RNA Extraction/Purification Reagents (Exosome)

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RNA contained in Exosomes

When first discovered, EVs, including exosomes, were thought to be a mechanism for the expulsion of unnecessary molecules from cells. However, their function as tools for intercellular communication has been revealed, attracting significant attention. EVs contain nucleic acids, proteins, metabolites, and lipids. Among these, RNA, alongside proteins, is considered a key functional molecule in intercellular communication, leading to active research in various fields including diagnostics and therapeutics.

The main types of RNA encapsulated in EVs are described below.

mRNA

EVs contain messenger RNAs (mRNAs), which encode proteins. According to the microarray analysis conducted by Valadi et al., mast cell-derived EVs contained about 1,300 types of mRNA¹⁾. Some of these mRNAs were shown to be translated in the recipient cells, indicating that RNA can be horizontally transferred from cell to cell via EVs. This discovery greatly surprised researchers. Subsequent studies in glioblastoma reported similar findings, in which approximately 27,000 types of mRNA were detected²⁾.

miRNA

MicroRNAs (miRNAs) are short RNAs, about 22 nucleotides in length, and are a type of non-coding RNA (ncRNA) that is not translated into proteins. Their primary function involves binding to mRNA, thereby regulating mRNA expression. In 2007, the presence of miRNA within EVs was reported¹⁾, and it became clear that the miRNA released could be taken up by recipient cells and function therein^3-5). Since then, miRNA has been recognized as a functional molecule in intercellular communication.

Research has revealed that miRNAs encapsulated in EVs possess characteristic sequences, known as “EXOmotifs.” The RNA-binding protein hnRNPA2B1 was found to interact with EXOmotif and assist in loading miRNAs into EVs. This discovery is expected to provide insights into the selective loading of miRNAs into EVs⁶⁾.

miRNAs within EVs have been implicated in various diseases. Particularly in cancer, there have been various reports regarding the role of miRNAs contained in EVs. For instance, miRNAs in K562 leukemia cell-derived EVs affect vascular endothelial cells, promoting cell motility and angiogenesis⁷⁾. Additionally, miRNAs in EVs released by breast cancer cells facilitate metastasis through altering glucose metabolism at the metastatic site⁸⁾.

lncRNA

Long non-coding RNAs (lncRNAs) are a type of non-coding RNA (ncRNA) that is over 200 nucleotides in length. It has various functions, including the regulation of gene expression and chromatin states. Although lncRNAs are known to be present in EVs, and specific lncRNAs characteristic of cancer cells have been identified⁹⁾, much remains unknown about their significance.

circRNA

Circular RNAs (circRNAs), as their name suggests, are a circular form of RNA and a type of non-coding RNA (ncRNA). They are more stable than linear RNAs due to the absence of terminal ends, and are believed to have functions such as the regulation of miRNA expression. The presence of circRNAs in EVs has been reported¹⁰⁾, and like other RNAs, circRNAs are thought to be involved in intercellular communication. However, as with lncRNAs, their detailed functions remain unclear, warranting further research.

In addition to the RNAs discussed in this article, EVs are known to contain other types of RNA, such as tRNA, rRNA, snRNA, snoRNA, and piRNA. It has been suggested that these RNAs could be involved in the regulation of protein expression¹¹⁾. mRNAs and miRNAs found in EVs are compiled in the ExoCarta database [http://www.exocarta.org].

EVs contain various types of RNA, and the variety and composition of RNA differ depending on the donor cell. The RNAs encapsulated within EVs are relatively stable, as it is less susceptible to degradation by nucleases. These characteristics have spurred active research in utilizing RNA within EVs as biomarkers for diagnostics. As research progresses, the identification of new RNAs and the elucidation of their functions are anticipated in the future.

References

1. Valadi, H. et al.: Nat. Cell Biol., 9(6), 654(2007).
   Exosome-mediated transfer of mRNAs and microRNAs is a novel mechanism of genetic exchange between cells
2. Skog, J. et al.: Nat. Cell Biol., 10(12), 1470(2008).
   Glioblastoma microvesicles transport RNA and proteins that promote tumour growth and provide diagnostic biomarkers
3. Kosaka, N. et al.: J. Biol. Chem., 285(23), 17442(2010).
   Secretory mechanisms and intercellular transfer of microRNAs in living cells
4. Pegtel, D. M. et al.: Proc. Natl. Acad. Sci. USA, 107(14), 6328(2010).
   Functional delivery of viral miRNAs via exosomes
5. Zhang, Y. et al.: Mol. Cell, 39(1), 133(2010).
   Secreted monocytic miR-150 enhances targeted endothelial cell migration
6. Villarroya-Beltri, C. et al.: Nat. Commun., 4(1), 2980(2013).
   Sumoylated hnRNPA2B1 controls the sorting of miRNAs into exosomes through binding to specific motifs
7. Umezu, T. et al: Oncogene, 32(22), 2747(2013).
   Leukemia cell to endothelial cell communication via exosomal miRNAs
8. Fong, M. Y. et al.: Nat. Cell Biol., 17(2), 183(2015).
   Breast-cancer-secreted miR-122 reprograms glucose metabolism in premetastatic niche to promote metastasis
9. Ahadi, A. et al.: Sci. Rep., 6(1), 24922(2016).
   Long non-coding RNAs harboring miRNA seed regions are enriched in prostate cancer exosomes
10. Bao, C. et al.: Mol. Cell. Oncol., 3(2), e1084443(2016).
    Circular RNA expands its territory
11. Nolte-’t Hoen, E. N. et al.: Nucleic Acids Res., 40(18), 9272(2012).
    Deep sequencing of RNA from immune cell-derived vesicles uncovers the selective incorporation of small non-coding RNA biotypes with potential regulatory functions