Sorting Mechanisms for MicroRNAs into Extracellular Vesicles and Their Associated Diseases

Abstract

Extracellular vesicles (EV) are secretory membranous elements used by cells to transport proteins, lipids, mRNAs, and microRNAs (miRNAs). While their existence has been known for many years, only recently has research begun to identify their function in intercellular communication and gene regulation. Importantly, cells have the ability to selectively sort miRNA into EVs for secretion to nearby or distant targets. These mechanisms broadly include RNA-binding proteins such as hnRNPA2B1 and Argonaute-2, but also membranous proteins involved in EV biogenesis such as Caveolin-1 and Neural Sphingomyelinase 2. Moreover, certain disease states have also identified dysregulated EV-miRNA content, shedding light on the potential role of selective sorting in pathogenesis. These pathologies include chronic lung disease, immune response, neuroinflammation, diabetes mellitus, cancer, and heart disease. In this review, we will overview the mechanisms whereby cells selectively sort miRNA into EVs and also outline disease states where EV-miRNAs become dysregulated.

Circulating Non-coding RNAs in Renal Cell Carcinoma—Pathogenesis and Potential Implications as Clinical Biomarkers

“…While the miRNA sorting mechanisms have not been completely elucidated, recent insight points toward the interaction with a series of RNA binding proteins, such as members of the heterogeneous nuclear ribonucleoprotein family that have been proven to form vesicle-specific miRNA-protein complexes (Villarroya-Beltri et al, 2013). Other proposed mechanisms are based on the activity of the neutral sphingomyelinase 2 and the affinity of miRNAs to membrane raft-like structures containing ceramide as a possible loading mechanism (Groot and Lee, 2020). While it is not yet clear whether the currently proposed miRNA exosomal sorting mechanisms have any clinical relevance, the presence of specific miRNA signatures is indicated to have both mechanistic (Squadrito et al, 2014) and biomarker implications in cancer (Qu et al, 2016;Hosseini et al, 2017;Jiang N. et al, 2019;Wang et al, 2019).…”

Exosomal miRNAs as Potential Diagnostic Biomarkers in Alzheimer’s Disease

“… Usually exosomal miRNAs have different expression patterns compared to cellular or free miRNAs [ 83 , 87 ]. Indeed, the miRNAs are packaged within the exosomes through specific mechanisms, although these specific mechanisms are not yet fully identified [ 95 ]. Exosomes are strongly enriched with miRNA, unlike the cells of origin and blood without cells.…”

microRNA-Based Biomarkers in Alzheimer’s Disease (AD)

“…One important limitation of the analysis of miRNAs in human CNS tissues, extracellular fluid (ECF) and CSF is that, apart from CNS biopsies, brain tissue samples must be obtained postmortem, and miRNAs have a relatively short post-mortem halflife in both human brain and retina, on the range of about ∼1 to 3 h for a typical 22 nucleotide (nt) single-stranded miRNA (∼45% G + T) in the human neocortical and hippocampal compartments that have been analyzed and for which there is experimental data (Sethi and Lukiw, 2009;Rüegger and Großhans, 2012;Pogue et al, 2014;Tudek et al, 2019). Very few studies have addressed miRNA half-life in vitro or in vivo but currently both miRNA and mRNA decay kinetics have been shown: (i) to follow the same AU-enrichment rules of singlestranded miRNA and mRNA stability that is, the more AUenriched elements (AREs) in the sncRNA, miRNA or mRNA, the shorter the half-life (Sethi and Lukiw, 2009;Rüegger and Großhans, 2012;Clement et al, 2016;Van Meter et al, 2020); (ii) to be stabilized in part by miRNA binding proteins (Zang et al, 2020); (iii) to be further stabilized by circularization (circRNA; Lukiw, 2013a,b;Zhao et al, 2016a,b;Xie et al, 2017;Kondo et al, 2020) and/or (iv) by their inclusion into exosomes or intracellular or extracellular micro-vesicles (Badhwar and Haqqani, 2020;Bitetto and Di Fonzo, 2020;Groot and Lee, 2020;Upadhya et al, 2020). Another indication of the usefulness of post-mortem material for molecular-genetic studies is that nuclei extracted from human brain biopsies or post-mortem brain tissues are able to fully support in vitro run-on transcription for up to ∼3-4 h after which there is a precipitous decline in polymerization activity (Cui et al, 2005;Rüegger and Großhans, 2012;Clement et al, 2016).…”

“…These miRNA "information packages" have recently been shown to be sequestered into extracellular, lipophilic microvesicles or exosomes that shuttle between cells and tissues and/or amongst ECF, CSF and blood serum compartments (Alexandrov et al, 2013;Jaber et al, 2017Jaber et al, , 2019Badhwar and Haqqani, 2020;Bitetto and Di Fonzo, 2020;Upadhya et al, 2020;Vanherle et al, 2020). Extracellularly secreted miRNAs circulating in the peripheral blood are referred to as "circulating miRNAs"; they are either encapsulated by extracellular vesicles such as exosomes and microvesicles, or bound to molecules such as the Argonaute protein, or HDL cholesterol (Zernecke et al, 2009;Arroyo et al, 2011;Vickers et al, 2011;Ishibe et al, 2018;Groot and Lee, 2020;Upadhya et al, 2020). In addition, miRNAs that leak from destroyed cells and apoptotic bodies are also found among circulating serum miRNAs and have been implicated in the spreading of AD neuropathology (Zernecke et al, 2009;.…”