*2.2.1 miRNAs*

*Update on Mesenchymal and Induced Pluripotent Stem Cells*

in exosome biogenesis involves the invagination of the membrane of the multivesicular body (MVB) to form membrane-bound vesicles in MVB lumens that are identified as intraluminal vesicles (ILVs) (**Figure 1**) [83, 84]. Various factors and signaling pathways have been considered in biogenesis, trafficking, and abscission of exosomes [85]. Of note, endosomal sorting complexes required for transport (ESCRT) machinery with four complexes, ESCRT-0, ESCRT-I, ESCRT-II, and ESCRT-III, participate in exosome formation and packing cargo incorporation with different accessory proteins (**Figure 1**) [81, 85, 86]. Noteworthy, the formation of MVBs in the absence of the ESCRT machinery is aborted. In this condition, oligodendroglial cell ceramide is a key molecule to induce inward budding of the limiting membrane of MVBs [83, 87]. After MVB formation, intracellular trafficking of vesicle systems was orchestrated by Rab-GTPase family proteins [81]. As shown in **Figure 1**, several Rab proteins specifically contribute to the transfer of vesicles in definitive pathways. Along with these factors, soluble NSF attachment protein receptor (SNARE) has been suggested to control the fusion of MVBs with the plasma membrane (**Figure 1**) [88]. At the intracellular level, three possible fates are considered to involve MVBs such as secretory, lysosomal, and back fusion pathways. Once secreted, exosomes can be received by neighboring cells by three possible

*Biogenesis, structure, and uptake of exosomes. Exosomes are producing during invagination process of MVB's membrane. ESCRT machinery and ESCRT-independent mechanisms (lipid rafts/tetraspanin) contribute to form exosomes and sort several molecules including proteins, miRNA, mRNA, DNA strands, and lipids into their lumen or limiting membrane of exosomes. Exosome cargoes are collected from materials received by endocytic pathway, Golgi apparatus, and cytoplasm. Rab-GTPase family proteins regulate intercellular trafficking and docking of MVBs. In the secretory pathway, MVBs actively fuse with the plasma membrane to release exosomes into the extracellular space. In alternative pathways, MVB could prefer binding to the lysosome or directly fuse back to the plasma membrane. Once secreted, exosomes enroll several mechanisms to arrive at the target cell: (I) enter through internalization process; (II) bind through receptor-ligand interactions, (III) direct fusion with the plasma membrane of the target cell. Exosomes are able to affect the biological processes of* 

**106**

*the target cells.*

**Figure 1.**

It seems that exosomal cargo such as cytokines and miRNAs could be easily transferred to recipient cells. Increasing evidence indicates that exosomal pro-angiogenic miRNAs (miRNA-125a, miRNA-30b, miRNA-30c, miRNA-424, miRNA-150, and let-7f) are important regulators of angiogenesis in the target sites [89, 94–96]. Data suggest that exosomal miR-150 is a key contributor to the pro-angiogenic activity of MSC exosomes following ischemic injuries [89, 96, 97]. In contrast, anti-angiogenic function on tumor cells was reported by a research group guided by Lee et al. They demonstrated the anti-angiogenic function of MSC exosomes on breast cancer cells governed by delivering miR-16 to suppress VEGF factor [91]. In a recent study conducted by Chen et al., they declared that exosome can be used as therapeutic transfer vesicles to carry miRNAs and genetic molecules to modulate VEGF content and control untamed angiogenesis in rheumatoid arthritis [98]. Based on the literature, the expression of VEGF, endothelial marker CD31, and matrix metalloproteinases-14 (MMP-14) activity is induced in patients with rheumatoid arthritis. The application of MSC-derived exosomes containing miRNA-150-5p (Exo-150) clearly decreased transcription of VEGF and MMP-14 in synovial fluid. Consistent with these changes, the pro-inflammatory response was blunted by decreasing IL-1β, transforming growth factor-β (TGF-β) and tumor necrosis factor-α (TNF-α) content in synovial fluid. This study has shown that MSC-derived Exo-150 can be used as bio-shuttle and magic bullet for inhibiting an exacerbated angiogenesis via the modulation of angiogenesis-related factors. However, some contradictory facts exist regarding the sole application of exosomes in the context of tumor cells.
