Exosoms

Exosomes are virus-sized (30-100 nm) secreted vesicles of cells and a bilayer membrane. They are constantly produced and secreted by all cells. Exosomes represent an effective way of intercellular communication. They contain a great deal of information from the parent cell in the form of proteins, lipids, enzymes, transcription factors, DNA fragments, mRNAs, micro RNAs and long non-coding RNAs (lncRNAs). Exosomes are released by various cell types, including erythrocytes, platelets, lymphocytes, dendritic cells (DCs), adipocytes, fibroblasts, brain cells, stem cells, and cancer cells. Exosomes can be detected in biofluids such as blood, plasma, urine, cerebrospinal fluid (CSF), milk, amniotic fluid, malignant ascites, saliva, and synovial fluid. They play important roles in signaling of normal and pathological processes, communication between cells, and transport of substances such as proteins and RNAs from donor cells to recipient cells (Gaurav I et al. 2021; Zhang Y et al. 2019). Exosomes are thus also involved in the pathogenesis of infectious and degenerative diseases.

Biogenesis of exosomes occurs by inward budding of the plasma membrane forming the endosome vesicle and multivesicular bodies (MVBs). MVBs fuse with lysosomes, degrade or fuse with the plasma membrane to form exosomes that are released from cells into the extracellular space (Gaurav I et al. 2021). Late endosomal structures containing numerous intraluminal vesicles (ILVs) are known as multivesicular bodies, which are eventually transported to the trans-Golgi network for endosome recycling, delivered to lysosomes for degradation of all entrained material, or fused to the plasma membrane and release exosomes into the extracellular space; this process is facilitated by Rab GTPases such as RAB11 and RAB35, which release exosomes enriched in flotillin and other cell-specific proteins (Yue B et al. (2020).

Thus, exosome biogenesis and secretion requires the formation of an endosomal sorting complex (ESCRT) required for transport (Patil AA et al. 2019). ESCRT consists of four complexes (ESCRT-0, ESCRT-I, ESCRT-II, and ESCRT-III) and associated proteins (VPS4, Tsg101, and ALIX). ESCRT-0 sorts ubiquitinated cargo proteins into the lipid domain; ESCRT-I and ESCRT-II cause membrane deformation to form the stable membrane neck, and recruitment of the Vps4 complex to ESCRT-III leads to cleavage of the vesicle neck and dissociation and recycling of the ESCRT-III complex (Patil AA et al. 2019).

Unlike ESCRT-sorted proteins, loading of RNA into exosomes is mediated by lipids and depends on self-assembling lipid and carrier motifs. Specific nucleotide sequences exhibit enhanced affinity for phospholipid bilayers, which depends on variables such as lipid morphology, hydrophobic modifications, and physiologically concentrated sphingosine in rafting membranes (O'Brien K et al. (2020).

Interestingly, the absence of an ESCRT machinery did not prevent the formation of MVB vesicles in mammalian cells but resulted in decreased processing of their cargo and changes in number and size (Hessvik NP et al. (2018). The methods of exosome entry into recipient cells have not been adequately explored. However, it has been shown that exosomes enter target cells via fusion with the plasma membrane, macropinocytosis, phagocytosis, and clathrin-dependent endocytosis, depending on the type of recipient cell.

The molecular mechanisms of exosome biogenesis and secretion are the subject of current research. It is possible to isolate intact exosomes from cell culture media or any body fluids using flexible and scalable reagents. These products and their associated protocols are ideal for a variety of experiments, including processing small sample volumes and handling multiple samples. Total exosomes enriched from cell cultures (using the total exosome isolation reagents or by ultracentrifugation) can be further purified into specific subpopulations by immunomagnetic capture.

1. Gaurav I et al (2021) Factors Affecting Extracellular Vesicles Based Drug Delivery Systems. Molecules 26:1544.
2. O'Brien K et al (2020) RNA Delivery by Extracellular Vesicles in Mammalian Cells and its Applications. Nat Rev Mol Cell Biol 21:585-606.
3. Hessvik NP et al (2018) Current Knowledge on Exosome Biogenesis and Release. Cell Mol Life Sci CMLS 75:193-208.
4. Patil AA et al. (2019) Exosomes: Biogenesis, Composition, Functions, and Their Role in Pre-Metastatic Niche Formation. Biotechnol Bioprocess Eng 24:689-701.
5. Wu X et al. (2021) The Roles of Exosomes as Future Therapeutic Agents and Diagnostic Tools for Glioma. Front Oncol 13: doi.org/10.3389/fonc.2021.733529.
6. Yue B et al (2020) Exosome biogenesis, secretion and function of exosomal miRNAs in skeletal muscle myogenesis. Cell Prolif 53:e12857.
7. Zhang Y et al (2019) Exosomes: Biogenesis, Biologic Function and Clinical Potential. Cell Biosci 9:19.