Both prokaryotic and eukaryotic cells are known to release RNA, and this release can be passive or active. The Endosomal Sorting Complex Required for Transport (ESCRT) machinery was previously considered as a possible mechanism for RNA secretion from the cell, but more recently research studying microRNA secretion in human embryonic kidney cells and Cercopithecus aethiops kidney cells identified neutral sphingomyelinase 2 (nSMase2), an enzyme involved in ceramide biosynthesis, as a regulator of microRNA secretion levels. ExRNAs are often found packaged within vesicles such as exosomes, ectosomes, prostasomes, microvesicles, and apoptotic bodies. Although RNAs can be excreted from the cell without an enveloping container, ribonucleases present in extracellular environments would eventually degrade the molecule.
Extracellular RNA should not be viewed as a category describing a set of RNAs with a specific biological function or belonging to a particular RNA family. Similar to the term "non-coding RNA", "extracellular RNA" defines a group of several types of RNAs whose functions are diverse, yet they share a common attribute which, in the case of exRNAs, is existence in an extracellular environment. The following types of RNA have been found outside the cell:
To function or even survive as full-length RNA in extracellular environments, exRNA must be protected from digestion by RNases. This requirement does not apply to prokaryotic syntrophy, where digested nucleotides are recycled. exRNA can be shielded from RNases by RNA binding proteins (RBPs), on their own or within/associated with lipoprotein particles and extracellular vesicles. Extracellular vesicles in particular are thought to be a way to transport RNA between cells, in a process that may be general or highly specific, for example, due to incorporation of markers of the parent cell that may be recognized by receptors on the recipient cell. Biochemical evidence supports the idea that exRNA uptake is a common process, suggesting new pathways for intercellular communication. As a result, the presence, absence, and relative abundance of certain exRNAs can be correlated with changes in cellular signaling and may indicate specific disease states.
Despite a limited understanding of exRNA biology, current research has shown the role of exRNAs to be multi-faceted. Extracellular miRNAs are capable of targeting mRNAs in the recipient cell through RNA interference pathways. In vitro experiments have shown the transfer of specific exRNAs into recipient cells inhibiting protein expression and preventing cancer cell growth. In addition to mRNAs being regulated by exRNAs, mRNAs can act as exRNAs to carry genetic information between cells. Messenger RNA contained in microvesicles secreted from glioblastomal cells were shown to generate a functional protein in recipient (human brain microvascular endothelial) cells in vitro. In another study of extracellular mRNAs, mRNAs transported by microvesicles from endothelial progenitor cells (EPCs) to human microvascular and macrovascular endothelial cells triggered angiogenesis in both the in vitro and in vivo setting. Work by Hunter et al. used Ingenuity Pathway Analysis (IPA) software that associated exRNAs found in human blood microvesicles with pathways involved in blood cell differentiation, metabolism, and immune function. These experimental and bioinformatics analyses favor the hypothesis that exRNAs play a role in numerous biological processes. These experimental and bioinformatics analyses favor the hypothesis that exRNAs play a role in numerous biological processes. The idea that mammalian cells might communicate over long distances by secreting RNAs is exciting, yet definitive evidence supporting this mode of intercellular regulation remains elusive, leaving the hypothesis without definitive data.
Several methods have been developed or adapted to detect, characterize, and quantify exRNA from biological samples. RT-PCR, cDNA microarrays, and RNA sequencing are common techniques for RNA analysis. Applying these methods to study exRNAs mainly differs from cellular RNA experiments in the RNA isolation and/or extraction steps.
For known exRNA nucleotide sequences, RT-PCR can be applied to detect their presence within a sample as well as quantify their abundance. This is done through first reverse transcribing the RNA sequence into cDNA. The cDNA then serves as a template for PCR amplification. The major benefits of using RT-PCR are its quantitative accuracy in a dynamic range and increased sensitivity compared to methods such as RNase protection assays and dot blot hybridization. The disadvantage to RT-PCR is the requirement of costly supplies, and the necessity of sound experimental design and an in-depth understanding of normalization techniques in order to obtain accurate results and conclusions.
Microarrays allow for larger-scale exRNA characterization and quantification. Microarrays used for RNA studies first generate different cDNA oligonucleotides (probes) that are attached to the microarray chip. An RNA sample can then be added to the chip, and RNAs with sequence complementarity to the cDNA probe will bind and generate a fluorescent signal that can be quantified. Micro RNA arrays have been used in exRNA studies to generate miRNA profiles of bodily fluids.
As growing evidence supports the function of exRNAs as intercellular communicators, research efforts are investigating the possibility of utilizing exRNAs in disease diagnosis, prognosis, and therapeutics.
The potential of extracellular RNAs to serve as biomarkers is significant not only because of their role in intercellular signaling but also due to developments in next generation sequencing that enable high throughput profiling. The simplest form of an exRNA biomarker is the presence (or absence) of a specific extracellular RNA. These biological signatures have been discovered in exRNA studies of cancer, diabetes, arthritis, and prion-related diseases. Recently, a bioinformatics analysis of extracellular vesicles extracted from Trypanosoma cruzi, in which SNPs were mined from transcriptomic data, suggested that exRNAs could be biomarkers of neglected diseases such as Chagas disease.
A major research area of interest for exRNA has been its role in cancer. The table below (adapted from Kosaka et al.) lists several types of cancer in which exRNAs have been shown to be associated:
Chen X, Ba Y, Ma L, Cai X, Yin Y, Wang K, Guo J, Zhang Y, Chen J, Guo X, Li Q, Li X, Wang W, Zhang Y, Wang J, Jiang X, Xiang Y, Xu C, Zheng P, Zhang J, Li R, Zhang H, Shang X, Gong T, Ning G, Wang J, Zen K, Zhang J, Zhang CY (October 2008). "Characterization of microRNAs in serum: a novel class of biomarkers for diagnosis of cancer and other diseases". Cell Research. 18 (10): 997–1006. doi:10.1038/cr.2008.282. PMID 18766170. https://doi.org/10.1038%2Fcr.2008.282
Michael, A; Bajracharya, SD; Yuen, PS; Zhou, H; Star, RA; Illei, GG; Alevizos, I (January 2010). "Exosomes from human saliva as a source of microRNA biomarkers". Oral Diseases. 16 (1): 34–8. doi:10.1111/j.1601-0825.2009.01604.x. PMC 2844919. PMID 19627513. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2844919
Kosaka, N; Izumi, H; Sekine, K; Ochiya, T (March 1, 2010). "microRNA as a new immune-regulatory agent in breast milk". Silence. 1 (1): 7. doi:10.1186/1758-907X-1-7. PMC 2847997. PMID 20226005. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2847997
Menke, TB; Warnecke, JM (June 2004). "Improved conditions for isolation and quantification of RNA in urine specimens". Annals of the New York Academy of Sciences. 1022 (1): 185–9. Bibcode:2004NYASA1022..185M. doi:10.1196/annals.1318.028. PMID 15251958. S2CID 25965962. /wiki/Bibcode_(identifier)
Zubakov D, Boersma AW, Choi Y, van Kuijk PF, Wiemer EA, Kayser M (May 2010). "MicroRNA markers for forensic body fluid identification obtained from microarray screening and quantitative RT-PCR confirmation". International Journal of Legal Medicine. 124 (3): 217–26. doi:10.1007/s00414-009-0402-3. PMC 2855015. PMID 20145944. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2855015
Hanson, EK; Lubenow, H; Ballantyne, J (April 15, 2009). "Identification of forensically relevant body fluids using a panel of differentially expressed microRNAs". Analytical Biochemistry. 387 (2): 303–14. doi:10.1016/j.ab.2009.01.037. PMID 19454234. /wiki/Doi_(identifier)
Demain, AL; Burg, RW; Hendlin, D (March 1965). "Excretion and Degradation of Ribonucleic Acid by Bacillus Subtilis". Journal of Bacteriology. 89 (3): 640–6. doi:10.1128/JB.89.3.640-646.1965. PMC 277514. PMID 14273638. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC277514
Iguchi, H; Kosaka, N; Ochiya, T (September 2010). "Secretory microRNAs as a versatile communication tool". Communicative & Integrative Biology. 3 (5): 478–81. doi:10.4161/cib.3.5.12693. PMC 2974086. PMID 21057646. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2974086
NIH, USA. "NIH Common Fund RFA for exRNA Communication". Retrieved 7 November 2012. http://commonfund.nih.gov/exrna/
NIH, USA. "ERCC2 Projects". Archived from the original on 22 May 2021. Retrieved 26 September 2019. https://web.archive.org/web/20210522001326/https://exrna.org/projects-ercc2/
Tucker, Ayanna (19 August 2019). "Research on Cellular 'Packages' Receives $900k in Federal Funding". Newsroom. Johns Hopkins Medicine. Retrieved 26 September 2019. https://www.hopkinsmedicine.org/news/newsroom/news-releases/research-on-cellular-packages-receives-900k-in-federal-funding
Demain, AL; Burg, RW; Hendlin, D (March 1965). "Excretion and Degradation of Ribonucleic Acid by Bacillus Subtilis". Journal of Bacteriology. 89 (3): 640–6. doi:10.1128/JB.89.3.640-646.1965. PMC 277514. PMID 14273638. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC277514
Iguchi, H; Kosaka, N; Ochiya, T (September 2010). "Secretory microRNAs as a versatile communication tool". Communicative & Integrative Biology. 3 (5): 478–81. doi:10.4161/cib.3.5.12693. PMC 2974086. PMID 21057646. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2974086
Deregibus MC, Cantaluppi V, Calogero R, Lo Iacono M, Tetta C, Biancone L, Bruno S, Bussolati B, Camussi G (October 1, 2007). "Endothelial progenitor cell derived microvesicles activate an angiogenic program in endothelial cells by a horizontal transfer of mRNA". Blood. 110 (7): 2440–8. doi:10.1182/blood-2007-03-078709. PMID 17536014. https://doi.org/10.1182%2Fblood-2007-03-078709
Wolfers, J; Lozier, A; Raposo, G; Regnault, A; Théry, C; Masurier, C; Flament, C; Pouzieux, S; Faure, F; Tursz, T; Angevin, E; Amigorena, S; Zitvogel, L (March 2001). "Tumor-derived exosomes are a source of shared tumor rejection antigens for CTL cross-priming". Nature Medicine. 7 (3): 297–303. doi:10.1038/85438. PMID 11231627. S2CID 24091247. /wiki/Clotilde_Th%C3%A9ry
Babiker, AA; Nilsson, B; Ronquist, G; Carlsson, L; Ekdahl, KN (February 1, 2005). "Transfer of functional prostasomal CD59 of metastatic prostatic cancer cell origin protects cells against complement attack". The Prostate. 62 (2): 105–14. doi:10.1002/pros.20102. PMID 15389819. S2CID 21127892. /wiki/Doi_(identifier)
Holmgren, L; Szeles, A; Rajnavölgyi, E; Folkman, J; Klein, G; Ernberg, I; Falk, KI (June 1, 1999). "Horizontal transfer of DNA by the uptake of apoptotic bodies". Blood. 93 (11): 3956–63. doi:10.1182/blood.V93.11.3956. PMID 10339505. /wiki/Doi_(identifier)
Valadi, H; Ekström, K; Bossios, A; Sjöstrand, M; Lee, JJ; Lötvall, JO (June 2007). "Exosome-mediated transfer of mRNAs and microRNAs is a novel mechanism of genetic exchange between cells". Nature Cell Biology. 9 (6): 654–9. doi:10.1038/ncb1596. PMID 17486113. S2CID 8599814. /wiki/Doi_(identifier)
Noerholm, M; Balaj, L; Limperg, T; Salehi, A; Zhu, LD; Hochberg, FH; Breakefield, XO; Carter, BS; Skog, J (Jan 17, 2012). "RNA expression patterns in serum microvesicles from patients with glioblastoma multiforme and controls". BMC Cancer. 12: 22. doi:10.1186/1471-2407-12-22. PMC 3329625. PMID 22251860. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3329625
Demain, AL; Burg, RW; Hendlin, D (March 1965). "Excretion and Degradation of Ribonucleic Acid by Bacillus Subtilis". Journal of Bacteriology. 89 (3): 640–6. doi:10.1128/JB.89.3.640-646.1965. PMC 277514. PMID 14273638. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC277514
Bellingham, SA; Coleman, BM; Hill, AF (November 2012). "Small RNA deep sequencing reveals a distinct miRNA signature released in exosomes from prion-infected neuronal cells". Nucleic Acids Research. 40 (21): 10937–49. doi:10.1093/nar/gks832. PMC 3505968. PMID 22965126. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3505968
Bellingham, SA; Coleman, BM; Hill, AF (November 2012). "Small RNA deep sequencing reveals a distinct miRNA signature released in exosomes from prion-infected neuronal cells". Nucleic Acids Research. 40 (21): 10937–49. doi:10.1093/nar/gks832. PMC 3505968. PMID 22965126. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3505968
Iguchi, H; Kosaka, N; Ochiya, T (June 2010). "Versatile applications of microRNA in anti-cancer drug discovery: from therapeutics to biomarkers". Current Drug Discovery Technologies. 7 (2): 95–105. doi:10.2174/157016310793180648. PMID 20836759. /wiki/Doi_(identifier)
Bellingham, SA; Guo, BB; Coleman, BM; Hill, AF (2012). "Exosomes: vehicles for the transfer of toxic proteins associated with neurodegenerative diseases?". Frontiers in Physiology. 3: 124. doi:10.3389/fphys.2012.00124. PMC 3342525. PMID 22563321. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3342525
Coleman, BM; Hanssen, E; Lawson, VA; Hill, AF (October 2012). "Prion-infected cells regulate the release of exosomes with distinct ultrastructural features". FASEB Journal. 26 (10): 4160–73. doi:10.1096/fj.11-202077. PMID 22767229. S2CID 364768. https://doi.org/10.1096%2Ffj.11-202077
Hessvik, NP; Phuyal, S; Brech, A; Sandvig, K; Llorente, A (November 2012). "Profiling of microRNAs in exosomes released from PC-3 prostate cancer cells". Biochimica et Biophysica Acta (BBA) - Gene Regulatory Mechanisms. 1819 (11–12): 1154–63. doi:10.1016/j.bbagrm.2012.08.016. PMID 22982408. /wiki/Doi_(identifier)
Iguchi, H; Kosaka, N; Ochiya, T (September 2010). "Secretory microRNAs as a versatile communication tool". Communicative & Integrative Biology. 3 (5): 478–81. doi:10.4161/cib.3.5.12693. PMC 2974086. PMID 21057646. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2974086
Kosaka, N; Iguchi, H; Ochiya, T (October 2010). "Circulating microRNA in body fluid: a new potential biomarker for cancer diagnosis and prognosis". Cancer Science. 101 (10): 2087–92. doi:10.1111/j.1349-7006.2010.01650.x. PMC 11159200. PMID 20624164. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11159200
Kosaka, N; Iguchi, H; Yoshioka, Y; Takeshita, F; Matsuki, Y; Ochiya, T (June 4, 2010). "Secretory mechanisms and intercellular transfer of microRNAs in living cells". The Journal of Biological Chemistry. 285 (23): 17442–52. doi:10.1074/jbc.M110.107821. PMC 2878508. PMID 20353945. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2878508
Deregibus MC, Cantaluppi V, Calogero R, Lo Iacono M, Tetta C, Biancone L, Bruno S, Bussolati B, Camussi G (October 1, 2007). "Endothelial progenitor cell derived microvesicles activate an angiogenic program in endothelial cells by a horizontal transfer of mRNA". Blood. 110 (7): 2440–8. doi:10.1182/blood-2007-03-078709. PMID 17536014. https://doi.org/10.1182%2Fblood-2007-03-078709
Skog J, Würdinger T, van Rijn S, Meijer DH, Gainche L, Sena-Esteves M, Curry WT, Carter BS, Krichevsky AM, Breakefield XO (December 2008). "Glioblastoma microvesicles transport RNA and proteins that promote tumour growth and provide diagnostic biomarkers". Nature Cell Biology. 10 (12): 1470–6. doi:10.1038/ncb1800. PMC 3423894. PMID 19011622. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3423894
Hunter, MP; Ismail, N; Zhang, X; Aguda, BD; Lee, EJ; Yu, L; Xiao, T; Schafer, J; Lee, ML; Schmittgen, TD; Nana-Sinkam, SP; Jarjoura, D; Marsh, CB (2008). "Detection of microRNA expression in human peripheral blood microvesicles". PLOS ONE. 3 (11): e3694. Bibcode:2008PLoSO...3.3694H. doi:10.1371/journal.pone.0003694. PMC 2577891. PMID 19002258. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2577891
Gruner, HN; McManus, MT (2021). "Examining the evidence for extracellular RNA function in mammals". Nature Reviews Genetics. 22: 448–456. doi:10.1038/s41576-021-00346-8. /wiki/Doi_(identifier)
Wong, ML; Medrano, JF (July 2005). "Real-time PCR for mRNA quantitation". BioTechniques. 39 (1): 75–85. doi:10.2144/05391rv01. PMID 16060372. https://doi.org/10.2144%2F05391rv01
Bellingham, SA; Coleman, BM; Hill, AF (November 2012). "Small RNA deep sequencing reveals a distinct miRNA signature released in exosomes from prion-infected neuronal cells". Nucleic Acids Research. 40 (21): 10937–49. doi:10.1093/nar/gks832. PMC 3505968. PMID 22965126. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3505968
Turchinovich, A; Weiz, L; Langheinz, A; Burwinkel, B (September 1, 2011). "Characterization of extracellular circulating microRNA". Nucleic Acids Research. 39 (16): 7223–33. doi:10.1093/nar/gkr254. PMC 3167594. PMID 21609964. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3167594
Ozsolak, F; Platt, AR; Jones, DR; Reifenberger, JG; Sass, LE; McInerney, P; Thompson, JF; Bowers, J; Jarosz, M; Milos, PM (October 8, 2009). "Direct RNA sequencing". Nature. 461 (7265): 814–8. Bibcode:2009Natur.461..814O. doi:10.1038/nature08390. PMID 19776739. S2CID 4426760. /wiki/Bibcode_(identifier)
Dillies MA, Rau A, Aubert J, Hennequet-Antier C, Jeanmougin M, Servant N, Keime C, Marot G, Castel D, Estelle J, Guernec G, Jagla B, Jouneau L, Laloë D, Le Gall C, Schaëffer B, Le Crom S, Guedj M, Jaffrézic F (September 17, 2012). "A comprehensive evaluation of normalization methods for Illumina high-throughput RNA sequencing data analysis". Briefings in Bioinformatics. 14 (6): 671–683. doi:10.1093/bib/bbs046. PMID 22988256. https://doi.org/10.1093%2Fbib%2Fbbs046
Wang, Z; Gerstein, M; Snyder, M (January 2009). "RNA-Seq: a revolutionary tool for transcriptomics". Nature Reviews Genetics. 10 (1): 57–63. doi:10.1038/nrg2484. PMC 2949280. PMID 19015660. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2949280
Chen X, Ba Y, Ma L, Cai X, Yin Y, Wang K, Guo J, Zhang Y, Chen J, Guo X, Li Q, Li X, Wang W, Zhang Y, Wang J, Jiang X, Xiang Y, Xu C, Zheng P, Zhang J, Li R, Zhang H, Shang X, Gong T, Ning G, Wang J, Zen K, Zhang J, Zhang CY (October 2008). "Characterization of microRNAs in serum: a novel class of biomarkers for diagnosis of cancer and other diseases". Cell Research. 18 (10): 997–1006. doi:10.1038/cr.2008.282. PMID 18766170. https://doi.org/10.1038%2Fcr.2008.282
Thind A, Wilson C (2016). "Exosomal miRNAs as cancer biomarkers and therapeutic targets". J Extracell Vesicles. 5: 31292. doi:10.3402/jev.v5.31292. PMC 4954869. PMID 27440105. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4954869
Cloonan, N; Xu, Q; Faulkner, GJ; Taylor, DF; Tang, DT; Kolle, G; Grimmond, SM (October 1, 2009). "RNA-MATE: a recursive mapping strategy for high-throughput RNA-sequencing data". Bioinformatics. 25 (19): 2615–6. doi:10.1093/bioinformatics/btp459. PMC 2752615. PMID 19648138. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2752615
Majewski, J; Pastinen, T (February 2011). "The study of eQTL variations by RNA-seq: from SNPs to phenotypes". Trends in Genetics. 27 (2): 72–9. doi:10.1016/j.tig.2010.10.006. PMID 21122937. /wiki/Doi_(identifier)
Chen X, Ba Y, Ma L, Cai X, Yin Y, Wang K, Guo J, Zhang Y, Chen J, Guo X, Li Q, Li X, Wang W, Zhang Y, Wang J, Jiang X, Xiang Y, Xu C, Zheng P, Zhang J, Li R, Zhang H, Shang X, Gong T, Ning G, Wang J, Zen K, Zhang J, Zhang CY (October 2008). "Characterization of microRNAs in serum: a novel class of biomarkers for diagnosis of cancer and other diseases". Cell Research. 18 (10): 997–1006. doi:10.1038/cr.2008.282. PMID 18766170. https://doi.org/10.1038%2Fcr.2008.282
Bellingham, SA; Coleman, BM; Hill, AF (November 2012). "Small RNA deep sequencing reveals a distinct miRNA signature released in exosomes from prion-infected neuronal cells". Nucleic Acids Research. 40 (21): 10937–49. doi:10.1093/nar/gks832. PMC 3505968. PMID 22965126. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3505968
Murata, K; Yoshitomi, H; Tanida, S; Ishikawa, M; Nishitani, K; Ito, H; Nakamura, T (2010). "Plasma and synovial fluid microRNAs as potential biomarkers of rheumatoid arthritis and osteoarthritis". Arthritis Research & Therapy. 12 (3): R86. doi:10.1186/ar3013. PMC 2911870. PMID 20470394. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2911870
Gaur, Pallavi; Chaturvedi, Anoop (2016-11-24). "Mining SNPs in extracellular vesicular transcriptome of Trypanosoma cruzi: a step closer to early diagnosis of neglected Chagas disease". PeerJ. 4:e2693: e2693. doi:10.7717/peerj.2693. PMC 5126619. PMID 27904804. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5126619
Kosaka, N; Iguchi, H; Ochiya, T (October 2010). "Circulating microRNA in body fluid: a new potential biomarker for cancer diagnosis and prognosis". Cancer Science. 101 (10): 2087–92. doi:10.1111/j.1349-7006.2010.01650.x. PMC 11159200. PMID 20624164. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11159200