Progress and limitations of exosomes

December 2, 2019 / BIOON / - -- it was first described in the late 1960s that there are vesicles around cells in mammalian tissues or liquids In 1983, pan and Johnstone discovered a kind of membrane vesicle from reticulocyte culture medium, which opened the door of exosomes world Johnstone named it exosomes in 1987 In 2011, the general term "extracellular vesicle" was proposed to define all the extracellular structures surrounded by lipid bilayers Exosomes are a type of extracellular vesicles Picture from frontier in genetics, 2018, DOI: 10.3389/fgene.2018.00092 Exosomes were originally thought to be a mechanism by which cells clear unwanted proteins However, studies in the late 1990s have shown that it can play a role in intercellular communication, especially in immune response and cancer Strong support for this concept emerged in 2007, when a team led by Swedish scientist Jan L ö TVall discovered that the exosomes contained mRNA and microRNA Moreover, in vitro experiments show that these mRNA can be translated and expressed into proteins in target cells, thus providing evidence for the first time that the exosomes secreted by cells contain mRNA and microRNA, which can transfer these genetic materials to other cells This extraordinary discovery not only indicates a new form of intercellular communication, but also suggests that in a sense exosomes behave like viruses because they carry genetic information that can be translated into proteins in the cells they "infect." 1 Exosomes are secreted by most types of cells Exosomes are small in size: about 150 nanometers in diameter or smaller They are formed in the compartments of cells called multivesicular bodies (MVB), also known as multivesicular endosomes (MVE) The formation of exosomes consists of the following steps: the first step is the formation of small inner vesicles which contain protein, mRNA and miRNA When the vesicles fused with the cell membrane, these inner vesicles were released as exosomes Alternatively, the multivesicles can fuse with lysosomes, and lysosomes can degrade the substances contained in the multivesicles Once the exosomes reach their destination, which is usually determined by the specific ligands on the surface of the exosomes, the exosomes can enter the target cells in two ways: ingestion into the cells by target cell endocytosis, or fusion into the target cell membrane, and directly release the substances it contains into the cytoplasm Cells also secrete extracellular vesicles from other membrane sources, such as extranuclear particles, exfoliated vesicles or microbubbles, which are also directly generated by budding from the cell membrane These vesicles are also known to carry active proteins and RNA, as well as some compounds that have never been described in exosomes before, but their effects on distant tissues are poorly understood The picture is from the scientist / Keith kasnot 2 According to the type and physiological state of secretory exosomes, the changes induced by the interaction between exosomes and its receptor cells can be very different, which can either help to prevent diseases or worsen diseases in some cases For example, exosomes secreted by mature dendritic cells carry antigens, or MHC peptide chain complexes embedded in their lipid bilayers can induce antigen-specific immune responses On the other hand, in the presence of immunosuppressive agents, exosomes secreted by mouse dendritic cells can promote immune tolerance Similarly, exosomes released by macrophages infected with non pathogenic mycobacteria carry bacterial antigens, which are then ingested by other antigen presenting cells to promote immune response On the contrary, exosomes released by macrophages infected by pathogenic mycobacterial strains inhibit macrophage activation and cytokine release, thus inhibiting immune responses to be induced by mycobacterial antigens The pathogen appears to have evolved at the exosome communication level to advantageously control the host's immune system, although details remain to be elucidated Exosomes secreted by cancer cells carry antigens from tumors, which can be captured by dendritic cells and used to present tumor antigens, thus activating immune cells against this cancer However, these exosomes also contain a variety of immunosuppressive molecules, which can either inactivate T lymphocytes or natural killer cells, or promote the differentiation of regulatory T lymphocytes or myeloid cells that inhibit immune response It is still unclear whether the net effect of these opposing effects is beneficial or harmful for individuals with cancer The results of several research groups have shown that tumor secreting exosomes promote tumor growth by inhibiting anti-tumor immune response, or by promoting angiogenesis or tumor migration to other parts of the body during tumor metastasis Recently, Mauro Poggio et al found that for many tumors that are resistant to immunotherapy, these tumors evade the immune system by transferring exosomes to the lymph nodes, where they inhibit the activation of immune cells remotely PD-L1 carried by exosomes does not shut down the immune response on the tumor surface, but suppresses the immune cells before the exosomes reach the lymph nodes through the lymphatic system or blood, so that the tumor is resistant to the existing immunosuppressive checkpoint inhibitors Picture from cell, 2019, DOI: 10.1016/j.cell.2019.02.016 Other tissues or cells also secrete exosomes carrying immunosuppressive molecules For example, placental derived vesicles carry inhibitory ligands against natural killer lymphocytes and other components of the immune system, which may prevent immune attack on the fetus Similarly, exosomes and other vesicles in BALF can transfer resistance to one allergen to other animals When exosomes purified from mice that have been resistant to an allergen are injected into natural mice, these mice that are sensitive to the allergen are also resistant to the allergic reaction caused by the allergen On the other hand, the exosomes in BALF can also enhance the pro-inflammatory response, because when the exosomes purified from asthmatic patients are cultured together with the bronchial epithelial cells, the bronchial epithelial cells react and secrete pro-inflammatory cytokines Recently, when exosomes collected from activated polymorphonuclear leukocytes (PMN) were infused into the lungs of healthy mice, Kristopher genschmer et al Caused lung tissue damage observed in patients with chronic obstructive pulmonary disease (COPD) It is worth noting that the researchers also collected exosomes from lung fluid of human COPD patients and newborn infants in intensive care unit with bronchopulmonary dysplasia; when these exosomes from human body are infused into the lungs of healthy mice, they also cause lung tissue damage of COPD This damage mainly comes from exosomes secreted by PMN cells in human lung In addition, Stephen E Flaherty III et al Found that fat cells not only release the fatty acid components of triglycerides, they also release intact triglycerides packaged into small particles These lipid filled particles called adipocyte exosomes (adexo) are absorbed by macrophages in adipose tissue Macrophages rapidly degrade triglycerides in adexo and release fatty acids These findings suggest that adexo seems to control the development of immune cells Picture from cell, DOI: 10.1016/j.cell.2018.12.002 It has also been found that eukaryotic parasites or pathogens secrete exosomes, which can contribute to host tolerance to invaders (by inhibiting immune response), or, conversely, promote host pathological inflammatory response in the presence of invaders In addition to the immune system, exosomes may affect other physiological functions Exosomes secreted by nerve cells, epithelial cells, muscle cells and stem cells may have functions such as promoting tissue repair, communication within the nervous system, and the formation and transfer of pathological proteins such as prion protein and amyloid deposition For example, neurons secrete exosomes carrying neurotransmitter receptors, so they can participate in the clearance of neurotransmitters in extracellular space to prevent signal transmission, and / or transfer these receptors to other cells, so that they respond to neurotransmitters 3 The limitations of exosomes research it has been observed that in various cell types, the same or larger extracellular vesicles as exosomes sprout from the plasma membrane or membrane extension (such as microvilli, filiform pseudopods, cilia and flagella) of the cell body The size, density and membrane location of extracellular vesicles are the same as those of exosomes, so the current methods can not distinguish them effectively The relative ratio of exosomes from polyvesicles released by cells to other small extracellular vesicles is highly variable, depending on cell type, environmental conditions and other factors (such as infection or artificially induced molecular expression), and through most current experimental methods The prepared exosomes may contain small exovesicles (exosomes) of endosome origin and small exovesicles of non endosome origin, as well as other lipid based non vesicular structures (such as multiple density lipoproteins, i.e medium density lipoproteins, high density lipoproteins, low density lipoproteins and very low density lipoproteins) and newly identified exomeres Recently, exosomes rich in advanced endosomes have been defined as a small extracellular vesicle subtype with tetratransmembrane protein CD63 (a protein accumulated in polyvesicles) and CD9 / CD81 (mainly on plasma membrane), but this definition needs to be verified in other cells and under his conditions In many pathophysiological situations, including cancer, immune response, cardiovascular disease, regeneration and stem cell therapy, extracellular vesicles play many functions Although many studies have described these functions as specific to exosomes, it has been found that many exosomes have potential subcellular origins, which indicates that many types of extracellular vesicles have a broader functional correlation The increased interest in extracellular vesicles is related to their ability to induce phenotypic changes in recipient cells The differences in the size and / or surface components of the extracellular vesicles are likely to affect the target cell's recognition and capture of them For example, micropinocytosis can theoretically capture isolated exosomes and small extracellular vesicles, but cannot capture too large or small extracellular vesicles In view of the diversity of extracellular vesicles, people's understanding of the fate of different extracellular vesicle subtypes, including exosomes, is still limited after they reach recipient cells In recent years, an important progress in the study of exosomes is that more and more people realize that the extracellular vesicles including exosomes contain many different granular subtypes, and each granular subtype may have an interesting function in intercellular communication Despite the fact that people