Bringing Light to Extracellular Vesicles

Origin of extracellular vesicles

Extracellular vesicles (EVs) are heterogeneous, membrane-encapsulated particles that are secreted by cells across the tree of life.^1–3 Though originally thought of as strictly a waste disposal system, EVs have been shown to have myriad cell communication functions. Two main classes of EVs exist: microvesicles are large (200-2000 nm diameter) vesicles produced by direct budding from the plasma membrane (PM), while exosomes are small (40-200 nm) and are primarily produced when a multivesicular body fuses with the PM, releasing intraluminal vesicles as exosomes. Exosomes directly budding from the PM has been observed in T-cells; however, the frequency of this phenomenon in other cell types is unclear.^2,4

Exosomes and microvesicles share several features; their proteomes resemble the originating cell but are enriched in certain proteins. Tetraspanins are membrane proteins that act as molecular organizers and include CD9 and CD81 in microvesicles and exosomes, while CD63 is a specific marker for exosomes.^1,2 Exosomes also contain a dense intraluminal core of scaffolding and aggregated protein bound with chaperones, primarily HSP70.² RNA and DNA are common cargoes in EVs and can elicit responses including gene transcription or suppression in recipient cells.^1,2,5,6

The tropism of EVs varies by their composition and the recipient cell type: the tetraspanins and other membrane proteins determine EV specificity by interacting with integrins, lectins, or extracellular matrix components.¹ Exosomes secreted by cortical neurons are taken up only by neurons, whereas exosomes from the N2a neuroblastoma cell line target neurons and glia.⁷ The mechanism of EV uptake –macropinocytosis, phagocytosis, clathrin or caveolin-dependent endocytosis, or direct membrane fusion—is also cell-type dependent.¹

Extracellular vesicles in disease and diagnostics

EVs carry cargoes of proteins, RNA, and DNA that function in the recipient cells.^1,2,5,6 Microvesicles secreted by cancer cells (sometimes called oncosomes) can transport oncogenic mRNA that promotes invasion and metastasis in breast cancer cells,⁸ and exosomes from pancreatic cancer cells initiate transformation in normal cells.⁹ EVs can also contribute to immunity and immunosuppression: antigen presenting cells secrete exosomes containing major histocompatibility complex class II bound with antigen, which can promote adaptive immunity over a distance,⁵ and tumor-derived EVs can promote immunosuppression within the tumor microenvironment.⁵ Viruses can use EVs to escape from cells and infect others; for example, rotavirus capsids hide within large secreted microvesicles and HIV Gag proteins use T-cell derived exosomes.^4,10

Exosomes have also been associated with neurodegeneration. Prion protein is present in brain-derived exosomes, where it plays roles in immunomodulation, plasticity, and normal brain function. Pathogenic prion protein in spongiform encephalopathies also gets carried by exosomes along with other factors—microRNAs in particular—that potentiate its neurotoxic effects.¹¹ MicroRNAs associated with the progression of Alzheimer’s Disease and Parkinson’s Disease have also been found in brain-derived exosomes. Exosomes’ roles in the pathogenesis of these and other neurodegenerative diseases remain to be determined.

Liquid Biopsies

Because EVs can travel far and wide through biofluids, one of the most exciting prospects is developing non-invasive “liquid biopsies” to monitor for disease or to sample hard-to-access organs such as the brain or pancreas.^6,12,13 Glypican-1 is seen in pancreatic cancer exosomes isolated from the blood of patients and can be used as a biomarker.¹⁴ Liquid biopsies could be used for early detection of cancers, neurodegenerative disease, pregnancy disorders, and other applications.¹²

The challenges of analyzing EVs

As small heterogeneous extracellular particles, EVs pose challenges to characterize; a principal confounder is the lack of standardized isolation and analysis methods.^5,15,16 EVs have classically been isolated by ultracentrifugation or sucrose gradient ultracentrifugation,^2,3 but the purity of isolated EVs is poor.¹⁵ New isolation techniques such as acoustic nanofiltering and microfluidic immuno-isolation improve purity, but require specialized equipment.

Scanning and transmission electron microscopy can readily visualize EVs, but sample preparation creates artifacts and, depending on the preparation method, can cause EV sampling bias.^2,4,16 Fluorescence imaging with lipophilic dyes in culture systems are confounded by other small particles in biofluids and cell culture media like serum proteins and lipoproteins.¹⁷ Mass spectroscopy, NGS and proteomics are commonly used to analyze EV content, but heterogeneity and purity are lingering concerns.

Analyzing exosomes with flow cytometry is a new field; one challenge is distinguishing exosomes from cellular debris. Fortunately, EVs have well-defined markers and properties that can be exploited:^1,16 CD9, CD63, and CD81 are markers for EVs, with CD63 being specific to exosomes. In certain types of EVs, Ep-CAM and HSP70 are useful targets.^1,5 For flow cytometry of EVs, it is crucial to avoid forming aggregates as the loss of individual resolution is disastrous for such a heterogeneous population.¹⁵  Antibodies validated for EV detection by flow cytometry are a key starting point. Membrane stains validated for flow cytometry are useful for measuring EV size by fluorescence intensity.

The Future of EVs

EVs represent an exciting and booming field of cell biology with life-changing diagnostic and therapeutic potential. Better understanding and characterization of EVs is critical in making this potential into reality. Biotium’s curated EV technology resource will assemble the latest validated tools for EV research. Without a doubt, EV research will drive advances in cell biology and medicine.

References

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