Abstract
Brain derived exosomes (BDEs) are extracellular nanovesicles that are collectively released by all cell lineages of the central nervous system and contain cargo from their original cells. They are emerging as key mediators of communication and waste management among neurons, glial cells and connective tissue during both physiological and pathological conditions in the brain. We review the rapidly growing frontier of BDEs biology in recent years including the involvement of exosomes in neuronal development, maintenance and communication through their multiple signaling functions. Particularly, we highlight the important role of exosomes in Alzheimer’s disease (AD), both as a pathogenic agent and as a disease biomarker. Our understanding of such unique nanovesicles may offer not only answers about the (patho) physiological course in AD and associated neurodegenerative diseases but also ideal methods to develop these vesicles as vehicles for drug delivery or as tools to monitor brain diseases in a non-invasive manner because crossing the blood brain barrier is an inherent capability of exosomes. BDEs have potential as biomarkers and as therapeutic tools for AD and related brain disorders in the near future.
Introduction
Exosomes are small extracellular nano-sized vesicles between 30 and 150 nm in diameter () that were first described in the 1980s (Johnstone et al., 1987). They consist of one type of EVs and are categorized on the basis of their biogenesis pathways (Pan and Johnstone, 1983). Since then, exosomes have been isolated from nearly all mammalian cell types, including cells in the CNS such as neurons, astrocytes, oligodendrocytes, microglia, and Schwann cells, as well as endothelial cells (; Sharma et al., 2013). Exosomes released from the nervous system are collectively called BDEs.
To form MVBs with ILVs, early endosomes undergo inward budding (). Then, ILVs are released into the extracellular environment as exosomes via fusion of MVBs with the plasma membrane (Heijnen et al., 1999; Mulcahy et al., 2014). Alternatively, exosomal formation can be regulated by sphingolipids, ceramides and tetraspanins (Trajkovic et al., 2008; Stuffers et al., 2009; van Niel et al., 2011). Otherwise, MVBs can fuse with the lysosomal membrane, resulting in degradation of ILVs and recycling of their content (Stuffers et al., 2009; Klumperman and Raposo, 2014). As a result of their origin and multifarious molecular cargo, including but not limited to gDNA, mRNA, other non-coding RNAs, lipids and proteins (Yokoi et al., 2019), the molecular species and relative amounts in exosomes are highly heterogeneous and complex in composition. Exosomes can randomly or selectively exhibit great variety depending on their membranes, cytosolic proteins, and nucleic acids compared with the cells that release them. Based on the proteomic and other comprehensive analyses, the heterogeneity of exosomes is conceptualized on the basis of their size, content (cargo), functional impact on recipient cells and cell of origin (source) (Kalluri and LeBleu, 2020). It is becoming increasingly clear that exosomes have specialized functions and play a key role in coagulation, intercellular signaling and waste management (van der Pol et al., 2012; Figure 1).
FIGURE 1
During the formation process, exosomes are comprised of enriched endosome-associated components such as flotillins and annexins (van Niel et al., 2006), ALIX, the ESCRT component, tumor susceptibility gene 101 protein (TSG101) (Lotvall et al., 2014) and lipid rafts, including cholesterol, sphingomyelin and ceramide (
Once the exosomes are secreted, they can be internalized from the extracellular space by recipient cells through several mechanisms including phagocytosis, micropinocytosis, endocytosis, and plasma membrane fusion (McKelvey et al., 2015; Figure 1). On the other hand, exosomes carrying multiple cargo with valuable biological information can also be released into most bodily fluids such as plasma, cerebrospinal fluid, urine, saliva, amniotic fluid, colostrum, breast milk, synovial fluid, semen and pleural ascites, not only in normal tissues (
Given the characteristic described above, exosomes and their constituents represent a novel class of therapeutic targets and such features also give them advantages as biomarkers to distinguish healthy and disease states (
Physiological Roles of BDEs in the CNS
Exosomes exert their effects on essential biological processes throughout the body including the CNS by different mechanisms. These mechanisms include cell surface receptors activation through direct binding to lipid ligands and proteins, exosomal membrane fusion contents with the recipient cell plasma membrane and effectors delivery. Some of these effectors are oncogenes, transcription factors, small and large non-coding regulatory RNAs (such as miRNAs) and mRNAs, as well as infectious particles such as amyloid-β (Aβ)−derived (
FIGURE 2

Intercellular communication of BDEs in the CNS in (patho) physiological course in AD. Exosomes secreted from oligodendrocytes, microglia, astrocytes and mesenchymal stem cells includes host cell derived cytosolic proteins, cytokines and transcription factors, Aβ scavenger enzymes, Aβ (Saeedi et al., 2019; You and Ikezu, 2019) and tau protein (
Particularly, in the CNS, exosomes can be released from virtually all cell types, including various types of neurons, astrocytes, oligodendrocytes, microglia, and Schwann cells, as well as endothelial cells (
TABLE 1
| Condition | Function | References | |
| Physiological roles | Neuron−glia communication | Neuron and microglia | |
| Neuron and astrocytes | Men et al., 2019; Pascua-Maestro et al., 2019 | ||
| Neuron and oligodendrocytes | Kramer-Albers et al., 2007 | ||
| Pathological roles in AD | Synaptic plasticity and neurotransmission | ||
| Enhancement of neuron protection | |||
| Improvement neuronal development | |||
| Carry the two hallmarks of AD brains, Aβ, and hyperphosphorylated tau | Sardar et al., 2018 | ||
| The spread of oligomers and neurotoxicity | Hamlett et al., 2018 | ||
| Carry the synaptic proteins | |||
| Carry the ceramide and sphingosine-1-phosphate (S1P) | Yuyama et al., 2012; | ||
| Containing tyrosine phosphorylated insulin receptor substrate 1 (IRS1) | Kapogiannis et al., 2015 | ||
Exosome is a double-edged sword in nervous system.
Neuron-Glia Communication via Exosome Secretion
Neurons and glial cells (a class of cells that mainly includes microglia, astrocytes, and oligodendrocytes) orchestrate CNS homeostasis via numerous mechanisms of intercellular communication. Exosomes might regulate the physiological condition of the recipient cells and interactions between various neural cells. For example, upon activation of glutamatergic synapses, cortical neuron-derived exosomes are selectively delivered to neurons but not glial cells (Koniusz et al., 2016). Recent studies using CD63-GFP positive intraluminal vesicles as exosomal reporter in mice have demonstrated that exosomes participate in mediating neuron to astroglia communication in the CNS (Men et al., 2019). Furthermore, miRNAs (especially miR-124a) in exosomes isolated from neuron-conditioned medium possess excitatory amino acid transporter 2, a necessary mediator of glutamate uptake via the internalization of exosomes into astrocytes (Morel et al., 2013). However, exosomes also participate in reciprocal oligodendrocyte-neuron communication and transfer cargo from oligodendrocytes to neurons (
Synaptic Plasticity and Neurotransmission by the Release of Exosomes
Maintenance and improvement of synaptic connectivity in the adult brain are crucial for cognitive function. Neural synaptic plasticity is mediated not only by neuron-specific progression but also by glial cells, such as astrocytes and microglia (Morris et al., 2013). Under resting conditions, synaptic vesicles are reposited in the cytoplasm of the nerve terminal. Many synaptic vesicles stick on some specialized sites at the presynaptic plasma membrane named active zones. During incoming action potentials, exocytosis of synaptic vesicles confirm how much transmitter is released from nerve terminals (Jahn and Fasshauer, 2012). In addition to typical synaptic neurotransmission, signal transduction of neurons via the secretion of exosomes can induce a range of neurobiological functions including synaptic plasticity (
Enhancement of Neuron Protection and Neuronal Development Through Exosomes
Although the understanding of astrocyte-neuron communication via exosomes remains unclear, evidence supports that it does occur, and this communication manner is required for neuronal cell survival (Figure 2; Pascua-Maestro et al., 2019; Luarte et al., 2020). Neuroprotective signaling is essential for neuronal growth and survival. PrP is a physiologically important receptor protein that protects against oxidative stress in the CNS. Protection of neurons through astrocyte-derived exosomes is dependent on astrocyte-derived exosomal PrP transport into neurons (
Pathological Roles of BDEs in AD
Increased secretion of exosomes is generally thought to occur in response to stress or pathological conditions (Urbanelli et al., 2013; Saxton and Sabatini, 2017;
Exosomes Containing Aβ in AD
Amyloid β-protein is a C-terminal cleavage product of the transmembrane APP produced by β- and γ-secretase (
Although most AD cases are sporadic, there is a minority patients from mutations in the genes encoding APP or during sequential cleavages by β- and γ-secretase enzyme activities (
In vivo, rodent exosomes can contain Aβ, BACE1, and presenilin 1 and 2 (Sharples et al., 2008). Exosomes isolated from bodily fluids of AD mouse model, including blood, CSF and urine, display indicative increase in the C-terminal fragments of APP (Laulagnier et al., 2018; Miranda et al., 2018). Exosomes isolated from bodily fluids of AD patients exhibit a remarkable increase soluble Aβ1–42 in
The above data suggested the hypothesis that exosomes could seed Aβ aggregation (
Exosomes Containing Hyperphosphorylated Tau in AD
The gradual deposition of hyperphosphorylated tau protein within specific neurons is pivotal to the tauopathy of AD (Saman et al., 2014; Takeda, 2019). Under normal physiological conditions, incorporation of neuronal microtubule-associated protein tau for microtubule elongation is a crucial event of neuronal synapse formation and synaptic plasticity. Additionally, intracellular tau also participates in neurite outgrowth, axonal transport, chromosome stability, regulation the cellular transcriptome and the structural architecture of heterochromatin (for more details see the review by Sotiropoulos et al., 2017). Extracellular tau is also secreted into brain interstitial fluid (Yamada et al., 2011) and may contribute to some characteristics of sleep (Lucey et al., 2019). The above functions rely on the site-specific phosphorylation of tau (Kapitein and Hoogenraad, 2015) in normal condition. However, hyperphosphorylation and aggregation of the microtubule-associated tau protein into intracellular neurofibrillary tangles is one of the classical pathological hallmarks of advanced-stage AD (Johnson and Stoothoff, 2004; Martin et al., 2011).
During the progressive accumulation of neurofibrillary tangles, tau becomes hyperphosphorylated in neurons. Meanwhile, the cellular clearance machinery takes up tau for degradation and packaging in exosomes (Saman et al., 2012;
Exosomes Containing Synaptic Proteins in AD
One consequence of AD is neuron loss and dysfunction. The levels of synaptic proteins, including synaptophysin, synaptotagmins, synaptobrevin, synaptopodin, Rab3A, GAP 43, and neurogranin, were decreased in the BDE cargo from the plasma of AD patients (
Ceramide and Sphingosine-1-Phosphate (S1P) in Exosomes in AD
Activated sphingolipids are signaling molecules that serve as intracellular second messengers and include ceramide, sphingosine, and their derivatives, 1-phosphates (C1P and S1P, respectively) (
Exosomes Containing Other AD-Associated Proteins
Dysregulation of insulin by the CNS and peripheral hyperinsulinemia have been reported as other events highly associated with AD (
Moreover, exosomes have the ability to spread toxic proteins through PrP activity (Hartmann et al., 2017). PrP is a cell surface-anchored protein that is highly related to AD pathology (Kellett and Hooper, 2009). Its pathological, misfolded form is associated with spongiform encephalopathy (Prusiner, 1982; Song et al., 2013). Studies in animal models of AD have demonstrated that the PrP receptor is essential for the cognitive impairment linked to Aβ (
Exosomal MiRNA as a Diagnostic Biomarker for AD
Generally, the biomarkers used for AD diagnosis include the expression of Aβ and pTau (
MiRNAs are a family of 18–22 nt single-stranded RNAs that post-translationally communicate with and regulate the expression of mature mRNAs. Single upregulated miRNAs can target various mRNAs to decrease their expression and multiple miRNAs can target a single mRNA (Sethi and Lukiw, 2009; Sarkar et al., 2016). Studies have demonstrated that mRNA and miRNA species are present in exosomes. It is possible that some mRNA sequences are definitely targeted for secretion by these vesicles (Valadi et al., 2007; Van Giau and An, 2016). Exosomal miRNAs play essential roles in intercellular communication between cell membranes in the CNS and in disease progression. Exosomal miRNAs are also ideal targets for use as potential biomarkers in clinical diagnostics or therapies as they can be analyzed through neuronal exosomes in the patient’s body fluids (A). Indeed, some research has illustrated that proteins and miRNAs can be transferred from glia to axons (Skog et al., 2008). It is assumed that miRNA signaling can impact neurodegenerative diseases via the dysregulation of tau, leading to neurotoxicity. One study convincingly demonstrated that, in brain tissues obtained at autopsy from AD patients and from those with severe primary age-related tau pathology, the level of the highly conserved miRNA-219 was decreased in the brain (Santa-Maria et al., 2015). Several reports have illustrated that high expression of tissue-specific miRNAs in the brain, such as miR-9, miR-29a/b, miR-107, miR-124, miR-128, miR-134, and miR-137, may result in defective neuronal development (Sempere et al., 2004; Kawase Koga et al., 2009; Huang et al., 2010). In addition, other miRNAs are also abnormal in brain tissues during neurodegenerative processes. These specific miRNAs, including miR-132 and miR-212, are among the most robustly declining miRNAs in neurodegenerative diseases, including AD (
Remarkably, miRNAs that were found to be greatly expressed in the brain were also detected in human body fluids such as the plasma, urine, and CSF. The levels of brain-enriched miRNAs including miR-9, miR-29a, miR-29b, and miR-137 have been found to be significantly decreased in plasma samples collected from AD patients (
Development of a Brain-Derived Exosomal Biomarker for AD
Currently, a mixed population of exosomes from various types of cells can be separated from biological fluids by multiple techniques such as classically differential ultracentrifugation, immunomagnetic beads and size exclusion chromatography (Li et al., 2017; Yu et al., 2018). Moreover, exosomes have a lipid bilayer to protect their cargo, which is used downstream, from RNAse treatment to be confirmed whether the miRNAs/mRNA analyzed are inside the exosomes or not (
Enrichment of a specific neuron-derived population of exosomes permits monitoring of target cells of interest (Table 2). Collectively, although exosome transfer of Aβ seems to mainly occur in AD and can be exploited as a helpful biomarker of the disease course, development of additional exosome biomarkers could contribute to a more accurate diagnosis of AD and discovering further close connections between the marker and mechanisms of the early stage of AD as well as other neurodegenerative diseases.
TABLE 2
| Exosomes isolated from different cell types | Biomarker | Function |
| Cortical neurons-derived exosomes; immature and mature hippocampal neurons exosomes | The GluR2/3 subunits of glutamate receptors | Neuronal markers and play key roles in virtually all excitatory neurotransmission in the brain |
| L1 cell adhesion molecule (L1CAM) | Neuronal markers, cell adhesion molecule with an important role in the development of the nervous system Lachenal et al., 2011. | |
| Microglia-derived exosomes | Ionized calcium binding adaptor molecule 1 (Iba1) | A microglia/macrophage-specific calcium-binding protein with actin-bundling activity that participates in membrane ruffling and phagocytosis in activated microglia Raffo-Romero et al., 2018. |
| Astrocytic-derived exosomes | Glutamine aspartate transporter (GLAST) | Selective markers of astrocytic plasma membranes Raffo-Romero et al., 2018. |
| Glial fibrillary acidic protein (GFAP) | A specific marker for astrocytes; the astrocytic cytoskeleton | |
| Glutamine synthetase (GS) | Astrocyte marker, it catalyzes the production of glutamine and 4-aminobutanoate | |
| Oligodendrocytes-derived exosomes | Myelin proteolipid protein (PLP) | Oligodendrocytes marker, it is the major myelin protein from the central nervous system. It plays an important role in the formation or maintenance of the multilamellar structure of myelin Kramer-Albers et al., 2007. |
| 2′, 3′-cyclic nucleotide 3′-phosphodiesterase (CNP) | Oligodendrocytes marker, it belongs to the cyclic nucleotide phosphodiesterase family Kramer-Albers et al., 2007. |
The biomarker of different neural derived exosomes (NDEs).
Exosomes as Novel AD Therapeutics
The BBB is a continuous endothelial membrane within brain microvessels and is sheathed by mural vascular cells and perivascular astrocyte end-feet, which seal the cell−to−cell contacts to prevent the transmission of potentially toxic compounds between the brain and the blood (Matsumoto et al., 2017). In addition to transmembrane diffusion of small (<400 Da) lipid-soluble molecules, the BBB permits selective transport of some compounds into and out of the brain (Sanchez-Covarrubias et al., 2014).
Exosomes have an inherent ability to cross the BBB, and because their properties remain active in the brain, they are ideal drug delivery vehicles. This BBB-penetrating capacity, which was first reported by
In contrast, while exosomes may play a role in the spreading of AD, some studies have shown a positive effect of introducing non-pathogenic exosomes to change disease duration and progression (Table 3). In animal studies, this therapeutic effect was found when exosomes from young mice were observed to significantly downregulate aging-associated signaling molecules such as IGF1R and upregulate telomerase-related genes such as Men1, Mre11a, Tep1, Terf2, Tert, and Tnks in aged mice (Lee et al., 2018). Furthermore, exosomes injected into the brain of transgenic mouse models of AD can help to decrease toxic oligomers and fibrils in a microglial-dependent manner following intracerebral administration, contributing to the clearance of Aβ in vivo (Yuyama et al., 2012, 2014; Yuyama and Igarashi, 2017). Other researchers have suggested that mesenchymal stromal-derived exosomes may have a therapeutic effect in vivo on the advancement of neurovascular plasticity in other neurodegenerative diseases such as stroke (Xin et al., 2013a).
TABLE 3
| Disease | Animal model | Source | Administration | Proposed mechanism and results | References | |||
| Concentration (Total amount) | Route | Period (time) | Sampling/Sacrifice | |||||
| APPsweInd | N2a cells | 2 mg / mL, 0.25 μL/h (168 μg) | Dentate gyrus | 14 days (continuously) | 14 days after surgery | Aβ clearence. Aβ level ↓; Amyloid deposit ↓; Synaptotoxicity ↓ | Yuyama et al., 2014, 2015 | |
| APPsweInd | Plateletfreeplasma | 3 μg / 3 μL (3 μg) | Dentate gyrus | Single injection | 3 days / 20 days after injection | Co-localization (exosome and Aβ) | Zheng et al., 2017 | |
| hiPSCs Injected mice | Tau mutation hiPSCs | 0.5 μg / 2 μL (0.5 μg) | Hippocampus | Single injection | 1 m / 2 m after injection | Tau propagation. Neurodegeneration ↑ | Winston et al., 2019 | |
| 5XFAD pups | Astrocyte | NA | Brain | Single injection | 48 h after injection | Aβ plaque ↓ by exosome ↓. nSMase2 ↓; exosome ↓; Aβ plaque ↓ | ||
| Aβ-derived diffusible ligands injected mice | N2a cells human CSF | 4 μg / 5 μL 4 μg | I.C.V. | Single injection | NA | Synaptic plasticity. LTP↑, Aβ action ↓ | ||
| AD mice | hUmbilical cord MSCs | 30 μg / 0.1 mL 120 μg | I.V. | 2 month (biweekly injection) | 1 m after injection | Neuron inflammation↓. Aβ deposit ↓; activation of microglia ↓; pro-inflammatory levels ↑; anti-inflamatory cytokines ↓ | ||
| APPsweInd | (hypoxia) PC-MSCs | 150 μg / 80 μL 1200 μg | I.V. | 4 month (biweekly injection) | 5 h after injection | Neuroprotection and Immunomodulation. Plaque deposition ↓; Aβ level ↓; activation of astrocytes ↓; activation of microglia ↓; TNF-α, IL-1β↓; IL-4 ↑ | ||
| Stroke | MCAo, rat | MSCs | 3 × 106 / mL 3 × 106 | I.V. | 24 h after surgery (single injection) | 14 days after surgery | White matter repair | |
| MCAo, rat | MSCs | 100 μg / 0.5 ml 100 μg | I.V. | 24 h after surgery (single injection) | 28 days after surgery | White matter repair | Xin et al., 2013a | |
| MCAo, rat | MSCs | 100 μg/0.5 mL 100 μg | I.V. | 24 h after surgery (single injection) | 28 days after surgery | White matter repair | Xin et al., 2017 | |
| Embolism, mouse | NSC EV | NA | I.V. | 2 / 14 / 38 h after surgery (triple injection) | 96 h after surgery | Immune modulation | Webb et al., 2018b | |
| MCAo, pig | NSC EV | NA | I.V. | 2 / 14 / 24 h after surgery (triple injection) | 1 / 84 days after surgery | Reduction in edema | Webb et al., 2018a | |
| ICH, rat | MSCs | NA | I.V. | 12 h after surgery (single injection) | 2 / 7 / 28 days after injection | Immunosuppression | Otero-Ortega et al., 2018 | |
| Rat | MSCs | NA | NA | NA | NA | White matter remodeling | ||
| TBI | CCI, rat | hMSCs | 100 μg / 0.5 ml 100 μg | I.V. | 12 h after surgery (single injection) | 35 days after surgery | Angiogenesis and neurogenesis | Zhang et al., 2017 |
| ICH, rat | MSCs | 3 × 106 MSCs 3 × 106 | I.V. | 12 h after surgery (single injection) | 12 days after surgery | Angiogenesis and neurogenesis | Kim et al., 2016 | |
| TBI, swine | MSCs | 1 × 1015; 1 × 1013 | I.V. | 6 h / 1 / 5 / 9 / 13 days | 30 days after surgery | Neuroprotection | Williams et al., 2019 | |
| Fetal hypoxia | OCD singleton fetuses | MSCs | 2.0 × 107 cell; 4.0 × 107 | I.V. | 1 h / 4 days (2) | 7 days after surgery | Neuroprotection | Williams et al., 2019 |
| ICH | ICH, rat | MSCs | 100 μg protein I.V., 200 μg | I.V. | 24 h after surgery (single injection) | 28 days after surgery | Neurovascular and white matter remodeling. | Han et al., 2018 |
| ICH, rat | MSCs | 100 μg protein I.V., 100 μg | I.V. | 24 h after surgery (single injection) | 28 days after surgery | White matter repair | Otero-Ortega et al., 2018 | |
| Focal ischemia | Photo thrombosis, mouse | BM-MSCs | NA | I.V. | 24 h after surgery (single injection) | 2 h after injection | Neurogenesis | Yang et al., 2017 |
| ASD | BTBR T+tf/J mouse | hMSCs | 3.81 × 108 particles/5 μL 1.9 ×109 | I.N. | 12 days (every other day injection) | Behavior test | Social interaction | Perets et al., 2018 |
| Inflammation | C57BL/6j mice | EL-4 T cell | 2 μg / 2 μL 10 μg / 10 μL | I.N. | 10 min (every 2 min injection) | After administration | Anti-inflammation | Zhuang et al., 2011 |
| Status epileoticus | C57BL/6J mice | MSCs | ∼ 5 μg 30 μg / 150 μL | I.N. | 18 h (every 5 min) | 4 days after administration | Neurogenesis and memory dysfunction | Long et al., 2017 |
| Bacterial infection | C57BL/6J mice | BMDCs | 25 μg / 30 μL 75 μg / 90 μL | I.N. | 2 week (three doses) | 2 week after immunization | Macrophage and dendritic cell activation | |
| C57BL/6J mice | Bone marrow cells | 25 μg / 30 μL 75 μg / 90 μL | I.N. | 2 week (three doses) | 2 week after dose | Potential mechanism for antigen cross-priming | ||
Exosome administration for the treatment of AD and other neurological disorders.
APPswelnd, mice expressing the human APP bearing the Swedish and Indiana (KM670/671NL, V717F) mutations (J20); ASD, autism spectrum disorders; BM-MSCs, bone marrow mesenchymal stem cells; CCI, controlled cortical impact; CSF, cerebrospinal fluid; hiPSCs, human induced pluripotent stem cells; EV, extracellular vesicles; ICH, intracerebral hemorrhage; I.C.V., intracerebroventricularly infusion; IL, interleukin; I.N., intranasal administration; I.V., intravenous injection; LTP, Long-term potentiation; MCAo, middle cerebral artery occlusion; MSCs, mesenchymal stem cells; NA, not applicable; NSC, Neural stem cell; OCD, occluder; PC-MSCs, preconditioned mesenchymal stem cells; TBI, traumatic brain injury; TNF-α.
Additionally, directed exosomal transmission systems for precision nanomedicine have attracted extensive interest across the fields of pharmaceutical sciences, molecular cell biology and nanoengineering (Zhu et al., 2018). Exosomes are also a promising type of novel drug delivery vehicle because of their ability to cross the BBB and shield their cargo from enzymatic and chemical degradation. Recent developments regarding nanoengineering using targeted exosomes for therapeutic purposes have been conducted by researchers, for example, Xin et al. (2013b), Tran et al. (2019).
Introducing exogenous exosomes into the CNS because they can effectively cross the BBB is a potentially novel strategy for AD therapies (
Opportunities and Challenges
Quantifying changes in EV cargo would be extremely difficult because of the lack of unique region-specific markers for circulating exosomes and the inaccessibility of specific brain tissue EVs from living patients. Interesting lines of research have examined both the induction of AD using pathogenic EVs and the sequestration of toxic plaques using exogenous healthy EVs.
It difficult to distinguish among EV types simply on the basis of protein markers or size alone. To better interpret and replicate the experimental results of exosome studies, combined exosome isolation methods as well as improved techniques for accurate purification and characterization are recommended. In addition, a crowdsourcing knowledgebase currently allows researchers in the EV field to track the latest EV biology and methodology (Lee et al., 2019).
In recent years, research has been focused on BDEs to attempt to solve questions of brain-associated disorders using blood biopsies. Exosomes isolated from plasma were used to enrich BDEs (Sun et al., 2017; Saeedi et al., 2019). This study demonstrated that both the number of neural-derived exosomes as well as the expression of Aβ, neurofilament light chain, and high-mobility group box 1 potentially act as biomarkers of neuropsychological impairment in HIV (Sun et al., 2017). BDEs from plasma have also been tested in a pilot study to examine protein biomarkers for patients with major depressive disorder (Kuwano et al., 2018). Moreover, in military personal with mild traumatic brain injuries, compared with controls, the levels of tau, Aβ42, and IL-10 deposited by BDEs were elevated (Kuwano et al., 2018). Cargo proteins and miRNA from astrocytic-derived exosomes have been analyzed to obtain mechanistic insight into AD (
Conclusion
Although the domain of exosome investigation, especially BDEs, remains relatively novel, attractive evidence from other fields demonstrates that investigation of exosomes can afford insight into the disease mechanisms and processes associated with AD and treatment responses. Currently, increased research on exosomes has focused on biomarkers of the course of AD and their ability to mediate cell-to-cell communication in the nervous system. However, additional work is needed with respect to the mechanisms of bi-directional transport of cargo-carrying exosomes across the BBB, the alterations in the number or size of exosomes secreted, changes in cargo constituents, and identification of differences in specific cell types. Meantime, it is necessary to take into consideration that several preparations may contain another type of EVs given the procedure used to obtain them. Exosomes derived from cells in the CNS have tremendous biomarker potential because they may reverse physiological changes in nervous system disorders, and these changes can be tested in the periphery.
Statements
Author contributions
CQ and ZS designed the project. ZS, YX, WD, LZ, HZ, YH, PY, YQ, and WZ performed a majority of writing the manuscript. All authors reviewed and revised the manuscript.
Funding
This work was supported by grants from the National Natural Science Foundation of China (Grant No. 81901114); the Fundamental Research Funds for the Central Universities (Grant No. 3332019091); and CAMS Innovation Fund for Medical Sciences (Grant Nos. 2019-I2M-1-003 and 2016-12M-2-006); and Young Elite Scientists Sponsorship Program by CAST (YESS) (Grant No. 2019QNRC001).
Acknowledgments
ZS wishes to extend her deepest appreciation and thanks to her husband, Mr. Xing Le, who has always supported her career and research.
Conflict of interest
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.
Abbreviations
- α -SYN
α -synuclein
- A β
amyloid β -protein
- AD
Alzheimer’s disease
- ALIX
ALG-2-interacting protein X
- AMPA
α -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
- Apo-E
apolipoprotein E
- APP
amyloid precursor protein
- ARF6
ADP ribosylation factor 6
- BACE1
beta-secretase 1
- BBB
blood brain barrier
- BDNF
brain derived neurotrophic factor
- BDEs
brain derived exosomes
- BIN1
Bridging I Ntegrator 1
- cAMP
cyclic adenosine monophosphate
- CD
Cluster of differentiation
- CNS
central nervous system
- CREB1
cAMP responsive element binding protein 1
- CSF
cerebrospinal fluid
- CT
computed tomography
- ESCRT
the endosomal sorting complex required for transport
- ECG
Electrocardiography
- EE
early endosome
- EGFR
epidermal growth factor receptor
- EVs
extracellular vesicles
- gDNA
genomic DNA
- GAP43
growth associated protein 43
- HSP
heat shock protein
- IRS1
insulin receptor substrate 1
- Iba1
ionized calcium-binding adapter molecule 1
- IGF1
insulin-like growth factor 1
- IL-1 β
interleukin 1 beta
- ILVs
intraluminal vesicles
- LE
late endosome
- L1CAM
neuronal L1 cell adhesion molecule
- LRP6
LDL receptor related protein
- MCI
mild cognitive impairment
- miRNA
micro ribonucleic acid
- MMPs
matrix metalloproteinase regulators
- MRI
magnetic resonance imaging
- mRNA
messenger ribonucleic acid
- MS
multiple sclerosis
- MSCs
mesenchymal stromal cells
- MVBs
multivesicular bodies
- MVEs
multivesicular endosomes
- Neuro-2a
Murine neuroblastoma
- nSMase2
neutral sphingomyelinase-2
- NSCs
neural stem cells
- NTA
nanoparticle tracking analysis
- PD
Parkinson’s disease
- PrP
prion protein
- p-S 396-tau
Tau phosphorylation at serine 396
- p-T 181-tau
Tau phosphorylation at threonine 181
- qPCR real-time
quantitative polymerase chain reaction
- REST
re1 silencing transcription factor
- S1P
sphingosine-1-phosphate
- S1PR
S1P receptor
- siRNA
small interfering ribonucleic acid
- SMS2
sphingomyelin synthase 2
- sMVBs
secretory multivesicular bodies
- TGF β
transforming growth factor beta
- TSG101
tumor susceptibility gene 101 protein.
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Summary
Keywords
brain derived exosomes, Alzheimer’s disease, cell-to-cell communication, biomarker, extracellular vesicles
Citation
Song Z, Xu Y, Deng W, Zhang L, Zhu H, Yu P, Qu Y, Zhao W, Han Y and Qin C (2020) Brain Derived Exosomes Are a Double-Edged Sword in Alzheimer’s Disease. Front. Mol. Neurosci. 13:79. doi: 10.3389/fnmol.2020.00079
Received
17 March 2020
Accepted
22 April 2020
Published
29 May 2020
Volume
13 - 2020
Edited by
Michele Papa, University of Campania Luigi Vanvitelli, Italy
Reviewed by
Ursula Wyneken, University of the Andes, Chile, Chile; Minho Moon, Konyang University, South Korea; Esperanza González, CIC bioGUNE, Spain
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© 2020 Song, Xu, Deng, Zhang, Zhu, Yu, Qu, Zhao, Han and Qin.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Chuan Qin, qinchuan@pumc.edu.cn
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