miRNAs mediate RNA silencing by targeting most protein-coding transcripts to modulate pathological processes associated with HF. Extracellular vesicles are natural membrane-bound nanoparticles released by all cells, have a wide range of diameters and are produced by different intracellular pathways. A growing body of literature shows that EVs are important mediators of intercellular and intra-organ communication in the pathogenesis of HF by transferring bioactive materials, including miRNAs. To better understand the various roles of EV miRNAs in the progression of HF it will be necessary to understand the biogenesis of miRNAs and EVs and the selection mechanisms of extracellular miRNAs.
2.1. miRNAs and Extracellular Vesicle Biogenesis
miRNA biogenesis is tightly controlled at all levels including transcription, processing, modification and Argonaute (AGO) protein loading as well as RNA decay [
22]. Increasing evidence supports the view that dysregulation of miRNA biogenesis is associated with human HF [23-26]. miRNA biogenesis can be regulated either by the canonical pathway (
Figure 1A) or non-canonical pathways (
Figure 1B,C,E). The canonical pathway comprises a series of steps:
First, miRNA transcription is carried out by RNA polymerase II (Pol II) and Pol II-associated transcription factors and epigenetic regulators to pri-miRNAs [27-30], followed by nuclear processing of pri-miRNAs by Drosha and DGCR8 (microprocessor complex) to pre-miRNAs [
31,
32].
Second, following nuclear events, nuclear export of pre-miRNAs is mediated by a transport complex comprising exportin 5 (XPO5) and GTP-binding RAN (Ras-related nuclear protein) in the cytoplasm [
33], where pre-miRNAs are processed by Dicer, one RNase III-type enzyme, and TRBP (HIV-1 transactivating response (TAR) RNA-binding protein) to miRNA-duplex [
34,
35].
Third, selective miRNA-duplexes are loaded onto AGO proteins to form an RNA-induced silencing complex (RISC), unwinding the miRNA-duplexes. Theoretically, if one miRNA strand is selectively loaded onto an AGO protein to form the mature RISC, which further cleaves target mRNA or represses mRNA translation by binding to 3’-UTR of mRNA [
36,
37], the other strand will be ejected from the RISC and subjected to degradation. However, increasing evidence not only suggests that miRNAs can bind to other regions of targets in addition to 3’-UTR including 5’-UTR, coding area and promoter regions to activate translation or regulate transcription [38-40], but also both miRNA strands were found to functionally co-exist and participate in the pathogenesis of HF [
41,
42].
In addition to the canonical pathway, there are several non-canonical pathways involved in miRNA biogenesis. These non-canonical pathways are generally divided into two groups: Drosha/DGCR8-independent (
Figure 1B,C) and Dicer-independent pathways (
Figure 1E). Mirtron is one type of pre-miRNAs produced from introns of mRNAs by splicing [
43]. The 7-methylguanosine (m
7G)-capped pre-miRNAs are also produced by Drosha/DGCR8-independent pathway [
44]. Both pre-miRNAs are still dependent on Dicer-mediated cytoplasmic maturation, but they differ in nuclear export. Mirtron is exported by XPO5, whereas m7G-capped pre-miRNA is exported by XPO1 [
22,
44]. In addition, some miRNAs are made through Dicer-independent pathways (e.g. miR-451). The product of pri-miRNA-451 cleavage by Drosha is too short to be processed by Dicer, and pre-miRNA-451 will be directly loaded onto AGO2 and then sliced into an AGO-cleaved pre-miRNA-451 (ac-pre-miR-451) which will be further matured by poly (A)-specific ribonuclease PARN-mediated trimming [
45].
Extracellular vesicles (EVs), as one type of membrane-enclosed nanoparticle, are attractive mediators of intercellular and inter-organ communication in various diseases including HF [46-49]. Currently, EVs are divided into three types of vesicles in terms of their origin and mechanisms of biogenesis [
50] (see
Figure 1): (1) Exosomes (EXOs) (50-160 nm); (2) Microvesicles (MVs) (100 nm-1000 nm) and (3) Apoptotic bodies (APO-EVs) (1-5 μm). EXOs are a type of EV with an endosomal origin made by sequential invagination of the cell membrane resulting in the formation of multivesicular bodies (MVBs). The MVBs will ultimately generate exosomes by fusing with the plasma membrane and undergoing exocytosis [
51,
52]. Increasing evidence suggests that the biogenesis of EXOs is tightly regulated not only by the endosomal sorting complex required for transport (ESCRT) machinery, Syndecan-syntenin-ALIX (Apoptosis-linked gene 2-interacting protein X), Tetraspanins and ceramides during the formation of MVBs, but also by cytoskeletal elements, molecular motors and Ras-associated binding GTPases (RABs) during the transport/docking of MVBs from cytoplasm to cell membrane. In addition, the final fusion of MVBs with the plasma membrane and the secretion of EXOs are driven by the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), sGTPases and calcium [
50,
52].
MVs are membrane-bound vesicles with a size range of 100 nm to 1000 nm, produced by direct budding and pinching of the plasma membrane. Although the biogenesis mechanisms are not as well understood, increasing evidence suggests that the formation of MVs are in addition to being highly related to the regulation of cytoskeletal elements by small GTPases, such as Rho (RAS homolog) family and ADP-ribosylation factors (ARFs), they are also associated with the recruitment of the Tumor susceptibility gene 101 (TSG101) by Arrestin domain-containing protein 1 (ARRDC1) to the plasma membrane facilitating the shedding and release of MVs [
53,
54].
Other than EXOs and MVs, apoptotic bodies are also EVs which are generated by dying cells during apoptosis and generally recognized and engulfed by phagocytes [
55]. However, recent studies suggested that apoptotic bodies are generated by budding from the plasma membrane. The formation of apoptotic bodies is regulated by apoptotic cell disassembly via several molecular regulators including ROCK, Pannexin-1 and Plexin-B2 (well-summarized in [
56]). In addition, another novel beads-on-a-string membrane structure is also involved in the formation of apoptotic bodies [
57]. Increasing evidence suggests that apoptotic bodies not only facilitate the clearance and degradation of apoptotic materials but also contain other biomolecular cargos (e.g. miRNAs, DNA, protein and lipids) to mediate intercellular communication [
58,
59].
Although these membrane-bound vesicles differ in size distribution and mechanism of biogenesis, they are secreted by cells under various pathophysiological conditions into the extracellular space where they mediate intercellular and inter-organ communication via EV bioactive substances including proteins, RNAs, DNAs and lipids. Accumulating evidence suggests that packaging of cargos into EVs is highly selective and tightly regulated. In this review, we will also focus on the selective packaging of miRNAs into EVs.
2.2. Mechanisms of miRNA Selection into EVs
Extracellular miRNAs (Ex-miRs) have been found to be stably transported by ribonucleoproteins (RNPs), lipoproteins and neutrophil extracellular traps [60-62]. However, it has become attractive to view EV function as intercellular and inter-organ mediators conveying their cargos including non-coding RNAs, in particular, miRNAs. Interestingly, the existence of EV-miRs in the circulation of patients with cardiovascular diseases (CVDs), including HF, has raised the possibility that EV-miRs serve as prognostic and diagnostic markers and potential therapeutic targets [63-65]. However, the underlying mechanisms by which miRNAs are selectively secreted remain unclear.
In 2010, a study by Kosaka et al. [
21] revealed that secretory miRNAs are regulated by the neutral sphingomyelinase 2 (nSMase 2) which regulates the biogenesis of ceramide and triggers exosome secretion, rather than by the ESCRT system, while the members of ESCRT regulate the biogenesis of EXOs. Increasing studies further suggest that the sorting of miRNA into EVs and their secretion are controlled by specific sequence motifs present in miRNAs that are recognized by RNA-binding Proteins (
Figure 1G) including Heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1) [
66] and hnRNPU [
67], Y-box protein 1 (YBX1) [
68,
69], Synaptotagmin-binding cytoplasmic RNA-interacting protein (SYNCRIP) [
70,
71], ELAV-like protein 1 (or HuR, human antigen R) [
72,
73], HSP90AB1, XPO5 and major vault protein (MVP) [
72,
74] and Serine/arginine splicing factor 1 (SRSF1) [
75]. Recently, another RNA-binding protein, Lupus La, has been identified to mediate the selective sorting of miRNAs into EVs, and in particular, it selectively sorts miRNA-122 through the specific motifs located at the 3’ end of miRNA-122 [
76]. Further studies by analyzing either EV-enriched (EXOmotifs) or Cell-enriched miRNA sequence (CELLmotifs) and structure revealed that the sorting sequences (EXOmotifs) present in miRNAs determine their secretion by EVs. Two novel RNA-binding proteins, Alyref and Fus, have also been identified to function as at least two RNA-binding proteins responsible for EXOmotif recognition and miRNA export into EVs [
19].
Additionally, the post-translational modifications of some RNA-Binding Proteins, such as SUMOylation [
66], oligomerization and ubiquitination [
73], Liquid-liquid phase separation (LLPS) [
77] and O-GlcNAcylation [
78], also control the sorting of miRNAs into EVs. Interestingly, cell activation-dependent alterations of miRNA targets promote the sorting of miRNAs into EVs [
79], and modifications of miRNAs also determine the distribution of miRNA in EVs, such as 3’-end uridylation rather than adenylation [
20]. In addition, the ALG-2-interacting protein X (Alix), an accessory protein of ESCRT, is also involved in the miRNA sorting to EVs by interacting with Ago2 and miRNAs during the EV biogenesis [
80]. Caveolin-1 (Cav-1) was identified as the first membrane protein to be involved in the selective sorting of miRNAs to EVs by directly interacting with hnRNPA2B1, and the phosphorylation of Cav-1 at Y14 not only promotes the O-GlcNAcylation of hnRNPA2B1 but also enhances the interaction between Cav-1 and O-GlcNAcylated hnRNPA2B1, subsequently facilitating the trafficking of the Cav-1/hnRNPA2B1/miRNAs complex into MVs [
78]. The studies and mechanisms cited above strongly support the conclusion that the sorting of miRNAs into EVs is highly selective and that EV secretion is tightly controlled, which may contribute to pathophysiological alterations in human diseases, including HF. For example, these EV sorting mechanisms of miRNAs have been potentially involved in the biogenesis of EV-miRNAs in heart failure (See
Table 1).