Liquid biopsy, that is, sampling and analysis of DNA, RNA and proteins found in blood or other body fluids, offers a potentially non-invasive and cost-effective way to gain insight into cancer and other disease detection and progression. Within blood, both healthy and diseased cells release small vesicles termed extracellular vesicles (EVs) that carry information from their parent donor cells to recipient cells in the form of nucleic acids and enzymes. EVs have been classified into four types based upon their origin and size (Table 1) (1).
Table 1. Comparison of extracellular vesicle types.
|EV type||Parent cell||Size (nm)||Formation|
|Microvesicles||Normal and diseased cells||100‒1000||Budding|
|Exosomes||Normal and diseased cells||40‒120||Multi-vesicular bodies|
|Apoptotic bodies||Apoptotic cells||50‒2000||Blebs|
|Oncosomes||Cancer cells||1000‒10,000||Budding, multi-vesicular bodies|
Analyses of RNA and miRNA from these structures have shown that exosome RNA content is distinct from that found in the cytoplasm of donor cells from which they were derived, thus suggesting active RNA sorting into these vesicles (2). The exact mechanism by which RNA and miRNA is sorted into exosomes is unclear. One suggested mechanism of exosomal miRNA sorting may involve association with the RNA-inducing silencing complex (RISC) (3). Argonaut 2 (Ago2) is a key component of the RISC, and binding of Ago2 to miRNAs results in the generation of Ago2-miRNA complexes. These complexes have been found both in the extracellular space as well as in secreted exosomes. McKenzie, A.J. et al. (2016), demonstrated that KRAS-dependent activation of MEK-ERK (mitogen-activated protein kinase kinase/extracellular-signal-regulated kinase) signaling in mesenchymal stem cells inhibited sorting of Ago2 and Ago2-dependent miRNAs into exosomes, and established a molecular mechanism for the regulation of Ago2 sorting and miRNA loading into exosomes (3).
Exosomes containing pre-sorted molecules are transported, sometimes distantly, from their cells of origin. The number of exosomes released and taken up by recipient cells is likely dependent on several factors, including cell type, physiological state, and micro-environmental conditions (1). Once integrated into recipient cells, exosomes and their cargo may interact with their recipient cells, playing roles in activities such as promoting cell proliferation, supporting induction of epithelial-to-mesenchymal transition, or enhancing the migration and metastasis of tumors (1).
Although much emphasis has been placed on the fate of recipient cells following exosome uptake, much less data is available for the fate of exosome donor cells, particularly regarding the effect of miRNA sorting and exosome export. A recent publication by Teng, Y. et al. (2016), examined the effect of major vault protein (MVP) and its role in exosomal sorting and export on donor mouse colon cancer cells. MVP has previously been suggested to interact with tumor suppressor miR-193a. In MVP knock-out experiments of their cell line, Teng, Y. et al. demonstrated that miR-193a accumulated in exosome donor cells instead of the exosomes themselves. Accumulated cellular miR-193a was then shown to target the 3’UTR of Caprin-1 mRNA, whose translated protein is known to regulate the cell cycle and cell proliferation. Thus, to summarize, the MVP knockout inhibited colon cancer cell division and progression. The paper suggests that miR-193a-mediated interruption of Caprin-1 complexes may be a potential target for development of colon cancer treatments.
Learn more about the QIAGEN exosomal technologies used in the Teng, Y. et al. (2016) publication.
Interested in learning more about new technologies for exosome research? Watch the 4-part webinar series on “Discover the Secrets of Exosomes – from Sample to Insight”:
- 1. Zaborowski, M.P., Balaj, L., Breakefield, X.O., Lai, C.P. (2015) Extracellular Vesicles: Composition, Biological Relevance, and Methods of Study. Bioscience 65, 783–797. Link
- 2. Baglio, S.R. et al. (2015) Human bone marrow- and adipose-mesenchymal stem cells secrete exosomes enriched in distinctive miRNA and tRNA species. Stem Cell Res. Ther. 6, doi:10.1186/s13287-015-0116-z. Link
- 3. McKenzie, A.J. et al. (2016) KRAS-MEK Signaling Controls Ago2 Sorting into Exosomes. Cell Reports 15, 978–987. Link
- 4. Teng, Y. et al. (2016) MVP-mediated exosomal sorting of miR-193a promotes colon cancer progression. Nature Comm. 8, doi: 10.1038/ncomms14448. Link