Understanding the extracellular vesicle surface for clinical molecular biology

Abstract

Extracellular vesicles (EVs) are lipid-membrane enclosed nanoparticles that play significant roles in health and disease. EVs are abundant in body fluids and carry an array of molecules (proteins, lipids, nucleic acids and glycans) that reflect the identity and activity of their cell-of-origin. While the advent of high throughput omics technologies has allowed in-depth characterisation of EV compositions, how these molecular species are spatially distributed within EV structures is not well appreciated. This is particularly true of the EV surface where a plethora of molecules are reported to be both integral and peripherally associated to the EV membrane. This coronal layer or 'atmosphere' that surrounds the EV membrane contributes to a large, highly interactive and dynamic surface area that is responsible for facilitating EV interactions with the extracellular environment. The EV coronal layer harbours surface molecules that reflect the identity of parent cells, which is likely a highly valuable property in the context of diagnostic liquid biopsies. In this review, we describe the current understanding of the mechanical, electrostatic and molecular properties of the EV surface that offer significant biomarker potential and contribute to a highly dynamic interactome.

FIGURE 1

The main lipid components and their general distribution across the two leaflets of the membrane bilayer of small and large‐EV sub‐types. EV membranes are typically comprised of phosphatidylcholine (PC) and sphingomyelin in the outer leaflet. PC and phosphatidylserine (PS) are phospholipids with a cylindrical shape and contribute to the formation of flat bilayer sheets. Approximately 60% of the total EV‐associated PS is externalised to the outer leaflet in larger EV sub‐types, while PS is localised on the inner leaflet of small‐EV sub‐types. Cholesterol is distributed across the two leaflets and is enriched two‐fold in the membranes of small‐EVs compared to larger‐EVs. Phosphatidylethanolamine (PE) is also enriched two‐fold in smaller‐EVs compared to larger‐EV sub‐types and is a conical lipid that exerts a positive curvature to membranes by encouraging the phospholipid heads to pack closer together during membrane bending. Phosphatidylinositol (PI) has an inverted conical shape and exerts a negative curvature to membrane bilayers.

FIGURE 2

Molecular components that comprise the EV surface. Molecules that are integral to the EV membrane include receptors (G‐protein‐coupled receptors, cytokine receptors, integrins, GPI‐anchored receptors and lipoprotein receptors), transporters, immunoglobulin superfamily proteins, major histocompatabilty complex, glycoproteins, enzymes and CD antigens. Peripheral molecules also contribute to the EV surface molecular repertoire by binding to the integral proteins or interacting with the EV lipid membrane. Molecules that interact with integral EV membrane proteins include cytokines, extracellular matrix components, plasminogen, lipoproteins and immunoglobulins. Molecules that have been shown to interact with the EV membrane include enzymes, DNA/RNA binding proteins, DNA, RNA, annexin, albumin and components of the coagulation cascade.