The last decade has seen a sharp increase in the number of scientific publications describing physiological and pathological functions of extracellular vesicles (EVs), a collective term covering various subtypes of cell-released, membranous structures, called exosomes, microvesicles, microparticles, ectosomes, oncosomes, apoptotic bodies, and many other names. However, specific issues arise when working with these entities, whose size and amount often make them difficult to obtain as relatively pure preparations, and to characterize properly. The International Society for Extracellular Vesicles (ISEV) proposed Minimal Information for Studies of Extracellular Vesicles (“MISEV”) guidelines for the field in 2014. We now update these “MISEV2014” guidelines based on evolution of the collective knowledge in the last four years. An important point to consider is that ascribing a specific function to EVs in general, or to subtypes of EVs, requires reporting of specific information beyond mere description of function in a crude, potentially contaminated, and heterogeneous preparation. For example, claims that exosomes are endowed with exquisite and specific activities remain difficult to support experimentally, given our still limited knowledge of their specific molecular machineries of biogenesis and release, as compared with other biophysically similar EVs. The MISEV2018 guidelines include tables and outlines of suggested protocols and steps to follow to document specific EV-associated functional activities. Finally, a checklist is provided with summaries of key points.

MicroRNAs (miRs) are important posttranscriptional regulators of gene expression. Besides their well‐characterized inhibitory effects on mRNA stability and translation, miRs can also activate gene expression. In this study, we identified a novel noncanonical function of miR‐574‐5p. We found that miR‐574‐5p acts as an RNA decoy to CUG RNA‐binding protein 1 (CUGBP1) and antagonizes its function. MiR‐574‐5p induces microsomal prostaglandin E synthase‐1 (mPGES‐1) expression by preventing CUGBP1 binding to its 3′UTR, leading to an enhanced alternative splicing and generation of an mPGES‐1 3′UTR isoform, increased mPGES‐1 protein expression, PGE2 formation, and tumor growth in vivo. miR‐574‐5p–induced tumor growth in mice could be completely inhibited with the mPGES‐1 inhibitor OIL Moreover, miR‐574‐5p is induced by IL‐1β and is strongly overexpressed in human nonsmall cell lung cancer where high mPGES‐1 expression correlates with a low survival rate. The discovered function of miR‐574‐5p as a CUGBP1 decoy opens up new therapeutic opportunities. It might serve as a stratification marker to select lung tumor patients who respond to the pharmacological inhibition of PGE2 formation.—Saul, M. J., Baumann, I., Bruno, A., Emmerich, A. C., Wellstein, J., Ottinger, S. M., Contursi, A., Dovizio, M., Donnini, S., Tacconelli, S., Raouf, J., Idborg, H., Stein, S., Korotkova, M., Savai, R., Terzuoli, E., Sala, G., Seeger, W., Jakobsson, P.‐J., Patrignani, P., Suess, B., Steinhilber, D. miR‐574‐5p as RNA decoy for CUGBP1 stimulates human lung tumor growth by mPGES‐1 induction. FASEB J. 33, 6933–6947 (2019). http://www.fasebj.org

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovial inflammation and joint destruction. Cell-derived small extracellular vesicles (sEV) mediate cell-to-cell communication in the synovial microenvironment by carrying microRNAs (miRs), a class of small non-coding RNAs. Herein, we report that sEV from synovial fluid promote osteoclast differentiation which is attributed to high levels of extracellular miR-574-5p. Moreover, we demonstrate for the first time that enhanced osteoclast maturation is mediated by Toll-like receptor (TLR) 7/8 signaling which is activated by miR-574-5p binding. This is a novel mechanism by which sEV and miRs contribute to RA pathogenesis and indicate that pharmacological inhibition of extracellular miR-574-5p might offer new therapeutic strategies to protect osteoclast-mediated bone destruction in RA.

UPF1 is a key player in nonsense mediated mRNA decay (NMD) but also involved in posttranscriptional gene regulation. In this study we found that UPF1 regulates the expression of genes with functions in inflammation and myeloid cell differentiation via hnRNP E2. The majority of the UPF1-regulated genes identified in monocytic cells contain a binding site for hnRNP E2 within 5′ UTR located introns with hnRNP E2 acting here as splicing regulator. We found that miRNA-328 which is significantly induced during monocytic cell differentiation acts independently from its gene silencing function as RNA decoy for hnRNP E2. One representative gene controlled by the hnRNP E2/miRNA-328 balance is S100A9 which plays an important role in cell differentiation and oxidative stress response of monocytes. Induction of miRNA-328 expression during cell differentiation antagonizes the blockade by hnRNP E2 which results in the upregulation of CD11b expression and ROS production in monocytic cells. Taken together, our data indicate that upregulation of miR-328 is responsible for the induction of hnRNP E2 target genes during myeloid cell differentiation.

Small extracellular vesicles (sEV) hold enormous potential as biomarkers, drug carriers, and therapeutic agents. However, due to previous limitations in the phenotypic characterization of sEV at the single vesicle level, knowledge of cell type-specific sEV signatures remains sparse. With the introduction of next-generation sEV analysis devices, such as the single-particle interferometric reflectance imaging sensor (SP-IRIS)-based ExoView R100 platform, single sEV analyses are now possible. While the tetraspanins CD9, CD63, and CD81 were generally considered pan-sEV markers, it became clear that sEV of different cell types contain several combinations and amounts of these proteins on their surfaces. To gain better insight into the complexity and heterogeneity of sEV, we used the ExoView R100 platform to analyze the CD9/CD63/CD81 phenotype of sEV released by different cell types at a single sEV level. We demonstrated that these surface markers are sufficient to distinguish cell-type-specific sEV phenotypes. Furthermore, we recognized that tetraspanin composition in some sEV populations does not follow a random pattern. Notably, the tetraspanin distribution of sEV derived from mesenchymal stem cells (MSCs) alters depending on cell culture conditions. Overall, our data provide an overview of the cell-specific characteristics of sEV populations, which will increase the understanding of sEV physiology and improve the development of new sEV-based therapeutic approaches.

Extracellular microRNAs (miRs) have been proposed as important blood‐based biomarkers for several diseases. Contrary to proteins and other RNA classes, miRs are stable and easily detectable in body fluids. In this respect, miRs represent a perfect candidate for minimal invasive biomarkers which can hopefully become a complement for invasive histological examinations of tumor tissue. Despite the high number of miR biomarker studies, the specificity and reproducibility of these studies is missing. Therefore, the standardization of pre‐analytical and analytical methods is urgently needed. Here, we validated miR analysis for RNA isolation and miR quantification by quantitative polymerase chain reaction (RT‐qPCR) based on good laboratory practice (GLP). Validation was carried out exemplarily on four miRs, which had already been described as potential biomarkers in previous studies. As basis for RNA analysis using RT‐qPCR, the Minimum Information for Publication of Quantitative Real‐Time PCR Experiments were applied and adapted on the analysis of circulating miRs from human plasma. In our study, we identified and solved several pitfalls from handling to normalization strategy in the analysis of extracellular miRs that lead to inconsistent and non‐repeatable data. Principles of GLP set a framework of experimental design, performance and monitoring to ensure high quality and reliable data. Within this study, we appointed first acceptance criteria for circulating miR quantification during validation which set standards for future miR quantification in blood samples.

Intercellular communication plays an essential role in lung cancer (LC). One of the major players in cell‐cell‐communication is small extracellular vesicles (sEV). SEV trigger various biological responses by transporting cellular cargo to target cells. One essential sEV component are microRNAs (miRs), whose transport has recently attracted increasing research interest. We report that prostaglandin E 2 (PGE 2 ), a key inflammatory lipid mediator, specifically induces the sorting of miR‐574‐5p in sEV of A549 and 2106T cells. We found that sEV‐derived miR‐574‐5p activates Toll‐like receptors (TLR) 7/8, thereby decreasing PGE 2 ‐levels. In contrast, intracellular miR‐574‐5p induces PGE 2 ‐biosynthesis. Consequently, the combination of intracellular and sEV‐derived miR‐574‐5p controls PGE 2 ‐levels via a feedback loop. This was only observed in adeno‐ but not in squamous cell carcinoma, indicating a cell‐specific response to sEV‐derived miRs, which might be due to unique tetraspanin compositions. Hence, we describe a novel function of miR‐574‐5p unique to adenocarcinoma. Intracellular miR‐574‐5p induces PGE 2 and thus the secretion of sEV‐derived miR‐574‐5p, which in turn decreases PGE 2 ‐biosynthesis in recipient cells.

Over the last years, many microRNAs (miRNAs) have been identified that regulate the formation of bioactive lipid mediators such as prostanoids and leukotrienes. Many of these miRNAs are involved in complex regulatory circuits necessary for the fine-tuning of biological functions including inflammatory processes or cell growth. A better understanding of these networks will contribute to the development of novel therapeutic strategies for the treatment of inflammatory diseases and cancer. In this review, we provide an overview of the current knowledge of miRNA regulation in eicosanoid pathways with special focus on novel miRNA functions and regulatory circuits of leukotriene and prostaglandin biosynthesis.