Exosoms

Exosomes are virus-sized (30-100 nm) secreted vesicles of the cells and a bilayer membrane. They are classified as etracellular vesicles . Exosomes are constantly produced and secreted by all cells. Exosomes represent an effective way of intercellular communication.

They contain a wealth of information from the mother 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 different cell types, e.g. 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 an important role in the signaling of normal and pathological processes, in communication between cells and in the 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, which forms the endosome vesicle and multivesicular bodies (MVBs). MVBs fuse with lysosomes, degrade or fuse with the plasma membrane and form exosomes that are released from the 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 with flotillin and other cell-specific proteins (Yue B et al. (2020).

Exosome biogenesis and secretion thus requires the formation of an endosomal sorting complex required for transport (ESCRT) (Patil AA et al. 2019). The 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).

In contrast to 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 depending on variables such as lipid morphology, hydrophobic changes and physiologically concentrated sphingosine in rafting membranes (O'Brien K et al. (2020).

Interestingly, the lack of ESCRT machinery did not prevent the formation of MVB vesicles in mammalian cells, but leads to reduced processing of their cargo and changes in number and size (Hessvik NP et al. 2018).

The methods of penetration of exosomes into recipient cells have not yet been sufficiently researched. However, it has been shown that, depending on the type of recipient cell, exosomes enter the target cells via fusion with the plasma membrane, macropinocytosis, phagocytosis and clathrin-dependent endocytosis.

The molecular mechanisms of exosome biogenesis and secretion are the subject of current research. Using flexible and scalable reagents, it is possible to isolate intact exosomes from cell culture media or any body fluids. These products and associated protocols are ideal for a variety of experiments, including low volume processing and handling of 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.