Review ArticleIntercellular Communication by Extracellular Vesicles and Their MicroRNAs in Asthma
Introduction
Cell-to-cell communication is one of the key mechanisms of lung disease biology. In multicellular organisms, intercellular communication is believed to involve the secretion of soluble factors such as cytokines and chemokines. Over the past decade, extracellular vesicles (EVs) have become the next focus of intensive scientific research as novel mediators of intercellular communication. In this review, we use the term EVs, in accordance with a recommendation of the International Society for Extracellular Vesicles, as an umbrella term for all types of vesicles present in the extracellular space, including exosomes and microvesicles.1
Although they have been known to exist for several decades, membrane vesicles have long been thought of as cell debris, signs of cell death, and/or structures that are very specific to a unique organ. However, recent studies have suggested that EVs might be important tools as diagnostic markers of, and as new potential therapeutic targets for the clinical treatment of, various lung diseases.2
EVs, especially exosomes, are small membrane vesicles released into the extracellular space after the fusion of multivesicular endosomes with the cell membrane.3 Exosomes contain enriched amounts of some specific markers, especially those of endosomal origin, including CD9, CD63, CD81, heat shock 70-kDa protein 4, and major histocompatibility complex class II (Figure 1).4, 5 Healthy cells shed microvesicles from the plasma membrane.6 Although EVs likely comprise both exosomes and microvesicles, it is difficult to fully discriminate between these 2 types of EV. Their difference is based on their diameters: Exosomes are in the range of 10 to 100 nm, and microvesicles are in the range of 100 to 1000 nm.7
A wide variety of cell types have been shown to release EVs, including immune cells, epithelial cells, and tumor cells. EVs are reportedly released from both immune and structural cells in the lungs, and they have recently been reported to play a role in allergies and asthma.8, 9 EVs have been isolated and characterized from different bodily fluids, such as plasma,10 urine,11 and bronchoalveolar lavage fluid (BALF).12
Previous studies have suggested that EVs might be involved in a broad range of biological processes, including immune system regulation, inflammation, and tumor development.3 It has been suggested that EVs carry communications between cells, allowing for cells to promote biological functions at distant sites.13
A breakthrough in EV research was the finding that their nucleic acid contents, such as mitochondrial DNA, messenger RNA (mRNA), and microRNA (miRNA), can be transported to recipient cells. In the EVs, these contents are protected from enzymatic degradation. In 2006, it was reported that EVs contain and transfer mRNAs to recipient cells, where these cargo mRNAs are translated into proteins.14 In 2007, Valadi et al15 reported that EVs are enriched in miRNAs called exosomal shuttle RNAs. miRNAs are emerging as novel therapeutic targets and diagnostic biomarkers for an array of disorders, including various lung diseases.16 EVs have garnered a huge amount of interest in recent years because of their crucial functions in maintaining homeostasis through intercellular communication in the lungs.
miRNAs are endogenous, single-strand, noncoding RNAs, 20 to 23 nucleotides in length, that regulate translation through their interactions with mRNA transcripts.17 After transcription, miRNAs inhibit gene expression through multiple mechanisms, all of which involve base-specific interactions with target mRNA transcripts. In the human genome, the transcripts of an estimated ~60% of all mRNAs are targeted by miRNAs. miRNAs are first transcribed for the most part by RNA polymerase II as a large primary miRNA, then processed by the endonuclease Drosha into a hairpin structure (precursor miRNA), and then further cleaved by the endonuclease Dicer into a single-strand, mature miRNA.18, 19 The mature miRNA is incorporated into a complex known as the RNA-induced silencing complex, which contains the proteins Argonaute 2 (Ago-2) and glycine-tryptophan 182 kDa (GW182). As a part of this complex, the mature miRNA regulates gene expression by binding to partially complementary sequences in the 3′-untranslated regions of the target mRNAs, leading to mRNA degradation or translation inhibition.20 A single miRNA may target dozens of mRNAs, and 1 mRNA can be regulated by multiple miRNAs. Currently, there is some evidence that miRNAs are involved in the asthmatic disease process. Some miRNAs reportedly regulate interleukin (IL)-13 and the T helper-2 (Th2) response, which are major components of the asthmatic response.16 They are key “fine-tuners” in the pathophysiology of asthma, such as inflammation, smooth muscle hypercontraction, and airway hyper-responsiveness.
For this review, we chose miRNAs that have been reported to have various roles in cell-to-cell communication and that also had biomarker potential. We summarize the significance of EVs and their miRNAs as new tools for intercellular communication in asthma. We believe that the discovery of EV miRNAs may bring fundamental changes to the understanding and therapeutic strategies of asthma. As mentioned earlier, the term microvesicles has also been used for exosome-like vesicles; clear distinction of exosome and microvesicles has not been adequately established. Therefore, only exosomal miRNAs are considered as EV miRNAs in this review.
Section snippets
Asthma
Asthma is a common chronic inflammatory disease involving the respiratory system in which the airways occasionally constrict and become inflamed, often in response to various stimuli, such as allergens, infections, and air pollutants.21 Asthma is a complex disease made up of disease variants with different underlying pathophysiologic aspects.22 The World Health Organization estimates that 300 million people are affected by the disease, and that by 2025, another 100 million will have been
Functions of miRNAs in Asthma
miRNAs play important functions in various lung diseases such as lung cancer, interstitial lung diseases, chronic obstructive pulmonary disease, and asthma.27 Evidence indicates that miRNAs are a promising technology for current and future therapeutic development in asthma.
The dysregulated expression of some miRNAs has been found in the airways or lymphocytes of asthmatic patients and an asthmatic murine model. Rodriguez et al28 reported that miR-155 is related to the development of
Function of Extracellular Vesicles in Asthma
The first evidence of EV function was reported by immunology researchers. In 1996, Raposo et al34 found that B lymphocyte–derived EVs function as immune system activators. After some groups demonstrated that dendritic cell (DC)-derived EVs modulated immune reactions by activating T and B lymphocytes,35, 36 important studies in the field of asthma immunology were initiated.
EVs are released from several cells that may be involved in allergies, including mast cells, DCs, T cells, and bronchial
Intercellular Communication with EV miRNAs in Asthma
To date, studies about EV miRNAs have provided considerable insight into the undefined mechanisms underlying various biological phenomena. A recent development in the EV field was provided by an article published in 2007. For the first time, Valadi et al15 reported that EVs derived from human and murine mast cell lines transported RNA to another mast cell, which was then translated, indicating that the transferred RNA was biologically active. In 2010, 3 groups independently reported that EV
Circulating miRNAs as Potential Biomarkers for Asthma
Intercellular miRNAs provide important functions in many biological processes. As described earlier, recent data have shown that miRNAs are present in the extracellular spaces, such as BALF, blood, urine, and saliva.48 This finding suggests that the miRNAs are potential noninvasive biomarkers for numerous diseases and conditions.49, 50, 51, 52 BALF is a useful research tool, but it is not suitable for clinical monitoring. In this context, serum or urine miRNAs are promising biomarkers in
Conclusions
With the amazing growth in the number of EV studies in recent years, it is clear that the intercellular messenger function of EVs now constitutes an exciting field. Investigating the biological functions of EVs is an emerging and rapidly progressing area in lung disease biology. Data from the studies cited in this review suggest that miRNAs play a significant role in the pathogenesis of asthma by the degradation of target mRNAs or by the inhibition of translation. Therefore, miRNA-based
Conflicts of Interest
This work was supported in part by a grant-in-aid for the Third-Term Comprehensive 10-Year Strategy for Cancer Control of Japan; Project for Development of Innovative Research on Cancer Therapeutics (P-Direct); Scientific Research on Priority Areas Cancer, Scientific Research on Innovative Areas (functional machinery for non-coding RNAs) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology; the National Cancer Center Research and Development Fund (23-A-2, 23-A-7,
Acknowlodgments
Dr. Fujita and Dr. Ochiya responsible to the concept of the review. Dr. Fujita responsible for the writing of the first draft of the manuscript. Dr. Yoshioka was responsible for assisting with sudy selection. Dr. Yoshioka, Drs. Ito, Drs. Araya, Drs. Kuwano and Dr. Ochiya were responsible for the review and critical comment of the manuscript.
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