Potential Roles of Exosomal MicroRNAs as Diagnostic Biomarkers and Therapeutic Application in Alzheimer’s Disease

Figure 1

The biogenesis pathway of exosomal miRNA and composition of exosome. Canonical miRNAs are initially transcribed from long (41 kb) endogenous precursors called primary miRNAs that are driven by RNA pol II promoters and cleaved by microprocessor, a multiprotein complex formed by the RNase type III Drosha (the Drosophila homolog of RNASEN in humans) and the protein DGCR8 (Di George Syndrome Critical Region 8). A hairpin structure (now known as pre-miRNA) with 60–80 nucleotides in length, which bears a two-nucleotide overhang at the 3′ end that is a mark left by the Drosha processing, is released in this step. Within neuronal nuclei, pri- and pre-miRNA may be stabilized by 3′-terminal adenylation performed by PAPD4. After recognizing the precursors by their overhangs, GTPase-dependent Ran-Exp5 complex exports the pre-miRNAs out of the nucleus to the cytoplasm. An alternative pathway needs splicing out of the miRNAs from introns located in other genes, then further lariat processing, and finally proper folding into a pre-miRNA structure. In the cytoplasm, cleavage of the pre-miRNAs takes place once they have been loaded onto the Dicer-TRBP (TAR-RNA binding protein) complex, which removes the loop from the pre-miRNA to produce a dsRNA duplex that contains both the mature miRNA (or leader strand) and the so-called passenger strand. The Dicer-TRBP complex rapidly transfers the duplex to the miRISC (miRNA-RNAi-induced silencing complex), which contains AGO (Argonaute proteins) as its core. The HSP90/HSC70 chaperone complex participates in the process. In the miRISC, the passenger strand is degraded by an unidentified mechanism, leaving a mature miRISC loaded with a fully mature single-stranded miRNA (19–22 nt) to bind canonically to non-fully complementary mRNA targets at their 3′ untranslated regions (UTR). Alternatively, pri-miRNAs and miRNA may be loaded with proteins of RNA transport granules. These molecules are then transported to specific neuronal compartments, where mature or precursor miRNAs are enveloped in exosomes to be released elsewhere. Mature miRNAs are sorted into exosomes via four potential mechanisms: (1) the neutral sphingomyelinase 2- (nSMase2-) dependent pathway; (2) the miRNA motif and sumoylated heterogeneous nuclear ribonucleoprotein- (hnRNP-) dependent pathway; (3) the 3′-end of the miRNA sequence-dependent pathway; and (4) the miRNA-induced silencing complex- (miRISC-) related pathway (not shown).