Fig. 1. MicroRNA (miRNA) biogenesis and secretion into extracellular space.

(A), Biogenesis. 1. Transcription of pri-miRNAs from genes in the nucleus by RNA polymerase II. 2. Processing of primary transcript into 70-nucleotide (nt)–long pre-miRNA by the Drosha-DGCR8 Microprocessor complex assisted by p68 (DDX5) and p72 (DDX17) DEAD-box RNA helicases that possibly act as scaffolding proteins to recruit cofactors. 3. Export into cytoplasm in a Ran-GTP– dependent manner through Exportin 5. 4. Cleavage of pre-miRNA by RNase III enzyme Dicer, into a 22-nt double-stranded RNA composed of the mature miRNA “guide” strand and the low-abundance miRNA* “passenger” strand; TRBP and protein activator of interferon-induced protein kinase PKR (PACT) are some of the molecules that regulate this step. 5. Incorporation of mature miRNA into the RISC, whose main components are AGO-2 and GW-182; partial complementarity of the seed region of miRNA to the 3′ untranslated region of target mRNAs causes translational repression while complete complementarity leads to degradation of the transcript. (B), Secretion of miRNAs into extracellular space by: 6. Packaging into multivesicular bodies (MVBs) that fuse with the plasma membrane and release as exosomes in a ceramide-dependent pathway positively regulated by nSMase2. 7. Encapsulation into HDL particles, a process which is repressed by nSMase2. 8. Binding to RNA-binding proteins, namely AGO-2 and nucleophosmin 1 (NPM1). 9. Incorporation into apoptotic bodies. 10. Pioneering studies describing exosomal miRNAs isolated from primary bone marrow derived mast cells; HDL-miRNA in human plasma; protein-bound miRNAs in human plasma (AGO-2) and human cell lines (NPM1 in HepG2 and A549); miR-126 from endothelial cell-derived apoptotic bodies in vitro in human umbilical vein endothelial cells (HUVEC). (C), Detection matrices. 11. Depiction of 13 different matrices where extracellular miRNAs have been discovered.