivery. Yet, in order to pursue therapies involving EVs and delivering their cargo, a better grasp of EV targeting is needed. Here, we review recent progress in understanding the molecular mechanisms underpinning EV uptake by receptor-ligand interactions with recipient cells, highlighting once again the overlap of EVs and viruses. Despite their highly heterogeneous nature, EVs require common viral entry pathways, and an unanticipated specificity for cargo delivery is being revealed. We discuss the challenges ahead in delineating specific roles for EV-associated ligands and cellular receptors.

FIG 1

Exosomes enter internal compartments of dendritic cells. Green fluorescent exosomes from EBV-infected B cells internalized by primary, activated monocyte-derived dendritic cells (MoDCs) are shown. The actin cytoskeleton is shown in red (phalloidin staining) and this highlights protrusions of the activated MoDCs.

FIG 2

Common mechanism of fusion. Here, fusion of a virus with the plasma membrane is shown, giving an example of the common (not virus-specific) mechanism of fusion. Initial contact is made through tethering. Involved molecules can contribute to target specificity and are regularly linked to fusion machinery. Tethering is followed by docking, as the fusion machinery connects the two membranes. Changes in tertiary structure of this machinery, noted as protein folding, culminate in fusion pore formation and subsequent dilatation. Insertion of fusion loops into the host cell membrane is necessary for entry.

FIG 3

HIV cell entry. HIV enters target cells via fusion with the concerted action of viral glycoproteins gp120 and gp41 and host cofactors such as CD4, CXCR4, and CCR5. The glycoproteins are visible as spikes on electron microscopy images.

FIG 4

Extracellular vesicle entry via fusion. Different viral and nonviral molecules can facilitate fusion of vesicles with the target cell. HCV-infected cell-derived exosomes are enriched in CD81 and HCV glycoproteins and could possibly be taken up via fusion (37). EBV-infected cell-derived exosomes are enriched for LMP1 and gp350, possibly facilitating fusion (98, 101). Furthermore, HSV gB, CD9, and other tetraspanins could have a role in EV entry via fusion (84, 93, 99).

FIG 5

Clathrin-mediated endocytosis used by EVs. EPS15, one of the two main integral components of a clathrin-coated pit, has a partial role in uptake of possible subtypes of EVs via CME. Blocking endocytic pathways partially inhibits EV uptake (34, 127). Villous trophoblast-derived exosomes seem to incorporate syncytins 1 and 2, which could facilitate endocytosis of these EVs (78, 103, 105).

FIG 6

HCV cell entry. HCV has a plethora of molecules involved in entry via receptor-mediated endocytosis. Initial attachment to the target cell is facilitated by HSPG, LDLR, or L-SIGN/DC-SIGN. Followed by interaction with the main receptors SR-B1 and CD81, TfR1 (not shown) could have a post-CD81 role in HCV entry, and SRFBP1 (not shown) is possibly recruited to CD81 during HCV uptake, coordinating host cell penetration. Late actors in the entry process are CLDN-1 and OCLN. NPC1L1 (not shown) might act in the entry process concerning cholesterol transport.

FIG 7

Apoptotic mimicry and receptor-mediated EV entry. (A) Tetraspanins and integrins. The Tspan8-CD49d (ITGα4β1) complex on exosomes possibly binds to VCAM-1 on target cells (84). In general, EVs could attach to the target cell via ITGα1β1, ICAM-1, or VCAM-1 (71). ICAM-1 on DC-derived exosomes could bind to LFA-1 (ITGαLβ2) on T cells/DCs and ITGαvβ3 on DCs (61, 72, 73). (B) Lectins. GM3 on B-cell-derived exosomes may bind to Siglec-1 for uptake by macrophages (67, 147). (C) Phosphatidylserine receptors. PtdSer is exposed on the outer leaflet of the exosomal membrane, and the Tam family of PtdSer receptors, which include but are not restricted to KIM-1, TIM-1, and TIM-4, mediates EV uptake. MFG-E8, enriched on exosomes, might mediate EV uptake by acting as a bridge between PtdSer and integrins or via Gas6 interactions (10, 176). (D) HSPG. Heparan sulfate proteoglycans act as internalizing receptors of cancer cell-derived EVs (152).