The neuron is the basic structural and functional unit of the CNS. The main function of neurons is to receive, integrate, and transport information to maintain the proper activity of the CNS. The release of exosomes has been demonstrated both in developing neurons and differentiated mature neurons. These neuron-derived exosomes (NDEs) contain some neuron-specific markers, including L1 cell adhesion molecule (L1CAM) and the GluR2/3 subunits of glutamate receptors (Fauré et al., 2006; Lachenal et al., 2011). Exosomes stem from MVBs, which can be transported in neurons and release exosomes by fusing with the PM of the soma, the dendrites, the axons, or the presynaptic membrane (Janas et al., 2016).

The secretion of exosomes is regulated by synaptic activity (Table 2). Fauré et al. (2006) proved that a high concentration of K⁺ stimulation could promote the secretion of NDEs, suggesting that exosome secretion can be modulated by depolarization. The team then confirmed that the release of exosomes from well-differentiated neurons was influenced by calcium influx and glutamatergic synaptic excitability (Lachenal et al., 2011). Interestingly, recent research shows that high-frequency electrical stimulation (HFS) can evoke a fast and immediate increase of intracellular calcium, while the fusion of MVBs and the PM has a time lag after HFS. The result suggests that, unlike synaptic vesicles, the fusion of MVBs and the PM is not directly coupled to the calcium influx and may involve a complex signal network. This study also found that the basic fibroblast growth factor (bFGF), a family of pleiotropic growth and differentiation factors, could increase the MVB-PM fusion and the release of exosomes in cultured hippocampal neurons (Kumar et al., 2020).

The binding of exosomes to neurons seems to be of specificity. Exosomes released by cortical neurons upon synaptic activation are reported to be selectively absorbed by neurons but not by glial cells, suggesting that NDEs can specifically interact with other neurons and this may be a novel aspect of neuronal communication (Chivet et al., 2014). Strikingly, it has been confirmed that some NDEs internalized into neurons can be re-secreted together with endogenous exosomes of the recipient neurons and this might result in a longer-distance action and increase the potential of exosomes for cell communication and widespread physiological or pathological signals (Polanco et al., 2018).

Microglia are the first line of defense against pathogens and brain injury in the CNS. By screening the brain parenchyma, microglia can quickly detect the signals of pathogen invasion, tissue damage, or internal environment disorders, and further participate in immune regulation, tissue repair, and remodeling to maintain the homeostasis of the CNS (Ginhoux et al., 2013; Caruso Bavisotto et al., 2019). There is increasing evidence in support of the presence of microglia-derived EVs, including exosomes and MVs (Bianco et al., 2005; Potolicchio et al., 2005). Analysis of protein content showed that microglia-derived exosomes (MDEs) contain several unique markers, including the aminopeptidase CD13 and the lactate transporter MCT-1 (Potolicchio et al., 2005).

Some studies have explored the stimulus and downstream signaling pathways that induce exosome shedding from microglia (Table 2). The secretion of MDEs is closely related to infection and neuroinflammation. Lipopolysaccharide (LPS) is a component of the cell wall of Gram-negative bacteria. Exposure to LPS or neuroinflammatory factors such as IL-4 has been identified to significantly increase the release of MDEs and modulate their biogenesis and composition, thereby regulating the neuroinflammatory (Cunha et al., 2016; Tian et al., 2019). Moreover, extracellular adenosine triphosphate (ATP) was showed to induce the secretion of MDEs and MVs by activating P2X7 receptors (P2X7R) (Bianco et al., 2005; Takenouchi et al., 2015). Another study has shown that ATP can further modify the protein composition of MDEs, promoting the sorting of proteins associated with the autophagy-lysosomal pathway, cellular metabolism, cell adhesion, and extracellular matrix organization, thereby influencing the function of MDEs (Drago et al., 2017). Neurotransmitter was also involved in the release of MDEs. Serotonin (5-HT) secreted by neurons was proved to stimulate the release of MDEs by binding to the 5-HT_2a,b, and 5-HT₄ receptors in microglia. The 5-HT₄ could transmit signals to adenylate cyclase (AC) via Gαs proteins to regulate the generation of cAMP then the cAMP activates GEF1/2 to transmit signals to phospholipase C (PLC) via Rap1. While 5-HT_2a,b transmit signals to PLC via Gαq proteins. PLC can further mediate the increase of cytosolic calcium, promoting the fusion of MVBs and cell membranes to release exosomes (Glebov et al., 2015). Additionally, Wnt3a was capable of stimulating the release of exosomes from microglia through a GSK3-independent mechanism without altering the release of primary cortical neuron exosomes. Wnt3a-induced MDEs contain a variety of proteins related to cellular structure, metabolism, protein synthesis, and decomposition (Hooper et al., 2012).

Astrocytes are the most abundant glial cells in the CNS and are essential for brain homeostasis maintenance (Colombo and Farina, 2016). Under physiological conditions, astrocytes play a variety of complex and important roles in the CNS, including providing metabolites and growth factors for neurons, promoting synapse formation, modulating synaptic transmission by removing or recycling neurotransmitters, and contributing to the maintenance of the BBB (Freeman and Rowitch, 2013). Moreover, astrocytes are also important regulatory cells of brain immune function and play an important part in controlling the transport and activation of immune cells (Colombo and Farina, 2016). Some studies provided preliminary evidence for the secretion of exosomes from cultured astrocytes (Taylor et al., 2007; Guescini et al., 2010). Some astrocyte-specific proteins including glial fibrillary acidic protein (GFAP), glutamine aspartate transporter (GLAST), and glutamine synthetase (GLUL) have been found in astrocyte-derived exosomes (ADEs) (Goetzl et al., 2016).

Similar to microglia, the release of ADEs is also influenced by the trophic stimulus, inflammatory stimulus, and other microenvironmental factors (Table 2). A recent study showed that ATP could induce astrocytes to release IL-1β in exosomes and MVs by binding to P2X7R and rapidly activating the sphingomyelinase (Bianco et al., 2009). In another recent study, both pro-inflammatory factors (IL-1β) and anti-inflammatory factors (IL-10) have been suggested to induce the release of EVs in astrocytes. Notably, the proteomic analysis found that EVs exposed to IL-10 or ATP were rich in proteins associated with axonal growth, dendrite branching, and synapse formation. By contrast, IL-1β-induced EVs contain proteins involved in regulating peripheral immune response and promoting the recruitment of immune cells to CNS, which potentially mediate the transmission of inflammatory signals (Datta Chaudhuri et al., 2020). This result suggests that these stimulating factors could induce the release of astrocyte EVs, as well as participate in the modification of the EVs cargos. Nonetheless, the specific signaling pathways still need to be further studied.

Stimulating with cytotoxic proteins such as HIV trans-activating regulatory protein (Tat) and amyloid peptide can promote the secretion of ADEs, thus mediating cell damage of the CNS (Wang et al., 2012; Yang L. et al., 2018). In the case of HIV infection, Tat-induced astrocytes have been observed to release exosomal miR-9 which was subsequently internalized by microglia and mediated microglial migration (Yang L. et al., 2018). It was reported that the amyloid peptide could induce astrocytes to secrete exosomes rich in ceramide and prostate apoptosis response 4 (PAR-4) and promote cell apoptosis. And the ceramide produced by neutral sphingomyelinase 2 (nSMase2) displays a crucial role in the formation and secretion of ADEs (Wang et al., 2012). Additionally, prion proteins (PrP) localized on the cell membrane were proved to be a positive regulator of exosome secretion and promote the release of ADEs by inhibiting the degradation pathway of MVBs. This process may depend on the role of PrP in facilitating the internalization of CAV1 and subsequently inhibiting the formation of the autophagy-related 12 (ATG12)-ATG5 complex, thus impairing the autophagy progression and protecting the MVBs from fusion with autophagosomes and their transport to lysosomes for degradation, thus promoting the release of ADEs (Dias et al., 2016). Nevertheless, some researches found that some cytotoxic proteins can reduce the release of ADEs, thereby inhibiting the neuroprotective effects of astrocytes (Abdullah et al., 2016; Hong et al., 2017). Beta-amyloid (Aβ) appears to suppress the secretion of ADEs by increasing the JNK phosphorylation and activating the JNK signal pathway. Aβ in the brain may impair exosome-mediated Aβ clearance through this mechanism (Abdullah et al., 2016). Moreover, αB-crystallin, a small heat shock protein that is abundant in astrocytes (Hong et al., 2017), has been shown to mediate exosomal secretion in cultured human retinal pigment epithelial cells (Gangalum et al., 2016). Mutant huntingtin (mHtt) has been found to inhibit the transcription of αB-crystallin and further reduce the secretion of ADEs, which potentially play a neuronal protective role in the brain of Huntington’s disease (Hong et al., 2017).

Oligodendrocytes are responsible for myelin formation, maintenance, and nutritional support for axons (Reiter and Bongarzone, 2020). Oligodendrocytes have been demonstrated to secrete high levels of exosome-like vesicles (Krämer-Albers et al., 2007; Frühbeis et al., 2013). Proteomic analysis showed that besides the typical exosomal markers such as Alix and Tsg101, oligodendrocyte-derived exosomes (ODEs) also contained some specific components of myelin, including myelin proteolipid protein (PLP), 2′3′-cyclic-nucleotide-phosphodiesterase (CNP), myelin basic protein (MBP), myelin oligodendrocyte glycoprotein (MOG), and typical myelin lipids (Krämer-Albers et al., 2007). Under electron microscopy, PLP-positive MVBs were observed present in the cytoplasm of oligodendrocytes near axons and could fuse with the PM and release exosomes to the extracellular matrix around axons, which may participate in the oligodendrocyte-axon interaction (Frühbeis et al., 2013).

The release of ODEs is closely relevant to the activity of neurons (Table 2). The glutamate released by neuron depolarization can mediate calcium influx by activating ionotropic glutamate receptors including N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on the surface of oligodendrocytes, which leads to increased intracellular calcium levels and triggers exosome secretion (Frühbeis et al., 2013). In addition, the Rab family guanosine triphosphatases (GTPases) serve as master regulators of membrane transport. Screening of a Rab GTPase-activating protein library identified that Rab35 and its activating proteins TBC1D10A-C can modulate the release of ODEs. Rab35 was detected at the surface of oligodendrocytes and can promote the recruitment of endosomes to the PM, while reducing the activity of Rab35 would lead to intracellular accumulation of endosomes and inhibit exosomal secretion, suggesting that Rab35 potentially promote exosome secretion by facilitating the docking of endosomes with oligodendrocyte membrane (Hsu et al., 2010).

Neural stem/progenitor cells are self-renewal multipotent cells and are capable of generating neurons, astrocytes, and oligodendrocytes during the development of the CNS (McKay, 1997). In adults, there are still a small number of NSCs in the sub-ventricular zone (SVZ) and the subgranular zone of the hippocampal dentate gyrus (Ma et al., 2009). As a special type of stem cell that only exist in the CNS, NSCs can differentiate into three types of neural cells to maintain and repair damaged brain tissue, thus playing an important role in the treatment of nervous system diseases (Vogel et al., 2018). NSCs can also secrete exosomes, which carry exosomal markers such as PDCD6IP (Alix), TSG101, CD63, and CD9 (Vogel et al., 2018). Through the next-generation sequencing technology, Stevanato et al. (2016) identified a set of specific miRNAs in NSC-derived exosomes. Among these, miR-1246, miR-4488, miR-4508, miR-4492, and miR-4516 were highly enriched exosomal miRNAs and further participated in the gene regulation of target cells (Stevanato et al., 2016). There are few studies on the stimulus and downstream pathways that induce exosomal secretion from NSCs. Cossetti et al. (2014) have shown that the stimulation of pro- and anti-inflammatory cytokines can alter the expression of proteins and RNAs enclosed in NSC-derived exosomes. It was observed that inflammatory cytokine IFN-γ could regulate the exosomal function by altering their miRNA composition, but not affect the secretion of NSC-derived exosomes (Zhang et al., 2020). NSC-derived exosomes have similar active components and functional characteristics to NSCs and have lower immunogenicity and almost zero possibility of malignant transformation (Vogel et al., 2018). Therefore, NSC-derived exosomes may serve as a safer and more effective therapeutic strategy for nervous system diseases.

Increasing evidence indicates that the transmission of exosomes between neurons may be implicated in the modulation of synaptic plasticity. For example, it has been observed that two miRNAs (Let7c and miR21) are present in the exosomes of the supernatant of cultured primary embryonic cortical neurons. Let7c and miR21 may be internalized to the target neurons through exosomes and interact with Toll-like receptor 7 (TLR7), restricting dendritic growth during development in a cell-autonomous manner (Liu et al., 2015). Moreover, the transmembrane protein proline-rich 7 (PRR7) is supposed to be the synapse-removal factor. PRR7 is secreted by neurons in an activity-dependent manner via exosomes. Through the fusion of exosomes and neuronal PM, exosomal PRR7 is taken up by neurons, and subsequently inhibits Wnt signaling, activates GSK3β signaling, and induces specific removal of excitatory synapses to maintain the synapse numbers under normal conditions (Lee et al., 2018). Furthermore, postsynaptic retrograde signaling is crucial to the regulation of synaptic growth and plasticity. It was found that the presynaptic membrane can transfer Synaptotagmin 4 (Syt4) to the postsynaptic cell via NDEs, thereby activating postsynaptic retrograde signaling and mediating increased neurotransmitter release and activity-dependent presynaptic growth (Korkut et al., 2013). These data demonstrate the significance of ADEs in signal transduction between neurons.

As the innate immune cells of the CNS, the communication between microglia and astrocyte is crucial for neuroinflammation modulation. Astrocyte-derived ATP can promote the EVs including exosomes and MVs shedding from microglia (Bianco et al., 2005; Takenouchi et al., 2015). Released MDEs and MVs can increase the expression of cytokines such as IL-1, IL-6, TNF-α in astrocytes, and promote astroglial activation (Drago et al., 2017). Activated astrocytes are extensively involved in the regulation of CNS inflammatory response (Colombo and Farina, 2016). However, how these MDEs and MVs regulate the function of astrocytes remains unclear. Astrocytes can also modulate the microglia-mediated inflammatory response through exosomes. After systemic immune activation, the expression of histocompatibility complex class I (MHC I) in astrocytes of mice medial prefrontal cortex increased. MHC I and its encoding gene H-2D may be transported to microglia cells and neurons through ADEs to activate microglia and reduce the density of the dendritic spine, thereby mediating nerve injury in mice. And these impairments were significantly alleviated with the treatment of an exosome synthesis inhibitor GW4869 (Sobue et al., 2018). Moreover, a recent study revealed that ADEs enabled to alleviate microglia-mediated neuroinflammation by promoting the transformation of microglia to the M2 phenotype. This process depended on the suppression of the NF-κB signaling pathway in microglia by exosomal miR-873a-5p (Long et al., 2020). Besides, the EVs secreted by astrocytes can also modulate the phagocytic activity of microglia. Long intergenic noncoding RNA (lincRNA)-Cox2 plays a critical role in the modulation of immune-related genes in microglia. EVs produced by morphine-stimulated astrocytes were found to be taken up by microglia and activate the TLR7-nuclear factor κB (NF-κB)-lincRNA-Cox2 signaling pathway, thus impairing microglial phagocytosis, which will affect the ability of microglia to phagocytize and remove pathogens or cell debris (Hu et al., 2018).

Fitzner et al. (2011) discovered that ODEs could be selectively internalized by microglia through macropinocytosis without causing the immune activation of microglia, suggesting that microglia may mediate the clearance of oligodendrocyte membrane in an immunologically “silent” manner through the uptake of ODEs. Microglia are involved in myelin damage and regeneration by regulating the function of oligodendrocyte precursor cells (OPCs) (Traiffort et al., 2020). Lombardi et al. (2019) have shown the function of microglia EVs in this process. Under different stimulations, activated microglia can secrete EVs with pro-inflammatory or pro-regenerative activities, which respectively inhibit or promote the differentiation of OPCs to myelin-forming cells (oligodendrocytes) (Lombardi et al., 2019). Furthermore, when cultured OPCs are exposed to inflammatory EVs released by microglia, maturation of OPCs is hindered only when astrocytes are present, suggesting that microglial EVs may suppress the remyelination by converting astrocytes into myelin sheath-injured cells. Additionally, although the role of EVs in regulating OPCs maturation is unclear, this study indicates that sphingosine 1 phosphate on the surface of EVs can stimulate OPCs migration. Therefore, the lipids of EVs may serve as a critical part in the modulation of remyelination (Lombardi et al., 2019).

Astrocytes are essential for oligodendrocyte survival and axonal myelin formation of axons and are also a source of nutritional support for maintaining myelin integrity (Nutma et al., 2020). Nevertheless, to our knowledge, few reports reveal the role of exosomes in astrocyte-to-oligodendrocyte communication. Astrocytes can also secrete a set of cytokines to promote the survival, proliferation, and maturation of OPCs (Nutma et al., 2020). A recent study found that EVs secreted by young astrocytes (cultured in vitro ≤4 weeks) could promote the differentiation of OPCs into oligodendrocytes, while this effect was reduced with EVs from old astrocytes (cultured in vitro ≥16 weeks). Further proteomic analysis showed that the proteins of EVs released by astrocytes also changed with the increase of culture time. This study supports the idea that astrocytes can provide support for oligodendrocyte differentiation through exosomes, and aging may reduce their ability to provide homeostasis support (Willis et al., 2020).

Oligodendrocytes can negatively regulate their differentiation and myelin formation in an autocrine manner. Exosomes isolated from rat primary oligodendrocytes were found to influence actomyosin contractility through activating the Rho-ROCK (Rho-associated kinase)-myosin signaling axis on the recipient oligodendrocytes, thus avoiding cell surface expansion and inhibiting the oligodendroglial differentiation and myelin formation. Interestingly, this study also showed that the secretion of self-inhibiting exosomes was significantly down-regulated by incubating oligodendrocytes with conditioned neuronal medium, indicating that neurons possibly participate in the regulation of oligodendrocyte differentiation and myelin formation by controlling the release of ODEs, which may contribute to the maintenance of brain homeostasis (Bakhti et al., 2011).

Neural stem/progenitor cells are involved in regulating the physiological activities of recipient cells through exosomes. EVs from both mouse and human NSCs contain asparaginase-like protein 1 (Asrgl1), which has L-asparaginase catalytic activity and can increase the level of aspartate in the extracellular microenvironment, thereby regulating the energy metabolism of surrounding cells (Iraci et al., 2017). Proinflammatory cytokine (Th1-like cells) induced NSCs can transfer IFN-γ to the target cells via exosomes, activating the STAT1 signaling pathway and mediating the activation of pro-inflammatory signaling in target cells (Cossetti et al., 2014). Stevanato et al. (2016) found that the exosomes secreted by human NSCs were significantly enriched in miR-1246, which has been shown to play an important role in regulating cell growth and apoptosis.

A few studies have preliminarily revealed the role of exosomes in the communication between NSCs and other neural cells. It was demonstrated that mild hypoxia can induce NSCs to secrete exosomes containing miR-210, which can be taken up by recipient NSCs and increase cell viability, whereas severe hypoxia or serious doses of miR-210 may reduce the viability of NSCs (Zhao et al., 2020). Another study has shown that ADEs can also promote the survival and proliferation of NSCs (Zhou et al., 2020). Moreover, NSC-derived exosomes play an important role in neurogenesis by mediating the differentiation of NSCs. A recent study showed that exosomes secreted by NSCs can promote the differentiation of recipient NSCs and subsequent maturation of neurons and glial cells through the miR-9-Hes1 pathway (Yuan et al., 2021). Stronati et al. (2019) also found that exosomes produced by differentiated NSCs may promote the differentiation of NSCs in a dose-dependent manner. MiR-21a was observed to be highly enriched in NSC-derived exosomes and could promote the differentiation of NSCs into neurons (Ma et al., 2019). Furthermore, several studies have highlighted the role of NSC-derived exosomes in regulating the physiological activity of neurons and microglia. NSC-derived exosomes have the effect of promoting nerve regeneration. An in vitro study has shown that exosomes released from NSCs can promote neuronal axonal elongation and neural network reconstruction under both normal and hypoxia-reperfusion injury conditions (Liu Q. et al., 2020). Morton et al. (2018) demonstrated that exosomes released by NSCs of neonatal SVZ can be selectively ingested by microglia. These exosomes contain miR-9, let-7, miR-26, and miR-181, which can regulate the morphology of microglia and increase the secretion of cytokines, thereby mediating negative feedback and reduce the proliferation of NSCs (Morton et al., 2018). Collectively, these results implied that NSC-derived exosomes have a potential role in neurodevelopment and homeostasis maintenance of the CNS.

Alzheimer’s disease (AD) is characterized by the deposition of the toxic beta-amyloid (Aβ) protein in the extracellular matrix between neurons, and the formation of neurofibrillary tangle due to the abnormal phosphorylation of tau protein, which could further induce neuroinflammation, oxidative stress, neurodegeneration and even neuronal apoptosis (Yin et al., 2020). Many recent studies have indicated that neural cell-derived exosomes have multiple roles in the occurrence, development, and early diagnosis of AD (Table 3).

Mounting evidence suggests that exosomes are capable of mediating the propagation of toxic proteins, both Aβ and hyperphosphorylated tau (p-tau) among neural cells, and further inducing apoptosis and promote the development of AD. For example, it has been confirmed that NDEs are implicated in the transmission of Aβ oligomers (oAβ) between neurons, and blocking the formation and secretion of NDEs can suppress the spread of oAβ (Sardar Sinha et al., 2018). Trans-synaptic transmission of tau may also be mediated by NDEs, and the release of NDEs containing tau is regulated by synaptic activity (Wang et al., 2017). Moreover, tau protein and Aβ were found existing in AD transgenic mice serum exosomes which possibly released from astrocytes, suggesting that ADEs may be important to the pathological transmission of AD (Rosas-Hernandez et al., 2019). Chiarini et al. (2017) found that exposure to Aβ can significantly increase the expression of tau and p-tau in astrocytes and promoted the release of ADEs carrying tau and p-tau, thereby promoting the spread of tau protein in the brain. Furthermore, MDEs also participate in the propagation of toxic proteins. Microglia can deliver the internalized tau to neurons through exosomes, promoting the pathological spread of tau. The transmission of tau via MDEs was proved to be more efficient than the naked form in vivo, and the microglial depletion and inhibition of MDEs synthesis can significantly reduce tau proliferation both in vivo and in vitro (Asai et al., 2015). Taken together, the described researches document that exosomes may be a key mediator in the propagation of toxic proteins and conduce to the neuronal damage in AD.

Neuroinflammation is viewed as a key mechanism of AD, and recent studies highlighted the role of MDEs and ADEs in AD-related neuroinflammation. Glutaminase C (GAC) was found to be abnormally elevated in the brain tissues of early AD mice. With the stimulation of pro-inflammatory factors (LPS, IL-4, and IL-10), GAC can be specifically up-regulated in microglia, thus inducing cell activation, exosome secretion, as well as the content modification of exosomes. The released MDEs contain up-regulated pro-inflammatory miRNAs and down-regulated anti-inflammatory miRNAs, thereby facilitating the transmission of neuroinflammation in AD (Gao et al., 2019). A1-type astrocytes that are up-regulated in AD patients have been reported to secrete exosomes with high levels of complement proteins (CPs), which may contribute to neuronal injury in the late inflammatory phase of AD (Goetzl et al., 2018b).

Besides the propagation of toxic proteins and neuroinflammation, neural cell-derived exosomes can also contribute to the neurodegeneration of AD through other pathways. It has been demonstrated that amyloid peptide-treated astrocytes can release exosomes containing proapoptotic proteins. These ADEs accumulated in the medium and bound to other astrocytes after reaching the proapoptotic concentration, thus promoting apoptosis by activating caspase 3. This study revealed a new mechanism of exosome-mediated astrocytes apoptosis under pathological conditions (Wang et al., 2012).

However, some other evidence indicated that the release of NDEs can also serve as a neuroprotective role to maintain neuronal homeostasis by promoting the clearance of toxic proteins. It was discovered that NDEs could promote the conformational transition of Aβ in the extracellular matrix and form non-toxic amyloid fibrils on the exosomal surface. Following the internalization of NDEs, the Aβ amyloid fibrils on the exosomal surface were internalized by microglia and further transported to lysosomes for degradation. Sphingomyelin synthase 2 (SMS2) has been shown to up-regulate the secretion of NDEs during this process (Yuyama et al., 2012). Moreover, with abundant glycosphingolipids, NDEs have a good ability to capture Aβ. The infusion of NDEs could reduce the deposition of Aβ in the brains of amyloid precursor protein (APP) transgenic mice, suggesting the crucial role of NDEs in Aβ clearance (Yuyama et al., 2015). Additionally, AD is believed to be closely related to the dysfunction of neuronal endolysosome mediated by phosphatidylinositol-3-phosphate (PI3P) deficiency. Neurons with endolysosomal dysfunction can compensatively secrete exosomes containing APP C-terminal fragments (APP-CTFs), thereby mediating the clearance of toxic proteins to maintain neuronal homeostasis (Miranda et al., 2018).

Multiple studies have revealed that neural-derived exosomes detected from peripheral body fluids carry specific proteins and genetic cargos, enabling to realize the early identification of AD at the preclinical stage. Regarding the exosomal protein biomarker of AD, the initial studies mainly focused on Aβ and Tau. It has been demonstrated that the levels of Aβ1–42, p-Tau 181, and p-Tau 396 in blood NDEs significantly increased in AD than healthy individuals and can predict the progression of AD 10 years before the clinical stage (Fiandaca et al., 2015; Winston et al., 2016). These pathogenic proteins in NDEs have also been found to be closely associated with the severity of clinical symptoms and are useful biomarkers for monitoring AD progression (Nam et al., 2020). In particular, it was confirmed that Aβ42, T-Tau, and P-T181-Tau in NDEs had the same ability to diagnose AD as those in CSF, thus providing a more convenient and less invasive method for the definite diagnosis of AD (Jia et al., 2019). Moreover, other protein biomarkers associated with synaptic pathology, lysosomal dysfunction, and decreased glucose metabolism or insulin resistance have also been discovered in NDEs of AD patients. These protein biomarkers have been observed to have a significant difference between AD and normal exosomes, and these changes are already present at the early stages of the disease (Goetzl et al., 2018a; Soares Martins et al., 2021). In addition, the possibility of proteins contained in ADEs as biomarkers has also been investigated. Studies by Goetzl et al. showed that the ADEs of AD patients load a variety of differentially expressed proteins, including β-site APP-cleaving enzyme 1 (BACE-1), soluble amyloid precursor protein (sAPP) β and glial-derived neurotrophic factor (GDNF). However, whether they can be used as biomarkers for early recognition of AD needs further study (Goetzl et al., 2016). The levels of inflammatory cytokines and CPs in plasma ADEs of AD patients were significantly higher than those in normal controls, which may be helpful for the early detection of AD (Goetzl et al., 2018b).

Gene cargo carried by exosomes also serves as biomarkers for early recognition of AD, in which exosomal miRNAs have been extensively studied and well described in some reviews (Manna et al., 2020; Soares Martins et al., 2021). At present, most studies on exosomal miRNA biomarkers have been carried out in total exosomes without identification of their origin. It has been described that miR-212 and miR-132 are decreased in plasma NDEs from AD patients, but their potential as AD biomarkers still should be further investigated (Cha et al., 2019). Future identification of the cellular origin of these exosomes will be necessary to understand the targeting mechanisms of miRNAs and to achieve a more accurate diagnosis.

The hallmarks of Parkinson’s disease (PD) pathology are the degeneration of dopamine neurons in the substantia nigra pars compacta and the progressive accumulation of α-synuclein (α-syn) and the formation of oligomers in neurons (Marchetti et al., 2020). Neural cell-derived exosomes are considered as important mediators for the transmission of PD pathology and may be pivotal in the occurrence and progression of PD (Table 4).

It is increasing evidence that the aggregation and propagation of α-syn are at least partially dependent on exosomes. A recent study reported that although α-syn can be released directly by neurons, pathogenic forms of α-syn may be more commonly secreted through exosomes and other EVs. EVs carrying α-syn may be more prone to internalized by the recipient cells, highlighting the unique advantage of EVs in the transmission of toxic proteins (Gustafsson et al., 2018). Neurons affected by toxic proteins can transfer α-syn to healthy neurons through NDEs, thus promoting the pathological transmission of PD (Alvarez-Erviti et al., 2011). Emmanouilidou et al. (2010) found that differentiated SH-SY5Y cells are capable of secreting exosomes containing α-syn in a calcium-dependent manner, and the released exosomes were observed to induce neuronal death. The potential role of microglia in the pathologic spreading of α-syn is revealed by Xia et al. (2019) and Guo et al. (2020) in their recent work. They respectively demonstrated that exposure to plasma exosomes from PD patients or human α-synuclein preformed fibrils can reduce the clearance efficiency of microglia and induce the release of α-syn-enriched exosomes, thereby facilitating α-syn propagation and the aggregation of α-syn in the target neurons (Xia et al., 2019; Guo et al., 2020).

Exosomes in the CNS also participated in microglial activation and neuroinflammatory, which aggravate the degeneration of dopamine neurons. It has been found that α-syn can stimulate microglia to produce inflammatory exosomes rich in MHC II and TNF-α, thereby inducing neuroinflammatory response and apoptosis of rat cortical neurons (Chang et al., 2013). Intriguingly, aging-related microglial dysfunction seems to be closely related to the pathogenesis of PD. Bliederhaeuser et al. (2016) found that compared with young mice, microglia in adult mice showed deficits in exosome-associated α-syn uptake and phagocytosis, which facilitates the diffusion of toxic proteins between neurons and exacerbates degenerative changes. Additionally, aging microglia can also release more exosomes containing TNF-α than young microglia, further damaging neurons by aggravating the inflammatory response. This observation illustrated that age is an important factor affecting the spreading of α-syn and neuroinflammation via exosomes (Bliederhaeuser et al., 2016).

The effect of exosomes on PD pathology may be mediated by specific miRNAs. Manganese exposure is thought to promote the progression of PD (Kwakye et al., 2015). Notably, it was found that MN9D cells (a dopaminergic cell model of PD) exposed to manganese upregulated the Rab27a, which was been demonstrate to promote the exosomal release. The released NDEs are rich in miRNAs associated with protein aggregation, autophagy, inflammation, and hypoxia, and may facilitate the progression of PD (Harischandra et al., 2018).

Exosomes from neural cells are not always harmful, and they can also show neuroprotective effects in AD under the influence of the extracellular environment and physiological state of cells. As an example, 1-methyl-4-phenylpyridinium (MPP⁺), a neurotoxin that could induce cell death, is widely used in models of PD both in vivo and in vitro. Exosomes released from normal astrocytes were found to significantly reduce MPP⁺-induced apoptosis of hippocampal neurons. This may be because miR-200A-3p in the ADEs can down-regulate the mitogen-activated protein kinase kinase 4 (MKK4) expression in MPP⁺-induced neurons and further inhibit the activation of c-jun n-terminal kinase (JNK), which is essential in the apoptotic signaling pathway (Shakespear et al., 2020).

In the last few years, mounting evidence points to a role for exosomes as biomarkers to assist in the clinical diagnosis and prognosis of PD. Multiple studies have examined the potential of exosome-carried α-syn and other proteins as biomarkers in different biological fluids of PD (Yu et al., 2020). Indeed, these exosomal protein biomarkers present in peripheral body fluids may be contaminated as they can also be secreted by peripheral tissues. Therefore, it is indispensable to identify the source of exosomes. Although some studies have analyzed the level of neuron-derived exosomal α-syn in the peripheral blood of PD patients, the results were inconsistent. Two studies on PD individuals’ plasma NDEs showed that the α-syn from NDEs was significantly higher than that of normal subjects, and the levels of α-syn increased with the progression of PD (Zhao et al., 2018; Niu et al., 2020). Jiang et al. also found similar results in serum NDEs (Jiang et al., 2020). In the study of Si et al. (2019), levels of neuron-derived exosomal α-syn in serum of PD patients were observed to be lower than those in controls. The lack of standardized protocol for identifying NDEs may be responsible for this difference. Studies of exosomal miRNAs from PD patients’ serum or CSF have demonstrated that the enrichment or deficiency of specific miRNAs may contribute to the diagnosis of PD (Gui et al., 2015; Cao et al., 2017; Wang and Zhang, 2020). However, it is challenging to identify the origin of these exosomes carrying functional miRNAs. Considering the prominent role of exosomes secreted by neural cells in PD pathology, it is to distinguish the neural origin of these exosomes in future biomarker studies.

Regarding amyotrophic lateral sclerosis (ALS), mutations of copper-zinc superoxide dismutase (SOD1) are one of the important causes of inherited ALS. Exosomes from neural cells seem to serve as an important part of the pathological development of ALS. Basso et al. (2013) found that exosomes secreted by mutant copper-zinc superoxide dismutase (SOD1) overexpressed astrocytes were shown to transport mutant SOD1 to cultured spinal neurons and selectively induce the injury of motor neurons. Neuroinflammation is one of the main pathogenic mechanisms of ALS, it is described that IL-6 in ADEs is up-regulated in sporadic ALS patients, and the disease progression rate of ALS is positively correlated with the level of IL-6 in ADEs. It is speculated that ADEs aggravate the ALS process by inducing inflammatory responses (Chen et al., 2019). Moreover, in a study by Pinto et al. (2017), exosomes secreted by NSC-34 motor neuron-like cells transfected with mutant SOD1 (G93A) were found containing miR-124. The miR-124-enriched NDEs were preferentially internalized by microglia, modulating the cell phenotype and triggering the microglial dysfunction by inducing the loss of phagocytic ability and the production of multiple inflammatory cytokines and MHC-II (Pinto et al., 2017). The damaged microglia have a lower ability to eliminate harmful substances and maintain neural homeostasis, as well as mediated the enhancement of neuroinflammation, thus further promoting the degeneration of motor neurons (Pinto et al., 2017).

Several researches point to a role for exosome-carried proteins as biomarkers of ALS (Chen P.C. et al., 2020; Hayashi et al., 2020). Similarly, studies of these exosomal biomarkers are based on total exosomes in biological fluids, few studies have focused on protein biomarkers from neuronal and glial cell-derived exosomes. Several studies have analyzed miRNAs carried by NDEs aiming to screen for potential biomarkers. Yelick et al. (2020) found that the elevation of exosome containing miR-124-3p from spinal neurons of patients is positively correlated with the severity of ALS, highlighting the potential role of exosome miR-124-3p as an indicator of ALS disease stage. Plasma NDEs-derived miRNAs were also identified by Banack et al. (2020) in the ALS group. In contrast to the controls, eight miRNA sequences were observed to be differentially expressed in NDEs of ALS, which may contribute to the early diagnosis of ALS (Banack et al., 2020).

Collectively, these studies demonstrated that exosomes from neurons, microglia, and astrocyte are involved in the occurrence and development of neurodegenerative diseases and this role is twofold, involving both nerve damage and neuroprotection. In the future, how to distinguish the beneficial or harmful exosomes under different physiological and pathological conditions is crucial for the development of new exosome therapies for neurodegenerative diseases. Oligodendrocytes are responsible for CNS myelination and provide nutritional support to axons and neurons. Dysfunction of oligodendrocytes and myelin may increase the vulnerability of neuronal degeneration (Ettle et al., 2016). However, the role of ODEs in neurodegenerative diseases still needs further investigation. Exosomes are also an important tool for the early non-invasive diagnosis of neurodegenerative diseases. However, most current studies are based on total exosomes in peripheral body fluids, and the role of neural cell-derived exosomes has not been fully explored. Although some studies have reported the exosomal origin of biomarkers, the current markers used to identify neural cell-derived exosomes seem to be unspecific. For instance, L1CAM, which is widely used in the identification of NDEs, may also be expressed by the kidney and adipose tissue. Further research on accurate identification of specific cell-derived exosomes is conducive to accurate diagnosis and prognosis prediction of neurodegenerative diseases.