Gliomas are the highest known primary malignant tumor that affects the brain. Among all the gliomas, glioblastoma (GBM) is the most prevalent type among all other types. It occupies 70% of all gliomas and has a median overall survival of 15 months (1). The United States’ incident rate is 3.20 per 100,000, and GBM handles 60%–70% of malignant glioma (2, 3). Glioma is the third highest cause of cancer deaths in patients aged 15 and 34 years, which is responsible for 2.5% of the worldwide cancer fatality rate. Glioblastoma multiforme is 50% of gliomas, with a higher prevalence of patients over 65 years of age (4).

Multiple occurrences such as high growth rate, widespread invasion, and genetic variations are characteristics of gliomas. The drug resistance of GBM, the poor prognosis of patients who harbor glioma, is significantly linked to the lack of insight into the molecular facts related to the initiation of glioma and the absence of sensitive diagnosis and accurate therapeutic agents (5). The glioma microenvironment comprises different cells other than tumor cells, such as astrocytes, microglia, endothelial cells, and immune cells. Currently, trimetazidine (TMZ) is the primary chemotherapy drug for GBM (6). Patients with GBM have relatively high treatment resistance, resulting in decreasing overall survival (7). Immunotherapy inhibits the immune checkpoint receptor of programmed cell protein 1 (PD-1), and bevacizumab that inhibits the vascular endothelial growth factor is now being studied to enhance the treatment outcome of patients treated with GBM, which is the most prevalent glioma type (8). The determination of GBM is primarily focused on imaging methods and biopsies of tissues. However, imaging methods cannot reliably distinguish lesions induced by tumor development from treatment-related pseudo-progression lesions that mimic tumor progression and may typically resolve with time (9). Liquid biopsies allow identifying circulating biomarkers and offer the advantage of being non-invasive, thus enabling serial sampling and tracking possible structural reforms in the tumor during therapy (10).

At present, the critical testing techniques for diagnosing gliomas are based on neurological tests and neuroimaging procedures are sometimes performed when gliomas are at the advanced level (11). Exosomes are extracellular vesicles first identified 30 years ago. They have since been connected to cell–cell communication, disease propagation, and drug development (12). Those exosomes comprise different bioactive molecules, such as microRNAs (miRNAs), messenger RNA (mRNA), and other vital protein compounds (13).

Exosomes are critical for cellular signaling in normal physiology and pathological conditions, most notably in cancer. Exosomes are potent progenitors capable of changing target cell phenotypes, most notably during carcinogenesis and development, by altering tumor microenvironments and assisting in establishing the pre-metastatic niche. Numerous features of exosomes point to them as a new method for identifying cancer biomarkers for early diagnosis and therapeutic targets and for utilizing exosomes’ inherent and modified properties as therapeutic tools to slow down disease development (14). The blood–brain barrier (BBB) protects the central nervous system, supplies nourishment, regulates homeostasis, and allows the brain and the rest of the human body to communicate through the serum. Because of the complex structure of the BBB, drug administration in the brain is a significant problem, necessitating the discovery of innovative methods to achieve improved drug delivery in the brain, either invasively or non-invasively (15). To solve the issue mentioned above, different studies have been proposed to use exosomes to deal with this issue due to their nano size. Also, various studies revealed that exosomes are highly or lowly expressed in different cancers, including glioma. In addition, exosomes are also involved in other main cancer events, such as cancer initiation and progression of various cancers (16); however, this active status made exosomes potential therapeutic and biomarkers tools for glioma (17). The primary purpose of this review is to highlight the roles of exosomes as future therapeutic agents and diagnostic tools for glioma

Exosomes are extracellular vesicles with a diameter of around 30–100 nm with a bilayer membrane (18, 19). Exosomes include several cargo types comprising proteins, lipids, enzymes, transcriptional factors, DNA fragments, mRNAs, micro-RNAs, and Long non-coding RNA (lncRNAs) (20, 21). Exosomes are released by different cell types, such as erythrocytes, platelets, lymphocytes, dendritic cells (DCs), adipocytes, fibroblasts, brain cells, stem cells, and cancer cells. Exosomes are detected in biofluids, including blood, plasma, urine, Cerebrospinal fluid (CSF), milk, amniotic fluid, malignant ascites, saliva, and synovial fluid ( Figure 1 ). They play a significant part in the signals of normal and pathological processes in communication between cells and transporting substances such as proteins and RNAs from donor cells to recipient cells (16, 22, 23).

The biogenesis of exosomes occurs by inward budding of the plasma membrane that forms the endosome vesicle and the multivesicular bodies (MVBs). MVBs fuse with lysosomes, degrade or fuse with the plasma membrane, and create exosomes released from cells into extracellular space (22). Late endosomal structures containing dozens of Intraluminal Vesicles (ILVs) are known as MVBs, which are eventually transported to the trans-Golgi network for endosome recycling, delivered to lysosomes for degradation of all carried material, or fused with the plasma membrane and release exosomes into the extracellular space; this process is facilitated by Rab GTPase, such as RAB11 and RAB35, which release exosomes enriched with flotillin and other cell-specific proteins (24). Thus, exosome biogenesis and secretion require forming an endosomal-sorting complex required for transport (ESCRT) (25). ESCRT comprises 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 induce membrane deformation to form the stable membrane neck, and recruitment of the Vps4 complex to ESCRT-III drives vesicle neck scission and the dissociation and recycling of the ESCRT-III complex (26, 27).

A broad range of research has indicated an ESCRT-independent route in the exosome synthesis and carrier packing of lipids and related proteins, including tetraspanine (28). In opposition to ESCRT-sorted proteins, the loading of RNA into exosomes becomes lipid-mediated, reliant on self-organizing fat and carrier motifs. Specific nucleotide sequences show improved phospholipid bilayer affinity that relies on variables such as lipid morphology, hydrophobic changes, and physiologically concentrated sphingosine within rafting membranes (29, 30). Lipid rafts are plasma membrane subdomains loaded in cholesterol, sphingolipids, and attached proteins of glycosyl-phosphatidylinositol (GPI) whose connection with proteins or compounds may help their release by the use of exosomes (31, 32). The availability on the limiting membrane of ceramide, lysophospholipid, and glycosphingolipid molecules promotes the impulsive entry method to create ILVs (33). The ceramide changes in the existence of ceramidase and sphingosine kinase into sphingosine and sphingosine phosphate and the continual stimulation of 1-phosphate sphingosine receptors on the limited membrane facilitate the kind of tetraspanin into ILV. Thirty-three, three-four tetraspanin is a cell surface protein superfamily member with four transmembrane domains. Tetraspanin organizes membrane microdomains termed TEMs with a broad range of transmembrane and cytosolic signaling proteins (34). As the first tetraspanin, CD63 works in ESCRT-independent ILV creation. Interestingly, the lack of an ESCRT machine did not prevent the formation of MVB vesicles in mammalian cells but led to decreased cargo processing and changes in ILV numbers and sizes (35), indicating that exosome biogenesis could be a synchronized process that involves ESCRT-dependent and ESCR-independent pathways. The methods of exosome penetration into recipient cells have not been adequately explored. Nevertheless, it has been demonstrated that, according on the recipient cell type, exosomes penetrate target cells via fusion with the plasma membrane, macropinocytosis, phagocytosis, and clathrin-dependent endocytosis.

Exosomes play a critical function in cell–cell communication by transporting bioactive materials from the donor cells to the receiving cells (36). Many pieces of research have revealed that cancer cells release more exosomes, both locally and at a distance, to share information with other cells (37). Cancer-derived exosomes contribute to pre-metastatic milieu creation, tumor development, progression, immune evasion, angiogenesis, anti-apoptotic signaling, and treatment resistance throughout their bioactive cargo. Meanwhile, healthy cell exosomes such as DCs, B cells, and T cells significantly prevent tumor growth (38). To date, many miRNAs, lncRNAs, and proteins have played a vital role in the development of cancer. Therefore, exosomes may play a dual function in controlling, preventing, or encouraging the development of cancer, depending on their cell of origin and bioactive cargo ( Figure 2 ).

It has been reported that over 48 h, a single glioma cell secretes around 10,000 EVs (39). Glioma cell exosomes transport different chemicals than regular glial cell exosomes (40). Cancer effectors such as a mutant oncoprotein, oncogenic transcripts, and miRNAs are among the elements involved in tumor formation (28). In addition, exosomes enhance communication between cancer cells and the stromal cells surrounding them, resulting in either the tumorigenesis of apparently normal cells or a change in their behavior, which offers a favorable environment for the tumor to grow (41).

Angiogenesis is a critical event in glioma development, and glioma-delivered exosomes have been reported to play an essential role in this important event of glioma progression. For instance, a study that aimed to determine the processes of the glioma cell–affected angiogenesis noted that glioma cells might stimulate angiogenesis by transferring Linc-CCAT2 to endothelial cells through exosomes (42). Also, Lang HL et al. showed that gliomas stimulated angiogenesis by producing exosomes with high linc-POU3F3 (43); linc-POU3F3 has been reported to be involved in the development of glioma (43). On the other hand, a study showed that exosomes generated from hypoxic GBM cells relative to normoxic circumstances are effective inducers of angiogenesis in vivo. Moreover, they do the same in vitro via endothelial cell regulation, which increases tumor progression (44). Also, a study conducted by Xu Sun et al. noted that exosomes generated from glioma stem cells (GSCs) enhance endothelial cells’ angiogenic capacity through the miR-21/VEGF signaling pathway (45).

Moreover, different studies have revealed that exosomes are also involved in promoting and facilitating metastasis, which is a significant event in the progression and development of various cancers, including glioma. For example, Q. Cai et al. showed that miR-148a carried by exosomes facilitated cancer cell proliferation and metastasis by directly targeting CADM1 to trigger the STAT3 pathway (46), which plays an essential role in the metastasis of different cancer types, including glioma (47). Also, Zhanjun Ma et al. found that U251 cell–derived exosomes promoted metastasis-related proteins such as MMP-2 and MMP-9 (48), which facilitated the development of glioma. Moreover, another study also showed that the exosome EpCAM promotes the metastasis of glioma by targeting the CD44 signaling molecule on the surface of glioma cells; this exosome influenced the progression of glioma (49).

On the other hand, another study by Karma R Pace et al. discovered that exosomal L1CAM enhances motility, proliferation, and invasion in GBM cells, adding to the intricacy of how exosomal L1CAM promotes cancer cells not just through soluble ectodomains but also by exosomes (50). Besides, by directly targeting FBXW7 and DKK3, Gang Peng and his colleagues found that exosomal miR−25−3p significantly promoted the proliferation and migration of glioma cells (51). Furthermore, a recent study revealed that exosomal microRNA-671-3p increases cell proliferation in glioma by directly targeting CKAP4; this affected the proliferation and significantly increased glioma cell migration, facilitating glioma growth (52). Epithelial–mesenchymal transition is another vital factor that plays a significant role in glioma progression, and exosomes have been reported to be involved in glioma’s EMT. For instance, a study showed that exosomal microRNA-708 repression increases cell proliferation and EMT in glioma via promoting the SPHK2/AKT/-catenin pathway (53). In addition, another study also discovered that TGF-1 treatment substantially increased miR-10b expression in GBM cells, and miR-10b upregulation increases GBM cell proliferation, migration, and EMT; in contrast, miR-10b deletion has an opposite impact (54). Conclusively, the above facts show that exosomes play a crucial role in many critical glioma occurrences, such as cell proliferation, metastasis, angiogenesis, and EMT. More details about the involvement of exosomes in the progression of glioma are presented in Table 1 .

Exosomes play a crucial function in tumor-immune cell cooperation. Besides, exosomes secreted by tumor cells carry tumor-specific antigens that, in extraordinary circumstances, enable and suppress the immune system and promote the proliferation, invasiveness, and chemoresistance of glioma. miRNAs released from the tumor-derived exosomes can regulate the differentiation and function of the immune cells. These tumor-derived exosomes have many physiopathological roles and act on a range of immune cells, including effector T cells, naturally occurring Treg cells, and natural killer cells synonymous with immune suppression and tumor progression (110, 111). Also, serum exosomes from GBM patients have been shown to cause M2 polarization in normal monocytes, indicating a tendency toward T-helper 2 cells (Th2). Th2 responses are considered unacceptable in tumor immunotherapy since they alleviate cytotoxic anti-tumor immune processes and help inhibit cell-mediated immunity (112). Microglia-derived exosomes often mediate essential immune responses to tumorigenesis, degeneration, and central nervous system infections (113). Antigen-dendritic cells induce T-cell activation upon incubation with genes with different expression levels (GDEs) and mediate cytotoxicity against in vitro (114). Hell Winkel and his colleagues observed that immunosuppressive phenotypes and elevated cytokine concentrations in exosomes extracted from tumors lead to decreased development of other cytokines, including interleukin 2 (IL-2) CD69, and T-cell function, obstructing the migration of lymphocytes and inhibiting the immunity of tumors (115). Exosomes derived from GBM GL26 cells reduced cytotoxic CD8+ T cells’ number and function, fostering tumor growth (116). GDEs help to classify peripheral blood monocytes into alternately triggered M2 tumor-supporting macrophages (117) and regulate the development of cytokines and mononuclear migratory ability mitogen-stimulated, healthy, peripheral blood cells.

A glioma’s TME is incredibly diverse, comprising various cancer and non-cancer cells, such as endothelial cells, immune cells, glioma stem-like cells, and atrocities’ non-cellular elements, including the extracellular matrix (118, 119). The tumor microenvironment is progressively recognized as a strong promoter of glioma advancement, playing a leading role in controlling tumor growth (120). Therefore, exosomes have been viewed as a necessary two-way contact between the tumor and the tumor microenvironment (121). Different studies have been conducted to reveal the linkage between exosomes and the glioma’s microenvironment. For instance, a study reported that the miR-340-5p-macrophage virtuous cycle changed GBM development and TM (122). Over the last few years, studies suggested EVs produced by GBM cells interact with the activation of this tumor-supporting Tumor-Associated Macrophage (TAM) phenotype modulation. For example, a study showed that EVs derived from GBMs’ primary cultures had manipulated the TAM phenotype in vitro, converting it into an M2-like anti-inflammatory phenotype that resembles a tumor-supporting phenotype found in patients. Phenotypic changes included changed expression of a wide range of cell surfaces and enhanced release of cytokines such as interleukin-6 (IL-6) and vascular endothelial growth factor. In addition, GBM-derived exosomes mediated and enhanced macrophages’ phagocytic activity, enhancing the extracellular matrix’s deterioration and promoting the migration of tumor cells (117). It has been shown that exosomes, isolated from GBM cell line U87 and the GSCs, primarily target monocytes to cause the restructuring of the actin cytoskeleton and immunosuppressive phenotype M2, with the release of cytokines like MCP-3 and CXCL1 (123). Exosomes are among the many ways GBM cells interact with the tumor microenvironment to their advantage. Based on the researches mentioned above, it is very clear that exosomes play essential roles in glioma TME. However, different advanced studies are needed to deeply highlight the extended functions of exosomes in gliomas’ tumor microenvironment.

Exosome research in gliomas is a new and fast-developing area. It is clear that exosomes can be used to create therapeutic strategies to prevent and treat glioma growth and development. Nevertheless, many issues remain unresolved, such as the lack of consistency and consistency in exosome detection, isolation, and purification methods. Table 5 summarizes the benefits and drawbacks of exosome isolation techniques. Second, the approach by which exosomes are taken up by recipient cells is unknown. Additionally, brain tissue collection is more complicated. Additional researches are necessary to thoroughly understand the pathophysiology of exosomes in gliomas and show exosomes’ involvement in the illness.

Exosomes are a novel mode of cell communication that facilitates communication between parent and target cells and changes the tumor microenvironment, promoting cancer progression. Recent research has highlighted their involvement in the pathways through which gliomas develop, infiltrate surrounding tissue, create resistance to therapy, and spread throughout the body. Continuously studying exosomes will alter and complement current understandings of carcinogenesis and development, thus contributing to a complete understanding of tumor-related molecular processes. It will also help discover efficient biomarkers and targeted tumor therapies using exosomes, improving efficacy and medication usage for glioma. However, more deep studies are needed that incorporate new advanced technologies such as machine learning, scRNA-seq, and high-throughput screening to enhance the characterization of exosomal drugs as carriers to get more reliable therapeutic and diagnostic results. In addition, more clinical trials are needed to prove exosomes’ usefulness as future daily therapeutic agents and biomarker tools for gliomas and other types of cancers.

XiaobW designed and drafted the manuscript. XingbW, JW, YH, FL, XinW, LY, and ZL discussed and revised the manuscript. All authors read and approved the final manuscript.

This study was supported by the Clinical Medicine Science and Technology Innovation Project of JINAN (No. 202019020).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.